Hot tear segregations as quality decisive factor for advanced steel ...

Hot tear segregations as quality decisivefactor for advanced steel gradesC. Bernhard 1) , G. Arth 1) , S. Ilie 2) and S.C. Michelic 1,3)1)Montanuniversitaet Leoben, Austria2)voestalpine Stahl, Linz, Austria3)INTECO Special Melting Technologies, Bruck, Austria8th Pacific Rim International Congress onAdvanced Materials and ProcessingAugust 4-9, 2013Hilton Waikoloa Village • Waikoloa, HawaiiA working group of COMOutline of presentation Advanced steel grades: A challenge for processmetallurgists Macro- and mesosegregation in continuous casting On the nature of hot tear segregations: Formation andconsequence on product quality Physical simulation of HTS-formation Numerical prediction of HTS-formation in slab casting:Concept and example Summary

Quality pyramid: Increasing quality demands forstate-of-the-art advanced steel gradesAdvanced steels: low production, lower economic pressure for steel producerbut high ancd mostly complex quality demandsAPI, GO-Si-steels, SEDDQ, Roller bearing steels, SHSS, ...Multiphase (DP, CP, TRIP), valve steels , spring steels ...Deep drawing steels (DDQ), ....High-strength low-alloy (HSLA), Press hardening, (PH), ...Reinforcing steels, structural steels, ....Commodity steel: high production, high economic pressure for steelproducer and low quality demandsPRICM8 Bernhard 3 Quality pyramid: Increasing quality demands forstate-of-the-art advanced steel gradesAdvanced steelsAPI, GO-Si-steels, SEDDQ, Roller bearing steels, SHSS, ...Quality demands:- Strength within small tolerances- Highest steel cleanness on macro- but also microscopic level- Homogeneity and internal soundness- High surface quality (Decarburization, inclusions, …)- …Commodity steelReinforcing steels, structural steels, ....Quality demand:- Minimum strengthPRICM8 Bernhard 4

Outline of presentation Advanced steel grades: A challenge for processmetallurgists Macro- and mesosegregation in continuous casting On the nature of hot tear segregations: Formation andconsequence on product quality Physical simulation of HTS-formation Numerical prediction of HTS-formation in slab casting:Concept and example Summary Importance of casting process for the homogeneityof the final product, example DP600, 210 mm slabPresslinger, H., S. Illie, A. Schiefermueller, A. Pissenberger, E.Parteder, and C. Bernhard: ISIJ, Vol. 46 (2006), No. 12, 1845-1851PRICM8 Bernhard 6

Macro- and Mesosegregations in ContinuousCasting of SlabsPorositiesV-type segregationSpot-like segregationCentre segregationPRICM8 Bernhard 7 Macro- and Mesosegregations in ContinuousCasting of Slabs, Example Mn in DP600Centre segregationPorositiesV-type segregationSpot-like segregationPresslinger, H., S. Illie, A. Schiefermueller, A. Pissenberger, E.Parteder, und C. Bernhard: 12 th ISIJ-VDEh-Seminar, November21-22 2005, Kitakyushu, Japan, 125-134PRICM8 Bernhard 8

Macro- and Mesosegregations in ContinuousCasting of Slabs, Mn in centre segregation of slabsPorositiesV-type segregationSpot-like segregationCentre segregationCentre segregation Mn Max/Mn Min4,03,53,02,52,01,51,00,50,0Soft reductionDP600/DP800TRIPHSLAAPIULCMC/HCHTS/SlabHTS/SSCT0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0Carbon content, wt.-%Illie, S. Et al.: steel research int. 78 (2007), No. 4.PRICM8 Bernhard 9 Macro- and Mesosegregations in ContinuousCasting of SlabsPorositiesV-type segregationSpot-like segregationCentre segregationSubsurface-off-corner „cracks“Triple-point „cracks“Halfway „cracks“,transversalHalfway „cracks“, longitudinalPRICM8 Bernhard 10

Macro- and Mesosegregations in ContinuousCasting of Slabs: Hot Tear SegregationSubsurface-off-corner cracksTriple-point cracksHalfway „cracks“,transversalHalfway „cracks“, longitudinalIlie, S.; Reiter, J.; Fluch, W.; Presslinger, H.; and C. Bernhard:6th European Continuous Casting Conference, June 4-6 2008,Riccione, Italy, S. Paper – 122PRICM8 Bernhard 11Outline of presentation Advanced steel grades: A challenge for processmetallurgists Macro- and mesosegregation in continuous casting On the nature of hot tear segregations: Formation andconsequence on product quality Physical simulation of HTS-formation Numerical prediction of HTS-formation in slab casting:Concept and example Summary

On the nature of hot tear segregations (HTS) HTS form during solidification inconsequence of an overcriticalstraining of the columnarsolidifying shell. HTS do not form betweendendrites, as commonly attributed,they form mainly along primarygrain boundaries.PRICM8 Bernhard 13 Formation of a hot tear segregation (HTS)Pierer, R. and C. Bernhard: AISTech 2010, Pittsburg, USAPRICM8 Bernhard 14

