Steel bar for heavy-load automobile crankshaft and production and preparation method of steel bar

Abstract

The invention relates to the technical field of special steel, in particular to a steel bar for a heavy-load automobile crankshaft and a production and preparation method of the steel bar, and the steel bar comprises the following chemical components in percentage by mass: 0.18-0.22% of C, 0.20-0.35% of Si, 0.60-0.90% of Mn, less than or equal to 0.012% of P, less than or equal to 0.005% of S, 0.40-0.60% of Cr, 0.20-0.30% of Mo, 0.15-0.20% of V, 0.01-0.02% of Nb, less than or equal to 0.15% of Cu, less than or equal to 0.25% of Ni and the balance of iron and inevitable impurities. The production and preparation method comprises the steps of converter smelting, LF refining, RH refining, continuous casting, casting blank annealing, heating, rolling process and heat treatment. According to the steel bar for the heavy-load automobile crankshaft, reasonable component design is adopted, and unique converter + LF + RH + continuous casting smelting process and rolling process are matched, so that the prepared steel bar for the heavy-load automobile crankshaft has the advantages that indexes such as mechanical property, impact property, high hardness and high steel purity of a finished steel product of the steel bar for the heavy-load automobile crankshaft are guaranteed; and the surface quality level of the steel bar for the heavy-load automobile crankshaft is guaranteed, and the requirement of a high-end product can be met.

Description

Technical Field

[0001] This invention relates to the field of special steel technology, and in particular to a steel bar for heavy-duty automobile crankshafts and its manufacturing method. Background Technology

[0002] The crankshaft is the power hub and skeleton of a car engine, and a crucial component of the vehicle's transmission system, responsible for converting and transmitting power. It is typically made of high-strength metal materials to ensure sufficient strength and rigidity to withstand enormous torque and loads. The steel used in automotive crankshafts needs to maintain high strength, high corrosion resistance, and other comprehensive performance requirements under high-speed operating conditions. Heavy-duty automotive crankshaft steel, as a vital component of heavy-duty vehicle transmissions, bears high torque loads under high-speed operating conditions.

[0003] The quality of steel used in the crankshaft of heavy-duty trucks directly affects the service life and stability of the vehicle's transmission system, and even the safety of the entire transmission system during operation.

[0004] Therefore, it is essential to develop heavy-duty automotive crankshaft steel bars with excellent mechanical properties, impact resistance, high hardness, and high steel purity to meet the requirements of high-end products. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, the technical problem solved by this invention is to provide a steel bar for heavy-duty automobile crankshafts and its production method. This invention adopts a reasonable composition design, combined with a unique converter + LF + RH + continuous casting smelting process and rolling process, so that the prepared steel bar for heavy-duty automobile crankshafts not only ensures the mechanical properties, impact performance, high hardness and high steel purity of the finished steel, but also ensures the surface quality level of the steel bar for heavy-duty automobile crankshafts, which can meet the requirements of high-end products.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A type of steel bar for heavy-duty automobile crankshafts has the following chemical composition by mass percentage: C: 0.18%–0.22%, Si: 0.20%–0.35%, Mn: 0.60%–0.90%, P: ≤0.012%, S: ≤0.005%, Cr: 0.40%–0.60%, Mo: 0.20%–0.30%, V: 0.15%–0.20%, Nb: 0.01%–0.02%, Cu: ≤0.15%, Ni: ≤0.25%, with the balance being iron and unavoidable impurities.

[0007] A method for producing steel bars for heavy-duty automobile crankshafts, comprising the following steps: converter smelting → LF refining → RH refining → continuous casting → billet annealing → heating → rolling process → heat treatment; The converter smelting process involves using scrap steel and molten iron, with scrap steel accounting for 15-20% and molten iron accounting for 80-85% by mass fraction. Of the scrap steel, heavy scrap accounts for 50-60%, and the remainder is light scrap. Oxygen oxidation is used, resulting in vigorous boiling. The tapping temperature is 1680-1690℃, the oxygen supply time is 12-13 minutes, the final oxygen concentration is 370-380 ppm, and the oxygen consumption per ton of steel is 48-49 mg / L. 3 / t, operating time 30-40min, C at tapping is 0.08-0.10%, P≤0.010%, slag, pre-deoxidizer, and ferroalloy are added when 1 / 4-1 / 3 of the steel is tapped; alloy addition per ton of steel: silicon manganese 8.0-8.1kg / t, high carbon ferrochrome 7.5-8.0kg / t, ferrosilicon 0.45-0.50kg / t, ferromolybdenum 3.5-3.8kg / t, nickel plate 4.5-4.8kg / t, aluminum granules 0.9-1.0kg / t, coke butadiene recarburizer 0.75-0.80kg / t; auxiliary material addition per ton of steel: dolomite 11.0-11.2kg / t, lime 13-14kg / t, limestone 22.0-23.0kg / t, sintered return ore 14.5-15.0kg / t; The LF refining process is as follows: Upon arrival at 1550–1560℃, after electrode heating for 5–7 minutes, a first sample is taken for chemical composition analysis. Alloys are added according to the target chemical composition values, and carbon is increased. Alloys and carbon powder should be added to the argon flow. The furnace clearance is 540–550 mm, slag thickness is 65–75 mm, argon blowing lasts 60–70 minutes, and the operation time is 90–100 minutes. When the molten steel temperature reaches 1620–1630℃, 1–3 kg / t of diffusion deoxidizer is added for diffusion deoxidation. The furnace door is closed for 10–15 minutes. When the molten steel temperature reaches 1650–1660℃, a second sample is taken for chemical composition analysis to confirm the deviation of each chemical element content from the target value. After the second sample is removed, diffusion deoxidizer is added again to continue the process. Slag adjustment and alloy addition: medium-carbon ferromanganese 2.8–2.9 kg / t, low-carbon ferrochrome 2.0–2.2 kg / t, ferrosilicon 0.3–0.32 kg / t, aluminum granules 0.75–0.77 kg / t, ferroniobium 0.37–0.4 kg / t, ferrovanadium (FeV50-B) 1.5–2.0 kg / t; maintain slag basicity pH of 2.5–3.5 and slag viscosity of 1.0–1.20 Pa·s, while continuing white slag smelting for 20–30 min. When the molten steel temperature reaches 1590–1600℃, perform final deoxidation and feed aluminum wire at 0.32–0.33 kg / t. After wire feeding, start static argon blowing and maintain for 10–20 min; add auxiliary materials: active lime 6.0–6.1 kg / t. The RH refining process is as follows: Arrival temperature 1645–1655℃; argon pressure controlled at 0.1–0.3 MPa before vacuum pump startup, ensuring the molten steel is not exposed by slight slag movement; timing begins when vacuum reaches 100–110 Pa, held for 10–15 minutes; net clearance 490–510 mm; slag thickness 65–75 mm; operation time 40–44 minutes. Alloy addition: aluminum granules 0.25–0.27 kg / t; simultaneously, argon pressure adjusted to 0.3–0.5 MPa; after static argon blowing, silicon-calcium alloy cored wire 1.1–1.2 meters / t is added. The continuous casting process is as follows: tundish temperature 1545–1555℃, casting speed 0.46–0.47 m / min, tundish H control ≤1.5 ppm, crystallizer electromagnetic stirring current 420–450 A, end electromagnetic stirring current 330–340 A, frequency 8–8.2 Hz; full-process protective casting, using medium carbon steel protective slag in the crystallizer; lightly pressing the 2nd, 3rd, and 4th rolls, with reductions of 3–3.2 mm, 4–4.2 mm, and 4–4.2 mm respectively; billet cutting is achieved through a combination of automatic and manual methods, with the continuously cast billet placed on a red steel base, and two heats of billet pressed on top to ensure flatness, with a holding time of 42–48 hours. The billet annealing process involves heating the billet to 600-620℃ for 2-2.5 hours, holding it at that temperature for 32-34 hours, and then allowing it to cool naturally in the furnace to ≤200℃ before tapping it out.

