WO2017104920A1 - Non-heat treated wire rod excellent in strength and cold workability and method for manufacturing same - Google Patents

Abstract

Disclosed is a non-heat treated wire rod, comprising, as percentage by weight: C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and the balance being Fe and unavoidable impurities, wherein the non-heat treated wire rod includes ferrite and pearlite microstructures, and wherein the phase fraction of the pearlite satisfies the following relational expressions 1 and 2, and the average lamellar spacing of the pearlite satisfies the following relational expressions 3 and 4. [Relational expression 1] VP₂/VP₁ ≤ 1.4 [Relational expression 2] 50 ≤ (15VP₁+VP₂)/16 ≤ 70 [Relational expression 3] DL₁/DL₂ ≤ 1.4 [Relational expression 4] 0.1 ≤ (15DL₁+DL₂)/16 ≤ 0.3 (Wherein VP1 and VP2, respectively, mean, in the cross-section perpendicular to the longitudinal direction of the wire rod, a pearlite fraction (area%) in the region from the surface of the wire rod to the 3/8D position in the diameter (D) direction of the wire rod, and a pearlite fraction (area%) in the region from the 3/8D position in the direction of the diameter (D) of the wire rod to the center of the wire rod, and DL1 and DL2, respectively, mean, in the cross-section perpendicular to the longitudinal direction of the wire rod, the average lamellar spacing (μm) of pearlite in the region from the surface of the wire rod to the 3/8D position in the diameter (D) direction of the wire rod, and the average lamellar spacing (μm) of pearlite in the region from the 3/8D position in the direction of the diameter (D) of the wire rod to the center of the wire rod.)

Description

Unstructured wire rod with excellent strength and cold workability and its manufacturing method

The present invention relates to a non-coarse wire rod having excellent strength and cold workability and a method for manufacturing the same, and more particularly, to a non-coarse wire rod having excellent strength and cold workability suitable for use as a material for mechanical parts, and a manufacturing method thereof. .

The cold working method is widely used in the manufacture of machine parts such as bolts and nuts because not only the productivity is excellent but also the effect of reducing the heat treatment cost is large compared with the hot working method and the mechanical cutting method.

However, in order to manufacture mechanical parts using the cold working method as described above, the cold workability of steel is essentially required to be excellent, and more specifically, the cold deformation is required to have low deformation resistance and excellent ductility. . This is because, if the deformation resistance of the steel is high, the life of the tool used in cold work is reduced, and if the ductility of the steel is low, breakage is likely to occur during cold work, which causes defects.

Accordingly, the conventional cold working steel is subjected to spheroidizing annealing heat treatment before cold working. This is because the steel material is softened during the spheroidizing annealing heat treatment, so that the deformation resistance is reduced, the ductility is improved, and the cold workability is improved. However, in this case, since additional costs are incurred and manufacturing efficiency is lowered, development of an unstructured wire rod that can secure excellent cold workability without additional heat treatment is required.

However, in the medium carbon steel containing 0.3 wt% or more of carbon, it is known that the cold workability is inferior due to the matrix strengthening by the pearlite structure when the pearlite fraction exceeds 50%. In particular, when segregation promoting elements such as Mn and Cr are used together to secure the strength, the variation of the structure between the center segregation area and the non-segregation area is increased, and in the case of non-tough steel which secures the strength by drawing, this deviation is greater after the drawing process. It is difficult to secure cold forging. In high-strength non-steel, more than medium carbon steel, the influence of the center oxide-based nonmetallic inclusions is greatly increased along with the structure imbalance due to the segregation of the center.

Moreover, when the matrix is strengthened by central segregation, the sensitivity of these nonmetallic inclusions becomes greater, which may adversely affect cold workability. Therefore, in the development of high strength non-coated steel of medium carbon steel or higher, it is necessary to examine the influence of the structure deviation and the center inclusions caused by such segregation.

One of the various objects of the present invention is to provide an unstructured wire rod and a method of manufacturing the same that can secure excellent strength and cold forging without additional heat treatment.

In order to achieve the above object, one aspect of the present invention, in weight%, C: 0.3 ~ 0.4%, Si: 0.05 ~ 0.3%, Mn: 0.8 ~ 1.8%, Cr: 0.5% or less, P: 0.02 % Or less, S: 0.02% or less, sol.Al: 0.01 to 0.05%, N: 0.01% or less and O: 0.0001 to 0.003%, Nb: 0.005 to 0.03% and V: 0.05 to 0.3% Including the above, the balance Fe and inevitable impurities, and contains a ferrite (pearlite) and perlite (pearlite) as a microstructure, the phase fraction of the pearlite satisfies the following relations 1 and 2, the average lamellar of the pearlite The spacing provides an uncoated wire rod that satisfies relations 3 and 4 below.

