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

Abstract

The present invention provides a steel sheet having a tensile strength of 1600 MPa or more, high L-shaped tensile strength after resistance welding, and excellent delayed fracture resistance after resistance welding. The steel sheet has a specified chemical composition, and the microstructure at the 1 / 4 position of the sheet thickness contains, by volume fraction, 0-5% ferrite, 0-5% retained austenite, 0-7% bainite, and 93% or more martensite, with an average grain size of 3 μm or less for the ferrite, 3 μm or less for the retained austenite, 5 μm or less for the bainite, and 7 μm or less for the martensite. The microstructure in a region 7-12 μm from the surface in the sheet thickness direction contains, by volume fraction, 30% or more for the ferrite, with an average grain size of 10 μm or less for the ferrite. 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 in the sheet thickness direction is 5 / 100 μm. 2 That's it, steel plate.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet, particularly a steel sheet suitable for use as a material for structural parts of automobiles, etc. The present invention also relates to a resistance spot welding method using the steel sheet, a resistance spot-welded component, and a method for manufacturing the steel sheet. [Background technology]

[0002] In recent years, CO2 emission regulations have become stricter due to growing environmental concerns, and in the automotive industry, there is a demand for lighter vehicle bodies to improve fuel efficiency. To address this, the use of high-strength steel sheets has been promoted to reduce the thickness of automotive parts, and in recent years, steel sheets with a high tensile strength (TS) of 1600 MPa or more have also been used.

[0003] When assembling an automobile, it is common practice to join parts made by press-forming steel sheets using resistance spot welding (hereinafter simply referred to as resistance welding) from the standpoints of cost and efficiency. Therefore, in order to ensure collision safety, it is necessary to increase the strength of the welded portion in addition to increasing the strength of the steel sheet (base steel sheet) used as the material.

[0004] Tensile shear strength (TSS) and cross tension strength (CTS) are widely used as indicators of the strength of resistance-welded joints. However, in actual automobile manufacturing, flanges of parts are often resistance-welded. Therefore, to effectively improve automobile crashworthiness, it is necessary to improve the strength in the L-shape tension test (L-shape tension strength (LTS)), which is suitable for evaluating the peel strength of flange welds.

[0005] Furthermore, steel sheets used in automotive parts and other applications are required to have excellent delayed fracture resistance after resistance welding to prevent delayed fracture due to hydrogen penetration from the operating environment. However, achieving a high tensile strength of 1600 MPa or more requires the addition of large amounts of alloying elements, which leads to a deterioration of delayed fracture resistance after resistance welding. Therefore, it has been difficult to achieve both high strength and excellent delayed fracture resistance after resistance welding.

[0006] To solve these problems, various techniques have been proposed.

[0007] For example, Patent Document 1 proposes a method for improving the peeling strength of a welded joint by inserting an insert plate having specific dimensions between multiple overlapping steel plates and performing resistance spot welding on the steel plates.

[0008] Furthermore, Patent Document 2 proposes a method of improving the strength of a welded portion by performing resistance spot welding to form the welded portion and then performing post-energization. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2020-151756 [Patent Document 2] Japanese Patent Publication No. 2022-140236 Summary of the Invention [Problem to be solved by the invention]

[0010] According to the methods proposed in Patent Documents 1 and 2, a certain degree of improvement in the strength of welded joints is observed. However, in both Patent Documents 1 and 2, the tensile strength of the steel plates used in the tests is up to about 1500 MPa, and it is unclear whether the method will be effective when applied to high-strength steel plates with a tensile strength of 1600 MPa or more.

[0011] Furthermore, as mentioned above, the problem of delayed fracture becomes prominent in high-strength steel sheets having a tensile strength of 1600 MPa or more, but Patent Documents 1 and 2 do not take the problem of delayed fracture into consideration.

[0012] As described above, with the conventional techniques proposed in Patent Documents 1 and 2, it remains difficult to simultaneously achieve a tensile strength of 1600 MPa or more, excellent L-tensile strength, and delayed fracture resistance.

[0013] Furthermore, the method of Patent Document 1 requires a step of preparing an insert that meets certain conditions and inserting it between the steel sheets when welding, which results in poor productivity. Similarly, the method of Patent Document 2 also requires post-welding after welding, which results in poor productivity. In order to obtain excellent properties without requiring additional steps that reduce productivity, it is necessary to improve the resistance weldability of the steel sheet itself.

[0014] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel sheet which has a tensile strength of 1600 MPa or more, a high L-shaped tensile strength after resistance welding, and also has excellent delayed fracture resistance after resistance welding. [Means for solving the problem]

[0015] As a result of our investigations, we have found that the above problems can be solved by controlling the microstructure at 1 / 4 of the steel plate thickness, the microstructure in the region 7-12 μm from the surface in the thickness direction, and the average number density of carbides in the region 50-100 μm from the surface in the thickness direction, in addition to the chemical composition of the steel plate. In particular, controlling the ferrite volume fraction in the region 7-12 μm from the surface in the thickness direction and the average number density of carbides in the region 50-100 μm from the surface in the thickness direction is considered to be important for improving properties after resistance welding. The findings of the inventors that led to the above conclusion are now explained.

[0016] In an L-shaped tensile test, a crack propagates in the fusion zone (nugget) of a resistance spot weld, leading to fracture. According to the inventors' research, the tensile strength in an L-shaped tensile test can be improved by changing the microstructure in the corona bond zone around the nugget. Specifically, softening the corona bond zone improves crack propagation resistance, resulting in high tensile strength in an L-shaped tensile test.

[0017] To achieve the above effect, it is sufficient to form a sufficient amount of ferrite in a region of 7 to 12 μm from the surface in the sheet thickness direction in the steel sheet before resistance welding. By controlling the structure of the surface layer of the steel sheet in this way, it is possible to effectively improve the tensile strength in an L-shaped tensile test after resistance welding while maintaining high base material strength.

[0018] Furthermore, the above-mentioned change in the microstructure in the corona bonded portion also has the effect of improving delayed fracture resistance after resistance welding. Therefore, in order to improve delayed fracture resistance after resistance welding, it is necessary to form a sufficient amount of ferrite in the region 7 to 12 μm deep from the surface in the plate thickness direction.

[0019] In addition, to sufficiently improve delayed fracture resistance after resistance welding, it is necessary to precipitate carbides at a sufficient density in a region 50 to 100 μm from the surface in the plate thickness direction. By precipitating a sufficient amount of C contained in the base material as carbides, the amount of solute C in the heat-affected zone (HAZ) after resistance welding is reduced, resulting in improved delayed fracture resistance.

[0020] The present invention is based on the above findings, and has the following gist and configuration.

[0021] 1. The component composition is in mass%: C: 0.22~0.38%, Si: 0.05 to 1.35% Mn: 2.4-3.5%, P: 0.02% or less, S: 0.002% or less, Al: 0.01 to 0.10%, N: 0.008% or less, B: 0.0002 to 0.0050%, and Contains at least one selected from the group consisting of Ti: 0.005 to 0.07%, Nb: 0.005 to 0.07%, and V: 0.005 to 0.07%, the balance being Fe and unavoidable impurities; The microstructure at the 1 / 4 position of the plate thickness is In terms of volume fraction, Ferrite: 0 to 5%, Retained austenite: 0 to 5% Bainite: 0-7% and Martensite: Contains 93% or more, Average grain size of ferrite: 3 μm or less, Average grain size of retained austenite: 3 μm or less, The average grain size of bainite is 5 μm or less, and The average grain size of martensite is 7 μm or less, The microstructure in the area 7 to 12 μm from the surface in the thickness direction is Volume fraction: ferrite: 30% or more, The average grain size of ferrite is 10 μm or less, The average number density of carbides with a grain size of 0.10 μm or more in the area 50 to 100 μm from the surface in the thickness direction is 5 / 100 μm. 2 That's it, steel plate.

[0022] 2. The component composition is, in mass%, Sb: 0.02% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less Sn: 0.30% or less, Ca: 0.0050% or less, and 2. The steel sheet according to 1 above, further containing at least one selected from the group consisting of REM: 0.0050% or less.

[0023] 3. The steel sheet according to 1 or 2 above, having a zinc-based plating layer on at least one surface.

[0024] 4. A resistance spot welding method in which a sheet assembly including at least one steel sheet according to any one of 1 to 3 above is clamped between a pair of welding electrodes and joined by passing current through the electrodes while applying pressure.

[0025] 5. A resistance spot welded component including at least one steel plate according to any one of 1 to 3 above in a plate assembly.

[0026] 6. Continuously cast the molten steel having the composition described in 1 or 2 above into a steel slab; The steel slab is cooled at an average cooling rate of 50°C / h or more in a temperature range up to 600°C, The cooled steel slab is reheated at a heating temperature of 1280 to 1400°C for a holding time at the heating temperature of 60 minutes or more, The reheated steel slab is hot-rolled under the condition of a finish rolling temperature of 850 to 950°C to obtain a hot-rolled steel sheet; The hot-rolled steel sheet is cooled to a cooling stop temperature of 460°C or less at an average cooling rate of 80°C / s or more, The cooled hot-rolled steel sheet is coiled at a coiling temperature of 460°C or less, The hot-rolled steel sheet after coiling is subjected to pickling. The pickled hot-rolled steel sheet is subjected to a heat treatment at a heat treatment temperature of 300 to 700 ° C to obtain a heat-treated hot-rolled steel sheet, The heat-treated hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is annealed under conditions in which the dew point is higher than -15 ° C in a temperature range of 600 to 980 ° C, In the annealing, the cold-rolled steel sheet is Average heating rate: 12℃ / s or more, heated to 650℃, Heat to an annealing temperature of 830 to 980°C at an average heating rate of less than 12°C / s. The annealing temperature is maintained for a holding time of 20 to 360 seconds, A method for producing a steel sheet, comprising cooling the steel sheet from the annealing temperature to room temperature at an average cooling rate of 3°C / s or more.

