Low-cost, high-toughness, ultrahigh-strength steel and preparation method therefor

Abstract

A low-cost, high-toughness, ultrahigh-strength steel, comprising the following alloy components in percentages by weight: C: 0.38-0.42%, Mn: 1.20-1.70%, Si: 1.50-2.00%, Cr: 1.10-1.50%, Ni: 0.15-0.40%, Mo: 0.15-0.40%, W: 0.05-0.50%, Nb ≤ 0.015%, B ≤ 0.002% of B, and the balance being Fe and inevitable impurities. The microstructure is composed of ultrafine bainite, lath martensite, and thin-film retained austenite. The preparation process comprises smelting-soaking-forging or hot rolling-solid solution-tempering, and the prepared low-cost, high-toughness, ultrahigh-strength steel has low costs and excellent performance.

Description

A low-cost, high-toughness, ultra-high-strength steel and its preparation method Technical Field

[0001] The present application relates to the technical field of ultra-high strength alloy steel, and in particular to a low-cost, high-toughness ultra-high strength steel and a preparation method thereof. Background Art

[0002] Key components in fields like aerospace have extremely high requirements for material performance, with ultra-high strength and good toughness being crucial. Components in these fields often face harsh operating environments and challenges, requiring materials with excellent mechanical properties.

[0003] Currently, 2GPa-class ultra-high strength steel is a class of materials that has attracted much attention. Its application in my country's aerospace and other engineering fields can significantly improve the safety level of materials and at the same time meet the requirements of some structural optimization. Traditional 2GPa-class ultra-high strength steel is usually based on a tempered martensite matrix and is mainly divided into three categories: 1. Low-temperature tempered, ε-carbide-strengthened medium-carbon low-alloy series, such as 300M steel (40CrNi2Si2MoVA), which can reach a tensile strength of about 1920MPa and a hardness of about 82MPa·m 1 / 2 Fracture toughness; 2. Ultra-low carbon martensitic matrix, intermetallic compound strengthened maraging steel, such as 18Ni series 350, 400 grades, although can achieve 2GPa level strength, but the plastic toughness is significantly reduced; 3. Medium carbon high CoNi martensitic matrix, nano M2C carbide precipitation strengthened secondary hardening steel, represented by AerMet310 / 340, the maximum strength can reach 2.2GPa, but the plastic toughness is still insufficient.

[0004] The three types of 2GPa-grade high-strength steel mentioned above all have high alloy material and manufacturing costs, which is a major constraint to their widespread application. At the same time, as component specifications increase, higher requirements are placed on the material's strength, toughness, and uniformity. Therefore, there is an urgent need to develop a low-cost, low-alloy ultra-high-strength steel that can meet the 2GPa-grade strength requirements while also providing high toughness and achieving low alloy and manufacturing costs.

[0005] Summary of the Invention

[0006] In view of the above analysis, the present application aims to provide a low-cost, high-toughness ultra-high strength steel and a preparation method thereof, so as to solve at least one of the problems of the existing 2GPa-grade ultra-high strength steel, namely, insufficient toughness and high cost.

[0007] On the one hand, the present application provides a low-cost, high-toughness, ultra-high-strength steel, the alloy composition of which is calculated by weight percentage as follows: C: 0.38-0.42%, Mn: 1.20-1.70%, Si: 1.50-2.00%, Cr: 1.10-1.50%, Ni: 0.15-0.40%, Mo: 0.15-0.40%, W: 0.05-0.50%, Nb≤0.015%, B≤0.002%, and the balance is Fe and unavoidable impurities.

[0008] Furthermore, the microstructure of the low-cost, high-toughness, ultra-high-strength steel consists of ultrafine bainite, lath martensite, and film-like retained austenite.

[0009] Furthermore, in the microstructure, the volume ratio of ultrafine bainite, lath martensite, and film-like retained austenite is 15-40:50-75:5-15.

[0010] Furthermore, in the microstructure, the spacing between the thin film retained austenite lamellae between the lath martensite is 100-200 nm; and the spacing between the thin film retained austenite lamellae between the ultrafine bainite is ≤100 nm.

[0011] On the other hand, the present application also provides a method for preparing low-cost, high-toughness, ultra-high-strength steel, which is used to prepare the above-mentioned low-cost, high-toughness, ultra-high-strength steel, comprising the following steps:

[0012] Step S1: smelting, preparing alloy according to component content, and smelting to obtain steel ingots or billets;

[0013] Step S2: Soaking: placing the steel ingot or billet into a heating furnace for soaking treatment;

[0014] Step S3: forging or hot rolling, forging or hot rolling the steel ingot or steel billet after the soaking treatment to obtain a forging or rolled piece;

[0015] Step S4: Solution treatment, heating the forging or rolled piece for solution treatment, keeping the temperature, and then cooling to room temperature. The cooling process is controlled in stages at a cooling rate.

