1900MPa-grade low-cost high-strength high-toughness steel and manufacturing method thereof

Abstract

The invention discloses 1900MPa-grade low-cost high-strength high-toughness steel and a manufacturing method thereof, belongs to the technical field of ultrahigh-strength steel, and aims to solve the problems of high alloy element content, high cost and poor obdurability matching property of existing ultrahigh-strength steel. The 1900 MPa grade high-strength and high-toughness steel is prepared from the following components in percentage by mass: 0.325 percent to 0.365 percent of C, 1.3 percent to 1.7 percent of Si, 0.5 percent to 0.9 percent of Mn, 0.8 percent to 1.2 percent of Ni, 3.25 percent to 3.6 percent of Cr, 0.35 percent to 0.6 percent of Mo, 0.4 percent to 1.0 percent of W, 0.01 percent to 0.04 percent of Nb and the balance of iron and inevitable impurities. The steel is low in cost and excellent in obdurability.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-high strength steel, and in particular to a 1900 MPa-level low-cost high-strength and high-toughness steel and a manufacturing method thereof. Background Art

[0002] As the research field of high-speed collision develops towards large-scale and ultra-high speed, higher strength and toughness matching requirements are also put forward for high-speed collision materials. At the same time, the materials are required to have good hardenability, low cost and process performance suitable for large-size forgings and mass production.

[0003] Existing high-alloy ultra-high-strength steels can achieve excellent strength-toughness matching with an impact energy of ≥52J while achieving a tensile strength of ≥1850MPa. However, this type of secondary hardening steel is mostly smelted using the high-threshold "double vacuum" (vacuum induction + vacuum consumable remelting) process, which is costly, has limited production capacity, and has severe segregation in large ingots. In addition, the content of precious metal elements is high, and the cost is high, which limits its practical application.

[0004] In order to control the overall cost, the existing medium and low alloy ultra-high strength steel is mostly smelted by electric furnace + refining outside the furnace + vacuum consumable remelting. Because ultra-high strength steel is very sensitive to impurities or second phases in the material and the microcracks caused by them, the ultra-high strength steel smelted by the traditional single vacuum process has a high content of gas and impurity elements, making it difficult to achieve the matching of ultra-high strength and high toughness. Therefore, it is necessary to start from multiple aspects such as composition design and process purification control to improve the comprehensive performance of low-cost ultra-high strength steel. Summary of the invention

[0005] In view of the above situation, the present invention aims to provide a 1900MPa grade low-cost high-strength and high-toughness steel and a manufacturing method thereof, which is used to solve at least one of the following problems: the existing ultra-high strength steel has a high alloy element content, high cost, and poor strength-toughness matching.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] On the one hand, the present invention provides a 1900MPa grade high-strength and high-toughness steel. The components of the 1900MPa grade high-strength and high-toughness steel include, by mass percentage: C: 0.325% to 0.365%, Si: 1.3% to 1.7%, Mn: 0.5% to 0.9%, Ni: 0.8% to 1.2%, Cr: 3.25% to 3.6%, Mo: 0.35% to 0.6%, W: 0.4% to 1.0%, Nb: 0.01% to 0.04%, and the remainder is iron and unavoidable impurities.

[0008] Furthermore, in the composition of 1900MPa grade high strength and high toughness steel, Si / C is 4.0 to 4.5, wherein C and Si refer to the mass percentages of the corresponding elements.

[0009] Furthermore, in the composition of 1900MPa grade high strength and high toughness steel, Cr / C is 9 to 11, wherein Cr and C refer to the mass percentages of the corresponding elements.

[0010] Furthermore, the components of 1900MPa grade high-strength and high-toughness steel include, by mass percentage: C: 0.33%~0.35%, Si: 1.4%~1.6%, Mn: 0.5%~0.8%, Ni: 0.9%~1.2%, Cr: 3.3%~3.5%, Mo: 0.4%~0.6%, W: 0.4%~0.6%, Nb: 0.01%~0.03%, and the remainder is iron and unavoidable impurities.

[0011] Furthermore, the microstructure of 1900MPa grade high-strength and high-toughness steel is a lath martensite matrix + a small amount of film residual austenite and fine dispersed ε-carbide.

[0012] The present invention also provides a method for manufacturing the above-mentioned 1900MPa grade high-strength and high-toughness steel, comprising: smelting in an electric furnace or a non-vacuum induction furnace; refining outside the furnace and casting electrodes; vacuum consumable remelting to obtain a steel ingot, annealing the steel ingot; heating and homogenizing the steel ingot, forging; and heat treatment.

[0013] Furthermore, in vacuum consumable remelting, the melting rate v in the steady-state melting stage and the ingot diameter D conform to the following relationship: v = (0.01-0.013) × D-3.

[0014] Furthermore, in the electric furnace or non-vacuum induction furnace smelting, the smelting oxidation temperature is ≥1580°C and the steel-out temperature is ≥1650°C.

[0015] Furthermore, the refining outside the furnace includes LF ladle refining and VD refining, and the LF ladle adopts a low-carbon steel ladle.

[0016] Furthermore, the start forging temperature of forging is ≥1050°C, and the final forging temperature is ≥850°C.

