100-kilogram-class ultra-high-strength galvanized steel sheet and method for producing the same

Abstract

An object of the present invention is to provide a 100-kilogram class ultra-high strength galvanized steel sheet having excellent resistance to liquid metal embrittlement (LME) cracking and excellent resistance spot weldability. The steel sheet contains, by weight percentage, C: 0.17 to 0.25%, Mn: 1.7 to 2.7%, Si: 0.35 to 1.5%, Al: 0.01 to 1.0%, 0.7% ≤ Si + Al ≤ 1.7%, and at least one of Nb, Ti, and B, with the balance being Fe and unavoidable impurities; provided that, with the thickness of the steel sheet being t, the resistivity of the steel sheet is 0 < R1 ≤ 50 μΩ·cm, and the resistivity of the steel sheet in the range of 0.010t or more to 0.035t or less in the direction of the steel sheet matrix from the interface between the plating layer and the steel sheet matrix is 0 < R2 ≤ 15 μΩ·cm, the resistivity of the steel sheet in the range of more than 0.035t to 0.065t or less is 0 < R3 ≤ 30 μΩ·cm, and (R2 / 2 + R3 / 3) ≤ (3.1R1 1 / 2 - 1.5) is satisfied.

Description

Technical Field

[0001] Technical Field The present invention relates to an ultra-high strength zinc-plated steel sheet and a method for manufacturing the same, and particularly to a 100-kilogram class ultra-high strength zinc-plated steel sheet excellent in resistance spot weldability and a method for manufacturing the same.

Background Art

[0002] Background Art Currently, against the backdrop of reducing energy consumption and pollutant emissions, weight reduction and corrosion resistance of the body are pursued in automobiles. Currently, high-strength steel sheets are widely used for the body in order to reduce the thickness of the body materials. High-strength steel with a tensile strength ≧ 780 MPa is usually called ultra-high strength steel. Ultra-high strength steel usually achieves improvement in material strength by utilizing a transformation strengthening mechanism, and in addition, more alloying elements in terms of content and type, such as Mn, Si, Al, Cr, Mo, Nb, Ti, V, etc. are added to ultra-high strength steel compared to steel grades with a low strength level. In order to improve corrosion resistance, the surface of ultra-high strength steel is plated with pure zinc (GI) or alloyed zinc (GA). Resistance spot welding of a surface plated with pure zinc (GI) or alloyed zinc (GA) is a welding method often applied in the automotive industry.

[0003] In electroplated ultra-high-strength steel, a phenomenon called liquid metal embrittlement (LME) occurs during the process of resistance spot welding. This is mainly because, under the action of stress, zinc on the material surface melts due to heat, and the liquid zinc infiltrates along the grain boundaries of the steel plate substrate, resulting in a decrease in the bonding force of the grain boundaries and the occurrence of cracks. LME is mainly affected by the state of the base material, the state of the plating, and the welding process. However, the state of the base material includes, but is not limited to, carbon equivalent, strength, resistivity, microstructure, etc.; the state of the plating includes, but is not limited to, the composition of the plating, the weight of the plating, etc.; the welding process includes, but is not limited to, stress, welding pressure, welding current, welding time, etc. During the welding process, the higher the strength of the material, the higher the tensile stress applied to the welded joint under the same welding conditions. When the welding current increases, welding spatter is likely to occur. Due to these factors, the probability of the LME problem occurring increases, and the risk of the welded joint breaking increases. However, compared with non-high-strength steel, ultra-high-strength zinc-plated steel is more likely to have the LME problem, which poses a safety risk during body manufacturing. Therefore, the widespread use of such materials for body lightweighting is severely restricted.

[0004] According to industry research, LME cracks in resistance spot welding are generally divided into four types: type A, type B, type C, and type D, as shown in Figure 1, according to the occurrence location. Among them, type A cracks occur at the site where the electrode is in direct contact with the material, and previous research has shown that they have little impact on the performance of the welded joint. However, type B and type C cracks occur on and between the base materials, and type D cracks occur at the shoulder part of the welded joint. Since they all affect the performance of the welded joint, it is necessary to control their number and length. In response to this, the following research has also been carried out.

[0005] For example, in a US patent document with a registration announcement number of US11299793B2 and a title of "Steel sheet having excellent resistance to liquid metal embrittlement cracks and method for manufacturing the same", a galvanized steel sheet with excellent resistance to liquid metal embrittlement cracks and a method for manufacturing the same have been invented. Its composition is in weight percentage: C: 0.04 - 0.35%, Al + Si: 0.99% or less, Mn: 3.5 - 10%, P: 0.05% or less (excluding 0%), S: 0.02% or less (excluding 0%), N: 0.02% or less (excluding 0%), and the balance is Fe and other inevitable impurities. In volume fraction, its microstructure includes 10% or more of retained austenite, 60% or more of annealed martensite, and 20% or less of α - martensite and ε - martensite, and the average thickness of the Mn depletion layer is 0.5 μm or more from the product surface. The invention has a high Mn content and is suitable for production by endless rolling processes such as ESP. In this patent, the crack resistance when applying the resistance spot welding process is not mentioned.

[0006] In the Chinese patent document with the publication number CN113227434A and the title "High-strength galvanized steel sheet excellent in resistance spot weldability and its manufacturing method", a high-strength galvanized steel sheet with a tensile strength of 490 MPa or more and its manufacturing method have been invented. Its composition is by weight percentage: C: 0.05 - 0.15%, Si ≤ 2.0%, Mn: 1.0 - 30%, acid-soluble aluminum: 3% or less, Cr: 2.5% or less, Mo: 1% or less, B: 0.005% or less, Nb: 0.2% or less, Ti: 0.2% or less, V: 0.2% or less, Sb + Sn + Bi: 0.1% or less, N: 0.01% or less, and the balance consists of Fe and inevitable impurities. The main feature of the steel sheet is that the decarburization rate in the region 35 μm deep from the steel sheet surface is 30% or more. The criterion for good spot weldability is that when the upper limit current at which welding spatter occurs, which is 0.5 kA and 1.0 kA, is not exceeded, type C cracks do not occur between the base metal plates, and the length of type B cracks at the shoulder part of the welded joint is 100 μm or less. The galvanized steel sheet disclosed in this patent is excellent in resistance spot weldability. However, in the production process of automobile bodies, welding spatter inevitably occurs. In the case of the upper limit current at which welding spatter occurs, and thus when the current exceeds this value, the cracking situation at the welded joint part is not described in this patent, and there is still a risk of serious welding cracks occurring in the production of the body.

