Non quenched and tempered steel and manufacturing process thereof

A process for producing a non quenched and tempered steel, the process comprises following steps, successively:

• (1) an electric furnace smelting step:
• providing iron raw materials having desired composition, a molten iron smelting is utilized in the electric furnace smelting, wherein final phosphorus content is ≦0.015%, final carbon content is 0.03% to 0.10%, and final temperature is 1620° C.-1700° C.;
• (2) a ladle furnace smelting step:
• wherein deoxidation is carried out using silicon carbide and/or ferrosilicon powder, white slag is made by adding lime, the white slag is held for 20 minutes or more;
• (3) a refining step:
• wherein deaeration is carried out, the refining time is controlled at 45 minutes or more, and the hydrogen content is controlled at 1.5 ppm or less;
• (4) a continuous casting step:
• the steel melt obtained in the refining step is subjected to continuous casting, the overheat of the steel melt is controlled at 20-35° C., the pulling speed is controlled at 0.5 m/min-0.6 m/min;
• (5) a heating step:
• the steel billet produced in the continuous casting step is placed in a furnace to be heated, wherein the preheating stage temperature is controlled at 850±30° C., the heating stage temperature is controlled at 1100±30° C., the soaking stage temperature is controlled at 1130±30° C., the total time of the soaking stage is not less than 2 hours;
• (6) a finish rolling step:
• wherein the temperature of the steel is controlled at ≦950° C. at the entry into the finish rolling step;
• the steel is subjected to low temperature rolling when the steel temperature is 850-900° C.;
• and
• (7) a cooling step:
• the steel is subjected to three stages of water cooling, wherein the first stage of water cooling uses intense cooling, the second stage of water cooling uses moderate cooling, and the third stage of water cooling uses intense cooling, so that the core temperature and the surface temperature become the same during the cooling.