[0017] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0018] a) In the 1900MPa grade high-strength and high-toughness steel of the present invention, the basic strength is ensured by controlling the C content, a certain amount of Si is used to inhibit the precipitation of brittle cementite during the tempering process, the Cr content is reasonably controlled, the occurrence of graphitization is inhibited, and the hardenability is effectively improved. At the same time, the Ni content is reduced, the Ac1 point of the steel is increased, and a better annealing softening effect is obtained. The present invention ensures the excellent performance of the steel by accurately controlling the types and contents of Cr, Ni, Mo, W, and Nb alloy elements, as well as the synergistic quantitative relationship of different elements. The present invention does not contain precious elements such as Co, and the contents of elements such as Ni and Nb are very low, thereby achieving the effect of preparing ultra-high strength steel at a low raw material cost.

[0019] b) The manufacturing method of the 1900MPa grade high-strength and high-toughness steel of the present invention realizes ultra-pure large ingot smelting in a single vacuum process by reasonably optimizing the electric furnace smelting and refining processes outside the furnace, accurately controlling the smelting temperature, ladle atmosphere and slag composition, and accurately controlling the vacuum degree, melting rate and cooling parameter matching in the consumable smelting stage. The S content of the finished product is ≤10ppm, the P content is ≤30ppm, the O content is ≤10ppm, the N content is ≤20ppm, and the H content is ≤1.5ppm. The limit O and N contents can be controlled within 10ppm, and the size and quantity of S, N and O inclusions are effectively controlled to ensure excellent toughness.

[0020] c) The preparation method of the present invention achieves excellent performance of the steel by precisely controlling process steps and parameters such as homogenization treatment and forging, avoiding the precipitation of harmful phases during the forging process that damages toughness, thereby ensuring the combination of ultra-high strength and ultra-high toughness.

[0021] d) The steel of the present invention has excellent strength and toughness properties and can meet the use requirements. For example, the tensile strength is above 1850MPa (e.g. 1884-1942MPa), the yield strength reaches above 1400MPa (e.g. 1455-1501MPa), the elongation A is above 10% (e.g. 10.5%-14.0%), the cross-sectional shrinkage Z is above 45% (e.g. 48%-60%), the impact energy KU2 is above 52J (e.g. 60-90J), and the fracture toughness K IC or K Q 105MPa·m 1 / 2 Above (e.g. 110-150 MPa·m 1 / 2 ).

[0022] Other features and advantages of the present invention will be described in the following description, and part of them will become obvious from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the contents particularly pointed out in the written description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0024] Figure 1 The microstructure diagram of the high-strength and high-toughness steel of Example 1;

[0025] Figure 2 The distribution diagram of undissolved carbides in the microstructure diagram of the high-strength and high-toughness steel of Example 1 is shown in FIG.

[0026] Figure 3 This is the morphology of retained austenite between martensite laths in the microstructure diagram of the high-strength and high-toughness steel of Example 1. DETAILED DESCRIPTION

[0027] Preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used to explain the principles of the present invention together with the embodiments of the present invention.

[0028] Ultra-high strength steel is very sensitive to impurities or second phases in the material and the microcracks caused by them. In order to improve the cleanliness of the material, avoid large-sized precipitation phases and thus improve toughness, high-alloy ultra-high strength steel is mostly smelted using the expensive "double vacuum" (vacuum induction + vacuum consumable remelting) process. In general, medium and low alloy ultra-high strength steel is smelted using a lower-cost process of electric furnace + refining outside the furnace + vacuum consumable / electroslag remelting to control the overall cost. This requires, on the one hand, fine control of the refining process to obtain highly clean steel ingots, and on the other hand, optimized design of the hot processing process to obtain high-quality original structure.

[0029] The present invention provides a 1900MPa high-strength and high-toughness steel. The components of the 1900MPa high-strength and high-toughness steel include, by mass percentage: C: 0.325%-0.365%, Si: 1.3%-1.7%, Mn: 0.5%-0.9%, Ni: 0.8%-1.2%, Cr: 3.25%-3.6%, Mo: 0.35%-0.6%, W: 0.4%-1.0%, Nb: 0.01%-0.04%, and the balance is iron and inevitable impurities.

[0030] Specifically, in the composition of the above 1900 MPa grade high strength and high toughness steel, Si / C is 4.0-4.5, wherein C and Si refer to the mass percentages of the corresponding elements.

[0031] Specifically, in the composition of the above 1900 MPa grade high strength and high toughness steel, Cr / C is 9 to 11, wherein Cr and C refer to the mass percentages of the corresponding elements.

[0032] The following is a detailed description of the effects and dosage of the components in the present invention:

[0033] Carbon (C): Carbon is the main strengthening element of steel. It improves the strength of steel by forming ε carbide through solid solution strengthening and tempering. Adding C can also increase the deformation hardening index of steel and reduce the yield strength ratio, thereby improving the dynamic properties of steel. However, too much C will reduce the fracture toughness and deteriorate the welding performance. Considering the strength and toughness level comprehensively, the C content in the present invention is controlled at 0.325% to 0.365%.

