1. ITRODUCTION

In recent years the energy crisis has created demand for high strength and good toughness in plain carbon and High Strength Low Alloy (HSLA) steel [1, 2]. The majority of the research and development efforts have aimed at developing steels with higher strength levels and improved toughness, formability and weldability. The investigations have resulted in recognition that with control of some parameters, resulted in certain morphologies of ferrite, the mechanical properties of steel with these ferrite structures are improved [3, 4]. The austenite to ferrite transformation, cooling rate and different ferrite morphologies are important because by controlling of these parameters, the mechanical properties of steel are improved [3, 4, 5]. It is generally agreed that the different transformation structures in continuous cooled system with decreasing temperature are as follow [6]. a) Grain boundary ferrite b) Polygonal ferrite c) Widmanstatten side plate ferrite d) Acicular ferrite e) Upper bainite f) Lower bainite and martensite Grain boundary ferrite nucleates at the austenite grain boundary and after that Widmanstatten side plate ferrite nucleates at the grain boundary ferrite, but grows along well-defined matrix planes, while intragranular ferrite nucleates entirely within the austenite grains. Observation of the mechanical property of materials with different ferrite microstructure has shown that Widmanstatten ferrite, upper bainite or grain boundary ferrite is detrimental to toughness, whereas intragranular or acicular ferrite improves it. A comparison between acicular ferrite and bainite is shown in Fig.1. The term acicular means shaped and pointed like a needle, but it is generally recognised tha