Relation 1 VP ₂ / VP ₁ ≤ 1.4

Equation 2 50≤ (15VP ₁ + VP ₂ ) / 16≤70

[Relationship 3] DL ₁ / DL ₂ ≤1.4

Relational Expression 4 0.1 ≦ (15DL ₁ + DL ₂ ) /16≦0.3

(Where VP ₁ and VP ₂ are each a pearlite fraction (area%) in the area from the surface of the wire rod to a position 3 / 8D in the diameter (D) direction of the wire rod in a cross section perpendicular to the longitudinal direction of the wire rod and the diameter of the wire rod (D ) Means the pearlite fraction (area%) in the area from the 3 / 8D position to the center of the wire rod, and each of DL ₁ and DL ₂ is the diameter of the wire rod (D) in the cross section perpendicular to the longitudinal direction of the wire rod. Mean lamellar spacing (μm) of pearlite in the area up to 3 / 8D) direction, and the average lamellar spacing (μm) of pearlite in the area from the 3 / 8D position in the direction of diameter (D) to the center of the wire. )

In addition, another aspect of the present invention, in weight%, C: 0.3 ~ 0.4%, Si: 0.05 ~ 0.3%, Mn: 0.8 ~ 1.8%, Cr: 0.5% or less, P: 0.02% or less, S: 0.02 % Or less, sol.Al: 0.01 to 0.05%, O: 0.0001 to 0.003% or less and N: 0.01% or less, and Nb: 0.005 to 0.03% and V: 0.05 to 0.3% Bloom containing a balance Fe and unavoidable impurities and having a carbon equivalent (Ceq) of 0.6 or more and 0.7 or less is heated to a heating temperature of 1200 to 1300 ° C., maintained at the heating temperature for 240 minutes or more, and then rolled into steel sheets to be billet. Obtaining a billet, After reheating the billet, Rolling the wire rod under the condition of the finish rolling temperature 750 ~ 900 ℃ to obtain a wire rod, and Winding the wire rod, and cooling at a rate of 0.3 ~ 1 ℃ / sec It provides a method of manufacturing non-structured wire comprising a.

According to the present invention, even if the spheroidized annealing heat treatment is omitted, it is possible to provide a high strength non-coarse wire rod which can sufficiently suppress the deformation resistance during cold working.

The present inventors examined from various angles to provide a wire rod that can secure excellent cold workability while having a predetermined strength and hardness after drawing, and as a result, optimized the alloy composition and manufacturing method in the medium-carbon steel wire rod The present invention finds that it is possible to provide a high-strength wire which does not deteriorate cold workability even after drawing, by securing a ferrite and pearlite composite structure with a microstructure of the wire rod, by appropriately controlling the pearlite phase fraction and the perlite lamellar spacing for each part of the wire rod. Came to complete.

Hereinafter, a non-coarse wire rod having excellent strength and cold workability, which is an aspect of the present invention, will be described in detail.

First, the alloy component and the composition range of the non-coarse wire rod will be described in detail. It is noted that the content of each component described below is based on weight unless otherwise specified.

C: 0.3 ~ 0.4%

Carbon serves to improve the strength of the wire rod. In order to exhibit such an effect in the present invention, it is preferable that 0.3% or more be included. However, when the content is excessive, the deformation resistance of the steel is rapidly increased, which causes a problem that the cold workability is deteriorated. Therefore, the upper limit of the carbon content is preferably 0.4%.

Si: 0.05-0.3%

Silicone is a useful element as a deoxidizer. In order to exhibit such an effect in the present invention, it is preferably included 0.05% or more. However, when the content is excessive, the deformation resistance of the steel is rapidly increased by the solid solution strengthening, which causes a problem that the cold workability is deteriorated. Therefore, the upper limit of the silicon content is preferably 0.3%, more preferably 0.25%.

Mn: 0.8-1.8%

Manganese is an element useful as a deoxidizer and a desulfurizer. In order to exhibit such an effect in the present invention, it is preferable to include 0.8% or more, and more preferably 1.0% or more. However, when the content thereof is excessive, the strength of the steel itself is excessively high, so that deformation resistance of the steel is rapidly increased, thereby deteriorating cold workability. Therefore, the upper limit of the manganese content is preferably 1.8%, more preferably 1.6%.

Cr: 0.5% or less (including 0%)

Chromium plays a role in promoting ferrite and pearlite transformation during hot rolling. In addition, while increasing the strength of the steel itself more than necessary, carbides in the steel can be precipitated to reduce the amount of solid solution carbon, thereby contributing to the reduction of the dynamic strain aging due to the solid solution carbon. There is no big obstacle. On the other hand, if the content is excessive, the strength of the steel itself is excessively high, the deformation resistance of the steel is rapidly increased, thereby causing a problem that the cold workability is deteriorated. It is preferable that it is 0.5% or less, and, as for the said chromium content, it is more preferable that it is 0.4% or less.