[0034] Silicon (Si): Si and Cr elements can inhibit the graphitization of steel, and at the same time inhibit the precipitation of cementite when the quenching cooling speed is slow and the lower bainite appears, which is more suitable for the overall heat treatment of large forgings. Si can also delay the occurrence of low-temperature temper brittleness and play a role in solid solution strengthening, but too high Si content will reduce the plasticity and coordinated deformation ability of steel. Considering the strength and toughness level comprehensively, the Si content in the present invention is controlled at 1.3% to 1.7%. At the same time, in order to ensure the synergistic effect of Si and C, the ratio of Si element content to C element content Si / C is controlled to be 4.0 to 4.5.

[0035] Manganese (Mn): Both Mn and Ni elements are beneficial to lowering the phase transition temperature of martensite, increasing the content of retained austenite, and improving the toughness of steel. At the same time, Mn element also has a certain solid solution strengthening effect, and can also be combined with S element to avoid the formation of harmful FeS. The Mn content in the steel of the present invention is controlled to be 0.5% to 0.9%.

[0036] Nickel (Ni): As one of the main elements of alloy steel, Ni can effectively improve hardenability and reduce the cold brittle transition temperature. It is also an indispensable element for the formation of residual austenite and an effective guarantee for the plasticity and toughness of steel. However, Ni is an expensive element. Too high Ni content will significantly increase material cost and reduce Ms point, which is not conducive to actual quenching operation. In the present invention, the Ni content is controlled to be 0.9% to 1.2%.

[0037] Molybdenum (Mo): Mo is the most effective element for improving hardenability and can also prevent the occurrence of temper brittleness. It can also reduce the activity of C, prevent C compounds from aggregating, and form a Mo-rich area, thereby improving the strength of the steel, especially the yield strength. Mo also has the effect of inhibiting the segregation of harmful impurity elements at the grain boundaries, thereby improving the toughness of the steel. However, as a carbide-forming element, too much Mo will lead to the residue of carbides, resulting in reduced toughness. Therefore, the present invention controls the Mo content to 0.35% to 0.6%.

[0038] Chromium (Cr): Cr is one of the main elements of the alloy steel. Cr can significantly improve the hardenability of steel, prevent the decomposition of residual austenite, and improve the toughness of steel. Cr can also inhibit the graphitization of steel. Therefore, considering the hardenability and other requirements, the ratio of Cr content to C content is controlled to be 9 to 11. A certain amount of Cr can also improve the high temperature oxidation resistance and corrosion resistance of steel. The present invention controls the Cr content to be 3.25% to 3.6%.

[0039] Tungsten (W): The W element can improve the hardness, strength and wear resistance of steel, especially can effectively improve the high temperature strength of the material. At the same time, similar to the Mo element, it can purify the grain boundary, improve the toughness of the material, and improve the hardenability. In the present invention, the W content is controlled between 0.4% and 1.0%.

[0040] Niobium (Nb): Nb can effectively pin grain boundaries, refine grains, and prevent grain growth by forming nano-scale NbC nitrides. However, too high Nb content will lead to the precipitation and growth of Nb-containing inclusions, affecting performance, so the Nb content is controlled to 0.01% to 0.04%.

[0041] In the present invention, in addition to the above-mentioned main element content requirements, in order to achieve the material strength and toughness matching requirements, the impurity element content P≤50ppm, S≤20ppm, O≤10ppm, N≤20ppm, H≤3ppm should also be guaranteed. The selection of alloy elements in the present invention takes into account the feasibility of the smelting production process and the comprehensive consideration of the cost of raw materials, and selects a specific content combination within a specific range to ensure that the finished product has excellent strength and toughness matching.

[0042] In order to further improve the comprehensive performance of the above-mentioned 1900MPa grade high-strength and high-toughness steel, the components of the above-mentioned 1900MPa grade high-strength and high-toughness steel are calculated by mass percentage as follows: C: 0.33%~0.35%, Si: 1.4%~1.6%, Mn: 0.5%~0.8%, Ni: 0.9%~1.2%, Cr: 3.3%~3.5%, Mo: 0.4%~0.6%, W: 0.4%~0.6%, Nb: 0.01%~0.03%, and the remainder is iron and unavoidable impurities.

[0043] Preferably, in the composition of the above 1900MPa grade high-strength and high-toughness steel, Si / C is controlled to be 4.1 to 4.5.

[0044] Preferably, in the composition of the above-mentioned 1900 MPa grade high-strength and high-toughness steel, Cr / C is 9.5 to 10.5.

[0045] Preferably, the contents of the impurity elements mentioned above are required to be P≤30ppm, S≤10ppm, O≤10ppm, N≤20ppm, and H≤1.5ppm.

[0046] The present invention also provides a method for manufacturing the above-mentioned 1900MPa grade high-strength and high-toughness steel, comprising:

[0047] Step 1, melting in an electric furnace or a non-vacuum induction furnace;

[0048] Step 2, refining outside the furnace and casting the electrode;

[0049] Step 3, vacuum consumable remelting to obtain a steel ingot, and annealing the steel ingot;

[0050] Step 4: heating and homogenizing the steel ingot and forging it;

[0051] Step 5: The forging is heat treated to obtain 1900MPa grade high-strength and high-toughness steel.

