Composite reinforced high-toughness steel and preparation method thereof

Abstract

The invention belongs to the technical field of alloy steel, and relates to composite reinforced high-toughness steel and a preparation method thereof. The steel comprises the following chemical components in percentage by weight: 6.0%-9.4% of Ni, 7.6%-8.9% of Co, 0.8%-3.8% of Cr, 1.0%-2.6% of Mo, 0-0.9% of W, 0.05%-0.60% of V, 0-0.10% of Nb and the balance of Fe. C and Al are added to serve as strengthening elements, and the balance is Fe and impurity elements. When 0.14% < = C < 0.20%, 1.2% < = Al < = 1.6%, 1.0% < = Mo + W / 2 < = 2.0%; when 0.20% < = C < = 0.35%, 0.6% < = Al < = 1.3%, and 2.0% < = Mo + W / 2 < = 3.0%. The alloy is subjected to vacuum induction melting or multi-time melting, homogenizing diffusion annealing and forging rolling to form a material, so that the tensile strength of the steel reaches 2200-2350 MPa or above, the yield strength reaches 2000-2150 MPa or above, and the steel has excellent strength, plasticity and toughness matching and can be used for components such as engine shafts, aircraft landing gears, high-performance shells, frame beams and molds.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy steel and relates to a composite reinforced high-toughness steel and a preparation method thereof. Background Art

[0002] With the development of aviation and aerospace industries, materials are required to have ultra-high strength and excellent strength-toughness combination to meet the demand for material strength of components with higher strength, smaller size and higher specific strength. Ultra-high strength steel of 2200MPa~2400MPa with good strength-toughness combination is the general trend.

[0003] The new generation of aircraft landing gear requires materials with higher specific strength and economic affordability to meet the needs of low-cost components, weight reduction and compact design in small spaces. When the strength of alloy steel reaches above 2200MPa, its specific strength far exceeds that of mature titanium alloys.

[0004] The low-pressure turbine shaft is one of the key core components of the engine. The low-pressure turbine shaft has a complex structure and is the part with the largest aspect ratio of the engine. It bears a large torsional moment and requires the material to have high strength, fatigue resistance and temperature resistance. From the development trend at home and abroad, ultra-high strength steel has replaced high-temperature alloys, titanium alloys and other materials to become the main material of the low-pressure turbine shaft of the large bypass ratio engine. As the engine thrust continues to increase, the engine main shaft is subject to an increasingly large load, and the demand for 2200MPa~2400MPa ultra-high strength steel is urgent.

[0005] Carpenter Company of the United States has developed the AerMet series alloys: AerMet100, AerMet310, and AerMet340. The design goal of AerMet340 steel is to increase the strength to more than 340ksi (2345MPa). The method adopted is to increase the Mo and C content and reduce the Cr content on the basis of the AerMet100 steel composition to increase the volume fraction of Mo2C, enhance the secondary hardening reaction strength, improve the alloy strength, maintain a high Ni content to obtain high dislocation density lath martensite, and increase the Co content to 15% to stabilize the dislocation substructure and improve toughness. The typical chemical composition of the steel is: 0.33C-12Ni-15Co-2.3Cr-1.8Mo (see Table 1), and the tensile strength reaches 2370MPa, but its strength and toughness matching needs to be further improved. At the same time, the high cobalt content of 15.6% and the nickel content of 12% further increase the cost of use.

[0006] The 18Ni series martensitic aging steel developed by International Nickel Company (INCO) has a very low carbon content (≤0.03%wt). It uses Ni as the main alloying element to form tough Fe-Ni based martensite. Through strengthening alloying elements such as Co, Mo and Ti, intermetallic compounds such as Fe2Mo and Ni(Fe)3Mo(Ti) are precipitated from the supersaturated solid solution during the aging process, making this type of steel have excellent comprehensive properties. Among them, the strength of C350 steel reaches more than 2400MPa, and its typical chemical composition is: 18Ni-12Co-1.8Mo-1.4Ti-0.1Al (see Table 1), but its plasticity is low, fatigue strength is not high, and the corrosion resistance is poor due to the lack of Cr. The high nickel and cobalt content also makes the cost high.

