JP7410437B2 - steel plate - Google Patents

Description

The present invention relates to steel plates, particularly high-strength steel plates to which high heat input welding is applied.

In recent years, the demands on steel frames for welded structures, such as high-rise buildings, have become more sophisticated from the viewpoints of larger buildings, higher construction efficiency, and improved safety against destruction during earthquakes (earthquake resistance). . Thick steel plates used for steel frames of welded structures are required not only to have high strength and thickness, but also to ensure toughness for high heat input welding HAZ. Note that "high heat input welding HAZ" means a weld heat affected zone (HAZ) formed by high heat input welding. Hereinafter, the high heat input welding HAZ may be simply referred to as the high heat input HAZ. High heat input welding is welding with large heat input, and examples include high-efficiency electroslag welding and submerged arc welding.

Conventionally, when applying the above-described large heat input welding to high-strength thick steel plates, it has been considered difficult to ensure good toughness in the HAZ. For example, the HAZ toughness of electroslag welds in thick steel plates with a tensile strength of 780 MPa class is shown in Non-Patent Document 1 and Non-Patent Document 2. According to FIG. 6 of Non-Patent Document 1, the average Charpy absorbed energy at the notch positions of the fusion line (FL), 1 mm from FL (HAZ1), 3 mm from FL (HAZ3), and 5 mm from FL (HAZ5) The value is 40J or less. Furthermore, according to FIGS. 3 and 5 of Non-Patent Document 2, the average value of Charpy absorbed energy at the notch position of the FL is 50 J or less.

To solve these problems, high heat input welding is achieved by applying two-phase region hardening treatment to the thick steel plate to reduce the yield ratio, and by distributing alloying elements such as Mn, Cu, and Ni at the boundary between ferrite and austenite. A thick steel plate having a tensile strength of 780 MPa with improved HAZ toughness has been proposed (see, for example, Patent Document 1). This technique increases the toughness of the HAZ by creating a concentration of alloying elements that improve hardenability, and by nucleating intragranular bainite in regions of the HAZ where the alloy concentration is low to refine the structure.

In addition, by using V carbonitride (V(C,N)), intragranular ferrite is generated in the HAZ and the structure is refined to improve the toughness of the HAZ, with a yield strength of 325 to 500 MPa. A thick steel plate has been proposed (see, for example, Patent Document 2). This technique reduces the N content and increases the C content, and utilizes MnS as a precipitation nucleus for precipitating V(C,N) in the high heat input HAZ.

Further, Patent Document 3 discloses a technique for improving the toughness of the HAZ by controlling the balance of the contents of Ti, Al, O, and N to refine crystal grains. This technology combines the effect of suppressing the growth of coarse austenite grains with fine TiN dispersed in the steel and the effect of promoting the precipitation of intragranular ferrite using Ti-containing inclusions as transformation nuclei. .

Patent Document 4 discloses a technology for suppressing the formation of a martensite-austenite constituent (MA) mixed phase (Martensite-Austenite constituent, MA) in the HAZ by dispersing fine C-enriched regions and improving HAZ toughness. There is. In Patent Document 4, the content of Si and P contained in the steel is reduced, and the conditions of accelerated cooling and heat treatment after hot rolling are controlled to suppress the formation of cementite and pearlite.

Japanese Patent Application Publication No. 2010-280976 Japanese Patent Application Publication No. 2007-327099 International Publication No. 2017/183720 Japanese Patent Application Publication No. 2017-155333

Kazushige TOKUNO et al, 780-N/mm2 Class High Tensile Strength Steel Plate with Large-Heat-Input-Weldability and Low-Weld-Cracking-Susceptibility for Architectural Construction, NIPPON STEEL THECHNICAL REPORT No.75 November 1997, p. 43～50 Minoru Hirota, and 5 others, “Low yield ratio 780 N/mm 2nd class steel material for architectural structures using online manufacturing process Part 3: Characteristics of high heat input welded joints”, Abstracts of Academic Lectures at the Architectural Institute of Japan Conference, 2012, No. 1017

In order to increase the strength of a steel plate, it is effective to increase the carbon equivalent CeqWES, which is an index of the hardenability of steel. However, when the content of alloying elements such as Mn is increased, the high heat input HAZ not only becomes a structure consisting mainly of coarse bainite, but also a martensite-austenite mixed phase (Martensite - Austenite constituent), which is a brittle phase. production of MA) is promoted. Since MA is a hard phase and serves as a starting point for fracture, the toughness of the HAZ produced by MA decreases.

Furthermore, since the high heat input HAZ is heated to a high temperature, austenite grain growth is promoted, the crystal grains of the steel become coarser, and the HAZ toughness decreases. As described above, when the carbon equivalent CeqWES is increased in order to strengthen the steel sheet, a coarse bainite-based structure generated by MA is formed in the high heat input HAZ, and the toughness tends to decrease.

As described above, in the case of thick steel plates containing Mn and Ni, which are alloying elements that increase strength, the toughness of the high heat input welding HAZ is significantly reduced due to the formation of MA and coarsening of prior austenite.
However, in the above-mentioned Patent Documents 1 to 3, although austenite grain growth can be suppressed, measures are not sufficient against other factors that reduce toughness.
In particular, when high heat input welding is performed on a steel plate with a thickness of 40 mm or more, the cooling rate of the weld is about 0.5° C./second or less, which is significantly different from the cooling of the weld with normal heat input. However, there have been no guidelines for component design assuming such welding conditions.
Further, in Patent Document 4, deterioration of HAZ toughness due to MA generation in the Mn segregation area is unavoidable, and it is difficult to stably secure HAZ toughness. Therefore, it has been difficult to simultaneously increase the strength of the steel plate (base material) and ensure the toughness of the high heat input HAZ based on the conventional guidelines for component design of thick steel plates.

The present invention was made in view of these circumstances, and proposes a new guideline for component design, and based on this, it is possible to simultaneously ensure the strength of the base metal and the toughness of the high heat input welding HAZ. The object of the present invention is to provide a steel plate (a high-strength steel plate for large heat input welding).

The present inventors have achieved both high strength of the steel plate (base material) and securing of the toughness of the high heat input HAZ from the viewpoint of suppressing the generation of MA that significantly embrittles the high heat input HAZ of high strength steel sheets. We conducted a study to make this possible. As a result, it was found that the generation of MA was caused by micro-segregation areas formed by local concentration of alloying elements such as Mn and Ni contained in the steel sheet. Specifically, it was found that when the micro-segregation area is heated by the effect of welding heat and cooled, part of the metal structure becomes MA due to phase transformation.
As a result of further study by the present inventors, it was found that setting Mn/Ni, which is the ratio of Mn content to Ni content in steel components (chemical composition), to 0.80 or less is effective in reducing MA in micro-segregation areas. We obtained the knowledge that it is effective for

Furthermore, in order to suppress the coarsening of crystal grains in the HAZ, it is effective to use Ti-based oxides that act as nuclei for intragranular transformation.

In this way, by suppressing the generation of MA using the method described above and using Ti-based oxides to refine the crystal grains, it is possible to achieve both the strength of the base material and the toughness of the high heat input HAZ. We obtained new knowledge that it is possible.

The present invention has been made based on such knowledge, and the gist thereof is as follows.

[ 1 ] The steel sheet according to one embodiment of the present invention has a chemical composition in mass %: C: 0.12% or more and 0.18% or less, Mn: 0.50% or more and 1.50% or less, Ni : 1.00% or more, 3.00% or less, Ti: 0.005% or more, 0.020% or less, O: 0.0010% or more, 0.0040% or less, B: 0.0003% or more, 0 .0030% or less , Cu: 0% or more, 2.0% or less, Cr: 0% or more, 1.0% or less, Mo: 0% or more, 1.00% or less, W: 0% or more, 1.00 % or less, Co: 0% or more, 1.0% or less, Nb: 0% or more, 0.100% or less, V: 0% or more, 0.10% or less, Ca: 0% or more, 0.0050% or less , REM: 0% or more, 0.0050% or less, Zr: 0% or more, 0.0050% or less, Si: 0.30% or less, P: 0.015% or less, S: 0.005% or less, N : 0.0010% or more, 0.0100% or less Al: Contains 0.0030% or less , the remainder consists of Fe and impurities, and the Mn/Ni content ratio of Mn and Ni is 0.80 or less. Yes, the carbon equivalent CeqWES calculated by the following formula (1) is 0.43% or more and 0.53% or less, the tensile strength is 780MPa or more and 930MPa or less, and the yield strength is 630MPa or more and 750MPa or less. The yield ratio is 85% or less, the plate thickness is 40 mm or more and 120 mm or less, and the Vickers hardness is measured at 225 points or more at a position 1/4 of the plate thickness from the surface. , Hvmin/Hvmax is 0.85 or less, where Hvmin is the average value of the values from the smallest to 20%, and Hvmax is the average value of the values from the largest to 20% .
CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)
Here, in formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents [mass%] of each element, and 0 is substituted for the terms of elements that are not contained.
[ 2 ] The steel plate according to [1] has the chemical composition, in mass%, Cu: 0.1% or more and 2.0% or less, Cr: 0.1% or more and 1.0% or less, Mo : 0.10% or more, 1.00% or less, W: 0.10% or more, 1.00% or less, Co: 0.1% or more, 1.0% or less, Nb: 0.005% or more, 0 .100% or less, V: 0.005% or more, 0.10% or less, Ca: 0.0001% or more, 0.0050% or less, REM: 0.0001% or more, 0.0050% or less, Zr: 0 It may contain at least one selected from the group consisting of 0.0001% or more and 0.0050% or less.
[ 3 ] The steel plate according to another aspect of the present invention has a chemical composition in mass%,
C: 0.030% or more, 0.080% or less,
Mn: 0.30% or more, 1.30% or less,
Ni: 1.30% or more, 7.00% or less,
Ti: 0.005% or more, 0.020% or less,
O: 0.0010% or more, 0.0040% or less,
B: 0% or more, 0.0050% or less,
Cu: 0.60% or more, 2.00% or less,
Cr: 0% or more, 1.0% or less,
Mo: 0% or more, 1.00% or less,
W: 0% or more, 1.00% or less,
Co: 0% or more, 1.0% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 0.10% or less,
Ca: 0% or more, 0.0050% or less,
REM: 0% or more, 0.0050% or less,
Zr: 0% or more, 0.0050% or less,
Si: 0.10% or less,
P: 0.010% or less,
S: 0.005% or less,
N: 0.0060% or less ,
Al: 0.0030% or less
Contains
The remainder consists of Fe and impurities,
Mn/Ni, which is the ratio of Mn and Ni content, is 0.80 or less,
The carbon equivalent CeqWES calculated by the following formula (1) is 0.45% or more and 0.70% or less,
CeqIIW calculated by the following formula (2) is 0.65% or more and 0.90% or less,
The tensile strength is 780 MPa or more and 930 MPa or less,
The yield strength is 630 MPa or more and 750 MPa or less,
The yield ratio is 85% or less,
The plate thickness is 40 mm or more and 120 mm or less,
Measure the Vickers hardness at 225 points or more at a position 1/4 of the plate thickness from the surface, and calculate the average value of the Vickers hardness values from the smallest to 20% as Hvmin, and the value from the largest to 20%. When the average value of is set as Hvmax, Hvmin/Hvmax is 0.85 or less.
CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)
CeqIIW (%) = C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5...(2)
Here, in formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents [mass%] of each element, and 0 is substituted for the terms of elements that are not contained.
Here, C, Mn, Cu, Ni, Cr, Mo, and V in the above formula (2) are the contents of each element in the steel sheet expressed in mass %, and 0 is substituted for the terms of elements that are not contained. do.
[ 4 ] The steel plate described in [ 3 ] may have a hardenability multiple DI of 10.0 inches or more and 21.0 inches or less, which is calculated by the following formula (3).
DI (inch)=0.5×fB×C ^0.5 ×(1+0.64×Si)×(1+4.1×Mn)×(1+0.27×Cu)×(1+0.52×Ni)×(1+2 .33×Cr)×(1+3.14×Mo)...(3)
Here, C, Si, Mn, Si, Cu, Ni, Cr, and Mo in the above formula (3) are the contents of each element in the steel sheet expressed in mass %, and the terms of elements not contained are Assign 0. fB is 1.0 when B is 0.0004% or less, and is 1.3 when B is over 0.0004%.
[ 5 ] The steel plate according to [ 3 ] or [ 4 ] has the chemical composition, in mass %, of Cr: 0.1% or more and 1.0% or less, Mo: 0.10% or more, and 1.00%. % or less, W: 0.10% or more, 1.00% or less, Co: 0.1% or more, 1.0% or less, Nb: 0.005% or more, 0.100% or less, V: 0.005 % or more, 0.10% or less, B: 0% or more, 0.0004% or less, Ca: 0.0001% or more, 0.0050% or less, REM: 0.0001% or more, 0.0050% or less, Zr : Any one or more selected from the group consisting of 0.0001% or more and 0.0050% or less may be contained.
[ 6 ] In the steel plate according to any one of [1] to [ 5 ], the chemical composition may contain P: 0.003% or more and 0.010% or less.
[ 7 ] In the steel plate according to any one of [1] to [ 6 ], the Hvmin and the Hvmax may satisfy the following formulas (8) and (9).
780≦0.25×Hvmin+1.07×Hvmax+387≦930...(8)
-0.00146×Hvmin+0.00246×Hvmax+0.659×Hvmin/Hvmax-0.163≦0.85...(9)
[ 8 ] The steel plate according to any one of [1] to [ 7 ] has a maximum Vickers hardness Hvs of 320 or less in a region up to 3 mm in the depth direction starting from the steel plate surface. good.
[ 9 ] The steel plate according to any one of [1] to [ 8 ] has a maximum Vickers hardness Hvs in a region up to 3 mm in the depth direction starting from the steel plate surface and a steel plate at a 1/4 thickness position. The difference ΔHv from the Vickers hardness Hvq may be 70 or less. However, ΔHv=Hvs−Hvq.
[ 10 ] The steel plate described in any one of [1] to [ 9 ] has a Charpy absorbed energy at 0°C in a simulated HAZ when subjected to a welding heat cycle corresponding to a heat input of 60 to 150 kJ/mm. may be 100 J or more on average.

