Gear based on low-cost high-toughness gear steel and method for manufacturing the same

Abstract

The application discloses a gear based on low-cost high-toughness gear steel and a preparation method thereof, which comprises the following components: C: 0.17%-0.23%, Si: 0.15%-0.35%, Mn: 0.70%-0.95%, Cr: 0.40%-0.70%, Mo: 0.10%-0.20%, Ni: 0.30%-0.70%, Al: 0.015%-0.035%, P: ≤0.010%, S: 0.015%-0.025%, Nb: 0.010%-0.030%, B: 0.0005%-0.0030%, and the rest is Fe. The content of C element in the gear steel is increased, and the B element is added, so that the hardenability of the gear steel and the core hardness of the gear after carburizing heat treatment are improved, the contents of alloy elements Cr, Ni and Mo which have the effect of improving the hardenability are reduced, and therefore the production cost of the gear steel is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metallurgical industry, and relates to a gear based on low-cost high-strength and high-toughness gear steel and a preparation method thereof. BACKGROUND

[0002] Heavy trucks have higher requirements for high efficiency, high reliability and light weight of transmission gears due to their large load and complex driving conditions. In particular, with the acceleration of electrification, new energy heavy trucks are developing rapidly. Since the required speed of pure electric vehicle gears is much higher than that of fuel vehicle gears, and the instantaneous torque is up to 10000 Nm, the performance requirements of high speed and high load of gears are greatly improved compared with traditional fuel vehicles.

[0003] Heavy truck transmission gears generally need to go through multiple cold and hot processing procedures such as forging, finishing, carburizing and heat treatment, in order to obtain high surface hardness and good core toughness, so that the gears have excellent properties such as wear resistance, fatigue resistance and corrosion resistance. In order to achieve high power density, high reliability and light weight, the gears must have: 1) sufficient strength and toughness in the core to ensure that the gears can withstand torque when transmitting power and impact during gear shifting, thereby adapting to high speed and heavy load; 2) high surface hardness to improve the contact fatigue performance and wear resistance of the gears, thereby improving the high reliability and long life of the gears.

[0004] Gear steel as the basic material for gear manufacturing, needs to have good toughness to meet the demand of high speed and heavy load performance of gear. In recent years, the technical personnel in the field have made a lot of improvement work on gear steel to improve the comprehensive performance of gear steel, such as micro alloying treatment of different elements such as Nb, V, Ti and so on. For the gear steel Cr-Ni-Mo system material commonly used in heavy truck transmission, the gear steel in the published patent CN115094309A: C: 0.15-0.19%, Si: 0.25-0.40%, Mn: 0.40-0.60%, Cr: 0.95-1.20%, Mo: 0.25-0.35%, Ni: 1.40-1.70%, Al: 0.025-0.040%, P: ≤0.020%, S: ≤0.015-0.030% is micro alloyed with Nb element, and the addition amount of Nb is 0.020-0.060%; The gear steel in the published patent CN113430461B: C: 0.15-0.19%, Si: 0.15-0.30%, Mn: 0.60-0.90%, Cr: 1.60-1.80%, Mo: 0.20-0.35%, Nb: 0.025-0.040%, Ni: 1.50-1.70%, Al: 0.020-0.040%, P: ≤0.010%, S: ≤0.010% is micro alloyed with Nb and V elements, the addition amount of Nb is 0.025-0.040%, and the addition amount of V is 0.05-0.20%; The gear steel in the published patent CN108866439B: C: 0.15-0.23%, Si: 0.10-0.40%, Mn: 0.45-0.90%, Cr: 1.50-1.80%, Mo: 0.15-0.55%, Ni: 1.40-1.70%, P: ≤0.020%, S: ≤0.020%, V: 0.05-0.15%, Ti: 0.05-0.15% is micro alloyed with Nb and Ti elements, the addition amount of Nb is 0.020-0.080%, and the addition amount of Ti is 0.015-0.08%; The gear steel in the published patent CN113416899B: C: 0.15-0.19%, Si: 0.15-0.30%, Mn: 0.60-0.90%, Cr: 1.60-1.80%, Mo: 0.20-0.35%, Ni: 1.50-1.70%, Al: 0.020-0.040%, P: ≤0.010%, S: ≤0.010% is micro alloyed with Nb and B elements, the addition amount of Nb is 0.025-0.040%, and the addition amount of B is 0.0005-0.0020%; The gear steel in the published patent CN113388783B: C: 0.15-0.19%, Si: 0.15-0.30%, Mn: 0.60-0.90%, Cr: 1.60-1.80%, Mo: 0.20-0.35%, Ni: 1.50~1.70%, Al: 0.020~0.040%, P: <=0.010%, S: <=0.010%, V: 0.05~0.15%, Ti 0.05~0.15%, Nb, V, Ti element micro-alloying treatment is carried out, the addition amount of Nb is 0.025~0.040%, the addition amount of V is 0.05~0.15%, and the addition amount of Ti is 0.05~0.15%; however, the grain of the Cr-Ni-Mo gear steel is refined by adding trace amounts of Nb, V, Ti and B elements in these patents, so as to improve the fatigue performance and high-temperature grain stability of the gear steel. However, the content of metal elements such as Cr, Ni and Mo added in the gear steel in these disclosed patents is high, which brings about the cost of the gear steel material, and for the market volume of nearly 1 million heavy truck transmissions per year, the comprehensive cost increases greatly, thereby affecting the production cost of enterprises. SUMMARY

[0005] In view of the problems in the prior art, the application provides a low-cost high-strength gear steel and a preparation method thereof, the hardenability of the gear steel is improved by increasing the content of C element, and the hardenability of the gear steel is improved by adding trace amounts of inexpensive B element, so as to improve the core hardness of the gear after carburizing heat treatment, and the content of alloy elements Cr, Ni and Mo which have a promoting effect on the hardenability is reduced, thereby reducing the production cost of the gear steel.

