Study On Longitudinal Cracks Of SPHC Hot Rolling Strip In CSP Plant

Using metallographic analysizing、SEM、electronic probe、electrolysising samples、ghost rolling and high temperature tensile means to study the SPHC hot-rolled strip steellongitudinal cracks on surface evolution and purity,the results show that:(1) The surface longitudinal cracks of the SPHC hot-rolled strip steel are shallow andwithin0.2mm, it is mainly formed in the subcutaneous slag exposing in the process ofrolling.(2) The total oxygen of the casting blank of SPHC steel is28～33×10-6,nitrogencontent is30～43×10-6,the purity in CSP plants of WISCO can meet the requirement.Reducing the nitrogen content can improve the rod cracks.From tundish to casting,thenumber of microscopic inclusions is reduced,but the large inclusions is increased,and thephenomenon of slag rolling up exists.(3) When the temperature is950℃~1250℃,the Z of SPHC steel is above60%;When the temperature is down from1000℃to800℃, the Z of SPHC steel is reduced;When the temperature is800℃, the Z of SPHC steel is the minimum（43.85%）。Collecting data in field,and analysising the affect of the process parameters of thecontinuous casting、the composition of molten steel,hot rolling process parameters,andsome effective control measures are put forward:(1) the composition of molten steel: Mn/S should be greater than84,the S、P contentare in the control limits, the Mn content is in the celling,these can increase ratio of Mn/S(2) the process parameters of the continuous casting:the degree of superheat intundish should be controlled in15~30℃,wide and narrow surface flow should becontrolled in2.2~2.5MW/m2and1.7MW/m2,casting speed should be4.5~5.0m/min.(3) hot rolling process:following the law,high heating temperature, high openingrolling temperature, high finishing rolling temperature and low coiling temperature,fourparameters should be in control:＞1140℃，＞1050℃、860~880℃and610~620℃,and theeffect of cracks controlling is good.