Manufacturing method of medium carbon steel wire and medium carbon steel wire manufactured thereby

Abstract

The present invention comprises the steps of: manufacturing a billet comprising, in weight percent, carbon (C): 0.45–0.48%, silicon (Si): 0.18–0.26%, manganese (Mn): 0.67–0.77%, phosphorus (P): 0.020% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), copper (Cu): 0.15% or less (excluding 0%), aluminum (Al): 0.01–0.04%, nickel (Ni): 0.1% or less (excluding 0%), chromium (Cr): 0.08–0.12%, titanium (Ti): 0.01% or less (excluding 0%), boron (B): 0.0015% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; and heating the billet in a furnace and then extracting it at 1,050°C or lower. A method for manufacturing a medium carbon steel wire rod is provided, comprising the step of manufacturing a wire rod by rough rolling and wire rod rolling the billet; and, during wire rod rolling, the step of extracting at 1,025°C or lower from a pre-finishing mill (PFM); and the step of rolling at a temperature of 900°C or higher at the entrance side of the material before entering a final rolling mill (RSM).

Description

Technology Field

[0001] The present invention relates to a method for manufacturing a medium-carbon steel wire rod and a medium-carbon steel wire rod manufactured thereby. More specifically, the invention relates to a method for manufacturing a medium-carbon steel wire rod that facilitates spheroidizing heat treatment during drawing by suppressing the formation of a mixed grain structure in the surface portion and the R / 2 portion, which is an intermediate portion between the surface portion and the center, and to a medium-carbon steel wire rod manufactured thereby. Background Technology

[0002] Medium-carbon special steel wire rods produced for cold heading undergo a drawing process that includes spheroidizing heat treatment to minimize die load during forging. Spheroidizing heat treatment refers to an annealing heat treatment that breaks down the lamellar and network cementite structures of the raw material into smaller, spherical cementite with lower hardness. This is because if these structures remain intact, their high hardness hinders cold forging and causes deformation and cracking during quenching. During spheroidizing heat treatment, the pearlite structure within the raw material breaks down to form spherical cementite, and the degree of spheroidization varies depending on the size of the pearlite. A coarse mixed-grain structure is considered a major cause of the low spheroidization rate because it hinders the smooth segmentation of cementite during the treatment. To increase the spheroidization rate of materials with mixed-grain structures, drawing companies perform re-heat treatment, which results in increased heat treatment process costs for the companies.

[0003] The mixed grain structure of medium carbon steel (SWRCH45K), specifically the surface layer and the R / 2 layer (the intermediate layer between the surface and the center), grows when rolling is performed at the recrystallization temperature of 1025°C to 1055°C. During the wire rod rolling process, a temperature rise due to rolling occurs at the downstream end of the PFM (Prefinishing Mill) and NTM (No Twisting Mill) facilities. Since a mixed grain structure is formed in the surface layer and the R / 2 layer (the intermediate layer between the surface and the center) when rolling is performed above the recrystallization temperature in this section, control is required to reduce the mixed grain structure. The problem to be solved

[0004] The present invention aims to solve such conventional problems by suppressing the formation of coarse mixed grain structures during wire rod production through controlled rolling, thereby minimizing structural imbalance. However, this objective is exemplary and does not limit the scope of the present invention. means of solving the problem

[0005] According to one aspect of the present invention, a method for manufacturing a medium carbon steel wire rod is provided.

[0006] The method for manufacturing the above medium-carbon steel wire rod comprises the steps of: manufacturing a billet comprising, in weight percent, carbon (C): 0.45~0.48%, silicon (Si): 0.18~0.26%, manganese (Mn): 0.67~0.77%, phosphorus (P): 0.020% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), copper (Cu): 0.15% or less (excluding 0%), aluminum (Al): 0.01~0.04%, nickel (Ni): 0.1% or less (excluding 0%), chromium (Cr): 0.08~0.12%, titanium (Ti): 0.01% or less (excluding 0%), boron (B): 0.0015% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; and heating the billet in a furnace and then extracting it at 1,050℃ or lower. The method may include the step of manufacturing a wire rod by rough rolling and wire rolling the billet; and, during wire rolling, the step of extracting at 1,025°C or lower from a pre-finishing mill (PFM); and the step of rolling at a temperature of 900°C or higher at the entrance of the material before it enters a final rolling mill (RSM).

[0007] According to one embodiment, the wire manufacturing step can be performed in a temperature range where no mixed structure occurs in the wire.

