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

1. 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

2. 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

3. Overviews

4. Overviews: Low carbon structural steel and low carbon strip steel • High strength low carbon steelsคือวัสดุประเภทไหน ? • Strengthเท่าไหร่ถึงจะเรียกว่า high strength low carbon steel? • High strength low carbon steelsมี strengthening mechanismอย่างไร, มีอะไรบ้าง? • High strength low carbon steelsใช้ทำอะไร, ใช้งานประเภทไหน ? • High strength low carbon steelsมีกรรมวิธีการผลิตอย่างไร ? • Physical metallurgyเกี่ยวข้องกับ high strength low carbon steelsได้อย่างไร ?

5. 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

6. General Steel Production Process

7. General Steel Production Process

8. Iron and Steel Making Process

9. Semi Finished Products

10. Overview

11. Overview

12. Overview

13. 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

14. Chemical compositions (mass%) and mechanical properties of the steels

15. Overview

16. Overview

17. Strengthening Mechanisms • Refining the ferrite grain size (Grain size effect) • Solid solution strengthening • Precipitation strengthening • Dislocation strengthening/Work hardening • Transformation strengthening

18. Refining the ferrite grain size(Grain size effect)

19. Refining the ferrite grain size(Grain size effect)

20. Solid solution strengthening

21. Precipitation strengthening

22. Low Carbon Structural Steel

23. 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)

24. 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

25. High Strength Low Alloy Steel (HSLA) And Thermo-mechanical Processing (TMCP)

26. 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)

27. 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)

28. Precipitate ของ Ti สามารถป้องกันการ growth ของเกรน austenite ได้ถึงอุณหภูมิ > 1250 ºC High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • Precipitate ของ Nb สามารถป้องกันการ growth ของเกรน austenite ได้ถึงอุณหภูมิ 1150 ºC • Precipitate ของ Al สามารถป้องกันการ growth ของเกรน austenite ได้ถึงอุณหภูมิ 1100 ºC • Precipitate ของ V สามารถป้องกันการ growth ของเกรน austenite ได้ถึงอุณหภูมิ 1000 ºC กลไกการเพิ่มความแข็งแรงหลักๆ ให้กับ HSLA steel คือ precipitation strengthening และ ferrite grain refining

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

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

31. Precipitation-Time-Temperature (PTT) Diagram ของ Nb(CN) ใน austenite หลังจากผ่านการรีดลดขนาด 50% ของความหนา ในขั้นตอนการรีดร้อน High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • Nb(CN) เกิด dynamic precipitation ได้ดีที่อุณหภูมิ ~ 900 ºC • %Mn ที่เพิ่มขึ้นมีผลให้การเกิด precipitation ช้าลง (shift PTT curve ไปทางด้านขวามือ) • Ps : Precipitation start • Pf : Precipitation finish

32. Precipitation-Time-Temperature (PTT) Diagram ของ Ti(CN) ใน austenite High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • Ti(CN) เกิด dynamic precipitation ได้ดีที่อุณหภูมิ ~ 1025 ºC (แต่จะส่งผลต่อ No-recrystallization temperature (Tnr) น้อยกว่า Nb(CN)) • %Mn ที่เพิ่มขึ้นมีผลให้การเกิด precipitation ช้าลง (shift PTT curve ไปทางด้านขวามือเช่นเดียวกันกับในกรณีของ HSLA steel ที่มีการเติมธาตุผสม Nb)

33. 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 High Strength Low Alloy Steel (HSLA) 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)

34. High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel) • Nb มีอิทธิพลต่ออุณหภูมิที่ไม่มีการตกผลึกใหม่ (No-recrystallization temperature; Tnr)มากที่สุด

35. 1. Outline process SRT ~ 1200-1250 ºC Controlled rolling/Thermo-mechanical processing (TMCP) Roughing rolling FT ~ 1000 ºC Hold/Delay No-recystallization temperature (Tnr) normalizing ~ 920 ºC Finishing rolling (Below Tnr) Austenite-elongated grain (pancake structure)

36. Controlled rolling/Thermo-mechanical processing (TMCP) 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)

37. Controlled rolling/Thermo-mechanical processing (TMCP) 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

38. Controlled rolling/Thermo-mechanical processing (TMCP) • 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

39. Controlled rolling/Thermo-mechanical processing (TMCP) - 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 3. Rolling

40. Controlled rolling/Thermo-mechanical processing (TMCP) - 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 3. Rolling

41. Controlled rolling/Thermo-mechanical processing (TMCP) • 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) 4. Transformation to ferrite

42. Controlled rolling/Thermo-mechanical processing (TMCP)

43. Controlled rolling/Thermo-mechanical processing (TMCP)

44. Low Carbon Strip Steel

45. 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

46. 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

47. Basic oxygen steelmaking (BOS) Process route 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. 560-710 ºC 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 Hot coiling 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 Zinc coating Temper rolling (Skin-passing) ~ 2% Deformed: For control of shape, surface texture, luder lines

48. 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

49. Formability of high-strength strip steels

50. Specimen: JIS 5L; Thickness: 0.8 mm Formability of high-strength strip steels