A tempered steel for crankshaft, a crankshaft and a production method

Abstract

The application relates to the technical field of metal smelting, and discloses a tempered steel for crankshafts, a crankshaft and a production method, the composition of the tempered steel for crankshafts is optimized, the internal quality of the steel is controlled, and the processing performance and the use performance of the crankshaft are ensured to meet the processing and use performance requirements of the crankshaft while the mechanical properties of the tempered steel for crankshafts are ensured according to the use characteristics of the crankshaft.

Description

Technical Field

[0001] This invention relates to the field of metal smelting technology, and more specifically, to a tempered and quenched crankshaft steel, a crankshaft, and a method for its production. Background Technology

[0002] As one of the core components of an engine, the quality of the crankshaft directly determines the engine's power, reliability, and service life. Because the crankshaft operates in an extremely harsh environment, enduring enormous alternating bending and torsional stresses, high-speed wear, and complex thermal loads, quality control is crucial at every stage, from raw material selection to finished product production. With increasingly fierce competition in the automotive industry, the requirements for steel used in crankshafts are becoming higher, demanding superior product quality. Simultaneously, the design and manufacturing process must consider not only the crankshaft's performance requirements but also subsequent crankshaft machining performance and internal quality properties, aiming to both reduce production costs and improve fatigue life.

[0003] Based on this, the present invention aims to propose a new type of quenched and tempered crankshaft steel, crankshaft, and production method to solve the problem that it is difficult to simultaneously achieve both quality and cost in the prior art. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a quenched and tempered crankshaft steel, a crankshaft, and a production method. The composition of the quenched and tempered crankshaft steel is optimized, the internal quality of the steel is controlled, and combined with the usage characteristics of the crankshaft, the mechanical properties of the quenched and tempered crankshaft steel are ensured, while the processing performance and service performance of the crankshaft are guaranteed to meet the processing and service performance requirements of the crankshaft.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution: A type of quenched and tempered crankshaft steel, by mass percentage, comprises: 0.38-0.45% carbon, 0.20-0.35% silicon, 0.65-0.85% manganese, ≤0.025% sulfur, ≤0.030% phosphorus, 1.05-1.20% chromium, 0.015-0.035% aluminum, ≤0.10% copper, ≤0.10% nickel, 0.20-0.30% molybdenum, with the balance being iron.

[0006] In one embodiment, the steel for quenched and tempered crankshafts contains, by mass percentage, 0.40-0.44% carbon, 0.70-0.80% manganese, 1.10-1.20% chromium, 0.21-0.24% molybdenum, 0.020-0.030% aluminum, 0.0030-0.0070% nitrogen, and 0.008-0.015% sulfur.

[0007] In one embodiment, the overall composition is controlled using carbon equivalent Ceq, where Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, and Ceq is controlled to be 0.80-0.86.

[0008] A method for producing steel for quenched and tempered crankshafts includes a billet smelting and continuous casting process, as follows: The final carbon content of the converter is controlled at 0.10-0.20%, and the final tapping temperature is ≥1595℃. When the steel reaches 1 / 3 of its weight, alloys and slag are added, of which 300kg±10kg of aluminum-iron alloy (40Al-Fe) is added. Slag detection, slag blocking with sliding plate, and steel retention operations are adopted to prevent slag from falling or entangled, and to control the purity of the initial molten steel. During the LF refining process, after the slag is melted, 100±20kg of silicon carbide is gradually added for diffusion deoxidation. Before sampling 1, the mixture is stirred for 4-6 minutes to ensure that the slag is yellowish-white before the initial sample is taken. The sulfur content of LF sample 1 is ≤0.005%. Silicon carbide is used to maintain the slag in the middle and later stages of smelting. The target Al content of LF sample 1 is 0.040%±0.002%. No further adjustments to aluminum are made to prevent excessive Al addition from affecting the purity of the molten steel. During the RH refining process, the high vacuum time of RH control vacuum degree ≤26Pa is ≥20min, and the soft blowing time after vacuum treatment is ≥30min to promote the flotation of fine inclusions. The above-mentioned molten steel is used to make a billet of quenched and tempered crankshaft steel, which is then rolled to obtain quenched and tempered crankshaft steel.

