Analysis And Countermeasures On Technological Problem Of High-quality Cold-rolling Product Production

Cold-rolling steel sheets were widely applied to every field of modern economic living. Their utilization ratio in the total consumption of steels was being increased, and their effect station was being more important with development of modern economy. How to improve the surface quality and the corrosion resistance of cold-rolling steel sheets were long-standing problems for steel makers. If those problems cannot be resolved better, the steel makers could face momentous economic loss and negative influence, and competitive ability and market share would decline. It has an important practical significance and economic benefit for increasing the recognition and capacity of competiion in market of products of the steel maker to analyze, improve, optimize the current production technologies or processes, and solve the problems about steel sheet quality and performance caused by technological bugs as well as produce high-quality cold rolled products.In this paper, various factors to affect the quality of cold rolled steel sheets was analyzed based on considering on-the-spot production condition of cold rolled steel sheets in Han Steel Company and from the perspective of cold rolling process. The feasibility measures to accord with spot practice were lodged in allusion to some spot practical problems. The technology which can substitute silane used to passivation of cold rolled steel sheet surface from chromate was explored. Multiplex film of silance with exelent performance was obtained to solve the harm problem from chromate and extend the product types of maker as well as satisfy the requests of different users through strengthening the performance of silance film by flozirconate. The surface micro-morphology and roughness, composition and distribution as well as chemical combinding state of element were systematically investigated by metallographic microscopy, Scanning Electron Microscopy with energy dispersive Spectroscopy (SEM/EDAX), Laser Scanning Confocal Microscopy (LSCM), Flourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS). The quantity of iron remained on the surface of cold rolled steel sheet was measured by spectrophotometer. The formability of cold rolled steel sheet was measured by metallic sheet forming machine. The corrosion resistibility and so on of cold rolled steel sheet were explored by the corrosion test methods such as whether exposure test, hydrothermal accelerating test, neutral salt spray test and electrochemical test and so onThe following main conclusions were obtained from the above analysis methods and the compositive consideration of spod conditions. For hot rolled steel coil during acid picking process, adding proper acid inhibitor to picking solution could observably improve the quality of steel sheet surface after pickled, reduce over-picking and iron loss and offer excellent substrate for downstream processing. During the cold rolling process, grains on the surface of steel sheets were elongated along rolling direction and broken out. The grain boundaries became blurry and grains on driving side and operating side were thinner and longer than those on middle position. For the grains after annealed, the morphologies on different position was well-defined and have very small difference, the sizes became larger, theirselves had no direction, grain boundaries were clear. The grains after temper rolled were coarser than those after cold rolled, had no direction and the morphologies on different position were accordant. No obvious change of grains on surface of specimens and on different position after finely rolled happened comparing with those after temper rolled. The electrochemical corrosion resistance of steel sheets after cold rolled was best, those of steel sheets after annealled declined, and those of steel sheets after temper rolled continue to decline, however that of steel sheets after finely rolled was similar to that after cold rolled. Annealing method affected the formability of steel sheets. The cupping value of cold rolled steel sheets after bell annealed related nearly to quantity and size of cementites in the microstructure. The volume fraction of the second phase and the average grain size of ferrites affected the ring-rolling ratio of cold rolled sheets. The conical cup value of cold rolled sheets was determined by the equiaxity degree of grains.The analysis on microstructues and morphologies showed that the defect such as oxidized color brought during bell annealing happened at near surface and didn’t affect the grain structure within steel sheets. The analysis from XPS showed that the defect such as oxidized color occurred because oxidation reaction between Fe and C, O during bell annealing process. The iron quantity on the surface of oxidation position was less than that on the surface of normal position. The valence state of iron on the surface of oxidation position was mainly Fe2+and Fe3+, however, that on the surface of normal position was mainly Fe2+. Standardization of equipment operation and strict control of process parameters could prevent or reduce the occurrance of the defects such as oxidized color.The quantity of total residue and iron residue on the surface of steel sheets were affected by the. technological specialty of each process. The quantity of total residue and iron residue on the surface of cold rolled steel sheets were more than those after the surfaces of annealed, temper rolled and finely rolled steel sheets. The quantity of total residue and iron residue on steel sheets after annealled were lest on steel sheets treated with the four processes. The quantity of total residue and iron residue on middle position was lest comparing with that on driving position and operating position for steel sheets produced by same process.The results of weather exposure test showed that the resistance to atmospheric corrosion for the steel sheets after cold rolled and finely rolled are best, however that for the steel sheets after annealed and finely rolled are worst. The quantity of total residue on surface of steel sheets affected severely the resistance to atmospheric corrosion in weather exposure test. More the quantity of total residue was, worser the resistance to corrosion was.The results of electrochemical analysis showed that the anti-corrosion of steel sheets was mainly affected by their surface state such as microstructure and clearness and so on. The resistance to electrochemical corrosion for cold rolled steel sheets was better than that for annealed, temper rolled and finely rolled. The quantity of total residue obviously affected the uniformity of resistance to electrochemical corrosion. For the same process, the resistance to electrochemical corrosion on operating position was better than that on driving and middle positions.The results of hydrothermal test showed that the resistance of cold rolled steel sheets to corrosion was best and that of annealed steel sheets was worst among the four types of steel sheets cold rolled, annealed, temper rolled and finely rolled.The passivation technology from silane could form a chromate-free passive film with excellent properties on electr-galvanized steel sheets. The silane composite film was shown to have outstanding anticorrosion and adhesional force. The heat-resistance and alkali-resistance of silane composite film were better than that of domestic commercial silane film, but its fingerprint-resistantance should be further improved. The silane composite film technology is an exellent green treatment tecknology as the substitution of chromate passivation film, and exhibited a promising application.