The last decade has seen an enormous increase in the stringency of the demands placed on steels. The main characteristics involved are higher strength and toughness, better suitability for welding and, in certain cases, corrosion resistance. The reason for these heightened demands resides in the higher strains to which the material is exposed in structural applications and in a greater need for safety. In many areas, the steel industry has succeeded in offering appropriate solutions through improved metallurgical and rolling techniques. Accelerated cooled steel grades are one example.
The manufacture of high-strength fine-grained structural steel requires extensive expertise in the rational utilization of metallurgical mechanisms and modem production plants, together with the use of innovative rolling and heat treatment techniques (Degenkolbe,1993a, Degenkolbe et ai, 1993b and Schulz, 1988). To ensure a high specific performance potential measured against the applicable criteria, normalized or normalizing rolled, thermomechanically rolled steels or steels which have undergone accelerated cooling after rolling, and quenched and tempered steels with varying alloying elements are used. Apart from very good strength and toughness properties, they are characterized by a high resistance to brittle fracture, good ductility and exceptional suitability for welding.
High-strength fine-grained structural steels are used in a wide range of applications and have proved successful in many cases because of their excellent processing characteristics and qualities in use (Musgen, 1985 and Baumgardt et ai, 1983). Figure 1 provides an overview of the main techniques for the manufacture of high-strength fine-grained structural steels, their typical microstructures, application areas and the resulting minimum yield strengths. When this technique is used, the thermomechanical rolling process is immediately followed by accelerated cooling, at an adequate cooling rate, to a specified cooling stop temperature. Further cooling then takes place in the atmosphere.