JP7159445B2 - Soft heat treatment time shortened cold forging wire and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a wire rod for cold heading with a shortened soft heat treatment time and its manufacture. The present invention relates to a wire rod for inter-heading and a method for manufacturing the same.

A spheroidizing heat treatment is generally performed to soften the wire. The spheroidizing heat treatment induces a homogeneous particle distribution by spheroidizing cementite to improve cold workability during cold forming. In addition, the hardness of the material to be processed can be made as soft as possible in order to improve the life of the working dies. In order to achieve the above two purposes, it is used as a material softening concept.
Such a spheroidizing heat treatment is roughly classified into two. One is a method of heating for a long time below the eutectoid temperature, which is mainly used for spheroidizing treatment of hot-rolled products (sub-critical annealing). The other is a method of heating between the eutectoid temperature and the austenitizing temperature followed by extremely slow cooling to obtain a spheroidized structure (inter-critical annealing).

When the initial structure is made of pearlite, the process of spheroidization at the spheroidizing heat treatment temperature is caused by defects in lamellar cementite due to diffusion at high temperature or carbon due to the difference in curvature with the flat interface at the end portion. It is known that a concentration gradient develops to segment lamellar cementite, which is then spheronized to reduce interfacial energy.
For such a spheroidizing softening treatment, it takes a number of separate steps, a lot of cost, and a long time, so it is preferable to shorten the process time as much as possible. Research is being conducted to develop technology to shorten the processing steps.

Korean Patent Application Publication No. 2018-0072965

The present invention comprises fine pro-eutectoid ferrite having a maximum crystal grain size of 5 μm or less with a pro-eutectoid ferrite fraction of 80% or more in the equilibrium phase, and a bainite/martensite fraction of 5% or less and the balance It is an object of the present invention to provide a wire rod for cold forging with a shortened softening heat treatment time and a method for producing the same by controlling with a composite structure including a pearlite structure.
The technical problems to be solved by the present invention are not limited to the technical problems mentioned above. can be clearly understood by those who have

A wire rod for cold heading capable of shortening the softening heat treatment time of the present invention to achieve the above object,
% by weight, C: 0.15-0.5%, Si: 0.02-0.4%, Mn: 0.3-1.2%, Al: 0.02-0.05%, P: 0.03% or less, S: less than 0.01%, N: less than 0.01%, and the remaining Fe and other inevitable impurities,
The internal fine structure contains 20 to 90 area% pro-eutectoid ferrite structure, 5 area% or less bainite and martensite structures, and the remaining pearlite structure, and 80% or more of the equilibrium pro-eutectoid ferrite fraction is average grains. A proeutectoid ferrite structure with a diameter of 5 μmm or less,
The ultrafine hardness for each microstructure is characterized by satisfying the following relational expressions 1 and 2.
[Relationship 1]
Proeutectoid ferrite: Hardness (Hv) ≥ 128 + 61 * ([C] + [Si] / 8 + [Mn] / 18)
[Relational expression 2]
Perlite: Hardness (Hv) ≤ 254 + 23 * ([C] + [Si] / 8 + [Mn] / 18)

It is preferable that the above C satisfies the range of 0.4 to 0.5%.

The method for producing a wire rod for cold heading that can shorten the softening heat treatment time of the present invention includes:
A step of heating the steel material having the above composition to 900° C. or higher and 1050° C. or lower and then maintaining the temperature within 180 minutes;
A step of controlling the austenite grain size (AGS) of the steel material in the range of 5 to 20 μm;
A step of finish hot rolling the AGS-controlled steel material into a wire rod shape at a temperature of Ae ₃ or less to 730 ° C. or more with a deformation amount of 0.3 to 2.0; cooling at a cooling rate of ~20°C/s.

The above C preferably satisfies the range of 0.4 to 0.5%.

