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

Dual Phase (DP), Quenching and Partitioning (QaP) 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, QaP and TWIP steels produced for auto construction. The following issues have been covered:1. Hydrogen influence on the mechanical properties of the DP, QaP and TWIP steels.2. Hydrogen diffusion and trapping in the key DP and QaP steels.3. Hydrogen concentration in some DP and QaP 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, QaP 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 QaP 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, morennegative charging potential, and decreasing stress rate. Hydrogen brittle fracture was associated with the hard martensite phase for DP and QaP 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 980DP-650YS, 980DP-700YS, 980QP and 950TW. Simulated crash situations caused HE for 980DP, 980QP and 950TW; their properties quickly reverted after the end of hydrogen charging. Two new HE mechanisms are proposed: (i) hydrogen enhanced macroscopic plasticity (HEMP) decreasing the yield stress, and (ii) hydrogen assisted micro-fracture (HAM) decreasing ductility at fracture at the ultimate tensile strength. The brittle features for 950TW were associated with mechanical twinning rather than martensitic transformation.Hydrogen diffusion and trapping were studied by permeability experiments for DP and QaP steels. The measured reversible hydrogen trap densities indicated that (i) trapping was less significant at a more negative potential, and (ii) the lattice diffusion coefficient of hydrogen could be measured from the partial transients at the most negative potentials. The reversible hydrogen trap densities evaluated from complete decays from m1.700VHg/HgO were s 2 t 1018 sites cmm3 , and were a factor of two higher than those from partial decay transients between m1.700VHg/HgO and m1.100VHg/HgO.The hydrogen concentration in DP and QaP, under simulated service corrosion and cathodic charging conditions was investigated through permeability experiments and hot extraction analysis. The hydrogen concentration in the steels increased with a more negative potential under cathodic charging. The hydrogen concentration under simulated service corrosions in 3 wt% NaCl solution was lower than that at the least negative charging potential of m1.100 VHg/HgO in 0.1M NaOH. Crevice corrosion introduces three times more hydrogen than the corrosion at the free corrosion potential.The equivalent hydrogen fugacity during electrochemical charging was determined using a new TDS apparatus for 980DP steel. The hydrogen concentration increased with (i) more negative charging potential, and (ii) increasing hydrogen gas pressure. The equivalent hydrogen fugacity related to the electrochemical charging overpotential was determined. The detrapping activation energies were 40.5 kJ mol-1 and 50.2 kJ mol-1 for the two desorption peaks, considered as hydrogen traps at boundary defects.