Strengthening-toughening Mechanism And Microstructural Control Of Ultra High Strength Aluminum-containing Medium Manganese Steel

Increasing requirements of energy conservation and environmental protection are constantly promoting the development of automotive lightweighting.Compared with light materials such as magnesium and aluminum,automotive steel is facing more challenges from full process green production,high strength,high plasticity and excellent formability.The third-generation advanced high-strength steel,which is represented by medium manganese steel and quenching&partitioning(Q&P)steel,has good competitiveness in automotive lightweight materials.Retained austenite is the key phase of the third-generation AHSS.Based on the inspiration from the austenite reverted transformation(ART)annealing and Q&P process,the ultra high strength aluminum-containing medium manganese steel with large amount of retained austenite is designed and prepared.The retained austenite characteristics,such as content,morphology,size and distribution of surrounding matrix phase are systematically studyed.And its relationship with the transformation induced plasticity(TRIP)effect is explored.The strengthening-toughening mechanism and microstructural control of the low-cost,short-process ultra-high strength is studied.The tensile strength is over 1300MPa,and product of strength and plasticity is over 35GPa·%.The low-cost "C-Si-Mn-Al" component design and energy saving process provide high-quality optional materials for automotive lightweighting.As the basic alloy system of 0.3C-1.5Si-4Mn,wt.%,the gradient aluminum content(1\2\4,wt.%)was designed to control the intercritical austenitizing temperature and process window of the medium manganese steel.Two types of ultra high strength microstructure was design,ie "ferritic +retained austenite" of aluminum containing medium manganese TRIP steel and "ferritic + tempered martensite + retained austenite" of aluminum containing medium manganese quenching and tempering partitioning(IQ-TL)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,combined with the in-situ deformation techniques were used to systematically analyze the multi-phase microstructure,strain coordination and strengthening-toughening mechanism of ultra high strength aluminum-containing medium manganese steel.The macroscopic element segregation behavior was analyzed by means of electron probe micro-analyzer EPMA.While the rules of microscopic element assignment were revealed by Thermo calc\DICTRA thermodynamics software and atom probe tomography(APT).By the reasonable adjustment of process parameters such as austenitizing temperature,heating rate and cold rolling reduction rate in intercritical section,the optimal configuration of retained austenite and other phases can be realized,and the plastic instability both yield platform and PLC can be improved.The corresponding research results are as follows:The phase diagram calculation and the thermal simulation results of the dilatometer show that the A1 element effectively broadens the intercritical austenitizing temperature process window.The DICTRA software simulates the elemental partitioning of the intercritical austenitization process with the same equilibrium state.The addition of Al element significantly enhances the diffusion efficiency of alloying elements and stabilization of retained austenite.The addition of high aluminum causes the delta ferrite to remain at room temperature,reducing the tensile strength and eliminating the PLC phenomenon.A large number of staggered dislocation slip bands in 8 ferrite in in-situ SEM demonstrate its good strain coordination ability.The intercritical austenitizing temperature is designed to achieve a multi-strength level of aluminum-containing medium manganese steel by adjusting the intercitical austenite fraction.Compared with aluminum-containing medium manganese TRIP steel,aluminum-containing medium manganese IQ-T~P steel with tempered martensite as the main matrix "skeleton" exhibits higher yield strength.XRD and APT detected the accumulation of carbon and manganese in the retained austenite and the segregation of manganese and aluminum at the interface.Then the local equilibrium(LE)of the alloying elements in the tempering phase was proved.The elemental partitioning of carbon and manganese during the intercritical austenitizing and tempering process under the IQ-T~P heat treatment promotes the stability of retained austenite,while the tempered martensite structure cuts the retained austenite grains and further increases its stability,and thus the aluminum-containing medium manganese IQ-T~P steel exhibits excellent mechanical properties.Taking 4MnlAl steel as an example,the hot-rolled IQ-T~P steel has a tensile strength of 1425±43 MPa and a total elongation of 25.9±3.8%,which are obviously better than the optimal mechanical properties with tensile strength of of 1345 MPa and an elongation of 1 8.9%in aluminum-containing medium manganese TRIP steel.The tensile strength of 4Mn2Al hot-rolled IQ-T~P steel is 1319±39MPa,and the elongation is 27.4±1.1%.The dilatometer thermal simulation and EPMA composition analysis confirmed that the abnormal growth of the cold-rolled annealed microstructure was controlled by the heating rate in the critical temperature range under the segregation of manganese and aluminum.The Al-rich Mn-lean region inhibits the nucleation of austenite,and the slow heating rate provides sufficient kinetic conditions for the recrystallization behavior and grain growth of the deformed martensite.Ultrafine grain cold-rolled aluminum-containing medium manganese TRIP steel has a clear yielding platform due to its smaller dislocation motion mean free path.The abnormally grown ferrite band provides a higher work hardening rate at the beginning stage of the deformation,which is beneficial to shorten the yield platform elongation of the material.However,aluminum-containing medium manganese IQ-T~P steel exhibits continuous yield characteristics due to the presence of martensite structure and geometrically necessary dislocations.The plasticity of the aluminum-containing medium manganese IQ-T~P steel is mainly due to the "lubricant" effect of the soft ferrite with lath morphology and the sustained TRIP effect of the retained austenite.The in-situ EBSD(kernel average misorientation,KAM)analysis and TEM characterization techniques were used to analyze the transformation of retained austenite during plastic deformation and the dislocation configuration of ferrite.The transformation of large-sized massive retained austenite at low strain stage in cold-rolled IQ-T~P steel leads to the growth of the "burst" geometrically necessary dislocations(GNDs)in the large-scale ferrite crystals.The "burst" GNDs also increase stress concentration near the interface,which deteriorates the mechanical properties.The higher stability of the lath-like retained austenite and the elongated lath ferrite in the hot-rolled IQ-TI steel promotes the continuous TRIP effect and the uniform distribution of dislocations during plastic deformation.The design of low cold rolling reduction(3 3%,LowCR)is beneficial to inhibit the recrystallization behavior of deformed martensite,and to obtain the improvement and refinement of the "long strip" microstructure,and finally achieve the excellent mechanical properties with tensile strength of about 1350 MPa and elongation of 19.5%between hot-rolled and cold rolled IQ-T~P steel.