In this paper, we present the results of investigations which form the basis for development of high-strength weldable bainite steels obtained using in-line thennomechanicaI treatment in the rolling mill and not requiring heat treatment with special heating. 777e thennomechanical treatment involves controlled rolling followed by regulated cooling. In order to achieve high strength properties than in steels with ferrite-pearlite structure, the alloying and cooling conditions for rolling in the thennomechanical treatment o, cle must ensure that lowtemperature austenite transformation products be obtained with a fine original austenite grain.Fine-grain steels with ferrite-pearlite structure, obtained by controlled rolling [1], have found broad application in fabrication of large-diameter pipes for gas lines and also other metal constructions, due to their favorable combination of strength properties, toughness, and weldability. However, for such steels there is a limiting strength level (ay -= 500-550 N/mm 2) above which the required technological properties are not ensured (especially weldability) for a ferrite-pearlite structure. In order to fabricate weldable steels with a higher yield stress (Cry > 590 N/mm2), usually quenching followed by high-temperature tempering is used. In this case, the steels should have a chemical composition which ensures formation of predominantly a martensite structure [2].We know that the strength of steel which contains bainite in its structure depends on the volume fraction and strength of the bainite. The strength of the bainite itself is affected by a number of its structural characteristics; among these, for constant carbon content the determining characteristics are the dimensions of the bainite rods, the dislocation density, and the dispersity of the carbides. As applied to low-carbon bainite, the strength is practically completely controlled by the distance between the slip barriers and also the elevated dislocation density. The length of the bainite rod depends on the diameter of the original grain of austenite and its width decreases as the transformation temperature decreases; in this case, the dislocation density simultaneously increases. The boundaries between bainite rods, grouped into packets, are low-angle [3]; they prevent the motion of dislocations but do not slow down the propagation of cracks. The motion of cracks is prevented only by highangle boundaries of the bainite packet or the original austenite grain. Thus the size of the bainite grain controls the strength, while the toughness and cold-resistance of bainite steel is determined by the structure of the austenite immediately before the 3' -* c~ transformation, which makes it necessary to use special thermomechanical treatment technology to achieve a set of properties with high values in production of such steels.We investigated more than thirty melts of steel with 0.1% C and different contents of Mn (up to 2.2%), Cr (up to 0.8%), Ni (up to 0.85%), Mo (up to 0.4%), microalloyed with Ti, V, and Nb.In Fig. 1 we...
