A hot press forming steel sheet and a method for manufacturing the same

Abstract

The present application relates to a kind of hot stamping forming steel plate, it is characterized in that, the chemical composition of the steel plate is C:0.14-0.22%, Si:0.25-0.65%, Mn:1.55-1.95%, P≤0.01%, S≤0.01%, Ti:0.03-0.07%, V:0.04-0.08%, Nb:0.02-0.045%, Al:0.22-0.47%, Sn:0.08-0.26% with mass percentage, the balance is Fe and inevitable impurities, and the content of Al and Sn satisfies ([Al] / 27) / ([Sn] / 119)=5.7-13.4, wherein [Al], [Sn] respectively represent the mass percentage content of Al and Sn.

Description

Technical Field

[0001] This invention relates to a hot stamping steel sheet, and more particularly to a hot stamping steel sheet that can be used for stamping and welding bridge housings. The steel sheet has excellent strength and plasticity, low anisotropy, no "earing" defects during the stamping process, and low risk of cracking during the stamping process and later service. Background Technology

[0002] As a load-bearing and force-transmitting component, the axle housing plays a crucial role in ensuring stable vehicle operation. For a long time, axle housings have primarily been manufactured using casting. While cast axle housings offer high strength, they are heavy, making transportation difficult. Furthermore, the manufacturing process is less environmentally friendly, prone to casting defects, resulting in a high scrap rate and a relatively high failure rate. In recent years, another manufacturing method—stamped axle housings—has gained market share. These are lighter, contributing to vehicle weight reduction, and allow for targeted reinforcement of key load-bearing areas. They also have a lower failure rate, and their manufacturing process is more environmentally friendly than casting. Moreover, with improvements in steel properties, stamped axle housings can achieve the same strength levels as cast axle housings.

[0003] In the long run, whether from the perspective of service reliability, lightweighting, or environmental protection, welded bridge housings are likely to replace cast bridge housings.

[0004] As the name suggests, stamped and welded bridge housings require two basic steps: stamping and welding. In actual production, due to the relatively thick steel plates used for the housings, the anisotropy after rolling during the stamping process easily leads to "earing" defects. More seriously, some steel develops cracks after stamping, resulting in a high scrap rate. Alternatively, some stamped and welded bridge housings, due to significant anisotropy, are prone to cracking during subsequent use, leading to service failure. Currently, to avoid anisotropy, the cold rolling reduction rate is usually increased. However, the steel plates used for stamped and welded bridge housings are usually hot-rolled steel plates without a cold rolling step. Anisotropy cannot be effectively controlled during the steel plate preparation stage, making "earing" defects easy to occur during the stamping process and cracking during subsequent service.

[0005] Therefore, how to obtain hot-rolled steel sheets with low anisotropy for hot stamping has become an important technical problem in this field. Summary of the Invention

[0006] This invention proposes a steel plate that not only has excellent strength and plasticity, but also low anisotropy (approaching isotropy), good stamping processing characteristics, does not produce "earing" defects after stamping, does not crack after stamping, and has good reliability during later service.

[0007] The technical solution of the present invention is as follows.

[0008] This invention provides a steel sheet for hot stamping, the chemical composition of which, by mass percentage, is C: 0.14-0.22%, Si: 0.25-0.65%, Mn: 1.55-1.95%, P≤0.01%, S≤0.01%, Ti: 0.03-0.07%, V: 0.04-0.08%, Nb: 0.02-0.045%, Al: 0.22-0.47%, Sn: 0.08-0.26%, and the contents of Al and Sn satisfy ([Al] / 27) / ([Sn] / 119)=5.7-13.4, where [Al] and [Sn] represent the mass percentage contents of Al and Sn, respectively.

[0009] The principles of the component design of this invention will be introduced below.

[0010] C: Carbon plays a solid solution strengthening role, and carbon produces a distribution during hot stamping, which can greatly improve the strength of the bridge shell. However, carbon will cause the weldability of the material to deteriorate. Since stamping and welding of bridge shells are required, the carbon content needs to be determined under the premise of balancing strength and weldability. In this invention, the carbon content is controlled at 0.14-0.22%.

[0011] Si: Similar to carbon, silicon is also an element that improves the strength of steel. However, silicon deteriorates the weldability of the material. Therefore, taking into account both strength and weldability, the silicon content of this invention is controlled at 0.30-0.80%.

