Boron-yttrium high-manganese steel and preparation method thereof

Abstract

The invention relates to the technical field of metallurgy, in particular to boron-yttrium high manganese steel and a preparation method thereof.The boron-yttrium high manganese steel comprises, by mass, less than or equal to 0.08% of C, less than or equal to 0.30% of Si, 6.0%-8.0% of Mn, less than or equal to 0.015% of P, less than or equal to 0.004% of S, 0.010%-0.040% of Al, 0.015%-0.035% of Nb, 0.0020%-0.0030% of B, 0.0012%-0.030% of Ti, 0.10%-0.20% of Y and the balance Fe and inevitable impurities. A low-carbon, high-manganese, micro-boron and yttrium rare earth microalloy system is adopted, boron and yttrium are combined with C and N in molten steel to generate stable borides, distribution of inclusions in the molten steel is improved, meanwhile, grain growth is inhibited, grains are refined, and the strength of the high-manganese steel is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the field of metallurgical technology, and specifically relates to a boron-yttrium high manganese steel and a preparation method thereof. Background Art

[0002] High-strength materials have significantly higher mechanical strength than traditional materials and are widely used in fields such as aerospace, automotive manufacturing, and military. High-strength materials can be divided into metal materials, inorganic non-metallic materials, polymer materials, and composite materials according to different material types. High-strength inorganic non-metallic materials represented by ceramics, carbon fibers, and glass have ultra-high hardness and high-temperature resistance, but low fracture toughness, poor impact resistance, and high processing difficulty; High-strength polymer materials represented by polyether ether ketone, ultra-high molecular weight polyethylene, and aramid fibers can be rapidly processed through processes such as injection molding and extrusion, and have the characteristics of easy forming, but poor high-temperature resistance; Composite materials represented by carbon fiber reinforced polymers, glass fiber reinforced plastics, and ceramic matrix composites have good high-temperature resistance and high specific strength, but are costly, have complex manufacturing processes, and are difficult to recycle; High-strength metal materials represented by high-strength steel, titanium alloy, and aluminum alloy have the characteristics of high strength, good high-temperature resistance, easy processing, and high recycling rate.

[0003] High-strength steel is significantly superior to titanium alloy and aluminum alloy in terms of absolute strength, comprehensive cost, high-temperature resistance, and processing performance. However, since the strengthening means to improve strength often limit dislocation movement and inhibit plastic deformation, it is difficult to simultaneously improve the strength and toughness of high-strength steel. Chinese Patent CN116637931A discloses a rolling method for a high-toughness high manganese austenite low-temperature steel plate. This patent expands the austenite phase region by increasing the manganese element content in the steel plate to improve the low-temperature toughness of the steel plate. However, too high a manganese content will lead to the formation of a large amount of oxides, resulting in a decrease in the tensile strength and yield strength of the steel. Summary of the Invention

[0004] Aiming at the technical problem that it is difficult to simultaneously improve the strength and toughness of existing high manganese steel, the present invention provides a boron-yttrium high manganese steel and a preparation method thereof. A low-carbon, high-manganese, micro-boron, yttrium rare earth micro-alloy alloy system is adopted, and boron and yttrium combine with C and N in the molten steel to form stable borides. While improving the distribution of inclusions in the molten steel, grain growth is inhibited and the grains are refined, significantly improving the strength of the high manganese steel.

[0005] The technical solution of the present invention is as follows: In a first aspect, a boron yttrium high manganese steel is provided, wherein the chemical composition of the boron yttrium high manganese steel is, by mass percentage, C ≤0.08%, Si ≤0.30%, Mn 6.0%-8.0%, P ≤0.015%, S ≤0.004%, Al 0.010%-0.040%, Nb0.015%-0.035%, B 0.0020%-0.0030%, Ti 0.0012%-0.030%, Y 0.10%-0.20%, and the rest is Fe and unavoidable impurities.

[0006] Chemical composition is one of the important factors affecting the comprehensive performance of the product. The present invention restricts the chemical composition of boron yttrium high manganese steel, as follows: C: The main solid solution strengthening element in steel, which can significantly improve the strength of steel plates, but a high carbon content is not good for steel plate welding, toughness and plasticity. In order to effectively reduce segregation, improve microstructural uniformity, avoid galvanic corrosion caused by the potential difference between different phases, reduce the corrosion resistance of steel, and consider economic efficiency; therefore, its mass percentage content is limited to C≤0.08%.

