High formability dual phase steel
To improve the formability of dual phase steels, the martensite phase is tempered. It may form a ferrite-carbide structure. The tempering step occurs after martensite has been formed in the dual phase steel. The tempering step can occur in a box annealing step or it can be performed in a continuous fashion, such as on a continuous annealing, continuous tempering heat treating, or continuous coating line. The tempering step can further comprise a temper rolling on a temper mill after the heating step.
1. A method of improving the formability of a dual phase steel strip comprising ferrite and martensite, the method comprising the step of temper heat treating the dual phase steel strip at a temperature and for a time sufficient to transform at least a portion of the martensite to ferrite and cementite, wherein the sufficient time and temperature is defined by a cumulative diffusion distance of carbon disposed within the dual phase steel strip after a batch process that is <1.1 micrometers when the dual phase steel strip has a nominal tensile strength of 780 MPa, or <1 micrometers when the dual phase steel strip has a nominal tensile strength of 980 MPa, wherein the sufficient time is one hour or more.
2. The method of claim 1 further comprising the step of temper rolling the dual phase steel after the temper heat treating step.
3. The method of claim 1 wherein the temper heat treating step occurs after the strip has been cold rolled.
4. The method of claim 1 wherein the temper heat treating step occurs after the strip has been coated with a coating.
5. The method of claim 1 wherein the temper heat treating step is a box annealing step.
6. The method of claim 1 wherein the temper heat treating step is a continuous temper heating step.
7. The method of claim 6 wherein the continuous temper heating is provided by induction heating.