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 the chemical composition described in the specification, satisfies f1 as defined by formula (1) to 0.34 to 0.45, and satisfies that prior austenite grains having a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area % or more of the prior austenite grains in the central part of the thickness of the steel sheet, satisfies that the area ratio S2 of the MA structure in the central part of the thickness of the steel sheet is greater than 0 to 0.50%, and satisfies that the area ratio S4 of the MA structure in the t / 4 part of the steel sheet is greater than 0 to 0.32%. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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.

Description

Technical Field

[0001] The present disclosure relates to a steel plate and a welded structure manufactured by welding the steel plate.

Background Art

[0002] As a column member of a building structure, a welded built-up box-section column (also referred to as a "welded four-sided box column") may be used. The welded four-sided box column is manufactured by combining and welding four steel plates (skin plates) so that the cross-section becomes rectangular.

[0003] Other members may be joined to the welded four-sided box column. For example, a steel plate (diaphragm) may be joined as a reinforcing member inside the column. In this case, the diaphragm is arranged in a direction perpendicular to the longitudinal direction of the welded four-sided box column to increase the rigidity and strength of the column. For example, further, a beam may be joined to the outside of the welded four-sided box column.

[0004] Specifically, when manufacturing such a welded four-sided box column, for example, the skin plates are welded by submerged arc welding with a large heat input. For example, further, the skin plate and the diaphragm are welded by electro-slag welding with a large heat input. When such welding with a large heat input is performed, the toughness may decrease in the heat affected zone (hereinafter referred to as HAZ (Heat Affected Zone)). Hereinafter, the toughness in the HAZ of the steel plate after welding is also simply referred to as "HAZ toughness".

[0005] So far, techniques for increasing the HAZ toughness of steel plates have been proposed in Japanese Patent Application Laid-Open No. 2007-126725 (Patent Document 1) and Japanese Patent Application Laid-Open 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 area (softening) may occur in some parts due to the heat input from welding and subsequent cooling. In other words, in the HAZ formed in the welded structure, a localized decrease in strength may result in non-uniform strength in the HAZ. However, it is preferable to suppress such non-uniformity of strength in the HAZ.

[0010] Patent documents 1 and 2 above propose techniques for improving the HAZ toughness of steel plates. However, the HAZ toughness of steel plates may also be improved by techniques other than those proposed in patent documents 1 and 2. Furthermore, patent documents 1 and 2 do not address the suppression of strength heterogeneity in the HAZ of welded structures.

[0011] The object of this disclosure is to provide a welded structure having excellent HAZ toughness and suppressed strength heterogeneity in the HAZ, and a steel sheet from which such a welded structure can be obtained. [Means for solving the problem]

[0012] The steel plate according to this disclosure is The chemical composition is expressed in mass percent. C: 0.030~0.150%, Si: 0.01~0.10%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Nb: 0.003~0.030%, Ti: 0.005~0.050%, Al: 0.005~0.100%, N: 0.0100% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~2.00%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Rare earth elements: 0 to 0.0500%, Zr: 0 to 0.050%, Hf: 0 to 0.020%, Sr: 0 to 0.020%, W: 0 to 0.100%, Co: 0 to 0.100%, B: 0 to 0.0050%, Sn: 0 to 0.100%, Sb: 0 to 0.050%, Zn: 0 to 0.020%, Bi: 0 to 0.100%, Se: 0 to 0.020%, Pb: 0 to 0.090%, As: 0 to 0.050%, Te: 0 to 0.050%, and, The balance consists of Fe and impurities, f1 defined by formula (1) satisfies 0.34 to 0.45, At the center of the plate thickness of the steel plate, Old austenite grains with a minor axis of 25 μm or less and a major axis of 50 μm or less satisfy 50 area% or more of the old austenite grains, The area ratio S2 of the MA structure (Martensite - Austenite Constituent) at the center of the plate thickness of the steel plate satisfies more than 0 to 0.50%, The area ratio S4 of the MA structure at the t / 4 part of the steel plate satisfies more than 0 to 0.32%. f1 = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) Here, in the element symbols in formula (1), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol.

[0013] The welded structure according to the present disclosure, Comprises a base metal steel plate and a welded part, The chemical composition of the base metal steel plate is, in mass%, C: 0.030~0.150%, Si: 0.01~0.10%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Nb: 0.003~0.030%, Ti: 0.005~0.050%, Al: 0.005~0.100%, N: 0.0100% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~2.00%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.200%, Ca: 0~0.0100%, Mg: 0~0.0100%, Rare earth elements: 0~0.0500%, Zr: 0~0.050%, Hf: 0~0.020%, Sr: 0~0.020%, W: 0~0.100%, Co: 0~0.100%, B: 0~0.0050%, 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 consists of Fe and impurities. The f1 defined in equation (1) satisfies 0.34 to 0.45, At the center of the thickness of the steel plate, Prior austenite grains that satisfy the conditions of having a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area or more of the prior austenite grains. The area ratio S2 of the MA structure (Martensite-Austenite Constituent) in the central part of the thickness of the base steel sheet satisfies the condition of being greater than 0 to 0.50%. The area ratio S4 of the MA structure in the t / 4 portion of the base steel sheet satisfies the value of greater than 0 to 0.32%. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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]

[0014] 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 can be used to manufacture the above-described welded structure. [Brief explanation of the drawing]

[0015] [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 schematic diagram showing an example of the vicinity of a weld formed by electroslag welding. [Modes for carrying out the invention]

[0016] 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.

[0017] Specifically, the inventors have determined the following by mass%, C: 0.030~0.150%, Si: 0.01~0.10%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Nb: 0.003~0.030%, Ti: 0.005~0.050%, Al: 0.005~0.100%, N: 0.0100% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~2.00%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.200%, Ca: 0~0.0100%, Mg: 0~0.0100%, rare earth elements: 0~0.050% We considered that a steel sheet having a chemical composition consisting of 0%, Zr:0~0.050%, Hf:0~0.020%, Sr:0~0.020%, W:0~0.100%, Co:0~0.100%, B:0~0.0050%, 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.

[0018] On the other hand, even with steel plates having the above-mentioned chemical composition, it was sometimes impossible to achieve both excellent HAZ toughness after welding and suppression of strength heterogeneity in the HAZ. Therefore, the inventors investigated various methods to suppress strength heterogeneity in the HAZ after welding and improve HAZ toughness for steel plates having the above-mentioned chemical composition. As a result, the inventors obtained the following findings.

[0019] In order to suppress the non-uniformity of strength in the heat-affected zone (HAZ) after welding for steel plates having the above-described chemical composition, it is sufficient to suppress the uneven distribution of hardness in the HAZ after welding. Therefore, the inventors have found that the hardness of the HAZ after welding can be stabilized by adjusting the chemical composition.

[0020] Specifically, assuming that the steel sheet according to this embodiment has the above-mentioned chemical composition, f1, as defined by formula (1), satisfies 0.34 to 0.45. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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.

[0021] f1 is an index of the hardness of the steel plate and the HAZ. If f1 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 f1 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 above-mentioned chemical composition, f1 is set to 0.34 to 0.45.

