CN118406971A - 1000 MPa-level structural steel and preparation method thereof - Google Patents

Abstract

The invention provides 1000 MPa-level structural steel and a preparation method thereof, and belongs to the field of steel preparation. The chemical components of the structural steel comprise: C. si, mn, P, S, al, ni, cr, mo, nb, V, ti and Fe; wherein, the content of C is 0.04 to 0.15 percent, the content of Si is 0.20 to 0.50 percent, the content of Mn is less than or equal to 2.00 percent, the content of P is less than or equal to 0.008 percent, the content of S is less than or equal to 0.003 percent, the content of Al is 0.020 to 0.050 percent, the content of Ni is 7.00 to 12.00 percent, the content of Cr is less than or equal to 2.00 percent, the content of Mo is less than or equal to 2.00 percent, the content of Nb is less than or equal to 0.100 percent, the content of V is less than or equal to 0.200 percent, and the content of Ti is less than or equal to 0.030 percent. The hardenability of the steel plate is ensured through the component design of adding Cr and Mo at low carbon, and the comprehensive matching of high strength, high plasticity and high toughness of the structural steel is ensured through obtaining tempered martensite, reverse transformed austenite and Nb, V and Ti precipitated phases. The concrete steps are as follows: the yield strength is more than 1050MPa, the tensile strength is more than 1180MPa, the elongation after fracture is more than or equal to 20.0 percent, the impact energy at the temperature of minus 85 ℃ is more than 200J, and the impact energy is obviously better than other steel plates in the steel plates with the same strength level.

Description

A 1000MPa grade structural steel and its preparation method

Technical Field

The present application relates to the technical field of steel preparation, and in particular to a 1000MPa grade structural steel and a preparation method thereof.

Background technique

1000MPa grade high-strength steel has broad application prospects in the fields of containers, engineering machinery, shipbuilding and offshore engineering. High-strength steel can effectively reduce the weight of engineering equipment, improve equipment manufacturing efficiency, and reduce material costs. Because 1000MPa grade steel is often used for key components, in addition to high strength, the steel plate is required to have high impact performance and good plasticity, especially for steel plates used for welding. In order to solve the problem of reduced impact toughness of welded joints, steel plates often require higher impact toughness reserves.

The patents that take into account the yield strength ≥ 1000MPa and meet the low-temperature toughness of -84℃ are mainly as follows: The component system with application number CN114058790A, CN114058960A, and CN114058790A adopts the technical idea of Cu precipitation strengthening. In order to achieve the strengthening effect, the Cu addition amount reaches 1.5% to 2.5%. As a precious alloy element, a large amount of Cu addition will significantly increase the manufacturing cost of the steel plate. At the same time, the Cu element is easy to cause cracks in the casting billet, affecting the yield rate. The yield strength of the steel plate disclosed in the patent application number CN 114196879A reaches 1000MPa level, and has high toughness and plasticity. The impact energy at -85℃ reaches> 100J, but the Ni content in the steel plate is between 10% and 15%, and the price of Ni alloy is relatively high. The higher Ni content will lead to higher alloy cost of the steel plate, increasing the manufacturing cost.

Other steels with a yield strength of 1000MPa have low impact toughness despite their yield strength reaching 1000MPa. For example, patent application numbers are CN106544590B, CN113444974A, CN 104561827A, CN109609848B, CN114134301A and CN 114277306 A. The yield strength of the steel plates in the above patents all reach 1000MPa, but the research temperature of the toughness of the steel plates is relatively low, and the toughness guarantee temperature is -40℃ or -60℃, which is quite different from the technical indicator of guaranteeing -85℃.

In summary, there is an urgent need to develop a steel plate with a yield strength of 1000 MPa, high plasticity and high toughness at -85°C.

Summary of the invention

The present application provides a 1000MPa grade structural steel and a preparation method thereof to solve the technical problem that existing high-strength structural steel plates cannot achieve high toughness and plasticity.

In a first aspect, the present application provides a 1000MPa grade structural steel, the chemical composition of which includes: C, Si, Mn, P, S, Al, Ni, Cr, Mo, Nb, V, Ti and Fe; wherein, in terms of mass fraction,

The C content is 0.04% to 0.15%, the Si content is 0.20% to 0.50%, the Mn content is ≤2.00%, the P content is ≤0.008%, the S content is ≤0.003%, the Al content is 0.020% to 0.050%, the Ni content is 7.00% to 12.00%, the Cr content is ≤2.00%, the Mo content is ≤2.00%, the Nb content is ≤0.100%, the V content is ≤0.200%, and the Ti content is ≤0.030%.

