WO2005098068A1

WO2005098068A1 - Thick high strength steel plate having excellent low temperature toughness in welding heat affected zone caused by high heat input welding

Info

Publication number: WO2005098068A1
Application number: PCT/JP2005/007109
Authority: WO
Inventors: Minoru Ito; Akihiko Kojima; Masanori Minagawa; Yoichi Tanaka; Toshiei Hasegawa; Jun Otani
Original assignee: Nippon Steel Corporation
Priority date: 2004-04-07
Filing date: 2005-04-06
Publication date: 2005-10-20
Also published as: NO20065095L; JP2005320624A; EP1736562A1; JP4660250B2; KR100839262B1; TWI295693B; TW200538561A; KR20060130700A; US20070181223A1; SG151274A1; EP1736562A4

Abstract

A thick high strength steel plate having excellent low temperature toughness in a welding heat affected zone caused by high heat input welding, which is characterized in that it has a chemical composition, in mass %, that C: 0.03 to 0.14 %, Si: 0.30 % or less, Mn: 0.8 to 2.0 %, P: 0.02 % or less, S: 0.005 % or less, Ni: 0.8 to 4.0 %, Nb: 0.003 to 0.040 %, Al: 0.001 to 0.040 %, N: 0.0010 to 0.0100 %, Ti: 0.005 to 0.030 %, with the proviso that the contents of Ni and Mn satisfy the following formula [1], and the balance: iron and inevitable impurities, Ni/Mn ≥ 10 X Ceq - 3 (0.36 < Ceq < 0.42) wherein Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. The use of the above thick high strength steel plate can achieve excellent welding HAZ toughness, even when a welding is carried out with a welding heat input of 20 to 100 kJ/mm on a steel plate having a thickness of 50 to 80 mm and exhibiting a base material tensile strength of about 490 to 570 Mpa.

Description

Thick, high-strength steel sheet with excellent low-temperature toughness in the heat-affected zone by large heat input welding

The present invention relates to a thick high-strength steel sheet excellent in low-temperature toughness of a heat affected zone (hereinafter referred to as HAZ) used for ships, offshore structures, middle-rise buildings, bridges, and the like. In particular, for welding with a plate thickness of 50 mm or more and a base metal tensile strength of 49 to 570 1 ^? & Grade, with a welding heat input of 20 to 10 books 0 kJ / mm It also relates to a steel sheet with excellent welded joints

Background art

In recent years, the demand for material properties of welding steel used for large structures such as ships, offshore structures, high-rise buildings, bridges, and the like has increased. In particular, among these structures, the use of thick steel plates with a plate thickness of more than 50 mm and a tensile strength of the base material of 570 MPa class is increasing. In addition, in order to promote the efficiency of welding, such thick high-strength steel sheets are welded by one-pass welding using large heat input welding methods such as the electoral gas welding method and the electroslag welding method. The requirements for HAZ toughness, as well as the toughness of the base material itself, are becoming more stringent. '

There have been many proposals focusing on the HAZ toughness of steel materials to which the large heat input welding method is applied. For example, Japanese Patent Publication No. 55-026164 discloses an invention in which a fine Ti nitride is secured in steel to reduce HAZ austenite grains and improve toughness. Has been. Also, Japanese Patent Application Laid-Open No. H03-2646414 proposes an invention for improving the toughness of HAZ by utilizing a composite precipitate of Ti nitride and MnS as a transformation nucleus of ferrite. Have been. Further, in Japanese Patent Application Laid-Open No. 04-1433246, a composite precipitate of Ti nitride and BN is used as a precipitation nucleus of grain boundary ferrite to improve HAZ toughness. An invention has been proposed.

However, the Ti nitride almost completely forms a solid solution in the vicinity of the boundary with the weld metal of the HAZ whose maximum temperature exceeds 140 ° C (hereinafter also referred to as the weld pound). There is a problem that the effect of improving toughness is reduced. For this reason, it is difficult for steel materials using Ti nitride as described above to achieve strict requirements for HAZ toughness in recent years and required properties of HAZ toughness in ultra-high heat input welding.

