Hot-Formed (HF) Steel

The implementation of press-hardening applications and the utilization of hardenable steels are promising alternatives for optimized part geometries with complex shapes and no springback issues. Boron-based hot-forming steels (between 0.002% and 0.005% boron) have been in use since the 1990s in body-in-white construction. A typical minimum temperature of 850 °C must be maintained during the forming process (austenitization) followed by a cooling rate greater than 50 °C/s to ensure that the desired mechanical properties are achieved.

Two types of press-hardening or hot forming applications are currently available:

• Direct Hot-Forming
• Indirect Hot-Forming

During Direct Hot-Forming, all deformation of the blank is done in the high temperature austenitic range followed by quenching. Indirect Hot-Forming preforms the blank at room temperature to a high percentage of the final part shape followed by additional high temperature forming and quenching.

Five process stages with different mechanical properties are important for Direct Hot-Forming:

• Stage 1 (ellipse 1): Room temperature blanking: Yield strengths at 340 - 480 MPa, tensile strengths up to 600 MPa, and elongations greater than 18 % must be considered for the design of blanking dies.
• Stage 2 (ellipse 1): Blank heating: The blank is heated to about 850 - 900 °C.
• Stage 3 (ellipse 2): High temperature forming in the die: High elongations (more than 50%) and low strengths (almost a constant 40-90 MPa true stress) at deformation temperatures allow extensive forming at low strengths.
• Stage 4 (ellipse 2): Quenching in the die: Following forming, tensile strengths above 1500 MPa and total elongations of 4 - 8% (martensitic microstructure) develop during part quenching in the die.
• Stage 5 (ellipse 3): Post-forming operations: Because of the very high strength, special processes are necessary when finishing the product (special cutting and trimming devices, etc.).

In contrast, most of the forming during Indirect Hot-Forming is accomplished at room temperature.

• Stage 1 (ellipse 1): Room temperature blanking,
• Stage 2 (ellipse 1): Preforming most of the final part shape at room temperature with a traditional press and die. As with all room temperature forming, as-received sheet metal properties (yield strengths of 340 - 480 MPa, tensile strengths up to 600 MPa, and elongations greater than 18 %) may constrain maximum formability. 
• Stage 3 (ellipse 1): Part heating: The part is heated to about 850 - 900 °C.
• Stage 4 (ellipse 2): Final high temperature part forming at low strength and high elongations.
• Stage 5 (ellipse 3): Quenching in the die: Complex parts with tensile strengths above 1500 MPa, total elongations of 6 - 8% (martensitic microstructure) and zero springback develop during quenching in the die.

True stress-strain curves for common boron-based HF steel in both the as-received room temperature condition and after quenching to final strength condition are located in AHSS Application Guidelines Figure 2-10. Additional information on the hot-forming process is available in AHSS Application Guidelines Section 2.C.3.e.