تغییر فاز در فولادهای میکرو آلیاژ کم آلیاژ استحکام بالا (HSLA)

Phase transformations in microalloyed high strength low alloy (HSLA) steels

تغییر فاز در فولادهای میکرو آلیاژ کم آلیاژ استحکام بالا (HSLA)

ABSTRACT

The addition of small amounts of Nb, Ti or V singly or in combination has been the key to  producing high strength low alloy (HSLA) steels with greatly improved mechanical properties. Such  microalloyed HSLA steels are now commonplace in a huge variety of applications. The combinations of  strength, toughness and formability that are possible are directly related to the way in which the steel is processed from the as- cast slab (usually) to the final plate or strip dimensions. The effects of microalloying result principally from three main effects on the austenite to ferrite  transformation. Firstly, the microalloying additions form precipitates in both austenite and ferrite; this factor alone can be exploited in various ways to control the microstructure, hence  mechanical properties, of the steel. Secondly, although present in relatively small quantities,there are discernible effects on ferrite transformations in terms of the hardenability of the steel. Finally, there are interactions between the various deformation sequences used in  steel production and the presence of microalloying elements as solutes or precipitates which can be  exploited to control microstructure. The interactions between processing, microstructural  development and the effect of microalloying additions on the austenite­ ferrite transformations are  the topic of this chapter. A brief history of the use of microalloying is also given as is some  indication of the range of mechanical properties that result from the current understanding of the role of microalloying additions.