KR20190063711A - Steel material and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a steel material and a manufacturing method thereof, wherein the steel material comprises: 0.08-0.13 wt% of C; 0.20 wt% or less of Si; 0.85-1.1 wt% of Mn; 0.08-0.11 wt% of P; 0.24-0.3 wt% of S; 0.005 wt% or less of Al; 0.03-0.06 wt% of Cu; 0.005-0.008 wt% of T.[O]; and the remainder consisting of Fe and inevitable impurities. Moreover, the steel material comprises a sulfide containing Cu and Mn, and can improve cutting ability without adding expensive alloying elements.

Description

Technical Field The present invention relates to a steel material and a manufacturing method thereof,

The present invention relates to a steel material and a manufacturing method thereof, and more particularly, to a steel material capable of improving machinability and workability and a manufacturing method thereof.

Free cutting steel is a steel material widely used in precision parts and the like, and it is a basic feature to have excellent machinability. The excellent machinability of the free cutting steel is obtained from metallic inclusions and nonmetallic inclusions present in the steel. When machining a steel material using a CNC lathe or other machining apparatus, manganese sulfide Non-metallic inclusions such as MnS act as a stress concentration source and generate voids at the inclusion and matrix interface, thereby promoting crack growth and reducing the force required for cutting.

On the other hand, a method of adding lead (melting point: 327 ° C), which is a low melting point element, is proposed as a method for improving the cutting performance of free cutting steel. Lead is melted during cutting, lubricating at the interface between the tool and the workpiece, reducing the cutting force and protecting the tool. Recently, however, environmental regulations have been strengthened, and many automobile and home appliance companies have regulated the use of lead, which is harmful to human body. Therefore, bismuth (melting point 273 ° C), which is a low melting point element and harmless to human body, did. However, bismuth inhibits the hot rolling property, and is expensive and has poor wettability when put into molten steel, so that it is difficult to uniformly adjust the components of molten steel. In addition, free-cutting steel contains a large amount of non-metallic inclusion manganese sulfide (MnS). When a low melting point element is added, manganese sulfide acts as a stress concentration source when stress concentrates for mechanical structure, There is a problem that the physical properties are further reduced.

1) KR1987-1319A 2) JP2005-523766A

The present invention provides a steel material capable of improving machinability without adding a low melting point alloy and a method of manufacturing the same.

The present invention provides a steel material capable of reducing manufacturing costs and a manufacturing method thereof.

The steel material according to the present invention comprises 0.08 to 0.13 wt% of C, 0.20 wt% or less of Si, 0.85 to 1.1 wt% of Mn, 0.08 to 0.11 wt% of P, 0.24 to 0.3 wt% of S, 0.005 to 0.008 wt% of Al, 0.005 wt% or less of Cu, 0.03 to 0.06 wt% of Cu, and 0.005 to 0.008 wt% of T. [O], and Fe and unavoidable impurities.

The sulfide may exist as a complex inclusion precipitated by using Si-Al-O as a nucleus.

The composite inclusion may have a size of 3 m or more and 150 to 250 / mm < 2 > or more.

The sulfide may be at least 500 / mm < 2 >.

The Mn and the Cu may be included so as to satisfy the following equations.

0.4? M? 0.6

(Where M is Cu (wt%) / (Mn (wt%) + Cu (wt%) x 100)

A method of manufacturing a steel material according to an embodiment of the present invention includes: a process of preparing a charcoal; Refining the molten iron to produce molten steel containing sulfides containing Cu and Mn; Casting the casting using the molten steel; And rolling the casting.

And a step of supplying scrap containing copper during at least one of a process of preparing the molten iron and a process of producing the molten steel.

The process of preparing the molten steel may be performed immediately after the process of preparing the molten iron.

The process of producing the molten steel may include a primary refining process of blowing oxygen to the molten iron to refine it; And a secondary refining step of adjusting the components of the molten steel in which the primary refining process is completed, wherein the primary refining process adjusts the dissolved oxygen in the molten steel to 300 ppm or less, Can be adjusted to 20 to 40 ppm.

