Physical Action Mechanism Of Formability And Microstructure Of 301L Stainless Steel Plate For Railway Vehicles

With the continuous development of urban rail transit,Metro coach manufacturing is more inclined to stainless steel materials.301L stainless steel is the main material for manufacturing the Metro coach.Because of the larger internal stress of cold-rolled plate,it is easy to produce poor forming performance,local cracks and wrinkles during plastic forming.In this paper,301L cold-rolled sheet produced by Jiuquan Iron and Steel company,witch reduces the internal stress of the sheet and improves its formability.The effects of different heat treatment parameters on mechanical properties,forming index,bulging and composite forming of 301L cold rolled sheet were studied,providing theoretical basis for actual manufacturing.The main results are as follows:1.After annealing,the yield strength and tensile strength of 301L-DLT、301L-ST、301L-MT cold rolled sheet tended to decrease,but the elongation increased,However,the yield strength and tensile strength of 301L-LT keep almost unchanged.The yield strength of 301L-HT cold rolled sheet tended to increase,but the elongation increased gradually with the increase of annealing temperature.The elongation increased by about 5%a fter holding for 2 hours at408℃,but the mechanical properties were not improved significantly by extend annealing time.After annealing,the thickness direction coefficient r of 301L cold rolled sheet increases gradually with the increase of temperature,and the hardening exponent n decreases.In addition the values of r and n do not change significantly with the prolongation of annealing time,and the higher the hardening grade,the smaller the value of n,the greater the value of plane orientationΔr.The yield strength and tensile strength of 301L-DLT,301L-ST and 301L-MT plates have almost no change along the rolling direction of 0、45°and90°,but the elongation along the rolling direction is the largest.It’s about 3（4）⁵（4）higher than that.After annealing,theΔr values of the three plates decrease and are greater than zero,which indicates that the three kinds of plates are prone to produce ear protrusions in the 0°and 90°directions.Therefore,the annealing temperature of408℃for 2h is the best annealing process for the stress relief of 301 L cold rolled sheet.2.With the increase of bending angle,the martensite content in 301L stainless steel is gradually increasing.The larger the thickness of the same grade,the easier it is to form martensite during plastic deformation.In addition,the higher the grade is at the same bending angle,the higher the phase transformation martensite content.After bending deformation,the shape and size of the grains hardly change,but the grain boundar y are not smooth after bending,and a large amount of phase transformation martensite is formed;the austenite peak is obviously weakened,and the martensite peak is strengthen ing.Moreover,ε-martensite was not found in all the bent pieces.3.The cupping test values of DLT（2.0 mm）,ST（2.02 mm）,MT（0.808 mm）and MT（2.54 mm）sheets decrease with the increase of annealing temperature,but the annealing time has little effect o n them.The cupping test values of unannealed sheets are the largest and the swelling forming properties are the best.Meanwhile,large cracks appeared on the top of all MT（0.808 mm）cup protrusions.Under the same other conditions,the stronger the hardening degree,the smaller the cupping value and the worse the swelling forming ability.Therefore,the swe lling forming ability DLT>ST>MT.At the same time,the bigger the thickness of the same grade in a certain thickness range,the better the swe lling forming ability.4.Conical cup tests were carried out on DLT（2.00 mm）,ST（2.02 mm）,MT₁（0.808 mm）and MT₂（0.788 mm）under different annealing processes.The relative cone value of DLT and ST grades increases with the increase of annealing temperature,while MT₁ decreases after the maximum value appears at 260℃.MT₂ is stable after the maximum value appears at 260℃.The higher the grade strength is at the same thickness,the larger the cone value,the better the composite forming performance DLT>ST.The grain size of the tissue is elongated and the grain boundary is not smooth after deformation on the microstructure figure.Four distinct austenite diffraction peaks{111}g,{200}g,{220}gand{311}gdisappeared completely after deformation in the original organization.Two distinct{200}α’and{211}α’martensite diffraction peaks appeared.The maximum martensite content in the top of ST cone cup wasα’=60.7（4）,and the fracture surfaces of DLT and ST were dimple-like,which belonged to ductile fracture.