Formation of Precipitates in Multiple Microalloyed Pipeline Steels

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Formation ofprecipitates inmultiplemicroalloyedpipeline steels

C. O. I. EmenikeJ. C. Billington

Introduction

An investigation has been carried out to identify the precipitates in multiplemicroalloyed steels. The microalloying elements and interstitials includedaluminium, niobium, titanium, vanadium, carbon, and nitrogen.. It was found thatthe precipitates are complex in nature and they were rationalised on the basis ofmutual solubility probably enhanced by non-stoichiometry. The precipitatemorphologies were interpreted mainly in terms of steel compositions. Steelsquenched from 1250 C contained titanium rich precipitates accompanied by theevolution of new niobium rich precipitates after hot rolling and quenching. Aparameter K[1 indicative of solute participation in the precipitation phenomenonwas established and showed excellent correlation between steel and precipitateanalyses. A sequence of precipitation in multiple microalloyed steels was achievedusing solubility relationships as a premise. MSTj803

1989 The Institute of Metals. Manuscript received 30 October 1987; in finalform 20 July 1988. At the time the work was carried out the authors were in theDepartment of Mechanical and Production Engineering, Aston University,Birmingham. Dr Billington is now a Consultant Metallurgist.

Experimental procedure

The worldwide increase in the demand for energy has led toa continuous increase in the production of steel pipelinesfor the transportation of oil and gas from production site topotential user. This has led to the demand for thinnerwalled, large diameter pipes which can resist hostileenvironments while increasing their fuel carrying capacitiesto meqt the economic restrictions imposed. These advanceshave~been made possible by the development of highstrength low alloy (HSLA) steels which form the basisof the materials used together with improvements in thethermomechanical treatments applied to achieve the finalproperties.

Microalloying additions, such as niobium, titanium, andvanadium which, in the presence of C and nitrogen, achievethe control of austenite grains (and, subsequently, ferritegrains) and precipitation strengthening of the ferrite bycontrolled formation of nitrides/carbides. For improvedgrain refinement, controlled rolling was introduced topermit the use of much lower finish rolling temperatures.Thus, modern pipeline steels are produced having relativelysmall ferrite grains, formed by the transformation of theoriginal austenite grains. Final properties can be achievedby the precipitation of vanadium (and niobium) carbo-nitrides below 900C improving strength and toughness.Knowledge of the interactions between the microalloying

elements and aluminium is far from complete, especially interms of their dependence upon processing variables. Thepresent work was undertaken to elucidate the sequence offormation of precipitates during the rolling of a series ofsteels containing fixed carbon and niobium contents withvarying (aluminium), titanium, and vanadium concentra-tions. The precipitates formed in as cast specimens andspecimens quenched from 1250C with and without defor-mation were examined.

Five 18 kg pipeline steel ingots were produced as describedpreviouslyl and their compositions are given in Table 1.The ingots were hot rolled in four passes, with interpassreheating, at 1250C to give an overall reduction of 66%and a final thickness of 11O'2 mm.

ELECTRON MICROSCOPYA scanning transmission electron microscope (STEM) withEDX attachment (Philips Model EM 400) was used tostudy the morphology and composition of the precipitates.Conventional carbon extraction replicas were preparedfrom selected specimens which were etched in 2% nitalbefore being carbon coated, scored, and stripped in 5%nital. Individual precipitates were quantitatively analysedin the Philips microscope for 200 live seconds and theelemental counts corrected for atomic numbers were sub-sequently processed through a computer to give the analy-ses presented in Tables 2-4. Because the objective of thiswork was to study the morphology and chemistry of theprecipitates, they were classified into three sizes only:coarse, intermediate, and fine.To elucidate further the production of precipitates and

their compositions during the various thermo mechanicaltreatments applied to the steels, a series of rolling andquenching experiments was made on steels 1 and 4. Twosamples from each cast steel (initial thickness 18mm) wereheated to 1250C, one sample was quenched after soakingand the other was hot rolled in a laboratory two-high millto give 17% reduction before quenching. The emergenttemperature of the specimens from the rolls was1150 10C. Specimens from the"sematerials were preparedfor electron microscopy. The remaining materials werereheated at 1250C and given a second pass of 23%

Table 1 Compositions of steels used in present investigation, wt-%

Cast no. C Si Mn P S Ni AI Cu N Nb Ti V 0

1* 009 024 122 0014 0002 003 0030 027 0007 0042 0007 0009 000512 009 020 113 0013 0002 004 0042 029 0012 0047 0007 0-073 000403 009 021 120 0015 0002 002 0-034 028 0-010 0047 0007 0043 0-00404 009 021 1-13 0-011 0-003 0-02 0059 026 0010 0043 0'074 000435 009 023 123 0-013 0003 003 0060 026 0011 0048 0-037 00032

* Base steel.