According to the present invention, there is provided a steel plate (high-strength steel plate for high heat input welding) that is capable of ensuring both the strength of the base material and the toughness of the high heat input welding HAZ based on new component design guidelines. I can do it.

It is a figure which shows the test piece shape of a thermal cycle test.

[First embodiment]
Hereinafter, a high-strength steel plate for large heat input welding (hereinafter sometimes simply referred to as a steel plate) according to a first embodiment of the present invention will be described. First, the results of the studies conducted by the inventors that led to the completion of the present invention and the new knowledge obtained will be described in detail.

The steel plate according to the first embodiment contains C, Mn, and Ni, which are alloying elements that improve hardenability. The steel plate according to the first embodiment is manufactured by hot rolling a steel piece obtained by melting and casting steel. The steel sheet manufactured in this manner has micro-segregation portions formed at the interface of the solidified structure due to solidification during casting. The concentration of alloying elements such as Mn and Ni in this micro-segregation area is difficult to eliminate by short-term heating such as the thermal effect of welding. Therefore, when high heat input welding is applied to a steel plate containing C, Mn, and Ni, the micro-segregation part of the HAZ becomes retained austenite with concentrated C due to heating, and becomes hard MA after cooling. Since such MA becomes a starting point for fracture and reduces HAZ toughness, in order to improve HAZ toughness, it is desirable to retain stable austenite, in other words, to suppress the formation of retained austenite. As a result of intensive studies, the present inventors have obtained a new finding that, compared to Ni, Mn delays the decomposition of retained austenite during cooling of a high heat input HAZ.

As described above, when a high heat input HAZ is cooled, if the retained austenite in the micro-segregation part is cooled to room temperature without being decomposed, this retained austenite becomes MA and the toughness of the HAZ deteriorates. Since Mn delays the decomposition of retained austenite compared to Ni, it is considered that Mn tends to cause an increase in MA. In other words, Ni is considered to have a smaller adverse effect on the toughness of the high heat input HAZ than Mn. Therefore, the present inventors focused on the balance between the Mn content and the Ni content in the steel, and by optimizing the ratio of the two, suppressed the amount of MA generated while increasing the hardenability of the steel. I thought it could be done. Specifically, the present inventors found that when Mn/Ni, which is the ratio of the Mn content divided by the Ni content in steel, becomes 0.80 or less, the amount of MA produced in the high heat input HAZ increases. We found a phenomenon that reduces This phenomenon is due to the effect of Mn atoms on the distribution behavior of C atoms at the interface of different phases when retained austenite, which is generated by concentration of C in micro-segregation areas during heating, is decomposed, that is, when retained austenite is transformed into ferrite and cementite. It is presumed that this is due to the difference in the influence between the Ni atoms and the Ni atoms.

In addition, the present inventors have found that when heated by high heat input welding, the higher the content of C that concentrates in the micro-segregation zone, the more the decomposition of retained austenite during cooling is promoted. , found that the production of MA was suppressed. The phenomenon in which the MA of the HAZ decreases as the C content in the steel increases is presumed to be due to the fact that C increases the driving force for producing cementite from retained austenite. As a result of further studies, the present inventors found that when Mn/Ni is set to 0.80 or less in the steel composition, the decomposition of retained austenite during cooling is promoted in a high heat input HAZ, and It was found that the production of MA could be further suppressed.

In addition, in high-strength steel plates for high heat input welding, coarsening of crystal grains causes deterioration of toughness of the high heat input HAZ. One effective method for suppressing the coarsening of HAZ crystal grains is the use of intragranular transformation using Ti-based oxides as production nuclei. The present inventors investigated the relationship between metal structure and toughness for steel subjected to a simulated thermal cycle simulating high heat input welding. As a result, by promoting intragranular transformation with Ti-based oxides dispersed in steel, grains are made finer, and by suppressing the formation of MA by setting Mn/Ni to 0.80 or less, large amounts of It has been found that the toughness of thermal HAZ can be significantly improved.

In this way, the present inventors dispersed Ti oxide by making the steel sheet contain 0.005% or more of Ti, thereby aiming to refine the crystal grains of the high heat input HAZ. Then, in order to make the crystal grains finer by using Ti oxide, the content of Al is limited. This is because as the Al content in the steel sheet increases, O in the steel sheet becomes more likely to be consumed to generate Al-based oxides, and the generation of Ti-based oxides is suppressed. Therefore, the amount of Al contained in the steel plate according to this embodiment is limited to 0.003% or less. On the other hand, by limiting the Ti content to 0.020% or less, generation of micron-sized coarse TiN can be avoided. Further, the content of O is limited to 0.0040% or less to prevent formation of coarse inclusions that would affect HAZ toughness.

Furthermore, in the steel plate according to the present embodiment, in order to suppress deterioration of toughness in the high heat input HAZ, the upper limit of the carbon equivalent CeqWES is limited, and the contents of C, Mn, Si, Ni, Cr, Mo, and V are set to a predetermined value. The range shall be . As a result of studies conducted by the present inventors, it has been found that the toughness of the high heat input HAZ can be ensured by limiting the carbon equivalent CeqWES to 0.70% or less. When CeqWES exceeds 0.70%, MA is generated and HAZ toughness is significantly deteriorated. The carbon equivalent CeqWES can be determined by the following formula (1) depending on the content of alloying elements.

CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14... (1)

Here, in formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents [mass %] of each element in the steel sheet, and 0 is substituted for the terms of elements that are not contained.

The chemical composition (steel composition) of the steel plate according to this embodiment will be explained below. In the description of each chemical composition below, mass % is simply expressed as %.
The steel plate according to the first embodiment has a chemical composition in mass%,
C: 0.03% or more, 0.18% or less,
Mn: 0.30% or more, 1.50% or less,
Ni: 1.00% or more, 7.00% or less,
Ti: 0.005% or more, 0.020% or less,
O: 0.0010% or more, 0.0040% or less,
B: 0% or more, 0.0050% or less,
Cu: 0% or more, 2.0% or less,
Cr: 0% or more, 1.0% or less,
Mo: 0% or more, 1.00% or less,
W: 0% or more, 1.00% or less,
Co: 0% or more, 1.0% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 0.10% or less,
Ca: 0% or more, 0.0050% or less,
REM: 0% or more, 0.0050% or less,
Zr: 0% or more, 0.0050% or less Si: 0.30% or less,
P: 0.015% or less,
S: 0.005% or less,
N: 0.0100% or less,
Contains Al: 0.0030% or less,
The remainder consists of Fe and impurities,
Mn/Ni, which is the ratio of Mn and Ni content, is 0.80 or less,
The carbon equivalent CeqWES calculated by the following formula (1) is 0.43% or more and 0.70% or less,
The tensile strength is 780 MPa or more and 930 MPa or less,
The yield strength is 630 MPa or more and 750 MPa or less,
The yield ratio is 85% or less,
The plate thickness is 40 mm or more and 120 mm or less,
Measure the Vickers hardness at 225 points or more at a position 1/4 of the plate thickness from the surface, and calculate the average value of the Vickers hardness values from the smallest to 20% as Hvmin, and the value from the largest to 20%. When the average value of is set as Hvmax, Hvmin/Hvmax is 0.85 or less.
CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)
Here, in formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents [mass%] of each element, and 0 is substituted for the terms of elements that are not contained.

(C: 0.03% or more, 0.18% or less)
C is an element that improves the hardenability of steel and contributes to high strength. In this embodiment, the C content is 0.03% or more. The content of C is preferably 0.035% or more, more preferably 0.04% or more. On the other hand, from the viewpoint of preventing excessive production of cementite and ensuring toughness, in this embodiment, the C content is 0.18% or less. The content of C is preferably 0.17% or less, more preferably 0.16% or less.

(Mn: 0.30% or more, 1.50% or less)
Mn is an element that improves the hardenability of steel and contributes to high strength, and in this embodiment, the Mn content is 0.30% or more. The Mn content is preferably 0.40% or more, more preferably 0.50% or more. On the other hand, in this embodiment, the Mn content is 1.50% or less from the viewpoint of suppressing the generation of MA in the high heat input HAZ and ensuring toughness. The Mn content is preferably 1.40% or less, more preferably 1.30% or less, and even more preferably 1.20% or less.

