Thermoformed steel and preparation method thereof

Abstract

The invention discloses hot-formed steel and a preparation method thereof. The hot-formed steel comprises the following chemical components in percentage by weight: 0.17%-0.25% of C, 0.8%-1.8% of Si, 0.8%-2.0% of Mn, 0.01%-0.06% of Al, 1.5%-4% of Cr, 0.02%-0.05% of Ti, 0.0015%-0.004% of B, 0.02%-0.08% of Nb, less than or equal to 0.25% of Mo, less than or equal to 0.008% of N, less than or equal to 0.025% of P, less than or equal to 0.01% of S and the balance of Fe and inevitable impurities. The hot-formed steel provided by the invention has good performance stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of metallurgy, in particular to a hot forming steel and a preparation method thereof. BACKGROUND

[0002] Automobile manufacturers must use thinner and higher strength materials to ensure passenger safety while reducing vehicle weight in order to meet increasingly stringent emission regulations and crash safety requirements. Hot forming steel has become an ideal solution for vehicle lightweighting due to its ultra-high strength and good forming performance. With the popularization and cost reduction of hot stamping technology, the proportion of hot forming parts in vehicle bodies has increased significantly. At the same time, as passive safety performance of vehicles is increasingly valued in crash energy absorption, vehicle parts not only need to have high strength, but also need to have high elongation and anti-bending fracture performance.

[0003] Hot gas expansion forming parts have higher torsional stiffness than traditional stamping open section parts by 50-100 times, and have better anti-collision fracture performance. Under the premise of the same strength material, there is about 20% weight reduction potential compared to traditional hot stamping vehicle bodies. However, hot gas expansion forming parts have only one surface that can contact the mold for heat transfer and cooling, and the contact conditions of the parts and the mold are far worse than the process conditions of the thousands of tons of pressure retention on the hot stamping mold. The conventional 22MNB5 material has the problems of incomplete quenching and unstable performance under such single-sided quenching process conditions. In addition, the ordinary 22MNB5 bare plate has a lot of residual iron oxide scale on the surface after hot forming, and the inside of the hot gas expansion part is difficult to be shot blasted, which affects the coating quality and the anti-corrosion ability of the part. Although the anti-corrosion ability of the part can be improved by adding a plating layer, the plating material has different plating layer resistance and lower melting point than the steel matrix, and is prone to flow during heating, which affects the uniformity of the part's electric heating. Therefore, the plating layer is not suitable for hot gas expansion forming process. The existing technology needs to be improved in view of the above problems. SUMMARY

[0004] In view of the above technical problems, the present application provides a hot forming steel and a preparation method thereof.

[0005] In a first aspect, the embodiments of the present application provide a hot forming steel, which comprises the following weight percentage chemical composition: C: 0.17% to 0.25%, Si: 0.8% to 1.8%, Mn: 0.8% to 2.0%, Al: 0.01% to 0.06%, Cr: 1.5% to 4%, Ti: 0.02% to 0.05%, B: 0.0015% to 0.004%, Nb: 0.02% to 0.08%, Mo≤0.25%, N≤0.008%, P≤0.025%, S≤0.01%, and the balance of Fe and unavoidable impurities.

[0006] According to the embodiments of the present application, the hot forming steel of the present application can effectively solve the problems of incomplete quenching and unstable performance in the hot gas expansion forming process through reasonable chemical composition design. The reasonable proportioning of carbon, silicon, manganese, chromium and other elements ensures that the steel has good strength and toughness during the hot forming process. The addition of aluminum, titanium, niobium and other elements further refines the grain and improves the comprehensive performance of the steel. The addition of boron significantly improves the hardenability of the steel, ensuring the performance stability of the steel under the condition of single-sided quenching process.

[0007] In some embodiments, the hot forming steel comprises the following chemical composition by weight percentage: C: 0.17% to 0.25%, Si: 0.8% to 1.8%, Mn: 0.8% to 2.0%, Al: 0.01% to 0.06%, Cr: 2.0% to 3.2%, Ti: 0.02% to 0.05%, B: 0.002% to 0.0036%, Nb: 0.04% to 0.07%, Mo≤0.25%, N≤0.008%, P≤0.025%, S≤0.01%, and the balance of Fe and unavoidable impurities.

[0008] In some embodiments, the yield strength of the hot forming steel is greater than or equal to 950 MPa.

[0009] In some embodiments, the elongation of the hot forming steel is greater than or equal to 5%.

[0010] In some embodiments, the tensile strength of the hot forming steel is greater than or equal to 1300 MPa.

