quantitative examination of segregation in slabs for...

International Pipeline Conference — Volume 1ASME 1996

QUANTITATIVE EXAMINATION OF SEGREGATION IN SLABS FOR THE PRODUCTION OF SOUR SERVICE LINEPIPE

Bernhard Hoh Technology Development OREGON STEEL MILLS

14400 N. Rivergate Blvd. Portland/Oregon 97208 Phone: (503) 978-6191 FAX: (503) 240 - 5795

Abstract

Segregation is a major problem to be overcome by producers of HIC resistant steels. Primary segregation is an inherent and unavoidable feature of the solidification process. The constitutive relationship between solid and liquid stage determines microsegregation and it is influenced by the chemical composition of the steel and its cooling rate. Macrosegregation occurs when microsegregated liquids collect and shift through liquid flow.

OREGON STEEL MILLS has conducted systematic measurements on pressure-cast slabs using a computer assisted micro analyzer. This microprobe measures element concentrations over a large area of the specimen. A statistical evaluation based on the frequency distribution of the concentrations forms the basis of a quantitative analysis. From this, characteristic parameters such as the segregation factor and maximum concentration can be derived.

This paper discussed the influence of carbon on segregation structure and on segregation properties of manganese, and compares the results with those of continuously cast slabs.

1. Introduction

DSAW pipe manufacturing in recent years has seen a steady rise in the proportion of pipes resistant to Hydrogen Induced Cracking (HIC). The method to be employed for testing a material’s resistance to sour gas is standardized in NACE TM0284. Many specifications make even more rigorous

demands in that they stipulate the use of a testing solution to NACE TM0177 with a pH value of 3.The HIC process passes through three stages:

• hydrogen absorption from the aqueous solution• trapping of hydrogen in defects and imperfections and

crack start• crack propagation in the steel matrix

Hydrogen absorption is determined by the chemical composition of the gases being transmitted and the operating conditions of the pipeline. Crack start and crack propagation on the other hand are determined solely by the chemical composition and production conditions of the steel used.

Factors relevant for the production of HIC resistant materials are:

• chemical composition of the steel• shape of non-metallic inclusions• distribution of critical precipitates

1.1 Chemical composition of the steel

Throughout the world sourgas grade products are marked by a low carbon content of approx. 0.05 %. The manganese content lies between approx. 1.0 and 1.4 % according the grade and wall thickness. Today, as a rule, pipe grades are rolled thermo-mechanically. Niobium is used for grain refinement, vanadium increases strength through precipitation hardening.

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1.2 Type and shape of non-metallic inclusions

Whereas the type of non-metallic inclusion has no demonstrable bearing on HIC resistance its shape is of paramount importance.

In this respect manganese sulfides must be mentioned specifically. Because of their low hardness sulfides may be rolled into long, flat sharp-ended inclusions, especially at the low temperature employed for TM-rolling, thus creating preeminently suitable conditions for hydrogen absorption and crack start. It is therefore a basic metallurgical requirement to avoid the formation of manganese sulfides by all means if high HIC resistance is to be achieved especially at the 3 pH level. Today this is achieved by desulfurizing steel to the lowest level and subsequently combining the residual sulfur with calcium to form sulfides or oxisulfides.

Other potential crack initiators are accumulations of carbonitrides and clusters of oxidic inclusions.

1.3 Microstructure and distribution of critical precipitates

Segregation is a natural and unavoidable phenomenon of the solidification process of molten steel.

One must differentiate between:

• primary segregation (micro and macro), and• secondary segregation

Primary segregation is segregation that occurs during the solidification of liquid steel. Different solubility values for the alloy and residual elements during the successive liquid and solid phases of the process give rise to localized concentration shifts. It is important to distinguish between microsegregation (inter-dendritic segregation) and macrosegregation (e.g. centerline segregation which occurs in continuously cast material). Microsegregation is determined by the chemical composition of the steel and its cooling speed. Macrosegregation is the dispersion of microsegregated liquids to different parts of the solidifying product by various mechanisms.