P: 0.02% or less

Phosphorus is an unavoidable impurity, and is an element which is segregated at grain boundaries to lower the toughness of steel and decreases delayed fracture resistance. Therefore, it is preferable to control the content as low as possible. In theory, the phosphorus content is advantageously controlled to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is controlled to 0.02%.

S: 0.02% or less

Sulfur is an inevitable impurity, which is segregated at grain boundaries and greatly reduces the ductility of steel and forms an emulsion in the steel, which is a major cause of deterioration in delayed fracture resistance and stress relaxation characteristics. It is preferable. In theory, the sulfur content is advantageously controlled to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is controlled to 0.02%.

sol.Al: 0.01 ~ 0.05%

Soluble aluminum is an element that functions as a deoxidizer and is added in an amount of 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. However, when the content is more than 0.05%, the austenite grain size miniaturization effect due to AlN formation becomes large and the cold workability is lowered. Therefore, in the present invention, the upper limit of the soluble aluminum content is controlled to 0.05%.

N: 0.01% or less

Nitrogen is inevitably an impurity to be contained. If the content is excessive, the amount of solid solution nitrogen increases, so that deformation resistance of the steel rapidly increases, which causes a problem that the cold workability is deteriorated. In theory, the nitrogen content is advantageously controlled to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, it is preferable to manage the upper limit of the nitrogen content at 0.01%, more preferably at 0.008%, and even more preferably at 0.007%.

O: 0.0001 to 0.003%

Oxygen is present in the wire rods in the form of nonmetallic inclusions, and typically contains at least 0.0001%. However, these non-metallic inclusions are the starting point of the failure to reduce the fatigue strength and cold forging of the steel, in particular, when the strength is secured by fresh processing, such as non-steel, fracture occurs based on the non-metallic inclusions in the center of the wire rod easy. In particular, according to the results of the inventors of the present invention, the amount of non-metallic inclusions increases in wire rods having an oxygen content of more than 0.003% in steel, and it is not sufficient to avoid disconnection in the workpiece used for strict applications. Therefore, in the present invention, the upper limit is controlled to 0.003%, more preferably 0.001%, even more preferably 0.0008%.

At least one of Nb: 0.005 to 0.03% and V: 0.05 to 0.3%

Niobium forms carbonitrides and adds 0.005% or more as an element that serves to limit grain boundary migration of austenite and ferrite. However, since the carbonitride acts as a starting point of destruction and can lower impact toughness, in particular, low temperature impact toughness, it is preferable to add the carbonitride in keeping with the solubility limit. Moreover, when the content is excessive, there is a problem of exceeding the solid solution limit to form coarse precipitates. Therefore, the content is preferably limited to 0.03% or less.

On the other hand, vanadium, like niobium, forms carbonitrides and adds 0.05% or more as an element that serves to limit grain boundary movement of austenite and ferrite. However, since the carbonitride acts as a starting point of destruction and may lower impact toughness, in particular, low temperature impact toughness, it is preferable to keep the solubility limit. Therefore, the content is preferably limited to 0.3% or less.

The remainder of the alloy composition is iron (Fe). In addition, the crude wire rod of the present invention may contain other impurities that may be included in the industrial production of steels in general. These impurities are known to those of ordinary skill in the art to which the present invention belongs, so the present invention does not particularly limit the type and content thereof.

However, Ti corresponds to a representative impurity that should be suppressed as much as possible in order to obtain the effect of the present invention.

Ti: 0.005% or less

Titanium is a carbonitride forming element and forms carbonitrides at temperatures higher than Nb and V. Therefore, although titanium may be included in steel, it may be advantageous to fix C and N. However, Nb and / or V may be precipitated using Ti carbon nitride as a nucleus to deteriorate cold workability by forming a large amount of coarse carbonitride in the matrix. have. Therefore, it is important to manage the upper limit, and in the present invention, it is preferable to manage the upper limit of the content of titanium to 0.005%, more preferably to 0.004%.

According to one embodiment, the carbon equivalent (Ceq) of the crude wire of the present invention may be 0.6 or more and 0.7 or less. Here, the carbon equivalent (Ceq) may be defined by the following formula (1). If the carbon equivalent (Ceq) is less than 0.6 or more than 0.7, it may be difficult to secure the target strength.

[Equation 1]

Ceq = [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12

(Where [C], [Si], [Mn] and [Cr] each represent the content of the element in%)

The crude wire rod of the present invention includes ferrite and pearlite as its microstructure.

In the crude wire of the present invention, the phase fraction (vol%) of pearlite satisfies the following relations (1) and (2).

