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.,,,Microalloyed Forging Steels for Automotive Applications,David K. Matlock Advanced Steel Processing and Products Research Center Department of Metallurgical and Materials Engineering Colorado School of Mines Golden, Colorado USA,.,,Microalloy Steel Applications,http://www.khulsey.com - ? Kevin Hulsey Illustration, Inc. (2005),Sheet steels for outer body and structural components,,Engine Components: Crank shafts Connecting rods,Suspension components,Transmission components,???Future Applications???,Gears, axles, shafts, hubs,,.,Ultra Light Steel Auto Suspension, AISI (2001), www.autosteel.org,Etc. 7%,Automotive Materials,,,,.,Example: Trends in Engine Design,,C. Lema?tre, AIST Bar Conf., Winter Park, Colorado (2006),New environmental legislation – continued reductions in allowable emissions Diesel: Increased injection pressures Gasoline: Increased use of turbo and/or compressors,European Data,.,Lighter weight powerplants for improved fuel economy Require higher strength materials Microalloyed bar steels for crankshafts Increased stiffness (compared to cast iron) Better fatigue resistance Reduced component weight Allow smaller and lighter flywheels and clutch systems,,AISI Report – 2004 www.autosteel.org,Example: Trends in Engine Design,.,Historical: Use of Microalloyed Forging Steels for Automotive Components,Initial applications in Europe – 1980’s Based on direct-cooled ferrite-pearlite steels Growth in use in the US – 1990’s Ferrite-pearlite steels Non-traditional bainitic steels with retained austenite With improved processing control and understanding of microalloying fundamentals, use continues to expand.,.,Requirements for New Microalloyed Bar and Forging Steels Alloying, Processing and Product Developments,Higher Strength … with … Excellent Mechanical Properties Formability, Toughness, Fatigue Resistance, Machinability …….. Product Benefits More efficient vehicle designs Higher torque-capacity components Higher operating stresses Safer vehicles Improved crashworthiness,.,,Presentation Overview,? Opportunities for use of Microalloyed Steels in Automobiles Reduce manufacturing costs Balance properties and performance Tailor materials for specific application Elements of interest Ti, Nb, V, N, C, … Al … and their T-dependent solubilities Presentation based on Selected Examples Automotive Springs Forgings for Gears Thermomechanical Processing Transmission Components,,.,,Microstructural Classes of Microalloyed Steels,Ferrite-Pearlite Steels – Direct Cooled Strengthened by: Pearlite volume fraction Ferrite grain size and substructure Precipitation strengthening of ferrite (e.g. V and Nb) Heat Treated Steels Through hardened (martensitic or bainitic) Induction hardened surfaces Surface modified and heat treated Carburized,.,,Direct-Cooled Forging Steels,Kaspar et al., Microalloying Forging Steels, TMS, 1996,Benefit of Direct-Cooling: Reduced processing time and cost Requirements: Equipment capable of controlled cooling,.,,,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar Steels,? Solubility Considerations Microstructural Control and Grain Growth during Processing Strengthening and Toughening Mechanisms,.,,Microalloy Precipitate Solubility,? Microalloy elements in metals: As solid solution alloy additions In precipitates At grain boundaries Within grains Precipitates form between solid solution elements and interstitials of C or N Microalloy Interstitials Typical Elements Precipitates Nb C NbC NbN V N VC VN Ti TiC TiN Al AlN Specific precipitates that form depend on composition and temperature,.,,Solubility Overview,? Apply equilibrium thermodynamics to predict state of microalloy elements in alloy – driving force for precipitate formation controlled by solubility Reactions of Interest: M + X = MX Example: Nb + C = NbC yM + zX = MyXz 4V +3C = V4C3 Reaction Rate: ks = Equilibrium constant ks