Steel plates and welded structures

Abstract

The present invention provides a welded structure having excellent heat-affected zone (HAZ) toughness and suppressed strength inhomogeneity in the HAZ, and a steel plate from which such a welded structure can be obtained. [Solution] The steel sheet according to this disclosure has a chemical composition in mass%, consisting of C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, O: 0.004% or less, and the remainder being Fe and impurities. The f1 specified in the specification is less than 0.0000, the f2 specified in the specification is 0.320~0.440, and the f3 specified in the specification is 150~205.

Description

[Technical Field]

[0001] This disclosure relates to steel plates and welded structures manufactured by welding steel plates. [Background technology]

[0002] In recent years, welded box-section columns (also known as "welded four-sided box columns") are sometimes used as column members in building structures. Welded four-sided box columns are manufactured by welding together four steel plates (skin plates) so that the cross-section is rectangular.

[0003] Welded four-sided box columns may have other members joined to them. For example, a welded four-sided box column may contain a steel plate (diaphragm) inside the column. In this case, the diaphragm is positioned perpendicular to the longitudinal direction of the welded four-sided box column, increasing its strength.

[0004] Specifically, when manufacturing such a welded four-sided box column, for example, the skin plates are welded together by submerged arc welding with high heat input. For example, the skin plates and diaphragm are further welded together by electroslag welding with high heat input. When welding with such high heat input is performed, the toughness may decrease in the heat-affected zone (HAZ). Hereafter, the toughness of the steel plate in the HAZ after welding will also be simply referred to as "HAZ toughness".

[0005] To date, techniques for improving the HAZ toughness of steel plates have been proposed in Japanese Patent Publication No. 2007-126725 (Patent Document 1) and Japanese Patent Publication No. 2007-327100 (Patent Document 2).

[0006] The steel sheet disclosed in Patent Document 1 is a high-tensile steel sheet with excellent toughness in the heat-affected zone of a large heat input weld, and is characterized by having, by mass%, C: 0.02~0.05%, Si: 0.05~0.20%, Mn: 1.0~2.5%, P: 0.02% or less, S: 0.005% or less, Al: 0.01~0.05%, Ni: 0.2~2.0%, Cr: 0.5~2.0%, Ti: 0.005~0.025%, N: 0.004~0.010%, and the remainder being Fe and impurities, and satisfying the following formulas (1) and (2). Patent Document 1 states that with this steel sheet, excellent HAZ toughness can be ensured even when performing submerged arc welding or electroslag welding with a large heat input exceeding 500 kJ / cm. 2.3≦(Mn+0.7×Ni+Cr)≦3.7 (1) [Cr / (Mn+0.7×Ni)]≧0.3 (2)

[0007] The steel sheet disclosed in Patent Document 2 is a thick steel sheet with excellent toughness in the heat-affected zone of a high-heat-input weld with a welding heat input of 20 to 150 kJ / mm, and is characterized by containing, by mass%, C: 0.03 to 0.2%, Si: 0.5% or less, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.001 to 0.005%, Al: 0.001 to 0.1%, V: 0.01 to 0.1%, B: less than 0.0001 to 0.0003%, N: 0.001 to 0.006%, O: 0.004% or less, with the remainder being Fe and impurities. Patent Document 2 states that with this steel sheet, good HAZ toughness can be ensured even when high-heat-input welding, which has a high welding efficiency, is performed. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2007-126725 [Patent Document 2] Japanese Patent Publication No. 2007-327100 [Overview of the project] [Problems that the invention aims to solve]

[0009] Incidentally, in the heat-affected zone (HAZ) of steel after welding, a low-strength region (softening) may occur in a part thereof due to the heat input by welding and subsequent cooling. That is, the HAZ formed in the welded structure may have non-uniform strength due to a locally reduced strength. However, it is preferable to suppress such non-uniformity of strength in the HAZ.

[0010] Patent Documents 1 and 2 propose techniques for enhancing the toughness of the HAZ of steel plates. However, the toughness of the HAZ of steel plates may be enhanced by techniques other than those proposed in Patent Documents 1 and 2. Furthermore, Patent Documents 1 and 2 do not consider at all suppressing the non-uniformity of strength in the HAZ of the welded structure.

[0011] An object of the present disclosure is to provide a welded structure having excellent HAZ toughness and in which non-uniformity of strength in the HAZ is suppressed, and a steel plate capable of obtaining such a welded structure.

Means for Solving the Problems

[0012] The steel plate according to the present disclosure is The chemical composition is, in mass%, C: 0.060 to 0.120%, Si: 0.05 to 0.55%, Mn: 0.70 to 1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02 to 0.30%, Ti: 0.007 to 0.016%, Nb: 0.005 to 0.030%, Al: 0.010 to 0.050%, N: 0.0040 to 0.0100%, [[ID=4O]] B: 0.0008 to 0.0030%, O: 0.004% or less, and The balance: consisting of Fe and impurities, f1 defined by formula (1) satisfies less than 0.0000, The f2A defined by equation (2A) satisfies 0.320 to 0.440, The value of f3A defined by equation (3A) satisfies 150 to 205. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2A=[C]+[Mn] / 6+[Si] / 24+[Mo] / 4 (2A) f3A = 99 + 330[Mo] (3A) Here, the elemental symbols in equations (1), (2A), and (3A) are substituted with the content of the corresponding element in units of mass%.

[0013] The steel plate according to this disclosure is The chemical composition is expressed in mass percent. C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, and, It contains 0.004% or less of O, and furthermore, Cu: 0.50% or less, Ni: 1.20% or less, Cr: 0.50% or less, W: 0.100% or less, Co:0.100% or less, V: 0.08% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Rare earth elements: 0.0100% or less, Zr: 0.050% or less, Hf: 0.020% or less, Ta: 0.100% or less, Sr: 0.020% or less, Sn: 0.100% or less, Sb: 0.050% or less, Zn: 0.020% or less, Bi: 0.100% or less, Se: 0.020% or less, Pb: 0.090% or less, As: 0.050% or less, and, It contains one or more elements selected from the group consisting of Te: 0.050% or less, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The f2 defined in equation (2) satisfies 0.320 to 0.440, The value of f3 defined in equation (3) satisfies the range of 150 to 205. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equations (1) to (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0014] The welded structure described herein is It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, O: 0.004% or less, and, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The f2A defined by equation (2A) satisfies 0.320 to 0.440, The value of f3A defined by equation (3A) satisfies 150 to 205. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2A=[C]+[Mn] / 6+[Si] / 24+[Mo] / 4 (2A) f3A = 99 + 330[Mo] (3A) Here, the elemental symbols in equations (1), (2A), and (3A) are substituted with the content of the corresponding element in units of mass%.

[0015] The welded structure described herein is It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, and, It contains 0.004% or less of O, and furthermore, Cu: 0.50% or less, Ni: 1.20% or less, Cr: 0.50% or less, W: 0.100% or less, Co:0.100% or less, V: 0.08% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Rare earth elements: 0.0100% or less, Zr: 0.050% or less, Hf: 0.020% or less, Ta: 0.100% or less, Sr: 0.020% or less, Sn: 0.100% or less, Sb: 0.050% or less, Zn: 0.020% or less, Bi: 0.100% or less, Se: 0.020% or less, Pb: 0.090% or less, As: 0.050% or less, and, It contains one or more elements selected from the group consisting of Te: 0.050% or less, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The f2 defined in equation (2) satisfies 0.320 to 0.440, The value of f3 defined in equation (3) satisfies the range of 150 to 205. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equations (1) to (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol. [Effects of the Invention]

[0016] The welded structure according to this disclosure has excellent heat-affected zone (HAZ) toughness and suppresses strength heterogeneity in the HAZ. The steel plate according to this disclosure makes it possible to obtain the above-mentioned welded structure. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 is a schematic perspective view of an example of a welded structure according to this embodiment. [Figure 2] Figure 2 is a schematic diagram showing a cross-section passing through the central axis of an example of a welded structure shown in Figure 1. [Figure 3] Figure 3 is a magnified view of a portion of region 100 shown in Figure 2. [Modes for carrying out the invention]

[0018] The inventors of this invention investigated steel plates that, when used as a material and welded, have excellent HAZ (heat-affected zone) toughness and suppressed strength heterogeneity in the HAZ, resulting in a welded structure.

[0019] First, the inventors focused on the chemical composition and considered how to achieve both excellent HAZ toughness and suppression of strength heterogeneity in the HAZ. Specifically, the inventors considered that it might be possible to achieve both excellent HAZ toughness and suppression of strength heterogeneity in the HAZ by utilizing boron (B). Therefore, the inventors considered increasing the B content to 0.0008 to 0.0030%.

[0020] More specifically, the inventors have determined, in mass%, that C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, O: 0.004% or less, Cu: 0~0.50%, Ni: 0~1.20%, Cr: 0~0.50%, W: 0~0.100%, Co: 0~0.100%, V: 0~0.08%, Mg: We considered that a steel sheet having a chemical composition consisting of 0-0.0100%, Ca:0-0.0100%, rare earth elements:0-0.0100%, Zr:0-0.050%, Hf:0-0.020%, Ta:0-0.100%, Sr:0-0.020%, Sn:0-0.100%, Sb:0-0.050%, Zn:0-0.020%, Bi:0-0.100%, Se:0-0.020%, Pb:0-0.090%, As:0-0.050%, Te:0-0.050%, and the remainder being Fe and impurities, could potentially achieve both excellent HAZ toughness and suppression of strength heterogeneity in the HAZ.

[0021] On the other hand, even with steel sheets having the above-mentioned chemical composition, it was sometimes impossible to achieve both excellent HAZ toughness and suppression of strength heterogeneity in the HAZ. Therefore, the inventors investigated various methods to improve the excellent HAZ toughness and suppress strength heterogeneity in the HAZ for steel sheets having the above-mentioned chemical composition. As a result, the inventors obtained the following findings.

[0022] As described above, the chemical composition of the steel sheet according to this embodiment has a B content increased to 0.0008-0.0030%. Here, B combines with nitrogen (N) to form B nitrides. Furthermore, in steel material subjected to welding heat, B nitrides can potentially act as nuclei for ferrite. Therefore, if a large number of B nitrides are formed in the steel sheet, fine ferrite grains may be formed in the heat-affected zone (HAZ), potentially increasing the HAZ toughness after welding. In other words, assuming a B content of 0.0008-0.0030%, precipitating a large number of B as nitrides may potentially increase the HAZ toughness after welding.

[0023] Specifically, assuming that the steel sheet according to this embodiment has the above-described chemical composition, f1 as defined by formula (1) is set to less than 0.0000. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%.

