A high torsional fatigue strength cold precision forged gear steel and its periodic spheroidizing annealing method and production method

Abstract

The invention discloses a high torsional fatigue strength cold precision forging gear steel and a periodic spheroidizing annealing method and a production method thereof. The gear steel contains the following chemical components: C, Si, Mn, Cr, Mo, Ni, Al, Ti, N, Fe, wherein Lm=C+(Cr+Mn+Si) / 4, and Lm: 0.573-0.723. The invention adopts relatively low Mn and Cr element addition, reduces the generation of alloy carbides during annealing, and reduces the influence of alloy elements on the periodic spheroidizing annealing rate and the uniformity of spheroidized product size. The periodic spheroidizing annealing process is adopted to avoid the phenomenon of uneven final spheroidized structure caused by local overheating of cementite and excessive austenitization degree caused by single temperature annealing. After annealing, the structure is a ferrite matrix surface with uniformly sized spheroidized carbides dispersedly distributed, austenite grains do not grow abnormally after carburizing, and the torsional fatigue strength is improved to ≥400MPa.

Description

Technical Field

[0001] The invention belongs to the field of gear steel, and in particular relates to a high torsional fatigue strength cold precision forged gear steel and a periodic spheroidizing annealing method and a production method thereof. Background Art

[0002] With the continuous development of gear processing technology, the cold precision forging process with low manufacturing cost, high yield rate and environmental friendliness has gradually replaced the traditional hot forged gear steel production. For cold precision forged differential gears, the overall production process is: hot rolling → warm forging → spheroidizing annealing → cold extrusion → carburizing → quenching → tempering, etc. The mechanical properties of parts can be effectively improved through cold processing forming methods. However, during the carburizing process of cold precision forged parts, austenite grains tend to grow unevenly, resulting in poor mechanical properties of gear steel and decreased fatigue resistance, which has become the biggest obstacle to the development of cold precision forging.

[0003] In addition, the rapid development of new energy vehicles is bound to bring about the renewal of automotive gear steel. Traditional gear steel can no longer meet the demand. For new energy gear steel with large instantaneous acceleration and greatly increased output power, its output torque is one of the most important considerations. At present, the torsional fatigue strength of gear steel can generally only reach about 300MPa, and there is still a lot of room for improvement. Therefore, it is necessary to develop a gear steel for new energy vehicles with higher torsional fatigue strength, and at the same time, develop a cold precision forging production process that matches this gear steel.

[0004] Chinese patent CN 112981228A published on June 18, 2021 discloses a steel bar for a transmission gear shaft with good cold forging performance and a manufacturing method. By optimizing the stoichiometric ratio of Ti / N in the steel and adopting a high-precision rolling process and a spheroidizing annealing process, a steel with good cold forging processing performance is obtained that can be directly used for cold forging processing, meeting the requirements of excellent cold forging processing performance. However, the patent does not have a targeted solution to the abnormal grain growth phenomenon during the carburizing process after cold forging.

[0005] Chinese patent CN 113755749A published on December 7, 2021 discloses a cold forged gear steel containing Nb and B and a preparation method thereof. By adding Nb and B elements and utilizing the pinning effect of the precipitate phase formed after cold forging and carburizing, the growth of austenite grains is prevented, thereby avoiding local coarse grain phenomenon. At the same time, it can compensate for the decrease in hardenability after reducing the Si and Mn content to meet the hardenability requirements of gears. However, the spheroidizing annealing process in the entire process is not adjusted. Nb and B can only play a partial role in the abnormal growth of austenite grains and cannot completely avoid it.

[0006] In summary, the high torque requirements of the differential of new energy vehicles make the existing cold forged gear steel and the corresponding supporting production process unable to meet the demand well. The abnormal grain growth phenomenon of cold forged gear steel during carburizing requires coordinated adjustment of alloy elements and pretreatment process. At present, there is still a gap in the relevant patents for the above problems. Therefore, it is necessary to develop a high torsional fatigue strength gear steel that meets the cold forging process and its matching pretreatment process to meet the continuous development needs of the automotive industry. Summary of the invention

[0007] The purpose of the present invention is to provide a high torsional fatigue strength cold forged gear steel, appropriately adjust the content of substitutional alloy elements such as Mn and Cr, reduce the formation of alloy cementite, reduce the unevenness of carbide size, and effectively control the carbide precipitation rate during the spheroidizing annealing process.

