| M3 microstructure(multi-phase,meta-stable retained austenite and multi-scale precipitates)was developed by 973 project(Microstructural Study of Third Generation of High Strength-High Toughness Low Alloy Steel).Experimental and thermodynamic analysis were adopted to study the formation and evolution mechanism of M3 microstructure during the multi-step intercritical annealing.A new heat treatment named multi-step partition process(MSP process)was proposed at here to improve the strength of intercritical annealed steels.Ferrite/bainite multi-phase tailored by developed TMCP(thermal mechanical control crocessing)in 973 project.The deformation behavior of ferrite/bainite multi-phase and the dislocation movement in lath-like microstructure were studied to reveal the special deformation behavior in lath-like structure.Two step intercritical annealing heat treatment was studied in 0.1C-3Mn high strength low alloy steel.Elements was enriched in reverted austenite during the first step intercritical annealing.The concentration of alloying elements in reverted austenite controlled the formation of martensite,which divided the reverted austenite and refined microstructure.The alloy elements concentrated at phase boundary results in martensite located at the center of reverted austenite and leads to the formation of film-like retained austenite.The uniform distribution of alloy elements leads to a random formation of martensite and results in the formation of rod-like retained austenite.The stability of retained austenite was highly improved by the concentration of alloy elements at phase boundary of retained austenite during the second step intercritical annealing.The concentration of alloy elements was attributed to two kinds of phase transformations:I)austenization at austenite/martensite phase boundary,?)ferrite transformation at austenite/ferrite phase boundary.Multi-step partition heat treatment was proposed to obtained martensite and C-Mn enriched retained austenite.In order to testify this heat treatment,0.1C-5Mn experimental steel was used to highlight the effect of Mn partition on the evolution of microstructure.Series of phase transformation and "three ways" partition of elements happened during heat treatment:1)reverted austenite was formed and enriched with C and Mn during intercritical annealing,?)austenitization company with elements partition from enriched zone to depleted zone happened during heating of flash process,lath martensite,?-martensite and twin marteniste formed during cooling of flash process,?)carbon partition to retained austenite during tempering.Concentration of alloy elements at the center of austenite results in kinds of martensite transformations and led to abnormal distribution of stability in retained austenite.The interaction of stress concentration and abnormal stability distribution of retained austenite results in an efficient TRIP effect.Deformation behavior of bainite/ferrite multi-phase microstructure was studied in pipeline steel.Excellent deformability of ferrite was accommodated by the rotation of bainite and led to stress and strain concentration at phase boundary.Rotation of lath structure also happened in bainite and martensite steels.The deformation of high Schmmid factor lathes was accommodated by the rotation of lathes with low Schmid factor.The rotation of bainitic lath is attributed the poor deformability of martenite/bainite lath on the normal direction of lath,which is governed by the length/thickness(l/t)ratio and presented as simple glided in lath.The probability of simple glide is increasing with l/t ratio,and the probability is higher than 50%when l/t>22.The laths parallel with load direction after necking.The contribution of lath rotation for area reduction is-40%-50%during necking.Microcracks formed normal to lath,but inclined to lath with the continued movement of dislocation along lath.The formation of inclined cracks was attributed to the simple glided in lath,which contributs to the formation of delamination phenomenon. |
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