Method for controlling layering defect of medium-wide strip steel for chain wheel

Abstract

The invention discloses a control method for layering defects of medium-wide strip steel for a chain wheel. The control method specifically comprises the following steps: 1) smelting low-sulfur control; (2) controlling the superheat degree of molten steel and equipment precision in the continuous casting process; (3) the continuous casting secondary cooling process is improved, the specific water flow is increased, and internal cracks caused by uneven cooling are avoided; and (4) a casting blank warm charging process: firstly carrying out stack cooling and slow cooling on the continuously cast casting blank for more than 24 hours, and charging after the casting blank is cooled to 100-300 DEG C, so as to finish initial diffusion and escape of hydrogen and stress relaxation of the casting blank. And 5) rough rolling and large rolling reduction: ensuring enough compression ratio in order to ensure that the internal structure of the casting blank is welded in the rolling process. Through multi-dimensional cooperative control of smelting purification, continuous casting homogenization and rolling densification, the internal quality of the steel is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical production technology, specifically relating to a method for controlling delamination defects in medium-wide strip steel used in sprockets. Background Technology

[0002] The 45mm sprocket is a key component in mechanical transmission systems, operating under harsh conditions, including cyclic impact loads, intense friction, and contact fatigue. Therefore, the steel used for sprockets must possess high strength, high toughness, and excellent fatigue performance. Currently, widely used medium-carbon alloy steels (such as 45, 45, and 42CrMo) are prone to a serious internal defect during production and subsequent heat treatment—delamination (also known as white spots or hairline cracks).

[0003] Delamination defects are essentially caused by the accumulation and combination of hydrogen atoms at microscopic defects in steel, generating enormous internal pressure and leading to microcracks within the steel. In a two-dimensional plane, this defect manifests as cracks parallel to the rolling surface, severely disrupting the continuity of the metal and significantly reducing the steel's lateral impact toughness, fatigue strength, and overall service life. For high-performance sprockets, delamination is an absolutely unacceptable defect.

[0004] In existing technologies, measures to control delamination mainly focus on reducing hydrogen content during the steelmaking process (such as hydrogen control in LF furnace refining and VD / VOD vacuum degassing) and slow cooling processes after rolling. However, practice has shown that controlling hydrogen and slow cooling alone cannot completely eliminate delamination, especially for steel used in large-section sprockets. The reason is as follows: 1. Central segregation and porosity: During continuous casting, severe elemental segregation (such as S and P) and shrinkage porosity are prone to occur in the central part of the billet. These microscopic defects provide "traps" for hydrogen accumulation and are the source of fractals.

[0005] 2. Internal cracks: Poor precision of continuous casting equipment and unreasonable secondary cooling process in continuous casting can lead to intermediate and central cracks in the billet, further aggravating the risk of delamination.

[0006] 3. Insufficient rolling compression ratio: If the billet is thin and the final product cross-section is large, the total compression ratio K value may be too small, and the internal defects of the billet cannot be fully eliminated through deformation during the rolling process.

[0007] Therefore, there is an urgent need for a comprehensive, systematic control technology covering the entire process from steelmaking to rolling, which can fundamentally improve the internal density of steel, eliminate hydrogen accumulation sites, and thus completely solve the delamination problem of steel used in sprockets. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for controlling delamination defects in sprocket steel. This method significantly improves the internal quality of the steel through multi-dimensional synergistic control of "smelting purification, continuous casting homogenization, and rolling densification."

[0009] To achieve the above objectives, the present invention employs the following technical solution: A method for controlling delamination defects in medium-width strip steel for sprockets, specifically including: 1) Low sulfur control: Through hot metal pretreatment and deep refining in an LF furnace, the sulfur content in the steel is stably controlled at an ultra-low level of ≤0.005wt%, or even below 0.003wt%. Low sulfur not only reduces the amount of harmful inclusions such as MnS, but more importantly, it eliminates the anisotropy and stress concentration points caused by MnS inclusions, which are common origins of fractal cracks.

