The Physical Metallurgy of Microalloyed Steels 2nd Edition PDF

Attention has been drawn to the factors involved in the development of microalloyed steels. The factors include the discovery, the development of the scientific and technological principles upon which these steels are based, and the need for these steels in terms of their ability to satisfy increasingly stringent demands for a variety of applications. These and the capability of the steel manufacturing industry to respond to these factors are summarised in Fig. 1.2. The benefits of weight savings are essentially cost related, as pointed out earlier. The pressures of safely legislation, energy conservation and material conservation, have all had their impact on the need for weight reduction, whether this arose from increased collapse resistant automobiles, lighter automobiles and structures, or simple conservation of steel and energy. A significant feature of the higher strength levels and materials savings is that the cost per unit of strength is reduced when using microalloying additions due to the small amounts of alloying element required, particularly when processing features such as hot rolling and accelerated cooling are optimised.

The need for the maintenance or improvement of ancillary properties such as weldability, toughness and formability when developing these higher strength steels has also been emphasised, as the conventional strengthening methods used prior to the discovery of microalloying would inevitably have led to deterioration of some or all of these properties.

The development of microalloyed steels also required the necessary alloy resources, and also a steel manufacturing industry that was capable of developing manufacturing methods to produce lower carbon, cleaner, tougher steels, together with a technological base from which to give customer advice on the utilisation of these new steels. Coupled with the development of microalloyed steels was the need to accommodate changes in the manufacturing process routes for cost reduction purposes such as continuous casting, as referred to earlier.

Also featured in Fig. 1.2 is the science-based input to both the steelmaking process developments, the metallurgical processing aspects, the behaviour of microalloying elements in steel, and the functions that they perform in controlling the mechanical properties of steels. The thermodynamic basis for many of the steelmaking developments were already in existence long before the development of microalloyed steels, but even here the necessary plant and operational procedures required considerable development. From a metallurgical processing and general physical metallurgy standpoint, the introduction of microalloyed steels coincided more or less with the development of a basic understanding of the relationship between the microstructure and properties of steels, and between the metallurgical processing and the evolution of the microstructure in steels. Most of the mechanical properties of steel are directly related to the microstructure, and any change in the microstructure will cause a corresponding change in the mechanical properties. The exception to this general statement is the elastic modulus. The Young modulus is essentially controlled by interatomic forces rather than by any form of dislocation movement, and substantial variations in elastic modulus are only attained by substantial changes in the chemical composition of steel - generally far greater than are encountered in the class of alloys known as low alloy steels. However, the yield strength is integrally related to the microstructure, depending as it does on the movement of dislocations (plastic deformation) which is highly sensitive to microstructure. The structure in this sense can range from atomic perturbations such as solute atoms, to microstructural features such as grain size, and the distribution of second phase particles.

Not all of the mechanical properties depend on the same microstructural features. For example, cleavage fracture is highly dependent upon grain size, but ductile fracture is highly dependent upon certain second phase particles and is virtually independent of grain size. Yield strength depends on many microstructural features, but not on widely spaced second phase particles, such as nonmetallic inclusions.

The purpose of this book is to draw together the scientific base of the structureproperty relationships in steel, and the relationships between metallurgical processing and the evolution of microstructure, that are relevant to micro alloyed steels. Such relationships are probably better established for microalloyed steels (and their antecedents, the plain carbon and carbon-manganese steels with ferritepearlite structures) than for many other classes of steel, and have been sufficiently well defined as to provide a systematic understanding upon which properties can be predicted almost as well as they can be measured. The studies that have led to this understanding have also generated a considerable scientific data base, most of which is well dispersed in scientific journals and conference proceedings. An attempt is made to draw these data together and to show how they are used in microstructural design. Theoretical developments in physical metallurgy that have taken place during the period of development of microalloyed steels are also reviewed. One of the aspects of these developments is that the effects of processing variables on microstructure and of microstructure on properties have been established in a substantially quantitative manner and, therefore, can be used predictively rather than phenomenologically, although it should be pointed out that there are still several gaps in the quantitative relationships as will become evident.

Edited by: T. Gladman