CN113388773A

CN113388773A - 1.5GPa grade high-formability hydrogen-embrittlement-resistant ultrahigh-strength automobile steel and preparation method thereof

Info

Publication number: CN113388773A
Application number: CN202110555869.2A
Authority: CN
Inventors: 张瑞坤; 徐鑫; 张南; 郝志强; 林利; 刘仁东; 苏洪英; 吕冬; 林春青; 王婷
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2021-05-21
Filing date: 2021-05-21
Publication date: 2021-09-14
Anticipated expiration: 2041-05-21
Also published as: CN113388773B

Abstract

The invention provides 1.5GPa grade high-formability hydrogen-embrittlement-resistance ultrahigh-strength automobile steel and a preparation method thereof, wherein the automobile steel plate comprises the following components in percentage by weight: c: 0.20-0.26%, Mn: 2.2% -2.9%, Si: 0.5% -2.0%, Al: 0.02% -1.5%, Cr: 0.20% -0.70%, Mo: 0.10-0.60%, Cu: 0.10-0.60%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, N is less than or equal to 0.005%, Nb: 0.01% -0.15%, V: 0-0.15%, Ti: 0.01 to 0.20 percent, and the balance of Fe and inevitable impurities. The preparation method comprises the following steps: smelting, continuous casting of medium and thin slabs, hot continuous rolling, acid pickling and cold rolling, continuous annealing and finishing; the yield strength of the 1.5GPa grade ultrahigh-strength automobile steel plate with enhanced formability and hydrogen embrittlement resistance prepared by the method is 1100-1350 MPa, the tensile strength is 1470-1650 MPa, the elongation after breakage of A80 is more than or equal to 7.0%, and the hole expansion rate is more than or equal to 35%.

Description

1.5GPa grade high-formability hydrogen-embrittlement-resistant ultrahigh-strength automobile steel and preparation method thereof

Technical Field

The invention belongs to the technical field of cold rolled steel, and relates to 1.5 GPa-grade ultrahigh-strength automobile steel with enhanced forming performance and hydrogen-induced delayed fracture resistance and a manufacturing method thereof.

Background

In the automobile industry, higher requirements on light weight of automobile bodies, emission limitation and safety standards are provided, and in order to better serve users, the automobile industry has more and more demands on parts with high formability. The conventional complex phase steel is difficult to satisfy the requirement of complex cup punching parts with high ductility, and the TRIP steel has limited wide use due to expensive production cost caused by high alloy content. The formability-enhanced complex phase steel (CH steel) has good forming performance due to the introduction of a certain amount of retained austenite, can overcome the defects of CP steel and TRIP steel in the application process, and further has remarkable advantages in the future steel application market. However, severe hydrogen embrittlement (hydrogen-induced delayed fracture) occurs during the service life of the ultra-high strength steel parts, and the hydrogen-induced delayed fracture sensitivity of the ultra-high strength steel is significantly increased with the increase of the strength grade. Since this phenomenon seriously affects the normal service of parts, the delayed fracture phenomenon of ultra-high strength steel is of great concern to manufacturers and users.

Patent document CN109778062A discloses a cold-rolled complex phase steel with 1200 MPa-level tensile strength and a preparation method thereof, and the cold-rolled complex phase steel mainly comprises the following chemical components: c: 0.1-0.15%, Si: 0.1-0.5%, Mn: 1.5-2.6%, Cr: 0.4-0.7%, Mo: 0.2-0.5%, Nb: 0.02 to 0.05%, Ti: 0.02-0.05%, P is less than or equal to 0.020%, S is less than or equal to 0.015%, and the balance is Fe and inevitable impurities. The invention adopts a cold rolling-continuous annealing production process to produce the cold-rolled complex phase steel with the strength grade of 1.2GPa, the product has poor forming performance and elongation after fracture, the strength grade is about 1200MPa and far reaches the grade of 1.5GPa, the requirements of stamping complex parts and the like are difficult to meet in the practical application process, and the problem of hydrogen embrittlement easily occurs.