[0010] 2) Continuous casting process equipment assurance: control the superheat of molten steel in the continuous casting process to 15-30℃, the fluctuation of the liquid level in the crystallizer to be within ±0.05mm, the accuracy of the continuous casting roll gap to be within ±0.20mm, and the fluctuation of the casting speed to be no more than 0.05m / min within 20s.

[0011] 3) Improved continuous casting secondary cooling process with increased specific water volume: Optimizing the cooling regime in the secondary cooling zone of continuous casting to suit the characteristics of sprocket steel. A segmented, precise control mode combining strong and weak cooling is adopted, appropriately increasing the total specific water volume to 1.0-1.4 L / kg (compared to 0.8-1.0 L / kg in conventional processes). Specifically: the total specific water volume is 1.0-1.4 L / kg; stronger cooling is used in the area before the straightening point, with water accounting for 90-95% of the total secondary cooling water volume, allowing rapid passage through the peritectic reaction zone and reducing billet shell thickness fluctuations; weak cooling is used after the straightening point and in the central area, with water accounting for 5-10% of the total secondary cooling water volume, slowing down the cooling rate, promoting equiaxed crystal growth, thereby effectively expanding the width of the central equiaxed crystal zone and reducing central segregation and porosity. Strictly control the water distribution in each cooling section before the straightening point (based on a total of 100%), including: foot roller section (20±2)%, bending section (10±2)%, fan-shaped section 1 (10±2)%, fan-shaped section 2 and 3 (25±2)%, fan-shaped section 4 and 5 (15±2)%, fan-shaped section 6 and 7 (10±2)%, and fan-shaped section 8 to 11 (10±2)%, to avoid internal cracks caused by uneven cooling.

[0012] 4) Slab warm charging process: The continuously cast slab is first subjected to slow cooling for >24 hours until it reaches 100-300℃ before being charged into the furnace, allowing for initial hydrogen diffusion and stress relaxation. Then, before entering the rolling furnace, the cold slab is preheated at a low temperature of 400-600℃ for ≥1 hour. The key purpose of this step is to further promote the diffusion and release of dissolved hydrogen in the steel without causing severe oxidation and grain growth, significantly reducing the total amount of hydrogen source and providing a "low-hydrogen billet" for subsequent rolling. Cooling the slab also promotes the transformation of austenite to ferrite and pearlite within the slab, which is beneficial for further refining the microstructure during subsequent rolling.

[0013] 5) Large Reduction in Rough Rolling: To ensure the internal structure of the cast billet is welded together during the rolling process, a sufficient reduction ratio is required, generally above 25%. However, since the billet shape is fixed and the rolling specifications need to be determined according to market demand, the reduction ratio often cannot fully meet the requirement of above 25%. Therefore, in the rough rolling stage, by optimizing the pass allocation, the single-pass reduction rate of the first pass is not less than 25%, and the single-pass reduction rate of the second pass is not less than 20%. The large reduction in the initial stage of rough rolling generates strong penetrating deformation, completely breaking the dendritic structure in the cast state, welding the central shrinkage cavity and microcracks, and fully compressing and diffusing the central porosity and segregation bands, thereby improving the density of the central part.

[0014] The strip steel is carbon structural steel, with the following chemical composition: C: 0.40wt%-0.50wt%, Mn: 0.50wt%-0.80wt%, Si: 0.17wt%-0.37wt%, S≤0.005wt%, P≤0.025wt%, Cr: 0wt%-1.0wt%. It is used in steel grades such as 45 and 40Cr.

[0015] Suitable for medium-width strip steel with a width of 600-750mm and a thickness of 6.0-9.0mm.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention, through the synergistic application of the above four core technologies, forms a complete layered control system for sprocket steel: 1. Low sulfur control reduces the causes of delamination (inclusions) from the material source.

[0017] 2. The continuous casting process equipment and optimized secondary cooling water distribution improved the central quality of the billet from the solidification source (reducing segregation and porosity).

[0018] 3. Heating the billet after casting actively reduces the hydrogen content (eliminating the driving force of fractals).

[0019] 4. The rough rolling and heavy reduction mechanically repaired the internal defects (densified welding).

