A multipurpose vehicle transmission gear steel and a method of manufacturing the same

Abstract

The present application relates to a kind of multipurpose vehicle gearbox gear steel and its manufacturing method.The chemical composition of steel material is as follows: C:0.20-0.24%, Si:≤0.03%, Mn:0.90-1.20%, Cr:0.50-0.60%, P:≤0.010%, S:0.012-0.018%, Mo:0.12-0.15%, Ni:0.12-0.15%, Al:0.021-0.035%, N:0.010-0.015%, Cu:0.03-0.05%, B:0.0001-0.0002%, and the rest is Fe and inevitable impurities.After the process steps of converter→refining→RH furnace vacuum degassing→continuous casting→heating furnace heating→rolling→oil quenching→hot drawing→oil cooling, the round steel can be used to manufacture multipurpose vehicle gearbox gear.After carburizing for 4 hours at 930℃, the size of grain in carburized layer is 6.0-7.0 grade;The depth of surface decarburization layer of round steel is ≤0.10mm;The depth of surface microcrack of round steel is ≤0.10mm;The flatness of round steel is ≤1.0mm / m;The longitudinal banded structure of round steel is 1.0-1.5 grade, and the width of ferrite strip is stably controlled to be ≤10um.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of special steel smelting, and particularly relates to a multipurpose vehicle gearbox gear steel and a manufacturing method thereof. BACKGROUND

[0002] MPV (multi-Purpose Vehicles) is evolved from a travel car, and has the functions of a travel car wide passenger space, a car comfort and a van. The MPV can seat 7-8 people, and is widely used in long-distance business activities or long-distance travel. The MPV gearbox gear needs to bear a long-time, high-strength and high-speed working condition. The MPV gearbox gear operates in a long-time, high-strength and high-speed environment. When the gear rotates for a long time, the gear surface is subjected to friction and generates a long-time pressure. When the gear is driven for a long time, each gear needs to bear a long-time tension. The long-time fatigue stress of the gear accelerates the wear and damage of the gear. In order to improve the fatigue life of the MPV gearbox gear, the high-end MPV gearbox gear has high strength, good size precision and excellent uniformity. Therefore, the present application manufactures a multipurpose vehicle gearbox gear steel. The steel has high strength, good surface quality and size precision, and excellent banded structure indexes. SUMMARY

[0003] The present application provides a multipurpose vehicle gearbox gear steel and a manufacturing method thereof, which solves the above problems. The prepared steel has high strength, good surface quality and size precision, and excellent banded structure indexes.

[0004] The technical scheme adopted by the present application to solve the above problems is as follows: a multipurpose vehicle gearbox gear steel. The chemical composition of the steel is as follows: C: 0.20-0.24%, Si: ≤0.03%, Mn: 0.90-1.20%, Cr: 0.50-0.60%, P: ≤0.010%, S: 0.012-0.018%, Mo: 0.12-0.15%, Ni: 0.12-0.15%, Al: 0.021-0.035%, N: 0.010-0.015%, Cu: 0.03-0.05%, B: 0.0001-0.0002%, and the balance is Fe and inevitable impurities.

[0005] The steel material is tested for hardenability according to GB / T 225-2006, the normalizing temperature of the end-quenching sample is 930 DEG C, the quenching temperature is 850 DEG C, and the end-quenching test result meets J5mm = 41-44HRC, J8mm: 33-36HRC, and J13mm: 27-30HRC. After the steel material is carburized at 930 DEG C for 4 hours, the size of the grains in the carburized layer is 6.0-7.0 grade; the surface decarburized layer depth of the round steel is less than or equal to 0.10 mm; the surface micro-crack depth of the round steel is less than or equal to 0.10 mm; the flatness of the round steel is less than or equal to 1.0 mm / m; and the longitudinal banded structure of the round steel is 1.0-1.5 grade, and the width of the ferrite band is stably controlled to be less than or equal to 10 um.