The wire rod manufactured by the above cooling is
The internal fine structure contains 20 to 90 area% pro-eutectoid ferrite structure, 5 area% or less bainite and martensite structures, and the remaining pearlite structure, and 80% or more of the equilibrium pro-eutectoid ferrite fraction is average grains. It is preferable that the proeutectoid ferrite structure has a diameter of 5 μmm or less, and the ultrafine hardness of each microstructure satisfies the following relational expression 1 and relational expression 2.
[Relationship 1]
Proeutectoid ferrite: Hardness (Hv) ≥ 128 + 61 * ([C] + [Si] / 8 + [Mn] / 18)
[Relational expression 2]
Perlite: Hardness (Hv) ≤ 254 + 23 * ([C] + [Si] / 8 + [Mn] / 18)

A step of subjecting the cooled wire to spheroidizing heat treatment may be further included.
The shape ratio of cementite in the pearlite after the spheroidizing heat treatment is preferably 2.5 or less.

According to the present invention having the above configuration, a wire having desired properties can be obtained in a relatively short softening heat treatment time by optimizing the microstructure of the manufactured wire, and accordingly, the manufacturing cost is reduced. And there is a useful effect that can reduce the time.

2 is a structural photograph showing AGS of a steel material before finish hot rolling, (a) showing invention example 1 and (b) showing comparative example 5. FIG. 2 is a microstructure photograph showing the microstructure of a wire rod obtained by cooling after wire rolling, (a) showing invention example 2 and (b) showing comparative example 1. FIG. 4 is a microstructure photograph showing the fine structure of the wire after spheroidizing heat treatment, (a) showing invention example 1 and (b) showing comparative example 1. FIG.

The present invention will be described below.
In the present invention, by weight %, C: 0.15 to 0.5%, Si: 0.02 to 0.4%, Mn: 0.3 to 1.2%, Al: 0.02 to 0.05 %, P: 0.03% or less, S: less than 0.01%, N: less than 0.01%, and the remaining Fe and other inevitable impurities are rolled to form proeutectoid ferrite, crystals The present invention relates to a method for producing a heat-treated shortened wire that induces grain refinement and obtains a soft wire by accelerating the diffusion of carbon during the soft heat treatment of the material.

The compositional components of the wire rod of the present invention and the reasons for restricting the content thereof will be explained. Here, % means weight % unless otherwise defined.

C: 0.15-0.5%
The reason why the content of carbon is limited to 0.15 to 0.5% is that if the content exceeds 0.5%, almost all the structure is composed of pearlite, and the desired pro-eutectoid ferrite On the other hand, if it is less than 0.15%, the crystal grains will not be fine due to an increase in the proeutectoid ferrite fraction, and it will be difficult to transform into a martensite microstructure during the QT heat treatment, and the above martensite structure. This is also because it is difficult to ensure sufficient strength due to the low carbon content.
In the present invention, it is more preferable to limit the carbon content to the range of 0.4 to 0.5%.

Si: 0.02-0.4%
The reason for limiting the content of silicon (Si) to 0.02 to 0.4% is as follows. Si, as a representative substitutional element, has a great effect on ensuring the strength of steel. If its content is less than 0.02%, it becomes difficult to ensure the strength of the steel, while if it exceeds 0.4%, it promotes the formation of a decarburized structure during wire rolling, resulting in additional removal costs. is required, and the strength at the time of forging increases, making it difficult to forge.

Mn: 0.3-1.2%
Manganese (Mn) forms a substitutional solid solution in the matrix structure, lowers the temperature of A1, and refines the gap between the pearlite layers. In order to increase the number of subgrains in the pro-eutectoid ferrite structure, the content is limited to 0.3-1.2%. If manganese is added in excess of 1.2%, manganese segregation will cause inhomogeneity in the structure and have a detrimental effect. Due to the segregation mechanism during solidification of steel, macro-segregation and micro-segregation are likely to occur. It is the main cause of the formation of core martensite. Moreover, when the manganese is added in an amount of less than 0.3%, it may become difficult to ensure sufficient hardenability for ensuring a martensite structure after QT.

Al: 0.02-0.05%
In the present invention, the aluminum content is preferably limited to 0.02 to 0.05%. This is because if the content is less than 0.02%, it becomes difficult to ensure sufficient deoxidizing power, while if it exceeds 0.05%, hard inclusions such as Al ₂ O ₃ increase. This is because there is a risk of nozzle clogging due to inclusions during continuous casting.

