KR101053409B1 - In-line spheroidized steel wire, and a manufacturing method thereof - Google Patents

Abstract

The present invention relates to a perforated steel steel which can be omitted spheroidizing heat treatment, a steel wire wired in-line spherical from the perforated steel steel, and a method for manufacturing the same. Steels that can be manufactured from super-vacuum steel wires that can be cold-processed as they are rolled, even without the spheroidizing process, which has a cementite having a spherical shape, and manufactures the steel wires manufactured from the steels and the steel wires It is about a method.

Steels of the present invention in weight%, carbon: 0.9-1.4%, silicon: 0.3-0.7%, manganese: 0.3-0.7%, aluminum: 0.75-1.45%, chromium: 1.3-1.7%, nickel: 0.3-0.7% , Titanium: 0.1-0.3%, niobium: 0.05-0.2%, vanadium: 0.05-1.5%, and the remaining Fe and other unavoidable impurities.

Spheroidizing heat treatment, omission, roughened steel, cold working

Description

HYPER-EUTECTOID STEEL, IN-LINE SPHEROIDIZED WIRE ROD AND MANUFACTURING METHOD FOR THE SAME}

The present invention relates to a perforated steel steel which can be omitted spheroidizing heat treatment, a steel wire wired in-line spherical from the perforated steel steel, and a method for manufacturing the same. Steels that can be manufactured from super-vacuum steel wires that can be cold-processed as they are rolled, even without the spheroidizing process, which has a cementite having a spherical shape, and manufactures the steel wires manufactured from the steels and the steel wires It is about a method.

The spheroidizing heat treatment of wire rod is suitable for increasing the share of ferrite structure by changing the plate-like cementite constituting pearlite into spherical cementite which does not cause resistance when processing because wire structure is poor in processability. Condition refers to a series of heat treatment processes that heat and cool steel.

As a result of the spheroidization heat treatment, the cementite constituting pearlite is changed into spherical and uniformly distributed cementite particles, so that the hardness of the material is reduced, thereby improving the life of the working die and improving the cold workability. In addition, even when cutting is required, the cutting performance can be better than that of conventional ferritic + cementite steel.

Such spheroidization heat treatment can be divided into two types based on the heat treatment temperature. The heat treatment temperature as a reference means a vacancy temperature, that is, a temperature at which pearlite can be produced. Therefore, one of the spheroidizing heat treatments has a so-called sub-critical annealing method for performing heat treatment below the vacancy temperature. The other one is an inte-critical annealing method in which the heat treatment is carried out at a temperature range of not less than the vacancy temperature, which is the temperature range in which austenite and ferrite coexist.

In the sub-critical annealing method, spheroidization is mainly performed by high temperature diffusion and mainly starts at defects or ends of lamellar cementite constituting pearlite, and lamellae by a gradient of carbon concentration due to a difference in curvature between an end portion and a flat interface next to it. Cementite has a segmented form and is known to be spheronized to reduce surface energy. As a result of the spheroidization, the proportion of cementite constituting the pearlite is reduced, so that a part of the pearlite is naturally transformed into ferrite. The spheroidized cementite is then increased in size by the so-called Oswald ripening effect and reduces the hardness of the material.

Intercritical annealing is slightly different from the subcritical annealing in its spheroidizing mechanism. That is, since the material is heated to the abnormal region where austenite and ferrite coexist, pearlite is mostly transformed into austenite. At this time, the cementite constituting the pearlite may not be completely dissolved in the austenite but exist in a partially undissolved state. The undissolved cementite acts as a spherical spherical nucleus to form coarse particles of cement when cooled. Will be. In this case, the pearlite may be regenerated or the ratio of spheroidized cementite may increase depending on how the cooling conditions are controlled during cooling. However, in the case of relatively slow cooling, the austenite to ferrite transformation may occur more stably. The ratio of cementite is also increased. In contrast, in the case of hard cooling, the proportion of pearlite is increased, so that the ratio of nodularity is reduced.

By any method, the cold workability can be improved by the above heat treatment. However, since the spheroidizing heat treatment has to undergo a process of heating the wire rod already rolled to the final shape only to improve the workability, the spheroidization heat treatment not only lowers the productivity but also increases the energy cost. Accordingly, wire rod manufacturers have proposed various methods to simplify or omit the spheroidization heat treatment.

Republic of Korea Patent Publication No. 2005-0000101 is an example of a method for producing a steel wire material can be omitted spheroidized heat treatment or softening heat treatment. The method contains at least 0.2% by weight of carbon and 0.5-1.5% by weight of Si, the structure consisting of ferrite and cementite, and the ferrite is ferrite produced by plastic organic dynamic transformation and ferrite produced by static transformation. And the cementite is distributed among the dynamic ferrites, and the softening heat treatment omitted steels having ferrites produced by the plastic organic dynamic transformation, wherein the cementite is spherical in shape. In order to obtain the plastic organic dynamic transformation process is introduced, it is required to condition that cementite is formed between the dynamic transformation ferrite. However, the plastic organic dynamic transformation process may not only control the rolling temperature precisely, but also may have a problem in that it is not easy to manufacture because the cementite formation conditions are difficult.

