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采用喷射成形快速凝固技术制备了M3型高速钢和以Nb代V的M3型高速钢.利用SEM(EDS),XRD,OM,TEM,HRTEM研究了Nb对M3型高速钢组织的影响和其组织演变.结果表明,喷射成形消除了宏观偏析,细化了组织,以Nb代V,可在共晶反应前析出MC型碳化物,使其球形化、均匀分布,由于消耗大量C,共晶M2C碳化物数量减少,促使更多W和Mo固溶进基体.均匀分布的高热稳定性含Nb-MC型碳化物能阻碍奥氏体化过程中晶粒长大,但难以固溶,使得回火过程中主要析出与基体共格的M2C型碳化物.喷射成形含Nb钢硬度和弯曲强度高于ASP23钢,大量硬质MC碳化物易于产生应力集中,使其韧性稍低于ASP23.

The billets of M3 high speed steel (HSS) with or without niobium addition were prepared via spray forming and compared with traditional cast steels with same composition,followed by hot forged and heat treated.The corresponding microstructure evolutions of steels induced by niobium have been investigated using SEM with EDS,XRD,OM,TEM and HRTEM.The results show that finer and uniformly-distributed grains without macrosegregation appear in the as-deposited HSS that are different to the as-cast HSS,1％ (mass fraction) niobium addition can promote the formation of primary MC-type carbides before onset of eutectic reaction,which can make the MC particles refined and evenly distributed.Niobium mainly contribute to the primary MC-type carbides by consuming carbon,the eutectic reaction is suppressed and the quantity of M2C eutectic carbides decrease,leading to more W and Mo atoms dissolve into matrix.Compared to spray formed M3 HSS,the niobium alloying M3 HSS possesses higher stability during austenitization,induced by the high stabilization of Nb-containing MC carbides,which can pin the grain boundaries and keep the grain size of primary austenite below that of spray formed M3 HSS.The quenched hardness of niobium-containing steel is remarkably higher,while the over tempering hardness of it is a little below than that of M3 HSS,it is related to the difference of dissolution rate of carbides during austenitization and the precipitation behavior of the secondary carbides after tempering.The amount of Nb-containing MC carbides are hard to dissolve into matrix,additionally,lower content of M2C carbides are in the as-deposited steel,leading to the larger numbers of nano-scaled M2C secondary carbides precipitate after tempering.Spray formed niobium-containing steel has a more advanced hardness and bending strength compared with ASP23,but possesses a lower impact toughness due to that the stress concentration can easily caused by mass of harder MC carbides distributed in matrix.