[0022] On the other hand, even with steel plates having the above-mentioned chemical composition and f1 satisfying 0.34 to 0.45, there were cases where the heterogeneity of strength in the HAZ after welding could not be sufficiently suppressed. Therefore, the inventors focused on the microstructure of the steel plate and investigated various methods to suppress the bias in the hardness distribution in the HAZ after welding. As a result, it became clear that in steel plates having the above-mentioned chemical composition and f1 satisfying 0.34 to 0.45, the prior austenite grain size of the steel plate and the distribution of MA structure (Martensite-Austenite Constituent) in the thickness direction of the steel plate influence the bias in the hardness distribution in the HAZ after welding.

[0023] Herein, prior austenite grains are also referred to as "prior γ grains" in this specification. In the microstructure of a steel sheet having the above-described chemical composition, if the prior γ grains are coarse, the HAZ toughness after welding may decrease. As a result of the inventors' investigations, it has become clear that prior γ grains with a short axis exceeding 25 μm and / or prior γ grains with a long axis exceeding 50 μm tend to reduce the HAZ toughness after welding. Therefore, in this embodiment, the area ratio of prior γ grains that satisfy the conditions of a short axis of 25 μm or less and a long axis of 50 μm or less in the central part of the steel sheet thickness is increased to 50% or more. In this case, the HAZ toughness after welding can be further and stably improved.

[0024] The steel sheet according to this embodiment further defines the distribution of the MA structure in the thickness direction of the steel sheet. Specifically, the MA structure is much harder than other phases (ferrite, pearlite, and bainite). Therefore, in regions where the area ratio of the MA structure is locally high, the strength may become locally too high. Accordingly, the steel sheet according to this embodiment defines the area ratio S2 of the MA structure in the center of the thickness of the steel sheet and the area ratio S4 of the MA structure in the t / 4 part of the steel sheet. As a result, unevenness in strength in the thickness direction of the steel sheet is suppressed, and it is possible to sufficiently suppress the heterogeneity of the strength of the HAZ while maintaining HAZ toughness. In this specification, the t / 4 part refers to a position at a depth of t / 4 from the surface of the steel sheet, where t is the thickness of the steel sheet.

[0025] As a result of detailed studies by the present inventors, it has become clear that in a steel sheet having the above-mentioned chemical composition and satisfying f1 of 0.34 to 0.45, if the area ratio S2 of the MA structure in the central part of the thickness of the steel sheet is greater than 0 to 0.50%, and furthermore, if the area ratio S4 of the MA structure in the t / 4 part of the steel sheet is greater than 0 to 0.32%, then the heterogeneity of the strength of the HAZ can be sufficiently suppressed while maintaining HAZ toughness. Therefore, the steel sheet according to this embodiment has the above-mentioned chemical composition, satisfies f1 of 0.34 to 0.45, satisfies that in the central part of the thickness of the steel sheet, prior γ grains with a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area % or more of the prior γ grains, satisfies that the area ratio S2 of the MA structure in the central part of the thickness of the steel sheet is greater than 0 to 0.50%, and satisfies that the area ratio S4 of the MA structure in the t / 4 part of the steel sheet is greater than 0 to 0.32%. As a result, the welded structure manufactured from steel plates according to this embodiment has excellent HAZ toughness, and the non-uniformity of strength in the HAZ is sufficiently suppressed.

[0026] Based on the above findings, the gist of the steel plate and welded structure according to this embodiment is as follows.

[0027] [1] It is a steel plate, The chemical composition is expressed in mass percent. C: 0.030~0.150%, Si: 0.01~0.10%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Nb: 0.003~0.030%, Ti: 0.005~0.050%, Al: 0.005~0.100%, N: 0.0100% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~2.00%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.200%, Ca: 0~0.0100%, Mg: 0~0.0100%, Rare earth elements: 0~0.0500%, Zr: 0~0.050%, Hf: 0~0.020%, Sr: 0~0.020%, W: 0~0.100%, Co: 0~0.100%, B: 0~0.0050%, 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 consists of Fe and impurities. The f1 defined in equation (1) satisfies 0.34 to 0.45, At the center of the thickness of the steel plate, Prior austenite grains that satisfy the conditions of having a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area or more of the prior austenite grains. The area ratio S2 of the MA structure (Martensite-Austenite Constituent) in the central part of the thickness of the steel plate satisfies the condition of being greater than 0 to 0.50%. The area ratio S4 of the MA structure in the t / 4 portion of the steel plate satisfies the condition of being greater than 0 to 0.32%. steel plate. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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.

[0028] [2] [1] The steel plate described above, The aforementioned chemical composition is Cu: 0.01~0.50%, Ni: 0.01~2.00%, Cr: 0.01~0.50%, Mo: 0.01~0.50%, V: 0.001~0.200%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Rare earth elements: 0.0001~0.0500%, Zr: 0.001~0.050%, Hf: 0.001~0.020%, Sr: 0.001~0.020%, W: 0.001~0.100%, Co: 0.001~0.100%, B: 0.0001~0.0050%, Sn: 0.001~0.100%, Sb: 0.001~0.050%, Zn: 0.001~0.020%, Bi: 0.001~0.100%, Se: 0.001~0.020%, Pb: 0.001~0.090%, As: 0.001~0.050%, and, Contains one or more elements selected from the group consisting of Te: 0.001 to 0.050%. steel plate.

[0029] [3] The steel plate described in [1] or [2], The aforementioned chemical composition satisfies the condition that f2, as defined by formula (2), is 120 or greater. steel plate. f2=119+6C+55Cu+27Ni+49Cr+95Mo (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.

[0030] [4] It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.030~0.150%, Si: 0.01~0.10%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Nb: 0.003~0.030%, Ti: 0.005~0.050%, Al: 0.005~0.100%, N: 0.0100% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~2.00%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.200%, Ca: 0~0.0100%, Mg: 0~0.0100%, Rare earth elements: 0~0.0500%, Zr: 0~0.050%, Hf: 0~0.020%, Sr: 0~0.020%, W: 0~0.100%, Co: 0~0.100%, B: 0~0.0050%, 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 consists of Fe and impurities. The f1 defined in equation (1) satisfies 0.34 to 0.45, At the center of the thickness of the steel plate, Prior austenite grains that satisfy the conditions of having a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area or more of the prior austenite grains. The area ratio S2 of the MA structure (Martensite-Austenite Constituent) in the central part of the thickness of the base steel sheet satisfies the condition of being greater than 0 to 0.50%. The area ratio S4 of the MA structure in the t / 4 portion of the base steel sheet satisfies the condition of being greater than 0 to 0.32%. Welded structure. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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.

[0031] [5] [4] The welded structure described above, The chemical composition of the base steel sheet is, Cu: 0.01~0.50%, Ni: 0.01~2.00%, Cr: 0.01~0.50%, Mo: 0.01~0.50%, V: 0.001~0.200%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Rare earth elements: 0.0001~0.0500%, Zr: 0.001~0.050%, Hf: 0.001~0.020%, Sr: 0.001~0.020%, W: 0.001~0.100%, Co: 0.001~0.100%, B: 0.0001~0.0050%, Sn: 0.001~0.100%, Sb: 0.001~0.050%, Zn: 0.001~0.020%, Bi: 0.001~0.100%, Se: 0.001~0.020%, Pb: 0.001~0.090%, As: 0.001~0.050%, and, Contains one or more elements selected from the group consisting of Te: 0.001 to 0.050%. Welded structure.