(Ni: 1.00% or more, 7.00% or less)
Ni is an element that improves the hardenability of steel and contributes to high strength, and at the same time, it is an element that increases the toughness of a high heat input HAZ. From the viewpoint of ensuring strength and toughness, in this embodiment, the Ni content is 1.00% or more. The Ni content is preferably 1.20% or more, more preferably 1.40% or more, and still more preferably 1.50% or more. On the other hand, Ni is an expensive element, and from the viewpoint of suppressing an increase in manufacturing costs, in this embodiment, the Ni content is 7.00% or less. The Ni content is preferably 6.50% or less, more preferably 6.00% or less, and still more preferably 5.50% or less.

(Mn/Ni: 0.80 or less)
Both Mn and Ni are elements that contribute to high strength of steel, but in a high heat input HAZ, Mn promotes the formation of MA more easily than Ni, so the Mn content is higher than the Ni content. It is also preferable that the amount is also small. From the viewpoint of ensuring toughness while increasing the strength of the large heat input HAZ, in the steel plate of this embodiment, the Mn/Ni ratio, which is the ratio of the Mn content divided by the Ni content in the steel, is 0. 80 or less. Mn/Ni is preferably 0.70 or less, more preferably 0.60 or less. Note that the lower limit of Mn/Ni may be a ratio obtained by dividing the lower limit of the Mn content by the upper limit of the Ni content, that is, it may be 0.17 or more. Mn/Ni may be 0.20 or more.

(B: 0% or more, 0.0050% or less)
B is an element that may be mixed into the steel plate as an impurity from scrap or the like. However, the lower limit of the content of B is not limited and may be 0%. B is an element that significantly improves the hardenability of steel, and even a small amount significantly improves the hardenability of steel, so the content of B is preferably 0.0003% or more. On the other hand, from the viewpoint of suppressing deterioration of toughness and weldability of the high heat input HAZ, in this embodiment, the content of B is 0.0050% or less. The content of B is preferably 0.0030% or less, more preferably 0.0020% or less. However, from the viewpoint of suppressing an increase in hardness of the surface layer of the steel sheet and preventing deterioration of surface properties and workability, the content of B may be 0.0004% or less.

(Ti: 0.005% or more, 0.020% or less)
Ti is an element that forms Ti oxide and TiN. TiN suppresses the coarsening of γ grains due to its pinning effect, and Ti oxides serve as intragranular transformation nuclei and contribute to the refinement of crystal grains in the HAZ. Further, since Ti forms TiN and suppresses the generation of BN, it is also effective in securing solid solution B that improves hardenability. In order to ensure the toughness of the large heat input HAZ, in this embodiment, the Ti content is 0.005% or more. The Ti content is preferably 0.007% or more. On the other hand, in this embodiment, the Ti content is 0.020% or less from the viewpoint of suppressing deterioration of the toughness of the base material and HAZ and deterioration of the surface quality of the slab. The Ti content is preferably 0.018% or less, more preferably 0.016% or less.

(N: 0.0100% or less)
The N content is 0.0100% or less from the viewpoint of suppressing the formation of BN to improve hardenability and suppressing deterioration of HAZ toughness due to nitrides. The N content is preferably 0.0080% or less, more preferably 0.0060% or less.

(O: 0.0010% or more, 0.0040% or less)
O is an element that combines with a deoxidizing element such as Ti to form an oxide. Ti oxide acts as an intragranular transformation nucleus and contributes to refinement of crystal grains. In order to obtain this effect, in the steel plate of this embodiment, the content of O is 0.0010% or more. However, from the viewpoint of suppressing the deterioration of the toughness of the base material and HAZ due to a decrease in the cleanliness of the steel, the content of O is 0.0040% or less. The O content is preferably 0.0030% or less.

(Al: 0.0030% or less)
Al is an element that forms oxides and is used for deoxidation. However, as the Al content increases, the generation of Ti-based oxides is suppressed. Therefore, the Al content is 0.0030% or less in this embodiment from the viewpoint of promoting the generation of Ti-based oxides. The Al content is preferably 0.0020% or less, more preferably 0.0010% or less. As mentioned above, Al is a deoxidizing element, but it can be deoxidized by Si, Mn, and Ti, and the Al content may be 0%.

(Si: 0.30% or less)
Si is an element contained in steel for deoxidation and high strength. On the other hand, Si is also an element that promotes the production of MA, and the present inventors have obtained the knowledge that Si has an extremely large effect on the production of MA in the micro-segregation part of a high heat input HAZ. Therefore, in order to ensure the toughness of the high heat input HAZ, it is necessary to limit the Si content, and in this embodiment, the Si content is 0.30% or less. The content of Si is preferably 0.25% or less, more preferably 0.20% or less, and still more preferably 0.15% or less. Although the lower limit of the Si content is not limited, from the viewpoint of manufacturing cost, the Si content may be 0.01% or more.

(P: 0.015% or less)
P is an impurity that is harmful to toughness. The content of P needs to be limited in order to stably ensure the toughness of the high heat input HAZ, and in this embodiment, it is 0.015% or less. The P content is preferably 0.010% or less. Although the lower limit of the P content is not limited, from the viewpoint of manufacturing cost, the P content may be 0.001% or more. Further, although P is an impurity harmful to toughness, it has the effect of improving the hardenability of the high heat input HAZ, making the crystal grain size finer, and improving the toughness of the high heat input HAZ. From the viewpoint of obtaining this effect, the P content may be set to 0.003% or more.

(S: 0.005% or less)
S is an impurity, and if contained in large amounts in steel, it may form coarse inclusions and reduce toughness. Therefore, the content of S needs to be limited in order to stably ensure the toughness of the high heat input HAZ, and in this embodiment, S is 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less. Although the lower limit of the amount of S is not limited, from the viewpoint of manufacturing cost, the content of S may be 0.0001% or more. The content of S may be 0.001% or more.

(Carbon equivalent CeqWES: 0.43% or more, 0.70% or less)
The carbon equivalent CeqWES is an index of hardenability that affects the strength of the steel plate (base material) and the hardness of the HAZ. In order to ensure the strength of the base material, in this embodiment, the carbon equivalent CeqWES is 0.43% or more. The carbon equivalent CeqWES is preferably 0.44% or more, more preferably 0.45% or more. On the other hand, from the viewpoint of ensuring high heat input toughness, the carbon equivalent CeqWES is 0.70% or less in this embodiment. The carbon equivalent CeqWES is preferably 0.65% or less.
Note that the carbon equivalent CeqWES is calculated by the following formula (1) based on the content of alloying elements.

CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)

Here, in formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents [mass%] of each element, and 0 is substituted for the terms of elements that are not contained.

The remainder of the chemical composition of the steel plate according to this embodiment is iron (Fe) and impurities. Impurities are components that are mixed in from raw materials such as ores and scraps and other factors during the industrial production of steel sheets, and are allowed within the range that does not adversely affect the steel sheet according to this embodiment. means. However, among the impurities, the upper limit of the content of P and S is limited as described above.

In order to improve the strength and toughness of the steel plate (base material), the steel plate according to this embodiment may include one of the following selected elements Cu, Cr, Mo, W, Co, Nb, and V, as necessary. Two or more types may be contained.

(Cu: 0% or more, 2.0% or less)
Cu is an element that may be mixed into steel sheets as an impurity from scraps and the like. However, the lower limit of the Cu content is not limited and may be 0%. Further, Cu has a small negative effect on weldability and HAZ toughness, and is also an element that improves the strength and toughness of the base metal. Therefore, in this embodiment, the content of Cu may be 0.1% or more. However, from the viewpoint of suppressing the occurrence of Cu cracks during hot rolling of the steel sheet, in this embodiment, the Cu content is 2.0% or less. The Cu content is preferably 1.0% or less, more preferably 0.7% or less, and still more preferably 0.5% or less.

(Cr: 0% or more, 1.0% or less)
Cr is an element that may be mixed into steel sheets as an impurity from scraps and the like. However, the lower limit of the Cr content is not limited and may be 0%. Cr is also an element that improves the strength of the base material. Therefore, in this embodiment, the Cr content may be 0.1% or more. The content of Cr is preferably 0.2% or more, more preferably 0.3% or more. However, from the viewpoint of suppressing deterioration of toughness and weldability of the high heat input HAZ, in this embodiment, the Cr content is 1.0% or less.The Cr content is preferably 0.8% or less. Yes, and more preferably 0.5% or less.

(Mo: 0% or more, 1.00% or less)
Mo is an element that may be mixed into steel sheets as an impurity from scraps and the like. However, the lower limit of the Mo content is not limited and may be 0%. Moreover, Mo is also an element that improves the strength and toughness of the base material. Therefore, in this embodiment, the content of Mo may be 0.10% or more. The Mo content is preferably 0.20% or more, more preferably 0.30% or more. However, in this embodiment, the Mo content is 1.00% or less from the viewpoint of suppressing deterioration of toughness and weldability of the high heat input HAZ and suppressing increase in alloy cost. The Mo content is preferably 0.50% or less.

(W: 0% or more, 1.00% or less)
W is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the W content is not limited and may be 0%. Further, W is also an element that increases the strength and toughness of the base material. Therefore, in this embodiment, the content of W may be 0.10% or more. The content of W is preferably 0.20% or more, more preferably 0.30% or more. However, in this embodiment, the W content is 1.00% or less from the viewpoint of suppressing deterioration of toughness and weldability of the high heat input HAZ and suppressing increase in alloy cost. The content of W is preferably 0.50% or less.

(Co: 0% or more, 1.0% or less)
Co is an element that may be mixed into steel sheets as an impurity from scraps and the like. However, the lower limit of the Co content is not limited and may be 0%. Further, Co has a small adverse effect on weldability and HAZ toughness, and is also an element that improves the strength and toughness of the base metal. Therefore, in this embodiment, the Co content may be 0.1% or more. However, from the viewpoint of suppressing an increase in alloy cost, in this embodiment, the Co content is 1.0% or less. The Co content is preferably 0.5% or less.

(Nb: 0% or more, 0.100% or less)
Nb is an element that may be mixed into steel sheets as an impurity from scraps and the like. However, the lower limit of the Nb content is not limited and may be 0%. Further, Nb is also an element that improves the strength and toughness of the base material. Therefore, in this embodiment, the Nb content may be 0.005% or more. However, from the viewpoint of suppressing deterioration of toughness and weldability of the high heat input HAZ, the Nb content is 0.100% or less. The Nb content is preferably 0.050% or less, more preferably 0.030% or less.

(V: 0% or more, 0.10% or less)
V is an element that may be mixed into the steel sheet as an impurity from scrap or the like. However, the lower limit of the V content is not limited and may be 0%. Further, V is also an element that improves the strength of the base material. Therefore, in this embodiment, the content of V may be 0.005% or more. However, from the viewpoint of suppressing deterioration of toughness and weldability of the high heat input HAZ and suppressing increase in alloy cost, the content of V is 0.10% or less. The V content is preferably 0.08% or less, more preferably 0.06% or less.

Further, the steel plate according to the present embodiment may contain one or more of the following selective elements Ca, REM, and Zr, as necessary, in order to control the form of inclusions.

(Ca: 0% or more, 0.0050% or less)
Ca is an element that forms oxides, sulfides, and oxysulfides, suppresses the formation of coarse inclusions, and increases the toughness of the base material and HAZ. Therefore, in this embodiment, the content of Ca may be 0.00010% or more. Preferably it is 0.0010% or more. However, in this embodiment, the Ca content is 0.0050% or less from the viewpoint of suppressing an increase in Ca-based inclusions that may act as a starting point for brittle fracture. The Ca content is preferably 0.0040% or less. Note that the content of Ca may be 0%.