[0020] Ultimately, this invention enables the production of high-end sprocket steel with virtually no delamination defects, good isotropy, and excellent overall performance. Attached Figure Description

[0021] Figure 1 This is a schematic diagram showing the process flow and main innovative points (in red) involved in this invention.

[0022] Figure 2 This is a diagram of the steel strip for sprockets produced according to the present invention.

[0023] Figure 3 Original design for producing sprockets and blanks.

[0024] Figure 4 This is a diagram of the sprocket produced according to the present invention. Detailed Implementation

[0025] The specific embodiments of the present invention will be further described below with reference to the examples. The following examples are used to specifically illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0026] Example 1: Taking 45 steel for producing chains as an example, the present invention will be described in detail.

[0027] Steel grade: 45 steel; Rolling specifications: 7.9mm*650mm.

[0028] 1. Initial production conditions: The composition requirements for 45 steel are: C: 0.46wt%, Mn: 0.58wt%, Si: 0.23wt%, S: 0.003wt%, P: 0.020wt%.

[0029] Molten iron conditions: Chemical composition: C: 4.3%, Si: 0.45%, S: 0.024%, P: 0.090%, Mn: 0.12%.

[0030] Main equipment parameters: KR desulfurization, 120-ton converter, RH furnace, LF furnace, slab continuous casting machine, walking beam furnace, 900mm rolling mill.

[0031] The original solution had a problem: the stamping process of the sprocket resulted in delamination, with a scrap rate of 3%.

[0032] 2. Implementation of the technical solution: 1) Desulfurization of molten iron. Molten iron undergoes desulfurization pretreatment using the KR method, resulting in a sulfur content of 0.002% in the desulfurized molten iron.

[0033] 2) Deep desulfurization in the LF furnace. The initial molten steel from the converter enters the LF furnace for refining. It is refined using high-basicity, strongly reducing white slag containing CaO: 56%, SiO2: 7.8%, Al2O3: 24.5%, MgO: 5.2%, (MnO+FeO): 0.6%. The sulfur content is controlled at 0.002% at the end of the refining process. After refining, a Ca wire is fed in for calcium treatment to spheroidize and modify inclusions.

[0034] 3) RH degassing. After refining in the LF furnace, the molten steel is subjected to RH vacuum degassing for 15 minutes to further remove gases. The measured [H] concentration is 1.2 ppm.

[0035] 4) Continuous casting. Before production, the roll gap accuracy is checked and maintained, and the roll gap deviation does not exceed ±0.18mm. The continuous casting process adopts full-process protective pouring, and the molten steel superheat is 21℃. The liquid level fluctuation in the crystallizer is ±0.04mm, and the casting speed is constant within a 40-minute pouring cycle, with a fluctuation range not exceeding 0.05m / min.

[0036] 5) Implement an improved secondary cooling water system, with a total water volume set at 1.19 / kg. The water distribution ratio for each section before the straightening point is as follows: foot roller section 19%, bending section 10%, fan-shaped section 1 10%, fan-shaped sections 2 and 3 23%, fan-shaped sections 4 and 5 13%, fan-shaped sections 6 and 7 8%, and fan-shaped sections 8 to 11 9%. The water volume before the straightening point accounts for 92% of the total water volume. The surface temperature of the cast billet is 967℃ at the straightening point.

[0037] 6) After the straightening point is reached in continuous casting and before cutting, continue to spray a small amount of cooling water (accounting for 8% of the total secondary cooling water) to avoid stress cracks caused by the surface temperature of the billet. After the billet is cut, it is slowly cooled at room temperature.

[0038] 7) Billet preheating. After being hot-sent to the rolling mill and stacked to cool to 198°C, the continuously cast billet enters the holding furnace for preheating at 520°C for 75 minutes.

[0039] 8) Billet heating. The preheated billet is sent to a walking beam furnace for heating for more than 186 minutes. The preheating section temperature is 950℃ and the preheating time is 67 minutes; the heating section temperature is 1245℃ and the heating time is 79 minutes; the soaking section temperature is 1246℃ and the soaking time is 40 minutes. The billet exiting the furnace is 1241℃.

