850MPa-grade hot-rolled automobile beam steel and preparation method thereof

Abstract

The invention discloses 850MPa-grade hot-rolled automobile beam steel and a preparation method thereof, and belongs to the field of high-strength automobile steel. The hot-rolled automobile beam steel comprises the following chemical components in percentage by mass: 0.050 to 0.090 percent of C, 0.04 to 0.12 percent of Si, 1.50 to 1.90 percent of Mn, less than or equal to 0.020 percent of P, less than or equal to 0.007 percent of S, 0.22 to 0.35 percent of Cr, 0.040 to 0.060 percent of Nb, 0.090 to 0.120 percent of Ti, 0.060 to 0.100 percent of V, 0.010 to 0.060 percent of Als, less than or equal to 0.0050 percent of N and the balance of Fe and impurities. According to the invention, component design is combined with a controlled rolling and controlled cooling process, so that the strength of the girder steel is improved, various comprehensive mechanical properties are ensured, coiling, uncoiling and subsequent user processing can be met, and meanwhile, excessive expensive alloy elements are not added, so that the production cost is controllable.

Description

Technical Field

[0001] The present invention belongs to the field of high-strength automobile steel and relates to an 850MPa-grade hot-rolled automobile beam steel and a preparation method thereof. Background Art

[0002] Automotive beam steel is primarily used in chassis components such as longitudinal beams, lining beams, crossbeams, and front and rear axles. These components bear virtually the entire weight of the vehicle during operation, and their overall performance is crucial to the vehicle's safety and service life. With the continued advancement of green and low-carbon development in the automotive industry, lightweighting has become a key trend, and high-strength automotive beam steel is a key material for simultaneously meeting safety and low-carbon requirements. However, as strength increases, the material's plasticity and toughness often suffer. Maintaining high strength while maintaining good overall mechanical properties remains a challenge for current technologies.

[0003] CN113322413A discloses a 900MPa grade hot-rolled automobile beam steel strip, which has a high Mn content and is added with alloying elements such as Mo, Nb, Ti, and V. A ferrite and ultrafine bainite composite structure is obtained through UFC intermediate billet cooling and controlled rolling process. This method has a high alloy cost, which is not conducive to large-scale promotion and application. CN117512436A discloses an 800MPa grade high-toughness and high-strength beam steel, which has a high C content and is added with elements such as Ti, Cr, and Mo. High strength is achieved through bainite phase transformation and nano-Ti precipitation strengthening. This method has a high carbon equivalent, poor welding performance, and a complex alloy system, which is not conducive to industrial production. CN119980058A discloses an 800MPa grade high-strength automobile beam steel, which uses Ti-Nb microalloying and achieves ultra-low sulfur control through LF deep desulfurization and RH vacuum treatment. The product produced by this method has a thin thickness (3~5mm), a limited scope of application, and difficult to control process parameters, resulting in poor production stability.

[0004] While existing technologies have achieved the development of 800-900 MPa grade automotive beam steel, they still suffer from common issues such as high alloy costs, complex processes, and poor production stability. This makes it difficult to balance comprehensive mechanical properties with production costs while ensuring high strength. Therefore, developing an 850 MPa grade hot-rolled automotive beam steel with excellent overall performance, low production costs, and suitability for industrial production is of great practical significance and market value. Summary of the Invention

[0005] The technical problem to be solved by the present invention is the poor comprehensive performance of the existing 850MPa grade hot-rolled automobile beam steel.

[0006] To achieve the above application objectives, the technical solutions adopted in this application are as follows: In a first aspect, the present invention provides an 850 MPa grade hot-rolled automobile beam steel, wherein the chemical composition thereof is, by mass percentage, as follows: C 0.050%-0.090%, Si 0.04%-0.12%, Mn 1.50%-1.90%, P≤0.020%, S≤0.007%, Cr0.22%-0.35%, Nb0.040%-0.060%, Ti 0.090%-0.120%, V 0.060%-0.100%, Als 0.010%-0.060%, N≤0.0050%, and the remainder is Fe and unavoidable impurities; The microstructure of the 850MPa grade hot-rolled automobile beam steel is mainly composed of ferrite and a small amount of pearlite, with a yield strength of 750~829MPa, a tensile strength of 860~900MPa, an elongation after fracture ≥18%, and a full-size impact energy of ≥80J at -20°C.

