JP7535490B2 - Steel plate and its manufacturing method - Google Patents

Description

This disclosure relates to steel plates and manufacturing methods thereof, and to steel plates that can be used for welded structures, such as in the civil engineering and architectural fields, including bridges.

In recent years, as welded structures have become larger, there has been a demand for higher strength steel plates for welded structures used in the civil engineering and architectural fields, including bridges, and high-strength steel plates with a yield strength of 440 MPa or more and a tensile strength of 590 MPa or more are used. When used in civil engineering and architectural structures, improved seismic safety is also required. Specifically, a low yield ratio is required to ensure the plastic deformability of the steel plate. In addition to a low yield ratio, a large uniform elongation is also desired. This is because in the unlikely event that a stress exceeding the design strength is applied to the steel plate due to a major earthquake or the like, the energy absorbed during deformation can be increased, ensuring plastic deformability.

In addition, high heat input welding exceeding 400 kJ/cm is applied to steel plates used in welded structures from the viewpoint of improving the construction efficiency of structures and reducing construction costs. Generally, when high heat input welding is performed, the area near the weld line, called the bond portion, is held in the high-temperature austenite (γ) region for a long time and then slowly cooled, which makes it easy for the structure to become coarse, as typified by the growth of γ grains during heating and the generation of coarse ferrite (α) grains during the cooling process, resulting in a problem of deterioration of the toughness of that area. From the viewpoint of safety, it is necessary to ensure a stable high level of toughness in such heat-affected zones (HAZ) (high heat input HAZ toughness) even when high heat input welding is performed.

In other words, high-strength steel plates used in welded structures are required to have excellent high-heat-input HAZ toughness. However, high-strength steel plates with a tensile strength of 590 MPa or more often contain large amounts of alloy elements to ensure strength, and the HAZ toughness (high-heat-input HAZ toughness) tends to decrease in the bond zone of high-heat-input welds.

For this reason, in the steel plate described in Patent Document 1, in addition to controlling the content of each element, for example by including 0.0006 mass% or more of B, a predetermined relationship is satisfied for multiple elements, and the steel plate is manufactured under predetermined conditions to ensure HAZ toughness. In addition, the yield ratio is controlled to 78% or less to prevent the collapse of high-rise buildings in the event of an earthquake.

Patent Document 2 relates to a low-yield-ratio, high-tensile steel with excellent weldability and a manufacturing method thereof. Specifically, a specific amount of carbonitride-forming alloy elements is added, and the amount of dissolved C in the state before the start of accelerated cooling from the austenite region after rolling is controlled, achieving both strength of 570 MPa or more (60 kg class) and toughness.

Patent Document 3 discloses a method for producing a steel plate having a tensile strength of 590 _MPa or more, a yield ratio of 80% or less, and excellent HAZ toughness even when large heat input welding is performed, by hot rolling a steel material having a predetermined composition so that the rolling end temperature is Ar3 point or more, and then performing an accelerated cooling treatment in which the steel material is cooled to a temperature of 600°C or less at a predetermined average cooling rate.

JP 2020-33585 A JP 2004-339550 A JP 2005-220379 A

However, none of Patent Documents 1 to 3 discloses how to ensure sufficient uniform elongation, such as 5% or more. In other words, due to the lack of sufficient uniform elongation, there is a risk that the absorption of energy during deformation due to an earthquake or the like will be insufficient, and that sufficient plastic deformability will not be ensured.

This disclosure has been made in light of these circumstances, and aims to provide a steel plate having high strength, a low yield ratio, high uniform elongation characteristics, and excellent high heat input HAZ toughness, as well as a manufacturing method thereof.

Aspect 1 of the present invention is
C: 0.04% by mass or more and 0.10% by mass or less,
Si: 0.10 mass% or less (excluding 0 mass%)
Mn: 1.00% by mass or more and 1.60% by mass or less,
P: 0.030% by mass or less (including 0% by mass),
S: 0.030% by mass or less (including 0% by mass),
Al: 0.020% by mass or more and 0.075% by mass or less,
Mo: 0.10% by mass or more and 0.50% by mass or less,
Nb: 0.005% by mass or more and 0.030% by mass or less,
Ti: 0.005% by mass or more and 0.020% by mass or less,
N: 0.0030% by mass or more and 0.0075% by mass or less,
Ca: 0.0005% by mass or more and 0.0035% by mass or less,
B: 0.0005% by mass or less (including 0% by mass), and one or more selected from the group consisting of Cr: 0.45% by mass or less (excluding 0% by mass) and V: 0.080% by mass or less (excluding 0% by mass),
The balance is Fe and unavoidable impurities,
The carbon equivalent C _eq defined by the following formula (1) is 0.400 mass% or more and 0.470 mass% or less,
The weld crack susceptibility composition _Pcm defined by the following formula (2) is 0.160 mass% or more and 0.220 mass% or less,
The θ parameter defined by the following formula (3) is 0.60 or more and 0.95 or less,
The metal structure at a position t/4 in the sheet thickness direction, which is a portion where the distance from the surface in the sheet thickness direction is a quarter of the sheet thickness, contains, in terms of area ratio, 75.0% or more of bainite and 1.0% or more and 15.0% or less of ferrite having a grain size of 7.5 μm or less,
The steel sheet has a number density of island martensite of 0.010 pieces/ ^μm2 or more and 0.040 pieces/ ^μm2 or less.

C _eq (mass%) = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
Here, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are the contents of C, Si, Mn, Ni, Cr, Mo and V, respectively, expressed in mass%.

P _cm (mass%) = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] (2)
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are the contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B, respectively, expressed in mass%.

θ parameter=[Si]/2.83+[Mn]/2.04+[Cr]/1.25 (3)
Here, [Si], [Mn] and [Cr] are the contents of Si, Mn and Cr expressed in mass %, respectively.

Aspect 2 of the present invention is a steel sheet according to aspect 1, further comprising one or more selected from the group consisting of Cu: 1.0 mass% or less (excluding 0 mass%), Ni: 1.0 mass% or less (excluding 0 mass%), REM: 0.010 mass% or less (excluding 0 mass%), and Zr: 0.010 mass% or less (excluding 0 mass%).

Aspect 3 of the present invention is
C: 0.04% by mass or more and 0.10% by mass or less,
Si: 0.10 mass% or less (excluding 0 mass%)
Mn: 1.00% by mass or more and 1.60% by mass or less,
P: 0.030% by mass or less (including 0% by mass),
S: 0.030% by mass or less (including 0% by mass),
Al: 0.020% by mass or more and 0.075% by mass or less,
Mo: 0.10% by mass or more and 0.50% by mass or less,
Nb: 0.005% by mass or more and 0.030% by mass or less,
Ti: 0.005% by mass or more and 0.020% by mass or less,
N: 0.0030% by mass or more and 0.0075% by mass or less,
Ca: 0.0005% by mass or more and 0.0035% by mass or less,
B: 0.0005% by mass or less (including 0% by mass), and one or more selected from the group consisting of Cr: 0.45% by mass or less (excluding 0% by mass) and V: 0.080% by mass or less (excluding 0% by mass),
and the balance being composed of Fe and inevitable impurities, a carbon equivalent C _eq defined by the following formula (1) being 0.400 mass% or more and 0.470 mass% or less, a weld crack susceptibility composition P _cm defined by the following formula (2) being 0.160 mass% or more and 0.220 mass% or less, and a θ parameter defined by the following formula (3) being 0.60 or more and 0.95 or less;
The steel material is heated to 1050°C or more and 1250°C or less, and then hot-rolled so that the cumulative reduction at 850°C or more and 980°C or less is 20% or more and 60% or less and the finish rolling temperature _Tfr satisfies the following formula (4), followed by water-cooling from an accelerated cooling start temperature _Tsc, which is a temperature equal to or higher than the A _r3 point shown in the following formula (5), to an accelerated cooling end temperature _Tsf , which is a temperature equal to or lower than 250°C, wherein the average cooling rate from the accelerated cooling start temperature _Tsc to 500°C is 3 to 20°C/sec;
The metal structure at a position t/4 in the sheet thickness direction, which is a portion where the distance from the surface in the sheet thickness direction is ^one- quarter of the sheet thickness, contains, in terms of area ratio, 75.0% or more of bainite and 1.0% to 15.0% of ferrite having a crystal grain size of 7.5 μm or less, and the number density of island martensite is 0.010 pieces/μm2 or more and 0.040 pieces/ ^μm2 or less.