The process for producing the molten steel includes the steps of: C: 0.08 to 0.13 wt%, Si: 0.20 wt% or less, Mn: 0.85 to 1.1 wt%, P: 0.08 to 0.11 wt%, S: 0.24 to 0.3 wt% , Molybdenum steel containing 0.03 to 0.06% by weight of Cu, 0.005 to 0.008% by weight of T. [O], balance Fe and unavoidable impurities can be produced.

The step of rolling the casting may include heat treating the casting at a temperature of 1100 to 1300 캜 for 3 hours to 4 hours and then rolling the casting.

The step of rolling the casting may include a step of heat-treating the casting at a temperature of 1100 to 1200 ° C for 2 to 4 hours and then rolling the casting.

According to the present invention, machinability can be improved without adding expensive alloying elements. That is, it is possible to improve the machinability by forming sulfides of Cu and Mn in the steel. In addition, it is possible to prevent deterioration of mechanical properties and to improve the service life of the manufactured product. In addition, when a large amount of uneven Cu and Mn sulfides are formed, the stress can be concentrated at a point where the components are uneven during the cutting operation for machining by external stress, so that the stress can be cut with a small amount of stress. In addition, by using the phosphorus (P) in the steel material, the effect of weakening the grain boundaries can be reversed and the workability can be improved. In addition, by using low-cost copper (Cu) -containing scraps, it is possible to omit the preliminary treatment process of the molten iron, thereby reducing the manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a photograph of the shape of inclusions and microstructures formed on a steel material according to an embodiment of the present invention. Fig.
2 is a view showing a result of a three-dimensional scanning electron microscope and X-ray spectroscopy (SEM-EDS) of inclusions formed on a steel material according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know. In the description, the same components are denoted by the same reference numerals, and the drawings are partially exaggerated in size to accurately describe the embodiments of the present invention, and the same reference numerals denote the same elements in the drawings.

The present invention provides a method for producing a free cutting steel for precision machining of a medium carbon machine structure excellent in cutting performance without adding a low melting point alloy such as lead and bismuth. The present invention can improve machinability by forming a large amount of nonmetallic inclusions in which (Mn, Cu) S sulfide and (Mn, Cu) S sulfide alone precipitated from the Si-Al-O oxide as a starting point. That is, it can induce the component nonuniformity of the (Mn, Cu) S sulfide compared to the conventional MnS alone, and it is possible to improve the machinability more easily than at the time of cutting. As a result, it is possible to dispense with expensive bismuth which is injected for the purpose of improving the machinability, or lead which may harm the health of the worker, thereby making it possible to produce a low cost and environmentally friendly tempering steel. In addition, it is possible to use low-cost scrap containing copper (Cu) as a copper-added steel, thereby reducing manufacturing costs. Generally, free cutting steels for machine structural use are difficult to secure excellent machinability by only MnS inclusions added to improve machinability. Therefore, it is very important to control the alloy component that the machinability is improved without the low melting point metal such as bismuth due to the external stress caused by the unevenness of the component of the (Mn, Cu) S sulfide.

For the production of free cutting steel for machine structural use excellent in cutting performance, the composition of free cutting steel for mechanical structure excellent in cutting ability and a manufacturing method thereof will be described below.

The steel according to the embodiment of the present invention comprises 0.08 to 0.13 wt% of C, 0.20 wt% or less of Si, 0.85 to 1.1 wt% of Mn, 0.08 to 0.11 wt% of P, 0.24 to 0.3 0.005 to 0.008% by weight of T. [O], the balance of Fe and unavoidable impurities, and a sulfide containing Cu and Mn, such as Cu , Mn) < / RTI > In addition, the cross-sectional area of the inclusions in the rolling direction section of the wire may contain 150 to 250 Si-Al-O composite inclusions having a size of 3 탆 ² or more per square mm.

