KR20220074475A - Non-heat treated steel with improved machinability and toughness and the method for manufacturing the same - Google Patents

Abstract

Disclosed are a medium-carbon non-tempered wire with improved machinability and impact toughness and a method for manufacturing the same.
The medium-carbon non-toughened wire according to the present invention is, by weight%, C: 0.28 to 0.5%, Si: 0.55 to 1.0%, Mn: 0.4 to 1.5%, P: 0.03% or less, S: 0.015 to 0.05%, sol. Al: 0.01 to 0.07%, N: 0.007% to 0.02%, including the remaining Fe and unavoidable impurities, including ferrite and pearlite as microstructures, and satisfy the following Relations 1 and 2.
[Relational Expression 1] 20 ≤ Mn/S ≤ 70
[Relational Expression 2] 1.4 ≤ Al/N ≤ 7

Description

Medium carbon non-tempered wire with improved machinability and impact toughness and manufacturing method therefor {NON-HEAT TREATED STEEL WITH IMPROVED MACHINABILITY AND TOUGHNESS AND THE METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a medium-carbon non-tempered wire rod with improved machinability and impact toughness and a method for manufacturing the same, and more particularly, to a medium-carbon non-tempered wire suitable for use as a material for automobiles or machine parts, and a method for manufacturing the same it's about

Structural steels used for mechanical structures or automobile parts are mostly quenched and tempered steels that have increased strength and toughness through reheating, quenching and annealing after hot working.

On the other hand, unlike tempered steel, Non-Heat Treated Steel refers to a steel that can obtain strength similar to that of heat treated (tempered) steel without heat treatment after hot working. It is also called micro-alloyed steel because it is manufactured with an alloy added as a basic composition.

As such, non-tempered wire rods omit the heat treatment process involved in manufacturing the existing tempered wire rods, thereby lowering the manufacturing cost of the material and providing excellent economic feasibility. This is suppressed and straightness is secured, so it is applied to many products.

In particular, the ferrite-pearlite medium carbon non-refined wire rod has the advantage of being able to design inexpensive components and stably obtaining a homogeneous structure in the wire rod manufacturing process. However, there is a problem in that the toughness of the ferrite-pearlite-based medium carbon non-refined wire rod is deteriorated as a large amount of MnS is generated by the addition of S, which is added for machinability.

One aspect of the present invention is to provide a non-tempered wire rod and a method for manufacturing the same, which can overcome the tough toughness compared to conventional tempered steel and secure machinability and impact toughness without additional heat treatment through addition of high S and high N .

The medium carbon non-tempered wire rod with improved machinability and impact toughness according to an embodiment of the present invention is, by weight, C: 0.28 to 0.5%, Si: 0.55 to 1.0%, Mn: 0.4 to 1.5%, P: 0.03% or less , S: 0.015 to 0.05%, sol.Al: 0.01 to 0.07%, N: 0.007% to 0.02%, including the remaining Fe and unavoidable impurities, including ferrite and pearlite as microstructures, the following Relations 1 and 2 is satisfied with

[Relational Expression 1] 20 ≤ Mn/S ≤ 70

[Relational Expression 2] 1.4 ≤ Al/N ≤ 7

According to an embodiment of the present invention, the medium carbon non-refined wire rod may satisfy the following relational expression (3).

[Relational Expression 3] 0.8 ≤ Si/C ≤ 2.0

According to an embodiment of the present invention, the average lamellar spacing of the pearlite may be 0.03 to 0.3㎛.

According to an embodiment of the present invention, the medium carbon non-refined wire rod may further include one or more of Nb: 0.1% or less and V: 0.2% or less.

According to an embodiment of the present invention, the medium carbon non-refined wire rod may further include Cr: 0.3% or less.

According to an embodiment of the present invention, the medium carbon non-refined wire rod may satisfy the following relational expression (4).