[0011] In a second aspect, the embodiments of the present application provide a preparation method of a hot forming steel, comprising the following steps:

[0012] The molten steel is smelted, and the molten steel comprises the following chemical composition by weight percentage: C: 0.17% to 0.25%, Si: 0.8% to 1.8%, Mn: 0.8% to 2.0%, Al: 0.01% to 0.06%, Cr: 1.5% to 4%, Ti: 0.02% to 0.05%, B: 0.0015% to 0.004%, Nb: 0.02% to 0.08%, Mo≤0.25%, N≤0.008%, P≤0.025%, S≤0.01%, and the balance of Fe and unavoidable impurities.

[0013] The smelted molten steel is subjected to continuous casting, hot rolling, pickling, cold rolling, annealing and skin passing to obtain the hot forming steel.

[0014] In some embodiments, in the continuous casting step, the tundish temperature during continuous casting is 1500°C to 1550°C.

[0015] In some embodiments, in the continuous casting step, the continuous casting speed is greater than or equal to 1 m / min.

[0016] In some embodiments, the continuous casting slab is heated at a temperature of 1180°C to 1280°C.

[0017] In some embodiments, the finishing rolling temperature is 880°C-950°C.

[0018] In some embodiments, the coiling temperature is 550°C-650°C.

[0019] In some embodiments, the pickling step has a pickling speed of less than or equal to 100 m / min.

[0020] In some embodiments, in the cold rolling step, the relative cold rolling reduction rate is 40%-70%.

[0021] In some embodiments, the annealing step includes any one of bell furnace annealing and continuous annealing.

[0022] In some embodiments, the bell-type furnace annealing temperature is 650° C.-720° C., and the holding time is greater than or equal to 10 hours.

[0023] In some embodiments, the continuous annealing has an annealing temperature of 740° C.-820° C. and a holding time of 100 s-200 s.

[0024] In some embodiments, the flattening step has a flattening elongation of 0.8%-1.5%. DETAILED DESCRIPTION

[0025] Below, the embodiments of the battery cells, battery devices, and electrical devices of the present application are disclosed in detail with appropriate reference to the detailed description. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the following description is provided to enable those skilled in the art to fully understand the present application and is not intended to limit the subject matter described in the claims.

[0026] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ , this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0027] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application.

[0028] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form new technical solutions, and such technical solutions should be deemed to be included in the disclosure of this application.

[0029] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0030] Unless otherwise specified, in this application, the terms "first", "second", etc. are used to distinguish different objects rather than to describe a specific order or a primary-secondary relationship.

[0031] In this application, the terms "plurality" and "multiple" refer to two or more.

[0032] In the description of the embodiments of the present application, unless otherwise specified, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium. Furthermore, a first feature being "above," "above," and "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is at a higher level than the second feature. A first feature being "below," "below," and "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is at a lower level than the second feature.

[0033] Unless otherwise specified, the terms used in this application have the common meanings that are commonly understood by those skilled in the art.

[0034] Unless otherwise stated, the numerical values ​​of the various parameters mentioned in this application can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in the examples of this application. Unless otherwise stated, the test temperature of each parameter is 25°C.

[0035] [Hot-formed steel]

[0036] An embodiment of the first aspect of the present application provides a hot-formed steel, whose chemical composition, by weight percentage, includes: C: 0.17% to 0.25%, Si: 0.8% to 1.8%, Mn: 0.8%-2.0%, Al: 0.01% to 0.06%, Cr: 1.5% to 4%, Ti: 0.02% to 0.05%, B: 0.0015% to 0.004%, Nb: 0.02% to 0.08%, Mo≤0.25%, N≤0.008%, P≤0.025%, S≤0.01%, and the balance is Fe and unavoidable impurities.

[0037] In the embodiment of the present application, carbon (C) ensures the martensite content and strength of the material after hot stamping.

[0038] Silicon (Si) can generate dense SiO2 in a high-temperature environment, isolating the diffusion of oxygen in the environment into the matrix, and significantly improving the material's resistance to high-temperature oxidation. Silicon can also improve the hardenability of steel and reduce the volume change during the transformation from austenite to martensite, thereby effectively controlling the generation of quenching cracks and ensuring the protective performance of steel in hot stamping applications.

