Steel plates, resistance spot welding methods, resistance spot welded components, and methods for manufacturing steel plates.

Abstract

Invention details;

Description

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

[0001] The present invention relates to a steel sheet, and in particular to a steel sheet that can be suitably used 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.

[0002] In recent years, CO ２ As emission regulations become stricter, reducing the weight of vehicle bodies to improve fuel efficiency has become an issue in the automotive industry. To address this issue, the use of high-strength steel sheets to reduce the thickness of automotive parts has been promoted, and in particular, the use of steel sheets with a tensile strength (TS) of 1450 MPa or more has been promoted.

[0003] On the other hand, high-strength steel sheets used in automotive structural and reinforcing components are required to have not only strength but also excellent formability. In particular, for use as materials for parts with complex shapes, steel sheets are required to have excellent hole expandability.

[0004] However, in order to ensure a tensile strength of 1450 MPa or more, it is necessary to add a large amount of alloying elements to the steel sheet, which reduces the hole expandability, making it difficult to achieve both high strength and excellent hole expandability.

[0005] Furthermore, resistance spot welding is widely used in automobile manufacturing from the viewpoints of cost and production efficiency. However, the addition of alloying elements reduces the resistance spot weldability (hereinafter simply referred to as "weldability") of steel sheets. Specifically, the addition of large amounts of alloying elements significantly narrows the appropriate current range for obtaining the required nugget diameter by resistance spot welding. Furthermore, the cross tensile strength (CTS) of the joint after resistance spot welding is significantly reduced. Therefore, it has been difficult to achieve both high strength and excellent weldability.

[0006] Therefore, various methods have been considered to solve the above problems.

[0007] For example, Patent Document 1 proposes a method for improving the weldability of high-strength steel plates by adjusting welding conditions.

[0008] Furthermore, Patent Document 2 proposes a method for improving the formability and weldability of high-strength steel sheets by controlling the microstructure.

[0009] JP 2011-005544 A JP 2008-231541 A

[0010] The method proposed in Patent Document 1 can suppress the occurrence of shrinkage defects and cracks in the weld metal zone. However, this method does not improve the appropriate current range or cross tensile strength. Furthermore, this method requires additional pressure and current application under specific conditions after welding current application, which increases the time required for welding and reduces productivity. Furthermore, the method proposed in Patent Document 1 adjusts the welding conditions and does not contribute to improving the formability of the steel sheet.

[0011] Furthermore, according to the method proposed in Patent Document 2, certain improvements in formability and weldability are observed. However, the tensile strength of the steel sheet obtained by the method of Patent Document 2 is less than 1000 MPa, and a tensile strength of 1450 MPa or more cannot be achieved. Furthermore, as mentioned above, excellent hole expandability is required for the manufacture of components having complex shapes. However, Patent Document 2 uses elongation (El) as an index of the formability of the steel sheet, and does not take hole expandability into consideration.

[0012] As such, the reality is that a steel sheet that combines a tensile strength of 1450 MPa or more with high levels of formability (hole expandability) and weldability (wide range of appropriate current, cross tensile strength) has yet to be realized.

[0013] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a steel sheet that combines a tensile strength of 1450 MPa or more with high levels of formability (hole expandability) and weldability (wide range of appropriate current, cross tensile strength).

[0014] As a result of investigations conducted to solve the above-mentioned problems, the inventors have found that a steel sheet that satisfies the above-mentioned requirements can be obtained by controlling the volume fraction of martensite in the surface layer of the steel sheet and the number density of fine Ti precipitates and Nb precipitates present in the steel, in addition to the component composition of the steel sheet.

[0015] The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

[0016] 1. The chemical composition contains, in mass%, C: 0.18 to 0.38%, Si: 0.2 to 1.8%, Mn: 2.4 to 3.5%, P: 0.03% or less, S: 0.003% or less, Al: 0.01 to 0.50%, N: 0.008% or less, one or both of Nb: 0.005 to 0.08% and Ti: 0.005 to 0.08%, and one or both of B: 0.0002 to 0.0050% and Sb: 0.001 to 0.012%, with the balance consisting of Fe and unavoidable impurities, and the A value defined by the following formula (1) is 0.9 to 6.0, and the microstructure at the 1 / 4 position of the plate thickness satisfies the following conditions: volume fraction of martensite: 93% or more, Volume fraction of ferrite: 0 to 5%, Volume fraction of retained austenite: 0 to 7%, Average grain size of martensite: 7 μm or less, Average grain size of ferrite: 3 μm or less, Average grain size of retained austenite: 3 μm or less, The microstructure in the range from the surface to a depth of 7 μm satisfies the following conditions: Volume fraction of martensite: 0 to 20%, In the range from the surface to a depth of 50 to 100 μm, the number density of Ti precipitates with a grain size of 0.005 μm or more and less than 0.10 μm and Nb-based precipitates with a grain size of 0.005 μm or more and less than 0.10 μm is 20 / 100 μm ２ A steel sheet that satisfies the above formula: A=([C]+[Si] / 8+[Mn] / 20) / (2×([Ti]+[Nb])+85×([B]+[Sb] / 20)...(1) where the parentheses in the formula (1) above represent the content (mass%) of the element in the parentheses, and if the element is not contained, the value is set to 0.

[0017] 2. The steel sheet according to item 1 above, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of V: ​​0.05% or less, Cu: 0.50% or less, Ni: 0.50% or less, Mo: 0.50% or less, Cr: 0.50% or less, Sn: 0.30% or less, Ca: 0.0050% or less, and REM: 0.0050% or less.

[0018] 3. The steel sheet according to 1 or 2 above, further comprising a zinc-based plating layer on at least one surface.

[0019] 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.

[0020] 5. A resistance spot welded component comprising at least one steel plate according to any one of 1 to 3 above in a plate assembly.

