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

Abstract

The present disclosure relates to a spheroidizing-annealed steel for the ball screw having high strength and resistance to low temperatures, wherein 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%, the balance is Fe and unavoidable impurities. In microstructure of the steel, cementite exists in a spheroidized state with a diameter of 0.1-0.5 μm, preferably 0.3-0.5 μm, a spheroidizing rate is 95% or more, and the rest is ferrite.

What is claimed is: 
     
         1 . 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.   
     
     
         2 . The method according to  claim 1 , wherein after quenching and tempering treatment, the steel has yield strength of ≥1380 MPa, tensile strength≥1500 MPa, elongation≥9%, impact energy AKU 2  at −40° C.≥27 J; when JIS G 0561 method is used to test end hardenability, J9 mm hardness≥58 HRC. 
     
     
         3 . The method according to  claim 1 , wherein in microstructure of the steel, cementite exists in a spheroidized state with a diameter of 0.1-0.5 μm, preferably 0.3-0.5 μm, a spheroidizing rate is 95% or more, and the rest is ferrite.