STEEL WIRE ROD HAVING EXCELLENT SPHEROIDIZING HEAT TREATMENT PROPERTIES AND METHOD OF MANUFACTURING SAME

Abstract

An embodiment of the present invention provides a wire rod and a method of manufacturing same. The wire rod comprises, by weight %, 0.3-0.5 wt % of C, 0.02-0.4 wt % of Si, 1.0-1.5 wt % of Mn, 0.3-0.7 wt % of Cr, 0.003 wt % or less (exclusive of 0 wt %) of B, less than 0.03 wt % (exclusive of 0 wt %) of Ti, 0.03 wt % or less (inclusive of 0 wt %) of P, 0.01 wt % or less (inclusive of 0 wt %) of S, 0.02-0.05 wt % of Al, 0.001-0.01 wt % of N, and the balance being Fe and inevitable impurities, wherein a microstructure is a complex structure having a main phase of ferrite+pearlite, with at least one of bainite or martensite accounting for 5 area % or less (inclusive of 0%), and has a cementite average aspect ratio of 35 or less in an area covering ⅖-⅗ of the diameter.

Claims

1. A steel wire rod having excellent spheroidizing heat treatment properties and a method of manufacturing same, comprising, by weight %: 0.3-0.5% of C, 0.02-0.4% of Si, 1.0-1.5% of Mn, 0.3-0.7% of Cr, 0.003% or less (excluding 0%) of B, less than 0.03% (excluding 0%) of Ti, 0.03% or less (including 0%) of P, 0.01% or less (including 0%) of S, 0.02-0.05% of Al, 0.001-0.01% of N, and a balance of Fe and inevitable impurities, wherein a microstructure is a complex structure having a main phase of ferrite+pearlite, with one or more of bainite or martensite accounting for 5 area % or less (including 0%), and has an average aspect ratio of cementite of 35 or less in a region at a ⅖-⅗ point of a diameter.

2. The steel wire rod having excellent spheroidizing heat treatment properties of claim 1, wherein a fraction of the ferrite is 50 area % or more.

3. The steel wire rod having excellent spheroidizing heat treatment properties of claim 1, wherein the steel wire rod has an average grain size of ferrite of 5 μm or less in the region of the ⅖ to ⅗ point of the diameter.

4. The steel wire rod having excellent spheroidizing heat treatment properties of claim 1, wherein the steel wire rod has an average aspect ratio of cementite of 2.5 or less after spheroidizing heat treatment is performed once.

5. A method for manufacturing a steel wire rod having excellent spheroidizing heat treatment properties, the method comprising operations of: preparing a billet including, by weight %, 0.3-0.5% of C, 0.02-0.4% of Si, 1.0-1.5% of Mn, 0.3-0.7% of Cr, 0.003% or less (excluding 0%) of B, less than 0.03% (excluding 0%) of Ti, 0.03% or less (including 0%) of P, 0.01% or less (including 0%) of S, 0.02-0.05% of Al, 0.001-0.01% of N, and a balance of Fe and inevitable impurities; heating the billet, and then extracting the billet at 950 to 1050° C.; performing secondary hot rolling on the extracted billet to obtain a steel wire rod; and cooling the steel wire rod to 2° C./sec or less, wherein the secondary hot rolling comprises operations of, intermediate finishing rolling the extracted billet; and finishing rolling the billet at 730° C.-Ae3 above a critical deformation amount expressed by the following Relational Equation 1,
critical deformation amount=−2.46 Ceq.sup.2+3.11 Ceq−0.39  [Relational Equation 1] (where, Ceq=C+Mn/6+Cr/5, wherein C, Mn, and Cr are expressed by wt %.

6. The method for manufacturing a steel wire rod having excellent spheroidizing heat treatment properties of claim 5, wherein the operation of preparing the billet comprises operations of, heating a steel material at 1200° C. or higher for 60 minutes or more, and then performing primary hot rolling to obtain a billet; air cooling the billet to 150 to 500° C.; and cooling the air-cooled billet to room temperature at a cooling rate of 5 to 30° C./sec.

