Multipurpose high-strength steel for automobiles and preparation method therefor

Abstract

The present invention belongs to high-strength steel for cold-rolled automobiles, and particularly relates to multipurpose high-strength steel for automobiles and a preparation method therefor. The steel comprises the following chemical components in percentages by mass: C: 0.12-0.16%, Mn: 1.80-2.30%, Si: 0.60-1.30%, Al: 0.015-0.5%, P: 0.007-0.012%, S: 0.001-0.004% and the balance of Fe and inevitable impurities. The method comprises the following steps: continuous casting, hot rolling, acid pickling, cold rolling and continuous annealing galvanizing. The present invention proposes that steel plates having the same or similar components are used to cover various types and strengths of steel by means of different heat treatment processes. On the production end, the casting of waste steel from a mixed casting section is reduced, such that the rolling production efficiency is improved, and the management process is optimized; and on the application end, the welding difficulty can be reduced, and the use range of a material is widened.

Description

Multi-purpose high-strength steel for automobiles and preparation method thereof Technical Field

[0001] The present invention belongs to the technical field of cold-rolled high-strength steel for automobiles, and in particular relates to a multi-purpose high-strength steel for automobiles and a preparation method thereof. Background Art

[0002] "Carbon peak" and "carbon neutrality" are the global consensus reached on sustainable human development and are essential prerequisites for the advancement of human science, technology, and civilization. Major CO2-emitting industries include cement, steel, and automobiles, with the automotive industry accounting for a higher proportion. The US automotive industry accounts for 28% of total CO2 emissions, the EU 18%, and China 6%. Considering global CO2 emissions, both the steel and automotive industries are high-carbon emitters, with the primary carbon emission pathway for the automotive industry concentrated in the production of automotive steel. Therefore, achieving "dual carbon" in automotive steel requires effective improvement measures. Among these, lightweighting vehicles is currently a major focus for the automotive industry and automotive steel manufacturers. Clearly, the use of high-strength steel in automotive steel products can reduce material usage, effectively reducing steel output while ensuring safety at the production end, and reducing weight and exhaust emissions at the application end. However, considering the current state of high-strength steel development, automotive high-strength steel demand remains limited, and demand for customized products requires a wide variety of product types. For example, QP980 steel includes products with high plasticity, high hole expansion, high plasticity, and galvanized QP980, as well as high hole expansion. A single steel type can include several, sometimes even dozens, of grades. The production of these different grades requires corresponding compositional design, significantly increasing production costs and, ultimately, carbon emissions. Therefore, addressing the wide variety of automotive high-strength steel production is crucial for reducing costs for steel companies and promoting carbon reduction.

[0003] Chinese patent CN11979488A discloses a 780 MPa-grade alloyed hot-dip galvanized DH steel and its preparation method. The steel's components, by weight, are as follows: C: 0.11-0.17%, Mn: 1.4-2.4%, Si: 0.15-0.60%, Al: 0.02-1.0%, Mo: 0.20-0.70%, P ≤ 0.03%, S ≤ 0.03%, B ≤ 0.005%, V ≤ 0.05%, Ti ≤ 0.05%, with the remainder being Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, pickling, cold rolling, and continuous annealing. The resulting DH galvanized steel exhibits tensile strength of 780-880 MPa and elongation of 20-25%. The concept behind this steel is to introduce retained austenite through an alloy design with high C and Mn ratios within the 780 MPa grade range, thereby improving the steel's plasticity.

[0004] Chinese patent CN110983198A discloses an alloyed hot-dip galvanized duplex steel and its preparation method. The steel's alloy composition is C: 0.10-0.15%, Si: 0.2-0.5%, Mn: 2.5-2.9%, Al: 0.02-0.05%, Ti: 0.015-0.03%, Nb: 0.015-0.03%, Cr: 0.4-0.6%, Mo: 0.1-0.3%, P ≤ 0.01%, S ≤ 0.01%, with the balance being Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, pickling, cold rolling, and continuous annealing to produce steel plates with a tensile strength exceeding 1180 MPa.