[0052] Specifically, in the above step 1, it is allowed to use the returned material of Ben Steel, and the raw materials are required to be clean and dry, and the metal alloys such as Ni plates need to be baked in advance.

[0053] Specifically, in the above step 1, during the melting period of the electric furnace, oxygen can be blown to assist melting after the charge has melted more than 70%. The P removal effect is ensured by controlling the end point C of the electric furnace melting (the end point C is controlled to be 0.08% to 0.15%), and sufficient P removal is achieved during the oxidation period. It is required that P ≤ 30ppm before the electric furnace tapping.

[0054] Specifically, in the above step 1, if a non-vacuum induction furnace is used for smelting, CaO and CaF2 slag should be added simultaneously during the smelting period, and the raw materials should be cleaned and the slag should be removed and replaced with new slag. Appropriate Al powder can be added during the process for diffusion deoxidation.

[0055] Specifically, in the above step 1, the smelting oxidation temperature is ≥1580°C, the tapping temperature is ≥1650°C, and the tapped steel is poured into a LF ladle.

[0056] Specifically, in the above step 1, 1.0-3.0 kg / t of aluminum ingots can be added to the LF ladle according to the mass of the molten steel during steel tapping, and lime and refined slag can be added at the same time. The main components of the lime and refined slag are CaO-CaF2, CaO-Al2O3-SiO2, etc.

[0057] Specifically, in order to ensure precise control of carbon composition, LF ladle cannot use magnesium-carbon ladle (carbon content of magnesium-carbon ladle is 10% to 11%) whose ladle carburization is difficult to predict, but should use low-carbon ladle, such as magnesium-chromium ladle (carbon content is about 2%).

[0058] Specifically, in the above step 2, the refining outside the furnace includes LF ladle refining and VD refining. The refining outside the furnace includes inserting graphite electrodes to submerge and heat the molten steel, blowing Ar gas to stir the whole process to accelerate the reaction of slag and remove impurities; adding Ca-Si powder, Al powder, etc. to enhance the deoxidation and desulfurization effect of slag, feeding Ca-Si wire and Al wire into the molten steel for deep deoxidation and desulfurization, and adding C powder in batches.

[0059] Specifically, in the above step 2, after smelting to slag white, Ca-Si powder and / or Al powder are continuously added for reduction, and Ca-Si wire and / or Al wire are fed for deep deoxidation reduction. The slag white time should be ≥30min, and a small amount of deoxidizer is used to maintain the reducing atmosphere. The temperature is ≥1620°C for steel tapping, ensuring that S≤0.0012%; the total evacuation time of the VD furnace is ≥35min, the ultimate vacuum holding time is ≥20min, and H≤1.5ppm after breaking the vacuum.

[0060] Specifically, in the above step 2, after LF is tapped, 50% of the slag is transferred into VD, and the ladle temperature of the VD furnace is 1520°C to 1540°C.

[0061] Specifically, since the P removal effect is assisted by the C component in the smelting stage, the carbon content in the molten steel is relatively low. Therefore, in the above step 2, C powder is added in batches during the LF ladle refining reduction period to enhance the deoxidation effect and ensure the reducibility of the molten steel on the one hand, and to accurately control the composition after deep deoxidation to ensure that the C content is in the ideal control range on the other hand.

[0062] Specifically, in the above step 2, among the slag components added during the LF ladle refining process, CaO is mainly used for S removal, and CaF2 is used for adsorbing inclusions and increasing the fluidity of the slag liquid. During the refining process, Ca acts as a strong deoxidizer. While deeply deoxidizing, Ca 2+ It can fully react with FeS to form CaS, fix the free S in the slag, and the appropriate slag basicity can ensure sufficient Ca 2+ It can fully react with S, and even take away S from MnS, so as to achieve the effect of inclusion modification, and avoid the formation of worm-shaped MnS inclusions in the subsequent smelting process, which will affect the toughness of the finished product. Because the basicity of refined slag affects the deoxidation, desulfurization and inclusion removal effects, the deoxidation effect is significantly improved when the basicity increases, and the inclusion adsorption effect is also improved; when the basicity is low, the adsorption effect on brittle phase inclusions is better. In the present invention, the basicity of LF slag is controlled to be 2.5-3.5 to achieve sufficient deoxidation and desulfurization effects. In actual operation, it is adjusted in time according to the color and fluidity of the slag. The white slag indicates that the FeO content is low and the slag has good reducibility. To ensure the full reduction effect, the slag whitening time should be no less than 30 minutes. During the smelting process, when the LF slag is sampled and S≤10ppm after whitening, the package can be adjusted to VD.

[0063] In the VD stage, the Ca-Si line and the Al line are fed for deep deoxidation and desulfurization. The Ca line can also denature the Al2O3 inclusions retained in the molten steel into liquid calcium aluminate and rapidly float up. Since the steel grade of the present invention has strict requirements on the N content (the actual VD control target is ≤20ppm), in order to ensure a sufficient degassing effect, the ultimate vacuum holding time should not be less than 20min. Keep blowing Ar throughout the process, and adjust the Ar to a small value after breaking the air to ensure that the molten steel is not exposed.

