KR20220169272A - Steel wire rod having excellent drawability and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a wire rod for a mechanical structure, which can be applied to automobiles, construction parts, etc., and a method for manufacturing the same, and relates to a wire rod having excellent drawability and a method for manufacturing the same. The wire rod of the present invention comprises 0.8 to 1.2 wt% of C, 0.01 to 0.6 wt% of Si, 0.1 to 0.6 wt% of Mn, 0.8 to 2.0 wt% of Cr, 0.01 to 0.06 wt% of Al, 0.02 wt% or less (except 0 wt%) of N, and the balance Fe and unavoidable impurities, and the microstructure includes pro-eutectoid cementite in a pearlite main structure.

Description

Wire rod with excellent drawing processability and its manufacturing method {STEEL WIRE ROD HAVING EXCELLENT DRAWABILITY AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a wire rod for mechanical structure applicable to automobiles, construction parts, etc., and a manufacturing method thereof, and relates to a wire rod having excellent wire-drawing performance and a method for manufacturing the same.

Steels for mechanical structures used in automobiles and construction parts, for example, parts such as bearings, are typically manufactured by drawing a rolled wire rod and cold-processing the wire rod into a complex shape.

However, since the above steel is a hypereutectoid steel and difficult to process, it is difficult to directly wire the rolled wire. To this end, a spherical soft nitronitride heat treatment is performed, material sizing is performed through wire drawing, and then a soft nitro nitride material is manufactured by correcting the increase in strength due to wire drawing through additional spherical soft nitro nitride heat treatment.

The spherical softening heat treatment as described above is intended to improve cold workability, spherical cementite in the microstructure and induces a homogeneous particle distribution. Through this, it is possible to prevent disconnection during wire drawing, improve the life of the processing die, and lower the hardness of the material to be processed.

However, when the spherical soft nitriding heat treatment as described above takes a lot of heat treatment cost and production time, it causes an increase in manufacturing cost. In addition, it does not meet the demand of the times to minimize energy consumption for carbon reduction. Therefore, in recent years, in providing wire rods used in bearings, etc., there is a demand for developing wire rods capable of securing excellent wire drawing characteristics while omitting or shortening the spheroidal softening heat treatment.

One aspect of the present invention is to provide a wire rod used for mechanical structural parts such as bearings and a manufacturing method thereof, specifically, a wire rod capable of omitting or shortening spherical soft nitriding heat treatment and securing excellent wire drawing characteristics and strength. And to provide a manufacturing method thereof.

The object of the present invention is not limited to the above. Those skilled in the art to which the present invention pertains will have no difficulty in understanding additional tasks of the present invention from the overall details of the present invention specification.

In one aspect of the present invention, in weight%, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: 0.8 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (excluding 0), the remainder including Fe and unavoidable impurities,

The microstructure includes proeutectoid cementite in the pearlite main structure,

Contains 20 or more AlN per unit area (μm ² ) with an average particle diameter of 30 nm or less,

Provided is a wire rod having excellent drawing workability including a microstructure that satisfies the following relational expression 1.

[Relationship 1]

(Average size of block crystal grains (㎛)) ² / (length of pro-eutectoid cementite (㎛/1200㎛ ² )) ≤ 0.5

Another aspect of the present invention, by weight%, C: 0.8 ~ 1.2%, Si: 0.01 ~ 0.6%, Mn: 0.1 ~ 0.6%, Cr: 0.8 ~ 2.0%, Al: 0.01 ~ 0.06%, N: 0.02% Hereafter (except for 0), the rest is heating a steel piece containing Fe and unavoidable impurities, and performing steel piece rolling to prepare a billet;

cooling the prepared billet;

heating the billet to 950-1050 °C;

manufacturing a wire rod by rolling the heated billet; and

Winding the wire rod, cooling at an average cooling rate of 3 ° C / sec or more up to 550 ~ 650 ° C, and cooling at an average cooling rate of 1 ° C / sec or less at a temperature of 550 ~ 650 ° C or less,

The wire rod rolling is performed so that the austenite grain size (AGS) before finish rolling is 5 to 20 μm, and the finish rolling is performed at a deformation amount of 0.3 or more in a temperature range of 730 ° C. to Acm. A method for manufacturing a wire rod is provided.

According to the present invention, it is possible to provide a wire rod for mechanical parts such as bearings having excellent strength and wire-drawing performance and a manufacturing method thereof, even though the spheroidal softening heat treatment can be omitted or shortened. Through this, cost reduction and carbon reduction effects in the manufacturing process can be obtained.

