Steel with a high hole expansion rate and extremely high plasticity and a method for producing the same

Abstract

The present invention provides a steel with a high hole expansion ratio and extremely high plasticity, and a method for manufacturing the same. The steel has the following components, and the weight percentages of the components are: C: 0.10 to 0.35%, Si: 0.8 to 2.0%, Mn: 1.0 to 2.5%, P ≤ 0.02%, S ≤ 0.005%, Al: 0.01 to 0.1%, N ≤ 0.005%, V: 0.10 to 0.50%, O ≤ 0.003%, and the balance is Fe and other inevitable impurities. The steel of the present invention has excellent mechanical properties, shows good harmony among yield strength, tensile strength, elongation and hole expansion ratio, and can be widely applied to components that require complex shapes such as those in commercial vehicles and passenger cars, or other parts that need to be thinned while maintaining high strength.

Description

Technical Field

[0001] The present invention belongs to the field of steel and its manufacturing methods, and particularly relates to steel with a high hole expansion rate and extremely high plasticity, and its manufacturing method.

Background Art

[0002] Background Automobiles play an important role in the development of the national economy. As is widely known, the lightweighting of passenger cars has always been a trend in the industry, and automobile manufacturers are constantly pursuing the lightweighting of the entire vehicle. This includes achieving lightweighting while maintaining high strength in various aspects such as the chassis, body, and seats, and further using other new materials such as aluminum alloys and carbon fibers. Compared with passenger cars, the lightweighting process of commercial vehicles is much slower. Considering that commercial vehicles may be overloaded during actual use, it is necessary to have sufficient margin in their structural design, which affects the lightweighting process of commercial vehicles.

[0003] Chassis parts of commercial vehicles such as fuel tank brackets, battery brackets, gas cylinder brackets, and front under lamp protection are currently produced with low-strength thick-plate ordinary steels such as Q345. Its typical composition includes: C 0.12 - 0.20%, Si 0.20 - 0.55%, Mn 1.20 - 1.60%, P below 0.03%, and S below 0.03%. Furthermore, the forming processes of these parts are relatively complex, some requiring bolt connection and some requiring welding. With the development of lightweighting in commercial vehicles, many users hope to use cold stamping for the integral forming of these parts in commercial vehicles, which reduces procedures and achieves lightweighting. As a result, the requirements for the performance of hot-rolled high-strength steels have increased, and higher elongation and better formability are required while ensuring high strength. When using conventional high-strength steels for stamping parts such as fuel tank brackets, cracks may occur in the large arc parts of the parts, and smooth stamping cannot be achieved. Therefore, the development of new high-strength steels with high tensile strength and excellent formability is desired. For the reasons mentioned above, hot-rolled advanced high-strength steels with various strength levels, characterized by high tensile strength, extremely high elongation, and high hole expansion ratio, have been developed. This steel enables the stamping and forming of complex parts for both passenger cars and commercial vehicles, showing the potential for promising applications and significant development prospects.

[0004] There are already several prior art solutions regarding steels with extremely high plasticity, most of which focus on the field of high-strength cold-rolled steels, but there are also those regarding hot-rolled steels with extremely high plasticity.

[0005] For example, Chinese Patent Application CN104233092A discloses a 780 MPa grade steel with extremely high plasticity, whose composition is designed to have a low carbon content, a high silicon content, and a certain amount of precious alloying elements such as Cr, Mo, and Nb. As a result, the alloy cost is relatively high.

[0006] Chinese Patent Application CN107815593A discloses a steel with low silicon content, high aluminum content, and extremely high plasticity. Its composition has low silicon content, high aluminum content, and a certain amount of precious element Cu. Its production process mainly includes heat treatment for 1 - 3 minutes in the two-phase region, followed by phase transformation in the bainite region to obtain a 780 MPa grade heat-treated steel with extremely high plasticity. However, this heat treatment process cannot be applied to the hot rolling production line.

