All-ferrite hot-rolled acid-washed high-strength steel and preparation method thereof

Abstract

The invention relates to full-ferrite hot-rolled acid-washed high-strength steel and a preparation method, and belongs to the technical field of steel preparation. The hot-rolled and pickled high-strength steel comprises the following chemical components in percentage by mass: 0.02%-0.1% of C, 0.1%-1.0% of Si, 0.5%-2.5% of Mn, less than or equal to 0.025% of P, less than or equal to 0.005% of S, 0.05%-0.15% of Ti, 0.01%-0.07% of Nb, less than or equal to 0.5% of Cr, 0.05%-0.5% of Mo, less than or equal to 0.005% of N, 0.0005%-0.0010% of Mg and the balance of Fe and inevitable impurities. Gt in the hot-rolled acid-washed high-strength steel; the number density of the inclusions of 10 microns is less than or equal to 0.6 / mm < 2 >, and the average size of the inclusions TiN.Al2O3 is 1t; 3.5 [mu] m; size lt; the volume fraction of crystal grains with the grain size of 3 microns is greater than or equal to 85%, the average size of effective crystal grains is 2.0 microns to 2.5 microns, and the maximum crystal grain size is lt; 15 [mu] m. By accurately controlling the content of all elements in the steel, controlling the size and number density of inclusions and controlling rolling, large-size grains are reduced, and the structure uniformity is improved, so that stress concentration points are reduced, the crack propagation resistance of the steel is improved, the sensitivity of edge cracks is effectively reduced, and the stamping forming performance of the steel is improved.

Description

Technical Field

[0001] The present application relates to the technical field of steel preparation, and in particular to a full ferrite hot-rolled pickled high-strength steel and a preparation method thereof. Background Art

[0002] The "dual carbon" goals have put forward new requirements for carbon emission assessments throughout the entire life cycle of automotive steel, from production, use, and recycling. Carbon emissions must be reduced at every stage, from raw material procurement to manufacturing, manufacturing, and transportation. Therefore, saving fuel and reducing vehicle exhaust emissions have become crucial priorities in the development of the automotive industry. Using high-strength steel in chassis structures and bodies can reduce vehicle weight by 25% to 30%. Replacing existing low-carbon steel with high-strength steel can reduce the thickness of a 10-meter-thick steel plate to 6-8mm, reducing weight by 30%. High-strength hot-rolled pickled steel is primarily used to manufacture safety structures, chassis components, seats, and beams. These components improve vehicle safety while reducing weight by up to 20%, keeping manufacturing costs roughly the same or slightly decreasing.

[0003] As ultra-high-strength hot-rolled pickled steel becomes increasingly widely used, customer requirements for its formability are also increasing. High hole expansion, high elongation, high strength, high surface quality, and low cost are the main development directions for ultra-high-strength hot-rolled pickled steel. Ferrite + martensite (F+M) has good elongation but poor hole expansion performance. Other microstructures, such as bainite single-phase steel and ferrite + bainite dual-phase steel, have high hole expansion rates but lower elongation. Existing products are insufficient to meet the performance requirements of high elongation and high hole expansion. JFE pioneered the successful development of ferrite single-phase high-strength steel plate, which is primarily strengthened by nanoscale carbide precipitation and exhibits high hole expansion rates and good elongation. In recent years, leading international steel companies such as Posco, Tata Steel, Baosteel, and Shougang have also developed similar products with excellent elongation and hole expansion rates. However, in actual use, these products suffer from high edge crack sensitivity, leading to stamping delamination or cracking. Summary of the Invention

[0004] The present application provides a full ferritic hot-rolled pickled high-strength steel and a preparation method thereof to solve the following technical problem: how to solve the problem of high edge crack sensitivity existing in full ferritic high-strength steel.

[0005] In a first aspect, an embodiment of the present application provides a fully ferritic hot-rolled pickled high-strength steel, wherein the chemical composition of the hot-rolled pickled high-strength steel is, by mass fraction, C: 0.02% to 0.1%, Si: 0.1% to 1.0%, Mn: 0.5% to 2.5%, P ≤ 0.025%, S ≤ 0.005%, Ti: 0.05% to 0.15%, Nb: 0.01% to 0.07%, Cr ≤ 0.5%, Mo: 0.05% to 0.5%, N ≤ 0.005%, Mg: 0.0005%% to 0.0010%, and the balance is Fe and unavoidable impurities;

[0006] In the hot-rolled pickled high-strength steel, the number density of inclusions >10 μm is ≤0.6 / mm 2 , the average size of inclusions TiN·Al2O3 is <3.5μm.