[0021] 6. A method for producing a steel sheet, comprising: continuously casting a molten steel having the chemical composition according to 1 or 2 above to form a steel slab; cooling the steel slab at an average cooling rate of 100°C / h or more in a temperature range up to 600°C; reheating the cooled steel slab by heating it to a heating temperature of 1250 to 1450°C and holding it at the heating temperature for 60 minutes or more; hot rolling the reheated steel slab under a finish rolling end temperature of 850 to 950°C to form a hot-rolled steel sheet; cooling the hot-rolled steel sheet at an average cooling rate of 80°C / s or more to a cooling stop temperature of 480°C or less; coiling the cooled hot-rolled steel sheet at a coiling temperature of 480°C or less; cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet; and annealing the cold-rolled steel sheet in an atmosphere having a dew point higher than -20°C, wherein in the annealing, the cold-rolled steel sheet is A method for producing a steel sheet, comprising: heating the steel sheet to an annealing temperature of 780 to 980°C at an average heating rate of 3 to 30°C / s; holding the steel sheet at the annealing temperature for a holding time of 15 to 360 seconds; and cooling the steel sheet from the annealing temperature to room temperature at an average cooling rate of 3°C / s or more.

[0022] 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.

[0023] 8. Continuously casting molten steel having the chemical composition described in 1 or 2 above to form a steel slab, cooling the steel slab at an average cooling rate of 100°C / h or more in a temperature range up to 600°C, reheating the cooled steel slab by heating it to a heating temperature of 1250 to 1450°C and holding it at the heating temperature for 60 minutes or more, hot rolling the reheated steel slab under a finish rolling end temperature of 850 to 950°C to form a hot-rolled steel sheet, cooling the hot-rolled steel sheet at an average cooling rate of 80°C / s or more to a cooling stop temperature of 480°C or less, coiling the cooled hot-rolled steel sheet at a coiling temperature of 480°C or less, cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet, and annealing the cold-rolled steel sheet in an atmosphere having a dew point higher than -20°C. the method for producing a steel sheet, comprising: immersing the annealed steel sheet in a hot-dip galvanizing bath to form a zinc-based plating layer on at least one surface of the steel sheet; and cooling the hot-dip galvanized steel sheet to room temperature at an average cooling rate of 3°C / s or more, wherein in the annealing, the cold-rolled steel sheet is heated to an annealing temperature of 780 to 980°C at an average heating rate of 3 to 30°C / s; held at the annealing temperature for a holding time of 15 to 360 seconds; and cooling from the annealing temperature to immersion in the hot-dip galvanizing bath at an average cooling rate of 3°C / s or more.

[0024] 9. The method for producing a steel sheet according to the above item 8, further comprising, after the hot dip coating, subjecting the steel sheet to an alloying treatment prior to cooling to room temperature.

[0025] According to the present invention, it is possible to provide a steel sheet that combines a tensile strength of 1450 MPa or more with high levels of formability (hole expandability) and weldability (wide range of appropriate current, cross tensile strength).

[0026] 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 "mass %" unless otherwise specified.

[0027] [Composition] The steel sheet of the present invention has the above-mentioned composition. The reasons for limiting the composition will be explained below.

[0028] C: 0.18 to 0.38%. C is an element that contributes to the formation of martensite, a hard phase, and its addition is essential to obtain the desired tensile strength. Furthermore, C reacts with Ti and Nb to form precipitates. Therefore, the addition of C is important to obtain the desired precipitate number density. If the C content is less than 0.18%, the required martensite volume fraction cannot be ensured. Therefore, the C content is set to 0.18% or more, preferably 0.19% or more. On the other hand, if the C content exceeds 0.38%, the toughness of the nugget after spot welding decreases, resulting in a decrease in cross tensile strength. Therefore, the C content is set to 0.38% or less, preferably 0.34% or less.

[0029] Si: 0.2 to 1.8% Si is an element that has the effect of improving weldability. This is because the addition of Si alleviates Mn segregation, thereby reducing the variation in hardness in the plate thickness direction. To achieve this effect, the Si content is set to 0.2% or more, preferably 0.3% or more. On the other hand, if the Si content exceeds 1.8%, liquid metal embrittlement occurs during resistance spot welding. Therefore, the Si content is set to 1.8% or less, preferably 1.6% or less.

[0030] Mn: 2.4 to 3.5% Mn is an element that contributes to improving the strength of steel sheets through solid solution strengthening and the formation of hard phases. Furthermore, Mn stabilizes austenite and is necessary for controlling the fraction of hard phases. To achieve this effect, the Mn content is set to 2.4% or more. On the other hand, if the Mn content is higher than 3.5%, the toughness of the nugget after spot welding decreases, resulting in a decrease in cross tensile strength. In addition, when hydrogen penetrates the steel sheet, the slip constraint at the grain boundaries increases, making cracks more likely to propagate at the grain boundaries. As a result, delayed fracture resistance after resistance welding decreases. Therefore, the Mn content is set to 3.5% or less, preferably 3.2% or less.

[0031] P: 0.03% or less If the P content is higher than 0.03%, segregation of P to grain boundaries becomes significant. As a result, the grain boundaries become embrittled and resistance weldability deteriorates. Therefore, the P content is set to 0.03% or less, preferably 0.02% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%. However, since excessive reduction of the P content increases steelmaking costs, the P content is preferably set to 0.005% or more.

[0032] S: 0.003% or less If the S content is higher than 0.003%, resistance weldability is reduced. This is because a large amount of sulfides such as MnS is generated, and cracks are generated from the sulfides when hydrogen penetrates. Therefore, the S content is set to 0.003% or less, preferably 0.002% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, since excessive reduction of the S content increases steelmaking costs, the S content is preferably set to 0.0002% or more.

[0033] Al: 0.01 to 0.50% 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 0.50%, the ferrite phase will be excessively formed during annealing, making it difficult to ensure strength. Therefore, the Al content is set to 0.50% or less, preferably 0.45% or less.

[0034] N: 0.008% or less N is an element that forms coarse nitrides and thereby deteriorates hole expandability. If the N content is higher than 0.008%, the deterioration of hole expandability becomes significant. Therefore, the N content is set to 0.008% or less, preferably 0.007% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, since excessive reduction of the N content increases steelmaking costs, the N content is preferably set to 0.001% or more.

[0035] The chemical composition of the steel sheet of the present invention contains one or both of Nb and Ti in the following amounts: Nb and Ti are elements that have the common function of improving resistance weldability by forming fine carbonitrides.

[0036] Nb: 0.005 to 0.08% Nb is an element that has the effect of improving resistance weldability by forming fine carbonitrides. When Nb is added to obtain this effect, the Nb content is set to 0.005% or more, preferably 0.01% or more. 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 0.08% or less, preferably 0.07% or less, and more preferably 0.05% or less.

