Characteristics of microstructure and mechanical properties in Al-bearing medium carbon martensitic steels

DOI
  • Shirakami Yusaku
    Graduate School of Engineering, Kyushu University
  • Masumura Takuro
    Department of Materials, Kyushu University International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University
  • Nanba Shigenobu
    Materials Research Laboratory, Kobe Steel, Ltd.
  • Tsuchiyama Toshihiro
    Department of Materials, Kyushu University International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University Research Center for Steel, Kyushu University

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<p>As-quenched α'-martensitic structure and its mechanical properties of Fe-1.5%Mn-0.5%C-Al alloys containing various concentrations of Al were investigated. Microstructural observations of specimens heat-treated under various conditions confirmed that a austenite single-phase region can be obtained in the temperature range from 1373 K to 1473 K when Al is increased up to 3%. Dilatometry tests showed that the Ms temperature increases at a rate of 25 K/mass% with increasing Al content. The hardness of α' tended to decrease due to the decrease in the concentration of carbon in solid solution by auto-tempering and the formation of coarse blocks near the prior austenite grain boundaries. The coarse blocks had a butterfly shape, and there were no lath structures or transformation twins inside them, but rather high-density dislocations and carbides. Tensile tests on the 2%Al steel showed that the above coarse blocks can produce a large strain, which suppresses early rupture, resulting in a significant increase in ductility without any loss of strength.</p>

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