ADVANCES IN STEELS FOR HIGH STRENGTH ERW LINEPIPE APPLICATION IN AUSTRALIA

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1 MATERIALS FORUM VOLUME Edited by J.M. Cairney and S.P. Ringer Institute of Materials Engineering Australasia Ltd ABSTRACT ADVANCES IN STEELS FOR HIGH STRENGTH ERW LINEPIPE APPLICATION IN AUSTRALIA J G Williams BlueScope Steel Limited, Australian and New Zealand Steel Manufacturing Businesses, Port Kembla Steelworks, Australia The importance of achieving high strength in pipes at low carbon equivalent levels so as not to compromise field weldability has been a major focus in the development of X70 and X80 grade ERW pipes in Australia. This paper outlines some aspects of the advances in alloy design and properties that have taken place for high strength X70 and X80 ERW pipeline steels at BlueScope Steel over about the past decade. It also outlines some current research activities that are aimed at reducing alloy levels for cost savings and making further improvements in the quality of linepipe steels to permit higher pipe body toughness and improved weld line toughness particularly in small diameter pipe made from centre-slit parent coils. 1. INTRODUCTION Over the last twenty years in Australia, the standard pipe grade has moved from API 5L X52 through to the present API 5L X70 with maximum operating pressures rising from 6.8 MPa to 15.3 MPa. These changes have been driven by the economic benefits of higher strength pipelines such as lower gas transportation costs, lower pipe procurement and transport-to-site costs and reduced welding costs due to smaller diameter and thinner wall. In Australia, X70 was supplied to a high-pressure gas transmission project for the first time in 1993 and since then BlueScope Steel has supplied over 400,000 tonnes of X70 skelp feed for many major domestic pipeline projects. The importance of achieving high strength in pipes without compromising field weldability was a major focus in the development of X70 grade ERW pipes and also laid the foundation for X80 grade development. Although no Australian pipelines have yet been designed in X80 grade, a demonstration segment (~2000 tonnes) was incorporated into an X65 Queensland looping line project in This paper reviews some of the advances in alloy design and properties that have taken place for ERW pipeline steels at BlueScope Steel in response to market requirements over about the past decade. It also outlines some current research activities that are aimed at making further improvements in the quality of linepipe steels which will permit higher pipe body toughness and improved weld line toughness particularly in small diameter pipe made from centre-slit parent coils. 2. PIPELINE STEEL DESIGN ASPECTS The achievement of pipe mechanical properties within the industry prevailing requirements of preheat free welding with cellulosic electrodes and other technical challenges (such as the requirement for high Charpy energy values to permit rich gas transportation and a low pipe yield strength range of less than 100 MPa above SMYS (specified minimum yield strength)) has required careful attention to alloy design and thermomechanical processing (TMCP) issues. Two key strategies have been pursued by BlueScope Steel to help meet the increasing demands on strength and toughness without compromising weldability, namely; o the use of the Mo-Nb-Ti microalloying system in traditional high Mn steels so as to achieve high pipe strength more comfortably at lower carbon equivalents compared to the formerly applied Nb-V steels. The broad approach taken by BlueScope Steel in the technical solution for X70 and X80 in recent years has been described in detail elsewhere [1]. Whilst large quantities of these steels have been supplied to many large ERW pipeline projects in Australia, the recent spiralling costs of certain alloying elements such as Mo and V has placed additional pressure on the use of these elements and efforts are now being directed towards their potential elimination, replacement or use at reduced levels. 1

2 Table 1. Typical Chemical Compositions of API 5L ERW Linepipe Steels API Chemical Composition, Mass % CEQ Steel Grade C Mn Si P S Al Nb V Cr Mo Ti N Ca IIW Pcm A X B X C X D X E X F X G X H X J X K X L X M* X * B added = % o the use of low Mn levels to improve pipe body and weld line toughness particularly in relation to small diameter pipes made from centre-slit parent coils. The low Mn levels have beneficial effects in terms of both reduced centreline segregation and reduced harmfulness of MnS inclusions on transverse Charpy energy values. Effectively, the steels have a higher tolerance to sulphur than for the more traditional high Mn steels such as referred to above. At the present time, these steels are subject to a comprehensive development program within BlueScope Steel. The low Mn alloy design approach has required that greater reliance be made on the use of other alloying elements such as Cr and Nb. The development program is also evaluating