| Carbide-free bainitic steel has become a typical representative of the new generation of advanced high strength steel due to its excellent mechanical properties,and has broad application prospects in various industries.The high plasticity of carbide-free bainitic steels is usually attributed to the transformation induced plasticity（TRIP）effect of retained austenite.In addition,the retained austenite microstructure has a significant effect on the stability of austenite,and even leads to changes in the microscopic deformation mechanism and affects the toughness.However,its long isothermal heat treatment time limits the large-scale industrial production.In order to explore and solve the problem of too long phase transformation time,the phase transformation and crystallographic characteristics of bainite nucleation and growth were studied from the perspective of crystallographic characteristics.At the same time,the microstructure evolution,deformation behavior and fracture mechanism of carbide-free bainitic steel during deformation are systematically expounded from macroscopic plastic deformation to microscopic plastic deformation and from quasi-static deformation to high-rate deformation by taking the internal relationship between the stability of retained austenite and deformation mechanism as a bridge,which provides an important theoretical reference for the analysis,evaluation and optimization of the service process of advanced high strength steel represented by carbide-free bainitic steel.The research results and conclusions are as follows:（1）The effect of bainite isothermal temperature on the nucleation and growth of bainite was investigated using high temperature confocal combined with EBSD characterization technology.The results show that the nucleation sites of bainite are original austenite grain boundary,pre-formed bainite lath side,intragranular inclusion,bainite lath end and sub-grain boundary.The growth rate of bainite lath formed at grain boundary is the largest,and the growth rate of bainite lath formed at intragranular inclusion is the smallest.The first variant formed in the high temperature bainite transformation is V15,and the final variant pair is dominated by the CP1 group（V1-V6）.The V10 variant is the first formed in the low temperature bainite transformation,and the distribution of the variants is wide.The preferentially formed V1/V2,V1/V4,V1/V8,V1/V10 and V1/V15 variants are helpful to increase the proportion of high angle grain boundaries.（2）The effect of retained austenite on the crack propagation and fracture behavior of carbide-free bainitic steel under impact deformation was investigated by impact test combined with EBSD characterization technology.The results show that the crack first starts from the block M/RA and propagates along the bainite sheaves with the same or similar orientation,and it was easy to turn when encountering different crystal orientations.During crack propagation,the thin film retained austenite will be passivated first,and then during crack propagation,the thin film retained austenite will be passivated first,and then propagation or terminated.When the blocky retained austenite is encountered,the stress promotes the TRIP effect of the retained austenite,and the relaxation stress is concentrated.The crack will pass through the retained austenite and then passivate,deflect or directly change the direction to continue to expand along the relatively weak position.（3）The relationship between blocky martensite/retained austenite（M/RA）and the initiation of micro-voids and microcracks was investigated by quasi-static tensile experiments,and the potential mechanism of block M/RA in the initiation/propagation of micro-voids and microcracks was revealed.The results show that the phase distribution of bainite and martensite is identified by EBSD characterization,which is highly consistent with the actual test results.It provides important evidence for identifying that the depolymerization reaction at the interface of martensite and bainitic ferrite is the main initiation mechanism of micropores and microcracks.In addition,the inhomogeneous plastic deformation in M/RA phase is related to the inhomogeneous distribution of dislocation density.Due to the large difference in carbon content between the boundary and the center,the M/RA undergoes persistent deformation,which affects the hardness and dislocation configuration.At a lower bainite transformation temperature,more variants are selected to make the austenite and bainite or martensite interface satisfy the K-S relationship,which promotes the movement of dislocations from martensite to adjacent RA.Dislocation movement promotes the synergistic deformation ability of the two phases and produces continuous plastic deformation by delaying or preventing the occurrence of micro-cracks.（4）The micro deformation mechanism of retained austenite in carbide-free bainitic steel at nano/micro-deformation scale was investigated by nanoindentation combined with EBSD technique,and the internal correlation of mechanical properties of various microstructures under micro-strain was explored.The results show that the dislocation density is a key factor affecting the stability of retained austenite.High dislocation density was enriched in RA with high KAM value.This leads to higher stability to delay the occurrence of pop-in phenomenon,i.e.,stress-induced phase transformation in nanoindentation.It is found by molecular dynamics that a typical pop-ins can be observed in the P-h curve at the loading stage.When the displacement reaches the critical value of the first pop-in,some atoms deviate from their initial structure and rearrange into BCC structure with a lower coordination number.Then,a sharp decrease in the load on the curve indicates that the permanent plastic deformation has begun,which is related to the nucleation and elastic-plastic transformation of dislocations.In addition,the indentation-induced temperature has a strong dependence on the indentation depth.As the indentation depth increases,more external energy drives the interaction between the dislocation and the indenter region,making the dislocation change more frequently,resulting in the release of elastic energy to increase the kinetic energy of the atom.（5）The relationship between microstructure evolution and strength and plasticity of carbide-free bainitic steel at high strain rate was investigated by high-speed tensile test,and the dependence of mechanical response on temperature and strain rate was revealed.The results show that the strength of steel increases with the increase of strain rate.At a strain rate of 500 s-1,the bainitic steel exhibited the optimal plasticity with a large elongation of 22%,showing certain degree of excellent plasticity,attributing to the strain rate hardening effect with the strain rate sensitivity exponent（m）.With the increase in strain rate,the intensity of the texture slightly increased,from unobvious texture to single Brass texture,and eventually evolved into a variety of textures dominated by Brass texture.Slip and movement of dislocations were blocked at high strain rates,and dislocation substructures cannot be annihilated and rearranged.The accumulation of slip band and dislocations led to the generation and evolution of LAGBs,and the line density of LAGBs increased significantly.（6）Split Hopkinson pressure bar（SHPB）was used to study the formation of dynamic adiabatic shear bands under high strain rates and to explore the mechanism of dynamic propagation of shear bands on crack formation and material fracture.The results show that the work hardening rate decreases with the increase of strain,but under the same decrease,the work hardening rate appears at 8000 s-1（29）2000 s-1（29）500 s-1.The increase of work hardening rate leads to the increase of sample strength,effectively alleviates the occurrence of fracture,and also improves the elongation.With the increase of strain rate,the content of retained austenite decreases obviously,and cracks and ASBs appear in turn with the increase of strain rate.Very fine equiaxed grains are formed in the ASBs formed at 8000 s-1,and more slender grains are observed around.The intersection of the dislocation source activated at high strain rate and the maximum shear direction（stress concentration region）in the specimen is the nucleation site of ASBs,which depends on the dislocation generation rate,local strain and strain rate. |
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