[0024] f1 corresponds to the amount (mass%) of B that does not form B nitride when considered in terms of stoichiometric ratio. Therefore, if the B content is similar, the smaller f1, the easier it is for B nitride to precipitate in the HAZ after welding. Specifically, if the B content is 0.0008% or more and f1 is less than 0.0000, a large amount of B nitride may precipitate in the HAZ after welding, potentially increasing the HAZ toughness. Accordingly, assuming that the steel sheet according to this embodiment has the above-described chemical composition, f1 is set to less than 0.0000.

[0025] On the other hand, in order to suppress the heterogeneity of strength in the heat-affected zone (HAZ) of steel plates having the above-mentioned chemical composition, it is sufficient to suppress the uneven distribution of hardness in the HAZ after welding. Therefore, the inventors investigated how to stabilize the hardness of steel plates by adjusting the chemical composition. As a result, the inventors obtained the following findings.

[0026] Specifically, the steel sheet according to this embodiment has the above-described chemical composition, and assuming that f1 is less than 0.0000, f2, as defined by formula (2), is set to 0.320 to 0.440. f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) Here, the elemental symbols in equation (2) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0027] f2 is an index of the hardness of the steel plate and the HAZ. If f2 is too low, the hardness of the steel plate cannot be sufficiently increased, and the strength of the HAZ tends to decrease. On the other hand, if f2 is too high, the hardness of the steel plate becomes too high, and the toughness of the HAZ decreases. Therefore, the steel plate according to this embodiment has the above-mentioned chemical composition, and assuming that f1 is less than 0.0000, f2 is set to 0.320 to 0.440.

[0028] The inventors further investigated methods to suppress strength heterogeneity in the HAZ after welding while maintaining HAZ toughness. As a result, it was found that in steel sheets with a B content of 0.0008 to 0.0030%, the uniformity of HAZ strength can be improved while maintaining HAZ toughness by controlling the content of copper (Cu), molybdenum (Mo), nickel (Ni), and chromium (Cr).

[0029] Specifically, the steel sheet according to this embodiment has the above-described chemical composition, and assuming that f1 is less than 0.0000 and f2 is between 0.320 and 0.440, f3, as defined by formula (3), is set to between 150 and 205. f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equation (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0030] f3 is an index of the hardness of the HAZ after welding. If f3 is too low, the hardness of the HAZ cannot be sufficiently increased, and the strength of the HAZ after welding tends to decrease. On the other hand, if f3 is too high, the hardness of the HAZ becomes too high, and the HAZ toughness decreases. Therefore, assuming that the steel plate according to this embodiment has the above-mentioned chemical composition, f1 is less than 0.0000, and f2 is 0.320 to 0.440, f3, as defined by formula (3), is set to 150 to 205.

[0031] In other words, the steel plate according to this embodiment has the above-described chemical composition, with f1 being less than 0.0000, f2 being 0.320 to 0.440, and f3 being 150 to 205. As a result, the welded structure according to this embodiment can achieve both excellent HAZ toughness and suppression of strength heterogeneity in the HAZ.

[0032] Based on the above findings, the gist of the steel plate and welded structure according to this embodiment is as follows.

[0033] [1] The chemical composition is expressed in mass percent. C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, O: 0.004% or less, and, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The f2A defined by equation (2A) satisfies 0.320 to 0.440, The f3A defined by equation (3A) satisfies 150 to 205. steel plate. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2A=[C]+[Mn] / 6+[Si] / 24+[Mo] / 4 (2A) f3A = 99 + 330[Mo] (3A) Here, the elemental symbols in equations (1), (2A), and (3A) are substituted with the content of the corresponding element in units of mass%.

[0034] [2] The chemical composition is expressed in mass percent. C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, and, It contains 0.004% or less of O, and furthermore, Cu: 0.50% or less, Ni: 1.20% or less, Cr: 0.50% or less, W: 0.100% or less, Co:0.100% or less, V: 0.08% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Rare earth elements: 0.0100% or less, Zr: 0.050% or less, Hf: 0.020% or less, Ta: 0.100% or less, Sr: 0.020% or less, Sn: 0.100% or less, Sb: 0.050% or less, Zn: 0.020% or less, Bi: 0.100% or less, Se: 0.020% or less, Pb: 0.090% or less, As: 0.050% or less, and, It contains one or more elements selected from the group consisting of Te: 0.050% or less, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The f2 defined in equation (2) satisfies 0.320 to 0.440, The f3 defined in equation (3) satisfies 150 to 205. steel plate. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equations (1) to (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0035] [3] The steel plate described in [1] or [2], In the aforementioned steel plate, B-containing particles with an equivalent circular diameter of 300 nm or less, 1.0 × 10 4 pieces / mm 2 Contains the above, steel plate.

[0036] [4] It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, O: 0.004% or less, and, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The f2A defined by equation (2A) satisfies 0.320 to 0.440, The f3A defined by equation (3A) satisfies 150 to 205. Welded structure. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2A=[C]+[Mn] / 6+[Si] / 24+[Mo] / 4 (2A) f3A = 99 + 330[Mo] (3A) Here, the elemental symbols in equations (1), (2A), and (3A) are substituted with the content of the corresponding element in units of mass%.

[0037] [5] It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050%, N: 0.0040~0.0100%, B: 0.0008~0.0030%, and, It contains 0.004% or less of O, and furthermore, Cu: 0.50% or less, Ni: 1.20% or less, Cr: 0.50% or less, W: 0.100% or less, Co:0.100% or less, V: 0.08% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Rare earth elements: 0.0100% or less, Zr: 0.050% or less, Hf: 0.020% or less, Ta: 0.100% or less, Sr: 0.020% or less, Sn: 0.100% or less, Sb: 0.050% or less, Zn: 0.020% or less, Bi: 0.100% or less, Se: 0.020% or less, Pb: 0.090% or less, As: 0.050% or less, and, It contains one or more elements selected from the group consisting of Te: 0.050% or less, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The f2 defined in equation (2) satisfies 0.320 to 0.440, The f3 defined in equation (3) satisfies 150 to 205. Welded structure. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equations (1) to (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0038] [6] A welded structure as described in [4] or [5], In the aforementioned base steel sheet, B-containing particles with an equivalent circular diameter of 300 nm or less, 1.0 × 10 4 pieces / mm 2 Contains the above, Welded structure.

[0039] [7] A welded structure described in any one of items [4] to [6], In the heat-affected zone of the aforementioned base steel plate, In the microstructure, the area ratio of ferrite is 90.0% or less. Welded structure.

[0040] [8] A welded structure described in any one of items [4] to [7], In the area affected by welding heat, The area percentage with a Vickers hardness of 150 Hv or less is 50.0% or less. Welded structure.

[0041] The configuration of the steel plate and welded structure according to this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass %.

[0042] [Steel plate] The steel plate of this embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition is within the range of this embodiment. (Feature 2) The value of f1 defined by equation (1) is less than 0.0000. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%. (Feature 3) The value of f2 defined by equation (2) satisfies 0.320 to 0.440. f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) Here, the elemental symbols in equation (2) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol. (Feature 4) The value of f3 defined in equation (3) satisfies the range of 150 to 205. f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equation (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol. The following describes each of its features.

[0043] [(Feature 1) Chemical Composition] The chemical composition of the steel sheet according to this embodiment contains the following elements:

[0044] C: 0.060~0.120% Carbon (C) enhances the hardenability of steel plates and increases the strength of the heat-affected zone (HAZ) after welding. If the C content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content is too high, even if the content of other elements is within the range of this embodiment, excessive carbides will be formed in the steel plate, reducing the toughness of the HAZ after welding. Therefore, the C content is 0.060 to 0.120%. The preferred lower limit of the C content is 0.065%, and more preferably 0.070%. The preferred upper limit of the C content is 0.115%, and more preferably 0.110%.

[0045] Si: 0.05~0.55% Silicon (Si) deoxidizes steel and increases the strength of the heat-affected zone (HAZ) after welding. If the Si content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content is too high, even if the content of other elements is within the range of this embodiment, MA (major oxidative) structure is more likely to form in the steel sheet after welding, and the HAZ toughness decreases. Therefore, the Si content is 0.05 to 0.55%. The preferred lower limit of the Si content is 0.08%, more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit of the Si content is 0.50%, more preferably 0.45%, and even more preferably 0.40%.

[0046] Mn: 0.70~1.60% Manganese (Mn) enhances the hardenability of steel plates and increases the strength of the heat-affected zone (HAZ) after welding. If the Mn content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content is too high, even if the content of other elements is within the range of this embodiment, MA (manganese-associated) structure is more likely to form in the steel plate after welding, reducing the HAZ toughness. Therefore, the Mn content is 0.70 to 1.60%. The preferred lower limit of the Mn content is 0.75%, and more preferably 0.80%. The preferred upper limit of the Mn content is 1.55%, and more preferably 1.50%.

[0047] P:0.020% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content is too high, even if the content of other elements is within the range of this embodiment, P will segregate at the grain boundaries, reducing the HAZ toughness after welding. Therefore, the P content is 0.020% or less. The preferred upper limit of the P content is 0.015%, and more preferably 0.010%. It is preferable to have as low a P content as possible. However, an extreme reduction in the P content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and still more preferably 0.003%.

[0048] S: 0.008% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is greater than 0%. If the S content is too high, even if the content of other elements is within the range of this embodiment, coarse sulfides will form in the steel sheet, reducing the HAZ toughness after welding. Therefore, the S content is 0.008% or less. The preferred upper limit of the S content is 0.007%, and more preferably 0.006%. It is preferable to have as low an S content as possible. However, an extreme reduction in the S content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.001%, and more preferably 0.002%.

[0049] Mo: 0.02~0.30% Molybdenum (Mo) enhances the hardenability of steel plates and suppresses strength inhomogeneity in the heat-affected zone (HAZ) after welding. If the Mo content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel plate becomes too high, and the HAZ toughness after welding decreases. Therefore, the Mo content is 0.02 to 0.30%. The preferred lower limit of the Mo content is 0.03%, more preferably 0.04%, and even more preferably 0.05%. The preferred upper limit of the Mo content is 0.25%, and even more preferably 0.20%.

[0050] Ti: 0.007~0.016% Titanium (Ti) combines with nitrogen to form Ti nitrides, which enhance HAZ toughness through a pinning effect. If the Ti content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ti content is too high, even if the content of other elements is within the range of this embodiment, coarse Ti nitrides are formed, and the HAZ toughness after welding decreases. Therefore, the Ti content is 0.007 to 0.016%. The preferred lower limit of the Ti content is 0.008%, more preferably 0.009%, and even more preferably 0.010%. The preferred upper limit of the Ti content is 0.015%, more preferably 0.014%, and even more preferably 0.013%.

