1. The 1.2GPa grade fatigue-resistant high-formability ultrahigh-strength automobile steel is characterized by comprising the following components in percentage by weight: c: 0.15% -0.23%, Mn: 1.7% -2.4%, Si: 0.2% -1.4%, Al: 0.02-1.2 percent of Cr, 0.02-0.60 percent of Cr, less than or equal to 0.01 percent of P, less than or equal to 0.01 percent of S, Nb: 0-0.10%, Ti: 0-0.10%, V: 0 to 0.10%, and the balance of Fe and inevitable impurities.

2. The 1.2GPa grade fatigue-resistant high-formability ultrahigh-strength automobile steel as recited in claim 1, wherein the microstructure of the steel plate is a bainite + martensite + ferrite + retained austenite structure; the microstructure of each phase of the steel plate is as follows: 30-60% of bainite, 10-30% of martensite, 10-30% of ferrite and 3-12% of residual austenite; the residual austenite in the steel plate 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 1.2GPa grade fatigue-resistant high-formability ultrahigh-strength automobile steel as claimed in claim 1, wherein the yield strength of the steel plate is 900-1100 MPa, the tensile strength is 1180-1300 MPa, the elongation after A80 fracture is more than or equal to 9.0%, the hole expansion rate is more than or equal to 50%, the minimum bending center radius of 180 degrees cold bending perpendicular to the rolling direction is less than or equal to 1.0t, t is the thickness of the steel plate, and the fatigue limit strength is more than or equal to 800 MPa.

4. A method for preparing 1.2GPa grade fatigue-resistant high-formability 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:

(1) smelting: the temperature of the molten steel in the converter is 1630-1720 ℃;

(2) continuous casting of medium and thin slabs: the casting temperature is 1550-1650 ℃, and the thickness of the continuous casting billet is 110-150 mm;

(3) hot continuous rolling: the charging temperature of a casting blank is 440-580 ℃, the heating temperature is 1150-1260 ℃, the initial rolling temperature is 1050-1160 ℃, the final rolling temperature is more than 890 ℃, and the coiling temperature is 520-640 ℃;

(4) acid pickling and cold rolling: the cold rolling reduction rate is 30-55 percent;

(5) and (3) continuous annealing: preheating at 320-480 ℃, annealing at 840-920 ℃, annealing for 10-600 s, slowly cooling to 660-730 ℃, and then rapidly cooling, wherein the rapid cooling rate is more than 31 ℃/s, the rapid cooling temperature is 380-550 ℃, the overaging temperature is 350-550 ℃, and the overaging time is 30-3600 s;

(6) finishing: the finishing elongation is controlled within the range of 0.3-0.7%.

5. The preparation method of the 1.2 GPa-grade fatigue-resistant high-formability ultrahigh-strength automobile steel as claimed in claim 4, wherein the preparation method comprises the following steps: in the step (3), the microstructure of the hot-rolled steel plate is ferrite, pearlite, bainite and cementite, and the microstructure of the steel plate is calculated by the volume percentage as follows: 30-60% of ferrite, 20-50% of pearlite, 5-20% of bainite and 1-5% of cementite.

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, the ultrahigh-strength steel key parts are often subjected to cyclic loading during service, so that the local load is higher than the yield strength of the material, and the fatigue failure phenomenon occurs. Since this phenomenon seriously affects the normal service of parts, the fatigue failure problem of ultra-high strength steel is of great concern to manufacturers and users. The patent develops a 1.2GPa grade enhanced formability ultrahigh-strength automobile steel and a production method thereof, aims to solve the problem that the ultrahigh-strength grade automobile steel is difficult to realize both the forming performance such as hole expansion and cold bending and the fatigue resistance, and provides a technical scheme for automobile manufacturers and steel companies.

Patent document CN109778062A discloses a cold-rolled complex phase steel with 1200 MPa-grade tensile strength and a production method thereof, and the steel comprises the following main 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 the production process of cold rolling-continuous annealing to produce the cold-rolled complex phase steel with the strength grade of 1.2GPa, the product of the invention has poor forming performance and elongation after fracture, and has great forming and cracking risks in the cold stamping application process.

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 and is difficult to have ultrahigh strength and better ductility, so that the product of the invention is difficult to meet the index requirements of a host factory on high forming performance and fatigue resistance.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to provide the enhanced formability ultrahigh strength automobile steel with good 1.2GPa grade fatigue resistance and the production method thereof, which can meet the production conditions of the traditional production line and simultaneously have excellent alloy cost control, hole expanding performance, cold bending forming performance, extensibility and fatigue resistance.

The purpose of the invention is realized as follows:

the 1.2GPa grade fatigue-resistant ultrahigh-strength automobile steel with enhanced formability comprises the following components in percentage by weight: c: 0.15% -0.23%, Mn: 1.7% -2.4%, Si: 0.2% -1.4%, Al: 0.02-1.2 percent of Cr, 0.02-0.60 percent of Cr, less than or equal to 0.01 percent of P, less than or equal to 0.01 percent of S, Nb: 0-0.10%, Ti: 0-0.10%, V: 0 to 0.10%, and the balance of Fe and inevitable impurities.