[0007] Furthermore, the grain size of the above-mentioned 850MPa grade hot-rolled automobile beam steel is 11.0~13.0.

[0008] Furthermore, the thickness specification of the above-mentioned 850MPa grade hot-rolled automobile beam steel is 3~10mm.

[0009] In a second aspect, the present invention provides a method for preparing 850MPa grade hot-rolled automobile beam steel: smelting according to the chemical composition of 850MPa grade hot-rolled automobile beam steel to obtain a casting billet, and then hot-rolling to obtain 850MPa grade hot-rolled automobile beam steel; The chemical composition is calculated by mass percentage as follows: C 0.050%-0.090%, Si 0.04%-0.12%, Mn 1.50%-1.90%, P≤0.020%, S≤0.007%, Cr 0.22%-0.35%, Nb 0.040%-0.060%, Ti 0.090%-0.120%, V 0.060%-0.100%, Als 0.010%-0.060%, N≤0.0050%, and the rest is Fe and unavoidable impurities; The smelting process includes converter → small platform → LF refining → RH vacuum refining → continuous casting; The hot rolling includes the steps of heating the casting billet → rough rolling → finishing rolling → laminar cooling → coiling; Among them, in the billet heating process, the billet out of the furnace temperature is 1250±20℃, and the time in the furnace is 180~400min; In the rough rolling process, 6 passes are carried out, with the deformation of each pass ≥ 20%, and odd passes are used for descaling; In the finishing rolling process, 7 passes of finishing rolling are carried out, the starting rolling temperature of finishing rolling is ≤1050℃, and the final rolling temperature is 850~900℃; In the coiling process, the coiling temperature is 570~620℃.

[0010] In the above converter process, slag blocking is used for tapping steel, and active lime is added along with the steel flow.

[0011] Among them, in the above-mentioned small platform process, aluminum wire is fed according to the oxygen content: when the oxygen content is ≤20ppm, no wire is fed; when the oxygen content is 20-100ppm, 150-250m of aluminum wire is fed; when the oxygen content is greater than 100ppm, more than 250m of aluminum wire is fed.

[0012] Among them, in the above-mentioned LF refining process, white slag is desulfurized, argon is blown throughout the process, and alloy Si, Mn and Nb are added after oxygen is blown to a carbon content of ≤0.050%.

[0013] Among them, in the above-mentioned RH vacuum refining process, the vacuum treatment time is ≥10 minutes, sponge titanium is used for alloying, the circulation time after alloying is ≥5 minutes, and 250~350m of iron-calcium wire is fed, and the blowing time after feeding the wire is ≥6 minutes.

[0014] In the above-mentioned continuous casting process, protective casting is adopted, the fluctuation of the liquid level in the crystallizer is controlled within ±10 mm, and a light pressure of 3 to 7 mm is applied, and the pulling speed is set to 0.8 to 1.6 m / min.

[0015] Furthermore, in the above-mentioned casting billet heating process, cold charging is adopted.

[0016] Furthermore, in the above rough rolling process, six rough rolling passes are performed.

[0017] Furthermore, in the above-mentioned rough rolling process, the descaling water pressure is ≥20MPa.

[0018] Furthermore, in the above-mentioned rough rolling process, the thickness of the intermediate billet is 39-58 mm.

[0019] Furthermore, in the above-mentioned rough rolling process, ultra-fast cooling of the intermediate billet is used.

[0020] Furthermore, in the above-mentioned finishing rolling process, 7 finishing rolling passes are performed.

[0021] Furthermore, in the above laminar cooling process, a front-stage cooling mode is adopted.

