Krahe, Tarentum, and Bernard M. Pittsburgh, Pa. This invention relates to a method for continuously annealing cold-reduced strip steel, particularly low carbon steel strip.
That is, the cold rolling procedure for a steel strip is carried out independ-ently from the continuous annealing procedure for the steel strip. That is, in order to connect the cold rol]ing process line with tne continuous annealing process line through an intermediate strip accumulator so as to provide one process line, it is necessary that the cold rolling mills can be operated at the above-mentioned speed. The cold rolling process line may comprise 6 tandem high speed cold rolling mills or 4 cold rolling rnills arranged in series and each consisting of a pair of top and bottom working rolls and two pairs of top and bottom backing up rolls, each working roll being backed up by two backing up rolls and, optionally, a cold rolling drawing mill.
Steel material hardens after cold rolling due to the dislocation tangling generated by plastic deformation. Annealing is therefore carried out to soften the material. The annealing process comprises heating, holding of the material at an elevated temperature soakingand cooling of the material.
Annealing is a process in which metals, glass and other materials are treated to render them less brittle and more workable. Continuous steel annealing subjects rolled strip product to a sequence of furnaces to elevate and profile the strip temperature according to grade and dimension. The end result is an increased ductility and removal of strains that lead to failures in service. Figure 1 shows the linear nature of a continuous annealing line.
Thus, the mechanical properties necessary for each application are achieved in a precisely controlled way. Water quenching and overaging may also be required process steps. The final product quality in terms of mechanical properties and surface quality is adjusted by an in-line skin pass mill.
This line facility changes the crystal structure of steel sheet by heat treatment, and improves properties such as hardness, strength, and elongation. It integrates the 5 processes of cleaning, cooling, heating, temper rolling, and refining, and carries them out in a single line, thus saving space and lowering costs. For this reason, we have a track record of delivering our line to many customers around the world.
Steel consumers and competitive market conditions are driving steel mills to produce higher quality products more cost effectively. In order to meet these demands, steel mills must implement better controls to manage the temperature and surface character of the steel strip during the annealing process. In the annealing process, a controlled time-temperature relationship is important to obtain the desired mechanical properties of the steel strip.
The cold rolling of steel is done at temperatures below the recrystallization temperature. During cold rolling process the reduction in thickness is due to plastic deformation which occurs by means of dislocation movement. Steel gets hardened because of the buildup of these dislocations.
The process of annealing steel strip continuously allows several indisputable quality advantages when compared to the batch process — in particular, uniformity of heating and cooling. Nowadays, continuous annealing is the only process that allows a very fast cooling rate for the steel after recrystallization. This high cooling rate is essential, at least in order to produce advanced high-strength steels without resorting to exotic grades and expensive steel chemistries, and it is the only option when final tensile strengths in excess of 1, MPa are required.