Large-size low-alloy ultra-high strength and toughness medium-manganese steel and its industrial production method

Abstract

Large-size low-alloy ultra-high strength and toughness medium-manganese steel and its industrial production method, by omitting high-cost alloying elements such as Mo, Cr, and Ni, and adopting a lower-cost smelting process route of electric furnace + ladle refining + vacuum degassing, which is more suitable for large-scale industrial production, is conducive to achieving the best balance between performance and cost. This provides a highly competitive new material for applications such as weaponry and aerospace. The large-size low-alloy ultra-high strength and toughness medium-manganese steel includes the following chemical composition and its wt% content: C=0.30~0.40, Mn=4.00~6.00, V=0.10~0.40, S≤0.0015, P≤0.005, O≤0.0007, N≤0.0015, H≤0.00005, Fe=balance. The microstructure with Fe as the matrix does not contain alloying elements other than C, Mn, and V.

Description

Technical Field

[0001] This invention relates to a large-size low-alloy ultra-high strength and toughness medium-manganese steel and its industrial production method, belonging to the field of alloy steel technology. Background Technology

[0002] Ultra-high strength steel (tensile strength ≥ 1500 MPa) is a core material for manufacturing weapons, aerospace key structural components, and other defense and high-end equipment. Currently, typical steel grades achieving a strength level of 1700 MPa, such as G31, DT300, and 30CrMnSiNi2A, generally employ a technical route combining multi-element alloying and special smelting processes. To ensure extremely high strength, hardenability, and a certain degree of toughness, these steel grades must incorporate alloying elements such as nickel (Ni), chromium (Cr), and molybdenum (Mo) to guarantee their strength and toughness. However, this technical route has two significant characteristics: First, the dependence on expensive alloying elements such as Ni, Mo, and Cr leads to high raw material costs, and their prices are highly susceptible to fluctuations in the international market. Second, to obtain pure molten steel to ensure performance, these steel grades typically require special smelting processes such as vacuum induction melting (VIM) and vacuum arc remelting (VAR). These processes have long production cycles and high energy consumption, further increasing the manufacturing cost of the final product.

[0003] Therefore, there is an urgent need in this field for an innovative technical solution that can fundamentally solve the problem of high cost of existing materials while ensuring ultra-high strength of 1700MPa and excellent ductility and toughness (e.g., total elongation ≥10%). An ideal solution is to design a completely new compositional system, eliminating high-cost alloying elements such as Mo, Cr, and Ni, and adopting a lower-cost smelting process route that combines electric furnace refining with ladle refining and vacuum degassing, which is more suitable for large-scale industrial production. This would achieve the best balance between performance and cost, providing a highly competitive new material for applications such as weaponry and aerospace. Summary of the Invention

[0004] This invention provides a large-size low-alloy ultra-high strength and toughness medium-manganese steel and its industrial production method. By omitting high-cost alloying elements such as Mo, Cr, and Ni, and adopting a lower-cost smelting process route of electric furnace + ladle refining + vacuum degassing, which is more suitable for large-scale industrial production, it is beneficial to achieve the best balance between performance and cost, thereby providing a highly competitive new material for applications such as weaponry and aerospace.

[0005] The technical solution of the present invention is as follows:

[0006] Large-size low-alloy ultra-high strength and toughness medium-manganese steel includes the following chemical composition and its wt% content: C=0.30~0.40, Mn=4.00~6.00, V=0.10~0.40, S≤0.0015, P≤0.005, O≤0.0007, N≤0.0015, H≤0.00005, Fe=balance. The microstructure with Fe as the matrix does not contain alloying elements other than C, Mn and V.

[0007] The medium manganese steel is a bar manufactured by smelting, forging and heat treatment processes. The metallographic sample of the bar is obtained by electron backscatter diffraction (EBSD). The phase distribution surface shows alternating distribution of martensite and austenite, with martensite accounting for 50-56%, austenite accounting for 38-45%, and cementite accounting for 3-5%.

[0008] The specifications and dimensions of the rod are as follows: diameter not less than 160mm, length not less than 8m; tensile strength of the rod is 1700~2100MPa, total elongation ≥10%, and impact toughness KU2≥20 J.

[0009] The industrial production method of the above-mentioned large-size low-alloy ultra-high strength and toughness medium-manganese steel includes the following steps:

[0010] Step 1: Obtain steel ingots through a smelting process, which includes smelting using an electric furnace + ladle refining + vacuum degassing + ingot casting.

[0011] Step 2: Obtain large-size forgings through a forging process, which includes two upsetting and two drawing operations on the steel ingot, plus precision forging.