As a method for improving the toughness in the vicinity of the weld bond, steels containing Ti oxide are used in various fields such as thick plates and section steels. For example, in the field of steel plates, as disclosed in Japanese Patent Application Laid-Open No. 61-079745 / Japanese Patent Application Laid-open No. 61-117245, Ti oxidation The steel containing the material is very effective in improving the toughness of large heat input welds, and its application to high tensile strength steel is promising. This principle is based on the assumption that Ti oxides, which are stable even at the melting point of steel, are used as precipitation sites, and Ti nitrides, MnS, etc. precipitate during the temperature drop after welding, and they are then added to the site. As a result, fine ferrite is generated, and as a result, the generation of coarse fly harmful to toughness is suppressed, and deterioration of toughness can be prevented.

However, such a Ti oxide has a problem that the number of the Ti oxides dispersed in the steel cannot be sufficiently increased. The cause is the coarsening and aggregation of Ti oxides.If the number of Ti oxides is to be increased, coarse Ti oxides of 5 / m or more, so-called inclusions, will occur. It is thought that it would increase. Inclusions of 5 μm or more should be avoided because they are harmful because they can be a starting point for structural rupture or cause a decrease in toughness. Therefore, in order to achieve further improvement in HAZ toughness, it was necessary to utilize an oxide which is less likely to be coarsened and aggregated and which is more finely dispersed than a Ti oxide. As a method of dispersing such Ti oxides in steel, there are many methods of adding Ti to molten steel substantially not containing a strong deoxidizing element such as A1. However, it is difficult to control the number of Ti oxides and the degree of dispersion in steel simply by adding Ti to molten steel, and furthermore, the number of precipitates such as TiN and MnS. However, it is also difficult to control the degree of dispersion. Therefore, in a steel in which Ti oxides are dispersed only by Ti deoxidation, for example, the number of Ti oxides cannot be obtained sufficiently, or the toughness of a thick plate in the thickness direction fluctuates. In order to solve such a problem, Japanese Patent Application Laid-Open No. H06-293939 / Japanese Patent Application Laid-Open No. Hei 10-1028395 states that A1 addition immediately after Ti addition, Alternatively, there is disclosed an invention utilizing a Ti_A1 composite oxide or a composite oxide of Ti, Al, and Ca produced by adding A1 and Ca composites. According to such an invention, large heat input welding HAZ toughness can be greatly improved. Disclosure of the invention

However, in the conventional methods described above, in which HAZ austenite grains are reduced or precipitates are used as transformation nuclei of ferrite to form ferrite, the base material strength is increased at a plate thickness of 50 mm or more. In order to secure a strength of 490 MPa or more, it is necessary to increase the alloying elements. In addition to the increase in the hardness of the welded HAZ, the formation of MA (Martensite—Austenite constituent), which deteriorates the toughness, becomes evident. However, it is not possible to ensure sufficient HAZ toughness in a stable manner. If the base metal strength exceeds 570 MPa in tensile strength, the required HAZ toughness cannot be obtained.

Therefore, the present invention is directed to a welding method using a steel plate having a thickness of 50 to 80 mm and a base metal tensile strength of 49 to 570 MPa, and a welding heat input of 20 to 100 kJ / mm. It is an object of the present invention to provide a thick high-strength steel sheet having excellent low-temperature toughness in the heat affected zone by large heat input welding, which can achieve excellent welding HAZ toughness even when welding is performed.

The present inventors have found that the above problems can be advantageously solved by defining the Ni addition amount and Ni / Mn, and completed the present invention only after further study. The gist is as follows.

(1) In mass%, C: 0.03 to 0.14%, Si: 0.30% or less, Mn: 0.8 to 2.0%, P: 0.02% or less, S: 0.05% or less, Α 1 ... 0.01 to 0.04 0%, Ν: 0.01 0 to 0.01 0 0%, Ni: 0.8 ~ 4.0%, T i: 0.005 ~ 0.03 30%, Nb: 0.03 ~ 0.040%, and N i and Mn are represented by the formula [1] Thick high-strength steel sheet excellent in low-temperature toughness of the heat affected zone by large heat input welding, characterized by satisfying the above conditions, with the balance being iron and inevitable impurities.

N i / M n ≥ 1 0 X C e q-3 (0.36 <C e q <0.