[0007] Mn: As the most important alloying element in steel, it has the functions of expanding the austenite phase region, reducing the Ac3, Ac1, Ar3, Ar1 point temperatures, refining the grain size and improving the low-temperature toughness of the steel plate; however, Mn is prone to segregation during the solidification of molten steel, especially when the Mn content is high, which not only makes the casting operation difficult, but also easily segregates with elements such as C, P, and S. Through research, the present invention limits the mass percentage of Mn to 6.0%-8.0%.

[0008] Si: It 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 damages the low-temperature toughness, elongation and weldability of the steel plate. Especially under the condition of large line energy welding, Si not only promotes the formation of MA islands, but also the formed MA islands are relatively large in size and unevenly distributed, which seriously damages the toughness of the welding heat affected zone (HAZ). Therefore, the Si content in the steel should be controlled as low as possible. Considering the economy and operability of the steelmaking process, the mass percentage of Si is controlled below 0.30% in the present invention.

[0009] P: is an element that is easily segregated, increases the cold brittleness of steel, and deteriorates welding performance. Considering the economy and operability of the steelmaking process, the mass percentage of P is controlled below 0.015% in the present invention.

[0010] S: It has a great detrimental effect on the low-temperature toughness of steel. More importantly, S combines with Mn in steel to form MnS inclusions. During the rolling process, MnS extends along the rolling direction, forming a MnS inclusion band, which seriously damages the low-temperature impact toughness, elongation, Z-direction performance, and weldability of the steel plate. At the same time, S is also the main element causing hot brittleness, and theoretically, the lower the better. However, considering the operability of steelmaking, steelmaking cost, and the principle of smooth logistics, for steel plates requiring excellent weldability and -40°C toughness, the mass percentage of S in this invention is controlled at ≤0.004%.

[0011] Nb: It is one of the important elements for fine grain strengthening. 1. It raises the austenite recrystallization temperature, prevents austenite recrystallization, and inhibits grain growth, refining the austenite grains; 2. The carbonitrides of niobium precipitate on dislocations and segregate at the austenite grain boundaries, enhancing strength and toughness. However, if the niobium content is too high, surface cracks are likely to occur in the continuous casting billet, and at the same time, the weldability deteriorates. In this invention, the niobium content is controlled at 0.015% - 0.035%.

[0012] Ti: It has a great affinity for N. When a small amount of Ti is added, N preferentially combines with Ti to form dispersed TiN particles, inhibiting the excessive growth of austenite grains during slab heating and hot rolling, and improving the low-temperature toughness of the steel plate. More importantly, to a certain extent, it inhibits the grain growth in the heat-affected zone (the area far from the fusion line) during welding with a large heat input, improving the toughness of the heat-affected zone. If the added 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 refining the grains of the steel plate and improving the weldability of the steel plate. Even when Ti / N is too large, it is not conducive to refining the grains of the steel plate and even deteriorates the weldability of the steel plate. Therefore, the appropriate Ti content range is 0.012% - 0.030%.

[0013] B: Compared with manganese, boron has a stronger binding ability with carbon, and due to the activity of boron, it has stronger adsorption than iron. After boron alloying treatment, the addition of boron makes it easy to combine with C and N in the molten steel to form stable borides. The formation of these compounds, on the one hand, improves the distribution of inclusions in the molten steel, and on the other hand, because the melting points and hardness of these compounds are high, they can act as the nuclei for heterogeneous nucleation, inhibiting grain growth and making the grains finer. Trace amounts of B in steel dissolve in austenite and segregate at the austenite grain boundaries, reducing the grain boundary energy of austenite. This makes it difficult for ferrite to nucleate at the austenite grain boundaries during the decomposition of austenite, prolonging the incubation period of the γ→α phase transformation and increasing the stability of austenite.

[0014] Y: The main role of rare earth yttrium in molten steel is to improve the inclusions in the molten steel, thereby purifying the molten steel and refining the grains, and then improving the properties of high manganese steel. After being modified by rare earth yttrium, due to the strong sulfur and oxygen affinity of rare earth yttrium, rare earth yttrium is easy to react with sulfur and oxygen in high manganese steel to form compounds with relatively high melting points (generally above 2000 °C), such as YS, Y2S3, YO2, Y2O3, and Y2O2S. The formation of these compounds, on the one hand, reduces the non-metallic inclusions in the molten steel, purifies the molten steel, reduces the segregation and associated inclusions (mainly MnS, aluminum-silicon oxides, etc.) in the center of the steel plate, and improves the low-temperature toughness of the steel plate; on the other hand, due to the high melting points and hardness of these compounds, they can act as nucleating agents to promote the formation of heterogeneous nucleation, thereby hindering the growth of grains and making them refined. Therefore, the addition of rare earth yttrium can improve the properties of high manganese steel.