[0007] The French company Auboduwa has developed ML340 steel, patent number US5424028, the patent composition range is as follows by weight: C: 0.18% ~ 0.30%, Cr: 2% ~ 5%, Ni: 10.5% ~ 15%, Co: 5% ~ 7%, Mo + W / 2: 1% ~ 4%, Al: 1% ~ 2%, Si ≤ 0.4%, Mn ≤ 0.4%, Nb ≤ 0.10%, the balance is iron. The typical chemical composition of ML340 steel is: 0.25C-14Ni-6Co-3Cr-1.5Mo-1.5Al (see Table 1). While achieving ultra-high strength of more than 2200MPa, the alloy reduces the cobalt content to less than 7%, controlling the alloy cost, but its strength and toughness matching needs to be further improved.

[0008] Table 1 Chemical composition and mechanical properties of ultra-high strength steel

[0009]

[0010] From the above, it can be seen that when the strength is as high as 2200-2400MPa or above, improving the plasticity and toughness of the alloy and achieving a good match between strength, plasticity and toughness are still the difficult problems faced by both China and abroad and the goals that are being pursued diligently. Summary of the invention

[0011] The purpose of the present invention is to provide a composite reinforced high-toughness steel and a preparation method thereof, which optimizes the composition ratio, undergoes high-purity smelting, homogenization diffusion annealing and fine-grain forging to form a material, and precipitates alloy carbides M2C and intermetallic compounds Ni(Fe)Al and other multi-type nano-scale precipitation phases during the heat treatment process, so that the tensile strength of the steel can reach above 2200MPa-2350MPa, the yield strength can reach above 2000MPa-2150MPa, and the steel has excellent strength, plasticity and toughness matching, the elongation can reach above 11%, the area reduction can reach above 55%, and the impact toughness can reach above 10%. kU Reach 50~65J / cm 2 Above, the fracture toughness reaches 40~65MPa.m1 / 2 above.

[0012] To solve this technical problem, the technical solution of the present invention is:

[0013] On the one hand, a composite strengthened high-toughness steel is provided, wherein the chemical composition weight percentage of the steel is: Ni: 6.0% to 9.4%, Co: 7.6% to 8.9%, Cr: 0.8% to 3.8%, Mo: 1.0% to 2.6%, W: 0 to 0.9%, V: 0.05% to 0.60%, Nb: 0 to 0.10%; Ti: 0 to 0.050%, C and Al are added as strengthening elements, and the balance is Fe and impurity elements;

[0014] When 0.14%≤C<0.20%, 1.2%≤Al≤1.6%, 1.0%≤Mo+W / 2≤2.0%;

[0015] When 0.20%≤C≤0.35%, 0.6%≤Al≤1.3%, and 2.0%≤Mo+W / 2≤3.0%.

[0016] Furthermore, the chemical composition of the steel is as follows by weight: Ni: 7.4% to 9.4%, Co: 7.6% to 8.9%, Cr: 0.8% to 3.0%, Mo: 1.0% to 2.6%, W: 0 to 0.90%, V: 0.05% to 0.30%, Nb: 0 to 0.05%; Ti: 0 to 0.050%, C and Al are added as strengthening elements, and the balance is Fe and impurity elements;

[0017] When 0.14%≤C<0.20%, 1.2%≤Al≤1.6%, 1.0%≤Mo+W / 2≤1.5%;

[0018] When 0.20%≤C≤0.35%, 0.6%≤Al<1.3%, 2.0%≤Mo+W / 2≤3.0%.

[0019] In a solution with better performance, the weight percentage of the chemical composition of the steel is: C: 0.20% ~ 0.35%, Ni: 8.0% ~ 9.4%, Co: 7.6% ~ 8.6%, Cr: 0.8% ~ 3.0%, Mo: 2.0% ~ 2.6%, W: 0 ~ 0.90%, Al: 0.6% ~ 1.3%, V: 0.05% ~ 0.30%, Nb: 0.02 ~ 0.05%; Ti: 0.01 ~ 0.03%, C and Al are added as strengthening elements, and the balance is Fe and impurity elements.