[Relational Expression 4] 0 ≤ Mn _c /Mn _f ≤ 3

(In the above formula, Mn _c means the average Mn content (at%) contained in the cementite in the pearlite, and Mn _f means the average Mn content (at%) contained in the ferrite in the pearlite)

According to an embodiment of the present invention, the method for manufacturing a medium-carbon non-tempered wire rod with improved machinability and impact toughness according to weight %, C: 0.28 to 0.5%, Si: 0.55 to 1.0%, Mn: 0.4 to 1.5%, P: Reheating the steel piece including 0.03% or less, S: 0.015 to 0.05%, sol.Al: 0.01 to 0.07%, N: 0.007% to 0.02%, the remaining Fe and unavoidable impurities; manufacturing a wire rod by finishing rolling the reheated steel piece at 750 to 850°C; and cooling the wire rod after winding, wherein the cooling step after winding includes: a first cooling step of cooling at an average cooling rate of 5 to 100° C./s from the finish rolling temperature to the winding temperature; a second cooling step of cooling at an average cooling rate of 2 to 5° C./s from the coiling temperature to 700° C. after the first cooling; and a third cooling step of cooling from 700° C. to 450° C. at an average cooling rate of 0.1 to 2° C./s after the second cooling.

According to an embodiment of the present invention, the steel piece may further include one or more of Nb: 0.1% or less, V: 0.2% or less.

According to an embodiment of the present invention, the steel slab may further include Cr: 0.3% or less.

The medium-carbon non-tempered wire rod with improved machinability and impact toughness according to an embodiment of the present invention utilizes MnS formation according to high S addition and nitride-forming elements by high N addition, so that even if heat treatment is omitted, high machinability and high toughness are required. It can be used suitably for parts, etc.

In addition, it is possible to provide a medium carbon non-refined wire rod with improved cold workability and a method for manufacturing the same by controlling the Mn distribution ratio in pearlite.

This specification does not describe all elements of the embodiments, and general content in the technical field to which the present invention pertains or content that overlaps among the embodiments is omitted.

Also, when a part "includes" a component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.

The singular expression includes the plural expression unless the context clearly dictates otherwise.

Hereinafter, the present invention will be described in detail.

The present inventors studied from various angles to provide a wire rod capable of securing machinability and impact toughness after wire drawing. It has been found that excellent impact toughness can be secured along with an increase in strength during wire drawing without heat treatment, and the present invention has been completed.

The medium carbon non-tempered wire rod with improved machinability and impact toughness according to an embodiment of the present invention is, by weight, C: 0.28 to 0.5%, Si: 0.55 to 1.0%, Mn: 0.4 to 1.5%, P: 0.03% or less , S: 0.015 to 0.05%, sol.Al: 0.01 to 0.07%, N: 0.007% to 0.02%, remaining Fe and unavoidable impurities, including ferrite and pearlite as microstructures.

In addition, the medium-carbon non-toughened wire according to the present invention may further include Nb: 0.1% or less, V: 0.2% or less.

In addition, the medium-carbon non-refined wire rod according to the present invention may further include Cr: 0.3% or less.

Hereinafter, the reason for numerical limitation of the alloying element content in an embodiment of the present invention will be described. Hereinafter, unless otherwise specified, the unit is % by weight.

The content of C is 0.28 to 0.5%.

C is an element that improves the strength of the wire rod. In order to exhibit the above-mentioned effect, it is preferable to contain C 0.28% or more. However, when the content is excessive, the deformation resistance of the steel rapidly increases and cold workability may deteriorate, so it is preferable to limit the upper limit of the C content to 0.5%.

The content of Si is 0.55 to 1.0%.

Si is an element useful as a deoxidizer and an element serving to improve strength. When the content of Si is less than 0.55%, the above-described effect cannot be exhibited, and when it exceeds 1.0%, the deformation resistance of the steel is rapidly increased due to solid solution strengthening, and cold workability may be deteriorated. Accordingly, the present invention limits its content.

The content of Mn is 0.4 to 1.5%.

Mn is an element useful as a deoxidizer and a desulfurizer. When the content of Mn is less than 0.4%, the above-described effect cannot be exhibited, and when the content of Mn exceeds 1.5%, the strength of the steel itself becomes excessively high and the deformation resistance of the steel rapidly increases, and cold workability may be deteriorated. Accordingly, the upper limit of the Mn content is preferably 1.5%, more preferably 1.3%.

The content of Cr is 0.3% or less.

Cr is an element that promotes ferrite and pearlite transformation during hot rolling. In addition, without increasing the strength of the steel itself more than necessary, carbides in the steel are precipitated to reduce the amount of solid-solution carbon, and contribute to the reduction of dynamic strain aging due to the solid-solution carbon. However, when the content of Cr exceeds 0.3%, the strength of the steel itself becomes excessively high, and the deformation resistance of the steel rapidly increases, which may deteriorate cold workability. Accordingly, the upper limit of the Cr content is preferably 0.3%, more preferably 0.2%.

