Preparation method of thin-gauge high-hole-expansion-rate hot-rolled galvanized steel plate

Abstract

The invention relates to a preparation method of a thin-gauge high-hole-expansion-rate hot-rolled galvanized steel plate, and belongs to the technical field of steel manufacturing. The method comprises the following steps: obtaining molten steel with set chemical components; performing continuous casting on the molten steel by adopting a thin slab continuous casting and rolling process to obtain a slab; the plate blank is sequentially subjected to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling, and a hot-rolled coil is obtained; the hot-rolled coil is sequentially subjected to acid pickling and nickel pre-plating, and a nickel pre-plated steel plate is obtained; the pre-nickel-plated steel plate is subjected to continuous hot galvanizing treatment, and a hot-rolled galvanized steel plate is obtained; wherein the continuous hot galvanizing treatment comprises a three-section continuous heating section, a cooling section, an induction heating section and a galvanizing section. Through a series of technical innovation and optimization, the problems that an existing hot-rolled galvanized steel plate is low in hole expansion rate and thin-specification products are difficult to produce are successfully solved, and a new development direction is provided for the iron and steel industry.

Description

Technical Field

[0001] The present application relates to the technical field of steel manufacturing, and in particular to a method for preparing a thin-gauge hot-rolled galvanized steel plate with a high hole expansion rate. Background Art

[0002] Hot-rolled galvanized sheet has relatively excellent corrosion resistance and forming performance, and gradually replaces hot-rolled pickled sheet for the manufacture of some chassis parts. With the continuous improvement of automobile lightweight and collision safety requirements, the shape of automobile parts tends to be complicated, and some parts have higher and higher requirements for hole expansion and flanging performance, which far exceeds the specified values ​​of product standards, resulting in conventional products being unable to meet the forming requirements. Products with high hole expansion performance must be developed.

[0003] In addition, due to the need to reduce carbon emissions and costs, the proportion of hot-rolled galvanized steel sheets replacing cold-rolled galvanized steel sheets will further expand. However, the current specifications of hot-rolled galvanized steel sheets are generally thicker, which has become an important bottleneck for "replacing cold with hot". Based on the above needs, it is urgent to develop a thin-gauge hot-rolled galvanized steel sheet with a high hole expansion rate. Summary of the invention

[0004] The present application provides a method for preparing a thin-gauge hot-rolled galvanized steel sheet with a high hole expansion rate, so as to solve the following technical problem: how to improve the hole expansion rate of the hot-rolled galvanized steel sheet on the basis of thin gauge.

[0005] The present application provides a method for preparing a thin-gauge high-hole expansion rate hot-rolled galvanized steel sheet, the method comprising:

[0006] Obtaining molten steel with a set chemical composition;

[0007] The molten steel is continuously casted by a thin slab continuous casting and rolling process to obtain a slab;

[0008] The slab is sequentially subjected to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil;

[0009] The hot-rolled coil is pickled and pre-nickel plated in sequence to obtain a pre-nickel plated steel sheet;

[0010] The pre-nickel-plated steel sheet is subjected to continuous hot-dip galvanizing treatment to obtain a hot-rolled galvanized steel sheet; wherein the continuous hot-dip galvanizing treatment includes: three-stage continuous heating, cooling, induction heating and galvanizing.

[0011] Optionally, the three-stage continuous heating includes: a preheating section, a heating section and a soaking section.

[0012] Optionally, the temperature of the preheating section is 210°C to 230°C, and the heating rate of the heating section is 1.5°C / s to 5°C / s.

[0013] Optionally, the temperature of the soaking section is 830° C. to 870° C., and the soaking time of the soaking section is 80s to 160s.

[0014] Optionally, the cooling section includes a slow cooling section and a fast cooling section.

[0015] Optionally, the slow cooling rate of the slow cooling section is 2°C / s to 6°C / s, and the terminal temperature of the slow cooling section is 740°C to 760°C.

[0016] Optionally, the rapid cooling rate of the rapid cooling section is 50°C / s to 75°C / s, and the terminal temperature of the rapid cooling section is 230°C to 270°C.

[0017] Optionally, the induction heating section includes an induction heating section 1, an aging section and an induction heating section 2.

[0018] Optionally, the heating rate of the induction heating stage 1 is 40°C / s to 60°C / s.

[0019] Optionally, the temperature of the aging section is 330° C. to 370° C., and the aging time of the aging section is 20s to 30s.

[0020] Optionally, the heating rate of the induction heating section 2 is 20°C / s to 35°C / s, and the strip temperature of the galvanizing section is 460°C.

[0021] Optionally, the continuous casting speed is 4m / min to 6m / min, and the thickness of the slab is 110mm to 125mm.

[0022] Optionally, the temperature of the heat treatment is 1150°C to 1200°C.

[0023] Optionally, the inlet temperature of the rough rolling is 1050°C to 1100°C, and the end temperature of the rough rolling is 950°C to 1000°C.

[0025] Optionally, the outlet temperature of the electromagnetic induction heating is ≥1200°C.

[0026] Optionally, the inlet temperature of the finishing rolling is 1150°C to 1200°C, and the final rolling temperature of the finishing rolling is 830°C to 870°C.

[0027] Optionally, the average cooling rate of the cooling is ≥10°C / s.

[0028] Optionally, the coiling temperature is 120° C. to 180° C., and the thickness of the hot-rolled coil is 1.0 mm to 2.0 mm.