Patent document CN110343971A discloses an ultrahigh strength hot-dip galvanized complex phase steel and a production method thereof, and the steel comprises the following main chemical components: : c: 0.09% -0.16%, Si: 0.2-0.5%, Mn: 1.7-2.5%, P is less than or equal to 0.025%, S is less than or equal to 0.005%, Mo: 0.20-0.60%, Al: 0.02% -0.08%, Nb: 0.010-0.070%, Ti: 0.030-0.070%, N is less than or equal to 0.006%, and the balance is Fe and other unavoidable impurities. The invention adopts a cold rolling-hot galvanizing production process to produce the hot galvanizing complex phase steel with the strength level of 1.1GPa, but the product has low strength and poor forming performance, is difficult to have ultrahigh strength and better ductility, and is easy to have service problems such as delayed fracture caused by hydrogen and forming problems such as stamping cracking in the practical application process.

Based on the current research situation, the problems of poor forming performance and hydrogen embrittlement of ultrahigh-strength automobile steel are urgently needed to be solved, the patent aims to develop a 1.5 GPa-grade novel ultrahigh-strength automobile steel strip with enhanced forming performance and hydrogen embrittlement resistance, the ultrahigh-strength automobile steel strip can be prepared on a traditional cold rolling production line at very low alloy cost, and a technical scheme is provided for automobile manufacturers and steel companies.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to provide the 1.5 GPa-grade ultrahigh-strength automobile steel with enhanced formability and hydrogen embrittlement resistance and the preparation method thereof, which can meet the production conditions of the traditional production line, control the alloy cost, and have excellent hole expanding performance, cold bending performance, extensibility and hydrogen-induced delayed fracture resistance.

The purpose of the invention is realized as follows:

the 1.5 GPa-grade ultrahigh-strength automobile steel with enhanced formability and hydrogen embrittlement resistance comprises the following components in percentage by weight: c: 0.20-0.26%, Mn: 2.2% -2.9%, Si: 0.5% -2.0%, Al: 0.02% -1.5%, Cr: 0.20% -0.70%, Mo: 0.10-0.60%, Cu: 0.10-0.60%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, N is less than or equal to 0.005%, Nb: 0.01% -0.15%, V: 0-0.15%, Ti: 0.01 to 0.20 percent, and the balance of Fe and inevitable impurities.

The steel plate microstructure comprises martensite, a bainite structure and residual austenite, wherein the steel plate microstructure comprises the following components in percentage by volume: 30 to 70 percent of martensite, 20 to 60 percent of bainite structure and 3 to 12 percent of residual austenite; the residual austenite in the steel plate is in two forms of block and film, the grain size is between 0.2 and 0.8 mu m, the block residual austenite is mainly distributed at the interface of martensite/ferrite and in the ferrite, and the film residual austenite is mainly distributed between bainite laths.

The yield strength of the steel plate is 1100-1350 MPa, the tensile strength is 1470-1650 MPa, the elongation after breakage of A80 is more than or equal to 7.0%, the hole expansion rate is more than or equal to 35%, the preset stress of 180-degree U-shaped bending forming is adopted, and the steel plate is soaked in 0.5mol/L HCl solution for 14 days without delayed breakage; meets the requirements of high strength and high plasticity, excellent forming performance and hydrogen-induced delayed cracking resistance of automobiles.

The reason for the alloy design of the present invention is as follows:

c: the carbon element guarantees the strength requirement of the steel through solid solution strengthening, and sufficient carbon element is beneficial to stabilizing austenite, thereby improving the forming performance of the steel. The content of the element C is too low, so that the mechanical property of the steel in the invention can not be obtained; too high a content can embrittle the steel, with the risk of delayed fracture due to hydrogen. Therefore, the content of the C element is controlled to be 0.20-0.26 percent in the invention.

Mn: manganese is an austenite stabilizing element in steel, can expand an austenite phase region, reduce the critical quenching speed of the steel, and can refine grains, thereby being beneficial to solid solution strengthening to improve the strength. The content of Mn element is too low, the super-cooled austenite is not stable enough, and the plasticity, the toughness and other processing performances of the steel plate are reduced; the excessively high content of the Mn element causes deterioration in the weldability of the steel sheet, and increases in the production cost, which is not favorable for industrial production. Therefore, the content of the Mn element is controlled to be 2.2-2.9 percent.

Si: the silicon element has a certain solid solution strengthening effect in ferrite, so that the steel has enough strength, and meanwhile, the Si can inhibit the decomposition of residual austenite and the precipitation of carbide, thereby reducing the inclusion in the steel. The Si element content is too low to play a role in strengthening; too high content of Si element may degrade the surface quality and weldability of the steel sheet. Therefore, the content of the Si element is controlled to be 0.5 to 2.0 percent in the invention.

