Steel for high temperature carburizing gear shaft and manufacturing method of the steel

Abstract

A high temperature carburized gear shaft steel and a method for producing the steel are disclosed. The high temperature carburized gear shaft steel has the following chemical compositions in mass percent: 0.17-0.22% C, 0.05-0.35% Si, 0.80-1.40% Mn, 0.010-0.035% S, 0.80-1.40% Cr, 0.020-0.046% Al, 0.006-0.020% N, 0.002-0.030% Nb, 0.02% V or less, and 0.01% Ti or less. Also disclosed is a method for producing the high temperature carburized gear shaft steel, including the steps of: smelting and casting; heating; forging or rolling; and final finishing. By rationally controlling the chemical element composition of the steel, the gear shaft steel in the present invention can maintain suitable austenite grain size and stability at high temperature, and maintain 5-8 grade austenite grain size before and after high temperature carburization at 940-1050℃, and can be effectively applied to high performance parts such as automotive gearboxes or new energy vehicle reduction gears and differentials, and has good application prospects and value.

Description

[Technical field]

[0001] Technical Field The present invention relates to the technical field of metallurgy, in particular to a high-temperature carburized gear shaft steel and a method for producing said steel. [Background technology]

[0002] background With the intensive progress of globalization of the automobile industry, the market and users' requirements for automobile safety, environmental protection and comfort are increasing, and the technical requirements for automobile parts are also increasing. Obtaining gear or shaft parts with high temperature stability, high fatigue life, easy machining and economic efficiency is one of the important development directions.

[0003] The surface of high-performance gears or shaft parts is usually treated by carburizing, quenching and tempering to obtain a surface with higher hardness and a core with better toughness, and finally to obtain excellent fatigue life and wear resistance. In recent years, in the face of the high technical requirements of gears in automobiles, especially the reduction gears and differentials of new energy vehicles, high temperature carburizing technology has been widely used, which can not only obtain carburized gears with excellent performance, but also greatly improve production efficiency, reduce gas emissions and protect the environment.

[0004] At present, the commonly used gas carburizing temperature at home and abroad is usually below 930°C, while the temperature of high-temperature vacuum carburizing can be as high as 960°C and even above 1000°C due to its oxygen-free processing environment. According to the carburizing principle, the carburizing time to obtain the same thickness of hardened layer can be shortened by about 50% by increasing the carburizing temperature by about 50°C. Therefore, if the carburizing temperature is increased from 930°C to 980°C, the carburizing time can be shortened to 50% of the initial carburizing time, and the production efficiency is significantly improved. In addition, the gear obtained by high-temperature vacuum carburizing has little or no intergranular oxidation on the surface, which can obviously improve the impact fracture resistance. Due to its own advantages, high-temperature vacuum carburizing technology is gradually becoming an inevitable choice to replace gas carburizing technology.

[0005] At present, the widely used MnCr-based carburized gear steel is also widely used in the reduction gear and differential of new energy vehicles due to its excellent overall cost-effectiveness. The main technical problem of MnCr-based high-temperature carburized gear steel is how to increase the carburizing temperature while avoiding the phenomenon of mixed crystals and coarse particles in the gear; once abnormal grain growth occurs, heat treatment deformation and early fatigue failure will easily occur, which may affect the transmission efficiency and cause traffic accidents. In addition, gas quenching with high-temperature vacuum carburizing is widely used to deal with the quenching and tempering of gears with complex shapes, and also puts higher requirements on the hardenability of gear steel.

[0006] Experimental studies have shown that the addition of elements such as Al, Nb, V, Ti, and N to MnCr-based carburizing gear steels can prevent grain coarsening during high-temperature carburizing by using carbonitrides. However, there are still problems that the grain coarsening temperature of gears is not high enough, and the grain size of gear steels obtained by mass production is unstable.

[0007] For example, China Invention Patent No. CN200610028265.8 describes a high-strength gear steel for automobiles, in which alloying elements such as Nb, V, and Al are compounded into the steel to refine the original austenite grains, and the steel contains the following components in mass percent: 0.20-0.40% C, 0.20-0.50% Si, 0.50-1.00% Mn, 0.80-1.30% Cr, 0.015-0.080% Nb, 0.030-0.090% V, 0.15-0.55% Mo, and 0.015-0.050% Al, with the balance being Fe and unavoidable impurities. By adding trace amounts of Nb and V, the grain size, hardenability, and zone of the gear steel are all significantly optimized; at the same time, the comprehensive mechanical properties of the gear steel are improved, and the service life is extended. However, this patent does not describe a specific carburizing temperature, and the addition of alloying elements such as Al, Nb and V can only satisfy the temperature requirements of conventional gas carburizing.

[0008] In another example, China Invention Patent No. CN201310301638.4 describes a NbTi composite micro-alloyed 20CrMnTi free-cutting gear steel, which contains the following components: 0.17-0.22% C, 0.20-0.35% Si, 0.9-1.10% Mn, 0.025% or less P, 0.020-0.035% S, 1.05-1.30% Cr, 0.015-0.035% Al, 0.02-0.06% Ti, and 0.02-0.06% Nb, with the balance being iron and unavoidable impurities. By controlling the content of micro-alloying elements such as Nb, Ti, and Al, the carburizing temperature of the gear can be increased or the carburizing time can be shortened, for example, 1050°C for 1h or 1000°C for 6h. In this patent, the addition of 0.02-0.06% Ti and Nb can increase the carburizing temperature up to 1000°C.

[0009] In another example, China Invention Patent No. CN202010128336.1 describes an ultra-pure high temperature fine-grained carburized gear steel with the following chemical compositions: 0.15-0.21% C, not more than 0.12% Si, 1.00-1.30% Mn, 1.00-1.30% Cr, 0.010-0.025% S, not more than 0.025% P, 0.70-1.00% Ni, 0.02-0.10% Mo, 0.0020-0.0040% B, not more than 0.20% Cu, not more than 0.05% Al, not more than 0.0005% Ca, not more than 0.003% Ti, and 0.0080-0.016% N, where N=(0.80-1.0)×(0.5%Al+0.7%B), with the balance being Fe and unavoidable impurities. The steel still has matrix grain size of 6 grade or more after high temperature carburization at 960℃ or more. In this patent, B element is added, and Al and B are well bonded to N to form AlN and BN particles, and thus obtain gear round steel with grain size of 6 grade or more after high temperature treatment at 1000℃ for 4h.