[0009] In one embodiment, the method for producing steel for quenched and tempered crankshafts further includes a billet heating and rolling process, as follows: The temperature of the second heating section and the soaking section is controlled at 1200℃±30℃, and the total heating time of the high temperature section and the soaking section is controlled at 100-200min. In the roughing stage of the continuously cast billet for tempered crankshaft steel, the pass reduction rate and the roller speed of the rolling stage are controlled. The rolling passes are 9, and the pass reduction rate of each pass is ≥15%. The rolling speed of the two-roll reversible roughing mill is reduced by 50% to ensure that the deformation process of each pass in the roughing process penetrates to the core, improves the shrinkage porosity and compositional segregation in the core, and improves the compositional uniformity.

[0010] A crankshaft made of the steel used for the quenched and tempered crankshaft described above.

[0011] A method for manufacturing a crankshaft includes a forging process and a quenching and tempering process, as follows: Forging process: The steel for quenched and tempered crankshafts is sawn into blanks. The sawn blanks are heated in a high-frequency heating furnace at a temperature of 1230±10℃ for 90 seconds. After heating, the blanks are pre-forged and die-forged. After trimming, they are placed in the air to cool naturally to room temperature. Quenching and tempering process: The crankshaft blank is heated in a continuous mesh belt furnace. The quenching temperature is 850℃±10℃ and the quenching heating time is controlled at (1.6±0.2)*D min, where D is the main journal diameter. The tempering temperature is 620±10℃ and the holding time is (2.0±0.2)*D min.

[0012] In summary, the present invention has the following beneficial effects: This invention optimizes the composition of the steel used for quenched and tempered crankshafts, controls the internal quality of the steel, and, in combination with the characteristics of crankshaft use, ensures that the mechanical properties of the steel used for quenched and tempered crankshafts are met, while also ensuring that the crankshaft's processing and performance meet the requirements of crankshaft processing and performance. Detailed Implementation

[0013] The present invention will now be described in detail with reference to the embodiments.

[0014] This invention proposes a quenched and tempered crankshaft steel, whose composition, calculated by mass percentage, includes: 0.38-0.45% carbon, 0.20-0.35% silicon, 0.65-0.85% manganese, ≤0.025% sulfur, ≤0.030% phosphorus, 1.05-1.20% chromium, 0.015-0.035% aluminum, ≤0.10% copper, ≤0.10% nickel, 0.20-0.30% molybdenum, with the balance being iron.

[0015] In this invention, the mechanical properties and microstructure requirements of the steel used for quenched and tempered crankshafts are as follows: Rp0.2 ≥ 950 MPa, Rm ≥ 1080 MPa, A ≥ 14%, Ku2 ≥ 62 J, J5mm: 53-60 HRC, J9mm: 52-60 HRC, J15mm: 50-58 HRC, J25mm: 36-46 HRC; banded grade ≤ 2.5. The performance and microstructure requirements of the crankshaft made from the steel used for quenched and tempered crankshafts of this invention are as follows: Rp0.2 ≥ 700 MPa, Rm ≥ 850 MPa, A ≥ 12%, Ku2 ≥ 75 J, hardness of the main shaft at the critical section: 265-300 HBW, and surface hardness of the journal ≥ 52 HRC.

[0016] Preferably, the composition is further adjusted as follows: carbon content is adjusted to 0.40-0.44%, manganese content to 0.70-0.80%, chromium content to 1.10-1.20%, molybdenum content to 0.21-0.24%, aluminum content to 0.020-0.030%, nitrogen content to 0.0030-0.0070%, and sulfur content to 0.008-0.015%. In this invention, a specific amount of sulfur is added to improve the crankshaft's machinability. This invention controls the sulfur content to 0.008-0.015%. If the sulfur content is too high, it will lead to worsening of steel segregation and increase the probability of crankshaft magnetic indentation failure.