[0005] Chinese patent CN111118397A discloses a 980 MPa quenched and partitioned steel and its preparation method. The steel's alloy composition is C: 0.20-0.25%, Si: 1.4-1.8%, Mn: 1.8-2.2%, V: 0-0.10%, Nb: 0-0.050%, Ti: 0-0.050%, P ≤ 0.010%, S ≤ 0.012%, Al ≤ 0.060%, with the remainder being Fe and other unavoidable impurities. This steel produces a cold-rolled, annealed steel sheet with a high strength of 980-1150 MPa and an elongation of 20-25%.

[0006] The aforementioned DH, DP, and QP steels have different alloy composition designs, exhibiting distinct strength-ductility balances. The design of a multi-purpose high-strength steel that achieves diverse mechanical properties while meeting specific application performance targets remains a niche topic.

[0007] Summary of the Invention

[0008] In order to solve the above technical problems, the present invention provides a multi-purpose high-strength steel for automobiles and a preparation method thereof, wherein the multi-purpose high-strength steel is prepared by using the same alloy component system and only changing the subsequent process.

[0009] In order to achieve the above object, the technical solution of the present invention is as follows:

[0010] In one aspect, the present invention provides a multi-purpose high-strength steel for automobiles, wherein the chemical composition of the steel comprises, by mass percentage:

[0011] C: 0.12-0.16%, Mn: 1.80-2.30%, Si: 0.60-1.30%, Al: 0.015-0.5%, P: 0.007-0.012%, S: 0.001-0.004%, and the balance is Fe and unavoidable impurities.

[0012] In the above technical solution, further, the chemical composition of the steel also includes one or more of Ni, Cr, Mo, Nb, and Ti, wherein, in terms of mass percentage, Ni: 0.10-0.30%, Cr: 0.10-0.30%, Mo: 0.05-0.30% and Mn+Ni+Cr+Mo≤2.50%, Nb: 0.015-0.025%, and Ti: 0.01-0.025%.

[0013] The selection principles and content design reasons of the chemical components of the steel of the present invention are as follows:

[0014] C: C is a common strengthening element in steel. As an interstitial solid solution atom, C dissolves in the matrix and increases the solid solution strength by causing lattice distortion. In the multi-purpose automotive high-strength steel of the present invention, the role of C in DH and QP steels is mainly to ensure the stability of retained austenite, and its role in DP steel is mainly to improve the hardenability of the critical zone and ensure the transformation content of martensite. However, the overall C content in the present invention is in the low C range, taking into account the resistance spot welding performance, hot rolling edge cracking problem, and cold rolling edge cracking problem. In the present invention, the C content is controlled at 0.12-0.16%.

[0015] Mn: Mn is low-cost, and Mn atoms strengthen the solid solution by inducing lattice distortion through substitutional solid solution. In the present invention, the addition of Mn content is the main element to obtain 780-980 MPa. However, the added content of Mn element should not exceed the scope of the present invention, considering the problem of C / Mn segregation caused by excessive Mn content, and the problem of inhibiting bainite formation due to increased hardenability caused by excessive Mn in DH steel. Therefore, the present invention comprehensively considers the composite addition content of Mn / Cr / Mo / Ni. In the present invention, the Mn content is controlled at 1.80-2.30%.

[0016] Si: Si is one of the important elements in the present invention. Sufficient Si addition to ferrite ensures the matrix strength of ferrite. At the same time, Si addition will increase the AC3 point of the steel plate, effectively adjust the annealing process window in the continuous annealing stage, and ensure the appropriate ferrite and austenite ratio in the critical zone at the industrial continuous annealing temperature. In addition, the role of Si addition is that sufficient Si addition can inhibit the formation of carbides in the over-aging stage, avoiding the reduction of performance of the steel plate due to carbide precipitation. In the present invention, Si belongs to the middle range of high-strength steel additions. It is considered that too high Si content will cause "leakage plating" on the galvanized surface and other problems that affect the surface quality, and too high Si content will increase the difficulty of pickling (due to the inner oxide layer and the grain boundary oxide layer). And the Si content cannot be too low. Too low will lead to the failure to inhibit the precipitation of carbides. This is the main application of QP steel and DH steel. In the present invention, the Si content is controlled at 0.60~1.30%.

