Study On Microstructure Control And Deformation Chracteristics Of Medium Mn Steel With High Product Of Tensile Strength And Ductility

With the continuous improvement of energy saving,environmental protection and safety requirements in the automobile industry,the automotive structural materials with high capacity of impact energy absorption are becoming more and more concerned,the further optimization of its chemical composition and improvement of its comprehensive mechanical properties has became one of the focuses worldwide.By summarizing and analyzing the chemical compositions and mechanical properties of medium Mn steel reported in recent years,we proposed the design principle of medium Mn steel with high product of tensile strength and elongation.That is:the volume fraction of metastable austenite should be increased as high as possible on the premise of ensuring cold-rolling,namely,control the M_s point at about 140?;meanwhile,the content of Al shall be controled at about 2%to improve the stability of metastable austenite and to control the stacking fault energy of austenite.Guided by this principle,we designed 7Mn(7%Mn-0.3%C-2%Al)and9Mn(9%Mn-0.3%C-2%Al)medium Mn steel.To verify the above-stated design principle and modify it if needed,we have studied the microstructure evolution and mechanical properties of hot-rolled and cold-rolled 7Mn and 9Mn steel.The hot-rolled and annealed 7Mn steel is composed of lamellar laths of ferrite and reversed austenite,the dislocation density in the microstructure is high,and it shows continuous yield during deformation.The average sizes of the austenitic laths are between 0.36~0.45?m after intercritical annealing(IA)at 680~720?for 1h,so the austenite stablity is high.After annealing at 720?for 1h,the product of tensile strength and elongation reached the highest value,namely 63GPa·%.Because of the higher Mn and C content,the macroscopic segregation band appears in the microstructure of hot-rolled 9Mn steel,which results in the bimodal distribution of metastable austenite after hot-rolled and annealing.Metastable austenite inside the segregation band presents large grain size with an average value of~2.5?m,while outside the segregation band the metastable austenite grains present an average size of~0.8?m.Because of the coarse grain size and low stability,the metastable austenite would rapidly transform into martensite during deformation,which results in the lower elongation and product of tensile strength and elongation(the highest value is only 42GPa·%).Therefore,different design principles should be adopted for the hot-rolled and annealed medium Mn steel.That is:on the premise of obtaining fine lath metastable austenite,the Mn and C content in the matrix should be increased as high as possible to obtain more metastable austenite,and control the M_s point at about240?.After cold-rolling and annealing,7Mn steel is mainly composed of equiaxed ferrite and reversed austenite.The dislocation density in the matrix is very low,and the specimen appears discontinuous yielding during straining.After IA at 700?for1h,the product of tensile strength and elongation reached the highest value,namely66GPa·%.As for the cold-rolled 9Mn steel,although the macro-segregation band is still retained after cold-rolling and annealing,the microstructure is much refined than the one hot-rolling and annealing.After IA at 680~720?for 1h,the average grain size of metastable austenite both inside and outside of segregation band are1.0~1.5?m and 0.46~0.63?m,respectively.Due to the moderate stability of metastable austenite and the appropriate value of stacking fault energy(SFE)where TWIP effect can be produced,the highest product of tensile strength and elongation in cold-rolling and annealing 9Mn steel is 88GPa·%,which is superior to conventional TWIP steel.This indicates the above-stated design principle is suitable for cold-rolled medium Mn steel.The PLC effect in medium manganese steel is produced in metastable austenite and the grain size of metastable austenite determines the occurance of PLC effect.If the austenite grain size is to fine then no sufficient dislocations generated before necking thus no PLC band appear during straining.It is the stability of austenite that determines the type of PLC band during straining.If the stability of austenite is moderate,the in-band transformed martensite is not very large,so the PLC band could move continuously.In contrast,if the stability of austenite is very low,then large fraction of in-band transformed martensite would be produced.In this case,the PLC band cannot easily overcome the resistance thus it had to hop frequently.The deformation and work hardening behaviors of each component phase in medium Mn steel were studied by using the constitutive model based on dislocation density.The results show that the stability of metastable austenite has a decisive effect on the tensile curve and work hardening rate of medium manganese steel.In the end,the influences of low temperature tempering(400?,20min)on the toughness of hot-rolled and cold-rolled 9Mn steel have also been studied,which showed the effect of Mn segregation on ferrite grain boundaries can not be ignored.In Fe-Mn alloy,Mn content in matrix,cooling rate and tempering time can significantly affect grain boundary Mn content.There is a critical value for grain boundary Mn concentration,when the actual grain boundary Mn concentration exceed the critical value,then the Fe-Mn alloy become brittle.The critical value at room temperature,-40?and-80?are 9.3 at.%,6.3 at.%and 4.3 at.%,respectively.