Preparation method of 700MPa-grade ultra-high-strength ultra-fine grain aseismic steel bar

Abstract

The invention discloses a preparation method of a 700MPa-grade ultra-fine grain aseismic steel bar with ultra-high strength. The 700 MPa grade ultra-high strength ultra-fine grain aseismic steel bar is prepared from the following chemical components in percentage by weight: 0.27 to 0.29 weight percent of C, 0.70 to 0.80 weight percent of Si, 0.60 to 0.70 weight percent of Mn, 1.05 to 1.15 weight percent of Cr, 0.165 to 0.180 weight percent of V, 0.012 to 0.016 weight percent of Nb, less than or equal to 0.030 weight percent of S, less than or equal to 0.030 weight percent of P, less than or equal to 0.0040 weight percent of O, 0.0300 to 0.0350 weight percent of N and the balance of Fe and inevitable impurities. The yield strength of the steel is 710-745 MPa, the tensile strength of the steel is 895-950 MPa, the maximum force total elongation of the steel is larger than or equal to 10.0, and the ferrite grain size of the steel is 12.5-14.0. According to the process, chemical component design, converter smelting, deoxidation alloying, argon station treatment, LF furnace refining, continuous casting, a steel rolling heating schedule, rolling temperature and a controlled cooling process are integrated and innovated, so that multiple strengthening effects of precipitation strengthening, fine grain strengthening, structure strengthening and the like are fully exerted; the produced reinforcing steel bar has the advantages of excellent and stable high-strength performance, fine and uniform microscopic structure, excellent corrosion resistance, excellent anti-seismic performance and the like, and has remarkable economic and social benefits.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting, and specifically relates to a method for preparing 700MPa-grade ultra-high strength ultra-fine-grained earthquake-resistant steel bars. Background Art

[0002] Ultra-high-strength earthquake-resistant steel bars are the main reinforcement materials for ultra-high-rise and ultra-large reinforced concrete building structures. They bear the stress and strain of tensile and compressive stress and strain in the structure. They are currently well used in Japan, South Korea and Europe. With the continuous development of my country's buildings towards ultra-high-rise and ultra-large span earthquake-resistant structures, the development of 700MPa ultra-high-strength earthquake-resistant steel bars with excellent comprehensive performance has become one of the important tasks for the steel industry to improve its technical level.

[0003] At present, there are related research reports on the production technology of 600MPa and 630MPa high-strength steel bars in China, which mainly adopt vanadium-nitrogen microalloying process. The seismic performance index of steel bars, strength-to-yield ratio (R°m / R°eL), is mostly 1.23-1.26, and the maximum force total elongation (Agt) is 7.5-9.0%. The seismic performance matching is not good, which restricts the use of products in super-high-rise and super-large building structures in high-intensity seismic areas. For the production and preparation technology of 700MPa ultra-high-strength seismic steel bars, there are currently no related patents and research papers in China.

[0004] The present invention aims to provide a method for preparing 700MPa-level ultra-high strength ultra-fine-grained earthquake-resistant steel bars. Summary of the invention

[0005] The object of the present invention is to provide a method for preparing 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bars.

[0006] The object of the present invention is achieved by a method for preparing a 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar, which is achieved by the following steps: A. Molten steel smelting: 140-155kg / t 钢 、15kg / t 钢 The cold material charging ratio is 900kg / t. Scrap steel and pig iron are added to the LD converter. 钢 Add molten iron with a temperature of ≥1280℃ and carry out conventional top and bottom composite blowing: according to 22-25kg / t 钢 , 15-18kg / t 钢 Conventional lime and light-burned dolomite are added to make slag, and the final carbon content is controlled to be ≥0.06wt%, and the tapping temperature is ≤1610℃; before tapping, 1.0kg / t is added to the bottom of the ladle. 钢 and 1.0kg / t 钢 Add slag washing material and active lime in the amounts required for slag washing; B. Deoxidation and alloying: The molten steel after smelting is tapped. When the amount of molten steel in the ladle is greater than 1 / 4, the deoxidation and alloying sequence is silicon-calcium-barium → ferrosilicon → silicon-manganese alloy → high carbon ferrochrome → silicon-nitrogen alloy → niobium-phosphorus-iron alloy → vanadium-nitrogen alloy, respectively, at a rate of 2.0 kg / t. 钢 、3.1~4.5kg / t 钢 、8.0~9.6kg / t 钢 , 19.5~21.5kg / t 钢 , 1.90~2.10kg / t 钢 , 0.26~0.40 kg / t 钢 , 0.95~1.16kg / t 钢 Add silicon calcium barium, ferrosilicon, silicon manganese alloy, high carbon ferrochrome, silicon nitrogen alloy, niobium phosphorus iron alloy and vanadium nitrogen alloy to the ladle in order, and add all the above alloys when the amount of molten steel in the ladle reaches 3 / 4; C. Argon treatment of molten steel: After the molten steel is tapped in step B, it is hoisted to the argon treatment station and passed through the alloy trough at a pressure of 15.0kg / t 钢 The amount of vanadium-containing pig iron having the following mass ratio is added: C 3.1wt%, Si 0.45wt%, Mn 2.16wt%, V 6.0wt%, P1.05wt%, S 0.075wt%, and the rest is Fe and inevitable impurities; the baking time of the vanadium-containing pig iron is 10 minutes, and the surface temperature of the pig iron is ≥300°C; after the vanadium-containing pig iron is added, nitrogen is turned on and nitrogen is blown to the molten steel with a flow rate of 25-30NL / min, and the blowing time is 3 minutes.

[0007] D. LF furnace refining of molten steel: The molten steel from step C is hoisted to the LF furnace for refining. After the refining, the molten steel is subjected to a small flow rate of soft nitrogen blowing. The blowing time is 3 minutes and the nitrogen flow rate is controlled to be 20-25 NL / min. After the blowing, a covering agent is added to the molten steel and the amount added is controlled to be 1.0 kg / t 钢 , and then lift the molten steel to the casting station.