[0017] Al: Al is used solely as a deoxidizer in this invention. This invention aims to address the "multiple uses of one steel" problem, so adding Al significantly increases costs and makes continuous casting more difficult. In this invention, the Al content is controlled within a range of 0.015-0.5%.

[0018] P: P is an impurity element in steel that easily segregates at grain boundaries. When the P content in steel is high, Fe2P particles are easily formed, which reduces the plasticity and toughness of the steel. Therefore, the lower the P content, the better. In the present invention, the P content is controlled to 0.007-0.012%.

[0019] S: S is an impurity element in steel and easily combines with Mn to form MnS inclusions, which deteriorates the plasticity of the steel plate. Therefore, the lower its content, the better. In the present invention, the S content is controlled to 0.001-0.004%.

[0020] You can also add elements:

[0021] Ni: Ni is a solid solution strengthening element. Like C and Mn, it improves austenite stability and also enhances the corrosion resistance of the steel plate to a certain extent. Ni can be added to the optional components of the present invention in appropriate amounts to improve corrosion resistance. In the present invention, the Ni content is controlled to 0.10-0.30%.

[0022] Cr and Mo: Cr and Mo are solid solution strengthening elements that strengthen the steel plate. In the present invention, Cr and Mo can improve the hardenability of the steel plate, delay the formation of pearlite and bainite in the cooling stage, and promote the formation of martensite. At the same time, Cr and Mo can change the type of iron oxide scale during the coiling process, limit the oxidation process in the steel plate, and improve the surface quality of the steel plate. In the present invention, Cr and Mo are added after Mn is added to balance the problems of hot-rolled edge cracks and cold-rolled edge cracks. In the present invention, the Cr content is controlled at 0.10-0.50%, and the Mo content is controlled at 0.05-0.30%.

[0023] As mentioned earlier, alloying elements such as Ni, Cr, and Mo are all substitutes for Mn. Their primary function in this invention is to stabilize and enhance austenite. However, considering multiple factors, including cost, difficulty in casting, hot rolling, and cold rolling, their overall addition should meet the unified goals of low cost, easy production, and high yield. Mn + Ni + Cr + Mo ≤ 2.6%.

[0024] Nb: Nb is a microalloying strengthening element. It is added to assist Ti in the production of DP and DH series products to refine grains, increase yield strength ratio, and thus improve the subsequent flanging performance of the steel plate. In the present invention, the Nb content is controlled to 0.015-0.025%.

[0025] Ti: In this invention, Ti is appropriately added to supplement strength. In the "one steel, multiple uses" strategy, some planned compositions may not meet strength requirements. Ti precipitation is used to refine the original austenite grains, strengthening them through grain refinement and precipitation strengthening to supplement strength. In this invention, the Ti content is controlled within a range of 0.015-0.025%.

[0026] Another aspect of the present invention provides a method for preparing the multi-purpose high-strength steel for automobiles, the method comprising the following steps: continuous casting, hot rolling, pickling, cold rolling, and continuous annealing. The specific steps of the method are as follows:

[0027] (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel;

[0028] (2) Hot rolling: The ingot is heated, and then subjected to rough rolling, finish rolling, and coiling into a hot-rolled coil;

[0029] (3) Pickling and cold rolling: cold rolling after pickling;

[0030] (4) Continuous annealing: The cold-rolled steel sheet is heated to 820-950°C, isothermal for 35-120 seconds, then slowly cooled to 700-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-470°C at a rate of 15-25°C / s, isothermal for 15-25 seconds, then placed in a zinc pot, and finally placed in an alloying furnace, with an alloying temperature of 480-560°C and an alloying time of 15-25 seconds. In the above technical solution, further, in step (1), the casting temperature is 1580-1620°C, and the thickness of the ingot is 220-280 mm.

