NEW MILLENNIUM STEELS FOR AUTOMOBILES

SAMBANTHAACHARI MANI1

M126133-3

ABSTRACT

The present day fuel crises and the environmental concerns of the globe

have put a lot of pressure on the automotive industry in this new millennium.

The demand for safer, lighter but stronger and cheaper vehicles with reduced

fuel consumption or with the renewable energy fuel is the driving force for the

development of this millennium automobile through innovative design, new

materials and optimized manufacturing technology. The material designers are

given the task of developing various types of steels with continuous increase in

the strength for various automotive applications, enabling the automobile

designer to reduce the thickness of sheet metal components. Micro Alloyed High

Strength Low Alloy Steels (HSLA) such as Advanced High Strength (AHSS) &

Ultra High Strength Steels (UHSS), Micro Alloyed Multiphase (MP) Alloy Steels

such as Dual Phase (DP STEEL) & TRIP Steels, Tailor Rolled blank & Tailor

Welded blank Steels, recent advancements in Powder Metallurgy Steels, steels

with better Acoustic properties are going to be the cost effective solutions for the

new Millennium Auto Steels.

1. INTRODUCTION

Today not only the Information Technology that bring name and fame to

India, the country should also be proud about the word STEEL and the fifth

global richest Indian origin steel man L.N.MITTAL who recently acquired

ARCELOR STEELS against all odds. Certainly Indian economy is going to gain

1 SCIENTIST ‘D’, LRDE, DRDO, MINISTRY OF DEFENCE, BANGALORE-560 093

much out of this global steel giants ARCELOR-MITTAL merger and its

proposed plan of huge investment of Rs.40 000 crores in Orissa and Jharkhand.

Technologically no doubt that, it is going to open gate for new millennium steels

in India especially for the Automobile industry where the demand and

competition is ever increasing.

In this new millennium in addition to the existing gasoline vehicles, with

the start of intensive research on gasoline to gas (Hydrogen Gas driven) Fuel Cell

Vehicles (FCVs), the strong contenders for steel material in automobile industry

is the lighter metals such as Aluminium alloys, Magnesium alloys and Composite

materials. The main reason for the substitution is their lightweight for the given

strength, which leads to fuel efficiency of the future automobiles. But this

advantage of saving in fuel cost is always with additional capital cost on these

lightweight materials. Hence today automobile industry has again turned back to

the steel industry for the cheaper but lighter steel material, which enjoys the

preference of the vehicle owners worldwide for its trusted safety. Parallel to the

demand for stronger steels, the formability of the newly developed steel types

acquired increasing importance, because the constructional requirements called

for growing complexity of the geometries which had to be implemented with

these materials. The vehicle durability, comfort and safety constituted a further

driving force for new steel material developments in this new millennium.

2. FUNCTIONAL REQUIREMENTS OF NEW MILLENNIUM VEHICLES

Increased Stiffness, Passive safety, Service life & Durability, Acoustics &

Comfort in addition to reduced mass leading to fuel savings are the functional

requirements of futuristic automobiles with reduced cost or at least with no

additional cost. This stringent and contradictory functional and market

requirement of new millennium vehicles has thrown a tough challenge for auto

material designers. Though the light metal alloys of aluminium, magnesium and

the composites replaced steel in the recent years with multi fold increase in the

cost, the recent technological developments in steel industry such as Thin Slab

Direct Rolling (TSDR), Micro alloying HSLA steel with V, Nb and Ti,

Multiphase (MP) Alloy Steels, Tailor rolled blank steel, Tailor welded blank steel

has once again re-established their unshakable cost effective position in the auto

industries in this new millennium. These new millennium steels have provided

the shot in the arm for auto material designers to fulfill the stringent and

contradictory functional and market requirement of new millennium vehicles.

3. MICRO ALLOYED HSLA STEELS

By substituting technologically more advanced steel material for the

inefficient, commodity carbon-manganese steels the profit and the market place

for the auto industry can be improved. Modern Micro Alloyed High Strength

Low-Alloyed Steels (MA-HSLA) fulfills this requirement by contributing to

significant weight reduction at low cost. Compared to C-Mn commodity steels,

MA HSLA steels exhibit superior engineering properties, especially formability,

toughness and weldability. Being two to three times stronger than C-Mn steels,

they may reduce weight by up to 30 to 40%. Their mechanical properties

particularly the high yield strength, are the result of micro structural changes i.e.

grain refinement and precipitation triggered during hot rolling by micro alloy

additives. The final properties are achieved in as hot-rolled condition, and no

costly alloys or heat treatment are required. The amount of micro alloying

elements usually niobium /vanadium/titanium is typically below 0.1% i.e. less

than one kilogram per metric ton, increasing the cost of carbon steel base by only

a meager 3 to 5% and hence the micro alloyed steels are uniquely cost effective.

