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

1. Contents 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

2. background
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

3. Relationship Between Yield and Tensile Strength Limits in API-5L PSL2
Yield strength
YR =
Tensile strength
TS_max
YS_max
TS_min
YS_min

4. Typical stress-strain curve
Strain hardening effect :
Ability of strain distribution more uniformly
Positive slope indicating increase in
resistance after yielding
UTS YS
strain
stress
cold work n
Uniform elongation

5. Discontinuous & Continuous stress-strain curve
Yield strength depends on σ-ε curve : microstructure, volume fraction of second phase

6. Behaviour of Load & Displacement
Different type of microstructure : PF+AF, PF+M(dual phase), PF+P

7. Behaviour of Load & Displacement
Yield strength depends on the microstructure

8. Hall-Petch and solid solution equation
Example for ferrite-pearlite steels containing up to 0.2 wt% C
- σy, MPa = ·CMn ·CSi +354·CMn ·d-1/2
- σt, MPa = ·CMn ·CSi ·Cpearlite + 7.7·d-1/2
- ITT, °C = ·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

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

10. Controlled rolling 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

11. Recrystallization Controlled Rolling(RCR)
Non-Recrystallization Temperature (Tnr)
Austenite conditioning : pancaked or elongated structure
Soaking
Rough rolling
Tnr
Ar3
Finish rolling
Laminar cooling

12. Recrystallization Controlled Rolling(RCR)
Prediction of non-recrystallization temperature (Tnr) :
Compression test, torsion test, flow stress
Soaking
Rough rolling
Tnr
Ar3
Finish rolling
Laminar cooling
Partial Recrystallization Temp.

13. chemistry Chemical composition and thickness
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
1.00
795
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 = (%C)-80(5Mn)-20(%Cu)-15(%Cr)-55(%Ni)-80(%Mo)+0.35(t-8)

14. Non-Recrystallization Temperature
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
J. Jonas equation & Boratto equation
Tnr (°C) = (%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) = C+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

15. Manufacturing facilities
Thermomechanical Processing (TMP) of Hot Rolled Coils
RHT
TBT FT CT
Roughing Mill
Reheating Furnace
Finishing Mill
Run Out Table
Coiling
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

16. Rolling temperature (°C)
Hot rolling concept
Thermomechanical Processing (TMP) of Hot Rolled Coils
Rolling temperature (°C)
Process position
1200
1000
800
600
Transfer_Bar Temperature
Steel A : above Tnr, near Tnr
Steel B : above Tnr
Steel C : near Tnr
holding
Steel A, B
Steel C

17. results
Effect of finishing temperature on the yield ratio

18. Tensile properties YS & TS : within the range of API-X65
Amount of increment of tensile strength is higher than yield strength

19. toughness properties
CVN Energy (J) : 0°C

20. microstructure Polygonal ferrite(PF)+Acicular ferrite (AC)+Pearlite(P)
Steel A_above Tnr
Steel A_near Tnr
Steel C_near Tnr

21. Microstructure (tem)
GB and sub_GB dislocation : steel A_near Tnr

22. discussion
Synergistic effects of Recovery, Recrystallization, Deformation, and Precipitation
RPTT
RECRYSTALLIZATION
PRECIPITATION
Driving force for ReX
Strain-induced ppt
DEFORMATION
(T, ε, έ)
Recovery rate
RECOVERY

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