[0016] Step S5: Tempering: Tempering the forged or rolled piece after solutionizing, and then air-cooling or furnace-cooling to room temperature.

[0017] Furthermore, in step S3, the initial forging or initial rolling temperature is 1150°C to 1220°C, and the final forging or final rolling temperature is 850°C to 900°C.

[0018] Furthermore, in step S4, the solution temperature is 900-980° C., and the holding time is 1-4 hours.

[0019] Furthermore, in step S4, the cooling process is controlled in stages to have a cooling rate of:

[0020] Solution temperature -800℃, cooling rate ≥0.5℃ / s;

[0021] 800℃-400℃, cooling rate ≥1℃ / s;

[0022] 400℃-300℃, cooling rate ≥0.2℃ / s;

[0023] 300℃-150℃, cooling rate ≥0.05℃ / s;

[0024] Air cool below 150℃ to room temperature.

[0025] Furthermore, in step S5, the tempering temperature is 200° C. to 300° C., and the tempering time is 2 to 10 hours.

[0026] Furthermore, an annealing process is also included between step S4 and step S5.

[0027] Compared with the prior art, this application can achieve at least one of the following beneficial effects:

[0028] 1. This application adopts a medium-carbon low-alloy design, uses a small amount of micro-alloying elements such as W and Mo to regulate the phase transformation behavior, and in terms of preparation technology, controls the cooling rate in the medium and low temperature sections to control the effective generation of bainite, thereby obtaining a high-strength and toughness structure.

[0029] 2. This application purifies the austenite grain boundaries by adding alloying elements such as W and Mo to prevent the formation of coarse bainite and ferrite structures during the cooling process, and ensures the appropriate size of the austenite grains by setting an appropriate solid solution temperature, so that the bainite can effectively cut the grains. Through the above-mentioned precise control of the alloy composition and the optimization of the grain boundaries, excellent strength and toughness are achieved under low alloy conditions, meeting the application requirements of high-strength materials in fields such as aerospace.

[0030] 3. This application controls the cooling process after solutionizing and adopts stage-by-stage cooling rate control to ensure the moderate growth of ultrafine bainite, so as to obtain a microstructure composed of ultrafine bainite, lath martensite, and film-like retained austenite. The ultrafine bainite is used to cut the austenite grains, thereby refining the microstructure and effectively improving the strength and toughness of the material, thereby making the material have better overall performance.

[0031] 4. In the low-cost, high-toughness, ultra-high-strength steel produced in this application, the content of alloying elements such as nickel, chromium, and molybdenum is precisely controlled to low levels, thereby reducing the material cost of the alloying elements. This optimized alloying element content balances the material's high toughness with manufacturing costs, achieving a dual performance and economic consideration.

[0032] 5. Compared with the existing technology, this application can significantly improve the strength and toughness of large structural parts with thick sections. When processing large structural parts with thick sections, the existing technology, even if oil cooling is used, the internal cooling rate is not high. If it is not reasonably designed in terms of composition, it may cause softening phases (such as upper bainite and ferrite) to appear in the material. If it is directly quenched to form martensite, the martensite laths formed by shear are relatively coarse and the effect of cutting austenite grains is not good, and the material strength cannot reach 2GPa. This application adopts ultrafine bainite cutting to cut large austenite grains into multiple small grains, so as to achieve material refinement and thus improve its strength and toughness.

[0033] In this application, the above-mentioned technical solutions can be combined with each other to achieve more preferred combination solutions. Other features and advantages of this application will be described in the subsequent description, and some advantages will become apparent from the description or be understood by practicing this application. The objectives and other advantages of this application can be achieved and obtained through the contents particularly pointed out in the description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings are only for the purpose of illustrating particular embodiments and are not to be considered as limiting the present application. Like reference symbols denote like components throughout the drawings.

[0035] FIG1 is a microstructure morphology diagram of Example 1 of the present application;

[0036] FIG2 is a transmission image of ultrafine bainite / austenite of Example 1 of the present application;

[0037] FIG3 is a microstructure morphology diagram of Comparative Example 2 of the present application;

[0038] Reference numerals: 1 - ultrafine bainite. DETAILED DESCRIPTION

[0039] The preferred embodiments of the present application are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present application and are used together with the embodiments of the present application to illustrate the principles of the present application, and are not used to limit the scope of the present application.

[0040] The specific embodiment of the present application discloses a low-cost, high-toughness, ultra-high-strength steel. The alloy composition of the low-cost, high-toughness, ultra-high-strength steel is calculated as follows by weight percentage: C: 0.38-0.42%, Mn: 1.20-1.70%, Si: 1.50-2.00%, Cr: 1.10-1.50%, Ni: 0.15-0.40%, Mo: 0.15-0.40%, W: 0.05-0.50%, Nb≤0.015%, B≤0.002%, and the balance is Fe and unavoidable impurities.

