Ultrafine grain corrosion-resistant bainite gear steel and production method thereof

Abstract

The invention provides ultra-fine grain corrosion-resistant bainite gear steel and a production method thereof, and belongs to the technical field of alloy machining. Comprising the following chemical components in percentage by mass (wt.%): 0.17-0.21% of C, less than or equal to 0.18% of Si, 1.10-1.30% of Mn, 1.00-1.20% of Cr, 0.00025-0.0020% of B, less than or equal to 0.06% of Mo, 0.02-0.03% of Nb, 0.10-0.20% of V, less than or equal to 0.035% of P + S, 0.027-0.054% of Al + N, less than or equal to 0.0002% of H, less than or equal to 0.0015% of O and the balance of Fe, and the steel is prepared through the processes of electric furnace primary smelting, LF external refining, VD vacuum degassing, nitrogen and sulfur control, continuous casting pouring, continuous casting billet heating, continuous rolling production and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy processing and relates to an ultrafine grain corrosion-resistant bainite gear steel and a production method thereof. Background Art

[0002] Metal corrosion causes enormous economic losses. Statistics show that annual metal waste due to corrosion accounts for 20% to 40% of annual metal production. For marine engineering steel, structural failure due to corrosion leads to significant economic losses in shipwrecks, oil pipelines, engineering platforms, and submarine oil and gas pipelines. Therefore, to mitigate the various hazards and economic losses caused by corrosion of marine steel, the development of low-alloy, seawater-resistant gear steel is crucial. Existing gear materials often suffer from deficiencies in mechanical properties, corrosion resistance, and wear resistance, significantly impacting the proper functioning of gear transmission systems and even posing a risk. Furthermore, high salt spray and humidity in marine environments accelerate pitting, crevice, and electrochemical corrosion. Corrosion rates in the seawater splash zone can reach 5-10 times those in inland environments, posing a serious threat to the structural integrity of gear steel. To address these challenges, material composition optimization and surface modification technologies are key breakthroughs.

[0003] Patent application CN117210766A discloses a seawater-corrosion-resistant marine steel and its preparation method. This steel utilizes a low-carbon, low-silicon, medium-chromium chemical composition, and employs Si and Zr-RE composite deoxidation to form fine, dispersed, and uniform composite oxysulfides. This significantly reduces the density of corrosive inclusions and significantly improves the seawater corrosion resistance of the steel for use in marine environments. This method, which uses rare earth elements to deoxidize the molten steel, not only increases steelmaking costs but also makes it prone to clogging of the submerged nozzle during continuous casting, leading to unplanned casting stops.

[0004] Patent application CN116590619A discloses a method for producing 10CrMoAl seawater corrosion-resistant steel. Through composition design and a combination of smelting, continuous casting, rolling, and heat treatment processes, the method produces a seawater corrosion-resistant steel with stable performance. However, the method does not describe parameters such as casting speed, secondary cooling water distribution, and sector roll gap control during the continuous casting process. Summary of the Invention

[0005] The purpose of the present invention is to provide an ultrafine grain corrosion-resistant bainite gear steel in response to the above-mentioned problems existing in the existing technology. The technical problem to be solved by the present invention is to provide a corrosion-resistant gear steel.

[0006] The object of the present invention can be achieved by the following technical solutions: It includes chemical components with the following mass percentages (wt.%): C: 0.17 - 0.21, Si ≤ 0.18, Mn: 1.10 - 1.30, Cr: 1.00 - 1.20, B: 0.00025 - 0.0020, Mo ≤ 0.06, Nb: 0.02 - 0.03, V: 0.10 - 0.20, P + S ≤ 0.035, Al + N: 0.027 - 0.054, H ≤ 0.0002, O ≤ 0.0015, and the rest is Fe.

[0007] It should be noted that in the composition of the ultra-fine grain corrosion-resistant bainitic gear steel provided by the present invention, the functions and content controls of each component are as follows:

[0008] C: C is the most basic and effective strengthening element in steel, the most effective element affecting hardenability, and has a relatively low cost. In order to ensure that the gear steel has sufficient strength and sufficient hardenability, a certain amount of carbon content should be contained. The present invention adopts a low carbon content. At the same time, in order to ensure sufficient strength and toughness at the core, the carbon content is controlled at 0.17 - 0.21 wt.%.

