Steel sheet, resistance spot welding method, resistance spot welded member, and method for manufacturing steel sheet

Abstract

This invention provides a steel plate that possesses both a tensile strength of over 1600 MPa and high L-shaped tensile strength after resistance welding, as well as excellent resistance to delayed fracture after resistance welding. The steel plate has a specified composition. The microstructure at 1 / 4 of the plate thickness comprises, by volume fraction, 0-5% ferrite, 0-5% retained austenite, 0-7% bainite, and 93% or more martensite. The average grain size of the ferrite is 3 μm or less, the average grain size of the retained austenite is 3 μm or less, the average grain size of the bainite is 5 μm or less, and the average grain size of the martensite is 7 μm or less. The microstructure in a region 7-12 μm from the surface towards the plate thickness comprises, by volume fraction, 30% or more ferrite with an average grain size of 10 μm or less. The average number density of carbides with a grain size of 0.10 μm or more in a region 50-100 μm from the surface towards the plate thickness is 5 carbides / 100 μm. 2 above.

Description

Technical Field

[0001] This invention relates to steel plates, particularly steel plates suitable for use as blanks in structural components of automobiles, etc. Furthermore, this invention relates to a resistance spot welding method using the aforementioned steel plates, resistance spot welding components, and a method for manufacturing the aforementioned steel plates. Background Technology

[0002] In recent years, due to increasing environmental problems, CO2 emission restrictions have become more stringent, and in the automotive industry, lightweighting of vehicle bodies is required to improve fuel efficiency. Therefore, the use of high-strength steel sheets to promote the thinning of automotive components has been promoted in recent years, and steel sheets with high tensile strength (TS) of over 1600 MPa have also been used.

[0003] In automobile assembly, considering cost and efficiency, components made by pressing steel sheets are typically joined using resistance spot welding (hereinafter referred to as resistance welding). Therefore, to ensure collision safety, in addition to increasing the strength of the steel sheet used as the raw material (base steel sheet), it is also necessary to increase the strength of the welded portion.

[0004] Tensile shear strength (TSS) and cross tensile strength (CTS) are widely used indicators of the strength of resistance-welded joints. However, in actual automobile manufacturing, resistance welding is often performed on the flange portions of components. Therefore, to effectively improve the crash safety of automobiles, it is necessary to improve the strength in the L-shape tensile test (LTS), which is suitable for evaluating the peel strength of flange welds.

[0005] Furthermore, for steel sheets used in automotive parts, excellent delayed fracture resistance after resistance welding is required to prevent delayed fracture caused by hydrogen intrusion from the operating environment. However, to achieve high tensile strengths of over 1600 MPa, a large amount of alloying elements needs to be added, but the addition of alloying elements leads to a deterioration of the delayed fracture resistance after resistance welding. Therefore, it is difficult to simultaneously achieve high strength and excellent delayed fracture resistance after resistance welding.

[0006] Various technologies have been proposed to solve these problems.

[0007] For example, Patent Document 1 proposes a method to improve the peeling strength of the weld by inserting an embedded plate of a specific size between the steel plates when multiple steel plates are overlapped and resistance spot welded.

[0008] In addition, Patent Document 2 proposes a method to improve the strength of the welded part by further applying current after the welded part is formed by resistance spot welding.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent Application Publication No. 2020-151756

[0012] Patent Document 2: Japanese Patent Application Publication No. 2022-140236 Summary of the Invention

[0013] According to the methods proposed in Patent Documents 1 and 2, the strength of the welded joint can be improved to a certain extent. However, in Patent Documents 1 and 2, the tensile strength of the steel plates used in the tests was at most around 1500 MPa, and the effect on high-strength steel plates with tensile strengths of 1600 MPa and above is still unclear.

[0014] Furthermore, as mentioned above, the problem of delayed fracture becomes significant in high-strength steel plates with tensile strengths of 1600 MPa or higher, but the problem of delayed fracture was not considered in Patent Documents 1 and 2.

[0015] Thus, in the prior art proposed in patent documents 1 and 2, it is still difficult to simultaneously achieve a tensile strength of over 1600 MPa, excellent L-shaped tensile strength, and resistance to delayed fracture.

[0016] Furthermore, the method in Patent Document 1 suffers from poor productivity because it requires preparing inserts that meet specified conditions and inserting them between the steel plates during welding. Similarly, the method in Patent Document 2 also suffers from poor productivity because it requires post-weld energization. In order to obtain excellent properties without adding additional processes that are the main cause of reduced productivity, it is necessary to improve the resistance weldability of the steel plates themselves.

[0017] The present invention was made in view of the above-mentioned actual situation, and its purpose is to provide a steel plate that has both a tensile strength of more than 1600MPa and a high L-shaped tensile strength after resistance welding, and also has excellent resistance to delayed fracture after resistance welding.

[0018] The inventors conducted research and found that, in addition to controlling the composition of the steel plate, they also controlled the microstructure at the 1 / 4 position of the plate thickness, the microstructure in a region of 7–12 μm from the surface towards the thickness, and the average number density of carbides in a region of 50–100 μm from the surface towards the thickness, thus solving the aforementioned problems. It was found that controlling the volume fraction of ferrite in the region of 7–12 μm from the surface towards the thickness and the average number density of carbides in the region of 50–100 μm from the surface towards the thickness is important for improving the properties after resistance welding. The inventors' insights in achieving the above conclusions will be explained.

[0019] In L-shaped tensile tests, cracks propagate through the molten portion (melt nugget) of the resistance spot weld, leading to fracture. According to the inventors' research, by altering the microstructure of the corona junction surrounding the weld nugget, the tensile strength in L-shaped tensile tests can be improved. Specifically, by softening the corona junction, crack propagation resistance is improved, resulting in high tensile strength in L-shaped tensile tests.

[0020] To achieve the above effects, it is sufficient to form a adequate amount of ferrite in the steel plate before resistance welding, within a region of 7–12 μm from the surface towards the plate thickness. By controlling the microstructure of the steel plate surface in this way, the tensile strength in the L-shaped tensile test after resistance welding can be effectively improved while maintaining a high base metal strength.

[0021] Furthermore, the microstructure changes at the aforementioned corona junction also contribute to improving the resistance to delayed fracture after resistance welding. Therefore, to enhance the resistance to delayed fracture after resistance welding, it is necessary to form a sufficient amount of ferrite in a region extending 7–12 μm from the surface towards the plate thickness.

[0022] Furthermore, to fully improve the resistance to delayed fracture after resistance welding, carbides need to be precipitated at a sufficient number density in a region of 50–100 μm from the surface towards the plate thickness. By fully precipitating the carbon contained in the base material in the form of carbides, the solid solution carbon in the heat-affected zone (HAZ) after resistance welding is reduced, resulting in improved resistance to delayed fracture.

[0023] Based on the above insights, the main structure of this invention is as follows.

[0024] 1. A steel plate, comprising, by mass percent, C: 0.22-0.38%, Si: 0.05-1.35%, Mn: 2.4-3.5%, P: less than 0.02%, S: less than 0.002%, Al: 0.01-0.10%, N: less than 0.008%, B: 0.0002-0.0050%, and at least one selected from Ti: 0.005-0.07%, Nb: 0.005-0.07%, and V: 0.005-0.07%.

[0025] The remainder consists of Fe and unavoidable impurities.

[0026] The microstructure at the 1 / 4 position of the plate thickness, by volume fraction, comprises ferrite: 0–5%, retained austenite: 0–5%, bainite: 0–7%, and martensite: ≥93%, with the average grain size of ferrite being ≤3 μm, the average grain size of retained austenite being ≤3 μm, the average grain size of bainite being ≤5 μm, and the average grain size of martensite being ≤7 μm.

[0027] The microstructure in the region from the surface to the thickness direction of the plate, ranging from 7 to 12 μm, contains ferrite at a volume fraction of more than 30%, and the average grain size of the ferrite is less than 10 μm.

[0028] The average number density of carbides with a particle size of 0.10 μm or larger in the region from the surface to the plate thickness in the 50–100 μm direction is 5 particles / 100 μm. 2 above.

[0029] 2. The steel plate according to claim 1 above, wherein the composition further comprises, by mass %, at least one selected from Sb: less than 0.02%, Cu: less than 0.50%, Ni: less than 0.50%, Cr: less than 0.50%, Mo: less than 0.50%, Sn: less than 0.30%, Ca: less than 0.0050%, and REM: less than 0.0050%.

[0030] 3. The steel plate according to 1 or 2 above, wherein at least one surface has a zinc-based coating.

[0031] 4. A resistance spot welding method, wherein a plate assembly comprising at least one steel plate as described in any one of 1 to 3 above is clamped by a pair of welding electrodes, and pressure is applied while electricity is applied to perform the bonding.

[0032] 5. A resistance spot welding component comprising at least one steel plate as described in any one of 1 to 3 above in a plate assembly.

[0033] 6. A method for manufacturing a steel plate,

[0034] Steel billets are produced by continuous casting of molten steel having the composition described in 1 or 2 above.

[0035] The steel billets were cooled at an average rate of 50°C / h or higher over a temperature range up to 600°C.

[0036] The cooled steel billet is then reheated at a temperature of 1280–1400°C for at least 60 minutes.

[0037] The reheated steel billet is then hot-rolled at a finishing rolling temperature of 850–950°C to produce hot-rolled steel sheet.

[0038] The hot-rolled steel sheet was cooled to a cooling stop temperature below 460°C at an average cooling rate of 80°C / s or higher.

[0039] The cooled hot-rolled steel sheet is then wound at a winding temperature below 460°C.

[0040] The hot-rolled steel sheets after being wound are pickled.

[0041] The above-mentioned pickled hot-rolled steel sheets are heat-treated at a temperature of 300–700°C to produce heat-treated hot-rolled steel sheets.

[0042] The above-mentioned heat-treated hot-rolled steel sheet is cold-rolled to produce cold-rolled steel sheet.

[0043] The above-mentioned cold-rolled steel sheet was annealed at a temperature range of 600–980°C under conditions where the dew point exceeded -15°C.

