Investigation On The High Strength-ductility And Its Mechanism Of High Carbon Low Alloy Quenching-Partitioning-tempering Steels

In 2007,quenching-partitioning-tempering(Q-P-T)process was proposed by Academician Zuyao Xu(T.Y.Hsu).After that,our group has systematically researched low carbon and medium carbon low alloy Q-P-T martensitic steels for 10 years.Both strength and ductility of Q-P-T martensitic steels were enhanced with rising carbon content ranging from low carbon to medium carbon.Whether both the strength and ductility of Q-P-T steels can be further enhanced by increasing carbon up to high carbon range or not,which is a big challenge our group faces to in recent years.For the sake,we designed several kinds of high carbon steels with different compositions,and treated them by different Q-P-T processes.The influence of microstructure on mechanical properties was focused,and three plasticity enhancement effects of retained austenite were concerned on:Transformation induced plasticity(TRIP)effect,dislocation absorption by retained austenite(DARA)effect and blocking crack propagation(BCP)effect.The mechanisms of high strength-ductility and toughness enhancement were revealed.The main conclusions are summarized as follows.(1)A high carbon containing Nb(Fe-0.63C-1.52Mn-1.49Si-0.62Cr-0.036Nb (wt.%),Abbreviation:Fe-0.63C-Cr)steel was designed.Then the samples were treated respectively by traditional quenching and tempering(Q&T)and novel Q-P-T heat treatment processes for comparison.The quenching temperature(T_q)of Q-P-T process was 160?and the partitioning/tempering temperature and duration were 400?and 60 seconds.The only difference betwee two processes lies only in quenching temperature.The result indicated that the volume fraction of retained fcc austenite(V_RA)was determined as 29.1%in Q-P-T sample and 7.1%in Q&T sample.The tensile tests of Q-P-T steel samples show a tensile strength of 1950 MPa,a total elongation of 12.4%and a high product of strength and elongation(PSE)of 24180 MPa%.The tensile tests of Q&T steel samples show a tensile strength of 2360 MPa,a total elongation of 6.1%and a low PSE of 14396 MPa%.It can be seen that novel Q-P-T process prominently improves the PSE of high carbon Q-P-T martensitic steel,comparing with traditional Q&T process.However,the PSE of high carbon steel is lower than that of low carbon or medium carbon Q-P-T steels,and this study cannot reach the expected objective.(2)The average dislocation densities in both martensite and retained austenite in Fe-0.63C-Cr Q-P-T samples at different strain stages were measured by XLPA(X-ray diffraction line profile analysis),indicating that DARA effect exists in high carbon Q-P-T steel.Although some twin-type martensitic plates embed in dislocation-type martensite matrix,but the few number of twin boundaries does not suppress DARA effect during deformation.It is wothty to point out that there is 7.1%of V_RA in Q&T sample,which is less than the lowest value of 10%for the generation of DARA effect,but there still is a weak DARA effect.(3)Normalization process was introduced for high carbon Fe-0.63C-Cr steel as a pretreatment of Q-P-T process so that relative dispersive and fine retained austenite can be obtained accompanying with the decrease of the brittle strain induced twin-type martensite during deformation.The high carbon Q-P-T steel with almost same V_RA of about 28%exhibits a tensile strength of 1890.8MPa and a total elongation of 28.9%and a ultrahigh PSE of 54.644 MPa%.The high strength-ductility mechanism of high carbon Q-P-T steel is revealed as:ultrahigh strength of high carbon Q-P-T steel stems from high carbon martensitic matrix,while high ductility is attributed to twofold softening of martensitic matrix,that is,a softening is caused by partition of carbon during Q-P-T process and another softening is caused by DARA effect during deformation.Twofold softening effects effectively enhance the deformation ability of martensitic matrix.(4)Based on the experimental fact that the strain-induced twin-type martensite seriously deteriorates the ducitily of high carbon Q-P-T martensitic steel,an idea of Anti-TRIP was proposed in this study,which is consistent with calculated results of Matlock and Speer,namely,more stability retained austenite is during deformation,higher the ductility of BCC matrix steel is.In addition to the normalizing process,the cold rolling as a pretreatment has been proved to improve the mechanical stability of retained austenite.The cold rolled high carbon Q-P-T steel exhibits tensile strength of 1880MPa and total elongation of 26.1%,which verifies the correctness of the design idea of Anti-TRIP effect used in high carbon martensitic steels again.(5)High carbon Q-P-T martensitic steel exhibits higher strength and ductility than medium and low carbon martensitic Q-P-T steels by means of either normalizing or cold-rolling as the pretreatment of Q-P-T processing,which realized the strength and ductility of the steel are both enhanced by the increase of carbon,and the related principle and mechanism were clarified,which reads a century-dream of investigators.(6)After finding the relation between the volume fraction of retained autenite and toughness,an idea of improving toughness was proposed,that is,the toughness is improved by reduction of the volume fraction of retained austenite.The combination of the removal of Cr element from Fe-0.63C-Cr steel with nomalization process as pretreatment of Q-P-T process effectively decreases chunky retained austenite by reducing V_RA and markedly enhances the mechanical stability of retained austenitem,which results in the toughness enhancement of high carbon Fe-0.67C-1.48Mn-1.53Si-0.038Nb(wt.%)martensitic steel.But the V_RA is still kept about 10%,which satisfies the generation condition of DARA effect.The PSE of high carbon Fe-0.67C-1.48Mn-1.53Si-0.038Nb martensitic steel reached to 51294MPa%.Besides,the average impact toughness value of three impact samples with notch reached to 33.2J/cm²,which is about quintuple that of high carbon steel containing Cr(Fe-0.63C-Cr).(7)The microstructure of a high carbon low alloyed Q-P-T low density steel(Fe-0.67C-1.55Mn-0.60Cr-3.98Al-0.038Nb(wt.%)),in which Si is replaced by4 wt.%Al,is?-ferrite+martensite+retained austenite.The?-ferrite Q-P-T steel shows a tensile strength of 1240MPa and a total elongation of 36.9%,accompanying with density of 7.48g/cm³,and thus the special product of strength and elongation(SPSE)of high carbon low density steel reached to6.1GPa%/gcm^-3.In the view of effect of tensile specimen size on elongation,elongation conversion based on ASTM E8 standard specimen geometry was used,and the elongation value of the?-ferrite Q-P-T steel(36.9%)decreases to25.3%,but the SPSE is still up to 4.2GPa%/gcm^-3 and is superior to the upper limit of the low-and medium-carbon low alloyed?-ferrite steels(2.7 to 4.0GPa%/gcm^-3)reported until now.