Production method of quenched and tempered silver bright steel for large-diameter mechanical transmission shaft

Abstract

The invention belongs to the field of metallurgical technology and metal material processing, and relates to a production method of quenched and tempered silver bright steel for a large-diameter mechanical transmission shaft. The adopted process route comprises a converter, an LF furnace, an RH furnace, continuous casting, hot rolling, online softening annealing, multi-roller straightening, tunnel gas furnace heating, precession type water cooling, roller hearth type continuous annealing furnace tempering, round steel fixed-length head and tail cutting, pressure straightening, skin turning, grinding, polishing and rust-proof packaging. According to the produced quenched and tempered round steel with the diameter of 180-250 mm, the properties of the 1 / 4 diameter position are that the tensile strength is larger than or equal to 1000 Mpa, the yield strength is larger than or equal to 800 Mpa, the ductility is larger than or equal to 20%, the percentage reduction of area is larger than or equal to 55%, the AKV2 at the temperature of-40 DEG C is larger than or equal to 27 J, the straightness is smaller than or equal to 1.0 mm / m, the grain size is larger than or equal to the grade 6, the inclusion grades A, B, C and D are all smaller than or equal to the grade 1.5, the tempered sorbite is larger than or equal to 90%, and the quenched and tempered round steel can be directly blanked and machined into the high-strength and long-fatigue-life mechanical transmission shaft with the diameter of 180-250 mm.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metallurgy and metal material processing, and relates to a production method of a quenched and tempered silver bright steel for a large-diameter mechanical transmission shaft. BACKGROUND

[0002] With the increasing requirement of the performance of transmission systems in the fields of heavy machinery, wind power, ships, engineering machinery and the like, higher requirements are put forward for the strength, toughness, fatigue life and dimensional accuracy of the steel for transmission shafts. The conventional transmission shafts are mostly made of medium-carbon alloy structural steel, such as 42CrMo, 40CrNiMo and the like. The steel is processed after quenching and tempering treatment, but the production process is complex, the energy consumption is high, the cost is large, and the large-diameter steel material is prone to problems such as uneven structure, insufficient hardenability and many internal defects, which affect the reliability of the final product.

[0003] At present, although some enterprises attempt to produce the steel for transmission shafts by using non-quenched and tempered steel or simplified process, it is still difficult to simultaneously ensure high strength, high toughness, high fatigue performance and good processing performance in the production of large-diameter steel, especially steel with a diameter of ≥180 mm. Therefore, it is of important industrial application value to develop a quenched and tempered silver bright steel suitable for large-diameter transmission shafts and having excellent comprehensive performance and economic benefit, and a production process thereof. SUMMARY

[0004] To achieve the above-mentioned object, the present application provides a production method of a quenched and tempered silver bright steel for a large-diameter mechanical transmission shaft, which realizes the overall improvement of the performance of the steel material by optimizing the component design and process flow, and meets the manufacturing requirements of high-end transmission shafts.

[0005] The technical scheme adopted by the present application is a production method of a quenched and tempered silver bright steel for a large-diameter mechanical transmission shaft, which comprises the following key steps:

[0006] S1: In the smelting stage, the converter tapping C is ≥0.08%, P is ≤0.010%, the LF+RH furnace refining process is adopted, argon is blown in the whole LF furnace refining process, slagging deoxidation is performed, the white slag holding time is ≥30 min, the process control Als is 0.040% to 0.055%, and the RH vacuum degassing treatment is performed under a vacuum degree of 0.5 tor or less, and the vacuum holding time is not less than 20 min, the outgoing molten steel H is ≤2 ppm, and O is ≤10 ppm;

[0007] S2: In the continuous casting stage, the whole-process protective casting is adopted, the continuous casting furnace overheat degree is ≤25℃, the starting furnace overheat degree is ≤35℃, and the billet size is 350*430 mm;

[0008] S3: In the heating stage, the billet heating time is 300 to 480 min, the heating section temperature is 1190 to 1240℃, and the soaking section temperature is 1210 to 1250℃;

[0009] S4: Rolling stage, the bar 1150 two-roll reciprocating mill is rolled with large reduction, the first and second pass reductions are controlled at 90 mm and 85 mm respectively; the starting rolling temperature is ≥1100℃; the finishing rolling is performed by a two-roll mill, and the finishing rolling temperature is 780-830℃;