[0041] The reasons for limiting the composition of the microalloyed high-toughness ultra-high strength steel and the method for producing the same in this application are explained below. Hereinafter, only weight percentages in the composition are expressed in %.

[0042] Carbon (C): Carbon is an important interstitial solid solution strengthening element that can reduce the phase transition temperature of bainite and martensite. At the same time, carbon can also stabilize the presence of retained austenite. It should be noted that a higher carbon content may lead to a decrease in the toughness of the material and increase the risk of cracking during the cooling process. Conversely, a too low carbon content may not fully exert the strengthening effect and may also produce low-strength phase transition products during the cooling process. Therefore, under the premise of ensuring the strength of the material, the toughness and preparation of the material should be ensured. In this application, the carbon content is controlled at 0.38-0.42%.

[0043] Manganese (Mn), chromium (Cr), nickel (Ni): The content of manganese, chromium and nickel in the alloy is critical for controlling the hardenability of the material and enhancing toughness. Manganese is a low-cost solid solution strengthening element that can improve the stability of austenite, but too high a manganese content may lead to element segregation. Therefore, while adding manganese to the alloy, it is also necessary to control the content of chromium and nickel. Chromium and nickel elements can not only enhance the toughness of the material, but also avoid cracking problems during the cooling process. It is worth noting that although nickel has excellent performance, its economic efficiency is poor. Taking all factors into consideration, in this application, the manganese content is controlled at 1.20~1.70%, the chromium content is controlled at 1.10~1.50%, and the nickel content is controlled at 0.15~0.40%.

[0044] Silicon (Si): The addition of silicon can inhibit the precipitation of cementite during cooling and tempering, improve tempering resistance, and expand the precipitation range of ε-carbide. This helps the material maintain high strength without reducing toughness. In addition, silicon also plays an important role in bainite growth, promoting the formation of slender bainite and preventing the formation of short and thick bainite. However, too high a silicon content may increase the brittleness of the material and increase the difficulty of high-temperature oxidation and preparation. Therefore, in this application, the silicon content is controlled at 1.50-2.00%.

[0045] Molybdenum (Mo) and tungsten (W): The addition of molybdenum and tungsten can play a role in solid solution strengthening and purify the austenite grain boundaries, thereby delaying the transformation of bainite. This can reduce the nucleation rate of bainite at the austenite grain boundaries, promote the segmentation of bainite, and improve the performance of the material. However, too much molybdenum may lead to the formation of coarse carbides, thereby reducing the economic efficiency of the material. Tungsten can improve the hydrogen embrittlement sensitivity of the material, but excessive addition may increase the risk of cracking. Therefore, in this application, the molybdenum content is controlled at 0.15-0.4%, and the tungsten content is controlled at 0.05-0.50%.

[0046] Niobium (Nb): Niobium is an important microalloying element that can achieve nano-precipitation strengthening. It also inhibits grain coarsening during austenitization, improving the material's performance consistency. Therefore, in this application, the niobium content should be controlled to ≤ 0.015%.

[0047] Boron (B): The addition of boron prevents the premature formation of ferrite and bainite in the material, thereby preventing a deterioration in strength and toughness. Furthermore, boron improves the material's hardenability and expands the heat treatment cooling range. However, excessive boron may affect the material's hot working properties. Therefore, in this application, the boron content is controlled to ≤ 0.002%.

[0048] Preferably, a low-cost, high-toughness, ultra-high-strength steel is provided, wherein the alloy composition of the low-cost, high-toughness, ultra-high-strength steel is as follows by weight percentage: C: 0.39-0.41%, Mn: 1.23-1.38%, Si: 1.50-1.75%, Cr: 1.18-1.48%, Ni: 0.33-0.40%, Mo: 0.15-0.40%, W: 0.13-0.22%, Nb≤0.009%, B≤0.0005%, and the balance is Fe and unavoidable impurities.

[0049] The specific embodiment of the present application further discloses a method for preparing low-cost, high-toughness, ultra-high-strength steel, which is used to prepare the above-mentioned low-cost, high-toughness, ultra-high-strength steel, comprising the following steps:

[0050] Step S1: smelting, preparing alloy according to component content, and smelting to obtain steel ingots or billets;

[0051] Step S2: Soaking: placing the steel ingot or billet into a heating furnace for soaking treatment;

[0052] Step S3: forging or hot rolling, forging or hot rolling the steel ingot or steel billet after the soaking treatment to obtain a forging or rolled piece;

[0053] Step S4: Solution treatment, heating the forging or rolled piece for solution treatment, keeping the temperature, and then cooling to room temperature. The cooling process is carried out in stages to control the cooling rate.