566 Materials Science and Technology June 1989 Vol. 5

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Emenike and Billington Formation of precipitates in pipeline steels 567

Table 2 STEM-EDX precipitate analyses of as caststeels 1, 2, and 4

Table 3 STEM-EDX precipitate analyses of steels 1-5as rolled at 1250C

Precipitate form

Analyses

wt-% at.-%Precipitate yieldquotient ~ Precipitate form

Analyses

wt-% at.-%

Steel 1Fine, spherical Nb 85 4 73 3

Ti 151 251AI 0'40'1 1204

Intermediate, spherical Nb 80 6 63 4Ti 15 12 23 2AI 5'30'8 142

20242143

1319052143177

Steel 1Fine, spherical

Intermediate, spherical

Nb 91 6Ti 71AI 31Nb 91 2Ti 90'2AI 0'60'1

81 5121482

832160'420'2

216795783

21671257

20

Results

reduction before quenching for subsequent microscopicexamination. Further experimental details are givenelsewhere.2

PRECIPITATE MORPHOLOGIESThe precipitates of the base steel 1 were predominantlyspherical (see Tables 2 and 3 and Fig. la), while variousshapes were found in the other steels (see Tables 2-4 andFig. Ib). Similar morphologies have been cited in theliterature.3-6

Steel 2Fine, spherical Nb 84 1

Ti 71'5AI 614V 41'1

Intermediate, spherical Nb 356Ti 31AI 604V 3'21

Steel 4Fine, spherical Nb 61 4

Ti 382AI 20'4

Intermediate, spherical Nb 592Ti 401AI 11 0'2

69911 2415462

142'521

826231

4335324'21

4225612504

178795713151

745357

142944

141951429

137254119

Steel 2Fine, spherical

Coarse, angular, hexagonal

Coarse, plate, cuboid

Steel 3Fine, spherical

Coarse, long plate (needle),

Coarse, plate (cuboid)

Steel 4Fine, spherical

Coarse, needle

Coarse, plate (cuboid)

Nb 895Ti 41AI 3'30'4V 6'31Nb 483Ti 202AI 461V 4'30'4Nb 542Ti 1'40'2AI 41 1V 403

Nb 569Ti 7'32AI 7314V 92Nb 432Ti 202AI 541V 1'20'3Nb 623Ti 21 0'3AI 331V 303

Nb 5711Ti 405AI 3414Nb 484Ti 51 2AI 1'30'3Nb 503Ti 492AI 11 0'2

7446'31

1021012211'40'2

73240'3

2611'30'2

6913203

5071322341431811'4O'1

8021 0'1

3422303

61 230'3

39153683'2

3236532'91

3426422'50'5

189454379

1501021214

1095102

114920097688

11911043215202915243

158829

131930097160

132654158

111668922

116366219

PRECIPITATE ANALYSESIt is evident from Tables 2 and 3 that the precipitates ofmultiple micro alloyed steels are of mixed composition, thussupporting previous work on similar steels.7-10

Steel 5Fine, spherical

Coarse, plate (cuboid)

Nb 566Ti 302AI 1'10'2Nb 555Ti 262AI 0'90'4

40541 32'50'5

405370'42'31

116781118

114670315

SOLUTE DISTRIBUTIONThe solute content of the precipitates tended to show sizedependence consistent with other findings. 7.8 The highestvanadium containing particles appeared to be fine andspherical (Table 3) which contrasted with the behaviour ofthe highest titanium and aluminium precipitates. Niobium

Table 4 STEM-EDX analyses of quenched steels 1 and 4

did not show such preferred morphologies. In the as rolledsteel 4, it could be argued that titanium was partitionedapproximately 50 : 50 in both fine and coarse particles anda possible explanation for this was that the Ti/N ratio washyperstoichiometric. Consequently, the excess titanium

Treatment

Steel 1Held at 1250C for 50 mins, WQ

Soaked + 17% reduction, WQ

Steel 4Held at 1250C for 50 min, WQ

Analyses

Precipitate form wt-% at.-% K1

Fine, dense, and spherical Nb 433 282 1024Ti 562 71 2 8000AI 0'70'2 1'60'5 23

Fine, dense, and spherical Nb 473 322 1119Ti 522 672 7429AI 0'60'2 1'40'5 20

Fine, less dense, and spherical Nb 81 4 683 1929Ti 181 302 2571AI 0'70'2 2'30'5 23

Coarse, plate (cuboid) Nb 261 160'5 605Ti 731 831 986AI 0'50'1 1001 9

Materials Science and Technology June 1989 Vol. 5

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568 Emenike and Billington