[0044] In the above annealing, the cold-rolled steel sheet is heated to 650°C at an average heating rate of 12°C / s or higher, and then heated to an annealing temperature of 830-980°C at an average heating rate of less than 12°C / s. The sheet is held at the above annealing temperature for a holding time of 20-360 seconds, and then cooled from the above annealing temperature to room temperature at an average cooling rate of 3°C / s or higher.

[0045] 7. The method for manufacturing a steel plate according to 6 above, wherein the annealed steel plate is further subjected to electroplating to form a zinc-based coating on at least one surface of the steel plate.

[0046] 8. A method for manufacturing a steel plate, comprising the following steps:

[0047] Steel billets are produced by continuous casting of molten steel having the composition described in 1 or 2 above.

[0048] The steel billets were cooled at an average rate of 50°C / h or higher over a temperature range up to 600°C.

[0049] The cooled steel billet is then reheated at a temperature of 1280–1400°C for at least 60 minutes.

[0050] The reheated steel billet is then hot-rolled at a finishing rolling temperature of 850–950°C to produce hot-rolled steel sheet.

[0051] The hot-rolled steel sheet was cooled to a cooling stop temperature below 460°C at an average cooling rate of 80°C / s or higher.

[0052] The cooled hot-rolled steel sheet is then wound at a winding temperature below 460°C.

[0053] The hot-rolled steel sheets after being wound are pickled.

[0054] The above-mentioned pickled hot-rolled steel sheets are heat-treated at a temperature of 300–700°C to produce heat-treated hot-rolled steel sheets.

[0055] The above-mentioned heat-treated hot-rolled steel sheet is cold-rolled to produce cold-rolled steel sheet.

[0056] The above-mentioned cold-rolled steel sheet was annealed at a temperature range of 600–980°C under conditions where the dew point exceeded -15°C.

[0057] The annealed cold-rolled steel sheet is subjected to hot-dip galvanizing to form a zinc-based coating on at least one surface of the steel sheet.

[0058] The hot-dip galvanized steel sheet was cooled to room temperature at an average cooling rate of 3°C / s or higher.

[0059] In the above annealing, the cold-rolled steel sheet is heated to 650°C at an average heating rate of 12°C / s or higher, and then heated to an annealing temperature of 830-980°C at an average heating rate of less than 12°C / s. The sheet is held at the above annealing temperature for 20-360 seconds, and then cooled from the above annealing temperature at an average cooling rate of 3°C / s or higher until it is immersed in a hot-dip galvanizing bath.

[0060] 9. The method for manufacturing the steel plate according to 8 above, wherein, after the hot-dip galvanizing and before the cooling to room temperature, a further alloying treatment is performed.

[0061] According to the present invention, a steel plate possessing a tensile strength of over 1600 MPa, high L-shaped tensile strength after resistance welding, and excellent resistance to delayed fracture after resistance welding can be provided. It should be noted that, in this specification, the aforementioned L-shaped tensile strength and resistance to delayed fracture are sometimes collectively referred to as resistance weldability. Detailed Implementation

[0062] The embodiments of the present invention will now be described. It should be noted that the following description illustrates a preferred embodiment of the present invention and is not intended to limit the scope of the invention in any way. Furthermore, unless otherwise specified, the unit of content, "%", represents "mass %".

[0063] [Composition]

[0064] The steel plate of the present invention has the above-described composition. The reasons for limiting the composition will be explained below.

[0065] C: 0.22–0.38%

[0066] Carbon (C) is an effective element for increasing the strength of steel plates and also contributes to the formation of martensite in this invention. Furthermore, C is also a component of carbides, one of the important elements in this invention. If the C content is less than 0.22%, the required strength and martensite volume fraction cannot be ensured. Therefore, the C content is 0.22% or more, preferably 0.23% or more, and more preferably 0.24% or more. On the other hand, if the C content is too high, the toughness of the weld nugget after resistance welding decreases, resulting in a decrease in L-shaped tensile strength. Therefore, the C content is 0.38% or less, preferably 0.34% or less, and more preferably 0.33% or less.

[0067] Si: 0.05–1.35%

[0068] Si is an element that improves resistance weldability. This is because the addition of Si mitigates the segregation of Mn, thereby mitigating the hardness deviation in the thickness direction of the steel plate. To achieve this effect, the Si content is 0.05% or more, preferably 0.15% or more, and more preferably 0.25% or more. On the other hand, excessive addition of Si can cause liquid metal embrittlement during resistance spot welding. Therefore, the Si content is 1.35% or less, preferably 1.25% or less, and more preferably 1.15% or less.

[0069] Mn: 2.4–3.5%

[0070] Mn is an element that improves the strength of steel plates through stabilization via martensite formation and solid solution strengthening. Furthermore, Mn stabilizes austenite and is essential for ensuring a sufficient volume fraction of martensite. To achieve these effects, the Mn content is 2.4% or higher. On the other hand, excessive Mn content reduces the toughness of the weld nugget after spot welding, resulting in a decrease in L-shaped tensile strength. Moreover, excessive Mn content increases grain boundary slip constraint when hydrogen enters the steel plate, making it easier for cracks at grain boundaries to propagate. Consequently, the resistance to delayed fracture after resistance welding is reduced. Therefore, the Mn content is 3.5% or less, preferably 3.2% or less.

[0071] P: below 0.02%

[0072] If phosphorus (P) is excessive, its segregation towards grain boundaries becomes significant, leading to grain boundary embrittlement. Consequently, resistance weldability decreases. Therefore, the P content is 0.02% or less, preferably 0.015% or less, and more preferably 0.012% or less. On the other hand, the lower limit of the P content is not particularly limited and can be 0%. However, excessive reduction will increase steelmaking costs. Therefore, the P content is preferably 0.002% or more.

[0073] S: below 0.002%

[0074] If sulfur (S) is excessive, resistance weldability decreases. This is because as the S content increases, the formation of sulfides such as MnS increases, and cracks form from these sulfides when hydrogen enters. Therefore, the S content is 0.002% or less, preferably 0.0015% or less, and more preferably 0.0012% or less. On the other hand, the lower limit of the S content is not particularly limited and can be 0%. However, excessive reduction will increase steelmaking costs. Therefore, the S content is preferably 0.0002% or more.

[0075] Al: 0.01–0.10%

[0076] Al is an element required for deoxidation. If the Al content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Al content is 0.01% or more, preferably 0.02%. On the other hand, if the Al content is higher than 0.10%, an excessive amount of ferrite phase is formed during annealing, making it difficult to ensure strength. Therefore, the Al content is 0.10% or less, preferably 0.08% or less, and more preferably 0.05% or less.

[0077] N: below 0.008%

[0078] If nitrogen (N) is excessive, resistance weldability decreases. This is because as the N content increases, the formation of coarse nitrides increases, and cracks form from these nitrides when hydrogen enters. This trend becomes significant when the N content is 0.008% or higher. Therefore, the N content is preferably 0.008% or lower, more preferably 0.007% or lower, and more preferably 0.005% or lower. On the other hand, the lower limit of the N content is not particularly limited and can be 0%. However, excessive reduction will increase steelmaking costs. Therefore, the N content is preferably 0.0005% or higher, more preferably 0.001% or higher.

[0079] B: 0.0002~0.0050%

[0080] Boron (B) is an element that contributes to high strength by improving hardenability and generating martensite. Furthermore, B improves hardenability without lowering the martensitic transformation initiation point, thus being useful for carbide formation. To obtain these effects, the B content is 0.0002% or more, preferably 0.0004% or more. On the other hand, if the B content exceeds 0.0050%, the effect saturates; therefore, the B content is 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0035% or less.

[0081] The steel plate of the present invention contains at least one selected from Ti, Nb, and V in the following amounts. Ti, Nb, and V are all elements that have the common function of improving resistance weldability by forming fine carbides.

[0082] Ti: 0.005~0.07%

[0083] Ti improves the resistance to delayed fracture after resistance welding by forming fine carbides. Furthermore, Ti also improves the resistance to delayed fracture after resistance welding by increasing the hydrogen overvoltage. To achieve these effects, the Ti content is 0.005% or more, preferably 0.008% or more. On the other hand, if a large amount of Ti is added, the elongation decreases significantly. Therefore, the Ti content is 0.07% or less, preferably 0.05% or less.

[0084] Nb: 0.005–0.07%

[0085] Nitrogen (Nb) improves the resistance to delayed fracture of resistance spot welds by forming fine carbides. To achieve this effect, the Nb content is 0.005% or more, preferably 0.01% or more. On the other hand, excessive Nb not only significantly reduces elongation but also causes slab cracking after continuous casting. Therefore, the Nb content is 0.07% or less, preferably 0.05% or less.

[0086] V: 0.005~0.07%

[0087] V improves the delayed fracture resistance of resistance spot welds by forming fine carbides. To achieve this effect, the V content is 0.005% or more when V is added. However, even with a large amount of V added, the strength increase is small for portions exceeding 0.07%, and the alloy cost also increases. Therefore, the V content is 0.07% or less, preferably 0.06% or less, and more preferably 0.05% or less.

[0088] In one embodiment of the present invention, the steel plate has a composition containing the above-mentioned components and the remainder consisting of Fe and unavoidable impurities.

[0089] It should be noted that, for example, Co, Zn, Ta, Mg, and Zr are unavoidable impurities. When Co is present as an unavoidable impurity, the Co content is preferably 0.10% or less. When Zn is present as an unavoidable impurity, the Zn content is preferably 0.10% or less. When Ta is present as an unavoidable impurity, the Ta content is preferably 0.10% or less. When Mg is present as an unavoidable impurity, the Mg content is preferably 0.10% or less. When Zr is present as an unavoidable impurity, the Zr content is preferably 0.10% or less.

[0090] In addition to the components described above, the steel plate in other embodiments of the present invention may further contain at least one of the following components in any further arbitrary manner. It should be noted that since the following elements are all elements that can be added arbitrarily, they are not mandatory. Therefore, the lower limit of their content can be 0%.