[0007] In view of the above drawbacks of the prior art, it is desired to obtain an ultra-high-strength galvanized steel sheet that has better resistance to liquid metal embrittlement LME cracks and can suppress the occurrence of type A, type B, type C, and type D cracks.

Summary of the Invention

Problems to be Solved by the Invention

[0008] Content of the Invention The object of the present invention is to provide a 100-kilogram class ultra-high-strength galvanized steel sheet that has excellent resistance to liquid metal embrittlement LME cracks and excellent resistance spot weldability. A further object of the present invention is to provide a 100-kilogram class ultra-high-strength galvanized steel sheet with excellent resistance spot weldability, and the welding current is the minimum current I at which spatter occurssplash When it is below, type B cracks and type C cracks do not occur. When type A cracks occur, the depth of type A cracks is 10% or less of the base material plate thickness, (I splash +I splash *25%) ≥ welding current ≥ I splash In this case, type B cracks and type C cracks do not occur. When type D cracks occur, the depth of type D cracks is 10% or less of the base material plate thickness.

Means for Solving the Problems

[0009] In order to achieve the above object, the present invention provides a 100-kilogram class ultra-high strength galvanized steel sheet. The steel sheet is in weight percentage, C: 0.17 to 0.25%, Mn: 1.7 to 2.7%, Si: 0.35 to 1.5%, Al: 0.01 to 1.0%, 0.7% ≤ Si + Al ≤ 1.7%, and at least one of Nb, Ti, and B, is contained, the balance is Fe and unavoidable impurities. However, let the thickness of the steel sheet be t, the resistivity R1 of the steel sheet is 0 < R1 ≤ 50 μΩ·cm, and from the interface between the plating layer and the steel sheet matrix to the steel sheet matrix direction, the resistivity R2 of the steel sheet in the range of 0.010t or more to 0.035t or less is 0 < R2 ≤ 15 μΩ·cm, and the resistivity R3 of the steel sheet material in the range of more than 0.035t to 0.065t or less in the steel sheet matrix direction is 0 < R3 ≤ 30 μΩ·cm, and (R2 / 2 + R3 / 3) ≤ (3.1R1 1 / 2 - 1.5) is satisfied.

[0010] Another embodiment of the present invention is a 100-kilogram class ultra-high strength galvanized steel sheet. In addition to 94% or more of Fe and other unavoidable impurities, the steel sheet further has C: 0.17 to 0.25%, Mn: 1.7 to 2.7%, Si: 0.35 to 1.5%, Al: 0.01 to 1.0%, 0.7%≦Si+Al≦1.7%, and at least one of Nb, Ti, and B; Contains The balance is Fe and unavoidable impurities, The thickness of the steel plate is t, and the resistivity R1 of the steel plate is 0 <R1≦50μΩ·cmであり、メッキ層と鋼板マトリックスとの界面から、鋼板マトリックス方向へ0.010t以上~0.035t以下の範囲における鋼板の抵抗率R2は0<R2≦15μΩ·cmであり、0.035t超~0.065t以下の範囲における鋼板の抵抗率R3は0<R3≦30μΩ·cmであり、且つ(R2 / 2+R3 / 3)≦(3.1R1 1 / 2 -1.5).

[0011] In the above technical solution of the present invention, the design principles of each chemical element are as follows: C: C is a solid solution strengthening element commonly used in steels. It has high solubility in austenite. In products that have undergone a quenching-distribution process, carbon in martensite is localized in the retained austenite, improving the stability of the retained austenite, and thus improving the elongation of the material by utilizing the TRIP effect of the retained austenite. The ultra-high strength galvanized steel sheet of the present invention has a tensile strength of 980 MPa or more and a fracture elongation of 20% or more. If the C content is too low, the ultra-high strength of the material cannot be ensured, and at the same time, it is disadvantageous to the formation of carbon-rich and stable retained austenite, which also affects the fracture elongation of the material. However, if the C content is too high, it significantly increases the carbon equivalent, which affects the resistance spot weldability, and at the same time, twins are more likely to occur, which increases the crack sensitivity. Therefore, in the present invention, the mass percentage of C is controlled to 0.17-0.25%.

[0012] Mn: Mn can improve the hardenability of steel, lower the critical transformation temperature, and improve the strength of steel. Mn can also significantly expand the austenite phase region and increase the A c3 , M s , and M. fIt can also reduce the points, improve the stability of austenite, and contribute to the improvement of the elongation of steel. However, if the Mn content is too high, it will significantly increase the carbon equivalent, be disadvantageous to resistance spot weldability, and at the same time severely exacerbate the tendency of grain coarsening, reduce the plasticity and toughness of the steel, and deteriorate the corrosion resistance. Therefore, in the present invention, the mass percentage of Mn is controlled to be 1.7 - 2.7%.

[0013] Si: Si has an extremely low solubility in carbides, strongly suppresses the precipitation of cementite, promotes the localization of carbon in retained austenite, improves the stability of retained austenite, and thereby can improve the strength and formability of the material. However, if the Si content is too high, Si-containing oxides are likely to form in the annealing process, resulting in the deterioration of the plating property of the steel sheet surface, being disadvantageous for obtaining a plating layer of good quality. Also, if the Si content is too high, the carbon equivalent will also increase, being disadvantageous to resistance spot weldability. Therefore, in the present invention, the mass percentage of Si is controlled to be 0.35 - 1.5%.

[0014] Al: Al has a similar effect to Si in suppressing cementite and improving the stability of retained austenite. At the same time, it can not only improve the mechanical stability of retained austenite but also improve the thermal stability of retained austenite. Furthermore, Al can form fine and dispersedly distributed insoluble particles with C and N to refine crystal grains, so Al can partially replace Si in the steel. However, the strengthening effect of Al is weaker than that of Si. If the Al content is too high, the A c3 temperature of the steel will rise significantly, increasing the difficulty of continuous casting and the risk of billet cracking. Therefore, in the present invention, the mass percentage of Al is controlled to be 0.01 - 1.0%.

