KR102178711B1 - Non-heat treated wire rod having excellent strength and impact toughness and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a wire rod suitable for a vehicle material or a mechanical component material. More specifically, the present invention relates to the non-tempered wire rod with excellent rigidity and impact toughness and a method for manufacturing the same. The present invention has a ferrite-pearlite layered structure in an L cross-section which is a cross-section parallel to the rolling direction.

Description

Non-tempered wire with excellent strength and impact toughness and its manufacturing method {NON-HEAT TREATED WIRE ROD HAVING EXCELLENT STRENGTH AND IMPACT TOUGHNESS AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a wire rod suitable as a material for automobiles or a material for mechanical parts, and more particularly, to a non-tempered wire rod having excellent strength and impact toughness, and a manufacturing method thereof.

Structural steels used for automobile parts or machine structures are mostly quenching and tempering steel (調質鋼), which has improved strength and toughness through processes such as reheating, quenching, and tempering after hot processing.

On the other hand, Non-Heat Treated Steel refers to steel that has similar strength to steel (tempered steel) that has been heat treated (tempered) without heat treatment after hot working, and such non-tempered steel is made by adding a trace amount of alloy. It is also called Micro-Alloyed Steel because it secures the material.

Typical tempered wire rods are obtained through the process of [hot rolling-cold drawing-spheroidization heat treatment-cold drawing-cold rolling-quenching and tempering] to obtain the final product, whereas non-tempered wire rods are the process of [hot rolling-cold drawing-cold rolling] It can only be manufactured.

In this way, the non-tempered wire is not only excellent in economics by lowering the manufacturing cost of the material by omitting the heat treatment process involved in the manufacture of the existing tempered wire, but also does not perform final quenching and tempering. Since the straightness is secured, its application is expanding to many products.

In particular, ferrite-pearlitic non-tempered wire rods have the advantage of being able to design inexpensive alloys, stably obtaining a homogeneous structure in the manufacturing process of the stellmor line, and the strength of the product as the amount of wire drawing increases. While is rising, there is a problem in that the ductility and toughness are rapidly deteriorated.

As a method to solve the above problems, a method of improving toughness through fine grain size by using an expensive micro-alloying element such as vanadium (V) has been proposed. It is difficult to overcome the decrease in toughness in the amount of wire drawing.

In addition, as the price of these microalloyings rapidly increases, there is a problem in that economic feasibility decreases for application to related industries.

Accordingly, there is an increasing need for a technology capable of optimizing the alloy design of a non-tempered wire rod to form a microstructure having excellent impact toughness resistance, and thereby simultaneously securing strength and impact toughness as well as wire drawing processability.

Korean Registered Announcement Patent No. 10-1639166

An aspect of the present invention is to provide a non-tempered wire rod having improved toughness and a method of manufacturing the same, without deterioration in strength even when a process such as an additional heat treatment is omitted, and using inexpensive elements.

The subject of the present invention is not limited to the above. The subject of the present invention will be understood from the general contents of the present specification, and those of ordinary skill in the art to which the present invention pertains will not have any difficulty in understanding the additional subject of the present invention.

One aspect of the present invention is, by weight %, carbon (C): 0.02 to 0.30%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 0.5 to 2.0%, chromium (Cr): 1.0% or less ( 0%), phosphorus (P): 0.04 to 0.2%, sulfur (S): less than 0.03%, aluminum (sol.Al): 0.01 to 0.07%, nitrogen (N): 0.01% or less, and niobium (Nb) : 0.1% or less, vanadium (V): 0.5% or less, and titanium (Ti): contains at least one selected from 0.1% or less, and contains the balance Fe and inevitable impurities,

It has a ferrite-pearlite layered structure in the L cross section, which is a parallel cross section in the rolling direction, and an average thickness of a ferrite band in the L cross section is 5 to 30 μm, providing a non-tempered wire having excellent strength and impact toughness.

Another aspect of the invention, the step of preparing a billet (billet) comprising the alloy composition described above; Reheating the billet at a temperature (T _r , °C) satisfying the following relational equation 1; Manufacturing a wire rod by finishing hot rolling the reheated billet at a temperature (T _f , °C) satisfying the following relational formula 2; And it provides a method of manufacturing a non-tempered wire rod excellent in strength and impact toughness, including the step of winding and cooling the wire rod.

