Abstract

For Poly-crystalline metals, Grain boundary design plays an important role to achieve desired mechanical properties in the final product form which may be a hot rolled or cold rolled coil. The fundamental mechanical properties are yield and tensile strengths, elongation and formability where grain refinement is particularly attractive mechanism for property design. Grain boundary strengthening provides benefits both in terms of fracture toughness and mechanical behavior at lower temperatures particularly in case of hot rolled high strength structural and line pipe steels. For cold rolled steels, grain boundary effects play a crucial role in critical automotive application steels such as Bake Hardening Index in Bake-hardening Steels. Additionally, deep drawability and formability characteristics are also strongly dependent upon grain size. Mechanical properties in steels are controlled majorly by two techniques - chemical composition and Steel processing parameters. Through first method of chemical composition control, during hot rolling, the control of austenite grain size is accomplished, alongside retardation of phase transformation to lower temperature. In second method, steel manufacturing process parameters i.e. hot rolling and coil cooling parameters, cold reduction, and subsequent heat treatment parameters such as annealing play an important role.

Highlights

  • Typical thermomechanical hot rolling process can be divided into five basic processes

  • The recrystallization rolling requires rolling at high temperatures that leads to recrystallization and control of grain size

  • The conventional controlled rolling approach requires rolling in no-recrystallization zone, leading to unrecrystallized grains which lead to finer sizes after phase transformation (Figure 2)

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Summary

Thermomechanical processing in hot rolling mill

Thermomechanical processing in hot rolling mill in general involves rolling of slabs or blooms to thinner sections which are classified as plate or coils by inducing reduction in thickness by rolling at controlled elevated temperatures to achieve desired mechanical properties.

Rough rolling
Types of hot rolling approaches
Recrystallization phenomenon during rolling
Determining rolling parameters for hot rolling
Modeling the mean flow stress to estimate the critical rolling parameters
Critical strain evaluation for static recrystallization
Grain size evaluation for static recrystallization
Grain size evaluation for dynamic recrystallization
Evaluation for work hardening and fractional softening
Evaluation for predicting mean flow stress (MFS) in each rolling pass
Effect of chemical composition
Addition of niobium, titanium, and vanadium
Manganese-based strengthening
Reheating temperatures at reheating furnace
Repeated recrystallization in roughing mill
Finish rolling at no-recrystallization temperatures
Accelerated cooling

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