JP2019502815A - Non-heat treated wire excellent in strength and cold workability and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-heat treated wire capable of ensuring excellent strength and cold forgeability without performing an additional heat treatment, and a method for producing the same.
[Solution]
In mass%, C: 0.3 to 0.4%, Si: 0.05 to 0.3%, Mn: 0.8 to 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%, and Nb: 0.005 to 0.03% and V: 0.05 -1% or more of ~ 0.3%, the balance Fe and unavoidable impurities, ferrite (ferrite) and pearlite as a fine structure, the phase fraction of the pearlite is represented by the following relational expressions 1 and 2 A non-heat treated wire that satisfies the relational expressions 3 and 4 is disclosed.

Description

  The present invention relates to a non-heat treated wire excellent in strength and cold workability and a method for producing the same, and more specifically, a non-heat treated wire excellent in strength and cold workability suitable for use as a material for machine parts. The present invention relates to a wire rod and a manufacturing method thereof.

The cold working method is not only more productive than the hot working method and machine cutting method, but also has a great effect on reducing heat treatment costs, so it is widely used in the manufacture of machine parts such as bolts and nuts. Has been.
However, as described above, in order to manufacture machine parts using the cold working method, it is essential that the cold workability of the steel material is essentially excellent. More specifically, it is required that the deformation resistance during cold working is low and the ductility is excellent. When the deformation resistance of steel is high, the tool life used during cold working is reduced, and when the ductility of steel is low, cracking during cold working is likely to occur, causing defective products. Because.
Therefore, ordinary steel for cold working is subjected to spheroidizing annealing heat treatment before cold working. This is because when the spheroidizing annealing heat treatment is performed, the steel material is softened, the deformation resistance is reduced, the ductility is improved, and the cold workability is improved. However, in this case, additional costs are incurred and the production efficiency is reduced, and therefore there is a demand for the development of a non-heat treated wire that can ensure excellent cold workability without additional heat treatment.

However, in general, in medium carbon steel containing 0.3% by mass or more of carbon, it is known that when the pearlite fraction exceeds 50%, the cold workability is inferior due to matrix strengthening by the pearlite structure. It has been. In particular, when segregation facilitating elements such as Mn and Cr are used together to ensure strength, the structural difference between the central segregation part and the non-segregation part increases, and the strength of non-heat treated steel that secures strength by wire drawing is increased. In such a case, the deviation after wire drawing is further increased, so that it is difficult to ensure cold forgeability. In addition, in the high-strength non-heat treated steel of medium carbon steel or higher, in addition to the structural imbalance due to segregation at the center, the influence of oxide-based nonmetallic inclusions at the center is very large.
Further, when matrix strengthening is performed by center segregation, the sensitivity of such non-metallic inclusions may be further increased and adversely affect cold workability. Therefore, in developing a high-strength non-tempered steel of medium carbon steel or higher, it is necessary to examine the structural difference due to such center segregation and the influence of center inclusions.

  An object of the present invention is to provide a non-heat treated wire capable of ensuring excellent strength and cold forgeability without performing an additional heat treatment, and a method for producing the same.

In order to achieve the above object, the present invention provides, in mass%, C: 0.3 to 0.4%, Si: 0.05 to 0.3%, Mn: 0.8 to 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%, and Nb: 0.005 to 0.03% and V: 0.05 -1% or more of ~ 0.3%, the balance is Fe and inevitable impurities, and the microstructure includes ferrite and pearlite, the phase fraction of the pearlite is represented by the following relational expression 1 and 2 and the average lamella spacing of the pearlite satisfies the following relational expressions 3 and 4.
[Relational expression 1] VP ₂ / VP ₁ ≦ 1.4
[Relational expression 2] 50 ≦ (15VP ₁ + VP ₂ ) / 16 ≦ 70
[Relational expression 3] DL ₁ / DL ₂ ≦ 1.4
[Relational Expression 4] 0.1 ≦ (15DL ₁ + DL ₂ ) /16≦0.3
(Here, VP ₁ and VP ₂ are each a cross section perpendicular to the length direction of the wire, and the pearlite fraction (area%) in the region from the surface of the wire to the position 3 / 8D in the diameter (D) direction of the wire. ), And the pearlite fraction (area%) in the region from the 3 / 8D position in the diameter (D) direction of the wire to the center of the wire, and DL ₁ and DL ₂ are each perpendicular to the length direction of the wire The average lamella spacing (μm) of pearlite in the region from the surface of the wire to the position of 3 / 8D in the diameter (D) direction of the wire, and the wire from the position of 3 / 8D in the diameter (D) direction of the wire Mean lamella spacing (μm) of pearlite in the region to the center of