[0010] S5: On-line softening annealing stage, after the round steel is sawn by the on-line metal saw, it is directly put into the slow cooling pit from the roller at high temperature, and is kept at 650℃ for 24 hours, and the hardness of the round steel is ≤280HBW;

[0011] S6: Finishing stage, the round steel is straightened by a multi-roll straightening machine, and the bending degree q is ≤2.0 mm / m;

[0012] S7: Tunnel gas furnace heating and rotary in-water cooling stage, a rotary bottom support roller is used, the round steel is rotated and advanced, the advancing speed is 0.006 m / s, 25 gas burners are used for heating, the water inlet temperature is 880-920℃, and the water outlet red return temperature is 80-150℃;

[0013] S8: Roller bottom type continuous annealing furnace tempering stage, the quenched round steel is tempered within 3 hours, the round steel is uniformly distributed on the roller, is arranged in one layer, and is uniformly advanced, natural gas is used for heating, the temperature is kept at 550-580℃ for 4-5 hours, and the round steel is discharged and air cooled;

[0014] S9: The round bar head and tail and the ultrasonic flaw detection blind area are cut off;

[0015] S10: Pressure straightening: q≤1.0 mm / m;

[0016] S11: Carriage, roller polishing, polishing and polishing: the tempering round steel q≤1.0 mm / m, the carriage diameter is reduced by 5 mm, and the polishing ensures that the surface roughness Ra≤0.6 μm;

[0017] S12: The round steel surface is sprayed with oil-based rust-proof oil, the round steel is bundled by using rubber-covered binding wire, then is wrapped and packaged by using waterproof plastic film, then is sealed by using transparent adhesive tape, then is sleeved by a woven bag, and finally is packed in a wooden box for protection.

[0018] Further, in the S7 tunnel gas furnace heating and rotary in-water cooling stage, the rotary in-water cooling uses 30 high-pressure water nozzles uniformly distributed as a cooling water ring, 6 water rings are arranged at intervals of 0.3 meters to cool the round steel, and the water pressure of a single water ring is 0.5-0.8 Mpa.

[0019] Further, in the S9, the round bar head and tail are cut off by 80±5 mm.

[0020] Further, the chemical composition of the steel is as follows: C: 0.41% to 0.43%, Si: 0.22% to 0.32%, Mn: 0.74% to 0.79%, P≤0.015%, S≤0.005%, Cr: 1.12% to 1.18%, Ni≤0.25%, Al: 0.035% to 0.060%, N: 0.004% to 0.008%, Cu≤0.20%, Mo: 0.18% to 0.22%, Ti≤0.01%, B≤0.0008%, H≤0.0002%, O≤0.0015%, and the rest is Fe and inevitable impurities.

[0021] Further, the performance of the quenched and tempered round steel with a diameter of 180 to 250 mm at 1 / 4 diameter reaches the following indexes: tensile strength≥1000Mpa, yield strength≥800Mpa, elongation≥20%, reduction of area≥55%, AKV2 at-40℃≥27J, flatness≤1.0mm / m, grain size≥6 levels, inclusion level A, B, C and D≤1.5 levels, and tempered sorbite≥90%, which can be directly machined into a high-strength high-fatigue-life mechanical transmission shaft with a diameter of 180 to 250 mm, and the fatigue limit reaches more than 450MPa.

[0022] The present application improves the performance of the steel from four aspects of component design, purity control, structure regulation and process optimization.

[0023] Component design: the present application adopts a medium-carbon-low-alloy-micro-alloyed system in component design, which takes into account strength, toughness, fatigue performance and processing performance.

[0024] The functions and control principles of each element are as follows:

[0025] C (carbon): controlled at 0.41% to 0.43%, which is in the medium-carbon range, can ensure sufficient hardenability and strength, and avoid the decline of toughness and welding performance caused by too high carbon content. Carbon is the basis for forming martensite, which directly affects the hardness and strength after quenching.

[0026] Cr (chromium): 1.12% to 1.18%, which significantly improves hardenability, enhances corrosion resistance and high-temperature strength, and cooperates with Mo to improve tempering stability.

