JP7506305B2 - High-strength steel plate for large heat input welding - Google Patents

Description

The present invention relates to a high-strength steel plate for high heat input welding that has excellent toughness in high heat input welds.

In recent years, as architectural structures have become taller and column-free spaces have expanded, the steel plates used in steel frames have become stronger and thicker. Furthermore, there is a growing demand for the fracture resistance of steel structures to ensure high safety even in the event of a large-scale earthquake. In addition, the application of highly efficient high-heat input welding has become common from the perspective of shortening construction periods and reducing construction costs, and there is also a growing demand for high toughness in high-heat input welding HAZ. Note that "high-heat input welding HAZ" refers to the heat affected zone (HAZ) formed by high-heat input welding, and is sometimes simply referred to as high-heat input HAZ.

Conventionally, when the above-mentioned high heat input welding is applied to high-strength thick steel plate, it has been difficult to ensure good toughness in the HAZ. For example, the HAZ toughness of electroslag welds in 780 MPa tensile strength thick steel plate is shown in Non-Patent Documents 1 and 2. According to Figure 6 of Non-Patent Document 1, the average Charpy absorbed energy at the notch positions of the fusion line (FL), 1 mm from the FL (HAZ1), 3 mm from the FL (HAZ3), and 5 mm from the FL (HAZ5) is 40 J or less. Also, according to Figures 3 and 5 of Non-Patent Document 2, the average Charpy absorbed energy at the FL notch position is 50 J or less.

In response to these problems, Patent Document 1 discloses a technology that simultaneously reduces the Si content and the P content (hereinafter sometimes referred to as "low Si" and "low P", respectively) to suppress the generation of MA in the HAZ and improve HAZ toughness. With this technology, it is said that it is possible to stably ensure toughness with an average Charpy absorbed energy of about 27 J or more at 0°C in the heat-affected zone of a welded joint with low to high heat input.

Patent Document 2 claims that MA can be reduced by adding a relatively high concentration of Mn in addition to the low Si and low P content of Patent Document 1. Furthermore, Patent Document 3 shows a technology in which fine sulfides with a particle size of 0.005 to 0.5 μm are dispersed in the steel, which has the effect of suppressing gamma grain growth in the HAZ and significantly improves HAZ toughness.

Patent No. 5862592 JP 2017-155333 A JP 2002-309337 A

Kazunari Tokunou and 7 others, "Large heat input welding type preheat reduction 780N/mm2 class high tensile steel plate for construction", Nippon Steel Technical Report, 1997, No. 365, pp. 37-43 Minoru Hirota and 5 others, "Low Yield Ratio 780N/mm2 Grade Steel for Building Structures by Online Manufacturing Process, Part 3: High Heat Input Welding Joint Properties", Abstracts of Academic Lectures by the Architectural Institute of Japan, 2012, No. 1017

However, with the techniques disclosed in Patent Document 1 and Patent Document 2, it is difficult to achieve high HAZ toughness or even higher toughness, such as an average Charpy absorbed energy of 100 J at 0°C, which is in line with the recent need for improved seismic resistance. Extremely reducing the P content (hereinafter, sometimes referred to as ultra-low P) leads to a longer refining process, which leads to longer manufacturing periods and increased costs. Furthermore, with the technique of Patent Document 2, deterioration of HAZ toughness due to MA generation in Mn segregation areas is unavoidable, making it difficult to stably ensure HAZ toughness. The technique shown in Patent Document 3, which finely disperses sulfides and suppresses gamma grain growth to refine the effective grain size of the HAZ and obtain high toughness, requires extremely high technology to control Mg and Ca on the ppm order, and is considered to be difficult to apply to stable and mass production.

It is known that the causes of the deterioration of the toughness of the high heat input welding HAZ in high strength steel plates are the generation of MA which is the initiation point of brittle fracture and the coarsening of crystal grains. The decrease in cooling rate associated with the increase in welding heat input due to the recent increase in thickness of steel plates leads to the coarsening of crystal grains and an increase in MA, making it even more difficult to ensure HAZ toughness.
The present invention has been made in consideration of the above circumstances, and has an object to propose a new guideline for component design and, based on this, to provide a high-strength steel plate for large heat input welding.

The inventors focused on the refinement of crystal grain size and the reduction of MA, which are the main controlling factors of toughness in high heat input welding HAZ, and conducted studies to achieve both high strength of steel plate (base material) and ensuring toughness of high heat input welding HAZ.
As a result, it was found that the grain size of the HAZ is significantly refined by increasing the hardenability of the steel plate to a certain level. Specifically, it was found that the grain size of the HAZ is significantly refined by setting the hardening factor DI to 17 or more. It was also confirmed that the refinement of the grains is accompanied by the refinement of the MA. In other words, it was found that the refinement of both the grains and the MA can be achieved by improving the hardenability.

On the other hand, it was also revealed that even if the hardenability is improved, only a small amount of highly tough lower bainite is formed, and it is not possible to expect high toughness due to the formation of lower bainite as previously thought. This is thought to be due to the effect of a decrease in the cooling rate accompanying an increase in welding heat input, and is a new finding that has become clear through the present invention.
In addition, it was found that Ni is extremely effective as an element for improving hardenability. It is generally known that increasing hardenability increases the amount of MA generated, but Ni can improve hardenability without increasing the amount of MA generated as much as other elements. Specifically, by adding Ni at 2.5% or more, the grain refining effect and the MA refinement effect due to the improvement of hardenability can be obtained.

On the other hand, the improvement of hardenability by Mn significantly increases the generation of MA. Therefore, when the Mn content in steel increases, the grain refinement effect due to the improvement of hardenability is easily offset by the increase in the amount of MA generated. For this reason, Mn needs to be suppressed to 1.0% or less.
The difference between Ni and Mn is presumably due to the difference in the ease of microsegregation and the difference in the effect on cementite formation.

It was also found that improving hardenability improves the performance of the steel plate (base material). Normally, when manufacturing steel plate with low hardenability, the two-phase heating temperature during heat treatment must be high and the tempering temperature must be low. In that case, the hardness of the steel plate surface increases and the hardness difference between the surface and the inside of the plate becomes large. These can induce surface cracks and reduce workability, so it is desirable to keep the hardness of the steel plate surface low and to make the hardness distribution in the plate thickness direction as uniform as possible.
In the present invention, the Vickers hardness of the steel plate surface can be 320 Hv or less, and the hardness difference between the steel plate surface and the 1/4 position of the plate thickness can be 70 or less.