[0010] Considering that the effect of V element in controlling high temperature austenite grain size is not obvious, square inclusions are easily formed after the addition of Ti element to affect fatigue life, and the higher content of B element is easy to cause segregation at grain boundary, so that in order to meet the increasingly higher technical requirements of carburized gear steel, it is extremely urgent to develop and manufacture large-sized MnCr-based carburized steel for gear shafts that is suitable for high temperature (vacuum) carburizing and free cutting. Summary of the Invention [Means for solving the problem]

[0011] summary In view of the above analysis, the present invention aims to provide a high temperature carburized gear shaft steel and a manufacturing method for said steel, so as to solve the problems existing in the prior art that the gear shaft steel can only meet the requirements of conventional carburizing temperature, and heat treatment deformation and early fatigue fracture caused by grain coarsening and grain size instability easily occur during high temperature carburizing.

[0012] The object of the present invention is to provide a high temperature carburized gear shaft steel. The gear shaft steel produced by using the elemental components of the present invention can maintain suitable austenite grain size and stability at high temperature, has a narrow hardenability band, is easy to process, and can effectively improve the production stability and use safety of the gear shaft steel. The gear shaft steel maintains 5-8 grade austenite grain size before and after high temperature carburization at 940-1050℃, and can be effectively applied to high performance parts such as automotive gearboxes or new energy vehicle reduction gears and differentials, and has good application prospects and value.

[0013] In order to achieve the above object, the present invention proposes a high temperature carburized gear shaft steel containing the following chemical components in mass percent: 0.17-0.22% C, 0.05-0.35% Si, 0.80-1.40% Mn, 0.010-0.035% S, 0.80-1.40% Cr, 0.020-0.046% Al, 0.006-0.020% N, 0.002-0.030% Nb, 0.02% or less V, and 0.01% or less Ti. In the high temperature carburized gear shaft steel according to the present invention, the design principle of each chemical component is specifically described as follows:

[0014] C: In the high temperature carburized gear shaft steel of the present invention, C is an essential component in the steel, and at the same time, C is also one of the most important elements affecting the hardenability of the steel. Carburized gear steel requires both high surface strength and sufficient core impact toughness, and if the content of C in the steel is too low, i.e., less than 0.17%, the strength of the steel is insufficient, and good hardenability is not guaranteed; therefore, the content of C element in the steel should not be too high. If the content of C element in the steel is too high, the requirement for the core toughness of the gear is not satisfied, and too high a content of C has a negative effect on the plasticity of the steel, especially the carburized gear steel with a high Mn content, and if the C content is more than 0.22%, it has a negative effect on the workability of the steel. Therefore, in the high temperature carburized gear shaft steel of the present invention, the mass percentage of C is controlled to be 0.17-0.22%.

[0015] Si: In the high-temperature carburized gear shaft steel of the present invention, the Si element can not only well eliminate the adverse effect of iron oxide on the steel, but also dissolve in ferrite, strengthen the ferrite, and improve the strength, hardness, wear resistance and elasticity as well as the elastic limit of the steel. At the same time, it should be noted that the Si element increases the Ac3 temperature of the steel and reduces the thermal conductivity, thus making the steel have the risk of cracking and the tendency of decarburization. Based on this, and taking into account the beneficial effects and adverse effects of Si in combination, in the high-temperature carburized gear shaft steel of the present invention, the mass percentage of Si is controlled to be 0.05-0.35%.

[0016] Mn: In the high temperature carburized gear shaft steel of the present invention, Mn is one of the main elements that affect the hardenability of steel. Mn element has good deoxidation ability, can reduce iron oxide in steel, and can effectively increase the yield of steel. Mn can dissolve in ferrite, can improve the strength and hardness of steel, and can make the steel have pearlite with finer thin layer and higher strength when the steel is cooled after hot rolling. In addition, Mn can also form MnS with S in steel, which can eliminate the harmful effect of S. Mn has the ability to form and stabilize austenite structure in steel, can strongly increase the hardenability of steel, and can also improve hot workability. When the content of Mn element in steel is less than 0.80%, the hardenability of steel is insufficient; when the content of Mn element in steel is too high, the thermoplasticity of steel is deteriorated, production is affected, and steel tends to crack during water quenching. Therefore, in the high temperature carburized gear shaft steel of the present invention, the mass percentage of Mn is controlled to be 0.80-1.40%.

[0017] S: In the high-temperature carburized gear shaft steel of the present invention, S generally exists as an impurity element in steel, and significantly reduces the plasticity and toughness of steel, a certain amount of S element can form non-metallic inclusions with Mn, and an appropriate amount of S can improve the cutting properties of steel. Based on this, in the high-temperature carburized gear shaft steel of the present invention, the mass percentage of S is controlled to be 0.010-0.035%.

[0018] Cr: In the high temperature carburized gear shaft steel of the present invention, Cr is one of the main alloying elements added to the steel of the present invention, and Cr can significantly improve the hardenability, strength, wear resistance, etc. of the steel. In addition, Cr can also reduce the activity of C element in the steel and prevent decarburization during heating, rolling and heat treatment, but too high content of Cr will significantly reduce the toughness of the quenched and tempered steel and form coarse carbides distributed along the grain boundaries. Therefore, in the high temperature carburized gear shaft steel of the present invention, the mass percentage of Cr element is controlled to be 0.80-1.40%.

[0019] Al: In the high temperature carburized gear shaft steel of the present invention, Al belongs to the element for refining grains. The combination of Al element with N can further refine grains and improve the toughness of steel. Grain refinement plays an important role in improving the mechanical properties of steel, especially strength and toughness, and meanwhile, grain refinement also helps to reduce the hydrogen embrittlement susceptibility of steel. However, it should be noted that the content of Al element in steel should not be too high, and too high content of Al easily increases the chance of generating inclusions in steel. Therefore, in the high temperature carburized gear shaft steel of the present invention, the mass percentage of Al element is controlled to be 0.020-0.046%.

