KR100605722B1 - Method of manufacturing graphite steel rod for machine structural use having excellent free cutting characteristics and cold forging characteristics - Google Patents

Abstract

The present invention relates to a method for manufacturing graphite steel, which is suitable for use in machine parts such as industrial machines or automobiles, and which has excellent cold forging and good machinability, and which has a shortened graphitization treatment time. The present invention relates to a method of manufacturing graphite steel which improves cold forging and free machinability by realizing in-line graphitization and minimizing the variation in strength of each part.

The method of the present invention by weight, C: 0.30-0.70%, Si: 2.0-4.0%, Mn: 0.1-1.0%, P: 0.01-0.15%, S: 0.01% or less, Se: 0.001-0.05% , Ti: 0.001% to 0.03%, B: 0.001% to 0.003%, Al: 0.002% to 0.01%, N: 0.004% to 0.008%, O: 0.005% or less, and include residual Fe and other impurities, and include Ti, A group of N, B, and Al and a group of Mn, Se, and S, respectively, to obtain a billet having a composition satisfying a specific relational expression, winding at 860 to 950 ° C immediately after rolling, and winding the wound wire to 1.8 ± 0.5 Cooling to 770 ± 30 ° C. at a cooling rate of ° C./sec, cooling to 620 ± 50 ° C. at a cooling rate of 0.4 ± 0.2 ° C./sec, and then cooling the air to obtain a graphitized wire; It features.

Graphite Steel, Machinability, Cold Forging, Graphitization, Temperature Deviation

Description

METHODS OF MANUFACTURING GRAPHITE STEEL ROD FOR MACHINE STRUCTURAL USE HAVING EXCELLENT FREE CUTTING CHARACTERISTICS AND COLD FORGING CHARACTERISTICS}

1 is a schematic view showing a typical wire rod rolling and cooling line,

2 is a conceptual diagram of cooling a wire rod coil by using a conventional stealmo cooling method;

3 is a schematic view showing the wire coil overlap state during cooling;

4 is a schematic view of installing a heat reflector on a cooling stand,

FIG. 5 is a view showing the heat reflector plate portion of FIG. 4 in more detail, illustrating a heat transfer mechanism in each portion; FIG.

6 is a conceptual diagram showing the detailed specifications of each part when the heat reflection plate is installed according to the present invention, and

It is a schematic diagram which shows the heat flow in a cooling zone, when the heat reflection board by this invention is installed.

The present invention relates to a method for manufacturing graphite steel, which is suitable for use in machine parts such as industrial machines or automobiles, and which has excellent cold forging and good machinability, and which has a shortened graphitization treatment time. The present invention relates to a method of manufacturing graphite steel which improves cold forging and free machinability by realizing in-line graphitization and minimizing the variation in strength of each part.

Mechanical parts used in industrial machines and automobiles are usually machined into complex shapes of the parts by either cutting or cold forging. However, in the case of the cutting process, in order to process the final shape of the raw material, there is a problem that the amount of processing increases, resulting in large parts loss and excessive production cost. In addition, the cold forging process has the advantage of relatively low component loss compared to the cutting process and can be carried out through a simple process. However, when the final shape of the component is complicated, the forging operation alone cannot realize the complicated shape. . Therefore, it is most suitable in terms of material saving, production cost reduction, etc., after machining into a near net shape by cold forging and final cutting into a complicated shape. In order to perform the cutting process after such cold forging, both machinability and cold forging of the material must be combined, and such steel cannot satisfy all of these properties.

That is, in order to improve the machinability of the material, a free-cutting steel containing machinable elements such as Pb, S, Bi, etc. can be used in steel. It is very excellent in terms of machinability, but cold forging is very poor in cold forging by using cracks caused by inclusions of machinability enhancing elements.

In addition, Pb, which has been conventionally used as a machinability granting element, emits harmful substances such as toxic fume during cutting, and thus is very harmful to the human body. Although S, Bi, Te, Sn, and the like have been proposed, Bi-added steels are easily cracked during manufacturing, which is very difficult to produce, and S, Te, Sn, etc. cause cracks during hot rolling. There is a problem.

Therefore, the free-cutting steel using the conventional machinability imparting element not only has very poor cold forging, but also has a problem of causing environmental problems and low production efficiency, making it difficult to use as a steel material having both cold forging and machinability. It was.

In addition, cold forged steel is excellent in toughness and ductility, so it is advantageous to process it by forging up to a degree similar to the final shape in the cold forging process, and to be similar to the final shape. ) There is a problem in that the machinability such as the treatment property and the surface roughness is very poor, making it difficult to use.

In order to solve the above problems, the proposed steel is graphite steel. Graphite steel is a carbon steel containing fine graphite grains inside, and has excellent impact toughness and ductility, and is excellent in cold forging, and also has good machinability as the internal fine graphite grains become a crack source during cutting. It is a river.

Japanese Unexamined Patent Publication No. 50-096416 discloses C (total): 0.45 to 0.90, Si: 1.0 to 2.5%, Mn: 0.1 to 0.7%, S: 0.015% or less, and one or two of Al and Ti 0.015 to Graphite free cutting steel containing 0.15%, containing 0.45 to 0.90% of graphite present in a distribution of 50 pieces / mm ² or more, and consisting of the balance Fe and impurities has been proposed. The invention has been proposed in terms of free machinability rather than cold forging, but can be referred to as an invention belonging to the invention of the invention for graphite-based free cutting steel. However, the composition of the present invention discloses that the silicon concentration should not exceed 2.3% and the minimum graphitization time accordingly requires about 10 hours. However, when demand is considered in the process, it is inevitable to perform graphitization heat treatment for about 10 hours in order to give free machinability. It is virtually impossible to implement.