the use of B instead of the more expensive Mo addition in the higher strength grades. Some important alloy design and TMCP aspects supporting the achievement of these two strategies in various high strength ERW linepipe steels are briefly outlined below: 2.1 High Manganese Mo-Nb-Ti Steels The Mo-Nb-Ti microalloying system has proven to be very effective in achieving the strength requirements of X70 and X80 pipes particularly in heavier wall thicknesses. In contrast, Nb-V steels require relatively higher carbon equivalent designs that can compromise their capability for preheat-free field welding with cellulosic consumables according to the guidelines specified in WTIA Technical Note 1 Recommendations [2]. This weldability benefit is evident in Figure 1, which depicts typical average production data for some major projects over recent years. Some typical chemical compositions of both Nb-V and high Mn-Mo-Nb-Ti steels supplied to various projects are included in Table 1 (Steels A,B and C-H respectively). Some other important aspects of the alloy design include the use of Ca for globular complexing of alumina inclusions (to eliminate hook cracks which deteriorate pipe UST performance), and micro-ti additions for enhanced weldability (reduced HAZ hardness [3] and resistance to HAZ cold cracking [4] ). The main structural factors contributing to the enhanced strengthening in Mo-Nb-Ti steels (in comparison to Nb or Nb-V steels) are; i) a significantly finer (often irregularly shaped) ferrite grain size, typically being about 2.5 or 2.0 µm in X70 and X80 steels respectively. These ferrite grains are also characterised by an extensive network of subgrains. ii) low temperature transformation products substantially replace pearlite, mainly bainite containing acicular carbide needles in the somewhat leaner alloyed X70 steels and martensite/ austenite (MA) regions in the more highly alloyed X80 steel. iii) enhanced precipitation hardening from the Mo-Nb-Ti system. A synergistic benefit of Ti on the strength of the Mo-Nb system is apparent from the data of Figure 2 even below the stoichiometric ratio with N when 2

3 Figure 2. Effect of Free Ti (not as TiN) on Strip YS of Mo-Nb X70 steels (Free Ti =0 when %Ti =3.4 x %N) Figure 1. Relative strengthening capacity of Mo-Nb versus Nb/V microalloying systems TiC strengthening would not be expected. It is thought that the addition of Ti promotes more efficient precipitation strengthening of ferrite in Mo- Nb steels. The removal of nitrogen as TiN should encourage the precipitation in ferrite of Nb carbide rather than Nb nitride. More precipitation hardening of ferrite should thus be possible because the carbide is more soluble in austenite than the nitride. Alternatively or in addition, the possibility of forming ultra fine (Ti,Mo)C has been recently reported as a significantly more efficient strengthening species in ferrite than TiC [5]. Such particles could be co-existing or complexed with Nb(C,N). inclusions can have especially deleterious effects on the mechanical properties of the ERW weld line. Specifically, the step of butting side edges of strip together in the ERW process diverts Mn rich centreline segregation bands exposed at the centre-slit side edge of strip into the plane of the weld that forms. The result is to align the Charpy fracture path in the plane of these low toughness microstructural features. In conventional high Mn steels, the abovementioned effects of elongated MnS inclusions have generally been controlled by use of low S levels and the sulphide shape controlling additions of Ca. In order to address the requirements for weld line toughness in small diameter pipes, low Mn alloy designs are currently under development at BlueScope Steel that specifically overcome the adverse effects of centreline segregation on ERW weld line toughness. This is achieved by dramatically reducing the occurrence of centreline segregation banding and the size of MnS 2.2 Low Manganese Steels A disadvantage of Mn as an alloying addition is that it has strong segregating tendencies that typically manifest in anomalous microstructures (with high hardness and low toughness) at the centreline of strip produced by hot rolling continuously cast steel slabs. The adverse effect of centreline segregation is exaggerated by the coincident occurrence of elongated MnS inclusions in the strip. The plasticity of MnS inclusions in the hot rolling process increases directly with increasing Mn level. Elongated MnS inclusions can then exert a controlling influence on the fracture toughness of both the pipe body and the weld line. In the case of linepipe produced from centre slit strip feed, the anomalous microstructures and elongated MnS 3 heavy segregation + stringer sulphides centre slit edge side light segregation + small sulphides Cv fusion line Hi Mn Cv rolled edge side Lo Mn Figure 3. Schematic representation of ERW pipe ring sample showing reduced effect of centreline segregation and sulphide inclusions on weld line Charpy toughness.