[0006] The application is realized by the following technical scheme:

[0007] A low-cost high-strength gear steel comprises the following chemical components in percentage by weight: C: 0.17%~0.23%, Si: 0.15%~0.35%, Mn: 0.70%~0.95%, Cr: 0.40%~0.70%, Mo: 0.10%~0.20%, Ni: 0.30%~0.70%, Al: 0.015%~0.035%, P: <=0.010%, S: 0.015%~0.025%, Nb: 0.010%~0.030%, B: 0.0005%~0.0030%, and the rest is Fe and inevitable impurity elements.

[0008] Preferably, the [O] in the gear steel is <=9ppm, the [H] is <=2ppm, and the [N] is 90-150ppm.

[0009] Preferably, the delivery state structure of the gear steel is ferrite+pearlite.

[0010] Preferably, the grain size of the gear steel is 12~16mu, and the grain size grade is at least 9.0 grade.

[0011] A gear based on low-cost high-toughness gear steel, prepared from the gear steel, wherein the core hardness of the gear ranges from 35.6 to 44.2 HRC.

[0012] A preparation method of a gear based on low-cost high-toughness gear steel, characterized in that it comprises,

[0013] The component materials are weighed according to the percentage by weight, C: 0.17%-0.23%, Si: 0.15%-0.35%, Mn: 0.70%-0.95%, Cr: 0.40%-0.70%, Mo: 0.10%-0.20%, Ni: 0.30%-0.70%, Al: 0.015%-0.035%, P: ≤0.010%, S: 0.015%-0.025%, Nb: 0.010%-0.030%, B: 0.0005%-0.0030%, and the rest is Fe and inevitable impurity elements. The component materials are added to an electric furnace, sequentially subjected to electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling treatment to obtain a round steel rod matrix. The round steel rod matrix is forged into a shape, normalized, and mechanically processed to obtain a gear semi-finished product. The gear semi-finished product is subjected to carburizing treatment to obtain the gear.

[0014] Preferably, the Nb is added in the form of ferro-niobium before tapping from the electric furnace, and the B is added in the form of ferro-boron in the early stage of LF refining.

[0015] Preferably, the specific process of forging the round steel rod matrix into a shape is as follows: the round steel rod is cut to the required size according to the part specifications, the cut round steel rod is heated to 1050-1250°C by medium-frequency induction, and the output shaft blank is obtained after being forged into a shape by die forging.

[0016] Preferably, the specific process of normalizing is as follows: the output shaft blank is annealed by heating to 900-930°C, holding for 2-5 h, cooling to 570-650°C, holding for 2-4 h, and then cooling after discharging, and the hardness of the annealed output shaft blank is required to be controlled at 150-180 HBW.

[0017] Preferably, the specific process of carburizing treatment is as follows: the processed gear semi-finished product is placed in a carburizing furnace for carburizing treatment, wherein the carburizing treatment carbon atmosphere is nitrogen and methanol, and the process sequentially comprises the stages of heating, strong carburizing, diffusion, holding and oil cooling, tempering, and air cooling to obtain the low-cost high-toughness gear steel.

[0018] Rapidly heating to 875-885 DEG C and keeping for 20-40 min, the carbon potential in heating stage is 0.70-0.80 C %;Heating to 915-935 DEG C for strong penetration, the time is 140-180 min, the carbon potential in strong penetration stage is 1.0-1.1 C %;Then diffusing at 915-935 DEG C, the time is 60-90 min, the carbon potential in diffusing stage is 0.75-0.95 C %;Finally, rapidly cooling to 855-865 DEG C and keeping for 30-60 min, the carbon potential in keeping stage is 0.65-0.85 C %, drawing from the back room to the front room of carburizing heat treatment furnace for oil quenching, the oil temperature of 70-90 DEG C is adopted for staged cooling, the stirring time is 30 min, the stirring speed is 100-200 rpm, and after oil quenching, it needs to drain oil for 15-25 min, and after cleaning, tempering heat treatment is carried out, the gear semi-finished product is heated to 150-180 DEG C again for tempering treatment, and after keeping in nitrogen atmosphere for 2h-5h, it is air cooled to room temperature, so that the low-cost high-strength and high-toughness gear steel is prepared;The nitrogen flow is 4.0-6.0 m 3 / h, and the methanol flow is 4.5-5.5 L / h.

[0019] Compared with the prior art, the low-cost high-strength and high-toughness gear steel and the preparation method thereof have the following beneficial technical effects:

[0020] The application provides a low-cost high-strength and high-toughness gear steel and a preparation method thereof, the hardenability of the gear steel is improved by increasing the content of C element, and the hardenability of the gear steel is further improved by adding a small amount of inexpensive B element, so that the core hardness of the gear after carburizing heat treatment is improved, the content of alloy elements Cr, Ni and Mo which can improve the hardenability is reduced, and therefore the production cost of the gear steel is reduced;Nb element which has the most obvious grain refining effect is added, the toughness of the gear steel is improved, the risk of high brittleness caused by high core hardness is avoided, and finally a gear steel material with high strength and toughness is obtained, and the gear obtains the good combination of high surface hardness, high core strength and high toughness, high contact fatigue performance and high bending fatigue strength by combining with the best carburizing heat treatment process.

[0021] C: C is a cheap and effective strengthening element in steel, the strength can be increased by about 450 MPa for every 0.1 % of solid solution C, C and alloy elements in steel form precipitated phases to play a precipitated strengthening effect;At the same time, it is the most effective element affecting the hardenability. However, with the increase of its content, the plasticity and toughness decrease, so the content of C is controlled to be 0.17 % to 0.23 %.

[0022] Si: Si improves the strength of the steel by solid solution strengthening, and can also improve the hardenability of the gear steel. Si is a deoxidizing agent, but in the carburizing process, it can easily cause the carburized layer to be easily oxidized, thereby reducing the toughness of the carburized layer and reducing the fatigue strength of the gear; therefore, in order to ensure the deoxidizing effect and improve the toughness of the carburized layer, the Si content is controlled to be 0.15% to 0.35%.