Various advantageous advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the process of describing specific embodiments of the present invention.

1 is a photograph of the microstructure of Inventive Example 1 in Examples of the present invention observed with a scanning electron microscope (Scanning Electron Microscope, SEM).
Figure 2 is a photograph of the microstructure of Comparative Example 5 in Examples of the present invention observed with a scanning electron microscope (Scanning Electron Microscope, SEM).
3 is a photograph of the microstructure of Inventive Example 1 in Examples of the present invention observed by EBSD (Electron Backscatter Diffraction).
4 is a photograph of the microstructure of Comparative Example 5 in Examples of the present invention observed by EBSD (Electron Backscatter Diffraction).

The terms used herein are intended to describe the present invention and are not intended to limit the present invention. Also, the singular forms used herein include the plural forms unless the related definition clearly dictates the contrary.

The meaning of "comprising" as used in the specification specifies a component, and does not exclude the presence or addition of other components.

Unless otherwise defined, all terms including technical terms and scientific terms used in this specification have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. The terms defined in the dictionary are interpreted to have a meaning consistent with the related technical literature and the currently disclosed content.

Hereinafter, the present invention will be described in detail. The inventors of the present invention recognized that in the case of performing spheroidal softening heat treatment on wire rods used for mechanical structural parts such as bearings, it takes a lot of heat treatment cost and time, and acts as an environmental burden. Therefore, even if the spherical soft nitriding heat treatment is shortened or omitted, a method for securing excellent wire drawing performance during wire drawing for manufacturing parts was studied in depth. As a result, it has led to the present invention.

First, a wire rod, which is one aspect of the present invention, will be described in detail.

In the wire rod of the present invention, by weight%, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: 0.8 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less ( 0), the remainder includes Fe and unavoidable impurities. Hereinafter, the role and content of each component will be described. % for each component below means weight%.

Carbon (C): 0.8 to 1.2%

The C is an element added to secure a certain level of strength. When the C content is less than 0.8%, it is difficult to secure sufficient strength even after spheroidal nitriding heat treatment and quenching and tempering heat treatment that proceeds after the forging process due to the decrease in strength of the base material, and when it exceeds 1.2% (FeCr ) Precipitates of new phases such as ₃ C may cause problems such as central segregation during solidification of cast steel such as bloom. Therefore, the content of C is preferably 0.8 to 1.2%, more preferably 0.9 to 1.1%.

Silicon (Si): 0.01 to 0.6%

Si is a typical substitutional element and is added to secure a certain level of strength. If Si is less than 0.01%, it is difficult to secure strength and sufficient hardenability of steel, and if it exceeds 0.6%, there is a disadvantage in that cold forging workability is deteriorated during forging after spheroidal soft nitriding heat treatment. Therefore, the Si content is preferably 0.01 to 0.6%.

Manganese (Mn): 0.1 to 0.6%

Mn is an element that forms a substitutional solid solution in the base structure to strengthen solid solution, and is an element capable of securing desired strength without deterioration of ductility, and is a representative austenite former. If the Mn is less than 0.1%, the strength by solid solution strengthening is not guaranteed, and it is difficult to expect the effect of improving toughness. In addition, when the Mn content exceeds 0.6%, since defects such as chevron cracks may occur due to MnS during forging after spherical soft nitriding heat treatment, the Mn content is 0.1 to 0.6% it is desirable

Chromium (Cr): 0.8~2.0%

Cr, like Mn, is an element that increases hardenability of steel. If the Cr is less than 0.8%, it is difficult to secure sufficient hardenability to obtain martensite during quenching and tempering heat treatment after the forging process, and if it exceeds 2.0%, a large amount of low-temperature structure in the wire is generated due to the promotion of center segregation Chances are higher. Accordingly, the content of Cr is preferably 0.8 to 2.0%, more preferably 1.0 to 2.0%.

Aluminum (Al): 0.01 to 0.06%

The Al is an element that not only has a deoxidizing effect, but also helps to suppress the growth of austenite grains by precipitating Al-based carbonitrides and to secure a pro-eutectoid ferrite fraction close to the equilibrium phase. When the Al content is less than 0.01%, most of the Al is dissolved due to insufficient solid-solution aluminum, so that aluminum nitride (AlN) is not sufficiently produced to suppress austenite grain growth during heat treatment, so it is preferably 0.01% or more. On the other hand, if it exceeds 0.06%, hard inclusions such as Al ₂ O ₃ may increase, and nozzle clogging due to inclusions may occur particularly during playing. Therefore, the Al content is preferably 0.01 to 0.06%.

