Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review

*Sanjeev Kumar and Erwin Povoden-Karadeniz*

## **Abstract**

The plastic deformation occurs in steels during metal forming processing such as rolling, forging, high-pressure torsion, etc. which modify mechanical properties of materials through the grain refinement, and the shape change of objects. Several phenomena in the scope of plastic deformation, such as hardening, recovery, and recrystallization are of great importance in designing thermomechanical processing. During the last decades, a focus of research groups has been devoted particularly to the field of metals processing of steel parts through plastic deformation combined with specific heat treatment conditions. In this review chapter, the current status of research work on the role of plastic deformation during manufacturing is illuminated.

**Keywords:** Plasticity, Ferrous metal, steel, SPD, Deformation, Strengthening, Flow Curves

## **1. Introduction**

In the modern era, the demand for lightweight material products is being increased in industries e.g. aerospace [1, 2], automobiles [3, 4], buildings [5, 6], trains [7, 8], forged connecting rods and pistons [9], bridges [10], naval [11–13], etc. for a high living standard (see **Figure 1**). The researchers are dedicating high effort to increase the strength to weight ratio by grain refinement through applying heat treatments [14–20], mechanical processing [21, 22], and a combination of both i.e. thermomechanical processing (TMP) [23–27]. TMP methods are being used in the manufacturing unit to fulfill requirements of grain refinement of materials and create optimum semi-finished and finished products for the applications. The grain size of steels is an important factor that affects all aspects of the mechanical, chemical, and physical behavior of metals to the surrounding media. It is well known that the smaller grains support an increase in grain boundaries in the matrix. In particular, according to the Hall-patch law, the reduction in grain size improves material properties like strength Eq. (1), hardness, and impact toughness except for the ductility of steels [28].

$$
\boldsymbol{\sigma}\_{\rm Y} = \boldsymbol{\sigma}\_{\rm i} + \mathbf{K}\_{\rm Y} / \sqrt{\mathbf{D}} \tag{1}
$$

Where: σi = friction stress, D = grain diameter, KY = yield coefficient or "locking parameter" that shows the relative hardening contribution of grain boundaries.

**Figure 1.** *Some important examples of industrial applications which developed using metal forming processes.*

Some of the major metal processing steps are often involved such as rolling, forging methods with wide temperature ranges (cold, warm and hot deformation temperature ranges) for the grain refinements [26, 27, 29–32]. The high-pressure torsion, equal channel angular pressing (ECAP), direct/indirect extrusion methods etc. are being used for ultrafine grains in which plastic transformation reaches over strain 1 through severe plastic deformation (SPD) [33–35]. In this SPD processes, the large shear stress involved usually results in a complex stress state resulting in a high defect density and homogeneous ultrafine grains.

During metal forming processing, the steel experiences different metallurgical phenomena like work hardening, dynamic recovery, dynamic recrystallization, flow instabilities, etc. [32, 36–38]. The effect of these metallurgical phenomena can be understood through the interpretation of flow curves [26, 31, 39, 40]. Where, the flow stress dependent on various processing parameters such as temperature, strain rate, and strain, etc. that can typically been described via constitutive equation.

This chapter focuses on plastic deformation behavior which can be controlled through processing parameters that affect microstructure refinement and associated mechanical properties of metals and steels during forming.