[0008] According to one embodiment, after the wire manufacturing step, the step of winding the wire at 810 to 880°C and then cooling may be further included.

[0009] According to one embodiment, in the wire rod manufacturing step, the temperature of the material at the exit side of the final rolling mill (RSM) may be characterized as being 800 to 950°C.

[0010] According to another aspect of the present invention, a medium carbon steel wire rod is provided, which is manufactured by the above-described manufacturing method and is characterized in that it does not contain a mixed structure in the surface portion and in the R / 2 portion which is an intermediate portion between the surface portion and the center. Effects of the invention

[0011] According to the embodiment of the present invention as described above, the reduction in the spheroidization rate caused by the mixed grain structure during spheroidization heat treatment is improved, and there is a cost reduction effect through the reduction of reheat treatment of the drawn yarn. In addition, quality problems during forging / processing caused by the mixed grain structure can be resolved.

[0012] Of course, the scope of the present invention is not limited by these effects. Brief explanation of the drawing

[0013] FIG. 1 is a process flowchart of a method for manufacturing a medium carbon steel wire rod according to an embodiment of the present invention. FIG. 2 is a diagram illustrating the temperature conditions during wire rod rolling in the method for manufacturing medium carbon steel wire rods according to the embodiments and comparative examples of the present invention. Figure 3 is a photograph showing the surface portion and the R / 2 portion, which is the intermediate portion between the surface portion and the center, of a medium carbon steel wire rod after rolling, which is implemented by the method for manufacturing a medium carbon steel wire rod according to the embodiment and comparative example of the present invention. Figure 4 is a photograph showing a comparison of the microstructure of a medium carbon steel wire rod produced by the method for manufacturing a medium carbon steel wire rod according to an embodiment of the present invention and the microstructure after spheroidizing heat treatment of the wire drawing. Specific details for implementing the invention

[0014] Hereinafter, various embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the spirit of the present invention to those skilled in the art. In addition, the thickness or size of each layer in the drawings is exaggerated for convenience and clarity of explanation.

[0015] FIG. 1 is a process flowchart of a method for manufacturing a medium carbon steel wire rod according to an embodiment of the present invention.

[0016] Referring to FIG. 1, a method for manufacturing a medium-carbon steel wire rod according to one embodiment of the present invention sequentially includes the steps of: manufacturing a billet (S10); heating the billet (S20); rough rolling and wire rod rolling the billet (S30); and winding and cooling the wire rod (S40).

[0017] To implement the step (S10) of manufacturing a billet, the steelmaking and continuous casting steps may be performed first. For example, a bloom may be manufactured by sequentially performing the steps of feeding molten iron and scrap into an electric furnace, refining using a ladle, fine-tuning the molten steel, performing a degassing process using the RH method, and performing a continuous casting process.

[0018] The above bloom may have a size of, for example, 390 mm X 530 mm. The above bloom may be heated using a heating furnace and rolled to form a billet (S10). The billet may have a size of, for example, 150 mm X 150 mm or 180 mm X 180 mm.

[0019] The above billet may contain, in weight percent, carbon (C): 0.45~0.48%, silicon (Si): 0.18~0.26%, manganese (Mn): 0.67~0.77%, phosphorus (P): 0.020% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), copper (Cu): 0.15% or less (excluding 0%), aluminum (Al): 0.01~0.04%, nickel (Ni): 0.1% or less (excluding 0%), chromium (Cr): 0.08~0.12%, titanium (Ti): 0.01% or less (excluding 0%), boron (B): 0.0015% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities.

[0020] Carbon (C): 0.45~0.48%

[0021] Carbon (C) is an element that is dissolved in the matrix and increases the strength of steel. To ensure sufficient strength of steel, at least 0.45% carbon (C) is required, but if the carbon (C) content exceeds 0.48%, toughness and ductility are significantly reduced due to the increase in strength, and cold forging performance is reduced. Therefore, to ensure the forging performance of medium-carbon steel, carbon (C) is added in an amount of 0.45% to 0.48%.

[0022] Silicon (Si): 0.18~0.26% ,

[0023] Silicon (Si) is a useful element for contributing to high strength and improving fatigue resistance by increasing softening resistance. However, if added in excess as an element that improves deformation resistance, it can significantly reduce cold forging performance. Considering this, silicon (Si) is added in an amount of 0.18% to 0.26% relative to the total weight of the medium carbon steel according to one embodiment of the present invention.

