Materials Research. 2015; 18(4): 843-852 © 2015 DOI: http://dx.doi.org/10.1590/1516-1439.000314

Microstructural and Micromechanical Effects of Cold Roll- on High Strength Dual Phase Steels

Meritxell Ruiz-Andresa, Ana Condea, Juan de Damboreneaa, Ignacio Garciaa*

aCentro Nacional de Investigaciones Metalúrgicas – CENIM-CSIC, Madrid, Spain

Received: June 18, 2015; Revised: June 29, 2015

In this work correlation between the 1000 MPa dual phase (DP) steel microstructure and the strain gained after roll-forming process have been studied by both microstructural and micromechanical analysis. The scanning electron microscope (SEM) inspection in the bent area reveals changes in the ferrite-martensitic microstructure. The plastic deformation of DP steels originates defects at the edges of bent sheet make them partly responsible for the damage caused. In addition, electron backscatter diffraction (EBSD) measurements have been carried out for an in-depth characterization after roll-forming. A high density of misorientation of the crystal lattice within the ferrite strained grains is observed, mainly concentrated in the ferrite/martensite grain boundaries. Furthermore, the ultramicrohardness tests exhibit little dependence between mechanical parameters and the material properties.

Keywords: dual phase steel, roll-forming, EBSD, ultramicrohardness, microstructure

1. Introduction High strength low alloy (HSLA) steels, specifically dual or cup . Luo & Wierzbicki15 analysed the failure phase (DP) steels, are widely used in the automotive industry behaviour of DP steels during stretch-bending operations due to the necessity of improving fuel efficiency1-3, with its and summarized the typical damage accumulation in this subsequent energy saving and environmental protection4. area. Wu-rong et al.8 found that DP steels show competitive In addition, these DP steels have been developed to increase formability after cup drawing tests, despite encountering the highly demanding collision safety standards5 in the consistent shear cracks on the damage surfaces. Wang & vehicle body frame parts like bumper beam, door beam and Wei 16 showed that in stretch-bending tests not only latitudinal panel reinforcement. but also longitudinal cracks were propagated along DP steels DP steels are characterized by a microstructure consisting in a mixed propagation mode, i.e. along ferrite-martensite of about 75-85% ferrite phase with the remainder being a interfaces and/or also across martensite grains. Subsequently, mixture of the martensite and lower amounts of other such Wang et al.4 discussed that the fracture mode experiences as bainite and retained austenite6,7. a transition from shear fracture to necking before fracture. Such a structure leads to different unique properties, as Mishra & Thuillier17 concluded that bending area of maximum high tensile strength, low yield strength ratio followed by strain is not highly localised, but more uniform. continuous yielding behavior (no-yield point elongation), high However, to the author’s best knowledge no studies work-hardening rate at early stages of plastic deformation have been reported on the DP steels evolution in cold as well as good ductility8-10. Furthermore, the absence of the roll‑. yield point elongation provides DP steels with a high crash In this work, microstructural and micromechanical study resistance, good formability and excellent surface finish11. have been performed in order to go deep in the understanding of Roll-forming is a bending process in which a the behaviour between the microstructure and the deformation is continuously and gradually formed into a profile with a during cold roll-forming processes and provides overall desired cross-section along the transverse direction by passing responses to the automobile manufacturing industry, which through a series of rolls arranged in tandem12. It can be is a subject of main concern, from both industry sector, in performed at high speeds and therefore has been considered, ever increasing new designs for car structural components, in recent years, very interesting for the manufacturing of and also in academic research. automobile components. However, during roll-forming the microstructure of the sheet metal can undergo serious 2. Experimental Procedure damage. The outer surface remains stretched consequently The chemical composition of the commercial high the inner surface is compressed. Thus, these outer surfaces, strength dual phase DP1000 used is given in Table 1. highly deformed, are preferential nucleation sites, growth The 1.5mm thick DP1000 sheet was roll-forming in hat‑section and coalescence of voids and/or cracks13,14. geometry profile by Autotech Engineering (AIE, Gestamp Several studies4,7,15-19 have been conducted on the DP R&D). The roll‑forming direction was parallel to the steels behaviour under formability tests, as bending, direction of the sheet and performed at a feed rate *e-mail: [email protected] of 50 m/min. Hat‑section was formed by 6-stages, in which 844 Ruiz-Andres et al. Materials Research

