Development of Line Pipe Steel at JSW

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Proceedings of “Steel Tech”, Vol 2, No 3, pp 33-37

167

Development of Line Pipe Steels at JSW Vijayanagar Works

D Satish Kumar, Madhusudhana R, S Manjini, P K Ghorui, P C Mahapatra,

Madhu Ranjan and A K Agarwal

Transportation of oil and natural gas through pipes to longer distances requires special grades of

line pipe steels and has been denominated as API grades (API X42, API X70, etc) based on strength

requirements in accordance with the American Petroleum Institute (API) standards. For enhanced

transport efficiency and reduced pipe laying costs, by the use of thinner wall pipes, the demand for

high strength line pipe steels is rapidly growing. The successful development and production of these

line pipe steels requires a close control of process parameters, alloy designing and cleanliness

controls. JSW Steel has started commercial production of line pipe steels since its inception. Over

the years, extensive work has been carried out to develop higher grade line pipe steels, to meet the

growing needs of customers in their endeavor to reduce weight and pipe laying costs. At JSW Steel,

the highest-strength grade of line pipes that have so far been brought to commercial application is

equivalent to API X70. This paper describes the development of line pipe steels, and addressesvarious issues of steel making and rolling to arrive at optimum structure and properties for its

specific applications.

Introduction

JSW Steel, Vijayanagar Works is presently a 4Mtpa integrated steel plant having two modules of

COREX C-2000 to produce 1.6 Mtpa and two Blast furnaces to produce 2.4 Mtpa of hot metal.

4Mtpa steel melt shop comprise three Basic oxygen furnaces, two Ladle heating furnaces, one RH

degasser and three single strand Slab casters and a Hot strip mill to produce 2.5 Mtpa of HR Coils.

The product range comprises of low carbon aluminium killed (LCAK), tube, structural, line pipe,micro-alloyed, and a range of medium carbon steels. It can be seen from the product mix distribution

(Fig 1) that the line pipe steels constitute of a major portion of value added products.

Fig 1: Product Mix at JSW Steel Ltd (2007-08)

JSW Steel has successfully developed a wide range of line pipe steels based on strength requirements

as per the API specifications (Table 1). Line pipe steels have a range of yield strengths to suit

applications of different criticality. In addition, the line pipe is required also to be excellent in

weldability in order to improve pipeline construction efficiency. These steels are produced with

various alloying elements [1,2] and tailored processes suitably to obtain the higher yield strength and

toughness. Since its inception in 2000, JSW has started commercial production of these high strength

steels (Fig 2). Till date, the highest grade of line pipe steel developed is equivalent to X70 and is inthe process of developing steels equivalent to X80 and steels for sour grade application.

COLD

ROLLING

4 1%

D R A W I N G

SLAB GRADE

12%

SPECIAL

8 %

STRUCTURAL

9 %

T U B E

12%




168

Table 1: API specifications for line pipe steels *

Grade

Yield

Strength,

Minimum

Yield

Strength,

Maximum

Ultimate

Tensile

Strength,

Minimum

Ultimate

Tensile

Strength,

Maximum

Mpa Mpa Mpa MpaX42 290 496 414 758

X52 359 531 455 758

X56 386 544 490 758

X60 414 565 517 758

X65 448 600 531 758

X70 483 621 565 758

X80 552 690 621 '827* API Specifications 5L, 43

rd edition, Mar 2004.

Fig 2: Commercial readiness of line pipe steels at JSW Steel Ltd

These line pipe steels when used for gas transportation, sometimes lead to some service leakages and

condensation/ freezing of gas under low temperature condition. This makes the low temperature

properties of line pipe grades more stringent. Combination of high tensile properties and low

temperature impact strength is controlled by optimal microstructure of steel, which is obtainable with

judicious effect of micro alloying and regimes of rolling.

The basic concept of producing line pipe steel includes:

• Lowering carbon equivalent level below 0.45

• Minimisation of sulphur and phosphorus contents in steel.

• Attainment of high strength with such low carbon by micro additions of carbonitride forming

elements (V, Nb, Ti);

• optimisation of casting, rolling and cooling conditions;

This paper explains the methods employed at JSW Steel, Vijayanagar Works to resolve metallurgical

and processing challenges for producing line pipe steels in the context of steel cleanness and

property requirement.

