Low-cost high-elongation 630MPa-grade high-strength steel bar and production process thereof

Abstract

The invention discloses a 630MPa-grade high-strength steel bar with low cost and high elongation and a production process thereof. The high-strength steel bar comprises the following chemical components in percentage by weight: 0.25 to 0.27 percent of C, 0.73 to 0.78 percent of Si, 1.50 to 1.59 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, 0.016 to 0.019 percent of Nb, 0.145 to 0.155 percent of V, 0.020 to 0.021 percent of N and the balance of Fe and inevitable impurities. The production process comprises the steps of KR pretreatment, converter smelting, refining, continuous casting, electromagnetic induction heating, rolling and cooling. The high-strength steel bar has good comprehensive mechanical properties, the yield strength is larger than or equal to 630 MPa, the tensile strength is larger than or equal to 815 MPa, the percentage elongation after fracture is larger than or equal to 13%, and the ratio of strength to yield is larger than or equal to 1.28.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical materials, and specifically relates to a low-cost, high-elongation, 630MPa-grade high-strength steel bar and a production process thereof. Background Art

[0002] In recent years, with the green transformation and high-quality development of my country's steel industry and construction industry, high-strength steel bars have become one of the important ways to improve building quality, save project costs, promote industrial transformation, cultivate new quality productivity, and achieve green and high-quality development. In the past, the steel grade required for large-scale construction facilities in China was 400MPa and some 500MPa steel bars, but this kind of steel bar has low strength, high consumption, lack of reliability, and greatly increases the construction period. The market's performance requirements for building materials are increasing. High-strength steel bars, especially steel bars above 600MPa, have been widely used in the structural design of high-rise buildings, long-span bridges and earthquake-prone areas due to their excellent mechanical properties; however, traditional high-strength steel bars often sacrifice elongation and strength-to-yield ratio while pursuing high yield strength, which limits their use effect in complex stress environments. In addition, the traditional hot rolling process of high-strength steel bars involves a heating furnace for heating the ingot, which is still an important constraint in the environment of environmental protection and cost reduction to advocate green steel manufacturing. In recent years, with the continuous development of new heating process equipment, such as the continuous advancement of electromagnetic induction heating equipment, the uniformity and efficiency of heating the ingot using this equipment have been significantly improved. Therefore, it is of great significance to use new heating equipment to change the existing traditional process and develop a low-cost, high-elongation 630MPa grade high-strength steel bar and its production process in a targeted manner.

[0003] The mainstream high-strength steel bars of 600MPa and above on the market, such as HRB600, HTRB600, T63, etc., have reached a high level in yield strength, but still need to be improved in terms of elongation and strength-to-yield ratio. These products usually adopt micro-alloying, controlled rolling and controlled cooling and other process methods to improve strength by refining grains and precipitation strengthening, but the plasticity of the material is often reduced during processing, and the elongation is insufficient, which makes it difficult to meet the requirements of complex structures for the comprehensive performance of the material. The production process of conventional high-strength steel bars mainly includes raw material preparation, smelting, continuous casting, heating, rolling, cooling and other steps; among them, the desulfurization process is a pretreatment technology for molten iron, which improves the kinetic conditions of desulfurization by controlling stirring, and is a key process for controlling the sulfur content of the final product. Secondly, the heating process uses traditional heating furnaces for heating, which cannot avoid problems such as low thermal efficiency, high maintenance costs, slow heating speed and environmental pollution; while electromagnetic induction heating solves the inherent defects of traditional heating furnaces. The use of electromagnetic heating will not cause fuel energy consumption, and will not produce pollution such as smoke and exhaust gas. It can realize green steel manufacturing, and it also has higher heat conduction efficiency, which can effectively shorten the heating time of the ingot, accurately control the temperature to reduce energy consumption, and comprehensively reduce costs. In addition, rolling controlled cooling technology is the key to improving the strength of steel bars. By controlling the rolling temperature and cooling rate, fine grains and dispersed precipitation phases can be formed inside the steel bars, thereby improving the yield strength and tensile strength. During the rolling and cooling process of high-strength steel bars, due to the refinement of grains and the formation of precipitation phases, the plastic deformation capacity of the material is limited, resulting in a significant decrease in elongation. While the traditional ingot heating and rolling process improves the yield strength, its tensile strength is not improved enough, resulting in a low strength-to-yield ratio; a low strength-to-yield ratio means that the material is quickly destroyed after reaching the yield point, which is not conducive to the overall stability and seismic performance of the structure. In addition, due to factors such as split rolling, uneven temperature distribution, and difficulty in accurately controlling the cooling rate during the rolling process, the internal structure of the steel bars is prone to unevenness, further affecting the stability of its performance. In this regard, the present invention adopts targeted physical metallurgical material mechanism analysis and equipment process optimization exploration to comprehensively solve the above-mentioned problems; it adopts new equipment and special adaptive processes to manufacture high-strength steel bars, which effectively realizes green manufacturing production while reducing costs. Summary of the invention

