recovery and results in improved elongation while strength is increased above the as cold rolled level. This must be caused by precipitation over-compensating the strength loss originating from partial recovery. It appears that a maximum of all acting strengthening effects is achieved by annealing at 550 °C. Preliminary estimations indicate that the precipitation strengthening effect is in

Using advanced characterization techniques on surface hardened steels builds a richer understanding of the underlying metallurgical phenomena.

Formed in January 2004 from the merger of the Iron & Steel Society and the Association of Iron and Steel Engineers, AIST is proud to be the #1 source for all your steel-related needs. We encourage you to explore all that we have to offer. From technology-based Operating Committees to our local Member Chapers, the opportunities to network with others and expand your technical know-how abound. Each year we offer two major conventions along with many specialty conferences that will help you get the most out of your membership.

Formed in January 2004 from the merger of the Iron & Steel Society and the Association of Iron and Steel Engineers, AIST is proud to be the #1 source for all your steel-related needs. We encourage you to explore all that we have to offer. From technology-based Operating Committees to our local Member Chapers, the opportunities to network with others and expand your technical know-how abound. Each year we offer two major conventions along with many specialty conferences that will help you get the most out of your membership.

Formed in January 2004 from the merger of the Iron & Steel Society and the Association of Iron and Steel Engineers, AIST is proud to be the #1 source for all your steel-related needs. We encourage you to explore all that we have to offer. From technology-based Operating Committees to our local Member Chapers, the opportunities to network with others and expand your technical know-how abound. Each year we offer two major conventions along with many specialty conferences that will help you get the most out of your membership.

Grey cast iron alloys for brake drum and brake disc applications are being developed with niobium additions and a range of equivalent carbon for commercial, passenger vehicle, and performance applications. The benefit of niobium in cast iron is based on the contribution of strength by matrix refinement for a given carbon equivalence that may permit the direct improvement of wear improvement or allow for an increase in carbon equivalence for a given strength. Proper carbon equivalency and pearlite stabilization contribute to an improved pearlite structure with an optimized distribution of graphite. These structures, when refined with niobium, demonstrate increased service life and reduced wear relative to their niobium-free equivalents as measured by lab dynamometer testing and by on-vehicle testing in passenger bus fleets. The increase in performance is attributed to both the presence of wear resistant carbides and refinement of pearlite interlamellar spacing with only minor refinement of graphite flake length.

Tensile behavior of advanced high strength steel (AHSS) grades with strengths up to 980 MPa has been extensively studied. However, limited data is found in literature on the tensile behavior of steels with tensile strengths of the order of 1180 MPa, especially at nominal strain rates up to 500/s. This paper examines tensile flow behavior to fracture of four different 1180 MPa grade steels at strain rates of 0.005/s, 0.5/s, 5/s, 50/s and 500/s using an experimental methodology that combines a servo-hydraulic tester and high speed digital image correlation. Even though the strength increase with the strain rate is consistent between the four different materials, the total elongation increase with the strain rate varies widely. Some insights as to why this occurs from examination of the steel microstructure and variation of retained austenite with strain are offered.

Implementation of 3rd generation advanced high strength steels (3GAHSS) in automotive structures requires careful consideration of joining process implications. Due to the presence of increased carbon equivalent in 3GAHSS such as 980 HF relative to commonly-applied 1st generation AHSS such as 980 DP, the resultant properties of the weld nugget and heat affected zones in resistance spot welded must be engineered to provide consistent and acceptable performance, specifically in cross-tension loading mode. Although the as-welded cross-tension strength (CTS) of 980 HF is less than 980 DP, there is a significant improvement of the CTS after paint baking in 980 HF welds. 980 DP steel welds do not exhibit such marked improvement upon baking. The mechanism of the strong and beneficial effect of paint baking on 980 HF in comparison to 980 DP is discussed in light of its effect on residual stress distributions and microstructures. Although residual stress (strain) reduction is observed via synchrotron x-ray diffraction for all tested steels, this effect is demonstrated as secondary to the performance restoration of the weld joint in cross tension. High-resolution electron channeling contrast imaging (ECCI) micrographs reveal that paint baking affects a more beneficial degree of martensite tempering in 980 HF as compared to 980 DP. The martensite tempering is also evidenced by both a reduction in nano-hardness of critical microstructural features and re-emergence of ductile fracture behavior following the paint baking process. The tempering of martensite is concluded to be the dominant mechanism in improving cross tension strength of 3GAHSS welds following paint baking.

Adiabatic heating during plastic straining can slow the diffusionless shear transformation of austenite to martensite in steels that exhibit transformation induced plasticity (TRIP). However, the extent to which the transformation is affected over a strain rate range of relevance to automotive stamping and vehicle impact events is unclear for most third-generation advanced high strength TRIP steels. In this study, an 1180MPa minimum tensile strength TRIP steel with carbide-free bainite is evaluated by measuring the variation of retained austenite volume fraction (RAVF) in fractured tensile specimens with position and strain. This requires a combination of servo-hydraulic load frame instrumented with high speed stereo digital image correlation for measurement of strains and ex-situ synchrotron x-ray diffraction for determination of RAVF in fractured tensile specimens. Specifically, the potentially competing effects of strain rate on austenite transformation to martensite were investigated to determine which predominate at nominal strain rates of 0.5 s-1, 5 s-1, 50 s-1 and 500 s-1. A corresponding decrease in austenite volume fraction at a fixed true strain with strain rate suggests that austenite transformation to martensite with strain is accelerated with increased strain rate despite potential energetic inhibition of the transformation due to adiabatic heating. Increased transformation rate to martensite results in increased work hardening rates, strengths, and elongations with strain rate increases to 500 s-1. Observations are discussed in the context of contributing mechanisms both favoring and inhibiting the strain-assisted transformation of austenite to martensite.