[0040] 9) Rough rolling. Large reduction rolling is performed on the roughing mill. In the first pass, the 180mm thick slab is reduced to 132mm (reduction of 45mm, reduction rate 26.6%), and in the second pass, it is reduced to 105mm (reduction of 27mm, reduction rate 20.4%), ensuring sufficient deformation of the core. After multiple passes, a 33mm thick intermediate slab is sent to the finishing mill, where it is finally rolled into a 7.9mm*650mm steel strip.

[0041] 3. In this case, a total of 126 tons of 45 steel were produced for medium-width sprockets. After flaw detection and cutting at the defect locations, the amount of scrap due to layered defects was 0.08t, and the scrap rate was 0.06%.

[0042] Table 1 Comparison of scrap rates of 45 steel used in sprockets before and after implementation of this invention Example 2: Taking 40Cr steel for producing chains as an example, the present invention will be described in detail.

[0043] Steel grade: 40Cr steel; Rolling specifications: 7.6mm*680mm.

[0044] 1. Initial production conditions: The composition requirements for 40Cr steel are: C: 0.40wt%, Mn: 0.65wt%, Si: 0.21wt%, S: 0.002wt%, P: 0.018wt%, Cr: 0.92wt%.

[0045] Molten iron conditions: Chemical composition: C: 4.25%, Si: 0.51%, S: 0.021%, P: 0.082%, Mn: 0.18%.

[0046] Main equipment parameters: KR desulfurization, 120-ton converter, RH furnace, LF furnace, slab continuous casting machine, walking beam furnace, 900mm rolling mill.

[0047] The original solution had a problem: the stamping process of the sprocket resulted in delamination, with a scrap rate of 3%.

[0048] 2. Implementation of the technical solution: 1) Desulfurization of molten iron. Molten iron undergoes desulfurization pretreatment using the KR method, resulting in a sulfur content of 0.003% in the desulfurized molten iron.

[0049] 2) Deep desulfurization in the LF furnace. The initial molten steel from the converter enters the LF furnace for refining. It is refined using high-basicity, strongly reducing white slag containing CaO: 57.5%, SiO2: 7.5%, Al2O3: 24.6%, MgO: 4.8%, (MnO+FeO): 0.65%. The sulfur content is controlled at 0.003% at the end of the refining process. After refining, a Ca wire is fed in for calcium treatment to spheroidize and modify inclusions.

[0050] 3) RH degassing. After refining in the LF furnace, the molten steel is subjected to RH vacuum degassing for 19 minutes to further remove gases. The [H] content was measured to be 1.1 ppm.

[0051] 4) Continuous casting. Before production, the roll gap accuracy is checked and maintained, and the roll gap deviation does not exceed ±0.17mm. The continuous casting process adopts full-process protective pouring, and the molten steel superheat is 18℃. The liquid level fluctuation in the crystallizer is ±0.035mm, and the casting speed is constant within the 38min pouring cycle, with the fluctuation range not exceeding 0.04m / min.

[0052] 5) Implement an improved secondary cooling water system, with a total water volume of 1.19 g / kg. The water distribution ratio for each section before the straightening point is as follows: foot roller section 20%, bending section 9%, fan-shaped section 1 10%, fan-shaped sections 2 and 3 24%, fan-shaped sections 4 and 5 13%, fan-shaped sections 6 and 7 9%, and fan-shaped sections 8 to 11 9%. The water volume before the straightening point accounts for 94% of the total water volume. The surface temperature of the cast billet is 958℃ at the straightening point.

[0053] 6) After the straightening point is reached in continuous casting and before cutting, continue to spray a small amount of cooling water (accounting for 6% of the total secondary cooling water) to avoid stress cracks caused by the surface temperature of the billet. After the billet is cut, it is slowly cooled at room temperature.

[0054] 7) Billet preheating. After being hot-sent to the rolling mill and stacked to cool to 175°C, the continuously cast billet enters the holding furnace for preheating. The preheating temperature is 515°C and the holding time is 65 minutes.

