| Popis: |
The relationship among strength, ductility, toughness and microstructure was studied in order to find out microstructure image of stronger and tougher steel. Initial samples with two different microstructures, ferrite-pearlite and martensite (and/or bainite), were prepared and then caliber rolling was conducted at warm working temperature. Two kinds of low carbon steel bars with ultrafine elongated grained (UFEG) structure in transverse grain size of 1.0 μm and 1.3 μm, respectively, were produced. For comparison, conventionally quenched and tempered 0.29%C steel and 1.03%C steel with a martensitic structure and low-carbon steel with ferrite (grain sizes, 10μm and 18μm)/pearlite structure were also prepared. The Charpy impact and static tensile tests were conducted at a temperature range from 200°C to －196°C. The reduction in area and the plastic deformation limit were used as a universal parameter of ductility. In the Charpy impact test, only the UFEG steels fractured with delamination crack, the delamination remarkably appeared near energy transition temperature and the impact energy becomes larger than all other steels. As a result, the UFEG steel with transverse grain size of 1.0 μm was best balance in correlation between strength and ductility and between strength and toughness. |
