A production method of non-quenched and tempered steel for automobile half shaft

Abstract

The invention belongs to the technical field of metallurgy and discloses a production method of non-quenched and tempered steel for automobile half shafts. The steel composition is: C 0.39-0.42%, Si 0.42-0.48%, Mn 1.22-1.30%, Cr 0.52-0.60%, P≤0.020%, S 0.018-0.028%, Al 0.018-0.032%, Ti 0.025-0.038%, V 0.11-0.13%, B 0.0016-0.0030%, Nb 0.030-0.055%. The smelting process (precise control of components in converter, LF refining, RH vacuum treatment process), continuous casting process and rolling process are reasonably optimized, so that the strength and toughness indexes of the material are optimized, the ability to prevent grain growth under high temperature state is improved, the non-equilibrium state organization is suppressed, and the comprehensive performance indexes of the product are leading in China.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and relates to a non-quenched and tempered steel for automobile half shaft and a production method thereof, and specifically relates to a steelmaking and rolling process of the non-quenched and tempered steel for automobile half shaft. Background Art

[0002] Non-quenched and tempered steel controls the cooling rate after rolling or forging through micro-alloying of components, so that the physical and chemical properties of non-quenched and tempered steel reach or approach the level of quenched and tempered steel after heat treatment. Therefore, it can replace quenched and tempered steel for wide application, save a heat treatment process, thereby reducing energy consumption and significantly improving production efficiency.

[0003] The cutting performance of non-quenched and tempered steel is also significantly improved after sulfur addition. Due to a series of advantages, it has been widely used in the fields of automobiles, machinery, electronics, etc. Germany developed the first non-quenched and tempered steel 49MnVS3 in the 1970s and used it for the manufacture of engine crankshafts, core components of automobiles. Then non-quenched and tempered steel has been rapidly developed and applied in the United States, Sweden, and Italy, developed in the West, and used for the manufacture of core parts such as automobile crankshafts, connecting rods, and steering knuckles. In the development and promotion of new varieties and new technologies of non-quenched and tempered steel, Japan has occupied a leading position in the world in recent years. At present, more than 90% of crankshafts and connecting rods in Japan's automobile manufacturing industry are made of non-quenched and tempered steel.

[0004] The non-quenched and tempered steels for passenger car connecting rods, crankshafts, steering knuckles, front axles, half shafts, etc. are developed and the number and scope of applications are expanded. The tensile strength is between 600 and 900 MPa. Compared with developed countries abroad, there is still a large gap in the number, types and high performance of applications, especially in the application field of high-performance hot-forged non-quenched and tempered steels for heavy-duty vehicles.

[0005] Due to the processing characteristics of the half shaft: the local surface is heated to 1200℃, and then forged into splines. However, in the heating transition zone, the temperature is high and there is no deformation, which easily leads to abnormal grain growth in the transition zone. In addition, the half shaft has high strength requirements and high alloy content, and segregation and abnormal martensite and bainite structures are prone to occur during the rolling process. These quality issues have become a major problem in the industry. Summary of the invention

[0006] In view of the technical shortcomings of non-quenched and tempered steel for half shafts, the purpose of the present invention is to provide a non-quenched and tempered steel (40MnSiVS) with uniform structure and capable of preventing high-temperature grain growth and its production process for producing heavy-duty commercial vehicle half shafts.

[0007] The purpose of the present invention is mainly achieved through the following technical solutions:

[0008] The invention relates to a non-quenched and tempered steel for automobile half-axles and a production method thereof. By optimizing and adjusting elements, matching appropriate continuous casting process parameters, high-temperature diffusion heating during rolling and low-temperature heating during secondary heating, and low-temperature rolling processes, the metallographic structure of the non-quenched and tempered steel for half-axles is made uniform, and the ability to prevent grain coarsening during high-temperature heating is enhanced.

