Tension leveling technology for high-strength strip

SMS group has developed tension levelers for cold as well as hot strip lines, that are capable to level steel strip with yield points much higher than 1,000 megapascals. The machines are also suited for installation in existing plants to make these facilities meet future material requirements.

In strip processing lines, tension levelers minimize flaws such as waviness, cambers or longitudinal and transverse bends by a combined tensioning and levelling action to create a flat final product. In pickling lines, where it is called scale breaker, the tension leveler has to perform an additional task which is to break the scale layers by alternate bending and stretching to assure better pickling medium efficiency and hence a faster pickling process. Market demands for higher material strengths lead to exacting changes in machine technology.

One aspect is higher forces in stretching and bending. SMS group has developed tension levelers for cold as well as hot strip lines, that are capable to level steel strip with yield points much higher than 1,000 megapascals. The machines are also suited for installation in existing plants to make these facilities meet future material requirements.

The newly developed tension leveler TL-40-V490 for high-strength cold strip is designed for strip tensions up to 550 kN. These strip tensions are capable of leveling third-generation AHSS grades. The machine is equipped with two bending roll units of 40-millimeter roll diameter to assure intense bending action. Furthermore, three correcting roll units are integrated to eliminate longitudinal and transverse bends in high-strength grades. The special arrangement makes sure the operating area is not coupled to the correcting area. Adjustment of the rolls as well as quick opening are achieved fully hydraulically, which permits the parameters to be flexibly adapted and thus to avoid non-flat areas. Material grades not to be leveled may pass the machine without contact. A special exchange system allows to quickly change the leveler cassettes during ongoing production.

At the end of 2019, Shougang Jingtang, China, successfully put on stream the new hot-dip galvanizing line from SMS group, including integrated tension leveler for high-strength strips. The line was specially designed for the processing of coated high-strength steels which are used, among others, as structural elements in the automotive industry. Referred to equal or even more stringent safety requirements, the new steel grades reduce the weight of the body in white. The main reason for placing the order with SMS group was the comprehensive expertise in processing lines for high-strength steel strip in addition to the positive experience they had with SMS group as plant supplier. The exit section of the line, including tension leveler, was designed to process steel strip with tensile strengths of up to 1,500 megapascals. One of the reasons behind was Shougang Jingtang’s intention to use the line for the development of new materials.

To meet the higher requirements in continuous pickling lines, SMS group developed tension leveler SB-80-V1050, a special machine for hot-rolled strip material. The machine is particularly made for use in pickling lines and designed for strip tensions of up to 1,100 kN. Usually, the required high tensions are established and released by at least two bridle roll sets upstream and downstream of the machine. Two bending and two correcting roll sets are installed to bend and stretch the material as needed. Further integrated in the machine are several backup rolls ensuring a uniform bending effect at the top and bottom strip sides.

This has the following advantages: a notable elongation referred to the strip tension applied, an increase in bending roll service life and a reduction in pickling time due to effectively pre-breaking the scale layer. Scale breaker adjustments are all hydraulic, and an exchange system permits the bending cassettes to be changed during ongoing operation.

The end of the second phase in Dece mber 2018 completed the revamp project of SSAB‘s continuous pickling line in the company’s Hämeenlinna works in Finland. The scope of the extensive modernization included the integration of a high-performance scale breaker. If so required, the maximum strip tension in the line can be increased to 900 kN, depending on the material grades. Besides the distinctly improved flatness, pickling times could be significantly shortened using the new scale breaker. For some materials it was possible to clearly increase the slow processing speed due to the pickling treatment, and hence to raise the output.