A 1000MPa grade steel and its preparation method

Abstract

This invention provides a 1000MPa grade steel and its preparation method, belonging to the field of steel production technology. The preparation method of the 1000MPa grade steel includes: obtaining a cast billet with a predetermined chemical composition; heating the cast billet in stages, then rough rolling the heated cast billet, and performing a first descaling treatment during the rough rolling process to obtain a rough-rolled plate; performing a second descaling treatment on the rough-rolled plate; and performing a third descaling treatment on the rough-rolled plate after the second descaling treatment, and performing a third descaling treatment during the finish rolling process to obtain a finish-rolled plate. This invention enables the steel to possess excellent mechanical properties, with a yield strength ≥900MPa, tensile strength 1000MPa~1500MPa, and elongation ≥10%. Based on high strength, the surface characteristics of the steel plate are improved, achieving the technical requirement of direct use without surface treatment or coating. This solves the technical problem that existing high-strength steels cannot meet the requirement of high surface quality.

Description

Technical Field

[0001] This application relates to the field of steel production technology, and in particular to a 1000MPa grade steel and its preparation method. Background Technology

[0002] China is a major producer of commercial vehicles, and the main components of these vehicles, including chassis beams, wheels, axle housings, fuel tank brackets, and cargo boxes, consume over 10 million tons of steel annually. Hot-rolled steel possesses high work hardening rate, high initial hardening rate, low yield strength, high tensile strength, high elongation, and good bake hardening properties, meeting the application requirements of various automotive components. Furthermore, the production process for hot-rolled steel is simple, with low production costs and large-scale production. Therefore, hot-rolled steel has gradually become one of the preferred steel grades for future automotive applications.

[0003] With the development of the automotive industry, high-strength automotive steel can meet users' requirements for lightweight, low-fuel-consumption, high-strength structural components, and high safety performance in commercial vehicles. Currently, the tensile strength levels of hot-rolled automotive steel research and application are mostly below 700 MPa, with few reports on hot-rolled automotive steel with tensile strengths of 1000 MPa and above. At the same time, automakers require hot-rolled automotive steel to have excellent surface quality, meeting the requirements for reduced or no pickling, and minimizing the environmental impact of the hot-rolling process. Therefore, to adapt to the development of the automotive industry, the research and development of high-strength, high-surface-quality automotive steel has become an important topic in current automotive steel research and development. Summary of the Invention

[0004] This application provides a 1000MPa grade steel and its preparation method to solve the technical problem that existing high-strength steels cannot meet the requirements for high surface quality.

[0005] In a first aspect, this application provides a method for preparing 1000MPa grade steel, the method comprising:

[0006] A cast billet with a set chemical composition is obtained;

[0007] The billet is heated in stages, and then the heated billet is rough rolled. During the rough rolling process, the billet undergoes a first descaling treatment to obtain a rough rolled plate.

[0008] The rough-rolled plate is subjected to a second descaling treatment;

[0009] The rough-rolled plate after the second descaling treatment is then subjected to finish rolling, and a third descaling treatment is performed during the finish rolling process to obtain a finish-rolled plate.

[0010] Optionally, the pressure of the first descaling treatment is 300 Bar to 500 Bar, the pressure of the second descaling treatment is 300 Bar to 500 Bar, and the pressure of the third descaling treatment is 200 Bar to 500 Bar.

[0011] Optionally, the pressure of the first descaling process is 350 Bar, the pressure of the second descaling process is 350 Bar, and the pressure of the third descaling process is 250 Bar.

[0012] Optionally, the heating process includes the following stages: a preheating section, a high-temperature section, and a homogenization section; wherein,

[0013] The temperature of the preheating section is 1000℃~1250℃, and the preheating time is 50min~60min. The temperature of the high-temperature section is 1260℃~1300℃, and the high-temperature time is 30min~50min. The temperature of the heat spreader section is 1270℃~1280℃, and the heat spreader time is 30min~50min.

[0014] Optionally, the specified chemical composition includes: C, Mn, P, S, Al, Ti, V, Cr, N, and Fe; in mass fraction,

[0015] The content of C is 0.03-0.06%, the content of Mn is 1.0-1.6%, the content of P is ≤0.015%, the content of S is ≤0.002%, the content of Al is 0.02-0.05%, the content of Ti is 0.20-0.25%, the content of V is 0.07-0.15%, the content of Cr is 0.40-0.80%, and the content of N is 0.007-0.010%.

