A low-temperature carburizing cold precision forging gear steel and its periodic spheroidizing annealing method and production method

Abstract

The present invention discloses a low-temperature carburizing cold precision forging gear steel and a periodic spheroidizing annealing method and production method thereof. The gear steel contains the following chemical components: C, Si, Mn, Cr, Mo, Ni, Al, Nb, Ti, V, N, and Fe. The present invention adopts relatively low addition of Mn and Cr elements, thereby reducing the generation of alloy carbides during annealing and reducing the influence of alloy elements on the periodic spheroidizing annealing rate and the uniformity of spheroidized product size. The periodic spheroidizing annealing process avoids the phenomenon of uneven final spheroidized structure caused by local overheating of cementite and excessive austenitization caused by single-temperature annealing. A sufficient amount of Ni is added to improve the low-temperature impact resistance of the gear steel. Nb and V are added as fine-grain elements to control the growth of austenite grains during high-temperature carburization of the gear steel so as to prevent abnormal growth. After periodic spheroidizing annealing, the gear steel has excellent low-temperature impact resistance.

Description

Technical Field

[0001] The invention belongs to the field of gear steel, and in particular relates to a low-temperature carburizing cold precision forging gear steel and a periodic spheroidizing annealing method and a production method thereof. Background Art

[0002] With the continuous advancement of gear processing technology, the cold precision forging process, known for its low manufacturing cost, high yield rate, and environmental friendliness, has gradually replaced traditional hot-forged gear steel production. The overall production process for cold precision forged differential gears is as follows: hot rolling → warm forging → spheroidizing annealing → cold extrusion → carburizing → quenching → tempering. This cold working method can effectively improve the mechanical properties of parts. However, the uneven growth of austenite grains in cold precision forged parts during the carburizing process leads to poor mechanical properties and decreased fatigue resistance in gear steel, a phenomenon that has become a major obstacle to the development of cold precision forging. The rapid development of high-temperature carburizing technology in gear heat treatment has exacerbated this problem. Unlike the traditional 930°C carburizing process, high-temperature carburizing at temperatures above 980°C has a relatively higher carburizing temperature, which increases the driving force for grain growth during the carburizing stage and further complicates the control of abnormal grain growth.

[0003] In addition, for gear steel used in new energy vehicles, which are increasingly exposed to a wide range of service environments, special attention needs to be paid to its special performance under extreme conditions. In particular, low-temperature impact resistance directly determines its toughness under low-temperature conditions. The impact energy of current gear steel at -40°C and -80°C can only reach 40J, which cannot well meet the needs of use under extreme conditions. At the same time, rotary bending fatigue performance is also one of the most important considerations for gear steel. At present, the rotary bending fatigue strength of gear steel can generally only reach 300-600MPa, which still has a lot of room for improvement. Therefore, it is necessary to develop a gear steel for new energy vehicles with higher low-temperature impact performance and rotary bending 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 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 high-precision rolling process and 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 its preparation method. 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 impact performance and high bending fatigue performance requirements under extreme conditions required by the differential of new energy vehicles make the existing cold-forged gear steel and the corresponding supporting production processes unable to meet the needs well. The use of high-temperature carburizing process to produce gear steel will make the abnormal grain growth that is prone to occur during the normal carburizing process of gear steel more prominent. These require coordinated adjustments in alloying elements and pretreatment processes. Currently, there are no relevant patents for the above-mentioned issues. Therefore, it is necessary to develop a low-temperature resistant cold-forged gear steel that meets the cold-forging process and its supporting pretreatment process. This gear steel needs to have high bending fatigue performance and no abnormal grain growth during high-temperature carburizing to meet the ever-evolving needs of the automotive industry. Summary of the Invention

[0007] The purpose of the present invention is to provide a low-temperature carburizing cold precision forging gear steel, which appropriately adjusts the content of substitutional alloying elements such as Mn and Cr, reduces the formation of alloy cementite, reduces the unevenness of carbide size, and effectively controls the carbide precipitation rate during the spheroidizing annealing process; on the other hand, adds a sufficient amount of Ni to improve the low-temperature impact resistance of the gear steel; and adds Nb and V fine-grain elements to control the growth of austenite grains during high-temperature carburizing of the gear steel to prevent abnormal growth.