[0027] Mo (molybdenum): 0.18% to 0.22%, which suppresses temper brittleness, improves high-temperature strength and creep resistance, and cooperates with Cr to enhance the overall comprehensive performance of the steel.

[0028] B (boron): added in a trace amount (≤0.0008%), which can significantly improve hardenability, especially suitable for large-section steel, and delay ferrite transformation by segregating at the austenite grain boundary.

[0029] Al (Aluminum): 0.035%~0.060%, as a strong deoxidizer, forms Al2O3 and AlN, refines grains, and improves toughness.

[0030] N (Nitrogen): 0.004%~0.008%, forms nitrides with Al, V, etc., plays a role in precipitation strengthening and grain refinement.

[0031] Low S, P control: S≤0.005%, P≤0.015%, maximally reduces harmful elements, reduces non-metallic inclusions, improves steel purity, improves toughness and fatigue performance.

[0032] Purity control principle:

[0033] The purity of steel directly affects the fatigue life and fracture toughness. The present application realizes ultra-low gas content and inclusion control through the following processes:

[0034] (1) LF+RH duplex refining: LF furnace through white slag refining, full argon blowing, effective deoxidation, desulfurization, control Als at 0.040%~0.055%, ensure the inclusion floating removal; RH vacuum treatment at ≤0.5torr for ≥20 minutes, significantly reduce [H]≤2ppm, [O]≤10ppm, avoid hydrogen-induced cracking and oxidation inclusions.

[0035] (2) Full protection casting: prevent secondary oxidation, control the tundish superheat ≤25℃, reduce the center segregation and shrinkage of the billet.

[0036] The advantages of high purity: reduce crack origin, improve fatigue limit (≥450MPa), enhance the service reliability of steel under alternating load.

[0037] Organization control principle:

[0038] (1) Through the precise cooperation of thermal mechanical control process (TMCP) and quenching and tempering treatment (quenching + tempering), realize the organization refinement and performance optimization:

[0039] (2) High temperature large reduction rolling: rolling temperature ≥1100℃, the first two passes reduction amount reaches 90mm, 85mm, effectively breaks the as-cast structure, refines austenite grains, provides uniform and fine original structure for subsequent quenching.

[0040] (3) Rotary water cooling quenching: round steel rotates forward, cooperates with multi-ring high pressure water nozzle (0.5~0.8MPa), realizes uniform and rapid cooling, obtains fine martensite structure, avoids soft point or quenching crack.

[0041] (4) Medium-high temperature tempering: keeping at 550-580℃ for 4-5 hours, so that the martensite is converted into tempered sorbite, carbide is uniformly precipitated, and the best matching of strength and toughness is achieved (tensile strength ≥1000MPa, elongation ≥20%, impact energy at-40℃ ≥27J).

[0042] The process design of the present application emphasizes continuity, automation and quality stability, and the key innovations include:

[0043] (1) On-line softening annealing: directly entering the 650℃ slow cooling pit for 24 hours after rolling, using high-temperature waste heat for heat preservation, energy saving and environmental protection, effectively eliminating internal stress and preventing quenching cracking.

[0044] (2) Multi-stage straightening and precision machining: multi-roll straightening + pressure straightening to ensure straightness ≤1.0mm / m; turning, polishing and polishing to achieve a high surface finish of Ra≤0.6μm, meeting the silver bright steel standard.

[0045] (3) Anti-rust packaging system: multi-layer protection of spraying anti-rust oil + waterproof film + woven bag + wooden box, ensuring no rust within one year, suitable for long-distance sea transportation and long-term storage.

[0046] The beneficial effects of the present application are: through the dual optimization of composition and process, the present application successfully produces a diameter 180-250mm quenched and tempered round steel, which has a tensile strength ≥1000Mpa, a yield strength ≥800Mpa, an elongation ≥20%, a reduction of area ≥55%, an AKV2 at-40℃ ≥27J, a straightness ≤1.0mm / m, a grain size ≥6 levels, a level of inclusions A, B, C and D ≤1.5 levels, and a tempered sorbite ≥90%. It can be directly machined into a diameter 180-250mm high-strength high-fatigue life mechanical transmission shaft, with a fatigue limit of more than 450MPa. BRIEF DESCRIPTION OF DRAWINGS

[0047] 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 needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0048] Figure 1 is the microstructure diagram of the steel produced by the method of embodiment 1 of the present application after quenching and tempering;

[0049] Figure 2 is the microstructure diagram of the steel produced by the method of embodiment 2 of the present application after quenching and tempering;

[0050] Figure 3is the microstructure diagram of the steel produced by the method of embodiment 3 of the present application. DETAILED DESCRIPTION

[0051] For the convenience of understanding of those skilled in the art, the present application is further described below in combination with embodiments and drawings, and the content mentioned in the embodiments is not a limitation on the present application.

