| The cast alloy die steel is mainly used to produce the large-scale cover panel, whichrequires the die steel exhibits excellent mechanical properties. In this study, the cold workingdie steel is fabricated by casting instead of forginghas characteristics of high hardness, goodwear resistance and no quenching crack. For Alloying and heat treatment are frequently usedto improve the mechanical properties of steel. That Rare Earth as alloying elements are used indie steel in recent years has drawn much attention. The addition of Rare Earth can significantlyrefine the grains, modify the morphology of inclusions at the grain boundary and enhance thetensile strength and impact toughness without hardness decrease.Based on the excellent characters of RE, in this study, a series of5Cr5MoVAl withdifferent contents of Rare Earth (0.010%,0.020%,0.032%,0.042%) were fabricated. The heattreatment includes Isothermal spheroidzing annealing, austenizing and tempering. The steelswere initially heated to860℃and held for120min, then cooled to760℃at a rate of10℃/min and held for270min in order to obtain the hardness value below250HB and meetthe machinability. The steels were austenitized for30min (holding time) at1040℃,1080℃and1120℃quenched in air and allowed to reach room temperature. Finally, they weretempered for120min at temperatures in the range of200-600℃(200℃，400℃，440℃，480℃,500℃,520℃,550℃,600℃), eventually air-cooled to room temperature, respectively. Thehardness of steels after different heat treatments is measured. According to the results, steelsquenching at1040℃,1080℃and tempering at500℃are selected to carry out the tensile andimpact tests. The composition, amount and distribution of the inclusions are studied by SEMand EDS. The results show that the inclusions in the microstructure changed from the Al2O3,AlN and MnS to sulfide, aluminate, manganese sulfide of rare earth and AlN after the additionof Rare Earth. Rare Earth addition inevitably results in the large size of RE inclusionsformation. As the independent core nucleation, Al2O3and CaO grow up to the large-size REinclusions under the adsorption of Rare Earth. SEM and metallographic analyses show that theaddition of RE has no influence on the substructure of martensite. However, RE addition causes the coarsening of the lath martenite, AlN and the rare earth inclusions.The industry standard of die steel is that the hardness of steel after annealing is lowerthan250HB, the hardness after tempering is higher than60HRC and the tensile strength ishigher than950MPa. The results of harness and tensile strength show that the hardness of steelafter annealing is all around200HB, and the steel with0.042%RE meets the servicerequirements after quenching at1040℃and tempering at500℃. The mechanical properties(62.1HRC,1035Mpa) are equivalent to that of the5Cr5MoVAl which the hardness and tensilestrength is62.8HRC and1121Mpa. The impact tests show that the impact toughness of steelwith0.042wt.%RE (6.16J/cm2) is below than5Cr5MoVAl (9.47J/cm2). In addition, theimpact toughness descends with increasing the quenching temperature. This is because thatrare earth mainly reacts to the Al2O3and S elements in the liquid and generates large rare earthinclusions. This weakens the micro-alloying effect of rare earth and leads to the impactductility decreasing. The SEM results show the fracture mode change from quasi cleavagefracture to cleavage fracture after RE addition. Steel without RE element has more tearingridges and secondary cracks. While，the steels with RE addition exhibit intergranular andpartial transgranular fracture characteristic. |
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