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The susceptibility of Advanced High Strength Steels (AHSS) to hydrogen embrittlement (HE) was evaluated on selected high strength sheet steels (DP 600, TRIP 780, TRIP 980, TWIP-Al, TWIP, and Martensitic M220) and the results were compared to data on a lower strength (300 MPa tensile strength) low carbon steel. Tensile samples were cathodically charged and then immediately tensile tested to failure to analyze the mechanical properties of the as-charged steel. The effects of hydrogen on deformation and fracture behavior were evaluated through analysis of tensile properties, necking geometry, and SEM images of fracture surfaces and metallographic samples of deformed tensile specimens. The two fully austenitic TWIP steels were resistant to hydrogen effects in the laboratory charged tensile samples.

The influence of coatings on fatigue behavior has been examined for the following commercially produced sheet steels: uncoated titanium stabilized interstitial-free (IF); electrogalvanized titanium stabilized IF; hot-dip galvanized aluminum killed, drawing quality (AKDQ); and galvannealed AKDQ. Fully reversed bending fatigue tests were conducted at ambient temperature on Krouse-type flexural fatigue machines. A dependence of crack development was observed and correlated to the microstructure and properties of the different coatings. Furthermore, a functional design relationship for each material was determined through stress-life analysis. The experimentally determined fatigue properties were compared to conventional estimates based on tensile properties which ignore coating effects. The results of this work suggest that ductile coatings may enhance fatigue resistance, while brittle coatings may reduce fatigue life.

High strain rate test methods to obtain strain-rate dependent sheet steel tensile properties are considered. A tensile test method for sheet steels was developed to obtain accurate stress-strain data over the strain rate range from 0.001 s-1 to 500 s-1 using a servo-hydraulic test machine and tensile samples instrumented with strain gages. Results on several different automotive sheet steels, including interstitial free (IF), high strength low alloy (HSLA), dual phase (DP), and transformation induced plasticity (TRIP) steels, are presented. The results show that strain rate response differs between the various alloy systems. These results are compared with previously published data on strain-rate dependent steel properties. The importance of stress-strain curve shapes, which depend on alloy system, on energy absorption calculations using areas under stress-strain curves are also described.

The effects of deep rolling were evaluated by reviewing the fatigue performance of three medium-carbon (0.4 C) bar steels representing microstructural classes characteristic of forging steels used for crankshaft and other automotive applications. Deep rolling is a surface deformation process whereby a radially symmetric work piece undergoes a surface deformation operation. The steel grades included a quenched and tempered alloy steel (4140) that demonstrated a high yield stress and low strain hardening rate, a non-traditional bainitic experimental grade (1.2 Mn, 0.72 Si) containing high amounts of retained austenite with low yield stress and high strain hardening rate, and a ferritic/pearlitic grade (1.3 Mn, 0.56 Si) with a low yield stress and medium strain rate hardening rate. A reproducible test methodology to assess fatigue behavior was developed, based on flex-beam, fully reversed, S-N type laboratory fatigue testing.

Quenching & Partitioning (Q&P) is receiving increased attention as a novel Advanced High Strength Steel (AHSS) processing route as promising tensile properties of the “third generation” have been reported. The current contribution reports hole expansion ratios (HER) of Q&P steels and compares the values with HERs obtained for “conventional” AHSS processing routes such as austempering and Quench & Tempering (Q&T). Intercritically annealed C-Mn-Al-Si-P and fully austenitized C-Mn-Si microstructures were studied. Optimum combinations of tensile strength and HER were obtained for fully austenitized C-Mn-Si Q&P samples. Higher HER values were obtained for Q&P than for Q&T steels for similar tempering/partitioning temperatures. Austempering following intercritical annealing results in higher HER than Q&P at similar tensile strength levels. In contrast, Q&P following full austenitization results in higher hole expansion than austempering even at higher strength levels.

Dynamic mechanical properties of TRIP steels with the same volume fraction but different stabilities of retained austenite were evaluated over a wide range of strain rates using a high-velocity hydraulic tensile testing machine. Tensile tests were performed at strain rates ranging from 10-2 to 6×102s-1 and ultimate tensile strength, strain hardening behavior, and absorbed energy were evaluated. Strain control during high speed tensile testing was accomplished using a “stopper” attachment designed to limit strain within the gage section to an amount preset before testing. Strain was controlled successfully up to the highest strain rate examined, 200 s-1. The methodology allowed, for the first time, the extent of austenite transformation to be monitored at incremental strains during a high-rate test. At all strain rates, the extent of martensite transformation was considerable after only a few strain percent, and was essentially complete well before the onset of necking.

The effects of prestrain and zinc coating type on the bending fatigue behavior of titanium-stabilized interstitial free steel were evaluated. From a single zinc bath chemistry, coated sheet steel samples were prepared with either a hot dip galvanized or galvannealed coating. Uniaxial tensile prestrains of 2 and 4 pct. were introduced parallel to the rolling direction on 12.7 cm wide strips. Krouse-type fatigue samples were machined both parallel and transverse to the rolling/prestrain direction. Reversed bending S-N fatigue data showed that the fatigue resistance depended on a complex interaction between the strength increase due to work hardening and fatigue crack development as altered by the presence of the coatings. For both coating types the fatigue resistance increased with prestrain. During prestrain, coating cracks oriented perpendicular to the tensile prestrain direction developed and the crack density was greater in the galvannealed materials.