[0009] Si: During the carburizing process of carburized gear steel, oxides are formed at the grain boundaries due to the reaction of CO2 and H2O with the steel. Silicon in the steel is extremely easy to oxidize. When oxygen invades from the surface, silicon near the grain boundaries or grains diffuses to the grain boundaries preferentially compared with other elements, combines with trace oxygen dissolved on the surface at the grain boundaries to form oxides, and distributes in a network shape. This reduces the alloy element content in the matrix near the oxides, deteriorates the hardenability of the structure, and thus forms non-martensite structure after quenching. The metallographic structure of the non-martensite structure on the carburized surface after corrosion is black, so it is usually called "black structure". These black structures usually distribute in a network shape and are mainly composed of alloy element oxides and non-martensite structures (bainite and pearlite). The adverse effects of internal oxides of silicon are as follows: ① The non-martensite structure reduces the surface hardness and fatigue strength of the parts; ② The residual stress distribution is uneven; ③ Weakening the grain boundaries increases the possibility of part cracking. Therefore, the silicon content should be minimized in carburized gear steel, and a low-silicon chemical composition is adopted to reduce the harm of internal oxidation to carburized gear steel.

[0010] Mn: Mn can expand the austenite phase region, stabilize the austenite structure, and improve the hardenability of the steel. However, excessive Mn can dissolve in ferrite, increase the hardness and strength of ferrite and austenite in the steel. At the same time, Mn can improve the stability of the austenite structure and significantly improve the hardenability of the steel. In the present invention, Mn is mainly used to lower the pearlite and ferrite phase transformation regions, raise the bainite phase transformation region, and improve the hardenability at the same time. However, excessive Mn will reduce the plasticity of the steel, and the toughness of the steel deteriorates during hot rolling. The Mn content is controlled at 1.10 - 1.30 wt.%.

[0011] Cr: Cr can improve the hardenability and strength of steel. In steel, Cr combines with carbon to form carbides. Since the gear steel is quenched and then tempered at low temperature, no large carbides precipitate, but all precipitate as fine carbides. The precipitated carbides accumulate between the martensite laths, inhibiting the movement of the laths under stress. The dislocations in the martensite can tangle, improving the strength and fatigue resistance. However, at the same time, excessive Cr will form a carbide film, affecting the carburizing effect and reducing the properties of the carburized layer. The Cr content is controlled at Cr: 1.00 - 1.20 wt.%.

[0012] B: B can strongly prevent the transformation of austenite into pearlite and ferrite, significantly expand the bainite transformation region, and at the same time, a small amount of B can greatly improve the hardenability of steel. The segregation of B at grain boundaries can improve the grain boundary strength, but excessive B is prone to form boride precipitates, leading to the deterioration of the plasticity and toughness of the steel, especially having a significant impact on the toughness of the steel under the hot rolling process. In this invention, B is mainly used to optimize the phase transformation range and enhance the hardenability, and at the same time, the content needs to be strictly controlled to avoid the risk of embrittlement. The B content is controlled at 0.00025 - 0.0020 wt.%.

[0013] Mo: Mo can significantly improve the hardenability of steel, prevent temper brittleness and overheating tendency. In addition, the reasonable combination of Mo element and Cr element in this invention can significantly improve the hardenability and tempering resistance, and Mo can refine the grains. However, if the Mo content is too low, the above effects are limited. If the Mo content is too high, it promotes the formation of grain boundary ferrite films, which is not conducive to the hot plasticity of the steel, increases the tendency of reheat cracking of the steel, and has a higher cost. Therefore, the Mo content is controlled at Mo ≤ 0.06 wt.%.

[0014] Nb: Nb element has a very strong affinity for carbon and oxygen, can refine the grains and structure, and can also produce solid solution strengthening. After heat treatment, it improves the strength and thermal sensitivity of the steel. When the Nb content is too high, its grain refinement effect and strength increment are not obvious, but it increases the additional cost. Therefore, the Nb content is controlled at 0.02 - 0.03 wt.%.