[0023] Mn: Mn is an effective deoxidizing agent and desulfurizing agent, and is also an element that ensures hardenability; therefore, the Mn content should be greater than 0.70%, but excessive Mn can reduce the toughness of the carburized layer and reduce the fatigue strength of the gear, so the Mn content should be less than 0.95%; therefore, the Mn content is controlled to be 0.70% to 0.95%.

[0024] Cr: Cr can effectively improve the hardenability and strength of the steel, and in the carburizing process, Cr combines with C to form fine Cr-rich carbides, and at the same time, fixes the free carbon in the steel, thereby reducing the carbon content in the martensite and improving the toughness of the material; therefore, Cr is at least ensured to be greater than 0.40%, but excessive Cr can deteriorate the cold workability of the steel, so Cr should not be higher than 0.70%; therefore, the Cr content is controlled to be 0.40% to 0.70%.

[0025] Mo: Mo can significantly improve the hardenability of the steel, prevent temper brittleness and overheating tendency; in addition, the reasonable combination of Mo and Cr elements in the present application can significantly improve the hardenability and temper resistance, and promote the precipitation of carbides in the material, but if the Mo content is too low, the above effects are limited, and if the Mo content is too high, it promotes the formation of grain boundary ferrite films, which is not conducive to the hot plasticity of the steel, increases the reheat cracking tendency of the steel, and the cost is higher; therefore, the Mo content is controlled to be 0.10% to 0.20%.

[0026] Nb: Nb is a strong carbide-forming element, and in the carburizing process, Nb forms carbides with C, thereby reducing the C content of the martensite in the carburized layer and the matrix, and improving the toughness of the martensite; therefore, the Nb content should not be less than 0.010%, and excessive Nb can result in the above effects not being obvious, and excessive Nb content can increase the manufacturing cost of the steel; therefore, in the present application, the Nb content is 0.010 to 0.030%.

[0027] Ni: Ni is an austenite-forming element that can effectively improve the toughness of the core of the steel, reduce the ductile-brittle transition temperature, and improve the low-temperature impact performance; the Ni content should be greater than 0.30%; excessive Ni content can reduce the machinability after hot working, and excessive Ni content can increase the manufacturing cost of the steel; therefore, the Ni content is controlled to be 0.30% to 0.70%.

[0028] B: trace B can multiply the hardenability of the steel, 0.0010-0.0030% B has the same effect on the hardenability as 0.6% Mn, 0.7% Cr, 0.5% Mo and 1.5% Ni; in addition, a small amount of B can improve the toughness of the case and thus the contact fatigue life of the material, therefore the B content should be greater than 0.0005%; however, higher B can cause the steel to be hot brittle and affect the processability of the steel; the B contained should also be prevented from combining with [N] to form BN and thus not play the above role; therefore the B content is controlled to be 0.0005%-0.0030%;

[0029] Al: Al is an effective deoxidizer and can form AlN to refine the grains, when the Al content is lower than 0.015%, the effect is not obvious, and when the Al content is higher than 0.035%, coarse inclusions are easily formed, which deteriorates the performance of the steel; therefore the Al content should be controlled to be 0.015%-0.035%.

[0030] P and S: S is easy to form MnS inclusions with Mn in the steel, which causes the steel to be hot brittle, however, a small amount of S can significantly improve the cutting performance of the gear steel without affecting the performance of the product, and MnS has the effect of refining the grains at the same time; P is an element with strong segregation tendency, which increases the cold brittleness of the steel, reduces the plasticity and is harmful to the uniformity of the structure and performance of the product; therefore P is controlled to be ≤0.010% and S is controlled to be 0.015%-0.025%.

[0031] Further, [N] in the gear steel of the present application can form compounds with B and Al, etc. to refine the grains, reasonable Al / [N] has obvious effect on grain refinement, and too high [N] can form bubbles and other continuous casting defects, therefore the [N] content should be controlled to be 80-120ppm, and Al / [N] is controlled to be 2.0-4.0; in addition, [O] forms oxide inclusions in the steel, and O is controlled to be ≤9ppm; [H] forms white spots in the steel, which seriously affects the performance of the product, and [H] is controlled to be ≤2.0ppm.

[0032] Further, in the preparation process of the present application, Nb is added in the form of niobium iron and B is added in the form of boron iron, and the niobium iron is added before tapping in the electric furnace and the boron iron is added in the early stage of LF refining. Since Nb has a high melting point, the added Nb is also difficult to burn, and in order to make Nb more uniform, the present application requires that the niobium iron alloy be added before tapping in the electric furnace to ensure the yield of Nb. A small amount of B can improve the hardenability, and B is not easy to burn, and generally the boron iron is required to be added in the LF furnace before tapping, and in order to ensure the full and uniform B, the present application requires that B be added in the early stage of the LF furnace.

[0033] Further, the gear steel of the present application has a core hardness range of 35.6-44.2 HRC and a grain size of 12-16 μm and a grain size grade of 9 or above after carburizing heat treatment. By adjusting the heat treatment process and shortening the carburizing process time, the technical index of the comparative steel can be reached, the cost can be reduced, and the pollution emission can be reduced. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 A schematic diagram of a gear part for testing;

[0035] Figure 2 A carburizing heat treatment process curve;

[0036] Figure 3 A curve diagram of effective hardening layer depth of the comparative steel and the gear steel after carburizing heat treatment;

[0037] Figure 4 A grain morphology diagram of the comparative steel and the gear steel, Fig. (a) is the comparative steel and Fig. (b) is the gear steel. DETAILED DESCRIPTION

[0038] The present application will be further described below in conjunction with specific embodiments, which are an explanation of the present application rather than a limitation.

[0039] In order to enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should belong to the scope of protection of the present application.

