High-strength high-hole-expansion steel and preparation method thereof

Abstract

The invention provides high-strength and high-hole-expansion steel. The high-strength and high-hole-expansion steel comprises, by mass, 0.04%-0.12% of carbon, smaller than or equal to 0.1% of silicon, 1.3%-1.7% of manganese, smaller than or equal to 0.002% of sulfur, smaller than or equal to 0.015% of phosphorus, 0.01%-0.03% of niobium, 0.0014%-0.06% of vanadium, 0.1%-0.15% of titanium, 0.08%-0.18% of molybdenum, 0.02%-0.05% of acid-soluble aluminum and smaller than or equal to 0.006% of nitrogen. The high-strength and high-hole-expansion steel prepared by the preparation method disclosed by the invention has relatively good hole expansion property while considering the strength, and is high in practicability and worthy of popularization.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to high-strength high-hole expansion steel and a preparation method thereof. Background Art

[0002] With the increasingly stringent national environmental laws and regulations and the implementation of national restrictions on automobile emissions, high-strength thinning or lightweight automobile structures have become an important research direction for international automobile manufacturers in the field of automobiles, especially passenger cars. From the perspective of energy conservation and emission reduction, there are two main ways to improve automobile fuel efficiency: one is to develop engines with higher fuel efficiency; the other is to use high-strength steel or light steel to reduce the weight of the car body and thus reduce emissions.

[0003] Due to the complex structure, the chassis and suspension components of passenger cars need to go through flanging and hole expansion processes, and usually require steel plates with a higher hole expansion rate to ensure that there is no cracking during the stamping process. The hole expansion and strength of steel plates are in a contradictory relationship, and high strength usually means lower hole expansion performance. Therefore, in order to cold-form components such as automobile chassis and suspension (high hole expansion), automobile manufacturers usually use high hole expansion steel with lower strength levels such as 440MPa and 590MPa, which limits the potential of high-strength thinning of these components.

[0004] Many foreign steel companies and R&D institutions have conducted research and development on high hole expansion steel. Kobe Steel of Japan developed 600MPa grade hot-rolled high hole expansion steel as early as the early 1990s. It is now widely used in automobile wheels and other parts, with significant weight reduction effect. However, due to the above-mentioned situation of mutual restraint between hole expansion and strength, hole expansion and strength still have low performance margins, making it difficult to achieve breakthrough progress. Summary of the invention

[0005] In view of this, the purpose of the present invention is to complete hardening measures such as solid solution strengthening, precipitation strengthening, dislocation strengthening, and fine grain strengthening within a limited deformation amount and temperature range based on metal deformation, phase transformation and other processes, to obtain a high-strength and high-hole expansion steel with a microscopic dual-phase structure of ferrite and bainite, a yield strength ≥600MPa, a tensile strength ≥780MPa, an elongation ≥12%, and a hole expansion rate ≥55%, so as to solve the technical problem in the prior art that hole expansion and strength restrict each other and it is difficult to achieve breakthrough progress.

[0006] To achieve the above-mentioned purpose, the present invention provides a high-strength high-hole expansion steel, whose chemical composition and mass fraction include: carbon 0.04% to 0.12%, silicon ≤0.1%, manganese 1.3% to 1.7%, sulfur ≤0.002%, phosphorus ≤0.015%, niobium 0.01% to 0.03%, vanadium 0.0014% to 0.06%, titanium 0.1% to 0.15%, molybdenum 0.08% to 0.18%, acid-soluble aluminum 0.02% to 0.05%, and nitrogen ≤0.006%.

[0007] Furthermore, the microstructure of the high-strength high-hole expansion steel is a dual-phase structure of ferrite and bainite.

[0008] Furthermore, the yield strength of the high-strength high-hole expansion steel is ≥600MPa, the tensile strength is ≥780MPa, the elongation is ≥12%, and the hole expansion ratio is ≥55%.

