High Strength Low Alloy Steel (hsla) 6j2019
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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 (TM) • 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 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.
Plain Carbon Steel Low-C steel
Alloy Steel
Medium-C steel
High-C steel
Low alloy steel
High alloy steel
C ≤ 0.2%
C = 0.2 – 0.5 %
C > 0.5%
Alloy elements ≤ 10%
Alloy elements > 10%
Flat products (rolled)
Machine parts
Tool steels
(some data: ≤ 5%)
(some data: > 5%)
Structural (rolled)
(Heat treatable)
(Wear, Abrasion, Heat resisting, Corrosion applications)
Applications - Body parts in automotive industry - Construction of building, bridge, pipeline, etc.
High strength low carbon steels - Cold-reduced products: YS > 220 MPa, TS > 330 MPa - Hot rolled products: YS > 280 MPa, TS > 370 MPa
Strengthening Mechanisms - Solid solution strengthening - Precipitation strengthening - Dislocation strengthening (Work hardening) - Transformation strengthening (Heat treatment) - Refining the ferrite grain size (Grain size effects)
Produced lighter wt. and higher strength
General Steel Production Process
General Steel Production Process
Iron and Steel Making Process
Semi Finished Products
Overview
Overview
Overview
Relation between tensile strength and elongation of HSS Currently, high strength steel products whose microstructure is reinforced for greater strength have been used. (DP steel, TRIP steel)
Conventional high strength sheet steel for automobiles used to be solid solution-hardened steel or precipitation-hardened steel with micro-alloy added.
Chemical compositions (mass%) and mechanical properties of the steels
Type of steel
Mn
Ti
0.05 0.01 0.24
-
241
384
43
B Solid solution 0.08 0.02 1.46 hardened steel
-
370
487
30
C DP steel
-
432
618
27
D Precipitation 0.09 0.01 0.80 0.07 hardened steel
539
636
22
E TRIP steel
510
644
37
A Mild steel
C
Si
Yield Tensile Elonstrength strength gation (Mpa) (Mpa) (%)
0.05 0.89 1.25
0.15 1.48 0.99
-
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-Pearlite microstructures) • 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 → “TM” • Improving history of structural steel for: Strength, Toughness, Weldability
High Strength Low Alloy Steel (HSLA) And Thermo-mechanical Processing (TM)
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)
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • 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
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • Precipitate ) Ti *กก growth ) ก austenite + ,# > 1250 ºC • Precipitate ) Nb *กก growth ) ก austenite + ,# 1150 ºC • Precipitate ) Al *กก growth ) ก austenite + ,# 1100 ºC • Precipitate ) V *กก growth ) ก austenite + ,# 1000 ºC
ก&กก -#(.)/. &ก0 ก HSLA steel precipitation strengthening .& ferrite grain refining
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel)
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel)
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) Precipitation-Time-Temperature (PTT) Diagram ) Nb(CN) austenite &ก%ก
&) 50% ) )1 'ก
• Nb(CN) ก# dynamic precipitation (+ ,# ~ 900 ºC • %Mn ( -#()1 %& ก ก# precipitation !& (shift PTT curve )) • Ps : Precipitation start • Pf : Precipitation finish
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) Precipitation-Time-Temperature (PTT) Diagram ) Ti(CN) austenite • Ti(CN) ก# dynamic precipitation (+ ,# ~ 1025 ºC (.'%&' No-recrystallization temperature (Tnr) ก Nb(CN)) • %Mn ( -#()1 %& ก ก# precipitation !& (shift PTT curve ) ! กก ก +) HSLA steel (ก '#$'% Nb)
High Strength Low Alloy Steel (HSLA) Recystallization-Time-Temperature (RTT) Diagram ) Nb microalloyed steel .& plain carbon steel a) . recystallization rate Nb microalloyed steel .& plain carbon steel 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 !& 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)
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • Nb #$#-&'+ ,#(ก 'ก %&ก (No-recrystallization temperature; Tnr) ก(
Controlled rolling/Thermo-mechanical processing (TM) 1. Outline process SRT ~ 1200-1250 ºC
FT ~ 1000 ºC
Roughing rolling Hold/Delay
normalizing ~ 920 ºC
No-recystallization temperature (Tnr) Finishing rolling (Below Tnr)
Austenite-elongated grain (pancake structure)
Controlled rolling/Thermo-mechanical processing (TM) 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)
Controlled rolling/Thermo-mechanical processing (TM) 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