[0032] [6] [4] The welded structure described above, The aforementioned chemical composition satisfies the condition that f2, as defined by formula (2), is 120 or greater. Welded structure. f2=119+6C+55Cu+27Ni+49Cr+95Mo (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.

[0033] [7] [5] The welded structure described above, The aforementioned chemical composition satisfies the condition that f2, as defined by formula (2), is 120 or greater. Welded structure. f2=119+6C+55Cu+27Ni+49Cr+95Mo (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.

[0034] [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 140 Hv or less is 30.0% or less. Welded structure.

[0035] [9] [8] The welded structure described above, Of the base steel plate, in the area affected by welding heat, In the microstructure, the area ratio of ferrite is 90.0% or less. Welded structure.

[0036] 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 %.

[0037] [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) satisfies 0.34 to 0.45. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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 3) In the central part of the steel plate thickness, prior austenite grains that satisfy the condition of having a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area or more of the total prior austenite grains. (Feature 4) The area ratio S2 of the MA structure in the center of the steel plate thickness is greater than 0 and satisfies 0 to 0.50%, and the area ratio S4 of the MA structure in the t / 4 portion of the steel plate is greater than 0 and satisfies 0 to 0.32%. The following describes each of its features.

[0038] [(Feature 1) Chemical Composition] The chemical composition of the steel sheet according to this embodiment contains the following elements:

[0039] C: 0.030~0.150% Carbon (C) enhances the hardenability of steel plates and increases the strength of the steel plate and 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 0.030 to 0.150%. The preferred lower limit of the C content is 0.032%, and more preferably 0.035%. The preferred upper limit of the C content is 0.130%, more preferably 0.110%, and still more preferably 0.100%.

[0040] Si: 0.01~0.10% 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 0.01 to 0.10%. The preferred lower limit of the Si content is 0.02%, and more preferably 0.03%. The preferred upper limit of the Si content is less than 0.10%, more preferably 0.09%, and still more preferably 0.08%.

[0041] Mn: 0.70~2.00% Manganese (Mn) enhances the hardenability of steel plates and increases the strength of the steel plate and 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 structures are more likely to form in the steel plate after welding, reducing the toughness of the HAZ. Therefore, the Mn content is 0.70 to 2.00%. The preferred lower limit of the Mn content is 0.75%, more preferably 0.80%, and even more preferably 0.85%. The preferred upper limit of the Mn content is 1.90%, and even more preferably 1.80%.

[0042] 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%.

[0043] S:0.0080% 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, and the HAZ toughness after welding will decrease. Therefore, the S content is 0.0080% or less. The preferred upper limit of the S content is 0.0060%, more preferably 0.0040%, and even more preferably 0.0030%. 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 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.

[0044] Nb: 0.003~0.030% Niobium (Nb) combines with carbon to form carbides and other compounds, thereby suppressing the heterogeneity of the strength in 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 0.003 to 0.030%. The preferred lower limit of the Nb content is 0.004%, more preferably 0.005%, and even more preferably 0.006%. The preferred upper limit of the Nb content is 0.028%, more preferably 0.025%, and even more preferably 0.023%.

[0045] Ti: 0.005~0.050% 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 0.005 to 0.050%. The preferred lower limit of the Ti content is 0.006%, more preferably 0.008%, and even more preferably 0.010%. The preferred upper limit of the Ti content is 0.045%, and even more preferably 0.040%.

[0046] Al: 0.005~0.100% 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 0.005 to 0.100%. The preferred lower limit of the Al content is 0.008%, and more preferably 0.010%. The preferred upper limit of the Al content is 0.090%, more preferably 0.080%, and still more preferably 0.070%.

[0047] N: 0.0100% or less Nitrogen (N) is inevitably present. That is, the lower limit of the N content is greater than 0%. N combines with Ti to form Ti nitrides, which enhance HAZ toughness through the pinning effect. 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 0.0100% or less. The preferred upper limit of the N content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%. The preferred lower limit of the N content to more effectively obtain the above effects is 0.0020%, more preferably 0.0030%, and even more preferably 0.0040%.

[0048] O: 0.0050% 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 0.0050% or less. The preferred upper limit of the O content is 0.0040%, and more preferably 0.0030%. It is preferable to have as low an O content as possible. However, an extreme reduction in O content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0005%, and more preferably 0.0010%.

[0049] 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.

[0050] [Optional element] The chemical composition of the steel sheet according to this embodiment further includes one or more elements selected from the group consisting of Cu, Ni, Cr, and Mo, 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.

[0051] 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 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. On the other hand, if the Cu content is too high, even if the content of other elements is within the range of this embodiment, MA structure is more likely to form in the steel sheet after welding, and the HAZ toughness decreases. 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%, more preferably 0.03%, and still more preferably 0.05%. The preferred upper limit of the Cu content is 0.48%, and more preferably 0.45%.

[0052] Ni: 0~2.00% 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 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. On the other hand, if the Ni content is too high, even if the content of other elements is within the range of this embodiment, MA structure is more likely to form in the steel sheet after welding, and the HAZ toughness will decrease. Therefore, the Ni content is 0 to 2.00%, and if included, it is 2.00% or less. The preferred lower limit of the Ni content is greater than 0%, more preferably 0.01%, more preferably 0.10%, more preferably 0.20%, more preferably 0.30%, and more preferably greater than 0.35%. The preferred upper limit of the Ni content is 1.80%, and more preferably 1.60%.

[0053] Cr: 0~0.50% Chromium (Cr) is an optional element and may not be present. That is, the Cr content may be 0%. When present, Cr enhances the hardenability of the steel sheet and suppresses the heterogeneity of strength in the heat-affected zone (HAZ) after welding. Even a small amount of Cr can 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%, and still more preferably 0.05%. The preferred upper limit of the Cr content is 0.45%, and more preferably 0.40%.

[0054] Mo: 0~0.50% Molybdenum (Mo) is an optional element and may not be included. That is, the Mo content may be 0%. When included, Mo 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 Mo is included, the above effects can be obtained to some extent. 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 will become too high and the HAZ toughness after welding will decrease. Therefore, the Mo content is 0 to 0.50%, and if included, it is 0.50% or less. The preferred lower limit of the Mo content is greater than 0%, more preferably 0.01%, more preferably 0.03%, and still more preferably 0.05%. The preferred upper limit of the Mo content is 0.45%, and more preferably 0.40%.

[0055] The chemical composition of the steel sheet according to this embodiment may further contain V instead of some of the Fe.

[0056] V: 0~0.200% 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, increasing the strength of the HAZ after welding. As a result, the heterogeneity of the strength in the HAZ after welding is suppressed. 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 0.200%, and if present, it is 0.200% or less. The preferred lower limit of the V 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 V content is 0.180%, more preferably 0.160%, and still more preferably 0.140%.

[0057] 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 Ca, Mg, rare earth elements, Zr, Hf, and Sr. Any of these elements are arbitrary and enhance the HAZ toughness after welding.

