High-strength and high-toughness multiphase high-manganese steel and preparation method thereof

Abstract

The invention provides high-strength and high-toughness multiphase high-manganese steel and a preparation method thereof. The steel comprises the following chemical components in percentage by weight: less than or equal to 0.08% of C, less than or equal to 0.30% of Si, 2.0%-5.0% of Mn, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.010%-0.031% of Al and 0.035%-0.050% of Nb. The method is low in production cost, simple in process and suitable for large-scale industrial stable production; the product has the characteristics of high strength and excellent low-temperature toughness.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgy, and in particular to a high-strength and high-toughness multiphase high-manganese steel and a preparation method thereof. Background Art

[0002] Multiphase steel is a high-strength steel with a complex microstructure, typically composed of multiple phases such as ferrite, martensite, bainite, and austenite. Due to its high strength and excellent formability, multiphase steel is widely used in the automotive, aerospace, and construction industries. Adding manganese to multiphase steel can improve its strength and hardness. However, when the manganese content is too high, the manganese in the multiphase steel will co-accumulate with other elements such as carbon, phosphorus, and sulfur at grain boundaries, phase boundaries, or other defects, resulting in co-segregation, which significantly reduces the toughness of the multiphase steel.

[0003] Chinese patent CN118979195A discloses a low yield ratio, high strength and toughness marine steel and a preparation method thereof, wherein the elemental composition thereof is, by mass percentage, C 0.08%-0.1%, Si 0.30%-0.35%, Mn 1.05%-1.25%, Ni 1.80%-2.10%, Mo 0.30%-0.35%, Cr 0.13%-0.16%, Ti 0.008%-0.010%, V 0.015%-0.030%, Nb0.045%-0.050%, B 0.0010%-0.0013%, Cu 0.08%-0.12%, Al 0.055%-0.060%, P≤0.010%, S≤0.0040%, carbon equivalent CET≤0.31, and the balance is Fe and unavoidable impurities. This patent improves the strength and toughness by adding Cr, Ni, Mo and Cu elements to the steel to produce dispersed second phase strengthening and fine grain strengthening, and produces marine steel with an elongation after fracture A ≥ 18%. Although this patent improves the toughness of the marine steel to a certain extent, the production cost is high. In addition, since the C and Mn content in the marine steel are both low, the yield strength of the marine steel produced by the method described in this patent is 703.5MPa-761.5MPa, and the tensile strength is 861.2MPa-880.3MPa. Both the yield strength and tensile strength are not high. In addition, the -40℃ impact energy of the marine steel produced by the method described in this patent is 80.35J-161.52J, and the low-temperature toughness is poor. Summary of the Invention

[0004] In response to the technical problem that existing technologies are difficult to synergistically improve the strength and toughness of multi-phase steel, the present invention provides a high-strength and high-toughness multi-phase high-manganese steel and a preparation method thereof. By controlling the ratios of elements such as C, Si, Mn, B, Nb, Ca and S in the multi-phase steel, the strength and toughness of the multi-phase steel are synergistically improved while reducing production costs.

[0005] The technical solutions of the present invention are as follows: In a first aspect, the present invention provides a high-strength and high-toughness multiphase high manganese steel, comprising the following chemical composition, in percentage by mass: C ≤0.08%, Si ≤0.30%, Mn 2.0%-5.0%, P ≤0.015%, S ≤0.005%, Al 0.010%-0.031%, Nb 0.035%-0.050%, B 0.0010%-0.0020%, Ti 0.0012%-0.030%, Ca 0.0025%-0.0040%, and the remainder being Fe and unavoidable impurities; Among them, Mn / C ≥ 55, 0.8 ≤ Ca / S ≤ 3.0; 9.5≤6×(%C)+1.3×(%Si)+2.1×(%Mn)+10×(%B)≤10.5; (%Ca)(%S) 1 / 6 ≤1.5×10 -3 ; %C, %Si, %Mn, %B, %Ca, and %S are the values before the percentage sign in the mass percentages of C, Si, Mn, B, Ca, and S, respectively.