[0006] The main roles of the corresponding chemical elements of the steel according to the application and the design basis are as follows:

[0007] C: Carbon is the most basic element to ensure the strength of the steel material, and is also an important element affecting the number and width of ferrite bands in the banded structure of the steel material. In order to ensure that the steel material has high strength, while controlling the number and width of ferrite bands in the banded structure of the steel material, the carbon content of the steel material needs to be increased. However, too high carbon content increases the core hardness of the gear and reduces the core toughness of the gear. Therefore, the carbon content of the steel material according to the application is selected in the range of 0.20-0.24%.

[0008] Si: The silicon content in the steel is strictly controlled. The increase of the silicon element in the steel will lead to the increase of the elastic limit of the steel material at 650 DEG C high temperature state. The high temperature elastic limit of the steel material refers to the ability of the steel material under high temperature environment to restore to the original state when the deformation disappears after the external force is removed. Significantly reducing the elastic limit of the steel material is beneficial to obtaining stable flatness and stable banded structure of the steel material after hot drawing. The flatness and banded structure of the steel material will not have the tendency to restore to the original state due to the disappearance of the external force. Therefore, the Si content is strictly controlled in the application, and the Si content is selected in the range of Si: ≤0.03%.

[0009] Mn: It plays a role in refining the pearlite in the steel, and can also improve the strength of the pearlite in the steel. However, too high manganese content will lead to high hardness of the steel material and reduce the hot drawing production efficiency of the hot-rolled round steel. Therefore, the Mn content in the application is selected in the range of 0.90-1.20%.

[0010] Cr: The main role is to improve the hardenability of the steel, and significantly improve the oxidation resistance of the steel material in high temperature environment, so as to prevent the steel material from producing too deep decarburization on the surface during the processes of heating, rolling, hot drawing and cooling.

[0011] Therefore, the Cr content in the application is selected in the range of 0.50-0.60%.

[0012] P: phosphorus is a harmful element in steel, increase the cold brittleness of steel, multipurpose vehicle gearbox gear use also face low temperature environment, winter cold start, gearbox gear need to withstand greater impact force in low temperature environment,

[0013] Therefore, the P content of the present application is selected in the range of P≤0.010%.

[0014] S: adding a small amount of sulfur in steel can improve the cutting performance of steel, but too high sulfur content leads to excessive sulfide in steel, therefore, the S content of the present application is selected in the range of 0.012-0.018%.

[0015] Mo, Ni: adding appropriate amount of molybdenum and nickel in steel can effectively improve the strength and hardenability of steel. But molybdenum and nickel are both precious metals, and excessive addition will significantly increase the cost of steel, and excessive addition of molybdenum and nickel will significantly increase the hardenability of steel, causing abnormal fluctuation of hardenability, so the content of molybdenum and nickel must be controlled within a narrow range. Therefore, the Mo and Ni content of the present application is selected in the range of 0.12-0.15%.

[0016] Al, N: the aluminum wire fed during refining process plays a role of deoxidization; after RH vacuum degassing, a certain amount of aluminum and nitrogen elements are added, a certain amount of uniform nitrogen and aluminum powder is blown into the molten steel through a long pipe, and argon gas is blown at the bottom of the ladle to stir the process, so that the formed AlN particles can be dispersed and uniformly distributed in the molten steel, which can effectively stabilize the grain size of the steel and improve the strength and toughness of the steel. The aluminum and nitrogen elements are added in the form of long pipe after RH vacuum degassing, the long pipe is inserted into the ladle, the uniform nitrogen and aluminum powder is blown into the molten steel through the long pipe, and the argon gas is blown at the bottom of the ladle to fully stir, so that the Al and N elements are distributed and homogenized in the molten steel. A certain amount of dispersed and uniformly distributed AlN particles can effectively refine the grain size of the steel, ensure that the grain size in the carburized layer is relatively small and the grain size is controlled within a narrow range after carburizing the steel at 930℃ for 4 hours, and the size of all grains in the carburized layer of the steel can be stably controlled in the range of 6.0-7.0, thereby improving the strength and stability of the carburized gear. Therefore, the Al and N content in the steel of the present application is selected in the range of Al: 0.021-0.035%, N: 0.010-0.015%.