[0051] Nb: 0.005~0.030% Niobium (Nb) combines with carbon to form carbides and other compounds, thereby increasing the strength of the heat-affected zone (HAZ) after welding. If the Nb content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, excessive carbides and other compounds will be formed, reducing the toughness of the HAZ after welding. Therefore, the Nb content is 0.005 to 0.030%. The preferred lower limit of the Nb content is 0.006%, more preferably 0.007%, and even more preferably 0.008%. The preferred upper limit of the Nb content is 0.028%, more preferably 0.025%, and even more preferably 0.023%.

[0052] Al: 0.010~0.050% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content is too high, even if the content of other elements is within the range of this embodiment, coarse oxide inclusions are formed, and the HAZ toughness after welding decreases. Therefore, the Al content is 0.010 to 0.050%. The preferred lower limit of the Al content is 0.012%, and more preferably 0.015%. The preferred upper limit of the Al content is 0.045%, more preferably 0.040%, and still more preferably 0.035%. As used herein, "Al" content refers to "acid-soluble Al," that is, the content of "sol.Al."

[0053] N: 0.0040~0.0100% Nitrogen (N) combines with Ti to form Ti nitrides, which enhance HAZ toughness through a pinning effect. If the N content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the N content is too high, even if the content of other elements is within the range of this embodiment, the amount of N dissolved in the steel sheet increases, and the HAZ toughness after welding decreases. Therefore, the N content is 0.0040 to 0.0100%. The preferred lower limit of the N content is 0.0045%, more preferably 0.0050%, and even more preferably 0.0055%. The preferred upper limit of the N content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0080%.

[0054] B: 0.0008~0.0030% Boron (B) forms B nitrides, which act as ferrite transformation nuclei, thereby increasing the toughness of the heat-affected zone (HAZ). Furthermore, fine B nitrides dissolve in the HAZ, forming solid solutions in the steel and improving hardenability. As a result, it may be possible to suppress the heterogeneity of the strength in the HAZ after welding. If the B content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the B content is too high, even if the content of other elements is within the range of this embodiment, coarse B nitrides are formed, and the toughness of the HAZ after welding decreases. Therefore, the B content is 0.0008 to 0.0030%. The preferred lower limit of the B content is 0.0009%, and more preferably 0.0010%. The preferred upper limit of the B content is 0.0028%, more preferably 0.0025%, and still more preferably 0.0020%.

[0055] O: 0.004% or less Oxygen (O) is an unavoidable impurity. That is, the lower limit of the O content is greater than 0%. If the O content is too high, oxides will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the O content is 0.004% or less. The preferred upper limit of the O content is less than 0.004%, and more preferably 0.003%. It is preferable to have as low an O content as possible. However, extreme reduction of the O content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.001%.

[0056] The remainder of the chemical composition of the steel sheet according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel sheet, and are acceptable within a range that does not adversely affect the steel sheet according to this embodiment.

[0057] [Optional element] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, Cr, W, and Co in place of a portion of Fe. All of these elements are arbitrary and enhance the hardenability of the steel sheet and suppress the heterogeneity of strength in the heat-affected zone (HAZ) after welding.

[0058] Cu: 0~0.50% Copper (Cu) is an optional element and may not be included. That is, the Cu content may be 0%. If included, Cu enhances the hardenability of the steel sheet and suppresses the heterogeneity of the strength in the HAZ after welding. Even if only a small amount of Cu is included, the above effects can be obtained to some extent. However, if the Cu content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel sheet will become too high and the HAZ toughness after welding will decrease. Therefore, the Cu content is 0 to 0.50%, and if included, the Cu content is 0.50% or less. The preferred lower limit of the Cu content is greater than 0%, more preferably 0.01%, and even more preferably 0.03%. The preferred upper limit of the Cu content is 0.48%, and even more preferably 0.45%.

[0059] Ni: 0~1.20% Nickel (Ni) is an optional element and may not be included. That is, the Ni content may be 0%. If included, Ni enhances the hardenability of the steel sheet and suppresses the heterogeneity of the strength in the HAZ after welding. Even if only a small amount of Ni is included, the above effects can be obtained to some extent. However, if the Ni content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel sheet will become too high and the HAZ toughness after welding will decrease. Therefore, the Ni content is 0 to 1.20%, and if included, it is 1.20% or less. The preferred lower limit of the Ni content is greater than 0%, more preferably 0.01%, even more preferably 0.02%, and even more preferably 0.05%. The preferred upper limit of the Ni content is 1.15%, and even more preferably 1.00%.

[0060] Cr: 0~0.50% Chromium (Cr) is an optional element and may not be present. That is, the Cr content may be 0%. If present, Cr enhances the hardenability of the steel sheet and suppresses the heterogeneity of strength in the HAZ after welding. Even a small amount of Cr will provide some of the above effects. However, if the Cr content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel sheet will become too high and the HAZ toughness after welding will decrease. Therefore, the Cr content is 0 to 0.50%, and if present, it is 0.50% or less. The preferred lower limit of the Cr content is greater than 0%, more preferably 0.01%, more preferably 0.03%, more preferably 0.05%, and more preferably 0.08%. The preferred upper limit of the Cr content is 0.45%, and more preferably 0.40%.

[0061] W: 0~0.100% Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. If included, W enhances the hardenability of the steel plate and suppresses the heterogeneity of the strength in the HAZ after welding. Even if only a small amount of W is included, the above effects can be obtained to some extent. However, if the W content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel plate will become too high and the HAZ toughness after welding will decrease. Therefore, the W content is 0 to 0.100%, and if included, it is 0.100% or less. The preferred lower limit of the W content is greater than 0%, more preferably 0.001%, and even more preferably 0.005%. The preferred upper limit of the W content is 0.090%, and even more preferably 0.080%.

[0062] Co: 0~0.100% Cobalt (Co) is an optional element and may not be present. In other words, the Co content may be 0%. When present, Co enhances the hardenability of the steel plate and suppresses the heterogeneity of the strength in the HAZ after welding. Even a small amount of Co will provide some of the above effects. However, if the Co content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel plate will become too high, and the HAZ toughness after welding will decrease. Therefore, the Co content is 0 to 0.100%, and if present, it is 0.100% or less. The preferred lower limit of the Co content is greater than 0%, more preferably 0.001%, and even more preferably 0.005%. The preferred upper limit of the Co content is 0.080%, more preferably 0.060%, and even more preferably 0.050%.

[0063] The chemical composition of the steel sheet according to this embodiment may further contain V instead of some of the Fe.

[0064] V: 0~0.08% Vanadium (V) is an optional element and may not be present. That is, the V content may be 0%. If present, V forms carbides and the like, suppressing the heterogeneity of the strength in the HAZ after welding. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content is too high, even if the content of other elements is within the range of this embodiment, excessive carbides and the like will be formed, and the toughness of the HAZ after welding will decrease. Therefore, the V content is 0 to 0.08%, and if present, it is 0.08% or less. The preferred lower limit of the V content is greater than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the V content is less than 0.08%, and even more preferably 0.07%.

[0065] The chemical composition of the steel sheet according to this embodiment may further contain, in place of some of the Fe, one or more elements selected from the group consisting of Mg, Ca, rare earth elements, Zr, Hf, Ta, and Sr. Any of these elements are arbitrary and enhance the HAZ toughness after welding.

[0066] Mg: 0~0.0100% Magnesium (Mg) is an optional element and may not be present. That is, the Mg content may be 0%. If present, Mg forms oxides, sulfides, and / or acid sulfides, thereby suppressing the formation of coarse inclusions and improving the HAZ toughness after welding. Mg further forms fine inclusions that suppress the coarsening of austenite grains in the HAZ, thereby improving the HAZ toughness after welding. Even a small amount of Mg can provide some of the above effects. However, if the Mg content is too high, coarse inclusions will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the Mg content is 0 to 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, even more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the Mg content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0040%.

[0067] Ca: 0~0.0100% Calcium (Ca) is an optional element and may not be present. That is, the Ca content may be 0%. If present, Ca forms oxides, sulfides, and / or acid sulfides, thereby suppressing the formation of coarse inclusions and improving the HAZ toughness after welding. Even a small amount of Ca present will provide some degree of the above effect. However, if the Ca content is too high, coarse inclusions will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the Ca content is 0 to 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the Ca content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%.

[0068] Rare earth elements (REM): 0~0.0100% Rare earth elements (REMs) are optional and may not be present. That is, the REM content may be 0%. If present, REMs form oxides, sulfides, and / or acid sulfides, thereby suppressing the formation of coarse inclusions and improving the HAZ toughness after welding. Even a small amount of REM can provide some of the above effect. However, if the REM content is too high, coarse inclusions will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the REM content is 0 to 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the REM content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. The preferred upper limit of the REM content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%.

[0069] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this specification, REM content refers to the total content of these elements.

[0070] Zr: 0~0.050% Zirconium (Zr) is an optional element and may not be present. That is, the Zr content may be 0%. If present, Zr forms oxides, sulfides, and / or acid sulfides, thereby suppressing the formation of coarse inclusions and improving the HAZ toughness after welding. Even a small amount of Zr present will provide some degree of the above effect. However, if the Zr content is too high, coarse inclusions will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the Zr content is 0 to 0.050%, and if present, it is 0.050% or less. The preferred lower limit of the Zr content is greater than 0%, more preferably 0.001%, and even more preferably 0.003%. The preferred upper limit of the Zr content is 0.040%, more preferably 0.020%, and even more preferably 0.010%.

[0071] Hf: 0~0.020% Hafnium (Hf) is an optional element and may not be present. That is, the Hf content may be 0%. If present, Hf forms oxides, sulfides, and / or acid sulfides, thereby suppressing the formation of coarse inclusions and improving the HAZ toughness after welding. Even a small amount of Hf present will provide some degree of the above effect. However, if the Hf content is too high, coarse inclusions will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the Hf content is 0 to 0.020%, and if present, it is 0.020% or less. The preferred lower limit of the Hf content is greater than 0%, more preferably 0.001%, and even more preferably 0.003%. The preferred upper limit of the Hf content is 0.015%, and even more preferably 0.010%.

[0072] Ta: 0~0.100% Tantalum (Ta) is an optional element and may not be present. That is, the Ta content may be 0%. If present, Ta forms oxides that suppress the formation of coarse inclusions, thereby improving the HAZ toughness after welding. Even a small amount of Ta present will provide some degree of the above effect. However, if the Ta content is too high, coarse inclusions will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the Ta content is 0 to 0.100%, and if present, it is 0.100% or less. The preferred lower limit of the Ta content is greater than 0%, more preferably 0.001%, and even more preferably 0.005%. The preferred upper limit of the Ta content is 0.080%, and even more preferably 0.060%.