[0054] Step S5: Tempering: Tempering the forged or rolled piece after solutionizing, and then air-cooling or furnace-cooling to room temperature.

[0055] Specifically, in step S2, the steel ingot or billet is placed in a heating furnace for soaking treatment at a soaking temperature of 1180-1250°C for 1-5 hours. This step can eliminate fine inclusions in the matrix as much as possible, ensure uniform distribution of the structure, and alleviate macro-segregation of elements.

[0056] Specifically, in step S3, the initial forging or initial rolling temperature is 1150°C to 1220°C, and the final forging or final rolling temperature is 800°C to 900°C. The higher initial forging or initial rolling temperature in this step can increase the deformation of the alloy and reduce the deformation resistance of the alloy. The final rolling temperature of 800°C to 900°C ensures sufficient recrystallization of the hot-deformed material and achieves grain refinement.

[0057] Specifically, in step S4, the solution temperature is 900-980°C, and the holding time is 1-4 hours. Within this temperature range, undissolved carbides can be effectively eliminated while avoiding the problem of excessively large austenite grains due to excessively high temperatures, thereby achieving optimized treatment of grain boundaries. Preferably, the solution temperature is 935-950°C.

[0058] Specifically, in step S4, the cooling rate is controlled in stages as follows: during the cooling process, from the solution temperature to 800°C, the cooling rate is ≥0.5°C / s. Too slow a cooling rate during the high-temperature stage may cause inclusions such as carbides to reprecipitate and coarsen, affecting the toughness of the material. The cooling rate from 800°C to 400°C is ≥1°C / s to prevent ferrite transformation or the formation of large amounts of upper bainite, which reduces material strength. The cooling rate from 400°C to 300°C is ≥0.2°C / s to further suppress the formation of excessive bainite. The cooling rate from 300°C to 150°C is ≥0.05°C / s to prevent the retention of excessive retained austenite, which reduces material strength. Below 150°C, air cooling is used. During the cooling process, the total cooling time from 400°C to 150°C is no less than 250 seconds to ensure a certain amount of bainite is formed, thus cutting the original austenite.

[0059] Specifically, in step S5, the tempering temperature is 200°C to 300°C, and the tempering time is 2 to 10 hours. Within this tempering temperature range, the formation of carbides can be effectively suppressed, avoiding a negative impact on toughness. At the same time, by suppressing dislocation recovery, the high strength of the matrix can still be maintained. Although adopting a high Si content can also suppress the formation of carbides that are detrimental to toughness, this is more expensive. Preferably, the tempering temperature is 220°C to 250°C, and the tempering time is 2 to 5 hours.

[0060] Preferably, between the forging or hot rolling and solutionizing steps, the forged or rolled piece is annealed at a temperature of 500°C to 600°C for 3 to 5 hours. Annealing helps to partially form cementite or ferrite, softening the matrix for easier processing and storage while also preventing cracking or hydrogen embrittlement.

[0061] This application prepares low-cost, high-toughness, ultra-high-strength steel through precise design of alloy composition, optimization of preparation process, and precise control of process parameters.

[0062] This application adjusts the carbon content to achieve the interstitial solid solution strengthening effect, reduces the phase transition temperature of bainite and martensite, and is conducive to the formation of a fine bainite structure within a suitable temperature range, thereby improving the matching of strength and toughness. At the same time, this application fully considers the influence of composition on the change of grain boundaries during the preparation process, optimizes the grain boundaries during the preparation process, purifies the austenite grain boundaries, suppresses the formation of coarse bainite, and considers using low-temperature bainite to cut the austenite grains to achieve a fine microstructure, thereby improving the strength and toughness of the material. Reduce the content of high-cost alloying elements such as nickel, molybdenum, and silicon, thereby reducing the manufacturing cost of the material, and at the same time maintain its positive impact on performance by precisely controlling the content. The present application comprehensively considers the interaction between various alloying elements to ensure that while optimizing a certain property, other properties will not be adversely affected; for example, the role of W and Mo elements is mainly to purify grain boundaries and inhibit the premature occurrence of bainite phase transformation, thereby preventing the formation of coarse bainite; Si element contributes to the formation of bainite and ferrite, but the presence of W and Mo elements offsets this effect. At the same time, Si element can also promote the slender growth of bainite and prevent it from widening, which makes bainite more likely to split austenite at the same bainite volume fraction, playing a significant role; C element and Mn, Cr, Ni elements have the effects of interstitial strengthening and solid solution strengthening, respectively, and can also regulate the thermodynamic process of bainite phase transformation and the martensitic phase transformation point. Based on the precise control of alloy composition, this application prepares low-cost, high-toughness ultra-high-strength steel through the preparation process of smelting-heat soaking-forging or hot rolling-solutionizing-tempering and the control of process parameters, so that the material maintains ultra-high strength while significantly improving the toughness, and successfully reduces the manufacturing cost of the material, meeting the stringent application requirements of high-strength materials in fields such as aerospace, and achieving a dual balance of performance and economy.