Hereinafter, the reasons for the addition of each alloy component constituting the steel, which is one of the main features of the present invention, and the appropriate content range thereof will be described. Here, the content of each component means weight%.

Carbon (C): 0.08 to 0.13 wt%

Carbon is an element that increases the strength and hardness of carbide by forming carbide. It is mostly used as pearlite in free carbon steel for free machining. It plays a role of giving proper strength to be used for heavy carbon machine structure. When the content of carbon is less than 0.08% by weight, strength and hardness can not be sufficiently secured, which is not preferable for a heavy carbon steel mechanical structure. On the other hand, when the content of carbon is more than 0.13 wt%, the hardness of the material is excessively increased and the tool life is greatly shortened. Therefore, the content of carbon is limited to 0.08 to 0.13% by weight.

Silicon (Si): 0.20 wt% or less

Silicon (Si) is used as a deoxidizing agent for molten steel, and can form a low melting point complex oxide inclusion capable of minimizing tool wear due to thermal diffusion during high-speed cutting. When the content of silicon (Si) exceeds 0.20%, hard SiO ₂ -single nonmetallic inclusions which are not Si - Al - O oxides are generated in a large amount, and the wear rate of the tool increases during cutting of the steel, . Therefore, the content of silicon (Si) is limited to 0.20% or less.

Manganese (Mn): 0.85-1.1 wt%

Manganese (Mn) is a very important alloying element important for forming manganese sulfide (MnS), which is a nonmetal inclusion. In the present invention, MnS inclusions can be efficiently extracted by adding 0.85% or more. In addition, the increase of manganese (Mn) widens the austenite region, thereby delaying the production of erosion ferrite during rolling and suppressing the increase of surface defects of the steel strip during hot rolling. However, when the content of manganese (Mn) exceeds 1.1%, there arises a problem that the Mn ratio in the composition of (Cu, Mn) S sulfide increases and the cutting performance decreases. Therefore, the content of manganese (Mn) is limited to 0.85-1.1 wt%.

Phosphorus (P): 0.08 to 0.11 wt%

Phosphorus (P) is an element capable of improving machinability and suppresses a build-up edge, which is present as a segregation or a compound at the grain boundaries of the material and is likely to be formed at the tip of the cutting tool. In addition, phosphorus (P) is an element that lowers mechanical properties and hot rolling property. Addition of 0.08% or more of phosphorus (P) results in weaker grain boundaries and improved cutting performance. However, when the content of phosphorus (P) exceeds 0.11% by weight, the mechanical properties of the steel are deteriorated and at the same time, the cold workability is deteriorated. Therefore, in order to secure mechanical properties and cold workability, it is preferable to control the content to 0.11% by weight or less. Therefore, the content of phosphorus (P) is limited to 0.08 to 0.11% by weight.

Conventional structural steels require a great deal of cost to remove phosphorus (P), which is an impurity element in the manufacturing process. However, since the steels of the present invention utilize phosphorus (P) which is an impurity element, The cost can be reduced, thereby enabling a low-cost operation.

Sulfur (S): 0.24 to 0.30 wt%

Sulfur (S) in free cutting steel forms MnS inclusions during solidification, which reduces wear of cutting tools and improves surface roughness of workpieces by inhibiting the formation of constituent edges during cutting operations of steel. When sulfur (S) is added more than 0.24%, cracks are generated in the MnS existing at the primary and secondary strain points formed at the tip of the cutting tool, thereby reducing the cutting stress and reducing the cutting tool wear and improving the surface roughness of the workpiece can do. However, if the content of sulfur (S) increases more than necessary, FeS having a relatively low melting point is produced. Such FeS deteriorates ductility at high temperature and the hot rolling property of the steel becomes worse. Therefore, Shall not exceed 0.33%. Therefore, the content of sulfur (S) is limited to 0.24 to 0.30% by weight.

