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, where f1 as defined by formula (1) is 0.35 to 0.40, f2 as defined by formula (2) is 120 or more, the microstructure in the center of the thickness of the steel sheet consists of 50 to 65% ferrite, 0 to 15% pearlite, 0 to 5.0% MA structure, and the remainder being bainite, and the ratio Rρ of the dislocation density ρ2 in the center of the thickness of the steel sheet to the dislocation density ρ4 in part t / 4 of the steel sheet, as defined by formula (3), is 0.60 to 1.00. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=119+6C+55Cu+27Ni+49Cr+95Mo (2) Rρ = ρ² / ρ₄ (3)

Description

[Technical Field]

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

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

[0003] Welded four-sided box columns may have other members joined to them. For example, a steel plate (diaphragm) may be joined to the inside of a welded four-sided box column as a reinforcing member. In this case, the diaphragm is positioned perpendicular to the longitudinal direction of the welded four-sided box column, increasing the rigidity and strength of the column. For example, a beam may also be joined to the outside of a welded four-sided box column.

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

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

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

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

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

[0009] By the way, in the steel material after welding, in the HAZ (heat affected zone of welding), a low-strength region (softening) may occur in a part thereof due to the heat input by welding and subsequent cooling. That is, in the HAZ formed in the welded structure, as a result of local strength reduction, the strength in the HAZ may become non-uniform. However, it is preferable to suppress such non-uniformity of strength in the HAZ.

[0010] The above Patent Documents 1 and 2 propose technologies for enhancing the toughness of the HAZ of steel plates. However, in the above Patent Documents 1 and 2, no consideration has been given to suppressing the non-uniformity of strength in the HAZ of the welded structure.

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

Means for Solving the Problems

[0012] The steel plate according to the present disclosure is The chemical composition is, in mass%, C: 0.060 to 0.200%, Si: 0.10 to 0.55%, Mn: 0.70 to 2.00%, P: leq 0.020%, S: leq 0.0080%, Ti: 0.007 to 0.020%, Al: 0.010 to 0.050%, [[ID=3P5]]N: leq 0.0070%, O: leq 0.0050%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, V: 0 to 0.080%, Nb: 0 to 0.030%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Rare earth elements: 0~0.0100%, Zr: 0~0.050%, Hf: 0~0.020%, Sr: 0~0.020%, W: 0~0.100%, Co: 0~0.100%, B: Less than 0-0.0008% 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 by equation (1) satisfies 0.35 to 0.40, If f2, as defined in equation (2), satisfies 120 or greater, At the center of the thickness of the steel plate, The microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure (Martensite-Austenite Constituent), and the remainder being bainite, by area percentage. The ratio Rρ of the dislocation density ρ2 at the center of the thickness of the steel plate, as defined by equation (3), to the dislocation density ρ4 at t / 4 of the steel plate, satisfies 0.60 to 1.00. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=119+6C+55Cu+27Ni+49Cr+95Mo (2) Rρ = ρ² / ρ₄ (3) Here, the elemental symbols in equations (1) and (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. In equation (3), ρ2 is the dislocation density at the center of the steel plate thickness in units of m -2Substituting this, ρ4 is the dislocation density in t / 4 of the steel plate, in units of m -2 It is substituted.

[0013] The welded structure described herein is It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060~0.200%, Si: 0.10~0.55%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Ti: 0.007~0.020%, Al: 0.010~0.050%, N: 0.0070% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~0.50%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.080%, Nb: 0~0.030%, Ca: 0~0.0100%, Mg: 0~0.0100%, Rare earth elements: 0~0.0100%, Zr: 0~0.050%, Hf: 0~0.020%, Sr: 0~0.020%, W: 0~0.100%, Co: 0~0.100%, B: Less than 0-0.0008% 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 by equation (1) satisfies 0.35 to 0.40, If f2, as defined in equation (2), satisfies 120 or greater, At the center of the thickness of the base steel plate, The microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure (Martensite-Austenite Constituent), and the remainder being bainite, by area percentage. The ratio Rρ of the dislocation density ρ2 at the center of the thickness of the base steel sheet, as defined by equation (3), to the dislocation density ρ4 at t / 4 of the base steel sheet satisfies 0.60 to 1.00. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=119+6C+55Cu+27Ni+49Cr+95Mo (2) Rρ = ρ² / ρ₄ (3) Here, the elemental symbols in equations (1) and (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. In equation (3), ρ2 is the dislocation density at the center of the thickness of the base steel sheet in units of m -2 Substituting this, ρ4 is the dislocation density in the t / 4 portion of the base steel sheet, in units of m -2 It is substituted. [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-mentioned 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. [Figure 4] Figure 4 is a schematic diagram showing how a Charpy impact test specimen is prepared from a welded joint created 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 of the present invention have determined the following composition ratios by mass: C: 0.060~0.200%, Si: 0.10~0.55%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Ti: 0.007~0.020%, Al: 0.010~0.050%, N: 0.0070% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~0.50%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.080%, Nb: 0~0.030%, Ca: 0~0.0100%, Mg: 0~0.0100%, rare earth elements: 0~0.0100%. We considered that a steel sheet having a chemical composition consisting of Zr: 0-0.050%, Hf: 0-0.020%, Sr: 0-0.020%, W: 0-0.100%, Co: 0-0.100%, B: less than 0-0.0008%, 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 this case, the distribution of hardness may be uneven in the HAZ after welding. That is, the hardness may decrease or increase locally. As a result, it is thought that heterogeneity in strength occurs in the HAZ after welding. In other words, in order to suppress the heterogeneity in strength in the HAZ after welding for steel plates having the above chemical composition, it is sufficient to suppress the uneven distribution of hardness in the HAZ after welding. The inventors have found that the hardness of the HAZ after welding can be stabilized by adjusting the chemical composition.

[0020] Specifically, assuming the steel plate according to this embodiment has the chemical composition described above, the inventors' investigations revealed that the hardness of the HAZ after welding can be stabilized if f1, as defined by formula (1), satisfies 0.35 to 0.40. 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 sheet. If f1 is too low, the hardness of the steel sheet 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 of the steel sheet becomes too high, and the HAZ toughness decreases. Therefore, assuming that the steel sheet according to this embodiment has the above-mentioned chemical composition, f1 is set to 0.35 to 0.40.

[0022] The inventors further investigated in detail a method to suppress the non-uniformity of strength in the HAZ after welding while maintaining HAZ toughness. As a result, it was found that in steel sheets having the above-mentioned chemical composition, if the content of carbon (C), copper (Cu), nickel (Ni), chromium (Cr), and molybdenum (Mo) is within the range where f1 satisfies 0.35 to 0.40, the uniformity of the strength of the HAZ can be improved while maintaining HAZ toughness.

[0023] Specifically, the steel plate according to this embodiment has the above-mentioned chemical composition, and assuming that f1 satisfies 0.35 to 0.40, the inventors' investigations revealed that the hardness of the HAZ after welding can be stabilized if f2, as defined by formula (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.

[0024] f2 is an index of the hardness of the HAZ after welding. If f2 is too low, the hardness of the HAZ cannot be sufficiently increased, and the strength of the HAZ after welding tends to decrease. As a result, a localized decrease in strength occurs in the HAZ after welding, and the heterogeneity of strength decreases. On the other hand, if f2 is too high, the hardness of the HAZ becomes too high, and the HAZ toughness decreases. Therefore, assuming that the steel plate according to this embodiment has the above chemical composition and f1 satisfies 0.35 to 0.40, f2, as defined by formula (2), is set to 120 or higher.

[0025] On the other hand, even with steel plates having the above-mentioned chemical composition, where f1 satisfies 0.35 to 0.40 and f2 satisfies 120 or more, 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 uneven distribution of hardness in the HAZ after welding. As a result, the inventors obtained the following findings.

[0026] First, we assume that in the central part of the thickness of a steel sheet having the above-mentioned chemical composition, where f1 is 0.35-0.40 and f2 is 120 or more, the microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure (Martensite-Austenite Constituent) by area percentage, and the remainder being bainite. In this case, it became clear that the distribution of dislocation density in the thickness direction of the steel sheet affects the bias in the hardness distribution in the HAZ after welding.