4 2) [1]

Where C eq = C + MnZ 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (2) Further, at a mass of ⁰ , C a: 0.0 003 to 0.0 050%, Mg: 0.0 003 to 0.0 050%, REM: 0. One or two or more of 0.01 to 0.030%, and O: 0.000 to 0.000%, and an equivalent circle diameter of 0 The low-temperature toughness of the heat affected zone by large heat input welding as described in (1) above, characterized by containing 100 to 0.5 μm of oxides of at least 100 Zmm ² . Excellent high-strength steel sheet.

(3) In addition, mass. /. And B: 0.0005 to 0.0005%, characterized in that the low-temperature toughness of the weld heat-affected zone by the large heat input welding according to the above (1) or (2), Excellent high-strength steel sheet.

(4) In addition, the mass ^{_{0/0, C r: 0.}} 1~ 0. 5%, M o: 0. 0 1~ 0. 5%, V: 0. 0 0 5~ 0. 1 0%, Cu: 0.;! Low temperature of the heat affected zone by large heat input welding according to any one of the above (1) to (3), characterized in that it contains one or more of the above-mentioned 1.0% to 1.0%. Thick high-strength steel sheet with excellent toughness. Brief Description of Drawings

FIG. 1 is a diagram showing a welding heat cycle equivalent to 45 kJ / mm. FIG. 2 is a diagram showing the relationship between Ni / Mn, Ceq, and reproduced HAZ toughness.

Figure 3 shows the effect of improving the reproducible HAZ toughness by dispersing fine oxides or using B. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

Until now, as a means of improving the HAZ toughness, as described above, suppression of the growth of austenite grains at high temperatures has been considered. That hand The most effective method for the step is to pin the austenite grain boundaries with dispersed particles and stop the movement of the grain boundaries. This is because the reheated austenite grains of HAZ are extremely effectively refined by pinning even when the welding heat input is as large as 20 to L0 kj / mm. However, in order to increase the strength of the base metal, the amount of alloy added was increased. In steels where the carbon equivalent (C eq), which indicates the chemical hardenability as well as the weldability of the steel, is 0.36 or more, the HA As the hardness of Z becomes higher, a new problem arises that sufficient HAZ toughness cannot be obtained even when the reheated austenite grains are refined by pin jung. When the hardness of the HAZ is high, it is necessary to improve the toughness of the ground steel itself.

Therefore, the inventors of the present invention need to improve the HAZ toughness when the thickness of the steel, which is required for thick high-strength steel, is as high as 0.36 or more and 0.42 or less. The optimal component system to improve toughness was studied diligently. It has been known that Ni is effective as an element that enhances the toughness of the matrix. However, as in this case, C eq is 0.36 or more and 0.42 or less, and it is difficult to know whether it is effective for improving the toughness of HAZ, and if so, what component conditions are effective. Not been. Therefore, first, the effect of the amount of Ni added was examined. In the study, it was assumed that the amount of Nb that is effective for securing the base metal strength was 0.003% or more. The HAZ toughness was evaluated by the ductility-brittle transition temperature (vT) in the Charpy impact test when a heat cycle equivalent to the electron gas welding (heat input 45 kJ / mm) shown in Fig. 1 was applied. rs)

As a result of examining the effect of the amount of Ni added, it was first found that the required toughness could not be obtained if 1 [1 was less than 0.8%. Also, even when Ni is added at 0.8% or more, the HAZ toughness is not improved. Some steels, on the other hand, had reduced HAZ toughness. Therefore, as a result of intensive studies including the relationship with other added elements and C eq, when C eq is 0.36 or more and 0.42 or less, as shown in FIG. Z toughness was found to be related by C eq and Ni / Mn. Figure 2 shows the HAZ toughness (vT rs) of the steel used in the study, stratified by C eq and plotted with the Ni / Mn ratio as the horizontal axis. Fig. 2

Good toughness of less than 115 ° C at vTrs was obtained for steels satisfying the relationship of Ni / Mn≥10XCeq-3 (1). Steel materials that do not satisfy equation [1] may not have sufficient HAZ toughness due to insufficient Ni addition and a small matrix toughening effect, or a high Ni content. It is considered that even in the case of containing, MA is generated in HAZ by excessive addition of Mn, and the toughening effect of Ni is lost. In addition, as a result of conducting the same study on the steel material used in the above study in a heat cycle equivalent to a heat input of 100 kJ / mm, the equation [ It has been confirmed that good reproducible HAZ toughness can be obtained for steel materials with the relationship [1].