[0012] Step 3: Obtain large-size low-alloy ultra-high strength and toughness medium-manganese steel products through heat treatment.

[0013] The smelting process in step 1 includes the following steps:

[0014] Step 1.1: Add the main raw materials, scrap steel, electrolytic manganese metal flakes, and ferrovanadium blocks to the electric furnace, and start heating by turning on the power;

[0015] Step 1.2: Heat and melt for 4 hours ± 10%, blow oxygen to steel and remove phosphorus, slag formation 3 times, slag dumping 2 times, for a total of 2 hours ± 10%. Through multiple slag replacement operations, low-P primary molten steel is obtained.

[0016] Step 1.3 involves refining the low-P primary steel in a refining furnace, including adjusting the alloy composition;

[0017] Step 1.4: After vacuum degassing for 20 minutes, pour the mixture, and then lay the heating plate after pouring.

[0018] Step 2, the forging process includes the following steps:

[0019] Step 2.1: Slowly heat the steel ingot to 400℃, hold for 3 hours, continue heating to 750℃, hold for 3.5 hours, then heat to 1200℃ and hold for 6 hours;

[0020] Step 2.2: Forge the steel ingot at an initial forging temperature of 1180±10℃, upsetting it to 2 / 5 of its original height and drawing it to 3 / 10 of its original height.

[0021] Step 2.3: Heat to 1180±10℃, thicken to 2 / 5 of the original height, and lengthen to 1 / 5 of the original height;

[0022] Step 2.4: Feed the product into the precision forging machine and precision forge it into bars of the corresponding size according to the product requirements. The final forging temperature is ≥750℃. After precision forging, air cool the product.

[0023] Step 3 involves a heat treatment process that includes either of the following two sets:

[0024] First set: Heat the forging to 850℃~880℃, hold for 1~1.2h and then air cool to room temperature, then heat to 180℃~200℃, hold for 0.5~2h, and then air cool to room temperature;

[0025] The second set: Heat the forgings to 670℃~700℃, hold for 0.5~1h, and then water cool to room temperature.

[0026] The technical effects of this invention are as follows: This invention relates to large-size low-alloy ultra-high strength and toughness medium-manganese steel and its industrial production method. By adding Mn and a small amount of V for alloying, Mn can improve the hardenability of the steel and retain more austenite during the cooling process after austenitization; a small amount of V can form nanoscale MC to hinder grain boundary migration, thereby refining the grains. Low-cost and large-scale industrial production is achieved through an electric furnace + ladle refining + vacuum degassing + die casting method, further reducing the manufacturing cost of the steel. After forging and heat treatment control, the microstructure of the steel is ensured to be uniform lath martensite + austenite + finely dispersed MC (i.e., M3C), giving the material both ultra-high strength and good toughness. While satisfying strength and toughness requirements, cost reduction is achieved, with a production price below 42,000 yuan / ton. The mechanical properties of the steel of this invention are subjected to quasi-static tensile testing according to national standards (GB / T 228.1-2018, GB / T229-2007). Heat treatment process 1: tensile strength reaches 2000MPa, total elongation reaches 10%, and room temperature U-notch impact energy reaches 20J. Heat treatment process 2: tensile strength reaches 1700MPa, elongation reaches 20%, and room temperature U-notch impact energy reaches 50J. Attached Figure Description

[0027] Figure 1 This is a physical image of the large-size low-alloy ultra-high strength and toughness medium-manganese steel bar of this invention. Figure 1 The dimensions of the medium bar stock are φ160mm×16m. Figure 1 Medium and large-sized low-alloy ultra-high strength and toughness medium-manganese steel bars are prepared by the industrial production method of this invention.

[0028] Figure 2 This is the stress-strain curve of sample No. 1#-1 in the large-size low-alloy ultra-high strength and toughness medium-manganese steel of this invention. Figure 2 The x-axis represents engineering strain (%, scale values ​​0, 2, ..., 16), and the y-axis represents engineering stress (MPa, scale values ​​0, 400, ..., 2400). The tensile strength / MPa of specimen number 1#-1 is 2101, the yield strength / MPa is 981, the total elongation / % is 11, and the impact toughness KU2 / J is 22.

[0029] Figure 3 This is the stress-strain curve of sample No. 2#-1 in the large-size low-alloy ultra-high strength and toughness medium-manganese steel of this invention. Figure 3 The x-axis represents engineering strain (%, scale values ​​0, 5, ..., 30), and the y-axis represents engineering stress (MPa, scale values ​​0, 400, ..., 2000). The tensile strength / MPa of specimen number 2#-1 is 1703, the yield strength / MPa is 403, the total elongation / % is 23, and the impact toughness KU2 / J is 53.

