The effect of crystallographic textures on the formability of high-strength steel sheets

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Abstract

The effect of crystallographic textures on the formability of high-strength steel sheets was investigated by using the elastic/crystalline viscoplastic finite element (FE) analysis. First, the preferred orientations, which correlate with the plastic anisotropy, were revealed by orientation fibers, skeleton lines and selected orientations in Euler angle space. Secondly, the mechanical properties such as R-values, yield loci and stress–strain relationships, and drawability were evaluated by crystallographic numerical analyses and experiments. It was confirmed that one of the reasons why high-strength steel sheets show low formability was due to lack of γ texture components. Then, a textural design method was proposed for improving the formability of high-strength steel sheets. An artificial γ fiber texture was specified in terms of a rotationally symmetric Gaussian distribution of misorientations with scatter widths along the given skeleton line. Traces of the artificial γ texture orientations in terms of volume fractions were randomly selected and assigned to the original high-strength steel model by using the orientation probability assignment method. Finally, from the standard limit dome height (LDH) test, it was found that there was distinct improvement on formability for the new high-strength steel sheets even with a little proportion of γ texture components.

Introduction

High-strength steel sheets, being largely driven by the needs of the automotive and steel industries for weight reduction of automotive body, possess strong anisotropy and lower formability. This plastic anisotropy is the product of the interplay between macroscale boundary conditions and microscale activity. So there is a great demand to introduce the crystalline plasticity theory and numerical simulation into the fields of sheet metal fabrication with refined microstructural detail, in order to evaluate and improve the formability of high-strength steel sheets.

The crystalline plasticity theory, based on the hardening and softening evolution equations defined on the slip systems of a crystal lattice cell, has been employed to describe the anisotropy and heterogeneity of meso-scale characterization of plasticity of sheet metal forming [1], [2], [3]. Our newly developed crystalline plasticity finite element (FE) code has enabled to predict the macroscopic anisotropic stress–strain response, shape changes and the evolution of crystallographic texture at meso-scale [4]. To bridge material processing and sheet metal fabrication, the numerical analysis is expected to be a powerful tool not only for the formability evaluation, but also for the textural design of sheet metal [5].

The purpose of this paper is as follows.

  • •

    It reveals the crystal morphological and the macroscopic mechanical properties of high-strength steel sheet (CP800) by experiment and orientation distribution function (ODF) analysis.

  • •

    It investigates the texture effect on formability, especially the influence of γ texture on strain localization in deep drawing operation.

  • •

    It proposes the texture design method for improving formability of high-strength steel sheets, in which the procedure of preparing a set of orientations from the artificial γ texture and the orientation assignment to FE integration point are presented.

  • •

    It predicts the formability of the designed material by using elastic/crystalline viscoplastic FE analyses of the standard limit dome height (LDH) tests.

Section snippets

ODF analyses

The crystal morphological properties of the steel sheets were investigated by using Bunge’s ODF analysis [6]. The ODFs of high-strength steel sheet (CP800) and mild steel sheet (DDQ) are shown in Fig. 1(a) and (b). The reduced Euler angle space with typical fibers and selected orientations is shown in Fig. 2, which is used as an orientation distribution map of BCC steel sheets.

Taking a comprehensive view of Fig. 1, Fig. 2, the orientation distinctions of CP800 steel and DDQ steel can be

Analytic representation of the artificial γ fiber texture

To model an artificial texture for the purpose of texture design, the orientation distributions can be considered as many orientation spreads with Gaussian distribution about a given skeleton line. So the orientations of the artificial γ fiber texture can be assumed to be the set of misorientations about the straight line from (1 1 1)[11̄0] to (1 1 1)[1̄1̄2] as shown in Fig. 2. Although not entirely realistic, physically the scatter is generally assumed to have rotational symmetry about the

Cylindrical cup deep drawing

The cylindrical cup deep drawing test with a hemispherical punch was adopted to investigate the texture effect on the strain localization and verify the availability of the crystalline plasticity FE modeling. Only a quarter of the sheet was analyzed. The material properties, tool dimensions and deep drawing process parameters are listed in Table 1, where τ0, H0 and τs are the initial reference shear stress, the initial hardening coefficient and the saturation shear stress, which were identified

Conclusions

The R-value, yield locus and drawability of high-strength steel sheet (CP800) were investigated by using crystallographic approach and FE analysis of deep drawing processes. Comparing with mild steel sheet (DDQ), it shows low formability, earlier localization and extreme anisotropy due to lack of γ texture components.

For the purpose of formability improvement, a textural design method was proposed. The artificial (candidate) γ fiber textures were setup, in which the misorientation angles about

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