The importance of material properties on the bendability of Q/QT steels

This master thesis work has been carried out at the company SSAB EMEA in Oxelösund which is a global leader in quenched and tempered high-strength steel. The aim is to examine the importance of the material properties for the bendability of the specific steel grade Weldox. There are many conditions for various applications such as mobile cranes and vehicles that need to be fulfilled in order for the material to be approved, where one main prerequisite is the bendability. It is very important to be able to bend the material without the occurrence of cracks. The purpose of this work has therefore been to investigate and try to find the relationship between the material properties and the critical bending radius. Furthermore has another aim been to find the most suitable treatment process for the composition of this specific steel grade.The work was divided into two parts, where the first part was to examine two equally treated Weldox 960 plates that differed significantly in properties such as bendability and toughness. In the second part seven different plates of the steel grade Weldox 1100 have been investigated where all the plates were treated in various ways. The leveling of the plates was performed in two different ways; either according to the standard leveling or to the property affected leveling (ERIK). The experimental part includes tests such as bending, hardness, toughness, tensile and inclusions measurements.The results have shown that the most significant material properties affecting the bendability are the yield strength and the purity close to the plate surface. Leveling with ERIK as the last treatment step has shown that the toughness of the material can be increased but it does not affect the bendability for these specific steel grades. Other properties such as hardness, ultimate tensile strength and elongation have no major impact on the bendability for this specific composition. The most suitable treatment proved to be tempering at 200°C as the last step in the process chain