[0015] In the design of the chemical composition of the present invention, it is necessary to further control the mass percentage content of Al and Si elements so that 0.7% ≤ (Al + Si) ≤ 1.7%. The reason for controlling this technical feature is that an appropriate amount of Al and Si can ensure a certain strengthening effect, promote the stabilization of retained austenite, and can not only ensure the strength and elongation rate of the material, but also there is no production obstacle due to the difficulty of continuous casting, billet cracking, and deterioration of plating properties.

[0016] The resistivity is closely related to any of alloying elements, crystal structure, crystal defects, and solid solution effect. The gradient change of resistivity in the thickness direction reflects the change of microstructure. When the resistivity decreases from the surface layer in a certain thickness direction, the total heat input in the welding process can be reduced, and the occurrence of cracks can be advantageously prevented. Therefore, in the ultra-high strength galvanized steel sheet according to the present invention, assuming the thickness of the steel sheet is t, the resistivity R1 of the steel sheet satisfies 0 < R1 ≤ 50 μΩ·cm, and the resistivity R2 of the steel sheet in the range of 0.010t or more to 0.035t or less from the interface between the plating layer and the steel sheet matrix in the steel sheet matrix direction satisfies 0 < R2 ≤ 15 μΩ·cm, and the resistivity R3 of the steel sheet material in the range of more than 0.035t to 0.065t or less satisfies 0 < R3 ≤ 30 μΩ·cm, and (R2 / 2 + R3 / 3) ≤ (3.1R1 1 / 2 -1.5). However, the ranges of R1, R2, R3 and their relationships represent the average resistivity level of the steel sheet and the resistivity level in the region where LME cracks may occur in the resistance spot welding process. In particular, when 0 < R2 ≤ 15 μΩ·cm, 0 < R3 ≤ 30 μΩ·cm, and (R2 / 2 + R3 / 3) ≤ (3.1R1 1 / 2 -1.5), the following can be ensured: when the welding current is less than I splash , type B cracks and type C cracks do not occur. When type A cracks occur, the depth of type A cracks is 10% or less of the base metal plate thickness, and when (I splash + I splash* 25%) ≥ welding current ≥ I splash , type B cracks and type C cracks do not occur. When type D cracks occur, the depth of type D cracks is 10% or less of the base metal plate thickness. When R2 > 15 μΩ·cm, R3 > 30 μΩ·cm, R2 / 2 + R3 / 3 > (3.1R11 / 2 If it is -1.5), it becomes difficult to avoid the occurrence of LME cracks, especially type C cracks. Preferably, the resistivity R1 of the steel sheet is 37 μΩ·cm ≤ R1 ≤ 50 μΩ·cm, and from the interface between the plating layer and the steel sheet matrix, the resistivity R2 of the steel sheet in the range of 0.010t or more to 0.035t or less in the steel sheet matrix direction is 9 μΩ·cm ≤ R2 ≤ 15 μΩ·cm, and the resistivity R3 of the steel sheet in the range of more than 0.035t to 0.065t or less is 24 μΩ·cm ≤ R3 ≤ 30 μΩ·cm, and (R2 / 2 + R3 / 3) ≤ (3.1R1 1 / 2 -1.5) is satisfied.

[0017] Furthermore, in the 100-kilogram-class ultra-high-strength zinc-plated steel sheet according to the present invention, when contained, the mass percentages of Nb, Ti, and B are: 0 < Nb ≤ 0.1%; 0 < Ti ≤ 0.1%; 0 < B ≤ 0.003%.

[0018] Furthermore, in the 100-kilogram-class ultra-high-strength zinc-plated steel sheet according to the present invention, when contained, the mass percentages of Nb, Ti, and B are: Nb: 0.03% - 0.05%; Ti: 0.001% - 0.02%; B: 0.0002% - 0.0025%.

[0019] Nb and Ti are carbide-forming elements, which can suppress the precipitation of cementite, and fine Nb and Ti carbides also have the effect of refining crystal grains and improving strength. However, Nb and Ti carbides are disadvantageous for the stabilization of retained austenite, and when a large amount of Nb and Ti is added, the cost of the material alloy increases. Therefore, in the present invention, the mass percentages of Nb and Ti are controlled to 0 - 0.1%.

[0020] B tends to be concentrated at the austenite grain boundaries. By suppressing the austenite transformation and improving the hardenability of the steel, the strength is enhanced. However, if the B content is too high, the formability of the steel deteriorates and the risk of cracking increases. Therefore, in the present invention, the mass percentage of B is controlled to be 0 to 0.003%.

[0021] Furthermore, among other inevitable impurities in the present invention, P ≤ 0.015%, S ≤ 0.010%, N ≤ 0.008%, and REM ≤ 0.01%.

[0022] P, S, and N are all inevitable impurity elements in the steel. If the P content is too high, the grain boundaries are weakened, the risk of brittle cracking increases, and the weldability decreases; S, as an impurity element, affects the formability and weldability of the steel; N can improve the stability of austenite and has a certain strengthening effect. However, if the N content is too high, the risk of brittle fracture increases, and at the same time, the precipitation of AlN is likely to occur, deteriorating the quality of continuous casting. Therefore, in the present invention, the mass percentages of P, S, and N are controlled to be 0.015% or less, 0.010% or less, and 0.008% or less, respectively.

[0023] REM is a general term for a class of elements. The rare earths commonly used in steel grades are mainly mixtures of La and Ce, but the inclusion of other rare earth elements other than La and Ce is not excluded. REM has the effect of purifying the grain boundaries and transforming inclusions. However, if its content is too high, the formability of the material is impaired. Therefore, in the present invention, the mass percentage of REM is controlled to be 0.01% or less.

[0024] Furthermore, in the 100-kilogram-class ultra-high-strength galvanized steel sheet according to the present invention, a GA sheet or a GI sheet is arranged as the steel sheet. Preferably, the thickness of the steel sheet is 0.8 to 2.5 mm.

[0025] Furthermore, in the 100 - class ultra - high - strength galvanized steel sheet according to the present invention, when a type III sample compliant with ISO 6892 - 1 standard perpendicular to the rolling direction is tensioned at room temperature, the tensile strength of the steel sheet is ≧980 MPa, the elongation at break is ≧20%, and the hole expansion rate is ≧20%.

[0026] Furthermore, in the 100 - class ultra - high - strength galvanized steel sheet according to the present invention, when the steel sheet is welded with a current of 1 to 1.25 times I splash no type B crack and type C crack occur. When type D crack occurs, the depth of the crack is 10% or less of the base metal plate thickness; when the steel sheet is welded with a current of 1 or less I splash no type B crack and type C crack occur. When type A crack occurs, the depth of the crack is 10% or less of the base metal plate thickness, provided that I splash is the minimum current when sputtering occurs.