[0007] Chinese Patent Application CN114107792A discloses a titanium-molybdenum steel with high hole expansion rate and low silicon, which is designed without vanadium in its composition. This steel exhibits high hole expansion characteristics in terms of performance. Since there is no retained austenite in its microstructure, a high level of elongation cannot be obtained.

Summary of the Invention

[0008] An object of the present invention is to provide a steel with a high hole expansion rate and extremely high plasticity and a manufacturing method thereof. The steel of the present invention with a high hole expansion rate and extremely high plasticity has excellent mechanical properties and can achieve a good balance among relatively high yield strength, high tensile strength, extremely high elongation, and hole expansion rate, and can be widely applied to components with complex shapes such as those in commercial vehicles or passenger cars, and other parts that are required to be thinned while maintaining high strength.

[0009] The present invention adopts a relatively economical composition design concept and can be produced on an existing hot rolling production line.

[0010] In a first aspect, the present invention provides a steel having the following components, and the weight percentages of the components are: C: 0.10 - 0.35%, Si: 0.8 - 2.0%, Mn: 1.0 - 2.5%, P ≤ 0.02%, S ≤ 0.005%, Al: 0.01 - 0.1%, N ≤ 0.005%, V: 0.10 - 0.50%, O ≤ 0.003%, and the balance is Fe and other inevitable impurities: When the C content is 0.10 - 0.25% and the V content is 0.10 - 0.30%, the steel has a tensile strength of 780 MPa or more, an elongation of 30% or more, and a hole expansion ratio of 65% or more; and When the C content is 0.25 - 0.35% and the V content is 0.30 - 0.50%, the steel has a tensile strength of 980 MPa or more, an elongation of 25% or more, and a hole expansion ratio of 50% or more.

[0011] Preferably, the above steel further has one or more selected from the group consisting of Mo, Cu, Ni, Cr, Nb, V and B; here, in wt%, Mo ≤ 0.5%, Cu ≤ 0.5%, Ni ≤ 0.5%, Cr ≤ 0.5%, Nb ≤ 0.06%, V ≤ 0.10%, B ≤ 0.001%.

[0012] Preferably, the inevitable impurities contain S at 0.003 wt% or less and N at 0.004 wt% or less.

[0013] Unless otherwise specified, the content of each element in the steel of the present invention means the mass fraction.

[0014] In the composition design of the steel of the present invention with a high hole expansion ratio and extremely high plasticity: Carbon is a basic element in steel and also one of the important elements in the present invention. Carbon can expand the austenite phase region and stabilize austenite. As an interstitial atom in steel, carbon plays an important role in improving the strength of steel, especially having the greatest influence on the yield strength and tensile strength of steel. Furthermore, as an effective element for stabilizing retained austenite, carbon typically needs to be present at a relatively high level. In the present invention, in order to obtain high-strength steel having different levels of tensile strength and a relatively large amount of stable retained austenite, the carbon content must be 0.10% or more. However, the carbon content should not exceed 0.35%. When the carbon content becomes excessive, it is likely to cause an increase in strength, a decrease in elongation, and deterioration of welding performance. Therefore, the carbon content needs to be controlled within 0.10 - 0.35%.

[0015] Silicon is a basic element in steel and also one of the important elements in the present invention. Increasing the silicon content not only enhances the solid solution strengthening effect, but more importantly, has the following two effects. First, it can significantly lower the non-recrystallization temperature of steel and cause dynamic recrystallization in a wide temperature range. Thereby, during actual rolling, the final rolling temperature can be controlled over a wider range, the difference in the microstructure between the transverse and longitudinal directions becomes smaller, the uniformity of the microstructure is improved, and it becomes easier to improve the strength and plasticity of the steel. The second important effect of silicon is to suppress the precipitation of cementite. In the present invention, in order to obtain a microstructure mainly composed of bainite and more stable retained austenite, it is necessary to add a relatively large amount of silicon. This carbide formation suppression effect of silicon becomes significant when the silicon content reaches 0.8% or more. However, if the silicon content in steel is too high, the rolling force load during the actual rolling process becomes too large and does not contribute to stable production. Therefore, the silicon content in the steel of the present invention is controlled within 0.8 - 2.0%.