Controlled rolling/Thermo-mechanical processing (TM) 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 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
Controlled rolling/Thermo-mechanical processing (TM) 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 recrystallization temperature than Al and V • At temperature between recrystallization temperature & Ar3 - temperature below 950 ºC
Controlled rolling/Thermo-mechanical processing (TM) 3. Rolling - strain induced precipitation of Nb(CN) or TiC is sufficient rapid to prevent recrystallization before the next (deformed-austenite providing nucleation sites of carbonitride 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
Controlled rolling/Thermo-mechanical processing (TM) 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)
Controlled rolling/Thermo-mechanical processing (TM)
Controlled rolling/Thermo-mechanical processing (TM)
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 (handoperated 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 , 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
Basic oxygen steelmaking (BOS) Secondary steelmaking (e.g. vacuum degassing) Al-killed steel (significant effect for good formability)
Ingot casting
Continuous casting AlN dissolved into solid solution and remain in this state after completion of hot rolling
At 1200-1250 ºC
Slab soaking F.T. 870-910 ºC
Hot rolling C.T. 710 ºC for CA: cool very slowly and have opportunity to precipitated of AlN
C.T. 560-710 ºC
Hot coiling
C.T. 560 ºC for BA: cool quickly and precipitated of AlN is suppressed and remain in solid solution on cooling to ambient temperature
Hot rolled strip
Pickling
Thickness > 2 mm Reduction ~ 65%
Cold rolling C.T. 560 ºC
C.T. 710 ºC
Batch annealing
Continuous annealing
Tin plate production
Temper rolling (Skin-ing) ~ 2% Deformed: For control of shape, surface texture, luder lines
Zinc coating
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 workSpecimen: JIS 5L; Thickness: 0.8 mm hardening coefficient)
Formability of high-strength strip steels
Formability of high-strength strip steels Specimen: JIS 5L; Thickness: 0.8 mm
Batch Annealing (BA) • SRT ,, FT , ก /'&( CT '(" (~560 ºC) SRT ,, FT , -( Al, N &&,#('#( .&" / /, CT '(" -(กก Al, N , solid solution ก precipitate ! )1!0 ) batch annealing
~ 700 ºC
Batch Annealing (BA) • 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
Batch Annealing (BA) • 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)
Batch Annealing (BA) - 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) • SRT '(" -( AlN && .&, CT , (~710 ºC) -( AlN 9'.& ก 9')1 (& #+ nitrogen free) ก1" continuous annealing .&' over-aging -(& carbon # solid solution
700-850 ºC (Holding for 40 sec.)
400-450 ºC (Holding ~ 3 min)
Heating up time < 1 min
Continuous Annealing (CA) • 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
Continuous Annealing (CA) • 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
Continuous Annealing (CA) • Due to rapid cooling rate that has little time for carbide precipitation and growth, therefore, overaging 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
Interstitial Free (IF) steel • 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 or temper rolling with ~2% strain (by creating new unlocked dislocations in each of grain in steel structure) • Skin- 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
Interstitial Free (IF) steel
Interstitial Free (IF) steel
Interstitial Free (IF) steel
Interstitial Free (IF) steel
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
Advance High Strength steel or Multi-phases steel
Advance High Strength steel or Multi-phases steel
Advance High Strength steel or Multi-phases 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
Dual Phases (DP) Steel • 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
Dual Phases (DP) Steel
Dual Phases (DP) Steel
Transformation Induced Plasticity (TRIP) Steel
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
Transformation Induced Plasticity (TRIP) Steel
Transformation Induced Plasticity (TRIP) Steel
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
Outline • Overviews • Low carbon structural steel • High strength low alloy steel (HSLA)/Micro-alloy steel and Thermo-mechanical control process (TM) • 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 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.