[0007] Optionally, the microstructure of the hot-rolled pickled high-strength steel is ferrite, wherein the volume fraction of grains with a size of less than 3 μm is ≥85%, and the average size of effective grains is 2.0 μm to 2.5 μm.

[0008] Optionally, the hot-rolled pickled high-strength steel meets at least one of the following properties: yield strength ≥700 MPa, tensile strength ≥750 MPa, elongation A80 ≥16%, hole expansion ratio λ ≥70%, and surface quality is FB grade.

[0009] In a second aspect, the present application provides a method for preparing the hot-rolled pickled high-strength steel described in the first aspect, the method comprising:

[0010] The molten iron is refined to obtain molten steel having the chemical composition; wherein the refining includes a magnesium treatment process;

[0011] Continuously casting the molten steel to obtain a slab;

[0012] The slab is sequentially heated, rough rolled and finish rolled to obtain a hot-rolled plate;

[0013] The hot-rolled plate is sequentially cooled, coiled, straightened and pickled to obtain a finished hot-rolled pickled high-strength steel.

[0014] Optionally, the magnesium treatment process adopts a wire feeding method, which includes feeding 200m to 400m of Mg-Si cored wire into the molten steel at a wire feeding speed of 2m / s to 6m / s.

[0015] Optionally, the target temperature of the heating is 1180° C. to 1280° C., and the heating time is ≥170 min.

[0016] Optionally, the final rolling temperature of the rough rolling is 1020° C. to 1100° C., and the final reduction rate of the rough rolling is >30%.

[0017] Optionally, the heated slab is subjected to rough rolling to obtain an intermediate slab, and the thickness of the intermediate slab is 25 mm to 45 mm.

[0018] Optionally, the finishing rolling includes stands F1 to F7, wherein the pass reduction ratio of stands F5 and F6 is ≥50%, and the final rolling temperature of the finishing rolling is 840° C. to 900° C.

[0019] Optionally, the coiling temperature is 580°C to 700°C.

[0020] Optionally, the elongation of the tension leveling is 0.3% to 0.8%, and the temperature of the raw material for the tension leveling is less than 80°C.

[0021] Optionally, the pickling strip running speed is 60m / min to 120m / min, the pickling temperature is 75°C to 85°C, and the acidity of the free acid of the pickling is 30g / L to 60g / L.

[0022] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0023] The present application provides a fully ferritic hot-rolled pickled high-strength steel. The chemical composition of the hot-rolled pickled high-strength steel is, by mass fraction, as follows: C: 0.02% to 0.1%, Si: 0.1% to 1.0%, Mn: 0.5% to 2.5%, P≤0.025%, S≤0.005%, Ti: 0.05% to 0.15%, Nb: 0.01% to 0.07%, Cr≤0.5%, Mo: 0.05% to 0.5%, N≤0.005%, Mg: 0.0005% to 0.0010%; the remainder is Fe and unavoidable impurities; in the hot-rolled pickled high-strength steel, the number density of inclusions >10 μm is ≤0.6 pieces / mm 2 The average size of the TiN·Al2O3 inclusions is <3.5μm. By precisely controlling the content of each element in the steel, the size and number density of the inclusions are controlled to reduce stress concentration points and improve the steel's resistance to crack growth, thereby effectively reducing the sensitivity of edge cracks and improving the steel's stamping performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] 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.

[0025] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0026] Figure 1 A schematic flow chart of a method for preparing all-ferrite hot-rolled pickled high-strength steel provided in an embodiment of the present application;

[0027] Figure 2 This is the microstructure diagram of the all-ferrite hot-rolled pickled high-strength steel provided in Example 1 of the present application;

[0028] Figure 3 This is the IPF-Z diagram of the all-ferritic hot-rolled pickled high-strength steel provided in Example 1 of the present application;

[0029] Figure 4 This is the inclusion distribution diagram of the all-ferritic hot-rolled pickled high-strength steel provided in Example 1 of the present application. DETAILED DESCRIPTION

[0030] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0031] 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, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range; for example, the range description from 1 to 6 should be considered to have 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 apply regardless of the range; in addition, whenever a numerical range is indicated in this document, it is meant to include any cited number (fractional or integer) within the indicated range.