From the above study, it has been found that the HAZ toughness can be improved by adding 0.8% or more of Ni, which satisfies the formula [1]. The improvement of toughness was studied. The following three methods were studied to improve the HAZ toughness. First, in high heat input welding, austenite grains are coarsened due to prolonged high-temperature residence time, which lowers HAZ toughness.This is a method of suppressing coarsening of austenite during high-temperature residence. . Secondly, in large heat input welding, the cooling time after welding is long, so ferrite generated from austenite grain boundaries becomes coarse, and this coarse grain boundary ferrite may cause a decrease in HAZ toughness. Therefore, it is a method to suppress the grain boundary ferrite from becoming coarse. Third,

This is a method for making the HAZ structure itself fine.

Regarding the first method for suppressing austenite grain coarsening, for example, as described in Patent Document 7, a method for dispersing fine oxides is effective. In Patent Document 7, in order to disperse fine oxides, the amount of dissolved oxygen in molten steel is adjusted by an equilibrium reaction with Si in a deoxidizing step, and then deoxidizing in the order of Ti, Al, and Ca. Then you are. According to this method, oxides having a particle size of 0.01 to 1.0 μm are dispersed at ⁵ × 10 ³ to 1 × 10 ⁵ particles / mm ² .

Therefore, the present inventors have found that when C eq is as high as 0.36 or more and 0.42 or less, in a system containing 0.003% of Nb and adding 0.8% or more of Ni. Then, a method for dispersing fine oxides and further improving the HAZ toughness was studied diligently. First, a method of dispersing fine oxides. In such a system, the amount of dissolved oxygen in the molten steel is adjusted to 0.010 to 0.050% in the deoxidation step, and thereafter, First, deoxidation is performed at T i, then deoxidation is performed at A 1, and then at least one of Ca, Mg, and REM is added, so that the equivalent circle diameter is 0.0. It has been found that fine oxides of 0.5 to 0.5111 can be dispersed at a rate of 100 / mm ² or more. In addition, due to the dispersion of the fine oxide, coarsening of austenite grains during high-temperature stagnation during welding was suppressed, and the HAZ toughness could be further improved. As an example, Fig. 3 shows the results of a comparison with the HAZ toughness with only proper addition of Ni. Note that the oxide produced is finer as the amount of N i is large, the number becomes large, the 1 weight 1. In the case of more than 5% 1 0 0 0 or mm ² or more. This is what we found this time. Furthermore, as for the Si content in the molten steel, when the Si content is large, it is difficult to form oxides, so the Si content is ◦0.30% or less, further 0.20% or less. It is clear from this study that I got it. On the other hand, if the dissolved oxygen content before Ti deoxidation exceeds 0.050% or the order of the deoxidizing elements is different, the oxides become coarse and fine oxides cannot be obtained sufficiently. The effect of suppressing coarsening of austenite grains is hardly obtained. The number of oxides with an equivalent circle diameter of 0.005 to 0.5 μιη was determined by preparing an extraction replica from a steel sheet as a base metal, and using an electron microscope to increase the number of oxides by 100,000. Observation was made from 0 visual fields or more (observed area is 1 Ο Ο Ο Ο μ ΐ η ² or more). Particles smaller than 0.1 μm were observed at an appropriately increased magnification. Elemental analysis was performed on each of the observed particles having a diameter of 0.05 to 0.5 μππι, and the particles that were oxides were counted.Then, as a method for improving HA toughness, the inventors described above. The second method and the third method, which were described as the suppression of grain boundary fly coarsening and the finer HA HA structure, were studied diligently. As a result, when C eq is as high as 0.36 or more and 0.42 or less, and in a system in which Ni is added at 0.8% or more, especially in the case of 20 to 100 kJ / mm as in this case, It has been found that the addition of B is effective for welding with a large heat input. The reason is that, in terms of suppressing grain boundary ferrite coarsening, segregation of solid solution B at the reheated austenite grain boundary suppresses generation of grain boundary ferrite. In addition, in terms of the refinement of the HAZ structure, when the cooling rate is low in large heat input welding as in this case, the addition of B causes the austenite grain boundaries and inclusions in the austenite grains to contain B. This is because the HAZ structure is refined due to the precipitation of nitrides and the formation of a large number of fine ferrite of several m / m with the nuclei at the austenite grain boundaries and in the grains. FIG. 3 shows the result of comparing the improvement of HAZ toughness by adding B with the HAZ toughness of only adding Ni properly. It can be seen that the HAZ toughness is further improved by the addition of B. Fig. 3 shows the addition of B The HAZ toughness in the case where the HAZ toughness is shown is shown, but the HAZ toughness is further improved by the fine oxide dispersion and the addition of B. This is probably because the number of oxides serving as BN precipitation sites increased, and the ferrite that nucleated the BN increased, resulting in a finer HAZ structure.