[0037] Ti: 0.005 to 0.08% Like Ti, Ti is an element that improves resistance weldability by forming fine carbonitrides. To achieve this effect, when Ti is added, 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 0.08% or less, preferably 0.07% or less. Ti also prevents B from reacting with N. Furthermore, the fine Ti carbonitrides act as hydrogen trap sites and increase hydrogen overvoltage, thereby improving delayed fracture resistance after resistance welding. From the perspective of achieving these effects, adding Ti at the above-mentioned content is preferable.

[0038] The steel sheet of the present invention contains one or both of B and Sb in the following amounts: B and Sb are elements that have the common function of improving cross tensile strength by strengthening grain boundaries.

[0039] B: 0.0002% to 0.0050% B is an element that segregates at grain boundaries to strengthen them, and adding B can improve cross-tensile strength. To achieve this effect, when B is added, the B content is set to 0.0002% or more, preferably 0.0004% or more. On the other hand, if the B content exceeds 0.0050%, the effect saturates, and the increased amount of B segregating at grain boundaries reduces hole expandability. Therefore, the B content is set to 0.0050% or less, preferably 0.0040% or less. B improves hardenability and contributes to further improving the strength of the steel sheet by generating a hard phase. Furthermore, B is an element that can improve hardenability without lowering the martensitic transformation start point. From the perspective of achieving these effects, adding B at the above content is preferable.

[0040] Sb: 0.001 to 0.012% Like B, Sb is an element that segregates at grain boundaries to strengthen them, and adding Sb can improve cross tensile strength. To achieve this effect, when Sb is added, the Sb content is set to 0.001% or more, preferably 0.002% or more. On the other hand, since Sb has the effect of preventing decarburization of the steel sheet surface, if the Sb content exceeds 0.012%, the desired surface steel sheet structure cannot be obtained, resulting in reduced resistance weldability. Therefore, the Sb content is set to 0.012% or less, preferably 0.010% or less.

[0041] 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.

[0042] When Co is contained as the unavoidable impurities, the Co content is preferably 0.10% or less.When Zn is contained as the unavoidable impurities, the Zn content is preferably 0.10% or less.

[0043] In addition, 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 the content may be 0%.

[0044] V: 0.05% or less V is an element that has the effect of further improving strength by forming fine carbonitrides. However, if the V content exceeds 0.05%, the effect of adding V becomes saturated, and it is not possible to obtain an effect commensurate with the increase in alloy cost. Therefore, when V is added, the V content is set to 0.05% or less, preferably 0.04% or less. On the other hand, from the viewpoint of enhancing the effect of adding V, it is preferable that the V content be 0.005% or more.

[0045] Cr: 0.50% or less Cr is an element that has the effect of further improving strength by generating a hard phase. However, if the Cr content exceeds 0.50%, planar 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.

[0046] Sn: 0.30% or less Sn is an element that increases the hydrogen overvoltage of the steel sheet, thereby further improving the 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.

[0047] Mo: 0.50% or less Like Cr, Mo is an element that has the effect of further improving strength by forming a hard phase. Mo also contributes to further increasing strength by forming carbides. However, if the Mo content exceeds 0.50%, the addition effect saturates and it is not possible to obtain an effect commensurate with the increase in alloy 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, more preferably 0.05% or more.

[0048] Cu: 0.50% or less Cu is an element that increases hydrogen overvoltage and, as a result, improves delayed fracture resistance after resistance welding. However, if the Cu content exceeds 0.50%, the effect of addition saturates and surface defects become more likely to occur. Therefore, 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.

[0049] Ni: 0.50% or less Like Cu, Ni is an element that has the effect of increasing hydrogen overvoltage and improving delayed fracture resistance. Furthermore, when added together with Cu, Ni has the effect of suppressing surface defects caused by Cu. However, when the Ni content exceeds 0.50%, the effect of addition is saturated. 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, the Ni content is preferably set to 0.005% or more.

[0050] Ca: 0.0050% or less Ca is an element that has the effect of further improving hole expandability by spheroidizing the shape of sulfides. Ca also contributes to improving delayed fracture resistance after resistance welding. However, when the Ca content exceeds 0.0050%, the effect saturates, so 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.

[0051] REM: 0.0050% or less Like Ca, REM (rare earth metal) is an element that has the effect of further improving hole expandability by spheroidizing the shape of sulfides. REM also contributes to improving delayed fracture resistance after resistance welding. 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.

[0052] A value: 0.9 to 6.0 The steel sheet of the present invention further has a chemical composition in which the A value defined by the following formula (1) is 0.9 to 6.0: A=([C]+[Si] / 8+[Mn] / 20) / (2×([Ti]+[Nb])+85×([B]+[Sb] / 20)...(1) where the parentheses in the formula (1) represent the content (mass%) of the element in the parentheses, and if the element is not contained, the value is set to 0.

[0053] If the A value is less than 0.9, the desired hole expandability cannot be obtained. This is because the amounts of Ti, Nb, B, and Sb added are excessive relative to the carbon equivalent. If too much Ti and Nb are added, Ti precipitates and Nb precipitates become excessive, resulting in reduced hole expandability. If too much B and Sb are added, B and Sb segregate excessively at the grain boundaries, reducing hole expandability. Therefore, the A value is set to 0.9 or more, preferably 1.0 or more. On the other hand, if the A value is greater than 6.0, weldability, particularly cross tensile strength, is reduced. This is because the amounts of Ti, Nb, B, and Sb added are insufficient relative to the carbon equivalent. Therefore, the A value is set to 6.0 or less, preferably 5.8 or less, and more preferably 5.0 or less.

[0054] [Microstructure] In the steel sheet of the present invention, the microstructure at the 1 / 4 position of the sheet thickness and the microstructure at a depth of 7 μm from the surface of the steel sheet must each satisfy specific conditions. The reasons for this will be explained below.

[0055] (Microstructure at 1 / 4 thickness position) M: 93% or more If the volume fraction of martensite (M) is less than 93%, the desired tensile strength and hole expandability cannot be obtained. Furthermore, if the total volume fraction is less than 93%, the delayed fracture resistance also deteriorates. Therefore, the volume fraction of martensite (M) at the 1 / 4 thickness position is set to 93% or more, preferably 95% or more. The upper limit of the volume fraction is not particularly limited, but may be 100%.