[0091] Sb: below 0.02%

[0092] Sb is an element that strengthens grain boundaries through grain boundary segregation. Adding Sb can further improve cross tensile strength. However, if the Sb content is higher than 0.02%, the formation of the ferrite phase in the surface layer of the steel sheet is suppressed, making it impossible to achieve the desired microstructure in the surface layer. Therefore, when adding Sb, the Sb content is preferably 0.02% or less, and more preferably 0.015% or less. On the other hand, there is no particular limitation on the lower limit of the Sb content, but from the viewpoint of improving the effect of Sb addition, the Sb content is preferably 0.001% or more, and more preferably 0.002% or more.

[0093] Cu: less than 0.50%

[0094] Cu is an element that improves hydrogen overvoltage and, consequently, enhances the resistance to delayed fracture after resistance welding. However, if the Cu content exceeds 0.50%, surface defects are easily generated in addition to the effect saturation. Therefore, when Cu is added, the Cu content is 0.50% or less. On the other hand, from the viewpoint of improving the effect of Cu addition, it is preferable to make the Cu content 0.005% or more.

[0095] Ni: below 0.50%

[0096] Like Cu, Ni is an element that improves hydrogen overvoltage and further enhances resistance to delayed fracture. Additionally, when added together with Cu, Ni helps suppress surface defects caused by Cu. However, the effect saturates if the Ni content exceeds 0.50%. Therefore, when adding Ni, the Ni content is 0.50% or less. On the other hand, from the viewpoint of maximizing the effect of Ni addition, it is preferable to have a Ni content of 0.005% or more.

[0097] Cr: less than 0.50%

[0098] Cr is an element that contributes to further high strength by forming a hard phase. However, if the Cr content exceeds 0.50%, surface defects are easily generated. Therefore, when Cr is added, the Cr content is 0.50% or less, preferably 0.45% or less. On the other hand, from the viewpoint of improving the effect of Cr addition, it is preferable to make the Cr content 0.02% or more, more preferably 0.05% or more.

[0099] Mo: 0.50% or less

[0100] Like Cr, Mo contributes to further strength enhancement by forming a hard phase. Additionally, some Mo also contributes to further strength enhancement by forming carbides. However, if the Mo content exceeds 0.50%, the effect saturates, and the effect cannot be matched by the increased cost. Therefore, when adding Mo, the Mo content is 0.50% or less, preferably 0.45% or less. On the other hand, from the viewpoint of improving the effect of Mo addition, it is preferable to have a Mo content of 0.02% or more, more preferably 0.05% or more.

[0101] Sn: below 0.30%

[0102] Sn is an element that increases the hydrogen overvoltage of steel sheets and thereby further improves their resistance to delayed fracture. However, if the Sn content exceeds 0.30%, in addition to effect saturation, ductility also decreases. Therefore, when adding Sn, the Sn content is 0.30% or less, preferably 0.25% or less. On the other hand, from the viewpoint of improving the effect of Sn addition, it is preferable to make the Sn content 0.005% or more, more preferably 0.01% or more.

[0103] Ca: below 0.0050%

[0104] Ca is an element that helps to further improve the delayed fracture resistance after resistance welding by making the sulfide shape spherical. However, if the Ca content exceeds 0.0050%, the effect saturates. Therefore, when adding Ca, the Ca content is kept below 0.0050%. On the other hand, from the viewpoint of improving the effect of Ca addition, it is preferable to keep the Ca content above 0.0005%.

[0105] REM: below 0.0050%

[0106] Like Ca, rare earth metals (REMs) further enhance the delayed fracture resistance after resistance welding by spherizing sulfides. However, the effect saturates if the REM content exceeds 0.0050%, therefore, the REM content is kept below 0.0050% when adding REM. On the other hand, from the viewpoint of maximizing the effect of REM addition, the REM content is preferably 0.0005% or more.

[0107] [Micro-organization]

[0108] In the steel plate of the present invention, the microstructure at the 1 / 4 thickness position and the microstructure in the region 7-12 μm from the surface of the steel plate in the thickness direction need to meet specific conditions respectively. The reasons for this are explained below. It should be noted that "1 / 4 thickness position" refers to the position at a depth of 1 / 4 of the thickness t of the steel plate from the surface of the steel plate, and is sometimes expressed as 1 / 4t position.

[0109] It should be noted that in this invention, the term "tempered martensite" is also included within the term "martensite." This is because it is difficult to distinguish between martensite and tempered martensite in the microstructure of the steel sheet of this invention. Furthermore, the term "tempered martensite" includes not only tempered martensite generated through self-tempering during the cooling process of annealing, but also tempered martensite generated through tempering after cooling to room temperature.

[0110] (Microstructure at 1 / 4 of the plate thickness)

[0111] Ferrite: 0–5%

[0112] If the volume fraction of ferrite at the 1 / 4 position of the plate thickness is higher than 5%, the desired tensile strength cannot be obtained. Therefore, the volume fraction of ferrite at the 1 / 4 position of the plate thickness is 5% or less, preferably 3% or less, and more preferably 1% or less. On the other hand, from the viewpoint of strength, the lower the volume fraction of ferrite, the better; therefore, the lower limit of the volume fraction of ferrite is 0%.

[0113] Average crystal grain size of ferrite: less than 3 μm

[0114] If the average grain size of ferrite at the 1 / 4 position of the plate thickness is greater than 3 μm, the resistance to delayed fracture deteriorates. This is because, after resistance welding, cracks due to hydrogen embrittlement easily form at the interface between ferrite and martensite from the softened HAZ to the base material. Therefore, the average grain size of ferrite at the 1 / 4 position of the plate thickness is 3 μm or less, preferably 2.5 μm or less. On the other hand, from the viewpoint of resistance to delayed fracture, the smaller the average grain size of ferrite, the better; therefore, the lower limit of the above-mentioned average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, it is preferable that the average grain size of ferrite is 0.2 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more.

[0115] Residual austenite: 0–5%

[0116] If the volume fraction of retained austenite at the 1 / 4 position of the plate thickness exceeds 5%, the resistance to delayed fracture deteriorates. This is because if the steel plate is cold-pressed, the retained austenite transforms into martensite with a high dislocation density, making it prone to hydrogen embrittlement cracking from the softened HAZ region to the base metal after resistance welding. Therefore, the volume fraction of retained austenite at the 1 / 4 position of the plate thickness is 5% or less, preferably 4% or less. On the other hand, from the viewpoint of resistance to delayed fracture, the lower the volume fraction of retained austenite, the better; therefore, the lower limit for the volume fraction of retained austenite is 0%.

[0117] Average grain size of retained austenite: less than 3 μm

[0118] If the average grain size of the retained austenite at the 1 / 4 position of the plate thickness is greater than 3 μm, the resistance to delayed fracture deteriorates. This is because martensite is easily generated during cold pressing due to the influence of the carbon distribution within the retained austenite. Therefore, the average grain size of the retained austenite at the 1 / 4 position of the plate thickness is 3 μm or less. On the other hand, there is no particular limitation on the lower limit of the above-mentioned average grain size. However, if the above-mentioned average grain size is 0.3 μm, it contributes significantly to the elongation; therefore, it is preferable that the above-mentioned average grain size is 0.3 μm or more, more preferably 1 μm or more, and even more preferably 2 μm or more.

[0119] Bainite: 0~7%

[0120] If the volume fraction of bainite at the 1 / 4 position of the plate thickness is higher than 7%, the desired tensile strength cannot be obtained. Therefore, the volume fraction of bainite at the 1 / 4 position of the plate thickness is 7% or less, preferably 5% or less. On the other hand, from the viewpoint of strength, the lower the volume fraction of bainite, the better; therefore, the lower limit of the volume fraction of bainite is 0%.

[0121] Average grain size of bainite: less than 5 μm

[0122] If the average grain size of bainite at the 1 / 4 position of the plate thickness is greater than 5 μm, the resistance to delayed fracture deteriorates. This is because, after resistance welding, cracks due to hydrogen embrittlement easily form at the interface between bainite and martensite from the softened HAZ to the base material. Therefore, the average grain size of bainite at the 1 / 4 position of the plate thickness is 5 μm or less, preferably 4 μm or less. On the other hand, from the viewpoint of resistance to delayed fracture, the smaller the average grain size of bainite, the better; therefore, the lower limit of the above-mentioned average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, it is preferable that the average grain size of bainite is 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more.

[0123] Martensite: 93% or more

[0124] To ensure the desired tensile strength, the volume fraction of martensite at the 1 / 4 mark of the plate thickness needs to be 93% or more. Therefore, the volume fraction of martensite at the 1 / 4 mark of the plate thickness is 93% or more, preferably 95% or more. On the other hand, there is no particular upper limit to the volume fraction of martensite at the 1 / 4 mark of the plate thickness, and it can be 100%.

[0125] Average martensite grain size: below 7 μm

[0126] If the average grain size of martensite at the 1 / 4 position of the plate thickness is greater than 7 μm, the grains will coarsen after resistance welding, thus reducing the L-shaped tensile strength. Therefore, the average grain size of martensite at the 1 / 4 position of the plate thickness is 7 μm or less, preferably 6 μm or less. On the other hand, from the viewpoint of L-shaped tensile strength, the smaller the average grain size of martensite, the better; therefore, the lower limit of the above-mentioned average grain size is not particularly limited. However, from the viewpoint of ease of manufacturing, it is preferable that the average grain size of martensite is 1 μm or more, more preferably 3 μm or more, and even more preferably 4 μm or more.

[0127] In one embodiment of the present invention, the microstructure at the 1 / 4 position of the plate thickness can be composed of ferrite: 0-5%, retained austenite: 0-5%, bainite: 0-7%, and martensite: 93% or more in volume fraction.

[0128] Furthermore, the aforementioned microstructure may further contain other microstructures as desired. Here, other microstructures refer to microstructures other than ferrite, retained austenite, bainite, and martensite. For example, these other microstructures may be pearlite. The volume fraction of these other microstructures may be 7% or less, preferably 3% or less.