[0029] Optionally, the set chemical composition includes, by mass fraction: C: 0.05%~0.10%, Si: 0.3%~1.0%, Mn: 1.8%~2.6%, Al: 0.01%~0.05%, P≤0.01%, S≤0.003%, Cr: 0.3%~0.8%, Mo≤0.05%, Cu≤0.03%, Ni≤0.03%, Ti: 0.01%~0.03%, Nb≤0.01%, V≤0.05%, and matrix element Fe.

[0030] Optionally, the microstructure of the hot-rolled galvanized steel sheet includes, by area fraction, ferrite: 20% to 40%, granular bainite: 40% to 60%, island martensite: 5% to 15%, and retained austenite ≤8%.

[0031] Optionally, the hot-rolled galvanized steel sheet satisfies at least one of the following properties: yield strength of 600MPa to 710MPa, tensile strength ≥800MPa, and hole expansion rate ≥85%.

[0032] Optionally, the scale breaking and straightening elongation of the pickling is 0.3% to 0.6%, the pickling speed is 150m / min to 200m / min, the acid concentration of the pickling is 30g / L to 35g / L, and the acid temperature of the pickling is 60°C to 80°C.

[0033] Optionally, the pre-nickel-plated steel plate contains a nickel-plated layer, and the nickel weight per unit area of ​​the nickel-plated layer is 800 mg / m 2 ~1000 mg / m 2 .

[0034] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0035] The embodiment of the present application provides a method for preparing a thin-gauge hot-rolled galvanized steel sheet with a high hole expansion rate, the method comprising: obtaining molten steel with a set chemical composition, continuously casting the molten steel by a thin slab continuous casting and rolling process to obtain a slab; performing soaking treatment, rough rolling, electromagnetic induction heating, finishing rolling, cooling and coiling on the slab in sequence to obtain a hot-rolled coil; performing pickling and pre-nickel plating on the hot-rolled coil in sequence to obtain a pre-nickel-plated steel sheet; performing continuous hot-dip galvanizing treatment on the pre-nickel-plated steel sheet to obtain a hot-rolled galvanized steel sheet; wherein the continuous hot-dip galvanizing treatment includes: three-stage continuous heating, cooling section, induction heating section and galvanizing section. By optimizing the chemical composition design of the molten steel and combining a specific rolling process (such as the application of electromagnetic induction heating before finishing rolling), the microstructure and texture of the steel sheet are effectively improved, thereby significantly improving the hole expansion rate of the hot-rolled galvanized steel sheet; this optimization not only meets the demand for high hole expansion performance of automotive parts, but also exceeds the standard specified value of conventional products. The thin slab continuous casting and rolling process is adopted to directly continuously cast the molten steel to obtain thin slabs, avoiding the increase in thickness caused by multiple heating and rolling in the traditional hot rolling process. At the same time, through precise rolling control and cooling process, thin-gauge hot-rolled galvanized steel sheets are successfully prepared, solving the problem that the existing hot-rolled galvanized sheets are generally thick. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0038] Figure 1 A schematic flow chart of a method for preparing a thin-gauge, high-hole expansion rate hot-rolled galvanized steel sheet provided in an embodiment of the present application;

[0039] Figure 2 A schematic diagram of a continuous hot-dip galvanizing process provided in an embodiment of the present application;

[0040] Figure 3 The microstructure of the thin-gauge high-pore expansion rate hot-rolled galvanized steel sheet provided in the embodiment of the present application. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0042] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range; in addition, whenever a numerical range is indicated in this document, it is meant to include any cited numbers (fractions or integers) within the indicated range.

[0043] In this document, the terms including "including" and "including" mean "including but not limited to". Relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. "And / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "plurality" means two or more; "at least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items; for example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, if the mass ratio of substance A, substance B and substance C is 1:2:3, then substance A, substance B and substance C should correspond to the proportional numbers in the proportional formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0044] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0045] Figure 1 A schematic flow chart of a method for preparing a thin-gauge, high-pore-expansion-ratio hot-rolled galvanized steel sheet provided in an embodiment of the present application.

[0046] like Figure 1 As shown, the embodiment of the present application provides a method for preparing a thin-gauge high-hole expansion rate hot-rolled galvanized steel sheet, comprising:

[0047] S1, obtaining molten steel with set chemical composition;

[0048] S2, continuously casting the molten steel by a thin slab continuous casting and rolling process to obtain a slab;

[0049] In some embodiments, the set chemical composition includes, by mass fraction: C: 0.05% to 0.10%, Si: 0.3% to 1.0%, Mn: 1.8% to 2.6%, Al: 0.01% to 0.05%, P≤0.01%, S≤0.003%, Cr: 0.3% to 0.8%, Mo≤0.05%, Cu≤0.03%, Ni≤0.03%, Ti: 0.01% to 0.03%, Nb≤0.01%, V≤0.05%, and matrix element Fe.

[0050] C is an interstitial solid solution atom and an element that improves hardenability. It can promote the formation of hard phase structures such as bainite and martensite, and mainly improves strength through solid solution strengthening and phase transformation strengthening. In order to obtain a tensile strength of not less than 800MPa in the embodiment of the present application, the mass fraction of C cannot be less than 0.05%; however, the mass fraction of C should not exceed 0.10%, otherwise the plasticity and hole expansion performance will deteriorate, and the welding performance will also be impaired. Exemplarily, the mass fraction of C can be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, etc.

