Nitrogen-containing microalloyed spring steel and preparation method thereof

Abstract

A nitrogen-containing microalloyed spring steel and a preparation method thereof are provided. The chemical components are: 0.45-0.52% of carbon, 0.15-0.35% of silicon, 0.90-1.10% of manganese, 0.90-1.15% of chromium, 0.10-0.25% of molybdenum, 0.10-0.20% of vanadium, 0.025-0.04% of niobium, 0.007-0.012% of nitrogen, less than or equal to 0.03% of lead, tin, zinc, antimony, and bismuth, less than or equal to 25 ppm of oxygen and hydrogen, less than or equal to 0.02% of sulfur and phosphorus, less than or equal to 0.2% of copper, less than or equal to 0.35% nickel, and a balance of iron. The spring steel has significantly improved properties, including high mechanical strength, large elongation, high reduction of area, and good anti-fatigue performance.

Claims

1. A nitrogen-containing microalloyed spring steel, comprising the following chemical components in a mass ratio: 0.45-0.52% of carbon, 0.15-0.27% of silicon, 0.90-1.10% of manganese, 0.90-1.15% of chromium, 0.10-0.25% of molybdenum, 0.10-0.20% of vanadium, 0.025-0.04% of niobium, 0.007-0.012% of nitrogen, less than or equal to 0.03% of sum of lead, tin, zinc, antimony, and bismuth, less than or equal to 25 ppm of sum of oxygen and hydrogen, less than or equal to 0.02% of sum of sulfur and phosphorus, less than or equal to 0.2% of copper, less than or equal to 0.35% nickel, and a balance of iron, wherein microstructures of the nitrogen-containing microalloyed spring steel comprises a distinct ferrite structure and a distinct pearlite structure wherein the spring steel has an elongation of at least 7%, a reduction of area of at least 25%, a tensile strength of at least 1756 MPa, a yield strength of at least 1648 MPa, and an average fatigue cycle of at least about 341,217 cycles.

2. The nitrogen-containing microalloyed spring steel of claim 1, wherein the spring steel is used to make a leaf spring.

3. A method for preparing the nitrogen-containing microalloyed spring steel according to claim 1, comprising: sequentially subjecting a spring steel raw material to a smelting, a refining, a vacuum degassing, and a continuous casting and cooling to obtain a steel ingot, and then subjecting the steel ingot to a peeling, a re-heating continuous rolling, a controlled cooling, a quenching, and a tempering to obtain the nitrogen-containing microalloyed spring steel.

4. The method for preparing the nitrogen-containing microalloyed spring steel according to claim 3, wherein, the smelting is conducted at a temperature of 1630-1700° C. for 25-60 min; the refining is conducted at a temperature of 1500-1550° C. for 20-60 min; an electromagnetic stirring is performed during the refining.

5. The method for preparing the nitrogen-containing microalloyed spring steel according to claim 3, wherein, during the vacuum degassing, a degree of vacuum is equal to or less than 130 Pa.

6. The method for preparing the nitrogen-containing microalloyed spring steel according to claim 3, wherein, the continuous casting and cooling comprises: first reducing a temperature to below 1150° C. at a rate of 25-35° C/min, and then naturally cooling to room temperature.

7. The method for preparing the nitrogen-containing microalloyed spring steel according to claim 3, wherein, in the peeling, the steel ingot is peeled with a depth of at least 3.0 mm.

8. The method for preparing the nitrogen-containing microalloyed spring steel according to claim 3, wherein, the re-heating continuous rolling starts at a temperature of 900-1100° C. and ends at a temperature of 850-900° C.

9. The method for preparing the nitrogen-containing microalloyed spring steel according to claim 3, wherein, the controlled cooling comprises: cooling to 600° C. at a speed of equal to or more than 30° C/min, and then cooling to room temperature at a speed of equal to or less than 10° C/min.

10. The method for preparing the nitrogen-containing microalloyed spring steel according to claim 3, wherein, the quenching is an oil quenching, wherein in the oil quenching, the steel ingot is processed at a temperature of 850-900° C. for 1.0-1.5 min per millimeter of the steel ingot, and is tempered at a temperature of 400-500° C.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) The present invention is further described hereinafter with reference to the embodiments. All raw materials used in the embodiments are commercially available unless otherwise specified.

