JOURNAL DE PHYSIQUE IV

ABSTRACT: The study of the microstructure is necessary so as to investigate the effects of the parameters of interrupted accelerated cooling (IAC) on the mechanical behaviour of 0.1 wt % C, Ti or Nb + Ti microalloyed steels. The paper deals with larger variations of the cooling rate and of thefinish cooling temperature than are usual in industrial practice. The work establishes: I) the favourable effects of the highest cooling rate and for the Nb-free materials of a lower than usual finish cooling temperature, 2) the unsuitable character of a 0.4 wt % addition of Ni in, IAC samples and 3) the chief advantage of a 0.0015 wt % microaddition of Nb. The metallurgical origins of these results are detailed. INTRODUCI'ION: The smngent control of all the stages of the hot-rolling process (reheating, rolling and cooling) is necessary so as to reach the best mechanical behaviour for high strength low alloy (HSLA) steels by means of the single optimization of their microstructure. In particular, a combination of controlled-rolling and subsequent accelerated cooling can strenghthen the steel while maintaining superior low-temperature toughness in spite of reduced contents of alloying additives or carbon in HSLA steels [I, 21. This double goal requires the systematic study both of the effects of the numerous parameters of the thermomechanica