[0027] Specifically, the dislocation density at the center of the thickness of the steel plate is ρ²(m -2 ) is defined as ρ4(m -2 ) is defined as follows. In this specification, the t / 4 portion of the plate thickness refers to the position at a depth of t / 4 from the surface of the steel plate, where t is the thickness of the steel plate. As a result of detailed studies by the inventors, it has become clear that if the ratio Rρ of the dislocation density ρ2 at the center of the steel plate thickness and the dislocation density ρ4 at the t / 4 portion of the steel plate is between 0.60 and 1.00, then the heterogeneity of the strength in the HAZ can be suppressed while maintaining the HAZ toughness after welding.

[0028] Here, if the ratio Rρ of the dislocation density ρ2 at the center of the steel plate thickness to the dislocation density ρ4 at the t / 4 portion of the steel plate is too small or too large, a bias in the distribution of dislocation density occurs in the thickness direction. In this case, if the strength of the HAZ after welding is too small or too large compared to the steel plate, it will exacerbate the heterogeneity of the strength. If Rρ is too small or too large, the bias in the distribution of dislocation density may become apparent. On the other hand, if the ratio Rρ of the dislocation density ρ2 at the center of the steel plate thickness to the dislocation density ρ4 at the t / 4 portion of the steel plate is between 0.60 and 1.00, the bias in the distribution of dislocation density is mitigated. Therefore, it may be possible to suppress the heterogeneity of the strength in the HAZ while maintaining the toughness of the HAZ after welding.

[0029] Therefore, the steel sheet according to this embodiment has the above-described chemical composition, with f1 satisfying 0.35 to 0.40 and f2 satisfying 120 or more. In the central part of the sheet thickness, the microstructure consists of 50 to 65% ferrite, 0 to 15% pearlite, 0 to 5.0% MA structure, and the remainder being bainite, in terms of area percent. Furthermore, the ratio Rρ of the dislocation density ρ2 in the central part of the sheet thickness to the dislocation density ρ4 in the t / 4 part of the sheet satisfies 0.60 to 1.00. As a result, the welded structure manufactured from the steel sheet according to this embodiment has excellent HAZ toughness, and the heterogeneity of strength in the HAZ is sufficiently suppressed.

[0030] Based on the above findings, the gist of the steel plate and welded structure according to this embodiment is as follows.

[0031] [1] It is a steel plate, The chemical composition is expressed in mass percent. C: 0.060~0.200%, Si: 0.10~0.55%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Ti: 0.007~0.020%, Al: 0.010~0.050%, N: 0.0070% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~0.50%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.080%, Nb: 0~0.030%, Ca: 0~0.0100%, Mg: 0~0.0100%, Rare earth elements: 0~0.0100%, Zr: 0~0.050%, Hf: 0~0.020%, Sr: 0~0.020%, W: 0~0.100%, Co: 0~0.100%, B: Less than 0-0.0008% 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 by equation (1) satisfies 0.35 to 0.40, If f2, as defined in equation (2), satisfies 120 or greater, At the center of the thickness of the steel plate, The microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure (Martensite-Austenite Constituent), and the remainder being bainite, by area percentage. The ratio Rρ of the dislocation density ρ2 at the center of the thickness of the steel plate, as defined by equation (3), to the dislocation density ρ4 at t / 4 of the steel plate satisfies 0.60 to 1.00. steel plate. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=119+6C+55Cu+27Ni+49Cr+95Mo (2) Rρ = ρ² / ρ₄ (3) Here, the elemental symbols in equations (1) and (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. In equation (3), ρ2 is the dislocation density at the center of the steel plate thickness in units of m -2 Substituting this, ρ4 is the dislocation density in t / 4 of the steel plate, in units of m -2 It is substituted.

[0032] [2] [1] The steel plate described above, The aforementioned chemical composition is Cu: 0.01~0.50%, Ni: 0.01~0.50%, Cr: 0.01~0.50%, Mo: 0.01~0.50%, V: 0.001~0.080%, Nb: 0.001~0.030%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Rare earth elements: 0.0001~0.0100%, 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: Less than 0.0001-0.0008% 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.

[0033] [3] The steel plate described in [1] or [2], In a plane including the rolling direction of the steel plate and the thickness direction of the steel plate, The total number of MA structures with an equivalent circle diameter of 5 μm or more and pearlite structures with an equivalent circle diameter of 5 μm or more contained within a virtual circle with a diameter of 30 μm is less than 1 or 10 or more. steel plate.

[0034] [4] It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060~0.200%, Si: 0.10~0.55%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Ti: 0.007~0.020%, Al: 0.010~0.050%, N: 0.0070% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~0.50%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.080%, Nb: 0~0.030%, Ca: 0~0.0100%, Mg: 0~0.0100%, Rare earth elements: 0~0.0100%, Zr: 0~0.050%, Hf: 0~0.020%, Sr: 0~0.020%, W: 0~0.100%, Co: 0~0.100%, B: Less than 0-0.0008% 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 by equation (1) satisfies 0.35 to 0.40, If f2, as defined in equation (2), satisfies 120 or greater, At the center of the thickness of the base steel plate, The microstructure consists of 50 - 65% ferrite, 0 - 15% pearlite, 0 - 5.0% MA structure (Martensite - Austenite Constituent) and the balance being bainite, by area percentage. The ratio Rρ of the dislocation density ρ2 at the center of the plate thickness of the base metal steel plate defined by formula (3) to the dislocation density ρ4 at the t / 4 part of the base metal steel plate satisfies 0.60 - 1.00. Welded structure. f1 = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) f2 = 119 + 6C + 55Cu + 27Ni + 49Cr + 95Mo (2) Rρ = ρ2 / ρ4 (3) Here, in formula (1) and formula (2), the element symbols are substituted with the contents of the corresponding elements in units of mass%. When the corresponding element is not contained, "0" is substituted into the element symbol. In ρ2 in formula (3), the dislocation density at the center of the plate thickness of the base metal steel plate is substituted in units of m -2 and in ρ4, the dislocation density at the t / 4 part of the base metal steel plate is substituted in units of m -2 is substituted.

[0035] [5] The welded structure according to [4], wherein the chemical composition of the base metal steel plate is Cu: 0.01 - 0.50%, Ni: 0.01 - 0.50%, Cr: 0.01 - 0.50%, Mo: 0.01 - 0.50%, V: 0.001 - 0.080%, Nb: 0.001 - 0.030%, Ca: 0.0001 - 0.0100%, Mg: 0.0001 - 0.0100%, Rare earth elements: 0.0001 - 0.0100%, 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: Less than 0.0001-0.0008% 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.

[0036] [6] [4] The welded structure described above, The aforementioned base steel plate is In a plane including the rolling direction of the base steel sheet and the thickness direction of the base steel sheet, The total number of MA structures with an equivalent circle diameter of 5 μm or more and pearlite structures with an equivalent circle diameter of 5 μm or more contained within a virtual circle with a diameter of 30 μm is less than 1 or 10 or more. Welded structure.

[0037] [7] [5] The welded structure described above, The aforementioned base steel plate is In a plane including the rolling direction of the base steel sheet and the thickness direction of the base steel sheet, The total number of MA structures with an equivalent circle diameter of 5 μm or more and pearlite structures with an equivalent circle diameter of 5 μm or more contained within a virtual circle with a diameter of 30 μm is less than 1 or 10 or more. Welded structure.

[0038] [8] A welded structure described in any one of items [4] to [7], The aforementioned welded structure is The base steel plate and, A diaphragm positioned perpendicular to the base steel plate, The base steel plate and the diaphragm are formed together, including a welded metal portion. Within the welded metal portion, the vertex is the intersection of the center line of the diaphragm's thickness and the extension line of the surface of the base steel plate, The thickness t mm of the base steel plate, In a rectangle whose sides are 21 mm long at the two intersections of the surface of the base steel plate and the welded metal portion, The area percentage with a Vickers hardness of 120 Hv or less is 30.0% or less. Welded structure.

[0039] [9] [8] The welded structure described above, In the heat-affected zone of the aforementioned base steel plate, In the microstructure, the area ratio of ferrite is 90.0% or less. Welded structure.

[0040] 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 %.