[0016] Optionally, the chemical composition satisfies the following relationship: ([Ti]+[V]) / ([C]+[N])=1.2~10.8,

[0017] In the formula, [Ti] represents the mass fraction of Ti, [V] represents the mass fraction of V, [C] represents the mass fraction of C, and [N] represents the mass fraction of N.

[0018] Optionally, the finishing temperature of the roughing roll and the finishing roll temperature are controlled; wherein,

[0019] The roughing rolling end temperature is 1080℃~1100℃, and the finishing rolling end temperature is 870℃~910℃.

[0020] Optionally, the method further includes:

[0021] The finished rolled plate is cooled in stages and then coiled to obtain 1000MPa grade steel; the cooling process includes the following stages: a first-stage cooling, air cooling after the first-stage cooling, and a second-stage cooling; wherein...

[0022] The temperature of the first stage of cooling is 600℃~660℃, the air cooling time is 4s~20s, and the temperature of the second stage of cooling is 550℃~610℃.

[0023] Optionally, the continuous casting employs an electromagnetic stirring process and a light reduction process.

[0024] Secondly, this application provides a 1000MPa grade steel, which is prepared by the method described in any one embodiment of the first aspect. The microstructure of the steel, by volume fraction, comprises 15-20% ferrite and 80-85% bainite; the grain size grade of the microstructure is 13-14.

[0025] Optionally, the steel meets the following conditions: yield strength ≥ 900 MPa, tensile strength 1000 MPa~1500 MPa, elongation ≥ 10%, and surface iron oxide scale thickness ≤ 6 μm.

[0026] The technical solutions provided in this application have the following advantages compared with the prior art:

[0027] The descaling process involves multiple descaling passes during roughing and finishing rolling to suppress the formation of iron oxide scale at high temperatures, reduce the thickness of the iron oxide scale, and improve surface quality. This process allows the steel to achieve the desired iron oxide scale thickness on its surface, and it can be used directly without surface treatment or coating. Attached Figure Description

[0028] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0029] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic flowchart illustrating a method for preparing 1000MPa grade steel, as provided in an embodiment of this application. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0032] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0033] In the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0034] 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.

[0035] Firstly, this application provides a method for preparing 1000MPa grade steel; please refer to [link to relevant documentation]. Figure 1The method may include:

[0036] S1. Obtain a cast billet with a set chemical composition;

[0037] In the embodiments of this application, before the above-mentioned S1 step, it may also include hot metal pretreatment, converter smelting, refining, continuous casting, etc.

[0038] In some embodiments, the continuous casting may employ an electromagnetic stirring process and a light reduction process.

[0039] In the embodiments of this application, the positive effects of using electromagnetic stirring and light reduction processes in the continuous casting process are: it can control the segregation of impurity elements in the core of the billet and ensure the plasticity of the final hot-rolled product.

[0040] In some embodiments, the specified chemical composition may include: C, Mn, P, S, Al, Ti, V, Cr, N, and Fe; in mass fractions,

[0041] The content of C can be 0.03-0.06%, the content of Mn can be 1.0-1.6%, the content of P can be ≤0.015%, the content of S can be ≤0.002%, the content of Al can be 0.02-0.05%, the content of Ti can be 0.20-0.25%, the content of V can be 0.07-0.15%, the content of Cr can be 0.40-0.80%, and the content of N can be 0.007-0.010%.

[0042] In the embodiments of this application, the positive effects of controlling the C content to be 0.03% to 0.06% are that C can enhance strength by forming mixed precipitates with elements such as Ti, V, and N. However, if the C content is too high, it can easily cause segregation in the core of the steel plate, which will adversely affect the elongation and toughness of the steel plate. Specifically, the C content can be 0.03%, 0.04%, 0.05%, 0.06%, etc.

[0043] The positive effects of controlling the Mn content to 1.0–1.6% include: Mn is the most effective element for improving strength and toughness, and it can improve the strength-elongation balance of steel. However, excessive Mn content will increase the hardenability of steel. Specifically, the Mn content can be 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, etc.

[0044] The positive effects of controlling the phosphorus (P) content to below 0.015% are as follows: P is a harmful element in steel, which easily causes segregation in the center of the cast billet, deteriorating cold bending performance and the toughness of the steel. Specifically, the P content can be 0%, 0.003%, 0.006%, 0.009%, 0.012%, 0.015%, etc.