According to one aspect of the present invention there is provided a steel having advantageous conditions to be able to produce a steel wire having a sufficient amount of spheroidized cementite through a relatively simple rolling process without a separate spheroidization heat treatment.

According to another aspect of the invention there is provided a steel wire produced from the steel.

According to another aspect of the invention there is provided a more preferred rolling method for producing the steel wire.

Steel material of the present invention for solving the above problems by weight, carbon: 0.9-1.4%, silicon: 0.3-0.7%, manganese: 0.3-0.7%, aluminum: 0.75-1.45%, chromium: 1.3-1.7%, Nickel: 0.3-0.7%, titanium: 0.1-0.3%, niobium: 0.05-0.2%, vanadium: 0.05-1.5% and the remaining Fe and other unavoidable impurities.

As another aspect of the present invention, the steel wire which can be used for cold working without spheroidization by itself is weight%, carbon: 0.9-1.4%, silicon: 0.3-0.7%, manganese: 0.3-0.7%, aluminum: 0.75 -1.45%, Chromium: 1.3-1.7%, Nickel: 0.3-0.7%, Titanium: 0.1-0.3%, Niobium: 0.05-0.2%, Vanadium: 0.05-1.5% and the remaining Fe and other unavoidable impurities The internal structure is composed of three phases of spheroidized cementite, ferrite and pearlite, and each area fraction has the values of spheroidized cementite 35-45%, ferrite 40-60%, and pearlite 5-15%, and the aspect ratio of cementite is 1.5 or less. It is characterized by having an average particle size of 3 ~ 10 ㎛.

In addition, another advantageous aspect of the present invention is a method for producing an advantageous steel wire is heated to a temperature of 1000 ~ 1100 ℃ 30 minutes ~ 1 hour 30 minutes; Cooling the heated steel at a cooling rate of 5 to 15 ° C./S; Rolling the steel at an A1 temperature or more to produce a steel wire shape; And it characterized in that it comprises the step of cooling the steel wire to 30 ℃ / s or less.

According to the present invention, since a steel wire spheroidized by a simple rolling process can be obtained, a manufacturing process for obtaining a steel wire with excellent workability is not complicated and energy costs can be reduced.

Hereinafter, the present invention will be described in detail.

In-line spheroidization as used in the present invention means that the wire is manufactured so as to have a spheroidized structure in a rolled state without a separate reheating process for spheroidizing heat treatment after the wire is manufactured.

The inventors of the present invention have studied in depth the conventional rolling method in order to provide a steel wire (or a wire wire capable of in-line spheroidization) that spheroidization heat treatment can be omitted, and the wire rod that was spheroidized and heat-treated in a rolled state in the conventional rolling method The reason why it could not be obtained was found and the present invention was reached.

That is, the wire rod is usually rolled from the billet (or steel strip), the billet must be heated to a predetermined temperature or more in order to reduce the rolling resistance and control the structure. Therefore, the temperature for heating the billet is usually 1000 ° C or more (1100 ° C or less).

By the way, as shown in Figure 1, the super-vacuum steel undergoes a process of transforming into pearlite + cementite → austenite + cementite → austenite single phase as the temperature increases when heated at room temperature. At this time, in the normal rough-steel, all cementite is transformed into austenite when heating the billet for rolling because the so-called Acm temperature, which is a temperature transformed into austenite + cementite → austenite single phase, is lower than 1000 ° C. There will be no cementite remaining. In this case, even if re-cooled after rolling, it is impossible to obtain a spheroidized wire rod because pearlite is formed again unless it is extremely cold.

Therefore, the inventors of the present invention control the component system of the steel in order to solve the above problems, so that even if the Acm temperature is higher than the heating temperature of the billet, even if the billet is heated, more than a certain level of cementite remains in the steel, By introducing the rolling method, it was possible to obtain a wire rod having excellent cold workability (particularly, cold rolling) in which spheroidized cementite of sufficient size and number is present even without additional spheroidization heat treatment.

In addition, another point to be considered in the present invention is that the cementite is present but such that an undesirable carbide phase such as M ₇ C ₃ is not formed. This is also a type of carbide, but because it is very different from cementite (Fe ₃ C), it is combined with other carbide-forming elements Nb, V, and Ti other than Fe, such as NbC, VC, and TiC, not NbC, VC and TiC. It is possible to form such forms as ₇ C ₃ , V ₇ C3 and Ti ₇ C ₃ . In this case, since the carbide becomes very high hardness, low strength to omit spheroidization cannot be secured, and tensile strength is increased more than necessary or ductility decreases. In addition, once produced, since a considerable time is required to dissolve through another heat treatment, it is to be avoided in the present invention.

Particularly, in the case of super-vacuum steel, since a large amount of carbon is added, conditions for forming carbides of the M ₇ C ₃ form are formed by reacting with the alloying element introduced to control Acm. Problems should be avoided.

In addition, in order to improve cold workability, in particular cold rolling, it is necessary to appropriately control the fraction of each structure present in the steel wire.