[0041] [Steel plate] The steel plate of this embodiment satisfies the following features 1 to 5. (Feature 1) The chemical composition is within the range of this embodiment. (Feature 2) The value of f1 defined by equation (1) satisfies 0.35 to 0.40. 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) 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 4) At the center of the thickness of the steel plate, The microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure, and the remainder being bainite, by area percentage. (Feature 5) The ratio Rρ of the dislocation density ρ2 at the center of the steel plate thickness, as defined by equation (3), to the dislocation density ρ4 at t / 4 of the steel plate satisfies 0.60 to 1.00. Rρ = ρ² / ρ₄ (3) Here, in equation (3), ρ2 represents the dislocation density at the center of the steel plate thickness, in units of m -2 Substituting this, ρ4 is the dislocation density in t / 4 of the steel plate, in units of m -2 It is substituted. The following describes each of its features.

[0042] [(Feature 1) Chemical Composition] The chemical composition of the steel sheet according to this embodiment contains the following elements:

[0043] C: 0.060~0.200% Carbon (C) enhances the hardenability of steel plates and increases the strength of the heat-affected zone (HAZ) after welding. If the C content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content is too high, even if the content of other elements is within the range of this embodiment, excessive carbides will be formed in the steel plate, making the strength of the steel plate too high. As a result, the HAZ toughness after welding will decrease. Therefore, the C content is 0.060 to 0.200%. The preferred lower limit of the C content is 0.065%, and more preferably 0.070%. The preferred upper limit of the C content is 0.195%, and more preferably 0.180%.

[0044] Si: 0.10~0.55% Silicon (Si) deoxidizes steel and increases the strength of the heat-affected zone (HAZ) after welding. If the Si content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content is too high, even if the content of other elements is within the range of this embodiment, MA (major oxidative) structure is more likely to form in the HAZ after welding, and the HAZ toughness decreases. Therefore, the Si content is 0.10 to 0.55%. The preferred lower limit of the Si content is 0.11%, more preferably 0.13%, and still more preferably 0.15%. The preferred upper limit of the Si content is 0.50%, more preferably 0.45%, and still more preferably 0.40%.

[0045] Mn: 0.70~2.00% Manganese (Mn) enhances the hardenability of steel plates and increases the strength of the heat-affected zone (HAZ) after welding. If the Mn content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content is too high, even if the content of other elements is within the range of this embodiment, MA structures are more likely to form in the HAZ 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%, and more preferably 0.80%. The preferred upper limit of the Mn content is 1.90%, and more preferably 1.80%.

[0046] P:0.020% or less Phosphorus (P) is an impurity. 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 should be 0.020% or less. The preferred upper limit for 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 for the P content is 0.001%, more preferably 0.002%, and still more preferably 0.003%.

[0047] S: 0.0080% or less Sulfur (S) is an impurity. If the S content is too high, even if the content of other elements is within the range of this embodiment, coarse sulfides will form in the steel sheet, reducing the HAZ toughness after welding. Therefore, the S content should be 0.0080% or less. The preferred upper limit for the S content is 0.0060%, and more preferably 0.0040%. 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 for the S content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%.

[0048] Ti: 0.007~0.020% Titanium (Ti) combines with nitrogen to form Ti nitrides, which enhance HAZ toughness through a pinning effect. If the Ti content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ti content is too high, even if the content of other elements is within the range of this embodiment, coarse Ti nitrides are formed, and the HAZ toughness after welding decreases. Therefore, the Ti content is 0.007 to 0.020%. The preferred lower limit of the Ti content is 0.008%, more preferably 0.009%, and even more preferably 0.010%. The preferred upper limit of the Ti content is 0.018%, and even more preferably 0.016%.

[0049] Al: 0.010~0.050% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content is too high, even if the content of other elements is within the range of this embodiment, coarse oxide inclusions are formed, and the HAZ toughness after welding decreases. Therefore, the Al content is 0.010 to 0.050%. The preferred lower limit of the Al content is 0.012%, and more preferably 0.015%. The preferred upper limit of the Al content is 0.045%, more preferably 0.040%, and still more preferably 0.035%.

[0050] N: 0.0070% or less Nitrogen (N) is inevitably present. N combines with Ti to form Ti nitrides, which enhance HAZ toughness through a 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, reducing the HAZ toughness after welding. Therefore, the N content is 0.0070% or less. The preferred upper limit of the N content is 0.0065%, more preferably 0.0060%, and even more preferably 0.0055%. 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%.

[0051] O: 0.0050% or less Oxygen (O) is an unavoidable impurity. If the O content is too high, oxides will form, reducing the HAZ toughness after welding, even if the content of other elements is within the range of this embodiment. Therefore, the O content should be 0.0050% or less. The preferred upper limit for the O content is 0.0040%, and more preferably 0.0030%. It is preferable to have as low an O content as possible. However, extreme reduction of the O content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the O content is 0.0005%, and more preferably 0.0010%.

[0052] [Optional element] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, Cr, and Mo. Any 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.

[0053] Cu: 0~0.50% Copper (Cu) may not be included; that is, the Cu content may be 0%. If included, Cu enhances the hardenability of the steel plate and suppresses the heterogeneity of strength in the HAZ after welding. Even a small amount of Cu can provide some of the above effects. 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 HAZ after welding, reducing the HAZ toughness. Therefore, the Cu content is 0 to 0.50%. 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%.

[0054] Ni: 0~0.50% Nickel (Ni) may not be included; that is, the Ni content may be 0%. If Ni is included, it 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 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 HAZ after welding, and the HAZ toughness will decrease. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni 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 Ni content is 0.45%, and more preferably 0.40%.

[0055] Cr: 0~0.50% Chromium (Cr) may not be included; that is, the Cr content may be 0%. If included, Cr enhances the hardenability of the steel sheet and suppresses the heterogeneity of strength in the HAZ after welding. Even a small amount of Cr can provide some of the above effects. On the other hand, 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%. The preferred lower limit of the Cr content is greater than 0%, more preferably 0.01%, more preferably 0.03%, more preferably 0.05%, and more preferably 0.08%. The preferred upper limit of the Cr content is 0.45%, more preferably 0.40%, and more preferably 0.28%.

[0056] Mo: 0~0.50% Molybdenum (Mo) may not be included. That is, the Mo content may be 0%. If 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%. The preferred lower limit of the Mo content is greater than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Mo content is 0.40%, more preferably 0.30%, and even more preferably 0.27%.

[0057] The chemical composition of the steel sheet according to this embodiment may further contain one or more elements selected from the group consisting of V and Nb. These elements are all arbitrary elements and form carbides, etc., to increase the strength of the heat-affected zone (HAZ) after welding.

[0058] V: 0~0.080% Vanadium (V) is an optional element and may not be present. That is, the V content may be 0%. When present, V forms carbides and other materials, 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 other materials will be formed, and the toughness of the HAZ after welding will decrease. Therefore, the V content is 0 to 0.080%. The preferred lower limit of the V content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the V content is 0.075%, more preferably 0.070%, and even more preferably 0.060%.

[0059] Nb: 0~0.030% Niobium (Nb) is an optional element and may not be present. That is, the V content may be 0%. If present, Nb forms carbides and other materials, 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 Nb is present, the above effect can be obtained to some extent. However, 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 materials will be formed, and the toughness of the HAZ after welding will decrease. Therefore, the Nb content is 0 to 0.030%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.001%, and even more preferably 0.003%. The preferred upper limit of the Nb content is 0.028%, more preferably 0.025%, and even more preferably 0.023%.

[0060] The chemical composition of the steel sheet according to this embodiment may further contain 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.

[0061] Ca: 0~0.0100% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. When present, Ca forms oxides and the like, suppressing the formation of coarse inclusions and thereby improving the HAZ toughness after welding. Even a small amount of Ca can provide the above effect to some extent. However, if the Ca content is too high, coarse inclusions will form even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the Ca content is 0 to 0.0100%. 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%.

[0062] 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, etc., 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%. The preferred lower limit of the Mg 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 Mg content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%.

[0063] Rare earth elements (REM): 0~0.0100% Rare earth elements (REMs) are optional and may not be present. That is, the REM content may be 0%. If present, REMs form oxides, sulfides, and / or acid sulfides, thereby suppressing the formation of coarse inclusions and improving the HAZ toughness after welding. Even a small amount of REM can provide some of the above effect. However, if the REM content is too high, coarse inclusions will form, even if the content of other elements is within the range of this embodiment, and the HAZ toughness after welding will decrease. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%. The preferred upper limit of the REM content is 0.0080%, more preferably 0.0060%, and more preferably 0.0040%.

