Study Of Microstructure Evolution And Mechanical Properties Of Fe-11Mn-2/6Al-0.2C TRIP Steels

Energy conservation,emission reduction and improve security have become the main development direction in modern automobile industry,and the emergence of the third generation advanced high strength steel make the call coming true.The comprehensive mechanical properties for steels are characterized by PSE,i.e.the product of ultimate tensile strength(UTS)and total elongation(TE).PSE is corresponding to the area surrounded by the tensile curve.Recently,a number of studies have been directed toward medium Mn-content(4～12%)steels,which are considered as promising third generation automotive steels with PSE>30GPa%.Microstructural evolution and mechanical properties of Fe-11Mn-2A1-0.2C TRIP steel(2Al steel)and Fe-11Mn-6Al-0.2C TRIP steel(6Al steel)have been investigated.And further investigate the plastic deformation mechanism of material.With the increase of Al content,the content of austenite decreased and the content of δ-ferrite increased.With the decrease of the tensile strength,the elongation increase.2 Al steel has a high tensile strength of 1407 MPa,6Al steel showed a high elongation rate of 65%.The high tensile strength of 2Al steel is the result of the new transformation of martensite with high hardness and PLC effect.The excellent extension of 6Al steel is the joint action of TRIP effect,the deformation of soft phase ferrite and TWIP effect.The increase of soft phase 8-ferrite leads to the increase of TE and decrease of UTS.The results of this study are as follows:(1)Microstructure of 2Al steel was changed with different quenching temperature.When the quenching temperature is in the range of 650～700℃,the microstructure is austenite and α-ferrite.When the quenching temperature is in the range of 750-800℃,the microstructure consists of austenite and martensite.The 2Al steel has excellent mechanical properties after quenching at 650℃(hereinafter referred to as 2Al-650),with elongation of 31.4%and tensile strength of 1407 MPa.(2)The excellent mechanical properties of 2Al-650 experimental steel are the common result of TRIP effect and PLC effect.TRIP effect is conducive to improve the elongation,while the PLC effect is not.The TRIP effect is mainly affected by the stability of austenite,but not the content of austenite.PLC effect is beneficial to the improvement of tensile strength of the steel.(3)6Al experimental steel is different from 2Al experimental steel,and its microstructure does not change with the change of quenching temperature.The microstructure is austenite and δ-ferrite.6Al steel quenched at 650℃(hereinafter referred to as 6Al-650)has a very high elongation of 64.7%,a lower tensile strength of 659 MPa.(4)The elongation of 6Al-650 experimental steel is the result of the interaction of TRIP effect and TWIP effect.In addition,the coordination deformation of δ-ferrite has greatly improved the elongation.However,due to the effect that δ-ferrite can effectively absorb the volume expansion of martensitic transformation in the process of deformation,the martensite has a lower hardness,and there is no PLC effect.So the tensile strength of 6Al steel is relatively low.