In addition, Japanese Patent Laid-Open No. 6-212351 discloses a method for producing graphite free-cutting steel that is graphitized by heat treatment in a relatively short time in order to solve the problem that the graphitization treatment, which is a problem shown in Japanese Patent Application Laid-Open No. 50-096416, takes a long time. Doing. The technique disclosed in the above publication is C: 0.3-1.0%, Si: 0.4-1.0%, Mn to shorten the graphitization treatment time; 0.3 ~ 1.0, P: 0.02% or less, S: 0.015 ~ 0.035, B: 0.001 ~ 0.004%, N: 0.002 ~ 0.008%, and hot-rolled steel bars immediately after hot rolling composed of the balance Fe and unavoidable impurities. The water cooling device installed before and after the line cools the cooling start temperature below Ac1 point, the cooling end temperature below Ms point, and the average cooling rate from 30 ° C / s to 100 ° C / s. After that, the graphitization treatment is then performed at a heating temperature of 600 to 700 ° C. This technique uses the grating distortion, a characteristic of martensitic phase, to facilitate the precipitation of carbon into graphite, which is a technical gist of at least 9-12. It was a technique that could reach 100% graphitization rate within time.

However, even with the above technique, the time required for graphitization is at least 9 hours, so that it is difficult for the demand to be applied to the process, and there is still a need for technical advancement required to shorten the graphitization time.

As a technique for further shortening the graphitization time, Japanese Patent Application Laid-Open No. 2000-063948 has a weight%, more than C: 1.00 to 1.50%, Si: 1.00 to 2.80%, Mn: 0.01 to 2.00%, and P: 0.050% or less. Cast iron or slab containing 0.10% or less, O: 0.0050% or less, N: 0.020% or less, consisting of residual iron (Fe) and unavoidable impurities, and having a graphitization index CE of 1.30 or more, 850 to 1150 ° C. After heating to a temperature in the range of, hot-rolled, and then cooled to room temperature, the hot-rolled steel thus obtained is heated to a temperature within the range of 600 to 1000 ° C. for 3 hours or less, followed by air cooling, and then averaged in the steel. A free-cutting steel, which precipitates 100 particles / mm ² or more of graphite having a particle diameter of 1.0 μm or more, and has a metal structure of 20% or more of ferrite and residual pearlite, or a ferrite-only structure, and has a Brinell hardness of 200 or less. Bar There is a description of the production method is described. If the above technique is used, the time required for graphitization can be shortened to at least 30 minutes, and thus it can be referred to as a technique of drastically improving the graphitization time compared to the prior art. However, even in the case of the above technique, it is still difficult to apply to the actual processing process because the graphitization treatment time, which is at least about 30 minutes and most of the time, is required.

In addition, even in the case of graphite steel having the above excellent properties, when graphite is coarse or unevenly distributed in steel, the free machinability and cold forging property are deteriorated. In other words, if the graphite is unevenly distributed, the water component cloth is uneven during cutting, resulting in poor chip treatment or surface roughness, shortening the tool life, and making it difficult to obtain the advantages of graphite steel, and when coarse graphite is present in the steel. In this case, the possibility of cracking increases, resulting in poor cold forging. Therefore, in order to prevent such a problem, uniform and fine graphite should be dispersed and precipitated in the steel, and the above-mentioned dispersion precipitation may allow the graphite grains to be easily precipitated through the appropriate wire rod cooling process as well as the composition of the graphite steel and internal fine inclusions. shall.

One of the most commonly used cooling methods in the world is the Stelmor cooling method, which uses a cold air blower to blow air while the wire coil is moving on a conveyor, or uses a slow cooling cover to perform ultra-low cooling. Way.

The stealmo cooling method in detail with reference to Figures 1 to 3, on the conveyor of the cooling table (7) as the rolled wire rod (9) is transferred in a ring-like state as the wire coil (9) The overlap density of is different for each part, divided into a high portion of the overlap density edge (12), a mid-edge (14) and a relatively low portion of the center (center) (13) do. When using this method, if the natural cooling proceeds without the device to control the cooling of each part of the material, that is, the wire coil (9), the wire overlap density difference is connected to the cooling temperature deviation.

However, when the cooling temperature deviation occurs when manufacturing the graphite steel, the phase transformation temperature difference is generated by the wire cooling speed difference, and as a result, the variation in tensile strength for each part of the wire rod is intensified. This leads to material deviation of the graphite steel and is a factor that significantly reduces the workability of the material in the subsequent cold forging and cutting operations.

Accordingly, an object of the present invention is to secure a good cold workability (or cold forging) while having similar free machinability without adding low melting point elements such as Pb, Bi, S, Sn, which are added to give high machinability. Its purpose is to provide a method that can produce graphite steel with graphite grains distributed inside the wire rod manufacturing process without any additional graphitization process and to minimize the material deviation of each part that may occur at this time. .

The production method of the present invention for achieving the above object, in weight%, C: 0.30-0.70%, Si: 2.0-4.0%, Mn: 0.1-1.0%, P: 0.01-0.15%, S: 0.01% or less , Se: 0.001 ~ 0.05%, Ti: 0.001 ~ 0.03%, B: 0.001 ~ 0.003%, Al: 0.002 ~ 0.01%, N: 0.004 ~ 0.008%, O: 0.005% or less, balance Fe and other impurities Wherein Ti, N, B, and Al satisfy the following relational formula 1, and Mn, Se, and S obtain a billet having a composition satisfying the following relational formula 2, which is wound up at 860 to 950 ° C immediately after the wire is rolled. Step, cooling the wound wire to 740 ~ 800 ℃ at a cooling rate of 1.3 ~ 2.3 ℃ / sec, cooling to 570 ~ 670 ℃ at a cooling rate of 0.2 ~ 0.6 ℃ / sec and then air-cooled graphitized wire Characterized in that it comprises the step of obtaining.

(However, Ti, N, B, Al, Mn, Se and S in the following relations 1 and 2, respectively, means the weight percent of the element.)

[Relationship 1]

2.0≤ (Ti + 5B + Al) /N≤5.5

[Relationship 2]

1.0≤ (Mn / 5 + Se) /5S≤3.0

At this time, in addition to the above composition Ni: 0.05 ~ 1.0%, Cu: 0.01 ~ 0.5%, Ca: 0.0001 ~ 0.05%, Zr: 0.0005 ~ 0.008%, REM (Rare Earth Metal, rare earth metal): 0.001 ~ 0.05% It is preferred to further include one or two or more selected from the group.

