| Austenitic stainless steels（ASSs）have broad application prospects in various industries for their excellent corrosion resistance,good plasticity and weldability.However,the low yield strength of ASSs（～200-350 MPa）seriously limits their application as structural materials.Increasing the strength of metals by various strengthening mechanisms is usually accompanied by decreasing in plasticity.Therefore,how to break through the“trade-off”relationship between strength and plasticity to obtain excellent strength-plasticity combination ASSs,and the applicability of high-strength-plasticity ASSs in harsh industrial environment have attracted widespread attention.In view of the above problems,Cr-Mn-N metastable ASS was studied in this paper.Austenitic stainless steel with high-strength and high-plasticity was obtained by cold deformation combined with annealing process.The strengthening mechanism and coordinated deformation mechanism of bimodal ultra-fine austenitic stainless steel during plastic deformation were systematically studied by scanning electron microscope（SEM）,transmission electron microscope（TEM）,electron backscatter diffraction（EBSD）,X-ray diffractometer（XRD）and nano-indentation.The effect of microstructure on hydrogen embrittlement sensitivity and high temperature mechanical properties of high strength ASS was investigated.The specific research contents and results are as follows:（1）The effect of cold rolling（CR）on microstructure evolution and mechanical properties of commercial Cr-Mn-N austenitic stainless steel was studied with various cold rolling reduction.The results indicated that the microstructure varied from equiaxed austenite in the as-received specimen to the coexistence of stacking faults（SFs）,deformation twins（DTs）and strain-induced martensite（SIM）in pre-deformed specimens.Furthermore,detailed TEM results showed that equiaxed dislocation cells,DT nets and dislocation-cell-typeα′-M occurred in the 60%CR specimen.Tensile tests revealed that the strength of ASS increased while the plasticity decreased with the increase of pre-reduction deformation.ASSs with good strength-plasticity combination can be obtained with 10%-30%cold rolled reduction,and their strong plastic product can achieve upto30GPa%.Such mechanical properties were confirmed to be related to the variations in microstructure dependent deformation mechanisms,varying from planar slip+DTs+SIM in the as-received specimen,to DTs+SIM with 30%reduction,and SIM+minor twinning with 60%reduction,respectively.（2）60%CR specimens were selected and then annealed at 700°C and 1000°C,respectively.The results indicated that the reversion of deformation-induced martensite（DIM）to austenite occurred via a combination of diffusion and shear mechanisms at700°C,while,at 1000°C,it was via a shear mechanism alone.Recrystallization occurred during the annealing process at both temperatures.The alterable DIM reversion behavior resulted in differences in the microstructure,with bimodal grains being observed in specimens annealed at 700°C,and unimodal grains in specimens annealed at 1000°C.The tensile-test results revealed that higher yield strength was obtained in specimens with a unimodal structure,while there was larger elongation in specimens with a bimodal structure of the same average grain size.Comprehensive analysis shows that the strength-plasticity combination for the specimens with a bimodal structure was higher.This superior plasticity is related to the formation of more geometrically necessary dislocations（GNDs）,deformation twins（DTs）,and DIM during tension.（3）In order to probe the superior nano/micron scale mechanical behaviors of ultra-fine grain（UFG）austenitic stainless steel with grain size of～1.3μm as compared with coarse grain（CG）with grain size of～14μm counterpart,systematic nanoindentation tests were carried out together with post-mortem EBSD characterizations.Relatively higher nano-hardness（Hn）was obtained in UFG in both grain interior and grain boundary.The higher Hn in the“grain interior”for UFG was result of the pre-existing dislocations,and in the case of“on grain boundary”,the higher Hn was due to the higher strain surrounded the grain boundary.The grain boundary effect coefficient k of UFG grain boundary was higher than that of CG.In addition,the strain rate sensitivity and activation volume obtained from nanoindentation had weak dependence on grain orientation but strong dependence on grain size.Therefore,the higher macron-strength of the UFG was mainly determined by the higher grain boundary strength,and the fine grain size with less contribution from the grain interior.The mechanical behavior could be governed by different grain boundary effects in microstructure.（4）ASS was strengthened by deformation strengthening and fine grain strengthening mechanisms,and the effects of different strengthening mechanisms and different grain sizes on the hydrogen brittleness of ASS were studied.The deformation-induced martensite formed during cold deformation can increase the hydrogen embrittlement sensitivity of ASS.The high hydrogen embrittlement sensitivity of cold rolled specimens is the result of deformation induced martensite and dislocation.Hydrogen embrittlement sensitivity experiments show that grain refinement not only increases the strength of ASS,but also improves its hydrogen embrittlement resistance.The content of diffusible hydrogen in Cr-Mn-N ASS decreases with the decrease of grain size.Hydrogen diffusion in ASS is controlled by the dual mechanism of rapid hydrogen diffusion along grain boundaries and dislocation as hydrogen trap to slow hydrogen diffusion.（5）Samples with different grain sizes were selected for high-temperature tensile at different strain rates to study the effects of grain refinement and bimodal grain size distribution on the mechanical properties of ASS at high temperature.The results show that the refinement strengthening mechanism is still applicable at 600℃.As the strain rate decreases from 10-3s-1 to 10-5s-1,the refinement strengthening effect weakens.Under different strain rates,the deformation mechanism of fine grain（FG）ASS at high temperature is strain-induced twin+dislocation slip+grain boundary slip associated with dynamic recrystallization.With high strain rate(10-3s-1),the recrystallization mechanism is mainly continuous dynamic recrystallization.Discontinuous dynamic recrystallization occurs at low strain rate(10-5s-1).The strain rate decreased from 10-3s-1 to 10-5s-1,and the elongation of fine austenitic stainless steel increased by about 2 times,which was attributed to the dynamic balance between strain hardening and recrystallization softening in a large strain range,which kept the strain hardening rate fluctuating at a high level.The elongation of bimodal samples with the same average grain size is lower after fracture,which is due to the dynamic recrystallization and grain refinement in the coarse grain region of bimodal samples during tensile process at high temperature,and the increase of grain boundary density inhibits dislocation slip. |
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