[0009] The objective of the present invention is achieved through the following technical solutions:

[0010] 1. A non-quenched and tempered steel for automobile half shafts, the alloy structural steel has the following components by weight percentage: C 0.39-0.43%, Si 0.40-0.50%, Mn 0.90-1.30%, P≤0.025%, S 0.010-0.030%, Al 0.015-0.035%, Cr 0.50-0.70%, Ti 0.020-0.040%, V 0.11-0.16%, B 0.0010-0.0040%, Nb 0.030-0.055%, and the balance is Fe and unavoidable impurities.

[0011] Preferably, the non-quenched and tempered steel has the following components by weight percentage: C 0.39-0.42%, Si 0.42-0.48%, Mn 1.22-1.30%, P≤0.020%, S 0.018-0.028%, Al 0.018-0.032%, Cr0.52-0.60%, Ti 0.025-0.038%, V 0.11-0.13%, B 0.0016-0.0030%, Nb 0.035-0.045%, and the balance is Fe and unavoidable impurities.

[0012] The addition of B and Nb in the present invention not only improves the hardenability of the raw materials, increases the depth of the hardened layer of the induction quenching of the half-axle parts, and improves the strength of the parts, but also prevents grain coarsening during the rolling process of the round steel and prevents the user from having coarse grains during forging heating.

[0013] The production process includes 120t converter smelting, LF refining, RH vacuum treatment, continuous casting and rolling (two-fired material), and the process flow is as follows:

[0014] (1) The converter uses clean and dry scrap steel and controls the scrap steel ratio to 10-15%, and the final carbon content to 0.06-0.30%, thereby preventing over-oxidation of the steel and preventing the carbon content of the finished product from being too high. The converter uses a slide plate to block the slag during the steel tapping process to prevent oxidation slag from falling off the steel, and controls the steel tapping volume to 120±2 tons, which is conducive to the accurate addition of the alloy quantity and ultimately achieves precise control of the composition;

[0015] (2) During the steel-making process of the converter, deoxidizing materials are added in sequence: 1.3-1.5 kg of aluminum cake per ton of steel, 1.4-1.7 kg of silicon carbide per ton of steel, to pre-deoxidize the molten steel; alloys are added: ferrosilicon, silicon manganese, and high-carbon ferrochrome, and finally slag is added.

[0016] (3) In the LF refining process, aluminum powder is used for rapid deoxidation in the early stage, and vanadium-nitrogen alloy is added to adjust the V content after the slag turns yellow and white, ensuring that the white slag time in the entire refining process is 25 to 50 minutes; in the middle and late stages of refining, silicon carbide is used to deoxidize the molten steel and maintain the fluidity of the slag, and then pyrite wire is fed to adjust the S content. The sulfur content of LF steel is controlled at 0.023 to 0.028%, and the aluminum content is 0.035 to 0.042%.

[0017] (4) The RH process strengthens the degassing and inclusion removal of the molten steel. After vacuum treatment, the B content is adjusted to control the B content of the finished product to 0.0020-0.0028%. Finally, the molten steel is subjected to silicon-calcium modification treatment. The first furnace is fed with 50-80 meters of silicon-calcium wire, and the continuous casting furnace is fed with 30-50 meters of silicon-calcium wire. Then, the molten steel is soft-blown with argon.

[0018] (5) Continuous casting uses 300*325mm 2 In order to improve the low-power quality, the continuous casting adopts slow pulling speed, and the pulling speed is controlled to 0.65m / min, and light pressure is used. Through simulation calculation, the maximum pressure reduction is 10mm with slow pulling speed. If the pulling speed is too low, the light pressure will not be able to press down, the pressure reduction will be less than 10mm, and the quality will deteriorate.

[0019] (6) The two-fire rolling process is adopted, and the high-temperature diffusion heating process is adopted for the first fire to fully homogenize the material structure, and the heating temperature is controlled at 1255-1285°C and the heating time is 400-500min. 160*160mm 2 The intermediate billet is sectioned and then subjected to internal ultrasonic testing, with the testing standard grade being C. The qualified products are subjected to surface peeling treatment and then flow into the next process for rolling.