[0008] The present invention also aims to provide a cyclic spheroidizing annealing method for low-temperature resistant carburized cold precision forging gear steel, wherein the metallographic structure after cyclic spheroidizing annealing is ferrite + spheroidal carbide; after cyclic spheroidizing annealing, the tensile strength is ≥1250MPa, the yield strength is ≥900MPa, the elongation is ≥20%, the low-temperature impact energy KV2 at -40°C is ≥55J, and the low-temperature impact energy at -80°C reaches KV2 ≥45J.

[0009] The present invention also aims to provide a method for producing low-temperature carburizing cold-forged gear steel. The low-temperature carburizing cold-forged gear steel produced by this production method has a bending fatigue strength of ≥750MPa.

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

[0011] A low-temperature-resistant carburized cold precision forging gear steel contains the following chemical components in percentage by weight: C: 0.18-0.21%, Si: 0.17-0.35%, Mn: 0.60-0.75%, Cr: 0.80-0.95%, Nb: 0.03-0.09%, V: 0.03-0.15%, Mo: 0.20-0.50%, Ni: 1.00-1.20%, Al: 0.030-0.050%, Ti: 0.02-0.05%, P: ≤0.010%, S: 0.005-0.035%, TO: ≤0.001%, [N]: 0.009-0.016%, and the remainder is Fe and unavoidable impurity elements.

[0012] The metallographic structure of the low-temperature carburizing cold precision forging gear steel after periodic spheroidizing annealing is ferrite + spheroidal carbide.

[0013] The low-temperature carburizing cold precision forging gear steel has a tensile strength of ≥1250MPa, a yield strength of ≥900MPa, an elongation of ≥20%, a low-temperature impact energy KV2 of ≥55J at -40°C, and a low-temperature impact energy KV2 of ≥45J at -80°C after periodic spheroidizing annealing.

[0014] After the low-temperature carburizing 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 rotational bending fatigue strength is ≥750MPa.

[0015] The present invention provides a cyclic spheroidizing annealing method for the low-temperature carburizing cold precision forging gear steel, which comprises the following steps: the steel is rapidly heated at a rate of 5-10°C / min to a temperature T1 between 20-60°C above the Ac1 temperature and kept warm for 30-80 minutes, then cooled at a cooling rate of 5-10°C / min to a temperature T1' between 10-50°C below the Ac1 temperature and kept warm for 30-80 minutes; then heated again to the Ac1 temperature The product is kept at a temperature of T2 between 20-60°C above the Ac1 temperature for 30-80 minutes, and then cooled to a temperature of T2' between 10-50°C below the Ac1 temperature for 30-80 minutes; then heated again to a temperature of T3 between 20-60°C above the Ac1 temperature for 30-80 minutes, and then cooled to a temperature of T3' between 10-50°C below the Ac1 temperature for 30-80 minutes; finally cooled to below 400°C, taken out of the furnace and air-cooled; wherein, T1>T2>T3; T1'>T2'>T3'.

[0016] The cyclic spheroidizing annealing method preferably includes the following steps: rapidly heating the steel to 770-790°C at a rate of 5-10°C / min and holding the temperature for 30-80min; subsequently cooling the steel to 710-730°C at a cooling rate of 5-10°C / min and holding the temperature for 30-80min; then heating the steel again to 760-780°C and holding the temperature for 30-80min, and then cooling the steel to 700-720°C and holding the temperature for 30-80min; then heating the steel again to 750-770°C and holding the temperature for 30-80min, and then cooling the steel to 690-710°C and holding the temperature for 30-80min; finally cooling the steel to below 400°C, taking the steel out of the furnace and air cooling the steel; air cooling the steel to 200-250°C, directly forging the steel, and then stack cooling the steel.