[0027] 7. The method for producing a steel sheet according to 6 above, further comprising electroplating the annealed steel sheet to form a zinc-based plating layer on at least one surface of the steel sheet.

[0028] 8. Continuously cast the molten steel having the composition described in 1 or 2 above into a steel slab; The steel slab is cooled at an average cooling rate of 50°C / h or more in a temperature range up to 600°C, The cooled steel slab is reheated at a heating temperature of 1280 to 1400°C for a holding time at the heating temperature of 60 minutes or more, The reheated steel slab is hot-rolled under the condition of a finish rolling temperature of 850 to 950°C to obtain a hot-rolled steel sheet; The hot-rolled steel sheet is cooled to a cooling stop temperature of 460°C or less at an average cooling rate of 80°C / s or more, The cooled hot-rolled steel sheet is coiled at a coiling temperature of 460°C or less, The hot-rolled steel sheet after coiling is subjected to pickling, The pickled hot-rolled steel sheet is subjected to a heat treatment at a heat treatment temperature of 300 to 700 ° C to obtain a heat-treated hot-rolled steel sheet; The heat-treated hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is annealed under conditions in which the dew point is higher than -15°C in a temperature range of 600 to 980°C, hot-dip galvanizing the annealed cold-rolled steel sheet to form a zinc-based plating layer on at least one surface of the steel sheet; A method for producing a steel sheet, wherein the steel sheet after hot dip coating is cooled to room temperature at an average cooling rate of 3 ° C. / s or more, In the annealing, the cold-rolled steel sheet is Average heating rate: 12℃ / s or more, heated to 650℃, Heat to an annealing temperature of 830 to 980°C at an average heating rate of less than 12°C / s. The annealing temperature is maintained for a holding time of 20 to 360 seconds, A method for producing a steel sheet, wherein the steel sheet is cooled at an average cooling rate of 3°C / s or more from the annealing temperature to immersion in a hot dip coating bath.

[0029] 9. The method for producing a steel sheet according to the above item 8, further comprising the step of subjecting the steel sheet to an alloying treatment after the hot dip coating and before the cooling to room temperature. [Effects of the Invention]

[0030] According to the present invention, it is possible to provide a steel sheet that has a tensile strength of 1600 MPa or more, a high L-shaped tensile strength after resistance welding, and also has excellent delayed fracture resistance after resistance welding. In this specification, the L-shaped tensile strength and the delayed fracture resistance may be collectively referred to as resistance weldability. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, an embodiment of the present invention will be described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited by the following description. Furthermore, the unit of content, "%", represents "% by mass" unless otherwise specified.

[0032] [Component composition] The steel sheet of the present invention has the above-mentioned chemical composition. The reasons for limiting the chemical composition will be explained below.

[0033] C: 0.22 to 0.38% C is an element effective in increasing the strength of steel sheets and also contributes to the formation of martensite in the present invention. Furthermore, C is also a component that forms carbides, which are an important element in the present invention. If the C content is less than 0.22%, the necessary strength and martensite volume fraction cannot be ensured. Therefore, the C content is set to 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 excessively high, the toughness of the nugget after resistance welding decreases, resulting in a decrease in L-shaped tensile strength. Therefore, the C content is set to 0.38% or less, preferably 0.34% or less, and more preferably 0.33% or less.

[0034] Si: 0.05 to 1.35% Si is an element that has the effect of improving resistance weldability. This is because the addition of Si alleviates Mn segregation, thereby alleviating the variation in hardness in the steel sheet's thickness direction. To achieve this effect, the Si content is set to 0.05% or more, preferably 0.15% or more, and more preferably 0.25% or more. On the other hand, excessive addition of Si causes liquid metal embrittlement during resistance spot welding. Therefore, the Si content is set to 1.35% or less, preferably 1.25% or less, and more preferably 1.15% or less.

[0035] Mn: 2.4 to 3.5% Mn is an element that improves the strength of steel sheets by stabilizing martensite formation and solid-solution strengthening. Furthermore, Mn stabilizes austenite and is an element necessary for ensuring the volume fraction of martensite. To achieve these effects, the Mn content is set to 2.4% or more. On the other hand, excessive Mn content reduces the toughness of the nugget after spot welding, resulting in a decrease in L-shaped tensile strength. Furthermore, excessive Mn content increases the slip constraint at grain boundaries when hydrogen penetrates the steel sheet, making cracks more likely to propagate at the grain boundaries. As a result, delayed fracture resistance after resistance welding is reduced. Therefore, the Mn content is set to 3.5% or less, preferably 3.2% or less.

[0036] P:0.02% or less If P is excessive, P segregates significantly at grain boundaries, embrittling the grain boundaries. As a result, resistance weldability deteriorates. Therefore, the P content is set to 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 may be 0%. However, excessive reduction increases steelmaking costs. Therefore, the P content is preferably 0.002% or more.

[0037] S: 0.002% or less Excessive S reduces resistance weldability. This is because an increase in S content increases the amount of sulfides such as MnS produced, and cracks form from these sulfides when hydrogen penetrates. Therefore, the S content is set to 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 may be 0%. However, excessive reduction increases steelmaking costs. Therefore, the S content is preferably set to 0.0002% or more.

[0038] Al: 0.01 to 0.10% Al is an element necessary for deoxidation. If the Al content is less than 0.01%, the deoxidation effect will be insufficient. Therefore, the Al content is set to 0.01% or more, preferably 0.02%. On the other hand, if the Al content is higher than 0.10%, the ferrite phase will be excessively generated during annealing, making it difficult to ensure strength. Therefore, the Al content is set to 0.10% or less, preferably 0.08% or less, and more preferably 0.05% or less.

[0039] N: 0.008% or less Excessive N reduces resistance weldability. This is because the amount of coarse nitrides formed increases with increasing N content, and cracks form from these nitrides when hydrogen penetrates. This tendency becomes more pronounced when the N content is 0.008% or more. Therefore, the N content is set to 0.008% or less, preferably 0.007% or less, and more preferably 0.005% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction increases steelmaking costs. Therefore, the N content is preferably set to 0.0005% or more, and more preferably 0.001% or more.

[0040] B: 0.0002 to 0.0050% B is an element that improves hardenability and contributes to high strength by forming martensite. Furthermore, B improves hardenability without lowering the martensite transformation starting point, making it useful for carbide formation. To achieve these effects, the B content is set to 0.0002% or more, preferably 0.0004% or more. However, if the B content exceeds 0.0050%, the effect saturates. Therefore, the B content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0035% or less.

[0041] The chemical composition of the steel sheet of the present invention contains at least one element selected from the group consisting of Ti, Nb, and V, each in the following amount. Ti, Nb, and V are elements that all have the common function of improving resistance weldability by forming fine carbides.

[0042] Ti: 0.005 to 0.07% Ti improves delayed fracture resistance after resistance welding by forming fine carbides. Furthermore, Ti also improves delayed fracture resistance after resistance welding by increasing the hydrogen overvoltage. When adding Ti, in order to achieve these effects, the Ti content is set to 0.005% or more, preferably 0.008% or more. On the other hand, adding a large amount of Ti significantly reduces elongation. Therefore, the Ti content is set to 0.07% or less, preferably 0.05% or less.

[0043] Nb: 0.005 to 0.07% Nb improves the delayed fracture resistance of resistance spot welds by forming fine carbides. When Nb is added, the Nb content is set to 0.005% or more, preferably 0.01% or more, to obtain this effect. On the other hand, adding a large amount of Nb not only significantly reduces elongation but also causes slab cracking after continuous casting. Therefore, the Nb content is set to 0.07% or less, preferably 0.05% or less.

[0044] V: 0.005 to 0.07% V improves the delayed fracture resistance of resistance spot welds by forming fine carbides. When V is added, the V content is set to 0.005% or more to obtain this effect. However, even if a large amount of V is added, the strength increase effect is small for amounts exceeding 0.07%, and the alloy cost also increases. Therefore, the V content is set to 0.07% or less, preferably 0.06% or less, and more preferably 0.05% or less.

[0045] A steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components with the balance being Fe and unavoidable impurities.

[0046] Examples of the unavoidable impurities include Co, Zn, Ta, Mg, and Zr. When Co is included as the unavoidable impurity, the Co content is preferably 0.10% or less. When Zn is included as the unavoidable impurity, the Zn content is preferably 0.10% or less. When Ta is included as the unavoidable impurity, the Ta content is preferably 0.10% or less. When Mg is included as the unavoidable impurity, the Mg content is preferably 0.10% or less. When Zr is included as the unavoidable impurity, the Zr content is preferably 0.10% or less.

[0047] Furthermore, the composition of the steel sheet according to another embodiment of the present invention may optionally contain at least one of the following elements in addition to the above-described elements. Note that all of the following elements can be added optionally, and therefore their inclusion is not essential. Therefore, the lower limit of their content may be 0%.

[0048] Sb: 0.02% or less Sb is an element that segregates at grain boundaries and thereby strengthens the grain boundaries. Adding Sb can further improve cross tensile strength. However, if the Sb content is higher than 0.02%, the formation of ferrite phase in the surface layer of the steel sheet is suppressed, and the microstructure in the surface layer cannot be made as desired. Therefore, when Sb is added, the Sb content is set to 0.02% or less, preferably 0.015% or less. On the other hand, although there is no particular lower limit for the Sb content, from the viewpoint of enhancing the effect of adding Sb, the Sb content is preferably set to 0.001% or more, and more preferably 0.002% or more.

