P. Romano Triguero1, S. Melzer1, J. Moerman2 Homogeneity in width of CR annealed steel sheet Texture and microstructure analysis of packaging steel Tata Steel RD & T (The Netherlands) 1 Ceramics Research Centre 2 Steel Metallurgy Department

Introduction Texture is a link between formability and processing for product development Tata Steel produces packaging steel mainly to be deep drawn in the form of food and beverage cans 1. Texture analysis is used by Tata for product and process development of packaging steel, battery steel and automotive steel sheet, especially for deep drawing applications. 2. Texture tools are used to characterise the formability of Corus' end products in relation to chemical composition and process data In order to ensure the same properties along the width of the steel sheet we need to trim the edges

Why do we need to trim the edges of the sheets? Tensile test (‘step size’ 15 mm) Edge region to 50 mm properties compared to the mean sheet width value: r-value 0.05-0.15 lower Rp 7-12 MPa higher Rm 2-4 MPa higher Ag & A50 1-3% lower

Why do we need to use EBSD? Light Microscopy after the tensile test (‘step size’ 15 mm) 0 15 30 45 60 mm Standard LOM: all samples ferrite GS ~ 7µm Not enough detail to see differences in grains

What can we do to save money? To reduce the amount of edge trimming Sample: cold rolled and annealed sheet with a full width of 1224 mm not previously edge trimmed. Standard trimming: 17 to 20 mm (Before cold rolling the coil was trimmed at the pickling line) Question: at which width do the properties become homogeneous? Standard LOM: ferrite grain structure ASTM 11.0 (= 7 mm) Experiment: investigate texture gradient (and possibly ferritic hot rolling)

Set up of experiments Large Electron Backscatter Diffraction (EBSD) scans are carried out from the edge towards the centre up to 60 mm into the sheet EBSD on normal plane at mid-thickness Scans of 430 mm (height) x 700 mm (length) with a step size of 1.5 mm are measured In total of 87 scans were measured and grouped in triplets to give more statistically reliable data: 18.000 grains per triplet TD RD Cross-section view 0 60 mm Normal section view

Data processing Merging of 3 maps Calculation of grain size distribution Mean grain size Fraction small and large grains Calculation of ODF Split ODF small and large grains Calculation of r-value profile r0, r45, r90 r_bar or normal anisotropy (r0 + 2*r45 + r90)/4 and Dr or planar anisotropy (r0 + 2*r45 + r90)/2

Inverse pole figure (IPF) map RD inverse pole figure of the first triplet starting from the edge and 2.1 mm into the sheet IPF legend

grain distribution size grain size distribution of the first triplet starting from the edge and 2.1 mm into the sheet RD each colour represents a fraction of grains with a particular size

orientation distribution map RD map showing grains with some of the well-known orientations orientation distribution map of the first triplet starting from the edge and 2.1 mm into the sheet E F J H G I Rotated cube ODF-45 Goss

Grain size distribution (I) Area (in mm): 0 – 2.1 4.2 – 6.3 10.5 – 12.6 18.9 – 21.0 46.2 – 48.3

Grain size distribution (II) Four different regions: 0 – 2 mm (edge) large grains 2 – 11 mm large gradient from small grain size to big grain size 11 – 30 mm slight increase of grain size >30 mm constant

ODF: orientation distribution function Undesirable grain orientation for deep drawing {001}uvw “cubic fibre” Desirable grain orientation for deep drawing {111}uvw “g-fibre” {hkl}110 “a-fibre”

Texture components g fibre- desirable cubic fibre- undesirable From the edge towards the centre the F/E ratio decreases. A value of around 1.5 is best for low earring. cubic fibre- undesirable

r-value The r-value or normal anisotropy deep drawing processing represents the resistance of plate material to thinning. The r-value is generally not equal in every direction. It means that the strain ratio in thickness and width differs. This causes a phenomenon called earing. The r-value is determined by uniaxial tensile tests in one or three (0º, 45º and 90º) directions to the rolling direction. The ratio of intensities of the g and the cubic fibre corresponds with the r-value.

r-value Four diff regions 0 – 2 mm 2 – 11 mm 11 – 30 mm >30 mm Taylor full constrain plasticity model using the van Houtte MTM-FHM software

Split ODFs at 11.5 mm Grains < 7 μm All grains Grains > 11 μm Small grains have unfavourable texture

Some background information… The deviating grain size and texture in the near edge region is believed to be related to the cooling rate of the coil in the hot strip mill The relatively lower coiling temperature in the edge region results in a smaller cementite particle size The morphology of cementite influences the texture formation during recrystallisation The smaller grain size near the edge region is a result of either more active nucleation sites in this area, or a lower amount of {111} grains Selective growth of {111} grains results in a shift of the grain size distribution [Ray, Jonas & Hook, 1994]

Properties start to change at 11.5 mm so in principle: Edge trimming Properties start to change at 11.5 mm so in principle: c edge trimming can be reduced, but: - only one measurement at one edge in one coil - measured at mid-thickness; V-shape possible Cross-section view Nominal edge trimming 17 – 20 mm

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