Ultra-high strength steel for automobile cold forming, and manufacturing method therefor

Abstract

Disclosed in the present invention is an ultra-high strength steel for automobile cold forming, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.16-0.21% of C, 0.5-1.0% of Si, 1.7-2.4% of Mn, 0.001-0.004% of B, 0.01-0.05% of Al and 0.045-0.085% of Ti; and the microstructure thereof has acicular bainite, residual austenite, and precipitated carbonitrides. Also disclosed in the present invention is a manufacturing method for the ultra-high strength steel, comprising the steps: smelting and casting; hot rolling; pickling and cold rolling; and annealing. The ultra-high strength steel obtains excellent mechanical strength and formability by means of a relatively low alloy composition design.

Description

Ultra-high strength steel for cold forming of automobile and manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to a steel plate and a manufacturing method thereof, in particular to an ultra-high strength steel and a manufacturing method thereof. BACKGROUND

[0002] With more and more automobiles pursuing weight reduction, on the one hand, the strength requirement of the steel used for parts is getting higher and higher, and on the other hand, the part design is required to be more delicate and complex to ensure that the safety of the whole vehicle can be ensured while the weight of the part is reduced. Although hot forming steel can meet the upgrading of part strength and the complexity of forming at the same time, the high carbon emission and high equipment cost of hot forming steel have become the bottleneck of the development of the automobile industry.

[0003] The material for automobile safety structure still needs to have ultra-high strength and excellent comprehensive forming performance of cold forming steel. However, the traditional 980MPa level ultra-high strength steel cannot meet the higher strength requirement of automobile structure in some application fields. Therefore, the cold forming ultra-high strength steel with a strength of 1180MPa level and excellent comprehensive forming performance has become a key material urgently needed for the development of the industry.

[0004] The existing 1180MPa level ultra-high strength steel is designed in the following two schemes:

[0005] One is to design a martensite structure to achieve a strength of 1180MPa level, and then introduce residual austenite or ferrite to realize that the steel has a certain fracture elongation. In this design, although the steel can have good drawing performance, due to the low yield strength, the hardness difference between ferrite or residual austenite and martensite is too large, which leads to poor flanging performance of the steel, and the comprehensive forming performance cannot meet the requirement of complex structure.

[0006] The other is to design high alloy elements, and add a large amount of Mn, Cr, Si, Al and Mo elements to realize the improvement of ultra-high strength and flanging performance, but on the one hand, due to the high content of alloy elements, the production cost is too high, and the practicability is extremely low; on the other hand, due to the excessive alloy elements, the steel has high carbon equivalent and poor weldability, which is not conducive to the production and assembly of automobile structure parts.

[0007] For example: the patent with publication number CN109778062A, publication date May 21, 2019, and the name of "a kind of tensile strength 1200MPa grade cold-rolled multiphase steel and its preparation method" discloses a kind of 1180MPa grade cold-rolled multiphase steel and its preparation method, the tensile strength reaches 1180MPa grade, and the elongation is ≥5%, but 0.6~1.2% Cr+Mo element is added, the high alloy addition makes that production cost and carbon equivalent are too high. SUMMARY

[0008] One of the purposes of the present application is to provide an ultra-high strength steel for automobile cold forming, which, by relatively low alloy composition design, regulates the generation of acicular bainite structure and residual austenite in the microstructure, and cooperates with the precipitation of nanoscale carbonitride, to obtain excellent mechanical strength and forming performance.

[0009] To achieve the above-mentioned purpose, the present application provides an ultra-high strength steel for automobile cold forming, which contains Fe and inevitable impurities, and further contains the following chemical elements in mass percentage:

[0010] C: 0.16-0.21%; Si: 0.5-1.0%; Mn: 1.7-2.4%; B: 0.001-0.004%; Al: 0.01-0.05%; Ti: 0.045-0.085%; the balance is Fe and inevitable impurities.

[0011] The microstructure thereof has acicular bainite, residual austenite and precipitated carbonitride.

