High-strength low-alloy steel and method for manufacturing same, and automobile component

Abstract

The present application relates to the technical field of steel metallurgy, in particular to a high carbon steel, a preparation method thereof and an automobile part. The high carbon steel provided by the present application is designed by alloying molybdenum, micro-chromium and micro-titanium, which significantly improves the hardenability of the steel J5~J10. Meanwhile, by limiting the content of B to be relatively low, the fluctuation of the hardenability and the quenching crack caused by the excessively high hardenability are avoided. On the other hand, by adopting a reasonable preparation method, the TiN inclusions are effectively inhibited, and the formation of the net ferrite is inhibited through the controlled cooling process, while the hardenability requirement of the steel is met. Tests show that the high carbon steel has the advantages of high hardenability, fine grains and high cleanliness. The hardenability of J3~J7 does not decrease significantly, and the hardenability of J10 is still high, which is 36~38 HRC. Meanwhile, due to the reasonable design of the controlled cooling process, the microstructure of the steel is uniform, the hardness is controlled to be 180~200 HBW, and the processing requirements of the customers are well met.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, specifically to a high-carbon steel, its preparation method, and automotive parts. Background Technology

[0002] The front axle is a critical core component of automobiles, classified as a safety part. China's annual steel consumption is approximately 600,000 tons, primarily consisting of 45 steel, 50 steel, 40Cr, 42CrMo, and bainitic non-quenched and tempered steel, with high-carbon steel accounting for over 30%. With the introduction of new front axle standards, the microstructure at J10 is required to meet Level 4 requirements of GB / T 13320. Currently, even meeting Level 4 requirements for hardenability at J5 with high-carbon steel is extremely difficult, with a hardenability depth typically around 4 mm. Hardenability at J5, J7, and J9 decreases significantly, making it difficult to meet the steel requirements for front axles. While many elemental compositions can improve hardenability, their effects are often highly variable, easily leading to excessively high hardenability causing poor machinability or making it difficult to achieve a suitable range of high hardenability. Summary of the Invention

[0003] In view of this, the technical problem to be solved by the present invention is to provide a high-quality carbon steel, its preparation method and automotive parts. The high-quality carbon steel provided by the present invention has high hardenability from J3 to J10 and can meet the processing requirements. Moreover, the hardenability has small fluctuations and good stability.

[0004] This invention provides a high-quality carbon steel, comprising, by weight percentage:

[0005] C: 0.48%~0.53%; Si: 0.17%~0.37%; Mn: 0.70%~0.80%; Cr: 0.15%~0.25%; P: 0~0.020%; S: 0.015%~0.030%; B :0~0.0008%; Mo: 0.05%~0.10%; Ti: 0.020%~0.040%; Ni: 0~0.20%; Cu: 0~0.20%; Al: 0.020%~0.060%; O: 0~15 ppm; N: 0~40 ppm; the balance is Fe.

[0006] The Cr content of this invention is 0.15%~0.25%, preferably 0.20%~0.25%, more preferably 0.21%~0.24%; the Mo content is 0.05%~0.10%, preferably 0.05%~0.08%, more preferably 0.05%~0.07%. Mo is a strong hardenability-enhancing element, especially for core hardness. Excessive Mo will significantly increase core hardness, leading to insufficient core toughness in the forging. If the core hardness at J10 exceeds 40 HRC, brittle fracture is likely, reducing the fatigue life of the steel. Insufficient Mo results in insufficient core hardness, with the core hardness at J10 being less than 32, which is insufficient to support the strength requirements of the steel and also easily leads to fatigue wear. This invention significantly improves the hardenability of high-carbon steel at J5-J10 through the rational design of added elements and their contents. The reasonable Mo and Cr contents enhance the hardenability of the steel, and the simultaneous addition of both elements strongly inhibits the transformation of supercooled austenite to pearlite, giving the steel better toughness.

[0007] The mass content of Ti in this invention is 0.020%~0.040%, preferably 0.020%~0.030%. The effect of Ti on hardenability in this invention shows a trend of first increasing and then decreasing. When Ti ≥ 0.045%, it does not significantly improve the hardenability of steel, but instead reduces it.

