High Strength Low Alloy Steel (HSLA)

8/7/2019 High Strength Low Alloy Steel (HSLA)

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High Strength Low Alloy Steel (HSLA)

Ultra Low Carbon Steel

Advance High Strength Steel

By

Panya Buahombura

School of Metallurgical EngineeringSuranaree University of Technology


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Outline Overviews Low carbon structural steel

High strength low alloy steel (HSLA)/Micro-alloy steeland Thermo-mechanical control process (TMCP)

Low carbon strip steel

Ultra-low carbon steel- Interstitial Free (IF) Steel

- Bake Hardening (BH) Steel

Advance high strength steel or Multi-phases steel

- Dual Phase (DP) Steel

- Transformation Induced Plasticity (TRIP) Steel


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Overviews


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Overviews: Low carbon structural steel

and low carbon strip steel

High strength low carbon steels?

StrengthFFFhigh strength low carbon steel? High strength low carbon steelsstrengthening

mechanismF,?

High strength low carbon steels,?

High strength low carbon steelsF?

Physical metallurgyhigh strength low carbonsteelsF?


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Steel

Alloy Steel

Low-C steel

Plain Carbon Steel

Low alloy steelHigh-C steelMedium-C steel High alloy steel

C, Si (up to 0.40%), Mn (up to 1.20%), S, P Nb, Ti, V, Al, Cr, Ni, Mo, Co, Cu, Mo, W, Mn, Si and etc.

C 0.2%

Flat products (rolled)

Structural (rolled)

C = 0.2 0.5 %

Machine parts

(Heat treatable)

C > 0.5%

Tool steels

(Wear, Abrasion, Heat resisting,

Corrosion applications)

Alloy elements 10%

(some data: 5%)

Alloy elements > 10%

(some data: > 5%)

Applications

- Body parts in automotive industry

- Construction of building, bridge, pipeline, etc.

High strength low carbon steels

Strengthening Mechanisms

Produced lighter wt. and higher strength

- Cold-reduced products: YS > 220 MPa, TS > 330 MPa

- Hot rolled products: YS > 280 MPa, TS > 370 MPa

- Solid solution strengthening

- Precipitation strengthening

- Dislocation strengthening (Work hardening)

- Transformation strengthening (Heat treatment)

- Refining the ferrite grain size (Grain size effects)


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General Steel Production Process


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General Steel Production Process


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Iron and Steel Making Process


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Semi Finished Products


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Overview


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Overview


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Overview


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Conventional high strength sheetsteel for automobiles used to besolid solution-hardened steel or

precipitation-hardened steel withmicro-alloy added.

Currently, high strength steelproducts whose microstructureis reinforced for greaterstrength have been used.

(DP steel, TRIP steel)

Relation between tensile strength and elongation of HSS


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Chemical compositions (mass%) and mechanical

properties of the steels

Yield Tensile Elon-

Type of steel C Si Mn Ti strength strength gation(Mpa) (Mpa) (%)

A Mild steel 0.05 0.01 0.24 - 241 384 43

B Solid solution 0.08 0.02 1.46 - 370 487 30hardened steel

C DP steel 0.05 0.89 1.25 - 432 618 27

D Precipitation 0.09 0.01 0.80 0.07 539 636 22

hardened steel

E TRIP steel 0.15 1.48 0.99 - 510 644 37


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Overview


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Overview


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Strengthening Mechanisms Refining the ferrite grain size

(Grain size effect)

Solid solution strengthening

Precipitation strengthening Dislocation strengthening/Work hardening

Transformation strengthening


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Refining the ferrite grain size

(Grain size effect)


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Refining the ferrite grain size

(Grain size effect)


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Solid solution strengthening


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Precipitation strengthening


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Low Carbon Structural Steel


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Overview: Low Carbon Structural Steel

Predominantly C-Mn steels (Ferrite-Pearlitemicrostructures)

Used in large quantities in civil and chemical engineering

General Y.S. up to 500 N/mm2 (low alloy grades whichquenched & tempered, Y.S. up to 700 N/mm2)

Applications: building, bridges, pressure vessels, ships,

offshore oil & gas platforms, pipeline (for weldability andtoughness which required low-carbon)

Early 1950s, designed of structural steel with concept ofrefinement of ferrite grain increase Y.S. & toughnessof ferrite-pearlite steels (Al-grain refined compositions

