Steels_ Metallurgy and Applications, Third Edition

82 Steels: Metallurgy and Applications

35

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Bainite transformation time (s)

Figure 1.93 Relationship between bainite transformation time and tensile properties

(After Itami et al. TM)

Ultra-high-strength steels

These are steels with a tensile strength above about 1000 N/mm 2, though steels
with a tensile strength above 800 or 900 N/mm 2 may sometimes be regarded
as ultra-high-strength steels. They are of two basic types, those in which the
microstructure contains a high proportion of martensite and those containing a
high proportion of bainite. Each may, however, contain smaller proportions of the
other phase, together with significant proportions of ferrite or retained austenite.
One approach to produce a steel with a tensile strength above 980 N/mm 2 was
to produce a dual-phase, ferrite-martensite structure, comprising a high hardness
martensite in a ductile ferrite containing a minimum of dissolved carbon. 142
The method involved intercritical, continuous annealing at the lowest possible
temperature subject to dissolving the carbides, cooling and then quenching from
just above the bainite start temperature to give a transformation product that is
enriched as far as possible with carbon. The highest elongation was obtained with
the highest silicon content used (1.9%) which led to a homogeneous distribution
of fine martensite islands. Quenching from just above the bainite start temperature
also led to the highest proportion of retained austenite. 143
A substantially complete martensite structure may be obtained by quenching
from the single-phase austenite region using, for example, steels containing
niobium and a high-carbon equivalent expressed as Ceq = C -I- Si/24 -I- Mn/6.144
It was found that the steels in the as-quenched condition softened during a
bake-hardening treatment at 170~ It was necessary, therefore, that the steel
was tempered to precipitate most of the carbon before use. The work showed,
as expected, that the tempering treatment reduced the strength, particularly for
tempering temperatures above about 300~ but also showed that there was a