[0016] Manganese is the most fundamental element in steel and also one of the most important elements in the present invention. Mn expands the austenite phase region, reduces the critical hardening rate of steel, stabilizes austenite, refines grains, and delays the transition from austenite to pearlite. Furthermore, during the heat treatment process, Mn undergoes partitioning and diffuses from bainite to retained austenite, further stabilizing the retained austenite and increasing its content. To achieve these effects, a manganese content of at least 1.0% is required. However, the manganese content should not be too high. If the manganese content exceeds 2.5%, it may lead to segregation in continuous casting slabs and the formation of a large amount of MnS inclusions. Therefore, the manganese content in the steel of the present invention is set to 1.0 - 2.5%.

[0017] Phosphorus is an impurity element in steel. P tends to segregate at grain boundaries. When the P content in steel is relatively high (0.1% or more), Fe2P is formed and precipitates around the grains, reducing the plasticity and toughness of the steel. Therefore, the lower the P content, the better. Generally, when the P content is 0.02% or less, the steel has good performance without increasing the steelmaking cost.

[0018] Sulfur is an impurity element in steel. In steel, typically, S combines with Mn to form MnS inclusions. Especially when the contents of both S and Mn are relatively high, a considerable amount of MnS will be formed in the steel. MnS itself has a certain degree of plasticity. MnS deforms along the rolling direction during the subsequent rolling process, not only reducing the transverse plasticity of the steel but also increasing the structural anisotropy and having an adverse effect on the hole expansion performance. Therefore, the lower the S content in the steel, the better. To minimize the content of MnS, it is necessary to strictly control the S content. The S content in the steel of the present invention is 0.005% or less, preferably 0.003% or less.

[0019] The main functions of aluminum in steel are deoxidation and nitrogen fixation. In the presence of strong carbide-forming elements such as Ti, aluminum mainly acts as a deoxidizer and a grain refiner. In the present invention, aluminum can be used as a conventional additive for deoxidation and nitrogen fixation, or its content can be increased to a certain level to promote the diffusion of carbon into retained austenite. As a general element for deoxidation and grain refinement, its content should be 0.01 - 0.08%. When used to promote the diffusion of carbon into austenite, in order to promote the diffusion of carbon atoms into austenite and play a role in grain refinement, the aluminum content is typically less than 0.1%. When the aluminum content exceeds 1.5%, the effect of promoting the diffusion and enrichment of carbon saturates. Therefore, the aluminum content in the steel of the present invention is 0.01 - 1.5%.

[0020] Nitrogen is an impurity element in the present invention. The lower the nitrogen content, the better. However, nitrogen is an element that cannot be avoided in the steelmaking process. Although in small amounts, nitrogen may combine with strong carbide-forming elements such as Ti to form TiN particles, which has an adverse effect on the performance of the steel. Therefore, in the present invention, the nitrogen content is controlled to be 0.005% or less, preferably 0.004% or less.

[0021] Vanadium is an important element in the present invention. The present invention relates to steel with extremely high plasticity having a high hole expansion rate characteristic. However, typically, the hole expansion rate and the elongation are inversely proportional. In order to achieve a high hole expansion rate on the premise of extremely high plasticity, it is necessary to minimize the difference between the yield strength and the tensile strength and improve the yield strength ratio. Therefore, by combining with an intermediate temperature coil winding process, vanadium is added in a relatively high content in the composition design. Utilizing the fact that the precipitation temperature of vanadium in bainite is relatively low, while forming bainite, nano-sized vanadium carbide is formed in bainite ferrite, thereby improving the yield strength ratio and obtaining advanced high-strength steel with extremely high plasticity having high hole expansion characteristics. Typically, in order to obtain a significant precipitation strengthening effect, a vanadium content of at least 0.10% is required. As the vanadium content increases, its precipitation strengthening effect gradually increases. When the vanadium content increases up to 0.50%, the precipitation strengthening effect of vanadium saturates. Therefore, the vanadium content in the steel of the present invention is 0.10 to 0.50%.

