The Study On Re-dissolution And Precipitation Behavior Of Second-Phase Particles In High Strength Hot-rolled Dual-phase Steel

The re-dissolution behavior of second-phase particles of the high strength hot-rolled dual-phase steel was carried out in box-tybe electrical resistance furnace of lab, the continuous cooling transformation of high strength hot-rolled dual-phase steel and hot-rolled simulation experiment in condition of different rolling parameters was carried out on MMS-200 thermal simulation experiment machine.The microstructure of high strength hot-rolled dual-phase steel was observed by the optical microscope technique and transmission electron microscope analytic technique.The precipitation behavior of second-phase particles of high strength hot-rolled dual-phase steel was studied by extraction-replica technique . The main results are as follows:1.The microstructure of test steel under different heating temperature and different holding time shows that: with the rise of temperature, the austenitic grain grows up and the precipitation amount of second-phase particles decreases, so did the size.with the rise of the holding time, austenitic grain grows up gradually, the precipitation amount of second- phase particles decreases, and the size of second-phase particles decreases. In the second-phase particles, the size of paticles is bigger and whose shape is rectangle and square, the composition is AlN,The size of second-phase paticles is smaller and whose shape is square and irregular,the composition is TiN by the EDX analysis.2.the continuous cooling transformation curve (CCT) of test steel is formed according to the experimental data of continuous cooling trans- formation and the metallographic microstructure observation of test steel. The high temperature transformation product of austenite is ferrite and pearlite, while the low temperature transformation product is martensite The dual-phase steel with ferrite and martensite was formed with the cooling speed over 5℃/ s. Under the same deformation, the micro- structure of multi-passes is clear finer than that of single pass.3.The microstructure was observed by optical microscope, which proved that: under the condition of double passes with finish rolling, the size of ferrite grain is finer and the volume fraction of martensite is samller , with the increase of deformation. Under the condition of double passes with rough rolling, the size of ferrite grain is finer and the volume fraction of martensite is larger,with the increase of deformation.Under the conditon of multi-passes with finish rolling, the size of ferrite grain is finer and the volume fraction of martensite is larger,with the increase of deformation. The size of ferrite grain becomes finer and the volume fraction of martensite is smaller with the decrease of the finish temperature.The size of ferrite grain becomes finer and the volume fraction of martensite is larger with the decrease of the cooling speed .4.The precipitation regularity of second-phase particles with different rolling process parameters was studied by extraction-replica technique, which showed that the size of second-phase particles decreased obviously with the increase of the deformation and the cooling speed, and the decrease of the finishing temperature; the shape of second-phase particles changed gradually from big rectangle ,square to fine circle, whose com- position is(Nb,Ti)C.