Nitrogen (N): 0.02% or less (excluding zero)

The N has a solid solution strengthening effect, but when it exceeds 0.02%, the toughness and ductility of the material may be lowered due to solid solution nitrogen that is not bonded to nitride, so the N content is preferably managed to 0.02% or less. .

The rest includes iron (Fe), and since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the art during the manufacturing process, not all of them are specifically mentioned in the present specification.

On the other hand, the microstructure of the wire rod, which is one aspect of the present invention, includes pro-eutectoid cementite in the pearlite main structure. Specifically, the pro-eutectoid cementite is formed in a network at the grain boundary along the old austenite grains, and complete pearlite is formed within the grains. As supersaturated carbon in austenite precipitates as Fe ₃ C during cooling, proeutectoid cementite is formed at the old austenite grain boundary, and proeutectoid cementite takes on a network shape due to refinement of crystal grains, which are diffusion paths of elements.

AlN is precipitated in the microstructure, and it is preferable that 20 or more AlNs having an average particle diameter of 30 nm or less are distributed per unit area (μm ² ). When the average particle diameter of the AlN exceeds 30 nm, since the effect of suppressing crystal grain growth by pinning is significantly reduced, it is preferable to have a size of 30 nm or less, and if it is less than 20 per unit area (μm ² ), , Even if AlN is generated, the number of AlN to suppress grain growth is not sufficient, and thus grains may be coarsened. The number of AlN having a size of 30 nm or less per unit area (μm ² ) is more preferably 50 or more.

On the other hand, the pearlite and pro-eutectoid cementite contain 10% or less of pro-eutectoid cementite in area fraction, preferably the remainder is pearlite, and incidentally contain one or more of pro-eutectoid ferrite, bainite and martensite of 5% or less can If the fraction of the pro-eutectoid cementite exceeds 10%, since toughness may rapidly deteriorate, it is preferable not to exceed 10%. On the other hand, one or more of pro-eutectoid ferrite, bainite, and martensite may partially occur in the wire rod manufacturing process, but if it exceeds 5%, breakage during drawing may easily occur, so it is preferable not to exceed 5%.

During the spheroidal nitrocarburization heat treatment, the grain boundary characteristics play a role in determining the total heat treatment time as a major factor determining the diffusion rate. During the soft nitriding heat treatment, the cementite in the pearlite structure changes its shape from a plate to a spherical shape, and the strength of the material decreases according to the degree of spheroidization.

During the soft nitriding heat treatment, metal atoms move in various diffusion paths through the defect space in the material, such as vacancy, which is an atomic unit defect, and dislocation or pipe, which is a kind of other line defects, and grain boundaries. spread through the Dislocations compared to atomic defects and grain boundaries are advantageous for rapid diffusion because their spaces are relatively wide.

In order to omit the softening heat treatment or shorten the time, it is preferable to increase the relative grain boundary area through crystal grain refinement, but adverse effects such as reduction in equipment life and productivity due to increased rolling load may occur. Accordingly, the wire rod of the present invention can obtain a wire rod having excellent drawing performance through a microstructure that satisfies the following relational expression 1, even if the spherical soft nitriding heat treatment is omitted or shortened.

[Relationship 1]

(Average size of block crystal grains (㎛)) ² / (length of pro-eutectoid cementite (㎛/1200㎛ ² )) ≤ 0.5

The block crystal grains mean crystal grains of a group having the same orientation of ferrite among cementite and ferrite constituting pearlite, and the average size means the average grain size of the crystal grains.

The length of pro-eutectoid cementite means the total length of pro-eutectoid cementite measured in a unit area (1200 μm ² ). As described above, since the proeutectoid cementite is formed along the old austenite grain boundary, the length of the proeutectoid cementite preferably means a length measured along the grain boundary.

The wire rod of the present invention can be drawn by 15% or more without spherical soft nitriding heat treatment before the drawing process, has a tensile strength (TS) of 1200 MPa or more, and a cross-sectional area reduction rate of 20% or more. The wire rod of the present invention can be wire-drawn even if the spherical softening heat treatment is omitted. Commonly used materials may cause defects such as chevron cracks even with a freshness of about 10% due to the coarse grain size. However, defects such as cracks do not occur inside the wire rod of the present invention even when the amount of drawing exceeds 15% and is about 30%. Since rotation of the colony is easy when applying the wire drawing, it relieves external stress and does not cause defects such as cracks with a small amount of wire drawing. In addition, as the wire drawing amount increases, pores such as dislocation and vacancy are generated, further promoting spheroidization behavior during spheroidal soft nitriding heat treatment after wire drawing.

