High-strength high-plasticity ferrite single-phase automobile structural steel and production method thereof

Abstract

The application discloses a high-strength and high-plasticity ferrite single-phase automobile structure steel and a production method thereof, and chemical components and weight percentage contents of the automobile structure steel are as follows: C 0.03-0.06%, Si 0.3-0.6%, Mn 0.5-0.9%, S less than or equal to 0.006%, Al 0.1-0.25%, Ti 0.10-0.13%, Nb 0.01-0.02%, Mo 0.0001-0.003%, and the balance of Fe and inevitable impurities. The automobile structure steel has a thickness of 2-10 mm, a lower yield strength greater than or equal to 600 MPa, a tensile strength of 700-850 MPa, an elongation greater than or equal to 25%, a hole expansion ratio greater than or equal to 75%, and a difference between transverse and longitudinal yield strengths less than or equal to 30 MPa, and a shearing section cold bending radius is half of the thickness, and a cold bending angle is 180 degrees, so that the hot-rolled steel plate has a single ferrite structure, and a surface layer has a high-ductility ferrite fine-grain layer, the steel plate has excellent forming performance, and meets the requirement of high formability of the steel plate when a vehicle enterprise is lightened.

Description

Technical Field

[0001] This invention relates to steel for the automotive industry and its production method, specifically to a high-strength, high-ductility ferritic single-phase automotive structural steel and its production method. Background Technology

[0002] To meet environmental regulations, major commercial vehicle manufacturers are vigorously promoting lightweighting of vehicles. A preliminary search revealed document CN202410477559.7, which discloses a low-temperature, high-toughness 700MPa high-strength beam steel and its production method. The chemical composition is: C 0.055%–0.085%, Si 0.05%–0.18%, Mn 1.20%–1.75%, P≤0.014%, S≤0.004%, Nb 0.035%–0.065%, Ti 0.07%–0.16%, Cr 0.15%–0.30%, Ni 0.12%–0.20%, Als 0.015%–0.045%, N≤0.0040%. The production method includes smelting, refining, continuous casting, heating, rough rolling, finish rolling, laminar flow cooling, and coiling. This design does not consider the microstructure and its impact on molding performance, making it unsuitable for manufacturing parts with high molding requirements.

[0003] Currently, the 700MPa grade hot-rolled high-strength steel used for cold forming in the domestic market has a complex phase transformation process and a multiphase structure due to its high carbon and alloy content. In particular, the surface structure is hardened, which leads to a series of quality problems for car manufacturers during the forming process, such as difficulty in forming parts, easy cracking, and poor punching cross-section. Summary of the Invention

[0004] To address the shortcomings of the existing technology, the main objective of this invention is to provide a high-strength, high-plasticity ferritic single-phase automotive structural steel and its production method. The resulting steel plate has a thickness of 2–10 mm, a lower yield strength ≥600 MPa, a tensile strength of 700–850 MPa, an elongation ≥25%, a hole expansion rate ≥75%, and a difference between transverse and longitudinal yield strengths within 30 MPa. The cold bending radius of the shear section is half the thickness, and the cold bending angle is 180 degrees.

[0005] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0006] A high-strength, high-plasticity ferritic single-phase automotive structural steel has the following chemical composition and weight percentage content: C 0.03-0.06%, Si 0.3-0.6%, Mn 0.5-0.9%, S≤0.006%, Al 0.1-0.25%, Ti 0.10-0.13%, Nb 0.01-0.02%, Mo 0.0001-0.003%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the weight percentage of C is preferably 0.04 to 0.058.

[0008] Furthermore, the weight percentage of Si is preferably 0.4 to 0.55.

[0009] Furthermore, the weight percentage of Mn is preferably 0.55 to 0.76.

[0010] Furthermore, the weight percentage of Ti is preferably 0.12 to 0.128.

[0011] The production method of the aforementioned high-strength, high-ductility ferritic single-phase automotive structural steel includes the following steps:

[0012] 1) After being smelted in a converter, the steel undergoes vacuum treatment for more than 8 minutes, after which it is continuously cast into billets.

