A 1900mpa grade low-cost high-strength high-toughness steel and a manufacturing method thereof

Abstract

This invention discloses a low-cost, high-strength, and high-toughness steel of 1900MPa grade and its manufacturing method, belonging to the field of ultra-high-strength steel technology. It addresses the problems of high alloy element content, high cost, and poor strength-toughness matching in existing ultra-high-strength steels. The composition of the 1900MPa grade high-strength and high-toughness steel, by mass percentage, includes: 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%, with the balance being iron and unavoidable impurities. The steel of this invention has low cost and excellent strength and toughness properties.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength steel technology, and in particular to a 1900MPa grade low-cost high-strength and high-toughness steel and its manufacturing method. Background Technology

[0002] As high-speed collision research moves towards larger scales and ultra-high speeds, higher requirements are being placed on the strength and toughness matching of materials used in high-speed collisions. At the same time, materials are required to have good hardenability, low cost, and processability suitable for large-scale forgings and mass production.

[0003] Existing high-alloy ultra-high-strength steels can achieve a tensile strength of ≥1850MPa and an impact energy of ≥52J, resulting in excellent strength and toughness matching. However, these secondary hardening steels are mostly smelted using the high-threshold "double vacuum" (vacuum induction + vacuum arc remelting) process, which is costly, has limited production capacity, and suffers from severe segregation in large ingots. Furthermore, the high content of precious metal elements further increases the cost, limiting their practical application.

[0004] To control overall costs, existing medium- and low-alloy ultra-high-strength steels are mostly smelted using an electric furnace + ladle refining + vacuum arc remelting method. Because ultra-high-strength steel is highly sensitive to impurities or second phases and the microcracks they induce, ultra-high-strength steel smelted using traditional single-vacuum processes has a high content of gaseous and impurity elements, making it difficult to achieve a balance between ultra-high strength and high toughness. Therefore, it is necessary to improve the comprehensive performance of cost-effective ultra-high-strength steel by addressing multiple aspects such as composition design and process purification control. Summary of the Invention

[0005] In view of the above, the present invention aims to provide a 1900MPa grade low-cost high-strength and high-toughness steel and its manufacturing method, in order to solve at least one of the following problems: existing ultra-high strength steels have high alloy element content, high cost, and poor strength-toughness matching.

[0006] The objective of this invention is mainly achieved through the following technical solutions:

[0007] On one hand, the present invention provides a 1900MPa grade high-strength and high-toughness steel. The composition of the 1900MPa grade high-strength and high-toughness steel, by mass percentage, includes: 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%, with the balance being iron and unavoidable impurities.

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

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

[0010] Furthermore, the composition of the 1900MPa grade high-strength and high-toughness steel, by mass percentage, includes: 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%, with the balance being iron and unavoidable impurities.

[0011] Furthermore, the microstructure of the 1900MPa grade high-strength and high-toughness steel consists of a lath martensite matrix, a small amount of thin film retained austenite, and finely dispersed ε-carbides.

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

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

[0014] Furthermore, in electric furnace or non-vacuum induction furnace smelting, the smelting oxidation temperature is ≥1580℃ and the tapping temperature is ≥1650℃.

[0015] Furthermore, ladle refining includes LF ladle refining and VD refining, with the LF ladle using a low-carbon steel ladle.

[0016] Furthermore, the initial forging temperature is ≥1050℃, and the final forging temperature is ≥850℃.

[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, and a certain amount of Si is used to suppress the precipitation of brittle cementite during the tempering process. The Cr content is reasonably controlled to suppress the occurrence of graphitization, while effectively improving hardenability. At the same time, the Ni content is reduced to increase the Ac1 point of the steel and obtain a better annealing softening effect. The present invention ensures the excellent performance of the steel by precisely controlling the types and contents of alloying elements such as Cr, Ni, Mo, W and Nb, 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, thus achieving the effect of preparing ultra-high strength steel with low raw material cost.