[0049] Cu: 0.50% or less Cu is an element that has the effect of increasing hydrogen overvoltage and, as a result, further improving delayed fracture resistance after resistance welding. However, if the Cu content exceeds 0.50%, the effect saturates and surface defects become more likely to occur. Therefore, when Cu is added, the Cu content is set to 0.50% or less. On the other hand, from the viewpoint of enhancing the effect of adding Cu, the Cu content is preferably set to 0.005% or more.

[0050] Ni: 0.50% or less Like Cu, Ni is an element that has the effect of increasing hydrogen overvoltage and further improving delayed fracture resistance. Furthermore, when Ni is added together with Cu, it has the effect of suppressing surface defects caused by Cu. However, when the Ni content exceeds 0.50%, the effect saturates. Therefore, when Ni is added, the Ni content is set to 0.50% or less. On the other hand, from the viewpoint of enhancing the effect of adding Ni, it is preferable that the Ni content be 0.005% or more.

[0051] Cr:0.50% or less Cr is an element that contributes to further increasing strength by generating hard phases. However, if the Cr content exceeds 0.50%, surface defects are likely to occur. Therefore, when Cr is added, the Cr content is set to 0.50% or less, preferably 0.45% or less. On the other hand, from the viewpoint of enhancing the effect of adding Cr, the Cr content is preferably set to 0.02% or more, more preferably 0.05% or more.

[0052] Mo: 0.50% or less Like Cr, Mo is an element that contributes to further strengthening by forming a hard phase. Furthermore, a portion of Mo also contributes to further strengthening by forming carbides. However, if the Mo content exceeds 0.50%, the effect saturates, and the effect cannot be justified by the increase in cost. Therefore, when Mo is added, the Mo content is set to 0.50% or less, preferably 0.45% or less. On the other hand, from the viewpoint of enhancing the effect of adding Mo, the Mo content is preferably set to 0.02% or more, and more preferably 0.05% or more.

[0053] Sn: 0.30% or less Sn is an element that increases the hydrogen overvoltage of steel sheet, thereby further improving delayed fracture resistance. However, if the Sn content exceeds 0.30%, the effect saturates and ductility decreases. Therefore, when Sn is added, the Sn content is set to 0.30% or less, preferably 0.25% or less. On the other hand, from the viewpoint of enhancing the effect of adding Sn, the Sn content is preferably set to 0.005% or more, more preferably 0.01% or more.

[0054] Ca:0.0050% or less Ca is an element that contributes to further improving delayed fracture resistance after resistance welding by making the shape of sulfides spheroidal. However, when the Ca content exceeds 0.0050%, the effect saturates. Therefore, when Ca is added, the Ca content is set to 0.0050% or less. On the other hand, from the viewpoint of enhancing the effect of adding Ca, the Ca content is preferably set to 0.0005% or more.

[0055] REM: 0.0050% or less Like Ca, REM (rare earth metal) is an element that further improves delayed fracture resistance after resistance welding by spheroidizing the shape of sulfides. However, if the REM content exceeds 0.0050%, the effect saturates, so when REM is added, the REM content is set to 0.0050% or less. On the other hand, from the viewpoint of enhancing the effect of adding REM, the REM content is preferably set to 0.0005% or more.

[0056] [Microstructure] In the steel sheet of the present invention, the microstructure at the 1 / 4 position in the sheet thickness direction and the microstructure at a position in the region 7 to 12 μm from the surface of the steel sheet in the sheet thickness direction must each satisfy specific conditions. The reasons for this are explained below. The "1 / 4 position in the sheet thickness direction" refers to a position at a depth of 1 / 4 of the sheet thickness t of the steel sheet from the surface of the steel sheet, and may also be referred to as the 1 / 4t position.

[0057] In the present invention, tempered martensite is also defined as being included in "martensite." This is because it is difficult to distinguish between martensite and tempered martensite in the microstructure of the steel sheet of the present invention. Furthermore, the "tempered martensite" includes not only tempered martensite formed by self-tempering during the cooling process in annealing, but also tempered martensite formed by performing a tempering treatment after cooling to room temperature.

[0058] (Microstructure at 1 / 4 of the plate thickness) Ferrite: 0 to 5% If the ferrite volume fraction at the 1 / 4 position in the sheet thickness direction is higher than 5%, the desired tensile strength cannot be obtained. Therefore, the ferrite volume fraction at the 1 / 4 position in the sheet thickness direction is set to 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 ferrite volume fraction, the better, so the lower limit of the ferrite volume fraction is set to 0%.

[0059] Average grain size of ferrite: 3 μm or less If the average grain size of ferrite at the 1 / 4 position in the sheet thickness direction is greater than 3 μm, the delayed fracture resistance deteriorates. This is because cracks due to hydrogen embrittlement are more likely to occur at the interface between ferrite and martensite from the softened HAZ after resistance welding to the base material. Therefore, the average grain size of ferrite at the 1 / 4 position in the sheet thickness direction is set to 3 μm or less, preferably 2.5 μm or less. On the other hand, from the viewpoint of delayed fracture resistance, a smaller average grain size of ferrite is preferable, so the lower limit of the average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, the average grain size of ferrite is preferably set to 0.2 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more.

[0060] Retained austenite: 0 to 5% If the volume fraction of retained austenite at the 1 / 4 thickness position is higher than 5%, the delayed fracture resistance deteriorates. This is because, when the steel sheet is cold-press formed, the retained austenite transforms into martensite with a high dislocation density, which makes it easier for cracks to form due to hydrogen embrittlement from the softened HAZ to the base material after resistance welding. Therefore, the volume fraction of retained austenite at the 1 / 4 thickness position is set to 5% or less, preferably 4% or less. On the other hand, from the viewpoint of delayed fracture resistance, the lower the volume fraction of retained austenite, the better, so the lower limit of the volume fraction of retained austenite is set to 0%.

[0061] Average grain size of retained austenite: 3 μm or less If the average grain size of the retained austenite at the 1 / 4 position in the sheet thickness direction is greater than 3 μm, the delayed fracture resistance deteriorates. This is because the C distribution in the retained austenite makes it easier for martensite to form during cold press forming. Therefore, the average grain size of the retained austenite at the 1 / 4 position in the sheet thickness direction is set to 3 μm or less. On the other hand, there is no particular lower limit for the average grain size. However, since an average grain size of 0.3 μm contributes significantly to elongation, the average grain size is preferably set to 0.3 μm or more, more preferably 1 μm or more, and even more preferably 2 μm or more.

[0062] Bainite: 0-7% If the volume fraction of bainite at the 1 / 4 position in the sheet thickness direction is higher than 7%, the desired tensile strength cannot be obtained. Therefore, the volume fraction of bainite at the 1 / 4 position in the sheet thickness direction is set to 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, so the lower limit of the volume fraction of bainite is set to 0%.

[0063] Average grain size of bainite: 5 μm or less If the average grain size of bainite at the 1 / 4 thickness position is greater than 5 μm, delayed fracture resistance deteriorates. This is because cracks due to hydrogen embrittlement are more likely to occur at the interface between bainite and martensite from the softened HAZ after resistance welding to the base material. Therefore, the average grain size of bainite at the 1 / 4 thickness position is set to 5 μm or less, preferably 4 μm or less. On the other hand, from the viewpoint of delayed fracture resistance, a smaller average grain size of bainite is preferable, so the lower limit of the average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, the average grain size of bainite is preferably 0.1 μm or more, more preferably 0.5 μm or more, and even more preferably 1.0 μm or more.

[0064] Martensite: 93% or more To ensure the desired tensile strength, the volume fraction of martensite at the 1 / 4 position in the sheet thickness must be 93% or more. Therefore, the volume fraction of martensite at the 1 / 4 position in the sheet thickness must be 93% or more, preferably 95% or more. On the other hand, the upper limit of the volume fraction of martensite at the 1 / 4 position in the sheet thickness is not particularly limited, but may be 100%.

[0065] Average grain size of martensite: 7 μm or less If the average grain size of martensite at the 1 / 4 position in the sheet thickness direction is greater than 7 μm, the grains will become coarse after resistance welding, resulting in a decrease in L-shaped tensile strength. Therefore, the average grain size of martensite at the 1 / 4 position in the sheet thickness direction is set to 7 μm or less, preferably 6 μm or less. On the other hand, from the viewpoint of L-shaped tensile strength, a smaller average grain size of martensite is preferable, so there is no particular lower limit for the average grain size. However, from the viewpoint of ease of manufacture, the average grain size of martensite is preferably 1 μm or more, more preferably 3 μm or more, and even more preferably 4 μm or more.

[0066] In one embodiment of the present invention, the microstructure at the 1 / 4 thickness position is In terms of volume fraction, Ferrite: 0 to 5%, Retained austenite: 0 to 5% Bainite: 0-7% and Martensite: 93% or more It may consist of:

[0067] The microstructure may further contain other structures. Here, the other structures are structures other than ferrite, retained austenite, bainite, and martensite. The other structures may be, for example, pearlite. The volume fraction of the other structures may be 7% or less, and preferably 3% or less.