Plain Carbon Steel Low-C steel
Alloy Steel
Medium-C steel
High-C steel
Low alloy steel
High alloy steel
C ≤ 0.2%
C = 0.2 – 0.5 %
C > 0.5%
Alloy elements ≤ 10%
Alloy elements > 10%
Flat products (rolled)
Machine parts
Tool steels
(some data: ≤ 5%)
(some data: > 5%)
Structural (rolled)
(Heat treatable)
(Wear, Abrasion, Heat resisting, Corrosion applications)
Applications - Body parts in automotive industry - Construction of building, bridge, pipeline, etc.
High strength low carbon steels - Cold-reduced products: YS > 220 MPa, TS > 330 MPa - Hot rolled products: YS > 280 MPa, TS > 370 MPa
Strengthening Mechanisms - Solid solution strengthening - Precipitation strengthening - Dislocation strengthening (Work hardening) - Transformation strengthening (Heat treatment) - Refining the ferrite grain size (Grain size effects)
Produced lighter wt. and higher strength
General Steel Production Process
General Steel Production Process
Iron and Steel Making Process
Semi Finished Products
Overview
Overview
Overview
Relation between tensile strength and elongation of HSS Currently, high strength steel products whose microstructure is reinforced for greater strength have been used. (DP steel, TRIP steel)
Conventional high strength sheet steel for automobiles used to be solid solution-hardened steel or precipitation-hardened steel with micro-alloy added.
Chemical compositions (mass%) and mechanical properties of the steels
Type of steel
Mn
Ti
0.05 0.01 0.24
-
241
384
43
B Solid solution 0.08 0.02 1.46 hardened steel
-
370
487
30
C DP steel
-
432
618
27
D Precipitation 0.09 0.01 0.80 0.07 hardened steel
539
636
22
E TRIP steel
510
644
37
A Mild steel
C
Si
Yield Tensile Elonstrength strength gation (Mpa) (Mpa) (%)
0.05 0.89 1.25
0.15 1.48 0.99
-
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-Pearlite microstructures) • 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 → “TM” • Improving history of structural steel for: Strength, Toughness, Weldability
High Strength Low Alloy Steel (HSLA) And Thermo-mechanical Processing (TM)
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)
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • 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
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • Precipitate ) Ti *กก growth ) ก austenite + ,# > 1250 ºC • Precipitate ) Nb *กก growth ) ก austenite + ,# 1150 ºC • Precipitate ) Al *กก growth ) ก austenite + ,# 1100 ºC • Precipitate ) V *กก growth ) ก austenite + ,# 1000 ºC
ก&กก -#(.)/. &ก0 ก HSLA steel precipitation strengthening .& ferrite grain refining
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel)
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel)
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) Precipitation-Time-Temperature (PTT) Diagram ) Nb(CN) austenite &ก%ก
&) 50% ) )1 'ก
• Nb(CN) ก# dynamic precipitation (+ ,# ~ 900 ºC • %Mn ( -#()1 %& ก ก# precipitation !& (shift PTT curve )) • Ps : Precipitation start • Pf : Precipitation finish
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) Precipitation-Time-Temperature (PTT) Diagram ) Ti(CN) austenite • Ti(CN) ก# dynamic precipitation (+ ,# ~ 1025 ºC (.'%&' No-recrystallization temperature (Tnr) ก Nb(CN)) • %Mn ( -#()1 %& ก ก# precipitation !& (shift PTT curve ) ! กก ก +) HSLA steel (ก '#$'% Nb)
High Strength Low Alloy Steel (HSLA) Recystallization-Time-Temperature (RTT) Diagram ) Nb microalloyed steel .& plain carbon steel a) . recystallization rate Nb microalloyed steel .& plain carbon steel b) .%&ก ) Nb (, &ก4+( 5 solute atom (solute effect only) (' recystallization rate (7(%&" ก ก# recystallization !&) ( ก ก + ) plain carbon steel c) . /ก ก#ก precipitation ) Nb(CN) %&'ก /))ก ก# recystallization !& Rs: Recystallization start, Rf: Recystallization finish Ps: Precipitation start, Pf: Precipitation finish (C): for plain carbon steel (S): for Nb microalloyed steel (solute effect only) (Nb): for Nb microalloyed steel (precipitation effect)
High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • Nb #$#-&'+ ,#(ก 'ก %&ก (No-recrystallization temperature; Tnr) ก(
Controlled rolling/Thermo-mechanical processing (TM) 1. Outline process SRT ~ 1200-1250 ºC
FT ~ 1000 ºC
Roughing rolling Hold/Delay
normalizing ~ 920 ºC
No-recystallization temperature (Tnr) Finishing rolling (Below Tnr)
Austenite-elongated grain (pancake structure)