[0020] In a particularly preferred embodiment, the typical composition and weight percentage of the steel are as follows:

[0021] Typical composition 1: C: 0.26% ~ 0.30%, Ni: 8.8% ~ 9.2%, Co: 7.8% ~ 8.2%, Cr: 0.8% ~ 1.0%, Mo: 2.0% ~ 2.2%, Al: 1.2% ~ 1.3%, V: 0.12% ~ 0.13%, Nb: 0.03%; the balance is Fe and impurity elements.

[0022] Typical composition 2: C: 0.15% ~ 0.19%, Ni: 8.6% ~ 9.0%, Co: 7.8% ~ 8.2%, Cr: 3.0% ~ 3.4%, Mo: 1.2% ~ 1.4%, Al: 1.4% ~ 1.5%, V: 0.24% ~ 0.28%, Nb: 0.03%; Ti: 0.02%, the balance is Fe and impurity elements.

[0023] Typical composition 3: C: 0.25% ~ 0.29%, Ni: 9.2% ~ 9.4%, Co: 7.9% ~ 8.1%, Cr: 0.8% ~ 1.0%, Mo: 2.0% ~ 2.2%, Al: 0.9% ~ 1.0%, V: 0.09% ~ 0.11%, Nb: 0.03%; Ti: 0.02%, the balance is Fe and impurity elements.

[0024] Typical composition 4: C: 0.14% ~ 0.18%, Ni: 8.9% ~ 9.3%, Co: 8.2% ~ 8.6%, Cr: 2.8% ~ 3.2%, Mo: 1.7% ~ 1.8%, W: 0.3% ~ 0.4%, Al: 1.5% ~ 1.6%, V: 0.30% ~ 0.34%, Nb: 0.03%; the balance is Fe and impurity elements.

[0025] Typical composition 5: C: 0.27% ~ 0.31%, Ni: 8.8% ~ 9.2%, Co: 8.0% ~ 8.2%, Cr: 0.9% ~ 1.1%, Mo: 2.4% ~ 2.6%, Al: 0.9% ~ 1.0%, V: 0.08% ~ 0.10%, Nb: 0.02%; the balance is Fe and impurity elements.

[0026] Typical composition 6: C: 0.21% ~ 0.25%, Ni: 8.8% ~ 9.2%, Co: 8.5% ~ 8.9%, Cr: 1.2% ~ 1.4%, Mo: 2.0% ~ 2.2%, W: 0.6% ~ 0.9%, Al: 1.0% ~ 1.1%, V: 0.11% ~ 0.12%, Nb: 0.02%; the balance is Fe and impurity elements.

[0027] On the other hand, a method for preparing the composite reinforced high-toughness steel is provided, the method comprising the following process steps:

[0028] Step 1, smelting: the smelting method adopts vacuum induction melting or multiple melting process;

[0029] Step 2: Homogenization diffusion annealing of steel ingot: heating temperature 1180℃~1220℃, holding time not less than 10h, furnace cooling to 600℃ and air cooling;

[0030] Step 3: hot processing the steel ingot into steel: the forging or rolling temperature is 900℃~1160℃;

[0031] Step 4: Preparatory heat treatment of steel, using high temperature tempering or normalizing + high temperature tempering treatment;

[0032] Step 5: Solution treatment: heating temperature 900℃~1040℃, holding time 1h~2h;

[0033] Step 6: Cold treatment and tempering: Cold treatment temperature: -70℃~-100℃, keep warm for 1h~3h, return to room temperature in air; tempering temperature: 480℃~550℃, keep warm for 2h~12h, air cooling or faster cooling.

[0034] In order to improve the toughness and fatigue performance of the steel, the mass content of impurity elements is controlled as follows during the smelting process in step 1: S≤0.0030%, P≤0.0060%, O≤0.0010%, N≤0.0020%, so as to achieve high-purity smelting of the alloy.