[0073] Sr: 0~0.020% Strontium (Sr) is an optional element and may not be present. That is, the Sr content may be 0%. If present, Sr forms oxides, sulfides, and / or acid sulfides, thereby suppressing the formation of coarse inclusions and improving the HAZ toughness after welding. Even a small amount of Sr present will provide some degree of the above effect. However, if the Sr content is too high, coarse inclusions will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the Sr content is 0 to 0.020%, and if present, it is 0.020% or less. The preferred lower limit of the Sr content is greater than 0%, and more preferably 0.001%. The preferred upper limit of the Sr content is 0.015%, more preferably 0.010%, and still more preferably 0.005%.

[0074] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of Sn, Sb, and Zn in place of a portion of Fe. Any of these elements are arbitrary and enhance the corrosion resistance of the steel sheet.

[0075] Sn: 0~0.100% Tin (Sn) is an optional element and may not be present. That is, the Sn content may be 0%. If present, Sn enhances the corrosion resistance of the steel sheet. Even a small amount of Sn will provide some degree of the above effect. However, if the Sn content is too high, the hot workability will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is 0 to 0.100%, and if present, it is 0.100% or less. The preferred lower limit of the Sn content is greater than 0%, more preferably 0.001%, more preferably 0.005%, and still more preferably 0.010%. The preferred upper limit of the Sn content is 0.080%, more preferably 0.060%, and still more preferably 0.040%.

[0076] Sb: 0~0.050% Antimony (Sb) is an optional element and may not be present. That is, the Sb content may be 0%. If present, Sb enhances the corrosion resistance of the steel sheet. Even a small amount of Sb will provide some degree of the above effect. However, if the Sb content is too high, the hot workability will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is 0 to 0.050%, and if present, it is 0.050% or less. The preferred lower limit of the Sb content is greater than 0%, more preferably 0.001%, and even more preferably 0.005%. The preferred upper limit of the Sb content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0077] Zn: 0~0.020% Zinc (Zn) is an optional element and may not be present. That is, the Zn content may be 0%. If present, Zn enhances the corrosion resistance of the steel sheet. Even a small amount of Zn will provide some degree of the above effect. However, if the Zn content is too high, the hot workability will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Zn content is 0 to 0.020%, and if present, it is 0.020% or less. The preferred lower limit of the Zn content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Zn content is 0.015%, and even more preferably 0.010%.

[0078] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of Bi, Se, Pb, As, and Te in place of a portion of Fe. Any of these elements are arbitrary and enhance the machinability of the steel sheet.

[0079] Bi: 0~0.100% Bismuth (Bi) is an optional element and may not be present. That is, the Bi content may be 0%. When present, Bi improves the machinability of the steel sheet. Even a small amount of Bi will provide some degree of the above effect. However, if the Bi content is too high, the hot workability will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Bi content is 0 to 0.100%, and if present, it is 0.100% or less. The preferred lower limit of the Bi content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Bi content is 0.080%, more preferably 0.060%, even more preferably 0.040%, and even more preferably 0.020%.

[0080] Se: 0~0.020% Selenium (Se) is an optional element and may not be present. That is, the Se content may be 0%. When present, Se improves the machinability of the steel sheet. Even a small amount of Se will provide some degree of the above effect. However, if the Se content is too high, the hot workability will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Se content is 0 to 0.020%, and if present, it is 0.020% or less. The preferred lower limit of the Se content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Se content is 0.015%, more preferably 0.010%, and even more preferably 0.007%.

[0081] Pb: 0~0.090% Lead (Pb) is an optional element and may not be present. That is, the Pb content may be 0%. When present, Pb improves the machinability of the steel sheet. Even a small amount of Pb will provide some degree of the above effect. However, if the Pb content is too high, the hot workability will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Pb content is 0 to 0.090%, and if present, it is 0.090% or less. The preferred lower limit of the Pb content is greater than 0%, and more preferably 0.001%. The preferred upper limit of the Pb content is 0.050%, more preferably 0.030%, and still more preferably 0.010%.

[0082] As: 0~0.050% Arsenic (As) is an optional element and may not be present. That is, the As content may be 0%. When present, As improves the machinability of the steel sheet. Even a small amount of As will provide some of the above effect. However, if the As content is too high, the hot workability will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the As content is 0 to 0.050%, and if present, it is 0.050% or less. The preferred lower limit of the As content is greater than 0%, and more preferably 0.001%. The preferred upper limit of the As content is 0.030%, more preferably 0.010%, and still more preferably 0.005%.

[0083] Te: 0~0.050% Tellurium (Te) is an optional element and may not be present. That is, the Te content may be 0%. If present, Te improves the machinability of the steel sheet. Even a small amount of Te will provide some of the above effect. However, if the Te content is too high, the hot workability will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Te content is 0 to 0.050%, and if present, it is 0.050% or less. The preferred lower limit of the Te content is greater than 0%, and more preferably 0.001%. The preferred upper limit of the Te content is 0.030%, more preferably 0.010%, and still more preferably 0.005%.

[0084] [(Feature 2) f1] Assuming that the steel plate according to this embodiment has the other features of this embodiment, f1 as defined by formula (1) is less than 0.0000. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%.

[0085] As mentioned above, f1 corresponds to the amount (mass%) of B that does not form B nitrides when considered in terms of stoichiometric ratios. B combines with nitrogen (N) to form B nitrides. On the other hand, N combines with titanium (Ti) to form Ti nitrides. Therefore, by fixing N with Ti, the formation of B nitrides is suppressed, and the amount of dissolved B increases. Thus, the amount of B dissolved in the steel sheet is estimated based on the B content, N content, and Ti content.

[0086] If the B content is 0.0008% or more and f1 is less than 0.0000, a large amount of B nitride may precipitate in the HAZ after welding, potentially increasing the HAZ toughness. Therefore, assuming that the steel sheet according to this embodiment has the other features of this embodiment, f1 is set to less than 0.0000.

[0087] The preferred upper limit of f1 is -0.0001, and more preferably -0.0002. The lower limit of f1 is not particularly limited, but is substantially -0.0055 when the chemical composition is as described in Feature 1. The lower limit of f1 may also be -0.0050, -0.0040, or -0.0035. Note that f1 is determined by rounding the value obtained from the element content and formula (1) to the fifth decimal place.

[0088] [(Feature 3) f2] Assuming that the steel plate according to this embodiment has the other features of this embodiment, the f2 defined by equation (2) satisfies 0.320 to 0.440. f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) Here, the elemental symbols in equation (2) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0089] As described above, f2 is an index of the hardness of the steel plate and the HAZ. If f2 is too low, the hardness of the steel plate cannot be sufficiently increased, and the strength of the HAZ tends to decrease. On the other hand, if f2 is too high, the hardness of the steel plate becomes too high, and the toughness of the HAZ decreases. Therefore, assuming that the steel plate according to this embodiment has the other features of this embodiment, f2 is set to 0.320 to 0.440.

[0090] The preferred lower limit of f2 is 0.325, more preferably 0.330, and even more preferably 0.335. The preferred upper limit of f2 is 0.435, more preferably 0.430, and even more preferably 0.425. Note that f2 is determined by rounding the value obtained from the element content and formula (2) to the fourth decimal place.

[0091] As stated above, in the chemical composition of the steel sheet according to this embodiment, Ni, Cr, and V are optional elements and may not be included. If Ni, Cr, and V are not included, f2 defined in formula (2) above can be rewritten as f2A defined in the following formula (2A). f2A=[C]+[Mn] / 6+[Si] / 24+[Mo] / 4 (2A) Here, the elemental symbols in equation (2A) are substituted with the content of the corresponding element in units of mass%.

[0092] [(Feature 4) f3] Assuming that the steel plate according to this embodiment has the other features of this embodiment, f3, as defined by formula (3), satisfies 150 to 205. f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equation (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0093] As described above, f3 is an index of the hardness of the HAZ after welding. Here, Mo in f3 interacts with B in the steel plate to suppress local softening in the HAZ. In addition, Cu, Ni, and Cr contribute to suppressing local softening in the HAZ. Therefore, by setting f3 to 150-205, it is possible to suppress the heterogeneity of strength in the HAZ after welding while maintaining the HAZ toughness. Accordingly, assuming that the steel plate according to this embodiment has the other features of this embodiment, f3 is set to 150-205.

[0094] The preferred lower limit of f3 is 151, more preferably 153, and even more preferably 155. The preferred upper limit of f3 is 204, more preferably 202, and even more preferably 200. Note that f3 is determined by rounding the first decimal place of the value obtained from the element content and formula (3).

[0095] As stated above, in the chemical composition of the steel sheet according to this embodiment, Cu, Ni, and Cr are optional elements and may not be included. If Cu, Ni, and Cr are not included, f3 defined in formula (3) above can be rewritten as f3A defined in the following formula (3A). f3A = 99 + 330[Mo] (3A) Here, the elemental symbols in equation (3A) are substituted with the content of the corresponding element in units of mass%.

[0096] [Any characteristic of steel plate] The steel plate of this embodiment may satisfy one or more of the following features 5 and 6. (Feature 5) The tensile strength TS is 590-740 MPa. (Feature 6) The number density of B-containing particles with an equivalent circular diameter of 300 nm or less is 1.0 × 10⁻⁶ 4 pieces / mm 2 That's all. The following describes each of its features.

[0097] [(Feature 5) Tensile Strength TS] The steel plate according to this embodiment preferably has a tensile strength of 590 to 740 MPa, provided that it has the other features of this embodiment. When the tensile strength TS is 590 to 740 MPa, the heterogeneity of the strength in the HAZ after welding of the welded structure can be stably suppressed, and the HAZ toughness of the welded structure can be stably increased. The preferred lower limit of the tensile strength TS is 595 MPa, more preferably 600 MPa, and even more preferably 605 MPa. The preferred upper limit of the tensile strength TS is 735 MPa, more preferably 730 MPa, and even more preferably 725 MPa.

[0098] In this embodiment, the yield stress YS, yield ratio YR, and tensile reduction in the thickness direction of the steel plate RAz are not particularly limited. The yield stress YS of the steel plate is preferably, for example, 440 to 540 MPa. The yield ratio YR of the steel plate is preferably, for example, 80% or less. The tensile reduction in the thickness direction of the steel plate RAz is preferably, for example, an average value of 25% or more, with each value being 15% or more. While these mechanical properties are not particularly limited, satisfying the above numerical ranges will result in greater stability, suppression of strength heterogeneity in the HAZ of the welded structure, and increased HAZ toughness of the welded structure.