In order to manufacture mechanical parts such as bearing steel with complex shapes, wire rods are manufactured into steel wires, and two rounds of spheroidal nitriding heat treatment and a wire drawing process for sizing of the material are usually applied. Conventional spherical soft nitriding heat treatment is performed at a temperature of Ae1 to Ae1 + 100 ° C, and after heat treatment, carbides having an average cementite aspect ratio of 3 or less are generated in the entire surface to center region. However, the wire rod of the present invention imparts a greater amount of drawing than conventional materials through improved wire drawing processability through the production of fine-grained wire rods, and promotes the generation of spheroidized cementite during spheroidizing heat treatment, so that only one spheroidizing heat treatment after drawing Since the average aspect ratio is 3 or less and a low tensile strength of 740 MPa or less is obtained, it is possible to facilitate cold pressing or cold forging for manufacturing final products.

Next, a method for manufacturing a wire rod, which is another aspect of the present invention, will be described in detail. As a preferred example of manufacturing the wire rod of the present invention, a billet is prepared by heating and rolling a steel piece having the above-described alloy composition, for example, bloom, and heating, wire rod rolling, winding and cooling the billet It can be manufactured by Each step is described in detail below.

First, a steel piece having the above-described alloy composition, for example, a bloom is prepared, and heated to 1100 to 1300 ° C. When the heating temperature of the slab is less than 1100° C., the temperature is low and is not sufficient to diffuse the elements in the slab, making it difficult to resolve the segregated layer generated during casting. On the other hand, when the temperature exceeds 1300 ° C., scale is rapidly formed on the surface of the slab, resulting in surface defects during rolling or reduced productivity due to material loss. On the other hand, the heating time of the steel piece is preferably 2 to 10 hours, and if the heating time of the steel piece is less than 2 hours, it is difficult to reach the target temperature to the inside of the steel piece, and if it exceeds 10 hours, the depth of the surface decarburized layer becomes thicker even after rolling is finished. A decarburized layer may remain, so it is preferable not to exceed 10 hours.

The heated steel piece is rolled into a steel piece to produce a billet. The billet produced after the steel strip rolling is generally cooled to room temperature through air cooling, but in the present invention, the billet at 500 ° C or higher is cooled at a cooling rate of 5 ° C / s or higher. For this purpose, water cooling is preferably performed, and as a specific example, it is preferable to prevent AlN precipitation and coarsening as much as possible by charging the AlN into a water cooling chamber. When the billet is water-cooled below 500° C., AlN is precipitated and coarsened, and AlN is not sufficiently dissolved during billet heating for wire rod manufacturing, which is the next step, making it difficult to secure AlN of 30 nm or less.

The prepared billet is heated to a temperature range of 950 to 1050 ° C. If the billet heating temperature is less than 950 ° C, the rollability is reduced, and if the billet heating temperature exceeds 1050 ° C, rapid cooling is required for rolling, so that not only is it difficult to control cooling, but also cracks occur, resulting in good Ensuring product quality can be difficult. The heating time is preferably 80 to 120 minutes. If the heating time is less than 80 minutes, it is difficult to reach the target temperature to the inside of the material, and an atmosphere in which reverse transformation is not partially completed may occur. If the time exceeds 120 minutes, the depth of the surface decarburized layer becomes thick, and the decarburized layer may remain after rolling is finished, which is not preferable.

The heated billet is wire rod rolled to obtain a wire rod. The wire rod rolling is preferably a ball-shaped rolling in which the billet has a shape of a wire rod. In the present invention, it is preferable to ensure that the austenite grain size (AGS) is 5 to 20 μm before finish rolling in order to refine the crystal grains during final finish rolling. Thereafter, finish rolling is preferably performed at a deformation amount of 0.3 or more in a temperature range of 730° C. to Acm. It is more preferable that the said distortion amount is 0.5 or more.

If the AGS before the finish rolling is less than 5㎛, since it is implemented through rough rolling at a low temperature, there is a problem that the roll load increases and the equipment life is shortened. It is difficult to manufacture wire rods with fine grains. In addition, when the finish rolling temperature is lower than 730 ° C., the rolling roll load increases and the life span of the equipment is shortened. When the finish rolling temperature is higher than Acm, phase transformation does not occur, making it difficult to manufacture fine-grained wire rods.

On the other hand, it is preferable to satisfy the condition of the following formula (2) during the rolling of the wire rod.