[0012] Further, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage of each chemical element is:

[0013] C: 0.16-0.21%; Si: 0.5-1.0%; Mn: 1.7-2.4%; B: 0.001-0.004%; Al: 0.01-0.05%; Ti: 0.045-0.085%; the balance is Fe and inevitable impurities.

[0014] In the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage of each chemical element is:

[0015] C: In the ultra-high strength steel for automobile cold forming described in the present application, the C element can not only control the phase transformation of the structure, but also form alloy carbonitride with other alloying elements, thereby affecting the strength, formability and performance uniformity of the steel sheet. When the mass percentage content of C element in the steel is too low, not only the strength of the steel cannot meet the target requirement, but also the structure of the steel cannot meet the design requirement, thereby the comprehensive forming performance is poor. Therefore, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage content of C element is controlled to be between 0.16-0.21%.

[0016] Si: In the ultra-high strength steel for automobile cold forming described in the present application, the Si element is not only a necessary element for steelmaking deoxidization and a solid solution strengthening element, but also directly affects the precipitation of carbonitride and the formation of residual austenite. When the mass percentage content of Si element in the steel is too low, residual austenite cannot be formed. When the mass percentage content of Si element in the steel is too high, the precipitation of carbonitride and the formation of acicular bainite are affected, and the comprehensive forming performance is affected. Therefore, in the ultra-high strength steel for automobile cold forming described in the present application, in order to obtain carbonitride precipitated phase with appropriate proportion and size, the mass percentage content of Si element is controlled to be between 0.5-1.0%.

[0017] Mn: In the ultra-high strength steel for automobile cold forming described in the present application, the Mn element not only affects the strength of the steel, but also controls the phase transformation, and directly affects the microstructure of bainite in the steel. When the mass percentage content of Mn element in the steel is too high, not only the corrosion resistance and welding performance are deteriorated, the formation of non-granular bainite structure of martensite is promoted, but also the bainite morphology transformation is easily caused, i.e. from acicular bainite to granular, lamellar and island-shaped morphology, thereby affecting the comprehensive forming performance of the steel. Therefore, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage content of Mn element is controlled to be between 1.7-2.4%.

[0018] B: In the ultra-high strength steel for automobile cold forming described in the present application, the B element not only benefits the formation of bainite in the steel, but also greatly affects the strength and hardness of the steel sheet. When the mass percentage content of B element in the steel is too low, the strength of the steel cannot meet the target requirement; when the mass percentage content of B element in the steel is too high, brittle borides are easily generated, thereby affecting the forming performance and performance uniformity of the steel sheet. Therefore, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage content of B element is controlled to be between 0.001-0.004%.

[0019] Al: In the ultra-high strength steel for automobile cold forming described in the present application, the Al element is only added to the steel as a deoxidizing element, which can remove the O element in the steel to ensure the performance and quality of the steel. Therefore, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage content of the Al element is controlled to be between 0.01-0.05%.

[0020] Ti: In the ultra-high strength steel for automobile cold forming described in the present application, the Ti element is a strong carbonitride forming element, which can refine the grains and greatly improve the yield strength of the steel, balance the strength difference between the phases in the steel plate, and improve the overall forming performance of the steel. Therefore, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage content of the Ti element is controlled to be between 0.045-0.085%.

[0021] Further, in the ultra-high strength steel for automobile cold forming described in the present application, it also contains at least one of the following elements: 0

[0022] In the present application, Mo, Cr, Nb, Cu and Ce can be selectively added to the steel as optional elements. Among them:

[0023] Mo, Cr: In the ultra-high strength steel for automobile cold forming described in the present application, the Mo element and the Cr element can increase the hardenability of the steel strip, increase the incubation period of pearlite and ferrite, inhibit the formation of pearlite and ferrite, and easily obtain bainite structure during cooling. When the mass percentage content of the Mo element and the Cr element is too low, it will lead to insufficient bainite formation. When the mass percentage content of the Mo element and the Cr element is too high, on the one hand, it is easy to form more martensite structure, which will deteriorate the forming performance of the steel strip; on the other hand, it is easy to form banded structure or center segregation in the steel strip, thereby deteriorating the uniformity and performance uniformity of the steel strip. Therefore, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage content of the Mo element can be controlled to be Mo≤0.3wt%, and the mass percentage content of the Cr element can be controlled to be Cr≤0.3wt%. In some embodiments, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage content of the Mo element is controlled to be 0.06-0.3%. In some embodiments, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage content of the Cr element is controlled to be ≤0.27%, for example, 0.08-0.27%.