[0129] That is, in one embodiment of the present invention, the microstructure at the 1 / 4 position of the plate thickness can be composed of ferrite: 0-5%, retained austenite: 0-5%, bainite: 0-7%, martensite: more than 93%, and other structures: 0-7% in volume fraction.

[0130] (Microstructure of the region from the surface to the thickness direction of the plate, 7–12 μm)

[0131] Ferrite: 30% or more

[0132] If the volume fraction of ferrite in the region 7–12 μm from the surface of the steel plate towards the thickness direction is less than 30%, the desired L-shaped tensile strength and resistance to delayed fracture characteristics cannot be obtained. This is believed to be because, as mentioned above, by pre-forming a sufficient amount of ferrite in the aforementioned region, the corona joint after resistance welding softens, resulting in improved resistance to crack propagation. Therefore, the volume fraction of ferrite in the region 7–12 μm from the surface of the steel plate towards the thickness direction is 30% or more, preferably 45% or more, and more preferably 60% or more. On the other hand, there is no particular upper limit to the volume fraction of ferrite, and it can be 100%. For example, the volume fraction of ferrite can be 90% or less, or 85% or less.

[0133] Average crystal grain size of ferrite: less than 10 μm

[0134] If the average grain size of ferrite in the region of 7–12 μm from the surface of the steel plate towards the thickness direction is greater than 10 μm, the microstructure of the corona joint after resistance welding becomes coarse, thus deteriorating the resistance to delayed fracture. Therefore, the average grain size of ferrite in the region of 7–12 μm from the surface of the steel plate towards the thickness direction is 10 μm or less, preferably 9 μm or less, and more preferably 8 μm or less. On the other hand, from the viewpoint of resistance to delayed fracture, the smaller the average grain size of ferrite, the better; therefore, the lower limit of the above-mentioned average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, it is preferable that the average grain size of ferrite is 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more.

[0135] Furthermore, the aforementioned microstructure may contain other structures. Here, other structures refer to structures other than ferrite, such as bainite, pearlite, retained austenite, martensite, and cementite. From the viewpoint of further improving tensile strength, it is preferable that at least 50% or more of the remaining portion other than ferrite is bainite and martensite. In other words, the total volume fraction of bainite and martensite in the remaining portion of the microstructure other than ferrite is preferably 50% or more. The upper limit of the total volume fraction of bainite and martensite is not particularly limited and can be 100%. In other words, in one embodiment of the present invention, the microstructure of the region from the surface to the plate thickness direction of 7 to 12 μm may be a microstructure consisting of ferrite with a volume fraction of 30% or more and one or both of the remaining portion of bainite and martensite.

[0136] [carbide]

[0137] Average number density: 5 per 100 μm 2 above

[0138] If the average number density of carbides with a particle size of 0.10 μm or larger in the region of 50–100 μm from the surface of the steel plate towards the thickness is less than 100 μm 2 If there are 5 cross-sections, the resistance welding's delayed fracture resistance deteriorates. This is because the carbides with a particle size of 0.10 μm or larger in the aforementioned region function as hydrogen traps, improving the hydrogen embrittlement resistance of the weld nugget and HAZ. Therefore, the average number density of carbides with a particle size of 0.10 μm or larger in the region from the surface of the steel plate to the thickness direction of 50–100 μm should be 5 particles / 100 μm. 2 The above is preferred, with 7 per 100μm. 2 More preferably, 10 per 100μm. 2 That's all. On the other hand, from the viewpoint of resistance to delayed fracture, the higher the average number density mentioned above, the better; therefore, there is no particular upper limit to the average number density. For example, the average number density could be 40 cells / 100 μm. 2 The following can also be 30 per 100μm 2 the following.

[0139] Here, the types of carbides are not particularly limited, but the steel plate of the present invention, as described above, contains at least one selected from Ti, Nb, and V. These elements readily form carbides, so in the steel plate of the present invention, in addition to Fe-based carbides (cementite), at least one of Ti-based carbides, Nb-based carbides, and V-based carbides may be included as the aforementioned carbides. It should be noted that the average number density of the aforementioned carbides can be determined by TEM (transmission electron microscopy) and EDS (Energy Dispersive X-ray Spectroscopy), and more specifically, by the methods described in the examples.

[0140] [Zinc-based coating]

[0141] The steel sheet of the present invention may be a cold-rolled steel sheet without a coating on the surface, but preferably has a zinc coating on at least one surface.

[0142] As the zinc-based coating described above, either a zinc coating or a zinc alloy coating can be used. In other words, the steel sheet of the present invention can be a galvanized steel sheet or a galvanized alloy steel sheet. As the zinc alloy coating described above, there is no particular limitation, and a coating composed of any zinc alloy can be used. As the zinc alloy coating described above, it is preferable to use a zinc alloy coating having a composition selected from Zn-Al, Zn-Al-Mg, Zn-Al-Si, Zn-Al-Mg-Si, and Zn-Al-Mg-Ni.

[0143] The zinc-based coating described above can be formed by any method. For example, the zinc-based coating can be any one of hot-dip galvanized coating, alloyed hot-dip galvanized coating, and electro-galvanized coating. In other words, the steel sheet of the present invention can be any one of hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and electro-galvanized steel sheet.

[0144] There is no particular limitation on the amount of zinc-based coating applied, but from the viewpoint of corrosion resistance and ease of coating control, the preferred coating amount is 25 g / m² per single side of the steel plate. 2 That's all. On the other hand, from the viewpoint of coating adhesion, the aforementioned adhesion amount is preferably 80 g / m² per single side of the steel sheet. 2 the following.

[0145] [Pre-plating]

[0146] When the steel plate has a zinc-based coating, a pre-coating layer can be further provided between the steel plate (base steel plate) and the zinc-based coating.

[0147] There are no particular limitations on the pre-plating layer mentioned above; any plating layer of any composition can be used, but it is preferred to be an Fe-based plating layer, and more preferably an Fe-based electroplating layer.

[0148] The aforementioned Fe-based coating can be, for example, an Fe coating or an Fe alloy coating. Here, an Fe coating refers to a coating composed of Fe and unavoidable impurities, also known as a "pure Fe coating." On the other hand, there is no particular limitation as to the aforementioned Fe alloy coating, and a coating composed of any Fe alloy can be used. The aforementioned Fe alloy coating can, for example, be a coating composed of at least one alloy selected from Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy.

[0149] In one embodiment of the present invention, the Fe-based electroplating layer preferably has the following composition: comprising a total of 10% or less of at least one selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder consisting of Fe and unavoidable impurities. By ensuring that the total amount of elements other than Fe is 10% or less, it is possible to prevent a decrease in electrolysis efficiency and form an Fe-based electroplating layer at low cost.

[0150] Fe-based coatings function as a soft layer, thus mitigating stress applied to the steel plate surface during welding. Furthermore, the presence of an Fe-based coating reduces residual stress in the resistance weld area and allows diffusible hydrogen to escape efficiently from the steel plate surface, thereby improving resistance to delayed fracture.

[0151] There is no particular limitation on the amount of Fe-based coating, but from the viewpoint of improving the above-mentioned effect, it is preferable to use 0.5 g / m² per single side of the steel plate. 2 The above, more preferably 1.0 g / m 2 That's all. On the other hand, from a cost perspective, the preferred adhesion amount of the Fe-based coating per single side is 60 g / m. 2 The following is more preferably 50g / m 2 The following is a further preferred value: 40 g / m 2 The preferred value is 30g / m 2 the following.

[0152] It should be noted that the adhesion amount of the Fe-based coating was determined as follows. A 10×15mm sample was taken from the steel plate after the zinc-based coating was formed and embedded in resin to create a cross-section of the embedded sample. The thickness of the Fe-based coating was determined by observing any three locations on this cross-section using a scanning electron microscope (SEM). An accelerating voltage of 15kV was sufficient for the SEM observation. Furthermore, the magnification during the SEM observation ranged from 2000 to 10000 times depending on the thickness of the Fe-based coating. The adhesion amount of the Fe-based coating per side was calculated by multiplying the average thickness measured at the three locations by the specific gravity of iron.

[0153] By satisfying the above conditions, the steel plate of the present invention possesses both a tensile strength of over 1600 MPa and a high L-shaped tensile strength after resistance welding, resulting in excellent resistance to delayed fracture. More specifically, a high L-shaped tensile strength of over 1.5 kN after resistance spot welding can be achieved. Furthermore, even if the resistance spot welded joint made using the steel plate of the present invention is subjected to hydrogen purging following the following treatment, delayed fracture will not occur.

[0154] • Let stand in the atmosphere at room temperature (20℃) for 24 hours.

[0155] • Immersed in a 3% NaCl + 0.5% NH4SCN aqueous solution, with a current density of 0.06 mA / cm² 2 Time: 72 hours for cathode electrolysis and hydrogen charging.

[0156] [Spot welding method]

[0157] In one embodiment of the spot welding method of the present invention, a plate assembly comprising at least one of the aforementioned steel plates is clamped by a pair of welding electrodes, and joined by applying pressure and electricity simultaneously. The conditions for spot welding are not particularly limited, and general welding conditions can be used.

[0158] For example, two steel plates are overlapped to form a plate assembly. Then, the plate assembly is clamped from above and below using a pair of welding electrodes, and while applying pressure and controlling the welding conditions to a specified level, an electric current is applied. This allows the steel plates constituting the plate assembly to be joined together to form a resistance spot-welded component. It should be noted that when forming a plate assembly by overlapping cold-rolled steel plates and galvanized steel plates, multiple steel plates can be overlapped with the zinc-coated side of the galvanized steel plate facing the cold-rolled steel plate.

[0159] In one embodiment of the present invention, the resistance spot welding method may include a main energizing process in which a weld nugget is formed by applying pressure and energizing simultaneously using the aforementioned pair of welding electrode clamping plates.

[0160] The energizing and pressurizing conditions for forming the melt nugget in the above-mentioned main energizing process are not particularly limited. From the viewpoint of structural components used in automobiles and the like, the energizing and pressurizing conditions are preferably adjusted to the following ranges.