[0039] Chromium (Cr) can generate dense Cr in high temperature environment x O yOxides can isolate the diffusion of oxygen in the environment into the matrix, inhibit the formation of loose FeO, and significantly improve the adhesion and stability of the oxide film on the surface of the parts; and chromium can improve the strength of steel by solid solution strengthening, making the structure uniform and increasing tempering stability; at the same time, chromium can improve the hardenability of the material, reduce the requirements for the cooling rate of parts in hot stamping, make the material suitable for the working conditions of discontinuous cooling of multi-pass hot stamping, and also reduce the difficulty and cost of hot stamping die design and development.

[0040] Niobium (Nb) refines the grain structure and forms hydrogen traps through a large number of Nb-containing precipitates, thereby improving the toughness and resistance to hydrogen-induced delayed cracking of the material.

[0041] Titanium (Ti) can preferentially form TiN high-temperature precipitates with N in molten steel, avoiding the combination of N and B, allowing B to exist in the form of solid solution, and playing the role of B in improving hardenability.

[0042] Boron (B) segregates at austenite grain boundaries, hindering ferrite nucleation. This improves the steel's hardenability, ensuring uniform mechanical properties across the cross-section of hot-stamped components and effectively increasing product strength. Furthermore, B significantly enhances the material's hardenability, reducing the required cooling rate for parts during hot stamping and making the material suitable for multi-pass hot stamping with discontinuous cooling. It also reduces the difficulty and cost of hot stamping die design and development.

[0043] Molybdenum (Mo) can improve the material's resistance to high-temperature oxidation and the toughness of hot stamping parts.

[0044] N, P, and S are harmful elements and their content needs to be controlled as low as possible to reduce the size and number of precipitates such as TiN and avoid them becoming crack sources.

[0045] According to the embodiment of the present application, the content of carbon (C) is controlled between 0.17% and 0.25% to ensure that the steel has sufficient strength and hardness. The content of silicon (Si) is 0.8% to 1.8%, which helps to improve the strength and wear resistance of the steel. The content of manganese (Mn) is between 0.8% and 2.0%, which can enhance the toughness and hardenability of the steel. The content of aluminum (Al) is 0.01% to 0.06%, which is mainly used for deoxidization and grain refinement. The content of chromium (Cr) is between 1.5% and 4%, which can improve the corrosion resistance and wear resistance of the steel. The content of titanium (Ti) is 0.02% to 0.050%, which helps to refine the grains and improve the strength. The content of boron (B) is 0.0015% to 0.004%, which can improve the hardenability of the steel. The content of niobium (Nb) is 0.02% to 0.08%, which helps to refine the grains and improve the strength. The content of molybdenum (Mo) is less than 0.25%, which can improve the strength and heat resistance of the steel. The content of nitrogen (N) is not more than 0.008%, the content of phosphorus (P) is not more than 0.025%, and the content of sulfur (S) is not more than 0.01%. The control of the content of these elements helps to reduce impurities in the steel and improve its purity. Through special alloy composition design, the embodiment of the present application enables the material to have excellent hardenability, and the finished part has stable tensile strength under the condition of single-sided contact heat transfer cooling process.

[0046] The hot forming steel of the present application can effectively solve the problems of incomplete quenching and unstable performance in the hot gas expansion forming process through reasonable chemical composition design. The reasonable proportioning of elements such as carbon, silicon, manganese, chromium, etc. ensures that the steel has good strength and toughness during hot forming. The addition of elements such as aluminum, titanium, niobium, etc. further refines the grains and improves the comprehensive performance of the steel. The addition of boron element significantly improves the hardenability of the steel, ensuring the performance stability of the steel under the condition of single-sided quenching process.

[0047] Therefore, the hot forming steel of the present application exhibits excellent performance in the hot gas expansion forming process, not only has high strength and good forming performance, but also can effectively resist high temperature oxidation, solving the technical problems of traditional 22MNB5 material in the hot gas expansion forming process. Through optimization of the chemical composition, the hot forming steel of the present application not only has high strength, but also has high elongation and anti-bending fracture performance, meeting the requirements of automobile lightweight and collision safety. Moreover, the hot forming steel provided by the embodiment of the present application has good high temperature oxidation resistance without additional plating, and under normal hot gas expansion processing conditions, the surface of the part does not produce iron oxide skin that will fall off, and the paintability of the formed part and the inner wall corrosion resistance are significantly enhanced. The hot forming steel provided by the present application has air cooling hardening capability, so the arrangement of the cooling pipeline of the hot gas expansion die only needs to ensure the safety and service life of the die, greatly reducing the development cost and cycle of the die.

