| The morphology and distribution of MnS significantly affect the cutting performance and stress concentration,can destroy the continuity of the matrix,making chip breaking easier in cutting process.On the other hand,MnS is a plastic inclusion,which is easy to deform along the rolling direction during rolling process,leading to the anisotropy of mechanical properties of steel.Therefore,it is necessary to control the morphology and distribution of MnS in order to achieve the performance matching of the mechanical properties and cutting performance of sulfur free-cutting steel.In this paper,the influence of deoxidation mode and magnesium treatment intensity on composition,quantity and particle size of sulfur free-cutting steel were investigated by vacuum induction furnace experiment and hot rolling simulation experiment.In addition,the mechanical properties and cutting properties of the rolled steel samples were tested,and the effects of magnesium treatment on the properties of ch2 free cutting steel were discussed in combination with the micro structure and inclusion state of the steel under different magnesium treatment intensities,and the main conclusions are as follows:There are two main forms of sulfide in as-cast steel:single-phase sulfide which exists alone and complex sulfide which precipitates out of oxide.In the aluminum deoxidized steel,the core oxide in the complex phase sulfide is mainly Al2O3 or MgAl2O4.With the increase of magnesium treatment strength,the content of magnesium in the core oxide increases and the ratio of magnesium to aluminum increases.The oxide changes from Al2O3 to MgAl2O4.Most of the sulfide compounds in as-cast steel are single phase II sulfide compounds distributed in clusters or chains along grain boundaries.With the increase of magnesium treatment strength,the number density of sulfide in steel decreases,the average particle size and area increase,and the average length-width ratio increases.The inclusion types of sulfide gradually changed from type ? and type ? to type ?.Sulfur compounds in sulfur free-cutting steels are elongated along the rolling direction after rolling.Magnesium treatment of sulfur free-cutting steel reduces the average length-to-width ratio of sulfur compounds in rolled steel,increasing the number of spindle-like sulfur compounds with ?L/?W less than 3,decreasing the number of elongated sulfur compounds with ?L/?W greater than 9,and increasing the dispersion of sulfur compounds in the steel.The rolling structure of sulfur free-cutting steel is "polygonal ferrite+pearlite".Increasing the treatment strength of magnesium can reduce the pearlite content,ferrite grain size,and improve the zoned distribution of pearlite and ferrite.In addition,magnesium treatment can change the decomposition products of supercooled austenite and degrade the morphology of pearlite to some extent.The fine dispersed magnesium inclusions produced after magnesium addition can effectively induce the formation of acicular tissue and refine grains to some extent.Magnesium treatment can improve the machinability of steel significantly without deteriorating its mechanical properties.With the increase of magnesium treatment strength,the impact toughness,tensile strength and yield strength of the experimental steel decreased,but the mechanical properties were within the range of the reference steel properties.After magnesium treatment,the cutting performance of the steel sample was significantly improved,and the chip morphology changed from"long and compact type" and "spiral type" to "C-type" chips,which were more easily broken.The number of broken chips per kilogram was greatly increased.In addition,magnesium treatment can improve the surface quality of the workpiece after processing. |
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