[0013] 2) The billet is charged to the furnace using a hot charging method, with an air-fuel ratio requirement of >1.0 and a charging temperature of ≥500℃. The billet is heated to 1220~1260℃ in the heating furnace and held for 100~120 minutes. The billet exiting the furnace should have a temperature difference between the surface and the core controlled within 30℃.

[0014] 3) The heated billet is rolled in sections: the roughing section is rolled in 5 passes, and the rolling temperature of the last pass is controlled at 1000-1040℃; the finishing section is rolled in 7 passes, and the rolling temperature of the last pass is controlled at 820-850℃.

[0015] 4) After rolling, the steel is subjected to high-speed cooling at a rate of 80-180℃ / s. After cooling to 660-680℃, it is air-cooled for 7-15 seconds, and then rapidly cooled to 500-570℃ before being coiled to obtain the high-strength, high-plasticity ferritic single-phase automotive structural steel.

[0016] Furthermore, the billet is hot-charged into the furnace, with an air-fuel ratio preferably of 1.25 to 1.6 and a charging temperature preferably of 520 to 650°C.

[0017] Furthermore, the roughing section is rolled in 5 passes, with the final pass preferably at 1010–1030℃.

[0018] Furthermore, the finishing rolling section is rolled in 7 passes, with the last pass preferably at 821–840℃.

[0019] Furthermore, the air-cooling temperature is preferably in the range of 665–678 °C / s.

[0020] The role of each component and the main process in this invention, and the reasons for their control:

[0021] Carbon: Carbon is an inexpensive solid solution strengthening element. If its content is less than 0.03%, the material strength requirements cannot be met; if its content is greater than 0.05%, the microstructure is prone to pearlite formation, which leads to a decrease in formability. Therefore, its content is limited to the range of 0.03% to 0.06%.

[0022] Silicon: Silicon is an inexpensive solid solution strengthening element. It also stabilizes the ferrite structure of the surface and matrix, improves the anisotropy of the microstructure, and enhances the material's plasticity. If its content is less than 0.3%, the requirements for material strength and anisotropy cannot be met; if its content is greater than 0.6%, it leads to a decrease in the surface quality of the material. Therefore, its content is limited to the range of 0.3% to 0.6%.

[0023] Manganese: Manganese is an effective solid solution strengthening element, but it also lowers the austenite transformation temperature, which is unfavorable for ferrite phase transformation. If its content is less than 0.5%, it cannot meet the material strength requirements; however, adding too much manganese will form pearlite structure, reducing the toughness of steel. Therefore, its upper limit is set at 0.9%, and its content is limited to the range of 0.5% to 0.9%.

[0024] Sulfur: Sulfur is a very harmful element. Sulfur in steel often exists in the form of manganese sulfides. These sulfide inclusions are very detrimental to the formability of steel and cause anisotropy in properties. Therefore, the sulfur content in steel should be controlled below 0.006%.

[0025] Aluminum: Aluminum is a good deoxidizing element and can refine ferrite grains, improving toughness. Its effect is not achieved when the Al content is less than 0.1%; on the other hand, adding too much aluminum reduces the toughness of the steel plate, so the upper limit for Al is specified as 0.25%. Therefore, the Al content is limited to the range of 0.1% to 0.25%.

[0026] Titanium: Titanium is an excellent grain refiner and precipitation strengthening element. When the Ti content is below 0.10%, the precipitates are insufficient and its effect is difficult to achieve. When the Ti content is above 0.13%, the precipitation is excessive and has an adverse effect on the toughness of the steel plate. Therefore, the Ti content is limited to 0.10-0.13%.

[0027] Niobium: Niobium is an excellent grain-refining strengthening element. However, its effect is minimal when the niobium content is below 0.01%, and it tends to form coarse precipitates during rolling, reducing the material's toughness, when the content is above 0.02%. Therefore, the Nb content is limited to 0.01–0.02%.

[0028] Molybdenum: Molybdenum is an excellent solid solution strengthening element. Combined with C, Ti, and Nb, it refines precipitate size and improves strength. However, its effect is minimal when the Mo content is below 0.0001%, and it reduces the toughness of the steel plate when the Mo content is above 0.003%. Therefore, the Mo content is limited to 0.0001–0.003%.

