Hot-rolled bridge shell steel and manufacturing process thereof

Abstract

The application discloses hot continuous rolling bridge shell steel, its main chemical component and mass percentage ratio include: C 0.15-0.18%, Si 0.10-0.30%, Mn 1.1-1.3%, Als 0.015-0.055%, Nb 0.008-0.015%, Ti 0.005-0.020%, P 0-0.025%, S 0-0.008%, N 0-0.005%, the rest is Fe and inevitable impurity.The application adopts medium carbon low silicon lower manganese component design, cooperates low Als and N content, appropriate Ti content, and combines improved smelting, casting, rolling and laminar flow cooling process, can effectively optimize finished steel organization structure, and significantly improves surface edge crack and forming crack and other problems, and the obtained hot continuous rolling bridge shell steel can consider low cost, good plate shape and good forming performance and other advantages, is suitable for popularization and application.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a low-cost, high-surface-quality, and high-formability hot-rolled bridge shell steel and its manufacturing process. Background Technology

[0002] As a major load-bearing component of automobiles, the axle is a large, complex tubular component with small transition radii on its cross-section. Current manufacturing processes primarily include casting and stamping / welding. Among these, stamping / welding axle housings are lightweight, have high material utilization, and low manufacturing costs, making them suitable for mass production and widely used in passenger cars, light vehicles, medium-sized vehicles, and heavy-duty vehicles.

[0003] Depending on the requirements of different stamping methods for the microstructure and properties of automotive axle housings, cold stamping typically employs one or more microalloying compositions from Nb-V-Ti, combined with low-carbon components, to obtain automotive axle housing steel with appropriate properties and microstructure. Hot stamping axle housing steel usually incorporates elements such as C, Si, Cr, or Mo to improve its mechanical properties at high temperatures and enhance its hot working performance. Cold stamping axles focus more on cold forming performance, while hot working processes, in addition to considering hot working performance, place higher demands on the shape of the steel sheet after blanking.

[0004] Because the axle housing houses the main reducer, differential, and drive wheel transmission system, it requires good shape stability after stamping. Therefore, the steel used for the axle housing should possess high strength, good cold bending and drawing performance, and excellent weldability. During composition design, while ensuring performance, appropriate controlled rolling and cooling processes are used to obtain a fine and uniform ferrite structure, thereby achieving higher strength and better formability. Ideal microstructure and properties are obtained under relatively low deformation resistance.

[0005] The composition design of bridge shell steel typically follows these principles: 1) The steel plate microstructure is ferrite + pearlite; to ensure formability, the ferrite grains cannot be too fine; 2) A low carbon equivalent composition design is beneficial for improving HAZ toughness, thereby enhancing toughness and weldability; 3) The use of Nb, Ti, or Ti microalloying allows fine undissolved particles to coarsen during high-temperature austenitization, improving the overall performance of the steel plate. However, existing medium-carbon, low-cost bridge shell steels are prone to frequent surface edge cracking and forming cracking defects during subsequent production. Surface edge cracking defects lead to high reversal rates, forming cracking objection compensation rates, and objection rates. Further research is needed on bridge shell steels and their preparation methods that can effectively improve edge cracking defects and enhance formability. Summary of the Invention

[0006] The purpose of this invention is to provide a hot-rolled bridge shell steel with low cost, good plate shape and good formability, and the preparation method involved is relatively simple and easy to operate, making it suitable for widespread application.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A hot-rolled bridge shell steel, the main chemical composition and its mass percentage include: C 0.15-0.18%, Si 0.10-0.30%, Mn 1.1-1.3%, Als 0.015-0.055%, Nb 0.008-0.015%, Ti 0.005-0.020%, P 0-0.025%, S 0-0.008%, N 0-0.005%, with the remainder being Fe and unavoidable impurities.

[0009] In the above scheme, the metallographic structure of the hot-rolled bridge shell steel includes ferrite and pearlite, wherein the ferrite content is 80-90 vol% and the pearlite content is 10-20 vol%.

[0010] In the above scheme, the thickness of the hot-rolled bridge shell steel is 6-18mm.