The crystalline segregates which form during the two phase (liquid-solid) process can give rise to carbon separation in the solid state as the steel cools down further. During rolling after casting the (micro and macro) segregates are stretched in the direction of rolling. Locations with higher concentrations no longer occur as patches or meshes but are now arranged in bands.

During the transformation of a homogeneous austenite, ferrite formation starts at the austenite grain boundaries. On the other hand an austenite rendered non-homogenous by crystal segregates behaves differently. In elements such as manganese (typically used in pipe steels) which lower the

A3 temperature, pre-eutectoidal ferrite precipitation starts in areas or bands of low concentration (dendrites) because there the As temperature is higher than in areas of higher concentration. Therefore a ferrite microstructure will be formed in dendrite areas whereas in locations of higher concentration a pearlitic microstructure will be formed in the dendrite arms after transformation (ferrite-pearlite bands). This can be remedied by accelerated cooling after rolling. In doing this the cooling rate must be chosen in such a way that carbon diffusion is restrained sufficiently (without causing formation of martensitic or other hard microstructures) for a largely band free structure to emerge.

The steel producer is solely responsible for setting the chemical composition and steel desulfurization to give the lowest sulfur values as well as for ensuring the bonding if the residual sulfur to become non-distorting calcium-oxi-sulfides.

Using accelerated cooling (ACC) after TM rolling influences secondary segregation and therefore implicitly ferrite-pearlite banding (1, 2). This can, however, only reduce propagation of hydrogen induced cracks.

The process of casting steel on the other hand presents entirely different problems, especially during primary segregation. Given the low carbon levels mentioned above, difficulties do not normally arise during micro segregation in the NACE test. The slab center, however, is extremely susceptible because of macro-segregation which is more or less unavoidable here. The occurrence of major cracking in pipes or plates during NACE testing is almost always due to centerline segregation in the slabs. All suppliers of material for welded pipe have in the past had to devote major efforts to improve casting facility (e.g. by using rigid casting rolls and soft reduction) in order to increase and guarantee uniform manufacturing quality of sourgas grade steel.

2. Testing for segregation

Since materials performing well in the NACE test have been developed, different methods to determine segregation properties have been devised. As mentioned above, the essential foundations for good HIC resistance are laid in the steel mill. The NACE test is without doubt an excellent - if very rigorous - test to determine the segregation properties of steel. The manufacturing risk for sourgas resistant steel is high as the corrosion test can only be carried out on the finished plate or pipe, which means there is a great time lapse between manufacture and test result. The test is not a real time test, and the results are too late for a direct evaluation of the manufacturing conditions during melting, during secondary metallurgical treatment as well as during casting.

The usual test carried out on slabs such as the Baumann-test (sulfur print) or macroetching provide only an imprecise picture of segregation properties. They have proved to be satisfactory but not conclusive.


With the development of automated electron beam microprobes (EMPA) a much more differentiated picture of segregation in cast products has become possible. The method enables the determination of the segregation- dependent maximum values for content of segregation elements over sufficiently dispersed locations (3). In order to describe segregation quantitatively it is, however necessary to derive characteristic parameters from the measurement data(4). Segregation morphology and intensity can be derived with this method.

3. Testing material

World-wide, the starting material for the manufacturing of DSAW sour service linepipe is produced by continuous casting. OREGON STEEL MILLS uses bottom-pressure casting (Amsted) to manufacture slabs which are rolled into plates for large-diameter pipe production.

In a study at the Mannesmann Research Center in Duisburg/Germany, EMPA was used for examining specimens from the segregation areas of pressure cast slabs.

The following illustration of OSM slabs describe the method employed to collect characteristic segregation data with the help of EMPA.