the arrangements of the roll bend angles were varied over Finally, for Electron Back Scatter Diffraction (EBSD) 15º → 30º → 45º→ 60º → 75º → 90º. analysis, the specimens were mounted in epoxy resin, grounded Specimens were taken from a DP1000 sheet after being in SiC and polished with diamond paste of 3 and 1 µm processed revealing the LT-ST plane (Figure 1). as it was above described. After this stage the specimens The specimens analysed are presented in Figure 2. were automatically polished with colloidal silica for 30 s. One of specimens was taken from an area free from any Then, the specimens were rinsed in distilled water, soaked strain - blank specimen-. Three specimens were taken from in ethanol and dried in direct warm air. Immediately after, the bent area characterized by three distinctive regions: the the specimens were etched with 2% Nital solution for 15 s. outer edge, the middle zone and the inner edge. SEM images and EBSD scans were carried out in a field Metallographic specimens were prepared according to emission gun scanning electron microscopy (FEG‑SEM) standard procedures. They were mounted in an epoxy resin, J8M6500F JEOL equipped with energy-dispersive spectroscopy ground down through successive grades of SiC paper to (EDS) facilities. A statistically relevant area of 322 x 257 µm 2000 grade, degreased with alkaline cleaner and rinsed in was scanned using step size of 0.1 µm. The EBSD raw data were post-processed in detail by making use of the HKL tap water followed by deionized water and finally polished Analysis Software. Grain boundaries were characterized by with diamond paste of 3 and 1 µm. a misorientation larger than 5º between among neighbouring For scanning electron microscopy (SEM) inspections, measurements points. the specimens were etched with 2% Nital solution for 15 s, Several parameters on the basis of EBSD, such as the given that Nital preferentially etches ferrite and outlines their image quality (IQ), the inverse pole figure (IPF) and the kernel grain boundaries leaving martensite undissolved. average misorientation (KAM) maps are powerful tools in In order to calculate the volume fraction of ferrite and order to characterize the material. However, the similarity martensite phases, metallographic analysis were conducted between the crystalline structure of the microconstituents in by using an image software analysis. Using a commercial dual-phase steels -ferrite (bcc) and martensite (bct, with a imaging software (Analysisdocu®) the martensite and ferrite low tretagonality)- limits the software in order to distinguish phases were automatically distinguished by adjusting the between both phases, specifically when ferrite is highly contrast and colour, so martensite grains turn black, while strained20. In this work, it has been indexed ferrite as the ferrite grains turn white. main phase. Consequently, the Kikuchi diffraction patterns that differ from the ferrite crystalline structure (bcc) will be considered not indexed, given as a dark point and related to strained ferrite, martensite phase or grain boundaries. Hardness measurements were performed by a NanoTest 550 Micro Materials Ltd nanoindenter, using a Berkovich diamond tip. Tests were carried out at two indentation depths of 500 nm and 5000 nm. The local hardness and elastic modulus results were estimated from the loading and unloading curves using the standard Oliver-Pharr methodology21.

Figure 1. DP1000 specimen (a) Hat-section profile; (b) Scheme 3. Results main directions. 3.1. SEM characterization Figure 3 clearly shows that the microstructure of DP1000 steel is comprised by harden island-shaped martensite inclusions randomly distributed (bright contrast) and a soft ferrite matrix (dark contrast). Figure 3a reveals the microstructure corresponding to the blank specimen. This DP1000 specimen analysed is characterized by a 26.5 vol.% of martensite phase. Ferrite grains size range from 1.2 µm to 5 µm, approximately. The SEM images gathered in Figure 3b-d correspond to the microstructure in the bent area; outer edge, middle zone and inner edge, respectively. The microstructure corresponding to the outer edge in the bent area, Figure 3b, Figure 2. Details of DP1000 specimen (a) selected areas to analysis; shows elongated ferrite grains, with the long axis parallel to (b) representative studied regions in the bent area. the bending direction. The length of these ferrite grains vary

Table 1. Chemical composition of DP1000 dual phase steel (wt.%).