[0022] Oxygen is an inevitable impurity element during the steelmaking process. In the present invention, the oxygen content in the steel can generally be 30 ppm or less after deoxidation, which does not have a significant adverse effect on the performance of the steel plate. The O content in the steel of the present invention is 30 ppm or less.

[0023] Molybdenum is one of the optional additive elements in the present invention. When molybdenum is added to steel, the transformation of pearlite can be significantly delayed, which is beneficial for obtaining a bainite structure. Molybdenum also has the effect of delaying ferrite transformation. When a small amount of molybdenum is contained in the alloy composition, by adding more aluminum element, the ferrite transformation can be accelerated. In addition, molybdenum has strong resistance to welding softening. The main purpose of the present invention is to obtain a microstructure mainly composed of ferrite, bainite and retained austenite. Since high-strength steel is prone to softening after welding, by adding an appropriate amount of molybdenum, the degree of welding softening can be effectively reduced. Considering that molybdenum is expensive, its addition amount needs to be controlled to meet the performance requirements while minimizing the alloy cost as much as possible. Therefore, the molybdenum content in the steel of the present invention is 0.5% or less.

[0024] Copper is one of the optional additive elements in the present invention. When copper is added to steel, the corrosion resistance of the steel can be improved, and when combined with phosphorus, the corrosion resistance effect of the steel is better. When the addition amount of Cu exceeds 1%, ε-Cu precipitation phase may be generated under specific conditions, and there is a relatively strong precipitation strengthening effect. However, the addition of Cu tends to cause the "Cu embrittlement" phenomenon during the rolling process. In order to avoid the significant "Cu embrittlement" phenomenon and fully utilize the corrosion resistance advantage of Cu in specific applications, the content of Cu is controlled to 0.5% or less in the present invention.

[0025] Nickel is one of the optional additive elements in the present invention. Adding nickel to steel provides a specific level of corrosion resistance, but its corrosion resistance effect is weaker than that of copper. Adding nickel to steel has little effect on the tensile properties of the steel, but it can refine the structure and precipitation phase of the steel and significantly improve the low-temperature toughness of the steel. Furthermore, in steel added with copper, the occurrence of "Cu embrittlement" can be suppressed by adding a small amount of nickel. Even if a relatively large amount of nickel is added, it will not have any significant adverse effect on the properties of the steel itself. Adding both copper and nickel not only improves the corrosion resistance, but also refines the structure of the steel and precipitates phases, so the low-temperature toughness is significantly improved. However, both copper and nickel are relatively expensive alloying elements. Therefore, in order to minimize the cost of alloy design, the addition amount of nickel is typically 0.5% or less.

[0026] Chromium is one of the optional additive elements in the present invention. When chromium is added to steel, its strength can be improved mainly by mechanisms such as solid solution strengthening or refinement of the microstructure. The microstructure of the present invention consists of fine bainitic ferrite and nano-precipitated cementite. Furthermore, after the high-temperature bell-type annealing process, the number of mobile dislocations in the microstructure decreases. Therefore, the ratio of the yield strength to the tensile strength of the steel (yield strength ratio) is relatively high, typically reaching 0.90 or more. Adding a small amount of chromium can moderately reduce the yield strength of the steel and decrease the yield strength ratio. Furthermore, adding a small amount of chromium can also improve the corrosion resistance. Overall, in the present invention, the addition amount of chromium is 0.5% or less.

