Effect Of Magnesium On Microstructure And Inclusions Of H13 Hot-working Die Steel

At present,both common steel production scale and quality in China has reached a higher level.But in special steel,especially die steels fields,there is still a big disparity compared with the developed countries.The most important reason for this is inclusions and carbide problem in steel.The domestic die steel have a uneven distribution of carbide and inclusions,coarse particles and other shortcomings which seriously affect the performance of the mold.Based on these reasons,this paper puts forward a new way to improve the quality of steel by adding magnesium to steel,observing the effect of magnesium on carbide and inclusions.In this paper,the H13 hot working die steel has been used as the research object,and the medium frequency furnace was used to smelt magnesium containing H13 steel.In order to study the effect of magnesium content on the carbides and inclusions in steel,0 0.001%,0.0025%,0.004% and 0.0055% of Mg were added to the steel respectively In the experiment.At the same time,the conditions of the refinement of the carbides were analyzed from the view of dynamics and thermodynamics,and analysed the principle modification of inclusions by magnesium.The conclusions are as follows.(1)The effect of magnesium on the carbides was studied through adding magnesium in steel.The result shows that the network carbide was broken after adding a small amount of magnesium in the steel.The shape and distribution of carbide in steel was improved,while the carbides in organization also become move evenly and tiny.There is a content range of magnesium refining carbides in H13 steel.In this range,the degree of carbide refinement become higher with the increase of magnesium,but when the addition of magnesium content reaches a certain level,the effect on refining carbides by magnesium becomes worse until the effect disappears finally.When the content of magnesium is 0.004%,the refining effect is best.(2)The condition of dynamics and thermodynamics of refining carbides by magnesium were analysed.It is found that the tendency of magnesium element to grain boundary is larger,and the degree of segregation is higher in the solidification cooling process of steel.When the magnesium element is segregated to the grain boundaries,the precipitated carbides form a carbide crystal with magnesium as the nucleation core.(3)The effect of magnesium on the segregation of main alloying elements in steel and the density of steel was investigated.The result show that the addition of magnesium to steel can improve the segregation of C,Cr,Mo,V and the density of the microstructure,and there is also an optimum range of magnesium content.In this range,with the increase of the content of magnesium in steel,the effect of magnesium on element segregation become better.When the content of magnesium beyond the optimum range,the effect of improving the element segregation reduce.When the content of magnesium in the steel is 0.004%,the effect of magnesium on the element segregation and the density of the microstructure is better,at the same time,the statistical segregation degree of each element is the smallest,the statistical uniformity reaches the maximum,and the tissue density is the highest.(4)The effect of magnesium on inclusions in steel was investigated,the results of which show that magnesium can change the size and distribution of inclusions in steel.With the increase of magnesium content in steel,the effect of improving inclusions become better when the magnesium content is in a certain range.And when the magnesium content continue to increase,the effect will become worse.In addition,the type of inclusions is modified to change when the magnesium is added into the steel.MgO?Al2O3 inclusions appear in steel after the treatment by magnesium.Since the binding ability of magnesium and sulfur is stronger than that of manganese and sulfur,when the magnesium are added into the steel,it will be combined with the sulfur first and restrain the formation of MnS inclusions,which makes the MnS inclusions in the steel change into MgS inclusions.