[0032] As used herein, the terms "including," "comprising," and the like mean "including but not limited to." Relational terms such as "first" and "second" are used solely to distinguish one entity or operation from another, and do not necessarily require or imply any actual relationship or order between these entities or operations. "And / or" describes the association relationship between associated objects, indicating that three relationships can exist. 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, "plurality" means two or more; "at least one," "at least one of the following" or similar expressions refers 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 each be single or plural. "Parts" notation, such as parts by weight or parts by mass, indicates the proportional relationship between components. In this article, the parameters described by ratio should be understood as the first term of the proportional formula, in the order in which they are described, and the proportional figures should be understood as the second term. For example, if the mass ratio of substances A, B, and C is 1:2:3, then substances A, B, and C should correspond to the proportional figures in the proportional formula, in the order in which they are described: that is, the mass of substance A:the mass of substance B:the mass of substance C = 1:2:3.

[0033] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0034] In a first aspect, an embodiment of the present application provides a fully ferritic hot-rolled pickled high-strength steel, wherein the chemical composition of the hot-rolled pickled high-strength steel is, by mass fraction, C: 0.02% to 0.1%, Si: 0.1% to 1.0%, Mn: 0.5% to 2.5%, P ≤ 0.025%, S ≤ 0.005%, Ti: 0.05% to 0.15%, Nb: 0.01% to 0.07%, Cr ≤ 0.5%, Mo: 0.05% to 0.5%, N ≤ 0.005%, Mg: 0.0005%% to 0.0010%, and the balance is Fe and unavoidable impurities;

[0035] In the hot-rolled pickled high-strength steel, the number density of inclusions >10 μm is ≤0.6 / mm 2 , the average size of inclusions TiN·Al2O3 is <3.5μm.

[0036] The positive effect of limiting the carbon (carbon) mass fraction to 0.02% to 0.1% is that carbon is one of the main strengthening elements in steel, increasing its strength and hardness. However, excessive carbon content (>0.1%) can reduce the steel's toughness and weldability. In the examples of this application, the carbon content is controlled within a relatively low range to ensure strength while maintaining good toughness and formability.

[0037] The positive effects of limiting the Si (silicon) mass fraction to 0.1% to 1.0% are: Silicon refines the steel matrix, increasing its strength and hardness. Silicon also improves the hardenability of steel, facilitating uniform structural transformation during heat treatment. In the embodiments of this application, the Si mass fraction is 0.1% to 1.0%, achieving a strengthening effect without excessively affecting the toughness and processing properties of the steel.

[0038] The positive effects of limiting the mass fraction of Mn (manganese) to 0.5% to 2.5% are as follows: Manganese is an important strengthening element in steel, significantly improving its strength and hardness. Manganese also refines the steel's microstructure, improving its hardenability and tempering stability. In the embodiments of this application, the manganese content range is relatively wide (0.5% to 2.5%) and can be adjusted to meet different performance requirements.

[0039] The positive effect of limiting P (phosphorus) to ≤ 0.025% is that phosphorus is considered a harmful element in steel because it can reduce the toughness and weldability of steel. Therefore, the phosphorus content is strictly controlled in the examples of this application to ensure the excellent performance of the steel.

[0040] The positive effect of limiting sulfur (S) to ≤ 0.005% is that sulfur, a harmful element, can embrittle steel at low temperatures, reducing its toughness and durability. In the examples of this application, the sulfur content is strictly controlled to an extremely low level (≤ 0.005%) to eliminate its adverse effects on steel properties.

[0041] The positive effect of limiting the mass fraction of titanium (titanium) to 0.05% to 0.15% is that titanium combines with nitrogen in steel to form fine TiN inclusions, thereby refining the steel's structure and improving its strength and hardness. Titanium also improves the steel's corrosion resistance. In the embodiments of this application, the mass fraction of titanium is 0.05% to 0.15%, which not only plays a role in refining the structure but also avoids the formation of excessive coarse inclusions.

[0042] The positive effects of limiting the Nb (niobium) mass fraction to 0.01% to 0.07% are: Niobium is a strong carbide-forming element that refines the steel matrix, increasing its strength and hardness. Niobium also improves creep strength and corrosion resistance. In the examples of this application, the niobium mass fraction is 0.01% to 0.07% to maximize its strengthening effect without affecting the steel's processability.

[0043] The positive effect of limiting Cr (chromium) to ≤ 0.5% is that chromium improves the hardenability, tempering stability, and corrosion resistance of steel. However, excessive chromium content (>0.5%) increases steel cost and brittleness. In the examples of this application, the chromium content is ≤ 0.5% to balance the performance and cost of the steel.

[0044] The positive effect of limiting the mass fraction of Mo (molybdenum) to 0.05% to 0.5% is that Mo can significantly improve the strength and hardness of steel, especially at high temperatures. At the same time, Mo can also improve the hardenability and corrosion resistance of steel. In the embodiments of this application, the Mo content range is relatively wide (0.05% to 0.5%) and can be adjusted according to different usage scenarios.