[0008] The heating process is as follows: the furnace inlet temperature is 450℃~500℃; the preheating section temperature is required to be 660℃~700℃ for 1.4~1.6h; the second heating section temperature is required to be 870℃~880℃ for 1.5~1.6h; the first heating section temperature is required to be 1210℃~1225℃ for 1.4~1.5h; the soaking section temperature is required to be 1200℃~1220℃ for 1.4~1.5h; and the tapping temperature is 1180℃~1210℃.

[0009] The rolling process includes primary rolling and continuous rolling; the primary rolling is as follows: a 1150mm BD primary rolling mill is used for billet preparation, and 7 passes are rolled. The rolling reduction is as follows: Turning the steel → Pass 1: Width 370-370.2mm, Height 405-405.2mm, Reduction 70-70.2mm → Pass 2: Width 390-390.2mm, Height 325-325.2mm, Reduction 80-80.2mm → Turning the steel → Pass 3: Width 348-348.2mm, Height 315-315.2mm, Reduction 74-74.2mm → Pass 4: Width 372 ... 72.2mm, height 234~234.2mm, reduction 81~81.2mm → turn over → 5th pass width 263~263.2mm, height 280~280.1mm, reduction 92~92.2mm → 6th pass width 272~272.2mm, height 250~250.2mm, reduction 30~30.2mm → turn over → 7th pass width 263~263.2mm, height 232~232.2mm, reduction 38~38.2mm, passing through 3 850mm rolling mills, and then entering a 9-stand continuous rolling mill for rolling; The continuous rolling process is as follows: The intermediate bar is rolled into a billet with a height of 268–268.2 mm and a width of 232–232.2 mm; the first pass has a height of 229–229.2 mm and a width of 245–245.2 mm, with a rolling speed of 0.27–0.272 m / s; the second pass has a height of 192–192.2 mm and a width of 235–235.1 mm, with a rolling speed of 0.31–0.312 m / s; the third pass has a height of 200–200.2 mm and a width of 195–195.2 mm, with a rolling speed of 0.35–0.352 m / s; the fourth pass has a height of 169–169.2 mm and a width of 208–208.2 mm, with a rolling speed of 0.358–0.360 m / s; and the fifth pass has a height of 177–177.2 mm. The first pass has a height of 180–180.2 mm and a width of 192–192.2 mm, with a rolling speed of 0.446–0.448 m / s; the 6th pass has a height of 140–140.2 mm and a width of 192–192.2 mm, with a rolling speed of 0.537–0.539 m / s; the 7th pass has a height of 150–150.2 mm and a width of 150–150.2 mm, with a rolling speed of 0.629–0.631 m / s; the 8th pass has a height of 114.5–114.7 mm and a width of 157.8–158 mm, with a rolling speed of 0.759–0.761 m / s; the 9th pass has a height of 126.702–126.722 mm and a width of 126.702–126.722 mm, with a rolling speed of 0.914–0.916 m / s; the finished product is produced after the 9th pass.

[0010] The heat treatment includes the following steps: 1) Quenching: 840~860℃, hold for 0.6~0.8h, oil cooling; 2) Tempering: 190~210℃, hold for 1.9~2.1h, then air cool.

[0011] In the converter smelting process, the dolomite particle size is 20-70 mm, the lime particle size is 10-70 mm, and the limestone particle size is 10-70 mm.

[0012] In the LF refining process, quicklime with a particle size of 10-70 mm is used for final deoxidation and then fed to aluminum wire with a diameter of φ10-16 mm.

[0013] In the RH refining process, silicon-calcium alloy cored wire with a diameter of 12-14 mm is added.

[0014] Compared with the prior art, the beneficial effects of the present invention are: This invention utilizes a multi-element synergy of C-Si-Mn-Cr-Mo-V-Nb and controlled rolling and cooling to ensure that the finished product possesses high hardness and strength while maintaining toughness, meeting the comprehensive requirements of heavy-duty crankshafts for high strength, toughness, and wear resistance. Low P, S, and inclusion modification reduce the brittle transition temperature and inclusion stress concentration, and combined with a fine-grained structure, significantly improves impact toughness and fatigue resistance at both room and low temperatures, adapting to high-load alternating conditions. Employing LF+RH dual refining, full-process protective casting, and white slag retention significantly reduces the content of O, N, H, and inclusions, forming a clean steel matrix, reducing the risk of pitting corrosion and intergranular failure, and improving service reliability. The optimized continuous casting-billet annealing-heating-rolling process effectively suppresses surface cracks, scale, and dimensional fluctuations, achieving good surface finish and flatness, which is beneficial for subsequent processing and assembly. Detailed Implementation

[0015] The specific embodiments of the present invention will be further described below: Example 1: The chemical composition of a heavy-duty automobile crankshaft steel bar in this embodiment, by mass percentage, is as follows: C: 0.19%, Si: 0.25%, Mn: 0.72%, P: 0.010%, S: 0.004%, Cr: 0.52%, Mo: 0.25%, V: 0.17%, Nb: 0.014%, Cu: 0.05%, Ni: 0.05%, balance being iron and unavoidable impurities.

[0016] A method for producing steel bars for heavy-duty automobile crankshafts includes the following steps: converter smelting → LF refining → RH refining → continuous casting → billet annealing → heating → rolling process → heat treatment.

[0017] in: (1) Converter smelting is as follows: The steelmaking process uses scrap steel and molten iron. By mass fraction, scrap steel accounts for 17% and molten iron accounts for 83%. Of the scrap steel, heavy scrap steel accounts for 55%, and the remainder is light scrap steel. Oxygen oxidation is used, resulting in vigorous boiling. The tapping temperature is 1684℃, the oxygen supply time is 12 minutes, the final oxygen concentration is 375 ppm, and the oxygen consumption per ton of steel is 48 mg / L. 3 / t, operating time 34min, C at tapping is 0.09%, P is 0.008%, slag, pre-deoxidizer, and ferroalloy are added when 1 / 4 of the steel is tapped. Alloy addition per ton of steel: silicon manganese 8.1kg / t, high-carbon ferrochrome 7.8kg / t, ferrosilicon 0.47kg / t, ferromolybdenum 3.6kg / t, nickel plate 4.6kg / t, aluminum granules 1.0kg / t, coke butadiene recarburizer 0.77kg / t. Auxiliary material addition per ton of steel: dolomite (30mm particle size) 11.0kg / t, lime (35mm particle size) 13kg / t, limestone (35mm particle size) 22.0kg / t, sintered return ore 14.8kg / t.