[0099] In the steel plate according to this embodiment, the tensile strength TS, yield stress YS, and yield ratio YR can be determined by the following method. Specifically, a round bar test specimen for tensile testing is prepared from the center of the thickness of the steel plate according to this embodiment. At this time, the axial direction of the round bar test specimen is parallel to the width direction of the steel plate. The round bar test specimen is a JIS 14A type as specified in JIS Z 2241:2022, and the diameter of the parallel part is 8.0 mm. A tensile test is performed on the round bar test specimen at room temperature in air in accordance with JIS Z 2241:2022. From the results of the tensile test, the tensile strength TS (MPa), yield stress YS (MPa), and yield ratio YR (%) are determined. The yield stress YS is defined as the 0.2% proof stress. The yield ratio YR (%) is defined as the ratio of the yield stress YS to the tensile strength TS (= YS / TS). Furthermore, the tensile strength TS (MPa), yield stress YS (MPa), and yield ratio YR (%) are obtained by rounding the obtained values ​​to the first decimal place.

[0100] In the steel plate according to this embodiment, the tensile reduction in size (RAz) in the thickness direction of the steel plate can be determined by the following method. Specifically, a round bar test specimen for tensile testing is prepared from the center of the thickness of the steel plate according to this embodiment. At this time, the axial direction of the round bar test specimen is parallel to the thickness direction of the steel plate. A round bar test specimen as specified in JIS G 3199:2021 is used, and for steel plates with a thickness of 25 mm or less, the diameter of the parallel section is 6.0 mm and the length of the parallel section is 9.0 mm. For steel plates with a thickness exceeding 25 mm, the diameter of the parallel section is 10.0 mm and the length of the parallel section is 15.0 mm. A tensile test is performed on the round bar test specimen at room temperature and in air in accordance with JIS Z 2241:2022. From the results of the tensile test, the tensile reduction in size (RAz) in the thickness direction of the steel plate is determined by the method specified in JIS G 3199:2021. The tensile reduction of area (RAz) in the thickness direction of the steel plate is calculated by rounding the obtained value to the first decimal place.

[0101] [(Feature 6) Fine B-containing particles] Assuming that the steel sheet according to this embodiment has the other features of this embodiment, the number density of B-containing particles with an equivalent circular diameter of 300 nm or less is 1.0 × 10 4particles / mm 2 It is preferably the above. In this specification, "B-containing particles" means particles in which the B content specified by the method described later is 5% or more in atomic percentage. In this specification, furthermore, B-containing particles having an equivalent circle diameter of 300 nm or less are also referred to as "fine B-containing particles".

[0102] As described above, the steel sheet according to this embodiment has a B content of 0.0008 to 0.0030% in terms of chemical composition. The steel sheet according to this embodiment further makes f1 less than 0.0000 to reduce the amount of B in solid solution. That is, in the steel sheet according to this embodiment, most of B is presumed to precipitate and / or crystallize as B nitride.

[0103] Here, B dissolves in steel to enhance the hardenability of the steel sheet. Therefore, if fine B-containing particles are dispersed in the steel sheet, the fine B-containing particles may dissolve during welding, and the hardenability may locally increase. If the hardenability increases, the hardness of the HAZ after welding increases, and the non-uniformity of strength in the HAZ is likely to be suppressed. In addition, due to the dispersion of a large number of fine B-containing particles, the regions where the hardenability increases are widely distributed, and the regions where the non-uniformity of strength in the HAZ is suppressed become wider.

[0104] In the steel sheet according to this embodiment having the above chemical composition, most of the B-containing particles are B nitrides. However, it is very difficult to identify that the fine B nitrides with an equivalent circle diameter of 300 nm or less are nitrides. Therefore, in this embodiment, B nitrides with an equivalent circle diameter of 300 nm or less are specified as B-containing particles with an equivalent circle diameter of 300 nm or less. The lower limit of the equivalent circle diameter of the fine B-containing particles according to this embodiment is not particularly limited, but the lower limit of the equivalent circle diameter that can be specified from the contrast is 20 nm. That is, in this embodiment, the fine B-containing particles correspond to B nitrides having an equivalent circle diameter of 20 to 300 nm.

[0105] Therefore, on the premise that the steel sheet according to this embodiment has other features of this embodiment, the number density of the fine B-containing particles is 1.0×10 4 particles / mm 2The above is preferable. As a result, the welded structure according to this embodiment has further suppressed strength heterogeneity in the HAZ.

[0106] The preferred lower limit for the number density of fine B-containing particles is 1.1 × 10⁻⁶. 4 pieces / mm 2 And more preferably 1.2 × 10 4 pieces / mm 2 And more preferably 1.3 × 10 4 pieces / mm 2 The number density of fine B-containing particles is not particularly limited, but for example, 6.0 × 10 4 pieces / mm 2 It may also be 5.0 × 10 4 pieces / mm 2 It may also be 3.0 × 10 4 pieces / mm 2 It may also be 2.8 × 10 4 pieces / mm 2 It may also be 2.5 × 10 4 pieces / mm 2 That's fine.

[0107] In the steel sheet according to this embodiment, the number density of fine B-containing particles can be determined by the following method. A test specimen for observing fine B-containing particles is prepared from the steel sheet according to this embodiment. The test specimen is prepared with the observation surface being a plane perpendicular to the thickness direction (including the rolling direction and the width direction) from the t / 4 portion of the steel sheet. The Z contrast image, also known as the COMPO image, of the backscattered electron detector is observed on the electrolytically polished observation surface using an electrolytic emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope). The observation magnification is set to 5000 to 10000 times. Furthermore, the total observation field area is 50000 μm² within the above observation field. 2 Set the number of observation fields so that the above conditions are met.

[0108] In each observation field, particles are identified based on contrast. Identifying particles based on contrast in each observation field is a matter of course for those skilled in the art. In this specification, "particles" are not limited to circular (spherical) particles, but may also be small pieces with angular shapes or elongated elliptical pieces. The equivalent circle diameter of a particle can be determined by image analysis of the observation image in the COMPO image. Using the above method, particles with an equivalent circle diameter of 300 nm or less are identified as "fine particles".

[0109] Furthermore, point analysis is performed on the identified fine particles (particles with an equivalent circular diameter of 300 nm or less) using energy dispersive X-ray spectroscopy (EDS). The elemental content in each particle is determined by the EDS point analysis. In the EDS point analysis, the acceleration voltage is set to 1-3 kV, and measurements are taken at any position within the fine particle. Based on the EDS analysis results for each fine particle, fine particles containing 5% or more of B in atomic percent are identified as "fine B-containing particles."

[0110] The number of fine B-containing particles in each observation field is counted. Based on the total number of fine B-containing particles and the total area of ​​the observation field, the number density of fine B-containing particles (particles / mm²) is calculated. 2 ) is calculated. Note that, as mentioned above, in this embodiment, the total area of ​​the observation field is 50,000 μm². 2 The number of observation fields is set to be as described above. In addition, in this embodiment, the number density of fine B-containing particles (particles / mm) 2 When calculating ), the unit is 10 4 pieces / mm 2 The mantissa of the number obtained by exponential notation for the number density is rounded to the second decimal place.

[0111] [Steel plate shape] The shape of the steel plate according to this embodiment is not particularly limited, but for example, the plate thickness is 19 to less than 40 mm. If the steel plate has a thickness of 19 to less than 40 mm, the above features 5 and 6 can be reliably obtained by the preferred manufacturing method described later.

[0112] [Microstructure of steel sheet] The microstructure of the steel sheet according to this embodiment is not particularly limited. On the other hand, the microstructure of the steel sheet having the above-described features 1 to 5 is a mixed structure of ferrite, pearlite, bainite, and MA structure (Martensite-Austenite Constituent). Preferably, the area ratio of ferrite in the microstructure of the steel sheet is 50% or less, more preferably 45% or less. The microstructure of the steel sheet may also contain small amounts of bainite and / or pearlite in addition to ferrite. In addition to ferrite, bainite, and pearlite, the microstructure of the steel sheet according to this embodiment may also contain trace amounts of precipitates, inclusions, etc. However, in the microstructure of the steel sheet according to this embodiment, the volume ratio of precipitates, inclusions, etc. is negligibly small compared to ferrite, bainite, and pearlite.

[0113] In this embodiment, the area ratio of ferrite can be determined by the following method. First, a test specimen is prepared from a t / 4 portion of the steel sheet according to this embodiment, with the observation surface being a surface that includes the rolling direction and the sheet width direction. After polishing the observation surface of the test specimen to a mirror finish, it is immersed in a Nital etching solution for about 10 seconds to reveal the microstructure by etching. The etched observation surface is observed using a scanning electron microscope (SEM) to obtain 10 fields of view as secondary electron images. The field of view area is, for example, 0.01 mm². 2 (Magnification 1000x). Ferrite is identified from the contrast in each field of view. It should be noted that identifying ferrite from the contrast in each observation field is naturally possible for those skilled in the art. The area ratio of the identified ferrite is determined. The method for determining the area ratio is not particularly limited and any well-known method may be used. For example, the area ratio of ferrite can be determined by image analysis. In this embodiment, the arithmetic mean of the area ratios of ferrite obtained in all fields of view is defined as the area ratio of ferrite.

[0114] [Welded Structures] The welded structure of this embodiment satisfies the following features 1 to 5. (Feature 1) It comprises a base steel plate and a welded joint. (Feature 2) The chemical composition of the base steel sheet is within the range of this embodiment. (Feature 3) The chemical composition of the base steel sheet satisfies the condition that f1, as defined by formula (1), is less than 0.0000. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in units of mass%. (Feature 4) The chemical composition of the base steel sheet satisfies the range of f², defined by equation (2), from 0.320 to 0.440. f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) Here, the elemental symbols in equation (2) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol. (Feature 5) The chemical composition of the base steel sheet satisfies the f3 value defined by formula (3) being between 150 and 205. f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equation (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol. The following describes each of its features.

[0115] [(Feature 1) Base steel plate and welded joint] The welded structure according to this embodiment comprises a base steel plate and a welded joint. Figure 1 is a schematic perspective view of an example of a welded structure according to this embodiment. Referring to Figure 1, the welded structure 1 according to this embodiment comprises a base steel plate 10 and a welded joint 20 on all four sides. As described above, in a welded four-sided box column, the base steel plate 10 is also called a "skin plate". As shown in Figure 1, the welded structure 1 is formed as a column with a rectangular cross-section by joining and welding the base steel plates (skin plates) 10 together.

[0116] Furthermore, the welded structure 1 according to this embodiment also includes a welded structure in which welded structures 1 (four-sided box columns) according to this embodiment are joined and welded together. The welded structure 1 according to this embodiment may further include components other than the base steel plate 10 and the welded portion 20. Specifically, the welded structure 1 may have another steel plate (diaphragm) placed inside the welded structure 1. The welded structure 1 may further have a beam joined to the outside of the welded structure 1. In other words, in the welded structure 1 according to this embodiment, the diaphragm and beam are arbitrary components.