[0030] Figure 4 This is the phase distribution diagram of sample No. 2#-1 in the large-size low-alloy ultra-high strength and toughness medium-manganese steel of this invention. Figure 4 The phase distribution surface of the metallographic phase is an electron backscatter diffraction (EBSD) pattern, where M (gray) represents martensite, A (red) represents austenite, and M3C (yellow) represents cementite. Martensite accounts for 52.4%, austenite accounts for 39.8%, and cementite accounts for 3.6%. Martensite and austenite are distributed alternately to achieve high strength and toughness. Figure 4 The 10µm is a scale, and 700℃×0.5h is the heat treatment process for forgings, which involves heating the forgings to 700℃, holding them at that temperature for 0.5h, and then water cooling them. Detailed Implementation

[0031] The following is in conjunction with the attached diagram ( Figures 1-4 The present invention will be described in conjunction with the embodiments.

[0032] Figure 1This is a physical image of the large-size low-alloy ultra-high strength and toughness medium-manganese steel bar of this invention. Figure 2 This is the stress-strain curve of sample No. 1#-1 in the large-size low-alloy ultra-high strength and toughness medium-manganese steel of this invention. Figure 3 This is the stress-strain curve of sample No. 2#-1 in the large-size low-alloy ultra-high strength and toughness medium-manganese steel of this invention. Figure 4 This is a phase distribution diagram of sample No. 2#-1 in the large-size low-alloy ultra-high strength and toughness medium-manganese steel of this invention. (Reference) Figures 1 to 4 As shown, the large-size low-alloy ultra-high strength and toughness medium-manganese steel includes the following chemical composition and its wt% content: C=0.30~0.40, Mn=4.00~6.00, V=0.10~0.40, S≤0.0015, P≤0.005, O≤0.0007, N≤0.0015, H≤0.00005, Fe=balance, and the matrix structure does not contain other alloying elements. The medium-manganese steel is a bar stock manufactured through a series of smelting, forging, and heat treatment processes. The metallographic sample of the bar stock, obtained by electron backscatter diffraction (EBSD), shows an alternating distribution of martensite and austenite on the phase surface, with martensite accounting for 50~56%, austenite accounting for 38~45%, and cementite accounting for 3~5%. The specifications and dimensions of the rod are as follows: diameter not less than 160mm (e.g., 160mm or 360mm in the embodiment), length not less than 8m (e.g., 10m or 8m in the embodiment); the tensile strength of the rod is 1700~2100MPa, total elongation ≥10%, and impact toughness KU2≥20 J.

[0033] The industrial production method of the aforementioned large-size low-alloy ultra-high strength and toughness medium-manganese steel includes the following steps: Step 1, obtaining steel ingots through a smelting process, wherein the smelting process includes melting using an electric furnace + ladle refining + vacuum degassing + die casting; Step 2, obtaining large-size forgings through a forging process, wherein the forging process includes two upsetting and two drawing of the steel ingots + precision forging; Step 3, obtaining large-size low-alloy ultra-high strength and toughness medium-manganese steel finished products through a heat treatment process.

[0034] The smelting process in step 1 includes the following steps: Step 1.1, adding the main raw material scrap steel, electrolytic manganese metal flakes and vanadium iron blocks to the electric furnace and starting to heat it; Step 1.2, heating and melting for 4h±10%, oxygen blowing for steelmaking to remove phosphorus, slag formation 3 times, slag dumping 2 times, for a total of 2h±10%, and obtaining low-P primary molten steel through multiple slag replacement operations; Step 1.3, refining the low-P primary molten steel in a refining furnace, including adjusting the alloy composition; Step 1.4, casting after vacuum degassing for 20min, and laying heating plates after casting.

[0035] Step 2, the forging process includes the following steps: Step 2.1, slowly heat the steel ingot to 400℃, hold for 3 hours, continue heating to 750℃, hold for 3.5 hours, then heat to 1200℃ and hold for 6 hours; Step 2.2, forge the steel ingot, with an initial forging temperature of 1180±10℃, upsetting to 2 / 5 of the original height and drawing to 3 / 10 of the original height; Step 2.3, raise the temperature to 1180±10℃, upsetting to 2 / 5 of the original height and drawing to 1 / 5 of the original height; Step 2.4, feed the ingot into a precision forging machine, precision forge it into bars of the corresponding dimensions according to product requirements, with a final forging temperature ≥750℃, and air-cool after precision forging.