[0040] The present application provides a low-cost high-strength and high-toughness gear steel such as Figure 1 as shown, comprising the following chemical components by weight percentage: C: 0.17-0.23%, Si: 0.15-0.35%, Mn: 0.70-0.95%, Cr: 0.40-0.70%, Mo: 0.10-0.20%, Ni: 0.30-0.70%, Al: 0.015-0.035%, P: ≤0.010%, S: 0.015-0.025%, Nb: 0.010-0.030%, B: 0.0005-0.0030%, [O]: ≤9ppm, [H]: ≤2ppm, [N]: 90-150ppm, and the rest is Fe and inevitable impurity elements. The delivery state structure of the gear steel is ferrite + pearlite, and the grain size grade is 9.0 or above.

[0041] C: C is a cheap and effective strengthening element in steel, and the strength can be increased by about 450 MPa for every 0.1% increase in solid solution C. C and alloying elements in steel form precipitates, which play a role in precipitation strengthening. Meanwhile, it is the most effective element affecting hardenability. However, as the content increases, the plasticity and toughness decrease, so the C content is controlled at 0.17% to 0.23%.

[0042] Si: Si improves the hardness of steel through solid solution strengthening, and can also improve the hardenability of gear steel. Si is a deoxidizer, but it is easy to oxidize the carburized layer during carburizing, thereby reducing the toughness of the carburized layer and the fatigue strength of the gear. Therefore, in order to ensure the deoxidizing effect and improve the toughness of the carburized layer, the Si content is controlled at 0.15% to 0.35%.

[0043] Mn: Mn is an effective deoxidizer and desulfurizer, and is also an element that ensures hardenability. Therefore, the Mn content should be greater than 0.70%. However, excessive Mn can reduce the toughness of the carburized layer and reduce the fatigue strength of the gear, so the Mn content should be less than 0.95%. The Mn content is controlled at 0.70% to 0.95%.

[0044] Cr: Cr can effectively improve the hardenability and strength of steel. During carburizing, Cr combines with C to form fine Cr-rich carbides, and at the same time, it fixes free carbon in the steel, reducing the carbon content in martensite and thereby improving the toughness of the material. Therefore, Cr is at least ensured to be greater than 0.40%. However, excessive Cr can deteriorate the cold workability of the steel, so Cr should not be higher than 0.70%. Therefore, the Cr content is controlled at 0.40% to 0.70%.

[0045] Mo: Mo can significantly improve the hardenability of steel, prevent temper brittleness and overheating tendency. In addition, the reasonable combination of Mo and Cr elements in the present application can significantly improve the hardenability and temper resistance, and promote the precipitation of carbides in the material. However, if the Mo content is too low, the above effects are limited, and if the Mo content is too high, it promotes the formation of grain boundary ferrite films, which is not conducive to the hot plasticity of the steel, increases the reheat cracking tendency of the steel, and the cost is higher. Therefore, the Mo content is controlled at 0.10% to 0.20%.

[0046] Nb: Nb is a strong carbide-forming element, and during carburizing, Nb forms carbides with C, thereby reducing the C content of martensite in the carburized layer and the matrix, and improving the toughness of the martensite. Therefore, the Nb content should not be less than 0.010%. Excessive Nb can result in the above effects not being obvious, and excessive Nb content can increase the manufacturing cost of the steel, so in the present application, the Nb content is 0.010 to 0.030%.

[0047] Ni: Ni is an austenite forming element, which can effectively improve the toughness of the core of the steel, reduce the ductile-brittle transition temperature, and improve the low temperature impact performance. The content of Ni should be greater than 0.30%. If the content of Ni is too high, the machinability after hot working will be reduced, and the manufacturing cost of the steel will be increased. Therefore, the content of Ni is controlled to be 0.30%-0.70%.

[0048] B: A small amount of B can increase the hardenability of the steel by several times. The effect of 0.0010-0.0030% B on the hardenability is equivalent to that of 0.6% Mn, 0.7% Cr, 0.5% Mo and 1.5% Ni. In addition, a small amount of B can improve the toughness of the diffusion layer and thus improve the contact fatigue life of the material, so the content of B should be greater than 0.0005%. However, higher B can cause the steel to be hot brittle, affecting the processing capacity of the steel. It should also be avoided that the contained B combines with [N] to form BN and cannot play the above role. Therefore, the content of B is controlled to be 0.0005%-0.0030%.

[0049] Al: Al is an effective deoxidizer and can form AlN to refine the grains. When the content of Al is less than 0.015%, the effect is not obvious, and when the content of Al is higher than 0.035%, coarse inclusions are easily formed, which deteriorates the performance of the steel. Therefore, the content of Al should be controlled to be 0.015%-0.035%.

[0050] P and S: S easily forms MnS inclusions with manganese in steel, causing the steel to be hot brittle. However, a small amount of S can significantly improve the machinability of the gear steel without affecting the performance of the product, and MnS also has the effect of refining the grains. P is an element with strong segregation tendency, which increases the cold brittleness of the steel, reduces the plasticity, and is harmful to the uniformity of the product structure and performance. The content of P is controlled to be ≤0.010%, and the content of S is controlled to be 0.015%-0.025%.

[0051] [N]: [N] can form compounds with B and Al to refine the grains. Reasonable Al / [N] has a significant effect on grain refinement, and too high [N] can form bubbles and other continuous casting defects. Therefore, the content of [N] should be controlled to be 80-120ppm, and Al / [N] is controlled to be 2.0-4.0.

[0052] [O] and [H]: [O] forms oxide inclusions in steel, and O is controlled to be ≤9ppm. [H] forms white spots in steel, which seriously affects the performance of the product, and [H] is controlled to be ≤2.0ppm.