4 Figure 4. X-ray Line Scan across centreline region of high and low Mn hot rolled strip samples. inclusions. Consequently, ERW weld line Cv toughness requirements can be much more consistently achieved. In addition, the general pipe body toughness of these steels is also improved at a given steel sulphur level because of the smaller less elongated sulphide inclusions. Figure 3 schematically shows how low Mn contents significantly reduce the harmful effects of anomalous microstructures occurring naturally in the strip centreline region when diverted towards the plane of the weld in the ERW process. The remarkable reduction in segregation at low Mn levels is highlighted in Figure 4 which compares the Mn levels measured by EPMA across the centreline of low and high Mn steels. Examples of trial X52 and X70 steel compositions that have been produced at the Port Kembla Steelworks for ERW pipe manufacture are shown in Table 1 (Steels J-M). Whilst the ability to achieve improved toughness in low Mn steels at higher S levels derives mainly from the smaller MnS inclusions that form during slab solidification, beneficial effects of Ti, Nb, V and Cr additions have also been noted at low Mn levels in terms of sulphide forming tendencies and the behaviour of the resultant inclusions in hot rolling. For example, Figure 5 demonstrates that, under certain conditions, Ti sulphide inclusions (formed during slab solidification) may resist deformation in the hot rolling process and retain a globular morphology. The current development program for low Mn steels is aimed at exploiting these effects. Nevertheless, whilst the requirements for steel desulphurising treatments can be significantly relaxed in low Mn steels, there is still a need to apply steel ladle Ca treatment to achieve globularisation of alumina clusters (and prevent hook cracks) as for the case of high Mn steels. Figure 5. Globular undeformed Ti Sulphide inclusion in centreline region of 6.4mm thickness X52 low Mn Nb- Ti steel strip rolled from a 230mm thick slab. In comparison to conventional high Mn steels, the low Mn steels have even lower CEQ values thereby providing ongoing assurance for preheat-free field welding with cellulosic consumables. In the lower strength API 5L grades (i.e. up to and including X60) it is expected that higher levels of Nb will largely compensate the strength loss arising from reduced alloy levels of Mn. However, for higher strength grades such as X70, compensating additions of other alloys such as Cr (and possibly B in lieu of the more expensive Mo addition) are also being actively evaluated. A key technical focus in the development of low Mn steels has been to understand that the austenite recrystallisation and austenite to ferrite transformation behaviour is significantly different to that of high Mn steels [6] and that there are significant resultant implications for thermomechanical processing and steel mechanical properties. In addition, significant changes to the levels of oxidisable elements in the low Mn steels such as Mn, Si and Cr require careful consideration of various quality aspects of ERW welding (e.g ensuring the formation of low melting point oxides for a clean weld line and modifying seam annealing temperatures for optimum microstructures). Field weldability aspects such as HACC resistance and HAZ toughness are additional important considerations by which the efficacy of the alloy design must be judged. 3. THERMOMECHANICAL PROCESSING, STRENGTH AND MICROSTRUCTURE Proper control of the entire rolling process from slab reheating to coiling is essential to develop the appropriate strength (and fracture toughness) requirements. In addition, there is a growing emphasis from pipeline operators for a controlled range of pipe strengths, such as SMYS MPa. The reason for this is to ensure 4

5 overmatching strength of girth weld metal to provide adequate protection of weld metal defects in the event of longitudinal in-service displacement of the pipe in regions Table 2. Effects of Mo Level, HSM Run-Out Table Cooling rate and Ti Level on Strength of High Mn X70 Grade Steels Steel ROT Cooling Rate* Pipe Size (mm) C Mn Si Nb Mo Ti N YS Pipe MPa High Mo low 6.4 x Med Mo high 5.6 x Low Mo high 5.6 x Low Mohigh Ti high 5.6 x * Low ROT cooling rate employed high pressure sprays; high cooling rate employed high flow laminar sprays TS Pipe MPa of unstable terrain. Overmatching the strength of weld metal is more difficult to achieve in higher strength pipe given the industry expectation in Australia for field welding with cellulosic electrodes. Although able to deliver higher strength welds, mechanised welding has not yet proven to be an economically viable alternative to manual girth welding of thin walled pipes. So