[0029] In addition to limiting the range of the above chemical components, from the point of view of improving the formability and economy of materials, this invention does not add expensive alloying elements such as Cu and Ni.

[0030] The key technical aspects of this invention are the control of the billet heating regime, air-fuel ratio, final rolling temperature, and post-rolling cooling, which facilitate the improvement of the plasticity of the matrix and surface layers. The billet heating temperature and time must ensure sufficient solid solution of alloying elements to guarantee matrix strength, while preventing overheating that leads to coarse grains and reduces the plasticity of the steel matrix. The combination of the billet air-fuel ratio and furnace time allows for the control of the carbon content in the billet surface layer within the heating furnace. Finish rolling ensures a thicker equiaxed ferrite layer on the surface, improving surface plasticity. During the cooling stage, a ferrite phase transformation occurs, precipitating a composite of C, N, Ti, Nb, and Mo elements, thus increasing the strength of the steel plate.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] 1. The high-strength, high-plasticity ferritic single-phase automotive structural steel provided by this invention has a single ferritic structure, with a lower yield strength ≥600MPa, tensile strength 700~850MPa, elongation ≥25%, hole expansion rate ≥75%, and the difference between transverse and longitudinal yield strength is within 30MPa. The cold bending radius of the shear section is half the thickness, and the cold bending angle is 180 degrees.

[0033] 2. The high-strength, high-plasticity ferritic single-phase automotive structural steel of the present invention has an equiaxed ferrite fine-grained layer with a thickness of 40-200 μm on both the upper and lower surfaces, and the surface ferrite region has a hardness of HV0.1<225. This achieves a single ferrite structure in the hot-rolled steel sheet and a highly ductile ferrite fine-grained layer on the surface, giving the steel sheet excellent formability and meeting the high formability requirements of steel sheets for lightweighting by automakers. Attached Figure Description

[0034] Figure 1 Metallographic diagram of the high-strength, high-plasticity ferritic single-phase automotive structural steel provided by this invention. Detailed Implementation

[0035] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the present invention is not limited to the following embodiments.

[0036] Example

[0037] Table 1 is a list of chemical components for each embodiment and comparative example of the present invention;

[0038] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0039] Table 3 lists the performance test results of each embodiment and comparative example of the present invention. Tensile properties were tested according to GB / T228, porosity was tested according to GB / T 24524, and surface hardness was tested according to GB / T 4340.

[0040] Each embodiment of the present invention is produced according to the following steps:

[0041] 1) After being smelted in a converter, the steel enters a vacuum treatment process for 12 to 15 minutes, after which the molten steel is continuously cast into billets.

[0042] 2) The billet is charged to the furnace using a hot charging method, with an air-fuel ratio requirement of >1.0 and a charging temperature of ≥500℃. The billet is heated to 1220~1260℃ in the heating furnace and held for 100~120 minutes. The billet is required to be discharged from the furnace at the following temperature. The temperature difference between the surface and the core of the billet is controlled within 30℃.

[0043] 3) The heated billet is rolled in sections: the roughing section is rolled in 5 passes, with the rolling temperature of the first pass controlled at 1060-1100℃ and the rolling temperature of the last pass controlled at 1000-1040℃; the finishing section is rolled in 7 passes, with the rolling temperature of the first pass controlled at 1030-1070℃ and the rolling temperature of the last pass controlled at 820-850℃.

[0044] 4) After rolling, the material is cooled at a high speed of 105-165℃ / s. After cooling to 660-680℃, it is air-cooled for 7-15 seconds, and then cooled to 500-570℃ before being coiled.

[0045] Table 1. List of chemical components of each embodiment and comparative example of the present invention.

[0046]

[0047] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0048]

[0049]

[0050] Table 3. List of performance test results for each embodiment and comparative example of the present invention.