Relation 1 VP ₂ / VP ₁ ≤ 1.4

Equation 2 50≤ (15VP ₁ + VP ₂ ) / 16≤70

(Where VP ₁ and VP ₂ are each a pearlite fraction (area%) in the area from the surface of the wire rod to a position 3 / 8D in the diameter (D) direction of the wire rod in a cross section perpendicular to the longitudinal direction of the wire rod and the diameter of the wire rod (D Means the pearlite fraction (area%) in the area from the 3 / 8D position to the center of the wire rod)

The relational equation 1 is a control formula related to the pearlite phase fractions of the wire rods. In general, when the segregation promoting elements such as Mn and Cr are actively used in the medium carbon steel of the present invention, the variation of the pearlite structure in the central segregation portion and the non-segregation portion is very large. In the case of non-coated steel which secures strength by drawing, this deviation becomes larger after drawing, resulting in deterioration of cold workability. In the present invention, excellent cold workability is secured by controlling VP ₂ / VP ₁ to 1.4 or less.

On the other hand, as described above, there may be various ways to control the VP ₂ / VP ₁ to 1.4 or less, so the independent claims of the present invention do not particularly limit it. However, as an example, VP ₂ / VP ₁ can be controlled to 1.4 or less as described above by appropriately controlling the bloom heating temperature and the holding time as described below.

The relational equation 2 is a control formula related to the average pearlite phase percentage of the wire rod, and if the (15VP ₁ + VP ₂ ) / 16 value is less than 50 or more than 70, it may be difficult to simultaneously secure the desired cold workability and strength.

In addition, in the non-coarse wire rod of the present invention, the average lamellar spacing (μm) of pearlite satisfies the following relations 3 and 4.

[Relationship 3] DL ₁ / DL ₂ ≤1.4

Relational Expression 4 0.1 ≦ (15DL ₁ + DL ₂ ) /16≦0.3

(Wherein DL ₁ and DL ₂ are each the average lamellar spacing (μm) of pearlite in the region from the surface of the wire rod to the 3 / 8D position in the diameter (D) direction of the wire rod in a cross section perpendicular to the longitudinal direction of the wire rod and the diameter of the wire rod. Mean average lamellar spacing (μm) of pearlite in the area from the 3 / 8D position in the direction (D) to the center of the wire rod)

The relational equation 3 is a control formula related to the spacing of pearlite lamellae for each part of the wire rod. In the medium carbon steel which actively utilizes the pearlite structure, the pearlite lamella spacing also has a great influence on physical properties. That is, the finer the lamellar spacing, the higher the strength of the wire rod, and the greater the difference between the lamellar spacing between the central segregation portion and the non-segregation portion, the worse the variation in physical properties. In the present invention, excellent cold workability is secured by controlling DL ₁ / DL ₂ to 1.4 or less.

On the other hand, as described above there can be a number of ways to control the DL ₁ / DL ₂ 1.4 or less in the independent claim of the present invention is not particularly limited. However, as an example, DL ₁ / DL ₂ can be controlled to 1.4 or less as described above by appropriately controlling the wire rolling temperature and the cooling rate as described below.

Equation 4 is a control equation relating to the average lamellar spacing of the wire rod, if the (15DL ₁ + DL ₂ ) / 16 value is less than 0.1 or more than 0.3, it may be difficult to simultaneously secure the desired cold workability and strength.

According to one example, the intensity deviation of the pearlite may satisfy the following relation 5.

[Relationship 5]

(VP ₂ / VP ₁ ) × (√ (DL ₁ / DL ₂ )) ≤1.5

As described above, in general, when Mn and Cr are actively used to secure strength and cold workability in a medium-carbon non-coated steel, variations in physical properties are caused over the wire C cross section due to segregation of the center of Mn and Cr. This is even greater after the draw process, which significantly increases the likelihood of internal cracking during forging for final product manufacture. Equation 5 is a control equation related to the intensity deviation of the pearlite for each part of the wire rod, the present inventors through a number of experiments when the (VP ₂ / VP ₁ ) × (√ (DL ₁ / DL ₂ )) value is 1.5 or less large fresh processing amount Nevertheless, it was confirmed that molding through cold forging was possible without the occurrence of internal cracking.

According to one example, the average composition of the oxide-based inclusions in the region from the position 3 / 8D in the diameter (D) direction of the wire rod to the center of the wire rod in a cross section perpendicular to the longitudinal direction of the wire rod may satisfy the following relations 6 to 8. .

