The present invention relates to linepipe steel, a method of producing linepipe steel strip, a method of producing linepipe from linepipe steel strip, and a linepipe complying with the specified requirements of the API 5L Grades made from linepipe steel.

In Australia, the electric resistance welding (ERW) method is the principal method for linepipe manufacture. ERW requires steel strip feed that has been sourced from a hot strip mill. ERW involves heating side edges of strip to temperatures above the melting point of the steel and thereafter butting and thereby welding the side edges together.

Mn levels in linepipe steels produced by the applicant have historically been in the range of 0.8-1.5 wt . % . The main purpose of the Mn addition is to provide solid solution strengthening.

However, one disadvantage of Mn 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 .

Such anomalous microstructures can have deleterious effects on the mechanical properties of the ERW weld line particularly when linepipes are produced from centre slit strip feed. Specifically, the step of butting side edges of strip together in ERW diverts Mn rich centreline segregation bands at the slit side edge of strip into the plane of the weld that forms . The result is to compromise the toughness of the weld line.

Such anomalous microstructures arising from

centreline segregation can also have deleterious effects on welds produced by welding together aligned ends of linepipes during construction of a line using upset welding processes such as MIAB and flash butt welding. These processes involve induction heating the rims of the aligned ends by and then butting the ends together to produce autogenous welds . Currently the MIAB welding process, which has some operational and economical advantages over conventionally used manual welding of line pipe, is not used widely in pipe line construction.

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 . The detrimental effects of MnS inclusions on ductile fracture propagation resistance of pipeline steels are well known. These inclusions exert a controlling influence on the fracture toughness of both the pipe body and the weld line . The adverse effect on the weld line toughness is particularly evident for the case of pipes made from centre slit strips .

Traditionally, the adverse effects of elongated MnS inclusions and centreline segregation have been controlled by limiting S concentrations in steel to be below 0.005% (or even lower limits) depending on the specific requirements of a pipeline .

In addition, some steel manufacturers, as an additional countermeasure , have the capability to achieve complete sulphide shape control using Ca injection processes .

However, there are significant capital and operating costs associated with both of the above measures , and the measures are not attractive options for

these reasons .

The present invention provides an alternative solution that is based on composition selection of linepipe steel that should not involve significant additional capital and operating expenditure. In addition to making it possible to produce improved quality linepipe, the linepipe steel is also specifically suited to on-site upset welding processes such as the MIAB and flash butt welding processes .

The solution is to use a steel having (a) a considerably reduced concentration of Mn (typically no more than 0.50 wt.%, preferably no more than 0.35%) than used conventionally for linepipe manufacture and (b) a small concentration of Ti (typically at least 0.01 wt.%).

Furthermore, for higher strength API linepipe grades , the solution may include additions of Cr in the steel . The applicant has found that alloying additions of Cr can also be effective in increasing the hardness and reducing the plasticity of MnS inclusions. In general, the specification toughness requirements of linepipe steels increase with increasing strength levels . For the higher strength API linepipe grades , the use of Cr additions in place of Mn will have the combined benefit of contributing to both increased strength and toughness .

The low Mn concentration reduces the degree of centreline segregation and therefore the anomalous microstructures that would otherwise be formed in the strip formed by hot rolling continuously cast slabs. In addition, the plasticity of MnS inclusions is greatly reduced at low Mn concentrations . The inclusions are relatively hard and remain in a largely globular form in a hot rolled strip product. The Ti addition further enhances the hardness of MnS inclusions, whilst also

assisting in achievement of improved surface quality and grain refinement in the steel and associated weld heat affected zones .

According to the present invention there is provided a linepipe steel having the following composition, in wt% :

C: up to 0.18; Mn: 0.10 to 0.50;

Ti: at least 0.01;

Si : up to 0.35 ;

Nb: up to 0.10;

Al: up to 0.05; Ca: up to 0.005;

S: up to 0.015;

P: up to 0.020;

Cr : up to 1.0;

Mo : up to 0.5; B: up to 0.002;

Ni: up to 0.35;

Cu: up to 0.35;

V: up to 0.06;

Fe : balance ; and incidental impurities .

The term "incidental impurities" is understood herein to mean impurities that are the result of the steelmaking process and the feed materials used in the steelmaking process and are not deliberate additions to the composition and are not already in the list of elements . Sn is one such element .

The linepipe steel includes Mn and Ti as deliberate additions to the composition.

The linepipe steel may also include additional

elements as deliberate additions to the composition.

Cr, Mo, B, Ni, Cu, and V are examples of additional elements .

Deliberate additions of elements may be required depending on the mechanical properties required for linepipe made from the steel . For example , for high strength linepipe grades, such as API 5L X65 and X70, which traditionally rely on relatively high Mn concentrations for strength purposes , Cr and Mo may be added to compensate for the low concentration of Mn. Furthermore, B may be added and be present in a protected solute form to enhance hardenability. It is noted that, when B is added, preferably the composition includes sufficient Ti to combine with all of the N in the composition and, hence, avoid the formation of BN.

In addition, deliberate additions of elements may be required depending on particular requirements relating to the end-use applications of the linepipe steel. For example, Ni and Cu may be required as elements in the composition for sour service applications .

Typically the steel composition includes less than 0.10 wt.% C.

Typically the steel composition includes at least 0.02 wt.% C.

Preferably the steel composition includes at least 0.03 wt.% C.