The present invention was made based on these findings, and its gist is as follows:

[1] A high-strength steel plate for large heat input welding according to one embodiment of the present invention has, in mass%, C: 0.08 to 0.14%, Mn: 0.3 to 1.0%, Ni: 2.5 to 7.0%, Al: 0.03 to 0.100%, Cu: 0 to 2.0%, Cr: 0 to 2.0%, Mo: 0 to 2.0%, Nb: 0 to 0.03%, V: 0 to 0.10%, W: 0 to 1.0%, Ti: 0 to 0.020%, B: 0 to 0.0050%, Ca: 0 to 0.005%, Mg: 0 to 0.005%, REM: 0 to 0.005%, Zr: 0 to 0.005%, The steel sheet has Si: 0.30% or less, P: 0.010% or less, S: 0.005% or less, N: 0.0060% or less, O: 0.0060% or less, with the balance being Fe and impurities, has a hardening multiple DI (inch) calculated by the following formula (1) of 17 to 35, a carbon equivalent CeqWES calculated by the following formula (2) of 0.600% to 0.900%, and a carbon equivalent CeqIIW calculated by the following formula (3) of 0.750% to 1.100%, and is applicable to welding with a welding heat input of 70 to 150 kJ/mm .
DI (inch) = 0.5 x fB x ^C0.5 x (1 + 0.64 x Si) x (1 + 4.1 x Mn) x (1 + 0.27 x Cu) x (1 + 0.52 x Ni) x (1 + 2.33 x Cr) x (1 + 3.14 x Mo) ... (1)
CeqWES(%)=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 ... (2)
CeqIIW(%)=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 ... (3)
Here, C, Mn, Si, Ni, Cr, Mo, V, and Cu in the above formulas (1) to (3) are the contents of each element in the steel sheet expressed in mass%, and 0 is substituted for the terms of elements that are not contained. fB is defined as follows depending on the B content: when the B content is 0.0004% or less, fB = 1.0, and when the B content is more than 0.0004% and 0.0050% or less, fB = 1.3.
[2] The high-strength steel plate for large heat input welding according to [1] may contain, by mass%, Ti: 0.003 to 0.020%.
[3] The high-strength steel plate for large heat input welding according to [1] or [2] may contain, by mass%, B: 0.0004 to 0.0050%.
[4] The high-strength steel plate for large heat input welding according to any one of the aspects of [1] to [3] may contain, by mass%, any one or more selected from the group consisting of Cu: 0.1 to 2.0%, Cr: 0.1 to 2.0%, and Mo: 0.1 to 2.0%.
[5] The high-strength steel plate for large heat input welding according to any one of the aspects of [1] to [4] may contain, by mass%, any one or more selected from the group consisting of Nb: 0.003 to 0.03%, V: 0.01 to 0.10%, and W: 0.10 to 1.0%.
[6] The high-strength steel plate for large heat input welding according to any one of [1] to [5] may contain, by mass%, any one or more selected from the group consisting of Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, REM: 0.0005 to 0.005%, and Zr: 0.0005 to 0.005% or less.
[7] The high-strength steel plate for large heat input welding according to any one of [1] to [6] may have a P content, in mass%, of 0.003 to 0.010%.
[8] The high-strength steel plate for large heat input welding according to any one of [1] to [7] may have a maximum Vickers hardness Hvs of 320 or less in a region extending from the surface of the steel plate to 3 mm in a depth direction.
[9] In the high-strength steel plate for large heat input welding according to any one of [1] to [8], a difference ΔHv between the maximum Vickers hardness Hvs and the Vickers hardness Hvq at a quarter thickness position of the steel plate may be 70 or less.
[10] The high-strength steel plate for large heat input welding according to any one of [1] to [9] may have a yield strength of 630 to 750 MPa, a tensile strength of 780 to 930 MPa, and a yield ratio of 85% or less.
[11] In the high-strength steel plate for large heat input welding according to any one of [1] to [10], in a welded joint having a welding heat input of 70 to 150 kJ/mm , the 0°C Charpy absorbed energy in a HAZ of the weld may be an average of 100 J or more.
[12] The high-strength steel plate for large heat-input welding according to any one of [1] to [11] may have a plate thickness t of 40 mm or more and 100 mm or less.
[13] The high-strength steel plate for large heat-input welding according to any one of [1] to [12] may be a steel plate for architectural steel box columns of high-rise buildings.

The present invention provides high-strength steel plates for high heat input welding based on new composition design guidelines.

FIG. 1 is a diagram showing a procedure for taking Charpy test specimens for electroslag welded T-joints.

Hereinafter, a high-strength steel plate for large heat input welding according to one embodiment of the present invention will be described. First, the results of the research conducted by the inventors that led to the completion of the present invention and the new findings obtained will be described in detail.

The high-strength steel plate for large heat input welding according to this embodiment (hereinafter, also simply referred to as "steel plate") contains C, Mn, and Ni, which are alloy elements that enhance hardenability. The steel plate according to this embodiment is manufactured by hot rolling a slab obtained by melting and casting steel. However, in the steel plate, microsegregation formed at the interface of the solidification structure due to solidification during casting remains, and further, this microsegregation is not eliminated by short-term heating such as welding heat effects, and remains locally in the large heat input HAZ. In this locally formed microsegregation, alloy elements such as Mn and Ni are concentrated. The inventors' study has revealed that Mn, compared to Ni, delays the decomposition of retained austenite during cooling of the large heat input HAZ, leading to an increase in the martensite-austenite mixed phase (MA).

In a high heat input HAZ, if the residual austenite in the microsegregation area is cooled to room temperature without decomposing, it turns into MA, which deteriorates toughness. For this reason, it is considered important to control the content of Mn, which delays the decomposition of the residual austenite, from the viewpoint of suppressing the amount of MA generated. In other words, since Ni is considered to have a smaller adverse effect on the toughness of a high heat input HAZ than Mn, it is considered that the amount of MA generated can be suppressed by optimizing the content of Mn and Ni in the steel. The inventors have discovered that by adding Ni to improve hardenability, the base material structure becomes finer and the deterioration of toughness due to MA is reduced.