[0015] Further, by mass percentage, the chemical composition of boron-yttrium high manganese steel is: C 0.06% - 0.07%, Si 0.21% - 0.24%, Mn 6.5% - 7.5%, P ≤0.013%, S ≤0.003%, Al 0.020% - 0.035%, Nb 0.020% - 0.030%, B 0.0022% - 0.0025%, Ti 0.015% - 0.018%, Y 0.13% - 0.15%, and the rest is Fe and unavoidable impurities.

[0016] Further, the microstructure of boron-yttrium high manganese steel consists of lath tempered martensite, retained austenite, ferrite, and pearlite.

[0017] Further, by volume fraction, in the microstructure of boron-yttrium high manganese steel, the content of lath tempered martensite is 90% - 95%, the content of retained austenite is 1% - 2%, and the rest is a small amount of ferrite and pearlite.

[0018] Further, the yield strength of boron-yttrium high manganese steel ≥1000 MPa, the tensile strength of boron-yttrium high manganese steel ≥1200 MPa, the elongation after fracture of boron-yttrium high manganese steel ≥14.0%, and the impact energy -20 °C KV2 of boron-yttrium high manganese steel at -20 °C ≥100 J.

[0019] Further, the yield strength of boron-yttrium high manganese steel ≥1043 MPa, the tensile strength of boron-yttrium high manganese steel ≥1278 MPa, the elongation after fracture of boron-yttrium high manganese steel ≥14.5%, and the impact energy -20 °C KV2 of boron-yttrium high manganese steel at -20 °C ≥134 J.

[0020] In the second aspect, a preparation method of the boron-yttrium high manganese steel as described above includes the following steps: Step 1: Rolling. Two-stage rolling is adopted. The first-stage rolling is rough rolling, which is carried out in the fully austenitic region; the second-stage rolling is finish rolling, which is completely carried out in the non-recrystallization region. The starting rolling temperature of the second-stage rolling is T_rolling + 3150 / H ± 5 °C, where T_rolling is the temperature of the rolled piece during rolling, with the unit of °C, and H is the finished thickness, with the unit of mm. T_rolling is calculated according to the following formula: T_rolling = 752 + 63×(%C) + 23×(%Mn) + 18×(%Ti) + 6×(%Al) + 370×(%Nb), where %C, %Mn, %Ti, %Al, and %Nb are the values before the percentage signs in the mass percentages of C, Mn, Ti, Al, and Nb respectively; Step 2: After rolling, water cooling treatment is carried out on the steel plate. The surface temperature of the steel plate at the start of cooling is controlled at 900 - 930 °C, and the surface temperature of the steel plate at the end of cooling is controlled at 200 - 300 °C. The surface temperature of the steel plate at the end of cooling refers to the surface temperature of the steel plate after the water cooling ends. The cooling rate of the water cooling is controlled at 20 - 30 °C / s; Step 3: After the water cooling treatment, air cooling treatment is carried out on the steel plate. During the air cooling treatment, there should be no obstacles around the steel plate.

[0021] The present invention utilizes water cooling treatment and self-tempering at 200 - 300 °C to promote the decomposition of retained austenite; it is produced by the controlled rolling and controlled cooling process, shortening the production cycle, reducing the production cost, and being conducive to popularization and application.

[0022] Furthermore, before rolling, the slab is heated. The main production process flow of the yttrium-boron high manganese steel is: hot metal pretreatment → BOF steelmaking → LF refining → RH vacuum treatment → continuous casting → slow cooling → heating → rolling → cooling.

[0023] Furthermore, the residence time of the slab in the furnace ≥ 9.5 min / mm, preferably 9.5 - 10.0 min / mm, and the tapping temperature of the slab ≥ 1150 °C, preferably 1150 - 1160 °C.

[0024] Furthermore, in Step 1, the starting rolling temperature of the finish rolling ≥ 980 °C, preferably 980 - 1095 °C.

