In this work, IF steel was used as raw material for HPT process, which was manufactured by the Pohang Steel Company (POSCO, Korea) with the composition of 0.0026 wt%C, 0.096 wt%Mn, 0.045 wt%Al and 0.041 wt%Ti. Cylindrical samples (20 mm diameter, 2.0 mm thickness) were machined after annealing heat-treatment (700˚C for 2 h followed by cooling in a furnace), resulted in sample grain size of 150 - 200 μm and hardness of about 80 HV. Surface pressure of 2.5 GPa was imposed on the disks at room temperature. The degree of revolutions during torsional stage were 0, 1/4, 2/4, 3/4, and 1turns, respectively.

An FM-700 micro-hardness tester was used to measure the hardness with pressure loading of 100 g for 10 s. On the symmetric section of disks’ thickness direction, the hardness is measured interval 0.5 mm distance from axial center to edge along with the radius direction. Micro-structures at different positions on the disks were observed using optical microscopy (Olympus U-TV0.5xc) taken from the disks after the HPT process.

A commercial rigid-plastic finite element method (FEM) code (DEFORM-3D) was used to simulate the plastic deformation of IF steel disks in HPT process. In the FEM simulations, the initial dimensions of the disks were 20 mm diameter and 2.0 mm thickness. The number of the initial mesh in the sample was 25521 and this number of elements was enough to show the local deformation of the sample by calculation without changing the number of elements, as shown in Figure 1. All the strain data taken from DEFORM-3D code was processed using the Origin software for linear presentation, and the effective strain is calculated automatically based on Von-Mises yield criterion.

Figure 2 shows the effective strain distribution on the selected line and the effective strain distribution of half HPT processed IF steel disks (2.5 GPa; 0, 2/4, and 1 turns).

The results clearly indicate that the degree of inhomogeneous deformation and the value of the effective strain increase as the revolutions increases, along the radial direction.

A line is selected near the surface to investigate the trend of effective strain along radial direction except the flying edge, as show in Figure 2(a). The results indicate that large deformation reduces gradually from the edge to the center along the radial direction at the early torsion stage of the HPT process due to the vertical wall constraint and the friction between disk and dies. In addition, the highest value of effective strain on the selected line increases from ~0.84 to ~8.18 with the revolutions increases to 1 turn. That means that, as the revolution increasing, the deformation of HPT processed disks is more inhomogeneous.

Hardness and micro-structure distributions on the transversal plane of the HPT processed IF steel disks (2.5 GPa; 0, 1/4, 2/4, 3/4 and 1 turns), as shown in Figure 3. The results indicates that the value of hardness is higher and the size of grain is smaller as number of revolution increasing, and the distributions of hardness and micro-structure are inhomogeneous.

As the Figure 3(a) shows, low hardness values in the center and high values at the edge: after 1 turn, the hardness values in the center, middle, and edge are 136.7, 189.6, 322.5 HV, respectively, exhibiting the same trend as in the literature [11] [17] [18] . In addition, the amplification of hardness from center to edge of disks is more and more big with the revolutions increase. Figure 3(c) clearly indicates that without torsional straining (0 turns), the grain boundaries are clearly observed and the grains are insufficiently equiaxed. In contrast, after an additional revolution in the same torsional direction to give 1 turn, the grain boundaries become obscure and the grains are reasonably equiaxed. That is to say, severe grain deformations and progressively larger deformation zone proceed gradually from the edge to the center with increasing the revolutions, which corresponds to the hardness distribution on the transversal testing plane. That means the micro-structure distribution of HPT processed disks is more inhomogeneous with the revolution increase, same as hardness distribution.

The hardness distribution of the experimentally compressed disks was reflected in the strain distribution of the simulations [19] [20] . Figure 3(b) provides a comparison between experimental hardness and simulation results of effective strain distribution in the compressed copper disk under the number of revolution of 3/4 turns, applied pressure of 2.5 GPa. The same distribution trend was indicated in both experimental and simulated results. Hence, the reliability of this computer simulation is verified.