Lecture of HSLA, Adv HSS and Ultra low C steel Completedmech-india.yolasite.com/resources/HSLA_Adv_HSS_Ultra_low_C.pdf · Chemical compositions (mass%) and mechanical properties of

High Strength Low Alloy Steel (HSLA)

Ultra Low Carbon Steel

Advance High Strength Steel

By

Panya Buahombura

School of Metallurgical Engineering

Suranaree University of Technology

Outline• Overviews

• Low carbon structural steel

• High strength low alloy steel (HSLA)/Micro-alloy steel and 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

Overviews

Overviews: Low carbon structural steel

and low carbon strip steel

• High strength low carbon steels �� ?

• Strength ��ก�� high strength low carbon steel?

• High strength low carbon steels �� strengthening mechanism ��, ��?

• High strength low carbon steels !��"��, !�� ?

• High strength low carbon steels ��ก��#$�ก�%&#'�� ?

• Physical metallurgy �ก�(��)��ก�� high strength low carbon steels �� ?

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)

General Steel Production Process

General Steel Production Process

Iron and Steel Making Process

Semi Finished Products

Overview

Overview

Overview

Conventional high strength sheet steel for automobiles used to be solid solution-hardened steel or precipitation-hardened steel with micro-alloy added.

Currently, high strength steel products whose microstructure is reinforced for greater strength have been used.

(DP steel, TRIP steel)

Relation between tensile strength and elongation of HSS

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 22hardened steel

E TRIP steel 0.15 1.48 0.99 - 510 644 37

Overview

Overview

Strengthening Mechanisms

• Refining the ferrite grain size

(Grain size effect)

• Solid solution strengthening

• Precipitation strengthening

• Dislocation strengthening/Work hardening

• Transformation strengthening

Refining the ferrite grain size

(Grain size effect)

Refining the ferrite grain size

(Grain size effect)

Solid solution strengthening

Precipitation strengthening

Low Carbon Structural Steel

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 which quenched & tempered, Y.S. up to 700 N/mm2)

• Applications: building, bridges, pressure vessels, ships, offshore oil & gas platforms, pipeline (for weldability and toughness which required low-carbon)

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

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

Overview: Low Carbon Structural Steel

• For higher strength steel, required precipitation strengthening by small addition of Nb, V, Ti to structural steel → Y.S. up to 500 N/mm2 (known

as “Micro-alloy steel” or “HSLA steel”)

• After 1950s and 1960s, new technique to produce structural steel → “Control Rolling”(fine-grained in as rolled conditions which eliminating of normalizing heat treatment)

• 1970s and 1980s, Control Rolling + Controlled Cooling → “TMCP”

• Improving history of structural steel for: Strength, Toughness, Weldability


And

Thermo-mechanical Processing (TMCP)

High Strength Low Alloy Steel (HSLA) (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 as

“High Strength Low Alloy (HSLA) steel”

• At slab soaking temperature ~ 1200 ºC

- 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)

• 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



• Precipitate )�� Ti ��*��ก��ก� growth )��ก� austenite ��+� ,�# > 1250 ºC

• Precipitate )�� Nb ��*��ก��ก� growth )��ก� austenite ��+� ,�# 1150 ºC

• Precipitate )�� V ��*��ก��ก� growth )��ก� austenite ��+� ,�# 1000 ºC

• Precipitate )�� Al ��*��ก��ก� growth )��ก� austenite ��+� ,�# 1100 ºC

ก&�กก��-#(��.)/�.��&�ก0 ��ก�� HSLA steel �� precipitation strengthening .&� ferrite grain refining




Precipitation-Time-Temperature (PTT) Diagram )�� Nb(CN) � austenite �&��ก%��ก��&�)�� 50% )�� )�1�'��ก��

• %Mn ��(�-#(�)�1��%& ��ก��ก#� precipitation !��&� (shift PTT curve ��)��)

• Nb(CN) �ก#� dynamic

precipitation ��(��+� ,�# ~ 900 ºC

• Ps : Precipitation start

• Pf : Precipitation finish


• Ti(CN) �ก#� dynamic

precipitation ��(��+� ,�# ~ 1025 ºC (.'��%&'�� No-recrystallizationtemperature (Tnr) ��ก�� Nb(CN))

