Hot-rolled galvanised dual phase steel and method for producing same

Abstract

The application relates to the field of advanced high-strength steel manufacturing technology, in particular to a hot-rolled galvanized dual-phase steel and a preparation method thereof. The method comprises the following steps: heating a slab with a set chemical composition, then performing rolling and controlling process parameters of the rolling to obtain a hot-rolled plate; performing stage-by-stage cooling on the hot-rolled plate, then performing coiling to obtain a hot-rolled coil; performing annealing on the hot-rolled coil, then performing hot-dip galvanizing to obtain the hot-rolled galvanized dual-phase steel; wherein the annealing comprises a fast cooling section and an aging section, and the fast cooling temperature of the fast cooling section and the aging temperature of the aging section are controlled. The yield strength of the hot-rolled galvanized dual-phase steel is greater than or equal to 500 MPa, the tensile strength is greater than or equal to 780 MPa, the elongation after fracture A 80 is greater than or equal to 21%, and the hole expansion ratio is greater than or equal to 40%. Compared with a traditional hot-rolled galvanized dual-phase steel (the elongation after fracture A 80 is about 16%), the elongation after fracture is greatly improved, and the forming performance is obviously improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of advanced high-strength steel manufacturing, and particularly relates to a hot-rolled galvanized dual-phase steel and a preparation method thereof. BACKGROUND

[0002] The microstructure of the dual-phase steel is composed of ferrite and martensite, has the characteristics of low yield strength ratio, high initial work hardening rate and continuous yield, and has excellent stamping forming performance. The application of high-strength grade dual-phase steel helps the lightweight of automobiles and is an important way to achieve carbon reduction and carbon neutralization in the automobile industry. At present, cold-rolled dual-phase steel has been maturely applied to 1200 MPa grade, including continuous annealing state and galvanized state.

[0003] However, the application of hot-rolled galvanized dual-phase steel is mainly at the level of 600 MPa. With the increase of the strength level, the elongation of the dual-phase steel decreases, which cannot meet the forming requirements of complex-shaped parts, and thus cannot replace low-strength grades to achieve lightweight. SUMMARY

[0004] The present application provides a hot-rolled galvanized dual-phase steel and a preparation method thereof to solve the technical problem of poor forming performance of the existing hot-rolled galvanized dual-phase steel.

[0005] In a first aspect, the present application provides a preparation method of a hot-rolled galvanized dual-phase steel, and the method comprises the following steps:

[0006] Heating a slab with a set chemical composition, then rolling, and controlling the process parameters of the rolling to obtain a hot-rolled plate;

[0007] Stage-by-stage cooling the hot-rolled plate, then coiling to obtain a hot-rolled coil;

[0008] Annealing the hot-rolled coil, then hot-dip galvanizing to obtain a hot-rolled galvanized dual-phase steel; wherein the annealing comprises a fast cooling section and an aging section, and the fast cooling temperature of the fast cooling section and the aging temperature of the aging section are controlled.

[0009] Optionally, the fast cooling end temperature of the fast cooling section is 180-250 DEG C.

[0010] Optionally, the aging temperature of the aging section is 340-420 DEG C.

[0011] Optionally, the process parameters of the rolling comprise: a rough rolling start temperature and a finish rolling temperature; wherein,

[0012] The finish rolling temperature is 960-1030 DEG C, and the finish rolling temperature is 870-910 DEG C.

[0013] Optionally, the stage-by-stage cooling of the hot-rolled plate, then coiling to obtain a hot-rolled coil comprises:

[0014] performing front-stage ultrafast cooling and rear-stage laminar water cooling on the hot-rolled plate, and then performing coiling to obtain a hot-rolled coil, wherein,

[0015] The cooling speed of the ultrafast cooling is ≥ 50℃ / s, and the cooling speed of the laminar water cooling is 10-20℃ / s.

[0016] Optionally, the temperature of the coiling is 280-380℃.

[0017] Optionally, the temperature of the heating is 1220-1260℃.