is referred to as the solubility product and is equal to the ratio of activities:,,,,,,,.,,Solubility Overview,? Temperature dependence of solubility product, ks, described by an Arrhenius equation; i.e. By convention ks given by: Example solubility diagram for NbC,,,,,,,,,,,,,.,,Example: NbC Solubility Diagram,Assume: T = 1100 oC [Nb] = 0.1 wt pct [C] = 0.2 wt pct,,,,,,,,,,,,,,Solubility data from: Narita, Trans ISIJ (1975),.,,Effects of Composition and Temperature,,,,,,,,,,,,,,,,Example: NbC Solubility Diagram,Solubility data from: Narita, Trans ISIJ (1975),.,,,,,,,,,,,,,Carbide/Nitride Solubility,Lower solubility product = Increased precipitate stability Examples: At 1200 oC TiN is more stable than NbC, VN At all temperatures VC has lowest stability, e.g. VC easily dissolves at higher temperatures,,,.,,From: Matlock, Krauss, and Speer, Microalloying ‘05,Importance of Understanding Solubility,Temperature ranges in which carbides, nitrides, and carbonitrides form and dissolve determine suitability for a given microalloying design. For example: TiN, stable at temperatures in excess of 1200 oC, used for austenite grain size control at high forging temperatures and during high-temperature carburizing VN (and carbonitrides) dissolve at low austenitizing temperatures V is available for fine-scale precipitation strengthening on cooling after forging NbC (and TiC) dissolve and precipitate at temperatures intermediate to TiN and V Precipitates prevent austenite recrystallization during finish hot rolling – results in fine ferrite grain sizes.,.,,,,,,,,,,,,,Carbide/Nitride Solubility,Compare Austenite and Ferrite,,,.,,Alloying and Process Control,Solubility controls driving force for precipitation, volume fraction, etc. Approach to use solubility products Specify alloy content Evaluate solubilities of constituents Determine which precipitates form/dissolve at temperature/composition of interest Predict austenite composition and precipitate volume fractions Design process methodology Approach will be illustrated with Nb-modified gear steels for high temperature carburizing.,,,,,,,,,,,,,.,,,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar Steels,? Solubility Considerations Microstructural Control and Grain Growth during Processing Strengthening and Toughening Mechanisms,.,,? Consider grain growth control During hot working During heat treating Grain growth limited by second phase particles R = stable grain size r = particle radius f = volume fraction particles Finer grain size = decrease r or increase f Size and volume fraction controlled by alloying and processing If too small – particles redissolve If too large – particles are ineffective Types of grain growth Normal grain growth Abnormal grain growth,,Particle Effects on Grain Growth,.,Steel: Controlled Rolled with 0.02Nb Time: 60 minutes Heating Rate: 145°C/min,NGG = Normal AGG = Abnormal IAGG = Initial Abnormal,Grain Growth Terminology,K. AlOgab, PhD, ASPPRC, CSM (2004),.,,Particle Effects on Grain Growth,[1] Moving Grain Boundary Approaches Particle,[3] Particle Retards Boundary Movement,,.,,Particle Effects on Grain Growth,From: Krauss, Steels, ASM, (2005),Plain Carbon Steels Normal grain growth (without particles),,Steel with Particles Grain growth slowed by particles,,.,,Particle Effects on Grain Growth,From: Krauss, Steels, ASM, (2005),Plain Carbon Steels Normal grain growth (without particles),,Steel with Particles Grain growth slowed by particles,,Particle dissolution at higher temperatures results in abnormal grain growth,,.,,Particle Effects on Grain Growth,From: AlOgab, PhD, ASPPRC, Colorado School of Mines (2004),Example Light Optical Micrographs,Abnormal Grain Growth,Normal Grain Growth,.,,Grain Growth Control – Ti, V, Al, Nb,From: Krauss, Steels, ASM, (2005),.,,From: Krauss, Steels, ASM, (2005),Grain Growth Control – Importance of Nb,.,,Grain Growth Control – Importance of Nb,From: