980 MPa level bainite hole-expanding steel and method for manufacturing the same

Abstract

A bainite high hole-expandability steel with a strength of 980 MPa and a manufacturing method thereof has a chemical composition, in weight percentages, of C 0.05-0.10%, Si 0.5-2.0%, Mn 1.0-2.0%, P≦0.02%, S≦0.003%, Al 0.02-0.08%, N≦0.004%, Mo≧0.1%, Ti 0.01-0.05%, Cr≦0.5%, B≦0.002%, O≦0.0030%, and the balance being Fe and other unavoidable impurities. The high hole-expandability steel of the present invention has a yield strength of ≧800 MPa, a tensile strength of ≧980 MPa, and good elongation (transverse A 50 It also has a strength of ≥ 11%) and hole expandability (hole expansion ratio ≥ 40%), making it suitable for use in passenger car chassis parts that require high strength and thin wall thickness, such as control arms and subframes.

Description

Technical Field

[0001] The present invention belongs to the field of high-strength steel, and particularly relates to bainitic high hole-expanding steel at the 980 MPa level and a manufacturing method thereof.

Background Art

[0002] With the development of the national economy, the production volume of automobiles has also increased significantly, and the usage amount of sheet materials has been continuously increasing. In the domestic automobile industry in China, in the original designs of parts of many vehicle types, such as chassis parts of automobiles, torsion beams, subframes of sedans, spokes and rims of wheels, front and rear axle assemblies, body structure parts, seats, clutches, seat belts, box panels of trucks, protection nets, beams of vehicles, etc., the use of hot-rolled sheets and pickled sheets is required. Among them, the steel for chassis also accounts for 24 - 34% of the total amount of steel used in automobiles.

[0003] The lightweighting of passenger cars is not only a trend in the automotive industry but also a requirement in regulations. Although fuel consumption is regulated in the regulations, this is essentially a requirement for the lightweighting of the body. When reflected in materials, it becomes requirements for high strength, thin thickness, and lightweight. High strength and lightweight are inevitable requirements for future new vehicles, which will inevitably lead to an improvement in the level of steel usage and a change in the chassis structure: for example, due to the complication of parts, requirements for the performance and surface of materials, as well as the progress of forming technologies such as hydroforming, hot stamping, and laser welding, resulting in requirements for the performance of materials such as high strength, stamping, flanging, springback, and fatigue.

[0004] Compared to overseas developments, the development of high-strength, high-hole-expandability steel in China is characterized by relatively low strength levels and poor performance stability. For example, most of the high-hole-expandability steel used by Chinese automotive parts manufacturers is high-hardness steel with a tensile strength of 600 MPa or less, and competition for high-hole-expandability steel with a tensile strength of 440 MPa or less is intensifying. Currently, high-hole-expandability steel with a tensile strength of 780 MPa is gradually being mass-produced, but the demands for elongation and hole expansion ratio, two important indicators of forming, are also increasing. On the other hand, high-hole-expandability steel with a tensile strength of 980 MPa is still in the research and development and certification stage and has not yet reached the mass production stage. However, 980 MPa high-hole-expandability steel, with its higher strength and higher hole expansion ratio, will inevitably be the direction of future development. In order to meet the potential needs of future users, the development of 980 MPa high-hole-expandability steel with good hole expansion properties is required.

[0005] Most existing patent documents relating to this invention concern high-hole-expandability steel at levels of 780 MPa or less. There are very few documents relating to high-hole-expandability steel at levels of 980 MPa. Chinese patent application CN106119702A discloses a hot-rolled high-hole-expandability steel at levels of 980 MPa, characterized by a low-carbon V-Ti fine alloy design with a microstructure of granular bainite and a small amount of martensite, and with trace amounts of Nb and Cr added as the main features of its composition. This differs significantly from the present invention in terms of composition, process, and microstructure. [Overview of the project] [Problems that the invention aims to solve]

[0006] According to the literature, the elongation of a material is usually inversely proportional to its hole-expanding rate; that is, the higher the elongation, the lower the hole-expanding rate, and conversely, the lower the elongation, the higher the hole-expanding rate. Therefore, it is extremely difficult to obtain a high-hole-expanding steel that possesses both high elongation and high hole-expanding properties, as well as high strength. Furthermore, with the same or similar strengthening mechanism, the higher the material's strength, the lower the hole-expanding rate.