[Relationship 1]

T ₁ ≤T _r

(Where, T ₁ = 757 + 606[C] + (80[Nb]/[C]) + 1023√[Nb] + 330[V]-100[P], and each element means a weight content, The unit of T _r is °C.)

[Relationship 2]

T ₂ ≤T _f ≤T ₃

(Where, T ₂ = 733 + 52[C] + 29.1[Si]-20.7[Mn] + 16.9[Cr]-80.6[Nb]-100[P], T ₃ = 962-300[C] + 24.6[ Si]-68.1[Mn]-75.6[Cr]-360.1[Nb]-20.7[V]-100[P], each element means the weight content, and the unit of T _f is ℃.)

Advantageous Effects of Invention According to the present invention, it is possible to provide a non-tempered wire rod excellent in impact toughness as well as strength by utilizing inexpensive elements without adding expensive elements.

1 shows a microstructure photograph measured in an L section, which is a parallel section in the rolling direction, for an invention steel according to an embodiment of the present invention.

The present inventors did a deep study in order to improve the toughness of the non-tempered steel by omitting the existing spheroidizing heat treatment and quenching-quenching heat treatment. As a result, while reducing the use of expensive trace elements, finding an inexpensive element that is advantageous for securing the strength and toughness of the non-tempered steel, and by using it, it was confirmed that the impact toughness as well as the strength of the non-tempered wire rod can be improved. It came to the completion of the invention.

Hereinafter, the present invention will be described in detail.

The non-tempered wire rod having excellent strength and impact toughness according to an aspect of the present invention is weight %, carbon (C): 0.02 to 0.30%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 0.5 to 2.0% , Chromium (Cr): 1.0% or less (including 0%), phosphorus (P): 0.04 to 0.2%, sulfur (S): less than 0.03%, aluminum (sol.Al): 0.01 to 0.07%, nitrogen (N) : May contain less than 0.01%.

Hereinafter, the reason for limiting the alloy composition of the wire rod provided by the present invention as described above will be described in detail.

On the other hand, unless specifically stated in the present invention, the content of each element is based on the weight, and the ratio of the structure is based on the area.

Carbon (C): 0.02~0.30%

Carbon (C) is an element advantageous in improving the strength of the wire rod, and may be included in an amount of 0.02% or more in order to secure a target level of strength. However, if the content is excessive, the deformation resistance of the steel increases rapidly, and there is a problem in that cold workability is deteriorated. In consideration of this, the C may be included in an amount of 0.30% or less.

Accordingly, the C may be included in an amount of 0.02 to 0.30%, more advantageously 0.05% or more, and even more advantageously 0.10% or more. Meanwhile, a more preferable upper limit of C may be 0.25%.

Silicon (Si): 0.05~0.5%

Silicon (Si) is a useful element as a deoxidizing agent for steel, and it may be contained in an amount of 0.05% or more in order to sufficiently obtain it. However, if the content is excessive, there is a problem that the deformation resistance of the steel increases rapidly due to solid solution strengthening and the cold workability deteriorates. In consideration of this, Si may be included in an amount of 0.5% or less.

Accordingly, the Si may be included in an amount of 0.05 to 0.5%, and more advantageously, it may be included in an amount of 0.1 to 0.4%.

Manganese (Mn): 0.5~2.0%

Manganese (Mn) is a useful source as a deoxidizer and a desulfurization agent, and may contain 0.5% or more in order to sufficiently obtain such an effect. However, if the content is excessive, the strength of the steel itself becomes too high, so that the deformation resistance of the steel increases rapidly, thereby deteriorating the cold workability. In consideration of this, Mn may be included in an amount of 2.0% or less.

Accordingly, the Mn may be included in an amount of 0.5 to 2.0%, and more advantageously, it may be included in an amount of 0.8% or more. Meanwhile, a more preferable upper limit of Mn may be 1.8%.

Chrome (Cr): 1.0% or less (including 0%)

Chromium (Cr) is an element that is advantageous for promoting transformation of ferrite and pearlite during hot rolling. In addition, without increasing the strength of the steel itself more than necessary, carbides are precipitated in the steel to reduce the amount of solid-solution carbon, thereby contributing to reducing the aging of dynamic damage caused by solid-solution carbon.

