A controlled rolling and controlled cooling method for small-size hot-rolled ribbed steel bars

Abstract

The application discloses a kind of for small size hot-rolled ribbed steel bar control rolling and cooling method, comprising the following steps: blank control: control the weight ratio of Si and Mn in blank is 0.31-0.39, smelting, casting forms continuous casting billet;Blank heating: continuous casting billet is sent into heating furnace heating, and then solid solution treatment is carried out after heat preservation;Temperature control rolling: after the deformation of multiple passes of rough rolling and intermediate rolling, the temperature for entering finishing rolling unit is controlled to be 850 DEG C-880 DEG C;Cooling after rolling: after finishing rolling, multistage water cooling is used, and after cooling, the red is returned to the ribbed steel bar by air cooling, and the red return temperature is stably controlled at 830 DEG C-850 DEG C, and finally hot-rolled ribbed steel bar is obtained.The application has the advantages that without reducing rolling speed and sacrificing strength, the ribbed steel bar obtains obvious physical yield platform, meets the requirement of anti-seismic standard, reduces alloy cost, and improves performance stability.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy and rolling technology, and in particular to a controlled rolling and cooling method for small-diameter hot-rolled ribbed steel bars. Background Technology

[0002] With the increasing demands for steel bar production efficiency in the construction industry, the rolling speed of high-speed bar production lines has generally reached over 35 m / s, with some advanced lines reaching 45 m / s, significantly increasing the production capacity of small-diameter steel bars. However, for small-diameter hot-rolled ribbed steel bars of Φ10mm~14mm, the high-speed rolling process, characterized by large deformation per pass and low final rolling temperature, leads to a sharp increase in austenite deformation energy storage, a violent deformation-induced ferrite transformation (DIFT) process, and ultimately, extremely refined grains and a significantly increased dislocation density within the steel bar. The pinning effect of Cottrell atmospheres on dislocations is weakened, and the tensile curve shows no obvious physical yield plateau, exhibiting continuous yield characteristics.

[0003] For seismic-resistant steel bars such as HRB400E and HRB500E used in buildings, the lack of a yield plateau makes it difficult to determine the yield strength value in structural design, and the steel bars lack a clear plastic hinge formation mechanism under seismic loads, posing potential safety hazards to building structures. Therefore, the GB 1499.2-2024 standard explicitly requires that seismic-resistant steel bars must have a clear yield point or a specified plastic elongation strength.

[0004] To address the issue of no yield plateau in small-diameter high-speed bars, existing technologies mainly employ two types of solutions: The first type is alloy strengthening: by increasing the Mn content and the amount of microalloying elements such as V and Nb, strength is ensured through solid solution strengthening and precipitation strengthening, while attempting to improve yield behavior. However, high Mn content exacerbates segregation at the center of the billet, and under the high cooling rate of high-speed rolling, it easily generates low-temperature phase transformation structures such as bainite and martensite, which further disrupts the pinning effect of the Cottrell atmosphere and exacerbates the no-yield plateau problem. At the same time, the high alloy content significantly increases production costs, which does not meet the production requirements of cost reduction and efficiency improvement.

[0005] The second type of existing technology is a process adjustment scheme: by reducing the rolling speed and increasing the final rolling temperature, the degree of deformation-induced ferrite phase transformation is weakened, thereby restoring the yield plateau. However, this scheme directly sacrifices the capacity advantage of high-speed bar lines, reducing production efficiency by more than 15%, and fluctuations in rolling speed can easily lead to poor uniformity of bar performance and a lower performance qualification rate. Summary of the Invention

[0006] The purpose of this invention is to provide a controlled rolling and cooling method for small-diameter hot-rolled ribbed steel bars, which solves the technical problems of no yield plateau, high alloy cost, and inability to balance performance and production capacity when producing Φ10mm~14mm seismic steel bars on high-speed bar lines. Without reducing rolling speed or sacrificing strength, the method enables the steel bars to obtain a significant physical yield plateau, meeting the seismic standard requirements, while reducing alloy cost and improving performance stability.

[0007] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0008] A controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars includes the following steps: Billet control: The weight ratio of Si to Mn in the billet is controlled to be 0.31 to 0.39, and the billet is smelted and cast to form a continuous casting billet; Billet heating: The continuously cast billet is sent into a heating furnace for heating, and then held at the temperature for solution treatment; Temperature-controlled rolling: After multiple passes of deformation in the roughing and intermediate rolling mills, the temperature of the rolling mill is controlled to be 850℃~880℃. Post-rolling cooling: After finishing rolling, multi-stage water cooling is adopted. After cooling, the steel bars are air-cooled to turn red. The turning temperature is stably controlled at 830℃~850℃, and finally hot-rolled ribbed steel bars are obtained.

[0009] Preferably, in the billet control step, the Si weight percentage of the continuously cast billet is 0.42% to 0.48%, and the Mn weight percentage is 1.29% to 1.33%.

