Smelting and rolling method for construction steel bars used in frozen soil environment

Abstract

The invention discloses a smelting and rolling method of construction steel bars for a frozen soil environment. The smelting and rolling method comprises the following steps: 1) smelting process; 2) rolling process; the steel bar comprises the following chemical components in percentage by mass: 0.09 to 0.11 percent of C, 0.24 to 0.27 percent of Si, 1.35 to 1.39 percent of Mn, 0.0011 to 0.010 percent of P, 0.002 to 0.010 percent of S, 0.34 to 0.37 percent of Cu, 0.25 to 0.29 percent of Cr, 0.26 to 0.28 percent of Ni, 0.025 to 0.030 percent of Nb, 0.042 to 0.045 percent of V, 0.0015 to 0.0020 percent of Ce and the balance of Fe and impurities. According to the invention, the impact toughness of the steel bar at low temperature can be obviously improved, excellent strength and yield ratio are guaranteed, the corrosion resistance is improved, and good welding performance is maintained.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, and particularly relates to a smelting and rolling method for building steel bars used in frozen soil environments. Background Technology

[0002] Because steel's toughness decreases significantly at extremely low temperatures, it is prone to brittle fracture. Furthermore, the complex chemical composition of soils in permafrost regions (such as saline soils and areas with high water / oxygen activity in freeze-thaw zones) makes reinforcing steel susceptible to pitting corrosion and crevice corrosion. Moreover, seasonal freeze-thaw cycles and frost heave can subject the foundation and reinforcing steel to repeated impacts and fatigue loads. Existing reinforcing steel cannot fully meet the stringent requirements of permafrost environments in terms of low-temperature toughness, weather resistance, and weldability, especially for steel used in cold regions (such as ordinary HRB400E or steel with a small amount of Nb / V). Therefore, a production method for reinforcing steel with superior overall performance and cost-effectiveness, specifically designed for permafrost environments, is needed.

[0003] This invention patent proposes that steel reinforcement for frozen soil environments possesses excellent low-temperature toughness: it exhibits high impact energy at temperatures of -40℃ or even lower, preventing low-temperature brittle fracture. It also possesses high corrosion resistance: resisting corrosion from meltwater (rich in chloride and sulfate ions), pH changes, and salt in frozen soil environments, reducing the risk of strength loss and stress corrosion cracking caused by rust. Furthermore, it boasts high strength and good comprehensive mechanical properties: meeting the load-bearing requirements of building structures on high-altitude / frozen soil foundations. Finally, it exhibits good weldability: ensuring reliable performance during on-site connections. Summary of the Invention

[0004] The purpose of this invention is to provide a smelting and rolling method for building steel bars in permafrost environments. By optimizing the chemical composition design and precisely controlling the smelting, rolling and cooling processes, the impact toughness (Akv) of the steel bars at low temperatures (≤-40℃) is significantly improved, ensuring excellent strength (yield strength / tensile strength) and strength-to-yield ratio, improving corrosion resistance and maintaining good weldability.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention discloses a method for smelting and rolling reinforcing steel bars for use in permafrost environments, comprising:

[0007] 1. Smelting

[0008] All raw materials and alloys used must meet the requirements of current raw material and alloy standards. A top-and-bottom blowing converter is used for smelting, with the final slag basicity controlled at 3.0. The final control targets are C ≥ 0.06% and T ≥ 1630℃. Ferromanganese and ferrosilicon are used for alloying, and final deoxidation is performed with Al, further alloyed according to composition requirements. Slag is blocked during tapping. Ar blowing is carried out throughout the process according to refining procedures, with desulfurization, composition fine-tuning, and temperature increases performed based on the converter steel composition and temperature. Vacuum degassing (VD) is required, with a VD vacuum degree ≤ 0.10 kPa, a target value ≤ 0.06 kPa, and a deep vacuum time ≥ 13 min. After breaking the vacuum, silicon-calcium wire is fed in, and rare earth alloys are added according to the required dosage. The soft blowing operation time (ideally without exposing the molten steel surface) is guaranteed to be ≥ 15 min. Superheat is controlled below 30℃, and the target casting speed is 2.1 m / min, ensuring constant casting speed operation. The cast billet must not exhibit severe squareness, bulging, or central shrinkage cavities. A hot acid low-magnification sample of the billet is taken from each heat for inspection. Defects such as subcutaneous cracks, subcutaneous bubbles, corner cracks, and central cracks must not exceed grade 1.0; any billet exceeding this standard must be reclassified. The O and N content of gases is tested for each heat. Continuously cast billets are placed in a slow-cooling pit for at least 48 hours after leaving the production line. This promotes homogenization of composition and temperature, and facilitates the flotation of inclusions. This involves optimizing the arrangement of permeable bricks and controlling the stirring intensity. Alloying elements (such as silicon-calcium wire), deoxidizers, and sulfide morphology control agents are precisely added. The main alloying elements required for reinforcing steel, such as Mn, C, and Si, as well as micro-alloying elements (Nb, V, Cr, Ni, Cu) added when necessary, are precisely controlled. Secondary oxidation of molten steel is prevented (e.g., through long nozzles, tundish seals, and covering agents), involving new refractory materials and sealing structure design. After continuous casting, the billet enters a dynamic light pressure and optimized secondary cooling zone to ensure good solidification quality at the end of solidification, improve the solidification structure, and reduce center segregation and porosity. Strict control of harmful impurity elements (S, P, O, H, N) greatly reduces grain boundary embrittlement.

