低碳铝镇静钢热镀锌表面粗晶缺陷形成原因及对策

低碳铝镇静钢热镀锌板表面易产生"指甲印"缺陷,缺陷主要分布在整卷带钢带中1/3长度位置、带钢表面中间位置。通过金相观察可以确认缺陷处因晶粒明显长大,形成粗晶,晶界处开裂表现为"指甲印"缺陷。低碳铝镇静钢在热轧工序生产时的卷取温度为730℃,这个温度恰好处于A_1温度以上,卷取后带钢仍处在奥氏体和铁素体两相区。在卷取后,带中1/3位置散热较慢,温度保持在A_1线以上,奥氏体晶粒在此温度下继续长大,形成粗大晶粒。在冷轧工序的生产过程中,带钢表面受压应力和切应力作用,粗大的晶粒沿边界开裂,表现为"指甲印"缺陷。通过降低热轧卷取温度,使带钢卷取后的温度处于A_1线以下,带钢组织由铁素体和渗碳体组成,使带钢晶粒在卷取后冷却的过程中不再长大形成粗晶,消除了低碳铝镇静钢热镀锌带钢表面"指甲印"缺陷。

Analysis and countermeasure of coarse grain defect on surface of low- carbon aluminium killed steel

On the galvanized sheet surface of low- carbon aluminium killed steel,the ’nails’ defect was easily generated,and it was distributed in the middle position of length and width.By observing the microstructure,it can be confirmed that the grain under the ’nails’ growed bigger than others,and the defect was cracks located on coarse grain boundaries.During hot rolling process,low- carbon aluminium killed steel ends at coiling temperature 730 ℃,which still beyond A1 temperature that the steel is still in α- γ phase area.After coiling,the strip in 1/3 middle position cooling slowly,the austenitic grains grow up to coarse grains.In the process of cold rolling,the coarse grain boundaries crack under compressive stress and shear stress,appearing like ’nails’.Through lowering the coiling temperature under A1 temperature,composed of ferrite and cementite,the strip grain could not be growed any more during the cooling process after coiling,and the ’nails’ defects was eliminated.