Metallurgy of High Strength Steel N. Yurioka

1. Metallurgy of High Strength SteelN. Yurioka Visiting Professor at Osaka University

2. Crystalline lattice structure BCC BCC BCC FCC HCP

3. Crystalline lattice structure • Face centered cubic (FCC) Steel (at high temp.), Austenitic stainless steel, Al, Cu,... • Body centered cubic (BCC) Steel (at low temp.), Ferritic stainless steel, Ti (at high temp.) • Hexagonally closed packed (HCP) Ti (at low temp.)

4. Fe-C Phase diagram Steel is an alloy of Iron and carbon Iron C < 0.02% Steel 0.02 C  0.21% Cast iron : 0.21% < C

5. Phase transformation in cooling - I

6. Pearlite (Composite of ferrite and cementite)aFe3C

7. Phase transformation in cooling - II

8. Line expansion (Dilatation)

9. Dilatometry-I

10. Dilatometry-II Transformation In heating Ac1: a to g start Ac3: a to g finish In cooling Ar3: g to a start Ar1: g to a finish In rapid cooling (quenching) Ms: M start Mf: M finish

11. Diffusion of carbon plays an important role inphase transformation

12. Microstructure of steels -I Martensite Lower bainite

13. Martensite and lower bainite

14. Microstructure of steels -II Rolling direction Upper bainite Ferrite and pearlite

15. Formation of upper bainite in cooling -I Nucleation of ferrite Growth of ferrite

16. Formation of upper bainite in cooling -II

17. Heat treatment of steels

18. Normalizing treatment of ferrite-pearlite steel Grain refining

19. Hot rolling processes

20. Microstructure of hot rolled steel As rolled Normalized TMCP-II Quenched & tempered

21. Features of steels • As rolled steel Ferrite –pearlite Low strength, Low YR • Normalized steel Grain-refined ferrite-pearlite • Higher strength and toughness • TMCP-II (controlled rolling and accelerated cooling) steel • Grain-refined ferrite + low temperature transformation product • High strength and toughness, low CE (better weldability) • Quenched and tempered steel • Tempered martensite, highest strength, high YR, high CE • (preheating) • Cautions for TMCP and QT steels: • Heat input limitation ( 4.5kJ/mm), No hot forming

22. Mild steels (JIS standard) • General structure SS series (SS400, SS490, etc…) • Welded structure SM series • Building construction SN series ( Tensile strength )

23. Steels for • Welded structures SM series

25. YR (Yield Ratio)

26. Steels for • Building construction SN series • High ratio decreases • the compliance of • structures such as • building .

27. Lamellar tear Reduction of area, RAZ in the thickness direction Reduction of P & S in steel Increase of RAz

28. Steels for • Building construction SN series

29. High strength steel • TS >= 490MPa • SM490, SM520, SM570….. • Reduction of weight of structures • Bridge, Storage tank, Pressure vessel • Submarine,…… • Increase of production efficiency • (Reduction of welding passes) • Pipeline,……. Welding of QT steel, TMCP steel Max allowable heat input 4.5kJ/mm to avoid HAZ softening, Low HAZ toughness

30. Steels for specific purposes • Lamellar tear resistant steel • Ex.Z25 grade (RA >= 25%) • Steel for very high heat input welding • Fire resistant steel • Hot-dip galvanizing crack resistant steel • Atmospheric corrosion resistant steel • (Weathering steel, SMA series)

31. Low temperature service steels • JIS SLA grade • Al-killed steel (N or QT or TMCP) • JIS SL grade • 3.5%Ni (NT, TMCP) • 5%Ni (NNT, TMCP) • 9%Ni (QQT, QLT, DQT) • Austenitic stainless steel • SUS304, SUS316 • Inver (34%Ni-Fe) Welding of low temperature steels (QT, TMCP) Low heat input welding (  35kJ/mm desired)

32.  -160oC

33. High temperature service steels • JIS G3103 SB series (C, Mo) • Boilers • JIS G3119 SBV series (Mn-Mo, Mn-Mo-Ni) • JIS G3120 SQV series (Mn-Mo, Mn-Mo-Ni) • Nuclear pressure vessels • JIS G4109 SCMV series (Cr-Mo) • 1%Cr-9%Cr • JIS 4110 SCMQ series (Cr-Mo-V-(W)) • 9-12%Cr

34. Weldability of steels

35. Welding heat input • Energy Input (AWS D1.1), Arc Energy(EN standard) • EI(J/mm) = 60 · (E·I/v) • E(V), I(A), v(mm/min) • 60·25·170/150  1700 (J/mm), 1.7(kJ/mm) • Heat Input • HI(J/mm) = h EI •  : Arc thermal efficiency 1.0 for SAW • 0.8 for SMAW, GMAW • 0.6 for autogenus TIG

36. Welding cooling rate, cooling time CR(oC/s) at 540oC t8/5(s): Cooling time between 800oC and 500oC 1.7kJ/mm on 20mm thick 7s in t8/5  

37. Cooling rate, Cooling time Heat input Plate thickness Joint shape (Butt-joint, fillet-joint) Preheat temperature Prediction of cooling time, t8/5 JWES IT-Center (http://www-it.jwes.or.jp/index_e.jsp)

38.  45mm

39. Microstructure of HAZ Normalizing heat treatment

40. CCT (Continuous Cooling Transformation) diagram

41. Cooling curve (log-scale)

42. CCT (Low-hardenability)

43. CCT (high hardenability)

44. HAZ maximum hardness

45. Hardness change against t8/5

46. Change in HAZ maximum hardness Martensite hardness = f(C) Hardenability Carbon equivalent CEIIW CEWES

47. Prediction of HAZ hardness • Welding conditions • Heat input • Plate thickness • Preheat temperature t8/5 HAZ hardness • Chemical composition of steel • C • Carbon Equivalent JWES IT-Center (http://www-it.jwes.or.jp/index_e.jsp)

48. Carbon equivalent CEIIW = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 CEWES = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14

49. Weld cracking • Hot cracking (>1200oC) • Solidification cracking • Liquation cracking • Cold cracking (<100oC) • (Hydrogen assisted cracking)

50. Hot cracking Solidification crack Liquation crack Stainless steel, Al