The content of P is 0.03% or less.

P is an unavoidably contained impurity and is an element that is segregated at grain boundaries to reduce the toughness of steel and reduce delayed fracture resistance. Therefore, in the present invention, it is preferable to control the content as low as possible. In theory, it is advantageous to control the content of P to 0%, but since it is inevitably contained in the manufacturing process, it is important to manage the upper limit, and in the present invention, the upper limit of the P content is managed as 0.03%.

The content of S is 0.015 to 0.05%.

S is an unavoidably contained impurity, which segregates at grain boundaries to greatly reduce the ductility of steel, and forms an emulsion in the steel, which is a major cause of deterioration of delayed fracture resistance and stress relaxation characteristics. Therefore, it is desirable to control the content as low as possible. However, since S is a very effective element for improving machinability by combining with Mn to form MnS, in the present invention, the content of S is reduced to 0.015% to It is preferable to control it to 0.05%.

The content of Sol.Al is 0.01 to 0.07%.

sol.Al is an element useful as a deoxidizer. In order to exhibit the above-described effect, sol.Al may be included in an amount of 0.01% or more, preferably 0.015% or more. However, when the content of Al exceeds 0.07%, the effect of refining the austenite grain size due to the AlN formation is increased, so that the cold forging formability may be deteriorated. Accordingly, in the present invention, the upper limit of the Al content is managed as 0.07%.

The content of Nb is 0.1% or less (including 0%).

Nb is an element that forms carbides and carbonitrides to limit grain boundary movement of austenite and ferrite. However, when the content of Nb exceeds 0.1%, coarse carbonitride precipitates can be formed, and Nb carbonitride can act as a fracture origin and reduce impact toughness. desirable. Therefore, in the present invention, it is preferable to limit the content of Nb to 0.1% or less.

The content of V is 0.2% or less (including 0%).

V is an element that, like Nb, forms carbides and carbonitrides to limit grain boundary movement of austenite and ferrite. However, when the content of V exceeds 0.2%, coarse carbonitride precipitates can be formed, and V carbonitride can act as a fracture origin to reduce impact toughness, so it is preferable to add it by keeping the solubility limit . Therefore, in the present invention, it is preferable to limit the content of V to 0.2% or less.

The content of N is 0.007 to 0.02%.

N is an essential element for implementing the effect of improving the impact toughness of the present invention. When the content of N is less than 0.007%, it is difficult to secure sufficient nitride, and the amount of precipitates such as Al, Nb, and V decreases, so that the toughness targeted in the present invention cannot be secured, and when the content of N exceeds 0.02% Solid solution nitrogen, which does not exist as a nitride precipitate, may increase, and thus the toughness and ductility of the wire rod may be reduced. Therefore, in the present invention, it is preferable to manage the N content to 0.007 to 0.02%.

The balance other than the alloy composition is Fe. The medium-carbon non-refined wire rod of the present invention may contain other impurities that may be included in the industrial production process of ordinary steel. These impurities are not particularly limited in the present invention, because the content can be known by anyone having ordinary knowledge in the technical field to which the present invention pertains.

The medium-carbon non-refined wire rod according to an embodiment of the present invention includes ferrite and pearlite as a microstructure.

The pearlite structure of the present invention may have an average lamellar spacing of 0.03 to 0.3 μm in a cross-section C, which is a cross-section perpendicular to the rolling direction. The finer the lamellar spacing of the pearlite structure, the greater the strength of the wire rod. However, if the lamellar spacing of the pearlite structure is less than 0.03㎛, the strength may increase significantly and the cold workability may deteriorate, and if it exceeds 0.3㎛, it may be difficult to secure the strength targeted in the present invention.

The medium-carbon non-refined wire rod according to an embodiment of the present invention may satisfy Relations 1 to 4.

In Relations 1 to 3, each of [Mn], [S], [Al], [N], [C], and [Si] means the content (% by weight) of the corresponding element

[Relational Expression 1] 20 ≤ [Mn]/[S] ≤ 70 (machinability)

Relation 1 is an expression related to machinability. In the present invention, MnS is formed due to the addition of high S and Mn. MnS, as a drawing inclusion, has a shape and directionality elongated in the rolling direction, and greatly improves the machinability of the medium-carbon non-refined wire rod according to the present invention. However, MnS acts as the initiation point and propagation path of cracks during impact, thereby lowering the impact toughness. If the Mn/S ratio is less than 20, machinability may not be sufficient, and if it exceeds 70, impact toughness may be reduced. Therefore, in the present invention, the Mn/S ratio is limited to 20 to 70.