To give an initial overview, the distribution of concentrations measured for selected segregation elements - in this case manganese - is represented in false color imaging (Fig.1).

Fig. 1: Manganese distribution (overview)

The following Figure shows the distribution of concentrations of the elements manganese (Fig. 2 a), niobium (Fig. 2 b), and phosphorus (Fig. 2 c) as they occur in the centerline segregation of the slab. The quantitative analysis of segregation is based on the normal distribution of the element concentrations and is shown below each element segregation picture.

Fig 2 a.: Distribution of manganese in centerline

Fig 2 b .: Distribution of niobium in centerline


To describe segregation in the specimen the segregation factor (SF) and the maximum element concentration (Cmax) can be derived from the statistical data:

SF = C m a x / C o D (ID

C m a x = C o S + 3 <7 (III)

where:

CoDCoSo

: average concentration in the dendrite field : average concentration in center-line segregation : standard deviation

3.1 Chemical composition of the test heats

For the test heats two different carbon contents were specified. We should explain that the specimen with the highe carbon content was only used to demonstrate the influence carbon has on segregation properties in pressure-cast slabs. Adequate MIC resistance properties were neither desired nor expected from it.

The production of the test heats took place in the course of an order for API grade X 65/70 pipe with HIC resistance to 5 pH. This batch had a wall thickness of 0.720” (19.3 mm).

The plates were rolled in OSM's rolling mill in Fontana, CA which has since been closed. The rolling mill did not have accelerated cooling facilities. Because the pipe was intended for laying off-shore it was necessary to ensure that the longitudinal tensile requirements conformed to API X 65 so that for the transverse tensile test had to measure up to X 70 values.

4. Test results

4.1 Chemical analysis

The two test heats (Table 1) differed in their carbon content. Heat B exceed the target value for carbon.

Fig 2 c.: Distribution of phosphorus in centerline

According to R.K. Poepperling and P. Schwaab (4) overall segregation in the specimen can be described with fair precision by the following formula:

where:

Q — Go + G* + G**

GGo

G*

G**

: overall segregation: element concentration in undisturbed dendrite field

(black): concentration in the weak centerline segregation

(green): strong centerline segregation (red)

Table 1 : Chemical analysis of two test heats


In the case of heat A the sulfur value is close to the threshold of 15 ppm. In addition to this, at a value significantly below 2 the heat’s Ca/S ratio is rather low.

4.2 Segregation test results

The slabs tested had a thickness of 8 inch (200 mm). Three slab slices were extracted from each of the two test heat parent slabs, each of which weight approx. 42 US tons (3 8 1).

Specimens for EMPA were prepared from each slab using the segregation areas: top, middle and bottom. Thus, 9 specimen per slab were tested.

The specimen size for the microprobe test was 3" x 4” x 0.4” (75 x 100 x 10 mm). For each specimen pictures providing a general overview were taken as well as a segregation map and data for the segregation elements manganese, phosphorus, and niobium. As part of this publication the pictures and test results for manganese will be presented.

Heat A (0.042 % C)

Heat B (0.080 % C)

Fig. 3: Segregation specimens (overview) from both test heats

Whereas the slab from the heat containing 0.04 % C shows favorable segregation results the heat containing 0.08 % C shows clear centerline segregation. This becomes more evident when one looks at the section showing the slab center (Fig. 4).

Heat B (0.080 % C)

Fig. 4: Selective enlargement of segregation specimensshowing the distribution of manganese segregation in the slab center

In the case of the higher carbon content the dendrites are much more delicate and show distinctly variable concentrations. At 0.04 % C on the other hand, the dendrites look blurred. The distance between the dendrite arms has considerably increased. The reason for this is in the solidification structure of 8-Fe. As opposed to the dense structure of face centered cubic (FCC) austenite the loose


structure of body centered cubic (BCC) 6-Fe offers good diffusion possibilities for carbon (and to a lesser extend also for manganese). The lower the carbon of a heat the more time is available for diffusion.