N: less than 0.01% In the present invention, the nitrogen content must be controlled at less than 0.01%. This is because if the content is 0.01% or more, there is a possibility that the toughness/ductility of the material may be lowered due to dissolved nitrogen that does not combine as precipitates.

P: 0.03% or less, S: less than 0.01% P and S are mixed as impurities, and P segregates at grain boundaries and reduces toughness, so the content is limited to 0.03% or less. preferably. S is an element with a low melting point, segregates at grain boundaries to reduce toughness, and produces sulfides, which adversely affect the product. .

In addition, the wire rod for cold heading of the present invention has an internal structure containing 20 to 90 area % of pro-eutectoid ferrite structure, 5 area % or less of bainite and martensite structure, and the remaining pearlite structure, and has equilibrium pro-eutectoid ferrite. 80% or more of the fraction is the proeutectoid ferrite structure with an average grain size of 5 μmm or less.

In the present invention, the equilibrium pro-eutectoid ferrite fraction means the pro-eutectoid ferrite fraction according to this principle at a temperature just above A1 in the phase diagram of each composition. In the present invention, Thermo calc. We utilized a phase diagram calculated using a software called

The present invention is characterized by having such a pro-eutectoid ferrite structure with an equilibrium pro-eutectoid ferrite fraction of 80% or more. The pro-eutectoid ferrite fraction of the steel of the present invention is higher than the pro-eutectoid ferrite in the wire rod that is formed and grown during normal cooling, and the pro-eutectoid ferrite during finish rolling is formed and grown at a temperature of Ae ₃ or less to 730 ° C. Since it grows and grows during cooling, it is higher than the pro-eutectoid ferrite fraction in wires of the same composition produced by conventional methods.

Further, the reason why the pro-eutectoid ferrite average grain size is limited to 5 μmm or less in the present invention is that the pro-eutectoid ferrite is rapidly generated during the finish rolling, thereby making the crystal grains finer. This is because the diffusion of carbon is accelerated by the fine crystal grains during the softening heat treatment in the process, and a spheroidized structure can be obtained in a shorter time than usual. The reason why the area ratio of the bainite and martensite structures is controlled to 5% or less is that if the above structures exist, the material may break during the wire drawing process or uncoiling before the softening heat treatment.

In addition, in the case of the wire of the present invention, the hardness of the wire due to the refinement of the crystal grains is higher than that of the ordinary material, and when each microstructure is measured by ultrafine hardness, the ferrite has a higher hardness than that of the ordinary material. In contrast, pearlite has a low hardness value. That is, in the present invention, it is preferable that the ultrafine hardness for each microstructure satisfies the following relational expressions 1 and 2. Satisfying the following relational expressions 1 and 2 has the effect of shortening the spheroidizing time during the same spheroidizing heat treatment. The hardness of each microstructure of the wire improves as the alloying elements (C, Si, Mn) increase. is characterized by a lower hardness value than ordinary materials, and is classified by the following relational expression.
[Relationship 1]
Proeutectoid ferrite: Hardness (Hv) ≥ 128 + 61 * ([C] + [Si] / 8 + [Mn] / 18)
[Relational expression 2]
Perlite: Hardness (Hv) ≤ 254 + 23 * ([C] + [Si] / 8 + [Mn] / 18)

Next, a detailed description will be given of a method for producing an ultra-fine grain wire capable of accelerating the softening heat treatment of the present invention.
A method for producing a wire rod for cold heading that can shorten the softening heat treatment time of the present invention includes a step of heating a steel material having the above-described composition in a range of 900 to 1050 ° C. and then maintaining the steel material for a certain period of time. A step of controlling the austenite grain size (AGS) in the range of 5 to 20 μm, and the steel material in which the AGS is controlled is Ae at a temperature of ₃ or less to 730 ° C. or more and a wire shape with a deformation amount of 0.3 to 2.0 and cooling the finish hot rolled wire rod at a cooling rate of 3 to 20° C./s.

First, in the present invention, a steel material having the above compositional components is heated in the range of 900 to 1050° C. and maintained within 180 minutes. This is because when the heating temperature exceeds 1050 ° C., AGS grows greatly, and there is a problem in inducing pro-eutectoid ferrite with a large amount of deformation during finish rolling and refining the crystal grains. On the other hand, if it is less than 900° C., the equipment will be overloaded due to an increase in rolling reduction during rough rolling. When the holding time exceeds 180 minutes, the AGS grows large for the above reason, and it is necessary to induce pro-eutectoid ferrite with a larger amount of deformation during finish rolling. This is because there is a problem.