[0048] In some embodiments, the chemical composition of the hot-formed steel includes the following weight percentages: C: 0.17% to 0.25%, Si: 0.8% to 1.8%, Mn: 0.8% to 2.0%, Al: 0.01% to 0.06%, Cr: 2.0% to 3.2%, Ti: 0.02% to 0.05%, B: 0.002% to 0.0036%, Nb: 0.04% to 0.07%, Mo ≤ 0.25%, N ≤ 0.008%, P ≤ 0.025%, S ≤ 0.01%, and the balance being Fe and unavoidable impurities.

[0049] According to embodiments of the present application, the content range of Cr is adjusted to 2.2% to 3.8%, the content range of B is adjusted to 0.002% to 0.0036%, and the content range of Nb is adjusted to 0.04% to 0.07%. Among them, the adjustment of Cr helps to improve the corrosion resistance and high temperature strength of the steel, the adjustment of B can optimize the hardenability and toughness of the steel, and the adjustment of Nb helps to refine the grain and improve the strength and toughness of the steel.

[0050] As a preferred implementation, the content of Cr can be further preferred to be 2.5% to 3.5% to achieve a better balance between corrosion resistance and processability. The content of B can be preferred to be 0.0025% to 0.0035% to further improve the hardenability and toughness of the steel. The content of Nb can be preferred to be 0.05% to 0.06% to more effectively refine the grain and improve the comprehensive performance of the steel.

[0051] Thus, the present application provides a hot-formed steel with higher strength, better toughness and corrosion resistance by optimizing the content range of Cr, B and Nb. The steel is particularly suitable for automotive lightweight design and can meet the increasingly stringent crash safety requirements while reducing the weight of the vehicle body. Compared with the prior art, the technical scheme of the present application further improves the toughness and corrosion resistance of the steel on the basis of ensuring the strength of the steel, solves the problem of unstable performance of traditional hot-formed steel under the condition of single-sided quenching process, and avoids the influence of residual iron oxide skin on the coating quality and anti-rust ability.

[0052] In some embodiments, the hot-formed steel has a yield strength greater than or equal to 950 MPa.

[0053] In some embodiments, the hot-formed steel has an elongation greater than or equal to 5%.

[0054] In some embodiments, the hot-formed steel has a tensile strength greater than or equal to 1300 MPa.

[0055] Specifically, the yield strength of hot-formed steel can be achieved by adjusting the chemical composition and heat treatment process. For example, by controlling the carbon content and the ratio of alloying elements, combined with appropriate heat treatment temperature and time, the material can reach the required yield strength when subjected to stress. The improvement of elongation can be achieved by optimizing the microstructure and processing technology of the material, such as by refining the grains and reducing internal defects, so that the material can exhibit higher ductility during the stretching process. The improvement of tensile strength can be achieved by strengthening the internal structure of the material and increasing the content of alloying elements. For example, by adding appropriate amounts of elements such as molybdenum and niobium, combined with appropriate heat treatment processes, so that the material can withstand greater stress during the stretching process.

[0056] To address this, the present embodiment sets minimum requirements for yield strength, elongation, and tensile strength for hot-formed steel, ensuring the material's reliability and stability in practical applications. Compared to existing technologies, this solution addresses the potential for insufficient strength and ductility in traditional hot-formed steel applications by clarifying the material's mechanical performance indicators, thereby improving the material's effectiveness in automotive lightweighting and safety performance.

[0057] [Preparation method of hot-formed steel]

[0058] An embodiment of the second aspect of the present application provides a method for preparing hot-formed steel, comprising the following steps:

[0059] Molten steel smelting, the molten steel comprising the following chemical composition in percentage by weight: C: 0.17% to 0.25%, Si: 0.8% to 1.8%, Mn: 0.8% to 2.0%, Al: 0.01% to 0.06%, Cr: 1.5% to 4%, Ti: 0.02% to 0.05%, B: 0.0015% to 0.004%, Nb: 0.02% to 0.08%, Mo≤0.25%, N≤0.008%, P≤0.025%, S≤0.01%, and the balance being Fe and unavoidable impurities;

[0060] The smelted molten steel is subjected to continuous casting, hot rolling, pickling, cold rolling, annealing and leveling to obtain hot-formed steel.

[0061] According to the embodiments of the present application, the content ranges of each element are strictly controlled in the molten steel smelting step to ensure that the chemical composition of the final hot forming steel meets the design requirements. In the continuous casting step, the control of the tundish temperature and the casting speed helps to obtain uniform billet quality. In the hot rolling step, the reasonable control of the heating temperature, the finishing temperature and the coiling temperature helps to optimize the microstructure and mechanical properties of the steel. In the pickling step, the control of the pickling speed helps to remove the surface oxide layer and improve the surface quality of the steel. In the cold rolling step, the control of the relative reduction rate helps to further refine the grain and improve the strength of the steel. In the annealing step, the control of the annealing temperature and the holding time helps to eliminate internal stress and improve the plasticity and toughness of the steel. In the leveling step, the control of the leveling elongation helps to improve the dimensional accuracy and surface flatness of the steel.