[0004] The purpose of the present invention is to provide a low-cost, high-elongation 630MPa grade high-strength steel bar and its production process. The high-strength steel bar provided has a yield strength of ≥630MPa, a tensile strength of ≥815MPa, an elongation after fracture of ≥13%, and a strength-to-yield ratio of ≥1.28.

[0005] To achieve the above purpose, the specific technical solutions adopted by the present invention are as follows: A low-cost, high-elongation 630MPa grade high-strength steel bar, whose chemical composition by weight percentage is: C: 0.25-0.27%, Si: 0.73-0.78%, Mn: 1.50-1.59%, P: ≤0.015%, S: ≤0.015%, Nb: 0.016-0.019%, V: 0.145-0.155%, N: 0.020-0.021%, and the balance is Fe and unavoidable impurities.

[0006] The mechanism of action of the chemical components in the low-cost, high-elongation 630MPa grade high-strength steel bar of the present invention is as follows: C: 0.25-0.27%. C is the most important strengthening element in steel and can effectively improve the strength and hardness of steel bars at low cost.

[0007] Si: 0.73-0.78%. Si can increase the strength and hardness of steel bars, while improving the toughness and corrosion resistance of steel.

[0008] Mn: 1.50-1.59%. Mn is an important strengthening element in steel bars. It can also deoxidize and desulfurize to improve the cold working performance and welding performance of steel.

[0009] P: ≤0.015%, P is generally regarded as a harmful element. Adding P will increase strength and hardness, but plasticity and toughness will decrease significantly.

[0010] S: ≤0.015%. S exists in steel in the form of non-metallic sulfide inclusions, which will reduce various mechanical properties of the steel.

[0011] Nb: 0.016-0.019%. Nb significantly improves the strength and toughness of steel bars by refining grains and strengthening through precipitation. It can also improve the welding performance and resistance to hydrogen-induced cracking of steel.

[0012] V: 0.145-0.155%. V is particularly effective in improving strength, while also improving plasticity and toughness.

[0013] N: 0.020~0.021%. N can form stable compounds with Nb, V, etc. in steel, which plays a role of precipitation strengthening. However, too high nitrogen content will increase the aging sensitivity of steel and affect the toughness and welding performance of steel.

[0014] Furthermore, the diameter of the 630MPa grade high-strength steel bar of the present invention is 12 to 32 mm.

[0015] Furthermore, the 630MPa grade high-strength steel bar of the present invention has a yield strength of ≥630MPa, a tensile strength of ≥815MPa, an elongation after fracture of ≥13%, and a strength-to-yield ratio of ≥1.28.

[0016] Furthermore, the 630MPa grade high-strength steel bar of the present invention is a pearlite and ferrite structure, with the pearlite accounting for 53% to 61% and the grain size being 8.0 to 9.5.

[0017] The production method of the high-strength steel bar with high yield ratio and high elongation of 630 MPa grade of the present invention comprises KR pretreatment→converter smelting→refining→continuous casting→electromagnetic induction heating→rolling→cooling.

[0018] Furthermore, in the KR pretreatment of the present invention, the S content after the KR pretreatment is less than 0.015%.

[0019] Furthermore, the converter smelting described in the present invention, through N2 blowing from the bottom of the converter, combined with the addition of VN alloy, V-Fe, and Nb-Fe for micro-alloying, accurately matches the scrap steel, molten iron, cold material conditions and slag retention operation, the quality of raw and auxiliary materials such as lime, splashing during the smelting process, and controls the converter end point at 1680-1690°C, etc., to reduce raw material consumption.

