三次渗碳体和游离渗碳体对低碳钢弯曲性能的影响

Influence of Tertiary and Free Cementites on Bending Performance of Low-carbon Steels

生产和使用中发生了两起低碳钢产品弯曲开裂:SPHC钢异形门框弯曲开裂和Q195钢热镀锌C形钢弯曲开裂。对开裂的异形门框和C形钢进行了宏观检验、金相检验和断口分析,并对热镀锌在线退火工艺进行了模拟试验。结果表明:低碳钢中的三次渗碳体和游离渗碳体与基体之间易形成孔隙和微裂纹,降低钢的弯曲性能,在弯曲力的作用下钢发生开裂;热镀锌SPHC和Q195钢带中的渗碳体来源于热轧钢带的组织遗传;为控制三次渗碳体的数量,热轧低碳钢带的终轧温度应控制在860~880℃,卷取温度控制在580~610℃,卷取后钢带应避免缓冷。

Two cases of bending cracking of low carbon steel products occurring in production and service comprised the following:the bending cracking of specially shaped door frame of SPHC steel and the bending cracking of hot-dip galvanized C-shaped Q195 steel.The macro check,metallographic examination and fracture test were performed on the cracked specially shaped door frame and the cracked C-shaped steel,as well as the simulated test for the online annealing process of hot-dip galvanizing was carried out.The results showed that(a)pores and microcracks were easy to form between the tertiary and free cementites and the substrate in low-carbon steels,and,as a result,the steel decreased in bending performance and cracked under the action of bending force;(b)the cementites in the hot-dip galvanized SPHC and Q195 steel strips were derived from the structure inheritance of hot-rolled steel strips;and(c)to control amount of tertiary cementites,the finishing temperature of hot-rolled low-carbon steel strip should be controlled within a range of 860 to 880℃,the coiling temperature should be controlled within a range of 680 to 610℃,and the steel strip should be immune from slow cooling after coiling.