Formation Mechanism And Control Of Linear Defects On The Edge Of Hot Rolled Coil

The linear defect at the edge is one of the most common defects of hot-rolled and cold-rolled coils.It is related to the defects of the corner of the continuous casting billet,the rolling process and equipment,etc.which seriously affects the hot rolled coil or even cold rolled sheet rate.In order to ascertain its formation mechanism and optimize process control measures,this study took a domestic steel plant to produce hot-rolled plate steel Q355,Q235 with different thicknesses as the research object,through the production process investigation and sampling,using pickling,field emission scanning electron microscope,energy spectrometer,optical microscope and other detection methods are used to detect and analyze the defects of the corners of the cast slab and hot-rolled sheet.Thermodynamic calculation of the behavior of precipitates and analysis of related process factors,the formation mechanism and control measures of linear defects on the edge of hot rolled coil are proposed.The main conclusions obtained are as follows:(1)The pickling inspection of Q355 and Q235 billet corners and peeled skin 3mm showed that no obvious subcutaneous defects were found,but the quality of the corners and corners of Q355 and Q235 billets was significantly different.The vibration marks of Q355 billets were relatively shallow and evenly distributed,and no corner cracks were found,while the vibration marks of Q235 billets were deep and the spacing was different(10-22mm),and there were obvious transverse cracks at the sides and corners.(2)The detection of the subsurface inclusion characteristics of 3mm peeled corners of Q355 and Q235 billets found that the distribution of subcutaneous inclusions was relatively uniform,and no large inclusions were found.Among them,the subcutaneous inclusions in titanium-containing Q355 steel were mainly Ti(C,N),Al-Ca-Mg-O composite inclusions exist under the skin of non-aluminum deoxidized Q235 steel,and contain a small amount of MnS solid solution in the composite inclusions.(3)Comparative analysis of the composition of Q355 and Q235 steels,continuous casting process parameters shows that the content of S and N in Q235 steel is relatively high and the value of Mn / S is relatively small.Among them,the casting speed and large fluctuations in the liquid level of the instrument may be the key factors causing its deep vibration marks,uneven spacing and transverse cracks in the corners.(4)The thermodynamic calculation of the precipitates of Q355 and Q235 steelsshows that the theoretical precipitation temperature of TiN,TiC,Al N and Mn S in the molten steel under the existing composition conditions are all lower than the liquidus temperature of the respective steel grades and will not precipitate;During the solidification of molten steel,when the solid phase ratio exceeds 0.44,TiN is precipitated in the aluminum deoxidized titanium-containing Q355 steel,which can effectively reduce the precipitation temperature of Al N in the steel and reduce the risk of cracks caused by Al N grain boundary precipitation.Rather than aluminum deoxidization Q235,the TiN is in the steel precipitates at the end of solidification,and the solid phase rate at the time of precipitation is 0.92.However,TiC,AlN,and MnS in the two steels will not precipitate,and the effect of the precipitates on the corner cracks of the two billets is not significantly different.(5)The linear defect surface and cross-section of Q355 and Q235 steel hot-rolled coils contain Ca,Si and continuous casting slag composition of K,Na.The iron oxide /inclusion layer exists on both sides of the crack on the defect cross-section,and high-temperature oxidation characteristics such as thick oxide dot layer,decarburization and grain growth.Therefore,the continuous casting mould protection slag involved in the formation of slag/subcutaneous slab corner of defects such as inclusions is the leading cause of hot rolled plate line defect.In addition,the horizontal cracks in the corners of the Q235 continuous casting billet are also an important cause of the linear defects of its hot-rolled coil,while the decarburization on both sides of the linear defect cracks of the Q355 steel is slight,and its linear defect may also be related to folding during the steel rolling process.(6)The difference between the surface width of Q355 and Q235 linear defects,the depth of cross-sectional crack extension,and the inclination angle extending deep into the substrate are mainly related to the thickness of the rolled product.Thick plates are easy to form linear defects,while thin plates are easy to form peeling defects.In addition,the depth of the transverse cracks in the corners is also an important reason for the crack extension of the Q235 linear defect cross-section.(7)The key to reducing the occurrence of linear defects on the edge of the hot rolled coil is to increase the steady-state pouring level,appropriately increase the viscosity of the protective slag,optimize the nitrogen and sulfur control level of the molten steel component,and reduce the edge cooling strength of the secondary coolingsection and adopt measures such as chamfering crystallizer and optimiz the wear resistance and hole shape of the roughing vertical roll.