[0045] The positive effects of controlling the sulfur (S) content to below 0.002%: Sulfur is a very harmful element, which is not conducive to the precipitation of precipitates and reduces the strength and corrosion resistance of steel plates. Specifically, the S content can be 0%, 0.001%, 0.002%, etc.

[0046] The positive effects of controlling the Al content to 0.02–0.05% include: Al is a good deoxidizing element that can shrink the austenite phase region. However, excessively high Al content can easily lead to the formation of alumina agglomerates and is detrimental to continuous casting processes. Specifically, the Al content can be 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0047] The positive effects of controlling the Ti content to 0.20–0.25% include: Ti is an excellent grain-refining strengthening element, and when the titanium carbonitride precipitates inside the steel plate reach a certain density, the corrosion resistance of the steel plate can be significantly improved. However, excessive Ti content can easily lead to the formation of Ti-containing metallic inclusions. Specifically, the Ti content can be 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, etc.

[0048] The positive effects of controlling the Cr content to 0.40–0.80% include: Cr exhibits good corrosion resistance and regulates the microstructure, especially bainite content. However, excessively high Cr content can easily lead to microstructure segregation defects. Specifically, the Cr content can be 0.40%, 0.50%, 0.60%, 0.70%, 0.80%, etc.

[0049] The positive effects of controlling the V content to 0.07–0.15% include: V has a strong grain-refining and precipitation-strengthening effect, which can improve the strength of steel plates and enhance corrosion resistance. However, excessive V content can easily lead to the formation of V-containing metallic inclusions. Specifically, the V content can be 0.07%, 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, etc.

[0050] The positive effect of controlling the N content to 0.007–0.01% is that N can promote the stable precipitation of V. Specifically, the V content can be 0.007%, 0.008%, 0.009%, 0.01%, etc.

[0051] In addition to limiting the range of the above chemical components, from the point of view of improving the formability and economy of materials, this invention does not add expensive alloying elements such as Cu, Ni, and Mo.

[0052] In some embodiments, the chemical composition satisfies the following relationship: ([Ti]+[V]) / ([C]+[N]) = 1.2~10.8,

[0053] In the formula, [Ti] represents the mass fraction of Ti, [V] represents the mass fraction of V, [C] represents the mass fraction of C, and [N] represents the mass fraction of N.

[0054] The positive effects of controlling the ratio of ([Ti]+[V]) / ([C]+[N]) in the chemical composition to be between 1.2 and 10.8: When (Ti+V) / (C+N) is within this range, Ti reacts with elements such as C and N to form a large number of complex precipitates, and the optimal precipitation effect is obtained by limiting the content of V, thereby improving strength and corrosion resistance. Specifically, the ratio of ([Ti]+[V]) / ([C]+[N]) in this chemical composition can be 1.2, 3.2, 5.2, 7.2, 9.2, 10.8, etc.

[0055] S2. The billet is heated in stages, and then the heated billet is rough rolled. During the rough rolling process, the billet undergoes a first descaling treatment to obtain a rough rolled plate.

[0056] S3. Perform a second descaling treatment on the rough-rolled plate;

[0057] S4. The rough-rolled plate after the second descaling treatment is subjected to finish rolling, and a third descaling treatment is performed during the finish rolling process to obtain a finish-rolled plate.

[0058] In some embodiments, the pressure of the first descaling process is 300 Bar to 500 Bar, the pressure of the second descaling process is 300 Bar to 500 Bar, and the pressure of the third descaling process is 200 Bar to 500 Bar.

[0059] In some embodiments, the pressure of the first descaling process is 350 Bar, the pressure of the second descaling process is 350 Bar, and the pressure of the third descaling process is 250 Bar.

[0060] In this embodiment, the positive effects of multi-pass descaling and descaling pressure control are: suppressing the formation of iron oxide scale at high temperatures, reducing the thickness of the iron oxide scale, improving surface quality, and ensuring that downstream users can use the product without a surface treatment step. Specifically, the first descaling pressure can be 350 Bar, 400 Bar, 450 Bar, 500 Bar, etc.; the second descaling pressure can be 350 Bar, 400 Bar, 450 Bar, 500 Bar, etc.; and the third descaling pressure can be 250 Bar, 300 Bar, 350 Bar, 400 Bar, 450 Bar, 500 Bar, etc.