[0053] 1) Material preparation

[0054] The preparation method of the gear steel provided by the application comprises the following steps: electric arc furnace smelting-LF refining-RH vacuum treatment-continuous casting-hot rolling-slow cooling. It should be noted that the addition of Nb is in the form of ferro-niobium, which is added before tapping in the electric furnace, and B is added in the form of ferro-boron before the early stage of LF furnace refining. Due to the high melting point of Nb, the added Nb is also difficult to burn, and in order to make Nb more uniform, the application requires that the ferro-niobium alloy be added before tapping in the electric furnace to ensure the yield of Nb. A small amount of B element can improve the hardenability, and B is not easy to burn, and generally requires that the ferro-boron be added before the LF furnace, and in order to ensure the full uniformity of B, the application requires that it be added in the early stage of the LF furnace.

[0055] 8620RH steel is produced as a comparison steel according to the same production process required in TES 047, and the comparison steel and the gear steel are subjected to the same preparation process to compare and analyze the influence of gear bending fatigue performance. Through the composition adjustment of the gear steel, the end quenching value detection at different positions shows that the hardenability of the gear steel is improved by 3~HRC compared with the comparison steel, because the addition of trace B and Nb elements improves the hardenability value.

[0056] Table 1 shows the chemical composition requirements of the comparison steel and the gear steel

[0057]

[0058] Table 2 shows the hardenability detection result range value of the comparison steel and the gear steel

[0059]

[0060] 2) forging

[0061] According to the part specifications, the round steel bar is cut and sheared into the required size, and then the cut round steel bar is heated to 1050℃~1250℃ by medium frequency induction, and is formed by die forging process.

[0062] 3) normalizing

[0063] In order to facilitate the finishing of the output shaft blank and improve the internal microstructure, the output shaft blank obtained in step 2) is subjected to annealing treatment, heated to 900℃~930℃, and kept for 2~5h, cooled to 570℃~650℃ with the furnace, kept for 2~4h, and cooled after discharging, and the hardness is required to be controlled at 150~180HBW.

[0064] 4) processing

[0065] The forged blank is sequentially subjected to rough turning, fine turning, boring and gear hobbing to obtain a gear semi-finished product.

[0066] 5) carburizing heat treatment

[0067] The processed gear semi-finished product is placed in a carburizing furnace for carburizing treatment, the carburizing atmosphere is nitrogen and methanol, and the gear is sequentially subjected to heating, strong carburizing, diffusion, holding and oil cooling stages, tempering, air cooling, and then the gear is obtained (see Figure 2 Process route); wherein:

[0068] The nitrogen flow rate is 4.0-6.0 m3 / h, and the methanol flow rate is 4.5-5.5 L / h.

[0069] The temperature is rapidly increased to 875-885°C and held for 20-40 min, the carbon potential in the heating stage is 0.70-0.80C%; the temperature is increased to 915-935°C for strong carburizing, the time is 140-180 min, the carbon potential in the strong carburizing stage is 1.0-1.1C%; then the temperature is increased to 915-935°C for diffusion, the time is 60-90 min, the carbon potential in the diffusion stage is 0.75-0.95C%; finally, the temperature is rapidly decreased to 855-865°C and held for 30-60 min, the carbon potential in the holding stage is 0.65-0.85C%.

[0070] The gear is pulled from the back chamber of the carburizing heat treatment furnace to the front chamber for oil quenching, the oil temperature used is 70-90°C of staged cooling oil, the stirring time is 30 min, and the stirring speed is 100-200 rpm. After oil quenching, the gear needs to be drained for 15-25 min. After cleaning, the gear is subjected to tempering heat treatment. The output shaft is heated to 150-180°C again for tempering treatment, held in a nitrogen atmosphere for 2-5 h, and then air cooled to room temperature to obtain the output shaft part.

[0071] After carburizing heat treatment, as shown in Figure 2 , the heat treatment detection indexes of the two materials show that the core hardness of the gear is in the range of 35.6-44.2 HRC, which is increased by about 5 HRC compared with the technical index of the comparative steel under the same process. The effective hardened layer depth is deepened by 0.37 mm (as shown in Figure 3 ) compared with the comparative steel. Subsequently, the carburizing process time can be shortened to achieve the technical index of the comparative steel, thereby reducing cost and reducing pollution emissions. The grain size is 12-16 μm, and the grain size grade is 9 grade or more. The grain size of the comparative steel is 23-25 μm, and the grain size grade is about 8 grade, as shown in Figure 4 .

[0072] The gear steel gear is tested according to the “GB / T-14230-2021 Gear Bending Fatigue Strength Test Method”, and the bending fatigue strength of the gear is increased by 15-20% compared with the comparative steel.

[0073] Example 1,

[0074] The component materials are weighed according to the following percentage, including C: 0.17%, Si: 0.35%, Mn: 0.70%, Cr: 0.70%, Mo: 0.10%, Ni: 0.70%, Al: 0.015%, P: 0.010%, S: 0.025%, Nb: 0.010%, B: 0.0030%, and the rest is Fe and inevitable impurity elements, the component materials are added into an electric furnace, and after arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling treatment, a round steel rod matrix is obtained, the round steel rod matrix is forged into a shape, normalized, and mechanically processed to obtain a gear semi-product, and the gear semi-product is subjected to carburizing treatment to obtain a low-cost high-toughness gear steel.

[0075] The Nb is added in the form of ferro-niobium and is added before tapping in the electric furnace; and the B is added in the form of ferro-boron and is added in the early stage of LF refining.

[0076] The specific process of forging the round steel rod matrix into a shape is as follows: the round steel rod is cut into the required size according to the part specifications, the cut round steel rod is heated to 1050 DEG C through medium-frequency induction, and the output shaft blank is obtained after being forged into a shape through die forging process.

[0077] The specific process of normalizing is as follows: the output shaft blank is annealed by heating to 900 DEG C, keeping for 2 hours, cooling to 570 DEG C, keeping for 2 hours, and then being discharged and cooled, and the hardness of the annealed output shaft blank is required to be controlled at 150 HBW.

[0078] The specific process of carburizing treatment is as follows: the processed gear semi-product is placed in a carburizing furnace for carburizing treatment, the carburizing treatment carbon atmosphere is nitrogen and methanol, and the gear semi-product is sequentially subjected to the stages of heating, strong carburizing, diffusion, keeping, oil cooling, tempering and air cooling to obtain the low-cost high-toughness gear steel.