Y.S. up to 300 N/mm2 which have good impact propertyand good welding characteristics)


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Overview: Low Carbon Structural Steel

For higher strength steel, required precipitationstrengthening by small addition of Nb, V, Ti tostructural steel Y.S. up to 500 N/mm2 (known

as Micro-alloy steel or HSLA steel)

After 1950s and 1960s, new technique toproduce structural steel Control Rolling

(fine-grained in as rolled conditions whicheliminating of normalizing heat treatment)

1970s and 1980s, Control Rolling + Controlled

Cooling TMCP Improving history of structural steel for: Strength,

Toughness, Weldability


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High Strength Low Alloy Steel (HSLA)And

Thermo-mechanical Processing (TMCP)


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(Precipitation strengthened/Grain refined steel) Addition of micro-alloy (carbide, nitride or carbo-nitride

forming elements) such as Nb, V, Ti in structural steel

and strip steel grades, the materials are known asHigh Strength Low Alloy (HSLA) steel

At slab soaking temperature ~ 1200 C

- undissolved particles (such as TiN, NbC and AlN)restricts the size of austenite grain (affect to inhibit

recrystallization during hot rolling produces fine

austenite grain size induces fine ferrite grain size)

- a proportion of micro-alloys are dissolved to solid

solution (affect to precipitate in later process in form of

fine carbide/carbonitride/nitride at austenite-ferrite

interface on cooling to room temperature)


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Hot rolled materials can be strengthened by separate

mechanisms of grain refine & precipitation strengthening

Magnitude of effects depend on:- type and amount of elements added

- base compositions

- soaking temperatures

- finishing and coiling temperatures

- cooling rate to room temperature

Strength increment up to 300 N/mm2 and Y.S. ~ 500-600

N/mm2 can be produced in hot rolled state

Y.S. ~ 350 N/mm2 are produced in cold-rolled strip

containing 0.06-0.10 %Nb


(Precipitation strengthened/Grain refined steel)


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(Precipitation strengthened/Grain refined steel) Precipitate Ti growth austenite > 1250 C

Precipitate Nb growth

austenite 1150 C

Precipitate V growth austenite 1000 C

Precipitate Al growth austenite 1100 C

HSLA steel precipitation strengthening ferrite grain refining


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High Strength Low Alloy Steel (HSLA)(Precipitation strengthened/Grain refined steel)

Precipitation-Time-Temperature (PTT) Diagram Nb(CN) austenite F 50%

%Mn precipitation (shiftPTT curve )

Nb(CN) dynamicprecipitation ~ 900 C

Ps : Precipitation start

Pf: Precipitation finish



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Ti(CN) dynamicprecipitation

~ 1025 C (FFFNo-recrystallizationtemperature (Tnr) F Nb(CN))

%Mn precipitation (shiftPTT curve F HSLAsteel Nb)

Precipitation-Time-Temperature (PTT) Diagram Ti(CN) austenite



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a) recystallization rate Nb microalloyed steel plaincarbon steel

Recystallization-Time-Temperature (RTT) Diagram

Nb microalloyed steel plain carbon steel

Rs: Recystallization start, Rf: Recystallization finish

Ps: Precipitation start, Pf: Precipitation finish

(C): for plain carbon steel

(S): for Nb microalloyed steel (solute effect only)

(Nb): for Nb microalloyed steel (precipitation effect)

b) Nb Fsolute atom (solute effect only) Frecystallization rate (recystallization ) plain carbon steel

c) F precipitation Nb(CN) FF/ recystallization

S S ( S )


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Nb FFF (No-recrystallization

temperature; Tnr)

Controlled rolling/Thermo mechanical processing


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Controlled rolling/Thermo-mechanical processing(TMCP)

1. Outline processSRT ~ 1200-1250 C

FT ~ 1000 C

normalizing ~ 920 C

Hold/Delay

Roughing rolling

Finishing rolling

(Below Tnr) Austenite-elongated grain

(pancake structure)

No-recystallization temperature (Tnr)

C t ll d lli /Th h i l i


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2. Slab Reheating

Importance of slab reheating stage

- control amount of micro-alloying element taken intosolution

- starting grain size

Re-solution temperature of micro-alloy precipitates- VC: complete solution ~ 920 C (normalizing temp.)