[Relational Expression 2] 1.4 ≤ [Al]/[N] ≤ 7 (toughness)

Relation 2 is an expression related to personality. In the present invention, AlN is formed due to the addition of high N and Al. Precipitation of fine AlN in steel improves the impact toughness of the medium-carbon non-refined wire rod according to the present invention by refining the crystal grains. In order to express the above-described effect, it is advantageous to generate as many AlN precipitates of 50 nm or less as possible, and for this purpose, it is preferable to control the Al/N ratio to be 1.4 to 7. When the Al/N ratio is less than 1.4, sufficient AlN precipitates cannot be formed, and when it exceeds 7, coarse AlN precipitates are formed, and the impact toughness may be rather poor. Therefore, in the present invention, the Al/N ratio is limited to 1.4 to 7.

[Relational Expression 3] 0.8 ≤ [Si]/[C] ≤ 2.0 (strength)

Relation 3 is an expression related to strength. Si and C are elements having a large solid solution strengthening effect. When the Si/C ratio is less than 0.8, it is difficult to secure the strength targeted in the present invention, and when it exceeds 2.0, work hardening is too high, and cold workability and impact toughness may be reduced. Therefore, in the present invention, the Si/C ratio is limited to 0.8 to 2.0.

[Relational Expression 4] 0 ≤ Mn _c /Mn _f ≤ 3 (cold workability)

Mn _c denotes an average Mn content (at%) contained in cementite in pearlite, and Mn _f denotes an average Mn content (at%) contained in ferrite in pearlite.

Relation 4 is an expression related to cold workability, and represents the Mn distribution ratio in perlite. The Mn distribution ratio in the pearlite is a value obtained by dividing the average Mn content (at%) contained in the cementite in the pearlite by the average Mn content (at%) contained in the ferrite in the pearlite. In the present invention, the Mn distribution ratio in pearlite is limited to 0 to 3. Through numerous experiments, the inventors have confirmed that cold workability is improved when the Mn distribution ratio in pearlite is 3 or less, and came to propose the present invention. Since Mn is an element with a strong tendency to segregate into cementite among pearlite, normal pearlite has an Mn distribution ratio of 5 or more. In order to control this Mn distribution ratio to 3 or less, it is necessary to suppress the diffusion of Mn into cementite in pearlite. The Mn distribution ratio in the pearlite can be achieved by a cooling process after winding in which a cooling rate is applied differently for each temperature section according to the present invention, which will be described later.

Next, a method for manufacturing a medium carbon non-refined wire rod according to an embodiment of the present invention will be described.

The medium carbon non-tempered wire rod with improved machinability and impact toughness according to the present invention can be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an embodiment, it may be manufactured by the following method.

According to an embodiment of the present invention, the method for manufacturing a medium-carbon non-tempered wire rod with improved machinability and impact toughness according to weight%, C: 0.28 to 0.5%, Si: 0.55 to 1.0%, Mn: 0.4 to 1.5%, P: 0.03% or less, S: 0.015 to 0.05%, sol.Al: 0.01 to 0.07%, N: 0.007% to 0.02%, reheating the steel piece containing the remaining Fe and unavoidable impurities; manufacturing a wire rod by hot rolling the reheated steel piece; and cooling the wire rod after winding.

In addition, the steel piece may further include Nb: 0.1% or less, V: 0.2% or less.

In addition, the steel slab may further include Cr: 0.3% or less.

Hereinafter, each manufacturing step will be described in more detail.

First, after heating the bloom that satisfies the above-mentioned component system, the steel slab is rolled to obtain a billet.

reheat stage

The reheating step is a step of reheating the rolled billet. It is a step to maintain the carbonitride formed by Al, Nb and V or a combination thereof in the component system in an undissolved state in the base material and to suppress the growth of the austenite grain size. .

At this time, the reheating may be performed at a temperature of 950 to 1,150 °C. If the reheating temperature of the steel piece is less than 950 ℃, the crystal grains are too fine and the hot rolling resistance may increase, whereas if it exceeds 1,150 ℃, the carbonitrides formed by Al, Nb and V or a combination thereof are completely re-dissolved and Excessive growth of the austenite structure may reduce ductility.

wire rod rolling stage

In the wire rod rolling step, the reheated steel piece is hot-rolled to manufacture a wire rod.