[0051] Si has a strong solid solution strengthening effect, which can strengthen the ferrite structure, reduce the hardness difference between ferrite and bainite / martensite, and is beneficial to improving the hole expansion performance. In addition, Si is a non-carbide forming element, which can inhibit the formation of cementite during the aging process of continuous hot-dip galvanizing. In order to form the above effects, the mass fraction of Si shall not be less than 0.3%. However, when the mass fraction of Si is too high, strip-shaped red rust is easily formed on the surface of the hot-rolled coil, and white strip-shaped color difference is formed after pickling, and it cannot be covered after galvanizing. Therefore, in the embodiment of the present application, the mass fraction of Si is first set at 0.3% to 1.0%. Exemplarily, the mass fraction of Si can be 0.3%, 0.5%, 0.7%, 0.9%, 1.0%, etc.

[0052] Mn has a strong solid solution strengthening effect and can also promote the phase transformation of bainite and martensite. It is an important component to ensure high yield and tensile strength. Therefore, the mass fraction of Mn is limited to 1.8% and above. However, when the mass fraction of Mn is too high, it is not only easy to cause Mn segregation, but also cause a significant decrease in plasticity. Therefore, the mass fraction of Mn in steel is set to no more than 2.6%. Exemplarily, the mass fraction of Mn can be 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, etc.

[0053] Al is a deoxidizing element that can reduce oxide inclusions in steel. However, too high a mass fraction of Al will increase the difficulty of continuous casting and the cost of the alloy, so the mass fraction of Al is limited to between 0.01% and 0.05%. For example, the mass fraction of Al can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0054] As an interstitial solid solution atom, P can appropriately improve the strength of the steel plate, but it is also easy to segregate at the grain boundary and deteriorate the plasticity and formability, so the upper limit of the mass fraction of P is set to 0.01%. For example, the mass fraction of P can be 0.002%, 0.004%, 0.006%, 0.008%, 0.01%, etc.

[0055] S is easy to combine with Mn to form coarse MnS inclusions, which deteriorates the forming properties of the steel plate such as hole expansion and flanging, so the upper limit of the mass fraction of S is set to 0.003%. For example, the mass fraction of S can be 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003%, etc.

[0056] Cr is an element that improves hardenability, delays pearlite transformation, promotes bainite formation, and is beneficial to improving strength. However, when the mass fraction of Cr exceeds 0.8%, the effect is no longer obvious and the cost will increase. Therefore, the mass fraction of Cr is controlled at 0.3% to 0.8%. For example, the mass fraction of Cr can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc.

[0057] Mo is also an element that improves hardenability, but Mo is an expensive alloy element. In order to reduce alloy costs, the mass fraction of Mo is controlled to be no more than 0.05%. For example, the mass fraction of Mo can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0058] Cu has the effect of improving the strength and corrosion resistance of steel, and can significantly eliminate the harmful effects of S in steel. However, the cost of Cu alloy is relatively high. In order to reduce the cost, the upper limit of the mass fraction of Cu is set to 0.03%. Exemplarily, the mass fraction of Cu can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0059] The effects of Ni and Cu on the quality and performance of steel are mutually influential. Generally, the two exist in steel at the same time, and Ni is also an element that improves the atmospheric corrosion resistance of steel. Due to the high cost of Ni alloys, the upper limit of the mass fraction of Ni is limited to 0.03%. In order to improve the corrosion resistance of the steel matrix, at least one of Cu≤0.03% and Ni≤0.03% can be added. Of course, the composition of Cu and Ni includes the case where both are 0, that is, the above elements can be omitted. Exemplarily, the mass fraction of Ni can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0060] Ti is a strong carbonitride-forming element, which can improve the hardness and strength of ferrite structure through precipitation strengthening. This can not only improve the yield strength of the steel plate, but also reduce the hardness difference between ferrite and bainite and martensite, which is beneficial to the improvement of local forming performance such as hole expansion and bending. In order to obtain excellent strength and forming performance, it is selected to add not less than 0.01% of Ti. However, since the embodiment of the present application adopts an induction heating process, the yield strength can be appropriately improved, and it is not necessary to rely on a large amount of micro-alloying elements to improve the strength, so the upper limit of the mass fraction of Ti is set to 0.03%. Exemplarily, the mass fraction of Ti can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0061] Nb can refine grains through solute drag, and is a strong carbide-forming element. It can improve the hardness and strength of ferrite structure through precipitation strengthening, which is beneficial to improve hole expansion performance. However, Nb will increase the recrystallization temperature, resulting in the formation of banded structure during hot rolling, which will deteriorate the hole expansion performance. In addition, the cost of Nb alloy is relatively high. Therefore, the upper limit of the mass fraction of Nb is set to 0.01%. For example, the mass fraction of Nb can be 0.002%, 0.004%, 0.006%, 0.008%, 0.01%, etc.

[0062] V can also improve strength and hole expansion performance through precipitation strengthening. However, under the process path of the embodiment of the present application, when the mass fraction of V exceeds 0.05%, the strengthening effect reaches saturation. Therefore, the mass fraction of V is limited to ≤ 0.05%. Exemplarily, the mass fraction of V can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0063] Fe is a matrix element. The specific content / content range of Fe can be obtained by the upper and lower limit formula of the component, that is:

[0064] The sum of the percentages of the contents of each component in a composition should be equal to 100%, and the content ranges of several components should meet the following conditions: the upper limit value of a component + the lower limit value of other components ≤ 100; the lower limit value of a component + the upper limit value of other components ≥ 100.