[0018] Optionally, the set chemical composition comprises:

[0019] C, Si, Mn, Al, Cr, Mo, B, P, S and Fe, wherein, in terms of mass fraction,

[0020] The content of C is 0.17-0.25%, the content of Si is 0.2-0.9%, the content of Mn is 1.4-2.5%, the content of Al is 0.2-0.6%, the content of Cr is 0-0.4%, the content of Mo is 0-0.2%, the content of B is 0.001-0.002%, the content of P is ≤ 0.01%, and the content of S is ≤ 0.003%.

[0021] In a second aspect, the present application provides a hot-rolled galvanized dual-phase steel, which is prepared by the above method.

[0022] Optionally, the microstructure of the hot-rolled galvanized dual-phase steel comprises ferrite, tempered martensite and residual austenite.

[0023] The area fraction of the ferrite is 30-50%, the area fraction of the tempered martensite is 45-65%, and the area fraction of the residual austenite is 5-12%.

[0024] The above technical solution provided by the embodiments of the present application has the following advantages compared with the prior art:

[0025] The above technical solution provided by the embodiments of the present application has the following advantages compared with the prior art:

[0026] The preparation method of the hot-dip galvanized dual-phase steel provided by the embodiment of the present application adjusts the microstructure of the hot-dip galvanized substrate to be martensite with high dislocation density through controlled rolling and post-rolling cooling, and further adjusts the fast cooling end point temperature in the annealing process to form appropriate proportions of martensite and untransformed austenite; and high-temperature aging treatment promotes the diffusion of C in the martensite to the untransformed austenite, improves the stability of the austenite, to obtain moderate proportions and stability of residual austenite, and improve the elongation after fracture and overall formability of the hot-dip galvanized steel plate. At the same time, the aging treatment eliminates the internal stress in the martensite, reduces the hardness difference between ferrite and martensite, and can significantly improve the hole expansion rate. The method not only improves the overall formability, but also improves the local formability. BRIEF DESCRIPTION OF DRAWINGS

[0027] The accompanying drawings, which are incorporated into and form a part of the specification, illustrate preferred embodiments of the present application and, together with the description, serve to explain the principles of the present application.

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings required to be used in the embodiments or prior art description will be briefly introduced. Obviously, for those skilled in the art, other drawings can also be obtained without creative labor based on these drawings.

[0029] Figure 1 A flowchart of a preparation method of a hot-dip galvanized dual-phase steel provided by the embodiment of the present application is shown in the figure.

[0030] Figure 2 An alloying galvanizing process curve diagram in the preparation method of a hot-dip galvanized dual-phase steel provided by the embodiment of the present application is shown in the figure. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0032] Various embodiments of the present application can exist in a range of forms; it should be understood that the description in a range form 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 all possible sub-ranges and single values within the range have been specifically disclosed. 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 values within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. In addition, whenever a numerical range is indicated herein, it refers to any cited number (fraction or integer) within the indicated range.

[0033] In the present application, the orientation words such as "upper" and "lower" are specifically the directions of the drawing surface in the drawings, unless otherwise stated. In addition, in the description of the present application, the terms "include", "contain" and the like mean "include but are not limited to". In the present text, the relational 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. In the present text, "and / or" describes the association between the associated objects, which means that there can be three relationships, for example, A and / or B can represent the cases of A alone, A and B together, and B alone. Where A and B can be singular or plural. In the present text, "one or more" means one or more, and "multiple" means two or more. "At least one", "at least one of the following" or the like means any combination of the items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can mean a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple.

[0034] Unless otherwise specifically stated, the various raw materials, reagents, instruments and equipment used in the present application can be purchased on the market or can be prepared by existing methods.

[0035] In a first aspect, the present application provides a method for preparing a hot-rolled galvanized dual-phase steel, please refer to Figure 1 , the method comprises:

[0036] S1, heating a slab with a set chemical composition, then rolling, and controlling the process parameters of the rolling to obtain a hot-rolled plate;

[0037] In some embodiments, the set chemical composition includes C, Si, Mn, Al, Cr, Mo, B, P, S and Fe, wherein the content of C is 0.17-0.25% by mass fraction, the content of Si is 0.2-0.9%, the content of Mn is 1.4-2.5%, the content of Al is 0.2-0.6%, the content of Cr is 0-0.4%, the content of Mo is 0-0.2%, the content of B is 0.001-0.002%, the content of P is ≤0.01%, and the content of S is ≤0.003%.

