International Journal of Minerals, Metallurgy and Materials Volume 21, Number 3, March 2014, Page 273 DOI: 10.1007/s12613-014-0905-x

Corresponding author: Florin Popa E-mail: florin.popa@stm.utcluj.ro © University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2014

Influence of high deformation on the microstructure of low-carbon steel

Florin Popa1), Ionel Chicinaş1), Dan Frunză1), Ioan Nicodim2), and Dorel Banabic2) 1) Materials Sciences and Engineering Department, Technical University of Cluj-Napoca, 103-105 Muncii Avenue, 400641, Cluj-Napoca, Romania 2) CERTETA Center, Technical University of Cluj-Napoca, Cluj Napoca 400114, Romania (Received: 10 September 2013; revised: 13 November 2013; accepted: 27 November 2013)

Abstract: Low-carbon steel sheets DC04 used in the automotive industry were subjected to cold rolling for thickness reduction from 20% to 89%. The desired thickness was achieved by successive reductions using a rolling mill. The influence of thickness reduction on the micro-structure was studied by scanning electron microscopy. Microstructure evolution was characterized by the distortion of grains and the occur-rence of the oriented grain structure for high cold work. A mechanism of grain restructuring for high cold work was described. The occur-rence of voids was discussed in relation with cold work. The evolution of voids at the grain boundaries and inside the grains was also consid-ered. To characterize the grain size, the Feret diameter was measured and the grain size distribution versus cold work was discussed. The chemical homogeneity of the sample was also analyzed.

Keywords: low carbon steel; deformation; microstructure; cold rolling

1. Introduction

Low-carbon steel is a widely used material in the field of machine building, well known but continuously studied be-cause of its tendency of creating new microstructures and developing new properties by heat treatment and/or proc-essing. This alloy is essential because of its high ductility, which allows it to be strongly deformed (improved deform-ability) and shaped in complex parts, to be welded [1], and to obtain finished products with small sections and light weight [2]. Microstructures determine the properties of the material on a large scale; the grain size has a strong influ-ence on the mechanical strength and ductility of the material [1, 3]. Processing methods such as cold forming and heat treatment can influence the grain structure. The grain de-formation and fragmentation due to the cold forming of the steel lead to smaller grains, which are more difficultly de-formed, and material strengthening is achieved. Extended studies concerning obtaining an extremely fine structure of low-carbon steel, especially by severe plastic deformation and heat treatment, were reported [4−8].

Combining deformation at high temperature with cooling at certain rates proved that an equiaxial structure can be re-tained up to 850°C [9−10]. Other studies observed a ten-

dency of the grain boundary to align with the shear direc-tions [11] and grain boundary migration by maintaining at high temperature [12]. After deformation, the microstruc-tures can be classified into regions with elongated micro-structures, newly generated ultrafine grains along with elongated grains, and newly generated ultrafine grains [12]. The elongated shapes of the ferrite grains can produce fa-vorable combinations of strength and ductility in materials, and offer new ranges of utilization of this material [2]. Plas-tic deformation can also lead to the initiation of cracks and voids in steel [13−14], which, in large number, can reduce or limit the potential use of the material.

For some practical applications, material anisotropy can assure useful properties. In the case of steels, anisotropy can be obtained by plastic deformation, and its effect on the texture development was studied [15−17].

In this study, microstructure evolution and void genera-tion/evolution in low-carbon steel are discussed at different cold works up to 90%.

2. Experimental

For the rolling process, a DC04 (C: 0.08%; Mn: 0.40%; P: max 0.03%; S: max 0.03%; and remaining: Fe) steel sheet

274 Int. J. Miner. Metall. Mater., Vol. 21, No. 3, Mar. 2014

with 0.85 mm of initial thickness was used. Table 1 lists the uniaxial mechanical parameters of the material such as yield stresses (σb

exp/σ0exp = 1.28) and anisotropy coefficients (rb

exp = 0.84), corresponding to equibiaxial loading along the rolling and transverse directions, respectively, determined on the samples cut at 0°, 45°, and 90° from the rolling direction (n and K represent the coefficients in the Hollomon’s harden-ing law) [18]. The values of yield stress are normalized by the yield stress associated with the 0° direction). The com-mercial DC04 steel sheet was cold rolled at room tempera-ture using a 100-mm rolling mill with 5% partial thickness reduction at each rolling pass. The straining degrees were in the range of 20%−89% thickness reduction. The deformed samples have been cut and embedded in resin for standard metallographic analysis. Abrasive paper and alumina pow-der (granulation below 0.5 µm) were used for polishing. The polished samples were etched with Nital reagent (10% HNO3 and 96% C2H5OH). For the analysis of the etched samples, scanning electron microscopy (SEM) and en-ergy-dispersive X-ray spectroscopy (EDX) microanalysis (JEOL scanning electron microscope—JSM 5600LV, equip-ped with an EDX spectrometer—Oxford Instruments, INCA 200 software) were used.