From the viewpoint of securing strength and improving corrosion resistance, the HAZ toughness when Cu, Cr, Mo, and V were added in addition to the above conditions was also examined. As a result, respectively, in the range of 0.1 to 0.4%, 0.1 to 0.5%, 0.01 to 0.2%, 0.005 to 0.050% It was found that the addition of HAZ did not significantly reduce the HAZ toughness.

The method for manufacturing the steel sheet of the present invention is not particularly limited, and may be manufactured according to a known method. For example, after the molten steel adjusted to the above preferable composition is formed into a slab by a continuous forming method, it is heated to 100 to 125 ° C. and then hot-rolled.

Next, the reasons for limiting the component composition of the steel material used in the present invention will be described. Hereinafter, the mass% in the composition is simply expressed as%.

C has a lower limit of 0.03% as an effective component for improving the strength of steel, and an excessive addition generates a large amount of carbides and MA and significantly lowers the HAZ toughness. It was set to 14%.

S i is a component necessary for securing the strength of the base material and deoxidizing, etc., but the upper limit is set to 0.30% in order to prevent the toughness from being reduced by the hardening of HAZ. Further, when oxides are used, the upper limit is preferably set to 0.20% or less in order to prevent a decrease in the oxygen concentration in the molten steel.

Mn is required to be added in an amount of 0.8% or more as an effective component for securing the strength and toughness of the base metal, but the upper limit is 2.0 as long as the toughness, cracking, etc. of the weld zone is acceptable. %. Further, regarding the upper limit of Mn, it is necessary to satisfy the equation [1] showing the relationship between Ceq, Mn amount, and Ni amount. This is because of the high C eq that was newly discovered in this study. In this case, the increase in Mn causes the formation of a large amount of MA in the HAZ structure, and the effect of Ni to improve the HAZ toughness is lost.

N i / M n ≥10 XC eq -3 [1] P is preferably as small as possible, but it costs a lot to reduce it industrially. 0 2 or less.

The content of S is preferably as small as possible, but it requires a great deal of cost to reduce it industrially. Therefore, the content range of S is set to 0.05 or less.

Ni is an important element in the present invention, and it is necessary to add at least 0.8%. Further, regarding the lower limit of N i, it is necessary to satisfy the equation [1] showing the relationship between C e q, M n amount, and N i amount. The upper limit was set at 4.0% from the viewpoint of manufacturing costs.

N i / M n ≥10 XC eq -3 [1] N b is an element that is effective for improving the strength of the base material by improving the hardenability. Add 3% or more. However, when Nb is added in a large amount, MA is easily generated in HAZ regardless of the Ni / Mn ratio, and when more than 0.040% is added, the major axis in HAZ is 5 μπι or more. The upper limit of Nb was set to 0.040% because a large number of coarse MAs may be generated and the HAZ toughness may be significantly reduced. In order to obtain higher toughness, coarse MA with a major axis of 5 μm or more is hardly generated when the NiZMn ratio satisfies the above equation [1]. It is preferable to suppress the Nb content to less than 10%. In order to obtain higher toughness more stably, in the case of the Ni / Mn ratio satisfying the above equation [1], almost no MA with a major axis of 3 μm or more is generated. It is preferable to suppress the Nb content to 0% or less. A 1 is an important deoxidizing element, and the lower limit was set to 0.001%. In addition, when A1 is present in a large amount, the surface quality of the piece is deteriorated. Therefore, the upper limit is set to 0.040%.

T i is added in an amount of not less than 0.05% in order to generate Ti nitrides and Ti-containing oxides, which are required to suppress coarsening of the reheated austenite grains. However, an excessive addition increases the amount of solid solution Ti and lowers the HAZ toughness. Therefore, the upper limit is 0.030%.