[0019] b) The manufacturing method of the 1900MPa grade high-strength and high-toughness steel of the present invention achieves ultra-pure large ingot smelting through single vacuum process by rationally optimizing the electric furnace smelting and ladle refining processes, precisely controlling the smelting temperature, ladle atmosphere and slag composition, and precisely controlling the matching of vacuum degree, melting rate and cooling parameters in the self-consumable melting stage. The finished product has S content ≤10ppm, P content ≤30ppm, O ≤10ppm, N content ≤20ppm, H content ≤1.5ppm, and the limit O and N contents can be controlled within 10ppm. This effectively controls the size and quantity of S and N and O inclusions, ensuring excellent toughness.

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

[0021] d) The steel of this invention exhibits excellent strength and toughness properties, meeting application requirements. For example, it has a tensile strength of 1850 MPa or higher (e.g., 1884–1942 MPa), a yield strength of 1400 MPa or higher (e.g., 1455–1501 MPa), an elongation A of 10% or higher (e.g., 10.5%–14.0%), a reduction of area Z of 45% or higher (e.g., 48%–60%), an impact energy KU2 of 52 J or higher (e.g., 60–90 J), and a fracture toughness K IC or K Q 105 MPa·m 1 / 2 Above (e.g., 110–150 MPa·m) 1 / 2 ).

[0022] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of what is particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0023] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0024] Figure 1 The image shows the microstructure of the high-strength, high-toughness steel from Example 1.

[0025] Figure 2 This is a microstructure diagram of the high-strength and high-toughness steel in Example 1, showing the distribution of undissolved carbides.

[0026] Figure 3 The image shows the morphology of residual austenite between martensite laths in the microstructure diagram of the high-strength and high-toughness steel of Example 1. Detailed Implementation

[0027] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0028] Ultra-high strength steel is highly sensitive to impurities or second phases and the microcracks they induce. To improve material cleanliness and avoid large-sized precipitates, thereby enhancing toughness, high-alloy ultra-high strength steel is often smelted using the costly "double vacuum" (vacuum induction + vacuum arc remelting) process. Generally, medium- and low-alloy ultra-high strength steel is smelted using a lower-cost process of electric furnace + ladle refining + vacuum arc remelting / electroslag remelting to control overall costs. This requires both meticulous control of the refining process to obtain highly clean steel ingots and optimized design of the hot working process to obtain a high-quality original microstructure.

[0029] This invention provides a 1900MPa grade high-strength and high-toughness steel. The composition of the above-mentioned 1900MPa grade high-strength and high-toughness steel, by mass percentage, includes: 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%, with the balance being iron and unavoidable impurities.

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

[0031] Specifically, in the composition of the aforementioned 1900MPa grade high-strength and high-toughness steel, the Cr / C ratio is 9 to 11, where Cr and C refer to the mass percentage of the corresponding elements.

[0032] The following details the function and dosage selection of the components contained in this invention:

[0033] Carbon (C): Carbon is the main strengthening element in steel, increasing its strength through solid solution strengthening and the formation of ε-carbides during tempering. Adding C also increases the strain hardening index and decreases the yield strength ratio, thereby improving the dynamic properties of the steel. However, excessive C can reduce fracture toughness and worsen weldability. Considering both strength and toughness levels, the C content in this invention is controlled between 0.325% and 0.365%.

[0034] Silicon (Si): Si and Cr elements can inhibit graphitization in steel and suppress the precipitation of cementite when lower bainite appears during slow quenching and cooling, making it more suitable for the overall heat treatment of large forgings. Si can also delay the onset of low-temperature tempering brittleness and play a role in solid solution strengthening. However, excessive Si content will reduce the plasticity and synergistic deformation ability of steel. Considering both strength and toughness, the Si content in this invention is controlled at 1.3% to 1.7%. At the same time, to ensure the synergistic effect of Si and C, the Si / C ratio is controlled at 4.0 to 4.5.

[0035] Manganese (Mn): Both Mn and Ni elements are beneficial for lowering the martensite transformation temperature, increasing the retained austenite content, and improving the toughness of steel. Mn also has a certain solid solution strengthening effect and can combine with sulfur to prevent the formation of harmful FeS. In this invention, the Mn content in the steel is controlled at 0.5% to 0.9%.

