Steel strip for stamping and cutting lines: hardness and bendability combined

Whether cutting, creasing, perforating, or punching paper, cardboard, textiles, leather, plastics, or other materials – the blades used to process these products must meet a variety of different requirements. On the one hand, a particularly hard steel material is required. The reason for this is that high levels of pressure are exerted on the blade cross-section during the stamping or cutting process, which the blade must withstand for as long a service life as possible without bending. Another special characteristic is the cutting edge, which requires a higher degree of hardness due to the necessary sharpening and its even smaller cross-section.

This is countered by another requirement, however: The holders in the cutting and stamping tools require the back of the blades to be formed in usually very tight 90° radii. Conventional, hard steel materials would break on the stretched outside of these tight radii during the forming process. In order to meet both requirements – bendability and tensile strength – Waelzholz manufactures bainitically hardened and tempered steel strip with different degrees of hardness within the strip cross-section thanks to a special annealing process.

“Our special open coil annealing process allows us to precisely define the carbon content in different levels across the cross-section of the material,” explains Stephan Müller from Quality Assurance at the Waelzholz production site in Wickede, Germany. “This involves decarburizing the strip surface, making it soft, while the material core remains hard and is made even stronger by the subsequent bainitic hardening and tempering.” Open coil refers to the fact that the coils of cold rolled steel strip are unwound on a special oscillating machine and then rewound with corrugated wires between the surfaces for the annealing process. This creates free space between the layers of coiled strip. The exposed strip surfaces can now be decarburized on both sides during the annealing process, thereby improving the bendability of the material, which remains hard at its core.

In addition to soft material surfaces for forming narrow blade radii, it is important for the customer’s further processing of the cold rolled steel strip that the edge of the subsequent blade can be easily sharpened and that it is extremely hard. For this purpose, the cold rolled steel strip is annealed via the open coil process at the Plettenberg production site and then bainitically hardened and tempered after being reduced to the desired strip thickness. Matthias Nagel, responsible for Quality Assurance at the Waelzholz production site in Plettenberg, Germany, explains: “In this process, it is particularly important to heat the steel strip slowly and evenly so that the decarburized surface does not ‘mix back together’ with carbon atoms from the material core. Water is also added to the annealing furnace to maintain the decarburization of the strip surface. This keeps the outer edges of the material soft, while the inside of the material hardens due to the transformation of the microstructure.” The final, extremely homogeneous bainitic structure is perfect for the grinding and subsequent heat treatment of the ground edge to produce hard martensite by the customer. One factor that sets Waelzholz apart is the ability to precisely tailor the hardness of the material core to customer requirements using bainitic hardening and tempering. This means that hardnesses of 375 – 425 HV are possible, for example. The harder the material that will be cut by the blade, the harder the strip core must be so that the blade does not bend during the cutting process.