Equation 6 30≤ [Al ₂ O ₃ ] ≤70

[Expression 7] 20≤ [SiO _2] ≤40

Equation 8 10≤CaO + MgO≤20

(Wherein [Al ₂ O ₃ ], [SiO ₂ ], [CaO] and [MgO] each represent the content (% by weight) of the inclusions)

Here, the reason for controlling the composition of the non-metallic inclusion is to provide a wire having further improved drawing and cold workability when drawing the wire continuously by reducing the hard inclusion (non-viscous inclusion) in the wire to a minimum. In particular, the present inventors confirmed that when the content of a specific oxide in the oxide inclusions inevitably mixed among steel materials increases, the inclusions become hard to deteriorate cold workability.

Hereinafter, the reason etc. which determined content of each oxide which comprises an oxide type interference | inclusion are explained in full detail. In order to reduce and soften the number of non-viscosity inclusions aimed at by the present invention, a combination of plural oxide compositions is required. First, combinations of ternary or higher oxides comprising at least one of CaO or MgO, including necessarily Al ₂ O ₃ and SiO ₂ , have been found to be optimal.

Al ₂ O ₃ : 30 ~ 70%

Al ₂ O ₃ is a useful component for making oxide inclusions lower melting point and softer. Al ₂ O ₃ inevitably exists in steel or slag, but if the Al ₂ O ₃ amount in the slag is properly managed, the melting point of the inclusions is lowered, which results in elongation and thus becomes fine during the rolling process. It is said to be advantageous for the soundness of the material. In order to effectively exert the above action, the Al ₂ O ₃ content is set to 30% or more. Preferably it is 35% or more, More preferably, it is 40% or more. However, when the Al ₂ O ₃ content is too large, then the hard and difficult to alumina-based inclusions fine, too, because the starting point of fracture or breakage is difficult to be finely divided in the hot rolling step, the upper limit of 70%. Preferably it is 65%, More preferably, it is 60%.

SiO ₂ : 20 ~ 40%

SiO ₂ is inevitably present in steel or slag together with Al ₂ O ₃ described above, and is an important oxide that forms the basis of a polycyclic oxide. If the content is less than 20%, a good combination with other oxides as an inclusion of the poly-based oxide cannot be obtained, and if it exceeds 40%, there is a high possibility that hard inclusions are formed. Therefore, it is preferable to make the lower limit into 20% and an upper limit into 40%.

CaO + MgO: 10-20%

MgO and CaO are components necessary for making the inclusions an optimal composite composition and lowering the melting point. Although both MgO and CaO alone have high melting points, there is an effect of lowering the melting point of the polycyclic oxide. In order to express the effect, it is necessary to contain 10% or more in total. However, if the sum of these contents is excessive, the melting point of inclusions increases, or MgO and CaO crystals are formed, which makes it difficult to refine the hot rolling process, which may be a starting point of breakage or damage. It should be less than%.

According to an example, the average diameter of the oxide-based inclusions may be 8 μm or less (excluding 0 μm), and the maximum diameter of the oxide-based inclusions may be 15 μm or less (excluding 0 μm).

In this way, by miniaturizing the non-metallic inclusion made of the oxide, the origin of destruction can be reduced. Here, the average diameter and the maximum diameter of the nonmetallic inclusions mean the average or maximum circular diameter of the particles detected by observing the longitudinal cross section of the wire rod, and the maximum diameter of the nonmetallic inclusions is determined as follows. It was. An optical microscope was used to observe 800 fields of view at 400 times, and the maximum diameter of the nonmetallic inclusions in each field of view was arranged on a Gumble probability sheet, and an extreme value equivalent to 50000 mm 2 was calculated to obtain the maximum diameter. It was.

On the other hand, the method of controlling the average composition and the diameter of the oxide inclusions as described above may be various, so the present invention is not particularly limited thereto. However, for example, by adjusting dissolved Al, Si, dissolved Mg, and Ca concentration in molten steel, the average composition and diameter of the oxide-based inclusions formed can be controlled.

The crude wire rod of the present invention described above can be produced by various methods, the manufacturing method is not particularly limited. However, it may be prepared by the following method as an embodiment.

Hereinafter, another aspect of the present invention will be described in detail with respect to the production method of non-coated wire having excellent strength and impact toughness.

First, a bloom satisfying the above component system is heated and then rolled into steel sheets to obtain a billet.

It is preferable that it is 1200-1300 degreeC, and, as for the heating temperature of a bloom, it is more preferable that it is 1200-1250 degreeC. If the heating temperature of the bloom is less than 1200 ° C, there is a possibility that the hot rolling property may be lowered. Furthermore, since the segregation of the central segregation elements such as C, Mn, and Cr is not sufficiently achieved, the variation of the structure of the segregation and non-segregation parts increases, resulting in cold workability. May cause deterioration. On the other hand, when it exceeds 1300 ℃ there is a fear that ductility deterioration due to coarsening of austenite.