[0058] 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, etc., suppressing the formation of coarse inclusions and thereby improving the HAZ toughness after welding. Even if only a small amount of Ca is present, the above effect can be obtained to some extent. However, if the Ca content is too high, coarse inclusions will be formed, 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%.

[0059] 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 and the like, suppressing the formation of coarse inclusions and thereby 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 if only a small amount of Mg is present, the above effects can be obtained to some extent. However, if the Mg content is too high, coarse inclusions will be formed, 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%.

[0060] Rare earth elements (REM): 0~0.0500% 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 0.0500%, and if present, it is 0.0500% or less. The preferred lower limit of the REM 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 REM content is 0.0400%, more preferably 0.0300%, more preferably 0.0200%, more preferably 0.0100%, and more preferably 0.0060%.

[0061] 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.

[0062] 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%.

[0063] 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%.

[0064] 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%.

[0065] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of W, Co, and B in place of a portion of Fe. These elements are all arbitrary elements and enhance the hardenability of the steel sheet and suppress the heterogeneity of strength in the heat-affected zone (HAZ) after welding.

[0066] 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%.

[0067] 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%.

[0068] B: 0~0.0050% Boron (B) is an optional element and may not be present. That is, the B content may be 0%. If present, B 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 B is present, the above effects can be obtained to some extent. However, if the B content is too high, even if the content of other elements is within the range of this embodiment, coarse inclusions will be formed, and the HAZ toughness after welding will decrease. Therefore, the B content is 0 to 0.0050%, and if present, it is 0.0050% or less. The preferred lower limit of the B content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, and still more preferably 0.0005%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, and still more preferably 0.0035%.

[0069] 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.

[0070] 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%.

[0071] 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%.

[0072] 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%.

[0073] 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.

[0074] 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%.

[0075] 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%.

[0076] 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%.

[0077] 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%.

[0078] 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%.

[0079] [(Feature 2) f1 defined by equation (1)] Assuming that the steel plate according to this embodiment has the other features of this embodiment, f1 as defined by formula (1) satisfies 0.34 to 0.45. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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.

[0080] As described above, f1 is an indicator of the hardness of the steel plate. If f1 is too low, the hardness cannot be sufficiently increased, and the strength of the heat-affected zone (HAZ) tends to decrease. On the other hand, if f1 is too high, the hardness becomes too high, and the HAZ toughness decreases. Therefore, assuming that the steel plate according to this embodiment has the other features of this embodiment, f1 is set to 0.34 to 0.45.

[0081] A preferred lower limit for f1 is 0.35, and more preferably 0.36. A preferred upper limit for f1 is 0.44, and more preferably 0.43. Note that f1 is determined by rounding the value obtained from the element content and formula (1) to the third decimal place.

[0082] [(Feature 3) Old austenite grains] In the steel sheet according to this embodiment, prior austenite grains satisfying the condition that a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area or more of the total prior austenite grains in the central part of the thickness of the steel sheet. As described above, in this specification, prior austenite grains are also referred to as "prior γ grains". In this specification, prior austenite grains satisfying the condition that a short axis of 25 μm or less and a long axis of 50 μm or less are also referred to as "specific prior γ grains".

[0083] As described above, if the prior γ grains in the center of the plate thickness are coarse, the HAZ toughness after welding may decrease. In particular, prior γ grains with a short axis exceeding 25 μm and / or a long axis exceeding 50 μm tend to reduce the HAZ toughness after welding. Therefore, in this embodiment, the area ratio of prior γ grains that satisfy the conditions of a short axis of 25 μm or less and a long axis of 50 μm or less (specific prior γ grains) is increased to 50% or more. In this case, the HAZ toughness after welding can be further and stably improved. Accordingly, the steel plate according to this embodiment has an area ratio of specific prior γ grains of 50% or more, assuming that it has the other features of this embodiment.

[0084] In this embodiment, the preferred lower limit of the area ratio of specific prior γ grains is 51%, more preferably 53%, and even more preferably 55%. In this embodiment, the upper limit of the area ratio of specific prior γ grains is not particularly limited and may be 100%. The upper limit of the area ratio of specific prior γ grains may be 95%, 90%, or 85%.

[0085] In the steel sheet according to this embodiment, the area ratio of specific prior γ grains can be determined by the following method. First, a test piece is prepared from the center of the thickness of the steel sheet according to this embodiment, with the observation surface being a plane that includes the rolling direction and the width direction. After wet polishing the observation surface of the test piece, buff polishing is performed to polish the observation surface to a mirror finish. Specifically, for wet polishing, it is preferable to use sandpaper of about #2000 grit. Also, for buff polishing, it is preferable to use alumina abrasive grains of φ3 μm. The mirror-polished observation surface is immersed in a Nital etching solution for about 10 seconds to reveal the microstructure by etching. The etched observation surface is observed in 10 fields of view using an optical microscope. The field of view area is, for example, 0.025 mm. 2 (Magnification 500x).

[0086] In each field of view, the old gamma grains are identified based on contrast. Identifying the old gamma grains based on contrast in each field of view is a matter of course for those skilled in the art. Furthermore, the short axis and long axis of the identified old gamma grains are determined. The method for determining the short axis and long axis of the old gamma grains is not particularly limited and any well-known method may be used. For example, the short axis and long axis of the old gamma grains can be determined by image analysis of the field of view. In each field of view, old gamma grains with a short axis of 25 μm or less and a long axis of 50 μm or less are defined as "specific old gamma grains." In each field of view, the area of ​​the specific old gamma grains is divided by the area of ​​all old gamma grains to obtain the area ratio of the specific old gamma grains. In this embodiment, the arithmetic mean of the area ratios of the specific old gamma grains obtained in all fields of view is defined as the area ratio of the specific old gamma grains. The area ratio of the specific old gamma grains is obtained by rounding the obtained value to the first decimal place.

[0087] [(Feature 4) MA Organization] In this embodiment, the area ratio S2 of the MA structure in the center of the thickness of the steel sheet satisfies the condition of being greater than 0 to 0.50%, and the area ratio S4 of the MA structure in the t / 4 portion of the steel sheet satisfies the condition of being greater than 0 to 0.32%. As stated above, in this specification, the t / 4 portion refers to a position at a depth of t / 4 from the surface of the steel sheet, where t is the thickness of the steel sheet.

[0088] As mentioned above, the MA structure in the microstructure is significantly harder than other phases (ferrite, pearlite, and bainite). Therefore, if the area ratio of the MA structure is too high, the strength becomes excessively high, reducing the toughness of the heat-affected zone (HAZ) after welding. Furthermore, in regions where the area ratio of the MA structure is locally high, the strength may become locally excessively high. For this reason, if the area ratio of the MA structure differs significantly between the center of the plate thickness and the t / 4 portion, the heterogeneity of the HAZ strength can be sufficiently suppressed.