[0024] In addition, as optional elements, Nb, Cu, Ce can refine the grain, improve the stability of austenite, and Cu is also an austenite stabilizing element. In the present application, although the addition of the above optional elements is beneficial to improving the proportion of acicular bainite and the proportion of residual austenite and improving the forming performance of the steel material, the upper limit of the addition amount of the above elements needs to be controlled from the perspective of cost control and ensuring weldability. Therefore, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage of the Nb element can be controlled to Nb≤0.06wt%, the mass percentage of the Cu element can be controlled to Cu≤0.3wt%, and the mass percentage of the Ce element can be controlled to Ce≤0.006wt%. In some embodiments, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage of the Nb element is controlled to 0.03-0.06%. In some embodiments, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage of the Cu element is controlled to 0.15-0.3%. In some embodiments, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage of the Ce element is controlled to 0.003-0.006%.

[0025] Further, in the ultra-high strength steel for automobile cold forming described in the present application, the mass percentage of each chemical element satisfies at least one of the following: 2.0≤Mn / Si≤3.5; 0.1≤(Ti+2Nb+Mo / 4) / Si≤0.2;

[0026] Wherein, each element symbol is substituted into the mass percentage value of the corresponding element.

[0027] In the present application, the control of Mn / S can be 2.0≤Mn / Si≤3.5 to obtain acicular bainite structure.

[0028] In the present application, Ti is a strong carbonitride forming element, Nb is also a strong carbonitride forming element, and Mo is a weak carbonitride forming element. The effects of Ti, Nb, and Mo elements on carbonitride precipitation and size need to be considered in coordination, and (Ti+2Nb+Mo / 4) / Si can be controlled to 0.1-0.2.

[0029] Further, in the unavoidable impurities of the ultra-high strength steel for automobile cold forming described in the present application, P≤0.012wt%, S≤0.003wt%, and N≤0.004wt%.

[0030] In the present application, the unavoidable impurities mainly include phosphorus (P), sulfur (S), and nitrogen (N), and it is desirable to have as little content as possible under the technical conditions.

[0031] Further, in the ultra-high strength steel for automobile cold forming according to the present application, the volume fraction of residual austenite in the microstructure is 2-9%.

[0032] Further, in the ultra-high strength steel for automobile cold forming according to the present application, the volume fraction of residual austenite in the microstructure is 2-9%.

[0033] Further, the microstructure of the ultra-high strength steel for automobile cold forming according to the present application optionally contains tempered martensite.

[0034] In some embodiments, the microstructure of the ultra-high strength steel for automobile cold forming according to the present application is acicular ferrite + residual austenite + carbonitride, or acicular ferrite + residual austenite + carbonitride + tempered martensite.

[0035] Further, in the microstructure of the ultra-high strength steel for automobile cold forming according to the present application, the number fraction of carbonitride with a size of ≤100 nm is ≥95%, and the number fraction of carbonitride with a size of ≤60 nm is ≥50%.

[0036] Further, in the ultra-high strength steel for automobile cold forming according to the present application, more than 95% of the carbonitride has a size of ≤100 nm, and more than 50% of the carbonitride has a size of ≤60 nm.

[0037] Further, in the ultra-high strength steel for automobile cold forming according to the present application, the yield strength is ≥950 MPa, the tensile strength is ≥1180 MPa, the elongation at break is ≥12%, and the hole expansion ratio is ≥45%.

[0038] In some embodiments, the yield strength of the ultra-high strength steel for automobile cold forming according to the present application is 950-1100 MPa.

