Fine-grained Ferritic Steels without Upper/Lower Yielding

Fine-grained ferritic steels without upper and lower yielding were created under caliber rolling in the range of C=0.050.44 mass% and N=0.001 mass%. The grain size of steels created was about 1 μm. Upper and lower yielding disappeared by adding the amount of Ti given by Ti>4C +3.43N, because C and N are fixed as TiC and TiN. Here, Ti, C and N are mass%. On the other hand, upper and lower yielding existed, when Ti<4C +3.43N. The difference in strength between fine-grained steels with and without yielding i.e. the solid solution strength due to C was estimated to be 100 MPa with the yield stress and 70 MPa with the tensile strength. TiC particles smaller than 10 nm precipitated, adjusting the thermo-mechanical process of creating fine-grained steels. The precipitate dispersion strength due to TiC was estimate to be 300 MPa at C_??_0.05 mass% and Ti_??_0.2 mass%, This strength is equivalent to 1500 MPa per 1 mass% Ti. As the results, the maximum tensile strength reached about 900 MPa at C=0.075 mass% and Ti_??_0.3 mass% in 2-μm fine-grained steels. Fine-grained steels with and without yielding also showed high toughness and high fatigue strength.