400mpa grade hot-rolled ribbed steel bar with ultra-low vanadium-nitrogen ratio and production process thereof

Abstract

This invention discloses an ultra-low vanadium-nitrogen ratio 400MPa grade hot-rolled ribbed steel bar and its production process. The steel bar's chemical composition, by mass percentage, is: C: 0.22~0.25%, Si: 0.40~0.50%, Mn: 1.35~1.55%, V: 0.009~0.012%, Nb: 0.009~0.012%, N: 0.015~0.022%, P≤0.045%, S≤0.045%, with the remainder being Fe and unavoidable impurities. The vanadium-nitrogen mass ratio (V / N) is 0.41~0.80. The advantages of this invention lie in the significant reduction of vanadium consumption and improved production efficiency through niobium-vanadium composite and precise nitrogen control.

Description

Technical Field

[0001] This invention relates to the field of hot-rolled ribbed steel bar production technology, and in particular to a 400MPa grade hot-rolled ribbed steel bar with an ultra-low vanadium-nitrogen ratio and its production process. Background Technology

[0002] 400MPa grade hot-rolled ribbed steel bars are the most widely used structural steel in my country's construction engineering field. Their strength, toughness, and weldability directly determine the safety and durability of building structures. Currently, HRB400 steel bar production mostly adopts vanadium-nitrogen microalloying, niobium microalloying, or conventional niobium-vanadium composite alloying processes. Through the fine-grain strengthening and precipitation strengthening effects of microalloying elements, the mechanical performance requirements of the steel bars are met.

[0003] A search of publicly available patents, industry standards, and industrial practices both domestically and internationally revealed that in the production of 400MPa grade hot-rolled ribbed steel bars, the vanadium-nitrogen mass ratio is generally controlled between 1.0 and 1.5. The industry generally believes that a V / N ratio ≥ 1 is more conducive to vanadium precipitation strengthening. To date, no publicly available technology has been found that systematically employs a niobium-vanadium composite microalloying process with a V / N ratio of 0.41–0.80 and has achieved stable industrial-scale production.

[0004] While the invention patent with application number CN202010870252.5 discloses a relatively low V / N ratio of 0.83, its process involves single vanadium-nitrogen microalloying without introducing niobium for composite strengthening or using multi-element nitride alloys for active nitrogen control. Therefore, its technical solution cannot solve the problem of insufficient strength in large-diameter steel bars such as Φ50mm. More importantly, the existing technology fails to reveal the decisive role of further reducing the V / N ratio from the conventional ≥0.8 to the 0.41~0.80 range within the process framework of "equal niobium-vanadium composite" and "multi-element nitride alloys" in achieving high strength and high grain size in large-diameter steel bars.

[0005] In addition, although the single niobium microalloying process can reduce the amount of vanadium alloy used, it is prone to problems such as cracks in the continuously cast billet and unstable control of steel grain size, which affects the stability of mass production. Conventional niobium-vanadium composite process still follows the traditional vanadium-nitrogen ratio principle, and fails to fully explore the synergistic promoting effect of nitrogen on microalloy precipitation, resulting in low vanadium utilization rate and inability to achieve stable strength matching with low vanadium addition.

[0006] Ti and Si in multi-element nitride alloys are both strong nitride-forming elements, capable of combining with free nitrogen in steel to form dispersed and stable composite precipitate nuclei such as (Ti,Si) and (C,N). On the one hand, this refines austenite grains; on the other hand, by utilizing Ti's preferential combination with nitrogen to form high-temperature stable TiN particles, nitrogen is effectively fixed and the formation of coarse V (C,N) is suppressed. Combined with the grain-refining effect of niobium, vanadium and niobium carbonitrides are induced to precipitate at high density and dispersedly during rolling. This is the key mechanism for achieving high strength at ultra-low V / N ratios.

[0007] Existing technologies have not yet disclosed how to precisely control the V / N ratio between 0.41 and 0.80 while introducing niobium-vanadium composite microalloying and achieving stable industrial production using multi-element nitride alloys. Summary of the Invention

[0008] The purpose of this invention is to provide a hot-rolled ribbed steel bar with an ultra-low vanadium-nitrogen ratio of 400MPa and its production process. By using niobium-vanadium composite and precise nitrogen control, vanadium consumption is significantly reduced and production efficiency is improved.

