casting steels for forging

7/27/2019 Casting Steels for Forging

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c. v. WhiteKettering UniversityG. Krauss and D.K. MatlockColorado School of Mines

INTRODUCTIONThe traditional steelproduction processconsisted of therefining and teemingof liquid steel intolarge ingots. These n-gots required exten-sive high temperaturesoaking and hot roll-ing to produce slabs,blooms and billets thatwere subsequentlyhotrolled to finishedproduct shapes. Although some ingotcasting capacity still exists, continuouscasting directly to slabs, blooms andbillets has largely displaced ingot cast-ing. Continuous casting has grown tomore than 80 percent of steel produc-tion in North America, and to more than85 percent in the United Kingdom.1,

The popularity of continuous castinghas been driven by improvements insurface quality, greater uniformity incomposition (due to the elimination oftop-to-bottom ingot segregation) en-ergy savings,high yields and increasedproduction efficiency. The history andevolution of continuous caster design

and technology have been described indetail by IMng.2

A concern exists hat smaller as-castsections of continuously caststeelprod-ucts may not receive sufficient hot workto produce microstructures and prop-erties characteristic of more highly de-formed steelproducts. This s especiallya concern in bar steels hat are forged tocomplex shapes.This article will reviewthe literature related to the effectsof hotreduction on the properties and perfor-mance of continuously cast bar steels.Diminished mechanical properties andfatigueperformance havebeen reportedin strand-cast steelssubjected o limited

reduction. Properties representative ofwrought steel eventually are attainedwith increasing hot reduction. Theamount of hot reduction required toachieve the transition from cast towrought structure and properties is ofgreat interest, especially in small con-tinuously cast billets that may receivelittle hot reduction to finished bar sizes.

The causesof reduced performanceas a function of limited hot work havebeen difficult to unravel. Performancedepends on a multitude of factors: thehomogenizing effectsof hot work; steel-making, which establishes he cleanli-ness and inclusion content of the prod-uct; caster design,which establishes hesizeand geometryof the as-castproduct;the nature ofhotwork or roll passdesignof the hot rolling standsused o producebars of various sizes;and, ultimately, themicrostructure of the heat-treatedbar orforgings, such as errite-pearlite or highstrength empered martensite.

Thus, the amount of hot work re-quired to produce wrought propertiesis caster- and mill-specific. A universalcriterion for acceptability, such as asingle value of the commonly used hotreduction ratio, based on reduction of

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Figure 3 The histogram shows the requency of inclusions with various shape actors as afunction of the hot reduction ratio.9 JO he shape actor is defined in the text.

and a central zone with equiaxed den-drites with random orientations. As dis-cussed here, extensive porosity may existin the central equiaxed zone. The relativeareas of the columnar and equiaxed zonesdepend on caster speed, which deter-mines thermal gradients and solidifica-tion velocity. Electromagnetic stirring isused to promote the size of the equiaxedzone, minimize centerline porosity andreduce the size of the columnar zone.

The scale of the dendritic structuredetermines the distribution of inter-dendrite chemical segregation andporosity due to solidification shrinkageor gas evolution. These factors maybe detrimental to mechanical proper-ties and performance. The fine-scale

and recrystallization of the austenite grainstructure is achieved by the same method.Despite this, microsegregation persistseven after extensive hot deformation.

Figure 6 presents evidence forinterdendritic microsegregation, al-though with increasingly finer spacing,in bars of 10V45 steel for reductionratios between 7:1 and 49:1.10 Thechemical segregation was revealed byetching with picric acid and sodiumtridecylbenzene. The austenite grainstructure and the ferrite-pearlite micro-structure formed on cooling of this steelwere superimposed on the chemicalvariations. When tested in torsional fa-tigue, all specimens showed wroughtbehavior and fatigue crack initiation atinclusions within the ferritic compo-nent of the microstructure.

Thus, in this experiment, microstruc-ture and inclusions controlled fatigueperformance. The coarser effects of so-lidification, such as columnar dendritecrystal orientation and porosity, weredissipated by a reduction ratio of 7: 1.

Residual effects of dendritic segrega-tion may contribute to ferritelpearlitebanding beyond association with as-caststructures! Also, residual chemical seg-regation effects rom the corners of squarebillets eventually may contribute to

distribution of porosity often is charac-terized by secondarydendrite arm spac-ing, which is directly related to heattransfer during solidification.

Figure 5 shows secondary dendritearm spacing as a function of the dis-tance from the chill surface for a num-ber of casters.14The larger the sectionsize and further the distance from thesurface, the coarser the dendrite spac-ing and associated micro segregationand porosity.M.CROSEGREGA T.ONCenter macroporosity and interdendritemicroporosity are healed by hot work,

1- Chill zone

Columnar one

3.~ .Low carbon steels2 .2 .Stainless steels (type 304) ....A

1.8 ~ ..-14 ..! 1.0= .6 ..

