A 1500MPa grade automotive lightweight cold-rolled hot-formed steel and its production method

Abstract

The present invention discloses a 1500 MPa grade automotive lightweight cold-rolled hot-forming steel. The mass percentage content of its chemical components is as follows: C: 0.20 - 0.25%, Si: 0.2 - 0.5%, Mn: 1.0 - 1.5%, P ≤ 0.020%, S ≤ 0.010%, Alt: 0.02 - 0.05%, Cr: 0.1 - 0.3%, Ti: 0.01 - 0.05%, B: 0.002 - 0.005%, and the rest is Fe and inevitable impurities. The production method thereof is also disclosed. The cold-rolled hot-forming steel of the present invention has the characteristics of high strength, high hardness, good strength and toughness matching, high ultimate sharp cold bending performance, etc. after hot stamping, meets the needs of automotive lightweight material selection, and the mechanical properties meet: yield strength R p0.2 ≥ 950 MPa, tensile strength R m : 1300 - 1700 MPa, elongation A 50 ≥ 5%.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgical sheets, and particularly to a 1500 MPa grade automotive lightweight cold-rolled hot-formed steel and a production method thereof. Background Art

[0002] The most challenging problems in the automotive manufacturing industry are: the improvement of safety and collision performance, the reduction of fuel consumption and environmental impact, and the improvement of inexpensive and reliable precision equipment. In these aspects, the development and application of high-strength steel (HSS) have successfully solved some problems. However, when the tensile strength of the steel sheet reaches above 980 MPa, phenomena such as cracking or obvious springback will occur during cold forming. For parts such as A, B, and C pillar reinforcement plates and bumpers in automotive design, the hot stamping forming technology is applied to deform the hot-formed steel sheet in the high-temperature austenite region, and then through processes such as pressure holding and rapid cooling, ultra-high-strength parts with martensite structure and stable dimensional accuracy are obtained at room temperature.

[0003] Automotive hot-formed steel is mainly of the 1500 MPa grade. The consumption of ordinary consumer-grade automobiles in the domestic market accounts for about 15% of the white body, and the design of new models approaches 20%. The general usage in European models reaches more than 30% of the white body. As the leading material for lightweight applications in the future Chinese automotive industry, hot-formed steel will become one of the automotive steel varieties with a large application volume. The strength, size, precision, and surface hardness of automotive parts manufactured by hot stamping technology have been greatly improved, effectively reducing the body weight and improving the safety of the vehicle.

[0004] Currently, in the composition design of the hot-formed steel produced by Wuhan Iron and Steel Co., Ltd. (a 1500 MPa grade low hydrogen-induced delayed cracking sensitivity hot-formed steel and a production method thereof, application number 201910423900.X), the contents of Mn (2.0 - 2.4%), Si (1.20 - 1.45%), and Nb (0.050 - 0.085%) are relatively high, which is different from the composition design of the present invention. When the Mn content is relatively high, it affects the welding performance of the steel strip. When the Si content is high, it is easy to cause the scale on the surface of the steel strip to be difficult to remove, increasing the pickling difficulty and affecting the surface quality of the steel strip. When the Nb content is high, it increases the cost per ton of steel. In the literature "Cold Bending Properties of Three Hot-Stamped Formed Parts of 22MnB5 Steel" in Volume 40, Issue 7 of Mechanical Engineering Materials in July 2016, it is reported that the bending angles of the three hot-stamped formed parts prepared from 22MnB5 are 45.3 - 50.8°, which is different from the design of the present invention. In the composition design of the hot-formed steel produced by Shougang Group Co., Ltd. (a steel for automotive stamping parts and its preparation method, automotive stamping parts, application number 202011060494.4), the contents of Mn (1.5 - 2.5%), Si (1.0 - 2.0%), Cr (0.5 - 3.0%), and Mo (0.10 - 0.30%) are relatively high, and the reduction ratio (75 - 80%) is relatively high, which is different from the composition and process design of the present invention. Summary of the Invention

[0005] The object of the present invention is to provide a 1500MPa - grade cold - rolled hot - forming steel for automotive lightweighting. The cold - rolled hot - forming steel of the present invention has the characteristics of high strength, high hardness, good strength - toughness matching, and high - limit sharp cold - bending performance after hot stamping, meeting the requirements of automotive lightweighting material selection. The mechanical properties meet: yield strength R p0.2 ≥950MPa, tensile strength R m : 1300 - 1700MPa, elongation A 50 ≥5%.

