Why is mild steel appear in black or bright form?

[ry2ncz]

Moderator note: The OP was warned that he needed to do more research before asking questions like this.
So mild steel appear to have black mild steel or bright mild steel... What are the differences between them... And bright mild steel appear to have higher quality than black mild steel... In comparisons, how much did mechanical properties altered and commercially too

 

[CWatters]

Google is your friend...
https://au.answers.yahoo.com/question/index?qid=20081022170611AA5X0S6

Technobuff answered 6 years ago

Black mild steel is steel direct from a hot rolling forming process, which still has a scale coating on its surface, is not precise in its dimensions, nor straightness or flatness. It is used for general non- critical work, especially where welding is performed. It does not contain any aditions for altering machining properties. It can contain some hard and soft spots, and is not considered good stock for machining parts from.

Bright mild steels are improved steels, they are free of scale, and have been cold worked (drawn or rolled) to form. They are produced to quite close dimensional tolerances, and straightness and flatness are better than black steels. The may have additives that improve machinability, without seriously affecting strength. They may, because of these additives, be unsuitable for welding operations. Lead, phosphor and sulphur are some of the possible additives. They are used for more precise operations, such as machinery shafting etc., and are widely used for machining mild steel components from. They have consistent hardness. They can be obtained in precision ground form if desired, or in their simple rolled/ drawn state.

 

[dean barry]

We used to refer to it as : BDMS (bright drawn mild steel)