A method for manufacturing a thin-gauge high-hole-expansion-rate hot-rolled galvanized steel sheet

Abstract

The application relates to a preparation method of a thin-gauge high-hole-expanding-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 a set chemical composition; adopting a thin slab continuous casting and rolling process to continuously cast the molten steel to obtain a slab; sequentially subjecting the slab to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil; sequentially subjecting the hot-rolled coil to pickling and pre-nickel plating to obtain a pre-nickel-plated steel plate; continuously hot-dip galvanizing the pre-nickel-plated steel plate to obtain a hot-rolled galvanized steel plate; and wherein the continuous hot-dip galvanizing process comprises a three-section continuous heating section, a cooling section, an induction heating section and a galvanizing section. Through a series of technical innovations and optimization, the problem that the hole-expanding rate of the existing hot-rolled galvanized steel plate is low and it is difficult to produce thin-gauge products is successfully solved, and a new development direction is provided for the steel industry.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of steel manufacturing, in particular to a preparation method of a thin-gauge high-hole-expanding-rate hot-rolled galvanized steel plate. BACKGROUND

[0002] The hot-rolled galvanized plate has relatively excellent corrosion resistance and forming performance, and gradually replaces the hot-rolled pickling plate for manufacturing of some chassis parts. With the increasing requirements for automobile lightweighting and collision safety, the shapes of automobile parts tend to be more complex, the requirements for hole expanding and flanging performance of some parts are higher and higher, and far exceed the specified values in product standards, so that the conventional products cannot meet the forming requirements, and a product with high hole expanding performance must be developed.

[0003] In addition, in order to reduce carbon and cost, the proportion of the hot-rolled galvanized plate replacing the cold-rolled galvanized plate, that is, the "hot instead of cold", will be further expanded. However, the current hot-rolled galvanized plate is generally thick, which becomes an important bottleneck of the "hot instead of cold". Based on the above requirements, a thin-gauge high-hole-expanding-rate hot-rolled galvanized steel plate needs to be developed. SUMMARY

[0004] The application provides a preparation method of a thin-gauge high-hole-expanding-rate hot-rolled galvanized steel plate, so as to solve the technical problem of how to improve the hole expanding rate of the hot-rolled galvanized steel plate on the basis of thin-gauge.

[0005] The application provides a preparation method of a thin-gauge high-hole-expanding-rate hot-rolled galvanized steel plate, so as to solve the technical problem of how to improve the hole expanding rate of the hot-rolled galvanized steel plate on the basis of thin-gauge.

[0006] The molten steel with a set chemical composition is obtained;

[0007] The thin-slab continuous casting and rolling process is used to continuously cast the molten steel, so as to obtain a slab;

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

[0009] The hot-rolled coil is sequentially subjected to pickling and pre-nickel plating, so as to obtain a pre-nickel plated steel plate;

[0010] The pre-nickel plated steel plate is subjected to continuous hot galvanizing treatment, so as to obtain a hot-rolled galvanized steel plate; wherein the continuous hot galvanizing treatment comprises a three-section continuous heating section, a cooling section, an induction heating section and a galvanizing section.

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

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

[0013] Optionally, the temperature of the soaking section is 830-870 DEG C, and the soaking time of the soaking section is 80-160 seconds.

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

[0015] Optionally, the slow cooling rate of the slow cooling section is 2-6 DEG C / s, and the end temperature of the slow cooling section is 740-760 DEG C.

[0016] Optionally, the fast cooling rate of the fast cooling section is 50-75 DEG C / s, and the end temperature of the fast cooling section is 230-270 DEG C.

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

[0018] Optionally, the heating rate of the induction heating 1 section is 40-60 DEG C / s.

[0019] Optionally, the temperature of the aging section is 330-370 DEG C, and the aging time of the aging section is 20-30 seconds.

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

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

[0022] Optionally, the temperature of the soaking treatment is 1150-1200 DEG C.

[0023] Optionally, the entry temperature of the rough rolling is 1050-1100 DEG C, and the end temperature of the rough rolling is 950-1000 DEG C.

[0024] Optionally, the exit temperature of the electromagnetic induction heating is greater than or equal to 1200 DEG C.

[0025] Optionally, the entry temperature of the finish rolling is 1150-1200 DEG C, and the finish rolling temperature is 830-870 DEG C.

[0026] Optionally, the average cooling speed of the cooling is greater than or equal to 10 DEG C / s.

[0027] Optionally, the temperature of the coiling is 120-180 DEG C, and the thickness of the hot-rolled coil is 1.0-2.0 mm.

[0028] Optionally, the set chemical composition includes, in 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 base element Fe.

[0029] Optionally, the microstructure of the hot-dip galvanized steel sheet includes, in area fraction, ferrite: 20% to 40%, granular bainite: 40% to 60%, island-shaped martensite: 5% to 15%, and residual austenite≤8%.