[0036] Nickel (Ni): As one of the main elements in alloy steel, Ni can effectively improve hardenability and lower the brittle-to-cold transition temperature. It is also an essential element for the formation of retained austenite, effectively ensuring the ductility and toughness of steel. However, Ni is an expensive element; excessively high Ni content will significantly increase material costs and lower the Ms point, which is detrimental to actual quenching operations. In this invention, the Ni content is controlled at 0.9%–1.2%.

[0037] Molybdenum (Mo): Mo is the most effective element for improving hardenability and can also prevent temper brittleness. It also reduces the activity of carbon (C), preventing carbide aggregation and forming Mo-rich regions, thereby increasing the strength of steel, especially its yield strength. Mo also inhibits the segregation of harmful impurity elements at grain boundaries, improving the toughness of steel. However, as a carbide-forming element, excessive Mo can lead to carbide residue, resulting in reduced toughness. Therefore, this invention controls the Mo content to be 0.35%–0.6%.

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

[0039] Tungsten (W): W can improve the hardness, strength and wear resistance of steel, especially effectively improve the high-temperature strength of the material. Similar to Mo, it can purify grain boundaries, improve the toughness of the material, and improve hardenability. In this 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 nanoscale NbC nitrides. However, excessive Nb can lead to the precipitation and growth of Nb-containing inclusions, affecting performance. Therefore, the Nb content is controlled at 0.01% to 0.04%.

[0041] In this invention, in addition to the aforementioned main element content requirements, to achieve the required balance of strength and toughness in the material, the impurity element contents should also be ensured to be P≤50ppm, S≤20ppm, O≤10ppm, N≤20ppm, and H≤3ppm. The selection of alloying elements in this invention takes into account both the feasibility of the smelting and production process and the overall cost of raw materials, selecting specific combinations within a specific range to ensure that the finished product exhibits excellent strength and toughness.

[0042] To further improve the overall performance of the aforementioned 1900MPa grade high-strength and high-toughness steel, the composition of the aforementioned 1900MPa grade high-strength and high-toughness steel, by mass percentage, is 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%, with the balance being iron and unavoidable impurities.

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

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

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

[0046] This 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 non-vacuum induction furnace;

[0048] Step 2: Refining outside the furnace and casting electrodes;

[0049] Step 3: Vacuum arc remelting is used to obtain steel ingots, which are then annealed.

[0050] Step 4: Heating and homogenizing the steel ingot, then forging;

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

[0052] Specifically, in step 1 above, recycled steel is allowed, but the raw materials must be clean and dry, and metal alloys such as Ni plates must be baked in advance.

[0053] Specifically, in step 1 above, during the electric furnace melting period, oxygen can be blown to assist melting after the furnace charge has melted more than 70%. The removal of phosphorus is ensured by controlling the end point C of the electric furnace melting (the control end point C is 0.08% to 0.15%), so as to achieve sufficient removal of phosphorus during the oxidation period. It is required that the phosphorus content be ≤30ppm before the electric furnace taps the steel.

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

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

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

[0057] Specifically, to ensure precise control of carbon composition, LF ladles should not use magnesium-carbon steel ladles (with a carbon content of 10% to 11%), which are difficult to predict due to carbon increase. Instead, low-carbon steel ladles, such as magnesium-chromium steel ladles (with a carbon content of around 2%), should be used.

[0058] Specifically, in step 2 above, ladle refining includes LF ladle refining and VD refining. Ladle refining involves inserting graphite electrodes to submerge the molten steel for heating, bottom-blowing Ar gas to accelerate the reaction and removal of impurities from the slag throughout the process; adding Ca-Si powder, Al powder, etc. to enhance the deoxidation and desulfurization effect of the 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 step 2 above, after smelting to white slag, Ca-Si powder and / or Al powder are added for reduction. Ca-Si wire and / or Al wire are fed in for deep deoxidation and reduction. The slag whitening time should be ≥30 min, and a small amount of deoxidizer is used to maintain the reducing atmosphere. The steel is tapped at a temperature ≥1620℃, and S ≤0.0012%. The total evacuation time of the VD furnace is ≥35 min, the ultimate vacuum holding time is ≥20 min, and H ≤1.5 ppm after breaking the vacuum.

