Effects of low temperature transformation products such as bainite and martensite on the mechanical properties of normalized low alloy high strength steels were studied using specimens with ferrite-pearlite structures including some mixed structures formed by varying the Mo content and cooling rate on normalizing.
It was shown that the presence of these products in the microstructure increased the tensile strength, but this led to a disappearence of the yield point on the stress-strain curve, and resulted in a marked decrease in the proof stress/tensile strength ratio. The tensile strength of the steels containing these products depended on their amounts. Charpy impact transition temperature was divided into two groups by the presence or absence of the low temperature transfomation products, and both of them were represented by straight lines against logarithm of ferrite grain size, and at any fixed grain size within the range studied, the transition temperature was raised about 50°C by the presence of low temperature transformation products regardless of their amounts. The fracture characteristics of specimens broken by impact were studied by microfractography.