Method for manufacturing a roll for low-ripple high-quality automotive sheet

Abstract

The application discloses a manufacturing method of a low-waved high-quality automobile plate special roller, which comprises smelting an electric slag ingot blank according to chemical components and weight percentages, forging a roller blank, preparing heat treatment and final heat treatment; the final heat treatment comprises preheat treatment, surface quenching treatment, cold treatment and tempering treatment; the surface quenching treatment adopts a movable double-frequency induction quenching treatment, and the specific method is as follows: the roller is vertically placed and rotated in the circumferential direction, the rotating speed is 25-40 r / min; meanwhile, the roller is uniformly and downwardly moved at a speed of 20-25 mm / min; the roller is sequentially heated by upper and lower induction rings and cooled by a water spraying ring; after the quenching is completed, the roller is quickly sunk into a circulating deep water pool underground to continue cooling for 1.5-3 h. The roller with good tissue uniformity and hardness uniformity and low roughness and grinding performance can be finally obtained, so that the roller can be used for rolling of the low-waved high-quality automobile plate.

Description

Technical Field

[0001] This invention belongs to the technical field of automotive sheet special rolling mill rolls, specifically relating to a manufacturing method of a low-wavy, high-quality automotive sheet special rolling mill roll. Background Technology

[0002] In recent years, due to considerations of environmental protection, energy consumption, and cost, and also impacted by new energy vehicles, more and more automakers (including new energy vehicle manufacturers) have begun to use a two-coat, one-bake (2C1B) painting process that eliminates the need for a primer. This painting process reduces the paint film thickness, thereby increasing the transfer of surface structure from the steel sheet to the paint surface. Therefore, under this new painting process, how to control the low waviness of the automotive sheet to ensure good appearance quality after the steel sheet is coated is a core technology that steel mills must master.

[0003] Currently, the requirement for low waviness is wsa≤0.35μm under 5% deformation. The key to achieving low waviness control of automotive steel sheets is the roll, because it directly transmits the surface morphology. Therefore, it has special requirements and must simultaneously possess: (1) good uniformity of structure, the roll surface must not have carbide segregation, inclusions and porosity defects; (2) good uniformity of hardness, the roll surface hardness uniformity is required to be ±0.3HSD; (3) low roughness grinding performance, that is, when the grinding roughness Ra<0.40μm, the roll surface still has no vibration marks, spot marks, bright spots, cracks and other defects, and ensures that this performance is maintained during long-term use on the machine and during the grinding process of the roll body after the machine.

[0004] Existing cold rolling rolls cannot simultaneously meet the above three requirements. Summary of the Invention

[0005] The purpose of this invention is to solve the above-mentioned problems and provide a method for manufacturing a low-wavy, high-quality automotive sheet roll that simultaneously possesses good microstructure uniformity, good hardness uniformity, and low roughness grinding performance.

[0006] The technical solution to achieve the purpose of this invention is: a method for manufacturing a low-wavy, high-quality automotive sheet special roll, comprising smelting an electroslag ingot blank and a forged roll blank according to chemical composition and weight percentage, pre-heat treatment, and final heat treatment.

[0007] Chemical composition and its content are crucial for obtaining good microstructure uniformity and low surface roughness grinding performance.

[0008] In order to obtain better uniformity of structure, the following requirements are made: (1) The carbon content should be appropriately reduced, and the content of strong carbide-forming elements chromium, molybdenum and vanadium should be strictly controlled. The total content of chromium + molybdenum + vanadium should not exceed 5.3%, which can significantly reduce carbide segregation; (2) The content of phosphorus and sulfur should be strictly controlled. The content of phosphorus + sulfur should not exceed 0.025%, which can significantly reduce inclusion defects; (3) The gas content should be strictly controlled. The content of [H] should be ≤2ppm, [O] should be ≤25ppm and [N] should be ≤120ppm, which can significantly reduce porosity defects; (4) The residual copper content should not exceed 0.25%.

[0009] In addition, in order to obtain grinding performance with low roughness, the following requirements are made: (1) appropriately reduce carbon content; (2) strictly control the content of vanadium and silicon. Vanadium carbides have very high hardness and are prone to forming vibration marks on the roller surface after grinding. Therefore, the vanadium content should not exceed 0.02%; silicon has high matrix strengthening performance, which will also affect high-quality grinding. However, silicon is indispensable in smelting. Therefore, the silicon content should be 0.20-0.35%; (3) strictly control nickel content. Nickel increases toughness and grinding abrasive grains are prone to forming pitting scratches on the roller surface. Therefore, the nickel content should not exceed 0.25%.

