JP7221478B6 - Cold heading wire rod for shortening soft heat treatment time and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a wire rod for cold heading and a method of manufacturing the same for shortening the time for softening heat treatment. It relates to a wire rod for cold heading and a method for manufacturing the same.

A spheroidizing heat treatment is generally performed to soften the wire. The spheroidizing heat treatment spheroidizes cementite to improve cold workability during cold forming and induces a homogeneous particle distribution. 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 Publication No. 2018-0072965

The present invention provides fine pro-eutectoid ferrite having a maximum crystal grain size of 5 μm or less in which the pro-eutectoid ferrite fraction of the wire rod structure after rolling is 80% or more in the equilibrium phase, and a bainite/martensite area fraction of 5% or less, It is an object of the present invention to provide a wire rod for cold heading and a method of manufacturing the same that can shorten the softening heat treatment time by controlling with a composite structure containing the remaining 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 soft heat treatment time of the present invention, which has been made 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 structure includes a pro-eutectoid ferrite structure of 20 to 90 area%, a bainite and martensite structure of 5 area% or less, and the remaining pearlite structure, and 80% or more of the equilibrium pro-eutectoid ferrite fraction has an average grain size of 5 The wire has a proeutectoid ferrite structure of μm or less, and the tensile strength of the wire satisfies the following relational expression 1.
[Relationship 1]
TS (MPa)≧279+864*([C]+[Si]/8+[Mn]/18)

In addition, the method for producing a wire rod for cold heading that can shorten the soft heat treatment time of the present invention includes:
A step of heating the steel material having the above composition in the range of 900 to 1050 ° C. and then maintaining it 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 tensile strength of the cooled wire rod satisfies the following relational expression 1.
[Relationship 1]
TS (MPa)≧279+864*([C]+[Si]/8+[Mn]/18)

In addition, the cooled wire is
The internal structure includes a pro-eutectoid ferrite structure of 20 to 90 area%, a bainite and martensite structure of 5 area% or less, and the remaining pearlite structure, and 80% or more of the equilibrium pro-eutectoid ferrite fraction has an average grain size of 5 A pro-eutectoid ferrite structure of μm or less is preferable.
Further, the cooled wire rod is not drawn, and the material is maintained in the temperature range of Ae1 to Ae1 + 40 ° C., then cooled to 660 ° C. at 15 to 30 ° C./hr, and the temperature maintenance and cooling time are measured. It is preferable to further include a spheroidizing heat treatment step for 10 to 15 hours.

According to the present invention having the above configuration, a wire rod having desired properties can be obtained even with a relatively short softening heat treatment time by optimizing the microstructure of the manufactured wire rod. And there is a useful effect that can reduce the time.

Fig. 3 is a structural photograph showing AGS of a steel material before finish hot rolling, in which (a) shows Inventive Example 2 and (b) shows Comparative Example 2; Fig. 3 shows a structure showing a fine structure of a wire rod obtained by cooling after wire rolling, (a) showing Inventive Example 4 and (b) showing Comparative Example 4. Figs.

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 rod characterized by inducing grain refinement and obtaining a soft wire rod by accelerating the diffusion of carbon during the soft heat treatment of the raw 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.

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. This is because the strength at the time of forging increases and forging becomes difficult.

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%. When manganese is added in excess of 1.2%, it has a detrimental effect due to structural heterogeneity due to manganese segregation. 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 content is less than 0.3%, it may become difficult to ensure sufficient hardenability for ensuring the 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 increase. This is because clogging of nozzles due to inclusions during continuous casting may occur.

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, solute nitrogen that does not combine as precipitates may cause deterioration in the toughness/ductility of the material.

P: 0.03% or less, S: less than 0.01% P and S are impurities, and since P segregates at grain boundaries and lowers toughness, the content should be limited to 0.03% or less. is preferred. Also, S is a low-melting element that segregates at grain boundaries to reduce toughness and forms 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 μm or less.
In the present invention, the equilibrium pro-eutectoid ferrite fraction means the pro-eutectoid ferrite fraction according to the principle of leverage at a temperature just above A1 in the phase diagram of each composition. In the present invention, Thermo calc. A phase diagram calculated using software was utilized.