[0052] The present application provides a production method of a quenched and tempered silver bright steel for a large-diameter mechanical transmission shaft, and the process route is: converter smelting + LF furnace refining + RH vacuum treatment + continuous casting + hot rolling + on-line softening annealing + multi-roll straightening + tunnel gas furnace heating + spin-in water quenching + roller bottom continuous annealing furnace tempering + fixed-length cutting + pressure straightening + car body + polishing + polishing + anti-rust packaging.

[0053] The production process includes the following key steps:

[0054] 1) Smelting: the converter tapping C≥0.08%, P≤0.010%, the LF+RH furnace refining process is adopted, argon is blown throughout the LF furnace refining process, slagging deoxidation is performed, the white slag maintaining time is≥30min, the process control Als=0.040%-0.055%, the RH vacuum degassing treatment is performed under a vacuum degree of 0.5tor or less, the vacuum maintaining time is not less than 20min, the tapping molten steel H≤2ppm, O≤10ppm;

[0055] 2) Continuous casting: full-process protective casting is adopted, the tundish superheat continuous furnace≤25℃, the tundish superheat starting furnace≤35℃, and the billet size is 350×430mm;

[0056] 3) Heating: the billet heating time is 300-480min, the heating section temperature is 1190-1240℃, and the soaking section temperature is 1210-1250℃;

[0057] 4) Rolling: the bar and wire 1150 two-high reciprocating mill adopts large reduction for rolling, and the one-two pass reduction amount is controlled at 90mm and 85mm respectively; the rough rolling temperature≥1100℃; the finish rolling is performed by using a two-high mill, and the finish rolling temperature is 780-830℃;

[0058] 5) On-line softening annealing: after the round steel is sawn by the on-line metal saw, it is directly put into the slow cooling pit from the roller in a high temperature state, the round steel hardness≤280HBW after 24 hours of 650℃ heat preservation. The stress of the round steel is eliminated, and the problem of round steel cracking in the quenching process is solved. The slow cooling pit inner wall is paved with heat preservation cotton, the heat preservation cotton is arranged with a pipeline, the pipeline is passed through with high temperature waste gas combustion of a heating furnace to supplement heat, the high temperature waste gas is finally backflowed into the heating furnace waste gas discharge port through the pipeline, and after filtration, the pollution-free emission is realized.

[0059] 6) Finishing: round steel is straightened by multi-roll straightening machine, and the bending degree q is less than or equal to 2.0 mm / m. The round steel is qualified according to GB / T 4162 B grade ultrasonic flaw detection, and qualified according to 0.3*20 mm magnetic flux leakage flaw detection.

[0060] 7) Tunnel gas furnace heating and rotary water cooling: the round steel is rotated forward by using a rotating bottom support roller, and the forward speed is 0.006 m / s. The round steel is heated by using 25 gas burners, and the water inlet temperature is 880-920℃. The rotary water cooling adopts 30 high-pressure water nozzles uniformly distributed as a cooling water ring. Six water rings are arranged at intervals of 0.3 meters to cool the round steel. The water pressure of a single water ring is 0.5-0.8 Mpa. The round steel outlet water red temperature is 80-150℃.

[0061] 8) Roller bottom type continuous annealing furnace tempering: the quenched round steel is tempered within 3 hours. The round steel is uniformly distributed on the roller, and is arranged in one layer and uniformly forwarded. The round steel is heated by natural gas, and is kept at 550-580℃ for 4-5 hours, and is discharged for air cooling.

[0062] 9) Round bar head and tail cutting: the round bar head and tail and the ultrasonic flaw detection blind area are cut off, and the head and tail cutting length is 80±5 mm.

[0063] 10) Pressure straightening: q≤1.0 mm / m.

