WIRE ROD HAVING ENHANCED STRENGTH AND IMPACT TOUGHNESS AND PREPARATION METHOD FOR SAME

Abstract

Provided is a wire rod having enhanced strength and impact toughness. The wire rod includes, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% to 5.0% or less, chromium (Cr): 0.5% to 2.0%, phosphorus (P): 0.020% or less, sulfur (S):0.020% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component, and inevitable impurities. The microstructure of the wire rod includes martensite in an area fraction of 95% or more, and retained austenite (y) as a residual component.

Claims

1. A wire rod having enhanced strength and impact toughness comprising, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): more than 3.5% to 5.0% or less, chromium (Cr): 0.5% to 2.0%, phosphorus (P): 0.020% or less, sulfur (S):0.020% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component, and inevitable impurities, wherein a microstructure includes martensite in an area fraction of 95% or more, and retained austenite (y) as a residual component.

2. The wire rod having enhanced strength and impact toughness of claim 1, wherein contents of Mn, Cr, and C satisfy Relational Expression 1 below,
4.0≦C(Mn+Cr).sup.5/50≦9.0.  [Relational Expression 1]

3. The wire rod having enhanced strength and impact toughness of claim 1, wherein contents of Mn and Si satisfy Relational Expression 2 below,
Mn/Si≧22.  [Relational Expression 2]

4. The wire rod having enhanced strength and impact toughness of claim 1, wherein the wire rod is provided with any cross section in which a ratio of a maximum concentration [Mn.sub.max] to a minimum concentration [Mn.sub.min] of Mn satisfies Relational Expression 3 below,
[Mn.sub.max]/[Mn.sub.min]≦4.  [Relational Expression 3]

5. A method of manufacturing a wire rod having enhanced strength and impact toughness, comprising: reheating a steel material including, by wt %, C: 0.05% to 0.15%, Si: 0.2% or less, Mn: more than 3.5% to 5.0% or less, Cr: 0.5% to 2.0%, P: 0.020% or less, S: 0.020% or less, Al: 0.010% to 0.050%, Fe as a residual component, and inevitable impurities; hot rolling the steel material, having been reheated; cooling the steel material at a rate of 0.2° C./s or higher at a temperature within a range of Mf° C. to Mf-50° C., after the hot rolling; and air cooling the steel material, having been cooled.

6. The method of claim 5, wherein contents of Mn, Cr, and C satisfy Relational Expression 1 below,
4.0≦C(Mn+Cr).sup.5/50≦9.0.  [Relational Expression 1]

7. The method of claim 5, wherein contents of Mn and Si satisfy Relational Expression 2 below,
Mn/Si≧22.  [Relational Expression 2]

8. The method of claim 5, wherein the reheating is performed at a temperature of 1000° C. to 1100° C.

9. The method of claim 5, wherein finish hot rolling of the hot rolling is performed at a temperature within a range of 850° C. to 950° C.

Description

EXEMPLARY EMBODIMENT

[0046] After a molten steel having a composition component of Table 1 below was cast, the cast steel was reheated at 1100° C., the cast steel was wire-rod rolled to have a diameter of 15 mm, the wire-rod was cooled to 150° C., below a temperature, Mf, at a cooling rate of Table 2, and the wire-rod was air cooled, thereby manufacturing a wire rod. Meanwhile, Mf, a martensite phase transformation end temperature, was measured using a dilatometer, slightly varies depending on a chemical composition, and exists in a range of 150° C. to 200° C.

[0047] In the wire rod manufactured using a method described above, a microstructure was analyzed, and analysis thereof was illustrated in Table 2. In addition, tensile strength and impact toughness thereof were measured and illustrated in Table 2. Meanwhile, a concentration of Mn was measured using electron probe micro-analysis (SPMA).

[0048] In addition, a room temperature tensile test was carried out for measurement, in which a crosshead speed was 0.9 mm/min to a yield point and was 6 mm/min thereafter. In addition, an impact test was carried out at room temperature for measurement, using an impact tester in which curvature of an edge portion of a striker impacting a specimen was 2 mm and test capacity was 500 J.