[0030] Optionally, the hot-dip galvanized steel sheet satisfies at least one of the following properties: yield strength: 600MPa to 710MPa, tensile strength≥800MPa, and hole expansion ratio≥85%.

[0031] Optionally, the pickling and straightening elongation of the pickling is 0.3% to 0.6%, the pickling speed is 150m / min to 200m / min, the pickling acid concentration is 30g / L to 35g / L, and the pickling acid temperature is 60°C to 80°C.

[0032] Optionally, the pre-plated nickel steel sheet contains a nickel plating layer, and the unit area weight of nickel in the nickel plating layer is 800mg / m 2 to 1000mg / m 2 .

[0033] Compared with the prior art, the above technical solution provided by the embodiments of the present application has the following advantages:

[0034] The embodiment of the present application provides a preparation method of a thin-gauge high-hole-expanding-rate hot-rolled galvanized steel plate, and the method comprises the following steps: obtaining molten steel with a set chemical composition; adopting a thin slab continuous casting and rolling process to continuously cast the molten steel to obtain a slab; sequentially performing the following steps on the slab, i.e., soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling, to obtain a hot-rolled coil; sequentially performing the following steps on the hot-rolled coil, i.e., pickling and pre-nickel plating, to obtain a pre-nickel plated steel plate; performing continuous hot galvanizing treatment on the pre-nickel plated steel plate to obtain a hot-rolled galvanized steel plate; wherein the continuous hot galvanizing treatment comprises three-stage continuous heating, a cooling section, an induction heating section and a galvanizing section. By optimizing the chemical composition design of the molten steel and combining with a specific rolling process (such as the application of electromagnetic induction heating before finish rolling), the microstructure and texture of the steel plate are effectively improved, so that the hole-expanding rate of the hot-rolled galvanized steel plate is significantly improved; the optimization not only meets the demand of automobile parts on the high-hole-expanding performance, but also exceeds the standard specified value of the conventional product. By adopting the thin slab continuous casting and rolling process, the molten steel is directly continuously cast to obtain a thin slab, so that the thickness increase caused by multiple heating and rolling in the traditional hot rolling process is avoided; meanwhile, by means of accurate rolling control and cooling process, the thin-gauge hot-rolled galvanized steel plate is successfully prepared, and the problem that the existing hot-rolled galvanized plate is generally thick is solved. BRIEF DESCRIPTION OF DRAWINGS

[0035] The drawings incorporated into the specification and forming a part thereof illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the application.

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced as follows, and obviously, other drawings can also be obtained by those skilled in the art without any creative labor under the premise of these drawings.

[0037] Figure 1 A flowchart of a preparation method of a thin-gauge high-hole-expanding-rate hot-rolled galvanized steel plate provided by the embodiment of the present application;

[0038] Figure 2 A flowchart of a continuous hot galvanizing treatment provided by the embodiment of the present application;

[0039] Figure 3 A microstructure of a thin-gauge high-hole-expanding-rate hot-rolled galvanized steel plate provided by the embodiment of the present application. DETAILED DESCRIPTION

[0040] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0041] Various embodiments of the present application can exist in the form of a range; it should be understood that the description in the form of a range is merely for the convenience and brevity, and should not be understood as a hard limitation on the scope of the present application; therefore, it should be considered that the described range has been specifically disclosed all possible sub-ranges and single 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 to any range; in addition, whenever a numerical range is indicated in this document, it refers to any cited number (fraction or integer) within the indicated range.

[0042] In this document, the term includes "includes" and the like means "including but not limited to". The relationship terms such as "first" and "second" are merely 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 the entities or operations. The "and / or" describes the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B can represent the cases of A alone, A and B together, B alone; where A and B can be singular or plural. "At least one" means one or more, "multiple" means two or more; "at least one", "at least one of the following" or the like means any combination of the items, including single item or combination of multiple items; for example, "at least one of a, b or c", or "at least one of a, b and c", can represent a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, where a, b, c can be single or multiple. The "parts ratio" such as weight parts, mass parts, etc. represents the proportional relationship between components. In the proportional relationship described in this document, the parameters that need to be described by proportion should be understood as the front item of the proportional form according to the order of description, and the proportional number should be understood as the rear item of the proportional form, for example, the mass ratio of substance A, substance B and substance C is 1:2:3, then substance A, substance B and substance C should be one-to-one corresponding in the proportional form according to the description order, i.e. the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0043] Unless otherwise specifically indicated, all materials, reagents, instruments and equipment used in the present application are commercially available or are prepared by known methods.

[0044] Figure 1 A flowchart of a preparation method of a thin-gauge high-hole-expansion-rate hot-rolled galvanized steel plate provided by the present application.