[0018] (2) LF is refined into: The arrival temperature is 1554℃. After the electrode is heated for 6 minutes, the first sample is taken for chemical composition analysis. Alloy is added according to the target chemical composition value, and carbon is added. Alloy and carbon powder should be added to the argon flow to promote rapid melting and homogenization of alloy and carbon powder. The furnace body clearance is 546mm, the slag thickness is 68mm, argon blowing is 63 minutes, and the operation time is 95 minutes. When the molten steel temperature reaches 1625℃, 1kg / t of diffusion deoxidizer is added for diffusion deoxidation. The furnace door is closed for 12 minutes. When the molten steel temperature reaches 1654℃, a second sample is taken for chemical composition analysis to confirm the deviation of the content of each chemical element from the target value. After the second sample is taken, diffusion deoxidizer is added to continue slag adjustment. The following alloys are added: medium carbon ferromanganese 2.8kg / t, low carbon ferrochrome 2.1kg / t, ferrosilicon 0.3kg / t, aluminum granules 0.76kg / t, ferroniobium 0.38kg / t, and ferrovanadium (FeV50-B) 1.7kg / t. Maintain the slag basicity (pH) at 2.9 and the slag viscosity at 1.10 Pa·s, while continuing white slag smelting for 25 minutes. When the molten steel temperature reaches 1595℃, perform final deoxidation and feed aluminum wire segments (φ12mm) at 0.32 kg / t. After feeding, begin static argon blowing for 15 minutes. Add auxiliary materials: 6.0 kg / t of active lime (35mm particle size).

[0019] (3) RH refining: The arrival temperature was 1648℃. Before starting the vacuum pump, the argon pressure was controlled at 0.1MPa to ensure the molten steel was not exposed due to slight slag movement. Timing began when the vacuum reached 104Pa and was maintained for 12 minutes. The net clearance was 498mm, the slag thickness was 68mm, and the operation time was 42 minutes. Alloy was added: 0.26kg / t of aluminum granules. Simultaneously, the argon pressure was adjusted to 0.3MPa. After static argon blowing, 1.1m / t of silicon-calcium alloy cored wire (φ12mm) was added.

[0020] (4) Continuous casting: The tundish temperature was 1547℃, the casting speed was 0.46m / min, the tundish H control was 1.3ppm, the crystallizer electromagnetic stirring current was 430A, the end electromagnetic stirring current was 333A, and the frequency was 8.0Hz. The entire process was protected during casting, and medium carbon steel protective slag was used in the crystallizer. The second, third, and fourth rollers were lightly pressed in, with reductions of 3mm, 4mm, and 4mm respectively. The billet cutting adopted a combination of automatic and manual methods. The continuous casting billet was placed on a red steel base, and the billet was pressed for two furnaces to ensure flatness. The holding time was 45h.

[0021] (5) Annealing of the cast billet: The billet is heated to 610℃ for 2 hours, held for 32 hours, and then allowed to cool naturally to 150℃ in the furnace before being tapped.

[0022] (6) Heating: The initial temperature in the heating furnace is 480℃; the preheating section temperature is required to be 670℃ for 1.5 hours; the second heating section temperature is required to be 874℃ for 1.5 hours; the first heating section temperature is required to be 1220℃ for 1.5 hours; the soaking section temperature is required to be 1215℃ for 1.5 hours; and the tapping temperature is 1190℃.

[0023] (7) Rolling process: Rolling processes include primary rolling and continuous rolling; The initial rolling process is as follows: A 1150mm BD initial rolling mill is used for billet preparation, with 7 rolling passes. The rolling reduction is as follows: Pass 1: Width 370mm, Height 405mm, Reduction 70.2mm → Pass 2: Width 390.2mm, Height 325mm, Reduction 80.2mm → Pass 3: Width 348mm, Height 315.2mm, Reduction 74mm → Pass 4: Width 372mm, Height 234.2mm, Reduction 81mm → Pass 5: Width 263.2mm, Height 280.1mm, Reduction 92.2mm → Pass 6: Width 272.2mm, Height 250mm, Reduction 30.2mm → Pass 7: Width 263.2mm, Height 232mm, Reduction 38mm. After passing through 3 850mm mills, the billet enters a 9-stand continuous rolling mill for further rolling.

[0024] The continuous rolling process is as follows: The intermediate bar is rolled into a billet with a height of 268 mm and a width of 232 mm. The first pass has a height of 229.2 mm and a width of 245.2 mm, with a rolling speed of 0.27 m / s; the second pass has a height of 192.2 mm and a width of 235.1 mm, with a rolling speed of 0.312 m / s; the third pass has a height of 200 mm and a width of 195 mm, with a rolling speed of 0.35 m / s; the fourth pass has a height of 169.2 mm and a width of 208.2 mm, with a rolling speed of 0.360 m / s; the fifth pass has a height of 1... 77.2mm high, 180mm wide, rolling speed 0.448m / s; 6th pass: 140mm high, 192.2mm wide, rolling speed 0.539m / s; 7th pass: 150mm high, 150mm wide, rolling speed 0.631m / s; 8th pass: 114.7mm high, 157.8mm wide, rolling speed 0.761m / s; 9th pass: 126.722mm high, 126.702mm wide, rolling speed 0.916m / s; finished product after 9th pass.

[0025] (8) Heat treatment: The heat-treated steel is then subjected to heat treatment, which includes the following steps: 1) Quenching: 850℃, hold for 0.7h, oil cooling; 2) Tempering: 200℃, hold for 2.0h, then air cool.

[0026] Example 2: The chemical composition of a heavy-duty automobile crankshaft steel bar in this embodiment, by mass percentage, is as follows: C: 0.20%, Si: 0.26%, Mn: 0.70%, P: 0.010%, S: 0.004%, Cr: 0.50%, Mo: 0.25%, V: 0.17%, Nb: 0.014%, Cu: 0.06%, Ni: 0.05%, balance being iron and unavoidable impurities.

[0027] A method for producing steel bars for heavy-duty automobile crankshafts includes the following steps: converter smelting → LF refining → RH refining → continuous casting → billet annealing → heating → rolling process → heat treatment.

[0028] in: (1) Converter smelting is as follows: The steelmaking process uses scrap steel and molten iron. By mass fraction, scrap steel accounts for 18% and molten iron accounts for 82%. Of the scrap steel, heavy scrap steel accounts for 56%, and the remainder is light scrap steel. Oxygen oxidation is used, resulting in vigorous boiling. The tapping temperature is 1686℃, the oxygen supply time is 13 minutes, the final oxygen concentration is 373 ppm, and the oxygen consumption per ton of steel is 48 mg / L. 3 / t, operating time 34min, C at tapping is 0.08%, P is 0.008%, slag, pre-deoxidizer, and ferroalloy are added when 1 / 4 of the steel is tapped. Alloy addition per ton of steel: silicon manganese 8.1kg / t, high carbon ferrochrome 7.6kg / t, ferrosilicon 0.47kg / t, ferromolybdenum 3.6kg / t, nickel plate 4.6kg / t, aluminum granules 1.0kg / t, coke butadiene recarburizer 0.78kg / t. Auxiliary material addition per ton of steel: dolomite (30mm particle size) 11.2kg / t, lime (35mm particle size) 14kg / t, limestone (35mm particle size) 23.0kg / t, sintered return ore 14.7kg / t.