[0039] In some embodiments, the tensile strength of the ultra-high strength steel for automobile cold forming according to the present application is 1180-1300 MPa.

[0040] In some embodiments, the elongation at break of the ultra-high strength steel for automobile cold forming according to the present application is 12-14%.

[0041] In some embodiments, the hole expansion ratio of the ultra-high strength steel for automobile cold forming according to the present application is 45-65%.

[0042] Another object of the present application is to provide a manufacturing method of an ultra-high strength steel for automobile cold forming, which can obtain an ultra-high strength steel with excellent forming performance and a low alloying amount.

[0043] To achieve the above object, the present application provides a manufacturing method of ultra-high strength steel for cold forming of automobile, comprising the steps of:

[0044] smelting and casting;

[0045] hot rolling;

[0046] pickling and cold rolling;

[0047] annealing: heating the strip steel to 840-900℃ and holding, then cooling to 680-760℃ at a slow cooling rate of 5-20℃ / s and holding, then cooling to 280-360℃ at a fast cooling rate of 30-80℃ and holding, then reheating to 420-470℃ at a heating rate of 5-30℃ / s and holding, then cooling to 300-380℃ at a second fast cooling rate of 5-50℃ / s, and then cooling to room temperature at a final cooling average cooling rate of ≤10℃ / s.

[0048] In the annealing step of the present application, controlling the heating temperature of the strip steel at 840-900℃ can ensure the full formation of austenite parent phase in the strip steel and the secondary resolubilization of carbonitride, refining the grain size and carbonitride size; cooling to 680-760℃ at a slow cooling rate of 5-20℃ / s and holding can avoid the formation of a large amount of ferrite on the one hand, and ensure the fine and dispersed precipitation of part of the carbonitride on the other hand; cooling to 280-360℃ at a fast cooling rate of 30-80℃ and holding can make the strip steel enter the martensite or bainite transformation region; reheating to 420-470℃ at a heating rate of 5-30℃ / s and holding can ensure the formation of residual austenite and the fine and dispersed precipitation of carbonitride. Cooling to 300-380℃ at a second fast cooling rate of 5-50℃ / s, and then cooling to room temperature at a final cooling average cooling rate of ≤10℃ / s can make the batch formation of acicular bainite.

[0049] Further, in the annealing step of the manufacturing method, the strip steel is heated to 840-900℃ and held for 90-180s, then cooled to 680-760℃ at a slow cooling rate of 5-20℃ / s and held for 5-40s, then cooled to 280-360℃ at a fast cooling rate of 30-80℃ and held for 1-10s, then reheated to 420-470℃ at a heating rate of 5-30℃ / s and held for 5-30s, then cooled to 300-380℃ at a second fast cooling rate of 5-50℃ / s, and then cooled to room temperature at a final cooling average cooling rate of ≤10℃ / s.

[0050] Further, in the annealing step of the manufacturing method, the process segment of reheating to 420-470℃ at a heating rate of 5-30℃ / s and holding completes the hot galvanizing of the surface of the strip steel.

[0051] Further, in the hot rolling step of the manufacturing method, the slab discharge temperature is controlled to be 1230-1300℃, the rough rolling temperature is controlled to be 1050-1120℃, and the finish rolling temperature is controlled to be 880-970℃.

[0052] In the hot rolling step of the manufacturing method, controlling the slab discharge temperature to be 1230-1300℃ can ensure that the carbonitride formed first in the slab can be fully redissolved; controlling the rough rolling temperature to be 1050-1120℃ and the finish rolling temperature to be 880-970℃ can avoid batch precipitation and coarsening of the carbonitride, and only form a small amount of fine carbonitride. In some embodiments, a high rough rolling temperature and finish rolling temperature combination can also be selected, in which case the carbonitride hardly precipitates.