[Relationship 2]

2500*([C]-1) ² +100000*([Al]-0.035) ² +(AGS-12.5) ⁴ /130+(finish rolling temperature-760) ² /65 ≤ 80

(In the relational expression (2), [C] and [Al] mean the content (wt%) of alloy composition C and Al, the unit of AGS is ㎛, and the unit of finish rolling temperature is ° C.)

Carbon content affects the production of cementite (Fe ₃ C) in the manufactured wire rod and spheroidized heat treatment material, which affects mechanical properties such as tensile strength, so it must contain appropriate carbon, and Al is the amount The smaller the amount, the smaller the amount of AlN precipitated, so that crystal grain growth cannot be suppressed, so an optimal amount is required. In addition, since the rolling amount and finish rolling temperature must be lowered in order to refine grains as the AGS before finish rolling increases, it is preferable to manage the appropriate AGS and finish rolling temperature from the viewpoint of process cost. The relational expression 2 reflects this technical point of view, and when the value of the relational expression 2 exceeds 80, it is difficult to expect proper cementite formation and grain refinement effect.

After the wire rod rolling, winding is performed and cooling is performed. The cooling is preferably performed at an average cooling rate of 3° C./sec or more to a temperature range of 550 to 650° C., and then cooled at an average cooling rate of 1° C./sec or less after 550 to 650° C. When the average cooling rate up to the temperature range of 550 to 650 ° C is less than 3 ° C / sec, it is difficult to maintain the fine grains obtained during rolling to the transformation point or less. On the other hand, after reaching 550 to 650 ° C, the cooling rate below that is preferably 1 ° C / sec or less in terms of suppressing low-temperature structures such as bainite and martensite.

In the present invention, after freshening the wire rod prepared as described above, heating at Ae1 to Ae1 + 100 ° C, holding for 5 to 15 hours, and then performing spheroidization heat treatment to cool at 20 ° C / hr or less to 660 ° C to prepare a spheroidized material can do. If the heating temperature is less than Ae1, there may be a disadvantage in that the spheroidization heat treatment time becomes long, and if it exceeds Ae1 + 100 ° C, the spheroidization carbide seed may be reduced and the effect of the spheroidization heat treatment may not be sufficient. If the holding time is less than 5 hours, there may be a disadvantage in that the aspect ratio of cementite increases because the spheroidization heat treatment does not sufficiently proceed, and if the holding time exceeds 15 hours, there may be a disadvantage in that the cost increases. If the cooling rate exceeds 20 °C / hr, there may be a disadvantage in that pearlite is formed again due to the fast cooling rate. After the spheroidization heat treatment, the wire rod has a low tensile strength of 740 MPa or less and an average cementite aspect ratio of 3 or less, so that cold pressing or cold forging processing for final product manufacturing can be facilitated.

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

(Example)

After preparing a steel piece (bloom) having the alloy composition (wt%, the rest being Fe and unavoidable impurities) of Table 1 below, steel piece rolling was performed to prepare a billet. The slab was subjected to homogenization heat treatment at 1200 ° C. for 4 hours after casting, and then slab rolling was performed at 1000 ° C. In the case of water cooling in the cooling method of Table 2 after the steel strip rolling, it was air-cooled to 500 ° C, then charged into a water cooling chamber and cooled at a cooling rate of 5 ° C / s or more. Thereafter, a wire rod having a diameter of 9 mm was manufactured from the produced billet under the wire rod manufacturing conditions in Table 2 below. The microstructure and mechanical properties of the wire rod thus prepared were measured, and the results are shown in Table 3. On the other hand, after the wire rod prepared above is wire-drawn, soft spheroidizing heat treatment is performed once (holding at 780 ° C for 8 hours and then cooling to 640 ° C at a cooling rate of 15 ° C / hr) to measure the average aspect ratio and tensile strength of cementite and the results are shown in Table 4.

On the other hand, in Table 2, the austenite grain size (AGS) before finish rolling was collected by cutting the material through a cutting crop performed before finish hot rolling and immediately quenching in water, using the ASTM E112 method. to measure AGS. For the specimens collected, 5 random points of 1/4 from the diameter were measured and expressed as the average value.

The average block grain size was measured using EBSD and ASTM E112 methods. A block is a region with the same crystal orientation of ferrite in pearlite, and the block size is defined as a size having a difference of more than 15 degrees in crystal orientation. Inventive Example 1 and Comparative Example 5 among the following Examples were observed and shown in FIGS. 3 and 4, respectively. The size of the block was quantified using the ASTM E112 method. The measured material was measured at 5 arbitrary points of 1/4 from the diameter of the sample collected after removing the uncooled part after rolling the wire, and then expressed as an average value. In addition, the length of pro-eutectoid cementite was taken by X3000 times using SEM at 1/4 random 5 points from the diameter of the sample taken after removing the uncooled portion after rolling the wire, and using Leica's Clemex vision software The total length was analyzed and the average of 5 points was obtained.