[0045] As shown in Figure 1 The present application provides a preparation method of a thin-gauge high-hole-expansion-rate hot-rolled galvanized steel plate, which comprises:

[0046] S1, obtaining molten steel with a set chemical composition;

[0047] S2, continuously casting the molten steel by using a thin slab continuous casting and rolling process to obtain a slab;

[0048] In some embodiments, the set chemical composition comprises, in 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 base element Fe.

[0049] C is an interstitial solid solution atom and an element for improving hardenability, which can promote the formation of hard phase structures such as bainite and martensite, and mainly improves the strength through solid solution strengthening and phase transformation strengthening. In order to obtain a tensile strength of not less than 800 MPa, 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 be poor, 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.

[0050] Si has strong solid solution strengthening effect, which can strengthen ferrite structure and reduce the hardness difference between ferrite and bainite / martensite, and is beneficial to improve the performance of hole expansion. In addition, Si is a non-carbide forming element, which can inhibit the formation of cementite during the aging process of continuous hot galvanizing. In order to achieve the above effects, the mass fraction of Si should not be less than 0.3%. However, when the mass fraction of Si is too high, red rust in the form of strip is easily formed on the surface of hot-rolled coil, and white strip color difference is formed after pickling, which cannot be covered after galvanizing. Therefore, in the embodiments of the present application, the mass fraction of Si is first set to 0.3% to 1.0%. For example, the mass fraction of Si can be 0.3%, 0.5%, 0.7%, 0.9%, 1.0%, etc.

[0051] Mn has strong solid solution strengthening effect, and can promote bainite and martensite transformation, which is an important element to ensure high yield and tensile strength. Therefore, the mass fraction of Mn is limited to 1.8% or more. However, when the mass fraction of Mn is too high, not only Mn segregation is easily caused, but also plasticity is significantly reduced, so the mass fraction of Mn in steel is set to not more than 2.6%. For example, the mass fraction of Mn can be 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, etc.

[0052] Al is a deoxidizing element, which can reduce oxide inclusions in steel. However, too high mass fraction of Al will increase the difficulty of continuous casting and alloy cost, so the mass fraction of Al is limited to 0.01% to 0.05%. For example, the mass fraction of Al can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0053] As a gap 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.

[0054] S is easy to combine with Mn to form coarse MnS inclusions, which deteriorates the forming performance 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.

[0055] Cr is an element to improve hardenability, which can delay pearlite transformation and promote bainite formation, and is beneficial to improve the strength. However, when the mass fraction of Cr exceeds 0.8%, the effect is no longer obvious and the cost is increased. Therefore, the mass fraction of Cr is controlled to be 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.

[0056] Mo is also an element for improving hardenability, but Mo is an expensive alloying element. In order to reduce the cost of the alloy, the mass fraction of Mo is controlled to be not more than 0.05%. For example, the mass fraction of Mo can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0057] Cu has the effect of improving the strength and corrosion resistance of the steel, and can significantly eliminate the harmful effects of S in the steel. However, the cost of Cu alloy is relatively high, and in order to reduce the cost, the upper limit of the mass fraction of Cu is set to 0.03%. For example, the mass fraction of Cu can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0058] The effects of Ni and Cu on the quality and performance of the steel are mutually influenced, and generally both of them exist in the steel. Ni is also an element for improving the atmospheric corrosion resistance of the steel. Since the cost of Ni alloy is high, 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 of 0, that is, the above-mentioned elements can not be added. For example, the mass fraction of Ni can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0059] Ti is a strong carbonitride forming element, which can improve the hardness and strength of the ferrite structure through precipitation strengthening, so as to not only improve the yield strength of the steel sheet, but also reduce the hardness difference between ferrite and bainite, martensite, which is beneficial to the improvement of local forming properties such as hole expansion and bending. In order to obtain excellent strength and forming properties, Ti not less than 0.01% is selected to be added. However, since the induction heating process is adopted in the embodiments of the present application, the yield strength can be appropriately improved, and it is not necessary to rely on the addition of 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%. For example, the mass fraction of Ti can be 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0060] Nb can refine the grain size through solute drag effect, and is a strong carbide forming element, which can improve the hardness and strength of the ferrite structure through precipitation strengthening, and is beneficial to the improvement of hole expansion performance. However, Nb can increase the recrystallization temperature, resulting in the formation of banded structure during hot rolling, which in turn deteriorates 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.

[0061] V can also enhance strength and porosity through precipitation strengthening. However, in the process path of this application embodiment, the strengthening effect reaches saturation when the mass fraction of V exceeds 0.05%. Therefore, the mass fraction of V is limited to ≤0.05%. For example, the mass fraction of V can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0062] Fe is a matrix element, and the specific content / range of Fe can be obtained through the upper and lower limit formulas of the component, that is:

[0063] The sum of the percentages of all components in a composition shall be equal to 100%, and the content ranges of several components shall meet the following conditions: the upper limit of a certain component + the lower limit of other components ≤ 100; the lower limit of a certain component + the upper limit of other components ≥ 100.

