Corrugated wire creates free spaces between layers of cold rolled steel strip
| Steel materials

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

Blades for cutting, stamping, and creasing lines place very special demands on the materials used. To produce the blades with tight radii, the cold rolled steel strip must be easy to bend but at the same time extremely rigid. In addition, the edge sharpness required for later use requires an exceptionally hard cutting edge. A special form of bainitically hardened and tempered steel strip from Waelzholz, which is annealed via a special open coil process, offers a solution for the synthesis of these contradictory properties.

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.

Open coil annealing process for different degrees of hardness

“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 a special coiling process, corrugated wire is inserted between the individual layers of steel strip to create open spaces. Hydrogen circulates through these free spaces during the annealing process, which reduces the amount of carbon in the material surfaces.

Soft material surfaces enable narrow blade radii

Stephan Müller explains: “In contrast to conventional annealing, where decarburization is prevented through the use of inert gas, we deliberately use steam in open coil annealing to reduce the carbon in the strip surfaces. One of our areas of expertise in this regard is precisely defining the carbon content in accordance with customer requirements and ensuring a high degree of uniformity across the entire length of the steel strip.” The decarburization depth can be specified exactly and can even go as far as complete decarburization. This level of accuracy when it comes to the degree of decarburization is made possible thanks to the company’s many years of experience with a sophisticated annealing process. “The precisely specified annealing time, perfectly matched temperature curves, and defined water quantities during the annealing process are just three of the many parameters that need to be mastered,” says the materials engineer. A homogeneous distribution of temperature and water vapor, among other factors, ensures that the degree of decarburization is uniform along the entire length of the strip. “In addition to the configuration of the process parameters, precisely winding the coils with the corrugated wires as spacer layers – which must enable a defined level of convection, i.e., air circulation between the individual layers of coiled strip – plays a role as well. We have a specially designed oscillating system for this purpose,” Müller notes.

Hard material core for durable stamping and cutting line blades

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.

Microscopic view of annealed bainitic hardened and tempered steel strip with decarburized surface

With a high level of process expertise in open coil annealing, Waelzholz is able to guarantee a defined decarburization depth in accordance with customer specifications.

Bainitic hardened and tempered steel strip in a bend testing machine with 90 degree bending

Waelzholz’s QA team bends the open-coil annealed and bainitically hardened and tempered steel strip by more than 90 degrees in a bending test machine. This ensures that the material will not tear during subsequent processing into a stamping or cutting blade.

Benefits of open coil annealed, bainitically hardened and tempered steel strip from Waelzholz

  • High tensile strength of the material core
  • Core hardness can be configured precisely to customer requirements
  • Subsequent heat treatment possible, for example after edge grinding to martensite
  • Edge decarburization depth can be precisely defined according to customer requirements
  • Excellent formability with extremely tight bending radii
Schematic diagram of different degrees of hardness of bainitically hardened and tempered steel strip

Blades for cutting, stamping, and creasing lines require cold rolled steel strip with different degrees of hardness: Soft material (decarburized) on the strip surfaces for tight bending radii of the blade backs, a hard material core (bainitically tempered) for high blade stability, and extremely hard material (martensitically hardened and tempered) for stable and durable cutting edges.

 

01  Decarburization (very soft) 

02  Bainitic microstructure (hard)

03  Martensitic structure (very hard)

Precisely defined straightness for ideal cutting results

In addition, the production process for blades used in cutting, stamping, and creasing lines places high demands on the straightness of the cold rolled steel strip being used. Matthias Nagel explains: “We manufacture our materials with a high degree of flatness and straightness so that the cutting blade can be sharpened precisely. On the other hand, the degree of straightness serves to ensure optimum clamping of the cutting blades in the subsequent tool and thus, in addition to other factors, also improves the cutting result. Optimally configured cutting blades also lead to an increase in tool life during subsequent use.”

One-of-a-kind composite material thanks to technical expertise and production equipment

“It’s fair to say that our cold rolled steel strip for cutting, stamping, and creasing lines is a composite material, as it has different hardnesses within the strip thickness,” says Nagel. "Our production expertise and our state-of-the-art equipment make it possible to precisely set the right carbon content at the right point in the material for the customer so that it exhibits the absolute best material properties for further processing and final use.”

Open coil annealing process at Waelzholz

Watch the accompanying video for insights into our special open coil annealing process, which produces cold rolled steel strip with different degrees of hardness. The process essentially consists of three steps:

  1. Inserting wire into the coil
    The space between the layers of strip created by the wire ensures that the water vapor can easily flow through during subsequent annealing.
     
  2. Bell annealing of the steel strip
    During annealing, steam is fed in to decarburize the material surface, making it soft, while the material core remains hard.
     
  3. Bend testing in quality assurance
    After open coil annealing and bainitic hardening and tempering, the steel strip can be bent by more than 90 degrees without cracking.

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