[0020] Furthermore, the refining of the present invention adopts low basicity (1-2.3) slag making with a cycle of 20-40 minutes.

[0021] Furthermore, the continuous casting of the present invention adopts 5-stream continuous casting, the pulling speed is 2-3m / min, the fluctuation is less than 0.2m / min, the secondary cooling adopts strong cooling (specific water volume 1.67-1.75L / kg) plus low superheat (superheat <35°C), and the continuous casting steel temperature is 1560-1590°C.

[0022] Furthermore, in the electromagnetic induction heating of the present invention, the continuous casting billet is conveyed to the electromagnetic induction heating for supplementary heating via a conveying roller with a cover and insulation, the temperature is maintained at 1170-1190° C., and the roller speed is 1-3 m / s.

[0023] Furthermore, the rolling described in the present invention includes rough rolling, intermediate rolling and finishing rolling; the starting temperature of the rough rolling is 1030-1130°C, and the roller speed is 0.5-3m / s; the slab is water-cooled before finishing rolling, the water cooling pressure is 0.8-1.2MPa, the roller speed is 8-11m / s, and after water cooling, it enters the finishing mill, and the starting temperature of the finishing rolling is 940-1040°C.

[0024] Furthermore, in the cooling of the present invention, the water pressure of the cooling equipment is 1.2-1.5 MPa, the roller speed is 8-15 m / s, the final cooling temperature is 820-880°C, the upper cooling bed return temperature is 900°C-950°C, and then air-cooled to room temperature on a walking cooling bed.

[0025] The beneficial effects of the technical solution of the present invention are: The present invention shortens the whole process by direct rolling after continuous casting, adopts electromagnetic induction to supplement heating of the ingot, controls the deformation temperature window by high-speed continuous rolling and adopts a multi-stage controlled cooling process, reduces the cost and waste gas problems generated by the heating furnace; and by optimizing the alloy composition and the post-rolling cooling microstructure phase change control process, without increasing the alloy composition, obtains 630MPa-grade high-strength steel bars with a diameter of 12 to 32mm, a yield strength of ≥630MPa, a tensile strength of ≥815MPa, an elongation after fracture of ≥13%, and a strength-to-yield ratio of ≥1.28, and the steel bars have excellent comprehensive performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The metallographic structure of the Φ12 specification 630MPa grade high-strength steel bar in Example 1; Figure 2 The metallographic structure of the Φ14 specification 630MPa grade high-strength steel bar in Example 2; Figure 3 The metallographic structure of the Φ16 specification 630MPa grade high-strength steel bar in Example 3; Figure 4 The metallographic structure of the Φ18 specification 630MPa grade high-strength steel bar in Example 4; Figure 5 The metallographic structure of the Φ20 specification 630MPa grade high-strength steel bar in Example 5; Figure 6 The metallographic structure of the Φ22 specification 630MPa grade high-strength steel bar in Example 6; Figure 7 The metallographic structure of the Φ25 specification 630MPa grade high-strength steel bar in Example 7; Figure 8 This is the metallographic structure of the Φ32 specification 630MPa grade high-strength steel bar in Example 8. DETAILED DESCRIPTION

[0027] The present invention is further described below in conjunction with specific implementation cases and drawings, but the present invention is not limited to these embodiments. Example 1

[0028] The diameter of the 630MPa grade high-strength steel bar in this embodiment is 12mm, and the chemical composition is shown in Table 1; its production process includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling; the specific process control is as follows: (1) KR pretreatment: S content after KR pretreatment is 0.015%.

[0029] (2) Converter smelting: N2 is blown from the bottom of the converter, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, and splashing occurs during the smelting process. The final converter temperature is 1683°C.

[0030] (3) Refining: low basicity slag making is adopted, the slag making basicity is 1, and the slag making cycle is 28 minutes.

[0031] (4) Continuous casting: 5-stream continuous casting is adopted, the casting speed is 2.9m / min, the fluctuation is less than 0.2m / min, the secondary cooling adopts strong cooling plus low superheat, the specific water volume is 1.70L / kg, the superheat is 32℃, and the continuous casting steel temperature is 1587℃.

[0032] (5) Electromagnetic induction heating: The continuous casting billet is sent to the electromagnetic induction heating machine for supplementary heating through a conveyor roller with a cover and insulation. The temperature is maintained at 1170°C and the roller speed is 3m / s, and then it enters the rolling process.