[0079] The temperature is rapidly increased to 875 DEG C and kept for 20 minutes, the carbon potential in the heating stage is 0.70C%, the temperature is increased to 915 DEG C for strong carburizing, the time is 140 minutes, the carbon potential in the strong carburizing stage is 1.0C%, then the temperature is increased to 915 DEG C for diffusion, the time is 60 minutes, the carbon potential in the diffusion stage is 0.75C%, finally the temperature is rapidly decreased to 855-865 DEG C and kept for 30 minutes, the carbon potential in the keeping stage is 0.65C%, the gear semi-product is pulled from the back chamber to the front chamber of the carburizing heat treatment furnace for oil quenching, the oil temperature of the fractional cooling oil used is 70 DEG C, the stirring time is 30 minutes, the stirring speed is 100 rpm, the gear semi-product needs to be drained for 15 minutes after oil quenching, is cleaned, is heated to 150 DEG C for tempering treatment again, is kept for 2 hours in a nitrogen atmosphere, and is air cooled to room temperature to obtain the low-cost high-toughness gear steel.

[0080] The nitrogen flow rate is 4.0 m 3 / h, and the methanol flow rate is 4.5 L / h.

[0081] Example 2,

[0082] The component materials are weighed according to the following percentages, including C: 0.23%, Si: 0.15%, Mn: 0.95%, Cr: 0.40%, Mo: 0.20%, Ni: 0.30%, Al: 0.035%, P: 0.009%, S: 0.015%, Nb: 0.030%, B: 0.0005%, and the rest is Fe and inevitable impurity elements. The component materials are added to an electric furnace, sequentially subjected to arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, hot rolling, and slow cooling treatment to obtain a round steel rod matrix. The round steel rod matrix is forged into a shape, normalized, and mechanically processed to obtain a gear semi-finished product. The gear semi-finished product is then subjected to carburizing treatment to obtain a low-cost high-toughness gear steel.

[0083] The Nb is added in the form of ferro-niobium before tapping from the electric furnace, and the B is added in the form of ferro-boron before the early stage of LF furnace refining.

[0084] The specific process of forging the round steel rod matrix into a shape is as follows: the round steel rod is cut to the required size according to the part specifications, and then the cut round steel rod is heated to 1250°C by medium-frequency induction, and is forged into a shape by die forging process to obtain an output shaft blank.

[0085] The specific process of normalizing is as follows: the output shaft blank is annealed by heating to 930°C, holding for 5h, cooling to 650°C, holding for 4h, and then cooling after tapping. The hardness of the annealed output shaft blank is required to be controlled at 180HBW.

[0086] The specific process of carburizing treatment is as follows: the machined gear semi-finished product is placed in a carburizing furnace for carburizing treatment. The carburizing treatment carbon atmosphere is nitrogen and methanol, and the process sequentially includes heating, strong carburizing, diffusion, holding, oil cooling, tempering, and air cooling to obtain a low-cost high-toughness gear steel.

[0087] Rapid heating to 885℃ and holding for 40min, the carbon potential of heating stage is 0.80C%; heating to 935℃ for strong infiltration, the time is 180min, the carbon potential of strong infiltration stage is 1.1C%; then diffusion at 935℃ for 90min, the carbon potential of diffusion stage is 0.95C%; finally, rapidly cooling to 865℃ and holding for 60min, the carbon potential of holding stage is 0.85C%, drawing from the back room to the front room of the carburizing heat treatment furnace for oil quenching, the oil temperature of 90℃ is used for fractional cooling oil, the stirring time is 30min, the stirring speed is 200rpm, and the oil needs to be drained for 25min after oil cooling, and the gear semi-finished product is heated to 180℃ for tempering treatment, and then the low-cost high-toughness gear steel is prepared after the tempering treatment in nitrogen atmosphere and air cooling to room temperature.

[0088] The nitrogen flow is 6.0m 3 / h, and the methanol flow is 5.5L / h.

[0089] Example 3,

[0090] The component materials are weighed according to the following weight percentages, including C: 0.19%, Si: 0.23%, Mn: 0.82%, Cr: 0.55%, Mo: 0.13%, Ni: 0.45%, Al: 0.25%, P: 0.008%, S: 0.018%, Nb: 0.018%, B: 0.0015%, and the rest is Fe and inevitable impurity elements. The component materials are added to an electric furnace, and after sequentially undergoing electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling treatment, a round steel rod matrix is obtained. The round steel rod matrix is forged and shaped, normalized, and mechanically processed to obtain a gear semi-finished product. The gear semi-finished product is subjected to carburizing treatment to obtain a low-cost high-toughness gear steel.

[0091] The Nb is added in the form of ferro-niobium before tapping, and the B is added in the form of ferro-boron before LF refining.

[0092] The specific process of forging and shaping the round steel rod matrix is as follows: the round steel rod is cut to the required size according to the part specifications, and then the cut round steel rod is heated to 1150℃ by medium-frequency induction, and is forged and shaped by die forging process to obtain an output shaft blank.

[0093] The specific process of normalizing is as follows: the output shaft blank is annealed by heating to 910℃ and holding for 3h, and then cooling to 570℃-650℃ and holding for 2.5h, and then cooling after discharging, and the hardness of the annealed output shaft blank is required to be controlled at 160HBW.

[0094] The specific process of the carburizing treatment is that the processed gear semi-finished product is placed in a carburizing furnace for carburizing treatment, wherein the carburizing treatment carbon atmosphere is nitrogen and methanol, and the carburizing treatment is sequentially subjected to a heating stage, a strong carburizing stage, a diffusion stage, a holding stage and an oil cooling stage, tempering, and air cooling to obtain the low-cost high-toughness gear steel.

