Effects Of Processing Technology On Microstructure And Mechanical Properties Of Austenitic Advanced High Strength Steels

With the increasing demand for safety and energy consumption, the automotive industry needs a kind of structural material with excellent comprehensive properties, which meets the requirements of lightweight, safety and energy saving at the same time. Austenitic stainless steel and twinning induced plasticity (TWIP) steel have been widely studied.In this work, the 301L stainless steel was treated by three kinds of treatment including cold rolling, warm rolling and annealing after cold rolling. The effects of cold rolling reduction, rolling temperature, annealing time and annealing temperature on the mechanical properties and microstructure of the 301L stainless steel were studied. The strengthening and toughening mechanisms of the materials in three kinds of treatment were analyzed and an optimization parameters were obtained. At the same time, the TWIP steel was treated by warm rolling and annealing treatment after cold rolling to discuss the effect of rolling reduction and annealing temperature on the microstructure and mechanical properties of TWIP steel, the strengthening and toughening mechanism was also discussed.There were high densities of martensite, dislocations and shear bands in 301L stainless steel after cold rolled. Their interactions could increase the yield strength, and decrease the plasticity. The warm rolling could also increase the yield strength, and decrease the plasticity by high density dislocations and shear bands without martensite in 301L stainless steel.In the process of annealing after cold rolling of 301L stainless steel, the yield strength increased with the annealing time increased until 24 h. The yield strength increased with the increase of annealing temperature until 450℃. The yield strength also increased with the increase of cold rolling reduction until 20%. The pinning effect on the dislocations by the interstitial atom was responsible on the increase of the yield strength of the material, while the martensitic transformation maintained the elongation of the material.Yield strength of 301L stainless steel annealed at 450℃ for 24 h after cold rolled 20% reached 1164 MPa while the elongation remained 28%. Compared to the samples with a 20% cold rolling reduction, the annealing produced an increase of yield strength as high as about 500 MPa without ductility delay. There were a large number of dislocations, shear bands and alpha martensite with precipitates in the annealed samples. The martensite could not grow up in the subsequent tensile deformation process due to precipitates.In the process of warm rolling, TWIP steel produced high density dislocations, resulting in the increase of the yield strength and the decrease of the plasticity. In the process of annealing, the interaction between the movable dislocations and the interstitial atoms increased the yield strength in certain degree. The formation of a large number of precipitates at grain boundaries and in grains is the main reason for the plasticity decrease.