[0006] To solve the above - mentioned technical problems, the present invention adopts the following technical solutions:

[0007] For a 1500MPa - grade cold - rolled hot - forming steel for automotive lightweighting of the present invention, the mass percentage content of its chemical components is: C: 0.20 - 0.25%, Si: 0.2 - 0.5%, Mn: 1.0 - 1.5%, P≤0.020%, S≤0.010%, Alt: 0.02 - 0.05%, Cr: 0.1 - 0.3%, Ti: 0.01 - 0.05%, B: 0.002 - 0.005%, and the rest is Fe and inevitable impurities.

[0008] Furthermore, the mass percentage content of its chemical components is: C: 0.22%, Si: 0.26%, Mn: 1.24%, P: 0.019%, S: 0.003%, Alt: 0.041%, Cr: 0.16%, Ti: 0.034%, B: 0.0031%, and the rest is Fe and inevitable impurities.

[0009] A production method of a 1500MPa - grade cold - rolled hot - forming steel for automotive lightweighting, including:

[0010] (1) The smelting - continuous casting production process flow: hot metal pretreatment - converter - LF refining - slab continuous casting; in the top - and - bottom combined blowing smelting of the converter, oxygen blowing is carried out for decarburization and temperature raising. During the tapping process of the converter, ferrosilicon, ferromanganese, etc. are added for deoxidation and alloying, controlling the P and S components to prevent the molten steel from being over - oxidized, and the tapping temperature ≥1620°C; the LF secondary refining adopts the whole - process argon - blowing process of LF, and adjusts to the target composition according to the molten steel composition; the superheat of the slab continuous casting is 25 - 40°C, and the casting speed is controlled at 1.0 - 1.8m / min;

[0011] (2) Hot rolling production process flow: continuous casting billet heating - rough rolling - finish rolling - coiling; the continuous casting billet is discharged from the furnace at a temperature of 1220 - 1280 °C, rough rolling is carried out on a 2-stand roughing mill with a 3+3 mode, finish rolling is carried out on a 7-stand finishing mill, the finish rolling starting temperature is 1000 - 1060 °C, the finish rolling ending temperature is 840 - 910 °C, the thickness of the hot rolled steel strip is 2.0 - 5.0 mm; cooling is carried out using laminar cooling equipment, and the coiling temperature is 590 - 650 °C;

[0012] (3) Pickling and cold rolling process flow: after the hot rolled steel strip is pickled to remove the surface scale, it is cold rolled on a 5-stand cold rolling mill, the cold rolling reduction rate is 50 - 70%, and the thickness is 1.0 - 2.0 mm;

[0013] (4) Continuous annealing process flow: after the cold hard steel strip is uncoiled, it is heated to 750 - 810 °C, slowly cooled to 600 - 660 °C, then cooled to 350 - 450 °C, and finally cooled to room temperature after over-aging treatment, and the temper rolling elongation is 0.8 - 1.3%;

[0014] (5) Quenching process flow: after the steel strip is sheared, it is austenitized under a protective atmosphere of pure nitrogen, the heating temperature is controlled at 900 - 940 °C, and it is held at this temperature, and the holding time is controlled at 5 - 10 min; after hot stamping forming, quenching is carried out, and the cooling rate is controlled at 40 - 70 °C / s.

[0015] Furthermore, the yield strength R of the formed steel produced p0.2 ≥950 MPa, the tensile strength R m : 1300 - 1700 MPa, the elongation A 50 ≥5%. The hardness (HV1) ≥400, and the ultimate sharp cold bending angle ≥55°.

[0016] Compared with the prior art, the beneficial technical effects of the present invention are:

[0017] The quenched microstructure of this cold-rolled hot-formed steel is mainly martensite, which has the characteristics of high strength, high hardness, good strength and toughness matching, and high ultimate sharp cold bending performance, meeting the needs of automotive lightweight material selection, and the mechanical properties: the yield strength R p0.2 ≥950 MPa, the tensile strength R m : 1300 - 1700 MPa, the elongation A 50 ≥5%. The hardness (HV1) ≥400, and the ultimate sharp cold bending angle ≥55°.

[0018] At the same time, the alloy cost of the present invention is low, the preparation method is simple, and it is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described below in conjunction with the drawings.

[0020] Figure 1 Microstructure diagram of Example 1 Specific implementation manners

[0021] The content of the present invention will be described in detail below through specific examples. The examples are only helpful for understanding the present invention and do not limit the content of the present invention.

[0022] Example

[0023] According to the above requirements of the steelmaking production process, the chemical composition of the actual slab is shown in Table 1.

[0024] Table 1 Chemical composition (wt%) of the example of the present invention

[0025]

[0026] According to the above requirements of the hot rolling process, the hot rolling thickness is 4.1 mm, and the actual process is shown in Table 2.

[0027] Table 2 Hot rolling process of the example of the present invention

[0028] Hot rolling thickness (mm) Final rolling temperature of finish rolling (℃) Coiling temperature (℃) 4.1 851～905 590～635

[0029] According to the above requirements of the cold rolling and annealing processes, the cold rolling thickness is 1.5 mm, and the actual process is shown in Table 3.