[0008] The mass content of boron in this invention is 0~0.0008%, preferably 0~0.0006%. While the boron content can significantly improve the hardenability of steel, it exhibits instability. This is mainly because the effective boron content in steel is difficult to control stably. The higher the boron content, the greater the fluctuation in its impact on hardenability, making it easy for the hardenability fluctuation range to become too large and difficult to guarantee the performance stability of the final product. Therefore, the boron content is controlled to be no greater than 0.0008% to avoid quenching cracks caused by excessively high hardenability.

[0009] The nitrogen content in the present invention is 0~40 ppm. During the preparation of the high-carbon steel described in this invention, by controlling the nitrogen content in the steel to ≤40 ppm, the final N content in the steel is also ≤40 ppm, effectively suppressing TiN inclusions, inhibiting the formation of network ferrite through controlled cooling, and simultaneously meeting the hardenability requirements of the steel.

[0010] In some embodiments of the present invention, the high-carbon steel of the present invention comprises, by weight percentage: C: 0.50%~0.52%; Si: 0.20%~0.30%; Mn: 0.74%~0.78%; Cr: 0.20%~0.25%; P: 0~0.015%; S: 0.015%~0.025%; B: 0~0.0006%; Mo: 0.05%~0.08%; Ti: 0.020%~0.030%; Ni: 0~0.20%; Cu: 0~0.20%; Al: 0.020%~0.045%; O: 0~15 ppm; N: 0~40 ppm; with the balance being Fe.

[0011] In other embodiments of the present invention, the high-carbon steel of the present invention comprises, by weight percentage: C: 0.50%~0.52%; Si: 0.23%~0.26%; Mn: 0.74%~0.77%; Cr: 0.21%~0.24%; P: 0.011%~0.014%; S: 0.018%~0.022%; B: 0%~0.0005%; Mo: 0.05%~0.07%; Ti: 0.023%~0.027%; Ni: 0.02%~0.03%; Cu: 0.02%~0.03%; Al: 0.025%~0.030%; O: 0~15 ppm; N: 0~40 ppm; with the balance being Fe.

[0012] The present invention also provides a method for preparing high-carbon steel according to any of the above technical solutions, comprising the following steps: smelting, continuous casting, rolling and cooling the raw materials in sequence to obtain the high-carbon steel.

[0013] This invention first smelts the raw materials; the smelting process involves sequentially performing converter smelting, LF refining, and RH vacuum treatment. The process parameters for converter smelting are as follows: the proportion of molten iron added in the converter is above 90%; the molten iron temperature is controlled at ≥1300℃ during the converter smelting process; the converter smelting ends with C=0.12 wt%~0.16 wt% and P=0.008 wt%~0.013 wt%, and the tapping temperature is 1620℃~1660℃. After converter smelting, LF refining is performed; specifically, LF refining enhances slag formation and desulfurization to control the sulfur content below 0.005 wt%. Finally, before tapping the LF-refined steel, high-titanium ferrophosphate (titanium content ≥70 wt%) and pure sulfur wire (sulfur content ≥90 wt%) are fed in. After LF refining, the present invention performs RH vacuum treatment; the RH vacuum treatment specifically involves maintaining the molten steel after LF refining at a high vacuum of ≤67 Pa for 16 min to 20 min, followed by soft argon blowing, wherein the argon blowing flow rate is 1200 NL / min to 1400 NL / min, and the soft argon blowing time is ≥20 min, preferably 23 min to 28 min; wherein maintaining a high vacuum ensures the gas content in the steel, and soft argon blowing ensures that inclusions are fully floated and removed.

[0014] After smelting, the present invention performs continuous casting. The process parameters for continuous casting of the present invention are as follows: the tundish temperature is controlled at 1500℃~1520℃; the casting speed is 0.50 m / min~0.60 m / min, preferably 0.53 m / min~0.58 m / min; the current intensity of the electromagnetic stirring in the crystallizer is 550 A~600 A and the current frequency is 2 Hz~3 Hz, the current intensity of the electromagnetic stirring at the end is 330 A~360 A and the current frequency is 6 Hz~7 Hz; the specific water content is 0.25 L / kg~0.27 L / kg, to ensure the quality of the cast billet.