[0020] N: In the high-temperature carburized gear shaft steel of the present invention, N is an interstitial atom that can bond with microalloys in the steel to form MN-type precipitates ("M" refers to alloying elements), which can fix grain boundaries at high temperatures and thereby inhibit the growth of austenite grains. When the content of N element in steel is low, MN is not formed much, and the fixing effect is not significant; when the content of N element in steel is too high, N element tends to be abundant in steel production, which reduces the toughness of steel. Therefore, in the high-temperature carburized gear shaft steel of the present invention, the mass percentage of N element is controlled to be 0.006-0.020%.

[0021] Nb: In the high-temperature carburized gear shaft steel of the present invention, the addition of Nb element in the steel can form fine precipitates, thereby inhibiting the recrystallization of the steel and effectively refine the particles. It should be noted that the content of Nb element in the steel should not be too high, and if the Nb content in the steel is too high, coarse NbC particles will be formed during the smelting process, which will reduce the impact toughness of the steel. Therefore, in the high-temperature carburized gear shaft steel of the present invention, the mass percentage of Nb element is controlled to be 0.002-0.030%.

[0022] V: In the high temperature carburized gear shaft steel of the present invention, V can effectively improve the hardenability of the steel. The V element can form precipitates with the C element or N element in the steel, thereby further improving the strength of the steel. If the content of the C element and the content of the V element are too high, coarse VC particles will be formed. Considering the production cost and competitiveness, the mass percentage of the V element is controlled to be not more than 0.02% in the high temperature carburized gear shaft steel of the present invention.

[0023] Ti: The addition of Ti to steel can form fine precipitates, but if the content of Ti element in steel is too high, coarse TiN particles with edges and corners will be formed during the smelting process, which will reduce the impact toughness of the steel. Therefore, the content of Ti element in the high temperature carburized gear shaft steel of the present invention is controlled to be less than 0.01%.

[0024] Preferably, the high temperature carburized gear shaft steel of the present invention may further contain at least one of the elements Ni, Mo and Cu, in mass percentages of 0.25% or less Ni, 0.10% or less Mo and 0.20% or less Cu.

[0025] In the present invention, the elements Ni, Mo and Cu can further improve the performance of the high temperature carburized gear shaft steel of the present invention.

[0026] Ni: In the high-temperature carburized gear shaft steel of the present invention, Ni exists in the form of solid solution in the steel, and can effectively improve the low-temperature impact performance of the steel. However, it should be noted that too high a Ni content will result in too high a content of retained austenite in the steel, thereby reducing the strength of the steel. Therefore, in consideration of production cost and competitiveness, the mass percentage of Ni in the high-temperature carburized gear shaft steel of the present invention can be preferably controlled to be not more than 0.25%.

[0027] Mo: In the high-temperature carburized gear shaft steel of the present invention, Mo can be dissolved in the steel, which is beneficial to improve the hardenability and strength of the steel. Tempering at a higher temperature can form fine carbides to further improve the strength of the steel; and the combined action of molybdenum and manganese can significantly improve the stability of austenite. Considering that Mo is a precious metal and its cost is high, in order to control the production cost, the mass percentage of Mo in the high-temperature carburized gear shaft steel of the present invention can be controlled to be preferably not more than 0.10%.

[0028] Cu: In the high temperature carburized gear shaft steel of the present invention, Cu can improve the strength of the steel, and is beneficial to improve the weather resistance and corrosion resistance of the steel. The content of Cu element in the steel should not be too high, and if the Cu content in the steel is too high, Cu will be enriched at the grain boundary during heating, resulting in weakening and cracking of the grain boundary. Therefore, in the high temperature carburized gear shaft steel of the present invention, the mass percentage of Cu can be preferably controlled to be less than or equal to 0.20%.

[0029] Preferably, in the high-temperature carburized gear shaft steel of the present invention, among the unavoidable impurities, the content of each impurity element satisfies the following requirements: P≦0.015%, O≦0.0020%, H≦0.0002%, B≦0.0010%, and Ca≦0.003%.

[0030] In the present invention, P, O, H, B and Ca are all impurity elements in steel, and the contents of impurity elements in steel should be reduced as much as possible when technical conditions permit, in order to obtain steel with better performance and better quality.

[0031] P: P is easily segregated at the grain boundaries in steel, which reduces the grain boundary binding energy and deteriorates the impact toughness of the steel. Therefore, in the high temperature carburized gear shaft steel of the present invention, the P content is controlled to be 0.015% or less.

[0032] O: O can form oxides and composite oxides with Al in the steel, and in order to ensure the uniformity of the steel structure and the low-temperature impact energy and fatigue performance, the content of O in the high-temperature carburized gear shaft steel of the present invention can be controlled to be 0.0020% or less.

[0033] H: H accumulates in defects in steel, and hydrogen-induced delayed fracture occurs in steel with tensile strength exceeding 1000 MPa. Therefore, in the high temperature carburized gear shaft steel of the present invention, the content of H element is controlled to be 0.0002% or less.

[0034] B: B is a more sensitive element to hardenability, and a small change in B content will cause a large variation in the hardenability of the steel. Because B element segregates easily, and the addition of B element to the gear shaft steel does not help to control the narrow amplitude of the hardenability band for the gear steel. Therefore, in the high temperature carburized gear shaft steel of the present invention, the content of B element is controlled to be less than 0.0010%.

[0035] Ca: In the high temperature carburized gear shaft steel of the present invention, Ca element easily forms inclusions, thereby affecting the fatigue performance of the final product. Therefore, the content of Ca element can be controlled to be not more than 0.003%.

[0036] Preferably, the present invention uses the microalloying element coefficient MX (X stands for C or N) to describe the fine dispersion of precipitates.

