A bridge shell steel for cold forming with high fatigue performance and its production method

Abstract

The present invention belongs to the technical field of bridge shell steel processing, and discloses a bridge shell steel for cold forming with high fatigue performance and a production method thereof. The chemical composition and weight percentage content of the steel are: C: 0.05-0.1%, Si: 0.6-1%, Mn: 1.2-1.7%, P≤0.02%, S≤0.005%, Als: 0.02-0.06%, Ti: 0.05-0.1%, RE: 0.001-0.003%, and the rest are Fe and impurities. The present invention adopts an economical C‑Si‑Mn‑Ti‑RE element system, fully utilizing the beneficial effects of Si, RE and other elements on the fatigue performance of steel, and through a dual strengthening mechanism combining the solid solution strengthening of Si and Mn elements with the precipitation strengthening of Ti, while ensuring the high strength and cold forming performance of the steel, it has excellent welding and fatigue performance, meeting all the use requirements of bridge shell steel.

Description

Technical Field

[0001] The invention belongs to the technical field of bridge shell steel processing, and in particular relates to a bridge shell steel for cold processing and forming with high fatigue performance and a production method thereof. Background Art

[0002] As an important component of the chassis system, one of the three major assemblies of commercial vehicles, the automobile axle housing steel currently accounts for about 4% of the total automobile steel plate consumption. The drive axle housing should have sufficient strength and rigidity, as well as good toughness and fatigue life. Most truck axles are made of steel plates that are welded after cold stamping or hot stamping. Therefore, the steel plates are required to have good stamping forming properties as well as good welding properties and fatigue properties. At present, the international standard for the strength of axle housing steel is up to Q460 level, and there is no higher grade technical standard.

[0003] Chinese patent CN104213019A discloses a 600MPa grade automobile bridge housing steel and a production method thereof, wherein 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 rest is Fe and unavoidable impurities, wherein V:N≤5:1, and by controlling the contents of V and N elements while controlling rolling and cooling, a 600MPa grade hot-rolled strip steel for automobile bridge housing is finally obtained, but in the design of this method: the C content is high and the welding performance is poor; the V content is high and the production cost is high, and the fatigue performance parameter design of the steel is not involved.

[0004] Chinese patent CN114015934A discloses a 600MPa grade hot-rolled dual-phase structure bridge shell steel and a production method, wherein the components and weight percentage contents are: 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%. The invention ensures that the tensile strength is at the 600MPa level, and the carbon content is low, the welding performance is excellent, and the performance is stable. However, the method adopts C-Mn-Nb component design strengthening. Due to the inclusion of Nb alloy, the cost is relatively high, and the fatigue performance parameter design of the steel is not involved.

[0005] In summary, the existing bridge shell steels with a tensile strength of 600MPa have technical difficulties such as high carbon equivalent, high production cost, and poor cold forming performance. There is no optimization plan for the fatigue performance of steel, and they cannot meet the needs of lightweighting of higher-end automobiles. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a bridge shell steel with high fatigue performance for cold working forming and a production method thereof in view of the deficiencies in the prior art. The economical C-Si-Mn-Ti-RE element system is adopted to make full use of the beneficial effects of elements such as Si and RE on the fatigue performance of steel. The dual strengthening mechanism of the solid solution strengthening of Si and Mn elements combined with the precipitation strengthening of Ti is used to ensure the high strength and cold forming performance of the steel while giving it excellent welding and fatigue performance, thus meeting all the use requirements of the bridge shell steel.

[0007] In order to solve the technical problem proposed by the present invention, the present invention provides a bridge shell steel for cold working forming with high fatigue performance, whose chemical composition and weight percentage content are: C: 0.05-0.1%, Si: 0.6-1%, Mn: 1.2-1.7%, P≤0.02%, S≤0.005%, Als: 0.02-0.06%, Ti: 0.05-0.1%, RE: 0.001-0.003%, and the rest are Fe and impurities.

[0008] Preferably, the chemical composition and weight percentage of the high fatigue performance cold-working bridge shell steel are as follows: C: 0.061-0.092%, Si: 0.61-0.93%, Mn: 1.311-1.633%, P≤0.02%, S≤0.005%, Als: 0.02-0.056%, Ti: 0.055-0.087%, RE: 0.0011-0.0028%, and the rest are Fe and impurities.