Images obtained by SEM were processed to enhance the grain boundaries and analyzed with Image J software for the

determination of the Feret diameter. To obtain reliable data, multiple images recorded at the same magnification were analyzed, and the mean results were used.

Table 1. Values of the uniaxial mechanical parameters de-termined on the samples cut at 0°, 45°, and 90° in the rolling direction.

Angle / (o) expbr exp exp

0/σ σθ n K / MPa

0 1.95 1.00 0.21 526.750

45 1.29 1.06 0.20 541.323

90 2.19 1.04 0.20 513.559

3. Results and discussion

The microstructure evolution of the 20%−89% deformed samples in the low-carbon steel during cold rolling is com-pared with the microstructure of the unstrained sample. At low magnification (Fig. 1), the microstructure of the un-strained sample contains equiaxial ferrite grains with po-lygonal shape (polygonal ferrite) and a small number of voids at the grain boundaries. The grain size distribution is highly inhomogeneous, with both large (up to 50 μm) and small values.

Fig. 1. Microstructures of the cold-worked samples obtained by SEM (the rolling direction (RD) is indicated for each deformation degree).

F. Popa et al., Influence of high deformation on the microstructure of low-carbon steel 275

Cold rolling of the steel sheet leads to a significant struc-tural modification of shape, size, and structure of each grain, depending on the plasticity of structural constituents. At small cold work (20%), the grains begin to deform and elongate in the rolling direction. For larger cold work (40%), the grain elongation degree along the deformation direction increases and, at 70% deformation, a highly oriented grain structure is visible, finalized at 89% with a fibrous structure, as the grains are fractured.

At higher magnifications (Fig. 2), the grain fragmenta-tion during the cold rolling and void apparition are easily visible. The voids have a notable influence on the fibrous structure formation in highly deformed samples. In the un-strained sample, voids exist inside the grains. In the sample with 20% cold work, new voids appear and, in certain

cases, they are aligned in the rolling direction. In the re-gion with a lower cohesive force, voids emerge at the grain boundary, not necessarily in the deformation direction. Generally, voids inside the grains are large, and are proba-bly original voids present in the undeformed material. The softness of the ferrite phase hinders void initiation. Con-tinuing the deformation, the voids evolve in the deforma-tion direction but, at 70% cold work, the voids start to de-velop inside the grains, leading to grain fragmentation. The grains evolution versus the thickness reduction suggests good deformability and plasticity of DC04 steel. The oc-currence of voids inside the ferrite grains can be explained by stress accumulation in the grains during the high defor-mation process, leading to a reduced influence of the grain boundaries [2].

Fig. 2. Microstructures of the samples obtained by SEM (the rolling direction (RD) is indicated for each deformation degree).

At 20% cold work, initial voids in the sample became elongated in the rolling direction and, at high magnification, the emergence of new grains due to the splitting of old grain boundaries following the dislocation mechanism can be ob-served [19]. The voids at this cold work increase their size in the rolling direction without coalescing, and the ferrite grain can be deformed without fracturing [14]. In addition, the grain boundary can split in the direction perpendicular to the rolling.

Fig. 3 shows the formation of new grains by continuous rolling at 40% cold work. Crack splitting and occurrence of three new grains along with a void that propagates from a

grain boundary are visible (Fig. 3(a)). A new behavior of the voids is observed: the voids group into families by creating connections between them (Fig. 3(b)), and the new crack propagates inside the larger grains (Figs. 3(c) and 3(d)). The void family formation leads to coalescence and emergence of large voids perpendicular to the deformation direction as a consequence of slip band apparition, as proved in fatigue tests [2, 13]. At this cold work, a second effect is the occur-rence of new grains inside the larger ones. The existence of an “island”-type grain inside a larger grain is unusual for metallographic structures (Fig. 4).

