A 700mpa grade cold formed axle housing steel with thickness ≤10mm and production method

Abstract

The application discloses a 700MPa grade bridge housing steel for cold forming with a thickness of less than 10mm, which comprises the following components and wt%: C: 0.05-0.09%, Si: 0.03-0.10%, Mn: 1.0-1.7%, P: less than or equal to 0.018%, S: less than or equal to 0.005%, Als: 0.015-0.060%, Nb: 0.020-0.050%, Ti: 0.030-0.050%, and N: less than or equal to 0.005%. The production method comprises the following steps: converter smelting, heating the casting blank after casting, rough rolling, finish rolling, cooling and coiling. The application combines the double strengthening mechanism of solid solution strengthening of Mn element and precipitated fine-grain strengthening of Nb and Ti, improves the solid solution content of Ti through the desulfurization mode in the LHF treatment process, guarantees the purity of the steel, controls the fine grains in the temperature range after the pre-stage cooling of the ultra-fast cooling, and makes the Rel of the steel plate greater than or equal to 600MPa, the Rm greater than or equal to 690MPa, the elongation A greater than or equal to 20%, and the cold bending performance D=a, 180° qualified, so that the steel plate has excellent cold forming and welding performance.

Description

Technical Field

[0001] This invention relates to automotive steel and its production method, specifically to a 700MPa grade cold-forming bridge housing steel and its production method. Background Technology

[0002] As a crucial load-bearing component of the chassis system, one of the three major assemblies of commercial vehicles, the axle housing accounts for approximately 4% of the total steel consumption in automobiles. Heavy-duty axle housings (10 tons and above) required for heavy trucks and large buses are generally produced using hot stamping, consuming approximately 200,000 tons of steel annually. Light-duty axle housings required for medium-duty trucks, light trucks, mini-trucks, and small buses are generally produced using cold stamping, consuming approximately 120,000 tons of steel annually. Automobile axle housings have high safety requirements, needing to meet stringent component fatigue performance standards. This translates to the steel plates used in axle housings, which must exhibit stable performance, resistance to low-temperature impact, and good weldability. Currently, the highest strength standard for axle housing steel plates in international standards is Q460, with no higher grade technical standards.

[0003] Search results:

[0004] Chinese Patent Publication No. CN104213019A discloses "A 600MPa Grade Automobile Axle Housing Steel and Its Production Method". The composition is as follows: C: 0.21%-0.26%; Si: 0.51%-0.6%; Mn: 1.1%-1.5%; Al: 0.01%-0.06%; P: ≤0.02%; S: ≤0.01%; V: 0.05%-0.06%; N: 0.012%-0.016%; the remainder being Fe and unavoidable impurities, with V:N ≤ 5:1. By controlling the content of V and N elements and controlling rolling and cooling, 600MPa grade hot-rolled strip steel for automobile axle housings is finally obtained. However, this method has drawbacks: the high C content leads to poor weldability, and the high V content makes N element control difficult, resulting in high production costs.

[0005] Chinese Patent Publication No. CN105239013A discloses "A cold-formed axle housing steel and its manufacturing method," with the following composition: C: 0.08-0.18%, Si: 0.10-0.45%, Mn: 1.2-2.5%, Ti: 0.01-0.05%, Al: 0.02-0.35%, P: ≤0.02%, S: ≤0.010%, V: 0.02-0.10%, Nb: 0.015-0.10%, Cr: 0.02-0.5%, with the remainder being Fe and unavoidable impurities. While this method achieves cost reduction while maintaining the weldability of the cold-formed axle housing steel, ultimately yielding a 600MPa grade automotive axle housing steel plate, the addition of Cr and V alloys, relying solely on the strengthening effect of alloying elements to improve steel strength, results in relatively high production costs, and the steel's yield strength is below 600MPa.