[0022] Furthermore, in the above laminar cooling process, the cooling manifolds are opened in a two-in-two mode or in a continuous and concentrated mode.

[0023] The beneficial effects of the present invention are: The application does not add expensive alloy elements such as Mo, Cu and Ni, and mainly relies on the composite precipitation and fine grain strengthening effect of alloy elements such as Nb, Ti and V to realize strengthening, so that the raw material cost is effectively reduced; the application ensures the accurate control of the composition and the high purity of the molten steel through smelting process control, and ensures the uniformity and stability of the organization through hot rolling process control; the application adopts a conventional hot rolling process, does not need additional heat treatment, and has lower production process cost, and is more suitable for large-scale industrial production and promotion.

[0024] The application obtains the organization mainly composed of fine ferrite and a small amount of pearlite through specific component design and combined control of rolling and cooling process, and the grain size reaches 11.0-13.0 grade. The yield strength of the final product is 750-829 MPa, the tensile strength is 860-900 MPa, the elongation after fracture is greater than or equal to 18%, the full-size impact energy at-20 DEG C is greater than or equal to 80 J, the product has excellent plasticity, low temperature toughness and forming performance, meets the application requirements of automobile beam steel, and has application and promotion value. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a metallographic structure diagram of the upper surface of the 850 MPa grade hot-rolled automobile beam steel prepared in example 1; Figure 2 is a metallographic structure diagram of the 1 / 4 thickness of the 850 MPa grade hot-rolled automobile beam steel prepared in example 1; Figure 3 is a metallographic structure diagram of the core of the 850 MPa grade hot-rolled automobile beam steel prepared in example 1. DETAILED DESCRIPTION

[0026] In order to make the technical problems, technical solutions and beneficial effects of the present application clearer, the present application will be further described in detail in combination with the embodiments. Unless otherwise defined, all the scientific and technical terms used in the present application have the same meanings as understood by those skilled in the art.

[0027] An 850MPa grade hot-rolled automobile beam steel has the following chemical compositions, calculated by weight percentage: C: 0.050%-0.090%, Si: 0.04%-0.12%, Mn: 1.50%-1.90%, P≤0.020%, S≤0.007%, Cr: 0.22%-0.35%, Nb: 0.040%-0.060%, Ti: 0.090%-0.120%, V: 0.060%-0.100%, Al: 0.010%-0.060%, N≤0.0050%, and the remainder being Fe and unavoidable impurities. The 850MPa grade hot-rolled automobile beam steel has a thickness specification of 3 to 10mm; the microstructure is mainly composed of ferrite and a small amount of pearlite, and the grain size grade is 11.0 to 13.0; the yield strength is 750 to 829MPa, the tensile strength is 860 to 900MPa, the elongation after fracture is ≥18%, and the full-size impact energy at -20°C is ≥80J.

[0028] The smelting method of the above-mentioned 850MPa grade hot-rolled automobile beam steel includes converter → small platform → LF → RH → continuous casting.

[0029] Among them, in the converter process, slag blocking is used for tapping, and active lime is added along with the steel flow to prevent slag from returning to phosphorus and ensure the purity of the molten steel.

[0030] Among them, in the small platform process, in order to avoid the formation of harmful oxide inclusions, precise deoxidation is achieved through fixed oxygen feeding. Specifically, when the oxygen content is ≤20ppm, no wire feeding is performed; when the oxygen content is between 20 and 100ppm, 150~250 meters of aluminum wire is fed; when the oxygen content is greater than 100ppm, >250 meters of aluminum wire is fed.

[0031] Among them, in the LF process, white slag is desulfurized, argon is blown throughout the process, and alloy Si, Mn, and Nb are added after oxygen is blown until C≤0.050%. The carbon content is controlled below the target range, and then the alloy is added to avoid Nb oxidation and burning.

