In the present paper, it is pointed out that the development of iron and steels is striding forward a new stage, and the discussion to this has been made It is shown that in the 20th century, about 30 years had been spent in studying the problem of the low tensile strengths σ s (σ b) of the practical materials of engineerings, and having recogniged and basically solved it; after that,the great efforts had been made to increasing the fracture strength σ f The roles played by the rare earth in steels are discussed, in combination with explaining the methods of increasing σ f In addition , the particularities of the rare earth in steels are also expounded, and it is pointed out that precisely these particularities determine the developing progress of the rare earth steels It is indicated that the demands for steels, in the 21st century, will change from unitary to synthatic; the problem of σ f,localized weakness and so on will be more pronounced due to the high strength and high-energy state of the steels The problems concerning "the super steels", the effects of enviroment and pollution on the ultra-pure material and the large size inclusios in the steels with ultra-fine grains and ultra-low concentration of impurities are also discussed The notable anisotropy of mechamical properties and the presence of a significant population of the inclusions with large size in the steels containing an ultra-low content of sulphur are described, and it is clearly shown tnat these have an universality and the roles of the modification properties of the rare earth are more remarkable in such
steels than in the general"steels treated with rare earth" The localized weakness in steels is discussed in detail And it is considered that controlling the weakness sources and decreasing the energy in the localized regions are as important as cleaning of steels, for reducing the localized weakening in steels; and the strong role suppressing the localized weakness of strength in steels played by the rare earth is presented It is indicated that all the processes occuring in the engineering materials are essentially associated with decreasing of the system energy, thus the microstructures in steels would play a prominent role in the processes due to the high-energy state of them It is considered that clarifying steels, modifying the inclusions in the clean steels, controlling microstructures and lowing the energy states of them, and so on would constitute a basic way to solve the problems of the steels in the 21st century; and the rare earth just play key roles in these respects It concludes that the addition of the rare earth to steels will be an important approach to develaping the steels in the 21st century; as a example,the fact that the "king of steels" in the 20th century, namely the 350 grade (245kg/mm 2) of maraging steel has been produced only by the adition of La, is given