• %Mn ��(�-#(�)�1��%& ��ก��ก#� precipitation !��&� (shift PTT curve ��)�� !��ก��ก�� ก+�)�� HSLA steel ��(��ก��'#�$�'�%�� Nb)

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


a) .�� recystallization rate � Nb microalloyed steel .&� plain carbon 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) .��%&ก��)�� Nb ��(��,� �&�ก4+��(�5� solute atom (solute effect only) ��(��'�� recystallization rate (7�(��%&�"� ��ก��ก#� recystallization !��&�) ��(��ก�� ก+�)�� plain carbon steel

c) .�� /��ก��ก#�ก� precipitation )�� Nb(CN) ��%&'��ก��/)��)��ก��ก#� recystallization ��!��&�


• Nb ��#�$#-&'��+� ,�#��(��ก�'ก%&�ก �� (No-recrystallizationtemperature; Tnr) ��ก��(��

Controlled rolling/Thermo-mechanical processing (TMCP)

1. Outline processSRT ~ 1200-1250 ºC

FT ~ 1000 ºC

normalizing ~ 920 ºC

Hold/Delay

Roughing rolling

Finishing rolling

(Below Tnr) Austenite-elongated grain

(pancake structure)

No-recystallization temperature (Tnr)

2. Slab Reheating

• Importance of slab reheating stage

- control amount of micro-alloying element taken into solution

- starting grain size

• Re-solution temperature of micro-alloy precipitates

- VC: complete solution ~ 920 ºC (normalizing temp.)

- VN: at somewhat higher temperature

- Nb(CN), AlN and TiN: around 1150-1300 ºC

- TiN (most stable compound) little dissolution at normal slab reheating temperature (SRT)


2. Slab Reheating

• Un-dissolved fine carbo-nitride (CN) particles

- maintain fine austenite grain size at slab reheating stage

• Micro-alloying elements taken into solution (which can be influence in later stage in process)

- control of recrystallization

- precipitation strengthening

• Multiple micro-alloy additions for above dual requirements


3. Rolling• Three distinct stages during controlled rolling.

- Deformation in the recrystallization (austenite phase) temperature range just below SRT

- Deformation in temperature range between recrystallization temperature and Ar3- Deformation in 2 phase (austenite-ferrite) temperature range between Ar3 & Ar1

• At temperature just below SRT

- rate of recrystallization is rapid

- provided the strain per pass exceeds a minimum critical level

- recrystallization is retarded by presence of solute atom Al, Nb, Ti, V (solute drag) → strain induced precipitation →form fine carbonitride during rolling process


3. Rolling

- rolling temperature decrease, recrystallization more difficult and reach a stage “recrystallization stop temperature (Trs or No-recrystallization temperature; Tnr)”(the temperature at which recrystallization is complete after 15 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 ºC


3. Rolling- strain induced precipitation of Nb(CN) or TiC is sufficient rapid to prevent recrystallization before the next pass (deformed-austenite providing nucleation sites of carbo-nitride precipitation and pins the substructure which inhibits recrystallization)

- finishing rolling below recystallizaion stop temperature

- can be obtain elongated-pancake morphology in the austenite structure

• At temperature between Ar3 & Ar1- further grain refinement

- mixed structures of polygonal-ferrite (transformed from deformed-austenite) and deformed-austenite during rolling process


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

- thickness of pancake-austenite grain

- alloying elements depress the austenite to ferrite transformation 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)




Low Carbon Strip Steel

Overview: Low Carbon Strip Steel

• The first hot strip mill was commissioned in 1923 in USA

- revolutionized steel industry and market for strip products

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

• 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 BA and CA furnace

Overview: Low Carbon Strip Steel

• 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 vehicle weigth, improve fuel consumption, corrosion in vehicle (increase in use of Zn-coated steel ~ 70% of strip required of most motor car)

• Building industry

- organic-coated

- galvanized sheet for architectural roofing, cladding

Process route

Continuous casting

Slab soaking

At 1200-1250 ºC

F.T. 870-910 ºC

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 ºC

C.T. 710 ºCC.T. 560 ºC

Reduction ~ 65%Thickness > 2 mm

AlN dissolved into solid solution and remain

in this state after completion of hot rolling

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

C.T. 560 ºC for BA: cool quickly and precipitated of AlN is suppressed and

remain in solid solution on cooling to ambient temperature

Basic oxygen steelmaking (BOS)