Optionally, the metallographic structure of the structural steel includes tempered martensite and reversed austenite, the volume fraction of the tempered martensite is 80% to 98%, and the volume fraction of the reversed austenite is 2% to 20%; wherein the average lath width of the tempered martensite is less than 0.3 μm.

Optionally, Nb, V, and Ti precipitate phases are dispersed in the matrix of the structural steel, and the size of the precipitate phases is less than 100 nm.

Optionally, the structural steel meets at least one of the following properties: yield strength of 1050MPa to 1200MPa, tensile strength of 1190MPa to 1300MPa, elongation ≥22%, and impact energy at -85°C >200J.

Optionally, the thickness of the structural steel is 10 mm to 50 mm.

In a second aspect, the present application provides a method for preparing the structural steel according to any one embodiment of the first aspect, the method comprising:

Obtaining a steel billet having the chemical composition;

The steel billet is sequentially subjected to pre-rolling heating, general hot rolling and post-rolling air cooling to obtain a hot-rolled steel plate;

The hot-rolled steel plate is heat-treated to obtain structural steel.

Optionally, the pre-rolling heating temperature is 1120° C. to 1180° C., and the pre-rolling heating insulation time is 240 min to 360 min.

Optionally, the general hot rolling is recrystallization zone rolling, the start rolling temperature of the recrystallization zone rolling is 1000°C to 1100°C, and the final rolling temperature of the recrystallization zone rolling is 900°C to 1000°C.

Optionally, the heat treatment includes: primary quenching, secondary quenching and tempering;

The primary quenching includes: heating the hot-rolled steel plate to 850°C to 900°C, keeping the temperature for 3d min, and then performing the first water quenching, wherein d is the thickness of the structural steel;

The secondary quenching comprises: heating the hot-rolled steel plate after the primary quenching to 650°C to 750°C, keeping the temperature for 3d min, and then performing a second water quenching, wherein d is the thickness of the structural steel;

The tempering comprises: heating the hot-rolled steel plate after secondary quenching to 500° C. to 600° C., keeping the temperature for 3 dmin, and then air-cooling to room temperature, wherein d is the thickness of the structural steel.

Optionally, the thickness of the steel billet is 120 mm to 240 mm.

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

This application rationally designs the composition of the alloy, ensures the hardenability of the steel plate by adding low-carbon Cr and Mo, and obtains tempered martensite, reversed austenite and Nb, V, Ti precipitation phases, thereby ensuring that the structural steel has a comprehensive match of high strength, high plasticity and high toughness. Specifically, it is reflected in: yield strength>1050MPa, tensile strength>1180MPa, elongation after fracture ≥20.0%, -85℃ impact energy>200J, and the impact energy is significantly better than other steel plates of the same strength level.

In addition, the present application can significantly reduce the addition of Ni alloy through appropriate composition design and process design, which is beneficial to reduce manufacturing costs and reduce the consumption of alloy resources.

BRIEF DESCRIPTION OF THE DRAWINGS

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.

In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

FIG1 is a schematic flow chart of a method for preparing 1000 MPa grade structural steel according to an embodiment of the present application;

FIG2 is a microstructure diagram of the structural steel provided in Example 1 of the present application;

FIG3 is a transmission electron microscope analysis diagram of tempered martensite of the structural steel provided in Example 1 of the present application;

FIG4 is a morphology diagram of Nb, V, and Ti nano-precipitates of the structural steel provided in Example 1 of the present application;

FIG5 is an energy spectrum analysis diagram of the Nb, V, and Ti nano-precipitation phases of the structural steel provided in Example 1 of the present application.

Detailed ways

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

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 herein, it is meant to include any cited number (fractional or integer) within the indicated range.

In addition, in the description of the specification of the present application, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second", etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. 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" means one or more, and "plurality" means two or more. "At least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of singular 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, a-b (i.e. a and b), a-c, b-c, or a-b-c, where a, b, c can be single or plural, respectively.