C _eq (mass%) = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
Here, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are the contents of C, Si, Mn, Ni, Cr, Mo and V, respectively, expressed in mass%.

P _cm (mass%) = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] (2)
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are the contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B, respectively, expressed in mass%.

θ parameter=[Si]/2.83+[Mn]/2.04+[Cr]/1.25 (3)
Here, [Si], [Mn] and [Cr] are the contents of Si, Mn and Cr expressed in mass %, respectively.

(800-t/2)≦T _fr (℃)≦(940-t/2) (4)
Here, t is the plate thickness (mm) after hot rolling.

Ar ₃ (°C) = 910-310×[C]-80×[Mn]-20×[Cu]-15×[Cr]-55×[Ni]-80×[Mo]-0.35×(t-8) (5)
Here, [C], [Mn], [Cu], [Cr], [Ni] and [Mo] are the contents of C, Mn, Cu, Cr, Ni and Mo, respectively, expressed in mass%, and t is the plate thickness (mm) after hot rolling.

Aspect 4 of the present invention is a method for producing a steel sheet according to aspect 3, in which the steel further contains one or more elements selected from the group consisting of Cu: 1.0 mass% or less (excluding 0 mass%), Ni: 1.0 mass% or less (excluding 0 mass%), REM: 0.010 mass% or less (excluding 0 mass%), and Zr: 0.010 mass% or less (excluding 0 mass%).

According to an embodiment of the present invention, it is possible to provide a steel plate having high strength, a low yield ratio, and high uniform elongation characteristics, as well as excellent high heat input HAZ toughness, and a manufacturing method thereof.

FIG. 2 is a schematic top view showing a groove shape used when preparing a welded joint for a joint test. FIG. 2 is a schematic top view showing the positions at which samples for Charpy impact testing were taken from the obtained welded joint.

In an embodiment of the present invention, specific examples of high strength, low yield ratio, high uniform elongation characteristics, and excellent HAZ toughness include tensile strength (TS): 590 to 740 MPa, yield strength (YS): 440 to 540 MPa, yield ratio (YR) ≦ 80%, uniform elongation (UE) ≧ 5%, and high heat input HAZ toughness: vE _0°C ≧ 27J (heat input ≧ 400 kJ/cm). The reason why the upper limit value of 740 MPa for the tensile strength and the upper limit value of 540 MPa for the yield strength are set is because the toughness decreases above these values.

The present inventors have conducted extensive research to simultaneously achieve these characteristics, and have arrived at an embodiment of the present invention.
While ensuring a tensile strength of 590 MPa or more, the controlling factors for obtaining a low yield ratio, high uniform elongation, and high high heat input HAZ toughness were examined.
Regarding strength, yield ratio, and uniform elongation, it was discovered that strength can be ensured by having a specified amount or more of bainite, which has a strength of more than 700 MPa, as the main metal structure, and that high strength, low yield ratio, and high uniform elongation can be obtained by dispersing appropriate amounts of ferrite and island martensite (MA) with a grain size of 7.5 μm or less.

It was found that it is important to reduce the amount of island martensite (MA amount) in the bond line generated by welding for the toughness of the high heat input HAZ. In order to obtain a certain amount of MA in the base metal, a common method is to add a large amount of alloying elements. However, this method also increases the amount of MA in the bond line. And there was a problem that the toughness of the high heat input HAZ decreases when the amount of MA in the bond line increases.
In the chemical composition of the steel plate according to the embodiment of the present invention, each element is controlled within a predetermined range, and the carbon equivalent _Ceq and the weld crack susceptibility composition _Pcm are controlled within appropriate ranges. In addition, the θ parameter, which is a parameter derived by the inventors, is controlled within an appropriate range in consideration of the solid solubility of the alloying elements in cementite and the diffusion rate in iron. As a result, it has been found that it is possible to achieve both an increase in the MA amount of the base metal and a decrease in the MA amount of the bond part, which were in a trade-off relationship in the past. As a result, the steel plate according to the embodiment of the present invention, which can realize high strength, low yield ratio, high uniform elongation, and high large heat input HAZ toughness, has been achieved.
The details of the embodiment of the present invention are given below.

<1. Chemical composition>
The steel sheet according to the embodiment of the present invention has a C content of 0.04 mass% or more and 0.10 mass% or less, a Si content of 0.10 mass% or less (excluding 0 mass%), and a Mn content of 1.00 mass% or less. % or more and 1.60% or less by mass, P: 0.030% or less by mass (including 0% by mass), S: 0.030% or less by mass (including 0% by mass), and Al: 0.020% or less by mass. % or more and 0.075% or less by mass, Mo: 0.10% or more by mass and 0.50% or less by mass, and Nb: 0.005% or more by mass. % or more and 0.030% or less by mass, Ti: 0.005% or more and 0.020% or less by mass, N: 0.0030% or more and 0.0075% or less by mass, and Ca: 0.0005% or more by mass. 0.0035% by mass or less, B: 0.0005% by mass or less (including 0% by mass), Cr: 0.45% by mass or less (excluding 0% by mass), and V: 0.080% by mass or less (excluding 0 mass %), and one or more selected from the group consisting of:
Furthermore, the carbon equivalent C _eq defined by the formula (1) described later is 0.400 mass % or more and 0.470 mass % or less, and the weld crack susceptibility composition P _cm defined by the formula (2) described later is 0 The content of C is 160 mass % or more and 0.220 mass % or less, and the θ parameter defined by the formula (3) described later is 0.60 or more and 0.95 or less.
The content of each element, the carbon equivalent C _eq , the weld crack susceptibility composition P _cm and the θ parameter will be described in detail below.

[1-1. Basic ingredients]
(C: 0.04% by mass or more and 0.10% by mass or less)
C has the effect of increasing the strength of the steel plate, but is also an element that deteriorates weldability such as weld crack resistance. If the C content is less than 0.04 mass%, it becomes difficult to ensure the necessary strength. Therefore, the C content is set to 0.04 mass% or more. The C content is preferably 0.05 mass% or more, more preferably 0.06 mass% or more. On the other hand, if the C content exceeds 0.10 mass%, it is easy to ensure the strength, but the MA of the HAZ becomes excessive, leading to deterioration of the high heat input HAZ toughness and deterioration of the weld crack resistance. Therefore, the C content is set to 0.10 mass% or less. The C content is preferably 0.095 mass% or less, more preferably 0.090 mass% or less.

(Si: 0.10 mass% or less (excluding 0 mass%))
Si is an element that is difficult to concentrate in cementite, and when austenite transforms into ferrite + cementite, diffusion of Si from the cementite interface to austenite is necessary. Therefore, the diffusion of Si is rate-limiting and makes it easier to delay the transformation, so that the amount of MA can be increased. In other words, the amount of MA can be increased by increasing the amount of Si added.
In order to obtain this effect, intentional addition of Si is necessary. In this specification, "not including 0 mass%" means that the material is intentionally added and contains an amount exceeding the impurity level. The material preferably contains 0.01 mass% or more, more preferably 0.02 mass% or more. However, if the Si content is excessive, the effect of deterioration of the high heat input HAZ toughness due to the increase in MA becomes significant, so that the upper limit of the Si content must be strictly controlled, and the Si content is set to 0.10 mass% or less. The Si content is preferably 0.08 mass% or less, more preferably 0.06 mass% or less.

