Steel plates and welded joints

Abstract

The steel plate has a predetermined chemical composition, wherein, when the plate thickness is t, the metal structure at the t / 4 position, which is a position t / 4 from the surface of a cross section in the plate thickness direction, comprises, by volume, ferrite of 20% or more, bainite and martensite of 40% or more in total, with the remainder being one or more selected from retained austenite and pearlite; when the manganese concentration is measured at a plurality of measurement points at intervals of 1 μm in a square region of the cross section in the plate thickness direction, having a side length of t / 4 and centered at the t / 4 position, the proportion of the measurement points having a manganese concentration of 1.1 times or more relative to the average value of the manganese concentration of all the plurality of measurement points is less than 10.0%; the steel plate has a tensile strength of 980 MPa or more, and a product of the tensile strength and the total elongation of 10,500 MPa·% or more.

Description

Technical Field

[0001] The present invention relates to steel plates and welded joints.

[0002] This application claims priority based on Japanese Patent Application No. 2021-051017 filed in Japan on March 25, 2021, and incorporates the contents thereof herein. Background Art

[0003] In recent years, the demand for improved fuel efficiency in automobiles has been growing due to greenhouse gas emission regulations implemented as part of global warming countermeasures. To ensure lightweight and crash safety, the use of high-strength steel sheets has been increasing. In particular, demand for high-strength steel sheets with a tensile strength of 980 MPa or higher has been increasing.

[0004] High-strength steel sheets for automotive parts require not only strength but also properties necessary for forming components, such as press formability. Properties required for press formability include ductility (elongation). Consequently, demand for steel sheets with both high strength and excellent ductility is increasing.

[0005] As a steel sheet capable of obtaining excellent ductility, a dual-phase steel sheet (hereinafter referred to as DP steel) mainly composed of a composite structure of a soft ferrite phase and a hard martensite phase is known (for example, Patent Documents 1 and 2).

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 6-128688

[0009] Patent Document 2: Japanese Patent No. 5305149 Summary of the Invention

[0010] Problems to be solved by the invention

[0011] As mentioned above, in the automotive field, high-strength steel sheets are used, and demand for high-strength steel sheets with excellent formability has been increasing, especially recently. However, in recent years, DP steels with a tensile strength of 980 MPa or more have been subject to a problem of reduced weld joint strength during welding.

[0012] To address this issue, DP steels are disclosed in Patent Documents 1 and 2, but the weld joint strength is not considered. As a result, no technology has been proposed to improve the strength of welded joints of DP steels having a tensile strength of 980 MPa or more.

[0013] An object of the present invention is to provide a steel plate having excellent formability, a tensile strength of 980 MPa or more, and sufficient weld joint strength after welding, and a welded joint obtained from the steel plate and having sufficient weld joint strength.

[0014] Means for solving problems

[0015] The inventors of the present invention investigated the reasons why weld joint strength decreases when welding DP steel with a tensile strength of 980 MPa or higher. They found that the presence of coarse martensite in the heat-affected zone (HAZ) generates residual stress in the plate thickness direction, suscepting cracking to occur near the zone. Furthermore, the inventors conducted further research and discovered that suppressing Mn segregation is effective in suppressing the coarsening of the original γ grain size in the HAZ.

[0016] The present invention has been made based on the above findings. The gist of the present invention is as follows.

[0017] [1] A steel plate according to one embodiment of the present invention has the following chemical composition: in mass %, C: 0.05% to 0.25%, Si: 0.05% to 2.00%, Mn: 1.50% to 3.00%, Al: 0.005% to 1.500%, P: 0% to 0.040%, S: 0% to 0.010%, N: 0% to 0.0100%, O: 0% to 0.0060%, Cr: 0% to 0.50%, Ni: 0% to 1.00%, Cu: 0% to 1.50%. %~1.00%, Mo: 0%~0.50%, Ti: 0%~0.200%, Nb: 0%~0.200%, V: 0%~0.500%, B: 0%~0.0100%, W: 0%~0.1000% , Ta: 0% ~ 0.1000%, Sn: 0% ~ 0.0500%, Co: 0% ~ 0.5000%, Sb: 0% ~ 0.0500%, As: 0% ~ 0.0500%, Mg: 0% ~ 0.0500% , Ca: 0% to 0.0400%, Y: 0% to 0.0500%, La: 0% to 0.0500%, Ce: 0% to 0.0500%, Zr: 0% to 0.0500% and the remainder: Fe and impurities, when the plate thickness is set to t, the metal structure at the position t / 4 from the surface in the cross section in the plate thickness direction, i.e., the position t / 4, comprises, by volume, ferrite: 20% or more, bainite and martensite: 40% or more in total, and the remainder is selected from residual The steel plate further comprises one or more of austenite and pearlite, and when the Mn concentration is measured at 1 μm intervals at a plurality of measurement points in a square area having a side length of t / 4 and centered at the t / 4 position in the cross section in the plate thickness direction, the proportion of the measurement points having a Mn concentration of 1.1 times or more with respect to the average value of the Mn concentration of all the plurality of measurement points is less than 10.0%, and the tensile strength of the steel plate is 980 MPa or more, and the product of the tensile strength and the total elongation is 10500 MPa·% or more.