At this time, the finish rolling temperature of the hot rolling may be 750 to 850 ℃. If the finish rolling temperature is less than 750 ℃, the deformation resistance may increase due to the increase in strength due to grain refinement, and if it exceeds 850 ℃, it may be difficult to secure the high toughness targeted in the present invention because the grains are coarse.

winding and cooling stage

The winding and cooling step is a step of winding the finish-rolled wire rod and cooling it to obtain the medium carbon non-refined wire rod according to the present invention. In order to control the Mn distribution ratio of cementite among pearlite to 3 or less, the diffusion of Mn should be suppressed as much as possible during the cooling process. In order to suppress the diffusion of Mn into cementite as much as possible, it is effective to apply different cooling rates for each temperature section.

First cooling step (CR1): finish rolling temperature ~ winding temperature

The first cooling step may be performed at an average cooling rate of 5 to 100° C./s from the finish rolling temperature to the coiling temperature. The temperature section of the first cooling step is a region in which diffusion of Mn occurs very rapidly. At a cooling rate of less than 5 °C/s, the Mn distribution ratio is highly likely to exceed 3 due to Mn diffusion, and the cooling rate exceeds 100 °C/s has limitations that are difficult to apply commercially. Therefore, the first cooling step is preferably performed at a cooling rate of 5 to 100 °C / s.

Second cooling step (CR2): coiling temperature ~ 700 ℃

The second cooling step may be performed at an average cooling rate of 2 to 5° C./s from the coiling temperature to 700° C. after the first cooling process. At a cooling rate of less than 2 °C/s, the Mn distribution ratio may exceed 3 due to Mn diffusion, and at a cooling rate exceeding 5 °C/s, material non-uniformity such as mixing may occur due to cooling non-uniformity. Accordingly, the second cooling step is preferably performed at a cooling rate of 2 to 5° C./s.

3rd cooling section (CR3): 700 ~ 450℃

The third cooling step may be performed at an average cooling rate of 0.1 to 2° C./s from 700° C. to 450° C. after the second cooling step. At a cooling rate of less than 0.1 °C/s, the pearlite lamellar spacing becomes coarse, making it difficult to secure the target strength in the present invention. have. Accordingly, the third cooling step is preferably performed at a cooling rate of 0.1 to 2° C./s.

Hereinafter, the present invention will be described in more detail through 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 matters reasonably inferred therefrom.

Example

After heating a bloom having an alloy composition as shown in Table 1 at 1,200° C. for 4 hours, the steel slab was rolled at a finish rolling temperature of 1,100° C. to obtain a billet. Thereafter, the billet was heated at 1,100° C. for 90 minutes, and then hot-rolled at a finish rolling temperature of 800° C. using a Ψ 25 mm roll to prepare a wire rod. Then, wire specimens of Inventive Steels 1 to 5 and Comparative Steels 1 to 5 were prepared by applying a three-step cooling process for each temperature section of CR1-CR2-CR3. Thereafter, the microstructure, tensile strength, and impact toughness of the cooled wire rod specimen were measured and shown in Table 2 below.

Here, the room temperature tensile strength was measured by sampling from the center of the non-tempered steel specimen at 25°C, and the room temperature impact toughness was obtained by performing a Charpy impact test on a specimen having a U-notch (U-notch standard sample, 10x10x55mm) at 25°C. The impact energy value was evaluated.