(Mn: 1.00 mass% or more and 1.60 mass% or less)
Mn is an element that easily concentrates in cementite, and when austenite transforms into ferrite + cementite, diffusion of Mn into cementite is necessary. For this reason, the diffusion of Mn determines the rate of transformation and tends to delay the transformation. By adding Mn, the amount of MA can be increased. In other words, Mn is an element that is effective in reducing the yield ratio of the base material. In addition, Mn stabilizes austenite and lowers the transformation temperature, Mn is an effective element for improving hardenability and ensuring both strength and toughness. In order to achieve this effect, Mn is contained in an amount of 1.00 mass% or more. The Mn content is preferably 1.00 mass% or more. However, if Mn is contained in excess, MnS becomes coarse and the toughness of the base material (toughness in a state not affected by heat from welding) decreases. The Mn content is preferably 1.55 mass% or less, and more preferably 1.50 mass% or less.

(P: 0.030% by mass or less (including 0% by mass))
P, an inevitable impurity, adversely affects the toughness of the base material and the weld. In order to avoid such inconvenience, the P content is suppressed to 0.030 mass% or less. The P content is preferably 0.015 mass% or less, more preferably 0.010 mass% or less. It is difficult to make the P content 0 mass% by the manufacturing method usually used industrially, and it is usually contained at about 0.002 mass% or more. In this specification, "containing 0 mass%" means including an embodiment in which the P content is only contained at the impurity level or less without intentional addition.

(S: 0.030% by mass or less (including 0% by mass))
S is an inevitable impurity and has a negative effect on toughness and ductility in the thickness direction of the steel plate, so the content is preferably low. From this viewpoint, the S content is suppressed to 0.030 mass% or less. The S content is preferably 0.020 mass% or less, more preferably 0.010 mass% or less, and most preferably 0.005 mass% or less. It is difficult to reduce the S content to 0 mass% in the manufacturing method usually used industrially, and the S content is usually about 0.001 mass% or more.

(Al: 0.020 mass% or more and 0.075 mass% or less)
Al is an element necessary for deoxidization, and also has the effect of fixing N in the steel and preventing deterioration of the base material toughness due to solute N. Furthermore, in the low Si steel according to the embodiment of the present invention, Al also has the effect of improving the cleanliness and the toughness of the large heat input HAZ. In order to exert such an effect, Al is contained in an amount of 0.020 mass% or more. The Al content is preferably 0.025 mass%. On the other hand, if Al is contained excessively, coarse alumina-based inclusions are formed and the toughness of the base material is reduced. The Al content is preferably 0.070 mass % or less, and more preferably 0.065 mass % or less.

(Mo: 0.10% by mass or more and 0.50% by mass or less)
Mo is an element that suppresses polygonal ferrite transformation in the base material and also suppresses grain boundary ferrite coarsening in the HAZ bond portion. In order to obtain these effects, Mo is contained in an amount of 0.10 mass % or more. Mo The content is preferably 0.15 mass% or more, more preferably 0.20 mass% or more, and further preferably 0.30 mass% or more. On the other hand, if the Mo content is excessive, the hardenability becomes excessive, As a result, the weld crack resistance is deteriorated, so the Mo content is set to 0.50 mass % or less, preferably 0.45 mass % or less, and more preferably 0.43 mass % or less.

(Nb: 0.005% by mass or more and 0.030% by mass or less)
Nb is an element that suppresses polygonal ferrite transformation in the base material and also suppresses grain boundary ferrite coarsening in the HAZ (bond zone). In order to obtain the above effects, the Nb content is set to 0.005 mass % or more. The Nb content is preferably 0.010 mass% or more, more preferably 0.014 mass% or more. On the other hand, if Nb is contained in excess, the toughness of the base material and the HAZ deteriorates. Therefore, the Nb content The Nb content is preferably 0.025% by mass or less, more preferably 0.021% by mass or less, and further preferably 0.020% by mass or less.

(Ti: 0.005% by mass or more and 0.020% by mass or less)
Ti is an element that combines with N to form TiN, prevents the coarsening of austenite grains (γ grains) in the HAZ, and contributes to improving the toughness of the large heat input HAZ. This also has the effect of preventing deterioration of the high heat input HAZ toughness due to solute N. In order to exert these effects, Ti is contained in an amount of 0.005 mass % or more. The Ti content is preferably 0.008 mass % or more. On the other hand, if the Ti content is excessive, TiN becomes coarse and the toughness of the base material and the HAZ deteriorates, so the Ti content is set to 0.020% by mass or less. The Ti content is preferably 0.018 mass % or less, and more preferably 0.015 mass % or less.

(N: 0.0030 mass% or more and 0.0075 mass% or less)
N is an element that produces TiN and AlN, prevents the coarsening of γ grains during heating before hot rolling and during welding, and is effective in improving the toughness of the base material and the toughness of the large heat input HAZ. If the N content is less than 0.0030% by mass, the TiN and other elements are insufficient, the γ grains become coarse, and the toughness of the base material deteriorates. Therefore, the N content is set to 0.0030% by mass or more. The N content is preferably 0.0035% by mass or more, and more preferably 0.0040% by mass or more. On the other hand, when the N content exceeds 0.0075% by mass and becomes excessive, the amount of dissolved N is reduced. An increase in N content leads to a deterioration in the toughness of the base material. Therefore, the N content is set to 0.0075 mass% or less. The N content is preferably 0.0070 mass% or less, and more preferably 0.0065 mass% or less. .

(Ca: 0.0005 mass% or more and 0.0035 mass% or less)
Ca contributes to the spheroidization of MnS and is an element effective in improving the toughness and ductility in the thickness direction of the base material. Furthermore, in low-Si steel such as the embodiment of the present invention, Ca improves the cleanliness. It also has the effect of improving the toughness of the large heat input HAZ. In order to achieve such an effect, the Ca content is set to 0.0005 mass% or more. The Ca content is preferably 0.0010 mass% or more, and more preferably 0.0020 mass% or more. The Ca content is preferably 0.0013% by mass or more. However, if the Ca content exceeds 0.0035% by mass and becomes excessive, the inclusions become coarse and the toughness of the base material deteriorates. Therefore, the Ca content is set to 0.0035% by mass or more. The Ca content is preferably 0.0030% by mass or less, and more preferably 0.0025% by mass or less.

(B: 0.0005% by mass or less (including 0% by mass))
Since B is an element that enhances hardenability and suppresses the precipitation of ferrite having a grain size of 7.5 μm or less in the base material, it may be added (or may be intentionally added) to reliably prevent the excessive precipitation of ferrite having a grain size of 7.5 μm or less. However, since the excessive precipitation of ferrite having a grain size of 7.5 μm or less can be prevented by optimizing the manufacturing conditions without adding B, the addition of B is not essential and B may not be added.
If the B content is excessive, the transformation suppression effect becomes too large, making it difficult to obtain a predetermined amount of ferrite with a grain size of 7.5 μm or less, so the B content is set to 0.0005 mass% or less. The B content is preferably 0.0003 mass% or less. When B is added, the B content is preferably 0.0001 mass% or more.

(One or more selected from the group consisting of Cr: 0.45 mass% or less (excluding 0 mass%) and V: 0.080 mass% or less (excluding 0 mass%))
Cr is an element that easily concentrates in cementite, similar to Mn, and diffusion of Cr into cementite is required when transforming from austenite to ferrite + cementite. Therefore, the diffusion of Cr determines the rate of transformation and makes it easier to delay the transformation, thereby increasing the amount of MA. Therefore, Cr is an effective element for reducing the yield ratio of the base metal.
In addition, Cr is an element that is effective in suppressing polygonal ferrite transformation in the base material.
V is also an element effective in suppressing polygonal ferrite transformation in the base material.
For this reason, one or both of Cr and V are added.