The wire drawing processability evaluation was evaluated by drawing the manufactured 9mm wire rod at a cross-sectional reduction rate of 5 to 50%, and photographing the center of the L section of the drawn material at 5000 times, and chevron cracks such as pearlite interface, proeutectoid cementite interface, etc. ) was confirmed, and the presence or absence was indicated by O/X.

On the other hand, after the one-time spheroidization heat treatment, the average aspect ratio of cementite was photographed with 3 fields of view at 1/4 to 1/2 points in the diameter direction of the wire rod with 3000 times SEM, and using an image measurement program, the long axis / The shortening was measured through statistical processing after automatic measurement.

steel grade C Si Mn Cr Al N steel grade 1 1.05 0.29 0.30 1.69 0.023 0.006 steel grade 2 1.01 0.28 0.35 1.33 0.024 0.005 steel grade 3 0.96 0.25 0.35 1.36 0.023 0.004 steel grade 4 1.00 0.25 0.33 1.36 0.027 0.006 steel grade 5 0.95 0.24 0.33 1.39 0.029 0.005 steel grade 6 1.00 0.30 0.27 1.51 0.025 0.003 steel grade 7 0.97 0.23 0.33 1.41 0.003 0.006 steel grade 8 0.50 0.20 0.27 1.48 0.023 0.006 steel grade 9 0.97 0.20 0.32 1.34 0.030 0.003 steel grade 10 1.04 0.22 0.27 1.57 0.030 0.003 steel grade 11 0.98 0.23 0.27 1.65 0.026 0.003 steel grade 12 0.98 0.28 0.30 1.50 0.024 0.007 steel grade 13 1.03 0.25 0.27 1.60 0.021 0.007

steel grade division Cooling method after steel strip rolling billet
heating temperature
(℃) billet
Heating time (minutes) AGS before finish rolling
(μm) finish rolling
Temperature
(℃) Wrap-up
rolling
amount of deformation Relation 2 Cooling rate up to 600℃
(℃/sec) Cooling rate after 600℃
(℃/sec) steel grade 1 Invention example 1 water cooling 960 83 10 779 0.6 28 3.2 0.6 steel grade 2 Invention example 2 water cooling 1030 92 12 769 One 14 4.7 One steel grade 3 Inventive example 3 water cooling 1050 105 8 761 0.5 22 4.9 One steel grade 4 Inventive Example 4 water cooling 1003 110 7 773 One 16 3.6 0.8 steel grade 5 Inventive Example 5 water cooling 955 97 10 754 0.5 11 4.4 0.7 steel grade 6 Comparative Example 1 air cooling 959 99 12 769 One 11 3.5 0.6 steel grade 7 Comparative Example 2 water cooling 1049 83 8 746 0.8 111 4.3 0.8 steel grade 8 Comparative Example 3 water cooling 1014 105 7 739 0.6 654 3.8 0.9 steel grade 9 Comparative Example 4 water cooling 1237 85 24 747 0.9 142 3.5 0.7 steel grade 10 Comparative Example 5 water cooling 1050 99 12 876 0.5 214 4.9 One steel grade 11 Comparative Example 6 water cooling 982 88 11 758 0.2 10 3 0.6 steel grade 12 Comparative Example 7 water cooling 977 94 12 760 0.5 13 1.2 0.5 steel grade 13 Comparative Example 8 water cooling 968 105 10 761 One 22 3.7 5

In Table 2, relational expression 2 is 2500*([C]-1) ² +100000*([Al]-0.035) ² +(AGS-12.5) ⁴ /130+(finish rolling temperature-760) ^2/65 Calculated, where [C] and [Al] are the contents (wt%) of C and Al in the alloy composition, AGS is the average grain size of austenite in μm, and the unit of finish rolling temperature is ℃

