| The present study is aimed at developing the high strength cold-rolled steel sheets with high ductility. The main emphasis of the present study has been placed on understanding the effects of Mn and Si on microstructure and properties of the cold-rolled steel sheets. Three kinds of 0.15C-Mn-Si cold-rolled steel sheets were prepared, and were intercritically annealed following isothermal treatment at bainitic region. Microstructural observation and tensile tests were conducted, and also the volume fractions of retained austenite were measured. In the case of the ECO-2 or ECO-3 steel sheet (high Mn content), the higher volume fractions of retained austenite were obtained than the ECO-1 steel sheet (low Mn content), but showed a dual phase stress-strain curve having continuous yielding behavior, high tensile strength over 1,000 MPa, and low elongation about 20%. The lower Si/Mn ratio, the steel sheets had the stronger band structure and the lower ductility. Therefore, Si/Mn ratio must be high for the increase of ductility. |
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