[0056] 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 martensite formed by self-tempering during cooling in annealing, from cooling below the martensitic transformation start point (Ms point) to cooling to room temperature, but also martensite formed by further tempering thereafter.

[0057] Average grain size of martensite: 7 μm or less If the average grain size of martensite is greater than 7 μm, voids formed at the interface between martensite and ferrite tend to connect, resulting in poor hole expandability. Furthermore, the grains become coarse after resistance welding, resulting in a decrease in cross tensile strength. Therefore, the average grain size of martensite at the 1 / 4 position in the plate thickness is set to 7 μm or less, preferably 6 μm or less. The lower limit of the average grain size of martensite is not particularly limited, but may be, for example, 2 μm or more.

[0058] F: 0 to 5% If the volume fraction of ferrite (F) exceeds 5%, the amount of voids generated during punching increases, resulting in a decrease in hole expandability. Furthermore, if the ferrite fraction is high, the hardness of martensite must be increased to ensure the desired strength. Therefore, the volume fraction of ferrite at the 1 / 4 thickness position is set to 5% or less, preferably 3% or less, and more preferably 1% or less. From the above perspective, the lower the ferrite volume fraction, the better, so the lower limit is set to 0%.

[0059] Average grain size of F: 3 μm or less If the average grain size of ferrite is larger than 3 μm, voids generated on the punched end surface during hole expansion tend to connect during hole expansion, making it impossible to obtain the desired hole expandability. Therefore, the average grain size of ferrite at the 1 / 4 position in the plate thickness is set to 3 μm or less, preferably 2.5 μm or less. On the other hand, the lower limit of the average grain size of ferrite is not particularly limited, but may be, for example, 1 μm or more, or 2 μm or more.

[0060] RA: 0 to 7% If the volume fraction of retained austenite (RA) exceeds 7%, martensite is generated during punching, increasing the amount of voids generated and reducing hole expandability. Therefore, the volume fraction of retained austenite at the 1 / 4 position in the plate thickness is set to 7% or less, preferably 5% or less. On the other hand, from the above viewpoint, the lower the volume fraction of retained austenite, the better, so the lower limit is set to 0%.

[0061] Average grain size of RA: 3 μm or less If the average grain size of retained austenite exceeds 3 μm, hole expandability decreases. This is because the amount of voids generated increases as a result of martensite formation during punching due to the influence of the C distribution in the retained austenite. Therefore, the average grain size of retained austenite is set to 3 μm or less, preferably 2.5 μm or less. On the other hand, although there is no particular lower limit, if the average grain size is 0.3 μm or more, elongation is further improved. Therefore, the average grain size of retained austenite is preferably set to 0.3 μm or more, more preferably 0.5 μm or more, and even more preferably 1 μm or more.

[0062] The microstructure at the 1 / 4 position of the plate thickness may contain, in addition to the above-mentioned structure, other structures, as desired. However, the volume fraction of the other structures must be 7% or less. The other structures may be, for example, at least one selected from the group consisting of bainite, cementite, and pearlite.

[0063] (Microstructure in the range from the surface to a depth of 7 μm) In the present invention, it is important to control the microstructure in the range from the surface of the steel sheet to a depth of 7 μm. The reason for this will be explained below.

[0064] The hole expansion ratio measured by a hole expansion test is generally used as an indicator of the hole expandability of a steel sheet. In this hole expansion test, a steel sheet is punched with a punch, and the punch movement is stopped the moment a crack penetrates the thickness of the test piece, and the hole diameter is measured. The crack propagates by gradual connection of voids generated in the center of the sheet thickness, eventually penetrating the surface layer of the steel sheet. The inventors have discovered that reducing the hard phase (martensite) in the surface layer of the steel sheet can prevent cracks from ultimately propagating to the surface layer. Furthermore, it has been found that this surface layer structure with reduced hard phase preferentially deforms when pressed by an electrode during spot welding, thereby suppressing spatter (dust) generated at high current densities. Furthermore, providing a surface layer structure with reduced hard phase also improves the strength of the corona bonded joint after spot welding.

[0065] M: 0 to 20% As mentioned above, in order to improve formability and weldability, it is important to reduce the hard phase in the surface layer of the steel sheet. Specifically, if the volume fraction of martensite in the range from the surface to a depth of 7 μm is higher than 20%, the hole expandability decreases, the appropriate current range during welding narrows, and the cross tensile strength decreases. Therefore, the volume fraction is set to 20% or less, preferably 15% or less. On the other hand, the lower limit of the volume fraction is set to 0%.

[0066] The microstructure in the range from the surface to a depth of 7 μm is not particularly limited as long as the volume fraction of martensite satisfies the above condition. However, when the steel sheet is manufactured using the manufacturing process described below, the microstructure in the range from the surface to a depth of 7 μm typically becomes a structure mainly composed of ferrite as a result of decarburization during the annealing process. Therefore, the microstructure in the range from the surface to a depth of 7 μm may be a structure mainly composed of ferrite. More specifically, the volume fraction of ferrite is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more.

[0067] The microstructure in the range from the surface to a depth of 7 μm may further contain 0 to 15% by volume of retained austenite. Furthermore, the microstructure may optionally contain other structures in addition to martensite, ferrite, and retained austenite. However, the volume fraction of the other structures is preferably 7% or less. The other structures may be, for example, at least one selected from the group consisting of bainite, cementite, and pearlite.

[0068] [Precipitates] The present inventors have found that dispersing fine Ti-based or Nb-based precipitates in the steel sheet structure refines the grain size at the nugget edge even after spot welding, thereby improving toughness. Therefore, it has been found that combining the above-mentioned control of the microstructure of the steel sheet surface layer with control of the number density of fine precipitates can extremely effectively improve cross tensile strength. Specifically, the number density of precipitates is controlled as follows.