Subsequently, in the present invention, the austenite grain size (AGS) of the steel material is controlled within the range of 5 to 20 μm immediately before finish hot rolling. The reason for controlling the austenite grain size (AGS) in this way is to induce pro-eutectoid ferrite and refine the grains even if the amount of deformation is 0.3 or more during finish rolling. If the size exceeds 20 μm, a larger amount of finish rolling is required, making it difficult to refine grains. Since a large amount of deformation is required, there is a problem of process constraints such as increasing the burette size or increasing the material transfer speed in order to reduce the interpass time.

In the present invention, the AGS-controlled steel material is finish hot rolled at a temperature of Ae ₃ or less to 730° C. or more into a wire rod shape having a deformation amount of 0.3 to 2.0.
At this time, it is preferable to control the hot finishing temperature range within a temperature range of Ae ₃ or less to 730° C. or more. This is because when the Ae ₃ temperature is exceeded, no proeutectoid ferrite is formed, which is unfavorable to grain refinement, while when it is less than 730 ° C, pearlite is formed during rolling, which is unfavorable to grain refinement. This is because the rolling temperature is low and the rolling rolls are overloaded.

It is preferable that the amount of deformation is 0.3 to 2.0. When this is 0.3 or less, the deformation amount is small and proeutectoid ferrite cannot be induced, and the crystal grains cannot be refined. On the other hand, when it is 2.0 or more, the deformation amount increases. This is because the amount of rolling is overloaded by the rolling, making it difficult to manufacture the desired material diameter.
Subsequently, in the present invention, the finish hot-rolled wire rod is cooled at a cooling rate of 3 to 20° C./s to obtain a wire rod in which the internal microstructure is controlled to fine grains as described above. be able to. At this time, the reason for controlling the cooling rate in the range of 3 to 20° C./s is to suppress the growth of ferrite grain size (FGS) to 5 μm or less after hot rolling.
Then, in the present invention, the cooled wire can be subjected to a spheroidizing heat treatment.

Specifically, the wire may be quenched after undergoing a 30% cross-sectional reduction wire drawing process, heat-treated at 690° C. for 2 hours, and then quenched. The wire rod of the present invention manufactured by such a method has an increased segmentation speed of cementite in pearlite, an excellent cementite shape ratio of 2.5 or less, and a hardness as low as 10 Hv.

The present invention will be described in detail below with reference to examples.
(Example)
A billet having the composition shown in Table 1 below was wire-rolled to a thickness of 9 mm. The invention examples satisfy the component ranges and production conditions of the present invention, and the comparative examples are outside the production conditions of the present invention.

＊表１の冷却条件は、線材の表面温度が５００℃に到達するまでの冷却速度（℃／ｓ）

*The cooling conditions in Table 1 are the cooling rate (°C/s) until the surface temperature of the wire reaches 500°C.

FIG. 1 is a micrograph showing AGS of a steel material before finish hot rolling, in which (a) shows Inventive Example 1 and (b) shows Comparative Example 5. FIG. AGS was measured utilizing the ASTM E112 method. In the case of Comparative Example 5, since the steel was heated at a higher heating temperature than under other conditions, it can be seen that the AGS before finish rolling is larger than under other conditions. On the other hand, a small AGS before finish rolling can generate a large amount of pro-eutectoid ferrite at grain boundaries due to the amount of deformation during finish rolling. Grain size can be reduced.

Table 2 below shows the microstructure of the wire manufactured under the above manufacturing conditions, and the microstructure and mechanical properties of the softened material subjected to the softening heat treatment.
On the other hand, in Table 2, the proeutectoid ferrite phase fraction was obtained by cutting, polishing, and etching the test piece, obtaining a microstructure photograph through an electron microscope, and identifying the corresponding phase through a program called image j'. The area was calculated from five SEM photographs at ×1000 times per corner condition, and the average value is shown.

