Crystallographic Textures of DP steels (dual phase steels)

The crystallographic texture of a sample can be determined by various types of diffraction methods that typically exploit the constructive interferrence of electromagnetic or electronic waves at groups of lattice planes. Some methods allow a quantitative analysis of the texture, while others are rather qualitative. Among the quantitative techniques, the most widely used is X-ray diffraction using texture goniometers, followed by the EBSD method (electron backscatter diffraction) in Scanning Electron Microscopes. Qualitative analysis can be done by Laue photography, simple X-ray diffraction or with a polarized microscope. Neutron and synchrotron high-energy X-ray diffraction are suitable for determining textures of bulk materials and in situ analysis, whereas laboratory x-ray diffraction instruments are more appropriate for analyzing textures of thin films.

Many material properties are anisotropic (direction dependent) already at the lattice structure / single crystal scale. Hence, if preferred orientation alignment prevails in a sample, so are the net / intergal properties of polycrystals.

Particularly material properties such as elastic anisotropy, yield and tensile strength, chemical reactivity, corrosion, sheet forming behaviour  and magnetic susceptibility can be highly dependent on the material’s crystal texture.

In many materials, properties are texture-specific, and development of unfavorable textures when the material is fabricated or in use can create weaknesses that can initiate or exacerbate failures.

Consequently, consideration of textures that are present in and that could form in engineered components while in use can be a critical when making decisions about the selection of some materials and methods employed to manufacture parts with those materials.

In manufacturing and product design textures matter particularly for the anisotropy of the mechanical formability of many metallic materials. An often cited example of this is the earing formation (in-plane sheet shape anisotropy) evolving during deep drawing and stretching operations of sheet metal, as well as the anisotropy of the magnetic properties of practically all soft magnetic (e.g. grain-oriented dynamo and transformer sheet, such as used in the engines and charging infrastructures of / for electric vehicles) and hard magnetic materials (e.g. anisotropic alnico magnets, samarium-cobalt alloys, anisotropic neodymium-iron-boron alloys, hard magnetic ferrites).

The notion DP steel (dual-phase steel) refers to all steels whose microstructure consists of a ferritic (soft) matrix in which a predominantly martensitic (strength-increasing) second phase is embedded in an island shape.

Depoendiung on the volume fraction of the harder phase (scaling the strength) percolation may occur. The proportion of martensite is between 10 and 40 %. With higher martensite content, a higher tensile strength can be achieved, but the elongation to fracture drops.

Materials with such microstructures have relatively low yield strength, which is favourable for the forming process, and high tensile strength, elongation and n-value. These properties are advantageous for complex deep-drawn parts. In addition, DP steel has a pronounced bake-hardening effect. Typical applications for these steels are flat stretch-formed profiles in car doors, strength-relevant carrier structures and energy-absorbing components as well as steel rims.

In DP steels it is an important to measure and to design properties through textures and microstructure features during processing.

In order to produce DP steel, it must first be heated or cooled in the two-phase area. Then one has to wait until the desired proportions of austenite (and the desired partitioning of carbon and manganese) and ferrite are obtained. Then cooling is carried out quickly, which transforms the austenite portion into hard martensite (depedning on the carbon content that is quenchedi in). The process can be carried out from the rolling heat by cooling from the γ (austenite phase) area to the two-phase area or by heating from room temperature to the α+γ area.

In a set of projects we conducted systematic through-process analysis studies of the crystallographic texture evolution in DP steels.

By using SEM/EBSD the substructure and texture evolution in dual phase steels in the first steps of the process chain, i.e. hot rolling, cold rolling, and following annealing were characterized.
The starting material was hot rolled steel with a chemical composition consisting of 0.147 wt. % C, 1.9 wt. % Mn, 0.4 wt. % Si.
In order to obtain dual phase steels with high ductility and high tensile strength an industrial process was reproduced by cold rolling of industrially hot rolled steel sheets of a thickness of 3.75 mm with ferrite and pearlite morphology down to a thickness of 1.75 mm and finally annealing at different temperatures. Such technique allows a compilation of ferrite and martensite morphology typical for dual phase steels. Due to the competition between recovery, recrystallization and phase transformation during annealing a variety of ferrite martensite morphologies was produced by promoting one of the mechanisms through the variation of technological parameters such as heating rate, intercritical annealing temperature, annealing time, cooling rate and the final annealing temperature. Annealing induced changes of the mechanical properties were determined by hardness measurements and are discussed on the basis of the results of the substructure investigations.
We found that in hot rolled sheets, ferrite and pearlite showed a band structure in the center and a heterogeneous distribution at the surface of the sheet. Texture analysis yielded a through-thickness texture inhomogeneity and a maximum plane-strain texture in the center and a maximum shear texture close to the surface of the sheet. Due to cold rolling the in-grain orientation gradients also increased, which is related to an increase of dislocation density and thereby an increase of the driving force for recrystallization. The through-thickness texture inhomogeneity was reduced by cold rolling.
Recovery and recrystallization were found to be dominant for annealing at ferritic temperatures and at low intercritical temperatures up to 695 °C. There is an overlap of recrystallization and phase transformation only at intercritical annealing temperatures exceeding 740 °C. A correlation between
the volume fraction of martensite and hardness was observed at an intercritical annealing temperature of 740 °C. Increasing the annealing time leads to an increasing martensite fractions and therefore to an increasing hardness.

By using EBSD the substructure and texture evolution in dual phase steels in the first steps along the process chain, i.e. hot rolling, cold rolling, and following annealing can be characterized. 
In order to obtain dual phase steels with high ductility and high tensile strength an industrial process can be reproduced at laboratory (or real) scale by cold rolling of industrially hot rolled steel sheets of a thickness of 3.75 mm with 
ferrite and pearlite morphology down to a thickness of 1.75 mm and finally annealing at different temperatures. Such technique allows a compilation of ferrite and martensite morphology typical for dual phase steels.

Due to the competition between recovery, recrystallization and phase transformation during annealing a variety of ferrite martensite morphologies was produced by promoting one of the mechanisms through the variation of technological parameters such as heating rate, intercritical annealing temperature, annealing time, cooling rate and the final annealing temperature.  
Annealing induced changes of the mechanical properties were determined by hardness measurements and are discussed on the basis of the results of the substructure investigations.