[0069] Number density of precipitates: 20 / 100 μm ２ The number density of Ti precipitates having a particle size of 0.005 μm or more and less than 0.10 μm and Nb-based precipitates having a particle size of 0.005 μm or more and less than 0.10 μm in a depth range of 50 to 100 μm from the surface of the steel plate is 20 / 100 μm ２ If the number density is less than 20 particles / 100 μm, the grain size at the nugget edge will be insufficient, resulting in a decrease in cross tensile strength. In addition, the function as a hydrogen trap site after resistance spot welding will also be insufficient, resulting in a decrease in delayed fracture resistance. ２ On the other hand, the upper limit of the number density is not particularly limited, but in order to ensure moldability, it is set to 250 pieces / 100 μm ２ It is preferable that the number of particles is 100 / 100 μm or less. ２ More preferably, it is 40 pieces / 100 μm or less. ２ More preferably, it is 30 pieces / 100 μm or less. ２ Most preferably, the following is true:

[0070] [Zinc-based plating layer] The steel sheet of the present invention may be a cold-rolled steel sheet having no plating layer on its surface, but it is preferable that the steel sheet has a zinc-based plating layer on at least one surface.

[0071] The zinc-based plating layer may be either a zinc plating layer or a zinc alloy plating layer. In other words, the steel sheet of the present invention may be either 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 may 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.

[0072] 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.

[0073] 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, it 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, it is preferable that the coating thickness is 80 g / m per one side of the steel sheet. ２ It is preferable that:

[0074] [Pre-plated Layer] When the steel sheet has a zinc-based plated layer, a pre-plated layer may be further provided between the steel sheet (base steel sheet) and the zinc-based plated layer.

[0075] 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.

[0076] 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 referred to as a "normal 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.

[0077] In one embodiment of the present invention, the Fe-based electroplating 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 balance 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 electroplating layer can be formed at low cost.

[0078] Since the Fe-based plating layer functions as a soft layer, providing the Fe-based plating layer can alleviate the stress applied to the steel sheet surface during welding. Furthermore, the presence of the Fe-based plating layer not only reduces the residual stress in the resistance weld, but also allows diffusible hydrogen to efficiently escape from the steel sheet surface, improving delayed fracture resistance.

[0079] The amount of the Fe-based plating 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 plating layer is 0.5 g / m per one 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 more 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 More preferably, it 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:

[0080] The deposition weight of the Fe-based plating layer is measured as follows. A 10 x 15 mm sample is taken from the steel sheet after the formation of the zinc-based plating layer 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 may be 15 kV. The magnification for the SEM observation is 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 deposition weight of the Fe-based plating layer per side.

[0081] [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 performing spot welding are not particularly limited, and general welding conditions can be adopted.

[0082] 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 applied while applying pressure and controlling it to achieve predetermined welding conditions. This joins the steel sheets that make up the sheet assembly, resulting in a resistance spot welded member. Note that 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.

[0083] 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.

[0084] 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.

[0085] 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 obtained stably. 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 sheet may increase, causing the molten weld to spill out of the gap between the sheets, resulting in a small nugget diameter.

[0086] 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.

[0087] The pressure conditions for the main current application process are preferably a pressure of 2.0 kN to 7.0 kN. If the pressure F in the main current application 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 F in the main current application process is too small, the current diameter will become smaller, making it more likely that expulsion will occur. Therefore, it is preferable to set the pressure F in the main current application process within the above-mentioned preferred range. Note that the pressure may be limited by the equipment capacity. However, as long as the pressure is such that the required nugget diameter can be obtained, the pressure F in the main current application process may be lower or higher than the above-mentioned preferred range.

[0088] In a resistance spot welding method according to an 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 current application time in the post-current application is preferably 1.0 second or less.

[0089] Furthermore, a tempering process may be performed after the main current application process to temper the periphery of the nugget. The conditions for the tempering process are not particularly limited, but the current value in the tempering process is preferably lower than the current value in the main current application process, specifically, preferably 0.9 times or less the current value in the main current application process. Furthermore, the current application time in the tempering process is preferably 2.0 seconds or less.

[0090] [Resistance Spot Welded Component] 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.

[0091] [Method for manufacturing steel sheet] 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.

[0092] First Embodiment In a first embodiment of the present invention, a steel sheet that satisfies 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. If no plating is applied 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) Coiling (7) Cold rolling (8) Annealing

[0093] (1) Continuous Casting First, molten steel having the above-mentioned component 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.

[0094] (2) Cooling 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 100°C / h, the weldability will be reduced. This is because Mn segregation is promoted and coarse Ti-based precipitates and Nb-based precipitates remain even after final annealing. Therefore, the steel slab is cooled under the condition of an average cooling rate of 100°C / h or more in the temperature range up to 600°C. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably 150°C / h or less, and more preferably 130°C / h or less.

[0095] 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.

[0096] (3) Reheating Next, the cooled steel slab is reheated. By reheating, precipitates such as Ti-based precipitates and Nb-based precipitates contained in the steel can be redissolved.

[0097] Heating temperature: 1250 to 1450°C. If the heating temperature in the reheating step is less than 1250°C, the precipitates cannot be sufficiently redissolved, and coarse precipitates remain even after final annealing. As a result, the number density of fine Ti precipitates and Nb precipitates cannot be set within the desired range, resulting in reduced weldability. Therefore, the heating temperature is set to 1250°C or higher, preferably 1270°C or higher. On the other hand, if the heating temperature is higher than 1450°C, the crystal grains become coarse. As a result, the desired crystal grain size cannot be obtained after final annealing, and hole expandability and resistance weldability are reduced. Therefore, the heating temperature is set to 1450°C or lower, preferably 1410°C or lower.

[0098] 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, the number density of fine Ti precipitates and Nb precipitates cannot be set within the desired range, and weldability deteriorates. 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 180 minutes or less, and more preferably 150 minutes or less.

[0099] (4) Hot Rolling Next, the reheated steel slab is hot rolled to obtain a hot-rolled steel sheet. In the hot rolling, the structure within the steel sheet is made uniform and the anisotropy of the material is reduced, thereby improving the elongation and hole expandability after annealing.

[0100] Finish rolling end temperature: 850 to 950°C. In order to obtain the above effect, it is necessary to end the hot rolling in the austenite single phase region. Therefore, the finish rolling end temperature is set to 850°C or higher. On the other hand, if the finish rolling end temperature is higher than 950°C, the structure of the hot-rolled steel sheet becomes coarse, and the properties after annealing deteriorate. Therefore, the finish rolling end temperature is set to 950°C or lower.