[0025] The beneficial effects of the present invention: The present invention provides a yttrium-boron high manganese steel and its preparation method. By reacting yttrium and boron with carbon and nitrogen in the high manganese steel, on the one hand, it can reduce the non-metallic inclusions in the molten steel, purifying the molten steel; on the other hand, it can use the high-melting-point and high-hardness compounds obtained from the reaction as the nuclei for nucleation, inhibiting grain growth and making the grains refined; the addition of yttrium and boron can also improve the stability of austenite, increase the hardenability of the high manganese steel, and the high-temperature stability of the microstructure.

[0026] By limiting the content of manganese element to 6.0% - 8.0%, it can not only expand the austenite phase region in the steel, improve the low-temperature toughness of the steel, but also prevent the segregation of manganese with elements such as carbon, phosphorus, and sulfur to a certain extent, avoiding difficulties in casting operations.

[0027] Reasonably match elements such as carbon, silicon, manganese, and boron to increase the martensite transformation temperature and the stability of retained austenite in the steel, which is beneficial to the formation of tempered martensite structure, retained austenite structure, ferrite structure, and pearlite structure. Specific embodiments

[0028] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0029] Example 1 A kind of yttrium-boron high manganese steel with a finished product thickness of 20 mm. In terms of mass percentage, the chemical composition of the yttrium-boron high manganese steel is: C 0.07%, Si 0.22%, Mn 7.3%, P 0.012%, S 0.002%, Al 0.029%, Nb 0.024%, B 0.0025%, Ti 0.015%, Y 0.15%, and the rest are Fe and unavoidable impurities.

[0030] The preparation method of the above yttrium-boron high manganese steel includes the following steps: Before rolling, the billet is subjected to hot metal pretreatment, BOF steelmaking, LF refining, RH vacuum treatment, continuous casting, slow cooling, and heating in sequence. During the heating process, the residence time of the billet in the furnace is 10.0 min / mm, and the billet tapping temperature is 1160 °C.

[0031] Step 1: Rolling. Two-stage rolling is adopted. The first-stage rolling is rough rolling, which is carried out in the fully austenite region. After rough rolling, an intermediate billet with a thickness of 75 mm is obtained. The second-stage rolling is finish rolling. First, calculate T_rolling according to the following formula: T_rolling = 752 + 63×(%C) + 23×(%Mn) + 18×(%Ti) + 6×(%Al) + 370×(%Nb) = 752 + 63×0.07 + 23×7.3 + 18×0.015 + 6×0.029 + 370×0.024 = 933.634 °C; Then substitute T_rolling = 933.634 °C into the following formula to calculate the finish rolling starting temperature: Finish rolling start temperature = Trolling + 3150 / H ± 5°C = 933.634°C + 3150 / 20 ± 5°C = 1094°C; The second stage of rolling is completely carried out in the non-recrystallization zone.

[0032] Step 2: After rolling, water-cool the steel plate, control the surface temperature of the steel plate at the start of water cooling at 920°C, control the surface temperature of the steel plate at the end of water cooling at 260°C, and control the cooling rate of water cooling at 26°C / s.

[0033] Step 3: After the water-cooling treatment, carry out air-cooling treatment on the steel plate to obtain the finished product of boron-yttrium high manganese steel. The thickness H of the finished product of boron-yttrium high manganese steel is 20 mm. During the air-cooling treatment, there should be no obstacles around the steel plate.

[0034] Use a metallographic microscope and a transmission electron microscope to observe the microstructure. The microstructure of boron-yttrium high manganese steel consists of lath tempered martensite, retained austenite, ferrite and pearlite. In terms of volume fraction, in the microstructure of boron-yttrium high manganese steel, it is estimated that the tissue content of lath tempered martensite is 93%, the tissue content of retained austenite is 1.5%, and the others are a small amount of ferrite and pearlite.

[0035] Test the yield strength, tensile strength, elongation after fracture and longitudinal impact energy of the finished product according to GB / T 228.1 and GB / T 229. After testing, the yield strength of boron-yttrium high manganese steel is 1098 MPa, the tensile strength of boron-yttrium high manganese steel is 1305 MPa, the elongation after fracture of boron-yttrium high manganese steel is 14.5%, and the longitudinal impact energy -20°C KV2 of boron-yttrium high manganese steel at -20°C are 168 J, 156 J, and 159 J respectively.

[0036] Example 2 A boron-yttrium high manganese steel with a finished product thickness of 40 mm. In terms of mass percentage, the chemical composition of boron-yttrium high manganese steel is: C 0.06%, Si 0.21%, Mn 6.8%, P 0.013%, S 0.001%, Al 0.031%, Nb 0.03%, B 0.0022%, Ti 0.016%, Y 0.13%, and the rest are Fe and unavoidable impurities.