[0009] 2. Rolling

[0010] The slowly cooled, qualified continuously cast billets are fed into a heating furnace, with the furnace temperature controlled at 1100-1200℃ in the soaking zone to ensure uniform heat penetration. Multi-pass rolling (>1000℃) is performed in the fully recrystallized austenite region. The deformation per pass is ≥15%, and the total compression ratio is ≥30, to fully refine the original austenite grains. The initial rolling temperature is 930-970℃, and the final rolling temperature is strictly controlled at 880-892℃ (below the non-recrystallization temperature but still within the austenite region). This temperature range is crucial for achieving non-recrystallization rolling / micro-recrystallization rolling. Immediately after finishing rolling, the steel bars are rapidly fed into a powerful water-cooling device to ensure the upper cooling bed temperature is 800-850℃; the goal of controlled cooling is to form a uniformly dispersed ferrite + pearlite + a very small amount of bainite microstructure across the entire cross-section. Ferrite grain size ≥10 (micrometer level), pearlite clusters are small and dispersed; after cooling to room temperature, heat dissipates from the core to the surface, resulting in self-tempering and reducing internal stress; thus, high-quality building steel reinforcement products with fine and uniform microstructure are obtained. Tensile testing, low-temperature impact performance testing, and corrosion resistance testing are performed after rolling.

[0011] The chemical composition of the steel reinforcement by mass percentage is as follows: C 0.09-0.11%, Si 0.24-0.27%, Mn 1.35-1.39%, P 0.0011-0.010%, S 0.002-0.010%, Cu 0.34-0.37%, Cr 0.25-0.29%, Ni 0.26-0.28%, Nb 0.025-0.030%, V 0.042-0.045%, Ce 0.0015-0.0020%, with the remainder being Fe and impurities.

[0012] Furthermore, the billet must not have severe squareness, bulging, or central shrinkage. A hot acid low-magnification sample of the billet must be taken from each furnace for inspection. The defect level of subcutaneous cracks, subcutaneous bubbles, corner cracks, and intermediate cracks must not exceed level 1.0. Any billet that exceeds the standard must be reclassified.

[0013] Furthermore, the chemical composition of the steel reinforcement by mass percentage is as follows: C 0.11%, Si 0.25%, Mn 1.35%, P 0.010%, S 0.010%, Cu 0.35%, Cr 0.27%, Ni 0.27%, Nb 0.030%, V 0.045%, Ce 0.0020%, with the remainder being Fe and impurities.

[0014] Furthermore, the chemical composition of the steel reinforcement by mass percentage is as follows: C 0.09%, Si 0.27%, Mn 1.36%, P 0.009%, S 0.002%, Cu 0.37%, Cr 0.25%, Ni 0.26%, Nb 0.025%, V 0.044%, Ce 0.0015%, with the remainder being Fe and impurities.

[0015] Furthermore, the chemical composition of the steel reinforcement by mass percentage is as follows: C 0.10%, Si 0.24%, Mn 1.39%, P 0.0011%, S 0.007%, Cu 0.34%, Cr 0.29%, Ni 0.28%, Nb 0.030%, V 0.042%, Ce 0.0019%, with the remainder being Fe and impurities.

[0016] Furthermore, after final rolling, controlled cooling is performed, with the outlet water temperature at 820±10℃.

[0017] Furthermore, the temperature of the upper cooling bed is 800±10℃.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0019] At -40℃, the full-size (10x10x55mm) impact absorption energy (Akv) of the V-notch is ≥80J, which is much higher than the low-temperature impact level of ordinary steel bars and simple controlled cooling processes.