[0089] Therefore, assuming that the steel sheet according to this embodiment has the other features of this embodiment, the area ratio of the MA structure is reduced in both the central part of the steel sheet thickness and the t / 4 part of the steel sheet. On the other hand, in a steel sheet having the above-mentioned chemical composition, it is extremely difficult to make the area ratio of the MA structure 0%. Therefore, the steel sheet according to this embodiment is assumed to contain an MA structure in its microstructure. In other words, assuming that the steel sheet according to this embodiment has the other features of this embodiment, the area ratio S2 of the MA structure in the central part of the steel sheet thickness is set to greater than 0 to 0.50%, and the area ratio S4 of the MA structure in the t / 4 part of the steel sheet is set to greater than 0 to 0.32%.

[0090] In this embodiment, the preferred upper limit of the area ratio S2 of the MA structure in the central part of the thickness of the steel sheet is 0.45%, more preferably 0.40%, and still more preferably 0.35%. The preferred lower limit of the area ratio S2 of the MA structure in the central part of the thickness of the steel sheet is 0.01%, and still more preferably 0.02%. In this embodiment, the preferred upper limit of the area ratio S4 of the MA structure in the t / 4 part of the steel sheet is 0.31%, more preferably 0.30%, and still more preferably 0.28%. The preferred lower limit of the area ratio S4 of the MA structure in the t / 4 part of the steel sheet is 0.01%, and still more preferably 0.02%.

[0091] In the microstructure of the steel sheet according to this embodiment, phases other than the MA structure are not particularly limited. However, in a steel sheet having the above-described chemical composition and satisfying f1 of 0.34 to 0.45, the microstructure in the center of the sheet thickness consists of 20 to 65% ferrite, 0 to 15% pearlite, more than 0 to 0.50% MA structure, and the remainder being bainite, in terms of area percent. Similarly, the microstructure in the t / 4 portion consists of 20 to 65% ferrite, 0 to 15% pearlite, more than 0 to 0.32% MA structure, and the remainder being bainite, in terms of area percent.

[0092] Furthermore, the microstructure of the steel sheet according to this embodiment may contain trace amounts of precipitates, inclusions, etc., in addition to ferrite, pearlite, MA structure, and bainite. However, in the microstructure of the steel sheet according to this embodiment, the area ratio of precipitates, inclusions, etc., is negligibly small compared to the sum of the area ratios of ferrite, pearlite, MA structure, and bainite.

[0093] In this embodiment, the area ratios S2 and S4 of the MA structure can be determined by the following method. First, a test piece is prepared from the center of the thickness of the steel sheet according to this embodiment, with the observation surface being a plane that includes the rolling direction and the width direction. Similarly, a test piece is prepared from t / 4 of the steel sheet according to this embodiment, with the observation surface being a plane that includes the rolling direction and the width direction. After wet polishing the observation surface of the test piece, the observation surface is polished to a mirror finish by buff polishing. Specifically, for wet polishing, it is preferable to use sandpaper of about #2000 grit. Also, for buff polishing, it is preferable to use alumina abrasive grains of φ3 μm. The microstructure is revealed by etching the mirror-polished observation surface. Etching is preferably performed by immersing, for example, in a Repera etching solution for about 10 seconds. The etched observation surface is observed in 10 fields of view using an optical microscope. The field of view area is, for example, 0.025 mm 2 (Magnification should be 500x). Note that observation of MA tissue may also be performed using a scanning electron microscope (SEM).

[0094] In each observation field, MA tissue is identified based on contrast. When performing optical microscopy observation, the areas with high brightness (i.e., white areas) within the field of view can be identified as MA tissue. The method for determining the area percentage of the identified MA tissue is not particularly limited and any well-known method may be used. For example, the area percentage can be determined by image analysis of the observation field. In this embodiment, the arithmetic mean of the area percentages of MA tissue obtained in all observation fields is defined as the area percentage of MA tissue. That is, the area percentage of MA tissue obtained using a test specimen prepared from the center of the plate thickness is defined as S2 (%). Similarly, the area percentage of MA tissue obtained using a test specimen prepared from t / 4 is defined as S4 (%). The area percentage of MA tissue is obtained by rounding the obtained value to the third decimal place.

[0095] [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 value of f2 defined by equation (2) is 120 or greater. f2=119+6C+55Cu+27Ni+49Cr+95Mo (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 6) The tensile strength TS is 550-740 MPa. The following describes these features.

[0096] [(Feature 5) f2 defined by equation (2)] Assuming that the steel plate according to this embodiment has the other features of this embodiment, f2 as defined by equation (2) satisfies 120 or more. f2=119+6C+55Cu+27Ni+49Cr+95Mo (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.

[0097] f2 is an index of the hardness of the heat-affected zone (HAZ) after welding. Here, C, Cu, Ni, Cr, and Mo in f2 contribute to improving the hardness of the HAZ. If f2 is 120 or higher, the hardness of the HAZ can be sufficiently increased, and the strength of the HAZ after welding tends to stabilize. In other words, if f2 is 120 or higher, the non-uniformity of the strength in the HAZ can be further stably suppressed while maintaining the toughness of the HAZ after welding. Therefore, assuming that the steel plate according to this embodiment has the other features of this embodiment, f2 is set to 120 or higher.

[0098] A preferred lower limit for f2 is 121, more preferably 123, and even more preferably 125. The upper limit for f2 is not particularly limited, but may be, for example, 200. A further preferred upper limit for f2 is 195, more preferably 190, and even more preferably 185. Note that f2 is determined by rounding the value obtained from the element content and formula (2) to the first decimal place.

[0099] [(Feature 6) Tensile Strength TS] The steel plate according to this embodiment preferably has a tensile strength of 550 to 740 MPa, provided that it has the other features of this embodiment. When the tensile strength TS is 550 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 555 MPa, more preferably 560 MPa, and still more preferably 565 MPa. The preferred upper limit of the tensile strength TS is 735 MPa, more preferably 730 MPa, and still more preferably 725 MPa.

[0100] In this embodiment, the yield stress YS and yield ratio YR of the steel plate are not particularly limited. The yield stress YS of the steel plate is preferably, for example, 385 to 540 MPa. The yield ratio YR of the steel plate is preferably, for example, 80% or less. 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.

[0101] 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 JIS No. 4 as specified in JIS Z 2241:2011. A tensile test is performed on the round bar test specimen at room temperature and in air, in accordance with JIS Z 2241:2011. 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.

[0102] [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 40 mm. If the steel plate has a thickness of 19 to 40 mm, the above features 1 to 6 can be reliably obtained by the preferred manufacturing method described later.

[0103] [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 value of f1 defined by equation (1) satisfies 0.34 to 0.45. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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) At the center of the thickness of the steel plate, Prior austenite grains that satisfy the conditions of having a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50% or more of the total area of ​​prior austenite grains. (Feature 5) The area ratio S2 of the MA structure in the center of the steel plate thickness is greater than 0 and satisfies 0 to 0.50%, and the area ratio S4 of the MA structure in the t / 4 portion of the steel plate is greater than 0 and satisfies 0 to 0.32%. The following describes each of its features.

[0104] [(Feature 1) Base steel plate and welded joint] The welded structure according to this embodiment comprises a base steel plate and a welded section. 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 section 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. 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.

[0105] 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.

[0106] 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.

[0107] [(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 the steel plate according to this embodiment as the base steel plate 10. 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. In short, the base steel plate 10 of the welded structure 1 according to this embodiment has the same characteristics as 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 characteristics as the steel plate according to this embodiment.