I 2= -.2o -.6

..I -1.0-2.4 -1.6 -0.8 -0.0 0.8 1.6 2.4

Log x (mm) Figure 6 A picric acid-sodiumtridecylbenzene etchJO epicts effectsassociated with residual interdendriticmicrosegregation in bars of 10V45 steelsubjected to hot reduction ratios of (a)7:1, (b) 10:1, (c) 27:1 and (d) 49:1.

Equiaxed zoneFigure 5 The secondary dendrite armspacingfor a number of dijJerent castersis shown as a unction of distance fromthe chill surface. !4

Figure 4 The diagram shows varioussolidification zones in a continuouslycast billet.12

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nonsyrnmetrical distortion during finalheat treatment of bars and forgings.15. 6 with discontinuities caused by speci-men machining.17Solidification porosity increaseswith

greater distance rom the surface of as-cast billets. As the dendritic structurecoarsens, the interior cooling rate de-creases, he ast iquid steelsolidifies andfinal shrinkage develops. Density mea-surements,asa unction of distance romthe surfacesof 140-mm-squarecast bil-lets, show porosity is low in columnardendrite solidification zones and in-creases o high evelsclose o and at billetcenterlines.18 rada has shown that cor-ners of square billets, where columnardendrite zones rom orthogonal surfaceimpinge, also are locations of high po-rosity in continuous cast billets.19

Figure 9 shows macroetched trans-verse sections of electromagneticallystirred and unstirred continuouslycastbillets of 4140 steel,171nhe unstirredbillet, the columnar dendrite zone occu-pies most of the cross section, and mac-roscopic porosity is present at thecenterline of the billet. Electromagnetic

SOLIDIFICATION POROSITYPorosity is a major consequence of so-lidification and has been shown o causediminished fatigue performance. Forexample, Figures 7 and 8 show fatiguecrack initiation at porosity in as-castcontinuously cast 152-mm-square bil-lets and hardened 4140 steeV 7Figure 7illustrates fatigue fracture surfaces de-veloped by axial fatigue testing ofspecimens emoved from the columnardendritic zone. Meanwhile, Figure 8illustrates fatigue in the equiaxedzone. The interdendritic morphologyof the porosity is revealed, especially nthe second example; the former re-vealed better fatigue life. Specimensremoved from bars of the same billetsubjected to hot reduction ratios of3.3: or greater showed marked im-provement in fatigue resistance.Fatigueinitiation of hardened specimens fromhot worked bars largely was associated

stirring produces an extensive equiaxeddendritic zone and eliminates macro-scopic centerline shrinkage. A light-etching band, because of solute deple-tion, marks the initiation of electromag-netic stirring.

In addition to stirring, porosity andthe extent of the equiaxed central zoneof billets also are affected by superheat.High superheat in unstirred billets in-creases the extent of columnar solidifi-cation by retarding the nucleation ofequiaxed dendrites:Q, 21High superheatsalso have been found to increaseinterdendrite segregation and internalporosity, requiring higher reductionratios for the elimination of porosity.21Electromagnetic stirring offsets the ef-fect of high superheats, but does notcompensate completely for the in-creased size of columnar solidificationzones at higher superheat temperatures.

Hot work eliminates porosity, butthe amount required to produce fulldensity in bars is a function of the meth-ods by which the hot work is applied.

~ ~10J.lrnI Figure 7 Porosity is a major conseqltence of solidification. The micrographs show theporosity at the fatigue crack initiation in a hardened 4140 steel specimen taken from thecolumnar dendritic zone of an as-cast sti"ed billet.ll

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etchingeffects ssociated ith chemicalsegregationre still apparent.

Figure 12, taken from the work of Fettand Ross, shows strength and ductilityparameters as a function of the reduc-tion ratio for normalized specimens of1040 steel,24 As noted previously, ulti-mate strength and hardness are notdegraded in as-cast structures, but elon-gation and reduction of area are re-duced by the effects of solidification.

Figure 13, from the same study, showsdependence of impact energy on thereduction ratio, similar to that of theductility parameters. Both the impactand ductility measures of fracture resis-tance reach constant values at the lowreduction ratio of 3:1. Morris et al.show similar sets of data for tensileproperties, but greater variation in im-pact properties in specimens is obtainedfrom billets subjected to low reductionratios of between 3:1 and 7:1.18

The studies of fatigue in as-cast andhot rolled continuously cast billets showthat specimens from as-cast billets failfor one of two reasons: coarse as-castcolumnar grain structure or porosityincorporated during solidification.17After relatively small amounts of hotreduction, the effects of residual solidi-fication no longer playa significant rolein the initiation of fatigue cracks. In-stead, fatigue initiates at inclusions orsurface defects, which are determinedby steel cleanliness, specimen prepara-tion and microstructure.