[0027] Niobium is one of the optional addition elements in the present invention. Similar to titanium, niobium is a strong carbide-forming element in steel. When niobium is added to steel, the non-recrystallization temperature of the steel can be significantly increased, enabling the formation of deformed austenite with a higher dislocation density during the final rolling stage, and the final phase transformation structure can be refined during the subsequent transformation process. However, the addition amount of niobium should not be too much. On the other hand, when the addition amount of niobium exceeds 0.06%, relatively coarse niobium-carbonitrides are likely to be formed in the structure, which does not contribute to the impact toughness of the steel. On the other hand, a large amount of niobium is likely to cause anisotropy in the hot-rolled austenite structure. Therefore, the niobium content in the steel of the present invention is set to 0.06% or less.

[0028] Titanium is one of the optional addition elements in the present invention. Similar to niobium and vanadium, titanium is also a strong carbide-forming element. However, titanium carbide has a high solubility or precipitation temperature and typically completely dissolves in austenite during the final rolling stage. Only when the phase transformation start temperature is relatively high, such as 550 °C or above, the precipitation kinetics is relatively fast and titanium carbides are formed in ferrite. However, in the present invention, since the coil winding temperature falls within a medium range, the precipitation strengthening effect of titanium is relatively weak. Therefore, the addition amount of titanium in the steel of the present invention is 0.10% or less.

[0029] Boron is one of the optional addition elements in the present invention. Boron can significantly improve the hardenability of steel and is beneficial for obtaining a martensite structure. Considering that the desired fine structure in the hot rolling stage of the present invention is bainite, it is necessary to strictly control the boron content in the steel to prevent the formation of martensite due to excessive addition of boron. Therefore, the addition amount of boron in the steel of the present invention is set to 0.001% or less.

[0030] Preferably, the present invention can provide two types of steel products with different mechanical properties by combining the C content and the V content in the steel. Specifically, when the C content is 0.10 - 0.25% and the V content is 0.10 - 0.30%, the steel has a tensile strength of 780 MPa or more, an elongation of 30% or more, and a hole expansion rate of 65% or more; and when the C content is 0.25 - 0.35% and the V content is 0.30 - 0.50%, the steel has a tensile strength of 980 MPa or more, an elongation of 25% or more, and a hole expansion rate of 50% or more.

[0031] Preferably, the steel has a fine structure of bainite and retained austenite, the content of austenite is 5% or more, preferably 5 - 15%, more preferably 5 - 12%, and the bainite contains nano-precipitated VC (vanadium carbide).

[0032] Unless otherwise specified, the content of each fine structure in the steel of the present invention means its volume fraction.

[0033] The main fine structures in the steel of the present invention are bainite and retained austenite. The bainitic ferrite in bainite contains nano-sized carbides. Bainite and nano-precipitates impart high yield strength and tensile strength to the steel sheet, and retained austenite imparts extremely high elongation to the steel sheet.

[0034] Preferably, the above steel has a yield strength of 700 MPa or more.

[0035] Considering the overall manufacturing cost of the steel, it is preferable that the steel of the present invention has a yield strength of 900 MPa or less, a tensile strength of 1100 MPa or less, an elongation of 25% or less, and a hole expansion rate of 100% or less.

[0036] Another aspect of the present invention provides a method for manufacturing the above steel, including the following steps: 1) Smelting and casting process After melting the steel according to the above components in a converter or an electric furnace, secondary refining is carried out in a vacuum furnace, and then it is cast into a casting blank or a casting ingot; 2) The step of reheating the casting blank or the casting ingot The heating temperature is 1100 °C or higher, and the holding time is 1 to 2 hours; 3) The step of hot rolling and cooling the casting blank or the casting ingot Here, the casting blank or the casting ingot is rolled at an initial rolling temperature of 1000 °C to 1100 °C, and then subjected to multi-pass rolling at a relatively large deformation rate of 50% or more at 1000 °C or higher to obtain an intermediate blank. After the intermediate blank reaches 950 °C to 1000 °C, it is subjected to final rolling of 3 to 5 passes at a cumulative deformation rate of 70% or more to obtain a steel strip, and the final rolling temperature is 800 to 950 °C; Here, the cooling is controlled cooling. After final rolling, the steel strip is water-cooled to a temperature of 400 to 550 °C at a cooling rate of 10 °C / s or more, wound into a coil, and then the steel coil is cooled to room temperature.