[0037] The preparation method of the above boron-yttrium high manganese steel includes the following steps: Before rolling, the cast billet is successively subjected to hot metal pretreatment, BOF steelmaking, LF refining, RH vacuum treatment, continuous casting, slow cooling, and heating. During the heating process, the residence time of the cast billet in the furnace is 9.5 min / mm, and the tapping temperature of the cast billet is 1150°C.

[0038] Step 1: Rolling. Two-stage rolling is adopted. The first-stage rolling is rough rolling, which is carried out in the fully austenitic region. After rough rolling, an intermediate billet with a thickness of 120 mm is obtained. The second-stage rolling is finish rolling. First, calculate T_rolling according to the following formula: T_rolling = 752 + 63×(%C) + 23×(%Mn) + 18×(%Ti) + 6×(%Al) + 370×(%Nb) = 752 + 63×0.06 + 23×6.8 + 18×0.016 + 6×0.031 + 370×0.03 = 923.754 °C; Then substitute T_rolling = 923.754 °C into the following formula to calculate the finish rolling starting temperature: Finish rolling starting temperature = T_rolling + 3150 / H ± 5 °C = 923.754 °C + 3150 / 40 ± 5 °C = 1005 °C; The second-stage rolling is completely carried out in the non-recrystallization region.

[0039] Step 2: After rolling, water-cool the steel plate. Control the surface temperature of the starting-cooling steel plate at 925 °C, control the surface temperature of the final-cooling steel plate at 220 °C, and control the cooling rate of water-cooling at 24 °C / s.

[0040] Step 3: After water-cooling treatment, air-cool the steel plate to obtain the finished product of yttrium-boron high manganese steel. The thickness H of the finished product of yttrium-boron high manganese steel is 40 mm. During the air-cooling process, there should be no obstacles around the steel plate.

[0041] Use a metallographic microscope and a transmission electron microscope to observe the microstructure. The microstructure of yttrium-boron high manganese steel consists of lath tempered martensite, retained austenite, ferrite and pearlite. In terms of volume fraction, it is estimated that in the microstructure of yttrium-boron high manganese steel, the content of lath tempered martensite is 92%, the content of retained austenite is 1.8%, and the others are a small amount of ferrite and pearlite.

[0042] Test the yield strength, tensile strength, elongation after fracture and longitudinal impact energy of the finished product according to GB / T 228.1 and GB / T 229. After testing, the yield strength of yttrium-boron high manganese steel is 1067 MPa, the tensile strength of yttrium-boron high manganese steel is 1289 MPa, the elongation after fracture of yttrium-boron high manganese steel is 15.5%, and the longitudinal impact energy -20 °C KV2 of yttrium-boron high manganese steel at -20 °C are 146 J, 153 J, 147 J respectively.

[0043] Example 3 A kind of yttrium-boron high manganese steel with a finished product thickness of 60 mm. In terms of mass percentage, the chemical composition of the yttrium-boron high manganese steel is: C 0.07%, Si 0.24%, Mn 7.00%, P 0.012%, S 0.002%, Al 0.033%, Nb 0.023%, B 0.0023%, Ti 0.017%, Y 0.14%, and the rest is Fe and unavoidable impurities.

[0044] The preparation method of the above yttrium-boron high manganese steel includes the following steps: Before rolling, the continuous casting billet is successively subjected to hot metal pretreatment, BOF steelmaking, LF refining, RH vacuum treatment, continuous casting, slow cooling, and heating. During the heating process, the residence time of the continuous casting billet in the furnace is 9.5 min / mm, and the tapping temperature of the continuous casting billet is 1150 °C.

[0045] Step 1: Rolling. Two-stage rolling is adopted. The first-stage rolling is rough rolling, which is carried out in the fully austenite region. After rough rolling, an intermediate billet with a thickness of 140 mm is obtained. The second-stage rolling is finish rolling. First, calculate T_rolling according to the following formula: T_rolling = 752 + 63×(%C) + 23×(%Mn) + 18×(%Ti) + 6×(%Al) + 370×(%Nb) = 752 + 63×0.07 + 23×7.00 + 18×0.017 + 6×0.033 + 370×0.023 = 926.424 °C; Then substitute T_rolling = 926.424 °C into the following formula to calculate the finish rolling starting temperature: Finish rolling starting temperature = T_rolling + 3150 / H ± 5 °C = 926.424 °C + 3150 / 60 ± 5 °C = 981 °C; The second-stage rolling is completely carried out in the non-recrystallization region.