[0064] In some embodiments, the continuous casting speed is 4 m / min to 6 m / min, and the slab thickness is 110 mm to 125 mm.

[0065] When the casting speed is below 4 m / min, the production efficiency is low; when the casting speed is above 6 m / min, strip breakage is likely to occur. For example, the casting speed can be 4 m / min, 4.5 m / min, 5 m / min, 5.5 m / min, 6 m / min, etc.

[0066] When the slab thickness is less than 110mm, the hot rolling reduction rate is low, and slab defects are difficult to eliminate; when the slab thickness is greater than 125mm, the cumulative reduction rate is high, and the deformation resistance is greater when rolling thin-gauge steel plates. For example, the slab thickness can be 110mm, 113mm, 116mm, 119mm, 122mm, 125mm, etc.

[0067] S3. The slab is subjected to homogenization heat treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling in sequence to obtain a hot-rolled coil;

[0068] In some embodiments, the temperature of the heat treatment is 1150°C to 1200°C.

[0069] When the homogenization temperature is below 1150℃, the homogenization effect is poor and it will lead to a low entry temperature for rough rolling; when the homogenization temperature is above 1200℃, the billet microstructure shows a significant growth trend. For example, the homogenization temperature can be 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃, etc.

[0070] In some embodiments, the inlet temperature of the roughing mill is 1050°C to 1100°C, and the end temperature of the roughing mill is 950°C to 1000°C.

[0071] When the inlet temperature of rough rolling is lower than 1050℃, the rolling resistance is large, which leads to difficult rolling; when the inlet temperature of rough rolling is higher than 1100℃, the structure is coarse. For example, the inlet temperature of rough rolling can be 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, 1100℃, etc.

[0072] In some embodiments, the outlet temperature of electromagnetic induction heating is ≥1200℃.

[0073] When the outlet temperature of induction heating is lower than 1200℃, the inlet temperature of finish rolling cannot be guaranteed to be greater than 1150℃, which leads to low rolling temperature of finish rolling and strengthens the structure inhomogeneity. For example, the outlet temperature of electromagnetic induction heating can be 1200℃, 1210℃, 1220℃, 1230℃, 1240℃, 1250℃, etc.

[0074] In some embodiments, the inlet temperature of finish rolling is 1150℃-1200℃, and the finish rolling temperature is 830℃-870℃.

[0075] When the inlet temperature of finish rolling is lower than 1150℃, the rolling temperature of finish rolling is low, which easily leads to structure inhomogeneity; when the inlet temperature of finish rolling is higher than 1200℃, the grain size is large, which easily leads to insufficient tensile strength of the final steel plate. For example, the inlet temperature of finish rolling can be 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃, etc.

[0076] When the finish rolling temperature is greater than 870℃, the austenite grain is coarse, which leads to increased grain size after cooling and phase change, high proportion of ferrite, and easily insufficient tensile strength of the steel plate; when the finish rolling temperature is lower than 830℃, the structure is obviously banded, and the structure inhomogeneity leads to decreased hole expansion performance of the steel plate; in addition, too low finish rolling temperature also leads to increased deformation resistance of the hot-rolled plate, which increases the rolling load of the hot rolling mill. For example, the finish rolling temperature can be 830℃, 840℃, 850℃, 860℃, 870℃, etc.

[0077] In some embodiments, the average cooling speed of cooling is ≥10℃ / s.

[0078] When the average cooling speed of cooling is less than 10℃ / s, too much ferrite or pearlite is formed during the cooling process, which is not conducive to phase change and recrystallization in the subsequent annealing process. For example, the average cooling speed of cooling can be 10℃ / s, 11℃ / s, 12℃ / s, 13℃ / s, 14℃ / s, 15℃ / s, etc.

[0079] In some embodiments, the temperature of coiling is 120℃-180℃, and the thickness of the hot-rolled coil is 1.0mm-2.0mm.

[0080] When the coiling temperature is greater than 180℃, the cooling rate of the hot-rolled coil after coiling is slow, the tempering characteristics are obvious, the recrystallization driving force decreases during subsequent annealing, and grain refinement is not conducive; when the coiling temperature is less than 120℃, the shape of the hot-rolled plate is poor, leading to coiling difficulty. Exemplarily, the temperature of coiling can be 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, etc.

[0081] S4, sequentially performing pickling and pre-plating nickel on the hot-rolled coil to obtain a pre-plated nickel steel plate;

[0082] In some embodiments, the pickling has a scale-breaking and straightening extension rate of 0.3% to 0.6%, a speed of 150 m / min to 200 m / min, an acid solution concentration of 30 g / L to 35 g / L, and an acid solution temperature of 60℃ to 80℃.

