Not all high-strength materials are created equal – if you define and select materials solely on the basis of the mechanical properties of yield strength, tensile strength, and elongation, you’ll miss out on the chance to obtain the material with the best overall potential for the desired purpose. After all, Waelzholz offers no less than six different material groups that all fall into the high-strength category. Yet their other properties that are relevant to further processing and the final application are all very different. This is where customers find a truly customized solution for their unique requirements.
When a steel grade has a yield strength of more than 800 MPa, it is referred to as high-strength steel. Some applications, for example clutch plates used in automotive manufacturing, require yield strengths of up to 1,200 MPa, which Waelzholz’s high-performance RAWAEL© material achieves. But the majority of applications across the high-strength spectrum lie in a range of yield strengths that are covered by very different materials, however. At Waelzholz, customers can choose from six different material groups (Figure 1). Each offers different capabilities, for example with respect to weldability, spring properties, isotropy, and longitudinal and transverse bendability. The advantage is that, in addition to strength, these properties can be included in the calculation to determine the best material for the application in the respective customer project. As a result, the customer receives a highly customized material solution.
Figure 1: Classification of the properties of Waelzholz high-strength materials
A wide range of high-strength steel grades
At Waelzholz, high-strength steel grades include bainite, martensite, SORBITEX©, cold rolled carbon steel as well as cold rolled, corrosion-resistant steel, and micro-alloyed fine-grain steel grades. Figure 2 shows a comparison of the key mechanical properties of yield strength, tensile strength, and elongation for these six material groups. The first three are the hardened and tempered steel strip materials bainite, martensite, and sorbite. Sorbite can still be shaped after hardening and tempering, whereby this can change the shape of the material by over 80%. This results in extremely high strength. The material then receives the suffix “-tex” and becomes SORBITEX©. The materials designated “Cold rolled +C” have a yield strength range RP0.2 from 600 MPa to 2,200 MPa and are brought into the strength range required by the customer by means of cold rolling. Corrosion-resistant grades already inherently offer extremely high levels of strength. The micro-alloyed grades are shown on the far right of the figure, which include Waelzholz’s proprietary material RAWAEL©.
Figure 2: Mechanical properties of Waelzholz high-strength materials
Selecting the most suitable high-strength material
But how does one select the right material from the Waelzholz range? This is best explained on the basis of an example: for a new automotive application in the field of safety technology, the component in question needed to be extremely stable with a precisely defined, low weight. As a result, the component would need to be made of steel from the high-strength group. The manufacturer’s requirements for the ideal steel material were, however, much more complex – it had to have high yield strength, but at the same time be easy to form and also capable of being welded during the manufacturing process.
The calculations performed on the basis of the forming and lightweight design requirements using the finite element method revealed that the material needed to have a yield strength Rp0.2 of at least 800 MPa in conjunction with an elongation A80 of at least 10%. Four of the six high-strength groups could have been selected on the basis of this analysis of the relevant physical properties: bainitic and martensitic hardened and tempered steel, corrosion-resistant steel, and micro-alloyed, fine-grain steel. However, only the corrosion-resistant steel strip and the micro-alloyed, fine-grain steel fulfilled the third – and in this case decisive – requirement, namely with respect to the material’s weldability. In light of its favorable cost-benefit ratio and excellent weldability, we supplied a sample of micro-alloyed steel strip from the RAWAEL® material group. As became apparent during prototype manufacturing, this was the perfect recommendation for cost-effective processing and a final product with exactly the right properties.