[0017] This invention further optimizes the composition of the steel used for quenched and tempered crankshafts, ensuring its mechanical properties while also guaranteeing its machinability and performance in use.

[0018] More preferably, in order to obtain stable strength, the carbon equivalent Ceq is used to control the overall composition. The carbon equivalent Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, and the Ceq is controlled to be 0.80-0.86.

[0019] In the existing technology, quenched and tempered crankshafts are prone to cracking during quenching. There are many reasons for cracking, among which the main reasons are the poor internal inclusions and compositional uniformity of the steel raw materials used in quenched and tempered crankshafts.

[0020] Based on the composition of the quenched and tempered crankshaft steel of the present invention, the present invention proposes a production method for the quenched and tempered crankshaft steel, including a billet smelting and continuous casting production process, as follows: To improve inclusions in steel, this invention controls the entire process and operation. The converter's final carbon content is controlled at 0.10-0.20%, and the final tapping temperature is ≥1595℃. When the steel reaches 1 / 3 of its weight, alloys and slag are added, with 300kg±10kg of aluminum-iron alloy (40Al-Fe) added. Slag detection, slag blocking using a sliding plate, and steel retention are employed to prevent slag accumulation and entrapment, controlling the initial purity of the molten steel. During the LF refining process, after the slag has melted, 100±20kg of silicon carbide is gradually added for diffusion deoxidation. Stirring is performed for 4-6 minutes before sampling to ensure the initial slag is yellowish-white. The sulfur content of LF sample 1 is ≤0.005%. Silicon carbide is used for slag maintenance in the later stages of smelting. The target Al content for LF sample 1 is 0.040%±0.002%, and subsequent adjustments to aluminum are prohibited to prevent excessive Al addition from affecting the purity of the molten steel. The high vacuum time (RH control vacuum degree ≤26Pa) is ≥20min, and the soft blowing time after vacuum treatment is ≥30min to promote the flotation of fine inclusions. The above molten steel is used to make a billet for quenched and tempered crankshaft steel, which is then rolled to obtain the quenched and tempered crankshaft steel. The remaining smelting process is the same as the existing technology.

[0021] Preferably, the production method of the quenched and tempered crankshaft steel of the present invention further includes a billet heating and rolling process, as follows: To improve the compositional uniformity of the billet, the temperature of the second heating section and the soaking section is controlled at 1200℃±30℃, and the total heating time of the high-temperature section and the soaking section is controlled at 100-200min. To further improve the segregation of quenched and tempered crankshaft steel and prevent severely segregated areas from being exposed on the crankshaft diameter and connecting rod journal surfaces, thus avoiding quenching cracks or magnetic marks leading to scrap, the following measures were taken during the roughing stage of the continuously cast billet for quenched and tempered crankshaft steel: the pass reduction rate and the roller speed during the rolling stage were controlled. The number of rolling passes was reduced from the conventional 11 passes to 9 passes, while ensuring that the pass reduction rate for each pass was ≥15%. The rolling speed of the two-roll reversible roughing mill was reduced to 50% of the conventional speed, ensuring that the deformation process in each pass of the roughing process penetrated to the core, improving the shrinkage porosity and compositional segregation in the core, and improving compositional uniformity. This method yields quenched and tempered crankshaft steel. The remaining heating and rolling processes are the same as existing technologies. The conventional rolling speed is determined according to the process; generally, the conventional rolling speed for each pass is controlled between 1.5 and 3.0 m / s.

[0022] Based on the quenched and tempered crankshaft steel obtained by this invention, this invention also proposes a method for producing crankshafts, including a forging process and a quenching and tempering process, as follows: Forging process: The steel for quenched and tempered crankshafts is sawn into a 380mm long blank and heated in a high-frequency heating furnace at a temperature of 1230±10℃ for 90s. After heating, it is pre-forged and die-forged, trimmed, and then allowed to cool naturally to room temperature in the air.