[0010] Preferably, the heating temperature of the continuously cast billet in the billet heating step is 1050℃~1150℃, and the holding time is ≥60min.

[0011] Preferably, in the temperature-controlled rolling step, the roughing mill exit temperature is 1000℃~1050℃, and the intermediate mill exit temperature is 920℃~950℃.

[0012] Preferably, the multi-stage water cooling in the post-rolling cooling step consists of 2 to 3 stages of water cooling, with an air-cooled return section between stages, and the cooling water pressure is 0.3 MPa to 0.6 MPa.

[0013] Preferably, in the post-rolling cooling step, the surface temperature of the steel bar drops to a final cooling temperature of 750℃~800℃ after multi-stage water cooling.

[0014] A hot-rolled ribbed steel bar is prepared using a controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars. Its chemical composition by weight percentage includes: C 0.22%–0.25%, Si 0.40%–0.50%, Mn 1.28%–1.35%, Nb 0.010%–0.015%, P≤0.045%, S≤0.045%, with the balance being Fe and unavoidable impurities.

[0015] Preferably, the hot-rolled ribbed steel bar is an HRB400E or HRB500E seismic steel bar with a nominal diameter of 10mm to 14mm.

[0016] Preferably, in the cross-sectional metallographic structure of the hot-rolled ribbed steel bar, the area ratio of polygonal ferrite is 65% to 75%, and the area ratio of bainite is ≤5%.

[0017] Preferably, the hot-rolled ribbed steel bar has a yield plateau elongation of ≥2.0%, a total elongation of ≥16.0%, and a strength-to-yield ratio of ≥1.25.

[0018] In summary, the present invention has the following beneficial effects: 1. Effectively solves the industry problem of no yield plateau under high-speed rolling: By synergistically controlling the Si / Mn ratio, the red-hot temperature, and the finishing rolling temperature, under high-speed rolling conditions above 35m / s, Φ10mm~14mm steel bars can obtain a significant physical yield plateau, with a yield plateau elongation ≥2.0% and a strength-to-yield ratio ≥1.25, fully meeting the requirements of seismic steel bars in GB 1499.2-2024, and resolving the contradiction of "reducing speed to maintain the plateau and improving efficiency to reduce performance" in existing technologies.

[0019] 2. Significantly reduce alloy production costs: By adopting a composition design that "increases silicon and reduces manganese," low-cost ferrosilicon replaces some of the high-cost ferromanganese, reducing the cost of alloy per ton of steel by 15 to 20 yuan; at the same time, there is no need to add vanadium-nitrogen alloys, further reducing raw material costs and resulting in significant economic benefits.

[0020] 3. Improved performance stability and uniformity of steel bars: The combination of narrow-range red-hot temperature control and stable finishing mill inlet temperature reduces the impact of rolling speed fluctuations on the phase transformation process, increasing the steel bar performance qualification rate to over 99%, and the yield strength difference of steel bars to ≤20MPa. The performance stability is significantly better than that of conventional processes.

[0021] 4. Overcoming technical biases in this field: It breaks the inherent perception that "high-speed rolling of small-diameter steel bars must rely on high manganese and high vanadium to ensure strength and yield behavior," and discovers the synergistic regulation mechanism of Si / Mn ratio and red-heat temperature on deformation-induced ferrite phase transformation, providing a brand-new technical approach for the production of small-diameter steel bars in high-speed bar mills. Attached Figure Description

[0022] Figure 1 This is a test diagram of the yield plateau of hot-rolled ribbed steel bars prepared according to an embodiment of the present invention; Figure 2 This is a test diagram of the yield plateau of the hot-rolled ribbed steel bar prepared according to the comparative example of the present invention. Detailed Implementation

[0023] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. These embodiments do not constitute a limitation on the present invention.

[0024] Test conditions: All examples and comparative examples were conducted on a high-speed bar production line with a rolling speed of 40 m / s. The continuous casting billet size was 150 mm × 150 mm, and the main steel bar size was Φ10 mm, supplemented by verification of Φ12 mm and Φ14 mm sizes. Tensile tests were performed according to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", metallographic structure testing was performed according to GB / T 13299-1991 "Methods for evaluating the microstructure of steel", and impact tests were performed according to GB / T 229-2020 "Metallic materials - Charpy pendulum impact test method".

[0025] Example and comparative parameters: The chemical composition and process parameters of each embodiment and comparative example are shown in Table 1 and Table 2, respectively, and the corresponding tissue and performance test results are shown in Table 3.

[0026] Table 1. Chemical composition (wt%) of each example and comparative example

[0027] Table 2. Process parameters for each embodiment and comparative example

[0028] Table 3. Results of tissue and performance tests for each embodiment and comparative example.