[0051]

[0052] As shown in Table 3, the high-strength, high-ductility ferritic single-phase automotive structural steel provided by this invention has a thickness ranging from 2 to 10 mm, a lower yield strength of 600 to 750 MPa, a tensile strength of 700 to 850 MPa, an elongation of 25 to 32%, and a porosity of 75 to 95%. The upper and lower surfaces have a fine-grained equiaxed ferrite layer with a thickness of 40 to 200 μm, and the surface ferrite region has a hardness HV of 180 to 225. Furthermore, the difference between the transverse and longitudinal yield strengths of the high-strength, high-ductility ferritic single-phase automotive structural steel provided by this invention is within 30 MPa, the cold bending radius of the shear section is half the thickness, and the cold bending angle is 180 degrees.

[0053] In summary, this invention achieves high strength and high plasticity in steel plates through composition and process design, thereby obtaining excellent formability and punching performance. Furthermore, the high-strength, high-plasticity ferritic single-phase automotive structural steel of this invention achieves a single ferritic microstructure in the hot-rolled steel plate, with a highly ductile fine-grained ferrite layer on the surface, giving the steel plate excellent formability and meeting the high formability requirements of automakers for lightweighting.

[0054] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A high-strength, high-ductility ferritic single-phase automotive structural steel, characterized in that, The chemical composition and weight percentage content are as follows: C 0.03~0.06%, Si 0.3~0.6%, Mn 0.5~0.9%, S≤0.006%, Al 0.1~0.18%, Ti 0.10~0.13%, Nb 0.01~0.02%, Mo 0.0001~0.003%, with the balance being Fe and unavoidable impurities; The high-strength, high-plasticity ferritic single-phase automotive structural steel has a single ferritic structure, with a fine-grained equiaxed ferrite layer on the surface with a thickness of 40-200 μm. The surface ferrite region has a hardness of HV180-225, an elongation of ≥25%, and a difference in yield strength between the transverse and longitudinal directions within 30 MPa.

2. The high-strength, high-ductility ferritic single-phase automotive structural steel according to claim 1, characterized in that, The weight percentage of C is 0.04 to 0.

058.

3. The high-strength, high-ductility ferritic single-phase automotive structural steel according to claim 1, characterized in that, The weight percentage of Si is 0.4 to 0.

55.

4. The high-strength, high-ductility ferritic single-phase automotive structural steel according to claim 1, characterized in that, The weight percentage of Mn is 0.55 to 0.

76.

5. The high-strength, high-ductility ferritic single-phase automotive structural steel according to claim 1, characterized in that, The weight percentage of Ti is 0.12 to 0.

128.

6. The method for producing high-strength, high-ductility ferritic single-phase automotive structural steel according to any one of claims 1 to 5, characterized in that, Includes the following steps: 1) After being smelted in a converter, the steel undergoes vacuum treatment for more than 8 minutes, after which it is continuously cast into billets. 2) The billet is charged to the furnace using a hot charging method, with an air-fuel ratio requirement of >1.0 and a charging temperature of ≥500℃. The billet is heated to 1220~1260℃ in the heating furnace and held for 100~120 minutes. The billet exiting the furnace should have a temperature difference between the surface and the core controlled within 30℃. 3) The heated billet is rolled in sections: the roughing section is rolled in 5 passes, and the rolling temperature of the last pass is controlled at 1000-1040℃; the finishing section is rolled in 7 passes, and the rolling temperature of the last pass is controlled at 820-850℃. 4) After rolling, the steel is subjected to high-speed cooling at a rate of 80-180℃ / s. After cooling to 660-680℃, it is air-cooled for 7-15 seconds, and then rapidly cooled to 500-570℃ before being coiled to obtain the high-strength, high-plasticity ferritic single-phase automotive structural steel.

7. The method for producing high-strength, high-ductility ferritic single-phase automotive structural steel according to claim 6, characterized in that, The billet is hot-charged into the furnace with an air-fuel ratio of 1.25 to 1.6 and a charging temperature of 520 to 650°C.

8. The high-strength, high-ductility ferritic single-phase automotive structural steel produced by the method of claim 6, characterized in that, The steel has a lower yield strength ≥600MPa, a tensile strength of 700~850MPa, a hole expansion rate ≥75%, a cold bending radius of half the thickness of the shear section, and a cold bending angle of 180 degrees.

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