[0064] Specifically, in the above step 2, carbon-free protective slag or ladle covering agent is added when pouring the electrode rod to ensure that the slag surface is completely covered.

[0065] It should be noted that in the above step 2, the pouring stage is the key stage of smelting control. The process usually adopts the method of adding husk or ladle covering agent, carbon-free protective slag, etc. in the ingot mold to ensure that the solidified molten steel is isolated from the air after pouring. The protective slag needs to be fully baked, and the baking time is ≥2h; at the same time, the steel mold should be fully filled with Ar before pouring to ensure the inert gas environment in the ingot mold; add a heating agent when the molten steel rises to about 1 / 3 of the riser line, and add a covering agent when pouring to the riser line to improve the fluidity of the molten steel, reduce casting defects, absorb impurities, and improve the quality of castings.

[0066] Specifically, in the above step 3, during the vacuum consumable remelting, the melting rate in the steady-state melting stage is controlled to be in the range of 4.5 to 11.5 kg / min, the hot vacuum degree is ≤1.0 Pa, and the leakage rate is ≤0.2 Pa / min.

[0067] Specifically, in the above step 3, in the vacuum consumable remelting or electroslag remelting smelting, in order to obtain a uniform and dense ingot solidification structure, the melting rate v (kg / min) in the steady-state smelting stage and the ingot diameter (i.e., crystallizer diameter, ingot diameter) D (mm) conform to the relationship v = (0.01~0.013) × D-3.

[0068] Preferably, when a Φ660 mm ingot is used, the steady-state melting rate is controlled at 4.5 kg to 5.5 kg / min, and when a Φ920 mm ingot is used, the melting rate should be controlled at 7.0 to 7.5 kg / min.

[0069] Specifically, in the above step 3, during the vacuum consumable remelting smelting, in order to further improve the cooling effect and avoid the molten pool from deepening significantly as the smelting time increases, helium filling is started when the weight of the steel ingot in the crystallizer is ≥10% of the total electrode weight, and the helium pressure is 300-500Pa, for example, 350Pa, 400Pa, 450Pa.

[0070] Specifically, in the above step 3, in order to fully degas and maximize the removal effect of inclusions, it is necessary to reasonably control the vacuum degree and melting rate in the vacuum consumable stage to ensure that the molten pool effectively removes inclusions and fully degasses. Therefore, it is necessary to reasonably control the vacuum degree and leakage rate indicators of the consumable furnace, and effectively control the melting rate.

[0071] Specifically, the above preparation method can be used to prepare large ingots with a size of Φ508 mm or larger, for example, Φ580 mm to Φ1200 mm.

[0072] Specifically, in the above step 3, the annealing of the steel ingot includes: cooling the steel ingot in the furnace for 1 to 6 hours before stripping, air cooling or hood cooling the steel ingot to a surface temperature of ≤150°C after stripping, and then annealing the steel ingot in time, with an annealing temperature of 650°C to 700°C and an annealing holding time of ≥24 hours.

[0073] It should be noted that the present invention realizes the precise control of alloy composition and the limit control of impurity elements through the above-mentioned electric furnace smelting, refining outside the furnace and vacuum consumable process, and produces steel ingots with ideal composition, high uniformity, high purity and low segregation, which provides a good material foundation for subsequent hot processing processes.

[0074] Specifically, the above step 4 includes:

[0075] S41, placing the steel ingot in a heating furnace for heating, with the insulation temperature being 1150-1250°C;

[0076] S42, forging, the starting forging temperature of the steel ingot is ≥1050℃, multiple upsetting and drawing forging is adopted, and the final forging temperature is ≥850℃.

[0077] Specifically, in the above S42, no less than 3 times of upsetting and drawing are adopted, and finally the steel is directly drawn and formed by a fast forging machine + a hammer or the steel is drawn and then transferred to a fine forging machine for final forming; the single upsetting reduction is ≥1 / 2H0 (original height), the operation amount of each fire is based on the temperature of the ingot, and the stop forging temperature is ≥850℃; the re-firing temperature is consistent with the heating temperature, and the re-firing time is not less than 2h; the deformation amount of the last fire is ≥30%.

[0078] It should be noted that, within the deformation temperature range of 1050°C to 1200°C, the steel of the present invention has a maximum tendency of dynamic recrystallization under conventional hot forging strain rate. Deformation within this temperature range can fully break up the cast structure and fully recrystallize. Therefore, the start forging temperature is controlled to be ≥1050°C, for example, 1050°C to 1150°C.

[0079] It should be noted that, according to the undissolved phase analysis of the steel of the present invention, the undissolved phases that may exist during the austenitization treatment of the steel of the present invention include Nb-rich MC phase and Mo-rich M6C phase, Cr-, Fe-, Mo-rich M7C3 phase, etc. Experiments combined with calculations show that the driving force of the M6C phase is positive at 850°C to 900°C, and the driving force of the M6C phase is negative above 900°C. If the M6C particles nucleate and grow, it may cause the formation of pores, resulting in macro cracks in the steel ingot and reducing toughness. Therefore, the stop forging temperature is controlled to be ≥850°C.