7. The method for manufacturing a steel wire rod having excellent spheroidizing heat treatment properties of claim 6, wherein, after the steel material is heated, the steel material has an average size of TiN of 500 μm or more.

8. The method for manufacturing a steel wire rod having excellent spheroidizing heat treatment properties of claim 6, wherein the cooled billet comprises 80% or more of TiN among all precipitates excluding oxidative inclusions.

9. The method for manufacturing a steel wire rod having excellent spheroidizing heat treatment properties of claim 5, wherein, after the intermediate finishing rolling, an average grain size of austenite of the steel wire rod is 5 to 20 μm.

Description

MODE FOR INVENTION

[0051] Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. The scope of the present disclosure may be determined by matters described in the claims and matters able to be reasonably inferred therefrom.

Embodiment

[0052] By casting using a 50 kg vacuum induction melting furnace, a steel material having an alloy composition shown in Table 1 were prepared. The steel material was heated at 1230° C. for 480 minutes, air-cooled to 300° C., and then cooled to room temperature at a cooling rate of 10° C./sec to prepare a billet. A steel wire rod was prepared from the prepared billet under the conditions shown in Table 2 below. For the steel wire rod prepared as described above, after measuring a microstructure, an average grain size of ferrite, and an average aspect ratio of cementite after spheroidizing heat treatment is performed once, the results thereof were shown in Table 3 below.

[0053] After intermediate finishing rolling, an average grain size of austenite (AGS) was measured through a cutting crop performed before finishing rolling.

[0054] Ae3 displayed values calculated using a JmatPro, a commercial program.

[0055] The average grain size of ferrite (FGS) was measured at 3 arbitrary points in a region at a ⅖-⅗ point from a diameter of a specimen collected after removing an uncooled portion after rolling the steel wire rod, which was expressed as the average value.

[0056] As for the average aspect ratio of cementite, 10 arbitrary points were selected from the same point as in the FGS measurement, a (long axis+short axis)/2 value of each colony was obtained, and then an average value of colony sizes was obtained.

[0057] Meanwhile, a spheroidizing heat treatment was directly performed on the specimen of the steel wire rod prepared as described above without a separate processing process. In this case, the spheroidizing heat treatment was performed by being heated to 760° C. at a heating rate of 100° C./Hr, maintained for 4 to 6 hours, cooled to 730° C. at a cooling rate of 50° C./Hr, and then cooled at a cooling rate of 10° C./Hr in a section between 730° C. and 670° C., and then maintained furnace cooling at a temperature, lower than that. After spheroidizing heat treatment is performed, the average aspect ratio of cementite was imaged in 3 fields of view of ¼-½ point in a diameter direction of the steel wire rod, and a long/short axis of cementite in the field of view was automatically measured using an image measurement program, and then was measured through statistical processing.

TABLE-US-00001 TABLE 1 Steel No. Alloy composition (weight %) type C Si Mn Cr P S Ti B Al N IS 1 0.32 0.21 1.2 0.45 0.018 0.006 0.015 0.002 0.03 0.004 IS 2 0.36 0.2 1.15 0.51 0.015 0.006 0.018 0.003 0.02 0.005 IS 3 0.43 0.15 1.3 0.38 0.01 0.008 0.025 0.0015 0.04 0.003 IS 4 0.38 0.25 1.18 0.62 0.011 0.003 0.015 0.0015 0.03 0.005 CS 1 0.34 0.54 1.05 0.6 0.016 0.004 0.023 0.002 0.03 0.004 CS 2 0.43 0.56 1.22 0.43 0.013 0.005 0.014 0.0018 0.03 0.004