[0064] In this specification, REM refers to the 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the 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.

[0065] 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 can 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%. 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%.

[0066] 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 can provide some 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%. 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%.

[0067] 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 can provide some 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%. 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%.

[0068] 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. 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.

[0069] W: 0~0.100% Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. When included, W enhances the hardenability of the steel plate and suppresses the heterogeneity of the strength in the HAZ after welding. Even a small amount of W can provide the above effects 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%. 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%.

[0070] Co: 0~0.100% Cobalt (Co) is an optional element and may not be included. That is, the Co content may be 0%. When included, 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%. 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%.

[0071] B: Less than 0-0.0008% 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 a small amount of B can provide some of the above effects. 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, reducing the HAZ toughness after welding. Therefore, the B content is 0 to less than 0.0008%. The preferred lower limit of the B content is greater than 0%, more preferably 0.0001%, and even more preferably 0.0003%. The preferred upper limit of the B content is 0.0007%, and even more preferably 0.0006%.

[0072] 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. Any of these elements are arbitrary and enhance the corrosion resistance of the steel sheet.

[0073] Sn: 0~0.100% Tin (Sn) is an optional element and may not be present. That is, the Sn content may be 0%. When 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%. The preferred lower limit of the Sn content is greater than 0%, more preferably 0.001%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Sn content is 0.080%, more preferably 0.060%, and more preferably 0.040%.

[0074] Sb: 0~0.050% Antimony (Sb) is an optional element and may not be present. That is, the Sb content may be 0%. When 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%. 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%.

[0075] 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%. 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%.

[0076] 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. Any of these elements are arbitrary and enhance the machinability of the steel sheet.

[0077] Bi: 0~0.100% Bismuth (Bi) is an optional element and may not be present. In other words, the Bi content may be 0%. When present, Bi improves the machinability of the steel sheet. Even a small amount of Bi will provide the above effect to some extent. 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%. 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%.

[0078] 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%. 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%.

[0079] Pb: 0~0.090% Lead (Pb) is an optional element and may not be present. In other words, 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%. 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%.

[0080] As: 0~0.050% Arsenic (As) is an optional element and may not be present. In other words, the As content may be 0%. When present, As improves the machinability of the steel sheet. Even a small amount of As can 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%. 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%.

[0081] Te: 0~0.050% Tellurium (Te) is an optional element and may not be present. That is, the Te content may be 0%. When 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%. 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%.

[0082] 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.

[0083] [(Feature 2) f1 defined by equation (1)] Assuming that the steel plate according to this embodiment has the other features of this embodiment, the f1 defined by formula (1) satisfies 0.35 to 0.40. 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.

[0084] 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. As a result, the heterogeneity of the strength in the HAZ decreases. 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 0.35 to 0.40.

[0085] The preferred lower limit of f1 is greater than 0.35, more preferably 0.36, and even more preferably 0.37. The preferred upper limit of f1 is less than 0.40, more preferably 0.39, and even more preferably 0.38. Note that f1 is determined by rounding the value obtained from the element content and formula (1) to the third decimal place.

[0086] [(Feature 3) 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.

[0087] As described above, f2 is an index of the hardness of the HAZ after welding. Here, C, Cu, Ni, Cr, and Mo in f2 contribute to improving the hardness of the HAZ. If f2 is too low, the hardness of the HAZ cannot be sufficiently increased, and the strength of the HAZ after welding tends to decrease. On the other hand, if f2 is too high, the hardness of the steel plate becomes too high, and the toughness of the HAZ decreases. Therefore, assuming that the steel plate according to this embodiment has the other features of this embodiment, f2 is set to 120 or higher.

[0088] A preferred lower limit for f2 is 121, more preferably 123, and even more preferably 125. The upper limit for f2 may be 230. A further preferred upper limit for f2 is 195, and even more preferably 185. Note that f2 is determined by rounding the first decimal place of the value obtained from the element content and formula (2).

[0089] [(Feature 4) Microstructure] Assuming the steel sheet according to this embodiment has other features of this embodiment, the microstructure in the central part of the thickness of the steel sheet consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure (Martensite-Austenite Constituent) by area percentage, and the remainder being bainite.

[0090] If the area ratio of ferrite in the microstructure is too high, the strength of the steel sheet will decrease, and the heterogeneity of the strength in the heat-affected zone (HAZ) after welding will not be sufficiently suppressed. On the other hand, if the area ratio of ferrite in the microstructure is too low, the hardness of the steel sheet will become too high, and the HAZ toughness after welding will decrease. Therefore, in this embodiment, the area ratio of ferrite in the microstructure of the steel sheet is set to 50-65%. Furthermore, if the area ratio of pearlite in the microstructure is too high, the strength of the steel sheet will be insufficient. Therefore, in this embodiment, the area ratio of pearlite in the microstructure of the steel sheet is set to 0-15%. In addition, if the area ratio of MA (matrix-acid) structure is too high in the microstructure of the central part of the steel sheet thickness, the strength of the steel sheet will become too high, and the HAZ toughness after welding will decrease. Therefore, in this embodiment, the area ratio of MA (matrix-acid) structure in the microstructure of the steel sheet is set to 0-5.0%.

[0091] Furthermore, the remainder of the microstructure of the steel sheet according to this embodiment is bainite. In other words, in the central part of the thickness of the steel sheet according to this embodiment, the microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure, and the remainder is bainite, in terms of area percentage.

[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 of ferrite, pearlite, and bainite are 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 surface that includes the rolling direction and the width direction. The observation surface of the test piece is polished to a mirror finish in the same manner as described above. Furthermore, the 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 for 10 fields using an optical microscope. Specifically, the field area is 0.025 mm². 2 (The magnification is 500x.)

[0094] In each observation field, ferrite, pearlite, and bainite are identified based on contrast. Specifically, a histogram of the brightness of each pixel is created from the digital image of the optical microscope. At this time, no image processing that drastically alters the histogram distribution is performed, and the brightness is adjusted so that the microstructure of the steel can be naturally distinguished. The image is also adjusted so that the minimum value of the brightness histogram of the entire image is black and the maximum value of the histogram is white, resulting in a grayscale image. Regions with brightness higher than the brightness at which the histogram shows the maximum peak (mode) are identified as ferrite, and regions with brightness lower than the brightness at which the histogram shows the maximum peak (mode) are identified as MA structure, pearlite, and bainite. Furthermore, the region from the minimum brightness to the median brightness of the histogram is identified as MA structure and pearlite. For the ferrite, bainite, and the sum of MA structure and pearlite identified as described above, the area ratio is calculated from the number of pixels.

[0095] In this embodiment, the area ratio of the MA structure is 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 microstructure is revealed by etching on the mirror-polished observation surface. Etching is performed by immersing in Repera etching solution for about 10 seconds. The etched observation surface is observed for 10 fields using an optical microscope. The field area is 0.025 mm². 2 (The magnification is 500x.)

[0096] The area ratio of MA (Magnesium Acetate) tissue is determined by image analysis of the obtained observation field. Specifically, a histogram of the brightness of each pixel is created from the digital image of the optical microscope for each observation field. At this time, no image processing that drastically alters the histogram distribution is performed, and the brightness is adjusted so that the microstructure of steel can be naturally distinguished. Furthermore, the image is adjusted so that the minimum value of the brightness histogram of the entire image is black and the maximum value of the histogram is white, resulting in a grayscale image. In this case, peaks of ferrite and bainite are formed in the mid-brightness region of the histogram. Furthermore, a peak of pearlite is formed in the region with lower brightness than the ferrite and bainite peaks. In addition, the MA tissue has a peak in the region with higher brightness than the ferrite and bainite peaks. Therefore, when the minimum value of the brightness of the entire histogram is set to 0% and the maximum value to 100%, the region from 72% to 100% is considered to be the MA tissue. The area ratio of the pearlite and MA tissue identified as described above is determined from the number of pixels.