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

The technical solution provided by this application is to solve the above technical problems, and the overall idea is as follows:

At present, there is a lack of high-toughness plastic structural steel plates with a yield strength of 1000MPa and their manufacturing methods. A few companies have developed steel with a yield strength of 1000MPa, but the low-temperature toughness reserve is insufficient. When welded, the impact toughness of the welded joint will be greatly reduced, and there is a risk of poor impact toughness of the welded joint. Other research institutes have successfully trial-produced 1000MPa high-toughness plastic steel using the strengthening route of Cu alloy precipitation, but the amount of Cu element added is relatively large, resulting in high manufacturing costs. A few companies have developed 1000MPa high-toughness structural steel using quenching and tempering processes, but a higher content of Ni alloy elements needs to be added, resulting in high manufacturing costs.

Based on this, the present application proposes a high-toughness and plasticity structural steel plate with a yield strength of 1000MPa and a low-cost manufacturing method. The hardenability of the steel plate is ensured by designing the composition of low-carbon additions of Cr and Mo, and the high strength, high plasticity and high toughness of the structural steel are ensured by obtaining tempered martensite, reverse transformation austenite and Nb, V, Ti precipitation phases. In the process, the first quenching temperature will be heated to complete austenitization and then quenched to obtain a first quenched martensite structure with uniform size and composition. The second quenching temperature is controlled at the temperature of the martensite and austenite two-phase region. The heating process before the second quenching causes elements such as Ni and Mn to be enriched in the austenite. At the same time, the Ni and Mn content in the martensite that has not undergone phase transformation will be reduced. Subsequently, during quenching, the austenite enriched with Ni and Mn elements will be transformed into a second quenched martensite enriched with Mn and Ni elements, and the lath size in the martensite is finer than that of the first quenched martensite. The secondary quenched martensite enriched with Ni and Mn elements will partially transform into reverse austenite structure during subsequent tempering, and the rest will form tempered martensite structure together with the primary quenched martensite. At the same time, the tempering heat treatment will promote the precipitation of the second phase of Nb, V, and Ti. The target structure is obtained by the above-mentioned general hot rolling + heat treatment process, and a high-toughness and plasticity structural steel plate with a yield strength of 1000MPa is obtained. The two quenching and tempering processes obtain tempered martensite and reverse austenite structures, and contain a large amount of second phases precipitated by Nb, V, and Ti. The reverse austenite is beneficial to improving the plasticity and toughness of the steel plate, and the tempered martensite structure and the second phases precipitated by a large amount of Nb, V, and Ti are beneficial to improving the strength.

The present application provides a 1000MPa grade structural steel, the chemical composition of which includes: C, Si, Mn, P, S, Al, Ni, Cr, Mo, Nb, V, Ti and Fe; wherein, in terms of mass fraction,

The C content is 0.04% to 0.15%, the Si content is 0.20% to 0.50%, the Mn content is ≤2.00%, the P content is ≤0.008%, the S content is ≤0.003%, the Al content is 0.020% to 0.050%, the Ni content is 7.00% to 12.00%, the Cr content is ≤2.00%, the Mo content is ≤2.00%, the Nb content is ≤0.100%, the V content is ≤0.200%, and the Ti content is ≤0.030%.

In some embodiments, the positive effect of controlling the C content to 0.04-0.15% is that the C element is an element that expands the austenite phase region and has a strong solid solution strengthening effect. At the same time, the C element can form a second phase with Nb and Ti elements to improve the strength of the steel plate through precipitation strengthening. However, if the C content is too high, the toughness and welding performance of the product are poor. Comprehensive consideration requires that the steel has excellent strength and toughness.

The positive effect of controlling the Si content to 0.20-0.50% is that Si does not form carbides with C, but exists in the steel in a solid solution form, and interacts with the stress field of movable dislocations to hinder dislocation movement and improve the strength of the steel plate. However, when the Si content is high, it is not good for the welding performance of the steel.

The positive effect of controlling the content of Mn to ≤2.00% is: Mn is an austenite-forming element and expands the austenite phase region. During the cooling process, Mn dissipates free energy through solute drag and inhibits diffusion-type phase transformation. By adding an appropriate amount of Mn, the microstructure of the steel plate can be controlled under appropriate process conditions. When the Mn content in austenite increases, the stability of austenite is greatly improved, creating conditions for obtaining reverse transformation austenite.

