Spheroidizing-annealed steel for ball screw having high strength and resistance to low temperatures and manufacturing method thereof

A method of manufacturing spheroidizing-annealed steel for ball screw having high strength and resistance to low temperatures, wherein comprises following steps:

• Step 1:
• preparing smelting raw materials according to the chemical composition of steel, the chemical composition of the steel in mass percentage is:
• C:
• 0.40-0.70%, Si:
• 1.20-1.80%, Mn:
• 1.00-1.60%, Cr:
• 0.80-1.20%, S:
• ≤0.025%, P≤0.025%, Ni:
• 0.10-0.60%, Cu:
• 0.30-0.80%, Mo:
• 0.10-0.40%, Al≤0.05%, Ca≤0.0010%, Ti≤0.003%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, a balance is Fe and unavoidable impurities;
• smelting raw materials by converter, refining and vacuum degassing in electric furnace or converting to obtain molten steel;
• in a refining process of electric furnace or converter, a content of C at a smelting end point is controlled within 0.05-0.25%, and a content of P at a end point is less than 0.025%;
• molten steel is cast into 390×510 mm or above continuous casting billets with the same chemical composition as the finished steel products by continuous casting process;
• a casting flow in the solidification process is subjected to electromagnetic stirring and light pressing, and a superheat of continuous casting is 15-35° C.;
• Step 2:
• the continuous casting billet is slowly cooled in a pit, a slow cooling lasts for not less than 48 hours;
• then the continuous casting billet is sent to a heating furnace with a neutral atmosphere or a weakly oxidative atmosphere for heating;
• then the continuous casting billet is rolled into an intermediate billet of 200 mm×200 mm to 300 mm×300 mm;
• a heating temperature of a continuous casting slab is 1000-1250° C., a heating time is more than 5 hours, in rolling a start rolling temperature is set at 1000° C.-1200° C., a final rolling temperature is ≥800° C., a compression ratio in rolling is greater than 5;
• the intermediate billet obtained by rolling is slowly cooled in a pit, a temperature of the intermediate billet entering the pit is ≥500° C., and the slow cooling lasts for ≥48 hours;
• Step 3:
• heating the intermediate billet again and rolling into a target size;
• the intermediate billet is heated as follows:
• in a section of preheating, the heating temperature is 650-900° C., in a section of heating, the heating temperature is 1000-1250° C., and in a section of soaking, the heating temperature is 1000-1250° C., a total heating time is more than 2 hours;
• a start rolling temperature in rolling is 1000° C.-1200° C., a final rolling temperature is ≥800° C.;
• after the rolling is completed, stack cooling;
• Step 4:
• spheroidizing annealing a rolled product;
• the spheroidizing annealing process is:
• firstly the rolled product is kept warm at 805±10° C. for more than 7 hours;
• then the rolled product is cooled to 745° C.±10° C. by water-mist cooling, and kept warm at this temperature for more than 5 hours;
• then the rolled product is cooled to 690±10° C. in the furnace, and kept warm at this temperature for more than 4.5 hours;
• finally the rolled product is cooled to 500±10° C. in the furnace and then taken out of the furnace;
• and
• Step 5:
• straightening and detecting the rolled product after spheroidizing annealing, so as to obtain a qualified product.