with these restrictions, alloy design and key rolling parameters need to be optimised to ensure a controlled range of strength in the hot strip. Some of the processing and metallurgical issues that need to be considered for consistency of critical property achievement can be outlined with reference to the case of high Mn Mo-Nb-Ti steels. As for any steel, the chemical composition and prior thermomechanical history will determine the hardenability prior to cooling start. The final strip properties are then essentially controlled by the manner in which the strip-cooling curve (which depends on such factors as the selection of spray banks applied, strip thickness and rolling speed) intersects the deformation CCT diagram (Figure 6). If rapid cooling on the run-out table is applied prior to the start of transformation, the strength may be enhanced by additional undercooling of the austenite to ferrite transformation which promotes grain refinement, and increased volume fraction ratio of acicular to polygonal ferrite phase (AF/PF). This aspect assumes even greater importance in the case of low Mn steels where the Ar 3 temperature is relatively high in direct response to the lower levels of Mn (a strong austenite stabiliser). The microstructures generated in hot strip rolling can show a variation in the relative volume fractions of component phases through the strip thickness as a result of local cooling rate and hardenability effects [7]. Generally speaking, a decreasing volume fraction of acicular and M-A (martensite/austenite) phases from strip surface regions (as well as an increase in grain size) towards the strip centre is observed particularly for thicker strips. This behaviour reflects the relatively low hardenability of these microalloyed steels so that hardenability drivers such as Mo level, % austenite non- Figure 6. Deformation CCT Curve for X80 Figure 7. Effect of % M-A on strength of X80 strip and 8.6 x 457mm pipe 5

6 Table 3. Typical Mechanical Properties for X70/X80 ERW Pipe Steels Strip Mechanical Properties Pipe Tensile Properties Tensile (Trans.) BDWTT Charpy V-Notch Toughness Ring Steel Strip 85% Spec'n Test En. 50% Pipe Expan. Thick. LYS TS El. FATT Size Temp. (J) FATT Dia. YS TS* (mm) (MPa) (MPa) (%) ( C) (mm) ( C) ( C) (mm) (MPa) (MPa) X70 Steel B < x (T) X70 Steel C < x < (T) X70 Steel D !0 x < (T) X70 Steel E < x < (T) X70 Steel F x (T) X80 Steel G < < x < < x < (T) < x < X80 Steel H < x < (T) X52 Steel J x (T) X52 Steel K x (T) X70 Steel L x X70 Steel M x recrystallising reduction, FRT (finish roll temperature)- Ar 3 and cooling rate can significantly influence the strip strength. Additionally, balancing the cooling intensity from top to bottom of the strip will ensure that the strip strength is not compromised by a softer bottom surface region. The effect of increasing %M-A (which is accompanied by an increasing ratio of acicular to polygonal ferrite) on strip and pipe strength of X80 grade (for varying strip rolling and cooling conditions) is shown in Figure 7. Low FRT and high % nonrecrystallising reduction in austenite tend to diminish the strength of high Mn Mo-Nb-Ti strip due to reduced austenite hardenability. Conversely, higher FRT and lower non-recrystallising reduction eventually reduces strip strength because of coarser transformation structures inherited from the resultant larger austenite grain sizes. However, strip strength variations for these steels have proven to be rather small in the range of finish rolling and cooling parameters normally selected for production conditions. This makes high Mn Mo-Nb- Ti steels well suited to the achievement of narrow strength ranges across large project orders. Notwithstanding these attractive features of high Mn Mo- Nb-Ti steels, the massive increase in the price of Mo that occurred in recent years * has placed considerable pressure on the use of this alloying addition. Recent developments at BlueScope Steel have therefore targeted reductions in Mo levels for the high Mn Mo-Nb-Ti steels. The data of Figure 2 suggests that a small increase in Ti level might be able to largely offset the loss in strengthening from reduced Mo levels. An indication that this can be successfully achieved (along with some alterations to the thermomechanical processing regime such as higher run out table cooling rates) is evident in the data of Table 2. As this work proceeds, it may prove possible to completely eliminate Mo additions, especially in pipes having a wall thickness below about 7mm. As noted earlier, this cost issue of Mo additions is also one of the reasons for consideration of the use of B instead of Mo in the development of low Mn X70 steels. * The price of western molyoxide was 4 USD/lb in Dec 2002; 47 USD/lb in June 2005 and 30 USD/lb in March

