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Hot rolling is the most used process to obtain steel products with the most suitable shape to be used in the manufacturing industry.
The semi-products obtained in the solidification process of the steel mill are not usable due to their fragility and low internal quality due to their coarse grain and porosity. To give it toughness and improve other properties, the section must be reduced by rolling or hot forging, compacting it and forming the internal fibers.
How is a steel with low properties transformed into one that responds to the applications of the demanding mechanical industry?
Taking advantage of the deformability of steel at high temperatures, the semi-product of the steel mill (bloom, ingot, etc…) is heated between 850 and 1,200ºC. and it is forced to pass under pressure between two rollers or cylinders that rotate at the same speed but in the opposite direction, reducing the section and lengthening the product, obtaining products with a constant section such as sheets, profiles, bars, etc.
In modern steel manufacturing facilities, one more function is incorporated into the process, the so-called controlled rolling. It consists of rolling at lower temperature ranges than the traditional ones close to 1,200ºC, so the final rolling temperature is close to that of the austenite-perlite transformation, where the grain stops growing. Properties close to those of the product can be obtained with Normalized heat treatment.
The minimum section reduction that must be applied between the semi-product and the final product is a function of the chemical composition of the steel and the internal quality of the steel mill’s product. In C steels it is usually at least 3.5 times the initial section. Depending on the alloy, this proportion will be higher. In highly alloyed steels it can be greater than 10 times. The reduction calculation is obtained by dividing the initial section of the steelmaking semi-product by the final section of the rolled product.
Written by: Iñaki Aldayturriaga