[0017] Cu: By controlling the Cu content in the ordinary scrap steel ≤0.35%, the copper content in the steel is reduced, and the instability of the residual element copper content will cause the abnormal fluctuation of the hardenability of the steel, which is not conducive to the stable control of the hardenability of the steel within a relatively narrow range. Because the Cu content in the ordinary scrap steel is unstable, when the Cu content in the ordinary scrap steel is low, it is necessary to actively add a small amount of noble metal copper to the steel to ensure the stability of the hardenability of the steel. When the Cu content of the ordinary scrap steel is in the range of 0.30-0.35%, it is not necessary to actively add Cu element to the molten steel. The key to the control of the copper content in the steel of the present application is that the range is narrow and the content is low. The selected range of Cu content is 0.03-0.05%.

[0018] B: Boron element is the element that has the greatest influence on the hardenability of steel, and very small amount of boron in steel can promote the rapid rise of the hardenability of steel. It is necessary to strictly control the boron content in the chromium iron and boron iron added in the refining process, and low-boron chromium iron and low-boron manganese iron are used to avoid the uncontrollability of the boron content in the steel. Therefore, the key to the control of the boron content in the steel of the present application is that the range is narrow and the content is very low. The selected range of B content is 0.0001-0.0002%.

[0019] The above-mentioned multipurpose automobile gearbox gear steel and its manufacturing method comprises the following process steps:

[0020] (1) Converter smelting, converter adding hot metal and ordinary scrap steel, the ratio of hot metal and ordinary scrap steel is 9:1, the Cu content in ordinary scrap steel is controlled to be ≤0.35%; the P content at the time of converter tapping is controlled to be ≤0.08%, because the P content in oxidized slag is relatively high, in order to prevent excessive steel rephosphorization in the later smelting process, the converter tapping must be strictly controlled, the amount of slag is strictly controlled to be ≤2.5 kg / t, the thickness of slag layer is controlled to be 20-30 mm, if the slag layer is too thin, the slag layer cannot play a good role in isolating air, and the molten steel is easy to be oxidized by air; if the slag layer is too thick, the slag layer is easy to sink into the molten steel under the action of gravity, which leads to a significant increase in the rephosphorization of molten steel, in order to ensure that the rephosphorization of molten steel in the subsequent smelting process can be stably controlled to be ≤0.02%, the thickness of slag layer and the amount of slag must be strictly controlled; low-boron chromium iron and low-boron manganese iron are added in the refining process to adjust the chemical composition of molten steel, and the boron content of molten steel is controlled in a relatively low and narrow range of 0.0001-0.0002%, at the same time, aluminum wire is fed in the refining process for deoxidation; a small amount of molybdenum iron, nickel plate and pyrite are added in the RH vacuum degassing process to further fine-tune the chemical composition of molten steel, after RH vacuum degassing, a long pipe is inserted into the ladle, uniform nitrogen gas and aluminum powder are blown into the molten steel through the long pipe, and argon gas is blown at the bottom of the ladle for sufficient stirring, the uniform and fine aluminum powder in the nitrogen gas rapidly dissolves in the molten steel, which promotes the dispersion and homogenization of Al and N elements in the molten steel, and a certain content of dispersed and uniformly distributed AlN particles can ensure that the austenite grain size in the steel is stably controlled to be 6.0-7.0 grade. The ladle is lifted to the continuous casting platform to cast continuous casting billets, and a layer of dense high-purity iron powder is sprayed on the surface of the continuous casting billets during the high-temperature offline process, which prevents the surface of the billets from being oxidized.

