Optimisation of the influence of boron on the properties of steel

1 SUMMARY The effect of boron and the need to control its location and chemical state are known since the 1940's. Early attempts to commercially produce heat treatable steels resulted in variable properties due difficulties in ensuring that boron was present in the desired form, i.e. as soluble boron in the case of steels where boron was present to enhance hardenability. Subsequently, the importance of protecting boron using strong nitride formers such as titanium was appreciated and these types of steels are now produced widely within the EU. However, problems are still encountered in obtaining a consistent effect as can be seen from the wider hardenability bands that are specified for boron treated steels compared to their CrMoNi equivalents. On the other hand, similar problems can be encountered in areas where an unprotected boron addition is made to modify other properties, for example, ductility or formability in low carbon steels. In this instance, the boron is added to deliberately tie up any free nitrogen in the steel as boron nitride, thereby reducing tensile strength and work hardening rate and increasing ductility. However, a certain amount of the unprotected boron addition can be lost as an oxide and some may be lost due to the formation of carbides, hence necessitating a boron addition greater than the level required for the stoichiometric formation of boron nitride. Hence, there is a need to better characterise and understand the precise role of boron in a range of steels. The purpose of this project is to develop models, combining statistical and empirical approaches, for optimising and predicting its effects in three areas, namely: Influence of boron on austenite grain size during thermal and mechanical treatments. (b) Influence of boron on the modification of hardenability in low alloy steels. Influence of boron on ferrite grain size, ductility and strain ageing in low carbon steels. This ECSC project was carried out to better characterise and understand the precise role of boron in a range of steels. Hence, the mechanism by which boron additions influence the development of austenitic and ferritic grain structure in heat treatable and plain C-Mn steels (0.005-0.2%C) during and following thermal and mechanical processing by examining the location and chemical state of protected and unprotected boron additions was studied. It has been shown that, as austenitising temperature increases, grain coarsening occurs first in the boron treated (unprotected) plain carbon steels, then the plain carbon steels and finally in the boron treated, titanium protected steel and the boron treated, niobium, aluminium or titanium protected microalloyed steels, because these elements combine with the nitrogen and the nitrides stop the increase of the grain size. The tendency of austenite grain coarsening in these experimental steels was successfully compared with the Gladman model [9] for the plain carbon steels but it was not applicable for more highly alloyed steels. A model for prediction of the shape of the Jominy profile of low alloyed boron steels was developed on the basis of the steel chemistry. Such model has been implemented by means of a software (Jominy Profile Predictor) with a friendly and easy to use graphical interface and provided to manufacturers to replace the manual Jominy end-quench test. This tool has been widely tested by the industrial partners to improve friendliness as well as accuracy.