EFFECT OF TEMPER ROLLING ON THE MECHANICAL BEHAVIOUR OF THIN STEEL SHEETS UNDER MONOTONOUS AND REVERSE SIMPLE SHEAR TESTS C.Luis1, 2, M. Gaspérini1, S. Bouvier1, J.J. Li2 1
2
LPMTM-CNRS, Université Paris 13, 99 Avenue JB Clément, 93430 Villetaneuse, France ArcelorMittal-Research and Development, Voie Romaine, BP30320, 57283 Maizières les Metz, France
ABSTRACT: Temper rolling is performed on ferritic steel thin sheets after cold rolling and annealing, in order to increase the material strength and to obtain the correct roughness and a good flatness. However, depending on the thickness reduction and on the material composition, this operation may induce specific strain-path changes effects during subsequent forming processes. This paper focuses on the analysis of the hardening behaviour during simple shear tests. The results are discussed in relation with the material compositions, the initial anisotropy and the macroscopic strain heterogeneities.
KEYWORDS: Temper rolling, Strain-path change, Hardening behaviour, Simple shear, Steel thin sheets
1 INTRODUCTION During sheet metal forming of low carbon steels, the material undergoes complex strain-paths up to large accumulated plastic strains. Therefore good ductility and high strength are required, which can be partly controlled by solid solution strengthening. However, non uniform plastic deformation patterns due to strain ageing, like Lüders bands or “stretcher-strains marks” on the surfaces of sheets during stamping operations, have to be avoided [1]. This is usually done in industry by temper rolling, which produces a few percent thickness reduction under tension at the final stage of a sheet rolling process. In the case of very thin sheets, temper rolling is also used to improve the material strength through higher reductions. Mechanical loading after temper rolling induces a strain-path change, which may promote specific effects on the material hardening. In low carbon steels, such effects have already been observed in tension after cold rolling [2], and during strain-path changes combining tensile and shear testing [3, 4, 5]. Then in order to obtain reliable numerical simulations of forming processes the effects of the strain-path changes have to be taken into account in the modelling of the material behaviour [6], which has not yet be done in case of temper rolling. It is worth noting that the commonly used uniaxial tensile test is not sufficient due to the early failure
specifically in case of very thing sheets, as already observed on aluminium alloys [7]. The simple shear test has proved to be an efficient technique to investigate and evaluate the mechanical properties of flat sample [8, 9]. Simple shear tests allow to characterize the mechanical behaviour up to large amount of plastic shear strains (larger than 80%), and to easily investigate the role of testing direction and of reverse loading. The present study is devoted to the analysis of the hardening behaviour during monotonic and reverse simple shear tests after different thickness reductions by temper-rolling of some representative ferritic steels. The aim is to contribute in a better understanding of the respective influence of the material anisotropy, the thickness reduction and the material composition on the macroscopic hardening stages during strain-path changes.
2 EXPERIMENTAL 2.1 Materials Steel sheets for packaging applications have been selected because of their thin thickness. One steel sheet without aging effect (IF) and another aged steel sheet were studied. a) A low carbon Interstitial Free (IF) steel. From the asreceived sheets (BR) with initial thickness t0=0,188 mm,
____________________ * LUIS Caroline: LPMTM-CNRS, Université Paris 13, 99 Avenue JB Clément, 93430 Villetaneuse, France, Phone : +33 (0)1.49.40.34.62, Fax : +33 (0)1.49.40.39.38, Email :
[email protected]
3 sets of samples were temper rolled with different thickness reduction r, hereafter called: - IF-r2 (t=0,185 mm), - IF-r8 (t=0,172mm), - IF-r17 (t=0,161 mm). b) 3 steels used after aging treatment at 200°C during 20mn, with typical thickness t=0,200-0,220 mm: - N-r8 (mainly N as interstitial), - NCa-r9 (C and N as interstitial), - NCb-r17 (C and N as interstitial). Typical microstructures obtained by EBSD in the rolling plane are shown in fig. 1. From EBSD measurements, mean Taylor factors were determined from OIM software, in order to qualitatively analyse the effect of the initial crystallographic texture. (a)
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Figure 1: Image quality maps from EBSD at midthickness in the rolling plane of (a) IF-r8 and (b) N-r8
2.2 Uniaxial tensile tests For these temper rolled steel sheets the commonly used uniaxial tensile test is not sufficient to characterize the mechanical behaviour due to the low ductility especially for the most temper rolled IF sheets and aged steels (figure 2). Uniaxial tensile test-0° 700 N-r8
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2.4 Strain-path changes To characterize a strain-path change, Schmitt et al. [10] have proposed the θ parameter defined as the cross product of the normalized strain rate tensors for the prestrain and for the subsequent deformation respectively. The monotonic, Bauschinger and orthogonal strain-path changes are defined by the values θ = 1, −1 and 0, respectively. Considering here temper-rolling and shear in α direction as first and second paths, θ = ½ sin 2 α. So, θ ranges from -0.5 to 0.5 with α, with zero value for α = 0° and 90°, and maximum 0.5 value for α = 45°.
(b)
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2*30*t0 mm3 sheared zone. The shear direction is indicated with the angle α from the rolling direction: α =0°, 45°, 90° were considered here. For all the materials, monotonic and reverse simple shear tests were performed up to γ=80% amount of accumulated shear. Reverse shear tests allow the investigation of the Bauschinger effect, and consists in a two-stages non proportional loading after the temper rolling. Macroscopic inhomogeneities in the center of the sheared zone can be detected during the tests from the anomalous curvature of the straight line used to monitor the simple shear test by videoextensometry technique.