Furthermore, in the high-strength steel plate for large heat input welding according to this embodiment, the lower limits of the carbon equivalents CeqWES and CeqIIW are restricted to ensure hardenability, and the contents of C, Mn, Si, Ni, Cr, Mo, and V are controlled. On the other hand, if the hardenability is excessively increased, the HAZ structure becomes martensite, which results in an extremely reduced ductility and reduced absorbed energy, so upper limits of CeqWES and CeqIIW are also required. As a result of the inventors' studies, it was found that the toughness of the large heat input HAZ can be ensured by limiting the carbon equivalent CeqWES calculated by the following formula (4) to 0.600% to 0.900%, the CeqIIW calculated by the following formula (5) to 0.750% to 1.100%, and the hardenability multiple DI (inch) calculated by the following formula (6) to 17 to 35.

CeqWES(%)=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 ... (4)
CeqIIW(%)=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 ... (5)
DI (inch) = 0.5 × fB × C ^0.5 × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 0.27 × Cu) × (1 + 0.52 × Ni) × (1 + 2.33 × Cr) × (1 + 3.14 × Mo) ... (6)
Here, C, Mn, Si, Ni, Cr, Mo, V, and Cu in the above formulas (4) to (6) are the contents of each element in the steel sheet expressed in mass%, and 0 is substituted for the terms of elements that are not contained. fB is defined as follows according to the B content: when the B content is 0.0004% or less, fB = 1.0, and when the B content is more than 0.0004% and 0.0050% or less, fB = 1.3.

The inventors also focused on the P content. P is prone to segregation at grain boundaries, and when it segregates at the grain boundaries of the heat-affected zone, it can cause intergranular cracking. Therefore, it is necessary to limit the content of P in order to stably ensure the toughness of the high heat-input HAZ. Reducing the P content suppresses the decrease in toughness of the high heat-input HAZ due to intergranular cracking, but on the other hand, it was found that the toughness of the high heat-input HAZ can be further improved by adding an appropriate amount of P. It is speculated that the increase in the bainite transformation temperature is suppressed and the grain size becomes smaller, which in turn reduces the effective grain size of the HAZ, thereby further suppressing the occurrence of brittle fracture.

The high-strength steel plate for high heat input welding according to this embodiment is described below.

First, the chemical composition (steel composition) of the high-strength steel plate for large heat input welding according to this embodiment will be described. In the following explanation of the chemical composition, mass% will simply be expressed as %.

(C: 0.08% or more, 0.14% or less)
C is an element that improves the hardenability of steel and contributes to high strength. Therefore, in this embodiment, the C content is 0.08% or more. However, an excessive increase in the C content leads to an increase in MA and cementite, which reduces toughness, so the upper limit is 0.14% or less.

(Si: 0.30% or less)
Although Si may be contained in steel for deoxidization and strength, it is also an element that promotes the generation of MA. As a result of the inventors' investigation into the harmfulness of Si on MA, it was confirmed that Si has a very large effect on the generation of MA in the microsegregation part of the large heat input HAZ. Therefore, in order to ensure the toughness of the large heat input HAZ, the Si content is 0.30% or less in this embodiment. The Si content is preferably 0.25% or less, more preferably 0.20% or less, and further preferably 0.15% or less. The lower limit of the Si content is not particularly limited, but from the viewpoint of manufacturing costs, the Si content is preferably 0.01% or more.

(Mn: 0.3% or more, 1.0% or less)
Mn is an element that improves the hardenability of steel and contributes to high strength, and in this embodiment, the Mn content is 0.3% or more. However, if the Mn content is excessively increased, it increases MA in the large heat input HAZ and significantly deteriorates toughness, so in this embodiment, the upper limit of the Mn content is 1.0% or less.

(P: 0.010% or less)
P is an impurity that is harmful to toughness. If the P content is excessively increased, grain boundary cracking occurs in the large heat input HAZ, and the toughness is significantly deteriorated. Therefore, in this embodiment, the P content is 0.010% or less. The lower limit of the P content is not particularly limited, but the P content may be 0.001% or more from the viewpoint of manufacturing costs. On the other hand, by adding P appropriately, the hardenability of the large heat input HAZ is improved, the grain size is refined, and the toughness of the large heat input HAZ is improved. Therefore, in this embodiment, the P content is preferably 0.003 to 0.010%, may be 0.004 to 0.010%, or may be 0.007 to 0.010%.

(S: 0.005% or less)
S is an impurity, and if contained in a large amount, it may form coarse inclusions and reduce toughness. Therefore, the content of S needs to be limited in order to stably ensure the toughness of the large heat input HAZ, and in this embodiment, S is 0.005% or less. The lower limit of the content of S is not particularly limited, but from the viewpoint of manufacturing costs, the content of S may be 0.0001% or more.

(Ni: 2.5% or more, 7.0% or less)
Ni is an element that improves the hardenability of steel and contributes to high strength, and at the same time, it is also an element that improves the toughness of the large heat input HAZ. Furthermore, from the research of the present inventors, it has been found that Ni has the characteristic of being less likely to generate MA than Mn in the large heat input HAZ. For these reasons, in this embodiment, the Ni content is 2.5% or more. On the other hand, Ni is an expensive element, and from the viewpoint of suppressing an increase in manufacturing costs, in this embodiment, the Ni content is 7.0% or less.

(Al: 0.03% or more, 0.100% or less)
Al is an important deoxidizing element, and when B is added, it forms AlN to fix N, thereby suppressing the precipitation of BN, and is an important element to be contained in order to ensure solute B that is effective for the hardenability of steel. In order to exert this effect, the Al content is 0.03% or more in this embodiment. On the other hand, from the viewpoint of suppressing the generation of coarse aluminum-based oxides that become fracture initiation points and reduce toughness, the Al content is 0.100% or less in this embodiment.

(N: 0.0060% or less)
N is an impurity, and coarse nitrides reduce the toughness of the base material and the high heat input HAZ. From the viewpoint of preventing the formation of coarse nitrides and ensuring toughness, the content of N is 0.0060% or less in this embodiment. In addition, when adding B, an excessive increase in the amount of N may significantly reduce the amount of solute B that generates BN and contributes to improving hardenability. Therefore, a small content of N is preferable, but from the viewpoint of manufacturing costs, the content of N may be 0.0001% or more.

(O: 0.0060% or less)
O is an impurity, and when coarse aluminum-based oxides are present overlapping with the microsegregation portion of the high heat-input HAZ, they act as fracture origins and exhibit extremely low toughness. Therefore, in this embodiment, the content of O is 0.0060% or less. A small content of O is preferable, and it may be 0%, but from the viewpoint of manufacturing costs, in this embodiment, the content of O may be 0.0001% or more.