[0009] In the above scheme, the yield strength Rel≥500MPa, tensile strength Rm≥600MPa, elongation A≥20% of the high fatigue performance cold-formed bridge shell steel, cold bending performance D=a, 180° qualified, and axial fatigue strength lower limit ≥220MPa.

[0010] The present invention also provides a method for producing a bridge shell steel with high fatigue performance for cold forming, the process flow of which includes converter smelting, RH treatment, continuous casting into billets, heating of the billets, rolling, cooling and coiling.

[0011] In the above scheme, the RH treatment adopts a light treatment mode, the vacuum degree is 0.1-8.0 KPa, and the total treatment time is ≥12 min.

[0012] In the above scheme, after the alloy components are added in the RH treatment process, rare earth element RE is added, and the addition amount of RE is 0.05-0.1 kg / t steel. After adding RE, the vacuum cycle is completed for more than 5 minutes.

[0013] In the above scheme, the ingot heating adopts a three-stage heating system, namely the first heating stage, the second heating stage and the soaking stage, wherein the heating temperatures of the second heating stage and the soaking stage are 1220-1280°C, the soaking time is ≥40min, and the total time in the furnace is ≥120min.

[0014] In the above scheme, the rolling includes rough rolling and finish rolling; the outlet temperature of rough rolling is 1020-1060°C; the start rolling temperature of finish rolling is 950-1050°C, the rolling speed is 2-5m / s, and the final rolling temperature is 810-880°C.

[0015] In the above scheme, the cooling process is first cooled to 680°C to 750°C at a cooling rate of 10 to 20°C / s, and then laminar cooled to the coiling temperature.

[0016] In the above scheme, the coiling temperature is 560-640°C.

[0017] The components and their content ranges of the present invention are mainly based on the following principles:

[0018] C: C can play a role in solid solution strengthening in steel. When the C content is controlled within a reasonable range, the steel decomposes a small amount from austenite to ferrite and cementite during the heating, rolling and cooling process, and can form needle-shaped or lath-shaped ferrite with excellent toughness and discontinuous carbides, which is beneficial to improve the fatigue life of the steel. Taking all factors into consideration, the appropriate C content is 0.05-0.1%, preferably 0.061-0.092%.

[0019] Si: Si has a good deoxidizing effect on molten steel. When its mass fraction is above 0.4%, it exists in the form of solid solution in ferrite or austenite, which can reduce the austenite phase area and improve the hardness and strength of ferrite and austenite. Its effect is stronger than that of manganese, nickel, chromium, tungsten, aluminum, vanadium, etc., and it significantly improves the elastic limit, yield strength and yield strength ratio of steel, and improves fatigue strength. Taking all factors into consideration, the appropriate Si content is 0.6-1%, preferably 0.61-0.93%.

[0020] Mn: Mn is a good deoxidizer and desulfurizer with low alloy price. The solid solution formed by manganese and iron can effectively improve the hardness and strength of ferrite and austenite in steel. At the same time, Mn is a carbide-forming element, which enters cementite to replace part of iron atoms, improves the strength and toughness of steel, and thus helps to improve fatigue strength. Taking all factors into consideration, the appropriate Mn content is 1.2-1.7%, preferably 1.311-1.633%.

[0021] Al: Al is a deoxidizing element. The AlN element formed can effectively refine the grains. When the content is below 0.01%, deoxidation is insufficient. When it exceeds 0.07%, it is not conducive to the toughness of the welding heat affected zone. Taking all factors into consideration, the appropriate Als content is 0.02-0.06%, preferably 0.02-0.056%.

[0022] Ti: Ti can combine with N to form stable TiN during the solidification process of steel. TiN precipitation is more stable than Nb and V carbonitrides. The formed carbonitrides precipitate in the matrix and pin to the grain boundaries of austenite, hindering the growth of austenite grains and thus refining 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. When the mass fraction of Ti is between 0.03% and 0.1%, the yield strength can be increased and the fatigue strength of steel can be enhanced. Taking all factors into consideration, the appropriate Ti content is 0.05-0.1%, preferably 0.055-0.087%.

[0023] RE: RE has a strong desulfurization, deoxidation and modification effect. The rare earth elements added to the steel first combine with oxygen to transform the original Al2O3 into rare earth aluminate inclusions (REAl 11 O 18 or REA l O3), it is compounded with MnS or distributed in the steel as fine particles. These modified sulfides do not change into spheres or break into small pieces during rolling, and the horizontal and vertical lengths are basically equal, thereby significantly improving the fatigue life of the steel; at the same time, RE will also refine the grain size, strengthen the matrix, and reduce the spacing between pearlite sheets, thereby improving the fatigue strength of the steel. However, the manifestation of these effects is closely related to its addition process. If it is not added properly, the number of inclusions in the steel will not only not decrease, but will increase. Comprehensively considered, the appropriate RE content is 0.001-0.003%, preferably 0.0011-0.0028%.

