Hot-rolled ultra-high strength steel for automobiles, method for manufacturing the same, and hot forming method and cold forming method

Abstract

The application discloses a hot-rolled ultra-high-strength automobile steel, a manufacturing method, a hot forming method and a cold forming method, and belongs to the technical field of automobile steels.The automobile steel comprises the following chemical components in percentage by weight: C: 0.22-0.27%, Si: 1.2-1.8%, Cr: 2.2-2.6%, Mn: 1.0-1.4%, Nb: 0.02-0.03%, Al: 0.02-0.05%, P: 0.020% or less, S: 0.010% or less, H: 0.0003% or less, and the balance of Fe and inevitable impurities.The automobile steel can be directly cold formed or hot formed, the tensile strength of the automobile steel can reach 1500 MPa or more when the automobile steel is cold formed, the tensile strength of the automobile steel can reach 1800 MPa or more after the automobile steel is hot formed, and no precious metal is added, so that the production cost is low.

Description

Technical Field

[0001] The present invention belongs to the technical field of automobile steel, and in particular relates to a hot-rolled ultra-high-strength automobile steel, a manufacturing method thereof, and a hot forming method and a cold forming method. Background Art

[0002] Energy conservation, environmental protection, and safety are the three major challenges facing the automotive industry today. Lightweighting can reduce vehicle weight and energy consumption without sacrificing safety. Furthermore, lightweighting can improve vehicle handling stability and, to a certain extent, collision safety. High-strength steel is a key material choice for lightweighting vehicles.

[0003] Currently, high-strength or ultra-high-strength steel for automotive applications is categorized into two categories: cold forming and hot forming. Cold forming involves stamping and rolling. Stamping involves placing the material into a mold and directly stamping it into shape. This has the advantages of simple processing and low cost, but it is subject to springback and forming limits for ultra-high-strength steel. Roll forming involves passing the strip through multiple sets of rollers, gradually deforming it. This gradual, small-increment deformation avoids concentrated stress that can cause cracking, making it suitable for ultra-high-strength steel parts with simple cross-sections and long dimensions. Hot forming, on the other hand, is a new process that separates forming and strengthening into two steps to produce ultra-high-strength automotive parts. It is suitable for the production of ultra-high-strength parts with strengths above 1500MPa. The parts produced have the advantages of ultra-high strength, high forming precision, and no springback, but the production process is relatively complex.

[0004] Usually, cold forming and hot forming are two different product series with quite different composition designs. They need to be produced with different steel grades, which increases procurement costs. In addition, the maximum strength of the currently mature cold-formed ultra-high-strength steel hot-rolled plates for automotive use is around 800~1000MPa. Strengths above 1000MPa often require advanced roll forming technology, while ultra-high-strength steels with even higher strengths above 1500MPa often need to be achieved through hot forming.

[0005] Chinese patent CN111996465A discloses a hot-rolled ultra-high-strength lightweight medium-manganese steel sheet for automotive use and its preparation method. Its chemical composition is 0.75-0.80% C, 7.6-8.0% Mn, 7.4-8.0% Al, and 0.05-0.10% Si, with the balance being Fe and unavoidable impurities. The C, Mn, and Al content of this sheet far exceeds that of existing automotive steels, resulting in high manufacturing costs and poor weldability. The sheet is cold-formed, making it unsuitable for large-scale deployment.

[0006] Chinese patent CN116200655A discloses an oxidation-resistant hot-formed steel and its production method. Its chemical composition is: C: 0.20-0.24%, Mn: 0.8-2.0%, Ti: 0.015-0.040%, P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.0050%, Si + Cr: 2.0-4.0%, B: 0.0015-0.0040%, and a rare earth content of 0.0015-0.0040%. The rare earth elements include, but are not limited to, one or more of Ce, Y, and La, with the remainder being Fe and unavoidable impurities. The steel is hot-formed, achieving a tensile strength of ≥1500 MPa after hot forming. The addition of 0.0015-0.0040% of rare earth elements to the chemical composition increases manufacturing cost and difficulty.