Controlled rolling/Thermo-mechanical processing (TM) 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)
Controlled rolling/Thermo-mechanical processing (TM) 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
Controlled rolling/Thermo-mechanical processing (TM) 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 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
Controlled rolling/Thermo-mechanical processing (TM) 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 recrystallization temperature than Al and V • At temperature between recrystallization temperature & Ar3 - temperature below 950 ºC
Controlled rolling/Thermo-mechanical processing (TM) 3. Rolling - strain induced precipitation of Nb(CN) or TiC is sufficient rapid to prevent recrystallization before the next (deformed-austenite providing nucleation sites of carbonitride 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
Controlled rolling/Thermo-mechanical processing (TM) 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)
Controlled rolling/Thermo-mechanical processing (TM)
Controlled rolling/Thermo-mechanical processing (TM)
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 (handoperated 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 , 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
Basic oxygen steelmaking (BOS) Secondary steelmaking (e.g. vacuum degassing) Al-killed steel (significant effect for good formability)
Ingot casting
Continuous casting AlN dissolved into solid solution and remain in this state after completion of hot rolling
At 1200-1250 ºC
Slab soaking F.T. 870-910 ºC
Hot rolling C.T. 710 ºC for CA: cool very slowly and have opportunity to precipitated of AlN
C.T. 560-710 ºC
Hot coiling
C.T. 560 ºC for BA: cool quickly and precipitated of AlN is suppressed and remain in solid solution on cooling to ambient temperature
Hot rolled strip
Pickling
Thickness > 2 mm Reduction ~ 65%
Cold rolling C.T. 560 ºC
C.T. 710 ºC
Batch annealing
Continuous annealing
Tin plate production
Temper rolling (Skin-ing) ~ 2% Deformed: For control of shape, surface texture, luder lines
Zinc coating
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 workSpecimen: JIS 5L; Thickness: 0.8 mm hardening coefficient)
Formability of high-strength strip steels
Formability of high-strength strip steels Specimen: JIS 5L; Thickness: 0.8 mm
Batch Annealing (BA) • SRT ,, FT , ก /'&( CT '(" (~560 ºC) SRT ,, FT , -( Al, N &&,#('#( .&" / /, CT '(" -(กก Al, N , solid solution ก precipitate ! )1!0 ) batch annealing
~ 700 ºC
Batch Annealing (BA) • 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
Batch Annealing (BA) • 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)
Batch Annealing (BA) - 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) • SRT '(" -( AlN && .&, CT , (~710 ºC) -( AlN 9'.& ก 9')1 (& #+ nitrogen free) ก1" continuous annealing .&' over-aging -(& carbon # solid solution
700-850 ºC (Holding for 40 sec.)
400-450 ºC (Holding ~ 3 min)
Heating up time < 1 min
Continuous Annealing (CA) • 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
Continuous Annealing (CA) • 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
Continuous Annealing (CA) • Due to rapid cooling rate that has little time for carbide precipitation and growth, therefore, overaging 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
Interstitial Free (IF) steel • 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 or temper rolling with ~2% strain (by creating new unlocked dislocations in each of grain in steel structure) • Skin- 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
Interstitial Free (IF) steel
Interstitial Free (IF) steel
Interstitial Free (IF) steel
Interstitial Free (IF) steel
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
Advance High Strength steel or Multi-phases steel
Advance High Strength steel or Multi-phases steel
Advance High Strength steel or Multi-phases 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
Dual Phases (DP) Steel • 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
Dual Phases (DP) Steel
Dual Phases (DP) Steel
Transformation Induced Plasticity (TRIP) Steel
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
Transformation Induced Plasticity (TRIP) Steel
Transformation Induced Plasticity (TRIP) Steel
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
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