(Carbon equivalent CeqWES: 0.600% or more, 0.900% or less)
The carbon equivalent CeqWES has a large effect on the strength of the steel plate (base material) and the grain size of the HAZ. In order to ensure the hardenability in the HAZ and to refine the grains, the carbon equivalent CeqWES is 0.600% or more in this embodiment. On the other hand, if the carbon equivalent CeqWES exceeds 0.900%, the HAZ becomes martensite and the toughness decreases. Therefore, in this embodiment, CeqWES is 0.900% or less. The carbon equivalent CeqWES is calculated according to the following formula (4) depending on the content of the alloy elements.

CeqWES=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 ... (4)
Here, C, Mn, Si, Ni, Cr, Mo, and V in formula (4) are the contents of each element in the steel sheet expressed in mass%, and when the selective elements Cr, Mo, and V described later are not contained, 0 is substituted for each term.

(Carbon equivalent CeqIIW: 0.750% or more, 1.100% or less)
The carbon equivalent CeqIIW has a large effect on the strength of the steel plate (base material) and the grain size of the HAZ. In order to ensure the hardenability in the HAZ and to refine the grains, the carbon equivalent CeqIIW is 0.750% or more in this embodiment. On the other hand, if the carbon equivalent CeqIIW exceeds 1.100%, the HAZ becomes martensite and the toughness decreases. Therefore, in this embodiment, CeqIIW is 1.100% or less. The carbon equivalent CeqIIW is calculated by the following formula (5) depending on the content of the alloy elements.

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

(Hardening factor DI: 17 inches or more, 35 inches or less)
The quenching multiple DI has a significant effect on the strength of the steel plate (base material) and the grain size of the HAZ.
In order to ensure the hardenability of the HAZ and to refine the crystal grains, the hardening factor DI is set to 17 inches or more. On the other hand, if the hardening factor DI exceeds 35 inches, the HAZ becomes martensite and the toughness decreases, so the hardening factor DI is set to 35 inches or less. The hardening factor DI is calculated according to the following formula (6) depending on the content of the alloy elements.

DI (inch) = 0.5 × fB × C ^0.5 × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 0.27 × Cu) × (1 + 0.52 × Ni) × (1 + 2.33 × Cr) × (1 + 3.14 × Mo) ... (6)
In the above formula (6), when the B content is 0.0004% or less, fB = 1.0, and when the B content is more than 0.0004% and 0.0050% or less, fB = 1.3.

The remainder of the chemical components of the high-strength steel plate according to this embodiment are iron (Fe) and impurities. Impurities refer to components that are mixed in due to raw materials such as ores and scraps and other factors during industrial production of steel, and are acceptable within a range that does not adversely affect the steel according to this embodiment. However, among the impurities, the upper limits of P, S, O, and N must be restricted as described above. Also, preferably, the lower limit of P may be restricted as described above.

The high-strength steel plate of this embodiment may contain one or more of the optional elements Cu, Cr, Mo, W, Nb, V, Ti, and B shown below, as necessary to improve the strength and toughness of the steel plate (base material).

(Cu: 0% or more, 2.0% or less)
Cu is an element that may be mixed into a steel sheet as an impurity from scrap or the like. However, the lower limit of the Cu content is not particularly limited and may be 0%. Cu also has a small adverse effect on weldability and HAZ toughness, and is an element that improves the strength and toughness of the base material.
Therefore, in this embodiment, the Cu content may be 0.1% or more. However, from the viewpoint of suppressing the occurrence of Cu cracks during hot rolling of the steel sheet and suppressing the deterioration of the toughness and weldability of the large heat input HAZ, the Cu content is 2.0% or less in this embodiment.

(Cr: 0% or more, 2.0% or less)
Cr is an element that may be mixed into a steel sheet as an impurity from scrap or the like. However, the lower limit of the Cr content is not particularly limited and may be 0%. Cr is also an element that improves the strength of the base material. Therefore, in this embodiment, the Cr content may be 0.1% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the high heat input HAZ, in this embodiment, the Cr content is 2.0% or less.

(Mo: 0% or more, 2.0% or less)
Mo is an element that may be mixed into the steel plate as an impurity from scrap or the like. However, the lower limit of the Mo content is not particularly limited and may be 0%. Mo is also an element that improves the strength and toughness of the base material. Therefore, in this embodiment, the Mo content may be 0.1% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the large heat input HAZ and suppressing an increase in the alloy cost, in this embodiment, the Mo content is 2.0% or less.

(Nb: 0% or more, 0.03% or less)
Nb is an element that may be mixed into the steel plate as an impurity from scrap or the like. However, the lower limit of the Nb content is not particularly limited and may be 0%. Nb is also an element that improves the strength and toughness of the base material. Therefore, in this embodiment, the Nb content may be 0.003% or more, or may be 0.005% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the large heat input HAZ, in this embodiment, the Nb content is 0.03% or less. Preferably, the Nb content is 0.02% or less, more preferably 0.01% or less.

(V: 0% or more, 0.10% or less)
V is an element that may be mixed in as an impurity from scrap or the like. However, the lower limit of the V content is not particularly limited and may be 0%. V is also an element that improves the strength of the base metal. Therefore, in this embodiment, the V content may be 0.005% or more, or may be 0.001% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the large heat input HAZ, in this embodiment, the V content is 0.10% or less. Preferably, the V content is 0.08% or less, more preferably 0.06% or less.

(W: 0% or more, 1.0% or less)
W is an element that may be mixed in as an impurity from scrap or the like. However, the lower limit of the W content is not particularly limited and may be 0%. W is also an element that improves the strength and toughness of the base material. Therefore, in this embodiment, the W content may be 0.1% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the large heat input HAZ and suppressing an increase in alloy costs, in this embodiment, the W content is 1.0% or less. Preferably, the W content is 0.5% or less.

(Ti: 0% or more, 0.020% or less)
Ti is an element effective for increasing the strength of the base material and for refining the grains. Therefore, in this embodiment, the Ti content may be 0.003% or more, or 0.005% or more. On the other hand, since Ti does not need to be added, the Ti content may be 0%. However, from the viewpoint of suppressing the adverse effect of coarse TiN on toughness, the Ti content is 0.020% or less in this embodiment.