[0083] The purpose of pickling in the embodiments of the present application is to remove the iron oxide scale and obtain a pickled plate with good surface state. When the scale-breaking and straightening extension rate is less than 0.3%, the effect of breaking the iron oxide scale on the surface of the hot-rolled plate is not good, resulting in residual scale after pickling, which affects the quality of the galvanized surface; when the scale-breaking and straightening extension rate is greater than 0.6%, the scale on the surface of the hot-rolled plate is excessively broken, the removal effect of the scale during pickling is good, but the acid solution easily penetrates through the iron oxide scale and reaches the steel substrate, which also affects the quality of the subsequent galvanizing, such as the formation of zinc flow lines. Exemplarily, the scale-breaking and straightening extension rate can be 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, etc.

[0084] In the embodiments of the present application, the running speed of the strip in the pickling tank is constant, and the speed is 150 m / min to 200 m / min. When the pickling speed is less than 150 m / 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 200 m / min, the removal effect of the iron oxide scale is not good. Exemplarily, the pickling speed can be 150 m / min, 160 m / min, 170 m / min, 180 m / min, 190 m / min, 200 m / min, etc.

[0085] The acid solution concentration directly affects the pickling effect of the hot-rolled plate surface. When the acid solution concentration is less than 30 g / L, the hot-rolled iron oxide scale is difficult to clean; when the acid solution concentration is higher than 35 g / L, the surface of the steel plate is easily over-pickled, which affects the quality of the galvanized surface. Exemplarily, the acid solution concentration can be 30 g / L, 31 g / L, 32 g / L, 33 g / L, 34 g / L, 35 g / L, etc.

[0086] The temperature of the acid solution is also an important factor affecting the pickling quality. When the temperature of the acid solution is lower than 60°C, the removal effect of the oxide scale on the surface of the steel plate is poor; when the temperature of the acid solution is higher than 80°C, over pickling is prone to occur. Through repeated experiments, it is found that when the temperature of the acid solution is 60°C-80°C, the surface quality of the zinc-plated steel plate is the best. For example, the temperature of the acid solution can be 60°C, 65°C, 70°C, 75°C, 80°C, etc.

[0087] In the embodiments of the present application, the steel strip is flattened on line after being discharged from the pickling tank, and the flattening rolling force is 2500kN-3500kN. When the flattening rolling force is lower than 2500kN, the surface morphology uniformity of the pickled steel plate is poor. By increasing the flattening rolling force, the transfer rate of the surface roughness of the flattening roller on the pickled steel strip can be increased, and thus the surface roughness morphology of the pickled steel strip is uniform, and good micro-zone plating uniformity can be obtained after zinc plating. In addition, increasing the flattening rolling force can also improve the flatness of the steel strip and improve the shape of the steel plate. When the flattening rolling force is higher than 3500kN, it is easy to cause the pickled steel plate to have a wavy shape and transverse roller marks.

[0088] In some embodiments, the pre-plated nickel steel plate contains a nickel plating layer, and the unit area weight of nickel in the nickel plating layer is 800mg / m 2 -1000mg / m 2 .

[0089] In the embodiments of the present application, the pre-plated nickel 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 embodiments of the present application is to improve the platability of the hot-rolled pickled steel plate. The diffusion coefficient of alloying elements in nickel is significantly lower than that in the matrix, which makes it difficult for alloying elements to diffuse outward to the surface and be oxidized, thereby inhibiting the outward oxidation behavior of alloying elements Si and Mn. When the unit area weight of nickel in the nickel layer on the surface of the substrate is lower than 800mg / m 2 , the nickel layer is thin, and the inhibition of outward oxidation is not good; when the unit area weight of nickel in the nickel layer is higher than 1000mg / m 2 , the nickel layer is thick, resulting in high cost. For example, the unit area weight of nickel in the nickel layer can be 800mg / m 2 , 850mg / m 2 , 900mg / m 2 , 950mg / m 2 , 1000mg / m 2 , etc.

[0090] S5, the pre-plated nickel steel plate is subjected to continuous hot dip galvanizing treatment to obtain a hot-rolled zinc-plated steel plate; wherein the continuous hot dip galvanizing treatment comprises a three-stage continuous heating, a cooling stage, an induction heating stage and a galvanizing stage.

[0091] Figure 2A flowchart of a continuous hot galvanizing process is provided for the embodiments of the present application.

[0092] In some embodiments, the three-stage continuous heating includes a preheating stage, a heating stage, and a soaking stage.

[0093] In some embodiments, the temperature of the preheating stage is 210-230℃, and the heating rate of the heating stage is 1.5-5℃ / s.

[0094] When the heating rate of the heating stage is lower than 1.5℃ / s, the hot-rolled structure stays in the low-temperature zone for a long time, the recovery characteristics are poor, and the phase transformation and recrystallization are not conducive. When the heating rate of the heating stage is higher than 5℃ / s, the temperature of the steel plate is not uniform, and internal stress and poor plate shape are prone to occur, which leads to furnace deviation. Exemplarily, the heating rate of the heating stage can be 1.5℃ / s, 2.5℃ / s, 3.5℃ / s, 4.5℃ / s, 5℃ / s, etc.

