Study On The Precipitation Law And Action Mechanism Of Nano-carbides In Low Carbon Ti/Ti-Mo Steels

Ti microalloyed high strength steel is a new type of steel material with low cost and high performance.The stability control of its strength has been an important topic in this field.With the progress of steelmaking technology,the problem of steel composition fluctuation has been basically solved,and the problem of performance fluctuation caused by the sensitivity of nano-precipitates to the TMCP process parameters is still very prominent.How to give full play to the precipitation strengthening effect and stably control the precipitation are the key to the development of Ti microalloyed high-strength steel.In this work,based on the composition of low carbon steel（labelled C-Mn steel）,the low carbon Ti steel and Ti-Mo steel were designed by adding trace amount of Ti and Mo elements.The precipitation rule of nano-carbide under the new TMCP was investigated,by means of the thermal simulation test machine,transmission electron microscope（TEM）and three-dimensional atomic probe（APT）.The interaction mechanism between the precipitation of nano-carbides and physical metallurgical behaviors such as austenite recrystallization and phase transformation was clarified.The strengthening mechanism and stabilization control mechanism of the steels were revealed,and the physical metallurgy data of"process-precipitation-strength"was established,which provided design ideas and theory for stable production of the low cost and high strength Ti microalloyed steel.The main work and conclusions are as follows:（1）The strain induced precipitation behavior and its relationship with recrystallization of austenite in Ti steel after rough rolling and finish rolling were studied by the stress relaxation method.The strain induced precipitation-temperature-time（PTT）curves were C-shaped with the nose temperature about 900℃.The introduction of roughing process accelerated the strain-induced precipitation,and reduced the nose temperature.The static recrystallization of deformed austenite was almost completely inhibited at the initial growth stage of strain induced TiC particles,and the softening occurred again after coarsening of TiC particles.（2）The relationship between austenite continuous cooling transformation and precipitation was studied.The interphase precipitation was very sensitive to the changes in the cooling rate,and was only formed in the Ti steels at very low cooling rates（V≤0.5℃/s）.When the cooling rate exceeded 3℃/s,the precipitation of carbides was completely suppressed.Compared with the C-Mn steel,the microhardness of the Ti steel and Ti-Mo steel was significantly improved,with the largest increase at 0.5℃/s,increasing by 132.4 HV and 118.5 HV,respectively.The maximum microhardness of Ti steel was 279.5±18.9 HV at 0.5℃/s in the cooling rate range of0.1-30℃/s.However,the microhardness of different ferrite grains varied greatly due to the inhomogeneity of precipitation.Compared with the Ti steel,the Ti-Mo steel did not show better precipitation strengthening effect,and even was weakened,due to its narrow precipitation temperature window.（3）The austenite isothermal transformation curves（TTT curves）of C-Mn steel,Ti steel and Ti-Mo steel were inverted double"C"shape.The addition of Ti and Mo elements decreased the nose temperature,and inhibited the austenite isothermal transformation rate.Furthermore,based on the TTT curves and isothermal yield strength increment method,the precipitation kinetics of carbides,and its relationship withγ→αisothermal transformation were studied.The results showed that the isothermal precipitation initiation curves of the Ti steel and Ti-Mo steel were both"C"shape,with the nose temperatures about 600℃and 625℃,respectively.The end curves were inverted half"C"shape.The addition of Mo prolonged the isothermal precipitation end time（P_S）of carbides.The precipitation of carbides mainly occurred in theγ→αisothermal transformation stage,that is,the phase transformation significantly promoted the carbides precipitation.The change of austenite transformation rate leaded to the coexistence of interphase precipitation and random precipitation,and the characteristics of interphase precipitation with different inter-sheet spacing in ferrite grain.（4）The results invested by APT and TEM showed that the precipitates were mainly spherical shaped,and kept Baker-Nutting（B-N）orientation relationship with ferrite matrix.The maximum density of precipitates in Ti steel and Ti-Mo steel were 4.38×10²³ m^-3 and 1.05×10²⁴m^-3steel,respectively,obtained at nose temperature for holding time of P_S.The corresponding average sizes of the particles were 5.2±0.6 nm and 4.6±0.5 nm,and the chemical formulas were close to that Ti₅₄C₄₆and(Ti₄₅,Mo₂₃)C₃₂,respectively.In the Ti-Mo steel,the precipitation of Ti-C occurred at first,and then the Mo replaced the Ti to participate in precipitation,which made the carbides size smaller with more uniform size distribution,showing more excellent resistance to coarsening.（5）During the austenite isothermal transformation process,the precipitation strengthening of nano-carbides was the main reason for the increase of the microhardness in Ti steel and Ti-Mo steel.The largest precipitation strengthening contribution and microhardness of the two test steels were obtained isothermally transformed at the nose temperature for Ps time,which were366 MPa,312±10.7 HV and 384 MPa,322±8.2 HV,and were 122 HV and 138 HV higher than that of C-Mn Steel under the same heat treatment,respectively.Compared with the C-Mn steel,the Ti steel and Ti-Mo steel have no obvious effect on grain refinement and dislocation density enhancement,and the maximum increment was about 62 MPa.Compared with the Ti steel,the strength and stability of the Ti-Mo steel both increased.But longer isothermal time was needed for carbides precipitation in the Ti-Mo steel.（6）The strain-induced precipitation in austenite weakened the precipitation strengthening effect in ferrite,resulting in a significant decrease in microhardness.In order to obtain the maximum strength,the strain-induced precipitation in austenite should be avoided,and the alloying elements should be retained and precipitated in the ferrite as much as possible.