Gear steel for low-speed and large-torque application scene and preparation method thereof

Abstract

The invention relates to gear steel for a low-speed and large-torque application scene and a preparation method thereof, and belongs to the technical field of steel. The gear steel comprises the following chemical components in percentage by mass: 0.12%-0.25% of C, 0.05%-0.70% of Si, 0.30%-2.00% of Mn, less than or equal to 0.03% of P, 0.015%-0.05% of S, 0.80%-2.0% of Cr, 1.0%-2.0% of Ni, 0.015%-0.055% of Al, less than or equal to 0.0150% of N and Fe, Al / N is greater than 3, and Al / N represents the mass ratio of Al to N. Through scientific design of a component system of the gear steel, the strength of the material is remarkably improved, impurities and harmful gas in molten steel are effectively removed through strict smelting and refining process control, and through strict control of temperature and time parameters in the rolling process, the yield of the gear steel is improved. And the ideal microstructure and mechanical properties of the material in the plastic deformation process are ensured. The prepared gear steel meets at least one of the following properties that the impact load impact energy is larger than 60 J, the tensile strength is larger than or equal to 1200 Mpa, the ductility is larger than or equal to 10%, the core hardness is larger than or equal to 33 HRC, the surface hardness ranges from 58 HRC to 68 HRC, and the retained austenite level is smaller than or equal to the second level.

Description

Technical Field

[0001] This application relates to the field of steel technology, and particularly to a gear steel for low-speed and high-torque application scenarios and a preparation method thereof. Background Art

[0002] In recent years, the field of low-speed and heavy-duty has risen rapidly, and there is a strong demand for low-speed and high-torque steel for heavy equipment manufacturing. Under conditions such as a rotational speed of 0.1 r / min to 600 r / min, a voltage of 380 v to 10 kv, and a torque of 1 knm to 1000 knm, how to meet the requirements of low-speed and high-torque application scenarios and thus replace imported materials has become a current research hotspot.

[0003] Taking the drive axle and gear of a high-power wheeled tractor as an example for the low-speed and high-torque application scenario, to meet the tillage and transportation requirements under different working conditions, the structural design of the drive axle and gear is relatively complex. It not only has a large size but also has various structural types such as different models, different diameters, different keyways, splines, and steps. Any change in its structural form will cause functional failure, reduce the performance of large equipment, and seriously affect the service effect of the equipment. During the working process of this part, the load bearing low-speed and high-torque is mainly an alternating load of bending and torsion, and the stress condition is complex. Low-speed and high-torque parts severely restrict the development of equipment such as wheeled tractors and have become a bottleneck in the development of equipment in this field. Summary of the Invention

[0004] This application provides a gear steel for low-speed and high-torque application scenarios and a preparation method thereof to solve the following technical problem: how to improve the anti-impact load capacity of gear steel for low-speed and high-torque application scenarios.

[0005] In the first aspect, this application provides a gear steel for low-speed and high-torque application scenarios. In terms of mass fraction, the chemical composition of the gear steel includes: C: 0.12% - 0.25%, Si: 0.05% - 0.70%, Mn: 0.30% - 2.00%, P ≤ 0.03%, S: 0.015% - 0.05%, Cr: 0.80% - 2.0%, Ni: 1.0% - 2.0%, Al: 0.015% - 0.055%, N ≤ 0.0150%, Fe, Al / N > 3, where Al / N represents the mass ratio of Al and N.

[0006] Optionally, in terms of mass fraction, the chemical composition of the gear steel includes: S: 0.015% - 0.035%, Al: 0.015% - 0.035%, N: 0.0070% - 0.0150%.

[0007] Optionally, the gear steel satisfies at least one of the following properties: impact work against impact load > 60 J, tensile strength ≥ 1200 Mpa, elongation ≥ 10%, core hardness ≥ 33 HRC, surface hardness 58 HRC - 68 HRC, retained austenite grade ≤ 2 grades.

[0008] In a second aspect, the present application provides a method for preparing the gear steel for low-speed and high-torque application scenarios described in the first aspect, the method comprising:

[0009] Smelting the molten iron to obtain molten steel for smelting;

[0010] Refining the molten steel for smelting to obtain refined molten steel;

[0011] Continuous casting the refined molten steel to obtain a continuous casting billet;

[0012] Successively heating and rolling the continuous casting billet to obtain a hot-rolled bar;

[0013] Cooling the hot-rolled bar to obtain gear steel.

[0014] Optionally, the oxygen activity at the tapping end of the smelting is 500 ppm - 600 ppm.

