Influence of hydrogen on some DP, Q&P and TWIP grades of advanced high strength steels for auto construction

Abstract

Dual Phase (DP), Quenching and Partitioning (Q&P) and Twinning-Induced Plasticity (TWIP) advanced high-strength steels (AHSS) have demonstrated the capability to meet the demands of fuel efficiency and improved safety performance in auto industry, attributed to their excellent combination of strength and ductility, good formability and high energy absorption capability. However, hydrogen embrittlement (HE) is particularly under concern for the stressed AHSS, causing the deterioration of steel mechanical properties, especially ductility. For the lifetime of a car body, the sources of this hydrogen are extensive, such as steel making, auto construction processes, and corrosion processes in actual service.Therefore, in this doctoral dissertation, efforts have been made to gain a deeper understanding of hydrogen influence on the key DP, Q&P and TWIP steels produced for auto construction. The following issues have been covered:1. Hydrogen influence on the mechanical properties of the DP, Q&P and TWIP steels.2. Hydrogen diffusion and trapping in the key DP and Q&P steels.3. Hydrogen concentration in some DP and Q&P steels.4. Equivalent hydrogen fugacity during electrochemical charging of 980DP steels.The following techniques were employed in this research: optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), electrochemical polarization, hydrogen permeation analysis, linearly increasing stress test (LIST), universal tensile machine (UTM), hot extraction analyser and thermal desorption spectroscopy (TDS).The hydrogen influence on six different DP, Q&P and TWIP steels was studied using LIST with cathodic hydrogen charging. The steels were designated as 980DP, 980DP-650YS, 980DP-700YS, 1200DP, 980QP and 950TW. 980DP referred to a DP steel with tensile strength of 980 MPa. 980DP-650YS and 980DP-700YS referred to a DP steel with tensile strength of 980 MPa and increased yield strength of 650MPa and 700MPa, respectively. 1200DP referred to a DP steel with tensile strength of 1200 MPa. 980QP referred to a Q&P steel with tensile strength of 980 MPa. 950TW referred to a TWIP steel with tensile strength of 950 MPa. All steels exhibited HE susceptibility, manifested by (i) decreased strength, and (ii) reduced ductility and a change from ductile cup and cone fracture to brittle transgranular and/or intergranular fracture. There was no sub-critical crack growth at stresses below the ultimate tensile strength. The hydrogen influence increased with increasing strength, more II negative charging potential, and decreasing stress rate. Hydrogen brittle fracture was associated with the hard martensite phase for DP and Q&P steels.The hydrogen influence was also studied using LIST and UTM (i) for immersion in 3 wt% NaCl, to simulate hydrogen picked in automobile service, and (ii) at substantial loading rates to simulate a crash. Simulated service corrosions caused minimal HE for 980DP and 1200DP, and some HE for...