Nitriding gear steel and method for manufacturing the same

Abstract

The present application relates to a kind of nitriding gear steel and its manufacturing method, belong to gear field.The chemical composition of steel is as follows: C:0.20~0.25%, Si:0.15~0.35%, Mn:1.25~1.55%, Cr:1.05~1.35%, P:≤0.015%, S:≤0.005%, Ti:0.015~0.035%, N:≤0.0020%, Al:0.005~0.015%, the balance is Fe and inevitable impurities.The manufacturing method involves hot metal desulfurization, converter smelting, refining, vacuum degassing, continuous casting, multi-pass rolling, flame cleaning, oil tank quenching, low temperature tempering, the size of TiN inclusion in the final steel organization is ≤5um, the size of Al2O3 inclusion is ≤50um;The hardness of the region from the surface of steel to 15mm away from the surface of steel is 33~36HRC, the organization in this region is as follows according to the area ratio in metallographic diagram: ferrite: 35~40%, martensite: 50~55%, bainite: 5~15%;The actual grain size of organization is ≥6 levels.

Description

Technical Field

[0001] This invention belongs to the technical field of special steel smelting, specifically relating to a nitriding gear steel and its manufacturing method. Background Technology

[0002] Compared to carburizing and quenching, nitriding provides gears with higher hardness, wear resistance, and anti-galling properties. Because nitriding occurs at a lower temperature (typically 480-580℃), the gears are not quenched after nitriding, thus eliminating the problem of quenching deformation. Furthermore, the nitrided layer exhibits very high hardness. Therefore, nitrided gear steel can be used to manufacture precision gears with high wear resistance, good fatigue strength, and minimal heat treatment deformation. Summary of the Invention

[0003] To meet the requirements for nitrided gear steel, this invention develops a nitrided gear steel and its manufacturing method. The steel surface is free of cracks and oxide layer defects; the hardness of the area from the steel surface to 15mm away from the steel surface is 33-36 HRC, and the microstructure (area percentage of each microstructure in the SEM image) in this area is ferrite (35-40%) + martensite (50-55%) + bainite (5-15%); the actual grain size of the steel is ≥6.

[0004] The technical solution adopted by the present invention to solve the above problems is as follows: a nitrided gear steel, the elemental composition of which is C: 0.20-0.25% by weight, Si: 0.15-0.35%, Mn: 1.25-1.55%, Cr: 1.05-1.35%, P: ≤0.015%, S: ≤0.005%, Ti: 0.015-0.035%, N: ≤0.0020%, Al: 0.005-0.015%, with the balance being Fe and unavoidable impurities.

[0005] The main functions and design basis of each chemical element in the steel of this invention are as follows:

[0006] C: Carbon is an important element affecting the microstructure transformation during the quenching process of steel. In this invention, the steel is cooled to 10°C below the critical ferrite transformation temperature Ar3 (760°C) before oil quenching. The lower carbon content is beneficial for the formation of a certain amount of ferrite microstructure on the steel surface and in the area up to 15mm from the steel surface during the cooling process. Therefore, the carbon content is selected in the range of 0.20% to 0.25%.

[0007] Si: During the refining process, ferrosilicon powder floats onto the surface of the molten steel, which can play a role in diffusion deoxidation. The silicon content is selected in the range of 0.15% to 0.35%.

[0008] Mn: Adding manganese to steel has a significant strengthening effect, improving its hardness, strength, and wear resistance. However, excessively high manganese content is detrimental to the formation of a certain amount of ferrite in the steel after oil quenching. Therefore, the manganese content is selected within the range of 1.25% to 1.55%.

[0009] Cr: Adding chromium to the steel of this invention can effectively reduce the critical cooling rate of the steel and increase the hardness of the steel surface to a range of 15 mm from the surface after oil quenching. Therefore, the chromium content is selected in the range of 1.05% to 1.35%.

[0010] P: Phosphorus is a harmful element in steel, increasing its cold brittleness and reducing its plasticity. The P content in this invention is ≤0.015%.