[0037] Preferably, the above method further includes step 4) pickling. Here, the strip steel is pickled at a running speed of 30 to 140 m / min, a pickling temperature of 75 to 85 °C, and a straightening rate of 3% or less, then washed with water at a temperature in the range of 35 to 50 °C, and the surface of the hot-rolled strip steel is dried at a temperature of 120 to 140 °C and oil is applied.

[0038] After the hot-rolled strip steel is pickled, pickled steel with high performance stability and extremely high hole expansion rate can be obtained.

[0039] The beneficial effects of the above method for manufacturing the steel of the present invention are as follows.

[0040] The present invention adopts an innovative composition design with medium to low carbon and high vanadium, which is combined with an innovative hot rolling medium-temperature coil winding control technology and a technology for coordinating nano-precipitation and bainite phase transformation. As a result, it is possible to produce advanced high-strength steels with various strength levels, characterized by high yield strength, high tensile strength, extremely high plasticity, and high hole expansion ratio. The relatively high carbon content is beneficial for obtaining high strength and provides a large amount of available carbon atoms that can diffuse into retained austenite, resulting in highly stable retained austenite. The main purpose of adding a relatively high silicon content is to suppress the formation of carbides during the bainite phase transformation process. By containing a relatively large amount of manganese, the stability of retained austenite can be further improved. Coil winding at 400 - 550°C after final rolling is mainly for obtaining a fine structure of bainite without carbides and retained austenite, and at the same time for adjusting the nano-precipitation of vanadium carbide.

[0041] Based on this innovative composition and process design, the present invention can achieve a good coordination of extremely high plasticity and high hole expansion ratio over different strength levels. As a result, while ensuring tensile strengths of 780 MPa or more and 980 MPa or more respectively, it meets the elongation requirements of 30% or more and 25% or more, as well as hole expansion ratios of 65% or more and 50% or more.

[0042] Compared with the prior art, the solution of the present invention has the following advantages or beneficial effects.

[0043] Compared with steels with extremely high plasticity in existing patent documents, the present invention adopts a composition design with medium to low carbon, high silicon, and low aluminum, which is different from the traditional design of low carbon, high or low silicon, and high aluminum for conventionally hot-rolled steels with extremely high plasticity.

[0044] In the steel disclosed in Chinese Patent Application CN104233092A, in addition to low carbon and high silicon, precious metal elements such as Cr, Mo, and Nb are added to the steel, resulting in a relatively high alloy cost.

[0045] The composition design of Chinese Patent Application CN107815593A has low silicon, high aluminum, and contains a certain amount of Cu. However, the process route for manufacturing this steel mainly involves heat treatment in the two-phase region and phase transformation in the bainite region, and it cannot be applied to the current hot rolling production line.

[0046] The composition design of Chinese Patent Application CN114107792A does not contain vanadium, and has low silicon, titanium, and molybdenum. Its performance is mainly characterized by a high hole expansion rate. However, since there is no retained austenite in its microstructure, a high level of elongation cannot be achieved.

[0047] The present invention adopts an innovative composition design with medium to low carbon and high vanadium, combined with a medium-temperature coil winding process. Through the cooperation of precise composition and process, an advanced high-strength steel with high yield strength, high hole expansion rate characteristics, and extremely high plasticity can be obtained.

[0048] The steel with a high hole expansion rate and extremely high plasticity obtained by using the technology provided by the present invention has a yield strength of 700 MPa or more, further 800 MPa or more, a tensile strength of 780 MPa or more, further 980 MPa or more. The hot-rolled pickled product of the present invention has a thickness of 1.5 - 8.0 mm, and shows a relatively high yield strength, extremely high elongation (A ≧ 25%, further reaching 30% or more), and a high hole expansion rate (50% or more, further reaching 65% or more). The steel of the present invention has excellent mechanical properties and shows harmony among yield strength, tensile strength, plasticity, hole expansion performance, and formability. This can be applied to the manufacture of various complex parts of passenger cars or commercial vehicles and has promising application prospects.