[0017] The cyclic spheroidizing annealing method more preferably includes the following steps: rapidly heating the steel to 780°C at a rate of 5-10°C / min and holding it for 60 minutes; subsequently cooling it to 720°C at a cooling rate of 5-10°C / min and holding it for 60 minutes; then heating it again to 770°C and holding it for 60 minutes, and then cooling it to 710°C and holding it for 60 minutes; then heating it to 760°C and holding it for 60 minutes, and then cooling it to 700°C and holding it for 60 minutes; finally cooling it to below 400°C, taking it out of the furnace and air cooling it; air cooling it to 200°C-250°C, then directly forging it and then stack cooling it.

[0018] The present invention provides a method for producing the low-temperature carburizing cold-forged gear steel, which 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 using the periodic spheroidizing annealing method described in the present invention.

[0019] 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 1120-1200° C., and the finishing rolling temperature is 930-970° C.

[0020] The soaking temperature of the steel billet in the heating furnace is controlled at 1200-1240℃, and the total time of preheating, heating and soaking is controlled at 5.0h-10.0h. When heated at a high temperature of 1200-1240℃, Al and N are both dissolved in the austenite, and in the subsequent slow cooling stage, they will be enriched on or around the austenite grain boundaries. Subsequently, although AlN precipitation and residual solid solution Al diffusion occur during the heating process during quenching, the Al segregated near the original coarse austenite grain boundaries moves in the lattice and is difficult to move in large quantities to the vicinity of the newly formed austenite grain boundaries. The amount of solid solution Al is relatively reduced, resulting in the amount of solid solution Al near the grains failing to reach the necessary content to sufficiently improve the hardenability. Therefore, the free aluminum enriched in the dendrite gaps is enriched at the original austenite grain boundaries after rolling and does 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 slow cooling in a pit, and the slow cooling time is ≥24h.

[0021] 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.

[0022] The carburizing treatment method is as follows: after the parts are put into the furnace, the temperature is raised to 970-990° C. within 3 hours, carburized for 4-5 hours, then cooled to 820-840° C., kept at this temperature for 3-4 days, and then quenched.

[0023] The low-temperature tempering treatment method is: keeping the temperature at 180-220° C. for 1.5-2.5 hours.

[0024] The low-temperature carburizing and 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.

[0025] The components of the low-temperature carburizing cold precision forging gear steel provided by the present invention have the following functions and controls:

[0026] 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 should not be less than 0.18. However, too high a carbon content will cause greater damage to toughness, so the carbon content is determined to be 0.18-0.21%.

[0027] Si: Si is a deoxidizer that increases the hardness of steel through solid solution strengthening and improves the hardenability of gear steel, thereby compensating for the impact of reduced Mn and Cr contents on hardenability. Furthermore, during the spheroidizing annealing and holding phase, Si distributes between ferrite and carbide, reducing the cementite growth rate. Therefore, the Si content cannot be lower than 0.17%. However, excessive Si increases the material's resistance to cold deformation, so the Si content cannot exceed 0.35%. The Si content is controlled within a range of 0.17-0.35%.

[0028] 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 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. 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%.

[0029] Cr: Cr is another important alloying element that improves hardenability, enhancing both the hardenability and strength of steel. In this invention, Cr acts similarly to Mn. During spheroidizing annealing, it readily combines with carbon to form carbides, which inhibit the dissolution and spheroidization kinetics of cementite. This is primarily due to the diffusion of Cr between cementite and austenite during cementite dissolution. Excessive Cr content can form a carbide film, impairing the carburizing effect and reducing the performance of the carburized layer. Therefore, the Cr content is controlled within a range of 0.80-0.95%.

[0030] Mo: Mo significantly improves the hardenability of steel, preventing temper brittleness and overheating. Furthermore, the rational combination of Mo and Cr in the present invention significantly enhances hardenability and temper resistance, and Mo also refines grain size. However, too low a Mo content limits these benefits. Excessive Mo content promotes the formation of ferrite films at grain boundaries, negatively impacting the steel's thermoplasticity, increasing the steel's reheat cracking tendency, and increasing cost. Therefore, the Mo content is controlled to 0.20-0.50%.