[0117] More specifically, the case in which a diaphragm is placed inside the welded structure 1 will be explained using drawings. Figure 2 is a schematic diagram showing a cross-section passing through the central axis of an example of the welded structure 1 shown in Figure 1. As shown in Figure 2, the inside of the welded structure 1 may contain a diaphragm 30 and a backing plate 40. As described above, the diaphragm 30 increases the rigidity and strength of the welded structure 1. The backing plate 40 is placed so as to sandwich the diaphragm 30. In particular, when joining the diaphragm 30 to the base steel plate (skin plate) 10 by electroslag welding, the molten slag and weld metal during welding are introduced into the gap formed by the backing plate 40. Therefore, the backing plate 40 can prevent the molten slag and weld metal from flowing out.

[0118] [(Feature 2)~(Feature 5)] The welded structure 1 according to this embodiment has a base steel plate 10 whose chemical composition is within the range of this embodiment. As described above, the welded structure 1 according to this embodiment is manufactured by welding using the steel plate 10 according to this embodiment as the material. Therefore, the base steel plate 10 of the welded structure 1 according to this embodiment has the same chemical composition as the steel plate according to this embodiment. That is, characteristic 2 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as characteristic 1 of the steel plate according to this embodiment. Similarly, characteristic 3 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as characteristic 2 of the steel plate according to this embodiment. Similarly, characteristic 4 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as characteristic 3 of the steel plate according to this embodiment. Similarly, characteristic 5 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as characteristic 4 of the steel plate according to this embodiment. In short, the base steel plate 10 of the welded structure 1 according to this embodiment has the same chemical composition as the steel plate according to this embodiment.

[0119] [Suppression of intensity heterogeneity in HAZs] The welded structure 1 according to this embodiment, which has the above-described features 1 to 5, suppresses strength heterogeneity in the HAZ. In this embodiment, suppression of strength heterogeneity in the HAZ is defined as follows. Specifically, a welded joint is fabricated by joining a diaphragm 30 to the base steel plate 10 (skin plate) of the welded structure 1 by electroslag welding.

[0120] Here, the diaphragm 30 is made of steel plate having the same chemical composition as the base steel plate 10. Electroslag welding can be performed by those skilled in the art using well-known welding materials and under well-known welding conditions, with appropriate adjustments. Specifically, the welding conditions can be set to an average heat input of 500-600 kJ / cm, a root spacing of 20-25 mm, a current of 350-400 A, and a voltage of 45-55 V. The welding material can be any well-known material as appropriate, depending on the welding conditions. Specifically, it is preferable to use the wire described in JIS Z3353:2013. The welding of the base steel plate 10 and the diaphragm 30 by electroslag welding will be explained more specifically below with reference to the drawings.

[0121] Figure 3 is a magnified view of a portion of the region 100 shown in Figure 2. In other words, Figure 3 shows an example of the vicinity of a weld made by electroslag welding. Referring to Figure 3, the base steel plate 10 with a thickness t (mm) and the diaphragm 30 are fixed perpendicularly to each other. Furthermore, two backing plates 40 are positioned to sandwich the diaphragm 30. Further, electroslag welding is performed to weld the weld metal portion 50. In this embodiment, we focus on the region 2tl shown as a shaded area in Figure 3 and determine the area ratio of the region with non-uniform strength. Referring to Figure 3, the upper right end of region 2tl represents the center of the weld metal portion 50. In this specification, "center of the weld metal portion 50" means the intersection of the center line of the thickness of the diaphragm 30 and the extension line of the surface of the base steel plate 10, within the weld metal portion 50. Furthermore, the length from the center of the weld metal portion 50 to one end of the weld metal portion 50 is l (mm). In other words, region 2tl is defined as a region with a length of 2l (mm) from the center of the weld metal portion 50 in the left-right direction of Figure 3, and a region with a thickness t (mm) of the base steel plate 10 in the up-down direction of Figure 3.

[0122] In the region 2tl defined above, a Vickers hardness test is performed in accordance with JIS Z2244:2020. In the Vickers hardness test, the measurement points are grid points arranged at 1 mm intervals in a direction parallel to the thickness t of the base steel plate 10 shown in Figure 3, and at 1 mm intervals in a direction parallel to the length l shown in Figure 3. The Vickers hardness is measured at each measurement point. At this time, the test force (Vickers load) is set to 1 kgf (9.8 N). Of the Vickers hardness values ​​obtained at each measurement point, the measurement points with a Vickers hardness of 150 Hv or less are counted. The ratio of the number of measurement points with a Vickers hardness of 150 Hv or less to the total number of measurement points is calculated and defined as the area ratio (%) of the strength heterogeneity region.

[0123] In this embodiment, the welded structure 1 has a strength heterogeneity area ratio of 50.0% or less within the region 2tl determined by the method described above. The strength heterogeneity area ratio (%) is calculated by rounding the obtained value to the second decimal place.

[0124] [HAZ toughness] The welded structure 1 according to this embodiment, which possesses the above-described features 1 to 5, has excellent HAZ toughness. In this embodiment, having excellent HAZ toughness is defined as follows: Similar to the evaluation of the non-uniformity of strength in the HAZ described above, a welded joint is fabricated by joining the diaphragm 30 to the base steel plate 10 (skin plate) of the welded structure 1 by electroslag welding.

[0125] A full-size V-notch specimen is prepared from the fabricated welded joint. The V-notch specimen is prepared so that the notch position coincides with the fusion line (hereinafter also referred to as "FL" (Fusion Line)) of the welded joint. Preferably, the V-notch specimen is prepared in a direction such that the longitudinal direction of the V-notch specimen is parallel to the center line of the diaphragm 30. Alternatively, if the specimen cannot be prepared as described above, it may be prepared near the backing plate 40 such that the longitudinal direction of the V-notch specimen makes an angle with respect to the center line of the diaphragm 30.

[0126] A Charpy impact test is performed on the prepared V-notch test specimen in accordance with JIS Z 2242:2023 to determine the absorbed energy (J) at 0°C. The welded structure 1 according to this embodiment has an absorbed energy of 70J or more at 0°C, as determined by the method described above. The absorbed energy (J) at 0°C is obtained by rounding the obtained value to the first decimal place.

[0127] [Microstructure of HAZ] The microstructure of the HAZ of the welded structure 1 according to this embodiment consists of ferrite and the remainder being bainite and / or pearlite. In other words, in this embodiment, the HAZ may have a microstructure consisting of ferrite and bainite, or a microstructure consisting of ferrite and pearlite, or a microstructure consisting of ferrite, bainite and pearlite. The microstructure of the steel sheet according to this embodiment may also contain trace amounts of precipitates, inclusions, etc., in addition to ferrite, bainite, and pearlite. However, in the microstructure of the steel sheet according to this embodiment, the volume fraction of precipitates, inclusions, etc., is negligibly small compared to ferrite, bainite, and pearlite.

[0128] In the microstructure of the HAZ of the welded structure 1 according to this embodiment, the area ratio of ferrite is preferably 90.0% or less. The lower limit of the ferrite area ratio is not particularly limited, but for example, it is 80.0%. In other words, in the microstructure of the HAZ, the sum of the area ratios of bainite and / or pearlite is preferably 10.0% or more, and more preferably 10.0 to 20.0%. In the HAZ of the welded structure 1 according to this embodiment having the above chemical composition, if the area ratio of ferrite is 80.0 to 90.0%, the heterogeneity of the strength in the HAZ can be suppressed more stably.

[0129] In this embodiment, the area ratio of ferrite in the HAZ can be determined by the same method as the method for determining the area ratio of ferrite in steel plates according to this embodiment described above. Specifically, a test specimen for microstructural observation is prepared from the HAZ of the welded structure according to this embodiment. After polishing the observation surface of the test specimen to a mirror finish, it is immersed in a Nital etching solution for about 10 seconds to reveal the microstructure by etching. The etched observation surface is observed using a SEM to obtain 10 fields of view as secondary electron images. The field of view area is, for example, 0.01 mm². 2 (Magnification 1000x). Ferrite is identified from the contrast in each field of view. It should be noted that identifying ferrite from the contrast in each observation field is naturally possible for those skilled in the art. The area ratio of the identified ferrite is determined. The method for determining the area ratio is not particularly limited and any well-known method may be used. For example, the area ratio of ferrite can be determined by image analysis. In this embodiment, the arithmetic mean of the area ratios of ferrite obtained in all fields of view is defined as the area ratio of ferrite.

[0130] [Manufacturing method] The following describes the manufacturing method of steel plates and the manufacturing method of welded structures according to this embodiment. The manufacturing methods of steel plates and welded structures described below are examples of methods for manufacturing steel plates and welded structures according to this embodiment. That is, steel plates according to this embodiment may be manufactured by methods other than those described below. Similarly, welded structures according to this embodiment may be manufactured by methods other than those described below. An example of the manufacturing method of steel plates according to this embodiment comprises the steps of preparing a slab and manufacturing a steel plate by hot rolling the slab.

[0131] [Slab preparation process] In the slab preparation process, a slab having the above-mentioned chemical composition is prepared. Specifically, in mass%, C: 0.060~0.120%, Si: 0.05~0.55%, Mn: 0.70~1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02~0.30%, Ti: 0.007~0.016%, Nb: 0.005~0.030%, Al: 0.010~0.050% , N: 0.0040~0.0100%, B: 0.0008~0.0030%, O: 0.004% or less, Cu: 0~0.50%, Ni: 0~1.20%, Cr:0~0.50%, W:0~0.100%, Co:0~0.100%, V:0~0.08%, Mg:0~0.0100%, Ca:0~0.0100% Prepare a slab having a chemical composition consisting of rare earth elements: 0-0.0100%, Zr: 0-0.050%, Hf: 0-0.020%, Ta: 0-0.100%, Sr: 0-0.020%, Sn: 0-0.100%, Sb: 0-0.050%, Zn: 0-0.020%, Bi: 0-0.100%, Se: 0-0.020%, Pb: 0-0.090%, As: 0-0.050%, Te: 0-0.050%, and the remainder being Fe and impurities, wherein f1 as defined by formula (1) is less than 0.0000, f2 as defined by formula (2) is between 0.320 and 0.440, and f3 as defined by formula (3) is between 150 and 205. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equations (1) to (3) are substituted with the content of the corresponding element in units of mass%. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0132] The method for preparing the slab is not particularly limited and any well-known method may be used. For example, a cast slab may be manufactured by continuous casting using molten steel having the chemical composition described above. In this case, the conditions of the continuous casting method are not particularly limited and can be set as appropriate. The slab is manufactured by the above process.

