1988 Volume 74 Issue 3 Pages 535-542
An influence of boron and nitrogen contents on the strength and toughness of 0.02%C-1.65%Mn0.04%Nb-0.015%Ti-B steel and 0.03%C-1.50%Mn-0.15%Mo-0.07%V-0.04%Nb-0.02%Ti-B steel finish-rolled at 720°C after heating at 1050°C was studied. It was found that the strength and toughness could be estimated by calculated free-boron (B*).
The microstructure changes from coarse ferrite and degenerated pearlite structure to fine-grained ferrite and isolated martensite structure as boron becomes effective for strengthening, consequently the tensite strength rises 5 to 6 kgf/mm2 and the yield strength to tensile strength ratio decreases. Furthermore, the microstructure changes to bainite structure by the addition of molybdenum and vanadium and the increase of boron in solution, rising the strength. The strength rises mainly because ferrite becomes fine. The critical boron content in solution which effectively increases tensile strength is 8 ppm as B* in 0.02%C-1.65%Mn-0.04%Nb-0.015%Ti-B steel. The critical boron in solution decreases to about 1 ppm by the addition of molybdenum and vanadium. In the region where boron contributes to strengthening, the tensile strength rises linearly with increasing B*. The degree of the strengthening rises by the addition of molybdenum and vanadium.