[0038] In the embodiments of the present application, the positive effect of controlling the content of C to be 0.17-0.25% is that C can improve the hardenability and is beneficial to obtaining high dislocation density of martensite after hot rolling and cooling. C is an extremely important austenite stabilizing element, which can quickly diffuse from martensite to untransformed austenite during the aging stage after annealing, thereby improving the stability of austenite. In addition, C is an important solid solution strengthening element, which can improve the strength of tempered martensite and ensure that the tensile strength is not less than 780 MPa. However, too high C content can damage the welding performance. Specifically, the content of C can be 0.17%, 0.19%, 0.21%, 0.23%, 0.25%, etc.

[0039] The positive effect of controlling the content of Si to be 0.2-0.9% is that Si is a non-carbide forming element, which can inhibit the formation of cementite and other carbides during aging, promote the diffusion of C from martensite to untransformed austenite, and improve the stability of austenite. In addition, Si has a strong solid solution strengthening effect, which can improve the strength of the ferrite matrix and reduce the hardness difference between ferrite and martensite, which is beneficial to improving the hole expansion performance. However, too high Si content can cause serious iron oxide scale on the surface of the hot-rolled steel plate, which cannot be removed by pickling, and thus can cause plating defects and dezincification defects of the plated steel plate. Specifically, the content of Si can be 0.2%, 0.5%, 0.9%, etc.

[0040] The positive effect of controlling the content of Mn to be 1.4-2.5% is that Mn has the effects of improving hardenability and solid solution strengthening, and is an important element for promoting the formation of low-temperature phases of martensite during cooling after rolling. However, too high Mn content can easily lead to element segregation, which is not conducive to local forming performance. Specifically, the content of Mn can be 1.4%, 2.0%, 2.5%, etc.

[0041] The positive effect of controlling the content of Al to be 0.2-0.6%: Al, like Si, can inhibit the precipitation of carbide, promote the diffusion of C from the martensite to the untransformed austenite, and improve the stability of the austenite, and cooperates with Si to achieve the effect of promoting the formation of austenite. However, when the content of Al is too high, it will likely significantly increase the Ac3 temperature, and in turn increase the annealing temperature in the two-phase region, leading to easy grain growth and coarsening. Specifically, the content of Al can be 0.2%, 0.4%, 0.6%, etc.

[0042] The positive effect of controlling the content of Cr to be 0-0.4%: Cr can improve the hardenability, delay the pearlite transformation during hot rolling, and promote the formation of martensite, which is beneficial to obtaining high dislocation density martensite. However, the alloy cost of Cr is high, and it is not suitable to be too high. Specifically, the content of Cr can be 0.2%, 0.4%, etc.

[0043] The positive effect of controlling the content of Mo to be 0-0.2%: Mo has a significant effect on improving hardenability, about 2-3 times that of Mn, so it is beneficial to replace Mn with a small amount of Mo to reduce the segregation of the structure. However, Mo is an expensive alloying element, and it is not suitable to be too high in order to save costs. Specifically, the content of Mo can be 0.2%, 0.15%, etc.

[0044] The positive effect of controlling the content of B to be 0.001-0.002%: during cooling after hot rolling, B segregates at the grain boundaries of austenite to inhibit the formation of ferrite and promote the transformation of martensite. However, the effect of excessive B content is saturated. Specifically, the content of B can be 0.001%, 0.002%, 0.0015%, etc.

[0045] The positive effect of controlling the content of P to be ≤0.01%: P can appropriately increase the strength of the steel plate, but it is also easy to segregate at the grain boundaries and deteriorate the plasticity, so its content should be as low as possible. Specifically, the content of B can be 0.01%, 0.008%, etc.

[0046] The positive effect of controlling the content of S to be ≤0.003%: S is easy to combine with Mn to form coarse MnS inclusions, which deteriorates the forming properties such as hole expansion and cold bending of the steel plate, so its content should be as low as possible. Specifically, the content of B can be 0.003%, 0.0018%, etc.

[0047] In the application examples of the company, the above-mentioned chemical composition can also include: Nb and or Ti; wherein the content of Nb is 0-0.04%, and the content of Ti is 0-0.06%.

