中国微合金化技术与含铌钢发展30周-of Evolution

中国微合金化技术与含铌钢发展30周-of Evolution

High Strength Microalloyed Linepipe: Half a Century

of Evolution

J. Malcolm Gray, F. Siciliano

Microalloyed Steel Institute

5100 Westheimer, Suite 540, Houston, TX 77056 USA

INTRODUCTION

Microalloyed low carbon steels, originally termed High Strength Low Alloy (HSLA) steels, were first introduced in the late 1930’s. The elements niobium vanadium and titanium were added singly, in amounts as low as 0.005 to 0.010 percent in early steels, but later in combination as strengths increased and the metallurgical approach became more refined. The predominant melting method at that time was the Siemens - Martin Open Hearth Process, the steels were ingot cast, usually semi-killed, and in many cases were normalized. Over time this manufacturing approach has dramatically changed. Very low carbon, fully killed, steels are now continuously cast than converted to plate and coil using complex thermomechanical controlled processing (TMCP) regimes. Via this revolution the rolling mill has emerged as a sophisticated metallurgical tool, not simply a means of achieving the final product shape.

Microalloying was first introduced in ship plate, beams, bridge steels, reinforcing bar and heat treated forgings and was not introduced into linepipe steels until 1959 [1]. However, the escalating technological demands of high pressure pipeline systems can be credited with the rapid evolution of HSLA technology since then.

Today’s pipeline designs consider all aspects of a steel’s performance, including strength, toughness, weldability, fatigue and collapse resistance, strain tolerance, as well as environmental degradation such as stress corrosion cracking, and resistance to sour hydrocarbons containing H2S and CO2, which combination of properties must be achieved at affordable prices.

Some of the technological gains and escalating end user expectations have evolved naturally, as competitive producers applied technology developed primarily for other steel products, whereas other requirements may have been provoked by catastrophic and costly failures. The time period of interest involves the last 50 years during which dramatic changes have taken place in steel manufacturing, plate rolling and pipe making technology. Nowadays the melt shop is concerned with ultra purification (impurities measured in ppm), rather than with

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raw productivity, plus all subsequent processing steps are metallurgically integrated since they have an influence on final mechanical properties of the pipe.

Emergence of HSLA (Microalloyed) Steels

The effect of vanadium in increasing strength of normalized steels is reported in German literature, circa 1945, whereas the effect of niobium in increasing the strength of hot-rolled carbon-manganese steels is reported in the patent literature as early as 1938-1939 [2-3]. The exact strengthening mechanism was not known but it was surmised that the benefit was predominantly due to beneficial grain refining effects of niobium and vanadium carbides and nitrides.

Other reports at the time [4] indicated that the new steels had poor notch toughness due to the formation of cementite networks on ferrite grain boundaries, or the formation of Widmanstatten ferrite during normal air cooling [5]. Later it was discovered that hardening or strengthening by the precipitation of vanadium or niobium carbonitrides could also harm toughness [6].

In time it was discovered that these problems could be eliminated by increasing manganese content [7] and refining the austenite grain size during hot rolling. In 1967 the effect of niobium in retarding austenite recrystallization was discovered [8-9] which later became the foundation for the large scale introduction of controlled rolling and other thermomechanical processing methods of austenite [10]. Other microalloying elements, plus molybdenum and aluminum, retard austenite recrystallization to some degree but niobium has been found to be most effective Figure 1 [11], and termed indispensable by Kosazu, et al [12].

Figure 1: Retardation of recrystallization by microalloying elements

In the ensuing years (1968-1975) the strengthening mechanisms operating in microalloyed steels became better understood and quantified [13-14] and optimized compositional and

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processing regimes were established. The available strengthening mechanisms used today can be depicted as simple incremental building blocks as illustrated in Figure 2.

Figure 2: Required combination of strengthening components for hot rolled plate and skelp. These strengthening components are all utilized to varying degrees in modern high strength linepipe.

Microalloyed Steels in Linepipe Applications

The first reported use of microalloying techniques in linepipe steels occurred in Europe (Mannesmann) in normalized API Grade X-52 vanadium grades around 1952. This was later extended to API Grade X-56 and X-60 in 1953 and 1962 respectively [15-16].

