A continuous casting and rolling production method for thickness transition of carbon structural steel

Abstract

The application discloses a kind of carbon structural steel thickness transition's continuous casting and rolling production method, belong to iron and steel production field, the carbon structural steel component design range is: C:0.03-0.06%, Si:0.10-0.30%, Mn:0.50-0.60%, S:≤0.006%, P:≤0.017%, N:≤0.005%, the balance is Fe and impurities;Process control method includes: continuous casting is according to pouring section control production thickness specification;In the coiling, temperature control is controlled at 580-530 DEG C according to strip thickness, and the greater the thickness, the higher the temperature.The application guarantees that pouring section kilometer number is promoted, and also meets the requirement of customer to strip surface quality.

Description

Technical Field

[0001] This invention is a divisional application of the invention patent "A method for continuous casting and rolling of thin plates with long kilometers of high-speed steel on a full-frame machine" (202111134894X), which relates to a steel production method, particularly a method for continuous casting and rolling of thin plates with thickness transition of carbon structural steel. Background Technology

[0002] Thin slab continuous casting and rolling production lines allow for stable production under uniform temperature and speed conditions, ensuring significantly superior dimensional accuracy, performance stability, and yield compared to conventional hot continuous rolling production lines. Longer casting cycles can significantly reduce refractory material and roll costs while increasing output and maximizing investment returns.

[0003] However, long-cycle casting inevitably leads to an increase in the tonnage and rolling kilometers per casting cycle. Therefore, the extended rolling kilometers pose a significant challenge to the uniform wear of the rolls, especially in the high-nickel-chromium rolls of the finishing section, where uneven wear is prone to occur with increasing mileage, resulting in localized high-point defects in the strip. Therefore, to ensure the strip's cross-sectional shape and surface quality, the rolls must be replaced after a certain rolling mileage. However, the timing of roll replacement is often difficult to determine. Replacing rolls too early increases roll consumption and affects output, while delaying roll replacement affects strip product quality and production stability, potentially leading to production interruptions.

[0004] Therefore, in existing thin slab continuous casting and rolling production lines, the work rolls in the later stages generally adopt high-speed steel rolls with good resistance to hot cracking, wear resistance, oxide film retention, and uniform microstructure. These rolls naturally control hot cracking, oxide film peeling, and high wear. Their stable application in thin slab continuous casting and rolling production lines is significant for increasing casting mileage, thereby increasing production capacity, and improving product surface quality and shape quality. However, in actual use, they exhibit characteristics such as large thermal expansion, high roll surface roughness, and sensitivity to temperature uniformity, resulting in low accident resistance. Cases of their use in the final stand are rare. Under current technical conditions, in the casting process of carbon structural steel, abnormal waviness caused by abnormal wear of high-nickel-chromium rolls in the later stages of casting leads to reduced rolling stability and poor product shape quality. In severe cases, this can result in scrap steel, transverse bends in subsequent processes, and the inability of cold rolling to eliminate shape problems. The production mileage for a single casting cycle does not exceed 150 km.

[0005] Therefore, there is an urgent need for a production process that can both ensure the online use of high-speed steel rolls on the entire stand and improve the long-term stable service of high-speed steel rolls, so as to achieve the goal of reducing costs and increasing efficiency. Summary of the Invention

[0006] The technical objective of this invention is to address the shortcomings of the prior art by providing a continuous casting and rolling production method for carbon structural steel with thickness transition. This method is used for the production of thin-gauge strip steel under the conditions of full-stand high-speed steel roll application in thin slab continuous casting and rolling production lines. Its continuous casting process, roll process, roll shape design, parameter control, and surface quality are all tailored to the characteristics and service characteristics of high-speed steel rolls. The single casting mileage is increased to 190km, ensuring that the casting mileage is increased while also meeting the customer's requirements for strip steel surface quality.

[0007] The technical solution of this invention to solve its technical problem is: a continuous casting and rolling production method for carbon structural steel with thickness transition, comprising steelmaking → continuous casting → slab heating → rough rolling → finish rolling → laminar flow cooling → coiling, characterized in that the carbon structural steel composition design range is: C: 0.03-0.06%, Si: 0.10-0.30%, Mn: 0.50-0.60%, S: ≤0.006%, P: ≤0.017%, N: ≤0.005%, with the balance being Fe and impurities; the process control method includes: continuous casting is controlled in stages according to the casting batch: within the first 300t of the casting batch, the continuous casting speed is increased from 3.5m / min to 4.8m / min; The roughing mill inlet temperature is not lower than 850℃. The intermediate billet in the roughing mill gradually transitions from 18mm to 13mm, and then passes through an induction heating furnace. The induction heating temperature is not higher than 1170℃. The finishing mill outlet temperature is between 830℃ and 860℃, maintaining a production thickness specification ≥2.0mm. After passing through 300t of steel, the drawing speed is increased to 5m / min, the intermediate billet transitions to 9-13mm, the induction heating furnace temperature is not lower than 1080℃, and the finishing mill outlet temperature is between 830℃ and 860℃, producing main rolled materials with a thickness specification <1.5mm in batches. During coiling, the temperature is controlled between 580-530℃ according to the strip thickness, with higher temperatures for thicker strips.

