输电铁塔用SWRCH15A钢螺栓冷镦断头原因分析及改进工艺

Cause Analysis on Bolt Cold-Heading Breakage of SWRCH15A Steel for Transmission Tower and Improvement Process

采用化学成分分析、金相检验、硬度分析、断口形貌分析和氧氮含量检测等方法,分析了SWRCH15A钢螺栓断头的原因。结果表明,合格螺栓T[O]和[N]分别为0.002 0%%~0.002 1%和0.006 2%~0.006 4%,而断头螺栓T[O]达0.006 3%~0.0064%,[N]为0.010 3%~0.0108%,导致材料在冷变形过程中塑性降低,螺栓头部HRB硬度值超过100,易在应力集中的头部和杆部过渡区断裂,是螺栓断头的主要原因。通过控制精炼渣(FeO+MnO)≤1.0%,防止钢水吸氧氮和二次氧化,待浇时间由原来的15~20 min降至5~10 min、加砂方式由人工投掷改为导管罐装加砂、加强包底清理等措施,钢包自动开浇率由原来的85.7%提高至98.9%,未再出现类似质量问题。

The reasons for SWRCH15 A steel bolt breakage are analyzed by chemical composition analysis,macroscopic examination,hardness analysis,fracture morphology analysis and oxygen and nitrogen content detection.The results show that qualified bolt T[O] and [N] are respectively 0.002 0%~0.002 1 % and 0.006 2%~0.006 4%,but the breakage bolt T[O] and [N] are respectively up to 0.006 3%~0.006 4% and 0.010 3% ~0.010 8% that leads to a decrease in the plasticity of the material during cold deformation,the HRB hardness value of bolt head is more than 100,it is easy to break in the stress-concentrated head and shank transition area and is the main reason for the break.With controlling refining end slag(FeO+MnO)≤1.0%,avoiding liguid absorbing oxygen and nitrogen and secondary oxidizig,reducing the casting waiting time from 15~20 min to 5 ~10 min,changing the way of adding sand from manual throwing to tube canning sand adding and strengthening the cleaning of the bottom of ladle,the automatic pouring rate of ladle has been raised from 85.7% to 98.9%,and no similar quality problems have occurred again.