Development of new continuously cooled steels with high strength and toughness combination for hot-forged automotive applications

SUMMARY Steel has a wide usage in industrial applications such as automotive, machinery and defense due to its relatively low production cost and high mechanical properties. Depending on performance requirements in its service condition, an additional heat-treatment is generally applied to steel materials. Quenching and tempering (Q&T) is one of the most commonly used heat treatments which provides an increament in strength, hardness, impact energy, etc. Q&T heat treatment is generally applied to the products that are produced by hot forging. Q&T heat treatment has three major processing stages; austenitization, quenching and tempering. These process steps involve usage of high temperature furnace and specialized quenching media. Since most of the forging companies do not have such facilities in their plants, the heat treatments are carried out by outsourcing the service. This requirement brings along an additional cost not only for process itself, but also for additional labor and logistic operations. Moreover, the heat treatment in conjunction with the other needs give rise to CO2 footprints of hot forged products. In previous studies, it is observed that the mechanical properties of hot-forged steels without applying any subsequent Q&T heat treatment generally result with insufficient high strength – toughness balance. Additionally, in many of these studies, industrial producibility and commercialization possibility of proposed steel chemical compositions are limited either by the need of specialized steelmaking operations or by their high alloying costs. In this study, it is aimed to develop a new steel chemical composition and process route that provide higher strength and toughness balance by using continuous cooling and eliminating the need of additional heat treatment. Firstly, preliminary studies were carried out by using computational thermodynamic and thermomechanical software in order to specify proper alloy designations. Then, selected chemical compositions were produced in laboratory scale. The results of microstructural examinations and mechanical tests of laboratory-scaled productions were analyzed by using different statistical approaches, such as Multiple Linear Regression (MLR) and Artificial Neural Network (ANN). According to estimations of ANN model, a new chemical composition and cooling rate that are able to provide the aims of the thesis was decided. The validation studies were performed in industrial scale by hot forging an example automotive part. It is found that the mechanical requirements of 41Cr4 steel in quenched and tempered condition according to DIN EN 10083-3 standard can be fulfilled by using proposed new chemical composition and process route. Even though the alloying cost of new developed steel design is 13% higher than 41Cr4 standard steel grade, it is thought that it is possible to decrease the overall production cost of hot forging products through eliminating the need of additional Q&T heat treatment. In addition to processing and other operational cost advantages, it is believed that the proposed material and process route have a great potential to reduce CO2 footprints of hot-forging products arising from additional heat treatment.