0

100

200

300

400

500

600

1

Year of Commercial Production

Y S , M p a

2001 2001 2004 2004 2005 2007 2008

~X42~X52

~X56

~X52~X65

~X70

~X80

P

L

A

N

N

E

D




169

HMPT BOF LHF CONTINIOUS CASTING HSM

Steel Making

Line pipe steels are produced at JSW Vijayanagar works, by the process route shown in Fig 3. The

processing of line pipe grade steels requires more meticulous control of parameters than the ordinary

cold rolling or galvanizing steels. The steelmaking and casting processes are required to not only

produce clean steel with morphology modified inclusions, but also to ensure ‘segregation and crack’free casting. In this respect the separate standard procedures were formulated to be followed during

the processing of these grades. This approach to steel cleanness was achieved by maintaining a

narrow range of processing parameters, casting temperature and chemical composition. The heats

made are of 130 t and sequence length is of 4/5 heats. All heats are de-sulphurised to 0.005 ‘S’

before being poured into the converter. The hot metal ratio is kept high with the use of minimum

scrap to avoid any ‘S’ inputs. Customized blowing patterns and addition strategy is followed for

these grades. Steel cleanliness is ensured with the use of dart at tapping and ladle slag detection

system at caster. Tapping practice involves complete killing of the bath to obtain maximum recovery

of the various alloying additions. Calcium is injected to modify the shape of alumina-based

inclusions from angular to round and to tie-up any dissolved sulphur as calcium sulphide. Higher

calcium / sulphur ratio is an important requirement in line pipe grades. This necessitates highercalcium addition in line pipe steel which sometimes, causes severe wear of the stopper rod in the

tundish and restricts the length of the sequence. This was resolved with usage of higher MgO

refractory material and optimization of Ca levels.

HMPT-Hot Metal Pre-Treatment, BOF-Basic Oxygen Furnace, LHF-ladle Heating Furnace, HSM-Hot Strip Mill

Fig 3: Process route for line pipe steels at JSW

Line pipe steels having low heat transfer phenomenon are one of the difficult to cast steels[3].

Separate mould powders with high viscosity are employed during its casting. Modified spray plans

with higher secondary cooling are used to avoid cracks and segregation. Macro-etched slab samples

show complete surface to centre columnar crystal structure with small equiaxed zone at the slab

centre. Macroscopic examination also confirmed very little segregation in the centerline region.

Some of the important changes or precautions made in the process during steelmaking are enlisted in

Table 2.

Table 2: Process specifications for line pipe steels

Hot MetalDirect ladle after treatment at HMDS (% S =0.005

max).

Charge MixDesulphurised hot metal. Minimum Scrap due to

low S requirement HM temp > 13000 C

Tapping chemistry C – 0.035 max, P – 0.010 max, S – 0.008 max

Tapping Procedure 1650-16600 C

% Slag FeO + MnO < 1.0 %

Super Heat 17-23 oC

Casting Powder For Ferrite potential: 1.05




170

Effect of Microalloying Elements

Line pipe steels are complex microalloyed grades and involve metallurgical concepts to include the

composition and precipitation of the carbide and nitride forming elements [2,3]. Their strength is

controlled by various mechanisms, viz. solid solution strengthening, grain refinement, precipitation

strengthening, dislocation strengthening and sub-structure strengthening [4]. First line pipe steels

were made with emphasis on ferrite-pearlite structure, which use additions of alloying elements suchas niobium and vanadium to increase strength of hot-rolled steel. Therefore steels equivalent up to

X56 were made with merely Nb & V combination. Niobium retards the austenite to ferrite

transformation and increase the strengthening by precipitation hardening. Addition of V increases the

TS without increasing the YS [5,6]. This helps in maintaining low YS/TS ratio. However for above

grades Ti, Cr and Ni were also added which lead to significant decrease in the carbon content thus

improving the weldability and impact properties without affecting the strength as shown in Fig 4.

Fig 4: Relation of YS & TS for line pipe steels

Additions of Ti, Cr and Ni enhance properties through mechanisms of precipitation strengthening,

suppression of grain growth and solute effects on transformation kinetics [2]. Titanium was added

also to bind nitrogen thereby preventing the precipitation of niobium carbonitride and making

niobium more effective for increasing the strength. With the increase in the grade, manganese

content was also increased which stabilizes the austenite phase and permits rolling to take place at

lower temperatures. Typical chemical compositions of line pipe steels made at JSW are enlisted in