7 For low Mn steels, significant technical challenges that have become evident in designing the thermomechanical processing regime include recognition that: o significantly higher Ar 3 temperatures will be encountered in comparison with high Mn steels with a potentially negative effect on ferrite grain refinement and concomitant increase in Cv 50% FATT and BDWTT 85% FATT. This means that special care needs to be applied to ensure that austenite recrystallisation is optimised and that rolling is completed and run out table cooling is commenced above the Ar 3 temperature. Low Mn steels are less forgiving of metallurgically undisciplined roughing and finish rolling practices than high Mn steels because they cannot depend on the same compensating or ameliorating benefits to grain refinement of low transformation temperatures. Consequently there is a tendency for mixed austenite grain sizes to have amplified effects in the transformed ferrite of low Mn steels (resulting in somewhat coarser grain sizes). o the effects of Nb on austenite recrystallisation could be different at low Mn levels because the solubility Figure 10. Predicted vs actual YS using empirical models for 19 major X70 projects (Mo-Nb-Ti steels) with sheet thickness in range of mm 4. PIPE PROPERTIES Strength: When the hot rolled strip is converted into ERW pipe, the pipe-forming and sizing strains can significantly modify the pipe yield strength by virtue of the Bauschinger effect and work hardening behaviour [8]. Material factors such as steel composition and microstructure (ratio of AF/PF, %MA, grain size etc), Luder s elongation and pipe processing factors (eg. the level of forming strain, t/d; pipe mill fin-pass and sizing set-up, etc) contribute in a complex manner to this strength change. Some indications of these changes are Figure 9. Bauschinger effect: Nb-V versus Mo-Nb Steels of Nb in austenite is reduced. Any such differences in austenite recrystallisation might significantly alter the balance between so-called pancaking and dynamic/post-dynamic recrystallisation during the finish rolling stage and adversely affect the extent to which mixed austenite grain sizes may be produced. o as is the case with high Mn steels, the AF/PF ratio is also an important contributing microstructural factor in controlling the strength and toughness properties of low Mn X70 steels. evident in Figure 8 [9]. For a given steel type and pipe size, the strength loss from strip to pipe generally increases with increasing strip strength thus warranting major consideration in the design of higher strength grades such as X70 and X80. During the initial development of X70 and X80 grades, the limitations of utilising more stringent rolling schedules (eg. finish rolling temperatures just below Ar 3 temperature) to increase strip yield strength and, pipe strength to X80 levels, proved an inappropriate approach particularly for the case of conventional Nb-V steels. As evident from Figure 9, lowering the finish rolling temperature below the Ar 3 temperature was effective in significantly increasing the strength of the hot strip. However, almost no further improvement in the corresponding pipe strength was recorded suggesting that two-phase rolling exacerbates the Bauschinger drop. This diminishing returns effect led to the consideration of molybdenum additions as a means for more comfortably reaching the higher strength levels required for X70 and X80 pipes. It is evident from Figure 9 that higher pipe strength levels for Mo-Nb-Ti steels (at a given strip strength level) are 7

8 Figure 11. Range of ring expansionys for 19 major X70 projects (range is for 90% of all test results); t: mm, dia: mm due in part to the lower Bauschinger drop. The level of the Bauschinger drop in various Mo-Nb-Ti steels, for a given pipe t/d, is clearly controlled by the Luder s elongation as shown in Figure 8. The importance of microstructural factors in this behaviour has been suggested in Figure 7 and underlines the critical interrelation of alloy design and thermomechanical processing factors. Whilst achieving a narrow range of strip strength is important for the reasons mentioned above, achievement of the targeted value for the average of the population is also critical in order to meet the requirements for all resultant pipe to be in a range of SMYS +100 MPa. Careful design and control of the steel chemical composition and an intimate knowledge of the effects of rolling parameters are thus indispensable when it comes to achieving a tight control of strip strength. At BlueScope Steel, this is achieved by the use of empirical predictive strength equations. In addition, a large history database for property shifts from strip to pipe at the