[0037]

number

[0038] Define, where

[0039]

number

[0040] And each chemical element in the formula is replaced by the numerical value before the percent sign of the mass percent content of the corresponding chemical element. In the present invention, Nb, V, Ti, and Al can all form MX micro-alloying precipitates, which play a certain role in refining austenite grains and maintaining grain stability. Research has found that under the temperature conditions used in the gear shaft steel of the present invention, in the process of forming nano-sized carbonitride precipitates MX, V and Nb have a competitive relationship, and further increasing the content of V element has no significant effect on controlling high-temperature austenite grain size, while Ti element itself easily forms inclusions with carbon and nitrogen elements, affecting the machinability of the steel, and Ti element is also easy to combine with Nb to form large inclusions during smelting, affecting the effect of Nb precipitates in refining austenite grains. Therefore, in the present invention, finely dispersed MX precipitates are mainly formed by controlling the amount of two elements Nb and Al, especially the micro-alloying element Nb, thus keeping austenite grains stable at high temperature. Based on the above analysis, the micro-alloying element coefficient of the present invention

[0041]

number

[0042] is calculated as above, and is in the range of 0.5-3.0. During the smelting process, the micro-alloying element coefficient needs to be controlled within an appropriate range: if the micro-alloying element coefficient is too large, coarse precipitates will easily form during the smelting process, reducing the impact toughness and fatigue life of the steel; and if the micro-alloying element coefficient is too small, an appropriate amount of fine precipitates will not be formed, which will fix the grain boundary, inhibit grain boundary migration, and thereby fail to achieve the purpose of inhibiting austenite grain growth.

[0043] One of the positive effects of the present invention is that by controlling the contents of micro-alloying elements as well as carbon and nitrogen elements and the micro-alloying element coefficient in the gear steel, the appropriate amount of Al and Nb can form precipitates with excess nitrogen and carbon elements, thus effectively inhibiting the austenite grain growth at high temperature stage.

[0044] Preferably, the high temperature carburized gear shaft steel of the present invention has a hardenability of 30-43 HRC at the representative position J9mm, and maintains 5-8 grade austenite grain size before and after high temperature vacuum carburization at 940-1050°C.

[0045] Another object of the present invention is to provide a method for producing high-temperature carburized gear shaft steel, which is simple to produce and has high adaptability, and the high-temperature carburized gear shaft steel produced by the method of the present invention has high-temperature austenite stability, narrow hardenability band, high toughness, free cutting, high dimensional accuracy, high fatigue performance, etc., and can be effectively applied to highly demanding parts such as automotive gearboxes or reduction gears and differentials of new energy vehicles, and has good application prospects and application value.

[0046] In order to achieve the above object, the present invention proposes a method for producing high-temperature carburized gear shaft steel, which includes the following steps: Smelting and foundry; heating; forged or rolled; and Final finishing touches.

[0047] The smelting in the smelting and casting step of the manufacturing method of the present invention can be carried out by electric furnace smelting or converter smelting, and refining and vacuum treatment such as external smelting and vacuum degassing are carried out. Of course, in some other embodiments, a vacuum induction furnace can be used for smelting. The furnace charge for electric furnace smelting may use low P and S scrap iron, end cuttings and high quality pig iron; the alloy may be ferrochromium, low phosphorus ferromanganese, ferromolybdenum, etc.; the reducing agent may include: calcium carbide, carbon powder and aluminum powder; during the oxidation period: slag frequently to remove P, and slag frequently means the process of removing P element by increasing the number of slag passes and the amount of steel slag, reducing the P content in the steel; the slag discharge conditions may be controlled as follows: the slag discharge temperature is 1630-1660°C; and [P]≦0.015%; and the tapping conditions may be controlled as follows: the tapping temperature is 1630-1650°C; [P]≦0.011%, and [C]≧0.03%.

[0048] After the completion of electric furnace smelting or converter smelting, it is necessary to refine the molten steel in a ladle refining furnace to remove harmful gases and inclusions in the steel. The control ladle pedestal, temperature measurement and analysis, and argon pressure can be adjusted according to the situation; the initial deoxidation of the LF can be achieved by supplying 0.04% Al, and then adding alloy block and stirring for 5-10 minutes. When the temperature of the molten steel is measured to be T=1650-1670℃, vacuum degassing can be carried out, and the vacuum degree of vacuum degassing can be controlled to be 66.7 Pa and held for more than 15 minutes, ensuring [O]≦0.0020% and [H]≦0.00015%. Furthermore, in this technical solution, the temperature of the crane ladle can be controlled to be 1550-1570℃, and the temperature of the crane ladle is lowered, so that the element diffusion is accelerated, which is beneficial to further reduce dendritic segregation.

[0049] Thus, casting can be carried out by die casting or continuous casting. During the continuous casting process, the hot molten steel in the steel ladle is poured into the tundish through a protective sleeve, where the superheat of the tundish is 20-40℃. The tundish is thoroughly cleaned before use, and the inner surface of the tundish is coated with a refractory coating and must not have cracks; and the molten steel in the tundish is stirred well by electromagnetic stirring using a continuous casting crystallizer, so that a qualified continuously cast billet with a cross-sectional dimension of 140mm×140mm~320mm×425mm can be obtained. In this technical solution, the casting speed can be controlled to be 0.6-2.1 m / min according to different square billet sizes. Then the continuous cast billet is slowly cooled in an annealing pit for an annealing time of more than 24 hours.

[0050] Furthermore, in the forging or rolling process of the manufacturing method of the present invention, if forging is performed, it can be directly forged into the final finished product size; if rolling is performed, the steel slab can either be directly rolled into the final finished product size, or the steel slab can be first rolled into a specified intermediate slab size, and then heated and rolled into the final finished product size. Among them, the heating temperature of the intermediate slab can be controlled to be 1050-1250°C, and the holding time can be controlled to be 3-24 hours.

[0051] Furthermore, in the final finishing step of the manufacturing method of the present invention, the final finishing process includes scalping and heat treatment of the round bar and non-destructive testing to ensure quality. In the final finishing step, the optional scalping process may include: rotary scalping or grinding wheel scalping, etc.; the optional heat treatment process may include annealing, isothermal annealing, etc.; the optional non-destructive testing may include ultrasonic testing, magnetic particle testing, etc.

