680MPa-grade hot-rolled high-hole-expansion steel and preparation method thereof

Abstract

The invention relates to 680MPa-grade hot-rolled high-hole-expansion steel and a preparation method thereof, and belongs to the technical field of hot-rolled strip steel production processes. The hot-rolled highly-expanded steel comprises the following chemical components in percentage by mass: 0.05%-0.12% of C, 0.1%-0.5% of Si, 0.8%-1.5% of Mn, less than or equal to 0.02% of P, less than or equal to 0.006% of S, 0.01%-0.06% of Nb, 0.05%-0.11% of Ti, 0.1%-0.8% of Cr and a matrix element Fe, the microstructure of the hot-rolled highly-reamed steel comprises, by volume fraction, 70%-90% of ferrite and 10%-30% of granular bainite. Through control over chemical components and control over technological parameters such as continuous casting and hot rolling, the microstructure of the produced strip steel is ferrite and bainite, due to the fact that ferrite has good plasticity and toughness, and bainite has high strength, the microstructure of ferrite and bainite can guarantee certain strength, and meanwhile the yield of the strip steel is greatly improved. And good toughness is maintained.

Description

Technical Field

[0001] The present application relates to the technical field of hot-rolled strip steel production technology, and in particular to a 680MPa grade hot-rolled high-hole expansion steel and a preparation method thereof. Background Art

[0002] In recent years, with the development of the automobile industry, passenger cars have higher and higher requirements for lightweight, safety and fuel economy. The use of high-strength steel and advanced high-strength steel is an inevitable trend to adapt to the development of the automobile industry. As a type of advanced high-strength steel (AHSS), hot-rolled pickled high-hole expansion steel has good cold formability, good hole expansion performance and fatigue performance due to the smaller hardness difference between the two phases (ferrite and bainite) in the matrix structure compared with traditional low-alloy high-strength, ferrite-martensite dual-phase steel. It has been widely used in the manufacture of automotive spokes, chassis, various frames of the body and other structural parts.

[0003] As a type of advanced high-strength steel, 680MPa-grade high-hole expansion steel has broad prospects in the manufacture of core parts such as passenger car wheel spokes, upper and lower plates of subframes, and triangular arms due to its high strength and high hole expansion performance. Taking the triangular arm as an example, due to the complex shape of the part, in addition to requiring a high elongation, there are large holes and flanging at key positions of the part, which requires high hole expansion formability. Ordinary high-strength steel is prone to hole expansion cracking, while high-hole expansion steel can solve this problem. In the existing hot-rolled pickled high-hole expansion steel production technology, when the tensile strength is ≥680MPa, high alloy elements are generally added, and the high carbon equivalent is not conducive to welding, and it is difficult to obtain a high hole expansion rate and elongation. Summary of the invention

[0004] The present application provides a 680MPa grade hot-rolled high hole expansion steel and a preparation method thereof to solve the following technical problem: how to improve the hole expansion rate of hot-rolled steel while ensuring strength.

[0005] In a first aspect, the present application provides a 680MPa grade hot-rolled high hole expansion steel, wherein the chemical composition of the hot-rolled high hole expansion steel comprises, by mass fraction: C: 0.05% to 0.12%, Si: 0.1% to 0.5%, Mn: 0.8% to 1.5%, P≤0.02%, S≤0.006%, Nb: 0.01% to 0.06%, Ti: 0.05% to 0.11%, Cr: 0.1% to 0.8%, and matrix element Fe;

[0006] Measured by volume fraction, the microstructure of the hot-rolled high hole expansion steel includes: ferrite: 70% to 90%, granular bainite: 10% to 30%.

[0007] Optionally, the microstructure of the hot-rolled high hole expansion steel satisfies: Hv 粒状贝氏体 -Hv铁素体 ≤50, where Hv 粒状贝氏体 Indicates the average hardness of granular bainite, Hv 铁素体 Indicates the average hardness of ferrite.

[0008] Optionally, the ferrite includes Ti(N,C) precipitates and TiN precipitates, the average diameter of the Ti(N,C) precipitates is <12 nm, and the maximum diameter of the TiN precipitates is ≤15 μm.