[0029] (2) LF is refined into: The arrival temperature is 1555℃. After the electrode is heated for 5 minutes, the first sample is taken for chemical composition analysis. Alloy is added according to the target chemical composition value, and carbon is added. Alloy and carbon powder should be added to the argon flow to promote rapid melting and homogenization of alloy and carbon powder. The furnace body clearance is 545mm, the slag thickness is 68mm, argon blowing is 65 minutes, and the operation time is 95 minutes. When the molten steel temperature reaches 1625℃, 1kg / t of diffusion deoxidizer is added for diffusion deoxidation. The furnace door is closed for 12 minutes. When the molten steel temperature reaches 1654℃, the second sample is taken for chemical composition analysis to confirm the deviation of the content of each chemical element from the target value. After the second sample is taken, diffusion deoxidizer is added to continue slag adjustment. The following alloys are added: medium carbon ferromanganese 2.8kg / t, low carbon ferrochrome 2.0kg / t, ferrosilicon 0.3kg / t, aluminum granules 0.76kg / t, ferroniobium 0.38kg / t, and ferrovanadium (FeV50-B) 1.7kg / t. Maintain the slag basicity (pH) at 2.9 and the slag viscosity at 1.1 Pa·s, while continuing white slag smelting for 25 minutes. When the molten steel temperature reaches 1595℃, perform final deoxidation and feed aluminum wire segments (φ12mm) at 0.32 kg / t. After feeding, begin static argon blowing for 13 minutes. Add auxiliary materials: 6.0 kg / t of active lime (35mm particle size).

[0030] (3) RH refining: The arrival temperature was 1648℃. Before starting the vacuum pump, the argon pressure was controlled at 0.1MPa to ensure the molten steel was not exposed due to slight slag movement. Timing began when the vacuum reached 104Pa and was maintained for 12 minutes. The net clearance was 497mm, the slag thickness was 67mm, and the operation time was 42 minutes. Alloy was added: 0.26kg / t of aluminum granules. Simultaneously, the argon pressure was adjusted to 0.3MPa. After static argon blowing, 1.1m / t of silicon-calcium alloy cored wire (φ12mm) was added.

[0031] (4) Continuous casting: The tundish temperature was 1549℃, the casting speed was 0.46m / min, the tundish H control was 1.2ppm, the crystallizer electromagnetic stirring current was 435A, the end electromagnetic stirring current was 334A, and the frequency was 8Hz. The entire casting process was protected, and medium carbon steel protective slag was used in the crystallizer. The 2nd, 3rd, and 4th rolls were lightly pressed in, with reductions of 3mm, 4.1mm, and 4mm, respectively. The billet cutting adopted a combination of automatic and manual methods. The continuous casting billet was placed on a red steel base, and the billet was pressed for two furnaces to ensure flatness. The holding time was 45h.

[0032] (5) Annealing of the cast billet: The billet is heated to 615℃ for 2 hours and held for 33 hours. After holding, the temperature in the furnace is naturally reduced to 150℃ before the steel is tapped.

[0033] (6) Heating: The initial temperature in the heating furnace is 475℃; the preheating section temperature is required to be 680℃ for 1.5 hours; the second heating section temperature is required to be 873℃ for 1.5 hours; the first heating section temperature is required to be 1222℃ for 1.5 hours; the soaking section temperature is required to be 1215℃ for 1.4 hours; and the tapping temperature is 1192℃.

[0034] (7) Rolling process: The rolling process includes primary rolling and continuous rolling; the primary rolling is as follows: a 1150mm BD primary rolling mill is used for billet preparation, and 7 passes are rolled. The rolling reduction is as follows: Turning the steel → Pass 1: Width 370mm, Height 405mm, Reduction 70.2mm → Pass 2: Width 390.2mm, Height 325mm, Reduction 80mm → Turning the steel → Pass 3: Width 348mm, Height 315mm, Reduction 74.2mm → Pass 4 ...48mm, Height 315mm, Reduction 74.2mm → Pass 4: 372mm wide, 234.2mm high, reduction 81.2mm → Turn over → 5th pass, width 263mm, height 280.1mm, reduction 92.2mm → 6th pass, width 272.2mm, height 250mm, reduction 30mm → Turn over → 7th pass, width 263.2mm, height 232mm, reduction 38.2mm, passes through 3 850mm rolling mills, and then enters a 9-stand continuous rolling mill for rolling.

[0035] The continuous rolling process is as follows: The intermediate bar is rolled into a billet with a height of 268 mm and a width of 232 mm. The first pass has a height of 229.2 mm and a width of 245 mm, with a rolling speed of 0.272 m / s; the second pass has a height of 192 mm and a width of 235.1 mm, with a rolling speed of 0.31 m / s; the third pass has a height of 200 mm and a width of 195 mm, with a rolling speed of 0.352 m / s; the fourth pass has a height of 169.2 mm and a width of 208 mm, with a rolling speed of 0.358 m / s; and the fifth pass has a height of 177.2 mm. The first pass has a height of 140 mm, a width of 180 mm, and a rolling speed of 0.448 m / s; the 6th pass has a height of 140 mm, a width of 192 mm, and a rolling speed of 0.539 m / s; the 7th pass has a height of 150.2 mm, a width of 150.2 mm, and a rolling speed of 0.629 m / s; the 8th pass has a height of 114.7 mm, a width of 157.8 mm, and a rolling speed of 0.761 m / s; the 9th pass has a height of 126.702 mm, a width of 126.722 mm, and a rolling speed of 0.916 m / s; the finished product is produced after the 9th pass.

[0036] (8) Heat treatment: The heat-treated steel is then subjected to heat treatment, which includes the following steps: 1) Quenching: 850℃, hold for 0.7h, oil cooling; 2) Tempering: 200℃, hold for 2.0h, then air cool.

[0037] Example 3: The chemical composition of a heavy-duty automobile crankshaft steel bar in this embodiment, by mass percentage, is as follows: C: 0.19%, Si: 0.26%, Mn: 0.75%, P: 0.010%, S: 0.004%, Cr: 0.50%, Mo: 0.25%, V: 0.17%, Nb: 0.012%, Cu: 0.05%, Ni: 0.05%, balance being iron and unavoidable impurities.

[0038] A method for producing steel bars for heavy-duty automobile crankshafts includes the following steps: converter smelting → LF refining → RH refining → continuous casting → billet annealing → heating → rolling process → heat treatment.

[0039] in: (1) Converter smelting is as follows: The converter smelting process involves using scrap steel and molten iron. By mass fraction, scrap steel accounts for 16% and molten iron accounts for 84%. Of the scrap steel, heavy scrap steel accounts for 53%, and the remainder is light scrap steel. Oxygen oxidation is used, resulting in vigorous boiling. The tapping temperature is 1684℃, the oxygen supply time is 12 minutes, the final oxygen concentration is 374 ppm, and the oxygen consumption per ton of steel is 49 mg / L. 3 / t, operating time 35min, C at tapping is 0.08%, P is 0.009%, slag, pre-deoxidizer, and ferroalloy are added when 1 / 4 of the steel is tapped. Alloy addition per ton of steel: silicon manganese 8.1kg / t, high carbon ferrochrome 7.7kg / t, ferrosilicon 0.48kg / t, ferromolybdenum 3.6kg / t, nickel plate 4.7kg / t, aluminum granules 0.9kg / t, coke butadiene recarburizer 0.78kg / t. Auxiliary material addition per ton of steel: dolomite (30mm particle size) 11.0kg / t, lime (35mm particle size) 14kg / t, limestone (35mm particle size) 23.0kg / t, sintered return ore 14.7kg / t.