[0097] In this embodiment, the area ratio of ferrite is defined as the value obtained by subtracting the area ratio of the MA structure from the arithmetic mean of the area ratios of ferrite obtained in all observation fields. Similarly, in this embodiment, the area ratio of pearlite is defined as the arithmetic mean of the area ratios of pearlite obtained in all observation fields. The area ratio of ferrite is obtained by rounding the obtained value to the first decimal place. The area ratio of pearlite is obtained by rounding the obtained value to the second decimal place. Furthermore, the remainder after subtracting the area ratios of ferrite, pearlite, and MA structure is defined as the area ratio of bainite.

[0098] [(Feature 5) Dislocation Density] Assuming that the steel plate according to this embodiment has other features of this embodiment, the ratio Rρ of the dislocation density ρ2 at the center of the thickness of the steel plate and the dislocation density ρ4 at t / 4 of the steel plate, as defined by equation (3), satisfies 0.60 to 1.00. Rρ = ρ² / ρ₄ (3) Here, in equation (3), ρ2 represents the dislocation density at the center of the steel plate thickness, in units of m -2Substituting this, ρ4 is the dislocation density in t / 4 of the steel plate, in units of m -2 It is substituted.

[0099] Rρ (=ρ2 / ρ4) is an index indicating the uniformity of strength in the thickness direction of a steel plate. If Rρ is too small or too large, there is a bias in the distribution of dislocation density in the thickness direction. In this case, the bias in the distribution of dislocation density in the base steel plate may manifest as heterogeneity in strength in the HAZ after welding. On the other hand, if the ratio Rρ of the dislocation density ρ2 at the center of the steel plate thickness to the dislocation density ρ4 at t / 4 of the steel plate is between 0.60 and 1.00, the bias in the distribution of dislocation density is mitigated. Therefore, heterogeneity in strength in the HAZ can be suppressed while maintaining the HAZ toughness after welding.

[0100] Therefore, the steel sheet according to this embodiment has an Rρ of 0.60 to 1.00. The preferred lower limit of Rρ is 0.61, more preferably 0.63, and even more preferably 0.65. It is preferable that Rρ be close to 1.00. However, in a steel sheet having the above chemical composition, the upper limit of Rρ may be 0.99, 0.98, 0.97, or 0.96.

[0101] In addition, in the steel plate according to this embodiment, the dislocation density ρ2(m) in the center of the plate thickness is -2 ) and the dislocation density ρ4(m) in the t / 4 portion of the steel plate -2 ) is not particularly limited as long as Rρ (=ρ2 / ρ4) satisfies 0.60 to 1.00. However, in a steel plate having other features of this embodiment, the dislocation density ρ2 in the center of the plate thickness is, for example, 0.60 × 10 14 ~1.60×10 14 m -2 Furthermore, in a steel sheet having other features of this embodiment, the dislocation density ρ4 in the t / 4 portion is, for example, 0.95 × 10⁻⁶. 14 ~2.30×10 14 m -2 That is the case.

[0102] In this embodiment, the dislocation density is determined by the following method. First, a test specimen for X-ray diffraction (XRD) measurement is prepared from the center of the thickness of the steel plate according to this embodiment. The test specimen is a plate-shaped specimen with an X-ray irradiation surface of 5 mm × 10 mm. The X-ray irradiation surface of the test specimen is a plane parallel to the plate width direction and the rolling direction.

[0103] The X-ray irradiation surface of the test specimen is mirror-polished in the same manner as described above. Furthermore, the X-ray irradiation surface is subjected to acid corrosion or electrolytic polishing using 10 vol% perchloric acid (acetic acid solvent) to remove strain from the surface region of the X-ray irradiation surface to a depth of 5 μm or more. The depth of the surface region from which strain is removed should be 50 μm or more. An XRD profile is obtained from the observation surface after electrolytic polishing using an X-ray diffractometer. In the XRD measurement, a Cu tube is used with a tube voltage of 40 kV and a tube current of 40 mA. From the obtained profile, the full width at half maximum β of the peaks of the (110), (211), and (220) planes of the body-centered cubic structure (iron) is determined. Note that since the X-ray diffraction intensity is the overlap of Kα1 and Kα2 lines with different wavelengths, the Lorentz function is used to separate them.

[0104] The strain ε of the specimen is determined by the least squares method from the full width at half maximum β obtained by the method described above and the Williamson-Hall equation (Equation (4)). β×cosθ / λ=0.9 / D+2ε×sinθ / λ (4) Here, in equation (4), θ represents the diffraction angle, λ represents the wavelength of the X-ray, and D represents the crystallite size.

[0105] Furthermore, using the obtained strain ε and equation (5), the dislocation density ρ(m -2 ) can be calculated. ρ = 14.4 × ε 2 / b 2 (5) Here, in equation (5), b is the Burgers vector for the body-centered cubic structure (iron).

[0106] [Any characteristic of steel plate] The steel plate of this embodiment may satisfy at least one of the following features 6 and 7. (Feature 6) The tensile strength TS is 550-740 MPa. (Feature 7) In a plane that includes the rolling direction of the steel plate and the thickness direction of the steel plate, Within a virtual circle with a diameter of 30 μm, the total number of MA structures with an equivalent circle diameter of 5 μm or more and perlite structures with an equivalent circle diameter of 5 μm or more is less than 1 or 10 or more. The following describes these features.

[0107] [(Feature 6) Tensile Strength TS] Assuming that the steel plate according to this embodiment has other features of this embodiment, it is preferable that the tensile strength TS is 550 to 740 MPa. If 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 further stably suppressed, and the HAZ toughness of the welded structure can be further 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 670 MPa, more preferably 665 MPa, still more preferably 660 MPa, and still more preferably 655 MPa.

[0108] 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 520 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-mentioned 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.

[0109] In the steel plate according to this embodiment, the tensile strength TS, yield stress YS, and yield ratio YR are 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.

[0110] [(Characteristic 7) Coarse, hard tissue] The steel sheet according to this embodiment, provided that it has other features of this embodiment, preferably has a microstructure in which, in a plane including the rolling direction of the steel sheet and the thickness direction of the steel sheet, the total number of MA structures with an equivalent circle diameter of 5 μm or more and pearlite structures with an equivalent circle diameter of 5 μm or more, contained within a virtual circle with a diameter of 30 μm, is less than 1 or 10 or more. In this specification, the virtual circle with a diameter of 30 μm is also referred to as a "specific circle".

[0111] As described above, the microstructure of the steel sheet according to this embodiment consists of 20-50% ferrite, 0-5.0% pearlite, 0-5.0% MA structure (Martensite-Austenite Constituent), and the remainder being bainite. Here, pearlite and MA structure are harder than ferrite and bainite. In particular, MA structure with an equivalent circle diameter of 5 μm or more and pearlite with an equivalent circle diameter of 5 μm or more tend to increase the hardness of the steel sheet. Therefore, if such coarse MA structure and pearlite are present locally, the hardness of the steel sheet tends to become heterogeneous. As a result, the heterogeneity of strength in the HAZ after welding decreases.

[0112] Therefore, in this embodiment, a virtual circle (specific circle) with a diameter of 30 μm is defined, and the number of coarse MA structures and pearlite contained within the specific circle is defined. If the total number of coarse MA structures and pearlite located inside the specific circle is less than 1, the distribution of hard structures is less uneven, and the hardness of the steel plate is less likely to increase locally. In other words, there is a possibility that the heterogeneity of strength in the HAZ after welding will increase. On the other hand, if the total number of coarse MA structures and pearlite located inside the specific circle is 10 or more, the structure will be mainly composed of ferrite, MA, and pearlite, and the strength of the base steel plate will be less likely to increase. In other words, there is a possibility that the heterogeneity of strength in the HAZ after welding will decrease.

[0113] Therefore, in the steel plate according to this embodiment, it is preferable that the total number of MA structures with an equivalent diameter of 5 μm or more and pearlite structures with an equivalent diameter of 5 μm or more contained within a specific circle is less than 1 or 10 or more. If the total number of MA structures with an equivalent diameter of 5 μm or more and pearlite structures with an equivalent diameter of 5 μm or more within the circle is less than 1 or 10 or more, the heterogeneity of the strength in the HAZ of the welded structure is suppressed more stably.