[0161] For example, to obtain a stable weld nugget diameter, the current value of the main energizing process can preferably be 3.0 to 15.0 kA. The weld nugget diameter used in spot welding of automotive steel sheets is typically 3.0√t to 6.0√t (where t is the thickness of the thinnest sheet in the group). If the current value is too low, the target weld nugget diameter cannot be consistently obtained. On the other hand, if the current value of the main energizing process increases and deviates from the above range, the weld nugget diameter may become too large, or the degree of melting of the steel sheet may increase, causing the molten weld to overflow from between the plates in the form of spatter, resulting in a smaller weld nugget diameter.

[0162] The energizing time of the main energizing process is preferably 0.18 to 1.0 s. This is the same as the current value of the main energizing process, representing the time required to obtain the target weld nugget diameter. If the energizing time of the main energizing process is less than 0.18 s, it is difficult to generate a weld nugget. On the other hand, if the energizing time of the main energizing process exceeds 1.0 s, the weld nugget diameter may become larger, raising concerns about reduced workability. However, as long as the necessary weld nugget diameter can be obtained, the energizing time tw of the main energizing process can be shorter or longer than the above-mentioned preferred range.

[0163] The pressure applied during the main energizing process is preferably between 2.0 kN and 9.0 kN. If the pressure applied during the main energizing process is too high, the energizing diameter will increase, making it difficult to ensure the weld nugget diameter. On the other hand, if the pressure applied during the main energizing process is too low, the energizing diameter will decrease, making splashing more likely. Therefore, the pressure F applied during the main energizing process is preferably within the above-mentioned preferred range. It should be noted that the above-mentioned pressure is sometimes limited by the device capability. However, as long as the pressure can achieve the necessary weld nugget diameter, the pressure F applied during the main energizing process can be lower or higher than the above-mentioned preferred range.

[0164] It should be noted that in the resistance spot welding method of one embodiment of the present invention, a subsequent energization can be performed after the main energization process described above. The subsequent energization is not particularly limited and can be performed under any conditions, but the current value in the subsequent energization is preferably higher than the current value in the main energization process. Specifically, it is preferably at least 1.1 times the current value in the main energization process. Furthermore, the welding time in the subsequent energization is preferably 1.0 second or less. The subsequent energization can be performed in multiple stages; in this case, it is preferable that the total energization time in the subsequent energization is 1.0 second or less.

[0165] In addition, a tempering process for tempering the periphery of the molten core can be performed after the main energizing process. The conditions for the tempering process are not particularly limited, but the current value in the tempering process is preferably lower than the current value in the main energizing process; specifically, it is preferably 0.9 times or less than the current value in the main energizing process. Furthermore, the energizing time in the tempering process is preferably 2.0 seconds or less.

[0166] [Resistance spot welded components]

[0167] In one embodiment of the present invention, the resistance spot welding component is a resistance spot welding component comprising at least one of the aforementioned steel plates in a plate assembly. As described above, the resistance spot welding component can be manufactured using conventional resistance spot welding methods.

[0168] [Methods for manufacturing steel plates]

[0169] Next, the manufacturing method of the steel sheet of the present invention will be described. As described above, the steel sheet of the present invention can be a cold-rolled steel sheet without a coating on its surface, or a galvanized steel sheet with a zinc coating on its surface. Moreover, the aforementioned galvanized steel sheet can be any one of electroplated steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet. Therefore, preferred manufacturing methods will be described below for various situations.

[0170] • First Implementation Method

[0171] In the first embodiment of the present invention, molten steel having the above-described composition is used as the starting material, and the following steps are performed sequentially, thereby enabling the manufacture of a steel sheet that meets the above conditions. It should be noted that if plating is not performed after annealing, a steel sheet (cold-rolled steel sheet) without a coating on its surface can be obtained.

[0172] (1) Continuous casting

[0173] (2) Cooling

[0174] (3) Reheating

[0175] (4) Hot rolling

[0176] (5) Cooling

[0177] (6) Winding

[0178] (7) Pickling

[0179] (8) Heat treatment

[0180] (9) Cold rolling

[0181] (10) Annealing

[0182] (1) Continuous casting

[0183] First, molten steel with the above-mentioned composition is continuously cast to produce steel billets. Continuous casting offers higher production efficiency compared to mold casting. Any continuous casting machine can be used for this process, but a vertical bending type continuous casting machine is preferred. Vertical bending type continuous casting machines offer an excellent balance between equipment cost and the surface quality of the resulting steel billets. Furthermore, vertical bending type continuous casting machines also exhibit excellent surface crack suppression.

[0184] (2) Cooling

[0185] • Average cooling rate: 50℃ / h or higher

[0186] Next, the steel billet obtained by the continuous casting process described above is cooled. During this cooling process, if the average cooling rate over the temperature range up to 600°C is less than 50°C / h, Mn segregation is promoted, thus deteriorating the delayed fracture resistance after resistance welding. Therefore, the steel billet is cooled under the condition that the average cooling rate over the temperature range up to 600°C is 50°C / h or higher.

[0187] In this invention, as described above, the average cooling rate only needs to be controlled within the temperature range up to 600°C, and the cooling stop temperature is not particularly limited. In other words, it can be cooled to any temperature below 600°C. For example, hot rolling can be performed by reheating after cooling to room temperature, or cooling can be stopped at a temperature above room temperature to form a warm sheet, and hot rolling can be performed by reheating from that state.

[0188] (3) Reheating

[0189] Next, the cooled steel billet is reheated. Reheating allows the Ti-based, Nb-based, and V-based precipitates contained in the steel to be dissolved again.

[0190] Heating temperature: 1280~1400℃

[0191] If the heating temperature during the reheating process is less than 1280°C, the precipitates cannot be fully dissolved, and coarse precipitates remain after final annealing. As a result, the resistance welding performance deteriorates. Therefore, the heating temperature is set to 1280°C or higher. On the other hand, if the heating temperature is higher than 1400°C, the grains become coarse. As a result, the desired grain size cannot be obtained after final annealing, and the resistance weldability decreases. Therefore, the heating temperature is set to 1400°C or lower, preferably 1350°C or lower.

[0192] • Duration: 60 minutes or more

[0193] If the holding time during the reheating process is less than 60 minutes, the precipitates cannot be fully re-dissolved, and large precipitates remain after the final annealing. As a result, resistance solderability decreases. Therefore, the holding time is 60 minutes or more. On the other hand, there is no particular upper limit to the holding time, but from a productivity point of view, it is preferable to be 180 minutes or less, and more preferably 150 minutes or less.

[0194] (4) Hot rolling

[0195] Next, the reheated steel billet is hot-rolled to produce a hot-rolled steel sheet. During this hot rolling process, the microstructure of the steel sheet is homogenized, reducing material anisotropy, thereby improving the L-shaped tensile strength after resistance welding.

[0196] Finishing rolling temperature: 850~950℃

[0197] To achieve the aforementioned effects, hot rolling must be completed within the austenitic single-phase region. Therefore, the finishing rolling temperature is above 850°C. On the other hand, if the finishing rolling temperature exceeds 950°C, the microstructure of the hot-rolled steel sheet becomes coarse, and the resistance to delayed fracture after resistance welding deteriorates. Therefore, the finishing rolling temperature is below 950°C.

[0198] (5) Cooling

[0199] Next, the hot-rolled steel sheet is cooled. The microstructure of the hot-rolled steel sheet is controlled by rapidly cooling it to a temperature range where bainitic transformation occurs but ferrite transformation does not. By controlling this homogenized hot-rolled microstructure, the microstructure of the final steel sheet is refined, primarily consisting of ferrite and martensite.

[0200] • Average cooling rate: 80℃ / s or higher

[0201] To achieve the above-mentioned effect, the average cooling rate should be 80°C / s or higher. If the average cooling rate is less than 80°C / s, a ferrite phase transformation begins, resulting in an uneven microstructure and reduced resistance weldability. On the other hand, there is no particular upper limit to the above-mentioned average cooling rate, but it is preferably 200°C / s or lower.

[0202] Cooling stop temperature: below 460℃

[0203] Similarly, to achieve the above-mentioned effects, the cooling stop temperature should be below 460°C. If the cooling stop temperature is above 460°C, excessive pearlite will form, resulting in an uneven microstructure of the hot-rolled steel sheet and reduced resistance weldability. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, but it is preferably above 250°C.

[0204] (6) Winding

[0205] • Winding temperature: below 460℃

[0206] Next, the cooled hot-rolled steel sheet is wound at a winding temperature of 460°C or below. If the winding temperature is higher than 460°C, excessive pearlite is formed, resulting in an uneven microstructure of the hot-rolled steel sheet and thus reduced resistance weldability. Therefore, the winding temperature is 460°C or below, preferably 440°C or below. There is no particular limitation on the lower limit of the winding temperature, but if the winding temperature is too low, excessive hard martensite is formed, increasing the cold rolling load. Therefore, the winding temperature is preferably 250°C or above.

[0207] (7) Pickling

[0208] The hot-rolled steel sheet after winding is then pickled. Pickling removes the oxide scale formed on the surface of the hot-rolled steel sheet. The pickling conditions are not particularly limited and can be performed using conventional methods.

[0209] (8) Heat treatment

[0210] • Heat treatment temperature: 300~700℃

[0211] Next, the pickled hot-rolled steel sheet is heat-treated at a temperature of 300–700°C to produce a heat-treated hot-rolled steel sheet. This heat treatment ensures a good precipitation state of carbides in the final steel sheet, improving its resistance to delayed fracture after resistance welding.

[0212] If the heat treatment temperature is below 300°C, carbides will not be fully precipitated, thus failing to obtain the desired resistance to delayed fracture. Therefore, the heat treatment temperature is 300°C or higher. On the other hand, if the heat treatment temperature is above 700°C, austenite will form and the elemental distribution will become uneven, making it impossible to achieve the desired average carbide number density. Therefore, the heat treatment temperature is 700°C or lower.