[0032] Among them, in the RH process, vacuum refining treatment is ≥10min to remove gases such as hydrogen, oxygen, and nitrogen and purify the molten steel; sponge titanium is used for alloying, and the vacuum refining cycle time after alloying is not less than 5min to ensure that the titanium element is evenly distributed and fully combined with nitrogen to form fine TiN. Too short a time will lead to uneven composition, coarse TiN or inadequate reaction; 250~350m of iron-calcium wire is fed, and after the wire feeding is completed, it needs to be blown for ≥6min.

[0033] Among them, in the continuous casting process, in order to prevent secondary oxidation of molten steel, open pouring is not allowed; the fluctuation of the liquid level in the crystallizer shall not be greater than 10mm to avoid the formation of inclusions; light pressure is used, and the pressure reduction is set to 3~7mm to reduce the center segregation of the casting and improve the uniformity of the composition; the pulling speed is set to 0.8~1.6m / min.

[0034] The hot rolling method of the above-mentioned 850MPa grade hot-rolled automobile beam steel comprises the steps of heating the ingot → rough rolling → finishing rolling → laminar cooling → coiling.

[0035] Among them, in the billet heating process, cold charging is adopted, and the billet discharge temperature is controlled to be 1250±20℃, so that the carbonitrides of Nb and Ti are partially dissolved, and the furnace time is controlled to be 180~400min.

[0036] Among them, in the rough rolling process, 6 rough rolling passes are carried out, and the deformation of a single pass is controlled to be ≥20%, so that the austenite grains are fully recrystallized and refined in the austenite recrystallization zone; odd passes are used for descaling, and the descaling water pressure is ≥20MPa; the thickness of the intermediate billet is 42~58mm, and ultra-fast cooling of the intermediate billet is used to prevent the growth of austenite grains during the waiting process.

[0037] Among them, in the finishing rolling process, 7 finishing rolling passes are carried out, the starting rolling temperature of the finishing rolling is ≤1050℃, and the final rolling temperature is 850~900℃. When rolling in this temperature range, the austenite is in the non-recrystallization zone, and the rolling deformation will form a large number of deformation bands and distortion energy, increase the phase deformation nucleus points, and obtain fine ferrite grains (grade 11~13), thereby achieving high strength and high toughness.

[0038] Among them, in the laminar cooling process, the front cooling mode is adopted to facilitate rapid passage through the pearlite transformation zone, inhibit the formation of coarse pearlite, and ensure that the phase transformation is completed before the coiling temperature.

[0039] During the coiling process, controlling the coiling temperature between 570°C and 620°C produces a uniform, fine F+P structure. V carbonitrides disperse and precipitate at this temperature, resulting in precipitation strengthening. Coiling temperatures that are too high can coarsen pearlite and reduce strength and toughness; while temperatures that are too low can introduce bainite / martensite, leading to a sudden drop in plasticity and toughness and flexural cracking.

[0040] Specific examples will be listed below to explain the scheme of the present invention. Those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Where specific techniques or conditions are not specified in the examples, they are carried out according to the techniques or conditions described in the literature in this area or according to the product specifications. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be obtained commercially.

[0041] The chemical compositions of the alloys described in Examples 1 to 3 of the present invention and Comparative Examples 1 to 2 are shown in Table 1, with the remainder being Fe and unavoidable impurities.

[0042] Table 1 Chemical composition (by mass percentage, %)

[0043] Example 1: Preparation of 850 MPa hot-rolled automobile beam steel The ingots are smelted according to the composition shown in Table 1, including converter → small platform → LF → RH → continuous casting. In the converter process, slag is blocked for tapping, and active lime is added along with the steel flow. In the small platform process, 200 meters of aluminum wire are fed. In the LF process, white slag is made for desulfurization, and argon is blown throughout the process. After oxygen is blown until C ≤ 0.040%, alloys Si, Mn, and Nb are added. In the RH process, vacuum refining treatment is carried out for 15 minutes, and sponge titanium is used for alloying. After alloying, vacuum refining is carried out for 5 minutes, and 265 meters of iron-calcium wire is fed. After the wire feeding is completed, the blow is continued for 10 minutes. In the continuous casting process, open pouring is not allowed, and the fluctuation of the liquid level in the crystallizer shall not be greater than 10 mm. Soft reduction is used, the reduction is set to 5 mm, and the pulling speed is set to 1 m / min.