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.

The reason why the pro-eutectoid ferrite average grain size is limited to 5 μm or less in the present invention is that the crystal grains are refined by the rapid formation of the pro-eutectoid ferrite during finish rolling. During the softening heat treatment of 1, diffusion of carbon is accelerated by the fine crystal grains, 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 be broken during the wire drawing process or uncoiling before the softening heat treatment. .

Further, in the present invention, it is preferable that the wire manufactured by the above cooling satisfies the TS parameters of the following relational expression 1. In the case of the present invention, the tensile strength of the manufactured wire is stronger than that of a normal wire due to the grain refinement, so that it has a higher tensile strength that satisfies the TS parameter of the following relational expression 1. It is possible to effectively shorten the softening heat treatment time by miniaturization.

The tensile strength of the wire increases as the alloying elements (C, Si, Mn) increase. It is a feature of the steel of the present invention that the is high. Compared with a normal material, it is classified by the following relational expression 1, and when satisfying the following relational expression 1, there is an effect that a low tensile strength can be obtained with the same spheroidized heat-treated material.
[Relationship 1]
TS (MPa)≧279+864*([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.
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 a steel material having the above composition in a range of 900 to 1050 ° C. 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, and the steel material in which the AGS is controlled is subjected to a wire rod with a deformation amount of 0.3 to 2.0 at a temperature of Ae ₃ or less to 730 ° C. or more. A step of finish hot rolling into a shape and a step of 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., the AGS grows large, and there is a problem in refining the crystal grains by inducing proeutectoid ferrite with a larger amount of deformation during finish rolling. , 900° C., the equipment will be overloaded due to an increase in rolling reduction during rough rolling. When the maintenance time exceeds 180 minutes, the AGS grows greatly for the above reason, and there is a problem in inducing pro-eutectoid ferrite with a larger amount of deformation during finish rolling and refining the crystal grains. Because there is

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 that the billet size must be increased or the material transfer speed must be increased 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 if the temperature exceeds Ae ₃ , proeutectoid ferrite is not formed, which is disadvantageous for grain refinement, and if it is lower than 730 ° C, pearlite is formed during rolling, which is disadvantageous for grain refinement. This is because the rolling temperature is low and the rolling rolls are overloaded.

The deformation amount is preferably 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, by cooling the finish hot-rolled wire at a cooling rate of 3 to 20° C./s, it is possible to obtain a wire in which the internal fine structure is controlled to fine grains. . 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 grains having a ferrite grain size (FGS) of 5 μm or less after completion of hot rolling.

In the present invention, it is preferable that the material is maintained at a temperature of Ae ₁ to Ae ₁ +40° C. and then cooled to 660° C. at a rate of 15 to 30° C./hr. A normal hypo-eutectoid steel wire spheroidizing heat treatment is produced by a method in which the temperature is maintained at a temperature of Ae ₁ to _{Ae 1 +} 40°C and then slowly cooled _. After maintaining in the temperature range, it is preferable to cool to 660° C. at 15 to 30° C./hr, and perform heat treatment for a total of 10 to 15 hours of temperature maintenance and cooling time. The wire rod of the present invention that has undergone the heat treatment process can exhibit a lower tensile strength than a wire rod manufactured by a conventional method due to the accelerated diffusion of C due to grain refinement.

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 2 and (b) shows Comparative Example 2. FIG. AGS was measured utilizing the ASTM E112 method. In the case of Comparative Example 2, it can be seen that the AGS before finish rolling is larger than under other conditions because the steel was heated for a longer time 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. size can be reduced.

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

In addition, the grain size was obtained by cutting, polishing, and etching the test piece, obtaining a microstructure photograph through an electron microscope, and measuring the grain size according to the ASTM E112 standard with 5 SEM photographs at a magnification of × 1000. It is the measured average value.
Also, the tensile strength shows the result of a tensile test performed at a speed of 10 mm/min by preparing a test piece according to ASTM E-8 standard.

