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.

Claims

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.

Description

DETAILED DESCRIPTION

[0050] The chemical compositions of Examples A1-10# according to the present disclosure and three Comparative Steel Grades B1-3# are shown in Table 1 below, and the specific manufacturing methods are as follows:

[0051] Examples A1-5# according to the present disclosure, and Comparative Steel Grades B1 and B2 alloys were smelted with the use of an electric furnace, and Example A6-10190 and Comparative Steel Grade B3 alloys were smelted with the use of a converter. Then, secondary refining was performed, wherein Examples A1-3#, A6-8#, and Comparative Steel Grade B1 alloys were treated with an LF furnace plus VD refining, while Examples A4-5#, A9-10#, Comparative Steel Grade B2, and B3 alloys were treated with LF plus RH. The structure and basicity of a synthetic slag were optimized. A1-6#, and B1 were vacuum degassed for 30 minutes, and A7-10#, B2, and B3 were vacuum degassed for 35 minutes. The final O content was controlled at 0.0005-0.0040%, the N content was 0.001-0.009%, and the H content was less than 2 ppm.

[0052] After smelting, A1-4# and B1 were cast into 300 mm round billets, A5-6# were cast into 450 mm round billets, A7-9# and B2 were cast into 320*420 mm square billets, and A10# and B3 were cast into 500mm square billets. A tundish covering agent and a casting mold with good sealing performance were used to protect the slag in the casting process. The blooming temperature for the A1-5# and B1 continuously cast billets was 1050° C., and the end face size of the rolled small square blanks was 115 mm. The heating temperature for A6-7# and B2 square billets was 1270° C., and the size of the rolled blanks was 125 mm. The heating temperature for A8-10# and B3 square billets was 1100° C., and the size of the rolled blanks was 170 mm.

[0053] The furnace temperature of the heating furnace for A1-4# and B1 was controlled at 920° C., and the holding time was 1.0 h. The temperature of the heating furnace for A5-10#, B2 and B3 was controlled at 1150° C., and the holding time was 3.0 h. In the high-speed wire rod rolling process, the rolling speed was controlled to be 15-115 m/s. The online temperature control scheme: for the A1-6# and B1 alloys, the inlet temperature of the finishing rolling unit was 880-950° C., the inlet temperature of the reducing and sizing unit was 840-950° C., and the silking temperature was 800-890° C.; for the A7-10#, B2 and B3 alloys, the inlet temperature of the finishing rolling unit was 950-1050° C., the inlet temperature of the reducing and sizing unit was 940-970° C., and the silking temperature was 870-950° C.

[0054] The dimensions of the A1-5#, B1 and B2 alloy rolled wire rods were Φ5-15mm respectively, and the rolling specifications of the A6-10# and B3 alloy wire rods were Φ16-28mm. After the rolling of A1-5# and B1 alloy wire rods, the Stelmor cooling process was: the air volume was 40% for fans F1-F4, 10% for fans F5-F7, 5% for fans F8-F12, and 40% for fans F13-F14. After the rolling of A6-10#, B2 and B3 alloy wire rods, the Stelmor cooling process was: the air volume was 50% for fans F1-F4, 20% for fans F5-F7, 15% for fans F8-F12, and 35% for fans F13-F14. The structure of the wire rods after the Stelmor cooling was sorbite plus a very small amount of ferrite.

[0055] The wire rods were drawn prior to heat treatment. The quenching and tempering treatment temperatures for the drawn steel wires were divided into three groups, wherein the heating temperature was 850° C. and the tempering temperature was 550° C. for A1-2190 and B1; the heating temperature was 980° C. and the tempering temperature was 470° C. for A3-7# and B2; and the heating temperature was 1100° C. and the tempering temperature was 370° C. for A8-10# and B3.

[0056] The mechanical properties of the high-strength springs of Examples A1-A10 and the Comparative Steel Grades B1-B3 are shown in Table 2 below. As can be seen from the table, the strength of the alloys all reach 2020 MPa or higher, higher than that of the samples of Comparative Examples B1-B3. At the same time, the area reduction rate of the materials can still reach 40% or higher. A good combination of plasticity and toughness is obtained. The high-strength springs according to the present disclosure and the comparative alloys were made into the same type of helical springs, and the fatigue life of the helical springs was measured using a spring fatigue testing machine according to GBT16947-2009 “Helical Spring Fatigue Testing Standard”. The results are shown in Table 3. Under the same conditions, the fatigue life of the high-strength spring steel according to the present disclosure is superior to that of the comparative steel.

TABLE-US-00001 TABLE 1 Chemical compositions (wt %) of Examples A1-10# according to the present disclosure and Comparative Steel Grades B1-3# Steel No. C Si Mn Cr V Nb Al N O P S A1 0.60 1.40 0.75 0.80 0.15 0.03 0.0030 0.001 0.004 0.015 0.008 A2 0.62 1.45 0.75 0.75 0.15 0.03 0.0027 0.001 0.003 0.010 0.008 A3 0.55 1.30 0.75 0.70 0.15 0.03 0.0030 0.001 0.0015 0.010 0.008 A4 0.58 1.35 0.60 0.60 0.10 0.03 0.0045 0.001 0.0015 0.010 0.008 A5 0.54 1.35 0.55 0.70 0.05 0.05 0.0045 0.001 0.0015 0.010 0.008 A6 0.56 1.35 0.30 0.67 0.02 0.05 0.0010 0.009 0.0015 0.008 0.008 A7 0.55 1.35 0.25 0.70 0.02 0.008 0.0010 0.009 0.0005 0.008 0.008 A8 0.52 1.35 0.60 0.65 0.02 0.05 0.0010 0.009 0.001 0.008 0.015 A9 0.53 1.20 0.60 0.30 0.01 0.05 0.0020 0.005 0.001 0.008 0.004 A10 0.52 1.45 0.60 0.60 0.05 0.001 0.0020 0.005 0.001 0.008 0.004 B1 0.55 1.50 0.70 0.75 0 0 0.0020 0.005 0.001 0.008 0.004 B2 0.65 1.35 0.70 1.05 0.2 0 0.0045 0.001 0.004 0.017 0.001 B3 0.5 1.6 0.55 0.8 0.15 0.08 0.003 0.001 0.006 0.015 0.015

TABLE-US-00002 TABLE 2 Alloy steel structure according to the present disclosure Original austenite Nitride-carbide Maximum width of grain size precipitate size monoparticle inclusions Steel No. (μm) (nm) (μm) A1 75 10-60 28 A2 60  5-55 30 A3 55 30-55 15 A4 54  5-45 19 A5 67 10-55 25 A6 80  7-45 18 A7 60 10-56 30 A8 34 23-60 10 A9 56 12-55 25 A10 77 12-60 30 B1 90 — 45 B2 50  15-100 50 B3 45  25-145 25

TABLE-US-00003 TABLE 3 Properties of alloy steel examples according to the present disclosure and comparative steel grades Tensile strength Area reduction rate Fatigue life Steel No. MPa % Number of cycles A1 2080 46 85 × 10.sup.4 A2 2110 42 90 × 10.sup.4 A3 2050 43 105 × 10.sup.4  A4 2090 40 99 × 10.sup.4 A5 2110 44 90 × 10.sup.4 A6 2075 41 98 × 10.sup.4 A7 2095 42 100 × 10.sup.4  A8 2130 44 110 × 10.sup.4  A9 2097 40 95 × 10.sup.4 A10 2089 45 97 × 10.sup.4 B1 1905 45 70 × 10.sup.4 B2 2080 37 57 × 10.sup.4 B3 2055 35 60 × 10.sup.4