high strength steel

1. DEVELOPMENT OF HIGH STRENGTH HOT
ROLLED STEELS FOR STRAIN-BASED DESIGN
IN LINEPIPE APPLICATIONS
April 17, 2016

2. No. 1
• Background
• Strain-based design concept
• Stress-strain curve
• Controlled rolling
• Controlled rolling concept
• Recrystallization stop temperature
• Manufacturing process and results :
• Rolling concept
• Behavior of tensile properties
• Microstructure
• Summary
CONTENTS

3. No. 2
• Strain-based design concept in development of line pipe steels
• Seismically active areas
• Arctic regions subject to frost-heave and thaw settlement cycles
• Buried pipeline is pressurized
• Experience strains due to displacement or bending caused by mud slides
or thermal expansion effects
• Required mechanical properties
• Excellent uniform elongation
• A higher strain hardening
• A low YR in the longitudinal direction of pipe
• Toughness
BACKGROUND

4. No. 3
Yield strength
YR =
Tensile strength
RELATIONSHIP BETWEEN YIELD AND TENSILE STRENGTH
LIMITS IN API-5L PSL2
TS_max
TS_min
YS_max
YS_min

5. No. 4
• Strain hardening effect :
Ability of strain distribution more uniformly
stress
n
TYPICAL STRESS-STRAIN CURVE
Positive slope indicating increase in
resistance after yielding
UTS
YS

6. No. 5
• Yield strength depends on σ-ε curve : microstructure, volume fraction of second
phase
DISCONTINUOUS & CONTINUOUS STRESS-STRAIN CURVE

7. No. 6
0
1000
2000
3000
4000
5000
6000
7000
8000
0 2 4 6 8 10
Load
(kgf)
Displacement (mm)
Load vs Displacement
F+AF+P F+M F+P
• Different type of microstructure : PF+AF, PF+M(dual phase), PF+P
BEHAVIOUR OF LOAD & DISPLACEMENT

8. No. 7
0
1000
2000
3000
4000
5000
6000
7000
8000
0 0.5 1 1.5 2 2.5 3
Load
(kgf)
Displacement (mm)
Load vs Displacement
F+AF+P F+M F+P
• Yield strength depends on the microstructure
BEHAVIOUR OF LOAD & DISPLACEMENT

9. No. 8
• Example for ferrite-pearlite steels containing up to 0.2 wt% C
- σy, MPa = 53.9 + 32.3·CMn + 83.2·CSi +354·CMn + 17.4·d-1/2
- σt, MPa = 294 +27.7·CMn + 83.2·CSi + 3.85·Cpearlite + 7.7·d-1/2
- ITT, °C = 19 + 44·CSi + 700·√CN + 2.2·Cpearlite – 11.5·d-1/2
σy, : yield strength, σt : tensile strength
CMn, CSi, CN, Cpearlite : weight % of Mn, Si, free soluble N and pearlite %, respectively
ITT : impact transition temperature
* Strength of pearlite is influenced by its interlamellar spacing
HALL-PETCH AND SOLID SOLUTION EQUATION

10. No. 9
FACTORS CONTRIBUTING TO THE STRENGTH
• Tensile strength
• Second phase volume fraction : Acicular ferrite, bainitic ferrite, bainite, martensite
• Pearlite : interlamellar spacing
• Ferrite grain size
• Solid solution hardening by Mn
• Yield strength
• Precipitation hardening by microalloy elements : Nb, Ti, V
• Ferrite grain size

11. No. 10
• Refining austenite and ferrite grain size
• Recrystallized austenite grain in roughing stage
• Deformation temperature effect on recrystallization
• Non recrystallization temperature : Tnr
• Recrystallization stop temperature : Tr
• Austenite conditioning in finishing stage
• Formation of deformation bands : dislocation substructure
• Nucleation sites for ferrite transformation
CONTROLLED ROLLING

12. No. 11
Soaking
Rough rolling
Tnr
Ar3
Finish rolling
Laminar cooling
Soaking
Rough rolling
Tnr
Ar3
Finish rolling
Laminar cooling
• Non-Recrystallization Temperature (Tnr)
• Austenite conditioning : pancaked or elongated structure
RECRYSTALLIZATION CONTROLLED ROLLING(RCR)

13. No. 12
Soaking
Rough rolling
Tnr
Ar3
Finish rolling
Laminar cooling
Partial Recrystallization Temp.
• Prediction of non-recrystallization temperature (Tnr) :
Compression test, torsion test, flow stress
RECRYSTALLIZATION CONTROLLED ROLLING(RCR)

