A PRESENTATION BY-

SHANTANU SINGH

B. TECH. 3RD YEAR MME

NIFFT, RANCHI

CONTENTS INTRODUCTION TO AHSS –DEFINING AHSS AND ITS NEED

IN AUTOMOTIVE INDUSTRY

VARIOUS GRADES OF AHSS AND ITS APPLICATION

FUTURE OF AHSS- RESEARCH AND ADVOCACY

INTRODUCTION TO AHSS

What is AHSS? AHSS are multiphase steels with excellent combination of

strength and formability.

By definition, steels with yield strengths (YS) greater than 550 MPa are called AHSS.

AHSS with YS> 780 MPa are called ULTRA HIGH STRENGTH STEELS.

AHSS with YS> 1000 MPa are called GIGAPASCAL STEELS.

WHY AHSS? AHSS grades allow excellent formability- Greater part

complexity - Fewer individual parts - Time, mass and cost effective.

High strength to weight ratio – Gauge reduction – Mass, Material and Fuel Saver.

Leave minimum carbon footprint – Environment friendly.

Steel is the most recycled material – Used in new automotive or other products.

Improved crash performance – Improved passenger safety.

MATERIAL SELECTIONCRASH PERFORMANCE

CRUMPLE ZONE SAFETY CAGE

REQUIREMENT High energy absorption over a distance in a crash event

No deformation during crash event

MATERIALPROPERTY NEEDED

High work hardening, strength, ductility ( large area under s-Є curve)

High yield strength (high UTS values on s-Є curve)

STIFFNESS- It is the rigidity of an object- extent to which material resists deformation under applied forces.

- Stiffness is a function of part geometry and elastic modulus, not YS or UTS.

- The load carrying capacity of the component can suffer from gauge reduction of the component.

FORMABILITY AND MANUFACTURABILITY-With increased strength of the materials, it becomes difficult, to form them into complex automotive parts.

-Thus, a combination of strength and ductility is required to effectively use a material in manufacturing of auto parts.

VARIOUS GRADES OF AHHS AND THEIR APPLICATION

STEEL TYPES

CONVENTIONAL HIGH STRENGTH STEELS

1. MILD STEEL- Very low strength but extremely high formability.

- Inexpensive, widely produced and used.

- Used in body structures, closures, and other ancillary parts.

2. INTERSTITIAL FREE STEEL - Ultra low carbon content, achieved by vacuum degassing of

CO, H2, N2, and other gases during steel making.

- Lack of interstitial atoms enables IF steels to have high ductility values, ideal for deep drawing.

- Low strength, but high work hardening rates and excellent formability.

- Used in elements of body structures and closures.

3. BAKE HARDENABLE STEEL- The bake hardening procedure increases the YS of the BH Steel.

- This increment occurs during low temperature (150-200oC) paint baking of already formed components.

- The steel is soft during forming stage, but picks up strength during subsequent paint baking stage.

- Used in closure panels like door outer, hoods, and decklids.

4. HIGH STRENTH LOW ALLOY STEEL- Finely dispersed alloy carbides and ferrite- pearlite aggregates sits in a

ferrite matrix, minimum alloying content.

- This complex structure mixed with grain refinement gives the steel strength.

- They are tough, corrosion resistant, formable, and weldable.

- It is mostly used in ancillary parts, body structures, suspensions, chassis parts and wheels.

ADVANCED HSS GRADES 1st generation ( ferrite based)

- Dual Phase (DP) Steel

- Ferritic Bainitic (FB) Steel

- Complex Phase (CP) Steel

- Martensitic (MS) Steel

- Transformation Induced Plasticity (TRIP) Steel

- Hot Formed (HF) Steel

2nd generation (austenite based)

- Twinning Induced Plasticity (TWIP) Steel

DUAL PHASE (DP) STEEL DP Steels consist of a continuous soft, ferrite matrix with dispersed harder

martensite phase, along with some retained austenite and bainite.

Alloy additions- Si, P strengthen the steel, but must be balanced for weldability, while Mn, Cr, V, Nb increase the hardenability.

The most common way of producing DP steels is by cold rolling of low alloy (LA) steels followed by intercritical annealing in a continuous annealing line, here referred to as CAL.

Properties - High strength meaning high resistance to fracture.

- High work hardening rates and high strain energy absorption meaning high UTS values and delayed necking.

- Excellent welding and elongation, meaning good manufacturability.

Applications - DP Steel is currently the most widely used AHSS in the automotive sector.

- It can be produced with low to high YS to UTS ratio, allowing a broad range of applications from crumple zone to body structures.

- Used in 1. Beams, cross members and structural components

2. Crash energy absorption in crumple zone

3. tailored blanks, and hydroformed tubes

FERRITIC BAINITIC STEEL

The microstructure of FB Steel consists of a softer ferrite matrix, with fine bainite as second phase.

The microstructure is finer than the typical DP steel.

The second hard- phase bainite and grain refinement make FB a strong material with excellent formability.

PROPERTIES FB steels have improved stretchability of sheared edges when compared to

DP and HSLA steels.

Higher strain hardening exponent (n-value) at the same level of strength.

They also show good fatigue properties and crash performances, allowing good performance under dynamic loading.

Applications Applications where High Hole Expansion (HHE) or Stretch Flangeable (SF)

capabilities are required for enhanced edge stretch capabilities involve the use of FB steels.

Used for - cross beams and reinforcements, and wheels.