After cutting, polishing, and etching the test piece, the ultrafine hardness was measured through an optical microscope to confirm the microstructure site, and the proeutectoid ferrite was measured under the pressure of 0.2942 N of pyramidal diamond pressure particles. After 10 pieces of perlite and 10 perlite were pressed in, the length of the press-in trace was measured, converted into a hardness value, and averaged.

The wire hardness value and the hardness value of the softened material after spheroidizing heat treatment are obtained by cutting, polishing, and etching the test piece, and then using an optical microscope to check the microstructure area for hardness measurement. After press-fitting 1/4 part of the C cross section 10 times with a pressure of 98.1 N of pressure particles, the length of the press-fitting trace was measured, converted into a hardness value, and the average value was obtained.
In addition, the aspect ratio is obtained by taking three SEM photographs at ×5000 times, measuring the long axis/shortened length of all cementite present in the SEM photographs, and calculating the length ratio of each. Average value.

＊表２のａは初析フェライト結晶粒サイズ（μｍ）、ｂは初析フェライト平衡分率（％）、ｃは初析フェライト分率（％）、ｄはフェライト超微細硬度（Ｈｖ）、ｅはフェライト超微細硬度パラメータ、Ｈａｒｄｎｅｓｓ（Ｈｖ）≧１２８＋６１＊（［Ｃ］＋［Ｓｉ］／８＋［Ｍｎ］／１８）、ｆはパーライト超微細硬度（Ｈｖ）、ｇはパーライト超微細硬度パラメータ、Ｈａｒｄｎｅｓｓ（Ｈｖ）≦２５４＋２３＊（［Ｃ］＋［Ｓｉ］／８＋［Ｍｎ］／１８）、ｉは熱処理後、セメンタイトのアスペクト比、そして、ｊは球状化熱処理後の硬さ（Ｈｖ）を示す。

* In Table 2, a is pro-eutectoid ferrite grain size (μm), b is pro-eutectoid ferrite equilibrium fraction (%), c is pro-eutectoid ferrite fraction (%), d is ferrite ultrafine hardness (Hv), e is the ferrite ultrafine hardness parameter, Hardness (Hv) ≥ 128 + 61 * ([C] + [Si] / 8 + [Mn] / 18), f is the pearlite ultrafine hardness (Hv), g is the pearlite ultrafine hardness parameter, Hardness (Hv)≦254+23*([C]+[Si]/8+[Mn]/18), i is the aspect ratio of cementite after heat treatment, and j is the hardness (Hv) after spheroidizing heat treatment.

Specifically, in Comparative Examples 1 and 2, pro-eutectoid ferrite was not induced because finish rolling was performed at a rolling temperature of 887° C. and 862° C., which is Ae ₃ or higher. In the case of Comparative Example 3, since the cooling rate at which the surface temperature of the wire reached 500° C. was as low as 1° C./s, the pro-eutectoid ferrite grew significantly. In the case of Comparative Example 4, since the amount of deformation during finish rolling was as small as 0.1, pro-eutectoid ferrite due to deformation could not be induced. In the case of Comparative Example 5, as described above, the heating temperature was 1120 ° C., which was higher than other conditions, so the AGS before finish rolling was larger than other conditions, and pro-eutectoid ferrite was sufficiently formed during rolling. was not induced.

FIG. 2 shows microstructures of wire rods obtained by cooling after wire rolling, (a) showing invention example 2 and (b) showing comparative example 1. FIG.
In Table 2, the inventive examples compared to the comparative examples are characterized by a small ferrite grain size and a high pro-eutectoid ferrite fraction of 80% or more of the equilibrium fraction. In addition, the ultrafine hardness of the ferrite of the invention example is characterized in that the hardness value of ferrite is higher than that of the comparative example, and the hardness value of pearlite is lower than that of the comparative example.

