Low-cost 980MPa-grade high-reaming steel and manufacturing method thereof

Abstract

The invention discloses a low-cost 980MPa-grade high hole expansion steel, which comprises the following chemical components in percentage by weight: 0.172 to 0.206 percent of C, 0.33 to 0.74 percent of Si, 1.52 to 1.95 percent of Mn, 0.008 to 0.024 percent of Nb, 0.018 to 0.048 percent of Ti, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S, less than or equal to 0.006 percent of N and the balance of Fe and inevitable impurities. By adopting the CSP production line, the production of strip steel with the thinnest thickness of 1.2 mm can be realized, the purpose that cold is replaced by hot pickling of automobile steel is realized, the tensile strength of a steel plate subjected to pickling is greater than or equal to 980MPa, the yield strength is greater than or equal to 750MPa, the ductility is greater than or equal to 14%, the requirements of automobile chassis parts on reaming and flanging performance are met, and the reaming rate is greater than or equal to 75%; and meanwhile, the extremely low-cost alloy design is adopted, the addition of alloy elements is reduced, the production cost is greatly reduced, and the final alloy cost is controlled to be 350-450 yuan / ton.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a low-cost 980MPa grade high-expansion steel and its manufacturing method. Background Technology

[0002] In the 21st century, reducing fuel consumption, CO2 emissions, and exhaust emissions has become a societal necessity for the automotive industry. To adapt to this trend, the steel industry, as a materials producer, has developed many types of ultra-high-strength steel sheets to help reduce vehicle weight and meet the new requirements of the automotive industry. An increasing number of pickled automotive steel products are being used to manufacture automotive parts, such as suspension components and chassis parts.

[0003] Many automotive parts, such as front / rear control arms and spring seats, typically have numerous round holes designed for high strength, lightweight construction, and assembly requirements. These holes need to be punched and then enlarged and flanged during the forming process to obtain the final part. Extensive data shows that a material's hole-enlarging performance is not entirely equivalent to its forming performance. Generally, the better the plasticity of steel, the better its forming performance, but this does not necessarily mean better hole-enlarging and flanging performance. For example, traditional duplex steel has good plasticity and forming performance, but its hole-enlarging and flanging performance is poor, making it unsuitable for producing automotive suspension and chassis parts that require hole enlargement.

[0004] Preliminary search revealed that patent CN114107794A describes a 980MPa grade ultra-low carbon martensitic steel with residual austenitic structure and its manufacturing method. The chemical composition by weight percentage is as follows: C 0.03%~0.06%, Si 0.8%~2.0%, Mn 1.0%~2.0%, P≤0.02%, S≤0.003%, Al 0.02%~0.08%, N≤0.004%, Mo 0.1%~0.5%, Ti 0.01%~0.05%, O≤0.0030%, with the remainder being Fe and other unavoidable impurities. The high-expansion steel described has a yield strength ≥800MPa, tensile strength ≥980MPa, elongation (transverse A50 ≥10%), cold bending performance (d≤4a, 180°), and expansion rate ≥80%. It can be applied to passenger vehicle chassis parts such as control arms and subframes where high strength and thinning are required. This patent describes a method for producing 980MPa-grade high-expansion steel using conventional hot rolling. The thinnest steel sheet produced is typically ≥2.0mm. The desired properties are achieved by adding the precious metal element Mo. Based on a target value of 0.3% Mo, the alloy cost of Mo is approximately 700 RMB / ton, and the total alloy cost is approximately 900 RMB / ton, which does not meet the user's demand for low-cost manufacturing. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a 980MPa grade high-expansion steel and its manufacturing method that utilizes a CSP production line and achieves extremely low cost. This method enables the production of strip steel as thin as 1.2mm, allowing for hot pickling of automotive steel instead of cold pickling. After pickling, the steel sheet exhibits a tensile strength ≥980MPa, a yield strength ≥750MPa, and an elongation ≥14%, meeting the requirements of automotive chassis components for expansion and flanging performance, with an expansion rate ≥75%. Simultaneously, an extremely low-cost alloy design is employed, reducing the addition of alloying elements and significantly lowering production costs, with the final alloy cost controlled at 350~450 yuan / ton.

[0006] To achieve the above objectives, the following technical solution is adopted: A low-cost 980MPa grade high-expansion steel has the following chemical composition and weight percentage content: C: 0.172~0.206%, Si: 0.33~0.74%, Mn: 1.52~1.95%, Nb: 0.008~0.024%, Ti: 0.018~0.048%, P≤0.015%, S≤0.005%, N≤0.006%, with the remainder being Fe and unavoidable impurities.

