Abstract
Keywords - Microalloyed steel, bainite, HSLA, continuous cooling, toughness.
Abstract - Automotive designers and suppliers have been faced with significant changes over the past few years in the demand for products that provide equivalent or higher functional performance as past products, while weighing less, and requiring less space. The driving force for using microalloy forgings for automotive products is lower cost components, which have equivalent strength properties as the heat-treated forgings that they are intended to replace. Nowadays, microalloyed steels are widely employed in different engine components as crankshafts, connecting rods, coil springs in suspension designs due to their excellent combination of properties. In many applications, the decision to replace plain carbon steels with higher strength microalloyed steel can improve the profitability of both, the steelmaker and the steel user. The main demands that this kind of materials must accomplish are divided into:
- Costumer demands: Excellent machinability, High degree of purity, Improved mechanical properties.
- Demands on forging operation: Low purchasing cost, Reduction of heat-treating operations.
Microalloyed steels have been introduced in the automotive industry in order to satisfy the previous demands, using different carbon contents depending on the application features of the final components.
Compared to martensite type, ordinary continuous-cooled bainite steel provides a more ductile and tougher material. Moreover, bainite type steel has advantage in cost due to the saving caused by the absence of quenching and tempering, and it is proper for a complex shaped components in which quenching distortion is not allowed.
The main purpose of this article is to characterize different types of bainitic microstructures of a medium-high carbon microalloyed steel, used commonly in car crankshafts, in order to establish which is the treatment that leads to a microstructure with improved mechanical properties. The main features of the bainitic transformation at each heat treatment performed are studied by means of optical microscopy, scanning and transmission electron microscopy.
Special attention is paid to the mechanisms of these transformations, emphasizing the relationship between microstructure and properties. Assessment of the properties of the different microstructures is accomplished through many different test procedures, tension (ASTM E8M-89 and A370-88a), impact (ASTM A370-88a and E23-88) and hardness testing(UNE7-423/1).