[0114] Within a specific circle, the upper limit of the total number of MA structures with an equivalent circular diameter of 5 μm or more and pearlite structures with an equivalent circular diameter of 5 μm or more is, for example, 15. In other words, the steel sheet according to this embodiment may have a total of 0 or 10 to 15 MA structures with an equivalent circular diameter of 5 μm or more and pearlite structures with an equivalent circular diameter of 5 μm or more contained within a specific circle.

[0115] In this embodiment, the sum of MA tissue with an equivalent diameter of 5 μm or more and perlite with an equivalent diameter of 5 μm or more within a specific circle is determined by the same method as described above for microstructural observation.

[0116] [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 feature 6 can be obtained more stably by the preferred manufacturing method described later.

[0117] [Welded Structures] The welded structure of this embodiment satisfies the following features 1 to 6. (Feature 1) It comprises a base steel plate and a welded joint. (Feature 2) The chemical composition of the base steel sheet is within the range of this embodiment. (Feature 3) The chemical composition of the base steel sheet satisfies the value of f1, as defined by equation (1), between 0.35 and 0.40. 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) The chemical composition of the base steel sheet satisfies the condition that f2, as 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 5) At the center of the thickness of the base steel plate, The microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure, and the remainder being bainite, by area percentage. (Feature 6) The ratio Rρ of the dislocation density ρ2 at the center of the thickness of the base steel sheet, as defined by equation (3), to the dislocation density ρ4 at t / 4 of the base steel sheet satisfies 0.60 to 1.00. Rρ = ρ² / ρ₄ (3) Here, in equation (3), ρ2 represents the dislocation density at the center of the thickness of the base steel plate, in units of m -2 Substituting this, ρ4 is the dislocation density in the t / 4 portion of the base steel sheet, in units of m -2 It is substituted. The following describes each of its features.

[0118] [(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.

[0119] 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.

[0120] 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.

[0121] [(Feature 2)~(Feature 6)] 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, feature 2 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as feature 1 of the steel plate according to this embodiment. Similarly, feature 3 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as feature 2 of the steel plate according to this embodiment. Similarly, feature 4 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as feature 3 of the steel plate according to this embodiment. Similarly, feature 5 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as feature 4 of the steel plate according to this embodiment. Similarly, feature 6 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as feature 5 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 configuration as the steel plate according to this embodiment.

[0122] [Intensity heterogeneity in HAZ] The welded structure 1 according to this embodiment, which has the above-described features 1 to 6, 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.

[0123] 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.

[0124] 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 point inside the weld metal portion 50 where the center line of the thickness of the diaphragm 30 intersects with the extension line of the surface of the base steel plate 10. Specifically, referring to Figure 3, the intersection point P of the center line of the thickness of the diaphragm 30 and the extension line of the surface of the base steel plate 10 is considered the center of the weld metal portion 50. Furthermore, the length of the two intersection points between the surface of the base steel plate 10 and the weld metal portion 50 is set to 2l (mm). Specifically, referring to Figure 3, the surface of the base steel plate 10 and the weld metal portion 50 have intersection points Q1 and Q2. Furthermore, the length between intersection points Q1 and Q2 is set to 2l (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.

[0125] 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 2l 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 120 Hv or less are counted. The ratio of the number of measurement points with a Vickers hardness of 120 Hv or less to the total number of measurement points is calculated and defined as the area ratio (%) of the strength heterogeneity region.

[0126] 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.

[0127] [HAZ toughness] The welded structure 1 according to this embodiment, which possesses the above-described features 1 to 6, 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.

[0128] A full-size V-notch test specimen is prepared from the fabricated welded joint. Specifically, Figure 4 is a schematic diagram showing how a Charpy impact test specimen is prepared from a welded joint fabricated by electroslag welding. Similar to Figure 3, in the welded joint shown in Figure 4, a base steel plate 10 with a thickness t (mm) and a diaphragm 30 are fixed perpendicularly to each other, and two backing plates 40 are positioned to sandwich the diaphragm 30. Furthermore, electroslag welding is performed to weld metal 50.

[0129] In this embodiment, the V-notch test specimen 70 is prepared so that the notch position of the V-notch test specimen 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 has an angle with respect to the center line of the diaphragm 30. The prepared V-notch test specimen is subjected to a Charpy impact test in accordance with JIS Z 2242:2018 to determine the absorbed energy (J) at 0°C. In this embodiment, the absorbed energy (J) at 0°C is determined 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 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.

[0130] [Any characteristic of a welded structure] The welded structure of this embodiment may satisfy at least one of the following features 7 and 8. (Feature 7) The tensile strength TS of the base steel plate is 550-740 MPa. (Feature 8) In a plane that includes the rolling direction of the base steel sheet and the thickness direction of the base steel sheet, Within a virtual circle with a diameter of 30 μm, the total number of MA structures with an equivalent circle diameter of 5 μm or more and perlite structures with an equivalent circle diameter of 5 μm or more is less than 1 or 10 or more.

[0131] [(Feature 7) and (Feature 8)] 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 characteristics as the steel plate according to this embodiment. That is, characteristic 7 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as characteristic 6 of the steel plate according to this embodiment. Similarly, characteristic 8 of the base steel plate 10 of the welded structure 1 according to this embodiment is the same as characteristic 7 of the steel plate according to this embodiment.

[0132] [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.

[0133] 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.

[0134] 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.

[0135] [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.

[0136] [Slab preparation process] In the slab preparation process, a slab having the above-mentioned chemical composition is prepared. Specifically, in mass percent, C: 0.060~0.200%, Si: 0.10~0.55%, Mn: 0.70~2.00%, P: 0.020% or less, S: 0.0080% or less, Ti: 0.007~0.020%, Al: 0.010~0.050%, N: 0.0070% or less, O: 0.0050% or less, Cu: 0~0.50%, Ni: 0~0.50%, Cr: 0~0.50%, Mo: 0~0.50%, V: 0~0.080%, Nb: 0~0.030%, Ca: 0~0.0100%, Mg: 0~0.0100%, rare earth elements: 0~0.0100%, Zr: 0~ Prepare a slab having a chemical composition consisting of 0.050%, Hf: 0~0.020%, Sr: 0~0.020%, W: 0~0.100%, Co: 0~0.100%, B: less than 0~0.0008%, Sn: 0~0.100%, Sb: 0~0.050%, Zn: 0~0.020%, Bi: 0~0.100%, Se: 0~0.020%, Pb: 0~0.090%, As: 0~0.050%, Te: 0~0.050%, and the remainder being Fe and impurities, wherein f1, as defined by formula (1), satisfies 0.35~0.40, and f2, as defined by formula (2), is 120 or greater. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=119+6C+55Cu+27Ni+49Cr+95Mo (2) Here, the elemental symbols in equations (1) and (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.

[0137] 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.

[0138] [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.

[0139] [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.

[0140] 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.

[0141] [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.

[0142] [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.

[0143] Preferably, in the second rolling process, the temperature of the slab during the initial rolling is set to 860-930°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 860-930°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.

[0144] 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.

[0145] Preferably, in the second rolling process, the slab temperature at the start of rolling for passes where the cumulative reduction ratio in the second rolling process exceeds 40% is set to 870-930°C. Here, the slab temperature at the start of rolling for passes where the cumulative reduction ratio in the second rolling process exceeds 40% means the slab temperature at a point in the second rolling process when the cumulative reduction ratio in the second rolling process exceeds 40%. Specifically, if the cumulative reduction ratio in the second rolling process up to the nth pass is 38%, and the cumulative reduction ratio in the second rolling process when the nth pass is completed is 42%, then this refers to the slab temperature at the start of the nth pass.

[0146] In regions where the slab temperature is low, the amount of strain introduced into the slab by hot rolling tends to be high. Therefore, if the slab temperature at the start of rolling in a pass where the cumulative reduction ratio in the second rolling process exceeds 40% is too low, the dislocation density of the manufactured steel sheet may become too high. Furthermore, if the slab temperature at the start of rolling in a pass where the cumulative reduction ratio in the second rolling process exceeds 40% is too low, the ferrite area ratio in the manufactured steel sheet may not be sufficiently increased. On the other hand, if the slab temperature at the start of rolling in a pass where the cumulative reduction ratio in the second rolling process exceeds 40% is too high, the dislocation density of the manufactured steel sheet may not be sufficiently increased. Furthermore, if the slab temperature at the start of rolling in a pass where the cumulative reduction ratio in the second rolling process exceeds 40% is too high, the ferrite area ratio in the manufactured steel sheet may become too high. Therefore, it is preferable that the temperature at the start of rolling in a pass where the cumulative reduction ratio in the second rolling process exceeds 40% is 870 to 930°C.

