41xx steel is a family of high-strength low-alloy (HSLA) steels, as specified by the Society of Automotive Engineers (SAE). Alloying elements include chromium and molybdenum, and as a result these materials are often referred to as chromoly steel, or cro-mo, or CRMO. They have an excellent strength to weight ratio, are easily welded and are considerably stronger and more durable than standard 1020 steel.
While these grades of steel do contain chromium, it is not in great enough quantities to provide the corrosion resistance found in stainless steel.
Examples of applications for 4130 include structural tubing, bicycle frames, AK-47 receivers, clutch and flywheel components, and roll cages. They are also used in aircraft and therefore 41xx grade structural tubing is sometimes referred to as "aircraft tubing".