Mechanism of Rolling to Improve Strength and Toughness of Steel
Steel is closely related to our life, because it is widely used in household items, bridges, large scale engineering, transportation and manufacturing etc. Therefore, how to improve the strength and toughness of steel is an important problem in the steel industry. Rolling is the most commonly used process to strengthen and toughen steel.
The strengthening and toughening of steel by rolling can be divided into two major processes at different stages. The first process is prior plastic deformation also known as work hardening, which occurs in hot plastic deformation, cold or warm plastic deformation stage. During this stage, the motion of dislocation and the change of grain size are two main factors that influence the performance of steel. During the second process also known as recrystallization, the dislocation and imperfection of original grains are corrected by recrystallized grains, and the process performance of steel is greatly improved.
Work hardening refers to the phenomenon that the hot plastic deformation can be further strengthened by cold or warm rolling. During hot rolling, the dislocation of steel is the most important factor that strengthen steel. At the beginning of hot rolling, some physical and chemical bonds in the material are broken, resulting in a large number of random dislocation of atoms in the crystal lattice of the material. When the applied pressure is less than a certain value, these dislocation move slowly and accumulate, resulting in a temporary increase in strength of the steel. This phenomenon is called dynamic recrystallization, and the strength of the steel is increased.
In the recrystallization process, the grains of steel are completely changed. The original grains composed of many dislocations are substituded by the recrystallized grains with fewer and superior dislocations. As the coalescence of recrystallized grains increases, the uniformity of grains increases, the number and direction of dislocations decrease, and the structure gradually becomes ideal. At the same time, the original internal stress is released and the strength and toughness of the steel are increased.
In conclusion, the two processes of work hardening and recrystallization play an important role in the rolling process. The work hardening process is responsible for the strengthening of steel and the recrystallization process is responsible for the release of internal stress, leading to an increase in the strength and toughness of the steel. This process is the mechanism of the rolling process to improve the strength and toughness of steel.