TABLE-US-00002 TABLE 1 Steel No. Alloy composition (weight %) type C Si Mn Cr P S Ti B Al N IS 1 0.32 0.21 1.2 0.45 0.018 0.006 0.015 0.002 0.03 0.004 IS 2 0.36 0.2 1.15 0.51 0.015 0.006 0.018 0.003 0.02 0.005 IS 3 0.43 0.15 1.3 0.38 0.01 0.008 0.025 0.0015 0.04 0.003 IS 4 0.38 0.25 1.18 0.62 0.011 0.003 0.015 0.0015 0.03 0.005 CS 1 0.34 0.54 1.05 0.6 0.016 0.004 0.023 0.002 0.03 0.004 CS 2 0.43 0.56 1.22 0.43 0.013 0.005 0.014 0.0018 0.03 0.004

TABLE-US-00003 TABLE 2 Cooling stop Cooling AGS temper- rate Billet after ature of extrac- inter- Finishing Finishing of steel tion mediate rolling Critical rolling steel wire Steel temper- finishing temper- deform- deform- wire rod Classi- type ature rolling Ae3 ature ation ation rod (° C.)/ fication No. (° C.) (μm) (° C.) (° C.) amount amount (° C.) sec) IE 1 IS 1 1032 12 792 742 0.61 0.82 0.82 0.8 IE 2 IS 2 1025 11 783 755 0.65 0.75 0.75 1.2 IE 3 IS 3 1034 14 766 764 0.72 0.92 0.92 0.9 IE 4 IS 4 1043 13 801 760 0.64 0.75 0.75 1.5 IE 5 IS 1 1021 12 778 752 0.72 0.85 0.85 1.8 IE 6 IS 2 1030 12 780 770 0.61 0.84 0.84 0.9 CE 1 CS 1 1024 12 792 802 0.61 0.43 0.43 0.4 CE 2 CS 2 1031 24 783 823 0.65 0.63 0.63 0.3 CE 3 CS 3 1034 22 766 790 0.72 0.48 0.48 1.5 CE 4 CS 4 1035 21 801 835 0.64 0.55 0.55 2.0 CE 5 CS 1 1011 15 778 804 0.72 0.52 0.52 2.4 CE 6 CS 2 1028 18 780 816 0.61 0.55 0.55 3.0 Critical deformation amount = −2.46Ceq.sup.2 + 3.11Ceq − 0.39 (where, Ceq = C + Mn/6 + Cr/5, wherein C, Mn, and Cr are expressed by weight %)

TABLE-US-00004 TABLE 3 Average Average aspect grain Average ratio size aspect of cementite of ratio after Classi- Microstructure (area %) ferrite of spherodizing fication F P B + M (μm) cementite heat treatment IE 1 57 41 2 3.8 29 2.1 IE 2 53 47 0 4.5 32 2.3 IE 3 51 45 4 4.8 27 2.4 IE 4 53 45 2 4.3 33 2.2 IE 5 56 42 2 4.7 30 2.4 IE 6 52 46 2 4.6 31 2.2 CE 1 47 51 2 9.4 36 3.1 CE 2 32 66 2 11 37 2.8 CE 3 46 52 2 9.5 35 3.3 CE 4 34 64 2 12 42 2.9 CE 5 48 50 2 11 44 3.1 CE 6 35 63 2 11 42 3.3 F: ferrite, P: pearlite, B: bainite, M: martensite

[0058] As can be seen from the Tables 1 to 3 above, in the case of Inventive Examples 1 to 6 satisfying the alloy composition and manufacturing conditions proposed by the present disclosure, it can be seen that the microstructure type and the fraction of the present disclosure and also fine grains were secured, such that, with only the spheroidization heat treatment performed once, the average aspect ratio of cementite was 2.5 or less.

[0059] However, in Comparative Examples 1 to 6 which did not satisfy the alloy composition or manufacturing conditions suggested in the present disclosure, it is indicated that the microstructure type and the fraction of the present disclosure were not satisfied, or fine grains were not secured, such that the cementite average aspect ratio was relatively high when the spheroidization heat treatment was performed once, and accordingly, to be applied to a final product, additional spheroidization heat treatment may be necessary.