[0021] (2) the cast blank with dense high-purity iron powder sprayed on the surface is heated in a heating furnace, the residual oxygen content in the heating furnace is controlled to be less than or equal to 2%, the furnace pressure of the heating furnace is controlled to be a slight positive pressure, the furnace pressure of the heating furnace is controlled to be 20-30 pa to prevent air outside the furnace from being introduced into the furnace and increase the residual oxygen content in the furnace, thereby increasing the decarburization depth of the surface of the steel; a slow temperature rising process with the furnace is adopted in the heating process of the heating furnace to prevent the steel from being cracked due to the large temperature difference between the inside and outside of the cast blank during the heating process, thereby causing the cast blank to generate a large stress and causing the surface stress of the steel to crack, the temperature rising rate of the preheating section is less than or equal to 2.5 ℃ / min, the temperature of the preheating section is 650-1000 ℃, the temperature rising rate of the heating section is less than or equal to 5 ℃, the temperature of the heating section is 1000-1150 ℃, the temperature rising rate of the soaking section is less than or equal to 5 ℃, the temperature of the soaking section is 1150-1200 ℃, the temperature rising rate of the heating section is greater than or equal to 2 times the temperature rising rate of the preheating section, the temperature rising rate of the soaking section is greater than or equal to 2 times the temperature rising rate of the preheating section, the temperature rising rate of the heating section and the temperature rising rate of the soaking section are controlled to be 2 times or more than the temperature rising rate of the preheating section to reduce the heating time of the steel in the high-temperature section of the furnace, although the steel organization in the heating section and the soaking section is austenite organization with good toughness, and the fast temperature rising rate of the heating section and the soaking section is also beneficial to avoid the formation of a too deep decarburization layer of the steel in the high-temperature section, but it is still necessary to prevent the heating speed of the steel from being too fast to cause micro-cracks on the surface of the cast blank during the heating process, therefore, the temperature rising rate of the heating section and the soaking section is controlled to be less than or equal to 5 ℃, the temperature rising rate of the heating section is greater than or equal to 2 times the temperature rising rate of the preheating section, and the temperature rising rate of the soaking section is greater than or equal to 2 times the temperature rising rate of the preheating section. In order to prevent the total heating time of the steel from being too long to cause a deep decarburization layer on the surface of the steel, the total heating time of the steel in the preheating section + the heating section + the soaking section is controlled to be 180-200 min.

[0022] (3) The heated billet is rolled into round steel. The high-temperature round steel after rolling is rapidly cooled to 650°C in cooling oil at a temperature of 30-40°C to prevent decarburization on the surface of the round steel, and at the same time, the austenite structure of the steel is rapidly cooled to form ferrite structure uniformly distributed in the steel. Then it is sent to a furnace at a temperature of 650°C, and the steel is hot drawn using hot drawing technology. The round steel is slowly passed through a round pipe blank with a hole diameter 1 mm smaller than the round steel itself and a flatness of ≤1 mm / m using hot drawing technology. The ferrite structure strip formed in the early stage of hot drawing is gradually narrowed under the action of the radial extrusion force applied by the round pipe blank, and the width of the ferrite strip is finally controlled to be ≤10 um. At the same time, the remaining austenite structure in the steel during hot drawing at 650°C will gradually transform into pearlite structure. Due to hot drawing, the pearlite structure will be extruded into the ferrite strip generated in the early stage, thereby reducing the number of ferrite strips across the metallographic field and reducing the metallographic rating of the banded structure. The banded structure rating of the steel is controlled to be 1.0-1.5 grade. Finally, due to the flatness of the round pipe blank through which the hot-drawn round steel is ≤1 mm / m, the flatness of the round steel after hot drawing is also ≤1 mm / m. The hot-drawn round steel is immediately rapidly cooled to room temperature in cooling oil at a temperature of 25°C to prevent the steel from bending and surface micro-cracks due to uneven local cooling intensity during the cooling process, and to prevent the steel from further decarburizing on the surface during the cooling process.