[0018] [2] The steel sheet according to [1], wherein the chemical composition may contain, in mass%, one or more elements selected from the group consisting of: Cr: 0.01% to 0.50%, Ni: 0.01% to 1.00%, Cu: 0.01% to 1.00%, Mo: 0.01% to 0.50%, Ti: 0.001% to 0.200%, Nb: 0.001% to 0.200%, V: 0.001% to 0.500%, B: 0.0001% to 0.0100%, W: 0.0005% to 0.1000%, Ta: 0.0001% to 0.200%, 5% to 0.1000%, Sn: 0.0010% to 0.0500%, Co: 0.0010% to 0.5000%, Sb: 0.0010% to 0.0500%, As: 0.0010% to 0.0500%, Mg: 0.0001% to 0.0500%, Ca: 0.0001% to 0.0400%, Y: 0.0001% to 0.0500%, La: 0.0001% to 0.0500%, Ce: 0.0001% to 0.0500% and Zr: 0.0001% to 0.0500%.

[0019] [3] The steel sheet according to [1] or [2] above may also have a hot-dip galvanized layer on the surface.

[0020] [4] The steel sheet according to [3] above, wherein the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

[0021] [5] Another embodiment of the present invention is a weld joint obtained by welding the steel plate described in any one of [1] to [4], which has a base material portion, a nugget portion and a heat-affected portion, wherein the base material portion other than the heat-affected portion and the nugget portion has the same chemical composition and metal structure as the steel plate, and the maximum value of the original γ grain size in the heat-affected portion is less than 30 μm.

[0022] Effects of the Invention

[0023] According to the above aspects of the present invention, it is possible to provide a steel plate having excellent formability, a tensile strength of 980 MPa or more, and sufficient weld joint strength after welding, and a welded joint obtained from the steel plate having sufficient weld joint strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a diagram explaining the observation area of ​​the structure in the cross section in the plate thickness direction and the measurement area of ​​the Mn concentration.

[0025] Figure 2 This is a schematic diagram illustrating the heat-affected zone of a weld joint.

[0026] Figure 3 It is a histogram showing an example of the Mn concentration distribution in the case where the holding at 1300° C. is not performed (0 h) and in the case where the holding is performed for 10 hours (10 h) in the first segregation step. DETAILED DESCRIPTION

[0027] A steel plate according to one embodiment of the present invention (the steel plate of the present embodiment) comprises: (a) a predetermined chemical composition; (b) a metal structure at a position t / 4, which is a distance t / 4 from the surface of a cross section in the plate thickness direction, comprising, by volume, ferrite of 20% or more, bainite and martensite of 40% or more in total, with the remainder being one or more selected from retained austenite and pearlite; (c) in a square region of the cross section in the plate thickness direction, centered at the t / 4 position and having a side length of t / 4, when the Mn concentration is measured at a plurality of measurement points at intervals of 1 μm, the proportion of the measurement points having a Mn concentration of 1.1 times or more relative to the average value of the Mn concentration of all the plurality of measurement points is less than 10.0%; and (d) the steel plate has a tensile strength of 980 MPa or more, and a product of the tensile strength and the total elongation of 10,500 MPa·% or more.

[0028] Furthermore, the weld joint of the present embodiment has (A) a predetermined chemical composition, and (B) a maximum value of the original γ grain size in the heat-affected zone of less than 30 μm.

[0029] The following describes each of them separately.

[0030] 1. Steel plate

[0031] First, the steel plate according to this embodiment will be described.

[0032] Chemical composition

[0033] The chemical composition of the steel sheet according to the present embodiment will be described. "%" of the content of each element represents "mass %" unless otherwise specified.

[0034] C: 0.05% to 0.25%

[0035] C (carbon) is an element essential for ensuring the strength of the steel sheet. By setting the C content to 0.05% or more, the desired high strength can be achieved. The C content may be 0.07% or more or 0.08% or more.

[0036] On the other hand, in order to ensure workability and weldability, the C content is set to 0.25% or less. The C content may be 0.23% or less, 0.22% or less, or 0.20% or less.

[0037] Si: 0.05% to 2.00%

[0038] Si (silicon) is an element that stabilizes ferrite. Specifically, Si increases the Ac3 point (Ac3 transformation point), allowing for the formation of a large amount of ferrite over a wide annealing temperature range. The inclusion of Si is desirable from the perspective of improving the microstructure controllability of the steel sheet. To achieve this effect, the Si content is set to 0.05% or more.

[0039] On the other hand, in order to ensure the weldability of the steel sheet, the Si content is set to 2.00% or less. The Si content may be 1.80% or less, 1.70% or less, or 1.50% or less.

[0040] Mn: 1.50% to 3.00%

[0041] Mn (manganese) is a strong austenite stabilizing element and is effective in increasing the strength of steel sheets. To achieve these effects, the Mn content is set to 1.50% or more. The Mn content can be 1.60% or more or 1.70% or more.

[0042] On the other hand, in order to ensure weldability and low-temperature toughness, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.70% or less, or 2.50% or less.

[0043] Al: 0.005%~1.500%

[0044] Al (aluminum) is an element used for deoxidation of steel. To achieve this effect, the Al content is set to 0.005% or more.

[0045] On the other hand, even if Al is contained excessively, the effect becomes saturated, leading not only to unnecessary cost increases but also to an increase in the steel's transformation temperature and an increase in the load during hot rolling. Therefore, the Al content is set to 1.500% or less. The Al content is preferably 1.200% or less, 1.000% or less, or 0.800% or less.

[0046] P: 0%~0.040%

[0047] Phosphorus (P) is a solid solution strengthening element and is effective for increasing the strength of steel plates. However, excessive P content degrades weldability and toughness. Therefore, the P content is set to 0.040% or less. The P content is preferably 0.035%, 0.030%, or 0.020%. The P content can be 0%, but extreme reductions in P content increase the cost of P removal. Therefore, for economic reasons, the P content can be set to 0.001% or more.