When the Cr content is excessive, the strength of the steel itself increases too much, so that the resistance to change of the steel increases rapidly, thereby deteriorating the cold workability. In consideration of this, the Cr may be included in an amount of 1.0% or less, and more advantageously, it may be included in an amount of 0.8% or less. However, even if the content is 0%, it is not unreasonable to secure physical properties intended in the present invention.

Phosphorus (P): 0.04~0.2%

In general, phosphorus (P) is an impurity that is inevitably contained in steel, and is well known as an element that is the main cause of segregating at grain boundaries to lower the toughness of the steel and to inhibit delayed fracture resistance. On the other hand, since phosphorus (P) has a very excellent solid solution strengthening effect, it is a very advantageous element in improving the strength of steel even with only a small amount of addition.

The inventors of the present invention have studied from various angles for a method capable of overcoming the decrease in toughness due to grain boundary segregation while securing strength by adding P to the steel. From that, it was found that when the Ferrite+Pearlite banded structure is uniformly formed in the L section, which is a parallel section in the rolling direction, the intended strength and toughness can be secured at the same time.

In order to achieve the above-described structure and physical properties, P may be included in an amount of 0.04% or more, and if the content is excessive, the toughness of the steel decreases, and thus, it may be included in an amount of 0.2% or less.

Therefore, the P may be included in an amount of 0.04 to 0.2%, and more advantageously, it may be included in an amount of 0.06% or more. Meanwhile, a more preferable upper limit of P may be 0.17%.

Sulfur (S): less than 0.03%

Sulfur (S) is an impurity that is inevitably contained in steel, and is segregated at grain boundaries, which greatly inhibits the ductility of the steel, and forms an emulsion in the steel, which causes the delayed fracture resistance and stress relaxation properties to deteriorate.

Therefore, it is desirable to control the content of S as low as possible. In theory, it is advantageous to control the S to 0% by weight, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and may include less than 0.03% in the present invention.

Aluminum (sol.Al): 0.01~0.07%

Aluminum (sol.Al, soluble aluminum) is a useful element as a deoxidizing agent, and for this purpose, it may contain 0.01% or more. When the content of sol.Al exceeds 0.07%, AlN precipitates are formed in the steel, resulting in excessive austenite particle size refining effect, resulting in deterioration of cold forging.

Therefore, the sol.Al may be included in an amount of 0.01 to 0.07%, more advantageously 0.015% or more, and even more advantageously 0.02% or more.

Nitrogen (N): 0.01% or less

Nitrogen (N) is an impurity that is inevitably contained in steel, and when the content is excessive, the amount of solid solution nitrogen increases, so that the deformation resistance of the steel increases rapidly, thereby deteriorating cold workability.

In theory, it is advantageous that the N content in the steel is 0%, but it is inevitably contained in the manufacturing process. Accordingly, it is important to manage the upper limit, and in the present invention, it may be included as 0.01% or less. More advantageously, it may contain 0.008% or less, and even more advantageously 0.007% or less.

In addition to the above-described alloy composition, the non-tempered wire rod of the present invention may further include at least one of Nb, V, and Ti for the purpose of improving physical properties.

Niobium (Nb): 0.1% or less

Niobium (Nb) is an element that forms carbides and carbonitrides in steel to limit the intergranular movement of austenite and ferrite. However, since the carbonitride acts as a fracture origin and can inhibit the impact toughness of the steel, it is advantageous to add the Nb while keeping the solubility limit.

When the content of Nb exceeds 0.1%, coarse precipitates are formed and the properties of the steel are deteriorated, so the content of Nb may be limited to 0.1% or less.

Vanadium (V): 0.5% or less

Vanadium (V), like Nb, serves to limit the intergranular movement of austenite and ferrite by forming carbides and carbonitrides. However, since the carbonitride acts as a fracture origin and may impair impact toughness, it is advantageous to add V while keeping the solubility limit.

When the content of V exceeds 0.5%, coarse precipitates are formed and the properties of the steel are deteriorated, so the content of V may be limited to 0.5% or less.

Titanium (Ti): 0.1% or less

Titanium (Ti) has the effect of limiting the grain size of austenite by combining carbon and nitrogen to form carbonitrides.