[0061] In some embodiments, the heating includes the following stages: a preheating section, a high-temperature section, and a soaking section; wherein,

[0062] The temperature of the preheating section can be 1000℃~1250℃, and the preheating time can be 50min~60min. The temperature of the high-temperature section can be 1260℃~1300℃, and the high-temperature time can be 30min~50min. The temperature of the heat spreader section can be 1270℃~1280℃, and the heat spreader time can be 30min~50min.

[0063] In this embodiment, the three-stage heating of the billet and the setting of the billet heating temperature to 1000℃~1300℃ have the following positive effects: It maximizes the complete solidification of alloying elements, ensuring strength; it allows elements such as C, S, and P to diffuse fully and distribute more evenly, reducing their harmful effects, such as core banding and inclusion segregation; and it homogenizes the microstructure of the surface and core, ensuring plasticity. This results in steel with excellent mechanical properties: yield strength ≥900MPa, tensile strength 1000MPa~1500MPa, and elongation ≥10%. Specifically, the preheating temperature of the billet in the preheating section can be 1000℃, 1050℃, 1100℃, 1150℃, 1200℃, 1250℃, etc., and the preheating time of the billet in the preheating section can be 50min, 55min, 60min, etc.; the heating temperature of the billet in the high-temperature section can be 1260℃, 1270℃, 1280℃, 1290℃, 1300℃, and the heating time of the billet in the high-temperature section can be 30min, 40min, 50min, etc.; the heating temperature and time of the billet in the soaking section can be 1270℃, 1275℃, 1280℃, etc., and the heating time of the billet in the soaking section can be 30min, 40min, 50min, etc.

[0064] In some embodiments, the finishing temperature of the roughing roll and the finishing roll temperature are controlled; wherein,

[0065] The roughing finishing temperature can be 1080℃~1100℃, and the finishing finishing temperature can be 870℃~910℃.

[0066] In this embodiment, the positive effects of controlling the roughing rolling end temperature to be 1080℃~1100℃ and the finishing rolling end temperature to be 870℃~910℃ are as follows: If the roughing rolling end temperature is below 1080℃, the finishing rolling end temperature cannot be guaranteed to reach the set value; if it is above 1100℃, it will lead to coarse grains and reduced plasticity. If the finishing rolling end temperature is below 870℃, it will reduce the brittleness of the surface iron oxide scale, leading to cracking and surface defects; if it is above 910℃, it will increase the thickness of the surface iron oxide scale, which will fall off in large quantities during subsequent finishing processes, deteriorating the surface quality. Specifically, the roughing rolling end temperature can be 1080℃, 1090℃, 1100℃, etc., and the finishing rolling end temperature can be 870℃, 880℃, 890℃, 900℃, 910℃, etc.

[0067] In some embodiments, the method further includes:

[0068] The finished rolled plate is cooled in stages and then coiled to obtain 1000MPa grade steel; the cooling process includes the following stages: a first-stage cooling, air cooling after the first-stage cooling, and a second-stage cooling; wherein...

[0069] The temperature of the first stage of cooling is 600℃~660℃, the air cooling time is 4s~20s, and the temperature of the second stage of cooling is 550℃~610℃.

[0070] In this embodiment, the hot-rolled steel utilizes a two-stage cooling system and air cooling with controlled cooling temperatures, achieving the following positive effects: The first stage cooling is to 600℃~660℃ to ensure sufficient ferrite precipitation, increasing plasticity. The second stage cooling is to 550℃~610℃ to ensure sufficient Ti precipitation, resulting in high strength. Air cooling is used to control ferrite transformation and precipitation. Specifically, the first stage cooling temperature can be 600℃, 620℃, 640℃, 660℃, etc.; the second stage cooling temperature can be 550℃, 570℃, 590℃, 610℃, etc. The air cooling time can be 4s, 8s, 12s, 16s, 20s, etc.

[0071] Secondly, this application provides a 1000MPa grade steel, which is prepared by the method described in any embodiment of the first aspect. The microstructure of the steel, by volume fraction, may include 15-20% ferrite and 80-85% bainite; the grain size grade of the microstructure may be 13-14.

[0072] In the embodiments of this application, the positive effects of a ferrite volume percentage of 15-20% are as follows: Ferrite has low strength and hardness, is easily deformed, and bears most of the strain during deformation, making it an important component phase for ensuring the plasticity and formability of multiphase steel. When the ferrite proportion is too low, the strength of the steel plate increases, but the plasticity and formability deteriorate; when the ferrite proportion is too high, it is difficult to guarantee a tensile strength of over 1000 MPa. Specifically, the area fraction of ferrite can be 15%, 16%, 17%, 18%, 19%, 20%, etc.