[0023] The application is directed to a multipurpose vehicle gearbox gear steel, which is designed with low silicon, low phosphorus, low copper and low boron, and the molybdenum, nickel, aluminum and nitrogen are strictly controlled within a narrow range to control the steel to have a narrow hardenability range interval. In the production process, the converter smelting process adds molten iron and ordinary scrap steel according to a ratio of 9:1, and controls the Cu content of the ordinary scrap steel to be ≤0.35%, and controls the amount of slag and the thickness of the slag layer during converter tapping to control the rephosphorization amount of the molten steel in the subsequent smelting process; in the refining process, low-boron chromium iron and low-boron manganese iron are added to adjust the chemical composition while the boron content of the molten steel is strictly controlled within a lower and narrower range; a small amount of molybdenum iron, nickel plate and pyrite is added in the RH vacuum degassing process to further fine-tune the chemical composition of the molten steel, and after vacuum degassing, uniform nitrogen gas and aluminum powder are blown into the molten steel using a long pipe, and at the same time, argon gas is blown into the bottom of the ladle for full stirring to promote the uniform distribution of Al and N elements in the molten steel. A layer of dense high-purity iron powder is sprayed on the surface of the continuous casting billet during the high-temperature off-line process to prevent the surface of the billet from being oxidized. The billet with dense high-purity iron powder sprayed on the surface is heated into a heating furnace, and the residual oxygen content and the furnace pressure stability in the heating furnace are controlled; the slow temperature rising process is adopted during the heating process of the heating furnace, and the temperature rising rate, temperature and total heating time of the preheating section, heating section and soaking section are controlled. The high-temperature round steel after rolling is rapidly cooled to 650 DEG C in the constant-temperature cooling oil, and then sent into a furnace with a temperature of 650 DEG C, and the steel is hot drawn by using the hot drawing process to control the size, banded structure grade and ferrite band width of the steel; the round steel after hot drawing is rapidly cooled in the constant-temperature cooling oil to prevent the steel from being bent and surface micro-cracks due to the slow cooling speed and the uneven local cooling intensity during the cooling process of the steel, and to prevent the steel from further surface decarburization during the cooling process. The round steel of the application can be used to manufacture multipurpose vehicle gearbox gear steel.

[0024] Compared with the prior art, the application has the advantages that:

[0025] (1) The application is designed with low silicon, low phosphorus, low copper and low boron, and the molybdenum, nickel, aluminum and nitrogen are strictly controlled within a narrow range to control the steel to have a narrow hardenability range and stable austenite grain grade.

[0026] (2) Converter smelting process according to the proportion of 9:1 to add hot metal and ordinary scrap steel, and control the Cu content of ordinary scrap steel ≤0.35%, control the amount of slag and slag layer thickness of converter tapping, to control the phosphorus content of molten steel in subsequent smelting process; In the refining process, low boron chromium iron and low boron manganese iron are added to adjust the chemical composition, while the boron content of molten steel is strictly controlled in a lower and narrower range; A small amount of molybdenum iron, nickel plate and pyrite is added in the RH vacuum degassing process to further fine-tune the chemical composition of molten steel, and after vacuum degassing, uniform nitrogen and aluminum powder are blown into the molten steel using a long guide pipe, and at the same time, argon is passed through the bottom of the ladle for full stirring, so that the Al element and N element are distributed and homogenized in the molten steel. A layer of dense high-purity iron powder is sprayed on the surface of the continuous casting billet during the high-temperature out-of-line process to prevent the surface of the billet from being oxidized.

[0027] (3) The billet with dense high-purity iron powder sprayed on the surface is heated into the heating furnace, and the residual oxygen content and the furnace pressure stability in the heating furnace are controlled; The slow temperature rising process is adopted in the heating process of the heating furnace, and the temperature rising rate, temperature and total heating time of the preheating section, heating section and soaking section are controlled. The high-temperature round steel after rolling is rapidly cooled to 650℃ in the constant-temperature cooling oil, and then sent into the furnace with a furnace temperature of 650℃, and the hot drawing process is adopted to draw the steel to control the size, banded structure grade and width of ferrite band of the steel; The round steel after hot drawing is rapidly cooled in the constant-temperature cooling oil to prevent the steel from being bent and having surface micro-cracks due to slow cooling speed and uneven local cooling intensity in the cooling process of the steel, and to prevent the steel from further producing surface decarburization in the cooling process.

[0028] (4) The round steel produced according to the method has the following characteristics: the steel is tested for hardenability according to GB / T 225-2006, the normalizing temperature of the end quenching sample is 930℃, the quenching temperature is 850℃, and the end quenching test result meets J5mm=41-44HRC, J8mm: 33-36HRC, J13mm: 27-30HRC; After the steel is carburized at 930℃ for 4 hours, the size of the grains in the carburized layer is 6.0-7.0 grade; The surface decarburization layer depth of the round steel is ≤0.10mm; The surface micro-crack depth of the round steel is ≤0.10mm; The flatness of the round steel is ≤1.0mm / m; The longitudinal banded structure of the round steel is 1.0-1.5 grade, and the width of the ferrite band is stably controlled to be ≤10um. DETAILED DESCRIPTION

[0029] The technical solutions of the present application are described in more detail in combination with the preferred embodiments of the present application. However, these embodiments are only descriptions of the preferred embodiments of the present application, and cannot have any limitation on the scope of the present application.