Krauss, Steels, ASM, (2005) andK. AlOgab, PhD, ASPPRC, CSM (2004),.,,From: Krauss, Steels, ASM, (2005),Summary: Grain Growth Control,Microalloying elements produce particles that suppress grain growth Particle dissolution leads to abnormal grain growth and steels with large average grain sizes Particle size, volume fraction, and distribution controlled by microalloy additions and temperature Ti and Nb effective in creating particles that suppress grain growth,.,,,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar Steels,? Solubility Considerations Microstructural Control and Grain Growth during Processing Strengthening and Toughening Mechanisms,.,,Strengthening Mechanisms,? Grain Size Refinement Increase strength Increase fatigue resistance Increase toughness Refines transformed microstructures Precipitation Strengthening Transformation Strengthening,.,,,,Hall- Petch Equation Grain boundary blocks slip band Stress concentrated at head of blocked slip band,Strengthening: Grain Size Effects,Barrett et al. (1973) and Gladman (1997),.,,Grain Size Refinement,From: Krauss, Steels, ASM, (2005),.,,Grain Size Refinement,Austenite refinement also modifies martensite packet size in quenched and tempered steels.,From: Krauss, Steels, ASM, (2005),Martensite packet size v. austenite grain size,Yield strength depends on packet size,.,,Fatigue Endurance Limits: Carburized Gear Steels,Cornelissen et al., ASM (2000),Modified 4320 and 8620 Gear Steels,,Finer Austenite Grain Size,,.,,Fatigue Endurance Limits: Carburized Gear Steels,Cornelissen et al., ASM (2000),Modified 4320 and 8620 Gear Steels,,Finer Austenite Grain Size,,,Refined Prior Austenite Grain Size Leads Directly to Improved Fatigue Resistance,.,Grain Size Effects on Toughness,Charpy V-Notch Data Fine grain size lowers transition temperature Fine grains resist cleavage fracture,From: Hertzberg (1989),.,Precipitation Strengthening,,Strength increases with smaller particle sizes or greater volume fractions,,Pinned Dislocation 0.1 C - 0.04 Nb (wt pct) Steel (From Gladman, 1985),,.,D. Ponge, www.materialsknowledge.org (2005),Combined Effects: Grain Refinement and Precipitation,.,,Direct-Cooled Microalloyed Bar Steels,Sawada et al., ISS-AIME, (1994),Direct cool after forging – ferrite-pearlite or “non-traditional bainite” microstructures High carbon critical for strength Microalloying ? precipitation hardening Strength Increase ? toughness decrease,.,,,Direct-Cooled Microalloyed Bar Steels,S. Thompson, ASPPRC, (2006),Pearlite + Interphase Precipitation,.,,Importance of Interlamellar Spacing,? Pearlite strength and toughness depend on interlamellar spacing,From: Gladman,in Microalloying Forging Steels, TMS, 1996,S = true interlamellar spacing t = pearlitic carbide thickness,.,,Summary: Strengthening Mechanisms,? Contributions of different strengthening mechanisms are additive Many equations available in the literature to summarize properties (e.g. see Gladman’s text) Opportunities exist to design materials with specific mechanical properties,From: Gladman,in Microalloying Forging Steels, TMS, 1996,.,,,Thermomechanical Processing of Microalloyed Bar Steels,.,,TMP Control of Microstructures,Example Reference: Boyd and Zhao, in New Developments in Long and Forged Products, AIST, (2006) Purpose of Study: Compare forging schedules designed to control austenite recrystallization with alloy content in modified 1541 alloys Variables: Forging history (temperature), cooling rate, microalloy additions,Boyd and Zhao, AIST, (2006),.,Boyd and Zhao, AIST, (2006),Warm forging: Designed to finish in ferrite-pearlite region,Forging Schedules Applied to Nb/V/Ti Steels,Goal: Produce refined ferrite and pearlite colonies + precipitation strengthening,Deformation in Austenite: Designed to produce non-recrystallized austenite,,.,Boyd and Zhao, AIST, (2006),,Warm Forged 1541 with Nb,SEM Micrographs,Result: successfully produced refined