Medium carbon gear steel for automobile synchronizer and manufacturing method

Abstract

The invention relates to medium carbon gear steel for an automobile synchronizer. The medium carbon gear steel comprises the following chemical components in percentage by weight: 0.35 to 0.40 percent of C, 0.10 to 0.35 percent of Si, 1.20 to 1.40 percent of Mn, 0.35 to 0.50 percent of Cr, 0.025 to 0.045 percent of S, less than or equal to 0.025 percent of P, less than or equal to 0.20 percent of Cu, less than or equal to 0.030 percent of Al, 0.03 to 0.06 percent of V, 0.0020 to 0.0060 percent of Ca and the balance of Fe and inevitable impurities. The mechanical properties of the controlled-rolling and controlled-rolling strip meet the conditions that Rm is larger than or equal to 850 MPa, Rel is larger than or equal to 700 MPa, A is larger than or equal to 12%, Z is larger than or equal to 50%, the surface hardness of the rolled strip is 180-240 HBW, the Ca / S ratio is controlled to reach a reasonable range, the sulfide form in the steel has the characteristics of being small, distributed in a dispersed mode, small in length-width ratio and the like, damage to metal continuity possibly caused by formation of thick sulfide is reduced, and the machinability of the steel is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and specifically relates to a medium-carbon gear steel material for automobile synchronizers and a manufacturing method thereof. Background Art

[0002] The main function of the automobile synchronizer is to make the gears that are about to mesh reach the same speed during gear shifting, so as to mesh smoothly. The synchronizer achieves this function through the friction plates arranged on the coupling sleeve and the gear set. Before the spur teeth and the vertical teeth of the disc come into contact, the energy of the faster speed side is transferred to the slower speed side through the early friction of the friction plate, thereby increasing the speed of the slower speed side and synchronizing it with the faster speed side. It can ensure smooth gear shifting and play a buffering role at the same time, reducing the impact during gear shifting and making the car run more smoothly.

[0003] The production materials of automobile synchronizers are required to have high strength, wear resistance, corrosion resistance and other properties. At present, the main materials of steel synchronous wheels are 45# steel, 40Cr, 42CrMo, 20CrMnTi, etc. The advantages are strong load-bearing capacity, good wear resistance and heat resistance, easy processing and relatively low cost. However, if the material of the synchronous wheel is too hard and brittle, it will not be resistant to impact and vibration, and it is easy to crack, wear, deform, etc., so it is easy to fail under harsh environments such as high load, high speed and high frequency. Carburizing steel such as 20CrMnTi is used. If the surface hardness reaches HRC58-62 and good strength, carbonitriding treatment is required, and low-temperature tempering treatment is performed after carbonitriding. However, the biggest problem of heat treatment is deformation after heat treatment, and especially important dimensions need to be processed after heat treatment. In addition, because the wall of the synchronizer gear sleeve is thin, it is very easy to cause the gear sleeve to flatten. If it is serious, the gear sleeve needs to be rounded. Therefore, low-carbon carburizing or carbonitriding steel is used, and the processing cost is high.

[0004] Patent CN1548571A discloses a gear steel for manufacturing synchronizer gears. Its weight percentage is, C: 0.25-0.30%, Si≤ 0.37%, Mn: 0.50-0.85%, P≤0.03, S: 0.020-0.060%, Cr: 0.50-0.85%, Ni≤0.10%, Mo≤0.05%, W≤0.05%, Ti: 0.04-0.10%, the rest is Fe and unavoidable impurities. The invention steel is to solve the problems existing in the current synchronizer gear manufacturing, such as large deformation, super-grade ferrite structure, poor cold processing performance, high manufacturing cost, etc., and provide a synchronizer gear steel with small heat treatment deformation and good mechanical processing performance.

[0005] The above-mentioned inventive steel may produce hard spot TiN in the steel after adding Ti, causing early failure and affecting its fatigue life, and the probability of gear deformation will also increase. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a medium carbon gear steel for automobile synchronizer and a manufacturing method in view of the above-mentioned prior art. The processed steel has good direct cutting performance and is easy to produce. After online controlled cooling on the rolling line, the structure is uniform and the strength is high. It can be directly used for the processing of synchronizer parts without deformation of the parts, and it saves users working procedures and reduces production costs.

