Control method for preventing medium carbon steel hot-rolled coil from being flattened

Abstract

The invention relates to a control method for preventing a medium carbon steel hot-rolled coil from being flattened. The control method comprises the following steps: 1) hot charging and hot delivery of a continuous casting billet; (2) putting a heat preservation cover and an edge heater into the middle roller way; (3) the laminar flow roller way and the coiling machine rotate at the set advance rate; 4) controlling the head temperature of the strip steel before coiling, and performing laminar cooling to shield the edge part; (5) after the coiling machine builds tension, constant tension needs to be formed between a coiling block and a finish rolling tail rack; (6) covering and coiling in a coiling machine after coiling; and (7) the steel coils are placed on the flat coil prevention steel coil frame after entering the coil warehouse, are stored in a single layer and are prevented from being placed at a ventilation position, and impact on the steel coils is avoided during hoisting. The technical process is optimized and improved, the measures that the temperature of a rolled piece is kept consistent as much as possible, it is guaranteed that a steel coil is not loosened after being coiled, the influence of expansion generated by phase change of the steel coil on interlayer gaps is reduced, and the shape of the steel coil is fixed to prevent the flat coil from being aggravated are taken, so that the flat coil rate is greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of hot rolling technology, and in particular to a method for controlling the flattening of hot-rolled medium carbon steel coils. Background Art

[0002] Flattened coils are a common appearance defect in hot-rolled strip steel (coils) during production or subsequent processing. They manifest as an uneven, elliptical, or locally flattened end face. Flattened coil defects can lead to the following consequences: 1) Difficulty uncoiling: Flattened coils may cause the strip to deviate or jam during uncoiling. 2) Edge damage: Flattened coils cause localized stress concentration, easily leading to cracks or warping during subsequent processing. 3) Reduced dimensional accuracy: Affects the positioning accuracy of stamping or slitting processes. For flattened coil defects, the difference between the major and minor axes of the steel coil is generally specified to be no more than 30mm.

[0003] Medium carbon hot-rolled steel coils are prone to flattening defects. After being unloaded from the coiler, they begin to gradually collapse. After a period of time, some coils cannot be uncoiled again, directly affecting delivery. At this time, the coils need to be sealed and uncoiled, and then recoiled on the slitting machine, which seriously affects the product yield and occupies the slitting machine, leading to a decrease in production efficiency.

[0004] Chinese patent application CN202410579742.8 discloses "a production method based on CSP process to avoid flat coil defects of medium and high carbon steel". It solves the problem of coil collapse defect after coiling of medium and high carbon steel by controlling coiling tension and coiling temperature. However, the main equipment configuration and rolling parameters of its production line are significantly different from those of this invention.

[0005] Chinese patent application CN201610645499.0 discloses a "method to prevent flattening of hot-rolled high-carbon steel coils". This method, through comprehensive control of coiling temperature and tension, prevents flattening of high-carbon steel coils at different coiling temperatures and effectively prevents problems such as cracking and narrowing of the coils. However, the main chemical composition of the steel and the measures taken to prevent flattening are significantly different from those of this invention.

[0006] Chinese patent application CN202411745205.2 discloses a "method to prevent hot-rolled high-carbon tool steel coils from flattening". The method involves controlling the furnace entry temperature of a continuously cast slab to ≥300℃, then heating it in a furnace to achieve an exit temperature of 1150℃~1250℃, resulting in a first slab. The first slab is then rough-rolled at a final rolling temperature of 1050℃~1150℃ to obtain an intermediate slab. The intermediate slab is then finish-rolled at a final rolling temperature of 800℃~950℃ to obtain finished steel. Laminar flow cooling is performed on the finished steel using a front-end centralized cooling method. The final fine-tuning water and front and rear side sprays of the laminar flow cooling are shut off to lower the temperature of the finished steel to the first target temperature, which is the phase transformation point, resulting in cooled steel. The cooled steel is then coiled at 590℃~690℃, with the coiling tension increased by 10%~20% during the coiling process, resulting in a hot-rolled high-carbon tool steel coil. However, the main chemical composition of its steel and the measures taken to prevent flattening are significantly different from those of the present invention. Summary of the Invention