[0007] To obtain steel materials with excellent plasticity and hole-expanding / flamming properties, it is necessary to strike a good balance between the two. Of course, the hole-expanding rate of a material is closely related to many factors, but among them, the homogeneity of the microstructure, the level of control of inclusions and segregation, the type of microstructure, and the measurement of the hole-expanding rate are the most important factors. Generally, a single, homogeneous microstructure is advantageous for improving the hole-expanding rate, while a two-phase or multi-phase microstructure is disadvantageous for improving the hole-expanding rate. [Means for solving the problem]

[0008] Content of the invention The object of the present invention is to provide bainite high hole-expanding steel with a yield strength of 980 MPa or higher and a method for manufacturing the same, wherein the high hole-expanding steel has a yield strength of ≥ 800 MPa, a tensile strength of ≥ 980 MPa, and good elongation (lateral elongation A 50 It also has hole-expanding properties (hole expansion ratio ≥ 40%) and can be used in passenger car chassis parts such as control arms and subframes that require high strength and thin walls.

[0009] To achieve the above objectives, the technical solution of the present invention is: According to the composition design of the present invention, the carbon content is designed to be low in order to ensure excellent weldability, good hole expansion and impact toughness of the resulting martensitic structure during use by the user; a lower carbon content is better in satisfying a tensile strength of ≥ 980 MPa; the silicon content is designed to be high in order to improve the plasticity of the material by obtaining more retained austenite according to the process; at the same time, the silicon content is designed to be high in order to improve the anisotropy of the steel structure, refine the size of the austenite crystal grains and the final bainite lath, and improve plasticity and hole expansion rate by contributing to lowering the non-recrystallization temperature of the steel and completing the dynamic recrystallization process in the steel within a wider final rolling temperature range.

[0010] Specifically, the 980 MPa level bainite hole-expanding steel according to the present invention has a chemical composition by weight percentage of: C 0.05~0.10%, Si 0.5~2.0%, Mn 1.0~2.0%, P≦0.02%, S≦0.003%, Al 0.02~0.08%, N≦0.004%, Mo≧0.1%, Ti 0.01~0.05%, Cr≦0.5%, B≦0.002%, O≦0.0030%, with the remainder being Fe and other unavoidable impurities.

[0011] Furthermore, the 980 MPa level bainite hole-expanding steel according to the present invention further contains one or more elements from among Nb ≤ 0.06%, V ≤ 0.05%, Cu ≤ 0.5%, Ni ≤ 0.5%, and Ca ≤ 0.005%.

[0012] In some embodiments, the Mo content is 0.1 to 0.55% by weight. In some embodiments, the 980 MPa level bainite hole-expanding steel according to the present invention has a chemical composition by weight percentage of: C 0.05~0.10%, Si 0.5~2.0%, Mn 1.0~2.0%, P≦0.02%, S≦0.003%, Al 0.02~0.08%, N≦0.004%, Mo≧0.1%, Ti The composition is 0.01-0.05%, Cr≦0.5%, B≦0.002%, O≦0.0030%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, Ni≦0.5%, Ca≦0.005%, with the remainder being Fe and other unavoidable impurities, and the bainite high hole-expanding steel at the 980 MPa level contains at least one of Nb, V, Cu, Ni, and Ca, preferably at least one or both of Cr and B.

[0013] The preferred content of Nb and V is ≤0.03% each; the preferred content of Cu and Ni is ≤0.3% each; and the preferred content of Ca is ≤0.002%.

[0014] In some embodiments, the 980 MPa level bainite hole-expanding steel according to the present invention has a yield strength of ≥800 MPa, preferably ≥830 MPa, more preferably ≥850 MPa, a tensile strength of ≥980 MPa, preferably ≥1000 MPa, more preferably ≥1020 MPa, and a transverse elongation A 50 The hole expansion rate is ≥11%, and preferably ≥40%, more preferably ≥50%.

[0015] Preferably, the microstructure of the 980 MPa level bainite hole-expanding steel according to the present invention consists of bainite and retained austenite. By volume, the retained austenite content in the 980 MPa level bainite hole-expanding steel is 1 to 5%.

[0016] In the design of the composition of the highly hole-expanding steel according to the present invention: Carbon is a fundamental element in steel and is also one of the important elements in this invention. Carbon expands the austenite phase region and stabilizes austenite. As a void atom in steel, carbon plays a very important role in improving the strength of steel, and has the greatest influence on the yield strength and tensile strength of steel. In this invention, the microstructure to be obtained is low-carbon bainite, so in order to obtain high-strength steel that reaches a tensile strength of 980 MPa, the carbon content must be ensured to be 0.05% or more. Otherwise, if the carbon content is less than 0.05%, the tensile strength cannot reach 980 MPa even if it is completely quenched to room temperature. However, a carbon content exceeding 0.10% is also unacceptable, as if the carbon content is too high, the strength of the formed bainite will be too high, and many island-like martensite-austenite constituents will easily form in the microstructure, which is unfavorable for both elongation and hole expansion rate. Therefore, the carbon content should be controlled between 0.05 and 0.10%, with a preferred range of 0.06 to 0.08%.

