High Strength Low Alloy Steel (HSLA) Ultra Low Carbon Steel

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 คือวัสดุประเภทไหน ? Strength เท่าไหร่ถึงจะเรียกว่า high strength low carbon steel? High strength low carbon steels มี strengthening mechanism อย่างไร, มีอะไรบ้าง? High strength low carbon steels ใช้ทำอะไร, ใช้งานประเภทไหน ? High strength low carbon steels มีกรรมวิธีการผลิตอย่างไร ? Physical metallurgy เกี่ยวข้องกับ high strength low carbon steels ได้อย่างไร ?

Steel Plain Carbon Steel Alloy Steel Low-C steel Medium-C 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 Alloy Steel Low-C steel Medium-C steel High-C steel Low alloy steel High alloy steel 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 - 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

Iron and Steel Making Process

Semi Finished Products

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

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)

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 → “TMCP” Improving history of structural steel for: Strength, Toughness, Weldability

High Strength Low Alloy Steel (HSLA) 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. ~ N/mm2 can be produced in hot rolled state Y.S. ~ 350 N/mm2 are produced in cold-rolled strip containing %Nb

Precipitate ของ Ti สามารถป้องกันการ growth ของเกรน austenite ได้ถึงอุณหภูมิ > 1250 ºC Precipitate ของ Nb สามารถป้องกันการ growth ของเกรน austenite ได้ถึงอุณหภูมิ ºC Precipitate ของ Al สามารถป้องกันการ growth ของเกรน austenite ได้ถึงอุณหภูมิ ºC Precipitate ของ V สามารถป้องกันการ growth ของเกรน austenite ได้ถึงอุณหภูมิ ºC กลไกการเพิ่มความแข็งแรงหลักๆ ให้กับ HSLA steel คือ precipitation strengthening และ ferrite grain refining

Precipitation-Time-Temperature (PTT) Diagram ของ Nb(CN) ใน austenite หลังจากผ่านการรีดลดขนาด 50% ของความหนา ในขั้นตอนการรีดร้อน Nb(CN) เกิด dynamic precipitation ได้ดีที่อุณหภูมิ ~ 900 ºC %Mn ที่เพิ่มขึ้นมีผลให้การเกิด precipitation ช้าลง (shift PTT curve ไปทางด้านขวามือ) Ps : Precipitation start Pf : Precipitation finish

Precipitation-Time-Temperature (PTT) Diagram ของ Ti(CN) ใน austenite Ti(CN) เกิด dynamic precipitation ได้ดีที่อุณหภูมิ ~ 1025 ºC (แต่จะส่งผลต่อ No-recrystallization temperature (Tnr) น้อยกว่า Nb(CN)) %Mn ที่เพิ่มขึ้นมีผลให้การเกิด precipitation ช้าลง (shift PTT curve ไปทางด้านขวามือ เช่นเดียวกันกับในกรณีของ HSLA steel ที่มีการเติมธาตุผสม Nb)

High Strength Low Alloy Steel (HSLA)
Recystallization-Time-Temperature (RTT) Diagram ของ Nb microalloyed steel และ plain carbon steel a) แสดง recystallization rate ใน Nb microalloyed steel และ plain carbon steel b) แสดงผลกระทบของ Nb ที่อยู่ในลักษณะที่เป็น solute atom (solute effect only) ที่มีต่อ recystallization rate (ซึ่งมีผลทำให้การเกิด recystallization ช้าลง) เมื่อเปรียบเทียบกับในกรณีของ plain carbon steel c) แสดงให้เห็นว่าการเกิดการ precipitation ของ Nb(CN) มีผลต่อการหน่วง/ขัดขวางการเกิด recystallization ให้ช้าลง 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)

Nb มีอิทธิพลต่ออุณหภูมิที่ไม่มีการตกผลึกใหม่ (No-recrystallization temperature; Tnr) มากที่สุด

Controlled rolling/Thermo-mechanical processing (TMCP)
1. Outline process SRT ~ ºC Roughing rolling FT ~ 1000 ºC Hold/Delay No-recystallization temperature (Tnr) normalizing ~ 920 ºC Finishing rolling (Below Tnr) Austenite-elongated grain (pancake structure)

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 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

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

- 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

- 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 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 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 ºC Slab soaking F.T ºC 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 C.T ºC Hot coiling Hot rolled strip Pickling Thickness > 2 mm Reduction ~ 65% Cold rolling C.T ºC C.T ºC Batch annealing Continuous annealing Tin plate production Zinc coating Temper rolling (Skin-passing) ~ 2% Deformed: For control of shape, surface texture, luder lines

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 steels
Specimen: JIS 5L; Thickness: 0.8 mm

Batch Annealing (BA) SRT สูง, FT สูง เร่งการเย็นตัวลงมาที่ CT ต่ำ (~560 ºC) SRT สูง, FT สูง เพื่อให้ Al, N ละลายอยู่อย่างอิ่มตัวยิ่งยวด และทำให้เย็นเร็วสู่ CT ต่ำเพื่อกักให้ Al, N อยู่ใน solid solution ก่อนจะมา precipitate ในช่วงอบให้ร้อนขึ้นช้าๆ ของ 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 ~ - 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 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 ~ 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 %Al and %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 ต่ำ เพื่อให้ AlN ไม่ละลาย และ, CT สูง (~710 ºC) เพื่อให้ AlN โตและเกรนโตขึ้น (ลดปริมาณ nitrogen free) จากนั้นทำ continuous annealing และตามด้วย over-aging เพื่อลด carbon อิสระใน solid solution ºC (Holding for 40 sec.) ºC (Holding ~ 3 min) Heating up time < 1 min

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 ºC for 40 sec) rapid cooling and then overaging (by holding at 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 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 % - CA about %

Ultra Low Carbon Steel Interstitial Free (IF) Steel
Bake Hardening (BH) Steel

(Solid solution strengthened steel)
Ultra Low Carbon Steel (Solid solution strengthened steel) Re-phosphorized steel - addition P up to 0.10 max. (normally %) - strengthening effect ~ 10 N/mm2 per 0.01%P - Y.S. in range 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-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 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 (~ 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%) ภายหลังจากการขึ้นรูปแล้วนำไปพ่นสีและทำการอบที่อุณหภูมิ 170 ºC เป็นเวลา 20 นาที เพื่อให้ C diffuse เข้าไปขัดขวางการเคลื่อนที่ของ dislocation (ร่วมกับ N ซึ่งจะสามารถ diffuse ได้ที่อุณหภูมิห้อง) ทำให้เมื่อจะนำไปขึ้นรูปหรือใช้งานต่อไปจะทำให้ความแข็งแรงสูงขึ้น

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

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

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

High Strength Low Alloy Steel (HSLA) Ultra Low Carbon Steel

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