EFFECTS OF N CONTENT ON MICROSTRUCTURE AND PROPERTIES OF 29Cr CASTING SUPER DUPLEX STAINLESS STEEL

Abstract

High manganese steels containing 15%—30%(mass fractions) Mn and additions of(2%—4%)Si and(2%—4%)A1 exhibit superior ductility and extraordinary strengthening behavior during plastic deformation,due to extensive twin formation under mechanical load(the so-called TWIP effect-twinning induced plasticity effect) or thee-martensite andα'-martensite transformation(the so- called TRIP effect-transformation induced plasticity effect).In this paper,different heat treatments were carried out for high-manganese TRIP steels of Fe-15Mn-4Si-2Al and Fe-20Mn-4Si-2Al,and the uniaxial tensile tests were performed.The results show that increase of holding time at 1373 K can increase elongation and strength.Through the analysis of stress-strain curves,the relationship between mechanical properties and deformation mechanism was established.The dependence of phase compositions in high manganese TRIP steels on the heat treatment parameters has been studied by metallographic characterization and XRD pattern.The transformation routes ofγ→e→α' andγ→α' were observed by TEM,which dominates the TRIP effects.Microstructural characterization ofe-martensite andα'-martensite indicated thatα'-martensite held the K-S orientation relationship with austenite,and the S-N(Shoji-Nishiyama) orientation relationship was observed between austenite ande-martensite,wherease-martensite andα'-martensite held the Burgers orientation relationship. Deformation behavior of the two kinds of high Mn steels has been clarified through the relationship between work hardening rate of dσ/deand true strain based on the true stress-true stain curves.It has been found that continuous transformation from austenite ore-martensite toα'-martensite can take place during deformation,which increases the work-hardening rate to improve the TRIP effect.