Niobium in Fire Resistant Structural Steels David K. Matlock and - PowerPoint PPT Presentation

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, 2012 * An NSF Industry/University Cooperative Research Center - Est. 1984 http://aspprc.mines.edu/

Acknowledgements This presentation based primarily on the following theses: Matthew S. Walp, “ Fire-Resistant Steels For Construction Applications,” MS Thesis, Colorado School of Mines, 2003. Justin C. Cross, “Effects of Microstructure On The Fire- Resistant Properties Of HSLA Structural Steels,” MS Thesis, Colorado School of Mines, 2006. Ryan W. Regier, “Thermomechanical Processing Effects on the Elevated Temperature Behavior of Niobium Containing Fire-Resistant Steel,” MS Thesis, Colorado School of Mines, 2008.

Fire Resistance in Structures Coatings Design Tokoname Gymnasium Design of Steel Frames to Eliminate Fire Protection , Nippon Steel Corp., 1993. Sprinkler Systems Fire Resistant Structural Materials Cost Effective Fire Resistant (FR) Steels www.armorfirepro.com

Background: Fire Resistant (FR) Steels • Significant Japanese Developments…. • Requirement: Guarantee 2/3 of room temperature yield strength at 600°C • Enhanced performance due to R. Wildt (2005) microstructural stability; alloy with R. Wildt, Fire Resistant Steel – A New Approach to Fire Safety, Proceedings of the Mo, Nb, Cr, ….. 7th World Congress, CTBUH, Council on Tall Buildings and Urban Habitat: Renewing the Urban Landscape, New York, 2005. ISBN: 978-0-939493-22-7. • Applications require specifications and building code acceptance

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

Experimental Methods Tensile Testing = f(T) Constant Load (conventional approach) “Accelerated Creep” (newly developed test) • Impose constant strain rate • Impose constant load • Vary heating rate to test T • Heat at constant rate = 100 to 1200 o C/hr Displacement or Strain Displacement or Strain σ σ T 1 T 1 T 2 T 2 Fail Fail T 3 T 3 Elastic Elastic Limit Limit Increase Temperature Increase Temperature ε ε Temperature or Time Temperature or Time

Alloy Matrix Alloying elements & Steel Alloys Composition (wt%) Base 0.1C - 1.0Mn - 0.2Si - 0.01N Nb Base + 0.02Nb Mo + Nb Base + 0.5Mo - 0.02Nb V + Nb Base + 0.05V - 0.02Nb 1.0Cu - 0.75Ni - 0.51Cr - 0.5 Mo - Cu 0.06V - 0.02Nb

Tensile Data - Examples Base alloy V + Nb alloy 100 100 V+Nb Alloy Base Alloy 25°C 25°C 600 600 400°C 100°C 100°C 200°C 80 80 200°C 300°C 400°C 300°C 25°C 300°C 300°C 400°C 400°C 25°C 500°C 200°C 500°C Eng. Stress (MPa) Eng. Stress (MPa) 600°C Eng. Stress (ksi) Eng. Stress (ksi) 500°C 60 60 600°C 700°C 400 400 500°C 200°C 700°C 100°C 100°C 600°C 600°C 40 40 200 200 700°C 700°C 20 20 0 0 0 0 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Plastic Eng. Strain Plastic Eng. Strain Strain Rate = 3.9x10 -3 s -1 Heating Rate = 600 o C/hr 15 minute hold @ T M. Walp, MS Thesis, 2003

Temperature Dependent Tensile Data Temperature (°F) 0 400 800 1200 100 Mo+Nb 600 Nb 80 V+Nb Base Eng. Stress (MPa) Eng. Stress (ksi) Mo+Nb 60 400 V+Nb Nb Base 40 200 Strain Rate = 3.9x10 -3 s -1 20 Heating Rate = 600 o C/hr 15 minute hold @ T 0 0 0 200 400 600 Temperature (°C) M. Walp, MS Thesis, 2003

Heating Rate Effects – Tensile Data Vary heating rate 100 to 1200 o C/hour Heating Rate (°F/Hr) 0 500 1000 1500 2000 80 Mo + Nb Alloy 300°C 400°C 60 400 100°C Yield Stress (MPa) 200°C Eng. Stress (MPa) Yield Stress (ksi) Eng. Stress (ksi) 500°C 600°C 40 200 20 3.9x10 -3 s -1 15 minute hold 0 0 0 400 800 1200 Heating Rate (°C/Hr) M. Walp, MS Thesis, 2003