[0027] The second aspect of the present invention is a method for manufacturing the above - mentioned 100 - class ultra - high - strength galvanized steel sheet, which is characterized by including the following steps.

[0028] (1) Perform refining and continuous casting to obtain a billet satisfying the above composition; (2) Hot rolling: Heat the billet in step (1), finish rolling, perform laminar flow cooling, and wind it up to obtain a hot - rolled coil. However, the heating temperature for heating the billet obtained in step (1) is in the range of 1100 - 1300 °C, the temperature for finish rolling is controlled in the range of A c3 ~1000 °C, the temperature for laminar flow cooling of the finish - rolled strip steel is controlled in the range of (A c1 ±45 °C), and the residence time of laminar flow cooling is 5 - 30 s. Further, it is cooled to 450 - 550 °C and wound up, and the winding temperature is T C The wound steel coil is kept warm for 30 - 300 min in the range of (T C ±30 °C) to obtain a hot - rolled coil; (3) Pickling and cold rolling: Pickle the hot - rolled coil obtained in step (2) and perform cold rolling to obtain a full - hard coil; (4) Continuous annealing: subjecting the fully hard coil obtained in step (3) to multi-stage heat treatment; (5) Zinc plating: putting the strip steel obtained in step (4) into a zinc pot at a temperature of (T ZP ±15 °C) to complete zinc plating and obtaining a zinc-plated steel sheet.

[0029] The above multi-stage heat treatment includes the following (a) to (d): (a) In the first-stage annealing, heating the fully hard coil in the first-stage temperature (preheating temperature) range of 600 °C or higher to (A c1 +40 °C) to obtain a steel coil; (b) In the second-stage annealing, continuously heating the steel coil obtained in (a) in the second-stage temperature (heating / soaking temperature) range of (A c1 +50 °C) to (A c3 +80 °C) or (A c1 +50 °C) to 900 °C, holding for 30 to 300 s (soaking time) to obtain a strip steel, and taking the smaller value of (A c3 +80 °C) and 900 °C as the upper limit of the second-stage temperature range; (c) In the third-stage annealing, heating the strip steel obtained in (b) at a cooling rate V of not less than a predetermined value 2-3 to the third-stage temperature (quenching temperature) range M s ~M f and holding for 10 to 200 s (quenching holding time); (d) In the fourth-stage annealing, heating the strip steel obtained in (c) again in the fourth-stage temperature (reheating temperature) range of (350 °C to T ZP ) and holding for 15 to 90 s (reheating time); However, in step (4), the atmosphere in (a) contains O2 with a volume content of 0.01 to 0.5%, and the balance is N2 and inevitable impurities; the atmosphere in (b) contains H2 with a volume content of at least 0.5%, the balance is N2 and inevitable impurities, and the dew point is -20 to 15 °C; V 2-3 in (c) represents the cooling rate and is 50 °C / s or higher, for example, 50 to 1000 °C / s; T ZP in (d) is the zinc pot temperature.

[0030] In the method for manufacturing an ultra-high strength galvanized steel sheet according to the present invention, in step (2), cooling the finish-rolled strip steel in the range of (A c1 ±45 °C) and holding for 5 to 30 s is to control the laminar cooling process to perform stepwise cooling, so as to cause as much ferrite transformation as possible in the hot-rolled strip steel within the residence time, reduce the variation in structure and performance in the plate width direction, and improve the quality of the strip steel plate shape. Holding the coiled steel coil in the range of (T C ±30 °C) for 30 to 300 min is to give sufficient time for the bainite transformation or pearlite transformation of the strip steel, reduce the generation of hard-phase martensite, and further reduce the strength of the strip steel, so as to facilitate cold rolling.

[0031] In the method for manufacturing an ultra-high strength galvanized steel sheet according to the present invention, the multi-stage annealing in step (4) is a quenching-partitioning process. In step (4), (a) the first-stage annealing is a process of preheating and pre-oxidizing the strip steel. By controlling the O2 content in the atmosphere, external oxidation of elements such as Si and Mn in the steel can be suppressed, and oxidation of Fe can be promoted. (b) The second-stage annealing is a heating / soaking process and also an internal oxidation process. By controlling the annealing temperature, the strip steel is heated in the austenite single-phase region or the ferrite + austenite two-phase region to obtain an appropriate ratio of austenite, and by controlling the annealing atmosphere and dew point, elements such as Si and Mn are internally oxidized. (c) The third-stage annealing is a quenching process. The strip steel is quenched to M s ~M f at a cooling rate higher than the cooling rate in the high-hydrogen cooling mode, so that the austenite generated in the heating / soaking stage is transformed into martensite + retained austenite, and the amount of martensite generated is determined by the height of the quenching temperature. (d) The fourth-stage annealing is a process of reheating and partitioning. By controlling the annealing temperature to (350 °C to T ZP ), it can not only play a role in promoting the diffusion and localization of carbon elements in martensite into austenite, but also avoid a significant decrease in strength caused by severe martensite tempering at too high a partitioning temperature.

[0032] The strip steel obtained after the fourth annealing in process (5) is put into a zinc pot at a temperature of (T ZP ±15 °C) to complete zinc plating, obtaining a zinc-plated steel sheet; when the zinc-plated steel sheet is a hot-dip galvanized layer steel sheet, after the steel sheet with a zinc-plated layer is taken out from the zinc pot, it is cooled to room temperature to obtain the final product; when the zinc-plated steel sheet is a hot-dip zinc-iron alloy plated layer steel sheet, after the strip steel is taken out from the zinc pot, a steel sheet with a hot-dip zinc-iron alloy plated layer is obtained, and then it is reheated for alloying; if the alloying temperature is too low, insufficient alloying will occur, but if the alloying temperature is too high, the stability of retained austenite will decrease and decompose, affecting the elongation rate of the final product, so the alloying temperature is controlled at (T ZP -20 °C) to (T ZP +35 °C); if the alloying time is too short, sufficient alloying of the steel sheet with a hot-dip zinc-iron alloy plated layer cannot be achieved, but if the alloying time is too long, the iron content in the plating layer is too high, deteriorating the choking resistance of the plating layer, so the alloying time is controlled at 5 to 30 s.

