So, in the marking of steel are the following elements of the notation:

N – Nickel, X – chromium, M – molybdenum, T – titanium, cobalt, D copper, silicon, G – manganese.
To specify the methods of deoxidation of steel are also used letter designations: SP – quiet steel, SS – balanced, KP – boiling.
Your notation there are different types of steel depending on the composition and appointment.
Non-alloy structural steel of ordinary quality are indicated by the letters ST. (for example, ART.4, where figure 4 after the letters indicates the carbon content in percent).
Non-alloy structural steel high quality are indicated by a two-digit number that indicates the average content of carbon in steel (ST.10)
Quality steel for manufacturing high pressure vessels and boilers represent as well as structural and non-alloy steel, add the letter K (for example, 20K).
Alloy structural steel designate an alphanumeric code. The numbers after the letters indicate the approximate content of the element (but the content of alloying elements less than 1.5% figure is not put).
A low content of impurities (sulphur, phosphate) are denoted by the letters A or W at the end of the markings (e.g., 17ХГ-W).
To the designation of casting of structural steel in the end is added the letter L.
The designation of structural steel there is a letter and numbers that correspond to the minimum yield point of steel. Additionally, at the end put T for heat-strengthened rolled, for superior corrosion resistance, D to denote a high content of copper (for example, 354).
Non-alloy tool steel divided into qualitative (denoted by the letter U and the numeral indicating the average carbon content, for example, U9) and high-quality in the marking which is placed the letter a at the end of the labeling (e.g., U9A).
Alloy tool steels indicate in the marking the same as a structural alloy (for example, 3Х2В5МФ).
Marking high-speed steels starts with the letter R, then there is a digit indicating the content of tungsten. If the vanadium content exceeds 2.5%, indicated by the letter f and the corresponding digit (e.g., Р5М6Ф3).
Knowing these rules, you can easily determine the composition of a particular steel, and an experienced professional transcript grades of steel and is simple and intuitive.