(REM: 0% or more, 0.0050% or less)
REM (rare earth elements) is a general term for two elements, Sc and Y, and 15 lanthanoid elements, such as La, Ce, and Nd. REM in this embodiment is composed of one or more selected from these rare earth elements, and the content of REM described below is the total content of rare earth elements.

Like Ca, REM is an element that forms oxides, sulfides, and oxysulfides to suppress the formation of coarse inclusions and improve the toughness of the base material and HAZ. Therefore, in this embodiment, the content of REM may be 0.0001% or more. However, in this embodiment, the REM content is 0.0050% or less from the viewpoint of suppressing the increase in REM-based inclusions that may act as a starting point for brittle fracture. The content of REM is preferably 0.0030% or less. Note that the content of REM may be 0%.

(Zr: 0% or more, 0.0050% or less)
Like Ca and REM, Zr is an element that forms oxides, sulfides, and oxysulfides to suppress the formation of coarse inclusions and improve the toughness of the base material and HAZ. Therefore, the content of Zr may be 0.0001% or more. However, in this embodiment, the Zr content is 0.0050% or less from the viewpoint of suppressing an increase in Zr-based inclusions that may act as a starting point for brittle fracture. The Zr content is preferably 0.0030% or less. Note that the Zr content may be 0%.

The steel plate according to this embodiment is suitable for applications requiring a thick steel plate with high strength. The steel plate according to the present embodiment is particularly suitable for applications where high heat input welding with high welding efficiency is performed and where the required level of HAZ toughness is high. Specifically, the steel plate according to this embodiment is suitable for high-strength thick steel plates that are subjected to diaphragm welding (electroslag welding) and require HAZ toughness, such as four-sided box columns for architectural steel frames.

(Plate thickness: 40mm or more, 120mm or less)
(Tensile strength: 780 MPa or more, 930 MPa or less)
(Yield strength: 630 MPa or more, 750 MPa or less)
(Yield ratio: 85% or less)
BACKGROUND OF THE INVENTION As buildings become larger, construction becomes more efficient, and safety is required, demands for thick steel plates for welded structures are becoming more sophisticated. Therefore, in the steel plate according to this embodiment, from the viewpoint of strength, the plate thickness is 40 mm or more and 120 mm or less, the tensile strength is 780 MPa or more and 930 MPa or less, and the yield strength is 630 MPa or more and 750 MPa or less. Further, from the viewpoint of earthquake resistance, the yield ratio of the steel plate according to this embodiment is 85% or less. The lower limit of the yield ratio is not limited, and for example, the yield ratio may be 70% or more. Furthermore, from the viewpoint of high construction efficiency and seismic resistance, it is preferable that the average value of Charpy absorbed energy (test temperature 0° C.) in the HAZ of the high heat input weld zone is 70 J or more. Note that high heat input welding includes, for example, electroslag welding and submerged arc welding.
More preferably, the average Charpy absorbed energy at 0° C. in the HAZ for high heat input welding with a heat input of 60 to 150 kJ/mm is 100 J or more.

The Charpy absorbed energy of the HAZ when welding is performed with a certain heat input can be evaluated by a thermal cycle test that provides a thermal history corresponding to the heat input.

In addition, in the steel plate according to this embodiment, the Vickers hardness is measured at 225 points or more at a position 1/4 of the plate thickness from the surface, and the average value of the values from the smallest to 20% of the Vickers hardness is determined. Hvmin/Hvmax is 0.85 or less, where Hvmin is the average value of 20% of the largest values.
When Hvmin/Hvmax is 0.85 or less, it becomes easier to satisfy the yield ratio of 85% or less. On the other hand, if Hvmin/Hvmax exceeds 0.85, it becomes difficult to satisfy the yield ratio of 85% or less, which is not preferable.

Hvmin and Hvmax are obtained as follows.
The L cross section (parallel to the rolling direction, thickness side) of the steel plate is mechanically polished, and a 15 x 15 lattice pattern is formed at 30 μm intervals from the surface in the thickness direction, centered at 1/4 of the thickness of the steel plate. The Vickers hardness (measuring load: 10 gf) in accordance with JIS Z 2244:2009 was measured for a total of 225 points.
Hvmin is calculated by arranging the obtained Vickers hardness values in descending order, and calculating the hardness values of measurement points from the smallest to 20% of the total number of measurement points (for example, if 500 points are measured, in order from the smallest to the smallest) It is obtained by averaging the 1st to 100th Vickers hardness).
Moreover, Hvmax is obtained by averaging the hardness values of measurement points from the largest to 20% of the total number of measurement points.

In the steel plate according to the present embodiment, it is preferable that Hvmin and Hvmax satisfy the following expressions (8) and (9).
780≦0.25×Hvmin+1.07×Hvmax+387≦930...(8)
-0.00146×Hvmin+0.00246×Hvmax+0.659×Hvmin/Hvmax-0.163≦0.85...(9)
Note that the fact that Hvmin and Hvmax satisfy the above relationship indicates that the metal structure is composed of martensite (or tempered martensite) and bainite.

The maximum value of Vickers hardness Hvs is 320 or less in a region up to 3 mm in the depth direction starting from the steel plate surface. It is preferable that the maximum value of Vickers hardness Hvs is 320 or less. This is because if the maximum Vickers hardness Hvs of the surface layer region exceeds 320, cracks are likely to occur when bending stress or tensile stress is applied.

The difference ΔHv between the maximum value Hvs of Vickers hardness in a region up to 3 mm in the depth direction from the steel plate surface and the Vickers hardness Hvq at the 1/4 thickness position of the steel plate is 70 or less. It is preferable that the difference ΔHv between the maximum hardness Hvs and the Vickers hardness Hvq at the 1/4 thickness position of the steel plate is 70 or less. If ΔHv exceeds 70, cracks may easily occur when bending stress or tensile stress is applied.

The hardness Hvs of the surface layer region is determined by mechanically polishing the L cross section (parallel to the rolling direction, thickness side) of the steel plate, and applying Vickers hardness according to JIS Z 2244:2009 at a position within 3 mm from the steel plate surface in the thickness direction. Measure the strength (measuring load: 10 kgf) at three points, and find the average value.
Vickers hardness Hvq at a position 1/4 of the plate thickness from the surface is determined by mechanically polishing the L cross section (parallel to the rolling direction, plate thickness side) of the steel plate, and measuring 1/4 of the plate thickness from the surface in the plate thickness direction. At the /4 position, the Vickers hardness (measuring load 10 kgf) based on JIS Z 2244:2009 was measured at three points, and the average value was taken.
The hardness difference ΔHv is calculated by the following equation (7) from Hvs and Hvq obtained by the above method.
ΔHv = Hvs - Hvq (7)

Next, a method for manufacturing a steel plate according to the first embodiment will be described.

The steel plate according to this embodiment is manufactured by melting steel, casting it to produce a steel billet, and hot rolling the obtained steel billet. The manufacturing method of the steel slab is not limited, and any known method may be used. For example, a steel billet is melted using a normal refining process such as a converter or an electric furnace, and then manufactured using a method such as a continuous casting method or an ingot-blowing method. After being hot-rolled, the steel billet may be directly subjected to controlled cooling such as water cooling, or may be air-cooled and then heat treated. Further, the steel billet may be manufactured by melting and casting steel, and then hot-rolled as it is. However, as will be described later, the steel billet is preferably cooled after casting, reheated to a temperature of Ac3 or higher, and hot rolled.

Preferred manufacturing conditions for the steel plate according to this embodiment will be described below.

A steel piece having a thickness of 200 mm or more and made of the above-mentioned chemical components and manufactured by a continuous casting method is once cooled to 400° C. or less. Thereafter, the steel slab is heated to a temperature range of 900° C. or higher and 1250° C. or lower, and hot rolled to produce a steel plate having a thickness of 50 mm or higher and 100 mm or lower. The steel plate is subjected to various heat treatments as necessary.

If a steel billet after continuous casting is charged into a heating furnace with a hot charge without being cooled to below 400°C, the coarse γ structure generated during casting will remain in the steel billet after heating, and the structure of the steel plate will deteriorate. may not be sufficiently refined and low-temperature toughness may deteriorate. Therefore, it is preferable that the steel billet after continuous casting is once cooled to 400° C. or lower.

The heating temperature of the slab is preferably 900° C. or higher in order to solutionize the carbides and nitrides precipitated in the steel slab after casting and promote the formation of TiN during hot rolling. In particular, when B is included, N in the heated steel billet forms TiN during hot rolling, and the generation of BN is suppressed. As a result, in the steel plate, a sufficient amount of solid solution B, which improves the hardenability of the steel, and TiN, which suppresses grain growth, are ensured. On the other hand, the heating temperature of the steel billet should be 1250°C or less from the viewpoint of suppressing coarsening of γ grains, refining the metal structure after hot rolling, and suppressing deterioration of low-temperature toughness. preferable. The heating temperature is more preferably 1200°C or lower.
In addition, when quenching is performed directly after hot rolling, the end temperature of hot rolling (finishing temperature) is preferably in the austenite (γ) single phase region, that is, the Ar3 transformation point at which ferrite transformation starts or higher. At this time, even if the temperature at the surface layer of the steel plate is in the austenite (γ)/ferrite (α) two-phase region at the end of hot rolling, if the temperature at the center in the thickness direction is in the γ single-phase region, there is no problem. do not have. The finishing temperature of hot rolling may be 750°C or higher. The finishing temperature of hot rolling is preferably 900° C. or lower from the viewpoint of refining the metal structure. The Ar3 transformation point (°C) can be determined by the following equation (4).

Ar3 transformation point = 868-396×C+24.6×Si-68.1×Mn-36.1×Ni-20.7×Cu-24.8×Cr+29.1×Mo...(4)

Here, C, Si, Mn, Ni, Cu, Cr, and Mo in the above formula (4) are the contents of each element in the steel sheet expressed in mass %, and 0 is substituted for the terms of elements that are not contained. do.

Furthermore, in the case of direct quenching after hot rolling, the hot rolling is finished in the γ single phase region, and water cooling is subsequently performed to adjust the material quality of the steel sheet. On the other hand, when the steel sheet is air cooled after hot rolling, the steel sheet is reheated to a γ single phase region and subsequently quenched (γ reheat quenching).

After hot rolling, a steel plate that has been subjected to direct quenching or gamma reheating quenching is subjected to various heat treatments in order to adjust the material quality. Specifically, steel sheets that have been subjected to these quenching treatments (direct quenching or γ reheat quenching) undergo hardening into a two-phase region where austenite (γ) and ferrite (α) coexist in order to lower the yield ratio. reheating followed by quenching (two-phase region quenching). Here, the two-phase region is a region above the Ac1 transformation point and below the Ac3 transformation point, and the Ac1 transformation point and the Ac3 transformation point can be determined by the following equations (5) and (6), respectively. The two-phase heat treatment temperature is preferably 720 to 810°C.

Ac1 transformation point = 750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23.0Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B... (5)
Ac3 transformation point = 910-203√C+44.7Si-30Mn-400Al-15.2Ni+104V+31.5Mo+13.1W+11Cr+20Cu-700P-400Ti...(6)

Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, B, W, and P in the above formulas (5) and (6) are each expressed in mass%. It is the content of the element in the steel plate, and 0 is substituted for the term of the element not contained.