[0015] V: V element has a very strong affinity for carbon and oxygen, can refine the grains and structure, and can also produce solid solution strengthening. After heat treatment, it improves the strength and thermal sensitivity of the steel. When the V content is too high, its grain refinement effect and strength increment are not obvious, but it increases the additional cost. Therefore, the V content is controlled at 0.10 - 0.20 wt.%.

[0016] P and S: Sulfur is easy to form MnS inclusions with manganese in steel, making the steel thermally brittle; P is an element with a strong segregation tendency, increasing the cold brittleness of the steel, reducing the plasticity, and being harmful to the uniformity of the product structure and properties. Control P + S ≤ 0.045 wt.%.

[0017] Al and N: As elements that significantly refine the grain size, Al and N mainly exist in the form of aluminum nitride precipitates in steel; aluminum nitride is mainly distributed at grain boundaries and prevents grain growth through the mechanism of pinning grain boundaries by particles. When the contents of Al and N in steel reach a certain level and meet a certain ratio, these two elements will combine into fine and dispersed aluminum nitride particles, which will pin the grain boundaries, thereby inhibiting grain growth, ensuring that the carburized gear steel has a fine and uniform grain size, and further improving the service life of the gear steel.

[0018] O and [H]: O forms oxide inclusions in steel, and O ≤ 0.0015 is controlled; [H] forms white spots in steel, seriously affecting product performance, and [H] ≤ 0.0002 is controlled.

[0019] During the design process, more optimally, for the gear steel of the present invention, its components meet the following requirements:

[0020] 2.8 ≤ K = [5C + 0.5Mn + 0.7Cr + 500B - 0.5Si - 2Mo - 15(P + S) + 20(Al + N)] / 1 + 10Nb + 5V ≤ 3.8

[0021] Based on the above design principles, the steel composition and dosage finally determined by the present invention are as follows: in terms of mass percentage (wt.%), C: 0.17 - 0.21, Si ≤ 0.18, Mn: 1.10 - 1.30, Cr: 1.00 - 1.20, B: 0.00025 - 0.0020, Mo ≤ 0.06, Nb: 0.02 - 0.03, V: 0.10 - 0.20, P + S ≤ 0.035, Al + N: 0.027 - 0.054, H ≤ 0.0002, O ≤ 0.0015, and the rest is Fe.

[0022] On the other hand, the present invention provides a production method for the above-mentioned ultra-fine grain corrosion-resistant bainitic gear steel, including the following steps:

[0023] S1. Primary melting in an electric furnace

[0024] The steel charge consists of 95% hot metal and 5% scrap steel. To control the gas content in the molten steel, the slag materials and alloys must be baked to remove moisture. At the same time of selecting low-rust chromium and low-phosphorus steel return materials for scrap steel; for each batch of slag materials added in the electric furnace, 500 - 700 kg of lime and 300 - 400 kg of dolomite are added, and the total consumption of slag materials is 60 - 80 kg / t;

[0025] Requirements for tapping from the electric furnace: C ≥ 0.08%, P ≤ 0.010%, the residual elements are qualified, the temperature ≥ 1660 °C. When tapping, slag blocking is strictly carried out, and 800 - 1000 kg of lime, 300 - 500 kg of accelerator, and 2.0 kg / t of steel of pure aluminum (for strengthening deoxidation) are added to the top slag.

[0026] S2, LF Secondary Refining

[0027] The temperature of the molten steel when LF arrives ≥ 1560°C; When electric heating ≥ 15 min and the temperature ≥ 1560°C, sample and analyze the chemical composition; In the early stage: the argon flow rate is 200 - 300 NL / min (the diameter of the bright circle ≥ Φ400 mm) to promote alloy dissolution; In the middle stage: the flow rate is 150 - 200 NL / min, and the white slag refining time ≥ 40 min (extend the desulfurization time);

[0028] In the later stage: the flow rate ≤ 100 NL / min to avoid molten steel from sucking in gas.