[0213] There is no particular limitation on the time for the above heat treatment (heat treatment time), but if it exceeds 96 hours, productivity will decrease significantly. Therefore, from the viewpoint of further improving productivity, the heat treatment time is preferably 96 hours or less.

[0214] (9) Cold rolling

[0215] Next, the hot-rolled steel sheet that has undergone heat treatment is cold-rolled to produce a cold-rolled steel sheet. The conditions for the cold rolling are not particularly limited and can be carried out according to conventional methods.

[0216] (10) Annealing

[0217] Next, the cold-rolled steel sheet is annealed. Recrystallization occurs through this annealing, forming the microstructure (martensite) required to achieve the desired strength.

[0218] • Dew point in the temperature range of 600–980℃: above -15℃

[0219] In the above-described annealing process, the carbon concentration on the surface of the steel sheet decreases due to the reaction between carbon (C) and moisture in the atmosphere. As a result, ferrite can be generated in a region of 7–12 μm from the surface of the steel sheet towards its thickness. However, if the dew point in the temperature range of 600–980°C is below -15°C, the microstructure of the steel sheet surface does not become the desired structure, resulting in reduced resistance weldability. Therefore, during the above-described annealing, the dew point in the temperature range of 600–980°C should be above -15°C, preferably above -10°C, and more preferably above -5°C. On the other hand, the upper limit of the dew point is not particularly limited, but from the viewpoint of improving the adhesion of the zinc-based coating on the steel sheet surface, it is preferably below 30°C. It should be noted that the reaction between carbon and moisture in the steel sheet surface mainly occurs at temperatures above 600°C. Therefore, the dew point control during annealing only needs to be within the temperature range of 600–980°C; the dew point in the temperature range below 600°C is not particularly limited.

[0220] In the above annealing process, heating, holding, and cooling are performed according to the following steps.

[0221] • Heating to 650°C at an average heating rate of 12°C / s or higher.

[0222] • Heat at an average heating rate of less than 12°C / s to the annealing temperature of 830–980°C.

[0223] • Hold at the above annealing temperature for 20 to 360 seconds (soaking).

[0224] • Cool from the above annealing temperature to room temperature at an average cooling rate of 3°C / s or higher.

[0225] That is, in this embodiment, heating to the annealing temperature is performed in two stages, and the average heating rate of each stage is controlled within a specific range. In the following description, for convenience, the heating in the first stage will be referred to as "first heating" and the heating in the second stage will be referred to as "second heating".

[0226] (First heating: up to 650℃)

[0227] • Average heating rate: 12℃ / s or higher

[0228] First, the cold-rolled steel sheet is heated to 650°C (first heating). If the average heating rate up to 650°C is less than 12°C / s, the microstructure of the steel sheet becomes coarse, and the desired average grain size cannot be obtained. Therefore, in the above annealing, the cold-rolled steel sheet is heated to 650°C at an average heating rate of 12°C / s or higher. There is no particular upper limit to the above average heating rate, but if the heating is too rapid, recrystallization becomes difficult. Therefore, the above average heating rate is preferably 30°C / s or less.

[0229] (Second heating)

[0230] • Average heating rate: less than 12℃ / s

[0231] Next, the cold-rolled steel sheet is heated to the annealing temperature (second heating). In this second heating, by heating to the annealing temperature at an average heating rate of less than 12°C / s, decarburization near the surface of the steel sheet can be promoted, resulting in the formation of a ferrite phase. If the average heating rate is 12°C / s or higher, decarburization is insufficient, and the volume fraction of ferrite in the region from the surface to the thickness direction of 7–12 μm cannot be within the desired range. Therefore, the average heating rate is less than 12°C / s. On the other hand, there is no particular limitation on the lower limit of the average heating rate, but to ensure tensile strength, it is preferable to make the average heating rate exceed 2°C / s.

[0232] Annealing temperature: 830~980℃

[0233] If the annealing temperature is less than 830°C, the ferrite content becomes too high, making it difficult to balance tensile strength and resistance weldability. Therefore, the annealing temperature is 830°C or higher, preferably 840°C or higher, and more preferably 850°C or higher. On the other hand, if the annealing temperature is too high, austenite grain growth becomes significant, resulting in coarser grains and reduced resistance spot weldability. Therefore, the annealing temperature is 980°C or lower, preferably 950°C or lower.

[0234] • Duration: 20–360 seconds

[0235] By holding the annealing temperature as described above, recrystallization occurs, and part or all of the microstructure undergoes an austenitic phase transformation. If the holding time at the annealing temperature is less than 20 seconds, the desired microstructure cannot be obtained. Therefore, the holding time is 20 seconds or more. On the other hand, if the holding time is greater than 360 seconds, the grains coarsen, thereby reducing resistance weldability. Therefore, the holding time is 360 seconds or less, preferably 300 seconds or less.

[0236] After holding at the above annealing temperature, cool to room temperature at an average cooling rate of 3°C / s or higher.

[0237] • Average cooling rate: 3°C / s or more

[0238] If the average cooling rate during the above cooling process is less than 3°C / s, the desired martensite volume fraction cannot be obtained, thus reducing the tensile strength. Therefore, the average cooling rate is made to be 3°C / s or more. On the other hand, there is no particular upper limit to the average cooling rate, but it is preferably less than 100°C / s.

[0239] Furthermore, quenching and tempering rolling can be performed after the above-mentioned annealing. This quenching and tempering rolling can be carried out under any conditions, but it is preferable to achieve an elongation of 0.05% to 2.0%.

[0240] Second implementation method

[0241] In a second embodiment of the present invention, the annealed steel sheet can be electroplated to form a zinc-based coating on at least one surface of the steel sheet. This method enables the production of electroplated zinc-based steel sheets.

[0242] (plating)

[0243] The electroplating described above is not particularly limited and can be carried out under any conditions. That is, in this invention, the desired properties are achieved by controlling the microstructure and precipitates of the base steel plate, therefore the plating treatment conditions are not limited and can be carried out according to conventional methods.

[0244] Third implementation method

[0245] In a third embodiment of the present invention, molten steel having the above-described composition is used as a starting material, and the following steps are performed sequentially, thereby enabling the manufacture of a steel sheet that meets the above conditions. According to this method, a hot-dip galvanized steel sheet with a hot-dip galvanized coating on its surface can be obtained.

[0246] (1) Continuous casting

[0247] (2) Cooling

[0248] (3) Reheating

[0249] (4) Hot rolling

[0250] (5) Cooling

[0251] (6) Winding

[0252] (7) Pickling

[0253] (8) Heat treatment

[0254] (9) Cold rolling

[0255] (10) Annealing

[0256] (11) Hot-dip galvanizing

[0257] (12) Cooling

[0258] In this embodiment, each of the steps (1) to (10) described above can be performed under the same conditions as in the first embodiment. However, in the cooling step of annealing (10), instead of cooling to room temperature, cooling is performed until immersion in the hot-dip plating bath. The average cooling rate in the above cooling is 3°C / s or more, just like in the first embodiment.

[0259] The remaining processes (11) and (12) will be explained below.

[0260] (11) Hot-dip galvanizing

[0261] In this embodiment, the annealed steel sheet is immersed in a hot-dip galvanizing bath to form a hot-dip zinc coating on at least one surface of the steel sheet. This method allows the production of hot-dip galvanized steel sheets.

[0262] The above-mentioned hot-dip galvanizing can be carried out by any method. That is, in this invention, the desired properties are achieved by controlling the microstructure and precipitates of the base steel plate, so the galvanizing conditions are not particularly limited and can be carried out according to conventional methods.

[0263] In the above-described hot-dip plating process, there are no particular limitations, and any hot-dip plating bath can be used, but a hot-dip plating bath with a composition consisting of Al, Zn, and unavoidable impurities is preferred. The Al concentration in the plating bath is not particularly limited, and can be, for example, 0.05% to 0.25%. If the Al concentration is 0.05% or higher, the formation of bottom dross is suppressed, thus preventing molten slag from adhering to the steel plate and becoming a defect. On the other hand, if the Al concentration is 0.25% or lower, the increase of top dross is suppressed, thus preventing molten slag from adhering to the steel plate and becoming a defect. Furthermore, by reducing the Al concentration, material costs can be reduced.

[0264] There are no particular restrictions on the other conditions for hot-dip galvanizing. For example, the temperature of the hot-dip galvanizing bath is preferably the general hot-dip galvanizing bath temperature, i.e., 440–500°C. In addition, the temperature of the steel plate when immersed in the hot-dip galvanizing bath (immersion plate temperature) is preferably 440–550°C.

[0265] Furthermore, the coating adhesion amount can be adjusted after hot-dip galvanizing. There are no particular limitations on the method for adjusting the coating adhesion amount; typically, it is adjusted by gas wiping. The coating adhesion amount is adjusted by modifying the gas wiping conditions, such as air pressure and the distance between the wiping nozzle and the steel plate.

[0266] (12) Cooling

[0267] Next, the hot-dip galvanized steel sheet is cooled to room temperature at an average cooling rate of 3°C / s or higher. If the average cooling rate during the cooling process is less than 3°C / s, the desired martensite volume fraction cannot be obtained, thus reducing the tensile strength. Therefore, the average cooling rate is set to 3°C / s or higher. On the other hand, there is no particular upper limit to the average cooling rate, but it is preferable to be less than 100°C / s.

[0268] Thus, in this embodiment, in order to obtain the desired microstructure, it is important that the average cooling rate of both the cooling during the annealing process and the cooling after hot-dip galvanizing is 3°C / s or more.

[0269] Fourth Implementation Method

[0270] In the fourth embodiment of the present invention, an alloying treatment is performed after the hot-dip galvanizing and before cooling to room temperature. This alloying treatment allows the hot-dip galvanized layer to be alloyed, resulting in an alloyed hot-dip galvanized steel sheet.