[0020] (7) The second-fired material heating adopts low-temperature heating and low-temperature rolling processes to control the core temperature and prevent the core temperature from being too high during the rolling process and before cooling after rolling, thereby preventing the cooling rate from being too fast and eventually causing abnormal core martensite and bainite structures. The specific low-temperature heating temperature control is: 1110~1150℃ in the soaking section, and the heating time is controlled to be 150~190min. The controlled rolling is carried out by running water before and after rolling and controlling the rolling speed to 4~5m / s to ensure that the final rolling temperature is 730~760℃ and the upper cooling bed temperature is 715~740℃.

[0021] The beneficial effects of the present invention are as follows: considering that high-strength non-quenched and tempered steel has high performance requirements and poor structural uniformity, mixed crystals and abnormal martensite and bainite structures are prone to occur during the user's processing. The present invention has done the following work:

[0022] ①. The chemical composition has been optimized and adjusted, especially the solid solution strengthening elements such as Si, Mn, Cr and precipitation strengthening elements such as V, Ti, Nb. Among them, Mn is further preferably 1.22-1.30%; Cr is preferably controlled at 0.52-0.60%, Si 0.25-0.35%, Ti 0.025-0.038%, V 0.12-0.14%, Nb 0.035-0.045% are designed according to the comprehensive requirements of the strength and toughness of this steel. Among them, the addition of Nb can prevent the user from having mixed crystal structure during high temperature heating.

[0023] ②. The converter, LF refining and RH vacuum treatment processes achieve precise control of the composition.

[0024] ③. The continuous casting process adopts a slow pulling speed and an appropriate amount of light reduction to improve the low-multiple quality of the ingot.

[0025] ④. The steel rolling process adopts two-fire heating, in which the first fire adopts high-temperature heating process to uniform the composition and structure. The intermediate billet adopts ultrasonic flaw detection to pick out the billet with defects in the core. The second fire rolling adopts low-temperature heating and low-temperature controlled rolling to prevent the high temperature in the core and the abnormal martensite and bainite structures in the cooling process after rolling.

[0026] Through the above efforts, the strength and toughness of the material are stabilized and improved, the emergence of abnormal tissues is prevented, and mixed crystals are prevented during the user's processing process, ultimately improving the user's performance of the material.

[0027] The present invention is described in detail below in conjunction with specific embodiments, wherein the conditions not limited thereto are conventional conditions of 40MnSiVS.

[0028] Embodiment 1:

[0029] A non-quenched and tempered steel for automobile half shafts, the chemical composition of which is calculated by weight percentage: C 0.40%, Si 0.42%, Mn 1.26%, Cr 0.54%, P 0.015%, S 0.021%, Al 0.022%, Ti 0.029%, V 0.11%, B 0.0022%, Nb 0.040%, and the balance is Fe.

[0030] The process includes 120t converter smelting, LF refining, RH vacuum treatment, continuous casting and rolling (secondary fire)

[0031] (1) Converter smelting: The scrap steel ratio entering the converter was 13.5%, the final carbon content was 0.12%, and the output of steel was 120.5 tons.

[0032] (2) During the steelmaking process of the converter, 160 kg of aluminum cake and 180 kg of silicon carbide were added.

[0033] (3) In the LF refining process, the white slag refining time is controlled at 30 min, the sulfur content of LF steel is controlled at 0.024%, and the aluminum content is controlled at 0.037%.

[0034] (4) After RH vacuum treatment, B is adjusted. After vacuum treatment, 50 meters of silicon-calcium wire is fed into the molten steel for modification treatment.

[0035] (5) Continuous casting: The continuous casting section is 300 mm × 325 mm, the continuous casting speed is controlled at 0.65 m / min, and the light pressure reduction is controlled at 10 mm.