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

[0061] Specifically, since the carbon content in the molten steel is low due to the carbon composition being used to assist in determining the phosphorus removal effect during the smelting stage, the carbon powder is added in batches during the refining and reduction period of the LF ladle in step 2 above. This is to enhance the deoxidation effect and ensure the reducibility of the molten steel, and to precisely control the composition after deep deoxidation to ensure that the carbon content is within the ideal control range.

[0062] Specifically, in step 2 above, among the slag components added during the LF ladle refining process, CaO is mainly used for sulfur removal, while CaF2 is used to adsorb inclusions and increase the fluidity of the slag liquid. During the refining process, Ca acts as a strong deoxidizer, deeply deoxidizing while... 2+ It can fully react and combine with FeS to form CaS, fixing free S in the slag, while the appropriate slag basicity ensures sufficient Ca. 2+ The slag reacts fully with sulfur (S), even capturing S from MnS, thus modifying inclusions and preventing the formation of worm-like MnS inclusions during subsequent smelting, which would affect the toughness of the finished product. Since the basicity of the refining slag affects the deoxidation, desulfurization, and inclusion removal effects, higher basicity significantly improves deoxidation and inclusion adsorption; lower basicity results in better adsorption of brittle phase inclusions. In this invention, the basicity of the LF slag is controlled at 2.5–3.5 to achieve sufficient deoxidation and desulfurization. In actual operation, adjustments are made based on slag color and fluidity. White slag indicates low FeO content and good reducing properties; to ensure sufficient reduction, the whitening time should be no less than 30 minutes. During smelting, once the S content is ≤10 ppm after the LF slag whitens, it can be transferred to the VD (Vacuum-Degrading) solution.

[0063] During the VD (Vacuum Degassing) stage, Ca-Si and Al wires are fed in for deep deoxidation and desulfurization. The Ca wire also transforms Al2O3 inclusions trapped in the molten steel into liquid calcium aluminate, which then floats rapidly. Because the steel grade of this invention has strict requirements for nitrogen content (actual VD control target ≤20ppm), the ultimate vacuum level should be maintained for at least 20 minutes to ensure sufficient degassing. Ar is continuously purged throughout the process; after venting, the Ar pressure is reduced to ensure no exposed molten steel.

[0064] Specifically, in step 2 above, carbon-free protective slag or ladle covering agent is added when casting the electrode rod to ensure complete coverage of the slag surface.

[0065] It should be noted that in step 2 above, the pouring stage is a critical stage for smelting control. In terms of process, rice husks or ladle covering agents and carbon-free protective slag are usually added to the ingot mold to ensure that the molten steel to be solidified after pouring is isolated from the air. The protective slag needs to be fully baked for ≥2 hours. At the same time, Ar should be fully filled into the steel mold before pouring to ensure an inert gas environment inside the ingot mold. When the molten steel rises to about 1 / 3 of the riser line, a heating agent is added, and when the steel is poured to the riser line, a covering agent is added to improve the fluidity of the molten steel, reduce casting defects, adsorb impurities, and improve the quality of the casting.

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

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

[0068] Preferably, when using Φ660mm ingots, the steady-state melting rate should be controlled at 4.5kg~5.5kg / min, and when using Φ920mm ingots, the melting rate should be controlled at 7.0~7.5kg / min.

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

[0070] Specifically, in step 3 above, in order to fully degas and improve the removal effect of inclusions as much as possible, the vacuum degree and melting rate need to be reasonably controlled in the vacuum self-consumption 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 of the self-consumption furnace, and at the same time effectively control the melting rate.

[0071] Specifically, the above preparation method can be used to prepare large ingots with a diameter of 508 mm or more, such as 580 mm to 1200 mm.

[0072] Specifically, in step 3 above, the ingot annealing includes: cooling the ingot in the furnace for 1 to 6 hours and then removing it from the furnace. After removing the ingot, it is air-cooled or covered and cooled until the surface temperature is ≤150°C and then annealed in time. The annealing temperature is 650°C to 700°C and the annealing holding time is ≥24 hours.