The planar frequency of manganese distribution for the two illustrations above can be seen in Ffg. 5.

Heat A (0.042 %)

Heat B (0.080 %)

Fig. 5: Distribution of area fractions of manganese concentration

The coefficients of the different G-curves are determined by a correlation computation and lie as a rule at > 0.99.

Differences between the curves especially in respect of the heavily segregated middle region are self-evident.

Table 2 summarizes all results from the different positions of the two slabs.

Heat A B

| P 1 ! I*j jL*» 0.080 v

Zo Mn 1.37 1.34Slab Location Cmax.{%) Cmax.(%) * SF •2/3 top 2.07 1.5 2.43 1.8

middle 2.07 1.5 2.45 1.8bottom 1.99 1.5 2.41 1.8

4/5 top 1.78 1.3 2.78 2.0middle 1.72 1.3 2.44 1.8bottom 2.40 1.8 2.27 1.7

5/6 top 2.00 1.5 2.51 1.9middle 1.91 1.4 2.82 2.1bottom 1.91 1.4 2.56 1.9

1.98 2.52 1.8

Table 2: Segregation factors

The mean values for the segregation factor of manganese derived from the two slabs from the two different heats stands at SF = 1.5 at 0.042 % C and SF = 1.9 at 0.080 % G respectively.

The influence of carbon on the segregation factors of manganese in continuously cast slabs is well known from the literature (5 ,6 ,7 ). Figure 6 shows the segregation values established by H. Tamehiro and H. Chino (7).

Fig. 6: Influence of carbon content on segregation factor of manganese


The range of values derived from the present test on OSM slabs lies within the scatter range of this curve. From this it becomes clear that segregation factors from pressure cast slabs correspond to those from continuously cast slabs.

Many elements form coarser non-metallic inclusions or precipitates. Notable in this context are manganese sulfides and the carbonitrides of the alloying elements niobium, vanadium and titanium. Concentrations of this order are not captured by the mathematical model described. The frequency curve for manganese in this example shown points to the presence of such particles - in this case manganese sulfides. Given the relatively high sulfur content, for steel intended for sour service product, of 15 ppm and in particular the Ca/S ratio of only 1.7, manganese sulfides are to be expected. Since these inclusions act as crack starters optimal values in the HIC test, especially for pH 3, cannot be expected.

4.3 HIC test results

The HIC test conducted in conformity with NACE TM0284 in the context of these researches used the enhanced test solution per NACE TM0177 (pH 3). The identification of hydrogen induced cracks was effected with the help of an automated ultrasonic test (8). This procedure for HIC tests is now generally used when making metallurgical evaluations in steelworks. The Crack Area Ratio (CAR) thus derived is expressed as a percentage of cracked surface to the total surface area of the specimen.

Crack Area Ration (CAR in %)

Carbon Content (%)

The HIC test was conducted on two plates specimens from each of the two different heats. The CAR values shown are in each case the mean values from three individual HIC specimens.

The strong influence of carbon on HIC susceptibility is well known from existing literature (9).

Figure 7 shows the relationship established by own experience.

The results from OSM plate and pipe production lie above this curve, especially in case of the heat with 0.08 % C. The reason for this lies in the pronounced ferrite-pearlite banding induced during rolling.

5. Production of sour service grades at OSM

The steel is melted in a 85 US tons (7 7 1) electric arc furnace. The necessary target sulfur content necessitates the use of good quality scrap. The adjacent ladle furnace equipped with EMS enables exact adjustment of the chemical analysis and casting temperature. Sulfide shaping takes place with air excluded in the tank degasser using CaSi wire.