[0064] 11) Carriage, roller light, polishing and polishing: the quenched round steel q≤1.0 mm / m, the carriage diameter is reduced by 5 mm, the diameter size tolerance is (0, -0.40) mm, and the polishing ensures the surface finish Ra≤0.6 μm.

[0065] 12) Anti-rust packaging: the round steel surface is sprayed with oil-based anti-rust oil, and is banded with rubber banding line. Then, the round steel is wrapped and packaged with waterproof plastic film to prevent oil leakage, and is sealed with transparent adhesive tape. Then, the round steel is sleeved with woven bag. Finally, the round steel is protected by being loaded into a wooden box. It is ensured that the material does not rust within 1 year after production.

[0066] The chemical composition of the quenched and tempered silver bright steel is as follows in terms of weight percentage: C: 0.41%-0.43%, Si: 0.22%-0.32%, Mn: 0.74%-0.79%, P≤0.015%, S≤0.005%, Cr: 1.12%-1.18%, Ni≤0.25%, Al: 0.035%-0.060%, N: 0.004%-0.008%, Cu≤0.20%, Mo: 0.18%-0.22%, Ti≤0.01%, B≤0.0008%, H≤0.0002%, O≤0.0015%, and the rest is Fe and unavoidable impurities.

[0067] The following is further illustrated by examples. The mechanical properties of the steels of the examples are shown in Table 1, and the impact energy, grain size, inclusion and fatigue performance test results are shown in Table 2.

[0068] Example 1:

[0069] The chemical composition of the tempered silver bright steel involved in this example is as follows in terms of percentage by weight: C: 0.41%, Si: 0.25%, Mn: 0.76%, P≤0.015%, S≤0.005%, Cr: 1.16%, Ni: 0.03%, Al: 0.040%, N: 0.005%, Cu≤0.20%, Mo: 0.20%, Ti≤0.01%, B: 0.0008%, H≤0.0002%, O≤0.0015%, and the rest is Fe and inevitable impurities.

[0070] The production process includes the following key steps:

[0071] 1) Smelting: C=0.08% and P=0.008% in converter tapping, LF+RH furnace refining process is adopted, white slag holding time is 35 min, process control Als=0.050%; RH vacuum degassing treatment is performed under vacuum degree of 0.5 torr or below, vacuum holding time is 25 min, outgoing molten steel H=1.5 ppm and O=8 ppm;

[0072] 2) Continuous casting: tundish superheat for continuous casting furnace is 22℃, and the billet size is 350×430 mm;

[0073] 3) Heating: billet heating time is 380 min, heating section temperature is 1230℃, and soaking section temperature is 1240℃;

[0074] 4) Rolling: large reduction is adopted for bar and wire 1150 two-high reciprocating mill, and the first and second pass reductions are controlled to be 90 mm and 85 mm respectively; opening rolling temperature is 1140℃; final rolling is performed by two-high rolling mill, and the final rolling temperature is 800℃;

[0075] 5) On-line softening annealing: after the round steel is sawn by on-line metal saw, it is directly put into the slow cooling pit from the roller in high temperature state, and the round steel hardness is 268 HBW after 24 hours of 650℃ heat preservation.

[0076] 6) Finishing: straightening is performed by multi-roller straightening machine, and the bending degree q is less than or equal to 2.0 mm / m. According to GB / T 4162, the ultrasonic flaw detection is qualified at level B, and the magnetic flux leakage 0.3*20 mm flaw detection is qualified.

[0077] 7) Tunnel gas furnace heating and rotary water cooling: the water inlet temperature is 900℃, and the round steel red temperature after water outlet is 100℃.

[0078] 8) Roller bottom type continuous annealing furnace tempering: the round steel is tempered within 3 hours after quenching, the heat preservation temperature is 570℃, and the holding time is 4.5 hours, and the round steel is air cooled after furnace discharge.

[0079] 9) Cutting off the head and tail of the fixed-length round steel: Cut off the head and tail of the round bar and the ultrasonic flaw detection blind area. The length of the head and tail cut off is 80±5mm.

[0080] 10) Pressure straightening: q≤1.0mm / m.

[0081] 11) Car body, roll finishing, grinding, polishing: Tempered round steel q≤1.0mm / m, car body diameter reduced by 5mm, diameter tolerance is (0, -0.40)mm, polishing ensures surface finish Ra≤0.6μm.