[0017] Silicon is a fundamental element in steel and is also one of the important elements in this invention. Increasing the Si content not only improves the solid solution strengthening effect, but more importantly, it can also play the following two roles. First, by significantly lowering the non-recrystallization temperature of steel, dynamic recrystallization can be completed in steel over a very wide temperature range. This allows rolling to be completed within a rolling completion temperature range such as 800-920°C during the actual rolling process, reducing the difference between the transverse and longitudinal microstructures, which contributes to improved strength and plasticity, as well as achieving a good hole expansion ratio. Another important role of Si is the suppression of cementite precipitation. Under appropriate rolling process conditions, especially when obtaining a bainite-dominant microstructure, a predetermined amount of retained austenite can be retained, contributing to improved elongation. This role of Si generally cannot be realized unless its content is 0.5% or more, but too much Si content is also unacceptable, otherwise the rolling force load during the actual rolling process will be excessive, which is detrimental to stable product production. Therefore, in steel, the Si content is usually controlled between 0.5 and 2.0%, with a preferred range being between 0.8 and 1.6%.

[0018] Manganese is the most fundamental element in steel and is also one of the most important elements in this invention. Mn is an important element that expands the austenite phase region, reduces the critical cooling rate of steel, stabilizes austenite, refines the crystal grains, and delays the transformation from austenite to pearlite. However, in this invention, a predetermined amount of molybdenum is added, and molybdenum plays a far greater role than manganese in delaying the formation of ferrite and pearlite and reducing the critical cooling rate. Therefore, the manganese content in steel can be appropriately reduced and is usually controlled to 1.0% or more, but the Mn content is usually not permitted to exceed 2.0%, otherwise Mn segregation is more likely to occur during steelmaking, and hot cracking is more likely to occur during continuous slab casting. Therefore, in steel, the Mn content is usually controlled to 1.0-2.0%, with a preferred range of 1.4-1.8%.

[0019] Phosphorus is an impurity element in steel. P tends to be extremely concentrated at grain boundaries, and if the P content in steel is high (≧0.1%), it forms Fe2P and precipitates around the grains, reducing the plasticity and toughness of the steel. Therefore, the lower the P content, the better, and it is generally preferable to control it to within 0.02%, without increasing steelmaking costs.

[0020] Sulfur is an impurity element in steel. In steel, sulfur (S) usually combines with manganese (Mn) to form MnS inclusions. In particular, when both S and Mn content are high, a large amount of MnS is formed in the steel. However, MnS itself has some plasticity, and in the subsequent rolling process, MnS deforms along the rolling direction, which not only reduces the lateral plasticity of the steel sheet but also increases the anisotropy of the microstructure, making it unfavorable for hole expansion. Therefore, the lower the S content in steel, the better. Considering that the Mn content must be at a high level in this invention, the S content must be strictly controlled in order to reduce the MnS content. The S content needs to be controlled to within 0.003%, and a preferred range is 0.0015% or less.

[0021] In steel, aluminum primarily plays a role in deoxidation and nitrogen fixation. Assuming the presence of strong carbide-forming elements such as Ti, Al primarily plays a role in deoxidation and grain refinement. In this invention, Al is a general deoxidizing and grain refinement element, and its content should generally be controlled to 0.02-0.08%. If the Al content is less than 0.02%, it cannot contribute to grain refinement, and similarly, if the Al content is 0.08% or more, its grain refinement effect becomes saturated. Therefore, in steel, the Al content should generally be controlled between 0.02-0.08%, but the preferred range is between 0.02-0.05%.

[0022] In this invention, nitrogen is an impurity element, and a lower nitrogen content is preferable. However, nitrogen is an unavoidable element in the steelmaking process. Although its content is low, when it combines with strong carbide-forming elements such as Ti, the formed TiN particles have a very negative impact on the performance of the steel, especially its hole-expanding properties. Furthermore, because TiN has a square shape, there is a large stress concentration between its sharp corners and the substrate. During the hole-expanding deformation process, this stress concentration between TiN and the substrate makes it prone to cracking, significantly reducing hole-expanding properties. Assuming that the nitrogen content is controlled as much as possible, it is preferable that the content of strong carbide-forming elements such as Ti be as low as possible. In this invention, the adverse effects of TiN are reduced as much as possible by adding a small amount of Ti to fix the nitrogen. Therefore, the nitrogen content should be controlled to 0.004% or less, and the preferred range is 0.003% or less.