[0045] The positive effect of limiting N (nitrogen) to ≤ 0.005% is that nitrogen in steel typically combines with elements such as titanium and niobium to form inclusions, which can affect the steel's properties. In the examples of this application, the nitrogen content is strictly controlled to reduce inclusion formation and improve the purity and performance of the steel.

[0046] The positive effect of limiting the mass fraction of Mg (magnesium) to 0.0005% to 0.0010% is that magnesium is an effective inclusion modifier, converting large inclusions in steel into small, dispersed inclusions, thereby improving the steel's forming stability and corrosion resistance. In the examples of this application, an appropriate amount of magnesium is added through a magnesium treatment process to improve the steel's properties.

[0047] Smaller inclusion sizes and lower density of large inclusions help reduce stress concentration points, thereby improving the stability of steel during the forming process. In addition, large inclusions are more likely to become crack sources or promote crack expansion, so controlling their density can effectively reduce the sensitivity of edge cracks and improve the stamping performance of steel. In the embodiment of this application, for inclusions larger than 10 μm, their density is strictly limited to ≤ 0.6 / mm 2 , the average size of TiN·Al2O3 inclusions is controlled below 3.5μm to reduce the adverse effects of large-sized inclusions on steel properties.

[0048] In some embodiments, the microstructure of the hot-rolled pickled high-strength steel is ferrite, wherein the volume fraction of grains with a size of less than 3 μm is ≥85%, and the average effective grain size is 2.0 μm to 2.5 μm.

[0049] Smaller grain size usually means higher strength and hardness, because refined grains can hinder the movement of dislocations, thereby improving the strength of the material. When the ferrite grains are refined to a scale of <3μm, their strength and hardness will be significantly improved. In the embodiment of the present application, the volume fraction of grains with a size of <3μm is ≥85%, which ensures that the steel has higher strength while maintaining a certain plasticity and toughness. Effective grains refer to grains with an orientation difference greater than 15° in hot-rolled pickled high-strength steel. The average effective grain size of 2.0μm to 2.5μm indicates that the steel has an ultrafine grain structure, which helps to improve the strength and hardness of the steel. The presence of coarse grains usually reduces the mechanical properties of steel, because dislocation pileups and stress concentration are easily formed inside the coarse grains, making the material more susceptible to plastic deformation or fracture when subjected to stress. In the embodiment of the present application, there are no coarse grains of 15μm and above in the hot-rolled pickled high-strength steel.

[0050] In some embodiments, the hot-rolled pickled high-strength steel meets at least one of the following properties: yield strength ≥700 MPa, tensile strength ≥750 MPa, elongation A80 ≥16%, hole expansion ratio λ ≥70%, and surface quality is FB grade.

[0051] FB-grade surface quality is a high standard requirement for automotive steel, which means that the steel plate surface is smooth and defect-free, which is conducive to improving the quality of subsequent painting and welding.

[0052] Figure 1 A schematic flow chart of a method for preparing all-ferritic hot-rolled pickled high-strength steel provided in an embodiment of the present application.

[0053] See Figure 1 In a second aspect, the present application provides a method for preparing the hot-rolled pickled high-strength steel described in the first aspect, the method comprising:

[0054] S1. Refining molten iron to obtain molten steel having the chemical composition; wherein the refining includes a magnesium treatment process;

[0055] In some embodiments, the magnesium treatment process adopts a wire feeding method, which includes feeding 200m to 400m of Mg-Si cored wire into the molten steel at a wire feeding speed of 2m / s to 6m / s.

[0056] The magnesium treatment process during the steel refining process is achieved through the wire feeding method. Its core is to precisely feed Mg-Si cored wire into the molten steel to improve the steel's cleanliness, inclusion morphology, and mechanical properties. The Mg-Si cored wire is made of a magnesium-silicon alloy powder wrapped around an outer layer of low-carbon steel strip. The steel strip protects the magnesium from oxidation while allowing it to melt rapidly in the molten steel. Magnesium has a high vapor pressure and low density, which makes it prone to forming bubbles and bubbling in the molten steel. Controlling the release rate of magnesium through the wire feeding method prevents sudden and violent evaporation, improving recovery. The wire also reacts with inclusions such as Al2O3 to form low-melting-point complexes (such as MgO·Al2O3), improving inclusion morphology.

[0057] S2, continuously casting the molten steel to obtain a slab;

[0058] S3, sequentially heating, rough rolling and finish rolling the slab to obtain a hot-rolled plate;

[0059] In some embodiments, the target temperature of the heating is 1180° C. to 1280° C., and the heating time is ≥170 min.