[0006] Chinese Patent Publication No. CN111235460A discloses "A Bridge Shell Steel Suitable for Induction Heating and Its Production Method," with the following composition: C: 0.15-0.25%; Si: 0.05-0.18%; Mn: 1.8-3.0%; Ti: 0.15-0.30%; Al: 0.02-0.35%; P: ≤0.015%; S: ≤0.005%; N ≤0.004%; the remainder being Fe and unavoidable impurities. The hot-rolled bridge shell steel in this document has a yield strength ≥650MPa, tensile strength ≥750MPa, elongation ≥18%, and impact energy ≥150J at -20℃. However, after induction heating, the yield strength of the bridge shell steel is in the 520MPa range, the tensile strength is in the 650MPa range, the elongation is ≥25%, and the impact energy at -20℃ is ≥100J. This means that the yield strength decreases significantly after heating, and the high C content results in poor weldability. The high Mn and Ti content also leads to low elongation performance, making it only suitable for the manufacture of hot-stamped automotive axle housings.

[0007] Chinese Patent Publication No. CN114015934A discloses "A 600MPa Grade Hot-Rolled Duplex Bridge Shell Steel and its Production Method," with the following composition and weight percentage content: C: 0.06-0.10%, Si: 0.05-0.20%, Mn: 1.30-1.60%, P≤0.015%, S≤0.005%, Als: 0.02-0.06%, Nb: 0.035-0.050%. This document uses a C-Mn- While Nb composition design strengthens steel, it has low carbon content, excellent weldability, and low alloy cost. However, it can only guarantee a yield strength of 500-600 MPa. The hot rolling process requires a complex secondary cooling process, and the proportion of coiling temperature controlled in the 380-480℃ range is only about 75% (while the proportion can reach more than 96% when the coiling temperature is controlled in the 620-660℃ range). This results in insufficient strength or elongation at the beginning and end of the steel coil, leading to its removal and scrapping. Consequently, the steel plate performance qualification rate and yield are low.

[0008] In summary, existing high-strength bridge shell steels with a strength of 600MPa or higher have shortcomings such as high carbon equivalent, poor cold forming performance, and low yield, making them unsuitable for the lightweight requirements of higher-end automobiles. Summary of the Invention

[0009] The present invention aims to overcome the shortcomings of the existing technology and provide a 700MPa grade cold-formed bridge shell steel with a thickness ≤10mm and a production method that not only has a yield strength Rel≥600MPa, tensile strength Rm≥690MPa, elongation A≥20%, and cold bending performance D=a, 180° qualified, and the yield strength fluctuation value of the whole roll does not exceed 50MPa.

[0010] Measures to achieve the above objectives:

[0011] A bridge shell steel of 700MPa grade with a thickness ≤10mm has the following composition and weight percentage content: C: 0.05~0.09%, Si: 0.03~0.10%, Mn: 1.0~1.7%, P≤0.018%, S≤0.005%, Als: 0.015~0.060%, Nb: 0.020~0.050%, Ti: 0.030~0.050%, N≤0.005%, with the remainder being Fe and impurities.

[0012] Preferably, the weight percentage content of C is 0.055% to 0.085%.

[0013] Preferably, the weight percentage content of Si is between 0.036% and 0.093%.

[0014] Preferably, the weight percentage content of Mn is 1.05% to 1.68%.

[0015] A method for producing 700MPa grade cold-formed bridge shell steel with a thickness ≤10mm, comprising the following steps:

[0016] 1) During the converter smelting process:

[0017] The LHF process employs a deep desulfurization treatment mode, controlling the slag basicity at CaO / SiO2 = 2.0–2.5. After the first batch of desulfurized slag, deoxidized alloy, and temperature-controlled scrap steel are added, the bottom-blown Ar gas flow rate is controlled at 70–90 Nm3 / h and maintained for 3–10 minutes.

[0018] After adding the Ti alloy, adjust the bottom-blown Ar gas flow rate to 15-30 Nm3 / h and maintain this flow rate for 3-5 minutes; control the total LHF treatment time to be no less than 18 minutes.

[0019] 2) After casting, the billet is heated. During this process, the heating temperature of the second heating section is controlled at 1220-1280℃, the soaking time in the soaking section is controlled at not less than 60 minutes, and the total time in the furnace is not less than 140 minutes.