[0036] (6) Rolling process: the temperature of the first-fire heating and soaking section is 1275°C, the heating time is 430 min, and the rolled billet is subjected to ultrasonic flaw detection. The second-fire rolling is controlled to have a heating and soaking section temperature of 1135°C, a heating time of 175 min, a final rolling temperature of 745°C, and an upper cooling bed temperature of 718°C. After rolling, a 50 mm round steel bar is obtained.

[0037] Embodiment 2:

[0038] A non-quenched and tempered steel for automobile half shafts, the chemical composition of which is calculated by weight percentage: C 0.41%, Si 0.44%, Mn 1.25%, Cr 0.53%, P 0.014%, S 0.022%, Al 0.025%, Ti 0.030%, V 0.12%, B 0.0024%, Nb 0.038%, and the balance is Fe.

[0039] The process includes 120t converter smelting, LF refining, RH vacuum treatment, continuous casting and rolling (secondary fire)

[0040] (1) Converter smelting: The scrap steel ratio entering the converter was 12.7%, the final carbon content was 0.15%, and the output of steel was 120.7 tons.

[0041] (2) During the steelmaking process of the converter, 170 kg of aluminum cake and 190 kg of silicon carbide were added.

[0042] (3) In the LF refining process, the white slag refining time is controlled at 37 min, the LF steelmaking sulfur content is controlled at 0.025%, and the aluminum content is controlled at 0.040%.

[0043] (4) After RH vacuum treatment, B is adjusted. After vacuum treatment, 50 meters of silicon-calcium wire is fed into the molten steel for modification treatment.

[0044] (5) Continuous casting: The continuous casting section is 300 mm × 325 mm, the continuous casting speed is controlled at 0.65 m / min, and the light pressure reduction is controlled at 10 mm.

[0045] (6) Rolling process: The temperature of the first-fire heating and soaking section is 1257°C, the heating time is 435 min, and the rolled billet is subjected to ultrasonic flaw detection. The second-fire rolling is controlled to have a heating and soaking section temperature of 1127°C, the heating time is 182 min, the final rolling temperature is 737°C, and the upper cooling bed temperature is 723°C. After rolling, a 50 mm round steel bar is obtained.

[0046] Embodiment 3:

[0047] A non-quenched and tempered steel for automobile half shafts, the chemical composition of which is calculated by weight percentage: C 0.40%, Si 0.43%, Mn 1.27%, Cr 0.53%, P 0.012%, S 0.025%, Al 0.020%, Ti 0.026%, V 0.11%, B 0.0025%, Nb 0.037%, and the balance is Fe.

[0048] The process includes 120t converter smelting, LF refining, RH vacuum treatment, continuous casting and rolling (secondary fire)

[0049] (1) Converter smelting: The scrap steel ratio entering the converter was 13.4%, the final carbon content was 0.21%, and the output of steel was 121.5 tons.

[0050] (2) During the converter steelmaking process, 180 kg of aluminum cake and 200 kg of silicon carbide were added.

[0051] (3) In the LF refining process, the white slag refining time is controlled at 42 min, the LF steelmaking sulfur content is controlled at 0.027%, and the aluminum content is controlled at 0.036%.

[0052] (4) After RH vacuum treatment, B is adjusted. After vacuum treatment, 45 meters of silicon-calcium wire is fed into the molten steel for modification treatment.

[0053] (5) Continuous casting: The continuous casting section is 300 mm × 325 mm, the continuous casting speed is controlled at 0.65 m / min, and the light pressure reduction is controlled at 10 mm.

[0054] (6) Rolling process: The temperature of the first-fire heating and soaking section is 1263°C, the heating time is 427 min, and the rolled billet is subjected to ultrasonic flaw detection. The second-fire rolling is controlled to have a heating and soaking section temperature of 1138°C, the heating time is 175 min, the final rolling temperature is 755°C, and the upper cooling bed temperature is 721°C. After rolling, 50 mm round steel is obtained.

[0055] Embodiment 4:

[0056] A non-quenched and tempered steel for automobile half shafts, the chemical composition of which is calculated by weight percentage: C 0.39%, Si 0.45%, Mn 1.23%, Cr 0.55%, P 0.012%, S 0.024%, Al 0.027%, Ti 0.032%, V 0.11%, B 0.0020%, Nb 0.041%, and the balance is Fe.