Embodiment 1

(2) In one embodiment, the nitrogen-containing microalloyed spring steel is prepared as follows. Molten iron is added to a 120-ton converter and smelted at 1680° C. for 45 minutes to obtain a steel. Then, 18% scrap steel is added to adjust the temperature to 1650° C. and then transferred to a refining furnace. Under electromagnetic stirring, a ferrosilicon, a ferromanganese, a chromium-molybdenum-iron alloy, a ferrovanadium, a ferroniobium, and manganese nitride are added. After adjusting the chemical composition at 1535±15° C. for 40 minutes, vacuum degassing (under a condition of vacuum degree ≤130 Pa) is performed, and then continuous casting is performed to obtain an ingot blank of 180×180. After cooling to 1150° C. at a rate of 28° C./min, the ingot blank is air-cooled to room temperature and then is peeled. After peeling 3.2 mm in depth, the peeled ingot blank is heated to 1200° C., and then is continuously rolled into a spring strip steel of 30×89 mm, with a starting rolling temperature of 1050° C. and a final rolling temperature of 890° C. After rolling, the spring strip steel is quickly cooled to 600° C. at a rate of 37° C./min, and then slowly cooled to room temperature at a rate of 8° C./min.

(3) Upon repeated testing, the spring strip steel of 30×89 mm obtained by the above method was found to yield the chemical compositions shown in Table 1. The spring strip steel is further processed into two leaf springs. After quenching at 880° C. and tempering at 460° C., tensile specimen processing and tensile tests are performed in accordance with the Chinese national standard GB/T228-2002. Meanwhile, the yield strength, elongation, and reduction of area are tested. The assembled leaf spring is subjected to a fatigue test in accordance with GB/T228-2002. The test results are shown in Table 2.

Embodiment 2

(4) In another embodiment, he nitrogen-containing microalloyed spring steel is prepared as follows.

(5) Molten iron is added to a 120-ton converter and smelted at 1630° C. for 60 minutes to obtain a steel. Then, 18% scrap steel is added to adjust the temperature to 1650° C., and then transferred to a refining furnace. Under electromagnetic stirring, a ferrosilicon, a ferromanganese, a chromium-molybdenum-iron alloy, a ferrovanadium, a ferroniobium, and manganese nitride are added. After adjusting the chemical composition at 1515±15° C. for 60 minutes, vacuum degassing (under a condition of vacuum degree ≤130 Pa) is performed, and then continuous casting is performed to obtain an ingot blank of 180×180. After cooling to 1150° C. at a rate of 30° C./min, the ingot blank is air-cooled to room temperature, and then is peeled. After peeling 3.5 mm in depth, the peeled ingot blank is heated to 1200° C., and then is continuously rolled into a spring strip steel of 30×89 mm, with a starting rolling temperature of 950° C. and a final rolling temperature of 850° C. After rolling, the spring strip steel is quickly cooled to 600° C. at a rate of 35° C./min, and then slowly cooled to room temperature at a rate of 10° C./min.

(6) Upon testing, the spring strip steel of 30×89 mm obtained by the above method was found to have the chemical compositions shown in Table 1. The spring strip steel is further processed into two leaf springs. After quenching at 850° C. and tempering at 480° C., tensile specimen processing and tensile tests are performed in accordance with GB/T228-2002. Meanwhile, the yield strength, elongation, and reduction of area are tested. The assembled leaf spring is subjected to a fatigue test in accordance with GB/T228-2002. The test results are shown in Table 2.

Embodiment 3

(7) In yet another embodiment, the nitrogen-containing microalloyed spring steel is prepared as follows.

(8) Molten iron is added to a 120-ton converter and smelted at 1700° C. for 25 minutes to obtain a steel. Then, 18% scrap steel is added to adjust the temperature to 1650° C., and then transferred to a refining furnace. Under electromagnetic stirring, a ferrosilicon, a ferromanganese, a chromium-molybdenum-iron alloy, a ferrovanadium, a ferroniobium, and manganese nitride are added. After adjusting the chemical composition at 1535±15° C. for 20 minutes, vacuum degassing (under a condition of vacuum degree ≤130 Pa) is performed, and then continuous casting is performed to obtain an ingot blank of 180×180. After cooling to 1150° C. at a rate of 35° C./min, the ingot blank is air-cooled to room temperature, and then the is peeled. After peeling 3.0 mm in depth, the peeled ingot blank is heated to 1200° C., and then continuously rolled into a spring strip steel of 30×89 mm, with a starting rolling temperature of 900° C. and a final rolling temperature of 900° C. After rolling, the spring strip steel is quickly cooled to 600° C. at a rate of 40° C./min, and then slowly cooled to room temperature at a rate of 9° C./min.

