| As the national defense industry,electric power,nuclear,rail transit and shipbuilding industries rapidly develop in recent years,the market demand for non-magnetic steels has been increasing.European developed countries,America,Japan and Russia all have already conducted an extensive and systematical investigation on the non-magnetic steels.Theydeveloped Cr-Ni-Mo,Cr-Ni-Mn-N,Cr-Mn-N non-magnetic stainless steels and Fe-Mn-Cr,Fe-Mn-Al non-magnetic high-Mn steels,almost covering all strength grades of non-magnetic steels.A lot of related patents were also authorized.Nevertheless,the initiation of domestic research on the non-magnetic steels was later than that of those developed countries,and the number of non-magnetic steel grades is also limited.Furthermore,the understanding on some aspects of strain hardening mechanism and strengthening method is still unclear for the non-magnetic steels.For this reason,the emphasis of this paper is put on the microstructural evolution and strengthening mechanism of high-Mn non-magnetic steels.This study is supported by the national natural science foundation of China "Formation mechanism of annealing twins in high manganese non-magnetic steels during asymmetrical hot rolling process",in order to provide the theoretical basis and technological support for developing new non-magnetic steel grade with high quality.The main work and the experimental result of this study are listed as follows:(1)The hot deformation properties of Fe-20/27Mn-4Al-0.3C non-magnetic steels with low-to-medium stacking fault energy(SFE)have been investigated.The SFE was calculated using a thermodynamic model and the influence of the increase in Mn content on the peak stress,dynamic recovery and recrystallization behavior during the hot deformation process was also analyzed.Besides,the high-temperature constitutive equation and resistance model for Fe-20/27Mn-4Al-0.3C non-magnetic steels were also established for the calculation of stress and strain filed during the ASHR process.The results showed that the influence of the increase in Mn content on the hot deformation behavior was primarily achieved by improving the SFE of high-Mn non-magnetic steels.As the Mn content was raised,the stacking fault energy was increased,favoring the fact that the reinforced softening effect of dynamic recovery exceeded the strain hardening effect caused by the increase of Mn content.For this reason,the peak stress was decreased,whereas the onset of dynamic recrystallization was also delayed with the increase of peak stain.(2)The strain hardening mechanism of high-Mn non-magnetic steels with low-to-medium SFE was investigated during the tensile deformation process.The effect of Mn content on the dislocation density,volume fraction and critical stress of deformation twins,and the mechanical properties was also analyzed,and the individual contribution of each strengthening mechanism to the strain hardening performance was estimated.The results showed that in the whole strain hardening process,the primary strengthening effect was caused by the dislocation strengthening,and meanwhile,the deformation substructures such as deformation twins and microbands were also developed during the tensile deformation process.Nevertheless,the strengthening effect of both the twinning induced plasticity(TWIP)and microbands induced plasticity mechanisms could be neglected in the experimental steels.Furthermore,with the increase of Mn content,the strength and total elongation of high-Mn non-magnetic steels was also decreased,but the impact toughness exhibited a significant improvement.The increase of Mn content could cause the increase of dislocation density during the tensile deformation,which enhanced the strain hardening rate of high-Mn non-magnetic steels.However,on the other hand,the increase of SFE dramatically inhibited the occurrence of deformation twinning,such that the TWIP effect was hence diminished.(3)The strategy of using asymmetrical hot rolling(ASHR)process to refine austenite grains and strengthen the high-Mn non-magnetic steels has been proposed.The influence of ASHR process parameters,such as speed ration,rolling temperature,roll speed and thickness reduction on the grain refinement,shear texture and recrystallization texture formation,microstructure gradient along thickness reduction and the mechanical propertieshas been investigated.Moreover,the uncommon texture component of rotated copper during asymmetrical rolling was found to be present and we explained its formation from the view of both slip system selection and the crystallographic relationship.The results showed that under symmetrical hot rolling condition the shear strain was distributed symmetrically and no shear deformation existed at the center of the plate.However,under the ASHR condition the shear strain was distributed asymmetrically and non-uniformly,which caused an obvious microstructural gradient throughout the plate thickness.With the increase of either the speed ratio or the rolling reduction,the shear strain level was improved,generating a fine-grained surface layer with average grain size of 3?5 ?m.Moreover,the penetration effect of shear deformation to the central layer of the plate was reinforced with the grains refined to 6?9 ?m.These fine grains enhance the strength by 100 MPa in comparison with the conventionally multi-pass hot rolling process.Both the grain refinement effect and strength grade of ASHR plates approach that of cold rolled plates afterrecrystallization annealing.(4)The proportion of annealing twin boundaries among the high-angle grain boundaries in Fe-20Mn-4Al-0.3C steel has been compared for three different processes of conventional hot rolling,ASHR and cold rolling followed by an annealing.The relationship of annealing twin density with annealing processing parameters and the grain size of austenite was also analyzed.Then the relationship of grain size with the high-angle grain boundary density and the annealing twin density was linearly fitted.The results showed that the formation of annealing twins was related to the migration of grain boundaries.The annealing twin density only depended on the grain size,independent of the processing parameters of both cold rolling and annealing.The proportion of annealing twin boundaries during the conventional hot rolling process was the highest,and the second highest proportion of annealing twins was obtained during the ASHR process.The lowest one was for cold rolling and annealing process.The cause for this could be probably ascribed to the difference in the nucleation and growth manner of recrystallized grains between the processes of ASHR and cold rolling followed by annealing.Therefore,although the cold rolling and annealing process could achieve a superior grain refinement effect,the formation of annealing twins in this process was suppressed in comparison with ASHR process.(5)The magnetic hysteresis loops of high-Mn non-magnetic steels subjected to different cold rolling reductions were measured by vibrating sample magnetometer.We found that when the cold rolling reduction exceeds 73%,the direct phase transformation from austenite to a'-martensite could still occur in Fe-20Mn-4Al-0.3C steel even with a low-to-medium stacking fault energy.There is no participation of ?-martensite.The primary nucleation site of a'-martensite is at the intersections of microbands and deformation twins.Such a small amount of stain induced a'-martensite resulted in a light magnetization of non-magnetic steel.On the other hand,the welding test of two high-Mn non-magnetic steels have been carried out,and there was no welding crack to be formed.The strength of welded joint could also meet the requirement of strength,but the impact toughness was comparatively low. |
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