[0095] In some embodiments, the temperature of the soaking stage is 830-870℃, and the soaking time of the soaking stage is 80-160s.

[0096] When the soaking temperature is lower than 830℃, the steel plate will enter the two-phase zone, and excessive ferrite will be generated during soaking, which leads to low strength after phase transformation. When the soaking temperature is higher than 870℃, the original austenite grains grow and coarsen, and decarburization is prone to occur on the surface of the strip, which leads to strength reduction and uneven structure after phase transformation. Exemplarily, the temperature of the soaking stage can be 830℃, 840℃, 850℃, 860℃, 870℃, etc. When the soaking time is less than 80s, the homogenization degree of the high-temperature zone structure is low, which easily leads to uneven performance. When the soaking time exceeds 160s, the grains grow excessively, which easily leads to insufficient tensile strength. Exemplarily, the soaking time of the soaking stage can be 80s, 100s, 120s, 140s, 160s, etc.

[0097] In some embodiments, the cooling stage includes a slow cooling stage and a fast cooling stage.

[0098] In some embodiments, the slow cooling rate of the slow cooling stage is 2-6℃ / s, and the end temperature of the slow cooling stage is 740-760℃.

[0099] When the slow cooling end temperature is lower than 740℃, a large amount of ferrite is prone to be formed during slow cooling, which leads to insufficient strength of the steel plate. When the slow cooling end temperature is higher than 760℃, the amount of ferrite generated is too low, which leads to low elongation after the stage. Exemplarily, the end temperature of the slow cooling stage can be 740℃, 745℃, 750℃, 755℃, 760℃, etc.

[0100] In some embodiments, the fast cooling rate of the fast cooling section is 50-75℃ / s, and the end temperature of the fast cooling section is 230-270℃.

[0101] When the fast cooling end temperature is lower than 230℃, too much martensite is generated, and the plasticity and hole expansion performance of the steel plate are both poor. When the fast cooling end temperature is higher than 270℃, 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 the hole expansion performance. Exemplarily, the end temperature of the fast cooling section can be 230℃, 240℃, 250℃, 260℃, 270℃, etc.

[0102] In some embodiments, the induction heating section comprises an induction heating 1 section, an aging section, and an induction heating 2 section.

[0103] In some embodiments, the heating rate of the induction heating 1 section is 40-60℃ / s.

[0104] In some embodiments, the temperature of the aging section is 330-370℃, and the aging time of the aging section is 20-30s.

[0105] When the aging temperature is lower than 330℃, the bainite and martensite do not recover well, the strength and hardness are high, and the hole expansion performance is poor. When the aging temperature is higher than 370℃, the bainite and martensite are obviously tempered, which easily leads to a tensile strength of less than 800MPa. Exemplarily, the temperature of the aging section can be 330℃, 340℃, 350℃, 360℃, 370℃, etc.

[0106] When the aging time is less than 20s, the internal stress of the bainite and martensite is not fully eliminated, and the formability is poor. When the aging time is greater than 30s, the bainite and martensite are excessively tempered, and the tensile strength decreases significantly. Exemplarily, the aging time of the aging section can be 20s, 22s, 24s, 26s, 28s, 30s, etc.

[0107] In some embodiments, the heating rate of the induction heating 2 section is 20-35℃ / s, and the strip temperature of the galvanizing section is 460℃.

[0108] In some embodiments, the microstructure of the hot-rolled galvanized steel plate comprises, in terms of area fraction, ferrite: 20-40%, granular bainite: 40-60%, island-shaped martensite: 5-15%, and residual austenite ≤8%.

[0109] Ferrite has low strength and hardness, and is easy to deform, and bears most of the strain in the deformation process, which is an important component to ensure the plasticity and formability of the steel plate. When the proportion of ferrite is less than 20%, the strength of the steel plate is improved, but the plasticity and formability are poor; when the proportion of ferrite is more than 40%, it is difficult to ensure the tensile strength of more than 800 MPa under the process path of induction heating. For example, the area fraction of ferrite can be 20%, 24%, 28%, 32%, 36%, 40%, etc.

[0110] Bainite is a hard phase organization to ensure strength. When the content of granular bainite is less than 40%, it is impossible to ensure the tensile strength of more than 800 MPa, and because the proportion of hard phase is relatively low, the hard phase organization is highly carbon-rich, which leads to a significant increase in microhardness, increases the hardness difference between soft and hard phases, and easily causes crack initiation during deformation, reducing the hole expansion performance. When the proportion of granular bainite is greater than 60%, the elongation after fracture of the steel plate is reduced, and the overall formability is insufficient. For example, the area fraction of granular bainite can be 40%, 45%, 50%, 55%, 60%, etc.

