Abstract:
The vanadium microalloyed tested steel was heated to 780 ℃, 800 ℃, 820 ℃ and 840 ℃, respectively and then held for a few minutes, followed by different cooling modes such as slow cooling, rapid cooling, simulated galvanizing, and air cooling to room temperature. Microstructure observation by SEM shows that microstructure uniformity is poor during annealing at 780 ℃ and undissolved carbides are found, leading to a weak combination of strength and plasticity. With the increase of annealing temperature, the size and fraction of ferrite gradually decrease, the volume fraction of mid-low temperature transformation products gradually increases and its size becomes coarser. When the annealing temperature was 800 ℃, the fraction of martensite reaches the maximum. When the annealing temperature further increases, the fraction of martensite decreases, while the fraction of bainite increases, resulting in a relatively stable tensile strength when annealing temperature of 800 ～840 ℃ range, however, the elongation firstly increases and then decreases. When the annealing temperature was 820 ℃, the steel strip obtains better mechanical properties. The yield strength, tensile strength, yield ratio, elongation after fracture A80 and hole expansion rate are 486 MPa, 835 MPa, 0.58, 16.0% and 27%, respectively.