[0053] It should be noted that in the hot rolling step of the manufacturing method, whether a small amount of fine carbonitride precipitates or the carbonitride is controlled to hardly precipitate, there are different contributions to the comprehensive forming performance. When the rough rolling temperature and finish rolling temperature are low, it is beneficial to the improvement of the elongation at break, making the drawing forming performance of the steel better; when the rough rolling temperature and finish rolling temperature are high, it is beneficial to the improvement of the hole expansion rate, making the flanging performance of the steel better.

[0054] Further, in the hot rolling step of the manufacturing method, the strip steel is cooled to 440-520℃ at a cooling rate of 30-80℃ / s after rolling and coiled.

[0055] In the hot rolling step of the manufacturing method, the above-mentioned cooling process after rolling can avoid the formation of uneven martensite structure in the hot rolling structure; controlling the coiling temperature to be 440-520℃ can avoid excessive precipitation and coarsening of the carbonitride in the hot rolling coil, thereby affecting the organization regulation during subsequent annealing.

[0056] Further, in the hot rolling step of the manufacturing method, the hot rolling coil after coiling is held at a temperature of 350-450℃ for 6-72h.

[0057] Further, in the pickling and cold rolling step of the manufacturing method, the cold rolling reduction can be controlled to be 1-20%.

[0058] The ultra-high strength steel for automobile cold forming and the manufacturing method thereof have the following advantages and beneficial effects:

[0059] The ultra-high strength steel for automobile cold forming has a relatively low alloy composition design, controls the formation of acicular bainite structure and residual austenite in the structure, and cooperates with the precipitation of nanoscale carbonitride to obtain excellent mechanical strength and forming performance.

[0060] In some embodiments, the automobile cold forming ultra-high strength steel described in the present application has a yield strength of ≥950 MPa, a tensile strength of ≥1180 MPa, a fracture elongation of ≥12%, and a hole expansion ratio of ≥45%, and can be widely used in the preparation of various automobile structural parts. BRIEF DESCRIPTION OF DRAWINGS

[0061] Figure 1 shows the microstructure morphology of the automobile cold forming ultra-high strength steel of Example 1. DETAILED DESCRIPTION

[0062] The automobile cold forming ultra-high strength steel and the manufacturing method thereof described in the present application will be further explained and described below in conjunction with specific examples, however, the explanation and description do not constitute undue limitations on the technical solutions of the present application.

[0063] Examples 1-6 and Comparative Examples 1-4

[0064] Table 1-1 and Table 1-2 list the mass percentages of each chemical element in the automobile cold forming ultra-high strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-4.

[0065] Table 1-1. (the balance is Fe and other unavoidable impurities other than P, S, and N)

[0066] Table 1-2. (the balance is Fe and other unavoidable impurities other than P, S, and N)

[0067] The automobile cold forming ultra-high strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-4 are prepared by the following steps:

[0068] (1) smelting and continuous casting;

[0069] (2) hot rolling: the slab discharge temperature is controlled at 1230-1300℃, the rough rolling temperature is 1050-1120℃, the final rolling temperature is 880-970℃, and after rolling, the strip steel is cooled to 440-520℃ at a cooling rate of 30-80℃ / s for coiling;

[0070] In some embodiments, the hot-rolled coil after coiling can be kept at a temperature of 350-450℃ for 6-72h.

[0071] (3) pickling and cold rolling;

[0072] (4) annealing: heating the strip to 840-900℃ and holding for 90-180s, then cooling to 680-760℃ at a slow cooling rate of 5-20℃ / s and holding for 5-40s, then cooling to 280-360℃ at a fast cooling rate of 30-80℃ / s and holding for 1-10s, then reheating to 420-470℃ at a heating rate of 5-30℃ / s and holding for 5-30s, then cooling to 300-380℃ at a second fast cooling rate of 5-50℃ / s, and then cooling to room temperature at a final cooling average cooling rate of ≤10℃ / s.

[0073] In addition, in some embodiments, the hot-dip galvanizing of the strip surface can be completed in the process segment of reheating to 420-470℃ at a heating rate of 5-30℃ / s and holding in step (4), so as to obtain a hot-dip galvanizing product.