C Mn Si Mo Cr Nb Ni V Fe DP1000 0.067 2.29 0.38 0.25 0.020 0.037 0.025 <0.02 Balanced 2015; 18(4) Microstructural and Micromechanical Effects of Cold Roll-forming on High Strength Dual Phase Steels 845 from 3µm to 10 µm, approximately. Conversely, the martensitic 3.2. EBSD measurements islands appear not to be deformed, but it is noteworthy the presence of some voids (A). These voids appear preferentially EBSD enables an exhaustive study related to the localized around the martensite particles as result of the stress crystallographic orientations based on the Kikuchi diffraction concentration and deformation mismatch22,23. Other voids patterns from the surface of the specimens24-26. Besides initiation (B) appears in the ferrite/martensite grain boundary lattice rotation and lattice strain at the surface of very small and are considered a normal separation of both phases22. volumes of material can be measured11, 27. Moving throughout the bent area, towards the inner Figure 4 depicts representative EBSD image quality maps edge, the appearance of the ferrite grains is less elongated (IQ) corresponding to the studied areas, where ferrite has and the presence of voids disappears. The microstructure been indexed as the main phase. The IQ maps describe the corresponding to the middle region of the bent area (Figure 3c) grain structure of the different phases24. Due to overlapping is comprised by ferrite grains apparently not deformed, keeping similar microstructure as in the blank specimen diffraction signals, coming from two grains at a grain boundary, (Figure 3a). Finally, at the compressed inner edge of the the contrast in the Kikuchi pattern faint or is even inexistent. bent specimen, Figure 3d, the ferrite grains vary their size As a consequence, a low value in the IQ map leads to a dark and morphology into smaller and more equiaxed grains. grain boundary or as a noticeable dark grain, constituting In this region voids are not distinguished. the martensite phase or highly strained ferrite20.

Figure 3. SEM images of DP1000 specimens etched with 2% Nital at 3000x (a) blank area; (b) outer bent edge; (c) middle bent zone; (d) inner bent edge. 846 Ruiz-Andres et al. Materials Research

Figure 4. IQ maps of DP1000 specimens in (a) blank area; (b) outer bent edge; (c) middle bent zone; (d) inner bent edge.

These maps reveal the very low quality of the EBSD patterns as black regions. Figure 4b-d present a noticeable difference in dark regions regarding the IQ map in the blank specimen (Figure 4a). These dark areas suggest extremely strained ferrite phase as a result of a high level of residual stresses due to the bending process. Also, martensite islands could appear as black. Furthermore IQ maps have been successfully used to determine the volume fraction of the microstructural constituents of several alloys, such as AHSS steels and aluminium alloys24. In the present work, the volume fraction of the phases varies depending on the region analysed, that is, blank specimen or bent regions. Figure 5 show the volume fraction calculated as a function of these areas. The volume fraction for high IQ values, related to the ferrite phase, is higher than the volume fraction for the so-called low IQ values, related to grain boundaries, strained ferrite and martensite. It is noteworthy Figure 5. Volume fraction of phases measured as of IQ maps of that the average volume fraction of the low IQ values, related DP1000. to the non-indexed areas (grain boundaries, martensite and deformed ferrite) slightly increases regarding the blank lattice are perceived, in Figure 6f there is an abrupt increase specimens. This is consequence of the increase of the strained of the misorientation suggesting the proximity to low IQ ferrite at the extremely deformed regions (outer and inner values that in this case, blank specimen, should correspond edges). A volume fraction of the low IQ values about 25% to the proximity of a martensite phase. has been found in the outer and inner bent areas, while in the blank specimen the volume fraction is about 16%. In the The histograms corresponding to selected grains within middle area is observed a similar low IQ value to the blank the bent area are shown in Figure 6g and h. In particular specimen coincidently with the neutral line which separates these histograms are associated to specific grains in the outer the areas submitted to tensile and compressive strains in the edge (3) and inner edge (4), respectively. Such values could outer and inner areas respectively. be consequence either of the presence of martensite next to Misorientation profiles, associated to selected ferrite these grains or highly strained ferrite grains. grains in blank specimens (1, 2) and bent specimens (3, 4), Figure 7 depicts the representative EBSD KAM maps, are depicted in Figures 6e-h. These histograms reveal the corresponding to the areas above studied. The misorientation misorientation distribution within each grain. For instance, between a data point and its neighbours is analysed by the Figure 6e and f gather the analysis performed in the areas KAM maps. Furthermore, these maps indicate the dislocation labelled as 1 and 2 in the blank specimen. These histograms density28 and the strain distribution on individual measurement show significant changes of the misorientation distribution. points29. In this work KAM scans were measured calculating While in Figure 6e, no differences in the orientation of the each point by considering up to its 3rd neighbours and 2015; 18(4) Microstructural and Micromechanical Effects of Cold Roll-forming on High Strength Dual Phase Steels 847