(B: 0% or more, 0.0050% or less)
B is an important element contained in order to ensure the hardenability of steel while limiting the carbon equivalent Ceq. Even a small amount of B significantly improves the hardenability of steel, and in order to exert this effect, the content of B may be 0.0003% or more, or may be 0.0004% or more. On the other hand, from the viewpoint of suppressing deterioration of the toughness and weldability of the high heat input HAZ, the content of B in this embodiment is 0.0050% or less.

Furthermore, in order to control the morphology of inclusions, the high-strength steel plate of this embodiment may contain one or more of the optional elements Mg, Ca, REM, and Zr shown below, as necessary.

(Ca: 0% or more, 0.005% or less)
Ca is an element that forms oxides, sulfides, and oxysulfides to suppress the generation of coarse inclusions and improve the toughness of the base material and HAZ. Therefore, in this embodiment, the Ca content may be 0.0001% or more, or may be 0.0005% or more. However, from the viewpoint of suppressing an increase in Ca-based inclusions that may act as the initiation point of brittle fracture, the Ca content is 0.005% or less in this embodiment. Preferably, the Ca content is 0.004% or less. The Ca content may be 0%.

(Mg: 0% or more, 0.005% or less)
Mg is an element that, like Ca, forms oxides, sulfides, and oxysulfides to suppress the generation of coarse inclusions and improve the toughness of the base material and HAZ. Therefore, in this embodiment, the Mg content may be 0.0001% or more, or may be 0.0005% or more. However, from the viewpoint of suppressing an increase in Mg-based inclusions that may act as the initiation point of brittle fracture, the Mg content is 0.005% or less.
The Mg content is preferably 0.003% or less. The Mg content may be 0%.

(REM: 0% or more, 0.005% or less)
REM (rare earth element) is a general term for two elements, Sc and Y, and 15 lanthanoid elements, such as La, Ce, and Nd. In this embodiment, REM is composed of one or more elements selected from these rare earth elements, and the content of REM described below is the total content of the rare earth elements.

Like Ca and Mg, REM is an element that forms oxides, sulfides, and oxysulfides to suppress the generation of coarse inclusions and increase the toughness of the base material and HAZ. Therefore, in this embodiment, the REM content may be 0.0001% or more, or may be 0.0005% or more. However, from the viewpoint of suppressing an increase in REM-based inclusions that may act as the initiation point of brittle fracture, in this embodiment, the REM content is 0.005% or less. Preferably, the REM content is 0.003% or less. The REM content may be 0%.

(Zr: 0% or more, 0.005% or less)
Zr, like Ca, Mg, and REM, is an element that forms oxides, sulfides, and oxysulfides to suppress the generation of coarse inclusions and improve the toughness of the base material and HAZ. Therefore, in this embodiment, the Zr content may be 0.0001% or more, or may be 0.0005% or more. However, from the viewpoint of suppressing an increase in Zr-based inclusions that may act as the initiation point of brittle fracture, the Zr content is 0.005% or less in this embodiment. Preferably, the Zr content is 0.003% or less.
The Zr content may be 0%.

(Thickness t: 40mm or more, 100mm or less)
(Yield strength: 630 MPa or more, 750 MPa or less)
(Tensile strength: 780 to 930 MPa)
(Yield ratio: 85% or less)
The high-strength steel plate for large heat input welding according to the present embodiment is particularly suitable for applications requiring high-strength and thick steel plate, particularly for applications requiring a high level of HAZ toughness when large heat input welding with high welding efficiency is performed. Specifically, the high-strength steel plate is suitable for high-strength thick steel plate requiring toughness of diaphragm welding HAZ (electroslag welding HAZ), such as four-sided box columns for architectural steel frames used in high-rise buildings.

As buildings become larger, construction becomes more efficient, and safety is improved, the requirements for thick steel plates for welded structures are becoming more stringent. Therefore, from the viewpoint of strength, the high-strength steel plate for large heat input welding according to this embodiment preferably has a plate thickness of 40 to 100 mm and a yield strength of 630 to 750 MPa. In addition, the tensile strength is preferably 780 to 930 MPa. In addition, from the viewpoint of workability, the yield ratio is preferably 85% or less.

Furthermore, from the viewpoint of increasing the efficiency of construction and improving safety against destruction during earthquakes, it is preferable that the average value of the Charpy absorbed energy (test temperature 0°C) in the HAZ of high heat input welds is 100 J or more. The average value of the Charpy absorbed energy (test temperature -20°C) in the HAZ of high heat input welds may be 27 J or more, or may be 40 J or more. Furthermore, it is more preferable that the fracture transition temperature (vTrs) in the HAZ of high heat input welds is 0°C or less. Examples of high heat input welding include electroslag welding and submerged arc welding.

In the high-strength steel plate for large heat-input welding according to this embodiment, in a region extending from the steel plate surface to a depth of 3 mm (sometimes referred to as a surface layer region), the maximum Vickers hardness Hvs is preferably 320 or less. This is because if the maximum Vickers hardness Hvs of the surface layer region exceeds 320, cracks are likely to occur when bending stress or tensile stress is applied.
In addition, in the high-strength steel plate for large heat input welding according to this embodiment, it is preferable that the difference ΔHv between the maximum Vickers hardness Hvs and the Vickers hardness Hvq at the 1/4 thickness position of the steel plate is 70 or less.

Next, we will explain the manufacturing method for high-strength steel plate for large heat input welding according to this embodiment.

The high-strength steel plate for large heat input welding according to the present embodiment is manufactured by melting steel, casting the steel to produce a steel slab, and hot rolling the obtained steel slab. The method for manufacturing the steel slab is not limited, and the steel slab may be manufactured by a known method. For example, the steel slab is melted by a normal refining process such as a converter or an electric furnace, and then manufactured by a known method such as a continuous casting method or an ingot-blooming method. After hot rolling, the steel slab may be directly subjected to controlled cooling such as water cooling, or may be air-cooled and then heat treated. In addition, the steel slab may be directly subjected to hot rolling after being manufactured by melting and casting the steel. However, as described later, the steel slab is preferably cooled after casting, reheated to a temperature of Ac ₃ or higher, and hot rolled.

The following describes the preferred manufacturing conditions for the high-strength steel plate for high heat input welding according to this embodiment.