[0033] To obtain better implementation effects, in process (2), the finish rolling temperature is controlled at (A c3 +20 °C) to 950 °C.

[0034] To obtain better implementation effects, in some preferred embodiments, in the laminar flow cooling process of the strip steel in process (2), the temperature is controlled in the range of (A c1 -20 °C) to (A c1 +30 °C), and the residence time of laminar flow cooling is 8 to 15 s.

[0035] To obtain better implementation effects, in some preferred embodiments, in the heat preservation process after coiling the strip steel in process (2), the heat preservation time is controlled at 60 to 210 min.

[0036] In order to obtain better implementation effects, in some preferred embodiments, in step (4), in the (b) second annealing process, the second annealing temperature is (A c1 +70 °C) to (A c3 +80 °C) or (A c1 +70 °C) to 900 °C, the heat preservation time is 35 to 120 s, and the upper limit of the second temperature range is the smaller one of (A c3 +80 °C) and 900 °C.

[0037] In order to obtain better implementation effects, in some preferred embodiments, in step (4), in the (b) second annealing process, the atmosphere of the second temperature range contains H2 with a volume content of 0.5 to 20%, and the balance is N2 and inevitable impurities, and the dew point is -10 to 10 °C.

[0038] In order to obtain better implementation effects, in some preferred embodiments, in the annealing process in step (4), the fourth-stage temperature is in the range of 350 °C to (T ZP -35 °C), and the heat preservation time is 30 to 60 s.

[0039] In order to obtain better implementation effects, in some preferred embodiments, in step (5), the strip steel obtained after the (d) fourth-stage annealing in step (4) is continuously heated and put into a zinc pot at a temperature of (T ZP ±10 °C).

[0040] In order to obtain better implementation effects, in some preferred embodiments, in the zinc plating in step (5), the alloying temperature is in the range of (T ZP -10 °C) to (T ZP +25 °C), and the heat preservation time is 10 to 20 s.

[0041] The 100-kilogram-class ultra-high-strength zinc-plated steel sheet excellent in resistance spot weldability and its manufacturing method according to the present invention have the following advantages and beneficial effects as compared with the prior art: (1) The present invention first achieves suppressing the occurrence of LME cracks during the resistance spot welding process by limiting the resistivity in different regions in the thickness direction of the steel sheet and the average resistivity of the steel sheet for a 100-kilogram class ultra-high strength galvanized steel sheet, and an ultra-high strength galvanized steel sheet with excellent resistance spot weldability can be obtained.

[0042] (2) In the 100-kilogram class ultra-high strength galvanized steel sheet according to the present invention, when the welding current is below the minimum current I splash at which spatter occurs, type B cracks and type C cracks do not occur. When type A cracks occur, the depth of type A cracks is 10% or less of the base metal plate thickness. When (I splash + I splash * 25%) ≥ welding current ≥ I splash , type B cracks and type C cracks do not occur. When type D cracks occur, the depth of type D cracks is 10% or less of the base metal plate thickness.

[0043] (3) The ultra-high strength galvanized steel sheet according to the present invention has a simple composition. Through precise control of the process in laminar flow cooling in hot rolling, coiling and heat preservation in hot rolling, and the continuous annealing process, the obtained steel sheet has a tensile strength of ≥ 980 MPa, an elongation at break of ≥ 20%, a hole expansion rate of ≥ 20%, and excellent resistance spot weldability.

[0044] (4) The ultra-high strength galvanized steel sheet according to the present invention has simple process equipment required for production, enables stable batch production, can also be used for manufacturing body parts with complex shapes due to the comprehensive performance of the material, and is excellent in both resistance spot weldability and corrosion resistance.

Brief Description of the Drawings

[0045]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0046] Specific embodiments Hereinafter, based on specific examples, the specific embodiments of the present invention will be further interpreted and described, but such interpretation and description do not limit the technical solution of the present invention.

[0047] In the present invention, zinc plating refers to hot-dip zinc plating or hot-dip zinc-iron alloy plating.

[0048] In the present invention, A c1 refers to the transformation temperature from pearlite to austenite during heating, and the unit is °C.

[0049] In the present invention, A c3 refers to the end temperature of the transformation to austenite during heating, and the unit is °C.

[0050] In the present invention, T C refers to the coiling temperature in step (2), and the unit is °C. In the present invention, M s is the temperature at which "martensite" appears, and M f is the temperature at which complete "martensite" transformation occurs, and the unit is °C.

[0051] In the present invention, T ZP represents the zinc pot temperature, and the unit is °C. In the present invention, V 2-3 represents the cooling rate.

Example

[0052] Examples 1 to 14 and Comparative Examples 1 to 2 The mass percentages of each chemical element in the ultra-high-strength zinc-plated steel sheets according to Examples 1 to 14 and the zinc-plated steel sheets according to Comparative Examples 1 to 2 are shown in Table 1.

[0053]

Table 1

[0054] The ultra-high strength zinc-plated steel sheets according to Examples 1 to 14 of the present invention were manufactured by the following steps: (1) Smelting and continuous casting: Obtained a billet whose chemical composition satisfies the composition of the present invention; (2) Hot rolling: The billet obtained in step (1) was heated at 1100 to 1300 °C and finish-rolled in the range of A c3 to 1000 °C, and the finish-rolled strip steel was cooled in the range of A c1 ±45 °C, and the residence time of laminar flow cooling was 5 to 30 s. Further, it was cooled to 450 to 550 °C and coiled, and the coiling temperature was T C . The coiled steel coil was retained in the range of T C ±30 °C for 30 to 300 min to obtain a hot-rolled coil; (3) Pickling and cold rolling: The hot-rolled coil obtained in step (2) was pickled and cold-rolled to obtain a full-hard coil; (4) Continuous annealing: The full-hard coil obtained in step (3) was subjected to multi-stage heat treatment; (a) In the first-stage annealing, the full-hard coil was heated to the first-stage temperature range of 600 °C or higher to (A c1 +40 °C) to obtain a steel coil; (b) In the second-stage annealing, the steel coil obtained in (a) was continuously heated to the second-stage temperature range of (A c1 +50 °C) to (A c3 +80 °C) or (A c1 +50 °C) to 900 °C, and held for 30 to 300 s to obtain a strip steel; (c) In the third-stage annealing, the strip steel obtained in (b) was heated to the third-stage temperature range M 2-3 at a cooling rate V s equal to or higher than a predetermined value, and held for 10 to 200 s; f (d) In the fourth-stage annealing, the strip steel obtained in (c) was heated again to the fourth-stage temperature range of (350 °C to T ZP ) and held for 15 to 90 s; ​However, in step (4), the atmosphere in (a) contains O2 with a volume content of 0.01 to 0.5%, and the balance is N2 and unavoidable impurities; the atmosphere in (b) contains H2 with a volume content of at least 0.5%, and the balance is N2 and unavoidable impurities, the dew point is -20 to 15°C, and the upper limit of the second-stage temperature range is the smaller of (A c3 +80°C) and 900°C; V in (c 2-3 represents the cooling rate and is 50°C / s or more; T in (d ZP is the zinc pot temperature.

