8/13/2019 Autobody Materials - Lecture 4

1/6

Automotive Materials

Autobody Materials

IF : Interst it ial Free steel

Interstitial-free (IF) steels, also referred to as ultra-low carbon (ULC) or extra-low carbon (ELC)

very low carbon and nitrogencontents

The processing and chemistry of IF steels produce very high ductility and formability, albeit at low

strength

8/13/2019 Autobody Materials - Lecture 4

2/6

Automotive Materials

Autobody Materials

IF : Interst it ial Free steel

Despite such low levels of interstitials, further alloying, by additions of titanium and/or niobium, are

made to remove carbon and nitrogen from solid solution by the precipitation of carbides, nitrides,

and other compounds

IF steels border on being almost pure iron ( almost ZERO PERCENTAGE of Carbon)

8/13/2019 Autobody Materials - Lecture 4

3/6

Automotive Materials

Autobody Materials

IF : Interst it ial Free steel - Produc t

Niobiumis considered only to combine with carbonNb + CNbC

Cold-rolledand annealedsheet steels annealed at 650 C

Two strengthening approaches are used:

solid-solution strengthening and bake hardening

Bake hardening refers to the increment of strength that develops in cold formed sheet steel during

baking of automotive panels.

Bake-hardening at 175 C for 20 minutes

Steels with very low interstitial contents exhibit excellent formability with low yield strength, high

elongation, and good deep drawability

Yield strengths typically range between 140 and 180 MPa (20 and 26 ksi) and

Tensile strengths range between 290 and 340 MPa (42 and 49 ksi)

8/13/2019 Autobody Materials - Lecture 4

4/6

Automotive Materials

Autobody Materials

TRIP : Transfo rmat ion Ind uced Plast ic i ty Steels Basic Concept

Similar to dual-phase steels, trip steels have emerged as an energy-absorbing high-strength

steel for the automobile.

The term tripis derived from the mechanism of t ransformation induced plasticity.

These steels contain a high percentage of retained austenite (1015%). ( austenitic structure @

RT).

The austenite transforms to martensite during the forming of the part (PLASTIC

DEFORMATIONDuring Crash), thus providing enhanced formability or transforms upon impact in

a crash.

8/13/2019 Autobody Materials - Lecture 4

5/6

Automotive Materials

Autobody Materials

TRIP : Microstru cture, Composit ion & Propert ies

Microstructure:

austenite remains in the ferritic-bainitic basic matrix,

The bainitic structure consists of acicular ferrite and retained austenite

Acicular ferrite is a microstructure of ferrite that is characterized by needle shaped crystallites or

grains when viewed in two dimensions

Composition:

8/13/2019 Autobody Materials - Lecture 4

6/6

Automotive Materials

Autobody Materials

TRIP : Microstru cture, Composit ion & Propert ies

Mechanical Property:

TRIP steels are intercritically annealed and isothermallytransformed.

The microstructure after heat-treatment consists of ferrite, bainite, retained austenite

Manganeseis used for hardenabilityand strength

Siliconplays an important role on the stabilisation of the retained austenite

590- 780 Mpa high strength steel sheet

superior formability

The excellent mechanical properties exhibited by the TRansformation Induced Plasticity

steels are mainly due to the martensitic transformation of the metastable retained austenite induced

by strain