The economics of micro alloyed steels is further enhanced by technological

developments such as growth of Electric Arc Furnace (EAF) steel making and

Thin Slab-Casting and Rolling technology. The rapid growth of EAF steel

making, accounting for almost 50% of world’s steel production, is motivated by

low capital investment, flexibility of operation, availability of scrap or scrap

substitutes and presence of more nitrogen in EAF steel which reduces the cost of

micro alloying with vanadium by 20 to 40%. Thin Slab-Casting and Rolling

technology, introduced during the recent past, has a revolutionary impact on

reducing the cost of steel making. The process converts in-line liquid steel to a

marketable product, significantly lowering the cost of hot-rolling. Vanadium

steels, made in an EAF and processed as a thin slab, offer low-cost, high strength

strip up to 12-17mm thick which the auto industries, after thickness reduction to

the suitable gauge can readily roll-form to produce structural or hollow shapes.

The technical and economic advantages of substitution of high strength

steels for C-Mn steels have been demonstrated by the internationally sponsored

project on “Ultra Light Steel Auto Body” (USLAB). The three objectives of the

project such as

1. Designing a stronger and hence the safer auto body,

2. Reducing the weight and

3. Lowering the cost

have been achieved. Successful completion of this phase of the project made high

strength steel the material of choice of the cost-conscious automotive industry in

this new millennium instead of the costly light metal alloys and composites.

Fig.1 shows the details of HSLA high strength steels and other materials, for the

new millennium lightweight BMW-sports wagon with more strength, safety and

speed.

FIG.- 1

4. MULTIPHASE STEELS

MP steels group consists of Dual-Phase (DP), Residual-austenite (RA),

Complex-phase (CP) and Martensite Phase (MS) steels. Among the well-known

hardening mechanisms like precipitation hardening, solid-solution hardening

and grain refinement this steel category makes use of the particular

characteristic of steel to transform into various microstructure in dependency on

the composition and temperature. The material properties are defined essentially

by the combination of the micro structural components with different degrees of

hardness.

4.1 DUAL PHASE (DP) STEELS

Hot rolled Dual-phase steels are the MP steels in which the soft ferritic-

phase and the very hard Martensite-phase co-exists together. They are

manufactured today with tensile strengths in the range of 500-600 MPa and in

thickness from 1.8 to 5mm. In this new millennium the auto industries use this

steel for making wheel disk & wheel rim instead of costly aluminium alloy

wheels. The micro alloyed version of this steel with the utilization of Nb,

compared with a P-alloyed variant with the same C, Mn and Cr content, gives a

significantly finer ferrite grain size of the average 2.5m instead of 4.2m. In

both variants a ferritic basic structure prevails with incorporated Martensite

islands but the Nb-alloyed variant exhibits greater strength values with good

elongation due to finer grain size.

4.2 RESIDUAL-AUSTENITE (RA) STEELS

RA steels are the one in which a small proportion of austenite remains in

the ferritic-bainitic basic matrix. The austenite transforms into hard martensite

during forming to a component that leads to a significant improvement of the

formability and high component strengths. Since this particular steel possesses

the Transformation-induced plasticity (TRIP) effect it is also known as TRIP

steels. In this steel the residual austenite is so stable that it does not transform

prematurely to martensite during the initial percentage of elongation of the

tensile test i.e. the transformation is delayed until higher stresses are applied,

shifting the onset of the reduction of area to higher elongation values. The micro

alloying of niobium (Nb) & Titanium (Ti) to RA steels further increase the

strength and the residual austenite characteristic i.e. increased resistance to

strain induced transformation to martensite.

4.3 COMPLEX PHASE (CP) STEELS

In this new millennium this class of CP steels are the appropriate

materials to effectively meet the demand of ultra high strength steels from

automotive industries with tensile strengths reaching up to 800 MPa with smaller

thickness sheets. The CP steels consists of bainite with some traces of ferrite and

martensite. Since ferritic-pearlitic microstructure can no longer meet the

demand of ultra high strength steels from automotive industries, the

development trend has gone in the direction of greater bainite content in the

present day environment. The desired small hot strip thickness of 1.5mm can be

achieved without any problem in both variants of CP steels i.e. plain CMnCr and

Ti micro alloyed CMnCr CP steels. In case of CMnCr CP steel without micro

alloying, maximum tensile strengths up to 750 MPa can be achieved and in case

of CMnCr CP steel with micro alloying of Ti and Si, maximum tensile strengths

up to 1000 MPa can be easily achieved. Today, this CP steels are used as low cost

& light weight (about 2mm thick) Bumper beams and door impact beams in

automobiles.