[0031] Ni: Ni can effectively increase the core toughness of steel, lower the ductile-brittle transition temperature, improve low-temperature impact resistance, and enhance the fatigue strength of steel. Another role of Ni in this project is to increase the stacking fault energy, improve the barrier for dislocations to cross, and enhance torsional resistance. However, Ni is relatively expensive, and excessive Ni content can reduce machinability after hot working. Therefore, the Ni content is controlled within 1.00-1.20%.

[0032] Al: Al is an effective deoxidizer and can form AlN to refine grains. When the Al content is below 0.030%, its effect is not significant. When it exceeds 0.050%, it tends to form coarse inclusions, deteriorating the steel's properties. Therefore, the timing of Al addition during the steelmaking process must be carefully adjusted to ensure that the Al content is controlled within the range of 0.030-0.050%.

[0033] Nb: Nb is a very effective microalloying element for grain refinement. Its carbonitrides can "pin" grain boundaries, hindering austenite grain growth and effectively reducing carburizing and quenching deformation. Its characteristic in steel is to increase the recrystallization temperature of austenite, thereby refining austenite grains and improving the steel's strength and toughness, but this also reduces the steel's hardenability. When the Nb content is less than 0.03%, the carburizing temperature exceeds 980°C and the holding time exceeds 10 hours, which cannot effectively meet the grain size requirements. Excessive Nb has little effect. Therefore, the Nb content is controlled between 0.03 and 0.09%.

[0034] V: Vanadium is a widely used microalloying element that inhibits austenite grain growth during heating. The addition of vanadium inhibits austenite grain growth through V (C, N) precipitation and grain boundary pinning by undissolved V (C, N) particles, thereby increasing the steel's strength and toughness, but also reducing its hardenability. Excessive V addition increases production costs, so the V content should be controlled within a range of 0.03-0.15%.

[0035] Ti: Ti has a strong affinity for carbon, oxygen, and nitrogen. The TiN and TiC phases precipitated by Ti in combination with carbon and nitrogen effectively hinder austenite grain growth, resulting in grain refinement. Furthermore, Ti in steel significantly lowers Ar3 (the final temperature at which the steel transforms to austenite upon heating), reducing structural stability and favoring martensite formation, thereby improving the hardenability of the test material. Titanium carbonitride precipitated in the steel also serves as a nucleation site for NbC, promoting the precipitation of Nb-containing carbides and increasing the volume fraction of the precipitated phase. Titanium carbonitride exhibits strong high-temperature resistance and can maintain a relatively fine size at temperatures between 1200°C and 1250°C. This helps to hinder grain growth during high-temperature carburization in gear steel, making the precipitation of the second phase more stable relative to austenite grain boundaries. Therefore, the Ti content is controlled between 0.02% and 0.05%.

[0036] P and S: Sulfur easily forms MnS inclusions with manganese in steel, causing hot brittleness. However, adding a small amount of S can significantly improve the cutting performance of gear steel without affecting product performance. MnS also has the effect of refining the grain size. P is an element with a strong tendency to segregate, increasing cold brittleness and reducing plasticity, which is detrimental to the uniformity of the product's structure and properties. Control P ≤ 0.010%, and S: 0.005-0.035%.

[0037] TO: TO forms oxide inclusions in steel, control TO: ≤0.001%.

[0038] [N]: It can form compounds with Nb and Al, refining grains. A reasonable Al / [N] ratio significantly contributes to grain refinement, while excessive [N] can cause continuous casting defects such as bubbles. Therefore, the [N] content should be controlled within 0.009-0.016%.