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Fig. 3. Microstructures of the samples cold rolled at 40 % by SEM: (a) grain fracture; (b) void grouping occurrence; (c) voids in-side grain; (d) voids at the grain boundary.

Fig. 4. Apparition of new grains inside the deformed grain at 40% cold work.

The presence of particles can be observed in some large voids at 40% and 70% cold work, but not for 89% for which the voids are smaller. The EDX elemental mapping analysis of the voids containing particles was performed and showed the absence of Fe at the void level and the presence of Al and O elements (data not shown). Alumina particles inside the voids do not play any role in the void generation or propagation. In all the recorded images, the white particles are alumina residues that could not be removed after polishing.

The grain splitting by void evolution is more evident at 70% cold work, in which the void number is larger than that

at smaller cold work. The deformation leads to the occur-rence of parallel structures in certain grains (Fig 5), whose origin could be the exceeding of the maximum shear stress supported by the material and the activation of a slip system [20].

Fig. 5. Apparition of parallel slip lines at 70% cold work.

A brief comparison of 40% and 70% cold works shows that at 40%, the voids are larger than at 70%. This difference can be understood by the fact that the larger voids created at smaller cold work create new grains with smaller size by flattening out. At 70% cold work, it is more difficult to cre-ate large voids in the smaller grains, but a large number of smaller voids can be created at higher cold works. As the

F. Popa et al., Influence of high deformation on the microstructure of low-carbon steel 277

material is strongly deformed, its strength increases by cold hardening mechanisms and a new deformation is more dif-ficultly propagated.

At the most intense cold work (89%), the grain structure contains both very elongated and extremely small grains. The elongated grains are aligned in the rolling direction, but the smaller ones do not follow a particular direction. The obtained fibrous grain and void structure is found to repre-sent a single initial phase (chemical map analysis) and not the result of some impurity phases formed/expressed by rolling. All the grains contain a large number of voids, but the grain boundaries contain the largest amount (Figs. 6(a) and 6(b)). To quantify the size of the deformed grains, the Feret diameter of the deformed samples was determined and its evolution with the cold work is shown in Fig. 7. The evolution of the Feret diameter shows that, up to 20% cold

work, the mean grain size has a small evolution (i.e., it re-mains approximately 29.5 μm). As the deformation in-creases, the mean grain size increases as well up to 33 μm in the sample deformed at 70%. The increase in the mean par-ticle size has its origin in the grain distribution of the un-strained sample containing small grains and few larger ones. As the deformation begins and increases, the larger grains are elongated and start to be fractured. Increasing the mate-rial strength upon deformation can produce new grains more difficultly, but it is possible to increase the size of the exist-ing ones by generating voids in the material. Finally, for 70% cold work, the number of grains is smaller but shows a more homogenous distribution than those at lower cold works.

At the highest cold work (89%), the Feret diameter is not calculated because the grain boundaries were not visible.

Fig. 6. Microstructures of the samples cold rolled at 89% (different magnifications are presented to evidence void distribution).

Fig. 7. Evolution of the Feret diameter versus the cold work (for comparison, the Feret diameter of the undeformed sample is presented, and the line is only a guide for eyes).

4. Conclusions

The deformation of the DC04 steel leads to structural

changes in the ferrite microstructure, from equiaxial ferrite grains to fibrous structure at 89% cold work. As the cold work increases, the grains are fragmented or elongated in the rolling direction. At low cold works, new grains form by splitting the old joint grains. At high cold work, new grains can emerge inside the old large grains.

Voids are generated in the structure during the rolling. For low cold works, voids are generated and elongated in the rolling direction. At high cold work, the voids start to propagate inside the grains, and this originates in the stress release inside the grains by generation of the slip bands. The existence of parallel structures in certain grains proves the occurrence of the slip bands.

By measuring the Feret diameter of the grains, an in-crease of the mean grain size for larger cold works is ob-served. For large cold work up to 70%, the number of grains is smaller but their diameter is higher and they are more homogeneously distributed. At extremely high cold work (89%), the Feret diameter was not calculated due to the dif-ficulty to visualize the grain boundaries.

278 Int. J. Miner. Metall. Mater., Vol. 21, No. 3, Mar. 2014

The carbon and iron distribution maps in deformed sam-ples do not show the occurrence of new phases, but all the samples exhibit chemical homogeneity.