[0063] The low-cost, high-toughness, ultra-high-strength steel prepared by the method of the present application has a microstructure composed of ultrafine bainite, lath martensite, and film-like retained austenite. As shown in Figure 1, the ultrafine bainite divides the matrix, the lath martensite exists in the divided block, and the film-like retained austenite exists between the ultrafine bainite and the lath martensite, and mainly exists between the ultrafine bainite. The volume ratio of ultrafine bainite, lath martensite, and film-like retained austenite in the microstructure is 15-40:50-75:5-15. The interlamellar spacing of the film-like retained austenite between the lath martensite is 100-200nm, and the interlamellar spacing of the film-like retained austenite between the ultrafine bainite is ≤100nm.

[0064] The low-cost, high-toughness ultra-high-strength steel of the present application has a tensile strength of ≥2GPa; a yield strength of ≥1500MPa; an elongation after fracture of ≥10%; a cross-sectional shrinkage of ≥36%; and a fracture toughness of ≥82MPa·m 1 / 2 .

[0065] Example 1

[0066] In this embodiment, the alloy composition, by weight percentage, is as follows: C: 0.39%, Mn: 1.27%, Si: 1.50%, Cr: 1.18%, Ni: 0.40%, Mo: 0.40%, W: 0.22%, Nb: 0.009%, B: 0.0005%, with the balance being Fe and unavoidable impurities. This meets the alloy composition range required by this application.

[0067] The preparation process includes:

[0068] S1: smelting, preparing alloy according to component content, and smelting in a 200kg vacuum induction furnace to obtain steel ingots;

[0069] S2: Soaking: Place the steel ingot in a heating furnace for soaking treatment at a temperature of 1200°C for 3 hours.

[0070] S3: Forging, forging the steel ingot after soaking treatment, the initial forging temperature is 1150℃, the final forging temperature is 880℃, and the final cross-section is 60×60mm 2 Square forgings;

[0071] S4: Annealing: cool the square forging to room temperature, then heat it to 500°C for annealing for 3 hours;

[0072] S5: Solution treatment, heating the square forging for solution treatment, keeping it warm, and then cooling it to room temperature. The cooling process is controlled in stages to control the cooling rate;

[0073] Among them, the solution temperature is 950℃ and the holding time is 1h;

[0074] 950-800℃, cooling rate 2℃ / s;

[0075] 800-400℃, cooling rate 2℃ / s;

[0076] 400-300℃, cooling rate 1℃ / s;

[0077] 300-150℃, cooling rate 0.5℃ / s;

[0078] 150℃-room temperature, air cooling;

[0079] S6: Tempering, tempering the forgings after solution treatment, and then air cooling to room temperature.

[0080] Among them, the tempering temperature is 250℃ and the tempering time is 1h.

[0081] Example 2

[0082] The alloy composition of this embodiment is the same as that of embodiment 1, as shown in Table 1.

[0083] The preparation method of this embodiment is similar to that of Example 1, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0084] Example 3

[0085] The alloy composition of this embodiment is the same as that of embodiment 1, as shown in Table 1.

[0086] The preparation method of this embodiment is similar to that of Example 1, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0087] Example 4

[0088] The alloy composition of this embodiment is the same as that of embodiment 1, as shown in Table 1.

[0089] The preparation method of this embodiment is similar to that of Example 1, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0090] Example 5

[0091] In this embodiment, the alloy composition, by weight percentage, is as follows: C: 0.41%, Mn: 1.38%, Si: 1.64%, Cr: 1.40%, Ni: 0.33%, Mo: 0.16%, W: 0.17%, Nb: 0.008%, B: 0.0001%, with the balance being Fe and unavoidable impurities. This meets the alloy composition range required by this application.

[0092] The preparation process includes:

[0093] S1: smelting, preparing alloy according to component content, and smelting in a 200kg vacuum induction furnace to obtain steel ingots;

[0094] S2: Soaking: Place the steel ingot in a heating furnace for soaking treatment at a temperature of 1200°C for 3 hours.

[0095] S3: rolling, rolling the soaked steel ingot at an initial rolling temperature of 1150°C and a final rolling temperature of 850°C, ultimately obtaining a plate with a thickness of 60 mm;

[0096] S4: Annealing: air-cool the plate to room temperature, then heat it to 600°C for annealing for 5 hours;

[0097] S5: Solution treatment, heating the plate for solution treatment, keeping it warm, and then cooling it to room temperature. The cooling process is controlled in stages to control the cooling rate;

[0098] Among them, the solution temperature is 950℃ and the holding time is 1h;

[0099] 950-800℃, cooling rate 2℃ / s;

[0100] 800-400℃, cooling rate 2℃ / s;

[0101] 400-300℃, cooling rate 1℃ / s;

[0102] 300-150℃, cooling rate 0.5℃ / s;

[0103] 150℃-room temperature, air cooling;

[0104] S6: Tempering, tempering the forgings after solution treatment, and then air cooling to room temperature.