＊表２のａ＊は、ＴＳパラメータの関係式１｛２７９＋８６４＊（［Ｃ］＋［Ｓｉ］／８＋［Ｍｎ］／１８）｝により計算された引張強度（ＭＰａ）。

* a* in Table 2 is the tensile strength (MPa) calculated by the TS parameter relational expression 1 {279+864*([C]+[Si]/8+[Mn]/18)}.

In the case of Comparative Example 1, 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 2, as described above, the heating furnace charging time was 207 minutes, which was longer than other conditions. No ferrite was induced.

In the case of Comparative Example 3, pro-eutectoid ferrite was not induced because the finish rolling was performed at a rolling temperature of 842° C. and a temperature of Ae ₃ or higher.
In the case of Comparative Example 4, pro-eutectoid ferrite grew significantly because the cooling rate at which the surface temperature of the wire reached 500° C. was as low as 1° C./s.
Therefore, in the case of Comparative Examples 1 to 4, the average pro-eutectoid ferrite grain size of the final wire is 10 μm or more, which is larger than that of the invention examples, which is the main reason why the wire strength is lower than that of the invention steel. became.

FIG. 2 shows microstructures of wire rods obtained by cooling after wire rolling, (a) showing invention example 4 and (b) showing comparative example 4. FIG.
On the other hand, considering the TS parameter (279+864*([C]+[Si]/8+[Mn]/18) in this example, the tensile strength of the wire of the present invention example is greater than the TS parameter, but the comparative example It can be seen that the tensile strength is smaller than the TS parameter.In other words, the wire of the above invention example has a remarkable tensile strength after spheroidization due to rapid carbon diffusion during the softening heat treatment due to the fine structure. It can be confirmed that it shows a low tensile strength in

As described above, the present invention has been described through limited examples and experimental examples, but the present invention is not limited by these, and a person having ordinary knowledge in the technical field to which the present invention belongs may understand the technical concept of the present invention. And, of course, various modifications and variations are possible within the equivalent scope of the claims.

Claims (4)

% 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 structure includes a pro-eutectoid ferrite structure of 20 to 90 area%, a bainite and martensite structure of 5 area% or less, and a remaining pearlite structure, and the proportion of equilibrium pro-eutectoid ferrite in the pro-eutectoid ferrite is 80%. The proeutectoid ferrite has an average particle size of 5 μm or less ,
A wire rod for cold heading capable of shortening the soft heat treatment time, wherein the tensile strength (TS (MPa)) of the wire rod satisfies the following relational expression 1.
[Relationship 1]
TS (MPa)≧279+864*([C]+[Si]/8+[Mn]/18)
(However, in relational expression 1, [C], [Si], and [Mn] mean weight percent of the corresponding elements.) % by weight, C: 0.15-0.5%, Si: 0.02-0.4%, Mn: 0.3-1.2%, Al: 0.02-0.05%, P: Within 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 in the range of 900 to 1050 ° C. process to maintain,
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,
A method for producing a wire for cold heading capable of shortening the soft heat treatment time, wherein the tensile strength (TS (MPa)) of the cooled wire satisfies the following relational expression 1.
[Relationship 1]
TS (MPa)≧279+864*([C]+[Si]/8+[Mn]/18)
(However, in relational expression 1, [C], [Si], and [Mn] mean weight percent of the corresponding elements.) The cooled wire is
The internal structure includes a pro-eutectoid ferrite structure of 20 to 90 area%, a bainite and martensite structure of 5 area% or less, and a remaining pearlite structure, and the equilibrium pro-eutectoid ferrite fraction of the pro-eutectoid ferrite is 80% or more. , and the proeutectoid ferrite has an average grain size of 5 μm or less. The cooled wire rod is maintained in the temperature range of Ae ₁ to Ae ₁ +40° C. without drawing, then cooled to 660° C. at 15 to 30° C./hr, maintained and cooled in the temperature range. 3. The method of claim 2, further comprising a spheroidizing heat treatment process for a total time of 10 to 15 hours.

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