[0015] Optionally, the refining uses the LF process, and the mass of Al at the end of the refining is 0.025% - 0.035% of the total mass of the molten steel.

[0016] Optionally, the oxygen activity of the molten steel at the end of the refining ≤ 3 ppm.

[0017] Optionally, the refining uses steel slag, and the basicity of the steel slag is 2 - 3.

[0018] Optionally, the MI index of the steel slag is 0.20 - 0.30.

[0019] Optionally, the steel slag contains Al2O3 and CaO, wherein the mass of Al2O3 is 20% - 35% of the total mass of the steel slag, and the mass of CaO is 45% - 65% of the total mass of the steel slag.

[0020] Optionally, the refining the molten steel for smelting to obtain refined molten steel includes:

[0021] Refining the molten steel for smelting to obtain initial refined molten steel;

[0022] Performing vacuum degassing on the initial refined molten steel to obtain refined molten steel; the refining uses the LF process, and the mass of Al at the end of the refining is 0.045% - 0.055% of the total mass of the molten steel.

[0023] Optionally, the heating temperature is 1150°C to 1200°C, and the heating time is 2.5 h to 3.5 h.

[0024] Optionally, the starting rolling temperature for rolling is 1050°C to 1100°C, and the finishing rolling temperature for rolling is 950°C to 1000°C.

[0025] Optionally, the grain size difference of the hot-rolled bar is ≤3, and the Rockwell hardness of the J10 point of the hardenability of the hot-rolled bar is 33 HRC to 42 HRC.

[0026] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0027] The present application provides a kind of gear steel for low-speed and high-torque application scenarios. In terms of mass fraction, the chemical components of the gear steel include: C: 0.12% to 0.25%, Si: 0.05% to 0.70%, Mn: 0.30% to 2.00%, P≤0.03%, S: 0.015% to 0.05%, Cr: 0.80% to 2.0%, Ni: 1.0% to 2.0%, Al: 0.015% to 0.055%, N≤0.0150%, Fe, Al / N>3, where Al / N represents the mass ratio of Al and N. By scientifically designing the component system of the gear steel, the strength of the material is significantly improved. Through strict control of the smelting and refining processes, impurities and harmful gases in the molten steel are effectively removed. By strictly controlling the temperature and time parameters during the rolling process, an ideal microstructure and mechanical properties are ensured during the plastic deformation of the material. The prepared gear steel meets at least one of the following properties: impact energy against impact load >60 J, tensile strength ≥1200 Mpa, elongation ≥10%, core hardness ≥33 HRC, surface hardness 58 HRC to 68 HRC, retained austenite grade ≤2 levels. It can withstand higher alternating impact loads and cyclic bending loads, meeting the stringent requirements of low-speed and high-torque equipment for material strength. Description of the Drawings

[0028] The drawings here are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present application, and are used together with the specification to explain the principles of the present application.

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0030] Figure 1It is a schematic flow chart of a preparation method of a gear steel for low-speed and high-torque application scenarios provided by an embodiment of the present application. Detailed implementation manners

[0031] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts shall fall within the protection scope of the present application.

[0032] The various embodiments of the present application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be construed as a rigid limitation on the scope of the present application; therefore, it should be considered that the described range description has specifically disclosed all possible sub-ranges and individual values within that range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and individual numbers within that range, such as 1, 2, 3, 4, 5, and 6, regardless of the range; in addition, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0033] In this text, terms including "comprising" and the like mean "including but not limited to". Relative terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. "And / or", which describes the associated relationship of associated objects, indicates that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "a plurality" means two or more; "at least one kind", "at least one of the following items" or similar expressions refer to any combination of these items, including any combination of single item or plural items; for example, "at least one of a, b, or c", or, "at least one of a, b, and c" can both represent: a, b, c, a-b (that is, a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple respectively. "Part number representation" such as weight part, mass part, etc. represents the proportional relationship between each component. In the proportional relationships involved in this text, the parameters that need to be described by proportion should be understood as the antecedents of the proportion formula in the order of description, and the proportional numbers should be understood as the consequents of the proportion formula. For example, if the weight ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one by one with the proportional numbers in the proportion formula in the order of description, that is, the weight of substance A: the weight of substance B: the weight of substance C = 1:2:3.

[0034] Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment, etc. used in this application can be obtained through market purchase or can be prepared by existing methods.

