Niobium in Fire Resistant Structural Steels

Transcription

1 Niobium in Fire Resistant Structural Steels David K. Matlock and John G. Speer Advanced Steel Processing and Products Research Center* Colorado School of Mines Golden, Colorado Steven G. Jansto CBMM Reference Metals Bridgeville, Pennsylvania Niobium in Structural Steels Armourers Hall, London July 6, 212 *An NSF Industry/University Cooperative Research Center - Est

2 Acknowledgements This presentation based primarily on the following theses: Matthew S. Walp, Fire-Resistant Steels For Construction Applications, MS Thesis, Colorado School of Mines, 23. Justin C. Cross, Effects of Microstructure On The Fire- Resistant Properties Of HSLA Structural Steels, MS Thesis, Colorado School of Mines, 26. Ryan W. Regier, Thermomechanical Processing Effects on the Elevated Temperature Behavior of Niobium Containing Fire-Resistant Steel, MS Thesis, Colorado School of Mines, 28.

3 Fire Resistance in Structures Coatings Design Tokoname Gymnasium Design of Steel Frames to Eliminate Fire Protection, Nippon Steel Corp., Sprinkler Systems Fire Resistant Structural Materials Cost Effective Fire Resistant (FR) Steels

4 Background: Fire Resistant (FR) Steels Significant Japanese Developments. Requirement: Guarantee 2/3 of room temperature yield strength at 6 C Enhanced performance due to microstructural stability; alloy with Mo, Nb, Cr,.. R. Wildt (25) R. Wildt, Fire Resistant Steel A New Approach to Fire Safety, Proceedings of the 7th World Congress, CTBUH, Council on Tall Buildings and Urban Habitat: Renewing the Urban Landscape, New York, 25. ISBN: Applications require specifications and building code acceptance

5 Recent Research at ASPPRC Microstructure/alloying parameters of interest Starting microstructure Hot rolled: ferrite-pearlite Control cooled: bainitic, martensitic Thermomechanically processed Microalloy precipitation Prior to fire exposure During exposure Evaluate testing methods for FR steels High temperature tensile Constant load test developed at ASPPRC

6 Experimental Methods Tensile Testing = f(t) (conventional approach) Impose constant strain rate Vary heating rate to test T Constant Load Accelerated Creep (newly developed test) Impose constant load Heat at constant rate = 1 to 12 o C/hr σ T 1 T 2 T 3 Increase Temperature ε Displacement or Strain Elastic Limit Temperature or Time Fail

7 Alloy Matrix Steel Alloys Alloying elements & Composition (wt%) Base.1C - 1.Mn -.2Si -.1N Nb Mo + Nb V + Nb Cu Base +.2Nb Base +.5Mo -.2Nb Base +.5V -.2Nb 1.Cu -.75Ni -.51Cr -.5 Mo -.6V -.2Nb

8 Tensile Data - Examples Base alloy V + Nb alloy Eng. Stress (ksi) C 25 C 4 C 3 C 5 C 2 C 1 C Base Alloy 25 C 1 C 2 C 3 C 4 C 5 C 6 C 7 C 6 4 Eng. Stress (MPa) Eng. Stress (ksi) C 2 C 5 C 6 C 3 C 25 C 1 C V+Nb Alloy 25 C 1 C 2 C 3 C 4 C 5 C 6 C 7 C 6 4 Eng. Stress (MPa) 2 7 C C Plastic Eng. Strain Plastic Eng. Strain Strain Rate = 3.9x1-3 s -1 Heating Rate = 6 o C/hr 15 minute T M. Walp, MS Thesis, 23

9 Temperature Dependent Tensile Data 1 Temperature ( F) Mo+Nb 6 8 Nb V+Nb Base Eng. Stress (ksi) 6 4 Nb Mo+Nb V+Nb Base 4 Eng. Stress (MPa) 2 Strain Rate = 3.9x1-3 s -1 Heating Rate = 6 o C/hr 15 minute T Temperature ( C) 2 M. Walp, MS Thesis, 23

10 Heating Rate Effects Tensile Data Vary heating rate 1 to 12 o C/hour 8 Heating Rate ( F/Hr) Mo + Nb Alloy 6 1 C 3 C 4 C 4 Eng. Yield Stress (ksi) 4 5 C 6 C 2 C 2 Eng. Yield Stress Stress (MPa) (MPa) 2 3.9x1-3 s minute hold Heating Rate ( C/Hr) M. Walp, MS Thesis, 23

11 Constant Load Accelerated Creep Temperature ( F) % 12 C/Hr Base Nb Mo+Nb V+Nb Nb V+Nb Base Mo+Nb.3 Plastic Displacement (in) Plastic Eng Strain Temperature ( C) -.1 M. Walp, MS Thesis, 23

12 Microstructure after Accelerated Creep Nb alloy Mo + Nb alloy 1 nm TEM Replicas Tested at 5% of room temp. yield stress of Nb alloy Heating rate = 3 C/hr J. Cross, MS Thesis, 26