[0035] Step 1: When the smelting process adopts multiple smelting processes, the preliminary smelting can adopt any of the methods such as vacuum induction melting, EAF (electric arc furnace) + LF (ladle refining furnace) + LVD (vacuum degassing), and the remelting process can adopt electroslag remelting, vacuum arc remelting or a combination of them, preferably vacuum arc remelting. The preliminary smelting and remelting process refers to the preliminary smelting and remelting in the multiple smelting process.

[0036] In order to improve the uniformity of the material, the steel ingot forging in step 3 adopts multiple upsetting and drawing processes, and the total forging ratio is not less than 8.

[0037] The steel ingot described in step 3 is forged or rolled into a product after being opened. In order to improve the performance of the material, the optimal forging temperature range is 1100°C to 900°C. Multiple forgings can be performed, and the forging temperature of the last forging is not higher than 1040°C, and the deformation is not less than 30%.

[0038] The normalizing process parameters in step 4 are: heating temperature 950° C. to 1070° C., holding time not less than 1 hour, air cooling or faster cooling.

[0039] The high temperature tempering process parameters in step 4 are: heating temperature 650° C. to 700° C., insulation time not less than 6 hours, and air cooling.

[0040] The solid solution heating equipment in step 5 may be a vacuum furnace, and the cooling method may be vacuum air cooling or vacuum oil cooling; when non-vacuum heating is used, the cooling method may be air cooling or faster cooling.

[0041] In order to eliminate the residual austenite in the alloy as much as possible, the step 6 may be tempered multiple times, and a cold treatment may be performed between each two temperings, or no cold treatment may be performed. The number of tempering and cold treatments is determined according to the carbon content and the residual austenite.

[0042] In order to eliminate stress, stabilize the structure, reduce hardness, and facilitate material processing, low-temperature tempering treatment can be performed after the first cold treatment in step 6, and its process parameters are: heating temperature 150℃~250℃, holding time 3h~12h, air cooling. After low-temperature tempering treatment, the tempering / cold treatment described in step 6 is performed again. During this process, a secondary hardening reaction occurs, achieving a strength of more than 2200MPa and excellent strength-toughness matching.

[0043] In order to eliminate the internal stress formed by material processing, etc., further tempering treatment can be performed after the final tempering treatment in step six, and its process parameters are: heating temperature 400°C to 500°C, holding time 2h to 6h, and air cooling.

[0044] In order to obtain excellent strength-toughness matching, when Mo+W / 2≤2.0, the solution heating temperature in step five is preferably 900℃~950℃, and the tempering temperature in step six is ​​preferably 480℃~520℃; when Mo+W / 2≥2.0, the solution heating temperature in step five is preferably 950℃~1040℃, and the tempering temperature in step six is ​​preferably 520℃~550℃.

[0045] The beneficial effects of the present invention are:

[0046] Adding C, Al, Mo / W elements to the Fe-Co-Ni matrix, aging precipitation of alloy carbide M2C and intermetallic compound Ni(Fe)Al phase, the composite strengthening effect of nano-scale precipitation phase makes the steel tensile strength reach above 2200-2350MPa, and the yield strength reaches above 2000MPa~2150MPa. By optimizing the composition range and composition ratio of C, Al, Mo / W elements, high-purity smelting, homogenization diffusion annealing, forging and rolling, and matching heat treatment process control, the alloy has excellent strength, plasticity and toughness matching, with elongation of more than 10%, area reduction of more than 55%, and impact toughness of a kU Reach 50~65J / cm 2 Above, the fracture toughness reaches 40~65MPa.m 1 / 2At the same time, the content of expensive metal elements such as Co, Ni, and Mo in the steel of the present invention is lower than that of AerMet340, C350, etc., which has a significant cost advantage.