[0020] 3) Perform rough rolling and control the rough rolling exit temperature at 1100~1160℃;

[0021] 4) Perform finish rolling, controlling the initial finishing rolling temperature at 1000-1100℃ and the final finishing rolling temperature at 840-880℃;

[0022] 5) Cooling: First, cool to 650℃~690℃ at a cooling rate of 20~30℃ / s, then cool to the winding temperature using laminar flow.

[0023] 6) Perform winding, and control the winding temperature at 620-660℃.

[0024] The functions and mechanisms of each raw material and main process in this invention are as follows:

[0025] C: C plays a solid solution strengthening role in steel. When the C content is low, during the heating, rolling, and cooling process, a small amount of C decomposes from austenite into ferrite and cementite phases, forming acicular or lath-shaped ferrite with excellent toughness and discontinuous carbides. Considering all factors, the suitable C content is 0.05–0.09%, preferably 0.055–0.085%.

[0026] Si: Si has a good deoxidizing effect on molten steel. When the Si content is low, it helps to reduce oxides on the surface of the steel plate and improve the surface quality of the steel plate. Taking all factors into consideration, the suitable Si content is 0.03-0.10%, preferably 0.036-0.093%.

[0027] Mn: Mn is an inexpensive and effective deoxidizer and desulfurizer for alloys. The solid solution formed by manganese and iron effectively improves the hardness and strength of ferrite and austenite in steel. Simultaneously, Mn enters the cementite and replaces some iron atoms, thus improving the strength and toughness of the steel. Considering all factors, the suitable Mn content is 1.0–1.7%, preferably 1.05–1.68%.

[0028] Al: Al is a deoxidizing element, and the resulting AlN element can effectively refine grains. When the content is below 0.01%, deoxidation is insufficient, and when it exceeds 0.070%, it is detrimental to the toughness of the weld heat-affected zone. Taking all factors into consideration, the suitable Al content is 0.015% to 0.060%.

[0029] Ti: During the solidification process of steel, Ti can combine with N to form TiN. The resulting carbonitrides precipitate in the matrix, pinning the austenite grain boundaries and hindering austenite grain growth, thereby refining the austenite grains. When Ti exists in ferrite in a solid solution state, its strengthening effect is higher than that of Al, Mn, Ni, Mo, etc. Considering all factors, the suitable Ti content is 0.03%–0.05%.

[0030] Nb: Nb is dissolved in the austenitic matrix and, due to the size effect, tends to segregate at grain boundaries. This hinders the movement of new grain boundaries during recrystallization after austenite deformation, greatly refining the grain size. During the cooling process after high-temperature deformation, the segregation of Nb atoms at grain boundaries significantly impedes the nucleation of new phases at the grain boundaries, shifting the proeutectoid ferrite formation region to the right, thus achieving a uniform bainitic structure over a wide range of cooling rates. Considering all factors, the suitable Nb content is 0.020–0.050%.

[0031] Nitrogen (N): Nitrogen dissolved in molten steel precipitates during solidification due to decreased solubility. It combines with Si and Al elements in the steel to form nitrides such as SiN and AlN. A small amount of nitrides can refine the grain size of the steel, while a large amount of nitrides will reduce the plasticity and toughness of the steel. Taking all factors into account, N ≤ 0.005%.

[0032] P and S: P in steel tends to segregate, reducing the toughness and weldability of the steel. S tends to form plastic sulfides, causing delamination defects. Therefore, the lower the P and S content, the better. Taking all factors into consideration, P ≤ 0.018% and S ≤ 0.005%.

[0033] The reason this invention employs a deep desulfurization treatment mode in the LHF process, with a slag basicity of CaO / SiO2 = 2.0–2.5; after the initial addition of desulfurization slag, deoxidizing alloy, and temperature-regulating scrap steel, bottom-blown Ar gas is applied at a rate of 70–90 Nm³ / h with stirring for 3–10 minutes; after the deep desulfurization treatment is completed and Ti alloy is added, bottom-blown Ar gas is applied at a rate of 15–30 Nm³ / h with stirring for 3–5 minutes, ending the LHF treatment with a total treatment time ≥18 minutes, is to reduce the oxidation of Ti by oxygen in the steel after the addition of Ti, increase the solid solution content of Ti, control the amount of Al₂O₃ in the steel, and ensure the purity of the steel.