[0023] Quenching and tempering process: To ensure the service life of the crankshaft, it is necessary to obtain appropriate strength and toughness while preventing cracking during the quenching process. Therefore, a quenching and tempering process is required to treat the crankshaft blank, so that the surface hardness and the hardness of the critical sections of the crankshaft can achieve the appropriate strength and toughness. Quenched and tempered crankshafts are prone to cracking during the quenching process, especially in stress concentration areas, such as the crankshaft parting surface. In addition, due to the uneven material flow during the forging process, areas with severe steel segregation are exposed at the crankshaft parting surface or on the inside of the connecting rod journal, increasing the probability of quenching cracking. In the crankshaft quenching process, quenching temperature, cooling rate, and the selection of cooling medium are the main factors leading to crankshaft quenching cracking. The crankshaft blank is heated in a continuous mesh belt furnace with a quenching temperature of 850℃±10℃ and a quenching heating time controlled at (1.6±0.2)*D (D is the main journal diameter) min. PAG coolant is used for quenching, and the cooling intensity of the coolant is controlled while ensuring the strength of the crankshaft. The tempering temperature is 620±10℃ and the holding time is (2.0±0.2)*D min.

[0024] The technical solution of the present invention will be described below through specific embodiments.

[0025] The components of Examples 1-3 are shown below, the production process of the steel for quenched and tempered crankshafts is shown in Table 1, and the production process of the crankshafts is shown in Table 2.

[0026] Example 1: The composition, by mass percentage, includes: 0.42% carbon, 0.23% silicon, 0.71% manganese, 0.013% phosphorus, 0.013% sulfur, 0.02% copper, 0.02% nickel, 1.14% chromium, 0.22% molybdenum, 0.004% vanadium, 0.028% aluminum, 54 ppm nitrogen, with the balance being iron, and a Ceq of 0.81.

[0027] Example 2: The composition, by mass percentage, includes: 0.42% carbon, 0.28% silicon, 0.77% manganese, 0.009% phosphorus, 0.009% sulfur, 0.01% copper, 0.01% nickel, 1.12% chromium, 0.23% molybdenum, 0.003% vanadium, 0.024% aluminum, 35 ppm nitrogen, with the balance being iron, and a Ceq of 0.82.

[0028] Example 3: The composition, calculated by mass percentage, includes: 0.43% carbon, 0.24% silicon, 0.74% manganese, 0.011% phosphorus, 0.001% sulfur, 0.04% copper, 0.02% nickel, 1.18% chromium, 0.22% molybdenum, 0.006% vanadium, 0.025% aluminum, 65 ppm nitrogen, with the balance being iron, and a Ceq of 0.84.

[0029] Table 1. Production process of tempered crankshaft steel in Examples 1-3

[0030] Table 2 Crankshaft manufacturing processes in Examples 1-3

[0031] The steel properties and microstructure of the quenched and tempered crankshaft steel prepared in Examples 1-3 were tested. The test samples were processed according to the following steps: a sample with a diameter of 40 mm was taken, quenched at 860±10℃, tempered at 600±15℃, and then forged into a blank with a diameter of 32 mm. The blank was then heated at 870±10℃ and held for 40-60 minutes for normalizing, and then quenched at 850±5℃.

[0032] The test results are shown in Table 3.

[0033] Table 3 Steel properties and microstructure

[0034] The crankshafts prepared in Examples 1-3 were tested, and the results are shown in Tables 4-5.

[0035] Table 4. Crankshaft microstructure and properties in Examples 1-3

[0036] Table 5. Crankshaft machining performance and quenching quality in Examples 1-3

[0037] Note: The rated machining capacity of a machining tool is the tool life, defined as Q.

[0038] Crankshaft quenching crack rate: When 10,000 parts are processed and cracked, the crack rate PPM = number of cracks / 10,000.