[0055] 8) Billet heating. The preheated billet is sent to a walking beam furnace for heating for more than 182 minutes. The preheating temperature is 940℃ and the preheating time is 62 minutes; the heating temperature is 1241℃ and the heating time is 75 minutes; the soaking temperature is 1248℃ and the soaking time is 45 minutes. The billet exiting the furnace is 1242℃.

[0056] 9) Rough rolling. Large reduction rolling is performed on the roughing mill. In the first pass, the 180mm thick slab is reduced to 132mm (reduction of 48mm, reduction rate 26.6%), and in the second pass, it is reduced to 104mm (reduction of 28mm, reduction rate 21.2%), ensuring sufficient deformation of the core. After multiple passes, a 33mm thick intermediate slab is sent to the finishing mill, where it is finally rolled into a 7.6mm*680mm steel strip.

[0057] 3. In this case, a total of 127 tons of 40Cr steel for medium-width sprockets were produced. After flaw detection and cutting at the defect locations, the amount of scrap due to layered defects was 0.07t, and the scrap rate was 0.05%.

[0058] Table 2 Comparison of scrap rates of 40Cr steel for sprockets before and after implementation of this invention Example 3: Taking 40Cr steel for producing chains as an example, the present invention will be described in detail.

[0059] Steel grade: 40Cr steel; Rolling specifications: 6.5mm*750mm.

[0060] 1. Initial production conditions: The composition requirements for 40Cr steel are: C: 0.40wt%, Mn: 0.72wt%, Si: 0.28wt%, S: 0.004wt%, P: 0.022wt%, Cr: 0.98wt%.

[0061] Molten iron conditions: Chemical composition: C: 4.41%, Si: 0.38%, S: 0.029%, P: 0.102%, Mn: 0.26%.

[0062] Main equipment parameters: KR desulfurization, 120-ton converter, RH furnace, LF furnace, slab continuous casting machine, walking beam furnace, 900mm rolling mill.

[0063] The original solution had a problem: the stamping process of the sprocket resulted in delamination, with a scrap rate of 3%.

[0064] 2. Implementation of the technical solution: 1) Desulfurization of molten iron. Molten iron undergoes desulfurization pretreatment using the KR method, resulting in a sulfur content of 0.005% in the desulfurized molten iron.

[0065] 2) Deep desulfurization in the LF furnace. The initial molten steel from the converter enters the LF furnace for refining. It is refined using high-basicity, strongly reducing white slag containing CaO: 56.2%, SiO2: 6.8%, Al2O3: 25.3%, MgO: 5.4%, (MnO+FeO): 0.83%. The S content is controlled at 0.004% at the end of the refining process. After refining, a Ca wire is fed in for calcium treatment to spheroidize and modify inclusions.

[0066] 3) RH degassing. After refining in the LF furnace, the molten steel is subjected to RH vacuum degassing treatment for 24 minutes to further remove gases. The [H] content was measured to be 0.8 ppm.

[0067] 4) Continuous casting. Before production, the roll gap accuracy is checked and maintained, and the roll gap deviation does not exceed ±0.17mm. The continuous casting process adopts full-process protective pouring, and the molten steel is superheated to 24℃. The liquid level fluctuation in the crystallizer is ±0.042mm, and the casting speed is constant within a 35-minute pouring cycle, with a fluctuation range not exceeding 0.05m / min.

[0068] 5) Implement an improved secondary cooling water system, with a total water volume of 1.19 g / kg. The water distribution ratio for each section before the straightening point is as follows: foot roller section 18%, bending section 10%, fan-shaped section 1 10%, fan-shaped sections 2 and 3 23%, fan-shaped sections 4 and 5 13%, fan-shaped sections 6 and 7 8%, and fan-shaped sections 8 to 11 8%. The water volume before the straightening point accounts for 90% of the total water volume. The surface temperature of the cast billet is 972℃ at the straightening point.

[0069] 6) After the straightening point is reached in continuous casting and before cutting, continue to spray a small amount of cooling water (accounting for 10% of the total secondary cooling water volume) to avoid stress cracks caused by the surface temperature of the billet. After the billet is cut, it is slowly cooled at room temperature.