[0040] (2) LF is refined into: The arrival temperature is 1556℃. After the electrode is heated for 6 minutes, the first sample is taken for chemical composition analysis. Alloy is added according to the target chemical composition value, and carbon is added. Alloy and carbon powder should be added to the argon flow to promote rapid melting and homogenization of alloy and carbon powder. The furnace body clearance is 545mm, the slag thickness is 70mm, argon blowing is 65 minutes, and the operation time is 96 minutes. When the molten steel temperature reaches 1626℃, 1kg / t of diffusion deoxidizer is added for diffusion deoxidation. The furnace door is closed for 12 minutes. When the molten steel temperature reaches 1654℃, the second sample is taken for chemical composition analysis to confirm the deviation of the content of each chemical element from the target value. After the second sample is taken, diffusion deoxidizer is added to continue slag adjustment. The following alloys are added: medium carbon ferromanganese 2.8kg / t, low carbon ferrochrome 2.0kg / t, ferrosilicon 0.32kg / t, aluminum granules 0.75kg / t, ferroniobium 0.4kg / t, and ferrovanadium (FeV50-B) 1.7kg / t. Maintain the slag basicity (pH) at 2.8 and the slag viscosity at 1.1 Pa·s, while continuing white slag smelting for 25 minutes. When the molten steel temperature reaches 1596℃, perform final deoxidation and feed aluminum wire segments (φ12mm) at 0.32 kg / t. After feeding, begin static argon blowing for 14 minutes. Add auxiliary materials: 6.0 kg / t of active lime (35mm particle size).

[0041] (3) RH refining: The arrival temperature was 1648℃. Before starting the vacuum pump, the argon pressure was controlled at 0.1MPa to ensure the molten steel was not exposed due to slight slag movement. Timing began when the vacuum reached 105Pa and was maintained for 12 minutes. The net clearance was 497mm, the slag thickness was 70mm, and the operation time was 42 minutes. Alloy was added: 0.26kg / t of aluminum granules. Simultaneously, the argon pressure was adjusted to 0.4MPa. After static argon blowing, 1.1m / t of silicon-calcium alloy cored wire (φ12mm) was added.

[0042] (4) Continuous casting: The tundish temperature was 1550℃, the casting speed was 0.46m / min, the tundish H control was 1.2ppm, the crystallizer electromagnetic stirring current was 440A, the end electromagnetic stirring current was 336A, and the frequency was 8Hz. The entire casting process was protected, and medium carbon steel protective slag was used for the crystallizer. The second, third, and fourth rollers were lightly pressed in, with reductions of 3mm, 4mm, and 4mm respectively. The billet cutting adopted a combination of automatic and manual methods. The continuous casting billet was placed on a red steel base, and the billet was pressed for two furnaces to ensure flatness. The holding time was 45h.

[0043] (5) Annealing of the cast billet: The billet is heated to 610℃ for 2 hours, held for 33 hours, and then allowed to cool naturally to 150℃ in the furnace before being tapped.

[0044] (6) Heating: The initial temperature in the heating furnace is 480℃; the preheating section temperature is required to be 675℃ for 1.5 hours; the second heating section temperature is required to be 874℃ for 1.5 hours; the first heating section temperature is required to be 1222℃ for 1.4 hours; the soaking section temperature is required to be 1215℃ for 1.4 hours; and the tapping temperature is 1192℃.

[0045] (7) Rolling process: The rolling process includes primary rolling and continuous rolling; the primary rolling is as follows: a 1150mm BD primary rolling mill is used for billet preparation, and 7 passes are rolled. The rolling reduction is as follows: Turning the steel → Pass 1: Width 370mm, Height 405mm, Reduction 70mm → Pass 2: Width 390mm, Height 325mm, Reduction 80mm → Turning the steel → Pass 3: Width 348mm, Height 315mm, Reduction 74mm → Pass 4: Width 370mm, Height 315mm, Reduction 74mm → 2mm wide, 234mm high, 81mm reduction → Turn over → 5th pass, 263mm wide, 280mm high, 92mm reduction → 6th pass, 272.2mm wide, 250.2mm high, 30.2mm reduction → Turn over → 7th pass, 263.2mm wide, 232.2mm high, 38.2mm reduction, passing through 3 850mm rolling mills, and then entering a 9-stand continuous rolling mill for rolling.

[0046] The continuous rolling process is as follows: The intermediate bar is rolled into a billet with a height of 268 mm and a width of 232.2 mm; the first pass has a height of 229.2 mm and a width of 245 mm, with a rolling speed of 0.272 m / s; the second pass has a height of 192.2 mm and a width of 235 mm, with a rolling speed of 0.312 m / s; the third pass has a height of 200 mm and a width of 195.2 mm, with a rolling speed of 0.352 m / s; the fourth pass has a height of 169.2 mm and a width of 208.2 mm, with a rolling speed of 0.358 m / s; the fifth pass has a height of 1... The first pass has a height of 77.2 mm, a width of 180.2 mm, and a rolling speed of 0.448 m / s; the 6th pass has a height of 140 mm, a width of 192.2 mm, and a rolling speed of 0.539 m / s; the 7th pass has a height of 150.2 mm, a width of 150 mm, and a rolling speed of 0.629 m / s; the 8th pass has a height of 114.7 mm, a width of 157.8 mm, and a rolling speed of 0.761 m / s; the 9th pass has a height of 126.702 mm, a width of 126.722 mm, and a rolling speed of 0.914 m / s; the finished product is produced after the 9th pass.

[0047] (8) Heat treatment: The heat-treated steel is then subjected to heat treatment, which includes the following steps: 1) Quenching: 850℃, hold for 0.7h, oil cooling; 2) Tempering: 200℃, hold for 2.0h, then air cool.

[0048] Example 4: The chemical composition of a heavy-duty automobile crankshaft steel bar in this embodiment, by mass percentage, is as follows: C: 0.20%, Si: 0.25%, Mn: 0.72%, P: 0.010%, S: 0.004%, Cr: 0.50%, Mo: 0.25%, V: 0.17%, Nb: 0.014%, Cu: 0.05%, Ni: 0.05%, balance being iron and unavoidable impurities.

[0049] A method for producing steel bars for heavy-duty automobile crankshafts includes the following steps: converter smelting → LF refining → RH refining → continuous casting → billet annealing → heating → rolling process → heat treatment.

[0050] in: (1) Converter smelting is as follows: The steelmaking process uses scrap steel and molten iron. By mass fraction, scrap steel accounts for 17% and molten iron accounts for 82%. Of the scrap steel, heavy scrap steel accounts for 54%, and the remainder is light scrap steel. Oxygen oxidation is used, resulting in vigorous boiling. The tapping temperature is 1685℃, the oxygen supply time is 12 minutes, the final oxygen concentration is 375 ppm, and the oxygen consumption per ton of steel is 48 mg / L. 3 / t, operating time 35min, C at tapping is 0.09%, P is 0.008%, slag, pre-deoxidizer, and ferroalloy are added when 1 / 4 of the steel is tapped. Alloy addition per ton of steel: silicon manganese 8.0kg / t, high carbon ferrochrome 7.7kg / t, ferrosilicon 0.47kg / t, ferromolybdenum 3.6kg / t, nickel plate 4.6kg / t, aluminum granules 0.9kg / t, coke butadiene recarburizer 0.78kg / t. Auxiliary material addition per ton of steel: dolomite (30mm particle size) 11.0kg / t, lime (35mm particle size) 14kg / t, limestone (35mm particle size) 22.0kg / t, sintered return ore 14.7kg / t.

