Steels_ Metallurgy and Applications, Third Edition

284 Steels: Metallurgy and Applications

traditional martensitic grades due to easier crack propagation in the predominantly
pearlitic structure.
In addition to compositional effects, the mechanical properties of these steels
are controlled by the soaking temperature, hot-working schedule and cooling
rate to ambient temperature. The cooling rate from the finishing temperature is
important since this controls the transformation temperature and the precipitation
of V(CN). In conventional forging steels, the components may be placed in a bin
after the forging operation and the slow cooling rates encountered in this situation
would cause the precipitates to overage, resulting in a substantial loss in strength.
Simple conveyer systems have therefore been introduced which enable the micro-
alloy steel forgings to cool freely in air or else the cooling rate is enhanced with
fan cooling.
The automotive components that are being produced in medium-carbon, micro-
alloy steels include crankshafts, connecting rods, steering knuckles, axle beams
and tension rods. Various manufacturers, including the Rover Group, have
claimed that very substantial cost savings have been achieved by the adoption of
these grades due to:

1. The lower cost of micro-alloy steels compared with the alloy grades that they
   replace.


2. The elimination of heat treatment costs.


3. The improved machining characteristics compared with traditional grades.

However, car makers in the UK have been reluctant to use these steels for
safety-critical components due to their low impact properties and the rate of
acceptance has been particularly slow in North America where manufacturers face
greater threats of litigation on improper application/product liability. Therefore
the further exploitation of these steels is very dependent on the development of
improved toughness via controlled processing or grain-refinement techniques.

Controlled processed bars

There is a limited demand for normalized or quenched and tempered bar prod-
ucts that can be machined directly to the finished component form. Given that
these heat treatments are expensive, there is obviously an incentive to develop
the required mechanical properties in the as-roUed condition, i.e. the incentive
is similar to that described in the previous section for the elimination of heat
treatment in automotive forgings.

Normalized steels

Normalizing is applied to bar products in order to refine the grain size and facil-

itate subsequent processing operations such as machining or cold forging. The

steel is reheated to just above the reaustenization temperature (Ac3) to achieve a

fine austenite grain size and the material is then allowed to cool freely in air. In

order to simulate such a structure in the 'as-rolled' condition, three options can