[0060] This temperature range is to ensure that the alloy is fully dissolved and the burnout is reasonable. If the heating temperature is too low (<1180℃), the alloy elements cannot be fully dissolved. If the temperature is too high (>1180℃), the original austenite grains will be too large and the burnout will be serious.

[0061] In some embodiments, the finishing temperature of the rough rolling is 1020° C. to 1100° C., and the final reduction ratio of the rough rolling is greater than 30%.

[0062] Within this temperature range, the austenite structure inside the steel billet can be fully recrystallized, which is conducive to the subsequent homogenization and refinement of the structure. If the temperature is too low (<1020℃), it may lead to insufficient recrystallization, affecting the toughness and strength of the final product; if the temperature is too high (>1100℃), it may cause excessive grain growth, which is also detrimental to the performance.

[0063] The final-pass reduction refers to the ratio (in %) of the reduction Δh in the final rolling pass to the billet thickness H before that pass. In this embodiment, the rough rolling process consists of five passes. A final-pass reduction of >30% helps further break up the as-cast structure within the billet, promoting recrystallization and laying a good foundation for the finishing stage. Furthermore, a final-pass reduction of >30% improves the billet's surface quality and reduces cracks and defects.

[0064] In some embodiments, the heated slab is subjected to rough rolling to obtain an intermediate billet, and the thickness of the intermediate billet is 25 mm to 45 mm.

[0065] The intermediate billet is in a transitional state between rough rolling and finishing rolling, and the selection of its thickness needs to comprehensively consider the equipment capacity, rolling efficiency, and the needs of subsequent finishing rolling. Within the thickness range of 25mm to 45mm, it can not only ensure effective deformation in the rough rolling stage, but also provide a suitable raw material size for the finishing rolling stage, ensuring the smooth progress of the finishing rolling process. In the embodiment of the present application, a coil box is used for the intermediate billet. The coil box is mainly used to insulate and buffer the intermediate billet between rough rolling and finishing rolling, reduce temperature loss and temperature difference between the head and tail, and ensure that the billet has a uniform and appropriate temperature before entering the finishing mill, which is crucial to improving the stability of the finishing process and the performance consistency of the final product.

[0066] In some embodiments, the finishing rolling includes stands F1 to F7, wherein the pass reduction ratio of stands F5 and F6 is ≥50%, and the final rolling temperature of the finishing rolling is 840° C. to 900° C.

[0067] In the embodiment of the present application, the finishing rolling adopts the strategy of load shifting. Load shifting means that in the finishing rolling process, a larger amount of reduction is arranged in the later passes. This strategy helps to further refine the steel structure in the later stage of finishing rolling and improve the strength and hardness of the product. At the same time, load shifting can also reduce the rolling force of the first few passes, reduce the equipment load, and extend the service life of the equipment. F5 and F6 are key passes in the finishing rolling stage. A reduction rate of ≥50% is conducive to further breaking up the internal structure of the steel and promoting the formation of ultrafine grains, which has a significant effect on improving the strength and hole expansion performance of the steel. At the same time, a reduction rate of ≥50% can also improve the surface quality of the steel and reduce the occurrence of defects such as cracks and wrinkles.

[0068] In the examples of this application, the final rolling temperature for finishing rolling is 840°C to 900°C. Completing finishing rolling within this temperature range facilitates obtaining a fine ferrite structure, improving the strength and hardness of the steel. Furthermore, a final rolling temperature of 840°C to 900°C also reduces uneven structural transformation during cooling, ensuring stable performance. Finishing rolling is performed at a constant speed, with the rolling speed varying depending on the thickness and controlled within a range of 5m / s to 12m / s.

[0069] S4. Cooling, coiling, straightening and pickling the hot-rolled plate in sequence to obtain finished hot-rolled pickled high-strength steel.

[0070] In some embodiments, the coiling temperature is 580°C to 700°C.

[0071] After finishing rolling, hot-rolled plates need to go through a cooling process to control the transformation of their internal structure and the development of their properties. Coiling within the temperature range of 580℃ to 700℃ is conducive to obtaining fine ferrite structure, which is the key to improving the strength and hardness of steel. The coiling temperature of 580℃ to 700℃ helps promote the nucleation and growth of ferrite, thereby refining the matrix structure; too low a temperature (<580℃) may cause steel embrittlement and affect the forming performance. In addition, during the coiling process, alloy steel elements in the steel may precipitate in the form of precipitates. The coiling temperature of 580℃ to 700℃ can control the morphology, distribution and quantity of the precipitates, thereby optimizing the performance of the steel. After coiling, the steel is placed in a slow cooling pit for 72 hours to 75 hours, and then naturally cooled to room temperature in air.