[0055] (5) Zinc plating: The strip steel obtained by the annealing in the fourth stage of (d) in step (4) is put into a zinc pot at a temperature of (T ZP ±15°C) to complete zinc plating, and a zinc-plated steel sheet is obtained; however, when the zinc-plated steel sheet is a hot-dip zinc-coated steel sheet, after the steel sheet having a zinc coating layer is taken out from the zinc pot, it is cooled to room temperature; when the zinc-plated steel sheet is a hot-dip zinc-iron alloy-coated steel sheet, after the steel sheet having a hot-dip zinc-iron alloy coating layer is taken out from the zinc pot, it is kept warm for 5 to 60 s in the range of (T ZP -20°C) to (T ZP +35°C) for alloying and then cooled to room temperature.

[0056] Specific process parameters of the ultra-high-strength zinc-plated steel sheets according to Examples 1 to 14 and the zinc-plated steel sheets according to Comparative Examples 1 to 2 are shown in Tables 2-1 and 2-2.

[0057]

Table 2-1

[0058]

Table 2-2

[0059] GI: The plating layer is a pure zinc layer, GA: The plating layer is a zinc-iron alloy.

[0060] Regarding the ultra-high strength zinc-plated steel sheets according to Examples 1 to 14, the mechanical properties and resistance spot weldability were measured, and the obtained measurement results are shown in Table 3.

[0061] However, the measurement methods for the mechanical properties, resistivity, and resistance spot weldability were as follows: Mechanical properties: In the direction perpendicular to the rolling direction of the steel sheet, tensile samples were processed according to the requirements of Type III samples in ISO 6892-1 standard, and tensile tests were conducted at room temperature to measure the tensile strength TS and elongation at break EL. It should be emphasized that due to differences in measurement methods and sample geometric dimensions, there are differences between the elongation at break measured according to ISO 6892-1 standard and the elongation at break measured according to JIS Z2241 standard or GB / T 228.1 standard. The differences in the measured values of the elongation at break due to differences in reference standards all fall within the protection scope of the ultra-high strength zinc-plated steel sheet according to the present invention.

[0062] Hole expansion ratio: Samples were processed according to ISO 16630 standard, and the hole expansion ratio HER was measured at room temperature according to the requirements of the standard. It should be emphasized that due to differences in measurement methods and sample geometric dimensions, there are differences between the hole expansion ratio measured according to ISO 16630 standard and the hole expansion ratio measured according to JFS T1001 standard or GB / T 15825.4 standard. The differences in the measured values of the hole expansion ratio due to differences in reference standards all fall within the protection scope of the ultra-high strength zinc-plated steel sheet according to the present invention.

[0063] Resistivity: Steel sheets with a predetermined area were sampled, and using processing means including but not limited to grinding machines, wire cutters, milling machines, etc., with the total thickness of the steel sheet being t, samples were taken in the range of 0.010t or more to 0.035t or less, and in the range of more than 0.035t to 0.065t or less in the steel sheet matrix direction, and the resistivities R1, R2, and R3 of the samples were measured with a resistivity measuring device.

[0064] LME cracks: After cutting, polishing, and corroding the welded joint, the cross-section of the joint was observed under a microscope (usually an optical microscope) at a predetermined magnification, and the lengths of various types of cracks in the observed welded joint were determined.

[0065]

Table 3

[0066] TS: Tensile strength, TEL: Elongation at break, HER: Hole expansion rate, N / D: Not detected.

[0067] For the galvanized steel sheets according to Comparative Examples 1 to 2, the resistance spot weldability was measured, and the obtained measurement results are shown in Table 4.

[0068]

Table 4

[0069] As can be seen from Tables 1 to 3, since the chemical elements of the steel grades used in Examples 1 to 14 of the present invention meet the requirements of the present invention, the ultra-high strength galvanized steel sheets obtained by the method according to the present invention in Examples 1 to 14 have a tensile strength of ≧980 MPa, an elongation at break of ≧20%, a hole expansion rate of ≧20%, and a resistivity that meets the requirements of the present invention. Therefore, in the steel sheets according to Examples 1 to 14, when resistance spot welding is performed with a current in the range of I splash ~I splash +I splash *25%, in all cases, type B and type C LME cracks did not occur, type D LME cracks did not appear, or when type D LME cracks appeared, the length of type D cracks was all 10% or less of the base metal plate thickness. For example, among the examples, in Example 7 where the length of type D LME cracks was the longest, the length of type D cracks was 7.6% of the base metal plate thickness.

[0070] In contrast, Comparative Example 1 and Comparative Example 2 are conventional steel plates, and the manufacturing method of the steel plate according to the present invention was not applied. In the steel type of Comparative Example 1, since the chemical elements B, Cr, Mo, and Sb did not meet the requirements of the present invention, as a result, when performing resistance spot welding on the steel plate according to Comparative Example 1, four types of cracks, namely Type A, Type B, Type C, and Type D, occurred, and the resistance spot weldability was poor.

[0071] In the steel type used in Comparative Example 2, since the contents of elements C, Mn, Al, and Ti were different from those of the present invention, in the steel plate obtained in Comparative Example 2, when performing resistance spot welding, LME cracks of Type C did not occur, but the length of Type D cracks was 37.9% of the base metal plate thickness. The length of Type D cracks occurring in the steel plate according to Comparative Example 2 was much larger than the length of Type D cracks occurring in the base metal in the examples of the present invention. The Type D cracks occurring in the steel plate according to Comparative Example 2 had a great influence on the base metal, and the resistance spot weldability was poor.

[0072] In the present invention, by optimizing the chemical elements and controlling the manufacturing process, the LME cracks of the ultra-high strength galvanized steel plates according to Examples 1 to 14 obtained were suppressed, and the resistance spot weldability was excellent.