[0009] Optionally, the hot-rolled high hole expansion steel meets at least one of the following properties: yield strength ≥ 550 MPa, tensile strength

[0010] ≥680MPa, hole expansion rate λ≥65%, elongation A≥16%.

[0011] In a second aspect, the present application provides a method for preparing the 680MPa grade hot-rolled high hole expansion steel described in the first aspect, the method comprising:

[0012] Obtaining molten steel having the chemical composition;

[0013] Continuously casting the molten steel to obtain a continuously cast billet;

[0014] The continuous casting billet is sequentially subjected to rough rolling, finish rolling, stage cooling and coiling to obtain a hot rolled coil; wherein the stage cooling includes: rapid cooling, air cooling and water cooling;

[0015] The hot-rolled coil is flattened to obtain a finished hot-rolled high-hole expansion steel.

[0016] Optionally, the temperature of the rapid cooling is 600°C to 660°C, the cooling rate of the rapid cooling is ≥50°C / s; and the time of the air cooling is 5s to 10s.

[0017] Optionally, the coiling temperature is 400°C to 460°C.

[0018] Optionally, the superheat of the molten steel is ≤20°C, the specific water content of the continuous casting is ≥0.6L / kg, and the light reduction at the solidification end of the continuous casting is ≥3.5mm.

[0019] Optionally, the final reduction rate of the second rough rolling mill R2 of the rough rolling is ≥35%.

[0020] Optionally, the final rolling temperature of the rough rolling is 1050°C to 1100°C.

[0021] Optionally, the rough rolling obtains an intermediate billet, and the thickness of the intermediate billet is 30 mm to 40 mm.

[0022] Optionally, the finishing rolling is constant speed rolling, and the rolling speed of the finishing rolling is 5m / s to 10m / s.

[0023] Optionally, the finish rolling obtains a hot-rolled plate, and the thickness of the hot-rolled plate is 1.8 mm to 5 mm.

[0024] Optionally, the final rolling temperature of the finish rolling is Ar3 to Ar3+80°C.

[0025] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0026] The embodiment of the present application provides a 680MPa grade hot-rolled high hole expansion steel. The chemical composition of the hot-rolled high hole expansion steel includes, by mass fraction, C: 0.05% to 0.12%, Si: 0.1% to 0.5%, Mn: 0.8% to 1.5%, P≤0.02%, S≤0.006%, Nb: 0.01% to 0.06%, Ti: 0.05% to 0.11%, Cr: 0.1% to 0.8%, and matrix element Fe; by volume fraction, the microstructure of the hot-rolled high hole expansion steel includes: ferrite: 70% to 90%, granular bainite: 10% to 30%. By controlling the chemical composition and microstructure, the strength of the steel is guaranteed and the hole expansion rate is improved. Since ferrite has good plasticity and toughness, and bainite has high strength, the microstructure of ferrite + bainite can maintain good toughness while ensuring a certain strength. The addition of microalloying elements further refines the microstructure of the steel and improves its strength and toughness. The combined effect of these factors enables the hot-rolled high hole expansion steel to have a high hole expansion rate while ensuring strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0028] In order to more clearly illustrate the embodiments of the present application 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, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0029] Figure 1 A schematic flow chart of a method for preparing a 680MPa grade hot-rolled high hole expansion steel provided in an embodiment of the present application;

[0030] Figure 2 The microstructure of the 680MPa grade hot-rolled high hole expansion steel provided in Example 1 of the present application. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0032] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range; in addition, whenever a numerical range is indicated in this document, it is meant to include any cited numbers (fractions or integers) within the indicated range.

[0033] In this document, the terms including "including" and "including" mean "including but not limited to". Relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. "And / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "plurality" means two or more; "at least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items; for example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, if the weight ratio of substance A, substance B and substance C is 1:2:3, then substance A, substance B and substance C should correspond to the proportional numbers in the proportional formula in the order of description, that is, the weight of substance A: the weight of substance B: the weight of substance C = 1:2:3.