More preferably the steel composition includes at least 0.04 wt.% C.

Typically the steel composition includes less

than 0. 35 wt . % Mn .

Typically the steel composition includes at least 0.15 wt.% Mn.

Preferably the steel composition includes at least 0.20 wt.% Mn.

More preferably the steel composition includes at least 0.25 wt.% Mn.

Typically the steel composition includes less than 0.05 wt.% Ti.

Preferably the steel composition includes less than 0.03 wt.% Ti.

More preferably the steel composition includes less than 0.04 wt.% Ti.

Typically the steel composition includes less than 0.25 wt.% Si.

Typically the steel composition includes at least 0.005 wt.% Si.

Typically the steel composition includes less than 0.08 wt.% Nb.

Typically the steel composition includes at least

0.001 wt.% Nb.

Preferably the steel composition includes at least 0.01 wt.% Nb.

Typically the steel composition includes at least 0.01 wt.% Al.

Typically the steel composition includes less than 0.001 wt.% Ca.

Typically the steel composition includes less than 0.012 wt.% S.

Typically the steel composition includes less than 0.01 wt.% S.

Typically the steel composition includes at least 0.005 wt.% S.

Typically the steel composition includes less than 0.020%wt. P.

Typically the steel composition includes less than 0.7 wt.% Cr.

Preferably the steel composition includes less than 0.5 wt.% Cr.

Typically the steel composition includes less than 0.3 wt.% Mo.

According to the present invention there is also provided a linepipe made from the above-described linepipe steel.

According to the present invention there is also provided a method of producing a coiled strip of the above-described linepipe steel that is suitable to be used as a feedstock for producing a linepipe, which method includes the steps of:

(a) casting a slab of the above-described linepipe steel;

(b) hot rolling the slab to form a strip having a required thickness, typically 5-10 mm; and

(c) coiling the strip.

Preferably the microstructure of the linepipe steel in the coiled strip produced by the above-described method is predominantly fine grained polygonal ferrite.

Preferably the microstructure includes a small (up to 15%) volume fraction of pearlite in the case of medium strength linepipe grades , such as API 5L X42 and X60.

Preferably the microstructure includes acicular ferrite and/or martensite/austenite in the case of high strength linepipe grades, such as API 5L X65 and X70.

According to the present invention there is also provided a method of producing a linepipe which includes electric resistance welding the above-described linepipe steel strip and forming the linepipe.

Apart from the narrow seam annealed region of the ERW weld zone, the microstructure of the linepipe is essentially unchanged by the pipe forming process and is the same as that of the linepipe steel in the above- described linepipe steel strip.

The applicant has carried out research work that has evaluated the extent of centerline segregation in linepipes made from the above-described electric resistance welded linepipe steel strip and linepipes made from electric resistance welded, conventional high Mn linepipe steel strip.

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Figure 1 illustrates the results of the research work.

Figure 1 comprises two graphs . Each graph plots the concentration of Mn (measured by Electron Probe Microanalysis) in the particular steel tested against distance from the centerline of the steel strip.

The upper graph of Figure 1 is for a conventional high Mn linepipe steel with a Mn concentration of 1.1 wt. %.

The lower graph of Figure 1 is for a linepipe steel in accordance with the present invention with a low Mn concentration of 0.3 wt.%.

It is readily apparent from a comparison of the two graphs that there was significantly less variation in Mn concentration in the vicinity of the strip centerline for the linepipe steel in accordance with the present invention. This indicates significantly less segregation in this linepipe steel. Consequently, the toughness of the ERW weld line for pipes made from centre slit strip can be significantly improved.

The improved toughness is illustrated by the results of further research work that are summarized in Table 1 below.

Table 1 provides the results of weld line Charpy

V impact tests on ^ΛΛ gull-winged" wall thickness specimens produced from 219 mm x 6.4 mm pipe made from a conventional high Mn linepipe steel and a low Mn linepipe steel in accordance with the present invention. The tests were made with the test specimens positioned with their notch locations coincident with the weld lines . The chemical compositions of both the high Mn and the low Mn

steel pipes tested in this way are provided in Table 2

Table 1. Weld Line Charpy V Impact Tests

Table 2 Chemical Composition of Pipes Subject to Weld Line Charpy V Tests (wt%)

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The results in Table 1 are typical weld line Charpy test results obtained by the applicant in the research work.

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It is evident from Table 1 that the linepipe steel of the present invention had weld line toughness consistently higher than for the conventional high Mn linepipe steel tested.

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The research work carried out by the applicant investigated further the effect of low Mn concentrations on the Charpy V impact energy of linepipe steel strip in accordance with the present invention.

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The results of the further research work are presented in Figure 2.

Figure 2 is a graph of Charpy V Energy at -15° C versus -the Mn concentrations (in wt.%) of a number of linepipe strips in which the C content was held in the range of 0.08-0.10% and the S content was varied over the range of 0.003 to 0.010%.

Figure 2 shows that low Mn steels of the present invention can tolerate higher concentrations of S than higher Mn steels to obtain a given toughness . This is an advantage from the viewpoint of the practical issue of making steel with low S concentrations. In other words, it is apparent from Figure 2 that the low Mn alloy design approach of the present invention permits considerably higher S concentrations to be used to achieve a given specification requirement for Charpy V impact energy.

Many modifications may be made to the present invention described above without departing from the spirit and scope of the invention.