[0029] The target composition of the slag system: CaO: 55% - 60%, SiO2: ≤ 8%, MgO: 5% - 8%, Al2O3: 25% - 30%; During the refining process, use carbon powder and aluminum powder for deoxidation;

[0030] S3, VD Vacuum Degassing

[0031] The temperature when entering the VD station ≥ 1620°C; Use an on-line hydrogen analyzer to measure H ≤ 1.5 ppm, [N] ≤ 100 ppm; The vacuum degree ≤ 40 Pascals, and the ultra-high vacuum holding time ≥ 25 min. The argon flow rate: 200 - 250 NL / min during the ultra-high vacuum stage to strengthen the floating of inclusions.

[0032] S4, Nitrogen and Sulfur Control

[0033] The nitrogen flow rate is 250 - 600 NL / min. The target of nitrogen blowing: N = 80 - 110 ppm. After blowing nitrogen for 5 min, sample and test the nitrogen content. After meeting the internal control requirements, change to soft blowing with argon: the flow rate is 60 - 90 L / min, and the time ≥ 30 min; At the same time, adjust the aluminum content in the steel to 0.020 - 0.040%; After adjusting the aluminum, add ferrosulfur for rough adjustment according to the S content in the steel, and feed sulfur wire for fine adjustment.

[0034] S5, Continuous Casting Pouring

[0035] After soft blowing, solidification control: casting speed 0.25 - 0.30 m / min, mold electromagnetic stirring (M-EMS): current 150 - 180 A, frequency 3.0 Hz, equiaxed crystal ratio ≥ 70%;

[0036] Requirements for ingot pit cooling of the continuous casting billet: the temperature when entering the pit ≥ 600°C, slow cooling time ≥ 72 h, to eliminate stress and inhibit premature transformation of bainite.

[0037] S6, Heating of Continuous Casting Billet

[0038] The temperature in the preheating section ≤ 850°C, the temperature in the first heating section 1000 - 1150°C, the temperature in the second heating section 1160 - 1220°C, the temperature in the soaking section 1200 - 1230°C; the total heating time of the continuous casting billet is 8 - 12 h, and the high-temperature diffusion time ≥ 6.0 h;

[0039] S7. Products of continuous rolling

[0040] Process temperature control: The starting rolling temperature ≥ 1080°C, the finishing rolling temperature 850 - 900°C, the total reduction ratio ≥ 70%, and the reduction ratio of the last three passes ≥ 18%; Rapid cooling after rolling: The cooling rate is 10 - 15°C / s. When it reaches 250 - 350°C, it is transferred to an isothermal furnace and kept warm for 2 - 4 h to form a bainite structure.

[0041] As a further preference of the technical solution of the present invention, in step S1, the requirements for the furnace body are as follows: The number of furnace charges ≥ 4 charges per time, the number of ladle charges ≥ 2 charges per time; The steel grades smelted in the furnace before the ladle are not allowed to be titanium-containing steel grades.

[0042] As a further preference of the technical solution of the present invention, in step S2, the argon gas is controlled during the argon blowing operation as follows: Appropriately increase the argon gas stirring in the early stage, the flow rate is 200 - 300 NL / min, and the target bright circle diameter ≥ Φ350 mm; Keep medium argon gas intensity in the middle stage, the flow rate is 150 - 250 NL / min; Reduce the argon gas intensity in the later stage to avoid oxidation of the molten steel, the flow rate is 60 - 100 NL / min. Avoid exposing the molten steel during argon blowing and stirring. The bright circle diameter is detected by an infrared thermal imager to monitor the stirring effect in real time and ensure that the desulfurization rate ≥ 60%.

[0043] As a further preference of the technical solution of the present invention, in step S3, the regulations for vacuum degassing operation are as follows: The holding time at a vacuum degree ≤ 40 Pa ≥ 20 minutes; The argon gas is controlled during degassing operation as follows: When in rough vacuum (>26666.4 Pa), the argon gas flow rate is 80 - 120 NL / min, and when in extremely high vacuum (≤ 67 Pa), the argon gas flow rate is 180 - 220 NL / min; The holding time in extremely high vacuum ≥ 25 minutes; The temperature of the molten steel after degassing ≥ 1580°C to avoid a decrease in nitrogen solubility due to too low temperature during subsequent nitrogen control.