[0111] Island-shaped martensite is a hard phase organization that can improve the strength of the steel plate, but the strength and hardness of island-shaped martensite are relatively high, and form a large hardness difference with ferrite, which is not conducive to hole expansion performance, so the upper limit of island-shaped martensite is limited to 15%. For example, the area fraction of island-shaped martensite can be 5%, 7%, 9%, 11%, 13%, 15%, etc.

[0112] During the formation of granular bainite and martensite, and during the aging process before galvanizing, austenite stabilizing elements gradually enrich in untransformed austenite, leading to an increase in its stability, and finally forming residual austenite. Residual austenite can appropriately improve the elongation after fracture through TRIP effect, but when the content is too high, it will increase the risk of delayed cracking, so the content of residual austenite is limited to within 8%. For example, the area fraction of residual austenite can be 2%, 4%, 6%, 8%, etc.

[0113] In the embodiments of the present application, the microstructure further includes fine and dispersed carbide precipitates, which include but are not limited to TiC, NbC, VC or (Nb, Ti)C.

[0114] In order to obtain higher yield strength and relatively lower hardness difference between soft and hard phases, it is necessary to form precipitate strengthening by adding V, Ti, Nb micro-alloy elements on the basis of induction heating process. By controlling the coiling temperature, soaking temperature, cooling rate and aging temperature, etc., the precipitates are fine, uniform and dispersedly distributed on the ferrite matrix.

[0115] In some embodiments, the hot-rolled galvanized steel sheet satisfies 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 ratio of ≥ 85%.

[0116] The embodiment of the present application innovatively uses thin slab continuous casting and rolling technology in the hot rolling stage, the thickness of the slab is half of the thickness of the conventional slab in the prior art; electromagnetic induction heating is introduced after rough rolling to improve the temperature of finish rolling, avoid high deformation resistance in the finish rolling stage, and eliminate the deformation zone 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 the thin steel sheet can be obtained without further cold rolling; finally, the galvanizing induction heating technology is used and the parameters are optimized, realizing the low-temperature rapid cooling + aging + hot-dip galvanizing step-by-step treatment of the steel strip, and more bainite and / or martensite hard phase structure can be obtained by low-temperature rapid cooling, and the internal stress of the hard phase structure is reduced during the subsequent aging process, the hardness difference between the soft ferrite and the hard phase structure is reduced, and the hole expansion ratio of more than 85% is obtained.

[0117] The present application will be further described in conjunction with specific examples. The experimental methods in the following examples are not specified, and are usually determined according to national standards / industry standards; if there is no corresponding national standard / industry standard, the general international standard, conventional conditions or the conditions recommended by the manufacturer are used.

[0118] The molten steel of Examples 1-8 and Comparative Examples 1-4 was obtained and cast into slabs, the casting speed of the slab was 5.5 m / min, the thickness of the slab was 120 mm, and the chemical composition of the molten steel is shown in Table 1.

[0119] Table 1 Chemical composition of molten steel (wt%)

[0120]

[0121]

[0122] Based on the chemical composition of the molten steel of the examples and comparative examples, the present embodiment also provides a method for preparing a thin-gauge high-hole-expansion-ratio hot-rolled galvanized steel sheet, comprising the following steps:

[0123] Obtaining molten steel with a set chemical composition, continuously casting the molten steel by using a thin slab continuous casting and rolling process to obtain a slab;

[0124] The slab is sequentially subjected to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil;

[0125] The hot-rolled coil is sequentially subjected to pickling and pre-nickel plating to obtain a pre-nickel plated steel sheet;

[0126] The pre-nickel-plated steel sheet is subjected to continuous hot-dip galvanizing treatment to obtain 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 rapid cooling section, an induction heating section 1, an aging section, an induction heating section 2, and a galvanizing section. The process parameters are shown in Tables 2 and 3.

[0127] Table 2

[0128]

[0129]

[0130] Table 3

[0131]

[0132] The microstructure of the steel plates was detected and analyzed using metallographic microscopy, scanning electron microscopy, and electron backscatter diffraction (EBSD); the mechanical properties and porosity of the steel plates were determined using a tensile testing machine and a forming testing machine. The microstructure and properties of each embodiment and comparative example are shown in Table 4.

[0133] Table 4. Microstructure and properties of each embodiment and comparative example.

[0134]

[0135] As shown in Tables 1-4, the hot-rolled galvanized steel sheets prepared in the examples have a yield strength of 600MPa-710MPa, a tensile strength ≥800MPa, and a hole expansion rate ≥85%.