Constant Load – Accelerated Creep Temperature (°F) 1000 1100 1200 1300 1400 0.3 50% YS @ 1200°C/Hr Base 0.3 Nb Nb Mo+Nb Mo+Nb V+Nb V+Nb 0.2 Base Plastic Displacement (in) 0.2 Plastic Eng Strain 0.1 0.1 0 0 -0.1 -0.1 550 600 650 700 750 Temperature (°C) M. Walp, MS Thesis, 2003

Microstructure after Accelerated Creep Nb alloy Mo + Nb alloy 100 nm • TEM Replicas • Tested at 50% of room temp. yield stress of Nb alloy • Heating rate = 300°C/hr J. Cross, MS Thesis, 2006

Precipitation during Heating Temperature (°F) 1000 1100 1200 1300 • 1 % Cu Steel – three 0.3 heat treat conditions: 50% YS @ 600°C/Hr 1% Cu steel Cu N 0.3 Cu P • Normalized (N) Cu O • Maximum precipitation 0.2 potential during test Plastic Displacement (in) 0.2 Cu P Plastic Eng Strain Cu O • Peak Aged (P) Cu N • Distribution of fine ppts 0.1 0.1 • Overaged (O) • Coarse ppts • minimum precipitation 0 0 Normalized potential -0.1 -0.1 500 550 600 650 700 750 Temperature (°C) M. Walp, MS Thesis, 2008

Importance of Base Microstructure • C-Mn Alloy: Three heat treat Temperature (°F) 1100 1200 1300 conditions 0.15 300°C/hr • Ferrite-Pearlite (F/P) 50% Nb RT YS 0.125 • Limited substructure in ferrite F/P • Bainite (B) 0.1 Plastic Strain Martensite • Martensite (M) 0.075 Bainite 0.05 • Result: • Substructure contributes to 0.025 FR properties • Improvement less than by 0 560 600 640 680 720 using Nb-alloy Temperature (°C) J. Cross, MS Thesis, 2006 Speer, et al., HSLA- 2005.

Evaluate Substructure Control by TMP • Nb Alloy • Laboratory Rolled • Vary Finishing Temperature, 650 to 900 o C 60 min @ 1100C 25% @ 1000C Temperature ( o C) 10% @ 900C ) A 3 800C 750C 700C A 1 650C Air Cool Time (min) R. Regier, MS Thesis, 2003

TMP: Microstructures – Nb Alloy Electron Back Scattered Diffraction Images (Combined IQ, IPF, and Misorientation Plots) 900°C 650°C 50 µm 50 µm RD R. Regier, MS Thesis, 2003

TMP: Tensile Properties – Nb Alloy Effect of Finishing Temperature: 650 to 900 o C Test T = 25°C 600°C Finish Rolling Temperature (°F) Finish Rolling Temperature (°F) 1200 1350 1500 1650 1200 1350 1500 1650 600 85 45 A 1 A 3 UTS UTS 300 550 UTS UTS A 1 75 A 3 Stress (MPa) Stress (MPa) 40 Stress (ksi) Stress (ksi) 500 α α+ γ γ � ��� � � � ��� 250 α α+ γ γ 65 450 35 YS YS YS YS 400 55 30 200 350 600 700 800 900 600 700 800 900 Finish Rolling Temperature (°C) Finish Rolling Temperature (°C) Heating rate = 600 o C/hr R. Regier, MS Thesis, 2003

TMP: Constant Load – Nb Alloy Effect of Finishing Temperature: 650 to 900 o C Temperature ( o F) 1000 1100 1200 0.3 50% Nb RT YS @ 600 o C/Hr o C 900 0.2 o C 750 Plastic Eng Strain o C 800 0.1 o C 700 1% offset strain 0 o C 650 Identical Applied loads -0.1 500 550 600 650 700 Temperature ( o C) R. Regier, MS Thesis, 2003

TMP: Constant Load – Nb Alloy Enhanced properties with sub-critical finishing temperature Finish Rolling YS Ratio Temp. ( ° C) (%) 650 ° C 63.1 700 ° C 57.1 750 ° C 53.1 800 ° C 57.6 900 ° C 59.1 R. Regier, MS Thesis, 2003

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

Current Status: United States • New ASTM Standard Approved - 2012 • A1077/A1077M-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.