  Moreover, this invention is mass%, 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%, and N: 0.01% or less, and Nb: 0.005 to 0.03% and V: 0.05 Heating temperature of 1200 ° C. to 1300 ° C., including one or more of ˜0.3%, the balance being Fe and inevitable impurities, and a carbon equivalent (Ceq) of 0.6 to 0.7 And after maintaining at the above heating temperature for 240 minutes or more, rolling the steel billet to obtain a billet, and after reheating the billet, the wire rod under conditions of a finish rolling temperature of 750 ° C. to 900 ° C. It includes a step of rolling to obtain a wire, and a step of cooling at a rate of 0.3 ° C. to 1 ° C./sec after winding the wire.

  ADVANTAGE OF THE INVENTION According to this invention, even if a spheroidizing annealing heat treatment is abbreviate | omitted, the non-heat treated wire which can fully suppress the deformation resistance at the time of cold working can be provided.

  The present inventors have studied from various angles in order to provide a wire having predetermined strength and hardness after wire drawing and capable of ensuring excellent cold workability. As a result, by optimizing the alloy composition and manufacturing method of the medium carbon steel wire, a composite structure of ferrite and pearlite is ensured as the microstructure of the wire, and the pearlite phase fraction and pearlite lamella by position of the wire are secured. The inventors have found that a high-strength wire that does not deteriorate cold workability even after wire drawing can be provided by appropriately controlling the spacing and the like, and the present invention has been completed.

Hereinafter, the non-heat treated wire excellent in strength and cold workability of the present invention will be described in detail.
First, the alloy components and composition range of the non-heat treated wire will be described in detail. The content of each component described below is based on mass unless otherwise specified.
C: 0.3 to 0.4%
Carbon plays a role of improving the strength of the wire. In this invention, in order to show this effect, it is preferable to contain 0.3% or more. However, when the content is excessive, there is a problem that the deformation resistance of the steel increases rapidly, and as a result, the cold workability deteriorates. Therefore, the upper limit of the carbon content is preferably 0.4%.

Si: 0.05-0.3%
Silicon is an element useful as a deoxidizer. In this invention, in order to show this effect, it is preferable to contain 0.05% or more. However, when the content is excessive, there is a problem that the deformation resistance of the steel rapidly increases due to solid solution strengthening, and as a result, the cold workability deteriorates. Therefore, the upper limit of the silicon content is preferably 0.3%, and more preferably 0.25%.

Mn: 0.8 to 1.8%
Manganese is an element useful as a deoxidizer and desulfurizer. In this invention, in order to show this effect, 0.8% or more is contained preferably, and 1.0% or more is more preferred. However, when the content is excessive, the strength of the steel itself becomes excessively high and the deformation resistance of the steel increases rapidly, resulting in a problem that cold workability deteriorates. Therefore, the upper limit of the manganese content is preferably 1.8%, and 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, the strength of the steel itself is not increased more than necessary, and carbides in the steel are precipitated to reduce the amount of dissolved carbon, thereby contributing to a decrease in dynamic strain aging due to the dissolved carbon. However, even if chromium is not added, there is no problem in securing the physical properties. On the other hand, when the content is excessive, the strength of the steel itself is excessively increased, and the deformation resistance of the steel is rapidly increased. As a result, there is a problem that cold workability is deteriorated. The chromium content is preferably 0.5% or less, and more preferably 0.4%.

P: 0.02% or less Phosphorus is an unavoidable impurity, and is an element that is segregated at the grain boundaries to lower the toughness of steel and reduce delayed fracture resistance. It is preferable to control the content as low as possible. Theoretically, the phosphorus content is advantageously controlled to 0%, but is necessarily contained in the production process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is managed to 0.02%.