[0036] The heat treatment process in step 3 includes either of the following two sets: Set 1: Heat the forging to 850℃~880℃, hold for 1~1.2h and then air cool to room temperature, then heat to 180℃~200℃, hold for 0.5~2h and then air cool to room temperature; Set 2: Heat the forging to 670℃~700℃, hold for 0.5~1h and then water cool to room temperature.

[0037] A composition and industrial production method for a large-scale, low-cost, ultra-high strength and toughness medium-manganese steel, comprising:

[0038] 1. The mass percentages of each component are: C: 0.30–0.40%, Mn: 4.00–6.00%, V: 0.10–0.40%, balance: Fe;

[0039] 2. A low-cost industrial manufacturing process is employed, utilizing an electric furnace + ladle refining + vacuum degassing + die casting + forging. Electric furnace smelting is used for oxygen blowing and dephosphorization treatment, followed by three slag-forming processes to obtain primary molten steel with low phosphorus content, resulting in a phosphorus content below 0.004%. This primary molten steel undergoes ladle refining and vacuum degassing to obtain molten steel with the target composition. This target composition steel is then cast into ingots, with heating plates applied for shrinkage after casting. After cooling to 600°C, the ingots are demolded and annealed to produce large-diameter ingots. These large-diameter ingots are then forged using a gradient heating method to reduce structural stress, prevent cracking, and improve microstructure uniformity. After forging, air cooling is performed to reduce structural stress from the surface to the core of the steel, further preventing cracking and ultimately yielding high-performance, large-diameter manganese steel bars. The finished product is a round bar with a diameter exceeding 160mm and a length exceeding 10m.

[0040] 3. By controlling the microstructure through various processes, the tensile strength of the steel can reach 1700-2100MPa, while also possessing excellent plasticity and toughness.

[0041] The specific preparation process is as follows:

[0042] Smelting process: Electric furnace + ladle refining + vacuum degassing smelting, and then casting into steel ingots;

[0043] Forging process: The smelted steel ingot undergoes a forging process of two upsetting and two drawing plus precision forging; the specific steps are as follows: Step 1: Gradual heating to 1200℃; Step 2: Initial forging temperature is 1180±10℃, upsetting to 2 / 5 of the original height, drawing to 3 / 10 of the original height; Step 3: Heating to 1180±10℃, upsetting to 2 / 5 of the original height, drawing to 1 / 5 of the original height; Step 4: Entering the precision forging machine, precision forging into bars of the corresponding size according to product requirements. The final forging temperature of the above process is ≥750℃, and air cooling is performed after precision forging.

[0044] Heat treatment process:

[0045] (1) Heat the forging to 850℃~880℃, hold for 1~1.2h and then air cool to room temperature. Then heat to 180℃~200℃, hold for 0.5~2h and then air cool to room temperature.

[0046] (2) Heat the forging to 670℃~700℃, hold for 0.5~1h and then cool to room temperature with water.

[0047] Example: Experimental steel was smelted using an electric furnace + ladle refining + vacuum degassing + ingot casting method. Each steel ingot weighed 5t, and the chemical composition is shown in Table 1.

[0048] The smelted steel ingots undergo a No. 1 forging process consisting of two upsetting and two drawing operations followed by precision forging. The specific steps are as follows: Step 1: Gradual heating, slowly heating the steel ingot to 400℃ and holding for 3 hours, then continuing to heat to 750℃ and holding for 3.5 hours, followed by heating to 1200℃ and holding for 6 hours; Step 2: Initial forging temperature is 1180±10℃, upsetting to 2 / 5 of the original height and drawing to 3 / 10 of the original height; Step 3: Heating to 1180±10℃, upsetting to 2 / 5 of the original height and drawing to 1 / 5 of the original height; Step 4: Entering the precision forging machine, precision forging into a φ160mm bar with a total length of 16m. The final forging temperature of the above process is ≥750℃, and the precision forging is followed by air cooling.

[0049] The smelted steel ingots undergo a No. 2 forging process consisting of two upsetting and two drawing operations followed by precision forging. The specific steps are as follows: Step 1: Gradual heating, slowly heating the steel ingot to 400℃ and holding for 3 hours, then continuing to heat to 750℃ and holding for 3.5 hours, followed by heating to 1200℃ and holding for 6 hours; Step 2: Initial forging temperature is 1180±10℃, upsetting to 2 / 5 of the original height and drawing to 3 / 10 of the original height; Step 3: Heating to 1180±10℃, upsetting to 2 / 5 of the original height and drawing to 1 / 5 of the original height; Step 4: Entering the precision forging machine, precision forging into a φ360mm bar with a total length of 8m. The final forging temperature of the above process is ≥750℃, and the precision forging is followed by air cooling.