[0271] (Alloying treatment)

[0272] The alloying treatment described above is not particularly limited and can be carried out under any conditions. That is, in this invention, the desired properties are achieved by controlling the microstructure and precipitates of the base steel plate, therefore the alloying treatment conditions are not limited and can be carried out according to conventional methods.

[0273] The alloying treatment described above is preferably performed at a temperature of 450°C to 600°C. By alloying at 450°C or higher, steel sheets with excellent pressability can be provided without leaving any η phase in the coating. Furthermore, by alloying at 600°C or lower, good coating adhesion can be obtained. It should be noted that the alloying time is preferably 5 to 60 seconds.

[0274] (Pre-plating)

[0275] Furthermore, in other embodiments of the present invention, pre-plating can be performed arbitrarily before plating to form a zinc-based coating, thereby forming a pre-plating layer on the steel plate surface.

[0276] There is no particular limitation on the timing of the pre-plating, as long as it is performed before the plating used to form the zinc-based coating. Typically, when the zinc-based coating is formed by electroplating, the pre-plating is preferably performed after the annealing and before the electroplating. That is, the cold-rolled steel sheet can be annealed, pre-plated, and then electroplated with zinc in sequence. On the other hand, when the zinc-based coating is formed by hot-dip galvanizing, the pre-plating is preferably performed after the cold rolling and before the annealing. That is, the process can be performed in the order of cold rolling, pre-plating, annealing, and hot-dip galvanizing. It should be noted that when alloying is performed, it can be done conventionally after the hot-dip galvanizing.

[0277] In the above pre-plating process, any pre-plating layer can be formed, but it is preferable to form an Fe-based plating layer. The formation of the Fe-based plating layer is preferably performed by electroplating. Hereinafter, the case of forming an Fe-based plating layer as a pre-plating layer by performing Fe-based electroplating will be described.

[0278] There are no particular limitations on the Fe-based electroplating treatment method. For example, any bath, such as a sulfuric acid bath, a hydrochloric acid bath, or a sulfuric acid + hydrochloric acid bath, can be used as the Fe-based electroplating bath.

[0279] The Fe ion content in the Fe-based electroplating bath before energization is determined by Fe. 2＋ The preferred concentration is 1.0 mol / L or higher. If the Fe ion content in the Fe-based electroplating bath is... 2＋A concentration of 1.0 mol / L or higher is sufficient to achieve adequate Fe adhesion. In addition to Fe ions and alloying elements such as B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, Fe-based electroplating baths may also contain conductive additives such as sodium sulfate and potassium sulfate as additives or impurities. It should be noted that metallic elements only need to be present in the form of metal ions, while non-metallic elements can be present in the form of boric acid, phosphoric acid, nitric acid, or a portion of organic acids. Furthermore, ferric sulfate plating solutions may also contain conductive additives, chelating agents, and pH buffers such as sodium sulfate and potassium sulfate.

[0280] It should be noted that Fe-based electroplating can also be performed on cold-rolled steel sheets without oxidation treatment in a preheating furnace or similar process.

[0281] Other conditions for the Fe-based electroplating bath are not particularly limited. Considering temperature stability, the bath temperature is preferably above 30°C. The pH of the Fe-based electroplating bath is not particularly specified, but considering the conductivity of the Fe-based electroplating bath, it is preferably below 3.0. The current density is not particularly limited, and is typically 10–150 A / dm³. 2 The board speed only needs to be between 5 mpm and 150 mpm. This is because when the board speed is less than 5 mpm, the productivity is poor, and on the other hand, when the board speed is above 150 mpm, it is difficult to stably control the amount of plating.

[0282] It should be noted that, as a pretreatment before Fe-based electroplating, degreasing and rinsing for cleaning the surface of the cold-rolled steel sheet, and pickling and rinsing for surface activation can be performed. Fe-based electroplating is preferably performed after these pretreatments. The methods for degreasing and rinsing are not particularly limited, and conventional methods can be used. In the pickling process, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Sulfuric acid, hydrochloric acid, or mixtures thereof are preferred. The concentration of the acid is not particularly specified, but considering the ability to remove the oxide film and to prevent surface roughness (surface defects) caused by over-pickling, it is preferably about 1 to 20% by mass. Furthermore, the pickling solution may also contain defoamers, pickling accelerators, pickling inhibitors, etc.

[0283] Example

[0284] To confirm the effectiveness of the present invention, steel sheets were manufactured according to the following steps, and their properties were evaluated. The manufactured steel sheets were of four types: cold-rolled steel sheet (CR), electro-galvanized steel sheet (EG), hot-dip galvanized steel sheet (GI), and alloyed hot-dip galvanized steel sheet (GA).

[0285] First, molten steel with the composition shown in Table 1 is continuously cast to form a steel billet, which is then cooled. The average cooling rate for the temperature range up to 600°C during the cooling process is shown in Table 2.

[0286] The cooled steel billet was reheated under the conditions shown in Table 2, and then hot-rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet. The hot-rolled steel sheet was cooled to the winding temperature under the conditions shown in Table 2 and wound into coils. Then, the hot-rolled steel sheet was pickled.

[0287] Next, the pickled hot-rolled steel sheet was subjected to heat treatment at the temperatures shown in Table 2 to produce a heat-treated hot-rolled steel sheet. The heat treatment time was 12 hours.

[0288] Then, the above-mentioned heat-treated hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet with a thickness of 1.4 mm.

[0289] In the manufacture of cold-rolled steel sheet (CR), the aforementioned cold-rolled steel sheet is annealed under the conditions shown in Table 2. In the annealing process, the cold-rolled steel sheet is homogenized at the annealing temperature and holding time shown in Table 2, and then cooled to room temperature at the average cooling rate shown in Table 2.

[0290] In the manufacture of electro-galvanized steel sheet (EG), the aforementioned cold-rolled steel sheet is first annealed under the conditions shown in Table 2, and then cooled to room temperature at the average cooling rate shown in Table 2. Then, electroplating is performed to form an electro-galvanized layer on the surface of the steel sheet. The electroplating uses a Zn solution containing 1.5 mol / L. 2＋ The plating is carried out using a sulfuric acid bath. The temperature of the plating bath is 50℃, and the pH is 1.5.

[0291] In the manufacture of hot-dip galvanized steel sheet (GI), the aforementioned cold-rolled steel sheet is first annealed under the conditions shown in Table 2, and then cooled at the average cooling rate shown in Table 2 until it is immersed in a hot-dip galvanizing bath. The steel sheet is then immersed in the hot-dip galvanizing bath to form a hot-dip galvanized layer on its surface. The hot-dip galvanizing bath used is a bath composed of Al, Zn, and unavoidable impurities, with an Al concentration of 0.14%. The temperature of the hot-dip galvanizing bath is 460°C. Then, it is cooled to room temperature at the average cooling rate shown in Table 2.

[0292] In the manufacture of alloyed hot-dip galvanized steel sheet (GA), the cold-rolled steel sheet is first annealed under the conditions shown in Table 2, and then cooled at the average cooling rate shown in Table 2 until it is immersed in a hot-dip galvanizing bath. Then, an alloying treatment is performed to alloy the hot-dip galvanized layer. The hot-dip galvanizing bath used is a bath with the same composition and temperature as the bath used in the manufacture of the hot-dip galvanized steel sheet (GI). The alloying treatment is performed at a temperature of 550°C. Then, it is cooled to room temperature at the average cooling rate shown in Table 2.

[0293] Next, the microstructure and average carbide number density of the obtained steel plate were determined according to the following steps. The results are shown in Table 3.

[0294] (Microstructure at 1 / 4 of the plate thickness)

[0295] Ferrite, Martensite, Bainite

[0296] The volume fractions of ferrite, martensite, and bainite in the steel plate were determined according to the following steps. First, a section of the plate thickness parallel to the rolling direction was ground and etched with 3% nitric acid ethanol to reveal the microstructure. Then, the section was observed at magnifications of 3000x and 10000x using SEM (scanning electron microscope), TEM (transmission electron microscope), and FE-SEM (field emission scanning electron microscope) to obtain microscopic images of the microstructure at one-quarter of the plate thickness. The area fractions of ferrite, martensite, and bainite in the microscopic images were calculated using the point counting method (according to ASTM E562-83 (1988)), and these area fractions were used as the volume fractions.

[0297] The average grain size of ferrite, martensite, and bainite was determined by image analysis of the aforementioned microscope images. Specifically, firstly, the area of ​​each grain of ferrite, martensite, and bainite in the microscope images was determined by image analysis. Then, the equivalent circle diameter of the grain was calculated from the area, and the average of these areas was taken as the average grain size. The image analysis was performed using ImagePro from Media Cybernetics.

[0298] Residual austenite

[0299] The volume fraction of retained austenite was determined by X-ray diffraction. Specifically, firstly, the steel plate was ground to one-quarter of its thickness, and the intensity of the diffracted X-rays at this one-quarter thickness plane was measured by X-ray diffraction. The measurement was performed using a Rigaku RINT2200 X-ray diffractometer, with Mo's Kα rays as the source and an accelerating voltage of 50 keV. The integrated intensities of the X-ray diffracted rays from the {200}, {211}, and {220} planes of ferrite and the {200}, {220}, and {311} planes of austenite were measured, and the volume fraction of retained austenite was calculated from the obtained integrated intensities. The calculation was performed using the formulas described on pages 26, 62-64 of the "X-ray Diffraction Handbook" (Rigaku Electric Co., Ltd., 2000).

[0300] Furthermore, the average grain size of the retained austenite grains was determined by image analysis of TEM images obtained from TEM observation after grinding the cross-section to one-quarter of the plate thickness and etching with 3% nitric acid ethanol. Specifically, firstly, TEM images of the microstructure at one-quarter of the plate thickness were obtained by observing the cross-section at 15000x magnification. The area of ​​each retained austenite grain was calculated by image analysis of the obtained TEM images. Then, the equivalent circle diameter of each grain was calculated from the area, and the average value was taken as the average grain size of the retained austenite. The above image analysis was performed using Image-Pro from MediaCybernetics.