[0044] The ingot is hot-rolled to produce 850MPa-grade hot-rolled automotive beam steel. The process includes ingot heating, rough rolling, finishing rolling, laminar cooling, and coiling. Cold charging is used during the ingot heating process, with the ingot exiting the furnace at 1253°C and a furnace time of 299 minutes. Rough rolling is performed in six passes, with deformations of 20.25%, 20.34%, 21.10%, 23.43%, 20.45%, and 26.34% respectively. Descaling is performed in every other pass, with a water pressure of ≥20MPa. The intermediate billet thickness is 53mm, and ultra-rapid cooling of the intermediate billet is used. Finishing rolling is performed in seven passes, with a start temperature of 1028°C and a final rolling temperature of 880°C. Laminar cooling utilizes a front-end cooling mode, with the cooling manifolds open in two sections, and distributed operation. The coiling temperature is 590°C.

[0045] According to the above composition and process, 850MPa grade hot-rolled automobile beam steel was prepared, and the microstructure of different parts is as follows: Figures 1 to 3 As shown in FIG. 2 , it can be seen that the microstructure of the 850 MPa grade hot-rolled automobile beam steel is mainly composed of ferrite and a small amount of pearlite, and the grain size is 12.0-13.0. The mechanical properties are shown in Table 2.

[0046] Example 2: Preparation of 850MPa hot-rolled automobile beam steel The ingots are smelted according to the composition shown in Table 1, including converter → small platform → LF → RH → continuous casting. In the converter process, slag is blocked for tapping, and active lime is added along with the steel flow. In the small platform process, 200 meters of aluminum wire are fed. In the LF process, white slag is made for desulfurization, and argon is blown throughout the process. After oxygen is blown until C ≤ 0.040%, alloys Si, Mn, and Nb are added. In the RH process, vacuum refining treatment is carried out for 16 minutes, and sponge titanium is used for alloying. After alloying, vacuum refining is carried out for 7 minutes, and 255 meters of iron-calcium wire is fed. After the wire feeding is completed, the blowing is continued for 8 minutes. In the continuous casting process, open pouring is not allowed, and the fluctuation of the liquid level in the crystallizer shall not be greater than 10 mm. Soft reduction is used, the reduction is set to 5 mm, and the pulling speed is set to 1.5 m / min.

[0047] The ingot is hot-rolled to produce 850MPa-grade hot-rolled automotive beam steel. The process involves ingot heating, rough rolling, finishing rolling, laminar cooling, and coiling. Cold charging is used during the ingot heating process, with the ingot exiting the furnace at 1256°C and a furnace time of 288 minutes. The rough rolling process involves six passes, with deformations of 20.26%, 20.50%, 21.28%, 23.58%, 20.45%, and 25.77%, respectively. Descaling is performed in every pass, with a water pressure of ≥20MPa. The intermediate bar thickness is 53mm, and ultra-rapid cooling of the intermediate bar is employed. The finishing rolling process involves seven passes, with a start temperature of 1034°C and a final rolling temperature of 880°C. Laminar cooling utilizes a front-end cooling mode with the cooling manifolds continuously and centrally open. The coiling temperature is 590°C.

[0048] According to the above composition and process, 850MPa grade hot-rolled automobile beam steel was prepared, and its mechanical properties are shown in Table 2.

[0049] Example 3: Preparation of 850 MPa hot-rolled automobile beam steel The ingots are smelted according to the composition shown in Table 1, including converter → small platform → LF → RH → continuous casting. In the converter process, slag is blocked for tapping, and active lime is added along with the steel flow. In the small platform process, 200 meters of aluminum wire are fed. In the LF process, white slag is desulfurized, and argon is blown throughout the process. After oxygen is blown until C ≤ 0.040%, alloys Si, Mn, and Nb are added. In the RH process, vacuum refining treatment is carried out for 15 minutes, and sponge titanium is used for alloying. After alloying, vacuum refining is carried out for 6 minutes, and 270 meters of iron-calcium wire is fed. After the wire feeding is completed, the blown steel is blown for 9 minutes. In the continuous casting process, open pouring is not allowed, the crystallizer liquid level fluctuation shall not be greater than 10 mm, and light reduction is used, with a set reduction of 5 mm and a pulling speed of 1.1 m / min.