Hot rolled steel utilizing microalloying concepts emerged in North America in 1959 [1, 17] and completely replaced normalizing in Europe by 1972 [15-16]. However normalizing of medium carbon (0.17%) microalloyed steel was still used in the Soviet Union until the mid 1990’s. The new hot-rolled semi-killed medium carbon steel [1] shown in Table 1 below, was substituted for steels having much higher carbon and manganese contents thereby improving field weldability.

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Table 1: Chemical Composition of semi-killed niobium steel [1].

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In retrospect the carbon content was still relatively high since carbon contents have steadily decreased, Figure 3, as strengths have increased and larger amounts of microalloying elements have become necessary. The current limit on the combined microalloying content Nb+V+Ti for steels with yield strength >60 ksi is 0.15 percent [18-19]. Individual limits are presented in Figure 4.

Figure 3: Relationship between carbon content and yield strength for high strength pipeline

steels [16].

Figure 4: Comparison of limits for vanadium and niobium for different pipe grades.

There has been a steady increase in available yield strengths from 52 ksi in the 1950’s to 120 ksi today Figure 5.

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Figure 5: Line pipe grade evolution

Despite the considerable amount of research on X-100 and X-120 steels since 1985 and 1998 respectively [20-32] pipes with strengths above 80 ksi have yet to see commercial service in pipelines, even though they have been incorporated into the latest revisions of International specifications [18-19]. Small quantities of X-100 linepipe have been installed in the Trans Canada System [33] to demonstrate construction feasibility, but the pipe nevertheless is operating at a relatively low stress (0.60 SMYS) relative to its real capability.

The metallurgy of X-90 to X-120 linepipe is based on very low carbon microalloyed steels, additionally alloyed with molybdenums, chromium, nickel, copper and occasionally boron, Table 2, combined with elegant TMCP cooling practices, Figures 6 & 7. Some of these concepts have been adopted in second generation X-70 and X-80 steels.

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Figure 6: Four processing methods utilized in TMCP, conventional ACC, intensified ACC, and intensified ACC with low FCT or normal coiling, and their characteristics in control of microstructures with reference to a CCT diagram. Figure 7: Relation between Pcm value and

tensile strength for various cooling methods.

Figure 7: Relation between Pcm value and tensile strength for various cooling methods.

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During evolution of linepipe steels over the period depicted in Figure 5, several essential events, chronicled below, can be credited with changing of specifications or with stimulating metallurgical developments in both linepipe and plate steel.

These factors are presented in Table 3 below:

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Table 3: Technology development incentives 1943 - Present

The trend toward higher strength levels (up to 120 ksi yield strength and above) has been led by developments in DSAW linepipe where the benefits translate into lighter wall thicknesses for high pressure, long distance onshore pipelines. Similar metallurgical approaches are used for HFERW linepipe, which is made from hot-rolled coiled skelp, but yield strength levels have not yet exceeded 80 ksi. In offshore pipelines the useful yield strength is limited to 65-70 ksi due to the need for negative buoyancy and to guard against plastic collapse. In the most extreme cases wall thicknesses in the range 1.125-1.875 inch are being installed in water depths >8000 ft. as detailed later. In the case of seamless linepipe, quenching and tempering is used to achieve the desired yield strength level which is now approaching 100 ksi in offshore riser applications. Examples of the chemical compositions and microalloying element contents used for each grade, product type, and application will be presented later.

It should be noted that the early lower strength steels presented in Figure 5 had very poor toughness from both a fracture energy absorption and impact transition temperature viewpoint. The low absorbed energies in the Charpy test were related to high carbon and sulfur contents and poor steel cleanness, especially in the case of semi-killed steels, whilst the high Charpy and Battelle Drop Weight Tear Test transition temperatures were caused by the coarse ferrite grain sizes and high carbon contents. As a consequence, the early high strength steels often relied on normalizing to achieve adequate grain refinement. In the mid to late 1960’s the benefits of low finish rolling temperatures were discovered and thermomechanical processing was quickly introduced on a broad scale. Simultaneously, lower carbon contents were

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introduced, which were stimulated both from an alloy design perspective, to aid niobium carbide solubility, and from an end user (specification) initiative to improve field weldability. This led to dramatically improved toughness in the X-60 and X-70 steels shown in Figure 8. These levels of toughness at low temperatures have been preserved/maintained as strengths have almost doubled.