[0073] In summary, the present invention achieves the suppression of the occurrence of LME cracks in the resistance spot welding process by limiting the resistivity in different regions in the thickness direction of the steel plate and the average resistivity of the steel plate for 100-kilogram-class ultra-high strength galvanized steel plates, and an ultra-high strength galvanized steel plate with excellent resistance spot weldability can be obtained. In this case, when the welding current is below the minimum current I splash at which spatter occurs, Type B cracks and Type C cracks do not occur. When Type A cracks occur, the depth of Type A cracks is 10% or less of the base metal plate thickness, and (I splash +I splash *25%) ≧ welding current ≧ I splashIn the case where type B cracks and type C cracks do not occur, but type D cracks occur, the depth of the type D cracks is 10% or less of the base metal plate thickness. The ultra-high strength galvanized steel sheet according to the present invention has a simple composition. Through precise control of the laminar cooling in hot rolling, coiling and heat preservation in hot rolling, and the process in the continuous annealing process, the obtained steel sheet has a tensile strength of ≧980 MPa, an elongation at break of ≧20%, a hole expansion rate of ≧20%, and excellent resistance spot weldability. The ultra-high strength galvanized steel sheet according to the present invention has simple process equipment required for production, enables stable batch production, can also be applied to the manufacture of body parts with complex shapes due to the comprehensive performance of the material, and also has excellent resistance spot weldability and corrosion resistance. The 100-kilogram class ultra-high strength galvanized steel sheet according to the present disclosure has excellent liquid metal embrittlement (LME) crack resistance and resistance spot weldability, can be effectively applied to the manufacture of automobile body structures, meets the current development needs of automotive steels regarding vehicle weight reduction and safety, and has good application prospects.

[0074] The prior art part within the protection scope of the present invention is not limited to the embodiments described in the application documents of this application. It should be explained that all prior arts (including but not limited to prior patent documents, prior published publications, prior public uses, etc.) that do not conflict with the solution of the present invention are incorporated into the protection scope of the present invention. Also, the combination of each technical feature in this application is not limited to the combination described in the claims of this application or the combination described in specific embodiments. As long as they do not conflict with each other, all the technical features described in this application can be freely combined or joined in any form.

[0075] Furthermore, it should also be noted that the above-mentioned embodiments are only specific embodiments of the present invention. The present invention is not limited to the above embodiments. It is obvious that any similar changes or modifications that those skilled in the art can directly derive from or easily conceive from the disclosure content of the present invention are included in the protection scope of the present invention.

Claims

1. A 100-kilogram class ultra-high strength galvanized steel sheet, wherein the steel sheet has the following weight percentages: C: 0.17 - 0.25%; Mn: 1.7 - 2.7%; Si: 0.35 - 1.5%; Al: 0.01 - 1.0%; 0.7% ≤ Si + Al ≤ 1.7%; and at least one of Nb, Ti, and B; containing; the balance being Fe and unavoidable impurities, provided that Let the thickness of the steel plate be \(t\), and the resistivity \(R\) of the steel plate １ satisfies \(0 \lt R\) １ \(\leq 50\ \mu\Omega\cdot cm\), and the resistivity \(R\) of the steel plate in the range of \(0.010t\) or more to \(0.035t\) or less in the steel plate matrix direction from the interface between the plating layer and the steel plate matrix ２ satisfies \(0 \lt R\) ２ \(\leq 15\ \mu\Omega\cdot cm\), and the resistivity \(R\) of the steel plate in the range of more than \(0.035t\) to \(0.065t\) or less ３ satisfies \(0 \lt R\) ３ \(\leq 30\ \mu\Omega\cdot cm\), and \((R\) ２ / 2 + R ３ / 3) \(\leq (3.1R\) １ １／２ - 1.5) is satisfied characterized in that it is a 100-kilogram class ultra-high strength galvanized steel sheet.

2. The Nb, Ti, and B are in mass percentages: 0 < Nb ≤ 0.1%; 0 < Ti ≤ 0.1%; 0 < B ≤ 0.003% characterized in that it is the 100-kilogram class ultra-high strength galvanized steel sheet according to Claim 1.

3. Other unavoidable impurities are in mass percentages: P ≤ 0.015%; S ≤ 0.010%; N ≤ 0.008%; REM ≤ 0.01% characterized in that it contains the same, and it is the 100-kilogram class ultra-high strength galvanized steel sheet according to Claim 1.

4. Characterized in that it contains at least one of the following components in mass percentages, and it is the 100-kilogram class ultra-high strength galvanized steel sheet according to Claim 1. Nb: 0.03% - 0.05%; Ti: 0.001% - 0.02%; B: 0.0002% - 0.0025%.

5. Let the thickness of the steel plate be \(t\), and the resistivity \(R\) of the steel plate １ is \(37\ \Omega\cdot cm\leq R\) １ \(\leq50\ \mu\Omega\cdot cm\), and from the interface between the plating layer and the steel plate matrix, the resistivity \(R\) of the steel plate in the range of \(0.010t\) or more to \(0.035t\) or less in the steel plate matrix direction ２ is \(9\ \Omega\cdot cm\leq R\) ２ \(\leq15\ \mu\Omega\cdot cm\), and the resistivity \(R\) of the steel plate material in the range exceeding \(0.035t\) to \(0.065t\) or less ３ is \(24\ \Omega\cdot cm\leq R\) ３ \(\leq30\ \mu\Omega\cdot cm\), and moreover \((R\) ２ / 2 + R\) ３ / 3)\(\leq(3.1R\) １ １／２ - 1.5)\) is satisfied. The 100 - kiloton - class ultra - high - strength zinc - plated steel plate according to claim 1.

6. The steel sheet is arranged as a GA sheet or a GI sheet, and preferably, the thickness of the steel sheet is 1 - 2 mm, characterized in that it is the 100-kilogram class ultra-high strength galvanized steel sheet according to Claim 1.

7. When a Type III sample compliant with ISO 6892-1 standard perpendicular to the rolling direction is tensioned at room temperature, the tensile strength of the steel sheet is ≥ 980 MPa, the elongation at break is ≥ 20%, and the hole expansion rate is ≥ 20%, characterized in that it is the 100-kilogram class ultra-high strength galvanized steel sheet according to Claim 1.