The microstructure of the steel plate is a bainite + martensite + ferrite + residual austenite structure; the microstructure of each phase of the steel plate is as follows: 30-60% of bainite, 10-30% of martensite, 10-30% of ferrite and 3-12% of residual 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.

The yield strength of the steel plate is 900-1100 MPa, the tensile strength is 1180-1300 MPa, the elongation after A80 fracture is more than or equal to 9.0%, the hole expansion rate is more than or equal to 50%, the minimum bending core radius of 180-degree cold bending sampled in a direction perpendicular to the rolling direction (transverse direction) is less than or equal to 1.0t, t is the thickness of the steel plate, and the fatigue limit strength is more than or equal to 800 MPa; the requirements of high strength, high plasticity, excellent forming performance, fatigue resistance and high reliability of the automobile are met.

The reason for the design of the components of the 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 0.15-0.23 percent in the invention.

Mn: manganese is an austenite stabilizing element in steel, can enlarge an austenite phase region, reduce the critical quenching speed of the steel, and meanwhile, can refine grains, delay the transformation from austenite to pearlite, and is 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 1.7-2.4 percent in the invention.

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 0.2 to 1.4 percent in the invention.

Al: the aluminum element contributes to deoxidation of the molten steel. It is also possible to suppress decomposition of residual austenite and precipitation of carbide, and to accelerate bainite transformation to improve the ability to coordinate transformation. Too high content of Al element not only increases production cost, but also causes difficulties in continuous casting production, etc. Therefore, the content of the Al element is controlled within the range of 0.02 to 1.2 percent in the invention.

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 0.02 to 0.60 percent in the invention.

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 deformability of the steel, and the lower the content of the P element is, the better the plasticity and the deformability of the steel are. 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.

Nb, V and Ti: the microalloying elements Nb, V and Ti are used for improving the comprehensive performance of the material through fine grain strengthening, and Nb (or V or Ti) of not more than 0.1 percent can be added according to actual conditions, and the microalloying elements Nb, V and Ti can be not added in order to control the production cost.

The second technical scheme of the invention also provides a production method of the 1.2GPa 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;

(1) smelting in a converter: smelting by a converter to obtain molten steel meeting the following component requirements in percentage by mass, C: 0.15% -0.23%, Mn: 1.7% -2.4%, Si: 0.2% -1.4%, Al: 0.02-1.2 percent of Cr, 0.02-0.60 percent of Cr, less than or equal to 0.01 percent of P, less than or equal to 0.01 percent of S, Nb: 0-0.10%, Ti: 0-0.10%, V: 0-0.10% of Fe and inevitable impurities as the rest, and the temperature of the molten steel is 1630-1720 ℃.

(2) Continuous casting of medium and thin slabs: the casting temperature is 1550-1650 ℃, and the thickness of the continuous casting billet is 110-150 mm.

(3) Hot continuous rolling: the charging temperature of the casting blank is 440-580 ℃, the heating temperature is 1150-1260 ℃, the initial rolling temperature is 1050-1160 ℃, the final rolling temperature is 890 ℃ or more, and the coiling temperature is 520-640 ℃. The thickness of the hot-rolled coil is 2-4 mm, the hot-rolled microstructure is ferrite, pearlite, bainite and cementite, and the steel plate structure is as follows according to volume percentage: 30-60% of ferrite, 20-50% of pearlite, 5-20% of bainite and 1-5% of cementite.

(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 30-55%. 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 320-480 ℃, the heating temperature is 840-920 ℃, the annealing time is 10-600 s, the annealing temperature is slowly cooled to 660-730 ℃, then the annealing temperature is rapidly cooled, the rapid cooling rate is more than 31 ℃/s, the annealing temperature is 380-550 ℃, the overaging temperature is 350-550 ℃, and the overaging time is 30-3600 s; the microstructure of the cold-rolled continuous annealing steel plate is a bainite + martensite + ferrite + residual austenite structure; the microstructure of each phase of the steel plate is as follows: 30-60% of bainite, 10-30% of martensite, 10-30% of ferrite and 3-12% of residual austenite; the residual austenite is in two forms of block and film, the grain size is between 0.2um and 0.8um, the block residual austenite is mainly distributed at the martensite/ferrite interface and in the ferrite, and the film residual austenite is mainly distributed between bainite laths. .

The all-austenite region: the annealing temperature is 840-920 ℃, 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.

Finishing: the finishing elongation is controlled within the range of 0.3-0.7%.

The method can be used for producing 1.2GPa grade fatigue-resistant reinforced formability ultrahigh-strength automobile steel with the yield strength of 900-1100 MPa, the tensile strength of 1180-1300 MPa, the elongation after A80 fracture of more than or equal to 9.0 percent, the hole expansion rate of more than or equal to 50 percent, the minimum bending radius of 180 DEG cold bending vertical to the rolling direction (transverse direction) of less than or equal to 1.0t and the fatigue limit strength of more than or equal to 800 MPa.