The positive effect of controlling the P content ≤ 0.008% is that phosphorus has a strong solid solution strengthening effect in steel. As an alloying element, it is added to low-alloy structural steel to improve its strength and atmospheric corrosion resistance. However, the biggest harm of phosphorus is that it has serious segregation, which significantly reduces the plasticity and toughness of steel, making it easy to crack during cold processing, which is the so-called "cold brittleness" phenomenon. Phosphorus also has an adverse effect on weldability. As a harmful element, phosphorus should be strictly controlled.

The positive effect of controlling the S content to ≤0.003% is that sulfur segregates seriously in steel, deteriorating the quality of steel. At high temperatures, it reduces the plasticity of steel. It exists in the form of FeS with a lower melting point. The melting point of FeS alone is only 1190°C, and the eutectic temperature of FeS in steel is even lower, only 988°C. When the steel solidifies, iron sulfide gathers at the primary grain boundaries. When steel is rolled at 1100°C to 1200°C, the FeS on the grain boundaries will melt, greatly weakening the bonding force between the grains and causing the hot brittleness of the steel. Therefore, sulfur should be strictly controlled.

The positive effects of controlling the Al content to 0.020-0.050% are as follows: Al increases the driving force for phase transformation, and Al interacts with N in the steel to form fine and dispersed AlN precipitation, which can inhibit grain growth, thereby achieving grain refinement and improving the toughness of the steel at low temperatures.

The positive effect of controlling the Cr content ≤ 2.00% is: Cr can prevent the graphitization tendency of Mo-added steel, is a stable austenite element, can greatly improve the hardenability of steel, and improve the strength of steel, but too high Cr will reduce the welding performance of steel.

The positive effect of controlling the Ni content to 7.00-12.00% is that Ni strengthens ferrite by forming a simple substitution solid solution, which can improve the strength of steel. At the same time, Ni is an austenite stabilizing element, which can significantly improve the low-temperature impact toughness of steel.

Positive effects of controlling the Mo content ≤ 2.00%: Mo is the most effective element for improving the high temperature strength of steel plates. Generally, the higher the Mo content, the greater the impact on the tensile strength than on the yield strength.

The positive effect of controlling the Nb content ≤ 0.100% and the Ti content ≤ 0.030% is that the Nb and Ti microalloying elements play a role in grain refinement and strengthening of the steel plate, and grain refinement is beneficial to improving the low-temperature toughness of the steel plate.

The positive effect of controlling the V content to ≤0.200% is that V is an element that refines grains and can significantly improve steel strength through V (C, N) dispersion precipitation. However, if the addition amount is too high, the toughness and welding performance of the material will be reduced.

Strictly control the purity of molten steel to avoid the adverse effects of impurity elements P and S on the low-temperature toughness of the steel plate.

It is strictly prohibited to intentionally add Cu and B elements to steel.

In some embodiments, the metallographic structure of the structural steel includes tempered martensite and reversed austenite, the volume fraction of the tempered martensite is 80% to 98%, and the volume fraction of the reversed austenite is 2% to 20%; wherein the average lath width of the tempered martensite is less than 0.3 μm.

In some embodiments, Nb, V, and Ti precipitate phases are dispersed in the matrix of the structural steel, and the size of the precipitate phases is less than 100 nm.

In some embodiments, the hardenability is ensured by designing the composition of low-carbon addition of Cr and Mo, and conditions are created for obtaining reverse transformation austenite by adding appropriate Mn and Ni elements, and a primary quenching martensite structure with uniform size and composition is obtained by primary quenching, and the secondary quenching temperature is controlled to be carried out at the temperature of the two-phase region of martensite and austenite. The heating process before the secondary quenching causes elements such as Ni and Mn to be enriched in austenite, and the Ni and Mn content in the martensite that has not undergone phase transformation will be reduced. The austenite enriched with Ni and Mn elements will be transformed into secondary quenching martensite enriched with Mn and Ni elements during subsequent quenching, and the size of its martensite lath is finer than that of the primary quenching martensite lath. The secondary quenching martensite enriched with Ni and Mn elements will be partially transformed into reverse transformation austenite structure during subsequent tempering, and the remaining part will form a tempered martensite structure together with the primary quenching martensite, and the tempering heat treatment will promote the precipitation of the second phase of Nb, V, and Ti. By adopting the general hot rolling + air cooling + heat treatment process, the target tempered martensite, reverse transformed austenite structure and Nb, V, Ti precipitation phases are obtained to ensure that the yield strength of the steel plate reaches 1000MPa level, and at the same time has high plasticity and high toughness.

