03_An Alloy Design Concept for Better Matching of Strength and Toughness in Pipeline Steel

7/30/2019 03_An Alloy Design Concept for Better Matching of Strength and Toughness in Pipeline Steel

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An Alloy Design Concept for Better Matching of Strength and

Toughness in Pipeline SteelsIts Development and Application

Pascoal Bordignon1, Klaus Hulka2

(1. Companhia Brasileira de Metalurgia e Minerao CBMM, Brazil; 2. Niobium Products Company GmbH NPC, Germany)

Abstract: The metallurgy of niobium microalloyed steels became to be better understood and spread worldwide in the 1970s. Based

on industrial trials in the 1970s and 1980s, an industrial demonstration heat was produced in 1985. It allowed intensifying the

deformation in the non-recrystallization region of austenite at higher rolling temperatures and thus, it was called the HTP (High

Temperature Processing) concept. The extended delay in austenite recrystallisation, obtained by the strong effect of niobium, allowed

more flexible rolling schedules in plate mills, making it possible to obtain high toughness levels, in addition to high strength, even in

plate mills with power limitation and having relatively low separating forces. In more recent years, with further development of

technology in the steel industry, very low carbon levels, well below 0.10%, became possible for applications like in line pipe steels.

Under this condition, the HTP steels combine an additional effect of niobium by lowering the transformation temperature, promoting

bainitic microstructures with excellent toughness and weldability at very high strength levels. As a result, the HTP steels became a

reality in the market, where combination of several properties at high levels are desired. For instance, those steels have been applied

in important pipeline projects, including the recently concluded, over 4,000 km long, West-East Pipeline in China. This paper

summarizes the development of the HTP steels, describing the metallurgical concept, more flexibility in thermomechanical

processing conditions, final product alloy design, microstructure and mechanical properties as well as its application in pipeline

projects.

Key words: niobium, microalloying, thermomechanical rolling, high temperature processing, pipeline steel

1 IntroductionWorld energy consumption is continuously increasing

and in the next decades two thirds of the total demand

still have to be covered by crude oil and natural gas.

The most economic way of transportation from the

well to the end user is via pipelines.

Since new wells are often located in arctic regions or

offshore and new resources can contain high amounts

of H2S or CO2, new demands in the material for large

diameter pipelines arose, such as high toughness at

low temperatures, thick wall and sour gas resistance.

Furthermore, the economy of a pipeline asks for high

transportation capacity and thus higher strength of the

steel in combination with a higher Charpy-V-notch

energy at operating temperature, in order to avoid

long running ductile cracks[1]

.

Starting already in the 1970s, but for sure since the

1980s, the most relevant pipe steel grades are X 65

and X 70. With the new demands mentioned before,

this steel grade had to be modified and also steel

grades with higher strength, such as X 80 are

considered in new pipeline projects.

2 Metallurgical BackgroundThermomechanical rolling is the standard means to

produce plate or strip for high strength large diameter

line pipe in order to fulfil the economic demands and

safety requirements of pipelines. It relies on

processing austenite in the temperature region of non-

recrystallisation and is the most efficient method for

achieving grain refinement and thus both, higher

strength and toughness.

All these steels are niobium microalloyed. If the

amount of solute niobium is increased, retardation of

austenite recrystallisation is observed at significantly

higher temperatures, Figure 1[2]

, thereby allowing the

thermo-mechanical rolling to occur already at higher

temperatures.

Several metallurgical mechanisms, such as grain

refinement, solid solution, dislocation or precipitation

hardening, but also the carbon and the free nitrogen

content influence the yield strength and the toughness

of steel. Figure 2[3]

shows some of the factors for a

steel with 0.08 %C 1.50 %Mn. The role of niobium

is to prepare a finer grain size and to reduce the free


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Fig.1 Retardation of austenite recrystallisation by

microalloying

nitrogen content. It is assisted or can be substituted to

a certain extent by lower finish rolling temperatures.

With this base composition the positive role of

niobium gets diminished at levels higher than about

0.05 % and in order to achieve high strength and good

low temperature toughness, low finish rolling

temperatures in the lower austenite region are needed.

However, there are metallurgical situations or facilitylimitations, where the necessary processing at

temperatures of the metastable austenite is not feasible

or advisable.