N is added as necessary to form Ti nitride and B nitride in austenite grain boundaries and in grains during cooling after welding. In order to combine with B to form a B nitride, the addition of 0.010% or more is necessary.However, excessive addition increases the amount of solute N and lowers the HAZ toughness. The upper limit was 0.1%.

Ca is added in an amount of 0.0003% or more as necessary to generate a Ca-based oxide serving as pinning particles necessary for suppressing coarsening of the reheated austenite grains. However, excessive addition causes coarse inclusions to be formed, so the upper limit was made 0.050%.

Mg is added in an amount of 0.0003% or more as necessary to generate Mg-based oxides that become pinning particles necessary for suppressing coarsening of the reheated austenite grains. However, excessive addition causes coarse inclusions to be formed, so the upper limit was made 0.050%.

REM is added in an amount of 0.001% or more as necessary to generate a REM-based oxide which becomes a pinning particle necessary for suppressing coarsening of the reheated austenite grains. However, an excessive addition generates coarse inclusions, so the upper limit was 0.030%. The REMs described here are Ce and La, and the added amount is the total amount of both. B is necessary as a solid solution B to segregate at austenite grain boundaries during cooling after welding to suppress generation of grain boundary ferrite, and to form BN at austenite grain boundaries and intragranular. Therefore, add 0.0005% or more. However, excessive addition increases the amount of solute B and greatly increases the HAZ hardness, resulting in a decrease in HAZ toughness. Therefore, the upper limit was set to 0.050%.

Cu is added in an amount of 0.1% or more as necessary to improve the strength and corrosion resistance of the steel material. Since the effect saturates at 1.0%, the upper limit is set to 1.0.However, if it exceeds 0.4%, MA is likely to be generated and the HAZ toughness is reduced. Is good.

Cr is added in an amount of 0.1% or more as necessary in order to improve the corrosion resistance of the steel material. However, an excessive addition causes a decrease in the HAZ toughness due to the generation of MA, so the upper limit is 0.5%.

Mo is an element effective for improving the strength and corrosion resistance of the base material, and is added at 0.01% or more as necessary. Since the effect saturates at 0.5%, the upper limit is set to 0.5%. However, since excessive addition causes reduction in HAZ toughness due to the formation of MA, the upper limit is preferably 0.2% or less.

V is an element effective for improving the strength of the base material, and is added at 0.05% as necessary. Since the effect saturates at 0.10%, the upper limit is set to 0.10% .However, excessive addition causes reduction in the HAZ toughness due to the formation of MA, so it is preferably 0.050%. The following is good. Example 1

A steel strip was prepared by continuously forming molten steel having the chemical components shown in Table 1. For D23 to D34 and D46 to D49, before introducing Ti, adjust the dissolved oxygen of the molten steel to 0.010 to 0% to 0.050% by Si, and then First, deoxidation with T i, and then with A 1, then C a, M g, RE Any of M was added for deacidification. After reheating these at 110 to 125 ° C, steel plates with a thickness of 50 to 80 mm were manufactured by the following two rolling methods. One is that after rolling at a surface temperature of 750 to 900 ° C, water cooling is performed until the temperature of the sheet surface after reheating reaches a temperature range of 200 to 400 ° C. The other method is hot rolling, water cooling to room temperature, and tempering in the range of 500 to 600 ° C (Table 2). DQ—T and 载). Table 2 shows the manufacturing conditions, thickness, and mechanical properties of the steel sheet. For D23 to D34 and D46 to D49, the number of fine oxides with an equivalent circle diameter of 0.05 to 0.5 m, measured at any point on the steel sheet, is also shown. did. For the number of oxides, an extracted replica was prepared from an arbitrary part of the steel sheet, and it was extracted with an electron microscope at a magnification of 100,000, and a field of view of 100 or more (100,000 μm in observation area) ² or more), and particles having a particle size of less than 0.1 μιη were observed at an appropriately increased magnification. Elemental analysis was performed on each of the observed particles with a diameter of 0.05 to 0.5 μm, and the values were determined by counting oxides. D 2 3~D 3 1, D 4 6~D 4 9 throat steel also, a circle equivalent diameter 0.0 1-1 fine oxide of 0. 5 im is of the range of the present invention 0 0 ZMM ² more dispersed Let me. From the comparison of D46, D47 and D48, D49, in which the elements other than Si are almost the same, the amount of ≤ 1 is less than 0.20%, and the smaller is the amount of oxide. It turns out that there are many.

