| Special steel occupies a very important position in the national economy and military industry,but there is huge gap in production level of special steel in China compared with the developed countries,especially high-quality special steel which is mainly dependent on import.As the typical special steels,D2 die steel and 300M ultrahigh-strength steel play an important role in the manufacturing of high-end equipment,but the properties and service lives of domestic D2 steel and 300M steel are shorter,and there is certain gap compared with international advanced level.The treatment with lanthanum and magnesium is one of the effective methods to improve the properties and service life of special steels.Firstly,D2 steel was taken as the research object and the effects of magnesium on inclusions,carbides and properties were studied systematically,and then 300M steel was also taken to systematically study the effects of lanthanum and magnesium on deoxidation,desulfurization,inclusions and properties.In this work,the thermodynamic analysises of dissolution of magnesium in D2 steel and 300M steel,as well as that of deoxidation,desulfurization and inclusion modifying process by lanthanum and magnesium were carried out.It can be finded that the solubility of magnesium in steel decreases with the increase of melting temperature,and the solubility of magnesium in 300M steel is higher than that in D2 steel because of the influence of content of alloy elements.In addition,deoxidation and desulfurization capacity of lanthanum and magnesium are strong and the desulfurization capacity of lanthanum is vastly superior to that of magnesium.Low melting temperature and low oxygen content are in favor of desulfurization of lanthanum and magnesium.Trace amount of lanthanum and magnesium can modify Al2O3 inclusion,but there must be sufficient lanthanum and magnesium to modify MnS inclusion,and oxygen content,sulfur content and lanthanum content in steel together determine the formation order and type of rare earth inclusions.When the aluminum contents in D2 steel and 300M steel by aluminum-magnesium complex deoxidization are same,D2 steel needs more dissolved magnesium for modifying Al2O3 at 1873 K.With the study of inclusions,carbides,and properties in D2 steel,it is found that the oxygen and sulfur contents significantly decrease after magnesium treatment,and a large number of smaller size inclusions containing-magnesium are formed,namely Mg-Al-O and(Mg-Al-O)+MnS.With the magnesium contents increase,the MgO-Al2O3 phase of inclusions transforms into MgO phase which has a smaller critical nucleation radius that results in easy nucleation and high nucleation rate,thus the inclusions are refined.Meanwhile,the MgO can be the nucleation sites of other subsequently precipitated inclusions,and the average size of inclusions decreases.The network eutectic carbides in the as-cast D2 steels by magnesium treated are interrupted and refined,and the carbides in the forged steels are spheroidized and the average size of that decreases with the increase of Mg content.Magnesium has a high segregation under the annealing temperature to hinder the growth of carbides,and the phase interface energy decreases with the increase of misfit caused by the magnesium which is into carbides.Meanwhile,the annealing aggregation tendency of carburizing carbides decreases due to the increase of chromium content in carbides caused by the improvement of the diffusion rate of chromium by magnesium,so that the carbids are refined and spheroidized.Finally,the thermoplasticity of D2 steel is significantly improved due to the decrease of oxygen and sulfur content and the modification of inclusions and carbides.The elongation and sectional shrinkage of steel that containing 0.0043%magnesium respectively increased by 14.95%and 18.90%.The study on effect of magnesium on cleanliness and properties of 300M steel shows that the oxygen content can be reduced to lower level and the sulfur content also significantly decreases,and using the alloy with lower magnesium content to treat steel can improve the yield of magnesium and reduce the waste.With the increase of magnesium content,the size of inclusions decreases and the phase of inclusions are changed with the route as MgO·Al2O3(0.0014%Mg),Mg0(0.0044%Mg),MgS(0.0084%).Magnesium treatment has little effect on the strength and plastic of 300M steel,but the impact toughness firstly increases and then decreases with the increase of magnesium content.The impact toughness is impoved because the MnS is harmless to the properties of steel due to small inclusions containing magnesium which is as the core of sulfide and the ability of resisting crack formation and expansion of incusions and matrix is improved,and the impact fracture of steel containing 0.0044%magnesium is the highest.The average spacing of inclusions of 3 00M steels containing magnesium increases and the volume fraction decrease,and the inclusions are fine oxide containing magnesium,so the fracture toughness increases and become to be highest when the magnesium content is 0.0044%.When the magnesium content is excessive,the fracture toughess decreases due to the increase of volume frcation of inclusions.In addition,when the steels are treated by self-made magnesium alloys with low magnesium content,the reactions are stable and the magnesium yield are high,and this is conducive to control the content of magnesium and reduce the loss of magnesium.Adding magnesium to steel liquid.with self-made magnesium alloys is a more effective and more reliable way.The results of study on 300M steel with rare earth lanthanum treatment indicate that the oxygen content and sulfur content can be respectively reduced to 0.0002%and 0.0011%by lanthanum,and the number of inclusions decreases,so the cleanliness of 300M steel is improved.The inclusions in steels with lanthanum addition are lanthanum oxysulfide,lanthanum sulfide and composite rare earth inclusions containing P and As which are formed during solidication due to the segregateion of P,As on lanthanum sulfide and oxysulfide which as the nucleation particle,the low oxygen content is beneficial to the form of inclusions containing P and As.The strength of 300M steels is slightly improved by the lanthanum treatment,but the plasticity and impact toughness are significantly improved in a certain range of lanthanum content due to the modification of inclusions and the purification of grain boundary.With the addition of a small amount of lanthanum,the fracture toughness is improved due to the modification of inclusion and the increase of average spacing of inclusions,and it is the highest when the lanthanum content is 0.018%,but the excessive lanthanum content can increase the volum fraction of inclusions,which leads to the decrease of fracture toughness. |
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