[0073] It should be noted that, through the above-mentioned electric furnace smelting, ladle refining and vacuum self-consumption processes, the present invention achieves precise control of alloy composition and extreme control of impurity elements, producing steel ingots with ideal composition, high uniformity, high purity and low segregation, providing a good material basis for subsequent hot working processes.

[0074] Specifically, step 4 above includes:

[0075] S41. The steel ingot is placed in a heating furnace for heating, and the holding temperature is 1150~1250℃;

[0076] S42. Forging: The initial forging temperature of the steel ingot is ≥1050℃, and multiple upsetting and drawing forging are adopted. The final forging temperature is ≥850℃.

[0077] Specifically, in the above S42, no less than 3 upsetting and drawing processes are adopted, and the final shape is directly drawn by a fast forging machine and a shovel, or the shape is drawn by a fast forging machine and then transferred to a precision forging machine for final shaping; the single upsetting reduction is ≥1 / 2H0 (original height), the operation amount per heat is based on the ingot temperature, and the stopping temperature is ≥850℃; the reheating temperature is the same as the heating temperature, and the reheating time is no less than 2 hours; the deformation amount in the final heat is ≥30%.

[0078] It should be noted that the steel of this invention has the greatest tendency for dynamic recrystallization under conventional hot forging strain rates within the deformation temperature range of 1050℃ to 1200℃. Deformation within this temperature range can fully break up the as-cast structure and fully recrystallize. Therefore, the initial forging temperature should be controlled at ≥1050℃, for example, 1050℃ to 1150℃.

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

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

[0081] Specifically, in step 5 above, the annealing holding temperature is 600-700℃, the quenching holding temperature is 900-960℃, and oil quenching is used for quenching; the tempering holding temperature is 200-300℃.

[0082] Specifically, in step 5 above, if the quenching temperature is too high, the grains will grow excessively. However, if 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, taking all factors into consideration, the quenching holding temperature should be controlled at 900-960℃, such as 920℃, 930℃, 940℃, 950℃, etc.

[0083] Specifically, in step 5 above, the tempering process is accompanied by the precipitation of supersaturated C in the quenched martensite, forming extremely fine nano-ε carbides (Fe). 2.4 C) Excessive tempering temperature can lead to cementite formation, severely reducing plasticity and toughness. Therefore, it is necessary to select a suitable tempering temperature, such as 230℃~260℃, to obtain superior strength and toughness. Precise control of these process parameters helps ensure that the final product possesses the required mechanical properties and microstructure, further enhancing the overall performance of the material.

[0084] Specifically, in step 5 above, the microstructure consists of a lath martensite matrix, a small amount of thin film-like retained austenite between the laths, and finely dispersed ε-carbides precipitated during tempering, in order to obtain a good combination of strength and toughness. The width of the thin film retained austenite is about 10 nm to 50 nm, and the volume fraction is about 1% to 5%.

[0085] Specifically, the 1900MPa grade high-strength and high-toughness steel of this invention exhibits excellent performance and can meet application requirements. For example, its tensile strength is above 1850MPa (e.g., 1884–1942MPa), its yield strength reaches above 1400MPa (e.g., 1455–1501MPa), its elongation A is above 10% (e.g., 10.5%–14.0%), its reduction of area Z is above 45% (e.g., 48%–60%), its impact energy KU2 is above 52J (e.g., 60–90J), and its fracture toughness K... IC or K Q 105 MPa·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 will be demonstrated below with specific embodiments and comparative examples.

[0087] Example 1

[0088] This embodiment provides a 1900MPa grade high-strength and high-toughness steel and its manufacturing method. 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 fed into the electric furnace and melted by high power. After the furnace charge is melted by more than 70%, oxygen is blown to assist melting. The oxidation temperature is 1590℃. Before tapping the steel, P=30ppm and the tapping temperature is 1680℃. When tapping the steel, blocky Al ingots, refining slag and lime are added to the ladle according to the weight of the molten steel.