Figure 6. IPF maps of DP1000 specimens in (a) blank area; (b) outer bent edge; (c) middle bent zone; (d) inner bent edge; (e)-(h) misorientation profiles of the selected grains taken from the same areas. 848 Ruiz-Andres et al. Materials Research

Figure 7. KAM maps of DP1000 specimens in (a) blank area; (b) outer bent edge; (c) middle bent zone; (d) inner bent edge.

without taking into account any misorientation higher than 5º, since this value was chosen to elucidate between grain boundaries and internal misorientation. The illustrated colour typed code determines the degree of local crystalline misorientation within each grain. Blue colour corresponds to the grains without any misorientation, while red colour would corresponds to the maximum. As it can be seen in Figure 7a, the blank specimen shows low misorientation. This specimen is characterized by low KAM values -green colour- visible at the ferrite-martensite interfaces. However, it is clearly observed that the KAM distribution shifts to higher values as the plastic deformation proceeds. Figure 7b-d reveal an evident increase of the local crystalline misorientation, higher areas coloured in green. In these figures, it is seen that the plastic deformation occurs in the outer and inner edges regarding the blank specimen, Figure 8. Light-optical microscope visualization of a representative varying substantially within the ferrite grain as result of Berkovich indentation in DP1000. the deformation. In particular, note that the ferrite with the highest level of misorientation -green to red colour- tends to concentrate at the ferrite/martensite grain boundaries. Figure 9c presents a typical load versus displacement 3.3. Ultramicrohardness curve at the indentation depths of 5000 nm, respectively. The curve exhibits a parabolic behaviour in the loading Figure 8 shows an optical micrograph depicting representative section and a power-law behavior in the unloading one21. ultramicrohardness indentations displayed in the DP1000 steel. Higher magnifications of several selected indentations, The elastic recovery exhibited during the unloading section 30-32 corresponding to two different indentation depths of 500 nm is small, as expected in metal alloys . and 5000 nm, are shown in Figure 9a and b, respectively. Figure 10 shows the hardness obtained for the DP1000 blank These SEM images show that the higher the indentation and bent specimens at the two analysed indentation depths of depth, the larger the Berkovich indentation. In addition, it 500 nm and 5000 nm. This depicts that the values obtained at is clearly observed that each indentation encompass both lower indentation depth are slightly higher. This response is ferrite and martensite phases. classically observed for the known indentation size effect33. 2015; 18(4) Microstructural and Micromechanical Effects of Cold Roll-forming on High Strength Dual Phase Steels 849

Figure 10. Variations in DP1000 specimens of hardness measured at indentation depths of 500 nm and 5000 nm.

field and /or the specimen size34. As shown in the literature34, nanohardness measurements in dual-phase steels (e.g. with 26% content of martensite) show different intrinsic hardness values, i.e. 2.96 ± 0.47 GPa for ferrite, and 6.99 ± 5.75 GPa for martensite. Therefore, current results obtained at an indentation depth of 500 nm show an average hardness value dominated by the martensite phase. This might be a direct consequence of the relationship between the indentation size and the martensite grain size. On the other hand, it is observed that the hardness values corresponding to the bent specimens are slightly higher than the blank specimen, regardless the indentation depth, suggesting a work-hardening after the roll-forming process. Furthermore, it can be noticed that the hardness measured in the bent area also depends on the specific region evaluated. The values corresponding to specimens with severe deformation -outer and inner edges- appear slightly higher than in the middle region.

4. Discussion The deformation behaviour of dual phase steels is considered to be quite complex. However, despite the lack of understanding of the interactions between the present constituents and their influence on mechanical properties some generalization has been made in the literature6. In general, localized plastic strain is resulting from the incompatible deformation between the soft ferritic matrix and the harder martensite phase8,35, which have dissimilar properties. This plastic deformation process is initially related to the dislocation density in the ferrite phase36. In the low strain range -during the first stages deformation- the work‑hardening rate is considerably high as a consequence of rapid dislocation Figure 9. SEM images of Berkovich indentations in DP1000 at multiplication and the back stresses resulting from the strain indentation depths (a) 500 nm; (b) 5000 nm. Representative P-h 37 curve at (c) 5000 nm. incompatibility . Then, when the ferrite phase reaches its maximum strained capacity (in the high strain range), ferrite matrix transfers Particularly, in dual-phase steels these size effects usually strain across the ferrite-martensite interface, leading to the result from the strain gradient induced by several sources onset of a plastic deformation in the martensite grains6,36. related to the microstructure -natural hardening of ferrite Subsequently, the work-hardening rate of the dual phase and reinforcement by martensite-, the applied deformation steel diminishes37. 850 Ruiz-Andres et al. Materials Research