[0047] The functions of the alloying elements in the steel of the present invention are as follows:

[0048] Co in the steel of the present invention is dissolved in the matrix to play a role of solid solution strengthening. Adding Co can improve the strength of the steel after aging treatment. Co is a non-carbide forming element dissolved in the matrix, which can increase the supersaturation of Mo and increase the power of precipitation. Co can inhibit the recovery of dislocation substructure in martensite, increase the dislocation density of martensite, and provide more nucleation sites for the subsequent formation of precipitation phase, so that the precipitation phase particles are finer and dispersed. Co is the only element in the alloy system that can increase the Ms point temperature or reduce the Ms point by a small amplitude, so Co can effectively regulate the Ms point temperature. The C and Al contents in the steel of the present invention are high. When the Co content is too high, it will promote the formation of twins, which is not good for the toughness of the steel. In order to obtain a good strength-toughness match and considering cost savings, the Co content is controlled at 7.6% to 8.9%.

[0049] Ni improves the hardenability of steel and produces solid solution strengthening. Ni increases the stacking fault energy, making cross slip easier to carry out. At the same time, Ni reduces the energy of interaction between dislocations and impurities, so that there are more movable screw dislocations in martensite, thereby improving plasticity and toughness, reducing the tendency of cleavage fracture, and reducing the toughness-brittle transition temperature. Ni is an austenite-forming element that expands the γ phase region, promotes the formation of reverse-transformed austenite during tempering, and improves the toughness of steel. Both Co and Ni can promote the dissolution of cementite, provide more carbon content for carbide formation, and increase the secondary hardening effect. The addition of Ni in the steel of the present invention can form NiAl intermetallic compounds, which have a strong aging strengthening effect. However, too high a Ni content will increase the amount of residual austenite in the steel and reduce the strength. The Ni content of the steel of the present invention is controlled at 6.0% to 9.4%.

[0050] Mo and W are both ferrite-forming elements, which can stabilize ferrite and improve the hardenability of steel. Mo and W have similar effects and are the main elements for forming M2C carbides. The combined effects of Mo, W and Cr can produce a strong secondary hardening effect. After adding W, the carbide formation temperature is relatively high, and a higher temperature is required to completely dissolve it. Increasing the Mo / W content can increase the hardness value of the secondary hardening peak and the yield strength; adding Mo / W can delay overaging, improve the tempering resistance and thermal strength of steel, and enable the steel to maintain a higher strength after being treated at a higher tempering temperature, which is conducive to the formation of reverse transformation austenite and plays a role in increasing strength and toughness. Adding excessive Mo / W will easily cause microsegregation of elements when the ingot solidifies, and will also cause coarse μ phases and Fe2Mo particles in the steel to have a strong tendency to precipitate, seriously damaging the plasticity and toughness of the steel. The Mo content of the steel of the present invention is controlled at 1.0% to 2.6%, and the W content is controlled at 0 to 0.9%.

[0051] Cr is a ferrite-forming element, which reduces the Ms point temperature and improves the hardenability of steel. Cr is the main carbide-forming element and can enhance the secondary hardening effect. Cr can be dissolved in Mo2C to reduce its stability, which can reduce the reaction temperature of secondary hardening and accelerate the over-aging reaction. Cr replaces Mo in M2C to reduce the lattice constant of M2C and make it closer to the matrix, thereby obtaining a better coherence and improving the toughness of the alloy. At the same time, Cr can improve corrosion resistance and oxidation resistance. The Cr content of the steel of the present invention is controlled at 0.8% to 3.8%.

[0052] V forms stable carbides with C. When the steel is heated in solid solution, VC is difficult to completely dissolve, thus playing a role in refining the grains; V dissolved in austenite increases the stability of supercooled austenite, and VC or V4C3 precipitates from the martensite matrix during tempering and causes secondary hardening. When a small amount of V is added to the steel, V is mainly dissolved in M2C to improve its stability, thereby improving the tempering resistance of the steel; adding excessive V will reduce the plasticity and toughness of the steel, and is also detrimental to the forgeability and processability of the steel. The V content of the steel of the present invention is controlled at 0.05% to 0.60%.