[0072] In some embodiments, the elongation of the tension-leveling is 0.3% to 0.8%, and the temperature of the raw material during the tension-leveling is less than 80°C.

[0073] Through proper stretching, the straightening process eliminates internal stresses generated during hot rolling and coiling, improving the strip's dimensional stability and shape accuracy. Straightening also reduces surface defects such as wavy strips, creating better conditions for the pickling process and improving the surface quality of the final product. The straightening elongation, defined as the ratio of the length of the strip stretched during the straightening process to its original length, is controlled within a range of 0.3% to 0.8%. This effectively adjusts the strip's shape, eliminates internal stresses, and improves its straightness and dimensional accuracy, preparing it for the subsequent pickling process. If the straightening elongation is too low (<0.3%), internal stresses may not be fully eliminated, resulting in persistent defects such as wavy strips after pickling. If the straightening elongation is too high (>0.8%), the strip may be overstretched, thinning, or even breaking, compromising product quality.

[0074] The temperature of the raw material for straightening refers to the temperature of the strip before it enters the straightening machine. It should be controlled below 80℃ to ensure that the strip will not soften due to excessive temperature during the straightening process, thereby maintaining good rigidity and shape stability, which is conducive to the realization of the straightening effect. If the temperature is too high (≥80℃), the strip will become soft and easily deform excessively during the straightening process, or even crack, affecting the straightening effect and the subsequent pickling process.

[0075] In some embodiments, the pickling strip running speed is 60m / min to 120m / min, the pickling temperature is 75°C to 85°C, and the acidity of the free acid of the pickling is 30g / L to 60g / L.

[0076] Controlling the pickling strip speed within the range of 60m / min to 120m / min can ensure that the acid solution has sufficient reaction time with the iron oxide scale on the strip surface, while avoiding excessive corrosion of the strip matrix, thereby meeting the FB-level surface quality requirements. Controlling the pickling temperature within the range of 75℃ to 85℃ can ensure that the acid solution and the iron oxide scale maintain a moderate reaction speed, which not only ensures the pickling effect, but also improves production efficiency and avoids damage to the equipment. Free acid acidity refers to the concentration of free acid (such as hydrochloric acid, sulfuric acid, etc.) in the pickling solution during the pickling process. Controlling the free acid acidity within the range of 30 to 40g / L can ensure the pickling effect while reducing corrosion to the equipment and reducing the cost of acid treatment, thereby ensuring that the produced strip meets the FB-level surface quality requirements.

[0077] The embodiment of the present application controls the chemical composition, the controlled rolling and controlled cooling process parameters, the pickling process parameters, the input of the coil box, etc., so that the microstructure of the produced strip steel is ferrite. The yield strength of the strip steel is ≥700MPa, the tensile strength is ≥750MPa, the elongation A80 is ≥16%, the hole expansion rate λ is ≥70%, and it has excellent strip steel surface quality. It also improves the problem of high edge crack sensitivity of this type of product.

[0078] The product prepared by the preparation method of the all-ferrite hot-rolled pickled high-strength steel is the above-mentioned all-ferrite hot-rolled pickled high-strength steel. Since the preparation method of the all-ferrite hot-rolled pickled high-strength steel adopts part or all of the technical solutions of the all-ferrite hot-rolled pickled high-strength steel embodiment, it at least has all the beneficial effects brought by the technical solutions of the all-ferrite hot-rolled pickled high-strength steel embodiment, which will not be repeated here one by one.

[0079] The present application is further described below with reference to specific examples. Experimental methods in the following examples where specific conditions are not specified are generally measured in accordance with national / industry standards. If there are no corresponding national / industry standards, the methods are carried out in accordance with commonly used international standards, conventional conditions, or conditions recommended by the manufacturer.

[0080] Example 1

[0081] The chemical composition of the all-ferritic hot-rolled pickled high-strength steel provided in this embodiment is, by mass fraction, C: 0.045%, Si: 0.1%, Mn: 1.4%, P: 0.015%, S: 0.0035%, Ti: 0.1%, Mo: 0.2%, Nb: 0.01%, Cr: 0.15%, N: 0.004%, Mg: 0.0008%, and the balance is Fe and unavoidable impurities.