[0009] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0010] A type of hot-rolled ribbed steel bar with an ultra-low vanadium-nitrogen ratio of 400MPa has the following chemical composition by mass percentage: C: 0.22~0.25%, Si: 0.40~0.50%, Mn: 1.35~1.55%, V: 0.009~0.012%, Nb: 0.009~0.012%, N: 0.015~0.022%, P≤0.045%, S≤0.045%, with the remainder being Fe and unavoidable impurities. The mass ratio of vanadium to nitrogen (V / N) is 0.41~0.80.

[0011] Preferably, the strength-to-yield ratio of the steel bar is ≥1.25, and the total elongation at maximum force is ≥7.5%.

[0012] Preferably, the mass ratio of vanadium to nitrogen (V / N) is 0.60~0.75, and the finished product has a grain size ≥9.5.

[0013] A production process for ultra-low vanadium-nitrogen ratio 400MPa grade hot-rolled ribbed steel bars includes the following steps: S1, Converter smelting: Using blast furnace hot metal mixed with high-quality scrap steel as raw materials, the converter is combined with top and bottom blowing to control the final carbon ratio and final temperature. S2, Alloying: During the tapping process, silicon manganese, silicon iron, silicon carbide balls, niobium iron, vanadium nitrogen alloy and multi-element nitriding alloy are added according to the element content of the finished product. During the addition process, bottom blowing argon is used for stirring. This order of addition is conducive to the first dissolution of niobium, which then interacts with the subsequently added vanadium and nitrogen elements to form a more uniform micro-alloying environment. After the alloy is added, bottom blowing argon is immediately used for stirring. S3, Continuous casting: Qualified molten steel is sent into the continuous casting process, and continuous casting billets are manufactured using a small billet continuous casting machine. S4, Rolling: The continuously cast billet is directly fed into the heating furnace and air-cooled to room temperature to obtain the finished product.

[0014] Preferably, in step S1, the carbon content at the endpoint is ≥0.06%, and the endpoint temperature is 1620~1650℃.

[0015] Preferably, during the tapping process in step S2, when the molten steel has been tapped to 1 / 3, silicon manganese, silicon iron and silicon carbide balls are added; when the molten steel has been tapped to 1 / 2, niobium iron and vanadium nitrogen alloy are added; and when the molten steel has been tapped to 2 / 3, a multi-element nitride alloy is added.

[0016] Preferably, in step S2, the Mn content in the silicon-manganese alloy is 65%, and the addition amount is 20.5 kg / t; the Si content in the silicon-iron alloy is 72%; the Si content in the silicon-carbon spheres is 32%; the C content is 25%; the Nb content in the niobium-iron alloy is 44%; the V content in the vanadium-nitrogen alloy is 77%; and the N content is 14%. The chemical composition of the multi-element nitride alloy, by mass percentage, includes: C≤3%, N≥20%, Ti 8~13%, Si 25~30%, P≤0.15%, S≤0.015%, moisture ≤1.5%, and the remainder is Fe and unavoidable impurities. The mass ratio of Ti to N is 3.5~4.5, which is used to form dispersed TiN particles with a size ≤50 nm.

[0017] Preferably, the argon flow rate for stirring in step S2 is 35 NL / (min·t).

[0018] Preferably, in step S3, the continuously cast billet is a small square billet of 170mm×170mm, the superheat of the tundish is controlled to be ≤25℃, and the casting speed is 2.6~3.0m / min. In order to suppress the hot cracking sensitivity of high nitrogen niobium microalloyed steel, the crystallizer adopts a concave taper design, the secondary cooling zone adopts air mist cooling, and the specific water volume of the secondary cooling zone is controlled at 0.8~1.0L / kg to ensure that the surface temperature recovery rate of the billet is ≤50℃ / s, avoid surface cracks of the continuously cast billet, and promote uniform solid solution of microalloying elements.

[0019] Preferably, in step S4, the temperature of the heating section in the heating furnace is 1080~1120℃, the temperature of the soaking section is 1100~1130℃, to ensure that the microalloying elements are fully dissolved and that no grain coarsening occurs. The initial rolling temperature is 990℃, the entry temperature of the finishing rolling is 920℃, and a two-stage controlled cooling is adopted after finishing rolling, with a final rolling temperature of 850~880℃.