For example, Figure 10 shows poros-ity is eliminated by relatively low reduc-tion ratios, and rolling schedules withheavy passes increase density morereadily than those with light passes.22Theheavy passes were accomplished withlarger diameter cylindrical rolls. A studyof the elimination of porosity in continu-ously cast slabs also emphasized that acombination of rolling reduction androlling shape factor (as related to radiusof the work rolls) must be considered forthe elimination of porosity.23

Another study was conducted on theeffectiveness of various deformationprocesses on consolidating centrallooseness and voids in bars producedfrom strand-cast billets. Morris et al.found that hammer forging restored fulldensity at a nominal reduction ratio of3:1.18 But, when continuous forging,groove rolling and flat rolling were usedfor hot deformation, reduction ratios ofat least 5:1 were required.

Figure 11, from the study by Schultz etal., shows in continuously cast 4140 barssubjected to a reduction ratio of 3:3,some residual center porosity persists inthe bar produced from an unstirred bil-let. 17No macroscopic evidence ofporos-ity is visible in the bar produced from anelectromagnetically stirred billet, but

MINIMUM HOT REDUCTIONRATIOSTable I lists a number of investigationsthat have been designed to evaluate hetransition from cast to wrought me-chanical behavior and structure in con-tinuously cast steels. The criteria, asidentified by each set of authors, dif-fered for an effective transition. Thecriteria included an elimination ofporosity, mechanical behavior charac-teristic of products produced from in-got cast steels or mechanical behaviorthat attained a stable high level as afunction of increasing hot reduction.

A number of the investigations ofmechanical properties determined bytensile testing show yield and tensilestrength are relatively insensitive tomicrostructural components intro-duced by solidification or hot work ofthe as-cast structures. 17, 8,21,24 ow-ever, the properties that measure duc-tility, such as reduction of area or totalelongation, are sensitive to solidifica-tion structures and improve with in-creasing hot reduction.

The ductility parameters are deter-mined by ductile fracture mechanismsrelated to pore and inclusion distribu-tions, and are adversely affected bythosedistributions in as-castspecimens.

0Heavy pass~ ~C')-

E()...s 01

I.02

/.03 1 15

Rolling Ratio2 2.5 3

Figure 10 Density differences betweencore and surface zones of cas t plates, shownas a unction of reduction ratio and rollingseverity, indicate that porosity is eliminatedby relatively low reduction ratios!2

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the Advanced Steel Processing and Prod-ucts Research Center, a National Sci-ence Foundation Industry-University Co-operative Research Center at theColorado School of Mines. ~

superheat and electromagnetic stirring.Deternlination of the success of theseefforts must then be made on a case-by-case basis. The mechanical propertiesmust be evaluated and demonstrate thatthe properties are adequate for the pro-posed application.

This article has reviewed a numberof technical papers on the effect of hotreduction on the properties of continu-ously cast steel. Two of the most com-prehensive papers regarding mill proc-essing were commissioned by theEuropean Commission.18, 21 Producersand users of steels should perform morework to be published and evaluated.This work would enable the establish-ment of clear limits of the minimumamounts of hot reduction for specificsteel compositions, steelmaking condi-tions and product applications.

ACKNOWLEDGMENTSC.V. White acknowledges the support ofAmerican Iron and Steel Institute, Deere& Company, Chaparral Texas Steel Re-cycling, Caterpil lar Inc. and Inland SteelCompany. This support made possiblea sabbatical leave from GMI Engineer-ing and Management Institute (nowKettering University) at the ColoradoSchool of Mines. The program on theeffects of the hot reduction ratio oncontinuous cast steels is supported by

ReferencesI. G. Millar , "Strand Cast Bar Steels in North America,"

Bar Applications Group/American Iron and SteelInstitute, 1995, pp. 3-4.2. W .R.lrving, "Continuous Casting of Steel, " Book 584,The Institute of Materials, 1993, pp. 1-20.

3. W .C. Leslie, "Inclusions and Mechanical Properties,"Trans. oftbe ISS, ISS, Vol. 2, 1983, pp. 1-24.

4. L.E. Samuels, OpticalMicroscopy ofCarbon Steels,ASM, Materials Park, OH, 1980.

5. D. Schauwinhold, "Characteristic Properties and Ap-plications ofProducts from Continuously Cast Steel,"4th Internationallron and Steel Conference, 1982 ,pp. 3/3-3/12.