When adding Cr, the Cr content is preferably 0.05 mass% or more to ensure the above effects. The Cr content is more preferably 0.10 mass% or more, even more preferably 0.15 mass% or more, and even more preferably 0.20 mass% or more. On the other hand, if Cr is contained in excess, the HAZ and MA of the base material will become excessive, and toughness will deteriorate. Therefore, the Cr content is set to 0.45 mass% or less. The Cr content is preferably 0.35 mass% or less, and more preferably 0.30 mass% or less.

When V is added, in order to reliably obtain the above effects, it is preferable that the V content be 0.005 mass% or more. The V content is more preferably 0.020 mass% or more, even more preferably 0.030 mass% or more, and even more preferably 0.040 mass% or more. On the other hand, if V is contained in excess, the toughness of the base material and HAZ decreases. Therefore, the V content is set to 0.080 mass% or less. The V content is preferably 0.070 mass% or less, and more preferably 0.060 mass% or less.

(Carbon equivalent C _eq : 0.400 mass% or more and 0.470 mass% or less)
In order to ensure high strength even in the case of a thick plate having a plate thickness of, for example, 100 mm or more, the carbon equivalent C _eq defined by the following formula (1) is set to 0.400 mass % or more. Carbon equivalent C _eq is preferably 0.420 mass% or more, more preferably 0.430 mass% or more. On the other hand, when a large amount of alloying elements is contained and the carbon equivalent C _eq exceeds 0.470 mass%, The HAZ toughness and weldability, particularly the weld crack resistance, are deteriorated. Therefore, the carbon equivalent C _eq is set to 0.470 mass% or less. The carbon equivalent C _eq is preferably set to 0.465 mass% or less, and more preferably set to 0. .460% by mass or less, more preferably 0.455% by mass or less

C _eq (mass%) = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
Here, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are C, Si, Mn, Ni, Cr, Mo, respectively, expressed in mass%. and the V content.
In addition, there are cases where only one of Cr and V is contained, but in such cases, the content of the element not contained (the element whose content is below the impurity level) is calculated as zero. Even if no Ni is contained, the Ni content is calculated as zero.

(Weld crack susceptibility composition P _cm : 0.160 mass% or more and 0.220 mass% or less)
In order to ensure good resistance to cold cracking even in the case of a thick plate having a thickness of, for example, 100 mm or more and weldability that does not require preheating, the following formula (2) is used. The weld crack susceptibility composition _Pcm is set to 0.220 mass% or less. This is because if a large amount of alloy elements is contained and the weld crack susceptibility composition _Pcm exceeds 0.220 mass%, weld cracks frequently occur. The weld crack susceptibility _Pcm is preferably 0.210 mass% or less, more preferably 0.200 mass% or less. The lower the weld crack susceptibility _Pcm , the better. Considering the composition of the components, the lower limit is approximately 0.1670 mass %.

P _cm (mass%) = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] (2)
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are the contents of C, Si, Mn, Mn, Cu, Ni, Cr, Mo, V and B, respectively, expressed in mass%. , Mn, Cu, Ni, Cr, Mo, V and B contents.
In addition, there are cases where either Cr or V, or B, is not contained, and in such cases, the content of the element not contained (element whose content is below the impurity level) is calculated as zero. Even if no Ni is contained, the Ni content is calculated as zero.

(θ parameter: 0.60 or more and 0.95 or less)
As described above, Si is an element that is difficult to concentrate in cementite, while Mn and Cr are elements that easily concentrate in cementite, and the diffusion of each element is rate-limiting, delaying the transformation from austenite to ferrite + cementite, and increasing the amount of MA. In order to properly manage the amount of MA in the base metal and HAZ, the inventors have found the θ parameter shown in the following formula (3) as a parameter that can comprehensively manage these three elements. The θ parameter was derived taking into consideration the solid solubility of Si, Mn, and Cr in cementite and the diffusion rate in iron.
In order to ensure the amount of MA in the base metal, the θ parameter is set to 0.60 or more. The θ parameter is preferably 0.65 or more, and more preferably 0.70 or more. On the other hand, an excessive amount of MA leads to a decrease in the toughness of the large heat input HAZ, so the θ parameter is set to 0.95 or less. The θ parameter is preferably 0.90 or less, and more preferably 0.85 or less.

θ parameter=[Si]/2.83+[Mn]/2.04+[Cr]/1.25 (3)
Here, [Si], [Mn] and [Cr] are the contents of Si, Mn and Cr expressed in mass %, respectively.
In addition, there are cases where Cr is not contained (when the content is below the impurity level), in which case the Cr content is calculated as zero.

The basic components are as described above, and in one preferred embodiment, the balance is iron and inevitable impurities. As inevitable impurities, the inclusion of elements brought in due to the conditions of raw materials, materials, manufacturing facilities, etc. is permitted. Representative examples of inevitable impurities include As, Sn, Sb, and H.
In addition, there are elements such as P and S that are usually treated as inevitable impurity elements, but whose composition ranges are separately defined as described above. Therefore, in this specification, when referring to "unavoidable impurities" that constitute the balance, this concept excludes elements whose composition ranges are separately defined.

[1-2. Selectively Added Elements]
Furthermore, in another preferred embodiment of the present invention, elements other than those described above may be added as necessary within a range that does not impair the action according to the embodiment of the present invention. Examples of the content of such selectively added elements include one or more selected from the group consisting of Cu: 1.0 mass% or less (excluding 0 mass%), Ni: 1.0 mass% or less (excluding 0 mass%), REM: 0.010 mass% or less (excluding 0 mass%), and Zr: 0.010 mass% or less (excluding 0 mass%).
These selectively added elements further improve the properties of the steel depending on the element contained. The effects of each selective element are shown below.

(Cu: 1.0 mass% or less (excluding 0 mass%))
Cu is an element that is effective in improving hardenability and increasing strength without significantly affecting the high heat input HAZ toughness. In order to obtain this effect more reliably, it is preferable to contain Cu at 0.05 mass% or more. The Cu content is more preferably 0.10 mass% or more, and even more preferably 0.20 mass% or more. However, if the Cu content exceeds 1.0 mass%, the toughness decreases, so the Cu content is set to 1.0 mass% or less. The Cu content is preferably 0.80 mass% or less, more preferably 0.75 mass% or less, and even more preferably 0.50 mass% or less.

(Ni: 1.0 mass% or less (excluding 0 mass%))
Ni is also an element that is effective in improving hardenability and increasing strength without significantly affecting the toughness of the high heat input HAZ. In order to obtain this effect more reliably, it is preferable to contain Ni at 0.05 mass% or more. The Ni content is more preferably 0.10 mass% or more, and even more preferably 0.20 mass% or more. However, if the Ni content exceeds 1.0 mass%, the toughness deteriorates, so the Ni content is set to 1.0 mass% or less. The Ni content is preferably 0.80 mass% or less, more preferably 0.75 mass% or less, and even more preferably 0.50 mass% or less.

(REM: 0.010% by mass or less (excluding 0% by mass))
REM (rare earth elements) are effective in preventing the coarsening of grains in the HAZ by forming oxides, sulfides, oxysulfides, etc. The lower limit of the REM content is preferably 0.0001 mass%, more preferably 0.0005 mass%, and even more preferably 0.0010 mass% in order to reliably obtain the above effect. On the other hand, if the REM content is excessive, the cleanliness decreases. Therefore, the REM content is set to 0.010 mass% or less, preferably 0.005 mass% or less, and even more preferably 0.003 mass% or less.
The REM content means the total content of 17 elements, including the two elements Sc and Y and the 15 elements from La to Lu, and containing REM means containing one or more elements selected from these 17 elements.

(Zr: 0.010 mass% or less (excluding 0 mass%))
Zr is effective in improving the toughness of the large heat input HAZ. The lower limit of the Zr content is preferably 0.0001 mass%, more preferably 0.0005 mass%, and even more preferably 0.0010 mass%, so that the above effect can be obtained more reliably. On the other hand, an excessive Zr content leads to a decrease in cleanliness. Therefore, the Zr content is set to 0.010 mass% or less, preferably 0.005 mass% or less, and even more preferably 0.003 mass% or less.