In some embodiments, the structural steel meets at least one of the following properties: yield strength of 1050MPa to 1200MPa, tensile strength of 1190MPa to 1300MPa, elongation ≥22%, and impact energy at -85°C>200J.

The structural steel provided in this application has a comprehensive match of high strength, high plasticity and high toughness.

In some embodiments, the thickness of the structural steel is 10 mm to 50 mm.

FIG1 is a schematic flow chart of a method for preparing 1000 MPa grade structural steel provided in an embodiment of the present application.

Referring to FIG. 1 , the present application provides a method for preparing structural steel, the method comprising:

S1, obtaining a steel billet having the chemical composition;

In some embodiments, before the above step S1, molten iron pretreatment, smelting and forging are also included.

In some embodiments, the smelting is performed using a 500Kg vacuum smelting furnace for steelmaking.

In some embodiments, the thickness of the steel billet is 120 mm to 240 mm.

S2, sequentially performing pre-rolling heating, general hot rolling and post-rolling air cooling on the steel billet to obtain a hot-rolled steel plate;

In some embodiments, the pre-rolling heating temperature is 1120° C. to 1180° C., and the pre-rolling heating holding time is 240 min to 360 min.

In some embodiments, the general hot rolling is recrystallization zone rolling, the start rolling temperature of the recrystallization zone rolling is 1000°C to 1100°C, and the finish rolling temperature of the recrystallization zone rolling is 900°C to 1000°C.

The present application obtains the target tempered martensite, reverse transformed austenite structure and Nb, V, Ti precipitation phases by adopting the general hot rolling + air cooling + heat treatment process, ensuring that the yield strength of the steel plate reaches 1000MPa level, and at the same time has high plasticity and high toughness.

S3. Heat-treating the hot-rolled steel plate to obtain structural steel.

In some embodiments, the heat treatment includes: primary quenching, secondary quenching and tempering;

The primary quenching includes: heating the hot-rolled steel plate to 850°C to 900°C, keeping the temperature for 3d min, and then performing the first water quenching, wherein d is the thickness of the structural steel;

The secondary quenching comprises: heating the hot-rolled steel plate after the primary quenching to 650°C to 750°C, keeping the temperature for 3d min, and then performing a second water quenching, wherein d is the thickness of the structural steel;

The tempering comprises: heating the hot-rolled steel plate after secondary quenching to 500° C. to 600° C., keeping the temperature for 3 dmin, and then air-cooling to room temperature, wherein d is the thickness of the structural steel.

The present application obtains a first quenched martensite structure with uniform size and composition through a single quenching, and the temperature of the second quenching is controlled at the temperature of the two-phase region of martensite and austenite. The heating process before the second quenching causes the elements such as Ni and Mn to be enriched in the austenite, and the Ni and Mn content in the martensite that has not undergone phase transformation will be reduced. During the subsequent quenching, the austenite enriched with Ni and Mn elements will be transformed into a second quenched martensite enriched with Mn and Ni elements, and the size of its martensite lath is finer than that of the first quenched martensite lath. The second quenched martensite enriched with Ni and Mn elements will be partially transformed into a reverse-transformed austenite structure during subsequent tempering, and the remaining part and the first quenched martensite will form a tempered martensite structure together, and the tempering heat treatment will promote the second phase precipitation of Nb, V, and Ti.

The preparation method of the structural steel is implemented based on the chemical composition of the above-mentioned structural steel. The specific chemical composition of the structural steel can be referred to the above-mentioned embodiment. Since the preparation method of the structural steel adopts part or all of the technical solutions of the above-mentioned embodiments, it has at least all the beneficial effects brought by the technical solutions of the above-mentioned embodiments, which will not be described one by one here.

The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

The molten steels of Examples 1 to 5 were prepared and cast into steel billets, the chemical compositions of which are shown in Table 1.