[0034] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0035] In a first aspect, the present application provides a 680MPa grade hot-rolled high hole expansion steel, wherein the chemical composition of the hot-rolled high hole expansion steel comprises, by mass fraction: C: 0.05% to 0.12%, Si: 0.1% to 0.5%, Mn: 0.8% to 1.5%, P≤0.02%, S≤0.006%, Nb: 0.01% to 0.06%, Ti: 0.05% to 0.11%, Cr: 0.1% to 0.8%, and matrix element Fe;

[0036] Measured by volume fraction, the microstructure of the hot-rolled high hole expansion steel includes: ferrite: 70% to 90%, granular bainite: 10% to 30%.

[0037] The positive effect of limiting the mass fraction of C to 0.05% to 0.12% is that C is the main strengthening element in steel, which can significantly improve the strength and hardness of steel. However, a mass fraction of C higher than 0.12% may reduce the plasticity and toughness of steel and increase the sensitivity of welding cracks. Within the range of 0.05% to 0.12%, the mass fraction of C can ensure the strength of steel while maintaining good plasticity and toughness. Exemplarily, the mass fraction of C can be 0.05%, 0.07%, 0.09%, 0.11%, 0.12%, etc.

[0038] The positive effect of limiting the mass fraction of Si to 0.1% to 0.5% is that Si plays a major role in solid solution strengthening in steel, which can improve the strength and hardness of steel. At the same time, Si can also improve the oxidation resistance and corrosion resistance of steel. The mass fraction of Si in the range of 0.1% to 0.5% helps to obtain good comprehensive mechanical properties. For example, the mass fraction of Si can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, etc.

[0039] The positive effect of limiting the mass fraction of Mn to 0.8% to 1.5%: Mn is an important deoxidizing and desulfurizing element in steel. It can improve the strength and hardness of steel, and improve the toughness and weldability of steel. The mass fraction of Mn in the range of 0.8% to 1.5% helps to obtain excellent mechanical properties and welding properties. For example, the mass fraction of Mn can be 0.8%, 1.1%, 1.3%, 1.5%, etc.

[0040] The positive effect of limiting P ≤ 0.02%: P is generally regarded as a harmful element in steel because it reduces the plasticity and toughness of steel and increases cold brittleness, but an appropriate amount of P can improve the strength of steel. The mass fraction of P ≤ 0.02% has little effect on the performance of steel. For example, the mass fraction of P can be 0.01%, 0.012%, 0.014%, 0.016%, 0.018%, 0.02%, etc.

[0041] The positive effect of limiting S≤0.006%: S is also a harmful element in steel. It will reduce the plasticity and toughness of steel and increase hot brittleness. The mass fraction of S≤0.006% helps to ensure the good performance of steel. For example, the mass fraction of S can be 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, etc.

[0042] The positive effect of limiting the mass fraction of Nb to 0.01% to 0.06% is that Nb is a microalloying element in steel, which can refine the grains and improve the strength and toughness of steel. The mass fraction of Nb in the range of 0.01% to 0.06% helps to obtain a good fine grain strengthening effect. For example, the mass fraction of Nb can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, etc.

[0043] The positive effect of limiting the mass fraction of Ti to 0.05% to 0.11% is that Ti can form compounds with elements such as carbon and nitrogen in steel, which can refine grains and fix free carbon and nitrogen, thereby improving the strength and toughness of steel. The mass fraction of Ti is 0.05% to 0.11%, which helps to obtain a good precipitation strengthening effect. For example, the mass fraction of Ti can be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, 0.11%, etc.

[0044] The positive effect of limiting the mass fraction of Cr to 0.1% to 0.8% is that Cr is an alloying element in steel, which can improve the strength and hardness of steel and improve the corrosion resistance of steel. The mass fraction of Cr is 0.1% to 0.8%, which helps to obtain good strength and corrosion resistance. Exemplarily, the mass fraction of Cr can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc.

