Effect Of Rolling Process Parameters On Microstructure And Inclusions Of Bismuth Free-cutting Steel

In this thesis,bismuth-containing free-cutting steel was studied and the thermal simulation experiments were carried out by thermal simulation tester under different rolling conditions.Optical microscope and scanning electron microscope were used to observe the evolution of microstructures and inclusions in bismuth-containing freecutting steel under different rolling conditions.The effects of rolling parameters on the microstructures,especially the morphology,size and distribution of inclusions in free-cutting steel with bismuth were studied.The main research contents and results are as follows:(1)The microstructure of the sample is mainly composed of ferrite,pearlite and inclusions.Ferritic content is more than 90%,pearlite content is less,generally exists in the grain boundary.Scanning electron microscopy(SEM)shows that the inclusions in steel are mainly spherical,rod-shaped and fusiform,and the proportion of rodshaped inclusions in steel decreases with the increase of heating rate.No inclusions exist in No.2 steel,and the size of inclusions in No.3 steel is the largest than the size of inclusions in the other three steels.All in all,70% of the inclusions in the other three steels are 2～3 μm in size.(2)The results of laboratory thermal simulation show that with the increase of deformation,the percentage of ferrite increases,while the grain size of ferrite decreases.The inclusions decrease with the increase of deformation.Among the 5 test steels,No.2 steel has no inclusions,and No.3 steel has the most inclusions.The other three inclusions are fine spherical,rod-shaped and fusiform inclusions with average grain size of about 2 μm.(3)The results of laboratory thermal simulation show that the microstructure of bismuth-containing free-cutting steel does not change with the change of rolling pass.The microstructure is still dominated by ferrite and contains a small amount of massive lamellar pearlite.The inclusions in the five kinds of steels are mainly spherical,rod-shaped and fusiform.There is no manganese sulfide inclusion in No.2steel,the average grain size of the inclusion in No.3 steel is about 7 μm,and the average grain size of the inclusion in the other three kinds of steel is about 2 μm.(4)The experimental results show that with the increase of cooling rate,the grain size of ferrite decreases obviously,and pearlite particles appear in the structure.The inclusions in steel are mainly spherical,rod-shaped and fusiform.The average size of inclusions in steel decreases with decreasing cooling rate.There are no inclusions in No.2 steel,only some white bismuth elemental highlights.Due to the low sulfur content in No.2 steel,manganese sulfide inclusion cannot be formed with manganese element.(5)According to the results of the thermal simulation test,it can be found that when the heating speed is 10 ℃/s,the deformation is 7 mm,the rolling pass is 3passes,and the cooling speed is 30 ℃/s,the shape of the inclusions in the steel is mainly spherical and fusiform,and the ferrite grains in the steel are the smallest.