[0090] (2) Ladle refining: C powder is added in batches in the LF furnace, and Al wire is fed in at the same time; Fe-Si powder + Al powder + refining slag are added during the smelting process for deep deoxidation and desulfurization, and the white slag time is 30 min. At the same time, argon blowing and stirring are used for deep desulfurization; when S≤10ppm, Al wire is fed in again, 50% slag is poured out, and steel is tapped and transferred to VD; the ultimate vacuum degree of VD is controlled at no more than 67Pa, the ultimate vacuum holding time is 20 min, and H is determined after the vacuum is broken. The actual measured H = 1ppm, and Ar gas is blown statically for 15 min; ladle covering agent is added, and steel is poured in a protective state;

[0091] (3) Vacuum self-consumable remelting: Cast Φ580mm electrode rods, vacuum self-consumable remelting 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, start charging helium gas, helium pressure is 400Pa; cool and remove the ingot in the furnace, air cool to below 100℃ on the surface and then anneal, annealing temperature is 680℃;

[0092] (4) Forging process: The forging holding temperature is 1200℃, the forging start temperature is 1080℃, the forging stop temperature is 880℃, and the forging is carried out by three forging and three drawing. The high-speed forging machine is sent to the precision forging machine to forge Φ310mm black bar stock, and the finished product is air-cooled and then moved back to the train for finishing.

[0093] (5) Heat treatment: Annealing at 680℃, oil quenching at 930℃, tempering at 260℃, quenching holding time 1 hour after heat penetration, and tempering holding time 2 hours after heat penetration.

[0094] Example 2

[0095] This embodiment provides a 1900MPa grade high-strength and high-toughness steel and its manufacturing method. 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, recycled scrap steel and other metal materials, which are put into the electric furnace and smelted by oxidation. When the steel is tapped, P = 30ppm and the tapping temperature = 1650℃.

[0097] (2) Ladle refining: C powder is added in batches to the LF furnace, and Al wire is fed in simultaneously; during the smelting process, Fe-Si powder + Al powder + refining slag are added for deep deoxidation and desulfurization. After the slag turns white, the refining time is 60 minutes, and the desulfurization is reduced to below 10 ppm. After slag removal, CaO and refining slag are used to form new slag, and Ca-Si wire is fed for deep deoxidation and desulfurization before transferring to VD. Evacuation is carried out for 30 minutes, and the ultimate vacuum is maintained for 15 minutes. The evacuation is then stopped and the H is set to 1 ppm. Ar gas is blown statically for 15 minutes, and the casting is carried out under Ar gas protection.

[0098] (3) Consumable remelting: Cast Φ580mm electrode rods, vacuum consumable remelting ingots of Φ660mm, steady-state melting rate 4.5kg / min, hot vacuum degree maintained ≤1.2Pa, air leakage rate ≤0.5Pa / min. After the consumable ingot mold cools for 3 hours, demold and send for hot annealing at 700℃;

[0099] (4) Forging process: The forging holding temperature is 1180℃, the forging opening temperature is 1050℃, the forging stopping temperature is 850℃, and the three-stage forging and three-stage drawing are adopted. The billet is opened and drawn by a fast forging machine, and the bar is forged by a precision forging machine. After air cooling, the bar is back-rolled and finished with Φ160mm specification.

[0100] (5) Heat treatment: Annealing at 680℃, quenching at 950℃, tempering at 240℃, quenching holding time 1 hour after heat penetration, quenching cooling medium oil, tempering holding time 2 hours after heat penetration.

[0101] Example 3

[0102] This embodiment provides a 1900MPa grade high-strength and high-toughness steel and its manufacturing method. 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 material, low impurity metal material, 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 the slag is cleared. The steel is tapped after the temperature is 1650℃ and the composition is within the control range.

