| Morphology,spatial distribution and segregation of sulfide in the S-bearing special steels are two key indexes for the evaluation of steel quality.Based on the cooperation projects of Shanghai University and Zhongtian Steel Co.,Ltd.,with Ma Steel Group,this work aims to carry out the study to find out the laws of sulfide precipitated in the billet,segregation of sulfide,spatial distribution of sulfide,and variation of morphology of sulfide,and also to investigate the effect of Mg and Te modification on the sulfide segregation in the billet,which provides a basis study for property and morphology of steel.Based on the high-S SAE1144 steel and medium-S 46Mn VS non-tempered steel billet,metallographic microscope,scanning electron microscope,non-aqueous electrolytic corrosion and other methods were used to analyze the type,spatial distribution and three dimensional morphology of sulfide in the two steels in this work;the modification effect of Mg and Te on the sulfide was compared;the segregation law of sulfide in the billet was summarized;the inner correlation and affected mechanism of segregation and spatial distribution of the sulfide were explored.The main conclusions are made as follows:(1)The type of sulfide in SAE1144 steel was the main(Mn,Fe)S.The sulfide in the chilled grain zone was mainly in single-type ellipsoid shape,with the sizes of 2.6?m in diameter,and 5?10?m in length.The number density of sulfide in the steel was1300 N/mm2.The shape of sulfide in columnar grain zone mainly included spherical,rodlike and irregular types.The area and length of the clustering sulfide in the steel increased from 39.2%to 49.1%and 67.4?m to 101.2?m,respectively.The percentage of the single-type sulfide area reduced from 49.7%to 41.6%.The length of the single-type sulfide gradually increased from 3.7?m to 6.0?m.However,the density of sulfide was reduced from 496 N/mm~2 to 444 N/mm~2.The chain-type sulfide occupied about14%of all,with small fluctuation.In the center equiaxed grain zone the sulfide type mainly included clustering,chain and single-type shapes.From the cooling boundary to center,the percentage of sulfide in clustering shape increased from 57.1%to 75.1%.The length of the single-type sulfide gradually increased from 105.8 to 167.9?m.The occupied percentage of sulfide in chain shape reduced from 14.8%to 9.7%.The single-type sulfide reduced from 28.1%to 15.1%,with the smaller fluctuation of size;the density of sulfide reduced from 287 N/mm~2 to 190 N/mm~2.(2)Two types of sulfide in 46Mn VS steel were:the main(Mn,Fe)S,with Fe atomic content of 15%?25%,and the composite inclusions formed by the nucleation of Al2O3.The three morphologies of sulfide in the chilled grain zone were mainly in irregular type,accounting for about 77%;a small number of sulfides were in regular ellipsoid and octahedral type,with the sizes of 5?10?m.The three morphologies of sulfide in the columnar grain zone mainly included octahedral,irregular and ellipsoid types.From the cooling boundary to center,the percentage of sulfide in octahedral shape gradually reduced from 53.5%to 36.4%,while that of sulfide in irregular and ellipsoid shape was respectively increased from 40.0%to 55.8%,and 6.5%to 7.8%,with the most sizes of about 30?m.In the mixed grain zone the morphologies of sulfide were mainly in irregular,octahedral and ellipsoidal shape,with the percentage of 65%,27%and 8%respectively;the sizes of sulfide were in range of 30?40?m.In the equiaxed grain zone the morphologies of sulfide included octahedral,lamellar,and irregular shapes.From the cooling boundary to center,the percentage of sulfide in octahedral shape gradually reduced from 17.6%to 6.8%,while that of sulfide in lamellar shape was increased from 11.3%to 20%;the percentage of sulfide in irregular shape gradually reduced from 71.1%to 62.4%,with the most sizes of above 30?m.(3)S element in SAE1144 as cast steel had a serious macro-segregation.In the center region the segregation index of S element was maximum,1.30.In the starting end of the equiaxed grain zone,the negative segregation index was maximum,0.89.In the center of 46Mn VS steel the segregation index was maximum,1.13,while in the 1/4mixed grain region of the steel the segregation index was 1.08,just smaller than that in the center region.After Mg treatment,the segregation index of S element in the steel reduced from 1.3 to 1.04.After Te modification,the segregation index of S element was reduced from 1.13 to 1.06.Thus,both Mg and Te modifications could remarkably reduce the segregation.With the micro-segregation DCG model,the micro-segregation of solute was calculated and the results show that under solid fraction of fs=0.46 and temperature of Tl=1458.8?,Mn S in liquid steel began to generate,at this time with S concentration of 0.435%in the residual liquid solute.Solidification was over at 1392.1?,timely with S concentration of 0.640%in the residual liquid solute,1.56 times the content of S in the initial liquid steel.It shows that S element was enriched in the final phase of solidification.However,for the numerical calculation of 46Mn VS steel,under fs=0.80and Tl=1425.0?Mn S in liquid steel began to generate,timely with S concentration of0.28%in the residual liquid solute,4.6 times the content of S in the initial liquid steel.The solidification was over at 1392.1?,timely with S concentration of 0.19%in the residual liquid solute,1.17 times the content of S in the initial liquid steel. |
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