Improvement of the Toughness and Wear Resistance by Decreasing Silicon and Vanadium Contents of a Hot Working Tools Steel

Middle carbon 5Cr-1Mo-V type steels such as SKD61 and AISI H13 have been widely and globally used for hot working applications from die casting to forging. As for V, although 1 % V steels are popular, 0.5 % V steels have been applied as well. In these 20 years, low Si and high Mo type steels have been developed and evaluated as high performance steels mainly for die casting fields. In forging applications, however, the effect of Si has not been fully clarified. Therefore, the steel with 0.06Si and 0.55V has been studied to make it clear the effect of Si and V on wear resistance and toughness especially in hot forging tools. The results obtained are as follows. (1) Softening resistance is almost the same in both steels of A and SKD61. The hardness of these steels tempered at 873 K and 973 K for 3.6 ks is 44 and 30 HRC, respectively. In spite of lower hardness, steel A with 49.5 HRC shows 28 % less wear than SKD61 with 52.7 HRC. Oxide film of steel A is thicker than that of SKD61. (2) The friction coefficient, μ, in metal-metal contact condition is approximately 0.7. In oxide film-metal contact condition, on the other hand, μ reduces to smaller than 0.35. Steel A shows lower μ value than SKD61 at the all oxidation temperature ranges. (3) In high temperature oxidized specimen, the higher contact pressure Pmax, the higher μ value. With Pmax less than 20 MPa, μ of SKD61 is much more sensitive to Pmax than that of steel A. The oxidized surface of steel A with small μ is hardly worn away without the adhesion of worn dust. (4) Charpy impact value of steel A is remarkably improved, which is caused by the suppress of coarse primary carbides sized 10 to 30 μm by reducing Si and V contents.