635MPa-grade high-strength high-ductility hot-rolled twisted steel and manufacturing method thereof

Abstract

The invention belongs to the technical field of metal material processing, and particularly discloses a 635MPa-grade high-strength and high-ductility hot-rolled twisted steel and a manufacturing method thereof. The steel bar comprises the following chemical components in percentage by mass: 0.26 to 0.29 percent of C, 0.70 to 0.75 percent of Si, 1.55 to 1.60 percent of Mn, 0.168 to 0.180 percent of V and 0.015 to 0.020 percent of Nb, and the contents of P, S and N and the carbon equivalent are limited. The manufacturing method comprises the working procedures of converter smelting, LF refining, continuous casting and rolling, and the excellent performance that the tensile strength of the steel bar is larger than or equal to 795 MPa, the yield strength is larger than or equal to 635 MPa, and the percentage elongation after fracture is larger than or equal to 16% is achieved by controlling key process parameters such as the converter end point C content, the LF soft blowing time, the continuous casting superheat degree and the rolling temperature. The technical problem that the strength and plasticity of the high-strength steel bar are difficult to consider at the same time is solved, and the method is suitable for production of steel bars of multiple specifications of 16-32 mm.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, and in particular to a 635MPa grade high-strength and high-ductility hot-rolled threaded steel bar and its manufacturing method. Background Technology

[0002] With the development of building structures towards higher heights and larger spans, the demand for high-strength steel bars is increasing. 635MPa grade high-strength steel bars can effectively reduce the amount of steel used in structures and lower construction costs, making them a key product for promotion. However, current high-strength steel bar production generally faces the technical bottleneck of "difficulty in balancing strength and ductility," especially in the production of large-diameter (e.g., 32mm) steel bars, where the elongation after fracture is often less than 16%, failing to meet seismic resistance and safety requirements.

[0003] The existing technology has three main problems: the proportion of V-Nb microalloying elements was not precisely controlled in the composition design; the smelting process was unstable, with excessive sulfur content in the molten iron and low carbon removal rate at the converter endpoint; and the rolling process parameters were poorly controlled, resulting in uneven microstructure. This invention solves these problems through optimization of all process parameters. Summary of the Invention

[0004] The purpose of this invention is to provide a 635MPa grade high-strength and high-ductility hot-rolled threaded steel bar and its manufacturing method, so as to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the basic solution provided by this invention is: a 635MPa grade high-strength and high-ductility hot-rolled threaded steel bar, the chemical composition of which, by mass percentage, includes: C: 0.26~0.29, Si: 0.70~0.75, Mn: 1.55~1.60, P≤0.025, S≤0.020, V: 0.168~0.180, Nb: 0.015~0.020, N: 0.015~0.020, Ceq≤0.60, with the balance being Fe and unavoidable impurities.

[0006] Furthermore, the mechanical properties of the steel bars meet the following requirements: tensile strength ≥ 795 MPa, yield strength ≥ 635 MPa, elongation after fracture ≥ 16%, and strength-to-yield ratio ≥ 1.25.

[0007] Furthermore, the specifications of the reinforcing bars are 16mm-32mm.

[0008] A method for manufacturing 635MPa grade high-strength and high-ductility hot-rolled threaded steel bars includes converter smelting, LF refining, continuous casting, and rolling processes. In the converter smelting process, the molten iron fed into the furnace must have an S content of ≤ 0.060%, and the final C content must be controlled between 0.06% and 0.16%. In the LF refining process, the soft blowing time is ≥5.5 min; In the continuous casting process, the superheat is controlled between 15 and 38°C; During the rolling process, the initial rolling temperature is controlled at 1050–1080℃. Furthermore, the tapping temperature of the converter smelting is 1640–1680°C.

[0009] Furthermore, the initial temperature of the LF refining is ≥1540℃, and the amount of slag added is 250-350kg / furnace of lime, 80-150kg / furnace of slag-forming agent, and 50-100kg / furnace of composite deoxidizer.

[0010] Furthermore, the continuous casting speed is 2.2–2.6 m / min, and the casting cycle is ≤32 min.

[0011] Furthermore, during the rolling process, the billet is in the furnace for ≥85 minutes, the cooling rate of 16-28mm steel bars is 5-8℃ / s, and the cooling rate of 32mm steel bars is 3-5℃ / s.

[0012] Furthermore, the dimensional tolerance of the rolled product is controlled within -2.0% to -2.5%.

[0013] Compared with the prior art, the advantages of this invention are: 1. Synergistic Performance Improvement: Through the "low carbon + V-Nb micro-alloying" composition design and full-process process control, the tensile strength of 635MPa grade steel bars is ≥795MPa, yield strength is ≥635MPa, elongation after fracture is ≥16%, and strength-to-yield ratio is ≥1.25, meeting the requirements for seismic resistance and safe use of building structures. In particular, the performance qualification rate of 32mm large-diameter steel bars has been increased from 70% to 100%.

[0014] 2. High process stability: Key control parameters for each process, including converter smelting, LF refining, continuous casting, and rolling, are clearly defined, such as molten iron S≤0.060%, converter carbon removal rate≥85%, continuous casting superheat 15-38℃, and rolling time in the furnace≥85min. The hit rate of parameters for each process is ≥80%, ensuring the stability of industrial mass production.

[0015] 3. Multi-specification coverage and cost optimization: It can stably produce steel bars of various specifications from 16mm to 32mm. By reducing the amount of alloy replenishment in the LF refining furnace and optimizing the rolling negative difference, the production cost is reduced by 8-10 yuan / ton. At the same time, the yield rate is ≥92.5%, which has significant economic benefits.

