超声波探伤不合格T91钢棒材的分析和工艺改进

试验T91钢(/%:0.10C,0.30Si,0.45Mn,0.012P,0.005S,8.90Cr,0.95Mo,0.08Nb,0.22V)的生产流程为60t UHP EAF-AOD-LF-VD-240 mm×240 mm坯连铸-加热-连轧。对超声波探伤不合格的T91钢Φ90 mm材低倍检验结果表明,钢材存在中心裂纹和孔洞;通过金相分析得出,缺陷出现三种特征:(1)过烧型孔洞和裂纹;(2)连铸坯带来的轴心晶间裂纹;(3)铸坯的缩孔。通过连铸Ⅰ、Ⅱ和Ⅲ区二冷水量分别由原28、31和17 L/min优化成15、18和10 L/min,并通过控制连铸坯加热温度,使连铸坯中心的等轴晶比率由原14%提高至25%以上,铸坯的中心裂纹级别由2.0降至0.5以下,T91钢材的超声波探伤不合格率由5%降至0.2%以下

An Analysis on Unqualified T91 Steel Bar by Ultrasonic Flaw Detection and Process Improvement

The production flowsheet of test steel T91 (/% :0.10C,0.30Si,0.45Mn,0.012P,0.005S, 8.90Cr, 0.95Mo ,0.08Nb ,0.22V) is 60 t UHP EAF-AOD-LF-VD-240 mm x 240 mm bloom casting- heating and continuous rolling. The examination results of macrostructure of unqualified steel T91 Φ90 mm products by ultrasonic flaw detection show that there are center cracks and hole in bar,by metallographic analysis it is obtained that the defects have three characteristics： (1) overheating hole and cracks； (2) axial intercrystalline cracks its source at casting bloom and (3) shrinkage cavity. With the technology measures including optimizing the casting I、U and HI zone secondary cooling water amount respectively from original 28、31 and 17 L/min to 15、18 and 10 L/min,and controlling the casting bloom heating temperature ,the equiaxial crystal ratio in casting bloom center increases from original 14% to more than 25% ,the rating of central cracks decreases from original 2.0 to less than 0. 5 ,and the unqualified ratio of steel T91 products by ultrasonic flaw detection decreases from original 5% to less than 0.2%.