[0007] This invention provides a control method to prevent flattening of hot-rolled medium carbon steel coils. The process is optimized and improved by taking measures such as keeping the temperature of the rolled parts as consistent as possible, ensuring that the coil does not loosen after winding, reducing the impact of expansion caused by phase transformation on the interlayer gap, and fixing the shape of the coil to prevent the flattening from increasing the weight. These measures significantly reduce the occurrence rate of flattening defects.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for controlling the flattening of hot-rolled medium carbon steel coils, wherein the production process of hot-rolled medium carbon steel coils includes heating, rough rolling, finish rolling, laminar flow cooling, and coiling, wherein the following process is controlled:

[0010] 1) The continuously cast billet is hot-charged and hot-delivered, with an entry temperature ≥300℃; starting from the entry of the continuously cast billet into the second heating section, the temperature difference between the upper and lower surfaces is controlled according to the ratio of upper surface temperature - lower surface temperature = 50~60℃.

[0011] 2) An insulation cover is placed in the intermediate roller table between the roughing and finishing mills to keep the rolled piece warm. Before finishing milling, an edge heater is placed to heat the edge of the rolled piece at a temperature of 70-90℃.

[0012] 3) After the strip exits the finishing mill stand, the laminar flow roller table and coiler rotate at a set lead rate. The lead rates of the pinch rolls, coiling drum, and auxiliary coiling rolls are 7.5%–8.5%, 9.5%–10.5%, and 11.5%–12.5%, respectively.

[0013] 4) Before coiling after laminar flow cooling, the temperature within 100m of the strip head should be controlled to be 70-100℃ higher than the target coiling temperature. Laminar flow cooling should be carried out in a decentralized manner and the edges of the strip should be shielded.

[0014] 5) After the coiler establishes tension, a constant tension is formed between the coil and the finishing mill stand, specifically set as follows:

[0015] When the strip thickness is <3.0mm, the drum tension is 25.0~26.9kN;

[0016] When the strip thickness is 3.0 to 3.5 mm (excluding 3.5 mm), the drum tension is 24.0 to 24.9 kN.

[0017] When the strip thickness is 3.5 to 4.0 mm (excluding 4.0 mm), the drum tension is 23.0 to 23.9 kN.

[0018] When the strip thickness is 4.0 to 4.5 mm (excluding 4.5 mm), the drum tension is 21.0 to 22.9 kN.

[0019] When the strip thickness is ≥4.5mm, the drum tension is 19.0~19.9kN;

[0020] 6) After winding, hold the coil in the winding machine for 30-60 seconds;

[0021] 7) After the steel coils are put into the coil storage, they should be placed on the anti-flattening steel coil rack and stored in a single layer. Avoid placing them in ventilated areas and avoid impacting the steel coils during hoisting.

[0022] The medium carbon steel hot-rolled coil has a carbon content of 0.20% to 0.60%, a silicon content of 0.17% to 0.37%, and a manganese content of 0.50% to 1.00% by mass percentage.

[0023] During the heating process, if the continuously cast billet cannot be immediately loaded into the furnace, it should be placed in a heat preservation pit for heat preservation, with the temperature of the heat preservation pit ≥400℃.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The process was optimized and improved by taking measures such as keeping the temperature of the rolled parts as consistent as possible, ensuring that the coil does not loosen after winding, reducing the impact of the expansion of the coil caused by phase transformation on the interlayer gap, and fixing the shape of the coil to prevent the flat coil from becoming heavier. As a result, the incidence of flat coil defects was reduced significantly from more than 10% before optimization to less than 2%. Detailed Implementation

[0026] This invention discloses a method for controlling the flattening of hot-rolled medium carbon steel coils. The hot-rolled medium carbon steel coils have a carbon content of 0.20%–0.60% (by mass percentage), a silicon content of 0.17%–0.37% (by mass percentage), and a manganese content of 0.50%–1.00%. The production process of the hot-rolled medium carbon steel coils includes heating, rough rolling, finish rolling, laminar flow cooling, and coiling, wherein the following processes are controlled:

[0027] 1) The continuously cast billet is hot-charged and hot-delivered, with an entry temperature ≥300℃; starting from the entry of the continuously cast billet into the second heating section, the temperature difference between the upper and lower surfaces is controlled according to the ratio of upper surface temperature - lower surface temperature = 50~60℃.