[0009] The present invention provides a method for preparing high-strength high-hole expansion steel, comprising the steps of:

[0010] S1. Heating the steel billet;

[0011] The chemical composition and mass fraction of the steel billet include: carbon 0.04% to 0.12%, silicon ≤0.1%, manganese 1.3% to 1.7%, sulfur ≤0.002%, phosphorus ≤0.015%, niobium 0.01% to 0.03%, vanadium 0.0014-0.06, titanium 0.1% to 0.15%, molybdenum 0.08-0.18, acid-soluble aluminum 0.02-0.05, nitrogen ≤0.006;

[0012] S2. rolling the heated billet;

[0013] The rolling process includes rough rolling and finish rolling in sequence to obtain a steel strip, wherein the starting rolling temperature of the rough rolling is 1150-1190°C, and the final rolling temperature of the rough rolling is 1020-1100°C; the starting rolling temperature of the finish rolling is 980-1060°C, and the final rolling temperature of the finish rolling is 850-890°C;

[0014] S3. Coiling the steel strip;

[0015] S4. Cooling to room temperature to obtain the high-strength high-hole expansion steel.

[0016] Furthermore, the preparation of the steel billet includes: the molten steel is sequentially subjected to KR desulfurization, converter smelting, LF refining, and RH vacuum treatment, and then continuously cast to obtain the steel billet; wherein the continuous casting speed is ≥0.85m / min.

[0017] Furthermore, in the step of rolling the heated steel billet, the heated steel billet enters a single stand for 5 to 7 passes of rough rolling, and then is rolled in a seven-stand finishing rolling unit.

[0018] Furthermore, in step S1, the heating temperature of the steel billet is 1230-1260° C., the heating time of the steel billet is 160-300 min, and the solution time of the steel billet is 20-30 min.

[0019] Furthermore, the coiling in step S3 includes: cooling the steel strip after the rolling process to a coiling temperature;

[0020] Wherein, the cooling method of cooling the steel strip after the rolling process to the coiling temperature includes laminar cooling, and the coiling temperature is 530-590°C.

[0021] Furthermore, during the LF refining process, the temperature of the LF outgoing station is 1600-1630° C., and the time is 40-90 minutes. The temperature of the molten steel after LF refining is greater than 1538° C. and is transferred to the RH vacuum treatment.

[0022] Furthermore, in the RH vacuum treatment, the vacuum degree is required to be no greater than 133 MPa, and the RH cycle time is 180 to 30 minutes.

[0023] Compared with the prior art, the present invention has at least the following advantages:

[0024] This application limits the carbon content in high-strength high-hole expansion steel to a range of 0.04% to 0.12%. A carbon content greater than 0.04% can avoid the formation of lath martensite and ensure the purity of the ferrite-bainite dual-phase structure. When the carbon content is greater than 0.12%, the low-temperature toughness, welding performance and corrosion resistance of the high-strength high-hole expansion steel will be deteriorated.

[0025] This application limits the silicon content in high-strength and high-hole expansion steel to less than 0.1%. The appropriate silicon content balances the strength and plasticity of the steel plate while playing a role in solid solution strengthening: on the one hand, silicon effectively reduces the solubility of carbon in austenite, thereby reducing the stability of supercooled austenite and accelerating the phase change process. On the other hand, silicon can inhibit the precipitation of carbides and increase the ferrite content, which is beneficial to improving the hole expansion performance and improving the elongation of the steel plate. However, a silicon content greater than 0.1% will affect the surface quality and welding performance of high-strength and high-hole expansion steel.

[0026] The present application controls the manganese content to 1.3% to 1.7%. Appropriate manganese content can improve hardenability and improve the strength of high-strength and high-expansion steel. However, manganese itself is a crystalline positive segregation element. When the manganese content is higher than 13.7%, banded structure will be produced, making the toughness and strength of the material uneven, seriously affecting product quality.

[0027] The present application controls the sulfur content to 0.002%. An appropriate sulfur content can reduce the number and size of sulfide inclusions, and modify the morphology of sulfides, thereby avoiding the formation of long sulfides and improving the material pore expansion rate. However, when the sulfur content is too high, MnS segregation and sulfide inclusions are easily generated.

[0028] This application controls the phosphorus content to below 0.015%. Appropriate phosphorus content can work together with sulfur and manganese to improve the strength and hardness of steel while improving the cutting performance of steel and the surface quality of the material. However, when the phosphorus content is higher than 0.015%, phosphorus is dissolved in ferrite, causing severe segregation and increased temper brittleness, significantly reducing the plasticity and toughness of steel, making the steel prone to brittle cracking during cold working.

