Production process of rare earth microalloying low-cost 550MPa-grade weathering steel based on CSP production line

Abstract

The invention discloses a production process of rare earth microalloying low-cost 550MPa-grade weathering steel based on a CSP production line, and belongs to the technical field of steel and iron material preparation. According to the process, the composition proportion is optimized, La / Ce composite microalloying is adopted, and the CSP short-process continuous casting and rolling technology is combined, so that low-cost production of the high-performance weathering resistant steel is achieved. The method specifically comprises the key procedures of molten steel component design, whole-course protective pouring, homogenizing annealing at the temperature of 1100-1300 DEG C, six-rack controlled rolling, layered cooling and the like. The thickness of the obtained steel plate is 1-5 mm, the yield strength is larger than or equal to 550 MPa, the tensile strength is 620-780 MPa, the percentage elongation after fracture is larger than or equal to 16%, and the weight loss ratio is reduced by 15% or above compared with that of rare earth-free steel after the steel plate is immersed and corroded by 3.5% NaCl for 72 h. Through rare earth purification of molten steel, grain refinement and improvement of rust layer compactness, the corrosion resistance is remarkably improved while the use amount of Ni / Cu is reduced, the technical problem that nodulation is prone to occurring when rare earth is added in a CSP production line is solved, and the rare earth metal powder has the comprehensive advantages of being high in production efficiency, low in cost and stable in product performance and is suitable for weather-proof structural parts such as buildings and bridges.

Description

Technical Field

[0001] The present invention relates to a production process of a 550 MPa grade weathering steel with low cost and rare earth microalloying based on a CSP production line, belonging to the technical field of production and preparation of steel materials. Background Art

[0002] Weathering steel, also known as atmospheric corrosion resistant steel, is a kind of low alloy high strength steel with few alloying elements, low price and good corrosion resistance to atmospheric corrosion. Under the action of the atmosphere, a dense and firmly attached corrosion product protective film will gradually form on the surface of the weathering steel, which will protect the matrix from further corrosion by corrosive media in the atmosphere and achieve "rusting without corrosion". At the same time, as a structural material, weathering steel not only requires good atmospheric corrosion resistance, but also requires good mechanical properties. Because the corrosion rate of weathering steel is significantly lower than that of ordinary carbon steel, and compared with stainless steel, the price of weathering steel is lower, with higher cost performance. Since the loss caused by corrosion failure of steel during service every year is huge, it is particularly important to develop high-performance weathering steel. As an important strategic resource, rare earth resources can play advantages such as purifying molten steel, modifying inclusions, refining grains and microalloying in steel materials. An appropriate rare earth addition amount can, through the coupling effect of various advantages, improve the atmospheric corrosion resistance on the premise of ensuring the mechanical properties of weathering steel.

[0003] The full name of the CSP process is the compact short process thin slab continuous casting and rolling process, which has the advantages of low production cost and high production efficiency. However, due to the small size of the mold in the CSP production line and the thin thickness of the continuous casting billet, the total reduction ratio during rolling is limited. Therefore, compared with the traditional hot continuous rolling production line, it is more difficult to produce high-performance plates on the CSP production line, and process parameters such as the reduction ratio per pass, rolling speed, cooling rate and coiling temperature need to be reasonably controlled. It cannot be ignored that rare earth elements are prone to nozzle caking during continuous casting, which leads to blockage of the pouring nozzle and affects the continuity of production. Moreover, the size of the mold in the CSP production line is small, and the risk of nozzle blockage during pouring is more prominent. Therefore, it is more important to control the reasonable rare earth addition amount and molten steel composition.

[0004] The invention patent "A production method of 235MPa grade rare earth weathering steel" discloses a weathering steel with the chemical composition: C: 0.06 - 0.08%, Si: 0.25 - 0.35%, Mn: 0.35 - 0.45%, P: ≤0.020%, S: ≤0.001%, Cu: 0.20 - 0.35%, Cr: 0.35 - 0.45%, Ni: 0.08 - 0.12%, Ti: 0.010 - 0.020%, La: 10 - 80 ppm and its production process. This weathering steel has the advantages of saving Cu, Ni, etc. However, in the weathering steel without La compared with the rare earth weathering steel in this patent, alloying elements such as Cu, Cr, Ni, etc. that improve corrosion resistance are not added. Therefore, it cannot be explained that the decrease in the corrosion weight loss rate is caused by La. And the difference in the corrosion weight loss rate among the weathering steels with different La contents is not significant.

