High Performance Steels: Alloy and Thermo-mechanical Process Design
Sponsored by: TMS Structural Materials Division, TMS: Steels Committee
Program Organizers: Ana Luiza Araujo, CBMM North America Inc.; C. Tasan, Massachusetts Institute of Technology; Jonah Kleem-Toole, Colorado School of Mines; Louis Hector, General Motors Global Technical Center; Tilmann Hickel, Max-Planck-Institut Fuer Eisenforschung; Benjamin Adam, Oregon State University

Thursday 8:30 AM
March 3, 2022
Room: 252C
Location: Anaheim Convention Center

Session Chair: Tomas Scuseria, Colorado School of Mines; Cem Tasan, MIT

8:30 AM  
Multi-stage Welding Cycles for Resistance Spot Welding of Advanced Martensitic Steels: Emmitt Fagerstrom1; Benjamin Hilpert2; Holger Schubert2; Bharat Balasubramanian1; Luke Brewer1; 1University of Alabama; 2Mercedes-Benz
    This research investigates the influences of in-process heat treatment on resistance spot welding (RSW) of next generation martensitic steels for automotive body applications. RSW is used extensively in the automotive industry, but with the advent of newer high strength steels (ultimate strengths > 1200 MPa), the standard process cycle cannot always achieve acceptable weld quality. The martensite formed by the high cooling rates associated with RSW can be ameliorated by the use of pre- or post- weld current pulses during joining. In this study, we have systematically investigated multi-stage welding cycles for RSW of a fully-martensitic 1500 MPa steel. The mechanical properties are evaluated through lap-shear tensile testing and microhardness testing. While initial microscopy does not show large differences in weld microstructure, the lap-shear results show a significant, positive increase in mechanical energy absorbed during joint failure. Further connections between martensite content and weld mechanical properties will be discussed.

8:50 AM  
Impact of Alloy Composition on the Hot Ductility of Steel during Continuous Casting: Alyssa Stubbers1; Thomas Balk1; 1University of Kentucky
    The connection between alloy content and high-temperature ductility behavior of steels is due to the inhibition of diffusion and precipitate formation caused by additional material present in alloys. This diffusion and precipitation behavior can cause low ductility regions to occur during crucial operations of a continuous casting process which may cause transverse cracking or total breakouts of steel. In this study, a range of alloys containing Nb, V, N, and Ti were examined to identify and characterize the influence of alloying elements on ductility behavior at temperatures above 700°C. Using a Gleeble 3500, samples were hot tensile tested and quenched at failure for subsequent analysis using SEM and TEM techniques to evaluate ferrite to austenite phase transformations and precipitate formation. Ductility profiles and microstructural information were then used to construct a clearer picture of how alloying elements interact within the steel microstructure to modify high-temperature material properties.

9:10 AM  
Custom Designed Tapered-rolling Process Enables Hard Steels with Mixed-mode Cracking Resistance: Gianluca Roscioli1; Cemal Tasan1; 1Massachusetts Institute of Technology
    Carbide-rich martensitic stainless steels are typically used for sharp edges to obtain high hardness and wear resistance while reducing costs. However, the characteristic size of the microstructural constituents is larger than the tip radius, leading to spatial heterogeneity in mechanical properties along the edge and, in turn, to mixed mode II/III cracking. To improve these materials, we designed a new manufacturing process that uses tapered rolls to locally promote solid-solutioning of ultrafine ferritic grains through plastic deformation. This process leads to a gradient-induced toughening effect and high strength near the sharp edge, sufficient to avoid cracking due to the compressive and bending loads. We characterized this graded material through electron backscattered diffraction and Picoindentation hardness tests. In addition, a sharp edge produced with this material was tested in in-situ SEM cutting experiments to show the improved mechanical response.

9:30 AM  
Large Strain Ambient Temperature Rolling Reduction of an Ultra-high Strength Steel: Joshua Edwards1; Thomas Kozmel2; Jeffrey Lin2; Suveen Mathaudhu1; 1Colorado School of Mines; 2QuesTek Innovations LLC.
    Ferrium® M54® is a medium-carbon ultra-high strength steel with the potential for achieving outstanding strength, toughness, and stress corrosion cracking resistance. A sheet rolling process involving repeated low-percentage rolling reductions, referred to as Skin Pass Rolling (SPR), was investigated to reveal the relationships between processing, microstructure, and mechanical properties. Rolling was conducted dry at 23 °C with final reductions of 10% - 60% at between 0.5% - 3% per-pass. Subsurface hardening was assessed via Rockwell Hardness and confirmed locally by Vickers Microhardness. After processing, mechanical performance and tensile testing was evaluated across the range of final reductions, with digital image correlation strain mapping. Microstructural investigations of the microstructure (phases present and block and lathe structures) point to the mechanisms for the low-temperature plasticity and resultant mechanical properties. The results forecast the ability to use Skin Pass Rolling to improve formability and enhance the performance of novel ultra-high strength steel products.

9:50 AM Break

10:05 AM  
Understanding Composition Dependence of Deformation Microstructure in Hydrogen Resistant Austenitic Steels: Quinten Yurek1; 1University of Illinois
    The development of hydrogen-resistant steels depends on understanding alloy composition and microstructure evolution during deformation. We analyzed the 21Cr-6Ni-9Mn (Nitronic 40) alloy and a series of four developmental alloys in which the Ni/Mn ratio is varied. The Ni/Mn balance can affect several critical materials parameters including stacking fault energy, slip-induced twinning and dislocation splitting. We probe the relationship between deformation and microstructure using tensile testing and nanoindentation tests. Conospherical indentations allow TEM lift-out samples in which the stress state of the lift-out can be determined. Therefore, a relationship between stress and microstructure can be drawn. Interrupted strain tests serve to compare an initial plastic microstructure between the alloys to determine dislocation density, burgers vector and stacking fault energy. How these microstructural aspects vary between alloys and in the presence of hydrogen are of importance in understanding the role of composition and embrittlement in the deformation microstructure.