[0029] This application controls the niobium content to 0.01% to 0.03%. As a carbide-forming element, Nb can increase the recrystallization temperature of austenite, making austenite more adaptable to high temperatures during rolling. An appropriate amount of Nb can also play a precipitation strengthening role in the process of controlling continuous cooling during rolling. Through the strain induction of Nb on carbonitrides, after the austenite grains are precipitated and the austenite grains are refined, a ferrite structure with fine grains is obtained, thereby improving the strength and low-temperature toughness of the material. When the niobium content is higher than 0.025%, it will make the control of the yield ratio more difficult.

[0030] The present application controls the mass fraction of vanadium to 0.0014% to 0.06%. As an excellent deoxidizer, an appropriate content of V can refine the grain size, thereby improving the strength and toughness of the material. At the same time, the carbides formed by vanadium and carbon can improve the resistance to hydrogen corrosion under high temperature and high pressure.

[0031] The present application controls the content of aluminum to be between 0.02% and 0.05%. An appropriate content of Al can promote the formation of acicular ferrite together with boron. However, when the content of aluminum is higher than 0.05%, the toughness of the material will be impaired. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0033] Figure 1 The electron microscope image of the surface layer of the high-strength high-expansion steel prepared in one embodiment of the present invention at a scale of 10 microns is as follows: Figure 1 As shown;

[0034] Figure 2 This is an electron microscope image of a cross section of a high-strength, high-hole expansion steel made in one embodiment of the present invention at a quarter of the distance from the edge in the width direction;

[0035] Figure 3 This is an electron microscope image of the central structure of the high-strength and high-hole expansion steel produced in one embodiment of the present invention;

[0036] Figure 4 This is a real picture of the hole expansion of the high-strength and high-hole expansion steel produced in one embodiment of the present invention, wherein from left to right and from top to bottom are the real pictures of the hole expansion of the first group to the sixth group respectively. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] Furthermore, the technical solutions between the various embodiments of the present invention may be combined with each other, but this must be based on the fact that they can be implemented by ordinary technicians in the field. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0039] When numerical ranges are given in the embodiments, it should be understood that, unless otherwise specified in the present invention, both endpoints of each numerical range and any value between the two endpoints can be selected. Unless otherwise defined, all technical and scientific terms used in the present invention are familiar to those skilled in the art and the description of the present invention, and any methods, equipment and materials of the prior art similar or equivalent to the methods, equipment and materials described in the embodiments of the present invention can also be used to implement the present invention.

[0040] The present application provides a high-strength high-hole expansion steel, whose chemical composition and mass fraction include: carbon 0.04% to 0.12%, silicon ≤0.1%, manganese 1.3% to 1.7%, sulfur ≤0.002%, phosphorus ≤0.015%, niobium 0.01%-0.03%, vanadium 0.0014%-0.06%, titanium 0.1% to 0.15%, molybdenum 0.08% to 0.18%, acid-soluble aluminum 0.02% to 0.05%, and nitrogen ≤0.006%.

[0041] Exemplarily, the mass fraction of carbon in high-strength high-hole expansion steel can be 0.05% to 0.012% or 0.04% to 0.10%. In some specific embodiments, the mass fraction of carbon can be 0.05% to 0.10%, and in some more specific embodiments, 0.05% to 0.07%.

[0042] Exemplarily, the mass fraction of silicon in high-strength high-hole expansion steel may be ≤0.8. In some specific embodiments, the mass fraction of silicon may be 0.1% to 0.8%. In some more specific embodiments, the mass fraction of silicon may be 0.2% to 0.5%.

[0043] Exemplarily, the mass fraction of manganese in high-strength high-hole expansion steel can be 1.3% to 1.6%. In some specific embodiments, the mass fraction of manganese can be 1.3% to 1.5%. In some more specific embodiments, the mass fraction of manganese can be 1.4% to 1.5%.

[0044] Exemplarily, the mass fraction of niobium in high-strength high-hole expansion steel can be 0.02% to 0.03%. In some specific embodiments, the mass fraction of niobium can be 0.02% to 0.025%. In some more specific embodiments, the mass fraction of niobium can be 0.25% to 0.03%.

[0045] Exemplarily, the mass fraction of vanadium in high-strength high-hole expansion steel can be 0.04%-0.06%. In some specific embodiments, the mass fraction of vanadium can be 0.0014% to 0.04%. In some more specific embodiments, the mass fraction of vanadium can be 0.0014% to 0.02%.