In addition, it is preferable that the size of the austenite grains after the hot rod hot rolling be 30 μm or less.

In addition, the wire rod preferably has a graphite grain size of 15 µm or less and a graphite grain phase fraction of 0.1% or more.

In addition, the wire rod is preferably hot rolled to a diameter of 5.5 ~ 14mm.

In order to minimize the temperature between the edge overlap of the wire rod and the center non-overlap part so as to have a homogeneous graphitized microstructure, it is preferable to install a heat reflex mirror directly under the upper surface of the cooling stand cover during the wire cooling. .

In this case, it is preferable that the ratio W _ref / W _rod of the width of the heat reflecting plate and the width of the wire coil is 0.88 to 0.93.

The length t _e of the downwardly bent end of the heat reflection plate is more preferably 25.0 to 27.5 mm.

In addition, it is preferable that the angle α of the end portion of the heat reflection plate is 25 to 30 °.

Hereinafter, the present invention will be described in detail.

[Lecture composition]

The reason for limiting the alloying elements in the present invention is as follows.

Carbon (C): 0.3-0.7 wt%

Carbon is an essential element for forming graphite, and is an important element for securing the strength of mechanical parts afterwards, but 0.3 to 0.7% by weight since the effect is less than 0.3% by weight and the effect is saturated at more than 0.7% by weight. It is limited to the range of.

Silicon (Si): 2.0-4.0 wt%

Silicon is a necessary component as a deoxidizer in molten steel and is an element that makes carbon carbide precipitate by making iron carbide (cementite) in steel unstable. In addition, silicone is actively added because it is a component that improves strength. Less than 2.0% by weight, the effect is insufficient. Even if more than 4.0% by weight of silicon is added, the effect of promoting graphitization is saturated and the temperature range where liquid phase is generated becomes low, so that the appropriate temperature range is narrowed during hot rolling. It was limited to the range of weight%.

Manganese (Mn): 0.1-1.0 wt%

Manganese is an effective element to secure the strength of steel and is also useful as a deoxidizer in molten steel manufacturing. In addition, it combines with S to form MnS, contributing to the improvement of machinability. However, when the content is less than 0.1% by weight, the effect of improving strength is small, and when the content is more than 1.0% by weight, the toughness is deteriorated. Therefore, the content is limited to the range of 0.1 to 1.0% by weight.

Phosphorus (P): 0.01 wt% or less

Phosphorus not only inhibits graphitization, but also segregates at the austenite grain boundary during the quenching treatment, thereby lowering the grain boundary strength, so that quenching cracks are easily generated.

Sulfur (S): 0.01 wt% or less

Sulfur forms MnS to improve chip breaking during cutting to improve machinability, especially as nucleation of graphitization to promote graphitization, but if the amount is more than 0.01% by weight, the effect is not only saturated. On the contrary, the upper limit was limited to 0.01% by weight because it delayed the graphitization and drastically lowered the toughness of the steel, thus adversely affecting the cold forging.

Selenium (Se): 0.001 ~ 0.05 wt%

Selenium improves chip breaking by forming MnSe in conjunction with manganese. At the same time, MnSe is an element that improves machinability by acting as a graphitization nucleus to promote graphitization. This effect is limited in the range of 0.001 to 0.05% by weight is less than 0.001% by weight and the effect is saturated at 0.05% by weight or more.

Titanium (Ti): 0.001-0.03 wt%

Titanium combines with nitrogen in steel to form TiN, making cementite unstable and at the same time becoming a nucleation site of graphite to promote graphitization. Moreover, since it functions effectively also as a deoxidizer, it adds actively. To attain the above useful effects, at least 0.001% by weight should be added. However, at 0.03% or more by weight, the graphitization is rather hindered, so it is limited to 0.001 to 0.03% by weight.

Boron (B): 0.001 to 0.003% by weight

Boron is an element that is actively added because it combines with N to form BN and acts as a crystal nucleus of graphite while interfering with stabilization of cementite, thereby promoting graphitization and enhancing quenchability. If it is less than 0.001% by weight, the effect of addition is insufficient, so it is necessary to add more than 0.001% by weight. On the contrary, when it is added more than 0.003% by weight, the effect can not be increased any more. The addition range was limited to 0.001 to 0.003% by weight because it lowered the hot workability.

Aluminum (Al): 0.002-0.03 wt%

Aluminum is a powerful deoxidation element and a useful element that not only contributes to deoxidation but also promotes graphitization. In the graphitization heat treatment, the decomposition of cementite is promoted, and at the same time, it binds with nitrogen to form AlN, thereby preventing the cementite from stabilizing. In addition, aluminum oxide produced in the steel by the addition of aluminum is effective in that it also becomes a precipitation nucleus of BN and promotes crystallization of graphite. In the present invention, aluminum is actively added, but if the content is less than 0.002% by weight, it is difficult to expect the effect of addition, and at 0.03% or more by weight, the graphitization promoting action is saturated and the hot deformation is significantly lowered. Limit to the range of.

Nitrogen (N): 0.004-0.008 wt%

Nitrogen is actively added because it combines with titanium, boron and aluminum to form nitrides and use these as nuclei to promote the crystallization of graphite. On the other hand, in order to form nitrides which are effective for promoting graphitization, it is preferable to add in stoichiometric amounts almost equivalent to those of titanium, boron and aluminum, but in order to uniformly finely disperse these nitrides, it is preferable to add slightly higher than the chemical equivalents. In addition, it is advantageous to add a little excessively because nitrogen improves chip treatability by dynamic strain aging. For this reason, it is necessary to add more than 0.004% by weight, but when added more than 0.008% by weight because the effect is saturated, it was limited to 0.004 ~ 0.008% by weight.

Oxygen (O): 0.005% by weight or less

In the present invention, the role of oxygen is important. Oxygen combines with aluminum to form oxides. The production of such oxides reduces the effective concentration of aluminum. As a result, the amount of AlN produced useful for the crystallization of graphite is reduced, thus causing the effect of substantially obstructing the graphitization action. In addition, alumina oxide formed by containing a large amount of oxygen damages the cutting tool during cutting, resulting in a decrease in machinability. For this reason it is desirable to manage as low as possible. However, if the oxygen is managed too low, it causes a refining load of the steelmaking process, and the upper limit is limited to 0.005% by weight or less because the problem caused by the oxygen is not so much up to 0.005% by weight.

