Steels_ Metallurgy and Applications, Third Edition

70 Steals: Metallurgy and Applications

on cooling. Detailed studies x2~ showed that a minimum cooling rate from 850~

greater than 30~ is necessary to obtain sufficient bake hardening for a niobium-

bearing steel with an Nb/C atomic ratio of unity, as illustrated in Figure 1.75.

Figure 1.76 illustrates the effect of annealing temperature on bake hardening

and r value for several steels containing different amounts of either titanium or

niobium. Figure 1.76(a) shows that the steel with an Nb/C atomic ratio of 0.3, a

high degree of bake hardening is obtained for all annealing temperatures, whereas

50 850~ x 40s ......

h5oc voc/s

~. 4op -i L_4oooc K\~

~ y-2oc/s~30 ~ / ~~_~

Ill / Nb/c= 1.o .... -Y. I

.'i

...= 2ol- /i I

~ I /i!

" 101- /! i

ca O" ~ ~r- , 1 ,, i '

5 10 30 50 100

Cooling rate V (~

Figure 1.75 Effect of cooling rate on the bake hardenability of Nb-added extra-low-C
steel. The Specimens were annealed at 850~ for 40 s (After Sakata et al. x~)

2.5

=~ 2.0

~. 1.5

1.0

70

o. 5O

-r. en 30

10

9 Nb/C = 0.3

0 Nb/C= 0.7

a,

..........

Iml

I

dk Ti*/C = 0.4

Z~ Ti*/C = 1.1

A Ti*/C = 4.6

#.

,, s ~ b

:/

I I I I I I I I I I

800 900 1000 800 900 1000

Soaking temperature (~

Figure 1.76 Effect of soaking temperature on bake hardenability (BH) and average r
value in Nb- and Ti-added extra-low-C steels (After Sakata et al. 1~)