[0048] The drag effect of the solid solution Nb atoms and the pinning effect of the Nb and Ti precipitates can inhibit the growth of austenite grains during the heating and rolling of the hot rolling, and the effect of refining the grains is obvious, which is beneficial to improving the tensile strength and forming performance of the final steel plate. In addition, the TiC can further inhibit the growth of the grains during the heating and soaking of the annealing. In order to obtain the above effects, at least one of Nb and Ti can be added. However, when the content of Nb or Ti is too high, not only the cost will be significantly increased, but also the effect will be saturated.

[0049] In some embodiments, the heating temperature is 1220-1260℃.

[0050] In some embodiments, the process parameters of the rolling include: a rough rolling start temperature and a rough rolling finish temperature; wherein the rough rolling start temperature is 960-1030℃, and the rough rolling finish temperature is 870-910℃.

[0051] The positive effect of controlling the process parameters of the rolling is to obtain fine martensite structure while ensuring the quality of the steel plate.

[0052] The positive effect of controlling the heating temperature to be 1220-1260℃ is that when the heating temperature is too low, the micro-alloying elements are not fully dissolved, and the hot rolling load is too high; when the heating temperature is too high, the grains are coarse, which is not conducive to obtaining fine martensite structure, and the high temperature also increases the surface iron oxide scale of the slab. Specifically, the heating temperature can be 1220℃, 1240℃, 1260℃, etc.

[0053] The positive effect of controlling the rough rolling start temperature to be 960-1030℃ is that when the rough rolling start temperature is too low, the hot rolling structure is prone to appear obvious deformation zone in the non-recrystallization zone during the rough rolling stage, which deteriorates the hole expansion performance; when the rough rolling start temperature is too high, the surface of the hot-rolled plate is prone to appear iron oxide scale, which affects the quality of the galvanized surface. Specifically, the rough rolling start temperature can be 960℃, 1000℃, 1030℃, etc.

[0054] The positive effect of controlling the rough rolling finish temperature to be 870-910℃ is that when the finish temperature is too low, the anisotropy of the hot-rolled steel plate increases, which leads to the decline of the local forming performance of the final galvanized steel plate; when the finish temperature is too high, the hot-rolled grain size is too large, and the risk of growth in the high temperature zone increases, which is not conducive to obtaining fine martensite structure after cooling. Specifically, the rough rolling finish temperature can be 870℃, 890℃, 910℃, etc.

[0055] S2, the hot-rolled plate is subjected to stage cooling, and then is coiled to obtain a hot-rolled coil;

[0056] In some embodiments, the stage cooling of the hot-rolled plate and then coiling to obtain a hot-rolled coil includes:

[0057] The hot-rolled plate is subjected to pre-stage ultra-fast cooling and post-stage laminar water cooling, and then is coiled to obtain a hot-rolled coil, wherein,

[0058] The cooling speed of the ultra-fast cooling is ≥50℃ / s, and the cooling speed of the laminar water cooling is 10-20℃ / s.

[0059] The positive effect of the stage cooling of the hot-rolled plate is to obtain a good microstructure.

[0060] The positive effect of controlling the cooling speed of the ultra-fast cooling to be ≥50℃ / s is that, in the temperature range from the end of the finish rolling to 500℃, if the cooling speed of the ultra-fast cooling is too low, non-desired structures such as ferrite or pearlite will be generated in the cooling process, and the grain growth trend is obvious in the low cooling speed state, so that coarse austenite grains will be obtained in the subsequent annealing process. Specifically, the cooling speed of the ultra-fast cooling can be 50℃ / s, 52℃ / s, 54℃ / s, etc.

[0061] The positive effect of controlling the cooling speed of the laminar water cooling to be 10-20℃ / s is that, if the cooling speed of the laminar water cooling is too low, a long bainite transformation region will be experienced in the range from 500℃ to the coiling temperature, resulting in the appearance of bainite in the final microstructure; and if the cooling speed of the laminar water cooling is too high, the cooling effect reaches saturation and it is difficult to control the coiling temperature. Specifically, the cooling speed of the laminar water cooling can be 10℃ / s, 15℃ / s, 20℃ / s, etc.

[0062] In some embodiments, the coiling temperature is 280-380℃.