[0020] In summary, this invention has the following beneficial effects: By combining niobium-vanadium composite microalloying with multi-element nitride alloy for precise nitrogen control, this invention achieves an ultra-low vanadium-nitrogen ratio of 0.41~0.80 (V / N), solving the problem of insufficient strength in large-diameter steel bars under low-vanadium systems. The multi-element nitride alloy is rich in nitrogen and strong nitride-forming elements (such as Ti and Si), which can promote the formation of dispersed precipitates under low-vanadium conditions. Detailed Implementation

[0021] The specific embodiments of the present invention will be further described below. These embodiments do not constitute a limitation on the present invention.

[0022] Example 1: Taking the production of 400MPa grade steel bars in a 190t converter as an example S1, Converter smelting: Using blast furnace molten iron and high-quality scrap steel as raw materials, the process is carried out by top and bottom composite blowing in the converter, with a final carbon content of 0.08% and a final temperature of 1630℃.

[0023] S2, Alloying: During the tapping process, when the molten steel reaches 1 / 3 of its capacity, silicon manganese, ferrosilicon, and silicon carbide spheres are added for full-process composite deoxidation and alloying, preventing secondary oxidation of the molten steel. The silicon manganese contains 65% Mn at an addition rate of 20.5 kg / t; the ferrosilicon contains 72% Si at an addition rate of 1.85 kg / t; and the silicon carbide spheres contain 32% Si and 25% C at an addition rate of 1.2 kg / t. When the molten steel reaches 1 / 2 of its capacity, ferroniobium and vanadium-nitrogen alloy are added. The ferroniobium contains 44% Nb at an addition rate of 0.27 kg / t; and the vanadium-nitrogen alloy contains 77% V and 14% N at an addition rate of 1.5 kg / t. This precise control of niobium content in the steel... Vanadium content is 0.011% in all cases, achieving niobium-vanadium equal-weight composite microalloying. When the molten steel reaches 2 / 3 of its volume, a multi-element nitride alloy is added. The chemical composition of the added multi-element nitride alloy, by mass percentage, includes: C≤3%, N≥20%, Ti 8~13%, Si 25~30%, P≤0.15%, S≤0.015%, moisture ≤1.5%, with the remainder being Fe and unavoidable impurities. The mass ratio of Ti to N is 3.5~4.5. The nitrogen content of the finished product is controlled at 0.018%, and V / N=0.61. During the alloy addition process, bottom blowing argon is used for stirring in the ladle, with the argon flow rate controlled at 35NL / (min·t) to ensure rapid melting and uniform composition distribution of the alloy.

[0024] S3, Continuous Casting: Refined and qualified molten steel is sent into the continuous casting process. The superheat of the molten steel in the tundish is controlled at 20°C. A 170mm×170mm small square billet continuous casting machine is used, and the casting speed is controlled at 2.8m / min. The crystallizer adopts a concave taper design, and the secondary cooling zone adopts air mist cooling. The specific water volume is controlled at 0.9L / kg to avoid surface cracks of the continuous casting billet and promote uniform solid solution of microalloying elements.

[0025] S4, Rolling: The continuously cast billet is directly fed into the heating furnace. The heating section temperature is 1100℃, the soaking section temperature is 1120℃, and the holding time is 65min to ensure that the microalloying elements are fully dissolved and that no grain coarsening occurs. The initial rolling temperature is 990℃, the finishing rolling inlet temperature is 920℃, and after finishing rolling, a two-stage controlled cooling is adopted. The final rolling temperature is controlled at 870℃, and the cooling bed is air-cooled to room temperature to obtain the finished HRB400 steel bar.

[0026] The following specifications of ultra-low vanadium-nitrogen ratio 400MPa grade hot-rolled ribbed steel bars were rolled according to the above method: φ12, φ18, φ22, φ32, and φ50.

[0027] Example 2: Taking the production of 400MPa grade steel bars in a 190t converter as an example 1. The raw materials, smelting, continuous casting, and rolling process parameters are exactly the same as in Example 1.

