Investigation On The Microstructural Control And Mechanical Properties Of Quenching Partitioning Steels

The combination of plastic deformation and heat treatment is recently considered as a preferential processing manner to improve mechanical properties of steels,in addition to the energy saving.Since being proposed ten years ago,the novel quenching&partitioning?Q&P?treatment has been investigated extensively both in the involved transformation theory and in the selection of processing parameters.However,most of the progresses are based on off-line processing and,thus,it is necessary to combine plastic deformation into Q&P treatment to develop the on-line manufacturing one.In this paper,both cold rolling prior to austenitization and hot rolling during austenitization were carried out before Q&P treatment to explore the microstructural evolution in a low carbon Fe-0.19C-1.47Mn-1.50Si Q&P steel,along with the changes in rolling parameters and cooling manners.The deformation behavior and associated mechanical properties including strength,plasticity,impact toughness and strain-rate dependence from quasi-static to dynamic ranges were studied with particular focus on the roles of retained austenite and carbon–depletion in martensitic matrix.The main results are summarized as follows.?1?Cold rolling prior to austenitization at 910°C was applied to refine prior austenite grains of the experimental low carbon steel.Then,either traditional quenching and tempering?Q&T?or typical two-step Q&P treatment was employed to obtain different specimens for comparison.It has been shown that because of the rolling in advance,the grain size of prior austenite was reduced to less than 10 percent of the unrolled state,resulting in a great decrease in packet/block size and an increase in dislocation density in martensite in both Q&T and Q&P specimens,but the lath size was not obviously changed.Meanwhile,cold-rolling showed little dependence on the state of retained austenite including its volume fraction in both Q&T or Q&P specimens.However,the carbon depletion in martensitic matrix was very serious in Q&P state.It is shown that refining martensitic microstructure enhances strength and impact toughness without scarifying elongation very much for both Q&T and Q&P specimens.In addition,Q&P specimens are softer than Q&T ones due to the existences of considerable amount of retained austenite and the carbon-depleted martensitic matrix.As a result,the higher elongation and impact toughness with relatively lower strength were exhibited in Q&P specimens.Analyses indicated that carbon depletion in martensitic matrix plays more critical role than the amount of retained austenite in affecting the plastic deformation behavior and mechanical properties of Q&P specimens,because martensitic matrix is dominant microstructure with volume fraction close to⁹0%.Through such a combination of rolling and Q&P processes,the optimized microstructure including refined martensitic matrix with carbon depletion and proper amount of retained austenite can be obtained,contributing to a best comprehensive mechanical property with high product of strength and elongation and low ductile-to-brittle transition temperature.?2?Hot-rolling plus direct quenching and partitioning?HDQ&P?process was designed to simulate the on-line manufacturing manner with particular focus on the effects of rolling temperature and the final cooling manner.By changing the rolling temperature,either fine-grained or deformed prior austenite can be obtained.After typical two-step Q&P treatment immediately following rolling,both packets and blocks were refined significantly when martensite formed in the fine-grained prior austenite,but only blocks were refined when formed in the deformed prior austenite.The amount of retained austenite,however,shows little relationship with respect to prior austenite state.By changing the cooling manner from two-step Q&P treatment to natural cooling after quenching to M_s temperature,the amount of retained austenite stabilized at room temperature was decreased dramatically and the associated degree of carbon depletion in lath matrix was decreased simultaneously.The refinement of martensitic microstructure including blocks and packets by hot rolling is benefit to improve both strength and toughness without scarifying plasticity,presumably due to the stronger strain-hardening and energy absorption capabilities provided by the existence of retained austenite and refined carbon-depleted lath matrix.Contrarily,the smaller amount of retained austenite and higher carbon concentration in lath matrix due to natural air cooling resulted in the higher strength but lower plasticity and impact toughness even though the martensitic microstructures including packet and block were refined effectively.In general,the specimen rolled at a temperature higher than the dynamic recrystallization temperature followed by typical two-step Q&P treatment exhibits the best comprehensive mechanical property,because of refined packet and block,considerable amount of retained austenite and lath matrix with serious carbon depletion.?3?Because it is difficult to operate two-step Q&P treatment under the condition of industrial application,partitioning of carbon may occur accompanying martensitic transformation during continuous cooling.By taking a medium-carbon Fe-0.38C-1.54Mn-1.58Si as reference,the effect of cooling rate below M_s temperature on the amount of retained austenite was investigated in the experimental low-carbon steel?Fe-0.20C-1.49Mn-1.54Si?.In water-quenched state?³00 ^oC/s?,the amount of retained austenite in the medium-carbon specimen is higher than in low-carbon one,although the actual volume fraction is very low?<0.5%as measured by magnetism measurement?in either one,which is in agreement with the previous results from other scholars.However,when the cooling rate decreases,e.g.cooling in air?^{5 o}C/s?or in furnace?⁰.5 ^oC/s?,the amount of retained austenite in both specimens increases but the increment in low-carbon one is much higher.As a result,the amount of retained austenite in low-carbon specimen is always higher than in medium carbon one for a given cooling rate.The thermodynamic and kinetic analyses have indicated that the higher M_s temperature of low-carbon specimen should be responsible for this difference.That is to say,the carbon diffusivity relies on temperature strongly and the lower cooling rate would detain the low-carbon specimen more time at relatively high temperature,assisting carbon atoms to escape from newly formed martensite into neighboring austenite with very quick velocity and,finally,stabilizing more at room temperature.Since carbon partitioning is inevitable in most cases during cooling even under the water-quenched state?for example,the cooling rate at the center of thick specimen is low?,the traditional equations describing the relationship between undercooling and amount of martensite formed,such as those proposed by Koistinen-Marburger?K-M?and Magee,are not accurate enough.Therefore,the modification by Hsu concerning the effect of carbon partitioning should be considered.The actual volume amount of martensite may exist between those predicted by K-M?Magee?and Hsu equations.?4?The dynamic compression properties of the above-mentioned low-carbon Q&P steel were investigated over the strain rate range of 500 to2500 s^-1 using split-Hopkinson pressure bar equipment.Traditional quenched and tempered?Q&T?specimen with an identified composition was used for comparison.For both specimen types,the flow stress and yield strength gradually increase as the strain rate increases from 500 to 2000 s^-1,implying that strain-rate hardening dominates the deformation behavior.At a higher strain rate(2500 s^-1),the flow stress and yield strength begin to decrease,indicating that the thermal softening of martensite matrix caused by adiabatic heating begins to overcome the effect of the strain-rate hardening.Based on the Johnson-Cook equation describing the deformation behavior and the parameters calculated by fitting with this equation,the Q&P specimens exhibit less pronounced strain-rate dependencies than the Q&T ones.In addition,the impact shearing tests demonstrated that adiabatic shear failure is less likely to occur in Q&P specimens than in Q&T ones.Analyses revealed that a considerable amount of retained austenite in Q&P specimen is responsible for these differences and,in turn,its stability was affected by the strain rate.At a relatively low strain rate,stress/strain induced martensitic transformation would occur easier.With higher strain rates,martensitic transformation might be suppressed because adiabatic heating decreases its driving force.