[0147] Preferably, in the second rolling process, the slab temperature at the start of the final pass is set to 850-915°C. If the slab temperature at the start of the final pass is too low, the dislocation density 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 dislocation density distribution of the manufactured steel sheet is likely to become uneven, and the dislocation density ratio Rρ may become small. Therefore, it is preferable to set the slab temperature at the start of the final pass in the second rolling process to 850-915°C.

[0148] [Cooling process] In the cooling process, the steel plate on which the second rolling process has been performed is cooled. The method for cooling the steel plate is not particularly limited. For example, cooling by water cooling is carried out. At this time, the cooling start temperature is preferably 780 to 860 °C. If the cooling start temperature is too low, the dislocation density of the manufactured steel plate may become too low. On the other hand, if the cooling start temperature is too high, the difference in the microstructure generated between the surface and the center of the steel plate may become large. Furthermore, if the cooling start temperature is too high, the dislocation density of the manufactured steel plate may become too high. Therefore, the cooling start temperature in the cooling process is preferably 780 to 860 °C.

[0149] Also, in the cooling process, the cooling stop temperature is preferably 430 to 630 °C. If the cooling stop temperature is too low, a bias in the distribution of the dislocation density is likely to occur, and the dislocation density ratio Rρ of the manufactured steel plate may become small. Furthermore, if the cooling stop temperature is too low, the ferrite area ratio in the manufactured steel plate may not be sufficiently increased. On the other hand, if the cooling stop temperature is too high, a bias in the distribution of the dislocation density is likely to occur, and the dislocation density ratio Rρ of the manufactured steel plate may become small. Furthermore, if the cooling stop temperature is too high, the ferrite area ratio in the manufactured steel plate may become too high. Therefore, the cooling stop temperature is preferably 430 to 630 °C.

[0150] By the above method, the steel plate according to this embodiment is manufactured. Furthermore, using the steel plate according to this embodiment, the welded structure according to this embodiment is manufactured. The manufacturing method of the welded structure includes a welding process. Hereinafter, the welding process will be described.

[0151] [Welding Process] In the welding process, steel plates are welded together to manufacture a welded structure. The welding conditions are not particularly limited and may be well-known methods. For example, when welding steel plates, submerged arc welding may be performed. For example, further, when introducing a diaphragm, electro-slag welding may be performed. In any case, a person skilled in the art can perform welding by appropriately adjusting the welding conditions as necessary. Hereinafter, the present invention will be described more specifically by way of examples.

Example

[0152] Slabs were manufactured from molten steel having the chemical compositions shown in Table 1A and Table 1B by continuous casting. The blanks in Table 1A and Table 1B mean that the content of each element is at the impurity level.

[0153]

Table 1A

[0154]

Table 1B

[0155] The manufactured slabs were subjected to a hot rolling process. First, the thickness (mm) of the slabs for each test number was as shown in Table 2. Each slab for each test number was heated in a heating furnace at the heating temperature (°C) shown in Table 2 for the heating time (minutes). At this time, the thickness (mm) of the slab at the end of the first rolling process was as shown in Table 2.

[0156]

Table 2

[0157] 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 start temperature (°C), final rolling start temperature (°C), temperature of the pass where the reduction ratio exceeds 40% (temperature at the start of rolling for the pass where the cumulative reduction ratio exceeds 40% in the second rolling process) (°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 final rolling mill in the second rolling process. Furthermore, the temperature of the pass where the reduction ratio exceeds 40% refers to the temperature of the slab at the start of the pass where the cumulative reduction ratio exceeds 40% in the second rolling process.

[0158] 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.

[0159] [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) Dislocation density measurement test (Test 3) Tensile Test (Test 4) HAZ Strength Heterogeneity Evaluation Test (Test 5) HAZ Microtissue Observation Test (Test 6) HAZ Toughness Evaluation Test

[0160] [(Test 1) Microtissue observation test] For each steel plate with a test number, microstructural observation was performed using the method described above to determine the area percentage of ferrite (%) and pearlite (%) in the center of the plate thickness. The area percentages of ferrite (%), pearlite (%), and MA structure (%) in the center of the plate thickness for each steel plate with a test number are shown in Table 3. Furthermore, the total amount of coarse MA structure and coarse pearlite inside a specific circle was determined using the method described above. The number obtained is shown in Table 3.

[0161] [Table 3]

[0162] [(Test 2) Dislocation density measurement test] For each steel plate with a test number, the dislocation density was measured by the above method, and the dislocation density (m -2 ) at the center of the plate thickness was obtained. The dislocation density (m -2 ) at the center of the plate thickness of each steel plate with the obtained test number is shown in the column of "Dislocation density ρ2 (×10 14 m -2 )" in Table 3. The dislocation density (m -2 ) at the t / 4 part of each steel plate with the obtained test number is shown in the column of "Dislocation density ρ4 (×10 14 m -2 )" in Table 3.

[0163] [(Test 3) Tensile test] For each steel plate with a test number, a tensile test was carried out by the above method, and the tensile strength TS (MPa) and the yield stress YS (MPa) were obtained. The obtained tensile strength TS (MPa) and yield stress YS (MPa) for each test number are shown in Table 3.

[0164] [Manufacture of welded structure] Prior to Tests 4 to 6, mock-ups simulating welded structures were manufactured. Specifically, steel plates with each test number having the thicknesses described in Table 4 were used as skin plates. Further, steel plates with the thicknesses described in Table 4 were prepared from steel plates having the same chemical composition and used as diaphragms. In addition, in electro-slag welding (ESW), a welding wire corresponding to YES562-S described in JIS Z3353:2013 (denoted as "A" in the "Welding wire" column in Table 4), or a welding wire corresponding to YES602-S described in JIS Z3353:2013 (denoted as "B" in the "Welding wire" column in Table 4) was used. Note that the welding wire used for each test number had a diameter of 1.6 mm. Further, the conditions of ESW carried out for each test number were as shown in Table 4. By ESW carried out under the above conditions, an electro-slag weld joint (ESW joint) between the skin plate and the diaphragm was manufactured.

[0165]

Table 4

[0166] [(Test 4) 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 120 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 4.

[0167] [(Test 5) 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 4.

[0168] [(Test 6) 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, as shown in Figure 4. The notch position of the V-notch specimen was prepared so that it 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, to determine the absorbed energy (J) at 0°C. The absorbed energy at 0°C for each test number obtained is shown in the "vE0°C(J)" column of Table 4.

[0169] [Evaluation Results] Referring to Tables 1A, 1B, 2, 3, and 4, the steel plates for test numbers 1 to 14 satisfied steel plate characteristics 1 to 5. As a result, the welded structures manufactured from these steel plates had a region of 30.0% 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 J or more at 0°C in the HAZ, demonstrating excellent HAZ toughness.

[0170] Steel plates from test numbers 2 to 14 further met the criteria of having less than 1 or more than 10 coarse MA and coarse perlite particles in a specific circle. As a result, the area ratio of the strength heterogeneity region in the HAZ was 26.0% or less, further suppressing the strength heterogeneity in the HAZ.

[0171] The steel plate in test number 15 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.

[0172] The steel plate in test number 16 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.

[0173] The steel plate in test number 17 had too high a Cr content. As a result, the welded structure made from this steel plate had a region of HAZ (High-Area Zone) strength heterogeneity exceeding 30.0%, 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 possess good HAZ toughness.

[0174] The steel plate in test number 18 had too high a molybdenum (Mo) content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 47 J at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0175] The steel plate in test number 19 had too high a V content. As a result, the welded structure produced from this steel plate had an absorbed energy (J) of less than 47 J at 0°C in the heat-affected zone (HAZ), and did not possess good HAZ toughness.

[0176] The steel plate used in test number 20 had too high 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 30.0%, 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 at 0°C in the HAZ, and did not possess good HAZ toughness.

[0177] The steel plate in test number 21 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 30.0%, 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.

[0178] The steel plate in test number 22 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 30.0%, 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.