Copper (Cu): 0.03 to 0.06 wt%

Copper (Cu) combines with manganese sulfide (MnS) to form sulfides. Component irregularity occurs in the sulfide, which is weaker than the existing MnS alone sulfide, so that the cutting ability can be remarkably improved. In addition, even when a low melting point element such as bismuth and lead is not added, excellent effects can be obtained especially on the tool life and chip segmentality. If the content of copper (Cu) is less than 0.03% by weight, the desired machinability effect can not be obtained. If the content of copper exceeds 0.06% by weight, surface scratches due to brittleness due to copper (Cu) may occur during rolling. Therefore, the content of copper (Cu) is limited to 0.03 to 0.06% by weight.

Aluminum (Al): 0.005 wt% or less

Aluminum (Al) not only contributes to the deoxidation of molten steel as a deoxidizing element of a strong molten steel, but also forms Si-Al-O complex inclusions and contributes to improvement of cutting performance of the steel. When the content of aluminum (Al) exceeds 0.005%, a large amount of hard Al ₂ O ₃ -based nonmetal inclusions can be generated, and the formation of Si - Al - O complex inclusions is remarkably deteriorated, Can be remarkably lowered. Therefore, the content of aluminum (Al) is limited to 0.005% or less.

Total oxygen amount (T. [O]): 0.005 to 0.008 wt%

Since the silicon (Si) is applied as the main deoxidizing element, the dissolved oxygen (O) during solidification bonds with the silicon (Si) in the mold to form a coarse Si-Al-O oxide and the Si- And serves as a nucleation site for purifying a sulfide containing Cu and Mn, that is, (Cu, Mn) S. Here, oxygen means the total amount of oxygen (T. [O], total oxygen) of the cast (or billet) that has been cast. If the content of oxygen (O) is less than 0.005% by weight, the Si-Al-O oxide is present in a fine size and the cutting property is deteriorated. If the content of oxygen (O) is too much and exceeds 0.008% by weight, Si-Al-O oxide is excessively formed to form a Si-Al-O oxide intermixture to form a coarse Si-Al-O oxide do. Mechanical properties may be deteriorated, and surface defects such as pin holes, blow holes, and the like are greatly increased in a solidified cast product. Therefore, the content of oxygen (O) is limited to 0.005 to 0.008% by weight.

The weight ratio of copper (Cu) and manganese (Mn) (M: (Cu / (Mn + Cu) x 100)): 4.0? M? 6.0

(Cu / (Cu + Mn) x 100) is controlled to be 4.0 or more on the basis of the weight%, in order to provide a free cutting steel for precision machining requiring proper hot rolling property. This results in the formation of sulfides ((Cu, Mn) S) containing Cu and Mn by bonding copper (Cu) and manganese (Mn) to sulfur (S), thereby increasing component nonuniformity, So that the workability can be secured. However, when M is 6.0 or more, sulfur (S) is sufficiently ensured, but the formation of sulfides containing Cu and Mn is reduced and the workability is poor. Therefore, the weight ratio of copper (Cu) to manganese (Mn) is limited to 4.0? Cu / (Mn + Cu) x 100? 6.0.

Si-Al-O complex inclusion: the number of inclusions having a diameter of 3 占 퐉 or more is? 150 to 250 per

Here, the Si-Al-O complex inclusion means a state including a sulfide (Cu, Mn) S) precipitated by using Si-Al-O oxide and Si-Al-O oxide as nuclei. According to the results of the optical microscope observation and the machinability evaluation, the Si-Al-O composite inclusion having a diameter of 3 탆 or more, preferably 3 to 100 탆, when the cross-sectional area of the inclusions in the cross- When 150 to 250 pieces were present, the cutting performance was the best. If it is less than 150, the machinability improvement effect can not be obtained. If it exceeds 250, the mechanical properties are decreased and it is difficult to be used for precision machining.