According to one example, upon heating of the bloom, the holding time at the heating temperature may be at least 240 minutes. If the holding time is less than 240 minutes, the homogenization treatment may not be sufficient. On the other hand, the longer the holding time at the heating temperature, the more favorable for homogenization and the lower the segregation. In the present invention, the upper limit of the holding time is not particularly limited.

Next, after reheating the billet, the wire rod is rolled to obtain a crude wire rod.

It is preferable that it is 1050-1250 degreeC, and, as for the reheating temperature of a billet, it is more preferable that it is 1100-1200 degreeC. If the reheating temperature of the billet is less than 1050 ° C., there is a concern that the heat deformation resistance may increase, leading to a decrease in productivity. On the other hand, if the heating temperature exceeds 1250 ° C., the ferrite grains may be excessively coarse to reduce ductility. There is concern.

According to one example, when reheating the billet, the holding time at the reheating temperature may be 60 to 240 minutes or more. If the holding time is less than 60 minutes, the homogenization treatment may not be sufficient. On the other hand, the longer the holding time at the reheating temperature is advantageous for the homogenization of segregation promoting elements, but the austenite structure may grow excessively and the ductility may be reduced, so the upper limit of the holding time may be limited to 240 minutes.

When the wire is rolled, the finish rolling temperature may be 750 ~ 900 ℃, preferably 800 ~ 880 ℃. If the finish rolling temperature is less than 750 ℃, there is a fear that the deformation resistance increases due to the increase in strength due to the refinement of ferrite grains, on the other hand, if it exceeds 900 ℃ ferrite grains are too coarse to deteriorate the ductility, the lamellar spacing of pearlite There is a fear that the fineness is reduced and the cold workability is deteriorated.

Thereafter, the uncoated wire rod is wound up and then cooled.

According to one example, the winding temperature of the non-coarse wire may be 750 ~ 900, more preferably 800 ~ 850. If the coiling temperature is less than 750, the martensite generated during the cooling may not be recovered by reheating, and some martensite is formed to form a hard and soft steel, which may reduce cold workability. On the other hand, when the coiling temperature exceeds 900, a thick scale is formed on the surface thereof, and troubles are easily generated during descaling, and cooling time is prolonged, which may lower productivity.

The cooling rate of the non-coated wire rod may be 0.3 ~ 1 / sec, preferably 0.3 ~ 0.8 / sec or less. This is to stably form a ferrite and pearlite composite structure. If the cooling rate is less than 0.3 / sec, the lamellar spacing of the pearlite tissue may be widened, leading to a lack of ductility, and if it exceeds 1 / sec, the ferrite fraction is reduced. In addition, there is a fear that the pearlite lamellar spacing becomes fine and the cold forging property deteriorates.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the description of these examples is only for illustrating the practice of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

( Example )

A bloom having an alloy composition as shown in Table 1 was heated at 1250 ° C. for 5 hours, and then rolled into steel sheets under a finish rolling temperature condition of 1150 ° C. to obtain a billet. Thereafter, the billet was heated at 1200 ° C. for 3 hours, and then hot-rolled to Φ 25 mm to prepare a wire. At this time, the finish rolling temperature was set at 850 ° C., and the rolling ratio was constant at 80%. Then, after winding up at a temperature of 800 ℃, it was cooled at a rate of 0.5 ℃ / sec.

Then, the pearlite fraction and lamellar spacing of the cooled wire, the composition and size of the inclusions were measured and shown in Tables 2 and 3.

In addition, the cold workability of the cooled wire rod was evaluated and shown in Table 4 together. For cold workability evaluation, the notched compression specimens were evaluated for the presence of cracks by the compression test of true strain 0.7, and if the cracks did not occur, "GO" and if the cracks occurred, "NG".

Meanwhile, steel wires were prepared by applying the amount of wire drawing of 10%, 15%, and 20% to each wire, respectively, and the cold workability of the manufactured steel wire was evaluated and shown in Table 4 below. The specific evaluation method is as above-mentioned.

Steel grade	Alloy composition (% by weight)												Ceq
	C	Si	Mn	P	S	Cr	Al	Nb	V	Ti	N	O
Inventive Steel 1	0.30	0.23	1.52	0.011	0.0042	0.00	0.03	0.025			0.0042	0.0007	0.630
Inventive Steel 2	0.33	0.21	1.48	0.011	0.0044	0.25	0.03		0.11		0.0045	0.0008	0.670
Invention Steel 3	0.35	0.17	1.33	0.010	0.0055	0.13	0.02	0.010	0.12		0.0044	0.0010	0.646
Inventive Steel 4	0.37	0.16	1.26	0.012	0.0043	0.11	0.04		0.09	0.003	0.0052	0.0005	0.649
Inventive Steel 5	0.39	0.15	1.02	0.010	0.0052	0.00	0.02	0.008	0.11	0.002	0.0044	0.0011	0.611
Comparative Steel 1	0.32	0.26	1.69	0.010	0.0058	0.00	0.03	0.023			0.0058	0.0027	0.687
Comparative Steel 2	0.34	0.24	1.51	0.010	0.0055	0.34	0.03		0.17		0.0055	0.0025	0.697
Comparative Steel 3	0.38	0.18	1.48	0.012	0.0062	0.22	0.02	0.018	0.14		0.0053	0.0019	0.714
Comparative Steel 4	0.42	0.16	1.45	0.010	0.0047	0.16	0.03		0.08	0.018	0.0045	0.0011	0.741
Comparative Steel 5	0.45	0.17	1.37	0.012	0.0053	0.00	0.02	0.013	0.11	0.015	0.0050	0.0020	0.743
Here, Ceq = [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12, wherein [C], [Si], [Mn] and [Cr] are each the content of the element ( Weight percent)