Concentration profile of S along a primary grain boundary,Example: 0.6% C, 0.005% S, orientation difference 30°C 0 : 0.6 %C0.5 %Si0.5 %Mn0.005 %P0.005 %Sh T : 500 Wm -2 K -130° angleMichelic, S.C. and C. Bernhard: TMS2013, San Antonio, USAPRICM8 Bernhard 15 Macro- and Mesosegregations in ContinuousCasting of SlabsCentre segregation Mn Max/Mn Min4,03,53,02,52,01,51,00,50,0DP600/DP800TRIPHSLAAPIULCMC/HCHTS/SlabHTS/SSCT0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0Halfway „cracks“, longitudinalSubsurface-off-corner cracksTriple-point cracksHalfway „cracks“,transversalCarbon content, wt.-%Ilie, S.; Reiter, J.; Fluch, W.; Presslinger, H.; and C. Bernhard: 6th European Continuous Casting Conference,June 4-6 2008, Riccione, Italy, S. Paper – 122PRICM8 Bernhard 16

HTS result in …. … the formation of undesirablephases (commonly hard and brittlephases like martensite). … inhomogeneous grain size in thefinal product. … the formation of inclusions(Sulphides but also primary carbonitrides). … may even result in surfacedefects on cold-rolled band („glossystripes“).PRICM8 Bernhard 17Outline of presentation Advanced steel grades: A challenge for processmetallurgists Macro- and mesosegregation in continuous casting On the nature of hot tear segregations: Formation andconsequence on product quality Physical simulation of HTS-formation Numerical prediction of HTS-formation in slab casting:Concept and example Summary

In-situ simulation of HTS - formation Parameters: Steel composition,solidification time, strain, strain rate.Pierer, R. and C. Bernhard: AISTech 2010, Pittsburg, USAPRICM8 Bernhard 19 In-situ simulation of HTS - formation Results confirm proposed mechanism: In contrast to thecommon opinion no clear boundary between „Cracks“ and „Nocracks“ exists. An increasing strain results in an increasingnumber of HTS and an more pronounced enrichment.Bernhard, C. and R. Pierer:Solidification 2007, Sheffield, UKPRICM8 Bernhard 20

In-situ simulation of HTS - formation The SSCT-experiment allows to assign the HTS-phenomenon to acertain range of solid fractions, commonly in the range of 0,96 – 1,0.Distance from surface, mmAccumulated strain, %Dierer, F.: Bachelor-thesis, MU-Leoben, 2013PRICM8 Bernhard 21 In-situ simulation of HTS - formation The SSCT-experiment allows to assign the HTS-phenomenon to acertain range of solid fractions, commonly in the range of 0,96 – 1,0.Distance from surface, mmAccumulated strain, %+ 79 ppm BDierer, F.: Bachelor-thesis, MU-Leoben, 2013PRICM8 Bernhard 22

Outline of presentation Advanced steel grades: A challenge for processmetallurgists Macro- and mesosegregation in continuous casting On the nature of hot tear segregations: Formation andconsequence on product quality Physical simulation of HTS-formation Numerical prediction of HTS-formation in slab casting:Concept and example SummaryHTS-Analysis of Casting Processes by Numerical and Physical SimulationBoundary conditionsHeat transfer (Mold)Measurement, ModelsCastingmachineLayout, steel grades,casting parametersPre-ProcessingThermophysicalpropertiesCATD-packagesHeat transfer(Secondary cooling)Nozzle measuring standMetallurgical modelse.g. Hot tearing:- Prediction of accumulated strain- Prediction of HTS-propability1D- and 2Dsolver,transientmodelsCommercialsoftwarepackagesPostprocessingPierer, R., S.C. Michelic and C. Bernhard: AISTech 2012, Atlanta, USA

Case study: slab casting of two steel grades, hard and soft coolingin the first secondary cooling zoneSteel AC Si Mn P S N Cr Ni0,17 0,43 1,54 0,015 0,007 0,0001 0 0Steel BC Si Mn P S N Cr NiSteel 0,15A; Hardcooling 0,015vs. Softcooling 1,109 0,008 0,007 0,0001 Steel B; 0Hardcooling 0vs. Softcooling120120Shell Thickness [mm]100806040Steel AShell Thickness [mm]100806040Steel B2020Stahl A - Hard - 0 MisAligStahl A - Soft - 0 MisAlig00,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0Accumulated Strain [%]Stahl B - Hard - 0 MisAligStahl B - Soft - 0 MisAlig00,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0Accumulated Strain [%]PRICM8 Bernhard 25Outline of presentation Advanced steel grades: A challenge for processmetallurgists Macro- and mesosegregation in continuous casting On the nature of hot tear segregations: Formation andconsequence on product quality Physical simulation of HTS-formation Numerical prediction of HTS-formation in slab casting:Concept and example Summary

ConclusionsTolerance for HT and HTSHot tearing sensitivity of steels> 2/3 of theproducedsteels areinsensitive forHT and HTSHTS-formationresults fromsingular eventsHTS-criticalsteelgradesExamples:Medium manganesesteels (0.1%C, 5% Mn)M³ HSLA (Mo, Nb, Ni, Al)B-alloyed steels,….Increasing complexity of quality demands,Increasing content of „critical“ elements in the steelPRICM8 Bernhard 27Hot tear segregations as quality decisivefactor for advanced steel gradesC. Bernhard 1) , G. Arth 1) , S. Ilie 2) and S.C. Michelic 1,3)1)Montanuniversitaet Leoben, Austria2)voestalpine Stahl, Linz, Austria3)INTECO Special Melting Technologies, Bruck, Austria8th Pacific Rim International Congress onAdvanced Materials and ProcessingAugust 4-9, 2013Hilton Waikoloa Village • Waikoloa, HawaiiA working group of COM