It is known that large TiN particles deteriorate the toughness of high-strength steels. The toughness of press-hardened steel (PHS) sheets with full-martensite microstructure for safety critical components of a car body has, however, seldom been studied. Consequently, the effect of TiN particles on either the toughness of PHS sheets or the performance of hot-stamped components have not been investigated. In this research, the size and distribution of TiN particles are quantitatively analyzed for two 22MnB5 PHS sheets containing nitrogen of 29ppm and 44ppm in mass. Higher populations of coarse TiN particles have a negligible effect on standard tensile properties, but significantly reduce impact toughness by Charpy V-notch impact tests. Hot-formed components with smaller fractions of TiN particles show greater fracture resistance by a three-point bending test.

The dependence of the plastic anisotropy on the nominal strain rate for a medium-manganese (10 wt.% Mn) transformation-induced plasticity (TRIP) steel with initial austenite volume fraction of 66% (balance ferrite) has been investigated. The material exhibited yield point elongation, propagative instabilities during hardening, and austenite transformation to α′-martensite either directly or through ε-martensite. Uniaxial strain rates within the range of 0.005–500 s−1 along the 0°, 45°, and 90° orientations were selected based upon their relevance to automotive applications. The plastic anisotropy (r) and normal anisotropy (rn) indices corresponding to each direction and strain rate were determined using strain fields obtained from stereo digital image correlation systems that enabled both quasistatic and dynamic measurements. The results provide evidence of significant, orientation-dependent strain rate effects on both the flow stress and the evolution of r and rn with strain. This has implications not only for material performance during forming but also for the development of future strain-rate-dependent anisotropic yield criteria. Since tensile data alone for the subject medium-manganese TRIP steel do not satisfactorily determine the microstructural mechanisms responsible for the macroscopic-scale behavior observed on tensile testing, additional tests that must supplement the mechanical test results presented herein are discussed.

Modem automotive body structures incorporate an increasing amount of press hardening steel (PHS) stampings to simultaneously increase vehicle safety and reduce mass for improved fuel economy. Recent advances in steel metallurgy have increased PHS tensile strengths from 1.5 GPa to 2.0 GPa by increasing carbon content to approximately 0.30-0.35 mass contents in %. However, increased carbon in martensite may lead to lower impact toughness and an increase in the ductile -to-brittle transformation temperature. This paper will examine additional toughening mechanisms that address these deficiencies by utilizing a customized Charpy V -notch (CVN) impact test to quantitatively assess the toughness of selected PHS materials. Microstructural features such as refined prior austenite grain size (PAGs), retained austenite, and surface decarburization zone are shown to enhance impact performance as measured by thickness -normalized CVN energy. A 15% to 50% improvement in upper shelf energy is achieved for selected alloys relative to 22MnB5 utilizing one or more of these toughening mechanisms. On the contrary, incomplete austenitization can dramatically reduce impact toughness. Higher nitrogen content in the boron-bearing steels also impairs toughness because large TiN particles can serve as crack initiation sites. Several toughness-enhanced PHS grades demonstrate a 25% increase in absorbed energy for crack initiation relative to the 22MnB5 as assessed by static 3-point bending tests of the hot stamped hat section components.

Retained austenite stability to both mechanically induced transformation and athermal transformation is of great importance to the fabrication and in-vehicle performance of automotive advanced high strength steels. Selected cold-rolled advanced high strength steels containing retained austenite with minimum tensile strengths of 980 MPa and 1180 MPa were pre-strained to pre-determined levels under uniaxial tension in the rolling direction and subsequently cooled to temperatures as low as 77 K. Room temperature uniaxial tensile results of pre-strained and cooled steels indicate that retained austenite is stable to athermal transformation to martensite at all tested temperatures and pre-strain levels. To evaluate the combined effects of temperature and pre-strain on impact behavior, stacked Charpy impact testing was conducted on the same 980 MPa minimum tensile strength steel following similar pre-straining in uniaxial tension. A reduction in absorbed energy was observed with decreasing temperature and increasing pre-strain, indicating that thermal effects on plasticity, not athermal transformation to martensite, predominantly account for the observed reductions in impact energy at reduced temperatures.

Impact toughness (or resistance to fracture) is a key material property for press hardened steel used in construction of the safety-critical elements of automotive body structures. Prior austenite grain size, as primarily controlled by the incoming microstructure and austenitization process, is a key microstructural feature that influences the impact toughness of press hardened steel. In this paper, a special Charpy V-notch impact test is developed to quantify the impact toughness of press hardened steel sheets with various prior austenite grain sizes, by stacking a number of thin sheets via mechanical riveting. Both the ductile-to-brittle transition temperature and upper shelf energy are analyzed in an effort to establish a correlation between impact toughness and prior austenite grain size. Within tested conditions, impact performance shows only a slight decrease as the prior austenitic grain size increases from 18 to 38 microns. On the other hand, grain refinement via micro-alloying in the new 1.8 GPa press hardening steel can significantly increase the impact toughness.