[0070] 7) Billet preheating. After being hot-sent to the rolling mill and stacked to cool to 108°C, the continuously cast billet enters the holding furnace for preheating at 419°C for 77 minutes.

[0071] 8) Billet heating. The preheated billet is sent to a walking beam furnace for heating for more than 165 minutes. The preheating temperature is 924℃ and the preheating time is 52 minutes; the heating temperature is 1226℃ and the heating time is 64 minutes; the soaking temperature is 1242℃ and the soaking time is 49 minutes. The billet exiting the furnace is 1228℃.

[0072] 9) Rough rolling. Large reduction rolling is performed on the roughing mill. In the first pass, the 180mm thick slab is reduced to 135mm (reduction of 45mm, reduction rate of 25%), and in the second pass, it is reduced to 107mm (reduction of 28mm, reduction rate of 20.7%), ensuring sufficient deformation of the core. After multiple passes, a 31mm thick intermediate slab is sent to the finishing mill, where it is finally rolled into a 6.5mm*750mm steel strip.

[0073] 3. In this case, a total of 124 tons of 40Cr steel for medium-width sprockets were produced. After flaw detection and cutting at the defect locations, the amount of scrap due to layered defects was 0.11t, and the scrap rate was 0.09%.

[0074] Table 3 Comparison of scrap rates of 40Cr steel for sprockets before and after implementation of this invention Example 4: Taking 45 steel for producing chains as an example, the present invention will be described in detail.

[0075] Steel grade: 45 steel; Rolling specifications: 9.0mm*600mm.

[0076] 1. Initial production conditions: The composition requirements for 45 steel are: C: 0.49wt%, Mn: 0.51wt%, Si: 0.30wt%, S: 0.005wt%, P: 0.024wt%.

[0077] Molten iron conditions: Chemical composition: C: 3.8%, Si: 0.65%, S: 0.017%, P: 0.098%, Mn: 0.22%.

[0078] Main equipment parameters: KR desulfurization, 120-ton converter, RH furnace, LF furnace, slab continuous casting machine, walking beam furnace, 900mm rolling mill.

[0079] The original solution had a problem: the stamping process of the sprocket resulted in delamination, with a scrap rate of 3%.

[0080] 2. Implementation of the technical solution: 1) Desulfurization of molten iron. Molten iron undergoes desulfurization pretreatment using the KR method, resulting in a sulfur content of 0.004% in the desulfurized molten iron.

[0081] 2) Deep desulfurization in the LF furnace. The initial molten steel from the converter enters the LF furnace for refining. It is refined using high-basicity, strongly reducing white slag containing CaO: 57.5%, SiO2: 8.2%, Al2O3: 23.1%, MgO: 4.6%, (MnO+FeO): 0.9%. The S content is controlled at 0.003% at the end of the refining process. After refining, a Ca wire is fed in for calcium treatment to spheroidize and modify inclusions.

[0082] 3) RH degassing. After refining in the LF furnace, the molten steel is subjected to RH vacuum degassing treatment for 22 minutes to further remove gases. The measured [H] concentration is 0.5 ppm.

[0083] 4) Continuous casting. Before production, the roll gap accuracy is checked and maintained, and the roll gap deviation does not exceed ±0.18mm. The continuous casting process adopts full-process protective pouring, and the molten steel is superheated to 16℃. The liquid level fluctuation in the crystallizer is ±0.042mm, and the casting speed is constant within the 46min pouring cycle, with the fluctuation range not exceeding 0.03m / min.

[0084] 5) Implement an improved secondary cooling water system, with a total water volume of 1.19 g / kg. The water distribution ratio for each section before the straightening point is as follows: foot roller section 22%, bending section 9%, fan-shaped section 1 10%, fan-shaped sections 2 and 3 23%, fan-shaped sections 4 and 5 13%, fan-shaped sections 6 and 7 9%, and fan-shaped sections 8 to 11 8%. The water volume before the straightening point accounts for 94% of the total water volume. The surface temperature of the cast billet is 954℃ at the straightening point.

[0085] 6) After the straightening point is reached in continuous casting and before cutting, continue to spray a small amount of cooling water (accounting for 6% of the total secondary cooling water) to avoid stress cracks caused by the surface temperature of the billet. After the billet is cut, it is slowly cooled at room temperature.