[0052] Preferably, in the heating process, the steel slab is first heated in a pre-heating section to a temperature not higher than 700°C, and then continuously heated in a first heating section to a temperature not higher than 980°C, and after maintaining at that temperature, continues to be heated to 950-1200°C in a second heating section, and then enters a soaking section having a temperature of 1050-1250°C, and after maintaining at that temperature, proceeds with subsequent rolling or forging.

[0053] In the above technical solution, compared with the prior art, the technical solution adopted in the heating step of the manufacturing method of the present invention has a higher temperature in the immersion section. The higher temperature in the immersion section can be beneficial to improve the compositional uniformity and structural uniformity of the continuously cast billet during the diffusion process of steel slab heating. At this temperature, the precipitates also have a faster solid solution rate, so that the high rolling heating temperature causes greater dissolution of the originally undissolved precipitate particles in the steel, increases the concentration of micro-alloying elements in the matrix, and precipitates more and more dispersed particles upon subsequent cooling. In addition, the final rolling temperature can be increased only after the rolling heating temperature has been increased, resulting in a more complete recovery and recrystallization of austenite after rolling, and a more uniform precipitate distribution.

[0054] Preferably, in the manufacturing method of the present invention, the final forging or rolling temperature in the forging or rolling step is controlled to be 900° C. or higher.

[0055] In the forging or rolling process of the manufacturing method of the present invention, after the steel slab is discharged from the furnace, high pressure water can be used to remove the scale and oxide film, and the initial forging or initial rolling temperature is controlled to be 1150-1250°C, and the final forging or final rolling temperature is controlled to be above 900°C. This is beneficial for N to dissolve out of gamma solid solution and combine with the micro-alloying elements in the steel to form nitrides under this process.

[0056] It should be noted that N has a lower solubility in α-Fe than in γ-Fe, and two peaks of precipitate amount arise due to the excitation of phase transformation. When the final forging or final rolling temperature is low, the peak precipitation of precipitates causes non-uniform distribution of precipitates, as well as insufficient recovery and recrystallization, resulting in anisotropy of the microstructure. Therefore, the final forging or final rolling temperature is 900 °C or higher, resulting in a uniformly dispersed distribution of precipitates. In addition, increasing the final forging or final rolling temperature leads to finer grains, which increases the difference between the average grain size of ferrite after the transformation of undercooled austenite and the space between the manganese-rich bands, and reduces the tendency of the manganese-rich bands to form pearlite, thereby reducing the banded structure.

[0057] The beneficial effects of the present invention are as follows: 1. According to the present invention, a gear shaft steel that can stably maintain austenite grains under the above-mentioned high temperature conditions can be obtained by rationally controlling the chemical components. In the present invention, the contents of micro-alloying elements Nb, Al and V as well as carbon and nitrogen elements are mainly rationally controlled to ensure that the carbonitride precipitates MX have an appropriate size and amount, which can limit the migration of austenite grain boundaries and enable the austenite grains of the carburized gear shaft steel of the present invention to maintain an appropriate grain size and stability at high temperatures. Among them, Nb and Al are the main elements for forming precipitates in the present invention, the effect of V and Ti elements in controlling the grain size of high temperature austenite in the present invention is not obvious, and V and Ti elements are easy to compound with Nb to form large inclusions, thereby affecting the properties of Nb precipitates, and therefore V and Ti elements are regarded as impurity elements that should be controlled to a low range in the present invention, thereby avoiding the occurrence of harmful inclusions of large particles in steel, ensuring the stable production quality of steel, reducing the production cost of steel, and realizing mass production on bar production line.

[0058] 2. The high-temperature carburized gear shaft steel of the present invention does not contain any precious metal elements such as Ni, Mo, Cu, V, or contains only small amounts of them, which can control the type and amount of alloying elements in the steel under the premise of ensuring high-temperature carburization, high hardenability, narrow band and free cutting, etc., thereby improving the applicability of the steel. The austenite grain size, hardenability and cost competitiveness of the high-temperature carburized gear shaft steel obtained by adopting the element composition and manufacturing method of the present invention are superior to those in the existing patent technology.

[0059] 3. In the present invention, by increasing the heat treatment temperature in the heating, forging or rolling stages, the recovery and recrystallization of austenite after forging or rolling is more sufficient, and the nano-sized carbonitride precipitates are uniformly dispersed in the matrix steel, and the grain stability of austenite during high temperature carburization is further improved.

[0060] 4. By using the technical solution of the present invention, a gear shaft steel can be obtained which can undergo vacuum carburization at high temperatures up to 960℃ and even above 1000℃, and can maintain the stability of austenite grains during carburization, and avoid the phenomenon of mixed crystals and coarse grains. The grain size of this steel after 4 hours of vacuum carburization at temperatures up to 1000℃ remains stable at 5-8 grades, and their properties reach the service performance index of gear shaft steel. By using the steel of the present invention, the carburization temperature of the steel can be as high as above 960℃, and carburization under such high temperature conditions can greatly shorten the carburization time of the gear shaft, reduce the production cost of the gear, reduce carbon dioxide emission, save energy and protect the environment, and have the prospect of wide industrial application. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] Detailed Description The embodiments of the present invention are described with specific embodiments, and other advantages and effects of the present invention will be readily apparent to those skilled in the art from the disclosure of this specification. Although the present invention is described in connection with a preferred embodiment, the features of the present invention are not intended to be limited to only this embodiment. On the contrary, the description of the present invention in connection with the embodiment is intended to cover other alternatives or modifications that may be derived based on the claims of the present invention. The following description includes many specific details to provide a thorough understanding of the present invention. The present invention may also be practiced without these details. Furthermore, some specific details are omitted in the description to avoid confusing or obscuring the focus of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments may be combined with each other without inconsistency.

[0062] Examples 1-8 and Comparative Examples 1-4 The high temperature carburized gear shaft steels in Examples 1-8 are all manufactured by using the following process: (1) Smelting and casting are carried out according to the chemical composition shown in Table 1 below: Wherein smelting can be carried out by using 50 kg vacuum induction furnace, 150 kg vacuum induction furnace, or 500 kg vacuum induction furnace, or smelting can also be carried out by using electric furnace smelting + external smelting + vacuum degassing, or smelting can be carried out by using converter smelting + external smelting + vacuum degassing, and casting can be carried out by using die casting or continuous casting.

