Causes And Control On Transverse Corner Cracks Of Microalloying Wide And Heavy Plate

During the manufacturing process of wide and heavy plate at some steel mill, four types of steel appear different degree surface defects (transverse corner crack and surface transverse crack) which seriously affected pass percent of steels. In this paper, the metallographic analyzing and secondary electron microscopy technology were used to make an in-depth study about cause mechanism of surface defects especially transverse corner cracks of microalloying wide and heavy plate. Meanwhile, through studied about the properties of mold flux, the mechanical properties of steel at high temperature and cold state characteristic of secondary cooling spray nozzles, specific measures to control surface defects were put forward and the cracking rate of various types was dramatically reduced. The major contents and conclusions are summarized as follows:(1) Gleeble3800 thermal simulated test machine was used to test the mechanical properties of steel at high temperature of four types of steel. The third steel high temperature brittleness zone of each steel was determined according to the stangard of percent reduction of area less than 60%. The third brittle zone of each steel are 650 between 875?? 600 between 900??600 between 950? and 600 between 850?. In order to ensure slabs surface temperature keep away from the third steel high temperature brittleness zone, secondary cooling system should be strictly controlled while casting these steels.(2) Metallographic analyzing and secondary electron microscopy technology used to analyse transverse corner cracks of LGNS1 steel. Through metallographic analyzing, we found the crack morphology irregular and found copper element in crack. Therefore, we can conclude that crack is caused by copper brittleness phenomenon. Meanwhile, Ferrite widmanstatten structure, which usually occurs on the condition of rapid cooling intensity, was found in slab corner substrate. It is mean that Slab corner cooling intensity is too large to slab surface temperature enter into the third steel high temperature brittleness zone when slab access straightening segment. As a consequence, cracks appeared in steel corner.(3) Through the cold state characteristics of spray nozzles testing we obtained flux characteristic, water flow density and spraying angle of spray nozzles. Meanwhile, through assessing secondary cooling nozzle layout we found that corner flow density too large to slab corner temperature lower than 900? when producted 1490mm×230mm?1570mm×230mm and 1520mm × 230mm cross-sections. Conversely, when producted 1270mm×230mm? 1890mm×230mm cross-sections, the corner flow density is less so that effectively avoid the third steel high temperature brittleness zone. Thus, it turned out than the flow density of slab corner in secondary cooling zone had important influence on cracks rate.(4) According to the value of Ferrite Potential, fine-tuning composition of steel to reduced cracks rate of steels which appeared surface defects. Meanwhile, in order to keep away from the third steel high temperature brittleness zone when slab enter into straightening segment, properties of mold flux was optimized and secondary cooling system was adjusted.