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 1.2GPa grade fatigue-resistant formability-enhanced ultrahigh-strength automobile steel plate prepared by the method disclosed by the invention is added with a certain proportion of residual austenite on the basis of the traditional cold-rolled complex-phase steel, and has the characteristics of high strength, high plasticity, high formability and fatigue resistance under the action of a transformation induced plasticity (TRIP) effect.

(4) The yield strength of the 1.2GPa grade fatigue-resistant enhanced formability ultrahigh-strength automobile steel plate prepared by the invention is 900-1100 MPa, the tensile strength is 1180-1300 MPa, the elongation after breakage of A80 is more than or equal to 9.0%, the hole expansion rate is more than or equal to 50%, the minimum bending core radius of 180 DEG cold bending vertical to the rolling direction (transverse direction) is less than or equal to 1.0t, and the fatigue limit strength is more than or equal to 800 MPa.

(5) The microstructure of the finished steel plate is 30-60% (volume ratio) bainite, 10-30% (volume ratio) martensite, 10-30% (volume ratio) ferrite and 3-12% (volume ratio) residual austenite; the residual austenite in the product of the invention is in two forms of block and film, the grain size is between 0.2um and 0.8um, 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 molten steel in the converter is 1630-1720 ℃;

(2) continuous casting of medium and thin slabs: the casting temperature is 1550-1650 ℃, and the thickness of the continuous casting billet is 110-150 mm;

(3) hot continuous rolling: the charging temperature of a casting blank is 440-580 ℃, the heating temperature is 1150-1260 ℃, the initial rolling temperature is 1050-1160 ℃, the final rolling temperature is more than 890 ℃, and the coiling temperature is 520-640 ℃;

the microstructure of the hot-rolled steel plate is ferrite, pearlite, bainite and cementite, and the steel plate structure comprises the following components in percentage by volume: 30-60% of ferrite, 20-50% of pearlite, 5-20% of bainite and 1-5% of cementite.

(4) Acid pickling and cold rolling: the cold rolling reduction rate is 30-55 percent;

(5) and (3) continuous annealing: preheating at 320-480 ℃, annealing at 840-920 ℃, annealing for 10-600 s, slowly cooling to 660-730 ℃, and then rapidly cooling, wherein the rapid cooling rate is more than 31 ℃/s, the rapid cooling temperature is 380-550 ℃, the overaging temperature is 350-550 ℃, and the overaging time is 30-3600 s;

(6) finishing: the finishing elongation is controlled within the range of 0.3-0.7%.

The compositions of the steels of the examples of the invention are shown in table 1. The main process parameters of the continuous casting and hot rolling of the steel of the embodiment of the invention are shown in Table 2. The main process parameters of the cold rolling annealing of the steel of the embodiment of the invention are shown in 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	Nb	V	Ti
											1	0.20	2.22	1.25	0.004	0.005	0.45	0.35	-	-	0.042
2	0.22	2.16	1.02	0.005	0.003	0.04	0.26	-	0.027	-
											3	0.21	2.35	0.72	0.004	0.004	0.55	0.47	0.083	-	-
4	0.19	2.44	0.86	0.003	0.003	0.67	0.05	-	0.025	0.030
											5	0.19	2.16	1.18	0.002	0.006	1.10	0.07	0.033	-	0.026
6	0.23	1.88	0.66	0.006	0.003	0.44	0.26	-	0.065	-
											7	0.18	2.30	1.13	0.006	0.006	0.86	0.55	0.047	0.036	0.035
8	0.22	1.92	0.8	0.001	0.002	0.06	0.05	-	-	-
											9	0.20	2.15	1.44	0.005	0.001	0.04	0.51	-	-	0.074
10	0.19	2.37	0.53	0.006	0.004	1.02	0.44	0.022	0.008	0.030

TABLE 2 continuous casting and Hot Rolling of steels according to examples of the invention

TABLE 3 main process parameters of cold rolling and continuous annealing of steel of the examples of the present invention

TABLE 4 Structure of inventive example steels

TABLE 5 Properties of steels of examples of the invention

Note: 180 degree cold bending minimum bend radius test: the sampling direction is longitudinal direction, and t is the thickness of the steel plate; 1180MPa grade high-strength steel fatigue limit strength sigma_-1(yield strength + tensile strength) × 0.23 ═ 900+1180 ═ 0.23 ═ 478 MPa; if the actually measured high cycle fatigue limit strength of the high-strength steel is far higher than sigma_-1The material is excellent in fatigue resistance.

The embodiment shows that the yield strength of the 1.2GPa grade enhanced formability ultrahigh-strength automobile steel prepared by adopting the component design, rolling and continuous annealing process is 900-1100 MPa, the tensile strength is 1180-1300 MPa, the elongation after A80 fracture is more than or equal to 9.0 percent, the hole expansion rate is more than or equal to 50 percent, the sampling minimum bending core radius of 180 DEG perpendicular to the rolling direction (transverse direction) is less than or equal to 1.0t, and the fatigue limit strength is more than or equal to 800 MPa; the requirements of high strength, high plasticity, excellent forming performance, fatigue resistance and high reliability of the automobile are met.

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.