Engineering steels 281Lagnenborg 47 has shown that the tensile strength of these steels can be expressed
as a function of (V + 5 x N)%. Nitrogen levels of up to 0.02% are therefore
incorporated in the steels in order to intensify the strengthening effect.
One of the disadvantages of these micro-alloy steels is that they display
significantly lower levels of toughness than the traditional quenched and tempered
martensitic grades and this has inhibited their large-scale commercial exploitation.
The low impact strength is related to the coarse pearlitic structure but this effect
is exacerbated by precipitation strengthening. Whereas this problem has been
overcome in structural steel plates with the use of low-temperature finishing
(controlled rolling), there is little scope for the adoption of this practice in drop-
forging operations due to the metal flow/die filling problems that occur at low
forging temperatures.
The impact strength of these grades can be improved by lowering the carbon
content and compensating for the loss in strength by increasing the manganese,
vanadium and nitrogen contents. Experience in Sweden and Germany has shown
that an improvement in toughness can also be obtained by increasing the silicon
content of the steels. However, attention has also been given to the potential of
grain-refining additions of titanium in pursuit of higher impact strength.
As indicated earlier, titanium has a low solubility in medium-carbon steels
but TiN is even less soluble than TiC. Particles of TiN are therefore present at
the normal soaking temperature for forging, namely 1150~ and will refine the
austenite grains provided the particles are present as a fine dispersion. According
to Gladman, 46 this is achieved by restricting the titanium content to below the
stoichiometric level required for reaction with nitrogen in TiN and the growth of
particles is also minimized by rapid solidification from the liquid state. In practice,
the titanium additions are restricted to levels of about 0.01% and the need for
rapid solidification is generally satisfied by continuous casting as opposed to
ingot casting.
Japanese steelmakers have expressed concern that the formation of TiN for
grain refinement can reduce the level of soluble nitrogen that is available for
precipitation strengthening by V(CN). However, this problem can be overcome
by adjusting the nitrogen content such that the free nitrogen (total nitrogen minus
nitrogen as TiN) exceeds 0.006%.
More recently, it has been reported that improved levels of toughness can
be obtained in medium-carbon, micro-alloy steels by the generation of bainitic
structures. However, according to Naylor 4s the benefits of this development are
not yet clear-cut and the need for alloy additions to achieve a bainitic structure
may detract from the viability of the approach.
Commercial exploitation
Korchynsky and Paules 49 have reviewed the various grades of micro-alloy forging
steels that are produced in Europe and Japan and their lists of compositions and
associated tensile properties are shown in Tables 3.26 and 3.27. Heading the list
is the German grade 49MnVS3, the first medium-carbon, micro-alloy steel to be
used commercially for air-cooled automotive forgings. Like the Swedish Volvo