[0029] As can be seen from the results of the above embodiments and comparative examples: Examples 1-5 using the technical solution of this invention all yielded a polygonal ferrite microstructure of 65%-75%, with bainite accounting for ≤5%, and the tensile curves exhibited a clear physical yield plateau, such as... Figure 1 As shown, the yield plateau elongation is ≥2.0%, the strength-to-yield ratio is ≥1.25, and the strength and toughness both meet the standard requirements of the corresponding grade of seismic steel bars, verifying the effectiveness of the present invention.

[0030] Comparative Example 1 uses a conventional high-manganese composition, with a Si / Mn ratio lower than the lower limit of this invention, and a bainite content as high as 13%, with no obvious yield plateau, proving that insufficient Si / Mn ratio cannot solve the problem of no yield plateau under high-speed rolling.

[0031] In Comparative Example 2, the Si content was too high, and the Si / Mn ratio exceeded the upper limit of this invention. Although the yield plateau was obvious, the impact energy was only 28J, and the toughness was insufficient, which could not meet the safety requirements of earthquake-resistant steel bars.

[0032] Comparative Examples 3 and 4 represent reddening temperatures below and above the range of this invention, respectively. The former exhibits insufficient strength and Widmanstätten structure, while the latter shows an increased proportion of bainite and an indistinct yield plateau. Figure 2 As shown, narrow-range reddening temperature control is a key process parameter for achieving microstructure and performance regulation.

[0033] In Comparative Example 5, the finishing mill temperature exceeded the upper limit, resulting in coarse austenite grains, insufficient deformation energy storage, a reduced proportion of polygonal ferrite, and a weak yield plateau. This verified the synergistic effect of finishing mill inlet temperature, composition, and red-heat temperature.

[0034] In summary, this invention, through precise matching and synergistic control of the Si / Mn ratio, finishing mill inlet temperature, and reheating temperature, successfully improved the yield plateau of small-diameter steel bars on a high-speed bar production line, while ensuring a balance between strength, toughness, and cost, demonstrating excellent technical effects and industrial application value.

[0035] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various modifications or equivalent substitutions to the present invention within the scope of its essence and protection. Such modifications or equivalent substitutions should also be considered to fall within the protection scope of the present invention.

Claims

1. A controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars, characterized in that, Includes the following steps: Billet control: The weight ratio of Si to Mn in the billet is controlled to be 0.31 to 0.39, and the billet is smelted and cast to form a continuous casting billet; Billet heating: The continuously cast billet is sent into a heating furnace for heating, and then held at the temperature for solution treatment; Temperature-controlled rolling: After multiple passes of deformation in the roughing and intermediate rolling mills, the temperature of the rolling mill is controlled to be 850℃~880℃. Post-rolling cooling: After finishing rolling, multi-stage water cooling is adopted. After cooling, the steel bars are air-cooled to turn red. The turning temperature is stably controlled at 830℃~850℃, and finally hot-rolled ribbed steel bars are obtained.

2. The controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars according to claim 1, characterized in that: In the billet control step, the Si weight percentage of the continuously cast billet is 0.42% to 0.48%, and the Mn weight percentage is 1.29% to 1.33%.

3. The controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars according to claim 1, characterized in that: In the billet heating step, the heating temperature of the continuously cast billet is 1050℃~1150℃, and the holding time is ≥60min.

4. The controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars according to claim 1, characterized in that: In the temperature-controlled rolling process, the roughing mill exit temperature is 1000℃~1050℃, and the intermediate mill exit temperature is 920℃~950℃.

5. The controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars according to claim 1, characterized in that: The post-rolling cooling step involves multi-stage water cooling with 2 to 3 stages, and an air-cooled reheating section is set between stages. The cooling water pressure is 0.3 MPa to 0.6 MPa.

6. The controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars according to claim 1, characterized in that: In the post-rolling cooling step, the surface temperature of the steel bar drops to a final cooling temperature of 750℃~800℃ after multi-stage water cooling.

7. A hot-rolled ribbed steel bar, characterized in that, It is prepared by the controlled rolling and controlled cooling method for small-diameter hot-rolled ribbed steel bars as described in any one of claims 1 to 6. Its chemical composition by weight percentage includes: C 0.22% to 0.25%, Si 0.40% to 0.50%, Mn 1.28% to 1.35%, Nb 0.010% to 0.015%, P≤0.045%, S≤0.045%, with the balance being Fe and unavoidable impurities.

8. A hot-rolled ribbed steel bar according to claim 7, characterized in that: The hot-rolled ribbed steel bars are HRB400E or HRB500E seismic steel bars with a nominal diameter of 10mm to 14mm.

9. A hot-rolled ribbed steel bar according to claim 7, characterized in that: In the cross-sectional metallographic structure of the hot-rolled ribbed steel bar, the area ratio of polygonal ferrite is 65% to 75%, and the area ratio of bainite is ≤5%.

10. A hot-rolled ribbed steel bar according to claim 7, characterized in that: The hot-rolled ribbed steel bars have a yield plateau elongation of ≥2.0%, a total elongation of ≥16.0%, and a strength-to-yield ratio of ≥1.25.

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