[0008] Furthermore, in the aforementioned finishing rolling process, different roll crown settings are used for different stands to ensure that the crown of the finished strip is controlled within 15-30µm.

[0009] Furthermore, in the above-mentioned finishing mill roll shape control, the roll crown of F1-2 is -400 to -650 μm, and that of F3-5 is -150 to -350 μm.

[0010] Furthermore, in the above-mentioned finishing rolling process, the stand rolling force is set as follows: the stand rolling force is set by gradually increasing control method of F1-2, with the rolling force range between 1.4MN and 2.8MN, and gradually decreasing trend of load of F3-5, with the rolling force controlled between 1.2MN and 2.1MN.

[0011] The optimization scheme also includes looper tension control: the looper tension gradually increases as the thickness decreases, with the tension set at 8-25 N / mm. 2between.

[0012] Furthermore, in the above-mentioned looper tension control settings, the thickness range of looper #1 is 12-4mm, and the tension is set to 8-15N / mm. 2 #2 looper thickness range 7-3.5mm, tension setting 11-18N / mm 2 The thickness range of the #3 slip ring is 5-2.4mm, and the tension setting is 14-21N / mm. 2 The thickness range of the 4# looper is 4-1.2mm, and the tension setting is 16-25N / mm. 2 .

[0013] Furthermore, in the above-mentioned winding process, the winding temperature is set to 580℃ for strip thickness ≥ 1.8mm, 550℃ for strip thickness 1.2~1.8mm, and 530℃ for strip thickness ≤ 1.2-1.8mm.

[0014] Compared with the prior art, the present invention has the following outstanding advantages:

[0015] 1. This invention is applicable to the production of carbon structural steel under the application conditions of high-speed steel rolls in a thin slab continuous casting and rolling production line with all stands. It can increase the single casting mileage of carbon structural steel to 190km and the proportion of thin specifications <1.5mm can reach 74%.

[0016] 2. It can achieve long-term stable service of high-speed steel rolls, and can still ensure good strip shape control even under 190km operation conditions;

[0017] 3. The number of heats cast in a single continuous casting cycle has been increased from 9.2 to 10.1, saving millions of yuan annually in costs for tundish baking, nozzle, SEN, and roll grinding. Detailed Implementation

[0018] The present invention will be further described below with reference to specific embodiments.

[0019] This invention provides a continuous casting and rolling production method for carbon structural steel with thickness transition, which can improve the shape control of carbon structural steel casting at the highest kilometer count.

[0020] The process flow of this invention includes: steelmaking → continuous casting → slab heating → rough rolling → finish rolling → laminar flow cooling → coiling.

[0021] This invention is applicable to the rolling of carbon structural steel, with a composition design range of: C: 0.03-0.06%, Si: 0.10-0.30%, Mn: 0.50-0.60%, S: ≤0.006%, P: ≤0.017%, N: ≤0.005%. The steel is then smelted in a converter, LF furnace, or RH furnace according to the above composition.

[0022] Process control methods include:

[0023] (1) Continuous casting and rolling are controlled in stages according to the number of castings.

[0024] Within the first 300 tons of casting, the continuous casting speed is increased from 3.5 m / min to 4.8 m / min; the roughing mill inlet temperature is not lower than 850℃, the roughing mill intermediate billet is gradually transitioned from 18 mm to 13 mm, and then passes through the induction heating furnace, with the induction heating temperature not exceeding 1170℃, and the finishing mill outlet temperature is between 830℃ and 860℃, maintaining the production thickness specification ≥2.0 mm.

[0025] After passing through 300 tons of steel, the drawing speed is increased to 5 m / min. Maintaining a high drawing speed and increasing the steel passing rhythm can effectively suppress the overall grain growth. The intermediate billet is transitioned to 9-13 mm. The temperature of the induction heating furnace is not lower than 1080℃. The finishing mill exit temperature is between 830℃ and 860℃. Mass production of main rolled materials with a thickness of <1.5 mm is carried out.

[0026] (2) Finishing mill roll shape control

[0027] Because the finishing mill uses high-speed steel rolls across all stands, the front section of the finishing mill uses a cosine concave roll profile, while the rear section uses a single-conical roll profile with continuous roll movement. Considering roll wear and thermal crown variations, and taking into account the similarity of the front and rear roll profiles, different roll crown settings are applied to different stands. Appropriate initial roll crown ensures that the actual strip crown meets quality requirements during rolling, guaranteeing that the finished strip crown is controlled within 15-30 μm. Specifically, the crown settings are: F1-2 roll crown is -400 to -650 μm, and F3-5 is -150 to -350 μm.

[0028] (3) Setting the rolling force of the stand

[0029] To ensure the stability of the plate shape when the high-speed steel is rolled on the whole stand, the rolling force setting adopts the control method of gradually increasing F1-2, with the rolling force range between 1.4MN and 2.8MN, and the load of F3-5 gradually decreasing, with the rolling force controlled between 1.2MN and 2.1MN.