[0023] Titanium is one of the important elements in this invention. Ti plays two main roles in this invention: firstly, it combines with impurity elements N in the steel to form TiN, playing a role in some "nitrogen fixation"; and secondly, it forms a certain number of dispersed fine TiN during the subsequent welding process of the material, suppressing the size of austenite crystal grains, refining the microstructure, and improving low-temperature toughness. Therefore, in steel, the Ti content is controlled to a range of 0.01 to 0.05%, with a preferred range of 0.01 to 0.03%.

[0024] Molybdenum is one of the important elements for the present invention. Adding molybdenum to steel can significantly delay the transformation of ferrite and pearlite, which is beneficial for obtaining a bainite structure in the intermediate and high temperature regions. Moreover, by adding molybdenum, the stability of the structure and performance of steel can be improved, and the crystal grains can also be refined. This role of molybdenum is beneficial for the adjustment of various processes in the actual rolling process. For example, at the end of rolling, stepwise cooling may be carried out, or air cooling can be followed by water cooling, etc. In the present invention, two methods of air cooling or direct cooling after rolling are adopted. During the air cooling process, by adding molybdenum, it can be ensured that structures such as ferrite and pearlite are not formed during the air cooling process. On the other hand, dynamic recovery of austenite deformed during the air cooling process occurs, contributing to the improvement of the homogeneity of the structure and performance, and being beneficial for hole expansion. In order to exert the inhibitory effect of molybdenum on the formation of ferrite and pearlite, its content needs to reach 0.10% or more. Therefore, the molybdenum content should be controlled at ≧0.10%, and the preferred range is ≧0.15%. In some embodiments, the molybdenum content is 0.1 - 0.55%.

[0025] Chromium is one of the important elements for the present invention. In the present invention, chromium is not for improving the hardenability of steel. Instead, it combines with the B phase and contributes to the formation of acicular ferrite structure in the heat-affected zone after welding, for significantly improving the low-temperature toughness of the heat-affected zone after welding. Since the final application parts according to the present invention are chassis system products of passenger cars, the low-temperature toughness of the heat-affected zone after welding is an important index. In addition to ensuring that the strength of the heat-affected zone after welding does not decrease too much, the low-temperature toughness of the heat-affected zone after welding also needs to meet certain requirements. Moreover, chromium itself also has a certain resistance to welding softening. Therefore, it is necessary to add a small amount of chromium element to steel, and the range is usually ≦0.5%, for example, 0.1 - 0.5%, and the preferred range is 0.2 - 0.4%.

[0026] The role of boron in steel is mainly to be unevenly distributed at the austenite grain boundaries and suppress the formation of primary ferrite; by adding boron to steel, the hardenability of the steel can also be greatly improved. However, in the present invention, the main purpose of adding trace amounts of boron element is not to improve the hardenability, but to combine with the chromium phase, improve the structure of the heat-affected zone of welding, and obtain a needle-like ferrite structure with excellent toughness. The boron element added to steel is usually controlled to be 0.002% or less, and the preferred range is between 0.0005% and 0.0015%.

[0027] Calcium is an element that can be added to the present invention. Calcium improves the form of sulfides such as MnS, changes sulfides such as long-strip MnS into spherical CaS, contributes to the improvement of the form of inclusions, and thereby can reduce the adverse effects of long-strip sulfides on the hole expansion property. However, if too much calcium is added, the number of calcium oxides will increase, which is disadvantageous for the hole expansion property. Therefore, in steel, the calcium addition amount is usually ≤0.005%, and the preferred range is ≤0.002%.

[0028] Oxygen is an inevitable element in the steelmaking process. For the present invention, the O content in steel can usually reach 30 ppm or less after deoxidation and does not have an obvious adverse effect on the performance of the steel plate. Therefore, in steel, the O content can be controlled within 30 ppm.

[0029] Niobium is one of the elements that can be added to the present invention. Niobium, like titanium, is a strong carbide-forming element in steel. By adding niobium to steel, the unrecrystallized temperature of the steel is significantly increased, resulting in deformed austenite with a higher dislocation density during the finishing rolling stage, and allowing for refinement of the final transformed structure during the subsequent transformation process. However, the amount of niobium added should not be too much. On the other hand, if the amount of niobium added exceeds 0.06%, it tends to form relatively coarse niobium carbonnitride in the structure, consuming some carbon atoms and reducing the precipitation strengthening effect by carbides. Furthermore, a high niobium content makes it easier for anisotropy to occur in the austenitic structure in the hot-rolled state, which is carried over to the final structure during the subsequent cooling transformation process, making it unfavorable for hole expansion. Therefore, in steel, the niobium content is usually controlled to ≤0.06%, and the preferred range is ≤0.03%.