[0136] In Comparative Example 1, the Mn content is outside the range of the embodiments of the present invention, and the tensile strength of the steel plate is only 768 MPa, and the expansion rate is only 72%. In Comparative Example 2, the finishing rolling temperature is outside the range of the embodiments of the present invention, and the tensile strength of the steel plate is only 779 MPa. In Comparative Example 3, no induction heating process is used, and the rapid cooling temperature, aging temperature, and rapid cooling rate are outside the range of the embodiments of the present invention, and the yield strength of the steel plate is only 546 MPa, and the expansion rate is only 41%. In Comparative Example 4, the induction heating process is one-step induction heating, and the rapid cooling temperature and aging temperature are outside the range of the embodiments of the present invention, and the expansion rate of the steel plate is only 58%.

[0137] Appendix Figure 3 Detailed explanation:

[0138] Figure 3 The microstructure of the thin-gauge hot-rolled galvanized steel sheet with high hole expansion ratio provided in the embodiments of this application; such as Figure 3As shown, the thin-gauge high-hole-expansion-rate hot-rolled galvanized steel plate provided by the embodiments of the present application has fine and uniform microstructure, mainly including ferrite, granular bainite, island-shaped martensite and residual austenite.

[0139] The one or more technical solutions in the embodiments of the present application have at least the following technical effects or advantages:

[0140] The thin-gauge high-hole-expansion-rate hot-rolled galvanized steel plate provided by the embodiments of the present application has the corrosion resistance greatly improved due to the zinc layer on the surface, compared with the same strength level pickling plate, on the basis of ensuring the same or better hole expansion performance, and can greatly prolong the service life of the part.

[0141] The steel plate provided by the embodiments of the present application 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 thus has better stamping forming performance, and the high-hole-expansion rate is obtained without relying on high yield strength ratio.

[0142] The above is only the specific embodiments of the present application, which enables those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, 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 of the present application.

Claims

1. A method for preparing a thin-gauge hot-rolled galvanized steel sheet with high hole expansion ratio, the method comprising: To obtain molten steel with a set chemical composition; The molten steel is continuously cast using a thin slab continuous casting and rolling process to obtain a slab; The slab is subjected to homogenization heat treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling in sequence to obtain a hot-rolled coil. The hot-rolled coil is sequentially pickled and pre-nickel plated to obtain a pre-nickel-plated steel sheet. The pre-nickel-plated steel sheet is subjected to continuous hot-dip galvanizing treatment to obtain hot-rolled galvanized steel sheet; wherein, the continuous hot-dip galvanizing treatment includes: a three-stage continuous heating stage, a cooling stage, an induction heating stage, and a galvanizing stage; The cooling section includes a slow cooling section and a rapid cooling section; wherein... The slow cooling rate of the slow cooling section is 2℃ / s to 6℃ / s, and the final temperature of the slow cooling section is 740℃ to 760℃. The rapid cooling rate of the rapid cooling section is 50℃ / s to 75℃ / s, and the final temperature of the rapid cooling section is 230℃ to 270℃. 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 first induction heating section is 40℃ / s to 60℃ / s; The temperature of the aging period is 330℃~370℃, and the aging time of the aging period is 20s~30s; The heating rate of the second induction heating section is 20℃ / s to 35℃ / s, and the strip temperature of the galvanizing section is 450℃ to 470℃. The specified chemical composition, by mass fraction, includes: 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 the matrix element Fe; The microstructure of the hot-rolled galvanized steel sheet, in terms of area fraction, includes: ferrite: 20%–40%, granular bainite: 40%–60%, island martensite: 5%–15%, and retained austenite ≤8%.

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℃~230℃, and the heating rate of the heating section is 1.5℃ / s~5℃ / s; The temperature of the heat exchange zone is 830℃~870℃, and the heat exchange time of the heat exchange zone is 80s~160s.

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

4. The method according to claim 1, characterized in that, The temperature for the homogenization heat treatment is 1150℃~1200℃; The inlet temperature of the roughing mill is 1050℃~1100℃, and the end temperature of the roughing mill is 950℃~1000℃. The outlet temperature of the electromagnetic induction heating is ≥1200℃; The entry temperature of the finishing mill is 1150℃~1200℃, and the finishing mill's final rolling temperature is 830℃~870℃. The average cooling rate is ≥10℃ / s; The winding temperature is 120℃~180℃, and the thickness of the hot-rolled coil is 1.0mm~2.0mm.

5. The method according to claim 1, characterized in that, The scaling elongation rate of the pickling is 0.3% to 0.6%, the pickling speed is 150 m / min to 200 m / min, the acid concentration is 30 g / L to 35 g / L, and the acid temperature is 60℃ to 80℃.

6. The method according to claim 1, characterized in that, The pre-nickel-plated steel sheet contains a nickel plating layer, and the nickel in the nickel plating layer has a unit area weight of 800 mg / m². 2 ~1000mg / m 2 .

7. The method according to any one of claims 1 to 6, characterized in that, The hot-rolled galvanized steel sheet meets at least one of the following properties: yield strength of 600MPa~710MPa, tensile strength ≥800MPa, and hole expansion rate ≥85%.

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