[0008] E. Molten steel casting: The tundish temperature is 1525-1540°C, the pulling speed is 3.0-3.2 m / min, and the crystallizer cooling water flow rate is 140-150 m 3 / h, and the secondary cooling water volume is 1.7-1.9L / kg, casting the molten steel in step D into a steel billet with a cross-section of 165mm×165mm; F. Billet heating: The billet in step E is sent to a heating furnace with a soaking section temperature of 1110-1160°C for heating for 90 minutes. The billet tapping temperature is 1050-1065°C, and then pushed to a fully continuous bar mill for rolling; G. Controlled rolling and cooling of steel billets: The steel billets in step F are rolled for 6 passes at a rolling speed of 0.8-1.2 m / s by the rough rolling unit, and then rolled for 4 passes at a rolling speed of 1.8-2.5 m / s by the intermediate rolling unit, and then rolled for 2-4 passes at a rolling speed of 7.0-9.0 m / s by the pre-finishing rolling unit; then the rolled pieces are rolled for 1-2 passes at a rolling speed of 10.0-11.0 m / s by the finishing rolling unit, and the temperature of the rolled pieces entering the finishing rolling unit is controlled at 920-950°C; before the rolled pieces enter the finishing rolling unit, they are controlled to cool by two water cooling devices with a length of 8 m each, and the cooling water volume is 70-80 m 3 / h, the temperature of the rolled piece entering the second finishing rolling group is controlled at 850-880℃; then the rolled piece passes through the second finishing rolling group at a rolling speed of 12.0-14.0m / s for 2 passes; after finishing rolling, the rolled piece passes through a group of water cooling devices with a length of 6m for post-finishing cooling, and the cooling water volume is 40-50m 3 / h; after controlled cooling, the temperature of the steel bar on the cooling bed is controlled at 860-880℃, and then the steel bar is naturally air-cooled to room temperature on the cooling bed, thus obtaining 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bars with a nominal diameter of 20-28mm.

[0009] The method of the present invention adds silicon-nitrogen alloy in the deoxidation alloying process of the steelmaking converter, significantly increases the nitrogen content in the molten steel, promotes the transfer of V and Nb from the solid solution state to the carbonitride precipitation phase in the rolling process, forms and precipitates a large amount of fine dispersed V(C,N) and Nb(C,N) precipitation phases, and significantly improves the precipitation strengthening effect of the steel; after the steel is tapped, a large amount of vanadium-containing pig iron is added to the molten steel at the argon station to replace the expensive vanadium-nitrogen alloy, thereby increasing the V content in the molten steel and greatly reducing the amount of vanadium alloy added and the micro-alloying cost in the steel tapping process; a certain amount of chromium is added to the steel, thereby improving the passivation corrosion resistance of the steel, and the corrosion rate is only 1 / 2 of that of ordinary HRB600 steel bars, while the hardenability and secondary hardening effect are significantly improved, thereby promoting the improvement of the tensile strength of the steel and significantly improving the seismic performance; a small amount of niobium is added to the steel, thereby changing the microstructure of the steel The microstructure ratio is improved to generate a small amount of granular bainite, which promotes the reasonable matching of the tensile strength and yield strength of the steel and improves the seismic resistance of the steel; the high rolling start temperature is controlled during steel rolling to promote the full solid solution of V and Nb microalloying elements; after the finished rolling, the rolled piece is rolled by two groups of finishing mills, and a longer water cooling device is used between the first and second groups of finishing rolling to control the cooling while controlling the lower finishing temperature; a longer water cooling device is used for cooling after finishing rolling; by adopting multiple graded controlled cooling processes such as low-temperature rolling with the finishing mill and controlled cooling after finishing rolling, the original austenite grains are refined, the formation and precipitation of a large number of fine and dispersed V (C, N) and Nb (C, N) precipitation phases are promoted, the nucleation rate of ferrite grains is increased, the ferrite grain size reaches above level 12.5, the ultrafine grain strengthening effect is significant, the strength of the steel is greatly improved, and the plasticity and toughness of the steel are improved. The process of the present invention fully exerts the multiple strengthening effects of precipitation strengthening, fine grain strengthening, and organizational strengthening through integrated innovation of chemical composition design, converter smelting, deoxidation alloying, argon station treatment, LF furnace refining, continuous casting, steel rolling heating system, rolling temperature, and controlled cooling process. The produced 700MPa grade earthquake-resistant steel bar has the advantages of excellent and stable high strength performance, fine and uniform microstructure, excellent corrosion resistance and earthquake resistance, etc., which improves the application scope and market competitiveness of the product. The process of the present invention has the characteristics of low production cost, strong process applicability and controllability, and has significant economic and social benefits. DETAILED DESCRIPTION

[0010] The present invention is further described in detail below in conjunction with the embodiments, but the present invention is not limited in any way. Any changes or improvements made based on the teachings of the present invention fall within the protection scope of the present invention.

[0011] The present invention provides a method for preparing 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bars, which is achieved by the following steps: A. Molten steel smelting: 140-155kg / t 钢 、15kg / t 钢 The cold material charging ratio is 900kg / t. Scrap steel and pig iron are added to the LD converter. 钢 Add molten iron with a temperature of ≥1280℃ and carry out conventional top and bottom composite blowing: according to 22-25kg / t 钢 , 15-18kg / t 钢 Conventional lime and light-burned dolomite are added to make slag, and the final carbon content is controlled to be ≥0.06wt%, and the tapping temperature is ≤1610℃; before tapping, 1.0kg / t is added to the bottom of the ladle. 钢 and 1.0kg / t 钢 Add slag washing material and active lime in the amounts required for slag washing; B. Deoxidation and alloying: The molten steel after smelting is tapped. When the amount of molten steel in the ladle is greater than 1 / 4, the deoxidation and alloying sequence is silicon-calcium-barium → ferrosilicon → silicon-manganese alloy → high carbon ferrochrome → silicon-nitrogen alloy → niobium-phosphorus-iron alloy → vanadium-nitrogen alloy, respectively, at a rate of 2.0 kg / t. 钢 、3.1~4.5kg / t 钢 、8.0~9.6kg / t 钢 , 19.5~21.5kg / t 钢 , 1.90~2.10kg / t 钢 , 0.26~0.40 kg / t 钢 , 0.95~1.16kg / t 钢Add silicon calcium barium, ferrosilicon, silicon manganese alloy, high carbon ferrochrome, silicon nitrogen alloy, niobium phosphorus iron alloy and vanadium nitrogen alloy to the ladle in order, and add all the above alloys when the amount of molten steel in the ladle reaches 3 / 4; C. Argon treatment of molten steel: After the molten steel is tapped in step B, it is hoisted to the argon treatment station and passed through the alloy trough at a pressure of 15.0kg / t 钢 The amount of vanadium-containing pig iron having the following mass ratio is added: C 3.1wt%, Si 0.45wt%, Mn 2.16wt%, V 6.0wt%, P1.05wt%, S 0.075wt%, and the rest is Fe and inevitable impurities; the baking time of the vanadium-containing pig iron is 10 minutes, and the surface temperature of the pig iron is ≥300°C; after the vanadium-containing pig iron is added, nitrogen is turned on and nitrogen is blown to the molten steel with a flow rate of 25-30NL / min, and the blowing time is 3 minutes.