[0008] The present invention also aims to provide a cyclic spheroidizing annealing method for cold-forged gear steel with high torsional fatigue strength, wherein the metallographic structure after cyclic spheroidizing annealing is ferrite + spheroidal carbide; the tensile strength after cyclic spheroidizing annealing is 1200MPa-1500MPa, the yield strength is ≥950MPa, the elongation is ≥17%, the cross-sectional shrinkage is ≥50%, and the impact energy KU2 is ≥60J.

[0009] The present invention also aims to provide a method for producing high torsional fatigue strength cold-forged gear steel. The high torsional fatigue strength cold-forged gear steel produced by this production method has a torsional fatigue strength of ≥400MPa.

[0010] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0011] A high torsional fatigue strength cold precision forged gear steel contains the following chemical components in weight percentage: C: 0.18-0.21%, Si: 0.17-0.35%, Mn: 0.60-0.75%, Cr: 0.80-0.95%, Mo: 0.20-0.50%, Ni: 0.20-0.30%, Al: 0.030-0.050%, Ti: 0.040-0.060%, P: ≤0.010%, S: 0.005-0.035%, TO: ≤0.001%, [N]: 0.009-0.016%, and the rest are Fe and unavoidable impurity elements, wherein Lm=C+(Cr+Mn+Si) / 4, and Lm: 0.573-0.723.

[0012] In the calculation formula of Lm, the value of each component is the content of each component in the gear steel × 100.

[0013] The metallographic structure of the high torsional fatigue strength cold precision forged gear steel after periodic spheroidizing annealing is ferrite + spheroidal carbide.

[0014] The high torsional fatigue strength cold precision forging gear steel has a tensile strength of 1200MPa-1500MPa, a yield strength of ≥950MPa, an elongation of ≥17%, a cross-sectional shrinkage of ≥50%, and an impact energy KU2 of ≥60J after periodic spheroidizing annealing.

[0015] After the high torsional fatigue strength cold precision forging gear steel is subjected to periodic spheroidizing annealing and carburizing treatment, the grain size is ≥8.0, the grains are uniform, and the torsional fatigue strength is ≥400MPa.

[0016] The cyclic spheroidizing annealing method of the high torsional fatigue strength cold precision forged gear steel provided by the present invention comprises the following steps: rapidly heating the steel material at a rate of 5-10°C / min to a temperature T1 between 20-60°C above the Ac1 temperature and keeping the temperature for 40-80min, wherein the heating rate and the holding time are mainly formulated to retain more residual carbide particles, and a too slow time and a higher temperature will result in a reduction in the residual carbides, and then cooling the steel material at a cooling rate of 5-10°C / min to a temperature T1' between 10-50°C below the Ac1 temperature and keeping the temperature for 40-80min. -80min; then heat again to T2 temperature between 20-60℃ above Ac1 temperature and keep warm for 40-80min, then cool to T2' temperature between 10-50℃ below Ac1 temperature and keep warm for 40-80min; then heat again to T3 temperature between 20-60℃ above Ac1 temperature and keep warm for 40-80min, then cool to T3' temperature between 10-50℃ below Ac1 temperature and keep warm for 40-80min; finally cool to below 400℃ and take out of the furnace and cool in air; wherein, T1>T2>T3; T1'>T2'>T3'.

[0017] The periodic spheroidizing annealing method preferably comprises the following steps: rapidly heating the steel to 770-790°C at a rate of 5-10°C / min and keeping the temperature for 40-80min; subsequently cooling the steel to 700-720°C at a cooling rate of 5-10°C / min and keeping the temperature for 40-80min; then heating the steel to 760-780°C again and keeping the temperature for 40-80min, then cooling the steel to 690-710°C and keeping the temperature for 40-80min; then heating the steel to 750-770°C again and keeping the temperature for 40-80min, then cooling the steel to 680-700°C and keeping the temperature for 40-80min; finally cooling the steel to below 400°C and air cooling the steel out of the furnace; air cooling the steel to 200-250°C and then directly forging the steel and stack cooling the steel.