[0048] S: 0%~0.010%

[0049] S (sulfur) is an element contained as an impurity. It forms MnS in steel, which degrades toughness and hole expandability. Therefore, the S content is set to 0.010% or less, as the degradation of toughness and hole expandability is not significant. The S content is preferably 0.005%, 0.004%, or 0.003%. The S content can be 0%, but extreme reductions in the S content increase desulfurization costs. Therefore, from an economical perspective, the S content can be set to 0.0001% or more.

[0050] N: 0%~0.0100%

[0051] Nitrogen (N) is an element contained as an impurity. If its content exceeds 0.0100%, it forms coarse nitrides in the steel, degrading bendability and hole expandability. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. While the N content can be 0%, extreme reductions in N content increase the cost of denitrification. Therefore, for economic reasons, the N content should be set to 0.00010% or more.

[0052] O: 0%~0.0060%

[0053] O (oxygen) is an element contained as an impurity. If its content exceeds 0.0060%, it forms coarse oxides in the steel, deteriorating bendability and hole expandability. Therefore, the O content is set to 0.0060% or less. The O content is preferably 0.0050% or less, or 0.0040% or less. The O content can be 0%, but from the perspective of manufacturing costs, it is recommended to set the O content to 0.0001% or more.

[0054] The basic chemical composition of the steel sheet of this embodiment includes the aforementioned elements (essential elements), with the remainder consisting of Fe and impurities. Here, "impurities" refer to components that may be introduced into the steel sheet during industrial production due to various factors, such as raw materials such as ore and scrap, and during the manufacturing process. These components are tolerated as long as they do not adversely affect the present invention.

[0055] However, the steel sheet may contain the following elements (optional elements) as needed to replace part of the Fe. These elements are not necessarily contained, so the lower limit is 0%. In addition, the following elements may be mixed from scrap materials, etc., but they may be contained as impurities as long as the content is below the upper limit described below.

[0056] Cr: 0%~0.50%

[0057] Ni: 0%~1.00%

[0058] Cu: 0%~1.00%

[0059] Cr (chromium), Ni (nickel), and Cu (copper) are all elements that contribute to increased strength. Therefore, one or more selected from these elements may be contained as needed. To achieve the aforementioned effects, the content of one or more selected from Cr, Ni, and Cu is preferably 0.01% or more, more preferably 0.10% or more.

[0060] On the other hand, a Cr content exceeding 0.50%, a Ni content exceeding 1.00%, or a Cu content exceeding 1.00% may reduce pickling properties, weldability, and hot workability. Therefore, the Cr content is set to 0.50% or less, the Ni content is set to 1.00% or less, and the Cu content is set to 1.00% or less. The Cr content may be 0.40% or less, 0.30% or less, or 0.10% or less. The Ni content may be 0.80% or less, 0.60% or less, or 0.20% or less. The Cu content may be 0.80% or less, 0.60% or less, or 0.20% or less.

[0061] Mo: 0%~0.50%

[0062] Mo (molybdenum) is an element that improves the hardenability of steel and contributes to the improvement of strength, similar to Mn. Therefore, Mo can be contained as needed. In order to obtain the above-mentioned effects, the Mo content is preferably 0.01% or more, preferably 0.10% or more.

[0063] On the other hand, if the Mo content exceeds 0.50%, hot workability may be reduced, and productivity may be reduced. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.40% or less, 0.30% or less, or 0.10% or less.

[0064] Ti: 0%~0.200%

[0065] Nb: 0%~0.200%

[0066] V: 0%~0.500%

[0067] Ti (titanium), Nb (nickel), and V (vanadium) are all elements that contribute to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening due to grain growth inhibition, and dislocation strengthening due to recrystallization inhibition. Therefore, one or more of these elements may be included as needed. To achieve the aforementioned effects, it is preferred that the steel sheet contain at least 0.001% of Ti, at least 0.0001% of Nb, and at least 0.001% of V.

[0068] On the other hand, a Ti content exceeding 0.200%, a Nb content exceeding 0.200%, or a V content exceeding 0.500% may cause coarse carbonitride precipitation and reduce formability. Therefore, the Ti content is set to 0.200% or less, the Nb content is set to 0.200% or less, and the V content is set to 0.500% or less. The Ti content can be set to 0.180% or less, 0.150% or less, or 0.100% or less. The Nb content can be set to 0.180% or less, 0.150% or less, or 0.100% or less. The V content can be set to 0.400% or less, 0.300% or less, or 0.100% or less.

[0069] B: 0%~0.0100%

[0070] Boron (B) is an element that segregates at austenite grain boundaries during welding, strengthening the grain boundaries and contributing to improved resistance to molten metal embrittlement cracking. Therefore, B may be contained as needed. To achieve the aforementioned effects, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, or 0.0008% or more.

[0071] On the other hand, if the B content exceeds 0.0100%, carbides and nitrides are formed, saturating the above-mentioned effects and reducing hot workability. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, 0.0050% or less, or 0.0030% or less.

[0072] W: 0%~0.1000%

[0073] Ta: 0%~0.1000%

[0074] Sn: 0%~0.0500%

[0075] Co: 0%~0.5000%

[0076] As: 0%~0.0500%

[0077] W (tungsten), Ta (tantalum), Sn (tin), Co (cobalt), and As (arsenic) are elements that contribute to improving the strength of the steel sheet by preventing precipitation strengthening and grain coarsening. Therefore, these elements may be contained. To achieve these effects, the W content may be set to 0.0005% or more, 0.0010% or more, 0.0050% or more, or 0.0100% or more. The Ta content may be set to 0.0005% or more, 0.0010% or more, 0.0050% or more, or 0.0100% or more. The Sn content may be set to 0.0010% or more, 0.0020% or more, or 0.0050% or more. The Co content may be set to 0.0010% or more, 0.0100% or more, or 0.0300% or more. The As content may be set to 0.0010% or more, 0.0020% or more, or 0.0050% or more.