[0046] Step 2: After rolling, the steel plate is subjected to water cooling treatment. The surface temperature of the steel plate at the start of cooling is controlled at 925 °C, the surface temperature of the steel plate at the end of cooling is controlled at 220 °C, and the cooling rate of water cooling is controlled at 22 °C / s.

[0047] Step 3: After the water cooling treatment, the steel plate is subjected to air cooling treatment to obtain the finished product of yttrium-boron high manganese steel. The thickness H of the finished product of yttrium-boron high manganese steel is 60 mm. During the air cooling treatment, there should be no obstacles around the steel plate.

[0048] The microstructure is observed by a metallographic microscope and a transmission electron microscope. The microstructure of the yttrium-boron high manganese steel consists of lath tempered martensite, retained austenite, ferrite, and pearlite. In terms of volume fraction, it is estimated that in the microstructure of the yttrium-boron high manganese steel, the content of lath tempered martensite is 92%, the content of retained austenite is 1.7%, and the rest is a small amount of ferrite and pearlite.

[0049] The yield strength, tensile strength, elongation after fracture, and longitudinal impact energy of the finished product were tested in accordance with GB / T 228.1 and GB / T 229. After testing, the yield strength of the boron-yttrium-high manganese steel was 1043 MPa, the tensile strength of the boron-yttrium-high manganese steel was 1278 MPa, the elongation after fracture of the boron-yttrium-high manganese steel was 15.0%, and the longitudinal impact energy -20℃KV2 of the boron-yttrium-high manganese steel at -20℃ was 138 J, 134 J, and 142 J respectively.

[0050] Comparative Example 1 A boron-yttrium-high manganese steel with a finished product thickness of 40 mm. In terms of mass percentage, the chemical composition of the boron-yttrium-high manganese steel is: C 0.06%, Si 0.23%, Mn 4.7%, P 0.011%, S 0.002%, Al 0.03%, Nb 0.028%, B 0.0018%, Ti 0.015%, Y 0%, and the rest are Fe and unavoidable impurities.

[0051] The preparation method of the above boron-yttrium-high manganese steel includes the following steps: Before rolling, the continuous casting billet is subjected to hot metal pretreatment, BOF steelmaking, LF refining, RH vacuum treatment, continuous casting, slow cooling, and heating in sequence. During the heating process, the residence time of the continuous casting billet in the furnace is 9.5 min / mm, and the tapping temperature of the continuous casting billet is 1150℃.

[0052] Step 1: Rolling. Two-stage rolling is adopted. The first-stage rolling is rough rolling, which is carried out in the fully austenite region. After rough rolling, an intermediate billet with a thickness of 40 mm is obtained. The second-stage rolling is finish rolling. First, calculate T rolling according to the following formula: T rolling = 752 + 63×(%C) + 23×(%Mn) + 18×(%Ti) + 6×(%Al) + 370×(%Nb) = 752 + 63×0.06 + 23×4.7 + 18×0.015 + 6×0.03 + 370×0.028 = 874.69℃; Then substitute T rolling = 874.69℃ into the following formula to calculate the finish rolling starting temperature: Finish rolling starting temperature = T rolling + 3150 / H ± 5℃ = 874.69℃ + 3150 / 40 ± 5℃ = 953.44℃ ≈ 953℃; The second-stage rolling is completely carried out in the non-recrystallization region.

[0053] Step 2: After rolling, the steel plate is subjected to water cooling treatment. The surface temperature of the starting cooled steel plate is controlled at 890℃, the surface temperature of the final cooled steel plate is controlled at 320℃, and the cooling rate of water cooling is controlled at 22℃ / s.

[0054] Step 3: After the water cooling treatment, the steel plate is subjected to air cooling treatment to obtain the finished product of yttrium-boron high manganese steel. The thickness H of the finished product of yttrium-boron high manganese steel is 40 mm. During the air cooling process, there should be no obstacles around the steel plate.

[0055] The microstructure was observed by optical microscopy and transmission electron microscopy. The microstructure of yttrium-boron high manganese steel consists of lath tempered martensite, retained austenite, ferrite and pearlite. In terms of volume fraction, it is estimated that in the microstructure of yttrium-boron high manganese steel, the content of lath tempered martensite is 98%, the content of retained austenite is 0.5%, and the rest is a small amount of ferrite.