S: 0.02% or less Sulfur is an impurity inevitably contained, segregates at the grain boundaries, greatly reduces the ductility of the steel, forms sulfides in the steel, delayed fracture resistance and stress Since it is an element that is a main cause of deteriorating the relaxation properties, it is preferable to control its content as low as possible. Theoretically, the sulfur content is advantageously controlled to 0%, but is necessarily contained in the production 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 to 0.05%
Soluble aluminum is an element usefully acting as a deoxidizer, and is added in an amount of 0.01% or more, preferably 0.015% or more, more preferably 0.02% or more. However, if the content exceeds 0.05%, the effect of refining the austenite grain size due to the formation of AlN is increased, and the cold workability is lowered. Therefore, in this invention, the upper limit of content of the said soluble aluminum is managed to 0.05%.

N: 0.01% or less Nitrogen is an inevitably contained impurity, and when its content is excessive, the amount of dissolved nitrogen increases and the deformation resistance of the steel increases rapidly. As a result, There is a problem that cold workability deteriorates. Theoretically, the nitrogen content is advantageously controlled to 0%, but is necessarily contained in the production process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the nitrogen content is preferably managed to 0.01%, more preferably 0.008%, more preferably 0.007 More preferably, it is controlled to%.

O: 0.0001 to 0.003%
Oxygen is present in the wire in the form of non-metallic inclusions and is usually contained in an amount of 0.0001% or more. By the way, such a non-metallic inclusion becomes a starting point of fracture, and reduces the fatigue strength and cold forgeability of steel. In particular, when strength is ensured by wire drawing as in the case of non-heat treated steel, breakage starting from non-metallic inclusions tends to occur at the center of the wire. In addition, according to the research results of the present inventors, in the case of a wire with an oxygen content exceeding 0.003% in steel, the amount of non-metallic inclusions increases, and in a processed material for strict use, The disconnection avoidance is not enough. Therefore, in the present invention, the upper limit of oxygen is controlled to 0.003%, more preferably 0.001%, and still more preferably 0.0008%.

One or more of Nb: 0.005 to 0.03% and V: 0.05 to 0.3% Niobium forms carbonitrides and plays a role in limiting grain boundary migration of austenite and ferrite It is an element, and 0.005% or more is added. However, since the carbonitride functions as a fracture starting point and may reduce impact toughness, particularly low temperature impact toughness, it is preferable to add the carbonitride while keeping the solubility limit. In addition, when the content is excessive, the solid solution limit is exceeded, so that there is a problem that coarse precipitates are formed. Therefore, the niobium content is preferably limited to 0.03% or less.

  On the other hand, vanadium is an element that forms carbonitrides like niobium and plays a role in restricting the grain boundary movement of austenite and ferrite, and is added at 0.05% or more. However, the carbonitride functions as a fracture starting point and may reduce impact toughness, particularly low temperature impact toughness. Therefore, it is preferable to add the carbonitride while keeping the solubility limit. Accordingly, the vanadium content is preferably limited to 0.3% or less.

The other balance of the alloy composition is iron (Fe). Furthermore, the non-heat treated wire of the present invention may be composed of other impurities during the industrial production process of normal steel. Since such impurities can be understood by anyone who has ordinary knowledge in the technical field to which the present invention belongs, the present invention does not particularly limit the type and content thereof.
However, since Ti corresponds to a typical impurity whose content must be suppressed as much as possible in order to obtain the effects of the present invention, this will be briefly described as follows.

Ti: 0.005% or less Titanium is an element forming carbonitride, and forms carbonitride at a temperature higher than Nb and V. Therefore, when titanium is contained in the steel, it may be advantageous for fixing C and N, but Nb and / or V precipitates with the carbonitride of Ti as a nucleus and coarse carbonitride in the base. Since a large amount of is formed, cold workability may be deteriorated. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the titanium content is preferably managed to 0.005%, more preferably 0.004%.