microstructures Strengthened also by dislocation substructure in ferrite,.,D. Ponge, www.materialsknowledge.org (2005),Thermomechancial Processing Also Enhances Precipitation,.,Boyd and Zhao, AIST, (2006),,Effect of TMP on Mechanical Properties,Warm Forged: 1541+ Nb,Forged to produce non-recrystallized austenite,Conventional Forging,,,,1541+ Ti/V,1541+ Nb,,.,Boyd and Zhao, AIST, (2006),,Summary – Thermomechanical Processing,Microstructures with refined ferrite and pearlite can be successfully produced to increase both strength and toughness Precipitates resulting from Nb additions successfully suppressed grain growth to yield the superior properties. Thermomechanical processing of microalloyed bar steels offers opportunities for property development in new steels.,.,,,Example Automotive Springs,.,,Example: Automotive Springs,? Design Requirements: Higher strength Improved fatigue resistance Improved toughness Lighter weight Historically, spring steels based on SAE 5160 and more recently, SAE 9259 Recently, microalloyed steels, based on Nb + V have led to improved materials,.,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,,Spring Steel Compositions,New Microalloyed Grade: Lower C, with Si, Nb, and V Use Thermomechanical Processing to control grain size,With (max values) 0.02 P; 0.021 S; 0.012 Ni (in wt pct),M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,Spring Steel Processing,Hot Rolled Formed Heat Treated Austenitize ~ 940 oC TEM Showed Nb Precipitates for Control of Austenite Grain Size,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,Front Suspension Coil Spring in North American Minivan: Improved with Microalloyed Steel,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,,Summary – Spring Steel Development,Controlled thermomechanical processing is critical in order to be able to utilize benefits of microalloying to control and refine final microstructure Final properties benefit from contributions of both Nb and V in alloys. Development of improved microalloy spring steels will continue.,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,,,Example: Vacuum Carburized Gears,.,,Bending Fatigue In Gears,.,Trends for Automotive Gear Steels,Carburize at higher T: 930 oC ? 1050 oC Shorter heat treat cycle = ?$$ Use alternate technologies Vacuum or low-pressure plasma carburizing Higher T gas carburizing Design alloys to respond to modified thermal histories ? use microalloy additions Improve material properties Fatigue resistance Bending fatigue Contact fatigue,,.,,Example Time Saving with “Vacuum” Carburizing,,Klinkenberg and Jansto, AIST, (2006),.,,Grain Size and “Vacuum” Carburizing,,Advanced Carburizing: Plasma and Vacuum may operate at higher temperatures = shorter cycle times, but……..,From Davidson et al. 2001,.,Austenite Grain Size – Importance to Fatigue of Carburized Steels,Fine Grain Size = Increased Endurance Limit,Cornelissen et al. (2000),.,Trends for Automotive Gear Steels,Methods to refine austenite grain sizes in carburized gear steel Reheat after carburize and quench -- Refine by transformation cycling Requires extra heat treat cycle = $$ Utilize microalloy precipitates to suppress grain growth Alloy designs based on solubility and process temperature considerations,,.,Laboratory Bending Fatigue Sample,Production Gear Set: Ring and Pinon,,Bending Fatigue of Carburized Steels,ASPPRC, Golden, CO (2006),.,Bending Fatigue Failure Mechanisms in Carburized Steels,Gas Carburized 4820 Steel,Matlock et al. ABM, S?o Paulo, Brazil (2004),.,Grain Refinement: Ti Additions,Titanium + Nitrogen,,Titanium Nitride Precipitates,,,,,,,,,,,,,,,,From Davidson et al. 2001,,,,.,200 mm,,Simulated Carburizing 8620 @ 927oC (no Ti),Plasma Carburizing 8620 @ 1093oC,Plasma Carburizing @ 1093oC 8620 Modified: Ti + N,,200 mm,,50 mm,d = 15 mm,d = 123 mm,d = 57 mm,From Davidson et al. (2001),,,,.,Ti additions only produce grain refinement To