A ferritic bainitic steel and a method of making the same

Abstract

The application provides a ferrite bainite steel and a preparation method thereof, and belongs to the field of steel preparation. The chemical composition of the ferrite bainite steel includes, in mass fraction, C: 0.18% to 0.22%, Si: 1.3% to 1.8%, Mn: 2.0% to 2.5%, P: less than or equal to 0.01%, S: less than or equal to 0.01%, N: less than or equal to 0.004%, and the balance of Fe and inevitable impurities. The metallographic structure of the ferrite bainite steel includes, in volume fraction, ferrite bainite: 50% to 65%, tempered martensite: 25% to 30%, MA island: 1% to 3%, and residual austenite: 7% to 10%. By reducing the coarse MA island and unstable blocky residual austenite, the crack initiation source is avoided, and the technical problems of poor hole expansion and elongation of the ferrite bainite steel in the prior art are solved.

Description

Technical Field

[0001] The present application relates to the technical field of steel preparation, and in particular to a ferrite bainite steel and a preparation method thereof. Background Art

[0002] With the ever-expanding use of advanced high-strength steel plates, performance requirements are becoming increasingly stringent, requiring not only high strength but also excellent formability. To meet these demands, improvements have been made to conventional annealing lines in recent years, with the construction of new dedicated lines. By introducing a hard phase matrix and improving microstructure uniformity, third-generation advanced high-strength steels with high formability have been developed. While these steels offer improved formability compared to conventional advanced high-strength steels, they still cannot fully meet the formability requirements of some specific users. The microstructure of third-generation transformation-induced plasticity (TIPP) high-strength steels consists of a ferrite-bainite hard phase matrix, MA islands, massive and membranous retained austenite, and a small amount of tempered martensite. Coarse MA islands can be sources of crack initiation, while unstable massive retained austenite easily transforms into brittle martensite, leading to the initiation and propagation of secondary cracks, thus compromising the formability of third-generation TIP high-strength steels.

[0003] Based on the above situation, according to the dedicated galvanized high-strength steel production line, we are looking for an optimal production method for bainitic ferritic steel with high elongation and high hole expandability, that is, to design a reasonable alloy composition system, and then determine a reasonable process method to match it, so as to reduce the coarse MA islands and massive retained austenite in the organization that deteriorate the formability, thereby obtaining excellent hole expandability and elongation. Summary of the Invention

[0004] The present application provides a ferritic bainite steel and a preparation method thereof, which reduces coarse MA islands and unstable blocky retained austenite, avoids crack initiation sources, and solves the technical problem of poor hole expandability and elongation of ferritic bainite steel in the prior art.

[0005] In a first aspect, the present application provides a ferritic bainite steel, wherein the chemical composition of the ferritic bainite steel comprises, by mass fraction: C: 0.18% to 0.22%, Si: 1.3% to 1.8%, Mn: 2.0% to 2.5%, P ≤ 0.01%, S ≤ 0.01%, N ≤ 0.004%, and the balance is Fe and unavoidable impurities;

[0006] Calculated by volume fraction, the metallographic structure of the ferrite bainite steel includes: ferrite bainite: 50% to 65%, tempered martensite: 25% to 30%, MA islands: 1% to 3%, and retained austenite: 7% to 10%.

[0007] Optionally, the thickness of the ferrite martensitic steel is 1.0 mm to 2.0 mm.

[0008] Optionally, the ferritic bainite steel satisfies at least one of the following properties: yield strength of 700MPa to 850MPa, tensile strength of 980MPa to 1100MPa, elongation of 19% to 22%, and hole expansion rate of 55% to 65%.

[0009] In a second aspect, the present application provides a method for preparing the ferrite bainite steel according to any one embodiment of the first aspect, the method comprising:

[0010] Continuously casting the molten steel having the chemical composition to obtain a slab;

[0011] Hot rolling and cold rolling the slab to obtain a chilled coil;

[0012] heat treating the chilled coil to obtain a steel coil;

[0013] The steel coil is subjected to partitioning treatment to obtain ferrite bainite steel.

