LNG储罐用9Ni钢夹杂物控制生产实践

Production practice of inclusion control in 9Ni steel for LNG storage tank

9Ni钢中厚板06Ni9DR在生产过程中发生较多批次超声波探伤不合格现象,采用金相显微镜和扫描电镜对缺陷部分试样检验分析,结果表明:造成探伤不合格的主要原因是钢板中存在大量的以CaO-Al_(2)O_(3)为主的长度为61~167μm的夹杂物,并伴有少量MgO和硫化物。由于LF脱氧时加入的Al不足,在精炼过程中Al烧损导致Al含量的进一步降低,在LF处理后期或RH处理过程中补Al,从而产生大量的此类夹杂物。通过控制转炉终点碳和氧含量、延长RH真空处理时间和软吹时间、保证足够的钢液镇静时间等措施,钢板中的夹杂物数量显著减少,超声波探伤不合格比例由8.11%降低到0.2%。

During the production of 9Ni steel medium-thick plates 06Ni9DR,many batches were unqualified in ultrasonic flaw detection.The defective parts of the samples were inspected and analyzed using the metallographic microscope and scanning electron microscope.The results showed that there were a large number of CaO-Al_(2)O_(3) inclusions(in length of 61-167μm)accompanied by a small amount of MgO and sulfide in the steel,which was main reason for the unqualified flaw detection.Due to insufficient addition of aluminum in LF deoxidation,the burning loss of aluminum during refining led to further reduction of aluminum content.Aluminum was added in the later stage of LF treatment or in the process of RH treatment,thus a large number of such inclusions were generated.After taking some measures such as controlling the end-point carbon and oxygen content of the converter,extending the RH vacuum treatment time and soft blowing time,and ensuring sufficient liquid steel sedation time,the quantity of inclusions in the steel plate had been significantly reduced.Meanwhile,the proportion of unqualified ultrasonic flaw detection decreased from 8.11%to 0.2%.