Abstract: When the manganese content in steel is 1.79%, the as-hot-rolled microstructure consists some pearlite, and there is physical yield plateau in the stress-strain curve. The lower manganese content, the more amount of pearlite, and the longer plateau of physical yield. But when the manganese content is more than 2.09 %, there is no product of non-martensitic transformation. In the microstructure of as-hot-rolled steel, the island field of martensite is composed of several parts. They are dislocated lath martensite area and twinned martensite area respectively.