[0046] Exemplarily, the mass fraction of molybdenum in high-strength high-hole expansion steel can be 0.08%-0.015%. In some specific embodiments, the mass fraction of molybdenum can be 0.08%-0.013%. In some more specific embodiments, the mass fraction of molybdenum can be 0.10%-0.013%.

[0047] Exemplarily, the mass fraction of nitrogen in high-strength high-hole expansion steel can be 0.001%-0.006%. In some specific embodiments, the mass fraction of nitrogen can be 0.001%~0.004%. In some more specific embodiments, the mass fraction of nitrogen can be 0.001%~0.0035%.

[0048] In some embodiments, the present application makes full use of the rolling and heat treatment control capabilities of the continuous rolling mill, and adopts a production method of controlled rolling and accelerated cooling; grains of appropriate size are obtained through deformation recrystallization of the high-temperature austenite zone, deformation of the low-temperature austenite non-recrystallization zone, and accelerated cooling after rolling.

[0049] From the perspective of organizational type, ferrite and bainite organizations usually have relatively high hole expansion performance, but due to the mutual restriction between hole expansion and strength, it is always difficult for this dual-phase steel to make breakthrough progress in strength. In common technologies, high-strength and high-hole expansion steels are also mainly concentrated in the two strength levels of 440MPa and 590MPa.

[0050] Although ferrite and martensite structures have lower yield ratio and higher strength, their hole expandability is the worst among high-strength steels with the same strength level but different structure types due to the large difference in performance between the two phases.

[0051] The present invention designs a steel material with both high strength and high hole expansion performance. The high strength and high hole expansion steel has a yield strength of ≥600MPa, a tensile strength of ≥780MPa, an elongation of ≥12%, and a hole expansion rate of ≥55%.

[0052] In some embodiments, the thickness of the high-strength high-hole expansion steel may be 1.5 to 6 mm.

[0053] The role of limiting the content of each element in this application: This application limits the carbon content in high-strength high-hole expansion steel to a range of 0.04% to 0.12%. A carbon content greater than 0.04% can avoid the formation of lath martensite and ensure the purity of the ferrite and bainite dual-phase structure. When the carbon content is greater than 0.12%, the low-temperature toughness, welding performance and corrosion resistance of the high-strength high-hole expansion steel will be deteriorated.

[0054] This application limits the silicon content in high-strength and high-hole expansion steel to less than 0.1%. The appropriate silicon content balances the strength and plasticity of the steel plate while playing a role in solid solution strengthening: on the one hand, silicon effectively reduces the solubility of carbon in austenite, thereby reducing the stability of supercooled austenite and accelerating the phase change process. On the other hand, silicon can inhibit the precipitation of carbides and increase the ferrite content, which is beneficial to improving the hole expansion performance and improving the elongation of the steel plate. However, a silicon content greater than 0.1% will affect the surface quality and welding performance of high-strength and high-hole expansion steel.

[0055] The present application controls the manganese content to 1.3% to 1.7%. Appropriate manganese content can improve hardenability and improve the strength of high-strength and high-expansion steel. However, manganese itself is a crystalline positive segregation element. When the manganese content is higher than 13.7%, banded structure will be produced, making the toughness and strength of the material uneven, seriously affecting product quality.

[0056] The present application controls the sulfur content to 0.002%. An appropriate sulfur content can reduce the number and size of sulfide inclusions, and modify the morphology of sulfides, thereby avoiding the formation of long sulfides and improving the material pore expansion rate. However, when the sulfur content is too high, MnS segregation and sulfide inclusions are easily generated.

[0057] This application controls the phosphorus content to below 0.015%. Appropriate phosphorus content can work together with sulfur and manganese to improve the strength and hardness of steel while improving the cutting performance of steel and the surface quality of the material. However, when the phosphorus content is higher than 0.015%, phosphorus is dissolved in ferrite, causing severe segregation and increased temper brittleness, significantly reducing the plasticity and toughness of steel, making the steel prone to brittle cracking during cold working.

