Study On The Control Of The Characteristics Of Manganese Sulfide Inclusions In Free-cutting Non-quenched And Tempered Steel F45MnVS

A large number of Mn S inclusions will precipitate in the production of free-cutting non-quenched and tempered steel.On the one hand,the existence of Mn S inclusions will improve the free-cutting performance of steel and reduce the wear of cutting tools during processing.On the other hand,Mn S inclusions are plastic inclusions that have been elongated during rolling,resulting in anisotropy of steel properties.Therefore,studying the evolution law and mechanism of Mn S inclusions in sulfur-containing free-cutting non-quenched and tempered steel during metallurgical processes such as solidification,rolling,and heat treatment is critical.Based on this,the precipitation of Mn S inclusions and the segregation of main solute elements during solidification in F45Mn VS non-quenched and tempered steel were theoretically calculated,and high temperature solidification,simulated rolling,and heat treatment experiments were carried out.The precipitation behavior of Mn S inclusions,as well as the morphological change law and mechanism in subsequent thermal processing,were systematically investigated.The results were are follows:（1）Mn S inclusions in steel are generally precipitated at the end of liquid steel solidification,according to thermodynamic calculations.The Clyne-Kurz and Ohnaka solidification segregation models’calculation results show that the effects of the segregation of the main solute elements in steel on the precipitation of Mn S inclusions are essentially the same,and the two models can be considered almost equivalent.（2）The results of solidification experiments show that the number density of Mn S inclusions increases with cooling rate,but the average size decreases.The nonlinear fitting curve equation describing the relationship between the cooling rate of molten steel and the average size of Mn S is obtained:d=10.726v^-0.438.The solidification model,which is based on two segregation models（Clyne-Kurz and Ohnaka）,is used to calculate the relationship between the average size of inclusions and the cooling rate,which is essentially consistent with the experimental results.（3）Mn S inclusions are mainly elongated during rolling,according to the results of simulated rolling experiments.In general,Mn S inclusions have low relative plasticity under large deformation（75%）,low rolling rate(0.01 s^-1),and high rolling temperature（1200°C）.As a result,rolling should be performed when the relative plasticity of Mn S inclusions is low,so that the deformation of inclusions after rolling is minimal,which helps to improve the mechanical properties of steel after rolling.（4）Through heat treatment experiments,it was found that the morphology of long strip Mn S inclusions changed in the order of strip→cylinderization→spindle→spheroidization during isothermal heating.The experimental and mathematical model calculations of the’steel matrix-Mn S phase’diffusion couple confirmed that the driving force for the transformation of Mn S inclusions is surface diffusion rather than volume diffusion under isothermal heating（1473 K for 0-10 h;1173-1573 K for 3 h）.