As already shown in figure 1, the best possibility to

carry out the austenite conditioning at higher

temperatures is to increase the amount of niobium in

solid solution. For a given reheating temperature, this

is possible by lowering the carbon content and by

fixing the nitrogen by titanium, an element, which

shows a higher affinity to nitrogen than niobium as

described already elsewhere [4]. In the following, steel

with niobium levels above 0.07 % (most typical 0.10

%), carbon levels below 0.06 % (most typical 0.03 %)

and a Ti/N treatment will be called HTP steel (HTP =

high temperature processing).

Modern high strength low alloy (HSLA) steel

typically comprise carbon levels, which avoid the

peritectic reaction during solidification, which may be

responsible for surface cracks formed already in the

crucible during continuous casting[5].

The lowering of the carbon content has also a positive

Fig.2 Mechanical properties of a 0.08 %C 1.50 %Mn

steel as a function of niobium content and finish rollingtemperature

influence on the homogeneity of the microstructure.

Figure 3 describes the tendency for segregation by

means of the Fe-C diagram: The peritectic reaction,

where the liquid steel and the already formed -ferrite

will be transformed into -iron, is connected with an

additional shrinkage and causes an interdendritic

inclusion of the remaining liquid, which is enriched

with alloying elements. This inhomogeneousdistribution of alloying elements, especially in

manganese, is the origin of banded microstructures in

the final product. This is another reason, why low

carbon levels are aimed for.

If the carbon content is below the threshold value of

0.09 %, the solidification goes via the -ferrite phase.

In this case not only any indendritic segregation is

avoided, but also the crystal segregation gets reduced

with lower the carbon content: The smaller interval of

the liquidus to the solidus temperature results in

reduced crystal segregation during solidification and

the bigger interval in the -region facilitates the post

solidification homogenization by diffusion.

It is well known that with lower carbon content many

properties are being improved, such as the ductile-

brittle fracture transition temperature, the impact

energy, the ductility and formability and last but not

least the weldability. Furthermore, the improved

homogeneity itself has a positive effect on the

resistance against hydrogen induced cracking.


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Fig.3 Part of the Fe-C diagram with classification of the

segregation severity

3 Results of a Demonstration Heat

Based on experience of earlier trials, an industrial

demonstration heat according to the HTP concept withthe chemical composition given in Table 1 was

produced in 1985. Since sour gas resistance was also

aimed for, the sulphur content was kept below 10 ppm

and proper calcium treatment was also applied.

Table 1 Chemical composition of an HTP

demonstration heat

Chemical composition in wt.%

C Si Mn P S Al

0.028 0.24 1.77 0.007 0.0008 0.030

Cu Cr Ni Nb N Ti

0.29 0.27 0.17 0.100 0.0035 0.014

Slabs of this heat have been distributed to many

experienced pipe plate and strip producers, who rolled

this material according to their experience and needs.

The results have already been published[4]

and just a

few are being summarized here again.

If air-cooling is applied after thermomecha-nical

rolling, one obtains X 70 properties with excellent

toughness, figure 4, even when the finish rolling

temperature is 150 C higher than typical. A small

impairment in the Batelle drop weight tear test

transition temperature is observed in that case, but the

absolute value is still outstanding. Furthermore, by

finish rolling in the two-phase region + the strained

ferrite brings up a huge strength increase, and X 80

properties are achieved. By this approach the BDWTT

transition temperature is not impaired only a certain

reduction in the Charpy-V-notch energy at subzero

temperatures occurs owing to separations.

Fig.4 Influence of finish rolling temperature on the

mechanical properties of air-cooled plate

It is worth mentioning that the described results were

based on a rolling schedule, where the final

deformation started with a sheetbar thickness of 3.5

times the final plate thickness. If the total deformationduring finish rolling is lower, e.g. 3.0 times the final

thickness, then both, the yield strength and the

transition temperature are impaired, barely sufficient

to guarantee X 70 properties[6]

.

Other than in conventional pipe plate with higher

carbon content, this alloy allows a relevant amount of

niobium to stay in solid solution at finish rolling

temperature and dependent on the processing

conditions up to 50 % of the niobium content are not

precipitated. The amount of niobium in solution is

higher with higher finish rolling temperature,


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promoting a higher volume fraction of bainite, figure 5.

