| With the development of the automobile industry and the increasing awareness of environmental protection,it is a critical way to meet the requirement of green development in the field of transportation technology by using optimal materials to realize the lightweight of the automobile on the premise of ensuring the strength and safety of the body.In the aspect of body structure manufacturing,compared with magnesium,aluminum,and other lightweight materials,advanced high strength steel(AHSS)has the advantages of low cost,high recovery efficiency,and green manufacturing process.As a representative of the third generation of AHSS,quenching and partitioning(Q&P)steel realizes the suitable combination of strength and plasticity by obtaining composite effect and transformation-induced plasticity(TRIP)effect through multiphase-metastable-multiscale design and carbon-richen retained austenite,leading to extraordinary competitiveness and potential among automobile lightweight materials and schemes.Meanwhile,Q&P steel also faces challenges in many aspects,such as composition improvement,microstructure control,and process optimization.Based on the composition of commercial 1180 MPa grade Q&P steel and referring to the thermodynamic and dynamic simulation results of Thermo-calc and Mucg83 software,the experimental steel(0.2C-(1.2-2.5)Si-2.6Mn)(wt.%)was designed and prepared by taking the content of Si,a stabilizing element of retained austenite,as a variable.Combined with thermodynamics and constrained carbon equilibrium(CCE)equation,the Q&P process was designed and implemented reasonably to find the best combination of strength and plasticity under the optimal composition system.In this study,electron microscopy(SEM),electron backscatter diffraction(EBSD),X-ray diffraction(XRD),and neutron diffraction were used for characterization and analysis.The evolution law of the lattice strain and microstructure in the deformation process of the Q&P steel was clarified using the(quasi)in-situ analysis,and the formation process of mechanical induced martensite was analyzed through variants.Based on the analysis of the composition,process,microstructure,and mechanical properties of Q&P steel with gradient Si content,the following results are obtained:The effect of Si element on transformation point and continuous cooling curve of test steel was analyzed using a dilatometer combined with microstructure analysis.Si element increases the transformation temperature of the intercritical region and decreases martensitic transformation temperature at the same time,and affects the range of phase transformation zone of undercooled austenite in continuous cooling transformation curve of test steel.Si expands the range of austenite(A)?martensite(M)phase transformation zone;meanwhile,the range of austenite(A)?bainite(B)phase transformation zone is reduced,which is beneficial to avoid the occurrence of bainite transformation.The relationship between quenching temperature and volume fraction of retained austenite was calculated by using the CCE model.The validity of the CCE model in predicting Q&P steel's optimum quenching temperature of Q&P steel was verified by experimental analysis.Aiming at the problem that the classical CCE model's prediction results are not suitable for isothermal Q&P steel in the intercritical region,the iterative calculation process of the CCE model was optimized by investigating the prior austenite grain size and alloy element diffusion conditions in the intercritical region.Thus,a good agreement between final calculation results and experimental data was realized.Based on CCE theoretical calculation and Q&P process design,the test steel's microstructure and mechanical properties were systematically analyzed.The results show that the Si element is beneficial to improve the test steel's comprehensive mechanical properties.Si content's influence on the phase transition point was avoided by adjusting the heating process in the intercritical region.It could be concluded that the improvement of mechanical properties is attributed to the stabilizing effect on austenite and the solution strengthening and work hardening caused by Si.The multiphase-metastable-multiscale Q&P steel with 16.2%(volume fraction)retained austenite,43.1%ferrite,and 40.7%through the optimization of the heat treatment process martensite was prepared.The tensile strength of Q&P steel is 1308 MPa,and the elongation is 21.5%.For the test steel with the best performance,the in-situ neutron diffraction technique was used to study each phase's lattice strain in the deformation process.The test steel's micromechanical behavior in tensile deformation can be divided into four stages:elastic stage,body-centered cubic(bcc)phase yield,face-centered cubic(fcc)phase yield,and plastic deformation.The macroscopic yield of test steel was caused by deformation-induced martensitic transformation,bcc phase yield,and fcc compatible deformation.In each stage of micromechanical behavior,the additional load was transferred between different phases and grains with different orientations to achieve coordinated deformation and ensure the material's continuity.The stability of retained austenite was discussed through quasi in-situ EBSD,and the variants of deformation-induced martensitic transformation were analyzed.The results show that the selectivity of variants is negatively correlated with the grain size of retained austenite,and when the grain size of austenite is small enough,the selection of variants is concentrated in the same CP(close-packed planes)group.Besides,when the orientation of equiaxed retained austenite near ferrite approximately conforms to one of 24 variants,this variant will be preferentially selected and form a small angle grain boundary,thus reducing the mechanical stability of retained austenite.Combined with actual test data,a dynamic model of mechanical induced martensitic transformation was established by using the O-C(Olson-Cohen)relationship.Under the premise of considering each phase's microstructural size,alloy composition,phase volume fraction,and dislocation density evolution,the constitutive relation of the test steel considering the mechanical induced martensite transformation was constructed quantitatively by using the iso-work theory.The strengthening mechanism of Q&P steel was discussed,and the relationship between microstructure and mechanical properties was analyzed. |
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