[0030] Example 1 and Example 2:

[0031] Steel for multi-purpose vehicle transmission gear and manufacturing method thereof: 100t converter→100t refining→RH furnace vacuum degassing→continuous casting small billet (200mm*200mm)→high-temperature off-line continuous casting billet surface iron powder spraying→three-stage heating of heating furnace→oil quenching to 650℃→650℃ furnace hot drawing→oil quenching to room temperature.

[0032] The converter is added with molten iron and ordinary scrap steel, the weight of the molten iron is 90t, the weight of the ordinary scrap steel is 10t, the Cu content in the ordinary scrap steel is 0.20% (Example 1) and 0.35% (Example 2), the P content at the converter tapping is controlled to be 0.06% (Example 1) and 0.08% (Example 2), the converter tapping is controlled to be strictly controlled under slag, the amount of the slag is 2.1kg / t (Example 1) and 2.5kg / t (Example 2), the slag layer thickness is controlled to be 24mm (Example 1) and 30mm (Example 2), the amount of the P returned in the subsequent smelting process is controlled to be 0.01% (Example 1) and 0.02% (Example 2), low-boron chromium iron and low-boron manganese iron are added in the refining process to adjust the chemical composition of the molten steel, and the boron content of the molten steel is controlled to be 0.0002% (Example 1) and 0.0001% (Example 2), and aluminum wire is fed at the same time for deoxidization, a small amount of copper plate, molybdenum iron, nickel plate and pyrite (Example 1) and molybdenum iron, nickel plate and pyrite (Example 2) are added in the RH vacuum degassing process to further fine-tune the chemical composition of the molten steel, a long pipe is inserted into the ladle after the RH vacuum degassing, uniform nitrogen gas and aluminum powder are blown into the molten steel through the long pipe, and argon gas is blown at the bottom of the ladle for sufficient stirring, so as to promote the dispersion and homogenization of Al and N elements in the molten steel. The ladle is lifted to the continuous casting platform to cast the continuous casting billet, a layer of dense high-purity iron powder is sprayed on the surface of the continuous casting billet during the high-temperature off-line process, so as to prevent the surface of the billet from being oxidized.

[0033] The cast blank is heated in a heating furnace with a surface sprayed with dense high-purity iron powder. The residual oxygen content of the heating furnace is controlled at 1.5% (Example 1) and 2% (Example 2). The furnace pressure of the heating furnace is controlled at a slight positive pressure. The furnace pressure of the heating furnace is controlled at 21 Pa (Example 1) and 30 Pa (Example 2) to prevent air outside the furnace from being introduced into the furnace and to increase the residual oxygen content in the furnace, thereby increasing the decarburization depth of the surface of the steel. The heating process of the heating furnace uses a slow temperature rising process with the furnace to prevent the cast blank from generating a large stress due to a large temperature difference between the inside and outside of the cast blank during the heating process, thereby causing stress cracking on the surface of the steel. The temperature rising rate of the preheating section is 2.3°C / min (Example 1) and 2.4°C / min (Example 2). The temperature of the preheating section is 650-1000°C. The temperature rising rate of the heating section is 4.7°C / min (Example 1) and 5.0°C / min (Example 2). The temperature of the heating section is 1000-1150°C. The temperature rising rate of the soaking section is 4.6°C / min (Example 1) and 4.9°C / min (Example 2). The temperature of the soaking section is 1150-1200°C. The total heating time is controlled at 195 min (Example 1) and 186 min (Example 2).