[0033] (6) Rolling: including rough rolling, intermediate rolling and finishing rolling. The starting temperature of rough rolling is 1127℃ and the roller speed is 3m / s. After intermediate rolling and before finishing rolling, the slab is water-cooled. The water cooling pressure is 1.2MPa and the roller speed is 11m / s. Finishing rolling is carried out after water cooling. The starting temperature of finishing rolling is 940℃ and cooling is carried out after final rolling.

[0034] (7) Cooling: The water pressure of the cooling equipment is 1.2 MPa, the roller speed is 15 m / s, the final cooling temperature is 832 °C, the return temperature of the upper cooling bed is 900 °C, and then it is air-cooled to room temperature on a step-by-step cooling bed.

[0035] The metallographic structure of the 630MPa grade high-strength steel bar produced in this embodiment is as follows: Figure 1 As shown by Figure 1 It can be seen that it is a pearlite and ferrite structure, with pearlite accounting for 58% and a grain size of 9.5.

[0036] The 630MPa grade high-strength steel bar produced in this embodiment has good comprehensive mechanical properties, with a tensile strength of 875MPa, a yield strength of 683MPa, an elongation after fracture of 13.0%, and a strength-to-yield ratio of 1.28. Example 2

[0037] The diameter of the 630MPa grade high-strength steel bar in this embodiment is 14mm, and the chemical composition is shown in Table 1; its production process includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling; the specific process control is as follows: (1) KR pretreatment: S content after KR pretreatment is 0.015%.

[0038] (2) Converter smelting: N2 is blown from the bottom of the converter, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, and splashing occurs during the smelting process. The final converter temperature is 1685°C.

[0039] (3) Refining: low basicity slag making is adopted, the slag making basicity is 1.3, and the slag making cycle is 20 minutes.

[0040] (4) Continuous casting: 5-stream continuous casting is adopted, the casting speed is 2m / min, the fluctuation is less than 0.2m / min, the secondary cooling adopts strong cooling plus low superheat, the specific water volume is 1.67L / kg, the superheat is 34℃, and the continuous casting steel temperature is 1590℃.

[0041] (5) Electromagnetic induction heating: The continuous casting billet is sent to the electromagnetic induction heating machine for supplementary heating through a conveyor roller with a cover and insulation. The temperature is maintained at 1178°C and the roller speed is 3m / s, and then it enters the rolling process.

[0042] (6) Rolling: including rough rolling, intermediate rolling and finishing rolling. The starting temperature of rough rolling is 1130℃ and the roller speed is 3m / s. After intermediate rolling and before finishing rolling, the slab is water-cooled. The water cooling pressure is 1.1MPa and the roller speed is 10m / s. Finishing rolling is carried out after water cooling. The starting temperature of finishing rolling is 942℃. Cooling is carried out after final rolling.

[0043] (7) Cooling: The water pressure of the cooling equipment is 1.2 MPa, the roller speed is 14 m / s, the final cooling temperature is 838 °C, the return temperature of the upper cooling bed is 907 °C, and then it is air-cooled to room temperature on a step-by-step cooling bed.

[0044] The metallographic structure of the 630MPa grade high-strength steel bar produced in this embodiment is as follows: Figure 2 As shown by Figure 2 It can be seen that it is a pearlite and ferrite structure, with pearlite accounting for 57% and a grain size of 9.0.

[0045] The 630MPa grade high-strength steel bars produced in this embodiment have good comprehensive mechanical properties, with a tensile strength of 885MPa, a yield strength of 678MPa, an elongation after fracture of 13.1%, and a strength-to-yield ratio of 1.30. Example 3

[0046] The diameter of the 630MPa grade high-strength steel bar in this embodiment is 16mm, and the chemical composition is shown in Table 1; its production process includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling; the specific process control is as follows: (1) KR pretreatment: S content after KR pretreatment is 0.015%.

[0047] (2) Converter smelting: N2 is blown from the bottom of the converter, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, and splashing occurs during the smelting process. The final converter temperature is 1680°C.

[0048] (3) Refining: low basicity slag making is adopted, the slag making basicity is 1.8, and the slag making cycle is 23 minutes.