Ingot casting

Secondary steelmaking (e.g. vacuum degassing)

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

Temper rolling (Skin-passing)

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

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

Batch Annealing (BA)

~ 700 ºC

• SRT �,�, FT �,� ��ก��/�'��&��( CT '("� (~560 ºC)

SRT �,�, FT �,� �-�(� �� Al, N &�&��,��#(�'��#(�� .&��"� ��/��/��,� CT '("��-�(�ก�ก �� Al, N ��,� � solid solution ก�� precipitate �!�� )�1�!��0 )�� batch annealing

• Deep drawing characteristic of low-carbon strip

are 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 favorable

texture

- large ferrite-grain size


• Al must be present in steel in solid solution prior to

annealing (BA) which will be coiled at low temperature

(560 ºC) 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 ºC (close to Ac1) which

recrystallization of cold worked structure will take

place in temperature range 500-550 ºC

- 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)


- 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


Continuous Annealing (CA)

700-850 ºC (Holding for 40 sec.)

400-450 ºC (Holding ~ 3 min)

Heating up time < 1 min

• SRT '("� �-�(� �� AlN ��&�&�� .&�, CT �,� (~710 ºC) �-�(� �� AlN 9'.&��ก�9')�1� (&�#��+ nitrogen free) ��ก��1��"� continuous annealing .&�'�� over-aging �-�(�&� carbon �#�� solid solution

• First application of CA by Armco Steel Corporation in USA for hot dip galvanized steel in 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


• Heat treatment cycle of CA

- rapid heating (less than 1 min), short soaking time (at 700-850 ºC for 40 sec) rapid cooling and then overaging (by holding at 400-450 ºC up to 3 min)

- process completed in 4-8 min

• Due to fast heating rate in CA, N would be remained in solid solution and lead to increase strength, reduced formability an susceptibility to strain aging

• In order to reduce level of N in solid solution, HB materials for CA will coiled at high temperatures (up to 710 ºC) to cool slowly in coil form and precipitate AlN and remove N from solid solution


• Due to rapid cooling rate that has little time for

carbide precipitation and growth, therefore, over-

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

will combine into the cycle in order to reduce C

content to low level

• Carbon content proper for BA and CA:

- BA about 0.04-0.05%

- CA about 0.02-0.03%


Ultra Low Carbon Steel• Interstitial Free (IF) Steel

• Bake Hardening (BH) Steel

Ultra Low Carbon Steel

(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

• 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 of

“Luder 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

Interstitial Free (IF) steel

• 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 of

carbon 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

Strain aging





Bake-hardened (BH) steel

• Bake-hardening process

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

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%)

• ��&��กก�)�1�,.&��"��-��.&��"�ก��(��+� ,�# 170 ºC �5��&� 20 �� -�(� �� C diffuse �)��)��)��ก��&�(��()�� dislocation (��ก�� N 7�(�� diffuse ��(��+� ,�#��) �"� ��(��"��)�1�,�� !��'��"� ��.)/�.��,�)�1�

Advance High Strength steel or Multi-phases steel

• Dual Phases (DP) Steel

• Transformation Induced Plasticity (TRIP) Steel




Dual Phases (DP) Steel

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

alloy steel that were heat treated to form a mixed microstructures 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 base

composition which increase hardenability of austenite

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

• 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




Transformation Induced Plasticity (TRIP) Steel


Si (ferrite stabilizer): retard the precipitation of Fe3C

(Carbon more dissolved in austenite)

Mn: austenite stabilizer and reduce transformation

temperature



Others High Strength Strip Steel

Work-hardened Steel

• 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 (recovery

annealed) or partial recrystallization

Transformation-strengthened Steel

• 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 can

occur in heat affected zone (HAZ) after welding

• Currently produced in very limited amounts

Documents

Lecture of HSLA, Adv HSS and Ultra low C steel Completedmech-india.yolasite.com/resources/HSLA_Adv_HSS_Ultra_low_C.pdf · Chemical compositions (mass%) and mechanical properties of