[0034] The heated cast blank is rolled into a round steel. The high-temperature round steel after rolling is rapidly cooled to 650°C in cooling oil at a temperature of 31°C (Example 1) and 40°C (Example 2). Then, the round steel is sent into a furnace at a temperature of 650°C. The steel is subjected to hot drawing. The round steel slowly passes through a hole with a diameter of 1 mm smaller than the diameter of the round steel (Example 1: diameter Φ 30 mm; Example 2: diameter Φ 65 mm) by using the hot drawing process. The round steel has a flatness of 0.9 mm / m (Example 1) and 0.8 mm / m (Example 2). The number of ferrite strips of the steel after hot drawing is reduced. The banded structure grade is controlled at 1.0 (Example 1) and 1.5 (Example 2). The width of the ferrite strips of the round steel after hot drawing is narrowed. The width of the ferrite strips is stably controlled at ≤10 um. The round steel after hot drawing is immediately rapidly cooled to room temperature in cooling oil at a temperature of 25°C to prevent the steel from generating bending and surface micro-cracks due to uneven local cooling intensity during the cooling process and to prevent the steel from further generating surface decarburization during the cooling process.

[0035] The round steels prepared in Example 1 and Example 2 have the chemical components shown in Table 1.

[0036] Table 1 (wt%)

[0037]

[0038] The round steels prepared in Example 1 and Example 2 are subjected to quenching and tempering according to GB / T 225-2006 to test the hardenability thereof. The normalizing temperature of the end-quenching sample is 930°C. The quenching temperature is 850°C. The end-quenching test results are shown in Table 2.

[0039] Table 2

[0040]

[0041] The round steel prepared in the example 1 and the example 2 has the grain size of 6.0-7.0 in the carburized layer after carburizing at 930℃ for 4 hours.

[0042] The surface decarburized layer depth, the surface micro crack depth, the flatness of the round steel, the longitudinal banded structure grade of the round steel and the maximum width of the ferrite band of the round steel prepared in the example 1 and the example 2 are shown in table 3.

[0043] Table 3

[0044]

[0045] The longitudinal banded structure grade of the round steel and the maximum width of the ferrite band of the round steel prepared in the example 1 and the example 2 are shown in table 4.

[0046] Table 4

[0047]

[0048] The present application adopts low silicon, low phosphorus, low copper and low boron design in composition, and strictly controls molybdenum, nickel, aluminum and nitrogen within a relatively narrow range to control the hardenability range of the steel. In the production process, a fixed proportion of molten iron and ordinary scrap steel is added in the converter smelting, and the amount of slag and the thickness of slag layer are controlled during the converter tapping; low boron alloy is added in the refining process to strictly control the boron content within a relatively low and narrow range; after RH vacuum degassing, uniform nitrogen gas and aluminum powder are blown into the molten steel through a long pipe, and uniform and dispersed AlN particles are formed by bottom argon stirring; a layer of dense high-purity iron powder is sprayed on the surface of the continuous casting billet during the high-temperature line process, and the residual oxygen content and the furnace pressure stability in the heating furnace are controlled during the heating process of the continuous casting billet; the slow temperature rising process is adopted during the heating process of the heating furnace, and the temperature rising rate, temperature and total heating time of the preheating section, heating section and soaking section are controlled. The high-temperature round steel after rolling is rapidly cooled to 650℃ in the constant temperature cooling oil, and then sent to the furnace with a furnace temperature of 650℃, and the steel is hot drawn by using the hot drawing process; the round steel after hot drawing is rapidly cooled in the constant temperature cooling oil. A multipurpose automobile gearbox gear steel is manufactured by reasonable steelmaking and rolling control cooling process, which fills the domestic blank.

