Selection by Component Requirements
Appearance and forming-oriented components: prefer cold-rolled steel, which has no oxide scale, a smooth surface that can be painted directly, and good soft ductility for complex cold forming such as deep drawing and multiple bending. Load-bearing and cost-oriented components: prefer hot-rolled steel, which has high toughness, low cost, and excellent weldability for parts that withstand impact and load in large volumes. Precision and rust-prevention-oriented components: prefer cold-rolled steel for small dimensional errors and strong coating adhesion, with galvanizing or coating for rust prevention.
Visible Appearance Requirement
For body panels and exterior trim, cold-rolled steel is a must. Hot-rolled steel has a dark brown scale and high roughness; even after pickling it cannot achieve a flawless finish when painted. Cold-rolled steel has a smooth, scale-free surface, giving uniform gloss after painting and meeting automotive exterior requirements for doors, hoods, and trunk lids. For hidden components such as chassis longitudinal rails and frame crossmembers, hot-rolled steel is preferred because appearance is not essential and costs are saved.
Complex Cold Forming Requirement
For deep drawing and multiple bending, cold-rolled steel is a must. Parts such as fuel tanks and instrument panel brackets rely on the soft ductility of annealed cold-rolled steel with fine grains and excellent plasticity. Hot-rolled steel has coarse grains and is susceptible to stress concentration cracking during cold forming; a 90-degree cold bend of hot-rolled steel may crack while a 180-degree bend of cold-rolled steel is easily achieved. For simple cutting and welding parts such as straight chassis sections, hot-rolled steel meets the requirements at lower cost.
High Load and Impact Requirement
For load-bearing structural parts, hot-rolled plate or hot-rolled high-strength base material is preferred because of its greater toughness and impact resistance, essential for chassis rails, truck frames, and collision structures. Cold-rolled plate has higher strength in the work-hardened state but lower toughness and is prone to brittle fracture under impact. The exception is precision load-bearing parts such as new energy vehicle battery pack frames, where cold-rolled high-strength plate balances precision and load-bearing at higher cost.
High-Precision Installation Requirement
For components with tight assembly clearances, cold-rolled steel is a must. Its thickness tolerance of plus or minus 0.01 to 0.05 mm and smooth plate shape prevent sticking or squeaking during assembly. Hot-rolled steel has wider tolerances of plus or minus 0.1 to 0.3 mm and is prone to warping and uneven gaps. For welded frame components where minor deviations can be adjusted during welding, hot-rolled steel provides adequate precision at lower cost.
Frequently Asked Questions
Q: Which steel is used for visible body panels?
Cold-rolled steel, because its smooth, scale-free surface gives a uniform painted finish that hot-rolled cannot match.
Q: Which steel suits deep drawing?
Annealed cold-rolled steel such as SPCE or DC04, whose fine grain and high ductility support complex deformation.
Q: Why is hot-rolled steel used for chassis rails?
It provides greater toughness and impact resistance for load-bearing structures at lower cost.
Q: What tolerance does cold-rolled steel hold?
Thickness tolerance of about plus or minus 0.01 to 0.05 mm with smooth, wave-free plate shape.
Q: When is cold-rolled high-strength steel used for load-bearing parts?
When precision and load-bearing are both required, such as battery pack frames, at higher cost.
Q: Can hot-rolled steel be used for simple cut-and-weld parts?
Yes, for parts requiring simple forming, where its lower cost is a significant advantage.