Ashby38 stated that the compatible deformation of a soft Qualitatively, the previously results in IQ maps are in matrix which contains hard particles requires the generation good agreement with the results displayed in the KAM maps. of plasticity gradients, such as statistically stored (SSDs) The misorientation differs to a great extent as the plastic and geometrically necessary dislocations (GNDs), within deformation increases. In general, the blank specimens the more deformable phase. The SSDs are created by simple reveal a weak misorientation within the grains. Whilst in work hardening of ferrite39, while the GNDs emerge from the the bent area the plastic strain is mainly localized next to necessity to maintain both ferrite and martensite in contact ferrite/martensite boundaries. during plastic deformation40. This theory has been widely In this work, hardness measurements indicate that the used to explain dislocation movements and deformation of plastic deformation resulting from the roll-forming process dual-phase steels38,41,42. applied on the specimens have small effects in the surface Speich & Miller43 ascertained this statement in dual-phase hardness. That suggests dependence among the mechanical steels. Also, the original Bergström dislocation theory44, but parameters (depth, hardness) and the material properties, adjusted for dual-phase steels45, is employed in order to justify such as the work-hardening resulting from the plastic flow the plastic deformation process in these steels. However, deformation. according to this theory, the deformation is considered to be entirely supported by the ferrite phase, conversely to several 5. Conclusions 6,36 authors who consider plastic deformation in martensite. In the present work the microstructure and micromechanical In this work, SEM images (Figure 3) confirm that the analysis of ultrafine grained DP1000 steel have been performed ultrafine grained DP1000 steel deformation mainly takes for two different representative areas of a constant hat-section place in the ferrite phase. As a result, an elongated ferritic profile obtained by means of continuous roll-forming process microstructure is observed in the outer edges from the bent with a high feed rate with the aim of providing responses area and compressed ferrite grains in the inner edges. to the behaviour of this high strength low alloy steel for the Moreover the presence of several voids in the specimen automotive industry. after the roll-forming process is also worth to note, Meanwhile in not deformed specimens the microstructure specifically in the outer edges (Figure 3b). It is well known of the DP steel reveals weak misorientation within the ferrite that the initiation of voids occurs at ferrite/ferrite grain grains, the microstructure corresponding to the bent area boundaries, martensite/martensite grain boundaries and in exhibits variations depending on the particular analysed ferrite/martensite ones22. region. The specimens from the edges of the bent specimen In addition, it is known that the distribution of voids -outer and inner- exhibit a severe plastic deformation mainly nucleation sites is dependent on the microstructure but localized within the ferritic matrix and in the vicinity of their growth and coalescence is controlled by local stress the ferrite/martensite grain boundaries. As a result, high state13. In this sense, the localization of the voids formed in misorientation is noticed, suggesting a large density of the outer edge and their shape suggest that they initiate and dislocation in these deformed zones. Stretched ferrite grow parallel to the bending direction. grains are found in the outer edges whereas compressed The IQ maps report significant differences of the evolution ferrite grains are observed in the inner edge. The presence of the microstructure during the deformation process. of several voids in the outer edge also indicates evident They indicate the degree of distortion of the crystal lattices damage resulting from the plastic deformation undergone in the diffraction patterns46, revealing the location of the in the roll forming process. grain boundaries and the martensite phase. Therefore, the The hardness measurements appear to indicate that the plastic deformation possess certain effects on the DP steel low IQ values in the bent area -outer and inner edges- suggest surface hardness. a severe deformation in ferrite due to the plastic stresses appeared predominantly within the ferritic matrix and in the vicinity of the ferrite/martensite boundaries. Acknowledgements On the other hand, as it is shown in the misorientation The authors gratefully acknowledge the funding profiles Figure( 6e-h), high values of misorientation degree are by Ministerio de Economía y Competitividad under concentrated within the ferrite grains which are surrounded project Innpacto IPT-020000-2010-020 and project by martensite. In particular, the maximum degree of this CONSOLIDER‑INGENIO 2010 CSD 2008-0023 variation is found close to the ferrite/martensite boundaries. FUNCOAT.

