Metallurgical Perspective of Line Pipe Steel

 

Santosh Kumar, N Mondal, Ajay Arora

R&D Centre for Iron and Steel, SAIL, Ranchi

 

ABSTRACT:

Over the past 30 years, severe demands have been placed on the pipe manufacturer with respect to the development and processing of materials to pipe line. Line pipe steel is put to extremely severe conditions like deep water, cold climate, corrosive and high pressure applications etc. Their quality requirements are becoming exceptionally stringent with respect to metallurgical properties i.e. cleanliness, physical and mechanical properties etc. In line with these requirements, significant improvements have been undertaken for achieving the desired chemistry and bringing down the level of non-metallic residuals to meet stringent specifications of customers. This paper briefly summarizes the technological requirements of line pipe steel.

 

 

 

INTRODUCTION:

The development of high strength line pipe steel grades is major concern for a large number of steel manufacturers. The most popular document specifying the line pipe material is the American Petrolium Institute (API) Standard API 5L (API, 2012) that specifies steel grades with a range of properties. These steels have an improved tensile strength, yield to tensile strength ratio, elongation, weld ability, susceptibility to hydrogen induced cracking particularly for sour service applications, low temperature impact toughness and ductile to brittle transition temperature.

 

Three general types of line pipes exist: gathering lines, transmission lines, and distribution lines. For higher transportation efficiency, the trend in line pipe design is to use larger diameters with higher operating pressures. High-pressure lines generally operate at pressures up to 10 MPa and are made of steel pipes welded or mechanically coupled together.

 

This results in a requirement for steels with high yield and tensile strengths. In addition to these, demand of high material toughness is required for line pipe used in colder regions and/or operative at low temperature.

 

The steels that have been used for line pipe range in yield strength from (API) grade B (245 MPa) to API 5L-X70 (485 MPa). These properties are basically acquired by the suitable additions of different micro alloying components, for example, Ti, B, P, N, S, Mn, V, Nb and through controlled thermo-mechanical processing and cooling rates. Line pipe wall thicknesses are based on the pressure in the line and on the allowable hoop stress levels. The allowable stress levels for gas line pipes vary from 40 to 72% of the specified minimum yield strength. The ability to pre-service test a line pipe to a high stress level is a unique characteristic of a linepipe. The lines are designed such that they can withstand this type of test without undergoing plastic deformation that would induce residual stresses or damage the pipe.

 

One of the failure problems unique to gas line pipes was the potential length of a failure. The pressurized gas contains an enormous amount of stored energy and during rupture, the energy-release rate is often slow relative to the speed of a propagating fracture, a fracture may propagate along the pipe axis for a considerable distance. Thus, line pipes are among the few structures for which the extent of fracture propagation is of concern.

 

Therefore, General requirements for line pipe steel are:

·        A relatively high design pressure of 15MPa compared to 10MPa or less that is characteristically used in most parts of the world;

·        The transmission of rich gas that places special demands upon fracture toughness.

 

1. The Technical Challenge:

The major challenges in line pipe steel design that need to holistically review and respond to, include a combination of:

·        The need to achieve the demanding pipe mechanical properties, in particular high strength,

·        Steels that can be welded using the existing procedures of preheat free welding with cellulosic electrodes,

·        Higher Charpy energy levels, due to the higher operating stresses, as well as to permit rich gas transportation and

·        A low Pipe yield strength range of less than 100 MPa above Specified Minimum Yield Strength.

·        Less susceptible to Hydrogen embrittlement, low crack sensitivity coefficient, low ductile to brittle transition temperature.

 

This combination of properties has required careful attention to alloy design, steel making and hot rolling practices.

 

1.1  Alloy Design:

The family of Linepipe steel grades are based on a low Carbon (C), Manganese (Mn), Niobium (Nb) and Titanium (Ti) alloy design with a overall total low alloy content or carbon equivalent.

 

A key strategy in the development of higher strength Linepipe steel grades is the use of a low Molybdenum (Mo) addition in conjunction with micro alloying with Niobium (Nb) and Titanium (Ti) components. The synergistic effect within this alloy system provides a very efficient strengthening capability.

 

Accordingly, this alloy design allows the achievement of high strength at lower carbon equivalents than traditional Niobium-Vanadium (Nb-V) steels. The Nb-V steels require relatively higher carbon equivalent designs, which can compromise their capability for preheat-free field welding with cellulosic consumables. Moreover, the efficient strengthening capability of the Mo-Nb-Ti system provided the basis for the development of API X80. Chemical composition for PSL-2 pipe of API grades steel (API Specification 5L, 2012) is given in Table: 1.

 

Some other aspects of the alloy design include the use of calcium silicide treatment for the purpose of controlling non-metallic inclusion type and morphology and lowering the sulphur content. Low sulphur levels and non-metallic inclusion control improve notch toughness.

 

2.2 Steel making and casting practices:

The steel making process for production of Linepipe steel grades is complex and requires a considerable amount of strictness. Production of Linepipe steel grades requires the utilisation of special steel making and casting techniques that have been developed and refined over many years of high strength steel production. The main steelmaking processes involved are shown schematically in Figure 1.

