These swords were made chiefly by smiths from China or Korea or by the early Japanese smiths. Although they were made of steel, the tempering was faulty. They were mostly of the straight (chokuto) type. The centers of the sword-making profession were in Yamato, Mutsu, and San-in. Although these swords were made in Japan, they were mere imitations of Chinese blades. High-ranking officers usually carried expensive swords made in China. This imitation of the Chinese sword was gradually developed into the typical samurai sword.
Source: Yumoto, J. M. (1992). The Samurai Sword: a Handbook. Tokyo: Charles E. Tuttle Company.