[0063] The positive effect of controlling the coiling temperature to be 280-380℃ is that, if the coiling temperature is too high, the phase transformation will enter the partial bainite region, resulting in non-desired microstructure; and if the coiling temperature is too low, the coiler load is increased and the plate shape of the steel plate is poor. Specifically, the coiling temperature can be 280℃, 340℃, 380℃, etc.

[0064] The microstructure of the hot-rolled coil is composed of high dislocation density martensite with an area fraction of more than 95% and residual austenite. Through various defects in the martensite, the recrystallization nucleation points in the subsequent annealing process are increased, and then refined austenite grains are obtained.

[0065] S3, annealing the hot-rolled coil, and then hot-dip galvanizing to obtain a hot-rolled galvanized dual-phase steel; wherein the annealing comprises a fast cooling stage and an aging stage, and the fast cooling temperature of the fast cooling stage and the aging temperature of the aging stage are controlled.

[0066] In some embodiments, the fast cooling temperature of the fast cooling stage is 180-250℃.

[0067] In some embodiments, the aging temperature of the aging section is 340-420℃.

[0068] The positive effect of the annealing process is that the martensite nucleation energy is retained, the recrystallization point is increased, the austenite and ferrite organizations formed by the intercritical annealing are refined, and the elongation and the hole expansion rate of the hot-rolled galvanized steel plate are improved. Before the S3 step, the hot-rolled coil is uncoiled, and the iron oxide scale is removed by pickling to obtain a pickled plate.

[0069] The positive effect of controlling the fast cooling temperature of the fast cooling section to be 180-250℃ is that the fast cooling end temperature is precisely controlled to form appropriate proportions of martensite and untransformed austenite. When the fast cooling temperature is too low, the proportion of quenched martensite is too high and the proportion of untransformed austenite is too low, resulting in a high proportion of tempered martensite and a low proportion of residual austenite in the final organization, which causes high tensile strength and insufficient elongation. When the fast cooling temperature is too high, the proportion of quenched martensite is too low and the proportion of untransformed austenite is too high, resulting in a low average C content in the untransformed austenite after aging, and some austenite with insufficient stability will be transformed into fresh martensite during the cooling process after galvanizing, ultimately causing insufficient elongation and decreased formability of the steel plate. Specifically, the fast cooling temperature of the fast cooling section can be 180℃, 230℃, 250℃, etc.

[0070] The positive effect of controlling the aging temperature of the aging section to be 340-420℃ is that the C in the martensite diffuses to the untransformed austenite, the stability of the austenite is improved, and a moderate proportion and stability of residual austenite are obtained, which improves the elongation after fracture and the overall formability of the hot-rolled galvanized steel plate. At the same time, the aging treatment eliminates the stress in the martensite and reduces the hardness difference between the ferrite and the martensite, which can significantly improve the hole expansion rate. When the aging temperature is too low, the C diffusion rate is slow, and the C diffusion from the martensite to the untransformed austenite is insufficient, resulting in insufficient stability of the austenite. When the aging temperature is too high, the martensite is excessively tempered, resulting in a significant decrease in tensile strength. Specifically, the aging temperature of the aging section can be 340℃, 380℃, 420℃, etc.

[0071] Specifically, the process parameters of the annealing include: the heating rate of the heating section is 5-10℃ / s, the soaking temperature of the soaking section is 780-820℃, the soaking time is 60-120s, the slow cooling rate of the slow cooling section is 3-8℃ / s, the slow cooling temperature is 720-740℃, the fast cooling rate of the fast cooling section is 20-40℃ / s, the fast cooling end temperature is 180-250℃, the aging temperature of the aging section is 340-420℃, and the aging time is 90-210s.

[0072] When the heating rate is too low, defects such as dislocations in the high dislocation density martensite formed by hot rolling will recover, which will reduce the nucleation sites and rate of austenite, resulting in a larger size of austenite formed by two-phase annealing.

[0073] When the soaking temperature is too low, the ferrite and austenite structures formed in the two-phase region are uneven and the proportion of ferrite is too high, which leads to a decrease in the final tensile strength. When the soaking temperature is too high, the region will be fully austenitized, and the austenite grains will be significantly coarsened, resulting in coarse ferrite and tempered martensite structures, which are detrimental to tensile strength and formability.

[0074] If the soaking time is too short, the austenite and ferrite will not be sufficiently homogenized; if the soaking time is too long, the grains will grow excessively and coarsen.