[0063] (2) Heating: The steel slab is first heated to be below 700℃ in the pre-heating section, and then continues to be heated in the first heating section, where the designated heating temperature is below 980℃. At this stage, the temperature of the steel slab is 600-980℃; after heating maintenance, it continues to be heated to 950-1200℃ in the second heating section, and enters the soaking section after heating maintenance. The temperature of the soaking section is 1050-1250℃, and the temperature of the core of the steel slab and the temperature of the surface of the steel slab are maintained at the same temperature by heating maintenance.

[0064] (3) Forging or rolling: The final forging or rolling temperature is controlled to be 900°C or higher.

[0065] (4) Final Finishing: Final finishing includes skinning or annealing or normalizing.

[0066] The specific processes of the high temperature carburized gear shaft steels in Examples 1-8 and Comparative Examples 1-4 are as follows: Example 1: Smelting is carried out in a 50 kg vacuum induction furnace according to the chemical composition shown in Table 1 below. The molten steel is cast into steel ingots, then heated, and forged into billets, and the steel ingots are first heated to 700°C in the pre-heating section, then continue to heat to 900°C in the first heating section. And after the heating is maintained, continue to heat to 1000°C in the second heating section. Then, after the heating is maintained, it goes into the soaking section with a temperature of 1100°C. Then, after the heating is maintained, it goes into the subsequent forging, and finally makes a bar with Φ60 mm, where the final forging temperature is controlled to be 910°C, and after forging, it is normalized at 920°C for 100 minutes.

[0067] Example 2: Smelting is carried out in a 150 kg vacuum induction furnace according to the chemical composition shown in Table 1 below. The molten steel is cast into steel ingots, heated, and forged into billets, and the steel ingots are first heated to 650°C in the pre-heating section, then continue to be heated to 950°C in the first heating section, and continue to be heated to 1100°C in the second heating section after the heating is maintained. Then, after the heating is maintained, it enters into a soaking section with a temperature of 1200°C, and after the heating is maintained, it proceeds to subsequent forging, and finally produces a bar with Φ75mm, where the final forging temperature is controlled to be 1000°C, and after forging, rotary scalping is carried out.

[0068] Example 3: Electric furnace smelting is carried out according to the chemical composition shown in Table 1, and then refining and vacuum treatment are carried out, and then cast into a 320 mm x 425 mm continuous casting billet, and the continuous casting billet is first heated to 600 ° C in the pre-heating section, then continues to be heated to 980 ° C in the first heating section. And after heating maintenance, continues to be heated to 1200 ° C in the second heating section. Then, after heating maintenance, enters the soaking section with a temperature of 1220 ° C, and after heating maintenance, continues rolling. The steel slab is discharged from the heating furnace, and starts to be rolled after high-pressure water descaling, and finally rolled into a bar with Φ120 mm, where the final rolling temperature is controlled to be 1000 ° C. After rolling, air cooling, annealing treatment at 650 ° C for 12 hours, and inspection by ultrasonic inspection, magnetic particle inspection, etc. are carried out.

[0069] Example 4: Electric furnace smelting is carried out according to the chemical composition shown in Table 1, and then refining and vacuum treatment are carried out, and then cast into a 280 mm x 280 mm continuous casting billet, and the continuous casting billet is first heated to 620 ° C in the pre-heating section, then continues to be heated to 950 ° C in the first heating section. And after heating maintenance, continues to be heated to 1150 ° C in the second heating section. Then, after heating maintenance, enters the soaking section with a temperature of 1200 ° C. And after heating maintenance, proceeds to subsequent rolling. The steel slab is discharged from the heating furnace, and starts to be rolled after high-pressure water descaling, and finally rolled into a bar with Φ90 mm, where the final rolling temperature is controlled to be 970 ° C. After rolling, air cooling, grinding wheel skinning, and inspection by ultrasonic inspection, magnetic particle inspection, etc. are carried out.

[0070] Example 5: Electric furnace smelting is carried out according to the chemical composition shown in Table 1, and refining and vacuum treatment are carried out, and then cast into a continuous casting billet of 320 mm x 425 mm, and the continuous casting billet is first heated to 600 ° C in the preheating section, and then continues to be heated to 950 ° C in the first heating section. And after the heating is maintained, it continues to be heated to 1200 ° C in the second heating section. Then, after the heating is maintained, it enters the soaking section with a temperature of 1230 ° C. And after the heating is maintained, it is followed by rolling. The steel slab is discharged from the heating furnace, and begins to be rolled into an intermediate slab after high pressure water descaling, where the first final rolling temperature is controlled to be 1050 ° C, and the intermediate slab has a size of 220 mm x 220 mm. The intermediate slab is then preheated to 680 ° C, and subsequently heated to 1050 ° C first, and then to 1200 ° C. Then, after maintaining the heating, soaking is performed, the soaking temperature is 1220°C, and the slab after soaking is discharged from the furnace, and begins to be rolled after high-pressure water descaling, thereby obtaining a finished product bar with a specification of Φ50mm, where the second final rolling temperature is controlled to be 950°C. After rolling, air cooling, isothermal annealing treatment is performed, that is, holding at 900°C for 90 minutes, followed by air cooling to 600°C, and holding for 120 minutes, then discharging from the furnace and air cooling, and then carrying out inspection such as ultrasonic inspection and magnetic particle inspection.