[0053] C and Al are the main strengthening elements of the alloy of the present invention. C is an austenite-forming element. C dissolved in austenite strongly reduces the Ms temperature and increases the residual austenite content and stability. In the steel of the present invention, C exists in the form of interstitial solid solution atoms and carbides, which plays a role in solid solution strengthening and precipitation strengthening. Al is often added to steel as a deoxidizer. At the same time, the addition of Al can also effectively refine the grains of steel and improve the oxidation resistance and corrosion resistance of steel. When the steel of the present invention is tempered, fine and dispersed Ni(Fe)Al intermetallic compounds are precipitated on the martensitic matrix. Ni(Fe)Al is an ordered phase with a strong ordered strengthening effect. At the same time, the lattice constant of the Ni(Fe)Al phase is similar to that of the matrix, and the low mismatch with the matrix effectively slows down the microscopic elastic distortion around the phase particles, improves the macroscopic uniform plastic deformation ability of the material, thereby achieving the purpose of increasing the strength of the material while increasing the plastic toughness. The present invention adds C, Al, and Mo / W, and adopts alloy carbide M2C and intermetallic compound Ni(Fe)Al polymorphic nano-precipitation phase for composite strengthening, so that the strength reaches 2200-2300MPa or above. However, when Al and C elements are excessively added or unreasonably matched, the plasticity and toughness of the alloy will be seriously deteriorated. Therefore, in the present invention, in order to improve the strength, plasticity and toughness matching of steel, C, Al, and Mo / W are component matched: when 0.14%≤C<0.20%, 1.2%≤Al≤1.6%, 1.0%≤Mo+W / 2≤2.0%; when 0.20%≤C≤0.35%, 0.6%≤Al≤1.3%, 2.0%≤Mo+W / 2≤3.0%. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0055] The features of various aspects of the embodiments of the present invention will be described in detail below. In the detailed description below, many specific details are proposed to fully understand the present invention. However, it is obvious to those skilled in the art that the present invention can also be implemented without these specific details. The following description of the embodiments is only for a better understanding of the present invention by illustrating examples of the present invention. The present invention is not limited to any specific settings and methods provided below, but covers all product structures, any improvements, replacements, etc. of methods covered without departing from the spirit of the present invention.

[0056] In the following description, well-known structures and techniques are not shown to avoid unnecessarily obscuring the present invention.

[0057] The steel materials with the alloy composition of the present invention and the steel materials with the comparative alloy composition were smelted by vacuum induction melting method. The chemical compositions of the steel materials of the present invention and the comparative steel materials are shown in Table 2.

[0058] After homogenization annealing, the steel ingot is forged or rolled into billets and square billets at 1150℃~950℃. After forging, it is tempered at 650℃~700℃ for 8h.

[0059] Cut tensile, impact and fracture toughness mechanical properties specimens from the forging blank and carry out final heat treatment. The final heat treatment is carried out according to the following steps:

[0060] (1) Solution heat treatment: The heating temperature of Examples 1#, 2#, 3#, 4#, 5#, 6#, 7#, and 8# is 950°C to 980°C, and the heating temperature of Examples 9#, 10# and Examples 1#, 2#, 3#, and 4# is 900°C to 930°C. The holding time is 1 hour, and oil cooling is used;

[0061] (2) Cold treatment: temperature is -70℃~-90℃, treatment time is 2h, and the temperature is returned to room temperature in air;

[0062] (3) Tempering: The heating temperature of Examples 1#, 2#, 3#, 4#, 5#, 6#, 7#, 8# and Comparative Examples 1# and 4# is 520°C to 550°C, kept at this temperature for 3h to 6h, and air-cooled. The heating temperature of Examples 9# and 10# and Comparative Examples 2# and 3# is 490°C to 510°C, kept at this temperature for 3h to 6h, and air-cooled.

[0063] The number of tempering and cold treatment is determined according to the residual austenite content of the sample. Examples 7#, 8#, 9#, and 10# use one cold treatment and one tempering, and Examples 1#, 2#, 3#, 4#, 5#, and 6# and Comparative Examples 1#, 2#, 3#, and 4# use two cold treatments and two temperings. The mechanical properties of the steel of the present invention and the comparative steel are shown in Table 3.

