| Hot-rolled seamless steel pipes,as an important branch of hot-rolled steel,are widely used in shipbuilding,oil and gas transportation and other fields.Because hot-rolled seamless steel pipes were processed at high temperature,it is impossible to refine austenite grain size by controlled rolling.After cooling to room temperature,coarse phase transformation structure occurs,which seriously deteriorates the comprehensive mechanical properties of steel pipes.Most of the research at home and abroad focused on on-line heat treatment and controlled cooling of steel pipes and these two methods had limited ability to refine the microstructure of steel pipes.So oxide metallurgy technology and controlled cooling are proposed in this study.Adding oxides which are stable at high temperature can induce intragranular ferrite transformation,and control the cooling stage after rolling to obtain more intragranular ferrite to refine the microstructure.In this paper,low carbon steels using traditional Al deoxidization and Ti-Mg-Ca-Ce composite deoxidization methods respectively were selected as the experimental materials.Phase transformation temperature and CCT curves of experimental steels with different compositions were measured by automatic dilatometry,which provided a foundation for subsequent heat treatment schedule.The experimental steels were processed by continuous cooling and isothermal treatment.The inclusions induced intragranular ferrite transformation under different treatments were analyzed by optical microscopy,field emission scanning electron microscopy and transmission electron microscopy.The inclusions in contrast steel were mainly Al2O3·MnS with large size.After composite deoxidization,the inclusions in 1#,2#,3#steel are(Ti-Mg-Ca-Ce-Al-O)with MnS.The size of inclusions were refined greatly.Owing to the addition of oxide,the original austenite grain size of composite deoxidized steel was refined to less than 100?m.The microstructure of contrast steel at low cooling rate mainly consisted of coarse lath bainite and Widmanstatten ferrite,while the microstructure of 1#,2#,3#steel mainly consisted of fine lath bainite and acicular ferrite.The cooling rate range of acicular ferrite formation was between 3K/s and 10K/s.Composite deoxidization treatment enlarges the ferrite zone and reduces the martensite zone of the experimental steel.With the decrease of isothermal temperature,the morphology of intragranular ferrite in steel 3#changed from equiaxed to acicular,and the optimum nucleation temperature of intragranular acicular ferrite was 500?.The inclusions inducing intragranular ferrite transformation in 3#steel are(Ti-Mg-Ca-Ce-Al-O)type composite oxides or their composite inclusions with MnS.The mechanism of inclusion-induced intragranular ferrite transformation is the Mn-depleted zone mechanism and the minimum mismatch mechanism.From the curve of isothermal transformation at 500? of 3#steel,it was observed that intragranular ferrite transformation was completed in about 10 seconds,and the grain misorientation angle distribution showed a double-peak feature with an absence of boundary angles between 21° to 47° like bainite transformation.It indicated that intragranular ferrite transformation mechanism is bainitic. |
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