[0007] In order to save the production cost of steel mills and the procurement cost of automobile factories, it is necessary to develop a hot-rolled ultra-high-strength automobile steel that can be used for both cold forming and hot forming. Summary of the Invention

[0008] In order to solve the above technical problems, the present invention provides a hot-rolled ultra-high-strength automotive steel, its manufacturing method, hot forming method and cold forming method. The automotive steel can be directly cold formed or hot formed. Regardless of the forming method used, its tensile strength can reach above 1500MPa. At the same time, no precious metals are added, and the production cost is low.

[0009] The technical solution adopted by the present invention is as follows:

[0010] A hot-rolled ultra-high-strength automotive steel, comprising the following chemical components in weight percentage: C: 0.22-0.27%, Si: 1.2-1.8%, Cr: 2.2-2.6%, Mn: 1.0-1.4%, Nb: 0.02-0.03%, Al: 0.02-0.05%, P≤0.020%, S≤0.010%, H≤0.0003%, and the balance being Fe and unavoidable impurities;

[0011] The metallographic structure of the hot-rolled plate of the ultra-high-strength automobile steel is martensite + a small amount of ferrite, wherein the volume percentage of ferrite is 5-15%, and the volume percentage of martensite is 85-95%.

[0012] The thickness of the iron oxide scale on the surface of the hot-rolled plate of the ultra-high-strength automobile steel is less than 20 μm, and a dense oxide film no thicker than 1 μm is formed on the surface of the steel plate after hot forming.

[0013] The tensile strength of the hot-rolled plate of the ultra-high-strength automobile steel is ≥1500 MPa, and the tensile strength after hot forming is ≥1800 MPa.

[0014] The present invention also provides a method for manufacturing hot-rolled ultra-high-strength automobile steel, which comprises the following steps: steelmaking → continuous casting → hot rolling → laminar cooling → coiling.

[0015] The steelmaking process specifically includes molten iron pretreatment → converter smelting → alloy fine-tuning station → LF refining → RH vacuum treatment. Other arbitrary steelmaking processes can also be used to smelt the molten steel with the composition requirements of the present invention.

[0016] During the continuous casting step, the fluctuation of the steel liquid level in the crystallizer is controlled within ±3 mm, dynamic soft reduction is used, and electromagnetic rollers are used for stirring to make impurities float up to obtain good internal quality of the ingot.

[0017] During the hot rolling step, in order to reduce cracks in the ingot and save energy, the ingot is hot-charged into the furnace for heating at a temperature of 1200-1250° C. and kept warm for 2-3 hours, thereby effectively reducing the rolling deformation resistance and reducing the thickness of the decarburization layer and the oxide layer on the surface of the ingot and the steel plate.

[0018] During the hot rolling process, rough rolling is performed in 3+3 passes with descaling fully enabled. The finishing rolling inlet temperature is 1030-1050°C, and the rolls are replaced before finishing. Finishing rolling of the automotive steel is performed within the first half of a rolling cycle to ensure the surface quality of the hot-rolled product. To achieve a fine and uniform microstructure and reduce the formation of iron oxide scale, the finishing rolling temperature is controlled at 860-930°C, and rolling is performed to the target thickness.

[0019] Further, it is rough rolled to a thickness of 30-50 mm.

[0020] In the laminar cooling step, cooling is performed in a front-stage cooling mode.

[0021] In the coiling step, the coiling temperature is 540-600°C.

[0022] The present invention also provides a hot forming method for hot-rolled ultra-high strength automobile steel, wherein the hot-rolled sheet of the automobile steel is heated to 860-930° C. for 210-450 seconds and then subjected to die quenching.

[0023] The metallographic structure of the hot-rolled ultra-high-strength automobile steel after hot forming is 100% martensite.

[0024] The present invention also provides a cold forming method for hot-rolled ultra-high-strength automotive steel, wherein the hot-rolled plate of the hot-rolled ultra-high-strength automotive steel is roll-cold-formed to a desired cross-sectional shape, wherein the deformation amount per roll-cold-forming process is ≤30°, and the relative bending radius R / t is ≥3, wherein R is the bending radius and t is the plate thickness.

[0025] The roll cold forming includes: uncoiling, leveling, progressive roll forming, and cut-to-length processes.

[0026] Furthermore, the flattening step further includes pre-punching; the progressive roll forming step further includes welding; and the cut-to-length step further includes shaping.