Further, in order to finally adjust the strength, yield ratio, and toughness of the steel plate, the steel plate is subjected to tempering. The tempering conditions may be appropriately selected depending on the target strength, yield ratio, and toughness (for example, 320 to 600°C), but when Hvmin, Hvmax, Hvs, ΔHv, etc. are in the above-mentioned preferred ranges, the tempering temperature is , preferably 350°C or higher and 600°C or lower.

Here, the above-mentioned hot rolling finishing temperature, γ reheating quenching temperature, two-phase region quenching temperature, and tempering temperature all refer to the temperature at the center in the thickness direction. The temperature at the center in the plate thickness direction can be determined by heat transfer calculation from the temperature of the steel plate surface measured with a radiation thermometer.

The steel plate according to the present embodiment can be manufactured by the above manufacturing method (a manufacturing method including direct quenching or γ reheat quenching + two-phase region quenching + tempering).

The steel plate according to the present embodiment maintains good HAZ toughness even when subjected to large heat input welding such as electroslag welding or submerged arc welding in which the welding heat input exceeds 60 kJ/mm.

Therefore, the steel plate according to this embodiment is suitable for building steel frames, and the steel plate according to this embodiment can promote the progress of buildings becoming taller and with larger spans, and further improves construction efficiency and seismic safety. You can improve your performance.

[Second embodiment]
Next, a high-strength steel plate for large heat input welding (hereinafter also simply referred to as "steel plate") according to a second embodiment of the present invention will be described. Note that descriptions of parts that overlap with the first embodiment will be omitted.

The steel plate according to the second embodiment contains C, Mn, and Ni, which are alloying elements that improve hardenability. The steel plate according to the second embodiment is manufactured by hot rolling a steel piece obtained by melting and casting steel. The steel sheet manufactured in this manner has micro-segregation portions formed at the interface of the solidified structure due to solidification during casting. The concentration of alloying elements such as Mn and Ni in this micro-segregation area is difficult to eliminate by short-term heating such as the thermal effect of welding. Therefore, when high heat input welding is applied to a steel plate containing C, Mn, and Ni, the micro-segregation part of the HAZ becomes retained austenite with concentrated C due to heating, and becomes hard MA after cooling. Since such MA becomes a starting point of fracture and reduces HAZ toughness, it is desirable to suppress the remaining of stable austenite, in other words, the generation of retained austenite. As a result of intensive studies, the present inventors have obtained a new finding that, compared to Ni, Mn delays the decomposition of retained austenite during cooling of a high heat input HAZ.

As mentioned above, when the high heat input HAZ is cooled, if the retained austenite in the micro-segregation part is not decomposed and the high heat input HAZ is cooled to room temperature, this retained austenite becomes MA and deteriorates the toughness of the HAZ. let Since Mn delays the decomposition of retained austenite compared to Ni, it is considered that Mn tends to cause an increase in MA. In other words, Ni is considered to have a smaller adverse effect on the toughness of the high heat input HAZ than Mn. Therefore, the present inventors focused on the balance between the Mn content and the Ni content in the steel, and by optimizing the ratio of the two, suppressed the amount of MA generated while increasing the hardenability of the steel. I thought it could be done. Specifically, the present inventors found that when Mn/Ni, which is the ratio of the Mn content divided by the Ni content in steel, becomes 0.80 or less, the amount of MA produced in the high heat input HAZ increases. We found a phenomenon that reduces This phenomenon is due to the effect of Mn atoms on the distribution behavior of C atoms at the interface of different phases when retained austenite, which is generated by concentration of C in micro-segregation areas during heating, is decomposed, that is, when retained austenite is transformed into ferrite and cementite. It is presumed that this is due to the difference in the influence between the Ni atoms and the Ni atoms.

Furthermore, the present inventors found that when heated by high heat input welding, as the content of C that concentrates in the micro-segregation zone increases, the decomposition of retained austenite during cooling is accelerated, and the high heat input HAZ It was found that the production of MA was suppressed. The phenomenon in which the MA of the HAZ decreases as the C content in the steel increases is presumed to be due to the fact that C increases the driving force for producing cementite from retained austenite. As a result of further studies, the present inventors found that in the steel composition, if Mn/Ni is limited to 0.80 or less and the C content is increased to 0.12% or more, in a high heat input HAZ, It was found that the decomposition of retained austenite during cooling was accelerated.

In addition, in high-strength steel plates for high heat input welding, coarsening of crystal grains causes deterioration of toughness of the high heat input HAZ. In order to ensure sufficient strength of the base material, it is necessary to reduce the content of alloying elements that increase hardenability, and to compensate for that amount of hardenability, use B, which significantly increases hardenability even in small amounts. is valid. On the other hand, one effective method for suppressing the coarsening of crystal grains in the HAZ is to utilize intragranular transformation using Ti-based oxides as production nuclei. The present inventors investigated the relationship between metal structure and toughness for steel subjected to a simulated thermal cycle simulating high heat input welding. As a result, by promoting intragranular transformation with Ti-based oxides dispersed in steel, grains are refined, and by limiting Mn/Ni to 0.80 or less and suppressing the formation of MA. It has been found that the toughness of a high heat input HAZ can be significantly improved.

In this way, the present inventors dispersed Ti oxide by making the steel sheet contain 0.005% or more of Ti, thereby aiming to refine the crystal grains of the high heat input HAZ. Then, in order to make the crystal grains finer by using Ti oxide, the content of Al is limited. This is because as the Al content in the steel sheet increases, O in the steel sheet becomes more likely to be consumed to generate Al-based oxides, and the generation of Ti-based oxides is suppressed. Therefore, the amount of Al contained in the steel plate according to this embodiment is limited to 0.003% or less. On the other hand, by limiting the Ti content to 0.020% or less, generation of micron-sized coarse TiN can be avoided. Further, the content of O is limited to 0.0040% or less to prevent formation of coarse inclusions that would affect HAZ toughness.

Furthermore, in the steel plate according to the present embodiment, in order to suppress deterioration of toughness in the high heat input HAZ, the upper limit of the carbon equivalent CeqWES is limited, and the contents of C, Mn, Si, Ni, Cr, Mo, and V are set to a predetermined value. The range shall be . As a result of studies by the present inventors, it was found that the toughness of the high heat input HAZ can be ensured by limiting the carbon equivalent CeqWES to 0.53% or less. When CeqWES exceeds 0.53%, MA is generated and HAZ toughness is significantly deteriorated. The method for determining the carbon equivalent CeqWES is the same as in the first embodiment.

Furthermore, by limiting the upper limit of the carbon equivalent CeqWES, there is a concern that the strength of the base material may be insufficient. Therefore, in the steel plate according to this embodiment, the strength of the base material is ensured by containing B. Note that the effect of B on improving hardenability is impaired by the formation of BN, but when Ti, which forms nitrides, is contained and N is fixed by forming TiN, the formation of BN is suppressed. That is, Ti nitride contributes to the expression of the effect of increasing the hardenability of B. However, as described above, it is desirable to avoid the formation of micron-sized coarse TiN that becomes a starting point for destruction.

Next, the chemical composition (steel composition) of the steel plate according to this embodiment will be explained in detail. In addition, in the following description of chemical components, mass % is simply expressed as %.

(C: 0.12% or more, 0.18% or less)
C is an element that improves the hardenability of steel, contributes to high strength, and also influences the production of MA. In this embodiment, the C content is 0.12% or more. As a result, in the high heat input HAZ, decomposition of retained austenite, that is, transformation to ferrite and precipitation of cementite are promoted, and generation of MA is suppressed. The content of C is preferably 0.13% or more, more preferably 0.14% or more. On the other hand, from the viewpoint of preventing excessive production of cementite and ensuring toughness, in this embodiment, the C content is 0.18% or less. The content of C is preferably 0.17% or less, more preferably 0.16% or less.

(Mn: 0.5% or more, 1.5% or less)
Mn is an element that improves the hardenability of steel and contributes to high strength, and in this embodiment, the Mn content is 0.5% or more. The content of Mn is preferably 0.8% or more. On the other hand, in this embodiment, the Mn content is 1.5% or less from the viewpoint of suppressing the generation of MA in the high heat input HAZ and ensuring toughness. The Mn content is preferably 1.4% or less, more preferably 1.3% or less, and still more preferably 1.2% or less.

(Ni: 1.0% or more, 3.0% or less)
Ni is an element that improves the hardenability of steel and contributes to high strength, and at the same time, it is an element that increases the toughness of a high heat input HAZ. From the viewpoint of ensuring strength and toughness, in this embodiment, the Ni content is 1.0% or more. The Ni content is preferably 1.2% or more, more preferably 1.4% or more, and still more preferably 1.5% or more. On the other hand, Ni is an expensive element, and from the viewpoint of suppressing an increase in manufacturing costs, in this embodiment, the Ni content is 3.0% or less. The Ni content is preferably 2.5% or less, more preferably 2.2% or less, even more preferably 2.0% or less.

(B: 0.0003% or more, 0.0030% or less)
B is an important element for ensuring the hardenability of steel while limiting the carbon equivalent CeqWES. B is an element that can significantly improve hardenability even if the content in steel is minute, and in this embodiment, the B content is 0.0003% or more. The content of B is preferably 0.0005% or more, more preferably 0.0007% or more. On the other hand, from the viewpoint of suppressing deterioration of toughness and weldability of the high heat input HAZ, in this embodiment, the content of B is 0.0030% or less. The content of B is preferably 0.0020% or less, more preferably 0.0015% or less.

(N: 0.0010% or more, 0.0100% or less)
N is an element constituting TiN. TiN suppresses coarsening of γ grains by pinning. In order to exhibit the effect of suppressing the growth of γ grains in the HAZ, the N content in the steel sheet of this embodiment is 0.0010% or more. On the other hand, the N content is 0.0100% or less from the viewpoint of suppressing the formation of BN to improve hardenability and suppressing a decrease in HAZ toughness due to nitrides. The N content is preferably 0.0080% or less, more preferably 0.0060% or less.

(Carbon equivalent CeqWES: 0.43% or more, 0.53% or less)
The carbon equivalent CeqWES is an index of hardenability that affects the strength of the steel plate (base material) and the hardness of the HAZ. In order to ensure the strength of the base material, in this embodiment, the carbon equivalent CeqWES is 0.43% or more. The carbon equivalent CeqWES is preferably 0.44% or more, more preferably 0.45% or more. On the other hand, from the viewpoint of ensuring toughness, in this embodiment, the carbon equivalent CeqWES is 0.53% or less. The carbon equivalent CeqWES is preferably 0.52% or less, more preferably 0.51% or less. Note that the method for determining the carbon equivalent CeqWES is the same as in the first embodiment.

The method for manufacturing a steel plate according to the second embodiment is the same as the method for manufacturing a steel plate according to the first embodiment, so a description thereof will be omitted.

[Third embodiment]

Next, a high-strength steel plate for large heat input welding (hereinafter also simply referred to as "steel plate") according to a third embodiment of the present invention will be described. First, the results of the studies conducted by the present inventors and the new findings obtained will be explained in detail. Note that descriptions of parts that overlap with the first embodiment and the second embodiment will be omitted.