[0082] 12) Rust-proof packaging: The round steel surface is sprayed with oil-based rust-preventive oil and bundled with rubber-coated binding wire. Waterproof plastic film is wrapped around the steel, and the opening is sealed with transparent tape. The steel is then placed in woven bags and packaged in a wooden crate for protection. This ensures the material will not rust within one year of production.

[0083] The microstructure of the steel produced by the above method after quenching and tempering is as follows: Figure 1 As shown.

[0084] Example 2:

[0085] The chemical composition of the tempered silver bright steel involved in this embodiment, by weight percentage, is as follows: C: 0.42%, Si: 0.32%, Mn: 0.75%, P≤0.015%, S≤0.005%, Cr: 1.12%, Ni: 0.04%, Al: 0.035%, N: 0.006%, Cu≤0.20%, Mo: 0.22%, Ti≤0.01%, B: 0.0006%, H: 0.0001%, O: 0.0010%, with the remainder being Fe and unavoidable impurities.

[0086] The production process includes the following key steps:

[0087] 1) Smelting: The steel tapped from the converter has C=0.12% and P=0.010%. The LF+RH furnace refining process is adopted, the white slag holding time is 36min, and the process control Als=0.055%; RH vacuum degassing treatment is carried out at a vacuum degree below 0.5tor for a vacuum holding time of 35min. The molten steel exiting the station has H=0.8ppm and O=10ppm.

[0088] 2) Continuous casting: The tundish is superheated in a continuous casting furnace at 20°C, and the billet size is 350×430mm;

[0089] 3) Heating: The billet is heated for 440 minutes, the temperature of the heating section is 1240℃, and the temperature of the soaking section is 1250℃;

[0090] 4) Rolling: The 1150 bar and wire rod two-roll reciprocating mill adopts a large reduction for rolling, with the reduction in the first and second passes controlled at 90mm and 85mm respectively; the initial rolling temperature is 1150℃; the final rolling is carried out using a two-roll mill with a final rolling temperature of 780℃.

[0091] 5) Online softening annealing: After the round steel is sawed by the online metal saw, it is directly put into the slow cooling pit from the roller way in high temperature state, and is kept at 650℃ for 24 hours, and the hardness of the round steel is 262HBW.

[0092] 6) Finishing: straightening by the multi-roller straightening machine, and the bending degree q is less than or equal to 2.0mm / m. The round steel is qualified according to the GB / T 4162B level ultrasonic flaw detection, and the magnetic flux leakage is 0.3*20mm.

[0093] 7) Tunnel gas furnace heating and rotary water cooling: the water inlet temperature is 880℃, and the round steel red temperature after water outlet is 80℃.

[0094] 8) Roller bottom type continuous annealing furnace tempering: the round steel is tempered within 3 hours after quenching, kept at 555℃ for 4.5 hours, and air cooled after outlet.

[0095] 9) Round steel head and tail cutting: the round bar head and tail and ultrasonic flaw detection blind area are cut, and the head and tail cutting lengths are each 80±5mm.

[0096] 10) Pressure straightening: q is less than or equal to 1.0mm / m.

[0097] 11) Carriage, roller light, polishing and polishing: the tempering round steel q is less than or equal to 1.0mm / m, the carriage diameter is reduced by 5mm, the diameter size tolerance is (0, -0.40)mm, and the polishing ensures the surface smoothness Ra≤0.6μm.

[0098] 12) Anti-rust packaging: the round steel surface is sprayed with oil-based anti-rust oil, and the rubber band is tied. The waterproof plastic film is wound, the transparent adhesive tape is adhered and sealed, the woven bag is sleeved, and the wood box is protected. It is ensured that the material does not rust within 1 year after production.

[0099] The microstructure of the steel after quenching and tempering produced according to the above method is shown in Figure 2 .

[0100] Example 3

[0101] The chemical composition of the quenching and tempering silver bright steel involved in this example is as follows in terms of weight percentage: C: 0.42%, Si: 0.28%, Mn: 0.77%, P≤0.015%, S≤0.005%, Cr: 1.15%, Ni: 0.05%, Al: 0.050%, N: 0.006%, Cu≤0.20%, Mo: 0.20%, Ti≤0.01%, B: 0.0006%, H: 0.0001%, O≤0.0015%, and the rest is Fe and inevitable impurities.

