Study On The Mechanism Of Strip Breaking And It's Control Method Of High Strength Thermal Forming Steel Used For Automobile During Cold Rolling

With the improvement of people's requirements for environmental protection and automobile quality,the application of ultra-high strength hot-formed steel has emerged.At present,the strength of hot-formed steel in China has exceeded 2GPa.In the process of mass production of 2GPa ultra-high strength hot forming steel,continuous cold rolling process needs to be carried out.However,the strip breaking problem of PHS2000 usually occurs during its continuous cold rolling after laser welding,which has a great impact on the stable production of the steel mill and also causes huge economic losses.Aiming at the rolling strip breaking problem of 2GPa ultra-high strength hot forming steel in the production process,the mechanism and control strategy of weld strip breaking problem of 2GPa hot forming steel has been studied in this thesis.The theoretical basis for the assurance of the production process has been proposed in this thesis.It provides theoretical basis for the production of ultra-high strength steel used for automobile.The conclusions are as follows:(1)The weld zone microstructure of PHS1500 is lath martensite,and the microstructure of the mixed grain zone is composed of ferrite and block martensite.The microstructure of PHS2000 weld zone is twin martensite,and the microstructure of mixed grain zone is mainly composed of ferrite and martensite.The microhardness and tensile strength of PHS2000 welded joint are much higher than that of PHS1500,but the elongation of PHS2000 weld joint is lower than that of PHS1500.When the rolling reduction ratio reaches 70%,PHS1500 welding joint has no crack occurs,PHS2000 welding joint crack originated from the mixed grain zone,propagate through the whole weld zone.(2)The microstructure of the welded joint can be effectively improved by the tempering heat treatment in the furnace after welding.When the rolling reduction ratio reaches 70%,the welded joint has no crack occurs,which provides a theoretical basis for solving the problem of strip breaking during the cold rolling.The microstructure and properties of the weld joint were improved and the problem of strip breaking of PHS2000 welded joint during the cold rolling was solved by additional pre-welding and post-welding heat treatment in the production site.(3)The carbon content of the weld zone can be effectively reduced,the weld microstructure can be improved,and the plasticity of the weld zone can be increased under the condition of compressed air and weld with disimillar material.By adopting high heat input,low power and low welding speed,the microhardness of the weld zone and the coarse grain heat-affected zone can be reduced and the plasticity of the weld zone can be improved,and the crack propagation can be prevented.These technical measures can be used to solve the strip breaking problem of PHS2000 welding joint during its cold rolling.