Study on Crack Propagation Rate of 600MPa High Strength Steel for Mn-Ti and Nb-V System

The microstructure of 600MPa Mn-Ti and Nb-V steels for cold stamping axle housing was investigated by means of optical microscope. On the low-frequency fatigue tester MTS-810 with frequency of 10 Hz, the crack propagation rate of two chemical components of the tested steels was tested by three-point bending method. The relationship between the propagation rate da/dN and the amplitude of stress intensity factor ΔK was given. The results showed that the microstructure all of two tested steels were ferrite and pearlite, the grain size of Mn-Ti steel and Nb-V steel was 5.8μm and 3.3μm respectively. The coefficient C and m in Paris equation of Mn-Ti steel and Nb-Vsteel were determined as m=2.6298, C=4.13×10^-12mm/cycle and m=2.6799, C=1.30×10^-12mm/cycle respectively, and the threshold value of crack propagation ΔK_th was 46.50 MPa·m^1/2 and 66.58 MPa·m^1/2, respectively. (paper)