[0024] Manganese (Mn): 0.67~0.77% ,

[0025] Manganese (Mn) is an element that is dissolved in the matrix to improve strength and increases hardenability as an austenite stabilizing element. However, since adding more than 0.77% lowers cold forgingability, manganese is added at a level of 0.67% to 0.77% relative to the total weight of medium carbon steel.

[0026] Phosphorus (P): 0.020% or less (excluding 0%) ,

[0027] Phosphorus (P) is an inevitably contained impurity that segregates at grain boundaries and is a major cause of reduced toughness and reduced resistance to delayed fracture in steel. Therefore, it is desirable to control its content to be as low as possible. In one embodiment, the phosphorus is contained in an amount greater than 0 and less than or equal to 200 ppm with respect to the total weight of the billet. When the phosphorus is contained in an amount exceeding 200 ppm, the phenomenon of segregation at grain boundaries, which reduces the toughness and resistance to delayed fracture in steel, may appear significantly.

[0028] Sulfur (S): 0.010% or less (excluding 0%) ,

[0029] When the sulfur (S) content is excessive, a problem of reduced strength may occur due to an increase in non-metallic inclusions. In one embodiment, the sulfur is included in an amount greater than 0 and less than or equal to 100 ppm with respect to the total weight of the billet. When the sulfur is included in an amount exceeding 100 ppm, the amount of non-metallic inclusions increases, which may lead to a decrease in strength.

[0030] Copper (Cu): 0.15% or less (excluding 0%) ,

[0031] Copper (Cu) is an element that contributes to improving the corrosion resistance of the ferrite structure. In one embodiment, the copper is included in an amount greater than 0 and greater than 0.15 weight percent with respect to the total weight of the billet. If the copper is included in an amount greater than 0.15 weight percent, the manufacturing cost may increase.

[0032] Aluminum (Al): 0.01~0.04% ,

[0033] Aluminum (Al) is an element that plays a role in refining the grain size of austenite by forming aluminum nitride (AlN). In one embodiment, the aluminum is included in an amount of 0.01 to 0.04 weight% relative to the total weight of the billet. If the aluminum is included in an amount less than 0.01 weight%, the effect of the addition is negligible, and if it is included in an amount exceeding 0.04 weight%, the effect of refining the austenite grain size becomes large, which may lead to a decrease in cold workability.

[0034] Nickel (Ni): 0.1% or less (excluding 0%) ,

[0035] Nickel (Ni) contributes to increasing hardenability and improving toughness. If the amount of nickel (Ni) added exceeds 0.1%, the cost may increase. Therefore, nickel is added at 0.1% or less relative to the total weight of medium carbon steel.

[0036] Chrome (Cr): 0.08~0.12%

[0037] Chromium (Cr) is an element that promotes ferrite and pearlite transformations during hot rolling. Additionally, Cr reduces the amount of dissolved carbon by precipitating carbides in the steel without increasing the strength of the steel itself beyond what is necessary, and contributes significantly to the reduction of dynamic strain aging caused by dissolved carbon. In one embodiment, the chromium is included in an amount of 0.08 to 0.12 weight% relative to the total weight of the billet. If the chromium is included in an amount less than 0.08 weight%, the effect of the addition is negligible, and if it is included in an amount exceeding 0.12 weight%, it may reduce forgeability.

[0038] Titanium (Ti): 0.01% or less (excluding 0%) ,

[0039] Titanium (Ti) is an element that forms nitrides, such as aluminum and niobium, and is generally added in small amounts to steels that contain boron to prevent the formation of BN inclusions. Generally, it is added in an amount 3.45 times that of nitrogen according to the stoichiometric ratio, and adding more than that increases production costs. Therefore, titanium is added at 0.01 weight% or less relative to the total weight of medium carbon steel.

[0040] Boron (B): 0.0015% or less (excluding 0%)

[0041] Boron (B) is a major element that improves the hardenability of steel by segregating at grain boundaries. However, if boron exceeds 0.0015%, it can react with nitrogen to precipitate BN inclusions. This lowers the fatigue properties of the steel. Therefore, boron is added at 0.0015% or less relative to the total weight of medium carbon steel.

[0042] In the present invention, a billet having the above composition range is heated in a furnace and then extracted at 1,050°C or lower (S20).

[0043] In the manufacturing method for medium-carbon steel wire rods, a billet is typically heated in a furnace to a temperature suitable for rolling, and then wire rods are produced through rough rolling, intermediate rolling, and wire rod rolling. Wire rod rolling involves the sequential execution of a Prefinishing Mill (PFM), No Twisting Mill (NTM), and Reducing Sizing Mill (RSM).

