Study On Sulfide Precipitation In Solidification Process Of Sulfur Containing Non-quenched And Tempered Steel

The precipitation mechanism and control method of sulfide in cast and forged steel had been studied systematically because of the low sulphide control level in domestic sulfur-containing non-quenched and tempered steel（F40MnVS）.The precipitation characterization of MnS,precipitation conditions of MnS+Al₂O₃ duplex inclusions and effects of sulfide on intragranular ferrite were investigated by using the Thermo-Calc and JMatPro thermodynamic software.The effects of different rate of Mn/S and cooling rates on morphology,quantity and distribution of sulfide inclusions in steel were observed and analyzed by optical microscope（OM）,scanning electron microscopy（SEM）,energy disperse spectroscopy（EDS）.The results were summarized as follows:（1）The thermodynamic calculation were carried out to evaluate the solidification process.The results show that MnS inclusions begin to precipitate at 1431℃.The precipitations of MnS account for 88.5%of total precipitations at the end of solidification.The remaining small parts are formed in the austenite,accounting for11.5%,while MnS inclusions does not precipitate in ferrite.（2）The dynamics of MnS inclusions in the experiment steels were calculated.The results indicate that MnS inclusions are mainly homogeneous nucleation and grain boundary nucleation during solidification process.With the increase of cooling rate,the critical precipitate radius of MnS can be reduced.The morphology and size of MnS can be controlled by changing the content of w[Mn]and w[S]and cooling rates in molten steels.It would be helpful to improve the properties of steel.（3）With the decrease of temperature,Al₂O₃ inclusions are solid in molten steel while MnS inclusions begin to precipitate at the end of solidification,which can improve to precipitate MnS+Al₂O₃ duplex inclusions.The relationship between the push-capture critical rate and the Al₂O₃ size are analyzed and calculated.Small size Al₂O₃ inclusions are easily trapped in the liquid phase of the solidification front,and become the hetero-structure core of MnS,which are useful to form MnS+Al₂O₃duplex inclusions.（4）The morphology,size and distribution of MnS inclusions were studied by changing the rate of Mn/S.The results show that when the rate of Mn/S is 37 in cast steel,the number of MnS inclusion is less;the average siez is smaller and the distribution is more diffuse.The proportion of MnS+Al₂O₃ duplex inclusion in steel is11.2%.The length-width ratio of MnS inclusions in forged steel are relatively smaller.Most of them are spindle shaped,and the spindle rate is 67%.The proper improvement of Mn/S is beneficial to improve the morphology,size and distribution of MnS inclusions in the cast and forged steel.（5）The morphology,size and distribution of MnS inclusions were researched by changing the conditions of cooling rate.The results indicate that the number of MnS inclusion in the air cooling is less;the size is smaller and the distribution is more diffuse by comparing with the furnace cooling.With the decrease of cooling rate,the average precipitation rate of MnS on Al₂O₃ is increased from 5.25%to 10.4%.Reasonable cooling rate would be helpful to increase the spindle rate and decrease detrimental factors on the properties of steel.（6）The precipitations of MnS inclusions in solidification process are effected by the rate of Mn/S and cooling rate,which can effect the microstructure of steel.The results show that the intragranular ferrite nucleates on the sulfide and grows up.It is considered to interacting between the inert interface energy mechanism and the poor Mn zone mechanism.MnS,MnS+Al₂O₃ and MnS-V（C,N）complex inclusions are found to act as effective nucleation sites to improve the formation of intragranular ferrite.