Table 3

Table 3: Chemical compositions of line pipe steels at JSW

Steel C Si Mn S P Al Nb Others Ceq Pcm

~ X42 0.11 0.22 0.95 0.01 0.01 0.05 - V 0.28 0.17

~ X52 0.14 0.23 0.98 0.008 0.008 0.04 0.028 V 0.32 0.18

~ X56 0.11 0.23 1.00 0.003 0.008 0.04 0.028 V, Ti 0.32 0.17

~ X60 0.095 0.22 1.25 0.003 0.006 0.03 0.035 V, Ti 0.34 0.18

~ X65 0.13 0.19 1.27 0.005 0.006 0.035 0.037V, Ti, Ni,

Cr0.34 0.18

~ X70 0.065 0.3 1.45 0.005 0.006 0.028 0.042V, Ti, Ni,

Cr0.37 0.17

Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5

Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B




171

X56 X60 X65

Due to high alloy additions, these grades are prone to centre segregation. The problem is well

controlled by the developing customized spray plans for casting process and the maintenance of

casting machine alignment.

Fig 5: Optical microstructures (200X) of base material of line pipe steels

Figure 5 shows typical microstructures of three types of line pipe steel. The microstructures of these

material show good homogeneity. Banded ferrite and pearlite and coarse ferrite grain size (ASTM

8–10) are the characteristic features of conventionally rolled and normalized X56 and X60

equivalent steels respectively. The microstructure of X65 steels are more uniform and the ferrite

grains are finer (ASTM 11–12). The improved properties of these steel can be attributed to its

uniform ferritic-pearlitic microstructure.

Rolling

Rolling parameters also play important roles in processing of line pipe steels. They control the final

microstructure through the kinetics of various physical and metallurgical processes, viz.

austenitization, recrystallization and precipitation behavior [7,8]. Due to increasing costs of alloying

elements the narrow range control of hot strip mill parameters enables achievement of a higher

degree of consistency in mechanical properties and microstructure.

The influential rolling parameters are:

- the slab reheating temperature for dissolution of the precipitated carbonitrides,

- the roughing temperatures for producing a fine grains by recrystallisation,

- the finishing temperature

- the degree of final deformation in this temperature range

- the coiling temperature

Finishing rolling temperatures and coiling temperatures influence ferrite grain size and morphology,

pearlite lamella thickness, grain boundary thickness and precipitate morphology [7]. By varying

finish rolling temperatures and run-out table cooling rates coiling temperature are controlled. These

parameters are set depending upon the chemistry and required final mechanical properties. Rolling

parameters employed for line pipe steels at JSW are enlisted in Table 4

Table 4: Rolling parameters for line pipe steels

~ X42 - X60 ~ X65 – X70

Heating Zone 1250-1270 Deg C 1250-1270 Deg C

Soaking Zone 1240-1260 Deg C 1240-1260 Deg CFurnace

Temperature RM Exit

Temperature1050-1100 Deg C 1050-1100 Deg C

Finishing MillFinishing

Temp.

830-850 Deg C

830-850 Deg C F1-F3,

Scale suppression were

used F4-F6, Strip cooling

were used

Coiling Coiling Temp. 600-630 Deg C(Normal Cooling)

570-600 Deg C(Accelerated Cooling)




172

Furnace temperatures and finishing temperatures are decided based on the optimum dissolution

temperatures of precipitates required for improving the yield stress. When finishing temperatures are

in the two-phase region a structure of deformed ferrite (which may recover or recrystallize), soft

ferrite or recrystallized ferrite and pearlite is formed. This microstructure produces highest strength.

For the composition of line pipe steels developed at JSW, coiling temperature of ~ 600-630°C was

found to be optimum. Too low coiling temperature lowered yield strength due to insufficient precipitation of Nb(CN) in ferrite and higher coiling temperature coarsen the precipitates lowering

their strengthening potential [8]. Table 5 shows the mean values of the mechanical properties

measured on the HR Coils. The measured tensile and the impact energy values conformed fully to

the specification requirements in all cases. The standard deviation for the yield and tensile strength

values was also very low.

Table 5: Mechanical properties of base material of JSW line pipe steels

GradeASTM

Grain size

YS,

MPa

UTS,

MPa

YS/UT

S% El

Impact, J

(-0oC)

~ X42 9-10 350-380 450-480 0.76 38 110

~ X52 9-10 460-490 550-590 0.85 33 130

~ X56 10 470-500 550-590 0.86 33 140

~ X60 11 480-510 540-570 0.85 38 180

~ X65 11-12 490-520 590-620 0.86 34 150

~ X70 11-12 500-530 600-630 0.85 36 280 (-40 oC)

Development of grade equivalent to API X70

Steel equivalent to X70 line pipe grade requires far higher production technology than an X60 or

X65 equivalent grade does. The development activities for the X70 line pipe in all the technical

fields such as pre-treatment, steelmaking, casting, and rolling was organized in an integrated manner.