[0035] In a first aspect, the present application provides a gear steel for low-speed and high-torque application scenarios. In terms of mass fraction, the chemical composition of the gear steel includes: C: 0.12% - 0.25%, Si: 0.05% - 0.70%, Mn: 0.30% - 2.00%, P ≤ 0.03%, S: 0.015% - 0.05%, Cr: 0.80% - 2.0%, Ni: 1.0% - 2.0%, Al: 0.015% - 0.055%, N ≤ 0.0150%, Fe, Al / N > 3, where Al / N represents the mass ratio of Al and N.

[0036] In some embodiments, in terms of mass fraction, the chemical composition of the gear steel includes: S: 0.015% - 0.035%, Al: 0.015% - 0.035%, N: 0.0070% - 0.0150%.

[0037] The gear steel provided by the present invention is designed with the chemical composition of the gear steel being designed to meet the special requirements of low-speed and high-torque application scenarios.

[0038] The positive effect of limiting the C content to 0.12% to 0.25% is that C is the key to ensuring the strength and hardness of gear steel, and is also beneficial to the subsequent heat treatment process. However, a C content greater than 0.25% will increase the brittleness of the steel and reduce its toughness. For example, the C content can be 0.12%, 0.15%, 0.18%, 0.21%, 0.23%, 0.25%, etc.

[0039] The positive effect of limiting the Si content to 0.05% to 0.70% is that the addition of Si can improve the strength and heat resistance of steel, and also help improve the deoxidation and desulfurization performance of steel. For example, the Si content can be 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.70%, etc.

[0040] The positive effect of limiting the content of Mn to 0.30% to 2.00%: Mn is one of the important alloying elements in gear steel. In gear steel, a Mn content of 0.30% to 2.00% can ensure that the gear maintains sufficient strength and rigidity when subjected to large torque, and prevent failure due to deformation or fracture. In addition, Mn has a significant effect on the hardenability of steel. Increasing the Mn content can expand the hardenability range of steel, so that the gear steel can obtain a deeper hardened layer during the quenching process, thereby improving the hardness and wear resistance of the core. This is especially important for gears that need to withstand high-frequency and high-load conditions. Mn can also improve the wear resistance and fatigue resistance of steel. In low-speed, high-torque application scenarios, gears need to withstand complex stress changes for a long time and are prone to fatigue damage. A Mn content of 0.30% to 2.00% can enhance the fatigue resistance of gear steel and extend its service life. For example, the content of Mn may be 0.30%, 0.80%, 1.30%, 1.80%, 2.00%, etc.

[0041] The positive effect of limiting the P content to ≤0.03%: Although P in steel can improve the strength and hardness of steel, it will also significantly reduce the plasticity and impact toughness of steel, especially in low temperature environments. This phenomenon is called "cold brittleness". For gear steel in low-speed and high-torque application scenarios, if the P content is higher than 0.03%, the gear may suddenly break under low temperature or high stress conditions, seriously affecting the safety and reliability of the equipment. Therefore, controlling the P content to ≤0.03% helps to avoid or reduce the occurrence of cold brittleness. Exemplarily, the P content can be 0.005%, 0.01%, 0.015%, 0.02%, 0.025%, 0.03%, etc.

[0042] Positive effects of limiting the S content to 0.015% - 0.05%: An S content of 0.015% - 0.05% can improve the cutting performance of the gear steel, making it easier to cut during processing, reducing tool wear, and improving production efficiency. Exemplarily, the S content can be 0.015%, 0.02%, 0.025%, 0.03%, 0.035%, 0.04%, 0.045%, 0.05%, etc.

[0043] Positive effects of limiting the Cr content to 0.80% - 2.0%: 0.80% - 2.0% of Cr can significantly improve the hardenability of the steel, enabling the gear to obtain a deeper hardened layer after quenching, thereby enhancing its load-bearing capacity and wear resistance. Additionally, in low-speed, high-torque application scenarios where the gear needs to withstand high stresses and long-term frictional wear, 0.80% - 2.0% of Cr can improve its strength and wear resistance while maintaining the toughness of the steel, ensuring the stable operation and long service life of the gear. Exemplarily, the Cr content can be 0.80%, 1.00%, 1.20%, 1.40%, 1.60%, 1.80%, 2.0%, etc.

[0044] Positive effects of limiting the Ni content to 1.0% - 2.0%: Ni is a strong hardenability element that can improve the hardenability of the steel, enabling the gear to obtain a more uniform and deeper hardened layer during quenching. This is particularly important for low-speed, high-torque gears that require high surface hardness and wear resistance. 1.0% - 2.0% of Ni can also improve the cutting performance and forgeability of the steel, making it easier to form and cut the gear during processing, reducing tool wear and processing time. Additionally, during carburizing heat treatment, Ni helps to promote the diffusion and uniform distribution of carbon, thereby obtaining a more ideal carburized layer and microstructure. This helps to improve the surface hardness and wear resistance of the gear while maintaining the toughness and strength of the core. Exemplarily, the Ni content can be 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, etc.

