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High School Teacher has some metal questions

I would like to know the major difference between hot rolled and cold rolled steel. Which is better for machining and galvanized sheet steel is made from which kind of steel and why. I am a High School Teacher so could you help me out.

Thanks

Dwain Mc Clure
High School - Keota, Iowa
2004

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First of two simultaneous responses --

Hot-rolled products obtain their dimensions and properties from rolling at elevated temperatures (> 800 C), while cold-rolled products obtain their from ambient rolling (nominally 20 C). Cold rolling is used when it is necessary to:

1. Improve the dimensional tolerances of a bar or sheet in terms of straightnes, flatness, etc.

2. Improve the surface roughness of a bar or sheet.

3. Improve the strength of a bar or sheet.

In general, cold-rolled products are used when surface quality is important, such as machining of bars or when surface coating (galvanizing, painting, etc.). However, hot-rolled steel is often galvanized when the application is not very critical in terms of surface appearance.

Toby Padfield
Automotive module supplier - Michigan
2004

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Second of two simultaneous responses --

As the names indicate, hot rolled is rolled into flat sheets or strip while hot, and the final rolling on cold rolled is done cold. Because of the temperature involved, hot rolled steel has a surface layer of black iron oxide, while cold rolled is clean and much smoother. Prior to galvanizing, it is necessary to remove the oxide on hot rolled by pickling, or by abrasive means. Cold rolled can be galvanized after simple cleaning and fluxing. Either one can be readily machined, but only cold rolled is suitable for cosmetic use, such a quality painting or plating.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2004