[0080] Specifically, in the above step 5, the forging is subjected to annealing, quenching and tempering to obtain 1900 MPa grade high-strength and high-toughness steel.

[0081] Specifically, in the above step 5, the annealing holding temperature is 600-700°C, the quenching holding temperature is 900-960°C, and the quenching is performed by oil quenching; the tempering holding temperature is 200-300°C.

[0082] Specifically, in the above step 5, if the quenching temperature is too high, the grains will grow excessively, but when the quenching temperature is too low, there will be a small amount of undissolved (Cr, Fe, Mo)7C3 carbides, which will damage the toughness. Therefore, based on comprehensive considerations, the quenching and holding temperature is controlled to be 900-960°C, for example, 920°C, 930°C, 940°C, 950°C, etc.

[0083] Specifically, in the above step 5, the tempering process is accompanied by the precipitation of supersaturated C in the quenched martensite to form extremely fine nano-ε carbides (Fe 2.4 C), when the tempering temperature is too high, cementite will be formed, which will seriously reduce the plastic toughness. Therefore, it is necessary to select a suitable tempering temperature, such as 230℃~260℃, to obtain superior high strength and toughness. The precise control of these process parameters helps to ensure that the final product has the required mechanical properties and microstructure, and further improves the comprehensive performance of the material.

[0084] Specifically, in the above step 5, the structure is a lath martensite matrix, a small amount of film-like residual austenite between the laths, and fine and dispersed ε-carbide precipitated by tempering to obtain a good combination of strength and toughness, wherein the width of the film-like residual austenite is about 10nm to 50nm, and the volume fraction is about 1% to 5%.

[0085] Specifically, the 1900MPa grade high-strength and high-toughness steel of the present invention has excellent performance and can meet the use requirements. For example, the tensile strength is above 1850MPa (for example, 1884-1942MPa), the yield strength reaches above 1400MPa (for example, 1455-1501MPa), the elongation A is above 10% (for example, 10.5%-14.0%), the cross-sectional shrinkage Z is above 45% (for example, 48%-60%), the impact energy KU2 is above 52J (for example, 60-90J), and the fracture toughness K IC or K Q 105MPa·m 1 / 2 Above (e.g. 110-150 MPa·m 1 / 2 ).

[0086] The advantages of precise control of the composition and process parameters of the steel of the present invention are demonstrated below with specific examples and comparative examples.

[0087] Example 1

[0088] This embodiment provides a 1900 MPa grade high-strength and high-toughness steel and a manufacturing method thereof. The composition of the steel in this embodiment is shown in Table 1 below. The manufacturing method includes:

[0089] (1) Electric furnace smelting: The raw materials are pig iron, high-quality scrap steel, metal materials, etc., which are put into an electric furnace and melted by high-power feeding. After the charge is melted by more than 70%, oxygen is blown to assist melting. The oxidation temperature is 1590°C. P = 30ppm before tapping from the electric furnace. The tapping temperature is 1680°C. When tapping, blocky Al ingots, refined slag and lime are added to the ladle according to the weight of the molten steel.

[0090] (2) Refining outside the furnace: C powder is added to the LF furnace in batches, and Al wire is fed at the same time; during the smelting process, Fe-Si powder + Al powder + refined slag are added for deep deoxidation and desulfurization, the white slag time is 30 minutes, and argon blowing and stirring are used for deep desulfurization; when S≤10ppm, continue to feed Al wire, pour 50% slag, and turn to VD for steel tapping; VD limit vacuum is controlled to be no more than 67Pa, and the vacuum is maintained for 20 minutes at the limit. After breaking the air, H is set, and the measured H=1ppm. Ar gas is blown quietly for 15 minutes; add ladle covering agent, and pour steel under protective conditions;

[0091] (3) Vacuum consumable remelting: pouring electrode rods with a specification of Φ580mm, vacuum consumable remelting consumable ingots with a specification of Φ660mm, steady-state melting rate of 4.8kg / min, hot vacuum degree control ≤1.0Pa, leakage rate ≤0.5Pa / min; when the weight of the steel ingot in the crystallizer reaches 10% of the total electrode weight, helium filling begins, and the helium pressure is 400Pa; the consumable ingot is cooled and stripped in the furnace, air-cooled to below 100°C on the surface, and then annealed, with an annealing temperature of 680°C;

[0092] (4) Forging process: forging holding temperature is 1200℃, start forging temperature is 1080℃, stop forging temperature is 880℃, three-pier and three-draw, fast forging machine and red feed fine forging machine are used to forge Φ310mm black bar, and the finished product is air-cooled and then spun off by train polishing;

[0093] (5) Heat treatment: annealing at 680℃, oil quenching at 930℃, tempering at 260℃, quenching and holding time for 1h after heating through, tempering and holding time for 2h after heating through.