[0064] Table 2 Chemical composition of the steel of the present invention and the comparative steel (mass fraction, %)

[0065]

[0066] Table 3 Mechanical properties of the steel of the present invention and the comparative steel

[0067]

[0068]

[0069] From Table 2 and Table 3, it can be seen that the C content and Al content of the 1# and 4# comparative examples are too high and improperly matched. Although the strength reaches 2300-2400MPa and the strength and plasticity are well matched, the toughness is low, especially the impact toughness of the 1# comparative example is only 23.5J / cm 2 ; The tensile strength of the 2# and 3# comparison examples reached 2200MPa, but their yield strength was slightly lower, and the strength-toughness match was not ideal. The steel of the present invention, based on Ni: 6.0% to 9.4%, Co: 7.6% to 8.9%, has higher strength (tensile strength reaches more than 2200-2350MPa, yield strength reaches more than 2000MPa to 2150MPa) and good plasticity and toughness, with an elongation of more than 11%, a surface reduction of more than 55%, and an impact toughness of a kU Reach 50~65J / cm 2 Above, the fracture toughness reaches 40~65MPa.m 1 / 2 above.

[0070] In addition, under the typical composition of the steel of the present invention (such as Examples 1#, 2#, 3#, 4#, 9#, 10#), the solution heat treatment process parameters in step 5 and the tempering and cold treatment processes in step 6 were studied, and we found that: when the Mo and W contents are high (Mo+W / 2≥2.0), such as Examples 1#, 2#, 3#, and 4#, the solution heating temperature is selected from 950℃ to 1040℃, and the tempering temperature is selected from 520℃ to 550℃, and the strength and toughness are best matched. If the solution heating temperature is too low, both the strength and toughness are reduced; if the tempering temperature is reduced, the strength is increased, but the toughness is significantly reduced. When the Mo and W contents are low (Mo+W / 2≤2.0), such as Examples 9# and 10#, the solution heating temperature is selected from 900℃ to 950℃, and the tempering temperature is selected from 490℃ to 510℃, and the strength and toughness are best matched. If the solution heating temperature is too high, both the yield strength and plasticity are reduced; if the tempering temperature is increased, the strength is significantly reduced. In addition, when the carbon content is high and the solution heating temperature is high (such as Examples 1#, 2#, 3#, and 4#), multiple tempering and cold treatments are required to eliminate the residual austenite as much as possible to improve strength and toughness.

[0071] Compared with ML340 steel (patent number US5424028), the steel of the present invention has higher yield strength and better strength-toughness matching (typical properties of ML340 steel are shown in Table 1 and Table 3 for comparative example 2). At the same strength level, the fracture toughness is increased by 20 MPa.m 1 / 2 The above, the improvement is as high as 50%.

[0072] Compared with C350 steel (performance shown in Table 1), the steel of the present invention has better plasticity, and the tensile elongation is increased by more than 50%. At the same time, it has lower content of expensive metal elements such as Co, Ni and Mo, thus greatly reducing the cost.

[0073] Compared with AerMet340 steel (performance see Table 1), the steel of the present invention has better strength-toughness matching, and the fracture toughness is improved by 10-20 MPa.m 1 / 2 The above increases by 25% to 55%, while significantly reducing the content of expensive metal elements such as Co and Ni, greatly reducing costs.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the field can easily think of various equivalent modifications or substitutions within the technical scope disclosed by the present invention, and these modifications or substitutions should all be covered within the protection scope of the present invention.

Claims

1. A composite reinforced high-toughness steel, characterized in that: The chemical composition of the steel is as follows: Ni: 6.0% to 9.4%, Co: 7.6% to 8.9%, Cr: 0.8% to 3.8%, Mo: 1.0% to 2.6%, W: 0 to 0.9%, V: 0.05% to 0.60%, Nb: 0 to 0.10%; Ti: 0 to 0.050%, C and Al are added as strengthening elements, and the balance is Fe and impurity elements; When 0.14%≤C<0.20%, 1.2%≤Al≤1.6%, 1.0%≤Mo+W / 2≤2.0%; When 0.20%≤C≤0.35%, 0.6%≤Al≤1.3%, and 2.0%≤Mo+W / 2≤3.0%.