[0015] After continuous casting, the present invention performs rolling. The rolling process parameters of the present invention are as follows: the soaking temperature is 1170℃~1220℃, preferably 1180℃~1210℃; the soaking time is 2.5 h~4.0 h, preferably 3.0 h~3.5 h; the initial rolling temperature is 1000℃~1100℃, preferably 1030℃~1080℃; and the final rolling temperature is 900℃~980℃, preferably 920℃~960℃.

[0016] After rolling, the steel is cooled. Specifically, the cooling process involves cooling the rolled steel at a rate of 0.8℃ / s to 1.5℃ / s using a six-tank water cooling system. The upper cooling bed temperature is 600℃ to 660℃, and the lower cooling bed temperature is 250℃ to 300℃. After the lower cooling bed, the steel is rapidly collected into a slow cooling pit for heat preservation and slow cooling, ensuring the temperature of the cooled steel is below 100℃, preferably 50℃ to 70℃. In the preparation of the high-carbon steel, this invention controls the nitrogen content of the steel to ≤40 ppm, which not only effectively suppresses TiN inclusions but also inhibits the formation of network ferrite through controlled cooling, while simultaneously meeting the hardenability requirements of the steel.

[0017] This invention also provides automotive parts made of the high-carbon steel described in any of the above-described technical solutions or the high-carbon steel obtained by the preparation method described in any of the above-described technical solutions. This invention does not particularly limit the automotive parts, but includes, but is not limited to, automotive front axles.

[0018] This invention provides a high-quality carbon steel, its preparation method, and automotive parts. The high-quality carbon steel provided by this invention employs a design involving molybdenum and micro-chromium / micro-titanium alloying, significantly improving the hardenability of steels J5-J10. Simultaneously, by limiting the boron content to a low level, it avoids excessive fluctuations in hardenability and quenching cracks caused by excessively high hardenability. Furthermore, by using a reasonable preparation method, the nitrogen content [N] in the steel is controlled to ≤40 ppm, effectively suppressing TiN inclusions. The controlled cooling process also suppresses the formation of network ferrite, while simultaneously meeting the steel's hardenability requirements. Experiments show that the high-quality carbon steel of this invention has advantages such as high hardenability, fine grains, and high cleanliness. Not only does the hardenability of J3-J7 not decrease significantly, but J10 also maintains high hardenability at 36 HRC-38 HRC. Simultaneously, due to the reasonable design of the controlled cooling process, the steel has a uniform microstructure, and the hardness is controlled at 180 HBW-200 HBW, well meeting the customer's processing requirements. Attached Figure Description

[0019] Figure 1 This is a metallographic micrograph of the steel in Example 1 of the present invention. Detailed Implementation

[0020] This invention discloses a high-quality carbon steel, its preparation method, and automotive parts. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments; those skilled in the art will clearly be able to modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0021] The specific embodiment of this invention takes the preparation of Φ120 mm steel as an example. The composition of the steel by mass percentage is as follows: C: 0.50%~0.52%, Si: 0.20%~0.30%, Mn: 0.74%~0.78%, Cr: 0.20%~0.25%, P: ≤0.015%, S: 0.015%~0.025%, B: ≤0.0006%, Ti: 0.022%~0.027%, Mo: 0.05%~0.08%, Ni: ≤0.20%, Cu: ≤0.20%, Al: 0.020%~0.045%, [O] ≤15 ppm, [N] ≤40 ppm, and the remainder is Fe and unavoidable impurities.

[0022] The production process steps are as follows:

[0023] 1) Smelting

[0024] The process employs a converter for smelting, with the proportion of molten iron added being ≥90%, the molten iron temperature being 1300~1400℃, the sulfur content of the molten iron being controlled below 0.020%, and the final tapping temperature of the converter being [C]=0.12~0.16%, [P]=0.008~0.013%, and the tapping temperature being 1620~1660℃.