[0031] In the above technical solution, further, in step (2), the heating temperature is 1230-1280°C, the furnace time is 180-240 min, the rough rolling temperature is 1150-1200°C, the intermediate billet thickness is 50-80 mm, the finishing rolling is divided into two stages, the recrystallization rolling temperature is 1070-1130°C, the final rolling temperature is above 920°C, the coiling temperature is between 450-520°C, and the hot rolled coil thickness is 2.8-3.5 mm.

[0032] The heating temperature is controlled between 1230°C and 1280°C, with a furnace time of 180 to 240 minutes, to promote full solid solution of the alloy and control the banded structure caused by segregation. The finishing rolling stage is divided into two stages to promote recrystallization of the original austenite grains and inhibit the coarsening of the unrecrystallized austenite grains. The coiling temperature is controlled between 450°C and 520°C to prevent the formation of Si-rich oxides on the surface of the steel plate due to the addition of Si, which in turn leads to the formation of internal oxide layers and grain boundary oxide layers.

[0033] In the above technical solution, further, in step (3), the thickness of the cold-rolled plate is 1.4 / 1.6 / 1.8 mm, the 1.4 mm plate thickness corresponds to the 2.8 mm hot-rolled steel plate, the 1.6 mm and 1.8 mm plate thicknesses correspond to the 3.0-3.5 mm hot-rolled steel plates, and the cold rolling reduction rate is 46.7-48.6%.

[0034] Too low a rolling reduction rate cannot ensure sufficient cold rolling deformation energy storage, resulting in insufficient ferrite recrystallization effect in the continuous annealing stage; too high a rolling reduction rate greatly increases the load of the cold rolling mill and cannot ensure the target thickness is achieved.

[0035] In the above technical solution, further, in step (4), the continuous galvanizing process is as follows: the cold-rolled steel sheet is heated to 820-850°C, isothermal for 60-105 seconds, then slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-460°C at a rate of 15-25°C / s, isothermal for 15-25 seconds, then placed in a zinc pot, and finally placed in an alloying furnace with an alloying temperature of 495-525°C and an alloying time of 15-25 seconds. The resulting product steel meets the requirements of the DH780 standard.

[0036] A high proportion of ferrite structure is obtained by isothermal treatment in the critical zone, and the C concentration in the austenite in the critical zone increases while ferrite is formed. After slow cooling, a relatively obvious concentration gradient appears in the supercooled austenite, which promotes the formation of bainite in the 380-460℃ stage while continuously enriching C in the surrounding supercooled austenite along the concentration gradient. After alloying, the supercooled austenite rich in C and with a smaller grain size is retained to room temperature to exist in the form of austenite, while the supercooled austenite poor in C and with a larger size undergoes martensitic phase transformation.

[0037] In the above technical solution, further, in step (4), the continuous galvanizing process is as follows: the cold-rolled steel sheet is heated to 860-900°C, isothermal for 35-45 seconds, then slowly cooled to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-460°C at a rate of 15-25°C / s, isothermal for 15-25 seconds, then placed in a zinc pot, and finally placed in an alloying furnace with an alloying temperature of 495-505°C and an alloying time of 15-25 seconds. The resulting product steel meets the requirements of the DH980 standard.

[0038] A high proportion of austenite is achieved through isothermal treatment in the critical zone. Slow cooling is used to suppress the formation of excessive epitaxial ferrite to ensure yield strength. Bainite formation is promoted between 380°C and 460°C, while carbon is continuously enriched in the surrounding undercooled austenite along a concentration gradient. After alloying, the carbon-rich, small-grained undercooled austenite is retained at room temperature as austenite, while the carbon-poor, larger undercooled austenite undergoes a martensitic transformation. The alloying temperature is adjusted to prevent excessive carbide formation, which can affect the strength of the steel plate.

[0039] In the above technical solution, further, in step (4), when the product steel meets the requirements of the DP980 standard, the continuous galvanizing process is as follows: heating the cold-rolled steel sheet to 860-900°C, isothermal for 100-120 seconds, then slowly cooling to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooling to 380-460°C at a rate of 15-25°C / s, isothermal for 15-25 seconds, then entering the zinc pot, and finally entering the alloying furnace, the alloying temperature is 530-560°C, and the alloying time is 15-25 seconds. The obtained product steel meets the requirements of the DP980 standard.