[0101] (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 final steel sheet structure, mainly ferrite and martensite.

[0102] Average cooling rate: 80°C / s or more To achieve the above 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 steel sheet structure and reduced hole expandability and resistance weldability after annealing. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably set to 200°C / s or less, more preferably to 150°C / s or less, and even more preferably to 120°C / s or less.

[0103] Cooling stop temperature: 480°C or less Similarly, to obtain the above effect, the cooling stop temperature is set to 480°C or less, preferably 450°C or less. If the cooling stop temperature is higher than 480°C, pearlite is produced in excess, the steel sheet structure of the hot-rolled steel sheet becomes inhomogeneous, and the hole expandability and resistance weldability after annealing deteriorate. 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 more.

[0104] (6) Coiling - Coiling temperature: 480°C or less Next, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 480°C or less. If the coiling temperature is higher than 480°C, Ti-based precipitates and Nb-based precipitates will coarsen, resulting in a decrease in resistance weldability. Therefore, the coiling temperature is set to 480°C or less, preferably 450°C or less. There is no particular restriction on the lower limit of the coiling temperature, but if the coiling temperature is too low, hard martensite will be excessively formed, increasing the cold rolling load. Therefore, the coiling temperature is preferably set to 250°C or more, more preferably 300°C or more, and even more preferably 350°C or more.

[0105] (7) Cold Rolling After the coiling, the hot-rolled steel sheet is 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 according to a conventional method.

[0106] Furthermore, it is preferable to carry out pickling prior to the cold rolling. By carrying out pickling, scale on the surface layer of the hot-rolled sheet can be removed. The conditions for the pickling are not particularly limited, and the pickling can be carried out according to a conventional method.

[0107] (8) Annealing Next, the cold-rolled steel sheet is annealed to promote recrystallization and form a structure (martensite) required to obtain the desired hardness.

[0108] Dew point: greater than -20°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 of the steel sheet. This reduces the volume fraction of martensite in the steel sheet surface. However, if the dew point is less than -20°C, the microstructure of the surface layer of the steel sheet will not be as desired, resulting in reduced resistance weldability. Therefore, the dew point of the annealing atmosphere is set to greater than -20°C, preferably -15°C or higher, and more preferably -10°C or higher. There are no particular restrictions on the upper limit of the dew point of the annealing atmosphere, but from the viewpoint of improving adhesion when a zinc-based plating layer is provided on the steel sheet surface, it is preferably set to 30°C or lower, more preferably 10°C or lower, and even more preferably 5°C or lower.

[0109] In the annealing, first, the cold-rolled steel sheet is heated to an annealing temperature of 780 to 980°C at an average heating rate of 3 to 30°C / s, and then held (soaked) at the annealing temperature for a holding time of 15 to 360 seconds.

[0110] Average heating rate: 3 to 30°C / s. By performing the annealing, the crystal grains can be refined. However, if the heating is rapid, recrystallization does not proceed sufficiently, making it impossible to obtain the desired crystal grain size. Therefore, the average heating rate in the heating is set to 30°C / s or less. On the other hand, if the heating rate is too slow, the Ti-based precipitates and Nb-based precipitates will coarsen, resulting in a decrease in resistance weldability. Therefore, the average heating rate is set to 3°C / s or more, preferably 5°C / s or more.

[0111] Annealing temperature: 760 to 980°C. The annealing must be performed in the two-phase region of ferrite and austenite or in the austenite single-phase region. If the annealing temperature is less than 760°C, the ferrite fraction increases, making it impossible to achieve strength, hole expandability, and resistance weldability simultaneously. Therefore, the annealing temperature is set to 760°C or higher, preferably 820°C or higher, and more preferably 850°C or higher. On the other hand, if the annealing temperature is too high, austenite grain growth becomes significant, resulting in coarsening of the grains and a decrease in resistance weldability. Therefore, the annealing temperature is set to 980°C or lower, preferably 950°C or lower.

[0112] Holding time: 15 to 360 seconds. Holding at the annealing temperature promotes recrystallization and transforms part or all of the structure into austenite. If the holding time is less than 15 seconds, the desired microstructure cannot be obtained. Therefore, the holding time is set to 15 seconds or more. On the other hand, if the holding time is longer than 360 seconds, the Ti-based precipitates or Nb-based precipitates will coarsen, resulting in a decrease in resistance weldability. Therefore, the holding time is set to 360 seconds or less, preferably 300 seconds or less.

[0113] After holding at the annealing temperature, the material is cooled to room temperature at an average cooling rate of 3°C / s or more.

[0114] Average cooling rate: 3°C / s or more If the average cooling rate during cooling is less than 3°C / s, the desired martensite volume fraction cannot be obtained, resulting in a decrease in strength and hole expandability. Therefore, the average cooling rate is set to 3°C / s or more. On the other hand, there is no particular upper limit to the average cooling rate, but it is preferably less than 100°C / s, more preferably 50°C / s or less, and even more preferably 20°C / s or less.

[0115] 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 0.05% to 2.0%.

[0116] 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. By this method, an electrogalvanized steel sheet can be obtained.

[0117] (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.

[0118] Third Embodiment In a third 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. 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) Coiling (7) Cold rolling (8) Annealing (9) Hot-dip galvanizing (10) Cooling

[0119] In this embodiment, the steps (1) to (7) can be carried out under the same conditions as in the first embodiment. Therefore, steps (8) to (10) will be described below.

[0120] (8) Annealing In this embodiment, the cold-rolled steel sheet obtained in the cold rolling step is annealed prior to hot-dip plating.

[0121] Dew point: greater than -20°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 of the steel sheet. This reduces the volume fraction of martensite in the steel sheet surface. However, if the dew point is less than -20°C, the microstructure of the surface layer of the steel sheet will not be as desired, resulting in reduced resistance weldability. Therefore, the dew point of the annealing atmosphere is set to greater than -20°C, preferably -15°C or higher, and more preferably -10°C or higher. There are no particular restrictions on the upper limit of the dew point of the annealing atmosphere, but from the viewpoint of improving adhesion when a zinc-based plating layer is provided on the steel sheet surface, it is preferably set to 30°C or lower, more preferably 10°C or lower, and even more preferably 5°C or lower.

