Method for improving surface bright line defect of high-aluminum steel cold-rolled coil

Abstract

The invention discloses a method for improving the surface bright line defect of a high-aluminum steel cold-rolled coil. The method comprises the steps of flame cleaning, heating and hot rolling. In the flame cleaning step, the state of burners of a flame cleaning machine is checked before flame cleaning, the burners are guaranteed to be smooth, and it is guaranteed that the gap between the burners is smaller than 0.6 mm; the flame cleaning depth ranges from 2.5 mm to 2.7 mm, and a rib larger than or equal to 1 m at a groove of the ignition pit is manually cleaned; in the heating step, the furnace temperature of a heating furnace is 1220-1260 DEG C, and the furnace staying time is 230-260 minutes; in the hot rolling step, the fixed width amount is controlled to be 20-50 mm. According to the method, by confirming the state of a burner before flame cleaning, optimally controlling the plate shape of the casting blank, thinning the cleaning depth, reasonably controlling the temperature of a hot rolling heating furnace and the in-furnace time and controlling the small fixed width amount, the phenomena of high-aluminum steel flame cleaning of flanges of the casting blank and missing cleaning are obviously improved, and the bright line defect on the surface of strip steel is stably controlled; and the bright line defect ratio of continuous unwinding is reduced from 6.8% to 0.03%.

Description

Technical Field

[0001] The invention relates to the technical field of steel rolling, in particular to a method for improving bright line defects on the surface of a high-aluminum steel cold-rolled coil. Background Art

[0002] During the hot rolling heating process, the surface is subjected to two-dimensional heating and oxidation, resulting in the formation of thick loose iron oxide scale. At the same time, due to the strong affinity between aluminum and oxygen, the defects are affected by thermal stress to form hot loose iron oxide scale, which preferentially contacts O2 in the air. In addition, there is a difference in the compression ratio between the defects and the normal matrix during the rolling process, resulting in large roughness and high surface activity at the defects, which leads to oxidation reaction at the defects to form bright lines of grayish-white Al2O3 particles, seriously affecting the surface quality of cold-rolled coil products.

[0003] At present, domestic and foreign research mainly focuses on the optimization of flame cleaning machine media and burner modification, or only analyzes and studies the hot rolling equipment accuracy and hot rolling heating furnace process. However, no research has been done on the pre-production confirmation of flame cleaning machine equipment, abnormal burner maintenance, hot rolling dephosphorization effect, especially for special steel grades with high aluminum content, high aluminum steel, the influence of flame cleaning temperature, cleaning depth and edge height control on the bright line defects of high aluminum steel.

[0004] The Chinese patent application with publication number CN113102501A discloses a method for improving the surface quality of automobile steel. The method optimizes the frequency of hot rolling roller rolling kilometers to perform finish rolling on the intermediate billet, and optimizes the side guide plate pressure to coil to improve the surface quality; however, no much research has been done on the equipment status and process parameters of the flame cleaning machine.

[0005] Chinese patent application with publication number CN104384209A discloses a method for eliminating machine-cleaning stripes. This method only studies the hot rolling heating process. The ingot is subjected to secondary oxidation in a hot rolling heating furnace so that the surface of the continuous casting ingot to be oxidized is loosely scaled. The loose scale is removed to obtain a continuous casting ingot with a smooth machine-cleaned surface. However, no control of the root cause of the machine-cleaning stripes or control of other hot rolling parameters is involved.

[0006] The Chinese patent application with publication number CN116559175A discloses an automatic grading and determination method for the surface quality of machine-cleaned billets, which collects continuous image data from the four sides of the billet through detection devices installed on the upper, lower, left and right sides of the output roller, and uses a computer to automatically identify and count the number, size and position of inclusions and bubbles per unit area of ​​the billet to grade the billet surface quality, thereby improving the hot charging and hot delivery rate of the billet. However, no research has been conducted on how to ensure the surface quality of the billet after machine cleaning.

[0007] The Chinese patent application with publication number CN2755441Y discloses a machine cleaning burner for preventing groove defects in casting billets. The machine cleaning burner greatly reduces the groove cleaning defects of the casting machine caused by excessive gaps between the shoe blocks by setting a knife-shaped airflow wall on the side wall of the shoe block, but other process parameters are not involved in detail.

[0008] "Research on New Defects of High-quality Automotive Steel Ingots at Anshan Iron and Steel Co., Ltd." (Proceedings of the 14th China Steel Annual Conference, pages 1-6), Anshan Iron and Steel Co., Ltd. improved the white strip defects of ultra-low carbon IF steel acid rolling by adjusting and improving the parameters and equipment of the flame cleaning machine. However, no research was conducted on the control of the "bright line" of aluminum-containing high-strength steel and the key parameters of hot rolling.

