热浸镀锌高强汽车板界面组织对其拉伸断裂行为的影响

Effect of the Interface Microstructure of Hot-Dip Galvanizing High-Strength Automobile Steel on Its Tensile Fracture Behaviors

对3种成分的高强汽车板进行相同的热浸镀锌工艺处理,研究表面氧化物的热力学成因及其对界面层组织的影响,并对不同变形量条件下的拉伸断裂行为进行原位分析。结合微观组织分析与热力学计算可以看出,钢板成分的不同会造成其表面氧化物的形成差异。当Mn2SiO4与SiO2为热力学稳定相时,界面处难以形成连续的Fe2Al5Zn0.4抑制层,锌液与Fe基体反应生成ζ-FeZn13脆性化合物,在拉伸过程中界面易开裂且裂纹会向基体扩展,导致力学性能下降;当少量MnO与Mn2SiO4为热力学稳定相时,形成以Fe2Al5Zn0.4抑制层为主的界面组织,拉伸裂纹在界面产生,而后向Zn层扩展,钢板的拉伸断裂主要由基体的失效所致;而当钢板表面形成大量MnO与FeO亚稳相时,通过铝热还原反应形成的Fe会与锌液接触,使Zn层内部形成脆性Γ-Fe11Zn40相,拉伸裂纹极易在Zn层产生并扩展,而界面未还原的MnO层与基体结合强度较高,拉伸断裂由基体失效所致。

Hot-dip galvanizing is the most economical way to improve the corrosion resistance of advanced high strength automobile steel.With the high trend of developing of automobile steel towards light-weight and high-strength,the contents of Si,Mn,Al alloy elements in steel increase accordingly.These alloy elements would be selectively oxidized during hot-dip galvanizing process,which affect in turn the wettability of steel surface and form different interface microstructures.However,its effect on the mechanical behavior of steel has never been known as clear as desired.Base on this point,the thermodynamics of the surface oxide and its effect on the interface microstructure of three high-strength automobile steels were studied after the same hot-dip galvanizing treatment as well as their tensile fracture behavior under different deformation conditions was in situ analyzed.Combined with the microstructure analysis and thermodynamic calculation,it can be concluded that different compositions of steel wouldproduce different kinds of oxide on its surface.When Mn2 Si O4 and Si O2 were formed as thermodynamic stable phases,it was difficult to form a continuous Fe2 Al5 Zn0.4 inhibition layer at the interface,zinc liquid could penetrate into the iron substrate and then form the brittle phase ζ-Fe Zn13,where the crack was easily to be obtained and expanded to the substrate,resulting in the decrease of mechanical properties.When Mn O and Mn2 Si O4 with a small amount were formed as thermodynamic stable phase,the Fe2 Al5 Zn0.4 inhibition layer can be obtained.Under the tensile stress,this crack generated at the interface and extended to the zinc layer.So,the fracture of the experimental steel was mainly resulted from the failure of substrate.When Mn O and Fe O were formed as metastable phase,Fe,as formed by aluminothermic reduction during hot-dip galvanizing,would reacted with zinc liquid to form Γ-Fe11 Zn40 phase in the zinc layer.The crack generated under the tensile stress and expanded in the zinc layer.Since the unreduced Mn O layer at the interface exhibited a higher bonding strength with substrate,tensile fracture of the experimental steel was caused by the failure of substrate.