Al: when Al: when the content is 0.02-0.1%, the aluminum element is mainly used for deoxidizing molten steel and purifying casting powder. When Al: 0.1 to 1.5 percent of aluminum element can inhibit the decomposition of residual austenite and the precipitation of carbide and accelerate the transformation of bainite to improve the capability of coordinated deformation. Too high content of Al element not only increases production cost, but also causes difficulties in continuous casting production, etc. Therefore, the content range of the Al element is controlled to be 0.02 to 1.5 percent,

cr: the chromium element can increase the hardenability of the steel to ensure the strength of the steel and stabilize the retained austenite, the hardenability of the steel is influenced by too low content of the Cr, and the production cost is increased by too high content of the Cr. Therefore, the content of Cr element is controlled within the range of 0.20 to 0.70 percent in the invention.

Mo: the molybdenum element is a strengthening element in the steel, is beneficial to stabilizing the retained austenite, has obvious effect on improving the hardenability of the steel, can form a large amount of TiMoC precipitates by matching Mo element with Ti, is beneficial to ensuring that the hydrogen diffused in the steel is in dispersion distribution, reduces the aggregation of the hydrogen diffused, and can take high strength and hydrogen embrittlement resistance into consideration. The invention controls the content range of Mo element at 0.10% -0.60%; and the mass percentage ratio of Mo to Ti is controlled between 8:1 and 13: 1.

Cu: the Cu element is a solid solution strengthening element, so that the hardenability of the steel can be improved, the thermodynamic stability of austenite can be effectively improved, the stable residual austenite at room temperature can be formed, and the plasticity, the delayed fracture resistance and the like of the material are improved; however, considering that the cost of the copper element is high, the content of the Cu element is controlled to be 0.10-0.60 percent.

P: the P element is a harmful element in steel, is easy to be deviated to a grain boundary to seriously reduce the plasticity and the deformation performance of the steel, and the lower the content is, the better the content is. In the invention, the content of the P element is controlled to be less than or equal to 0.01 percent in consideration of the cost.

S: the S element is a harmful element in steel, sulfur and manganese elements are easily combined to form MnS inclusions, the transverse performance of the material is obviously reduced after rolling deformation, the formability of the steel is seriously influenced, and the lower the content of the S element is, the better the formability of the steel is. In the invention, the content of the S element is controlled to be less than or equal to 0.01 percent in consideration of the cost.

N: the N element is easy to react with Ti to separate out TiN large particles, and the TiN large particles serve as crack sources in the deformation process and are unfavorable for resisting hydrogen brittleness, so that the content of the N element in the steel needs to be strictly controlled. The invention controls the content of N to be less than or equal to 0.005 percent.

Nb: the microalloy element Nb forms a compound with carbon and nitrogen, is favorable for delaying recrystallization of the material in the hot rolling process, has the functions of refining the grain size, and obviously improving the obdurability and the anti-fatigue failure of the material, and in the invention, the content of the Nb element is controlled to be 0.01-0.15 percent.

V: the microalloying element vanadium mainly exists in a VC form, the strength and the fatigue resistance of the material are improved through fine grain strengthening and dispersion strengthening, and undissolved VC particles can pin ferrite grain boundaries in the hot galvanizing annealing heating process, so that the effect of refining grains is achieved; when the annealing temperature is increased to a two-phase region, the VC is low in dissolving temperature, so that the VC is fully dissolved in a matrix, and solid-solution C atoms are enriched into austenite to improve the stability of the VC; during the annealing process, VC in the ferrite is separated out again, thereby producing obvious precipitation strengthening. Therefore, in the present invention, the content of V element is controlled to 0 to 0.15%.

Ti: the small amount of Ti element can be added to refine the grain size, the precipitates can pin dislocation to play a role in delaying the expansion of a crack source and obviously improve the toughness of the material, and in the invention, the content of the Ti element is controlled to be 0.01-0.20 percent.

The second technical scheme of the invention is to provide a preparation method of 1.5GPa grade enhanced formability ultrahigh strength automobile steel, which comprises the following steps: smelting, continuous casting of medium and thin slabs, hot continuous rolling, acid pickling and cold rolling, continuous annealing and finishing. The preparation process comprises the following specific steps:

(1) smelting: smelting by a converter to obtain molten steel C which meets the following component requirements in percentage by mass: 0.20-0.26%, Mn: 2.2% -2.9%, Si: 0.5% -2.0%, Al: 0.02% -1.5%, Cr: 0.20% -0.70%, Mo: 0.10-0.60%, Cu: 0.10-0.60%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, N is less than or equal to 0.005%, Nb: 0.01% -0.15%, V: 0-0.15%, Ti: 0.01 to 0.20 percent, and the balance of Fe and inevitable impurities, wherein the temperature of the molten steel is 1650 to 1750 ℃.