As can be seen from above, in order to promote graphitization, first, a carbon source necessary for graphitization is sufficiently maintained in the steel, and the cementite is destabilized so that carbon in the cementite can be easily diffused into the steel. Carbon must be able to grow in graphite on various types of nitride or compound inclusions, which are non-homogeneous nucleation sites in steel.

However, the composition alone may not provide a sufficient nucleation site for graphitization, and the nitride- or emulsion-based inclusions generated by the composition may be effectively finely dispersed in a large amount in the steel to supply a sufficient nucleation site. Accordingly, the graphitization time can be significantly shortened and fine graphite can be produced.

In the present invention, in order to significantly shorten the graphitization and graphitization time, the ratio of the composition ratio between alloy elements is 2.0≤ (Ti + 5B + 2Al) /N≤5.5, 1.0≤ (Mn / 5 + Se) /5S≤3.0 The reason for this is as follows.

It is preferable that (Ti + 5B + Al) / N ratio is 2.0 <(Ti + 5B + Al) / N <= 5.5.

If the (Ti + 5B + Al) / N ratio is less than 2.0, the number of TiN and BN precipitates that contribute to the nucleation of graphite grains due to the lack of Ti, B, Al is insufficient, and the amount of (Ti + 5B + Al) / N When the ratio exceeds 5.5, Ti, B, and Al are sufficient, but the amount of nitrogen is insufficient, so the number of precipitates of TiN, BN, and AlN necessary for graphite nucleation is not increased anymore, but due to excess nitrogen, The amount of nitrogen that is added increases and adversely affects the graphitization rate.

The value of (Mn / 5 + Se) / 5S is preferably 1.0 ≦ (Mn / 5 + Se) /5S≦3.0.

If the value of (Mn / 5 + Se) / 5S is less than 1.0, the number of MnSe precipitates contributing to nucleation of graphite grains and the effective MnS inclusions for machinability are insufficient, and when (Mn / 5 + Se) / 5S ratio exceeds 3.0, Not only the number of MnSe precipitates and MnS precipitates required for graphite grain nucleation is saturated, but the amount of sulfur dissolved in the base material increases, causing grain boundary segregation, which adversely affects mechanical properties.

One or more of the group consisting of 0.05 to 1.0% by weight of nickel, 0.01 to 0.05% by weight of copper, 0.0001 to 0.05% by weight of calcium, 0.0005 to 0.008% by weight of zirconium, and 0.001 to 0.05% by weight of rare earth metal. Two or more kinds are selectively added, and the reason for component limitation is explained.

Nickel (Ni): 0.05-1.0 wt%

Nickel improves the hardenability of steel and is effective in improving the strength of steel by hardening and at the same time assisting and promoting graphitization. The effect was insufficient at 0.05% by weight or less, and at 1.0% by weight or more, the effect was saturated and limited to 0.005 to 1.0% by weight because it was not economical as an expensive element.

Copper (Cu): 0.01-0.5 wt%

Since copper is effective for promoting graphitization by making cementite unstable, not only improves machinability but also has an effect of increasing strength at the time of hardening of steel by the effect of enhancing the hardening of the steel and the strengthening of precipitation. In addition, when copper is added, the corrosion resistance of the steel can be improved. If it is less than 0.01% by weight, the effect of promoting graphitization and improvement of corrosion resistance is insufficient. If it exceeds 0.5% by weight, the improvement effect is saturated and the melting point is lowered at the grain boundary segregation. It was limited to 0.01 to 0.5% by weight because of the high probability of occurrence of surface defects due to grain embrittlement during charging and the impact toughness in the final product.

Calcium (Ca): 0.0001 ~ 0.05 wt%

Calcium improves machinability by forming Ca-Al-based oxides in the steel composition of the present invention, which acts as a nucleus of graphitization to promote graphitization. This effect is less than 0.0001% by weight, and in 0.05% by weight or more coarse oxide-based non-metallic inclusions are generated in a large amount to reduce the fatigue strength of the mechanical parts were limited to the range of 0.0001 ~ 0.05% by weight.

Zirconium (Zr): 0.0005 ~ 0.008% by weight

Zirconium finely disperses oxides such as CaO and Ti2O3 and MnS sulfides. These oxides and sulfides serve as precipitation sites of the graphite to effectively disperse the graphite particles and reduce the graphitization time. However, if the amount of zirconium added is less than 0.0005% by weight, the effect is insufficient. If the amount of zirconium is more than 0.008% by weight, coarse Zr-based sulfides and carbonitrides are formed to reduce the miniaturization effect of oxides by Zr and to deteriorate fracture toughness. It was limited to the range of 0.008% by weight.

Rare Earth Metal (REM): 0.001 ~ 0.05% by weight

Rare earth metals are added for the purpose of improving the hot workability of steel and further promoting graphitization. This action is useful to use La, Ce, etc., but the content is less than 0.001% by weight because the effect is insufficient, and at 0.05% or more because the effect is saturated, it was limited to the range of 0.001 to 0.05% by weight. .

[Manufacturing method of wire rod]

Wire Rod Rolling Process

The wire rod rolling step in the method for producing the graphite steel of the present invention can be carried out in accordance with the usual rolling step. However, it is preferable that the austenite grain size after rolling is 30 micrometers. The reason is that when the size of the austenite grains exceeds 30 µm, the graphitization rate is affected, and it is difficult to control the size of the graphite grains in the wire rod to 15 µm or less and the graphite grain phase fraction to 0.1% or more.

In addition, it is preferable that the diameter of the wire rod manufactured therefrom is 5.5 to 14 mm. If the diameter is smaller than 5.5 mm, the cooling rate of the wire rod is too high in the subsequent graphitization process, so that it is difficult to proceed with the graphitization. This is because it is difficult to manufacture the internal grain size finely due to the insufficient amount of reduced wire rod.