[0006] Furthermore, the following chemical components are included, in terms of mass percentage: C 0.06%-0.08%, Si 0.21%-0.30%, Mn 4.0%-5.0%, P 0.012%-0.015%, S 0.001%-0.005%, Al 0.029%-0.031%, Nb 0.043%-0.050%, B 0.0015%-0.0020%, Ti 0.015%-0.030%, Ca 0.0028%-0.0036%, and the rest are Fe and unavoidable impurities.

[0007] Chemical composition is one of the important factors affecting the comprehensive performance of steel. The present invention defines the chemical composition of high-strength, high-toughness, multiphase, high-manganese steel as follows: Carbon (C): Carbon is the primary solid-solution strengthening element in steel, significantly increasing its strength. However, a high carbon content is detrimental to weldability, toughness, and ductility. To effectively reduce segregation, improve microstructural uniformity, and avoid potential differences between different phases that cause galvanic corrosion and reduce the corrosion resistance of the steel, while also considering economic efficiency, the present invention limits the carbon content to ≤ 0.08% by mass.

[0008] Mn: Manganese, as the most important alloying element in steel, has the function of expanding the austenite phase, lowering the Ac3, Ac1, Ar3, and Ar1 point temperatures, and refining the grain size, thereby improving the low-temperature toughness of the steel plate. However, Mn is prone to segregation during the solidification process of molten steel. Especially when the Mn content is high, the Mn element not only makes the casting operation difficult, but also easily segregates with elements such as C, P, and S. To avoid the adverse effects of high Mn content, the present invention limits the mass percentage of Mn to 2.0%-5.0%.

[0009] Si: Silicon is one of the effective deoxidizing and exothermic elements in the steelmaking process and has a certain solid solution strengthening effect. However, high Si content seriously impairs the low-temperature toughness, elongation and weldability of the steel plate. Especially under high line energy welding conditions, Si can promote the formation of MA islands. The MA islands formed are relatively large and unevenly distributed, seriously impairing the toughness of the weld heat-affected zone (HAZ). Therefore, the Si content in steel should be kept as low as possible. Considering the economy and operability of the steelmaking process, the present invention controls the Si content to below 0.30%.

[0010] P: Phosphorus is an element that easily segregates, which increases the cold brittleness of steel and deteriorates welding performance. Considering the economy and operability of the steelmaking process, the present invention controls the P content to below 0.015%.

[0011] S: Sulfur significantly impairs the low-temperature toughness of steel. More importantly, S combines with Mn in steel to form MnS inclusions. During rolling, MnS extends along the rolling direction, forming a MnS inclusion band, which severely impairs the low-temperature impact toughness, elongation, Z-axis performance, and weldability of the steel plate. S is also the primary element responsible for hot brittleness, and theoretically, the lower the S content, the better. However, considering the operability, cost, and smooth logistics of the steelmaking process, and to ensure excellent weldability and -40°C low-temperature toughness of the steel plate, the present invention controls the S content to ≤0.005%.

[0012] Nb: Niobium is a key element for grain refinement. On the one hand, niobium can increase the austenite recrystallization temperature, inhibiting recrystallization and grain growth, thereby refining the austenite grains. On the other hand, niobium carbonitrides precipitate on dislocations and segregate at austenite grain boundaries, contributing to improved strength and toughness. However, excessive niobium content can easily cause surface cracks in the ingot and degrade weldability. Therefore, the present invention limits the niobium content to 0.035%-0.050%.