[0078] On the other hand, if these elements are present in large amounts, the various properties of the steel sheet may be impaired. Therefore, the W content is set to 0.1000% or less, the Ta content is set to 0.1000% or less, the Sn content is set to 0.0500% or less, the Co content is set to 0.5000% or less, the Sb content is set to 0.0500% or less, and the As content is set to 0.0500% or less. The W content may be set to 0.0800% or less, 0.0500% or less, or 0.0300% or less. The Ta content may be set to 0.080% or less, 0.050% or less, or 0.0300% or less. The Sn content may be set to 0.0400% or less, 0.0300% or less, or 0.0100% or less. The Co content may be set to 0.4000% or less, 0.3000% or less, or 0.1000% or less. The As content can be set to 0.0400% or less, 0.0300% or less, or 0.0100% or less.

[0079] Mg: 0%~0.0500%

[0080] Ca: 0% to 0.0400%

[0081] Y: 0%~0.0500%

[0082] La: 0% to 0.0500%

[0083] Ce: 0%~0.0500%

[0084] Zr: 0%~0.0500%

[0085] Sb: 0%~0.0500%

[0086] Ca (calcium), Mg (magnesium), Y (yttrium), La (lanthanum), Ce (cerium), Zr (zirconium), and Sb (antimony) are all elements that contribute to improved formability. Therefore, one or more selected from these elements may be contained as needed. To achieve the aforementioned effects, the content of one or more selected from Mg, Ca, Y, La, Ce, Zr, and Sb is more preferably 0.0001% or more or 0.0010% or more. The Sb content is more preferably 0.0020% or more or 0.0050% or more.

[0087] On the other hand, contents of Mg, Y, La, Ce, Zr, or Sb exceeding 0.050%, or Ca exceeding 0.040%, may reduce pickling properties, weldability, and hot workability. Therefore, the contents of Mg, Y, La, Ce, Zr, and Sb are all set to 0.0500% or less, and the Ca content is set to 0.0400% or less. The contents of Mg, Ca, Y, La, Ce, Zr, and Sb are each preferably 0.0350% or less, 0.0300% or less, or 0.0100% or less.

[0088] As described above, the chemical composition of the steel sheet of this embodiment includes the basic elements and the balance includes Fe and impurities, or includes the basic elements and one or more optional elements and the balance includes Fe and impurities.

[0089] <Metallic structure at position t / 4, which is t / 4 from the surface in the thickness direction cross section, when the plate thickness is set to t>

[0090] [Ferrite: 20% by volume or more]

[0091] Ferrite is a soft structure with excellent ductility. In order to ensure the elongation of the steel sheet, the volume fraction of ferrite is set to 20% or more.

[0092] On the other hand, if the volume fraction of ferrite becomes excessive, it becomes difficult to ensure the desired strength of the steel sheet. Therefore, the volume fraction of ferrite is set to 60% or less. The volume fraction of ferrite can be 50% or less.

[0093] [Bainite and martensite: 40% by volume or more in total]

[0094] To improve strength, the combined volume fraction of bainite and martensite is set to 40% or more. As long as this requirement is met, the volume fractions of martensite and bainite do not need to be specified individually, but the tempered martensite content may be 10%, 15%, or 20% or more by volume.

[0095] In order to ensure 20% or more of ferrite, the volume fractions of bainite and martensite are set to 80% or less.

[0096] In the steel sheet of the present embodiment, martensite includes fresh martensite and tempered martensite.

[0097] [Remainder: one or more selected from retained austenite and pearlite]

[0098] The remainder other than ferrite, bainite, and martensite may include one or more selected from retained austenite and pearlite. The volume fraction of the remainder is, for example, 10% or less or 5% or less. The volume fraction of the remainder may also be 0%.

[0099] The volume fraction of martensite at the t / 4 position is determined by the following method.

[0100] The observation surface of the sample was etched with nitric acid solution. Figure 1 As shown in A of , a 100 μm × 100 μm area is observed at a magnification of 3000 times using FE-SEM within a range of 1 / 8 to 3 / 8 of the thickness of the plate, centered at a position where the distance from the surface to the surface is 1 / 4 of the thickness of the plate in the thickness direction cross section. In nitric acid corrosion, martensite and retained austenite are not corroded, so the area ratio of the uncorroded area is the total area ratio of martensite and retained austenite. In addition, in the present embodiment, the total area ratio of martensite and retained austenite is regarded as their total volume ratio. The volume ratio of martensite is calculated by subtracting the volume ratio of retained austenite measured by the method described later from the area ratio (i.e., volume ratio) of the uncorroded area.

[0101] The volume fraction of retained austenite can be calculated by measurement using an X-ray diffractometer. In this measurement, the region from the plate surface (rolled surface) of the sample to a depth of 1 / 4 of the plate thickness is first removed by mechanical and chemical polishing. Next, using MoKα radiation as the characteristic X-ray, the integrated intensity ratios of the diffraction peaks of the bcc phase (200), (211) and the fcc phase (200), (220), and (311) are determined in the surface at a depth of 1 / 4 of the plate thickness t. Based on these integrated intensity ratios, the volume fraction of retained austenite can be calculated.

[0102] The volume fractions of ferrite, bainite, and pearlite at the t / 4 position are determined by the following method.