[0012] D. LF furnace refining of molten steel: The molten steel from step C is hoisted to the LF furnace for refining. After the refining, the molten steel is subjected to a small flow rate of soft nitrogen blowing. The blowing time is 3 minutes and the nitrogen flow rate is controlled to be 20-25 NL / min. After the blowing, a covering agent is added to the molten steel and the amount added is controlled to be 1.0 kg / t 钢 , and then lift the molten steel to the casting station.

[0013] E. Molten steel casting: The tundish temperature is 1525-1540°C, the pulling speed is 3.0-3.2 m / min, and the crystallizer cooling water flow rate is 140-150 m 3 / h, and the secondary cooling water volume is 1.7-1.9L / kg, casting the molten steel in step D into a steel billet with a cross-section of 165mm×165mm; F. Billet heating: The billet in step E is sent to a heating furnace with a soaking section temperature of 1110-1160°C for heating for 90 minutes. The billet tapping temperature is 1050-1065°C, and then pushed to a fully continuous bar mill for rolling; G. Controlled rolling and cooling of steel billets: The steel billets in step F are rolled for 6 passes at a rolling speed of 0.8-1.2 m / s by the rough rolling unit, and then rolled for 4 passes at a rolling speed of 1.8-2.5 m / s by the intermediate rolling unit, and then rolled for 2-4 passes at a rolling speed of 7.0-9.0 m / s by the pre-finishing rolling unit; then the rolled pieces are rolled for 1-2 passes at a rolling speed of 10.0-11.0 m / s by the finishing rolling unit, and the temperature of the rolled pieces entering the finishing rolling unit is controlled at 920-950°C; before the rolled pieces enter the finishing rolling unit, they are controlled to cool by two water cooling devices with a length of 8 m each, and the cooling water volume is 70-80 m 3 / h, the temperature of the rolled piece entering the second finishing rolling group is controlled at 850-880℃; then the rolled piece passes through the second finishing rolling group at a rolling speed of 12.0-14.0m / s for 2 passes; after finishing rolling, the rolled piece passes through a group of water cooling devices with a length of 6m for post-finishing cooling, and the cooling water volume is 40-50m 3 / h; after controlled cooling, the temperature of the steel bar on the cooling bed is controlled at 860-880℃, and then the steel bar is naturally air-cooled to room temperature on the cooling bed, thus obtaining 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bars with a nominal diameter of 20-28mm.

[0014] In step A, the chemical composition of the scrap steel is: C 0.21-0.25wt%, Si 0.28-0.55wt%, Mn 0.87-1.52wt%, P 0.017-0.045wt%, S 0.018-0.045wt%, and the rest is Fe and inevitable impurities; the chemical composition of the pig iron is: C 3.0-3.3wt%, Si 0.15-0.40wt%, Mn0.20-0.40wt%, P 0.080-0.112wt%, S 0.020-0.042wt%, and the rest is Fe and inevitable impurities. The chemical composition of the molten iron is: C 4.0-4.3wt%, Si 0.20-0.45wt%, Mn 0.20-0.42wt%, P 0.072-0.090wt%, S≤0.035wt%, and the rest is Fe and inevitable impurities. The chemical composition mass ratio of the slag washing material is: Al2O320.6wt%, SiO26.7wt%, CaO 43.5wt%, Al8.8wt%, MgO4.9wt%, and the rest is Fe and inevitable impurities.

[0015] In step A, the whole process of bottom blowing nitrogen is adopted in the process of molten steel tapping, and the nitrogen flow rate is controlled at 25-30NL / min; In step B, the chemical composition of silicon calcium barium is: Si 54.5wt%, Ca 10.15wt%, Ba 12.50wt%, Al2.85wt%, and the rest is Fe and inevitable impurities; the chemical composition of ferrosilicon is: Si 73.5wt%, and the rest is Fe and inevitable impurities; the chemical composition of silicon manganese alloy is: Mn65.2wt%, Si 17.6wt%, C1.5wt%, and the rest is Fe and inevitable impurities; the chemical composition of high carbon ferrochrome is: Cr 54.7wt%, C 7.2wt%, P0.085wt%, S 0.088wt%, and the rest is Fe and inevitable impurities; the chemical composition of silicon nitrogen alloy is: Si 57.8wt%, N 42.5wt%, C1.65wt%, P0.075wt%, S 0.045wt%, and the rest is Fe and inevitable impurities; the chemical composition of niobium phosphorus iron alloy is: Nb The chemical composition of vanadium-nitrogen alloy is: V 77.3wt%, N 15.5wt%, C 3.14wt%, P0.089wt%, S 0.057wt%, and the rest are Fe and inevitable impurities.

[0016] In step D, during the refining process, nitrogen gas with a flow rate of 20-25 NL / min is used to blow nitrogen into the molten steel, and then the lower electrode is slag-free at gears 7-9; after 3 minutes of slag-freeing, the electrode is lifted to observe the slag-freeing in the furnace; if the slag sample is too thin and the color is too black, lime is added at 3.5-4.5 kg / t. 钢 Adjust the slag, otherwise add pre-melted refined slag 1.0kg / t 钢 Adjustment; then measure temperature and take samples. According to the steel sample and temperature test results, add alloy to adjust the steel liquid composition and lower electrode temperature to ensure that the composition and temperature are qualified.

[0017] In step E, R9m straight arc continuous straightening 5-machine 5-strand billet casting machine is used for casting.

[0018] In step E, the straightening temperature of the ingot out of the straightening machine is controlled to be 1010-1030°C.

[0019] The present invention further provides a 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar obtained based on the preparation method. It has the following chemical composition in weight percentage: C 0.27-0.29wt%, Si 0.70-0.80wt%, Mn 0.60-0.70wt%, Cr 1.05-1.15wt%, V 0.165-0.180wt%, Nb 0.012-0.016wt%, S≤0.030wt%, P≤0.030wt%, O≤0.0040wt%, N 0.0300-0.0350wt%, and the rest is Fe and inevitable impurities; its process mechanical properties, microstructure, Vickers hardness difference and corrosion resistance are shown in Tables 1 and 2.