[0049] (4) Continuous casting: 5-stream continuous casting is adopted, the casting speed is 2m / min, the fluctuation is less than 0.2m / min, the secondary cooling adopts strong cooling plus low superheat, the specific water volume is 1.67L / kg, the superheat is 34℃, and the continuous casting steel temperature is 1562℃.

[0050] (5) Electromagnetic induction heating: The continuous casting billet is sent to the electromagnetic induction heating machine for supplementary heating through a conveyor roller with a cover and insulation. The temperature is maintained at 1170°C and the roller speed is 2m / s, and then it enters the rolling process.

[0051] (6) Rolling: including rough rolling, intermediate rolling and finishing rolling. The starting temperature of rough rolling is 1095℃ and the roller speed is 2m / s. After intermediate rolling and before finishing rolling, the slab is water-cooled. The water cooling pressure is 1.0MPa and the roller speed is 10m / s. Finishing rolling is carried out after water cooling. The starting temperature of finishing rolling is 943℃ and cooling is carried out after final rolling.

[0052] (7) Cooling: The water pressure of the cooling equipment is 1.3 MPa, the roller speed is 11 m / s, the final cooling temperature is 820 °C, the return temperature of the upper cooling bed is 903 °C, and then it is air-cooled to room temperature on a step-by-step cooling bed.

[0053] The metallographic structure of the 630MPa grade high-strength steel bar produced in this embodiment is as follows: Figure 3 As shown by Figure 3 It can be seen that it is a pearlite and ferrite structure, with pearlite accounting for 56% and a grain size of 9.0.

[0054] The 630MPa grade high-strength steel bar produced in this embodiment has good comprehensive mechanical properties, with a tensile strength of 878MPa, a yield strength of 679MPa, an elongation after fracture of 13.8%, and a strength-to-yield ratio of 1.29. Example 4

[0055] The diameter of the 630MPa grade high-strength steel bar in this embodiment is 18mm, and the chemical composition is shown in Table 1; its production process includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling; the specific process control is as follows: (1) KR pretreatment: S content after KR pretreatment is 0.015%.

[0056] (2) Converter smelting: N2 is blown from the bottom of the converter, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, and splashing occurs during the smelting process. The final converter temperature is 1690°C.

[0057] (3) Refining: low basicity slag making is adopted, the slag making basicity is 1.8, and the slag making cycle is 40 minutes.

[0058] (4) Continuous casting: 5-stream continuous casting is adopted, the casting speed is 2.3m / min, the fluctuation is less than 0.2m / min, the secondary cooling adopts strong cooling plus low superheat, the specific water volume is 1.71L / kg, the superheat is 28℃, and the continuous casting steel temperature is 1566℃.

[0059] (5) Electromagnetic induction heating: The continuous casting billet is sent to the electromagnetic induction heating machine for supplementary heating through a conveyor roller with a cover and insulation. The temperature is maintained at 1174°C and the roller speed is 2m / s, and then it enters the rolling process.

[0060] (6) Rolling: including rough rolling, intermediate rolling and finishing rolling. The starting temperature of rough rolling is 1106℃ and the roller speed is 2m / s. After intermediate rolling and before finishing rolling, the slab is water-cooled. The water cooling pressure is 1.2MPa and the roller speed is 10m / s. Finishing rolling is carried out after water cooling. The starting temperature of finishing rolling is 961℃ and cooling is carried out after final rolling.

[0061] (7) Cooling: The water pressure of the cooling equipment is 1.2 MPa, the roller speed is 12 m / s, the final cooling temperature is 847 °C, the return temperature of the upper cooling bed is 930 °C, and then it is air-cooled to room temperature on a step-by-step cooling bed.

[0062] The metallographic structure of the 630MPa grade high-strength steel bar produced in this embodiment is as follows: Figure 4 As shown by Figure 4 It can be seen that it is a pearlite and ferrite structure, with pearlite accounting for 53% and a grain size of 9.0.

[0063] The 630MPa grade high-strength steel bars produced in this embodiment have good comprehensive mechanical properties, with a tensile strength of 891MPa, a yield strength of 675MPa, an elongation after fracture of 13.3%, and a strength-to-yield ratio of 1.31. Example 5

[0064] The diameter of the 630MPa grade high-strength steel bar in this embodiment is 20mm, and the chemical composition is shown in Table 1; its production process includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling; the specific process control is as follows: (1) KR pretreatment: S content after KR pretreatment is 0.015%.