[0034] The reason why the temperature of the second heating section and the final soaking section are controlled at 1220-1280℃, and the heating and soaking time is not less than 60 minutes, and the total furnace time is controlled to be not less than 140 minutes, is that the austenite grains will become significantly coarser when heated above 1280℃, while below 1220℃, the core temperature of the slab cannot be guaranteed to be consistent with the surface temperature. In order to ensure complete solid solution and full austenitization of alloying elements, the original austenite grains are controlled to be fine, and the temperature of the slab is guaranteed to be uniform.

[0035] The reason why the roughing exit temperature is controlled at 1100-1160℃ in this invention is to refine the austenite grains and perform roughing in the recrystallization temperature range.

[0036] The reason why the finishing rolling start temperature is controlled at 1000-1100℃ and the finishing rolling finish temperature is 840-880℃ is to adapt to the finishing rolling equipment, reduce the roll load, ensure uniform plastic deformation, and reduce the internal stress of the rolled steel plate.

[0037] The reason why the present invention cools to 650°C to 690°C at a cooling rate of 20 to 30°C / s in the first stage and then cools to the coiling temperature in a laminar flow manner is to obtain a refined ferrite structure in the first stage of cooling and to obtain the effect of grain strengthening by precipitation of Ti elements in the laminar flow cooling coiling stage.

[0038] The reason why the winding temperature is controlled at 620-660℃ is that the precipitation strengthening effect of Ti can be maximized within this range, and the controlled temperature range accounts for more than 96%.

[0039] Compared with existing technologies, this invention utilizes a C-Si-Mn-Nb-Ti elemental system, fully leveraging the dual strengthening mechanism of solid solution strengthening by Mn combined with precipitation grain refinement strengthening by Nb and Ti. Furthermore, by controlling the desulfurization mode and slag basicity during LHF treatment, as well as the timing of Ti alloy addition and the parameters and duration of bottom-blown Ar gas, the solid solution content of Ti is increased, ensuring the purity of the steel and improving its strength, plasticity, and toughness. Through grain refinement control within the temperature range after the initial cooling stage of ultra-fast cooling, the yield strength Rel ≥ 600 MPa, tensile strength Rm ≥ 690 MPa, elongation A ≥ 20%, and cold bending performance D = a and 180° are all satisfactory. This ensures high strength while also providing excellent cold forming and welding performance, meeting the requirements for high-strength bridge shell steel. Moreover, the process is simple, low-cost, and can be implemented under existing production conditions. Attached Figure Description

[0040] Figure 1 This is a metallographic diagram of the present invention. Detailed Implementation

[0041] The present invention will now be described in detail:

[0042] Table 1 is a list of chemical components of the various embodiments and comparative examples of the present invention;

[0043] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0044] Table 3 is a list of performance test results for each embodiment and comparative example of the present invention.

[0045] Each embodiment of the present invention is produced according to the following steps:

[0046] 1) During the converter smelting process:

[0047] The LHF process employs a deep desulfurization treatment mode, controlling the slag basicity at CaO / SiO2 = 2.0–2.5. After the first batch of desulfurized slag, deoxidized alloy, and temperature-controlled scrap steel are added, the bottom-blown Ar gas flow rate is controlled at 70–90 Nm3 / h and maintained for 3–10 minutes.

[0048] After adding the Ti alloy, adjust the bottom-blown Ar gas flow rate to 15-30 Nm3 / h and maintain this flow rate for 3-5 minutes; control the total LHF treatment time to be no less than 18 minutes.

[0049] 2) After casting, the billet is heated. During this process, the heating temperature of the second heating section is controlled at 1220-1280℃, the soaking time in the soaking section is controlled at not less than 60 minutes, and the total time in the furnace is not less than 140 minutes.

[0050] 3) Perform rough rolling and control the rough rolling exit temperature at 1100~1160℃;

[0051] 4) Perform finish rolling, controlling the initial finishing rolling temperature at 1000-1100℃ and the final finishing rolling temperature at 840-880℃;

[0052] 5) Cooling: First, cool to 650℃~690℃ at a cooling rate of 20~30℃ / s, then cool to the winding temperature using laminar flow.