[0073] The positive effects of a bainite volume percentage of 80%–85%: Bainite is the hard phase structure in steel that guarantees strength. If the bainite content is too low, a tensile strength above 1000 MPa cannot be guaranteed. Furthermore, due to the low proportion of the hard phase, the hard phase structure becomes highly carbon-rich, leading to a significant increase in microhardness and widening the hardness difference between the soft and hard phases. This increases the likelihood of crack initiation during deformation and reduces hole-expanding performance. If the bainite content is too high, the elongation after fracture of the steel plate decreases significantly, resulting in insufficient formability. Specifically, the bainite volume percentage can be 80%, 81%, 82%, 83%, 84%, 85%, etc.

[0074] The positive effects of a grain size grade of 13-14: Within this range, the grain boundary area is larger, resulting in better mechanical properties in the steel. This application's embodiments utilize ultrafine grain control technology to obtain high-strength steel on conventional continuous casting and hot rolling production lines.

[0075] In some embodiments, the steel may meet the following conditions: yield strength ≥ 900 MPa, tensile strength 1000 MPa~1500 MPa, elongation ≥ 10%, and surface iron oxide scale thickness ≤ 6 μm.

[0076] In the embodiments of this application, the steel has excellent mechanical properties and a relatively thin iron oxide scale thickness.

[0077] The 1000MPa grade steel is achieved based on the above-described preparation method for 1000MPa grade steel. The specific steps of the preparation method for 1000MPa grade steel can be referred to the above embodiments. Since the 1000MPa grade steel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0078] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0079] The molten steel of Examples 1-8 and Comparative Examples 1-4 was prepared and cast into slabs. The chemical composition of the slabs is shown in Table 1.

[0080] Table 1 shows the chemical composition (wt%) of each example and comparative example, with the remainder being Fe and unavoidable impurities.

[0081] Group C Mn P S Al Ti V Cr N ([Ti]+[V]) / ([C]+[N]) Example 1 0.03 1.0 0.015 0.001 0.025 0.20 0.07 0.40 0.007 7.30 Example 2 0.032 1.2 0.010 0.002 0.030 0.21 0.15 0.80 0.008 9.00 Example 3 0.034 1.4 0.012 0.001 0.045 0.23 0.13 0.70 0.01 8.18 Example 4 0.040 1.1 0.004 0.002 0.050 0.25 0.14 0.50 0.009 7.96 Example 5 0.045 1.6 0.007 0.002 0.035 0.23 0.12 0.60 0.008 6.60 Example 6 0.050 1.5 0.008 0.001 0.040 0.24 0.08 0.50 0.009 5.42 Example 7 0.055 1.3 0.001 0.001 0.048 0.21 0.09 0.80 0.007 4.84 Example 8 0.060 1.6 0.009 0.002 0.50 0.23 0.10 0.70 0.01 4.71 Comparative Example 1 0.20 1.0 0.011 0.001 0.04 0.21 0.01 0.50 0.008 1.06 Comparative Example 2 0.040 1.3 0.003 0.002 0.03 0.23 0.14 0.70 0.009 7.55 Comparative Example 3 0.040 1.4 0.009 0.002 0.03 0.24 0.10 0.60 0.007 7.23 Comparative Example 4 0.040 1.6 0.006 0.001 0.05 0.23 0.11 0.50 0.007 7.23

[0082] Based on the above chemical composition of 1000MPa grade steel, this application provides a method for preparing 1000MPa grade steel, the method comprising:

[0083] S11. Obtain a cast billet with a set chemical composition;

[0084] S21. The billet is heated in stages, and then the heated billet is rough rolled. During the rough rolling process, the billet undergoes a first descaling treatment to obtain a rough rolled plate.

[0085] S31. Perform a second descaling treatment on the rough-rolled plate;

[0086] S41. The rough-rolled plate after the second descaling treatment is subjected to finish rolling, and a third descaling treatment is performed during the finish rolling process to obtain a finish-rolled plate.

[0087] S51. The finished rolled plate is cooled in stages and then coiled to obtain 1000MPa grade steel. Please refer to Table 2 for specific manufacturing process parameters.

[0088] Table 2. Main process parameters for each embodiment and comparative example.