[0062] Therefore, the hot forming steel preparation method proposed in the present application can effectively ensure the chemical composition, microstructure and mechanical properties of the hot forming steel by strictly controlling the process parameters of each step, thereby meeting the demand for high-strength and high-forming performance materials for automobile lightweight. Compared with the prior art, the method has the advantages of strong controllability of process parameters, stable product quality, etc., and can effectively solve the problems of incomplete quenching and unstable performance in the hot gas expansion forming process.

[0063] In some embodiments, in the continuous casting step, the continuous casting tundish temperature is 1500-1550°C; and / or the continuous casting casting speed is greater than or equal to 1 m / min.

[0064] The continuous casting tundish temperature refers to the temperature of the molten steel when it enters the tundish during continuous casting. This temperature range can ensure good flowability of the molten steel during continuous casting and avoid defects caused by premature solidification or excessively high temperature. The continuous casting casting speed refers to the pulling speed of the billet during continuous casting. A casting speed greater than or equal to 1 m / min can improve production efficiency while ensuring the quality and uniformity of the billet.

[0065] The control of the continuous casting tundish temperature can be achieved by adjusting the heating temperature of the molten steel in the smelting furnace and the heat preservation measures of the tundish. The control of the continuous casting casting speed can be optimized by adjusting the billet pulling device and cooling system of the continuous casting machine. As a preferred embodiment, the continuous casting tundish temperature can be further controlled to be between 1520°C and 1540°C, and the continuous casting casting speed can be further controlled to be between 1.2 m / min and 1.5 m / min, so as to further improve the quality of the billet while ensuring production efficiency.

[0066] According to the embodiments of this application, precise control of the continuous casting tundish temperature and casting speed resolves the problems of unstable billet quality and low production efficiency caused by improper temperature or casting speed in traditional continuous casting processes. Compared with existing technologies, this solution effectively improves billet uniformity and production efficiency while reducing defects, providing a high-quality raw material foundation for subsequent hot and cold rolling processes.

[0067] In some embodiments, in the hot rolling step, the continuous casting slab heating temperature is 1180°C-1280°C; the final rolling temperature is 880°C-950°C; and the coiling temperature is 550°C-650°C.

[0068] The heating temperature of the continuous casting slab is selected to ensure good plasticity and deformability during the hot rolling process, while avoiding grain coarsening caused by excessively high temperatures. The finishing temperature is set to control the steel's microstructure, ensuring a uniform austenitic structure during the subsequent cooling process, thereby improving the steel's strength and toughness. The coiling temperature is adjusted to maintain the steel's shape stability during cooling, avoiding deformation or cracking caused by excessively high or low temperatures.

[0069] The continuous casting slab heating temperature range is 1180℃-1280℃, which effectively ensures the steel's plastic deformation ability during hot rolling while preventing grain coarsening caused by excessively high temperatures. The final rolling temperature range is 880℃-950℃, which ensures the steel forms a uniform austenitic structure during cooling, thereby improving its strength and toughness. The coiling temperature range is 550℃-650℃, which effectively maintains the steel's shape stability and avoids deformation or cracking caused by excessively high or low temperatures.

[0070] As a preferred embodiment, the continuous casting slab heating temperature can be further preferably 1200°C-1250°C, the final rolling temperature can be further preferably 900°C-930°C, and the coiling temperature can be further preferably 600°C-630°C. These preferred temperature ranges can further improve the performance of the steel, ensuring its stability during the hot rolling process and reliability in subsequent use.

[0071] Therefore, by rationally controlling the heating temperature, finishing temperature, and coiling temperature during the hot rolling process, this application effectively improves the strength, toughness, and shape stability of steel, resolving the problem of unstable steel properties caused by improper temperature control in the prior art. Compared with the prior art, the technical solution of this application offers higher temperature control accuracy and better steel properties, meeting the demand for high-strength steel in lightweight vehicles.

[0072] In some embodiments, the pickling speed is less than or equal to 100 m / min in the pickling step. Controlling the pickling speed is one of the key factors to ensure the quality of the steel surface treatment. If the pickling speed is too fast, the acid solution may not react sufficiently with the steel surface, which can not effectively remove the iron oxide scale, thereby affecting the quality of subsequent process steps. If the pickling speed is too slow, it can reduce production efficiency and increase production costs. Therefore, by controlling the pickling speed in the range of less than or equal to 100 m / min, the pickling effect can be ensured while maintaining high production efficiency.