[0020] Table 1 Metallographic structure and Vickers hardness of 700MPa ultra-high strength ultra-fine grain earthquake-resistant steel bars produced by the present invention Table 2 Mechanical properties and corrosion resistance of 700MPa ultra-high strength ultra-fine grain earthquake-resistant steel bars produced by the present invention Example 1 A. Molten steel smelting: 140kg / t 钢 、15kg / t 钢 The cold material charging ratio is as follows: the scrap steel (chemical composition: C 0.21wt%, Si 0.28wt%, Mn 0.87wt%, P 0.017wt%, S0.018wt%, the rest is Fe and inevitable impurities) and pig iron (chemical composition: C 3.0wt%, Si 0.15wt%, Mn 0.20wt%, P0.080wt%, S0.020wt%, the rest is Fe and inevitable impurities) in the LD converter are added; then the 900kg / t 钢The molten iron charging ratio is as follows: molten iron of the following temperature and mass ratio is added to the LD converter: molten iron temperature is 1280℃, molten iron composition is C 4.0wt%, Si0.20wt%, Mn 0.20wt%, P 0.072wt%, S 0.021wt%, and the rest is Fe and inevitable impurities; after the scrap steel, pig iron and molten iron are added to the LD converter, conventional top and bottom composite blowing is carried out, conventional lime and light-burned dolomite are added to make slag, the amount of lime added is 22kg / t steel, the amount of light-burned dolomite added is 15kg / t steel, the terminal carbon content is controlled to be 0.06wt%, and the tapping temperature is 1602℃; before tapping, slag washing material (chemical composition mass ratio: Al2O320.6wt%, SiO26.7wt%, CaO 43.5wt%, Al 8.8wt%, MgO4.9wt%, the rest are Fe and inevitable impurities) and active lime for slag washing. The steelmaking process adopts the full bottom blowing nitrogen process, and the nitrogen flow rate is controlled at 25NL / min.

[0021] B. Deoxidation and alloying: After the molten steel smelted in step A is tapped, when the amount of molten steel in the ladle is greater than 1 / 4, add the following materials to the ladle in the following order of deoxidation and alloying: silicon calcium barium → ferrosilicon → silicon manganese alloy → high carbon ferrochrome → silicon nitrogen alloy → niobium phosphorus iron alloy → vanadium nitrogen alloy: 2.0 kg / t 钢 The following mass ratios of silicon, calcium and barium are added: Si 54.5wt%, Ca 10.15wt%, Ba 12.50wt%, Al 2.85wt%, and the rest is Fe and unavoidable impurities; 3.1kg / t 钢 Add the following mass ratio of ferrosilicon: Si 73.5wt%, the rest is Fe and inevitable impurities; at 8.0kg / t 钢 Add the following mass ratio of silicon-manganese alloy: Mn 65.2wt%, Si 17.6wt%, C1.5wt%, the rest is Fe and inevitable impurities; according to 19.5kg / t 钢 The amount of high carbon ferrochrome in the following mass ratio is added: Cr 54.7wt%, C7.2wt%, P 0.085wt%, S0.088wt%, and the rest is Fe and unavoidable impurities; at 1.90kg / t 钢 The amount of silicon-nitrogen alloy with the following mass ratio is added: Si 57.8wt%, N 42.5wt%, C 1.65wt%, P 0.075wt%, S 0.045wt%, and the rest is Fe and unavoidable impurities; at 0.26kg / t 钢The following mass ratio of niobium-phosphorus-iron alloy is added: Nb 43.5wt%, Si 2.7wt%, C1.25wt%, P 1.85wt%, S 0.065wt%, the rest is Fe and unavoidable impurities; at 0.95kg / t 钢 The following vanadium-nitrogen alloys are added in the following mass ratios: V 77.3wt%, N 15.5wt%, C 3.14wt%, P 0.089wt%, S0.057wt%, and the rest are Fe and inevitable impurities; the above alloys are added when the amount of molten steel in the ladle reaches 3 / 4; after the steel is tapped, the molten steel is hoisted to the argon station for adding vanadium-containing pig iron and nitrogen blowing refining treatment.

[0022] C. Molten steel argon station treatment: The molten steel from step B is hoisted to the argon station treatment station and passed through the alloy trough at a pressure of 15.0kg / t 钢 The following vanadium-containing pig iron in mass ratio is added: components C 3.1wt%, Si 0.45wt%, Mn 2.16wt%, V 6.0wt%, P1.05wt%, S 0.075wt%, and the rest are Fe and inevitable impurities. The vanadium-containing pig iron is baked for 10 minutes, and the surface temperature of the pig iron is 300°C. After the vanadium-containing pig iron is added, nitrogen is turned on and nitrogen is blown to the molten steel at a flow rate of 25NL / min for 3 minutes.

[0023] D. LF furnace refining of molten steel: Lift the molten steel from step C to the LF furnace refining station and connect the nitrogen belt. Turn on the nitrogen and use a nitrogen flow rate of 20NL / min to blow nitrogen into the molten steel. Then, use the 7th to 9th gear of the lower electrode to slag. After 3 minutes of slag slag slag, lift the electrode to observe the slag slag situation in the furnace. If the slag sample is too thin and the color is too black, add 3.5kg / t lime. 钢 Adjust the slag, otherwise add pre-melted refined slag 1.0kg / t 钢 Adjustment; then measure temperature and take samples, add alloy to adjust the composition of molten steel and raise the temperature of lower electrode according to the steel sample and temperature test results to ensure that the composition and temperature are qualified; after refining, perform soft nitrogen blowing of molten steel at a small flow rate for 3 minutes, and the nitrogen flow rate is controlled to 20NL / min; after nitrogen blowing, add molten steel covering agent, and the amount added is controlled to 1.0 kg / t 钢 , and then lift the molten steel to the casting station.

[0024] E. Molten steel casting: The tundish temperature is 1540℃, the pulling speed is 3.0m / min, and the crystallizer cooling water flow rate is 140m 3 / h, and the secondary cooling water volume is 1.7L / kg. The molten steel in step C is cast into billets with a cross-section of 165mm×165mm using an R9m straight arc continuous straightening 5-machine 5-strand small square billet casting machine. The straightening temperature of the billet leaving the straightening machine is controlled at 1010℃.

[0025] F. Billet heating: The billet in step E is fed into a heating furnace with a temperature of 1110°C in the soaking section and heated for 90 minutes. The billet tapping temperature is 1065°C and then pushed to a fully continuous bar mill for rolling.

[0026] G. Controlled rolling and cooling of steel billets: The steel billets in step F are rolled for 6 passes at a speed of 1.2 m / s by the rough rolling unit; then the rolled pieces are rolled for 4 passes at a speed of 2.5 m / s by the intermediate rolling unit; then the rolled pieces are rolled for 4 passes at a speed of 9.0 m / s by the pre-finishing rolling unit; then the rolled pieces are controlled to be 950°C by the finishing rolling unit; the rolled pieces are rolled for 2 passes at a speed of 11.0 m / s by the finishing rolling unit; before the rolled pieces enter the finishing rolling unit II, they are controlled to be cooled by 2 water cooling devices with a length of 8 m respectively, and the cooling water volume is 70 m 3 / h, the temperature of the rolled piece entering the second finishing rolling unit is controlled at 880℃; then the rolled piece passes through the second finishing rolling unit at a rolling speed of 14.0m / s for 2 passes; after finishing rolling, the rolled piece passes through a set of water cooling devices with a length of 6m for post-finishing cooling, and the cooling water volume is 40m 3 / h; after controlled cooling, the temperature of the cooling bed on the steel bar is controlled to 880℃, and then the steel bar is naturally air-cooled to room temperature on the cooling bed, and a 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar with a nominal diameter of 20mm and a chemical composition of the following weight percentage is obtained: C 0.27wt%, Si 0.70wt%, Mn 0.60wt%, Cr 1.05wt%, V 0.165wt%, Nb 0.012wt%, S 0.026wt%, P 0.018wt%, O 0.0028wt%, N 0.0300wt%, and the rest is Fe and unavoidable impurities. Its process mechanical properties, microstructure, Vickers hardness difference and corrosion resistance are shown in Tables 3 and 4.