[0039] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A quenched and tempered crankshaft steel, characterized in that, by mass percentage, its composition comprises: 0.38-0.45% carbon, 0.20-0.35% silicon, 0.65-0.85% manganese, ≤0.025% sulfur, and ≤0.030% phosphorus. 1.05-1.20% chromium, 0.015-0.035% aluminum, ≤0.10% copper, ≤0.10% nickel, 0.20-0.30% molybdenum, balance iron.

2. The quenched and tempered crankshaft steel as described in claim 1, characterized in that, In the steel used for quenched and tempered crankshafts, the carbon content is 0.40-0.44%, the manganese content is 0.70-0.80%, the chromium content is 1.10-1.20%, the molybdenum content is 0.21-0.24%, the aluminum content is 0.020-0.030%, the nitrogen content is 0.0030-0.0070%, and the sulfur content is 0.008-0.015% by mass percentage.

3. The quenched and tempered crankshaft steel as described in claim 1, characterized in that, The overall composition was controlled using carbon equivalent Ceq, where Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, and Ceq was controlled to be 0.80-0.

86.

4. A method for producing steel for quenched and tempered crankshafts, characterized in that, The production of quenched and tempered crankshaft steel as described in any one of claims 1-3 includes a billet smelting and continuous casting production process, as follows: The final carbon content of the converter is controlled at 0.10-0.20%, and the final tapping temperature is ≥1595℃. When the steel reaches 1 / 3 of its weight, alloys and slag are added, of which 300kg±10kg of aluminum-iron alloy (40Al-Fe) is added. Slag detection, slag blocking with sliding plate, and steel retention operations are adopted to prevent slag from falling or entangled, and to control the purity of the initial molten steel. During the LF refining process, after the slag is melted, 100±20kg of silicon carbide is gradually added for diffusion deoxidation. Before sampling 1, the mixture is stirred for 4-6 minutes to ensure that the slag is yellowish-white before the initial sample is taken. The sulfur content of LF sample 1 is ≤0.005%. Silicon carbide is used to maintain the slag in the middle and later stages of smelting. The target Al content of LF sample 1 is 0.040%±0.002%. No further adjustments to aluminum are made to prevent excessive Al addition from affecting the purity of the molten steel. During the RH refining process, the high vacuum time of RH control vacuum degree ≤26Pa is ≥20min, and the soft blowing time after vacuum treatment is ≥30min to promote the flotation of fine inclusions. The above-mentioned molten steel is used to make a billet of quenched and tempered crankshaft steel, which is then rolled to obtain quenched and tempered crankshaft steel.

5. The method for producing quenched and tempered crankshaft steel as described in claim 4, characterized in that, This also includes the billet heating and rolling processes, as follows: The temperature of the second heating section and the soaking section is controlled at 1200℃±30℃, and the total heating time of the high temperature section and the soaking section is controlled at 100-200min. In the roughing stage of the continuously cast billet for tempered crankshaft steel, the pass reduction rate and the roller speed of the rolling stage are controlled. The rolling passes are 9, and the pass reduction rate of each pass is ≥15%. The rolling speed of the two-roll reversible roughing mill is reduced by 50% to ensure that the deformation process of each pass in the roughing process penetrates to the core, improves the shrinkage porosity and compositional segregation in the core, and improves the compositional uniformity.

6. A crankshaft, characterized in that, It is made of the quenched and tempered crankshaft steel as described in any one of claims 1-3.

7. A method for producing a crankshaft, characterized in that, The production of the crankshaft as described in claim 6 includes a forging process and a quenching and tempering process, as follows: Forging process: The steel for quenched and tempered crankshafts is sawn into blanks. The sawn blanks are heated in a high-frequency heating furnace at a temperature of 1230±10℃ for 90 seconds. After heating, the blanks are pre-forged and die-forged. After trimming, they are placed in the air to cool naturally to room temperature. Quenching and tempering process: The crankshaft blank is heated in a continuous mesh belt furnace. The quenching temperature is 850℃±10℃ and the quenching heating time is controlled at (1.6±0.2)*D min, where D is the main journal diameter. The tempering temperature is 620±10℃ and the holding time is (2.0±0.2)*D min.

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