In addition, since the temperature for heating the billet is less than Acm, even if undissolved cementite is present, it is more preferable to set the rolling conditions so that the cemented spheroidite is properly formed in the subsequent rolling process. . Of course, those skilled in the art can manufacture the steel wire intended by the present invention by appropriately using a conventional method, but in the present invention by providing a new rolling method more efficient and desirable It was possible to effectively produce the advantageous steel wire.

Therefore, the following description will be divided into the manufacturing method for providing a steel wire having the characteristics and the advantageous features of the steel and steel wire.

(Composition of steel)

Carbon: 0.9-1.4% (less than weight%)

Carbon is an element which improves the strength of steel and is an element which is added indispensably to maintain steel in super-vacuum composition. In addition, carbon also has an effect of increasing the Acm temperature, it is necessary to properly adjust the content. At a content of less than 0.9%, the Acm synergy is not great, and if it exceeds 1.4%, a new phase such as M ₇ C ₃ is not preferable.

Silicon: 0.3-0.7%

The reason for limiting the content of silicon to 0.3-0.7% is as follows. If the silicon content exceeds 0.7%, the work hardening phenomenon occurs during the cold drawing and pressing process, which is a problem in the workability, and less than 0.3% can not secure sufficient strength of the bolt.

Manganese: 0.3-0.7%

Manganese is an element that forms a solid solution to form a solid solution to strengthen the solid solution to obtain a required strength without a decrease in ductility, the content is limited to 0.3-0.7%. When the manganese is added in excess of 0.7%, it is more harmful to the product characteristics due to the manganese segregation than the solid solution strengthening effect. Macro segregation and micro segregation are more likely to occur depending on the segregation mechanism during steel solidification. Due to the relatively low diffusion coefficient, the segregation zone is promoted and the hardenability improvement is the main cause of the core martensite. In addition, when the manganese is added less than 0.3%, the effect of segregation zone due to manganese segregation is hardly expected, but the effect of guaranteeing strength and improving toughness by solid solution strengthening is hardly expected.

Aluminum: 0.75-1.45%

When calculating the Fe-C state diagram according to the aluminum content, as the aluminum content increases, the austenite region becomes smaller, so that it cannot exist as an austenite single phase at 1.45% or more. Therefore, this is limited and the addition is less than 0.75%, it is limited because the increase in the Acm temperature designed in the present invention can not be increased to more than 1000 ℃.

Chromium: 1.3-1.7%

If the chromium content is 1.7% or more, M ₇ C ₃ is formed in the carbon content range 1.1 to 1.4% C of the roughened steel. In order to suppress the formation of such M ₇ C _3, the amount of Cr added is added at 1.7% or less, and if it is added at less than 1.3%, it is limited because there is no influence on the increase in Acm temperature.

Nickel: 0.3-0.7%

Nickel is similar in shape to aluminum and the austenite region expands with increasing content. When it exceeds 0.7%, the austenite region disappears, and when it is added below 0.3%, nickel has no effect of increasing Acm temperature. It is not suitable for the present invention because no hash cementite can be present.

Titanium: 0.1-0.3%

Titanium is a strong carbide forming element, which is considered to be a seed for forming cementite during rolling and cooling by forming carbide formed prior to cementite formation. These carbides act as cementite nucleation sites because they are formed at high temperatures above 1000 ° C. If the content exceeds 0.3%, the softness of the material is reduced due to excessive solidification and the formation effect of M ₇ C ₃ in addition to the purpose of carbide formation, and it is difficult to form carbide when added below 0.1%.

Niobium: 0.05-0.2%

Niobium is also a strong carbide forming element, the same as titanium addition. When the content exceeds 0.2%, the softness of the material is reduced due to excessive solid solution and the formation effect of M ₇ C ₃ in addition to the purpose of carbide formation, and it is difficult to form carbide when added below 0.05%.

Vanadium: 0.05-1.5%

Vanadium also acts as a strong carbide forming element, with the same addition of titanium. If it exceeds 1.5%, in addition to the purpose of carbide formation, excessive solid solution and the effect of the formation of M ₇ C ₃ to reduce the ductility of the material, there are difficulties in terms of cost, it is difficult to form carbide when added below 0.05%.

Therefore, the super masonry steel of the present invention is by weight%, carbon: 0.9-1.4%, silicon: 0.3-0.7%, manganese: 0.3-0.7%, aluminum: 0.75-1.45%, chromium: 1.3-1.7%, nickel: Steel with a composition comprising 0.3-0.7%, titanium: 0.1-0.3%, niobium: 0.05-0.2%, vanadium: 0.05-1.5% and the remaining Fe and other unavoidable impurities.

Even if the steel is reheated to a temperature of 1000 ° C. or higher in order to roll the steel, cementite is not completely dissolved and remains as it can act as a nucleus of spheroidized cementite in the subsequent rolling process. Further, as described above, carbides of titanium, niobium, or vanadium may be used. Since it acts as a nucleus of the nodular cementite, the ratio of nodular cementite can increase.