A steel slab with a thickness of 200 mm or more, which is made from the above-mentioned chemical components and produced by the continuous casting method, is first cooled to 400°C or less. The steel slab is then heated to a temperature range of 900°C to 1250°C and hot-rolled to produce a steel plate with a thickness of 40 mm or more and 100 mm or less. The steel plate is subjected to various heat treatments as necessary.

If the steel slab after continuous casting is hot-charged into a heating furnace without being cooled to below 400°C, the coarse gamma structure formed during casting will remain in the steel slab after heating, and the structure of the steel plate may not be sufficiently refined, resulting in a deterioration of low-temperature toughness. For this reason, it is preferable to once cool the steel slab after continuous casting to below 400°C.

The heating temperature of the steel slab is preferably 900°C or higher in order to bring the BN precipitated in the steel slab after casting into solution. The N in the heated steel slab is fixed as AlN during hot rolling, suppressing the formation of BN. As a result, the steel plate is provided with sufficient dissolved B, which improves the hardenability of the steel. On the other hand, the heating temperature of the steel slab is preferably 1250°C or lower from the viewpoint of suppressing the coarsening of γ grains, refining the metal structure after hot rolling, and suppressing the deterioration of low-temperature toughness. The heating temperature is more preferably 1200°C or lower.

In addition, when quenching is performed directly after hot rolling, the end temperature of the hot rolling (finishing temperature) is preferably the austenite (γ) single phase region, i.e., the _Ar3 transformation point at which ferrite transformation starts, or higher. The end temperature of the hot rolling is more preferably 750°C or higher and 900°C or lower. The _Ar3 transformation point (°C) can be calculated by the following formula (7).

_Ar3 transformation point (°C) = 868-396 x C + 24.6 x Si-68.1 x Mn-36.1 x Ni-20.7 x Cu-24.8 x Cr + 29.1 x Mo ... (7)

Here, C, Si, Mn, Ni, Cu, Cr, and Mo in the above formula (7) are the contents of each element in the steel sheet expressed in mass%, and 0 is substituted for the terms of elements that are not contained.

After hot rolling, the steel plate is air-cooled or directly quenched, and various heat treatments are then performed to develop the properties of the base material. If air-cooling is performed after hot rolling, it is then reheated to the gamma single-phase region and quenched, at which point the quenching effect of B is exerted. On the other hand, if quenching is performed directly after hot rolling, as described above, hot rolling is completed in the gamma single-phase region, and the subsequent water cooling exerts the quenching effect of B. In this case, even if the temperature of the surface layer of the steel plate is in the gamma/ferrite (α) two-phase region at the end of hot rolling, there is no problem as long as the temperature inside the plate is in the gamma single-phase region.

In order to stably obtain a low yield ratio, the steel plate subjected to these quenching treatments may be reheated to a two-phase region where γ and ferrite (α) coexist, followed by quenching (two-phase region quenching). Here, the two-phase region is equal to or higher than the Ac ₁ transformation point and lower than the Ac ₃ transformation point, and the Ac ₁ transformation point and the Ac ₃ transformation point can be calculated by the following formulas (8) and (9), respectively.

Ac ₁ transformation point (°C) = 723 + 29.1Si - 10.7Mn - 16.9Ni + 6.38W + 16.9Cr ... (8)
Ac ₃ transformation point (°C) = 910 - 203√C + 44.7Si - 30Mn - 400Al - 15.2Ni + 104V + 31.5Mo + 13.1W + 11Cr + 20Cu - 700P - 400Ti ... (9)
Here, in the above formulas (8) and (9), Si, Mn, Ni, W, Cr, C, Al, V, Mo, Cu, P, and Ti are the contents of each element in the steel sheet expressed in mass%, and 0 is substituted for the terms of elements that are not contained.

Furthermore, the steel plate may be tempered to finally adjust the strength, yield ratio, and toughness of the steel plate. When tempering is performed, it is preferable to set the tempering temperature to 350 to 600°C.

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

The above manufacturing method allows the production of high-strength steel plate for large heat input welding according to this embodiment.

The high-strength steel plate for high heat input welding according to this embodiment can ensure good HAZ toughness even when high heat input welding, such as electroslag welding or submerged arc welding, is performed in which the welding heat input exceeds 50 kJ/mm.

In addition, the high-strength steel plate for large heat input welding according to this embodiment can be stably supplied, preferably having a yield strength of 630 MPa or more and an average Charpy absorbed energy (test temperature 0°C) of 100 J or more in the HAZ of a large heat input weld (e.g., an electroslag weld). Therefore, the high-strength steel plate for large heat input welding according to this embodiment is suitable for architectural steel frames, and can promote the progress of taller and larger span buildings, as well as improve construction efficiency and seismic safety.

The following is an example of the present invention. However, the following example is merely an example of the present invention, and the present invention is not limited to the example described below.

The thickness of the steel slab produced by melting steel in a converter and continuous casting is 300 mm. After continuous casting, the steel slab is cooled to room temperature, reheated to a temperature range of 1000°C to 1200°C, and hot rolled. The finishing temperature for hot rolling is 700°C to 900°C.

Next, the hot-rolled steel sheets were subjected to heat treatment under the conditions shown in Table 2-1. In Table 2-1, "quenching temperature" refers to the quenching temperature when the steel sheets are air-cooled, reheated to the γ single-phase region (at least the _Ac3 transformation point), and quenched, and "two-phase region quenching temperature" refers to the quenching temperature when the steel sheets are water-cooled or reheated to the γ single-phase region, quenched, and then reheated to the γ/α2 phase region (at least the _Ac1 transformation point and less than the _Ac3 transformation point).

Samples were taken from the steel plates manufactured in this way and chemical analysis was performed. The chemical composition of each steel plate is shown in Table 1-1 and Table 1-2, and the plate thickness is shown in Table 2-1. The carbon equivalents CeqWES, CeqIIW, and DI shown in Table 1-2 were calculated using the following formulas (10), (11), and (12), respectively.

CeqWES(%)=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 …(10)

CeqIIW(%)=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 …(11)

DI (inch) = 0.5 × fB × C ^0.5 × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 0.27 × Cu) × (1 + 0.52 × Ni) × (1 + 2.33 × Cr) × (1 + 3.14 × Mo) ... (12)

Here, in the above formulas (10) to (12), C, Mn, Si, Ni, Cr, Mo, V, and Cu are the contents of each element in the steel sheet expressed in mass%, and 0 is substituted for the terms of elements that are not contained. When the B content is 0.0004% or less, fB = 1.0, and when the B content is more than 0.0004% and 0.0050% or less, fB = 1.3.