- VN: at somewhat higher temperature

- Nb(CN), AlN and TiN: around 1150-1300 C- TiN (most stable compound) little dissolution at normalslab reheating temperature (SRT)



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2. Slab Reheating

Un-dissolved fine carbo-nitride (CN) particles

- maintain fine austenite grain size at slab reheatingstage

Micro-alloying elements taken into solution (which canbe influence in later stage in process)

- control of recrystallization

- precipitation strengthening

Multiple micro-alloy additions for above dual

requirements

Controlled rolling/Thermo-mechanical processing

(TMCP)



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3. Rolling Three distinct stages during controlled rolling.

- Deformation in the recrystallization (austenite phase)

temperature range just below SRT- Deformation in temperature range betweenrecrystallization temperature and Ar3- Deformation in 2 phase (austenite-ferrite) temperaturerange between Ar3 & Ar1

At temperature just below SRT

- rate of recrystallization is rapid

- provided the strain per pass exceeds a minimum criticallevel

- recrystallization is retarded by presence of solute atom Al,Nb, Ti, V (solute drag) strain induced precipitation

form fine carbonitride during rolling process




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3. Rolling

- rolling temperature decrease, recrystallization moredifficult and reach a stage recrystallization stop

temperature (Trs or No-recrystallization temperature; Tnr)(the temperature at which recrystallization is complete after15 s. after particular rolling sequence)

- Nb is powerfull retardation effect which depend on

solubilities in austenite- Nb lease soluble

- largest driving force for precipitation

- creating greater effect in increasing of recrystallizationtemperature than Al and V

At temperature between recrystallization temperature & Ar3- temperature below 950 C




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3. Rolling- strain induced precipitation of Nb(CN) or TiC is sufficientrapid to prevent recrystallization before the next pass

(deformed-austenite providing nucleation sites of carbo-nitride precipitation and pins the substructure which inhibitsrecrystallization)

- finishing rolling below recystallizaion stop temperature

- can be obtain elongated-pancake morphology in theaustenite structure

At temperature between Ar3 & Ar1- further grain refinement

- mixed structures of polygonal-ferrite (transformed fromdeformed-austenite) and deformed-austenite during rollingprocess




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4. Transformation to ferrite Mean ferrite grain size relate to:

- thickness of pancake-austenite grain

- alloying elements depress the austenite to ferritetransformation which decrease ferrite-grain size

- cooling rate from austenite or austenite-ferrite region(accelerate cooling)

increase strength

achieve strength level by lower alloy content

- direct quenching

refine ferrite-grain formation of bainite and martensite (required

tempering)

Controlled rolling/Thermo mechanical processing(TMCP)



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Controlled rolling/Thermo mechanical processing(TMCP)


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Low Carbon Strip Steel

Overview: Low Carbon Strip Steel


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The first hot strip mill was commissioned in 1923 inUSA

- revolutionized steel industry and market forstrip products

- made available wide steel strip in lower price &superior properties than the old process (hand-operated mills) which resulted in dramatic growth ofautomotive industry (major product develop in striparea)

Produced both hot rolled and cold rolled conditions

- hot rolled materials can be produced in

thickness ~ 2.0 mm (in present down to 1.0-1.2 mm)- main demand cold rolled and softened in BAand CA furnace

O i L C b St i St l


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Main properties:

- high level of cold formability

- strip is produced with C < 0.05%, Mn < 0.20%

High strength steel for automotive industry

- down-gauging of body panel, reduce vehicleweigth, improve fuel consumption, corrosion in

vehicle (increase in use of Zn-coated steel ~ 70% ofstrip required of most motor car)

Building industry

- organic-coated- galvanized sheet for architectural roofing,

cladding

Process route Basic oxygen steelmaking (BOS)


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Continuous casting

Slab soaking

At 1200-1250 C

F.T. 870-910 C

Al-killed steel (significant effect for good formability)

Hot coiling

Hot rolling

Pickling

Cold rolling

Hot rolled strip

Batch annealing Continuous annealing Tin plate production Zinc coating

C.T. 560-710 C

C.T. 710 CC.T. 560 C

Reduction ~ 65%Thickness > 2 mm

AlN dissolved into solid solution and remain

in this state after completion of hot rolling

C.T. 710 C for CA: cool very slowly and have opportunity to precipitated of AlN

C.T. 560 C for BA: cool quickly and precipitated of AlN is suppressed andremain in solid solution on cooling to ambient temperature

