Effect of tempering temperature on stretch-flangeability of maltensitic steels

The effect of microstructures on the stretch-flangeability has been investigated using high-strength martensitic steels tempered at various temperatures. For the index of the stretch-flangeability, hole-expanding ratio A was measured using plate specimens with a hole introduced by a punching process. Tensile tests were also carried out using the same tempered specimens. A increased with increasing tempering temperature, and took the maximum value at 773K, and then decreased in the specimens tempered at temperatures higher than 773K. On the other hand, in the tensile tests, total elongations or local elongations increase with increasing tempering temperature up to 973K, and decreased in the specimen tempered at 1073K where typical dual phase microstructure was observed. Microstructural observations around the holes formed by punching process suggest that the maximum value of λ is determined by the balance between the recovery enhancing ductility and the growth of cementite size causing void formation during the punching process. It is to be noted that grain refinement due to severe plastic deformation during punching process was observed around the holes, which might contribute to enhancing the stretch-flangeability.