[0108] [Intensity heterogeneity in HAZ] 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.

[0109] 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.

[0110] Figure 3 is a schematic diagram showing an example of the vicinity of a weld formed 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.

[0111] 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 10 kgf (98 N). Of the Vickers hardness obtained at each measurement point, the measurement points with a Vickers hardness of 140 Hv or less are counted. The ratio of the number of measurement points with a Vickers hardness of 140 Hv or less to the total number of measurement points is calculated and defined as the area ratio (%) of the strength heterogeneity region.

[0112] In this embodiment, the welded structure 1 has a strength heterogeneity area ratio of 30.0% 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.

[0113] [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.

[0114] A full-size V-notch test specimen is prepared from the fabricated welded joint. The V-notch test 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 test specimen is prepared in a direction such that the longitudinal direction of the V-notch test specimen is parallel to the center line of the diaphragm 30. Alternatively, if the test 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 test specimen makes an angle with respect to the center line of the diaphragm 30. A Charpy impact test in accordance with JIS Z 2242:2018 is performed on the prepared V-notch test specimen to determine the absorbed energy (J) at 0°C. In this embodiment, the absorbed energy (J) at 0°C is obtained by rounding the obtained value to the first decimal place. The welded structure 1 according to this embodiment has an absorbed energy of 47 J 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.

[0115] [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 and / or MA structure. In other words, in this embodiment, the HAZ may have a microstructure consisting of ferrite and bainite, a microstructure consisting of ferrite and pearlite, a microstructure consisting of ferrite and MA structure, or a microstructure consisting of ferrite, bainite, pearlite, and MA structure. In addition to ferrite, bainite, pearlite, and MA structure, 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 fraction of precipitates, inclusions, etc. is negligibly small compared to ferrite, bainite, pearlite, and MA structure.

[0116] 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 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 after welding can be suppressed more stably.

[0117] In this embodiment, the area ratio of ferrite in the heat-affected zone (HAZ) can be determined in the same manner as the method for determining the area ratio of ferrite in the steel plate according to this embodiment described above. Specifically, a test piece for microstructural observation is prepared from the HAZ of the welded structure according to this embodiment. Microstructural exposure is performed in the same manner as described above, and the observation surface is observed in 10 fields using an optical microscope. The area ratio of ferrite identified from the contrast is determined in the same manner as described above. It should be noted that identifying ferrite from the contrast in each observation field is a matter of course for those skilled in the art. In this embodiment, the arithmetic mean of the ferrite area ratios obtained in all fields is defined as the ferrite area ratio. The ferrite area ratio is obtained by rounding the obtained value to the second decimal place.

[0118] [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.

[0119] [Slab preparation process] In the slab preparation process, a slab having the above-mentioned chemical composition is prepared. Specifically, in mass%, C: 0.030~0.150%, Si: 0.01~0.10%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Nb: 0.003~0.030%, Ti: 0.005~0.050%, Al: 0.005~0.100%, N: 0.0100% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~2.00%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.200%, Ca: 0~0.0100%, Mg: 0~0.0100%, rare earth elements: Prepare a slab having a chemical composition consisting of 0-0.0500%, Zr:0-0.050%, Hf:0-0.020%, Sr:0-0.020%, W:0-0.100%, Co:0-0.100%, B:0-0.0050%, 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, and satisfying f1, as defined by formula (1), to be between 0.34 and 0.45. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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.

[0120] 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.

[0121] [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.

[0122] [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 preferably 1100 to 1200°C. If the heating temperature 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 is too high, the austenite grains may become coarse, and the desired mechanical properties may not be obtained. Therefore, the heating temperature is preferably 1100 to 1200°C.

[0123] In the heating process, it is preferable to hold the material at the above heating temperature for 60 to 360 minutes. If the heating time is too short, 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 time is too long, the above effect will saturate. Therefore, in the heating process according to this embodiment, it is preferable to hold the material at 1100 to 1200°C for 60 to 360 minutes.

[0124] [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.

[0125] [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.

[0126] Preferably, in the second rolling process, the temperature of the slab during the initial rolling is set to 860-940°C. In this embodiment, the temperature during the initial rolling refers to the temperature of the slab when it is introduced into the first rolling mill of the second rolling process. If the temperature during the initial rolling is too low, many deformation strips formed by processing may remain in the steel sheet, potentially resulting in excessively high strength. Therefore, it is preferable that the temperature of the slab during the initial rolling is set to 860-940°C. The method for adjusting the temperature during the initial hot rolling is not particularly limited and can be adjusted as appropriate. For example, the temperature of the slab introduced into the first rolling mill of the second rolling process may be adjusted by adjusting the time it takes to transport the slab after the first rolling process.

[0127] Furthermore, it is preferable that the cumulative reduction ratio in the second rolling process be 40% or more. If the cumulative reduction ratio is too low, there may be too few processed austenite grains or deformed bands. Therefore, it is preferable that the cumulative reduction ratio in the second rolling process be 40% or more.

[0128] Preferably, in the second rolling process, the slab temperature when the reduction ratio exceeds 40% is set to 820-900°C. Here, the slab temperature when the reduction ratio exceeds 40% refers to the slab temperature when the reduction ratio exceeds 40% during the second rolling process. Specifically, if the cumulative reduction ratio before the nth pass is 38%, and the cumulative reduction ratio when the nth pass is completed is 42%, this refers to the slab temperature when the nth pass begins.

[0129] In the low-temperature region of the slab, the amount of strain introduced into the slab by hot rolling tends to be greater. Therefore, if the slab temperature is too low when the reduction ratio exceeds 40%, the strength of the manufactured steel sheet may become too high. On the other hand, if the slab temperature is too high when the reduction ratio exceeds 40%, the area ratio of specific γ grains in the manufactured steel sheet may decrease. For this reason, it is preferable to set the temperature at which the reduction ratio exceeds 40% in the second rolling process to 820-900°C.

[0130] Preferably, in the second rolling process, the slab temperature at the start of the final pass is set to 820-890°C. If the slab temperature at the start of the final pass is too low, the strength of the steel sheet may become too high. On the other hand, if the slab temperature at the start of the final pass is too high, the area ratio of specific γ grains in the manufactured steel sheet may decrease. Therefore, it is preferable to set the slab temperature at the start of the final pass in the second rolling process to 820-890°C.

[0131] [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, water cooling is performed. In this case, the cooling start temperature is preferably set to 780 to 830°C. If the cooling start temperature is too low, the area ratio of specific γ grains in the manufactured steel sheet may decrease. On the other hand, if the cooling start temperature is too high, the strength of the manufactured steel sheet may become too high. Therefore, the cooling start temperature in the cooling process is preferably set to 780 to 830°C.

[0132] Furthermore, in the cooling process, it is preferable to set the cooling stop temperature to 430-630°C. If the cooling stop temperature is too low, the strength of the manufactured steel sheet may become too high. On the other hand, if the cooling stop temperature is too high, the area ratio of the MA structure in the manufactured steel sheet may become too high. Therefore, it is preferable to set the cooling stop temperature to 430-630°C.

[0133] 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.