[0020] The reinforcing steel bars offer a high balance of strength and good ductility, with a stable strength-to-yield ratio ≥1.25 and elongation meeting standard requirements. The addition of Cu, Ni, Cr, and RE creates a protective oxide film (dense and stable rust layer). This dense and uniform matrix reduces the tendency for corrosion microcells, particularly enhancing resistance to saline soils and freeze-thaw cycles. The cyclic wetting rate is 55% higher than that of HEB400E. Attached Figure Description

[0021] The present invention will be further described below with reference to the accompanying drawings.

[0022] Figure 1 The microstructure is shown in Example 1. Detailed Implementation

[0023] A method for smelting and rolling reinforcing steel bars for use in permafrost environments stipulates that the raw materials and alloys used must meet the requirements of current standards for raw materials and alloys. All materials, including iron balls and pure scrap steel, must be dry and free of inclusions to minimize the introduction of hydrogen into the steel. The conditions for molten iron entering the furnace are: Si: 0.42%, S: 0.010%, P: 0.115%, Ti: 0.08%, T: 1310℃. The slag content is 0.42%. Ni: 0.005%, Cu: 0.006%. A converter with combined blowing is used for smelting, with the final slag basicity controlled at 3.0. The final steel composition at tapping is: C: 0.10%, P: 0.015%, S: 0.003%, T: 16610℃. The converter charge is strictly controlled at 94 tons, and self-charged small-circulation scrap steel is preferred. The converter's final charge should not exceed one hit or one additional blowing cycle.

[0024] The converter steel is alloyed by adding a carburizing agent and supplementing the alloying with some metallic manganese. Two tapping stages are used to strictly control the slag discharge. Ar blowing is performed throughout the refining process according to the refining specifications. Desulfurization, composition fine-tuning, and temperature increases are performed based on the converter steel composition and temperature. The vacuum degassing vacuum degree is 0.07 kPa. The deep vacuum time is 18 minutes. After breaking the vacuum, high-calcium wire is fed in, and rare earth alloy is added at a rate of 50 ppm. The soft blowing time is maintained at 15 minutes.

[0025] Continuous casting maintains a constant casting speed, with a target speed of 2.1 m / min. In special production situations, speed reduction or flow shut-off operations can be implemented. During normal casting, the liquid level in the tundish is controlled at 800-810 mm. Protective casting is used throughout the entire casting process. A special protective slag is used. The billet undergoes hot acid low-magnification inspection; defects such as subcutaneous cracks, subcutaneous bubbles, corner cracks, and intermediate cracks are all classified as level 0.5. The H, O, and N content of the billet gases is inspected for each heat; specific finished product gas values ​​are shown in Table 2. The billet is slow-cooled promptly after leaving the production line, and then sent to the rolling mill for rolling after 48 hours of slow cooling. The billet composition is shown in Table 1.

[0026] Table 1 Chemical composition / %

[0027]

[0028]

[0029] Table 2 Gas content / ppm

[0030] Example H O N Example 1 0.9 20 56 Example 2 1.2 14 67 Example 3 1.5 15 68

[0031] Before rolling, the surface quality of the cast billets was inspected, and billets with surface defects were removed. The heating furnace and rolling process are shown in Tables 3 and 4. After final rolling, controlled cooling was performed, with the outlet water tank temperature at 820±10℃ and the upper cooling bed temperature at 800±10℃. After shearing, the billets were promptly placed in the slow cooling pit, ensuring an entry temperature ≥300℃. Cooling was promptly implemented when rolling was stopped to reduce rolling line accidents and prevent excessive time spent in the furnace. Performance testing was conducted after the billets exited the slow cooling pit, and the results are shown in Tables 5 and 6. The microstructure of Example 1 is shown in Table 5. Figure 1 As shown.

[0032] Table 3 Heating Regulation

[0033]

[0034] Table 4 Finishing rolling process parameters

[0035]

[0036]

[0037] Table 5. Test results of mechanical properties and low-temperature impact performance

[0038]

[0039] Table 6 Requirements / Grades for Non-metallic Inclusions

[0040]

[0041] At -40℃, the full-size (10x10x55mm) V-notch impact energy (Akv) is ≥80J, far exceeding the low-temperature impact level of ordinary steel bars and simple controlled cooling processes. Table 4 shows that the steel bar exhibits a high strength and good plasticity, with a stable strength-to-yield ratio ≥1.25 and elongation meeting standard requirements. Due to the addition of Cu, Ni, Cr, and RE, a protective oxide film (dense and stable rust layer) is formed. The dense and uniform matrix reduces the tendency for corrosion microcells, especially enhancing resistance to saline soils and freeze-thaw cycles. The cyclic wetting rate is 55% higher than that of HEB400E.