<Mechanical properties of base material>
The test pieces used for evaluation of the mechanical properties of the base material, i.e., the tensile test and the Charpy impact test, were taken from a position corresponding to 1/4 of the plate thickness of the thick steel plate.

The tensile test was performed using two test pieces in accordance with JIS Z 2241:2011. YS (0.2% yield strength) and TS (tensile strength) are the average values of two test pieces. YR (yield ratio) is the ratio of YS to TS, and is expressed as a percentage, i.e., 100 x (YS/TS). The unit of YR (yield ratio) is %.

The Charpy impact test was conducted in accordance with JIS Z 2242:2018 using three V-notch test pieces, and the absorbed energy was measured. The test temperature was 0°C. The average (arithmetic mean) of the absorbed energies obtained for the three pieces was taken as the absorbed energy of the base material (vE0).

Surface hardness Hvs was determined by mechanically polishing the L-section of the steel plate (parallel to the rolling direction, plate thickness surface), measuring the Vickers hardness (measurement load 10 kgf) in accordance with JIS Z 2244 at three points within 3 mm from the steel plate surface in the plate thickness direction, and calculating the average value.

The hardness difference ΔHv is calculated by the following formula (13). Hvq is calculated by mechanically polishing the L-section of the steel plate (parallel to the rolling direction, plate thickness surface) and measuring the Vickers hardness (measurement load 10 kgf) according to JIS Z 2244 at three points at a position 1/4 thickness from the steel plate surface in the plate thickness direction, and averaging the results.

ΔHv = Hvs - Hvq ... (13)

<HAZ toughness of welded joint>
The HAZ toughness of the welded joints was evaluated using welded joints of each thick steel plate prepared by electroslag welding (ESW).

The T-joint illustrated in Figure 1 was produced using electroslag welding (ESW). Welding was performed in one pass, and high heat input welding with a welding heat input of 70 to 150 kJ/mm was applied. The heat input is the average heat input over the entire weld length of the T-joint shown in Figure 1.

The T-joint in FIG. 1 is fabricated by ESW as follows. First, a diaphragm 2 made of a thick steel plate is placed in a T-shape on a skin plate 1 made of a thick steel plate with a gap therebetween, and backing metals 3 and 4 are placed along the diaphragm 2 so as to sandwich the gap from the longitudinal direction of the skin plate 1. The backing metals 3 and 4 surround the gap so that the molten slag and molten metal during welding do not flow out of the welded part. Then, a welding wire is supplied into the molten slag bath inside the gap, and the welding wire is melted mainly by the resistance heat of the molten slag to form a welded metal part 5, thereby fabricating a T-joint. At the welded part of this T-joint, a test piece 7 for a Charpy impact test was taken along the center line of the plate thickness of the diaphragm 2. Specifically, as shown in FIG. 1, the test piece 7 was taken from the part from the welded metal part 5 beyond the fusion line (FL), through the welded heat affected zone (HAZ) 6 on the skin plate 1 side, and to the inside of the skin plate 1. Skin plate 1 and diaphragm 2 are made of the same steel type and have the same plate thickness.

The grain size is the grain size on the HAZ side of the FL. In the L cross section of the test piece 7, the crystal orientation is measured in the range from the FL to 0.5 mm on the HAZ side using an EBSD (electron backscatter diffraction) device. The grain size is the average circle equivalent diameter of the grains with circle equivalent diameters in the top 0.2% of grains with circle equivalent diameters of more than 1 μm, when the grain boundary is defined as a 15° high angle grain boundary.

dMA is the circle equivalent diameter of MA (Martensite-Austenite Constituent). MA is revealed in the same cross section as the sample where the grain size was measured, and the circle equivalent diameter of MA with a circle equivalent diameter of over 0.5 μm is averaged among MA with circle equivalent diameters within the top 1%. Possible methods for revealing MA include measuring the MA by binarizing the image using image processing after Lepera etching, or measuring the austenite phase using EBSD.

Test piece 7, taken as shown in Figure 1, was notched above the fusion part (FL) to obtain a V-notch test piece. Using each V-notch test piece, a Charpy impact test was performed in accordance with JIS Z 2242 at 0°C and -20°C. Three tests were performed, and the average value (arithmetic mean) of the absorbed energy was adopted. Tables 2-1 and 2-2 show the plate thickness t of the thick steel plate, the mechanical properties of the base material, and the HAZ toughness of the electroslag welded joint. Note that "vE0@FL" is the test result at 0°C when the notch position is on the FL, and "vE-20@FL" is the test result at -20°C when the notch position is on the FL.

Furthermore, a notch was made on the fusion part (FL) of the test piece 7 taken as shown in Figure 1 to make a V-notch test piece, and the fracture transition temperature (vTrs) was measured using each V-notch test piece. The fracture transition temperature (vTrs) was evaluated in accordance with JIS Z 2242:2005.

As shown in Tables 2-1 and 2-2, the steel plates of the present invention had a yield strength (YS) of 630 MPa or more and a yield ratio (YR) of 85% or less in a plate thickness of 40 to 100 mm, and when used as an ESW joint, had excellent HAZ toughness of 100 J or more at 0°C. Furthermore, the steel of the present invention had very good HAZ toughness of 27 J or more even at a test temperature of -20°C.

In addition, in Examples 1 to 10, 12, and 13, in which the P content was in the range of 0.003 to 0.010%, the fracture appearance transition temperature (vTrs) was below 0°C when used in ESW joints, and they had superior HAZ toughness. On the other hand, Example 11, in which the P content was 0.002%, had a slightly inferior fracture appearance transition temperature (vTrs).

On the other hand, as shown in Tables 1-1, 1-2, 2-1 and 2-2, the comparative examples had chemical compositions outside the range of the present invention, and therefore the mechanical properties of the base material and the HAZ toughness of the ESW joints were inferior.

Comparative Example 1 had poor HAZ toughness because the Ni content was too low, and Comparative Example 3 had poor HAZ toughness because the Si content was too high.
In Comparative Example 2, the C content was too high, and in Comparative Example 8, the DI was too high, so that the tensile strength was excessive, the surface hardness and hardness difference were large, and the HAZ toughness was poor.
Comparative Example 4 had poor HAZ toughness because the Mn and P contents were too high.
Comparative Example 5 had poor HAZ toughness because the S content and Al content were too high, and Comparative Example 6 had a too high N content and too low CeqWES.
In Comparative Example 7, CeqWES, CeqIIW and DI were too low, and in Comparative Example 9, CeqWES was too low, so the surface hardness and hardness difference were large and the HAZ toughness was poor.
In Comparative Example 10, the C content and CeqWES were too high, so that the surface hardness and hardness difference were large and the HAZ toughness was poor.
Comparative Example 11 had poor HAZ toughness because the Al content was too high.