[0010] Therefore, the chemical composition and weight percentage of this application are as follows: carbon 0.75-0.85%, silicon 0.20-0.35%, manganese 0.30-0.50%, phosphorus ≤0.015%, sulfur ≤0.010%, chromium 4.60-5.00%, nickel ≤0.25%, molybdenum 0.15-0.30%, vanadium ≤0.02%, copper ≤0.25%, [H] ≤2ppm, [O] ≤25ppm, [N] ≤120ppm, and the remainder is iron and unavoidable impurities.

[0011] The electroslag remelting process for ingots includes electric furnace smelting, ladle refining, vacuum degassing, casting electrode rods, and electroslag remelting. Electroslag remelting is crucial for achieving good microstructure uniformity. Due to its favorable solidification and crystallization conditions and strong slag washing effect, electroslag remelting has become a commonly used refining method for high-quality steel. If electroslag steel is not properly controlled, a common defect is wavy segregation, which appears as multiple layers of distinct rings on the surface of the electroslag remelted ingot, commonly known as "waistbands." After forging deformation, multiple layers of wavy black and white rings appear on the surface of the roll blank. This wavy texture affects the quality of the final high-quality automotive steel sheet. Therefore, low-wavy, high-quality automotive steel sheet rolls must not have this defect. Specific optimization techniques include:

[0012] ① Consumable electrode: A single consumable electrode rod must be used for electroslag remelting, meaning the electrode cannot be replaced during the process and must be electroslag remelted in one go; otherwise, severe watermarks will form at the electrode replacement point.

[0013] ② Electroslag remelting slag system: In order to better filter out inclusions and reduce segregation, a slag system with calcium fluoride as the main component is adopted. The components and their weight percentages are: calcium fluoride 40-50%, calcium oxide 15-35%, and alumina 25-40%.

[0014] ③ Electroslag remelting process: The remelting process adopts a low voltage and high current. During the electroslag remelting process, a voltage of 55-65V and a current of 18000-23000A are used.

[0015] An automatic voltage control system is used in the electroslag remelting process to ensure that voltage fluctuations are ≤3V and current fluctuations are ≤800A. Such fluctuations prevent component segregation.

[0016] The forging of the roll blank and the preheating treatment are both conventional methods in the art.

[0017] The final heat treatment includes preheating, surface hardening, cold treatment, and tempering. Among these, surface hardening is crucial for obtaining good hardness uniformity.

[0018] The surface hardening treatment of this application adopts a mobile dual-frequency induction hardening treatment. The frequencies of the two induction coils are 45-55Hz and 250-380Hz, respectively. The distance between the two induction coils is 100-200mm. The height of the two induction coils (i.e., the heating width) is 50% higher than that of the conventional type, reaching 100-200mm. The water pressure of the water spray ring is increased by 1 / 3 compared with the conventional type, reaching 0.15-0.25MPa.

[0019] The specific method of the mobile dual-frequency induction hardening process is as follows: The roll is placed vertically and rotated in the circumferential direction at a speed of 25-40 r / min, while moving downward at a uniform speed of 15-25% slower than the conventional speed, reaching 20-25 mm / min; it is then heated by the upper induction coil, heated by the lower induction coil, and cooled by the water spray ring.

[0020] The surface quenching temperature of this application is 930-1000℃, and the specific heating process is as follows: first, rapidly heat up to 930-955℃ → hold at that temperature → slightly decrease the temperature by 15-30℃ → then heat up to 980-1000℃ → hold at that temperature again → slightly decrease the temperature by 5-15℃ → water spray quenching.

[0021] After quenching, the roll is quickly submerged in an underground circulating deep water pool for 1.5 to 3 hours to continue cooling. This ensures that the roll body cools down quickly, thereby effectively improving the surface hardness of the roll body, and also avoids uneven cooling of the roll body, thereby effectively improving the uniformity of hardness.

[0022] The preheating treatment temperature is 250-350℃, and the time is 15-30h.

[0023] The cold treatment temperature is -80 to -60°C, and the time is 3 to 6 hours;

[0024] The tempering treatment temperature is 120–160°C, and the time is 60–100 h.

[0025] The positive effects of this invention are as follows: (1) By optimizing the chemical composition and content, and by optimizing the electroslag remelting process, this invention can ensure that there are no carbide segregation, inclusion defects, porosity defects, or wavy segregation on the surface of the roll, thus achieving good microstructure uniformity. The optimization of chemical composition and content can also achieve low roughness grinding performance. (2) By optimizing the final heat treatment process, especially the surface quenching process, this invention can ultimately achieve a hardness uniformity of ±0.3HSD. (3) This invention can ultimately obtain a roll with good microstructure uniformity, hardness uniformity, and low roughness grinding performance, thus enabling it to be used for rolling low-wavy, high-quality automotive sheets. Detailed Implementation

[0026] (Example 1)

[0027] The manufacturing method of the low-wavy, high-quality automotive sheet special roll in this embodiment includes the following steps:

[0028] S1: Electroslag ingots are smelted according to the following chemical composition and weight percentages: carbon 0.80%, silicon 0.27%, manganese 0.40%, phosphorus ≤0.015%, sulfur ≤0.010%, chromium 4.80%, nickel 0.18%, molybdenum 0.23%, vanadium 0.01%, copper 0.18%, [H] ≤2ppm, [O] ≤25ppm, [N] ≤120ppm, the remainder being iron and unavoidable impurities.