optimize alloy to resist grain growth, use both Nb and Ti to further suppress austenite grain growth,Summary: Ti Additions,,.,Precipitate Control of Austenite Grain Size During Carburizing,Microalloy Design: Ti ? TiN then Nb ? NbC to further suppress austenite grain growth Alloys: Base: 8620 with 0.03Ti 0.02Nb, 0.06Nb, 0.11Nb Lab melted Heats (Timken) – hot or controlled rolled,AlOgab et al. ISIJ (2007),.,Add Ti to precipitate all N as TiN,Review Basis for Alloy Design,,AlOgab et al. ISIJ (2007),.,Design of Nb Alloys,,,With N as TiN, Evaluate NbC solubility,AlOgab et al. ISIJ (2007),.,Effect of carbon increase during carburizing for 0.1 and 0.3 Nb alloy additions,Review Basis for Alloy Design,,AlOgab et al. ISIJ (2007),.,SAE 8620 steel with 0.03 Ti,Review Basis for Alloy Design,,AlOgab et al. ISIJ (2007),.,Predictions of dissolved Nb content and NbC contents as a function of carburizing temperature,Review Basis for Alloy Design,,Ti-modified SAE 8620 Steel with 0.06 Nb wt pct,AlOgab et al. ISIJ (2007),.,Solubilities at finishing rolling finishing temperatures Hot Rolled: Finish at 1100 oC Controlled Rolled: Finish at 850 oC Solubility Considerations predicts higher precipitate volume fraction in controlled rolled material,Importance of Thermomechanical Processing,,Ti-modified SAE 8620 Steel with 0.06 Nb wt pct,AlOgab et al. ISIJ (2007),.,500 nm,,,,,HR 0.06Nb Steel,CR 0.06Nb Steel,Importance of Thermomechanical Processing,Ti-modified SAE 8620 Steel 0.06 Nb wt pct,AlOgab et al. ISIJ (2007),.,,100 mm,Nb-Free – 60 min.,Hot Rolled 0.02Nb – 240 min.,Influence of Nb Additions: At 950 oC,K. AlOgab, PhD, ASPPRC, CSM (2004),.,Pseudo Carburizing Microstructures: @ 950oC,,,,,,Base Alloy (T),HR 0.02Nb,60,90,240,360 minutes,,,,100 mm,,,,,,K. AlOgab, PhD, ASPPRC, CSM (2004),.,Pseudo Carburizing Results: Grain Growth,,Hot Rolled,Control Rolled,,,,Ti-only,Nb,,Increased Nb,K. AlOgab, PhD, ASPPRC, CSM (2004),.,Fatigue Performance of Nb-Modified Carburized Gear Steels,Fatigue Test at Room Temperature Utilize modified Brugger Sample Vacuum Carburized at 1050 oC Frequency = 30 Hz Base alloy = SAE 8620,Ti-modified SAE 8620 steel With 0.8 Mn, 0.6 Cr, 0.43 Ni, 0.2 Mo (wt pct),R. Thompson et al., ASPPRC, CSM SAE (2007),.,0.06 Nb,0.1 Nb,0.02 Nb,R. Thompson et al., ASPPRC, SAE (2007),Fatigue of Vacuum Carburized (1050 oC) Steel Modified with Nb,.,SummaryGrain Size Control in Carburized Steels,Nb + Ti additions effective in suppressing grain growth during carburizing Opportunities exist for new alloy development for high T carburizing Microstructural control critical to fatigue performance,K. AlOgab, PhD, ASPPRC, CSM (2004),.,,,Example: Microalloyed Steel Tubing for Induction Hardened Transmission Components,.,http://www.khulsey.com ? Kevin Hulsey Illustration, Inc. (2005),Automatic Transmission: One-Way Clutch Race,,,,,,,Automatic Transmission: One-Way Clutch Race,http://www.innerauto.com (2005),.,http://www.len-ind.com (2005),Automatic Transmission: One-Way Clutch Race Q: Can part be manufactured form hot-rolled microalloyed tubing and processed by induction hardening?,.,Automatic Transmission: One-Way Clutch Race,Design requirements Core: 22 to 30 HRC Surface: HRC 60 (aim – after tempering), 58 (min) Machinable Manufacturing methods (typical) Machined from medium carbon alloy steel Carburized and heat treated (twice) to achieve desired core and case properties,.,From: G. Krauss, ASM, 1980,Property Development: Minimum Carbon Content to Achieve Desired Surface Hardness by Induction Hardening + Tempering,,,Requires carbon content greater than 0.5 wt. % to insure properties after tempering,Aim: As-Quenched 61 - 62,,,.,Property Development: Alloying to Produce Core Ferrite-Pearlite Core Hardness of 22 to 30 HRC (equivalent to UTS of 800 to 1000 MPa),Sawada et al., 1994,,,,,Plain carbon steels – not strong enough!! Use Microalloyed Steel,.,Automatic Transmission: One-Way Clutch Race,