[0013] Ti: Titanium has a strong affinity for nitrogen. When a small amount of Ti is added, nitrogen preferentially combines with Ti to form dispersed TiN particles, which inhibit excessive growth of austenite grains during slab heating and hot rolling, thereby improving the low-temperature toughness of the steel plate. More importantly, the addition of Ti can, to a certain extent, inhibit grain growth in the heat-affected zone (the area farther from the fusion line) during high heat input welding, thereby improving the toughness of the heat-affected zone. If the Ti content is too low (≤0.006%), the effect is not significant. When the Ti content exceeds 0.030%, further increasing the Ti content in the steel has little effect on grain refinement and weldability. In fact, when the Ti / N ratio is too large, it is not conducive to grain refinement and may even worsen the weldability of the steel plate. Therefore, the present invention controls the Ti content within the range of 0.012%-0.030%.

[0014] Boron (B): Compared to manganese, boron has a stronger binding ability with carbon, and its reactivity makes it more absorbable than iron. After boron alloying, the addition of boron makes it easier to combine with carbon and nitrogen in the molten steel to form stable borides. The formation of these compounds not only improves the distribution of inclusions in the molten steel, but also, due to their high melting points and hardness, they can serve as nucleation sites, resulting in heterogeneous nucleation, inhibiting grain growth and refining the grains. Trace amounts of boron in the steel dissolve in austenite and segregate at austenite grain boundaries, thereby reducing the grain boundary energy. This makes it difficult for ferrite to nucleate at austenite grain boundaries during austenite decomposition, prolonging the incubation period for the γ→α phase transformation and improving austenite stability.

[0015] Furthermore, the microstructure of the high-strength and high-toughness multiphase high manganese steel is a multiphase structure containing lath martensite, retained austenite, tempered troostite, acicular ferrite and pearlite.

[0016] Furthermore, in terms of volume fraction, the microstructure of the high-strength and high-toughness multiphase high manganese steel comprises 80%-88% lath martensite, 3.0%-5.5% retained austenite, 3.0%-5.0% tempered troostite, 4.0%-6.0% acicular ferrite, and a small amount of pearlite.

[0017] Furthermore, the yield strength of the high-strength and high-toughness multiphase high manganese steel is ≥690MPa, the tensile strength of the high-strength and high-toughness multiphase high manganese steel is ≥910MPa, the elongation after fracture of the high-strength and high-toughness multiphase high manganese steel is ≥18.0%, and the impact energy of the high-strength and high-toughness multiphase high manganese steel at -40℃ KV2 is ≥180J.

[0018] Furthermore, the yield strength of the high-strength and high-toughness multiphase high manganese steel is ≥721MPa, the tensile strength of the high-strength and high-toughness multiphase high manganese steel is ≥965MPa, the elongation after fracture of the high-strength and high-toughness multiphase high manganese steel is ≥21.0%, and the impact energy of the high-strength and high-toughness multiphase high manganese steel at -40℃ KV2 is ≥226J.

[0019] In a second aspect, the present invention further provides a method for preparing the high-strength and high-toughness multiphase high manganese steel as described above, comprising the following steps: Step 1: rolling, adopting two-stage rolling, the first stage rolling is rough rolling, and the first stage rolling is carried out in the complete recrystallization zone; the second stage rolling is finish rolling, and the second stage rolling is carried out completely in the non-recrystallization zone; the starting rolling temperature of the second stage rolling is Trolling ± 20 °C; wherein Trolling is the temperature of the rolled piece during rolling, and the unit of Trolling is °C; Trolling is calculated according to the following formula: T rolling = 562+163×(%C)+83×(%Mn)+18×(%Ti)+36×(%Al)+1370×(%Nb), Wherein, %Ti, %Al, and %Nb are the values before the percentage sign in the mass percentages of Ti, Al, and Nb, respectively; Step 2: After rolling, the steel plate surface temperature is cooled to ≥900℃; Step 3: Two-stage water cooling is adopted. After the first stage of water cooling, the surface temperature of the steel plate is 400-430°C, and the cooling rate of the first stage of water cooling is 18-25°C / s; after the first stage of water cooling for 30-60s, the second stage of water cooling is carried out. After the second stage of water cooling, the surface temperature of the steel plate is 200-250°C, and the cooling rate of the second stage of water cooling is 15-20°C / s. After the second stage of water cooling is completed, the finished product is obtained.