[0011] Sulfur exists in steel as a non-metallic inclusion, which significantly reduces the fatigue life of the steel. In this invention, the sulfur content is ≤0.005%.

[0012] Ti and N: In the early stages of refining, titanium-iron wire is added. This promotes the combination of titanium and nitrogen in the steel to form large TiN inclusions. During refining and vacuum degassing, these large TiN inclusions have sufficient time to float and be adsorbed by the steel slag. This process effectively reduces the N content in the molten steel and removes large TiN inclusions. Although TiN in the solid austenite phase begins to precipitate slowly at 1481℃ during continuous casting solidification, the N content in the steel is below 0.0020% at this point, and the precipitation rate is slow. Therefore, the number of precipitated TiN particles is small, and their morphology is fine and dispersed. These small, finely dispersed TiN particles do not affect the fatigue strength of the steel. After rolling and cooling, Ti in the steel mainly exists in an atomic state. During the gear nitriding heat treatment, the nitrogen atoms diffused into the steel surface will combine with titanium atoms to form a large amount of highly dispersed molecular TiN, thereby increasing the hardness and strength of the nitrided layer on the steel surface. Therefore, the titanium content of this invention is selected in the range of 0.015-0.035%, and the nitrogen content is selected in the range of N: ≤0.0020%.

[0013] Al is added to the steel as a deoxidizer. Because aluminum wire is fed into the steel for precipitation deoxidation while ferrosilicon powder is simultaneously introduced for diffusion deoxidation, only a small amount of aluminum wire is needed for sufficient deoxidation. Excessive aluminum content will cause aluminum to be oxidized by oxygen in the air during subsequent production processes, leading to the formation of aluminum oxide non-metallic inclusions in the steel. Therefore, the aluminum content in this invention is selected within the range of Al: 0.005-0.015%.

[0014] The aforementioned nitriding gear steel and its manufacturing method include the following process steps:

[0015] (1) Steelmaking and Billet Casting: The process involves desulfurization of molten iron, converter smelting, and pre-refining treatment with manganese alloy, chromium alloy, and titanium-iron wire. During refining, ferrosilicon powder is introduced onto the surface of the molten steel for diffusion deoxidation, while aluminum wire is fed in for precipitation deoxidation. After refining, the molten steel undergoes vacuum degassing, using inert argon as a booster gas to prevent an increase in nitrogen and oxygen content. After vacuum degassing, argon is blown into the bottom of the molten steel for at least 20 minutes to promote the further floating of large non-metallic inclusions (TiN) to the slag layer. The continuous casting process uses a crystallizer to cast the billet. The remaining steel in the ladle is preferably controlled at 6 tons. When there is a remaining 6 tons of molten steel in the ladle, the ladle automatically exits the casting process, ensuring that slag from the ladle does not enter the tundish or crystallizer. This prevents phosphorus, nitrogen, aluminum, and non-metallic inclusions such as titanium nitride and aluminum oxide from the slag from entering the billet and affecting its chemical composition and purity.

[0016] (2) The continuous casting billet is removed from the high temperature line. The design obtains the continuous casting billet at a high temperature of 1050-1100℃. In this way, the continuous casting billet does not need to be heated by the steel rolling furnace. Eliminating the heating process of the steel rolling furnace can greatly reduce the cost of steel rolling. The high temperature billet is descaled by high pressure water to completely remove the oxide layer on its surface.

[0017] The slab with the oxide layer removed undergoes several rolling passes. The initial passes use a high compression ratio to induce dynamic recrystallization, refining the actual grain size on the steel surface and internally. The intermediate passes use a medium rolling compression ratio, and the final passes use a finishing rolling compression ratio. Both the medium and finishing rolling compression ratios are relatively low, further refining the actual grain size on the steel surface. The time from the end of rough rolling to the start of medium rolling, and from the end of medium rolling to the start of finishing rolling, is ≤2 seconds. Effectively controlling this rolling process time prevents the austenite grains within the workpiece from continuously growing at high temperatures (>900℃) due to subdynamic crystallization and static recrystallization. Reducing the rolling process time promotes the formation of fine actual grains in the rolled steel. The final rolling temperature is controlled at 850-900℃. A lower final rolling temperature prevents rapid grain growth at high temperatures, ensuring a stable grain size control of ≥6 levels.