[0179] The steel plate in test number 23 had too high an oxygen content. As a result, the welded structure made from this steel plate had a region of over 30.0% of the area of ​​strength heterogeneity in the heat-affected zone (HAZ), 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 possess good HAZ toughness.

[0180] The steel plate in test number 24 had an excessively high cooling start temperature during the cooling process. As a result, this steel plate had an excessively high ferrite area ratio. Consequently, the welded structure produced from this steel plate had an area ratio of over 30.0% of the HAZ (Heat Hazard Zone) strength heterogeneity, 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 at 0°C in the HAZ, indicating a lack of good HAZ toughness.

[0181] The steel plates in test numbers 25 and 26 had their cooling stop temperature too low during the cooling process. As a result, these steel plates had a dislocation density ratio Rρ that was too low. Consequently, the welded structures produced from these steel plates had an area ratio of strength heterogeneity in the HAZ exceeding 30.0%, and the strength heterogeneity in the HAZ was not suppressed. Furthermore, the welded structures produced from these steel plates had an absorbed energy (J) of less than 47 J in the HAZ at 0°C, indicating that they did not possess good HAZ toughness.

[0182] The steel plate in test number 27 had a cooling start temperature that was too low during the cooling process. As a result, this steel plate had an excessively high dislocation density ratio (Rρ). Consequently, the welded structure produced from this steel plate had an area ratio of strength heterogeneity in the heat-affected zone (HAZ) exceeding 30.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0183] In test number 28, the starting temperature for the final rolling in the second rolling process was too high. As a result, this steel sheet had an excessively high ferrite area ratio and an excessively low dislocation density ratio Rρ. Consequently, the welded structure produced from this steel sheet had an area ratio of strength heterogeneity in the heat-affected zone (HAZ) exceeding 30.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0184] In test number 29, the temperature was too low when the reduction ratio was 40% during the second rolling process. As a result, the ferrite area ratio in this steel sheet was too low. Consequently, the welded structure produced from this steel sheet had an area ratio of strength heterogeneity in the heat-affected zone (HAZ) exceeding 30.0%, and the strength heterogeneity in the HAZ was not suppressed. Furthermore, the welded structure produced from this steel sheet had an absorbed energy (J) of less than 47 J in the HAZ at 0°C, indicating that it did not possess good HAZ toughness.

[0185] The steel plate in test number 30 had an f1 value that was too low. As a result, the welded structure produced from this steel plate had a strength heterogeneity area ratio in the HAZ exceeding 30.0%, and the strength heterogeneity in the HAZ was not suppressed.

[0186] 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]

[0187] 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: 0.060-0.200%, Si: 0.10 to 0.55%, Mn: 0.70-2.00%, P: 0.020% or less, S: 0.0080% or less, Ti: 0.007 to 0.020%, Al: 0.010-0.050%, N: 0.0070% or less, O: 0.0050% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, V: 0 to 0.080%, Nb: 0 to 0.030%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, Zr: 0 to 0.050%, Hf: 0 to 0.020%, Sr: 0 to 0.020%, W: 0-0.100%, Co: 0-0.100%, B: Less than 0-0.0008%, 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 0.35 to 0.40, The f2 defined in equation (2) satisfies 120 or more, At the center of the thickness of the steel plate, The microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure (Martensite-Austenite Continent) by area percentage, and the remainder being bainite. Dislocation density ρ at the center of the thickness of the steel plate as defined by equation (3) 2 The dislocation density ρ in the t / 4 portion of the steel plate. 4 The ratio Rρ satisfies 0.60 to 1.

00. steel plate. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=119+6C+55Cu+27Ni+49Cr+95Mo (2) Rr=r 2 / r 4 (3) Here, the elemental symbols in equations (1) and (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. ρ in equation (3) 2 The dislocation density in the center of the steel plate thickness is given in units of m -2 Substituted with ρ 4 The dislocation density in the t / 4 portion of the steel plate is given in units of m -2 It is substituted.

2. A steel plate according to claim 1, The aforementioned chemical composition is Cu: 0.01 to 0.50%, Ni: 0.01-0.50%, Cr: 0.01-0.50%, Mo: 0.01-0.50%, V: 0.001-0.080%, Nb: 0.001-0.030%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Rare earth elements: 0.0001 to 0.0100%, 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: Less than 0.0001% to 0.0008% Sn: 0.001 to 0.100%, Sb: 0.001 to 0.050%, Zn: 0.001 to 0.020%, Bi: 0.001-0.100%, Se: 0.001-0.020%, 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, In a plane including the rolling direction of the steel plate and the thickness direction of the steel plate, The total number of MA structures with an equivalent circle diameter of 5 μm or more and pearlite structures with an equivalent circle diameter of 5 μm or more contained within a virtual circle with a diameter of 30 μm is less than 1 or 10 or more. steel plate.

4. It comprises a base steel plate and a welded joint, The chemical composition of the aforementioned base steel sheet is, in mass%, C: 0.060-0.200%, Si: 0.10 to 0.55%, Mn: 0.70-2.00%, P: 0.020% or less, S: 0.0080% or less, Ti: 0.007 to 0.020%, Al: 0.010-0.050%, N: 0.0070% or less, O: 0.0050% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, V: 0 to 0.080%, Nb: 0 to 0.030%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, Zr: 0 to 0.050%, Hf: 0 to 0.020%, Sr: 0 to 0.020%, W: 0-0.100%, Co: 0-0.100%, B: Less than 0-0.0008%, 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 0.35 to 0.40, The f2 defined in equation (2) satisfies 120 or more, At the center of the thickness of the base steel plate, The microstructure consists of 50-65% ferrite, 0-15% pearlite, 0-5.0% MA structure (Martensite-Austenite Continent) by area percentage, and the remainder being bainite. The dislocation density ρ at the center of the thickness of the base steel plate defined by formula (3) 2 and the dislocation density ρ at the t / 4 part of the base steel plate 4 where the ratio Rρ thereof satisfies 0.60 to 1.00 Welded structure. f1=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14 (1) f2=119+6C+55Cu+27Ni+49Cr+95Mo (2) Rr=r 2 / r 4 (3) Here, the elemental symbols in equations (1) and (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. ρ in equation (3) 2 This includes the dislocation density at the center of the thickness of the base steel plate, in units of m -2 Substituted with ρ 4 The dislocation density in the t / 4 portion of the base steel plate is given in units of m -2 It is substituted.

5. A welded structure according to claim 4, The chemical composition of the base steel sheet is, Cu: 0.01 to 0.50%, Ni: 0.01-0.50%, Cr: 0.01-0.50%, Mo: 0.01-0.50%, V: 0.001-0.080%, Nb: 0.001-0.030%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Rare earth elements: 0.0001 to 0.0100%, 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: Less than 0.0001% to 0.0008% Sn: 0.001 to 0.100%, Sb: 0.001 to 0.050%, Zn: 0.001 to 0.020%, Bi: 0.001-0.100%, Se: 0.001-0.020%, 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 base steel plate is In a plane including the rolling direction of the base steel sheet and the thickness direction of the base steel sheet, The total number of MA structures with an equivalent circle diameter of 5 μm or more and pearlite structures with an equivalent circle diameter of 5 μm or more contained within a virtual circle with a diameter of 30 μm is less than 1 or 10 or more. Welded structure.

7. A welded structure according to claim 5, The aforementioned base steel plate is In a plane including the rolling direction of the base steel sheet and the thickness direction of the base steel sheet, The total number of MA structures with an equivalent circle diameter of 5 μm or more and pearlite structures with an equivalent circle diameter of 5 μm or more contained within a virtual circle with a diameter of 30 μm is less than 1 or 10 or more. Welded structure.

8. A welded structure according to any one of claims 4 to 7, The aforementioned welded structure is The base steel plate and, A diaphragm positioned perpendicular to the base steel plate, The base steel plate and the diaphragm are formed together, including a welded metal portion. Within the welded metal portion, the vertex is the intersection of the center line of the diaphragm's thickness and the extension line of the surface of the base steel plate, The thickness t mm of the base steel plate, In a rectangle whose sides are 2l mm long at the two intersections of the surface of the base steel plate and the welded metal portion, The area percentage with a Vickers hardness of 120 Hv or less is 30.0% or less. Welded structure.

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

Images