(Organization of Steel Wire)

Hereinafter, the preferable conditions of the steel wire material manufactured from the said steel material are demonstrated. The steel wire rod has the same composition as the steel rod. However, since it is a steel wire material inline spheroidization by the rolling process, it is characterized in that spheroidized cementite is preferably formed therein. Therefore, steel wire rods obtained by rolling steel materials under the above-described conditions, as rolled, do not require any further in-line spheroidization, and their inner structure is composed of three phases of spheroidized cementite, ferrite and pearlite, and each area fraction is spheroidized. Cementite has a value of 35 to 45%, ferrite 40 to 60%, pearlite 5 to 15%, the aspect ratio of cementite is 1.5 or less, characterized in that the average particle size of 3 ~ 10 ㎛.

If the area fraction of the spheroidized cementite is too low, it is difficult to improve the cold pressure composition. On the contrary, if the ratio of the spheroidized cementite is too high, the size of the spheroidized cementite may become coarse. There is also a limit to the amount of carbon that is limited to the above range. In addition, when the pearlite area ratio exceeds 15%, the deformation resistance of the steel wire increases, which is not preferable because the cold rolling is deteriorated, and the pearlite of about 5% inevitably remains. Ferrite ratio refers to a range that is naturally determined by the carbon content, the spheroidized cementite ratio, and the residual pearlite ratio.

In addition, although the shape of the cementite needs to be close to a spherical shape, in the case of plate-shaped cementite, it is not preferable for improving the cold workability of steel materials. Therefore, it is preferable that the aspect ratio of the spherical cementite is 1.5 or less. (It is important to note that the aspect ratio is the ratio between the width and the length, so that the larger one becomes the numerator, 1 is the minimum value.)

In addition, the average size of cementite is limited to 3-10 micrometers. This is because when the particle size is 3 μm or less, the particles are too fine to obtain a sufficiently low strength to omit the spheroidizing heat treatment, and the number thereof increases considerably as well as the fineness. It can be obtained by maintaining it for a long time at high temperature than conditions. Since the actual rolling condition is a continuous operation, it is almost impossible in the case of 10 μm or more, and since the grain size of the ferrite, which is a known structure, is greatly increased due to such long-term maintenance, the ductility decrease is also caused.

(Manufacturing method)

The steel wire of the present invention having the above advantageous conditions can be produced from the steel of the composition. Herein, a more preferable and novel manufacturing method will be described in comparison with the manufacturing method conventionally provided for manufacturing the steel wire. Briefly described, the manufacturing method of the present invention, as shown in Figure 2, consists of a process of heating the steel to an appropriate temperature for a predetermined time, cooling and then rolling and slow cooling. Hereinafter, the heating temperature, the heating time, the cooling conditions after heating, the rolling temperature, and the cooling conditions after rolling of the steel forming the manufacturing conditions of the present invention will be described in detail.

Heating temperature of steel: 1000 ~ 1100 ℃

Heating at this temperature prevents austenite grains from coarsening excessively while austenite and undissolved cementite are present together in the steel having the advantageous composition of the present invention. If the heating temperature is too high, not only the austenite grains become very coarse, but also the amount of cementite dissolved increases, so that the amount of remaining cementite may be insufficient.

In addition, when the heating temperature is too low, the amount of undissolved cementite may be too large and rolling may be difficult.

Heating time of steel: 30 minutes ~ 1 hour 30 minutes

Since the heating of the steel is to facilitate the subsequent rolling process and to allow the cementite to dissolve properly, it is necessary to heat the steel for 30 minutes or more to make the temperature inside the steel uniform. However, when heating for too long, not only the possibility of coarsening of austenite grains increases but also the productivity decreases considerably, which limits the heating range.

Cooling rate after heating: 5 ~ 15 ℃ / s

The cooling rate is limited to the purpose of minimizing the transformation of the microstructure in the cooling step before hot rolling. If the cooling rate before hot rolling is 5 ° C / s or less, productivity is reduced, and additional equipment is required to maintain slow cooling. In addition, as in the case where the heating time is maintained for a long time, there is a fear that the strength and toughness of the wire rod after the hot rolling is completed. On the other hand, when the cooling rate exceeds 15 ° C / s, the driving force of the transformation of the billet before rolling increases, which increases the possibility of the appearance of new microstructures during rolling, and such microstructures are extremely low temperature (for example, 600). It is not possible to remove it by rolling at C), but it is not preferable because the low temperature rolling is not possible with a conventional rolling equipment.

Rolling temperature: A ₁ or more

When the rolling temperature is less than the A ₁ , which is the transformation temperature to ferrite + pearlite, the rolling is made to be A ₁ or more because new microstructures such as pearlite may be generated during rolling. Naturally, the upper limit of the rolling temperature is not particularly limited, because the rolling temperature is lower than the cooling temperature.

Cooling rate after rolling: 30 ℃ / s or less

If the cooling rate is more than 30 ℃ / s rapidly quenched austenite in the microstructure of austenite and undissolved cementite is transformed to bainite or martensite to achieve the purpose. In addition, if the slow cooling at a cooling rate of 30 ℃ / s or less can be sufficient transformation can be achieved, any cooling rate of 30 ℃ / s or less can obtain a microstructure designed in the present invention, the slower the cooling rate is very useful. The cooling rate is more preferably controlled until the temperature of the steel is 500 ° C or less.