[0007] In the optimized scheme, the chemical composition and weight percentage content are as follows: C: 0.181~0.204%, Si: 0.33~0.74%, Mn: 1.67~1.83%, Nb: 0.013~0.017%, Ti: 0.018~0.048%, P≤0.015%, S≤0.005%, N≤0.006%, and the remainder is Fe and unavoidable impurities.

[0008] The method for manufacturing the aforementioned low-cost 980MPa grade high-expansion steel using a CSP production line includes the following steps: During the billet heating stage, the billet temperature is controlled at 820–960℃, the time in the furnace is 31–48 min, and the exit temperature is 1200–1240℃. During the rolling stage, the final rolling temperature is controlled at 870–900℃. During the cooling and coiling stage, the cooling rate in the first stage is ≥135℃ / s, and coiling is carried out at 467–502℃.

[0009] The optimized solution, which uses a CSP production line to produce the aforementioned low-cost 980MPa grade high-expansion steel, includes the following steps: 1) A dual refining process of "LF heating + RH" is adopted, followed by calcium treatment after RH treatment to obtain molten steel with the target composition; 2) Continuous casting is carried out; 3) The billet is heated, and the billet temperature is controlled at 820-960℃, the time in the furnace is 31-48min, and the exit temperature is 1200-1240℃; 4) High-pressure water descaling is carried out before rolling; 5) Rolling is carried out, and the final rolling temperature is controlled at 870-900℃; 6) Post-rolling cooling is carried out using a front-stage cooling method, with a front-stage cooling rate ≥135℃ / s; 7) Coiling is carried out, and the coiling temperature is controlled at 467-502℃. After coiling, the temperature is cooled to room temperature at a cooling rate ≤20℃ / h; 8) Continuous pickling is carried out to obtain the low-cost 980MPa grade high-expansion steel.

[0010] A more optimized approach involves using a CSP production line to produce the aforementioned low-cost 980MPa grade high-expansion steel, which includes the following steps: 1) A dual refining process of "LF heating + RH" is adopted, followed by calcium treatment after RH treatment to obtain molten steel with the target composition; 2) Continuous casting is carried out, with a billet pulling speed of 3.2–3.8. 3) Heat the billet, controlling the billet temperature at 820-960℃, the time in the furnace at 31-48 min, and the exit temperature at 1200-1240℃; 4) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 290-430 bar; 5) Perform rolling, controlling the final rolling temperature at 870-900℃; 6) Use front-stage cooling for post-rolling cooling, with a front-stage cooling rate ≥135℃ / s; 7) Perform coiling, controlling the coiling temperature at 467-502℃, and cooling to room temperature at a cooling rate ≤20℃ / h after coiling; 8) Perform continuous pickling, controlling the pickling temperature between 72-85℃, and the tensile elongation ≤2%, to obtain the low-cost 980MPa grade high-expansion steel.

[0011] In the optimized scheme, the furnace entry temperature of the billet is controlled at 835-948℃, the furnace time is 35-46 minutes, and the furnace exit temperature is 1210-1237℃.

[0012] In the optimized scheme, the final rolling temperature is controlled between 880 and 890℃. In the optimized scheme, the winding temperature is between 471 and 498℃.

[0013] The above scheme was used to prepare a low-cost 980MPa grade high-expansion steel with a thickness of 1.2mm~5.0mm, tensile strength ≥980MPa, yield strength ≥750MPa, elongation ≥14%, and expansion rate ≥75%; the metallographic structure is 80~95% bainite and 5~20% ferrite by volume percentage.

[0014] The roles and mechanisms of each raw material and main process in this invention: Carbon (C) is a crucial element affecting strength and hole-expanding performance. As a solid solution strengthening element, C is the most important strength-enhancing element in this invention. To ensure that the steel plate achieves a tensile strength of over 980 MPa, the carbon content must be at least 0.17% to further improve the strength of the solid solution-strengthened bainitic structure; otherwise, the strength requirements will be difficult to meet. On the other hand, carbon content directly affects hole-expanding performance. If the carbon content exceeds 0.21%, the hole-expanding deformation ability of the bainitic structure weakens drastically, and the hole-expanding rate cannot meet the application requirements. Therefore, considering the comprehensive influence of carbon on strength and hole-expanding performance, the carbon content in the steel should be controlled between 0.172% and 0.206%, preferably 0.181% to 0.204%.