[0042] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method of smelting and rolling of construction reinforcement for permafrost environments, characterized in that, include: 1) Smelting The raw materials and alloys used must meet the requirements of the current standards for raw materials and alloys. The smelting process was carried out using a top-and-bottom blown converter, and the final slag basicity was controlled at 3.

0. The final control targets are C≥0.06% and T≥1630℃; ferromanganese and ferrosilicon manganese alloying are used, and final deoxidation is performed with Al deoxidation, and alloying is carried out according to composition requirements; slag blocking is implemented at tapping; Ar blowing is carried out throughout the refining process, and desulfurization, composition fine-tuning, and temperature increase are performed according to the converter steel composition and temperature; VD vacuum degassing is required, with a VD vacuum degree ≤0.10kPa, a target value ≤0.06kPa, and a deep vacuum time ≥13min; after breaking the vacuum, silicon-calcium wire is fed in, and rare earth alloys are added according to the required amount; the soft blowing operation time is guaranteed to be ≥15min; the superheat is controlled within 30℃, and the casting speed is controlled to a target of 2.0-2.2m / min, ensuring constant casting speed operation; the continuous casting billet is placed in a slow cooling pit for slow cooling for more than 48 hours after casting; after continuous casting, the billet enters a dynamic light pressure and optimized secondary cooling zone to ensure good solidification quality at the end of the billet solidification, improve the solidification structure, and reduce central segregation and porosity; 2) Rolling The slowly cooled, qualified continuous casting billet is fed into a heating furnace, with the furnace temperature controlled at 1100-1200℃ in the soaking zone to ensure uniform heat treatment. Multi-pass rolling is performed in the fully recrystallized austenite zone; the deformation per pass is ≥15%, and the total compression ratio is ≥30, to fully refine the original austenite grains; the initial rolling temperature is 930-970℃, and the final rolling temperature is strictly controlled at 880-892℃; immediately after finishing rolling, the steel bar is rapidly fed into a powerful water-cooling device to ensure the upper cooling bed temperature is 800-850℃. The chemical composition of the steel reinforcement by mass percentage is as follows: C 0.09-0.11%, Si 0.24-0.27%, Mn 1.35-1.39%, P 0.0011-0.010%, S 0.002-0.010%, Cu 0.34-0.37%, Cr 0.25-0.29%, Ni 0.26-0.28%, Nb 0.025-0.030%, V 0.042-0.045%, Ce 0.0015-0.0020%, with the remainder being Fe and impurities.

2. The method of smelting and rolling of construction reinforcement for permafrost conditions according to claim 1, characterized in that, The billet must not have serious squareness, bulging, or central shrinkage. A hot acid low-magnification sample of the billet should be taken from each furnace for inspection. The defect level of subcutaneous cracks, subcutaneous bubbles, corner cracks, and intermediate cracks must not be greater than level 1.

0. Any billet that exceeds the standard must be re-evaluated.

3. The method of smelting and rolling of construction reinforcement for permafrost conditions according to claim 1, characterized in that, The chemical composition of the steel reinforcement by mass percentage is as follows: C 0.11%, Si 0.25%, Mn 1.35%, P 0.010%, S 0.010%, Cu 0.35%, Cr 0.27%, Ni 0.27%, Nb 0.030%, V 0.045%, Ce 0.0020%, with the remainder being Fe and impurities.

4. The method of smelting and rolling of construction reinforcement for permafrost conditions according to claim 1, characterized in that, The chemical composition of the steel reinforcement by mass percentage is as follows: C 0.09%, Si 0.27%, Mn 1.36%, P 0.009%, S 0.002%, Cu 0.37%, Cr 0.25%, Ni 0.26%, Nb 0.025%, V 0.044%, Ce 0.0015%, with the remainder being Fe and impurities.

5. The smelting and rolling method for reinforcing steel bars used in frozen soil environments according to claim 1, characterized in that, The chemical composition of the steel reinforcement by mass percentage is as follows: C 0.10%, Si 0.24%, Mn 1.39%, P 0.0011%, S 0.007%, Cu 0.34%, Cr 0.29%, Ni 0.28%, Nb 0.030%, V 0.042%, Ce 0.0019%, with the remainder being Fe and impurities.

6. The smelting and rolling method for reinforcing steel bars used in frozen soil environments according to claim 1, characterized in that, After final rolling, controlled cooling is performed, with the outlet water temperature at 820±10℃.

7. The method of smelting and rolling of construction reinforcement for permafrost conditions according to claim 1, characterized in that, The temperature of the upper cooling bed is 800±10℃.

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