[0012] Mn: Similar to carbon and silicon mentioned above, manganese also has the effect of increasing strength and worsening weldability. Therefore, taking into account both strength and weldability, the manganese content of the present invention is controlled at 1.55-1.95%.

[0013] Ti: Titanium reacts with carbon to form titanium carbide, which precipitates as micron- and nano-sized particles during rolling and hot stamping, exerting a strong precipitation strengthening effect and significantly improving the strength of the steel. Titanium carbide also acts as a pinning agent, effectively inhibiting grain growth and contributing to increased strength. However, excessive titanium content can lead to excessive precipitates, affecting the plasticity of the steel plate and making it prone to defects such as cracks during stamping. Considering both the strength and plasticity requirements of the steel, the titanium content in this invention is controlled at 0.03-0.07%.

[0014] V: Vanadium reacts with carbon to form vanadium carbide during rolling and hot stamping, thereby strengthening the steel through dispersed precipitation and significantly improving its strength. However, excessive vanadium content can lead to a deterioration in the plasticity and toughness of the steel plate. Considering the strength, plasticity and toughness requirements of the steel, the vanadium content of this invention is controlled at 0.04-0.08%.

[0015] Nb: During rolling and hot stamping, niobium reacts with carbon to form micron- and nano-sized niobium carbides, which precipitate into the steel matrix, thus significantly improving the strength of the steel plate. At the same time, niobium carbides have a pinning effect, which can effectively inhibit grain growth, thereby ensuring that the steel plate has excellent strength properties. However, if the niobium content is too high, the plasticity of the steel plate will decrease sharply. Taking into account the strength and plasticity requirements of steel, the niobium content of this invention is controlled at 0.02-0.045%.

[0016] P and S: Phosphorus and sulfur are both common impurity elements in steel, and their content is related to many factors such as raw material conditions and smelting level. The presence of phosphorus and sulfur will deteriorate the performance of steel. Therefore, it is necessary to control the content of phosphorus and sulfur at the lowest possible level. This invention controls both phosphorus and sulfur to below 0.01%. However, the lower the content of phosphorus and sulfur, the higher the cost. Therefore, it is necessary to set the lower limit of phosphorus and sulfur content based on actual cost. While balancing performance and cost, the lower limit of phosphorus and sulfur content can usually be set at 0.002%.

[0017] Al, Sn, and ([Al] / 27) / ([Sn] / 119): The addition of aluminum is beneficial for refining the grain size of the steel plate and can effectively promote carbon distribution during hot stamping, which is crucial for ensuring the excellent strength and plasticity of the steel plate. This invention controls the aluminum content to be 0.22-0.47%. Tin is also beneficial for grain refinement, thus significantly ensuring the strength and plasticity of the steel plate. This invention controls the tin content to be 0.08-0.26%. If the content of aluminum and tin is too high, the plasticity effect will be insufficient, and if the content of aluminum and tin is too low, the strength cannot be guaranteed. In addition, the inventors have found that when aluminum and tin are controlled within the above content range, and the content relationship between the two is further controlled in a synergistic way, an unexpectedly low anisotropy can be obtained, thereby ensuring that there is no "earing" phenomenon in the steel plate during the stamping process and that it is not easy to crack during the stamping process and subsequent use. Through extensive experimental research and summarization, the inventors discovered that when ([Al] / 27) / ([Sn] / 119) is controlled within the range of 5.7-13.4, the anisotropy of the steel plate is particularly low, and the stamping performance is outstanding within ±0.1, thus giving the steel plate excellent strength, plasticity and anisotropy.

[0018] While the exact mechanism by which the addition of Al and Sn, and the control of their content relationship, achieve low anisotropy is not fully understood, it is speculated that the control of the amount and content of Al and Sn can effectively prevent the formation of columnar crystals, thereby enabling the steel sheet to obtain an equiaxed crystal structure. Therefore, the final material exhibits very low anisotropy, making it particularly suitable for hot stamping forming.

[0019] This invention achieves high strength, high plasticity, and extremely low anisotropy by optimizing the composition and controlling the content and ratio of Al and Sn. The steel plate exhibits excellent hot stamping performance, and there is no "earing" defect after stamping. It is not easy to crack after stamping, and its stability and reliability are extremely high during subsequent service.