(2) Continuous casting of medium and thin slabs: the casting temperature is 1580-1660 ℃, and the thickness of the continuous casting billet is 110-140 mm.

(3) Hot continuous rolling: the charging temperature of the casting blank is 510-670 ℃, the heating temperature is 1170-1290 ℃, the initial rolling temperature is 1080-1160 ℃, the final rolling temperature is above 920 ℃, and the coiling temperature is 580-690 ℃. The thickness of the hot rolled coil is 2-4 mm;

the microstructure of the hot-rolled steel plate is ferrite, pearlite, bainite and cementite; wherein the steel plate structure comprises the following components in percentage by volume: 20-40% of ferrite, 25-50% of pearlite, 10-30% of bainite and 1-5% of cementite; meanwhile, the grain size of the hot-rolled steel plate is more than 7.0 grade.

(4) Acid pickling and cold rolling: the iron scale on the surface of the steel coil is removed by acid liquor before cold rolling, and the cold rolling reduction rate is 30-50%. The rolling reduction is too high, so that the deformation resistance is too high, and the rolling is difficult to reach the target thickness; the reduction ratio is too low, resulting in a decrease in the elongation of the cold-rolled steel sheet.

(5) And (3) continuous annealing: the preheating temperature is controlled to be 450-610 ℃, the heating temperature is controlled to be 850-950 ℃, the annealing temperature is 850-950 ℃, the annealing time is 10-600 s, the annealing is slowly cooled to 720-780 ℃, then the annealing is rapidly cooled, the rapid cooling rate is more than 40 ℃/s, the annealing is rapidly cooled to be 380-550 ℃, the overaging temperature is 350-550 ℃, and the overaging time is 60-3600 s.

The all-austenite region: the annealing temperature is 850-950 ℃, and if the annealing temperature is too high, the ductility of the steel is reduced; if the annealing temperature is too low, the final material will exhibit soft phase ferrite and thus it is difficult to meet the strength requirements of the material. The annealing time is 10-500 s, if the annealing time is too long, the grains of the steel plate are coarse, the annealing time is too short, and the steel plate does not finish the annealing and recrystallization processes quickly, so that the elongation of the steel plate is reduced.

(6) Finishing: the finishing elongation is controlled within the range of 0.05-0.25%.

The microstructure of the steel plate produced by applying the technical scheme of the invention is martensite, bainite and residual austenite, wherein the microstructure of the steel plate is calculated by the volume percentage: 30 to 70 percent of martensite, 20 to 60 percent of bainite structure and 3 to 12 percent of residual austenite; the residual austenite in the steel plate is in two forms of block and film, the grain size is between 0.2 and 0.8 mu m, the block residual austenite is mainly distributed at the interface of martensite/ferrite and in the ferrite, and the film residual austenite is mainly distributed between bainite laths.

The preparation method of the 1.5GPa grade reinforced formability ultrahigh-strength automobile steel with yield strength of 1100-1350 MPa, tensile strength of 1470-1650 MPa, elongation of A80 after fracture of more than or equal to 7.0%, hole expansion rate of more than or equal to 35% and good hydrogen-induced delayed cracking resistance can be obtained by the method.

The invention has the beneficial effects that:

(1) the steel material of the invention mainly takes C, Mn and Si as main elements, and has lower original cost.

(2) The invention adopts the production process of converter smelting, medium and thin slab continuous casting and rolling, acid pickling and cold rolling and continuous annealing, can realize the industrial production of the automobile steel on the traditional production line, and has the advantages of low cost, no need of adding new production equipment and stable production process.

(3) The 1.5GPa grade ultrahigh-strength automobile steel plate with enhanced formability and hydrogen embrittlement resistance, which is prepared by the invention, is added with a certain proportion of retained austenite on the basis of the traditional cold-rolled complex phase steel, and has the characteristics of high strength, high plasticity and high hole expanding performance under the action of a transformation induced plasticity (TRIP) effect.