[0027] The control and effects of the chemical components in the automotive steel provided by the present invention are as follows:

[0028] C: Carbon is the main element that improves the strength of martensite. Too low a carbon content will result in insufficient steel plate strength, while too high a carbon content will be detrimental to weldability. In the present invention, the carbon content is controlled to be 0.22-0.27%.

[0029] Si and Cr: Silicon is a preferentially oxidized element and has a high diffusion coefficient. Si and Cr are easily co-enriched on the surface of the steel plate. When heated at 900℃~1100℃, surface element enrichment and oxidation mainly occur. The surface-enriched Si and Cr are first oxidized and grow flatly at the interface, forming a continuous and dense oxide film (SiO2 and Cr2O3), which can isolate oxygen and play an antioxidant role. At the same time, the Cr element can significantly improve the hardenability of the material, so that the finished product has higher strength. The preferred Si content of the present invention is controlled at 1.2~1.8%, and the Cr content is controlled at 2.2~2.6%.

[0030] Mn: The Mn element can improve the strength of steel through solid solution strengthening, and at the same time can promote the dissolution of carbonitride precipitation phase during heating, inhibit the precipitation phase during rolling, which is beneficial to keep more precipitation elements precipitated in ferrite during the cooling process after rolling, thereby enhancing precipitation strengthening. In addition, Mn can also expand the austenite phase region, reduce the transformation temperature of the supercooled austenite phase, and be beneficial to the refinement of the phase transformation structure. However, the Mn content should not be too high. When it is higher than 1.40%, Mn segregation is likely to occur in steelmaking, and edge cracks are likely to occur during slab continuous casting. At the same time, the segregation of the steel structure will be aggravated to form a banded structure, which will reduce the plasticity and toughness of the steel. In order to ensure the strength of the steel plate and the quality of the finished product, the Mn content should be reasonably controlled within 1.0~1.4%.

[0031] Nb: Its purpose is to refine grains and improve toughness by forming dispersed, fine precipitates during heating and rolling. A Nb content that is too low will have no effect and the finished product strength cannot be guaranteed. A high Nb content will increase costs and hinder expanded applications. The preferred Nb content in this invention is 0.02-0.03%.

[0032] Al: Adding a small amount of aluminum during smelting can be used for deoxidation, but too much Al will block the nozzle during continuous casting, increasing the difficulty of continuous casting. Therefore, the Al content should be controlled at 0.02~0.05%.

[0033] P and S are impurity elements in steel. The lower the content, the better. However, too low a P content will increase steelmaking costs. Therefore, the P content is controlled below 0.02% to meet production cost and product requirements. S usually combines with Mn in steel to form MnS inclusions. The amount and form of sulfides in steel directly affect the hole expansion and tensile properties of complex phase steel. Therefore, the S content is controlled below 0.01% in actual production.

[0034] H: H is a harmful element in steel and will cause adverse effects such as porosity during welding. Therefore, the lower its content, the better. However, too low H content will increase steelmaking costs, so the H content is controlled below 0.0003%.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The tensile strength of the hot-rolled sheet of automobile steel produced by the solution of the present invention reaches above 1500 MPa, and after direct cold forming, it can meet the requirements of automobile steel with a tensile strength of 1500 MPa.

[0037] The automobile steel provided by the present invention can achieve a strength of more than 1800 MPa after hot forming, which can better achieve the lightweighting and weight reduction effect of automobiles.

[0038] The automotive steel provided by this invention incorporates Cr and Si. When hot-formed at temperatures above 900°C, a dense oxide film (SiO2 and Cr2O3) less than 1 μm thick forms on the steel surface, protecting the substrate from high-temperature oxidation and providing excellent high-temperature corrosion resistance. This process eliminates the need for shot blasting after hot forming, contributing to environmental friendliness and cost reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the metallographic structure diagram of the hot-rolled plate of automobile steel in Example 1;

[0040] Figure 2 This is an SEM image of the hot-rolled plate of the automobile steel in Example 1;

[0041] Figure 3 This is the metallographic structure diagram of the hot-rolled plate of automobile steel after hot forming in Example 1;

[0042] Figure 4 This is a SEM image of the hot-rolled sheet of automobile steel in Example 1 after hot forming. DETAILED DESCRIPTION

[0043] The present invention is described in detail below with reference to the embodiments.