[0058] This application controls the niobium content to 0.01% to 0.03%. As a carbide-forming element, Nb can increase the recrystallization temperature of austenite, making austenite more adaptable to high temperatures during rolling. An appropriate amount of Nb can also play a precipitation strengthening role in the process of controlling continuous cooling during rolling. Through the strain induction of Nb on carbonitrides, after the austenite grains are precipitated and the austenite grains are refined, a ferrite structure with fine grains is obtained, thereby improving the strength and low-temperature toughness of the material. When the niobium content is higher than 0.025%, it will make the control of the yield ratio more difficult.

[0059] The present application controls the mass fraction of vanadium to 0.0014% to 0.06%. As an excellent deoxidizer, an appropriate content of V can refine the grain size, thereby improving the strength and toughness of the material. At the same time, the carbides formed by vanadium and carbon can improve the resistance to hydrogen corrosion under high temperature and high pressure.

[0060] The present application controls the content of aluminum to be between 0.02% and 0.05%. An appropriate content of Al can promote the formation of acicular ferrite together with boron. However, when the content of aluminum is higher than 0.05%, the toughness of the material will be impaired.

[0061] Gaseous elements such as nitrogen, oxygen, and hydrogen are extremely detrimental to the toughness of materials. The N element can be dissolved in iron to form an interstitial solid solution. It can also be combined with elements such as Al, V, and Ti to form extremely stable nitrides to achieve precipitation strengthening effects. However, while the N element improves the strength and hardness of the material, it also reduces the toughness of the material, increases the brittleness of the material, increases notch sensitivity, and reduces fatigue performance. At the same time, excessive residual nitrogen in the material will also lead to loose macrostructure or pores.

[0062] The present application also provides a method for preparing high-strength high-hole expansion steel, comprising the steps of:

[0063] S1. Heating the steel billet.

[0064] The chemical composition and mass fraction of the steel billet include: carbon 0.04%-0.12%, silicon ≤0.1%, manganese 1.3%-1.7%, sulfur ≤0.002%, phosphorus ≤0.015%, niobium 0.01%-0.03%, vanadium 0.0014-0.06, titanium 0.1%-0.15%, molybdenum 0.08-0.18, acid-soluble aluminum 0.02-0.05, and nitrogen ≤0.006.

[0065] Exemplarily, the mass fraction of carbon in the steel billet may be 0.05% to 0.012% or 0.04% to 0.10%. In some specific embodiments, the mass fraction of carbon may be 0.05% to 0.10%, and in some more specific embodiments, 0.05% to 0.07%.

[0066] Exemplarily, the mass fraction of silicon in the steel billet may be ≤0.8. In some specific embodiments, the mass fraction of silicon may be 0.1% to 0.8%. In some more specific embodiments, the mass fraction of silicon may be 0.2% to 0.5%.

[0067] Exemplarily, the mass fraction of manganese in the steel billet may be 1.3% to 1.6%. In some specific embodiments, the mass fraction of manganese may be 1.3% to 1.5%. In some more specific embodiments, the mass fraction of manganese may be 1.4% to 1.5%.

[0068] Exemplarily, the mass fraction of niobium in the steel billet may be 0.02% to 0.03%. In some specific embodiments, the mass fraction of niobium may be 0.02% to 0.025%. In some more specific embodiments, the mass fraction of niobium may be 0.25% to 0.03%.

[0069] Exemplarily, the mass fraction of vanadium in the steel billet may be 0.04%-0.06%. In some specific embodiments, the mass fraction of vanadium may be 0.0014%-0.04%. In some more specific embodiments, the mass fraction of vanadium may be 0.0014%-0.02%.

[0070] Exemplarily, the mass fraction of molybdenum in the steel billet may be 0.08%-0.015%. In some specific embodiments, the mass fraction of molybdenum may be 0.08%-0.013%. In some more specific embodiments, the mass fraction of molybdenum may be 0.10%-0.013%.

[0071] Exemplarily, the mass fraction of nitrogen in the steel billet may be 0.001%-0.006%. In some specific embodiments, the mass fraction of nitrogen may be 0.001%-0.004%. In some more specific embodiments, the mass fraction of nitrogen may be 0.001%-0.0035%.

[0072] In some embodiments, the preparation of the steel billet includes: the molten steel is sequentially subjected to KR desulfurization, converter smelting, LF refining, and RH vacuum treatment, and then continuously cast to obtain the steel billet.

[0073] Further refinement:

[0074] KR desulfurization: remove the sulfur content of molten steel to below 0.003%.