[0007] The technical solution adopted by the present invention to solve the above problems is: a medium carbon gear steel for automobile synchronizer, the chemical composition of the steel and its mass percentage are: C: 0.35-0.40%, Si: 0.10-0.35%, Mn: 1.20-1.40%, Cr: 0.35-0.50%, S: 0.025-0.045%, P≤0.025%, Cu≤0.20%, Al≤0.030%, V: 0.03-0.06%, Ca: 0.0020-0.0060%, and the balance is Fe and unavoidable impurities.

[0008] The design principles of the chemical elements of the medium carbon gear steel for automobile synchronizer of the present invention are as follows: C is the most basic element in steel and also the most economical strength-enhancing element. In order to ensure that the synchronizer gear has good toughness after normalizing or quenching and tempering, the carbon content of the steel of the present invention is determined to be in the range of 0.35-0.40%.

[0009] Si is a deoxidizing element and also a basic solid solution strengthening element. The Si content range is determined to be 0.10~0.35%.

[0010] Mn has a solid solution strengthening effect on steel. The content of Mn in the steel of the present invention is determined to be within the range of 1.20-1.40% by utilizing the characteristics of strongly improving the hardenability of steel and having low cost.

[0011] Cr has a solid solution strengthening effect on steel. The content of Cr in the steel of the present invention is determined to be in the range of 0.35-0.50% by utilizing the characteristics of improving the hardenability of steel and having low cost.

[0012] The addition of S is mainly to improve the cutting performance of steel, while ensuring that the addition of sulfur has no harmful effect on the plasticity and toughness of steel. Therefore, based on the above factors, the S content range is determined to be 0.025~0.045%.

[0013] In addition to deoxidation, Al in steel forms dispersed fine aluminum nitride particles with N to refine grains. The appropriate Al content is determined by combining it with residual N in steel to be ≤0.03%.

[0014] V exists in the steel in the form of carbides, and its main function is to refine the structure and grains of the steel and improve the strength and toughness of the steel. The present invention controls the vanadium content to be 0.03-0.06%.

[0015] The role of Ca in the steel of the present invention is to modify the MnS formed after sulfur addition in the steel, so that the Ca / S ratio in the steel reaches a reasonable range and the cutting performance of the steel is improved. However, a high content will increase the number and size of point-like oxides in the steel, cause nozzle nodules when casting steel, and also deteriorate the performance of the steel.

[0016] The present invention controls the Ca / S ratio to reach a reasonable range of Ca / S≥0.12, and the sulfide morphology in the steel has the characteristics of being fine, dispersedly distributed and having a small aspect ratio, and can meet the aspect ratio of the sulfide ≤8, thereby reducing the damage to the continuity of the metal that may be caused by the formation of coarse and long strips of sulfide, and improving the machinability of the steel.

[0017] The mechanical properties of the rolled material after controlled rolling and controlled cooling of the present invention meet the following requirements: Rm≥850MPa, Rel≥700MPa, A≥12%, Z≥50%, and the surface hardness of the rolled material is 180-240HBW.

[0018] In the gear steel, Ca / S≥0.12.

[0019] The microstructure of the gear steel is ferrite and pearlite, wherein the ferrite content is controlled at 35-50%.

[0020] Another object of the present application is to provide a method for manufacturing ultra-high strength spring steel for automobile torque converters, the specific production process is: 100t converter - refining outside the furnace - RH vacuum degassing - 240X240mm continuous casting billet - reheating and rolling the casting billet into a product and controlled cooling to achieve a suitable steel structure. The main steps are as follows: (1) Converter smelting The raw materials entering the furnace are molten iron accounting for 85-90%, and the rest is selected high-quality scrap steel; during the steel-out process, the steel-retaining operation is adopted to prevent the oxidation slag; (2) LF furnace refining After the LF furnace is powered on for 10 minutes, 200 kg of lime is added, and the forced deoxidation of the molten steel is strengthened by feeding Al wire. FeSi powder is added to the surface of the refined slag for diffusion deoxidation to maintain good slag fluidity. The LF refining time is controlled at 40-50 minutes. (3) RH vacuum degassing and soft argon blowing The steel flow circulation is maintained for more than 5-10 minutes under high vacuum of 133Pa; sampling and analysis are performed after breaking the air, and the content of all elements is adjusted to meet the internal control requirements; the soft argon blowing time is controlled within 15-30 minutes after RH is discharged; (4) Continuous casting The continuous casting process is protected by setting the crystallizer frequency to 2-4Hz and the current to 100-400A; the secondary cooling section frequency to 8-10Hz and the electromagnetic stirring parameters to 50-200A; the continuous casting molten steel superheat target is controlled at 15-30℃, and the casting speed is controlled at 1.0-1.25m / min; (5) Rolling The continuous casting ingot is heated in a heating furnace with a low oxidizing atmosphere. The ingot is descaled with high-pressure water before rolling. The starting rolling temperature is 1010℃-1050℃. The finishing section is rolled using a KOCKS three-roll mill to ensure that the dimensional accuracy of the rolled material is controlled within ±0.15mm. The final rolling temperature is 870℃-920℃.