[0027] Table 3 Metallographic structure and Vickers hardness of 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bars with nominal diameter of 20mm produced in Example 1 Table 4 Mechanical properties and corrosion resistance of 700MPa grade ultra-high strength ultra-fine grain seismic steel bars with nominal diameter of 20mm produced in Example 1 Example 2 A. Molten steel smelting: 151kg / t 钢 、15kg / t 钢The cold material charging ratio is as follows: scrap steel (chemical composition: C 0.24wt%, Si 0.41wt%, Mn 1.27wt%, P 0.032wt%, S0.029wt%, the rest is Fe and inevitable impurities) and pig iron (chemical composition: C 3.1wt%, Si 0.29wt%, Mn 0.32wt%, P0.094wt%, S0.030wt%, the rest is Fe and inevitable impurities) are added to the LD converter in the following mass ratio; then the 900kg / t 钢 The molten iron charging ratio is as follows: molten iron of the following temperature and mass ratio is added to the LD converter: molten iron temperature 1305℃, molten iron composition C 4.1wt%, Si 0.32wt%, Mn 0.30wt%, P 0.080wt%, S 0.032wt%, and the rest is Fe and inevitable impurities; after the scrap steel, pig iron and molten iron are added to the LD converter, conventional top and bottom composite blowing is carried out, conventional lime and light-burned dolomite are added to make slag, and the amount of lime added is 24 kg / t 钢 , the amount of light-burned dolomite added is 16kg / t 钢 , control the final carbon content to 0.07wt%, the tapping temperature to 1606℃; before tapping, press 1.0kg / t 钢 and 1.0kg / t 钢 The amount of slag washing material (chemical composition mass ratio: Al2O320.6wt%, SiO26.7wt%, CaO 43.5wt%, Al 8.8wt%, MgO4.9wt%, the rest are Fe and inevitable impurities) and active lime are added for slag washing. The steelmaking process adopts the full bottom blowing nitrogen process, and the nitrogen flow rate is controlled at 30NL / min.

[0028] B. Deoxidation and alloying: After the molten steel smelted in step A is tapped, when the amount of molten steel in the ladle is greater than 1 / 4, add the following materials to the ladle in the following order of deoxidation and alloying: silicon calcium barium → ferrosilicon → silicon manganese alloy → high carbon ferrochrome → silicon nitrogen alloy → niobium phosphorus iron alloy → vanadium nitrogen alloy: 2.0 kg / t 钢 Add the following mass ratio of silicon, calcium and barium: Si 54.5wt%, Ca 10.15wt%, Ba 12.50wt%, Al 2.85wt%, the rest is Fe and unavoidable impurities; at 3.8kg / t 钢 Add the following mass ratio of ferrosilicon: Si 73.5wt%, the rest is Fe and inevitable impurities; at 8.9kg / t 钢 Add the following mass ratio of silicon-manganese alloy: Mn 65.2wt%, Si 17.6wt%, C1.5wt%, the rest is Fe and unavoidable impurities; according to 20.4kg / t 钢The amount of high carbon ferrochrome in the following mass ratio is added: Cr 54.7wt%, C7.2wt%, P 0.085wt%, S0.088wt%, and the rest is Fe and unavoidable impurities; at 2.00kg / t 钢 The amount of silicon-nitrogen alloy with the following mass ratio is added: Si 57.8wt%, N 42.5wt%, C 1.65wt%, P 0.075wt%, S0.045wt%, and the rest is Fe and unavoidable impurities; at 0.32kg / t 钢 The following mass ratio of niobium-phosphorus-iron alloy is added: Nb 43.5wt%, Si 2.7wt%, C 1.25wt%, P 1.85wt%, S 0.065wt%, the rest is Fe and unavoidable impurities; at 1.04kg / t 钢 The following vanadium-nitrogen alloys are added in the following mass ratios: V 77.3wt%, N 15.5wt%, C 3.14wt%, P 0.089wt%, S0.057wt%, and the rest are Fe and inevitable impurities; the above alloys are added when the amount of molten steel in the ladle reaches 3 / 4; after the steel is tapped, the molten steel is hoisted to the argon station for adding vanadium-containing pig iron and nitrogen blowing refining treatment.

[0029] C. Molten steel argon station treatment: The molten steel from step B is hoisted to the argon station treatment station and passed through the alloy trough at a pressure of 15.0kg / t 钢 The following vanadium-containing pig iron in mass ratio is added: components C 3.1wt%, Si 0.45wt%, Mn 2.16wt%, V 6.0wt%, P1.05wt%, S 0.075wt%, and the rest are Fe and inevitable impurities. The vanadium-containing pig iron is baked for 10 minutes, and the surface temperature of the pig iron is 326°C. After the vanadium-containing pig iron is added, nitrogen is turned on and nitrogen is blown to the molten steel at a flow rate of 30NL / min for 3 minutes.

[0030] D. LF furnace refining of molten steel: Lift the molten steel from step C to the LF furnace refining station and connect the nitrogen belt. Turn on the nitrogen and use a nitrogen flow rate of 25NL / min to blow nitrogen into the molten steel. Then, use the 7th to 9th gear of the lower electrode to slag. After 3 minutes of slag slag slag, lift the electrode to observe the slag slag situation in the furnace. If the slag sample is too thin and the color is too black, add 4.0kg / t lime. 钢 Adjust the slag, otherwise add pre-melted refined slag 1.0kg / t 钢 Adjustment; then measure temperature and take samples, add alloy to adjust the composition of molten steel and raise the temperature of lower electrode according to the steel sample and temperature test results to ensure that the composition and temperature are qualified; after refining, perform soft nitrogen blowing of molten steel at a small flow rate for 3 minutes, and the nitrogen flow rate is controlled to 20NL / min; after nitrogen blowing, add molten steel covering agent, and the amount added is controlled to 1.0 kg / t 钢, and then lift the molten steel to the casting station.