[0301] (Microstructure of the region from the surface to the thickness direction of the plate, 7–12 μm)

[0302] The microstructure of the steel plate was observed under a microscope at a depth of 7–12 μm from the surface, and the volume fraction of ferrite and the average grain size were calculated. It should be noted that when the test subject is a coated steel plate (EG, GI, and GA), the measurement is performed on the surface of the steel plate using glow discharge luminescence analysis, and points where Fe exceeds 50 mass% are considered the surface of the steel plate. Microscopic observation and the calculation of the volume fraction and average grain size based on the microscopic images were performed using the same method as the measurement of the microstructure at the 1 / 4 thickness position of the plate.

[0303] (Number density of precipitates)

[0304] The average number density of carbides with a particle size of 0.10 μm or larger in the region 50–100 μm from the surface towards the plate thickness was determined by TEM observation. Specifically, firstly, the L-section of the steel plate was observed using a TEM at 10,000x magnification, obtaining TEM images at 10 randomly selected locations within a depth range of 50–100 μm from the surface of the steel plate. Next, Image-Pro was used to analyze the TEM images, and the area of ​​each carbide was calculated. The equivalent circle diameter of each particle was then calculated from the area. It should be noted that the identification of carbides present in the TEM images was performed using EDS (Energy Dispersive X-ray Spectroscopy). Then, the number of carbides with an equivalent circle diameter of 0.10 μm or larger was counted and divided by the area of ​​the observed range to determine the carbide number density. The same procedure was followed to calculate the number density of precipitates from the TEM images at the aforementioned 10 locations, and the average value was taken as the average number density of carbides.

[0305] Next, the tensile strength and resistance weldability of the obtained steel plate are evaluated according to the following steps.

[0306] (Tensile strength)

[0307] JIS No. 5 tensile test specimens were collected from the steel plate with the rolling orthogonal direction as the long side direction (tensile direction). Then, tensile tests were performed using these specimens to determine the tensile strength (TS) of the steel plate. The tensile tests were conducted according to JIS Z2241 (1998).

[0308] (Resistance welding properties)

[0309] As an indicator of weldability, the L-shaped tensile strength and delayed fracture characteristics after resistance spot welding are evaluated.

[0310] L-shaped tensile strength

[0311] First, an L-shaped tensile test piece for evaluating L-shaped tensile strength is prepared. Specifically, two 50×150mm test pieces are cut from the aforementioned steel plate, and the two test pieces are each bent at a 90° V-bend with the welded surface dimensions being 50×50mm. Resistance spot welding is performed at the center of the welded surface to weld the two bent test pieces together, thus forming an L-shaped tensile test piece.

[0312] The resistance spot welding described above was performed using a servo motor-driven, pressurized single-phase AC (50Hz) resistance welding machine. A DR-type electrode with alumina-dispersed copper at the tip, with a radius of curvature R40mm and a tip diameter of 6mm, was used as the electrode head. The welding conditions were as follows.

[0313] • Pressure applied: 4500N

[0314] • Power-on time: 20 cycles (50Hz)

[0315] • Hold time: 5 cycles (50Hz)

[0316] • Melt core diameter: 5.0√t (mm)

[0317] Here, t represents the thickness (mm) of the steel plate used.

[0318] Using the obtained L-shaped tensile test specimens, tensile tests were conducted at a tensile speed (long side direction) of 10 mm / min to determine the L-shaped tensile strength. Cases with an L-shaped tensile strength of 1.5 kN or higher were deemed "qualified," while those less than 1.5 kN were deemed "unqualified." The results are shown in Table 3.

[0319] • Resistance to delayed fracture

[0320] First, a welded joint for evaluating resistance to delayed fracture was fabricated. Specifically, two 50×150mm test pieces were cut from the aforementioned steel plate. Next, the two test pieces were overlapped with a 50mm×50mm spacer with a thickness of 1.4mm sandwiched between their two ends, and a temporary weld was performed. Then, the center of the temporarily welded test pieces was welded together to form a welded joint. The welding conditions were the same as those used in the fabrication of the L-shaped tensile test piece, with a weld nugget diameter of 5.0√t (mm).

[0321] The resulting welded joint was left to stand in the atmosphere at room temperature (20°C) for 24 hours. Then, the welded joint was immersed in a 3% NaCl + 1.0% NH4SCN aqueous solution at a concentration of 0.06 mA / cm². 2 The current density was used to perform cathodic electrolysis and hydrogen charging for 72 hours. Then, it was confirmed whether delayed fracture occurred in the weld joint. The weld joint that did not experience delayed fracture was judged as "acceptable", and the weld joint that did experience delayed fracture was judged as "unacceptable". The judgment results are shown in Table 3.

[0322] As shown in Table 3, the steel plate that meets the conditions of the present invention has both a tensile strength of over 1600 MPa and a high L-shaped tensile strength after resistance welding, and thus has excellent resistance to delayed fracture after resistance welding.

[0323] [Table 1]

[0324]

[0325] [Table 2]

[0326]

[0327] [Table 3]

[0328]

Claims

1. A steel plate, comprising, by mass percent, C: 0.22-0.38%, Si: 0.05-1.35%, Mn: 2.4-3.5%, P: less than 0.02%, S: less than 0.002%, Al: 0.01-0.10%, N: less than 0.008%, B: 0.0002-0.0050%, and at least one selected from Ti: 0.005-0.07%, Nb: 0.005-0.07%, and V: 0.005-0.07%. The remainder consists of Fe and unavoidable impurities. The microstructure at the 1 / 4 position of the plate thickness, by volume fraction, comprises ferrite: 0–5%, retained austenite: 0–5%, bainite: 0–7%, and martensite: ≥93%, with the average grain size of ferrite being ≤3 μm, the average grain size of retained austenite being ≤3 μm, the average grain size of bainite being ≤5 μm, and the average grain size of martensite being ≤7 μm. The microstructure in the region from the surface to the thickness direction of the plate, ranging from 7 to 12 μm, contains more than 30% ferrite by volume fraction, and the average grain size of the ferrite is less than 10 μm. The average number density of carbides with a particle size of 0.10 μm or larger in the region from the surface to the plate thickness of 50–100 μm is 5 particles / 100 μm. 2 above.

2. The steel plate according to claim 1, wherein, The composition further comprises, by mass%, at least one selected from the group consisting of less than 0.02% Sb, less than 0.50% Cu, less than 0.50% Ni, less than 0.50% Cr, less than 0.50% Mo, less than 0.50% Sn, less than 0.30% Ca, less than 0.0050% and less than 0.0050% REM.

3. The steel plate according to claim 1 or 2, wherein, It has a zinc-based coating on at least one surface.

4. A resistance spot welding method, wherein a plate assembly comprising at least one steel plate as described in any one of claims 1 to 3 is clamped with a pair of welding electrodes and joined by applying pressure and energizing simultaneously.

5. A resistance spot welding component comprising at least one steel plate as described in any one of claims 1 to 3 in a plate assembly.

6. A method for manufacturing a steel plate, comprising continuously casting molten steel having the composition described in claim 1 or 2 to produce a steel billet. The steel billet is cooled at an average cooling rate of 50°C / h or more over a temperature range up to 600°C. The cooled steel billet is then reheated at a temperature of 1280–1400°C for at least 60 minutes. The reheated steel billet is hot-rolled at a finishing rolling temperature of 850–950°C to produce a hot-rolled steel plate. The hot-rolled steel sheet is cooled to a cooling stop temperature below 460°C at an average cooling rate of 80°C / s or higher. The cooled hot-rolled steel sheet is wound at a winding temperature below 460°C. The wound hot-rolled steel sheet is pickled. The pickled hot-rolled steel sheet is heat-treated at a temperature of 300–700°C to produce a heat-treated hot-rolled steel sheet. The heat-treated hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. The cold-rolled steel sheet is annealed at a temperature range of 600–980°C under conditions where the dew point exceeds -15°C. In the annealing process, the cold-rolled steel sheet is heated to 650°C at an average heating rate of 12°C / s or higher, and then heated to an annealing temperature of 830-980°C at an average heating rate of less than 12°C / s. The annealing temperature is held for 20-360 seconds, and then cooled to room temperature at an average cooling rate of 3°C / s or higher.

7. The method for manufacturing a steel plate according to claim 6, wherein, The annealed steel sheet is further subjected to electroplating to form a zinc-based coating on at least one surface of the steel sheet.

8. A method for manufacturing a steel plate, comprising continuously casting molten steel having the composition described in claim 1 or 2 to produce a steel billet. The steel billet is cooled at an average cooling rate of 50°C / h or more over a temperature range up to 600°C. The cooled steel billet is then reheated at a temperature of 1280–1400°C for at least 60 minutes. The reheated steel billet is hot-rolled at a finishing rolling temperature of 850–950°C to produce a hot-rolled steel plate. The hot-rolled steel sheet is cooled to a cooling stop temperature below 460°C at an average cooling rate of 80°C / s or higher. The cooled hot-rolled steel sheet is wound at a winding temperature below 460°C. The wound hot-rolled steel sheet is pickled. The pickled hot-rolled steel sheet is heat-treated at a temperature of 300–700°C to produce a heat-treated hot-rolled steel sheet. The heat-treated hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. The cold-rolled steel sheet is annealed at a temperature range of 600–980°C under conditions where the dew point exceeds -15°C. The annealed cold-rolled steel sheet is subjected to hot-dip galvanizing to form a zinc-based coating on at least one surface of the steel sheet. The hot-dip galvanized steel sheet is cooled to room temperature at an average cooling rate of 3°C / s or higher. In the annealing process, the cold-rolled steel sheet is heated to 650°C at an average heating rate of 12°C / s or higher, and then heated to an annealing temperature of 830-980°C at an average heating rate of less than 12°C / s. The annealing temperature is held for 20-360 seconds, and then cooled from the annealing temperature at an average cooling rate of 3°C / s or higher until it is immersed in a hot-dip galvanizing bath.

9. The method for manufacturing a steel plate according to claim 8, wherein, After the hot-dip plating and before cooling to room temperature, a further alloying process is performed.

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