[0044] During wire rod rolling, a mixed grain structure is formed in the surface portion and the R / 2 portion, which is the intermediate portion between the surface portion and the center, within the recrystallization temperature zone. This mixed grain structure is primarily formed as it passes through the recrystallization temperature zone of 1,025℃ to 1,055℃ at the exit side of the PFM facility and the exit side of the NTM facility. The medium carbon steel wire rod produced in the present invention has a shape that is a rod or a line, wherein the cross-section perpendicular to the longitudinal direction is circular. Here, the surface portion refers to the perimeter surface. If R is defined as the distance from the perimeter surface to the center, which is the center of the cross-section, in units of mm, the R / 2 portion, which is the intermediate portion between the surface portion and the center, refers to a position at a depth that is half the radius R.

[0045] In the present invention, in order to perform PFM / NTM operation below the recrystallization temperature of medium carbon steel during wire rod rolling, the furnace extraction temperature is set to 1,050°C or lower to prevent the formation of mixed grain structures in the surface portion and R / 2 portion. Here, if the extraction temperature is 900°C or lower, the rolling effect may be reduced during hot rolling.

[0046] Subsequently, the hot rolling step (S30) is performed by extracting the material from the PFM at 1,025°C or lower, and rolling it at a temperature of 900°C or higher before entering the final rolling mill (RSM). This corresponds to a temperature range where no mixed grain structure occurs in the wire rod.

[0047] During the wire rod rolling process, a temperature rise due to rolling occurs at the exit sides of the PFM and NTM equipment. If rolling occurs above the recrystallization temperature in these sections, a mixed grain structure develops in the surface and R / 2 sections. This mixed grain structure in the surface and R / 2 sections reduces the spheroidization rate due to coarse pearlite during spheroidization heat treatment, leading to various problems such as the need for re-heat treatment. Therefore, temperature control by rolling section is necessary to avoid the raw material rolling temperature rising to the recrystallization temperature during wire rod rolling.

[0048] In the present invention, the temperature control on the PFM exit side, which has a small amount of process deformation, was performed through approximate analysis of the rolling force during the rolling section. Since the temperature on the NTM exit side also affects the mixed grain structure, the temperature on the RSM inlet side after NTM is completed was managed to improve the mixed grain structure of the raw material surface and R / 2 sections.

[0049] Meanwhile, in the present invention, the rough rolling and intermediate rolling processes can be performed, for example, at a rough rolling (RM) temperature of 900 to 1,025°C and an intermediate rolling (IM) temperature of 900 to 1,025°C to avoid the recrystallization temperature zone.

[0050] Afterwards, the step (S40) of winding the above wire at 810~880℃ and then cooling is performed.

[0051] After undergoing final rolling (RSM), the temperature of the exit-side material is set to 800~950℃, more preferably 850~900℃, and the temperature of the material before coiling is set to 810~880℃, thereby minimizing the difference between the final rolling temperature and the coiling temperature, which can suppress the formation of microstructure in the surface and R / 2 regions.

[0052] Preferred experimental examples are presented below to aid in understanding the present invention. However, the following experimental examples are intended only to aid in understanding the present invention and do not limit the present invention. Details not described herein can be sufficiently technically inferred by those skilled in the art, so their description is omitted.

[0053] <Example>

[0054] A medium carbon steel billet containing carbon: 0.48%, silicon: 0.20%, manganese: 0.70%, phosphorus: 0.001%, sulfur: 0.007%, copper: 0.1%, aluminum: 0.02%, nickel: 0.1%, boron: 0.001%, chromium: 0.08%, titanium: 0.01%, and the remainder being Fe and other unavoidable impurities was heated, intermediate rolled, wire rolled, and coiled under the conditions shown in Table 1 below to produce a wire rod (Φ10).

[0055] FIG. 2 is a diagram illustrating the temperature conditions according to the example and comparative example 2 of Table 1 below. As shown in FIG. 2, in comparative example 2, rolling is carried out in the recrystallization temperature range of 1025°C to 1055°C, whereas in the example, rolling takes place at a temperature below the recrystallization temperature range.

[0056] The microstructure of the wire prepared as described above was observed, and the results are shown in Table 1 and Figures 3 and 4 below.