[0014] Optionally, heat treating the chilled coil to obtain the steel coil comprises:

[0015] The chilled coil is subjected to a three-stage continuous heating process, wherein the parameters of the first heating process include: a heating rate of 8°C / s to 12°C / s, and a temperature at the end of the process of 210°C to 230°C; the parameters of the second heating process include: a heating rate of 3°C / s to 8°C / s, and a temperature at the end of the process of 640°C to 660°C; the parameters of the third heating process include: a heating rate of 1°C / s to 4°C / s, and a temperature at the end of the process of 840°C to 870°C;

[0016] The chilled coil after the heating stage is subjected to a holding stage treatment under the temperature condition at the end of the third heating stage, wherein the holding stage treatment time is 60s to 150s;

[0017] The chilled coil after the insulation section treatment is subjected to three consecutive cooling section treatments to obtain a steel coil, wherein the parameters of the first cooling section include: a cooling rate of 2°C / s to 6°C / s, and a section end temperature of 780°C to 820°C; the parameters of the second cooling section include: a cooling rate of 20°C / s to 30°C / s, and a section end temperature of 300°C to 380°C; the parameters of the third cooling section include: a cooling rate of 50°C / s to 60°C / s, and a section end temperature of 200°C to 270°C.

[0018] Optionally, the parameters of the distribution treatment include: a holding temperature of 380° C. to 420° C., and a holding time of 60s to 120s.

[0019] Optionally, hot rolling and cold rolling the slab to obtain a chilled coil comprises:

[0020] Heating, rough rolling, finish rolling and coiling the slab to obtain a hot-rolled coil;

[0021] The hot rolled coil is cold rolled to obtain a chilled coil.

[0022] Optionally, the heating temperature is 1150°C to 1280°C, the finishing temperature of the finish rolling is 870°C to 920°C, and the coiling temperature is 550°C to 620°C.

[0023] Optionally, the total reduction rate of the cold rolling is 50% to 60%.

[0024] Optionally, before continuously casting the molten steel having the chemical composition to obtain a slab, the method further comprises:

[0025] Molten steel with the chemical composition is obtained through converter smelting; the terminal temperature of the converter smelting is 1650°C to 1670°C, the deoxidizer used in the converter smelting includes: 200kg / t·steel to 800kg / t·steel of lime, 0~1000kg / t·steel of pre-melted slag and 0~400kg / t·steel of fluorite, the slag amount during tapping of the converter smelting is ≤80mm, and the tapping time of the converter smelting is 4min to 9min.

[0026] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0027] This application provides a ferritic bainite steel. By adopting a high-Si, high-Mn composition system, a high-strength steel is obtained with a hard ferritic bainite matrix and a mixed refined structure of tempered martensite, film-like retained austenite, and MA islands. In this structure, the ferritic bainite hard phase matrix provides strength, the film-like retained austenite TRIP effect provides ductility, and the fine, uniform overall structure provides a high hole expansion rate. The overall structure is relatively uniform and does not cause local strain concentration. Compared with third-generation transformation-induced plasticity high-strength steel, the content of coarse, massive MA islands and unstable, massive retained austenite is significantly reduced. The ferritic bainite steel provided by the present invention has a yield strength of 700MPa to 850MPa, a tensile strength of 980MPa to 1100MPa, an elongation of 19% to 22%, and a hole expansion rate of 55% to 65%. The high elongation and excellent hole expansion performance can meet the special processing performance requirements of automotive parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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.

[0029] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces 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.

[0030] Figure 1 A schematic flow chart of a method for preparing ferrite bainite steel provided in an embodiment of the present application;

[0031] Figure 2 This is a metallographic structure diagram of a ferrite-bainite steel provided in an embodiment of the present application. DETAILED DESCRIPTION

[0032] 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.

[0033] 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 hard 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 numbers within the range. For example, the description of a range 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 applies regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0034] In addition, in the description of the specification of this application, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second" are merely 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. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" refers to one or more, and "plurality" refers to 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, respectively.

[0035] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0036] The creative idea of ​​this application is: adopting the C-Si-Mn component system and high temperature heating-bainite transformation-quenching and partitioning annealing process to obtain a ferritic bainite steel with a mixed refined structure of bainitic ferrite + tempered martensite hard matrix and film-like retained austenite. A certain content of ferrite bainite hard phase is obtained by utilizing the bainite transformation process, while increasing the carbon enrichment in the austenite. Next, part of the unstable blocky retained austenite is transformed into a martensite phase through the quenching process. Finally, tempered martensite and ferritic bainite are obtained through the partitioning process, and carbon is enriched in the austenite to further improve the stability of the austenite, thereby reducing the content of blocky unstable retained austenite and coarse MA islands, avoiding the brittle martensitic transformation of unstable blocky retained austenite during the strain process and the initiation source of MA island cracks. The developed hard phase matrix bainitic ferritic steel has better elongation and hole expansion than the third generation ferritic bainite steel.

