Structural Changes in HSLA Steel during Controlled Rolling

Thermomechanical Control processing (TMCP) has been extensively applied to steel production, especially that of plate products. In TMCP, a steel is hot rolled to a necessary amount of reduction at a proper temperature. The primary objective of controlled rolling on HSLA steel is to achieve a very fine, uniform ferrite grain size resulting in an increase in yield strength and an improvement in toughness properties due to grain size refinement. Structural changes in steels during controlled rolling are schematically illustrated in Figure 1. These changes are related to deformation at three regions.

(1)  Deformation in recrystallization region—In this region, coarse austenite grain a is refined by repeated deformation and recrystallization, which produce the recrystallized grains b. During cooling these grains would transform into relatively coarse feritte b’. (2) Deformation in non-recrystallization region— In this region, deformation bands are formed in elongated, unrecrystallized austenite c. During cooling, ferrite would nucleate on the deformation bands as well as gamma-grain boundaries, giving fine alpha grain c’.(3)    Deformation in the gamma–alpha region—In this region, deformation bands continue to be formed and also the deformed ferrite produces a substructure d. After deformation, during cooling, unrecrystallized austenite transforms into equiaxed alpha- grains, while the deformed ferrite changes into the subgrains d’.

Figure 1 Schematic illustration of changes in microstructure with deformation during controlled rolling.

The formation of the deformation bands is one of the principle features of controlled rolling. In the conventional hot rolling, alpha-grains nucleate exclusively at the gamma-grain boundaries, whereas in the controlled rolling, the alpha-grain nucleation occurs at both the grain interiors and grain boundaries, since the deformation band is equivalent to gamma-grain boundary with regard to the ferrite nucleation, the gamma -grain can be considered as divided into several blocks by deformation bands. This division allows one to produce much more refined grain structure. The second important feature of the controlled rolling is a formation of subgrain structure during deformation in two-phase region. The smaller the subgrain size, the stronger its strengthening effect is.

Industries: Steel Plant; line pipe Mill; Heat Exchanger; Pressure Vessel; Welding Fabrication; Automotive; Defence; etc.

Material: High strength low alloy steels, API grade Steels, structural Steels, various grade of steels; etc.  

Rishikesh kumar

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