Development of Acicular ferrite/Bainitic microstructures in pipeline grade microalloyed steel

V. Rajinikanth a Indian Institute of Technology, IIT Kanpur, India

b MST Division, CSIR-National Metallurgical Laboratory, India

Acicular ferrite (AF)/ bainite (B) is often reported as the most desirable microstructure for development of high strength pipeline grade micro alloyed steels. Therefore the research objective is aimed at obtaining desired microstructural combinations( AF/ B) and its quantification by optimizing thermo mechanical processing (TMP) schedule using Gleeble 3800 thermo mechanical simulator. The TMP schedule needs to be designed based on determination of critical temperatures of transformation including no recrystallisation temperature ( Tnr ) of microalloyed steels. The CCT diagram was simulated using JMATPRO as well as by performing dilatometry in Gleeble to determine the transformation temperatures like Bs, Bf etc., Tnr of the steel was ~1010°C, based on deviation in the slope of mean flow stress vs. temperature plot, determined using multihit deformation method. The samples were deformed in plain strain condition at three different temperatures chosen based on the Tnr studies , i.e., (i) 860°C – well below Tnr and above intercritical region; (ii) 950°C – just below Tnr ; (iii) 1050°C – well above Tnr – complete recrystallisation regime. The deformed samples were examined in EBSD and TEM to obtain the texture and microstructural information. It is also observed that formation of acicular ferrite in this steel is very much dependent on the deformation strain levels in the (PAG) austenite matrix prior to transformation. The grain boundary misorientation angle distribution obtained from EBSD analysis can be a very important parameter to distinguish different microstructural constituents i.e., (i) bainite – parallel plates with grain boundary misorientation < 15°; (ii) martensite – parallel plates with grain boundaries misorientation peak around 58-60°; (iii) acicular ferrite – non-parallel plates with grain boundary misorientation peak around 54°.

Abstract

Determination of Critical Temperatures

The thermodynamic analysis was performed to determine the NbC dissolution temperature (Soaking Temp) as well as transformation temperatures like Ar3 etc., CCT was determined by both simulation(Jmatpro) and dilatometry (CCT dilatometry). The multihit deformation tests were carried out to determine the Tnr based on slope change in the mean flow stress Vs 1/T plot. Based on above analysis, the soaking temperature was kept at 1200°C, and cooling rate of ~20°C/s is sufficient to form bainite/Af and Tnr is ~1010°C.

EBSD and TEM

Conclusions

600 800 1000 1200 1400 16000.01

0.1

Mas

s Fr

actio

n

Temperature,oC

NbC TiN

0.0 0.4 0.80

100

200

300

True

Stre

ss, M

Pa

True Strain

860 950 1050

Strain rate = 5.0

0.0 0.3 0.6 0.90

100

200

300

True

Stre

ss, M

Pa

True Strain

860 950 1050

Strain rate = 0.5

0.0 0.4 0.80

100

200

True

Stre

ss, M

Pa

True Strain

860 950 1050

Strain rate =0.05

60 80 100 120 140 160 180 200 220 240 260 280 3000

5

10

15

20

25

30

35

40

-dθ/d

σ

Flow Stress, MPa

850-5s-1

850-0.05s-1

860-0.5s-1

950-5s-1

950-0.05s-1

950-0.5s-1

1050-0.05s-1

1050-5.0s-1

1050-0.5s-1

Strain = 0.8 860°C 950°C 1050°C

5 s-1 4.35 µm 10.49 µm 22.00 µm 0.5 s-1 10.28 µm 13.31 µm 50.85 µm 0.05 s-1 8.10 µm 12.50 µm 61.86 µm

EBSD misorientation profile- 1050 deg

A

B

C

A

B C acicular ferrite

bainite

Composition (% max.) C Mn P S Si Cu Ni Mo Cr Nb+V+Ti

0.12 1.85 0.025 0.015 0.45 0.50 1.00 0.50 0.50 0.15

0.04 1.75 .009 .003 0.26 0.80(Ni+Cu+Mo) 0.085Nb,0.015 Ti

Yield strength : 555 MPa (min.) and 705 MPa (max) Tensile strength : 625 MPa (min.) and 825 (max.)

Elongation: 16% (min.) Yield to tensile ratio : 0.93

API

5L

X80

Recrytallization and Microstructural parameters @ stain rate 5 860 950 1050

Critical Stress,σc 262 235 186

Critical Strain, εc 0.167 0.176 0.169 Peak Stress,σp 301 253 200 Peak Strain,εp 0.593 0.587 0.349

The flow curves were analysed to determine the dynamic recrystallization parameters like critical stress, peak stress for all the deformation conditions and by using hyperbolic sine law, apparent activation energy of hot deformation of the experimental steel was calculated to be 515.97 KJ/mol.

weld pool – acicular ferrite

X80 – acicular ferrite Simulated

1050°C 950°C 860°C

Strainfree

strained strain free

acicular ferrite bainite

NbC

NbC NbC

MA

MA

EBSD GBCD showed that the acicular ferrite and bainite microstructures corresponds to large fraction of high angle and low angle boundaries respectively. It is also observed that formation of acicular ferrite and bainite microstructures is very much dependent on the deformation strain levels in the austenite matrix prior to transformation. The acicular ferrite of weld pool corresponds to acicular ferrite microstructure of X80 grades with a characteristic GBM peak at ~54°. The TEM analysis revealed that the steel contains acicular ferrite plates with MA constituents and fine NbC precipitates within the laths. The evolution of typical fcc deformation texture for Tdef < Tnr evidences the pancaking of deformed austenite grains.

The critical temperatures including Tnr of the given microalloyed grade X80 pipeline steel was determined based on thermodynamic analyis , CCT dilatometry and multihit deformation using PSC in Gleeble3800 with Hydrawedge facility. The recystallization and microstructural parameters were determined based on flow curve analysis and metallography. EBSD analysis showed that Acicular ferrite has a characteristic GBM at ~54° similar to that of weld pool acicular ferrite microstructure and bainite is predominantly of low angle boundaries with GBM <12°. TEM analysis showed both acicular ferrite and bainite are plate type with distributed MA constituents and fine NbC within the plates. The strain has role on the formation of acicular ferrite. Rotated cube was obtained at Tdef > Tnr and ND fiber for Tdef < Tnr based on ODF analysis.

Soaking Temp

Plain strain set up -Gleeble Plain strain compression -schematic Plain strain samples

before

after

Montage –PSC sample

Tantalum Foil

Pancaked austenite

Strain free zone

860-0.05

860-0.5

860-5.0 950-0.05 1050-0.05

950-0.5

950-5.0

1050-0.5

1050-5.0

Flow Curves of single hit plain strain compression at 860°C, 950°C & 1050°C Critical strain for DRX

grain boundary misorientation

bainite

bainite acicular ferrite

TEM analysis

GBCD