Table 1 Chemical composition mass percentage (wt%) of the steel billets of each embodiment and comparative example, the remainder being Fe and unavoidable impurities

Group C Si Mn P S Al Ni Cr Mo Nb V Ti Example 1 0.10 0.35 1.60 0.007 0.003 0.031 7.2 0.55 0.70 0.045 0.060 0.013 Example 2 0.11 0.31 0.65 0.006 0.003 0.035 7.5 0.55 0.70 0.055 0.060 0.015 Example 3 0.10 0.28 1.77 0.006 0.002 0.030 8 0.60 0.80 0.070 0.070 0.015 Example 4 0.10 0.30 1.75 0.005 0.001 0.027 9.5 0.80 1.00 0.072 0.095 0.017 Example 5 0.12 0.33 1.80 0.005 0.001 0.029 11.6 0.80 1.00 0.080 0.085 0.016

Based on the chemical composition of the above structural steel, an embodiment of the present application provides a method for preparing structural steel, the method comprising the following steps:

S11, obtaining a steel billet having the chemical composition;

S21, sequentially performing pre-rolling heating, general hot rolling and post-rolling air cooling on the steel billet to obtain a hot-rolled steel plate;

S31. The hot-rolled steel plate is subjected to primary quenching, secondary quenching and tempering to obtain structural steel. The rolling process parameters are shown in Table 2, and the heat treatment process parameters are shown in Table 3.

Table 2 Rolling process parameters

Group Billet thickness, mm Heating temperature, °C Heating time, min Rolling temperature, ℃ Finishing rolling temperature, ℃ Example 1 120 1160 240 1085 970 Example 2 120 1160 240 1085 950 Example 3 180 1170 270 1080 940 Example 4 200 1170 300 1060 925 Example 5 240 1175 360 1050 910

Table 3 Heat treatment process parameters

The mechanical properties of the structural steels obtained in Examples 1 to 5 were tested, and the results are shown in Table 4. The mechanical properties were tested by measuring the tensile properties of the steel plate according to GB/T228 "Metallic Materials Room Temperature Tensile Test Method", and measuring the impact properties of the steel plate at 1/4 of the thickness according to GB/T229-2007 "Metallic Materials Charpy Pendulum Impact Test Method".

Table 4 Mechanical properties of structural steel plates

Group Yield strength, MPa Tensile strength, MPa Elongation, % -85℃ impact energy, J Example 1 1165 1285 23.5 268 Example 2 1190 1296 22.0 249 Example 3 1122 1250 25.0 256 Example 4 1075 1192 25.5 230 Example 5 1055 1199 26.0 231

As can be seen from Tables 1-4, Examples 1-5 of the present invention ensure hardenability by designing the composition of low-carbon addition of Cr and Mo, create conditions for obtaining reverse transformation austenite by adding appropriate Mn and Ni elements, obtain a first quenched martensite structure with uniform size and composition by a first quenching, and control the temperature of the second quenching to be carried out at the temperature of the two-phase region of martensite and austenite. The heating process before the second quenching causes elements such as Ni and Mn to be enriched in austenite, and the content of Ni and Mn in the martensite that has not undergone phase transformation will be reduced. The austenite enriched with Ni and Mn elements will be transformed into a second quenched martensite enriched with Mn and Ni elements during subsequent quenching, and the size of its martensite lath is finer than that of the first quenched martensite lath. The second quenched martensite enriched with Ni and Mn elements will be partially transformed into a reverse transformation austenite structure during subsequent tempering, and the remaining part and the first quenched martensite will form a tempered martensite structure together, and the tempering heat treatment will promote the second phase precipitation of Nb, V, and Ti. By adopting the general hot rolling + air cooling + heat treatment process, the target tempered martensite, reverse transformed austenite structure and Nb, V, Ti precipitation phases are obtained to ensure that the yield strength of the steel plate reaches 1000MPa level, and at the same time has high plasticity and high toughness.

Under the composition design and process conditions of the present invention, the structural steels of Examples 1-5 have excellent mechanical properties, with yield strength ≥1055MPa, tensile strength ≥1192MPa, elongation after fracture ≥22%, and impact energy at -85°C >200J. The developed structural steels have good mechanical indicators.

Detailed description of attached drawings 2-5:

FIG. 2 is a microstructure diagram of the structural steel provided in Example 1 of the present application.

As can be seen from FIG. 2 , in the metallographic structure of the structural steel provided in Example 1, the volume fraction of tempered martensite is 87.2%, and the volume fraction of reversed austenite is 12.8%.

FIG3 is a transmission electron microscope analysis diagram of the tempered martensite of the structural steel provided in Example 1 of the present application.

