Once it warms up, it is no longer brittle (in general).
Mete is right, it depends on the steel chemistry and previous heat treatment.
This is not the place to give lectures but it may be the place to pass along references.
If you can find a book called
Introduction to Manufacturing Processes by John A. Schey, all your questions will be answered. The original slim volume is preferred - Second Edition is too thick, looks like a textbook (because it *is* a textbook), and the original one is much more accessible and less intimidating. The ISBN is 0-07-055274-6.
An absolute classic is
Steels: Heat Treatment and Processing Principles by George Krauss. ISBN 0-87170-370-X, published by ASM International (formerly American Society for Metals). This is another classic but not a quick read.
There is a great softbound called
Engineer to Win by Carroll Smith, ISBN 0-87938-186-8. It is aimed at the Auto Racing community but has the answers you are looking for. It is old - look on bibliofind or one of those.
If you are anywhere near a University library, look for the Metals Handbook from ASM International. It is a multi-volume compendium but Volume 1 (of any Edition) contains a wealth of information on irons and steels, and it is accessible (technical but it is in plain enough language you will not have a problem).
On the Web, look at
http://www.steelynx.net/ and dig around.
I apologise for going on at such length without answering your question, but I love this field and I think you will gain more by looking up some answers than by my giving you half-baked, over-simplified misinformation.
Purrrs,
BobCat