The challenge of minimum 483 Mpa YS and minimum 565 Mpa UTS with the available facilities

was successfully taken up by conducting short sequence trials with varying chemistries. The

approach to increase the strength of the steel was aimed at the distribution and type of

microstructural constituents and at achieving additional solid solution hardening. The classical

composition of the niobium vanadium-type steel, used for grade equivalent to X65 pipe, was

modified by increasing the concentrations of copper & chromium in the steel. Both Copper andchromium increases the strength by solid solution strengthening. Two short sequences were

experimented.

1. Cu / Nb / V combination, and

2. Cr / Nb / V combination

To optimize the rolling parameters in the commercial production, the trial slabs were rolled at

different thickness to examine the variation in properties. Steel plates of these different chemical

compositions and thickness were cut and analyzed. Compared to copper, chromium based chemistry

has been found to better match all the specifications and was accepted for commercial production.

Based on the properties achieved in the individual trials, final parameters were designed to be the

‘most cost effective’. Changes in rolling parameters were done to achieve the desired mechanical




173

100 X 400 X

properties consistently. Finishing temperature was maintained just above A3 temperature to have

single phase structure. Accelerated cooling was followed to have fine grain uniform microstructure.

Slabs were rolled and supplied at 3 different thicknesses and were found to match all specifications

at the customers end.

Fig 6: Micrographs of the rolled samples

The microstructures of rolled X70 equivalent (Fig 6) steel is uniform and the ferrite grains are finer

(ASTM 11–12) with uniform distribution of precipitates. Table 6 shows the mechanical properties of

API X70 equivalent grade rolled coils at JSW, which matches well with the specifications.

Table 6: Mechanical Properties of API X70 equivalent gradeCoil

Thickness,

mm

Direction FT,oC CT,

oC YS, MPa

UTS,

MPaYS/UTS % El

Impact, J (-

40oC)

6.4 Transverse 860-870 590-610 540 614 0.87 35 296

9.53 Transverse 820-830 610-620 530 626 0.85 38 270

12 Transverse 835-845 605-615 524 620 0.84 34 274

Future Plans

With the commissioning of RH degasser and proposed slab conditioning systems, JSW is planning to

develop steels equivalent to API X80 and Sour grades. Further increases in strength and toughness,

required for the development of X80 steel, can only be attained by changing the microstructure of the

steel matrix from ferrite-pearlite to ferrite-bainite. X80 steel has a further reduced carbon content,

reduced grain size and an increased dislocation density. Optimum compositions and processing

parameters are being developed to meet the X80 category requirements. Sour service grades for line

pipe with resistance to hydrogen induced cracking (HIC) needs more increased cleanliness. To meet

these requirements, JSW has started introducing new technologies in steel making and rolling.




174

Conclusions

The development and economic production line pipe steels have stringent challenges for controlling

metallurgical and mechanical properties. The control of the steel cleanness in these grades requires

narrow range of processing parameters which often affects the productivity. JSW Steels, Vijayanagar

works have developed wide range of Line Pipe steels (from X42 to X70) through severalimprovements and standardization of the processes for its economic production. In-house

development of these high strength steels have been made by controlling inclusion characteristics,

precipitation behavior and solidification phenomenon. HR coils performance at the customers end

w.r.t the chemical and physical property requirement before and after pipe making were satisfactory

to the API 5L specifications. Development of X80 equivalent and sour grades are also envisaged in

the coming years.

Acknowledgement

Authors acknowledges the steelmaking and rolling team of JSW Steel who directly or in-directly

contributed to the development of these steels. Authors also thank management of JSW for theirencouragement to publish the paper.

References

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No 4, P-325.

3. D. Simpson, Z. Tritsiniotis and L. G. Moore, Ironmaking and Steelmaking, 2003, Vol. 30,

No. 2, P-158.

4. Hans-Georg Hillenbrand, Michael Gräf, Christoph Kalwa, Niobium 2001, Orlando, Florida,

USA, December 02-05, 2001.

5. Yoshio Terada, Akihito Kiyose and Naoki Doi, Nippon Steel Technical Report No. 90, July

2004.

6. Young Min Kim, Sang Yong Shin, Hakcheol Lee, Byoungchul Hwang, Sunghak Lee, and

Nack J. Kim, Metallurgical And Materials Transactions A, Volume 38A, August 2007, P-

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B K Panigrahi, Bull. Mater. Sci., Vol. 24, No. 4, August 2001, P-361

Development of Line Pipe Steel at JSW

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