[0057] division Heater extraction temperature (°C) Intermediate rolling temperature (°C) Wire rod rolling temperature (°C) Presence or absence of mixed tissue IM extraction PFM extraction NTM entry side NTM extraction RSM entry side Kwon Chi Comparative Example 1 1160 1036 1070 968 1027 951 853 generation Comparative Example 2 1183 1040 1088 952 1027 819 842 generation Comparative Example 3 975 964 1036 913 996 829 818 generation Examples 975 964 991 996 930 910 861 Non-occurrence

[0058] As shown in Table 1 above, it can be seen that in the case of the wire rod manufactured according to the present invention (Example), a mixed structure is not formed. On the other hand, it can be seen that in the case of the wire rod manufactured under conditions outside the scope of the present invention (Comparative Examples 1-3), a mixed structure is formed.

[0059] Figure 3 is a photograph showing the surface portion and the R / 2 portion mixed structure after rolling of a medium carbon steel wire rod manufactured by the method for manufacturing a medium carbon steel wire rod according to the embodiment and comparative example 2 of the present invention.

[0060] As shown in Figure 3, a mixed structure of the surface and R / 2 parts was confirmed from both ends of the head / tail of the wire coil according to Comparative Example 2 up to the 10th ring, but a mixed structure of the surface and R / 2 parts did not occur in the head / tail of the wire coil according to the embodiment.

[0061] In other words, it can be seen that there is a high possibility of mixed grain structure occurring when the furnace extraction temperature exceeds 1,050℃, the PFM extraction temperature and NTM extraction temperature exceed the recrystallization temperature of 1,025℃, and the RSM inlet temperature is less than 900℃.

[0062] Figure 4 is a photograph showing a comparison of the microstructure of a medium carbon steel wire rod produced by the method for manufacturing a medium carbon steel wire rod according to an embodiment of the present invention and the microstructure after spheroidizing heat treatment of the wire drawing.

[0063] As shown in Fig. 4, the wire rod (Example) manufactured according to the manufacturing method of the present invention was confirmed to have no mixed structure as a result of raw material structure inspection and spheroidizing heat treatment.

[0064] From these results, it can be seen that, according to an embodiment of the present invention, there is an effect of improving the reduction in the spheroidization rate caused by the mixed grain structure during spheroidization heat treatment, reducing the reheat treatment of the wire, and simultaneously resolving quality problems during forging / processing caused by the mixed grain structure.

[0065] The present invention has been described with reference to the embodiments illustrated in the drawings, but this is merely illustrative, and those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible therefrom. Accordingly, the true technical scope of protection of the present invention should be determined by the technical spirit of the appended claims.

Claims

Claim 1 A step of manufacturing a billet comprising, in weight percent, carbon (C): 0.45–0.48%, silicon (Si): 0.18–0.26%, manganese (Mn): 0.67–0.77%, phosphorus (P): 0.020% or less (excluding 0%), sulfur (S): 0.010% or less (excluding 0%), copper (Cu): 0.15% or less (excluding 0%), aluminum (Al): 0.01–0.04%, nickel (Ni): 0.1% or less (excluding 0%), chromium (Cr): 0.08–0.12%, titanium (Ti): 0.01% or less (excluding 0%), boron (B): 0.0015% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; a step of heating the billet in a furnace and then extracting it at 1,050℃ or lower; A method for manufacturing a medium carbon steel wire rod, comprising: a step of manufacturing a wire rod by rough rolling and wire rod rolling the billet; a step of extracting at 1,025°C or lower from a pre-finishing mill (PFM) during wire rod rolling; and a step of rolling with the temperature of the material at the inlet side at 900°C or higher before entering a final rolling mill (RSM). Claim 2 A method for manufacturing a medium carbon steel wire rod according to claim 1, wherein the wire rod manufacturing step is performed in a temperature range in which mixed grain structure does not occur in the wire rod. Claim 3 A method for manufacturing a medium carbon steel wire rod according to claim 1, further comprising, after the wire rod manufacturing step, a step of coiling the wire rod at 810 to 880°C and then cooling it. Claim 4 A method for manufacturing a medium carbon steel wire rod according to claim 1, characterized in that, in the wire rod manufacturing step, the temperature of the material at the exit side of the final rolling mill (RSM) is 800 to 950°C. Claim 5 A medium carbon steel wire rod manufactured according to any one of claims 1 to 4, characterized in that it does not contain a mixed structure in the surface portion and in the R / 2 portion which is an intermediate portion between the surface portion and the center.

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