退火工艺对780 MPa冷轧多相钢组织及性能影响

 为了研究退火工艺对780 MPa冷轧多相钢组织及性能影响,采用万能试验机、场发射扫描电镜(SEM)、综合成形试验机及电子探针(EPMA)对两种具有相同成分、抗拉强度均为780 MPa但具有不同屈强比和扩孔性能的多相钢进行了相关研究,结果表明热处理工艺对试验钢组织、力学性能及扩孔性能有着重要影响,采用两相区退火时马氏体岛呈现连续的和条带状分布特征,马氏体岛平均晶粒尺寸为3.7 μm,且碳和锰具有明显的聚集分布现象,而全奥氏体化退火时组织呈弥散细小分布,马氏体岛平均晶粒尺寸为1.4 μm,碳和锰元素没有明显富集,且具有更高的屈强比和扩孔率。另外还对扩孔失效机制及微观形貌进行了分析,组织中条带状马氏体岛促使裂纹沿软硬相界面快速扩展,而细小弥散分布的硬质相促使应变向多方向扩展,避免在软硬相界面处形成较大的应力集中而产生开裂,同时高扩孔率材料表现出具有更高比例的光亮带及大尺寸韧窝。

Effect of annealing process on microstructure and properties of 780 MPa cold rolled multiphase steel

In order to study the effect of annealing process on the microstructure and properties of 780 MPa cold rolled multiphase steel,the universal testing machine,Scanning electron microscope (SEM),comprehensive forming testing machine and Electron probe micro-analyzer(EPMA) were used to research two kinds of multiphase steel with the same composition and tensile strength of 780 MPa but with different yield ratio and hole expansion ratio. The results shown that the annealing process has an important influence on the microstructure,mechanical properties and hole expansion performance of the experimental steel. The martensite islands with obvious segregation distribution of carbon and manganese shown continuous and banded distribution under intercritical annealing,the average grain size of martensite islands is 3.7 μm. However,there is no obvious enrichment of carbon and manganese in the martensite islands which are dispersed and finely distributed in the microstructure during full austenitizing annealing,the average grain size of martensite islands is 1.4 μm,and it exhibited higher yield ratio and hole expansion ratio. Moreover,the failure mechanism and fracture morphology of the hole expansion were analyzed. The banded martensite promoting the crack to propagate rapidly along the interface between soft and hard phases. While the fine dispersed hard phase promoting the strain to expand in multi-direction therefore avoided cracking due to large stress concentration at the interface of soft and hard phases. Furthermore,the steel with higher hole expansion ratio shown a higher proportion of sheared area and large-scale dimples.