[0028] After medium carbon steel continuously cast billets are transported from the continuous casting process to the hot-rolled slab warehouse, they should be immediately loaded into the furnace to ensure an entry temperature ≥300℃. If immediate loading is not possible, they should be placed in a heat preservation pit with a temperature ≥400℃. When the continuously cast billets enter the secondary heating section after loading, the temperature difference between the upper and lower surfaces should initially be controlled at 50-60℃, based on the upper surface temperature minus the lower surface temperature. This is primarily to prevent uneven temperature distribution in the rolled product during subsequent production, which could lead to inconsistent phase transformation processes.

[0029] 2) An insulation cover is placed in the intermediate roller table between the roughing and finishing mills to keep the rolled piece warm. Before finishing milling, an edge heater is placed to heat the edge of the rolled piece at a temperature of 70-90°C to compensate for the temperature loss at the edge and ensure that the temperature difference of the rolled piece is within a small range.

[0030] 3) After the strip exits the finishing mill stand, the laminar flow roller table and coiler rotate at a set lead rate. The lead rates of the pinch rolls, coiling drum, and auxiliary coiling rolls are 7.5%–8.5%, 9.5%–10.5%, and 11.5%–12.5%, respectively.

[0031] If the lead rate of the laminar flow rollers, pinch rollers, and coiling aid rollers is not set properly when establishing tension in the strip, the strip head may slip slightly, resulting in a longer time to establish tension than normal coiling. At this time, the strip head is in a relatively loose state, and the layers of the strip are not rolled tightly enough during coiling. In addition, the calculation of coil diameter and tension will also be incorrect, ultimately leading to a mismatch between the speed of the steel coil and the coiler, which can easily result in flattening.

[0032] In this invention, after the strip exits the finishing mill stand, the laminar flow roller conveyor and coiler rotate sequentially with a certain lead rate, causing the strip head to generate tension on the laminar flow roller conveyor and "straighten". The lead rates of the pinch rolls and assist rolls must be matched with those of the coiling drum to avoid slippage in the core during the coiling process. Preferably, the lead rates of the pinch rolls, coiling drum, and assist rolls are 8%, 10%, and 12%, respectively.

[0033] 4) Before coiling after laminar flow cooling, the temperature within 100m of the strip head should be controlled to be 70-100℃ higher than the target coiling temperature. Laminar flow cooling should be carried out in a decentralized manner and the edges of the strip should be shielded.

[0034] When the strip head comes into contact with the cooler drum, the cooling rate is higher, which easily leads to the formation of bainite. During laminar flow cooling, the strip edges are cooled at higher rates due to repeated water flow, also easily forming bainite. This results in inconsistent phase transformation processes throughout the coil, causing interlayer misalignment during volume expansion, leading to loose coiling, and then flattening due to its own weight. Therefore, it is necessary to control the temperature of the head and edges to ensure a consistent phase transformation process throughout the coil.

[0035] 5) After the coiler establishes tension, a constant tension must be maintained between the coil drum and the finishing mill stand to ensure the strip is tightly wound onto the drum and maintains a good coil shape. If the tension is set too low, the tension between the mandrel and the finishing mill stand will decrease, resulting in gaps between layers. After uncoiling, because the core of the coil has no external support, it will slowly collapse downwards under its own weight, forming a flattened coil. This flattening defect becomes increasingly severe as the coil is transferred. The specific settings for the constant tension between the coil drum and the finishing mill stand are as follows:

[0036] When the strip thickness is <3.0mm, the drum tension is 25.0~26.9kN;

[0037] When the strip thickness is 3.0 to 3.5 mm (excluding 3.5 mm), the drum tension is 24.0 to 24.9 kN.

[0038] When the strip thickness is 3.5 to 4.0 mm (excluding 4.0 mm), the drum tension is 23.0 to 23.9 kN.

[0039] When the strip thickness is 4.0 to 4.5 mm (excluding 4.5 mm), the drum tension is 21.0 to 22.9 kN.