Sulfides containing Cu and Mn ((Cu, Mn) S): ㎟ More than 500 per

Here, the sulfide ((Cu, Mn) S) containing Cu and Mn means that it is not precipitated together with the Si-Al-O oxide but is precipitated singly. The greater the number of sulfides, 500 or more, preferably < RTI ID = 0.0 > The cutting performance can be improved when 500 to 2000 pieces are present.

Hereinafter, a method of manufacturing a steel material according to an embodiment of the present invention will be described.

A forced manufacturing method according to an embodiment of the present invention includes a process of preparing a molten iron, a primary refining process of charging a molten iron into a converter and blowing and refining oxygen, a process of pouring molten steel having been subjected to the primary refining into a ladle, A second refining process for refining the molten steel after the ladle containing the molten steel is transferred to a ladle furnace (LF), a step for transferring the refined molten steel to a continuous casting machine for casting, and a step for casting the casting, Casting the billet, rolling the billet into a billet, and then processing the billet again into a wire rod.

Hereinafter, detailed conditions for each step will be described.

Charter arrangement

Chartered wire can be produced in a blast furnace or an electric furnace. At this time, the molten iron may contain a large amount of impurities such as carbon (C), phosphorus (P), sulfur (S), silicon (Si), and manganese (Mn). In the present invention, since phosphorus (P), sulfur (S) and copper (Cu) are used in the impurities, a separate pretreatment process does not need to be performed during the charcoal preparation process.

Primary refining

The primary refining process is a process which can remove carbon, silicon, phosphorus, etc. contained in the molten metal by air or slag by blowing oxygen supersonic into the molten metal in the converter. In the present invention, the amount of silicon (Si) to be used for deoxidation can be reduced by controlling the dissolved oxygen in the molten steel in the converter to 300 ppm or less. In addition, in order to carry out the primary refining process, a low-cost scrap containing copper (Cu) may be injected to adjust the copper (Cu) component when the charcoal is charged into the converter. Here, it is described that scrap containing copper (Cu) is charged in the primary refining process, but scrap containing copper (Cu) may be added during the production of the molten iron.

Course

The molten steel in which the oxygen blowing is completed is introduced into the ladle in a metic acid state not deoxidized. The reason for tapping in the state of metic acid is that when the oxygen content is high, a large amount of MnO is formed, and since sulfur can not be dissolved due to oxygen, the inclusion can be increased by reacting with sulfur and MnO. However, in some cases, additives such as carbon powder, Fe-Mn, Fe-P, Fe-S, and the like may be added to control the molten steel and slag to an appropriate range during the lapping process.

Second refining process

The secondary refining is carried out to finely control the composition of the molten steel. However, in order to prevent the molten metal from solidifying, an electric arc can be supplied to the molten metal through the carbon electrode to raise the temperature of the molten metal. However, in the process of heating the molten metal, the oxygen compound in the slag and the oxygen in the atmosphere are decomposed by the electric arc, and the molten metal flows into the molten metal to increase the free oxygen concentration. During the heating, additional iron or additive may be added to obtain desired components. In the ladle, it is preferable that the refining is finished in the range of 20 to 40 ppm of dissolved oxygen in the molten steel. When the refining is terminated when the dissolved oxygen is less than 20 ppm, the subsequent Si-Al-O complex inclusions are present in a minute size, making it difficult to secure cutting performance. When the concentration of oxygen exceeds 40 ppm, the size of the Si-Al-O complex inclusion becomes large, and the mechanical properties are deteriorated.

Continuous casting process

The refined molten metal is transferred to the turn-dish to carry out the continuous casting to cast a bloom or a billet.