Ingot casting

Secondary steelmaking (e.g. vacuum degassing)

~ 2% Deformed: For control of shape, surface texture, luder lines

Temper rolling (Skin-passing)

Sheet Formability


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Sheet Formability Draw-ability

rm-value or r-bar value or Lankford value

(plastic strain ratio) which represents plastic

anisotropy of the material

Stretch-ability

n-value (strain hardening exponent or work-

hardening coefficient)Specimen: JIS 5L; Thickness: 0.8 mm

Formability of high-strength strip steels


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Formability of high-strength strip steelsSpecimen: JIS 5L; Thickness: 0.8 mm

Batch Annealing (BA)


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~ 700 C

SRT , FT F CT (~560 C)

SRT , FT Al, N FFF CT Al, N F solid solutionF precipitate F batch annealing



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Deep drawing characteristic of low-carbon stripare influenced signification by crystallographic

texture

- good drawability strong {111} cube and

reduction of {100} cube

- rimming steel: rm

-value ~ 1.0-1.2

- Al-killed steel: rm-value ~ 1.8

Addition of Al is beneficial to

- formability due to generate of a favorabletexture

- large ferrite-grain size




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Al must be present in steel in solid solution prior toannealing (BA) which will be coiled at low temperature

(560 C) in order to avoid the precipitation of AlN

Heat treatment cycle in batch annealing

- very slow heating and cooling rate

- heated slowly to about 700 C (close to Ac1) which

recrystallization of cold worked structure will takeplace in temperature range 500-550 C

- during initial heating process, AlN precipitate on the

deformation sub-grain boundary which retard the

recrystallization process, inhibiting the nucleation of

new grains an thereby producing a large grain size

(ASTM ~ 5-6, grain size ~ 40-60 micron)

g ( )



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- AlN also induces the formation of a strong {111}

texture which depend on heating rate and

proportions of Al and N (highest rm-value are

produced in steels containing 0.025-0.04 %Al and

0.005-0.01 %N

Cooling rate:

- slow Carbon in solid solution is precipitated,therefore BA of Al-killed steel is characterized by:

- strong {111} texture

- large ferrite grain size- low solute Carbon and Nitrogen content

- can adjusted to retain some Carbon in solid

solution which offer to bake hardening process

g ( )

Continuous Annealing (CA)


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700-850 C (Holding for 40 sec.)

400-450 C (Holding ~ 3 min)

Heating up time < 1 min

SRT AlN F, CT (~710 C) AlN ( nitrogen free) continuous annealing over-aging carbon solid solution



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First application of CA by Armco SteelCorporation in USA for hot dip galvanized steelin 1936 (later apply for aluminized steel, tinplate,

stainless steel and non-oriented Si steel) CA advantages:

- more uniform properties

- cleaner surface- shorter production times

but still lack of cold forming properties and

resistance to aging when compare to BA Early 1970s, Japanese steel-maker incorporated

and over aging treatment in the CA process and

then improved the properties

g ( )



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Heat treatment cycle of CA- rapid heating (less than 1 min), short soakingtime (at 700-850 C for 40 sec) rapid cooling and

then overaging (by holding at 400-450 C up to 3min)

- process completed in 4-8 min

Due to fast heating rate in CA, N would be

remained in solid solution and lead to increasestrength, reduced formability an susceptibility tostrain aging

In order to reduce level of N in solid solution, HBmaterials for CA will coiled at high temperatures(up to 710 C) to cool slowly in coil form andprecipitate AlN and remove N from solid solution

g ( )



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Due to rapid cooling rate that has little time for

carbide precipitation and growth, therefore, over-

aging stage (holding at 400-450 C up to 3 min)

will combine into the cycle in order to reduce Ccontent to low level

Carbon content proper for BA and CA:

- BA about 0.04-0.05%

- CA about 0.02-0.03%



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Ultra Low Carbon Steel Interstitial Free (IF) Steel

Bake Hardening (BH) Steel

Ultra Low Carbon Steel

(Solid solution strengthened steel)


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(Solid solution strengthened steel)

Re-phosphorized steel

- addition P up to 0.10 max. (normally 0.005-0.01%)