[0044] As a further preference of the technical solution of the present invention, in step S4, the soft argon gas flow rate is 70 - 90 L / min, and the time ≥ 20 min.

[0045] As a further preference of the technical solution of the present invention, in step S5, the target casting speed is controlled at 0.28 - 0.32 m / min; The conditions for electromagnetic stirring are: M-EMS, the current is 120 - 150 A, and the frequency is 2.5 - 3.0 Hz; F-EMS, the current is 850 - 950 A, and the frequency is 8. Hz.

[0046] As a further preference of the technical solution of the present invention, in step S7, the control of water descaling for continuous casting billets before rolling: high-pressure water descaling by spraying water with multiple nozzles in a single pass, the water descaling pressure ≥ 30 MPa, and the descaling rate ≥ 98%.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] (1) A superfine-grained corrosion-resistant bainitic gear steel and its production method provided by the present invention ensure that the steel has good hardenability and stability.

[0049] (2) The present invention adds a small amount of B to the steel to improve the corrosion resistance of the steel, reduce the corrosion degree of the gear during the preparation and use processes, and thus improve the service life of the gear.

[0050] (3) The present invention adopts rapid cooling after rolling and low-temperature isothermal treatment to directly promote the formation of a uniform bainite structure.

[0051] (4) During the continuous casting process of the present invention, electromagnetic stirring is adopted to enhance the flow of molten steel, promote the formation of equiaxed crystals, and at the same time, the dynamic soft reduction technology is adopted to compensate for solidification shrinkage. This design meets the production requirements of different specifications of billets, reduces central segregation and porosity, improves the material density, and provides high-quality billets for the subsequent rolling process. Description of the Drawings

[0052] Figure 1 It is the matrix structure diagram of the gear steel in the embodiment. Detailed Embodiments

[0053] The following are specific embodiments of the present invention and in combination with the drawings, the technical solutions of the present invention are further described, but the present invention is not limited to these embodiments.

[0054] S1. Electric furnace primary melting

[0055] The steel materials consist of 95% hot metal and 5% scrap steel. To control the gas content in the molten steel, the slag materials and alloys must be baked to remove moisture. At the same time, low-rust chromium and low-phosphorus steel return materials are selected for the scrap steel; 600 kg of lime and 350 kg of dolomite are added to each batch of electric furnace slag materials, and the total consumption of slag materials is 60 kg / t;

[0056] Requirements for tapping from the electric furnace: C ≥ 0.08%, P ≤ 0.010%, the residual elements are qualified, the temperature ≥ 1660 °C. During tapping, slag blocking is strictly carried out, and 800 kg of lime, 350 kg of accelerator, and 2.0 kg / t of steel of pure aluminum (strengthening deoxidation) are added to the top slag.

[0057] S2. LF furnace refining

[0058] The molten steel temperature at LF arrival ≥ 1560°C; when the electric heating is ≥ 15 min and the temperature ≥ 1560°C, sample and analyze the chemical composition; in the early stage: the argon flow rate is 200 NL / min (the bright circle diameter ≥ Φ400 mm) to promote alloy dissolution; in the middle stage: the flow rate is 150 NL / min, and the white slag refining time ≥ 40 min (extend the desulfurization time);

[0059] In the later stage: the flow rate ≤ 100 NL / min to avoid molten steel from inhaling gas.

[0060] The target composition of the slag system: CaO: 55% - 60%, SiO2: ≤ 8%, MgO: 5% - 8%, Al2O3: 25% - 30%; use carbon powder and aluminum powder for deoxidation during the refining process;

[0061] S3, VD vacuum degassing

[0062] The temperature when entering the VD station ≥ 1620°C; use an on-line hydrogen analyzer to measure H ≤ 1.5 ppm, [N] ≤ 100 ppm;; the vacuum degree ≤ 40 Pa, the ultra-high vacuum holding time ≥ 25 min, the argon flow rate: 225 NL / min in the ultra-high vacuum stage to enhance the floating of inclusions.