[0044] The present invention provides a hot-rolled ultra-high-strength automobile steel, comprising the following chemical components in weight percentage: C: 0.22-0.27%, Si: 1.2-1.8%, Cr: 2.2-2.6%, Mn: 1.0-1.4%, Nb: 0.02-0.03%, Al: 0.02-0.05%, P≤0.020%, S≤0.010%, H≤0.0003%, and the balance being Fe and unavoidable impurities.

[0045] The method for manufacturing hot-rolled ultra-high-strength automobile steel comprises the following steps: molten iron pretreatment → converter smelting → alloy fine-tuning station → LF refining → RH vacuum treatment → continuous casting → hot rolling → laminar cooling → coiling.

[0046] During the continuous casting step, the fluctuation of the steel liquid level in the crystallizer is controlled within ±3 mm, dynamic soft reduction is used, and electromagnetic rollers are used for stirring to make impurities float up to obtain good internal quality of the ingot.

[0047] In the hot rolling step, the slab is hot-charged into the furnace for heating at a temperature of 1200-1250° C. and kept warm for 2-3 hours.

[0048] In the hot rolling step, the rough rolling is performed in 3+3 passes with the descaling machine fully open, and the rough rolling is performed to a thickness of 30-50 mm; the finishing rolling inlet temperature is 1030-1050° C., and the final rolling temperature is controlled at 860-930° C. to roll to the target thickness.

[0049] In the laminar cooling step, cooling is performed in a front-stage cooling mode.

[0050] In the coiling step, the coiling temperature is 540-600°C.

[0051] The present invention is described in detail below with reference to the embodiments.

[0052] The chemical composition and weight percentage of the automobile steel in each embodiment and comparative example are shown in Table 1, with the remainder being iron and unavoidable impurities.

[0053]

[0054] The production process parameters of the automotive steel in each embodiment and comparative example are shown in Table 2.

[0055]

[0056] The mechanical properties and metallographic structures of the hot-rolled plates of automobile steel obtained in each embodiment and comparative example are shown in Table 3. The thickness of the hot-rolled plates of automobile steel obtained in each embodiment and comparative example is 2 mm.

[0057]

[0058] As can be seen from Table 3, the tensile strength of the hot-rolled steel plate for automobiles produced by the solution of the present invention reaches above 1500 MPa.

[0059] The hot-rolled plates of automobile steel obtained in each embodiment and comparative example were heated to 860-930° C. and kept at this temperature for 210s to 450s, and then die-quenched and hot-formed. The mechanical properties and metallographic structures after hot forming are shown in Table 4.

[0060]

[0061] Table 4 shows that the strength of the automobile steel provided by the present invention can reach above 1800 MPa after hot forming, which can better achieve the lightweighting effect of the automobile.

[0062] The automotive steel provided by the present invention can also be directly cold-formed. The specific method is: the hot-rolled plate of the automotive steel is roll-cold-formed to the desired cross-sectional shape, wherein the deformation amount of each roll-cold-forming process is ≤30°, and the relative bending radius R / t is ≥3, wherein R is the bending radius and t is the plate thickness.

[0063] The roll cold forming process specifically includes: uncoiling, leveling, pre-punching, progressive roll forming, welding, cutting to length, and shaping processes.

[0064] The aforementioned cold forming process does not alter the original strength of the hot-rolled sheet of automotive steel. The strength of cold-formed automotive steel generally refers to the strength of the substrate. The cold-formed hot-rolled ultra-high-strength steel for automotive use described herein has a tensile strength of ≥1500 MPa. During the cold forming process, some areas undergo work hardening, increasing their strength, while the strength of the undeformed areas remains unchanged. Therefore, the entire cold forming process does not result in a reduction in strength. Therefore, the hot-rolled ultra-high-strength steel described herein can meet the 1500 MPa tensile strength requirement for automotive steel after direct cold forming.