[0016] 4. Wide applicability: The product is suitable for building scenarios with high requirements for steel strength and plasticity, such as high-rise residential buildings, long-span bridges, and nuclear power plants. It can replace some higher-strength steel bars, reducing the amount of steel used in construction and construction costs. Detailed Implementation

[0017] The present invention will be further described in detail below through specific embodiments: The equipment used in the following examples is a 150t converter production line, an LF refining furnace, a 150mm×150mm continuous casting machine, and a bar rolling production line of a steel plant.

[0018] Example 1: 16mm diameter 635MPa grade threaded steel bar (HG6E / C) Chemical composition (mass percentage %): C 0.275%, Si 0.73%, Mn 1.58%, P 0.023%, S 0.019%, V 0.171%, Nb 0.018%, N 0.019%, Ceq 0.58%.

[0019] Manufacturing process parameters: converter tapping temperature 1655℃, LF refining soft blowing time 7.2min, continuous casting superheat 28℃, casting speed 2.43m / min, rolling start temperature 1070℃, bed temperature 1010℃, furnace time 123min.

[0020] Mechanical properties: yield strength 670-685MPa, tensile strength 835-860MPa, elongation after fracture 17-19%, strength-to-yield ratio 1.25-1.26, total elongation at maximum force 11.1-12.9%, performance qualification rate 100%, yield rate 92.51%.

[0021] Example 2: 25mm diameter 635MPa grade threaded steel bar (HG6E / C) Chemical composition (mass percentage %): C 0.278%, Si 0.72%, Mn 1.57%, P 0.020%, S 0.017%, V 0.170%, Nb 0.017%, N 0.018%, Ceq 0.57%.

[0022] Manufacturing process parameters: converter tapping temperature 1644℃, LF refining soft blowing time 6.0min, continuous casting superheat 32℃, casting speed 2.51m / min, rolling start temperature 1065℃, bed temperature 1005℃, furnace time 98min.

[0023] Mechanical properties: yield strength 660-690MPa, tensile strength 830-875MPa, elongation after fracture 16-20%, strength-to-yield ratio 1.26-1.28, total elongation at maximum force 10.5-14.7%, performance qualification rate 100%, yield rate 97.18%.

[0024] Example 3: 32mm diameter 635MPa grade threaded steel bar (HG6E / C) Chemical composition (mass percentage %): C 0.282%, Si 0.74%, Mn 1.59%, P 0.019%, S 0.015%, V 0.178%, Nb 0.019%, N 0.020%, Ceq 0.59%.

[0025] Manufacturing process parameters: converter tapping temperature 1660℃, LF refining soft blowing time 8.7min, continuous casting superheat 35℃, casting speed 2.29m / min, rolling start temperature 1075℃, bed temperature 1015℃, furnace time 95min, cooling rate 4℃ / s.

[0026] Mechanical properties: yield strength 675-695MPa, tensile strength 850-880MPa, elongation after fracture 16-17%, strength-to-yield ratio 1.26-1.28, total elongation at maximum force 11.2-13.4%, performance qualification rate 100% (30 percentage points higher than existing processes), yield rate 97.46%.

[0027] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A 635MPa grade high-strength, high-ductility hot-rolled threaded steel bar, characterized in that, Its chemical composition, by mass percentage, includes: C: 0.26–0.29, Si: 0.70–0.75, Mn: 1.55–1.60, P≤0.025, S≤0.020, V: 0.168–0.180, Nb: 0.015–0.020, N: 0.015–0.020, Ceq≤0.60, with the balance being Fe and unavoidable impurities.

2. The 635MPa grade high-strength, high-ductility hot-rolled threaded steel bar according to claim 1, characterized in that, The mechanical properties of the steel bars meet the following requirements: tensile strength ≥ 795 MPa, yield strength ≥ 635 MPa, elongation after fracture ≥ 16%, and strength-to-yield ratio ≥ 1.

25.

3. The 635MPa grade high-strength, high-ductility hot-rolled threaded steel bar according to claim 1, characterized in that, The steel bars are 16mm-32mm in diameter.

4. A method for manufacturing 635MPa grade high-strength, high-ductility hot-rolled threaded steel bars as described in any one of claims 1-3, comprising converter smelting, LF refining, continuous casting, and rolling processes, characterized in that: In the converter smelting process, the molten iron fed into the furnace must have an S content of ≤0.060%, and the final C content must be controlled between 0.06% and 0.16%. In the LF refining process, the soft blowing time is ≥5.5 min; In the continuous casting process, the superheat is controlled between 15 and 38°C; During the rolling process, the initial rolling temperature is controlled at 1050–1080℃.

5. The method according to claim 4, characterized in that, The tapping temperature of the converter smelting is 1640–1680℃.

6. The method according to claim 4, characterized in that, The initial temperature of the LF refining process is ≥1540℃, and the amount of slag added is 250-350kg / furnace of lime, 80-150kg / furnace of slag-forming agent, and 50-100kg / furnace of composite deoxidizer.

7. The method according to claim 4, characterized in that, The continuous casting speed is 2.2 to 2.6 m / min, and the casting cycle is ≤32 min.

8. The method according to claim 4, characterized in that, During the rolling process, the billet is in the furnace for ≥85 minutes, the cooling rate of 16-28mm steel bars is 5-8℃ / s, and the cooling rate of 32mm steel bars is 3-5℃ / s.

9. The method according to claim 4, characterized in that, The dimensional tolerance of the rolled product is controlled within -2.0% to -2.5%.