The steel plate according to the third embodiment contains C, Mn, Ni, and Cu, which are alloying elements that improve hardenability. The steel plate according to this embodiment is manufactured by hot rolling a steel piece obtained by melting and casting steel. As described above, the steel sheet manufactured in this manner has micro-segregation portions formed at the interface of the solidified structure by solidification during casting. The concentration of alloying elements such as Mn and Ni in this micro-segregation area is difficult to eliminate by short-term heating such as the thermal effect of welding. Therefore, when high heat input welding is applied to a steel plate containing C, Mn, and Ni, micro-segregation parts tend to remain in the high heat input HAZ. The micro-segregation part of the HAZ that is locally formed in this way becomes residual austenite with concentrated C when heated, and becomes hard MA after cooling. Since such MA becomes a starting point of fracture and reduces HAZ toughness, it is desirable to suppress the remaining of stable austenite, in other words, the generation of retained austenite. As a result of intensive studies, the present inventors have obtained a new finding that, compared to Ni, Mn delays the decomposition of retained austenite during cooling of a high heat input HAZ.

As mentioned above, when the high heat input HAZ is cooled, if the retained austenite in the micro-segregation part is not decomposed and the high heat input HAZ is cooled to room temperature, this retained austenite becomes MA and deteriorates the toughness of the HAZ. let Since Mn delays the decomposition of retained austenite compared to Ni, it is considered that Mn tends to cause an increase in MA. In other words, Ni is considered to have a smaller adverse effect on the toughness of the high heat input HAZ than Mn. Therefore, the present inventors focused on the balance between the Mn content and the Ni content in the steel, and by optimizing the ratio of the two, suppressed the amount of MA generated while increasing the hardenability of the steel. I thought it could be done. Specifically, the present inventors found that when Mn/Ni, which is the ratio of the Mn content divided by the Ni content in steel, becomes 0.80 or less, the amount of MA produced in the high heat input HAZ increases. We found a phenomenon that reduces This phenomenon is due to the effect of Mn atoms on the distribution behavior of C atoms at the interface of different phases when retained austenite, which is generated by concentration of C in micro-segregation areas during heating, is decomposed, that is, when retained austenite is transformed into ferrite and cementite. It is presumed that this is due to the difference in the influence between the Ni atoms and the Ni atoms.

In addition, the present inventors have found that the concentration of C at the start of decomposition of retained austenite in the micro-segregation zone has a greater influence on the type and amount of alloying elements such as Mn and Ni contained in the steel sheet than on the C content of the entire steel sheet. I found out that it can be done. In other words, the present inventors have found that as the content of C in the steel component is reduced, bainite transformation is promoted, the amount of retained austenite that becomes MA decreases, and the size of the retained austenite tends to become finer. This phenomenon is presumed to be related to the mechanism of incomplete transformation of bainite. In addition, as a result of further studies, the present inventors found that by limiting Mn/Ni to 0.80 or less and C content to 0.080% or less, micro-segregation It has been found that the production of MA can be suppressed.

In addition, in high-strength steel plates for high heat input welding, coarsening of crystal grains causes deterioration of toughness of the high heat input HAZ. One effective method for suppressing the coarsening of HAZ crystal grains is the use of intragranular transformation using Ti-based oxides as production nuclei. The present inventors investigated the relationship between metal structure and toughness for steel subjected to a simulated thermal cycle simulating high heat input welding. As a result, by promoting intragranular transformation with Ti-based oxides dispersed in steel, grains are refined, and by limiting Mn/Ni to 0.80 or less and suppressing the formation of MA. It has been found that the toughness of a high heat input HAZ can be significantly improved.

In this manner, the present inventors dispersed Ti-based oxides by setting the Ti content of the steel sheet to 0.005% or more, thereby achieving refinement of the crystal grains of the high heat input HAZ. Then, in order to make the crystal grains finer by using the Ti-based oxide, the content of Al is limited. This is because when the Al content in the steel sheet increases, O (oxygen) in the steel sheet is more likely to be consumed to generate Al-based oxides, and the generation of Ti-based oxides is suppressed. Therefore, the amount of Al contained in the steel plate according to this embodiment is limited to 0.003% or less. On the other hand, generation of micron-sized coarse TiN can be avoided by limiting the Ti content to 0.02% or less. Further, the content of O is limited to 0.0040% or less to prevent formation of coarse inclusions that would affect HAZ toughness.

Furthermore, in the third embodiment, the crystal grains can be made finer by increasing the content of Cu and Ni and ensuring sufficient hardenability. Specifically, the lower limits of the carbon equivalents CeqWES and CeqIIW are appropriately set, and preferably the lower limit of the hardenability multiple DI is appropriately set. On the other hand, in order to suppress an extreme decrease in ductility or absorption energy, the upper limits of CeqWES and CeqIIW are appropriately set, and preferably the upper limit of DI is appropriately set. The carbon equivalents CeqWES and CeqIIW and DI are controlled by the contents of C, Mn, Si, Cu, Ni, Cr, Mo, and V. As a result of intensive studies, the present inventors controlled the carbon equivalent CeqWES to 0.45% or more and 0.70% or less, and controlled CeqIIW determined by the following equation (2) to 0.65% or more and 0.90% or less. It was found that by doing so, the toughness of the high heat input HAZ can be ensured. Preferably, in order to refine the crystal grains, the hardenability multiple DI determined by the following equation (3) is controlled within the range of 10.0 inches or more and 21.0 inches or less.

CeqIIW (%) = C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5...(2)
DI (inch)=0.5×fB×C ^0.5 ×(1+0.64×Si)×(1+4.1×Mn)×(1+0.27×Cu)×(1+0.52×Ni)×(1+2 .33×Cr)×(1+3.14×Mo)...(3)
Here, C, Mn, Si, Ni, Cr, Mo, V, and Cu in the above formulas (2) and (3) are the content of each element in the steel sheet expressed in mass %, and the elements not contained Assign 0 to the term. fB is 1.0 when B is 0.0004% or less, and is 1.3 when B is over 0.0004%.

Next, the chemical composition (steel composition) of the steel plate according to the third embodiment will be explained. In addition, in the following description of chemical components, mass % is simply expressed as %.

(C: 0.030% or more, 0.080% or less)
C is an element that improves the hardenability of steel and contributes to high strength. In this embodiment, the C content is 0.030% or more. The content of C is preferably 0.035% or more, more preferably 0.040% or more. On the other hand, from the viewpoint of ensuring toughness, the C content is 0.080% or less. The content of C is preferably 0.075% or less, more preferably 0.070% or less.

(Mn: 0.30% or more, 1.30% or less)
Mn is an element that improves the hardenability of steel and contributes to high strength, and in this embodiment, the Mn content is 0.30% or more. The Mn content is preferably 0.40% or more, more preferably 0.50% or more. On the other hand, from the viewpoint of suppressing excessive production of MA in the high heat input HAZ, in this embodiment, the Mn content is 1.30% or less. The Mn content is preferably 1.20% or less, more preferably 1.10% or less.

(Ni: 1.30% or more, 7.00% or less)
Ni is an element that improves the hardenability of steel and contributes to high strength, and at the same time, it is an element that increases the toughness of a high heat input HAZ. From the viewpoint of ensuring strength and toughness, in this embodiment, the Ni content is 1.30% or more. The Ni content is preferably 1.80% or more, more preferably 2.00% or more, and still more preferably 2.50% or more. On the other hand, Ni is an expensive element, and from the viewpoint of suppressing an increase in manufacturing costs, in this embodiment, the Ni content is 7.00% or less. The Ni content is preferably 6.50% or less, more preferably 6.00% or less, and still more preferably 5.50% or less.

(Cu: 0.60% or more, 2.00% or less)
Cu has a small adverse effect on weldability and HAZ toughness, and is an element that improves the hardenability of steel and improves the strength and toughness of the base metal. From the viewpoint of ensuring strength and toughness, the content of Cu in the steel plate of this embodiment is 0.60% or more. The Cu content is preferably 0.70% or more, more preferably 0.80% or more. On the other hand, from the viewpoint of suppressing the occurrence of Cu cracks during hot rolling of a steel plate, in this embodiment, the Cu content is 2.00% or less. The Cu content is preferably 1.90% or less, more preferably 1.80% or less.

(Si: 0.10% or less)
Si is an element contained in steel for deoxidation and high strength. On the other hand, Si is also an element that promotes the production of MA, and the present inventors have obtained the knowledge that Si has an extremely large effect on the production of MA in the micro-segregation part of a high heat input HAZ. Therefore, in order to ensure the toughness of the high heat input HAZ, it is necessary to limit the Si content, and in this embodiment, the Si content is 0.10% or less. The content of Si is preferably 0.08% or less, more preferably 0.05% or less. Although the lower limit of the Si content is not particularly limited, from the viewpoint of manufacturing cost, the Si content may be 0.01% or more.

(P: 0.010% or less)
P is an impurity that is harmful to toughness. The content of P needs to be limited in order to stably ensure the toughness of the high heat input HAZ, and in this embodiment, it is 0.010% or less. The P content is preferably 0.008% or less. Although the lower limit of the P content is not limited, from the viewpoint of manufacturing cost, the P content may be 0.001% or more.

(N: 0.0060% or less)
N is an element constituting TiN. Fine TiN suppresses the coarsening of γ grains by pinning, but coarse TiN may become a starting point for fracture in the HAZ and reduce toughness. From the viewpoint of suppressing the formation of coarse TiN and ensuring HAZ toughness, in this embodiment, the N content is 0.0060% or less. The N content is preferably 0.0040% or less. In this embodiment, the lower limit of the N content is not limited, but from the viewpoint of manufacturing cost, the N content may be 0.0001% or more. From the viewpoint of exhibiting the effect of suppressing the growth of γ grains in the HAZ, the N content is preferably 0.0010% or more.

(Carbon equivalent CeqWES: 0.45% or more, 0.70% or less)
The carbon equivalent CeqWES is an index of hardenability that greatly affects the strength of the steel plate (base material) and the grain size of the HAZ. In this embodiment, the carbon equivalent CeqWES is 0.45% or more from the viewpoint of increasing the hardenability of the steel, ensuring the strength of the base material, and refining the crystal grains of the HAZ. The carbon equivalent CeqWES is preferably 0.50% or more, more preferably 0.55% or more. On the other hand, from the viewpoint of ensuring toughness, the carbon equivalent CeqWES is 0.70% or less in this embodiment. The carbon equivalent CeqWES is preferably 0.65% or less. Note that the method for determining the carbon equivalent CeqWES is the same as in the first embodiment and the second embodiment.

(Carbon equivalent CeqIIW: 0.65% or more, 0.90% or less)
The carbon equivalent CeqIIW is an index of hardenability that greatly affects the strength of the steel plate (base material) and the grain size of the HAZ. Since the steel plate according to this embodiment contains Cu, the carbon equivalent CeqIIW is an important index. In this embodiment, the carbon equivalent CeqIIW is 0.65% or more from the viewpoint of increasing the hardenability of the steel, ensuring the strength of the base material, and refining the crystal grains of the HAZ. The carbon equivalent CeqIIW is preferably 0.70% or more, more preferably 0.75% or more. On the other hand, from the viewpoint of ensuring toughness, the carbon equivalent CeqIIW is 0.90% or less in this embodiment. The carbon equivalent CeqIIW is preferably 0.89% or less, more preferably 0.85% or less. Note that the carbon equivalent CeqWES is calculated by the following formula (2) based on the content of alloying elements.