[0045] Fe is a matrix element. The specific content / content range of Fe can be obtained by the upper and lower limit formula of the component, that is:

[0046] The sum of the percentages of the contents of each component in a composition should be equal to 100%, and the content ranges of several components should meet the following conditions: the upper limit value of a component + the lower limit value of other components ≤ 100; the lower limit value of a component + the upper limit value of other components ≥ 100.

[0047] In some embodiments, the microstructure of the hot-rolled high hole expansion steel satisfies: Hv 粒状贝氏体 -Hv 铁素体 ≤50, where Hv 粒状贝氏体 Indicates the average hardness of granular bainite, Hv 铁素体 Indicates the average hardness of ferrite.

[0048] In the embodiment of the present application, the microstructure of the hot-rolled high hole expansion steel meets the specific hardness condition, that is, the average hardness of the granular bainite (Hv 粒状贝氏体 ) and the average hardness of ferrite (Hv 铁素体) does not exceed 50. Granular bainite is a complex phase structure composed of ferrite and island-shaped residual austenite, martensite or MA components (martensite-austenite components) distributed therein. It has high strength and good toughness and is a common strengthening phase in hot-rolled steel. Granular bainite usually has a higher hardness because it contains hard martensite or MA components. Ferrite is a soft phase in steel with lower hardness and good plasticity. It is one of the main constituent phases in steel and makes an important contribution to the toughness and plasticity of steel.

[0049] When the difference between the average hardness of granular bainite and the average hardness of ferrite does not exceed 50, it means that the distribution of soft and hard phases in the steel is relatively uniform. This uniform distribution helps the steel to maintain high strength while having good plasticity and toughness when subjected to external forces. In addition, the uniform microstructure also helps to improve the hole expansion performance of the steel. During the hole expansion process, the coordinated deformation between the soft and hard phases can reduce stress concentration and the generation of cracks, thereby improving the success rate and quality of hole expansion.

[0050] In some embodiments, the ferrite includes Ti(N,C) precipitates and TiN precipitates, the average diameter of the Ti(N,C) precipitates is <12 nm, and the maximum diameter of the TiN precipitates is ≤15 μm.

[0051] In the embodiment of the present application, the ferrite in the hot-rolled high hole expansion steel contains specific precipitates, namely Ti(N,C) precipitates and TiN precipitates. Ti(N,C) precipitates are compounds of titanium, nitrogen and carbon, which exist in ferrite in the form of fine particles. In the embodiment of the present application, the average diameter of the Ti(N,C) precipitates is less than 12nm. Such fine precipitates help to refine the grains of the steel, thereby improving the strength and toughness of the steel. The fine precipitates can also pin the grain boundaries more effectively and inhibit grain growth, thereby maintaining the stability of the steel at high temperatures. Ti(N,C) precipitates improve the strength and hardness of the steel through the precipitation strengthening mechanism. At the same time, they can also improve the corrosion resistance and oxidation resistance of the steel. Exemplarily, the average diameter of the Ti(N,C) precipitates can be 2nm, 4nm, 6nm, 8nm, 10nm, etc.

[0052] TiN precipitates are compounds of titanium and nitrogen, which are usually present in steel in the form of larger particles. An appropriate amount of TiN precipitates can improve the hardness and wear resistance of steel. However, excessively large TiN precipitates may become a source of cracks in steel, reducing the toughness and hole expansion performance of steel. Therefore, the size of TiN precipitates needs to be strictly controlled. In an embodiment of the present application, the maximum diameter of the TiN precipitates does not exceed 15μm, and this size control can ensure that they do not have a negative impact on the performance of the steel. Exemplarily, the maximum diameter of the TiN precipitates can be 3μm, 5μm, 7μm, 9μm, 11μm, 13μm, 15μm, etc.

[0053] In some embodiments, the hot-rolled high hole expansion steel meets at least one of the following properties: yield strength ≥550 MPa, tensile strength ≥680 MPa, hole expansion ratio λ ≥65%, and elongation A ≥16%.

[0054] Yield strength is the maximum stress that steel can withstand before plastic deformation, which reflects the ability of steel to resist plastic deformation. For hot-rolled high-expansion steel, a yield strength of 550MPa or more means that the steel has high stability when subjected to external forces and is not prone to plastic deformation, thus ensuring the safety and reliability of the structure.