[0030] Vanadium is an element that can be added to the present invention. Like titanium and niobium, vanadium is a strong carbide-forming element. However, vanadium carbides have low solid solution and precipitation temperatures, and are usually all dissolved in austenite during the finishing rolling stage. Vanadium only begins to form in ferrite when the temperature drops and transformation begins. Since the solid solubility of vanadium carbides in ferrite is greater than that of niobium and titanium, vanadium carbides form in larger sizes in ferrite, which is unfavorable for precipitation strengthening, and its contribution to steel strength is far less than that of titanium. Furthermore, the formation of vanadium carbides consumes some carbon atoms, which is unfavorable for improving steel strength. Therefore, the amount of vanadium added to steel is usually ≤0.05%, and the preferred range is ≤0.03%.

[0031] Copper is one of the elements that can be added to the present invention. By adding copper to steel, the corrosion resistance of the steel can be improved, and when added together with the element P, the corrosion resistance is even better. When the amount of Cu added exceeds 1%, an ε-Cu precipitate phase can be formed under certain conditions, resulting in a strong precipitation strengthening effect. However, the addition of Cu makes it easy for the "Cu embrittlement" phenomenon to occur during the rolling process. In order to fully utilize the corrosion resistance improvement effect of Cu without significantly causing the "Cu embrittlement" phenomenon in certain application environments, the Cu element content is usually controlled to within 0.5%, and the preferred range is within 0.3%.

[0032] Nickel is one of the elements that can be added to the present invention. Adding nickel to steel provides some degree of corrosion resistance, but the corrosion resistance effect is weaker than that of copper. Adding nickel to steel does not significantly affect the tensile performance of the steel, but it can refine the steel's microstructure and precipitate phases, and significantly improve the steel's low-temperature toughness. In addition, adding a small amount of nickel to steel with added copper can suppress the occurrence of "Cu embrittlement". Adding a large amount of nickel does not have a clear adverse effect on the performance of the steel itself. Adding copper and nickel simultaneously not only improves corrosion resistance but also refines the steel's microstructure and precipitate phases, and significantly improves low-temperature toughness. However, since both copper and nickel are relatively expensive alloying elements, the amount of nickel added is usually ≤0.5%, and the preferred range is ≤0.3%, in order to minimize the cost of alloy design.

[0033] The method for producing bainite high-hole-expandability steel at a level of 980 MPa according to the present invention includes the following steps: 1) Smelting and casting The mixture is smelted in a converter or electric furnace according to the specified composition, then refined in a vacuum furnace, and finally cast into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100-1200°C for 1-2 hours; 3) Hot rolling With the primary objective of refining the austenite crystal grains, the rolling start temperature is set to 950-1100°C, and 3-5 passes are performed at 950°C or higher under high pressure, with a cumulative deformation of ≥50%, preferably ≥60%; then, after raising the temperature of the intermediate billet to 920-950°C, 3-7 final passes are performed, with a cumulative deformation of ≥70%, preferably ≥85%; and the rolling end temperature is set to 800-920°C; 4) Cooling To dynamically restore the deformed austenite to a more homogenized state, the strip is first air-cooled for 0-10 seconds, then water-cooled to 400-550°C at a cooling rate of ≥10°C / s, preferably ≥30°C / s, before being wound up and allowed to cool naturally to room temperature. 5) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 100 m / min, the pickling temperature is controlled between 75 and 85°C, the tensile straightening ratio is controlled to ≤2% to reduce elongation loss of the steel strip, then it is rinsed, the surface of the steel strip is dried, and oil is applied.

[0034] Preferably, after pickling in step 5), the strip is rinsed in a temperature range of 35 to 50°C to ensure surface quality, the surface of the strip is dried between 120 and 140°C, and then oil is applied.

[0035] The innovative aspects of this invention are as follows: According to the composition design of the present invention, the carbon content is designed to be low in order to ensure excellent weldability, good hole expansion and impact toughness of the resulting martensitic structure during use by the user; a lower carbon content is better in satisfying a tensile strength of ≥ 980 MPa; the silicon content is designed to be high in order to improve the plasticity of the material by obtaining more retained austenite according to the process; at the same time, the silicon content is designed to be high in order to improve the anisotropy of the steel structure, refine the size of the austenite crystal grains and the final bainite lath, and improve plasticity and hole expansion rate by contributing to lowering the non-recrystallization temperature of the steel and completing the dynamic recrystallization process in the steel within a wider final rolling temperature range.

