Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 57

40

.o c:^30

C 0

2O

400 600 800 1000

Tensile strength (MPa)

Figure 1.60 Variation of tensile elongation with tensile strength for various steel types
(After Hayami and Furukawa 95)

2.5

2.0

i~ 1.5

1.0

I

0 10

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20 30 40 50 60 70 80 90 100

Tensile strength (kgf/mm 2)

Figure 1.61 Relationship between r value and tensile strength (After Takech~ ~)

strengths up to and above 600 N/ram 2. These steels have relatively high n values
for their strength but the r values are low.
Bake-hardening steels, usually with strengthening by phosphorus, develop
a substantial increase in yield stress during the paint stoving treatment after
painting. These steels, with yield stresses up to about 300 N/mm 2 in the as-
supplied condition, have the formability, therefore, of a relatively low-strength,
solid solution-strengthened steel but the performance in use of a much higher-
strength steel.
The development of highly formable low-strength IF steels subsequently led to
the generation of solid solution-strengthened versions of these steels with values
of tensile strength up to about 440 N/mm 2. It was found, however, that some
of these steels could be potentially affected by a phenomenon called secondary
cold work embrittlement when subjected to fairly severe drawing strains. This
tendency has been reduced by the addition of boron and is considered further in
a section below.
Steels with very high strength up to above 800 N/ram 2 have been developed
using the 'so-called' TRIP mechanism. This refers to trahsformation-induced