Therefore, the steel wire manufacturing method of the present invention comprises the steps of heating the steel of the above-described advantageous composition at a temperature of 1000 ~ 1100 ℃ 30 minutes ~ 1 hour 30 minutes; Cooling the heated steel at a cooling rate of 5 to 15 ° C./S; Rolling the steel at an A1 temperature or more to produce a steel wire shape; And it characterized in that it comprises the step of cooling the steel wire to 30 ℃ / s or less.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following examples are intended to illustrate the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

A billet having the composition shown in Table 1 below was prepared and rolled under the conditions shown in Table 2 to prepare a steel wire. In Table 1, the content of each component means weight%, and in Table 2, various temperatures mean ℃, and cooling rate means ℃ / s.

In Table 1, Acm was derived using a thermodynamic calculation method, and the accuracy was found to be ± 5 ° C.

division C Si Mn Al Cr Ni Ti Nb V Acm (℃) Comparative Material 1 0.8 0.3 0.3 0.75 1.3 0.3 0.1 0.05 0.05 827 Comparative Material 2 0.9 0.1 0.3 0.75 1.3 0.3 0.1 0.05 0.05 798 Comparative Material 3 0.9 0.3 0.1 0.75 1.3 0.3 0.1 0.05 0.05 923 Comparative Material 4 0.9 0.3 0.3 0.2 1.3 0.3 0.1 0.05 0.05 895 Comparative Material 5 0.9 0.3 0.3 0.75 0.1 0.3 0.1 0.05 0.05 986 Comparative Material 6 0.9 0.3 0.3 0.75 1.3 0.1 0.1 0.05 0.05 906 Comparative Material7 0.9 0.3 0.3 0.75 1.3 0.3 - 0.05 0.05 899 Comparative Material 8 0.9 0.3 0.3 0.75 1.3 0.3 0.1 - 0.05 962 Comparative Material 9 0.9 0.3 0.3 0.75 1.3 0.3 0.1 0.05 - 981 Invention 1 0.9 0.3 0.3 0.75 1.3 0.3 0.1 0.05 0.05 1035 Invention 2 1.0 0.3 0.3 0.75 1.3 0.3 0.1 0.05 0.05 1042 Invention 3 1.2 0.7 0.7 1.45 1.7 0.7 0.3 0.2 1.5 1078 Invention 4 1.4 0.7 0.7 1.45 1.7 0.7 0.3 0.2 1.5 1096 Comparative Material 10 1.6 0.7 0.7 1.45 1.7 0.7 0.3 0.2 1.5 1099 Comparative Material 11 1.4 1.0 0.7 1.45 1.7 0.7 0.3 0.2 1.5 1102 Comparative Material 12 1.4 0.7 1.0 1.45 1.7 0.7 0.3 0.2 1.5 1111 Comparative Material 13 1.4 0.7 0.7 2.0 1.7 0.7 0.3 0.2 1.5 1085 Comparative Material14 1.4 0.7 0.7 1.45 2.0 0.7 0.3 0.2 1.5 1023 Comparative Material 15 1.4 0.7 0.7 1.45 1.7 1.0 0.3 0.2 1.5 996 Comparative Material 16 1.4 0.7 0.7 1.45 1.7 0.7 0.5 0.2 1.5 989 Comparative Material17 1.4 0.7 0.7 1.45 1.7 0.7 0.3 0.5 1.5 1025 Comparative Material 18 1.4 0.7 0.7 1.45 1.7 0.7 0.3 0.2 2.0 1108

division Heating temperature Heating time Cooling rate after heating Rolling temperature Cooling rate after rolling (up to 500 ℃) Comparative Material 1 1007 85 minutes 8 889 27 Comparative Material 2 1010 85 minutes 9 913 13 Comparative Material 3 1003 82 minutes 11 902 12 Comparative Material 4 1009 87 minutes 9 945 21 Comparative Material 5 1010 90 minutes 10 941 22 Comparative Material 6 1013 85 minutes 8 899 23 Comparative Material7 1011 90 minutes 12 923 21 Comparative Material 8 1001 90 minutes 7 961 26 Comparative Material 9 1019 90 minutes 14 887 18 Invention 1 1009 85 minutes 7 946 7 Invention 2 1013 90 minutes 8 967 14 Invention 3 1008 87 minutes 11 956 15 Invention 4 1008 90 minutes 12 923 11 Comparative Material 10 1050 90 minutes 10 915 22 Comparative Material 11 1018 90 minutes 9 935 18 Comparative Material 12 1010 85 minutes 12 942 12 Comparative Material 13 1019 90 minutes 13 901 23 Comparative Material14 1000 90 minutes 10 957 21 Comparative Material 15 1001 85 minutes 9 921 19 Comparative Material 16 1003 87 minutes 15 897 18 Comparative Material17 1003 90 minutes 14 899 18 Comparative Material 18 1011 90 minutes 10 942 16

As described in Table 1, the invention is a steel material that satisfies all the composition ranges required by the present invention, but in the case of Comparative Material 1, the carbon content is less than the range required by the present invention, and Comparative Material 2 is the silicon content. In this case, the comparative material 3 shows a case where the manganese content is insufficient, and the comparative material 4 shows a case where the aluminum content is insufficient. Comparative materials 5, 6, 7, 8, and 9 represent cases where chromium, nickel, titanium, niobium, and vanadium do not meet the content conditions defined in the present invention, respectively. In the case of the comparative material 10, the carbon content exceeds the range required by the present invention, the comparative material 11 is the case where the silicon content is excessive, the comparative material 12 is the case where the manganese content is excessive, and the comparative material 13 is aluminum The case where content exceeds the range prescribed | regulated by this invention is shown. In addition, the comparative materials 14, 15, 16, 17 and 18 represent a case where chromium, nickel, titanium, niobium and vanadium exceed the content conditions specified in the present invention, respectively.