The slab dimensions are 100” x 400” x 8” (2,540 x 10,160 x 200 mm). Pouring the slabs in the pressure caster takes about 8 minutes. After casting all slabs are as a matter of standard practice cooled slowly under covers. This is done primarily to avoid stress cracking. Stacking temperatures are around 2,000° F (1,100° C). The slabs remain covered for 24 hours and are unstacked at about 550°F (300° C). Although longer times and higher temperatures are needed for “slab soaking”, as it is called, we can safely assume that the above procedure makes some diffusion compensation of the carbon and the segregation elements possible.

Next the slabs are cut according to charging size and rolled. With the commissioning of the new Steckel/plate rolling mill and the accelerated cooling facility in the second half of 1996 it will be possible to obtain optimal properties of strength and toughness. The ACC will enable homogeneous microstructuring of the plate which will have a positive effect on HIC values.

The rolled plate is formed and welded into pipe using UOE process at NAPA PIPE in Napa Valley, CA.

Using the production route described OSM/NAPA PIPE have since 1994 delivered a total of around 200,000 tons of DSAW pipe meeting NACE test (pH 3 and pH 5) to various customers (Table 3).

Fig. 7: Influence of carbon content on HIC susceptibility


Year Dimension Grade Tons pH1994 24" x 0.500" (12.7 mm) X 60/65 23,200 3

24" x 0.688" (17.5 mm) X 60/65 1,000 328" x 0.312" (7.9 mm) X 65/70 4,200 528" x 0.469" (11.9 mm) X 65/70 16,400 536" x 0.720” (18.3 mm) X 65/70 67,200 536" x 0.785" (19.9 mm) X 65/70 32,800 536" x 0.811" (20.6 mm) X 52 19,600 520" x 0.348" (8.8 mm) X 60 2,500 5

1995 16" x 0.437" (11.1 mm) X 65 8,000 520" x 0.500" (12.7 mm) X 65 18,000 518" x 0.280" (7.1mm) X 52 3,500 5

Table 3: Production of DSAW pipe for sour service

Pipe 24" x 0.500" (12.7 mm)

36" x 0.720" (18.3 mm)

Orientation trans. long. trans. long.Yield (MPa)

Tens. (MPa) Y/T-ratio

485 495 496 496563 548 579 5580.86 0.86

Toughness CVN, test temp. 0° CBase (J) Weld (J) HAZ (J)

330 31271 68

313 307

Table 5: Production results on API grade X60/65 and X65/70 pipe for sour service

The underlying analysis and the mechanical properties The HIC test were conducted in conformity with NACEattained are now discussed. TM0284. The results of the HIC test using the pH 5 test

solution and the test solution per NACE TM0177 (pH 3) are The mean chemical composition of two large diameter pipe given in Fig. 8.orders is given in Table 4.

Pipe 24" x 0.500" 36" x 0.720"(12.7 mm) (18.3 mm)

C 0.042 0.041Mn 1.19 1.20Si 0.27 0.28Cu 0.23 0.24Ni 0.11 0.13Nb 0.049 0.044V 0.04 0.04Ti 0.015 0.014Al 0.034 0.034N 0.0071 0.0073P 0.005 0.005S 0.0018 0.0020

Ca 0.0023 0.0027Carbon Equivalent

IIW 0.275 0.287pern 0.124 0.132

Frequency (%)100

80

60

40

20

1« 2 - r x 12 .7 m n n (p H 3)

111 3 6 " x 1 8 .3 m m (p H 5 )

1 - 4 = - 4 - ■4- - 1 - —1—2 4 6 8 10

Crack Length Ratio (CLR in %)

Table 4: Chemical analysis of API grade X60/65 and X65/70 for sour service

Fig. 8: HIC test results on X60/65 and X65/70 production pipe

The following Table 5 shows the principle mechanical properties. All requirements specified for each order were met.

All sour service requirements specified were fully met.


6. Summary

Unavoidable segregation within cast slabs is a significant impediment to the development of alloys for welded linepipe for the transportation of sour media (gas or crude). When discussing segregation phenomena it must be noted that various forms of segregation occur which also have distinct effects on the various stages of the HIC process.