[0082] The production method includes: molten iron pretreatment, converter smelting, refining, continuous casting, hot rolling, straightening, and pickling. The rough rolling finish temperature is 1060°C, the finishing temperature is 860°C, and the finishing rolling speed is 8 m / s. After rolling, the strip is cooled to 650°C, coiled, and slowly cooled in a slow cooling pit for 72 hours before cooling to room temperature. The strip, cooled to room temperature, is straightened, with an elongation controlled at 0.5%. Pickling is performed using shallow trough turbulent flow pickling at a speed of 80 m / min. After pickling, the strip's surface color is off-white, resulting in fully ferritic hot-rolled pickled high-strength steel. After drying and double-sided electrostatic spraying with anti-rust oil, it is coiled and packaged for use as passenger car chassis steel.

[0083] The strip steel produced in this embodiment has a microstructure of ferrite, a yield strength of 720 MPa, a tensile strength of 818 MPa, an elongation A80 of 17%, a hole expansion ratio λ of 87%, and a surface quality of FB grade.

[0084] Example 2

[0085] Measured by mass fraction, the chemical composition of the all-ferritic hot-rolled pickled high-strength steel provided in this embodiment is: C: 0.05%, Si: 0.1%, Mn: 1.5%, P: 0.015%, S: 0.0035%, Ti: 0.1%, Mo: 0.2%, Nb: 0.012%, N: 0.003%, Mg: 0.001%, and the balance is Fe and unavoidable impurities.

[0086] The production method includes: molten iron pretreatment, converter smelting, refining, continuous casting, hot rolling, straightening, and pickling. The rough rolling finish temperature is 1080°C, the finishing temperature is 850°C, and the finishing rolling speed is 9 m / s. After rolling, the strip is cooled to 630°C, coiled, and slowly cooled in a slow cooling pit for 72 hours before cooling to room temperature. The strip, cooled to room temperature, is straightened, with an elongation controlled at 0.5%. Pickling is performed using shallow trough turbulent flow pickling at a speed of 80 m / min. After pickling, the strip's surface color is off-white, resulting in fully ferritic hot-rolled pickled high-strength steel. After drying and double-sided electrostatic spraying with rust-proof oil, it is coiled and packaged for use as passenger car chassis steel.

[0087] The strip steel produced in this embodiment has a microstructure of ferrite, a yield strength of 708 MPa, a tensile strength of 796 MPa, an elongation A80 of 20%, a hole expansion ratio λ of 92%, and a surface quality of FB grade.

[0088] Example 3

[0089] The chemical composition of the all-ferritic hot-rolled pickled high-strength steel provided in this embodiment is, by mass fraction, C: 0.05%, Si: 0.12%, Mn: 1.6%, P: 0.018%, S: 0.005%, Ti: 0.11%, Mo: 0.18%, Nb: 0.015%, Cr: 0.15%, N: 0.004%, Mg: 0.0006%, and the balance is Fe and unavoidable impurities.

[0090] The production method includes: molten iron pretreatment, converter smelting, refining, continuous casting, hot rolling, straightening, and pickling. The rough rolling finish temperature is 1090°C, the finishing temperature is 880°C, and the finishing rolling speed is 9.5 m / s. After rolling, the strip is cooled to 620°C, then coiled, slow-cooled in a slow cooling pit for 72 hours, and then cooled to room temperature. The strip cooled to room temperature is straightened, with an elongation controlled at 0.5%. Pickling is performed using shallow trough turbulent flow pickling at a speed of 80 m / min. After pickling, the strip surface is off-white, resulting in fully ferritic hot-rolled pickled high-strength steel. After drying and double-sided electrostatic spraying of anti-rust oil, it is coiled and packaged for use as passenger car chassis steel.

[0091] The strip steel produced in this embodiment has a microstructure of ferrite, a yield strength of 718 MPa, a tensile strength of 819 MPa, an elongation A80 of 16%, a hole expansion ratio λ of 79%, and a surface quality of FB grade.

[0092] Attachment Figure 2-4 Detailed description:

[0093] Figure 2 The microstructure diagram of the all-ferrite hot-rolled pickled high-strength steel provided in Example 1 of the present application; Figure 2 As shown, the microstructure is ferrite.

[0094] Figure 3 This is the IPF-Z diagram of the all-ferrite hot-rolled pickled high-strength steel provided in Example 1 of the present application; Figure 3 As shown, the average size of effective grains with an orientation difference greater than 15° is 2.1 μm, the volume fraction of ultrafine ferrite grains smaller than 3 μm is not less than 85%, and there are no coarse grains larger than 15 μm.

[0095] Figure 4 This is the inclusion distribution diagram of the all-ferrite hot-rolled pickled high-strength steel provided in Example 1 of the present application; Figure 4 As shown in the figure, the inclusions in the steel are relatively small, the average size of TiN·Al2O3 inclusions is less than 3.5μm, and the density of inclusions larger than 10μm is not higher than 0.6 / mm 2 .

