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

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

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

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

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

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

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

Behaviour of Load & Displacement Yield strength depends on the microstructure

Hall-Petch and solid solution equation 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

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

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

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

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.

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 = 910-310(%C)-80(5Mn)-20(%Cu)-15(%Cr)-55(%Ni)-80(%Mo)+0.35(t-8)  

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) = 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

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

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

results Effect of finishing temperature on the yield ratio

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

toughness properties CVN Energy (J) : 0°C

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

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

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

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