[0073] The pickling speed can be achieved by adjusting the speed of the conveyor belt of the pickling equipment or the spray pressure of the acid solution. For example, in a continuous pickling production line, the pickling speed can be precisely controlled by adjusting the speed of the conveyor belt. In addition, the spray pressure of the acid solution can also be controlled by adjusting the power of the pump or the aperture of the nozzle to ensure that the acid solution uniformly covers the surface of the steel, achieving the desired pickling effect.

[0074] By controlling the pickling speed in the range of less than or equal to 100 m / min, the iron oxide scale on the surface of the steel can be effectively removed, providing good surface quality for subsequent cold rolling, annealing and leveling steps. This technical solution not only solves the quality problem caused by improper speed control in traditional pickling processes, but also improves production efficiency and reduces production costs.

[0075] In some embodiments, the cold rolling relative reduction is 40%-70% in the cold rolling step. The cold rolling relative reduction refers to the ratio of the thickness reduction of the material during cold rolling to the original thickness, which is set to be in the range of 40%-70% to ensure that the material obtains sufficient deformation strengthening during cold rolling, while avoiding excessive deformation that can cause a decrease in material performance. Specifically, the cold rolling relative reduction can be achieved by adjusting the roll gap and rolling speed, for example, by precisely controlling the roll gap so that the thickness of the material decreases by 40%-70% when passing through the rolls. As a preferred embodiment, the cold rolling relative reduction can be set to 50%-60% to ensure the strength of the material while optimizing its forming performance.

[0076] According to the embodiments of the present application, by controlling the cold rolling relative reduction, the problems of insufficient deformation strengthening or excessive deformation of the material during cold rolling are solved. By setting the cold rolling relative reduction in the range of 40%-70%, the material can obtain moderate deformation strengthening during cold rolling, thereby improving its strength and hardness, while avoiding a decrease in material performance caused by excessive deformation. Compared with the prior art, the technical solution of the present application optimizes the cold rolling process parameters while ensuring the performance of the material, thereby improving production efficiency and product quality.

[0077] In some embodiments, in the annealing step, the bell furnace annealing temperature is 650° C.-720° C., and the holding time is greater than or equal to 10 hours; the continuous annealing temperature is 740° C.-820° C., and the holding time is 100 seconds to 200 seconds.

[0078] Specifically, bell-type annealing is a common annealing method suitable for mass-produced hot-formed steel. By holding the steel in a bell-type furnace for an extended period at temperatures between 650°C and 720°C, internal stresses can be effectively eliminated, improving its microstructure and mechanical properties. A holding time of 10 hours or longer ensures a fully and evenly recrystallized steel, resulting in stable performance.

[0079] Continuous annealing is an annealing method suitable for continuous production lines. During continuous annealing, the steel passes through the annealing furnace at a constant speed, with the annealing temperature controlled between 740°C and 820°C for 100 to 200 seconds. This annealing method offers high production efficiency, completing the annealing process in a relatively short time while ensuring that the steel's performance meets the required standards.

[0080] As a preferred embodiment, bell-type annealing can be further divided into multiple stages, such as a heating phase, a holding phase, and a cooling phase, to ensure uniform temperature variation during the annealing process and avoid uneven performance due to temperature gradients. Continuous annealing can further optimize the annealing effect by adjusting the steel's throughput speed and the temperature distribution of the annealing furnace.

[0081] Therefore, the technical solution of this application ensures that the hot-formed steel achieves a uniform microstructure and stable mechanical properties during the annealing process by rationally controlling the annealing temperature and holding time. Compared with existing technologies, the technical solution of this application can effectively solve the problem of uneven performance that may occur during the annealing process of hot-formed steel, while also improving production efficiency and being suitable for large-scale production needs.

[0082] In some embodiments, during the leveling step, the leveling elongation is 0.8%-1.5%. Leveling elongation refers to the degree of deformation experienced by a material during the leveling process. By controlling the leveling elongation between 0.8% and 1.5%, the surface quality and dimensional accuracy of the material can be effectively improved while ensuring that the material's mechanical properties are not affected. Specifically, the leveling elongation can be controlled by adjusting the roller gap, rolling speed, and leveling pressure of the leveling machine. For example, as a preferred embodiment, the gap between the rollers can be precisely controlled to ensure that the material is evenly stressed as it passes through the leveling machine, thereby achieving the desired elongation.

