How To Distinguish Between Hot-rolled Steel And Cold-rolled Steel?

Hot-rolled steel and cold-rolled steel can be distinguished by the following key characteristics:
1. Processing differences
Temperature conditions
Hot rolling is carried out at temperatures above 1000°C (above the recrystallization temperature), while cold rolling is done at room temperature (usually below 200°C).
Process flow
Hot rolling is a single-forming process, while cold rolling requires secondary processing using hot-rolled plates as raw materials.
2. Appearance feature comparison
Surface condition
Hot-rolled steel: rough surface with oxide scale (Ra 6.3 - 12.5 μm), dark gray or brown color, edge rounded radius ≥ 2mm
Cold-rolled steel: smooth without oxide scale (Ra 0.8 - 1.6 μm), silver-white or mirror-like effect, edge straight-edge cutting accuracy ± 0.05mm
Thickness specification
Hot-rolled steel: thickness ≥ 6mm (commonly 6 - 200mm)
Cold-rolled steel: thickness ≤ 8mm (typical 0.1 - 3.5mm)
3. Performance parameter differences
Mechanical properties
Strength: Cold-rolled steel has a yield strength of 400 - 600 MPa, which is 20% - 50% higher than that of hot-rolled steel (300 - 500 MPa)
Plasticity: The elongation of hot-rolled steel is higher than that of cold-rolled steel by more than 30%
Microstructure
Cold-rolled steel has grain size of 10 - 20 μm (with high dislocation density), while hot-rolled steel has 50 - 100 μm
Hardness testing
The hardness of cold-rolled steel is usually higher than that of hot-rolled steel by 20% - 50%, and it can be quickly verified using a portable hardness tester
Bending test
Hot-rolled steel has better ductility and is less likely to crack during bending; cold-rolled steel is prone to brittle fracture due to work hardening during processing
4. Process trace identification
Processing traces
Cold-rolled steel may have rolling mill marks or annealing marks on the surface, while hot-rolled steel often has longitudinal raised stripes
Oxide layer detection
The oxide iron scale on the surface of hot-rolled steel needs to be removed by acid washing, while cold-rolled steel has no oxide layer and can be directly used
5. Professional testing methods
Metallographic analysis
The grain size of hot-rolled steel is 50 - 100 μm, while that of cold-rolled steel is 10 - 20 μm and has a high dislocation density
Quality certificate verification
The most reliable method is to check the process annotations in the quality certificate of the steel products at the factory exit