steel grade Alloy composition (wt%) relation note C Si Mn P S Cr Al Nb V N Mn/S
(①) Al/N
(②) Si/C
(③) Invention lecture 1 0.32 0.63 1.32 0.012 0.024 - 0.018 - 0.110 0.0095 55 1.89 1.97 Invention Example 1 Invention lecture 2 0.37 0.72 0.87 0.010 0.026 0.14 0.026 0.010 0.062 0.0102 33.5 2.55 1.95 Invention example 2 Invention lecture 3 0.40 0.68 0.78 0.009 0.032 - 0.033 - - 0.0113 24.4 2.92 1.70 Invention example 3 Invention lecture 4 0.43 0.61 1.02 0.011 0.022 0.11 0.038 - - 0.0106 46.4 3.58 1.42 Invention example 4 Invention River 5 0.48 0.57 0.63 0.010 0.034 0.06 0.045 - 0.054 0.0098 26.3 4.59 1.19 Invention Example 5 Comparative lecture 1 0.30 0.33 1.32 0.011 0.005 - 0.025 0.015 - 0.0031 264.0 8.06 1.10 Comparative Example 1 Comparative lecture 2 0.36 0.38 1.24 0.012 0.011 0.11 0.035 - 0.078 0.0051 112.7 6.86 1.06 Comparative Example 2 Comparative lecture 3 0.41 0.32 1.15 0.009 0.026 - 0.041 - - 0.0043 44.2 9.53 0.78 Comparative Example 3 Comparative lecture 4 0.47 0.20 1.02 0.010 0.047 0.25 0.036 0.006 0.043 0.0050 21.7 7.20 0.43 Comparative Example 4 Comparative Steel 5 0.52 0.24 0.96 0.009 0.035 - 0.024 - - 0.0049 27.4 4.90 0.46 Comparative Example 5 Here, ① 1.4 ≤ Al/N ≤ 7, ② 20 ≤ Mn/S ≤ 70, ③ 0.8 ≤ Si/C ≤ 2.0
Each of [C], [Si], [Mn], [S], [Al] and [N] means the content (% by weight) of the corresponding element

steel grade Cooling rate (℃/s) Mn distribution ratio
(④) Pealite
fraction
(%) Pealite
lamella
interval
(μm) wire rod
ts
(MPa) wire rod
impact toughness
(J) note CR1 CR2 CR3 Invention lecture 1 5.7 2.9 0.5 2.6 56 0.14 777 123 Invention Example 1 Invention lecture 2 20.1 4.8 1.2 0.7 64 0.26 749 129 Invention example 2 Invention lecture 3 10.3 4.2 0.9 1.0 67 0.23 726 121 Invention example 3 Invention lecture 4 7.5 3.7 1.5 1.7 70 0.2 755 106 Invention Example 4 Invention River 5 6.8 3.5 1.8 1.9 74 0.18 803 93 Invention Example 5 Comparative lecture 1 5.4 4.9 0.6 2.9 61 0.25 724 71 Comparative Example 1 Comparative lecture 2 5.1 1.8 2.7 3.4 72 0.29 768 60 Comparative Example 2 Comparative lecture 3 4.1 3.6 1.3 4.1 74 0.24 752 64 Comparative Example 3 Comparative lecture 4 2.3 1.2 0.8 5.7 80 0.36 793 56 Comparative Example 4 Comparative Steel 5 3.5 1.7 2.4 4.5 84 0.32 789 51 Comparative Example 5 where, ④ 0 ≤ Mn _c /Mn _f ≤ 3
* Mn _c : Average Mn content contained in cementite in pearlite (at%), Mn _f : Average Mn content contained in ferrite in pearlite (at%)

As can be seen from Table 2, in the case of invention steels 1 to 5 that satisfy the alloy composition and manufacturing conditions of the present invention, it is possible to secure a wire rod tensile strength of 726 MPa or more and a wire rod impact toughness of 93J or more. It was confirmed that the toughness was excellent. On the other hand, in the case of Comparative Steels 1 to 5, which did not satisfy at least one of Relations 1 to 3, the tensile strength of the wire rod was secured, but it was confirmed that the wire rod impact toughness was inferior to that of the invention steel. there was.

Then, wire specimens satisfying the alloy composition and manufacturing conditions of Tables 1 and 2 were wire-drawn at a working rate of 10 to 30%, and the presence or absence of wire breakage, room temperature tensile strength, and room temperature impact toughness were measured, and the results are shown in Table 3 below. shown together. Cold workability was indicated by ○ when no disconnection occurred during wire drawing, and X when disconnection occurred more than once.

Psalter
No. 10%
fresh processing 20%
fresh processing 30%
fresh processing Seal
burglar
(MPa) Shock
tenacity
(J) cold
machinability Seal
burglar
(MPa) Shock
tenacity
(J) cold
machinability Seal
burglar
(MPa) Shock
tenacity
(J) cold
machinability Invention lecture 1 875 102 ○ 933 93 ○ 987 98 ○ Invention lecture 2 841 106 ○ 904 95 ○ 961 94 ○ Invention lecture 3 826 98 ○ 875 89 ○ 935 83 ○ Invention lecture 4 843 87 ○ 899 76 ○ 954 72 ○ Invention River 5 907 71 ○ 962 62 ○ 1014 55 ○ Comparative lecture 1 825 50 ○ 866 38 ○ 912 31 ○ Comparative lecture 2 857 41 ○ 909 32 ○ 963 23 ○ Comparative lecture 3 844 43 ○ 895 34 ○ 942 24 ○ Comparative lecture 4 892 34 ○ 940 25 ○ 985 17 X Comparative steel 5 886 29 ○ 931 20 X 977 14 X