[0021] Preferably, in step (1), the key points of the converter smelting and steel-tapping process are controlled as follows: the converter is subjected to terminal steel-tapping carbon control, [C] terminal is not less than 0.10%, [P] ≤ 0.015%; FeS powder and alloy components are added to the ladle at a target of 0.045% to meet the internal control lower limit requirements, and 50% synthetic slag and 50% lime are quantitatively added, and 1.5 kg / t of aluminum and iron are added.

[0022] Preferably, in step (2), the binary basicity is controlled at 1.5-2.2; the first chemical composition sample is taken for refining, and Cr, Mn and Si are adjusted to the internal control target. After analyzing the second sample, all components are fine-tuned to the target value, and fine-tuned according to the [S] content, and the FeS wire is fed according to the standard lower limit + 0.010%. Thereafter, the weight of the CaSi wire fed is determined according to the S content in the steel, and the steel is continued to be refined for about 5 minutes to tap the steel.

[0023] Preferably, in step (5), the temperature of the preheating section is controlled at 600°C-800°C, the temperature of the heating section is controlled at 1160°C-1190°C, and the temperature of the soaking section is controlled at 1170°C-1200°C, and the temperature is kept at this temperature for 160-180 minutes.

[0024] Preferably, in step (5), after the final rolling, the steel is controlled to cool through a two-stage cooling water tank, and the steel is forced to cool rapidly to 700-760°C and placed on a cooling bed and slowly cooled with a heat preservation cover, and the cooling rate of the steel is controlled to be ≤10°C / min. After the steel temperature is ≤500°C, it is taken out of the heat preservation cover and then placed on a cooling bed for stack cooling.

[0025] Compared with the prior art, the advantages of the present invention are: (1) The method of adding sulfur is different from the common method of feeding the FeS line after vacuum degassing. High-purity sulfur powder is added to the steel from the primary furnace and the slag basicity is controlled during the refining period to ensure the recovery rate of S. At the same time, the risk of secondary contamination of molten steel caused by feeding the FeS line in the later stage is reduced. (2) The calcium treatment process of sulfide is advanced to the refining process. On the one hand, it is to reduce the calcium-oxygen products generated by calcium treatment, which can float up during the refining period to reduce inclusions in the steel. On the other hand, it can fully combine calcium and sulfur to better achieve the purpose of modifying manganese sulfide. (3) By controlling the rapid cooling after final rolling and online isothermal treatment, the content of proeutectoid ferrite can be reduced and a balanced structure of ferrite and pearlite can be obtained in the hot-rolled state. The ferrite content can be controlled at 35-50%, meeting the hardness requirements for direct processing of parts, thereby eliminating the deformation of parts caused by quenching and tempering treatment, saving gear production costs, meeting the needs of energy saving, environmental protection, and improving efficiency, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the microstructure diagram of the hot-rolled steel material in Example 1 of the present invention, ×100.

[0027] Figure 2 This is the microstructure diagram of the hot-rolled steel material in Example 2 of the present invention, ×100.

[0028] Figure 3 This is the microstructure diagram of the hot-rolled steel material in Example 3 of the present invention, ×100.

[0029] Figure 4 This is the sulfide and morphology of the hot-rolled steel in Example 1 of the present invention, ×100.

[0030] Figure 5 This is the sulfide and morphology of the hot-rolled steel in Example 2 of the present invention, ×100.

[0031] Figure 6 This is the sulfide and morphology of the hot-rolled steel in Example 3 of the present invention, ×100. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0033] The three embodiments provided by the present invention are all carried out according to the above production process: (1) Converter smelting 1) 85-90% of the raw materials entering the furnace are molten iron, and the rest are selected high-quality scrap steel; 2) Control of key points in converter smelting and steel-tapping process: The final carbon of converter is controlled, [C] final value is not less than 0.10%, [P] ≤ 0.015%; FeS powder and alloy components are added to the ladle at a target of 0.045% to meet the internal control lower limit requirements, and 50% synthetic slag and 50% lime are added in a quantitative manner, and 1.5kg / t of aluminum and iron are added. During the steel-tapping process, steel retention operation is used to prevent oxidation slag.