According to an example, the carbon equivalent (Ceq) of the non-heat treated wire of the present invention is 0.6 or more and 0.7 or less. Here, the carbon equivalent (Ceq) is defined by the following formula 1. If the carbon equivalent (Ceq) is less than 0.6 or exceeds 0.7, it is difficult to ensure the target strength.
[Formula 1] Ceq = [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12
(Here, [C], [Si], [Mn] and [Cr] each mean the content (%) of the corresponding element)

The non-heat treated wire of the present invention includes ferrite and pearlite as its microstructure.
In the non-heat treated wire of the present invention, the pearlite phase fraction (% by volume) satisfies the following relational expressions 1 and 2.
[Relational expression 1] VP ₂ / VP ₁ ≦ 1.4
[Relational expression 2] 50 ≦ (15VP ₁ + VP ₂ ) / 16 ≦ 70
(Here, VP ₁ and VP ₂ are each a cross section perpendicular to the length direction of the wire, and the pearlite fraction (area%) in the region from the surface of the wire to the position 3 / 8D in the diameter (D) direction of the wire. ), And the pearlite fraction (area%) in the region from the 3 / 8D position in the diameter (D) direction of the wire to the center of the wire)

The relational expression 1 is a control expression related to the phase fraction of pearlite for each position of the wire. In general, in the case of medium carbon steel such as the present invention, when the segregation facilitating elements such as Mn and Cr are actively used, the structural difference between the central segregation part and the non-segregation part becomes very large, and the strength is increased by wire drawing. The non-heat treated steel to be secured further increases the deviation after the wire drawing. As a result, the cold workability is deteriorated. Therefore, in the present invention, excellent cold workability is ensured by controlling VP ₂ / VP ₁ to 1.4 or less.
On the other hand, since there are various methods for controlling VP ₂ / VP ₁ to 1.4 or less as described above, there is no particular limitation in the independent claims of the present invention. However, as an example, VP ₂ / VP ₁ can be controlled to 1.4 or less as described above by appropriately controlling the heating temperature and holding time of the bloom as described later.

The relational expression 2 is a control expression relating to the phase fraction of the average pearlite of the wire. If the value of (15VP ₁ + VP ₂ ) / 16 is less than 50 or exceeds 70, it is difficult to ensure both the desired cold workability and strength.
In the non-heat treated wire of the present invention, the average lamella spacing (μm) of pearlite satisfies the following relational expressions 3 and 4.
[Relational expression 3] DL ₁ / DL ₂ ≦ 1.4
[Relational Expression 4] 0.1 ≦ (15DL ₁ + DL ₂ ) /16≦0.3
(Here, DL ₁ and DL ₂ are each a cross section perpendicular to the length direction of the wire, and the average lamella spacing of pearlite in the region from the surface of the wire to the position 3 / 8D in the diameter (D) direction of the wire ( μm), and the average lamella spacing (μm) of pearlite in the region from the 3 / 8D position in the diameter (D) direction of the wire to the center of the wire)

The relational expression 3 is a control expression relating to the lamella spacing of the pearlite for each position of the wire. In medium-carbon steel that actively uses the pearlite structure, the pearlite lamella spacing has a great influence on the physical properties as well as the pearlite fraction. That is, the finer the lamella spacing, the greater the strength of the wire, and the greater the difference in the lamella spacing between the central segregation part and the non-segregation part, the greater the difference in physical properties. Therefore, in the present _invention, by controlling the DL 1 / DL ₂ to 1.4 or less, to ensure excellent cold workability.
On the other hand, since there are various methods for controlling DL ₁ / DL ₂ to 1.4 or less as described above, there is no particular limitation in the independent claims of the present invention. However, as an example, DL ₁ / DL ₂ can be controlled to 1.4 or less as described above by appropriately controlling the rolling temperature and cooling rate of the wire as described later.