<2. Metal structure>
In the steel plate according to the embodiment of the present invention, the metal structure at a portion where the distance from the surface in the plate thickness direction is a quarter of the plate thickness (hereinafter simply referred to as the "plate thickness direction t/4 position", where t is the plate thickness) contains, by area ratio, 75.0% or more of bainite and 1.0% to 15.0% or less of ferrite with a grain size of 7.5 μm or less. Furthermore, at the plate thickness direction t/4 position, the number density of island martensite (MA) is 0.010 pieces/ ^μm2 to 0.040 pieces/ ^μm2 .
The metal structure is described in detail below.

(Bainite is 75.0% or more by area ratio)
In order to obtain the desired strength, the structure must be mainly bainite, and the area ratio of bainite must be 75.0% or more at the t/4 position in the sheet thickness direction, which is the portion where a typical metal structure of a steel sheet appears. The area ratio of bainite is preferably 80.0% or more, more preferably 85.0% or more, and even more preferably 90.0% or more.

(1.0% to 15.0% of ferrite with a grain size of 7.5 μm or less by area ratio)
In order to have a low yield ratio, high uniform elongation and high strength, it is necessary to mix an appropriate amount of ferrite with a grain size of 7.5 μm or less. Specifically, at the t/4 position in the sheet thickness direction, the area ratio of ferrite with a grain size of 7.5 μm or less is 1.0 to 15.0%. If the area ratio of the ferrite structure with a grain size of 7.5 μm or less is less than 1.0%, it will lead to an increase in the yield ratio, or/and a decrease in uniform elongation, or/and an increase in strength. The area ratio of ferrite with a grain size of 7.5 μm or less is preferably 1.5% or more, more preferably 2.0% or more. On the other hand, if the area ratio of ferrite with a grain size of 7.5 μm or less exceeds 15.0%, the desired strength cannot be ensured. The area ratio of ferrite with a grain size of 7.5 μm or less is preferably 13.0% or less, more preferably 10.0% or less.
In addition, if a large amount of ferrite having a grain size exceeding 7.5 μm (so-called polygonal ferrite) is contained, the desired strength cannot be obtained. Therefore, in a preferred embodiment, polygonal ferrite is not contained. In another preferred embodiment in which polygonal ferrite is contained, the area ratio is limited to 5.0% or less at the position t/4 in the sheet thickness direction.
As the crystal grain size of ferrite, the circle equivalent diameter of ferrite determined by metal structure observation as described in detail in the examples below may be used.

The metal structure may be composed of the above-mentioned bainite and ferrite having a grain size of 7.5 μm or less, or may be composed of bainite, ferrite having a grain size of 7.5 μm or less, and polygonal ferrite. Alternatively, it may contain other metal structures in an area ratio of more than 0% and 5.0% or less. That is, at the position t/4 in the sheet thickness direction, it contains other metal structures (remainder metal structure) in an area ratio of 0 to 5.0%. Also, preferably, the remaining metal structure is 0 to 3.0% in area ratio.
Preferred examples of the remaining metal structure include one or both of pearlite and martensite. When one or both of pearlite and martensite are included, the remaining metal structure may consist of one or both of pearlite and martensite, or may contain structures other than pearlite and martensite in an area ratio of 1.0% or less.

(The number density of island martensite (MA) is 0.010 pieces/ ^μm2 or more and 0.040 pieces/ ^μm2 or less)
In order to obtain a low yield ratio, the number density of island martensite (MA) is set to 0.010 pieces/μm2 or more and 0.040 pieces/ ^μm2 or ^less at the t/4 position in the sheet thickness direction in order to have an appropriate amount of MA in bainite or on the interface between ferrite and bainite. Most of the MA are formed in bainite or on the interface between ferrite and bainite. Then, mobile dislocations are introduced at the interface between the MA and the parent phase during the formation of the MA. The yield ratio can be reduced by increasing the number of mobile dislocations. In order to increase the number of mobile dislocations, it is important to increase the surface area of the interface between the MA and the parent phase, which is the starting point of mobile dislocation generation, that is, to increase the number density of MA. In order to obtain a desired low yield ratio, the number density of MA is set to 0.010 pieces/ ^μm2 or more and 0.040 pieces/ ^μm2 or less.
If the MA number density is less than 0.010 pieces/ ^μm2 , the yield ratio will increase. The MA number density at the t/4 position in the sheet thickness direction is preferably 0.013 pieces/ ^μm2 or more, more preferably 0.015 pieces/ ^μm2 or more. If the MA number density exceeds 0.040 pieces/ ^μm2 , the base material toughness will deteriorate. The MA number density at the t/4 position in the sheet thickness direction is preferably 0.035 pieces/ ^μm2 or less, more preferably 0.030 pieces/ ^μm2 or less.
As described above, almost all of the MA is formed in bainite or at the interface between ferrite and bainite, and therefore the area ratio of bainite in this specification is a value including the MA portion.

<3. Manufacturing method>
As will be described in detail below, the steel plate according to the embodiment of the present invention can be manufactured by preparing a steel material such as a slab having a predetermined composition, heating this steel material to an appropriate temperature, controlling the cumulative reduction rate at 850°C or higher and 980°C or lower during hot rolling, setting the hot rolling end temperature in an appropriate range, and further water-cooling the steel material from a predetermined accelerated cooling start temperature that is higher than the Ar ₃ point temperature to a predetermined accelerated cooling end temperature, and cooling the steel material at a predetermined cooling rate from the accelerated cooling start temperature to 500°C.

[3-1. Preparation of steel material having a specified chemical composition]
A steel material having the composition shown in "1. Chemical composition" above is prepared for hot rolling in the next rolling step. The steel material may be a steel material normally used in hot rolling of thick plates. An example of such a steel material is a cast piece. Examples of cast pieces include a slab obtained by a continuous casting method and an ingot obtained by an ingot casting method using a mold. If necessary, these slabs and ingots may be subjected to surface treatment, heat treatment, processing treatment, etc. to prepare a steel material for rolling.

[3-2. Rolling]
The above-mentioned steel material is hot rolled.
(heating)
The steel material is heated to 1050°C or more and 1250°C or less. If the heating temperature during hot rolling is lower than 1050°C, Nb does not completely dissolve, and the effect of suppressing polygonal ferrite precipitation cannot be fully exhibited, resulting in an increase in yield ratio and/or a decrease in uniform elongation and/or an increase in strength. The lower limit of the heating temperature is preferably 1080°C. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains become coarse, so the upper limit is set to 1250°C. The upper limit of the heating temperature is preferably 1200°C.

(Hot rolling)
The heated steel material is hot-rolled. The hot-rolling is performed such that the cumulative rolling reduction at 850° C. or more and 980° C. or less is 20% or more and 60% or less, and the finish rolling temperature _Tfr satisfies the following formula (4).

(800-t/2)≦T _fr (℃)≦(940-t/2) (4)
Here, t is the plate thickness (mm) after hot rolling.

If the cumulative reduction ratio at 850°C to 980°C is too small, the prior austenite grain size is not refined and there are insufficient ferrite transformation nucleation sites, so that ferrite with a grain size of 7.5 μm or less cannot be obtained sufficiently, resulting in an increase in yield ratio and/or a decrease in uniform elongation and/or an increase in strength. Therefore, the cumulative reduction ratio at 850°C to 980°C is set to 20% or more. The lower limit of the cumulative reduction ratio in this temperature range is preferably 30% or more, more preferably 40% or more. On the other hand, if the cumulative reduction ratio at 850°C to 980°C exceeds 60%, the prior austenite grain size is refined excessively, which reduces the hardenability, thereby reducing the bainite fraction and causing a decrease in strength. Therefore, the upper limit of the cumulative reduction ratio in this temperature range is 60%, preferably 55%.
The cumulative reduction ratio specified above is usually achieved by performing multiple passes of rolling in this temperature range, but it is not excluded that it can be achieved by one pass. Also, usually, after the above heating, one or multiple passes of hot rolling are performed in a temperature range higher than 980° C., and then continuous hot rolling is performed at a temperature range of 850° C. to 980° C. with a cumulative reduction ratio of 20% to 60%, but it is not excluded that no hot rolling is performed in a temperature range higher than 980° C.
Furthermore, the carrying out of one or more passes of rolling at temperatures below 850° C. is not excluded.