At this time, the fine structure of the wire rod of the example of the present invention is the ferrite ultrafine hardness parameter, Hardness (Hv) ≥ 128 + 61 * ([C] + [Si] / 8 + [Mn] / 18), the pearlite ultrafine hardness parameter , Hardness (Hv)≦254+23*([C]+[Si]/8+[Mn]/18).
On the other hand, the wires of the present invention showed a wire hardness higher than that of the comparative example by about 10 Hv due to the refinement of the crystal grains, but the hardness of the material spheroidized after heat treatment at 690 ° C. for 2 hours was lower than that of the wires of the invention. 10Hv lower than

FIG. 3 is a structure showing the microstructure of the wire rod after the spheroidizing heat treatment, (a) showing Inventive Example 1 and (b) showing Comparative Example 1. FIG. As shown in FIG. 3, since the aspect ratio of cementite is 2.5 or less, it can be said that the cementite in the pearlite was sufficiently segmented by crystal grain refinement even at 690° C. for a short time of 2 hours. I can confirm.

As described above, the present invention has been illustrated by limited examples and experimental examples, but the present invention is not limited thereto, and a person having ordinary knowledge in the technical field to which the present invention belongs can understand the techniques of the present invention. It goes without saying that various modifications and variations are possible within the equivalent scope of the concept and claims.

Claims (6)

% by weight, C: 0.15-0.5%, Si: 0.02-0.4%, Mn: 0.3-1.2%, Al: 0.02-0.05%, P: 0.03% or less, S: less than 0.01%, N: less than 0.01%, and the remaining Fe and other inevitable impurities,
The internal fine structure contains 20 to 90 area% pro-eutectoid ferrite structure, 5 area% or less bainite and martensite structures, and the remaining pearlite structure, and 80% or more of the equilibrium pro-eutectoid ferrite fraction is average grains. It has a pro-eutectoid ferrite structure with a diameter of 5 μmm or less, and the ultrafine hardness of each microstructure satisfies the following relational expression 1 and relational expression 2. Cold heat treatment time can be shortened. Wire rod for forging.
[Relationship 1]
Proeutectoid ferrite: Hardness (Hv) ≥ 128 + 61 * ([C] + [Si] / 8 + [Mn] / 18)
[Relational expression 2]
Perlite: Hardness (Hv) ≤ 254 + 23 * ([C] + [Si] / 8 + [Mn] / 18) The wire rod for cold heading according to claim 1, wherein the C content satisfies the range of 0.4 to 0.5%, and the softening heat treatment time can be reduced. % by weight, C: 0.15-0.5%, Si: 0.02-0.4%, Mn: 0.3-1.2%, Al: 0.02-0.05%, P: 180 minutes after heating a steel material containing 0.03% or less, S: less than 0.01%, N: less than 0.01%, and remaining Fe and other inevitable impurities to 900 ° C. or higher and 1050 ° C. or lower. process to maintain within
A step of controlling the austenite grain size (AGS) of the steel material in the range of 5 to 20 μm;
A step of finish hot rolling the AGS-controlled steel material into a wire rod shape at a temperature of Ae 3 or less to 730 ° C. or more with a deformation amount of 0.3 to 2.0; cooling at a cooling rate of 20 ° C./s,
The wire rod manufactured by the cooling is
The internal fine structure contains 20 to 90 area% pro-eutectoid ferrite structure, 5 area% or less bainite and martensite structures, and the remaining pearlite structure, and 80% or more of the equilibrium pro-eutectoid ferrite fraction is average grains. A proeutectoid ferrite structure having a diameter of 5 μmm or less,
A method for producing a wire rod for cold heading capable of shortening the softening heat treatment time, wherein the ultrafine hardness for each microstructure satisfies the following relational expressions 1 and 2 .
[Relationship 1]
Proeutectoid ferrite: Hardness (Hv) ≥ 128 + 61 * ([C] + [Si] / 8 + [Mn] / 18)
[Relational expression 2]
Perlite: Hardness (Hv) ≤ 254 + 23 * ([C] + [Si] / 8 + [Mn] / 18) 4. The method for producing a wire rod for cold forging capable of shortening the softening heat treatment time according to claim 3, wherein the C satisfies the range of 0.4 to 0.5%. 4. The method of claim 3, further comprising subjecting the cooled wire to a spheroidizing heat treatment to shorten the softening heat treatment time. 6. The method of manufacturing a wire rod for cold heading according to claim 5 , wherein the cementite in the pearlite after the spheroidizing heat treatment has an aspect ratio of 2.5 or less.

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