division microstructure The number of AlNs of 30 nm or less per μm ² average block grain size
(μm) cornerstone cementite length
(㎛/1200㎛ ² ) Relation 1 The tensile strength
(MPa) section reduction rate
(%) Invention example 1 Cornerstone C + P 67 4.7 212 0.10 1270 28 Invention example 2 Cornerstone C + P 90 3.2 206 0.05 1251 35 Inventive example 3 Cornerstone C + P 68 6.4 225 0.18 1239 26 Inventive Example 4 Cornerstone C + P 69 3 241 0.04 1262 37 Inventive Example 5 Cornerstone C + P 71 7.2 156 0.33 1292 31 Comparative Example 1 Cornerstone C + P 13 10.2 63 1.65 1178 17 Comparative Example 2 Cornerstone C + P 16 9.7 67 1.40 1182 19 Comparative Example 3 Cornerstone C + P 76 4.3 230 0.08 850 31 Comparative Example 4 Cornerstone C + P 60 12.4 70 2.20 1157 17 Comparative Example 5 Cornerstone C + P 86 11.6 62 2.17 1162 19 Comparative Example 6 Cornerstone C + P 79 12.1 57 2.57 1171 16 Comparative Example 7 Cornerstone C + P 69 9 112 0.72 1192 19 Comparative Example 8 Cornerstone C + P + B + M 48 not measurable 192 not measurable 1491 12

In Table 3, pro-eutectoid C denotes pro-eutectoid cementite, P denotes perlite, B denotes bainite, and M denotes martensite. In addition, relational expression 1 means (average size of block crystal grains (μm)) ² / (length of pro-eutectoid cementite (μm/1200 μm ² )).

division Whether or not internal cracks occur in the material according to the amount of freshness (%) Microstructure and mechanical properties after applied freshness and spheroidization heat treatment 5 10 15 20 30 40 50 Freshness amount before spheroidization heat treatment (%) Average aspect ratio of cementite after spheroidization heat treatment Tensile strength after spheroidization heat treatment (MPa) Invention example 1 ○ ○ ○ ○ ○ ○ X 30 1.7 716 Invention example 2 ○ ○ ○ ○ ○ ○ ○ 20 2.3 724 Inventive example 3 ○ ○ ○ ○ ○ ○ ○ 40 2.5 721 Inventive example 4 ○ ○ ○ ○ ○ X X 30 2.5 730 Inventive Example 5 ○ ○ ○ ○ ○ ○ X 40 1.6 726 Comparative Example 1 ○ X X X X X X 5 5.7 811 Comparative Example 2 ○ ○ X X X X X 10 6.1 807 Comparative Example 3 ○ ○ ○ ○ ○ ○ X 40 2.3 607 Comparative Example 4 ○ X X X X X X 5 7.3 813 Comparative Example 5 ○ X X X X X X 5 6.2 827 Comparative Example 6 ○ X X X X X X 5 6.9 832 Comparative Example 7 ○ ○ X X X X X 10 5.8 789 Comparative Example 8 X X X X X X X 0 7.6 812

As can be seen from Tables 1 to 4, the wire rods of Inventive Examples 1 to 5 satisfying the conditions proposed by the present invention secure excellent drawing workability with a reduction area of 20% or more without spherical soft nitriding heat treatment, and at the same time , high strength of 1200 MPa or more can be secured. In addition, it is possible to provide a wire rod having an average cementite aspect ratio of 3 or less by performing only one spheroidization heat treatment after wire drawing. In particular, FIG. 1 is a photograph of the wire rod microstructure of Inventive Example 1 observed with a scanning electron microscope (SEM). Referring to FIG. 1, Inventive Example 1 is composed of proeutectoid cementite and complete pearlite, and the arrow in FIG. 1 indicates proeutectoid cementite. As shown in FIG. 1, it can be confirmed that the pro-eutectoid cementite is formed along the old austenite grain boundary. 3 is an EBSD photograph of Example 1, wherein the red line indicates a grain orientation difference of 2 to 5 degrees, a green line indicates a grain orientation difference of 5 to 15 degrees, and a blue line indicates a grain orientation difference of more than 15 degrees. Accordingly, it was confirmed that the average block grain size of Inventive Example 1 was about 4.7 μm, which was very small compared to normal manufacturing conditions.

On the other hand, in Comparative Example 1, air cooling was performed after steel strip rolling, resulting in coarsening of AlN in the steel, and in case of Comparative Example 2, almost no AlN was produced due to the low Al content in the steel composition. As a result, in the wires of Comparative Examples 1 and 2, the number of AlNs having a size of 30 nm or less per ㎛ ² was 20 or less, and the size of block grains was not controlled because grain growth could not be suppressed during wire cooling. In Comparative Example 3, the carbon content is low and pro-eutectoid ferrite remains in the wire rod, so the wire drawing characteristics are superior to other comparative examples, but the strength is low due to the low carbon content, which is unsuitable for use due to the low strength of the material even after spheroidization heat treatment. difficult to use properly.