[0039] The cyclic spheroidizing annealing method for low-temperature carburized cold-forged gear steel provided by the present invention is mainly divided into the steps of heating and holding, slow cooling, and isothermal treatment. Due to the addition of Nb, Ti, and V, the steel is heated to a temperature above Ac1 (740°C) and held between 20°C and 60°C for 30-80 minutes, so that the cementite dissolves. 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 precipitation and dissolution degree of pearlite lamellae and the distribution of carbides inside the ferrite matrix are crucial to the subsequent carbide growth and also determine the uniformity of the structure after spheroidizing annealing. Unlike the traditional single heating and holding process, the present invention adopts a gradient decreasing holding method to avoid the single temperature in the granulation process causing local overheating of cementite and excessive austenitization, which produces lamellar pearlite defects in the subsequent cooling and holding process. In addition, a single temperature during the spheroidization growth process will cause 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] Isothermal stage also adopts the isothermal means of gradient reduction, and the small-particle carbide inside ferrite and undissolved carbide all become the center of spontaneous nucleation in this stage, coordinate the interpolation of alloying element Mn, Cr substitutional element, in the high temperature holding stage, because too late to diffuse and remain in carbide, cause the Mn, Cr element content in carbide to be higher, be solid-dissolved in Fe3C in, can replace part Fe, form alloy cementite (Fe, Mn / Cr)3C.The isothermal stage subsequently becomes the center of spontaneous nucleation, and Cr, Mn element do not undergo long-range diffusion, just are enriched in the interface of ferrite and cementite, form certain concentration peak, therefore the growth rate of cementite is faster.That is, appropriate interpolation Mn, Cr can suppress the growth of carbide in the present invention.Meanwhile, the interpolation of silicon element also slows down the growth rate of carbide in isothermal stage to a certain extent.Finally obtain the spheroidized structure of uniformly distributed carbide on ferrite matrix, in subsequent cold forging process, tissue uniformity is better and deformation is more even, avoids the uneven growth of grain in subsequent process.

[0041] Adding a certain amount of Ni can effectively improve the core toughness of steel, reduce the ductile-brittle transition temperature, and improve low-temperature impact performance, resulting in greatly improved low-temperature impact performance of the material and enhanced low-temperature resistance; on the other hand, the increase of Ni elements also improves the hardenability of gear steel, and at the same time can increase the stacking fault energy, so that the dislocation crosses the potential barrier when the metal is subjected to stress, and the rotational bending fatigue strength is improved.

[0042] During the high-temperature carburizing process, the addition of Nb and V acts within the steel, inhibiting the abnormal growth of austenite grains during high-temperature carburization and improving the material's bending fatigue performance. Nb and V are the most commonly used grain-refining elements. On the one hand, they can form carbonitrides with carbon and nitrogen atoms, strengthening the steel through dissolution and precipitation during the heating and cooling process. On the other hand, they can exist as replacement solute atoms at dislocation lines, hindering recrystallization and refining the grains while improving bending fatigue strength.

[0043] During the production process, both Nb and V precipitate as Nb(C,N) and V(C,N). These carbonitrides pin grain boundaries during high-temperature carburizing, inhibiting recrystallization. The pinning effect of V(C,N) primarily occurs below 900°C, while Nb(C,N) is difficult to dissolve below 1100°C. Furthermore, both the solid solution and precipitated forms inhibit grain boundary growth.