[0103] The observation surface of the sample was etched with nitric acid solution. Figure 1As shown in A of FIG, a 100 μm × 100 μm area is observed at a magnification of 3000 times using FE-SEM within a range of 1 / 8 to 3 / 8 of the thickness of the plate, centered at a position where the distance from the surface to the surface is 1 / 4 of the thickness of the plate. The area without cementite in the crystal is judged to be ferrite, the area containing cementite in the crystal and the cementite is arranged in layers is judged to be pearlite, and the area containing cementite in the crystal and the cementite having multiple variants is judged to be bainite. The area ratio is calculated by the point counting method (based on ASTM E562). Since the area ratio is equivalent to the volume ratio, the area ratio obtained for each organization is used as the volume ratio.

[0104] <When the Mn concentration is measured at multiple measurement points at 1 μm intervals in a square region centered at position t / 4 and having one side t / 4 in a cross section in the plate thickness direction, the proportion of measurement points where the Mn concentration is 1.1 times or greater relative to the average value of the Mn concentration at the multiple measurement points (all measurement points) is less than 10.0%>

[0105] As described above, if coarse retained austenite or fresh martensite exists in the heat-affected zone, these serve as starting points for cracking, making cracking more likely to occur.

[0106] In order to suppress such cracking, it is effective to refine the retained austenite (γ). However, since coarse retained γ is generated in the Mn segregation portion, it is effective to suppress the Mn segregation.

[0107] Specifically, in Figure 1 When measuring the Mn concentration using EPMA (Electron Probe Micro Analyzer) at multiple measurement points at 1 μm intervals in a square area centered at position t / 4 and with one side t / 4 in a cross section in the plate thickness direction, as shown in FIG. B, the ratio (number ratio) of measurement points having a Mn concentration of 1.1 times or more (or 1.1 times or more when the average value is 1.0) relative to the average Mn concentration of all measurement points (multiple measurement points) must be less than 10.0%. In other words, when the segregation degree is defined as "concentration at each measurement point / average concentration at all measurement points in the measurement area," the ratio of measurement points having a Mn concentration of 1.1 or more must be less than 10.0%.

[0108] Mechanical properties

[0109] Considering the contribution to the lightweighting of automobile bodies, the tensile strength of the steel sheet of this embodiment is set to 980 MPa or more. In addition, in the steel sheet of this embodiment, the tensile strength × total elongation (TS × tEl) is set to 10500 MPa·% or more.

[0110] The tensile strength (TS) and total elongation (tEl) were determined by taking JIS No. 5 tensile test pieces from the steel plates in a direction perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z 2241:2011.

[0111] [Plating]

[0112] The steel sheet of the present embodiment described above may have a hot-dip galvanized layer on the surface. The presence of the hot-dip galvanized layer on the surface improves corrosion resistance.

[0113] For example, when the steel sheet is used in an environment that corrodes, there are concerns about opening holes, so sometimes even if it is strengthened, it cannot be thinned to below a certain plate thickness. One of the purposes of strengthening steel sheets is to achieve lightweighting through thinning. Therefore, even if high-strength steel sheets are developed, if the corrosion resistance is low, the application areas will be limited. Therefore, it is considered to apply a coating such as hot-dip galvanizing with high corrosion resistance to the steel sheet. The coating is, for example, a zinc coating such as a hot-dip galvanizing layer or an electroplated zinc coating. In addition, the zinc coating may also be a coating containing Si, Al and / or Mg in addition to Zn.

[0114] The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer. In the alloyed hot-dip galvanized layer, Fe is incorporated into the hot-dip galvanized layer by the alloying treatment, thereby achieving excellent weldability and paintability.

[0115] In addition, for the purpose of improving paintability and weldability, an upper layer plating may be applied to the zinc coating. In addition, for the cold-rolled steel sheet of this embodiment, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, and weldability improvement treatment may be applied to the hot-dip galvanized layer.

[0116] 2. Welding joints

[0117] Next, the weld joint according to this embodiment will be described.

[0118] The welded joint of this embodiment is obtained by welding the steel plates of this embodiment described above. Therefore, it comprises a base metal portion, a nugget formed by welding, and a heat-affected zone formed around the nugget. The steel plate of this embodiment is joined to another steel plate (which may also be a steel plate of this embodiment) via the nugget.

[0119] The base material portion (excluding the heat-affected zone and the nugget) has the same chemical composition and metal structure as the steel plate of this embodiment. The heat-affected zone has the same chemical composition as the steel plate of this embodiment, but its metal structure is mainly martensite and has the following characteristics.

[0120] <Maximum value of original γ grain size in the heat-affected zone: less than 30 μm>

[0121] In order to reduce residual stress in the heat-affected zone of a welded joint, the maximum value of the prior γ grain size, which corresponds to the block size of the martensite structure, should be less than 30 μm. If the prior γ (austenite) grain size is 30 μm or larger, residual stress will be generated in the thickness direction, making cracking more likely to occur near the grain size.

[0122] In the weld joint of the present embodiment, the heat-affected zone is defined as follows.

[0123] like Figure 2 As shown in the figure, in a weld joint having a steel plate S and a weld nugget (weld nugget portion) N joined to the steel plate S, the hardness (Vickers hardness with a load of 0.1 kgf: implemented in accordance with JISZ 2244) is measured at a position that is only 1 / 4 of the plate thickness away from the overlapping surface SF of the steel plates in the plate thickness direction (in the figure, there is a gap between the steel plates S and the steel plates S, but they can also be tightly fitted) on the overlapping surface side of the steel plates and in a range that is 8 mm away from the center O of the weld nugget of the spot weld in the direction perpendicular to the plate thickness direction (left and right direction of the paper), and the transition area in which the hardness is different from the hardness of the weld nugget portion and the hardness of the base material portion (the same hardness as the steel plate before welding) is referred to as the heat affected zone (HAZ).