[0055] Tensile strength is the maximum stress that steel can withstand before breaking in a tensile test, which reflects the strength and toughness of the steel. The tensile strength of hot-rolled high-expansion steel reaches or exceeds 680MPa, indicating that the steel has a high strength level and can withstand large external forces without breaking, which is of great significance for ensuring the safety and durability of the structure.

[0056] Hole expansion rate is an important indicator to measure the hole expansion performance of steel, which reflects the ability of steel to increase the hole diameter during the hole expansion process. The hole expansion rate of hot-rolled high hole expansion steel reaches or exceeds 65%, which means that the steel can maintain good plasticity and toughness during the hole expansion process, and is not prone to cracks and fractures, thereby improving the success rate and quality of hole expansion.

[0057] Elongation is an important indicator to measure the plasticity of steel, which reflects the ability of steel to undergo plastic deformation during the stretching process. The elongation of hot-rolled high-hole expansion steel reaches or exceeds 16%, indicating that the steel has good plasticity and can undergo large plastic deformation without breaking during the stretching process, which is of great significance for improving the processing performance and forming performance of steel.

[0058] Figure 1 A schematic flow chart of a method for preparing 680MPa grade hot-rolled high hole expansion steel provided in an embodiment of the present application.

[0059] See also Figure 1In a second aspect, an embodiment of the present application provides a method for preparing the 680MPa grade hot-rolled high hole expansion steel described in the first aspect, the method comprising:

[0060] S1, obtaining molten steel having the chemical composition;

[0061] S2, continuously casting the molten steel to obtain a continuously cast billet;

[0062] In some embodiments, the superheat of the molten steel is ≤20°C, the specific water content of the continuous casting is ≥0.6L / kg, and the soft reduction at the solidification end of the continuous casting is ≥3.5mm.

[0063] The superheat of molten steel refers to the degree to which the temperature of molten steel before pouring is higher than its liquidus temperature (i.e., melting point). In the embodiments of the present application, the superheat of molten steel ≤ 20°C helps to reduce the gas and inclusion content in the molten steel, because the fluidity of molten steel is enhanced at high temperature, and it is easier to bring gas and inclusions into the ingot. By controlling the superheat, the formation of these defects can be reduced, thereby improving the purity and quality of the steel. Exemplarily, the superheat of molten steel can be 10°C, 12°C, 14°C, 16°C, 18°C, 20°C, etc.

[0064] The specific water volume is an indicator of the intensity of the water spray for the secondary cooling of continuous casting, which indicates the ratio of the total amount of water consumed per unit time to the mass of the ingot passing through the secondary cooling zone per unit time. In the embodiment of the present application, the specific water volume is ≥0.6L / kg, in order to ensure that the ingot is sufficiently cooled in the secondary cooling zone, thereby avoiding defects such as cracks and segregation. Exemplarily, the specific water volume of continuous casting can be ≥0.6L / kg, 0.7L / kg, 0.8L / kg, 0.9L / kg, 1.0L / kg, etc.

[0065] Continuous casting light reduction refers to the pressure processing technology of the solidification process of the billet, which uses local clamping rollers or other special equipment in a suitable two-phase zone at the end of the solidification of the billet to offset the volume shrinkage at the end of the solidification of the billet, avoid the formation of central shrinkage (looseness); inhibit the flow and accumulation of concentrated molten steel caused by solidification shrinkage, and reduce the degree of central macro segregation. By implementing a light reduction of ≥3.5mm at the end of solidification, the central segregation and central looseness defects of the billet can be significantly reduced. This is because the appropriate reduction can enhance convection and stirring of the solidification front of the enriched solute, promote dendrite shedding and remelting, thereby increasing the crystallization core and solidification undercooling, and making the organization and composition of the last solidified central hot zone more uniform. At the same time, the implementation of a light reduction of ≥3.5mm at the end of solidification can also compensate for the volume shrinkage of the residual molten steel at the end of solidification, enhance the shrinkage filling capacity of the billet, and further reduce the degree of central looseness. For example, the soft reduction at the end of solidification of continuous casting can be 3.5 mm, 3.6 mm, 3.7 mm, 3.8 mm, 3.9 mm, 4.0 mm, etc.