[0036] In the microstructure design, a low-carbon bainite design concept is adopted, and by adding more silicon, cementite formation is suppressed and reduced, while the pre-recrystallization temperature is lowered and the rolling completion temperature range is expanded. By direct cooling after rolling or recooling after a certain period of air cooling, a bainite structure with fine and homogeneous crystal grains can be obtained while containing a small amount of retained austenite. The combination of a bainite structure that gives high strength to the steel sheet and retained austenite that gives high plasticity to the steel sheet results in a steel sheet that exhibits good strength, plasticity, and hole expansion ratio compatibility.

[0037] In designing the rolling process, the rough rolling and finish rolling stages should be completed as quickly as possible. After rolling is complete, the material should first be air-cooled for a predetermined time before water-cooling, or directly water-cooled. The main purpose of air cooling is to design the composition to include predetermined amounts of manganese and molybdenum. Manganese is an element that stabilizes austenite, and molybdenum significantly delays ferrite and pearlite transformations while promoting bainite transformations. Therefore, during the short-duration air-cooling process, the rolled deformed austenite undergoes a dynamic recovery process without transformation, i.e., without forming a ferrite structure. Dislocations within the austenite crystal grains that have undergone dynamic recovery are significantly reduced, the austenite structure becomes more homogeneous, and the bainite formed in the subsequent transformation process also becomes more homogeneous. To avoid the formation of ferrite during the continuous cooling process, the water-cooling rate of the steel strip should be ≥10°C / s.

[0038] To obtain a single-phase, homogeneous bainite structure, the steel strip needs to be cooled to the bainite transformation temperature range. In this invention, the bainite transformation temperature range is 400-550°C depending on the composition. In this temperature range, as the winding temperature decreases, the bainite lath becomes finer, the structure becomes relatively homogeneous, and the strength improves while the plasticity decreases; conversely, as the winding temperature increases, some of the bainite lath in the structure transforms into granular bainite, the strength decreases, while the plasticity improves. Theoretical calculations and experiments have demonstrated that cooling the steel strip to the range of 400-550°C yields a bainite structure with excellent overall performance. When the winding temperature is ≥ 550°C, a relatively coarse upper bainite is formed in the structure, and the strength requirement of 980 MPa or more cannot be met. However, when the winding temperature is ≤ 400°C, the structure transforms into martensite. For these reasons, it is necessary to control the winding temperature between 400-550°C. Based on such innovative components and process design concepts, the present invention makes it possible to obtain a high-hole-expandability steel with excellent strength, plasticity, and hole-expandability at a level of 980 MPa. After winding, the steel coil is allowed to cool naturally to obtain a microstructure of bainite and retained austenite. Typically, the cooling rate for natural cooling is ≤20°C / h, preferably ≤15°C / h.

[0039] The advantageous effects of the present invention are, (1) By adopting a relatively economical composition design concept (for example, by not adding precious metal elements or adding them in small amounts) and employing an innovative cooling process route, we obtained a high-hole-expandability steel at the 980 MPa level with excellent performance in terms of strength, plasticity, toughness, cold bending, and hole-expandability; (2) The steel coil or steel sheet has excellent strength, plasticity and compatibility with hole-expanding and flanging performance, with a yield strength of ≥800 MPa and a tensile strength of ≥980 MPa, as well as good elongation (lateral A 50 It also possesses hole-expanding properties (hole expansion ratio ≥ 40%) and can be used in the manufacture of automotive chassis and subframe parts that require high strength, thin walls, hole expansion, and flanging, and is expected to have a very wide range of applications. [Brief explanation of the drawing]

[0040] [Figure 1] Figure 1 is a process flowchart of the method for manufacturing bainite high-hole-expandability steel at a level of 980 MPa according to the present invention. [Figure 2] Figure 2 is a conceptual diagram of the rolling process in the method for producing bainite hole-expandable steel at a level of 980 MPa according to the present invention. [Figure 3] Figure 3 is a conceptual diagram of the cooling process in the method for producing bainite hole-expandable steel at a level of 980 MPa according to the present invention. [Figure 4] Figure 4 is a typical metallographic image of Example 2 of the 980 MPa level bainite hole-expanding steel according to the present invention. [Figure 5] Figure 5 is a typical metallographic image of Example 4 of the 980 MPa level bainite hole-expanding steel according to the present invention. [Figure 6] Figure 6 is a typical metallographic image of Example 6 of the 980 MPa level bainite hole-expanding steel according to the present invention. [Figure 7] Figure 7 is a typical metallographic image of Example 8 of the 980 MPa level bainite hole-expanding steel according to the present invention. [Modes for carrying out the invention]