[0015] Si: A large amount of C is added in this invention to increase strength, which easily leads to cementite precipitation during cooling. This cementite is detrimental to pore-expanding performance. Si can effectively inhibit cementite precipitation; therefore, adding more than 0.33% Si can effectively suppress cementite precipitation and improve pore-expanding performance. However, when the Si content exceeds 0.74%, it affects the surface quality of the steel plate. The Si content in this invention is controlled between 0.33% and 0.74%.

[0016] Mn: Mn is one of the most effective elements for improving strength. Mn can stabilize austenite and reduce the critical quenching rate of steel, improving the hardenability of the plate. To ensure that the present invention can obtain a bainitic structure of more than 80% after cooling, the Mn content should be at least 1.5%; at the same time, if the Mn content exceeds 2.0%, the risk of slab cracking will be significantly increased. Therefore, the Mn content should be controlled between 1.52% and 1.95%, preferably 1.67% to 1.83%.

[0017] S: S is an impurity element in steel. S in steel often exists in the form of manganese sulfides. These sulfide inclusions are very detrimental to the expansion and flanging performance of steel. Therefore, the sulfur content in steel should be controlled as low as possible. Thus, the sulfur content in steel should be controlled below 0.005%.

[0018] P: P is an impurity element in steel. During the solidification of the steel billet, it precipitates a Fe2P eutectic structure and leads to cold brittleness. Therefore, the lower the P content, the better. In actual production, it is generally controlled below 0.015%.

[0019] Nb: Nb is one of the key alloying elements used in this invention. Adding Nb without adding the precious metal Mo can effectively refine the original austenite grain size, resulting in a finer bainite structure with better coordinated deformation capabilities and thus better hole expansion and flanging performance. Therefore, an addition of 0.008% or more of Nb is required. However, research shows that when the Nb content exceeds 0.025%, the effect of further improving the hole expansion rate is not significant and instead leads to increased costs. Therefore, the Nb content is controlled between 0.008% and 0.024%.

[0020] Ti: Ti is one of the important alloying elements used in this invention. Ti can improve strength by precipitating TiC and other precipitates, and it can also refine grains and improve toughness. If the Ti content is too high, it will increase the number of coarse carbide and nitride inclusions, thus affecting the overall mechanical properties. Therefore, the Ti content should be controlled between 0.018% and 0.048%.

[0021] In addition to limiting the range of the above chemical components, from the point of view of improving economic efficiency, this invention does not add expensive alloying elements such as Mo, V, Cu, and Ni. This invention adopts an extremely low-cost composition design, and the alloy cost is 180-560 yuan / ton lower than that of traditional 980MPa grade high-expansion steel.

[0022] The experiments of this invention demonstrate that heating the billet to 1200–1240°C and holding it at that temperature for 31–48 minutes ensures that the temperature of the steel plate meets the requirements of each stage of the rolling process. Excessive heating temperature and holding time result in coarse austenite grains before rolling, preventing the subsequent rolling process from achieving complete austenite recrystallization and the acquisition of fine bainite during cooling. Conversely, insufficient heating temperature and holding time lead to excessively small initial austenite grains before rolling, resulting in excessive deformation resistance exceeding the limits of the rolling equipment and preventing the completion of the entire rolling process.

[0023] The reason why the final rolling temperature is controlled at 870-900℃ in this invention is that if the final rolling temperature is too high, exceeding 900℃, the austenite will recrystallize at high temperatures, resulting in coarse austenite grains, and fine bainite structure cannot be obtained during the cooling process; if the final rolling temperature is below 870℃, it is not conducive to the precipitation of Ti, resulting in the strength of the steel plate being lower than the standard requirements.

[0024] After rolling, the steel plate undergoes a pre-cooling process with a cooling rate of no less than 135℃ / s. This aims to suppress grain growth after rolling, ensuring a fine and uniform grain structure before coiling, resulting in a final steel product with a fine grain size. Coiling is then performed at 467–502℃ to ensure the coiled steel plate has a microstructure of 80–95% bainite and 5–20% ferrite. If the coiling temperature is too low, the bainite strength increases, while the plasticity and expansion properties decrease, which is detrimental to part forming and expansion / flanging performance. Conversely, if the coiling temperature is too high, the bainite strength decreases, failing to meet the tensile strength requirement of ≥980MPa.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention employs a reasonable chemical composition and hot-rolled pickling process to obtain a low-cost 980MPa grade high-expansion steel through a CSP production line. Specifically, the thickness is 1.2mm~5.0mm. After pickling, the steel plate has a tensile strength ≥980MPa, a yield strength ≥750MPa, and an elongation ≥14%, which meets the requirements of automotive chassis parts for expansion and flanging performance. The expansion rate of the pickled steel plate is ≥75%.