pipe-mill has provided assurance that the Bauschinger effect can be confidently predicted for the various manufactured pipe sizes. Whilst variation in pipe forming conditions can cause scattering of the strip to pipe strength shift from rolling to rolling, the inherent corrective nature of the Bauschinger effect generally operates to produce a narrower range of strength than was produced in the original strip population. For example, coils with strength at the top end of the strength population have a natural tendency to undergo a greater Bauschinger strength drop than coils with lower strength. Overall, the accuracy of these empirical equations has proven to be more than adequate to achieve the required level and range of strip and pipe strengths. This has obviated the need for more complex fundamentally based Figure 12. Effect of sulphur on Charpy impact toughness of API X65/X70 Steel types. metallurgical and non-linear neural networks models that have occupied many researchers over the past decade [10]. An example of the excellent predictive capability of BlueScope Steel s simple empirical models is shown in Figure 10 with the achieved pipe strength ranges for various X70 projects shown in Figure 11. Combined with a knowledge of strength changes during pipemaking, this empirical model has enabled BlueScope Steel to comfortably achieve demanding specified strength ranges for skelp, such as 90% of pipe yield strength values falling in the range of SMYS+100 MPa (Figure 11). Further examples of typical strip and pipe properties obtained for various X70 and X80 projects and pipe sizes are shown in Table 3. Application of the Mo-Nb-Ti alloy system to the development and production of large quantities of X70 and trial quantities of X80 ERW pipe has given excellent results to date from both steel property and pipeline construction aspects. In relation to the strength properties of low Mn steels, pipe making trials to date have been successful in achieving the strength requirements for API 5L grades of up to and including X70 (see Steels J-M, Table 3). In this case the potential for substitution of B for the higher cost Mo is indicated although other aspects of steel manufacture, processing and other property considerations of this option are still being evaluated. 8

9 processing window in some grades and sections it is possible that there will need to be limitations on how low the Mn content can be practically set. Figure 13. Influence of Mn on Charpy Energy for varying S levels (average data at each sulphur level for steels with carbon generally in range of 0.07 to 0.10%C) Fracture Toughness: Resistance to both brittle and ductile fracture propagation are prime requirements for linepipe steels. Modern low carbon alloy designs combined with the high non-recrystallising deformation levels and the high cooling rates achievable on hot strip mills ensure the base strip and accordingly pipe body has an extremely high resistance to brittle fracture. Typically the Charpy 50% FATT and the Battelle DWTT 85% FATT for high Mn steels are lower than -100 C and -60 C respectively, Table 3. Accordingly, for the case of high Mn steels, most attention is given to ensuring the appropriate level of toughness for resistance to ductile fracture propagation that is effectively a specification minimum for Charpy upper shelf energy. The Charpy upper shelf energy is primarily controlled via the sulphur and carbon levels while the hot rolling conditions can also contribute. The effect of the sulphur content on the Charpy toughness for high Mn-Mo-Nb-Ti X70 is shown in Figure 12. Whilst variation in Mn content has only a small effect on the Cv USE values when Mn is in the range above about 0.8% Mn, there are clear benefits available in steels with levels of Mn below about 0.5% Mn, as indicated in Figure 13. As noted above, the significantly higher Ar 3 transformation that accompanies low Mn steels means that some deterioration in Charpy 50% FATT and BDWTT 85% FATT is inevitable. Whilst these effects can be suitably managed by appropriate design and control of the thermomechanical In most instances with the current high Mn linepipe steels, the fracture toughness requirements to avoid ductile fracture propagation in pipelines transporting lean gas can be comfortably achieved in ERW API Grades X60 to X70 pipe with a 0.005% maximum sulphur requirement. In the case of pipelines required to transport rich gas, higher levels of ductile fracture propagation resistance (Charpy energy) are often sought in the steel. This requirement arises from the lower decompression rate of rich gas compared to methane. Together with the prospect of design factors being raised from 72-80% SMYS, and a coincident shift to higher strengths such as