[0075] Converter smelting: After converter smelting, argon is blown through the ladle, and the terminal temperature of the argon station is controlled to be greater than 1535° C. For example, converter slag blocking steelmaking or double slag smelting can be adopted.

[0076] LF refining: LF outlet temperature is 1600-1630℃, and refining time is 40-90 minutes. Refining is used to fine-tune the composition of molten steel to the target composition (carbon 0.04%-0.12%, silicon ≤0.1%, manganese 1.3%-1.7%, sulfur ≤0.002%, phosphorus ≤0.015%, niobium 0.01%-0.03%, vanadium 0.0014%-0.06%, titanium 0.1%-0.15%, molybdenum 0.08%-0.18%, acid-soluble aluminum 0.02%-0.05%, nitrogen ≤0.006%). During the argon blowing and stirring of molten steel in the LF furnace, the contact interface of slag is increased, further achieving deep desulfurization.

[0077] RH vacuum treatment: When the molten steel temperature is greater than 1538℃, it is transferred to RH for vacuum treatment. The vacuum degree is required to be ≤133MPa and the RH cycle time is 18-30min.

[0078] Continuous casting: The continuous casting speed is controlled at ≥0.85m / min, and the steel billet is continuously cast in a light pressure mode.

[0079] In other embodiments, the billet may be heated by furnace heating, the billet heating temperature may be 1230-1260° C., the heating time may be 160-300 min, and the solution time may be 20-30 min.

[0080] S2. The heated steel billet is subjected to rolling treatment.

[0081] The rolling process includes rough rolling and finish rolling in sequence to obtain a steel strip. The starting rolling temperature of the rough rolling can be 1150-1190°C, and the final rolling temperature of the rough rolling can be 1020-1100°C; the starting rolling temperature of the finish rolling can be 980-1060°C, and the final rolling temperature of the finish rolling can be 850-890°C.

[0082] To further refine, the rolling process can be limited to entering a single stand for 5-7 passes of rough rolling, and then rolling through a 7-stand finishing mill, and after rolling entering a laminar cooling system for tissue control.

[0083] Hot rolling is based on metal deformation, phase transformation and other processes. Under the specified deformation and temperature conditions, solid solution strengthening, precipitation strengthening, dislocation strengthening, fine grain strengthening and other hardening measures are completed to obtain steel plates with excellent comprehensive performance. Reduce the use of precious alloys, reduce manufacturing costs, make full use of the rolling and heat treatment control capabilities of the continuous rolling unit, adopt controlled rolling and accelerated cooling to produce, and obtain grains of appropriate size through deformation recrystallization in the high-temperature austenite zone, deformation in the low-temperature austenite non-recrystallization zone, and accelerated cooling after rolling.

[0084] S3. Coiling the steel strip.

[0085] Specifically, the steel strip is coiled after being cooled to the coiling temperature.

[0086] In some embodiments, the cooling method of cooling the rolled steel strip to the coiling temperature includes laminar cooling, and the coiling temperature can be controlled at 530-590°C.

[0087] S4. Cooling to room temperature to obtain the high-strength high-hole expansion steel.

[0088] In order to facilitate those skilled in the art to further understand the present invention, examples are given below:

[0089] Embodiment 1:

[0090] The molten steel is desulfurized by KR and then smelted in a 210-ton converter, and then argon is blown into a ladle, refined in a LF furnace, vacuum treated in RH, and continuously cast into a 230mm billet. The billet is pile-cooled, then heated, and rough-rolled for 5-7 times. It is then thermomechanically controlled rolled and controlled cooled in a 7-stand 2250 continuous rolling unit to obtain high-strength and high-hole expansion steel.

[0091] The thickness specifications of high-strength and high-hole expansion steel are 4mm, 3.5mm and 3mm. Its composition is shown in Table 1, the controlled rolling and controlled cooling process parameters are shown in Table 2, and the mechanical properties are shown in Table 3.

[0092] Table 1 Chemical composition of high strength hot rolled high hole expansion steel

[0093]

[0094]

[0095] Table 2 Controlled rolling and controlled cooling process parameters

[0096]

[0097] Table 3 Mechanical properties

[0098]

[0099]

[0100] The electron microscope images of the surface of the high-strength and high-expansion steel obtained in each group at a scale of 10 microns are as follows: Figure 1 As shown; the cross-sectional electron microscope images of the high-strength and high-hole expansion steels made in each group at a quarter of the width from the edge are shown in Figure 2 As shown; the electron microscope images of the central structure of the high-strength and high-hole expansion steel prepared in each group are shown Figure 3 As shown; the actual hole expansion pictures of the high-strength and high-hole expansion steels prepared in each group are shown in Figure 4 shown.

