Annealing and galvanizing lines

The production of modern AHSS grades places high demands on the thermal process in annealing and continuous galvanizing lines. In particular, the automotive industry requires homogeneous properties of the steel strips from coil to coil and along the coil length. In order to achieve this, information on the microstructure of the strip is necessary to control the process and compensate possible deviations caused in the upstream processes.

Therefore, SMS group developed the I-Furnace (Intelligent Furnace) including a smart annealing process and a production optimization model. Here, the combination of various tools leads to an optimized heat treatment and production process. Furnace control, online strength measurement, and a model to predict the material properties after annealing are combined and linked. However, the control of the process with a measurement system in the exit section of the line, far behind the process, has a long reaction time and at that position final mechanical properties are already adjusted. Thus, a further measurement system including a control model has been developed in addition to the already existing tools to complete the I-Furnace.

SMS group, Drever International, and IMS Messsysteme jointly developed X-CAP^® (X-ray Controlled Annealing Process), permitting the steel structure to be measured within the annealing process and hence the mechanical properties to be controlled in a closed loop first time, direct in the relevant process section.
 

To reach the highest cooling rates with gas cooling Drever developed the advanced gas jet cooling system UFCplus. With this cooling system, higher cooling rates of up to 150 kelvin per second per millimeter are obtained by adding pure hydrogen to the cooling chamber without increasing the total hydrogen consumption of the line. A patented hydrogen migration technology limits the diffusion of hydrogen into the adjacent chambers. Thus, a hydrogen content in the cooling chamber of up to 50 % can be reached while keeping the hydrogen content in the other sections still under 5 %.

The water spray rapid cooling system involves immersing the strip in demineralized water while special nozzles spray it from both sides at high pressure. Here, the cooling rate is more than 1,000 kelvin per second and millimeter of strip thickness, enough for manufacturing tough dual and complex-phase grades and martensitic grades with tensile strengths of 1,500 megapascals and more.

Wettability problems in the conventional continuous galvanizing process due to high alloyed materials can be prevented by a specific oxidation and reduction process, for which SMS group offers the PrOBOX ^®-technology. PrOBOX ^®-technology has become widely accepted as the best process for continuous galvanizing of high-strength steel grades with high silicon and manganese contents. For this purpose, on the strip surface the targeted reference oxide layer is adjusted during the annealing process at temperatures between 600 and 700 °C.

Tension levelers minimize irregularities such as waviness, cambers, or length- and crossbows using a combined tensioning and leveling action to create a flat final product. 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 of leveling steel strip with yield points much higher than 1,000 megapascals. The machines are also suitable for installation in existing plants to make these facilities meet future material requirements.

In principle, electromagnetic strip stabilization systems feature electromagnets arranged on both sides of the strip at the same height above the air knives. A position measuring system is installed below on each magnet position, which continuously measures the position to the strip in a non-contact manner. The strip position determined by this sensor is compared to the reference position. Due to this deflection, the electrical current is prescribed to the stabilization coils on the top or bottom side and results in the magnetic attraction of the steel strip. In this way, the crossbow of the steel strip is minimized and vibrations are reduced. Consequently, the distance of air knife lip to strip surface can be reduced. To satisfy today’s requirements on surface quality, an integrated solution has been developed, which brings air knife and strip stabilization closer together.

The X-Roll^® laser welder is designed to join hard-to-weld strip material. The laser welder comes with a patented, inductive heat treatment system including automatic welding parameter calculation system, a quality assurance system, and an exchangeable laser source. The laser welder has been installed in new plants as well as in modernization projects.