[0071] Example 6: Electric furnace smelting is carried out according to the chemical composition shown in Table 1, and refining and vacuum treatment are carried out, and then cast into a 280 mm x 280 mm continuous casting billet, and the continuous casting billet is first heated to 680 ° C in the pre-heating section, and then continues to be heated to 900 ° C in the first heating section. And after the heating is maintained, it continues to be heated to 1180 ° C in the second heating section. Then, after the heating is maintained, it enters the soaking section with a temperature of 1200 ° C. And after the heating is maintained, it is followed by rolling. The steel slab is discharged from the heating furnace, and begins to be rolled into an intermediate slab after high pressure water descaling, where the first final rolling temperature is controlled to be 1000 ° C, and the intermediate slab has a size of 140 mm x 140 mm. The intermediate slab is then pre-heated to 700 ° C, and subsequently heated to 1100 ° C, and then heated to 1220 ° C. Then, after maintaining the heating, soaking is performed, the soaking temperature is 1220°C, and the slab after soaking is discharged from the furnace, and begins to be rolled into a finished product bar with a specification of Φ20mm after high-pressure water descaling, where the second final rolling temperature is controlled to be 920°C. After rolling, air cooling, rotary skinning, and inspection by ultrasonic inspection, magnetic particle inspection, etc. are performed.

[0072] Example 7: Converter smelting is carried out according to the chemical composition shown in Table 1, and then refining and vacuum treatment are carried out, and then cast into a die-casting slab, and the cast slab is first heated to 620°C in the pre-heating section, and then continues to be heated to 950°C in the first heating section. And after the heating is maintained, it continues to be heated to 1150°C in the second heating section. Then, after the heating is maintained, it enters the soaking section with a temperature of 1200°C. And after the heating is maintained, it is followed by rolling. The steel slab is discharged from the heating furnace, and starts to be rolled after high-pressure water descaling, and finally rolled into a bar with Φ110 mm, where the final rolling temperature is controlled to be 970°C. After rolling, it is air-cooled, grinding wheel peeled, and inspected by ultrasonic inspection, magnetic particle inspection, etc.

[0073] Example 8: Converter smelting is carried out according to the chemical composition shown in Table 1, and refining and vacuum treatment are carried out, and then cast into a die-casting slab, and the cast slab is first heated to 600°C in the pre-heating section, and then continues to be heated to 950°C in the first heating section. And after the heating is maintained, it continues to be heated to 1200°C in the second heating section. Then, after the heating is maintained, it enters the soaking section with a temperature of 1230°C. And after the heating is maintained, it is followed by rolling. The steel slab is discharged from the heating furnace, and begins to be rolled into an intermediate slab after high-pressure water descaling, where the first final rolling temperature is controlled to be 1050°C, and the intermediate slab has a size of 260 mm x 260 mm. The intermediate slab is then pre-heated to 680°C, and subsequently heated to 1050°C first, and then to 1200°C. Then, after maintaining the heating, soaking is performed, the soaking temperature is 1220°C, and the slab after soaking is discharged from the furnace, and begins to be rolled into a finished product bar with a specification of Φ60mm after high-pressure water descaling, where the second final rolling temperature is controlled to be 950°C. After rolling, air cooling is performed, and then inspection is performed by ultrasonic inspection, magnetic particle inspection, etc.

[0074] The steels in Comparative Examples 1 and 2 are from commercial sources.

[0075] Comparative Example 3: The implementation method is the same as that in Example 1: smelting in a 50 kg vacuum induction furnace according to the chemical composition shown in Table 1, casting the molten steel into a steel ingot, heating and forging into a billet, and the steel ingot is first heated to 700°C in a pre-heating section, then continues to be heated to 900°C in a first heating section. And after maintaining the heat, continues to be heated to 1000°C in a second heating section. Then, after maintaining the heat, enters into a soaking section with a temperature of 1100°C. And after maintaining the heat, carries out subsequent rolling, and finally rolls into a bar with Φ60 mm, where the final forging temperature is controlled to be 910°C, and after forging, normalizes at 920°C for 100 minutes.

[0076] Comparative Example 4: The implementation method is the same as that in Example 5: electric furnace smelting is carried out according to the chemical composition shown in Table 1, and refining and vacuum treatment are carried out, and then cast into a 320 mm x 425 mm continuous casting billet, and the continuous casting billet is heated to 600 ° C in the preheating section, and then continues to be heated to 950 ° C in the first heating section. And after the heating is maintained, it continues to be heated to 1200 ° C in the second heating section. Then, after the heating is maintained, it enters the soaking section with a temperature of 1230 ° C. And after the heating is maintained, it carries out subsequent rolling. The steel slab is discharged from the heating furnace, and begins to be rolled into an intermediate slab after high-pressure water descaling, where the first final rolling temperature is controlled to be 1050 ° C, and the intermediate slab has a size of 220 mm x 220 mm. The intermediate slab is then preheated to 680 ° C, and subsequently heated to 1050 ° C first, and then to 1200 ° C. Then, after maintaining the heating, soaking is performed, the soaking temperature is 1220°C, and the slab after soaking is discharged from the furnace, and after high pressure water descaling, it starts to be rolled into a finished product bar with a specification of Φ50mm, where the second final rolling temperature is controlled to be 950°C. After rolling, it is air-cooled, isothermal annealed, that is, held at 900°C for 90 minutes, followed by air-cooling to 600°C, and held for 120 minutes, then discharged from the furnace, and air-cooled, and then inspected by ultrasonic inspection, magnetic particle inspection, etc.

[0077] Table 1 shows the mass percentages of the chemical elements in the high-temperature carburized gear shaft steels in Examples 1-8 and the comparative steels in Comparative Examples 1-4, as well as the micro-alloying element coefficients r M / X List the following.

[0078] Table 2 lists the specific process parameters for the above process steps of the high temperature carburized gear shaft steels in Examples 1-8 and the comparative steels in Comparative Examples 1-4.

[0079] [Table 1]

[0080] [Table 2]

[0081] In the above Table 2, Examples 5, 6, and 8 and Comparative Example 4 have two columns for parameters in step (2) and step (3) in the above process of the present invention, because during rolling in the above three examples, the steel slab is first rolled to a specific intermediate slab size, and then heated and rolled again to the final finished product size.

[0082] The high-temperature carburized gear shaft steels obtained in Examples 1-8 and the comparative steels in Comparative Examples 1-4 were each sampled and subjected to a simulated carburized quenching test, a hardenability test and a hardness test. The test results of the steels obtained in the Examples and Comparative Examples are shown in Table 3.