[0075] When the slow cooling temperature is too low, the proportion of ferrite is too high, resulting in insufficient tensile strength; when the slow cooling temperature is too high, the proportion of ferrite is too low, resulting in a decrease in the average C content in the remaining austenite, insufficient austenite stability, and easy formation of bainite and other structures during the cooling process.

[0076] If the aging time is too short (below a certain value), carbon will not diffuse sufficiently into the austenite, resulting in insufficient austenite stability and an inability to retain a large amount of it at room temperature. If the aging time is too long, not only will the carbon distribution effect reach saturation, but the martensite will also soften severely during tempering, leading to insufficient tensile strength.

[0077] The above hot-dip galvanizing process parameters include: the zinc pot temperature in the zinc pot section is 450℃~470℃, and the temperature is cooled to room temperature after hot-dip galvanizing.

[0078] Furthermore, after hot-dip galvanizing, alloying can be carried out. The alloying temperature in the alloying stage is 500℃~540℃, and the alloying time is 10s~30s, followed by cooling to room temperature. This alloying galvanizing process can be found in [reference needed]. Figure 2 .

[0079] When the alloying temperature is too low, the diffusion reaction is insufficient and the degree of alloying is inadequate; when the alloying temperature is too high, the coating's resistance to powdering decreases, and the coating is prone to damage or powdering during forming.

[0080] If the alloying time is too short, the alloying effect will be poor; if the alloying time is too long, it will easily lead to the decomposition of austenite, which cannot be retained in large quantities at room temperature to improve elongation.

[0081] The corrosion resistance of this steel plate is significantly improved compared to traditional uncoated hot-rolled steel plates, and the cost is significantly reduced compared to pickled plates electroplating zinc or electroplating zinc-nickel processes. In addition, the process is shorter and less polluting, which is more in line with the development trend of high-strength steel for advanced automobiles.

[0082] Further, the heating section can be provided with a pre-oxidation bin, the pre-oxidation temperature is 640-660℃, and the pre-oxidation time is 10-20s.

[0083] In a second aspect, the present application provides a hot-rolled galvanized dual-phase steel prepared by the above method.

[0084] In some embodiments, the microstructure of the hot-rolled galvanized dual-phase steel comprises ferrite, tempered martensite and residual austenite; wherein the area fraction of ferrite is 30-50%, the area fraction of tempered martensite is 45-65%, and the area fraction of residual austenite is 5-12%.

[0085] The positive effect of controlling the area fraction of ferrite to be 30-50%: ferrite is an important constituent phase to ensure the plasticity of dual-phase steel. When the area fraction of ferrite is too low, the tensile strength of the steel plate is too high, and the elongation decreases significantly; when the area fraction of ferrite is too high, it is difficult to ensure the tensile strength of no less than 780MPa. Specifically, the area fraction of ferrite can be 30%, 40%, 50%, etc.

[0086] The positive effect of controlling the area fraction of tempered martensite to be 45-65%: to ensure the tensile strength and post-break elongation of the steel plate. When the area fraction of tempered martensite is too low, the tensile strength of the steel plate is too low; when the area fraction of tempered martensite is too high, the tensile strength of the steel plate increases significantly, and the post-break elongation decreases. Specifically, the area fraction of tempered martensite can be 45%, 55%, 65%, etc.

[0087] The positive effect of controlling the area fraction of residual austenite to be 5-12%: to ensure the post-break elongation and hole expansion rate of the steel plate. When the area fraction of residual austenite is too low, it cannot provide a post-break elongation higher than 21%; when the area fraction of residual austenite is too high, the hole expansion rate of the steel plate decreases, and the risk of delayed fracture increases. Specifically, the area fraction of residual austenite can be 5%, 8%, 12%, etc.

[0088] The hot-rolled galvanized dual-phase steel is realized based on the preparation method of the hot-rolled galvanized dual-phase steel described above. The specific steps of the preparation method of the hot-rolled galvanized dual-phase steel can refer to the above embodiments. Since the XX system adopts part or all of the technical solutions of the above embodiments, it at least has all the beneficial effects brought by the technical solutions of the above embodiments, which will not be repeated here.

