Investigation Of Microstructural Design And Mechanical Behavior For Fe-30Mn-0.11C Steel

For the purpose of energy saving and high safety requirements,development of highstrength and high-toughness steels has always been the hot research topic in cryogenic materials science.Previous research on binary Fe-Mn austenitic steel has been shown that when the Mn content increases to 30~40%,the main deformation mechanism is dislocation slip and martensite transformation is inhibited,both at room temperature(RT)and at liquid nitrogen temperature(LN).The tensile elongation of these steels is more than 40% at RT and the impact energy is higher than 200 J at LN.However,the yield strength of this kind of high manganese steels at RT is low(<200 MPa),which is difficult to meet the requirements of cryogenic applications.Therefore,it is of great significance to obtain high strength and good plasticity at RT for Fe-30Mn-0.11 C steel by microstructural control.In this paper,specimens with different grain sizes of 0.054,0.43,0.48,0.77,0.8,1,4.3,8.5,47,52 and 67 ?m were successfully prepared by combining cold rolling with annealing treatments and solution treatments.Mechanical performance of specimens with different grain sizes were investigated by tensile tests at RT and LN,respectively.The results show that grain sizes have a significant effect on the tensile behavior of Fe-30Mn-0.11 C steel.When the grain sizes are less than 0.43 ?m,the tensile curve showed continuous flow.When the grain size is the range of 0.43~4.3 ?m,the tensile curves showed discontinuous feature.When the grain sizes are larger than 4.3 ?m,the tensile curves return to a continuous flow.In addition,the grain size and temperature effected the deformation mechanism of Fe-30Mn-0.11 C steel.Grain refinement suppresses martensite transformation at RT and grain refinement inhibits deformation twins at LN.It was also found that the temperature effected the deformation mechanism of Fe-30Mn-0.11 C steel by change the stacking fault energy.The dominant plastic deformation was dislocation slip at RT while the main plastic deformation was twinning at LN.Additional of cold rolling of 3~5% was applied the ultra-fine grained structure with the yielding platforms.Such a slight deformation rolling has introduced tangel dislocations in the structure,which leded to eliminate the discontinuous yielding phenomenon.The Hall-Petch relationship was established.In the larger grain size range(d = 4.3~67 ?m),a low Hall-Petch slope was obtained by linear fitting,while in the fine grain size range(d = 0.43~1 ?m),a very high Hall-Petch slope and a negative intercept was obtained.The strength of a laminated composite structure consisting of recrystallized grain and recoverd lamellar structures was calculated utilizing the rule of mixture to estimate the contributions of constraint effect and dislocation source strengthening.It showed that for the laminated composite structure(average grain size of 0.48 ?m)with a yield stress of 565 MPa and a uniform elongation of 25.2% at RT,the strength contribution originated from the constrain effect is about 47 MPa and dislocation source strengthening is aboout 46 MPa.