When the content of Ti exceeds 0.1%, there is a problem in that coarse precipitates are formed, thereby increasing the possibility of acting as a major crack generation source for inclusion breakage. Therefore, the Ti may be limited to 0.1% or less.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from the raw material or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone of ordinary skill in the manufacturing process, all the contents are not specifically mentioned in the present specification.

The non-tempered wire rod of the present invention containing the above alloy composition may have a composite structure of ferrite and pearlite as a microstructure.

The ferrite may be included in an area fraction of 30% or more, excluding 100%. The pearlite may be included in an area fraction of 5% or more.

When the non-tempered wire rod of the present invention has a composite structure of ferrite and pearlite as a microstructure, it is preferable to form a ferrite-pearlite layered structure in an L section whose shape is parallel to the rolling direction.

Specifically, the structure of a layered structure in which ferrite-pearlite-ferrite is continuous has an effect on improving the impact toughness of steel. For example, when an impact is applied in a right angle direction of the layered structure and the crack is propagated, the crack is broken at the interface of the layered structure and the propagation of the crack is dispersed at various angles, from which the impact toughness of the steel. This improved effect can be obtained.

In the present invention, the layered structure may be classified based on the boundary between ferrite and pearlite, and for example, in FIG. 1, the layered structure may be defined based on the boundary point between the region a and the region b.

In addition, in the present invention, a ferrite phase may be partially included in the pearlite structure, but if the ferrite phase in the pearlite structure is less than 30% of the total area, it has been revealed in advance that a pearlite structure appears.

It is preferable that the non-tempered wire rod of the present invention having the structure of the layered structure described above has an average thickness of a ferrite band of 5 to 30 μm in an L cross section which is a parallel cross section in the rolling direction.

If the average thickness of the ferrite band in the L cross section is less than 5 μm, the strength increases and cold workability may be deteriorated. On the other hand, if the thickness exceeds 30 μm, it may be difficult to secure the impact toughness to the target level.

Here, the ferrite band refers to a ferrite region of one layer in a layered structure in which ferrite-pearlite-ferrite is continuous. For example, referring to FIG. 1, it means area a of FIG. 1.

On the other hand, the non-tempered wire rod of the present invention preferably has an average ferrite particle diameter of 3 to 20 μm in a cross section C, which is a right angle cross section in the rolling direction, and a pearlite lamella spacing, between section C, which is a perpendicular cross section in the rolling direction, of 0.03 to 0.3 μm.

If the average ferrite particle diameter in the C cross-section is less than 3 μm, the strength may be excessively increased due to grain boundary refinement, and cold forging may decrease, and if it exceeds 20 μm, the strength may be reduced due to coarse grains. . More preferably, the average particle diameter of the ferrite may be 3 ~ 15㎛.

Since the average particle diameter of pearlite is affected by the average particle diameter of the ferrite, it is not particularly limited.

Here, the average particle diameter refers to an average circular diameter of particles detected by observing a cross section of the wire rod.

In addition, the finer the lamellar spacing of the pearlite structure in the C cross-section increases the strength of the wire rod. However, if the interval is less than 0.03 µm, there is a concern that cold workability may deteriorate, whereas if it exceeds 0.3 µm, the target level of strength cannot be secured.

The non-tempered wire of the present invention forms a structure of a well-developed ferrite + pearlite layered structure in the L cross section, which is a parallel cross section in the rolling direction, as described above, so that even if the steel contains a certain amount of phosphorus (P) or more, it has strength and impact toughness. At the same time, there is an advantage that can be secured excellently.

Hereinafter, a method of manufacturing a non-tempered wire having excellent strength and impact toughness according to another aspect of the present invention will be described in detail.

After preparing the billet including the alloy composition proposed in the present invention, the non-tempered wire rod according to the present invention can be manufactured through the process of [reheating-hot rolling-winding-cooling].

As described above, the present invention does not perform a separate heat treatment process (eg, spheroidization heat treatment, etc.) after the cooling process, so it will be economically useful.

In particular, in the present invention, there is a technical significance in optimizing each process in order to secure intended physical properties by forming a structure of a well-developed ferrite + pearlite layer structure in the rolling direction.

Hereinafter, each process condition will be described in detail.

