New ultra high strength (UHS) steels have been developed in order to get advantages in machine design and construction. Following benefits can be obtained for example:
- less material usage due to lighter constructions;
- better payload and less fuel consumption in vehicle industry;
- energy saving in material production.
A rough distinction of structural steels can be defined to ductile steels, with tensile strength less than 300 MPa, and high strength steels, up to 700 Mpa. A steel material can be defined as UHS steel when the tensile strength exceeds 700 MPa. Steels with yield strength of 1500 Mpa have been developed so far. UHS steels can also be divided into structural steels and wear resistant steels. With the tensile strength also the hardness increases and the tensile strain decreases. That causes several difficulties when the material is processed into products. Especially mechanical processing like bending, machining and shearing gets difficult as the material strength increases. That causes problems for the construction material users to find the proper manufacturing methods in production.
In Oulu University Production Technology Laboratory material processing tests have been performed during several years in co-operation with the local steel manufacturer. The usability tests comprise mainly of bending and machining tests. Shearing and welding tests have been made to a smaller extent. Also laser treatment has been used for local heat conditioning in order to improve the bending and shearing properties, but these techniques are not yet widely used in production.
The bending tests are carried out with standard bending tools and test steel plates with standard dimensions. The plate thickness varies depending on the test material. The target is to determine the reliable minimum bending radiuses whereby the plate can be bent without failure, from both sides and along the rolling direction and orthogonally to that. Also the springback angle is measured and the bent surfaces are evaluated according to several criteria. When necessary, also the mechanical testing of the formed material is carried out.
The machining tests are made mainly by drilling. Also some milling tests have been performed. Drilling is a convenient way of machining testing because a substantial amount of holes can be drilled in one test plate. The drilling power can be observed precisely by monitoring the spindle power. Also a variety of different tool types can be used, from uncoated HSS drills to boring tools with indexable inserts. The optimal machining parameters (feed and speed) will be defined according to maximum tool life and minimum machining costs.