[0104] (2) Ladle refining: Al ingots are added to the LF ladle, and lime and fluorite (CaF2) are used to form slag. Ca-Si powder and Al powder are used for slag surface deoxidation and reduction. Al wire or Ca wire is fed for deep deoxidation. After the slag turns white, the refining time is 30 minutes, and the desulfurization is reduced to 5 ppm. After 50% of the slag is turned over, Ca wire and Al wire are fed for deep deoxidation and desulfurization. The ladle is then moved to the VD station. The ladle is evacuated for 40 minutes and held at the ultimate vacuum degree (≤133Pa) for 20 minutes. The vacuum degree is broken and the H is set at 0.8 ppm. Ar is blown statically for 20 minutes. The ladle is then cast and tapped under Ar gas protection.

[0105] (3) Consumable remelting: Cast Φ810mm electrode rods, 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, air leakage rate ≤0.5Pa / min; after the consumable ingot mold is cooled for 6 hours, demold and air-cooled, then annealed at 680℃.

[0106] (4) Forging process: The forging holding temperature is 1200℃, the forging opening temperature is 1080℃, the forging stopping temperature is 850℃. After three forgings and three drawings using a fast forging machine, the last forging is directly drawn and shaped by throwing. The deformation amount of the last forging is 50%. After air cooling, the machine is moved back to the train to process the Φ340mm finished product.

[0107] (5) Heat treatment: Annealing at 680℃, quenching at 930℃, tempering at 260℃, quenching holding time 1 hour after heat penetration, quenching cooling medium oil, tempering holding time 2 hours after heat penetration.

[0108] The microstructure of the steel in Examples 1-3 consists of a lath martensitic matrix, a small amount of thin film-like retained austenite between the laths, and finely dispersed ε-carbides precipitated during tempering, to achieve a good combination of strength and toughness. The width of the thin film retained austenite is about 10 nm to 50 nm, and the volume fraction is 1% to 5%.

[0109] Figure 1 This is a microstructure diagram of the high-strength, high-toughness steel from Example 1. Figure 2 This is a microstructure diagram of the high-strength, high-toughness steel from Example 1, showing the distribution of undissolved carbides. Figure 3 The image shows the morphology of residual austenite between martensite laths in the microstructure diagram of the high-strength and high-toughness steel of Example 1.

[0110] The inventors conducted extensive experimental research during the research process, and some unsatisfactory solutions are now presented 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 in 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 control of raw materials and processes, the S element reached 24 ppm and the P element reached 59 ppm, which does not meet the requirements of this invention. This resulted in a serious decrease in the plasticity and toughness of the finished bar and could not reach the minimum design value of this invention.

[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 in Example 1.

[0116] The main difference between this comparative example and Example 1 lies in the composition design. In this comparative example, all elemental compositions meet the range requirements. However, because the C content is 0.36%, the Si content is 1.41%, and the Si / C ratio is 3.91, it exceeds the silicon-carbon ratio control requirements. The low Si content (close to the lower limit of the composition) and the high C content (close to the upper limit of the composition) do not match, and cannot effectively inhibit the precipitation of cementite during the quenching cooling process and the precipitation and growth of carbides during the tempering process. This does not meet the requirements of the present invention, resulting in the ductility and 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, derived from the same non-vacuum induction furnace as Example 3. Two Φ810mm electrode rods were cast in one furnace for both Example 3 and Comparative Example 3, and were subjected to consumable remelting. The chemical composition is the same as that of Example 3, and the preparation method is mostly the same as that of Example 3. The difference lies in that the melting rate during the steady-state melting stage of the vacuum consumable remelting of the same ingot type is set at 9.8 kg / 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) during the steady-state melting stage specified in this invention.

[0119] Because the ingot in this comparative example is an ultra-large ingot (Φ920mm), the solidification and cooling are relatively slow, resulting in a severe tendency for segregation. At the same time, the relatively fast melting rate leads to an excessively deep molten pool during the vacuum self-consumption remelting process, causing severe microsegregation. Subsequent forging heating and heat treatment processes cannot completely eliminate segregation and fully dissolve 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 steels in the examples and comparative examples are shown in Table 2.

