Research On Effect Of Variable Gauge Rolling Process On Al-Si Coating Of Hot Stamping Steel

With the increasingly serious environmental problems,Lightweight of vehicle body has become an important theme.TRB is one of the most important ways to reduce body weight.The TRB has the following advantages:thickness can be controlled and optimized,and thickness distribution can be designed according to part requirements.It is very convenient to produce multi section thickness sheet,to minimize the weight of the parts or to reduce the reinforcement parts.The Al-Si coating is a highly effective protective material and is widely used in the protection of hot stamping steel.This experiment was based on GMW14400-2014 and TL4225-EN-2016 to study the performance of Al-Si coating of TRB,and studied the performance change of the coating in the process of variable gauge rolling,evaluated the product standard and gave guidance to factory production.(1)The basic characteristics of the coating morphology,thickness and weight of the samples with different reducing rate were investigated experimentally.The research showed that with increases of reduction rate,the thickness of the coating decreased linearly,coating showed a tearing tendency when the rate of reduction was larger(?40%),but continuity remained intact,and there was no fault cracking phenomenon;The three points and single point test results conformed to the product standard.It showed that TRB within the range of the experimental process met the production requirements.(2)The experiment followed the standard JIS Z23 71:2000.The change of macro morphology and transformation of microstructure during the corrosion process were analyzed.The evolution process of the corrosion process was analyzed microscopically.The 30h corrosion grades of plates with different reduction rates were rated,indicating that the increase of reduction rate would reduce the protective performance of Al-Si coatings.In 30 hours the corrosion test,the reduction rate was smaller(?20%),the corrosion grade was 9,the reduction rate was larger(?30%),the corrosion grade was 8.Corrosion began with the boundary of Fe rich phase and Si rich phase.(3)Thermal simulation experiment process based on the actual production of hot forming steel:The parts were heated to 920? at 10?/s,and cooled to 800? at 10?/s after heat preservation 5min.The changes of coating properties during hot stamping process were evaluated isothermal tensile tests with different applied tensile strains(2%,4%,6%,8%).The experimental content was mainly for the change of the coating morphology,thickness and roughness after the thermal simulation,as well as the change of the phase composition of the coating.The influence of the reduction rate and the tensile deformation on the crack was analyzed,and the law of crack initiation and evolution during the process of tensile deformation was deduced.The research showed that the coating thickness accorded with the range of 15?m?50?m met the product standard.The line roughness accorded with the product standard of Ra?1.8 after thermal simulation.Crack generation in hot stamping process:micro cracks formed along the A path,and the larger tensile strength led to the stress concentration of cracks and the substrate.The crack met with the substrate when the tensile strength was larger.When the stress concentration was less than the strength of substrate,the crack began to expand along the B path,which led to the tearing and stripping of the coating.(4)The adhesion of the coating was quantified by determination of tensile test.The adhesive force of the experiment was greater than the standard of 16MPa.The tape adhesion test for paint finishes,results showed that the adhesion level is the 0.