[0045] Positive effects of limiting the Al content to 0.015% - 0.055%: Al is an effective grain-refining element in steel. By adding 0.015% - 0.055% of Al, the growth of austenite grains can be inhibited, thus obtaining a finer grain structure during the rolling process. In addition, Al can combine with oxygen in steel to form alumina inclusions, which can improve the cutting performance and wear resistance of steel to a certain extent. An Al content higher than 0.055% will result in excessive inclusions, affecting the properties of steel. By controlling the Al content within the range of 0.015% - 0.055% and combining with the refining process to reduce the oxygen content in steel, the number and size of inclusions can be reduced, improving the purity and quality of steel. Exemplarily, the content of Al can be 0.015%, 0.02%, 0.025%, 0.03%, 0.035%, 0.04%, 0.045%, 0.05%, 0.055%, etc.

[0046] Positive effects of limiting the N content ≤ 0.0150%: N in steel easily combines with various elements to form nitrides, which may have an adverse impact on the mechanical properties of steel. Exemplarily, the content of N can be 0.0030%, 0.0050%, 0.0070%, 0.0090%, 0.0110%, 0.0130%, 0.0150%, etc.

[0047] Positive effects of limiting Al / N > 3: By controlling the N content and ensuring that the Al / N ratio is greater than 3, the adverse impact of N on hardenability can be avoided, thus ensuring that the gear steel can obtain a uniform hardened layer after carburizing or quenching.

[0048] The specific content / content range of Fe can be obtained through the upper and lower limit formulas of the components, i.e.:

[0049] The sum of the percentage contents of each component in a composition should be equal to 100%. The content ranges of several components should meet the following conditions: the upper limit value of a certain component + the lower limit values of other components ≤ 100; the lower limit value of a certain component + the upper limit values of other components ≥ 100.

[0050] In some embodiments, the gear steel satisfies at least one of the following properties: impact energy against impact load > 60 J, tensile strength ≥ 1200 Mpa, elongation ≥ 10%, core hardness ≥ 33 HRC, surface hardness of 58 HRC - 68 HRC, retained austenite grade ≤ 2 levels.

[0051] Through precise chemical composition design, reasonable smelting and heat treatment processes, and strict quality control, the properties of the gear steel are achieved to meet the requirements that the impact energy against impact load is > 60 J, the tensile strength is ≥ 1200 Mpa, the elongation is ≥ 10%, the core hardness is ≥ 33 HRC, the surface hardness is 58 HRC - 68 HRC, and the retained austenite level is ≤ 2 levels, which can meet the material strength selection requirements of equipment with over 100 horsepower within 40 km of load-bearing.

[0052] The gear steel meeting the requirement that the impact energy against impact load is > 60 J means that when suffering from sudden high impact loads, it can maintain the integrity and stability of the structure and is not prone to fracture or damage. Exemplarily, the impact energy against impact load can be 60 J, 65 J, 70 J, 75 J, 80 J, etc. The tensile strength of the gear steel ≥ 1200 MPa ensures the strength and stability under high stress and is suitable for transmission systems bearing large torques. Exemplarily, the tensile strength can be 1200 MPa, 1210 MPa, 1220 MPa, 1230 MPa, 1240 MPa, 1250 MPa, etc. The elongation ≥ 10% indicates that the gear steel has good plastic deformation ability and can undergo a certain degree of deformation without immediate fracture when subjected to external forces, improving its safety and reliability. Exemplarily, the elongation can be 10%, 11%, 12%, 13%, 14%, 15%, etc. The core hardness ≥ 33 HRC, while the surface hardness is 58 HRC - 68 HRC. This hardness gradient distribution not only ensures the toughness of the core of the gear steel but also meets the requirement of high surface wear resistance. Exemplarily, the core hardness can be 33 HRC, 34 HRC, 35 HRC, 36 HRC, 37 HRC, etc.; the surface hardness can be 58 HRC, 60 HRC, 62 HRC, 64 HRC, 66 HRC, 68 HRC, etc. The retained austenite level ≤ 2 levels means that the transformation of austenite to other phases during the heat treatment process is relatively complete, reducing the content of retained austenite, thereby improving the hardness and stability of the gear steel.

[0053] Figure 1 The flowchart shows a preparation method of gear steel for low-speed and large-torque application scenarios provided by an embodiment of the present application.