[0009] "Formation Mechanism and Prevention Measures of Machine Cleaning Streaks on Hot-rolled Coil Surface" (China Metallurgy, Issue 7, July 2018, pp. 46-49), this paper comprehensively analyzes the improvement of machine cleaning streaks on the surface of hot-rolled coils by replacing machine cleaning burners, transforming natural gas, optimizing machine cleaning processes, etc.; however, it does not involve much about the hot rolling process.

[0010] "Analysis on the Morphology and Formation of Machine Cleaning Streaks on the Surface of Dual-Phase High-Strength Steel" (China Metallurgy, Issue 12, December 2016, pp. 26-30). This paper only analyzes the causes of machine cleaning streaks on the surface of dual-phase high-strength steel; however, no research was conducted on how to improve the machine cleaning streak defects.

[0011] "Classification and Analysis of Linear Defects on the Surface of Cold-Rolled Plates" (Steel Rolling, July 2018 Supplement 2, pages 20-25), this paper conducted research by replacing machine cleaning burners, transforming natural gas, optimizing production organization, etc.; however, no analysis was made on the improvement of machine cleaning stripes by hot rolling process.

[0012] The above prior art also only analyzes the machine-cleared streak defects, but no research has been done on the white bright line defects on the surface of high-aluminum steel strips with relatively high aluminum content. Summary of the invention

[0013] The technical problem to be solved by the present invention is to provide a method for improving bright line defects on the surface of a high-aluminum steel cold-rolled coil, so as to improve the surface quality of the high-aluminum steel.

[0014] In order to solve the above technical problems, the technical solution adopted by the present invention is: comprising flame cleaning, heating and hot rolling steps; The flame cleaning step includes: checking the burner status of the flame cleaning machine before flame cleaning to ensure that the burner is unobstructed and the gap between the burners is less than 0.6 mm; the flame cleaning depth is 2.5 to 2.7 mm, and manually cleaning the ridges ≥ 1 m at the ignition pit groove; The heating step comprises a heating furnace temperature of 1220 to 1260° C. and a furnace time of 230 to 260 minutes; The hot rolling step: the fixed width is controlled at 20 to 50 mm.

[0015] Furthermore, in the flame cleaning step, if the gap between the burners is ≥0.6 mm, a thin copper plate is used to block the gap.

[0016] Furthermore, in the flame cleaning step, the preheating time of the flame cleaning is 28 to 30 seconds.

[0017] Furthermore, in the hot rolling step, the dephosphorization pressure is controlled at 190-230 bar.

[0018] The beneficial effects of adopting the above technical scheme are: through the confirmation of the burner state before flame cleaning, the optimization control of the ingot plate shape, the thinning of the cleaning depth, the reasonable control of the hot rolling heating furnace temperature and the time in the furnace and the control of the small fixed width, the convex ridges and leakage of high-aluminum steel flame cleaning ingots are significantly improved, the "bright line" defects on the surface of the strip are stably controlled, and the proportion of bright line defects in continuous unwinding is reduced from 6.8% to 0.03%. The surface quality of high-aluminum steel products has achieved a qualitative leap and won high praise from customers. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0020] Figure 1 This is the actual surface appearance of the high aluminum steel ingot after cleaning obtained by conventional method; Figure 2 This is the actual surface appearance of the high aluminum steel hot-rolled coil obtained by conventional methods; Figure 3 This is the actual surface appearance of the high aluminum steel cold-rolled coil obtained by conventional method; Figure 4 This is a real picture of the surface of the high aluminum steel ingot after cleaning obtained in Example 1 of the present invention; Figure 5 This is a real picture of the surface of the high aluminum steel hot-rolled coil obtained in Example 1 of the present invention; Figure 6 This is a real picture of the surface of the high-aluminum steel cold-rolled coil obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] The method for improving the bright line defects on the surface of high-aluminum steel cold-rolled coils includes flame cleaning, heating and hot rolling steps; the design idea is: (1) Flame cleaning: The aluminum content of high-aluminum steel is 0.6% to 0.8% (wt), and its aluminum content is relatively high. If the inherited strip edges and leaking areas of high-aluminum steel ingots after flame cleaning are ≥2.8mm, or the ignition pit ridges during flame cleaning preheating are ≥1mm, when entering the hot rolling heating furnace for heating, the oxide scale formed in the above ridge areas is thicker than the normal matrix; because the surface oxide scale is affected by the matrix structure, thermal stress is easily generated at the edge of the ridge, forming hot oxide scale cracks to promote the entry of O2 in the air; at the same time, due to the high edge position of the ridge, there is a difference in the compression ratio with the normal matrix during rolling, resulting in the edge position of the ridge being rougher than the normal matrix; in addition, aluminum and oxygen have a strong affinity, and the oxide scale in the ridge area falls off and contacts the air first. The roughness of this area is large and the surface activity is high, resulting in the formation of gray-white Al2O3 particles as oxidation reaction products with O2 in the air.