14. No. 13
Steel C Si Mn P S Al Others
Thick.
(mm)
*Ceq *Pcm Ar3
A 0.04
0.2~0.3
1.05
< 0.008 < 0.005
0.02~0.06
Nb, V, Ti,
Mo, Cr, Ni,
Ca
9.5 < 0.36 < 0.18
796
B 0.04 1.00 795
C 0.09 1.35 < 0.015 < 0.001 765
• Ceq = C + Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
• Pcm = C + Si/30 + (Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B
* Ar3 = 910-310(%C)-80(5Mn)-20(%Cu)-15(%Cr)-55(%Ni)-80(%Mo)+0.35(t-8)
• Chemical composition and thickness
CHEMISTRY

15. No. 14
• J. Jonas equation & Boratto equation
Tnr (°C) = 887 +464(%C)+[6445(%Nb)-644√(%Nb)]+[732(%V)-230√(%V)+890(%Ti)+363(%Al)-375(%Si)
• Bai equation
Tnr (°C) = 174 log[Nb(C+0.857N)]+1444, N is the free N remaining after TiN precipitation
• Fretcher equation (1) is ignoring pass strain
Tnr (°C) = 849-349C+676√Nb+337V (R2
= 0.72)
• Fletcher equation (2) based on pass strain
Tnr (°C) = 203 – 310C - 149√V + 657√Nb + 683e-0.36ε
• Sim’s equation
F = w σ √(R·ΔH)·Q
F : roll force, σ : flow stress, w : width, R : roll radius, Δh : reduction in thickness
√RΔh : projected arc of contact, Q : shape factor, hf ; final thickness, r : percentage reduction
Steel
Tnr (°C)
Jonas & Boratto Bai equ. Fletcher equation (1) Fletcher equation (2)
A 979 970 994 976
B 1056 991 1017 996
C 1016 1031 978 962
NON-RECRYSTALLIZATION TEMPERATURE

16. No. 15
• Thermomechanical Processing (TMP) of Hot Rolled Coils
Reheating
Furnace
Roughing Mill Finishing Mill
Run Out Table
Coiling
RHT TBT FT CT
Steel
HSM parameter
RDT FDT CT
A
950~1080°C
Case I : 870~ 900
Case II : 850~870
570~630
B
C
• Transfer Bar Temperature :
Above Tnr and near Tnr
MANUFACTURING FACILITIES

17. No. 16
Rolling
temperature
(°C)
Process position
1200
1000
800
600
• Thermomechanical Processing (TMP) of Hot Rolled Coils
Steel A : above Tnr, near Tnr
Steel B : above Tnr
Steel C : near Tnr
Steel A, B
Steel C
• Transfer_Bar Temperature
holding
HOT ROLLING CONCEPT

18. No. 17
• Effect of finishing temperature on the yield ratio
0.74
0.76
0.78
0.8
0.82
0.84
0.86
0.88
0.9
0.92
0.94
830
840
850
860
870
880
890
900
B B B B B B B A A A A A A A A A A A A A A A A A A A A A A A C C C C C C
1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627282930313233343536
Yield
Ratio(YR)
Finishing
Temp.(°C)
Steels
API-X65M
Sum of FDT
Sum of YR
RESULTS

19. No. 18
0.78
0.8
0.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
400
450
500
550
600
650
700
B B B B B B B A A A A A A A A A A A A A A A A A A A A A A A C C C C C C
1 2 3 4 5 6 7 8 9 101112131415161718192021222324252627282930313233343536
YS
&
TS
(MPa)
Steels
API-X65M
Sum of YS
Sum of TS
Sum of YR
• YS & TS : within the range of API-X65
• Amount of increment of tensile strength is higher than yield strength
TENSILE PROPERTIES

20. No. 19
150
170
190
210
230
250
270
290
310
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
Energy
(Joule)
Number of point
Charphy Notch Test
Sum of CVN_Avg
Sum of CVN_Min
• CVN Energy (J) : 0°C
TOUGHNESS PROPERTIES

21. No. 20
• Polygonal ferrite(PF)+Acicular ferrite (AC)+Pearlite(P)
Steel A_near Tnr
Steel A_above Tnr
Steel C_near Tnr
MICROSTRUCTURE

22. No. 21
• GB and sub_GB dislocation : steel A_near Tnr
MICROSTRUCTURE (TEM)

23. No. 22
• Synergistic effects of Recovery, Recrystallization, Deformation, and Precipitation
RECRYSTALLIZATION PRECIPITATION
RECOVERY
DEFORMATION
(T, ε, έ)
Strain-induced ppt
Driving force for ReX
Recovery rate
RPTT
DISCUSSION

24. No. 23
SUMMARY
• Controlled rolling in roughing stage affects recrystallized austenite grain size, resulting in
variations of YS/UTS ratio to levels between 0.81 and 0.92
• The higher deformation temperature based on Tnr shows higher YS/UTS ratio compared
than those of a lower deformation temperature in roughing stage
• The YS/UTS ratio below 0.85 can be obtained by the adaptation of transformed AF
microstructure with lower coiling temperature
• Optimized TMCP processes between roughing and finishing stage showed to be effective
processing routes in order to produce steels with lower YS/UTS ratio and sufficient
toughness