As can be seen from Table 3, in the case of Inventive Steels 1 to 5 satisfying the alloy composition and manufacturing conditions proposed in the present invention, tensile strength of 826 MPa or more, and impact of 55 J or more during wire drawing at 10, 20% and 30% working rates It was confirmed that the toughness and cold workability were satisfied. Inventive Examples 1 to 5 not only satisfied all the conditions of Relations 1 to 3, but also the microstructure of the wire rod was composed of ferrite and pearlite, and the pearlite lamellar spacing and Mn distribution in the pearlite. It was confirmed that the ratio satisfies all the conditions suggested in the present invention, so that the impact toughness after wire drawing is excellent, and no cracks are generated inside.

On the other hand, in the case of Comparative Steels 1 to 5, which do not satisfy at least one of the conditions proposed in the present invention, the tensile strength of the wire rod was secured, but the machinability and impact toughness were inferior, and cracks were generated inside after wire drawing, so that the cold workability was achieved. inferiority was also confirmed.

In the above bar, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art may not depart from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (9)

In wt%, C: 0.28 to 0.5%, Si: 0.55 to 1.0%, Mn: 0.4 to 1.5%, P: 0.03% or less, S: 0.015 to 0.05%, sol.Al: 0.01 to 0.07%, N: 0.007 % to 0.02%, the remainder Fe and unavoidable impurities,
It contains ferrite and pearlite as a microstructure,
A medium carbon non-tempered wire rod with improved machinability and impact toughness satisfying the following Relations 1 and 2.
[Relational Expression 1] 20 ≤ Mn/S ≤ 70
[Relational Expression 2] 1.4 ≤ Al/N ≤ 7 The method of claim 1,
A medium carbon non-tempered wire rod with improved machinability and impact toughness satisfying the following Relational Equation 3.
[Relational Expression 3] 0.8 ≤ Si/C ≤ 2.0 The method of claim 1,
The average lamellar spacing of the pearlite is,
Medium carbon non-tempered wire with improved machinability and impact toughness of 0.03 to 0.3 μm. The method of claim 1,
Nb: 0.1% or less, V: medium carbon non-tempered wire rod with improved machinability and impact toughness further comprising at least one of 0.2% or less. The method of claim 1,
Cr: A medium carbon non-tempered wire with improved machinability and impact toughness containing 0.3% or less. The method of claim 1,
A medium-carbon non-tempered wire rod with improved machinability and impact toughness satisfying the following Relational Equation 4.
[Relational Expression 4] 0 ≤ Mn _c /Mn _f ≤ 3
(In the above relation 4, Mn _c means the average Mn content (at%) contained in the cementite in the pearlite, and Mn _f means the average Mn content (at%) contained in the ferrite in the pearlite) In wt%, C: 0.28 to 0.5%, Si: 0.55 to 1.0%, Mn: 0.4 to 1.5%, P: 0.03% or less, S: 0.015 to 0.05%, sol.Al: 0.01 to 0.07%, N: 0.007 % to 0.02%, reheating the steel piece containing the remaining Fe and unavoidable impurities;
manufacturing a wire rod by finish-rolling the reheated steel piece at 750 to 850°C; and
Including; cooling the wire rod after winding
The cooling step after the winding is a first cooling step of cooling from the finish rolling temperature to the winding temperature at an average cooling rate of 5 to 100 ℃ / s;
a second cooling step of cooling at an average cooling rate of 2 to 5°C/s from the coiling temperature to 700°C after the first cooling; and
After the second cooling, a third cooling step of cooling at an average cooling rate of 0.1 to 2° C./s from 700° C. to 450° C. 8. The method of claim 7,
The steel piece is
Nb: 0.1% or less, V: A method of manufacturing a medium carbon non-tempered wire rod with improved machinability and impact toughness further comprising at least one of 0.2% or less. 8. The method of claim 7,
The steel piece is
Cr: A method of manufacturing a medium carbon non-tempered wire rod with improved machinability and impact toughness further comprising 0.3% or less.