The steel material of the composition of Table 1 was rolled on condition of Table 2, and the steel wire material was manufactured. Table 2 above is to set the rolling conditions to be rolled according to the rolling conditions of the present invention regardless of the invention material and the comparative material. Table 3 shows the results of examining the physical properties of the steel wires manufactured by rolling each steel material satisfying the composition of Table 1 under the conditions of Table 2.

It was confirmed that the invention materials of the present invention all satisfied the conditions required by the present invention. In particular, referring to FIG. 3, which is a micrograph of the structure of the inventive material 3, it was confirmed that spheroidized cementite having a spherical average size of about 3 to 10 μm was uniformly distributed therein.

Table 3 shows the results of all of the materials shown in Table 1 prepared under the conditions shown in Table 2. As shown in Table 2, Comparative Materials 1 to 9 are unsuitable for remaining fine cementite nuclei when the Acm temperature is less than 1000 ° C., so that spheroidized cementite in the tissue is difficult to form. That is, as shown in FIG. 4, which is a photograph of a steel material having a composition of Comparative Material 1 at a heating temperature and a heating time of Table 2 and then quenched to observe its structure, fine cementite was hardly observed at all. If fine cementite does not remain at high temperatures, it is difficult to form spheroidized cementite as a nucleus and thus does not meet the conditions of the present invention. In addition, although not specifically shown in the drawings, the remaining comparative materials 1, 3 and 9 also had the same structure.

FIG. 5 shows a photograph of observing the tissue after quenching after heating Example 2 satisfying the component conditions of the present invention to the heating temperature and heating time shown in Table 2. FIG. As can be seen in Figure 5, even when the steel is heated, cementite is not completely dissolved but finely remaining, so that it can be developed into spheroidized cementite in the process of cooling, rolling, etc., thereby eliminating the need for additional spheroidizing heat treatment.

6 shows the results of preparing Comparative Material 3 under the conditions of Table 2. As described above, even in Comparative Material 3, since no fine cementite was present during heating, the tissues were all transformed into normal pearlite structures, which are not suitable for cold working. These results were confirmed to be the same in the comparative comparative materials 1, 2, 4-9.

Comparative material 11 is a case where the content of silicon is excessive compared to the conditions of the present invention. The silicon is an element that promotes work hardening, and when the content thereof is excessive, the silicon is not suitable for the material for cold pressing targeted in the present invention. That is, the comparative material 11 is difficult to use in a cold press because the extreme work hardening phenomenon occurs during processing. That is, compared to the invention material 3 prepared under the conditions of Table 2, the microstructure is similar, the tensile strength of the wire rod after rolling and cooling is also proposed in the present invention as 720 ~ 780MPa level similar to the invention materials 1-4 It belongs to the range. However, when cold working about 30% of the wire rod was performed, the tensile strength of the inventive materials 1 to 4 was 750 to 780 MPa, and there was almost no increase in tensile strength. However, the comparative material 11 was cold worked as its tensile strength was 850 MPa. It was confirmed that the extreme work hardening occurred by. If the work hardening is severe as described above it is not preferable because the molding load increases and causes early wear of the die. Therefore, Comparative Example 11 is not suitable.

In the case of Comparative Materials 10 and 14, some of the carbides were present in the form of M ₇ C ₃ , which is not suitable because of high strength and rapid ductility. 7 shows a photograph of M ₇ C ₃ and general cementite observed in Comparative Material 10 using a transmission electron microscope, which is almost identical to the naked eye, but can be distinguished by component analysis.

Comparative material 12 is a case of high manganese content. In this case, segregation by manganese is observed in a considerable amount, and it is not preferable because segregation may result in formation of a hard tissue such as martensite in the center. Table 4 shows the results of segregation analysis for each of the inventive and comparative materials. In the table, Mn (C) represents the manganese content in the center of the surface perpendicular to the length direction of the wire rod manufactured under the conditions of Table 2, and Mn (S) represents the manganese content analyzed at the middle point between the center and the surface. As can be seen in the table, the core manganese content of Comparative Material 12 is about 1.8% by weight, and in general, if the manganese content is so high, martensite may be generated through air cooling, which is not preferable.