[0044] At the same time, Nb and V exist in steel as replacement solute atoms, tending to segregate along dislocation lines, exerting a strong drag on dislocation climb, inhibiting recrystallization nucleation and thus strongly inhibiting recrystallization. During high-temperature carburizing, the effect of Nb and V on recrystallization manifests itself as a solute drag mechanism, which affects grain boundary expansion during carburizing. During carburizing, a large number of carbon atoms enter the carburized layer, increasing the probability of interstitial Nb and V atoms forming MCs and increasing dislocation density. After carburizing, slow cooling and reheating to above the austenitizing temperature allow intracrystalline solutes ample time to migrate to the newly formed MCs on dislocations, nurturing new grain boundaries. This results in finer grains when quenched again. Ni not only effectively improves the core toughness of steel and lowers the ductile-brittle transition temperature, but also enhances low-temperature impact properties, effectively increasing the fatigue strength of the steel. Ni increases the stacking fault energy, improves the potential for dislocations to cross barriers, and enhances torsional resistance.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The present invention adopts relatively low addition of Mn and Cr elements, which reduces the formation of alloy carbides during annealing and reduces the influence of alloy elements on the cyclic spheroidizing annealing rate and the uniformity of spheroidized product size. At the same time, the cyclic spheroidizing annealing process avoids the phenomenon of uneven final spheroidized structure caused by local overheating of cementite and excessive austenitization caused by single temperature annealing. On the other hand, adding an appropriate amount of Ni element effectively improves the core toughness of steel, reduces the ductile-brittle transition temperature, and improves low-temperature impact performance. At the same time, adding Nb and V controls the uniformity of gear steel grains and inhibits abnormal growth of austenite grains during high-temperature carburizing. The present invention provides a coupling process of cyclic spheroidizing annealing and appropriate addition of Mn / Cr / Si / V / Ti / Nb / Ni elements. The process is suitable for cold precision forging production. Under this process, the low-temperature impact performance of the steel of the present invention is greatly improved. At the same time, the grain size after high-temperature carburizing is greater than level 8.0, the comprehensive mechanical properties of the material are improved, and the bending fatigue strength is increased. Finally, after heat treatment, the tensile strength is ≥1250MPa, the yield strength is ≥900MPa, the elongation is ≥20%, the low-temperature impact energy KV2 at -40℃ is ≥55J, the low-temperature impact energy at -80℃ reaches KV2 ≥45J, and the rotational bending fatigue strength is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Schematic diagram of the periodic spheroidizing annealing process used for the gear steel in Examples 1 to 3 and Comparative Example 2;

[0048] Figure 2 This is the grain map of the gear steel after carburizing treatment in Example 1;

[0049] Figure 3This is the grain map of the gear steel after carburizing treatment in Comparative Example 1;

[0050] Figure 4 This is the grain diagram of the gear steel after carburizing treatment in Comparative Example 2. DETAILED DESCRIPTION

[0051] The present invention provides a low-temperature resistant carburizing cold precision forging 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%, Nb: 0.03-0.09%, V: 0.03-0.15%, Mo: 0.20-0.50%, Ni: 1.00-1.20%, Al: 0.030-0.050%, Ti: 0.02-0.05%, 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.

[0052] The production method of the low-temperature carburizing cold precision forging 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 using the periodic spheroidizing annealing method described in the present invention.

[0053] 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 1120-1200° C., and the finishing rolling temperature is 930-970° C.

[0054] The soaking temperature of the steel billet in the heating furnace is controlled at 1200-1240°C, and the total time for preheating, heating and soaking is controlled at 5.0h-10.0h; the slow cooling is to cool it to 600-650°C on the cooling bed and then enter 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.

[0055] The cyclic spheroidizing annealing method 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 holding the temperature for 30-80 minutes, 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 holding the temperature for 30-80 minutes; then heating the steel material again to a temperature T2 between 20-60°C above the Ac1 temperature and holding the temperature for 30-80 minutes, then cooling the steel material to a temperature T2' between 10-50°C below the Ac1 temperature and holding the temperature for 30-80 minutes; then heating the steel material again to a temperature T3 between 20-60°C above the Ac1 temperature and holding the temperature for 30-80 minutes, then cooling the steel material to a temperature T3' between 10-50°C below the Ac1 temperature and holding the temperature for 30-80 minutes; finally cooling the steel material to below 400°C and removing the steel material from the furnace for air cooling; wherein, T1>T2>T3; and T1'>T2'>T3'.

[0056] The carburizing treatment method is as follows: after the parts are put into the furnace, the temperature is raised to 970-990° C. within 3 hours, carburized for 4-5 hours, then cooled to 820-840° C., kept at this temperature for 3-4 days, and then quenched.

[0057] The low-temperature tempering treatment method is: keeping the temperature at 180-220° C. for 1.5-2.5 hours.

[0058] The present invention is described in detail below with reference to the embodiments.

[0059] The chemical composition and weight percentage of the gear steel in the embodiment and comparative example are shown in Table 1.