References 3. Cui X, Zhang H, Wang S, Zhang L and Ko J. Design of lightweight multi-material automotive bodies using new material performance 1. Meng Q, Li J and Zheng H. High-efficiency fast-heating annealing indices of thin-walled beams for the material selection with of a cold-rolled dual-phase steel. Materials & Design. 2014; crashworthiness consideration. Materials & Design. 2011; 58:194-197. http://dx.doi.org/10.1016/j.matdes.2014.01.055. 32(2):815-821. http://dx.doi.org/10.1016/j.matdes.2010.07.018. 2. Han Q, Kang Y, Zhao X, Lü C and Gao L. Microstructure and 4. Wang W, Li M, Zhao Y and Wei X. Study on stretch bendability Properties of Mo Microalloyed Cold Rolled DP1000 Steels. and shear fracture of 800 MPa dual phase steel sheet. Materials Journal of Iron and Steel Research International. 2011; 18(5):52- & Design. 2014; 56:907-913. http://dx.doi.org/10.1016/j. 58. http://dx.doi.org/10.1016/S1006-706X(11)60065-4. matdes.2013.12.004. 2015; 18(4) Microstructural and Micromechanical Effects of Cold Roll-forming on High Strength Dual Phase Steels 851

5. Futarama Y, Miura M and Tsunezawa M. Characteristics of 19. Hassannejadasl A, Green DE, Golovashchenko SF, Samei J and Highly Formable 590-980MPa Grade Hot-dip Galvannealed Maris C. Numerical modelling of electrohydraulic free-forming Steel Sheets for Automobiles. Kobelco Technology Review. and -forming of DP590 steel. Journal of Manufacturing 2011; 30:80-84. Processes. 2014; 16(3):391-404. http://dx.doi.org/10.1016/j. 6. Rashid MS. Dual Phase Steels. Annual Review of Materials jmapro.2014.04.004. Science. 1981; 11(1):245-266. http://dx.doi.org/10.1146/annurev. 20. Dillien S, Seefeldt M, Allain S, Bouaziz O and Van Houtte P. EBSD ms.11.080181.001333. study of the substructure development with cold deformation of dual phase steel. Materials Science and Engineering A. 2010; 7. Hayat F and Uzun H. Effect of Heat Treatment on Microstructure, 527(4-5):947-953. http://dx.doi.org/10.1016/j.msea.2009.09.009. Mechanical Properties and Fracture Behaviour of Ship and Dual Phase Steels. Journal of Iron and Steel Research 21. Oliver WC and Pharr GM. Measurements of hardness and International. 2011; 18(8):65-72. http://dx.doi.org/10.1016/ elastic modulus by instrumented indentation: Advances in S1006-706X(11)60106-4. understanding and refinements to methodology. Journal of Materials Research. 2004; 19(1):3-20. http://dx.doi.org/10.1557/ 8. Wu-Rong W, Chang-Wei H, Zhong-Hua Z and Xi-Cheng W. jmr.2004.19.1.3. The limit drawing ratio and formability prediction of advanced high strength dual-phase steels. Materials & Design. 2011; 22. Kadkhodapour J, Butz A, Ziaei-Rad S and Schmauder S. A 32(6):3320-3327. http://dx.doi.org/10.1016/j.matdes.2011.02.021. micro mechanical study on failure initiation of dual phase steels under tension using single crystal plasticity model. 9. Ozturk F, Toros S and Kilic S. Tensile and Spring-Back Behavior International Journal of Plasticity. 2011; 27(7):1103-1125. of DP600 Advanced High Strength Steel at Warm Temperatures. http://dx.doi.org/10.1016/j.ijplas.2010.12.001. Journal of Iron and Steel Research International. 2009; 16(6):41- 46. http://dx.doi.org/10.1016/S1006-706X(10)60025-8. 23. Kadkhodapour J, Butz A and Ziaei-Rad S. Mechanisms of void formation during tensile testing in a commercial, dual-phase 10. Gillard AJ, Golovashchenko SF and Mamutov AV. Effect steel. Acta Materialia. 2011; 59(7):2575-2588. http://dx.doi. of quasi-static prestrain on the formability of dual phase org/10.1016/j.actamat.2010.12.039. steels in electrohydraulic forming. Journal of Manufacturing 24. Randle V. Electron backscatter diffraction: strategies for reliable Processes. 2013; 15(2):201-218. http://dx.doi.org/10.1016/j. data acquisition and processing. Materials Characterization. 2009; jmapro.2012.12.005. 60(9):913-922. http://dx.doi.org/10.1016/j.matchar.2009.05.011. 11. Ramazani A, Mukherjee K, Scwedt A, Goravanchi P, Prahl U 25. Randle V and Engler O. Introduction to texture analysis: and Bleck W. Quantification of the effect of transformation- macrotexture, microtexture and orientation mapping. 