Winding process

Next, the wire is wound, but the winding temperature at this time is preferably 860 ~ 950 ℃. If the finish rolling temperature is high, the coiling temperature is ensured by spraying the coolant immediately after rolling the wire. This is because the time required for graphitization is insufficient when the coiling temperature is less than 860 ° C., and when the coiling temperature is higher than 950 ° C., the winding failure due to high temperature winding is likely to occur.

[Cooling process]

As pointed out in the above-described problems of the prior art, when using the prior art, variation in the cooling rate for each part between the wire overlapping portion and the non-overlapping portion occurs, and thus, the difference in strength of the final product occurs. In this invention, the cooling process which can implement uniform cooling was introduced in order to eliminate such a problem. Cooling the wire rod by installing a heat reflex mirror directly under the upper surface of the cooling stand cover to show the effect of the present invention, first cooled to a cooling rate of 1.3 ~ 2.3 ℃ / sec to 740 ~ 800 ℃. The conditions of cooling temperature and cooling rate are taken into account the difference in the degree of cooling of wire rod integrated state, ie overlapping part and non-overlapping part. That is, if the cooling temperature is higher than 800 ℃ at the cooling rate of 1.3 ~ 2.3 ℃ / sec, the appropriate graphitization transformation time in the cooling zone is insufficient, the graphitization rate is reduced. In addition, when the cooling temperature is less than 740 ° C, ferrite or perlite transformation is more likely to occur than graphitized tissue.

Next, it cools to 0.2-0.6 degreeC / sec to 570-670 degreeC. If the cooling temperature is higher than 670 ℃ the wire graphitization rate is lowered, if less than 570 ℃ temperature control is meaningless because the graphitization in the wire is completed. In addition, if the cooling rate is faster than 0.6 ℃ / sec, the graphitization rate can be reduced, and if it is slower than 0.2 ℃ / sec, the wire rod is cooled before securing the appropriate cooling temperature range of 570 ~ 670 ℃ due to the limitation of the length of the cooling equipment It is difficult to be discharged out of the facility, reducing the wire graphitization rate.

Next, air-cooling, since the transformation is complete, the change in cooling rate has no effect on the tissue. In the wire rod obtained by cooling, the graphite grain size is 15 µm or less, and its phase fraction is preferably 0.1% or more. When the graphite grain size is more than 15 μm, surface defects may be caused rather than the effect of improving cold forming property. In addition, when the graphite grain phase fraction is less than 0.1%, there is no tissue softening effect for improving cold forming.

Cooling deviation control method

Next, in order to show the effect of the present invention in the cooling process after winding the wire rod, as shown in Figure 1, a heat reflex mirror is installed directly under the upper surface of the cooling stand cover, so that the thickness of the heat reflecting plate, the width of the heat reflecting plate, and the width of the wire coil are By controlling the ratio of the ratio, the length of the end of the heat reflector, and the angle of the end of the heat reflector, the cooling speed variation between the wire overlapping edge overlapping portion and the center of relatively low overlapping density is minimized.

An installation example of a heat reflex mirror on the upper surface of the cooling stand cover for showing the effect of the present invention will be described with reference to FIGS. 4 and 5.

As shown in FIGS. 4 and 5, a reflex mirror 15 mounted at the bottom of the slow cooling cover 8 absorbs the radiant heat generated from the high temperature wire rod (or wire coil) 9. It minimizes the temperature reduction rate in the center of the wire rod by reflecting as much as possible to the center where the coil overlap density is relatively lower than the side part.

On the other hand, the side part, which has a relatively high overlap density, has the same cooling state as the conventional method, so that the heat loss due to conduction and convection is smaller than that of the center part. It becomes possible.

6 shows the detailed shape of the heat reflector and the shape of the wire rod that receives heat from the wire rod placed on the cooling table and reflects and recovers radiant heat to a central portion having a relatively low wire overlap density. As the material of the reflecting plate used, stainless steel having low thermal conductivity (for example, SUS 316L) is preferably used.

On the other hand, in order to achieve the object of the present invention, it is necessary to set various parameters and ratios between them for the heat reflector shape design. The reason for limiting the range of the above parameters and ratios for the geometric shape design of the invention is as follows.

The slow cooling cover thickness (t _cover ) is preferably 10 to 15 mm since it is directly related to heat loss due to convection outside the center of the slow cooling cover.

The reason for limiting the thickness (t _ref ) of the central part of the reflector to 13 to 18 mm is to prevent the deformation of the reflector when the reflection proceeds in the state of thermal balance after the high temperature wire radiation is absorbed by the reflector. . Therefore, when the thickness of the central part of the reflector is less than 13mm, the thermal equilibrium state is difficult to be achieved in the reflector itself, and thus heat conduction to the cover becomes easy, and convection often occurs out of the cover, and thus it is difficult to function properly as a heat reflector. In other words, if the thickness of the central part of the reflector is more than 13mm, it will lead to increase of the weight of the reflector and cause deflection of the slow cooling cover.

The reason for limiting the ratio of the width of the reflector to the width of the wire coil (W _ref / W _rod ) is 0.88 ~ 0.93. This is to prevent reflection from the side part. Therefore, when the ratio of the width of the reflector to the width of the wire coil is less than 0.88, it only reflects the radiation heat to about half of the wire. .

It is necessary to limit the maximum size (t _e ) of the bent portion at the tip of the reflector to the range of 25.0 to 27.5 mm. The reason for this is that the reflectance of the reflected radiant heat to the center of the wire coil becomes smaller at 25.0 mm or less, and the effect of maximizing re-reflection is saturated at 27.5 mm or more.

The angle of the end of the reflector (α) is limited to the range of 25 to 30 ^o because the reflectance of the reflected radiant heat to the center of the wire coil becomes smaller at 25 ^o or less, and at 30 ^o or more, the effect of maximizing the re-reflectance is more effective. Because it is saturated.