[0054] Please refer to Figure 1 , Second, the present application provides a preparation method of the gear steel for low-speed and large-torque application scenarios described in the first aspect, and the method includes:

[0055] S1. Smelt the hot metal to obtain smelted molten steel;

[0056] In some embodiments, smelting can be carried out by means of electric furnace smelting or converter smelting. After the steel is smelted, lime, fluorite, silicon carbide, aluminum ingots, refining slag, quartz sand, etc. are added, and then the LF process is used for refining to further remove impurities and gases in the steel. The refining ladle can be fed with Al wire to control the aluminum content. According to the required target nitrogen content, operations such as adding nitrogen-increasing alloy raw materials are selected to ensure that the AL:N ratio during continuous casting reaches more than 3.

[0057] In some embodiments, the oxygen activity at the end of steel tapping in the smelting is 500 ppm to 600 ppm.

[0058] The control of oxygen activity is an important part of quality control during the smelting process. By precisely controlling the oxygen activity, the quality of the molten steel produced can be ensured to be stable, providing a good foundation for subsequent processes such as refining, continuous casting, and rolling. This helps to reduce the scrap rate during production and improve production efficiency. Exemplarily, the oxygen activity at the end of steel tapping in the smelting can be 500 ppm, 520 ppm, 540 ppm, 560 ppm, 580 ppm, 600 ppm, etc.

[0059] S2. Refine the smelted molten steel to obtain refined molten steel;

[0060] In some embodiments, the refining uses the LF process, and the mass of Al at the end of refining is 0.025% to 0.035% of the total mass of the molten steel.

[0061] Al is a strong deoxidizer and can effectively react with the dissolved oxygen in the molten steel during the refining process to form alumina (Al2O3) inclusions that are insoluble in the molten steel. By controlling the mass of Al at the end of refining to be 0.025% to 0.035% of the total mass of the molten steel, the oxygen content in the molten steel can be effectively reduced, thereby improving the purity and quality of the steel. Exemplarily, the mass of Al at the end of refining can be 0.025%, 0.027%, 0.029%, 0.031%, 0.033%, 0.035%, etc. of the total mass of the molten steel.

[0062] In some embodiments, the oxygen activity of the molten steel at the end of refining is ≤ 3 ppm.

[0063] The positive effect of limiting the oxygen activity of the molten steel at the end of refining to ≤ 3 ppm: Oxygen activity is one of the important indicators for evaluating the cleanliness of molten steel. By controlling the oxygen activity of the molten steel after refining to be below 3 ppm, the content of dissolved oxygen and oxide inclusions in the steel can be significantly reduced, thereby improving the purity of the steel. Exemplarily, the oxygen activity of the molten steel at the end of refining can be 0.5 ppm, 1 ppm, 1.5 ppm, 2 ppm, 2.5 ppm, 3 ppm, etc.

[0064] In some embodiments, the refining uses steel slag, and the basicity of the steel slag is 2-3.

[0065] The positive effect of limiting the basicity of the steel slag for refining to 2-3: Controlling the basicity of the steel slag within the range of 2-3 helps to promote the combination of impurity elements (such as phosphorus, sulfur, etc.) in the molten steel with oxides (such as CaO, Al2O3, etc.) in the steel slag to form stable compounds and precipitate into the steel slag. In this way, the content of harmful elements in the molten steel can be significantly reduced, the purity of the molten steel can be improved, and thus the final performance of the gear steel can be improved. In addition, the basicity within the range of 2-3 helps to improve the fluidity of the steel slag, enabling the steel slag to flow and separate more smoothly during the refining process, and reducing the pollution of the molten steel by the steel slag.

[0066] In some embodiments, the MI index of the steel slag is 0.20-0.30.

[0067] The MI index (melting index) of the steel slag is an important indicator to measure the fluidity of the steel slag. An MI index of 0.20-0.30 means that the steel slag has good fluidity, which helps to promote the full contact and reaction at the slag-steel interface during the refining process and improve the smelting efficiency. The steel slag with high fluidity can also better absorb and remove non-metallic inclusions in the molten steel, which have an adverse effect on the performance of the steel. By improving the fluidity of the steel slag, the floating and removal of inclusions can be promoted, thereby improving the purity and quality of the molten steel. Exemplarily, the MI index of the refined steel slag can be 0.20, 0.22, 0.24, 0.26, 0.28, 0.30, etc.

[0068] In some embodiments, the steel slag contains Al2O3 and CaO, wherein the mass of Al2O3 is 20%-35% of the total mass of the steel slag, and the mass of CaO is 45%-65% of the total mass of the steel slag.