[0068] That is, the microstructure at the 1 / 4 position of the plate thickness in one embodiment of the present invention is In terms of volume fraction, Ferrite: 0 to 5%, Retained austenite: 0 to 5% Bainite: 0-7% Martensite: 93% or more, and Other organizations: 0-7% It may consist of:

[0069] (Microstructure in the region 7 to 12 μm from the surface in the thickness direction) Ferrite: 30% or more If the volume fraction of ferrite in the region 7 to 12 μm from the surface of the steel sheet in the thickness direction is less than 30%, the desired L-shaped tensile strength and delayed fracture resistance cannot be obtained. As mentioned above, this is thought to be because forming a sufficient amount of ferrite in this region softens the corona bond after resistance welding, resulting in improved crack propagation resistance. Therefore, the volume fraction of ferrite in the region 7 to 12 μm from the surface of the steel sheet in the thickness direction is set to 30% or more, preferably 45% or more, and more preferably 60% or more. Meanwhile, the upper limit of the ferrite volume fraction is not particularly limited and may be 100%. The ferrite volume fraction may be, for example, 90% or less, or 85% or less.

[0070] Average grain size of ferrite: 10 μm or less If the average grain size of ferrite in the region 7 to 12 μm from the surface of the steel sheet in the thickness direction is greater than 10 μm, the microstructure in the corona bonded portion after resistance welding will become coarse, resulting in a deterioration in delayed fracture resistance. Therefore, the average grain size of ferrite in the region 7 to 12 μm from the surface of the steel sheet in the thickness direction is set to 10 μm or less, preferably 9 μm or less, and more preferably 8 μm or less. On the other hand, from the viewpoint of delayed fracture resistance, a smaller average grain size of ferrite is preferable, so the lower limit of the average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, the average grain size of ferrite is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more.

[0071] The microstructure may also contain other structures. Here, the other structures are structures other than ferrite, such as bainite, pearlite, retained austenite, martensite, and cementite. From the viewpoint of further increasing tensile strength, it is preferable that 50% or more of the remainder other than ferrite is at least one of bainite and martensite. In other words, it is preferable that the total volume fraction of bainite and martensite in the remainder other than ferrite is 50% or more. The upper limit of the total volume fraction of bainite and martensite is not particularly limited and may be 100%. In other words, in one embodiment of the present invention, the microstructure in a region 7 to 12 μm from the surface in the sheet thickness direction may be a structure consisting of 30% or more ferrite by volume fraction and one or both of bainite and martensite as the remainder.

[0072] [carbide] Average density: 5 pieces / 100μm 2 End The average number density of carbides with a particle size of 0.10 μm or more in the region 50 to 100 μm from the surface of the steel plate in the plate thickness direction is 2If the number of carbides is less than 5 per 100μm, the delayed fracture resistance after resistance welding will be deteriorated. This is because the carbides with a grain size of 0.10μm or more present in this region function as hydrogen traps, improving the hydrogen embrittlement resistance of the nugget and HAZ. Therefore, the average number density of carbides with a grain size of 0.10μm or more in the region 50 to 100μm from the surface of the steel sheet in the sheet thickness direction should be set to 5 / 100μm. 2 More than 7 / 100μm 2 More preferably, 10 pieces / 100μm 2 On the other hand, from the viewpoint of delayed fracture resistance, the higher the average number density, the better, so there is no particular upper limit to the average number density. 2 May be less than 30 pieces / 100 μm 2 It may be the following:

[0073] Here, the type of the carbide is not particularly limited, but as described above, the steel sheet of the present invention contains at least one selected from the group consisting of Ti, Nb, and V. Since these elements easily form carbides, the steel sheet of the present invention may contain, as the carbide, at least one of Ti-based carbide, Nb-based carbide, and V-based carbide in addition to Fe-based carbide (cementite). The average number density of the carbides can be measured by TEM (transmission electron microscope) and EDS (energy dispersive X-ray spectroscopy), and more specifically, can be measured by the method described in the examples.

[0074] [Zinc-based plating layer] The steel sheet of the present invention may be a cold-rolled steel sheet having no plating layer on the surface, but it is preferable that the steel sheet has a zinc-based plating layer on at least one surface.

[0075] The zinc-based plating layer can be either a zinc plating layer or a zinc alloy plating layer. In other words, the steel sheet of the present invention may be a zinc-plated steel sheet or a zinc alloy-plated steel sheet. The zinc alloy plating layer is not particularly limited, and a plating layer made of any zinc alloy can be used. As the zinc alloy plating layer, it is preferable to use a zinc alloy plating layer having a composition selected from the group consisting of Zn-Al, Zn-Al-Mg, Zn-Al-Si, Zn-Al-Mg-Si, and Zn-Al-Mg-Ni.

[0076] The zinc-based plating layer can be formed by any method. For example, the zinc-based plating layer may be any of a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrogalvanized layer. In other words, the steel sheet of the present invention may be any of a hot-dip galvanized steel sheet, a galvannealed hot-dip galvanized steel sheet, and an electrogalvanized steel sheet.

[0077] The coating weight of the zinc-based coating layer is not particularly limited, but from the viewpoint of corrosion resistance and ease of coating weight control, the coating weight is preferably 25 g / m per one side of the steel sheet. 2 On the other hand, from the viewpoint of adhesion of the coating layer, the coating amount is preferably 80 g / m per one side of the steel sheet. 2 It is preferable that:

[0078] [Pre-plated layer] When the steel sheet has a zinc-based plating layer, a pre-plating layer may be further provided between the steel sheet (base steel sheet) and the zinc-based plating layer.

[0079] The pre-plating layer is not particularly limited and a plating layer of any composition can be used, but is preferably an Fe-based plating layer, and more preferably an Fe-based electroplating layer.

[0080] The Fe-based plating layer may be, for example, an Fe plating layer or an Fe alloy plating layer. Here, the Fe plating layer is a plating layer made of Fe and unavoidable impurities, and is also called a "pure Fe plating layer." Meanwhile, the Fe alloy plating layer is not particularly limited, and a plating layer made of any Fe alloy can be used. The Fe alloy plating layer may be, for example, a plating layer made of at least one alloy selected from the group consisting of an Fe-B alloy, an Fe-C alloy, an Fe-P alloy, an Fe-N alloy, an Fe-O alloy, an Fe-Ni alloy, an Fe-Mn alloy, an Fe-Mo alloy, and an Fe-W alloy.

[0081] In one embodiment of the present invention, the Fe-based electroplated layer preferably has a composition containing a total of 10% or less of at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder being Fe and unavoidable impurities. By keeping the total amount of elements other than Fe to 10% or less, a decrease in electrolysis efficiency can be prevented and the Fe-based electroplated layer can be formed at low cost.

[0082] The Fe-based plating layer functions as a soft layer, so by providing an Fe-based plating layer, it is possible to alleviate the stress applied to the steel sheet surface during welding. Furthermore, the presence of an Fe-based plating layer not only reduces residual stress in resistance welds, but also improves delayed fracture resistance by efficiently diffusible hydrogen escaping from the steel sheet surface.

[0083] The amount of the Fe-based coating layer is not particularly limited, but from the viewpoint of enhancing the above-mentioned effect, it is preferable that the amount of the Fe-based coating layer be 0.5 g / m per side of the steel sheet. 2 It is preferable that the content is 1.0 g / m or more. 2 On the other hand, from the viewpoint of cost, it is preferable that the coating weight of the Fe-based plating layer is 60 g / m per side. 2 It is preferable that the density is 50 g / m or less. 2 It is more preferable that the density is 40 g / m or less. 2 It is more preferable that the density is 30 g / m or less. 2 Most preferably, the following is true:

[0084] The coating weight of the Fe-based plating layer is measured as follows. After the zinc-based plating layer is formed, a 10 × 15 mm sample is taken from the steel sheet and embedded in resin to obtain a cross-section embedded sample. Three arbitrary locations on the cross-section are observed using a scanning electron microscope (SEM) to determine the thickness of the Fe-based plating layer. The acceleration voltage for the SEM observation is set to 15 kV. The magnification for the SEM observation is set to 2,000 to 10,000 times depending on the thickness of the Fe-based plating layer. The average thickness measured at the three locations is multiplied by the specific gravity of iron to convert it into the coating weight of the Fe-based plating layer per side.

[0085] By satisfying the above conditions, the steel sheet of the present invention has both a tensile strength of 1600 MPa or more and high L-shaped tensile strength after resistance welding, and also has excellent delayed fracture resistance after resistance welding. More specifically, a high L-shaped tensile strength of 1.5 kN or more can be achieved after resistance spot welding. Furthermore, resistance spot-welded joints produced using the steel sheet of the present invention do not suffer from delayed fracture even when subjected to the following treatment and hydrogen charging. - Leave in the air at room temperature (20°C) for 24 hours. Current density: 0.06mA / cm when immersed in a 3% NaCl + 0.5% NHCl SCN aqueous solution 2 , Time: 72h cathodic electrolytic charging

[0086] [Spot welding method] In a spot welding method according to one embodiment of the present invention, a plate assembly including at least one of the above-described steel plates is clamped between a pair of welding electrodes and joined by applying current while applying pressure. The conditions for spot welding are not particularly limited, and general welding conditions can be adopted.

[0087] For example, two steel sheets are overlapped to form a sheet assembly. Next, the sheet assembly is clamped from above and below with a pair of welding electrodes, and current is passed through the sheet assembly while applying pressure and controlling the welding conditions to achieve predetermined conditions. This joins the steel sheets that make up the sheet assembly, resulting in a resistance spot welded member. When a cold-rolled steel sheet and a zinc-based plated steel sheet are overlapped to form a sheet assembly, the steel sheets may be overlapped so that the surface of the zinc-based plated steel sheet having the zinc-based plating layer faces the cold-rolled steel sheet.

[0088] A resistance spot welding method according to one embodiment of the present invention can include a main current application step of clamping the sheet assembly using the pair of welding electrodes and applying current while applying pressure to form a nugget.