[0074] It should be noted that the components and processes of the embodiments 1-6 of the present application all meet the requirements of the present application, the components of the comparative example 1 are the same as those of the embodiment 1, and the components of the comparative example 2 are the same as those of the embodiment 6, but the process parameters of the comparative examples 1-2 do not meet the present application; although the process parameters of the comparative examples 3-4 meet the design of the present application, the component design does not meet the design of the present application.

[0075] Table 2-1, Table 2-2 and Table 2-3 list the specific process parameters of the ultra-high strength steel for automobile cold forming of the embodiments 1-6 and the comparative steels of the comparative examples 1-4.

[0076] Table 2-1.

[0077] Table 2-2 Process parameters of step (4)

[0078] Table 2-3 Process parameters of step (4)

[0079] In order to verify the implementation effect of the present application, the automobile cold forming ultra-high strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-4 are sampled, the metallographic sample after polishing is etched by 4% nitric acid alcohol solution for about 10s, and then the observation results are listed in Table 3.

[0080] Table 3 lists the microstructure observation results of the automobile cold forming ultra-high strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3.

[0081] Table 3.

[0082] As can be seen from the above Table 3, the microstructure of the ultra-high strength steel for automobile cold forming of the embodiments 1-6 of the present application has acicular bainite, residual austenite and precipitated carbonitride. Among them, the volume phase ratio of residual austenite is between 2-9%, the volume phase ratio of acicular bainite is ≥80%, and the volume phase ratio of carbonitride is between 3-10%. At the same time, the number ratio of carbonitride with a size ≤100nm is greater than 95%. The number ratio of carbonitride with a size ≤60nm is greater than 50%.

[0083] In addition, Figure 1 shows the microstructure morphology of Example 1.

[0084] As described in Figure 1, the microstructure of the ultra-high strength steel has acicular bainite, residual austenite and precipitated carbonitride. Among them, the volume phase ratio of residual austenite is 5%, the volume phase ratio of acicular bainite is 85%, and the volume phase ratio of carbonitride is 7%. At the same time, the number ratio of carbonitride with a size ≤100nm is 98%. The number ratio of carbonitride with a size ≤60nm is 80%.

[0085] The ultra-high strength steel for automobile cold forming of Examples 1-6 and the comparative steels of Comparative Examples 1-4 were resampled, and various performance tests were performed, and the test results are listed in Table 4. Among them:

[0086] Tensile property test: GB / T228.1-2021 "Metallic materials-tensile testing-Part 1: Method of test at room temperature" was used.

[0087] Hole expansion rate test and test method: The hole expansion rate test method specified in the standard of national standard GB / T 24524-2021 "Metallic materials-thin sheet and strip hole expansion test method" was executed, and the center original hole was in the form of a punched hole (corresponding to the worst processing method of the original hole edge).

[0088] Table 4.

[0089] As can be seen from the above Table 4, the yield strength of the ultra-high strength steel for automobile cold forming of the embodiments 1-6 of the present application is greater than 950MPa, the tensile strength is greater than 1180MPa, the fracture elongation is greater than or equal to 12%, and the hole expansion rate is greater than or equal to 45%.

[0090] It should be noted that the combination of the technical features in the present case is not limited to the combination mode described in the claims of the present case or the combination mode described in the specific embodiments. All technical features described in the present case can be freely combined or combined in any way, unless contradictory to each other.

[0091] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made in accordance with the disclosure of the present application are directly derived or easily thought of by those skilled in the art, and should all belong to the protection scope of the present application.

Claims

1. A type of ultra-high strength steel for automotive cold forming, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following mass percentages: C: 0.16–0.21%; Si: 0.5–1.0%; Mn: 1.7–2.4%; B: 0.001~0.004%; Al: 0.01~0.05%; Ti: 0.045~0.085%; Its microstructure consists of acicular bainite, retained austenite, and precipitated carbonitrides.

2. The ultra-high strength steel for automotive cold forming as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.16–0.21%; Si: 0.5–1.0%; Mn: 1.7–2.4%; B: 0.001–0.004%; Al: 0.01–0.05%; Ti: 0.045–0.085%; balance is Fe and unavoidable impurities.

