Post on 05-Apr-2018
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High Strength Steels ForAutomobiles
By
Prof. D. K. Mondal
NATIONAL INSTITUTE OF TECHNOLOGY DURGAPUR
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Property requirements and controlling
factors of various vehicle parts
Parts Properties
Panel
Rigidity
Dent
Resistance
Member
Rigidity
Fatigue
Strength
Crash
Strength
Outer Panels Door outer, etc.
Inner panels Floor, etc.
Structural parts
Front rail, rear pillar, etc.
Front side member, side
sill, etc.
Door reinforcement, etc.
Underbody parts Suspension arm, discwheel, etc.
Controlling factors Youngs
Modulus
Yield
Strength
Youngs
Modulus
Ultimate
Tensile
Strength
Ultimate
Tensile
Strength
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Classes of Steel Strength for different members
Members Classes
Exposed panels 340 MPa class
Structural members (pillars and other) 440-550 MPa class
Reinforcement Ultra high strength steel>1000MPa class
Chassis 490-590 MPa class
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Steel sheets for automobiles are generally press
formed between the punch and die after
blanking to obtain final components. The pressformability can be classified into four modes of
deformation.
a) Deep drawingb) Stretching
c) Stretch flanging
d) Bending
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In deep drawing, the flange part is required toreduce its volume. Shrinking is essential for this part.This type of deformation is represented by theLimiting Draw Ratio (a ratio between the maximumdisk radius and the radius of a cylindrical cup towhich the disc can be drawn successfully).
LDR is interrelated with the plastic anisotropy, r-value(a ratio between true strain in width direction andtrue strain in thickness direction, W/t) of a steelsheet.
Steels with good r-value must have low carbon andmicroalloyed with Ti and Nb to arrest any individualcarbon and nitrogen atom. In fact, it is difficult toimprove r-value in high strength steels (TS>500MPa).
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Stretching is represented by Limiting Domeheight. LDH is interrelated with ductility (UE).
Since large r-value is impossible in highstrength steel, stretching or ductility is animportant index of press formability of highstrength steels.
Stretch flangeability is evaluated by the holeexpansion ratio [= 100 x (d-d0)/d0], which, inturn, depends on the local ductility and
uniformity in microstructure. Bendability is also interpreted similar to
stretch flangeability.
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High strength steel sheets for panels
Panels are formed by deep drawing Interstitial
Free (IF) steels which are ultra low-carbon andmicroalloyed with Ti and Nb.
Extremely formable steel sheet has r=2.5 and
work hardening coefficient n=0.27.( = kn, ln=lnk+nln).
High r-value is obtained by grain refinement offerrite through: a) High reduction hot rolling at
low temperature followed by fast cooling, and b)High reduction cold rolling followed byrecrystallization annealing.
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High bake hardenable (BH) steel sheet
Exposed panels are required to have rigidity and
dent resistance. Increase in YS can improve dent
resistance, but it is necessary to maintain YS
below 240MPa to make press-forming easy. So
BH steels have been developed that arecharacterised by low YS before press forming.
However, once deep-drawn, then coated with
paint and subjected to a bake heat treatment at1700C for 20 min, it is found that BH parts have a
YS that increased considerably, thus giving them
good dent resistance.
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High strength steel sheets with excellent deep
drawability for outer panels:
(i) High strength steels (YS440MPa) with goodpress formability and TE=38%, n-value=0.24,
r-value=1.95 have been developed by solid
solution hardening Nb-Ti bearing ultra lowcarbon steel containing P, Mn and Si.
(ii) A high r-value (1.9), high strength
(YS590MPa) steel sheet has been developed bystrengthening Ti-bearing IF steel by cold rolling
and annealing, with copper precipitation through
successive heat treatment at ~6000C
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High strength steels for structural parts andreinforcement
Many Structural parts are used to protectpassengers in case of a car crash. These partssecure a survival space for passengers on a frontalcollision by absorbing energy through buckling and
bending of parts.
In case of side collision, the plastic deformation ofparts reduces the passenger survival space.
Therefore, structural parts should be as rigid aspossible to minimize thrust into the cabin.
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Highly ductile multiphase steel sheets
Structural parts, expected to absorbenergy on collision, are required tohave good formability.
Stretchability, which corresponds toductility, plays an important role forhigh strength steels in press-forming.
Suitable steels are: 1) Galvanised Dual-
phase (ferrite + martensitemicrostructure) steel sheets, and2) Low alloy TRIP (TransformationInduced Plasticity containing ferrite +bainite + residual austenite) type
multiphase steel.Both the steels exhibit high workhardenability throughout the range ofstrain, and thus shows a goodstretchability.
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Ultra high strength steel sheets
1. For bumper reinforcement and components
facing side crash, ultra high strength steelsheets with TS of 980 MPa or more areapplied. Not only elongation (ductility) butalso bendability and stretch flangeability are
important factors of formability.2. Cold rolled DP steels with TS up to 1180MPa
are also in use.
Mixed microstructure is effective to improveelongation. Whereas, homogeneity ofmicrostructure is needed to improve thebendability.
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High strength steel sheets for chasis
Characterics: High fatigure strength, corrosion
resistance, panel rigidity. Usually hot rolledbecause of thicker ganges.
Highly ductile hot rolled high strength steelsheets: Low alloy TRIP steel hot rolled cooled coiled at 4000C, allowing bainitetransformation proceed to stabilise austenite.Because of mixed microstrure, it has inferior
stretch-flangeability. However, stretchflangeability can be improved by controllingchemical composition and microstructure ofthe steel.
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High burring hot rolled steel sheets
High strength steel sheets with enhanced
stretch flangability. They are obtained bycontrolling the microstructure to ferrite+bainit
or bainite only.
High burring steels with 590MPa in TS andkeeping comentite particles as refined as
possible.
Low carbon high strength steel sheets(780MPa class) with high stretch flangeability.
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High strength steel sheets with good weldabilityWhen arc welding is used to join high strength
sheets, strength could be reduced by softening at
HAZ. So, antisoftening agents Moand Nb are
added which form (Nb, Mo) C complex
precipitates and interact with dislocations
preventing their annihilation.
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High fatigue strength steel sheets
Underbody structural parts including wheel discs andsuspension arms are required to have high fatigue strength.
DP steels with mixed microstructure of ferrite and finalydispered martensite islands exhibit high fatigue strength(cyclic Y.S). They should have good combination ofductility and fatigue strength by decreasing the bulkybainite content and abnormal mixture of fine and coarsegrains near the surface of the sheet.
Low alloy TRIP steels also show high fatigue strength dueto solid solution hardening of ferrite and also due tocompressive stresses introduced by the expansion
following austenite martensite trasformation duringcyclic loading.
Steels with copper in solid solution also provides veryhigh fatigue limit ratio (fatigue strength / TS).
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