[0095] rapidly heated to 880℃ and held for 30 min, the carbon potential in the heating stage is 0.73C%; heated to 925℃ for strong carburizing, the time is 160 min, the carbon potential in the strong carburizing stage is 1.05C%; then diffused at 925℃ for 75 min, the carbon potential in the diffusion stage is 0.85C%; finally, rapidly cooled to 860℃ and held for 45 min, the carbon potential in the holding stage is 0.75C%, pulled from the back chamber to the front chamber of the carburizing heat treatment furnace for oil quenching, the oil temperature of the fractional cooling oil used is 80℃, the stirring time is 30 min, the stirring speed is 150 rpm, and the oil cooling needs to be drained for 20 min after the oil cooling is completed, and then cleaned and subjected to tempering heat treatment, the gear semi-finished product is heated to 165℃ again for tempering treatment, held in a nitrogen atmosphere for 3h, and then air cooled to room temperature to obtain the low-cost high-toughness gear steel.

[0096] The nitrogen flow is 5m 3 / h, and the methanol flow is 5L / h.

[0097] Example 4,

[0098] The component materials are weighed according to the following weight percentages, including C: 0.21%, Si: 0.31%, Mn: 0.87%, Cr: 0.65%, Mo: 0.18%, Ni: 0.65%, Al: 0.3%, P: 0.009%, S: 0.022%, Nb: 0.025%, B: 0.0024%, and the rest is Fe and unavoidable impurity elements, the component materials are added to an electric furnace, sequentially subjected to electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, hot rolling and slow cooling treatment to obtain a round steel rod matrix, the round steel rod matrix is forged into a shape, normalized, and mechanically processed to obtain a gear semi-finished product; and the gear semi-finished product is subjected to carburizing treatment to obtain the low-cost high-toughness gear steel.

[0099] The Nb is added in the form of ferrocolumbium before tapping in the electric furnace, and the B is added in the form of ferroboron before the early stage of LF furnace refining.

[0100] The specific process of the round steel rod matrix for forging into a shape is that the round steel rod is cut to the required size according to the part specifications, the cut round steel rod is heated to 1200℃ by medium-frequency induction, and then forged into a shape by die forging process to obtain the output shaft blank.

[0101] The specific process of the normalizing treatment is that the output shaft blank is annealed, heated to 925 DEG C, cooled to 630 DEG C after 4h of holding, then held for 3h, discharged and cooled, and the hardness of the output shaft blank after annealing is required to be controlled at 170HBW.

[0102] The specific process of the carburizing treatment is that the processed gear semi-finished product is placed in a carburizing furnace for carburizing treatment, wherein the carburizing treatment carbon atmosphere is nitrogen and methanol, and the carburizing treatment is sequentially subjected to the stages of heating, strong carburizing, diffusion, holding and oil cooling, tempering and air cooling to obtain the low-cost high-toughness gear steel, and the specific process is as follows:

[0103] rapid heating to 880 DEG C and holding for 35min, the carbon potential in the heating stage is 0.76C%; heating to 930 DEG C for strong carburizing, the time is 170min, the carbon potential in the strong carburizing stage is 1.08C%; then diffusion at 925 DEG C for 80min, the carbon potential in the diffusion stage is 0.90C%; finally, rapid cooling to 860 DEG C and holding for 50min, the carbon potential in the holding stage is 0.80C%, oil quenching is carried out by pulling from the back chamber to the front chamber of the carburizing heat treatment furnace, the oil temperature of the fractional cooling oil used is 85 DEG C, the stirring time is 30min, the stirring speed is 180rpm, and after the oil quenching is completed, the oil needs to be drained for 25min, and then the gear semi-finished product is cleaned and subjected to tempering heat treatment, and the gear semi-finished product is heated to 170 DEG C again for tempering treatment, held in a nitrogen atmosphere for 4h, and then air-cooled to room temperature to prepare the low-cost high-toughness gear steel.

[0104] The nitrogen flow is 5.5m 3 / h, and the methanol flow is 5L / h.

[0105] At present, the high-performance Cr-Ni-Mo gear steel in the market is basically designed by low-carbon + high-alloy composition, and such a composition design can make the material have good comprehensive performance and meet the performance use requirements. However, due to the scarcity of precious metal element resources, the production cost of the gear steel with the conventional composition design is high, the profit obtained is small, and the enterprise cannot reduce the cost and increase the benefit. The present application provides a low-cost high-toughness heavy truck transmission gear steel and a method for preparing a gear therefrom, by means of low-cost element design, especially adding the low-cost B element to improve the hardenability and the Nb element to refine the grain, and combining the developed carburizing heat treatment process, the strength and toughness of the gear core are improved, and the heat treatment quenching deformation is reduced, and the strength and precision of the gear are improved.

[0106] The above merely describes preferred embodiments of the present application, and is not intended to limit the present application in any form; any person skilled in the art can easily implement the present application according to the drawings and the above description; however, any person skilled in the art can make some changes, modifications and equivalent changes within the scope of the technical solutions of the present application, and the equivalent embodiments of the present application are still within the protection scope of the technical solutions of the present application.

Claims

1. A gear based on low-cost, high-strength, and high-toughness gear steel, characterized in that, The gear steel comprises the following chemical composition by weight percentage: C: 0.17%, Si: 0.23%, Mn: 0.95%, Cr: 0.70%, Mo: 0.10%–0.13%, Ni: 0.30%, Al: 0.015%–0.035%, P: ≤0.010%, S: 0.015%–0.025%, Nb: 0.010%–0.018%, B: 0.0024%–0.0030%, with the balance being Fe and unavoidable impurity elements; the gear steel contains [O]: ≤9ppm, [H]: ≤2ppm, and [N]: 90%. 150ppm; The gear steel has a grain size of 12~16μm and a grain size grade of at least 9.

