Effect of Hot-dip Galvanizing Process on the Microstructure and Mechanical Properties of a 600 MPa Hot-dip Galvanized Dual Phase Steel

摘要

In the paper,a 600 MPa dual phase steel was soaked at 800 ℃ for 90 s,either rapidly cooled to 450 ℃ and held for 30 s (process A) or rapidly cooled to 350 ℃ and then reheated to 450 ℃ and held for 30 s (process B) to simulate the hot-dip galvanizing process.The influence of hot-dip galvanizing process on the microstructure and mechanical properties of the 600 MPa hot-dip galvanized dual phase steel was investigated by using the microscope,SEM,tensile test and so on.The results showed that,for process A,the microstructure of the hot-dip galvanized dual phase steel was composed of ferrite,martensite and a small amount of bainite.The granular bainite was formed in the hot-dip galvanizing stage,and the martensite island was formed in the final cooling stage after hot-dip galvanizing.While for process B,the microstructure of the hot-dip galvanized dual phase steel was composed of ferrite,martensite,bainite and cementite.The formation of cementite resulted from the decomposition of martensite during hot-dip galvanizing at 450 ℃.In addition,compared with the process A,the yield strength of the hot-dip galvanized dual phase steel for process B was increased by about 50 MPa due to the tempering of martensite by which the martensite was decomposed into ferrite and carbides,and the mobile dislocations located in the ferrite disappeared.The work hardening coefficient (n value) of dual phase steel for process B was decreased obviously because the increment of yield strength affected the computation result of n value.However,the ultimate tensile strength and elongation of dual phase steel for process B showed little change compared with the process A.Therefore,the hot-dip galvanized dual phase steel with excellent comprehensive mechanical properties (YS=362 MPa,UTS=638 MPa,A80=24.3％,n(10-Ag％)=0.17) was obtained for process A.