 

Figure-1: Processes of line pipe steelmaking

Table-1: Chemical composition of PSL 2 welded pipe (API Specification 5L, 2012)

Grades	C	Mn	Si	P	S	Nb	V	Ti		Mo	N, ppm
	% Maximum
API X46	0.22	1.30	0.45	0.025	0.015	0.05		0.05	0.04	0.15*	60
API X52	0.22	1.40	0.45	0.025	0.015	Nb + V +Ti ≤ 0.15				0.15 *	60
API X56	0.22	1.40	0.45	0.025	0.015	Nb + V +Ti ≤ 0.15				0.15 *	60
API X60	0.12	1.60	0.45	0.025	0.015	Nb + V +Ti ≤ 0.15*				0.50*	40
API X70	0.12	1.70	0.45	0.025	0.015	Nb + V +Ti ≤ 0.15*				0.50*	40
API X80	0.12	1.85	0.45	0.025	0.015	Nb + V +Ti ≤ 0.15*				0.50*	40
API X100	0.10	2.10	0.55	0.020	0.010	Nb + V +Ti ≤ 0.15*				0.50*	40
API X120	0.10	2.10	0.55	0.020	0.010	Nb + V +Ti ≤ 0.15*				0.50*	40

* Unless otherwise agreed.

Carbon equivalent:     For C>0.12%, CE_IIW <0.43 &

For C≤ 0.12% CE_pcm<0.25

 

In primary steel making, carry over slag and reblow practice is most detrimental for making cleaner steel. Converter carryover slag and synthetic slag additions are two elements of the ladle steelmaking system that can be considered together, as they are the principal components which go to make up the 'Ladle Top Slag'. The ladle top slag floats on the surface of the steel in the ladle and has a profound impact on the inclusion population in the steel. Steel making slag produced in the primary section and the secondary sections are very different in nature, even though the basicity of both is around 3. Therefore, the carry over slag from primary steel making furnace always causes some detrimental effect on the processing of liquid steel. Carry over Slag during tapping is minimised by good tap hole maintenance (6-8 min tap time), slag stopper system (like DART etc.), infrared slag identification System etc.

Another one is minimisation of re-blow practice at converter. During reblow, bath carbon content is low and there is no vigorous C-O reaction to flush out the nitrogen. Also increasing the oxygen in bath, may result in increase in consumption of de-oxidiser. Hence reblow is harmful and can cause an increase of nitrogen content by as much as 30 ppm.

 

It may be seen that nitrogen pick during tapping is about 10-13 ppm. Somehow, nitrogen pick-up during tapping is controlled by controlled addition of deoxidiser during tapping and controlled bottom stirring of ladle. Nitrogen pick up in the range 18 to 25 ppm occurs during casting and can be controlled by shrouding of the metal stream during casting for all the heats consistently.

 

A major evolution in steelmaking has been the development of secondary steel treatments to further refine the steel, accurately control alloy additions and to provide a high level of steel cleanliness in respect to non-metallic inclusions. Such secondary steelmaking practices include injection of the steel with calcium silicide powder and degassing the steel by cycling it under a vacuum. In particular, these post steelmaking refining treatments enable the achievement of low sulphur levels, reduced levels of dissolved gases such as hydrogen, fine adjustments to the composition, particularly the micro alloying elements and condition the steel to control the non-metallic inclusion type, volume and morphology.

 

Detrimental effects of phosphorus in steel include various forms of embrittlement which reduce the toughness and ductility. The phosphorus requirement for linepipe ranges up to about 0.02%, but P_max 0.01% is desirable. In primary steel making process, phosphorus can be controlled by adopting double de-slag practices at EAF/BOF and minimisation of carry over slag practices during tapping.

 

The tight compositional control achieved by utilising vacuum degassing ensures a narrow range of carbon equivalent that ensures consistent weldability. Vacuum degassing also allows for fine adjustments to be made to the chemical composition, achieve the required hydrogen content, and to minimise the content of non-metallic inclusions.

 

During the continuous casting process ceramic shrouds and inert gas are used to protect the liquid metal stream and avoid oxidation of the steel. This aids compositional control, steel cleanliness and mechanical property control.

 

2.3 Thermo-mechanical processing:

Control of the entire rolling process from slab reheating to coiling is essential to ensure the appropriate strength and fracture toughness requirements are achieved in the hot rolled strip. The refinement of the ferrite grain size of the final strip simultaneously increases the strength and toughness of the steel. Hence, one of the main aims of the thermo mechanical controlled processing of the slab to strip is to maximize the ferrite grain refinement.

 

The slabs are reheated to the rolling temperature in a walking beam furnace. The slab extraction rate and furnace zone set points are specified in order to ensure uniformity of heat and therefore, efficient dissolution of micro alloys. The roughing mill pass schedule is designed to give reductions that will maximise austenite grain refinement.

 

The entry temperature and total finishing mill reduction are specified in order to control the metallurgical conditioning of the austenite as dictated by the mechanical property requirements. Similarly, the strip finish rolling temperature is specified above the austenite – ferrite transformation temperature.

