Whoever said; “Cuts like butter, or, Machines like butter”, may not have meant that was a good thing. Visualize butter; not cryogenic butter. The thing is, you need a little hardness (read as strength) if you are expecting decent cutting and machining.
Generally, cold finished steel; which takes into consideration both Cold Rolled and Cold Drawn, provides higher hardness and higher strength than Hot Rolled Steel. Hot Rolled Steel here, means Hot Rolled Annealed (softened) Steel. Hot Rolled Q&T (Hardened) Steel, on the other hand, is most often harder than Cold Finished Steel. A ball park assessment of the comparable hardness would be something like this;
Cold Drawn/Cold Rolled = 20RC Hot Rolled Annealed = 18RC Hot Rolled Q&T = 30RC
If you want a rough idea of the relative strength of each, convert RC roughly to Brinell (BHN) hardness (RC times 10), and divide by two. So, 20RC = 200bhn = 100,000psi Tensile. 30RC = 300 bhn = 150,000psi Tensile, and so on. Now, don’t be using this for engineering. It is simply a rough comparison for reference sake.
Cold finished steels are worked at room temp. Hot finished steels are worked hot, almost 2000°F. “Cold Rolled” typically refers to flat bar, sheet, and plate. “Cold drawn” generally refers to bar shapes. Cold working steel strain or “work” hardens it. The higher hardness elevates the strength. But, since it is not a thermally hardened material, the cold working process also imparts stress. That stress is retained in the steel. It may cause movement during subsequent machining or grinding. Q&T Hot Rolled Steels respond to thermal treatment, so, they may be stress relieved at some point of their production. That process relieves stress that may have been retained from processing.
With all of the changes that are happening in the world of steel availability, the question may soon be, which one is available in the size you need. More and more hot rolled shapes, that were once considered staples of the industry, are disappearing. There is a far greater variety of bar shapes available in cold finished product that in hot rolled product; especially hot rolled Q&T.
Also, cold finished bars are provided in much more accurate cross sections and closer tolerances than hot rolled. So, Cold Finished bars are more available, better surface condition, greater aesthetics, stronger, and offer greater opportunity to obtain the size and shape you need. That is why Cold rolled squares and rectangles (flats) are ubiquitously used as rails for automation. They are often machined to accept custom wheels, cams, and mounting hardware. Cold finished alloy bars may be surface hardened to minimize greatly extend wear resistance and service life.
Hot Rolled Annealed steels are more malleable. They tend to retain less stress which translates to less movement. Hot Rolled Q&T steels, especially Alloy steels, offer much higher strength, but, they are somewhat less available than the great variety of sizes available in Cold Finished products, have less cross sectional accuracy, and generally are used where accuracy is not critical and/or a portion of the surface will be removed.
High grade cold finished alloy bars are clean and strong. Better service centers will employ subsequent straightening, even with bars from their general stock. They will also provide fabrication services to prep the mill bars for specific automation rail applications. This may involve; further straightening, drilling and countersinking, surface hardening (multiple or single surface), and custom end preparation.
-Howard Thomas, August 19th 2019