Rolling process

When the cast bloom is cast in the continuous casting process, the billet can be manufactured by rolling the billet. At this time, in the case of the rolling of the billet, when the rolling is performed under a low temperature of the bloom, the surface of the billet produced may be seriously damaged. Therefore, the bloom is maintained in a heating furnace at 1100 to 1300 ° C or higher for 3 to 4 hours, Can be performed. When the temperature of the heating furnace is less than 1100 ° C, it is difficult to obtain a billet having a good surface quality even if it is maintained in the heating furnace for a long time. When the temperature of the heating furnace is higher than 1300 ° C, burnt phenomenon occurs in which the grain boundary of the material extremely melts. Therefore, it is not desirable to keep the temperature of the heating furnace too high. In addition, if the heating furnace retention time is less than 3 hours, it is difficult to obtain a good billet surface quality in the indicated temperature range, and even if the heating time exceeds 4 hours, the grain boundary of the material is extremely melted, It is difficult to obtain.

In addition, when the billet is cast in the continuous casting process, the billet can be rolled and processed into a wire rod. At this time, in the case of the wire rolling, the billet can be maintained in a heating furnace at about 1100 to 1200 ° C for 2 to 4 hours, that is, after heat treatment. When the temperature of the heating furnace is lower than 1100 DEG C, it is difficult to obtain a wire rod having good surface quality even if it is maintained for a long time in the heating furnace. When the temperature of the heating furnace is higher than 1200 ° C., burnt phenomenon occurs in which the grain boundary of the material extremely melts. Therefore, it is not desirable to keep the temperature of the heating furnace too high. Also, when the heating furnace holding time is less than 2 hours, it is difficult to obtain good surface quality of wire in the prescribed temperature range, and even when the heating time exceeds 4 hours, the grain boundary of the raw material melts extremely, resulting in excellent wire surface quality it's difficult.

2 is a scanning electron micrograph and an X-ray spectroscopy (SEM-EDS) image of inclusions formed on a steel material according to an embodiment of the present invention; FIG. 2 is a graph showing the inclusion shape and microstructure of a steel material according to an embodiment of the present invention; Fig.

As a result of the production of the steel material as described above, it was found that the (S, Mn) S sulfide precipitated in the steel material. As shown in Fig. 1, the (Cu, Mn) S sulfide may be precipitated singly or precipitated from Si-Al-O oxide.

FIG. 2 shows the results of X-ray spectroscopic analysis of the contents of inclusions formed on the steel material. It is confirmed that the (Cu, Mn) S sulfide precipitates from Si-Al-O oxide as a starting point I could.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the present invention by way of illustration and not to limit the scope of the present invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

A bloom having a size of 500 mm × 600 mm and having a size of 500 mm × 600 mm and satisfying the composition of the following Table 1 was cast through a continuous casting process through a 300-ton converter and an LF, and the bloom was processed into a billet having a size of 160 mm × 160 mm , A wire material having a diameter of 27 mm was produced, and then a wire material was processed to produce a cold-drawn bar having a diameter of 25 mm.

Then, an experiment was conducted to evaluate the machinability of a round bar on a CNC lathe.

The machinability evaluation was a turning test using a turning insert. The turning tool had a diamond shape with a corner angle of 55 ° and an allowance angle of 7 °, a nose radius of 0.4 mm, and a tool type cermet.

A round bar having a diameter of 25 mm was processed to a diameter of 23 mm and a length of 20 mm to prepare a specimen. Cutting speeds of 200m / min, feed rate of 0.075mm / rev and cutting depth of 0.5mm were adopted, and cutting test was carried out in wet condition using cutting oil.