[0037] The present application provides a ferritic bainite steel, wherein the chemical composition of the ferritic bainite steel comprises, by mass fraction, C: 0.18% to 0.22%, Si: 1.3% to 1.8%, Mn: 2.0% to 2.5%, P ≤ 0.01%, S ≤ 0.01%, N ≤ 0.004%, and the balance is Fe and unavoidable impurities;

[0038] Calculated by volume fraction, the metallographic structure of the ferrite bainite steel includes: ferrite bainite: 50% to 65%, tempered martensite: 25% to 30%, MA islands: 1% to 3%, and retained austenite: 7% to 10%.

[0039] In the embodiment of this application, the functions of each element are as follows:

[0040] C: C is the most effective solid solution strengthening element and is crucial for ensuring the hard phase content of steel. Therefore, the C content should be controlled within a range of 0.18% to 0.22% by weight. A lower C content will not guarantee the hard phase content and make it difficult to achieve the desired strength, while an excessive C content can deteriorate weldability. For example, the C content can be 0.18%, 0.19%, 0.20%, 0.21%, 0.22%, etc.

[0041] Si: Si is an important element that inhibits cementite precipitation. Therefore, the Si content should be controlled within a range of 1.3% to 1.8% by weight. If the Si content is too low, it will be difficult to inhibit cementite precipitation, resulting in a small amount of retained austenite, which will affect the ductility of the steel. If the Si content is too high, a large amount of unstable retained austenite will be generated, which will deteriorate the flanging performance. For example, the Si content can be 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, etc.

[0042] Mn: Mn is a solid solution strengthening element and an important element for stabilizing austenite. Therefore, the present invention controls the Mn content by weight to 2.0% to 2.5%. Too low a content makes it difficult to maintain a hard phase in the steel and achieve high strength, while too high a content deteriorates workability and weldability. For example, the Mn content can be 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, etc.

[0043] Phosphorus (P): Phosphorus can significantly reduce the plasticity and toughness of steel, so its content should be kept as low as possible, with the P content controlled below 0.01% by weight. For example, the P content can be 0.003%, 0.004%, 0.006%, 0.008%, 0.009%, 0.01%, etc.

[0044] S: S is a harmful impurity element in steel, causing hot brittleness, reducing ductility and toughness, and causing cracks during forging and rolling. Therefore, the S content is controlled below 0.01% by weight. For example, the S content can be 0.003%, 0.004%, 0.006%, 0.008%, 0.009%, 0.01%, etc.

[0045] N: Nitrogen, like carbon, is also a solid-solution element. As the nitrogen content in steel increases, its stamping performance will deteriorate. At the same time, solid-solution nitrogen is the main cause of aging of the finished galvanized sheet, especially for the strain aging effect after flattening, the influence of nitrogen is particularly large, so the nitrogen content is required to be as low as possible. For the tinplate of the present invention, the nitrogen content in the steel should be controlled below 0.004%. For example, the nitrogen content can be 0.001%, 0.002%, 0.003%, 0.004%, etc.

[0046] In the embodiments of the present application, the ferrite-bainite phase serves as the matrix phase. The retained austenite is in the form of a thin film and is evenly dispersed within the ferrite-bainite matrix. This reduces the hardness difference between the hard and soft phases, resulting in a uniform microstructure for the ferrite-bainite steel. This also blunts crack propagation, improving hole expansion performance and increasing the elongation of the ferrite-bainite steel. Compared to third-generation transformation-induced plasticity high-strength steels, this significantly reduces the presence of coarse MA islands and massive retained austenite, thus avoiding sources of crack initiation. Exemplarily, the content of ferrite bainite can be 50%, 52%, 55%, 58%, 60%, 62%, 65%, etc.; the content of tempered martensite can be 25%, 26%, 27%, 28%, 29%, 30%, etc.; the content of MA islands can be 1%, 1.2%, 1.5%, 1.8%, 2.0%, 2.5%, 3%, etc.; the content of retained austenite can be 7%, 8%, 9%, 10%, etc.

[0047] In some embodiments, the ferrite-martensitic steel has a thickness of 1.0 mm to 2.0 mm.