[0020] The steel sheet of the present invention possesses excellent strength, plasticity, and very low anisotropy. Preferably, the hot-stamping steel sheet of the present invention has a yield strength ≥ 800 MPa, a tensile strength ≥ 980 MPa, an elongation ≥ 25%, and an anisotropy Δr within ±0.1, exhibiting excellent hot-stamping performance. Δr is calculated according to GB / T5027-2016, the determination of the plastic strain ratio (r value) of thin sheets and strips of metallic materials, Δr = 1 / 2(r0 + r 90 -2r 45 ).

[0021] Because the steel plate of the present invention has high strength and plasticity, extremely low anisotropy, and outstanding hot stamping performance, it is particularly suitable for manufacturing stamped and welded bridge shells.

[0022] As a non-limiting description, the preparation process of the hot stamping steel sheet of the present invention includes the following steps: S1: melting to obtain the corresponding composition and casting to obtain a slab; S2: homogenizing the slab; S3: hot rolling the homogenized slab; S4: laminar cooling after rolling; S5: coiling.

[0023] Preferably, the homogenization temperature in step S2 is 1200-1280℃, and the homogenization time is 2-5 hours. Suitable homogenization temperature and time have a significant impact on obtaining high-performance steel plates. If the homogenization temperature is too low, sufficient and effective solution cannot be achieved, resulting in decreased strength; if the homogenization temperature is too high, overheating is likely, and significant grain coarsening occurs, leading to insufficient strength; if the homogenization time is too long, the material will also overheat, resulting in grain coarsening and decreased strength; if the homogenization time is too short, sufficient and effective solution cannot be achieved, and sufficient strength cannot be guaranteed.

[0024] As an optional technical solution, the initial rolling temperature of the S3 hot rolling is 950-1000℃, and the final rolling temperature is 850-910℃. A suitable rolling temperature is beneficial for obtaining a good microstructure, thereby further improving the strength and plasticity of the steel plate and reducing anisotropy. If the rolling temperature is too high, the grain coarsening tendency is obvious; if the rolling temperature is too low, the rolling energy consumption is too high and defects such as rolling cracks are easily generated.

[0025] As a further preferred technical solution, the cooling rate of S4 post-rolling laminar flow cooling is controlled at 10-40℃ / s. A suitable cooling rate is beneficial for refining the microstructure and effectively improving the strength, plasticity, and hot stamping forming characteristics of the steel plate.

[0026] As a further preferred technical solution, controlling the winding temperature of S5 within a suitable range is beneficial for obtaining a good balance between strength and plasticity. If the winding temperature is too high, the strength decreases significantly, making it impossible to guarantee the acquisition of high strength characteristics; if the winding temperature is too low, the elongation is insufficient, and the plasticity deteriorates. Therefore, this invention controls the winding temperature at 580-640℃, which is beneficial for obtaining steel plates that combine strength and plasticity.

[0027] As a further preferred technical solution, after S5 is coiled, it is cooled to room temperature to obtain hot-rolled steel sheet, and the cooling method can be air cooling.

[0028] As a further preferred technical solution, since the steel plate of the present invention has high strength and plasticity, extremely low anisotropy, and outstanding hot stamping performance, the hot stamping steel plate prepared by the present invention is particularly suitable for the preparation of stamped and welded bridge shells.

[0029] The present invention has the following beneficial effects.

[0030] This invention, through optimized composition design and by adding specific amounts of Al and Sn and synergistically controlling their content relationship, has discovered a way to obtain high-strength, high-ductility steel sheets, particularly those with extremely low anisotropy. Due to their low anisotropy, the steel sheets obtained by this invention exhibit excellent stamping performance, without "earing" defects or cracks during the stamping process, and demonstrate good stability and high reliability during subsequent use. In summary, the steel sheets of this invention possess the characteristics of high strength, high ductility, low anisotropy, and low cost, making them particularly suitable for the preparation of high-strength, high-ductility stamped welded bridge housings. Detailed Implementation

[0031] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, the following detailed explanation is provided in conjunction with specific experimental examples.