[0031] E. Molten steel casting: The tundish temperature is 1535℃, the pulling speed is 3.1m / min, and the crystallizer cooling water flow rate is 145m 3 / h, and the secondary cooling water volume was 1.8L / kg. The molten steel from step C was cast into billets with a cross-section of 165mm×165mm using an R9m straight arc continuous straightening 5-machine 5-strand billet casting machine. The straightening temperature of the billet leaving the straightening machine was controlled at 1022℃.

[0032] F. Billet heating: The billet in step E is fed into a heating furnace with a temperature of 1140°C in the soaking section and heated for 90 minutes. The billet tapping temperature is 1060°C and then pushed to a fully continuous bar mill for rolling.

[0033] G. Controlled rolling and controlled cooling of steel billets: The steel billets in step F are rolled for 6 passes at a speed of 1.0 m / s by the rough rolling unit; then the rolled pieces are rolled for 4 passes at a speed of 2.1 m / s by the intermediate rolling unit; then the rolled pieces are rolled for 3 passes at a speed of 8.0 m / s by the pre-finishing rolling unit; then the rolled pieces are controlled to be 940°C by the first finishing rolling unit; the rolled pieces are rolled for 1 pass at a speed of 10.0 m / s by the first finishing rolling unit; before the rolled pieces enter the second finishing rolling unit, they are controlled to be cooled by 2 water cooling devices with a length of 8 m respectively, and the cooling water volume is 75 m 3 / h, the temperature of the rolled piece entering the second finishing rolling group is controlled at 865℃; then the rolled piece passes through the second finishing rolling group at a rolling speed of 13.0m / s for 2 passes; after finishing rolling, the rolled piece passes through a group of water cooling devices with a length of 6m for post-finishing cooling, and the cooling water volume is 45m 3 / h; after controlled cooling, the temperature of the cooling bed on the steel bar is controlled to 875℃, and then the steel bar is naturally air-cooled to room temperature on the cooling bed, and a 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar with a nominal diameter of 20mm and a chemical composition of the following weight percentage is obtained: C 0.28wt%, Si 0.76wt%, Mn 0.64wt%, Cr 1.11wt%, V 0.173wt%, Nb 0.014wt%, S 0.028wt%, P 0.025wt%, O 0.0034wt%, N 0.0320wt%, and the rest is Fe and unavoidable impurities. Its process mechanical properties, microstructure, Vickers hardness difference and corrosion resistance are shown in Tables 5 and 6.

[0034] Table 5 Metallographic structure and Vickers hardness of 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bars with nominal diameter of 25mm produced in Example 2 Table 6 Mechanical properties and corrosion resistance of 700MPa grade ultra-high strength ultra-fine grain seismic steel bars with nominal diameter of 25mm produced in Example 2 Example 3 A. Molten steel smelting: 155kg / t 钢 、15kg / t 钢 The cold material charging ratio is as follows: the scrap steel (chemical composition: C0.25wt%, Si 0.55wt%, Mn 1.52wt%, P 0.045wt%, S0.045wt%, the rest is Fe and inevitable impurities) and pig iron (chemical composition: C 3.3wt%, Si 0.40wt%, Mn 0.40wt%, P0.112wt%, S0.042wt%, the rest is Fe and inevitable impurities) in the LD converter are added; then the 900kg / t 钢 The molten iron charging ratio is as follows: molten iron of the following temperature and mass ratio is added to the LD converter: molten iron temperature is 1308℃, molten iron composition is C 4.3wt%, Si0.45wt%, Mn 0.42wt%, P 0.090wt%, S 0.035wt%, and the rest is Fe and inevitable impurities; after the scrap steel, pig iron and molten iron are added to the LD converter, conventional top and bottom composite blowing is carried out, conventional lime and light-burned dolomite are added to make slag, and the amount of lime added is 25 kg / t 钢 , the amount of light-burned dolomite added is 18kg / t 钢 , control the final carbon content to 0.09wt%, the tapping temperature to 1610℃; before tapping, press 1.0kg / t 钢 and 1.0kg / t 钢 The amount of slag washing material (chemical composition mass ratio: Al2O320.6wt%, SiO26.7wt%, CaO 43.5wt%, Al 8.8wt%, MgO4.9wt%, the rest are Fe and inevitable impurities) and active lime are added for slag washing. The steelmaking process adopts the bottom blowing nitrogen process throughout the process, and the nitrogen flow rate is controlled at 25NL / min.

[0035] B. Deoxidation and alloying: After the molten steel smelted in step A is tapped, when the amount of molten steel in the ladle is greater than 1 / 4, add the following materials to the ladle in the following order of deoxidation and alloying: silicon calcium barium → ferrosilicon → silicon manganese alloy → high carbon ferrochrome → silicon nitrogen alloy → niobium phosphorus iron alloy → vanadium nitrogen alloy: 2.0 kg / t 钢 Add the following mass ratio of silicon, calcium and barium: Si 54.5wt%, Ca 10.15wt%, Ba 12.50wt%, Al 2.85wt%, the rest is Fe and unavoidable impurities; at 4.5kg / t钢 Add the following mass ratio of ferrosilicon: Si 73.5wt%, the rest is Fe and inevitable impurities; at 9.6kg / t 钢 Add the following mass ratio of silicon-manganese alloy: Mn 65.2wt%, Si 17.6wt%, C1.5wt%, the rest is Fe and unavoidable impurities; according to 21.5kg / t 钢 Add the following mass ratio of high carbon ferrochrome: Cr 54.7wt%, C7.2wt%, P 0.085wt%, S0.088wt%, the rest is Fe and unavoidable impurities; at 1.90~2.10kg / t 钢 The amount of silicon-nitrogen alloy is added in the following mass ratio: Si 57.8wt%, N 42.5wt%, C 1.65wt%, P0.075wt%, S 0.045wt%, and the rest is Fe and unavoidable impurities; at 0.40kg / t 钢 The following mass ratio of niobium-phosphorus-iron alloy is added: Nb43.5wt%, Si 2.7wt%, C1.25wt%, P1.85wt%, S 0.065wt%, the rest is Fe and unavoidable impurities; at 1.16kg / t 钢 The following vanadium-nitrogen alloys are added in the following mass ratios: V 77.3wt%, N 15.5wt%, C 3.14wt%, P0.089wt%, S 0.057wt%, and the rest are Fe and inevitable impurities; the above alloys are added when the amount of molten steel in the ladle reaches 3 / 4; after the steel is tapped, the molten steel is hoisted to the argon station for adding vanadium-containing pig iron and nitrogen blowing refining treatment.