[0060] Table 1

[0061]

[0062]

[0063] Using the gear steel with the composition in each embodiment and comparative example in Table 1, a total of 3 heats of the present invention steel and 2 heats 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 1200-1240°C for ≥5h and then rolled into round steel. The starting rolling temperature was 1120-1200°C, the finishing rolling temperature was 930-970°C, and after rolling, it was cooled to 600-650°C on a cooling bed and then slowly cooled in a pit for 48h. The production process parameters of each embodiment and comparative example are shown in Table 2.

[0064] Table 2

[0065]

[0066]

[0067] The hot-rolled steels in Examples 1 to 3 and Comparative Example 2 were subjected to a cyclic spheroidizing annealing treatment, specifically: rapid heating to 780°C at a rate of 5-10°C / min and holding for 60 min; subsequently cooling to 720°C at a cooling rate of 5-10°C / min and holding for 60 min; then heating again to 770°C and holding for 60 min, and then cooling to 710°C and holding for 60 min; then heating to 760°C and holding for 60 min, and then cooling to 700°C and holding for 60 min; finally cooling to below 400°C, taking out of the furnace and air cooling; air cooling to 200°C-250°C, and then directly forging and stack cooling.

[0068] The hot-rolled steel in Comparative Example 1 was subjected to conventional annealing treatment, specifically: the steel was heated to 780°C and held for 3 hours, then cooled to 670°C and held 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.

[0069] Table 3 shows the average size of the precipitates after annealing of the gear steels of different embodiments and comparative examples. The average value of the precipitate diameters under 10 viewing fields of the same magnification and different positions is calculated. This value mainly reflects the size of the precipitates under different embodiments; at the same time, the variance of the precipitates of different embodiments is calculated to indicate the uniformity of the precipitate size.

[0070] Table 3 Average size and variance of precipitated phases in Examples and Comparative Examples

[0071] Average size of precipitated phase d / nm Precipitated phase difference Example 1 572 1.34 Example 2 573 1.29 Example 3 585 1.30 Comparative Example 1 885 3.35 Comparative Example 2 950 4.16

[0072] 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 -40°C impact energy KU2 and -60°C impact energy KV2.

[0073] Table 4 Mechanical properties

[0074]

[0075] The annealed gear steel from the above examples and comparative examples was cold worked and then subjected to carburization and gas quenching at 980°C followed by low-temperature tempering before testing. The carburization process involved heating the parts to 980°C 3 hours after entering the furnace, carburizing for 4.5 hours, then lowering the temperature to 830°C and holding for 3.5 hours before quenching and removal from the furnace. The entire process had a production cycle of 10-12 hours. The low-temperature tempering temperature was 200°C and held for 2 hours. The carburization and tempering processes used in the comparative example were identical. The test results are shown in Table 5.

[0076] Table 5 shows that the gear steels in Examples 1-3 of the present invention, after carburizing and gas quenching at 930°C followed by low-temperature tempering, have a grain size ≥8.0, uniform grains, and a bending fatigue strength exceeding 780 MPa. In contrast, the gear steels in the comparative example, after carburizing and gas quenching at 980°C followed by low-temperature tempering, have uneven grains and lower bending fatigue strength.

[0077] Table 5 Carburized gas quenching + low temperature tempering grain size, grain uniformity and torsional fatigue strength of the embodiment of the present invention

[0078]

[0079]

[0080] In the table, the test of rotational bending fatigue strength refers to GB / T 4337 "Metal Rotational Bending Fatigue Test Method".

[0081] The detailed description of a low-temperature carburizing 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. 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 low-temperature carburizing cold precision forging 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%, Nb: 0.03-0.09%, V: 0.03-0.15%, Mo: 0.20-0.50%, Ni: 1.00-1.20%, Al: 0.030-0.050%, Ti: 0.02-0.05%, P: ≤0.010%, S: 0.005-0.035%, TO: ≤0.001%, [N]: 0.009-0.016%, and the rest is Fe and unavoidable impurity elements; The production method of the low-temperature carburizing cold precision forging 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 cyclic spheroidizing annealing method 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 holding the temperature for 30-80 minutes, 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 holding the temperature for 30-80 minutes; then heating the steel material again to a temperature T2 between 20-60°C above the Ac1 temperature and holding the temperature for 30-80 minutes, then cooling the steel material to a temperature T2' between 10-50°C below the Ac1 temperature and holding the temperature for 30-80 minutes; then heating the steel material again to a temperature T3 between 20-60°C above the Ac1 temperature and holding the temperature for 30-80 minutes, then cooling the steel material to a temperature T3' between 10-50°C below the Ac1 temperature and holding the temperature for 30-80 minutes; finally cooling the steel material to below 400°C and removing the steel material from the furnace for air cooling; wherein, T1>T2>T3; and T1'>T2'>T3'.