[0056] The yield strength, tensile strength, elongation after fracture and longitudinal impact energy of the finished product were tested according to GB / T 228.1 and GB / T 229. After testing, the yield strength of yttrium-boron high manganese steel is 1268 MPa, the tensile strength of yttrium-boron high manganese steel is 1345 MPa, the elongation after fracture of yttrium-boron high manganese steel is 8.5%, and the longitudinal impact energy -20℃KV2 of yttrium-boron high manganese steel at -20℃ is 34 J, 45 J, 39 J respectively.

[0057] In Comparative Example 1, yttrium was not added, and the segregation of C, Mn, P, and S during the solidification of molten steel could not be well inhibited. The segregation and the content of associated inclusions (mainly MnS, aluminum-silicon oxides, etc.) in the central part of the steel plate were relatively high; the contents of manganese and boron were low. Under this controlled rolling and controlled cooling process, there was little retained austenite, and most of the retained austenite decomposed after self-tempering. The final structure was mainly tempered martensite, resulting in high strength, but a large reduction in plasticity and toughness, and the comprehensive performance index of the steel plate became worse.

[0058] Comparative Example 2 A yttrium-boron high manganese steel with a finished product thickness of 60 mm. In terms of mass percentage, the chemical composition of yttrium-boron high manganese steel is: C 0.07%, Si 0.22%, Mn 4.9%, P 0.012%, S 0.002%, Al 0.032%, Nb 0.026%, B 0%, Ti 0.016%, Y 0.12%, and the rest is Fe and unavoidable impurities.

[0059] The preparation method of the above yttrium-boron high manganese steel includes the following steps: Before rolling, the cast billet is subjected to hot metal pretreatment, BOF steelmaking, LF refining, RH vacuum treatment, continuous casting, slow cooling, and heating in sequence. During the heating process, the residence time of the cast billet in the furnace is 9.5 min / mm, and the tapping temperature of the cast billet is 1150℃.

[0060] Step 1: Rolling is carried out in two stages. The first stage of rolling is rough rolling, which is carried out in the fully austenitic region. After rough rolling, an intermediate billet with a thickness of 140 mm is obtained; the second stage of rolling is finish rolling. First, calculate T_rolling according to the following formula: T_rolling = 752 + 63×(%C) + 23×(%Mn) + 18×(%Ti) + 6×(%Al) + 370×(%Nb) = 752 + 63×0.07 + 23×4.9 + 18×0.016 + 6×0.032 + 370×0.026 = 879.21 °C; Then substitute T_rolling = 879.21 °C into the following formula to calculate the finishing rolling starting temperature: Finishing rolling starting temperature = T_rolling + 3150 / H ± 5 °C = 879.21 °C + 3150 / 60 ± 5 °C = 929 °C; The second stage of rolling is completely carried out in the non-recrystallization zone.

[0061] Step 2: After rolling, water-cool the steel plate, control the surface temperature of the starting water-cooling steel plate at 880 °C, control the surface temperature of the final water-cooling steel plate at 320 °C, and control the cooling rate of water-cooling at 20 °C / s.

[0062] Step 3: After the water-cooling treatment, air-cool the steel plate to obtain the finished product of yttrium-boron high manganese steel. The thickness H of the finished product of yttrium-boron high manganese steel is 60 mm. During the air-cooling process, there should be no obstacles around the steel plate.

[0063] Use an optical microscope and a transmission electron microscope to observe the microstructure to test the microstructure of the finished product. The microstructure of yttrium-boron high manganese steel consists of lath tempered martensite, retained austenite, ferrite and pearlite. In terms of volume fraction, it is estimated that in the microstructure of yttrium-boron high manganese steel, the tissue content of lath tempered martensite is 98%, the tissue content of retained austenite is 0.4%, and the others are a small amount of ferrite.

[0064] Test the yield strength, tensile strength, elongation after fracture and longitudinal impact energy of the finished product according to the methods of GB / T 228.1 and GB / T 229. After testing, the yield strength of yttrium-boron high manganese steel is 1246 MPa, the tensile strength of yttrium-boron high manganese steel is 1350 MPa, the elongation after fracture of yttrium-boron high manganese steel is 9.0%, and the longitudinal impact energy -20 °C KV2 of yttrium-boron high manganese steel at -20 °C are 35 J, 40 J, and 38 J respectively.

[0065] Although yttrium was added in Comparative Example 2, boron was not added and the manganese content was low. Under this controlled rolling and controlled cooling process, the stability of retained austenite is poor, and there is basically no retained austenite after self-tempering, resulting in high strength, but a significant reduction in plasticity and toughness, and the comprehensive performance index of the steel plate becomes worse.

