Spring steel having superior fatigue life, and manufacturing method for same

Abstract

A spring steel having a superior fatigue life, and a manufacturing method for the same. The chemical components thereof are as follows in weight percentage: C: 0.52-0.62%, Si: 1.20-1.45%, Mn: 0.25-0.75%, Cr: 0.30-0.80%, V: 0.01-0.15%, Nb: 0.001-0.05%, N: 0.001-0.009%, O: 0.0005-0.0040%, P: ≤0.015%, S: ≤0.015%, and Al: ≤0.0045%, with the remainder being Fe and incidental impurities, wherein the following condition is also met 0.02≤(2Nb+V)/(20N+C)≤0.40. The spring steel of the present invention has a microstructure of tempered troostite+tempered sorbite, a prior austenite grain size less than 80 um, a size of alloy nitride and carbide precipitates being 5-60 nm, and a maximum width of single-grain inclusions being less than 30 pm. The spring steel has a handling strength greater than 2020 MPa, superior ductility and toughness (the reduction of area≥40%), and a fatigue life≥800,000 times, thereby meeting application requirements of high-stress springs in industries, such as automobiles, machinery, and the like.

1 . A spring steel having a superior fatigue life, wherein its chemical composition based on weight percentage is:
 C: 0.52-0.62%;   Si: 1.20-1.45%;   Mn: 0.25-0.75%;   Cr: 0.30-0.80%;   V: 0.01-0.15%;   Nb: 0.001-0.05%;   N: 0.001-0.009%;   O: 0.0005-0.0040%;   P: ≤0:015%;   S: ≤0:015%;   Al: ≤0:0045%;
 a balance of Fe and unavoidable impurities, wherein the following relationship is satisfied: 0.02≤(2Nb+V)/(20N+C)≤0.40. 
   
     
     
         2 . The spring steel having a superior fatigue life according to  claim 1 , wherein the spring steel has a microstructure that is a tempered troostite+sorbite structure, an original austenite grain size≤80 μm, a size of alloying nitride and carbide precipitates in the range of 5-60 nm, and a maximum width of monoparticle inclusions≤30 um. 
     
     
         3 . The spring steel having a superior fatigue life according to  claim 1 , wherein the spring steel has a machining strength≥2020 MPa, an area reduction rate≥40%, and a fatigue life≥800000 cycles. 
     
     
         4 . A method for manufacturing the spring steel having a superior fatigue life according to  claim 1 , comprising: smelting, continuous casting, rough rolling, high-speed wire rolling, Stelmor controlled cooling, wire rod drawing, and quenching and tempering treatment, wherein
 an electric furnace or a converter is used for the smelting; after the smelting, secondary refining is performed with the use of an LF furnace plus VD or RH degassing treatment; during the LF refining, the composition and basicity of a synthetic slag are adjusted to control the contents of the P and S elements in the steel to be lower than 0.015% and 0.015%; stirring in the presence of argon is performed to allow for full reaction between a refining slag and inclusions in the molten steel to realize denaturation and removal of the inclusions; VD or RH vacuum degassing time is more than 30 minutes to control a final O content at 0.0005-0.0040%, a final N content at 0.0010-0.0090%, and a H content of less than 2 ppm; killing time of the ladle is more than 15 min at the end of the refining to facilitate floating of large particle inclusions, so that the size of inclusions in molten steel is smaller than 30 μm;   in the high-speed wire rolling, heating of a heating furnace is controlled at 920-1150° C., and holding time is 1.0-3.0 h; a rolling speed is controlled at 15-115 m/s in the high-speed wire rod rolling process; an online temperature control scheme is as follows:   an inlet temperature of a finishing rolling unit is 880-1050° C., an inlet temperature of a reducing-sizing unit is 840-970° C., and a silking temperature is 800-950° C.   
     
     
         5 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 4 , wherein a continuous casting machine is used to cast a round or square billet having a size of 320-500 mm; during the continuous casting process, a drawing speed is controlled in the range of 0.5-0.8 m/min, and a tail end soft reduction is controlled to be greater than 10 mm, so as to control carbon segregation in a core of the billet to achieve a target of lower than 1.08. 
     
     
         6 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 4 , wherein the rough rolling adopts a twice-heating production process, wherein a cast billet is bloomed into a 115-170 mm square or round blank at a temperature of 1050-1270° C., and a total rolling reduction is higher than 40%. 
     
     
         7 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 4 , wherein when the wire rod is drawn, a drawing speed is not higher than 3.5 m/min. 
     
     
         8 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 4 , wherein in the quenching and tempering treatment, a heating temperature prior to the quenching and tempering treatment of the drawn steel wire is controlled in the range of 850-1100° C.; oil or water is used as a quenching medium; a temperature of the quenching medium is controlled at 15-40° C.; and a tempering temperature is controlled at 370-550° C., so that a size of nitride and carbide precipitates in a finished steel wire is controlled in the range of 5-60 nm. 
     
     
         9 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 4 , wherein in the Stelmor controlled cooling, air volumes of 14 fans on a Stelmor line are adjusted in the following ranges: fans F1-F7 have an air volume of 10-100%, fans F8-F12 have an air volume of 0-50%, and fans F13-F14 have an air volume of 0-50%. 
     
     
         10 . The spring steel having a superior fatigue life according to  claim 2 , wherein the spring steel has a machining strength≥2020 MPa, an area reduction rate≥40%, and a fatigue life≥800000 cycles. 
     
     
         11 . The spring steel having a superior fatigue life according to  claim 1 , wherein 0.045≤(2Nb+V)/(20N+C)≤0.37. 
     
     
         12 . The spring steel having a superior fatigue life according to  claim 11 , wherein 0.15≤(2Nb+V)/(20N+C)≤0.37. 
     
     
         13 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 4 , wherein the spring steel has a microstructure that is a tempered troostite+sorbite structure, an original austenite grain size≤80 μm, a size of alloying nitride and carbide precipitates in the range of 5-60 nm, and a maximum width of monoparticle inclusions≤30 um. 
     
     
         14 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 4 , wherein the spring steel has a machining strength≥2020 MPa, an area reduction rate≥40%, and a fatigue life≥800000 cycles. 
     
     
         15 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 13 , wherein the spring steel has a machining strength≥2020 MPa, an area reduction rate≥40%, and a fatigue life≥800000 cycles. 
     
     
         16 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 4 , wherein in the chemical composition of the spring steel, 0.045≤(2Nb+V)/(20N+C)≤0.37. 
     
     
         17 . The method for manufacturing the spring steel having a superior fatigue life according to  claim 16 , wherein 0.15≤(2Nb+V)/(20N+C)≤0.37.