[0066] S3, sequentially performing rough rolling, finish rolling, stage cooling and coiling on the continuous casting billet to obtain a hot rolled coil; wherein the stage cooling includes: rapid cooling, air cooling and water cooling;

[0067] In some embodiments, the final rolling reduction ratio of the second roughing mill R2 of the rough rolling is ≥35%.

[0068] Rough rolling is an important step in the hot rolling process. Its main purpose is to make preliminary shape and size adjustments to the continuous casting billet. At the same time, through multiple passes of reduction, the grains are refined to improve the strength and toughness of the steel. The final reduction rate of the second rough rolling mill R2 refers to the ratio of the thickness reduction of the steel to the original thickness in the last pass of the rough rolling mill. In the embodiment of the present application, the final reduction rate of the second rough rolling mill R2 is ≥35%. This requirement is intended to ensure that the steel is fully reduced during the rough rolling process, thereby refining the grains and improving the strength and toughness of the steel. Exemplarily, the final reduction rate of R2 can be 35%, 36%, 37%, 38%, 39%, 40%, etc.

[0069] In some embodiments, the final rolling temperature of the rough rolling is 1050°C to 1100°C.

[0070] Rolling within this temperature range can effectively refine the grains of the steel. Grain refinement helps to improve the strength and toughness of the steel, so that it can meet the requirements of high-performance steel. For example, the final rolling temperature of the rough rolling can be 1050°C, 1060°C, 1070°C, 1080°C, 1090°C, 1100°C, etc.

[0071] In some embodiments, the rough rolling obtains an intermediate billet having a thickness of 30 mm to 40 mm.

[0072] In some embodiments, the finish rolling is constant speed rolling, and the rolling speed of the finish rolling is 5 m / s to 10 m / s.

[0073] Finishing rolling is carried out after rough rolling, with the aim of further improving the shape, size and performance of the steel. Constant speed rolling, that is, the biting, rolling and throwing speeds are kept consistent. This method can ensure that the steel is uniformly deformed during the rolling process, thereby obtaining better dimensional accuracy and surface quality. In the embodiment of the present application, the rolling speed is determined according to the thickness of the hot-rolled plate and is controlled within the range of 5m / s to 10m / s. Exemplarily, the rolling speed of finishing rolling can be 5m / s, 6m / s, 7m / s, 8m / s, 9m / s, 10m / s, etc.

[0074] In some embodiments, the finish rolling produces a hot-rolled plate having a thickness of 1.8 mm to 5 mm.

[0075] In some embodiments, the final rolling temperature of the finish rolling is Ar3 to Ar3+80°C.

[0076] Ar3 is the temperature at which austenite begins to form during the heating process of steel, also known as the starting temperature of austenite transformation. It is an important critical point, marking the transformation of steel from low-temperature structures such as ferrite and pearlite to high-temperature structure of austenite. Controlling the final rolling temperature of fine rolling within the range of Ar3 to Ar3+80°C can ensure that the steel is deformed in the austenite + ferrite two-phase region, thereby obtaining an ideal microstructure. Preferably, the final rolling temperature of fine rolling is Ar3 to Ar3+50°C. Exemplarily, the final rolling temperature of fine rolling can be Ar3, Ar3+20°C, Ar3+40°C, Ar3+60°C, Ar3+80°C, etc.

[0077] In some embodiments, the temperature of the rapid cooling is 600° C. to 660° C., the cooling rate of the rapid cooling is ≥50° C. / s; and the time of the air cooling is 5s to 10s.