[0133] [Hot rolling process] In the hot rolling process, a slab is hot-rolled to produce a steel plate. The hot rolling process may include a heating process, a first rolling process, a second rolling process, and a cooling process. Each of these processes will be described below.

[0134] [Heating process] In the heating process, the slab is heated in a heating furnace. In this embodiment, the heating temperature in the heating process is denoted as Tc (°C). The heating temperature Tc is preferably 1050 to 1250°C. If the heating temperature Tc is too low, the carbonitrides and other materials in the slab may not be sufficiently dissolved, and the desired mechanical properties may not be obtained. On the other hand, if the heating temperature Tc is too high, the austenite grains may become coarse, and the desired mechanical properties may not be obtained. Therefore, the heating temperature is preferably 1050 to 1250°C.

[0135] [First Rolling Process] In the first rolling process, hot rolling is performed on the heated slab. In the first rolling process, a hot rolling mill is used to apply a reduction of multiple passes to the slab. At this time, it is preferable that the cumulative reduction rate in the first rolling process be 50% or more. If the cumulative reduction rate is too low, the porosity inside the slab may not be sufficiently compressed. In this case, the desired mechanical properties cannot be obtained. Therefore, it is preferable that the cumulative reduction rate in the first rolling process be 50% or more.

[0136] [Second Rolling Process] In the second rolling process, a well-known hot rolling method is performed on the slab that has undergone the first rolling process. In other words, the second rolling process means that the slab after the first rolling process is further hot-rolled. Here, the method of hot rolling in the second rolling process is not particularly limited and any well-known method is acceptable. Specifically, in the second rolling process, reverse rolling may be performed, which involves moving the slab back and forth in the rolling mill to impart a reduction of multiple passes.

[0137] In the second rolling step according to this embodiment, it is preferable to limit the upper limit Tsmax (°C) of the starting temperature for hot rolling and the lower limit Tfmin (°C) of the ending temperature for hot rolling as follows. First, using the heating temperature Tc (°C) in the heating step described above, the solubility product K of the B nitride is defined by the following equation (A). K=10 (5.24-13970 / (Tc+273)) (A) Here, in equation (A), Tc is substituted with the heating temperature in the heating process, in units of °C.

[0138] Next, using f1 and the solubility product K of the B nitride, the amount of solid-dissolved B [rB] during heating is defined by the following equation (B). [Sound B] = ([f1] + ([f1] 2 +4 × (11 / 14) × K) 0.5 ) / 2 (B) Here, in equation (B), [f1] is replaced with f1 as defined in equation (1) above, and K in equation (B) is replaced with the solubility product K of the B nitride as defined in equation (A). However, if the [rB] calculated by equation (B) is greater than the B content [B] of the steel plate, the amount of dissolved B [rB] during heating is defined by the following equation (C). [rB]=[B] (C) Here, in equation (C), [B] is substituted with the B content of the steel plate in units of mass%.

[0139] Furthermore, the effective amount of N [fN] is defined by the following equation (D). [fN]=[N]-(11 / 48)×[Ti] (D) Here, in formula (D), the N content and Ti content in the steel sheet are substituted with the N content and T content of the steel sheet in units of mass%.

[0140] Furthermore, using the B content [B] of the steel sheet, the amount of dissolved B [rB] during heating, and the effective N amount [fN], the amount of dissolved N [rN] during heating is defined by the following formula (E). [rN]=[fN]-(14 / 11)×([B]-[rB]) (E)

[0141] Here, using the amount of dissolved B [rB] during heating and the amount of dissolved N [rN] during heating, the upper limit of the starting temperature Tsmax of hot rolling in the second rolling process is defined by the following formula (F). Tsmax=760-5.90×104×[rB]+1.85×104×[rN] (F)

[0142] Also, the lower limit of the finishing temperature Tfmin of hot rolling in the second rolling process is defined by the following formula (G). Tfmin=Tsmax-40 (G)

[0143] That is, in the second rolling process according to the present embodiment, it is preferable that the starting temperature of hot rolling is set to be equal to or lower than the upper limit of the starting temperature Tsmax (°C) of hot rolling defined by formula (F). In the second rolling process according to the present embodiment, it is further preferable that the finishing temperature of hot rolling is set to be equal to or higher than the lower limit of the finishing temperature Tfmin (°C) of hot rolling defined by formula (G). By controlling the temperature range of hot rolling in the second rolling process as described above, a large number of fine B nitrides precipitate in the slab. As a result, in the manufactured steel sheet, the number density of fine B-containing particles can be sufficiently increased.

[0144] In the second rolling process, rolling is further performed in a plurality of passes. Here, when the plate thickness after rolling in the n-th pass of hot rolling in the second rolling process is t n the reduction ratio in the n-th pass is defined by the following formula (H). Also, the reduction ratio in the n-th pass is preferably 15 to 20%. Reduction ratio in the n-th pass=(t n-1 -t n ) / tn (H) Here, in equation (H), t n-1 The thickness of the plate after the (n-1)th pass is substituted in units of mm. Similarly, the t in equation (H) n The value of the plate thickness after the nth pass is substituted in millimeters. Note that n is a natural number.

[0145] In the second rolling process, it is preferable to control the time between each pass as follows: Excluding the time between the pass immediately preceding the final pass and the final pass itself, the time between odd-numbered passes and even-numbered passes should be within 6 seconds, and the time between even-numbered passes and odd-numbered passes should be 10 to 20 seconds. Furthermore, the time between the pass immediately preceding the final pass and the final pass should be 20 to 30 seconds. By controlling the time between each pass in the second rolling process as described above, a large number of fine B nitrides are precipitated in the slab. As a result, the number density of fine B-containing particles in the manufactured steel sheet can be sufficiently increased.

[0146] [Cooling process] In the cooling process, the steel sheet that has undergone the second rolling process is cooled. The method of cooling the steel sheet is not particularly limited, but for example, accelerated cooling by water cooling is performed. In this case, it is preferable that the cooling start temperature be 680°C or higher. If the cooling start temperature is too low, the strength of the steel sheet may decrease.

[0147] Furthermore, the cooling stop temperature is preferably set to 250-400°C. If the cooling stop temperature is too low, the tensile strength of the steel plate may become too high. On the other hand, if the cooling stop temperature is too high, the yield strength may become too high. If the cooling stop temperature is too high, the yield ratio may become too high. Therefore, the cooling stop temperature is preferably set to 250-400°C.

[0148] By the method described above, a steel plate according to this embodiment is manufactured. Furthermore, a welded structure according to this embodiment is manufactured using the steel plate according to this embodiment. The manufacturing method of the welded structure includes a welding process. The welding process will be described below.

[0149] [Welding Process] In the welding process, steel plates are welded together to manufacture a welded structure. The welding conditions are not particularly limited and can be any well-known method. For example, when welding steel plates together, submerged arc welding may be performed. For example, when introducing a diaphragm, electroslag welding may be performed. In any case, a person skilled in the art can adjust the welding conditions as needed to perform the welding. The present invention will be described in more detail below with reference to examples. [Examples]

[0150] Slabs were manufactured by continuous casting from molten steel having the chemical compositions shown in Tables 1A and 1B. In Table 1B, a "-" indicates that the content of each element is at the impurity level.

[0151] [Table 1A]

[0152] [Table 1B]

[0153] A hot rolling process was performed on the manufactured slabs. First, the thickness (mm) of each test slab was as shown in Table 2A. Each test slab was heated in a heating furnace at the heating temperature Tc (°C) shown in Table 2A for the heating time (minutes).

[0154] [Table 2A]

[0155] For the heated slab, the first rolling process was carried out at the cumulative reduction rate (%) shown in Table 2A. The transfer thickness (mm), which is the thickness of the slab after the first rolling process, was as shown in Table 2A. Further, the second rolling process was carried out at the starting temperature (°C), ending temperature (°C), and cumulative reduction rate (%) shown in Table 2A. Note that the reduction rate (%) in each pass of the second rolling process and the time between passes were as shown in Table 2B.

[0156]

Table 2B

[0157] Furthermore, the solubility product K of B nitride, the amount of dissolved B [rB] (mass%) during heating, the effective N amount [fN] (mass%), the amount of dissolved N [rN] (mass%) during heating, the upper limit of the starting temperature Tsmax (°C) of hot rolling in the second rolling process, and the lower limit of the ending temperature Tfmin (°C) of hot rolling in the second rolling process, which were obtained from the above formulas (A) to (G), are shown in Table 3.

[0158]

Table 3

[0159] For the steel plate after the second rolling process, cooling was carried out under the conditions shown in Table 2A to produce a steel plate with the thickness shown in Table 2A.

[0160] [Evaluation Test] The following evaluation tests were carried out on the produced steel plates of each test number. (Test 1) Tensile test (Test 2) Test for measuring the number density of fine B-containing particles (Test 3) HAZ strength inhomogeneity evaluation test (Test 4) HAZ microstructure observation test (Test 5) HAZ toughness evaluation test

[0161] [(Test 1) Tensile test] Tensile tests were performed on each steel plate with the specified test number using the method described above to determine the tensile strength TS (MPa), yield stress YS (MPa), yield ratio YR (%), and tensile reduction of area RAz (%) in the thickness direction of the steel plate. The obtained tensile strength TS (MPa), yield stress YS (MPa), yield ratio YR (%), and tensile reduction of area RAz (%) in the thickness direction of the steel plate for each test number are shown in Table 4.

[0162] [Table 4]

[0163] [(Test 2) Number density measurement test of fine B-containing particles] For each steel plate with a test number, the number density measurement test of fine B-containing particles was performed using the method described above, and the number density (particles / mm²) of B-containing particles with an equivalent circular diameter of 300 nm or less was determined. 2 The following was determined. In this example, the observation surface included the t / 4 portion of the steel plate and was perpendicular to the plate thickness direction (including the rolling direction and the plate width direction), with a total observation field area of ​​50,000 μm². 2 Observations were conducted to achieve the above results. Furthermore, the lower limit of the equivalent circular diameter of B-containing particles was 20 nm. The number density of the obtained fine B-containing particles (particles / mm²) was also determined. 2 ) are shown in Table 4.

[0164] [Manufacturing of welded structures] Prior to tests 3-5, a simulated welded structure was manufactured. Specifically, steel plates with the thicknesses listed in Table 2A for each test number were used as skin plates. Furthermore, steel plates with the thicknesses listed in Table 4 were prepared from steel plates with similar chemical composition and used as diaphragms. In addition, for electroslag welding (ESW), a welding wire equivalent to YES562-S (indicated as "A" in the "Welding Wire" column in Table 4) or a welding wire equivalent to YES602-S (indicated as "B" in the "Welding Wire" column in Table 4) described in JIS Z3353:2013 was used. The welding wire used for each test number all had a diameter of 1.6 mm. Furthermore, the ESW conditions performed for each test number are shown in Table 4. Welded joints between the skin plates and diaphragms were manufactured by welding performed under the above conditions.

