HRB400热轧带肋钢筋冷弯断裂原因的分析

国内西北某钢厂生产的HRB400热轧带肋钢筋在冬天作业时常常出现冷弯断裂。通过扫描电镜-能谱仪(SEM-EDS)、Thermo-Calc热力学软件以及光学显微镜(OM)等,对HRB400螺纹钢冷弯断裂的原因进行了研究,论述了非金属夹杂物、磷含量、钒氮比及显微组织对钢筋断裂的影响。结果表明:试验钢断口为脆性断口;破碎状的大尺寸夹杂物可能是裂纹产生的源头;钢中P含量偏高使得韧-脆转变温度升高,且自由氮的存在加剧了该温度的进一步升高,由于西北地区冬天气温低,钢筋作业时的温度很可能低于韧-脆转变温度,使钢筋处于脆性状态;魏氏组织的存在增加了钢筋的脆性,为裂纹的扩展提供了可能性。由于国内西北、东北地区温度较低,对在西北、东北地区使用的HRB400热轧带肋钢筋应提出更高的要求,对P、S、O、N的含量应有新的标准,以满足冬天作业时使用温度高于韧-脆转变温度的需要。

Analysis on cause of cold bending fracture of HRB400 hot-rolled ribbed bar

In view of the fact that the HRB400 hot-rolled ribbed steel bars produced by steel mills often break in cold bend operations during winter, the cause of cold bending fracture of HRB400 hot-rolled ribbed bar was analyzed by means of scanning electron microscope equipped with energy dispersive spectrometer (SEM-EDS),Thermo-Calc thermodynamic software and optical microscope (OM). The effects of non-metallic inclusion, P content, V/N ratio and metallographic microstructure on fracture of the steel were discussed. The results show that the fracture of the tested steel belongs to brittle fracture, and the cracked large size inclusion is the source of the cracks. The high content of P in the steel increases the ductile-brittle transition temperature and the existence of free nitrogen intensifies the temperature further. Due to the low temperature in winter in northwest China, the working temperature of the steel is likely to be lower than the ductile-brittle transition temperature, which makes the reinforcement in a brittle state. The existence of widmanstatten microstructure increases the brittleness of the steel and provides the possibility for crack propagation. Due to the low temperature in northwest and northeast China, higher requirements should be put forward for the HRB400 hot-rolled ribbed bar used in northwest and northeast China, and new standards should be set for P, S, O and N contents in order to meet the needs of working and using temperature higher than ductile-brittle transformation temperature in winter.