[Billette reheating]

Prior to manufacturing the billet into a wire rod by rolling the billet, the billet may be reheated, and at this time, reheating is performed at a temperature (T _r , °C) that satisfies the following relational formula 1, that is, at a temperature higher than T ₁ according to the following relational formula 1. It is desirable. The reheating may be performed in a heating furnace, for example.

In the present invention, when the billet is reheated, the carbonitride formed by Nb, V, or a combination thereof in the alloy composition can be re-dissolved in the base material by performing at a temperature of T ₁ or higher according to the relational equation 1. If the carbonitride obtained by the Nb, V or a combination thereof remains undissolved in the reheating process in the heating furnace, it becomes difficult to refine ferrite grains in the subsequent rolling process due to continuous coarsening during high temperature maintenance, There is a concern that a mixed structure may be formed upon cooling.

[Relationship 1]

T ₁ ≤T _r

(Where, T ₁ = 757 + 606[C] + (80[Nb]/[C]) + 1023√[Nb] + 330[V]-100[P], and each element means a weight content, The unit of T _r is °C.)

[Hot Rolled]

The reheated billet according to the above may be hot-rolled, that is, wire rod-rolling to manufacture a wire rod, and at this time, it is preferable to perform finish hot rolling at a temperature (T _f , °C) that satisfies the following relational formula 2.

Since the rolling temperature during the finish hot rolling affects the formation of the microstructure, it is necessary to control the rolling temperature in order to form the ferrite-pearlite layered structure intended in the present invention. Accordingly, the present invention is set to a temperature range (T _f ) according to the following relational equation 2 at the time of finish hot rolling, and the intended structure can be advantageously formed therefrom.

If the temperature at the time of the finish hot rolling is less than T ₂ according to the following relational equation 2, the deformation resistance due to ferrite grain boundary micronization increases, and cold forging is likely to be inferior.On the other hand, when the temperature exceeds T ₃ , the ferrite-pearlite layer The structure is not sufficiently formed.

[Relationship 2]

T ₂ ≤T _f ≤T ₃

(Where, T ₂ = 733 + 52[C] + 29.1[Si]-20.7[Mn] + 16.9[Cr]-80.6[Nb]-100[P], T ₃ = 962-300[C] + 24.6[ Si]-68.1[Mn]-75.6[Cr]-360.1[Nb]-20.7[V]-100[P], each element means the weight content, and the unit of T _f is ℃.)

[Wind up]

A process of winding the wire rod manufactured according to the above in a coil shape may be performed, and the winding temperature may be 750 to 900°C.

If the coiling temperature is less than 750° C., there is a problem in that martensite in the surface layer generated during cooling cannot be recovered by reheating, and tempered martensite is generated, thereby increasing the possibility of causing surface defects during wire drawing. On the other hand, when the temperature exceeds 900°C, a thick scale is formed on the surface of the wire rod, so that surface defects are likely to occur during descaling, and there is a concern that the cooling time becomes excessive during subsequent cooling, resulting in a decrease in productivity.

[Cooling]

The wire rod manufactured according to the above can be cooled, and it is preferable to perform at an average cooling rate of 0.1 to 2.0°C/s.

If the average cooling rate at the time of cooling is less than 0.1°C/s, the lamella spacing in the pearlite structure may widen and thus the ductility may be inferior, whereas if it exceeds 2.0°C/s, a low-temperature structure is created and the strength of the steel is excessively increased. There is a fear of sharply deteriorating toughness. More preferably, cooling can be performed at an average cooling rate of 0.3 to 1.0°C/s.

The present invention may further include the step of drawing the wire rod after the cooling is completed, and at this time, the drawing may be performed at a total reduction rate of 30 to 70%.

If the total reduction rate is less than 30%, the target strength cannot be secured. On the other hand, if it exceeds 70%, there is a problem that the cold-rolling composition is deteriorated.

The non-tempered wire rod of the present invention has excellent toughness with an impact toughness at room temperature of 130J or more after performing the wire drawing.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

( Example )

After preparing the billet having the alloy composition of Table 1 below, the billet was reheated for 3 hours at the temperature (T _r ) shown in Table 2 below, and then finished hot-rolled at T _f to prepare a wire rod having a wire diameter of 20 mm. Thereafter, each of the wire rods was wound and then cooled to room temperature at the cooling rate shown in Table 2.

