Steel wire rod having high strength and ductility and method for producing same

Abstract

There are provided a steel wire rod for ultra-high-strength parts such as automobile engine bolts or structural mechanical parts, and a method for producing the steel wire rod. The steel wire rod having high strength and ductility includes, by wt %, carbon (C): 0.7% to 0.9%, manganese (Mn): 13% to 17%, copper (Cu): 1% to 3%, and the balance of iron (Fe) and inevitable impurities.

Claims

1. A steel wire rod, the steel wire rod comprising, by wt %, carbon (C): 0.7% to 0.9%, manganese (Mn): 13% to 17%, copper (Cu): 1% to 3%, and the balance of iron (Fe) and inevitable impurities, wherein after a cold drawing process, the steel wire rod comprises twins having a thickness of 10 nm to 50 nm in an area fraction of 60% to 80%.

2. The steel wire rod of claim 1, wherein after a hot rolling process, the steel wire rod comprises an austenite single phase structure having a grain size of 10 m to 100 m.

3. The steel wire rod of claim 1, wherein the steel wire rod has a tensile strength of 1800 MPa or greater and an elongation of 15% or greater.

4. A method for producing a steel wire rod having high strength and ductility, the method comprising: reheating a steel ingot to a temperature of Ae3+150 C. to Ae3+250 C., the steel ingot comprising, by wt %, carbon (C): 0.7% to 0.9%, manganese (Mn): 13% to 17%, copper (Cu): 1% to 3%, and the balance of iron (Fe) and inevitable impurities; cooling the reheated steel ingot and hot-rolling the cooled steel ingot within a temperature range of Ae3+50 C. to Ae3+150 C., so as to form a hot-rolled wire rod; cooling the hot-rolled wire rod to a temperature of 600 C. or lower at a cooling rate of 1 C./s to 5 C./s; and cold-drawing the cooled hot-rolled wire rod at an area reduction ratio of 60% to 80% so as to form a steel wire rod.

5. The method of claim 4, wherein the reheating of the steel ingot is performed for 30 minutes to one and a half hours.

6. The method of claim 4, wherein the cooling of the reheated steel ingot is performed at a cooling rate of 5 C./s to 15 C./s.

7. The method of claim 4, wherein the cold-drawing of the cooled and hot-rolled wire rod is performed using a cold-drawing die having a die angle of 10 to 13.

Description

BREIF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an image illustrating the microstructure of a hot-rolled wire rod according to an exemplary embodiment of the present disclosure; and

(2) FIG. 2 is an image illustrating the microstructure of the hot-rolled wire rod of FIG. 1 after a final cold drawing process is performed on the hot-rolled wire rod.

DETAILED DESCRIPTION OF THE INVENTION

(3) In the present disclosure, the term steel wire rodrefers to a final product obtained after the completion of a cold drawing process, and the term hot-rolled wire rodrefers to a wire rod obtained through a hot rolling process. In addition, a product obtained by cooling a hot-rolled wire rod is referred to as an intermediate product.

(4) Hereinafter, a steel wire rod will be described in detail according to an exemplary embodiment of the present disclosure. First, the composition of the steel wire rod will be described in detail according to the exemplary embodiment of the present disclosure (hereinafter, percent (%) refers to wt %).

(5) Carbon (C): 0.7% to 0.9%

(6) In the exemplary embodiment of the present disclosure, if the content of carbon (C) in the steel wire rod is less than 0.7%, twins of the steel wire rod may not behave in desired manner, and thus it may be difficult to obtain desired strength and ductility. That is, if the carbon content of the steel wire rod is low, stacking fault energy (SFE) decreases during multiplication of dislocation or deformation, and thus -martensite may be formed during a cold drawing process or a cold forming process. If -martensite is formed during a forming process, the strength of the steel wire rod may become lower than a degree of strength obtainable by twins, and the ductility of the steel wire rod may be steeply decreased. On the other hand, if the content of carbon (C) in the steel wire rod is greater than 0.9%, excessive carbon (C) may increase the possibility of carbide formation at grain boundaries during a cooling process. If carbides are formed at grain boundaries, grain boundary embrittlement may occur, resulting in a large decrease in the ductility of the steel wire rod. Therefore, the carbon content of the steel wire rod may be maintained to be equal to or lower than 0.9%.

(7) Manganese (Mn): 13% to 17%

(8) In the exemplary embodiment of the present disclosure, manganese (Mn) is dissolved in the microstructure of the steel wire rod to form a substitutional solid solution and is related to the stability of an austenite single phase structure. If the content of manganese (Mn) in the steel wire rod is less than 13%, although the rate of work hardening is increased, SFE is decreased, and thus the possibility of the formation of -martensite increases during a cold drawing process or a cold forming process. In addition, if the content of manganese (Mn) in the steel wire rod is greater than 17%, it is economically unfavorable, and the surface quality of the steel wire rod may be worsened due to severe internal oxidation occurring during a reheating process for hot-rolling. Therefore, it may be preferable that the content of manganese (Mn) in the steel wire rod be maintained to be within the range of 13% to 17%.

