HIgh Strength Steels for Automobiles

7/31/2019 HIgh Strength Steels for Automobiles

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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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HIgh Strength Steels for Automobiles