[0086] 7) Billet preheating. After being hot-sent to the rolling mill and stacked to cool to 286°C, the continuously cast billet enters the holding furnace for preheating at 595°C for 61 minutes.

[0087] 8) Billet heating. The preheated billet is sent to a walking beam furnace for heating for more than 164 minutes, including a preheating section temperature of 964℃ and a preheating time of 61 minutes; a heating section temperature of 1236℃ and a heating time of 58 minutes; a soaking section temperature of 1243℃ and a soaking time of 45 minutes; and a billet exiting the furnace at a temperature of 1241℃.

[0088] 9) Rough rolling. Large reduction rolling is performed on the roughing mill. In the first pass, the 180mm thick slab is reduced to 130mm (reduction of 50mm, reduction rate 27.7%), and in the second pass, it is reduced to 100mm (reduction of 30mm, reduction rate 23%), ensuring sufficient deformation of the core. After multiple passes, a 32mm thick intermediate slab is sent to the finishing mill, where it is finally rolled into a 9.0mm*600mm steel strip.

[0089] 3. In this case, a total of 125 tons of 45 steel were produced for medium-width sprockets. After flaw detection and cutting at the defect locations, the amount of scrap due to layered defects was 0.03t, and the scrap rate was 0.02%.

[0090] Table 1 Comparison of scrap rates of 45 steel used in sprockets before and after implementation of this invention

Claims

1. A method for controlling delamination defects in medium-width strip steel for sprockets, characterized in that, Specifically, it includes: 1) Secondary cooling in continuous casting: The total water volume is 1.0-1.4 L / kg; the water volume in the area before the straightening point accounts for 90%-95% of the total secondary cooling water volume, and the water volume in the area after the straightening point and in the central area accounts for 5%-10% of the total secondary cooling water volume. The water volume distribution of each section of the secondary cooling before the straightening point is calculated as 100% of the total volume, of which: foot roll section (20±2)%, bending section (10±2)%, fan-shaped section 1 (10±2)%, fan-shaped section 2 and 3 25±2%, fan-shaped section 4 and 5 (15±2)%, fan-shaped section 6 and 7 (10±2)%, fan-shaped section 8 to 11 (10±2)%; 2) The continuously cast billet is piled up and slowly cooled. After the billet is cooled to 100-300℃, it is loaded into the furnace. Before entering the rolling heating furnace, the cold billet is preheated at a low temperature. The preheating temperature is controlled at 400-600℃ and the holding time is ≥1 hour. 3) During the roughing stage, ensure that the reduction rate of the first pass is not less than 25% and the reduction rate of the second pass is not less than 20%.

2. The method for controlling delamination defects in medium-width strip steel for sprockets according to claim 1, characterized in that, Through hot metal pretreatment and deep refining in an LF furnace, the sulfur content in the steel is controlled to ≤0.005wt%.

3. The method for controlling delamination defects in medium-width strip steel for sprockets according to claim 1, characterized in that, Control the superheat of molten steel during continuous casting to 15-30℃.

4. A method for controlling delamination defects in medium-width strip steel for sprockets according to claim 1 or 3, characterized in that, During continuous casting, the fluctuation of the liquid level in the crystallizer should be controlled within ±0.05mm, the accuracy of the continuous casting roll gap should be within ±0.20mm, and the casting speed fluctuation should not exceed 0.05m / min within 20s.

5. The method for controlling delamination defects in medium-width strip steel for sprockets according to claim 1, characterized in that, The strip steel is carbon structural steel, and the chemical composition of the steel is as follows: C: 0.40wt%-0.50wt%, Mn: 0.50wt%-0.80wt%, Si: 0.17wt%-0.37wt%, S≤0.005wt%, P≤0.025wt%, Cr: 0wt%-1.0wt%.

6. The method for controlling delamination defects in medium-width strip steel for sprockets according to claim 1, characterized in that, Suitable for medium-width strip steel with a width of 600-750mm and a thickness of 6.0-9.0mm.

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