[0122] In the annealing, first, the cold-rolled steel sheet is heated to an annealing temperature of 780 to 980°C at an average heating rate of 3 to 30°C / s, and then held (soaked) at the annealing temperature for a holding time of 15 to 360 seconds.

[0123] Average heating rate: 3 to 30°C / s. By performing the annealing, the crystal grains can be refined. However, if the heating is rapid, recrystallization does not proceed sufficiently, making it impossible to obtain the desired crystal grain size. Therefore, the average heating rate in the heating is set to 30°C / s or less. On the other hand, if the heating rate is too slow, the Ti-based precipitates and Nb-based precipitates will coarsen, resulting in a decrease in resistance weldability. Therefore, the average heating rate is set to 3°C / s or more, preferably 5°C / s or more.

[0124] Annealing temperature: 760 to 980°C. The annealing must be performed in the two-phase region of ferrite and austenite or in the austenite single-phase region. If the annealing temperature is less than 760°C, the ferrite fraction increases, making it impossible to achieve strength, hole expandability, and resistance weldability simultaneously. Therefore, the annealing temperature is set to 760°C or higher, preferably 820°C or higher, and more preferably 850°C or higher. On the other hand, if the annealing temperature is too high, austenite grain growth becomes significant, resulting in coarsening of the grains and a decrease in resistance weldability. Therefore, the annealing temperature is set to 980°C or lower, preferably 950°C or lower.

[0125] Holding time: 15 to 360 seconds. Holding at the annealing temperature promotes recrystallization and transforms part or all of the structure into austenite. If the holding time is less than 15 seconds, the desired microstructure cannot be obtained. Therefore, the holding time is set to 15 seconds or more. On the other hand, if the holding time is longer than 360 seconds, the Ti-based precipitates or Nb-based precipitates will coarsen, resulting in a decrease in resistance weldability. Therefore, the holding time is set to 360 seconds or less, preferably 300 seconds or less.

[0126] Average cooling rate: 3°C / s or more After the holding, the steel is cooled from the annealing temperature to the immersion in the hot-dip galvanizing bath at an average cooling rate of 3°C / 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 strength and hole expandability. Therefore, the average cooling rate is set to 3°C / s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably less than 100°C / s, more preferably 50°C / s or less, and even more preferably 20°C / s or less.

[0127] (9) Hot-dip galvanization In this embodiment, the annealed steel sheet is subjected to hot-dip galvanization 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.

[0128] The hot-dip galvanizing 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.

[0129] 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 can reduce material costs.

[0130] 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. In addition, the temperature of the steel sheet when entering the hot-dip galvanizing bath (entry sheet temperature) is preferably 440 to 550°C.

[0131] Furthermore, after the hot-dip galvanizing treatment, the coating weight may be adjusted. Although the method for adjusting the coating weight is not particularly limited, 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.

[0132] (10) Cooling Next, the steel sheet after the hot dip coating is cooled to room temperature at an average cooling rate of 3°C / 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 strength and hole expandability. Therefore, the average cooling rate is set to 3°C / s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but it is preferably less than 100°C / s, more preferably 50°C / s or less, and even more preferably 20°C / s or less.

[0133] As described above, 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 a desired microstructure.

[0134] In a fourth embodiment of the present invention, after the hot-dip galvanizing, 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.

[0135] (Alloying Treatment) The above-mentioned 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.

[0136] 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 an 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.

[0137] (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 steel sheet surface.

[0138] 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.

[0139] 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 plating layer is formed as a pre-plating layer by performing an Fe-based electroplating process will be described.

[0140] 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.

[0141] The Fe ion content in the Fe-based electroplating bath before the start of current application is Fe ２＋ The Fe ion content 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 concentration 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 contained as metal ions, and nonmetal elements may be contained 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.

[0142] 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.

[0143] Other conditions for the Fe-based electroplating bath are not particularly limited. The bath temperature is preferably 30°C or higher, taking into account 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 account 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 ２ The sheet threading speed may be 5 mpm or more and 150 mpm or less. If the sheet threading speed is less than 5 mpm, productivity is poor, while if the sheet threading speed is 150 mpm or more, it is difficult to stably control the coating weight.

[0144] Prior to the Fe-based electroplating treatment, the cold-rolled steel sheet may be subjected to a degreasing treatment and water rinsing to clean the surface, followed by a pickling treatment and water rinsing to activate the surface. These pretreatments are preferably followed by the Fe-based electroplating treatment. The degreasing and water rinsing 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 specified, but is preferably approximately 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 treatment solution may also contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0145] In order to confirm the effects of the present invention, steel sheets were prepared by the following procedure, and their properties were evaluated. The steel sheets prepared were four types: cold-rolled steel sheet (CR), electrogalvanized steel sheet (EG), hot-dip galvanized steel sheet (GI), and galvannealed steel sheet (GA).

[0146] First, molten steel having the chemical composition shown in Table 1 was continuously cast into a steel slab, which was then cooled at the average cooling rate shown in Table 2.

[0147] 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 a coiling temperature (CT) under the conditions shown in Table 2 and wound into a coil. Thereafter, the hot-rolled steel sheet was pickled and cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet having a thickness of 1.4 mm.

[0148] In producing the cold-rolled steel sheets (CR), the cold-rolled steel sheets were annealed under the conditions shown in Table 2 and cooled to room temperature at the average cooling rate shown in Table 2.

[0149] 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 bath temperature was 50° C. and the pH was 1.5.

[0150] 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 a 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 coating bath containing Al, Zn, and unavoidable impurities, with an Al concentration of 0.14% in the bath. The temperature of the hot-dip galvanizing bath was 460°C.

[0151] 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 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.

[0152] Next, the microstructure and precipitate density of the obtained steel sheets were measured by the following procedure. The obtained results are shown in Table 3.

[0153] (Microstructure at 1 / 4 position of plate thickness) Ferrite, martensite The volume fractions of ferrite and martensite in the steel plate were measured by the following procedure. First, a cross section of the plate thickness parallel to the rolling direction of the steel plate was polished and etched with 3% nital to reveal the structure. Then, the cross section was observed at 3000x magnification using an SEM (scanning electron microscope), and an SEM image of the microstructure at 1 / 4 position of the plate thickness was obtained. The area fractions of ferrite and martensite in the SEM image were calculated by the point counting method (in accordance with ASTM E562-83 (1988)), and these area fractions were taken as volume fractions.