[0124] The original γ grain size is measured using the following method at a location on the overlapping surface of the steel plates, 1 / 4 of the thickness away from the steel plate surface and at the center of the heat-affected zone (in the horizontal direction of the paper) perpendicular to the plate thickness. The observation surface of the sample is polished and etched with Nital. Three fields of view are observed using an FE-SEM at 1000x magnification, with each field measuring 500 μm x 500 μm. The resulting microstructure photograph is used to calculate the original γ grain size using the line segment method.

[0125] The maximum original γ grain size in each of the three observed fields was averaged, and the obtained value was taken as the maximum value of the original γ grain size in the heat-affected zone.

[0126] [Joint strength]

[0127] In consideration of weldability during assembly of automobile bodies, the steel plate of this embodiment preferably has a joint strength exceeding 6.0 kN.

[0128] The joint strength was determined by collecting test pieces according to JIS Z 3137:1999 from the steel plate in a direction perpendicular to the rolling direction, welding the pieces using a servo motor pressurized single-phase AC spot welder (power supply frequency 50 Hz), and then conducting a cross tensile test according to JIS Z 3137:1999.

[0129] <Manufacturing method>

[0130] The steel plate of the present embodiment can be produced by a production method including the following steps.

[0131] (I) a first Mn segregation reduction step of holding a slab obtained by continuous casting or the like at 1300° C. or higher for 5.0 hours or longer and cooling the slab to 200° C. or lower at an average cooling rate of 20° C. / hour to 80° C. / hour;

[0132] (II) a second Mn segregation step of heating the slab and maintaining it at a temperature of 1200° C. or higher for 1.0 hour or longer;

[0133] (III) a hot rolling step of hot-rolling the slab after the second Mn segregation reducing step to produce a hot-rolled steel sheet;

[0134] (IV) a coiling step of coiling the hot-rolled steel sheet;

[0135] (V) a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step to produce a cold-rolled steel sheet; and

[0136] (VI) An annealing step of annealing the cold-rolled steel sheet.

[0137] The welded joint of the present embodiment can be produced by further performing the following steps on the steel plate of the present embodiment obtained by the production method including (I) to (VI).

[0138] (VII) A welding process for welding the steel plates.

[0139] Hereinafter, each step will be described.

[0140] [First Mn segregation reduction step]

[0141] In the first Mn segregation reduction step, the slab obtained by continuous casting or the like is held at 1300°C or higher for 5.0 hours or more before the hot rolling step, and is cooled to 200°C or lower at an average cooling rate of 20°C / hour to 80°C / hour.

[0142] By holding the slab at a high temperature of 1300°C or higher for more than 5.0 hours, the diffusion rate of Mn is increased and the segregation of Mn is reduced. However, this holding alone does not sufficiently reduce the Mn segregation. It is necessary to further cool the slab to below 200°C at an average cooling rate of 20°C / hour or higher. By cooling the slab to below 200°C at an average cooling rate of 20°C / hour or higher, dislocations caused by thermal shrinkage differences are introduced. These dislocations become high-speed diffusion paths for Mn during heating in the second Mn segregation reduction step in the next step, thereby efficiently diffusing Mn and reducing the Mn segregation degree.

[0143] The faster the average cooling rate, the more dislocations can be introduced. However, if the cooling rate is too fast, the thermal contraction difference becomes excessive, increasing the risk of slab cracking. Therefore, the average cooling rate is set to 80°C / hour or less.

[0144] If the heating temperature is too high, the production cost will increase, and if the heating time is prolonged, the productivity will deteriorate. From these perspectives, the slab heating temperature is set to 1400°C or less, and the holding time at 1300°C or above can be set to 50.0 hours or less.

[0145] [Second Mn Segregation Reduction Step]

[0146] In the second Mn segregation reducing step, the slab after the first Mn segregation reducing step is heated in a heating furnace to 1200° C. or higher and maintained within this temperature range for 1.0 hour or longer.

[0147] After the first Mn segregation step, by maintaining the slab at 1200°C or higher for 1.0 hour or longer, dislocations introduced into the slab can be used as high-speed diffusion paths to diffuse Mn, thereby further reducing Mn segregation.

[0148] Excessively increasing the heating temperature increases production costs, and extending the heating time degrades productivity. Considering these considerations, the slab heating temperature is set to 1300°C or lower, and the holding time at 1200°C or higher can be set to 5.0 hours or less.

[0149] This second Mn segregation reducing step may be performed in a hot rolling heating furnace as heating for hot rolling.

[0150] [Hot rolling process]

[0151] In the hot rolling step, the slab heated to 1200° C. or higher and held for 1.0 hour or longer in the hot rolling heating furnace in the second Mn segregation reducing step is hot rolled to obtain a hot-rolled steel sheet.

[0152] Hot rolling conditions are not particularly limited. For example, the finish rolling step may be completed at 800°C to 980°C, followed by cooling to 600°C to 750°C at an average cooling rate of 2.5°C / s or higher, and then to a coiling temperature of 600°C or lower.

[0153] [Coiling process]

[0154] [Cold rolling process]

[0155] The hot-rolled steel sheet after the hot rolling process is coiled under known conditions to form a hot-rolled coil, and then cold-rolled under known conditions to form a cold-rolled steel sheet. For example, the total reduction ratio can be set to 20% to 85%.

[0156] [Annealing process]

[0157] In the annealing step, in order to set the tensile strength to 980 MPa or more after the annealing step, the metal structure is annealed under conditions such that the ferrite volume fraction is 20% or more and the total of the bainite volume fraction and the martensite volume fraction is 40% or more.

[0158] Specifically, a continuous annealing furnace is used to heat the cold-rolled steel sheet to a soaking temperature of (Ac3-100)°C or higher and less than 900°C, hold the soaking temperature for 5 to 600 seconds, and cool the sheet so that the average cooling rate in the temperature range of 500°C to 750°C is 2.5°C / second to 30.0°C / second. The cooling rate in the temperature range below 500°C is not particularly specified, and reheating and isothermal holding may be performed as long as the temperature is below 500°C.