[0134] [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]

[0135] Slabs were manufactured by continuous casting from molten steel having the chemical compositions shown in Tables 1A and 1B. The blank spaces in Table 1B indicate that the content of each element is at the impurity level.

[0136] [Table 1A]

[0137] [Table 1B]

[0138] A hot rolling process was performed on the manufactured slabs. First, the thickness (mm) of each test slab was as shown in Table 2. Each test slab was heated in a heating furnace at the heating temperature (°C) and for the heating time (minutes) shown in Table 2. At the end of the first rolling process, the thickness (mm) of the slab was as shown in Table 2.

[0139] [Table 2]

[0140] The first rolling process was carried out on the heated slab at the cumulative reduction ratio (%) shown in Table 2. Furthermore, the second rolling process was carried out at the starting temperature (°C), final rolling start temperature (°C), temperature at 40% reduction ratio (°C), and cumulative reduction ratio (%) shown in Table 2. Note that the final rolling start temperature in the second rolling process refers to the temperature of the slab immediately before it is introduced into the first rolling mill in the second rolling process. Furthermore, the temperature at 40% reduction ratio refers to the temperature of the slab at the stage in the second rolling process when the reduction ratio exceeds 40%.

[0141] The steel plates that underwent the second rolling process were cooled under the conditions described in Table 2 to produce steel plates of the thicknesses described in Table 2.

[0142] [Evaluation Test] The following evaluation tests were performed on each steel plate with the specified test number that was manufactured. (Test 1) Microtissue observation test (Test 2) Tensile test (Test 3) HAZ Strength Heterogeneity Evaluation Test (Test 4) HAZ Microtissue Observation Test (Test 5) HAZ Toughness Evaluation Test

[0143] [(Test 1) Microtissue observation test] For each steel plate with a given test number, microstructural observation was performed using the method described above to determine the area ratio S2 of the MA structure in the center of the steel plate thickness and the area ratio S4 of the MA structure in the t / 4 portion of the steel plate thickness. For each steel plate with a given test number, the area ratio of prior austenite grains (specific prior γ grains) satisfying the condition of having a short axis of 25 μm or less and a long axis of 50 μm or less in the center of the steel plate thickness was further determined using the method described above. The obtained area ratio S4 (%) of the MA structure in the t / 4 portion for each test number is shown in the "MA area ratio in t / 4 portion" column of Table 3. The obtained area ratio S2 (%) of the MA structure in the center of the steel plate thickness for each test number is shown in the "MA area ratio in t / 2 portion" column of Table 3. The obtained area ratio (%) of specific prior γ grains in the center of the steel plate thickness for each test number is shown in Table 3.

[0144] [Table 3]

[0145] [(Test 2) 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) and yield stress YS (MPa). The obtained tensile strengths TS (MPa) and yield stresses YS (MPa) for each test number are shown in Table 3.

[0146] [Manufacturing of welded structures] Prior to tests 3-5, a simulated welded structure was manufactured. Specifically, steel plates with the thicknesses listed in Table 3 for each test number were used as skin plates. Furthermore, steel plates with the thicknesses listed in Table 3 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 3) or a welding wire equivalent to YES602-S (indicated as "B" in the "Welding Wire" column in Table 3) was used. The welding wire used in each test number all had a diameter of 1.6 mm. Furthermore, the conditions for ESW performed in each test number are shown in Table 3. An electroslag welded joint (ESW joint) between the skin plate and the diaphragm was manufactured by ESW performed under the above conditions.

[0147] [(Test 3) Evaluation test of strength heterogeneity of HAZ] From the welded joints of each test number, region 2tl, indicated as the shaded area in Figure 3, was identified. Region 2tl was defined using the method described above. Vickers hardness tests were performed on region 2tl at 1 mm intervals in the left-right direction and 1 mm intervals in the up-down direction of Figure 3, using the method described above. From the obtained Vickers hardness test results, the ratio of the number of measurement points with a Vickers hardness of 140 Hv or less to the total number of measurement points was calculated and defined as the area ratio (%) of the strength heterogeneity region. The area ratio (%) of the strength heterogeneity region for each test number obtained is shown in Table 3.

[0148] [(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 an optical microscope. 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 percentage (%) of the second phase for each test number obtained is shown in Table 3.

[0149] [(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 Charpy impact testing 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. The prepared V-notch specimens were subjected to Charpy impact testing in accordance with JIS Z 2242:2018 as described above, and the absorbed energy (J) at 0°C was determined. The absorbed energy at 0°C obtained for each test number is shown in the "vE0°C(J)" column of Table 3.

[0150] [Evaluation Results] Referring to Tables 1A, 1B, 2, and 3, the steel plates for test numbers 1 to 18 satisfied steel plate characteristics 1 to 5. 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 47 or more at 0°C in the HAZ, demonstrating excellent HAZ toughness.

[0151] [Evaluation Results] Referring to Tables 1A, 1B, 2, and 3, the steel plates for test numbers 1 to 18 satisfied steel plate characteristics 1 to 5. 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 47 or more at 0°C in the HAZ, demonstrating excellent HAZ toughness.

[0152] On the other hand, the steel plate of test number 19 had too high a carbon content and too high an f1 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.

[0153] The steel plate in test number 20 had too low a carbon content and too low an area ratio of specific prior γ grains. 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.

[0154] The steel plate used in test number 21 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.

[0155] The steel plate in test number 22 had too low a Nb content and too low an area ratio of specific prior γ grains. 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. Furthermore, the welded structure produced from this steel plate had an absorbed energy (J) of less than 47 J in the HAZ at 0°C, and did not have good HAZ toughness.

[0156] The steel plate in test number 23 had too low a Ti 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.

[0157] The steel plate in test number 24 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 47 J at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0158] The steel plate in test number 25 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 47 J at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0159] The steel plate in test number 26 had too high a Ca content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 47 J at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0160] The steel plate in test number 27 had too low an aluminum content. As a result, the welded structure made 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. Furthermore, the welded structure made from this steel plate had an absorbed energy (J) of less than 47 J at 0°C in the HAZ, and did not have good HAZ toughness.

[0161] The steel plate in test number 28 had too high a nitrogen content. As a result, the welded structure made 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. Furthermore, the welded structure made from this steel plate had an absorbed energy (J) of less than 47 J at 0°C in the HAZ, and did not have good HAZ toughness.

[0162] The steel plate in test number 29 had too high a Cr content and too high an f1. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 47 J at 0°C in the HAZ, and did not possess good HAZ toughness.

[0163] The steel plate of Test No. 30 had too high Mo content and too high f1. As a result, the welded structure manufactured from this steel plate had an absorbed energy (J) at 0 °C in the HAZ of less than 47 J and did not have excellent HAZ toughness.

[0164] The steel plate of Test No. 31 had too high V content and too low area ratio of specific prior γ grains. As a result, the welded structure manufactured from this steel plate had an absorbed energy (J) at 0 °C in the HAZ of less than 47 J and did not have excellent HAZ toughness.

[0165] The steel plate of Test No. 32 had too high f1. As a result, the welded structure manufactured from this steel plate had an absorbed energy (J) at 0 °C in the HAZ of less than 47 J and did not have excellent HAZ toughness.