[0083] The relevant methods for the simulated carburizing and quenching test, hardenability test and hardness test are listed below: Simulated carburizing and quenching test: hold at 940℃ for 5 hours; hold at 960℃, 980℃ and 1000℃ for 4 hours respectively; hold at 1020℃ for 3 hours; and hold at 1050℃ for 2 hours, then water quenching, and take samples to observe the structure of the steels in the examples and comparative examples, and evaluate their austenite grain size according to the standard ASTM E112.

[0084] Hardenability test: For the steels in the examples and the comparative examples, samples are taken and prepared from hot-rolled round steel according to national standard GB / T 225, and subjected to end hardenability test (Jominy test) with reference to GB / T 5216, where the normalizing temperature is controlled to be 920±10°C, and the quenching temperature is controlled to be 870±5°C. Then, Rockwell hardness test is performed according to GB / T 230.2 to obtain the hardness value (HRC) at a specific position, such as the hardness at 9 mm from the quenching end, i.e., J9 mm. The above process parameters can also be determined by negotiation.

[0085] Table 3 lists the test results of the high temperature carburized gear shaft steels in Examples 1-8 and the comparative steels in Comparative Examples 1-4.

[0086] [Table 3]

[0087] As can be seen from Table 3, after the high temperature carburized gear shaft steels in Examples 1-8 of the present invention are subjected to simulated carburizing at four temperatures not exceeding 1000°C in the simulated carburizing quenching test, the austenite grain size is maintained within the 5-8 grade, and phenomena such as mixed crystals or abnormal coarse particles are not observed. And the workability of the obtained steels meets the technical requirements, where the steels in Examples 1 and 3 have a grain size of 5 grade after heating at 1040°C for 2h.

[0088] The mixed crystal phenomenon (1 grade) is observed after the comparative steel in Comparative Example 2 is subjected to simulated carburization and quenching at a temperature of 960°C, where 6(1) represents an average grain size of 6 grades and abnormal coarsening of 1 grades occurring in localized areas. After continuing to increase the simulated carburization temperature of the comparative steels in Comparative Examples 1, 3, and 4 to above 980°C, the abnormal growth of austenite grains becomes intense, where 5.5(1) represents an average grain size of 5.5 grades and abnormal coarsening of 1 grades occurring in localized areas. It can be seen that in Comparative Example 3, TiN type inclusions are present in the steel, adversely affecting the fatigue performance. The comparative steel in Comparative Example 1 has a lower hardenability and does not meet the requirements for 20MnCrS5H high hardenability gear steel specified in EN 10084-2008.

[0089] In summary, in the present invention, through the rational chemical composition design and optimization process, it can be found that the high temperature carburized gear shaft steel according to the present invention can have high temperature austenite grain stability, high hardenability, narrow hardenability band and good high temperature grain stability. It is also free-cutting and suitable for high temperature carburizing. And it has a hardenability of 30-43 HRC at the representative position J9mm, and maintains 5-8 grade austenite grain size before and after high temperature vacuum carburizing up to 1000℃. The bar rolled or forged with high hardenability steel for gear shaft can be effectively processed into gears, and has suitable strength and toughness after heat treatment such as high temperature carburizing. The gear shaft steel can be effectively applied to high performance parts such as gearboxes for automobiles or reduction gears and differentials of new energy vehicles, and has good application prospects and value.

[0090] Furthermore, the combinations of various technical features in the present invention are not limited to the combinations described in the claims of the present invention or the combinations described in the specific examples, and all technical features described in the present invention can be freely combined or combined in any manner, as long as there is no contradiction between the technical features.

[0091] It should also be noted that the above listed embodiments are merely specific examples of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar modifications or variations made therein can be directly derived or easily associated by those skilled in the art from the contents disclosed by the present invention, and they should be included in the protection scope of the present invention.

Claims

1. 1. A high-temperature carburizing gear shaft steel having the following chemical composition in mass percent: 0.17-0.22% C, 0.05-0.35% Si, 0.80-1.40% Mn, 0.010-0.035% S, 0.80-1.40% Cr, 0.020-0.046% Al, 0.006-0.020% N, 0.002-0.030% Nb, not more than 0.02% V, and not more than 0.01% Ti, and further containing the elements Ni, Mo, and Cu in mass percent not more than 0.25% Ni, not more than 0.10% Mo, and 0.06-0.20% Cu, the balance being Fe and unavoidable impurities.

2. A high-temperature carburized gear shaft steel as described in claim 1, wherein the content of each impurity element among the unavoidable impurities satisfies the following mass percents: P 0.015% or less, O 0.0020% or less, H 0.0002% or less, B 0.0010% or less, and Ca 0.003% or less.

3. The contents of elements Nb, V, Al, N and C in high temperature carburized gear shaft steel are calculated by the following formula: Micro-alloying element coefficient [Equation 1] and the micro-alloying element coefficient [Equation 2] 2. The high-temperature carburized gear shaft steel according to claim 1, wherein x is in the range of 0.5 to 3.0, and wherein each chemical element in said formula is replaced by the numerical value before the percent sign of the mass percent content of the corresponding chemical element.

4. 2. The high-temperature carburized gear shaft steel according to claim 1, which has a hardenability of 30-43 HRC at a representative position J9 mm and maintains a 5-8 grade austenite grain size before and after high-temperature vacuum carburizing.

5. 2. A method for producing a high-temperature carburized steel for gear shafts according to claim 1, comprising the following steps: smelting and foundry; heating; forged or rolled; and Final finishing touches.

6. 6. The method for producing high-temperature carburized steel for gear shafts according to claim 5, wherein in the heating step, the heating temperature of the preheating section is 700°C or less, the temperature of the first heating section is 980°C or less, the temperature of the second heating section is 950-1200°C, and the temperature of the soaking section is 1050-1250°C.

7. 6. The method for producing high-temperature carburized steel for gear shafts according to claim 5, wherein the final forging temperature or final rolling temperature in the forging or rolling step is 900°C or higher.

8. 6. The method for producing high temperature carburized gear shaft steel according to claim 5, wherein the final finishing step includes at least one of scalping, annealing, and tempering.

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