- strengthening effect ~ 10 N/mm2 per 0.01%P

- Y.S. in range 220-260 N/mm2

- rm-value ~ 1.6

IF steel (Interstitial-Free Steel)

- good cold formability

- low level of C & N content (add Ti and Nb)

IF-HSS steel

- strengthen IF steel with small additions of P, Mn, Si

- maintained rm-value ~ 2.0

- T.S. similar to Al-killed and Re-phosphorized grade

Interstitial Free (IF) steel


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Free of interstitial Carbon and Nitrogen atoms

IF steel used for producing of auto-body

The presence of interstitial atoms (C and N), lead to the

discontinuous yield behavior of steel by appearance ofLuder bands

Luder bands are usually not hidden by coating and

painting Conventional method of avoiding luder bands is by skin-

pass or temper rolling with ~2% strain (by creating new

unlocked dislocations in each of grain in steel structure)

Skin-pass process does not preclude the return of

discontinuous yield phenomenon if steel contains an

excessive amonut of interstitial elements



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Interstitial atoms are attracted by elastic strains

surrounding the dislocations, and subsequently arrive at

the dislocation core

The return of the yield point caused by the segregation ofcarbon and nitrogen atoms to the dislocation core is know

as strain aging

Strain aging produces 2 kinds of changes in mechanical

properties of steel:

- Strain age-hardening: increasing of Y.S. and T.S.

- Strain age-embrittlement: increasing of impact transition

temperature

Stretcher strain/Luder band/Yield point elongation


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Strain aging


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Strain aging



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Bake-hardened (BH) steel

Bake hardening process


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Bake-hardening process

Cold forming (auto-body) Painting Heat-treating (at 170 C

for 20 min) Increasing of Y.S. due to aging effect (~ 40-50

N/mm2)

Supply to cold-reduced conditions with Y.S. 250 N/mm2 max. BH strengthening increase with increasing solute carbon (C

content of base steel is reduced to below 0.02%)

F 170 C 20 C diffuse dislocation (F N

diffuse )F


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Advance High Strength steel or Multi-phases steel Dual Phases (DP) Steel

Transformation Induced Plasticity (TRIP) Steel

Advance High Strength steel or Multi-phases steel


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Dual Phases (DP) Steel


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After 1970s, major interest was generated in USA in low

alloy steel that were heat treated to form a mixedmicrostructures of ferrite and martensite Dual Phase

Steel Low Y.S., high work-hardening rate and high n-value

(strain hardening exponent) and elongation

Discovered of DP steel; Rashid, found mixtures of ferrite& martensite could be produced in 0.15% CNbV by

annealing in the intercritical (two phase ferrite+austenite

region, between Ac1 and Ac3), carbon can diffuse from

ferrite to austenite that level higher than nominal basecomposition which increase hardenability of austenite

(martensite can form on cooling to ambient temperature) mixtures of soft ferrite & hard martensite



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T.S. of DP steel depend on martensite content (typically~ 15%) which can develop T.S. in excess of 800 N/mm2

High n-value, low rm-value (~1.0)

DP steel can be produced in hot-rolled and cold-rolled(by continuous annealing furnace) product by apply rapid

cooling rate from intercritical annealing temperature to

form martensite structure

Addition of Si, Mn and Cr sometime incorporated in DP

in order to provide sufficient hardenability to ensure the

formation of matensite

Trend of DP steel expensive and large-scale usage



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Si (ferrite stabilizer): retard the precipitation of Fe3C

(Carbon more dissolved in austenite)

Mn: austenite stabilizer and reduce transformation

temperature



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Others High Strength Strip Steel

Work-hardened Steel


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Limited potential in area of high strength strip steel

Due to cold work increasing strength but major

loss in ductility Use in moderate forming requirement

Ductility of work-hardened steel can be improved

by heat treatment that produce recovery (recoveryannealed) or partial recrystallization

Transformation-strengthened Steel


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Can be produced structures as acicular ferrite,

bainite or martensite which depending upon

composition of the strip and cooling rate from

austenitic region

Y.S. up to 1400 N/mm2

Limited in cold formability and softening canoccur in heat affected zone (HAZ) after welding

Currently produced in very limited amounts

Documents

High Strength Low Alloy Steel (HSLA)