(4) The yield strength of the 1.5GPa grade ultrahigh-strength automobile steel plate with enhanced formability and hydrogen embrittlement resistance prepared by the method is 1100-1350 MPa, the tensile strength is 1470-1650 MPa, the elongation after breakage of A80 is more than or equal to 7.0%, the hole expansion rate is more than or equal to 35%, the preset stress of 180-degree U-shaped bending forming is adopted, and the ultrahigh-strength automobile steel plate is soaked in 0.5mol/L HCl solution for 14 days without delayed fracture.

(5) The microstructure of the finished steel plate is 30-70% (volume ratio) martensite, 20-60% (volume ratio) bainite and 3-12% (volume ratio) retained austenite; the residual austenite in the product of the invention is in two forms of block and film, the grain size is between 0.2 mu m and 0.8 mu m, the block residual austenite is mainly distributed at the interface of martensite/ferrite and in the ferrite, and the film residual austenite is mainly distributed between bainite laths.

Drawings

FIG. 1 is a metallographic microstructure of a steel sheet of example 1;

FIG. 2 is an engineering stress-strain curve of example 1.

Detailed Description

The present invention is further illustrated by the following examples.

According to the component proportion of the technical scheme, the embodiment of the invention carries out smelting, medium and thin slab continuous casting, hot continuous rolling, acid pickling and cold rolling, continuous annealing and finishing.

(1) Smelting: the temperature of the converter smelting molten steel is 1650-1750 ℃;

(2) continuous casting of medium and thin slabs: the casting temperature is 1580-1660 ℃, and the thickness of the continuous casting billet is 110-140 mm;

(3) hot continuous rolling: the charging temperature of a casting blank is 510-670 ℃, the heating temperature is 1170-1290 ℃, the initial rolling temperature is 1080-1160 ℃, the final rolling temperature is above 920 ℃, and the coiling temperature is 580-690 ℃;

the microstructure of the hot-rolled steel plate is ferrite, pearlite, bainite and cementite; wherein the steel plate structure comprises the following components in percentage by volume: 20-40% of ferrite, 25-50% of pearlite, 10-30% of bainite and 1-5% of cementite; meanwhile, the grain size of the hot-rolled steel plate is more than 7.0 grade;

(4) acid pickling and cold rolling: the cold rolling reduction rate is 30-50%;

(5) and (3) continuous annealing: preheating at 450-610 ℃, annealing at 850-950 ℃, annealing for 10-600 s, slowly cooling to 720-780 ℃, then rapidly cooling at a rapid cooling rate of more than 40 ℃/s to 380-550 ℃, overaging at 350-550 ℃, and overaging for 60-3600 s;

The compositions of the steels of the examples of the invention are shown in table 1. The main process parameters of the steel continuous casting and hot rolling of the embodiment of the invention are shown in Table 2. The main process parameters of the cold rolling and annealing of the steel of the embodiment of the invention are shown in the table 3. The structure of the steel of the examples of the present invention is shown in Table 4. The properties of the steels of the examples of the invention are shown in Table 5.

TABLE 1 composition (wt%) of steels of examples of the present invention

Examples	C	Mn	Si	P	S	Al	Cr	Mo	Cu	Nb	V	Ti	Mo/Ti
														1	0.23	2.44	1.38	0.002	0.002	0.05	0.25	0.52	0.11	0.025	-	0.047	11
2	0.25	2.26	1.73	0.003	0.002	0.27	0.57	0.13	0.45	0.032	0.031	0.012	11
														3	0.22	2.36	1.11	0.005	0.005	0.06	0.36	0.24	0.57	0.092	-	0.023	10
4	0.26	2.25	1.26	0.004	0.004	1.15	0.28	0.46	0.23	0.015	0.137	0.043	11
														5	0.21	2.60	1.58	0.005	0.005	0.85	0.65	0.38	0.17	0.045	0.064	0.041	9
6	0.20	2.35	1.06	0.006	0.002	0.56	0.58	0.27	0.36	0.073	0.075	0.030	9
														7	0.24	2.43	0.88	0.003	0.001	0.69	0.40	0.59	0.25	0.066	-	0.072	8
8	0.20	2.67	0.55	0.002	0.005	0.04	0.52	0.18	0.44	0.104	0.022	0.021	9
														9	0.23	2.30	0.79	0.005	0.004	0.96	0.36	0.25	0.51	0.133	0.014	0.027	9
10	0.25	2.65	0.93	0.003	0.006	1.22	0.46	0.33	0.36	0.038	0.072	0.035	9