(9) Copper (Cu): 1% to 3%

(10) Copper (Cu) is a main element stabilizing austenite and considerably contributes to the formation of twins and the multiplication of dislocation during a cold drawing process. If the content of copper (Cu) in the steel wire rod is less than 1%, the effect of copper (Cu) is very low, and a cold drawing process may not be easily performed due to frequent breakages. On the other hand, if the content of copper (Cu) in the steel wire rod is greater than 3%, it is economically unfavorable, and unlike carbon (C), copper (Cu) causes a decrease in the tensile strength of the steel wire rod. Therefore, it may be preferable that the content of copper (Cu) in the steel wire rod be maintained to be equal to or less than 3%.

(11) In addition, the steel wire rod includes iron (Fe) and inevitable impurities. In the exemplary embodiment of the present disclosure, the inclusion of other elements in the steel wire rod is not excluded. Impurities of raw materials or manufacturing environments may be inevitably included in steel during iron and steel making processes, and such impurities may not be removed from the steel wire rod. Those of skill in the iron and steel manufacturing field will understand the inclusion of inevitable impurities.

(12) Such inevitable impurities include phosphorus (P) and sulfur (S). Phosphorus (P) and sulfur (S) will now be described.

(13) Phosphorus (P): 0.035% or less, and sulfur (S): 0.040% or less

(14) Phosphorus (P) segregates at grain boundaries and thus decreases the ductility of the steel wire rod. Therefore, it may be preferable that the upper limit of the phosphorous content in the steel wire rod be 0.035%. Sulfur (S) has a low melting point and segregates at grain boundaries, thereby decreasing the ductility of the steel wire rod and forming sulfides. Sulfides lower the resistance to delayed fracture and worsen stress relaxation characteristics of the steel wire rod. Therefore, it may be preferable that the upper limit of the sulfur content in the steel wire rod be 0.040%.

(15) According to the exemplary embodiment of the present disclosure, after a hot-rolling process, the steel wire rod (hot-rolled wire rod) may have an austenite single phase structure with a grain size of 10 m to 100 m. The austenite single phase structure formed in the hot-rolled wire rod by the hot-rolling process is maintained in an intermediate product obtained by cooling the hot-rolled wire rod. An example of the hot-rolled wire rod is illustrated in FIG. 1. FIG. 1 illustrates an austenite single phase structure having an average grain size of about 18 m. The formation of twins is related to the size of grains. Thus, if the size of grains is less than 10 m, twins may not be readily formed, and if the size of grains is greater than 100 m, the ductility and fatigue characteristics of the steel wire rod may be worsened. Therefore, it may be preferable that the grain size be maintained to be within the range of 10 m to 100 m.

(16) Preferably, the steel wire rod, a final product produced through a cold drawing process, may have a microstructure in which twins having a thickness of 10 nm to 50 nm are formed in an area fraction of 60% to 80%. FIG. 2 illustrates the microstructure of a steel wire rod obtained by performing a cold drawing process on the hot-rolled wire rod illustrated in FIG. 1 at a ratio of about 60%. Referring to FIG. 2, the steel wire rod is twinned (please refer to black bands within grains) while being work-hardened during the cold drawing process, and the area fraction of twins in the steel wire rod is within the range of 60% to 80%. If the amount of drawing is increased in the cold drawing process, the thickness and area fraction of internal twins are increased. However, if the amount of drawing is insufficient during the cold drawing process, the thickness and area fraction of twins of the steel wire rod may be outside of the above-mentioned ranges, and thus the strength of the steel wire rod may not have strength within a range proposed in the present disclosure. On the other hand, if the amount of drawing in the cold drawing process is excessive, the thickness and area fraction of twins of the steel wire rod may be excessively increased. In this case, although the steel wire rod may have a very high tensile strength, the ductility of the steel wire rod may be markedly decreased, and thus it may be difficult to form the steel wire rod into structural mechanical parts due to brittleness. Therefore, in the exemplary embodiment of the present disclosure, the thickness and area fraction of twins of the steel wire rod may be maintained to be within the above-mentioned ranges.

(17) According to the exemplary embodiment of the present disclosure, the steel wire rod may have an ultra-high degree of strength within the range of 1800 MPa or greater and a high degree of ductility within the range of 15% or greater.

(18) Hereinafter, a method for producing the steel wire rod will be described in detail according to an exemplary embodiment of the present disclosure.

(19) A steel ingot having the above-mentioned composition is reheated. The steel ingot refers to a steel billet for forming a steel wire rod. Preferably, the reheating of the steel ingot may be performed within the temperature range of Ae3+150 C. to Ae3+250 C. for 30 minutes to one and a half hours.