8. 1 to 1.25 times I ｓｐｌａｓｈ When welding the steel sheet with a current of ｓｐｌａｓｈ , if type B cracks and type C cracks do not occur and type D cracks occur, the depth of the crack is 10% or less of the base metal thickness; when welding the steel sheet with a current of 1 times or less I ｓｐｌａｓｈ When welding the steel sheet with a current of ｓｐｌａｓｈ , if type B cracks and type C cracks do not occur and type A cracks occur, the depth of the crack is 10% or less of the base metal thickness, provided that I ｓｐｌａｓｈ is the minimum current when sputtering occurs, and the 100-kilogram-class ultra-high-strength galvanized steel sheet according to claim 1.

9. Characterized in that it includes the following steps, and it is a method for manufacturing a 100-kilogram class ultra-high strength galvanized steel sheet according to any one of Claims 1 - 8. (1) Perform refining and continuous casting to obtain a billet that satisfies the composition of the steel sheet according to any one of Claims 1 - 4; (2) Hot rolling: Heat the billet in step (1), finish rolling, perform laminar flow cooling, and wind it up to obtain a hot rolled coil; (3) Pickling and cold rolling: Pickle the hot rolled coil obtained in step (2) and perform cold rolling to obtain a full hard coil; (4) Continuous annealing: Perform multi-stage heat treatment on the full hard coil obtained in step (3) to obtain a strip steel; (5) Zinc plating: The strip steel obtained in step (4) is placed in a zinc pot at a temperature of (zinc pot temperature T ＺＰ ±15 °C) to complete zinc plating and obtain a zinc-plated steel sheet.

10. The method for manufacturing a 100-kilogram class ultra-high strength galvanized steel sheet according to claim 9, characterized in that the multi-stage annealing treatment includes the following (a) to (d). In the first-stage annealing, the fully hard coil was heated to a first-stage temperature range of 600 °C or higher to (A ｃ１ + 40 °C) to obtain a steel coil; In the second annealing step, the steel coil obtained in (a) is subsequently heated to a second temperature range of (A ｃ１ + 50 °C) to (A ｃ３ + 80 °C) or (A ｃ１ + 50 °C) to 900 °C, held for 30 to 300 s to obtain a strip steel, and the upper limit of the second temperature range is taken as the smaller of (A ｃ３ + 80 °C) and 900 °C; In the annealing of the third stage, the strip steel obtained in (b) is heated at a cooling rate V equal to or higher than a predetermined value ２－３ to the third-stage temperature range M ｓ to M ｆ and held for 10 to 200 s; In the fourth-stage annealing, the strip steel obtained in (c) is heated again to the fourth-stage temperature range of (350°C to the zinc pot temperature) and held for 15 to 90 s; However, A ｃ１ is the transformation temperature from pearlite to austenite during heating, and A ｃ３ is the end temperature of the transformation to austenite during heating. M ｓ is the temperature at which martensite appears, and M ｆ is the temperature at which complete martensite transformation occurs. V ２－３ represents the cooling rate and is 50°C / s or more.

11. In (b), the annealing temperature of the second stage is controlled to be (A ｃ１ + 70°C) to (A ｃ３ + 80°C) or (A ｃ１ + 70°C) to 900°C, the holding time is 35 to 120 s, and the upper limit of the second-stage temperature range takes the smaller value of (A ｃ３ + 80°C) and 900°C; and / or In (d), the temperature of the fourth stage is in the range of 350°C to (T ＺＰ - 35°C), and the heat preservation time is 30 to 60 s The method for manufacturing a 100-kilogram class ultra-high strength galvanized steel sheet according to claim 10, characterized by the above.

12. In step (4), the atmosphere in (a) contains O with a volume content of 0.01 to 0.5%, and the balance is N ２ and inevitable impurities; the atmosphere in (b) contains H with a volume content of at least 0.5%, and the balance is N ２ and inevitable impurities, and the dew point is -20 to 15°C; ２ and inevitable impurities; ２ and the dew point is -20 to 15°C; Preferably, the atmosphere in the second temperature range contains H with a volume content of 0.5 to 20%, and the balance is N ２ and inevitable impurities, and the dew point is -10 to 10 °C ２ and inevitable impurities, and the dew point is -10 to 10 °C The method for manufacturing a 100-kilogram class ultra-high strength galvanized steel sheet according to claim 9, characterized by the above.

13. In the step (2), the heating temperature is in the range of 1100 to 1300°C, The temperature for finish rolling is A ｃ３ ~1000 °C, and The heat preservation temperature for laminar flow cooling is in the range of (A ｃ１ ± 45°C), and the residence time of laminar flow cooling is 5 to 30 s. Further, it is cooled to 450 to 550°C for coiling, and the coiled steel coil is heat-preserved for 30 to 300 min in the range of (coiling temperature T Ｃ ± 30°C); Preferably, the temperature for finish rolling is (A ｃ３ + 20°C) to 950°C; Preferably, in the laminar flow cooling process of the strip steel, the temperature is controlled within the range of (A ｃ１ - 20°C) to (A ｃ１ + 30°C), and the residence time of laminar flow cooling is 8 to 15 s; preferably, in the heat preservation process after coiling the strip steel, the heat preservation time is controlled to be 60 to 210 min The method for manufacturing a 100-kilogram class ultra-high strength galvanized steel sheet according to claim 9, characterized by the above.

14. In the step (5), when the galvanized steel sheet is a hot-dip galvanized steel sheet, after taking out the steel sheet having a galvanized layer from the zinc pot, it is cooled to room temperature; when the galvanized steel sheet is a hot-dip zinc-iron alloy galvanized steel sheet, after taking out the steel sheet having a hot-dip zinc-iron alloy galvanized layer from the zinc pot, (zinc pot temperature T ＺＰ -20 °C) to (zinc pot temperature T ＺＰ +35 °C) and keeping it warm for 5 to 60 s for alloying, and then cooling it to room temperature. The method for manufacturing a 100-kilogram-class ultra-high-strength galvanized steel sheet according to claim 9, characterized in that.

15. In the step (5), the strip steel is put into a zinc pot at a temperature of T ＺＰ ±10°C, alloyed in the temperature range of (T ＺＰ -10°C) to (T ＺＰ +25°C), and the heat preservation time is 10 - 20 s. The method for manufacturing a 100-kilogram-class ultra-high-strength zinc-plated steel sheet according to claim 9, characterized in that.

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