This explains one result shown in Figure 4, i.e. a

certain strength increase is observed with higher

finishing temperatures. In all cases the microstructure

consists of poligonal ferrite and low carbon bainite

(=acicular ferrite) only, and this in different portions

dependent on the processing conditions, but no

pearlite or martensite at all.

Fig.5 Correlation between niobium in solid solution,cooling rate and microstructure

If accelerated cooling is applied - a typical cooling

rate for 18 mm plate is around 15 C/s - the

microstructure is almost 100 % bainitic, with the result

that the yield strength is higher than 600 MPa I and

also the low temperature toughness is further

improved, figure 6. Since niobium in solid solution is

even more effective in lowering the transformation

temperature when involving accelerated cooling, the

HTP alloy design is especially suitable for that

production route[7]

. With higher cooling rates, such as

50 C/s, even higher strength is achieved[8]

.

Niobium in solid solution can also add to further

strength increase by precipitation hardening the ferrite

after transformation. If a slow cooling rate at 550 to

500 C is applied (stack cooling or coiling), this

strength increase amounts to about 50 MPa.

4 Pipe Forming, Welding and HIC-Resistance

Fig.6 Influence of cooling conditions during and after to

transformation on the mechanical properties

Plate with (partial) bainitic microstructure exhibits

almost no Lders elongation in the stress-strain curve.

Therefore also the drop in yield strength, often

observed in pipe forming - the Bauschinger effect -, is

not observed with this steel; in most cases the

involved cold deformation even results in a small yield

strength increase.

Welding simulation of the grain-coarsened heat

affected zone (HAZ) showed, that one obtains a

bainitic microstructure for a wide range of cooling

rates (=welding processes). The low carbon bainitic

microstructure guarantees excellent toughness. This is

in line with fundamental considerations, figure 7[9]

,

showing that besides the dominating influence of the

carbon content, also the overall alloy content

determines the HAZ toughness. Since higher alloy

content lowers the transformation temperature, the

final microstructure changes from coarse ferrite side

plates, via granular bainite to acicular bainite,

corresponding to a continuously finer effective grain.

However, when the alloy content gets too high,

martensite islands may be formed, impairing the

toughness. The HTP alloy concept corresponds to a

chemical composition guaranteeing the optimum HAZ

toughness for a big variety of welding processes,

including submerged arc welding and field welding.

Test results of submerged arc weldments confirm the


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Fig.7 Impact toughness in the simulated HAZ

excellent toughness data in the base metal and the heat

affected zone. The relatively lowest toughness occurs

in the weld metal, figure 8. These data show that the

best results were obtained with a wire guaranteeing a

bainitic microstructure used together with an alumina

based basic flux, suitable to pick up the base

composition.

When the necessary low sulphur content and the

correct Ca/S-treatment are guaranteed, the low carbon

content of HTP steel should be beneficial avoiding

hydrogen induced cracks (HIC). Test results shown inFigure 9 confirm, that even prolonged testing

conditions (typical are 96 hours) do not lead to any

cracks.

Fig.8 Toughness of submerged arc weldment

Fig.9 Results of HIC testing in pH=3.2 solution

5 Recent Experience in Pipeline Projects

In the recent years several of large diameter pipe

projects applied the HTP concept. In this context it

should be mentioned, that already in 1971/72 a 0.04

%C 1.60 %Mn - 0.25 %Mo 0.06 %Nb steel had

been used for a Canadian pipeline[10]

. However, most

of the actual application differ from this historic

example by the fact that plate production made use of

the installed accelerated cooling device.

Table 2 gives an overview about pipeline projectsapplying the HTP concept and the chemical

composition used.

In 1997/98, Pemex applied this concept for an 84 km

offshore gas pipeline in the Gulf of Mexico, the

Cantarell project, which had to guarantee HIC

resistance. The production of these pipes involved

various companies, with Ispat Mexicana for

steelmaking, Bethlehem Steel (US) for plate rolling

and the Mexican pipe mill PMT [8]. Remaining slabs

from this order were distributed by CBMM to a large

number of clients around the world, who developed

their own experience with this alloy concept. The

results were collected and data regarding the influence

of the carbon and manganese content and the

processing conditions have been summarized[8]

.