TABLE 2 Main Process parameters for continuous casting and Hot Rolling of steels according to examples of the invention

TABLE 3 Main Process parameters for the Cold-Rolling annealing of the steels of the examples of the invention

TABLE 4 Structure of inventive example steels

TABLE 5 Properties of steels of examples of the invention

Note: the hydrogen embrittlement resistance (delayed fracture resistance) performance evaluation adopts U-shaped bending soaking evaluation, the 180-degree cold bending radius is 5mm, each group of 5 parallel samples are placed in 0.5mol/L HCl solution to be soaked for 14 days, if no fracture occurs, the sample is judged to have no delayed fracture risk, and O is marked; if fracture occurs, the specimen is judged to have delayed fracture risk and marked x.

The embodiment shows that the yield strength of the 1.5GPa grade enhanced formability ultrahigh-strength automobile steel prepared by adopting the component design, rolling and continuous annealing process is 1100-1350 MPa, the tensile strength is 1470-1650 MPa, the elongation after A80 fracture is more than or equal to 7.0 percent, the hole expansion rate is more than or equal to 35 percent, the preset stress of 180-degree U-shaped bending forming is adopted, and the steel is soaked in 0.5mol/L HCl solution for 14 days without delayed fracture; meets the requirements of high strength and high plasticity, excellent forming performance and hydrogen-induced delayed cracking resistance of automobiles.

In order to express the present invention, the above embodiments are properly and fully described by way of examples, and the above embodiments are only used for illustrating the present invention and not for limiting the present invention, and those skilled in the relevant art can make various changes and modifications without departing from the spirit and scope of the present invention, and any modifications, equivalent substitutions, improvements, etc. made by the persons skilled in the relevant art should be included in the protection scope of the present invention, and the protection scope of the present invention should be defined by the claims.

Claims

1. The 1.5GPa grade high-formability hydrogen-embrittlement-resistant ultrahigh-strength automobile steel is characterized by comprising the following components in percentage by weight: c: 0.20-0.26%, Mn: 2.2% -2.9%, Si: 0.5% -2.0%, Al: 0.02% -1.5%, Cr: 0.20% -0.70%, Mo: 0.10-0.60%, Cu: 0.10-0.60%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, N is less than or equal to 0.005%, Nb: 0.01% -0.15%, V: 0-0.15%, Ti: 0.01 to 0.20 percent, and the balance of Fe and inevitable impurities.

2. The 1.5GPa grade high-formability hydrogen-embrittlement-resistant ultra-high-strength automobile steel as claimed in claim 1, wherein the steel plate microstructure is martensite + bainite + retained austenite, and the steel plate microstructure comprises, by volume percent: 30 to 70 percent of martensite, 20 to 60 percent of bainite structure and 3 to 12 percent of residual austenite; the residual austenite is in two forms of block and film, the grain size is 0.2-0.8 mu m, the block residual austenite is distributed at the martensite/ferrite interface and in the ferrite, and the film residual austenite is distributed between bainite laths.

3. The 1.5GPa grade high-formability hydrogen-embrittlement-resistant ultrahigh-strength automobile steel as claimed in claim 1, wherein the yield strength of the steel plate is 1100-1350 MPa, the tensile strength is 1470-1650 MPa, the elongation after fracture of A80 is not less than 7.0%, and the hole expansion rate is not less than 35%.

4. A method for preparing 1.5GPa grade high-formability hydrogen-brittleness-resistant ultrahigh-strength automobile steel as set forth in any one of claims 1 to 3, which comprises smelting, medium and thin slab continuous casting, hot continuous rolling, acid pickling and cold rolling, continuous annealing and finishing; the method is characterized in that:

(4) acid pickling and cold rolling: the cold rolling reduction rate is 30-50%;

5. The preparation method of the 1.5GPa grade high-formability hydrogen-embrittlement-resistance ultrahigh-strength automobile steel, which is characterized by comprising the following steps: in the step (3), in the hot continuous rolling, the microstructure of the hot-rolled steel plate is ferrite, pearlite, bainite and cementite; wherein the steel plate structure comprises the following components in percentage by volume: 20-40% of ferrite, 25-50% of pearlite, 10-30% of bainite and 1-5% of cementite; meanwhile, the grain size of the hot-rolled steel plate is more than 7.0 grade.