Page 163 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

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

steels have been produced, with a typical microalloyed steel containing 0.5% C, 0.8%
Mn, and 0.1% V When subjected to carefully controlled cooling (usually in air), these
materials develop improved and uniform strength. Compared to medium-carbon
steels, microalloyed steels also can provide cost savings of as much as 10%, since the
manufacturing steps of quenching, tempering, and stress relieving are not required.

Nanoalloyed Steels. Now under development, these steels have extremely small
grain sizes (10-100 nm) and are produced using metallic glasses (Section 6.14) as a
precursor. The metallic glass is subjected to a carefully controlled vitrification (crys-
tallization) process at a high nucleation rate, resulting in fine nanoscale phases. (See
also Section 8.8.)

5.5.7 Ultra-high-strength Steels
Ultra-high-strength steels are defined by AISI as those with an ultimate tensile
strength higher than 700 MPa. There are five important types of ultra-high-strength
steel: dual-phase, TRIP, TWIP, complex phase, and martensitic. The main applica-
tion of these steels is for crashworthy design of automobiles. The use of stronger
steels allows for smaller cross sections in structural components, thus resulting in
weight savings and fuel economy increases without compromising safety. The signif-
icant drawbacks of all these steels are higher cost, tool and die wear, forming loads,
and springback.
Dual-phase steels are processed specially to have a mixed ferrite and marten-
site structure. Developed in the late 1960s, these steels have a high work-hardening
exponent [n in Eq. (2.8)], which improves their ductility and formability.
TRIP steels consist of a ferrite-bainite matrix and 5-20% retained austenite.
During forming, the austenite progressively transforms into martensite. Thus, TRIP
steels have both excellent ductility because of the austenite and high strength after
forming. As a result, these steels can be used to produce more complicated parts
than other high-strength steels.
TWIP steels (from TV(/inning-Induced Plasticity) are austenitic and have high
manganese content (17-20%). These steels derive their properties from the genera-
tion of twins during deformation (see Section 1.4) without a phase change, resulting
in very high strain hardening and avoiding necking during processing. As can be
seen in Fig. 5.5, TWIP steels combine high strength with high formability.
Complex-phase grades (CP grades) are very fine grained microstructures of
ferrite and a high volume fraction of hard phases (martensite and bainite). These
steels can provide ultimate tensile strengths as high as 800 MPa and are therefore of
interest for automotive crash applications such as bumpers and roof supports.
Martensitic grades are also available, consisting of high fractions of martensite to at-
tain tensile strengths as high as 1500 MPa.

EXAMPLE 5.1 Advanced High-strength Steels in Automobiles

Increasing fuel economy in automobiles has received structural elements of automobiles. For example, the
considerable attention in recent years for both envi- application of steel in the Ford 500 automobile is
ronmental and economic reasons. Regulatory require- shown in Fig. 5.6. Note that although 60% of the
ments call for automobile manufacturers to achieve steel in this automobile is mild steel and is associated
corporate average fuel economy (CAFE) standards. with body panels and transmission and engine com-
To achieve higher fuel economy without compromis- ponents, structural components are exploiting the
ing performance or safety, manufacturers have in- higher strength-to-weight ratios of advanced high-
creasingly applied advanced high-strength steels in strength steels.