[0028] 2. During tapping, the vanadium and nitrogen addition amounts are the same as in Example 1, so that the finished product [V] = 0.011%. The amount of multi-element nitride alloy added is adjusted to finally control [V] = 0.011% and [N] = 0.0268% in the finished steel, and the calculated V / N = 0.41.

[0029] 3. Roll steel bars of the same specifications (Φ12, Φ18, Φ22, Φ32, Φ50) and test their performance.

[0030] Comparative Example 1 (Traditional High V / N): Taking the production of 400MPa grade steel bars in a 190t converter as an example.

[0031] 1. Converter smelting and primary deoxidation: Using blast furnace molten iron and high-quality scrap steel as raw materials, the process is carried out by top and bottom composite blowing in the converter, with a final carbon content of 0.075% and a final temperature of 1625℃. 2. During the tapping process, when the molten steel has been tapped to 1 / 3, silicon manganese, silicon ferrosilicon, and silicon carbon balls are added to carry out full-process composite deoxidation and alloying to prevent secondary oxidation of the molten steel. When the steel has been tapped to 3 / 4, all deoxidizers and basic alloys are added.

[0032] 3. Vanadium composite alloying: Vanadium and nitrogen are added simultaneously during the mid-to-late stages of steelmaking, precisely controlling the vanadium content in the steel to 0.022%. During alloy addition, bottom-blowing argon stirring is used in conjunction with the ladle, with the argon flow rate controlled at 35 NL / (min·t) to ensure rapid alloy melting and uniform composition distribution. Nitrogen is also used for bottom-blowing in the ladle, resulting in a finished product nitrogen content of 0.011% and a V / N ratio of 2.0.

[0033] 4. Continuous casting billet preparation: Refined qualified molten steel is sent to the continuous casting process. The superheat of the molten steel in the tundish is controlled at 20℃. The casting speed of the 170mm×170mm small square billet continuous casting machine is controlled at 2.8m / min. The crystallizer adopts a weak cooling process, and the secondary cooling zone adopts air mist cooling. The specific water volume is controlled at 1.3L / kg to avoid surface cracks of the continuous casting billet and promote uniform solidification of microalloying elements.

[0034] 5. Rolling and controlled cooling forming: The continuously cast billet is directly fed into the heating furnace. The heating section temperature is 1100℃, the soaking section temperature is 1120℃, and the holding time is 65min to ensure that the microalloying elements are fully dissolved and that no grain coarsening occurs. The initial rolling temperature is 990℃, the finishing rolling inlet temperature is 920℃, and after finishing rolling, a two-stage controlled cooling is adopted. The final rolling temperature is controlled at 870℃, and the cooling bed is air-cooled to room temperature to obtain the finished HRB400 steel bars.

[0035] 6. Rolled specifications: φ12, φ18, φ22, φ32, φ50 respectively.

[0036] Comparative Example 2 (Niobium-free Ultra-low Vanadium): Taking the production of 400MPa grade steel bars in a 190t converter as an example.

[0037] 1. Converter smelting and primary deoxidation: Using blast furnace molten iron and high-quality scrap steel as raw materials, the process is carried out by top and bottom composite blowing in the converter, with a final carbon content of 0.082% and a final temperature of 1628℃. 2. During the tapping process, when the molten steel has been tapped to 1 / 3, silicon manganese, silicon ferrosilicon, and silicon carbon balls are added to carry out full-process composite deoxidation and alloying to prevent secondary oxidation of the molten steel. When the steel has been tapped to 3 / 4, all deoxidizers and basic alloys are added.

[0038] 3. Vanadium composite alloying. Vanadium and nitrogen are added simultaneously in the middle and late stages of steelmaking, and the vanadium content in the steel is precisely controlled to be 0.010%. During the alloying process, bottom blowing argon is used for stirring in the ladle, and the argon flow rate is controlled at 35NL / (min·t) to ensure rapid melting of the alloy and uniform distribution of composition. The nitrogen content of the finished product is 0.012, and V / N=0.83.

[0039] 4. Continuous casting billet preparation: Refined qualified molten steel is sent to the continuous casting process. The superheat of the molten steel in the tundish is controlled at 20℃. The casting speed of the 170mm×170mm small square billet continuous casting machine is controlled at 2.8m / min. The crystallizer adopts a weak cooling process, and the secondary cooling zone adopts air mist cooling. The specific water volume is controlled at 1.3L / kg to avoid surface cracks of the continuous casting billet and promote uniform solidification of microalloying elements.

