1.What are the aging phenomena of ordinary cold-rolled coils?
Harmful defects that should be avoided
The occurrence time is spontaneous during storage or transportation at room temperature.
The core mechanism is the slow diffusion of carbon and nitrogen atoms at room temperature, pinning dislocations.
The result is the formation of a yield plateau, leading to tensile strain marks (surface defects) during stamping and reduced formability.
Negative impacts on molding: It damages surface quality and affects molding stability.
2.What are the properties of bake-hardening (BH) steel?
Properties: Beneficial, designed characteristics
Activated during high-temperature baking (170°C, painting line).
Core mechanism: Carbon atoms diffuse rapidly at high temperatures, resulting in a large number of dislocations generated by pinning and stamping.
Results: Strength increased by 30-80 MPa, enhancing the dent resistance of parts.
Maintain low strength during molding (easy to mold), and achieve high strength (good performance) after molding.
3.How to reserve "hardener"?
In the production of BH steel, through precise chemical composition design (such as the addition of elements like Nb and Ti) and rigorous heat treatment processes, a small portion of "free" carbon atoms are intentionally retained in the steel's microstructure. These carbon atoms are dissolved in the iron lattice, serving as "seeds" for subsequent hardening.
4.How to create "hardened spots" through stamping?
In the stamping workshop, steel sheets are pressed into complex parts (such as car doors). This drastic deformation process generates a large number of dislocations inside the steel. You can think of dislocations as the dense "defect lines" or "scars" inside a crystal.
5.How to bake and activate the material to complete the "pinning hardening" process?
After molding, the parts enter the painting line and are "baked" in an oven at about 170°C for 20-30 minutes. The high temperature provides energy to the reserved "free" carbon atoms, enabling them to diffuse rapidly and automatically "run" around the large number of dislocations generated during the stamping process, forming so-called "Cochrie atmospheres," which firmly hold these dislocations in place like nails.