The finish rolling temperature (also called "hot rolling completion temperature"; the steel sheet surface temperature immediately before the final rolling pass of hot rolling) _Tfr satisfies the following formula (4).

(800-t/2)≦T _fr (℃)≦(940-t/2) (4)
Here, t is the plate thickness (mm) after hot rolling.

If the finish rolling temperature _Tfr is too high, the refinement of the prior austenite grain size does not proceed, and the ferrite transformation nucleation sites are insufficient, so that ferrite with a grain size of 7.5 μm or less cannot be obtained sufficiently, resulting in an increase in the yield ratio and/or a decrease in uniform elongation and/or an increase in strength. Therefore, the finish rolling temperature is aimed at such that the temperature inside the steel sheet is 940°C or less. However, since it is difficult to measure the temperature inside the steel sheet during rolling, the steel sheet surface temperature is used as the finish rolling temperature, and the steel surface temperature is set to (940-t/2)°C or less (where t is the sheet thickness (mm)) (because there is a difference of about t/2 between the steel sheet inside and surface temperatures). On the other hand, if the finish rolling temperature is too low, the number of ferrite transformation nucleation sites increases too much, resulting in a decrease in hardenability, a decrease in the bainite fraction, and a decrease in strength. Therefore, the internal temperature of the steel sheet is aimed at 800°C or more, and the steel surface temperature (i.e., the finish rolling temperature) is set to (800-t/2)°C or more.
Since it is preferable to aim for an internal temperature of 820°C or higher, the lower limit of the finish rolling temperature _Tfr is preferably (820-t/2)°C.

(Accelerated cooling after rolling)
The steel material (rolled steel sheet) after hot rolling is accelerated cooled under the following conditions.
Accelerated cooling is performed by water cooling between an accelerated cooling start temperature _Tsc , which is a temperature equal to or higher than the _Ar3 point shown in the following formula (5), and an accelerated cooling end temperature _Tsf , which is a temperature not higher than 250° C. As will be described in detail later, this accelerated cooling by water cooling may be any type of cooling as long as i) between the cooling start temperature _Tsc and 500° C., the average cooling rate is 3 to 20° C./sec, while ii) between 500° C. and the accelerated cooling end temperature _Tsf, which is a temperature not higher than 250° C.

Ar ₃ (°C) = 910-310×[C]-80×[Mn]-20×[Cu]-15×[Cr]-55×[Ni]-80×[Mo]-0.35×(t-8) (5)
Here, [C], [Mn], [Cu], [Cr], [Ni] and [Mo] are the contents of C, Mn, Cu, Cr, Ni and Mo, respectively, expressed in mass%, and t is the plate thickness (mm) after hot rolling.

i) Water cooling from the accelerated cooling start temperature _Tsc to 500°C If the accelerated cooling start temperature _Tsc is lower than the _Ar3 point, coarse polygonal ferrite is generated, resulting in a decrease in strength. Therefore, the accelerated cooling start temperature _Tsc is set to be equal to or higher than the _Ar3 transformation point. Note that cooling from the completion of hot rolling to the accelerated cooling start temperature _Tsc may be performed by any means, such as air cooling.
If the average cooling rate from the accelerated cooling start temperature _Tsc to 500°C is slower than 3°C/sec, a large amount of polygonal ferrite precipitates, resulting in insufficient strength. Therefore, the lower limit of the average cooling rate is set to 3°C/sec. On the other hand, if the average cooling rate from the accelerated cooling start temperature _Tsc to 500°C is too fast, a sufficient amount of ferrite with a grain size of 7.5 μm or less cannot be obtained, resulting in an increase in yield ratio and/or a decrease in uniform elongation and/or an increase in strength. Therefore, the average cooling rate is set to 20°C/sec or less. The lower limit of the average cooling rate is preferably 4°C/sec, more preferably 5°C/sec. The upper limit of the average cooling rate is preferably 18°C/sec, more preferably 15°C/sec.
Regarding water cooling, any method that can be used for water cooling of hot rolled material can be used as long as the above average cooling rate can be realized. Examples of such water cooling methods include spray cooling, laminar cooling, jet cooling, mist cooling, and immersion cooling. In these methods, the desired cooling rate can be realized by adjusting, for example, the water flow rate and the cooling time.

ii) Water cooling from 500°C to the accelerated cooling end temperature _Tsf If the accelerated cooling end temperature _Tsf is higher than 250°C, MA decomposes during air cooling after the accelerated cooling, resulting in an increase in the yield ratio. Therefore, the accelerated cooling end temperature _Tsf is set to 250°C or less. However, since the decomposition of MA can be prevented by slow cooling such as air cooling, there is no need to specify the cooling rate as long as the cooling is performed by water cooling. For water cooling, any method that can be used for water cooling of hot rolled materials can be used. Examples of such water cooling methods include spray cooling, laminar cooling, jet cooling, mist cooling, and immersion cooling.
From the viewpoint of simplifying the management during hot rolling operation, one preferred embodiment is to make the water flow density in water cooling from the above-mentioned accelerated cooling start temperature _Tsc to 500°C the same as the water flow density in water cooling from 500°C to the accelerated cooling end temperature _Tsf .
Cooling in a temperature range lower than the accelerated cooling end temperature _Tsf may be performed by any cooling method such as normal air cooling or air jet cooling. Alternatively, the accelerated cooling end temperature _Tsf may be room temperature, and the accelerated cooling start temperature _Tsc may be cooled to room temperature by water cooling.

The temperature of the steel material described in the rolling process described above may be measured using a non-contact thermometer such as a radiation thermometer, or a contact thermometer such as a thermocouple. It may also be confirmed by simulation, etc.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can be modified as appropriate within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present invention.

1. Sample Preparation Steels having the chemical compositions shown in Table 1 were melted in a converter. These molten steels were used to obtain slabs by continuous casting. The thickness of the obtained slabs was 250 mm or 277 mm.
The notation "-" in Table 1 indicates that it is not intentionally added, and if it is contained, it is contained only at the impurity level or less. Table 1 also shows the carbon equivalent C _eq calculated by formula (1), the weld crack susceptibility composition P _cm calculated by formula (2), and the θ parameter calculated by formula (3). In Table 1, values that deviate from the embodiment of the present invention are underlined.

The obtained steel slab was heated to the heating temperature shown in Table 2 and then hot rolled to obtain a steel plate sample (hot-rolled plate) having the plate thickness shown in Table 2. More detailed hot rolling conditions, such as the cumulative reduction at 850° C. or higher and 980° C. or lower, and the finish rolling temperature _Tfr , are shown in Table 2.
The hot-rolled steel sheet was subjected to accelerated cooling by water cooling. Table 2 shows the accelerated cooling start temperature T _sc , the accelerated cooling end temperature T _sf , and the average cooling rate from the accelerated cooling start temperature T _sc to 500° C. (referred to as "average cooling rate up to 500° C." in Table 2). For reference, the A _r3 point calculated using formula (5) is also shown. Sample No. 14 was air-cooled to room temperature without accelerated cooling. Therefore, the columns for the accelerated cooling start temperature T _sc , the accelerated cooling end temperature T _sf , and the average cooling rate are marked with "-".
In Table 2, conditions that deviate from the conditions of the embodiment of the present invention are underlined.