[0101] The above technical solutions of the present invention are only preferred embodiments of the present invention, and the patent scope of the present invention is not limited thereto. All equivalent structural changes made by using the contents of the present invention specification and drawings under the technical concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A high-strength high-hole expansion steel, characterized in that: Its chemical composition and mass fraction include: carbon 0.04% ~ 0.12%, silicon ≤0.1%, manganese 1.3% ~ 1.7%, sulfur ≤0.002%, phosphorus ≤0.015%, niobium 0.01% ~ 0.03%, vanadium 0.0014% ~ 0.06%, titanium 0.1% ~ 0.15%, molybdenum 0.08% ~ 0.18%, acid-soluble aluminum 0.02% ~ 0.05%, and nitrogen ≤0.006%.

2. The high-strength high-hole expansion steel according to claim 1, characterized in that: The microstructure of the high-strength high-hole expansion steel is a ferrite-bainite dual-phase structure.

3. The high-strength high-hole expansion steel according to claim 1, characterized in that: The high-strength high-hole expansion steel has a yield strength of ≥600 MPa, a tensile strength of ≥780 MPa, an elongation of ≥12%, and a hole expansion rate of ≥55%.

4. A method for preparing high-strength high-hole expansion steel, characterized in that: Includes steps: S1. Heating the steel billet; The chemical composition and mass fraction of the steel billet include: carbon 0.04% to 0.12%, silicon ≤0.1%, manganese 1.3-1.7%, sulfur ≤0.002%, phosphorus ≤0.015%, niobium 0.01% to 0.03%, vanadium 0.0014-0.06, titanium 0.1% to 0.15%, molybdenum 0.08-0.18, acid-soluble aluminum 0.02-0.05, nitrogen ≤0.006; S2. rolling the heated billet; The rolling process includes rough rolling and finish rolling in sequence to obtain a steel strip, wherein the starting rolling temperature of the rough rolling is 1150-1190°C, and the final rolling temperature of the rough rolling is 1020-1100°C; the starting rolling temperature of the finish rolling is 980-1060°C, and the final rolling temperature of the finish rolling is 850-890°C; S3. Coiling the steel strip; S4. Cooling to room temperature to obtain the high-strength high-hole expansion steel.

5. The method for preparing high-strength high-hole expansion steel according to claim 4, characterized in that: The preparation of the steel billet comprises: the molten steel is sequentially subjected to KR desulfurization, converter smelting, LF refining, and RH vacuum treatment, and then continuously cast to obtain the steel billet; wherein the continuous casting speed is ≥0.85 m / min.

6. The method for preparing high-strength high-hole expansion steel according to claim 5, characterized in that: In the step of rolling the heated steel billet, the heated steel billet enters a single stand for 5 to 7 passes of rough rolling, and then is rolled by a seven-stand finishing rolling unit.

7. The method for preparing high-strength high-hole expansion steel according to claim 5, characterized in that: In the step S1, the heating temperature of the steel billet is 1230-1260°C, the heating time of the steel billet is 160-300 minutes, and the solution time of the steel billet is 20-30 minutes.

8. The method for preparing high-strength high-hole expansion steel according to claim 5, characterized in that: The coiling in step S3 includes: cooling the steel strip after the rolling process to a coiling temperature; The cooling method of cooling the rolled steel strip to the coiling temperature includes laminar cooling, and the coiling temperature is 530-590°C.

9. The method for preparing high-strength high-hole expansion steel according to claim 5, characterized in that: During the LF refining process, the temperature of the LF outgoing station is 1600-1630° C., and the time is 40-90 minutes. The temperature of the molten steel after LF refining is greater than 1538° C. and is transferred to the RH vacuum treatment.

10. The method for preparing high-strength high-hole expansion steel according to claim 9, characterized in that: In the RH vacuum treatment, the vacuum degree is required to be no greater than 133 MPa, and the RH cycle time is 180 to 30 minutes.

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