Study On Process And Forming Parts Properties Of A Hot Stamping Steel

High strength steel and ultra-high strength steel is widely applied in the automotive industry with its light weight and high strength.However the cold forming of high strength steel is very difficult,and the traditional cold stamping method is only used to produce parts that is simple shaped.The technology of hot stamping can solve the contradiction between the high strength and good forming ability.During the hot stamping process,the plate forming in the condition of high temperature firstly,then hardening.It can be used to produce parts with complex shape.The main principle is that the sheet preheated above to the austenitizing temperature,holding this temperature for certain seconds,then transferred to the mold forming and rapid quenching.The parts with high strength and size precision,Hot stamping is a new technology to produce ultra-high strength steel specially.Nowadays,between the thickness of 1.2mm to 1.8mm,the original sheet of hot stamping steel mostly is cool-rolled annealed sheet,the application of hot-rolled pickling steel is very few.In this paper,the process of hot-rolled pickling sheet and cold-rolled annealed sheet of hot stamping steel were studied.On the basis of that,we designed different process parameters and carried out the test stamping of three automotive parts.We studied the influences of different hot stamping process on micro structure and properties of hot stamping steel.What's more,we analyzed the bending properties of hot-rolled pickling sheet and cold-rolled annealing sheet.The main research results are shown as follows:(1)The static continuous cooling transformation of two types of hot stamping steel are studied.The micro structure and properties of hot-rolled pickling plate is consistent with the cold-rolled annealing plate:the temperature of Ac3 is about 830?,the critical cooling rate of whole martensite transformation is 25?/s.To avoid the ferrite precipitation,the time interval between sheet out of furnace and being hot-formed should be as short as possible.At the same time,the cooling and quenching rate must be fast enough in practical hot stamping process to ensure the final micro structure is whole martensite.(2)The forming parts of hot-rolled pickling sheet have the similar micro structure and properties with the cold-rolled annealing sheet:the microstructure of forming parts is full martensite and the grain size is in the same level,the tensile strength is about 1500MPa with its elongation near 7%.(3)The micro structure and properties hot stamped forming parts of 1.2mm cold-rolled annealing sheet is consistent with the specification of 1.7mm sheet:the micro structure is whole martensite,the grain size is close,the tensile strength is 1500MPa and the elongation is 6.5%.(4)The three-point bending properties:the bending properties in transverse direction slightly better than the rolling direction(about 10 degrees).The bending degrees of inner panel are greater than 60° and the bending degrees of A-pillar are greater than 80°.The existence of decarburized layer is good for the forming ability.The smaller the grain size,the better the forming properties of the material.In practical hot stamping process,under the condition of full austenite,the heating temperature should not be too high to avoid coarse grain.