[0024] P, S: P in steel is easy to segregate and reduce the toughness and welding performance of steel. S is easy to form plastic sulfide and produce delamination defects. S higher than 0.005% will significantly reduce the inclusion modification effect of RE element. Considering comprehensively, P≤0.02%, S≤0.005%.

[0025] The technical conception of the main process parameters of the manufacturing method of the present invention is as follows:

[0026] The present invention selects mode parameters during RH treatment, and controls the timing, amount and vacuum cycle duration of RE elements, in order to minimize the number of inclusions in steel after rare earth elements are added, and ensure excellent fatigue performance of steel. The heating temperature of the second heating section and the soaking section of the billet heating process is controlled to 1220-1280°C, because the austenite grains will be significantly coarse when heated to above 1280°C, and the temperature of the core of the slab cannot be guaranteed to be consistent with the surface below 1220°C. The outlet temperature of rough rolling is controlled to 1020-1060°C, in order to refine the austenite grains and perform rough rolling in the recrystallization temperature zone. The control of finishing rolling parameters is to adapt to finishing rolling equipment, reduce the roller load, ensure uniform plastic deformation, and reduce the internal stress of the steel plate after rolling. The control of the cooling program is to obtain a refined ferrite soft phase structure in the front cooling stage, and to precipitate Ti elements in the laminar cooling coiling stage to obtain the effect of grain strengthening.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1) The present invention adopts an economical C-Si-Mn-Ti-RE element system, fully utilizes the beneficial effects of elements such as Si and RE on the fatigue properties of steel, and through a dual strengthening mechanism combining solid solution strengthening of Si and Mn elements with precipitation strengthening of Ti, while ensuring the high strength and cold forming properties of the steel, it also has excellent welding and fatigue properties. The obtained bridge shell steel has a yield strength Rel≥500MPa, a tensile strength Rm≥600MPa, an elongation A≥20%, a cold bending performance D=a, 180° qualified, and an axial fatigue strength lower limit ≥220MPa, which meets all the use requirements of cold-formed bridge shell steel, especially the fatigue performance is better than that of hot-rolled bridge shell steel of the same level of strength.

[0029] 2) The present invention innovatively controls the selection of mode parameters during the RH treatment process, as well as the timing and amount of RE element addition and the vacuum cycle duration, thereby ensuring that the RE content and inclusion level in the final steel will not reduce the fatigue performance of the steel. The process is simple and the cost is low, and improved production can be carried out under existing production conditions. DETAILED DESCRIPTION

[0030] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0031] Examples 1-10

[0032] The chemical composition and weight percentage of the bridge shell steel for high fatigue performance cold working forming in Examples 1-10 are shown in Table 1.

[0033] Table 1

[0034]

[0035] In Example 1-10, the production method of bridge shell steel for cold forming with high fatigue performance comprises the process flow of converter smelting, RH treatment, continuous casting into billets, heating of the billets, rolling, cooling and coiling.

[0036] Among them, the RH treatment adopts a light treatment mode, the vacuum degree is 0.1~8.0KPa, and the total treatment time is ≥12min; after the alloy components are added in the RH treatment process, the rare earth element RE is added, and the addition amount of RE is 0.05~0.1Kg / t steel. After adding RE, the vacuum cycle is ended after more than 5min of vacuum cycle; the main parameters are shown in Table 2.

[0037] Table 2

[0038]

[0039] The heating of the ingot adopts a three-stage heating system, namely the first heating stage, the second heating stage and the soaking stage, wherein the heating temperature of the second heating stage and the soaking stage is 1220-1280°C, the soaking time is ≥40min, and the total time in the furnace is ≥120min. The main parameters are shown in Table 3.

[0040] Table 3

[0041]

[0042] Among them, rolling includes rough rolling and finish rolling; the outlet temperature of rough rolling is 1020-1060°C; the starting temperature of finish rolling is 950-1050°C, the rolling speed is 2-5m / s, and the final rolling temperature is 810-880°C; the main parameters are shown in Table 4.