The comparative material 13 shows a case where the aluminum content is very high. If the aluminum content is excessive, austenite cannot exist in a single phase, and the coexistence area of austenite and ferrite is wide. In addition, it is difficult to pass the three-phase coexistence zone of austenite, cementite and ferrite, rather than passing the state diagram of austenite and salt cementite during cooling. In addition, this not only increases the amount of ferrite much more than that of cementite, but also the strength cannot be lowered because a considerable amount of carbon is dissolved in the transformed ferrite. The method proposed in the present invention is intended to reduce the strength by spheroidizing the layered cementite in the layered pearlite structure within an appropriate range, which is not suitable in this case.

Comparative materials 16 to 18 are cases where titanium, niobium, and vanadium exceed the content conditions specified in the present invention, and the titanium carbide, niobium carbide, and vanadium carbide are present in a large amount of M ₇ C ₃ to have high strength and low ductility. This is not the case. As a result, all of the comparative materials 16 to 18 are not suitable in the present invention because the tensile strength is more than 1000 MPa, the ductility is also not suitable because it is rapidly reduced to 10% or less, which is 50% of the invention material. Carbide formation form is observed in the form of M ₇ C _{3 as} shown in FIG.

Table 3 compares the various microstructure and mechanical properties after preparing the materials having the contents of Table 1 according to the conditions of Table 2. In addition, the remarks described in each case the reason that is not suitable for the present invention.

In general, steel is suitable for cold pressing when the tensile strength of 800MPa or less and elongation of 15% or more. As shown in Table 1, all of the invention materials, which are steels suitable for the present invention, satisfied the above conditions. In the comparative material, the comparative material 11 and the comparative material 12 satisfy the above tensile strength conditions, but these are cases where the work hardening or the segregation is severe as described above, which is not suitable for the conditions of the present invention. Therefore, only the invention materials were found to be suitable as the cold-rolled steel for the purpose of the present invention, the observation of their common characteristics, spheroidized cementite 35-45%, ferrite 40-60%, pearlite 5-15% It can be seen that the case has a value of and the aspect ratio of cementite is 1.5 or less. In addition, when the average particle size of the spheroidized cementite is 3 μm or less, it was difficult to obtain the effect by spheroidization, but it was found that the result was to increase the tensile strength. The tensile strength could be reduced, but it was almost impossible to manufacture by commercial methods.

In addition, it should be noted that, as defined in Table 3, the ratio of cementite and ferrite tissue does not mean that the pearlite is included, but rather means an existing tissue. This is because cementite and ferrite present in pearlite were observed in pearlite. In addition, in the case of Comparative Materials 1 to 9, since most of them were transformed into pearlite structures, the cementite shape also referred to only the shape of cementite independently existing in spherical form.

division
Organization rate Cementite geometry The tensile strength
Elongation
Remarks
Cementite ferrite Pearlite Aspect ratio Average particle size Comparative Material 1 - - 100 - - 1205 12 Acm lack of temperature Comparative Material 2 - - 100 - - 1243 14 Acm lack of temperature Comparative Material 3 - - 100 - - 1356 16 Acm lack of temperature Comparative Material 4 - - 100 - - 1402 15 Acm lack of temperature Comparative Material 5 - - 100 - - 1289 11 Acm lack of temperature Comparative Material 6 - - 100 - - 1264 12 Acm lack of temperature Comparative Material7 - - 100 - - 1321 13 Acm lack of temperature Comparative Material 8 - - 100 - - 1416 12 Acm lack of temperature Comparative Material 9 - - 100 - - 1379 14 Acm lack of temperature Invention 1 45 40 15 1.3 3 768 16 Suitable for the present invention Invention 2 36 52 12 1.2 7 772 18 Suitable for the present invention Invention 3 39 48 13 1.1 8 750 19 Suitable for the present invention Invention 4 35 40 25 1.5 10 781 18 Suitable for the present invention Comparative Material 10 58 28 14 1068 3 M ₇ C ₃ Formation Comparative Material 11 36 52 12 1.2 7 763 17 Work hardening Comparative Material 12 36 52 12 1.2 7 778 9 Center Segregation Extreme Comparative Material 13 48 39 13 1.5 3 1026 5 Austenite phase disappeared Comparative Material14 49 31 30 1.1 2 1256 7 M ₇ C ₃ Formation Comparative Material 15 52 36 12 1.5 6 1123 3 Austenite phase disappeared Comparative Material 16 49 18 33 1.8 One 1325 9 M ₇ C ₃ Formation Comparative Material17 51 16 33 1.2 2 1232 8 M ₇ C ₃ Formation Comparative Material 18 50 21 29 1.3 3 1161 5 M ₇ C ₃ Formation