Steel grade	Microstructure	①	②	③	④	⑤	Remarks
Inventive Steel 1	F + P	1.06	58.3	1.33	0.22	1.22	Inventive Example 1
Inventive Steel 2	F + P	1.14	60.6	1.28	0.17	1.28	Inventive Example 2
Invention Steel 3	F + P	1.20	62.7	1.22	0.19	1.32	Inventive Example 3
Inventive Steel 4	F + P	1.27	64.1	1.16	0.15	1.36	Inventive Example 4
Inventive Steel 5	F + P	1.35	66.9	1.05	0.12	1.38	Inventive Example 5
Comparative Steel 1	F + P	1.17	59.8	1.45	0.23	1.40	Comparative Example 1
Comparative Steel 2	F + P	1.25	61.6	1.41	0.18	1.48	Comparative Example 2
Comparative Steel 3	F + P	1.34	65.3	1.33	0.14	1.54	Comparative Example 3
Comparative Steel 4	F + P	1.46	70.2	1.27	0.12	1.64	Comparative Example 4
Comparative Steel 5	F + P	1.55	72.5	1.19	0.09	1.69	Comparative Example 5
Herein, F means ferrite and P means pearlite. ① = VP 2 / VP 1 and ② = (15VP 1 + VP 2 ) / 16 ③ = DL 1 / DL 2 , ④ = (15DL 1 + DL 2 ) / 16, and ⑤ = (VP 2 / VP 1 ) × (√ (DL 1 / DL 2 )) Means.

Steel grade	Inclusion composition (% by weight)					Inclusion average diameter (μm)	Inclusion diameter (μm)	Remarks
	Al 2 O 3	SiO 2	CaO	MgO	Sum
Inventive Steel 1	64	22	7	6	99	7.1	9.1	Inventive Example 1
Inventive Steel 2	55	25	8	5	93	7.5	7.3	Inventive Example 2
Invention Steel 3	40	28	5	7	80	5.8	10.5	Inventive Example 3
Inventive Steel 4	36	21	8	8	73	6.5	11.3	Inventive Example 4
Inventive Steel 5	32	26	10	4	72	4.6	9.8	Inventive Example 5
Comparative Steel 1	82	11	2	3	98	6.2	16.7	Comparative Example 1
Comparative Steel 2	63	17	One	5	86	7.6	15.6	Comparative Example 2
Comparative Steel 3	52	23	5	2	82	8.8	11.5	Comparative Example 3
Comparative Steel 4	37	30	7	3	77	9.4	10.4	Comparative Example 4
Comparative Steel 5	22	35	10	5	72	11.3	12.2	Comparative Example 5

Steel grade	Cold workability				Remarks
	Wire rod	Steel wire (10%)	Steel wire (15%)	Steel wire (20%)
Inventive Steel 1	GO	GO	GO	GO	Inventive Example 1
Inventive Steel 2	GO	GO	GO	GO	Inventive Example 2
Invention Steel 3	GO	GO	GO	GO	Inventive Example 3
Inventive Steel 4	GO	GO	GO	GO	Inventive Example 4
Inventive Steel 5	GO	GO	GO	GO	Inventive Example 5
Comparative Steel 1	GO	GO	NG	NG	Comparative Example 1
Comparative Steel 2	GO	GO	NG	NG	Comparative Example 2
Comparative Steel 3	GO	GO	GO	NG	Comparative Example 3
Comparative Steel 4	GO	GO	GO	NG	Comparative Example 4
Comparative Steel 5	GO	GO	GO	NG	Comparative Example 5

As can be seen from Table 4, in the case of Inventive Examples 1 to 8 satisfying the alloy composition and manufacturing conditions proposed in the present invention, not only satisfy the conditions of the relational formula 1 to 5, but also the composition, average diameter and maximum diameter of the non-metallic inclusions This was controlled under the conditions proposed in the present invention, so that no cracks were generated inside the wire after drawing, thereby securing strength and excellent cold workability. On the other hand, in the case of Comparative Examples 1 to 5, cracks were generated after the fresh processing as not satisfying at least one or more of the conditions proposed by the present invention, and the cold workability was inferior to the invention examples.