The relational expression 4 is a control expression related to the average lamella spacing of the wire. If the value of (15DL ₁ + DL ₂ ) / 16 is less than 0.1 or exceeds 0.3, it is difficult to ensure both the desired cold workability and strength.
According to an example, the intensity difference of the pearlite can satisfy the following relational expression 5.
[Relational Expression 5] (VP ₂ / VP ₁ ) × (√ (DL ₁ / DL ₂ )) ≦ 1.5

As described above, in general medium carbon non-tempered steel, when Mn and Cr are actively used to ensure strength and cold workability, the C of the wire is caused by segregation at the center of Mn and Cr. Differences in physical properties are introduced across the cross section. This becomes larger after the wire drawing process, and greatly increases the possibility of occurrence of internal cracks during the forging process for manufacturing the final product. The relational expression 5 is a control expression related to the difference in the intensity of pearlite according to the position of the wire. Through many experiments, the present inventors have found that the value of (VP ₂ / VP ₁ ) × (√ (DL ₁ / DL ₂ )) is 1.5 or less, but internal cracks despite a large amount of wire drawing. It was confirmed that forming by cold forging was possible without the occurrence of.

According to an example, the average composition of the oxide inclusions in the region from the position of 3 / 8D in the diameter (D) direction of the wire to the center of the wire in the cross section perpendicular to the length direction of the wire is expressed by the following relational expression 6 to Satisfy 8
[Relational Expression 6] 30 ≦ [Al ₂ O ₃ ] ≦ 70
[Relational Expression 7] 20 ≦ [SiO ₂ ] ≦ 40
[Relational Expression 8] 10 ≦ [CaO] + [MgO] ≦ 20
(Here, [Al ₂ O ₃ ], [SiO ₂ ], [CaO] and [MgO] each mean the content (mass%) of the corresponding inclusion)

  At this time, the reason for controlling the composition of the non-metallic inclusions is that the hard inclusions (non-viscous inclusions) in the wire are reduced to the minimum, and when the steel wire is continuously drawn to the wire. In order to provide a wire having further improved drawability and cold workability. In particular, the present inventors have confirmed that among oxide-based inclusions inevitably mixed in steel materials, inclusions harden and cold workability deteriorates when the content of specific oxides increases. did.

Below, the reason etc. which determined content of each oxide which comprises an oxide type inclusion are demonstrated in detail. In order to reduce and soften the number of non-viscous inclusions aimed at in the present invention, a combination of the composition of multi-component oxides is necessary. First, a combination of ternary or higher oxides that always contains Al ₂ O ₃ and SiO ₂ and contains either CaO or MgO is optimal.

Al ₂ O ₃ : 30 to 70%
Al ₂ O ₃ is a useful component for making the oxide inclusions have a lower melting point and a softness. Al ₂ O ₃ is inevitably present in steel or slag, but when the content of Al ₂ O ₃ in the slag is appropriately controlled, the melting point of inclusions is lowered, and as a result, stretchability is reduced. It is known that it is secured and becomes fine during the rolling process, which is advantageous for the soundness of the final material. In order to make such an effect effective, the content of Al ₂ O ₃ is set to 30% or more, preferably 35% or more, more preferably 40% or more. However, if the content of Al ₂ O ₃ is too large, alumina inclusions that are hard and difficult to be miniaturized are obtained, and are difficult to miniaturize in the hot rolling process, which becomes the starting point of destruction or breakage. %, Preferably 65%, more preferably 60%.

SiO _2: 20~40%
SiO ₂ is inevitably present in the steel or slag together with the Al ₂ O ₃ and is an important oxide serving as a base of the multi-component oxide. When the content of SiO ₂ is less than 20%, an excellent combination with other oxides cannot be obtained as inclusions of multi-component oxides. On the other hand, if it exceeds 40%, there is a high possibility that hard inclusions are formed. Therefore, it is preferable that the lower limit is 20% and the upper limit is 40%.

CaO + MgO: 10-20%
MgO and CaO are components necessary for making inclusions into an optimal composite composition and lowering the melting point. Both MgO and CaO have a high melting point, but have the effect of lowering the melting point of the multi-component oxide. In order to exhibit such an effect, it is necessary to contain 10% or more in total. However, if the total content of CaO + MgO is too large, the melting point of inclusions is increased, or crystals of MgO and CaO are generated, and it is difficult to refine in the hot rolling process, which can be the starting point of destruction or breakage. The upper limit of the total content is 20% or less.
According to an example, the average diameter of the oxide inclusions may be 8 μm or less (excluding 0 μm), and the maximum diameter of the oxide inclusions may be 15 μm or less (excluding 0 μm).