[0029] Electric furnace smelting, ladle refining, vacuum degassing, and casting of electrode rods are all conventional methods in this field. Electroslag remelting uses a slag system mainly composed of calcium fluoride, with the following specific composition and weight percentages: calcium fluoride 45%, calcium oxide 25%, and alumina 30%. This slag system can better filter out inclusions and reduce segregation.

[0030] The electroslag remelting process uses a low-voltage, high-current remelting process, namely 60V voltage and 20,000-21,000A current. During the electroslag remelting process, an automatic voltage control system is used to control the voltage fluctuation value to ≤3V and the current fluctuation value to ≤800A. Such fluctuations can significantly avoid component segregation.

[0031] During the electroslag remelting process, a single consumable electrode rod must be used for electroslag remelting. That is, the electrode cannot be replaced during the electroslag remelting process. The electrode must be electroslag remelted and formed in one go. Otherwise, severe watermarks will be formed at the electrode replacement point.

[0032] S2: Forge the electroslag ingot obtained from step S1 into a roll blank.

[0033] S3: Preliminary heat treatment, including quenching and tempering at 900±5℃ for 15 hours, followed by high-temperature tempering at 600±5℃ for 30 hours.

[0034] S4: Final heat treatment, as follows:

[0035] S31: Preheating treatment.

[0036] The entire process is preheated in a box furnace at a temperature of 300±10℃ for 20 hours.

[0037] S32: Surface hardening treatment.

[0038] The process employs a mobile dual-frequency induction hardening treatment. The frequencies of the two induction coils are 50Hz and 300Hz, respectively. The distance between the two induction coils is 150mm, the height of the two induction coils is 150mm, and the water pressure of the spray ring is 0.20MPa.

[0039] The specific method is as follows: The roller is placed vertically and rotated in the circumferential direction at a speed of 30 r / min, while moving downwards at a uniform speed of 22 mm / min; it is then heated by the upper induction coil, heated by the lower induction coil, and cooled by the water spray ring.

[0040] The specific heating process is as follows: first, rapidly heat up to 930-955℃ → hold at that temperature → slightly decrease the temperature by 15-30℃ → then heat up to 980-1000℃ → hold at that temperature again → slightly decrease the temperature by 5-15℃ → water spray quenching.

[0041] After quenching, the roll is quickly submerged in an underground circulating deep water pool for 1.5 to 3 hours to continue cooling. This ensures that the roll body cools down quickly, thereby effectively improving the surface hardness of the roll body, and also avoids uneven cooling of the roll body, thereby effectively improving the uniformity of hardness.

[0042] S33: Cold treatment.

[0043] Perform cold treatment at -70℃ for 4 hours.

[0044] S34: Tempering treatment.

[0045] A long-term low-temperature tempering treatment of 90 h was carried out at 140±5℃.

[0046] The surface hardness and surface roughness of the low-wavy, high-quality automotive sheet roll prepared in Example 1 were tested, and the results are shown in Table 1.

[0047] In Table 1, point C is the middle of the roller body, points A and E are the ends of the roller body, and points B and D are the middle of points A and C and points C and E, respectively. Points A to E are all on the same generatrix.