The present invention is applied to thick steel plates manufactured in the steel industry. The present invention can also be applied to steel products other than thick steel plates, such as shaped steel. The high-strength, thick steel plate to which the present invention is applied is mainly used as the steel frame of a high-rise building, and is particularly suitable as the steel frame of a four-sided box column roughly composed of four skin plates and a diaphragm arranged inside. The joining of each member of the four-sided box column is performed by so-called high heat input welding, which involves a large welding heat input. For example, diaphragm welding for attaching a diaphragm to a skin plate and corner welding for assembling a skin plate are each performed by highly efficient high heat input welding such as electroslag welding and submerged arc welding. The high-strength steel plate for high heat input welding according to the present invention can also be used for welded structures such as bridges, ships, tanks, marine structures, and line pipes.

The high-strength steel plate for large heat input welding according to the present invention is suitable for use in high-strength, thick steel plates, where large heat input welding with high welding efficiency is performed, and where a high level of HAZ toughness is required. Specifically, the high-strength steel plate according to the present invention is a thick steel plate for large heat input welding, which has a yield strength of 630 MPa or more, a plate thickness of 40 to 100 mm, and an average value of Charpy absorbed energy (test temperature 0°C) in the HAZ of electroslag welds (box column diaphragm welds) of 100 J or more. Therefore, the high-strength steel plate for large heat input welding according to the present invention is suitable for high-strength thick steel plates requiring the toughness of the four-sided box column diaphragm weld HAZ (electroslag welding HAZ) of steel-framed buildings.

1: skin plate, 2: diaphragm, 3, 4: backing metal, 5: welded metal part, 6: welded heat affected zone (HAZ),
7. Test piece

Claims (13)

In mass percent,
C: 0.08 to 0.14%,
Mn: 0.3 to 1.0%,
Ni: 2.5 to 7.0%,
Al: 0.03 to 0.100%,
Cu: 0 to 2.0%,
Cr: 0 to 2.0%,
Mo: 0 to 2.0%,
Nb: 0 to 0.03%,
V: 0 to 0.10%,
W: 0 to 1.0%,
Ti: 0 to 0.020%,
B: 0 to 0.0050%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
REM: 0 to 0.005%,
Zr: 0 to 0.005%,
Si: 0.30% or less,
P: 0.010% or less,
S: 0.005% or less,
N: 0.0060% or less,
O: 0.0060% or less,
The balance is Fe and impurities,
The hardening factor DI (inch) calculated by the following formula (1) is 17 to 35,
The carbon equivalent CeqWES calculated by the following formula (2) is 0.600% to 0.900%,
The carbon equivalent CeqIIW calculated by the following formula (3) is 0.750% to 1.100%,
A high-strength steel plate for large heat input welding , characterized in that it is applicable to welding with a welding heat input of 70 to 150 kJ/mm .
DI (inch) = 0.5 × fB × C ^0.5 × (1 + 0.64 × Si) × (1 + 4.1 × Mn) × (1 + 0.27 × Cu) × (1 + 0.52 × Ni) × (1 + 2.33 × Cr) × (1 + 3.14 × Mo) ... (1)
CeqWES(%)=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 ... (2)
CeqIIW(%)=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 ... (3)
Here, in the above formulas (1) to (3), C, Mn, Si, Ni, Cr, Mo, V, and Cu are the contents of each element in the steel sheet expressed in mass%, and 0 is substituted for the term of an element that is not contained. When the B content is 0.0004% or less, fB = 1.0, and when the B content is more than 0.0004% and 0.0050% or less, fB = 1.3. In mass percent,
Ti: 0.003 to 0.020%
The high strength steel plate for large heat input welding according to claim 1, further comprising: In mass percent,
B: 0.0004 to 0.0050%
3. The high strength steel plate for large heat input welding according to claim 1, further comprising: In mass percent,
Cu: 0.1 to 2.0%,
Cr: 0.1 to 2.0%,
Mo: 0.1 to 2.0%,
The high-strength steel plate for large heat input welding according to any one of claims 1 to 3, further comprising at least one selected from the group consisting of: In mass percent,
Nb: 0.003 to 0.03%,
V: 0.01 to 0.10%,
W: 0.10 to 1.0%
The high-strength steel plate for large heat input welding according to any one of claims 1 to 4, further comprising at least one selected from the group consisting of: In mass percent,
Ca: 0.0005 to 0.005%,
Mg: 0.0005 to 0.005%,
REM: 0.0005-0.005%,
The high-strength steel plate for large heat input welding according to any one of claims 1 to 5, further comprising one or more elements selected from the group consisting of Zr: 0.0005 to 0.005% or less. A high-strength steel plate for large heat input welding according to any one of claims 1 to 6, characterized in that the P content is 0.003 to 0.010% by mass. A high-strength steel plate for large heat input welding according to any one of claims 1 to 7, characterized in that the maximum Vickers hardness Hvs is 320 or less in a region extending from the surface of the steel plate to a depth of 3 mm. A high-strength steel plate for large heat input welding according to any one of claims 1 to 8, characterized in that the difference ΔHv between the maximum Vickers hardness Hvs and the Vickers hardness Hvq at the 1/4 thickness position of the steel plate is 70 or less. Yield strength is 630 to 750 MPa,
Tensile strength: 780-930 MPa;
The high strength steel plate for large heat input welding according to any one of claims 1 to 9, characterized in that the yield ratio is 85% or less. The high-strength steel plate for large heat input welding according to any one of claims 1 to 10, characterized in that in a welded joint with a welding heat input of 70 to 150 kJ/mm , the 0°C Charpy absorbed energy in the HAZ of the weld is an average of 100 J or more. A high-strength steel plate for large heat input welding according to any one of claims 1 to 11, characterized in that the plate thickness t is 40 mm or more and 100 mm or less. A high-strength steel plate for large heat input welding according to any one of claims 1 to 12, characterized in that it is a steel plate for architectural steel box columns of high-rise buildings.

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