Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 55

Figure 1.58, which shows how the carbon content, as reflected in the ageing
index, varies with time for different overageing temperatures. The reprecipitation
proceeds quickly to a final high level at temperatures close to 450~ due to a high
diffusion coefficient but proceeds very slowly to a lower level at temperatures
close to 250~ due to a low diffusion coefficient. Certain relatively short over-
ageing processes, therefore, involve cooling to below 300~ to take advantage of
the nucleation of closely spaced carbides and then include reheating to a higher
temperature such as 350~ to take advantage of a higher diffusion rate. 92
The early continuous annealing lines for strip gauge steels used gas cooling,
gas jet cooling or cold water quenching to cool the strip down to the overageing
temperature. These cooling methods did not, however, provide the optimum
cooling rate. The methods mentioned previously of hot-water quenching, roll
cooling, gas-roll cooling, water-mist cooling and high-hydrogen gas-jet cooling
were, therefore, developed to give cooling rates closer to the optimum.

Ferritic rolling

It is clear from the above sections that the most formable properties from
both interstitial-free and aluminium-killed steels are usually obtained when
the finishing temperature is in the single-phase austenite region. For many
applications, however, the best properties are not required and this gives scope
for the use of cheaper processes. One such opportunity currently being developed
is ferritic rolling, whereby the roughing sequence is carried out at a lower
temperature than previously, but still usually in the single-phase austenite region,
and the finish rolling is carried out in the single-phase ferrite region. This process
takes advantage of the fact that the resistance to hot deformation is similar in the
ferrite region below 800~ to the resistance in the austenite region above 900~
as illustrated in Figure 1.59.
When extra-low-carbon steel was ferritically rolled in the laboratory, using
a finishing temperature of 757~ and a high coiling temperature of 650~ a
yield stress of 166 N/mm 2 was obtained for the as-hot rolled condition compared

~= Boo .~1 , iLABORA t o TOm" t RE~,UL'TS ' t' l l--~-, I

~, -- i I I I I I t t I t I I I I

r,~ I ~ ' " I I

9 .~ 500".~":I'~" ,-..p-.-..IELC III I-,,---- .-P-

~LC=-TI!_ :,j[ I

d~ ~ll. I i i I i I I

400 ""---'---'---'----' =.11 tl I I I I .... F-" I

9 ~-- v ~4 ~"11 i! 9 I I I I I I! I I I

=m~~l!

o 300 .... , C , ~" ," .... -t---.1., , .... {-, .... ,1""

o ~ I It i I I I

200,-'F- I "",'~--'N,'--! -', .... i T .... l l 1----r--!

Q-j I I I i

t~., ~1~ ,,.__I I! I I I I I ~'1~=f ~- I! ~ I I I

l UUl --T'. .... --"T-. ---~.---"~. ~---- .~r --

G) / / I I I I t t I I I I I I I

/ t I I I t!!

0 500 600 700 800 9()0 Y N0 i iO0

Deformation temperature (~

Figure 1.59 Mean hot deformation resistance of ELC and ULC-H steels (After Herman
et aL 93 )