Figure 8: Progress of development showing simultaneous increase in toughness with strength and passage of time.

A representative controlled rolled X-60 steel composition, circa 1980, is presented in Table 4 below. Such steels were used for construction of several major pipelines in Saudi Arabia and the North Sea.

Table 4: Typical X-60 Steel Composition

Steels of API Grade X-70 were successfully used in the construction of the Alyeska oil pipeline in 1973. A typical chemical composition for such steels, which were produced in Japan, is shown in Table 5 below:

Table 5: Example of X-70 Steel Used in Alaska, circa 1972

As mentioned earlier linepipe steels with strengths over X-80 have not been used except in

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small (5 km) demonstration projects. The workhorse grades for onshore pipelines are currently API Grade X-70 (482 MPa) and API Grade X-80 (550 MPa) with the latter taking on increasing importance.

The chemical compositions in current use are presented in Table 6 below. Niobium and vanadium may be used in combination with molybdenum, chromium, copper and nickel depending on pipe wall thickness, alloy costs and available manufacturing equipment. Each pipe mill thus arrives at an optimized steel composition and production cost matched to it’s engineering capability.

* S<0.005 N<0.008 Al 0.02-0.05

Table 6: Examples of X-80 Steels in Current Use.

The Nb-Cr alloy design has emerged in the past few years as a favorite of Chinese [34-44] and other pipe producers. The 0.10 percent niobium (HTP) steel has a much wider processing window than conventional Nb-V type steels, Figures 9-11 [45] which results in less variation in toughness and yield strength, the later being an important consideration in the current regulatory climate in the United States.

Figure 9: Wide Processing Window for Yield Strength of 22 mm Plates ACC at 6ºC/s [45].

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Figure 10: Wide Processing Window for Tensile Strength of 22 mm Plates ACC at 6ºC/s [45].

Figure 11: Wide Processing Window for Charpy Toughness of 22 mm Plates ACC at 6ºC/s

[45].

Figure 12: Tensile Strength of 3,200 HTP Plates, ArcelorMittal Burns Harbor Plant, USA

[45].

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Industry specification have been updated in many countries to accommodate the HTP concept

[1819] and over 5 million tons of this very low carbon steel have been installed since 1972

[48]. However, there are still opportunities for changing restrictive microalloying element limits in specifications in Iran, India, CIS and elsewhere.

Figure 13: Progress in adopting X-80 Steel [33].

The rate of adoption of API Grade X-80 since 1985 is presented in Figure 13 above. A list of the projects comprising Figure 13 is presented in Table 7 [47]. The growth is becoming exponential as API Grade X-80 is being utilized in China for major portions of the 9250 km Second West-East Pipeline Project [33].

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The steels used for production of large diameter DSAW linepipe are processed on conventional plate mills [30, 47, 48], Steckel mill facilities [49], or heavy strip mills (for 24 inch diameter spiral seam linepipe), whereas skelp for HFERW linepipe is produced either on conventional hot strip mills or increasingly by direct conversion of thin (50-125 mm) slabs. Typical steel compositions for Grade X-80 linepipe produced on a conventional hot strip mill are presented in Table 8 [45-46].

Table 8: Chemical compositions of API Grade X-80 ERW linepipe.

Seamless product for use in linepipe and oil and gas riser systems is becoming available in strengths up to the 90-100 ksi level and is invariably produced by quenching and tempering. In recent years carbon contents have been reduced to below 0.10 percent to improve field weldability, to improve weld heat affected zone (HAZ) toughness and fatigue resistance, and to reduce HAZ hardness so as to improve SSC resistance.

Examples of the chemical compositions of seamless linepipe are presented Table 9 below

[52-54]:

Table 9:Chemical composition of API Grade X-80 Seamless linepipe. The aforementioned examples of linepipe steel compositions mainly relate to product required for low temperature or sweet (i.e. treated non-corrosive) gas service. However, many developments have occurred in regions producing gas containing H2S, CO2 and chlorides. Depending on the concentrations of each of the above, the moisture content and ease and feasibility of inhibition, the end user may elect to use either carbon steels or more highly alloyed corrosion resistant grades. Examples of modern sour service carbon steels used for production of DSAW linepipe for the Black Sea Pipeline are presented in Table 10 below

[56].