[0020] The present invention utilizes first-stage water cooling and second-stage water cooling to promote the steel plate to form a multiphase structure containing retained austenite, lath martensite, troostite and acicular ferrite.

[0021] Furthermore, before rolling, the slab is cast, heated in a furnace and descaled.

[0022] Furthermore, during the casting process, the surface temperature of the cast billet out of the furnace is 1160-1190°C.

[0023] The beneficial effects of the present invention are: The present invention provides a high-strength and high-toughness multiphase high-manganese steel and a preparation method thereof. By limiting the chemical composition and the corresponding mass percentages of the high-strength and high-toughness multiphase high-manganese steel, and limiting the relationship between the contents of the C element, Si element, Mn element and B element to: 9.5≤6×(%C)+1.3×(%Si)+2.1×(%Mn)+10×(%B)≤10.5, on the one hand, the segregation of C, Mn, P and S during the solidification of molten steel is effectively suppressed, the segregation in the center of the steel plate and the associated inclusions (the associated inclusions are mainly MnS, aluminum silicon oxides, etc.) are reduced, and the low-temperature toughness of the steel plate is improved; on the other hand, the martensitic phase transformation temperature and the stability of retained austenite are increased.

[0024] By limiting Mn / C to ≥55, the low-temperature toughness of the steel plate is ensured, so that the fracture fiber rate of the Charpy impact specimen of the steel plate at -40°C is at least higher than 60%, and the pearlite crystals in the microstructure are small in size and evenly distributed.

[0025] By limiting the relationship between the content of Ca and S elements, the following effects are achieved: Ca treatment of steel can, on the one hand, further purify the molten steel, and on the other hand, denature the sulfides in the steel, turning them into non-deformable, stable, and fine spherical sulfides, suppressing the thermal brittleness of S, and improving the low-temperature toughness, elongation, and Z-direction properties of the steel plate. The amount of Ca added depends on the S content in the steel. If the amount of Ca added is too low, the treatment effect will be small; if the amount of Ca added is too high, the size of the Ca(O,S) will be too large, and the brittleness of the steel will also increase, which may become the starting point of fracture cracks, reducing the low-temperature toughness and elongation of the steel, while also reducing the purity of the steel, polluting the molten steel, and deteriorating the crack arrest properties of the steel plate. The present invention limits 0.8≤Ca / S≤3.0, and at the same time limits (%Ca)(%S) 1 / 6 ≤1.5×10 -3 , ensuring the spheroidization of sulfides in the steel to improve the low-temperature toughness, elongation and Z-direction properties of the steel plate.

[0026] In summary, the present invention utilizes boron to combine with C and N in molten steel to generate stable borides, thereby improving the distribution of inclusions in the molten steel. Borides with high melting point and high hardness are used as nucleation cores to inhibit grain growth, refine grains, improve the stability of austenite, and reduce the addition cost.

[0027] The reasonable matching relationship of carbon, silicon and manganese (9.5≤6×(%C)+1.3×(%Si)+2.1×(%Mn)+10×(%B)≤10.5) is used to improve the martensite phase transformation temperature and the stability of retained austenite. The first-stage water cooling and the second-stage water cooling are used to promote the formation of a multiphase structure of a small amount of lath martensite + retained austenite + tempered troostite + acicular ferrite + pearlite.

[0028] The high manganese steel designed by the present invention is produced through a controlled rolling and controlled cooling process, has a short production cycle and low cost, and is conducive to popularization and application. DETAILED DESCRIPTION

[0029] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention are clearly and completely described below. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work should fall within the scope of protection of the present invention.

