The role of molybdenum additions and prior deformation on acicular ferrite formation in microalloyed Nb–Ti low-carbon line-pipe steels

Abstract Microstructures in Nb–Ti-microalloyed line-pipe steels with various molybdenum additions, consisted mostly of acicular ferrite plus polygonal ferrite after hot rolling and rapid cooling. Structure-sensitive surface relief after etching on shadowed extraction replicas, allowed quantification of the acicular and polygonal ferrite contents. Continuous cooling transformation diagrams of two alloys, one Mo-free and the other containing 0.22% Mo, were determined for cooling rates from 0.1 to 40 °C s− 1 without and with prior deformation of the austenite below the nil-recrystallisation temperature. Molybdenum additions slightly enhanced the acicular ferrite formation in the strain-free austenite whereas prior deformation had a much greater effect, and strongly promoted acicular ferrite formation in both alloys. Thin foil electron microscopy of acicular ferrite in these low-inclusion content alloys showed a preference for parallel acicular ferrite laths with less “chaotically” nucleated laths.

[1]  H. W. Kerr,et al.  Inclusion phases and the nucleation of acicular ferrite in submerged arc welds in high strength low alloy steels , 1986 .

[2]  H. W. Kerr,et al.  Observations on inclusions and acicular ferrite nucleation in submerged arc HSLA welds , 1983 .

[3]  H. Bhadeshia,et al.  Solid-state nucleation of acicular ferrite on minerals added to molten steel , 1997 .

[4]  N. Kim.  The Physical Metallurgy of HSLA Linepipe Steels—A Review , 1983 .

[5]  R. Ricks,et al.  The nature of acicular ferrite in HSLA steel weld metals , 1982 .

[6]  D. Kwon,et al.  Transformation strengthening by thermomechanical treatments in C-Mn-Ni-Nb steels , 1995 .

[7]  H. Bhadeshia,et al.  Thermodynamics of acicular ferrite nucleation , 1994 .

[8]  S. Court,et al.  Inclusion chemistry and morphology in shielded metal arc (SMA) steel weld deposits , 1989 .

[9]  H. Bhadeshia,et al.  The mechanical stabilisation of Widmanstätten ferrite , 1997 .

[10]  V. Bišs,et al.  Martensite and retained austenite in hot-rolled, low-carbon bainitic steels , 1971 .

[11]  H. Bhadeshia,et al.  Lower acicular ferrite , 1989 .

[12]  H. Bhadeshia.  Diffusional and displacive transformations , 1987 .

[13]  I. Gutiérrez,et al.  Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancement by CuS , 1998 .

[14]  H. K. D. H. Bhadeshia,et al.  Transition from bainite to acicular ferrite in reheated Fe–Cr–C weld deposits , 1990 .

[15]  W. Stumpf,et al.  Measurement of particle density and volume fraction from extraction replicas , 1968 .

[16]  K. Easterling,et al.  Formation of acicular ferrite at oxide particles in steels , 1989 .

[17]  G. Tither,et al.  Beneficial stress-strain behavior of moly-columbium steel line pipe , 1975 .

[18]  H. K. D. H. Bhadeshia,et al.  Effect of plastic deformation on the formation of acicular ferrite , 2003 .

[19]  R. A. Farrar,et al.  Acicular ferrite in carbon-manganese weld metals: An overview , 1987 .

[20]  I. Gutiérrez,et al.  Analysis of different acicular ferrite microstructures generated in a medium-carbon molybdenum steel , 2003 .

[21]  I. Gutiérrez,et al.  Acicular Ferrite Microstructures and Mechanical Properties in a Medium Carbon Forging Steel , 1998 .

[22]  H. Aaronson,et al.  Another visit to the three definitions of bainite , 1987 .

[23]  I. Gutiérrez,et al.  Nucleation of acicular ferrite enhanced by the precipitation of CuS on MnS particles , 1997 .

[24]  H. Bhadeshia,et al.  Acicular ferrite morphologies in a medium-carbon microalloyed steel , 2001 .

[25]  G. Pollard,et al.  Microanalysis of weld metal inclusions , 1985 .

[26]  Xie Changsheng,et al.  Influence of Mo content on microstructure and mechanical properties of high strength pipeline steel , 2004 .

[27]  H. Bhadeshia,et al.  Orientation relationships between adjacent plates of acicular ferrite in steel weld deposits , 1989 .

[28]  Ke Yang,et al.  Continuous cooling transformation of undeformed and deformed low carbon pipeline steels , 2003 .

[29]  Ke Yang,et al.  The effects of thermo-mechanical control process on microstructures and mechanical properties of a commercial pipeline steel , 2002 .

[30]  R. Edwards,et al.  The Morphology and Mechanical Properties of Bainite Formed from Deformed Austenite , 1978 .

[31]  H. Bhadeshia,et al.  Stress and the acicular ferrite transformation , 1992 .

[32]  Ke Yang,et al.  Acicular ferritic microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel , 2005 .

[33]  I. Gutiérrez,et al.  Acicular Ferrite Microstructures and Mechanical Properties in a Low Carbon Wrought Steel , 1998 .

[34]  S. Fisher,et al.  Copper sulphide Cu1.8S (Digenite I) precipitation in mild steel , 1982, Nature.

[35]  Toshihiko Takahashi,et al.  Intragranular ferrite nucleation in medium-carbon vanadium steels , 1994 .

[36]  R. Farrar,et al.  Role of non-metallic inclusions in formation of acicular ferrite in low alloy weld metals , 1996 .

[37]  C. Y. Huang,et al.  Effect of Compressive Deformation on the Transformation Behavior of an Ultra-Low-Carbon Bainitic Steel , 1993 .