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With the development of lightweight in body-in-white (BIW), tailor welded blanks (TWBs) of dual phase steels have been widely used in the BIW. Stamping performance of the TWBs was changed dramatically after laser welded. The stamping performance of the HC340/590DP dual phase steel in the TWBs was studied from the uniaxial tension test and the Erichsen test. The results showed that the yield strength of the TWBs is higher than its base material above 52.3%. The elongation of the TWBs is less about 68.7%. Meanwhile, the IE value of the TWBs is less than its base material above 20%. The TWBs with different thickness has the weakest formability in the various welding combination at same grade steel. Stamping performance of dual phase steel in the TWBs is weaker than base materials.

The microstructure, tensile property and hole-expansion property of 780MPa grade cold rolled galvanized dual phase steel were studied; Scanning electron microscopy was used to analyze the crack formation and propagation behavior during the hole-expansion process. The results show that: DP780 steel was mainly composed of ferrite and martensite, and the plastic deformation behavior can be divided into two distinct stages; the average hole-expansion rate was 30.7%, and DP780 shown a excellent overall performance; the crack formation and expansion were mainly micro void coalescence fracture mode; the bifurcation of crack and deformation of ferrite are conducive to weaken the local stress concentration near the crack, improving the hole-expansion performance.

A dynamic tensile experiment was performed at various strain rates from 10^-4 to 10³ s^-1 on a dual phase steel, and the strain rate sensitivity on their tensile properties was studied. Results showed that the dual phase steel exhibited an obvious strain rate effect on strength. There were two stages in the effect of strain rate on strength. In the first stage, there was small sensitivity of strength to strain rate. In the second stage, there was obvious strain rate sensitivity, and the strength increased rapidly with the increase in strain rate. The strain rate sensitivity of the dual phase steel is considered to be originated from thermal activated mechanism. There is enriched thermal activated region in the first stage, and a lack thermal activated region in the second stage.

In this paper, a series of quasi static and dynamic mechanical properties tests to DP780 (1.4mm) based on different strain path were carried out under room temperature condition. The test results show that the yield strength of DP780 increases evidently compared to raw material through the 1% and 2% pre-strain treatment under either quasi-static or high strain velocity conditions, but the ultimate strength of DP780 increases slightly.

The selective oxidation behaviors of a DP780 steel during recrystallization under different annealing atmospheres have been investigated in a hot dip process simulator (HDPS). The surface chemistry and morphology were analyzed by glow discharge optical emission spectroscopy (GDOES) and scanning electron microscopy (SEM) respectively. The results indicate that the dew point and hydrogen content in the annealing atmosphere have great effects on selective oxidation, and the influence of dew point is slightly stronger than that of hydrogen content. Under the higher dew point and lower hydrogen content, resulting from the increased tendency of internal oxidization of Mn and Cr, the amount and size of oxides dispersed on the external surface decrease, which finally improves the wetting between the annealed substrate and the Zn bath.

The tension-compression fatigue properties of dual-phase steel after pre-compression deformation of 0 %, 5 % and 10 % were investigated. The fatigue property under load ratio of r = -1 was measured. The fatigue limit was calculated by using of national standard and SAE standard. The microstructure and fatigue fracture were observed. The results show that the fatigue property under high stress and the fatigue limit of 50 % survival rate are obviously improved after the 5 % and 10 % pre-compression. But effect of 5 % pre-compression on the fatigue property is better than that of 10 % pre-compression. This result is related to the stress state produced by the pre-compression strain and the effect it has on the mechanical properties of the material. The typical fatigue fractures in three states are analyzed and compared, and the fracture morphology and fatigue property have a good correspondence.

This paper investigates the development of dual phase steel in terms of chemical additives used; their impact on the resulting microstructure and tensile property; as well as their fatigue property. For study of fatigue damage; S-N tests were undertaken according to GB/T 3075-2008 for DP600 with thickness of 1.0mm; Dynamic strain-stress curves for DP590 were obtained by fitting the data with Swift-Hockett-Sherby constitutive equation; the forming limit curve (FLC) traditionally used to evaluate formability is not sufficient to predict potential edge crack, hence, the edge stretchability is studied hole expansion test that can emulate the stress/strain conditions formed during hole flanging under hole expansion test. The result of the Dynamic strain-stress curves obtained has shown that DP590 could achieve a maximum strain rate as high as 1000 /s, and with this increase of the strain rate, the strength also increases accordingly. This opens up dual phase (DP) steel for use as the first generation automotive steels apart from martensitic steel.

The effects of pre-compression deformation 5 %, 10 % on tension property of DP590 are investigated. The pre-compression deformation 5 %, 10 % were carried out on specific fixture in order to prevent buckling deformation. The samples were machined to 5 time standard tensile samples after pre-compression deformation. Tensile flow curves were measured. The metallographic structures are observed by OM. The results show that the tension yield strength after 5 % pre-compressed strain decreases, the n value increases when the strain is 2 %, but the n value decreases when the strain is 2-6 %, and the total elongation increases slightly. When the pre-strain was 10 %, the n value had a similar variation, but the yield strength increased slightly and the total elongation decreased significantly. The effects of pre-compression strain on mechanical properties of dual phase steel are discussed considering microstructure, fracture morphology, Bausinger effect and working hardening of dual phase steel.