[0036] C. Molten steel argon station treatment: The molten steel from step B is hoisted to the argon station treatment station and passed through the alloy trough at a pressure of 15.0kg / t 钢 The following vanadium-containing pig iron in mass ratio is added: components C 3.1wt%, Si 0.45wt%, Mn 2.16wt%, V 6.0wt%, P1.05wt%, S 0.075wt%, and the rest are Fe and inevitable impurities. The vanadium-containing pig iron is baked for 10 minutes, and the surface temperature of the pig iron is 360°C. After the vanadium-containing pig iron is added, nitrogen is turned on and nitrogen is blown to the molten steel at a flow rate of 30NL / min for 3 minutes.

[0037] D. LF furnace refining of molten steel: Lift the molten steel from step C to the LF furnace refining station and connect the nitrogen belt. Turn on the nitrogen and use nitrogen with a flow rate of 25NL / min to blow nitrogen into the molten steel. Then, use the 7th to 9th gear of the lower electrode to slag. After 3 minutes of slag slag slag, lift the electrode to observe the slag slag situation in the furnace. If the slag sample is too thin and the color is too black, add 4.5kg / t lime. 钢Adjust the slag, otherwise add pre-melted refined slag 1.0kg / t 钢 Adjustment; then measure temperature and take samples, add alloy to adjust the steel liquid composition and lower electrode temperature according to the steel sample and temperature test results to ensure that the composition and temperature are qualified; after refining, perform low-flow soft nitrogen blowing on the molten steel for 3 minutes, and the nitrogen flow rate is controlled to 25NL / min; after nitrogen blowing, add molten steel covering agent, and the amount added is controlled to 1.0 kg / t 钢 , and then lift the molten steel to the casting station.

[0038] E. Molten steel casting: The tundish temperature is 1525℃, the pulling speed is 3.2m / min, and the crystallizer cooling water flow rate is 150m 3 / h, and the secondary cooling water volume was 1.9L / kg. The molten steel from step C was cast into billets with a cross-section of 165mm×165mm using an R9m straight arc continuous straightening 5-machine 5-strand billet casting machine. The straightening temperature of the billet leaving the straightening machine was controlled at 1030℃.

[0039] F. Billet heating: The billet in step E is fed into a heating furnace with a temperature of 1160°C in the soaking section and heated for 90 minutes. The billet tapping temperature is 1065°C and then pushed to a fully continuous bar mill for rolling.

[0040] G. Controlled rolling and controlled cooling of steel billets: The steel billets in step F are rolled for 6 passes at a speed of 0.8 m / s by the rough rolling unit; then the rolled pieces are rolled for 4 passes at a speed of 1.8 m / s by the intermediate rolling unit; then the rolled pieces are rolled for 2 passes at a speed of 7.0 m / s by the pre-finishing rolling unit; then the rolled pieces are controlled to be 920°C by the finishing rolling unit; the rolled pieces are rolled for 1 pass by the finishing rolling unit at a speed of 10.0 m / s; before the rolled pieces enter the finishing rolling unit II, they are controlled to be cooled by 2 water cooling devices with a length of 8 m respectively, and the cooling water volume is 80 m 3 / h, the temperature of the rolled piece entering the second finishing rolling unit is controlled at 850℃; then the rolled piece passes through the second finishing rolling unit at a rolling speed of 12.0m / s for 2 passes; after finishing rolling, the rolled piece passes through a set of water cooling devices with a length of 6m for post-finishing cooling, and the cooling water volume is 50m 3 / h; after controlled cooling, the temperature of the cooling bed on the steel bar is controlled to 860℃, and then the steel bar is naturally air-cooled to room temperature on the cooling bed, and a 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar with a nominal diameter of 28mm and a chemical composition of the following weight percentage is obtained: C 0.29wt%, Si 0.80wt%, Mn 0.70wt%, Cr 1.15wt%, V 0.180wt%, Nb 0.016wt%, S 0.030wt%, P 0.030wt%, O 0.0040wt%, N 0.0350wt%, and the rest is Fe and unavoidable impurities. Its process mechanical properties, microstructure, Vickers hardness difference and corrosion resistance are shown in Tables 7 and 8.

[0041] Table 7 Microstructure and Vickers hardness of 700MPa ultra-high strength ultra-fine grain seismic steel bars with nominal diameter of 28mm produced in Example 3 Table 8 Mechanical properties and corrosion resistance of 700MPa grade ultra-high strength ultra-fine grain seismic steel bars with nominal diameter of 28mm produced in Example 3 .

Claims

1. A method for preparing 700MPa ultra-high strength ultra-fine grain earthquake-resistant steel bars, characterized in that: To do this, follow these steps: A. Molten steel smelting: 140-155kg / t 钢 、15kg / t 钢 The cold material charging ratio is 900kg / t. Scrap steel and pig iron are added to the LD converter. 钢 Add molten iron with a temperature of ≥1280℃ and carry out conventional top and bottom composite blowing: according to 22-25kg / t 钢 , 15-18kg / t 钢 Conventional lime and light-burned dolomite are added to make slag, and the final carbon content is controlled to be ≥0.06wt%, and the tapping temperature is ≤1610℃; before tapping, 1.0kg / t is added to the bottom of the ladle. 钢 and 1.0kg / t 钢 Add slag washing material and active lime in the amounts required for slag washing; B. Deoxidation and alloying: The molten steel after smelting is tapped. When the amount of molten steel in the ladle is greater than 1 / 4, the deoxidation and alloying sequence is silicon-calcium-barium → ferrosilicon → silicon-manganese alloy → high carbon ferrochrome → silicon-nitrogen alloy → niobium-phosphorus-iron alloy → vanadium-nitrogen alloy, respectively, at a rate of 2.0 kg / t. 钢 , 3.1~4.5kg / t 钢 、8.0~9.6kg / t 钢 , 19.5~21.5kg / t 钢 , 1.90~2.10kg / t 钢 , 0.26~0.40 kg / t 钢 , 0.95~1.16kg / t 钢 Add silicon calcium barium, ferrosilicon, silicon manganese alloy, high carbon ferrochrome, silicon nitrogen alloy, niobium phosphorus iron alloy and vanadium nitrogen alloy to the ladle in order, and add all the above alloys when the amount of molten steel in the ladle reaches 3 / 4; C. Argon treatment of molten steel: After the molten steel is tapped in step B, it is hoisted to the argon treatment station and passed through the alloy trough at a pressure of 15.0kg / t 钢 The amount of vanadium-containing pig iron having the following mass ratio is added: C 3.1wt%, Si 0.45wt%, Mn 2.16wt%, V 6.0wt%, P1.05wt%, S 0.075wt%, and the rest is Fe and inevitable impurities; the baking time of the vanadium-containing pig iron is 10 minutes, and the surface temperature of the pig iron is ≥300°C; after the vanadium-containing pig iron is added, the molten steel is subjected to nitrogen blowing treatment; D. LF furnace refining of molten steel: The molten steel in step C is hoisted to the LF furnace for refining. After the refining, a small flow rate of soft nitrogen blowing is performed on the molten steel. After the nitrogen blowing is completed, a molten steel covering agent is added, and then the molten steel is hoisted to the casting station; E. Molten steel casting: The tundish temperature is 1525-1540°C, the pulling speed is 3.0-3.2 m / min, and the crystallizer cooling water flow rate is 140-150 m 3 / h, and the secondary cooling water volume is 1.7-1.9L / kg, casting the molten steel in step D into a steel billet with a cross-section of 165mm×165mm; F. Billet heating: The billet in step E is sent to a heating furnace with a soaking section temperature of 1110-1160°C for heating for 90 minutes. The billet tapping temperature is 1050-1065°C, and then pushed to a fully continuous bar mill for rolling; G. Controlled rolling and cooling of steel billets: The steel billets in step F are rolled for 6 passes at a rolling speed of 0.8-1.2 m / s by the rough rolling unit, and then rolled for 4 passes at a rolling speed of 1.8-2.5 m / s by the intermediate rolling unit, and then rolled for 2-4 passes at a rolling speed of 7.0-9.0 m / s by the pre-finishing rolling unit; then the rolled pieces are rolled for 1-2 passes at a rolling speed of 10.0-11.0 m / s by the finishing rolling unit, and the temperature of the rolled pieces entering the finishing rolling unit is controlled at 920-950°C; before the rolled pieces enter the finishing rolling unit, they are controlled to cool by two water cooling devices with a length of 8 m each, and the cooling water volume is 70-80 m 3 / h, the temperature of the rolled piece entering the second finishing rolling group is controlled at 850-880℃; then the rolled piece passes through the second finishing rolling group at a rolling speed of 12.0-14.0m / s for 2 passes; after finishing rolling, the rolled piece passes through a group of water cooling devices with a length of 6m for post-finishing cooling, and the cooling water volume is 40-50m 3 / h; after controlled cooling, the temperature of the steel bar on the cooling bed is controlled at 860-880℃, and then the steel bar is naturally air-cooled to room temperature on the cooling bed, thus obtaining 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bars with a nominal diameter of 20-28mm.