In this way, by starting to refine non-metallic inclusions made of oxide, the starting point of destruction can be reduced. Here, the average diameter and the maximum diameter of non-metallic inclusions mean the average or maximum equivalent circular diameter of particles detected by observing one cross section in the length direction of the wire. The maximum diameter of nonmetallic inclusions was determined using the following method. An 800 field of view was observed with an optical microscope at a magnification of 400 times, the maximum diameter of non-metallic inclusions in each field of view was shown on a Gumbel probability paper, and an extreme value equivalent to 50000 mm ² was calculated to be the maximum diameter.

On the other hand, as described above, since there are various methods for controlling the average composition and diameter of the oxide inclusions, the present invention is not particularly limited. However, as an example, the average composition and diameter of the oxide inclusions formed can be controlled by adjusting the concentrations of dissolved Al and Si in the molten steel and the concentrations of dissolved Mg and Ca.
The non-heat treated wire of the present invention described above can be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an example, it can be manufactured by the following method.

Hereinafter, the manufacturing method of the non-heat-treated wire excellent in strength and impact toughness of the present invention will be described in detail.
First, a bloom satisfying the above component system is heated, and then billet is obtained by rolling a billet.
The heating temperature of the bloom is preferably 1200 ° C to 1300 ° C, and more preferably 1200 ° C to 1250 ° C. If the heating temperature of the bloom is less than 1200 ° C., the hot rollability may be reduced. Further, the diffusion of elements such as C, Mn, and Cr that promote segregation in the central portion is not sufficiently performed, and the structural difference between the segregated portion and the non-segregated portion may increase, resulting in deterioration of cold workability. On the other hand, when the temperature exceeds 1300 ° C., ductility may be deteriorated due to coarsening of austenite.

  According to one example, when the bloom is heated, the holding time at the heating temperature is 240 minutes or more. If the holding time is less than 240 minutes, homogenization may not be sufficient. On the other hand, the longer the holding time at the heating temperature, the more advantageous for homogenization and the reduction of segregation. Therefore, in the present invention, the upper limit of the holding time is not particularly limited.

Next, after re-heating the billet, the wire rod is rolled to obtain a non-heat treated wire.
The reheating temperature of the billet is preferably 1050 ° C to 1250 ° C, and more preferably 1100 ° C to 1200 ° C. If the reheating temperature of the billet is less than 1050 ° C., the hot deformation resistance increases, which may cause a decrease in productivity. On the other hand, when the reheating temperature exceeds 1250 ° C., the ferrite crystal grains become excessively coarse and the ductility may be lowered.

  According to an example, when the billet is reheated, the holding time at the reheating temperature is 60 to 240 minutes or more. If the retention time is less than 60 minutes, the homogenization process may not be sufficient. On the other hand, the longer the holding time at the reheating temperature, the more advantageous for the homogenization of elements that promote segregation, but the austenite structure grows excessively and the ductility may decrease, so the upper limit of the holding time. Is limited to 240 minutes.

  The finish rolling temperature during wire rod rolling is 750 to 900 ° C, preferably 800 to 880 ° C. When the finish rolling temperature is less than 750 ° C., the ferrite crystal grain refinement may increase the strength and increase the deformation resistance. On the other hand, when it exceeds 900 ° C., ferrite crystal grains are excessively coarse and ductility is deteriorated, and the lamella spacing of pearlite is fine and cold workability may be deteriorated.

Thereafter, the non-heat treated wire is wound up and then cooled.
According to an example, the winding temperature of the non-heat treated wire is 750 to 900, and more preferably 800 to 850. When the coiling temperature is less than 750, martensite in the surface layer generated during cooling is not recovered by recuperation, and tempered martensite is generated to form a hard and brittle steel, resulting in a decrease in cold workability. There is a risk. On the other hand, when the coiling temperature exceeds 900, a thick scale is formed on the surface, and not only trouble is likely to occur at the time of descaling, but also the cooling time becomes long, and the productivity may be lowered.