[0041] Specific Embodiments Referring to Figures 1 to 3, the method for producing bainite high-hole-expandability steel at a level of 980 MPa according to the present invention includes the following steps: 1) Smelting and casting The mixture is smelted in a converter or electric furnace according to the specified composition, then refined in a vacuum furnace, and finally cast into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100-1200°C for 1-2 hours; 3) Hot rolling The rolling process begins at a temperature of 950-1100°C, followed by 3-5 passes at 950°C or higher under high pressure, with a cumulative deformation of ≥50%; then, the intermediate billet temperature is raised to 920-950°C, followed by 3-7 final rolling passes, with a cumulative deformation of ≥70%; and the rolling process ends at a temperature of 800-920°C. 4) Cooling To achieve dynamic recovery and further homogenize the deformed austenite, the strip is first air-cooled for 0-10 seconds, then water-cooled to 400-550°C at a cooling rate of ≥10°C / s, wound up, and then allowed to cool naturally to room temperature. 5) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 100 m / min, the pickling temperature is controlled between 75 and 85°C, the tensile straightening rate is controlled to ≤2%, rinsing is performed in the temperature range of 35 to 50°C, the surface is dried between 120 and 140°C, and oil is applied.

[0042] Table 1 shows the composition of the examples of high hole-expanding steel according to the present invention, and Tables 2-3 show the production process parameters of the examples of steel according to the present invention, provided that the thickness of the steel billet in the rolling process is 120 mm; Table 4 shows the mechanical performance of the steel sheet according to the examples of the present invention. In the examples, tensile performance (yield strength, tensile strength, elongation) was measured according to the international standard ISO 6892-2-2018, and the hole expansion ratio was measured according to the international standard ISO 16630-2017.

[0043] As can be seen from Table 4, the steel coil has a yield strength of ≥800 MPa, a tensile strength of ≥980 MPa, an elongation of 10-13%, and a hole expansion ratio of ≥40%.

[0044] Figures 4-7 show typical metallographic images of Examples 2, 4, 6, and 8. As can be seen, the typical microstructure is bainite, with a small amount of retained austenite present.

[0045] As can be seen from the above examples, the 980MPa high hole-expanding steel according to the present invention has good strength, plasticity and hole-expanding properties, and is particularly suitable for manufacturing parts such as control arms and other automotive chassis parts that require high strength, thinning, and hole-expanding / flamming. It can also be applied to parts such as wheels that require hole-flamming, and its wide range of applications is expected.

[0046] [Table 1]

[0047] [Table 2]

[0048] [Table 3]

[0049] [Table 4]

Claims

1. A bainite hot-rolled pickled steel sheet with a tensile strength of 980 MPa or more and high hole-expanding properties, The steel sheet has a chemical composition by weight percentage of C 0.05-0.10%, Si 0.5-2.0%, Mn 1.0-2.0%, P ≤ 0.02%, S ≤ 0.003%, Al 0.02-0.08%, N ≤ 0.004%, Mo 0.1-0.55%, Ti 0.01-0.05%, Cr ≤ 0.5%, B ≤ 0.002%, O ≤ 0.0030%, with the remainder being Fe and other unavoidable impurities, and the microstructure of the steel sheet is bainite and retained austenite, with a retained austenite content of 1-5% by volume, and is a bainite hot-rolled pickled steel sheet with a tensile strength of 980 MPa or more and high hole-expanding properties.

2. A bainite hot-rolled pickled steel sheet with high hole-expanding properties having a tensile strength of 980 MPa or more, further comprising one or more elements from Nb ≤ 0.06%, V ≤ 0.05%, Cu ≤ 0.5%, Ni ≤ 0.5%, and Ca ≤ 0.005%, as described in claim 1.

3. The bainite hot-rolled pickled steel sheet with high hole-expanding properties having a tensile strength of 980 MPa or more, wherein the content of Nb and V is each ≤0.03%, the content of Cu and Ni is each ≤0.3%, and the content of Ca is ≤0.002%.

4. The chemical composition of the steel sheet is as follows, by weight percentage: C 0.05-0.10%, Si 0.5-2.0%, Mn 1.0-2.0%, P ≤ 0.02%, S ≤ 0.003%, Al 0.02-0.08%, N ≤ 0.004%, Mo 0.1-0.55%, Ti The bainite hot-rolled pickled steel sheet having a tensile strength of 980 MPa or more, comprising 0.01 to 0.05%, Cr ≤ 0.5%, B ≤ 0.002%, O ≤ 0.0030%, Nb ≤ 0.06%, V ≤ 0.05%, Cu ≤ 0.5%, Ni ≤ 0.5%, Ca ≤ 0.005%, with the remainder being Fe and other unavoidable impurities, wherein the steel sheet contains at least one of Nb, V, Cu, Ni, and Ca.