For each wire rod manufactured according to the above, using an electron microscope, the type and fraction of the microstructure, the average thickness of the ferrite band in the L section, which is a parallel section in the rolling direction, and C, which is a right angle section in the rolling direction, using an electron microscope. The average ferrite particle diameter and pearlite lamella spacing in the cross section were observed. The results are shown in Table 3 below.

On the other hand, the wires were wired by applying a wire cutting amount of 0%, 35%, 45%, and 55% to each wire, and then the cold workability was evaluated and shown in Table 4 below. At this time, the specimen having a U-notch at 25°C was subjected to a Charpy impact test to obtain an impact toughness value, and the center of each specimen was taken to measure the tensile strength at room temperature at 25°C.

Steel grade Alloy composition (% by weight) Ceq C Si Mn P S Cr sol.Al Nb V Ti N Invention Lesson 1 0.06 0.22 1.61 0.060 0.0040 0.15 0.033 0.048 0 0 0.0052 0.419 Invention Lesson 2 0.11 0.18 1.52 0.110 0.0046 0 0.038 0.041 0 0 0.0051 0.434 Invention Lesson 3 0.18 0.25 1.42 0.160 0.0051 0 0.031 0.036 0 0.011 0.0048 0.492 Comparative Steel 1 0.10 0.21 1.14 0.009 0.0047 0.37 0.042 0.020 0.154 0 0.0046 0.382 Comparative lecture 2 0.17 0.13 1.41 0.012 0.0052 0.22 0.037 0.027 0.106 0 0.0056 0.485 Comparative lecture 3 0.26 0.16 1.52 0.011 0.0048 0.14 0.044 0.025 0.055 0 0.0053 0.593 Here, Ceq=[C]+[Si]/9+[Mn]/5+[Cr]/12 is calculated, and each element means a weight content (%).

Steel grade Reheating (℃) Relation 1
Satisfaction
Whether Finish hot rolling (℃) Relationship 2
Satisfaction
Whether Winding
Temperature
(℃) Average
Cooling rate
(℃/s) T _r T ₁ T _f T ₂ T ₃ Invention
One 1112 1075 ○ 789 702 805 ○ 796 1.6 Invention
2 1072 1050 ○ 792 698 804 ○ 800 1.1 Invention
3 1083 1060 ○ 772 701 788 ○ 785 0.7 Comparative steel
One 1021 1028 × 826 724 820 × 819 0.4 Comparative steel
2 1094 1075 ○ 837 717 788 × 797 1.4 Comparative steel
3 1071 1101 × 813 719 763 × 778 2.3

Steel grade Organization
Configuration F fraction
(%) C cross section
F average particle diameter
(㎛) L cross section
F band average
Thickness(㎛) C cross section
P lamella
Spacing (㎛) Invention Lecture 1 F+P 86 14 23 0.23 Invention Lecture 2 F+P 79 10 21 0.25 Invention Lecture 3 F+P 71 7 18 0.24 Comparative lecture 1 F+P 78 19 31 0.32 Comparative lecture 2 F+P 73 22 34 0.27 Comparative lecture 3 F+P 61 15 32 0.35 F stands for ferrite, and P stands for pearlite.

Steel grade Fresh processing (%) 0% 35% 45% 55% The tensile strength
(MPa) Impact toughness
(J) The tensile strength
(MPa) Impact toughness
(J) The tensile strength
(MPa) Impact toughness
(J) The tensile strength
(MPa) Impact toughness
(J) Invention Lecture 1 532 321 731 202 781 211 824 218 Invention Lecture 2 578 278 787 186 854 195 912 204 Invention Lecture 3 646 231 852 153 905 165 953 173 Comparative lecture 1 595 227 812 129 872 116 931 98 Comparative lecture 2 619 216 824 118 873 102 914 81 Comparative lecture 3 677 176 889 95 938 78 964 53

As shown in Tables 1 to 3, Inventive Steels 1 to 3 satisfying all of the alloy composition and manufacturing conditions proposed in the present invention are as intended by the structure of the F+P layered structure in the L section, which is a parallel section in the rolling direction. Was formed. Accordingly, as shown in Table 4, excellent strength and impact toughness were secured even after wire drawing.