  The cooling rate during cooling of the non-heat treated wire is 0.3 to 1 / sec, preferably 0.3 to 0.8 / sec or less. This is to stably form a composite structure of ferrite and pearlite. If the cooling rate is less than 0.3 / sec, the lamella spacing of the pearlite structure may be widened and the ductility may be insufficient. If it exceeds 1 / sec, the ferrite fraction decreases and the pearlite lamella spacing is reduced. There is a possibility that the cold forgeability may be deteriorated due to the fineness.

Hereinafter, examples of the present invention will be described in more detail. However, the description of the embodiment is for illustrating the practice of the present invention, and the present invention is not limited by the description of the embodiment. This is because the scope of rights of the present invention is determined by matters described in the claims and 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 a billet 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 produce a Φ25 mm wire. At this time, the finish rolling temperature of 850 ° C. and the rolling ratio of 80% were kept constant. Then, after winding up at the temperature of 800 degreeC, it cooled at the speed | rate of 0.5 degree-C / sec.
Thereafter, the ferrite fraction, the lamellar spacing, the composition and size of inclusions, etc. of the cooled wire were measured and shown in Tables 2 and 3.

Moreover, the cold workability of the cooled wire was evaluated and is shown in Table 4. Cold workability evaluated whether the crack generate | occur | produced by performing the compression test of true distortion 0.7 with respect to the notch compression test piece. At this time, the case where no cracks occurred was evaluated as “GO”, and the case where cracks occurred was evaluated as “NG”.
On the other hand, a steel wire was manufactured by applying a wire drawing amount of 10%, 15%, and 20% to each wire. Moreover, the cold workability of the manufactured steel wire was evaluated and is shown in Table 4 together. The specific evaluation method is as described above.

  As shown in Table 4, in the case of Invention Examples 1 to 8 satisfying the alloy composition and manufacturing conditions proposed in the present invention, not only all the conditions of the relational expressions 1 to 5 are satisfied, but also the composition and average of nonmetallic inclusions The diameter and the maximum diameter were controlled under the conditions proposed in the present invention, and no cracks were generated in the interior after wire drawing, so it was confirmed that excellent strength and cold workability could be secured. On the other hand, Comparative Examples 1 to 5 are cases in which at least one of the conditions proposed in the present invention is not satisfied, and cracks occurred inside after wire drawing, so that cold workability is higher than that of the inventive examples. You can see that it is inferior.

Hereinafter, the manufacturing method of the non-heat treated wire excellent in strength and cold workability of the present invention will be described in detail.
First, a bloom satisfying the above component system is heated, and then billet is obtained by rolling a billet.
The heating temperature of the bloom is preferably 1200 ° C to 1300 ° C, and more preferably 1200 ° C to 1250 ° C. If the heating temperature of the bloom is less than 1200 ° C., the hot rollability may be reduced. Further, the diffusion of elements such as C, Mn, and Cr that promote segregation in the central portion is not sufficiently performed, and the structural difference between the segregated portion and the non-segregated portion may increase, resulting in deterioration of cold workability. On the other hand, when the temperature exceeds 1300 ° C., ductility may be deteriorated due to coarsening of austenite.

Thereafter, the non-heat treated wire is wound up and then cooled.
According to one example, the coiling temperature of microalloyed wire is 750 to 900 ° C., more preferably 800 to 850 ° C.. When the coiling temperature is less than 750 ° C. , the martensite in the surface layer generated during cooling is not recovered by recuperation, and tempered martensite is generated to form a hard and brittle steel. May decrease. On the other hand, when the coiling temperature exceeds 900 ° C. , a thick scale is formed on the surface, and not only troubles are likely to occur at the time of descaling, but also the cooling time becomes long and productivity may be lowered.

Cooling rate during cooling of microalloyed wire is 0.3 to 1 ° C. / sec, preferably not more than 0.3 to 0.8 ° C. / sec. This is to stably form a composite structure of ferrite and pearlite. If the cooling rate is less than 0.3 ° C / sec, the lamella spacing of the pearlite structure may be widened and the ductility may be insufficient. If it exceeds 1 ° C / sec, the ferrite fraction decreases, There is a possibility that the lamella spacing becomes fine and the cold forgeability deteriorates.