[0057] The process includes 120t converter smelting, LF refining, RH vacuum treatment, continuous casting and rolling (second fire) procedures.

[0058] (1) Converter smelting: The scrap steel ratio entering the converter was 11.9%, the final carbon content was 0.13%, and the output of steel was 119.8 tons.

[0059] (2) During the steelmaking process of the converter, 160 kg of aluminum cake and 180 kg of silicon carbide were added.

[0060] (3) In the LF refining process, the white slag refining time is controlled at 39 min, the LF steelmaking sulfur content is controlled at 0.026%, and the aluminum content is controlled at 0.041%.

[0061] (4) After RH vacuum treatment, B is adjusted. After vacuum treatment, 45 meters of silicon-calcium wire is fed into the molten steel for modification treatment.

[0062] (5) Continuous casting: The continuous casting section is 300 mm × 325 mm, the continuous casting speed is controlled at 0.65 m / min, and the light pressure reduction is controlled at 10 mm.

[0063] (6) Rolling process: The temperature of the first-fire heating and soaking section is 1274°C, the heating time is 456 min, and the rolled billet is subjected to ultrasonic flaw detection. The second-fire rolling is controlled to have a heating and soaking section temperature of 1130°C, a heating time of 169 min, a final rolling temperature of 740°C, and an upper cooling bed temperature of 731°C. After rolling, a 50 mm round steel bar is obtained.

[0064] Comparative Example 1

[0065] The difference between Comparative Example 1 and Example 1 is that the Nb content is reduced to 0.020%, and the other operations are the same as Example 1.

[0066] Comparative Example 2

[0067] The difference between Comparative Example 2 and Example 1 is that 100 m of silicon-calcium wire is fed, and other operations are the same as Example 1.

[0068] Comparative Example 3

[0069] Compared with Example 1, Comparative Example 3 differs in that the actual pulling speed is controlled to be 0.8 m / min, and other operations are the same as those in Example 1.

[0070] Comparative Example 4

[0071] Compared with Example 1, Comparative Example 4 differs in that: the high-temperature heating process is not used in the first-fire blanking, the heating temperature of the soaking section in the first fire is reduced to 1160°C, and the other operations are the same as in Example 1.

[0072] Comparative Example 5

[0073] Compared with Example 1, Comparative Example 5 is different in that low-temperature rolling is not used in the second heat, the final rolling temperature is controlled to be 870°C, and other operations are the same as Example 1.

[0074] The comprehensive performance indicators of Examples 1 to 4 of the present invention and Comparative Examples 1 to 5 are compared as shown in Table 1 below:

[0075] Table 1 Comprehensive performance comparison

[0076]

[0077] The yield strength, tensile strength and impact energy in Table 1 are all test results of round steel in hot-rolled state.

[0078] The results show that through the rational optimization design of the composition, the strength and toughness indexes have been greatly improved, especially the grains have been refined at high temperatures. Through the rational optimization of the continuous casting and rolling process, the material structure has been uniform, and the non-equilibrium structure: bainite and martensite have been effectively controlled. Through the flaw detection of the rolled billet, the final yield rate of the finished product has also been effectively improved. The various indicators of the material are leading in China and can fully meet the user's requirements.

[0079] The raw materials and equipment used in the present invention, unless otherwise specified, are all commonly used raw materials and equipment in the art; the methods used in the present invention, unless otherwise specified, are all conventional methods in the art. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification made to the above embodiments based on the technical essence of the present invention is included in the protection scope of the present invention.