[0018] The periodic spheroidizing annealing method more preferably comprises the following steps: rapidly heating the steel to 780°C at a rate of 5-10°C / min and keeping the temperature for 40-80min; subsequently cooling the steel to 720°C at a cooling rate of 5-10°C / min and keeping the temperature for 40-80min; then heating the steel to 770°C again and keeping the temperature for 40-80min, and then cooling the steel to 710°C and keeping the temperature for 40-80min; then heating the steel to 760°C and keeping the temperature for 40-80min, and then cooling the steel to 700°C and keeping the temperature for 40-80min; finally cooling the steel to below 400°C and air cooling the steel out of the furnace; air cooling the steel to 200°C-250°C, and then directly forging the steel and stack cooling the steel.

[0019] The production method of the high torsional fatigue strength cold-forged gear steel provided by the present invention comprises the following steps: electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - hot rolling - periodic spheroidizing annealing - forging - carburizing treatment - low temperature tempering treatment; the periodic spheroidizing annealing is carried out by the periodic spheroidizing annealing method described in the present invention.

[0020] In the hot rolling step, the steel billet is heated in a heating furnace and then rolled, and then slowly cooled after the rolling is completed; the starting rolling temperature of the rolling is 1100-1150°C, and the final rolling temperature is 830-870°C.

[0021] The soaking temperature of the steel billet in the heating furnace is controlled at 1150-1180°C, and the total time for preheating, heating and soaking is controlled at 5.0h-10.0h. When heated at a high temperature of 1150-1180°C, Al and [N] are both dissolved in austenite. In the subsequent slow cooling stage, they will be enriched on or around the austenite grain boundaries and enriched in the dendrite gaps. After rolling, the free aluminum is enriched at the original austenite grain boundaries and will not change with the changes in the grain boundaries after heat treatment. The slow cooling is cooling to 600-650°C on a cooling bed and then entering the pit for slow cooling, and the slow cooling time is ≥24h.

[0022] After slow cooling out of the pit, the process also includes grinding and peeling to ensure that there is no decarburization and zero defects on the surface.

[0023] The carburizing method is as follows: after the parts are put into the furnace, the temperature is raised to 920-940° C. within 3 hours for carburizing treatment for 4-5 hours, and then the temperature is lowered to 820-840° C. for keeping the temperature for 3-4 hours before quenching.

[0024] The method of low temperature tempering treatment is: keeping warm at 180-220° C. for 1.5-2.5 hours.

[0025] The present invention also provides an application of the high torsional fatigue strength cold-forged gear steel as a new energy vehicle gear steel. The high torsional fatigue strength cold-forged gear steel can fully meet the high torque requirements of new energy gear steel with large instantaneous acceleration and greatly improved output power.

[0026] The components of the high torsional fatigue strength cold precision forged gear steel provided by the present invention have the following functions and controls:

[0027] C: C is the most basic and effective strengthening element in steel, and is also the most effective element affecting hardenability, and has a low cost. In order to ensure that gear steel has sufficient strength and hardenability, the C content needs to be increased; at the same time, the C element is also the main component factor of carbides formed during spheroidizing annealing. The main type of carbides precipitated during spheroidizing annealing is M3C, which is crucial to the final effect of spheroidizing annealing. Therefore, the carbon content is not less than 0.18, but too high a carbon content will cause greater damage to toughness, so the carbon content is determined to be 0.18-0.21%.

[0028] Si: Si is a deoxidizer. It can improve the hardness of steel through solid solution strengthening and the hardenability of gear steel, thus compensating for the effect of the reduction of Mn and Cr content on hardenability. At the same time, Si is distributed between ferrite and carbide during the spheroidizing annealing and heat preservation stage, resulting in a decrease in the growth rate of cementite. The Si content cannot be lower than 0.17%. However, excessive Si will increase the cold deformation resistance of the material, so the Si content cannot be higher than 0.35%, and the Si content is controlled at 0.17-0.35%.

[0029] Mn: Mn can improve the stability of austenite structure and significantly improve the hardenability of steel. In the present invention, during the spheroidizing annealing process, the main type of carbide is M3C, and the diffusion of alloy elements will form alloy cementite (Fe, Mn)3C. Mn is solid dissolved in Fe3C and can replace part of Fe. During the pearlite transformation, not only the diffusion and redistribution of carbon in austenite is required, but also the diffusion and redistribution of carbide-forming elements in austenite is required. The diffusion rate of carbide-forming elements in austenite is several orders of magnitude lower than that of carbon. The Mn element affects the carbide spheroidization rate to a certain extent. At the same time, excessive Mn will reduce the plasticity of steel, and the toughness of steel will deteriorate during hot rolling. Therefore, the Mn content is controlled at 0.60-0.75%.