The reason for limiting the distance h between the heat reflector and the material coil to one tenth (50 to 60 mm) of the normal distance is as follows. In general, wire rod plants around the world that manufacture wire rods have a distance between the Strollmore cooling table table rollers and a slow cooling cover of 500 to 600 mm. However, the present invention is to reduce the amount of temperature reduction in the center portion of the wire rod through heat reflection as much as possible, and thus the efficiency of the heat reflection plate should be increased. Thus, the distance between the heat reflection plate and the material coil should be maintained at 60 mm or less in order to increase the efficiency of the heat reflection plate. However, when the rear end of the material coil comes out of the laying head (a device wound in the form of a coil after wire rolling), the height is often increased due to uneven coiling, and thus the material coil and heat If the spacing of the reflector is too small, the rear end of the material coil may be damaged by hitting the reflector. Thus, the gap h should be 50 mm or more to prevent such an accident.

7 is a schematic diagram of the heat transfer flow in the cooling table when the present invention is used. Since the heat flux q ₂ due to conduction and convection in the cover side portion is greater than the heat flux q ₁ in the center portion of the cover, the temperature reduction of the wire coil side portion is higher than the center portion. So it is possible to homogenize the temperature distribution in the central and side sections.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The billet having the component system of the inventive steel or the comparative steel as shown in Table 1 and Table 2 is heated at a heating rate of 20 ° C./min to the Ac1 transformation point temperature, and then heated at a heating rate of 10 ° C./min after the abnormal pass, and then heated after the Ac3 transformation point. Heated at 15 ℃ / min heating rate and maintained at 1050 ℃ heating rate and maintained for 40 minutes, and then rolled by high speed wire in a conventional manner to produce 14mm wire rod, and wound right after rolling the wire under the conditions of Table 3 and Table 4. And cooled and air cooled.

Steel grade C, wt% Si, wt% Mn, wt% S, wt% P, wt% Ti, wt% B, ppm Al, wt% Se, wt% N, ppm O, ppm Ni, wt% Cu, wt% Ca, wt% Zr, ppm REM, ppm foot                                                  persons                                                  River One 0.51 3.1 0.51 0.008 0.011 0.007 20 0.005 0.004 60 20 0.1 2 0.52 3.1 0.40 0.009 0.015 0.004 10 0.004 0.003 55 21 0.1 0.05 3 0.50 3.0 0.48 0.007 0.009 0.010 21 0.005 0.002 56 19 0.01 4 0.49 2.9 0.55 0.009 0.008 0.010 20 0.003 0.010 60 24 10 5 0.53 3.2 0.53 0.008 0.009 0.009 30 0.005 0.004 65 22 10 6 0.7 3.1 0.48 0.007 0.008 0.004 13 0.003 0.002 75 17 0.09 0.04 7 0.31 3.2 0.51 0.008 0.011 0.010 26 0.004 0.002 66 29 0.16 0.06 8 0.53 4.0 0.55 0.009 0.009 0.003 17 0.004 0.005 60 23 0.20 0.09 9 0.49 2.8 1.00 0.008 0.015 0.011 29 0.015 0.005 55 30 0.25 0.11 10 0.66 2.05 0.44 0.009 0.009 0.003 20 0.010 0.006 80 40 0.54 0.21 ratio                                                  School                                                  River One 0.51 3.0 0.51 0.008 0.011 0.001 10 0.003 0.004 65 19 0.13 2 0.52 3.1 0.48 0.009 0.008 0.010 20 0.005 0.002 40 23 0.12 0.04 3 0.50 3.0 0.22 0.007 0.130 0.004 10 0.004 0.001 55 17 0.02 4 0.49 2.9 0.71 0.009 0.009 0.003 17 0.004 0.021 60 21 20 5 0.53 3.2 0.19 0.008 0.129 0.001 10 0.003 0.001 65 20 10 6 0.8 3.1 0.70 0.007 0.009 0.010 20 0.005 0.019 40 28 0.21 0.10 7 0.31 3.2 0.69 0.008 0.009 0.001 10 0.003 0.020 65 15 0.51 0.23 8 0.53 4.0 0.20 0.009 0.131 0.010 20 0.005 0.001 40 34 0.24 9 0.49 1.5 0.52 0.008 0.015 0.011 15 0.015 30 25 10 0.66 2.5 0.44 0.009 0.009 0.044 21 0.03 100 32

Steel grade Relationship 1 Relation 2 Austenitic Average Crystal Grain Size (㎛) (Ti + 5B + Al) / N (Mn / 5 + Se) / 5S Inventive Steel 1 4.5 1.9 25 Inventive Steel 2 3.1 1.1 15 Inventive Steel 3 5.5 2.2 20 Inventive Steel 4 4.3 3.0 30 Inventive Steel 5 5.4 2.4 25 Inventive Steel 6 2.1 2.5 15 Inventive Steel 7 4.1 1.9 30 Comparative Steel 1 1.4 1.9 40 Comparative Steel 2 6.5 2.5 55 Comparative Steel 3 3.1 0.6 45

In order to measure the cooling temperature variation in the center and side part of the overlapping density of each wire part, the temperature change of each wire part was measured 10 times according to the change of time after the start of cooling by using a contact thermometer (Thermo Couple) of diameter 1.6mm. It is expressed as an average value.

Tensile experiments were performed at a cross head speed of 5 min / sec to measure tensile strength of the materials prepared as described above. Tensile test specimens were cut by 200 mm for each part of the wire coil without any specific specifications, and then subjected to an experiment immediately by being bitten by the tension machine without additional machining. Tensile strength measurement position was measured at 8 equal parts of wire cross section, and the measured value was based on the average value.

In addition, the graphitized tissue phase fraction of the wire rods prepared was measured using an image analyzer, wherein the test surface was based on 300 mm ² .

The temperature deviation for each site was measured for the inventive examples and comparative examples prepared as described above, and the results are shown in Tables 3 and 4.