[0063] S4, controlling nitrogen and sulfur

[0064] The nitrogen flow rate is 400 NL / min, the blowing nitrogen target: N = 90 ppm. After blowing nitrogen for 5 min, sample and test the nitrogen content. After meeting the internal control requirements, change to soft blowing with argon: the flow rate is 75 L / min, and the time ≥ 30 min; at the same time, adjust the aluminum content in the steel to 0.025%; after the aluminum adjustment is completed, add ferrosulfur for rough adjustment according to the S content in the steel, and feed the sulfur wire for fine adjustment.

[0065] S5, continuous casting

[0066] After soft blowing, solidification control: the casting speed is 0.25 m / min, the mold electromagnetic stirring (M-EMS): the current is 160 A, the frequency is 3.0 Hz, and the equiaxed crystal ratio ≥ 70%;

[0067] The requirements for the casting billet to be pit-cooled: the temperature when entering the pit ≥ 600°C, the slow cooling time ≥ 72 h to eliminate stress and inhibit the premature phase transformation of bainite.

[0068] S6, heating the continuous casting billet

[0069] The temperature in the preheating section ≤ 850°C, the temperature in the first heating section is 1050°C, the temperature in the second heating section is 1155°C, and the temperature in the soaking section is 1200°C; the total heating time of the continuous casting billet is 9 h, and the soaking section time ≥ 6.0 h;

[0070] S7, continuous rolling to produce materials

[0071] Process temperature control: The starting rolling temperature ≥ 1080 °C, the finishing rolling temperature 875 °C, the total reduction ratio ≥ 70%, and the reduction ratio of the last three passes ≥ 18%; Rapid cooling after rolling: The cooling rate is 12 °C / s. When it reaches 300 °C, it is transferred to an isothermal furnace and kept at 300 °C for 4 h to form a bainite structure.

[0072] A group of ultra-fine grain corrosion-resistant bainite wind power gear steels are prepared according to the above production method. Calculated as 100%, the component contents except Fe are as follows:

[0073]

[0074] Perform performance tests on the ultra-fine grain bainite gear steel products prepared in the examples. The detection items include: the austenite grain grade of the rolled material, mechanical properties, and end hardenability. Specifically as follows:

[0075] Test the austenite grain grade of the ultra-fine grain corrosion-resistant bainite steel rolled material. It is known through detection that the austenite grains of the corrosion-resistant bainite steel rolled material obtained in the present invention are uniform: the austenite grains ≥ 10.0 grades, and the difference in austenite grain size in the cross-section ≤ 1.0 grade.

[0076] Test the mechanical properties of the ultra-fine grain corrosion-resistant bainite steel. It is known through detection that the tensile strength Rm of the ultra-fine grain corrosion-resistant bainite steel obtained in the present invention ≥ 1200 MPa (average Rm = 1256 MPa), the yield strength Rel ≥ 1000 MPa (average Rel = 1077 MPa), the elongation at break A ≥ 16%, the reduction of area Z ≥ 52%, and the impact energy AKu ≥ 60 J.

[0077] Test the end hardenability of the ultra-fine grain corrosion-resistant bainite steel. It is known through detection that the end hardenability values of the ultra-fine grain corrosion-resistant bainite steel obtained in the present invention are J9 ≥ 45 HRC, J15 ≥ 39 HRC, and the difference in hardness values at the same distance ≤ 3.

[0078] The specific conditions of the austenite grain grade of the rolled material, mechanical properties, and end hardenability are shown in the figure:

[0079]

[0080] Observe the microstructure of the ultra-fine grain corrosion-resistant bainite steel rolled material prepared in the present invention. The structure of the steel is bainite + martensite + retained austenite + carbide structure, as Figure 1 shown.

[0081] Therefore, when the ultra-fine grain corrosion-resistant bainite steel prepared in the present invention is used for the gears of offshore wind power boxes, due to the good tissue performance and fatigue performance of the material, it can fully meet the performance requirements of the gears of offshore wind power boxes.