[0105] Among them, the tempering temperature is 250℃ and the tempering time is 1h.

[0106] Example 6

[0107] The alloy composition of this embodiment is the same as that of embodiment 5, as shown in Table 1.

[0108] The preparation method of this embodiment is similar to that of Example 5, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0109] Example 7

[0110] The alloy composition of this embodiment is the same as that of embodiment 5, as shown in Table 1.

[0111] The preparation method of this embodiment is similar to that of Example 5, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0112] Example 8

[0113] The alloy composition of this embodiment is the same as that of embodiment 5, as shown in Table 1.

[0114] The preparation method of this embodiment is similar to that of Example 5, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0115] Example 9

[0116] In this embodiment, the alloy composition, by weight percentage, is as follows: C: 0.40%, Mn: 1.23%, Si: 1.75%, Cr: 1.48%, Ni: 0.38%, Mo: 0.15%, W: 0.13%, Nb: 0.002%, with the balance being Fe and unavoidable impurities. This meets the alloy composition range required by this application.

[0117] The preparation process includes:

[0118] S1: smelting, preparing alloy according to component content, and smelting in a 200kg vacuum induction furnace to obtain steel ingots;

[0119] S2: Soaking: Place the steel ingot in a heating furnace for soaking treatment at a temperature of 1200°C for 2 hours.

[0120] S3: Forging, forging the steel ingot after soaking treatment, the initial forging temperature is 1180℃, the final forging temperature is 900℃, and the final cross-section is 80×80mm 2 Square forgings;

[0121] S4: Annealing: cool the square forging to room temperature and then heat it to 580°C for annealing for 3 hours;

[0122] S5: Solution treatment, heating the square forging for solution treatment, keeping it warm, and then cooling it to room temperature. The cooling process is controlled in stages to control the cooling rate;

[0123] Among them, the solution temperature is 950℃ and the holding time is 1h;

[0124] 950-800℃, cooling rate 2℃ / s;

[0125] 800-400℃, cooling rate 2℃ / s;

[0126] 400-300℃, cooling rate 1℃ / s;

[0127] 300-150℃, cooling rate 0.5℃ / s;

[0128] 150℃-room temperature, air cooling;

[0129] S6: Tempering, tempering the forgings after solution treatment, and then air cooling to room temperature.

[0130] Among them, the tempering temperature is 250℃ and the tempering time is 1h.

[0131] Example 10

[0132] The alloy composition of this embodiment is the same as that of embodiment 9, as shown in Table 1.

[0133] The preparation method of this embodiment is similar to that of Example 9, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0134] Example 11

[0135] The alloy composition of this embodiment is the same as that of embodiment 9, as shown in Table 1.

[0136] The preparation method of this embodiment is similar to that of Example 9, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0137] Comparative Example 1

[0138] The alloy composition of this comparative example is the same as that of Example 1, as shown in Table 1.

[0139] The preparation method of this comparative example is similar to that of Example 1, except for the process parameters in the preparation process; some process parameters in the preparation process do not meet the requirements of this application, as shown in Table 2.

[0140] Comparative Example 2

[0141] The alloy composition of this comparative example is the same as that of Example 1, as shown in Table 1.

[0142] The preparation method of this comparative example is similar to that of Example 1, except that water cooling is used for cooling after solutionizing during the preparation process; as shown in Table 2.

[0143] Comparative Example 3

[0144] The alloy composition of this comparative example is the same as that of Example 5, as shown in Table 1.

[0145] The preparation method of this comparative example is similar to that of Example 5, except that water cooling is used for cooling after solutionizing during the preparation process; as shown in Table 2.

[0146] Comparative Example 4

[0147] The alloy composition of this comparative example is the same as that of Example 9, as shown in Table 1.

[0148] The preparation method of this comparative example is similar to that of Example 9, except for the process parameters in the preparation process; some process parameters in the preparation process do not meet the requirements of this application, as shown in Table 2.

[0149] Comparative Example 5

[0150] The alloy composition of this comparative example is the same as that of Example 9, as shown in Table 1.

[0151] The preparation method of this comparative example is similar to that of Example 9, except that water cooling is used for cooling after solutionizing during the preparation process; as shown in Table 2.

