Non quenched and tempered steel and manufacturing process thereof

Abstract

A non-quenched and tempered steel and manufacturing process thereof. The process alters the cooling mode before finish rolling in previous production of non-quenched and tempered steel; the process at least has a cooling step after the finish rolling step; the process utilizes alternate intense cooling and moderate cooling. The intense cooling can ensure the surface temperature of the steel to decrease rapidly; and the moderate cooling allows the core temperature of the steel to dissipate gradually to the surface; a further intense cooling is carried out to allow rapid heat dissipation. The intense cooling and the moderate cooling can be carried out alternately several times accordingly. A water cooling mode combining intense cooling and moderate cooling allows the core temperature and the surface temperature of the steel to become the same within a short time, the uniformity of the mechanical properties of the steel.

1 . A non quenched and tempered steel, comprising following chemical components by weight percent: carbon 0.42 to 0.50, silicon 0.20 to 0.40, manganese 0.50 to 1.0, chromium 0.00 to 0.30, aluminum 0.010 to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus 0.000 to 0.025, sulfur 0.00 to 0.025, and balance iron. 
     
     
         2 . The non quenched and tempered steel according to  claim 1 , comprising following chemical components by weight percent: carbon 0.42 to 0.45, silicon 0.20 to 0.30, manganese 0.50 to 0.7, chromium 0.00 to 0.30, aluminum 0.010 to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus 0.000 to 0.025, sulfur 0.00 to 0.025, and balance iron. 
     
     
         3 . The non quenched and tempered steel according to  claim 1 , comprising following chemical components by weight percent: carbon 0.42 to 0.47, silicon 0.25 to 0.35, manganese 0.60 to 1.0, chromium 0.00 to 0.30, aluminum 0.010 to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus 0.000 to 0.025, sulfur 0.00 to 0.025, and balance iron. 
     
     
         4 . The non quenched and tempered steel according to  claim 1 , comprising following chemical components by weight percent: carbon 0.45 to 0.50, silicon 0.25 to 0.35, manganese 0.60 to 0.9, chromium 0.00 to 0.30, aluminum 0.010 to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus 0.000 to 0.015, sulfur 0.00 to 0.015, and balance iron. 
     
     
         5 . A process for producing the non quenched and tempered steel of  claim 1 , comprising a cooling step carried out at least after a finishing step, characterized in that: in said cooling step, an intense cooling and a moderate cooling are carried out alternately to allow the steel to undergo at least two stages of water cooling, so that the core temperature and the surface temperature of the steel become the same within a specified time. 
     
     
         6 . The process for producing non quenched and tempered steel according to  claim 5 , characterized in that: in said 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. 
     
     
         7 . The process for producing the non quenched and tempered steel according to  claim 5 , characterized in that: in said cooling step, the cooling intensity is controlled by controlling the opening degree of valve(s) of a water cooling unit. 
     
     
         8 . The process for producing non quenched and tempered steel according to  claim 5 , characterized in that: in said cooling step, the temperature of the steel decreases 100° C.-400° C. in 4-7 seconds after water cooling, and the temperature of the steel further decreases 50° C.-100° C. after temperature reversion. 
     
     
         9 . The process for producing non quenched and tempered steel according to  claim 8 , characterized in that: in said cooling step, the first stage valve opening degree is controlled to be 30%-40%, the second stage valve opening degree is controlled to be 20%, and the third stage valve opening degree is controlled to be 30%-40%, to ensure the surface temperature of the steel to decrease 100-400° C. in 4-7 seconds. 
     
     
         10 . The process for producing non quenched and tempered steel according to  claim 8 , characterized in that: in said cooling step, the steel is subjected to temperature-dropping cooling by means of spray cooling after temperature reversion. 
     
     
         11 . The process for producing non quenched and tempered steel according to  claim 8 , characterized in that: after said temperature-dropping cooling, the steel is separately disposed on a cold bed and subjected to air cooling for 10-12 minutes. 
     
     
         12 . The process for producing non quenched and tempered steel according to  claim 11 , characterized in that: after said air cooling, the steel is stacked and subjected to shield cooling. 
     
     
         13 . The process for producing non quenched and tempered steel according to  claim 5 , characterized in that: the process further comprises a finish rolling step before the cooling step, in said finish rolling step, 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° C.-900° C. 
     
     
         14 . The process for producing non quenched and tempered steel according to  claim 5 , characterized in that: the process further comprises a smelting step before the finish rolling step, the smelting step comprises electric furnace smelting step, ladle furnace smelting step and refining step carried out sequentially. 
     
     
         15 . The process for producing non quenched and tempered steel according to  claim 14 , characterized in that: a molten iron smelting is utilized in the electric furnace smelting, wherein the final phosphorus content is ≦0.015%, the final carbon content is 0.03% to 0.10%, and the final temperature is 1620-1700° C. 
     
     
         16 . The process for producing non quenched and tempered steel according to  claim 14 , characterized in that: in the ladle furnace smelting step and/or the refining step, silicon carbide, ferrosilicon powder are used to deoxidize. 
     
     
         17 . The process for producing non quenched and tempered steel according to  claim 14 , characterized in that: the ladle furnace smelting step comprises making white slag and holding the white slag for 20 minutes or more. 
     
     
         18 . The process for producing non quenched and tempered steel according to  claim 14 , characterized in that: in the refining step, the refining time is not less than 45 minutes, and the hydrogen content is controlled at 1.5 ppm or less. 
     
     
         19 . The process for producing non quenched and tempered steel according to  claim 18 , characterized in that: the process further comprises a continuous casting step after the refining step, in said continuous casting step, an overheat is controlled at 20-35° C., a pulling speed is controlled at 0.5 m/min-0.6 m/min. 
     
     
         20 . The process for producing non quenched and tempered steel according to  claim 19 , characterized in that: the process further comprises a heating step after the continuous casting step, in said heating step, the steel billet is placed in a heating 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., and the total time of the soaking stage is not less than 2 hours. 
     
     
         21 . A process for producing non quenched and tempered steel, the process comprises following steps, successively:
 (1) an electric furnace smelting step: providing iron raw materials having desired steel 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, an overheat of the steel melt is controlled at 20-35° C., a 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: wherein an intense cooling and a moderate cooling are carried out alternately to allow the steel to undergo at least two stages of water cooling, so that the core temperature and the surface temperature become the same during the cooling.