[0159] If the soaking temperature is too low, austenite cannot be obtained, and the volume fraction of bainite and martensite cannot be sufficiently obtained by subsequent cooling. Therefore, the soaking temperature is set to (Ac3-100)°C or higher. On the other hand, if the soaking temperature is too high, the manufacturing cost increases, so the soaking temperature is preferably set to 900°C or lower.

[0160] The soaking time is not limited, but if the soaking time is less than 5 seconds, austenitization may not proceed sufficiently. Therefore, the soaking time is preferably 5 seconds or longer. On the other hand, if the soaking time exceeds 600 seconds, productivity decreases, so the soaking time is preferably 600 seconds or shorter.

[0161] If the average cooling rate at 500-750°C is less than 2.5°C / s, the volume fraction of ferrite becomes too high, and the volume fractions of bainite and martensite cannot be sufficiently obtained. On the other hand, if the average cooling rate exceeds 50.0°C / s, the volume fraction of ferrite cannot be sufficiently obtained.

[0162] The Ac3 point is obtained by the following method.

[0163] Ac3(℃)=910-203×√[C]+44.7×[Si]-30×[Mn]+700×[P]-20×[Cu]-15.2×[Ni]-11×[Cr]+31.5×[Mo]+400×[Ti]+104×[V]+120×[Al]

[0164] Among them, [C], [Si], [Mn], [P], [Cu], [Ni], [Cr], [Mo], [Ti], [V] and [Al] are the contents (mass %) of the respective elements contained in the slab.

[0165] [Hot-dip galvanizing process]

[0166] [Alloying process]

[0167] After the annealing process, the cold-rolled steel sheet can be immersed in a hot-dip galvanizing bath to produce a hot-dip galvanized steel sheet with a hot-dip galvanized layer on the surface. Alternatively, the hot-dip galvanized steel sheet can be alloyed to produce an alloyed hot-dip galvanized steel sheet. In this case, the heat applied to the steel sheet during hot-dip galvanizing and alloying can be utilized to maintain the temperature of the steel sheet as described above. Any of the conditions can be applied under known conditions.

[0168] [Welding process]

[0169] The welded joint of the present embodiment is obtained by welding the steel plates of the present embodiment obtained through the above-mentioned steps.

[0170] The welding conditions may be known conditions, but the following conditions can be exemplified: using a servo motor pressure-type single-phase AC spot welder (power supply frequency is 50 Hz), the electrode diameter is set to 6 mm, the pressure during welding is set to 4 kN, the welding current is set to 6.0 kA to 9.0 kA, the power-on time is set to 0.4 seconds, the holding time is set to 0.1 seconds, and welding is performed so that the nugget diameter becomes 5√t (t: plate thickness).

[0171] Example

[0172] Slabs (Steel Nos. A to Z) having the chemical compositions shown in Tables 1-1 and 1-2 (unit: mass %, the remainder being Fe and impurities) were produced by continuous casting.

[0173] These slabs were heated and held as shown in Table 2-1 and Table 2-2, and then cooled to 200° C. or lower.

[0174] The slab was then further heated and held again as shown in Tables 2-1 and 2-2, and then hot rolled so that finish rolling was completed at 800 to 980°C. The slab was then cooled to a coiling temperature of 600°C or less so that the average cooling rate to a temperature of 600°C to 750°C was 2.5°C / second or more, and coiled at 600°C or less to obtain a hot-rolled steel sheet with a thickness of 2.0 to 4.0 mm.

[0175] Furthermore, these hot-rolled steel sheets were cold-rolled at a reduction ratio of 20 to 85% to obtain cold-rolled steel sheets with a thickness of 0.8 to 2.0 mm.

[0176] These cold-rolled steel sheets were annealed under the conditions shown in Tables 2-1 and 2-2 (however, the steps after hot rolling were not performed for the examples in which the slabs cracked).

[0177] In addition, as shown in Table 2-1 and Table 2-2, some of the cold-rolled steel sheets were subjected to hot-dip galvanizing, and some of the cold-rolled steel sheets were further subjected to alloying treatment.

[0178] Samples were collected from the obtained cold-rolled steel sheets (including plated steel sheets) according to the above-mentioned procedure, and the microstructure was observed to determine the volume fraction of ferrite, the total volume fraction of martensite and bainite, and the volume fraction of others (retained austenite and / or pearlite).

[0179] Furthermore, the Mn concentration was measured using EPMA according to the above-mentioned procedure, and the ratio of measurement points at which (concentration at measurement point) / (average concentration at all measurement points in the measurement area) (ie, segregation degree) was 1.1 or greater was determined.

[0180] The results are shown in Table 3-1 and Table 3-2.

[0181] Furthermore, JIS No. 5 tensile test pieces were collected from the obtained cold-rolled steel sheets in a direction perpendicular to the rolling direction, and tensile tests were performed in accordance with JIS Z 2241:2011 to determine the tensile strength and total elongation.

[0182] If the tensile strength (TS) is 980 MPa or more and the tensile strength×total elongation (TS×tEl) is 10500 MPa·% or more, it is judged to be high strength and excellent formability.

[0183] The results are shown in Table 4-1 and Table 4-2.