4.4 MARTENSITE PHASE (MS) STEELS

The MS steel has the highest tensile strength of all multi-phase steels (up

to 1500MPa). The micro alloying of Titanium results in still higher tensile

properties and better elongation values. High strength MS steel are suitable for

manufacturing crash-relevant components and at the same time for producing

components which are made today of steels with high carbon content and obtain

their component properties by subsequent heat treatment e.g. seat belt

connectors. The heat treatment is no longer necessary when utilizing MS steel.

This avoids post heat treatment problems such as geometric distortion and

surface impairment. Compared with many C-steels, the welding behaviour of

MS steel is considerably better.

5. POWDER METALLURGY (P/M) STEELS

The recent developments of iron based P/M materials have increased

opportunities for their usage, especially for cost effective high performance

automotive applications. In this new millennium prime automotive components

such as connecting rods, highly loaded gear box components and transmission

gears have created a need for the development of high performance P/M alloys

that reduce the excessive costs associated with the machining of these

components. Micro alloy additions of Nb, V, Cr, Ni, Mo and Mn have been

developed for cast and wrought alloys, showing large improvements in alloy

performance. In addition to the successes associated with the engine P/M steel

components they also found application in the complex electromechanical

systems such as steering, braking, power transmission and emission controls. In

North America, automotive applications accounts for 72% of the consumption of

ferrous powders, in Europe 80% and in Japan 88%. Hence, automotive

applications are going to be the dominant aspect of ferrous P/M in the years to

come.

6. TAILOR ROLLED BLANK (TRB) STEELS

Structural engineers are currently exploring the use of tailor rolled blank (TRB) technology in the manufacturing process, and it could result in vehicle weight savings of up to 40 percent on the body structure of future Ford vehicles. TRB allows engineers to do something they've not been able to do in the past -- vary the thickness of a part in a single process, giving strength where it is needed and eliminating excess weight where it is not.

"TRB lets us optimize the design in that we put the metal exactly where we need it right in the blank, not by welding-in a reinforcement as a secondary operation," said Karen Mianzo, supervisor, Body Structures.

Traditionally, is stamped into a part.

In sheet metal forming it is most common practice to stamp a part using a piece

of sheet metal called a "blank". The thickness of the part is constant along the

length of the part, regardless of whether or not it ideally needs to be.

7. TAILOR WELDED BLANK (TWB) STEELS

8. SANDWICHED VISCOELASTIC LAYER STEELS

The automotive industry's drive to create a quieter vehicle has become as

important as performance and styling. Power train vibration and road noise

resonate through vehicles creating unpleasant sounds that eventually enter the

passenger compartment. The recent research works has developed a series of

unique material solutions for use in Body-In-White applications that directly

damp vibration before it has an opportunity to penetrate the interior of the

vehicle.

Extensive acoustical testing of ‘sandwiched viscoelastic layer steel’

developed by a research center in US has indicated a substantial reduction in

passenger compartment ambient noise. This steel consists of an engineered

viscoelastic layer sandwiched between two cold-rolled layers of steel. Already

automotive industries such as FORD and GM started experimenting this steel. In

addition to improved acoustical performance, this Steel possesses good forming,

welding & 100% recycling characteristics offering potential weight reduction

opportunities and associated cost savings.

The objective is to isolate noise and vibration, reduce costs, and

streamline processes. This class of steels can be used in a variety of components

such as Power seat tracks, Window lift mechanisms, Power door systems, Fan

and blower housings to eliminate covers, minimize greases, and improve

performance. The dramatic reductions in noise and vibration in sheet metal

panels made from this class of Steels are due to the very high levels of damping

inherent in the material. The effect of damping is to reduce the vibration

amplitude of a system that is being excited at resonant frequency. Since it is the

vibration of a panel that generates noise, the reduction in vibration amplitude

due to damping directly leads to less noise radiated from the panel.

9. CONCLUSION

By the continuous research and development, the steel industry has

attracted the automotive industry, by meeting difficult challenges faced by the

later in this new millennium. The rapid growth of electric furnace (EAF) in steel

making & introduction of thin slab direct rolling (TSDR) has lead to a revolution

in the manufacture of flat steel products providing cost effective solutions. The

further study on micro alloyed HSLA high strength steels & multi phase steels

and the large scale implementation of tailor rolled blank (TRB) and tailor

welded blank (TWB) steels may lead to greater impact in the production of

cheap, superior and safe automobiles in the future.

REFERENCES

POWDER METALLURGY OF IRON AND STEEL BY RANDALL

M.GERMAN

MICRO ALLOYED STEELS 2002-CONFERENCE PROCEEDINGS

FROM MATERIALS SOLUTIONS 2002 EDITED BY RIAD I.ASFAHANI,

RICHARD L. BODNAR, MATTHEW J. MERWIN

http://www.matsci.com

http://www.autosteel.org

technical@ieindia.orgIn quadruplicate to

Deputy Director (Technical), The Institution of Engineers (India)

8,Gokhale Road, Kolkata 700 020 (31-07-2006)