[0011] The above-mentioned method for preparing hot-rolled bridge shell steel includes hot metal desulfurization, converter smelting, ladle refining (LF), continuous casting, slab heating, hot continuous rolling, laminar flow cooling, and coiling steps. The heating temperature is 1180-1265℃, the roughing mill exit temperature is 1020-1080℃, the final rolling temperature is 820-850℃, the steel billet in the pre-finishing section is kept warm in an insulation hood at a temperature of 980-1020℃, and the coiling temperature is 610-660℃.

[0012] In the above scheme, the desulfurization step of molten iron (molten iron pretreatment desulfurization) is controlled to S≤0.003%.

[0013] In the above scheme, the converter smelting step adopts a combined blowing and post-stirring process, with a target carbon content of ≤0.050%, and the weight percentage content of P in the steel after smelting is controlled at 0-0.020%; the scrap ratio is 0.18-0.28 (when the scrap ratio is 0.18-0.28, the corresponding scrap amount is 50-70t because the converter output is fixed at 250-270t), and the alloying adopts a weak-to-strong alloy addition sequence, controlling the nitrogen increase in the steel output to ≤20ppm.

[0014] In the above scheme, after argon blowing during steel tapping, a refining LF treatment is carried out. The ladle furnace adopts a refining process with high reducing and high basicity slag, adding diffusion deoxidizers such as aluminum shot to generate high basicity reducing slag. The basicity value of the high basicity reducing slag is 3.0-4.0, which improves the desulfurization effect and effectively reduces the nitrogen increase caused by stirring in deep desulfurization.

[0015] In the above scheme, the slab continuous casting process involves steel (peritectic steel) with a low casting temperature and small shell shrinkage in the crystallizer, making it a crack-sensitive steel. The window for key process parameters such as the protective slag, crystallizer cooling, and taper is narrow. Specific process optimization conditions include: ① Crystallizer taper designed to be 1.1-1.2; ② Use of low-melting-point, low-viscosity, high-basicity granular protective slag, with a basicity of 1.35-1.45, a melting point of 1110-1130℃, and a viscosity of 0.086-0.095 Pa·s; ③ Weak cooling of the crystallizer and secondary cooling water, with the crystallizer inlet water temperature controlled above 28℃ and the crystallizer heat flux controlled at 1.4-1.5 mW / m. 2 The heat flux in the narrow face is controlled at 1.15-1.2 mW / m. 2 Furthermore, the heat transfer difference between the symmetrical planes is controlled within 0.05 mW / m. 2 Within this range, the secondary cooling water volume is controlled at 0.5-0.55 L / t to ensure that the corner of the billet reaches a stable temperature of over 850°C before entering the straightening section. This reduces the likelihood of the billet surface falling into the brittle zone III for bending and straightening, which could lead to billet cracks.

[0016] In the above scheme, the low-melting-point, low-viscosity, and high-alkalinity granular protective slag can be selected from peritectic steel protective slag that meets the above-mentioned alkalinity, melting point, and viscosity requirements. This can control surface cracks and depressions of the billet, improve lubrication performance, and enhance the stability of molten steel casting.

[0017] In the above scheme, the laminar cooling step adopts a front-end cooling process. The FTZ value in the laminar cooling model is 40-55%, and the CS temperature in the middle section of laminar cooling is controlled at 650-700℃. This can reduce the cooling intensity of the front ultra-fast cooling section and avoid the formation of upper bainite structure on the surface due to overcooling, which would affect performance. At the same time, it also avoids the huge thermal stress generated by excessively fast cooling rate from affecting the plate shape.

[0018] Furthermore, the water volume for laminar flow cooling (upper and lower cooling) is 900-1000 m³ / h respectively. 3 / min and 1350-1900m 3 / min, fully taking into account the different laminar cooling rates of the upper and lower surfaces, to ensure that the strip maintains a basically uniform temperature on the upper and lower surfaces during the laminar cooling process.