[0040] 5. Rolling and controlled cooling forming: The continuously cast billet is directly fed into the heating furnace. The heating section temperature is 1100℃, the soaking section temperature is 1120℃, and the holding time is 65min to ensure that the microalloying elements are fully dissolved and that no grain coarsening occurs. The initial rolling temperature is 990℃, the finishing rolling inlet temperature is 920℃, and after finishing rolling, a two-stage controlled cooling is adopted. The final rolling temperature is controlled at 870℃, and the cooling bed is air-cooled to room temperature to obtain the finished HRB400 steel bars.

[0041] 6. Rolled specifications: φ12, φ18, φ22, φ32, φ50 respectively.

[0042] Comparative Example 3 (Niobium-vanadium composite, V / N=0.85): Taking the production of 400MPa grade steel bars in a 190t converter as an example.

[0043] 1. The raw materials, smelting, continuous casting, and rolling process parameters are exactly the same as in Example 1.

[0044] 2. During steel tapping, the amount of ferroniobium added is the same as in Example 1, so that the finished product [Nb] = 0.011%. The amount of multi-element nitride alloy added is adjusted to weaken its nitrogen-increasing effect, and finally the [V] in the finished steel is controlled to be 0.011% and [N] = 0.013%, and the calculated V / N = 0.85.

[0045] 3. Roll steel bars of the same specifications (Φ12, Φ18, Φ22, Φ32, Φ50) and test their performance.

[0046] The table below compares the chemical composition, mechanical properties, and cost calculations between the embodiments of the present invention and the comparative examples.

[0047] Table 1 Comparison of Chemical Composition (%)

[0048] Table 2 Comparison of Mechanical Properties

[0049] Table 3: Comparison of Cost Calculations for 3 Tons of Steel Alloy

[0050] The data in the table above shows that the present invention has the following advantages: 1. Innovative Proportioning and Synergistic Strengthening Mechanism: For the first time, a process combining "equal-amount composite addition of niobium and vanadium" with "precise nitrogen control in multi-element nitriding alloys" was successfully adopted, successfully maintaining the V / N ratio stably within the ultra-low range of 0.41~0.80. Comparative Example 3 demonstrates that even under the same niobium-vanadium composite and multi-element nitriding alloying process, if the V / N ratio is only slightly higher than 0.80 (e.g., 0.85), the yield strength (395MPa) of Φ50mm large-diameter steel bars still cannot meet the standard requirement of ≥400MPa, while the strength reaches 455MPa under the scheme of this invention (V / N=0.61). This strongly proves that controlling the V / N ratio within the specific range of this invention is the key to solving the problem of low-vanadium, high-strength large-diameter steel bars.

[0051] 2. Significant economic benefits: As shown in Table 3, compared with the traditional high-vanadium process (Comparative Example 1), the present invention reduces the amount of vanadium-nitrogen alloy by 50% and replaces it with some multi-element nitride alloys, thereby reducing the cost of alloy per ton of steel by about 16 yuan while ensuring performance, resulting in significant economic benefits.

[0052] 3. Excellent performance and universality of specifications: Through the above process, the yield strength of the produced steel bars is stable at 420~460MPa, the grain size is ≥9.5 grade, and the performance meets the standards for all specifications from Φ12mm to Φ50mm. This solves the technical problem that some existing low V / N processes cannot be applied to large-specification steel bars (such as comparative examples 2 and 3). The product fully meets and exceeds the requirements of GB / T1499.2-2024 standard.

[0053] 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. An ultra-low vanadium-to-nitrogen ratio 400 MPa grade hot-rolled ribbed steel bar, characterized by, The chemical composition, by mass percentage, is as follows: C: 0.22~0.25%, Si: 0.40~0.50%, Mn: 1.35~1.55%, V: 0.009~0.012%, Nb: 0.009~0.012%, N: 0.015~0.022%, P≤0.045%, S≤0.045%, with the remainder being Fe and unavoidable impurities. The mass ratio of vanadium to nitrogen (V / N) is 0.41~0.

80.

