Multi-scale Microstructure Control And Strengthening-Toughening Mechanism Of Ultra-high Strength Austenitic Steel

In recent years,many developed countries have conducted considerable research work on ultra-high strength and toughness(UHST)steel materials and mastered a large number of core preparation technologies.Successfully increased the yield strength of UHST steel to 2.0 GPa,while having an elongation of nearly 20%.These breakthrough research results have made steel materials with a body-centered cubic(BCC)structural phase as the main microstructure to obtain an ideal match between strength and plasticity.However,it is difficult to design UHST steels with elongation requirements exceeding 40%by using the BCC phase as the main microstructure.Both twinning induced plasticity steel and austenitic stainless steel take the austenite phase with a face-centered cubic structure as the main microstructure and have excellent plasticity.However,the yield strength of these steels is generally low(250?550 MPa),and their torsional deformation resistance is seriously insufficient,which severely limits their application as high-strength structural materials.In this paper,the FCC structural austenitic stainless steel was used as the research object,and the design concept of multi-scale nano/ultrafine-grained microstructure and the coupling control technology of deformation and phase transformation were used to achieve ultra-high strength and toughness of austenitic steel.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron backscatter diffraction(EBSD),X-ray diffractometer(XRD)and other microstructural characterization techniques or phase analysis methods were used to systematically analyze the composite strengthening mechanism of the multi-scale nano/ultrafine-grained microstructure and the coordinated deformation mechanism during plastic deformation.The corresponding research results are as follows:Under the design goals of avoiding using too much expensive alloying element Ni and refining the study of the influence of Nb element on the microstructural stability,this paper designed a low-cost Cr-Mn austenitic stainless steel with higher Md temperature and lower stacking fault energy,and different content of alloying element Nb was added for microalloying.It was easy to obtain enough volume fraction of strain-induced martensite(SIM,80%)without severe cold-rolling deformation.Subsequently,bimodal nano/ultrafine-grained(BM)steel was successfully fabricated by using the reverse transformation of SIM and recrystallization of deformed austenite during the annealing process.The annealing process was crucial for obtaining BM steel.The BM microstructure of ONb steel was obtained during the whole annealing process(1s?1000 s)at 700?800?.In Nb bearing steels,due to the better thermal stability of Nb(C,N)and the larger amount of Nb(C,N)in the fine-grained zone(FGZ),the difference between driving force and resistance in the FGZ couldn't lead the coarse-grained zone(CGZ)in a large extent.The grains in the FGZ couldn't obtain sufficient growth advantages so that the bimodal characteristic could be maintained for a long time when annealing at 900?.Thus,the addition of alloying element Nb greatly broadened the process window for obtaining the BM structure.BM steel had excellent comprehensive mechanical properties,its yield strength was maintained at 715 MPa approximately,tensile strength was floating at 1165 MPa approximately,uniform elongation was kept at 45%approximately,and total elongation was maintained at 55.5%approximately.With the increase of Nb content,the strength of BM steel increased gradually and the elongation decreased slightly.In addition,the linear relationship between the yield strength and the negative half power of the microstructure entropy of BM steel was obtained by using the grain self-similar evolution model.Due to the existence of nano-grains and lamellar interface of FGZ/CGZ,the effective combination of fine-grain strengthening and back-stress strengthening contributed to the excellent yield strength of BM steel.Under different strain levels,TWIP and the two-stage TRIP effect played a leading role in a different order,so that the work-hardening rate continued to remain at a high level,and BM steel had excellent plasticity.The heterogeneous nano/ultrafine-grained(H-N/UG)steel based on BM steel was obtained by using the secondary cold-rolling and annealing processes.H-N/UG steel had super-high yield strength of up to 1221 MPa,while still had a uniform elongation of 37.8%and a total elongation of 45.3%.The significant grain refinement of H-N/UG steel greatly improved the contribution of fine-grain strengthening to the yield strength.Besides,the stronger strain gradient caused by the large increase in the lamellar interface of FGZ/CGZ produced higher back stress,which also improved the yield strength.The strain distribution caused by the multi-scale characteristics of H-N/UG steel made the FGZ and the CGZ have excellent coordinate deformation ability and delayed the occurrence of the TRIP effect,resulted in a good connection between the TWIP effect and the TRIP effect.So that H-N/UG steel had ideal elongation.In order to further improve the strength and plasticity of this steel,a simple process of "cold rolling+ instantaneous annealing+low-temperature tempering(D&RT)" was proposed,in which the dense dislocations in the SIM were retained in the reversed austenite through reverse shear transformation,and the heterogeneous nano/ultrafine-grained(D&RT)steel based on high-density dislocations was prepared.Using this preparation technique,the combination of the multi-scale nano/ultrafine grains,high-density dislocations(1015 m-2),and dense precipitates was combined in 0.2Nb steel.The successful combination of fine-grain strengthening,dislocation strengthening,and precipitation strengthening made the yield strength of D&RT760 steel even reach 1.44 GPa.Meanwhile,it had a uniform elongation of 35.3%and an ultra-high product of strength and ductility of 68.7 GPa%.Due to the existence of multi-scale nano/ultrafine grains,the TRIP effect of coarse-grained austenite was delayed.And the presence of high-density dislocations made the strain-induced martensite transformation greatly promoted in the middle and late stages of plastic deformation.The good combination of dense dislocation slip,TWIP effect and TRIP effect was beneficial for D&RT steel to maintain an excellent work-hardening rate at different deformation stages,and its comprehensive mechanical properties had achieved great breakthroughs.