TABLE-US-00001 TABLE 1 Composition component (weight %) Relational Relational No. C Si Mn Cr P S Al Expression 1 Expression 2 1 0.07 0.17 4.1 1.0 0.017 0.020 0.024 4.83 24.1 2 0.09 0.19 3.9 1.4 0.014 0.017 0.029 7.53 20.5 3 0.08 0.15 3.8 0.9 0.011 0.015 0.035 3.67 25.3 4 0.06 0.16 4.7 0.7 0.016 0.013 0.018 5.51 29.4 5 0.12 0.14 3.6 1.5 0.015 0.014 0.034 8.28 25.7 6 0.14 0.20 4.3 1.2 0.011 0.012 0.026 14.09 21.5 7 0.07 0.08 3.7 1.8 0.019 0.013 0.043 7.05 46.2 8 0.11 0.18 4.5 0.8 0.015 0.016 0.015 9.20 25.0 9 0.07 0.16 3.7 2.5 0.014 0.013 0.038 12.83 23.1 10 0.18 0.16 4.2 0.5 0.011 0.015 0.033 8.26 26.3 11 0.11 0.17 5.3 0.8 0.018 0.014 0.027 18.58 31.2 12 0.06 0.15 2.6 1.5 0.016 0.017 0.021 1.39 17.3 13 0.10 0.24 3.8 1.8 0.012 0.011 0.025 11.01 15.8 14 0.08 0.14 3.6 1.4 0.015 0.012 0.032 5.00 25.7 15 0.09 0.18 4.3 0.2 0.017 0.016 0.036 3.32 23.9 (In Table 1, Relational Expression 1 is C (Mn + Cr).sup.5/50, Relational Expression 2 is Mn/Si, and a residual component thereof is Fe and inevitable impurities)

TABLE-US-00002 TABLE 2 Yield Tensile Cooling rate strength strength Elongation Impact Relational Classification No. (° C./s) (MPa) (MPa) percentage value (J) Expression 3 Inventive 1 6 718 1081 17 96 3.2 Example 2 10 739 1123 16 84 3.1 3 2 721 1105 16 85 3.1 4 20 702 1077 15 96 3.7 5 5 755 1155 16 125 2.9 6 3 756 1171 14 82 3.3 7 0.6 697 1056 19 142 3.0 8 8 720 1080 17 88 3.5 Comparative 9 3 787 1173 12 70 3.1 Example 10 5 815 1232 8 35 3.4 11 2 801 1203 10 41 4.6 12 0.5 514 826 25 120 2.2 13 8 837 1224 8 36 3.0 14 0.05 653 988 13 120 2.9 15 1 721 1125 12 60 3.5 (In Table 2, Relational Expression 3 is [Mn.sub.max]/[Mn.sub.min].)

[0049] As illustrated in Tables 1 and 2, in the cases of Inventive Examples 1 to 8 satisfying a steel composition and a manufacturing method thereof according to the present disclosure, a martensite structure of 95 area % or more may be obtained therefrom. Thus, it can be confirmed that tensile strength of 1000 MPa to 1200 MPa and excellent impact toughness of 80 J or higher are provided.

[0050] In the case of Inventive Example 7, a content of Si is 0.1 wt % or less. It can be confirmed that significantly excellent impact toughness and elongation percentage may be secured, compared to other Inventive Examples. Among Inventive Examples, in the cases of Inventive Examples 1, 4, 5, and 7, satisfying Relational Expression 1 (4.0≦C (Mn+Cr) .sup.5/50≦9.0) of contents of Mn, Cr, and C, in addition to Relational Expression 2 (Mn/Si≧22.0) of Mn and Si, it can be confirmed that impact toughness is further excellent, compared to different cases.

[0051] In other words, among Inventive Examples, in the cases of Inventive Examples 2, 3, 6, and 8, not satisfying Relational Expression 1 (4.0≦C(Mn+Cr).sup.5/.sub.50≦9.0) and/or Relational Expression 2 (Mn/Si≧22.0), it can be confirmed that impact toughness is somewhat reduced.

[0052] Comparative Example 9 is a case in which a component of Cr is outside of a range of the present disclosure and illustrates that toughness is increased, but ductility is decreased, so that impact toughness is reduced. Comparative Example 10 is a case in which a content of C exceeds the range of the present disclosure and has a problem in which toughness is significantly increased, due to a solid solution strengthening improvement effect of a martensite base of C, but impact toughness is significantly reduced.

[0053] Comparative Example 11 is a case in which a component of Mn is outside of the range of the present disclosure and illustrates that toughness is increased, but ductility is decreased, so that impact toughness is reduced. In addition, it can be confirmed that Mn is segregated in steel, and a locally uneven structure is formed, so that impact toughness may be reduced.

[0054] Comparative Example 12 is a case in which Mn is added in an amount less than a composition range of the present disclosure. Since hardenability is relatively low, a bainite structure, rather than a martensite structure, is formed when a cooling rate is relatively low. Thus, it is confirmed that impact toughness is increased, but strength is reduced. In addition, Comparative Example 13 is a case in which Si is contained in an amount exceeding the composition range of the present disclosure. It can be confirmed that, when an addition amount thereof is 0.52%, tensile strength is significantly increased, while impact toughness is significantly reduced.

[0055] Comparative Example 14 satisfies a composition component of steel of the present disclosure. However, when a cooling rate is significantly low, the bainite structure, rather than the martensite structure, is formed. Thus, it is confirmed that impact toughness is increased, but strength is reduced. Furthermore, it can confirmed that Comparative Example 15 containing a relatively small amount of Cr has relatively poor impact toughness.

[0056] While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.