[0089] The energization conditions and pressure application conditions for forming the nugget in the main energization step are not particularly limited. From the viewpoint of application to structural parts of automobiles and the like, it is preferable that the energization conditions and pressure application conditions be adjusted to the following ranges.

[0090] For example, the current value in the main current application step can be preferably set to 3.0 to 15.0 kA to obtain a stable nugget diameter. The nugget diameters used in spot welds of automotive steel sheets are generally between 3.0√t and 6.0√t (where t is the thinnest sheet thickness in the sheet assembly), and if the current value is too small, the target nugget diameter cannot be consistently obtained. On the other hand, if the current value in the main current application step is set outside the above range, the nugget diameter may become too large, or the degree of melting of the steel sheets may increase, causing the molten weld to spill out of the gap between the sheets, resulting in a small nugget diameter.

[0091] The current application time of the main current application step is preferably 0.18 to 1.0 seconds. This is the time required to obtain the target nugget diameter, similar to the current value of the main current application step. If the current application time of the main current application step is less than 0.18 seconds, it becomes difficult to form a nugget. On the other hand, if the current application time of the main current application step exceeds 1.0 seconds, the nugget diameter may become large, and there is a concern that workability may be reduced. However, as long as the required nugget diameter is obtained, the current application time tw of the main current application step may be shorter or longer than the above-mentioned preferred range.

[0092] The pressure applied in the main current process is preferably 2.0 kN to 9.0 kN. If the pressure applied in the main current process is too large, the current diameter will increase, making it difficult to ensure the desired nugget diameter. On the other hand, if the pressure applied in the main current process is too small, the current diameter will decrease, making it more likely that expulsion will occur. Therefore, it is preferable that the pressure applied in the main current process be within the above-mentioned preferred range. Note that the pressure applied may be limited by the capacity of the equipment. However, as long as the pressure applied is sufficient to obtain the required nugget diameter, the pressure applied in the main current process may be lower or higher than the above-mentioned preferred range.

[0093] In a resistance spot welding method according to one embodiment of the present invention, a post-current may be applied after the main current application step. The post-current application can be performed under any conditions without particular limitations, but the current value in the post-current application is preferably higher than the current value in the main current application step. Specifically, the current value is preferably 1.1 times or more the current value in the main current application step. The welding time in the post-current application is preferably 1.0 second or less. The post-current application can be performed in multiple stages, and in this case, the total application time in the post-current application is preferably 1.0 second or less.

[0094] Furthermore, after the main current application step, a tempering step may be performed to temper the periphery of the nugget. The conditions for the tempering step are not particularly limited, but the current value in the tempering step is preferably lower than the current value in the main current application step, specifically, preferably 0.9 times or less the current value in the main current application step. Furthermore, the current application time in the tempering step is preferably 2.0 seconds or less.

[0095] [Resistance spot welding components] A resistance spot welded component according to one embodiment of the present invention is a resistance spot welded component that includes at least one of the above-described steel plates in a plate assembly. As described above, the resistance spot welded component can be manufactured by a general resistance spot welding method.

[0096] [Steel plate manufacturing method] Next, a method for manufacturing the steel sheet of the present invention will be described. As described above, the steel sheet of the present invention may be a cold-rolled steel sheet having no plating layer on its surface, or a zinc-based plated steel sheet having a zinc-based plating layer on its surface. The zinc-based plated steel sheet may be any of an electroplated steel sheet, a hot-dip plated steel sheet, and an alloyed hot-dip plated steel sheet. Therefore, a suitable manufacturing method for each case will be described below.

[0097] First embodiment In the first embodiment of the present invention, a steel sheet satisfying the above-mentioned conditions can be manufactured by using molten steel having the above-mentioned composition as a starting material and sequentially carrying out the following steps. Note that if no plating is performed after annealing, a steel sheet (cold-rolled steel sheet) without a plating layer on the surface can be obtained. (1) Continuous casting (2) Cooling (3) Reheating (4) Hot rolling (5) Cooling (6) Winding (7) Pickling (8) Heat treatment (9) Cold rolling (10) Annealing

[0098] (1) Continuous casting First, molten steel having the above-mentioned composition is continuously cast into a steel slab. Continuous casting has higher production efficiency than mold casting. The continuous casting can be performed using any continuous casting machine, but it is preferable to use a vertical bending type continuous casting machine. A vertical bending type continuous casting machine has an excellent balance between equipment cost and the surface quality of the resulting steel slab. Furthermore, a vertical bending type continuous casting machine is also excellent in suppressing surface cracks.

[0099] (2) Cooling ·Average cooling rate: 50℃ / h or more Next, the steel slab obtained by the continuous casting is cooled. If the average cooling rate in the temperature range up to 600°C is less than 50°C / h, the segregation of Mn is promoted, which deteriorates the delayed fracture resistance after resistance welding. Therefore, the steel slab is cooled under the condition of an average cooling rate of 50°C / h or more in the temperature range up to 600°C.

[0100] In the present invention, as described above, it is sufficient to control the average cooling rate in the temperature range up to 600°C, and the cooling stop temperature is not particularly limited. In other words, the material can be cooled to any temperature below 600°C. For example, the material may be cooled to room temperature, reheated, and hot-rolled, or the cooling may be stopped at a temperature higher than room temperature to form a hot slab, which is then reheated and hot-rolled.

[0101] (3) Reheating Next, the cooled steel slab is reheated, which allows the Ti-based precipitates, Nb-based precipitates, and V-based precipitates contained in the steel to be redissolved.

[0102] ·Heating temperature: 1280~1400℃ If the reheating temperature is less than 1280°C, the precipitates cannot be sufficiently redissolved, and coarse precipitates remain even after final annealing. As a result, the delayed fracture resistance after resistance welding 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 crystal grains become coarse. As a result, the desired crystal grain size cannot be obtained after final annealing, and resistance weldability deteriorates. Therefore, the heating temperature is set to 1400°C or lower, preferably 1350°C or lower.

[0103] ·Holding time: 60 minutes or more If the holding time in the reheating is less than 60 minutes, the precipitates cannot be sufficiently redissolved, and coarse precipitates remain even after final annealing. As a result, resistance weldability is reduced. Therefore, the holding time is set to 60 minutes or more. On the other hand, although there is no particular upper limit to the holding time, from the viewpoint of productivity, it is preferably set to 180 minutes or less, and more preferably set to 150 minutes or less.

[0104] (4) Hot rolling The reheated steel slab is then hot-rolled to produce a hot-rolled steel sheet. The hot-rolling process homogenizes the structure within the steel sheet and reduces the anisotropy of the material, thereby improving the L-shaped tensile strength after resistance welding.

[0105] Finishing rolling temperature: 850~950℃ To achieve this effect, it is necessary to finish hot rolling in the austenite single-phase region. Therefore, the finish rolling temperature is set to 850°C or higher. On the other hand, if the finish rolling temperature is higher than 950°C, the structure of the hot-rolled steel sheet becomes coarse, and the delayed fracture resistance after resistance welding deteriorates. Therefore, the finish rolling temperature is set to 950°C or lower.

[0106] (5) Cooling Next, the hot-rolled steel sheet is cooled. The steel sheet structure of the hot-rolled steel sheet is controlled by quenching to a temperature range where bainite transformation occurs without causing ferrite transformation. This homogenized hot-rolled structure control has the effect of refining the microstructure of the finally obtained steel sheet, mainly ferrite and martensite.

[0107] ·Average cooling rate: 80℃ / s or more To achieve this effect, the average cooling rate is set to 80°C / s or more. If the average cooling rate is less than 80°C / s, ferrite transformation will begin, resulting in a heterogeneous microstructure and reduced resistance weldability. On the other hand, although there is no particular upper limit for the average cooling rate, it is preferably set to 200°C / s or less.

[0108] ·Cooling stop temperature: 460℃ or less Similarly, to obtain the above effect, the cooling stop temperature is set to 460°C or lower. If the cooling stop temperature is higher than 460°C, pearlite is formed in excess, the steel sheet structure of the hot-rolled steel sheet becomes inhomogeneous, and the resistance weldability deteriorates. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, but it is preferably set to 250°C or higher.

[0109] (6) Winding · Winding temperature: 460℃ or less Next, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 460°C or less. If the coiling temperature is higher than 460°C, pearlite is formed in excess, and the steel sheet structure of the hot-rolled steel sheet becomes inhomogeneous, resulting in reduced resistance weldability. Therefore, the coiling temperature is set to 460°C or less, preferably 440°C or less. There are no particular restrictions on the lower limit of the coiling temperature, but if the coiling temperature is too low, hard martensite is formed in excess, increasing the cold rolling load. Therefore, the coiling temperature is preferably set to 250°C or more.

[0110] (7) Pickling The hot-rolled steel sheet after coiling is subjected to pickling. By performing pickling, scale formed on the surface layer of the hot-rolled steel sheet can be removed. The conditions for the pickling are not particularly limited, and the pickling can be performed according to a conventional method.

[0111] (8) Heat treatment Heat treatment temperature: 300~700℃ Next, the pickled hot-rolled steel sheet is subjected to heat treatment at a heat treatment temperature of 300 to 700°C to obtain a heat-treated hot-rolled steel sheet. By performing the heat treatment, the precipitation state of carbides in the finally obtained steel sheet can be improved, and delayed fracture resistance after resistance welding can be improved.

[0112] If the heat treatment temperature is less than 300°C, carbides will not precipitate sufficiently, making it impossible to obtain the desired delayed fracture resistance. Therefore, the heat treatment temperature is set to 300°C or higher. On the other hand, if the heat treatment temperature is higher than 700°C, austenite will be generated, resulting in non-uniform element distribution and making it impossible to achieve the average number density of carbides within the desired range. Therefore, the heat treatment temperature is set to 700°C or lower.