[0040] A high proportion of austenite structure is obtained by isothermal treatment in the critical zone, and increasing the isothermal time promotes coarsening and balanced C concentration. Excessive epitaxial ferrite formation is suppressed by slow cooling to ensure yield strength, and the alloying temperature is controlled to promote the formation of secondary martensite.

[0041] In the above technical solution, further, in step (4), the continuous galvanizing process is as follows: the cold-rolled steel sheet is heated to 920-950°C, isothermal for 60-100 seconds, then slowly cooled to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 340-400°C at a rate of 15-25°C / s, raised to 450-470°C, isothermal for 15-25 seconds, then placed in a zinc pot, and finally placed in an alloying furnace with an alloying temperature of 480-495°C and an alloying time of 15-25 seconds. The obtained product steel meets the requirements of QP980 standard.

[0042] The fully austenitic structure is obtained by isothermal treatment in the critical zone, and the formation of excessive epitaxial ferrite is suppressed by slow cooling to ensure the yield strength. The martensitic phase transformation is promoted in the 340-400°C stage while retaining the corresponding content of retained austenite. The temperature is raised and isothermal treatment is carried out, and the martensite is tempered to form tempered martensite. The retained austenite is rich in carbon to improve its stability. The alloying process is continued to prevent the precipitation of carbides in the tempered martensite due to excessive alloying temperature.

[0043] The beneficial effects of the present invention are:

[0044] The present invention proposes to use steel plates with the same or similar composition to achieve coverage of multiple categories and strengths of steel through different heat treatment processes. On the production side, it will reduce scrap steel in the casting and mixing sections, improve rolling production efficiency, and optimize management processes; on the application side, it can reduce welding difficulty and broaden the application range of the material.

[0045] The application of the present invention can achieve energy conservation and emission reduction in the production and application processes, and is in line with the national "dual carbon" policy. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG1 is a microstructure diagram of four steels prepared from component 1 according to the present invention, a is DH780-1, b is DH980-1, c is DP980-1, and d is QP980-1. DETAILED DESCRIPTION

[0047] The present invention will be described in more detail by way of examples. These examples are merely descriptions of the best mode of carrying out the present invention and do not limit the scope of the present invention in any way.

[0048] Table 1 lists the chemical compositions of the example steels.

[0049] Table 1 Chemical composition of example steel, wt%

[0050] A multi-purpose high-strength steel for automobiles, the preparation method comprising the following steps:

[0051] (1) Continuous casting: Continuous casting is carried out according to the chemical composition of the steel, with a casting temperature of 1580-1620°C and a billet thickness of 220-280 mm;

[0052] (2) Hot rolling: heating temperature is between 1230 and 1280 ° C, furnace time is 180 to 240 min, rough rolling temperature is 1150 to 1200 ° C, intermediate billet thickness is 50 to 80 mm, finishing rolling is divided into two stages, recrystallization rolling temperature is between 1070 and 1130 ° C, final rolling temperature is above 920 ° C, coiling temperature is between 450 and 520 ° C, and hot rolled coil thickness is between 2.8 and 3.5 mm;

[0053] (3) Pickling and cold rolling: After pickling, cold rolling is carried out. The thickness of the cold rolled steel plate is 1.4 / 1.6 / 1.8 mm. The thickness of 1.4 mm corresponds to the 2.8 mm hot rolled steel plate, and the thickness of 1.6 and 1.8 mm corresponds to the 3.0-3.5 mm hot rolled steel plate. The cold rolling reduction is controlled at 46.7-48.6%.

[0054] (4) Continuous annealing: The cold-rolled steel plate is heated to 820-850°C, kept isothermal for 60-105 seconds, then slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-460°C at a rate of 15-25°C / s, kept isothermal for 15-25 seconds, then placed in a zinc pot, and finally placed in an alloying furnace with an alloying temperature of 495-525°C and an alloying time of 15-25 seconds. The resulting product steel meets the DH780 standard requirements.