[0030] (4) Loop tension control

[0031] To ensure the shape and rolling stability of the high-speed steel on the entire stand, the looper tension is gradually increased as the thickness decreases, as shown below.

[0032] 1# Loop 12-4 8-15 2# Loop 7-3.5 11-18 3# Loop 5-2.4 14-21 4# Loop 4-1.2 16-25

[0033] (5) Laminar flow cooling control

[0034] In the laminar flow coarse adjustment section, groups 8-30 are equipped with fault valves to ensure that the front section is fully open, or normally open fault valves are installed in the front section to ensure dense cooling in the front section.

[0035] (6) Winding temperature control

[0036] The coiling temperature is controlled between 580-530℃ according to the strip thickness. See the table below for specific settings.

[0037] H≥1.8 580 1.2＜H＜1.8 550 H≤1.2 530

[0038] In the existing process, for steel grades with the same composition, design and thickness, the production mileage for a single casting cannot exceed 150km. Exceeding this will result in reduced rolling stability and poor product shape quality. With an operation of 160km, the defect rate is as high as 1.48%.

[0039] Using the process of this invention, the single casting mileage for carbon structural steel can be increased to 190km, and the number of continuous casting heats per single casting cycle can be increased from 9.2 heats to 10.1 heats. Even at a 190km operating range, good plate shape control can still be guaranteed, with a defect rate of less than 0.01%.

[0040] The results above show that by using the high-speed steel rolls of the whole frame and adjusting the process of each process, the present invention can effectively reduce the problems of large waviness and uncontrolled strip shape in the later stage of casting caused by the large wear of high nickel-chromium rolls, and increase the single casting mileage to 190km. While ensuring the increase in casting mileage, it also meets the customer's requirements for the surface quality of strip steel.

[0041] It should be noted that the specific embodiments of the present invention have been described in detail. For those skilled in the art, various obvious changes made to it without departing from the spirit and scope of the present invention are within the protection scope of the present invention.

Claims

1. A continuous casting and rolling production method for carbon structural steel with thickness transition, comprising steelmaking → continuous casting → slab heating → rough rolling → finish rolling → laminar flow cooling → coiling, characterized in that, The carbon structural steel composition design range is: C: 0.03-0.06%, Si: 0.10-0.30%, Mn: 0.50-0.60%, S: ≤0.006%, P: ≤0.017%, N: ≤0.005%, with the balance being Fe and impurities; The process control methods include: continuous casting is controlled in stages according to the casting batch: within the first 300t of casting, the continuous casting speed is increased from 3.5m / min to 4.8m / min; the roughing mill inlet temperature is not lower than 850℃, the roughing mill intermediate billet gradually transitions from 18mm to 13mm, and then passes through the induction heating furnace, with the induction heating temperature not exceeding 1170℃, and the finishing mill outlet temperature between 830℃ and 860℃, maintaining a production thickness specification ≥2.0mm; after 300t of steel, the casting speed is increased to 5m / min, the intermediate billet transitions to 9-13mm, the induction heating furnace temperature is not lower than 1080℃, and the finishing mill outlet temperature is between 830℃ and 860℃, producing batches of main rolled materials with a thickness specification <1.5mm; during coiling, the temperature is controlled between 580-530℃ according to the strip thickness, with higher temperatures for thicker strips. It also includes looper tension control: the looper tension control gradually increases as the thickness decreases, with the tension set between 8-25 N / mm²; in the looper tension control settings, the thickness range of looper #1 is 12-4 mm, and the tension is set to 8-15 N / mm²; the thickness range of looper #2 is 7-3.5 mm, and the tension is set to 11-18 N / mm²; the thickness range of looper #3 is 5-2.4 mm, and the tension is set to 14-21 N / mm²; the thickness range of looper #4 is 4-1.2 mm, and the tension is set to 16-25 N / mm². In the aforementioned winding process, the winding temperature is set to 580℃ for strip thickness ≥ 1.8mm, 550℃ for strip thickness 1.2~1.8mm, and 530℃ for strip thickness ≤ 1.2-1.8mm.

2. The continuous casting and rolling production method for carbon structural steel thickness transition according to claim 1, characterized in that: In the finishing rolling process, different roll crown settings are used for different stands to ensure that the crown of the finished strip is controlled within 15-30µm.

3. The continuous casting and rolling production method for carbon structural steel thickness transition according to claim 2, characterized in that: In the control of the roll shape of the finishing mill roll, the crown of roll F1-2 is -400 to -650 μm, and that of roll F3-5 is -150 to -350 μm.

4. The continuous casting and rolling production method for carbon structural steel thickness transition according to claim 2, characterized in that: In the finishing rolling process, the stand rolling force is set as follows: the stand rolling force is set by gradually increasing F1-2, with the rolling force range between 1.4MN and 2.8MN, and gradually decreasing F3-5, with the rolling force controlled between 1.2MN and 2.1MN.