[0051] (2) LF is refined into: The arrival temperature is 1556℃. After the electrode is heated for 6 minutes, the first sample is taken for chemical composition analysis. Alloy is added according to the target chemical composition value, and carbon is added. Alloy and carbon powder should be added to the argon flow to promote rapid melting and homogenization of alloy and carbon powder. The furnace body clearance is 545mm, the slag thickness is 70mm, argon blowing is 65 minutes, and the operation time is 95 minutes. When the molten steel temperature reaches 1625℃, 1kg / t of diffusion deoxidizer is added for diffusion deoxidation. The furnace door is closed for 12 minutes. When the molten steel temperature reaches 1657℃, the second sample is taken for chemical composition analysis to confirm the deviation of the content of each chemical element from the required target value. After the second sample is taken, diffusion deoxidizer is added to continue slag adjustment. The following alloys are added: medium carbon ferromanganese 2.9kg / t, low carbon ferrochrome 2.1kg / t, ferrosilicon 0.32kg / t, aluminum granules 0.76kg / t, ferroniobium 0.38kg / t, and ferrovanadium (FeV50-B) 1.7kg / t. Maintain the slag basicity (pH) at 3.0 and the slag viscosity at 1.1 Pa·s, while continuing white slag smelting for 25 minutes. When the molten steel temperature reaches 1595℃, perform final deoxidation and feed aluminum wire segments (φ12mm) at 0.32 kg / t. After feeding, begin static argon blowing for 14 minutes. Add auxiliary materials: 6.0 kg / t of active lime (35mm particle size).

[0052] (3) RH refining: The arrival temperature is 1650℃. Before starting the vacuum pump, the argon pressure is controlled at 0.1MPa to ensure the molten steel is not exposed and the slag surface is slightly moved. Timing begins when the vacuum reaches 105Pa and is maintained for 11 minutes. The clearance is 500mm, the slag thickness is 70mm, and the operation time is 42 minutes. Alloy is added: 0.26kg / t of aluminum granules. At the same time, the argon pressure is adjusted to 0.4MPa. After static argon blowing is completed, 1.1m / t of silicon-calcium alloy cored wire (φ12mm) is added.

[0053] (4) Continuous casting: The tundish temperature was 1548℃, the casting speed was 0.46m / min, the tundish H control was 1.2ppm, the crystallizer electromagnetic stirring current was 435A, the end electromagnetic stirring current was 335A, and the frequency was 8Hz. The entire casting process was protected, and medium carbon steel protective slag was used in the crystallizer. The 2nd, 3rd, and 4th rolls were lightly pressed in, with reductions of 3.2mm, 4.2mm, and 4mm, respectively. The billet cutting adopted a combination of automatic and manual methods. The continuous casting billet was placed on a red steel base, and the billet was pressed for two furnaces to ensure flatness. The holding time was 45h.

[0054] (5) Annealing of the cast billet: The billet is heated to 612℃ for 2 hours, held for 32 hours, and then allowed to cool naturally to 150℃ in the furnace before being tapped.

[0055] (6) Heating: The initial temperature in the heating furnace is 480℃; the preheating section temperature is required to be 665℃ for 1.5 hours; the second heating section temperature is required to be 875℃ for 1.5 hours; the first heating section temperature is required to be 1221℃ for 1.4 hours; the soaking section temperature is required to be 1218℃ for 1.4 hours; and the tapping temperature is 1190℃.

[0056] (7) Rolling process: The rolling process includes primary rolling and continuous rolling; the primary rolling is as follows: a 1150mm BD primary rolling mill is used for billet preparation, and 7 passes are rolled. The rolling reduction is as follows: Turning the steel → Pass 1: Width 370mm, Height 405mm, Reduction 70.2mm → Pass 2: Width 390mm, Height 325.2mm, Reduction 80mm → Turning the steel → Pass 3: Width 348.2mm, Height 315mm, Reduction 74.2mm → Pass 4 372mm wide, 234mm high, reduction 81.2mm → Turn over → 5th pass: 263.2mm wide, 280mm high, reduction 92.2mm → 6th pass: 272.2mm wide, 250mm high, reduction 30.2mm → Turn over → 7th pass: 263mm wide, 232.2mm high, reduction 38.2mm. After passing through 3 850mm rolling mills, it enters a 9-stand continuous rolling mill for rolling.

[0057] The continuous rolling process is as follows: The intermediate bar is rolled into a billet with a height of 268 mm and a width of 232 mm; the first pass has a height of 229 mm and a width of 245.2 mm, with a rolling speed of 0.27 m / s; the second pass has a height of 192.2 mm and a width of 235.1 mm, with a rolling speed of 0.312 m / s; the third pass has a height of 200 mm and a width of 195.2 mm, with a rolling speed of 0.35 m / s; the fourth pass has a height of 169 mm and a width of 208.2 mm, with a rolling speed of 0.360 m / s; the fifth pass has a height of... 177.2mm high, 180mm wide, rolling speed 0.446m / s; 6th pass: 140mm high, 192mm wide, rolling speed 0.539m / s; 7th pass: 150.2mm high, 150mm wide, rolling speed 0.629m / s; 8th pass: 114.7mm high, 158mm wide, rolling speed 0.759m / s; 9th pass: 126.722mm high, 126.702mm wide, rolling speed 0.916m / s; finished product after 9th pass.

[0058] (8) Heat treatment: The heat-treated steel is then subjected to heat treatment, which includes the following steps: 1) Quenching: 850℃, hold for 0.7h, oil cooling; 2) Tempering: 200℃, hold for 2.0h, then air cool.

[0059] Example 5: The chemical composition of a heavy-duty automobile crankshaft steel bar in this embodiment, by mass percentage, is as follows: C: 0.19%, Si: 0.25%, Mn: 0.73%, P: 0.010%, S: 0.005%, Cr: 0.50%, Mo: 0.25%, V: 0.17%, Nb: 0.015%, Cu: 0.05%, Ni: 0.04%, balance being iron and unavoidable impurities.

[0060] A method for producing steel bars for heavy-duty automobile crankshafts includes the following steps: converter smelting → LF refining → RH refining → continuous casting → billet annealing → heating → rolling process → heat treatment.

[0061] in: (1) Converter smelting is as follows: The steel composition by mass is 17% scrap steel and 83% molten iron. Of the scrap steel, 55% is heavy scrap steel and the remainder is light scrap steel. Oxygen oxidation is used, resulting in vigorous boiling. The tapping temperature is 1685℃, the oxygen supply time is 12 minutes, the final oxygen concentration is 375 ppm, and the oxygen consumption per ton of steel is 48 mg / L. 3 / t, operating time 35min, C at tapping is 0.09%, P is 0.009%, slag, pre-deoxidizer, and ferroalloy are added when 1 / 4 of the steel is tapped. Alloy addition per ton of steel: silicon manganese 8.0kg / t, high carbon ferrochrome 7.7kg / t, ferrosilicon 0.47kg / t, ferromolybdenum 3.6kg / t, nickel plate 4.6kg / t, aluminum granules 0.9kg / t, coke butadiene recarburizer 0.78kg / t. Auxiliary material addition per ton of steel: dolomite (30mm particle size) 11.0kg / t, lime (35mm particle size) 13kg / t, limestone (35mm particle size) 22.0kg / t, sintered return ore 14.7kg / t.