Claims

1. A method for producing non-quenched and tempered steel for automobile axles, characterized in that: The non-quenched and tempered steel has the following components by weight percentage: C 0.39-0.43%, Si 0.40-0.50%, Mn 0.90-1.30%, P≤0.025%, S 0.010-0.030%, Al 0.015-0.035%, Cr 0.50-0.70%, Ti 0.020-0.040%, V 0.11-0.16%, B 0.0010-0.0040%, Nb 0.030-0.055%, and the balance is Fe and unavoidable impurities; The production method of non-quenched and tempered steel for automobile half shafts includes 120t converter smelting, LF refining, RH vacuum treatment, continuous casting and two-fire rolling. The specific steps are: (1) In converter, the scrap steel mass ratio in the raw material is controlled to be 10-15%, the final carbon content is controlled to be 0.06-0.30%, and the tapping process of the converter uses a slide plate to block the slag to prevent oxidation slag from falling off the tapping, and the tapping amount is controlled to be 120±2 tons; (2) During the steel tapping process of the converter, deoxidizing materials are added in sequence: 1.3-1.5 kg / ton of steel of aluminum cake and 1.4-1.7 kg / ton of steel of silicon carbide to pre-deoxidize the molten steel; alloys are added: ferrosilicon, silicon manganese, and high carbon ferrochrome, and finally slag is added; (3) In the LF refining process, aluminum powder is used for deoxidation in the early stage, and vanadium-nitrogen alloy is added to adjust the V content after the slag turns yellow and white, ensuring that the white slag time during the entire refining process is 25 to 50 minutes; silicon carbide is used to deoxidize the molten steel and maintain the fluidity of the slag in the middle and late stages of refining, and then sulfur iron wire is fed to adjust the S content. The sulfur content of LF steel is controlled at 0.023 to 0.028%, and the aluminum content is 0.035 to 0.042%; (4) The RH vacuum treatment process strengthens the degassing and inclusion removal of the molten steel. After the vacuum treatment, the B content is adjusted to control the B content of the finished product to 0.0020-0.0028%. Finally, the molten steel is subjected to silicon-calcium modification treatment. The first furnace is fed with 50-80 meters of silicon-calcium wire, and the continuous casting furnace is fed with 30-50 meters of silicon-calcium wire. Then, the molten steel is soft-blown; (5) Continuous casting uses 300*325mm 2 For the cross section, the continuous casting adopts slow drawing speed, and under the slow drawing speed state, the maximum reduction is adopted; (6) The second-fire rolling process is adopted, and the first-fire billet is made by high-temperature diffusion heating process, the heating temperature is controlled at 1255-1285°C, the heating time is 400-500min, and the 160*160mm 2 The intermediate billet is sectioned and then subjected to internal ultrasonic flaw detection. The qualified products are subjected to surface peeling treatment and then flow into the next process for rolling; (7) The second-heat finished product is heated by low-temperature heating and low-temperature rolling technology, with the heating time controlled at 150-190 min. Water is passed before and after rolling and the rolling speed is controlled at 4-5 m / s to perform controlled air cooling to ensure that the final rolling temperature is 730-760 °C, and then the product is placed on a cooling bed.

2. The method for producing non-quenched and tempered steel for automobile half shafts according to claim 1, characterized in that: The composition of non-quenched and tempered steel by weight percentage is: C 0.39~0.42%, Si 0.42~0.48%, Mn 1.22~1.30%, Cr 0.52~0.60%, P≤0.020%, S 0.018~0.028%, Al 0.018~0.032%, Ti0.025~0.038%, V 0.11~0.13%, B 0.0016~0.0030%, Nb 0.035~0.045%, and the balance is Fe and inevitable impurities.

3. The method for producing non-quenched and tempered steel for automobile half shaft according to claim 1, characterized in that: In step (5), the continuous casting adopts a slow pulling speed, which is controlled at 0.65 m / min and combined with a reduction of 10 mm.

4. The method for producing non-quenched and tempered steel for automobile half shaft according to claim 1, characterized in that: Step (7) Controlling the low temperature heating temperature: the soaking section is 1110-1150°C.

5. The method for producing non-quenched and tempered steel for automobile half shaft according to claim 1, characterized in that: In step (7), the cooling bed temperature is specifically 715-740°C.

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