[0065] The following is an introduction to the roll forming process of the bumper anti-collision beam using the hot-rolled steel plate of the present invention for automobile steel:

[0066] First, the 2.0mm hot-rolled steel plate is uncoiled and leveled with a leveler; then pre-punched with a hydraulic punch, and then progressively rolled. The symmetrical cross-section roll forming is carried out in three stages:

[0067] Stage 1 (passes 1-8): Bend the strip gradually upwards to 90° on both sides to form side walls, with a bend radius of 6 mm.

[0068] Stage 2 (passes 9-24): Bend the sidewalls inward again by two 90° steps to form a B-shaped section with a bend radius of 6 mm.

[0069] Phase 3 (passes 25-28): high-frequency welding of longitudinal seams + straightening and shaping.

[0070] Finally, it is cut to length and bent into shape to reach the final design shape.

[0071] The above-mentioned detailed description of a hot-rolled ultra-high-strength automotive steel, its manufacturing method, and hot forming and cold forming methods with reference to the embodiments is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A hot-rolled ultra-high strength automobile steel, characterized in that: The ultra-high-strength automotive steel comprises the following chemical components in weight percentage: C: 0.22-0.27%, Si: 1.2-1.8%, Cr: 2.4-2.6%, Mn: 1.2-1.3%, Nb: 0.023-0.027%, Al: 0.03-0.042%, P ≤ 0.020%, S ≤ 0.010%, H ≤ 0.0003%, and the balance is Fe and unavoidable impurities; the hot-rolled plate of the ultra-high-strength automotive steel has a metallographic structure of martensite + a small amount of ferrite, wherein the volume percentage of ferrite is 5-15% and the volume percentage of martensite is 85-95%, and the tensile strength of the hot-rolled plate reaches above 1500 MPa; The hot-rolled ultra-high-strength automotive steel is cold-formed to a target cross-section, and its tensile strength after cold forming meets the requirements of 1500 MPa-grade automotive steel; or the hot-rolled ultra-high-strength automotive steel is hot-formed by quenching at 860-930°C, and the metallographic structure after quenching and hot forming is 100% martensite, and the tensile strength after hot forming is ≥1800 MPa.

2. The hot-rolled ultra-high strength automobile steel according to claim 1, characterized in that: The thickness of the iron oxide scale on the surface of the hot-rolled plate of the ultra-high-strength automobile steel is less than 20 μm, and a dense oxide film no thicker than 1 μm is formed on the surface of the steel plate after hot forming.

3. The method for producing hot-rolled ultra-high strength automobile steel according to claim 1 or 2, comprising the steps of steelmaking, continuous casting, hot rolling, laminar cooling, and coiling, wherein: In the hot rolling step, the slab is hot-charged into the furnace for heating at a temperature of 1200-1250° C. and kept warm for 2-3 hours.

4. The method for producing hot-rolled ultra-high strength automobile steel according to claim 3, characterized in that: In the hot rolling step, rough rolling is performed in 3+3 passes with descaling fully on; the finishing rolling inlet temperature is 1030-1050°C, the rolls are replaced before finishing rolling, and the automotive steel is finished in the first 1 / 2 cycle of a rolling cycle, and the final rolling temperature is controlled at 860-930°C.

5. The manufacturing method according to claim 3, characterized in that During the continuous casting step, the fluctuation of the steel liquid level in the crystallizer is controlled within ±3 mm, dynamic soft reduction is used, and electromagnetic rollers are used for stirring.

6. The manufacturing method according to claim 3, characterized in that In the laminar cooling step, cooling is performed in a front-stage cooling mode.

7. The manufacturing method according to claim 3, characterized in that In the coiling step, the coiling temperature is 540-600°C.

8. A hot forming method for hot-rolled ultra-high strength automobile steel, characterized in that: The hot-rolled plate of the hot-rolled ultra-high-strength automotive steel according to claim 1 is heated to 860-930° C. and kept warm for 210-450 seconds, and then die quenched; the metallographic structure of the hot-rolled ultra-high-strength automotive steel after hot forming is 100% martensite.

9. A cold forming method for hot-rolled ultra-high strength automobile steel, characterized in that: The hot-rolled plate of the hot-rolled ultra-high-strength automotive steel according to claim 1 is roll-cold-formed to a desired cross-sectional shape, wherein the deformation amount per roll-cold-forming process is ≤30°, and the relative bending radius R / t is ≥3, wherein R is the bending radius and t is the plate thickness.

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