 Steel grade Winding temperature (℃) Primary cooling temperature (℃) Cooling rate deviation up to the first cooling temperature (℃ / sec) 2nd cooling temperature (℃) Cooling rate deviation up to secondary cooling temperature (℃ / sec) Wire rod overlapped graphite granules (%) Wire rod non-overlapping graphite grain phase percentage (%) Tensile strength distribution in wire ring (kg / mm ² ) (based on 16mm) Inventive Steel 1 900 740 2.3 ± 1.0 670 0.9 ± 0.7 2.0 0.7 64 ± 11 Inventive Steel 2 860 700 2.8 ± 1.0 700 0.7 ± 0.5 2.0 0.9 65 ± 12 Inventive Steel 3 900 740 2.8 ± 1.4 640 0.9 ± 0.7 2.0 0.8 63 ± 10 Inventive Steel 4 950 800 2.8 ± 1.5 660 0.9 ± 0.7 2.0 0.9 61 ± 12 Inventive Steel 5 900 740 2.8 ± 1.4 680 0.9 ± 0.7 2.0 0.8 68 ± 13 Inventive Steel 6 900 800 2.3 ± 1.4 570 0.7 ± 0.5 2.0 0.9 64 ± 11 Inventive Steel 7 950 770 2.8 ± 1.4 660 0.9 ± 0.7 2.0 0.9 66 ± 10 Comparative Steel 1 900 740 2.3 ± 1.0 670 0.9 ± 0.7 0 0 101 ± 13 Comparative Steel 2 900 740 2.3 ± 1.0 670 0.9 ± 0.7 0.7 0 92 ± 11 Comparative Steel 3 900 740 2.3 ± 1.0 670 0.7 ± 0.7 0.5 0 92 ± 12

division Details (Inventive Lecture 2) Heat reflection width and wire coil width ratio (Wref / Wrod) Heat reflector end length (t _e ) Heat reflector end angle (α) Cooling rate deviation up to the first cooling temperature (℃ / sec) Cooling rate deviation up to secondary cooling temperature (℃ / sec) Wire rod overlapped graphite granules (%) Wire rod overlapped graphite granules (%) Tensile strength distribution in wire ring (kg / mm ² ) (based on 16mm) foot                                                  persons                                                  Yes Inventive Example 1 0.88 25.0 30.0 1.8 ± 0.4 0.4 ± 0.2 2.0 1.8 55 ± 4 Inventive Example 2 0.89 25.5 29.0 1.7 ± 0.4 0.4 ± 0.2 2.0 1.7 56 ± 3 Inventive Example 3 0.90 26.0 28.0 1.8 ± 0.4 0.4 ± 0.2 2.0 1.9 54 ± 3 Inventive Example 4 0.91 26.5 27.0 1.8 ± 0.5 0.4 ± 0.2 2.0 1.7 55 ± 2 Inventive Example 5 0.92 27.0 26.0 1.7 ± 0.4 0.4 ± 0.2 2.0 1.8 55 ± 3 Inventive Example 6 0.93 27.5 25.0 1.7 ± 0.4 0.4 ± 0.2 2.0 1.6 54 ± 2 Comparative Example Comparative Example 1 0.86 24.0 24.0 2.4 ± 1.1 0.8 ± 0.5 2.0 0.8 62 ± 11 Comparative Example 2 0.87 24.5 23.0 2.5 ± 1.2 0.8 ± 0.6 2.0 0.7 61 ± 12 Comparative Example 3 0.94 27.5 22.0 2.3 ± 1.0 0.7 ± 0.5 2.0 0.9 60 ± 10 Comparative Example 4 0.95 28.0 21.0 2.4 ± 1.0 0.8 ± 0.5 2.0 0.8 60 ± 11

When the wire rod is cooled without using the heat reflection plate on the upper part of the wire cooling stand to show the effect of the invention, as shown in Table 3, cooling to the first and second cooling temperatures of the inventive steels 1 to 7 according to the present invention. While the speed deviation range was about 0.7 ~ 1.5 ℃ / sec and the tensile strength deviation in the wire ring was 50 ~ 77kg / mm ² , the variation range of cooling speed up to the first and second cooling temperature for the comparative steel is the present invention. Similarly, the tensile strength variation in the wire ring of about 0.7 ~ 1.5 ℃ / sec is 80 ~ 114kg / mm ² It can be seen that the tensile strength is significantly higher than the invention steels.

In addition, the inventive steels (1 ~ 7) showed a graphite grain phase ratio of 1.7 to 2.2% in the wire microstructure after cooling the wire, whereas the comparative steels (1 ~ 3) saw graphite grain phase ratio in the range of 0 to 0.4%. It can be seen that the graphitization rate or graphitization rate is very slow or small compared to the inventive steels. Therefore, it can be seen that the inventive steels are very effective in promoting graphitization. In view of the results shown in Table 3, even though the composition of the comparative steel is similar to the invention steel and the cooling pattern, since the graphitization rate is low, there is a large amount of solid solution carbon in the structure, thus increasing the tensile strength of the wire rod and increasing the graphitization rate. It can be seen that it is also unsuitable for producing graphite steel having excellent cold forging property and free machinability to be achieved by the present invention.

Table 4 shows a case in which a heat reflection plate is applied to the upper part of the wire cooling plate when the wire is manufactured by using the billet having the composition of Inventive Steel 2 in Table 1 to show the effect of the present invention. (1 ~ 6) ranges from 1.3 ° C / sec to 2.3 ° C / sec in the primary, and 0.2 ° C / sec to 0.6 ° C / sec in the secondary for cooling rate deviations up to the primary and secondary cooling temperatures. On the other hand, Comparative Examples (1 to 4) show a range of 1.3 ° C./sec to 3.7 ° C./sec, and in the second case, 0.2 ° C./sec to 1.4 ° C./sec. It can be seen that it can be reduced. Due to this, the tensile strength deviation in the wire ring after wire rod cooling showed that the examples of the present invention showed a range of 48 kg / mm ² to 60 kg / mm ² , but the comparative examples showed a range of 49 kg / mm ² to 73 kg / mm ^2. It can be seen that the steel is very effective in improving the tensile strength variation in the wire ring while significantly lowering the tensile strength required for cold forming.

In addition, as shown in Table 4, the ratio of the width of the heat reflector to the wire coil width W _ref / W _rod is 0.88 to 0.93, the maximum thickness t _e of the end of the heat reflector is 40 to 42.5, and the end of the heat reflector is It can be seen that when a is between 25 and 30 degrees, the cooling temperature deviation between the wire coil edge and the center can be minimized significantly.