[0065] In some embodiments, the continuous casting speed is 4 m / min to 6 m / min, and the thickness of the slab is 110 mm to 125 mm.

[0066] When the continuous casting speed is lower than 4 m / min, the production efficiency is low; when the continuous casting speed is higher than 6 m / min, the strip is prone to breakage. For example, the continuous casting speed can be 4 m / min, 4.5 m / min, 5 m / min, 5.5 m / min, 6 m / min, etc.

[0067] When the slab thickness is less than 110 mm, the hot rolling reduction rate is low and the slab defects are difficult to eliminate; when the slab thickness is greater than 125 mm, the cumulative reduction rate is high, and the deformation resistance is large when rolling thin gauge steel plates. For example, the thickness of the slab can be 110 mm, 113 mm, 116 mm, 119 mm, 122 mm, 125 mm, etc.

[0068] S3, sequentially performing soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling on the slab to obtain a hot-rolled coil;

[0069] In some embodiments, the soaking treatment is performed at a temperature of 1150°C to 1200°C.

[0070] When the soaking temperature is lower than 1150°C, the homogenization effect is poor and the rough rolling inlet temperature is low; when the soaking temperature is higher than 1200°C, the ingot microstructure tends to grow significantly. For example, the soaking temperature can be 1150°C, 1160°C, 1170°C, 1180°C, 1190°C, 1200°C, etc.

[0071] In some embodiments, the inlet temperature of the rough rolling is 1050°C to 1100°C, and the end temperature of the rough rolling is 950°C to 1000°C.

[0072] When the inlet temperature of rough rolling is lower than 1050°C, the rolling resistance is too large, resulting in difficulty in rolling; when the inlet temperature of rough rolling is higher than 1100°C, the structure is coarse. For example, the inlet temperature of rough rolling may be 1050°C, 1060°C, 1070°C, 1080°C, 1090°C, 1100°C, etc.

[0073] In some embodiments, the outlet temperature of the electromagnetic induction heating is ≥ 1200°C.

[0074] When the outlet temperature of induction heating is lower than 1200°C, the finishing inlet temperature greater than 1150°C cannot be guaranteed, which leads to a low finishing rolling temperature and increased structural inhomogeneity. For example, the outlet temperature of electromagnetic induction heating can be 1200°C, 1210°C, 1220°C, 1230°C, 1240°C, 1250°C, etc.

[0075] In some embodiments, the inlet temperature of the finish rolling is 1150°C to 1200°C, and the final rolling temperature of the finish rolling is 830°C to 870°C.

[0076] When the inlet temperature of the finishing rolling is lower than 1150°C, the finishing rolling temperature is too low, which may easily lead to uneven structure; when the inlet temperature of the finishing rolling is higher than 1200°C, the grain size is too large, which may easily lead to insufficient tensile strength of the final steel plate. For example, the inlet temperature of the finishing rolling may be 1150°C, 1160°C, 1170°C, 1180°C, 1190°C, 1200°C, etc.

[0077] When the final rolling temperature is greater than 870°C, the austenite grains are coarse, which increases the grain size after cooling phase transformation, and the ferrite ratio is high, which easily leads to insufficient tensile strength of the steel plate; when the final rolling temperature is lower than 830°C, the structure is obviously striped, and the uneven structure leads to a decrease in the hole expansion performance of the steel plate; in addition, too low a final rolling temperature will also increase the deformation resistance of the hot-rolled plate, which will increase the rolling load of the hot rolling mill. For example, the final rolling temperature of the finishing rolling can be 830°C, 840°C, 850°C, 860°C, 870°C, etc.

[0078] In some embodiments, the average cooling rate of the cooling is ≥10°C / s.

[0079] When the average cooling rate of cooling is less than 10°C / s, too much ferrite or pearlite will be formed during the cooling process, which is not conducive to phase transformation and recrystallization during the subsequent annealing process. Exemplarily, the average cooling rate of cooling can be 10°C / s, 11°C / s, 12°C / s, 13°C / s, 14°C / s, 15°C / s, etc.

[0080] In some embodiments, the coiling temperature is 120° C. to 180° C., and the thickness of the hot-rolled coil is 1.0 mm to 2.0 mm.

[0081] When the coiling temperature is greater than 180°C, the cooling rate of the hot rolled coil after coiling is slow, the tempering characteristics are obvious, and the recrystallization driving force decreases during subsequent annealing, which is not conducive to grain refinement; when the coiling temperature is less than 120°C, the shape of the hot rolled plate deteriorates, resulting in difficulty in coiling. For example, the coiling temperature can be 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, etc.

[0082] S4, sequentially pickling and pre-nickel-plating the hot-rolled coil to obtain a pre-nickel-plated steel plate;

[0083] In some embodiments, the scale breaking and straightening elongation of the pickling is 0.3% to 0.6%, the pickling speed is 150m / min to 200m / min, the acid concentration of the pickling is 30g / L to 35g / L, and the acid temperature of the pickling is 60°C to 80°C.