Claims (11)

In mass%, C: 0.3 to 0.4%, Si: 0.05 to 0.3%, Mn: 0.8 to 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%, and Nb: 0.005 to 0.03% and V: 0.05 Including one or more of ~ 0.3%, the balance consisting of Fe and inevitable impurities,
As a microstructure, ferrite (ferrite) and pearlite (pearlite),
The pearlite phase fraction satisfies the following relational expressions 1 and 2, and the average lamella spacing of the pearlite satisfies the following relational expressions 3 and 4.
[Relational expression 1] VP ₂ / VP ₁ ≦ 1.4
[Relational expression 2] 50 ≦ (15VP ₁ + VP ₂ ) / 16 ≦ 70
[Relational expression 3] DL ₁ / DL ₂ ≦ 1.4
[Relational Expression 4] 0.1 ≦ (15DL ₁ + DL ₂ ) /16≦0.3
(Here, VP ₁ and VP ₂ are each a cross section perpendicular to the length direction of the wire, and the pearlite fraction (area%) in the region from the surface of the wire to the position 3 / 8D in the diameter (D) direction of the wire. ), And the pearlite fraction (area%) in the region from the 3 / 8D position in the diameter (D) direction of the wire to the center of the wire, and DL ₁ and DL ₂ are each perpendicular to the length direction of the wire The average lamella spacing (μm) of pearlite in the region from the surface of the wire to the position of 3 / 8D in the diameter (D) direction of the wire, and the wire from the position of 3 / 8D in the diameter (D) direction of the wire Mean lamella spacing (μm) of pearlite in the region to the center of The non-heat treated wire according to claim 1, wherein the intensity difference of the pearlite satisfies the following relational expression 5.
[Relational Expression 5] (VP ₂ / VP ₁ ) × (√ (DL ₁ / DL ₂ )) ≦ 1.5   2. The non-heat treated wire according to claim 1, wherein the inevitable impurities include Ti and are suppressed to Ti: 0.005% or less by mass%.   The non-heat treated wire according to claim 1, wherein a carbon equivalent (Ceq) is 0.6 or more and 0.7 or less. The average composition of oxide inclusions in the region from the position of 3 / 8D in the diameter (D) direction of the wire to the center of the wire in the cross section perpendicular to the length direction of the wire satisfies the following relational expressions 6-8. The non-heat treated wire according to claim 1.
[Relational Expression 6] 30 ≦ [Al ₂ O ₃ ] ≦ 70
[Relational Expression 7] 20 ≦ [SiO ₂ ] ≦ 40
[Relational Expression 8] 10 ≦ [CaO] + [MgO] ≦ 20
(Here, [Al ₂ O ₃ ], [SiO ₂ ], [CaO] and [MgO] each mean the content (mass%) of the corresponding inclusion)   6. The non-heat treated wire according to claim 5, wherein an average diameter of the oxide inclusions is 8 μm or less.   The non-heat treated wire according to claim 5, wherein the oxide inclusions have a maximum diameter of 15 µm or less. In mass%, C: 0.3 to 0.4%, Si: 0.05 to 0.3%, Mn: 0.8 to 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%, and N: 0.01% or less, and Nb: 0.005 to 0.03% and V: 0.05 A heating temperature of 1200 ° C. to 1300 ° C. with a bloom containing at least one of ˜0.3%, the balance being Fe and inevitable impurities, and a carbon equivalent (Ceq) of 0.6 to 0.7 And maintaining at the heating temperature for 240 minutes or more, then rolling the steel billet to obtain a billet;
Reheating the billet, and then rolling the wire under a finish rolling temperature of 750 ° C to 900 ° C to obtain a wire,
And a step of cooling at a rate of 0.3 ° C. to 1 ° C./sec after winding the wire.   The non-heat treated wire according to claim 8, wherein the inevitable impurities include Ti and are suppressed to Ti: 0.005% or less by mass%.   The non-heat treated wire according to claim 8, wherein a reheating temperature of the billet is 1050C to 1200C.   The non-heat treated wire according to claim 8, wherein a winding temperature of the wire is 750C to 900C.