[0053] 6) Perform winding, and control the winding temperature at 620-660℃.

[0054] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0055] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0056]

[0057]

[0058] Continued from Table 2

[0059]

[0060] Table 3. List of mechanical property test results for each embodiment and comparative example of the present invention.

[0061]

[0062] Note: Table 3 is based on tests conducted according to national standards GB / T228 and GB / T231.

[0063] As shown in Table 3, the bridge shell steel obtained in Examples 1-10 has a yield strength Rel ≥ 600 MPa, tensile strength Rm ≥ 690 MPa, elongation A ≥ 20%, and cold bending performance D = a, 180° is qualified. The steel in Comparative Example 1 has higher strength, but its elongation and cold bending performance are lower than those of the examples; the steel in Comparative Example 2 has a higher carbon content, and its elongation is comparable to that of the examples, but its yield strength and cold bending performance are lower than those of the examples.

[0064] This specific embodiment is merely a best example and is not intended to limit the implementation of the technical solution of the present invention.

Claims

1. A type of 700MPa grade cold-formed bridge shell steel with a thickness ≤10mm, comprising the following components and weight percentages: C: 0.05~0.09%, Si: 0.03~0.046%, Mn: 1.0~1.45%, P≤0.018%, S≤0.005%, Als: 0.055~0.060%, Nb: 0.020~0.05%, Ti: 0.036~0.050%, N≤0.005%, with the remainder being Fe and impurities; Production method: 1) Deep desulfurization treatment mode is adopted in LHF, and the slag basicity is controlled at CaO / SiO2=2.0~2.5; after the first batch of desulfurized slag, deoxidized alloy and temperature-controlled scrap steel are added, the bottom blowing Ar gas flow rate is controlled at 70~90Nm3 / h and maintained for 3~10min. After adding the Ti alloy, adjust the bottom-blown Ar gas flow rate to 15~30 Nm3 / h and maintain this flow rate for 3~5 min; control the total LHF treatment time to be no less than 18 min. 2) After casting, the billet is heated. During this process, the heating temperature of the second heating section is controlled at 1252~1280℃, the soaking time in the soaking section is controlled at no less than 66min, and the total time in the furnace is 162~167min. 3) Perform rough rolling and control the rough rolling exit temperature at 1109~1160℃; 4) Perform finish rolling, controlling the initial finishing rolling temperature at 1000~1100℃ and the final finishing rolling temperature at 840~847℃; 5) Cooling: First, cool to 650-690℃ at a cooling rate of 20-30℃ / s, then cool to the winding temperature using laminar flow. 6) Perform winding, controlling the winding temperature at 620~660℃.

2. The method for producing 700MPa grade cold-forming bridge shell steel with a thickness ≤10mm as described in claim 1, comprising the following steps: 1) Deep desulfurization treatment mode is adopted in LHF, and the slag basicity is controlled at CaO / SiO2=2.0~2.5; after the first batch of desulfurized slag, deoxidized alloy and temperature-controlled scrap steel are added, the bottom blowing Ar gas flow rate is controlled at 70~90Nm3 / h and maintained for 3~10min. After adding the Ti alloy, adjust the bottom-blown Ar gas flow rate to 15~30 Nm3 / h and maintain this flow rate for 3~5 min; control the total LHF treatment time to be no less than 18 min. 2) After casting, the billet is heated. During this process, the heating temperature of the second heating section is controlled at 1252~1280℃, the soaking time in the soaking section is controlled at no less than 66min, and the total time in the furnace is 162~167min. 3) Perform rough rolling and control the rough rolling exit temperature at 1109~1160℃; 4) Perform finish rolling, controlling the initial finishing rolling temperature at 1000~1100℃ and the final finishing rolling temperature at 840~847℃; 5) Cooling: First, cool to 650-690℃ at a cooling rate of 20-30℃ / s, then cool to the winding temperature using laminar flow. 6) Perform winding, controlling the winding temperature at 620~660℃.

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