[0048] Illustratively, the thickness of the ferrite martensitic steel may be 1.0 mm, 1.2 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2.0 mm, etc.

[0049] In some embodiments, the ferritic bainite steel satisfies at least one of the following properties: yield strength of 700 MPa to 850 MPa, tensile strength of 980 MPa to 1100 MPa, elongation of 19% to 22%, and hole expansion rate of 55% to 65%.

[0050] The hard-phase matrix bainitic ferrite steel developed in this application has better elongation and hole expansion than the third-generation transformation-induced plasticity steel. For example, the yield strength can be 700MPa, 720MPa, 750MPa, 780MPa, 800MPa, 820MPa, 850MPa, etc.; the tensile strength can be 980MPa, 1000MPa, 1020MPa, 1040MPa, 1060MPa1, 080MPa, 1100MPa, etc.; the elongation can be 19%, 20%, 21%, 22%, etc.; the hole expansion rate can be 55%, 58%, 60%, 62%, 65%, etc.

[0051] Figure 1 A schematic flow chart of a method for preparing ferritic bainite steel provided in an embodiment of the present application.

[0052] See Figure 1 , the present application provides a method for preparing ferrite bainite steel, the method comprising:

[0053] S1. Continuously casting the molten steel having the chemical composition to obtain a slab;

[0054] In some embodiments, before continuously casting the molten steel having the chemical composition to obtain a slab, the method further comprises:

[0055] Molten steel with the chemical composition is obtained through converter smelting; the terminal temperature of the converter smelting is 1650°C to 1670°C, the deoxidizer used in the converter smelting includes: 200kg / t·steel to 800kg / t·steel of lime, 0~1000kg / t·steel of pre-melted slag and 0~400kg / t·steel of fluorite, the slag amount during tapping of the converter smelting is ≤80mm, and the tapping time of the converter smelting is 4min to 9min.

[0056] Exemplarily, the terminal temperature of the converter smelting is 1650°C, 1655°C, 1660°C, 1665°C, 1670°C, etc.

[0057] Illustratively, the amount of lime can be 200kg / t·steel, 300kg / t·steel, 400kg / t·steel, 500kg / t·steel, 600kg / t·steel, 700kg / t·steel, 800kg / t·steel, etc., the amount of pre-melted slag can be 0, 200kg / t·steel, 400kg / t·steel, 600kg / t·steel, 800kg / t·steel, 1000kg / t·steel, etc., and the amount of fluorite can be 0, 50kg / t·steel, 100kg / t·steel, 200kg / t·steel, 250kg / t·steel, 300kg / t·steel, 350kg / t·steel, 400kg / t·steel, etc.

[0058] For example, the slag amount during tapping of the converter smelting can be 20 mm, 30 mm, 40 mm, 50 mm, 60 mm, 70 mm, 80 mm, etc.

[0059] Exemplarily, the tapping time of the converter smelting can be 4 minutes, 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, etc.

[0060] S2, hot rolling and cold rolling the slab to obtain a chilled coil;

[0061] In some embodiments, hot rolling and cold rolling the slab to obtain a chilled coil comprises:

[0062] Heating, rough rolling, finish rolling and coiling the slab to obtain a hot-rolled coil;

[0063] The hot rolled coil is cold rolled to obtain a chilled coil.

[0064] In some embodiments, the heating temperature is 1150°C to 1280°C, the finishing temperature of the finish rolling is 870°C to 920°C, and the coiling temperature is 550°C to 620°C.

[0065] Controlling the slab heating temperature to 1150°C to 1280°C has the following positive effects: If the slab heating temperature is lower than 1150°C, the nitrogen carbides cannot be completely dissolved, affecting the required strength and elongation. Conversely, if the heating temperature is higher than 1280°C, the hot working plasticity is deteriorated. For example, the slab heating temperature can be 1150°C, 1180°C, 1200°C, 1220°C, 1240°C, 1260°C, 1280°C, etc.

[0066] Controlling the finishing temperature of the finishing rolling process to 870°C to 920°C has the following positive effects: If the finishing rolling finishing temperature is lower than 870°C, coarse ferrite will be generated during hot rolling, affecting subsequent elongation. Conversely, if the finishing rolling finishing temperature is higher than 920°C, coarse austenite will be generated during hot rolling, affecting subsequent strength. For example, the finishing rolling finishing temperature can be 870°C, 880°C, 890°C, 900°C, 910°C, 920°C, etc.