X80, questions are now being raised not only in regard to the required level of Charpy energy (and the relevance of the Charpy test for predicted values for full size specimens above about 100J) but also as to the technical feasibility of producing steels with sufficiently high energy values. These issues are currently receiving the attention of the Australian pipeline industry and full scale pipe burst testing is being considered to help resolve the requirements for ductile fracture control. To this end the current development of low Mn steels has the potential to provide some further scope for meeting the increasing demands for higher levels of ductile fracture resistance. For instance as can be seen from Figure 13, at a given S level, significantly higher Cv values can be expected from steels with lower Mn levels. A further limitation of the Charpy test concerns the conversion of upper shelf energy values between Charpy specimen sizes. In view of the pipe wall thicknesses commonly associated with small diameter ERW pipe, the Charpy specimen size is normally sub-size. It has been found that for modern high toughness pipeline steels, the energy absorbed per unit ligament area decreases with decreasing specimen thickness [11]. Thus, converting upper shelf energy values from a full size to a sub size specimen via a pro-rata calculation will overestimate the energy requirement. The use of experimentally determined conversion correlations should thus be considered in determining the appropriate values applying to sub-size specimens. 9

10 Table 4. ERW Weld Line Charpy Test Results for Pipe made from Centre Slit Strip Steel Type Low Mn X52* (Steel J, 0.08% C- 0.38% Mn %S)) Low Mn X52 # (Steel K, 0.08%C-0.24%Mn %S) High Mn X65* (0.08% C 1.07% Mn-0.003%S) *Gull-winged test specimens from 219x 6.4mm pipe # Gull-winged test specimen from 219 x 8.0mm pipe Test Pipe Number Charpy V Impact Energy (J) at 0 0 C (3 tests per pipe) As noted earlier, ERW weld line toughness in pipe made from centre slit coils can be adversely affected by anomalous microstructures and elongated MnS inclusions formed in the centreline segregation region of the slab. Low Mn alloy design largely overcomes this problem and an indication of the much improved weld line toughness in pipe produced from centre-slit low Mn X52 steel compared to high Mn X52 is shown in Table 4. Notably the benefits of low Mn alloy design are evident even for the case of S levels as high as 0.010%. 5. SUMMARY COMMENTS BlueScope Steel has continued to be responsive to the unique needs of the pipeline industry in Australia as outlined by the developments in low carbon equivalent high strength X70 and X80 grades and the current work in progress to reduce alloy costs and establish a range of new alloy designs involving low Mn steels. The low Mn steels are anticipated to provide good improvements to both ERW weld line and pipe body fracture toughness. References 1. JG Williams, CR Killmore, FJ Barbaro, J Piper and L Fletcher, High Strength ERW Linepipe Manufacture in Australia, Materials Forum (20), 1996, Welding Technology Institute of Australia (WTIA) Technical Note 1, GF Bowie, FJ Barbaro, RH Smith and JG Williams, The Influence of Micro Titanium Additions on the Weld Heat Affected Zone Hardness of C-Mn Structural Steels, Proc. Welding 90, International Conf. Geestacht, Germany, Oct 1990, L Fletcher, Field weldability of Large Diameter Thick Walled Linepipe, Australian Welding Journal, First Quarter, 1989, Y Funakawa, T Shiozaki, K Tomita, T Yamamoto and E Maeda, Development of High Strength Hotrolled Sheet Steel Consisting of Ferrite and Nanometer-sized Carbides, ISIJ International, Vol 44, 2004,No.11, S-H Cho, K-B Kang and JJ Jonas, Effect of Manganese on Recrystallisation Kineticsof Niobium Microalloyed Steel, Materials Science and Technology, April 2002, Vol 18, JG Williams, CR Killmore, PD Edwards and PG Kelly, Thermomechanical Processing of Mo-Nb High Strength Steels For Application to X70 and X80 ERW Linepipe Proc International Conference Thermec 97, University of Wollongong July 1997, ed. By T. Chandra et al., pp K Nakajima,W Mizutani, T Kikuma and H Matomoto The Bauschinger Effect in Pipe Forming Transactions ISIJ (15), 1975, JG Williams and L Fletcher, Development and Use of High Strength Steels in Pipelines, Pipeline & Corrosion Management Seminar, Melbourne, May, DM Jones, J Watton and KJ Brown, Comparison of Hot Rolled Steel Mechanical Property Prediction Using Various Models, Ironmaking and Steelmaking, Vol 32, No 5, 2005, CR Killmore, FJ Barbaro, JG Williams and AB Rothwell, Limitations of the Charpy Test for Specifying Fracture Propagation Resistance, International Seminar on Fracture Control in Gas Pipelines, WTIA/APIA, Sydney, June