The heating temperature is the atmospheric temperature in the heating furnace, and the time in the furnace (residence time) is set to be a time or more that satisfies the average slab temperature within ±20°C of the atmospheric temperature in the furnace, which was determined by a prior study. The finish rolling temperature _Tfr is the steel sheet surface temperature immediately before the final pass of hot rolling, and was measured using a radiation thermometer. The cooling stop temperature was obtained by measuring the steel sheet surface temperature at the time when reheating was completed after cooling was finished using a radiation thermometer.
The average cooling rate up to 500°C was calculated by the following method. Accelerated cooling was performed in advance for various plate thicknesses, and the average cooling rate was calculated by subtracting 500°C from the accelerated cooling start temperature (i.e., temperature difference) and dividing the value by the time required for cooling from the accelerated cooling start temperature to 500°C, thereby clarifying the relationship between plate thickness and average cooling rate.
Based on the obtained relationship between plate thickness and average cooling rate, the average cooling rate for the target plate thickness was calculated.

2. Sample Evaluation Each steel plate was subjected to microstructural observation, tensile testing, and high heat input HAZ toughness evaluation by the methods detailed below.

Observation of Metal Structure
For each steel plate sample, observation of the metal structure, specifically, observation of the ferrite, bainite and remaining structures was carried out according to the following procedure.
A sample for observing the metal structure was taken from the above steel plate sample so that a cross section of the plate thickness including the front and back surfaces of the steel plate, parallel to the rolling direction and perpendicular to the surface of the steel plate, could be observed.
The observation surface was mirror-finished by polishing with wet emery paper (#150 to #1500) or by polishing with an abrasive having an equivalent function, for example, diamond slurry or other such abrasive.

The polished samples were etched with a 3% Nital solution to reveal the grain boundaries, and with a Lepera solution to reveal the MA.
At the t/4 position in the sheet thickness direction, the structure in which the grain boundaries were revealed was photographed at a magnification of 400 times to obtain a 6 cm x 8 cm photograph. Using this photograph, the area ratios of bainite and ferrite with a grain size of 7.5 μm or less were calculated. In addition, for samples in which the total area ratio of bainite and ferrite with a grain size of 7.5 μm or less was not 100%, the area ratio of the portion other than bainite and ferrite with a grain size of 7.5 μm or less, i.e., the remaining metal structure, was measured, and the identity of the remaining metal structure was also confirmed.

The structure in which MA was revealed was photographed at 1000x magnification to obtain a 6cm x 8cm photograph. This photograph was used to measure the MA number density. Specifically, white lumps were defined as MA, and the number density was calculated from the number of MA that appeared in the photograph and the field area (60μm x 80μm) of the photograph. All of the MA precipitated in the bainite or on the interface between ferrite and bainite.
The results are shown in Table 3.

<Tensile test>
A round bar tensile test piece was taken from the t/4 position in the plate thickness direction so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and a tensile test was performed according to JIS Z 2241 (2011) to measure the tensile strength, yield strength, yield ratio, and uniform elongation. The measurement results are shown in Table 3. If the tensile strength was within the range of 590 to 740 MPa, it was considered to be acceptable, if the yield strength was within the range of 440 to 550 MPa, it was considered to be acceptable, if the yield ratio was 80% or less, it was considered to be acceptable, and if the uniform elongation was 5% or more, it was considered to be acceptable. If all of these items were acceptable, it was determined that the specimen had high strength, low yield ratio, and excellent uniform elongation.

<Large heat input HAZ toughness evaluation>
The evaluation of the high heat input HAZ toughness was performed by two methods, 1) a joint test and 2) a simulated thermal cycle test, which are described in detail below.

1) Joint test FIG. 1 is a schematic top view showing the groove shape used when preparing a welded joint for a joint test. The obtained steel plate sample was cut to 500 mm x 700 mm (length in the direction perpendicular to the surface of the figure), and a skin plate 1 with a plate thickness of skin plate thickness t _s (skin plate thickness of each sample is shown in Table 2) was prepared, and a diaphragm 3 (size 250 mm x 700 mm x diaphragm thickness t _d (mm)) having a diaphragm thickness t _d shown in Table 2 was placed 25 mm away from the surface. The end of the diaphragm 3 on the skin plate 1 side was sandwiched between two support plates 2 with dimensions of 50 mm x 700 mm x 25 mm. One end of the two support plates 2 was brought into contact with the surface of the skin plate 1. This formed a groove portion 4.

Then, a welded joint was created by filling the groove 4 by electroslag welding (ESW) (large heat input welding) with the welding heat input shown in Table 2. The welding wire used was ES-60ST (1.6φ) and the flux was MF-38.
FIG. 2 is a schematic top view showing the positions at which samples 10 for Charpy impact testing were taken from the obtained welded joint.
According to "Standard Test Manual for Mechanical Properties of Steel Welded Joints in Architectural Steel Structures - Tensile Test/Charpy Impact Test - JSS IV 13-2016" (issued by the Japan Steel Construction Association), a V-notch test piece 10 was taken so that the outer peripheral portion of the weld heat-affected zone (HAZ) 5 at a depth of 6 mm from the surface of the welded side of the skin plate 1 was included in the notch 10A and the tangent direction of the outer peripheral portion was parallel to the notch 10A. Using the obtained V-notch test piece, a Charpy impact test was performed at 0°C according to JIS Z 2242 (2005). For each steel plate sample, the impact test was performed three times, and the average value of the absorbed energy was taken as the high heat input HAZ toughness vE ₀ (J). The results are shown in Table 3.
The joint test was performed only on samples Nos. 2, 3, 5, 10 and 12.

2) Reproduced heat cycle test A test piece of 55 mm (rolling direction) x 323 mm (direction perpendicular to rolling direction and plate thickness direction) x 12.5 mm (plate thickness direction) was taken from the t/4 position in the plate thickness direction of the steel plate sample (taken so that the center of the plate thickness direction of the test piece was at the t/4 position in the plate thickness direction). The obtained test piece was held at 1420 ° C for 40 seconds, and then cooled at a controlled rate so that the cooling time from 800 ° C to 500 ° C was 600 seconds. This is a heat cycle simulating the case where the large heat input welding (welding heat input of about 1000 kJ / cm) of the above-mentioned ESW was performed. From these test pieces, three full-sized Charpy impact test pieces (JIS Z 2202 V-notch test pieces) were taken, and Charpy impact tests were performed three times for each steel plate sample at 0 ° C, and the absorbed energy was measured. The average value of the absorbed energy was taken as the large heat input HAZ toughness vE ₀ (J). The obtained results are shown in Table 3.
The replicated thermal cycle test was conducted only for Samples Nos. 1, 4, 6 to 9, and 11, which are marked with "◯" in the "Reproduced Thermal Cycle" column in Table 2.

Samples in which the large heat input HAZ toughness vE ₀ was 27 J or more in both the joint test and the thermal cycle test were determined to have excellent large heat input HAZ toughness.
Among the properties shown in Table 3, those that deviate from the embodiments of the present invention are underlined.

From Tables 1 to 3, the following can be considered.
Steel plate samples No. 1 to 7 all satisfy all of the requirements for chemical composition and manufacturing conditions stipulated in the embodiment of the present invention. As a result, as shown in Table 3, they satisfy the requirements for metal structure such as the area ratio of bainite, the area ratio of ferrite having a grain size of 7.5 μm or less, and the remaining metal structure containing one or both of pearlite and martensite, the requirements for mechanical properties such as yield strength, yield strength, yield ratio, and uniform elongation, and the HAZ toughness.

On the other hand, steel sheet samples Nos. 8 to 16 do not satisfy the requirements according to the embodiment of the present invention in the following respects.
Steel plate sample No. 8 had an excessive Mn content and did not provide excellent HAZ toughness because the amount of MA in the HAZ increased.
Steel plate sample No. 9, which does not contain Nb, did not exhibit excellent HAZ toughness because the grain boundary ferrite in the HAZ became coarse.
Steel plate sample No. 10 had an excessive B content, and was unable to obtain a sufficient amount of ferrite with a grain size of 7.5 μm or less in the base material. As a result, the tensile strength and yield strength were excessively large, the yield ratio was excessively large, and sufficient uniform elongation was not obtained.