2. The low-temperature carburizing cold-forged gear steel according to claim 1, characterized in that: The metallographic structure of the low-temperature carburizing cold precision forging gear steel after periodic spheroidizing annealing is ferrite + spheroidal carbide.

3. The low temperature carburizing cold forging gear steel according to claim 1, characterized in that: The low temperature carburized cold precision forging gear steel has a tensile strength of ≥1250MPa, a yield strength of ≥900MPa, an elongation of ≥20%, a low temperature impact energy KV2 of ≥55J at -40℃, and a low temperature impact energy of ≥100J at -80℃ after periodic spheroidizing annealing. KV2≥45J.

4. The low temperature carburizing cold forging gear steel according to claim 1, characterized in that: After the low-temperature carburizing cold precision forging gear steel is subjected to periodic spheroidizing annealing and carburizing treatment, the grain size is ≥8.0 and the grains are uniform; Bending fatigue strength ≥750MPa.

5. The cyclic spheroidizing annealing method for low-temperature carburizing cold-forged gear steel according to any one of claims 1 to 4, characterized in that: The cyclic spheroidizing annealing method 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 holding the temperature for 30-80 minutes, 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 holding the temperature for 30-80 minutes; then heating the steel material again to a temperature T2 between 20-60°C above the Ac1 temperature and holding the temperature for 30-80 minutes, then cooling the steel material to a temperature T2' between 10-50°C below the Ac1 temperature and holding the temperature for 30-80 minutes; then heating the steel material again to a temperature T3 between 20-60°C above the Ac1 temperature and holding the temperature for 30-80 minutes, then cooling the steel material to a temperature T3' between 10-50°C below the Ac1 temperature and holding the temperature for 30-80 minutes; finally cooling the steel material to below 400°C and removing the steel material from the furnace for air cooling; wherein, T1>T2>T3; and T1'>T2'>T3'.

6. The cyclic spheroidizing annealing method for low-temperature carburizing cold-forged gear steel according to any one of claims 1 to 4, characterized in that: The cyclic spheroidizing annealing method comprises the following steps: rapidly heating the steel to 770-790°C at a rate of 5-10°C / min and holding the temperature for 30-80min; subsequently cooling the steel to 710-730°C at a cooling rate of 5-10°C / min and holding the temperature for 30-80min; subsequently heating the steel again to 760-780°C and holding the temperature for 30-80min, then cooling the steel to 700-720°C and holding the temperature for 30-80min; then heating the steel again to 750-770°C and holding the temperature for 30-80min, then cooling the steel to 690-710°C and holding the temperature for 30-80min; finally cooling the steel to below 400°C, taking the steel out of the furnace and air cooling the steel; air cooling the steel to 200-250°C, then directly forging the steel and stack cooling the steel.

7. The method for producing low-temperature carburizing 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 according to claim 5 or 6.

8. The production method according to claim 7, characterized in that 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 1120-1200° C., and the finishing rolling temperature is 930-970° C.

9. The production method according to claim 8, characterized in that The soaking temperature of the steel billet in the heating furnace is controlled at 1200-1240°C, and the total time of preheating, heating and soaking is controlled at 5.0h-10.0h; the slow cooling is cooling to 600-650°C on the 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 treatment method is: after the parts are put into the furnace, the temperature is raised to 970-990°C within 3 hours and carburized for 4-5 hours, then the temperature is lowered to 820-840°C and kept warm for 3-4 days before quenching; the low-temperature tempering treatment method is: kept warm at 180-220°C for 1.5-2.5 hours.

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