[0030] Example 1 A high-strength and high-toughness multiphase high-manganese steel, comprising, by mass percentage, the following chemical composition: C 0.06%, Si 0.21%, Mn 4.5%, P 0.013%, S 0.001%, Al 0.031%, Nb 0.043%, B 0.0018%, Ti 0.016%, Ca 0.0028%, with the remainder being Fe and unavoidable impurities; Among them, Mn / C=75, Ca / S=2.8; 6×(%C)+1.3×(%Si)+2.1×(%Mn)+10×(%B)≈10.10; (%Ca)(%S) 1 / 6 =0.89×10 -3 ; %C, %Si, %Mn, %B, %Ca, and %S are the values before the percentage sign in the mass percentages of C, Si, Mn, B, Ca, and S, respectively.

[0031] The microstructure of the high-strength and high-toughness multiphase high manganese steel is a multiphase structure containing lath martensite, retained austenite, tempered troostite, acicular ferrite and pearlite. In terms of volume fraction, the microstructure of the high-strength and high-toughness multiphase high manganese steel has a tissue content of 83% of lath martensite, a tissue content of 4.3% of retained austenite, a tissue content of 3.3% of tempered troostite, a tissue content of 4.2% of acicular ferrite, and the rest is pearlite.

[0032] The thickness, yield strength, tensile strength, elongation after fracture and yield strength ratio of the high-strength and high-toughness multiphase high manganese steel are shown in Table 1. The impact energy of the high-strength and high-toughness multiphase high manganese steel at -40°C KV2 is shown in Table 2.

[0033] The method for preparing the high-strength and high-toughness multiphase high manganese steel as described above comprises the following steps: Before rolling, the casting process is followed by furnace heating and descaling. During the casting process, the surface temperature of the cast billet out of the furnace is 1160°C, and the billet specifications (thickness × width) are 200 × 2230 mm. During the furnace heating process, the billet is heated in the furnace for 195 minutes.

[0034] Step 1: Rolling, using two-stage rolling. The first stage is rough rolling, which is carried out in the fully recrystallized zone. After the first stage, the intermediate billet thickness is 75mm. The second stage is finishing rolling, which is carried out completely in the non-recrystallized zone. T rolling is calculated according to the following formula: T rolling = 562 + 163 × (% C) + 83 × (% Mn) + 18 × (% Ti) + 36 × (% Al) + 1370 × (% Nb) ≈ 1005 ° C; Among them, Troll is the temperature of the rolled piece during rolling, and the unit of Troll is ℃; %Ti, %Al, and %Nb are the values before the percentage sign in the mass percentages of Ti, Al, and Nb, respectively. The starting temperature of the finishing rolling is determined to be 1000°C based on T rolling.

[0035] After finishing rolling, the rolled product is obtained, and the specifications of the rolled product (thickness × width) are 20 × 2800 mm.

[0036] Step 2: After rolling, the steel plate surface temperature is cooled to 910℃; Step 3: Two-stage water cooling is adopted. After the first stage of water cooling, the surface temperature of the final cooled steel plate is 415°C, and the cooling rate of the first stage of water cooling is 22°C / s. After the first stage of water cooling is carried out for 50s, the second stage of water cooling is carried out. After the second stage of water cooling, the surface temperature of the final cooled steel plate is 250°C, and the cooling rate of the second stage of water cooling is 18°C / s. After the second stage of water cooling is completed, the finished product is obtained.

[0037] Example 2 A high-strength and high-toughness multiphase high-manganese steel, comprising, by mass percentage, the following chemical composition: C 0.06%, Si 0.26%, Mn 4.2%, P 0.012%, S 0.002%, Al 0.029%, Nb 0.044%, B 0.0015%, Ti 0.015%, Ca 0.0036%, with the remainder being Fe and unavoidable impurities; Among them, Mn / C=70, Ca / S=1.8; 6×(%C)+1.3×(%Si)+2.1×(%Mn)+10×(%B)≈9.53; (%Ca)(%S) 1 / 6 ≈1.27×10 -3 ; %C, %Si, %Mn, %B, %Ca, and %S are the values before the percentage sign in the mass percentages of C, Si, Mn, B, Ca, and S, respectively.