[0025] LF refining enhances slag formation and desulfurization processes to control sulfur content below 0.005%. High-titanium ferrophosphate (titanium content ≥70%) and pure sulfur wire (sulfur content ≥90%) are fed in before tapping.

[0026] RH vacuum treatment: the steel is held under a vacuum of 50~67 Pa for 16~20 min to ensure the gas content in the steel. After vacuum treatment, the soft argon blowing time is 23~28 min, and the argon blowing flow rate is 1200~1400 NL / min to ensure that the inclusions are fully floated and removed.

[0027] 2) Casting

[0028] The billet is cast using continuous casting. The tundish temperature is controlled at 1500~1520℃. The casting speed for 320 mm×420 mm billets is 0.53~0.58 m / min. The crystallizer is electromagnetically stirred at 550~600 A and 2~3 Hz, and the end electromagnetically stirred at 330~360 A and 6~7 Hz. The specific water content is controlled at 0.25~0.27 L / kg to ensure the quality of the billet.

[0029] 3) Rolling

[0030] The heating furnace is heated to a uniform temperature of 1180~1210℃ for 3.0~3.5 hours, the initial rolling temperature is 1030~1080℃, and the final rolling temperature is 920~960℃. After rolling, the steel is cooled by water through six water tanks at a cooling rate of 1.0~1.3℃ / s. The upper cooling bed temperature is 610~640℃, and the lower cooling bed temperature is 260~320℃. After cooling, the steel is collected in time and placed in a pit for slow cooling. The temperature of the steel leaving the pit after slow cooling is ≤80℃, thus producing hot-rolled round steel.

[0031] The present invention will be further described below with reference to the embodiments:

[0032] Examples 1-3 and Comparative Examples 1-2

[0033] The steels of Examples 1-3 and Comparative Examples 1-2 were prepared according to the chemical compositions shown in Table 1:

[0034] Table 1

[0035]

[0036] The steels of Examples 1-3 and Comparative Examples 1-2 were prepared according to the key process parameters of smelting and continuous casting shown in Table 2:

[0037] Table 2

[0038]

[0039] The steels of Examples 1-3 and Comparative Examples 1-2 were prepared according to the key process parameters of the rolling process shown in Table 3:

[0040] Table 3

[0041]

[0042] The steels prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to performance tests, and the results are shown in Tables 4-6. Table 4 shows the results of high-magnification microstructure testing, Table 5 shows the results of hardenability testing, and Table 6 shows the results of hardness testing. Additionally, the steel from Example 1 was subjected to metallographic microscopy, and the results are shown in Table 6. Figure 1 As shown, Figure 1 This is a metallographic micrograph of the steel in Example 1 of the present invention.

[0043] Table 4

[0044]

[0045] Table 5

[0046]

[0047] Table 6

[0048]

[0049] From Tables 4-6 and Figure 1 It is evident that the high-carbon steel described in this invention possesses advantages such as high hardenability, fine grain size, and high cleanliness. J3=60~62 HRC, J5=58~60 HRC, and J7=56~58 HRC show no significant decrease in hardenability, and J10=36~38 HRC still maintains high hardenability. Compared to conventional high-carbon steel products, J10 hardenability is improved by approximately 10 HRC. The grain size is no coarser than grade 7.5; gas inclusions and other indicators are well controlled, resulting in high steel cleanliness. Furthermore, due to the rational design of the controlled cooling process, the steel has a uniform microstructure and a hardness controlled at 180~200 HBW, effectively meeting customer processing requirements.

[0050] Comparative Example 1 shows that with high Mo and B content, the hardenability is extremely high, exceeding 40 HRC at J10. The core hardness is high, but the steel has poor toughness and is prone to brittle fracture. The hardness is consistently above 260 HBW, posing certain difficulties for machining.

[0051] As can be seen from Comparative Example 2, under the condition of extremely low Mo content, the decrease at J7 and J10 is obvious. After quenching heat treatment, J10 is difficult to meet the product's requirements for microstructure. The core hardness is low and the supporting effect is not strong, which is not conducive to the strength and wear resistance of the steel.