3. The ultra-high strength steel for automotive cold forming as described in claim 1, characterized in that, It also contains at least one of the following elements: 0 < Mo ≤ 0.3 wt%; 0 < Cr ≤ 0.3 wt%; 0 < Nb ≤ 0.06 wt%; 0 < Cu ≤ 0.3 wt%; 0 < Ce ≤ 0.006 wt%.

4. The ultra-high strength steel for automotive cold forming as described in claim 1, characterized in that, The mass percentage content of each chemical element satisfies at least one of the following conditions: 2.0≤Mn / Si≤3.5; 0.1≤(Ti+2Nb+Mo / 4) / Si≤0.

2.

5. The ultra-high strength steel for automotive cold forming as described in claim 1, characterized in that, Of its unavoidable impurities, P ≤ 0.012 wt%, S ≤ 0.003 wt%, and N ≤ 0.004 wt%.

6. The ultra-high strength steel for automotive cold forming as described in claim 1, characterized in that, The volume proportion of retained austenite in its microstructure is 2-9%; the volume proportion of acicular bainite is ≥80%.

7. The ultra-high strength steel for automotive cold forming as described in claim 1, characterized in that, The volume proportion of carbonitrides in its microstructure is 3-10%.

8. The ultra-high strength steel for automotive cold forming as described in claim 1, characterized in that, In its microstructure, the proportion of carbonitrides with a size ≤100nm is ≥95%, and the proportion of carbonitrides with a size ≤60nm is ≥50%.

9. The ultra-high strength steel for automotive cold forming as described in claim 1, characterized in that, Its yield strength is ≥950MPa, tensile strength is ≥1180MPa, elongation at break is ≥12%, and porosity is ≥45%.

10. The method for manufacturing ultra-high strength steel for automotive cold forming as described in any one of claims 1-9, characterized in that, It includes the following steps: Smelting and casting; Hot-rolled; Pickling and cold rolling; Annealing: Heat the strip steel to 840-900℃ and hold it at that temperature. Then, cool it to 680-760℃ at a slow cooling rate of 5-20℃ / s and hold it at that temperature. Next, cool it to 280-360℃ at a rapid cooling rate of 30-80℃ and hold it at that temperature. Then, reheat it to 420-470℃ at a heating rate of 5-30℃ / s and hold it at that temperature. Then, cool it to 300-380℃ at a secondary rapid cooling rate of 5-50℃ / s. Finally, cool it to room temperature at a final average cooling rate of ≤10℃ / s.

11. The manufacturing method as described in claim 10, characterized in that, In the annealing step, the strip steel is heated to 840-900℃ and held for 90-180s, then cooled to 680-760℃ at a slow cooling rate of 5-20℃ / s and held for 5-40s, then cooled to 280-360℃ at a rapid cooling rate of 30-80℃ and held for 1-10s, then reheated to 420-470℃ at a heating rate of 5-30℃ / s and held for 5-30s, then cooled to 300-380℃ at a secondary rapid cooling rate of 5-50℃ / s, and finally cooled to room temperature at a final average cooling rate of ≤10℃ / s.

12. The manufacturing method as described in claim 10, characterized in that, In the annealing step, the strip steel surface is hot-dip galvanized by reheating to 420-470℃ at a heating rate of 5-30℃ / s and holding at that temperature.

13. The manufacturing method as described in claim 10, characterized in that, In the hot rolling process, the slab exit temperature is controlled at 1230-1300℃, the rough rolling temperature at 1050-1120℃, and the finishing rolling temperature at 880-970℃.

14. The manufacturing method as described in claim 13, characterized in that, In the hot rolling process, the strip is cooled to 440-520°C at a cooling rate of 30-80°C / s after rolling and then coiled.

15. The manufacturing method as described in claim 13, characterized in that, In the hot rolling process, the hot-rolled coil is held at 350–450°C for 6–72 hours.

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