[0050] The ingot is hot-rolled to produce 850MPa-grade hot-rolled automotive beam steel. The process includes ingot heating, rough rolling, finishing rolling, laminar cooling, and coiling. Cold charging is used during the ingot heating process, with the ingot exiting the furnace at 1256°C and a furnace time of 295 minutes. The rough rolling process involves six passes, with deformations of 22.68%, 21.27%, 21.31%, 30.15%, 32.54%, and 29.08% respectively. Descaling is performed in every pass, with a water pressure of ≥20MPa. The intermediate bar thickness is 39mm, and ultra-rapid cooling of the intermediate bar is used. The finishing rolling process involves seven passes, with a start temperature of 1048°C and a final rolling temperature of 870°C. Laminar cooling utilizes a front-end cooling mode with the cooling manifolds continuously and centrally open. The coiling temperature is 600°C.

[0051] According to the above composition and process, 850MPa grade hot-rolled automobile beam steel was prepared, and its mechanical properties are shown in Table 2.

[0052] Comparative Example 1: Ingots were smelted according to the composition shown in Table 1, and the ingots were hot-rolled to obtain hot-rolled steel. The smelting and hot-rolling processes were controlled according to the requirements of the present invention. The mechanical properties are shown in Table 2.

[0053] Comparative Example 2: Ingots were smelted according to the composition shown in Table 1, and the ingots were hot-rolled to obtain hot-rolled steel. The smelting and hot-rolling processes were controlled according to the requirements of the present invention. The mechanical properties are shown in Table 2.

[0054] The yield strength, tensile strength and elongation of the 850MPa grade hot-rolled automobile beam steel are tested in accordance with the "Tensile Test of Metallic Materials Part 1: Room Temperature Test Method" (GB / T 228.1); the bending performance is tested in accordance with the "Bending Test Method for Metallic Materials" (GB / T232); and the impact performance is tested in accordance with the "Charpy Pendulum Impact Test Method for Metallic Materials" (GB / T 229).

[0055] Table 2 Mechanical properties

[0056] As shown in Tables 1 and 2, the microalloying content of Comparative Example 1 exceeds the lower limit specified in the present invention, the harmful element content is not controlled satisfactorily, the microalloying strengthening effect is not significant, and the strength does not reach the 850 MPa level. The microalloying content of Comparative Example 2 exceeds the upper limit specified in the present invention, resulting in an unbalanced balance of strength and ductility, a high yield strength ratio, and failure to pass the 180° bend test.

[0057] By comparing the above embodiments and comparative examples, it can be seen that the hot-rolled steel plate prepared according to the composition and method of the present invention has a strength of more than 850 MPa, and at the same time has good comprehensive mechanical properties, meets the application requirements of high-strength automobile beam steel, and has good application and promotion value.

Claims

1. 850MPa grade hot-rolled automobile beam steel, characterized in that: Its chemical composition is calculated by mass percentage as follows: C 0.050%~0.090%, Si 0.04%~0.12%, Mn 1.50%~1.90%, P≤0.020%, S≤0.007%, Cr 0.22%~0.35%, Nb0.040%~0.060%, Ti 0.090%~0.120%, V 0.060%~0.100%, Als 0.010%~0.060%, N≤0.0050%, and the rest is Fe and unavoidable impurities; The microstructure of the 850MPa grade hot-rolled automobile beam steel is mainly composed of ferrite and a small amount of pearlite, with a yield strength of 750~829MPa, a tensile strength of 860~900MPa, an elongation after fracture ≥18%, and a full-size impact energy of ≥80J at -20°C.