[0018] (3) The steel after final rolling is immediately cleaned with a flame to remove defects on the surface of the steel. After removing the surface defects, the steel is air-cooled to 10°C below the phase transformation point Ar3 and then quenched in an oil bath. In this case, the starting temperature of ferrite transformation during cooling is 760°C. The steel is air-cooled from 760°C to 750°C. The steel has a relatively slow cooling rate and a small degree of undercooling. Carbon atoms can diffuse fully and the undercooled austenite can be transformed into ferrite. After air-cooling to 750°C, the steel is immediately quenched in an oil bath. Under the fast cooling rate and large degree of undercooling, a small portion of carbon atoms do not diffuse sufficiently and the undercooled austenite is transformed into bainite. Most carbon atoms do not have enough time to diffuse due to the fast cooling rate and the undercooled austenite is transformed into martensite. After oil quenching, the hardness of the area from the surface of the steel to 15 mm away from the surface of the steel is 33-36 HRC, and the microstructure of this area is ferrite (35-40%) + martensite (50-55%) + bainite (5-15%).

[0019] (4) The oil-quenched steel should be immediately heated for low-temperature tempering (tempering temperature is 200℃) to eliminate internal stress. Quenched steel has significant stress, and if it is not tempered in time, stress deformation or even surface stress cracking will occur. At the same time, the low-temperature tempering process can also promote the evaporation of oil on the surface of the oil-quenched steel. The residual oil content on the steel surface should be controlled at 1.5-1.8 g / mm. 2 A small amount of residual oil on the steel surface can effectively prevent rust from forming, but excessive oil content can pollute the environment of the gear processing site during subsequent gear processing.

[0020] Preferably, in step (2), the billet with the oxide layer removed is rolled in 9 passes. The rolling compression ratios of the first three passes are fixed at 40%, 30%, and 20%. The larger rolling compression ratio of the rough rolling is conducive to the steel to produce larger plastic deformation, induce dynamic recrystallization, and refine the actual grain size of the steel surface and interior. The intermediate rolling compression ratios of the middle three passes are fixed at 10%, 8%, and 7%, and the finishing rolling compression ratios of the last three passes are (6-10%), (5-10%), and (4-10%).

[0021] Compared with existing technologies, the advantages of this invention are as follows: The nitrided gear steel of this application adopts a low-carbon, low-sulfur, low-titanium, low-aluminum, and low-nitrogen design. Furthermore, by employing a reasonable smelting and continuous casting process, the size of large inclusions of titanium nitride and aluminum oxide in the steel is reduced, and the chemical composition of the steel is stabilized. The heating process in the rolling process is eliminated, reducing the production cost. By controlling the rolling compression ratio of each pass, the actual grain size of the steel is ultimately refined. After rolling, a flame cleaning process is used to remove surface defects. After flame cleaning, the steel is air-cooled to 10°C below Ar3 and then oil-quenched. After oil quenching, the steel surface and the area within 15mm from the surface have stable hardness and microstructure. The oil-quenched steel is immediately heated to 200°C for low-temperature tempering to eliminate quenching stress and promote the evaporation of oil from the steel surface, reducing the residual oil content. The holding time for low-temperature tempering is based on a final residual oil content of 1.5-1.8g / mm². 2 To design.