[0022] Therefore, this method is based on the equipment status of the flame cleaning machine. Before cleaning the high-aluminum steel ingot, the residual slag or dirt blocking the burner pores should be removed to ensure smooth pores and good air permeability. The aluminum content in high-aluminum steel is high and the viscosity is high. After the flame cleaning, the slag remaining on the surface, especially the residues in uneven places such as ridges and depressions, adheres to the gaps and is not easy to remove. A large amount of high-pressure water is required, and the high-pressure water nozzle must be smooth to granulate the slag produced by the melting reaction during flame cleaning and flush it away.

[0023] The flame cleaning preheating time is 28 to 30 seconds. The flame cleaning depth of high-aluminum steel ingots is reduced from 2.8 to 3.0 mm to 2.5 to 2.7 mm, and the ridges ≥ 1m at the ignition pit groove are manually cleaned to ensure that the ridges are less than 1.0 mm and have a smooth transition, so as to reduce the hidden dangers of missing cleaning and gray-white Al2O3 particles at the ridges due to the affinity of aluminum with oxygen in the air.

[0024] Ensure that the gap between the burners of the flame cleaning equipment is less than 0.6mm to avoid the risk of Al2O3 particle light strip defects caused by excessive gap between the burners; if the gap between the burners is ≥0.6mm, use a thin copper plate to block the gap to make the gap between the burners less than 0.6mm to avoid the risk of high edge defects of the cast billet after machine cleaning caused by excessive gap between the burners; the thickness of the thin copper plate is 0.4-0.6mm.

[0025] (2) Heating: The furnace temperature is 1220-1260℃, and the furnace time is extended to 230-260min, so that the ridges and leak defects on the surface of the ingot after flame cleaning are fully oxidized to form a relatively thick and loose iron oxide scale, which is then removed by high-pressure dephosphorization to make the ridges and leak defects thinner, smoother and flatter, avoiding the widening and extension of subsequent rolling defects.

[0026] (3) Hot rolling: A small fixed width of 20 to 50 mm is adopted for hot rolling to improve the risk of extrusion of the head preheating pit and ridge; the hot rolling dephosphorization pressure is controlled at 190 to 230 bar.

[0027] Example 1: The method for improving bright line defects on the surface of high-aluminum steel cold-rolled coil is specifically described as follows.

[0028] (1) Flame cleaning: 6 high-aluminum steel ingots from one heat were selected for flame cleaning. The aluminum content of the furnace was 0.71%. The ingot shape was well controlled and no buckling or warping was found. In order to avoid burner blockage and excessive gap, which may cause defects such as leakage and high edges after flame cleaning of the ingot, the burner status should be checked before flame cleaning. The burner should be unobstructed and the gap between the burners should be 0.4mm, indicating that the burner is in good condition. The flame cleaning preheating time of 6 high-aluminum steel ingots is 29s, 30s, 28s, 28s, 30s, and 29s; The flame cleaning depths of the ingots are 2.5mm, 2.6mm, 2.7mm, 2.6mm, 2.6mm, and 2.7mm respectively; A depth gauge was used to measure the ridges at the groove of the ignition pit of 6 high-aluminum steel ingots after flame cleaning, and the results were 0.5mm, 0.7mm, 0.6mm, 0.7mm, 0.6mm, and 0.8mm respectively.

[0029] (2) Heating: High-temperature heating was carried out in a heating furnace. The temperatures of the heating furnace were 1246°C, 1239°C, 1256°C, 1241°C, 1238°C and 1229°C respectively. The time in the heating furnace was controlled at 251min, 245min, 239min, 258min, 260min and 232min respectively.

[0030] (3) Hot rolling: The dephosphorization pressures of hot rolling are 223 bar, 212 bar, 206 bar, 215 bar, 198 bar and 230 bar respectively. In order to improve the risk of extrusion of the head preheating pit ridge, hot rolling is carried out with a small fixed width, which are 36 mm, 42 mm, 35 mm, 22 mm, 50 mm and 37 mm respectively.