13 Precipitation during Heating Temperature ( F) 1 % Cu Steel three heat treat conditions: Normalized (N) Maximum precipitation potential during test Peak Aged (P) Distribution of fine ppts Overaged (O) Coarse ppts minimum precipitation potential Plastic Displacement (in) % 6 C/Hr Cu N Cu P Cu O 1% Cu steel Cu P Cu O Cu N Normalized Plastic Eng Strain Temperature ( C) M. Walp, MS Thesis, 28

14 Importance of Base Microstructure C-Mn Alloy: Three heat treat conditions Ferrite-Pearlite (F/P) Limited substructure in ferrite Bainite (B) Martensite (M) Result: Substructure contributes to FR properties Improvement less than by using Nb-alloy Plastic Strain Temperature ( F) F/P Bainite 3 C/hr 5% Nb RT YS Martensite Temperature ( C) J. Cross, MS Thesis, 26 Speer, et al., HSLA- 25.

15 Evaluate Substructure Control by TMP Nb Alloy Laboratory Rolled Vary Finishing Temperature, 65 to 9 o C Temperature ( o C) ) 6 11C A 3 A 1 1C 9C 8C 75C 7C 65C Air Cool Time (min) R. Regier, MS Thesis, 23

16 TMP: Microstructures Nb Alloy Electron Back Scattered Diffraction Images (Combined IQ, IPF, and Misorientation Plots) 9 C 65 C 5 µm 5 µm RD R. Regier, MS Thesis, 23

17 TMP: Tensile Properties Nb Alloy Effect of Finishing Temperature: 65 to 9 o C Stress (MPa) Test T = 25 C 6 C Finish Rolling Temperature( F) UTS A 1 A 3 α α+ γ γ YS Finish Rolling Temperature ( C) Stress (ksi) Stress (MPa) 3 25 Finish Rolling Temperature ( F) A 1 A 3 UTS α α+ γ γ YS YS Finish Rolling Temperature ( C) Stress (ksi) Heating rate = 6 o C/hr R. Regier, MS Thesis, 23

18 TMP: Constant Load Nb Alloy Effect of Finishing Temperature: 65 to 9 o C.3 Temperature ( o F) % Nb RT 6 o C/Hr.2 9 o C Plastic Eng Strain.1 75 o C 8 o C 7 o C Identical Applied loads 1% offset strain 65 o C Temperature ( o C) R. Regier, MS Thesis, 23

19 TMP: Constant Load Nb Alloy Enhanced properties with sub-critical finishing temperature Finish Rolling Temp. YS Ratio ( C) (%) 65 C C C C C 59.1 R. Regier, MS Thesis, 23

20 Closing Comments: Fire Resistant Steels Mo + Nb steels = improved FR properties with suitable manufacturabilty Substructural refinement leads to improved FR properties Bainite Warm worked ferrite Precipitation during heating may provide active fire protection Precipitate stability in Mo + Nb alloys is under consideration in ongoing ASPPRC research

21 Current Status: United States New ASTM Standard Approved A177/A177M-12 Standard Specification for Structural Steel with Improved Yield Strength at High Temperature for Use in Buildings Codes need to recognize FR steels in design guidelines.

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23 Selected References M.S. Walp, J.G. Speer, and D.K. Matlock, Fire-Resistant Steels, Advanced Materials and Processes, Vol. 162, No. 1, October 24, pp J.G. Speer, S.G. Jansto, J.C. Cross, M.S. Walp, and David K. Matlock, Elevated Temperature Properties of Niobium Microalloyed Steels for Fire-Resistant Structural Applications, Proceedings of the Joint International Conference of HSLA Steels 25 and ISUGS 25, Iron and Steel Supplement, The Chinese Society for Metals, Beijing, China, Vol. 4, 25, pp R.W. Regier, J.G. Speer, D.K. Matlock, A.J. Bailey, and S.G. Jansto, Thermomechanical Processing Effects on the Elevated Temperature Behavior of Niobium Bearing Fire-Resistant Steel, in CD STEEL: Recent Developments in Steel Processing, Proceedings of Materials Science and Technology (MS&T 7), edited by Matthew J. Merwin, Detroit, MI, USA, (27), pp ; also published in Proceedings of Steel Properties and Applications Conference, edited by L.C. Oldham, AIST, Warrendale, PA, 27, pp John G. Speer, Ryan W. Regier, David K. Matlock, and S. G. Jansto, Elevated Temperature Properties of Nb-Microalloyed Fire Resistant Constructional Steels, Proceedings, New Developments on Metallurgy and Applications of High Strength Steels, edited by Teresa Perez, published by Tenaris, Ternium, and Argentina Association of Materials, Buenos Aires, Argentina, 28, Paper #117, 13 pages; also published by TMS, Warrendale, PA, 28, pp R.W. Regier, J.G. Speer, D.K. Matlock and S.G. Jansto, Ferrite Substructure as an Elevated Temperature Strengthening Mechanism for Fire-Resistant Structural Steel, Materials Science and Technology (MS&T) 28, 28, pp J.G. Speer, R.W. Regier, D.K. Matlock, and S.G Jansto, "Nb-Microalloyed Fire Resistant Constructional Steels," Niobium Bearing Structural Steels, ed. by S.G. Jansto and J. Patel, TMS, Warrendale, PA, 21, pp