Cold-rolled hard coil, a key form of steel, has a hardness that is a key indicator of its quality and performance. Typically, the hardness value range of cold-rolled hard coil is between 50-90 HRB. This value is based on the Rockwell hardness test method, which determines the hardness of a material by measuring the depth to which a steel ball presses into the surface of a specimen.
The hardness of cold-rolled hard coil is affected by a variety of factors, including raw material quality, the cold rolling process, and the heat treatment process. Since cold-rolled hard coil is not annealed, its hardness is relatively high, but this also results in poor machinability.

Hard cold rolled coil is a semi-finished product made by continuously cold-rolling hot-rolled coil at room temperature after pickling to remove the oxide scale. Hard cold rolled coil typically has a hardness exceeding 90 HRB (Rockwell hardness) and has very poor machinability, requiring only simple, directional bends of less than 90 degrees (perpendicular to the coiling direction). Therefore, it is primarily used for parts requiring simple deformation or as feedstock for hot-dip galvanizing plants.

Factors Affecting Hardness
1. Material Composition:
Every 0.1% increase in carbon content increases hardness by approximately 10 HV; alloying elements such as manganese and silicon also enhance the hardening effect.
2. Rolling Process:
Reduction: The greater the rolling reduction, the higher the hardness (for example, a 50% reduction yields a 15-20% increase in hardness compared to a 30% reduction).
Annealing Temperature: The hardness of fully hardened (unannealed) cold-rolled coil can reach 180 HV, but after annealing at 700°C, it drops below 120 HV.

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