(20) Preferably, the temperature of reheating may be maintained to be within an austenite single phase temperature range equal to or higher than Ae3+150 C. so as to effectively dissolve remaining segregates, carbides, and inclusions. If the temperature of reheating is higher than Ae3+250 C., coarse austenite grains may be formed, and after cooling, a coarse microstructure may be finally formed. In this case, high strength and ductility may not be obtained.

(21) In addition, if the period of reheating is shorter than 30 minutes, the temperature of the steel ingot may not become uniform. On the other hand, if the period of reheating is longer than one and a half hours, coarse austenite grains may be readily formed, and productivity may be markedly decreased.

(22) Then, the reheated steel ingot is subjected to a cooling process and a hot-rolling process so as to produce a hot-rolled wire rod.

(23) Preferably, the cooling process may be performed at a cooling rate of 5 C./s to 15 C./s. The cooling rate is proposed to minimize the transformation of the microstructure of the steel ingot during the cooling process performed before the hot-rolling process. Before the hot-rolling process, if the cooling rate is lower than 5 C./s, productivity may decrease, and an additional apparatus may be required to maintain the cooling rate at a low level. Furthermore, in this case, since the period of reheating is substantially extended, after the hot-rolling process, the hot-rolled wire rod may have relatively low strength and ductility. On the other hand, if the cooling rate is greater than 15 C./s, the steel ingot may have a large degree of driving force for transformation, and thus the possibility of formation of a new microstructure may be increased during the hot-rolling process. In this case, the temperature of the hot-rolling process may have to be reset.

(24) Preferably, the hot-rolling process may be performed within the temperature range of Ae3+50 C. to Ae3+150 C. If the hot-rolling process is performed within the temperature range, the presence of a microstructure caused by deformation is suppressed, and recrystallization may not occur. That is, only the effect of sizing may be obtained through the hot-rolling process. If the temperature of the hot-rolling process is lower than Ae3+50 C., the temperature of the hot-rolling process is close to a dynamic recrystallization temperature, and thus grains may be elongated in the direction of hot rolling instead of being formed in a circular shape. Such elongated grains may cause undesired mechanical anisotropy. If the temperature of the hot-rolling process is higher than Ae3+150 C., the steel ingot is deformed due to high temperature, and thus even though dynamic recrystallization occurs, coarse grains may be formed due to rapid growth of grains at high temperature. Such coarse grains may also decrease the ductility of the hot-rolled wire rod, and an additional apparatus and energy may be required to cool the hot-rolled wire rod at a high cooling rate.

(25) The hot-rolled wire rod is cooled to 600 C. or lower at a cooling rate of 1 C. to 5 C. (such a wire rod cooled after the hot-rolling process is an intermediate product). At the above-mentioned cooling rate, the diffusion of carbon may be effectively suppressed by manganese, and thus unnecessary carbides may not be formed along grain boundaries of single-phase austenite. If the cooling rate is lower than 1 C./s, the cooling rate is too low to perform the cooling process with practical productivity. In addition, carbides may be formed along grain boundaries, and thus the ductility of the wire rod may be lowered. On the other hand, if the cooling rate is greater than 5 C./s, the wire rod may undergo thermal deformation due to rapid cooling, and thus a coiling and cooling method which is a unique cooling method for steel wire rods may not be used. In addition, as is known, it is difficult to obtain a desired cooling rate when performing a cold forging process on general steel wire rods having a diameter (wire diameter) of 10 mm to 20 mm.

(26) After the hot-rolled wire rod is cooled, a cold drawing process is performed on the cooled, hot-rolled wire rod to form a steel wire rod. The cold drawing process may be performed using a wedge-shaped cold drawing die to reduce the cross-sectional area of the hot-rolled wire rod and increase the tensile strength of the hot-rolled wire rod by the effect of work hardening.

(27) The cold drawing process is performed using the cold forming die having a die angle of 10 to 13 for reducing the cross-sectional area of the hot-rolled wire rod and imparting cold forming characteristics to the hot-rolled wire rod. It may be preferable that the cold drawing process be performed at an area reduction ratio of 60% to 80%. The area reduction ratio is calculated based on an initial wire diameter and a wire diameter after the die as follows.
Area reduction ratio=100(initial cross-sectional areacross-sectional area after cold drawing)/(initial cross-sectional area)

(28) In the exemplary embodiment of the present disclosure, if the area reduction ratio is less than 60%, it may be difficult to obtain a high degree of strength, for example, a tensile strength of 1800 MPa to 2100 MPa. On the other hand, if the area reduction ratio is greater than 80%, although a desired degree of tensile strength is obtained, the wire rod may be embrittled due to a large amount of cold forming, and thus breakage or fracture may occur.