A Chinese pipeline was built in the early 2000s,

bringing natural gas from Tarim in the West of China

over 4,000 km to Shanghai. Several companies were

involved in supplying longitudinal or spiral welded

pipes. The 26.6 mm thick wall pipes were supplied by

Europipe, with steelmaking and plate had some


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specific demands: for the first time the microstructure

was defined to be acicular rolling at the shareholders

Dillinger Htte and Mannermannrhrenwerke[11]

. The

project had some specific demands: for the first time

the microstructure was defined to be acicular ferrite

and also the demands in toughness for the HAZ and

the weld metal were higher than typical for X70

onshore lines. The suppliers decided to follow the

HTP concept instead of adding either Mo or Cu+Ni to

a 0.045 % Nb steel, since the overall properties were

better and the small increase in niobium was also the

cheaper solution.

Table 2 Examples of HTP projects

Project Steel grade wall Mean Chemical Composition in %

mm C Si Mn N Ti N b Cu Cr Ni

Cantarell X 70 sour gas 22.9 0.028 0.16 1.46 0.0042 0.011 0.100 0.27 0.27 0.16

WestEast China X 70, high weldment toughness 26.6 0.050 0.25 1.68 0.0050 0.018 0.077

Cheyenne Plains X 80 11.8 0.050 0.15 1.58


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minimum yield strength of 550 MPa for the

automotive industry[17]

, guaranteeing a narrow scatter-

band of mechanical properties and the excellent hole

expansion behaviour as result of the homogeneous

microstructure.

The excellent property combination and the ease of

fabrication allows the assumption that the HTP

concept will be used more extensively in the near

future. The application might spread to thick plates

and shapes for welded construction and also to new

welding processes such as laser or electron beam

welding.

References:

[1] Proceedings of an International Seminar onFracture in Gas Pipelines, Moscow (USSR), 1984,

published by CBMM, Sao Paulo (Brazil).

[2] L.J. Cuddy, Thermomechanical Processing of

Microalloyed Austenite; TMS, Warrendale (Pa), 1982,

p. 129.

[3] K. Hulka, B. Bergmann, A. Streisselberger and F.

Heisterkamp, Processing, Microstructure and

Properties of Microalloyed and Other Modern High

Strength Low Alloy Steels, ISS, Warrendal (Pa), 1992,

p. 177.[4] K. Hulka, J.M. Gray and F. Heisterkamp,

Niobium Technical Report NbTR 16/90, CBMM,

Sao Paulo (Brazil), 1990.

[5] R. Hammer et al., Stahl und Eisen 109 (1989), Nr.

6, p. 277.

[6] K. Hulka, J.M. Gray and F. Heisterkamp, Pipeline

Technology,Volume II, Brgge (Belgium), 2000.

[7] S. Okaguchi, T. Hashimoto and H. Ohtani,

Thermec 88, ISIJ, Tokyo (Japan), 1988, p. 330.

[8] K. Hulka, P. Bordignon and J.M. Gray, Niobium

Technical Report No 1-04, CBMM, Sao Paulo (Brazil),

August 2004.

[9] K. Hulka and F. Heisterkamp, HSLA Steels 95,

The Chinese Society of Metals, Beijing (China), 1995,

p. 543-551.

[10] R.L Cryderman et al., Proceedings of the 14th

Mechanical Working and Steel Processing Conference,

AIME, 1972, p. 114 .

[11] M. Grf, J. Schrder, V. Schwinn and K. Hulka,

Pipe Dreamers Conf. Proc., Yokohama (Japan), 2002,p.323.

[12] D. Stalheim, private communication, Dec. 2003

to May 2004.

[13] O.A. Bagmet and Yu.I. Matrosov, private

communication in 2005.

[14] H. Tamehiro et al., OMAE 1993, ASME, New

York (NY), 1993, Vol. V, p. 319.

[15] K. Hulka, H.G. Hillenbrand, F. Heisterkamp and

K. Niederhoff, Microalloying 95, ISS, Warrendale

(Pa), 1995, p. 235.[16] R. Grill and R. Schimbck, private

communication 2003.

[17] W. Hnsch and C. Klinkenberg, TMP 2004

Conference Proceedings, Verlag Stahleisen,

Dsseldorf (Germany), p. 115.

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03_An Alloy Design Concept for Better Matching of Strength and Toughness in Pipeline Steel