[0124] Table 2 Performance Test Results

[0125]

[0126] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-strength and high-ductility steel of grade 1900 MPa, characterized in that, The composition of the 1900MPa grade high-strength and high-toughness steel, by mass percentage, includes: 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%, S≤10ppm, P≤30ppm, O≤10ppm, N≤20ppm, H≤1.5ppm, with the balance being iron and unavoidable impurities; The composition of the 1900MPa grade high-strength and high-toughness steel has a Si / C ratio of 4.0~4.5, where C and Si refer to the mass percentage of the corresponding elements. In the composition of the 1900MPa grade high-strength and high-toughness steel, the Cr / C ratio is 9~11, where Cr and C refer to the mass percentage of the corresponding elements; The manufacturing method of the 1900MPa grade high-strength and high-toughness steel includes: melting in an electric furnace or non-vacuum induction furnace; refining outside the furnace and casting electrodes; vacuum consumable remelting to obtain steel ingots; annealing of steel ingots; heating and homogenizing treatment of steel ingots; forging; and heat treatment. During the electric arc furnace smelting melting period, oxygen is blown to assist melting after the furnace charge has melted more than 70%. The final carbon content is controlled at 0.08%~0.15%, and the phosphorus content is ≤30ppm before tapping the steel from the electric arc furnace. Ladle refining includes LF ladle refining and VD refining. During ladle refining, after smelting to white slag, Ca-Si powder and / or Al powder are added for reduction. Ca-Si wire and / or Al wire are fed for deep deoxidation and reduction. The slag whitening time is ≥30 min, and a small amount of deoxidizer is used to maintain the reducing atmosphere. The steel is tapped at a temperature ≥1620℃, ensuring S≤0.0012%. The total evacuation time of the VD furnace is ≥35 min, the ultimate vacuum holding time is ≥20 min, and H≤1.5 ppm after evacuation. The basicity of the LF slag is controlled at 2.5~3.

5. In vacuum consumable remelting, the melting rate v and the ingot diameter D in the steady-state melting stage conform to the following relationship: v = (0.01~0.013) × D - 3; when the weight of the steel ingot in the crystallizer is ≥10% of the total electrode weight, helium gas is started to be filled, and the helium pressure is 300~500 Pa. The microstructure of the 1900MPa grade high-strength and high-toughness steel is a lath martensite matrix with a small amount of thin film retained austenite and fine dispersed ε-carbides, and the volume fraction of thin film retained austenite is 1%~5%. The impact energy KU2 of the 1900 MPa grade high-strength high-toughness steel is 52 J or more, and the fracture toughness is 105 MPa·m 1 / 2 The above.

2. The 1900 MPa grade high strength high toughness steel as claimed in claim 1, wherein, The Si / C ratio in the 1900MPa grade high-strength and high-toughness steel is 4.1~4.

5.

3. The 1900MPa grade high-strength and high-toughness steel according to claim 1, characterized in that, The composition of the 1900MPa grade high-strength and high-toughness steel has a Cr / C ratio of 9.5 to 11.

4. The 1900 MPa grade high strength high toughness steel as claimed in claim 1, wherein, The composition of the 1900MPa grade high-strength and high-toughness steel, by mass percentage, includes: 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%, with the balance being iron and unavoidable impurities.

5. A method for manufacturing 1900MPa grade high-strength and high-toughness steel according to any one of claims 1 to 4, characterized in that, include: Smelting in an electric furnace or a non-vacuum induction furnace; Outside the furnace, refining and electrode casting are performed. Steel ingots are obtained by vacuum arc remelting, followed by annealing, heating and homogenization, and forging. Heat treatment.

6. The production method according to claim 5, wherein In the vacuum self-consuming remelting process, the melting rate v during the steady-state melting stage is related to the ingot diameter D as follows: v = (0.01~0.013) × D - 3.

7. The production method according to claim 5, wherein In the electric furnace or non-vacuum induction furnace smelting, the smelting oxidation temperature is ≥1580℃ and the tapping temperature is ≥1650℃.

8. The production method according to claim 5, wherein The ladle refining includes LF ladle refining and VD refining, wherein the LF ladle uses a low-carbon steel ladle.

9. The production method according to any one of claims 5 to 8, characterized by, The forging temperature is ≥1050℃ for the initial forging and ≥850℃ for the final forging.

Images