2. The 850MPa grade hot-rolled automobile beam steel according to claim 1, characterized in that: The grain size of the 850MPa grade hot-rolled automobile beam steel is 11.0-13.

0.

3. The 850MPa grade hot-rolled automobile beam steel according to claim 1, characterized in that: The thickness specification of the 850MPa grade hot-rolled automobile beam steel is 3-10mm. The method for preparing 4.850MPa grade hot-rolled automobile beam steel is characterized by: The smelting process is carried out according to the chemical composition of 850MPa grade hot-rolled automobile beam steel to obtain a casting billet, and then hot-rolled to obtain 850MPa grade hot-rolled automobile beam steel; The chemical composition is calculated by mass percentage as follows: C 0.050%-0.090%, Si 0.04%-0.12%, Mn 1.50%-1.90%, P≤0.020%, S≤0.007%, Cr 0.22%-0.35%, Nb 0.040%-0.060%, Ti 0.090%-0.120%, V 0.060%-0.100%, Als 0.010%-0.060%, N≤0.0050%, and the rest is Fe and unavoidable impurities; The smelting process includes converter → small platform → LF refining → RH vacuum refining → continuous casting; The hot rolling includes the steps of heating the casting billet → rough rolling → finishing rolling → laminar cooling → coiling; Among them, in the billet heating process, the billet out of the furnace temperature is 1250±20℃, and the time in the furnace is 180~400min; In the rough rolling process, 6 passes are carried out, with the deformation of each pass ≥ 20%, and odd passes are used for descaling; In the finishing rolling process, 7 passes of finishing rolling are carried out, the starting rolling temperature of finishing rolling is ≤1050℃, and the final rolling temperature is 850~900℃; In the coiling process, the coiling temperature is 570~620℃.

5. The method for preparing 850MPa grade hot-rolled automobile beam steel according to claim 4, characterized in that: In the converter process, slag blocking is adopted for tapping steel, and active lime is added along with the steel flow.

6. The method for preparing 850 MPa grade hot-rolled automobile beam steel according to claim 4, characterized in that: In the small platform process, aluminum wire is fed according to the oxygen content: when the oxygen content is ≤20ppm, no wire is fed; when the oxygen content is 20-100ppm, 150-250m of aluminum wire is fed; when the oxygen content is greater than 100ppm, more than 250m of aluminum wire is fed.

7. The method for preparing 850 MPa grade hot-rolled automobile beam steel according to claim 4, characterized in that: In the LF refining process, white slag is desulfurized, argon is blown throughout the process, and alloy Si, Mn, and Nb are added after oxygen is blown until the carbon content is ≤0.050%.

8. The method for preparing 850 MPa grade hot-rolled automobile beam steel according to claim 4, characterized in that: In the RH vacuum refining process, the vacuum treatment time is ≥10 min, titanium sponge is used for alloying, the circulation time after alloying is ≥5 min, 250-350 m of iron-calcium wire is fed, and the blowing time after feeding the wire is ≥6 min.

9. The method for preparing 850 MPa grade hot-rolled automobile beam steel according to claim 4, characterized in that: In the continuous casting process, protective casting is adopted, the fluctuation of the liquid level in the crystallizer is controlled within ±10 mm, a light pressure of 3 to 7 mm is applied, and the casting speed is set to 0.8 to 1.6 m / min.

10. The method for preparing 850MPa grade hot-rolled automobile beam steel according to claim 4, characterized in that: The hot rolling satisfies at least one of the following: In the billet heating process, cold charging is adopted; In the rough rolling process, 6 passes of rough rolling are performed; In the rough rolling process, the descaling water pressure is ≥20MPa; In the rough rolling process, the intermediate billet thickness is 39~58mm; In the rough rolling process, ultra-fast cooling of the intermediate billet is used; In the finishing rolling process, 7 passes of finishing rolling are performed; In the laminar cooling process, the front cooling mode is adopted.

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