Methods And Mechanisms For Improving The Microstructures And Properties Of Twinning Induced Plasticity Steels

Twinning Induced Plasticity(TWIP)steels are new alloy steels with excellent mechanical properties,showing extensive application prospects in aerospace,militaries,communications and so on.The high ductility of TWIP steels originates from the formation of large quantities of deformation twins during the plastic deformation as well as resultant dynamic Hall-Petch effect,which is closely related to the grain orientation,grain size,grain morphology and dislocation configuration.Therefore,tailoring the grain microstructures would be one of the most effective ways to improve the mechanical properties of TWIP steels.Besides,the corrosion resistance of TWIP steels is relatively weak due to lack of passivation forming elements,which thus limits the applications or affects the performance of TWIP steels.In view of the above problems,a Fe-30Mn-3Al-3Si(wt.%)TWIP steel was used as the objective material,and the methods for changing the grains and related mechanisms were studied in the present dissertation.And a surface covering technique and resultant microstructures were also investigated and characterized.The main results are as follows:The fabric microstructures with a small amount of fine equiaxed grains were produced by warm forging.The TWIP steel in this state showed significantly increased strength,particularly yield strength,but the ductility was decreased.As the annealing temperature was elevated after the forging,the fine equiaxed grains continuously decreased while the most fabric grains grew and parts of them became lath ones.Meanwhile,the strength of TWIP steel gradually decayed but the ductility increased.The optimum mechanical properties were obtained in 750? annealing,that is the tensile strength 727 MPa,the yield strength 562 MPa and the elongation 67.3%,obviously higher than those of common TWIP steels with single equiaxed grains.The TWIP steel in the warm-forged and low-temperature annealed states showed very similar mechanical behaviors.The changes of strain hardening rate against true strain can be divided into four stages.Compared with common TWIP steels with equiaxed grains,the present TWIP steel with fabric grains exhibited apparent yield point phenomenon.The rapid generation and multiplication of dislocations together with the pinning effect of dislocation substructures,small angle grain boundaries and twin boundaries should be responsible for the appearance of yield-point phenomenon.Fine equiaxed grains were realized with varied sizes by severely hot rolling and recrystallization annealing at different temperatures.It is shown that,in the initial stage of annealing,there appeared high-density dislocation walls,entanglements and networks in the microstructures.The dislocation walls induced the appearance of sub-structures in the grains,while the dislocation entanglements and networks provided the nucleation substrates for the recrystallized grains.Therefore,as the annealing process was continued,the dislocations and initial fine grains were progressively merged into relatively large grains,forming full equiaxed grains with similar sizes.Annealed at 650?,the TWIP steel had the finest average grains,around 8?m,and showed the highest strength,722 MPa,but the lowest ductility,53.0%.When annealed at the highest temperature,1050?,the grains grew to 45 ?m in diameter,and the steel showed lowest strength,578 MPa,but highest ductility,88.5%.It is therefore suggested that the higher the annealing temperature,the better the ductility,and vice versa.When the annealing temperature was over 850?,there appeared an obvious yield point phenomenon.This can be attributed to pinning effect of Cottrell atmosphere to dislocations and the de-pinning of dislocations.Single columnar grain microstructure was formed in unidirectionally solidified TWIP steel.As the solidification rate was increased,the segregation of alloy elements and dendrite spacing were gradually decreased,and the columnar grains became much clear.In the lowest solidification rate,60 ?m/s,the TWIP steel showed the best ductility and the elongation was as high as 97.5%,while in the highest solidification rate,240?m/s,the TWIP steel had the highest strength,563 MPa.For the TWIP steel with single columnar grains,the curves of stain hardening rate against true strain can be divided into five stages.In stage A and E,the stain hardening rate was rapidly dropped,and in stage B and D,the stain hardening rate was almost constant while in stage C,the stain hardening rate was continuously increased.A coating layer containing several intermetallic compounds was prepared on the surface of TWIP steel through hot dipping aluminum and subsequently thermal diffusion treatment.The initial coating layer consisted of two Al-rich compounds,Fe2Al5 and FeA13,and they were all transformed to Fe-rich phase,Fe3Al,after the diffusion treatment at 1050?.The electrochemical test in NaCl solution showed that the Fe3Al can obviously improve the corrosion resistance.This is because the coating layer containing Fe3Al is quite stable and its corrosion rate is much lower than that of TWIP steel substrate.