 

Accelerated cooling is applied to the strip immediately to achieve very high cooling rates to enhance the steel strengthening potential. The amount of cooling applied and the coiling temperature are specified to produce as fine a ferrite grain size as possible whilst still enabling strengthening of the ferrite by the micro alloys. The thermo-mechanical rolling conditions have a major influence on the control of the final strip properties.

 

2.4 Pipe properties:

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. Pipe yield strengths can be either significantly increased or decreased.

 

Steel microstructure is a factor in this behaviour and underlines the critical interrelationship of alloy design and thermo mechanical processing factors.

 

2.4.1 Relativity of yield and tensile strength:

The yield and tensile strength relativities required by API 5L PSL2 present some particular challenges. Figure-2 demonstrates how the allowable range for both yield and tensile strength reduces with increasing the API 5L grade. It difficult to produce coil with sufficient tensile strength without exceeding the maximum allowable yield strength in the pipe, particularly given that the strength increase during forming is more pronounced for yield strength than for tensile strength.

 

Table-2: Tensile requirement for API 5L PSL2 grade steel

Grade	YS (MPa) min	YS (MPa) Max	UTS (MPa) min	UTS (MPa) max
API Gr B	245	450	415	655
API X42	290	495	415	655
API X46	320	525	435	655
API X52	360	530	460	760
API X56	390	545	490	760
API X60	415	565	520	760
API X65	450	600	535	760
API X70	485	635	570	760
API X80	555	705	625	825
API X100	690	840	760	990
API X120	830	1050	915	1145

 

 

Figure-2: Allowable range for yield and tensile strength

 

2.4.2 Yield/tensile strength ratio:

A critical design factor for API grade Steel is to manage the YS/TS ratio (0.93 (max) for API X60/ X70/ X80, 0.97 (max) for X100 and 0.99 (max) for X120). Low C steels rely on micro alloying and thermo mechanical processing to achieve strength, tend to have higher YS/TS ratios than higher carbon steel grades. The tensile strength reduced by lowering C level and yield strength increases by ferrite grain refinement and precipitation hardening, resulting a higher yield/tensile strength ratio.

 

2.4.3 Fracture toughness:

Traditionally, steelmakers have sought to control toughness by restricting sulphur levels to be below 0.005% (or even lower limits) depending on the specific requirements of a linepipe. Some steel manufacturers, as an additional countermeasure, have the capability to achieve complete sulphide shape control using Ca wire injection processes.

 

3 Recent Developments:

To cope with market requirement for enhancing strength, effort have been put to the development of Grade X80, X100 and X120 in recent time.  As per design parameter of linepipe by using grade X80 linepipe can led to material saving of about 10-12% as compared with X70 linepipe. Similarly material saving of around 25% and 35% in case of X100 and X120 can be achieved compared with X70 grade steel.

The development of high strength steel is shown in figure-3. Material up to X70 is produced that are microalloyed with Nb and V and having reduced carbon content. By improvement in processing method, consisting of thermo-mechanical rolling plus subsequent accelerated cooling, it has become possible to produce higher strength material like X80. By further decreasing C content and additions of Mo, Cu and Ni enable the strength level to raised to that of grade X100 and X120 when the steel is processed to plate by thermo-mechanical rolling plus modified accelerated cooling.

 

Figure-3: Development of API grade steel

 

 

4 CONCLUSIONS:

The growth in energy consumption in coming decades necessitates severe efforts for transporting large amounts of natural gas to the end user. Large diameter linepipes serve as the best and safest means of transport. For the reason of technical feasibility and cost effective production, it is necessary in the future to redefine material requirement for linepipe. API grade steel is largely accepted material for linepipe. After reaching grade X70 the next step in the development of the high grade linepipe led to X80, X100 and X120. The main objective of this development is the design of low carbon, low sulphur, low phosphorous and V, Nb, Ti, Mo, B microalloyed steel with Thermo-mechanical rolling and Moderate to accelerated cooling.

 

5 REFERENCES:

1.       F. Grimpe et al, “Development, production and application of heavy plates in grades up to X120”, 1^st International Conference Super-High Strength Steels, November 2-4, 2005, Rome, Italy

2.       H G Hillenbrand et al, “Development of Grade X120 pipe material for high pressure gas transmission lines”, 4^th International conference on linepipe Technology, May 9-12 2004, Ostend, Belgium

3.       Tariq Mehmood et al, “Development of api grade linepipe steels at Saudi Iron and Steel Company, Hadeed”, The 6^th Saudi Engineering Conference, December 2002, KFUPM, Dhahran

4.       Christoph Kalwa, et al., “High Strength steel pipes: New Developments and applications”, Onshore Linepipe Conference, June 10-11 2002, Houston, Germany

5.       J. Malcolm Gray , “An independent view of linepipe and linepipe steel for high strength linepipes” , API X-80 Linepipe Cost Workshop, Oct 2002, Hobart, Australia

6.       Santosh Kumar et al. “Development of API X 70 grade HR coils at BSL”, The Banaras Metallurgist, Vol. 16, 201

 

 

 

 

Received on 23.02.2017                             Accepted on 29.04.2017

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