2. The 400 MPa grade hot-rolled ribbed steel bar with ultra-low vanadium-nitrogen ratio according to claim 1, characterized in that: The strength-to-yield ratio of the steel bars is ≥1.25, and the total elongation at maximum force is ≥7.5%.

3. The 400 MPa grade hot-rolled ribbed steel bar with ultra-low vanadium-nitrogen ratio according to claim 1, characterized in that: The mass ratio of vanadium to nitrogen (V / N) is 0.60~0.75, and the finished product has a grain size ≥9.

5.

4. The production process of a 400MPa grade hot-rolled ribbed steel bar with ultra-low vanadium-nitrogen ratio according to any one of claims 1 to 3, characterized in that: Includes the following steps: S1, Converter smelting: Using blast furnace hot metal mixed with high-quality scrap steel as raw materials, the converter is combined with top and bottom blowing to control the final carbon ratio and final temperature. S2, Alloying: During the tapping process, silicon manganese, silicon iron, silicon carbide balls, niobium iron, vanadium nitrogen alloy and multi-element nitriding alloy are added according to the element content of the finished product. Argon blowing from the bottom of the ladle is used for stirring during the addition process. S3, Continuous casting: Qualified molten steel is sent into the continuous casting process, and continuous casting billets are manufactured using a small billet continuous casting machine. S4, Rolling: The continuously cast billet is directly fed into the heating furnace and air-cooled to room temperature to obtain the finished product.

5. The production process of a 400MPa grade hot-rolled ribbed steel bar with ultra-low vanadium-nitrogen ratio according to claim 4, characterized in that: In step S1, the carbon content at the endpoint is ≥0.06%, and the endpoint temperature is 1620~1650℃.

6. The production process of 400 MPa grade hot-rolled ribbed steel bars with ultra-low vanadium / nitrogen ratio according to claim 4, characterized in that: In step S2, during the tapping process, when the molten steel reaches 1 / 3 of its volume, silicon manganese, silicon ferrosilicon, and silicon carbon balls are added; when the molten steel reaches 1 / 2 of its volume, niobium ferroalloy and vanadium-nitrogen alloy are added; and when the molten steel reaches 2 / 3 of its volume, a multi-element nitride alloy is added.

7. The production process of 400 MPa grade hot-rolled ribbed steel bars with ultra-low vanadium / nitrogen ratio according to claim 4, characterized by the fact that: In step S2, the Mn content in the silicon-manganese alloy is 65%, and the addition amount is 20.5 kg / t. The Si content in the silicon-iron alloy is 72%, the Si content in the silicon-carbon spheres is 32%, the C content is 25%, the Nb content in the niobium-iron alloy is 44%, and the V content in the vanadium-nitrogen alloy is 77% and the N content is 14%. The chemical composition of the multi-element nitride alloy, by mass percentage, includes: C≤3%, N≥20%, Ti 8~13%, Si 25~30%, P≤0.15%, S≤0.015%, moisture ≤1.5%, and the remainder is Fe and unavoidable impurities. The mass ratio of Ti to N is 3.5~4.

5.

8. The production process of 400 MPa grade hot-rolled ribbed steel bars with ultra-low vanadium / nitrogen ratio according to claim 4, characterized by the fact that: In step S2, the argon flow rate for stirring is 35 NL / (min·t).

9. The production process of 400 MPa grade hot-rolled ribbed steel bars with ultra-low vanadium / nitrogen ratio according to claim 4, characterized by the fact that: In step S3, the continuously cast billet is a small square billet of 170mm×170mm. The superheat of the tundish is controlled to be ≤25℃, the casting speed is 2.6~3.0m / min, the crystallizer adopts a concave taper design, the secondary cooling zone adopts air mist cooling, the water content of the secondary cooling zone is controlled at 0.8~1.0L / kg, and the surface temperature recovery rate of the billet is ensured to be ≤50℃ / s.

10. The production process of 400 MPa grade hot-rolled ribbed steel bars with ultra-low vanadium and nitrogen ratio according to claim 4, characterized in that: In step S4, the temperature of the heating section in the heating furnace is 1080~1120℃, the temperature of the soaking section is 1100~1130℃, the initial rolling temperature is 990℃, the entry temperature of the finishing rolling is 920℃, and after finishing rolling, a two-stage controlled cooling is adopted, with a final rolling temperature of 850~880℃.

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