[0094] Example 2

[0095] This embodiment provides a 1900 MPa grade high-strength and high-toughness steel and a manufacturing method thereof. The composition of the steel in this embodiment is shown in Table 1 below. The manufacturing method includes:

[0096] (1) Electric furnace smelting: The raw materials are molten iron, carbon steel, returned scrap steel and other metal materials, which are put into an electric furnace for oxidation smelting. When the steel is tapped, P = 30ppm and the tapping temperature = 1650°C;

[0097] (2) Refining outside the furnace: C powder is added to the LF furnace in batches, and Al wire is fed at the same time; Fe-Si powder + Al powder + refined slag are added during the smelting process for deep deoxidation and desulfurization. The refining time after slag whitening is 60 minutes, and the desulfurization is below 10ppm. After slag removal, CaO and refined slag are used to make slag again, and Ca-Si wire is fed for deep deoxidation and desulfurization, and then transferred to VD. Evacuate for 30 minutes, keep the time under ultimate vacuum for 15 minutes, and break the air to set H = 1ppm. Silica blow Ar gas for 15 minutes, and Ar gas is used to protect the pouring.

[0098] (3) Consumable remelting: pouring Φ580mm electrode rods, vacuum consumable remelting consumable ingots Φ660mm, steady melting rate 4.5kg / min, hot vacuum degree ≤1.2Pa, leakage rate ≤0.5Pa / min. After the consumable ingot mold is cooled for 3h, it is demoulded and sent for annealing, and the annealing temperature is 700℃;

[0099] (4) Forging process: forging holding temperature is 1180℃, start forging temperature is 1050℃, stop forging temperature is 850℃, three forging and three drawing are adopted, fast forging machine forging and drawing, fine forging machine forging and bar forming, air cooling and back-up lathe polishing to produce Φ160mm finished product;

[0100] (5) Heat treatment: annealing at 680℃, quenching at 950℃, tempering at 240℃, quenching and holding time for 1h after heating through, quenching cooling medium is oil, and tempering and holding time is 2h after heating through.

[0101] Example 3

[0102] This embodiment provides a 1900 MPa grade high-strength and high-toughness steel and a manufacturing method thereof. The composition of the steel in this embodiment is shown in Table 1 below. The manufacturing method includes:

[0103] (1) Non-vacuum induction furnace smelting: The raw materials are pure iron, Benxi Steel return materials, low-impurity metal materials, etc., which are added to the non-vacuum induction furnace for smelting. At the same time, CaO and CaF2 slag are added, and the slag is replaced after cleaning. The temperature is 1650℃ and the composition is within the control range.

[0104] (2) Refining outside the furnace: Al ingot is added to the LF ladle, lime, fluorite (CaF2) and other materials are used to make slag, Ca-Si powder and Al powder are used for slag surface deoxidation and reduction, and Al wire or Ca wire is used for deep deoxidation; the refining time after slag whitening is 30 minutes, desulfurization to 5ppm, slag turning 50% and Ca wire and Al wire are used for deep deoxidation and desulfurization, and then transferred to the VD station; evacuation for 40 minutes, holding time at the ultimate vacuum (≤133Pa) for 20 minutes, breaking the air to set H0.8ppm; static blowing of Ar for 20 minutes, pouring and tapping under Ar gas protection;

[0105] (3) Consumable remelting: pouring electrode rods with a specification of Φ810mm, vacuum consumable remelting consumable ingots with a specification of Φ920mm, steady-state melting rate of 7.5kg / min, hot vacuum degree controlled at ≤1.0Pa, leakage rate ≤0.5Pa / min; the consumable ingot mold is cooled for 6h, then demolded and air-cooled, and then annealed at a temperature of 680℃;

[0106] (4) Forging process: forging holding temperature is 1200℃, start forging temperature is 1080℃, stop forging temperature is 850℃, after three forgings and three drawing by fast forging machine, the last fire is directly drawn and formed by throwing, the deformation of the last fire is 50%, and the finished product with specification of Φ340mm is processed by air cooling and then lathe polishing.

[0107] (5) Heat treatment: annealing at 680℃, quenching at 930℃, tempering at 260℃, quenching and holding time for 1 hour after heating through, quenching cooling medium is oil, and tempering and holding time is 2 hours after heating through.

[0108] The microstructure of the steel of Examples 1-3 is a lath martensite matrix, a small amount of film-like retained austenite between the laths, and fine and dispersed ε-carbides precipitated by tempering to obtain a good combination of strength and toughness, wherein the width of the film-like retained austenite is about 10nm to 50nm, and the volume fraction is 1% to 5%.

[0109] Figure 1 is the microstructure diagram of the high-strength and high-toughness steel of Example 1, Figure 2 is the distribution diagram of undissolved carbides in the microstructure diagram of the high-strength and high-toughness steel of Example 1, Figure 3 This is the morphology of retained austenite between martensite laths in the microstructure diagram of the high-strength and high-toughness steel of Example 1.

[0110] The inventor has conducted a large number of experimental studies during the research process, and now uses some bad solutions as comparative examples.

[0111] Comparative Example 1

[0112] This comparative example provides an ultra-high strength steel, the chemical composition of which is shown in Table 1 above, and the preparation method is the same as that of Example 1.

[0113] The main difference between this comparative example and Example 1 lies in the composition design. The composition control of this comparative example is shown in Table 1. The main elements all meet the range requirements. However, due to poor raw material and process control, the S element reaches 24ppm and the P element is as high as 59ppm, which do not meet the requirements of the present invention, resulting in a serious decrease in the plastic toughness of the finished rod, and the minimum design value of the present invention cannot be reached.