- suspension and chassis

Because FB has good fatigue properties in dynamic load conditions, it is an outstanding candidate for shock towers and control arms.

COMPLEX PHASE STEEL

PROPERTIES High elongation at same tensile strengths than DP Steel.

Good wear characteristics and fatigue strength.

APPLICATIONS The high YS and elongation enables high energy absorption, also making it a good

choice for crash safety components.

Used for B-pillar reinforcements, door impact beams & parts with heavy wear.

•Mixed microstructure with a ferrite/bainite matrix containing bits of martensite, retained austenite, and pearlite. •Microalloying elements like titanium and niobium are often precipitated.•Delayed recrystallization is often employed to develop very small grain.

MARTENSITIC STEEL Produced from austenite phase by rapid quenching.

Nearly all austenite is transformed to martensite (Lathe martensite).

Increasing the carbon content and alloying with Mn, Si, Cr, Mo, B, V and Ni in various combinations increases hardenability.

PROPERTIES MS Steel is known for its extremely high strength but very poor ductility.

It can be improved by post quench tempering.

APPLICATIONS It has very high strength to weight ratio, thus used in making strong light

weight components.

Used for - bumper reinforcement and door intrusion beams

- rocker panel inners and reinforcements

- springs, and clips

Limitations include low elongation and high springback which is handled by roll forming in multiple operations and over bending.

TRANSFORMATION INDUCED PLASTICITY (TRIP) STEEL The microstructure consists of embedded retained austenite in a primary

ferrite matrix.

It has high carbon content to stabilise austenite below ambient temp.

During plastic strain, the austenite transforms to martensite.

•The strain at which the transformation occurs depends on the stability of austenite, which in turn is controlled by the carbon content of the steel.•It is stabilised to delay the transformation until high strain values are reached , beyond forming process, usually in a crash event.

PROPERTIES High work hardening rates.

High energy absorption during strain.

High formability prior to the transformation.

High fatigue endurance limit meaning high durability.

PRODUCTION PROCESS-A two-stage heat treatment with intercritical annealing in the temperature range 780–880 °C followed by cooling and isothermal annealing in the range 350–450 °C and cooling to ambient temperature is applied .- After bainitic transformation, the microstructure contains around 50–60% of ferrite, 25–40% of bainite and 5–15% of retained austenite.

APPLICATONS It is mainly used for forming complicated parts, and in weight reduction.

It is often selected for components that require high crash energy management.

Used for - cross members

- longitudinal beams

- A- and B- pillar reinforcements

- sills and bumper reinforcements

It has a few limitations:

- poor local and edge stretchability

- high alloying requirement

HOT FORMED STEEL HF steel is typically boron-based (0.002-0.005% B).

Microstructure is similar to martensite.

Two type of forming processes are available-

- direct hot forming

- indirect hot forming

Transitions in HF STEEL1: Initial, room temperature state where the steel is blanked.2: Raised temperature state where

forming is completed.3: Final strength-elongation achieved after rapid cooling.

PROPERTIES Negligible springback as cooling occurs indie itself. Very high strength.

APPLICATIONS Complex parts requiring high strength. Used for - A- and B-pillars,

- roof bows- side-wall members- beams for crash management structures

LIMITATIONS high initial investment in equipment. limited repairability.

TWINNING INDUCED PLASTICITY (TWIP) STEEL

Austenite based, these steels contain 17-24% Mn content.

Microstructure is fully austenitic at room temperature

The name comes from its particular mode of deformation.

Slip causes formation of twin boundaries which restrict the movement of dislocations and thereby, strengthen the material.

PROPERTIES High value of instantaneous strain hardening.

Excellent formability and good weldability.

Excellent combination of strength and ductility.

Good corrosion and wear resistance.

APPLICATIONS Very high strength parts, with complex geometry.

Used for - Door impact beam

- Wheel, lower control arm, front and rear bumper beams

- Floor cross-member

CURRENT EXAMPLES HONDA and BMW are the two major car manufacturers,

which have extensively used AHSS grades in the past few yearsTwo of Honda’s major initiatives in recent years have been safety and environmental leadership, both which are supported through the use of AHSS .

In the new Chevrolet Volt, 25 percent of the steel in the Volt is AHSS (GENERAL MOTORS).

The steel content in the 2011 Jeep Grand Cherokee has increased from 28 percent to 50 percent HSS and AHSS.

Ford similarly reports that the share of HSS and AHSS has increased in its vehicles, such as the Explorer and Fiesta.

Volvo and other European automakers make extensive use of hot stamped boron steels, as well.

Use of various AHSS grades in HONDA CR-Z

FUTURE OF AHSS AUTOMOTIVE APPLICATONS HSS and AHSS grades are already being widely used in vehicles.

Upcoming regulations (2017-2025) will impose stricter safety , performance and environmental standard which will promote use of AHSS grades.

RESEARCH Development of

austenite based AHSS.

Improvement in

already available

AHSS grades.

ADVOCACY Communicating the advantages of AHSS to the many communities who

could benefit from its use.

A close partnership between steel producers and automakers is essential.

SMDI, Auto/Steel Partnership, and other groups facilitate such work.

Conferences around the world, such as the Great Designs in Steel Seminar provide a venue for discussion and transfer of technical knowledge and ideas.

Policy makers are another vital audience for the message of AHSS.

The real goal of reducing carbon emissions can be easily obscured by the hype about producing the lightest weight vehicles possible.