[0067] Controlling the coiling temperature to 550°C to 620°C has the following positive effects: Coiling temperatures below 550°C result in a relatively high yield strength. This increases rolling force during cold rolling, hindering the cold rolling process. Coiling temperatures above 620°C can cause banding in the hot-rolled sheet due to high Mn content, increasing the difficulty of subsequent processing. For example, the coiling temperature can be 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, etc.

[0068] In some embodiments, the total reduction ratio of the cold rolling is 50% to 60%.

[0069] Controlling the total cold rolling reduction to 50% to 60% has the following positive effects: The total cold rolling reduction cannot be too high, otherwise the cold rolling process will be difficult to carry out; if the total cold rolling reduction is too low, the target thickness cannot be achieved. For example, the total cold rolling reduction can be 50%, 52%, 55%, 56%, 58%, 60%, etc.

[0070] S3, heat treating the chilled coil to obtain a steel coil;

[0071] In some embodiments, heat treating the chilled coil to obtain the steel coil comprises:

[0072] The chilled coil is subjected to a three-stage continuous heating process, wherein the parameters of the first heating process include: a heating rate of 8°C / s to 12°C / s, and a temperature at the end of the process of 210°C to 230°C; the parameters of the second heating process include: a heating rate of 3°C / s to 8°C / s, and a temperature at the end of the process of 640°C to 660°C; the parameters of the third heating process include: a heating rate of 1°C / s to 4°C / s, and a temperature at the end of the process of 840°C to 870°C;

[0073] The chilled coil after the heating stage is subjected to a holding stage treatment under the temperature condition at the end of the third heating stage, wherein the holding stage treatment time is 60s to 150s;

[0074] The chilled coil after the insulation section treatment is subjected to three consecutive cooling section treatments to obtain a steel coil, wherein the parameters of the first cooling section include: a cooling rate of 2°C / s to 6°C / s, and a section end temperature of 780°C to 820°C; the parameters of the second cooling section include: a cooling rate of 20°C / s to 30°C / s, and a section end temperature of 300°C to 380°C; the parameters of the third cooling section include: a cooling rate of 50°C / s to 60°C / s, and a section end temperature of 200°C to 270°C.

[0075] In the embodiments of the application, the first heating stage allows the cold-deformed ferrite produced during the cold rolling process to recover. The second heating stage allows the strip to be pre-oxidized, preventing plating problems caused by excessive levels of easily oxidized elements such as Si and Al. The third heating stage allows the cold-rolled ferrite structure to recrystallize, with pearlite first transforming into austenite and then growing toward ferrite. Exemplarily, the end temperature of the first heating section may be 210°C, 215°C, 220°C, 225°C, 230°C, etc., and the heating rate of the first heating section may be 8°C / s, 9°C / s, 10°C / s, 11°C / s, 12°C / s, etc.; the end temperature of the second heating section may be 640°C, 645°C, 650°C, 655°C, 660°C, etc., and the heating rate of the second heating section may be 3°C / s, 4°C / s, 5°C / s, 6°C / s, 7°C / s, 8°C / s, the end temperature of the third heating section may be 840°C, 845°C, 850°C, 855°C, 860°C, 865°C, 870°C, etc., and the heating rate of the third heating section may be 1°C / s, 2°C / s, 3°C / s, 3.5°C / s, 4°C / s, etc.

[0076] The holding process can achieve full or partial austenitization and obtain more austenite. At the same time, it effectively controls the austenite grains and effectively improves the hole expandability. In this heating process, if the holding temperature is too high or the holding time is too long, coarse austenite grains will be formed, which will affect the grain size of the subsequent structure and deteriorate the performance of the steel. On the contrary, if the holding temperature is too low or the holding time is too short, uneven original structure will be formed, which will also affect the performance of the subsequent steel. For example, the temperature of the holding section can be 840°C, 850°C, 860°C, 870°C, etc., and the time of the holding section can be 60s, 70s, 80s, 90s, 100s, 120s, 140s, 150s, etc.