[0083] The technical scheme of the present application solves the problems of surface unevenness, size deviation and unstable mechanical properties of the hot-formed steel in the skin pass process by precisely controlling the skin pass elongation. Thus, the surface quality of the material is significantly improved, the dimensional accuracy is ensured, and the mechanical properties of the material are also maintained within a stable range. Compared with the prior art, the technical scheme of the present application can better control the elongation deformation of the material in the skin pass process, thereby improving the overall quality and performance of the hot-formed steel.

[0084] Example

[0085] The present disclosure is described in more detail by the following examples, which are merely illustrative and not limiting, as various modifications and changes in the examples clearly fall within the scope of the present disclosure. Unless otherwise stated, all parts, percentages and ratios reported in the following examples are based on mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and used directly without further purification, and the instruments used in the examples are commercially available.

[0086] Example 1

[0087] The preparation method of the hot-formed steel is as follows:

[0088] After the converter smelting of the molten steel, the molten steel is sequentially subjected to refining, continuous casting, hot rolling, pickling, single-stand cold rolling, bell annealing, skin pass, and finishing to obtain a hot-formed steel with a thickness of 2 mm. The chemical composition of the hot-formed steel is shown in Table 1, and the specific process parameters are shown in Table 2.

[0089] In Table 1, the content of each alloying element is a mass percentage.

[0090] Examples 2-4

[0091] The preparation method of the hot-formed steel is similar to that of Example 1, except that the chemical composition of the hot-formed steel is adjusted, and the specific parameter adjustment is shown in Table 1.

[0092] Comparative Examples 1-4

[0093] The preparation method of the hot-formed steel is similar to that of Example 1, except that the chemical composition of the hot-formed steel is adjusted, and the specific parameter adjustment is shown in Table 1.

[0094] Table 1

[0095] No. C Si Mn Al Cr Ti B Nb Mo N P S Example 1 0.22 1.25 1.31 0.042 3.0 0.035 0.0022 0.057 0.08 0.003 0.009 0.002 Example 2 0.23 1.6 1.2 0.03 1.5 0.03 0.0015 0.02 Lower limit 0.005 0.008 0.002 Example 3 0.23 1.6 1.2 0.03 4 0.03 0.004 0.08 0.1 0.003 0.007 0.004 Comparative Example 1 0.22 1.25 1.31 0.042 3.0 0.035 0.0022 0 0.08 0.003 0.009 0.002 Comparative Example 2 0.22 1.25 1.31 0.042 0 0.035 0.0022 0.057 0 0.003 0.009 0.002 Comparative Example 3 0.22 1.25 1.31 0.042 1.2 0.035 0.001 0.01 0 0.003 0.009 0.002 Comparative Example 4 0.22 1.25 1.31 0.042 4.5 0.035 0.006 0.10 0.12 0.003 0.009 0.002

[0096] Examples 4-5

[0097] The chemical composition of the hot-formed steel is the same as that of Example 1, except that the preparation parameters for preparing the hot-formed steel are adjusted, and the specific parameter adjustments are shown in Table 2.

[0098] Table 2

[0099]

[0100] Test section

[0101] The hot-formed steel prepared in each of the above examples and comparative examples is subjected to hot stamping to form a part according to the following method:

[0102] S10, slitting: The steel coil is slit into a narrow strip with a width (16 cm) required for part processing;

[0103] S20, pipe welding and slitting: the narrow strip is formed by continuous rolling, welded into a circular tube by laser welding, and then slitted into the required length (90 cm);

[0104] S30, pre-forming: the steel tube is completely formed according to the part design shape and matched with the hot gas expansion die;

[0105] S40, resistance heating: the electrodes are connected to both ends of the formed steel tube, and the current is turned on to heat the part itself to 950°C for 20s;

[0106] S50, hot gas expansion forming: after the mold is closed, high-pressure air is introduced into both ends of the steel tube to make the hot tube expand and form a part while cooling.

[0107] The prepared part is subjected to performance testing according to the following method:

[0108] 1. Yield strength

[0109] Referring to the tensile test scheme for tubular parts in GB / T228.1-2010, the sample shape is S8 (the total length of the sample is 120 mm, the original gauge length is 50 mm, and the sample cross-sectional area is calculated), and the sample is stretched after the ends are added with plug heads. If the yield platform of the sample is not obvious, Rp 0.2 is taken as the yield strength value.