[0102] The production process includes the following key steps:

[0103] 1) Smelting: C = 0.10%, P = 0.010% in converter tapping, LF + RH furnace refining process, white slag holding time 30 min, process control Als = 0.045%; RH vacuum degassing treatment, under vacuum degree 0.5 tor, vacuum holding time 24 min, outgoing molten steel H = 0.5 ppm, O = 10 ppm;

[0104] 2) Continuous casting: tundish superheat 25°C, continuously cast slab size 350 x 430 mm;

[0105] 3) Heating: cast slab heating time 480 min, heating section temperature 1220°C, soaking section temperature 1230°C;

[0106] 4) Rolling: bar and wire 1150 two-high reciprocating mill adopts large reduction to roll, first and second pass reductions are controlled at 90 mm and 85 mm respectively; breakdown rolling temperature 1130°C; finish rolling is carried out by two-high mill, finish rolling temperature 800°C;

[0107] 5) On-line softening annealing: after round bar is sawn by on-line metal saw, it is directly put into slow cooling pit at high temperature, 650°C for 24 hours, round bar hardness 276 HBW.

[0108] 6) Finishing: multi-roll straightening machine straightening, bending degree q ≤ 2.0 mm / m. According to GB / T 4162, B-grade ultrasonic flaw detection is qualified, magnetic flux leakage 0.3*20 mm flaw detection is qualified.

[0109] 7) Tunnel gas furnace heating, rotary in-water cooling: water inlet temperature 900°C, round bar red temperature after water outlet 110°C.

[0110] 8) Roller hearth continuous annealing furnace tempering: round bar is tempered within 3 hours after quenching, 570°C for 4.5 hours, air cooling after furnace outlet.

[0111] 9) Round bar head and tail cutting: round bar head and tail and ultrasonic flaw detection blind area are cut, head and tail cutting length each 80 ± 5 mm.

[0112] 10) Pressure straightening: q ≤ 1.0 mm / m.

[0113] 11) Car, roller light, polishing, polishing: tempered round bar q ≤ 1.0 mm / m, car diameter is reduced by 5 mm, diameter size tolerance is (0, -0.40) mm, polishing ensures surface smoothness Ra ≤ 0.6 μm.

[0114] 12) Rust-proof packaging: round bar surface is sprayed with oil-based rust-proof oil, and is banded with rubber-covered binding wire. Water-proof plastic film is wound, transparent adhesive tape is used for sealing, woven bag is sleeved, and is packed in wooden box for protection. It is ensured that the material does not rust within 1 year after production.

[0115] The microstructure of the steel produced according to the above method after quenching and tempering is shown in Table 1. Figure 3

[0116] The mechanical properties of the steel of each example in Table 1

[0117]

[0118] The impact energy, grain size, inclusion and fatigue performance test results of each example in Table 2

[0119]

[0120] As can be seen from the data results in Table 1 and Table 2, the quenched and tempered silver bright steel for mechanical transmission shaft produced by the method of the present application has excellent test indexes, low gas content and non-metallic inclusions, and can well meet the use requirements of high strength, high toughness and fatigue life of the mechanical transmission shaft product.

[0121] The above examples are the preferred implementation of the present application, in addition to this, the present application can also be implemented in other ways, without departing from the technical solution concept of the present application, any obvious replacement is within the protection scope of the present application.

[0122] In order to make the ordinary skilled in the art more convenient to understand the improvement of the present application relative to the prior art, some drawings and descriptions of the present application have been simplified, and some other elements have also been omitted in the present application file for the sake of clarity, and the ordinary skilled in the art should realize that these omitted elements can also constitute the content of the present application.​