[0166] The steel plate of Test No. 33 had too low final rolling start temperature in the second rolling process and too low cooling start temperature in the cooling process. As a result, this steel plate had too low area ratio of specific prior γ grains. As a result, the welded structure manufactured from this steel plate had an area ratio of the strength inhomogeneous region in the HAZ exceeding 50.0%, and the inhomogeneity of the strength in the HAZ was not suppressed.

[0167] The steel plate of Test No. 34 had too high cooling stop temperature in the cooling process. As a result, this steel plate had too high area ratio of MA structure in the t / 4 part and too high area ratio of MA structure in the t / 2 part. As a result, the welded structure manufactured from this steel plate had an absorbed energy (J) at 0 °C in the HAZ of less than 47 J and did not have excellent HAZ toughness.

[0168] The steel plate of Test No. 35 had too high final rolling start temperature in the second rolling process. As a result, this steel plate had too low area ratio of specific prior γ grains. As a result, the welded structure manufactured from this steel plate had an absorbed energy (J) at 0 °C in the HAZ of less than 47 J and did not have excellent HAZ toughness.

[0169] In test number 36, the temperature was too high when the reduction ratio was 40% during the second rolling process. As a result, the area ratio of specific prior γ grains in this steel sheet was too low. Consequently, the welded structure produced from this steel sheet had an absorbed energy (J) of less than 47 J at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0170] 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]

[0171] 1. Welded Structure 10. Base material steel plate (skin plate) 20 Welded parts 30 diaphragms 40 Prize money 50 Weld metal part

Claims

1. It is a steel plate, The chemical composition is expressed in mass percent. C: 0.030-0.150%, Si: 0.01 to 0.10%, Mn: 0.70-2.00%, P: 0.020% or less, S: 0.0080% or less, Nb: 0.003 to 0.030%, Ti: 0.005 to 0.050%, Al: 0.005-0.100%, N: 0.0100% or less, O: 0.0050% or less, Cu: 0 to 0.50%, Ni: 0-2.00%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, V: 0-0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Rare earth elements: 0 to 0.0500%, Zr: 0 to 0.050%, Hf: 0 to 0.020%, Sr: 0 to 0.020%, W: 0-0.100%, Co: 0-0.100%, B: 0 to 0.0050%, Sn: 0-0.100%, Sb: 0 to 0.050%, Zn: 0 to 0.020%, Bi: 0-0.100%, Se: 0 to 0.020%, Pb: 0 to 0.090%, As: 0 to 0.050%, Te: 0-0.050%, and, The remainder consists of Fe and impurities. The f1 defined in equation (1) satisfies 0.34 to 0.45, At the center of the thickness of the steel plate, Prior austenite grains that satisfy the conditions of having a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area or more of the prior austenite grains. Area ratio S of the MA structure (Martensite-Austenite Continent) in the central part of the thickness of the steel plate. 2 If the percentage is greater than 0 and satisfies 0.50%, Area ratio S of MA structure in t / 4 portion of the steel plate 4 The value is greater than 0 and satisfies 0.32%. steel plate. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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.

2. A steel plate according to claim 1, The aforementioned chemical composition is Cu: 0.01 to 0.50%, Ni: 0.01-2.00%, Cr: 0.01-0.50%, Mo: 0.01-0.50%, V: 0.001-0.200%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Rare earth elements: 0.0001-0.0500%, Zr: 0.001 to 0.050%, Hf: 0.001-0.020%, Sr: 0.001-0.020%, W: 0.001-0.100%, Co: 0.001 to 0.100%, B: 0.0001 to 0.0050%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.050%, Zn: 0.001 to 0.020%, Bi: 0.001-0.100%, Se: 0.001 to 0.020%, Pb: 0.001-0.090%, As: 0.001 to 0.050%, and, Contains one or more elements selected from the group consisting of Te: 0.001 to 0.050%, steel plate.

3. A steel plate according to claim 1 or claim 2, The aforementioned chemical composition satisfies the condition that f2, as defined by formula (2), is 120 or greater. steel plate. f2=119+6C+55Cu+27Ni+49Cr+95Mo (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.

4. It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.030-0.150%, Si: 0.01 to 0.10%, Mn: 0.70-2.00%, P: 0.020% or less, S: 0.0080% or less, Nb: 0.003 to 0.030%, Ti: 0.005 to 0.050%, Al: 0.005-0.100%, N: 0.0100% or less, O: 0.0050% or less, Cu: 0 to 0.50%, Ni: 0-2.00%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, V: 0-0.200%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Rare earth elements: 0 to 0.0500%, Zr: 0 to 0.050%, Hf: 0 to 0.020%, Sr: 0 to 0.020%, W: 0-0.100%, Co: 0-0.100%, B: 0 to 0.0050%, Sn: 0-0.100%, Sb: 0 to 0.050%, Zn: 0 to 0.020%, Bi: 0-0.100%, Se: 0 to 0.020%, Pb: 0 to 0.090%, As: 0 to 0.050%, Te: 0-0.050%, and, The remainder consists of Fe and impurities. The f1 defined in equation (1) satisfies 0.34 to 0.45, At the center of the thickness of the steel plate, Prior austenite grains that satisfy the conditions of having a short axis of 25 μm or less and a long axis of 50 μm or less constitute 50 area or more of the prior austenite grains. Area ratio S of the MA structure (Martensite-Austenite Continent) in the central part of the thickness of the base steel sheet. 2 If the percentage is greater than 0 and satisfies 0.50%, Area ratio S of the MA structure in the t / 4 portion of the base steel sheet 4 The value is greater than 0 and satisfies 0.32%. Welded structure. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) Here, the elemental symbols in equation (1) 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.

5. A welded structure according to claim 4, The chemical composition of the base steel sheet is, Cu: 0.01 to 0.50%, Ni: 0.01-2.00%, Cr: 0.01-0.50%, Mo: 0.01-0.50%, V: 0.001-0.200%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Rare earth elements: 0.0001-0.0500%, Zr: 0.001 to 0.050%, Hf: 0.001-0.020%, Sr: 0.001-0.020%, W: 0.001-0.100%, Co: 0.001 to 0.100%, B: 0.0001 to 0.0050%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.050%, Zn: 0.001 to 0.020%, Bi: 0.001-0.100%, Se: 0.001 to 0.020%, Pb: 0.001-0.090%, As: 0.001 to 0.050%, and, Contains one or more elements selected from the group consisting of Te: 0.001 to 0.050%, Welded structure.

6. A welded structure according to claim 4, The aforementioned chemical composition satisfies the condition that f2, as defined by formula (2), is 120 or greater. Welded structure. f2=119+6C+55Cu+27Ni+49Cr+95Mo (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.

7. A welded structure according to claim 5, The aforementioned chemical composition satisfies the condition that f2, as defined by formula (2), is 120 or greater. Welded structure. f2=119+6C+55Cu+27Ni+49Cr+95Mo (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.

8. A welded structure according to any one of claims 4 to 7, In the area affected by welding heat, The area percentage with a Vickers hardness of 140 Hv or less is 30.0% or less. Welded structure.

9. A welded structure according to claim 8, Of the base steel plate, in the area affected by welding heat, In the microstructure, the area ratio of ferrite is 90.0% or less. Welded structure.

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