On the other hand, Comparative Steel 1 is a case where the P content in the steel is insufficient and the conditions during reheating and hot rolling deviate from the present invention. Comparative steel 1 has no problem in securing strength after wire drawing due to the addition of V in the steel, but it can be seen that the impact toughness decreases as the amount of wire drawing increases.

Comparative steels 2 and 3 are cases in which the P content in the steel is insufficient and the manufacturing process deviates from the present invention, and it can be confirmed that the impact toughness after wire drawing is poor as the microstructure intended in the present invention is not formed.

1 shows a microstructure photograph measured in the L section, which is a parallel cross section in the rolling direction of Inventive Steel 3, and it can be seen that it is formed in a ferrite-pearlite layered structure.

Claims (8)

In% by weight, carbon (C): 0.02 to 0.30%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 0.5 to 2.0%, chromium (Cr): 1.0% or less (including 0%), phosphorus ( P): 0.04 to 0.2%, sulfur (S): less than 0.03%, aluminum (sol.Al): 0.01 to 0.07%, nitrogen (N): 0.01% or less, niobium (Nb): 0.1% or less, vanadium ( V): 0.5% or less and titanium (Ti): contains at least one selected from 0.1% or less, and contains the balance Fe and inevitable impurities,
It has a ferrite-pearlite layered structure in the L section, which is a parallel section in the rolling direction,
A non-tempered wire rod having excellent strength and impact toughness having an average thickness of a ferrite band of 5 to 30 μm in the L cross section.
The method of claim 1,
The wire rod is a non-tempered wire rod having excellent strength and impact toughness, including ferrite and residual pearlite having an area fraction of 30% or more.
The method of claim 1,
The wire rod is a non-tempered wire rod having excellent strength and impact toughness having an average ferrite particle diameter of 3 to 20 μm in a cross-section C, which is a cross section at a right angle in the rolling direction.
The method of claim 1,
The wire rod is a non-tempered wire rod having excellent strength and impact toughness having a pearlite lamella spacing of 0.03 to 0.3 μm in a C cross section, which is a cross-section perpendicular to the rolling direction.
In% by weight, carbon (C): 0.02 to 0.30%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 0.5 to 2.0%, chromium (Cr): 1.0% or less (including 0%), phosphorus ( P): 0.04 to 0.2%, sulfur (S): less than 0.03%, aluminum (sol.Al): 0.01 to 0.07%, nitrogen (N): 0.01% or less, niobium (Nb): 0.1% or less, vanadium ( V): preparing a billet containing at least one selected from among 0.5% or less and titanium (Ti): 0.1% or less, the balance Fe and unavoidable impurities;
Reheating the billet at a temperature (T _r , °C) satisfying the following relational equation 1;
Manufacturing a wire rod by finishing hot rolling the reheated billet at a temperature (T _f , °C) satisfying the following relational formula 2; And
A method of manufacturing a non-tempered wire rod having excellent strength and impact toughness, including the step of winding and cooling the wire rod.

[Relationship 1]
T ₁ ≤T _r
(Where, T ₁ = 757 + 606[C] + (80[Nb]/[C]) + 1023√[Nb] + 330[V]-100[P], and each element means a weight content, The unit of Tr is ℃.)

[Relationship 2]
T ₂ ≤T _f ≤T ₃
(Where, T ₂ = 733 + 52[C] + 29.1[Si]-20.7[Mn] + 16.9[Cr]-80.6[Nb]-100[P], T ₃ = 962-300[C] + 24.6[ Si]-68.1[Mn]-75.6[Cr]-360.1[Nb]-20.7[V]-100[P], each element means the weight content, and the unit of T _f is ℃.)
The method of claim 5,
The winding is performed in a temperature range of 750 to 900° C. A method of manufacturing a non-tempered wire rod having excellent strength and impact toughness.
The method of claim 5,
The cooling is carried out at an average cooling rate of 0.1 ~ 2.0 ℃ / s, the method of manufacturing a non-tempered wire rod excellent in strength and impact toughness.
The method of claim 5,
Further comprising the step of drawing the wire rod having completed cooling to 30-70%,
After the wire drawing, a method of manufacturing a non-tempered wire rod having excellent strength and impact toughness having an impact toughness at room temperature of 130J or more.

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