[0038] The microstructure of the high-strength and high-toughness multiphase high manganese steel is a multiphase structure containing lath martensite, retained austenite, tempered troostite, acicular ferrite and pearlite. In terms of volume fraction, the microstructure of the high-strength and high-toughness multiphase high manganese steel is composed of 84% lath martensite, 3.6% retained austenite, 3.9% tempered troostite, 5.3% acicular ferrite, and the rest is pearlite.

[0039] The thickness, yield strength, tensile strength, elongation after fracture and yield strength ratio of the high-strength and high-toughness multiphase high manganese steel are shown in Table 1. The impact energy of the high-strength and high-toughness multiphase high manganese steel at -40°C KV2 is shown in Table 2.

[0040] The method for preparing the high-strength and high-toughness multiphase high manganese steel as described above comprises the following steps: Before rolling, the casting process is followed by furnace heating and descaling. During the casting process, the surface temperature of the cast billet out of the furnace is 1166°C, and the billet specifications (thickness × width) are 300 × 2200 mm. During the furnace heating process, the billet is heated in the furnace for 331 minutes.

[0041] Step 1: Rolling, using two-stage rolling. The first stage is rough rolling, which is carried out in the fully recrystallized zone. After the first stage, the intermediate billet thickness is 140mm. The second stage is finishing rolling, which is carried out completely in the non-recrystallized zone. T rolling is calculated according to the following formula: T rolling = 562 + 163 × (% C) + 83 × (% Mn) + 18 × (% Ti) + 36 × (% Al) + 1370 × (% Nb) ≈ 982 ° C; Among them, Troll is the temperature of the rolled piece during rolling, and the unit of Troll is ℃; %Ti, %Al, and %Nb are the values before the percentage sign in the mass percentages of Ti, Al, and Nb, respectively. The starting temperature of the finishing rolling is determined to be 980°C based on T rolling.

[0042] After finishing rolling, the rolled product is obtained, and the specifications of the rolled product (thickness × width) are 50×2770mm.

[0043] Step 2: After rolling, the steel plate surface temperature is cooled to 915°C; Step 3: Two-stage water cooling is adopted. After the first stage of water cooling, the surface temperature of the final cooled steel plate is 422°C, and the cooling rate of the first stage of water cooling is 20°C / s. After the first stage of water cooling is carried out for 55 seconds, the second stage of water cooling is carried out. After the second stage of water cooling, the surface temperature of the final cooled steel plate is 210°C, and the cooling rate of the second stage of water cooling is 16°C / s. After the second stage of water cooling is completed, the finished product is obtained.

[0044] Comparative Example 1 A multiphase high manganese steel comprising, by mass percentage, the following chemical composition: C 0.07%, Si 0.24%, Mn 3.5%, P 0.012%, S 0.003%, Al 0.035%, Nb 0.043%, B 0.0023%, Ti 0.017%, Ca 0.005%, and the remainder being Fe and unavoidable impurities; Among them, Mn / C=50, Ca / S≈1.67; 6×(%C)+1.3×(%Si)+2.1×(%Mn)+10×(%B)=8.11; (%Ca)(%S) 1 / 6 ≈1.90×10 -3 ; %C, %Si, %Mn, %B, %Ca, and %S are the values before the percentage sign in the mass percentages of C, Si, Mn, B, Ca, and S, respectively.

[0045] The microstructure of the high-strength and high-toughness multiphase high manganese steel is a multiphase structure containing lath martensite, retained austenite, tempered troostite, and acicular ferrite. In terms of volume fraction, the microstructure of the high-strength and high-toughness multiphase high manganese steel has a tissue content of 92% of lath martensite, a tissue content of 1% of retained austenite, a tissue content of 6.4% of tempered troostite, and the rest is ferrite.