[0031] (4) After the high aluminum steel of this embodiment is cleaned and rolled using the above process, no bright line defects occur on the surface of the cold-rolled coil steel. Figure 1 This is the actual surface appearance of the high aluminum steel ingot after cleaning obtained by conventional method; Figure 2 This is the actual surface appearance of the high aluminum steel hot-rolled coil obtained by conventional methods; Figure 3 This is the actual surface appearance of the high aluminum steel cold-rolled coil obtained by conventional method; Figure 4 This is a real picture of the surface of the ingot after high aluminum steel machine cleaning in this embodiment; Figure 5This is a real picture of the surface of the hot-rolled high-aluminum steel strip in this embodiment; Figure 6 is a real picture of the surface of the cold-rolled high-aluminum steel strip in this embodiment; Figure 1-6 It can be seen that the high-strength steel treated by this method effectively eliminates the surface bright line defects and the surface quality of high-aluminum steel products is significantly improved.

[0032] Example 2: The method for improving bright line defects on the surface of high-aluminum steel cold-rolled coil is specifically described as follows.

[0033] (1) Flame cleaning: 9 high-aluminum steel ingots from one heat were selected for flame cleaning. The aluminum content of the furnace was 0.68%. The ingot shape was checked and was well controlled, with no buckling or warping of the ingots. In order to avoid burner blockage and excessive gap, which may cause defects such as leakage and high edges after flame cleaning of the ingot, the burner status should be checked before flame cleaning. The burner should be unobstructed and the gap between the burners should be 0.3mm, indicating that the burner is in good condition. The flame cleaning preheating time of 9 high aluminum steel ingots is 28s, 29s, 28s, 30s, 28s, 29s, 29s, 29s, 30s; The flame cleaning depths of the ingots are 2.6mm, 2.5mm, 2.5mm, 2.7mm, 2.5mm, 2.6mm, 2.6mm, 2.7mm, and 2.6mm respectively; The ridges at the ignition pit groove of 9 high-aluminum steel ingots after flame cleaning were measured with a depth gauge, and they were 0.6mm, 0.8mm, 0.5mm, 0.7mm, 0.8mm, 0.9mm, 1.0mm, 0.7mm, and 1.2mm respectively. For two ingots with ridges ≥ 1.0mm at the ignition pit groove, 1.0mm and 1.2mm were cleaned by manual gun pulling, and the surface was smooth without ridges after manual gun pulling.

[0034] (2) Heating: High-temperature heating was carried out in a heating furnace. The temperatures of the heating furnace were 1236°C, 1229°C, 1232°C, 1251°C, 1240°C, 1228°C, 1220°C, 1238°C and 1249°C respectively. The time in the heating furnace was controlled at 239min, 247min, 232min, 259min, 253min, 241min, 235min, 230min and 238min respectively.

[0035] (3) Hot rolling: The dephosphorization pressures of hot rolling are 190 bar, 213 bar, 209 bar, 218 bar, 225 bar, 230 bar, 198 bar, 208 bar, and 221 bar respectively. In order to improve the risk of extrusion of the head preheating pit ridge, hot rolling is carried out with a small fixed width, and the fixed widths are 26 mm, 38 mm, 31 mm, 20 mm, 42 mm, 36 mm, 41 mm, 23 mm, and 29 mm respectively.

[0036] (4) After the high aluminum steel of this embodiment is cleaned and rolled using the above process, no "bright line" defect occurs on the surface of the cold-rolled coil steel.

[0037] Example 3: The method for improving bright line defects on the surface of high-aluminum steel cold-rolled coil is specifically described as follows.

[0038] (1) Flame cleaning: 7 high-aluminum steel ingots from one heat were selected for flame cleaning. The aluminum content of the furnace was 0.65%. The ingot shape was well controlled and no buckling or warping was found. In order to avoid burner blockage and excessive gap, which may cause defects such as leakage and high edges after flame cleaning of the ingot, the burner status should be checked before flame cleaning. The burner should be unobstructed and the gap between the burners should be 0.5mm, indicating that the burner is in good condition. The flame cleaning preheating time of 7 high-aluminum steel ingots is 29s, 28s, 28s, 30s, 28s, 28s, 29s; The flame cleaning depths of the ingots are 2.7mm, 2.5mm, 2.6mm, 2.5mm, 2.5mm, 2.6mm, and 2.6mm respectively; A depth gauge was used to measure the ridges at the groove of the ignition pit of 7 high-aluminum steel ingots after flame cleaning, and the results were as follows: 0.5mm, 0.7mm, 0.5mm, 0.8mm, 0.9mm, 0.9mm, and 0.7mm respectively.