[0043] Table 4

[0044]

[0045] The cooling process is first cooled to 680°C to 750°C at a cooling rate of 10 to 20°C / s, and then laminar cooled to the coiling temperature; the coiling temperature is 560 to 640°C; the main parameters are shown in Table 5.

[0046] Table 5

[0047]

[0048] According to GB / T228, GB / T232 and GBT26077 national standards, various properties of the steel in Examples 1-10 and Comparative Examples 1-2 were tested, wherein the lower limit of axial fatigue strength was stress ratio R=0.15, given a cycle of 1.0×10 7The test results are shown in Table 6.

[0049] Table 6

[0050]

[0051] As can be seen from Table 6, the yield strength Rel≥500MPa, tensile strength Rm≥600MPa, elongation A≥20% of the bridge housing steel obtained in Examples 1-10, cold bending performance D=a, 180° qualified, axial fatigue strength lower limit ≥220MPa, meet all the use requirements of cold-formed bridge housing steel. The steel strength in Comparative Example 1 is higher, but the elongation, cold bending performance, and axial fatigue strength lower limit strength are lower than those in the embodiment; the steel in Comparative Example 2 has a high carbon content, resulting in a low yield strength, and the elongation is equivalent to that in the embodiment, but the cold bending performance and axial fatigue strength lower limit strength are lower than those in the embodiment.

[0052] The above embodiments are merely examples for clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above descriptions. It is not necessary and impossible to list all the implementation methods here, and the obvious changes or modifications derived therefrom are still within the protection scope of the invention.

Claims

1. A bridge housing steel for cold forming with high fatigue performance, characterized in that: Its chemical composition and weight percentage content are: C: 0.05-0.1%, Si: 0.6-1%, Mn: 1.2-1.477%, P≤0.02%, S≤0.005%, Als: 0.02-0.06%, Ti: 0.05-0.1%, RE: 0.001~0.003%, the rest is Fe and impurities; The production process of the bridge shell steel for cold forming with high fatigue performance includes converter smelting, RH treatment, continuous casting into billets, heating of the cast billets, rolling, cooling and coiling, wherein: the RH treatment adopts a light treatment mode, the vacuum degree is 0.1-8.0 kPa, and the total treatment time is ≥12 min; in the RH treatment process, rare earth elements RE are added after the alloy components are added, and the addition amount of RE is 0.05-0.1 kg / t steel. After adding RE, the vacuum cycle is ended after the vacuum cycle is more than 5 minutes.

2. The high fatigue performance cold-working bridge shell steel according to claim 1, characterized in that: The yield strength Rel of the bridge shell steel is ≥500MPa, the tensile strength Rm is ≥600MPa, the elongation A is ≥20%, the cold bending performance D=a, 180° is qualified, and the lower limit of axial fatigue strength is ≥220MPa.

3. A method for producing a bridge shell steel for cold forming with high fatigue performance as claimed in any one of claims 1 to 2, wherein the process comprises converter smelting, RH treatment, continuous casting into billets, billet heating, rolling, cooling and coiling, wherein: The RH treatment adopts a light treatment mode, the vacuum degree is 0.1-8.0 kPa, and the total treatment time is ≥12 min. In the RH treatment process, rare earth element RE is added after the alloy components are added, and the addition amount of RE is 0.05-0.1 kg / t steel. After adding RE, the vacuum cycle is completed for more than 5 minutes.

4. A method for producing a bridge shell steel for cold forming with high fatigue performance as claimed in claim 3, characterized in that: The casting heating adopts a three-stage heating system, wherein the heating temperature of the second heating stage and the soaking stage is 1220-1280°C, the soaking time is ≥40min, and the total time in the furnace is ≥120min.

5. A method for producing a bridge shell steel for cold forming with high fatigue performance as claimed in claim 3, characterized in that: The rolling includes rough rolling and finish rolling; the outlet temperature of the rough rolling is 1020-1060°C; the start rolling temperature of the finish rolling is 950-1050°C, the rolling speed is 2-5m / s, and the final rolling temperature is 810-880°C.

6. A method for producing a bridge shell steel for cold forming with high fatigue performance as claimed in claim 3, characterized in that: The cooling process is firstly to cool to 680°C to 750°C at a cooling rate of 10 to 20°C / s, and then to laminar cooling to the coiling temperature.

7. A method for producing a bridge shell steel for cold forming with high fatigue performance as claimed in claim 3, characterized in that: The coiling temperature is 560-640°C.

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