The steel plates were butt-butted and subjected to one-pass welding using electrogas welding (EGW) or electroslag welding (ESW) with a welding heat input of 20 to 100 kJ Zmm. Then, in the HAZ located at the center of the plate thickness (tZ2), notches were inserted at two locations, HAZ and FL, 1 mm away from FL, and a Charby impact test was performed at 140 ° C. . Table 2 shows the welding conditions and HAZ toughness. This In this Charpy impact test, a JIS No. 2 full-size test piece with a 2 mm V notch was used. Table 2 also shows the prior austenite particle size between FL and HA Z l mm. The former austenite grain size between FL and HAZ 1 mm described here is 2 mm in the thickness direction centered on the center of the thickness and the former austenite included in the plane containing FL to HAZ 1 mm. This is the average particle size of the stenite particles measured by the cross-sectional method. Here, the measurement was performed using the massive ferrite connected in a net shape as the grain boundary of the former austenite grains.

D 1 -D49 is the steel of the present invention. Since the chemical composition of the steel is properly controlled, high heat input HAZ toughness at 140 ° C is satisfactory while satisfying the required base metal performance. D23 to D34 and D46 to D49 in which fine oxides are dispersed have a prior austenite particle size of less than 200 μππι between FL and HA It is fine grained, and the high heat input at 140 ° C HA has higher toughness. In addition, D20, to which B was added to reduce the size of the HAZ structure, had better HAZ toughness than D19, which did not contain B and had the same amount of added elements other than B, and Large heat input at-40 ° C HAZ toughness also shows high values.

On the other hand, the Cl-17 of the comparative steel has a large heat input HAZ toughness because it does not contain enough Ni to satisfy Equation [1] or because the chemical composition of the steel is properly controlled. Is insufficient.

/ vu 6mi / -ooisld / O 890860sosAV

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Continue 1

* If N i / M n ≥ 1 OXC eq—3 is satisfied, mark 〇. If not, mark X.

2

2 Continuation 1 Butting 1 Haus welding condition ³ ) FL ~ HAZImm ⁴⁾ HAZ toughness ⁵ ) Category Welding method Heat input (kJ / mm) Average r grain size (m) FL / vE— ₄₀ (J) FL + 1mm / vE_ ₄₀ (J)

EGW 39 480 140 128

EGW 42 520 135 124

ESW 85 770 116 106

ESW 73 660 123 113

ESW 67 605 127 117

EGW 42 520 135 124

ESW 85 770 116 106

EGW 51 640 124 114

EGW 39 480 140 128

ESW 79 715 119 109

EGW 48 600 128 117

EGW 51 640 124 114

EGW 35 440 144 132

ESW 79 715 119 109

ESW 85 770 116 106

EGW 42 520 135 124

■ ¾s day S class EGW 45 560 131 120

EGW 48 600 128 117

EGW 45 560 131 120

EGW 45 560 171 156

ES 85 770 116 106

EGW 45 560 184 180

ESW 79 180 207 189

EGW 39 165 214 196

EGW 45 152 221 203

EGW 51 185 204 18

EGW 45 180 207 189

ESW 79 167 213 195

EGW 39 184 205 188

EGW 42 165 214 196

EGW 39 184 205 188

EGW 45 180 207 189

EGW 42 164 214 197

ESW 98 180 196 180

ESW 85 660 123 113

Table 2 continued 2 Mother: O (t / 2 part) ^U oxide number ²⁾ Classification a ⁾ "Manufacturing method ^Plate thickness (mm) Tensile strength (MPa) Yield stress (Mpa) ^VcL -40 (J) (pcs / mm ² )