[0039] (2) Heating: High-temperature heating was carried out in a heating furnace. The temperatures of the heating furnace were 1226°C, 1234°C, 1231°C, 1256°C, 1243°C, 1226°C, and 1230°C respectively. The time in the heating furnace was controlled at 249min, 241min, 235min, 253min, 252min, 247min, and 233min respectively.

[0040] (3) Hot rolling: The dephosphorization pressures of hot rolling are 230 bar, 221 bar, 208 bar, 211 bar, 229 bar, 230 bar, and 196 bar respectively. In order to improve the risk of extrusion of the head preheating pit ridge, hot rolling is carried out with a small fixed width, which are 20 mm, 31 mm, 39 mm, 40 mm, 43 mm, 31 mm, and 45 mm respectively.

[0041] (4) After the high aluminum steel of this embodiment is cleaned and rolled using the above process, no "bright line" defect occurs on the surface of the cold-rolled coil steel.

[0042] Example 4: The method for improving bright line defects on the surface of high-aluminum steel cold-rolled coil is specifically described as follows.

[0043] (1) Flame cleaning: 8 high-aluminum steel ingots from one heat were selected for flame cleaning. The aluminum content of the furnace was 0.73%. The ingot shape was checked and was well controlled, with no buckling or warping of the ingots. In order to avoid burner blockage and excessive gap, which may cause defects such as leakage and high edges after flame cleaning of the ingot, the burner status was checked before flame cleaning. The burner was unobstructed, and the gap between the burners was 0.6mm. A 0.5mm thin copper plate was used to block the gap, and the status was good. The flame cleaning preheating time of 8 high-aluminum steel ingots is 30s, 29s, 30s, 29s, 29s, 28s, 30s, 29s; The flame cleaning depths of the ingots are 2.6mm, 2.6mm, 2.6mm, 2.7mm, 2.5mm, 2.6mm, 2.7mm, and 2.6mm respectively; The ridges at the ignition pit groove of 8 high-aluminum steel billets after flame cleaning were measured with a depth gauge, and the ridges were 0.6mm, 0.7mm, 0.8mm, 0.7mm, 0.9mm, 0.9mm, 1.3mm, and 0.7mm respectively. The billet with a ridge of 1.3mm at the ignition pit groove was cleaned manually with a manual gun pull. After manual gun pull treatment, the groove was smooth without slag flow and ridges.

[0044] (2) Heating: High-temperature heating was carried out in a heating furnace. The temperatures of the heating furnace were 1221°C, 1232°C, 1235°C, 1243°C, 1229°C, 1258°C, 1252°C and 1247°C respectively. The time in the heating furnace was controlled at 241min, 230min, 233min, 251min, 245min, 231min, 260min and 240min respectively.

[0045] (3) Hot rolling: The dephosphorization pressures of hot rolling are 195 bar, 203 bar, 219 bar, 213 bar, 205 bar, 220 bar, 193 bar, and 228 bar. In order to improve the risk of extrusion of the head preheating pit ridge, hot rolling is carried out with a small fixed width, which are 36 mm, 28 mm, 41 mm, 23 mm, 45 mm, 37 mm, 45 mm, and 33 mm respectively.

[0046] (4) After the high aluminum steel of this embodiment is cleaned and rolled using the above process, no "bright line" defect occurs on the surface of the cold-rolled coil steel.

Claims

1. A method for improving bright line defects on the surface of high aluminum steel cold rolled coil, characterized in that: including flame cleaning, heating and hot rolling steps; The flame cleaning step includes: checking the burner status of the flame cleaning machine before flame cleaning to ensure that the burner is unobstructed and the gap between the burners is less than 0.6 mm; the flame cleaning depth is 2.5 to 2.7 mm, and manually cleaning the ridges ≥ 1 m at the ignition pit groove; The heating step comprises a heating furnace temperature of 1220 to 1260° C. and a furnace time of 230 to 260 minutes; The hot rolling step: the fixed width is controlled at 20 to 50 mm.

2. The method for improving bright line defects on the surface of high aluminum steel cold rolled coil according to claim 1, characterized in that: In the flame cleaning step, if the gap between the burners is ≥0.6 mm, a thin copper plate is used to block the gap.

3. The method for improving bright line defects on the surface of high aluminum steel cold rolled coil according to claim 1, characterized in that: In the flame cleaning step, the preheating time of the flame cleaning is 28 to 30 seconds.

4. A method for improving bright line defects on the surface of high aluminum steel cold rolled coil according to claim 1, 2 or 3, characterized in that: In the hot rolling step, the dephosphorization pressure is controlled at 190-230 bar.

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