[0113] The time for which the heat treatment is carried out (heat treatment time) is not particularly limited, but if it exceeds 96 hours, productivity will drop significantly. Therefore, from the viewpoint of further improving productivity, the heat treatment time is preferably 96 hours or less.

[0114] (9) Cold rolling The heat-treated hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The conditions for the cold-rolling are not particularly limited, and the cold-rolling can be carried out in accordance with a conventional method.

[0115] (10) Annealing Next, the cold-rolled steel sheet is annealed, which promotes recrystallization and forms a structure (martensite) necessary to obtain the desired strength.

[0116] Dew point in the temperature range of 600-980°C: above -15°C During the annealing, C in the surface layer of the steel sheet reacts with moisture in the atmosphere, thereby reducing the C concentration in the surface layer. As a result, ferrite can be formed in a region of 7 to 12 μm from the surface of the steel sheet in the sheet thickness direction. However, if the dew point is −15°C or lower in the temperature range of 600 to 980°C, the microstructure of the surface layer of the steel sheet will not be as desired, resulting in reduced resistance weldability. Therefore, when performing the annealing, the dew point in the temperature range of 600 to 980°C is set to be higher than −15°C, preferably −10°C or higher, and more preferably −5°C or higher. While there are no particular limitations on the upper limit of the dew point, it is preferably set to 30°C or lower from the viewpoint of improving adhesion when a zinc-based plating layer is formed on the steel sheet surface. The reaction between C in the surface layer of the steel sheet and moisture mainly proceeds at temperatures of 600°C or higher. Therefore, the dew point during annealing may be controlled in the temperature range of 600 to 980°C, and the dew point in the temperature range below 600°C is not particularly limited.

[0117] In the annealing, heating, holding, and cooling are carried out in the following order. Average heating rate: 12°C / s or more up to 650°C. Average heating rate: less than 12℃ / s, annealing temperature: heated to 830-980℃. Hold (soak) at the annealing temperature for 20 to 360 seconds. Cool from the annealing temperature to room temperature at an average cooling rate of 3°C / s or more.

[0118] That is, in this embodiment, heating to the annealing temperature is performed in two stages, and the average heating rate in each stage is controlled within a specific range. In the following description, for convenience, the first heating stage will be referred to as "first heating" and the second heating stage will be referred to as "second heating."

[0119] (First heating: up to 650℃) ·Average heating speed: 12℃ / s or more First, the cold-rolled steel sheet is heated to 650°C (first heating). If the average heating rate to 650°C is less than 12°C / s, the structure of the steel sheet becomes coarse, making it impossible to obtain the desired average crystal grain size. Therefore, in the annealing, the cold-rolled steel sheet is heated to 650°C at an average heating rate of 12°C / s or more. There is no particular upper limit to the average heating rate, but if the steel sheet is heated too quickly, recrystallization will not proceed easily. Therefore, the average heating rate is preferably 30°C / s or less.

[0120] (Second heating) Average heating rate: less than 12°C / s Next, the cold-rolled steel sheet is heated to the annealing temperature (second heating). In this second heating, heating to the annealing temperature at an average heating rate of less than 12°C / s promotes decarburization near the surface of the steel sheet, resulting in the formation of a ferrite phase. If the average heating rate is 12°C / s or more, decarburization becomes insufficient, and the volume fraction of ferrite in the region 7 to 12 μm from the steel sheet surface in the thickness direction cannot be set within the desired range. Therefore, the average heating rate is set to less than 12°C / s. On the other hand, although there is no particular lower limit for the average heating rate, it is preferable that the average heating rate be more than 2°C / s in order to ensure tensile strength.

[0121] Annealing temperature: 830~980℃ If the annealing temperature is less than 830°C, the ferrite fraction becomes too high, making it difficult to achieve both tensile strength and resistance weldability. Therefore, the annealing temperature is set to 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, causing the grains to coarsen, resulting in a decrease in resistance spot weldability. Therefore, the annealing temperature is set to 980°C or lower, preferably 950°C or lower.

[0122] ·Holding time: 20~360 seconds By holding at the annealing temperature, recrystallization is promoted and part or all of the structure is transformed into austenite. If the holding time at the annealing temperature is less than 20 seconds, the desired microstructure cannot be obtained. Therefore, the holding time is set to 20 seconds or more. On the other hand, if the holding time is longer than 360 seconds, the crystal grains become coarse, resulting in a decrease in resistance weldability. Therefore, the holding time is set to 360 seconds or less, preferably 300 seconds or less.

[0123] After holding at the annealing temperature, the material is cooled to room temperature at an average cooling rate of 3°C / s or more.

[0124] ·Average cooling rate: 3℃ / s or more If the average cooling rate during the cooling is less than 3°C / s, the desired martensite volume fraction cannot be obtained, resulting in a decrease in tensile strength. Therefore, the average cooling rate is set to 3°C / s or more. On the other hand, although there is no particular upper limit for the average cooling rate, it is preferably set to less than 100°C / s.

[0125] Furthermore, after the annealing, temper rolling may be further carried out. The temper rolling may be carried out under any conditions, but it is preferable that the elongation ratio is set to 0.05% to 2.0%.

[0126] Second embodiment In a second embodiment of the present invention, the annealed steel sheet may be electroplated to form a zinc-based plating layer on at least one surface of the steel sheet, thereby obtaining an electrogalvanized steel sheet.

[0127] (Electroplating) The electroplating can be carried out under any conditions without any particular limitations. That is, in the present invention, the desired properties are achieved by controlling the microstructure and precipitates of the base steel sheet, so the plating treatment conditions are not limited and can be carried out according to a conventional method.

[0128] Third embodiment In a third embodiment of the present invention, a steel sheet satisfying the above-described conditions can be manufactured by using molten steel having the above-described chemical composition as a starting material and sequentially carrying out the following steps. According to this method, a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface can be obtained. (1) Continuous casting (2) Cooling (3) Reheating (4) Hot rolling (5) Cooling (6) Winding (7) Pickling (8) Heat treatment (9) Cold rolling (10) Annealing (11) Hot-dip plating (12) Cooling

[0129] In this embodiment, the steps (1) to (10) can be performed under the same conditions as in the first embodiment. However, in the cooling step (10) of annealing, instead of cooling to room temperature, cooling may be performed until the steel sheet is ready to enter the hot-dip galvanizing bath. The average cooling rate in the cooling step is 3°C / s or more, as in the first embodiment.

[0130] The remaining steps (11) and (12) will be explained below.

[0131] (11) Hot-dip plating In this embodiment, the annealed steel sheet is subjected to hot-dip galvanization by immersing it in a hot-dip galvanizing bath to form a hot-dip galvanized layer on at least one surface of the steel sheet. By this method, a hot-dip galvanized steel sheet can be obtained.

[0132] The hot dip coating can be carried out by any method. That is, in the present invention, the desired properties are achieved by controlling the microstructure and precipitates of the base steel sheet, so the plating treatment conditions are not particularly limited and can be carried out according to a conventional method.

[0133] In the hot-dip coating, any hot-dip coating bath can be used without any particular limitation, but it is preferable to use a hot-dip coating bath having a composition consisting of Al, Zn, and unavoidable impurities. The Al concentration in the coating bath is not particularly limited, but may be, for example, 0.05% or more and 0.25% or less. If the Al concentration is 0.05% or more, the generation of bottom dross is suppressed, thereby preventing the dross from adhering to the steel sheet and causing defects. On the other hand, if the Al concentration is 0.25% or less, the increase in top dross is suppressed, thereby preventing the dross from adhering to the steel sheet and causing defects. Furthermore, lowering the Al concentration reduces material costs.

[0134] Other conditions for the hot-dip galvanizing are not particularly limited. For example, the temperature of the hot-dip galvanizing bath is preferably 440 to 500°C, which is the bath temperature in general hot-dip galvanizing. Furthermore, the temperature of the steel sheet when entering the hot-dip galvanizing bath (entry sheet temperature) is preferably 440 to 550°C.

[0135] Furthermore, after the hot-dip galvanizing treatment, the coating weight may be adjusted. Although there is no particular limitation on the method for adjusting the coating weight, the coating weight is typically adjusted by gas wiping. The coating weight is adjusted by adjusting the gas wiping conditions, such as the gas pressure and the distance between the wiping nozzle and the steel sheet.

[0136] (12) Cooling Next, the hot-dip galvanized steel sheet is cooled to room temperature at an average cooling rate of 3°C / s or more. If the average cooling rate is less than 3°C / s, the desired martensite volume fraction cannot be obtained, resulting in a decrease in tensile strength. Therefore, the average cooling rate is set to 3°C / s or more. On the other hand, although there are no particular limitations on the upper limit of the average cooling rate, it is preferably set to less than 100°C / s.

[0137] Thus, in this embodiment, it is important to set the average cooling rate to 3°C / s or more in both the annealing process and the cooling after hot dip plating in order to obtain the desired microstructure.

[0138] Fourth embodiment In a fourth embodiment of the present invention, after the hot-dip galvanization, an alloying treatment is performed prior to the cooling to room temperature. The hot-dip galvanized layer is alloyed by the alloying treatment, and a galvannealed steel sheet can be obtained.

[0139] (Alloying treatment) The alloying treatment can be carried out under any conditions without any particular limitations. That is, in the present invention, the desired properties are achieved by controlling the microstructure and precipitates of the base steel sheet, so the alloying treatment conditions are not limited and can be carried out according to a conventional method.

