汽车用IF钢连铸坯第Ⅲ脆性区及其形成机理分析

The Third Brittle Region and Its Mechanism Analysis of IF Steel Slab Used in Automobile

探明连铸坯第Ⅲ脆性区及其形成机理对避免铸坯裂纹的产生、有效调控铸坯性能具有重要指导作用。采用Gleeble热/力模拟试验机对汽车用IF钢铸坯的高温力学性能进行了试验研究,结合热膨胀测试及组织分析手段,探究了IF钢铸坯第Ⅲ脆性区的形成机理。结果表明:IF钢的高温热塑性较优异,第Ⅲ脆性区温度范围为850~950℃,二冷低延性区为875~925℃,1250℃以上抗拉强度和屈服强度分别低于30 MPa和20 MPa,从变形和受力两个角度综合考虑,矫直区温度应控制在950~1250℃。第二相含量在950℃左右达到最高,大量微米级的TiN和少量TiN与MnS复合第二相会导致热塑性下降。950℃以下大量微米级第二相、A_(e3)温度附近形变诱导铁素体以及A_(ff)~A_(r3)温度范围内奥氏体分解的共同作用是导致IF钢连铸坯第Ⅲ脆性区形成的根本原因,不同冷速下IF钢铸坯二冷低延性区的温度范围与T_(α)^(40%)(C_(R))~A_(e3)基本一致。

Exploring the third brittle region of continuous casting slab and its mechanism plays an important guiding role in avoiding slab cracks and effectively regulating slab properties.The high-temperature mechanical properties of interstitial-free steel(IF)slab for automobiles have been studied by using Gleeble thermal/mechanical simulation testing machine.Combined with thermal expansion test and microstructure analysis,the formation mechanism of the third brittle region of IF steel slab has been explored.The results show that the IF steel has excellent thermoplasticity,the temperature range of the third brittle region is 850—950℃,the second cooling low ductility zone is 875—925℃,and the tensile strength and yield strength above 1250℃are lower than 30 MPa and 20 MPa,respectively.From the perspective of deformation and stress,the temperature of the straightening zone should be controlled between 950 and 1250℃.The content of the second phase reaches the highest level around 950℃,which is mainly composed of a large amount of micron-sized TiN and a small amount of composite second phases of TiN and MnS,which lead to a decrease in thermoplasticity.This combined effect,which a large number of micron-sized second phases,deformation-induced ferrite near the A_(e3)temperature,and austenite decomposition in the A ff—A_(r3)temperature range,is the root cause of the third brittle region of the IF steel slab.The IF steel second cooling low ductility zones under different cooling rates can be calculated according to T_(α)^(40%)(C_(R))—A_(e3).