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Figure 2: Engineering stress-strain curves for tensile test in the rolling direction of IF-r8 and N-r8
2.3 Simple shear tests Details on the simple shear tests can be found elsewhere [9]; the sample geometry was 30*18*t0 mm3, with
3 MECHANICAL BEHAVIOUR DURING SHEAR TESTS For both IF steel and aged materials, the final macroscopic deformation after monotonic and reverse shear tests is clearly homogeneous. A slight buckling tendency appears for γ > 30% in some cases. 3.1 Monotonous simple shear after temper rolling 3.1.1 Interstitial Free steel The shear stress-shear strain curves are given in figure 3 with a γs shift value on the x axis that corresponds to the temper rolling reduction. The amount of the shift is estimated using the von Mises equivalent strain (γs =2r). The curves clearly show the effect of temper rolling on the hardening behaviour of the IF steel during the monotonous shear tests: - an increase of the yield shear stress with r, - occurence of work softening, leading to a subsequent abrupt yield drop and a “plateau” for the higher temper rolled material (IF-r17) - a gradual resumption of the work-hardening after the transitory regime. These results are similar to those observed for an “orthogonal” strain-path change [3]. However, the material exhibits a poor plastic anisotropy and similar results are obtained for α = 45°, where the strain path change parameter θ = 0.5.
significantly lower than that for the monotonic curve. After the transitory regime, resumption of strain hardening occurs and a permanent softening remains even after large amount of reverse shear strain.
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Figure 3: Shear stress-shear strain curves of IF steel for α = 0° after different temper rolling reductions (IF -BR, IFr2, IF-r8 and IF-r17).
3.1.2 Aged materials For aged materials, figure 4 clearly shows that the mechanical behaviour of these industrial steels during the strain-path change temper rolling/simple shear greatly differs from IF steel: transient regime occurs at the beginning of loading, followed by a progressive increase of the yield stress towards a saturation value. Aged steels
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Figure 5: Shear stress vs accumulated shear strain for simple and reverse shear test at 0° of IF-r8.
3.2.2 Aged materials For aged materials, the Bauschinger effect is rather small: limited work-hardening stagnation is observed on figure 6 and stresses during the reversed deformation can reach higher values than for the monotonic curve.
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Figure 4: Shear stress-shear strain curves of industrial aged temper rolled steels for α = 0°.
Figure 6: Shear stress vs accumulated shear strain for simple and reverse shear test at 0° of N-r8
The effect of testing direction is more pronounced than for IF materials. Indeed, α = 45° shear test presents higher yield stress than α = 0° and 90° for all these materials which can not be explained by the θ parameter only, suggesting the effect of the differences in crystallographic textures.
For both IF steels and aged materials, there are no macroscopic inhomogeneities during the second path of the reverse shear test.
3.2 Reverse shear tests after temper rolling 3.2.1 Interstitial Free steel The Bauschinger effect and persistent softening are clearly seen on figure 5 during the reversed deformation. For all the IF steels and all amounts of forward shear strain (10, 20 and 30%), a rapid strain hardening is followed by another transient period (presence of a plateau), where the strain hardening becomes
4 DISCUSSION The analysis of the hardening behaviour during monotonic and reverse simple shear tests after different thickness reductions by temper-rolling of some representative ferritic steels shows that apparent transient periods occur during the hardening behaviour, which involve different phenomena: 1) Strain-path change effects: - Yield stress increase followed by softening then resumption of hardening has been already observed for “orthogonal” strain-path changes
[2, 3, 5]. Here this behaviour is evidenced on figure 3 for IF steels. However, even when θ=0.5 (α=45°), the behaviour is similar, suggesting that the observed transitory regime is the macroscopic manifestation of combined effects. Moreover, after high temper rolling reduction, the yield drop seems to lead to inhomogeneous deformation which has to be confirmed by further investigation. - Hardening stagnation after strain reversal, already observed in IF steels, is shown to be less pronounced in aged materials. In all cases, macroscopic deformation remains almost homogeneous in the sheared zone of the samples. 2) Strain aging phenomena, evidenced by Lüders bands, present in tensile tests of aged materials (figure 2), and suspected at the beginning of shear test (figure 4). Aged steels present higher yield stresses than IF steel, which can be linked to combine effects of solid solution strengthening and grain size effect: the average grain size is about 12µm in IF steel and 5µm in aged steels. Concerning the effect of the initial crystallographic texture, the figure 7 shows the evolution of the Taylor’s factor for the different materials, calculated from EBSD, for temper rolling (TR) loading and for simple shear (SS) at α = 0° and α = 45° . For IF steel, shifting from temper rolling to simple shear corresponds to a “textural” softening whereas it is not obvious for aged materials. In addition, IF steels present a poor plastic anisotropy during simple shear while this anisotropy is more pronounced for aged materials, which is consistent with the relative position of the shear stress-strain curves for α=0° and α=45°, especially for NCa and NCb materials, suggesting the need for a detailed analysis of the crystallographic texture of the different materials. Taylor's factor 3,6
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Figure 7: Taylor’s factor for temper rolling (TR) and simple shear (SS) at α = 0° and α = 45° for IF steel and aged materials.
5 CONCLUSION The present works show that complex hardening behaviour is exhibited in thin ferritic steels sheets submitted to shear tests up to large plastic deformation after temper rolling, combining transient strain-path effects, texture softening and strain ageing phenomena. These results suggest the need for modelling the observed behaviour, which is currently in progress.
ACKNOWLEDGEMENT The authors are grateful to the Arcelormittal Research and Development for supporting this work and for authorizing the publication of the results.
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