[0096] One or more technical solutions in the embodiments of the present application may have at least the following technical effects or advantages:

[0097] The full ferrite hot-rolled pickled high-strength steel provided in the embodiment of the present application has excellent mechanical properties, yield strength

[0098] ≥700MPa, tensile strength ≥750MPa, elongation A80 ≥16%, and hole expansion ratio λ ≥70%, meeting the high elongation and hole expansion requirements of high-strength steel. The strip surface quality reaches FB grade, meeting the automotive industry's high surface quality requirements for steel, and helping to improve the appearance and corrosion resistance of the final product.

[0099] The embodiments of the present application increase the proportion of ultrafine grains in the structure through fine control of the controlled rolling and controlled cooling process parameters, effectively improve the problem of high sensitivity to edge cracks, make the material less likely to delaminate or crack during the stamping process, and improve the forming stability.

[0100] The all-ferritic hot-rolled pickled high-strength steel provided in the embodiments of the present application is applied to automobile chassis and body structures, which can significantly reduce the weight of the vehicle, thereby reducing fuel consumption and reducing exhaust emissions, which is beneficial to environmental protection and sustainable development and meets the requirements of the "dual carbon" goals.

[0101] The foregoing is merely a list of specific embodiments of the present application, intended to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but rather is intended to conform to the broadest scope consistent with the principles and novel features claimed herein.

Claims

1. A fully ferritic hot-rolled pickled high-strength steel, wherein the chemical composition of the hot-rolled pickled high-strength steel is, by mass fraction, as follows: C: 0.02% to 0.1%, Si: 0.1% to 1.0%, Mn: 0.5% to 2.5%, P ≤ 0.025%, S ≤ 0.005%, Ti: 0.05% to 0.15%, Nb: 0.01% to 0.07%, Cr ≤ 0.5%, Mo: 0.05% to 0.5%, N ≤ 0.005%, Mg: 0.0005% to 0.0010%, and the balance being Fe and unavoidable impurities; In the hot-rolled pickled high-strength steel, the number density of inclusions >10 μm is ≤0.6 / mm 2 , the average size of inclusions TiN·Al2O3 is <3.5μm.

2. The hot-rolled pickled high-strength steel according to claim 1, characterized in that: The microstructure of the hot-rolled pickled high-strength steel is ferrite, wherein the volume fraction of grains with a size less than 3 μm is ≥85%, and the average size of effective grains is 2.0 μm to 2.5 μm.

3. The hot-rolled pickled high-strength steel according to claim 1, characterized in that: The hot-rolled pickled high-strength steel meets at least one of the following properties: yield strength ≥700 MPa, tensile strength ≥750 MPa, elongation A80 ≥16%, hole expansion ratio λ ≥70%, and surface quality is FB grade.

4. A method for preparing the hot-rolled pickled high-strength steel according to any one of claims 1 to 3, comprising: The molten iron is refined to obtain molten steel having the chemical composition; wherein the refining includes a magnesium treatment process; Continuously casting the molten steel to obtain a slab; The slab is sequentially heated, rough rolled and finish rolled to obtain a hot-rolled plate; The hot-rolled plate is sequentially cooled, coiled, straightened and pickled to obtain a finished hot-rolled pickled high-strength steel.

5. The method according to claim 4, characterized in that The magnesium treatment process adopts a wire feeding method, which includes feeding 200m to 400m of Mg-Si cored wire into molten steel at a wire feeding speed of 2m / s to 6m / s.

6. The method according to claim 4, characterized in that The target temperature of the heating is 1180° C. to 1280° C., and the heating time is ≥170 min.

7. The method according to claim 4, characterized in that The finishing temperature of the rough rolling is 1020° C. to 1100° C., and the final reduction ratio of the rough rolling is greater than 30%; and / or, The heated slab is subjected to rough rolling to obtain an intermediate slab, wherein the thickness of the intermediate slab is 25 mm to 45 mm; and / or, The finishing rolling includes stands F1 to F7, wherein the pass reduction ratio of stands F5 and F6 is ≥50%, and the final rolling temperature of the finishing rolling is 840° C. to 900° C.

8. The method according to claim 4, characterized in that The coiling temperature is 580°C to 700°C.

9. The method according to claim 4, characterized in that The elongation of the tension leveling is 0.3% to 0.8%, and the temperature of the raw material for the tension leveling is less than 80°C.

10. The method according to claim 4, characterized in that The pickling strip running speed is 60m / min-120m / min, the pickling temperature is 75°C-85°C, and the acidity of the free acid in the pickling is 30g / L-60g / L.

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