[0078] Controlling the rapid cooling temperature within the range of 600℃~660℃ can ensure that the steel is rapidly cooled at an appropriate temperature, thereby inhibiting the growth of austenite grains and promoting the formation of favorable structures such as bainite. Exemplarily, the rapid cooling temperature can be 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, etc. Controlling the cooling rate at ≥50℃ / s can ensure that the steel is rapidly cooled in a short time, thereby further refining the grains and improving the strength and toughness of the steel. Exemplarily, the cooling rate of rapid cooling can be 50℃ / s, 52℃ / s, 54℃ / s, 56℃ / s, 58℃ / s, 60℃ / s, etc. Air cooling time refers to the time for the steel to cool naturally after rapid cooling and before water cooling. The main purpose of the air cooling stage is to achieve an appropriate temperature gradient and promote the formation of favorable structures such as bainite. Through short air cooling, a more uniform temperature distribution can be provided for the subsequent water cooling stage, thereby avoiding the structural inhomogeneity caused by too fast or too slow cooling rate. For example, the air cooling time can be 5s, 6s, 7s, 8s, 9s, 10s, etc. Finally, water cooling is used to cool the steel to the coiling temperature.

[0079] In some embodiments, the coiling temperature is 400°C to 460°C.

[0080] The coiling temperature of 400°C to 460°C is helpful to obtain an ideal mixed structure of ferrite and bainite, and further improve the comprehensive performance of the steel. For example, the coiling temperature can be 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, etc.

[0081] S4, flattening the hot-rolled coil to obtain a finished hot-rolled high-hole expansion steel.

[0082] The product prepared by the preparation method of 680MPa grade hot-rolled high hole expansion steel is the above-mentioned 680MPa grade hot-rolled high hole expansion steel. Since the preparation method of 680MPa grade hot-rolled high hole expansion steel adopts part or all of the technical solutions of the 680MPa grade hot-rolled high hole expansion steel embodiment, it has at least all the beneficial effects brought by the technical solutions of the 680MPa grade hot-rolled high hole expansion steel embodiment, which will not be repeated here one by one.

[0083] The present application is further described below in conjunction with specific embodiments. The experimental methods in the following embodiments that do not specify specific conditions are usually measured in accordance with national standards / industry standards; if there are no corresponding national standards / industry standards, they are measured in accordance with common international standards, conventional conditions, or conditions recommended by the manufacturer.

[0084] The chemical composition of molten steel is shown in Table 1.

[0085] Table 1 Chemical composition of molten steel (wt%)

[0086] Group C Si Mn P S Nb Ti Cr Example 1 0.07％ 0.30％ 1.1％ 0.015％ 0.005％ 0.050％ 0.080％ 0.60％ Example 2 0.06％ 0.35％ 1.2％ 0.012％ 0.003％ 0.045％ 0.075％ 0.62％ Example 3 0.08％ 0.25％ 1.3％ 0.014％ 0.002％ 0.055％ 0.085％ 0.58％

[0087] Based on the chemical composition of the molten steel in the embodiment, this embodiment also provides a method for preparing 680MPa grade hot-rolled high hole expansion steel, comprising the following steps:

[0088] Obtaining molten steel having the chemical composition;

[0089] Continuously casting the molten steel to obtain a continuously cast billet;

[0090] The continuous casting billet is sequentially subjected to rough rolling, finish rolling, stage cooling and coiling to obtain a hot rolled coil; wherein the stage cooling includes: rapid cooling, air cooling and water cooling;

[0091] The hot rolled coil is flattened to obtain a finished hot rolled high hole expansion steel. Please refer to Table 2 for the preparation process parameters.

[0092] Table 2 Preparation process parameters of the embodiment

[0093]

[0094]

[0095] The properties of the finished hot-rolled high hole expansion steel obtained in the embodiment were tested, and the test results are shown in Table 3.

[0096] Table 3

[0097] Example 1 Example 2 Example 3 Yield strength(MPa) 580 574 595 Tensile strength(MPa) 702 688 714 Elongation(%) 21.5 23 21.5 Hole expansion rate (%) 70 76 74 Volume fraction of ferrite (%) 75 78 73 Volume fraction of granular bainite (%) 25 22 28 <![CDATA[Hv 粒状贝氏体 -What 铁素体 ]]> 40 38 37 Average diameter of Ti(N,C) precipitates (nm) 12 10 10 Maximum diameter of TiN precipitates (μm) 15 11 13

[0098] It can be seen from Tables 1 to 3 that the chemical composition and preparation process parameters of the examples are all within the required range of the present invention, with yield strength ≥550 MPa, tensile strength ≥680 MPa, hole expansion rate λ ≥65%, and elongation A ≥16%.