[0040] When the strip thickness is ≥4.5mm, the drum tension is 19.0~19.9kN;

[0041] 6) After winding, hold the coil in the winding machine for 30-60 seconds;

[0042] The mandrel of the coiler is kept in the steel coil for 30 to 60 seconds, which allows the steel coil to approach the completion of phase transformation. The expansion caused by phase transformation and the contraction caused by cooling cancel each other out, thereby eliminating the flattening phenomenon. At the same time, it can also reduce the interlayer gap generated after the phase transformation of the steel coil and reduce the influence of gravity.

[0043] 7) After the steel coils are put into the coil storage, they should be placed on the anti-flattening steel coil rack and stored in a single layer. Avoid placing them in ventilated areas and avoid impacting the steel coils during hoisting.

[0044] After the steel coils are transported into the warehouse, they are placed on anti-flattening steel coil racks (existing equipment) to fix their shape and prevent them from flattening. During the hoisting and warehousing process, the steel coils should be protected from violent impacts with other objects, which could exacerbate flattening. Thinner steel coils are more prone to flattening; therefore, choose a sheltered warehouse location to slow down cooling and use single-layer stacking to prevent the coils from being flattened under gravity.

[0045] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0046] Example:

[0047] The chemical composition of the steel in each embodiment is shown in Table 1, the hot rolling process parameters are shown in Table 2, and the cooling and coiling process parameters are shown in Table 3.

[0048] Table 1 Chemical composition of steel (wt%)

[0049]

[0050] Table 2 Hot Rolling Process Parameters

[0051]

[0052] Table 3 Cooling and winding process parameters

[0053]

[0054] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for controlling the flattening of hot-rolled medium carbon steel coils, characterized in that, The production process of medium carbon steel hot-rolled coils includes heating, rough rolling, finish rolling, laminar flow cooling, and coiling, with the following processes controlled: 1) The continuously cast billet is hot-charged and hot-delivered, with an entry temperature ≥300℃; starting from the entry of the continuously cast billet into the second heating section, the temperature difference between the upper and lower surfaces is controlled according to the ratio of upper surface temperature - lower surface temperature = 50~60℃. 2) An insulation cover is placed in the intermediate roller table between the roughing and finishing mills to keep the rolled piece warm. Before finishing milling, an edge heater is placed to heat the edge of the rolled piece at a temperature of 70-90℃. 3) After the strip exits the finishing mill stand, the laminar flow roller table and coiler rotate at a set lead rate. The lead rates of the pinch rolls, coiling drum, and auxiliary coiling rolls are 7.5%–8.5%, 9.5%–10.5%, and 11.5%–12.5%, respectively. 4) Before coiling after laminar flow cooling, the temperature within 100m of the strip head should be controlled to be 70-100℃ higher than the target coiling temperature. Laminar flow cooling should be carried out in a decentralized manner and the edges of the strip should be shielded. 5) After the coiler establishes tension, a constant tension is formed between the coil and the finishing mill stand, specifically set as follows: When the strip thickness is <3.0mm, the drum tension is 25.0~26.9kN; When the strip thickness is 3.0 to 3.5 mm (excluding 3.5 mm), the drum tension is 24.0 to 24.9 kN. When the strip thickness is 3.5 to 4.0 mm (excluding 4.0 mm), the drum tension is 23.0 to 23.9 kN. When the strip thickness is 4.0 to 4.5 mm (excluding 4.5 mm), the drum tension is 21.0 to 22.9 kN. When the strip thickness is ≥4.5mm, the drum tension is 19.0~19.9kN; 6) After winding, hold the coil in the winding machine for 30-60 seconds; 7) After the steel coils are put into the coil storage, they should be placed on the anti-flattening steel coil rack and stored in a single layer. Avoid placing them in ventilated areas and avoid impacting the steel coils during hoisting.

2. The method for controlling the flattening of hot-rolled medium carbon steel coils according to claim 1, characterized in that, The medium carbon steel hot-rolled coil has a carbon content of 0.20% to 0.60%, a silicon content of 0.17% to 0.37%, and a manganese content of 0.50% to 1.00% by mass percentage.

3. The method for controlling the flattening of hot-rolled medium carbon steel coils according to claim 1, characterized in that, During the heating process, if the continuously cast billet cannot be immediately loaded into the furnace, it should be placed in a heat preservation pit for heat preservation, with the temperature of the heat preservation pit ≥400℃.

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