Microstructure And Properties Of Advanced High Strength Steels Based On Control Of Meta-stable Austenite

With the shortage of fossil fuels and serious environment problems,automobile lightening has gradually been one of development directions in automobile industry,and received more attentions from both industry and academia.Automobile lightening means that the vehicle mass should be decreased on the premise of security restriction in order to reduce fuel consumptions and low exhaust emissions.Since steels account for about 60 percent of the total vehicle's body weight,it is an effective way to achieve lightweight using thinner steel sheet with higher strength instead of common steel.Both the conventional high strength steels(HSS)and the 1st generation advanced high strength steels(AHSS)cannot meet the entire requirements of mechanical performance for automobile steel in the future,and the cost of the 2nd generation AHSS,such as austenitic stainless steel and twinning induced plasticity(TWIP)steel,is quite higher mainly because of the high level of alloy elements.Therefore,it is quite essential to research and develop the 3rd generation AHSS with both low-cost and superior comprehensive mechanical performance.Based on the results of strengthening and toughening mechanism for high strength steels,the main concept of the 3rd generation AHSS is to guarantee high strength by conventional mechanisms,that is,phase transforming strengthening,grain refinement and dislocation strengthening et al.,and bring in meta-stable austenite in matrix,whose transformation induced plasticity(TRIP)effect can benefit formability.Presently,quenching and partitioning steel(Q&P)and medium manganese steel(medium Mn)are widely considered promising candidates for the 3rd generation AHSS.On the basis of above background and reasons,study on Q&P steel and medium Mn steel containing meta-stable austenite was performed in this paper,which is funded by the National Natrual Science Foundation of China.Effect of heat treatment process parameters on microstructure and mechanical properties was mainly studied,and the focus was drawn to the morphology,distribution and content of meta-stable austenite in these two steels.Mo-Nb and Ni-Mo-Ti microalloyed Q&P steel and Cu-bearing medium Mn steel were designed,and then the effect of alloying elements on micro structure and mechanical properties was respectively discussed.The chief original work and results are as follows:(1)The difference of micro structure and properties between C-Mn-Si steel treated by Q&P process after a full and intercritical austenitization(FQP and IQP for short)was systemically investigated.The former microstructure contains 3.5?9.6 vol.%retained austenite with mainly film-like morphology,while the later consists of 3.5?9.6 vol.%retained austenite with blocky shape.Additionally,retained austenite in the IQP sample transforms into martensite more easily due to higher nucleation rate,which means lower austenite stability.The product of strength and elongation(PSE)for the FQP and IQP samples is 15?20 GPa%and 20?28 GPa%,respectively.Notably,the one-step IQP samples show an excellent strength-ductility balance,whose tensile strength and total elongation are 1140?1280 MPa and 17?22%.Research shows that the tensile strength of Q&P steels is not influenced by quenching temperature(QT)but decreases with partitioning temperature(PT)and partitioning time(Pt)increasing,demonstrating that martensite tempering controls tensile strength of Q&P steels.The bake hardening(BH)value of Q&P steels is relatively higher(?100 MPa),and baking treatment do not influence the amount of retained austenite.(2)A novel integrated IQP and hot dip galvanizing(IQP-HDG)process was devised and carried out,which shows that carbon partition into untransformed austentie can be fulfilled simultaneously during the hot dip galvanizing.The low-cost C-Mn-Si steel treated by IQP-HDG process exhibit a superior strength-ductility balance with the highest PSE value of 30.0 GPa%,which increases about 50%compared with the same steel treated by Q&P process after a full austenitization.(3)Based on the results of C-Mn-Si steel,the microstructure and properties of Mo-Nb and Ni-Mo-Ti microalloyed steels treated by Q&P process were studied as well.The results show that the addition of alloying elements can refine prior austenite grains and then the thickness of martensitic lath,but not change the thickness of film-like retained austenite.Additionally,the amount of retained austenite is increased due to the addition of alloying elements,but the carbon concentration of retained austenite decreases correspondingly.The PSE value of Mo-Nb and Ni-Mo-Ti microalloyed steels is close to or higher than 20 GPa%,which means that the fluctuation of mechanical performance is reduced.It is beneficial to broaden the process window and ensure product quality during the manufacturing process.(4)A medium Mn steel added Cu element was designed,and the experimental results validated that Cu-rich particles can precipitate within the tempered marensitic lath or recrystallized ferritic grain,then they strengthen the alloy.Under the same condition of heat treatments,the tensile strength of Cu-bearing steel is 90?155 MPa higher than that of Cu-free steel,and the highest PSE value is up to 42.3 GPa%.The tensile strength of medium Mn steel increases at first and then remains stable with annealing time increasing,but it is the opposite for the yield strength.It can be attributed to the enrichment of C,Mn atoms into the reversed austenite and the decreasing of dislocation density in martensitic lath during annealing,which strengthens the reversed austenite but softens the tempered martensite.Compared with the annealing samples of as-quenched martensite,the annealing samples of deformed martensite shows slightly higher tensile strength and dramatically higher yield strength,but exhibites a large yield point elongation.(5)As for as-quenched martensite of medium Mn steel,the final microstructure after an intercritical annealing is also lath-like,and the reversed austenite grains are mainly acicular,with the thickness of 120?270 nm,which shows a Kurdjumov-Sachs(K-S)orientation relationship with martensitic matrix.For deformed martensite,the final microstructure after annealing consists of sub-micron globular ferrite and austenite grains.And interestingly,a high intensity of the y-fiber(?111?//ND)texture is formed for the equiaxed ferrite,and a weak texture of<110>//ND for the globular austenite.Additionally,the amount of reversed austenite is not affected by the initial microstructure,but there is a difference for austenite stability,that is,the globular austenite transforms into martensite more easily.(6)Dynamic tensile behavior of Q&P and medium Mn steels was studied.The results indicate that at high strain rates,the comprehensive mechanical performance of Q&P and medium Mn steels is excellent as well,and both steels show ductile mechanism.The variation of mechanical properties with strain rate increasing can be attributed to the integrative effect of dislocation slip resistance increasing,softening and change of TRIP effect due to adiabatic temperature rise.