5. The bainite hot-rolled pickled steel sheet with high hole-expanding properties having a tensile strength of 980 MPa or more, wherein the C content is 0.06 to 0.08%.

6. The bainite hot-rolled pickled steel sheet with high hole-expanding properties according to claim 1, wherein the Si content is 0.8 to 1.6%.

7. The bainite hot-rolled pickled steel sheet with high hole-expanding properties having a tensile strength of 980 MPa or more, wherein the Mn content is 1.4 to 1.8%.

8. The hot-rolled, pickled steel sheet of bainite hole-expandable steel having a tensile strength of 980 MPa or more, wherein the S content is controlled to 0.0015% or less, and / or the N content is controlled to 0.003% or less, according to claim 1.

9. The bainite hot-rolled pickled steel sheet with high hole-expanding properties according to claim 1, wherein the Al content is 0.02 to 0.05%.

10. The bainite hot-rolled pickled steel sheet with high hole-expanding properties having a tensile strength of 980 MPa or more, wherein the Ti content is 0.01 to 0.03%.

11. The bainite hot-rolled pickled steel sheet with high hole-expanding properties having a tensile strength of 980 MPa or more, wherein the Mo content is 0.15 to 0.55%.

12. The bainite hot-rolled pickled steel sheet having a tensile strength of 980 MPa or more, wherein the Cr content is 0.2 to 0.4%, and / or the B content is 0.0005 to 0.0015%.

13. The steel plate has a yield strength of ≥ 800 MPa, a tensile strength of ≥ 980 MPa, and a transverse elongation of A ５０ A bainite hot-rolled pickled steel sheet with high hole-expanding properties, having a tensile strength of 980 MPa or more, as described in claim 1, wherein the hole expansion ratio is ≥10% and the hole expansion ratio is ≥40%.

14. The steel plate has a yield strength of ≥ 850 MPa, a tensile strength of ≥ 1000 MPa, and a transverse elongation of A ５０ A bainite hot-rolled pickled steel sheet with high hole-expanding properties having a tensile strength of 980 MPa or more, characterized by having a tensile strength of ≥11% and a hole expansion ratio of ≥50%, as described in claim 1.

15. A method for producing a bainite hot-rolled pickled steel sheet with a tensile strength of 980 MPa or more and hole-expanding properties, as described in any one of claims 1 to 14, characterized by comprising the following steps. 1) Smelting and casting The mixture is smelted in a converter or electric furnace according to any one of claims 1 to 12, then secondary refined in a vacuum furnace, and finally cast into a billet or ingot; 2) Reheat the billet or ingot at a heating temperature of 1100-1200°C for 1-2 hours; 3) Hot rolling The rolling start temperature is set to 950-1100°C, and 3-5 passes are performed at 950°C or higher under high pressure, with a cumulative deformation of ≥50%; next, the temperature of the intermediate billet is raised to 920-950°C, and finally 3-7 passes of rolling are performed, with a cumulative deformation of ≥70%; the rolling end temperature is set to 800-920°C; 4) Cooling First, the strip is air-cooled for 0-10 seconds, then water-cooled to 400-550°C at a cooling rate of ≥10°C / s, wound up, and then allowed to cool naturally to room temperature. 5) Pickling The pickling speed of the steel strip is adjusted to within the range of 30 to 100 m / min, the pickling temperature is controlled between 75 and 85°C, the tensile straightening rate is controlled to ≤2%, then it is rinsed, the surface of the steel strip is dried, and oil is applied.

16. A method for producing a bainite hot-rolled pickled steel sheet with a tensile strength of 980 MPa or more and high hole-expanding properties, as described in claim 15, characterized in that, after pickling in step 5), the sheet is rinsed in a temperature range of 35 to 50°C, the surface is dried between 120 and 140°C, and oil is applied.

17. In step 3), after 3 to 5 passes at 950°C or higher under high pressure, the cumulative deformation amount is set to ≥60%, and finally, after 3 to 7 passes of rolling, the cumulative deformation amount is set to ≥85%. A method for producing a bainite hot-rolled pickled steel sheet with a tensile strength of 980 MPa or more and high hole-expanding properties, according to claim 15, wherein in step 4) the cooling rate is ≥ 30°C / s.

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