[0077] The first cooling section causes part of the austenite to transfer to ferrite, and elements such as C and Mn gather in the austenite. During the second cooling section, part of the austenite is transformed into the ferrite bainite phase, further enriching C in the austenite. During the third cooling section, part of the unstable austenite is further transformed into the martensite phase to supplement the insufficient strength. The martensite phase formed in this process provides a nucleation position, which promotes the subsequent bainite transformation. If the rapid cooling temperature is too low, the content of retained austenite will be reduced, and at the same time, the carbon content in the retained austenite will be reduced, so that the steel obtains ultra-high strength, but the elongation will be deteriorated, the microstructure will be poorly uniform, and the hole expansion rate will be reduced; on the contrary, if the rapid cooling temperature is too high, the MA island content will be increased, resulting in poor microstructure uniformity and a reduction in hole expansion. At the same time, the carbon content in the retained austenite is reduced, affecting the elongation. If the third cooling rate is too fast, ultra-high strength will be obtained while deteriorating the elongation. On the contrary, if it is too slow, the required hard phase martensite content will not be obtained. Exemplarily, the cooling rate of the first cooling section is 2°C / s, 3°C / s, 4°C / s, 5°C / s, 6°C / s, etc., and the end temperature of the first cooling section can be 780°C, 790°C, 800°C, 810°C, 820°C, etc.; the cooling rate of the second cooling section can be 20°C / s, 22°C / s, 23°C / s, 25°C / s, 26°C / s, 28°C / s, 30°C / s, etc., and the end temperature of the second cooling section can be 300°C, 320°C, 340°C, 360°C, 380°C, etc.; the cooling rate of the third cooling section can be 50°C / s, 52°C / s, 54°C / s, 56°C / s, 58°C / s, 60°C / s, etc., and the end temperature of the third cooling section can be 200°C, 210°C, 230°C, 250°C, 270°C, etc.

[0078] S4. Partitioning the steel coil to obtain ferrite-bainite steel.

[0079] In some embodiments, the parameters of the distribution process include: a holding temperature of 380° C. to 420° C., and a holding time of 60 s to 120 s.

[0080] Partitioning treatment can make elements such as C and Mn further gather in austenite, obtain a good match between the retained austenite content and its carbon content. If the partitioning temperature is too low, the content of retained austenite will be reduced, and the carbon content in the retained austenite will be reduced at the same time, so that the steel obtains ultra-high strength, but the elongation is worsened, the uniformity of the structure is poor, and the hole expansion rate is reduced; on the contrary, if the partitioning temperature is too high, cementite will be precipitated, the content of retained austenite will be reduced, and the carbon content in the retained austenite will be reduced at the same time, which will affect the elongation and fail to meet the strength of the steel of the embodiment of the present invention, with poor uniformity of the structure and low hole expansion rate. If the holding time is too long, some carbides will be precipitated, which will reduce the carbon content in the retained austenite and the retained austenite, thereby making the ductility of the steel poor and the hole expansion rate high. On the contrary, if the holding time is too short, elements such as C and Mn cannot further gather in austenite, which will also reduce the carbon content in the retained austenite and the retained austenite, and will also reduce the ductility of the steel. For example, the holding temperature of the distribution treatment can be 380°C, 390°C, 400°C, 410°C, 420°C, etc., and the holding time can be 60s, 70s, 80s, 90s, 110s, 120s, etc.

[0081] The product prepared by the method for preparing ferritic bainite steel is the above-mentioned ferritic bainite steel. The chemical composition and microstructure of the ferritic bainite steel prepared by the method for preparing ferritic bainite steel can refer to the above-mentioned embodiment. Since the method for preparing ferritic bainite steel adopts part or all of the technical solutions of the ferritic bainite steel embodiment, it has at least all the beneficial effects brought about by the technical solutions of the ferritic bainite steel embodiment, which will not be described one by one here.

[0082] On the other hand, an embodiment of the present invention further provides an application of ferrite bainite steel, wherein the ferrite bainite steel is used as a substrate for manufacturing a galvanized sheet.

[0083] The martensitic steel obtained above is heated to a galvanizing temperature of 450-460°C, and after galvanizing is completed, it is cooled to 420-430°C by air knife scraping. During this process, the residence time of the strip in the equalization section, the furnace nose and the zinc pot is minimized by heating to avoid partial decomposition of austenite at high temperature. Heating can be carried out by induction heating.

[0084] The front end between the air knife and the top roller is air-cooled and the rear end is air-cooled, and finally cooled to 250-300℃, with a cooling rate of about 6-9℃ / s. During this process, a very small part of the austenite phase transforms into martensite.