CeqIIW (%) = C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5... (2)
Here, C, Mn, Cu, Ni, Cr, Mo, and V in the above formula (2) are the contents of each element in the steel sheet expressed in mass %, and 0 is substituted for the terms of elements that are not contained. do.

(Hardenability multiple DI: 10.0 inch or more, 21.0 inch or less)
The hardenability multiple DI is an index of hardenability that affects the strength of the steel plate (base material) and the crystal grain size of the HAZ. From the viewpoint of increasing the hardenability of the steel, ensuring the strength of the base material, and refining the crystal grains of the HAZ, the hardenability multiple DI is preferably 10.0 inches or more. The hardenability multiple DI is more preferably 12.0 inches or more, and even more preferably 14.0 inches or more. On the other hand, from the viewpoint of ensuring toughness, the hardenability multiple DI is preferably 21.0 inches or less. The hardenability multiple DI is more preferably 20.0 inches or less, and still more preferably 19.0 inches or less. The hardenability multiple DI is calculated by the following formula (3) based on the content of alloying elements. fB is a coefficient indicating the influence of B on hardenability, and changes depending on the B content. When the B content is 0.0004% or less, the influence of B on hardenability is not considered, and fB is 1.0. When the B content exceeds 0.0004%, the effect of B on hardenability is taken into consideration, and fB is 1.3.

DI (inch)=0.5×fB×C ^0.5 ×(1+0.64×Si)×(1+4.1×Mn)×(1+0.27×Cu)×(1+0.52×Ni)×(1+2 .33×Cr)×(1+3.14×Mo)・・・(3)
Here, C, Si, Mn, Cu, Ni, Cr, and Mo in the above formula (3) are the contents of each element in the steel sheet expressed in mass %, and do not contain the selected elements Cr and Mo described later. In this case, assign 0 to each term. fB is 1.0 when B is 0.0004% or less, and is 1.3 when B is over 0.0004%.

The remainder of the chemical composition of the steel plate according to this embodiment is iron (Fe) and impurities, similar to the first and second embodiments. Impurities are components that are mixed in from raw materials such as ores and scraps and other factors during the industrial production of steel sheets, and are allowed within the range that does not adversely affect the steel sheet according to this embodiment. means. However, among the impurities, the upper limit of the content of P and S is limited as described above.

The method for manufacturing a steel plate according to the third embodiment is the same as the method for manufacturing a steel plate according to the first and second embodiments, so a description thereof will be omitted.

Examples of the present invention are shown below. However, the example shown below is an example of the present invention, and the present invention is not limited to the example described below.

Steel slabs with a thickness of 300 mm were manufactured by melting steel in a converter and continuous casting. After continuous casting, the steel slab was cooled to room temperature, reheated to a temperature range of 1000°C or higher and 1200°C or lower, and hot rolled. Note that the finishing temperature of hot rolling was 750°C or higher and 900°C or lower. When the hot-rolled steel plate is directly quenched, the hot-rolling finishing temperature is in the γ single phase range (Ar3 transformation point or higher).
Next, the hot rolled steel sheets were heat treated under the conditions shown in Tables 3 and 4. In Tables 3 and 4, "γ reheating quenching temperature" is the heating temperature when γ reheating quenching is performed on a steel plate that has been air cooled after hot rolling. On the other hand, the "two-phase region quenching temperature" is the heating temperature when direct quenching or γ reheating quenching is performed after hot rolling, and further two-phase region quenching is performed.
Samples were taken from the thick steel plates produced in this way and chemically analyzed. The chemical composition of each thick steel plate is shown in Tables 1 and 2, and the plate thickness is shown in Tables 5 and 6. In addition, the carbon equivalent CeqWES shown in Table 1 and Table 2 was calculated|required by the following formula (1).

CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)

Here, C, Mn, Si, Ni, Cr, Mo, and V in formula (1) are the contents [mass%] of each element, and 0 was substituted for the terms of elements not contained.

<Mechanical properties of base material>
The mechanical properties of the base material of the produced thick steel plates were evaluated.
Test pieces used for evaluation of the mechanical properties of the base material, that is, tensile tests and Charpy impact tests, were taken from a position of 1/4 of the thickness of the thick steel plate.
The tensile test was conducted at room temperature using two test pieces in accordance with JIS Z 2241:2011. YS (0.2% yield strength) and TS (tensile strength) are the average values of two test pieces, respectively. YR (yield ratio) is the ratio of YS to TS and is expressed as a percentage, ie, 100×(YS/TS). The unit of YR (yield ratio) is %.
The Charpy impact test was conducted using three V-notch test pieces in accordance with JIS Z 2242:2018, and the absorbed energy was measured. The test temperature is 0°C. The absorbed energy (KV ₂ (0° C.)) of the base material is the average value (arithmetic mean) of the absorbed energy of the three test pieces measured in this way.

A sample in which a thickness section (L section) of a steel plate parallel to the rolling direction was mechanically polished was prepared, and its Vickers hardness was measured in accordance with JIS Z 2244:2009. The Vickers hardness was measured at a position within 3 mm from the surface of the steel plate in the thickness direction, and at a position 1/4 thickness from the surface of the steel plate in the thickness direction. The measurement load for Vickers hardness was 10 kgf, and measurements were made at three points at each position. The average value of the measurement results at a position within 3 mm from the steel plate surface in the plate thickness direction was defined as the surface hardness Hvs. Further, the average value of the measurement results at a position 1/4 thickness away from the steel plate surface in the plate thickness direction was defined as Hvq. The hardness difference ΔHv was calculated using the following equation (7).

ΔHv = Hvs - Hvq (7)

Hvmin and Hvmax were obtained as follows.
The L cross section (parallel to the rolling direction, thickness side) of the steel plate is mechanically polished, and a 15 x 15 lattice pattern is formed at 30 μm intervals from the surface in the thickness direction, centered at 1/4 of the thickness of the steel plate. The Vickers hardness (measuring load: 10 gf) was measured at a total of 225 points in accordance with JIS Z 2244:2009.
Hvmin is calculated by arranging the obtained Vickers hardness values in descending order, and calculating the hardness values of measurement points from the smallest to 20% of the total number of measurement points (for example, if 500 points are measured, in order from the smallest to the smallest) It was obtained by averaging the 1st to 100th Vickers hardness).
Moreover, Hvmax was obtained by averaging the hardness values of measurement points from the largest to 20% of the total number of measurement points.

<Heat cycle test>
In addition, a thermal cycle test was conducted on the manufactured thick steel plate in order to evaluate the toughness of the high heat input HAZ.
The heat cycle test simulates the thermal history experienced by a region 1 mm from the fusion line (FL) to the base metal side (FL + 1 mm) when electroslag welding is applied to a test piece with the shape shown in Figure 1 taken from a thick steel plate. A structure resembling a high heat input HAZ structure was obtained by applying a thermal cycle. Specifically, as a thermal history, the temperature was raised from room temperature to 1400°C at an average heating rate of 10°C/s, then held at 1400°C for 60 seconds, and then the average cooling rate to 1000°C was 3°C/s. Cooling was performed such that the average cooling rate from 1000°C to room temperature was 0.5°C/s.
Thereafter, a test piece for Charpy impact test was taken from the heat cycled test piece, and the absorbed energy at 0°C and -20°C was measured in accordance with JIS Z 2242:2018.
Tables 5 and 6 show the thickness of the steel plate, the mechanical properties of the base material, the equivalent heat input in the thermal cycle test, and the HAZ toughness. KV ₂ (0°C) and KV ₂ (-20°C) are the absorbed energy at 0°C and -20°C, respectively, and the absorbed energy of the three test pieces measured at each temperature. is the average value (arithmetic mean).

As shown in Table 5, the steel plate of the present invention has a tensile strength (TS) of 780 MPa or more and 930 MPa or less, and a yield strength (YS) of 630 MPa or more and 750 MPa or less, when the plate thickness is 40 mm or more and 120 mm or less. ) and a yield ratio (YR) of 85% or less. Furthermore, the steel plate of the present invention example has excellent HAZ toughness of 100 J or more on average at 0° C. after a thermal cycle test simulating high heat input welding. Furthermore, even when the test temperature is -20°C, it has an extremely excellent HAZ toughness of 40 J or more.

On the other hand, as shown in Table 6, the chemical composition of the conventional steel plate (comparative steel) was outside the range of the present invention, so the mechanical properties and/or HAZ toughness of the base material were poor.

Since the code B2 has too high a C content, the HAZ toughness is poor. Code B4 has poor HAZ toughness because the Mn content is too high. Code B5 has poor HAZ toughness because the amount of Ni is too low.

The HAZ toughness is poor in B8 because the Ti amount is too low, and in B9 because the Ti amount is too high. The HAZ toughness is poor because B13 has too high an O content, B14 has too high a Si content, B15 has too high a P content, and B17 has too high an Al content.

Code B19 has poor HAZ toughness because CeqWES is too high. Code B20 has poor HAZ toughness because Mn/Ni is too high.

In case of code B1', since the amount of C was too low, YS, TS and Hvmin/Hvmax were outside the range of the present invention. Since the amount of Mn in code B3' is too low, YS, TS, and Hvmin/Hvmax are outside the range of the present invention, and the HAZ toughness is poor. In the case of B6' and B7', the B content is too high, so the YS and TS are outside the range of the present invention, and the HAZ toughness is poor.

In the case of code B10', the N amount is too high, so Hvmin/Hvmax is outside the range of the present invention, and the HAZ toughness is poor. Since the number B11' has too high an amount of N, the HAZ toughness is poor. In the case of code B12', since the amount of O is too low, the TS is outside the range of the present invention, and the HAZ toughness is poor.

B16' has an excessively high amount of S, and therefore has poor HAZ toughness. Since the code B18' has too low CeqWES, YS and TS are outside the scope of the present invention, and HAZ toughness is poor. Since the code B21' was not subjected to two-phase region quenching, YR and Hvmin/Hvmax were outside the scope of the present invention. Since the code B19' has too high CeqWES, YS, TS, and YR are outside the scope of the present invention, and the HAZ toughness is poor. Since the code B22' had a low two-phase region quenching temperature, YR and Hvmin/Hvmax were outside the range of the present invention.

Steel slabs with a thickness of 300 mm were manufactured by melting steel in a converter and continuous casting. After continuous casting, the steel pieces were cooled to room temperature, reheated to a temperature range of 1000°C or higher and 1200°C or lower, and hot rolled. Note that the finishing temperature of hot rolling was 750°C or higher and 900°C or lower. When the hot-rolled steel plate is directly quenched, the hot-rolling finishing temperature is in the γ single phase range (Ar3 transformation point or higher).
Next, the hot rolled steel sheets were heat treated under the conditions shown in Tables 9 and 10. In Tables 9 and 10, "γ reheating quenching temperature" is the heating temperature when γ reheating quenching is performed on a steel plate that has been air cooled after hot rolling. On the other hand, the "two-phase region quenching temperature" is the heating temperature when direct quenching or γ reheating quenching is performed after hot rolling, and further two-phase region quenching is performed.

Samples were taken from the thick steel plates produced in this way and chemically analyzed. The chemical composition of each thick steel plate is shown in Tables 7 and 8, and the plate thickness is shown in Tables 11 and 12. Note that the carbon equivalent CeqWES shown in Tables 7 and 8 was determined by the same method as in Example 1, and CeqIIW and DI were determined by the following formulas (2) and (3), respectively.