Claims

1. A method for producing tempered bright silver steel for large-diameter mechanical transmission shafts, characterized in that, This includes the following key steps: S1: In the smelting stage, the steel tapped from the converter has C ≥ 0.08% and P ≤ 0.010%. The LF+RH furnace refining process is adopted. Argon is blown throughout the LF furnace refining process for slag formation and deoxidation. The white slag holding time is ≥ 30 min. The process control Als = 0.040%~0.055%. The RH vacuum degassing treatment is carried out at a vacuum degree below 0.5 tor, and the vacuum holding time is not less than 20 min. The molten steel exiting the station has H ≤ 2 ppm and O ≤ 10 ppm. S2: In the continuous casting stage, full-process protective casting is adopted. The superheat of the tundish in the continuous casting furnace is ≤25℃, and the furnace temperature is ≤35℃. The billet size is 350×430mm. S3: Heating stage, billet heating time 300-480 min, heating section temperature 1190-1240℃, soaking section temperature 1210-1250℃; S4: In the rolling stage, the 1150 bar and wire rod two-roll reciprocating mill adopts large reduction for rolling, and the reduction in the first and second passes is controlled at 90mm and 85mm respectively; the initial rolling temperature is ≥1100℃; the final rolling adopts a two-roll mill, and the final rolling temperature is 780-830℃. S5: In the online softening and annealing stage, after the round steel is cut by the online metal saw, it is directly entered into the slow cooling pit from the roller conveyor at high temperature and kept at 650℃ for 24 hours. The hardness of the round steel is ≤280HBW. S6: In the finishing stage, round steel bars are straightened using a multi-roller straightener, with a curvature q ≤ 2.0 mm / m; S7: The tunnel gas furnace heating and swirling water cooling stage uses rotating bottom rollers, with round steel rotating forward at a speed of 0.006m / s. It uses 25 gas burners for heating, with an inlet water temperature of 880-920℃ and an outlet water temperature of 80-150℃. S8: Tempering stage of roller hearth continuous annealing furnace. The round steel is tempered within 3 hours after quenching. The round steel is evenly distributed on the roller conveyor, one layer of material is laid, and it moves forward at a constant speed. It is heated by natural gas and held at 550-580℃ for 4-5 hours. It is then air-cooled after being taken out of the furnace. S9: Cut off the beginning and end of the round bar and the blind area of ​​ultrasonic flaw detection; S10: Pressure straightening: q≤1.0mm / m; S11: Rolling, grinding, polishing: Tempered round steel q≤1.0mm / m, rolling diameter reduced by 5mm, polishing ensures surface finish Ra≤0.6μm; S12: The round steel is sprayed with oil-based anti-rust oil, bundled with rubber-coated binding wire, wrapped with waterproof plastic film, sealed with transparent tape, then put into a woven bag, and finally placed in a wooden box for protection.

2. The method for producing tempered bright silver steel for large-diameter mechanical transmission shafts as described in claim 1, characterized in that, In the S7 tunnel gas furnace heating and vortex water cooling stage, the vortex water cooling uses 30 evenly distributed high-pressure water nozzles as a cooling water ring, with 6 water rings arranged at 0.3-meter intervals to cool the round steel. The water pressure of a single water ring is 0.5-0.8 MPa.

3. The method for producing tempered bright silver steel for large-diameter mechanical transmission shafts as described in claim 2, characterized in that, In S9, the length of each end of the round bar cut off is 80±5mm.

4. The method for producing tempered bright silver steel for large-diameter mechanical transmission shafts as described in claim 3, characterized in that, The chemical composition of the steel by weight percentage is as follows: C: 0.41%–0.43%, Si: 0.22%–0.32%, Mn: 0.74%–0.79%, P≤0.015%, S≤0.005%, Cr: 1.12%–1.18%, Ni≤0.25%, Al: 0.035%–0.060%, N: 0.004%–0.008%, Cu≤0.20%, Mo: 0.18%–0.22%, Ti≤0.01%, B≤0.0008%, H≤0.0002%, O≤0.0015%, with the remainder being Fe and essential impurities.

5. The method for producing tempered bright silver steel for large-diameter mechanical transmission shafts as described in claim 4, characterized in that, The produced 180-250mm diameter quenched and tempered round steel has the following properties at 1 / 4 diameter: tensile strength ≥1000Mpa, yield strength ≥800Mpa, elongation ≥20%, reduction of area ≥55%, AKV2 ≥27J at -40℃, straightness ≤1.0mm / m, grain size ≥6 grade, inclusion grades A, B, C, and D are all ≤1.5 grade, and tempered sorbite ≥90%. It can be directly machined into high-strength, high-fatigue-life mechanical transmission shafts with a diameter of 180-250mm, and the fatigue limit reaches over 450MPa.

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