0. The microstructure of the gear steel in the supplied state is ferrite + pearlite; The core hardness of the gear ranges from 35.6 to 44.2 HRC; The method for manufacturing gears based on low-cost, high-strength, and high-toughness gear steel includes, The following components were weighed according to their weight percentages: C: 0.17%, Si: 0.23%, Mn: 0.95%, Cr: 0.70%, Mo: 0.10%, Ni: 0.30%, Al: 0.015%–0.035%, P: ≤0.010%, S: 0.015%–0.025%, Nb: 0.010%–0.018%, B: 0.0024%–0.0030%, with the remainder being Fe and unavoidable impurities. These components were added to an electric furnace and sequentially smelted in an electric arc furnace, refined using an LF furnace, vacuum treated with an RH furnace, continuously cast, hot rolled, and slowly cooled to obtain a round steel bar matrix. This matrix was then forged, normalized, and machined to obtain a gear semi-finished product. Finally, the gear semi-finished product was carburized to obtain the gear itself. The specific process of the carburizing treatment is as follows: the processed gear semi-finished product is placed in a carburizing furnace for carburizing treatment. The carburizing atmosphere is nitrogen and methanol. The treatment involves heating, strong carburizing, diffusion, heat preservation and oil cooling stages, tempering, and air cooling to obtain low-cost, high-strength and tough gear steel. The specific process is as follows: The temperature is rapidly increased to 875-885℃ and held for 20-40 minutes, with a carbon potential of 0.70-0.80C during the heating phase; then, the temperature is increased to 915-935℃ for strong infiltration for 140-180 minutes, with a carbon potential of 1.0-1.1C during the strong infiltration phase; followed by diffusion at 915-935℃ for 60-90 minutes, with a carbon potential of 0.75-0.95C during the diffusion phase; finally, the temperature is rapidly decreased to 855-865℃ and held for 30-60 minutes, with a carbon potential of 0.6C during the holding phase. 5~0.85C%, the steel is drawn from the rear chamber of the carburizing heat treatment furnace to the front chamber for oil quenching. Graded cooling oil with a temperature of 70~90℃ is used, with a stirring time of 30 minutes and a stirring speed of 100~200 rpm. After oil quenching, the oil needs to be drained for 15~25 minutes. After cleaning, tempering heat treatment is performed. The gear semi-finished product is reheated to 150~180℃ for tempering, held in a nitrogen atmosphere for 2h~5h, and then air-cooled to room temperature to obtain low-cost, high-strength, and tough gear steel. The nitrogen flow rate is 4.0~6.0 m³ / h. 3 The methanol flow rate is 4.5~5.5 L / h.

2. A method for preparing a gear based on low-cost, high-strength, and high-toughness gear steel as described in claim 1, characterized in that, include, The following components were weighed according to their weight percentages: C: 0.17%, Si: 0.23%, Mn: 0.95%, Cr: 0.70%, Mo: 0.10%, Ni: 0.30%, Al: 0.015%–0.035%, P: ≤0.010%, S: 0.015%–0.025%, Nb: 0.010%–0.018%, B: 0.0024%–0.0030%, with the remainder being Fe and unavoidable impurities. These components were added to an electric furnace and sequentially smelted in an electric arc furnace, refined using an LF furnace, vacuum treated with an RH furnace, continuously cast, hot rolled, and slowly cooled to obtain a round steel bar matrix. This matrix was then forged, normalized, and machined to obtain a gear semi-finished product. Finally, the gear semi-finished product was carburized to obtain the gear itself. The specific process of the carburizing treatment is as follows: the processed gear semi-finished product is placed in a carburizing furnace for carburizing treatment. The carburizing atmosphere is nitrogen and methanol. The treatment involves heating, strong carburizing, diffusion, heat preservation and oil cooling stages, tempering, and air cooling to obtain low-cost, high-strength and tough gear steel. The specific process is as follows: The temperature is rapidly increased to 875-885℃ and held for 20-40 minutes, with a carbon potential of 0.70-0.80C during the heating phase; then, the temperature is increased to 915-935℃ for strong infiltration for 140-180 minutes, with a carbon potential of 1.0-1.1C during the strong infiltration phase; followed by diffusion at 915-935℃ for 60-90 minutes, with a carbon potential of 0.75-0.95C during the diffusion phase; finally, the temperature is rapidly decreased to 855-865℃ and held for 30-60 minutes, with a carbon potential of 0.6C during the holding phase. 5~0.85C%, the steel is drawn from the rear chamber of the carburizing heat treatment furnace to the front chamber for oil quenching. Graded cooling oil with a temperature of 70~90℃ is used, with a stirring time of 30 minutes and a stirring speed of 100~200 rpm. After oil quenching, the oil needs to be drained for 15~25 minutes. After cleaning, tempering heat treatment is performed. The gear semi-finished product is reheated to 150~180℃ for tempering, held in a nitrogen atmosphere for 2h~5h, and then air-cooled to room temperature to obtain low-cost, high-strength, and tough gear steel. The nitrogen flow rate is 4.0~6.0 m³ / h. 3 The methanol flow rate is 4.5~5.5 L / h.

3. The method for preparing a gear based on low-cost, high-strength, and high-toughness gear steel according to claim 2, characterized in that, The Nb is added in the form of ferroniobium and is added before tapping from the electric furnace; B is added in the form of ferroboron and is added in the early stage of LF refining.

4. The method for preparing a gear based on low-cost, high-strength, and high-toughness gear steel according to claim 2, characterized in that, The specific process of forging the round steel bar base is as follows: the round steel bar is cut into the required size according to the part specifications, and then the cut round steel bar is heated to 1050℃~1250℃ by medium frequency induction heating, and then forged into a blank through die forging process.

5. The method for preparing a gear based on low-cost, high-strength, and high-toughness gear steel according to claim 2, characterized in that, The specific process of normalizing is as follows: the blank is annealed, heated to 900℃~930℃, held for 2~5 hours, cooled to 570℃~650℃, held for another 2~4 hours, and then cooled after being taken out of the furnace. The hardness of the blank after annealing is required to be controlled at 150~180HBW.

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