.590 北京科技大学学报 第29卷 Effect of Nb on deformation enhanced ferrite transformation in low carbon steel GUO Shouzhen,YANG Wangyue),CHEN Guoan,SUN Zuqing2) 1)Materials Science and Engineering School.University of Science and Technology Beijing,Beijing 100083.China 2)State Key Laboratory for Advanced Metals and Materials.University of Science and Technology Beijing.Beijing 100083.China ABSTRACI The microstructure evolutions during deformation enhanced transformation of undercooled austen- ite of Nb microalloyed steel and low carbon steel with the similar contents of C and Mn were investigated by hot compression simulation experiment.The effect of Nb on the deformation -enhanced ferrite transformation was studied,and the transformation kinetics was analyzed.The results show that Nb(CN)precipitation promotes ferrite nucleation significantly before deformation enhanced ferrite transformation.Ultra fine ferrite with a grain size of about 1.9Im is formed in Nb-microalloyed steel during deformation of undercooled austenite between A3-A,3.Deformation-enhanced transformation is the dominated process in the early stage of deformation:as ferrite transformation accomplished,the ferrite grain size of Nb microalloyed steel is smaller than that of low carbon steel. KEY WORDS Nb-microalloyed steel;deformation-enhanced transformation:transformation kinetics:Nb(CN) precipitation (上接第585页) Effect of electromagnetic stirring at secondary cooling area on central segregation of a continuously cast slab LIU Yang:WANG Xinhua Metallurgical and Ecological Engineering School,University of Science and Technology Beijing.Beijing 100083,China ABSTRACI Using layer-by-layer shave steel scrap,the effect of electromagnetic stirring frequency parameter at secondary cooling area on the central segregation and equiaxed crystal ratio of a 1450mmX230mm slab was investigated by chemical analysis and macrostructure.On the basis of data analysis,three conclusions are draw n: (1)the central segregation is obviously improved when electromagnetic stirring is applied at secondary cooling area,and the positive segregation coefficients of C.P and Mn are lower than those of no electromagnetic stir- ring:(2)the level of central segregation is all B0.5 when the electromagnetic stirring frequencies are 2,5 and 8 Hz;and(3)electromagnetic stirring frequency has slight influence on the central equiaxed crystal ratio,and the largest equiaxed crystal ratio appeased in the case of 5 Hz frequency. KEY WORDS continuously cast slab;electromagnetic stirring:central segregation;equiaxed crystal ratioEffect of Nb on deformation-enhanced ferrite transformation in low carbon steel GUO Shouz hen 1）‚Y A NG Wangyue 1）‚CHEN Guoan 1）‚SUN Zuqing 2） 1） Materials Science and Engineering School‚University of Science and Technology Beijing‚Beijing100083‚China 2） State Key Laboratory for Advanced Metals and Materials‚University of Science and Technology Beijing‚Beijing100083‚China ABSTRACT The microstructure evolutions during deformation-enhanced transformation of undercooled austen￾ite of Nb-microalloyed steel and low carbon steel with the similar contents of C and Mn were investigated by hot compression simulation experiment．The effect of Nb on the deformation-enhanced ferrite transformation was studied‚and the transformation kinetics was analyzed．The results show that Nb（CN） precipitation promotes ferrite nucleation significantly before deformation-enhanced ferrite transformation．Ultra-fine ferrite with a grain size of about 1∙9μm is formed in Nb-microalloyed steel during deformation of undercooled austenite between A3—A r3．Deformation-enhanced transformation is the dominated process in the early stage of deformation；as ferrite transformation accomplished‚the ferrite grain size of Nb-microalloyed steel is smaller than that of low carbon steel． KEY WORDS Nb-microalloyed steel；deformation-enhanced transformation；transformation kinetics；Nb（CN） precipitation （上接第585页） Effect of electromagnetic stirring at secondary cooling area on central segregation of a continuously cast slab LIU Y ang‚WA NG Xinhua Metallurgical and Ecological Engineering School‚University of Science and Technology Beijing‚Beijing100083‚China ABSTRACT Using layer-by-layer shave steel scrap‚the effect of electromagnetic stirring frequency parameter at secondary cooling area on the central segregation and equiaxed crystal ratio of a1450mm×230mm slab was investigated by chemical analysis and macrostructure．On the basis of data analysis‚three conclusions are drawn： （1） the central segregation is obviously improved when electromagnetic stirring is applied at secondary cooling area‚and the positive segregation coefficients of C‚P and Mn are lower than those of no electromagnetic stir￾ring；（2） the level of central segregation is all B0∙5when the electromagnetic stirring frequencies are2‚5and8 Hz；and （3） electromagnetic stirring frequency has slight influence on the central equiaxed crystal ratio‚and the largest equiaxed crystal ratio appeased in the case of 5Hz frequency． KEY WORDS continuously cast slab；electromagnetic stirring；central segregation；equiaxed crystal ratio ·590· 北 京 科 技 大 学 学 报 第29卷