In Comparative Example 4, the AGS size before finish rolling is larger than that of the inventive examples due to the high billet heating temperature. Since grain refinement of coarse AGS can be achieved through a high critical strain amount, an insufficient amount of finish rolling strain eventually causes coarse grains to appear in the wire rod, resulting in poor drawing performance. In Comparative Example 5, fine crystal grains were not obtained due to the high finish rolling temperature, and the drawing characteristics were not excellent due to coarse crystal grains, similar to Comparative Example 4. 2 is a photograph of the wire rod microstructure of Comparative Example 5 observed by SEM, and it can be confirmed that the grain size is larger than that of FIG. 1, and the length of pro-eutectoid cementite produced along the old austenite grain boundary is short. FIG. 4 is an EBSD photograph of Comparative Example 5, and the grain orientation difference is classified as in FIG. 3. As shown in FIG. 4, it can be seen that the block grain size of Comparative Example 5 is coarse.

In Comparative Example 6, fine crystal grains were not obtained due to a small amount of finish rolling, and coarse crystal grains appeared on the wire rod, resulting in poor drawing characteristics. In the wire rod of Comparative Example 7, fine crystal grains made by rolling were coarsened due to a low cooling rate at the beginning, so that fine wire rod crystal grains could not be obtained, resulting in poor drawing characteristics. In the case of Comparative Example 8, due to the fast cooling rate, martensite and bainite appeared, and it was confirmed that internal cracks occurred even with 5% wire drawing.

Claims (10)

In % by weight, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: 0.8 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (excluding 0), The remainder contains Fe and unavoidable impurities,
The microstructure includes proeutectoid cementite in the pearlite main structure,
Contains 20 or more AlN per unit area (μm ² ) with an average particle diameter of 30 nm or less,
A wire rod having excellent drawing workability including a microstructure that satisfies the following relational expression 1.
[Relationship 1]
(Average size of block crystal grains (㎛)) ² / (length of pro-eutectoid cementite (㎛/1200㎛ ² )) ≤ 0.5
The method of claim 1,
The pro-eutectoid cementite is formed at the grain boundaries along the old austenite crystal grains, and is formed in a network shape.
The method of claim 1,
The microstructure is a wire rod having excellent drawing processability of 10% or less of pro-eutectoid cementite and the rest of pearlite in area fraction.
The method of claim 1,
The wire rod has a tensile strength of 1200 MPa or more and a cross-sectional reduction rate of 20% or more, and excellent in drawing workability.
The method of claim 1,
The wire rod is a wire rod with excellent drawing processability that is drawn by 15% or more during drawing without spherical softening heat treatment before the drawing process.
The method of claim 1,
The wire rod is a wire rod with excellent drawing workability having an average aspect ratio of 3 or less of cementite after wire drawing and spheroidization heat treatment.
In % by weight, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: 0.8 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (excluding 0), Heating a slab containing the rest of Fe and unavoidable impurities, and performing slab rolling to produce a billet;
cooling the prepared billet;
heating the billet to 950-1050 °C;
manufacturing a wire rod by rolling the heated billet; and
Winding the wire rod, cooling at an average cooling rate of 3 ° C / sec or more up to 550 ~ 650 ° C, and cooling at an average cooling rate of 1 ° C / sec or less at a temperature of 550 ~ 650 ° C or less,
The wire rod rolling is performed so that the austenite grain size (AGS) before finish rolling is 5 to 20 μm, and the finish rolling is performed at a deformation amount of 0.3 or more in a temperature range of 730 ° C. to Acm. Manufacturing method of wire rod.
The method of claim 7,
The hot rolling is a method for producing a wire rod having excellent wire drawing performance so as to satisfy the condition of the following formula (2).
[Relationship 2]
2500*([C]-1) ² +100000*([Al]-0.035) ² +(AGS-12.5) ⁴ /130+(finish rolling temperature-760) ² /65 ≤ 80
(In the relational expression (2), [C] and [Al] mean the content (wt%) of alloy composition C and Al, the unit of AGS is ㎛, and the unit of finish rolling temperature is ° C.)
The method of claim 7,
A method for producing a wire rod having excellent wire workability by heating the slab at a temperature range of 1100 to 1300 ° C for 2 to 10 hours and cooling the billet at 500 ° C or higher at a cooling rate of 5 ° C / s or higher after rolling the slab.
The method of claim 7,
The billet heating time is 80 to 120 minutes. Method for producing a wire rod with excellent wire processability.

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