[0084] The purpose of pickling in the embodiment of the present application is to remove the scale and obtain a pickled plate with a good surface state. When the scale breaking and straightening elongation is lower than 0.3%, the scale on the surface of the hot-rolled plate is not broken well, resulting in residual scale after pickling, affecting the surface quality of galvanized plate; when the scale breaking and straightening elongation is higher than 0.6%, the scale on the surface of the hot-rolled plate is excessively broken. Although the scale removal effect is better during the pickling process, the acid can easily penetrate the scale into the steel matrix, which will also affect the subsequent galvanizing quality, such as the easy formation of zinc flow lines. Exemplarily, the scale breaking and straightening elongation can be 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, etc.

[0085] In the embodiment of the present application, the strip runs at a constant speed in the pickling tank, which is 150m / min to 200m / min. When the pickling speed is lower than 150m / min, the strip stays in the pickling tank for a long time, which easily leads to over-pickling and low production efficiency; when the pickling speed is higher than 200m / min, it easily leads to poor removal of iron oxide scale. For example, the pickling speed can be 150m / min, 160m / min, 170m / min, 180m / min, 190m / min, 200m / min, etc.

[0086] The acid concentration directly affects the pickling effect of the hot-rolled plate surface. When the acid concentration is lower than 30 g / L, the hot-rolled iron oxide scale is difficult to clean; when the acid concentration is higher than 35 g / L, for the embodiment of the present application, it is easy to cause the steel plate surface to be over-pickled, thereby affecting the galvanized surface quality. Exemplarily, the acid concentration can be 30 g / L, 31 g / L, 32 g / L, 33 g / L, 34 g / L, 35 g / L, etc.

[0087] The acid solution temperature is also an important factor affecting the pickling quality. When the acid solution temperature is lower than 60°C, the oxide scale removal effect on the steel plate surface is poor; when the acid solution temperature is higher than 80°C, it is also easy to cause over-pickling. After repeated tests, for the steel plate of the embodiment of the present application, when the acid solution temperature is 60°C to 80°C, the surface quality after galvanizing is the best. Exemplarily, the acid solution temperature can be 60°C, 65°C, 70°C, 75°C, 80°C, etc.

[0088] In the embodiment of the present application, the strip is leveled online after exiting the pickling tank, and the leveling rolling force is 2500kN to 3500kN. When the leveling rolling force is lower than 2500kN, the surface morphology uniformity of the pickled plate is poor. By increasing the leveling rolling force, the transfer rate of the surface roughness of the leveling roller on the pickled strip can be increased, thereby making the surface roughness morphology of the pickled strip uniform, and obtaining good micro-area coating uniformity after galvanizing; in addition, increasing the leveling rolling force will also increase the straightness of the strip and improve the strip shape. When the leveling rolling force is higher than 3500kN, it is easy to cause the pickled plate to have a wave shape and a transverse roller mark.

[0089] In some embodiments, the pre-nickel-plated steel sheet comprises a nickel-plated layer, and the nickel weight per unit area of ​​the nickel-plated layer is 800 mg / m 2 ~1000 mg / m 2 .

[0090] In the embodiment of the present application, the pre-nickel plating is an electroplating process, and the nickel plating current density is 0.4A / dm 2 The purpose of the flash nickel plating process in the embodiment of the present application is to improve the plating property of the hot-rolled pickled steel plate. The diffusion coefficient of the alloying elements in nickel is significantly lower than that in the matrix, which makes it difficult for the alloying elements to diffuse outward to the surface and be oxidized, thereby inhibiting the external oxidation behavior of the alloying elements Si and Mn. When the unit area weight of the nickel layer on the surface of the substrate is less than 800 mg / m 2 When the nickel layer is thin, the effect of inhibiting external oxidation is not good; when the nickel weight per unit area of ​​the nickel layer is higher than 1000mg / m 2 When the nickel layer is thick, the cost is high. For example, the nickel weight per unit area of ​​the nickel layer can be 800 mg / m 2 , 850mg / m 2 , 900mg / m 2 , 950mg / m 2 , 1000mg / m 2 wait.

[0091] S5. Perform continuous hot-dip galvanizing treatment on the pre-nickel-plated steel sheet to obtain a hot-rolled galvanized steel sheet; wherein the continuous hot-dip galvanizing treatment includes: three-stage continuous heating, cooling, induction heating and galvanizing.

[0092] Figure 2A schematic diagram of the process of continuous hot-dip galvanizing treatment provided in an embodiment of the present application.

[0093] In some embodiments, the three-stage continuous heating includes: a preheating stage, a heating stage, and a soaking stage.

[0094] In some embodiments, the temperature of the preheating section is 210° C. to 230° C., and the heating rate of the heating section is 1.5° C. / s to 5° C. / s.

[0095] When the heating rate of the heating section is lower than 1.5℃ / s, the hot rolling structure is in the low temperature zone for a long time, and the recovery characteristics are detailed, which is not conducive to phase transformation and recrystallization; when the heating rate is higher than 5℃ / s, the temperature of the steel plate is uneven, which is easy to produce internal stress and poor plate shape, resulting in deviation of the furnace zone. For example, the heating rate of the heating section can be 1.5℃ / s, 2.5℃ / s, 3.5℃ / s, 4.5℃ / s, 5℃ / s, etc.

[0096] In some embodiments, the temperature of the soaking section is 830° C. to 870° C., and the soaking time of the soaking section is 80 s to 160 s.