6. R.L. Widner, "Value Creation Through Investmentand Continuous Improvement," SAE, Warrendale,PA, 1992, Paper No.921678.

7. J.D. StoverandR.V. Kolarikll, "Air-MeltedSteel WithUltra-Low I nclusion Content Further ImpToves Bear-ing Fatigue Life," SME, Paper No.871208.

8. N .Islam, "Developments in the continuous casting ofsteel," Metals and Materials, 1989, pp. 392-396.

9. B. Rittgers et al., "Effect of Hot Working ReductionRatios on the Torsional Fatigue of Strand Cast AISI4140 Steel," 31st Mechanical Working and SteelProcessing Proceedings, ISS, Vol. XXVII, 1989, pp.99-113.

10. J. Dyek et al., "Effects of Hot Reduction and BarDiameter on Torsional Fatigue of a Strand-CastMicroalloyed St eel," 30thMechanical Working andSteel Processing Proceedings, ISS, 1988, Vol. VVXI,pp. 83-94.

II. G.R. Speich and W.A. Spitzig, "Ef fect of VolumeFraction and Shape of Sulfide Inclusions on Through-Thickness Ductility and Impact Energy ofHigh Strength4340 Plate Steels," Metall. TransactionsA, Vo113A,1982, pp 2,239-2,257.

12. Y. Tomita, "Effect of Hot Rolling Reduction on Shapeof Sulfide Inclusions and Fracture Toughness of A/SI4340 mtrahigh Strength Steel," Metall. Transac-tionsA, Vol. 19A, 1988, pp. 1,555-1,561.

13. T. Ototani, Calcium Clean Steel, Springer Verlag,Berlin, 1986.

14. A.W. Cramb, "Secondary Dendrite Arm Spacingsin Continuous Cast, Thin Steel Sections," Castingof Near Net Shape Products, TMS, 1988, pp.673-682.

15. S. Gunnarson, "Effect of Strand Casting on Distortionof Carburized Steel Crown Wheels," Harterei-TechniscbeMitteilungen, Vol. 46,1991, p. 216.

16. H. Mallender, "Dimension and Shape Changes byCarburizing," Einsatzharten, J. Grosch and J.Wunning, Editors, AWT, 1989, pp. 285-303.

17. E.J. Schultz et al., "The Effect of Hot-RoU ReductionRatio on the Axial Fatigue of Cuntinuously-Cast andHardened 4140 Steel," 34th Mechanical Workingand Steel Processing Proceedings, ISS, Vol. XXX,1992, pp. 309-319.

18. P.W. Morris, S.P. Ryalls and B.A. Wade, "Opti miza-tiun of the defonnation process for continuously castbillets to provide the most appropriate materi:j! prop-erties," Final Report, Contract No. 7210.EB/804 (DI-D5.5/88), European Commission, 1994.

19. G. Brada, "Characterization ofContinuouslyCastAISI4130 Steel and the Effects of Hot-Reductiun Ratio onStructure and Axial Fatigue," M.S. Thesis, ColoradoSchool of Mines, 1993.

20. D.J. Hurtuk, "Aberrations Observed in the Relation-ship ofDendrite Size-Alloying Elements for Low AlloySteel," Proceedings of the International Confer-ence on Solidification, The Metals Society, London,England, 1979, pp. 21-29.

21. F. Fattorini and B. Grifoni, "Effect of the RollingReduction Ratio on the Quality and Properties ofRolled Engineering Steel Bars Deriving from CastBillets and Blooms via C. C. Route," Final Report,Contract No. 7210-EB/402 (1.7.1985-30.6.1988),European Commission, 1990.

22. J-C. Brunet, "Reduction Ratios In Continuous Cast-ing: How Important Are They? ," Metal Progress, Vol.XX, 1985, pp. 45-53.

23. N. Okumura et al., "Hot Roll ing Conditions in Cun-tinuous Cast Slabs," pp. 217-228.

24. G.A. FettandJ.W. Ross, "The Effect of Reduction Ratioon the Mechanical Properties of Strand Cast Steel,"Impact of Improved Material Quality on Proper-ties, Product Performance and Design, ASME, MDVol. 28,1991, pp. 1-14.

25. R.H. McCreery , "Effects of Reduction On the MinimillSteel," Metal Progress, 1984, pp. 29-31.

26. H.B. Emerick, "Evaluatiun of Continuous Cast Steelfor Seamless Tube Production," Continuous Cast-ing, Gordun and Breach, 1962, pp. 197-208.

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casting steels for forging

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