[0034] (2) LF furnace refining 1) After the LF furnace is powered on for 10 minutes, 200 kg of lime is added, and the forced deoxidation of the molten steel is strengthened by feeding Al wire. FeSi powder is added to the surface of the refined slag for diffusion deoxidation to maintain good slag fluidity (binary basicity is controlled at 1.5-2.2); 2) Refining: Take the first chemical composition sample, adjust Cr, Mn, and Si to the internal control targets, analyze the second sample, fine-tune all components to the target values, make fine adjustments based on the [S] content, and feed the FeS line with additional content according to the standard lower limit + 0.010% (if the S content has already reached the internal control, no additional content is required), then determine the weight of the CaSi line to be fed based on the S content in the steel, and continue refining for about 5 minutes to tap the steel; the LF refining time is controlled at 40-50 minutes.

[0035] (3) RH vacuum degassing and soft argon blowing 1) The steel flow circulation is maintained for more than 5 to 10 minutes under high vacuum (133Pa); 2) Take samples for analysis after breaking the air, and adjust the content of all elements to meet the internal control requirements; 3) After RH is released, the soft argon blowing time is controlled within 15-30 minutes.

[0036] (4) Continuous casting 1) Protect casting during the entire continuous casting process and set the electromagnetic stirring parameters of the crystallizer (frequency 2-4Hz, current 100-400A) and the secondary cooling section (frequency 8-10Hz, 50-200A); 2) The superheat target of continuous casting molten steel is controlled at 15-30°C, and the casting speed is controlled at 1.0-1.25m / min.

[0037] (5) Rolling 1) The continuous casting billet is heated in a heating furnace with a low oxidizing atmosphere. The temperature of the preheating section is controlled at 600℃-800℃, the temperature of the heating section is controlled at 1160℃~1190℃, and the temperature of the soaking section is controlled at 1170℃~1200℃, and the temperature is kept at this temperature for 160-180 minutes; 2) The ingot is descaled by high-pressure water before rolling. The starting rolling temperature is 1010℃-1050℃. The finishing section is rolled using a KOCKS three-roll mill to ensure that the dimensional accuracy of the rolled material is controlled within ±0.15mm. The final rolling temperature is 870℃-920℃.

[0038] 3) After final rolling, the steel is cooled by the second-stage cooling water tank, and the steel is forced to cool quickly to 700~760℃ and placed on a cooling bed with a heat preservation cover for slow cooling. It depends on the phase transformation temperature of the steel. Taking into account the online "isothermal" treatment and temperature drop, it is controlled at 30-50℃ above the Ar1 temperature, and the cooling rate of the steel is controlled to be ≤10℃ / min. This is to carry out online isothermal treatment at this temperature so that the rolled material can be transformed into a balanced ferrite and pearlite structure. When the steel temperature is ≤500℃, it is taken out of the heat preservation cover and then placed on the cooling bed for stack cooling.

[0039] The chemical compositions (wt%) of the various embodiments of the present invention are shown in Table 1.

[0040] Example C Si Mn P S Cr Cu Al V Ca Ca / S 1 0.37 0.18 1.32 0.017 0.032 0.48 0.02 0.022 0.05 0.0041 0.128 2 0.39 0.27 1.36 0.012 0.027 0.42 0.01 0.016 0.04 0.0034 0.126 3 0.40 0.22 1.35 0.014 0.035 0.44 0.02 0.020 0.04 0.0044 0.126 Example Rolling material specifications, mm <![CDATA[R m ,MPa]]> <![CDATA[R el ,MPa]]> A,% Z,% Rolled material hardness, HBW 1 φ40 902 768 16 62 198 2 φ50 930 780 15 58 224 3 φ45 925 785 15 60 206 Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A medium carbon gear steel for automobile synchronizer, the chemical composition of the steel and its mass percentage are: C: 0.35-0.40%, Si: 0.10-0.35%, Mn: 1.20-1.40%, Cr: 0.35-0.50%, S: 0.025-0.045%, P≤0.025%, Cu≤0.20%, Al≤0.030%, V: 0.03-0.06%, Ca: 0.0020-0.0060%, and the balance is Fe and unavoidable impurities.

2. The medium carbon gear steel for automobile synchronizer according to claim 1, characterized in that: In the gear steel, Ca / S≥0.

12.

