Page 161 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications

+ Low- High-strength Ultra-high-strength
strength steels steels (UTS>7OO MPa)
V steels
UTS<27O MPa)
L
Interstitial free
Interstital free/
high strength

Dual Phase/
Complex Phase
` Bake
Hardenable

0
0 500 1000 1500 2000
Tensile strength (MPa)

FIGURE 5.5 Comparison of advanced high-strength steels.

steels are typically produced in sheet form by microalloying followed by controlled
hot rolling (Chapter 13). Plates, bars, and structural shapes are made from these
steels. HSLA steels have a low carbon content (usually less than O.30%) and are
characterized by a microstructure consisting of fine-grain ferrite as one phase and a
hard second phase of martensite and austenite. These steels have high strength and
energy-absorption capabilities as compared to conventional steels. The ductility,
formability, and weldability of HSLA steels, however, generally are inferior to those
of conventional low-alloy steels (see Fig. 5.5). To improve these properties, several
ultra-high-strength steels have been developed, as described in Section 5.5.7.
Sheet products of HSLA steels typically are used for parts of automobile bod-
ies and other transportation equipment (in order to reduce weight and, hence, fuel
consumption) and in mining, agricultural, and various other industrial applications.
HSLA plates are used in ships, bridges, building construction, and for shapes such as
I-beams, channels, and angles used in buildings and in various structures.
Mechanical properties for selected HSLA steels are given in Table 5 .3.
Designations. Three categories compose the system of AISI designations for high-
strength sheet steel (Table 5.4). Structural quality (S) includes the elements C, Mn,
P, and N. Low alloys (X) contain Nb, Cr, Cu, Mo, Ni, Si, Ti, V, and Zr, either singly
or in combination. Weathering steels (W) have environmental-corrosion resistance
that is approximately four times greater than that of conventional low-carbon
steels and contain Si, P, Cu, Ni, and Cr in various combinations. In addition, the
formability of these sheet steels is graded by the letters F (excellent), K (good), and
O (fair).
Another designation scheme in wide use is that defined by the Ultralight Steel
Auto Body Consortium (ULSAB). The ULSAB practice is to define both the type of
a steel and its yield and tensile strengths in a compact designation, in the form XX
aaa/bbb, where XX is the type of steel, aaa is the yield strength in MPa, and bbb is

)
tion (%
Total
elonga