Microalloyed Steels Flashcards
What are micro-alloyed steels, and what are the improvement factors?
Low carbon (~ 0.05 – 0.10 wt% C) plain carbon steels with a very small amount (< 1%) of strong carbide and nitride forming alloying additions, eg. Nb (Niobium), Ti and V.
These additions in conjunction with carefully controlled rolling practices, can produce great improvements in mechanical properties at low materials cost.
Main improvement factors:
1. Refinement of the ferrite grain size, by formation of a fine sub
grain structure.
2. Strain induced precipitation of carbides and nitrides. (example of
dispersion strengthening)
What is the process of precipitation strengthening of the ferrite?
➢ Before hot rolling the ingots are “soaked” at > 1230˚C. (Most of
the carbides and nitrides then dissolve, Austenitizing).
➢ Coarse austenite grains (from soaking) recrystallise during
deformation (via hot working).
➢ As the temperature reduces during rolling, the carbides and
nitrides become insoluble and precipitate out in the austenite
during hot rolling.
➢The precipitated carbides and nitrides “pin” the boundaries of the
“new” recrystallised austenite grains, thus limiting their size.
Describe work hardening for micro alloyed steels.
➢ In rolling the austenite grains are deformed and flattened. A
“pancake” grain structure is formed with a high, grain boundary
area.
➢ As the temperature falls further, ferrite nucleates, mainly on the
austenite grain boundaries. (Dispersion strengthening)
➢ Since there is a high proportion of austenite grain boundary there
are many nucleation sites and thus the resulting ferrite grain size
is small.
➢ Any microalloying elements left unprecipitated will now
precipitate as the ferrite cools, further adding to the strength.
In summary, what are micro alloyed steels strengthened as a combination of?
- Grain refinement
- Sub grain formation (means grains are very small & orientation
between them is very small 0.5-1˚). - Precipitation hardening.