[0030] Cr: Cr is another important alloying element for improving hardenability, which can improve the hardenability and strength of steel. In this invention, the role of Cr is similar to that of Mn. It is easy to combine with C to form carbides during spheroidizing annealing, which has an inhibitory effect on the dissolution and spheroidization dynamics of cementite, and the dissolution and spheroidization of cementite. This is mainly caused by the diffusion of Cr between cementite and austenite during the dissolution of cementite. At the same time, too high Cr will form a carbide film to affect the carburizing effect and reduce the performance of the carburized layer, so the Cr content is controlled at 0.80-0.95%.

[0031] Mo: Mo can significantly improve the hardenability of steel, prevent temper brittleness and overheating tendency. In addition, the reasonable combination of Mo and Cr in the present invention can significantly improve the hardenability and tempering resistance, and Mo can refine the grains. However, if the Mo content is too low, the above effects are limited. If the Mo content is too high, it promotes the formation of ferrite film at the grain boundary, which is not conducive to the thermoplasticity of steel, increases the reheat crack tendency of steel, and has a high cost. Therefore, the Mo content is controlled to be Mo: 0.20-0.50%.

[0032] Ni: Ni can effectively improve the toughness of the core of steel, reduce the tough-to-brittle transition temperature, improve low-temperature impact performance, and improve the fatigue strength of steel materials. Another role of Ni in this project is to increase the stacking fault energy, improve the dislocation crossing barrier, and improve the torsion resistance. However, Ni is relatively expensive, and too high Ni content will reduce the machinability after hot working. Therefore, the Ni content is controlled within 0.20-0.30%.

[0033] Al: Al is an effective deoxidizer and can form AlN to refine grains. When the Al content is lower than 0.030%, the effect is not obvious. When it is higher than 0.050%, it is easy to form coarse inclusions, which deteriorates the performance of steel. Therefore, it is necessary to adjust the timing of adding Al in the steelmaking process to ensure that the Al content should be controlled at 0.030-0.050%.

[0034] Ti: Ti has a strong affinity with C, O, and N. The TiN and TiC phases precipitated by Ti in combination with C and N effectively hinder the growth of austenite grain size and play a role in grain refinement. At the same time, Ti in steel can significantly reduce the Ar3 (the final temperature of transformation to austenite when heated) temperature, reduce the stability of the organization, and is conducive to the formation of martensite, thereby improving the hardenability of the test material. Titanium carbonitride precipitated in steel can also serve as a nucleation point for NbC, thereby promoting the precipitation of Nb-containing carbides and increasing the volume fraction of the precipitated phase. Titanium carbonitride has a strong high temperature resistance and can maintain a relatively fine size at 1200℃-1250℃, which is conducive to hindering the grain growth of gear steel during high temperature carburizing, making the precipitation of the second phase more stable to the austenite grain boundary. Therefore, the Ti content is controlled between 0.040 and 0.060%.

[0035] P and S: Sulfur easily forms MnS inclusions with manganese in steel, making the steel hot and brittle. However, adding a small amount of S can significantly improve the cutting performance of gear steel without affecting product performance, and MnS also has the effect of refining grains. P is an element with a strong tendency to segregate, which increases the cold brittleness of steel, reduces plasticity, and is harmful to the uniformity of product organization and performance. Control P≤0.010%, S: 0.005~0.035%.

[0036] TO: TO forms oxide inclusions in steel, control TO: ≤0.001%.

[0037] [N]: It can form compounds with Nb and Al, etc., and refine the grains. Reasonable Al / [N] has a significant effect on grain refinement, while too high N will form continuous casting defects such as bubbles. Therefore, the [N] content should be controlled at 0.009-0.016%.

[0038] Lm: C, Mn, Cr and Si have a great influence on the precipitation and growth of carbides during spheroidizing annealing. Therefore, a carbide correlation coefficient for spheroidizing annealing is specified, Lm = C + (Cr + Mn + Si) / 4, and the range of this coefficient Lm is: 0.573 ~ 0.723. The carbide size obtained by the alloy element ratio beyond this range is relatively coarse, and the spheroidizing annealing effect of steel below this range is not ideal.