[0022] The final produced nitrided gear steel is characterized by: TiN inclusions ≤5µm in size and Al2O3 inclusions ≤50µm in size; no cracks or oxide layer defects on the steel surface; a hardness of 33-36 HRC in the area from the steel surface to 15mm below the surface, with a microstructure of ferrite (35-40%) + martensite (50-55%) + bainite (5-15%); and an actual grain size ≥6. The residual oil content on the steel surface is 1.5-1.8g / mm². 2 This nitrided gear fills a gap in domestic steel production. Attached Figure Description

[0023] Figure 1 Metallographic image (×500x) of the microstructure of the steel surface to a region 15mm from the steel surface in Embodiment 1 of the present invention;

[0024] Figure 2 This is a metallographic image (×500x) of the microstructure of the steel surface to a region 15mm away from the steel surface in Embodiment 2 of the present invention. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0026] Example 1 and Example 2:

[0027] The two embodiments involve a manufacturing method for nitrided gear steel: KR desulfurization → 100t converter → 100t refining → RH vacuum degassing treatment → continuous casting of square billet (300mm*340mm) → high-temperature continuous casting billet high-pressure water descaling → 9-pass rolling → flame cleaning → oil bath quenching → low-temperature tempering. Two batches of nitrided gear steel were manufactured.

[0028] The steelmaking process employs hot metal desulfurization, converter smelting, and pre-refining furnace alloying with manganese alloy, chromium alloy, and titanium-iron wire. Ferrosilicon powder is diffused onto the surface of the molten steel for deoxidation, while aluminum wire is fed in for precipitation deoxidation. The RH vacuum degassing process uses inert argon as the lifting gas, followed by bottom argon blowing for 20 minutes after vacuum degassing. The continuous casting process uses a crystallizer to cast continuous casting billets, with the remaining steel in the ladle controlled at 6 tons. After continuous casting, the billets at high temperatures (1055℃, Example 1) and 1098℃ (Example 2) are descaled using high-pressure water to remove the oxide layer from the surface, followed by nine rolling passes. The roughing compression ratios for the first three passes are fixed at 40%, 30%, and 20%, the intermediate rolling compression ratios for the middle three passes are fixed at 10%, 8%, and 7%, and the finishing rolling compression ratios for the last three passes are 6%, 5%, and 4% (Example 1), and 10%, 10%, and 10% (Example 2). The time from the end of roughing rolling to the start of intermediate rolling and from the end of intermediate rolling to the start of finishing rolling are controlled at 0.9s and 0.8s (Example 1), and 1.9s and 1.7s (Example 2), respectively. The final rolling temperature is controlled at 859℃ (Example 1) and 887℃ (Example 2). After final rolling, the steel is immediately flame-cleaned to remove surface defects, then air-cooled to 750℃ (the Ar3 temperature for this steel grade is 760℃), and then immediately quenched in an oil bath. The oil-quenched steel was immediately heated to 200℃ for low-temperature tempering to relieve quenching stress and promote the evaporation of oil from the steel surface. The residual oil content on the steel surface was 1.55 g / mm². 2 (Example 1), 1.78 g / mm 2 (Example 2)

[0029] The smelting composition of the steels obtained in Examples 1 and 2 is shown in Table 1.

[0030] Table 1 Smelting composition (wt%)

[0031] Example 1 0.21 0.20 1.28 1.09 0.012 0.001 0.018 0.0012 0.007 Example 2 0.25 0.29 1.53 1.33 0.008 0.005 0.034 0.0019 0.015

[0032] The maximum sizes of TiN and Al2O3 in the steels obtained in Examples 1 and 2 are shown in Table 2.

[0033] Table 2

[0034] Example 1 2.3um 28um Example 2 4.7um 47um

[0035] The steel materials prepared in Examples 1 and 2 were free of cracks and oxide layer defects on their surfaces.

[0036] The steels obtained in Examples 1 and 2 had a hardness of 33.6 HRC (Example 1) and 35.5 HRC (Example 2) in the region from the surface to 15 mm from the steel surface, respectively; the microstructure in this region consisted of 40% ferrite, 50% martensite, and 10% bainite. See the attached photograph for details. Figure 1 (Example 1), 37% ferrite + 55% martensite + 8% bainite, see detailed microstructure photographs. Figure 2 (Example 2); The actual grain size of the steel is grade 7.0 (Example 1) and grade 7.5 (Example 2).

