微量稀土元素对冷成型超高强度汽车钢组织性能的影响

Effects of Trace Rare Earth Elements on Microstructure and Properties of Cold Formed Ultra High Strength Automobile Steel

随着新能源汽车产销量的快速崛起,超高强度汽车钢的需求量越来越大。而超高强度汽车钢冷成型过程中的回弹和变形开裂问题成为制约超高强度汽车钢应用的主要因素。因此,开发一种强度高且成型性能优良的汽车钢具有实际应用价值,市场潜力巨大。许多研究发现,在汽车钢中加入稀土元素可以提高其综合性能。文章设计了两组成分方案,采用相同的逆相变退火(ART)工艺参数,通过拉伸试验、扫描电镜以及XRD等手段研究了稀土元素对冷成型超高强度汽车钢的组织性能影响机制。结果表明,在ART退火工艺下,试验钢微观组织均为马氏体、残余奥氏体和微量铁素体,但两种成分方案中添加稀土试验钢的残余奥氏体含量略高于不含稀土试验钢,为后续成型过程中的TRIP效应提供了组织基础。在800℃淬火保温5 min,随后645℃退火保温15 min的最佳热处理工艺参数下,方案一中含稀土试验钢的残余奥氏体含量比不含稀土的提高了8.1%,断后伸长率提高了21.8%,强塑积提高了17.2%,达到了28.47 GPa·%;方案二中含稀土试验钢残余奥氏体含量比不含稀土的提高4%,断后伸长率提高了2.8%,强塑积提高了5.1%,达到了34.8 GPa·%,说明添加稀土元素可以不同程度地改善超高强度汽车钢的综合性能。

With the rapid rise of production and sales volume for new energy automobile,the quantity demanded of ultra high strength automobile steel is increasing.The problems such as spring back,deformation and cracking in cold forming process of ultra high strength automobile steel become the main factors restricting its applications.Therefore,it is of practical application values and huge market potential to develop a kind of automobile steel with high strength and excellent formability.It is found in many studies that the comprehensive properties of automobile steel could be improved by adding rare earth elements.In this paper,two groups of composition schemes are designed and the same process parameters of annealing of reverse transformation(ART)are used.The influencing mechanisms of rare earth elements on the microstructure and properties of cold formed ultra high strength automobile steel are studied by means of tensile test,scanning electron microscopy and X-ray diffraction(XRD).The results showed that the microstructure of experimental steel was martensite,retained austenite and trace ferrite under the ART process,but content of retained austenite of experimental steel with rare earth in the two composition schemes was slightly higher than that of experimental steel without rare earth,which provided the microstructure basis for transformation-induced plasticity(TRIP)effect in subsequent forming process.Under the optimum heat treatment process parameters of quenching at 800℃and holding for 5 min.,then annealing at 645℃and holding for 15 min.,the content of retained austenite in experimental steel with rare earth in the first scheme is increased by 8.1%compared with that of experimental steel without rare earth,percentage elongation after fracture is increased by 21.8%as well as product of strength and elongation(PSE)is increased by 17.2%and reaches 28.47 GPa·%;in the second scheme,the content of retained austenite in experimental steel with rare earth is increased by 4%compared with that of experimental steel without rare earth,percentage elongation after fracture is increased by 2.8%as well as PSE is increased by 5.1%and reaches 34.8 GPa·%,which illustrate that the comprehensive properties of ultra high strength automobile steel could be improved to different extent by adding rare earth elements.