D36 DQ-T 70 584 464 215

D37 TMCP 65 578 478 214

D38 TMGP 60 581 481 226

D39 TMCP 70 571 471 208

D40 DQ-T 80 570 450 203

D41 TMCP 70 565 465 214

D42 TMCP 65 576 476 216

Invention steel

D43 TMCP 60 589 489 217

D44 TMGP 65 577 477 215

D45 DQ-T 60 605 485 221

D46 TMCP 70 553 465 217 900

D47 TMCP 70 579 481 21 7 400

D48 TMCP 60 578 485 221 2300

D49 TMCP 60 592 485 221 1500

C1 TMCP 70 556 456 225

C2 DQ-T 60 595 475 233

C3 TMCP 75 544 444 224

C4 TMCP 60 574 474 234

C5 TMCP 60 576 476 232

C6 TMCP 55 586 486 236

C7 DQ-T 60 601 481 226

C8 TMCP 60 580 480 227

Comparative steel C9 TMGP 60 581 481 226

C10 TMCP 60 580 480 227

C1 1 DQ-T 70 585 465 214

C12 TMCP 60 585 485 221

C13 TMCP 70 564 464 215

C14 TMCP 65 575 475 218

C15 TMCP 55 598 498 222

C16 DQ-T 65 588 468 226

C17 TMCP 60 581 481 226

Continued 2 3

1) Sheet thickness center position, Y S and T S are the average values of two test pieces, and the Charpy absorbed energy at 140 ° C (VE _ 40) is the average value of three test pieces.

2) Make an extraction replica from any part of the steel plate. Observed with a scanning electron microscope at a magnification of 100,000 or more (100,000 _μm ² or more in observation area).

However, for particles smaller than 0.1 μm, observe the sample by appropriately increasing the magnification.

Elemental analysis of particles with an equivalent circle diameter of 0.005 to 0.5 μm, which includes oxidation, was counted and converted to the number per l mm ² . 3) E GW: Electro-port gas welding, ESW: Electro-slag welding, welding heat input is the average value of the entire welding length, and common welding materials are used for each welding method.

4) Average grain size of old austenite included in the plane containing 2 mm in the thickness direction centered on the center of the thickness and 1 mm from FL to HAZ.

Measured by section method. The ferrite connected in a net shape is measured as the grain boundary of the former austenite grains.

5) The FL notch is drawn so that WM and HAZ are equally divided, and vE—40—at each notch position is the average value of three test pieces.

Industrial applicability

The present invention provides a thick steel plate that satisfies the strict toughness requirements for crushing of ships, marine structures, and middle- and high-rise buildings, and has an extremely large effect in this kind of industrial field. Contribution to society is very large in terms of gender.

Claims

The scope of the claims

1. In mass%,

C: 0.03 to 0.14%,

S i: 0.30% or less,

Mn: 0.8 to 2.0%,

P: 0.02% or less,

S: 0.005% or less,

A 1: 0.00001 to 0.040%,

N: 0.00.01 0 to 0.0100.0%,

Ni: 0.8 to 4.0%,

T i: 0.005 to 0.030%,

Nb: 0.03 to 0.04 0%

Containing, and! ^! ! Satisfies the formula [1], and the balance is iron and unavoidable impurities. A thick high-strength steel sheet with excellent low-temperature toughness in the heat affected zone by large heat input welding.

N i ZMn ≥ 1 0 X C e q — 3 (0.36

4 2) [1]

Where C e q = C + Mn / 6 + (C r + M o + V) / 5 + (N i + C u) / 15

2. In addition, in mass%,

C a: 0.00 0 3 to 0.0 0 5 0%,

g: 0.0 0 0 3 to 0.0 0 5 0%,

R EM: 0.000-1 to 0.030%

Contains one or more of the above, and

O: 0.00.01 0 ~ 0.00.050%

And an oxide having a circle equivalent diameter of 0.05 to 0.5 μπι The thick high-strength steel sheet according to claim 1, wherein the high-heat-input welded zone has excellent low-temperature toughness at a low temperature toughness, characterized in that the steel sheet contains at least 1 piece / mm ² .

To 3. of al, the mass ^_0/0,

B: 0.0 0 0 5 to 0.0 0 5 0%

3. The thick high-strength steel sheet according to claim 1 or 2, wherein the high heat input welded zone has excellent low-temperature toughness at low temperature toughness.

4. In addition, in mass%,

C r: 0.1 to 0.5%,

M o: 0.01 to 0.5%,

V: 0.005-0.10%,

Cu: 0 .:! ~ 1.0%

4.A thick wall having excellent low-temperature toughness of a heat affected zone by large heat input welding according to claim 1, characterized by containing one or more of the above. Strength steel plate.