[0114] Comparative Example 2

[0115] This comparative example provides an ultra-high strength steel, the chemical composition of which is shown in Table 1 above, and the preparation method is the same as that of Example 1.

[0116] The main difference between this comparative example and Example 1 lies in the composition design. The components of each element in this comparative example meet the range requirements. However, due to the C content of 0.36%, the Si content of 1.41%, and Si / C=3.91, which exceed the silicon-carbon ratio control requirements, the lower Si content (close to the lower limit of the composition) and the higher C content (close to the upper limit of the composition) do not match, and cannot effectively inhibit the precipitation of cementite during the quenching and cooling process and the precipitation and growth of carbides during the tempering process. The requirements of the present invention are not met, resulting in the plastic toughness of this comparative example failing to reach the minimum design value of the present invention.

[0117] Comparative Example 3

[0118] This comparative example provides an ultra-high strength steel, which comes from the same non-vacuum induction furnace mother furnace as Example 3, and casts two Φ810mm electrode rods in one furnace, which are used in Example 3 and Comparative Example 3, and are consumable remelted respectively. The chemical composition is the same as that of Example 3, and the preparation method is mostly the same as that of Example 3, except that: the melting rate in the steady-state melting stage of vacuum consumable remelting of the same ingot is set to 9.8kg / min, which exceeds the upper limit of the relationship between the melting rate v (kg / min) and the ingot diameter (i.e., the crystallizer diameter, the ingot diameter) D (mm) in the steady-state melting stage specified in the present invention.

[0119] In this comparative example, the ingot itself is an ultra-large ingot (Φ920mm), the solidification and cooling are slow, and the segregation tendency is serious. At the same time, the faster melting rate causes the molten pool in the vacuum consumable remelting process to be too deep, resulting in heavier micro-segregation. The subsequent forging heating and heat treatment process cannot completely eliminate the segregation and fully dissolve the carbides, resulting in a slight decrease in strength and a significant decrease in toughness.

[0120] Table 1 Chemical composition, wt%

[0121]

[0122]

[0123] The main performance test results of the steel of the embodiments and comparative examples are shown in Table 2.

[0124] Table 2 Performance test results

[0125]

[0126] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A 1900MPa grade high strength and high toughness steel, characterized in that: The components of the 1900MPa grade high-strength and high-toughness steel include, by mass percentage, C: 0.325% to 0.365%, Si: 1.3% to 1.7%, Mn: 0.5% to 0.9%, Ni: 0.8% to 1.2%, Cr: 3.25% to 3.6%, Mo: 0.35% to 0.6%, W: 0.4% to 1.0%, Nb: 0.01% to 0.04%, and the remainder is iron and unavoidable impurities.

2. The 1900MPa high-strength and high-toughness steel according to claim 1, characterized in that: In the composition of the 1900 MPa grade high strength and high toughness steel, Si / C is 4.0-4.5, wherein C and Si refer to the mass percentages of the corresponding elements.

3. The 1900MPa high-strength and high-toughness steel according to claim 1, characterized in that: In the composition of the 1900 MPa grade high strength and high toughness steel, Cr / C is 9 to 11, wherein Cr and C refer to the mass percentages of the corresponding elements.

4. The 1900MPa high-strength and high-toughness steel according to claim 1, characterized in that: The components of the 1900MPa grade high-strength and high-toughness steel include, by mass percentage, C: 0.33% to 0.35%, Si: 1.4% to 1.6%, Mn: 0.5% to 0.8%, Ni: 0.9% to 1.2%, Cr: 3.3% to 3.5%, Mo: 0.4% to 0.6%, W: 0.4% to 0.6%, Nb: 0.01% to 0.03%, and the remainder is iron and unavoidable impurities.

5. The 1900MPa high-strength and high-toughness steel according to any one of claims 1 to 4, characterized in that: The microstructure of the 1900MPa grade high-strength and high-toughness steel is a lath martensite matrix + a small amount of film residual austenite and fine dispersed ε-carbides.

6. A method for manufacturing the 1900 MPa grade high-strength and high-toughness steel according to any one of claims 1 to 5, characterized in that: include: Electric furnace or non-vacuum induction furnace melting; Refining outside the furnace and casting electrodes; Vacuum consumable remelting to obtain steel ingots, annealing the steel ingots; heating and homogenization of the steel ingots, and forging; Heat treatment.

7. The manufacturing method according to claim 6, characterized in that: In the vacuum consumable remelting smelting, the melting rate v in the steady-state melting stage and the ingot diameter D meet the following relationship: v=(0.01-0.013)×D-3.

8. The manufacturing method according to claim 6, characterized in that: In the electric furnace or non-vacuum induction furnace smelting, the smelting oxidation temperature is ≥1580°C and the steel tapping temperature is ≥1650°C.

9. The manufacturing method according to claim 6, characterized in that: The off-furnace refining includes LF ladle refining and VD refining, and the LF ladle is a low-carbon ladle.

10. The manufacturing method according to any one of claims 6 to 9, characterized in that: The starting forging temperature of the forging is ≥1050°C, and the final forging temperature is ≥850°C.

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