Steels_ Metallurgy and Applications, Third Edition

12 Steels: Metallurgy and Applications

unit cell to the a-axis depends on the carbon content. Martensite is the hardest

structure that can be produced in steel, the strength increasing with the carbon

content. The temperature at which martensite can start to form is called the

martensite start temperature (Ms) and depends on the carbon content and other

alloy additions made to the steel. The following equation gives an estimate of

the Ms temperature for certain steels where the symbols in brackets again refer

to weight percentages. 19

Ms(*C) = 539- 423[C]- 30.3[Mn] - 12.1[Cr] - 17.7[Ni] - 7.5[Mo]

In steels, the martensite finish temperature M f is usually about 200~ below the
Ms temperature.
Further details on the effects of alloying elements and cooling rate on the
formation of martensite are given in the chapter on Engineering steels.
Whereas each of the strengthening reactions described above is employed
singly or together to produce high-strength grades, they will also decrease the
ductility. However, a very important consideration in the production of cold-
rolled and annealed steels is the development of the preferred crystallographic
orientation texture. If grains of every possible orientation are present in a steel
with equal volume fraction, the steel is said to possess a random orientation
texture. Such a texture is difficult to achieve completely and it is usual to find that
certain orientations are present to a greater extent than others. The steel is then
said to possess a preferred orientation texture. The strength of any component in
a texture is often given in 'times random' units which is the ratio of the strength
of the component in the texture divided by the strength that would exist in a
similar steel with a random texture.
Orientation textures may be studied using X-ray diffraction and may be
expressed in several ways. The first, called a pole figure, gives a plot, on a
stereographic projection, of the strength of a particular crystallographic plane in
each direction. Pole figures may be plotted for each relevant plane including, for
example, (100), (110) or (11 l) planes and an example is illustrated in Figure 1.7,
which also gives the positions of planes for several ideal orientations. Pole figures
have the limitation that they do not give precise information concerning the
strength of any component nor the way in which directions within each plane are
oriented.
The second method of expressing texture is called an inverse pole figure.
This gives the intensity of planes parallel with the strip surface, again in times
random units. This has the limitation that it gives no information concerning
the orientation of directions within the rolling plane. Nevertheless, inverse pole
figures are useful since they give the precise total intensity of planes that are
parallel to the surface. These intensities largely determine the mean r value (see
next section).
A complete characterization of any texture may be given by means of an
orientation distribution function which is calculated from several pole figures.
An orientation distribution function gives a plot of intensity of each orientation
in Euler space using angles that relate each orientation to the orientation of the
strip. The method was originally described independently by Roe 21 and Bunge 22
using different but related angles. In the Bunge notation, the Euler angles ~bl,