[0039] The cyclic spheroidizing annealing method of the cold precision forged gear steel with high torsional fatigue strength provided by the present invention, in the heating and heat preservation stage, the steel is heated to 20℃-60℃ above the Ac1 temperature (740℃) and kept warm for 40-80min, and the temperature of each heating and heat preservation is gradually decreased, so that the cementite is dissolved. This stage mainly completes the precipitation and dissolution process of thin cementite lamellae; at the same time, the diffusion activation energy and nucleation driving force in the heating stage are large, and carbon atoms are more likely to diffuse into the crystal to form small particles of carbide. The distribution of carbides inside the matrix is ​​crucial to the subsequent carbide growth, and also determines the uniformity of the structure after spheroidizing annealing. Different from the traditional single heating and heat preservation process, the present invention adopts a gradually decreasing heat preservation method to avoid the single temperature in the granulation process causing the cementite to be locally overheated and the degree of austenitization to be too sufficient, resulting in the defect of lamellar pearlite in the subsequent cooling and heat preservation process. In addition, a single temperature during the spheroidization growth process will cause the carbides containing a certain alloy element to precipitate too fully during the divorced eutectoid transformation, resulting in uneven spheroidization structure. Therefore, the present invention adopts a periodic spheroidization annealing process.

[0040] In the isothermal stage, the small-particle carbides and undissolved carbides inside the ferrite can all become the center of spontaneous nucleation in this stage, and the addition of the alloying elements Mn and Cr substitutional elements is not enough to diffuse and remain in the carbide in the high temperature insulation stage, so that the Mn and Cr element contents in the carbide are relatively high, and are solid-dissolved in Fe3C, which can replace part of Fe to form alloy cementite (Fe, Mn / Cr)3C. The subsequent isothermal stage becomes the center of spontaneous nucleation, and the Cr and Mn elements do not diffuse over a long distance, but are only enriched at the interface of ferrite and cementite, forming a certain concentration peak, so the growth rate of cementite is faster. That is, an appropriate amount of addition of Mn and Cr in the present invention can suppress the growth of carbides. Meanwhile, the addition of silicon element also slows down the growth rate of carbides in the isothermal stage to a certain extent. The spheroidized structure of uniformly distributed carbides on the ferrite matrix is ​​finally obtained, and the uniformity of organization is better and the deformation is more uniform in the subsequent cold forging process, which avoids the uneven growth of grains in the subsequent process.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] The present invention adopts relatively low addition of Mn and Cr elements, reduces the generation of alloy carbides during annealing, and reduces the influence of alloy elements on the periodic spheroidizing annealing rate and the uniformity of spheroidized product size; adopts the periodic spheroidizing annealing process, avoids the phenomenon of uneven final spheroidized structure caused by local overheating of cementite and excessive austenitization degree caused by single temperature annealing, and the structure after annealing is ferrite matrix surface with uniformly sized spheroidized carbides dispersed, and austenite grains do not grow abnormally after carburizing. The present invention provides a coupling process of improved periodic spheroidizing annealing and appropriate Mn / Cr / Si element addition, which is suitable for the production of cold precision forging gear steel. Under this process, the gear steel of the present invention has uniform grains after carburizing, the comprehensive mechanical properties of the material are improved, and the torsional fatigue strength is increased to ≥400MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the periodic spheroidizing annealing process used for the gear steel in Examples 1 to 3 and Comparative Example 2;

[0044] Figure 2 This is the grain diagram of the gear steel after carburizing treatment in Example 1;

[0045] Figure 3 This is the grain diagram of the gear steel after carburizing treatment in Comparative Example 1;

[0046] Figure 4 This is the grain diagram of the gear steel after carburizing treatment in Comparative Example 2. DETAILED DESCRIPTION

[0047] The invention provides a high torsional fatigue strength cold precision forged gear steel, which contains the following chemical components in weight percentage: C: 0.18-0.21%, Si: 0.17-0.35%, Mn: 0.60-0.75%, Cr: 0.80-0.95%, Mo: 0.20-0.50%, Ni: 0.20-0.30%, Al: 0.030-0.050%, Ti: 0.040-0.060%, P: ≤0.010%, S: 0.005-0.035%, TO: ≤0.001%, [N]: 0.009-0.016%, and the rest are Fe and unavoidable impurity elements, wherein Lm=C+(Cr+Mn+Si) / 4, and Lm: 0.573-0.723.

