Mechanical properties and retained austenite in intercritically heat-treated bainite-transformed steel and their variation with Si and Mn additions |
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Authors: | Yasuharu Sakuma Osamu Matsumura Hiroshi Takechi |
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Affiliation: | (1) Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO;(2) Sheet and Coil Laboratory, R&D-II, Nippon Steel Corporation, Japan;(3) Sheet and Coil Laboratory, R&D-II, Nippon Steel Corporation, 229 Sagamihara, Japan |
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Abstract: | Processing peculiarities and functions of alloying elements, such as Si and Mn, were studied for improving formability of
steel sheets with mixed microstructures. Annealing a sheet steel with 0.2 pct C in the intercritical range produced very fine
particles of retained austenite which were moderately stabilized due to C enrichment by subsequent holding in the bainite
transformation range. Its strength-ductility balance is greatly superior to that of other dual-phase steels due to transformation-induced
plasticity (TRIP). The holding time in the bainite transformation range varies with temperature, depending on the activation energy of C diffusion
in austenite, and shifts to longer times with an increase of Si or Mn additions. The optimum cooling rate from the intercritical
region is reduced with an increase of Mn content but is not influenced by Si content. Additional Mn makes the retained austenite
content larger, although uniform elongation remains the same. In this case, the product of tensile strength and total elongation
is increased due to an increase in the tensile strength. Contrary to Mn, Si does not affect retained austenite content but
improves the uniform elongation by increasing its stability. |
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