[0082] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Those skilled in the art to which the present invention pertains may make various modifications or supplements to the described specific embodiments or use similar means for substitution, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. An ultra-fine grain corrosion-resistant bainitic gear steel, characterized in that, By mass percentage, it includes C: 0.17 - 0.21, Si ≤ 0.18, Mn: 1.10 - 1.30, Cr: 1.00 - 1.20, B: 0.00025 - 0.0020, Mo ≤ 0.06, Nb: 0.02 - 0.03, V: 0.10 - 0.20, P + S ≤ 0.035, Al + N: 0.027 - 0.054, H ≤ 0.0002, O ≤ 0.0015, and the balance is Fe.

2. The ultra-fine grain corrosion-resistant bainitic gear steel according to claim 1, characterized in that, Its components meet the following requirements: 2.8 ≤ K = [5C + 0.5Mn + 0.7Cr + 500B - 0.5Si - 2Mo - 15(P + S) + 20(Al + N)] / 1 + 10Nb + 5V ≤ 3.

8.

3. The ultra-fine grain corrosion-resistant bainitic gear steel according to claim 1, characterized in that By mass percentage, the proportion of each component is: C: 0.17 - 0.21, Si ≤ 0.18, Mn: 1.10 - 1.30, Cr: 1.00 - 1.20, B: 0.00025 - 0.0020, Mo ≤ 0.06, Nb: 0.02 - 0.03, V: 0.10 - 0.20, P + S ≤ 0.035, Al + N: 0.027 - 0.054, H ≤ 0.0002, O ≤ 0.0015, and the balance is Fe.

4. A method for producing the ultrafine-grained corrosion-resistant bainitic gear steel according to any one of claims 1-3, characterized in that, It includes the following steps: S1. Primary melting in an electric furnace The steel materials consist of 95% hot metal and 5% scrap steel. To control the gas content in the molten steel, the slag materials and alloys must be baked to remove moisture. At the same time, low-rust chromium and low-phosphorus steel return materials are selected for the scrap steel; for each batch of electric furnace slag materials, 500 - 700 kg of lime and 300 - 400 kg of dolomite are added, and the total consumption of slag materials is 60 - 80 kg / t; Requirements for tapping from the electric furnace: C ≥ 0.08%, P ≤ 0.010%, the residual elements are qualified, the temperature ≥ 1660 °C. During tapping, slag blocking is strictly carried out, and 800 - 1000 kg of lime, 300 - 500 kg of accelerator, and 2.0 kg / t of steel of pure aluminum are added to the top slag; S2. LF refining outside the furnace The temperature of the molten steel in the LF when it arrives ≥ 1560 °C; when the electric heating is ≥ 15 min and the temperature ≥ 1560 °C, the chemical composition is sampled and analyzed; in the early stage: the argon flow rate is 200 - 300 NL / min to promote the dissolution of the alloy; In the middle stage: the flow rate is 150 - 200 NL / min, and the white slag refining time is maintained ≥ 40 min; In the later stage: the flow rate ≤ 100 NL / min to avoid the molten steel from inhaling gas; The target composition of the slag system: CaO: 55% - 60%, SiO2: ≤ 8%, MgO: 5% - 8%, Al2O3: 25% - 30%; during the refining process, carbon powder and aluminum powder are used for deoxidation; S3. VD vacuum degassing The temperature when entering the VD station ≥ 1620 °C; the online hydrogen analyzer measures H ≤ 1.5 ppm, [N] ≤ 100 ppm;; the vacuum degree ≤ 40 Pascals, and the ultra-high vacuum holding time ≥ 25 min. The argon flow rate: in the ultra-high vacuum stage, it is 200 - 250 NL / min to strengthen the floating of inclusions; S4. Controlling nitrogen and sulfur The nitrogen flow rate is 250 - 600 NL / min. The nitrogen blowing target: N = 80 - 110 ppm. After blowing nitrogen for 5 minutes, sample and test the nitrogen content. After meeting the internal control requirements, change to argon soft blowing: the flow rate is 60 - 90 L / min, and the time is ≥ 30 min; meanwhile, adjust the aluminum content in the steel to 0.020 - 0.040%; after adjusting aluminum, add ferrosulfur for rough adjustment according to the S content in the steel, and feed sulfur wire for fine adjustment; S5. Continuous casting pouring After soft blowing ends, solidification control: casting speed 0.25 - 0.30 m / min, mold electromagnetic stirring (M - EMS): current 150 - 180 A, frequency 3.0 Hz, equiaxed crystal ratio ≥ 70%; Requirements for ingot pit cooling: the ingot entering temperature ≥ 600 °C, slow cooling time ≥ 72 h, to eliminate stress and inhibit premature bainite phase transformation; S6. Heating of continuous casting billet The temperature of the preheating section ≤ 850 °C, the temperature of heating section 1 is 1000 - 1150 °C, the temperature of heating section 2 is 1160 - 1220 °C, and the temperature of soaking section is 1200 - 1230 °C; the total heating time of the continuous casting billet is 8 - 12 h, and the high - temperature diffusion time ≥ 6.0 h; S7. Continuous rolling production Process temperature control: starting rolling temperature ≥ 1080 °C, finishing rolling temperature 850 - 900 °C, total reduction ratio ≥ 70%, reduction ratio of the last three passes ≥ 18%; rapid cooling after rolling: cooling rate 10 - 15 °C / s, when it reaches 250 - 350 °C, transfer to an isothermal furnace and hold for 2 - 4 h to form bainite structure.