[0140] The alloying treatment is preferably carried out at a temperature of 450°C or higher and 600°C or lower. By carrying out the alloying treatment at 450°C or higher, it is possible to provide a steel sheet with excellent press formability without leaving any η phase in the coating layer. Furthermore, by carrying out the alloying treatment at 600°C or lower, good coating adhesion can be obtained. The alloying time is preferably 5 to 60 seconds.

[0141] (pre-plating) Furthermore, in another embodiment of the present invention, prior to plating to form a zinc-based plating layer, pre-plating may be optionally further carried out to form a pre-plated layer on the surface of the steel sheet.

[0142] The timing of the pre-plating is not particularly limited, as long as it is performed before plating for forming a zinc-based plating layer. Typically, when a zinc-based plating layer is formed by electroplating, it is preferable that the pre-plating be performed after the annealing and before the electroplating. That is, annealing, pre-plating, and zinc-based electroplating may be performed sequentially on the cold-rolled steel sheet. On the other hand, when a zinc-based plating layer is formed by hot-dip plating, it is preferable that the pre-plating be performed after the cold rolling and before the annealing. That is, the treatments may be performed in the order of cold rolling, pre-plating, annealing, and hot-dip galvanizing. Note that, when an alloying treatment is performed, it may be performed after the hot-dip galvanizing, as usual.

[0143] In the pre-plating, any pre-plating layer can be formed, but it is preferable to form an Fe-based plating layer. The Fe-based plating layer is preferably formed by electroplating. Hereinafter, a case where an Fe-based electroplating process is performed to form an Fe-based plating layer as a pre-plating layer will be described.

[0144] The Fe-based electroplating method is not particularly limited. 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.

[0145] The Fe ion content in the Fe-based electroplating bath before the start of current application is 2+ The content of Fe ions in the Fe-based electroplating bath is preferably 1.0 mol / L or more. 2+ A sufficient Fe deposition amount can be obtained if the iron content is 1.0 mol / L or more. The Fe-based electroplating bath may contain Fe ions and alloying elements such as B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, as well as conductivity enhancers such as sodium sulfate and potassium sulfate as additives or impurities. Metal elements may be present as metal ions, while nonmetal elements may be present as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. The iron sulfate plating solution may also contain conductivity enhancers such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0146] It is also possible to subject the cold-rolled steel sheet to Fe-based electroplating treatment without subjecting it to oxidation treatment in a preheating furnace or the like.

[0147] Other conditions for the Fe-based electroplating bath are not particularly limited. The bath temperature is preferably 30°C or higher, taking into consideration the ability to maintain a constant temperature. The pH of the Fe-based electroplating bath is not particularly specified, but is preferably 3.0 or less, taking into consideration the electrical conductivity of the Fe-based electroplating bath. The current density is also not particularly limited, but is usually 10 to 150 A / dm 2The strip threading speed should be 5 mpm or more and 150 mpm or less. If the strip threading speed is less than 5 mpm, productivity is poor, while if the strip threading speed is 150 mpm or more, it is difficult to stably control the coating weight.

[0148] Prior to the Fe-based electroplating treatment, the cold-rolled steel sheet may be subjected to a degreasing treatment and water washing to clean the surface, and further to a pickling treatment and water washing to activate the surface. It is preferable to perform these pretreatments followed by the Fe-based electroplating treatment. The degreasing and water washing methods are not particularly limited, and conventional methods can be used. Various acids can be used in the pickling treatment, such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof. Among these, sulfuric acid, hydrochloric acid, or mixtures thereof are preferred. The acid concentration is not particularly limited, but is preferably about 1 to 20% by mass, taking into consideration the ability to remove oxide films and the prevention of surface roughness (surface defects) due to excessive pickling. The pickling solution may also contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc. [Example]

[0149] To confirm the effects of the present invention, steel sheets were prepared by the following procedure and their properties were evaluated. Four types of steel sheets were prepared: cold-rolled steel sheet (CR), electrogalvanized steel sheet (EG), hot-dip galvanized steel sheet (GI), and galvannealed steel sheet (GA).

[0150] First, molten steel having the chemical composition shown in Table 1 was continuously cast into a steel slab, which was then cooled. The average cooling rate in the temperature range up to 600°C during the cooling was as shown in Table 2.

[0151] The cooled steel slab was reheated under the conditions shown in Table 2 and then hot rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was cooled to the coiling temperature under the conditions shown in Table 2 and coiled into a coil. Thereafter, the hot-rolled steel sheet was pickled.

[0152] Next, the pickled hot-rolled steel sheets were subjected to heat treatment at the heat treatment temperatures shown in Table 2 to obtain heat-treated hot-rolled steel sheets. The heat treatment time was 12 hours.

[0153] Thereafter, the heat-treated hot-rolled steel sheet was subjected to cold rolling to obtain a cold-rolled steel sheet having a thickness of 1.4 mm.

[0154] In producing the cold-rolled steel sheet (CR), the cold-rolled steel sheet was annealed under the conditions shown in Table 2. In the annealing, the cold-rolled steel sheet was soaked 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.

[0155] In the production of electrogalvanized steel sheets (EG), the cold-rolled steel sheets were 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 was performed to form an electrogalvanized layer on the surface of the steel sheets. 2+ The plating was carried out using a sulfuric acid bath containing 1.5 mol / L of HCl. The temperature of the plating bath was 50°C and the pH was 1.5.

[0156] In the production of hot-dip galvanized steel sheets (GI), the cold-rolled steel sheets were first annealed under the conditions shown in Table 2 and then cooled to the point of immersion in a hot-dip galvanizing bath at the average cooling rate shown in Table 2. The steel sheets were then immersed in the hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the steel sheets. The hot-dip galvanizing bath used was a bath containing Al, Zn, and unavoidable impurities, with an Al concentration of 0.14%. The temperature of the hot-dip galvanizing bath was 460°C. The steel sheets were then cooled to room temperature at the average cooling rate shown in Table 2.

[0157] In the production of the galvannealed steel sheet (GA), the cold-rolled steel sheet was first annealed under the conditions shown in Table 2 and then cooled to the point of immersion in a hot-dip galvanizing bath at the average cooling rate shown in Table 2. Thereafter, an alloying treatment was performed to alloy the hot-dip galvanized layer. The hot-dip galvanizing bath used had the same composition as the galvanizing bath used in the production of the hot-dip galvanized steel sheet (GI), and the temperature of the galvannealed bath was also the same. The alloying treatment was performed at a temperature of 550°C. The cold-rolled steel sheet was then cooled to room temperature at the average cooling rate shown in Table 2.

[0158] Next, the microstructure and average number density of carbides of the obtained steel sheets were measured by the following procedure. The results are shown in Table 3.

[0159] (Microstructure at 1 / 4 of the plate thickness) Ferrite, martensite, bainite The volume fractions of ferrite, martensite, and bainite in the steel sheet were measured using the following procedure. First, a cross-section of the steel sheet parallel to the rolling direction was polished and etched with 3% nital to reveal the structure. The cross-section was then observed at magnifications of 3,000x and 10,000x using an SEM (scanning electron microscope), a TEM (transmission electron microscope), and an FE-SEM (field emission scanning electron microscope), and microscopic images of the microstructure at a position 1 / 4 of the sheet thickness were obtained. The area fractions of ferrite, martensite, and bainite in the microscopic images were calculated using the point counting method (in accordance with ASTM E562-83 (1988)), and these area fractions were used as volume fractions.

[0160] The average grain sizes of ferrite, martensite, and bainite were determined by image analysis of the microscopic images. Specifically, the area of each grain of ferrite, martensite, and bainite in the microscopic images was first determined by image analysis. Next, the equivalent circle diameters of the grains were calculated from the areas, and the average value was used as the average grain size. Image-Pro from Media Cybernetics was used for the image analysis.

[0161] ·Residual austenite The volume fraction of retained austenite was determined by X-ray diffraction. Specifically, the steel sheet was first polished to a quarter of the thickness direction, and the diffracted X-ray intensity at the quarter-thickness plane was measured by X-ray diffraction. The measurement was performed using an X-ray diffractometer RINT2200 manufactured by Rigaku Corporation, using Mo Kα radiation as a radiation source at an acceleration voltage of 50 keV. The integrated intensities of X-ray diffraction lines 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. For the calculation, a formula described on pages 26, 62-64 of "X-Ray Diffraction Handbook" (2000, Rigaku Corporation) was used.

[0162] The average grain size of the retained austenite grains was determined by polishing the cross section to the 1 / 4th of the plate thickness, etching with 3% nital, and then analyzing the TEM image obtained by TEM observation. Specifically, first, a TEM was used to observe at a magnification of 15,000 times to obtain a TEM image of the microstructure at the 1 / 4th of the plate thickness position. The obtained TEM image was analyzed to determine the area of each retained austenite grain. Next, the circle-equivalent diameter of each grain was calculated from the area, and the average value was used as the average grain size of the retained austenite. For the image analysis, Image-Pro from Media Cybernetics was used.

[0163] (Microstructure in the region 7 to 12 μm from the surface in the thickness direction) The microstructure of the steel sheet in the range of 7 to 12 μm deep from the surface was observed under a microscope, and the ferrite volume fraction and average crystal grain size were calculated. When the steel sheet to be measured had a coating layer (EG, GI, and GA), measurement by glow discharge optical emission spectroscopy was performed from the surface of the steel sheet, and the point where Fe exceeded 50 mass% was considered to be the surface of the steel sheet. The microscopic observation and calculation of the volume fraction and average crystal grain size from the microscopic image were performed in the same manner as in the measurement of the microstructure at the 1 / 4 position of the sheet thickness.