[0050] Table 1. Chemical composition (wt, %) of embodiments of the present invention

[0051]

[0052] Table 2, Heat Treatment Process Parameters

[0053]

[0054] Table 3, Quasi-static Mechanical Properties

[0055]

[0056] In Tables 2 and 3 above, 1#-1 refers to the large-size low-alloy ultra-high strength and toughness medium-manganese steel bar of the present invention with a diameter of 160mm×16m and using the first heat treatment process; 1#-2 refers to the large-size low-alloy ultra-high strength and toughness medium-manganese steel bar of the present invention with a diameter of 160mm×16m and using the second heat treatment process; 2#-1 refers to the large-size low-alloy ultra-high strength and toughness medium-manganese steel bar of the present invention with a diameter of 360mm×8m and using the first heat treatment process; and 2#-2 refers to the large-size low-alloy ultra-high strength and toughness medium-manganese steel bar of the present invention with a diameter of 360mm×8m and using the second heat treatment process.

[0057] Contents not described in detail in this specification are existing technologies known to those skilled in the art. It is hereby indicated that the above description is intended to help those skilled in the art understand the present invention, but does not limit the scope of protection of the present invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essence of the present invention fall within the scope of protection of the present invention.

Claims

1. Large-size low-alloy ultra-high strength and toughness medium-manganese steel, characterized in that, The chemical composition and its wt% content include the following: C=0.30~0.40, Mn=4.00~6.00, V=0.10~0.40, S≤0.0015, P≤0.005, O≤0.0007, N≤0.0015, H≤0.00005, Fe=balance. The Fe-based microstructure does not contain alloying elements other than C, Mn and V. The medium manganese steel is a bar manufactured through a series of smelting, forging, and heat treatment processes. The metallographic sample of the bar is obtained by electron backscatter diffraction (EBSD). The phase distribution on the surface shows an alternating distribution of martensite and austenite, with martensite accounting for 50-56%, austenite accounting for 39.8-45%, and cementite accounting for 3-5%. The specifications and dimensions of the rod are as follows: diameter not less than 160mm, length not less than 8m; tensile strength of the rod is 1700~2100MPa, total elongation ≥10%, and impact toughness KU2≥20 J.

2. The industrial production method of large-size low-alloy ultra-high strength and toughness medium-manganese steel as described in claim 1, characterized in that, Includes the following steps: Step 1: Obtain steel ingots through a smelting process, which includes smelting using an electric furnace + ladle refining + vacuum degassing + ingot casting. Step 2: Obtain large-size forgings through a forging process, which includes two upsetting and two drawing operations on the steel ingot, plus precision forging. Step 3: Obtain large-size low-alloy ultra-high strength and toughness medium-manganese steel finished products through heat treatment process.

3. The industrial production method according to claim 2, characterized in that, The smelting process in step 1 includes the following steps: Step 1.1: Add the main raw materials, scrap steel, electrolytic manganese metal flakes, and ferrovanadium blocks to the electric furnace, and start heating by turning on the power; Step 1.2: Heat and melt for 4 hours ± 10%, blow oxygen to steel and remove phosphorus, slag formation 3 times, slag dumping 2 times, for a total of 2 hours ± 10%. Through multiple slag replacement operations, low-P primary steel is obtained. Step 1.3 involves refining the low-P primary steel in a refining furnace, including adjusting the alloy composition; Step 1.4: After vacuum degassing for 20 minutes, pour the mixture, and then lay the heating plate after pouring.

4. The industrial production method according to claim 2, characterized in that, Step 2, the forging process includes the following steps: Step 2.1: Slowly heat the steel ingot to 400℃, hold for 3 hours, continue heating to 750℃, hold for 3.5 hours, then heat to 1200℃ and hold for 6 hours; Step 2.2: Forge the steel ingot at an initial forging temperature of 1180±10℃, upsetting it to 2 / 5 of its original height and drawing it to 3 / 10 of its original height. Step 2.3: Heat to 1180±10℃, thicken to 2 / 5 of the original height, and lengthen to 1 / 5 of the original height; Step 2.4: Feed the product into the precision forging machine and precision forge it into bars of the corresponding size according to the product requirements. The final forging temperature is ≥750℃. After precision forging, air cool the product.

5. The industrial production method according to claim 2, characterized in that, Step 3 involves a heat treatment process that includes either of the following two sets: First set: Heat the forging to 850℃~880℃, hold for 1~1.2h and then air cool to room temperature, then heat to 180℃~200℃, hold for 0.5~2h, and then air cool to room temperature; The second set: Heat the forgings to 670℃~700℃, hold for 0.5~1h, and then water cool to room temperature.

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