[0099] Attached Figure 2 Detailed description:

[0100] Figure 2 The microstructure of the 680MPa grade hot-rolled high hole expansion steel provided in Example 1 of the present application; Figure 2 As shown, the microstructure is ferrite + bainite.

[0101] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0102] The embodiments of the present invention improve the welding performance of steel by reducing carbon equivalent and optimizing chemical composition.

[0103] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. It will be apparent to those skilled in the art that various modifications to these embodiments are possible, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown in the present application, but will conform to the widest range consistent with the principles and novel features applied for by the present application.

Claims

1. A 680MPa grade hot-rolled high hole expansion steel, wherein the chemical composition of the hot-rolled high hole expansion steel comprises, by mass fraction: C： 0.05%~0.12%, Si: 0.1%~0.5%, Mn: 0.8%~1.5%, P≤0.02%, S≤0.006%, Nb: 0.01%~0.06%, Ti: 0.05%~0.11%, Cr: 0.1%~0.8%, and matrix element Fe; In terms of volume fraction, the microstructure of the hot-rolled high hole expansion steel includes: ferrite: 70% to 90%, granular bainite: 10% to 30%.

2. The hot-rolled high hole expansion steel according to claim 1, characterized in that: The microstructure of the hot-rolled high hole expansion steel meets the following requirements: Hv 粒状贝氏体 -Hv 铁素体 ≤50, where Hv 粒状贝氏体 Indicates the average hardness of granular bainite, Hv 铁素体 Indicates the average hardness of ferrite.

3. The hot-rolled high hole expansion steel according to claim 1, characterized in that: The ferrite includes Ti(N, C) precipitates and TiN precipitates, the average diameter of the Ti(N, C) precipitates is less than 12 nm, and the maximum diameter of the TiN precipitates is less than or equal to 15 μm.

4. The hot-rolled high hole expansion steel according to claim 1, characterized in that: The hot-rolled high hole expansion steel meets at least one of the following properties: yield strength ≥550MPa, tensile strength ≥680MPa, hole expansion rate λ ≥65%, and elongation A ≥16%.

5. A method for preparing the hot-rolled high hole expansion steel according to claims 1 to 4, the method comprising: Obtaining molten steel having the chemical composition; Continuously casting the molten steel to obtain a continuously cast billet; The continuous casting billet is sequentially subjected to rough rolling, finish rolling, stage cooling and coiling to obtain a hot rolled coil; wherein the stage cooling includes: rapid cooling, air cooling and water cooling; The hot-rolled coil is flattened to obtain a finished hot-rolled high-hole expansion steel.

6. The method according to claim 5, characterized in that The temperature of the rapid cooling is 600° C. to 660° C., the cooling rate of the rapid cooling is ≥50° C. / s; and the time of the air cooling is 5s to 10s.

7. The method according to claim 5, characterized in that The coiling temperature is 400°C to 460°C.

8. The method according to claim 5, characterized in that The superheat of the molten steel is ≤20°C, the specific water content of the continuous casting is ≥0.6L / kg, and the soft reduction at the solidification end of the continuous casting is ≥3.5mm.

9. The method according to claim 5, characterized in that The final rolling reduction ratio of the second roughing mill R2 of the roughing rolling is ≥35%; and / or, The final rolling temperature of the rough rolling is 1050°C to 1100°C; and / or, The rough rolling produces an intermediate billet having a thickness of 30 mm to 40 mm.

10. The method according to claim 5, characterized in that The finishing rolling is constant speed rolling, and the rolling speed of the finishing rolling is 5m / s to 10m / s; and / or, The finish rolling produces a hot-rolled plate, the thickness of the hot-rolled plate is 1.8 mm to 5 mm; and / or, The final rolling temperature of the finish rolling is Ar3~Ar3+80℃.

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