[0164] (number density of precipitates) The average number density of carbides with a particle size of 0.10 μm or more in a region 50 to 100 μm from the surface in the sheet thickness direction was determined by TEM observation. Specifically, first, the L-section of the steel sheet was observed using a TEM at 10,000x magnification, and TEM images were obtained from 10 locations randomly selected from a range 50 to 100 μm deep from the surface of the steel sheet. Next, the TEM images were analyzed using Image-Pro to determine the area of each carbide, and the circle-equivalent diameter of each particle was calculated from the area. Carbides present in the TEM images were identified using EDS (Energy Dispersive X-ray Spectroscopy). The number of carbides with a circle-equivalent diameter of 0.10 μm or more was then counted and divided by the area of the observed region to determine the number density of the carbides. The number density of precipitates was calculated for the 10 TEM images using the same procedure, and the average value was used as the average number density of carbides.

[0165] Next, the tensile strength and resistance weldability of the obtained steel sheets were evaluated by the following procedures.

[0166] (tensile strength) JIS No. 5 tensile test pieces were taken from the steel sheets so that the direction perpendicular to the rolling direction was the longitudinal direction (tensile direction). Next, a tensile test was performed using the test pieces to measure the tensile strength (TS) of the steel sheets. The tensile test was performed in accordance with JIS Z2241 (1998).

[0167] (resistance weldability) As an index of weldability, L-shaped tensile strength and delayed fracture properties after resistance spot welding were evaluated.

[0168] ·L-shaped tensile strength First, an L-shaped tensile test piece was prepared to evaluate the L-shaped tensile strength. Specifically, two 50 × 150 mm test pieces were cut out from the steel plate, and each of the two test pieces was bent at 90° in a V-bend so that the size of the welded surface was 50 × 50 mm. The two bent test pieces were welded together by resistance spot welding at the center of the welded surface to prepare an L-shaped tensile test piece.

[0169] The resistance spot welding was performed using a servomotor-pressurized single-phase AC (50 Hz) resistance welding machine. The electrode tip used was a DR-type electrode made of alumina-dispersed copper with a tip curvature radius of 40 mm and a tip diameter of 6 mm. The welding conditions were as follows: Pressure: 4500N Current duration: 20 cycles (50Hz) Hold time: 5 cycles (50Hz) Nugget diameter: 5.0√t(mm) Here, t is the thickness (mm) of the steel plate used.

[0170] Using the obtained L-shaped tensile test piece, a tensile test was performed at a tensile speed (longitudinal direction) of 10 mm / min to measure the L-shaped tensile strength. An L-shaped tensile strength of 1.5 kN or more was judged as "pass", and an L-shaped tensile strength of less than 1.5 kN was judged as "fail". The judgment results are shown in Table 3.

[0171] - Delayed fracture resistance First, a welded joint to be used for evaluating delayed fracture resistance was prepared. Specifically, two 50 × 150 mm test pieces were cut out from the steel plate. Next, the two test pieces were overlapped with a 50 mm × 50 mm spacer with a thickness of 1.4 mm sandwiched between both ends of the test pieces, and tack-welded. Next, the centers of the tack-welded test pieces were welded to form a welded joint. The welding conditions were the same as those for preparing the L-shaped tensile test pieces, so that the nugget diameter was 5.0√t (mm).

[0172] The obtained welded joint was left standing in the air at room temperature (20°C) for 24 hours. Next, the right-hand bent joint was immersed in a 3% NaCl + 1.0% NHSCN aqueous solution and subjected to a current of 0.06 mA / cm 2 Cathodic electrolytic charging was carried out for 72 hours at a current density of 1000 kJ / s. After that, it was checked whether delayed fracture had occurred in the welded joint. If delayed fracture had not occurred in the welded joint, it was judged as "pass", and if it had occurred, it was judged as "fail". The judgment results are shown in Table 3.

[0173] As can be seen from the results shown in Table 3, the steel sheets satisfying the conditions of the present invention had a tensile strength of 1600 MPa or more and high L-shaped tensile strength after resistance welding, and furthermore, had excellent delayed fracture resistance after resistance welding.

[0174] [Table 1]

[0175] [Table 2]

[0176] [Table 3]

Claims

1. The component composition is, in mass%, C: 0.22-0.38%, Si: 0.05-1.35%, Mn: 2.4 to 3.5%, P: 0.02% or less, S: 0.002% or less, Al: 0.01-0.10%, N: 0.008% or less, B: 0.0002 to 0.0050%, and Contains at least one selected from the group consisting of Ti: 0.005 to 0.07%, Nb: 0.005 to 0.07%, and V: 0.005 to 0.07%, the balance being Fe and unavoidable impurities; The microstructure at the 1 / 4 position of the plate thickness is In terms of volume fraction, Ferrite: 0 to 5%, Retained austenite: 0 to 5% Bainite: 0-7%; and Martensite: Contains 93% or more, Average grain size of ferrite: 3 μm or less, Average grain size of retained austenite: 3 μm or less, The average grain size of bainite is 5 μm or less, and Average grain size of martensite: 7 μm or less, The microstructure in the region of 7 to 12 μm from the surface in the plate thickness direction is The volume fraction of ferrite is 30% or more, The average grain size of ferrite is 10 μm or less, The average number density of carbides with a particle size of 0.10 μm or more in the region 50 to 100 μm from the surface in the plate thickness direction is 5 / 100 μm ２ That's it, steel plate.

2. The component composition is, in mass%, Sb: 0.02% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Sn: 0.30% or less, Ca: 0.0050% or less, and The steel plate according to claim 1, further comprising at least one selected from the group consisting of REM: 0.0050% or less.

3. The steel sheet according to claim 1 or 2, having a zinc-based plating layer on at least one surface.

4. A resistance spot welding method comprising: clamping a plate assembly including at least one steel plate according to claim 1 or 2 between a pair of welding electrodes; and applying current while applying pressure to the plate assembly to join the plate assembly.

5. A resistance spot welding method in which a plate assembly including at least one steel plate described in claim 3 is clamped between a pair of welding electrodes and joined by passing electricity through it while applying pressure.

6. A resistance spot welded component comprising at least one steel plate according to claim 1 or 2 in a plate assembly.

7. A resistance spot welded component comprising at least one steel plate according to claim 3 in a plate assembly.

8. 3. A steel slab is formed by continuously casting a molten steel having the chemical composition according to claim 1 or 2. The steel slab is cooled at an average cooling rate of 50°C / h or more in a temperature range up to 600°C, The cooled steel slab is reheated under the conditions of a heating temperature of 1280 to 1400°C and a holding time at the heating temperature of 60 minutes or more, The reheated steel slab is hot-rolled under the condition of a finish rolling end temperature of 850 to 950°C to obtain a hot-rolled steel sheet; The hot-rolled steel sheet is cooled to a cooling stop temperature of 460°C or less at an average cooling rate of 80°C / s or more, The cooled hot-rolled steel sheet is coiled at a coiling temperature of 460°C or less. The hot-rolled steel sheet after coiling is subjected to pickling, The pickled hot-rolled steel sheet is subjected to a heat treatment at a heat treatment temperature of 300 to 700°C to obtain a heat-treated hot-rolled steel sheet; The heat-treated hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is annealed under conditions in which the dew point is higher than −15° C. in a temperature range of 600 to 980° C., In the annealing, the cold-rolled steel sheet is Average heating rate: 12 ° C. / s or more to 650 ° C. Heating to an annealing temperature of 830 to 980°C at an average heating rate of less than 12°C / s, Holding at the annealing temperature for a holding time of 20 to 360 seconds; The method for producing a steel sheet comprises cooling the steel sheet from the annealing temperature to room temperature at an average cooling rate of 3°C / s or more.

9. The method for producing a steel sheet according to claim 8, further comprising electroplating the annealed steel sheet to form a zinc-based plating layer on at least one surface of the steel sheet.

10. 3. A steel slab is formed by continuously casting a molten steel having the chemical composition according to claim 1 or 2. The steel slab is cooled at an average cooling rate of 50°C / h or more in a temperature range up to 600°C, The cooled steel slab is reheated under the conditions of a heating temperature of 1280 to 1400°C and a holding time at the heating temperature of 60 minutes or more, The reheated steel slab is hot-rolled under the condition of a finish rolling end temperature of 850 to 950°C to obtain a hot-rolled steel sheet; The hot-rolled steel sheet is cooled to a cooling stop temperature of 460°C or less at an average cooling rate of 80°C / s or more, The cooled hot-rolled steel sheet is coiled at a coiling temperature of 460°C or less. The hot-rolled steel sheet after coiling is subjected to pickling, The pickled hot-rolled steel sheet is subjected to a heat treatment at a heat treatment temperature of 300 to 700°C to obtain a heat-treated hot-rolled steel sheet; The heat-treated hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is annealed under conditions in which the dew point is higher than −15° C. in a temperature range of 600 to 980° C., hot-dip galvanizing the annealed cold-rolled steel sheet to form a zinc-based plating layer on at least one surface of the steel sheet; A method for producing a steel sheet, comprising cooling the hot-dip coated steel sheet to room temperature at an average cooling rate of 3 ° C. / s or more, In the annealing, the cold-rolled steel sheet is Average heating rate: 12 ° C. / s or more to 650 ° C. Heating to an annealing temperature of 830 to 980°C at an average heating rate of less than 12°C / s, Holding at the annealing temperature for a holding time of 20 to 360 seconds; A method for producing a steel sheet, wherein the steel sheet is cooled at an average cooling rate of 3 ° C. / s or more from the annealing temperature to immersion in a hot dip coating bath.

11. The method for producing a steel sheet according to claim 10, further comprising the step of: subjecting the steel sheet to an alloying treatment after the hot dip coating and before the cooling to room temperature.