Effects of Matrix Structure and Nitrogen Content on Fatigue Properties of Ultrahigh-Strength Low Alloy TRIP-Aided Steels

Abstract

To develop ultrahigh-strength steels for automotive impact safety parts, the effects of the microstructure and nitrogen content on the fatigue properties of ultrahigh-strength low alloy transformation-induced plasticity (TRIP)-aided steels with martensite (TM), bainitic ferrite-martensite (TBM), and bainitic ferrite (TBF) matrices were investigated. Compared to TBF steels, both the TM and TBM steels achieved high tensile strength, of more than 980 MPa, and excellent fatigue properties. This results from the suppression of crack propagation due to the effective TRIP of the relatively stable interlath retained austenite and the increase in tensile and yield strengths attributed to the low isothermal transformation treatment. The fatigue strengths of the ultrahigh-strength low alloy TRIP-aided steels were slightly increased by the addition of 100 ppm of nitrogen. The increase in fatigue strength of TM, TBM, and TBF steels with 100 ppm of nitrogen was caused by the fine and uniform martensite and bainitic ferrite matrices and retained austenite, along with the increase in carbon concentration in the retained austenite due to the precipitation of AlN.

Effects of Retained Austenite on Hydrogen Embrittlement in TRIP-aided Bainitic Ferrite Steel Sheet

“…In the sheet austempered for 500 s, a blocky structure is observed. Although this blocky structure was described as γ R in the research report by Hojo et al, 20) it is possible that a part of the blocky γ R was transformed into blocky martensite-austenite constituent structure by oil quenching after short austempering in this study. For austempering time of 1 000 s, a small amount of blocky structure is also observed although it is unclear.…”

Effect of Hydrogen on Fatigue Life and Fracture Morphologies of TRIP-Aided Martensitic Steels with Added Nitrogen

“…Tensile tests were carried out on a tensile testing machine at a crosshead speed of 10 mm/min (initial strain rate of 8.33 × 10 −3 /s) at 25 • C in ambient atmospheres with and without hydrogen using tensile specimens with a gauge length of 15 mm, width of 6 mm, and thickness of 1.2 mm at the parallel part, as shown in Figure 2a [7]. The tensile tests were performed twice for each tensile testing condition.…”

“…Load-controlled fatigue tests were conducted on a servohydraulic fatigue testing machine. A sinusoidal waveform of 10 Hz was applied at a stress ratio of 0.1 and 25 • C on a fatigue specimen with a width of 5 mm, radius of 2.5 mm at the notch root, and thickness of 1.2 mm, as shown in Figure 2b [7]. The stress concentration factor at the notch tip was 1.6.…”

“…Hydrogen embrittlement is caused by hydrogen introduced during production processes and service use, such as pickling, plating, and corrosion, through contact with airborne sea salt and splashes containing antifreeze salts. Sugimoto et al systematically investigated the tensile properties [3][4][5], press formabilities [6], fatigue properties [7], V-bendabilities [6], hydrogen embrittlement properties [4,5], and impact properties [8] of high-strength TRIP-aided steels with tensile strengths exceeding 1000 MPa. They found that excellent properties were obtained in these steels due to the effective TRIP effect of retained austenite.…”

“…However, the effect of nitrogen addition on the hydrogen embrittlement behavior of the TRIP-aided steels possessing metastable retained austenite is not revealed sufficiently. In addition, because the fatigue strength of metallic materials increases with their tensile strength [14], nitrogen addition could also improve the fatigue properties of high-strength TRIP-aided steels [7].…”