Enhancing grain refinement of austenitic steel with Ti additions by melt treatment sequence optimization

Transcription

1 Enhancing grain refinement of austenitic steel with Ti additions by melt treatment sequence optimization Jun Ge, Simon Lekakh, Von Richards, Ron O Malley Missouri University of Science and Technology Peaslee Steel Manufacturing Research Center (PSRMC)

2 Objective Grain size control is one of the most important aspects in the cast alloys processing Objective: To develop an advanced solidification process for controlling the crystallization structure of austenitic grades of cast steels in heavy sections Columnar structure Equiaxed structure High alloyed austenitic steels are used in both the as-cast condition and heat treated conditions The as cast grain size is important for all applications as it determines the minimum grain size achievable in these cast alloys

3 Phases, wt.% Potential heterogeneous nuclei in base steel Factor stability in the melt 100 Liquid 10 FCC Sigma Laves Potential 1??? Not effected Temperature, 0 C 19Cr18Ni superaustenitic stainless steel

4 Phases, wt % Phases, wt % Phases, wt % Phases, wt % Calculated stability of potential nuclei 100 Austenite Liquid 100 Austenite Liquid Zr>0.2 Ti>0.2 1 TiN Temperature, o C ZrN ZrC Temperature, o C 100 Austenite Liquid 100 Austenite Liquid 10 Hf> Nb>2% HfN Temperature, o C Oxides 1 NbN Temperature, o C

5 Effect of additional de-oxidation What is the sequence of reactions during processing? How sequence of treatment will change composition of reaction products? What will be optimal sequence of melt treatment? One step Complex Sequential Refiner [S] [O] [N] De-ox +Refiner [Al] [Ca] [Refiner] De-ox Refiner

6 Simulated cases and experimental heats Heat # Treatment First Second B - Al, Ca T1 Ti Al, Ca T2 Al, Ca Ti T3 Ca Al, Mg, Ti

7 Effect of treatment sequences on precipitations T1 (Ti+Al,Ca) B0 (Al+Ca) T2 (Ti+Al,Ca) T3 (Ca+Al,Mg,Ti)

8 Experimental procedure: heavy section casting Empty 0.0

9 Macrostructure B2 base T1 (IF: Ti, Ladle: deox) T2 (Ladle: deox + Ti)

10 Chemistry of precipitates START Mn S Diameter Range µm 2-5 µm 5-10 µm All Ti Al Ca N Si Si Mn Mg Al S START Diameter Range µm 2-5 µm 5-10 µm CaTi % of Total Mn % % % All % Si N Si B2 base T1 (Ti + deox) T2 (Deox + Ti) Al S Diameter Range µm 2-5 µm 5-10 µm Ca % of Total % % 7.43 % All 57.5 %

11 Phases, wt.% Solidification CET Co-precipitation grain refinement approach Ca2Ti2O5 MgAl2O4 MgO TiN (0.12N) TiN (0.08N) TiN (0.06N) TiN (0.04N) MgAl 2 O Ca 2 Ti 2 O 5 MgO Temperature, 0 C Compound Lattice parameter ɑ 0 at 2800 F, Å Planar disregistry with TiN, % TiN MgAl 2 O MgO Ti 2 O Al 2 O J. S. Park, Steel Research Int., 85 (2014) No. 9999

12 Experimental heat T3 for concept verification Treatment Heat # IF Ladle T3 Ca Al, Mg, Ti

13 Mn Precipitations - Heat T3 S Diameter Range µm 2-5 µm 5-40 µm All % Ti Al Ti MgAl 2 O 4 with TiN Mg N

14 TEM of complex precipitate µm 2,6,7 TiN 1, 3, 5 Spinel FeFCC matrix 9,10 Ca-Si modified spinel MnS

15 Spinel TiN boundary 2 Bulk: - Constant zone axes angles - The same [011] planes in beam direction - Very close orientation Boundary: -Flat topology - Almost epitaxial 1

16 High resolution Matrix-TiN-Spinel line scan Step size 5 nm, spot size 10 nm

17 Comparison of achieved grain refinement B0 T2 T3 R Equiaxed grains,, mm G2 (base) T2 (N+Deox+Ti) T3 (Al+Mg+Ti)

18 Findings Optimized sequence of de-oxidation and refinement provides increase grain refinement efficiency Co-precipitation of targeted nucleation sites by inclusion engineering is the effective way to enhance heterogeneous nucleation The well-refined as-cast structure was achieved in experimental heavy section castings

19 Acknowledgements This study is supported by Kent Peaslee Steel Manufacturing Research Center The authors gradually thank to the members of industrial advisers committee of this project: - Jack Lilley (Metaltek) chairman - John Cory (Bradken) - Michael Holt (Bradken) - Rod Grozdanich (Spokane Industries) - Sony Mascreen (Harrison Steel) - Jim Galaske (Metaltek)

20 Presenter Simon N. Lekakh Research Professor Missouri University of Science and Technology Kent Peaslee Steel Manufacturing Research Center Thank You