[0048] Table 1

[0049] Example 1 Example 2 Example 3 Example 4 Example 5 carbon 0.80% 0.77% 0.82% 0.80% 0.80% silicon 0.27% 0.23% 0.31% 0.27% 0.27% manganese 0.40% 0.33% 0.45% 0.40% 0.40% chromium 4.80% 4.65% 4.95% 4.80% 4.80% molybdenum 0.23% 0.26% 0.18% 0.23% 0.23% vanadium 0.01% 0.01% 0.01% 0.01% 0.01% nickel 0.18% 0.22% 0.15% 0.18% 0.18% copper 0.17% 0.21% 0.16% 0.17% 0.17% Preheating temperature and time 300±10℃ / 20h 300±10℃ / 20h 300±10℃ / 20h 320±10℃ / 18h 280±10℃ / 25h Cold treatment temperature and time -70℃ / 4h -70℃ / 4h -70℃ / 4h -80℃ / 3h -60℃ / 5h Tempering temperature and time 140±5℃ / 90h 140±5℃ / 90h 140±5℃ / 90h 120±5℃ / 100h 150±5℃ / 80h Hardness at point A on the roller body surface 94.8HSD 94.3HSD 95.0 HSD 94.9 HSD 94.6 HSD Hardness of point B on the roller body surface 95.0 HSD 94.5HSD 95.1HSD 95.2HSD 94.7HSD Hardness at point C on the roller body surface 95.2HSD 94.5HSD 95.3HSD 95.2HSD 94.9 HSD Hardness at point D on the roller body surface 94.1HSD 94.2HSD 95.4HSD 95.0 HSD 94.8HSD Hardness at point E on the roller body surface 94.9 HSD 94.0 HSD 95.1HSD 94.7HSD 94.3HSD Hardness uniformity 0.4HSD 0.5HSD 0.4HSD 0.5HSD 0.6HSD Roller surface roughness Ra 0.38～0.39 0.37～0.38 0.39～0.40 0.38～0.39 0.37～0.38

[0050] (Examples 2-3)

[0051] The manufacturing methods of the low-wavy, high-quality automotive sheet rolls in Examples 2 and 3 are the same as those in Example 1, except that the chemical composition and weight percentage are as shown in Table 1.

[0052] (Examples 4-5)

[0053] The chemical composition and weight percentage of the low-wavy, high-quality automotive sheet rolls in Examples 4 and 5 are the same as those in Example 1, and the manufacturing methods are basically the same as those in Example 1. The difference lies in the final heat treatment process parameters, which are shown in Table 1.

[0054] The surface hardness and surface roughness of the low-wavy, high-quality automotive sheet rolls prepared in Examples 2 to 5 were tested, and the results are shown in Table 1.

Claims

1. A method for manufacturing a low-wavy, high-quality automotive sheet special roll, comprising smelting an electroslag ingot and a forged roll blank according to chemical composition and weight percentage, pre-heat treatment, and final heat treatment; wherein the final heat treatment includes preheating treatment, surface quenching treatment, cold treatment, and tempering treatment; characterized in that: The chemical composition and weight percentages are as follows: carbon 0.75-0.85%, silicon 0.20-0.35%, manganese 0.30-0.50%, phosphorus ≤0.015%, sulfur ≤0.010%, chromium 4.60-5.00%, nickel ≤0.25%, molybdenum 0.15-0.30%, vanadium ≤0.02%, copper ≤0.25%, [H] ≤2ppm, [O] ≤25ppm, [N] ≤120ppm, the remainder being iron and unavoidable impurities; The electroslag ingot smelting process includes electroslag remelting; the electroslag remelting uses a single consumable electrode rod, without replacing the electrode during the process, and is formed in one electroslag remelting step; the slag system used in the electroslag remelting and its weight percentages are: calcium fluoride 40-50%, calcium oxide 15-35%, and alumina 25-40%; the electroslag remelting uses a low-voltage, high-current remelting process, where the low voltage is 55-65V and the high current is 18000-23000A; during the electroslag remelting process, the voltage fluctuation value is ≤3V and the current fluctuation value is ≤800A; The surface hardening treatment adopts a mobile dual-frequency induction hardening treatment, and the specific method is as follows: the roll is placed vertically and rotated in the circumferential direction at a speed of 25-40 r / min; at the same time, it moves downward at a uniform speed of 20-25 mm / min; it is heated by the upper induction coil, heated by the lower induction coil, and cooled by the water spray ring in sequence; after the hardening is completed, the roll is quickly immersed in the underground circulating deep water pool for continued cooling for 1.5-3 hours.

2. The manufacturing method of the low-wavy, high-quality automotive sheet special roll according to claim 1, characterized in that: The surface quenching temperature is 930-1000℃, and the specific heating process is as follows: first, rapidly heat up to 930-955℃ → hold at that temperature → slightly decrease the temperature by 15-30℃ → then heat up to 980-1000℃ → hold at that temperature again → slightly decrease the temperature by 5-15℃ → water spray quenching.

3. The manufacturing method of the low-wavy, high-quality automotive sheet special roll according to claim 1, characterized in that: The distance between the two induction coils is 100-200mm, the frequencies of the two induction coils are 45-55Hz and 250-380Hz respectively, the height of the two induction coils is 100-200mm, and the water pressure of the spray ring is 0.15-0.25MPa.

4. The manufacturing method of the low-wavy, high-quality automotive sheet special roll according to claim 1, characterized in that: The preheating treatment temperature is 250–350℃, and the time is 15–30 h; the cold treatment temperature is -80–-60℃, and the time is 3–6 h; the tempering treatment temperature is 120–160℃, and the time is 60–100 h.