[0005] The invention patent "A 550MPa grade nickel-saving rare earth weathering steel" discloses a rare earth weathering steel, whose chemical composition is: C: 0.05 - 0.16%, Si: 0.10 - 0.40%, Mn: 1.10 - 1.20%, P: 0.008 - 0.016%, S ≤0.005%, Cr: 0.55 - 0.65%, Cu: 0.10 - 0.15%, Nb: 0.03 - 0.05%, Ti: 0.015 - 0.025%; Al: 0.025 - 0.050%, O: 0.0005 - 0.0030%, La: 0.005 - 0.015%, La / S ≥5, and the rest are Fe and inevitable impurities. This invention has the advantages of reducing the consumption of precious alloys such as nickel and copper in weathering steel, reducing the production cost, and having good performance. However, a relatively small amount of Ni element is added to the rare earth weathering steel used as a control in this patent, and the production of the weathering steel in this patent is completed on a traditional hot continuous rolling production line, without attempting on the CSP production line with lower production cost.

[0006] In the invention patent "A Rare Earth Composite Nb, Ti Microalloyed High-Strength Weathering Steel with 800 MPa Grade and Its Preparation Method", a rare earth weathering steel is disclosed. Its chemical composition is as follows: C: 0.080 - 0.090%, Si: 0.500 - 0.650%, Mn: 0.750 - 0.800%, P ≤ 0.020%, S ≤ 0.006%, N: 0.004 - 0.006%, Al: 0.025 - 0.035%, Cr: 0.500 - 0.550%, Ni: 0.100 - 0.150%, Cu: 0.20 - 0.30%, Nb: 0.020 - 0.030%, Ti: 0.10 - 0.12%, Ce: 0.010 - 0.030%, and the balance is Fe and inevitable impurity elements. By adding trace rare earth element Ce to the weathering steel in this invention, and utilizing the interaction relationship between rare earth element Ce and microalloying elements such as Nb and Ti, the precipitation of fine and dispersed microalloy precipitation phases is promoted. On the premise of ensuring good plasticity of the steel, the strength level of the weathering steel is improved. However, the production process described in this patent is carried out on a traditional hot strip mill production line. Due to the relatively high addition amount of rare earth Ce in the patent, there is a great risk of nozzle caking and blocking the nozzle in the continuous casting process, affecting the smooth pouring. Moreover, the corrosion resistance performance is evaluated by using the weathering performance index I value, which belongs to the method of using empirical formulas and there is no actual experimental data to reflect the influence of rare earth Ce on the atmospheric corrosion resistance performance. Summary of the Invention

[0007] Referring to the advantages and disadvantages of the existing related technologies, the purpose of the present invention is to provide a production process of a rare earth microalloyed low-cost 550 MPa grade weathering steel based on the CSP production line, which can produce a 550 MPa grade rare earth microalloyed weathering steel with good atmospheric corrosion resistance on the CSP production line. The whole process has the characteristics of high production efficiency, low production cost, and safe and easy operation.

[0008] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows: A production process of a rare earth microalloyed low-cost 550 MPa grade weathering steel based on the CSP production line, characterized by comprising the following steps: (1) Design of the steel billet composition: By weight percentage, the composition is as follows: C: 0.03 - 0.08%, Si: 0.2 - 0.6%, Mn: 0.8 - 1.2%, P: 0.01 - 0.03%, S: ≤0.007%, Cu: 0.1 - 0.4%, Cr: 0.3 - 0.7%, Ni: 0.09 - 0.3%, Ti: 0.04 - 0.09%, Nb: 0.01 - 0.04%, Al: 0.03 - 0.06%, RE: 0.0005 - 0.007%, the balance being Fe and unavoidable impurities; RE is Ce or La: added as a composite in a mass ratio of Ce = 1: 1.2 - 4.5; The rare earth elements La and Ce are added during secondary refining through La - Fe and Ce - Fe master alloys. In the La - Fe and Ce - Fe master alloys, the mass fraction of La or Ce is 10% - 40%.