[0019] In the above scheme, the winding temperature is 610-660℃; the winter and summer processes are distinguished, with the winter process increasing the temperature by 10℃ compared to the summer process.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1) This invention adopts a medium-carbon, low-silicon, and low-manganese composition design to avoid the crack-sensitive area of ​​the billet as much as possible. Combined with low Al and N content and appropriate Ti content, it improves high-temperature plasticity and reduces edge cracking. By appropriately increasing the C content, it avoids the peritectic steel range as much as possible; appropriately reducing the Si content (Si reduces the carbon equivalent and is more likely to enter the peritectic region) and Mn content (increasing crack sensitivity), low Al and N content, and appropriate Ti element, it reduces the precipitation of fine AlN particles, improves high-temperature plasticity, and suppresses the tendency of surface cracking of the billet; in addition, there is no need to introduce additional microalloying elements such as V, which effectively reduces the alloy cost.

[0022] 2) In the smelting process, the nitrogen content is controlled by high alkalinity; in the casting process, the transverse temperature difference of the billet is reduced by optimizing the protective slag and the crystallizer + secondary cooling water supply, thereby increasing the billet temperature before straightening, reducing billet cracks, and improving edge crack defects; in the rolling process, the final product structure is improved by optimizing the rolling and laminar flow cooling processes, avoiding the formation of bainite on the surface, reducing elongation, and reducing forming cracks.

[0023] 3) The hot-rolled bridge shell steel of the present invention has the characteristics of low cost, good plate shape and good forming performance, and the preparation method involved is relatively simple and easy to operate, which is suitable for promotion and application. Attached Figure Description

[0024] Figure 1 The image shows the metallographic structure of the bridge shell steel obtained in Example 4;

[0025] Figure 2 The image shows the metallographic structure of the bridge shell steel obtained in Comparative Example 1. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to embodiments, so as to facilitate a clearer understanding of the present invention, but these embodiments do not constitute a limitation on the present invention.

[0027] Examples 1-7

[0028] Table 1 shows the chemical composition and weight percentage of the bridge shell steel described in Examples 1 to 7 of the present invention, with the balance being Fe and unavoidable impurities;

[0029] The specific preparation steps are as follows:

[0030] 1) Desulfurization of molten iron pretreatment, S≤0.003%;

[0031] 2) Converter smelting adopts a combined blowing + post-stirring smelting process to achieve stable and continuous production with a target carbon content of ≤0.050% in the tapped steel. The weight percentage content of P in the tapped steel is controlled at 0-0.020%, and the scrap ratio is 0.18-0.28 (the tapped steel output of the converter is fixed at 250-270t, and the specific scrap steel amount is controlled at 50-70t). Alloying adopts the order of adding weak alloys first and then strong alloys, and the nitrogen increase in the tapped steel is controlled at ≤20ppm.

[0032] 3) After argon blowing during tapping, the steel is refined in LF treatment. The ladle furnace adopts a refining process with high basicity reducing slag, and aluminum shot diffusion deoxidizer is added to generate high reducing basicity slag with a basicity value of 3.0-4.0.

[0033] 4) The production process adopted in the slab casting process is as follows: ① The taper of the crystallizer is designed to be 1.1-1.2; ② A low-melting-point, low-viscosity, and high-basicity granular protective slag (peritectic steel protective slag provided by Henan Xibao Metallurgical Materials Group Co., Ltd.) is used. The basicity of the protective slag is 1.35-1.45, the melting point is 1110-1130℃, and the viscosity is 0.086-0.095 Pa·s; ③ The crystallizer is weakly cooled, and the secondary cooling water is weakly cooled. Among them, the crystallizer inlet water temperature is controlled above 28℃, and the crystallizer heat flux is controlled at 1.4-1.5 mW / m 2 The heat flux in the narrow face is controlled at 1.15-1.2 mW / m. 2 Furthermore, the heat transfer difference between the symmetrical planes is controlled within 0.05 mW / m. 2 The secondary cooling water volume is controlled at 0.50-0.55 L / t; to ensure that the corner of the billet reaches a stable temperature of over 850°C before entering the straightening section, thereby reducing the risk of the billet surface falling into the third brittle zone for bending and straightening, which could lead to billet cracks.

[0034] 5) The slab heating temperature is 1190-1265℃, the roughing mill exit temperature is 1020-1080℃, the final rolling temperature is 820-850℃, the steel billet in the pre-finishing mill is kept in the heat insulation cover at a temperature of 980-1020℃, and the coiling temperature is 610-660℃ (distinguishing between winter and summer processes, with the winter process being 10℃ higher than the summer process).