[0066] Although the present invention has been described in detail by combining preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, those of ordinary skill in the art can make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions should all be within the scope covered by the present invention / Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered by the protection scope of the present invention.

Claims

1. A yttrium-boron high manganese steel, characterized in that, By mass percentage, the chemical composition of the boron-yttrium high manganese steel is: C ≤ 0.08%, Si ≤ 0.30%, Mn 6.0% - 8.0%, P ≤ 0.015%, S ≤ 0.004%, Al 0.010% - 0.040%, Nb 0.015% - 0.035%, B 0.0020% - 0.0030%, Ti 0.0012% - 0.030%, Y 0.10% - 0.20%, and the balance is Fe and unavoidable impurities.

2. A yttrium-boron high manganese steel according to claim 1, characterized in that, By mass percentage, the chemical composition of the boron-yttrium high manganese steel is: C 0.06% - 0.07%, Si 0.21% - 0.24%, Mn 6.5% - 7.5%, P ≤ 0.013%, S ≤ 0.003%, Al 0.020% - 0.035%, Nb 0.020% - 0.030%, B 0.0022% - 0.0025%, Ti 0.015% - 0.018%, Y 0.13% - 0.15%, and the balance is Fe and unavoidable impurities.

3. A yttrium-boron high manganese steel as claimed in claim 1, wherein, The microstructure of the boron-yttrium high manganese steel consists of lath tempered martensite, retained austenite, ferrite, and pearlite.

4. A yttrium boron high manganese steel as claimed in claim 3, characterized in that, By volume fraction, in the microstructure of the boron-yttrium high manganese steel, the content of lath tempered martensite is 90% - 95%, the content of retained austenite is 1% - 2%, and the rest is ferrite and pearlite.

5. A yttrium-boron high manganese steel as claimed in claim 1, characterized in that, The yield strength of the boron-yttrium high manganese steel ≥ 1000 MPa, the tensile strength of the boron-yttrium high manganese steel ≥ 1200 MPa, the elongation after fracture of the boron-yttrium high manganese steel ≥ 14.0%, and the impact energy -20℃KV2 of the boron-yttrium high manganese steel at -20℃ ≥ 100 J.

6. The yttrium-boron high manganese steel according to claim 5, characterized in that, The yield strength of the boron-yttrium high manganese steel ≥ 1043 MPa, the tensile strength of the boron-yttrium high manganese steel ≥ 1278 MPa, the elongation after fracture of the boron-yttrium high manganese steel ≥ 14.5%, and the impact energy -20℃KV2 of the boron-yttrium high manganese steel at -20℃ ≥ 134 J.

7. A preparation method of the yttrium-boron high manganese steel according to any one of claims 1-6, characterized in that, It includes the following steps: Step 1: Rolling. Two-stage rolling is adopted. The first-stage rolling is rough rolling, which is carried out in the fully austenitic region; the second-stage rolling is finish rolling, which is completely carried out in the non-recrystallization region. The starting rolling temperature of the second-stage rolling is T_rolling + 3150 / H ± 5℃; where, T_rolling is the temperature of the rolled piece during rolling, and the unit of T_rolling is ℃; H is the finished thickness, and the unit of H is mm; T_rolling is calculated according to the following formula: T_rolling = 752 + 63×(%C) + 23×(%Mn) + 18×(%Ti) + 6×(%Al) + 370×(%Nb), where %C, %Mn, %Ti, %Al, %Nb are the values before the percentage sign in the mass percentages of C, Mn, Ti, Al, and Nb respectively; Step 2: After rolling, water cooling treatment is carried out on the steel plate. The surface temperature of the starting-cooling steel plate is controlled at 900 - 930℃, the surface temperature of the final-cooling steel plate is controlled at 200 - 300℃, and the cooling rate of water cooling is controlled at 20 - 30℃ / s; Step 3: After the water cooling treatment, air cooling treatment is carried out on the steel plate. During the air cooling treatment, there should be no obstacles around the steel plate.

8. The preparation method according to claim 7, characterized in that, Before rolling, the continuous casting billet is heated.

9. The preparation method according to claim 8, characterized in that, The residence time of the continuous casting billet in the furnace is ≥9.5 min / mm, and the tapping temperature of the continuous casting billet is ≥1150 °C.

10. The preparation method according to claim 7, characterized in that, In step one, the finishing rolling starting temperature is ≥980 °C.