[0009] (2) Continuous casting of steel: Continuously cast the molten steel with full protection casting. The argon flow rate is 5 - 12 m³ / h, the casting speed is 3.5 - 5.0 m / min, the liquid level fluctuation in the mold is controlled within ±3 mm, and the thickness of the continuously cast steel billet is 40 - 80 mm; (3) Homogenization annealing: Anneal the continuously cast steel billet at 1100 - 1300 °C for homogenization; (4) Rolling process of the steel billet: Control rolling is carried out on a 6 - stand CSP (Compact Strip Production) production line for the steel billet (ingot) after homogenization annealing and descaling with high - pressure water. The rolling speed at the exit stand during the control rolling process is 4.5 - 15.0 m / s, the starting rolling temperature is 1000 °C - 1230 °C, the final rolling temperature is 800 °C - 960 °C, and the total reduction ratio is 88% - 99%. The reduction ratios for each pass are as shown in the table: The first pass (F1): 55 - 65%, the second pass (F2): 49 - 54%, the third pass (F3): 39 - 48%, the fourth pass (F4): 32 - 38%, the fifth pass (F5): 24 - 31%, the sixth pass (F6): 12 - 23%.

[0010] (5) Laminar flow cooling: The rolled steel plate is cooled by laminar flow at 10 - 50 °C / s; (6) Coiling: The coiling temperature is 500 - 700 °C, and the thickness of the finished steel plate is 1 - 5 mm.

[0011] The mechanical properties of the weathering steel meet the following requirements: yield strength ≥550 MPa, tensile strength 620 - 780 MPa, elongation after fracture ≥16%; the corrosion resistance meets the following requirements: after immersion in a 3.5% NaCl solution for 72 h in a cyclic immersion accelerated corrosion test, the corrosion weight loss rate is reduced by ≥15% compared with the weathering steel without rare earth addition.

[0012] The present invention can produce 550 MPa weathering steel with good atmospheric corrosion resistance on a CSP production line with low production cost and high production efficiency by using trace rare earth elements. Without increasing the usage amount of precious alloying elements such as Cr, Ni, and Cu, the 72-hour immersion corrosion weight loss rate of the weathering steel can be reduced by ≥15% through trace rare earth Ce or La+Ce elements. Therefore, the present invention has the characteristics of high production efficiency, low production cost, good service performance of products, and safe and easy operation in the production process, and has good application prospects.

[0013] Advantages of the present invention: The present invention provides a production process for rare earth microalloyed low-cost 550 MPa weathering steel based on a CSP production line, achieving significant technological breakthroughs and economic benefits through innovative composition design and process optimization. This process uses CSP short-process continuous casting and rolling technology, adding trace amounts of rare earth elements (La / Ce) in combination, and cooperating with Ti / Nb microalloying. While reducing the usage amount of precious alloys such as Cr and Ni, the yield strength of the steel can be stably reached above 550 MPa, the tensile strength is 620-780 MPa, and the elongation after fracture is ≥16%, with excellent comprehensive mechanical properties. The introduction of rare earth elements not only improves the strength and toughness of the material by refining grains and purifying the molten steel, but also promotes the formation of a dense corrosion product film, reducing the weight loss rate of the steel after 72 hours of immersion corrosion in 3.5% NaCl by more than 15% compared with traditional weathering steel, significantly extending the service life. Technologically, by precisely controlling parameters such as the liquid level fluctuation in the mold, layer cooling, and coiling temperature, the industry problem of nozzle coking easily caused by rare earth addition in the CSP production line is solved, ensuring the stability of continuous production. Compared with the traditional hot rolling process, the present invention has both the energy consumption advantage of the short process (omitting the reheating process) and the cost advantage of rare earth microalloying. The finished product thickness is adjustable, suitable for fields with strict weather resistance requirements such as buildings, bridges, and containers, and has broad market application prospects and environmental protection value. Brief description of the drawings

[0014] Figure 1 Microstructure of the steel plate produced in Example 1; Figure 2 Microstructure of the steel plate produced in Example 2. Detailed description of the specific implementation

[0015] The following further details the specific implementation of the present invention in conjunction with the examples.