[0110] 2. Tensile strength

[0111] Referring to the tensile test scheme for tubular parts in GB / T228.1-2010, the sample shape is S8 (the total length of the sample is 120 mm, the original gauge length is 50 mm, and the sample cross-sectional area is calculated), and the sample is stretched after the ends are added with plug heads. The highest point of the tensile curve is the tensile strength value.

[0112] 3. Elongation A50

[0113] According to the tensile test scheme of tubular parts in GB / T228.1-2010, the shape of the sample is S8 (the total length of the sample is 120 mm, and the original gauge length is 50 mm), the sample is stretched after adding a plug head at both ends. The length of the 50 mm gauge length containing the fracture after stretching is calculated to obtain the elongation after fracture.

[0114] 4. The product of strength and ductility

[0115] According to the tensile test scheme of tubular parts in GB / T228.1-2010, the shape of the sample is S8 (the total length of the sample is 120 mm, and the original gauge length is 50 mm), the sample is stretched after adding a plug head at both ends. The length of the 50 mm gauge length containing the fracture after stretching is calculated to obtain the elongation after fracture.

[0116] The test results are shown in Table 3.

[0117] Table 3

[0118]

[0119] From the above test results, it can be seen that the hot-formed steel provided by the embodiments of the present application has better performance.

[0120] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solutions of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications that can be thought of by those skilled in the art are applied to the embodiments, and other ways constructed by combining part of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A hot-formed steel, characterized in that: The chemical composition includes the following weight percentages: C: 0.17%~0.25%, Si: 0.8%~1.8%, Mn: 0.8%-2.0%, Al: 0.01%~0.06%, Cr: 1.5%~4%, Ti: 0.02%-0.05%, B: 0.0015%~0.004%, Nb: 0.02%~0.08%, Mo≤0.25%, N≤0.008%, P≤0.025%, S≤0.01%, and the balance is Fe and unavoidable impurities.

2. The hot-formed steel according to claim 1, characterized in that The chemical composition includes the following weight percentages: C: 0.17%~0.25%, Si: 0.8%~1.8%, Mn: 0.8%-2.0%, Al: 0.01%~0.06%, Cr: 2.0%~3.2%, Ti: 0.02%-0.05%, B: 0.002%~0.0036%, Nb: 0.04%~0.07%, Mo≤0.25%, N≤0.008%, P≤0.025%, S≤0.01%, and the balance is Fe and unavoidable impurities.

3. The hot-formed steel according to claim 1 or 2, characterized in that: The hot-formed steel meets at least one of the following conditions: (1) The yield strength of the hot-formed steel is greater than or equal to 950 MPa; (2) The elongation of the hot-formed steel is greater than or equal to 5%; (3) The tensile strength of the hot-formed steel is greater than or equal to 1300 MPa.

4. A method for preparing hot-formed steel, characterized in that: The steps include: Molten steel smelting, the molten steel comprising the following chemical composition in percentage by weight: C: 0.17% to 0.25%, Si: 0.8% to 1.8%, Mn: 0.8% to 2.0%, Al: 0.01% to 0.06%, Cr: 1.5% to 4%, Ti: 0.02% to 0.05%, B: 0.0015% to 0.004%, Nb: 0.02% to 0.08%, Mo≤0.25%, N≤0.008%, P≤0.025%, S≤0.01%, and the balance being Fe and unavoidable impurities; The smelted molten steel is subjected to continuous casting, hot rolling, pickling, cold rolling, annealing and leveling to obtain hot-formed steel.

5. The preparation method according to claim 4, characterized in that In the continuous casting step, the temperature of the continuous casting ladle is 1500° C.-1550° C.; and / or The continuous casting speed is greater than or equal to 1m / min.

6. The preparation method according to claim 4, characterized in that The hot rolling step, The continuous casting slab heating temperature is 1180°C-1280°C; and / or Finish rolling temperature is 880℃-950℃; and / or The coiling temperature is 550℃-650℃.

7. The preparation method according to claim 4, characterized in that In the pickling step, the pickling speed is less than or equal to 100 m / min.

8. The preparation method according to claim 4, characterized in that In the cold rolling step, the relative cold rolling reduction rate is 40%-70%.

9. The preparation method according to claim 4, characterized in that The annealing step includes any one of bell furnace annealing and continuous annealing, wherein: The bell-type furnace annealing temperature is 650° C.-720° C. and the holding time is greater than or equal to 10 hours; and / or The annealing temperature of the continuous annealing is 740° C.-820° C., and the holding time is 100s-200s.

10. The preparation method according to claim 4, characterized in that In the leveling step, the leveling elongation is 0.8%-1.5%.

Images