Steels_ Metallurgy and Applications, Third Edition

142 Steels: Metallurgy and Applications

+300

n +225

~+150

"o

._ +75

.~_

e,-^0

r r

-75

C and N

1 /P

Silicon

Copper

'Manganese

9 Molybdenum

C_ hromlum Nickel an-d aluminlum

0.5 1.0 1.5 2.0 2.5

Alloy weight (%)

Figure 2.2 Solid solution strengthening effects in ferrite-pearlite high-strength low-alloy

steels (After Pickering 3)

Table 2.1

Element N/mm 2 per 1 Wt %

C and N 5544

P 678

Si 83

Cu 39

Mn 32

Mo 11

Ni ~0

Cr -31

like carbon and nitrogen, phosphorus has a detrimental effect on toughness and

therefore it is not used as a strengthening agent per se in structural steels. On

the other hand, phosphorus is added to the so-called weathering grades because

of its beneficial effect on atmospheric corrosion resistance. These steels will

be discussed later in this chapter. Of the remaining elements, only silicon and

manganese are cost-effective as solid solution strengtheners but silicon is added

to steels primarily as a deoxidizing agent.

Precipi~tion strengthening

Precipitation strengthening can be induced by a variety of elements but, in the

context of ferrite-pearlite steels, the systems of commercial significance are

those involving niobium, vanadium and titanium. These elements have a strong

affinity for carbon and nitrogen and, consequently, they have only a limited solid

solubility in steel. Therefore they are added to steels in small amounts, e.g. up

to about 0.06% Nb or 0.15% V, and as stated earlier are often referred to as

micro-alloying elements.

As illustrated in Figure 2.3(a), a substantial amount of niobium will be taken

into solution at a slab or bloom reheating temperature of 1250~ On cooling,