Claims (11)

1. By weight%, C: 0.3-0.4%, Si: 0.05-0.3%, Mn: 0.8-1.8%, Cr: 0.5% or less, P: 0.02% or less, S: 0.02% or less, sol.Al: 0.01-0.05 %, N: 0.01% or less and O: 0.0001% to 0.003%, Nb: 0.005% to 0.03%, and V: 0.05% to 0.3%, and include residual Fe and unavoidable impurities,

   Microstructures include ferrite and pearlite,

   The phase fraction of the pearlite satisfies the following relations 1 and 2, the average lamellar spacing of the pearlite satisfies the following relations 3 and 4.

   Relation 1 VP ₂ / VP ₁ ≤ 1.4

   Equation 2 50≤ (15VP ₁ + VP ₂ ) / 16≤70

   [Relationship 3] DL ₁ / DL ₂ ≤1.4

   Relational Expression 4 0.1 ≦ (15DL ₁ + DL ₂ ) /16≦0.3

   (Where VP ₁ and VP ₂ are each a pearlite fraction (area%) in the area from the surface of the wire rod to a position 3 / 8D in the diameter (D) direction of the wire rod in a cross section perpendicular to the longitudinal direction of the wire rod and the diameter of the wire rod (D ) Means the pearlite fraction (area%) in the area from the 3 / 8D position to the center of the wire rod, and each of DL ₁ and DL ₂ is the diameter of the wire rod (D) in the cross section perpendicular to the longitudinal direction of the wire rod. Mean lamellar spacing (μm) of pearlite in the area up to 3 / 8D) direction, and the average lamellar spacing (μm) of pearlite in the area from the 3 / 8D position in the direction of diameter (D) to the center of the wire. )
2. The method of claim 1,

   The strength variation of the pearlite is an unstructured wire rod that satisfies the following relational formula 5.

   [Relationship 5]

   (VP ₂ / VP ₁ ) × (√ (DL ₁ / DL ₂ )) ≤1.5
3. The method of claim 1,

   The unavoidable impurity includes Ti, and by weight%, Ti: 0.005% or less of the crude wire rod.
4. The method of claim 1,

   A crude wire having a carbon equivalent (Ceq) of 0.6 or more and 0.7 or less.
5. The method of claim 1,

   An unstructured wire rod in which the average composition of the oxide-based inclusions in the region from the position 3 / 8D in the diameter (D) direction of the wire rod to the center of the wire rod in a cross section perpendicular to the longitudinal direction of the wire rod satisfies the following relations 6 to 8.

   [Relationship 6]

   30≤ [Al ₂ O ₃ ] ≤70

   [Relationship 7]

   20≤ [SiO ₂ ] ≤40

   [Relationship 8]

   10≤ [CaO] + [MgO] ≤20

   (Wherein [Al ₂ O ₃ ], [SiO ₂ ], [CaO] and [MgO] each represent the content (% by weight) of the inclusions)
6. The method of claim 5,

   The crude wire rod having an average diameter of the oxide-based inclusions is 8 μm or less.
7. The method of claim 5,

   The non-coarse wire rod having a maximum diameter of the oxide-based inclusions is 15 μm or less.
8. By weight%, C: 0.3-0.4%, Si: 0.05-0.3%, Mn: 0.8-1.8%, Cr: 0.5% or less, P: 0.02% or less, S: 0.02% or less, sol.Al: 0.01-0.05 %, O: 0.0001 to 0.003% or less, and N: 0.01% or less, Nb: 0.005 to 0.03%, and V: 0.05 to 0.3% of one or more, including the balance Fe and unavoidable impurities, carbon Heating a bloom having an equivalent weight (Ceq) of 0.6 or more and 0.7 or less to a heating temperature of 1200 to 1300 ° C., maintaining the heating temperature for 240 minutes or more, and then rolling the steel sheet to obtain a billet;

   After reheating the billet, rolling the wire under conditions of a finish rolling temperature of 750 to 900 ° C. to obtain a wire; And

   After winding the wire, cooling at a rate of 0.3 ~ 1 ℃ / sec;

   Method for producing a non-coarse wire rod comprising a.
9. The method of claim 8,

   The unavoidable impurity includes Ti, and by weight%, Ti: 0.005% or less of the crude wire rod.
10. The method of claim 8,

    The reheating temperature of the billet is 1050 ~ 1200 ℃ uncoated wire.
11. The method of claim 8,

    The winding temperature of the wire rod is 750 ~ 900 ℃ non-coarse wire rod.