(9) Upon testing, the spring strip steel of 30×89 mm obtained by the above method has the chemical compositions shown in Table 1. The spring strip steel is further processed into two leaf springs. After quenching at 900° C. and tempering at 500° C., tensile specimen processing and tensile tests are performed in accordance with GB/T228-2002. Meanwhile, the yield strength, elongation, and reduction of area are tested. The assembled leaf spring is subjected to a fatigue test in accordance with GB/T228-2002. The test results are shown in Table 2.

Comparative Embodiment 1

(10) The chemical compositions of standard steel 9260 are shown in Table 1. The standard steel 9260 is further processed into two leaf springs. After quenching at 900° C. and tempering at 500° C., tensile specimen processing and tensile tests are performed in accordance with GB/T228-2002. The assembled leaf spring is subjected to a fatigue test in accordance with GB/T228-2002. In addition, the yield strength, elongation, and reduction of area are measured. The test results are shown in Table 2.

Comparative Embodiment 2

(11) The chemical compositions of standard steel 5160 are shown in Table 1. The standard steel 5160 is further processed into two leaf springs. After quenching at 900° C. and tempering at 500° C., tensile specimen processing and tensile tests are performed in accordance with GB/T228-2002. The assembled leaf spring is subjected to a fatigue test in accordance with GB/T228-2002. In addition, the yield strength, elongation, and reduction of area are measured. The test results are shown in Table 2.

Comparative Embodiment 3

(12) The chemical compositions of standard steel 6150 are shown in Table 1. The standard steel 6150 is further processed into two leaf springs. After quenching at 900° C. and tempering at 500° C., tensile specimen processing and tensile tests are performed in accordance with GB/T228-2002. The assembled leaf spring is subjected to a fatigue test in accordance with GB/T228-2002. In addition, the yield strength, elongation, and reduction of area are measured. The test results are shown in Table 2.

(13) TABLE-US-00001 TABLE 1 Comparison of Chemical Compositions of Embodiments 1-3 and Comparative Embodiments 1-3 Mark C Si Mn Cr Mo Ni V Nb N Embodiment 1 0.46 0.25 1.08 1.08 0.13 0.11 0.13 0.027 0.011 Embodiment 2 0.48 0.20 1.02 1.03 0.15 0.13 0.15 0.033 0.010 Embodiment 3 0.50 0.27 0.91 1.11 0.25 0.19 0.20 0.037 0.009 Comparative 0.62 1.72 0.76 0.11 0.010 0.02 <0.01 <0.001 <0.001 Embodiment 1 Comparative 0.61 0.28 0.82 0.79 0.011 0.02 <0.01 <0.001 <0.001 Embodiment 2 Comparative 0.47 0.31 0.72 0.95 0.011 0.02 0.15 <0.001 <0.001 Embodiment 3

(14) TABLE-US-00002 TABLE 2 Test Results Average Rp.sub.0.2 Rm A Z fatigue cycle Mark (MPa) (MPa) (%) (%) (Cycle) Embodiment 1 1648 1756 7.9 33 341, 217 Embodiment 2 1652 1783 7.8 32 372, 381 Embodiment 3 1657 1796 7.7 31 388, 862 Comparative 940 980 8.2 31  43, 265 Embodiment 1 Comparative 980 1070 8.3 32  67, 352 Embodiment 2 Comparative 1030 1100 8.6 34  81, 210 Embodiment 3

(15) According to the results, under the conditions of similarity in plasticity, toughness, reduction of area Z, and elongation A, the strength of the spring steel in the present invention, including the yield strength (Rp.sub.0,2) and the tensile strength (Rm), is significantly improved. In particular, the fatigue strength is increased by more than 400%, which is especially applicable to the manufacturing of lightweight leaf springs.