[0165] [(Test 3) HAZ Strength Heterogeneity Evaluation Test] From the welded joints of each test number, region 2tl, indicated as the shaded area in Figure 3, was identified. Region 2tl was identified using the method described above. For region 2tl, Vickers hardness tests were performed using the method described above, at 1 mm intervals in the left-right direction and 1 mm intervals in the up-down direction of Figure 3. From the obtained Vickers hardness test results, the ratio of the number of measurement points with a Vickers hardness of 150 Hv or less to the total number of measurement points was calculated and defined as the area percentage (%) of the strength heterogeneity region. The area percentage (%) of the strength heterogeneity region for each test number obtained is shown in the "Heterogeneity Region (Area %)" column of Table 4.

[0166] [(Test 4) HAZ Microtissue Observation Test] The heat-affected zone (HAZ) was identified from the welded joints of each test number, and microstructural observation was performed. Test specimens were prepared using the method described above, and microstructural observation was performed using a scanning electron microscope (SEM). Ferrite was identified from the contrast, and its area percentage was determined. In this example, phases other than ferrite (bainite and / or pearlite) were defined as the "second phase," and the area percentage of the second phase was determined. The area percentages (%) of the second phase for each test number are shown in Table 4.

[0167] [(Test 5) HAZ Toughness Evaluation Test] The heat-affected zone (HAZ) was identified from the welded joints for each test number, and the HAZ toughness was evaluated by a Charpy impact test performed using the method described above. Specifically, full-size V-notch specimens were prepared from the HAZ of the welded joints for each test number using the method described above. The V-notch specimens were prepared so that the notch position coincided with the FL of the welded joint. A Charpy impact test was performed on the prepared V-notch specimens in accordance with JIS Z 2242:2023 as described above, and the absorbed energy (J) at 0°C was determined. The absorbed energy at 0°C for each test number obtained is shown in the "KV2(J)" column of Table 4.

[0168] [Evaluation Results] Referring to Tables 1A, 1B, 2A, 2B, 3, and 4, the steel plates for test numbers 1 to 23 satisfied steel plate characteristics 1 to 4. As a result, the welded structures manufactured from these steel plates had a region of 50.0% or less of strength heterogeneity in the HAZ, suppressing strength heterogeneity in the HAZ. Furthermore, the welded structures manufactured from these steel plates had an absorbed energy (J) of 70 or more at 0°C in the HAZ, demonstrating excellent HAZ toughness.

[0169] The steel plates from test numbers 1 to 22 also met the tensile strength TS described in Feature 5. As a result, the welded structures manufactured from these steel plates had a strength heterogeneity area ratio of 45.0% or less in the HAZ, further suppressing strength heterogeneity in the HAZ.

[0170] Steel plates from test numbers 1-14 and 19-22 further satisfied the number density of fine B-containing particles described in Feature 6. As a result, welded structures manufactured from these steel plates had an area ratio of 40.0% or less of the strength heterogeneity region in the HAZ, and strength heterogeneity in the HAZ was extremely suppressed.

[0171] On the other hand, the steel plate of test number 24 had too low an f2 value. As a result, the welded structure manufactured from this steel plate had an area ratio of strength heterogeneity in the HAZ exceeding 50.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0172] The steel plate in test number 25 had too high an f2 value. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the HAZ, and did not possess good HAZ toughness.

[0173] The steel plate in test number 26 had an f3 value that was too low. As a result, the welded structure produced from this steel plate had an area ratio of strength heterogeneity in the HAZ exceeding 50.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0174] The steel plate in test number 27 had an excessively high f3. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the HAZ, and did not possess good HAZ toughness.

[0175] The steel plate in test number 28 had an excessively high f1. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the HAZ, and did not possess good HAZ toughness.

[0176] The steel plate in test number 29 had too low a molybdenum (Mo) content. As a result, the welded structure produced from this steel plate had an area ratio of strength heterogeneity in the heat-affected zone (HAZ) exceeding 50.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0177] The steel plate in test number 30 had too high a molybdenum (Mo) content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0178] The steel plate in test number 31 had too low a carbon content. As a result, the welded structure produced from this steel plate had a strength heterogeneity area ratio in the heat-affected zone (HAZ) exceeding 50.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0179] The steel plate in test number 32 had too high a carbon content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0180] The steel plate in test number 33 had too low a Si content. As a result, the welded structure produced from this steel plate had an area ratio of strength heterogeneity in the heat-affected zone (HAZ) exceeding 50.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0181] The steel plate in test number 34 had too high a Si content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0182] The steel plate used in test number 35 had too low a manganese content. As a result, the welded structure produced from this steel plate had a strength heterogeneity area ratio in the heat-affected zone (HAZ) exceeding 50.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0183] The steel plate in test number 36 had too high a manganese content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0184] The steel plate in test number 37 had too high a Cu content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0185] The steel plate in test number 38 had too high a Ni content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0186] The steel plate in test number 39 had too high a chromium content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0187] The steel plate in test number 40 had too high a V content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0188] The steel plate in test number 41 had too low a Nb content. As a result, the welded structure produced from this steel plate had a strength heterogeneity area ratio in the heat-affected zone (HAZ) exceeding 50.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0189] The steel plate in test number 42 had too high a Nb content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 70 at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0190] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure. [Explanation of symbols]

[0191] 1. Welded Structure 10. Base material steel plate (skin plate) 20 Welded parts 30 diaphragms 40 Prize money 50 Weld metal part

Claims

1. The chemical composition is expressed in mass percent. C: 0.060-0.120%, Si: 0.05-0.55%, Mn: 0.70-1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02-0.30%, Ti: 0.007 to 0.016%, Nb: 0.005-0.030%, Al: 0.010-0.050%, N: 0.0040-0.0100%, B: 0.0008-0.0030%, O: 0.004% or less, and The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The value of f2A defined by equation (2A) satisfies 0.320 to 0.

440. The f3A defined by equation (3A) satisfies 150 to 205. steel plate. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2A=[C]+[Mn] / 6+[Si] / 24+[Mo] / 4 (2A) f3A=99+330[Mo] (3A) Here, the elemental symbols in equations (1), (2A), and (3A) are substituted with the content of the corresponding element in units of mass%.

2. The chemical composition is expressed in mass percent. C: 0.060-0.120%, Si: 0.05-0.55%, Mn: 0.70-1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02-0.30%, Ti: 0.007 to 0.016%, Nb: 0.005-0.030%, Al: 0.010-0.050%, N: 0.0040-0.0100%, B: 0.0008 to 0.0030%, and, O: Contains 0.004% or less, and further, Cu: 0.50% or less, Ni: 1.20% or less, Cr: 0.50% or less, W: 0.100% or less, Co: 0.100% or less, V: 0.08% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Rare earth elements: 0.0100% or less, Zr: 0.050% or less, Hf: 0.020% or less, Ta: 0.100% or less, Sr: 0.020% or less, Sn: 0.100% or less, Sb: 0.050% or less, Zn: 0.020% or less, Bi: 0.100% or less, Se: 0.020% or less, Pb: 0.090% or less, As: 0.050% or less, It contains one or more elements selected from the group consisting of Te: 0.050% or less, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The value of f2 defined in equation (2) satisfies 0.320 to 0.440, The f3 defined in equation (3) satisfies 150 to 205. steel plate. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equations (1) to (3) are substituted with the content of the corresponding element in units of mass percent. If the corresponding element is not present, "0" is substituted for that elemental symbol.

3. A steel plate according to claim 1 or claim 2, In the aforementioned steel plate, B-containing particles with an equivalent diameter of 300 nm or less, 1.0 × 10 ４ pieces / mm ２ Contains the above, steel plate.

4. It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060-0.120%, Si: 0.05-0.55%, Mn: 0.70-1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02-0.30%, Ti: 0.007 to 0.016%, Nb: 0.005-0.030%, Al: 0.010-0.050%, N: 0.0040-0.0100%, B: 0.0008-0.0030%, O: 0.004% or less, and The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The value of f2A defined by equation (2A) satisfies 0.320 to 0.

440. The f3A defined by equation (3A) satisfies 150 to 205. Welded structure. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2A=[C]+[Mn] / 6+[Si] / 24+[Mo] / 4 (2A) f3A=99+330[Mo] (3A) Here, the elemental symbols in equations (1), (2A), and (3A) are substituted with the content of the corresponding element in units of mass%.

5. It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060-0.120%, Si: 0.05-0.55%, Mn: 0.70-1.60%, P: 0.020% or less, S: 0.008% or less, Mo: 0.02-0.30%, Ti: 0.007 to 0.016%, Nb: 0.005-0.030%, Al: 0.010-0.050%, N: 0.0040-0.0100%, B: 0.0008 to 0.0030%, and, O: Contains 0.004% or less, and further, Cu: 0.50% or less, Ni: 1.20% or less, Cr: 0.50% or less, W: 0.100% or less, Co: 0.100% or less, V: 0.08% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Rare earth elements: 0.0100% or less, Zr: 0.050% or less, Hf: 0.020% or less, Ta: 0.100% or less, Sr: 0.020% or less, Sn: 0.100% or less, Sb: 0.050% or less, Zn: 0.020% or less, Bi: 0.100% or less, Se: 0.020% or less, Pb: 0.090% or less, As: 0.050% or less, It contains one or more elements selected from the group consisting of Te: 0.050% or less, The remainder consists of Fe and impurities. If f1, as defined in equation (1), satisfies the condition less than 0.0000, The value of f2 defined in equation (2) satisfies 0.320 to 0.440, The f3 defined in equation (3) satisfies 150 to 205. Welded structure. f1=[B]-(11 / 14)×([N]-(14 / 48)×[Ti]) (1) f2=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (2) f3=99+55[Cu]+27[Ni]+49[Cr]+330[Mo] (3) Here, the elemental symbols in equations (1) to (3) are substituted with the content of the corresponding element in units of mass percent. If the corresponding element is not present, "0" is substituted for that elemental symbol.

6. A welded structure according to claim 4 or claim 5, In the aforementioned base steel sheet, B-containing particles with an equivalent diameter of 300 nm or less, 1.0 × 10 ４ pieces / mm ２ Contains the above, Welded structure.

7. A welded structure according to claim 4 or claim 5, In the heat-affected zone of the aforementioned base steel plate, In the microstructure, the area ratio of ferrite is 90.0% or less. Welded structure.

8. A welded structure according to claim 7, In the area affected by welding heat, The area ratio with a Vickers hardness of 150 Hv or less is 50.0% or less. Welded structure.

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