[0069] Al2O3 in the steel slag can effectively adsorb non-metallic inclusions in the molten steel, especially the inclusions formed by Al2O3 itself, thereby improving the purity of the molten steel. In addition, Al2O3 in the steel slag reacts with sulfur in the molten steel to form sulfides and discharge them with the slag, which helps to improve the desulfurization efficiency. Exemplarily, the mass of Al2O3 in the steel slag can be 20%, 23%, 26%, 29%, 32%, 35%, etc. of the total mass of the steel slag.

[0070] As the main basic oxide in the steel slag, CaO helps to neutralize acidic oxides and impurities in the molten steel, thereby reducing the content of harmful elements in the molten steel and improving the purity of the molten steel. The molten steel with high purity is beneficial to producing gear steel with more stable quality. Exemplarily, the mass of CaO in the steel slag can be 45%, 49%, 53%, 57%, 61%, 65%, etc. of the total mass of the steel slag.

[0071] In some embodiments, refining the molten steel obtained from smelting to obtain refined molten steel includes:

[0072] Refining the molten steel obtained from smelting to obtain initial refined molten steel;

[0073] Performing vacuum degassing on the initial refined molten steel to obtain refined molten steel; the refining adopts the LF process, and the mass of Al at the outgoing station during refining is 0.045% - 0.055% of the total mass of the molten steel.

[0074] Before continuous casting after refining, vacuum degassing treatment can be performed to further reduce the gas content in the steel and improve the purity of the steel. Vacuum degassing can adopt the VD process or the RH process. At this time, the mass of Al at the outgoing station during refining can be adjusted to 0.045% - 0.055% of the total mass of the molten steel.

[0075] S3. Performing continuous casting on the refined molten steel to obtain a continuous casting billet;

[0076] In some embodiments, during continuous casting, the molten steel is protected throughout the process. Protected casting can effectively prevent the direct contact of the molten steel with oxygen in the air, reduce the oxygen content and inclusion generation in the steel, and improve the cleanliness of the molten steel and the surface quality of the casting billet. The casting temperature is 1500°C - 1580°C, and a two-stage electromagnetic stirring + reasonable cooling system is adopted to solidify and obtain a continuous casting billet. The superheat of the molten steel is 25 ± 5°C.

[0077] S4. Heating and rolling the continuous casting billet in sequence to obtain hot-rolled bars;

[0078] The continuous casting billet enters a heating furnace for heating, is descaled with high-pressure water after leaving the heating furnace, and enters a hot continuous rolling mill for rolling after descaling.

[0079] In some embodiments, the heating temperature is 1150°C - 1200°C, and the heating time is 2.5h - 3.5h.

[0080] The positive effect of limiting the heating temperature to 1150°C - 1200°C: It can ensure that the steel billet has good plastic deformation ability during rolling, reduce the rolling resistance and energy consumption, and improve the rolling efficiency. Exemplarily, the heating temperature can be 1150°C, 1160°C, 1170°C, 1180°C, 1190°C, 1200°C, etc.

[0081] Positive effects of limiting the heating time to 2.5 h to 3.5 h: The heating time of 2.5 h to 3.5 h can avoid the phenomenon of grain coarsening caused by too long heating time of the steel billet, and ensure the tissue performance of the product after rolling. Exemplarily, the heating time can be 2.5 h, 2.7 h, 2.9 h, 3.1 h, 3.3 h, 3.5 h, etc.

[0082] In some embodiments, the starting rolling temperature of the rolling is 1050 °C to 1100 °C, and the finish rolling temperature of the rolling is 950 °C to 1000 °C.

[0083] Positive effects of limiting the starting rolling temperature of the rolling to 1050 °C to 1100 °C: Starting rolling within the temperature range of 1050 °C to 1100 °C, the steel is in a good plastic state, which is conducive to the smooth progress of the rolling process. Exemplarily, the starting rolling temperature can be 1050 °C, 1060 °C, 1070 °C, 1080 °C, 1090 °C, 1100 °C, etc.

[0084] Positive effects of limiting the finish rolling temperature of the rolling to 950 °C to 1000 °C: Finish rolling within the temperature range of 950 °C to 1000 °C helps to refine and homogenize the internal structure of the steel. The high temperature and deformation during the finish rolling process can promote further refinement of austenite grains and reduce internal defects such as inclusions and pores, thereby improving the mechanical properties and wear resistance of the steel. Exemplarily, the finish rolling temperature can be 950 °C, 960 °C, 970 °C, 980 °C, 990 °C, 1000 °C, etc.