[0032] The molten steel was smelted according to the composition in Table 1, with P and S both controlled at 0.008% ± 0.001%. The molten steel was then continuously cast to obtain a slab with a thickness of 200 mm and a width of 1000 mm. The slab was subjected to a homogenization heat treatment at 1250℃ for 3 hours, and then hot-rolled at an initial rolling temperature of 980℃ and a final rolling temperature of 870℃ to obtain an 18 mm thick hot-rolled slab. The hot-rolled slab was then subjected to laminar flow cooling at a cooling rate of 25℃ / s. The slab was cooled to 600℃ before being coiled, and then air-cooled to room temperature. In Table 1, F1 = ([Al] / 27) / ([Sn] / 119).

[0033] The strength and elongation of the above steel plates were tested, and the anisotropy Δr was also tested. The elongation, tensile strength and yield strength at room temperature were tested according to GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature". The anisotropy Δr was determined according to the requirements of GB / T5027-2016 "Metallic materials - Determination of plastic strain ratio (r value) of thin plates and strips". The test results are recorded in Table 2.

[0034] Table 1 Chemical composition of each steel plate (by mass percentage, balance Fe).

[0035] Table 2 Performance test results of each steel plate.

[0036]

[0037] The following analysis will be conducted in conjunction with Tables 1 and 2.

[0038]

[0039] The steel plates in test numbers 1-6 in Table 1 all meet the composition requirements of this invention. The yield strength, tensile strength, and elongation of the final steel plates all meet the requirements of this invention, exhibiting high strength and high plasticity. The anisotropy Δr of the steel plates in test numbers 1-6 is controlled within ±0.1, demonstrating good stamping characteristics. Since the composition and properties of test numbers 1-6 meet the requirements of this invention, they are embodiments of this invention.

[0040] In Table 1, the steel plates in test numbers 7-12 have at least one of the following compositions: Al, Sn, and ([Al] / 27) / ([Sn] / 119). Consequently, at least one of their strength, elongation, and anisotropy fails to meet the requirements of this invention. Therefore, test numbers 7-12 are all comparative examples of this invention.

[0041] The following is a detailed analysis of each comparative example.

[0042] Comparative Example 7 is a comparative example of Example 1. Under the premise that other components are the same, the Sn content was adjusted. The adjusted Sn content is still within the range required by this invention, but ([Al] / 27) / ([Sn] / 119) no longer meets the requirements of the invention. The test results in Table 2 confirm that the anisotropy Δr of this steel plate is higher than the ±0.1 range required by the invention. It can be expected that the "earing" phenomenon will be severe after stamping, resulting in poor stamping performance and a high risk of stamping cracking or cracking during later use, thus failing to meet the requirements of the invention. Therefore, controlling ([Al] / 27) / ([Sn] / 119) within a suitable range is beneficial for obtaining a low anisotropy Δr in the steel plate, thereby achieving excellent stamping forming characteristics.

[0043] Comparative Example 8 is a comparative example of Example 2. Under the premise that other components are the same, the Al content was adjusted. The adjusted Al content is still within the range required by this invention, but ([Al] / 27) / ([Sn] / 119) no longer meets the requirements of the invention. The test results in Table 2 confirm that the anisotropy Δr of this steel plate is higher than the ±0.1 range required by the invention. It can be expected that the "earing" phenomenon will be severe after stamping, resulting in poor stamping performance and a high risk of stamping cracking or cracking during later use, thus failing to meet the requirements of the invention. Therefore, controlling ([Al] / 27) / ([Sn] / 119) within a suitable range is beneficial for obtaining a low anisotropy Δr in the steel plate, thereby achieving excellent stamping forming characteristics.

[0044] Comparative Example 9 is a comparative example of Example 3. With other components remaining the same, the Sn content was adjusted. The adjusted Sn content exceeded the Sn content range required by this invention and no longer met the requirements. The test results in Table 2 confirm that although the yield strength and tensile strength of the steel plate were improved, the elongation decreased and the plasticity deteriorated, failing to meet the requirements of the invention. Therefore, controlling Sn within a suitable range is beneficial for achieving excellent plasticity in the steel plate.

[0045] Comparative Example 10 is a comparative example of Example 6. With other components remaining the same, the Al content was adjusted. The adjusted Al content exceeded the Al content range required by this invention and no longer met the requirements. The test results in Table 2 confirm that although the yield strength and tensile strength of the steel plate were improved, the elongation decreased and the plasticity deteriorated, failing to meet the requirements of the invention. Therefore, controlling Al within a suitable range is beneficial for achieving excellent plasticity in the steel plate.