Table 5 shows the machinability and cold forging test results for all steel grades shown in Table 1. In order to evaluate the machinability, the graphitized wire rod was cut into an automatic lathe according to a preferred embodiment of the present invention. The machinability was determined by chip treatment and tool life. Determination of chip throughput was excellent when the chip was divided into two volumes or less, and poor when the chips were divided into three to six volumes, and poor when seven or more books were divided. In the test of tool life, the tool life was measured by measuring the time from the tip of the die tip to the inability to cut when cutting in the environment using cutting oil at cutting speed of 150m / min and 0.20mm / rev.

Cold forging was made by using the test piece manufactured with diameter 10mm x height 15mm as the criterion for the critical cold forging rate. At this time, the critical cold forging rate was performed 10 times, and the remaining value except the maximum minimum value was evaluated as the average value. The results of the test are shown in Table 5.

division Graphitization properties Cold Forging Machinability Graphitization Time (min) Graphite fraction (%) Graphite Grain Average Size (㎛) Critical cold forging rate (%) Chip Processing Tool life (min) Inventive Steel 1 5 2.1 7 130 Great 120 Inventive Steel 2 6 2.2 8 140 Great 110 Inventive Steel 3 5 2.0 9 130 Great 130 Inventive Steel 4 7 1.9 7 135 Great 115 Inventive Steel 5 7 2.1 6 140 Great 140 Inventive Steel 6 6 2.5 8 130 Great 120 Inventive Steel 7 7 1.2 7 140 Great 130 Inventive Steel 8 5 2.1 7 135 Great 125 Inventive Steel 9 6 2.0 6 135 Great 140 Inventive Steel 10 5 2.7 9 140 Great 135 Comparative Steel 1 350 2.0 25 70 usually 40 Comparative Steel 2 400 2.0 30 75 usually 35 Comparative Steel 3 10 2.1 20 80 usually 50 Comparative Steel 4 9 2.2 17 90 usually 60 Comparative Steel 5 600 2.1 21 85 usually 40 Comparative Steel 6 440 2.9 33 70 usually 30 Comparative Steel 7 530 1.3 20 80 usually 15 Comparative Steel 8 380 2.0 19 80 usually 45 Comparative Steel 9 3600 2.2 24 75 usually 20 Comparative Steel 10 3900 2.6 35 65 usually 25

As shown in Table 5, the graphitic grain percentage of the non-overlapping portion of the wire rod shows a range of 0 to 0.3% for the comparative steels, while the steels of the present invention are almost completely graphitized to 1.9 to 2.7%. Graphite grain size related to free machinability also showed that the inventive steels had a fine distribution of 10 μm or less.

Meanwhile, in the case of cold forging, the cold critical forging of the comparative steels is in the range of 65 to 80%, while the inventive examples are considerably superior in the range of 125 to 135%. In addition, in the case of free cutting, it can be seen that the present invention steels are remarkably improved as compared with the comparative steels in terms of chip treatment and tool life.

As can be seen from the above, according to the present invention, by using a wire having a high graphitization rate and a high graphitization rate, it is possible to provide an inline graphitization technique which can be graphitized immediately in the cooling process after rolling the wire. Graphitized wire rod by the invention can minimize the material deviation due to the uniform cooling rate, it is possible to improve the workability during cold forging work and cutting work.

Claims (9)

By weight%, C: 0.30 ~ 0.70%, Si: 2.0 ~ 4.0%, Mn: 0.1 ~ 1.0%, P: 0.01 ~ 0.15%, S: 0.01% or less, Se: 0.001 ~ 0.05%, Ti: 0.001 ~ 0.03 %, B: 0.001% to 0.003%, Al: 0.002% to 0.01%, N: 0.004% to 0.008%, O: 0.005% or less, and the balance is made of Fe and other impurities, and the Ti, N, B and Al are Obtaining a billet having a composition satisfying the following Expression 1 and Mn, Se and S satisfying the following Expression 2; Winding at 860 ~ 950 ° C. immediately after the wire has been rolled; Cooling the wound wire to 740-800 ° C. at a cooling rate of 1.3-2.3 ° C./sec; Cooling to 570-670 ° C. at a cooling rate of 0.2-0.6 ° C./sec; And Air cooling to obtain a graphitized wire; The wire rod is made of graphite, the size of the graphite grains 15㎛ or less, the graphite grain phase fraction is 0.1% or more characterized in that the cold forging and excellent machinability excellent mechanical structure manufacturing wire rod manufacturing method. (However, Ti, N, B, Al, Mn, Se and S in the following relations 1 and 2, respectively, means the weight percent of the element.) [Relationship 1] 2.0≤ (Ti + 5B + Al) /N≤5.5 [Relationship 2] 1.0≤ (Mn / 5 + Se) /5S≤3.0 The method according to claim 1, wherein in addition to the billet composition, Ni: 0.05 to 1.0%, Cu: 0.01 to 0.5%, Ca: 0.0001 to 0.05%, Zr: 0.0005 to 0.008%, REM (Rare Earth Metal): A method for producing graphitized wire for mechanical structure with excellent cold forging and good machinability, comprising at least one selected from the group consisting of 0.001% to 0.05%. The method of claim 1 or 2, wherein the austenite grain size is 30 µm or less after the hot rolling of the wire rod. delete The method of claim 1 or 2, wherein the wire rod is hot rolled to a diameter of 5.5 ~ 14mm. The method of claim 1 or 2, wherein a heat reflex mirror is disposed directly below the upper surface of the cooling stand cover during cooling of the wire rod to minimize homogeneity by minimizing the temperature deviation between the edge overlapping portion and the center non-overlap portion of the wire coil. A method for producing a graphitized wire for mechanical structure with excellent cold forging and free machinability, characterized by having a graphitized microstructure. The method of claim 6, wherein the ratio of the width of the heat reflecting plate to the width of the wire coil is W _ref / W _rod is 0.88 ~ 0.93 characterized in that the cold forging and excellent machinability excellent mechanical structure. 7. The method of claim 6, wherein the length t _e of the downwardly bent end of the heat reflection plate is 25.0 to 27.5 mm. The method of claim 6, wherein the angle α of the end of the heat reflection plate is 25 to 30 degrees.

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