[0065] (2) Converter smelting: N2 is blown from the bottom of the converter, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, and splashing occurs during the smelting process. The final converter temperature is 1685°C.

[0066] (3) Refining: low basicity slag making is adopted, the slag making basicity is 1.8, and the slag making cycle is 30 minutes.

[0067] (4) Continuous casting: 5-stream continuous casting is adopted, the casting speed is 2.6 m / min, the fluctuation is less than 0.2 m / min, the secondary cooling adopts strong cooling plus low superheat, the specific water volume is 1.73 L / kg, the superheat is 34 °C, and the continuous casting steel temperature is 1587 °C.

[0068] (5) Electromagnetic induction heating: The continuous casting billet is sent to the electromagnetic induction heating machine for supplementary heating through a conveyor roller with a cover and insulation. The temperature is maintained at 1166°C and the roller speed is 3m / s, and then it enters the rolling process.

[0069] (6) Rolling: including rough rolling, intermediate rolling and finishing rolling. The starting temperature of rough rolling is 1128℃ and the roller speed is 3m / s. After intermediate rolling and before finishing rolling, the slab is water-cooled. The water cooling pressure is 1.2MPa and the roller speed is 11m / s. Finishing rolling is carried out after water cooling. The starting temperature of finishing rolling is 967℃ and cooling is carried out after final rolling.

[0070] (7) Cooling: The water pressure of the cooling equipment is 1.3 MPa, the roller speed is 9 m / s, the final cooling temperature is 868 °C, the return temperature of the upper cooling bed is 936 °C, and then it is air-cooled to room temperature on a step-by-step cooling bed.

[0071] The metallographic structure of the 630MPa grade high-strength steel bar produced in this embodiment is as follows: Figure 5 As shown by Figure 5 It can be seen that it is a pearlite and ferrite structure, with pearlite accounting for 55% and a grain size of 8.5.

[0072] The 630MPa grade high-strength steel bar produced in this embodiment has good comprehensive mechanical properties, with a tensile strength of 832MPa, a yield strength of 645MPa, an elongation after fracture of 13.8%, and a strength-to-yield ratio of 1.29. Example 6

[0073] The diameter of the 630MPa grade high-strength steel bar in this embodiment is 22mm, and the chemical composition is shown in Table 1; its production process includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling; the specific process control is as follows: (1) KR pretreatment: S content after KR pretreatment is 0.014%.

[0074] (2) Converter smelting: N2 is blown from the bottom of the converter, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, and splashing occurs during the smelting process. The final converter temperature is 1688°C.

[0075] (3) Refining: Low basicity slag making is adopted, the slag making basicity is 1.7, and the slag making cycle is 32 minutes.

[0076] (4) Continuous casting: 5-stream continuous casting is adopted, the casting speed is 3m / min, the fluctuation is less than 0.2m / min, the secondary cooling adopts strong cooling and low superheat, the specific water volume is 1.75L / kg, the superheat is 34℃, and the continuous casting steel temperature is 1588℃.

[0077] (5) Electromagnetic induction heating: The continuous casting billet is sent to the electromagnetic induction heating machine for supplementary heating through a conveyor roller with a cover and insulation. The temperature is maintained at 1170°C and the roller speed is 2m / s, and then it enters the rolling process.

[0078] (6) Rolling: including rough rolling, intermediate rolling and finishing rolling. The starting temperature of rough rolling is 1090℃ and the roller speed is 2m / s. After intermediate rolling and before finishing rolling, the slab is water-cooled. The water cooling pressure is 1.0MPa and the roller speed is 10m / s. Finishing rolling is carried out after water cooling. The starting temperature of finishing rolling is 1040℃ and cooling is carried out after final rolling.

[0079] (7) Cooling: The water pressure of the cooling equipment is 1.2 MPa, the roller speed is 10 m / s, the final cooling temperature is 880 °C, the return temperature of the upper cooling bed is 950 °C, and then it is air-cooled to room temperature on a step-by-step cooling bed.

[0080] The metallographic structure of the 630MPa grade high-strength steel bar produced in this embodiment is as follows: Figure 6 As shown by Figure 6 It can be seen that it is a pearlite and ferrite structure, with pearlite accounting for 58% and a grain size of 8.5.