The essential basic conditions for good HIC resistance in the finished product are laid during steel production and casting. Rolling can make only an insignificant contribution to reducing the extent of cracking.

The steel production process itself must guarantee minimum tolerances in chemical composition in respect of carbon content and alloy elements. The guiding principle must be "as little as possible" (because of the impact on segregation) but also "as much as it takes” (on account of the necessary strength properties).

To avoid formation of manganese sulfides, desulfurization to minimal values followed by Calcium treatment for shape control are indispensable.

Low carbon content minimizes microsegregation by diffusion in the 8-phase. It also has a positive effect on segregation properties of the allying elements.

Special requirements and provisions need to be met when the steel is cast. Macrosegregation is process-specific and unavoidable. All available measures must be taken to minimize them. The need to monitor the casting process, especially for sour service products, is permanently there.

A computer-aided microprobe is used to measure the concentration of elements covering a large specimen area. Statistical evaluation of the frequency of concentrations of elements enables a quantitative statement to be made as segregation.

It was shown with great clarity that segregation factors in pressure cast slabs correlate to those in continuously cast slabs.

The segregation factor established using EMPA is well suited for comparative tests, even tough it is neither the only nor the most important factor influencing HIC resistance.

The essential controlling parameter for HIC resistance is the carbon content. The low carbon content typical of heats for sour service, together with secondary metallurgical processes to avoid manganese sulfides, have proved to be sufficiently rigorous measures to keep an acceptable minimum any minor

weakness in the notoriously difficult to control segregation evidenced by HIC tests.

References

(1) H.-G. Hillenbrand, W.M, Hof, B. Hoh, “Modern Line Pipe Steels and the Effect of Accelerated Cooling", 4th International Steel Rolling Conference, Deauville, France, June 1-3, 1987

(2) V. Schwinn, A. Streisselberger and J. Bauer, “Various Approaches to Different Demands of Low Alloy Steels with specified HIC Resistance”, Corrosion 95, Paper 66

(3) T. Saeki, T. Komai, K. Miyamura, S. Mizoguchi and H. Kajioka, “Application of Spot Segregation Evaluating Methods in Continuous Cast Slabs”, In: Proceedings: 68th Steelmaking Conference, Detroit, April 14.-17., 1985, ISS-AIME, pages 229-235

(4) R.K. Poepperling and P. Schwaab, “Quantitative Untersuchung der Seigerung von Strangguss”, steel research , No. 9 (1990), pages 416-418

(5) K. Hulka, J.M. Gray and F. Heisterkamp,“Metallurgical Concept and Full-scale Testing of a High Tougness, H2S Resistant 0.03 % C - 0.10 % Nb Steel", Niobium Technical Report, NbTR -16/90, August 1990

(6) I. Nagakawa, T. Obinata, J. Kudo, F. Kawabata, M. Kimura and K. Amano, “Production of Line Pipe Steel with High Resistance to Hydrogen Induced Cracking", In Proceedings: Metallurgy of Vacuum- Degassed Steel Products, Indianapolis, Indiana, October 3-5, 1989, The Minerals, Metals & Material Society, 1990, pages 437-449

(7) H. Tamehiro and H. Chino, “The Progress in Pipeline Material Properties”, Nippon Steel Corporation, April 1991

(8) H.-J. Baethmann, M. Graef, B. Hoh, J. Kleinen and R. Poepperling, “Metallurgical Influences and Testing on HIC-resistance of Big Inch Line Pipe", Corrosion 84, New Orleans, April 2-6, 1984, Paper 203

(9) W. Haumann , W. Heller, H.-A. Jungblut, H. Pircher,R. Poepperling, W. Schwenk, “ Der Einfluss von Wasserstoff auf die Gebrauchseigenschaften von unlegierten und niedriglegierten Staehlen", Stahl und Eisen, Heft 12, Juni 1987


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