[0026] The CSP production line of this invention produces 980MPa grade high-expansion steel at extremely low cost, which greatly meets the requirements of automotive chassis components for low cost and high expansion and flanging performance. Compared with traditional 980MPa grade high-expansion steel, it adopts an extremely low-cost composition design, reducing alloy costs by 180-560 yuan / ton. The thin-gauge high-strength high-expansion pickled steel produced by the CSP thin slab continuous casting and rolling process reduces the need for cold rolling and annealing processes, which can further reduce production and manufacturing costs. Attached Figure Description

[0027] Figure 1 Metallographic structure of the low-cost 980MPa grade high-expansion steel of this invention. Detailed Implementation

[0028] The following embodiments further illustrate the technical solution of the present invention, but are not intended to limit the scope of protection of the present invention.

[0029] Examples 1-5 Table 1 lists the chemical components of each embodiment and comparative example.

[0030] Table 2 lists the main process parameters for each embodiment and comparative example.

[0031] Table 3 is a list of performance test data for each embodiment and comparative example.

[0032] The embodiments and comparative examples of this invention are produced according to the following steps: 1) A "LF heating + RH" dual refining process is adopted. After the RH treatment, calcium treatment is performed to obtain molten steel with the target composition, as shown in Table 1; 2) Continuous casting is performed, with a billet pulling speed of 3.2 to 3.8. 3) Heat the billet, controlling the billet temperature at 820-960℃, the time in the furnace at 31-48 min, and the exit temperature at 1200-1240℃; 4) Perform high-pressure water descaling before rolling, and control the descaling water pressure at 290-430 bar; 5) Perform rolling, controlling the final rolling temperature at 870-900℃; 6) Use front-stage cooling for post-rolling cooling, with a front-stage cooling rate ≥135℃ / s; 7) Perform coiling, controlling the coiling temperature at 467-502℃, and cooling to room temperature at a cooling rate ≤20℃ / h after coiling; 8) Perform continuous pickling, controlling the pickling temperature between 72-85℃, and the tensile elongation ≤2%, to obtain the low-cost 980MPa grade high-expansion steel.

[0033] Table 1 Chemical Composition List (wt%)

[0034] Table 2 Main Process Parameters

[0035] Table 3 Performance Test Data List

[0036] The hole expansion test, conducted according to national standards, involves punching a 10mm center hole in the center of a 150mm x 150mm square sheet. A conical hole-expanding punch of specified shape and size is inserted into the punched hole of a metal sheet sample to expand the hole until cracks penetrating the sample thickness appear at the edge of the hole. The punching is then stopped, and the limiting hole expansion rate is measured. The hole expansion rate λ = In the formula, λ is the limiting expansion ratio, %; D0 is the initial diameter of the punched circular hole (D0 = 10 mm); D H The average diameter of the hole after rupture is expressed in millimeters (mm).

[0037] The metallographic structure of the low-cost 980MPa grade high-expansion steel obtained in Example 1 is shown in the appendix. Figure 1 As shown.

[0038] As can be clearly seen from Table 3, by employing the chemical composition and hot rolling and pickling processes of this invention, an extremely low-cost 980MPa grade high-expansion steel for CSP production lines can be obtained. Specifically, the thickness is 1.2mm~5.0mm. After pickling, the steel plate has a tensile strength ≥980MPa, a yield strength ≥750MPa, and an elongation ≥14%. Observation shows no surface defects, indicating good surface quality, meeting the requirements of automotive chassis components for expansion and flanging performance. The expansion rate of the pickled steel plate is ≥75%. This invention adopts an extremely low-cost composition design, and the alloy cost is 180~560 yuan / ton lower than that of traditional 980MPa grade high-expansion steel.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the inventive concept of the present invention, and these modifications all fall within the protection scope of the present invention.

