| Austenitic stainless steels?ASSs?have been widely used in various industries because of their excellent corrosion resistance,good plasticity and cold working ability.However,their use as structural components is limited by their low yield strength.To meet the requirements of low carbon development in contemporary society,developing ASSs with high strength and high plasticity has become an inevitable choice.At present,there are many methods to strengthen the ASSs.However,most of them are accompanied by a rapid decline in plasticity comes along with an increase in strength.Obviously,that is not fit for the development of high-strength and high-plasticity ASSs.To develop high-strength and high-plasticity ASSs,it is necessary to investigate their strengthening and plasticizing mechanism.Therefore,this paper took commercial Fe-18Cr-8Ni ASS with low yield strength as the research object.High-strength and high-plasticity ASSs were prepared by combining cold deformation and annealing process to refine grains.The microstructure evolution and mechanical properties during cold deformation and annealing,as well as the strengthening-plasticizing mechanism were analyzed.The research contents and results are as follows:?1?The ASS was tensile tested at different temperatures?25°C,200°C,400°C,600°C and 800°C?and at different strain rates(10-3 s-1,10-2 s-1,0.1 s-1,1 s-1,10 s-1 and 100 s-1),respectively.The deformation structures of commercial ASSs after tensile test at different temperatures and strain rates were characterized by EBSD,TEM,XRD and OM.The effect of tensile temperature and strain rate on deformation mechanism and mechanical properties of ASS were studied.The results indicate that when the strain rate was increased from 10-3 s-1 to 100 s-1,the deformation mechanism of ASS changed from TRIP mechanism to TRIP+TWIP mechanism.The yield strength of ASS increased,but the tensile strength and elongation to fracture first decreased and then increased with the strain rate increased.The reason is that when the strain rate is increased,martensite transformation is suppressed,and formation of mechanical twin was facilitated.When the tensile temperature was 25°C,200°C and 600°C,the deformation mechanism of ASS was TRIP mechanism,TWIP mechanism+dislocation slip mechanism and dislocation slip mechanism,respectively,and the corresponding tensile strength,yield strength,and elongation of ASS decreased.With the temperature increased to 800°C,the main deformation mechanism of ASS was recrystallization and grain boundary slip induced plasticity.Meanwhile,the tensile strength and yield strength further decreased but the elongation increased sharply.?2?The ASS samples were cold rolled with different reduction to investigate the effect of cold reduction on mechanical properties of ASS.The results indicated that the yield strength of ASS increased from 281 MPa to 780-968 MPa with a reasonable elongation of 30.8-27.4%through 20-30%cold deformation.The content of deformation-induced martensite increased and the size of untransformed austenite decreased with the increase of cold reduction,resulting in the increase in yield and tensile strength of ASS,and the decrease of elongation to fracture.The reasonable elongation of high-strength cold-rolled ASS samples with small deformation was associated with their good strain hardening ability after the cold rolling.?3?Three kinds of cold-rolled samples with 30%,60%and 90%cold reduction were annealed at 900°C to investigate the effects of cold reduction on the microstructure and mechanical properties of reverse ASSs.The results indicated that the martensite in 30%,60%and 90%cold-rolled samples disappeared after annealing at 900°C for 10 minutes,5minutes and 3 minutes,respectively.Compared with those of original ASS samples,the yield strength,tensile strength and elongation to fracture of cold-rolled samples after annealing were increased.Compared with 30%and 60%cold-rolled samples after annealing,90%cold-rolled sample after annealing has a more uniform and finer grain size,and the sample has a higher yield strength and elongation combination.?4?The 90%cold-rolled ASS sample was selected and annealed at different temperatures between 600 and 1000°C to investigate the effects of annealing temperature and time on the microstructure evolution and mechanical properties of ASS.The results indicated that the fine austenite grains formed in 90%cold-rolled samples during annealing through phase reversion of strain-induced martensite and recrystallization of deformed untransformed austenite.The refinement effect of grains formed through phase reversion is obviously stronger than that of grains formed through recrystallization.When the annealing temperatures were at 800°C,900°C and 1000°C,the complete austenite structure could be obtained after annealing for 5 minutes,3 minutes and 1.5 minutes,respectively.With the increase of annealing temperature and time,the grain boundary density decreased and the average grain size increased gradually.Meanwhile,the yield strength decreased and elongation increased.The ASS with high yield strength and high elongation can be obtained through appropriate annealing treatment.?5?Ultrafine-grained?UFG?,fine-grained?FG?and coarse-grained?CG?ASSs were prepared by combining 90%cold rolling and annealing process.The plasticizing mechanism of ASS was studied by analyzing strain hardening and fracture behavior.The results indicated that the deformation mechanism of ASS changes from TRIP mechanism to TWIP+TRIP mechanism with the grain size refined from CG to UFG.In UFG samples,a large number of mechanical twins formed in the early stage of strain,and a large amount of martensite formed in the later stage of strain.The high plasticity of the UFG,FG and CG samples is attributed to the three-stage strain hardening behavior and ductile fracture behavior during the tensile process.Meanwhile,with the grain size decreased,the range of each strain interval changed slightly and the void size of the fracture was decreased accordingly.The difference in strain hardening behaviors between samples with different grain sizes is attributed to the transformation of deformation mechanism caused by grain refinement,and difference in fracture morphology of samples with different grain sizes ascribed to the increased content of martensite and mechanical twin formed during the tension,providing more nucleation points for voids.?6?Fine-grained?FG?samples were selected and tensile tested at different temperatures and strain rates.The strengthening effect and plasticizing mechanism of FG samples at different strain rates and temperatures were studied by comparing those of commercial ASSs with coarse-grained structure.The results indicated that the effect of strain rate on FG samples was similar to that on the original ASSs.The yield strength was increased and the elongation decreased with the increase of strain rate from 10-4 s-1 to 0.1s-1.The tensile strength decreased first and then increased slowly with the increasing strain rate.This is also attributed to the suppression of martensite transformation and promotion of mechanical twin formation.Grain refinement strengthening existed at strain rates ranging from 10-4 s-1 to 0.1 s-1.With the tensile temperature increased from 25°C to 200°C,600°C and 800°C,the variation tendency of deformation mechanism in FG sample was TRIP+TWIP mechanism?TWIP mechanism?dislocation gliding mechanism?grain boundary gliding mechanism.Compared with the original ASS,these deformation mechanisms occurring in FG sample were at a larger temperature range.With the increase of tensile temperature,the yield strength and tensile strength of FG samples decreased,and the elongation first decreased and then at 600°C increased.Grain refinement strengthening was still obvious when the temperature was not higher than 600°C.At600°C,the elongation of FG samples was higher than that of original ASS,which could be attributed to the grain boundary migration occurring in FG samples.Since the deformation mechanism of both samples was grain boundary slip induced plasticity at 800°C,the tensile strength and yield strength of the FG sample were respectively close to those of the original ASS,indicating that the grain refinement strengthening effect was not obvious.The both kinds of the samples at this temperature have excellent elongation.With the increase of strain rate or tensile temperature,the strain hardening rate curves of FG samples and original ASSs both changed. |
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