[0062] (2) LF is refined into: The arrival temperature is 1556℃. After the electrode is heated for 5 minutes, the first sample is taken for chemical composition analysis. Alloy is added according to the target chemical composition value, and carbon is added. Alloy and carbon powder should be added to the argon flow to promote rapid melting and homogenization of alloy and carbon powder. The furnace body clearance is 545mm, the slag thickness is 70mm, argon blowing is 64 minutes, and the operation time is 95 minutes. When the molten steel temperature reaches 1626℃, 1kg / t of diffusion deoxidizer is added for diffusion deoxidation. The furnace door is closed for 13 minutes. When the molten steel temperature reaches 1658℃, the second sample is taken for chemical composition analysis to confirm the deviation of the content of each chemical element from the target value. After the second sample is taken, diffusion deoxidizer is added to continue slag adjustment. The following alloys are added: medium carbon ferromanganese 2.8kg / t, low carbon ferrochrome 2.1kg / t, ferrosilicon 0.3kg / t, aluminum granules 0.76kg / t, ferroniobium 0.38kg / t, and ferrovanadium (FeV50-B) 1.8kg / t. Maintain the slag basicity (pH) at 2.8 and the slag viscosity at 1.1 Pa·s, while continuing white slag smelting for 24 minutes. When the molten steel temperature reaches 1593℃, perform final deoxidation and feed aluminum wire segments (φ12mm) at 0.32 kg / t. After feeding, begin static argon blowing for 14 minutes. Add auxiliary materials: 6.0 kg / t of active lime (35mm particle size).

[0063] (3) RH refining: The arrival temperature was 1650℃. Before starting the vacuum pump, the argon pressure was controlled at 0.1MPa to ensure the molten steel was not exposed due to slight slag movement. Timing began when the vacuum reached 104Pa and was maintained for 13 minutes. The net clearance was 497mm, the slag thickness was 70mm, and the operation time was 41 minutes. Alloy was added: 0.26kg / t of aluminum granules. Simultaneously, the argon pressure was adjusted to 0.3MPa. After static argon blowing, 1.1m / t of silicon-calcium alloy cored wire (φ12) was added.

[0064] (4) Continuous casting: The tundish temperature was 1552℃, the casting speed was 0.46m / min, the tundish H control was 1.2ppm, the crystallizer electromagnetic stirring current was 440A, the end electromagnetic stirring current was 335A, and the frequency was 8Hz. The entire casting process was protected, and medium carbon steel protective slag was used in the crystallizer. The 2nd, 3rd, and 4th rollers were lightly pressed in, with reductions of 3mm, 4mm, and 4mm respectively. The billet cutting adopted a combination of automatic and manual methods. The continuous casting billet was placed on a red steel base, and the billet was pressed for two furnaces to ensure flatness. The holding time was 45h.

[0065] (5) Annealing of the cast billet: The billet is heated to 610℃ for 2 hours, held for 33 hours, and then allowed to cool naturally to 160℃ in the furnace before being tapped.

[0066] (6) Heating: The initial temperature in the heating furnace is 480℃; the preheating section temperature is required to be 670℃ for 1.5 hours; the second heating section temperature is required to be 875℃ for 1.5 hours; the first heating section temperature is required to be 1223℃ for 1.5 hours; the soaking section temperature is required to be 1215℃ for 1.5 hours; and the tapping temperature is 1192℃.

[0067] (7) Rolling process: The rolling process includes primary rolling and continuous rolling; the primary rolling is as follows: a 1150mm BD primary rolling mill is used for billet preparation, and 7 passes are rolled. The rolling reduction is as follows: Turning steel → Pass 1: Width 370.2mm, Height 405mm, Reduction 70.2mm → Pass 2: Width 390mm, Height 325.2mm, Reduction 80.2mm → Turning steel → Pass 3: Width 348mm, Height 315.2mm, Reduction 74.2mm → Pass 4: Width 390mm, Height 325.2mm, Reduction 80.2mm → Turning steel → Pass 4: Width 348mm, Height 315.2mm, Reduction 74.2mm → Pass 5: Width 390mm, Height 325.2mm, Reduction 80.2mm → Turning steel → Pass 5: Width 348mm, Height 315.2mm, Reduction 74.2mm → Pass 6: Width 390mm, Height 325.2mm, Reduction 80.2mm → Pass 7 ... The steel is rolled through the following stages: 4th pass (width 372.2mm, height 234mm, reduction 81.2mm) → 5th pass (width 263.2mm, height 280.1mm, reduction 92.2mm) → 6th pass (width 272mm, height 250mm, reduction 30mm) → 7th pass (width 263mm, height 232.2mm, reduction 38mm). After passing through 3 850mm rolling mills, it enters a 9-stand continuous rolling mill for further rolling.

[0068] The continuous rolling process is as follows: The intermediate bar is rolled into a billet with a height of 268 mm and a width of 232 mm; the first pass has a height of 229 mm and a width of 245 mm, with a rolling speed of 0.27 m / s; the second pass has a height of 192.2 mm and a width of 235 mm, with a rolling speed of 0.31 m / s; the third pass has a height of 200 mm and a width of 195 mm, with a rolling speed of 0.352 m / s; the fourth pass has a height of 169.2 mm and a width of 208.2 mm, with a rolling speed of 0.358 m / s; and the fifth pass has a height of 177 mm. The first pass has a height of 2mm, a width of 180.2mm, and a rolling speed of 0.448m / s; the 6th pass has a height of 140.2mm, a width of 192mm, and a rolling speed of 0.539m / s; the 7th pass has a height of 150mm, a width of 150mm, and a rolling speed of 0.631m / s; the 8th pass has a height of 114.5mm, a width of 157.8mm, and a rolling speed of 0.761m / s; the 9th pass has a height of 126.722mm, a width of 126.702mm, and a rolling speed of 0.914m / s; the finished product is produced after the 9th pass.

[0069] (8) Heat treatment: The heat-treated steel is then subjected to heat treatment, which includes the following steps: 1) Quenching: 850℃, hold for 0.7h, oil cooling; 2) Tempering: 200℃, hold for 2.0h, then air cool.

[0070] Inspection results of finished products in Examples 1-5: (1) Low magnification structure of Φ120mm steel (2) Non-metallic inclusions in Φ120mm steel specifications (3) Impact performance of Φ120mm specification The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention. Furthermore, it should be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not describe the various possible combinations separately. In addition, various different embodiments of the present invention can also be arbitrarily combined, as long as they do not violate the spirit of the present invention, they should also be considered as the content disclosed by the present invention.

Claims

1. A steel bar for heavy-duty automobile crankshafts, characterized in that, Its chemical composition, by mass percentage, is as follows: C: 0.18%–0.22%, Si: 0.20%–0.35%, Mn: 0.60%–0.90%, P: ≤0.012%, S: ≤0.005%, Cr: 0.40%–0.60%, Mo: 0.20%–0.30%, V: 0.15%–0.20%, Nb: 0.01%–0.02%, Cu: ≤0.15%, Ni: ≤0.25%, with the balance being iron and unavoidable impurities.