[0046] Comparative Example 11 is a comparative example of Example 4. With other components remaining the same, the Al content was adjusted. The adjusted Al content was below the Al content range required by this invention and no longer met the requirements. The test results in Table 2 confirm that although the elongation of the steel plate was improved, the yield strength and tensile strength decreased significantly, failing to meet the requirements of the invention. It is evident that controlling Al within a suitable range is beneficial for achieving excellent strength in the steel plate.

[0047] Comparative Example 12 is a comparative example of Example 5. Under the premise that other components are the same, the Sn content was adjusted. The adjusted Sn content is lower than the Sn content range required by this invention, and ([Al] / 27) / ([Sn] / 119) no longer meets the requirements of the invention. The test results in Table 2 confirm that the yield strength and tensile strength of the steel plate are both reduced and lower than the requirements of the invention. The anisotropy Δr is higher than the ±0.1 range required by the invention. It can be expected that the "earing" phenomenon will be severe after stamping, the stamping performance will be poor, and the risk of stamping cracking or later use cracking will be high, thus failing to meet the requirements of the invention. It is evident that controlling the Sn content and ([Al] / 27) / ([Sn] / 119) within a suitable range is beneficial for obtaining a low anisotropy Δr in the steel plate, thereby obtaining excellent stamping forming characteristics, and is crucial for ensuring the high strength characteristics of the steel plate.

[0048] Through the comparison of the above embodiments and comparative examples, it is easy to confirm that by adding Al and Sn to the steel plate composition and synergistically controlling the relationship between Al and Sn, the present invention helps to ensure that the steel plate has both high strength and high plasticity, and exhibits extremely low anisotropy. Therefore, it is easy to expect that the steel plate will not have "earing" defects during hot stamping, thus demonstrating good hot stamping characteristics. Furthermore, the stamped product is not prone to cracking, and its stability and reliability during use are extremely high, making it particularly suitable for the manufacture of stamped and welded bridge housing products.

[0049] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0050] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A steel sheet for hot stamping, characterized in that, The chemical composition of the steel plate, by mass percentage, is: C: 0.14-0.22%, Si: 0.25-0.65%, Mn: 1.55-1.95%, P≤0.01%, S≤0.01%, Ti: 0.03-0.07%, V: 0.04-0.08%, Nb: 0.02-0.045%, Al: 0.22-0.47%, Sn: 0.08-0.26%, with the balance being Fe and unavoidable impurities. The contents of Al and Sn satisfy ([Al] / 27) / ([Sn] / 119) = 5.7-13.4, where [Al] and [Sn] represent the mass percentage contents of Al and Sn, respectively.

2. The steel sheet for hot stamping according to claim 1, characterized in that, The hot-stamped steel sheet has a yield strength ≥800MPa, a tensile strength ≥980MPa, and an elongation ≥25%.

3. A hot-stamped steel sheet according to any one of claims 1-2, characterized in that, The anisotropy Δr of the hot stamping steel sheet is between ±0.

1.

4. A hot-stamped steel sheet according to any one of claims 1-3, characterized in that, The hot-stamped steel plate is used for the preparation of the welded bridge shell.

5. The manufacturing process of the hot-stamped steel sheet according to any one of claims 1-4, characterized in that, Includes the following steps: S1: Melt to obtain the corresponding composition and cast to obtain a slab; S2: Homogenize the slab; S3: Hot rolling of homogenized slabs; S4: Laminar flow cooling after rolling; S5: Wind-up.

6. The manufacturing process of the hot-stamped steel sheet according to claim 5, characterized in that, The homogenization temperature in S2 is 1200-1280℃, and the homogenization time is 2-5h.

7. The process for preparing hot-stamped steel sheet according to any one of claims 5-6, characterized in that, The initial rolling temperature of the S3 hot rolling is 950-1000℃, and the final rolling temperature is 850-910℃.

8. The manufacturing process of the hot-stamped steel sheet according to any one of claims 5-7, characterized in that, The post-rolling laminar flow cooling of S4 has a cooling rate of 10-40℃ / s.

9. The manufacturing process of the hot-stamped steel sheet according to any one of claims 5-8, characterized in that, The winding temperature of S5 is 580-640℃.

10. The manufacturing process of the hot-stamped steel sheet according to any one of claims 5-9, characterized in that, The hot-stamped steel sheet prepared by the aforementioned process is used for the preparation of welded bridge shells.

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