[0048] The production method of high torsional fatigue strength cold precision forged gear steel comprises the following steps: electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - hot rolling - periodic spheroidizing annealing - forging - carburizing treatment - low temperature tempering treatment; the periodic spheroidizing annealing is carried out by the periodic spheroidizing annealing method described in the present invention.

[0049] In the hot rolling step, the steel billet is heated in a heating furnace and then rolled, and then slowly cooled after the rolling is completed; the starting rolling temperature of the rolling is 1100-1150°C, and the final rolling temperature is 830-870°C.

[0050] The soaking temperature of the steel billet in the heating furnace is controlled at 1150-1180°C, and the total time for preheating, heating and soaking is controlled at 5.0h-10.0h; the slow cooling is cooling to 600-650°C on a cooling bed and then entering the pit for slow cooling, and the slow cooling time is ≥24h; after slow cooling out of the pit, it also includes the steps of grinding and peeling to ensure that there is no decarburization and zero defects on the surface.

[0051] The periodic spheroidizing annealing method comprises the following steps: rapidly heating the steel material to a temperature T1 between 20°C and 60°C above the Ac1 temperature at a rate of 5-10°C / min and keeping the temperature for 40-80min, then cooling the steel material to a temperature T1' between 10-50°C below the Ac1 temperature at a cooling rate of 5-10°C / min and keeping the temperature for 40-80min; then heating the steel material again to a temperature T2 between 20°C and 60°C above the Ac1 temperature and keeping the temperature for 40-80min, then cooling the steel material to a temperature T2' between 10-50°C below the Ac1 temperature and keeping the temperature for 40-80min; then heating the steel material again to a temperature T3 between 20°C and 60°C above the Ac1 temperature and keeping the temperature for 40-80min, then cooling the steel material to a temperature T3' between 10-50°C below the Ac1 temperature and keeping the temperature for 40-80min; finally cooling the steel material to below 400°C and air cooling the steel material out of the furnace; wherein, T1>T2>T3; T1'>T2'>T3'.

[0052] The carburizing method is as follows: after the parts are put into the furnace, the temperature is raised to 920-940° C. within 3 hours for carburizing treatment for 4-5 hours, and then the temperature is lowered to 820-840° C. for keeping the temperature for 3-4 hours before quenching.

[0053] The method of low temperature tempering treatment is: keeping warm at 180-220° C. for 1.5-2.5 hours.

[0054] The present invention is described in detail below in conjunction with the embodiments.

[0055] The chemical composition and weight percentage of the gear steel in the embodiments and comparative examples are shown in Table 1.

[0056] Table 1

[0057]

[0058]

[0059] Using the gear steel with the composition in each embodiment and comparative example in Table 1, a total of 3 furnaces of the steel of the present invention and 2 furnaces of comparative steel were produced, and the hot-rolled steel was obtained by electric arc furnace smelting-LF refining-RH vacuum treatment-continuous casting-rolling-finishing. Specifically, the continuous casting billet was heated and kept at 1150-1180°C for ≥5h and then rolled into round steel, with a starting rolling temperature of 1100-1150°C and a final rolling temperature of 830-870°C; after rolling, it was cooled to 600-650°C on a cooling bed and then slowly cooled in a pit for 40h. The production process parameters of each embodiment and comparative example are shown in Table 2.

[0060] Table 2

[0061]

[0062] The hot-rolled steels in Examples 1 to 3 and Comparative Example 2 were subjected to a periodic spheroidizing annealing treatment, specifically: the hot-rolled steel was rapidly heated to 780°C at a rate of 10°C / min and kept warm for 60 min; then cooled to 720°C at a cooling rate of 10°C / min and kept warm for 60 min; then heated to 770°C again and kept warm for 60 min, and then cooled to 710°C and kept warm for 60 min; then heated to 760°C and kept warm for 60 min, and then cooled to 700°C and kept warm for 60 min; finally cooled to 400°C and air-cooled out of the furnace; air-cooled to 200°C, directly forged and then pile-cooled.

[0063] The hot rolled steel in comparative example 1 was subjected to ordinary annealing treatment, specifically: the steel was heated to 780°C and maintained for 3 hours, then cooled to 670°C and maintained for 5 hours, and finally cooled to 400°C and air-cooled after being taken out of the furnace; after air-cooling to 200°C, it was directly forged and then pile-cooled.