2. A method for producing steel bars for heavy-duty automobile crankshafts as described in claim 1, characterized in that, The specific steps are: converter smelting → LF refining → RH refining → continuous casting → billet annealing → heating → rolling process → heat treatment; The converter smelting process involves using scrap steel and molten iron, with scrap steel accounting for 15-20% and molten iron accounting for 80-85% by mass fraction. Of the scrap steel, heavy scrap accounts for 50-60%, and the remainder is light scrap. Oxygen oxidation is used, resulting in vigorous boiling. The tapping temperature is 1680-1690℃, the oxygen supply time is 12-13 minutes, the final oxygen concentration is 370-380 ppm, and the oxygen consumption per ton of steel is 48-49 mg / L. 3 / t, operation time 30-40min, C at tapping is 0.08-0.10%, P≤0.010%, slag, pre-deoxidizer, and ferroalloy are added when 1 / 4 to 1 / 3 of the steel is tapped; alloy addition per ton of steel: silicon manganese 8.0-8.1kg / t, high carbon ferrochrome 7.5-8.0kg / t, ferrosilicon 0.45-0.50kg / t, ferromolybdenum 3.5-3.8kg / t, nickel plate 4.5-4.8kg / t, aluminum granules 0.9-1.0kg / t, coke butadiene recarburizer 0.75-0.80kg / t; auxiliary material addition per ton of steel: dolomite 11.0-11.2Kg / t, lime 13-14Kg / t, limestone 22.0-23.0kg / t, sintered return ore 14.5-15.0kg / t; The LF refining process is as follows: Upon arrival at 1550–1560℃, after electrode heating for 5–7 minutes, a first sample is taken for chemical composition analysis. Alloys are added according to the target chemical composition values, and carbon is increased. Alloys and carbon powder should be added to the argon flow. The furnace clearance is 540–550 mm, slag thickness is 65–75 mm, argon blowing lasts 60–70 minutes, and the operation time is 90–100 minutes. When the molten steel temperature reaches 1620–1630℃, 1–3 kg / t of diffusion deoxidizer is added for diffusion deoxidation. The furnace door is closed for 10–15 minutes. When the molten steel temperature reaches 1650–1660℃, a second sample is taken for chemical composition analysis to confirm the deviation of each chemical element content from the target value. After the second sample is removed, diffusion deoxidizer is added again to continue the process. Slag adjustment and alloy addition: medium-carbon ferromanganese 2.8–2.9 kg / t, low-carbon ferrochrome 2.0–2.2 kg / t, ferrosilicon 0.3–0.32 kg / t, aluminum granules 0.75–0.77 kg / t, ferroniobium 0.37–0.4 kg / t, ferrovanadium (FeV50-B) 1.5–2.0 kg / t; maintain slag basicity pH of 2.5–3.5 and slag viscosity of 1.0–1.20 Pa·s, while continuing white slag smelting for 20–30 min. When the molten steel temperature reaches 1590–1600℃, perform final deoxidation and feed aluminum wire at 0.32–0.33 kg / t. After wire feeding, start static argon blowing and maintain for 10–20 min; add auxiliary materials: active lime 6.0–6.1 kg / t. The RH refining process is as follows: Arrival temperature 1645–1655℃; argon pressure controlled at 0.1–0.3 MPa before vacuum pump startup, ensuring the molten steel is not exposed by slight slag movement; timing begins when vacuum reaches 100–110 Pa, held for 10–15 minutes; net clearance 490–510 mm; slag thickness 65–75 mm; operation time 40–44 minutes. Alloy addition: aluminum granules 0.25–0.27 kg / t; simultaneously, argon pressure adjusted to 0.3–0.5 MPa; after static argon blowing, silicon-calcium alloy cored wire 1.1–1.2 meters / t is added. The continuous casting process is as follows: tundish temperature 1545–1555℃, casting speed 0.46–0.47 m / min, tundish H control ≤1.5 ppm, crystallizer electromagnetic stirring current 420–450 A, end electromagnetic stirring current 330–340 A, frequency 8–8.2 Hz; full-process protective casting, using medium carbon steel protective slag in the crystallizer; lightly pressing the 2nd, 3rd, and 4th rolls, with reductions of 3–3.2 mm, 4–4.2 mm, and 4–4.2 mm respectively; billet cutting is achieved through a combination of automatic and manual methods, with the continuously cast billet placed on a red steel base, and two heats of billet pressed on top to ensure flatness, with a holding time of 42–48 hours. The billet annealing process involves heating the billet to 600-620℃ for 2-2.5 hours, holding it at that temperature for 32-34 hours, and then allowing it to cool naturally in the furnace to ≤200℃ before tapping it out.

3. The method for producing steel bars for heavy-duty automobile crankshafts according to claim 2, characterized in that, The heating process is as follows: the furnace inlet temperature is 450℃~500℃; the preheating section temperature is required to be 660℃~700℃ for 1.4~1.6h; the second heating section temperature is required to be 870℃~880℃ for 1.5~1.6h; the first heating section temperature is required to be 1210℃~1225℃ for 1.4~1.5h; the soaking section temperature is required to be 1200℃~1220℃ for 1.4~1.5h; and the tapping temperature is 1180℃~1210℃.

4. The method for producing steel bars for heavy-duty automobile crankshafts according to claim 2, characterized in that, The rolling process includes primary rolling and continuous rolling; the primary rolling is as follows: a 1150mm BD primary rolling mill is used for billet preparation, and 7 passes are rolled. The rolling reduction is as follows: Turning the steel → Pass 1: Width 370-370.2mm, Height 405-405.2mm, Reduction 70-70.2mm → Pass 2: Width 390-390.2mm, Height 325-325.2mm, Reduction 80-80.2mm → Turning the steel → Pass 3: Width 348-348.2mm, Height 315-315.2mm, Reduction 74-74.2mm → Pass 4: Width 372 ... 72.2mm, height 234~234.2mm, reduction 81~81.2mm → turn over → 5th pass width 263~263.2mm, height 280~280.1mm, reduction 92~92.2mm → 6th pass width 272~272.2mm, height 250~250.2mm, reduction 30~30.2mm → turn over → 7th pass width 263~263.2mm, height 232~232.2mm, reduction 38~38.2mm, passing through 3 850mm rolling mills, and then entering a 9-stand continuous rolling mill for rolling; The continuous rolling process is as follows: The intermediate bar is rolled into a billet with a height of 268–268.2 mm and a width of 232–232.2 mm; the first pass has a height of 229–229.2 mm and a width of 245–245.2 mm, with a rolling speed of 0.27–0.272 m / s; the second pass has a height of 192–192.2 mm and a width of 235–235.1 mm, with a rolling speed of 0.31–0.312 m / s; the third pass has a height of 200–200.2 mm and a width of 195–195.2 mm, with a rolling speed of 0.35–0.352 m / s; the fourth pass has a height of 169–169.2 mm and a width of 208–208.2 mm, with a rolling speed of 0.358–0.360 m / s; and the fifth pass has a height of 177–177.2 mm. The first pass has a height of 180–180.2 mm and a width of 192–192.2 mm, with a rolling speed of 0.446–0.448 m / s; the 6th pass has a height of 140–140.2 mm and a width of 192–192.2 mm, with a rolling speed of 0.537–0.539 m / s; the 7th pass has a height of 150–150.2 mm and a width of 150–150.2 mm, with a rolling speed of 0.629–0.631 m / s; the 8th pass has a height of 114.5–114.7 mm and a width of 157.8–158 mm, with a rolling speed of 0.759–0.761 m / s; the 9th pass has a height of 126.702–126.722 mm and a width of 126.702–126.722 mm, with a rolling speed of 0.914–0.916 m / s; the finished product is produced after the 9th pass.

5. The method for producing steel bars for heavy-duty automobile crankshafts according to claim 2, characterized in that, The heat treatment includes the following steps: 1) Quenching: 840~860℃, hold for 0.6~0.8h, oil cooling; 2) Tempering: 190~210℃, hold for 1.9~2.1h, then air cool.

6. The method for producing steel bars for heavy-duty automobile crankshafts according to claim 2, characterized in that, In the converter smelting process, the dolomite particle size is 20-70 mm, the lime particle size is 10-70 mm, and the limestone particle size is 10-70 mm.

7. The method for producing steel bars for heavy-duty automobile crankshafts according to claim 2, characterized in that, In the LF refining process, the active lime has a particle size of 10-70 mm, and the aluminum wire used for final deoxidation is φ10-16 mm.

8. The method for producing steel bars for heavy-duty automobile crankshafts according to claim 2, characterized in that, In the RH refining process, silicon-calcium alloy cored wire with a diameter of 12-14 mm is added.

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