[0154] The average grain size of ferrite and martensite was determined by image analysis of the SEM image obtained by the SEM observation. Specifically, the area of ​​each grain of ferrite and martensite in the SEM image was first determined by image analysis. Next, the equivalent circle diameter of the grain was calculated from the area, and the average value was used as the average grain size. Image-Pro from Media Cybernetics was used for the image analysis.

[0155] - Retained Austenite The volume fraction of retained austenite was determined by X-ray diffraction. Specifically, the steel sheet was first polished up to ¼ of the surface in the sheet thickness direction, and the diffracted X-ray intensity at the ¼ surface 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 calculation formula described on pages 26, 62-64 of "X-Ray Diffraction Handbook" (2000, Rigaku Corporation) was used.

[0156] The average grain size of the retained austenite grains was determined by polishing the cross section up to 1 / 4 of the plate thickness, etching with 3% nital, and then analyzing the TEM image obtained by TEM observation (transmission electron microscope). Specifically, first, a TEM image of the microstructure at the 1 / 4 plate thickness position was obtained by observation at a magnification of 15,000 times using a TEM. 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. Image-Pro from Media Cybernetics was used for the image analysis.

[0157] (Microstructure in the range from the surface of the steel sheet to a depth of 7 μm) The microstructure in the range from the surface of the steel sheet to a depth of 7 μm was observed using an SEM, and the volume fraction of martensite was calculated. When the steel sheet to be measured had a plating layer (EG, GI, and GA), the intensity profile was measured from the surface of the steel sheet using glow discharge optical emission spectroscopy, and the point where the intensity profile of Fe exceeded the intensity profile of Zn was defined as the steel sheet surface. The martensite volume fraction was calculated from the SEM image using the same method as for calculating the martensite volume fraction at the 1 / 4 position in the sheet thickness.

[0158] (Number Density of Precipitates) The number densities of Ti-based precipitates and Nb-based precipitates were determined by TEM-EDS (energy dispersive X-ray analysis). Specifically, first, the L-section of the steel sheet was observed at 10,000x magnification using a TEM, and TEM images were obtained at 10 locations randomly selected from a range of 5 to 100 μm deep from the surface of the steel sheet. Next, Ti-based precipitates and Nb-based precipitates in the TEM images were identified by EDS. The TEM images were analyzed using Image-Pro, and the areas of individual Ti-based precipitates and Nb-based precipitates were determined. From the areas, the circle-equivalent diameters of individual particles were calculated. Thereafter, the number of Ti-based precipitates and Nb-based precipitates with a circle-equivalent diameter of 0.005 μm or more and less than 0.10 μm was counted, and the number was divided by the area of ​​the observed range to determine the number density of the precipitates. The number densities of precipitates were calculated in the same manner for the TEM images obtained by observing the above 10 locations, and the average values ​​were taken as the number densities of Ti-based precipitates and Nb-based precipitates.

[0159] The strength, formability, and weldability of the steel sheets thus obtained were then evaluated by the following procedures. The results are shown in Table 3.

[0160] (Strength) A JIS No. 5 tensile test piece was taken from the steel plate so that the tensile direction was the direction perpendicular to the rolling direction. Next, a tensile test was performed using the test piece to measure the tensile strength (TS) of the steel plate. The tensile test was performed in accordance with JIS Z2241 (1998).

[0161] (Formability) The hole expansion ratio was measured as an index of the formability (hole expandability) of the steel sheet. The measurement was performed in accordance with the Japan Iron and Steel Federation standard (JIS Z2256 (2010)). Specifically, a 10 mm diameter hole was punched with a clearance of 12.5%. Next, the punched test piece was set in a testing machine so that the burr was on the die side, and then formed with a 60° conical punch to measure the hole expansion ratio (λ). A λ (%) of 25% or more was considered to have good hole expandability.

[0162] (Weldability) As indicators of weldability, the appropriate current range in resistance spot welding and cross tensile strength were evaluated.

[0163] Appropriate Current Range The appropriate current range was evaluated using the following procedure. First, a 20 mm × 50 mm test piece was cut out from the steel sheet. Next, two identical test pieces were combined into one plate and resistance spot welded. For the resistance spot welding, a servo motor pressure-type single-phase AC (50 Hz) resistance welding machine was used. 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 a pressure of 4000 N, a current application time of 20 cycles (50 Hz), and a hold time of 5 cycles (50 Hz).

[0164] The welding current was increased from 3.0 kA in 0.1 kA increments, and the maximum current value at which spatter (flash) did not occur was recorded. The nugget diameter was measured by observing the cross section of the welded part of the test piece after welding, and the difference between the minimum current at which the nugget diameter was 4.0√t (mm) or more relative to the plate thickness t (mm) and the maximum current value at which flash did not occur was determined to be the appropriate current range for welding. An appropriate current range of 1.2 kA or more was rated "good," and one below 1.2 kA was rated "poor."

[0165] Cross Tensile Strength: The cross tensile strength after resistance spot welding was measured based on the cross tensile test method (JIS Z 3137). For this measurement, a 50 × 150 mm cross tensile test specimen cut from a steel plate was used, and resistance spot welding was performed on a plate assembly consisting of two identical test specimens. Resistance spot welding was performed using a servomotor-pressurized, single-phase AC (50 Hz) resistance welding machine attached to a welding gun, to prepare a tensile test specimen with a resistance weld. The pair of electrode tips used were alumina-dispersed copper DR-type electrodes with a tip curvature radius R of 40 mm and a tip diameter of 6 mm. The welding conditions were a pressure of 4000 N, a current application time of 20 cycles (50 Hz), and a hold time of 5 cycles (50 Hz). The current value was set to the value that resulted in a nugget diameter of 4.5√t (mm) based on the above-mentioned optimum current range test. If the strength was 5.5 kN or more, the cross tensile strength after resistance spot welding was rated as "good," and if it was less than 5.5 kN, the cross tensile strength after resistance spot welding was rated as "poor."

[0166] 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 1450 MPa or more, and high levels of formability (hole expandability) and weldability (wide range of appropriate current, cross tensile strength).

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