[0097] When the soaking temperature is lower than 830°C, the steel plate will enter the two-phase region, and too much ferrite will be produced during soaking, resulting in low strength after cooling and phase transformation; when the soaking temperature is higher than 870°C, the original austenite grains grow and coarsen, and the surface of the strip is prone to decarburization, resulting in a decrease in strength and uneven structure after phase transformation. Exemplarily, the temperature of the soaking section can be 830°C, 840°C, 850°C, 860°C, 870°C, etc. When the soaking time is less than 80s, the degree of homogenization of the high temperature zone is low, which can easily lead to uneven performance; when the soaking time exceeds 160s, the grains grow excessively, which can easily lead to insufficient tensile strength. Exemplarily, the soaking time of the soaking section can be 80s, 100s, 120s, 140s, 160s, etc.

[0098] In some embodiments, the cooling section includes a slow cooling section and a fast cooling section.

[0099] In some embodiments, the slow cooling rate of the slow cooling section is 2°C / s to 6°C / s, and the terminal temperature of the slow cooling section is 740°C to 760°C.

[0100] When the slow cooling end temperature is lower than 740°C, a large amount of ferrite is easily formed during the slow cooling process, resulting in insufficient strength of the steel plate; when the slow cooling end temperature is higher than 760°C, the amount of ferrite generated is too low, which will result in a low elongation after the stage. For example, the end temperature of the slow cooling stage can be 740°C, 745°C, 750°C, 755°C, 760°C, etc.

[0101] In some embodiments, the rapid cooling rate of the rapid cooling section is 50°C / s to 75°C / s, and the terminal temperature of the rapid cooling section is 230°C to 270°C.

[0102] When the terminal temperature of rapid cooling is lower than 230°C, too much martensite will be generated, which will be detrimental to the plasticity and hole expansion performance of the steel plate; when the terminal temperature of rapid cooling is higher than 270°C, the amount of bainite and martensite generated is low, and the content of untransformed austenite is high, which will be transformed into fresh martensite during the subsequent final cooling to room temperature, resulting in a decrease in hole expansion performance. For example, the terminal temperature of the rapid cooling stage can be 230°C, 240°C, 250°C, 260°C, 270°C, etc.

[0103] In some embodiments, the induction heating section includes an induction heating section 1, an aging section, and an induction heating section 2.

[0104] In some embodiments, the heating rate of the induction heating stage 1 is 40°C / s to 60°C / s.

[0105] In some embodiments, the temperature of the aging stage is 330° C. to 370° C., and the aging time of the aging stage is 20 s to 30 s.

[0106] When the aging temperature is lower than 330°C, the recovery effect of bainite and martensite is poor, the strength and hardness are high, and the hole expansion performance is poor; when the aging temperature is higher than 370°C, the tempering of bainite and martensite is obvious, which easily leads to a tensile strength of less than 800MPa. For example, the temperature of the aging stage can be 330°C, 340°C, 350°C, 360°C, 370°C, etc.

[0107] When the aging time is less than 20s, the stress in bainite and martensite is not fully eliminated, and the forming performance is poor; when the aging time is greater than 30s, bainite and martensite are over-tempered, and the tensile strength decreases significantly. For example, the aging time of the aging stage can be 20s, 22s, 24s, 26s, 28s, 30s, etc.

[0108] In some embodiments, the heating rate of the induction heating section 2 is 20°C / s to 35°C / s, and the strip temperature of the galvanizing section is 460°C.

[0109] In some embodiments, the microstructure of the hot-rolled galvanized steel sheet includes, by area fraction, ferrite: 20% to 40%, granular bainite: 40% to 60%, island martensite: 5% to 15%, and retained austenite ≤8%.

[0110] Ferrite has low strength, low hardness, and is easy to deform. It bears most of the strain during the deformation process and is an important component phase to ensure the plasticity and formability of the steel plate. When the ferrite ratio is less than 20%, the strength of the steel plate increases, but the plasticity and formability deteriorate; when the ferrite ratio exceeds 40%, it is difficult to ensure a tensile strength of more than 800MPa under the induction heating process path. Exemplarily, the area fraction of ferrite can be 20%, 24%, 28%, 32%, 36%, 40%, etc.

[0111] Bainite is a hard phase structure that ensures strength. When the granular bainite content is less than 40%, the tensile strength of more than 800MPa cannot be guaranteed, and due to the low proportion of hard phase, the hard phase structure will be highly carbon-rich, resulting in a significant increase in microhardness, increasing the hardness difference between the soft and hard phases, which is easy to cause crack initiation during deformation and reduce hole expansion performance. When the proportion of granular bainite is greater than 60%, the elongation of the steel plate after fracture is reduced, and the global formability is insufficient. Exemplarily, the area fraction of granular bainite can be 40%, 45%, 50%, 55%, 60%, etc.

[0112] Island martensite is a hard phase structure that can improve the strength of the steel plate. However, the strength and hardness of island martensite are relatively high, and a large hardness difference is formed with ferrite, which is not conducive to hole expansion performance. Therefore, the upper limit of island martensite is limited to 15%. For example, the area fraction of island martensite can be 5%, 7%, 9%, 11%, 13%, 15%, etc.

[0113] During the formation of granular bainite and martensite, as well as the aging process before galvanizing, austenite stabilizing elements gradually enrich the untransformed austenite, resulting in improved stability and eventually the formation of retained austenite. Retained austenite can appropriately increase the elongation after fracture through the TRIP effect, but when the content is too high, it will increase the risk of delayed cracking, so the content of retained austenite is limited to less than 8%. For example, the area fraction of retained austenite can be 2%, 4%, 6%, 8%, etc.