[0184] In addition, test pieces described in JIS Z 3137:1999 were collected from the annealed cold-rolled steel sheets in a direction perpendicular to the rolling direction. Using a servo motor pressure-type single-phase AC spot welder (power supply frequency of 50 Hz), the electrode diameter was set to 6 mm, the welding pressure was set to 4 kN, the welding current was set to 6.0 kA to 9.0 kA, the power-on time was set to 0.4 seconds, and the holding time was set to 0.1 seconds. Welding was performed so that the nugget diameter became 5√t (t: plate thickness). Then, a cross tensile test was performed in accordance with JIS Z 3137:1999 to determine the joint strength.

[0185] If the joint strength exceeds 6.0 kN, the welded joint strength is judged to be excellent.

[0186] Furthermore, the maximum value of the original γ grain size in the heat-affected zone of the weld joint was determined according to the above-mentioned procedure.

[0187] The results are shown in Table 3-1, Table 3-2, Table 4-1, and Table 4-2.

[0188] [Table 1-1]

[0189]

[0190] [Table 1-2]

[0191]

[0192] [Table 2-1]

[0193]

[0194] [Table 2-2]

[0195]

[0196] [Table 3-1]

[0197]

[0198] [Table 3-2]

[0199]

[0200] [Table 4-1]

[0201]

[0202] [Table 4-2]

[0203]

[0204] As shown in Tables 1-1 to 4-2, the examples of the present invention all exhibit excellent formability and a tensile strength of 980 MPa or greater. Furthermore, the welded joints obtained by welding these steel plates exhibited a maximum original γ grain size of less than 30 μm in the heat-affected zone, demonstrating high weld joint strength.

[0205] On the other hand, in the comparative examples in which at least one of the chemical composition, the volume ratio of each phase in the microstructure, the maximum grain size of retained austenite, and the ratio of measurement points where the segregation degree is 1.1 or more in the steel plate does not meet the range of the present invention, one or more of the tensile strength and formability does not meet the target value, or in the welded joint obtained using the steel plate, the maximum value of the original γ grain size is large and the welded joint strength is low.

[0206] Industrial applicability

[0207] The present invention provides a steel plate having excellent formability, a tensile strength of 980 MPa or greater, and sufficient weld joint strength after welding, and a welded joint obtained from the steel plate having sufficient weld joint strength. Therefore, the present invention has high industrial applicability.

[0208] Explanation of symbols

[0209] Observation area of ​​tissue A (100 μm × 100 μm area within the range of t / 8 to 3t / 8 centered at position t / 4)

[0210] B Mn concentration measurement area (a square area with the t / 4 position as the center and a side length of t / 4)

[0211] t Plate thickness

[0212] ONugget Center

[0213] N nugget

[0214] S steel plate (base steel plate)

[0215] SF coincidence surface

Claims

1. A steel plate having the following chemical composition: in mass %: C: 0.05% to 0.25%, Si: 0.05% to 2.00%, Mn: 1.50% to 3.00%, Al：0.005％～1.500％、 P：0％～0.040％、 S：0％～0.010％、 N：0％～0.0100％、 O：0％～0.0060％、 Cr：0％～0.50％、 Ni: 0% to 1.00%, Cu: 0% to 1.00%, Mo: 0% to 0.50%, Ti: 0% to 0.200%, Nb: 0% to 0.200%, V：0％～0.500％、 B：0％～0.0100％、 W：0％～0.1000％、 Ta: 0%~0.1000%, Sn: 0% to 0.0500%, Co: 0% to 0.5000%, Sb: 0% to 0.0500%, As: 0%~0.0500%, Mg: 0% to 0.0500%, Ca: 0% to 0.0400%, Y：0％～0.0500％、 La: 0% to 0.0500%, Ce: 0% to 0.0500%, Zr: 0% to 0.0500%, and The rest: Fe and impurities, When the plate thickness is set to t, the metal structure at the position t / 4, which is t / 4 away from the surface in the cross section in the plate thickness direction, contains, by volume: Ferrite: more than 20%, Bainite and martensite: more than 40% in total, The remainder is one or more selected from retained austenite and pearlite, When the Mn concentration is measured at a plurality of measurement points at intervals of 1 μm in a square region of the plate thickness cross section having a side length of t / 4 and centered at the t / 4 position, the proportion of measurement points having a Mn concentration of 1.1 times or greater relative to the average value of the Mn concentrations at all of the plurality of measurement points is less than 10.0%. The tensile strength of the steel plate is above 980 MPa. The product of tensile strength and total elongation is 10500 MPa·% or more.

2. The steel plate according to claim 1, wherein The chemical composition contains, in mass %, one or more elements selected from the following: Cr：0.01％～0.50％、 Ni: 0.01% to 1.00%, Cu: 0.01% to 1.00%, Mo: 0.01% to 0.50%, Ti: 0.001% to 0.200%, Nb: 0.001% to 0.200%, V：0.001％～0.500％、 B：0.0001％～0.0100％、 W：0.0005％～0.1000％、 Ta: 0.0005%~0.1000%, Sn: 0.0010% to 0.0500%, Co: 0.0010% to 0.5000%, Sb: 0.0010% to 0.0500%, As: 0.0010%~0.0500%, Mg: 0.0001% to 0.0500%, Ca: 0.0001% to 0.0400%, Y：0.0001％～0.0500％、 La: 0.0001% to 0.0500%, Ce: 0.0001% to 0.0500%, and Zr：0.0001％～0.0500％。 The steel sheet according to claim 1 or 2, comprising a hot-dip galvanized layer on the surface.

4. The steel plate according to claim 3, wherein The hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

5. A welded joint obtained by welding the steel plates according to any one of claims 1 to 4, comprising a base metal portion, a nugget portion, and a heat-affected zone. The base material portion excluding the heat-affected zone and the nugget zone has the same chemical composition and metal structure as the steel plate. The maximum value of the original γ grain size in the heat-affected zone is less than 30 μm.

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