[0089]

[0090]

[0091] Table 3 Mechanical properties and iron oxide thickness of each embodiment and comparative example

[0092]

[0093] Table 3 shows that the automotive steel in this embodiment has a yield strength ≥900MPa, a tensile strength of 1000~1500MPa, and an elongation ≥10%, exhibiting excellent mechanical properties. The thickness of the iron oxide scale on the surface of the steel is ≤6um, ensuring that downstream users do not need to perform surface treatment.

[0094] In Comparative Example 1, the C and V contents are not within the scope of the embodiments of this application, and the yield strength of the steel plate is only 900 MPa and the tensile strength is only 880 MPa.

[0095] In Comparative Example 2, the billet was only heated in the high-temperature section, without the preheating section and the soaking section. The resulting steel plate had a yield strength of only 800 MPa and a tensile strength of only 850 MPa.

[0096] In Comparative Example 3, the billet was descaled only in the rough rolling section and not in the finish rolling section, resulting in a steel plate with a surface iron oxide scale thickness of 10.5 μm.

[0097] In Comparative Example 4, the billet was repeatedly heated at temperatures outside the range of the embodiments of this application, resulting in a steel plate with a yield strength of only 740 MPa and a tensile strength of only 800 MPa.

[0098] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this 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 this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing 1000 MPa grade steel, characterized in that, The method includes: A cast billet with a set chemical composition is obtained; The billet is heated in stages, and then the heated billet is rough rolled. During the rough rolling process, the billet undergoes a first descaling treatment to obtain a rough rolled plate. The rough-rolled plate is subjected to a second descaling treatment; The rough-rolled plate after the second descaling treatment is then subjected to finish rolling, and a third descaling treatment is performed during the finish rolling process to obtain a finish-rolled plate. The heating process includes the following stages: a preheating section, a high-temperature section, and a homogenization section; wherein... The temperature of the preheating section is 1000 ℃~1250 ℃, and the preheating time is 50 min~60 min; the temperature of the high temperature section is 1260 ℃~1300 ℃, and the high temperature time is 30 min~50 min; the temperature of the heat spreader section is 1270 ℃~1280 ℃, and the heat spreader time is 30 min~50 min. The specified chemical composition includes: C, Mn, P, S, Al, Ti, V, Cr, N, and Fe; by mass fraction, The content of C is 0.03~0.06%, the content of Mn is 1.0~1.6%, the content of P is ≤0.015%, the content of S is ≤0.002%, the content of Al is 0.02~0.05%, the content of Ti is 0.20~0.25%, the content of V is 0.07~0.15%, the content of Cr is 0.40~0.80%, and the content of N is 0.007~0.010%. The chemical composition satisfies the following relationship: ([Ti]+[V]) / ([C]+[N])=1.2~10.8, In the formula, [Ti] represents the mass fraction of Ti, [V] represents the mass fraction of V, [C] represents the mass fraction of C, and [N] represents the mass fraction of N.

2. The method according to claim 1, characterized in that, The pressure of the first descaling treatment is 300 Bar to 500 Bar, the pressure of the second descaling treatment is 300 Bar to 500 Bar, and the pressure of the third descaling treatment is 200 Bar to 500 Bar.

3. The method according to claim 2, characterized in that, The pressure of the first descaling treatment is 350 Bar, the pressure of the second descaling treatment is 350 Bar, and the pressure of the third descaling treatment is 250 Bar.

4. The method according to claim 1, characterized in that, The finishing temperature of the roughing roll and the finishing roll are controlled; wherein, The roughing rolling end temperature is 1080 ℃~1100 ℃, and the finishing rolling end temperature is 870 ℃~910 ℃.

5. The method according to claim 1, characterized in that, The method further includes: The finished rolled plate is cooled in stages and then coiled to obtain 1000 MPa grade steel; the cooling process includes the following stages: first-stage cooling, air cooling after the first-stage cooling, and second-stage cooling; wherein... The temperature of the first stage of cooling is 600 ℃~660 ℃, the air cooling time is 4s~20s, and the temperature of the second stage of cooling is 550 ℃~610 ℃.

6. A 1000 MPa grade steel, characterized in that, The steel is prepared by the method according to any one of claims 1-5, and the microstructure of the steel, by volume fraction, comprises: 15-20% ferrite and 80-85% bainite; the grain size grade of the microstructure is 13-14.

7. The 1000 MPa grade steel according to claim 6, characterized in that, The steel meets the following conditions: yield strength ≥ 900 MPa, tensile strength 1000 MPa~1500 MPa, elongation ≥ 10%, and surface iron oxide scale thickness ≤ 6 μm.

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