[0114] In the embodiment of the present application, the microstructure also includes fine dispersed carbide precipitation phases, and these carbide precipitation phases include but are not limited to TiC, NbC, VC or (Nb, Ti)C.

[0115] In order to obtain a higher yield strength and a relatively lower hardness difference between the soft and hard phases, it is necessary to add V, Ti, and Nb microalloying elements to form precipitation strengthening on the basis of the induction heating process. By controlling the coiling temperature, soaking temperature, cooling rate, and aging temperature, the precipitated phase is made fine and uniform and dispersed on the ferrite matrix.

[0116] In some embodiments, the hot-rolled galvanized steel sheet satisfies at least one of the following properties: yield strength of 600 MPa to 710 MPa, tensile strength ≥ 800 MPa, and hole expansion ratio ≥ 85%.

[0117] The embodiment of the present application innovatively adopts thin ingot continuous casting and rolling technology in the hot rolling stage, and the thickness of the slab is half of the thickness of the conventional ingot in the prior art; electromagnetic induction heating is introduced after the rough rolling to increase the finishing rolling temperature, avoid excessive deformation resistance in the finishing rolling stage, and eliminate the deformation band caused by low-temperature rolling, thereby improving the uniformity of the structure; the thickness of the hot-rolled coil is 1.0-2.0 mm, and a thin steel plate can be obtained without cold rolling; finally, galvanizing induction heating technology is adopted and the parameters are optimized to achieve low-temperature rapid cooling + aging + hot-dip galvanizing in steps. Low-temperature rapid cooling can obtain more bainite and / or martensite hard phase structures, which can be recovered in the subsequent aging process. While ensuring the tensile strength of 800 MPa, the internal stress of the hard phase structure is reduced, and the hardness difference between the soft phase ferrite and the hard phase structure is reduced, thereby obtaining a hole expansion rate of more than 85%.

[0118] The present application is further described below in conjunction with specific examples. The experimental methods in the following examples that do not specify specific conditions are usually measured in accordance with national standards / industry standards; if there are no corresponding national standards / industry standards, they are measured in accordance with common international standards, conventional conditions or conditions recommended by the manufacturer.

[0119] Molten steel of Examples 1 to 8 and Comparative Examples 1 to 4 was obtained and cast into slabs. The slab continuous casting speed was 5.5 m / min, the slab thickness was 120 mm, and the chemical composition of the molten steel was as shown in Table 1.

[0120] Table 1 Chemical composition of molten steel (wt%)

[0121]

[0122]

[0123] Based on the chemical composition of the molten steel in the embodiment and the comparative example, this embodiment also provides a method for preparing a thin-gauge high-pore expansion rate hot-rolled galvanized steel sheet, comprising the following steps:

[0124] obtaining molten steel with a set chemical composition, and continuously casting the molten steel by a thin slab continuous casting and rolling process to obtain a slab;

[0125] The slab is sequentially subjected to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil;

[0126] The hot-rolled coil is pickled and pre-nickel plated in sequence to obtain a pre-nickel plated steel sheet;

[0127] The pre-nickel-plated steel sheet is subjected to continuous hot-dip galvanizing treatment to obtain a hot-rolled galvanized steel sheet; wherein the continuous hot-dip galvanizing treatment includes: a preheating section, a heating section, a soaking section, a slow cooling section, a fast cooling section, an induction heating section 1, an aging section, an induction heating section 2 and a galvanizing section. The process parameters of the preparation are shown in Tables 2 and 3.

[0128] Table 2

[0129]

[0130]

[0131] Table 3

[0132]

[0133] The microstructure of the steel plate was detected and analyzed by metallographic microscope, scanning electron microscope and electron backscatter diffraction (EBSD); the mechanical properties and hole expansion rate of the steel plate were measured by tensile testing machine and forming testing machine. The microstructure and properties of each embodiment and comparative example are shown in Table 4.

[0134] Table 4 Microstructure and properties of each embodiment and comparative example

[0135]

[0136] It can be seen from Tables 1 to 4 that the hot-rolled galvanized steel sheet prepared in the embodiment has a yield strength of 600 MPa to 710 MPa, a tensile strength of ≥800 MPa, and a hole expansion rate of ≥85%.

[0137] In Comparative Example 1, the Mn content is not within the range of the embodiments of the present invention, and the tensile strength of the steel plate obtained is only 768 MPa, and the hole expansion rate is only 72%; in Comparative Example 2, the final rolling temperature of the fine rolling is not within the range of the embodiments of the present invention, and the tensile strength of the steel plate obtained is only 779 MPa; in Comparative Example 3, the induction heating process is not adopted, the rapid cooling temperature, the aging temperature and the rapid cooling rate are not within the range of the embodiments of the present invention, and the yield strength of the steel plate obtained is only 546 MPa, and the hole expansion rate is only 41%; in Comparative Example 4, the induction heating process is one-step induction heating, the rapid cooling temperature and the aging temperature are not within the range of the embodiments of the present invention, and the hole expansion rate of the steel plate obtained is only 58%.

[0138] Attached Figure 3 Detailed description:

[0139] Figure 3 The microstructure of the thin gauge high hole expansion rate hot-rolled galvanized steel sheet provided in the embodiment of the present application; Figure 3As shown, the microstructure of the thin-gauge high-hole expansion rate hot-rolled galvanized steel sheet provided in the embodiment of the present application is fine and uniform, and mainly includes ferrite, granular bainite, island martensite and residual austenite.