[0049] Although the preferred embodiments of the present application have been described in detail above, it should be understood that various modifications and changes can be made by those skilled in the art without departing from the spirit and principles of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method of manufacturing a steel for a gear of a transmission of a utility vehicle, characterized in that the steel has a chemical composition in weight percent of C: 0.20-0.24%, Si: <0.03%, Mn: 0.90-1.20%, Cr: 0.50-0.60%, P: <0.010%, S: 0.012-0.018%, Mo: 0.12-0.15%, Ni: 0.12-0.15%, Al: 0.021-0.035%, N: 0.010-0.015%, Cu: 0.03-0.05%, B: 0.0001-0.0002% and the balance being Fe and unavoidable impurities. The method comprises the following steps: in a converter smelting process, adding hot metal and ordinary scrap steel in a ratio of 9:1, controlling the Cu content of the ordinary scrap steel to be less than or equal to 0.35%, controlling the P content when the converter is tapped to be less than or equal to 0.08%, controlling the amount of slag to be less than or equal to 2.5 kg / t, and controlling the thickness of the slag layer to be 20-30 mm; in a refining process, low-boron chromium iron and low-boron manganese iron are added to adjust the chemical composition, and the boron content of the molten steel is controlled to be in a relatively low and narrow range of 0.0001-0.0002%; in an RH vacuum degassing process, a small amount of molybdenum iron, nickel plate and pyrite are added to further adjust the chemical composition of the molten steel; after vacuum degassing, uniform nitrogen gas and aluminum powder are blown into the molten steel by using a long nozzle, and argon gas is blown at the bottom of the ladle to fully stir the molten steel, so that the Al element and the N element are uniformly distributed in the molten steel; during the high-temperature out-of-mould process of the continuous casting billet, a layer of dense high-purity iron powder is sprayed on the surface of the billet to prevent the surface of the billet from being oxidized; the billet with the dense high-purity iron powder sprayed on the surface is put into a heating furnace for heating, the residual oxygen content of the heating furnace and the stability of the furnace pressure of the heating furnace are controlled, the residual oxygen content of the heating furnace is controlled to be less than or equal to 2%, and the furnace pressure of the heating furnace is controlled to be a micro-positive pressure, i.e. 20-30 pa; in the heating process of the heating furnace, a furnace temperature rising process is adopted, the furnace temperature rising process is specifically that the temperature rising rate of a preheating section is less than or equal to 2.5 ℃ / min, the temperature of the preheating section is 650-1000 ℃, the temperature rising rate of a heating section is less than or equal to 5 ℃, the temperature of the heating section is 1000-1150 ℃, the temperature rising rate of a soaking section is less than or equal to 5 ℃, the temperature of the soaking section is 1150-1200 ℃, the temperature rising rate of the heating section is greater than or equal to 2 times the temperature rising rate of the preheating section, the temperature rising rate of the soaking section is greater than or equal to 2 times the temperature rising rate of the preheating section, and the total heating time of the preheating section, the heating section and the soaking section is controlled to be 180-200 min; the high-temperature round steel after rolling is rapidly cooled to 650 ℃ in temperature-invariable cooling oil, the temperature-invariable cooling oil is controlled to be 30-40 ℃, then the round steel is put into a furnace with a temperature of 650 ℃, a hot drawing process is adopted to draw the steel, so as to control the size of the steel, the banded structure grade and the width of ferrite strips, the hot drawing process is that a round pipe blank with a hole diameter smaller than the round steel by 1 mm and a flatness less than or equal to 1 mm / m is used, the ferrite strip formed in the early stage of the hot drawing process is gradually narrowed in width under the action of the radial extrusion force of the round pipe blank, and the width of the ferrite strip is finally controlled to be less than or equal to 10 um; meanwhile, the remaining austenite structure is gradually transformed into pearlite structure under the condition of 650 ℃ during the hot drawing process of the steel, the round steel after the hot drawing process is immediately rapidly cooled to room temperature in temperature-invariable cooling oil with a temperature of 25 ℃; the round steel after the hot drawing process is rapidly cooled in temperature-invariable cooling oil, so as to prevent the steel from being bent and having surface micro-cracks due to the non-uniform local cooling intensity during the cooling process of the steel, and to prevent the steel from further producing surface decarburization during the cooling process.

2. A method of manufacturing a multi-purpose vehicle transmission gear steel according to claim 1, characterized in that The steel is tested for hardenability according to GB / T 225-2006, the normalizing temperature of the end-quenching sample is 930 DEG C, the quenching temperature is 850 DEG C, and the end-quenching test result meets J5mm=41-44HRC, J8mm:33-36HRC, J13mm:27-30HRC.

3. A method of manufacturing a multi-purpose vehicle transmission gear steel according to claim 1, characterized in that After the steel is carburized at 930 DEG C for 4 hours, the grain size in the carburized layer is 6.0-7.0 grade; the surface decarburized layer depth of the round steel is less than or equal to 0.10 mm; the surface micro-crack depth of the round steel is less than or equal to 0.10 mm; the flatness of the round steel is less than or equal to 1.0 mm / m; the longitudinal banded structure of the round steel is 1.0-1.5 grade, and the width of the ferrite band is stably controlled to be less than or equal to 10 um.

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