[0085] The present application will be further described below in conjunction with specific examples. It should be understood that these examples are intended to illustrate the present application only and are not intended to limit the scope of the present application. The experimental methods in the following examples where specific conditions are not specified are generally measured according to industry standards. If there are no corresponding industry standards, then the methods are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0086] Examples 1-4 and Comparative Example 1

[0087] Examples 1-4 and Comparative Example 1 provide a ferrite bainite steel and a preparation method thereof, and the preparation process thereof is specifically as follows:

[0088] S11, after smelting molten steel, continuous casting is performed to obtain slabs. The chemical composition of the slabs is shown in Table 1, and the remainder is Fe and unavoidable impurities;

[0089] S21. After heating the slab, rough rolling, finish rolling, laminar cooling, and coiling are sequentially performed to obtain a hot-rolled coil. The process control of heating, finish rolling, and coiling is shown in Table 2. The hot-rolled coil is pickled and then cold-rolled to obtain a cold-rolled coil. The reduction ratio of the cold rolling process is shown in Table 2.

[0090] S31. The cold-rolled coil is sequentially subjected to first heating, second heating, third heating, heat preservation, first cooling, second cooling (air cooling), and third cooling (air cooling) to obtain a heat-treated coil. The process control of the first heating, second heating, third heating, heat preservation, first cooling, second cooling, and third cooling in this step is shown in Table 4.

[0091] S41. The heat-treated coil is heated to the partitioning temperature and kept at this temperature for a period of time, and then subjected to aging treatment to obtain ferrite bainite steel. The process control in this step is shown in Table 4.

[0092] Table 1 Chemical composition of slab (wt)

[0093]

[0094] Table 2 Process parameters of hot rolling and cold rolling

[0095]

[0096] Table 3 Process parameters of heat treatment and partitioning treatment

[0097]

[0098]

[0099] The ferritic bainite steels provided in Examples 1-4 of the present invention and Comparative Example 1 were subjected to microstructure testing and mechanical property testing according to the national standard (GB / T 228.1-2010). The results are shown in Table 4. Holes were then expanded, and the number of samples that did not crack after expansion was counted, and the yield rate was calculated as shown in Table 4.

[0100] Table 4 Metallographic structure and mechanical properties of ferrite bainite steels of Examples 1-4 and Comparative Example 1

[0101]

[0102] In Table 4, λ is the hole expansion ratio. The higher λ is, the better the hole expansion performance of the ferritic bainite steel is.

[0103] As can be seen from Table 4, the ferritic bainite steel provided by Examples 1-4 of the present invention has a yield strength of 700 MPa to 850 MPa, a tensile strength of 980 MPa to 1100 MPa, an elongation of 19% to 22%, and a hole expansion rate of 55% to 65%, with good hole expansion performance.

[0104] The microstructure of the third-generation transformation-induced plasticity steel provided in Comparative Example 1 is ferrite bainite, MA islands, tempered martensite, and massive and film-like retained austenite. The yield strength is 623 MPa, the tensile strength is 1008 MPa, the elongation is 19.5%, and the yield rate after hole expansion is 40%. The yield strength and hole expansion performance are worse than those of Examples 1-4 of the present invention.

[0105] Figure 2 The metallographic structure diagram of the ferrite bainite steel provided in the embodiment of the present invention is Figure 2 It can be seen that the microstructure of the ferrite bainite steel provided by the embodiment of the present invention is based on a ferrite bainite hard phase and contains film-like retained austenite and tempered martensite.

[0106] The present invention provides a ferritic bainite steel and its preparation method. This steel utilizes a high-Si, high-Mn composition system and a high-temperature heating-bainite transformation-quenching partitioning pattern to produce a high-strength steel with a hard ferritic bainite matrix and a mixed, refined structure consisting of tempered martensite, retained austenite, and a minimal amount of MA islands. The ferritic bainite hard phase matrix provides strength, the film-like retained austenite TRIP effect provides ductility, and the fine, uniform overall structure provides a high hole expansion ratio. The overall structure is relatively uniform, avoiding local strain concentration. Compared with third-generation transformation-induced plasticity steels, the content of coarse, massive MA islands and unstable, massive retained austenite is significantly reduced. The ferritic bainite steel provided by the present invention has a yield strength of 700 MPa to 850 MPa, a tensile strength of 980 MPa to 1100 MPa, an elongation of 19% to 22%, and a hole expansion ratio of 55% to 65%. This steel exhibits excellent hole expansion performance, meeting the specialized processing requirements of automotive parts.