[0152] Comparative Example 6

[0153] In this comparative example, the alloy composition, by weight percentage, is as follows: C: 0.42%, Mn: 1.10%, Si: 1.4%, Cr: 1.0%, Ni: 0.7%, Mo: 0.55%, W: 0.05%, Nb: 0.004%, B: 0.0002%, with the balance being Fe and unavoidable impurities. This does not meet the alloy composition range required by this application.

[0154] The preparation process includes:

[0155] S1: smelting, preparing alloy according to component content, and smelting in a 200kg vacuum induction furnace to obtain steel ingots;

[0156] S2: Soaking: Place the steel ingot in a heating furnace for soaking treatment at a soaking temperature of 1180°C for 3 hours.

[0157] S3: rolling, rolling the steel ingot after soaking treatment, with an initial rolling temperature of 1150°C and a final rolling temperature of 890°C, to finally obtain a plate with a thickness of 80 mm;

[0158] S4: Annealing: air-cool the plate to room temperature, then heat it to 600°C for annealing for 5 hours;

[0159] S5: Solution treatment, heating the plate for solution treatment, keeping it warm, and then cooling it to room temperature. The cooling process is controlled in stages to control the cooling rate;

[0160] Among them, the solution temperature is 950℃ and the holding time is 1h;

[0161] 950-800℃, cooling rate 2℃ / s;

[0162] 800-400℃, cooling rate 2℃ / s;

[0163] 400-300℃, cooling rate 1℃ / s;

[0164] 300-150℃, cooling rate 0.5℃ / s;

[0165] 150℃-room temperature, air cooling;

[0166] S6: Tempering, tempering the forgings after solution treatment, and then air cooling to room temperature.

[0167] Among them, the tempering temperature is 250℃ and the tempering time is 1h.

[0168] Comparative Example 7

[0169] The alloy composition of this comparative example is the same as that of comparative example 6, as shown in Table 1, and does not meet the alloy composition range required by this application.

[0170] The preparation method of this comparative example is similar to that of comparative example 6, except for the process parameters in the preparation process; the process parameters in the preparation process all meet the requirements of this application, as shown in Table 2.

[0171] Table 1 Composition of alloys of Examples and Comparative Examples (wt.%)

[0172] Table 2 Process parameters of the preparation process of Examples and Comparative Examples

[0173] Table 3 Mechanical properties of the steels of the examples and comparative examples

[0174] Figure 1 shows the microstructure of Example 1. As can be seen in the figure, the microstructure of the material is composed of ultrafine bainite penetrating the grains and dividing the original austenite grains, and the rest is composed of lath martensite.

[0175] Figure 2 shows a transmission image of the ultrafine bainite / austenite structure from Example 1. The image shows thin film-like retained austenite lamellae separated by ultrafine bainite laminae, with spacing at the 50nm level. These ultrafine lamellae enhance the stability of the thin film austenite, effectively increasing the length of the phase interface and significantly improving the structural strength and toughness.

[0176] Figure 3 shows the microstructure of Comparative Example 2. As can be seen from the figure, the microstructure matrix after water quenching is mainly martensite laths, and no ultrafine bainite is generated.

[0177] Forgings prepared in Examples 1-4 were made using the same alloy composition. The parameters during the preparation process were different, but all within the scope required by this application. The prepared forgings had excellent strength and toughness.

[0178] The plates prepared in Examples 5-8 use the same alloy composition and different parameters in the preparation process, but all are within the scope required by this application. The prepared plates have excellent strength and toughness.

[0179] Forgings prepared in Examples 9-11 utilize the same alloy composition. Compared to Examples 1-4, the Mo and W contents are relatively reduced, yet still meet the requirements of this application. The parameters used in the preparation processes of Examples 9-11 vary, but all fall within the requirements of this application. The resulting forgings exhibit slightly lower toughness than those in Examples 1-4, but slightly higher strength, maintaining excellent strength and toughness.

[0180] Comparative Example 1 uses the same alloy composition as Example 1 and meets the requirements of this application. However, the cooling rate during the post-solutionizing cooling process does not meet the requirements of this application, cooling is relatively slow. Although higher toughness is achieved, the strength is significantly reduced, failing to reach the 2 GPa level. This is primarily due to the slow cooling rate in the intermediate temperature range, which leads to the formation of upper bainite / ferrite, softening the material, and reducing strength.

[0181] Comparative Example 2 uses the same alloy composition as Example 1 and meets the requirements of this application. However, the cooling process after solutionizing was not controlled in stages, but was rapidly cooled by water cooling. The total cooling time from solutionizing to room temperature was less than 180 seconds, resulting in significantly reduced strength and toughness, failing to reach 2 GPa. This indicates that the matrix after bainite segmentation has advantages over the martensite matrix in terms of strength and toughness.