[0089] The present application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods not specified in the following examples are generally determined according to national standards. If there is no corresponding national standard, it is determined according to the general international standard, the conventional condition, or the condition suggested by the manufacturer.

[0090] The hot-dip galvanizing dual-phase steel of Examples 1-6 was prepared by the above method, and Comparative Examples 1-3 were analyzed and compared. The specific process parameters of the hot-dip galvanizing dual-phase steel can be seen in Tables 1-3, and the comprehensive performance of the hot-dip galvanizing dual-phase steel can be seen in Table 4.

[0091] Table 1 Chemical composition of hot-dip galvanizing dual-phase steel (wt%)

[0092]

[0093]

[0094] Table 2 Rolling and cooling process parameters of hot-dip galvanizing dual-phase steel

[0095]

[0096] Table 3 Annealing and galvanizing process parameters of hot-dip galvanizing dual-phase steel

[0097]

[0098]

[0099] Table 4 Microstructure and mechanical properties of hot-dip galvanizing dual-phase steel

[0100]

[0101]

[0102] From Tables 1-4, the content of C in Comparative Example 1 is too low, the area fraction of residual austenite in the microstructure of the steel plate is too low, and the elongation after fracture is too low; the fast cooling temperature in Comparative Example 2 is too low, the area fraction of residual austenite in the microstructure of the steel plate is too low, and the elongation after fracture is too low; the aging temperature in Comparative Example 3 is too low, the area fraction of residual austenite in the microstructure of the steel plate is too low, the elongation after fracture is too low, and the hole expansion rate is too low.

[0103] As can be seen from Table 4, the yield strength of the hot-dip galvanizing dual-phase steel prepared in the examples of the present application is ≥500 MPa, the tensile strength

[0104] ≥780 MPa, the elongation after fracture A 80 ≥21%, and the hole expansion rate ≥40%. Compared with the conventional hot-dip galvanizing dual-phase steel (the elongation after fracture A 80 is about 16%), the elongation after fracture is greatly improved, and the formability is obviously improved.

[0105] The foregoing detailed description of the application has been presented for purposes of illustration and description. Various modifications and changes can be made to these embodiments without departing from the spirit and scope of the application. It is intended that the scope of the application should not be limited by the particular representative embodiments described above.

Claims

1. A method for preparing hot-rolled galvanized duplex steel, characterized in that, The method includes: A slab with a predetermined chemical composition is heated and then rolled, and the rolling process parameters are controlled to obtain a hot-rolled plate. The predetermined chemical composition, by mass fraction, includes: C: 0.17-0.25%, Si: 0.2-0.9%, Mn: 1.4-2.5%, Al: 0.2-0.6%, B: 0.001-0.002%, P≤0.01%, S≤0.003%, with the balance being Fe and unavoidable impurities. The hot-rolled plate is subjected to front-end ultra-fast cooling and rear-end laminar flow water cooling, and then coiled to obtain a hot-rolled coil. The cooling rate of ultra-fast cooling is ≥50℃ / s, the cooling rate of laminar flow water cooling is 10~20℃ / s, and the coiling temperature is 280~380℃. The hot-rolled coil is annealed and then hot-dip galvanized to obtain hot-rolled galvanized duplex steel; wherein the annealing includes a rapid cooling section and an aging section, and the rapid cooling temperature of the rapid cooling section is controlled to be 180-250℃ and the aging temperature of the aging section is controlled to be 340-420℃.

2. The method according to claim 1, characterized in that, The rolling process parameters include: the initial finishing rolling temperature and the final finishing rolling temperature; wherein, The initial rolling temperature of the finishing mill is 960–1030℃, and the final rolling temperature of the finishing mill is 870–910℃.

3. The method according to claim 1, characterized in that, The heating temperature is 1220–1260°C.

4. A hot-rolled galvanized duplex steel, characterized in that, The hot-rolled galvanized duplex steel is prepared by the method described in any one of claims 1-3.

5. The hot-rolled galvanized duplex steel according to claim 4, characterized in that, The microstructure of the hot-rolled galvanized duplex steel includes: ferrite, tempered martensite, and retained austenite; wherein... The area fraction of ferrite is 30%–50%, the area fraction of tempered martensite is 45%–65%, and the area fraction of retained austenite is... The area fraction of the body is 5-12%.

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