[0081] The 630MPa grade high-strength steel bars produced in this embodiment have good comprehensive mechanical properties, with a tensile strength of 827MPa, a yield strength of 640MPa, an elongation after fracture of 14.6%, and a strength-to-yield ratio of 1.29. Example 7

[0082] The diameter of the 630MPa grade high-strength steel bar in this embodiment is 25mm, and the chemical composition is shown in Table 1; its production process includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling; the specific process control is as follows: (1) KR pretreatment: S content after KR pretreatment is 0.014%.

[0083] (2) Converter smelting: N2 is blown from the bottom of the converter, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, and splashing occurs during the smelting process. The final converter temperature is 1681°C.

[0084] (3) Refining: low basicity slag making is adopted, the slag making basicity is 1.5, and the slag making cycle is 30 minutes.

[0085] (4) Continuous casting: 5-stream continuous casting is adopted, the casting speed is 2.5m / min, the fluctuation is less than 0.2m / min, the secondary cooling adopts strong cooling plus low superheat, the specific water volume is 1.71L / kg, the superheat is 27℃, and the continuous casting steel temperature is 1569℃.

[0086] (5) Electromagnetic induction heating: The continuous casting billet is sent to the electromagnetic induction heating machine for supplementary heating through a covered and insulated conveyor roller. The temperature is maintained at 1181°C and the roller speed is 1m / s, and then it enters the rolling process.

[0087] (6) Rolling: including rough rolling, intermediate rolling and finishing rolling. The starting temperature of rough rolling is 1030℃ and the roller speed is 1m / s. After intermediate rolling and before finishing rolling, the slab is water-cooled. The water cooling pressure is 0.9MPa and the roller speed is 8m / s. Finishing rolling is carried out after water cooling. The starting temperature of finishing rolling is 1004℃ and cooling is carried out after final rolling.

[0088] (7) Cooling: The water pressure of the cooling equipment is 1.4 MPa, the roller speed is 8 m / s, the final cooling temperature is 850°C, the return temperature of the upper cooling bed is 940°C, and then it is air-cooled to room temperature on a step-by-step cooling bed.

[0089] The metallographic structure of the 630MPa grade high-strength steel bar produced in this embodiment is as follows: Figure 7 As shown by Figure 7 It can be seen that it is a pearlite and ferrite structure, with pearlite accounting for 61% and a grain size of 8.0.

[0090] The 630MPa grade high-strength steel bar produced in this embodiment has good comprehensive mechanical properties, with a tensile strength of 825MPa, a yield strength of 630MPa, an elongation after fracture of 13.4%, and a strength-to-yield ratio of 1.31. Example 8

[0091] The diameter of the 630MPa grade high-strength steel bar in this embodiment is 32mm, and the chemical composition is shown in Table 1; its production process includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling; the specific process control is as follows: (1) KR pretreatment: S content after KR pretreatment is 0.013%.

[0092] (2) Converter smelting: N2 is blown from the bottom of the converter, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, and splashing occurs during the smelting process. The final converter temperature is 1686°C.

[0093] (3) Refining: Low basicity slag making is adopted, the slag making basicity is 1.6, and the slag making cycle is 31 minutes.

[0094] (4) Continuous casting: 5-stream continuous casting is adopted, the casting speed is 2.5m / min, the fluctuation is less than 0.2m / min, the secondary cooling adopts strong cooling plus low superheat, the specific water volume is 1.71L / kg, the superheat is 28℃, and the continuous casting steel temperature is 1560℃.

[0095] (5) Electromagnetic induction heating: The continuous casting billet is sent to the electromagnetic induction heating machine for supplementary heating through a conveyor roller with a cover and insulation. The temperature is maintained at 1185°C and the roller speed is 1m / s, and then it enters the rolling process.

[0096] (6) Rolling: including rough rolling, intermediate rolling and finishing rolling. The starting temperature of rough rolling is 1133℃ and the roller speed is 1m / s. After intermediate rolling and before finishing rolling, the slab is water-cooled. The water cooling pressure is 0.8MPa and the roller speed is 8m / s. Finishing rolling is carried out after water cooling. The starting temperature of finishing rolling is 1019℃ and cooling is carried out after final rolling.