[0055] The cold-rolled steel plate is heated to 860-900°C, kept isothermal for 35-45 seconds, then slowly cooled to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-460°C at a rate of 15-25°C / s, kept isothermal for 15-25 seconds, then placed in a zinc pot, and finally placed in an alloying furnace. The alloying temperature is 495-505°C, and the alloying time is 15-25 seconds. The obtained product steel meets the requirements of DH980 standard.

[0056] The cold-rolled steel plate is heated to 860-900°C, kept isothermal for 100-120 seconds, then slowly cooled to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-460°C at a rate of 15-25°C / s, kept isothermal for 15-25 seconds, then placed in a zinc pot, and finally placed in an alloying furnace. The alloying temperature is 530-560°C, and the alloying time is 15-25 seconds. The resulting product steel meets the requirements of the DP980 standard.

[0057] The cold-rolled steel plate is heated to 920-950°C, kept isothermal for 60-100 seconds, then slowly cooled to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 340-400°C at a rate of 15-25°C / s, raised to 450-470°C, kept isothermal for 15-25 seconds, then placed in a zinc pot, and finally placed in an alloying furnace. The alloying temperature is 480-495°C, and the alloying time is 15-25 seconds. The obtained product steel meets the requirements of QP980 standard.

[0058] According to the above preparation method, DH780-1, DH980-1, DP980-1, QP980-1, and so on were prepared using component-1 in Table 1.

[0059] Table 2 lists the continuous casting and hot rolling process parameters of the example steel, and Table 3 lists the cold rolling and continuous annealing galvanizing process parameters of the example steel.

[0060] Table 2 Continuous casting and hot rolling process parameters of example steel

[0061] Table 3 Cold rolling and continuous annealing galvanizing process parameters of the example steel

[0062] Table 4 gives the mechanical properties of the example steels.

[0063] Table 4 Mechanical properties of example steel

[0064] As can be seen from Table 4, the microstructure of DH780 is ferrite + retained austenite + bainite + martensite, in which the ferrite content is 50-60%, the martensite content is 10-25%, the bainite content is 5-10%, and the retained austenite content is 5-7.5%; the yield strength of the steel is above 440 MPa, the tensile strength is above 780 MPa, the elongation is above 23%, and the hole expansion rate is above 45%.

[0065] In DH980, the microstructure is ferrite + retained austenite + bainite + martensite, of which the ferrite content is 35-45%, the martensite content is 25-35%, the bainite content is 5-10%, and the retained austenite content is 5-7.5%; the yield strength of the steel is above 700MPa, the tensile strength is above 980MPa, the elongation is above 16%, and the hole expansion rate is above 30%.

[0066] In DP980, the microstructure is ferrite + martensite, with the ferrite content being 45% to 65% and the martensite content being 35% to 55%; the steel has a yield strength of more than 550 MPa, a tensile strength of more than 980 MPa, an elongation of more than 16%, and a hole expansion rate of more than 20%.

[0067] In QPHE980, the microstructure is ferrite + retained austenite + bainite + tempered martensite, of which the ferrite content is 5-15%, the tempered martensite content is 65-75%, the bainite content is 5-10%, and the retained austenite content is 10-12.5%; the steel has a yield strength of more than 780MPa, a tensile strength of more than 980MPa, an elongation of more than 14%, and a hole expansion rate of more than 65%.

[0068] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. The scope of protection of the present invention shall be determined by the scope defined in the claims. Other variations or modifications may be made based on the above description. Obvious variations or modifications derived therefrom shall remain within the scope of protection of the present invention.

Claims

1. A multi-purpose high-strength steel for automobiles, characterized in that: The chemical composition of the steel includes by mass percentage: C: 0.12~0.16%, Mn: 1.80~2.30%, Si: 0.60~1.30%, Al: 0.015~0.5%, P: 0.007~0.012%, S: 0.001~0.004%, and the balance is Fe and inevitable impurities.