[0064] Table 3 shows the average size of the precipitated phases after annealing of the gear steel in different embodiments and comparative examples. The average value of the diameter of the precipitated phase under 10 viewing fields of the same magnification and different positions is calculated. This value mainly reflects the size of the precipitated phase under different embodiments; at the same time, the variance of the precipitated phases of different embodiments is calculated to indicate the uniformity of the size of the precipitated phases.

[0065] Table 3 Average size and variance of precipitated phases in the examples and comparative examples

[0066] Average size of precipitated phase d / nm Precipitated phase difference Example 1 570 1.29 Example 2 565 1.31 Example 3 550 1.32 Comparative Example 1 880 3.22 Comparative Example 2 930 4.15

[0067] Table 4 is a table of mechanical properties of gear steels of different embodiments and comparative examples after annealing, including tensile strength, yield strength, elongation, cross-sectional shrinkage and room temperature impact energy KU2.

[0068] Table 4 Mechanical properties

[0069]

[0070]

[0071] The gear steel after annealing in the above embodiment and comparative example was cold worked and then subjected to 930℃ carburizing gas quenching + low temperature tempering and then tested. The carburizing process was: 3 hours after the parts were put into the furnace, the temperature was raised to 930℃ and carburized for 4.5 hours, then the temperature was lowered to 830℃ and kept for 3.5 hours before quenching and taking out of the furnace. The whole process ended with a production cycle of 10 to 12 hours; the low temperature tempering temperature parameter was: 200℃ and kept for 2 hours. The carburizing and tempering processes adopted in the comparative example were the same. The test results are shown in Table 5.

[0072] It can be seen from Table 5 that after carburizing and gas quenching at 930°C + low temperature tempering, the gear steels in Examples 1 to 3 of the present invention have a grain size of ≥8.0 and uniform grains. The torsional fatigue strength exceeds 400MPa. However, after carburizing and gas quenching at 930°C + low temperature tempering, the gear steels in the comparative examples have uneven grains and low torsional fatigue strength.

[0073] Table 5 Carburization gas quenching + low temperature tempering grain size, grain uniformity and torsional fatigue strength of the embodiment of the present invention

[0074]

[0075] In the table, for the test of torsional fatigue strength, refer to GB / T 12443 "Torque Controlled Fatigue Test Method for Metallic Materials".

[0076] The detailed description of a high torsional fatigue strength cold-forged gear steel and its periodic spheroidizing annealing method and production method with reference to the above-mentioned embodiments is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A high torsional fatigue strength cold forged gear steel, characterized in that: Containing the following chemical composition in weight percentage: C: 0.18-0.21%, Si: 0.17-0.35%, Mn: 0.60-0.75%, Cr: 0.80-0.95%, Mo: 0.20-0.50%, Ni: 0.20-0.30%, Al: 0.030~0.050%, Ti: 0.040~0.060%, P: ≤0.010%, S: 0.005~0.035%, TO: ≤0.001%, [N]: 0.009-0.016%, the rest are Fe and unavoidable impurity elements, wherein Lm=C+(Cr+Mn+Si) / 4, and Lm: 0.573~0.723; The cyclic spheroidizing annealing method of the cold precision forged gear steel with high torsional fatigue strength comprises the following steps: rapidly heating the steel to a temperature T1 between 20 and 60°C above the Ac1 temperature at a rate of 5-10°C / min and keeping the temperature for 40-80min, then cooling the steel to a temperature T1' between 10 and 50°C below the Ac1 temperature at a cooling rate of 5-10°C / min and keeping the temperature for 40-80min; then heating the steel again to a temperature T2 between 20 and 60°C above the Ac1 temperature and keeping the temperature for 40-80min, then cooling the steel to a temperature T2' between 10 and 50°C below the Ac1 temperature and keeping the temperature for 40-80min; then heating the steel again to a temperature T3 between 20 and 60°C above the Ac1 temperature and keeping the temperature for 40-80min, then cooling the steel to a temperature T3' between 10 and 50°C below the Ac1 temperature and keeping the temperature for 40-80min; finally cooling the steel to below 400°C and air cooling the steel out of the furnace; wherein, T1>T2>T3; T1'>T2'>T3'.

2. The high torsional fatigue strength cold forged gear steel according to claim 1, characterized in that: The metallographic structure of the high torsional fatigue strength cold precision forged gear steel after periodic spheroidizing annealing is ferrite + spheroidal carbide.