[0046] The thickness, yield strength, tensile strength, elongation after fracture and yield strength ratio of the high-strength and high-toughness multiphase high manganese steel are shown in Table 1. The impact energy of the high-strength and high-toughness multiphase high manganese steel at -40°C KV2 is shown in Table 2.

[0047] The method for preparing the high-strength and high-toughness multiphase high manganese steel as described above comprises the following steps: Before rolling, the casting process is followed by furnace heating and descaling. During the casting process, the surface temperature of the cast billet out of the furnace is 1166°C, and the billet specifications (thickness × width) are 200 × 2230 mm. During the furnace heating process, the billet is heated in the furnace for 196 minutes.

[0048] Step 1: Rolling, using two-stage rolling. The first stage is rough rolling, which is carried out in the fully recrystallized zone. After the first stage, the intermediate billet thickness is 75mm. The second stage is finishing rolling, which is carried out completely in the non-recrystallized zone. The starting rolling temperature is 930℃. T rolling is calculated according to the following formula: T rolling = 562 + 163 × (% C) + 83 × (% Mn) + 18 × (% Ti) + 36 × (% Al) + 1370 × (% Nb) ≈ 924 ° C; Among them, Troll is the temperature of the rolled piece during rolling, and the unit of Troll is ℃; %Ti, %Al, and %Nb are the values before the percentage sign in the mass percentages of Ti, Al, and Nb, respectively.

[0049] After finishing rolling, the rolled product is obtained, and the specifications of the rolled product (thickness × width) are 20 × 2800 mm.

[0050] Step 2: After rolling, the steel plate surface temperature is cooled to 840°C; Step 3: One-stage water cooling is adopted. After the first stage of water cooling, the surface temperature of the final cooled steel plate is 246°C. The cooling rate of the first stage of water cooling is 23°C / s. After the first stage of water cooling is completed, the finished product is obtained.

[0051] Table 1 Tensile properties of finished steel plates obtained in Example 1, Example 2 and Comparative Example 1

[0052] Table 2 Low temperature toughness of finished steel plates obtained in Example 1, Example 2 and Comparative Example 1

[0053] In Comparative Example 1, 6×(%C)+1.3×(%Si)+2.1×(%Mn)+10×(%B)=8.11, which is a relatively low value, and the austenite stability is poorer than that of Examples 1 and 2.

[0054] (%Ca)(%S) 1 / 6 ≈1.90×10 -3 , the value is large, and the purity of the molten steel is worse than that of Example 1 and Example 2.

[0055] Comparative Example 1 adopts conventional low-temperature controlled rolling and one-stage water cooling process to generate martensite and retained austenite. The retained austenite is unstable and decomposes into ferrite and carbide. There is too much hard phase structure and too little soft phase structure to absorb energy during the impact process.

[0056] The above results in that the comparative example 1 has high strength and poor toughness, and the overall performance index is far lower than that of the examples 1 and 2.

[0057] Although the present invention has been described in detail with reference to preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and substance of the present invention, persons of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any changes or substitutions that can be readily conceived by persons skilled in the art within the technical scope disclosed in the present invention shall be within the scope of protection of the present invention.

Claims

1. A high-strength and high-toughness multiphase high-manganese steel, characterized in that: Calculated in percentage by mass, it includes the following chemical compositions: C ≤0.08%, Si ≤0.30%, Mn 2.0%-5.0%, P ≤0.015%, S ≤0.005%, Al 0.010%-0.031%, Nb 0.035%-0.050%, B 0.0010%-0.0020%, Ti 0.0012%-0.030%, Ca 0.0025%-0.0040%, and the remainder is Fe and unavoidable impurities; Among them, Mn / C ≥ 55, 0.8 ≤ Ca / S ≤ 3.0; 9.5≤6×(%C)+1.3×(%Si)+2.1×(%Mn)+10×(%B)≤10.5; (%Ca)(%S) 1 / 6 ≤1.5×10 -3 ; %C, %Si, %Mn, %B, %Ca, and %S are the values before the percentage sign in the mass percentages of C, Si, Mn, B, Ca, and S, respectively.