CeqIIW (%) = C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5...(2)
DI (inch) = 0.5 × fB × C ^0.5 × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 0.27 × Cu) × (1 + 0.52 × Ni) × (1 + 2 .33×Cr)×(1+3.14×Mo)...(3)
Here, C, Mn, Si, Ni, Cr, Mo, V, and Cu in the above formulas (2) and (3) are the contents of each element in the steel sheet expressed in mass%, and the elements that are not contained are 0 was assigned to the term. fB in equation (3) is 1.0 when B is 0.0004% or less, and is 1.3 when B is over 0.0004%.

The mechanical properties of the base material and the toughness of the HAZ were measured in the same manner as in Example 1.

As shown in Table 11, the steel plate of the present invention has a thickness of 40 mm or more and 120 mm or less, a tensile strength (TS) of 780 MPa or more and 930 MPa or less, and a yield strength (YS) of 630 MPa or more and 750 MPa or less, It had a yield ratio (YR) of 85% or less. Further, the steel plates of the examples of the present invention had excellent HAZ toughness of 100 J or more on average at 0° C. after a thermal cycle test simulating high heat input welding. Furthermore, even when the test temperature was -20°C, it had an extremely excellent HAZ toughness of 40 J or more.

On the other hand, as shown in Table 12, the chemical composition of the conventional steel plate (comparative steel) was outside the range of the present invention, so the mechanical properties and/or HAZ toughness of the base material were poor.

Code D1 has poor yield strength because the amount of C is too low. Code D8 has a too low Ti content, code D11 has a too low O content, and code D12 has a too high O content, resulting in poor HAZ toughness. Since the code D16 has too high an amount of Al, the HAZ toughness is poor.

Since the code D18 has too high CeqWES, the HAZ toughness is poor. Since the code D21 has too high Mn/Ni, the HAZ toughness is poor.

Code D2' has poor HAZ toughness because the amount of C is too high. The specimen D3' has poor yield strength because the amount of Mn is too low. Code D4' has poor HAZ toughness because the Mn content is too high. Code D5' has poor HAZ toughness because the amount of Ni is too low. Codes D6' and D7' have poor HAZ toughness because the amount of Cu is too high.

Code D9' has poor HAZ toughness because the amount of Ti is too high. Code D10' has poor HAZ toughness because the amount of N is too high. Code D13' has poor HAZ toughness because the amount of Si is too high. Code D14' has poor HAZ toughness because the amount of P is too high. Code D15' has poor HAZ toughness because the amount of S is too high. Code D17' has poor HAZ toughness because CeqWES is too high.

Codes D19' and D20' have poor HAZ toughness because CeqWES is too low. The specimen D22' had an inferior yield ratio because the two-phase region quenching was not performed. The specimen D23' had an inferior yield ratio because the quenching temperature in the two-phase region was low.

The present invention is applied to thick steel plates manufactured in the steel industry. Further, the present invention can also be applied to steel products other than thick steel plates, such as shaped steel. The high-strength, thick steel plate to which the present invention is applied is mainly used as a steel frame for high-rise buildings, and is particularly suitable as a steel frame for four-sided box columns, which generally consists of four skin plates and a diaphragm placed inside. be. When joining each member of the four-sided box column, so-called large heat input welding, which involves a large welding heat input, is performed. For example, highly efficient high heat input welding such as electroslag welding or submerged arc welding is applied to diaphragm welding, which attaches a diaphragm to a skin plate, and corner welding, which assembles a skin plate, respectively. Moreover, the steel plate according to the present invention can also be used for welded structures such as bridges, shipbuilding, tanks, marine structures, and line pipes.

The steel plate according to the present invention is suitable for high-strength, thick steel plates subjected to high heat input welding with high welding efficiency and when the required level of HAZ toughness is high. Specifically, the steel plate according to the present invention has a tensile strength of 780 MPa or more and 930 MPa or less, a yield strength of 630 MPa or more and 750 MPa or less, a yield ratio of 85% or less, and a plate thickness of 40 mm or more and 120 mm or less. Therefore, the steel plate according to the present invention is suitable for high-strength thick steel plates that require high heat input HAZ toughness, such as building steel four-sided box column diaphragms to which large heat inputs such as electroslag welding are applied.

Claims (10)

The chemical composition is in mass%,
C: 0.12% or more, 0.18% or less,
Mn: 0.50% or more, 1.50% or less,
Ni: 1.00% or more, 3.00% or less,
Ti: 0.005% or more, 0.020% or less,
O: 0.0010% or more, 0.0040% or less,
B: 0.0003% or more, 0.0030% or less,
Cu: 0% or more, 2.0% or less,
Cr: 0% or more, 1.0% or less,
Mo: 0% or more, 1.00% or less,
W: 0% or more, 1.00% or less,
Co: 0% or more, 1.0% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 0.10% or less,
Ca: 0% or more, 0.0050% or less,
REM: 0% or more, 0.0050% or less,
Zr: 0% or more, 0.0050% or less,
Si: 0.30% or less,
P: 0.015% or less,
S: 0.005% or less,
N: 0.0010% or more, 0.0100% or less ,
Al: 0.0030% or less
Contains
The remainder consists of Fe and impurities,
Mn/Ni, which is the ratio of Mn and Ni content, is 0.80 or less,
The carbon equivalent CeqWES calculated by the following formula (1) is 0.43% or more and 0.53% or less,
The tensile strength is 780 MPa or more and 930 MPa or less,
The yield strength is 630 MPa or more and 750 MPa or less,
The yield ratio is 85% or less,
The plate thickness is 40 mm or more and 120 mm or less,
Measure the Vickers hardness at 225 points or more at a position 1/4 of the plate thickness from the surface, and calculate the average value of the Vickers hardness values from the smallest to 20% as Hvmin, and the value from the largest to 20%. A steel plate characterized in that Hvmin/Hvmax is 0.85 or less, where Hvmax is the average value of Hvmax .
CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)
Here, in formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents [mass%] of each element, and 0 is substituted for the terms of elements that are not contained. The chemical composition is in mass%,
Cu: 0.1% or more, 2.0% or less,
Cr: 0.1% or more, 1.0% or less,
Mo: 0.10% or more, 1.00% or less,
W: 0.10% or more, 1.00% or less,
Co: 0.1% or more, 1.0% or less,
Nb: 0.005% or more, 0.100% or less,
V: 0.005% or more, 0.10% or less,
Ca: 0.0001% or more, 0.0050% or less,
REM: 0.0001% or more, 0.0050% or less,
The steel plate according to claim 1 , containing at least one selected from the group consisting of Zr: 0.0001% or more and 0.0050% or less. The chemical composition is in mass%,
C: 0.030% or more, 0.080% or less,
Mn: 0.30% or more, 1.30% or less,
Ni: 1.30% or more, 7.00% or less,
Ti: 0.005% or more, 0.020% or less,
O: 0.0010% or more, 0.0040% or less,
B: 0% or more, 0.0050% or less,
Cu: 0.60% or more, 2.00% or less,
Cr: 0% or more, 1.0% or less,
Mo: 0% or more, 1.00% or less,
W: 0% or more, 1.00% or less,
Co: 0% or more, 1.0% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 0.10% or less,
Ca: 0% or more, 0.0050% or less,
REM: 0% or more, 0.0050% or less,
Zr: 0% or more, 0.0050% or less,
Si: 0.10% or less,
P: 0.010% or less,
S: 0.005% or less,
N: 0.0060% or less ,
Al: 0.0030% or less
Contains
The remainder consists of Fe and impurities,
Mn/Ni, which is the ratio of Mn and Ni content, is 0.80 or less,
The carbon equivalent CeqWES calculated by the following formula (1) is 0.45% or more and 0.70% or less,
CeqIIW calculated by the following formula (2) is 0.65% or more and 0.90% or less,
The tensile strength is 780 MPa or more and 930 MPa or less,
The yield strength is 630 MPa or more and 750 MPa or less,
The yield ratio is 85% or less,
The plate thickness is 40 mm or more and 120 mm or less,
Measure the Vickers hardness at 225 points or more at a position 1/4 of the plate thickness from the surface, and calculate the average value of the Vickers hardness values from the smallest to 20% as Hvmin, and the value from the largest to 20%. A steel plate characterized in that Hvmin/Hvmax is 0.85 or less, where Hvmax is the average value of .
CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14...(1)
CeqIIW (%) = C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5...(2)
Here, in formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents [mass%] of each element, and 0 is substituted for the terms of elements that are not contained.
Here, C, Mn, Cu, Ni, Cr, Mo, and V in the above formula (2) are the contents of each element in the steel sheet expressed in mass %, and 0 is substituted for the terms of elements that are not contained. do. The steel plate according to claim 3, wherein the hardenability multiple DI calculated by the following formula ( 3 ) is 10.0 inches or more and 21.0 inches or less.
DI (inch)=0.5×fB×C ^0.5 ×(1+0.64×Si)×(1+4.1×Mn)×(1+0.27×Cu)×(1+0.52×Ni)×(1+2 .33×Cr)×(1+3.14×Mo)...(3)
Here, C, Si, Mn, Si, Cu, Ni, Cr, and Mo in the above formula (3) are the contents of each element in the steel sheet expressed in mass %, and the terms of elements not contained are Assign 0. fB is 1.0 when B is 0.0004% or less, and is 1.3 when B is over 0.0004%. The chemical composition is in mass%,
Cr: 0.1% or more, 1.0% or less,
Mo: 0.10% or more, 1.00% or less,
W: 0.10% or more, 1.00% or less,
Co: 0.1% or more, 1.0% or less,
Nb: 0.005% or more, 0.100% or less,
V: 0.005% or more, 0.10% or less,
B: 0% or more, 0.0004% or less,
Ca: 0.0001% or more, 0.0050% or less,
REM: 0.0001% or more, 0.0050% or less,
Zr: 0.0001% or more, 0.0050% or less,
The steel plate according to claim 3 or 4 , containing any one or more selected from the group consisting of: The steel plate according to any one of claims 1 to 5 , wherein the chemical composition contains P: 0.003% or more and 0.010% or less. The steel plate according to any one of claims 1 to 6 , wherein the Hvmin and the Hvmax satisfy the following formulas (8) and (9).
780≦0.25×Hvmin+1.07×Hvmax+387≦930...(8)
-0.00146×Hvmin+0.00246×Hvmax+0.659×Hvmin/Hvmax-0.163≦0.85...(9) The steel plate according to any one of claims 1 to 7 , characterized in that the maximum value of Vickers hardness Hvs is 320 or less in a region up to 3 mm in the depth direction from the steel plate surface. Claim 1 characterized in that the difference ΔHv between the maximum value Hvs of Vickers hardness in a region up to 3 mm in the depth direction from the surface of the steel plate and the Vickers hardness Hvq at a position of 1/4 thickness of the steel plate is 70 or less. - The steel plate according to any one of 8 .
However, ΔHv=Hvs−Hvq. Any one of claims 1 to 9 , characterized in that the average Charpy absorbed energy at 0°C in the simulated HAZ is 100 J or more when subjected to a welding heat cycle corresponding to a heat input of 60 to 150 kJ/mm. Steel plate described in .

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