Materials for engineering, 3rd Edition - (Malestrom)

104
Materials for engineering
1600
δ
(ferrite)
Liquid
1400
Temperature (°C)
1200
1000
800
G
γ
(austenite)
γ + α
S
E
γ + liquid
γ + Fe 3 C
(austenite + cementite)
Liquid
+ Fe 3 C
600
α
(ferrite)
400
α + Fe 3 C
(α-ferrite + cementite)
200
0 1 2 3 4 5
Wt% C
3.21 The Fe–C phase diagram.
to a poor surface finish due to the presence of Lüders lines. Additions of
niobium or titanium can be made to ‘getter’ the C and N solute atoms to form
carbides or nitrides, thus improving the cold-forming response, the steels
being known as interstitial-free (IF).
With mild steels of higher C content, small Nb, V or Ti additions lead to
sufficient precipitation of finely divided precipitates during cooling from the
hot-rolling temperature to give rise to significant precipitation strengthening.
These materials are known as high-strength low-alloy (HSLA) steels.
Other approaches to achieving high strength in low-carbon strip steels
include solution strengthening, notably by the addition of up to 0.1%
phosphorus for use in automobile body pressings, and dual-phase steels
which are heat-treated to form a mixed microstructure of ferrite and martensite
(see later in this section). The latter have a low yield strength but a high
work-hardening rate, leading to excellent formability, but the alloying elements
needed to promote this microstructure make them relatively expensive.