Brief Description of the Drawings

[0049]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0050] Detailed Description The present invention will be further described below with reference to examples and drawings.

[0051] The compositions of the steel in the examples and comparative examples of the present invention are shown in Table 1. The remainder in Table 1 is Fe and other inevitable impurities. Table 2 shows the production process parameters of the steel in the examples of the present invention. Table 3 shows the performance parameters of the steel in the examples of the present invention. The manufacturing process of the steel in the examples of the present invention is as follows. 1) Smelting and casting: According to the composition in Table 1, the molten steel was smelted in a converter or an electric furnace, then secondary refined in a vacuum furnace, and then cast into a casting blank or a casting ingot. 2) The casting blank or casting ingot was reheated at a heating temperature of 1100 °C or higher and held for 1 to 2 hours. 3) Hot rolling and cooling of the casting blank or casting ingot: The casting blank or casting ingot was rolled at an initial rolling temperature of 1000 °C to 1100 °C, and then subjected to multi-pass rolling with a relatively large reduction rate of 50% or more at 1000 °C or higher to obtain an intermediate blank. After the intermediate blank reached 950 °C to 1000 °C, it was subjected to final rolling with 3 to 5 passes at a cumulative reduction rate of 70% or more to obtain a steel strip. The final rolling temperature was 800 to 950 °C.

[0052] The cooling was controlled cooling. After final rolling, the steel strip was water-cooled to a temperature of 400 - 550°C at a cooling rate of 10°C / s or more and wound into a coil. Then, the steel coil was cooled to room temperature.

[0053] The above process is shown in Figure 1.

[0054] Table 2 shows the steel production process parameters in the examples of the present invention.

[0055] The steels of Comparative Examples 1 - 3 were selected from CN104233092A. The steel of Comparative Example 4 was selected from CN114107792A, another patent application by the applicant of the present application.

[0056] Table 3 shows the steel performance parameters in the examples and comparative examples of the present invention.

[0057] As shown in Table 3, the yield strength, tensile strength, and elongation of the steel were tested in accordance with GB / T 228.1 - 2021 "Tensile testing of metallic materials - Part 1: Method of test at room temperature".

[0058] The hole expansion rate of the steel was tested in accordance with GB / T 24524 - 2021 "Metallic materials, sheet and strip, method of hole expansion test".

[0059] As can be seen from Table 1, in the examples of the present invention, a composition design with high silicon and high vanadium is adopted, while in the comparative examples, a design with high vanadium is not adopted. The composition designs of the two are different.

[0060] Comparative Examples 1 - 2 adopted a composition design with low carbon, and Comparative Example 4 adopted a composition design with low silicon and low manganese. The composition designs of all comparative examples are different from those of the examples.

[0061] As can be seen from Table 3, the tensile strengths of Comparative Examples 1 to 3 are the same as those of the Examples, but their yield strengths are even lower. From the viewpoints of elongation and hole expansion ratio, the elongation of the Comparative Examples is lower than that of the Examples of the present invention, and no hole expansion ratio data are provided for Comparative Examples 1 to 3. The microstructures of the Comparative Examples do not contain retained austenite, but the Examples of the present invention contain 5% or more of retained austenite, and the two have completely different microstructure types.

[0062] Figures 2 to 5 show the metallographic micrographs of the hot-rolled steels of Examples 1, 3, 5, and 7, respectively. These figures clearly show that the composition and process design according to the present invention achieve a microstructure mainly composed of bainite and retained austenite. This microstructure provides a good balance of high yield strength, high tensile strength, extremely high plasticity, and high hole expansion ratio, resulting in excellent overall performance.