2nd ed. induced geometrically necessary dislocations on the flow- Boca Raton: CRC Press; 2010. 488 p. curve modelling of dual-phase steels. International Journal of Plasticity. 2013; 43:128-152. http://dx.doi.org/10.1016/j. 26. Britton TB, Maurice C, Fortunier R, Driver JH, Day AP, Meaden ijplas.2012.11.003. G, et al. Factors affecting the accuracy of high resolution electron backscatter diffraction when using simulated patterns. 12. Larrañaga J, Galdos L, Uncilla L and Etxaleku A. Development Ultramicroscopy. 2010; 110(12):1443-1453. http://dx.doi. and validation of a numerical model for sheet metal forming. org/10.1016/j.ultramic.2010.08.001. PMid:20888125. International Journal of Material Forming. 2010; 3(1):151-154. 27. Ohashi T, Barabash RI, Pang JWL, Ice GE and Barabash OM. http://dx.doi.org/10.1007/s12289-010-0729-9. X-ray micro-diffraction and strain gradient crystal plasticity 13. Erdogan M and Tekeli S. The effect of martensite particle studies of geometrically necessary dislocations near a Ni bicrystal size on tensile fracture of surface-carburised AISI 8620 grain boundary. International Journal of Plasticity. 2009; steel with dual phase core microstructure. Materials & 25(5):920-941. http://dx.doi.org/10.1016/j.ijplas.2008.04.009. Design. 2002; 23(7):597-604. ­http://dx.doi.org/10.1016/ 28. Li H, Hsu E, Szpunar J, Utsunomiya H and Sakai T. Deformation S0261-3069(02)00065-1. mechanism and texture and microstructure evolution during 14. Abouei V, Saghafian H, Kheirandish S and Ranjbar K. An high-speed rolling of AZ31B Mg sheets. Journal of Materials investigation of the wear behaviour of 0.2% C dual phase steels. Science. 2008; 43(22):7148-7156. http://dx.doi.org/10.1007/ Journal of Materials Processing Technology. 2008; 203(1- s10853-008-3021-3. 3):107-112. http://dx.doi.org/10.1016/j.jmatprotec.2007.09.044. 29. Wright SI, Nowell MM and Field DP. A review of strain 15. Luo M and Wierzbicki T. Numerical failure analysis of a stretch- analysis using electron backscatter diffraction. Microscopy and bending test on dual-phase steel sheets using a phenomenological Microanalysis. 2011; 17(3):316-329. http://dx.doi.org/10.1017/ fracture model. International Journal of Solids and Structures. S1431927611000055. PMid:21418731. 2010; 47(22-23):3084-3102. http://dx.doi.org/10.1016/j. 30. Khan MK, Hainsworth SV, Fitzpatrick ME and Edwards L. A ijsolstr.2010.07.010. combined experimental and finite element approach for determining 16. Wang W and Wei X. The effect of martensite volume and mechanical properties of aluminium alloys by nanoindentation. distribution on shear fracture propagation of 600-1000MPa dual Computational Materials Science. 2010; 49(4):751-760. http:// phase sheet steels in the process of deep drawing. International dx.doi.org/10.1016/j.commatsci.2010.06.018. Journal of Mechanical Sciences. 2013; 67(0):100-107. http:// 31. Liu Z, Sun J and Shen W. Study of plowing and friction at the dx.doi.org/10.1016/j.ijmecsci.2012.12.011. surfaces of plastic deformed metals. Tribology International. 17. Mishra A and Thuillier S. Investigation of the rupture in tension 2002; 35(8):511-522. http://dx.doi.org/10.1016/S0301- and bending of DP980 steel sheet. International Journal of 679X(02)00046-4. Mechanical Sciences. 2014; 84(0):171-181. http://dx.doi. 32. Chang SH, Chen JZ, Hsiao SH and Lin GW. Nanohardness, org/10.1016/j.ijmecsci.2014.04.023. corrosion and protein adsorption properties of CuAlO2 films 18. Wang Z, Zhao A, Zhao Z, Ye J, Chen J and He J. Precipitation deposited on 316L for biomedical applications. Behavior and textural evolution of cold-rolled high strength deep Applied Surface Science. 2014; 289(0):455-461. http://dx.doi. drawing dual-phase steels. Journal of Iron and Steel Research org/10.1016/j.apsusc.2013.11.004. International. 2013; 20(12):61-68. http://dx.doi.org/10.1016/ 33. Nix WD and Gao H. Indentation size effects in crystalline S1006-706X(13)60217-4. materials: A law for strain gradient plasticity. Journal of the 852 Ruiz-Andres et al. Materials Research