2. The chemical composition of the steel is: Ni: 7.4% to 9.4%, Cr: 0.8% to 3.0%, W: 0 to 0.90%, V: 0.05% to 0.30%, Nb: 0 to 0.05%; 0.14% ≤ C < 0.20%, 1.2% ≤ Al ≤ 1.6%, 1.0% ≤ Mo + W / 2 ≤ 1.5%.

3. The chemical composition of the steel is: C: 0.20% ~ 0.35%, Ni: 8.0% ~ 9.4%, Co: 7.6% ~ 8.6%, Cr: 0.8% ~ 3.0%, Mo: 2.0% ~ 2.6%, W: 0 ~ 0.90%, Al: 0.6% ~ 1.3%, V: 0.05% ~ 0.30%, Nb: 0.02 ~ 0.05%; Ti: 0.01 ~ 0.03%.

4. A method for preparing the composite reinforced high-toughness steel according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following process steps: Step 1, smelting: the smelting method adopts vacuum induction melting or multiple melting process; Step 2: Homogenization diffusion annealing of steel ingot: heating temperature 1180℃~1220℃, holding time not less than 10h, furnace cooling to 600℃ and air cooling; Step 3: hot processing the steel ingot into steel: the forging or rolling temperature is 900℃~1160℃; Step 4: Preparatory heat treatment of steel, using high temperature tempering or normalizing + high temperature tempering treatment; Step 5: Solution treatment: heating temperature 900℃~1040℃, holding time 1h~2h; Step 6: Cold treatment and tempering: Cold treatment temperature: -70℃~-100℃, keep warm for 1h~3h, return to room temperature in air; tempering temperature: 480℃~550℃, keep warm for 2h~12h, air cooling or faster cooling.

5. The preparation method according to claim 4, characterized in that: In order to improve the toughness and fatigue performance of the steel, the mass content of impurity elements is controlled as follows during the smelting process in step 1: S≤0.0030%, P≤0.0060%, O≤0.0010%, N≤0.0020%, so as to achieve high-purity smelting of the alloy.

6. The preparation method according to claim 4, characterized in that: When the smelting process of step 1 adopts multiple melting processes, the preliminary smelting adopts one of the vacuum induction melting and EAF+LF+LVD methods, and the remelting process adopts electroslag remelting, vacuum arc remelting or a combination thereof.

7. The preparation method according to claim 4, characterized in that: In order to improve the uniformity of the material, the steel ingot forging in step 3 adopts multiple upsetting and drawing processes, and the total forging ratio is not less than 8.

8. The preparation method according to claim 4, characterized in that: The steel ingot described in step 3 is forged or rolled into a product after being opened. In order to improve the performance of the material, the optimal forging temperature range is 1100°C to 900°C. Multiple forgings can be performed, and the forging temperature of the last forging is not higher than 1040°C, and the deformation is not less than 30%.

9. The preparation method according to claim 4, characterized in that: The normalizing process parameters in step 4 are: heating temperature 950° C. to 1070° C., holding time not less than 1 hour, air cooling or faster cooling.

10. The preparation method according to claim 4, characterized in that: The high temperature tempering process parameters in step 4 are: heating temperature 650° C. to 700° C., insulation time not less than 6 hours, and air cooling.

11. The preparation method according to claim 4, characterized in that: The solid solution heating equipment in step 5 may be a vacuum furnace, and the cooling method may be vacuum air cooling or vacuum oil cooling; when non-vacuum heating is used, the cooling method may be air cooling or faster cooling.

12. The preparation method according to claim 4, characterized in that: In step six, tempering can be performed multiple times, and a cold treatment can be performed between each two temperings.

13. The preparation method according to claim 4, characterized in that: Between the first cold treatment and the first tempering in step six, a low-temperature tempering treatment can be performed to eliminate stress and stabilize the structure. The process parameters are: heating temperature 150° C. to 250° C., holding time 3 h to 12 h, and air cooling.

14. The preparation method according to claim 4, characterized in that: After the tempering treatment in step six, further tempering treatment may be performed, and the process parameters are: heating temperature 400° C. to 500° C., holding time 2 h to 6 h, and air cooling.

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