[0182] Comparative Example 3 uses the same alloy composition as Example 8 and meets the requirements of this application. However, the cooling process after solutionizing is not controlled in stages, but is instead rapidly cooled by water cooling. The total cooling time from solutionizing to room temperature is less than 180 seconds. Although the strength reaches 2 GPa, the toughness is significantly insufficient, which does not meet the requirements of high-toughness ultra-high-strength steel.

[0183] Comparative Example 4 uses the same alloy composition as Example 9 and meets the requirements of this application. However, the cooling rate during the cooling process after solutionization does not meet the requirements of this application, cooling is relatively slow. Although the toughness is improved, the tensile strength is significantly reduced, far from the 2 GPa level.

[0184] Comparative Example 5 uses the same alloy composition as Example 11 and meets the requirements of this application. However, the cooling process after solutionizing was not controlled in stages, but was rapidly cooled by water cooling. The total cooling time from solutionizing to room temperature was less than 180 seconds. The yield strength was slightly improved, but the tensile strength decreased, failing to reach 2 GPa.

[0185] In the alloy composition of Comparative Example 6, the contents of Cr, Ni, Mn, Mo and Si elements do not meet the requirements of this application. Although the preparation process meets the requirements of this application, good toughness is still not achieved.

[0186] In the alloy composition of Comparative Example 7, the contents of Cr, Ni, Mn, Mo and Si elements do not meet the requirements of this application. Although the preparation process meets the requirements of this application, the strength of 2 GPa cannot be achieved.

[0187] The above is only a preferred specific implementation method of the present application, but the scope of protection of the present application is not limited to this. The alloy design and preparation process of the present application are also applicable to the production of large-scale high-toughness ultra-high strength steel. Any changes or replacements that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed in this application should be covered by the scope of protection of this application.

Claims

1. A low-cost, high-toughness, ultra-high-strength steel, characterized in that: The alloy composition of the low-cost, high-toughness and ultra-high-strength steel is calculated by weight percentage as follows: C: 0.38-0.42%, Mn: 1.20-1.70%, Si: 1.50-2.00%, Cr: 1.10-1.50%, Ni: 0.15-0.40%, Mo: 0.15-0.40%, W: 0.05-0.50%, Nb≤0.015%, B≤0.002%, and the balance is Fe and unavoidable impurities.

2. The low-cost, high-toughness, ultra-high-strength steel according to claim 1, characterized in that: The microstructure of the low-cost, high-toughness and ultra-high-strength steel consists of ultra-fine bainite, lath martensite and film-like retained austenite.

3. The low-cost, high-toughness, ultra-high-strength steel according to claim 2, characterized in that: In the microstructure, the volume ratio of ultrafine bainite, lath martensite and film-like retained austenite is 15-40:50-75:5-15.

4. The low-cost, high-toughness, ultra-high-strength steel according to claim 3, characterized in that: In the microstructure, the spacing between the film-like retained austenite lamellae between the lath martensite is 100-200nm; the spacing between the film-like retained austenite lamellae between the ultrafine bainite is ≤100nm.

5. A method for preparing low-cost, high-toughness, ultra-high-strength steel, characterized in that: The method for preparing the low-cost, high-toughness and ultra-high-strength steel according to any one of claims 1 to 4 comprises the following steps: Step S1: smelting, preparing alloy according to component content, and smelting to obtain steel ingot or steel billet; Step S2: Soaking, placing the steel ingot or billet into a heating furnace for soaking treatment; Step S3: forging or hot rolling, forging or hot rolling the steel ingot or steel billet after the soaking treatment to obtain a forging or rolled product; Step S4: solution treatment, heating the forging or rolling piece for solution treatment, keeping the temperature, and then cooling to room temperature. The cooling process controls the cooling rate in stages; Step S5: Tempering, subjecting the forged or rolled product after solution treatment to tempering treatment, and then air cooling or furnace cooling to room temperature.

6. The preparation method according to claim 5, characterized in that: In step S3, the initial forging or initial rolling temperature is 1150°C to 1220°C, and the final forging or final rolling temperature is 850°C to 900°C.

7. The preparation method according to claim 6, characterized in that: In the step S4, the solution temperature is 900-980° C., and the holding time is 1-4 hours.

8. The preparation method according to claim 7, characterized in that: In step S4, the cooling process is controlled in stages to have a cooling rate of: Solution temperature -800℃, cooling rate ≥0.5℃ / s; 800℃-400℃, cooling rate ≥1℃ / s; 400℃-300℃, cooling rate ≥0.2℃ / s; 300℃-150℃, cooling rate ≥0.05℃ / s.

9. The preparation method according to claim 8, characterized in that: In the step S5, the tempering temperature is 200° C. to 300° C., and the tempering time is 2 to 10 hours.

10. The preparation method according to claim 5, characterized in that: Annealing treatment is also included between step S4 and step S5.

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