[0016] Example 1 A production process for rare earth microalloyed low-cost 550 MPa weathering steel based on a CSP production line, the specific steps are as follows: (1)Steel billet composition design requirements: The composition of the rare earth microalloyed low-cost Q550NH high-strength weathering steel billet in this embodiment is shown in Table 1; The remaining components in the steel billet (ingot) are Fe and other inevitable impurity elements.

[0017] (2)Continuous casting of the above-mentioned component molten steel, with the argon flow rate of the protective casing being 5 - 12 m 3 / h, the drawing speed is maintained at 3.5 - 5.0 m / min, and the liquid level fluctuation in the mold is controlled within ±3 mm, obtaining a continuous casting steel billet with a thickness of 52 mm; (3)Homogenizing annealing of the above-mentioned continuous casting steel billet in a heating furnace, with the soaking temperature being 1190 ± 10 °C; (4)Controlled rolling of the steel billet (ingot) after homogenizing annealing and descaling with high-pressure water on a 6-stand CSP (Compact Strip Production) production line. The rolling rate at the exit stand: 4.5 - 15.0 m / s, the starting rolling temperature is 1100 °C ± 20 °C, the final rolling temperature is 910 °C ± 10 °C, and the total reduction ratio is 96.15%; The reduction ratio for each pass is shown in Table 2 respectively: (5)Laminar cooling of the rolled steel plate, with the cooling rate controlled within 10 °C / s - 25 °C / s; (6)Using a coiler to coil the cooled steel plate with a thickness of 2 mm into a steel coil, with the coiling temperature being 630 °C ± 10 °C, and storing it in the warehouse after coiling is completed.

[0018] The microstructure of the steel plate produced in this embodiment is as Figure 1 shown, and the room temperature tensile mechanical properties and the 72-hour cyclic immersion corrosion weight loss rate are shown in Table 3. It can be seen that the mechanical properties of the steel plates in Example 1 and the comparative example meet the national standard requirements. After 72 hours of cyclic immersion corrosion, the corrosion weight loss rate of the steel plate added with 0.001% La and 0.003% Ce decreased by about 20.01% compared with the comparative example steel plate without adding RE.

[0019] Example 2 A production process for rare earth microalloyed low-cost 550 MPa grade weathering steel based on a CSP production line, and the specific steps are as follows: (1)Steel billet composition design requirements: The composition of the rare earth microalloyed low-cost Q550NH high-strength weathering steel billet involved in the present invention is shown in Table 4; The remaining components in the steel billet (ingot) are Fe and other inevitable impurity elements.

[0020] (2) Continuously cast the above-mentioned molten steel of the components, with the argon flow rate of the protective casing being 5 - 12 m 3 / h, the casting speed being maintained at 3.5 - 5.0 m / min, and the liquid level fluctuation of the mold being controlled within ±3 mm. The thickness of the continuously cast steel billet is 70 mm to obtain the continuously cast steel billet; (3) Homogenize and anneal the above-mentioned continuously cast steel billet in a heating furnace, with the soaking temperature being 1190 ± 10 °C; (4) Control-roll the steel billet (ingot) after homogenization annealing and descaling with high-pressure water on a 6-stand CSP (Compact Strip Production) production line. The rolling rate at the exit stand: 4.5 - 15.0 m / s, the starting rolling temperature being 1120 °C ± 20 °C, the finishing rolling temperature being 880 °C ± 10 °C, and the total reduction ratio being 95%; The reduction ratio per pass is shown in Table 5 respectively: (5) Conduct laminar cooling on the rolled steel plate, with the cooling rate being controlled within 26 °C / s - 40 °C / s; (6) Use a coiler to coil the cooled steel plate with a thickness of 3.5 mm into a steel coil, with the coiling temperature being 650 °C ± 10 °C, and store it in the warehouse after coiling is completed.