2. The high-strength and high-toughness multiphase high manganese steel according to claim 1, characterized in that: Calculated in mass percentage, it includes the following chemical components: C 0.06%-0.08%, Si 0.21%-0.30%, Mn 4.0%-5.0%, P 0.012%-0.015%, S0.001%-0.005%, Al 0.029%-0.031%, Nb 0.043%-0.050%, B 0.0015%-0.0020%, Ti 0.015%-0.030%, Ca 0.0028%-0.0036%, and the rest is Fe and unavoidable impurities.

3. A high-strength and high-toughness multiphase high manganese steel according to claim 1 or 2, characterized in that: The microstructure of high-strength and high-toughness multiphase high manganese steel is a multiphase structure containing lath martensite, retained austenite, tempered troostite, acicular ferrite and pearlite.

4. The high-strength and high-toughness multiphase high-manganese steel according to claim 3, characterized in that: Calculated by volume fraction, the microstructure of high-strength and high-toughness multiphase high manganese steel has a tissue content of lath martensite of 80%-88%, a tissue content of retained austenite of 3.0%-5.5%, a tissue content of tempered troostite of 3.0%-5.0%, a tissue content of acicular ferrite of 4.0%-6.0%, and the rest is a small amount of pearlite.

5. The high-strength and high-toughness multiphase high-manganese steel according to claim 1, characterized in that: The yield strength of high-strength and high-toughness multiphase high manganese steel is ≥690MPa, the tensile strength of high-strength and high-toughness multiphase high manganese steel is ≥910MPa, the elongation after fracture of high-strength and high-toughness multiphase high manganese steel is ≥18.0%, and the impact energy of high-strength and high-toughness multiphase high manganese steel at -40℃ KV2 is ≥180J.

6. The high-strength and high-toughness multiphase high-manganese steel according to claim 5, characterized in that: The yield strength of high-strength and high-toughness multiphase high manganese steel is ≥721MPa, the tensile strength of high-strength and high-toughness multiphase high manganese steel is ≥965MPa, the elongation after fracture of high-strength and high-toughness multiphase high manganese steel is ≥21.0%, and the impact energy of high-strength and high-toughness multiphase high manganese steel at -40℃ KV2 is ≥226J.

7. A method for preparing the high-strength and high-toughness multiphase high-manganese steel according to any one of claims 1 to 6, characterized in that: The steps include: Step 1: rolling, adopting two-stage rolling, the first stage rolling is rough rolling, the first stage rolling is carried out in the complete recrystallization zone; the second stage rolling is finish rolling, the second stage rolling is carried out completely in the non-recrystallization zone; the starting rolling temperature of the second stage rolling is Trolling ± 20 °C; wherein Trolling is the temperature of the rolled piece during rolling, and the unit of Trolling is °C; Trolling is calculated according to the following formula: T rolling = 562+163×(%C)+83×(%Mn)+18×(%Ti)+36×(%Al)+1370×(%Nb), Wherein, %Ti, %Al, and %Nb are the values before the percentage sign in the mass percentages of Ti, Al, and Nb, respectively; Step 2: After rolling, the steel plate surface temperature is cooled to ≥900℃; Step 3: Two-stage water cooling is adopted. After the first stage of water cooling, the surface temperature of the steel plate is 400-430°C, and the cooling rate of the first stage of water cooling is 18-25°C / s; after the first stage of water cooling for 30-60s, the second stage of water cooling is carried out. After the second stage of water cooling, the surface temperature of the steel plate is 200-250°C, and the cooling rate of the second stage of water cooling is 15-20°C / s. After the second stage of water cooling is completed, the finished product is obtained.

8. The preparation method according to claim 7, wherein Before rolling, the billet is cast, heated in a furnace and descaled.

9. The preparation method according to claim 8, wherein During the casting process, the surface temperature of the casting billet out of the furnace is 1160-1190℃.

Images