The Research On Microstructure And Properties Of Cold Rolled Low Carbon Microalloyed Steel By CSP Process

Compact strip production(CSP)is one of the most widely used technologies in thin slab continuous casting and rolling technology,it is a short-flow production line combining casting and rolling and it has many advantages,such as low cost,adaptable,high productivity and low energy consumption.It is widely used by domestic and foreign steel enterprises to produce cold rolled strips.However,owing to CSP have some special characteristics,such as fast casting speed,direct heating and fast cooling speed,which makes CSP process to produce steel materials have new research value.Using low-carbon microalloyed steel produced by each process of CSP production line as the research objects,optical microscope,field emission scanning electron microscope,automatic tensile testing machine and other equipments were used to observe the change of microstructure and performance during the slab heating process,coilling temperature and annealing processes,hope to provide guidance for actual production.The main findings are as follows:The effect of heating process on the austenite grain size and the dissolving of microalloying elements in experimental steel slabs was studied in laboratory conditions.The results showed that the experiment steel's austenitic coarsening temperature is 1050 ? and the austenite begins to roughen with hold time afer 15 min.The Nb and Ti microalloyed elements exist in matrix in the form of(Ti,Nb,Mn)C compound particles.When the heating temperature is lower than 1100 ?,most of the second phase particles fail to redissolue.When the heating temperature is higher than 1100 ?,most of the second phase particles are redissolved.The effect of coilling temperature on the microstructure and properties of hotrolled low-carbon microalloyed steel was studied and the strengthening mechanism of experimental steel at different coilling temperatures was analyzed.The results show that when coilling at 580~600 ?,the experimental steel structure is mainly composed of quasi-polygonal ferrite,granular ferrite and a small amount of pearlites with a small sheet spacing.When coilling at 620~640 ?,the experimental steel structure is mainly composed of polygonal ferrite and a large number of pearlites with a large sheet spacing.There are two sizes of(Ti,Nb)C second-phase particles distributed in the experimental steel,both of which are spherical.The first type is(Ti,Nb)C particles with a larger size(70~90 nm),there is almost no effect;the second type is(Ti,Nb)C particles of nanometer size(4~10 nm),which can significantly improve the performance of experimental steel.The main strengthening methods of experimental steel are fine grain strengthening,solid solution strengthening and precipitation strengthening.The effects of the annealing process on the microstructure,performance and yield ratio of cold-rolled low-carbon microalloyed steel were studied under laboratory conditions.The results showed that the recrystallization temperature of experimental steel is about 670 ?,when the cold strips are annealed between 490 ? and 580 ?,the ferrite grain morphology change little,it is still elongated,the number of cementite distributed in the structure is small and its yield strength and tensile strength change little,yield ratio remains above 0.9,the elongation is about 1 %;when the cold strips are annealed between 610 ? and 700 ?,undistorted recrystallized grains appear around the deformed ferrite,the recrystallized grains will grow into equiaxed ferrite with the increase of annealing temperature,the spheroidization process of cementite leads to an increase in the number of freely distributed granular cementite in the structure and its yield strength and tensile strength rapidly decrease,the elongation rate rapidly increases,the yield ratio shows a downward trend,reaching a minimum of 0.86 at 670 ?;when the cold strips are annealed between 730 ? and 760 ?,the ferrite grains grow sufficiently,the grain morphology changes little,there is almost no freely distributed cementite in the structure,cementite aggregates and distributes only in grain boundaries and its yield strength and tensile strength slowly decrease,the elongation slowly increases,the yield ratio increases,reaching a maximum of 0.91 at 760?.