[0021] The microstructure of the steel plate produced in this embodiment is as Figure 2 shown, and the room-temperature tensile mechanical properties and the 72-hour cyclic immersion corrosion weight loss rate are shown in Table 6. It can be seen that the mechanical properties of the steel plates in Example 2 and the comparative example meet the national standard requirements. After 72 hours of cyclic immersion corrosion, the corrosion weight loss rate of the steel plate added with 0.0045% Ce decreased by about 21.47% compared with the comparative example steel plate without adding RE.

[0022] The above embodiments are merely examples given to clearly illustrate the present invention and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes can be made based on the above descriptions. And the obvious changes derived therefrom all fall within the protection scope of the present invention.

Claims

1. A production process of a low-cost 550MPa weathering steel with rare earth microalloying based on a CSP production line, characterized in that, It includes the following steps: (1) Design of the billet composition: By weight percentage, the composition is as follows: C: 0.03 - 0.08%, Si: 0.2 - 0.6%, Mn: 0.8 - 1.2%, P: 0.01 - 0.03%, S: ≤0.007%, Cu: 0.1 - 0.4%, Cr: 0.3 - 0.7%, Ni: 0.09 - 0.3%, Ti: 0.04 - 0.09%, Nb: 0.01 - 0.04%, Al: 0.03 - 0.06%, RE: 0.0005 - 0.007%, and the balance is Fe and inevitable impurities; RE is a composite addition of Ce or a mixture of La and Ce in a mass ratio of 1:1.2 - 4.5; (2) Continuous casting of steel: Continuously cast the molten steel, adopt full - protection casting, with an argon flow rate of 5 - 12 m³ / h, a drawing speed of 3.5 - 5.0 m / min, the liquid level fluctuation in the mold is controlled within ±3 mm, and the thickness of the continuously cast billet is 40 - 80 mm; (3) Homogenization annealing: Anneal the continuously cast billet at 1100 - 1300 °C; (4) Controlled rolling: Roll on a 6 - stand CSP production line, with an initial rolling temperature of 1000 - 1230 °C, a final rolling temperature of 800 - 960 °C, a total reduction ratio of 88% - 99%, and the reduction ratios for each pass are F1: 55 - 65%, F2: 49 - 54%, F3: 39 - 48%, F4: 32 - 38%, F5: 24 - 31%, F6: 12 - 23% in sequence; (5) Laminar flow cooling: After rolling, the steel plate is cooled by laminar flow at a rate of 10 - 50 °C / s; (6) Coiling: The coiling temperature is 500 - 700 °C, and the thickness of the finished steel plate is 1 - 5 mm.

2. The production process according to claim 1, characterized in that, The mechanical properties of the weathering steel meet the following: yield strength ≥550 MPa, tensile strength is between 620 MPa and 780 MPa, and elongation after fracture ≥16%; The corrosion resistance of the weathering steel meets the following: when undergoing cyclic immersion accelerated corrosion in a 3.5% NaCl solution for 72 h, the corrosion weight loss rate is reduced by ≥15% compared to the weathering steel without rare earth addition.

3. The production process according to claim 1, characterized in that, The rare earth elements La and Ce are added during ladle refining through La - Fe and Ce - Fe master alloys. In the La - Fe and Ce - Fe master alloys, the mass fraction of La or Ce is 10% - 40%.

4. The production process according to claim 1, characterized in that, The rolling rate during the controlled rolling process: 4.5 - 15.0 m / s, and the liquid level fluctuation in the mold is controlled within ±3 mm.

5. The production process according to claim 1, characterized in that, The homogenization annealing temperature is 1190 ± 10 °C.

6. The production process according to claim 1, characterized in that, The laminar flow cooling rate is divided into two grades: for 1 - 3 mm thin - gauge steel plates: 10 - 25 °C / s; for 3 - 5 mm thick - gauge steel plates: 26 - 40 °C / s.

7. The production process according to claim 1, characterized in that, When only Ce is added as RE, the Ce content is 0.003 - 0.007%; when RE is a composite addition of La + Ce, the La content is 0.0005 - 0.0015%, and the Ce content is 0.002 - 0.005%.

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