[0030] Table 3 Annealing process of the example of the present invention

[0031]

[0032]

[0033] According to the above requirements of the quenching process, the actual process is shown in Table 4.

[0034] Table 4 Quenching process of the example of the present invention

[0035]

[0036] The mechanical properties, microhardness, and ultimate sharp cold bending angle of the steel strip of the example of the present invention were inspected, and the inspection results are shown in Table 5.

[0037] Table 5 Mechanical properties, microhardness, and cold bending properties of the steel strip of the example of the present invention

[0038]

[0039] As can be seen from the data in Table 5, the mechanical properties of a 1500 MPa grade automotive lightweight cold-rolled hot-formed steel of the present invention: yield strength R p0.2 : 1066 - 1142 MPa, tensile strength R m: 1472~1507 MPa, elongation A 50 : 7.1~7.3%. Hardness (HV1): 451~459, ultimate sharp cold bending angle: 66.7~73.3°. The steel strip products obtained by the above process are suitable in performance after being used by users and can be promoted for use.

[0040] The embodiments described above are only descriptions of the preferred modes of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A 1500MPa grade automotive lightweight cold-rolled hot-formed steel, characterized in that: The mass percentage content of its chemical components is as follows: C: 0.20 - 0.25%, Si: 0.2 - 0.5%, Mn: 1.0 - 1.5%, P ≤ 0.020%, S ≤ 0.010%, Alt: 0.02 - 0.05%, Cr: 0.1 - 0.3%, Ti: 0.01 - 0.05%, B: 0.002 - 0.005%, and the rest is Fe and inevitable impurities; It is produced by a method including the following steps: (1) Smelting - continuous casting production process flow: hot metal pretreatment - converter - LF refining - slab continuous casting; in the top and bottom combined blowing smelting of the converter, oxygen blowing is carried out for decarburization and temperature raising. During the tapping process of the converter, ferrosilicon aluminum, ferrosilicon, and ferromanganese are added for deoxidation alloying to control the P and S components and prevent the molten steel from being overoxidized. The tapping temperature ≥ 1620°C; the LF secondary refining adopts the whole - process argon blowing process in the LF furnace, and is adjusted to the target composition according to the molten steel composition; the superheat of the slab continuous casting is 25 - 40°C, and the casting speed is controlled at 1.0 - 1.8 m / min; (2) Hot rolling production process flow: billet heating - rough rolling - finish rolling - coiling; the billet heating furnace outlet temperature is 1220 - 1280°C, the rough rolling adopts a 3 + 3 mode 2 - stand rolling mill for rough rolling, the finish rolling adopts a 7 - stand rolling mill for finish rolling, the finish rolling starting rolling temperature is 1000 - 1060°C, the finish rolling final rolling temperature is 840 - 910°C, and the thickness of the hot - rolled steel strip is 2.0 - 5.0 mm; the cooling adopts a laminar cooling device, and the coiling temperature is 590 - 650°C; (3) Pickling and cold rolling process flow: after the hot - rolled steel strip is pickled to remove the surface scale, it is cold - rolled by a 5 - stand cold rolling mill, the cold - rolling reduction rate is 50 - 70%, and the thickness is 1.0 - 2.0 mm; (4) Continuous annealing process flow: the cold - hard steel strip is uncoiled and heated to 750 - 810°C, slowly cooled to 600 - 660°C, then cooled to 350 - 450°C, and finally cooled to room temperature after over - aging treatment, and the temper rolling elongation is 0.8 - 1.3%; (5) Quenching process flow: after the steel strip is sheared, it is austenitized under the protective atmosphere of pure nitrogen, the heating temperature is controlled at 900 - 940°C, and it is held at this temperature, and the holding time is controlled at 5 - 10 min; after hot stamping forming, quenching is carried out, and its cooling rate is controlled at 40 - 70°C / s.

2. The 1500MPa grade cold-rolled hot forming steel for automobile lightweight according to claim 1, wherein: The mass percentage content of its chemical components is as follows: C: 0.22%, Si: 0.26%, Mn: 1.24%, P: 0.019%, S: 0.003%, Alt: 0.041%, Cr: 0.16%, Ti: 0.034%, B: 0.0031%, and the rest is Fe and inevitable impurities.

3. The 1500MPa-class automotive lightweight cold-rolled hot-formed steel according to claim 1, wherein: The yield strength R of the formed steel produced p0.2 ≥ 950 MPa, the tensile strength R m : 1300 - 1700 MPa, the elongation A 50 ≥ 5%, the hardness HV1 ≥ 400, the ultimate sharp cold bending angle ≥ 55°.

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