Study On Dissolution And Precipitation Mechanism Of Manganese Sulfide In Steel

In today’s society,high-quality,high-performance sour steel plays an important role in the development of the construction of our country,such as free cutting steel,and non-modulated steel and so on,manganese sulfide spindle in sour steel can lubricate the cutting tool in the cutting process to improve the steel cutting performance;However,the inclusion of long strip manganese sulfide in sulphur-containing steel will seriously affect the properties of steel and deteriorate the quality of steel.Therefore,finding out the formation mechanism of manganese sulfide inclusion in sulfur steel and adopting effective production measures and control methods can not only reduce the amount of manganese sulfide in steel,control the form and distribution of manganese sulfide,but also effectively enhance the performance of sulfur steel and solve many problems caused by manganese sulfide in the production process,which is of great importance for improving the steel quality of sulfur steel.In this thesis,manganese sulfide inclusions in sulfur steel are taken as the research object.First,two groups of experimental steels with different sulfur and manganese contents are prepared by high-temperature tube furnace smelting.Then,the manganese sulfide inclusions are analyzed by energy dispersion spectrometer to determine their composition content.The relationship between the contents of manganese and sulfur and the morphology,size and distribution of manganese sulfide was obtained.High Temperature Confocal Laser Scanning Microscope（CLSM）was used to observe the changing behavior of manganese sulfide in the melting and solidification process of experimental steel,and analyze the dissolution and formation law of manganese sulfide in the process of heating and cooling.Thermodynamic model and kinetic model were used to calculate and analyze the control factors affecting the dissolution and precipitation process of manganese sulfide.Materials Studio simulation software was used to construct and optimize the structure of manganese sulfide clusters（Mn S）_n（n=1～10）,and the binding energy and Gibbs free energy of these ten clusters were calculated.The nucleation and growth mode of manganese sulfide from atom to crystal was proved by combining the two-step nucleation theory.The results are as follows:（1）The smelting experiment results show that the higher the sulfur content in steel,the more spherical the shape of manganese sulfide,the higher the spindle rate of manganese sulfide in steel,the larger the size of manganese sulfide in steel,the more manganese sulfide.The distribution of large size manganese sulfide is dispersive,and the distribution of small and medium size manganese sulfide is more dense,and the distribution is chain.（2）The results of in-situ observation show that the melting temperature of manganese sulfide with a diameter of 50μm in 2#steel is 1517.6℃,and when the temperature rises to 1524.3℃,the manganese sulfide in steel is completely dissolved and disappeared.During the solidification process,manganese sulfide begins to precipitate along austenitic grain boundaries at 1307.1℃and increases with the decrease of temperature.When the temperature drops to 202.5℃,manganese sulfide basically precipitates completely.Most of the manganese sulfide in the center of the experimental steel is spherical,and some of the manganese sulfide in the edge of the experimental steel is strip shape.The dot and face scan analysis of the two types of inclusions by energy spectrometer shows that the content of manganese in strip type manganese sulfide is lower than that in spherical type manganese sulfide.（3）The thermodynamic calculation results show that the equilibrium concentration of manganese sulfide dissolution increases with the increase of temperature,indicating that temperature rise is conducive to the dissolution of manganese sulfide.With the increase of sulfur content in steel,the precipitation temperature of manganese sulfide increases,indicating that the increase of sulfur content helps advance the precipitation time of manganese sulfide.The kinetic calculation results show that the dissolution time of large size manganese sulfide is longer,the dissolution rate decreases gradually with time,and the size temperature of manganese sulfide during precipitation decreases and increases.The larger the cooling rate is,the smaller the final size of manganese sulfide is.The results of segregation model show that the segregation of manganese sulfide solute increases with the increase of element concentration,and the increase of carbon silicon content can increase the activity of sulfur element and lead to the precipitation of manganese sulfide in advance.The theoretical values of manganese sulfide precipitation temperature in two groups of experimental steel are 1508.10℃and 1521.32℃,respectively.（4）Ten cluster structures of manganese sulfide were constructed and optimized using Materials Studio software,and the binding energy and Gibbs free energy of the clusters were calculated.The results showed that manganese sulfide clusters（Mn S）₁had linear structure,（Mn S）₂and（Mn S）₃had sheet structure,and the rest clusters were cage structure.With the increase of Mn-sulfur atoms in the cluster,the size of the cluster increases,the bond length of the cluster structure becomes more and more similar to the bond length of Mn-sulfide crystal,the binding energy of the cluster decreases,and the energy difference between the cluster and crystal also decreases,which indicates that Mn-sulfide cluster structure tends to change to crystal structure.The Gibbs free energy calculation results show that manganese sulfide clusters can stably exist at low temperature and transform into stable crystal structure.The lower the temperature,the stronger the driving force of transformation,which proves that the evolution process of manganese sulfide from atom to cluster to crystal conforms to the law of thermodynamics.