Claims

1. A nitriding gear steel, characterized in that: The chemical composition of the steel by weight percentage is: C: 0.20-0.25%, Si: 0.15-0.35%, Mn: 1.25-1.55%, Cr: 1.05-1.35%, P: ≤0.015%, S: ≤0.005%, Ti: 0.015-0.035%, N: ≤0.0020%, Al: 0.005-0.015%, with the balance being Fe and unavoidable impurities; the size of TiN inclusions in the steel microstructure is ≤5µm, and the size of Al2O3 inclusions is ≤50µm; the steel surface is free of cracks and oxide layer defects; the hardness of the area from the steel surface to 15mm from the steel surface is 33-36 HRC, and the microstructure in this area, according to the area ratio in the metallographic diagram, is ferrite: 35-40%, martensite: 50-55%, bainite: 5-15%; the actual grain size of the microstructure is ≥6.

2. The nitriding gear steel according to claim 1, characterized in that: The residual oil content on the steel surface is 1.5-1.8 g / mm. 2 .

3. A method for manufacturing the nitrided gear steel of claim 1, characterized in that: include Step 1: Steelmaking is designed according to chemical composition. Steelmaking involves desulfurization of molten iron, converter smelting, pre-refining furnace addition of manganese and chromium alloys, and titanium-iron wire alloying. Ferrosilicon powder is floated onto the surface of the molten steel for diffusion deoxidation, and aluminum wire is fed in for precipitation deoxidation. Argon is used as the lifting gas in the vacuum degassing process to prevent the increase of nitrogen and oxygen content in the molten steel. After vacuum degassing, argon is blown into the bottom of the molten steel for more than 20 minutes to promote the further floating of large non-metallic inclusions TiN to the slag layer. The molten steel is then cast into a continuous casting billet using a crystallizer. Step 2: After the continuous casting billet comes off the line, a continuous casting billet at a high temperature of 1050-1100℃ is obtained. The high-temperature billet is descaled with high-pressure water to completely remove the oxide layer on the surface of the billet. The billet with the oxide layer removed is rolled in several passes. The first few passes use a large compression ratio to induce dynamic recrystallization and refine the actual grain size of the steel surface and interior. The middle few passes use a medium rolling compression ratio, and the last few passes use a finishing rolling compression ratio. The medium rolling and finishing rolling refine the actual grain size of the surface. The process time from the end of rough rolling to the start of medium rolling and from the end of medium rolling to the start of finishing rolling is ≤2 seconds. The final rolling temperature is controlled at 850-900℃, and the actual grain size of the steel is stably controlled at ≥6 levels. Step 3: Immediately after final rolling, the steel is cleaned with a flame to remove defects on the steel surface. The steel is then air-cooled to 10°C below the phase transformation point Ar3 and then quenched in an oil bath. After quenching, the hardness of the area from the steel surface to 15mm from the steel surface is 33-36 HRC, and the microstructure of this area is 35-40% ferrite, 50-55% martensite and 5-15% bainite. Step 4: Immediately heat the quenched steel for low-temperature tempering to eliminate internal stress and promote the evaporation of oil from the surface of the oil-quenched steel.

4. The method according to claim 3, characterized in that: Step 1: During the continuous casting process, the remaining steel in the ladle is more than 6 tons. After the molten steel in the ladle drops to the remaining steel, the ladle automatically exits the casting process to ensure that the slag in the ladle does not enter the tundish or the crystallizer.

5. The method according to claim 3, characterized in that: Step 2: The billet is rolled in 9 passes. The rolling compression ratios of the first three passes are fixed at 40%, 30%, and 20%, the intermediate rolling compression ratios of the middle three passes are fixed at 10%, 8%, and 7%, and the finishing rolling compression ratios of the last three passes are 6-10%, 5-10%, and 4-10%, respectively.

6. The method according to claim 3, characterized in that: Step 3: After removing surface defects, the steel is air-cooled to 750°C. The starting temperature for ferrite transformation during cooling is 760°C.

7. The method according to claim 3, characterized in that: Step four: The tempering temperature is 200℃, and the residual oil content on the steel surface is controlled at 1.5-1.8 g / mm. 2 .

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