Acknowledgements

The authors would like to thank the Romanian Ministry of Education and Research for CNCS Grant, project number PCCE ID100/2010.

References

[1] M. Durand-Charre, Microstructure of Steels and Cast Irons, Springer-Verlag, 2004.

[2] G. Krauss, Steels, Processing, Structure and Performance, ASM Int., 2005.

[3] H.K.D.H. Bhadeshia and R.W.K. Honeycombe, Steels: Mi-crostructure and Properties, 3rd Ed., Elsevier Ltd., 2006.

[4] J. Zrnik, I. Mamuzić, S.V. Dobatkin, Z. Stejskal, and L. Kraus, Low carbon steel processed by equal channel angular warm pressing, Metalurgija, 46(2006), No. 1, p. 21.

[5] Z.Y. Cheng, J. Zhu, X. H. Chen, H.H. Zhou, X. J. Sun, and Q.Y. Liu, The netted ferrite at grain boundaries in the steels with thermomechanical processing, Mater. Letter., 59(2005), No. 19−20, p. 2570.

[6] T. Senuma, M. Kameda, and M. Suehiro, Influence of hot rolling and cooling conditions on the grain refinement of hot rolled extra low-carbon steel bands, ISIJ Int., 38(1998), No. 6, p. 587.

[7] Z.M. Yang and R.Z. Wang, Formation of ultra-fine grain structure of plain low carbon steel through deformation in-duced ferrite transformation, ISIJ Int., 43(2003), No. 5, p. 761.

[8] M.R. Barnett and J.J. Jonas, Influence of ferrite rolling tem-perature on microstructure and texture in deformed low C and IF steels, ISIJ Int., 37(1997), No. 7, p. 697.

[9] K. Shibata and K. Asakura, Transformation behavior and mi-crostructures in ultra-low carbon steels, ISIJ Int., 35(1995), No. 8, p. 982.

[10] R.K. Ray, M.P. Butron-Guillen, J.J. Jonas, and G.E. Ruddle, Effect of controlled rolling on texture development in a plain carbon and a Nb microalloyed steel, ISIJ Int., 32(1992), No. 2, p. 203.

[11] G. Angella, B.P. Wynne, W.M. Rainforth, and J.H. Beynon, Microstructure evolution of AISI 316L in torsion at high temperature, Acta Mater., 53(2005), No. 5, p. 1263.

[12] Y. Yogo, H. Takeuchi, T. Ishikawa, N. Iwata, and K. Naka-nishi, Strain-induced boundary migration of carbon steel at high temperatures, Scripta Mater., 61(2009), No. 11, p. 1001.

[13] N. Narasaiah and K.K. Ray, Initiation and growth of micro- cracks under cyclic loading, Mater. Sci. Eng. A, 474(2008), No. 1−2, p. 48.

[14] G. Avramovic-Cingara, Ch.A.R. Saleh, M.K. Jain, and D.S. Wilkinson, Void nucleation and growth in dual-phase Steel 600 during uniaxial tensile testing, Metall. Mater. Trans. A, 40(2009), No. 13, p. 3117.

[15] K. Kowalczyk and W. Gambin, Model of plastic anisotropy evolution with texture-dependent yield surface, Int. J. Plast., 20(2004), No. 1, p. 19.

[16] D. Raabe, P. Klose, B. Engl, K.P. Imlau, F. Friedel, and F. Roters, Concepts for integrating plastic anisotropy into metal forming simulations, Adv. Eng. Mater., 4(2002), p. 169.

[17] D. Banabic, T. Kuwabara, T. Balan, and D.S. Comsa, An anisotropic yield criterion for sheet metals, J. Mater. Process. Technol., 157−158(2004), p. 462.

[18] L. Paraianu, D.S. Comşa, I. Bichiş, and D. Banabic, Influence of the mechanical parameters on the forming limit curve, Steel Res. Int., (2011), Spec. Ed., p. 744.

[19] S.D. Lu, Z.B. Wang, and K. Lu, Strain-induced microstructure refinement in a tool steel subjected to surface mechanical attri-tion treatment, J. Mater. Sci. Technol., 26(2010), No. 3, p. 258.

[20] P. Balke and J.Th.M. De Hosson, Orientation imaging mi-croscopic observations of in situ deformed ultra low carbon steel, Scripta Mater., 44(2001), No. 3, p. 461.