2. The method for preparing the 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar according to claim 1, characterized in that: In step A, the chemical composition of the scrap steel is: C 0.21-0.25wt%, Si 0.28-0.55wt%, Mn 0.87-1.52wt%, P0.017-0.045wt%, S 0.018-0.045wt%, and the rest is Fe and inevitable impurities; the chemical composition of the pig iron is: C 3.0-3.3wt%, Si 0.15-0.40wt%, Mn 0.20-0.40wt%, P 0.080-0.112wt%, S 0.020-0.042wt%, and the rest is Fe and inevitable impurities.

3. The method for preparing the 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar according to claim 1, characterized in that: In step A, the chemical composition of the molten iron is: C 4.0-4.3wt%, Si 0.20-0.45wt%, Mn 0.20-0.42wt%, P0.072-0.090wt%, S≤0.035wt%, and the rest is Fe and inevitable impurities.

4. The method for preparing the 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar according to claim 1, characterized in that: In step A, the chemical composition mass ratio of the slag washing material is: Al2O3 20.6wt%, SiO26.7wt%, CaO 43.5wt%, Al8.8wt%, MgO4.9wt%, and the rest is Fe and inevitable impurities.

5. The method for preparing the 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar according to claim 1, characterized in that: In step A, the molten steel tapping process adopts the whole bottom blowing nitrogen process, and the nitrogen flow rate is controlled at 25-30NL / min.

6. The method for preparing the 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar according to claim 1, characterized in that: In step B, the chemical composition of silicon calcium barium is: Si 54.5wt%, Ca 10.15wt%, Ba 12.50wt%, Al 2.85wt%, and the rest is Fe and inevitable impurities; the chemical composition of ferrosilicon is: Si 73.5wt%, and the rest is Fe and inevitable impurities; the chemical composition of silicon manganese alloy is: Mn 65.2wt%, Si 17.6wt%, C1.5wt%, and the rest is Fe and inevitable impurities; the chemical composition of high carbon ferrochrome is: Cr 54.7wt%, C 7.2wt%, P 0.085wt%, S 0.088wt%, and the rest is Fe and inevitable impurities; the chemical composition of silicon nitrogen alloy is: Si 57.8wt%, N 42.5wt%, C 1.65wt%, P0.075wt%, S 0.045wt%, the rest are Fe and inevitable impurities; the chemical composition of niobium-phosphorus-iron alloy is: Nb43.5wt%, Si 2.7wt%, C1.25wt%, P 1.85wt%, S 0.065wt%, the rest are Fe and inevitable impurities; the chemical composition of vanadium-nitrogen alloy is: V 77.3wt%, N 15.5wt%, C 3.14wt%, P 0.089wt%, S 0.057wt%, the rest are Fe and inevitable impurities.

7. The method for preparing the 700MPa ultra-high strength ultra-fine grain earthquake-resistant steel bar according to claim 1, characterized in that: In step D, during the refining process, nitrogen gas with a flow rate of 20-25 NL / min is used to blow nitrogen into the molten steel, and then the lower electrode is slag-free at gears 7-9; after 3 minutes of slag-freeing, the electrode is lifted to observe the slag-freeing in the furnace; if the slag sample is too thin and the color is too black, lime is added at 3.5-4.5 kg / t 钢 Adjust the slag, otherwise add pre-melted refined slag 1.0kg / t 钢 Adjustment; then measure temperature and take samples. According to the steel sample and temperature test results, add alloy to adjust the steel liquid composition and lower electrode temperature to ensure that the composition and temperature are qualified.

8. The method for preparing the 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar according to claim 1, characterized in that: In step E, R9m straight arc continuous straightening 5-machine 5-strand billet casting machine is used for casting.

9. The method for preparing the 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar according to claim 1, characterized in that: In step E, the straightening temperature of the ingot out of the straightening machine is controlled to be 1010-1030°C.

10. A 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel obtained by the preparation method according to any one of claims 1 to 9 The tendon is characterized in that The 700MPa grade ultra-high strength ultra-fine grain earthquake-resistant steel bar has the following chemical composition in weight percentage: C 0.27-0.29wt%, Si 0.70-0.80wt%, Mn 0.60-0.70wt%, Cr 1.05-1.15wt%, V0.165-0.180wt%, Nb 0.012-0.016wt%, S≤0.030wt%, P≤0.030wt%, O≤0.0040wt%, N0.0300-0.0350wt%, and the rest is Fe and inevitable impurities; its yield strength is 710-745MPa, the tensile strength is 895-950MPa, the maximum force total elongation is ≥10.0, and the ferrite grain size is 12.5-14.0.

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