[0140] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0141] A thin-gauge, high-hole expansion rate hot-rolled galvanized steel plate provided in an embodiment of the present invention has significantly improved corrosion resistance due to the zinc layer on the surface while ensuring equivalent or better hole expansion performance compared to pickled plates of the same strength level, thereby greatly extending the service life of parts.

[0142] The steel plate provided by the embodiment of the present invention has a yield strength of 600-710 MPa and a yield strength ratio of 0.75-0.88, which is lower than that of ordinary high hole expansion steel and does not rely on a high yield strength ratio to obtain a high hole expansion rate, so it has better stamping forming performance. In addition, by "replacing cold with heat" for thin gauges, a significant cost reduction effect can be achieved.

[0143] The above is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. It will be apparent to those skilled in the art that various modifications to these embodiments are possible, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown in the present application, but will conform to the widest scope consistent with the principles and novel features applied for by the present application.

Claims

1. A method for preparing a thin-gauge high-hole expansion rate hot-rolled galvanized steel sheet, the method comprising: Obtaining molten steel with a set chemical composition; The molten steel is continuously casted by a thin slab continuous casting and rolling process to obtain a slab; The slab is sequentially subjected to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil; The hot-rolled coil is pickled and pre-nickel plated in sequence to obtain a pre-nickel plated steel sheet; The pre-nickel-plated steel sheet is subjected to continuous hot-dip galvanizing treatment to obtain a hot-rolled galvanized steel sheet; wherein the continuous hot-dip galvanizing treatment includes: three-stage continuous heating, cooling, induction heating and galvanizing.

2. The method according to claim 1, characterized in that The three-stage continuous heating includes: a preheating stage, a heating stage and a soaking stage; wherein, The temperature of the preheating section is 210°C to 230°C, and the heating rate of the heating section is 1.5°C / s to 5°C / s; and / or, The temperature of the soaking section is 830° C. to 870° C., and the soaking time of the soaking section is 80s to 160s.

3. The method according to claim 1, characterized in that The cooling section includes a slow cooling section and a fast cooling section; wherein, The slow cooling rate of the slow cooling section is 2°C / s to 6°C / s, and the terminal temperature of the slow cooling section is 740°C to 760°C; and / or, The rapid cooling rate of the rapid cooling section is 50°C / s to 75°C / s, and the terminal temperature of the rapid cooling section is 230°C to 270°C.

4. The method according to claim 1, characterized in that: The induction heating section includes an induction heating section 1, an aging section and an induction heating section 2; wherein, The heating rate of the induction heating stage 1 is 40°C / s to 60°C / s; and / or, The temperature of the aging section is 330° C. to 370° C., and the aging time of the aging section is 20s to 30s; and / or, The heating rate of the induction heating section 2 is 20°C / s to 35°C / s, and the strip temperature of the galvanizing section is 450°C to 470°C.

5. The method according to claim 1, characterized in that The continuous casting speed is 4m / min to 6m / min, and the thickness of the slab is 110mm to 125mm.

6. The method according to claim 1, characterized in that The soaking treatment temperature is 1150°C to 1200°C; and / or, The inlet temperature of the rough rolling is 1050°C to 1100°C, and the end temperature of the rough rolling is 950°C to 1000°C; and / or, The outlet temperature of the electromagnetic induction heating is ≥1200°C; and / or, The inlet temperature of the finishing rolling is 1150°C to 1200°C, and the final rolling temperature of the finishing rolling is 830°C to 870°C; and / or, The average cooling rate of the cooling is ≥10°C / s; and / or, The coiling temperature is 120° C. to 180° C., and the thickness of the hot-rolled coil is 1.0 mm to 2.0 mm.

7. The method according to claim 1, characterized in that The scale breaking and straightening elongation of the pickling is 0.3% to 0.6%, the pickling speed is 150m / min to 200m / min, the acid concentration of the pickling is 30g / L to 35g / L, and the acid temperature of the pickling is 60°C to 80°C.

8. The method according to claim 1, characterized in that The pre-nickel-plated steel sheet comprises a nickel-plated layer, wherein the nickel weight per unit area of ​​the nickel-plated layer is 800 mg / m 2 ~1000 mg / m 2 .

9. The method according to claim 1, characterized in that: In terms of mass fraction, the set chemical composition includes: C: 0.05% to 0.10%, Si: 0.3% to 1.0%, Mn: 1.8% to 2.6%, Al: 0.01% to 0.05%, P≤0.01%, S≤0.003%, Cr: 0.3% to 0.8%, Mo≤0.05%, Cu≤0.03%, Ni≤0.03%, Ti: 0.01% to 0.03%, Nb≤0.01%, V≤0.05%, and matrix element Fe; and / or, Measured by area fraction, the microstructure of the hot-rolled galvanized steel sheet includes: ferrite: 20% to 40%, granular bainite: 40% to 60%, island martensite: 5% to 15%, and retained austenite ≤8%.

10. The method according to any one of claims 1 to 9, characterized in that: The hot-rolled galvanized steel sheet meets at least one of the following properties: a yield strength of 600 MPa to 710 MPa, a tensile strength of ≥800 MPa, and a hole expansion rate of ≥85%.

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