[0097] (7) Cooling: The water pressure of the cooling equipment is 1.5 MPa, the roller speed is 8 m / s, the final cooling temperature is 835 °C, the return temperature of the upper cooling bed is 934 °C, and then it is air-cooled to room temperature on a step-by-step cooling bed.

[0098] The metallographic structure of the 630MPa grade high-strength steel bar produced in this embodiment is as follows: Figure 8 As shown by Figure 8 It can be seen that it is a pearlite and ferrite structure, with pearlite accounting for 57% and a grain size of 8.0.

[0099] The 630MPa grade high-strength steel bars produced in this embodiment have good comprehensive mechanical properties, with a tensile strength of 815MPa, a yield strength of 635MPa, an elongation after fracture of 13.2%, and a strength-to-yield ratio of 1.28.

[0100] Table 1 Chemical composition of high-strength steel bars in various examples (%)

[0101] The rest in Table 1 is Fe and unavoidable impurities.

Claims

1. A low-cost, high-elongation 630MPa grade high-strength steel bar, characterized in that: The chemical composition of the high-strength steel bar is as follows by weight: C: 0.25-0.27%, Si: 0.73-0.78%, Mn: 1.50-1.59%, P: ≤0.015%, S: ≤0.015%, Nb: 0.016-0.019%, V: 0.145-0.155%, N: 0.020-0.021%, and the remainder is Fe and unavoidable impurities.

2. A low-cost, high-elongation 630MPa grade high-strength steel bar according to claim 1, characterized in that: The 630MPa high-strength steel bar has a yield strength of ≥630MPa, a tensile strength of ≥815MPa, an elongation after fracture of ≥13%, and a strength-to-yield ratio of ≥1.

28.

3. A production process of low-cost, high-elongation 630MPa grade high-strength steel bars according to claim 1 or 2, characterized in that: It includes KR pretreatment → converter smelting → refining → continuous casting → electromagnetic induction heating → rolling → cooling.

4. The production process of a low-cost, high-elongation 630MPa grade high-strength steel bar according to claim 3, characterized in that: The KR pretreatment, the S content after the KR pretreatment is lower than 0.015%.

5. The production process of a low-cost, high-elongation 630MPa grade high-strength steel bar according to claim 3, characterized in that: In the converter smelting, N2 is blown from the converter bottom, VN alloy, V-Fe and Nb-Fe are added for micro-alloying, splashing occurs during the smelting process, and the converter end point is stably controlled at 1680-1690°C.

6. The production process of a low-cost, high-elongation 630MPa grade high-strength steel bar according to claim 3, characterized in that: The refining adopts low basicity slag making, the slag making basicity is 1 to 2.3, and the slag making cycle is 20 to 40 minutes.

7. The production process of a low-cost, high-elongation 630MPa grade high-strength steel bar according to claim 3, characterized in that: The continuous casting has a pulling speed of 2 to 3 m / min with a fluctuation of less than 0.2 m / min. The secondary cooling adopts strong cooling with low superheat. The water content of the strong cooling is 1.67 to 1.75 L / kg, the superheat is controlled to be less than 35°C, and the continuous casting steelmaking temperature is 1560 to 1590°C.

8. The production process of a low-cost, high-elongation 630MPa grade high-strength steel bar according to claim 3, characterized in that: The electromagnetic induction heating is to convey the continuous casting billet through a conveying roller with a cover and insulation to the electromagnetic induction heating for supplementary heating, with the temperature maintained at 1170-1190° C. and the roller speed at 1-3 m / s.

9. The production process of a low-cost, high-elongation 630MPa grade high-strength steel bar according to claim 3, characterized in that: The rolling includes rough rolling, intermediate rolling and finishing rolling; the starting temperature of the rough rolling is 1030-1130°C, and the roller speed is 0.5-3m / s; the slab is water-cooled before finishing rolling, the water cooling pressure is 0.8-1.2MPa, the roller speed is 8-11m / s, and after water cooling, it enters the finishing mill, and the starting temperature of the finishing rolling is 940-1040°C.

10. The production process of a low-cost, high-elongation 630MPa grade high-strength steel bar according to claim 3, characterized in that: The cooling equipment has a water pressure of 1.2-1.5 MPa, a roller speed of 8-15 m / s, a final cooling temperature of 820-880° C., a return temperature of the upper cooling bed of 900° C.-950° C., and then air cooling to room temperature on a walking cooling bed.

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