[0107] 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 is intended to conform to the broadest scope consistent with the principles and novel features of the present application.

Claims

1. A ferritic bainite steel, characterized in that: The chemical composition of the ferritic bainite steel includes, by mass fraction, C: 0.18% to 0.22%, Si: 1.3% to 1.8%, Mn: 2.0% to 2.5%, P≤0.01%, S≤0.01%, N≤0.004%, and the balance is Fe and unavoidable impurities; The metallographic structure of the ferritic bainite steel comprises, by volume fraction, ferritic bainite: 50% to 65%, tempered martensite: 25% to 30%, MA islands: 1% to 3%, and retained austenite: 7% to 10%; The preparation method of the ferrite bainite steel comprises: Continuously casting the molten steel having the chemical composition to obtain a slab; Hot rolling and cold rolling the slab to obtain a chilled coil; heat treating the chilled coil to obtain a steel coil; Partitioning the steel coil to obtain ferrite bainite steel; The step of heat treating the chilled coil to obtain a steel coil comprises: The chilled coil is subjected to a three-stage continuous heating process, wherein the parameters of the first heating process include: a heating rate of 8°C / s to 12°C / s, and a temperature at the end of the process of 210°C to 230°C; the parameters of the second heating process include: a heating rate of 3°C / s to 8°C / s, and a temperature at the end of the process of 640°C to 660°C; the parameters of the third heating process include: a heating rate of 1°C / s to 4°C / s, and a temperature at the end of the process of 840°C to 870°C; The chilled coil after the heating stage is subjected to a holding stage treatment under the temperature condition at the end of the third heating stage, wherein the holding stage treatment time is 60s to 150s; The chilled coil after the holding stage is subjected to three consecutive cooling stages to obtain a steel coil, wherein the parameters of the first cooling stage include: a cooling rate of 2°C / s to 6°C / s, and a temperature at the end of the stage of 780°C to 820°C; the parameters of the second cooling stage include: a cooling rate of 20°C / s to 30°C / s, and a temperature at the end of the stage of 300°C to 380°C; the parameters of the third cooling stage include: a cooling rate of 50°C / s to 60°C / s, and a temperature at the end of the stage of 200°C to 270°C; The parameters of the distribution process include: a holding temperature of 380° C. to 420° C., and a holding time of 60s to 120s.

2. The ferrite-bainite steel according to claim 1, characterized in that The thickness of the ferrite-martensitic steel is 1.0 mm to 2.0 mm.

3. The ferrite-bainite steel according to claim 1, characterized in that The ferritic bainite steel satisfies at least one of the following properties: yield strength of 700 MPa to 850 MPa, tensile strength of 980 MPa to 1100 MPa, elongation of 19% to 22%, and hole expansion rate of 55% to 65%.

4. A method for preparing the ferrite-bainite steel according to any one of claims 1 to 3, characterized in that: The method comprises: Continuously casting the molten steel having the chemical composition to obtain a slab; Hot rolling and cold rolling the slab to obtain a chilled coil; heat treating the chilled coil to obtain a steel coil; The steel coil is subjected to partitioning treatment to obtain ferrite bainite steel.

5. The method according to claim 4, characterized in that The step of hot rolling and cold rolling the slab to obtain a chilled coil comprises: Heating, rough rolling, finish rolling and coiling the slab to obtain a hot-rolled coil; The hot rolled coil is cold rolled to obtain a chilled coil.

6. The method according to claim 5, characterized in that The heating temperature is 1150°C to 1280°C, the finishing temperature of the finish rolling is 870°C to 920°C, and the coiling temperature is 550°C to 620°C.

7. The method according to claim 5, characterized in that The total reduction ratio of the cold rolling is 50% to 60%.

8. The method according to claim 4, characterized in that Before continuously casting the molten steel having the chemical composition to obtain a slab, the method further comprises: Molten steel with the chemical composition is obtained through converter smelting; the terminal temperature of the converter smelting is 1650° C. to 1670° C., the deoxidizer used in the converter smelting includes: 200 kg / t·steel to 800 kg / t·steel of lime, 0 to 1000 kg / t·steel of pre-melted slag and 0 to 400 kg / t·steel of fluorite, the slag amount during tapping of the converter smelting is ≤80 mm, and the tapping time of the converter smelting is 4 min to 9 min.

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