| The present study is concered with the processing of ultra-fine steels by thermo-mechanical processes and the grain refinement mechanism. Three steels were deformed at high strain rates using a pilot plant rolling mill, and exhibited good tensile properties and low temperature impact toughness, although they contained only about 0.01% carbon. Especially in the Ni added steel, room temperature tensile strength reached to 850 MPa, and Charpy impact energy at -120℃ showed 78J, indicating the potent effect of Ni addition. Although the microstructure was identified to be mainly acicular ferrite by optical microscopy, TEM microstructures of ultra-fine steels were composed of very fine and highly dislocated grains whose shapes were changed from elongated to equiaxed ones as alloying elements were added. The formation of this microstructure might be explained by a possible mechanism, i.e., a repeated dynamic strain-induced transformation mechanism. These findings suggest that ultra-low carbon steels with high strength and toughness can be processed by grain refining combined with microalloying. |
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