Based on the principle of FSW, two innovative welding methods for steels were developed. The first is to weld below the A1 temperature. A high carbon steel is generally difficult to weld due to martensite formation which causes cracks during the welding. Using this method, carbon steels, even including hypereutectoid steel, can be welded without any pre- or post-heat treatment, independent of the carbon content. In the second method, the welding temperature exceeds the A1 temperature. In this case, brittle martensite is generally formed. However, it was found that the austenite can be significantly stabilized by utilizing the severe plastic deformation during the FSW. Using this method, martensite is not formed because austenite is retained even after the welding. Due to the retained austenite, the welded steel has extraordinary properties, namely, over 1000MPa strength and 50% elongation for a typical steel due to the artificial TRIP effect.