[0085] In some embodiments, the grain size difference of the hot-rolled bar is ≤3, and the Rockwell hardness of the hardenability J10 point of the hot-rolled bar is 33 HRC to 42 HRC.

[0086] The grain size difference ≤3 means that the internal grain size distribution of the material is more uniform, which helps to reduce the mechanical property fluctuations caused by grain size differences and makes the overall performance of the material more stable and reliable. The uniform grain structure helps to improve the mechanical property indexes such as the tensile strength, yield strength and elongation rate of the material.

[0087] Hardenability refers to the ability of a material to obtain a predetermined hardness during the quenching process. By limiting the Rockwell hardness range at the J10 point, it can be ensured that hot-rolled bars from different batches and different positions have similar hardness after quenching, thus ensuring the consistency of material properties. For gear steels used in low-speed, high-torque application scenarios, they need to have high strength and toughness to withstand complex working loads. By limiting the Rockwell hardness range at the J10 point of the hot-rolled bars, it can be ensured that the final products meet these specific performance requirements. Exemplarily, the Rockwell hardness at the J10 point of the hot-rolled bars can be 33HRC, 35HRC, 37HRC, 39HRC, 41HRC, 42HRC, etc.

[0088] S5. Cool the hot-rolled bars to obtain gear steel.

[0089] In some embodiments, the cooling method is air cooling.

[0090] The following further elaborates on this application in conjunction with specific embodiments. For the experimental methods without specific conditions noted in the following embodiments, they are generally determined according to industry standards; if there is no corresponding industry standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0091] Example 1

[0092] In this example, by mass fraction, the chemical composition of the gear steel includes: C: 0.17%, Si: 0.23%, Mn: 0.60%, P: 0.008%, S: 0.025%, Cr: 1.30%, Ni: 1.2%, Al: 0.025%, N: 0.0075%. Electric furnace smelting is adopted, the C content at the end of tapping is 0.06%, the P content is 0.006%, and the tapping oxygen activity is 550 ppm; after tapping, lime, fluorite, silicon carbide, aluminum ingots, refining slag, quartz sand, etc. are added, 5 m / t of Al wire is fed into the ladle during refining, the Al content is controlled at 0.035%, and the VD process is used for vacuum degassing. Al wire is supplemented before vacuum degassing to reach an Al content of 0.045%; the basicity of the refining slag is 2.5, and the oxygen activity of the molten steel after refining and tapping is 2 ppm. The content of Al2O3 in the refining slag is 25%, and the content of CaO is 55%; the molten steel is protected during the whole process of casting, the casting temperature is 1580 °C, and two-stage electromagnetic stirring + a reasonable cooling system are adopted to solidify and obtain continuous casting billets. The continuous casting billets are put into a heating furnace, the heating temperature is 1160 °C, and the heating time is 2.5 h; after leaving the heating furnace, high-pressure water descaling is carried out; after descaling, it enters the hot continuous rolling mill, the starting rolling temperature is 1080 °C, and the finishing rolling temperature is 980 °C.

[0093] The grain size difference of the obtained hot-rolled bar is 2.5 grades. The Rockwell hardness at the J10 point of the hardenability of the hot-rolled bar is 35 HRC, the impact energy against impact load is 87 J, the tensile strength is 1270 Mpa, and the elongation is 12%; after the hot-rolled bar is cooled, the surface hardness reaches 63 HRC, the core hardness is 35 HRC, and the retained austenite level is grade 1.

[0094] Comparative Example 1

[0095] The comparison steel grade is 20CrMnTi. The chemical composition of 20CrMnTi includes: C: 0.20%, Si: 0.28%, Mn: 0.93%, P: 0.015%, S: 0.016%, Cr: 1.050%, Al: 0.015%, N: 0.0080%. Electric furnace smelting is adopted, the C content at the end of tapping is 0.06%, the P content is 0.012%, and the oxygen activity at tapping is 660 ppm; lime, fluorite, silicon carbide, aluminum ingot, refining slag, quartz sand, etc. are added at tapping. 3 m / t of Al wire is fed into the ladle during refining to control the Al content to be 0.035%. VD process is used for vacuum degassing, and Al wire is supplemented before vacuum degassing to reach an Al content of 0.045%; the basicity of the refined steel slag is 2.5, and the oxygen activity of the refined molten steel at tapping is 7 ppm. The content of Al2O3 in the refined steel slag is 15%, and the content of CaO is 45%; the molten steel is protected from pouring throughout the process, the pouring temperature is 1580 °C, and two-stage electromagnetic stirring + reasonable cooling system are adopted to obtain continuous casting billets by solidification. The continuous casting billets are put into the heating furnace, the heating temperature is 1160 °C, and the heating time is 2.5 h; high-pressure water descaling is carried out after leaving the heating furnace; after descaling, it enters the hot continuous rolling mill, the starting rolling temperature is 1000 °C, and the finishing rolling temperature is 900 °C.