As can be seen from FIG3 , the average lath width of the tempered martensite in the structural steel of Example 1 is 0.18 μm.

FIG4 is a morphology diagram of the Nb, V, and Ti nano-precipitates of the structural steel provided in Example 1 of the present application.

As can be seen from FIG. 4 , the size of the Nb, V, and Ti nano-precipitates of the structural steel obtained in Example 1 is less than 100 nm.

FIG5 is an energy spectrum analysis diagram of the Nb, V, and Ti nano-precipitation phases of the structural steel provided in Example 1 of the present application.

As can be seen from FIG5 , the structural steel of Example 1 contains Nb, V, and Ti nano-precipitates.

In addition, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

In the embodiments of the present invention, the structural steel achieves a comprehensive match of high strength, high plasticity and high toughness, and the higher toughness provides a performance reserve for subsequent welding; compared with precipitation-strengthened steel with the addition of Cu, the method of the present invention eliminates the addition of the Cu element, and compared with some high-toughness steels with the addition of a higher Ni element, the present invention ensures the high strength and high-toughness plasticity of the steel plate by refining the martensite laths through secondary quenching and combining Nb, V, and Ti precipitation strengthening, and has a more cost-effective approach.

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

Claims (10)

1. A 1000MPa grade structural steel, characterized in that the chemical composition of the structural steel includes: C, Si, Mn, P, S, Al, Ni, Cr, Mo, Nb, V, Ti and Fe; wherein, in terms of mass fraction, The C content is 0.04% to 0.15%, the Si content is 0.20% to 0.50%, the Mn content is ≤2.00%, the P content is ≤0.008%, the S content is ≤0.003%, the Al content is 0.020% to 0.050%, the Ni content is 7.00% to 12.00%, the Cr content is ≤2.00%, the Mo content is ≤2.00%, the Nb content is ≤0.100%, the V content is ≤0.200%, and the Ti content is ≤0.030%. 2. The structural steel according to claim 1 is characterized in that the metallographic structure of the structural steel includes tempered martensite and reversed austenite, the volume fraction of the tempered martensite is 80% to 98%, and the volume fraction of the reversed austenite is 2% to 20%; wherein the average width of the tempered martensite lath is less than 0.3 μm. 3. The structural steel according to claim 1 is characterized in that Nb, V, and Ti precipitate phases are dispersed in the matrix of the structural steel, and the size of the precipitate phases is less than 100 nm. 4. The structural steel according to claim 1 is characterized in that the structural steel meets at least one of the following properties: yield strength of 1050MPa to 1200MPa, tensile strength of 1190MPa to 1300MPa, elongation ≥22%, and impact energy at -85°C >200J. 5. The structural steel according to claim 1, characterized in that the thickness of the structural steel is 10 mm to 50 mm. 6. A method for preparing the structural steel according to any one of claims 1 to 5, characterized in that the method comprises: Obtaining a steel billet having the chemical composition; The steel billet is sequentially subjected to pre-rolling heating, general hot rolling and post-rolling air cooling to obtain a hot-rolled steel plate; The hot-rolled steel plate is heat-treated to obtain structural steel. 7. The method according to claim 6 is characterized in that the temperature of the pre-rolling heating is 1120°C to 1180°C, and the holding time of the pre-rolling heating is 240min to 360min. 8. The method according to claim 6 is characterized in that the general hot rolling is recrystallization zone rolling, the starting rolling temperature of the recrystallization zone rolling is 1000°C to 1100°C, and the finishing rolling temperature of the recrystallization zone rolling is 900°C to 1000°C. 9. The method according to claim 6, characterized in that the heat treatment comprises: primary quenching, secondary quenching and tempering; The primary quenching includes: heating the hot-rolled steel plate to 850°C to 900°C, keeping the temperature for 3d min, and then performing the first water quenching, wherein d is the thickness of the structural steel; The secondary quenching comprises: heating the hot-rolled steel plate after the primary quenching to 650°C to 750°C, keeping the temperature for 3dmin, and then performing a second water quenching, wherein d is the thickness of the structural steel; The tempering comprises: heating the hot-rolled steel plate after secondary quenching to 500° C. to 600° C., keeping the temperature for 3d min, and then air-cooling to room temperature, wherein d is the thickness of the structural steel. 10. The method according to claim 6, characterized in that the thickness of the steel billet is 120 mm to 240 mm.