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For some
steelmakers, adding nitrogen would seem to be the fastest way
to send a heat to the scrap yard. So-called "free" nitrogen
causes strain aging in carbon steels, increasing yield strength
and brittleness after cold working. Strain aging is particularly
detrimental in sheet products where it reduces formability.
However,
when a nitride former such as vanadium is present, nitrogen becomes
an extremely useful element. In high-strength, low-alloy steels,
nitrogen combines with vanadium to become a very cost-effective
strengthener.
Of the three
nitride-forming elements - vanadium, aluminum, and titanium -
vanadium is the only element that effectively strengthens steel
by combining with nitrogen.
COMMERCIAL
VERIFICATION
A technical
paper presented at ISSTech 2003 in Indianapolis in April 2003
showed that vanadium sharply reduced strain aging in commercial
HSLA steels. As shown in the graph below, plain-carbon steels
containing as little as 0.006% nitrogen showed significant strain
aging after simulated coil cooling. On the other hand, strain
aging was virtually eliminated in vanadium-strengthened HSLA steels
containing as much as 0.020% nitrogen.
This
is good news for steelmakers using electric-arc furnaces since
this process can yield over twice as much nitrogen as the basic-oxygen
furnace. With an ever-increasing percentage of steel being made
in the electric-arc furnace, the average nitrogen level of steel
products is constantly rising. Using vanadium to strengthen high-strength,
low-alloy steels converts nitrogen from an undesirable impurity
into an effective alloy.
Steelmakers
melting in basic-oxygen furnaces can also safely obtain additional
strengthening from their vanadium additions by raising the nitrogen
level of steel. In the presence of vanadium, this higher nitrogen
does not create a strain-aging problem.
Optimum
Strengthening
Slow
cooling after austenite is transformed into ferrite will optimize
strengthening from vanadium and nitrogen. This practice maximizes
the precipitation of the vanadium nitrides that provide strengthening
while eliminating strain aging. For strip steels, coiling temperatures
of 600 to 630 deg. C (1,100 to 1,150 deg. F.) followed by slow
cooling in the coil optimizes vanadium-nitride precipitation.
Data Sheet on Strain Aging (.PDF)
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