[0052] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing high-carbon steel, characterized in that, Includes the following steps: The raw materials are sequentially smelted, continuously cast, rolled, and cooled to obtain the high-carbon steel. The cooling process specifically involves cooling the rolled steel at a rate of 0.8℃ / s to 1.5℃ / s, controlling the temperature of the upper cooling bed to be 600℃ to 660℃ and the temperature of the lower cooling bed to be 250℃ to 300℃, and then holding it in place for slow cooling so that the temperature of the cooled steel is below 100℃. The high-carbon steel, by weight percentage, comprises: C: 0.48%~0.53%; Si: 0.17%~0.37%; Mn: 0.70%~0.80%; Cr: 0.15%~0.25%; P: 0~0.020%; S: 0.015%~0.030%; B :0~0.0008%; Mo: 0.05%~0.10%; Ti: 0.020%~0.040%; Ni: 0~0.20%; Cu: 0~0.20%; Al: 0.020%~0.060%; O: 0~15 ppm; N: 0~40 ppm; the balance is Fe.

2. The preparation method according to claim 1, characterized in that, The Cr content is 0.20%~0.25%; The content of Mo is 0.05%~0.08%.

3. The preparation method according to claim 1, characterized in that, The content of Ti is 0.020%~0.030%.

4. The preparation method according to claim 1, characterized in that, The high-carbon steel, by weight percentage, comprises: C: 0.50%~0.52%; Si: 0.20%~0.30%; Mn: 0.74%~0.78%; Cr: 0.20%~0.25%; P: 0~0.015%; S: 0.015%~0.025%; B :0~0.0006%; Mo: 0.05%~0.08%; Ti: 0.020%~0.030%; Ni: 0~0.20%; Cu: 0~0.20%; Al: 0.020%~0.045%; O: 0~15 ppm; N: 0~40 ppm; the balance is Fe.

5. The preparation method according to claim 1, characterized in that, The high-carbon steel, by weight percentage, comprises: C: 0.50%~0.52%; Si: 0.23%~0.26%; Mn: 0.74%~0.77%; Cr: 0.21%~0.24%; P: 0.011%~0.014%; S: 0.018%~0.022%; B: 0 ~0.0005%; Mo: 0.05%~0.07%; Ti: 0.023%~0.027%; Ni: 0.02%~0.03%; Cu: 0.02%~0.03%; Al: 0.025%~0.030%; O: 0~15 ppm; N: 0~40 ppm; the balance is Fe.

6. The preparation method according to claim 1, characterized in that, The smelting process involves sequentially performing converter smelting, LF refining, and RH vacuum treatment. The process parameters for the converter smelting are as follows: the proportion of molten iron added in the converter smelting is more than 90%; the converter smelting uses C=0.12 wt%~0.16 wt% and P=0.008 wt%~0.013 wt% as the endpoint of the converter tapping, and the tapping temperature is 1620℃~1660℃; The RH vacuum treatment specifically involves maintaining the molten steel after LF refining at a high vacuum of ≤67 Pa for 16 min to 20 min, followed by soft argon blowing for ≥20 min.

7. The preparation method according to claim 1, characterized in that, The process parameters for continuous casting are as follows: The intermediate ladle temperature is controlled at 1500℃~1520℃; the casting speed is 0.50 m / min~0.60 m / min; the current intensity of the electromagnetic stirring in the crystallizer is 550 A~600 A and the current frequency is 2 Hz~3 Hz, the current intensity of the electromagnetic stirring at the end is 330 A~360 A and the current frequency is 6 Hz~7 Hz; the specific water content is 0.25 L / kg~0.27 L / kg.

8. The preparation method according to claim 1, characterized in that, The rolling process parameters are as follows: The soaking temperature is 1170℃~1220℃, the soaking time is 2.5 h~4.0 h; the initial rolling temperature is 1000℃~1100℃, and the final rolling temperature is 900℃~980℃.

9. An automotive component, characterized in that, It is made of high-carbon steel obtained by any of the preparation methods described in claims 1 to 8.

Images