[0035] 6) When using laminar flow cooling, front-end cooling is adopted. The FTZ value in the laminar cooling model is 40-55%, and the CS temperature in the middle section of laminar cooling is controlled at 650-700℃. This reduces the cooling intensity of the front ultra-fast cooling section and avoids the formation of bainite structure on the surface due to overcooling, which would affect performance. It also avoids the huge thermal stress generated by excessively fast cooling rate from affecting the plate shape.

[0036] 7) The water volume for laminar flow cooling (upper and lower) is 900-1000 m³ respectively. 3 / min and 1350-1900m 3 / min;

[0037] The specific values ​​of the relevant process parameters in Examples 1-7 are shown in Table 2 below, and the performance results are shown in Table 3. Metallographic analysis was performed on the bridge shell steel obtained in Example 4 and Comparative Example 1, and the results are as follows: Figure 1 (Ferrite + Pearlite, wherein the ferrite content is 80-90 vol% and the pearlite content is 10 vol%) and Figure 2 As shown in (bainite + pearlite).

[0038] Table 1. Steel composition (wt%) of the bridge shell steel described in Examples 1-7 and Comparative Examples.

[0039]

[0040]

[0041] Table 2 Main process parameters of the bridge shell steel described in Examples 1-7 and Comparative Examples

[0042]

[0043] Table 3. Performance test results of the bridge shell steel described in Examples 1-7 and Comparative Examples.

[0044]

[0045] This invention is not limited to the embodiments described above. Those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A manufacturing process of hot continuous rolling of axle housing steel, characterized in that, The hot continuous rolling bridge shell steel comprises the following main chemical components and their mass percentages: C 0.15-0.18%, Si 0.10-0.30%, Mn 1.1-1.264%, Als 0.015-0.055%, Nb 0.008-0.015%, Ti 0.005-0.020%, P 0-0.025%, S 0-0.008%, N 0-0.005%, and the rest is Fe and inevitable impurities. The refining LF treatment is carried out after argon blowing, and the ladle furnace adopts a refining treatment process of high-alkalinity reducing slag, and the alkalinity value of the high-alkalinity reducing slag is 3.0-4.

0. The metallographic structure of the hot continuous rolling bridge shell steel comprises ferrite and pearlite, wherein the content of the ferrite is 80-90vol%, and the content of the pearlite is 10-20vol%. The specific production process optimization conditions of slab continuous casting include: ① The taper of the crystallizer is designed as 1.1-1.2; ② The low-melting-point, low-viscosity and high-basicity granular protective slag is used, the basicity of which is 1.35-1.45, the melting point is 1110-1130℃, and the viscosity is 0.086-0.095 Pa.s; ③ The crystallizer is weakly cooled, and the secondary cooling water is weakly cooled, wherein: the inlet water temperature of the crystallizer is controlled to be above 28℃, the heat flow of the crystallizer is controlled to be 1.4-1.5 mW / m 2 , the heat flow of the narrow surface is controlled to be 1.15-1.2 mW / m 2 , and the difference of the heat transfer of the symmetric surface is controlled to be within 0.05 mW / m 2 , and the water quantity of the secondary cooling water is controlled to be 0.5-0.55 L / t, so that the corner of the cast slab is stabilized to be above 850℃ before entering the straightening section.

2. The manufacturing process of claim 1, wherein, The thickness of the hot continuous rolling bridge shell steel is 6-18mm.

3. The manufacturing process of claim 1, wherein, The converter smelting step adopts a combined blowing and post-stirring process, the target carbon is ≤0.050%, the weight percentage content of P after smelting and tapping is controlled to be 0-0.020%, the scrap steel ratio is 0.18-0.28, the alloying adopts a weak-then-strong alloy adding sequence, and the nitrogen increase amount after tapping is controlled to be ≤20ppm.

4. The manufacturing process of claim 1, wherein, ​ 5. The manufacturing process of claim 1, wherein, The water quantity of laminar cooling upper and lower cooling is 900-1000m 3 / min and 1350-1900m 3 / min respectively.

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