5. The production method of an ultra-fine grain corrosion-resistant bainitic gear steel according to claim 1, characterized in that, In step S1, the requirements for the furnace body are as follows: the number of furnace body operations ≥ 4 times per furnace, the number of ladle operations ≥ 2 times per furnace; the steel grade smelted in the furnace before the ladle is not allowed to be a titanium - containing steel grade.

6. The production method of an ultra-fine grain corrosion-resistant bainitic gear steel according to claim 1, characterized in that, In step S2, during the argon blowing operation, argon is controlled as follows: appropriately increase the argon stirring in the early stage, the flow rate is 200 - 300 NL / min, the target bright circle diameter ≥ Φ350 mm; maintain medium argon intensity in the middle stage, the flow rate is 150 - 250 NL / min; reduce the argon intensity in the later stage to avoid oxidation of the molten steel, the flow rate is 60 - 100 NL / min, and avoid exposing the molten steel during argon blowing stirring; the bright circle diameter is detected by an infrared thermal imager to monitor the stirring effect in real - time and ensure that the desulfurization rate ≥ 60%.

7. The production method of an ultra-fine grain corrosion-resistant bainitic gear steel according to claim 1, characterized in that, In step S3, the regulations for vacuum degassing operation: the holding time at a vacuum degree ≤ 40 Pa ≥ 20 minutes; during degassing operation, argon is controlled as: the argon flow rate during rough vacuum is 80 - 120 NL / min, the argon flow rate during extremely high vacuum is 180 - 220 NL / min; the holding time of extremely high vacuum ≥ 25 minutes; the temperature of the molten steel after degassing ≥ 1580 °C to avoid a decrease in nitrogen solubility due to too low temperature during subsequent nitrogen control.

8. The production method of an ultra-fine grain corrosion-resistant bainitic gear steel according to claim 1, characterized in that, In step S4, the soft blowing argon flow rate is 70 - 90 L / min, and the time is ≥ 20 min.

9. The production method of an ultra-fine grain corrosion-resistant bainitic gear steel according to claim 1, characterized in that, In step S5, the target casting speed is controlled at 0.28 - 0.32 m / min; the electromagnetic stirring conditions are: M - EMS, current 120 - 150 A, frequency 2.5 - 3.0 Hz; F - EMS, current 850 - 950 A, frequency 8.0 Hz.

10. The production method of an ultra-fine grain corrosion-resistant bainitic gear steel according to claim 1, characterized in that, In step S7, before rolling, water descaling control for continuous casting billets: multi - nozzle high - pressure water descaling in a single pass, water descaling pressure ≥ 30 MPa, descaling rate ≥ 98%.

Images