In the steel plate sample No. 11, the C content and the Si content were excessive, Ti was not added, and the weld crack susceptibility composition _Pcm was excessive, and therefore excellent HAZ toughness was not obtained. This is because the MA in the HAZ increased and the prior austenite grain size in the HAZ became coarse.
Steel plate sample No. 12 has excessive Si content, Mn content, Nb content and Cr content, and the θ parameter is also excessive. In addition, Mo is not added. Therefore, excellent HAZ toughness was not obtained. This is because MA in the HAZ increased and the grain boundary ferrite in the HAZ became coarse.

In the steel plate sample No. 13, the rolling reduction ratio at 850° C. or more and 980° C. or less was too small, and ferrite having a grain size of 7.5 μm or less could not be obtained. As a result, the yield strength became too large, and a low yield ratio could not be obtained.
In the steel plate sample No. 14, accelerated cooling was not performed, and polygonal ferrite was excessively precipitated, and the amount of ferrite having a grain size of 7.5 μm or less was also excessive, and the amount of bainite was insufficient, so sufficient tensile strength and yield strength were not obtained.

In the steel plate sample No. 15, the cooling rate during accelerated cooling was too high, and a sufficient amount of ferrite having a grain size of 7.5 μm or less could not be obtained, resulting in excessive tensile strength and yield strength.
In the case of steel plate sample No. 16, the cooling stop temperature during accelerated cooling was too high, and the MA number density was too low, so a low yield ratio could not be obtained.

Claims (4)

C: 0.04% by mass or more and 0.10% by mass or less,
Si: 0.10 mass% or less (excluding 0 mass%)
Mn: 1.00% by mass or more and 1.60% by mass or less,
P: 0.030% by mass or less (including 0% by mass),
S: 0.030% by mass or less (including 0% by mass),
Al: 0.020% by mass or more and 0.075% by mass or less,
Mo: 0.10% by mass or more and 0.50% by mass or less,
Nb: 0.005% by mass or more and 0.030% by mass or less,
Ti: 0.005% by mass or more and 0.020% by mass or less,
N: 0.0030% by mass or more and 0.0075% by mass or less,
Ca: 0.0005% by mass or more and 0.0035% by mass or less,
B: 0.0005% by mass or less (including 0% by mass), and Cr: 0.45% by mass or less (excluding 0% by mass) and V: 0.080% by mass or less (excluding 0% by mass) One or more selected from the group consisting of:
The balance is Fe and unavoidable impurities,
The carbon equivalent C _eq defined by the following formula (1) is 0.400 mass% or more and 0.470 mass% or less,
The weld crack susceptibility composition _Pcm defined by the following formula (2) is 0.160 mass% or more and 0.220 mass% or less,
The θ parameter defined by the following formula (3) is 0.60 or more and 0.95 or less,
The metal structure at the t/4 position in the sheet thickness direction, which is a quarter of the sheet thickness from the surface, is 75.0% or more bainite in area ratio and 7.5 μm or less grain size. and ferrite of 1.0% or more and 15.0% or less,
A steel sheet having a number density of island martensite of 0.010 pieces/ ^μm2 or more and 0.040 pieces/ ^μm2 or less.

C _eq (mass%) = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
Here, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are C, Si, Mn, Ni, Cr, Mo, respectively, expressed in mass%. and the V content.

P _cm (mass%) = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] (2)
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are the contents of C, Si, Mn, Mn, Cu, Ni, Cr, Mo, V and B, respectively, expressed in mass%. , Mn, Cu, Ni, Cr, Mo, V and B contents.

θ parameter=[Si]/2.83+[Mn]/2.04+[Cr]/1.25 (3)
Here, [Si], [Mn] and [Cr] are the contents of Si, Mn and Cr expressed in mass %, respectively. The steel sheet according to claim 1, further comprising one or more selected from the group consisting of Cu: 1.0 mass% or less (excluding 0 mass%), Ni: 1.0 mass% or less (excluding 0 mass%), REM: 0.010 mass% or less (excluding 0 mass%), and Zr: 0.010 mass% or less (excluding 0 mass%). C: 0.04% by mass or more and 0.10% by mass or less,
Si: 0.10 mass% or less (excluding 0 mass%)
Mn: 1.00% by mass or more and 1.60% by mass or less,
P: 0.030% by mass or less (including 0% by mass),
S: 0.030% by mass or less (including 0% by mass),
Al: 0.020% by mass or more and 0.075% by mass or less,
Mo: 0.10% by mass or more and 0.50% by mass or less,
Nb: 0.005% by mass or more and 0.030% by mass or less,
Ti: 0.005% by mass or more and 0.020% by mass or less,
N: 0.0030% by mass or more and 0.0075% by mass or less,
Ca: 0.0005% by mass or more and 0.0035% by mass or less,
B: 0.0005% by mass or less (including 0% by mass), and Cr: 0.45% by mass or less (excluding 0% by mass) and V: 0.080% by mass or less (excluding 0% by mass) One or more selected from the group consisting of:
The balance is Fe and unavoidable impurities, and the carbon equivalent C _eq defined by the following formula (1) is 0.400 mass% or more and 0.470 mass% or less, and the carbon equivalent C eq defined by the following formula (2) is A process of preparing a steel material having a weld crack susceptibility composition _Pcm of 0.160 mass% or more and 0.220 mass% or less and a θ parameter defined by the following formula (3) of 0.60 or more and 0.95 or less. and,
After heating the steel material to 1050°C or more and 1250°C or less, the steel _material is heated so that the cumulative rolling reduction rate at 850°C or more and 980°C or less is 20% or more and 60% or less, and the finish rolling temperature Tfr satisfies the following formula (4). Then, the steel sheet is water-cooled from an accelerated cooling start temperature _Tsc, which is equal to or higher than the _Ar3 point, to an accelerated cooling end temperature _Tsf , which is equal to or lower than 250°C, as shown in the following formula (5): a cooling step in which the average cooling rate from the accelerated cooling start temperature _Tsc to 500°C is 3 to 20°C/sec;
The metal structure at the t/4 position in the sheet thickness direction, which is a quarter of the sheet thickness from the surface, is 75.0% or more bainite in area ratio and 7.0% or more grain size. A method for manufacturing a steel plate containing 1.0% or more and 15.0% or less ferrite of .5 μm or less, and having a number density of island martensite of 0.010 pieces/ ^μm2 or more and 0.040 pieces/ ^μm2 or less.

C _eq (mass%) = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (1)
Here, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are C, Si, Mn, Ni, Cr, Mo, respectively, expressed in mass%. and the V content.

P _cm (mass%) = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] (2)
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are the contents of C, Si, Mn, Mn, Cu, Ni, Cr, Mo, V and B, respectively, expressed in mass%. , Mn, Cu, Ni, Cr, Mo, V and B contents.

θ parameter=[Si]/2.83+[Mn]/2.04+[Cr]/1.25 (3)
Here, [Si], [Mn] and [Cr] are the contents of Si, Mn and Cr expressed in mass %, respectively.

(800-t/2)≦T _fr (℃)≦(940-t/2) (4)
Here, t is the plate thickness (mm) after hot rolling.

Ar ₃ (°C) = 910-310×[C]-80×[Mn]-20×[Cu]-15×[Cr]-55×[Ni]-80×[Mo]-0.35×(t -8) (5)
Here, [C], [Mn], [Cu], [Cr], [Ni] and [Mo] are the contents of C, Mn, Cu, Cr, Ni and Mo, respectively, expressed in mass%. and t is the plate thickness (mm) after hot rolling. The method for manufacturing a steel sheet according to claim 3, wherein the steel further contains one or more selected from the group consisting of Cu: 1.0 mass% or less (excluding 0 mass%), Ni: 1.0 mass% or less (excluding 0 mass%), REM: 0.010 mass% or less (excluding 0 mass%), and Zr: 0.010 mass% or less (excluding 0 mass%).

Images