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. 2022 Oct 25;15(21):7482.
doi: 10.3390/ma15217482.

Effect of Cold Rolling on Microstructural and Mechanical Properties of a Dual-Phase Steel for Automotive Field

Emilio Bassini  1   2 , Giulio Marchese  1 , Antonio Sivo  1 , Pietro Antonio Martelli  1 , Alessio Gullino  1 , Daniele Ugues  1   2
Affiliations

Effect of Cold Rolling on Microstructural and Mechanical Properties of a Dual-Phase Steel for Automotive Field

Emilio Bassini et al. Materials (Basel). .

Abstract

A new advanced dual-phase (DP) steel characterized by ferrite and bainite presence in equal fractions has been studied within this paper. The anisotropy change of this steel was assessed as a progressively more severe cold rolling process was introduced. Specifically, tensile tests were used to build a strain-hardening curve, which describes the evolution of this DP steel's mechanical properties as the thinning level increases from 20 to 70% with 10% step increments. As expected, the cold rolling process increases mechanical properties, profoundly altering the material's microstructure, which was assessed in depth using Electron Backscatter Diffraction (EBSD) analysis coupled with the Kernel Average Misorientation (KAM) maps. At the same time, the process strongly modifies the material planar anisotropy. Microstructural and mechanical assessment and the Kocks-Mecking model applied to this steel evidenced that a 50% strain hardening makes the DP steel isotropic. The material retains or resumes anisotropic behavior for a lower or higher degree of deformation. Furthermore, the paper evaluated the forming limit of this DP steel and introduced geometric limitations to testing the thin steel plates' mechanical properties.

Keywords: Kernel Average Misorientation (KAM); advanced high-strength steel; bainitic–ferritic steel; cold rolling; dual-phase steel.

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Conflict of interest statement

All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

Figures

Figure 1
Figure 1
High-resolution micrography showing bainitic islands (indicated as B) surrounded by ferritic matrix (indicated as F). Carbide platelets are indicated with a C.
Figure 2
Figure 2
Microstructures of the DP steel as the thinning level is increased, samples with longitudinal orientation observed with SEM. Thinning level is increased as follows 20, 30, 40, 50, 60, 70% represented in (a–f) panels.
Figure 3
Figure 3
Microstructures of the DP steel as thinning level is increased, samples with transversal orientation observed with SEM. Thinning level is increased as follows 20, 30, 40, 50, 60, 70% represented in (a–f) panels.
Figure 4
Figure 4
KAM (a) as calculated from EBSD detector; and (b) after conversion to a binary image using 0.6 as KAM threshold value.
Figure 5
Figure 5
(a) Nano-hardness grid and (b) the resulting contour map. (c) Summary of results from (a,b) in a histogram showing a bimodal distribution.
Figure 6
Figure 6
Bainite fragmentation: carbides appear more homogenously distributed as the strain hardening level is increased.
Figure 7
Figure 7
DP steel after 50% cold rolling observed with EBSD. Grains were observed in the x, y and z directions. The texture due to cold rolling is clearly visible. Pole figures and IPF are also shown in the bottom part of the figure.
Figure 8
Figure 8
Bar plots of the mechanical properties of the DP steel in all three metallurgical orientations as a function of the thinning levels. (a) Young modulus; (b) yield stress; (c) ultimate tensile stress; (d) elongation before sample striction; (e) elongation at break; (f) springback effect.
Figure 9
Figure 9
Mechanical properties as a function of metallurgical orientation after thinning the DP steel by 50, 60 and 70% with w/t = 4.5. (a) yield stress, (b) ultimate tensile stress, (c) elongation ant break and (d) springback effect.
Figure 10
Figure 10
Evolution of mechanical properties and springback for longitudinal samples as a function of strain hardening level (orange circles and squares indicate samples with reduced w/t ratio). (a) Young modulus, (b) yield stress, (c) deformation and (d) springback effect.
Figure 11
Figure 11
Anisotropy measured using Equations (1), (2) and (3): (a) Normal R and (b) planar ΔR.
Figure 12
Figure 12
Fractured surface of longitudinal samples with increasing work hardening levels of (a) 20, (b) 30, (c) 40, (d) 50, (e) 60 and (f) 70%.
Figure 13
Figure 13
Fractured surface of transversal samples with increasing work hardening levels of: (a) 20, (b) 30, (c) 40, (d) 50, (e) 60 and (f) 70%.
Figure 14
Figure 14
Schematic showing the change in shape and organization of bainitic islands in ferrite matrix passing from 20 to 50% strain hardening level.
Figure 15
Figure 15
Strain hardening rate and net flow stress plots for samples with (a) 20 and (b) 50% strain hardening level; and (σ − Y)Θ versus net flow stress (σ − Y) plots for DP steel with (c) 20 and (d) 50% strain hardening level.
Figure 16
Figure 16
(a) Strain hardening rate and net flow stress plots and (b) (σ − Y)Θ versus net flow stress (σ − Y) for samples with 70% strain hardening level.
See this image and copyright information in PMC

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