[0063] Also, from Table 3, it can be seen that the yield strengths of the steels of the present invention meet the requirements of 700 MPa or more and 800 MPa or more, the tensile strengths are 780 MPa or more and 980 MPa or more, the elongations are 30% or more and 25% or more, and the hole expansion ratios are 65% or more and 50% or more, respectively.

[0064] In summary, the steel of the present invention having a high hole expansion ratio and extremely high plasticity has a good balance of strength, extremely high plasticity, and high hole expansion ratio, and is particularly suitable for complex formed parts such as automobile chassis structures, and a wide range of applications can be expected.

[0065]

Table 1

[0066]

Table 2

[0067]

Table 3

Claims

1. A steel having the following components, wherein the weight percentages of the components are: C: 0.10 to 0.35%, Si: 0.8 to 2.0%, Mn: 1.0 to 2.5%, P ≤ 0.02%, S ≤ 0.005%, Al: 0.01 to 0.1%, N ≤ 0.005%, V: 0.10 to 0.50%, O ≤ 0.003%, and the balance is Fe and other inevitable impurities; When the C content is 0.10 to 0.25% and the V content is 0.10 to 0.30%, the steel has a tensile strength of 780 MPa or more, an elongation of 30% or more, and a hole expansion rate of 65% or more; and When the C content is 0.25 to 0.35% and the V content is 0.30 to 0.50%, the steel has a tensile strength of 980 MPa or more, an elongation of 25% or more, and a hole expansion rate of 50% or more, The steel as described above.

2. The steel according to claim 1, further comprising one or more selected from the group consisting of Mo, Cu, Ni, Cr, Nb, V, and B; wherein Mo ≤ 0.5%, Cu ≤ 0.5%, Ni ≤ 0.5%, Cr ≤ 0.5%, Nb ≤ 0.06%, V ≤ 0.10%, B ≤ 0.001%.

3. The steel according to claim 1, wherein the inevitable impurity contains S at 0.003 wt% or less.

4. The steel according to claim 1, wherein the inevitable impurity contains N at 0.004 wt% or less.

5. The steel according to any one of claims 1 to 4, having a microstructure of bainite and retained austenite, wherein the content of the austenite is 5% or more, and the bainite contains nano-precipitated VC.

6. The steel according to any one of claims 1 to 5, having a yield strength of 700 MPa or more.

7. A method for manufacturing the steel according to any one of claims 1 to 6, the method including the following steps, the steps being: 1) A smelting and casting step, According to the components specified in any one of claims 1 to 6, after smelting molten steel in a converter or an electric furnace, secondary refining is performed in a vacuum furnace, and then casting is performed into a casting blank or a casting ingot; 2) A reheating step of the casting blank or the casting ingot, The heating temperature is 1100 °C or more, and the holding time is 1 to 2 hours; 3) A hot rolling and cooling step of the casting blank or the casting ingot, Here, the cast blank or the cast ingot is rolled at an initial rolling temperature of 1000°C to 1100°C, and then subjected to multi-pass rolling at a deformation rate of 50% or more and at a temperature of 1000°C or higher to obtain an intermediate blank. After the intermediate blank reaches 950°C to 1000°C, it is subjected to final rolling of 3 to 5 passes at a cumulative deformation rate of 70% or more to obtain a steel strip. Here, the final rolling temperature is 800 to 950°C; Here, the cooling is controlled cooling. After final rolling, the steel strip is water-cooled at a cooling rate of 10°C / s or more to a temperature of 400 to 550°C, and after being coiled, the steel coil is cooled to room temperature to obtain a hot-rolled strip steel. The method.

8. The method according to claim 7, wherein the method further has step 4) pickling. Here, the hot-rolled strip steel is pickled at a running speed of 30 to 140 m / min, a pickling temperature of 75 to 85°C, and a straightening rate of 3% or less, then washed with water at a temperature in the range of 35 to 50°C, and the surface of the hot-rolled strip steel is dried at a temperature of 120 to 140°C and oil is applied.

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