(29) [Mode for Invention]

(30) Hereinafter, examples of the present disclosure will be described in detail. The following examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure.

EXAMPLES

(31) Steel ingots (billets) having compositions shown in Table 1 below were manufactured, and transformation points of the steel ingots were measured to about 910 C. Then, process temperatures were applied to the examples as follows. The steel ingots having the compositions shown in Table 1 below were reheated to about 1100 C., and were hot-rolled at about 1000 C. to form hot-rolled wire rods. The hot-rolled wire rods were cooled to about 520 C. at a cooling rate of about 3 C./s to form intermediate products.

(32) Thereafter, the intermediate products were cold-drawn according to amounts of cold drawing (area reduction ratios) shown in Tables 2 and 3 so as to form steel wire rods, and the tensile strength and elongation of the steel wire rods were measured as shown in Tables 2 and 3.

(33) TABLE-US-00001 TABLE 1 Examples C Si Mn Cr V Al Cu Notes *CE 1 0.82 0.25 0.7 0.05 Conventional product for cold drawing CE 2 0.92 0.25 0.7 0.2 Conventional product for cold drawing CE 3 0.6 18 1.5 Al-containing commercial product CE 4 0.9 15 CMn based high manganese steel CE 5 0.5 17 1.5 Insufficient carbon content compared to inventive examples CE 6 1.2 15 2.0 Excessive carbon content compared to inventive examples CE 7 0.8 10 1.5 Insufficient manganese content compared to inventive examples CE 8 0.8 20 1.5 Excessive manganese content compared to inventive examples CE 9 0.8 17 0.5 Insufficient copper content compared to inventive examples CE 10 0.9 13 4.0 Excessive copper content compared to inventive examples **IE 1 0.7 17 1.5 Composition according to present disclosure IE 2 0.8 17 1.5 Composition according to present disclosure IE 3 0.9 13 2.0 Composition according to present disclosure IE 4 0.9 13 3.0 Composition according to present disclosure *CE: Comparative Example, **IE: Inventive Example

(34) TABLE-US-00002 TABLE 2 Tensile strength (MPa) according to cold drawing amounts (%) Examples 0 12 20 28 46 58 64 79 Notes *CE 1 1170 1236 1386 1423 1498 1568 1620 1685 Insufficient strength CE 2 1210 1298 1350 1463 1503 1571 1653 1691 Insufficient strength CE 3 802 1196 1302 1426 X Breakage during drawing CE 4 882 1035 1298 1506 X Breakage during drawing CE 5 920 1103 1236 1302 1468 1529 1690 1732 Insufficient strength CE 6 945 X Breakage during drawing CE 7 889 X Breakage during drawing CE 8 821 965 X Breakage during drawing CE 9 886 1012 X Breakage during drawing CE 10 801 1098 1169 1253 1405 1638 1789 1802 Insufficient strength **IE 1 842 1156 1418 1502 1652 1865 2109 2135 Within inventive range IE 2 853 1201 1369 1489 1752 1902 2122 2136 Within inventive range IE 3 882 1196 1356 1523 1625 1898 2109 2156 Within inventive range IE 4 896 1163 1374 1489 1698 1869 2112 2145 Within inventive range *CE: Comparative Example, **IE: Inventive Example

(35) TABLE-US-00003 TABLE 3 Maximum elongation (%) according to cold drawing amounts (%) Examples 0 12 20 28 46 58 64 79 Notes *CE 1 17 11 9 8 9 8 8 8 Insufficient ductility for structural mechanical parts CE 2 16 14 10 9 8 8 6 7 Insufficient ductility for structural mechanical parts CE 3 76 58 40 32 X Breakage during drawing CE 4 75 52 49 38 X Breakage during drawing CE 5 81 72 60 40 32 21 19 16 Sufficient ductility but insufficient strength CE 6 52 X Breakage during drawing CE 7 67 X Breakage during drawing CE 8 72 35 X Breakage during drawing CE 9 76 45 X Breakage during drawing CE 10 81 71 59 45 32 19 18 15 Sufficient ductility but insufficient strength **IE 1 88 65 52 36 21 19 18 16 Within inventive range IE 2 88 67 56 38 23 20 18 15 Within inventive range IE 3 81 63 53 35 22 21 16 16 Within inventive range IE 4 86 58 49 39 19 18 17 15 Within inventive range *CE: Comparative Example, **IE: Inventive Example

(36) Referring to Tables 2 and 3, inventive examples satisfying conditions of the present disclosure have high degrees of tensile strength equal to or greater than 1800 MPa and high degrees of elongation equal to or greater than 15%.

(37) However, it is difficult to obtain ultra-high strength and high ductility from comparative examples which are commercially available products of the related art or products not including copper (Cu) and do not satisfy conditions of the present disclosure.