Rolling Process For Steel
Steel undergoes the process of rolling to reduce the thickness of the metal as well as make it uniform. In this process, steel is placed between two/multiple rolls where the thickness is altered by manipulating the length direction of the steel rather than the width direction. This is because the point of contact between rolls along the length is much smaller than the width of the steel sheet that is pushed in between. This makes sure that the length of the steel is worked on and the working of the roll along the width is restrained.
Types of rolling
There are two types of rolling process: hot and cold rolling process. The basic distinction between these two processes is the difference in the processing temperature with regards to the recrystallization of the steel. When the processing temperature of the deformation of steel is above the recrystallization temperature, it hot rolling. On the other hand, if the processing temperature is below the recrystallization temperature, it is cold rolling.
It makes the steel soft and ductile. The hardness of steel cannot be modified after the hot rolling process. The hardness is related to chemical composition of the metal as well as the rate of cooling. Hot rolling, however, may lead to loss of steel because of oxidation and result in poor surface finish.
In case of cold rolling, the hardness is much more than the hot rolling process. This process helps to obtain physical qualities of metal that are usually not obtainable by the hot rolling process. This can eliminate the errors like that of shrinkage. In short, cold rolling results in a final metal sheet that is more accurate in terms of dimensions and properties than the one that is obtained from hot rolling process. The metal sheet has a smoother surface and the strength of the steel is also increased. However, the ductility decreases. The strain makes the steel more brittle. Further, the final metal obtained needs to undergo heating regularly to make it workable.
The basic of rolling process of steel
Both horizontal and vertical stresses caused by the rolls compresses as well as restrain the steel. The rolls exert a vertical stress on the piece of steel and at the same time the price of metal exerts pressure back on the rolls. The rolls are exposed to elastic deformation because of the pressure induced by the piece of metal.
It leads to uneven distribution of thickness of the steel in such a way that the thickness is greater in the center and reduces as it moves along the edges of the metal. A thicker center indicates that the edges of the steel are more elongated. This implies that the edges are subjected to more stress and there is also more tension in the center of the working metal. The uneven thickness can lead to flawed final sheet of steel. It can cause warping, cracking or wrinkling of the steel sheet.
To counter this, back-up rolls are used. Otherwise, you can also use materials like sintered carbide that are high elastic modulus. Sometimes, cambered rolls are also used to counter elastic deflection of rolls. Depending upon the width of steel involved in the rolling and the stress flow, the degree of cambering can be decided. Insufficient cambering or absence of cambering can result in steel that has thicker center. On the other hand, if over cambering happens, it will make the edges thicker than center, which just the exact opposite of insufficient cambering. This can lead to splitting, wrinkling and cracking of edges of the final sheet of steel.
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