3. The medium carbon gear steel for automobile synchronizer according to claim 1, characterized in that: The mechanical properties of the gear steel meet the following requirements: Rm≥850MPa, Rel≥700MPa, A≥12%, Z≥50%, and the surface hardness of the rolled material is 180-240HBW.

4. The medium carbon gear steel for automobile synchronizer according to claim 1, characterized in that: The microstructure of the gear steel is ferrite and pearlite, wherein the ferrite content is controlled at 35-50%.

5. A method for producing a medium carbon gear steel for automobile synchronizer according to claim 1, characterized in that: The method comprises the following steps: smelting in an electric furnace or a converter, refining outside the furnace and vacuum degassing, and then casting into a continuous casting billet, which is then heated and rolled into a finished steel product. After the final rolling, the cooling rate of the rolled product needs to be controlled so that the steel has a balanced F+P structure.

6. The method for producing medium carbon gear steel for automobile synchronizer according to claim 5, characterized in that: The method specifically comprises the following steps: (1) Converter smelting The raw materials entering the furnace are molten iron accounting for 85-90%, and the rest is selected high-quality scrap steel; during the steel-out process, the steel-retaining operation is adopted to prevent the oxidation slag; (2) LF furnace refining After the LF furnace is powered on for 10 minutes, 200 kg of lime is added, and the forced deoxidation of the molten steel is strengthened by feeding Al wire. FeSi powder is added to the surface of the refined slag for diffusion deoxidation to maintain good slag fluidity. The LF refining time is controlled at 40-50 minutes. (3) RH vacuum degassing and soft argon blowing The steel flow circulation is maintained for more than 5-10 minutes under high vacuum of 133Pa; sampling and analysis are performed after breaking the air, and the content of all elements is adjusted to meet the internal control requirements; the soft argon blowing time is controlled within 15-30 minutes after RH is discharged; (4) Continuous casting The continuous casting process is protected by setting the crystallizer frequency to 2-4Hz and the current to 100-400A; the secondary cooling section frequency to 8-10Hz and the electromagnetic stirring parameters to 50-200A; the continuous casting molten steel superheat target is controlled at 15-30℃, and the casting speed is controlled at 1.0-1.25m / min; (5) Rolling The continuous casting ingot is heated in a heating furnace with a low oxidizing atmosphere. The ingot is descaled with high-pressure water before rolling. The starting rolling temperature is 1010℃-1050℃. The finishing section is rolled using a KOCKS three-roll mill to ensure that the dimensional accuracy of the rolled material is controlled within ±0.15mm. The final rolling temperature is 870℃-920℃.

7. The method for producing medium carbon gear steel for automobile synchronizer according to claim 6, characterized in that: Step (1) Control of key points in converter smelting and steel-tapping process: Control the final carbon content of converter steel-tapping, [C] final content is not less than 0.10%, [P] ≤ 0.015%; FeS powder and alloy components are added to the ladle at a target of 0.045% to meet the internal control lower limit requirements, and 50% synthetic slag and 50% lime are added in a quantitative manner, and 1.5 kg / t of aluminum and iron are added.

8. The method for producing medium carbon gear steel for automobile synchronizer according to claim 6, characterized in that: In step (2), the binary basicity is controlled at 1.5-2.2; the first chemical composition sample is taken during refining, and Cr, Mn and Si are adjusted to the internal control targets. After analyzing the second sample, all components are fine-tuned to the target values, and fine-tuned according to the [S] content, and the FeS wire is fed according to the standard lower limit + 0.010%. Thereafter, the weight of the CaSi wire fed is determined according to the S content in the steel, and the steel is discharged after refining for about 5 minutes.

9. The method for producing medium carbon gear steel for automobile synchronizer according to claim 6, characterized in that: In step (5), the temperature of the preheating section is controlled at 600°C-800°C, the temperature of the heating section is controlled at 1160°C-1190°C, and the temperature of the soaking section is controlled at 1170°C-1200°C, and the temperature is kept at this temperature for 160-180 minutes.

10. The method for producing medium carbon gear steel for automobile synchronizer according to claim 6, characterized in that: In step (5), the steel after final rolling is controlled to cool through a two-stage cooling water tank, and the steel is forced to cool quickly to 700-760°C and placed on a cooling bed and slowly cooled with a heat preservation cover. The cooling rate of the steel is controlled to be ≤10°C / min. When the temperature of the steel is ≤500°C, it is taken out of the heat preservation cover and then placed on the cooling bed for stack cooling.

Images