[0096] The Rockwell hardness at the J10 point of the hardenability of the obtained hot-rolled bar is 28 HRC, the impact energy against impact load is 45 J, the tensile strength is 1080 Mpa, and the elongation is 9%; after the hot-rolled bar is cooled, the core hardness is 30 HRC.

[0097] By comprehensively comparing the examples of the present application with the comparative examples, the examples of the present application have been improved in terms of impact energy against impact load, elongation, tensile strength, etc., meeting the stringent requirements of low-speed high-torque equipment for material strength.

[0098] One or more technical solutions in the embodiments of the present invention at least further have the following technical effects or advantages:

[0099] The gear steel provided by the examples of the present application meets at least one of the following performances: impact energy against impact load > 60 J, tensile strength ≥ 1200 Mpa, elongation ≥ 10%, core hardness ≥ 33 HRC, surface hardness is 58 HRC - 68 HRC, and retained austenite level ≤ 2 grades.

[0100] The above are only specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown in the present application, but rather to the broadest scope consistent with the principles and novel features claimed in the present application.

Claims

1. A gear steel for low speed and high torque applications, characterized in that: Measured in mass fraction, the chemical composition of the gear steel includes: C: 0.12%~0.25%, Si: 0.05%~0.70%, Mn: 0.30%~2.00%, P≤0.03%, S: 0.015%~0.05%, Cr: 0.80%~2.0%, Ni: 1.0%~2.0%, Al: 0.015%~0.055%, N≤0.0150%, Fe, Al / N>3, wherein Al / N represents the mass ratio of Al to N.

2. The gear steel according to claim 1, characterized in that: Measured by mass fraction, the chemical composition of the gear steel includes: S: 0.015% to 0.035%, Al: 0.015% to 0.035%, and N: 0.0070% to 0.0150%.

3. The gear steel according to claim 1, characterized in that: The gear steel meets at least one of the following properties: impact load impact energy> 60J, tensile strength≥1200Mpa, elongation≥10%, core hardness≥33HRC, surface hardness of 58HRC~68HRC, and retained austenite level≤2.

4. A method for preparing gear steel according to any one of claims 1 to 3, characterized in that: The method comprises: Smelting the molten iron to obtain molten steel; Refining the smelted molten steel to obtain refined molten steel; Continuously casting the refined molten steel to obtain a continuously cast billet; The continuous casting billet is sequentially heated and rolled to obtain a hot-rolled bar; The hot rolled bar is cooled to obtain gear steel.

5. The method according to claim 4, characterized in that The oxygen activity at the steel tapping end point of the smelting is 500ppm to 600ppm.

6. The method according to claim 4, characterized in that The refining adopts LF process, and the mass of Al out of the refining station is 0.025% to 0.035% of the total mass of the molten steel; and / or, The oxygen activity of the refined molten steel is ≤3ppm.

7. The method according to claim 4, characterized in that The refining uses steel slag, and the basicity of the steel slag is 2 to 3; and / or, The MI index of the steel slag is 0.20 to 0.30; and / or, The steel slag contains Al2O3 and CaO, wherein the mass of the Al2O3 accounts for 20% to 35% of the total mass of the steel slag, and the mass of the CaO accounts for 45% to 65% of the total mass of the steel slag.

8. The method according to claim 4, characterized in that The smelting molten steel is refined to obtain refined molten steel, comprising: Refining the smelted molten steel to obtain initial refined molten steel; The initial refined molten steel is subjected to vacuum degassing to obtain refined molten steel; the refining adopts LF process, and the mass of Al out of the refined station is 0.045% to 0.055% of the total mass of the molten steel.

9. The method according to claim 4, characterized in that The heating temperature is 1150° C. to 1200° C., and the heating time is 2.5 h to 3.5 h; and / or, The starting rolling temperature is 1050°C to 1100°C, and the finishing rolling temperature is 950°C to 1000°C.

10. The method according to claim 4, characterized in that The grain size range of the hot-rolled bar is ≤3, and the Rockwell hardness of the hot-rolled bar at J10 point is 33HRC to 42HRC.

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