division Total manganese content
(wt.%) Mn (C)
(wt.%) Mn (S)
(wt.%) Mn (C) / Mn (S) Remarks Comparative Material 1 0.3 0.31 0.29 1.1 Segregation is good Comparative Material 2 0.3 0.30 0.30 1.0 Segregation is good Comparative Material 3 0.1 0.12 0.09 1.3 Segregation is good Comparative Material 4 0.3 0.30 0.30 1.0 Segregation is good Comparative Material 5 0.3 0.30 0.28 1.1 Segregation is good Comparative Material 6 0.3 0.32 0.28 1.1 Segregation is good Comparative Material7 0.3 0.35 0.31 1.1 Segregation is good Comparative Material 8 0.3 0.32 0.30 1.1 Segregation is good Comparative Material 9 0.3 0.31 0.28 1.1 Segregation is good Invention 1 0.3 0.30 0.29 1.0 Segregation is good Invention 2 0.3 0.34 0.28 1.2 Segregation is good Invention 3 0.7 0.75 0.68 1.1 Segregation is good Invention 4 0.7 0.76 0.62 1.2 Segregation is good Comparative Material 10 0.7 0.79 0.70 1.1 Segregation is good Comparative Material 11 0.7 0.72 0.71 1.0 Segregation is good Comparative Material 12 1.0 1.82 0.82 2.2 Segregation extremes Comparative Material 13 0.7 0.73 0.69 1.1 Segregation is good Comparative Material14 0.7 0.73 0.65 1.1 Segregation is good Comparative Material 15 0.7 0.75 0.68 1.1 Segregation is good Comparative Material 16 0.7 0.72 0.70 1.0 Segregation is good Comparative Material17 0.7 0.78 0.71 1.1 Segregation is good Comparative Material 18 0.7 0.74 0.69 1.1 Segregation is good

In order to observe the influence of the production conditions of the present invention, the influence of the steel heating time on the steel of the invention materials 1 to 4 suitable for the composition conditions of the present invention was observed. In the experiment, the heating temperature was set to about 1000 ° C, and after heating for the heating time described in FIG. .

As shown in the graph of FIG. 8, it was confirmed that the austenite grains rapidly became coarse when heated for more than 1 hour 30 minutes (90 minutes). On the contrary, when heating to less than 30 minutes, as a result of the analysis by the inventors, the preheating time is insufficient, so that the temperature of the entire steel (billet) is not uniform. Hot rolling could not be performed.

Next, in order to observe the effect of cooling rate after rolling of the wire rod, the steel (billet) having the composition of Inventive Material 3 was cooled to a heating temperature of about 1003 ° C, a heating time of 84 minutes, and an average cooling rate of 11 ° C / s, and then 921 ° C. The result of observing the ratio of each structure in the wire rod after cooling to 500 degreeC by each cooling rate condition after rolling at is shown in FIG. As can be seen from the figure, it can be seen that the rate of pearlite exceeds 15% while the cooling rate exceeds 30 ℃ / s. In addition, when the cooling rate is further increased, it is confirmed that even in the cold working material such as bainite or martensite, undesired tissues are generated and deformation resistance of the steel is very high.

Therefore, it was confirmed that the cooling rate is preferably 30 ° C / s or less.

1 is a Fe-C equilibrium diagram for explaining the concept of the present invention,

2 is a conceptual diagram showing a preferred embodiment of the steel wire manufacturing method of the present invention,

3 is an electron micrograph of the internal structure of the steel wire produced by the present invention,

4 is an electron micrograph of cement 2 observed in cementite by quenching the comparative material 2 after the heating temperature and time conditions of Table 2;

5 is an electron micrograph of the invention material 2 observed in cementite by quenching after the heating temperature and time conditions of Table 2,

6 is an electron micrograph showing the final microstructure of Comparative Material 3,

7 is a transmission electron microscope photograph of the observed form of M ₇ C ₃ and general cementite of Comparative Material 10,

8 is a graph showing that the grain size changes with heating time, and

9 is a graph showing that the ratio of the internal structure of the steel wire is changed according to the cooling rate.

Claims (3)

By weight, carbon: 0.9-1.4%, silicon: 0.3-0.7%, manganese: 0.3-0.7%, aluminum: 0.75-1.45%, chromium: 1.3-1.7%, nickel: 0.3-0.7%, titanium: 0.1- 0.3%, niobium: 0.05-0.2%, vanadium: 0.05-1.5% and the remaining masonry steels comprising the remaining Fe and other unavoidable impurities. By weight, carbon: 0.9-1.4%, silicon: 0.3-0.7%, manganese: 0.3-0.7%, aluminum: 0.75-1.45%, chromium: 1.3-1.7%, nickel: 0.3-0.7%, titanium: 0.1- 0.3%, niobium: 0.05-0.2%, vanadium: 0.05-1.5% and the composition containing the remaining Fe and other unavoidable impurities, The internal structure is composed of three phases of spheroidized cementite, ferrite and pearlite, and each area fraction has the values of spheroidized cementite 35-45%, ferrite 40-60%, and pearlite 5-15%. An in-line spherical steel wire having an aspect ratio of cementite of 1.5 or less and an average particle size of 3 to 10 µm. By weight, carbon: 0.9-1.4%, silicon: 0.3-0.7%, manganese: 0.3-0.7%, aluminum: 0.75-1.45%, chromium: 1.3-1.7%, nickel: 0.3-0.7%, titanium: 0.1- Heating the steel containing 0.3%, niobium: 0.05-0.2%, vanadium: 0.05-1.5% and the remaining Fe and other unavoidable impurities at a temperature of 1000-1100 ° C. for 30 minutes to 1 hour 30 minutes; Cooling the heated steel at a cooling rate of 5 to 15 ° C./S; Rolling the steel at an A1 temperature or more to produce a steel wire shape; And cooling the steel wire to 30 ° C / s or less.

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