Claims

1. A low-cost 980MPa grade high-expansion steel, characterized in that... The chemical composition and weight percentage content are as follows: C: 0.172~0.206%, Si: 0.33~0.74%, Mn: 1.52~1.95%, Nb: 0.008~0.024%, Ti: 0.018~0.048%, P≤0.015%, S≤0.005%, N≤0.006%, with the remainder being Fe and unavoidable impurities.

2. The low-cost 980MPa grade high-expansion steel as described in claim 1, characterized in that... The chemical composition and weight percentage content are as follows: C: 0.181~0.204%, Si: 0.33~0.74%, Mn: 1.67~1.83%, Nb: 0.013~0.017%, Ti: 0.018~0.048%, P≤0.015%, S≤0.005%, N≤0.006%, with the remainder being Fe and unavoidable impurities.

3. A method for manufacturing low-cost 980MPa grade high-expansion steel as described in claim 1 using a CSP production line, characterized in that... include: During the billet heating stage, the billet temperature is controlled at 820–960℃, the time in the furnace is 31–48 min, and the exit temperature is 1200–1240℃. During the rolling stage, the final rolling temperature is controlled at 870–900℃. During the cooling and coiling stage, the cooling rate in the first stage is ≥135℃ / s, and coiling is carried out at 467–502℃.

4. The method for manufacturing low-cost 980MPa grade high-expansion steel on the CSP production line according to claim 3, characterized in that... Includes the following steps: 1) The "LF heating + RH" dual refining process is adopted. After the RH treatment, calcium treatment is carried out to obtain molten steel with the target composition. 2) Perform continuous casting; 3) Heat the billet, controlling the billet temperature at 820-960℃ when it enters the furnace, the time in the furnace at 31-48 minutes, and the temperature at 1200-1240℃ when it exits the furnace; 4) High-pressure water descaling before rolling; 5) Roll the material, controlling the final rolling temperature at 870–900℃; 6) Post-rolling cooling is performed using a front-stage cooling method, with a front-stage cooling rate ≥135℃ / s; 7) Perform winding, controlling the winding temperature between 467 and 502℃, and cool to room temperature at a rate of ≤20℃ / h after winding; 8) Perform continuous pickling to obtain the low-cost 980MPa grade high-expansion steel.

5. The method for manufacturing low-cost 980MPa grade high-expansion steel on the CSP production line according to claim 3, characterized in that... Includes the following steps: 1) The "LF heating + RH" dual refining process is adopted. After the RH treatment, calcium treatment is carried out to obtain molten steel with the target composition. 2) Continuous casting is carried out, with a billet pulling speed of 3.2–3.8 m / min; 3) Heat the billet, controlling the billet temperature at 820-960℃ when it enters the furnace, the time in the furnace at 31-48 minutes, and the temperature at 1200-1240℃ when it exits the furnace; 4) Perform high-pressure water descaling before rolling, and control the descaling water pressure between 290 and 430 bar; 5) Roll the material, controlling the final rolling temperature at 870–900℃; 6) Post-rolling cooling is performed using a front-stage cooling method, with a front-stage cooling rate ≥135℃ / s; 7) Perform winding, controlling the winding temperature between 467 and 502℃, and cool to room temperature at a rate of ≤20℃ / h after winding; 8) Perform continuous pickling, with the pickling temperature controlled between 72 and 85°C and the tensile elongation ≤2%, to obtain the low-cost 980MPa grade high-expansion steel.

6. The method for manufacturing low-cost 980MPa grade high-expansion steel on the CSP production line as described in claim 5, characterized in that... The temperature of the billet entering the furnace is controlled at 835-948℃, the time in the furnace is 35-46 minutes, and the temperature at the exit of the furnace is 1210-1237℃.

7. The method for manufacturing low-cost 980MPa grade high-expansion steel on the CSP production line as described in claim 5, characterized in that... Control the final rolling temperature at 880–890℃.

8. The method for manufacturing low-cost 980MPa grade high-expansion steel on the CSP production line as described in claim 5, characterized in that... The winding temperature is between 471 and 498°C.

9. The low-cost 980MPa high-expansion steel manufactured by the method according to any one of claims 3 to 5, characterized in that, The 980MPa grade high-expansion steel has a thickness of 1.2mm~5.0mm, tensile strength ≥980MPa, yield strength ≥750MPa, elongation ≥14%, and expansion rate ≥75%.

10. The low-cost 980MPa grade high-expansion steel manufactured by the method according to claim 9, characterized in that, The metallographic structure of the 980MPa grade high-expansion steel comprises 80-95% bainite and 5-20% ferrite by volume percentage.

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