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Table 10: Chemical Composition of 30” O.D. X-65 31.8 mm Linepipe (Blue Stream Project)

[55].

Steelmaking and Manufacturing Developments

At their inception microalloyed (HSLA) steels were based on Open Hearth or Basic Electric Arc steelmaking. Thus there were limitations on reducing carbon contents (to improve solubility of niobium and titanium) and impurity levels were very high. Typical sulfur and phosphorus levels were around 0.025 percent and sometimes twice this level. This level of sulfur had a very detrimental on notch toughness especially when combined with the very high oxide contents of semi-killed steel.

Sequential technical demands for improved linepipe referenced earlier in Table 3 eventually led to dramatic improvements in steel cleanness, reduction in impurities such as phosphorus and sulfur, as well as reduced nitrogen and hydrogen contents. These developments occurred very rapidly in the 1970’s and 1980’s especially in Japan [56-58] Figure 14 and were similarly adopted in Europe. The benefit of reduced sulfur on Charpy shelf energy is presented in Figure 15.

Figure 14: Progress in improving impurity removal through 1985.

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Figure 15: Effect of sulfur content on toughness of linepipe.

Other manufacturing developments occurring since 1959/60 in response to end user demands, production economics, new test methods (e.g. HIC testing) and other factors discussed in this and other* papers are presented chronologically in Table 11 below:

Table 11: Manufacturing Developments 1960-2009

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Examples of the chemical compositions of X-65 sour service linepipe produced with the new technology (circa 1995) are presented in Table 12.

Table 12: Chemical Composition of API Grade X-65 for Sour Service

Evolution of Linepipe Producers & Production

As stated earlier manufacturing technologies for all HSLA steels improved rapidly from 1970 onward often due to demands from the linepipe arena. The demands occurred regionally, first in Alaska and later in the North Sea, followed by the demand for massive volumes of 56” OD X-70 (K60) linepipe from the former Soviet Union from 1970 through 1992 at which point the market collapsed.

Middle East demands from Saudi Arabia, Oman, Iran etc were for sour service X-60 and X-65 grades. In North America Canadian gas transmission companies such as TCPL and West Coast Energy led the move to higher strength levels.

For example: X-70 Nb-Mo steel was adopted by TCPL in 1970 produced at that time by IPSCO from semi-killed ingot steel [59]. Nowadays the pipe is produced from fully-killed continuously cast very low carbon (0.035%) skelp [60].

In the 1990’s there was diversified demand on pipe makers in traditional steelmaking regions from projects offshore China and Brazil, from the Gulf of Mexico and the Middle East. Starting in 1999 Indian and Chinese pipe producers emerged mostly equipped with modern steelmaking and pipe making facilities. India became a major exporter of line pipe to North America, whereas Chinese demand was generated internally. Between 1999 and 2005 Chinese producers developed API Grade X-70 and utilized it for the First West East Pipeline Project [33] which extends 4500 km from the autonomous regions of Lunnan and Hao’er’guosi in the West to Shanghai in the East. After 2005 API X-80 was systematically developed and in a crowning achievement is being used for approximately half of the Second West East Pipeline Project (18.4 to 25.0 mm x 48” OD). A list of major large diameter pipe mills in China with details of their estimated production capacities and size ranges is presented in Table 13. A listing of Indian pipe mills is shown in Table 14.

Globalization of Procurement

In the early 1970’s pipe manufacturer were located close to traditional steelmaking factories in Germany, UK, USA, Canada and Japan and Italy. Later, pipe making became more Internationalized and increasingly non-integrated see Table 15 below and 16 for longitudinal seam and helical respectively. For example countries such as Turkey, Greece and India have built pipe manufacturing facilities based on imported coils or plate, meanwhile procurement

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became less nationalistic. In the period since 2000 North American pipeline construction has been based to a major extent (>50%) on imported pipe from producers in India and elsewhere. This has recently stimulated the construction of 8 or 9 green field spiral seam pipe manufacturing plants in the USA which will once again affect the dynamics of the market place.

Table 13: Chinese (CNPC and Sinopec) Pipe Mills

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Table 14: Compilation of Indian DSAW Pipe Mills

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Table 15: Compilation of Large OD Longitudinal Seam Pipe Mills Worldwide

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