56 Steels: Metallurgy and Applicationswith 238 N/mm 2 for conventional rolling. 93 The difference was due to a grain
size effect. Under production conditions, ferritically hot-rolled steel developed
similar properties and was considered to be suitable for direct application or for
cold rolling and annealing to give an Fe P01-type product. It was also found,
however, that a finishing temperature in the range 700-750"C, combined with
a low coiling temperature at least below 600"C, led to a hot band structure
that retained a deformed structure that could be directly annealed without an
intermediate cold reduction. For this process, a yield stress in the final product of
either 250 or 130 N/mm 2 was obtained depending on whether the steel was an
extra-low-carbon steel or an IF steel. The rm value of the IF steel was, however,
low at 1.1. It was considered that this low rm value was due to the formation of
a deleterious shear texture close to the surface of the strip, but this shear texture
could be minimized by the use of lubrication on the hot mill. (93)
High-strength steels
The low-strength steels considered in the previous section have been in use for
many years, but changes mainly in the automobile market have prompted the
development of higher-strength cold-reduced steels which have penetrated the
market for the more traditional steels. The first higher-strength cold-rolled steels
considered were the previous mild steels but with a higher degree of temper
rolling to increase the yield strength. 94 The main problem, however, was the low
formability which restricted use. Nevertheless, strengthening by cold reduction
is an effective way of strengthening when a very low degree of formabilitY is
acceptable. Cold-reduced steels continue to be used, therefore, for many strapping
applications and in the zinc coated condition for corrugated roofing panels.
Other early cold-rolled high-strength steels were obtained by cold rolling
and annealing the hot rolled type of micro-alloyed steels already available but
with some modification to chemistry to provide specific strength requirements
after annealing. These steels, with yield stresses up to about 400 N/mm 2, were
strengthened mainly by grain refinement and were more formable than the steel
with a high degree of temper rolling, but they had low r values.
The main emphasis subsequently was the development of steels for which the
loss in formability with increasing strength was minimized. Substitutional solid
solution-strengthened steels were developed and the main reason, as illustrated
in Figure 1.60, was that the loss in elongation per unit strength increase is less
for a solid solution-strengthened steel than for a micro-alloyed steel. These rela-
tionships are similar to those for n value given in Figure 1.25. The equivalent
relationships between r value and strength are given in Figure 1.61.
The strength increase that could be obtained with a rephosphorized addition
was, however, limited by the detrimental effect of phosphorus on welding. These
steels were restricted, therefore, to relatively modest strength increases with
minimum yield stresses up to but usually well below 300 N/mm 2. The steels
were clearly, therefore, highly formable.
Dual-phase steels, based on a ferrite matrix but containing up to about 20%
of dispersed martensite islands, have been developed which generate tensile