Mechanics and Physics of Solids. 1998; 46(3):411-425. http:// 40. Calcagnotto M, Ponge D, Demir E and Raabe D. Orientation dx.doi.org/10.1016/S0022-5096(97)00086-0. gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD. Materials 34. Delincé M, Jacques PJ and Pardoen T. Separation of size-dependent Science and Engineering A. 2010; 527(10-11):2738-2746. http:// strengthening contributions in fine-grained Dual Phase steels dx.doi.org/10.1016/j.msea.2010.01.004. by nanoindentation. Acta Materialia. 2006; 54(12):3395-3404. http://dx.doi.org/10.1016/j.actamat.2006.03.031. 41. Mughrabi H. On the current understanding of strain gradient plasticity. Materials Science and Engineering: A. 2004; 387- 35. Imandoust A, Zarei-Hanzaki A, Heshmati-Manesh S, Moemeni 389:209-213. http://dx.doi.org/10.1016/j.msea.2004.01.086. S and Changizian P. Effects of ferrite volume fraction on the 42. Jiang Z, Guan Z and Lian J. Effects of microstructural variables tensile deformation characteristics of dual phase twinning on the deformation behaviour of dual-phase steels. Materials induced plasticity steel. Materials & Design. 2014; 53:99-105. Science and Engineering. 1995; 190(1-2):55-64. http://dx.doi. http://dx.doi.org/10.1016/j.matdes.2013.06.033. org/10.1016/0921-5093(94)09594-M. 36. Sun S and Pugh M. Properties of thermomechanically processed 43. Speich GR and Miller RL. Mechanical properties of ferrite- dual-phase steels containing fibrous martensite. Materials martensite steels. In: Morris JW and Kot PA, editors. Structure Science and Engineering A. 2002; 335(1-2):298-308. http:// and properties of dual-phase steels. New Orleans: Tms; 1979. dx.doi.org/10.1016/S0921-5093(01)01942-6. p. 145-182. 37. Calcagnotto M, Adachi Y, Ponge D and Raabe D. Deformation 44. Bergström Y. The plastic deformation of metals: a dislocation and fracture mechanisms in fine- and ultrafine-grained ferrite/ model and its applicability. Reviews on Powder martensite dual-phase steels and the effect of aging. Acta and Physical Ceramics. 1983; 2(2-3):79-265. Materialia. 2011; 59(2):658-670. http://dx.doi.org/10.1016/j. 45. Bergström Y, Granbom Y and Sterkenburg D. A dislocation actamat.2010.10.002. based theory for the deformation hardening behavior of DP steels-Impact of martensite content and ferrite grain 38. Ashby MF. The deformation of plastically non-homogeneous size. Journal of Metallurgy. 2010; 2010:1-16. http://dx.doi. materials. Philosophical Magazine. 1970; 21(170):399-424. org/10.1155/2010/647198. http://dx.doi.org/10.1080/14786437008238426. 46. Chen Y, Hjelen J and Roven HJ. Application of EBSD technique 39. Demir E, Raabe D, Zaafarani N and Zaefferer S. Investigation to ultrafine grained and nanostructured materials processed by of the indentation size effect through the measurement of the severe plastic deformation: sample preparation, parameters geometrically necessary dislocations beneath small indents of optimization and analysis. Transactions of Nonferrous Metals different depths using EBSD tomography. Acta Materialia. 2009; Society of China. 2012; 22(8):1801-1809. http://dx.doi. 57(2):559-569. http://dx.doi.org/10.1016/j.actamat.2008.09.039. org/10.1016/S1003-6326(11)61390-3.