3. The high torsional fatigue strength cold forged gear steel according to claim 1, characterized in that: The high torsional fatigue strength cold precision forged gear steel has a tensile strength of 1200MPa-1500MPa, a yield strength of ≥950MPa, an elongation of ≥17%, a cross-sectional shrinkage of ≥50%, and an impact energy KU2 of ≥60J after periodic spheroidizing annealing.

4. The high torsional fatigue strength cold forged gear steel according to claim 1, characterized in that: After the high torsional fatigue strength cold precision forging gear steel is subjected to periodic spheroidizing annealing and carburizing treatment, the grain size is ≥8.0, the grains are uniform, and the torsional fatigue strength is ≥400MPa.

5. The periodic spheroidizing annealing method for cold-forged gear steel with high torsional fatigue strength according to any one of claims 1 to 4, characterized in that: The periodic spheroidizing annealing method comprises the following steps: rapidly heating the steel material to a temperature T1 between 20 and 60°C above the Ac1 temperature at a rate of 5-10°C / min and keeping the temperature for 40-80min, then cooling the steel material to a temperature T1' between 10 and 50°C below the Ac1 temperature at a cooling rate of 5-10°C / min and keeping the temperature for 40-80min; then heating the steel material again to a temperature T2 between 20 and 60°C above the Ac1 temperature and keeping the temperature for 40-80min, then cooling the steel material to a temperature T2' between 10 and 50°C below the Ac1 temperature and keeping the temperature for 40-80min; then heating the steel material again to a temperature T3 between 20 and 60°C above the Ac1 temperature and keeping the temperature for 40-80min, then cooling the steel material to a temperature T3' between 10 and 50°C below the Ac1 temperature and keeping the temperature for 40-80min; finally cooling the steel material to below 400°C and air cooling the steel material out of the furnace; wherein, T1>T2>T3; T1'>T2'>T3'.

6. The periodic spheroidizing annealing method for cold-forged gear steel with high torsional fatigue strength according to any one of claims 1 to 4, characterized in that: The periodic spheroidizing annealing method comprises the following steps: rapidly heating the steel to 770-790°C at a rate of 5-10°C / min and keeping the temperature for 40-80min; subsequently cooling the steel to 700-720°C at a cooling rate of 5-10°C / min and keeping the temperature for 40-80min; then heating the steel to 760-780°C again and keeping the temperature for 40-80min, then cooling the steel to 690-710°C and keeping the temperature for 40-80min; then heating the steel to 750-770°C again and keeping the temperature for 40-80min, then cooling the steel to 680-700°C and keeping the temperature for 40-80min; finally cooling the steel to below 400°C and air cooling the steel out of the furnace; air cooling the steel to 200-250°C and then directly forging the steel and stack cooling the steel.

7. The method for producing high torsional fatigue strength cold forged gear steel according to any one of claims 1 to 4, characterized in that: The production method comprises the following steps: electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - hot rolling - periodic spheroidizing annealing - forging - carburizing treatment - low temperature tempering treatment; the periodic spheroidizing annealing is carried out by the periodic spheroidizing annealing method described in claim 5 or 6.

8. The production method according to claim 7, characterized in that: In the hot rolling step, the steel billet is rolled after being heated in a heating furnace, and then slowly cooled after the rolling is completed; the starting rolling temperature of the rolling is 1100-1150° C., and the final rolling temperature is 830-870° C.

9. The production method according to claim 8, characterized in that: The absorptive temperature of the steel billet in the heating furnace is controlled at 1150-1180°C, and the total time for preheating, heating and absorptive heating is controlled at 5.0h-10.0h; the slow cooling is cooling to 600-650°C on a cooling bed and then entering the pit for slow cooling, and the slow cooling time is ≥24h.

10. The production method according to claim 7, characterized in that: The carburizing method is as follows: after the parts are put into the furnace, the temperature is raised to 920-940° C. within 3 hours for carburizing treatment for 4-5 hours, and then the temperature is lowered to 820-840° C. for keeping the temperature for 3-4 days before quenching.

11. The production method according to claim 7, characterized in that: The method of low temperature tempering treatment is: keeping warm at 180-220°C for 1.5-2.5h.

12. Application of the high torsional fatigue strength cold forged gear steel as claimed in any one of claims 1 to 4 as gear steel for new energy vehicles.

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