2. The multi-purpose high-strength steel for automobiles according to claim 1, characterized in that: The chemical composition of the steel also includes one or more of Ni, Cr, Mo, Nb, and Ti, wherein, by mass percentage, Ni: 0.10-0.30%, Cr: 0.10-0.30%, Mo: 0.0.5-0.30% and Mn+Ni+Cr+Mo≤2.50%, Nb: 0.015-0.025%, and Ti: 0.01-0.025%.

3. A method for preparing the multi-purpose high-strength steel for automobiles according to any one of claims 1 to 2, characterized in that: The method comprises the following steps: continuous casting, hot rolling, pickling, cold rolling, and continuous galvanizing; The specific steps of the method are as follows: (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel; (2) Hot rolling: The ingot is heated, and then rough rolled, finish rolled, and coiled into a hot rolled coil; (3) Pickling and cold rolling: cold rolling after pickling; (4) Continuous annealing: The cold-rolled steel sheet is heated to 820-950°C, isothermal for 35-120 seconds, then slowly cooled to 700-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-470°C at a rate of 15-25°C / s, isothermal for 15-25 seconds, then enters the zinc pot, and finally enters the alloying furnace, the alloying temperature is 480-560°C, and the alloying time is 15-25 seconds.

4. The preparation method according to claim 3, characterized in that: In step (1), the casting temperature is 1580-1620°C, and the thickness of the ingot is 220-280 mm.

5. The preparation method according to claim 3, characterized in that: In step (2), the heating temperature is 1230-1280°C, the furnace time is 180-240 min, the rough rolling temperature is 1150-1200°C, the intermediate billet thickness is 50-80 mm, the finishing rolling is divided into two stages, the recrystallization rolling temperature is 1070-1130°C, the final rolling temperature is above 920°C, the coiling temperature is 450-520°C, and the hot rolled coil thickness is 2.8-3.5 mm.

6. The preparation method according to claim 3, characterized in that: In step (3), the thickness of the cold-rolled plate is 1.4 / 1.6 / 1.8 mm, the 1.4 mm plate thickness corresponds to the 2.8 mm hot-rolled steel plate, the 1.6 mm and 1.8 mm plate thicknesses correspond to the 3.0-3.5 mm hot-rolled steel plates, and the cold-rolling reduction ratio is 46.7-48.6%.

7. The preparation method according to claim 3, characterized in that: In step (4), the continuous galvanizing process is as follows: the cold-rolled steel sheet is heated to 820-850°C, isothermally heated for 60-105 seconds, then slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-460°C at a rate of 15-25°C / s, isothermally heated for 15-25 seconds, then enters the zinc pot, and finally enters the alloying furnace, the alloying temperature is 495-525°C, and the alloying time is 15-25 seconds.

8. The preparation method according to claim 3, characterized in that: In step (4), the continuous galvanizing process is as follows: the cold-rolled steel sheet is heated to 860-900°C, isothermally heated for 35-45 seconds, then slowly cooled to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-460°C at a rate of 15-25°C / s, isothermally heated for 15-25 seconds, then enters the zinc pot, and finally enters the alloying furnace, the alloying temperature is 495-505°C, and the alloying time is 15-25 seconds.

9. The preparation method according to claim 3, characterized in that: In step (4), the continuous galvanizing process is as follows: the cold-rolled steel sheet is heated to 860-900°C, isothermally heated for 100-120 seconds, then slowly cooled to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 380-460°C at a rate of 15-25°C / s, isothermally heated for 15-25 seconds, then enters the zinc pot, and finally enters the alloying furnace, the alloying temperature is 530-560°C, and the alloying time is 15-25 seconds.

10. The preparation method according to claim 3, characterized in that: In step (4), the continuous galvanizing process is as follows: the cold-rolled steel sheet is heated to 920-950°C, isothermally heated for 60-100s, then slowly cooled to 750-780°C at a cooling rate of 1.2-3.6°C / s, then cooled to 340-400°C at a rate of 15-25°C / s, raised to 450-470°C, isothermally heated for 15-25s, then entered into a zinc pot, and finally entered into an alloying furnace, with an alloying temperature of 480-495°C and an alloying time of 15-25s.

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