For example, the BH values are 46 MPa and 52 MPa
for the steels annealed at 800 1C and 900 1C, respectively, at the
quenching temperature of 600 1C. The BH values decrease to
30 MPa and 23 MPa when the quenching temperature reaches
800 1C. The positive BH values remain until quenching up to about
670 1C and 710 1C for the steels annealed at 800 1C and 900 1C,
respectively.
In the DP steel, the microstructure evolution during baking
includes three stages [11]. Therefore, the BH value can be calculated
by the following equation:
BH ¼ ΔσCottrell þΔσprecipitation þΔσtempering ð1Þ
where ΔσCottrell is the hardening caused by the formation of
Cottrell atmosphere; Δσprecipitation is the hardening because of
the precipitation of carbides in the ferrite; Δσtempering is the
variation due to the tempering of martensite phase including
martensite decomposition and relief of residual stresses. Usually,
ΔσCottrell and Δσprecipitation are positive, while Δσtempering is negative.
In addition, the residual stresses include thermal stresses due
to the temperature variation and structural stress because of the
martensite formation [13]. The residual stresses have a pronounced
influence on the mechanical properties of the samples,
even leading to distortion [14]. The residual stresses mainly
depend on various parameters, including quenching temperature,
martensite volume fraction, cooling rate and thermal conductivity