C
(wt%) Si
(wt%) Mn
(wt%) P
(wt%) S
(wt%) Cu
(wt%) Al
(wt%) T. [O]
(wt%) Cu / Mn + Cu) * 100 Example 1 0.11 0.18 0.91 0.101 0.28 0.039 0.004 0.0062 4.1 Example 2 0.11 0.2 0.95 0.105 0.25 0.058 0.003 0.0076 5.8 Example 3 0.12 0.19 0.92 0.103 0.26 0.045 0.005 0.0059 4.7 Example 4 0.1 0.17 0.93 0.095 0.26 0.049 0.004 0.0053 5.0 Example 5 0.09 0.15 0.95 0.096 0.29 0.21 0.003 0.0065 18.1 Example 6 0.1 0.16 0.96 0.093 0.24 0.01 0.005 0.0054 1.0 Example 7 0.11 0.17 0.93 0.95 0.29 0.051 0.05 0.0021 5.2 Example 8 0.12 0.18 0.92 0.91 0.03 0.043 0.003 0.0056 4.5

The results of the measurement of the number of Al-Si-O complex inclusions and the number of (Cu, Mn) S sulfides from each specimen after the test, and the measurement results of cutting quality and surface quality are shown in Table 2 below. The machinability was judged by the tool life and the surface quality was judged by measuring the surface roughness. At this time, the tool life was generally determined by measuring the degree of flank wear according to the widely used cutting time. The machinability can be judged through the degree of flank wear and the degree of flank wear is not represented by numerical values, but the smaller the flank wear degree, the better the cutting performance. The surface quality can be judged from the surface roughness, and the surface roughness is not also represented by the numerical value. However, the smaller the numerical value, the better the surface quality and the rolling property.

(Si-Al-O) (number / mm2) (Cu, Mn) S number (number / mm2) Surface quality Machinability Example 1 215 1265 Good Very good Example 2 220 1351 Good Very good Example 3 183 1129 Good Great Example 4 192 1265 Good Very good Example 5 231 1350 Very low Great Example 6 176 890 Good Dread Example 7 35 1302 Good Dread Example 8 230 286 Good Very low

Referring to Table 2, Examples 1, 2, and 4 are manufactured to have the composition range of the components defined in the present invention, and exhibit extremely excellent machinability. However, in the case of Example 3, the Si-Al-O composite inclusions were manufactured to have the component composition ranges defined by the present invention, but the number of Si-Al-O complex inclusions was relatively small as compared with Examples 1, 2 and 4,

On the other hand, Examples 5 to 8 were produced so as to deviate from the component composition range defined in the present invention, and showed poor cutting performance or surface quality. In the case of Example 5, copper (Cu) was contained in a range exceeding the range proposed in the present invention, but the machinability was comparatively good, but the surface quality due to the failure of rolling was extremely poor due to the brittleness of copper. In the case of Example 6, copper (Cu, Mn) S was not formed as much as desired because the copper was less than the range suggested in the present invention, so that the rolling property was good but the cutting property was extremely poor. In the case of Example 7, aluminum (Al) was added in excess of the range suggested in the present invention, so that the number of Si-Al-O oxides was remarkably small, so that Cu and Mn The number of sulfides contained in the steel was relatively small and the machinability was poor. In the case of Example 8, it can be seen that sulfur (S) is added in a smaller amount than the range suggested in the present invention, and (Cu, Mn) S per unit area is formed to be less than the range suggested in the present invention, . In Examples 7 and 8, even if the value of M (Cu / (Cu + Mm) x 100) defined in the present invention is included in the range, if the composition range proposed in the present invention is not satisfied, .

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be construed as limited to the embodiments set forth herein. Those skilled in the art will appreciate that various modifications and equivalent embodiments may be possible. Accordingly, the technical scope of the present invention should be defined by the following claims.

Claims (12)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises 0.08 to 0.13% by weight of C, 0.20% by weight or less of Si, 0.85 to 1.1% by weight of Mn, 0.08 to 0.11% by weight of P, 0.24 to 0.3% 0.03 to 0.06% by weight of Cu, 0.005 to 0.008% by weight of T. [O], the balance of Fe and unavoidable impurities,
Steel containing sulfides containing Cu and Mn. The method according to claim 1,
The sulfide is present as a complex inclusion precipitated by using Si-Al-O as nuclei. The method of claim 2,
The composite inclusion may be formed,
And a size of not less than 3 占 퐉 of 150 to 250 pieces / mm2 or more. The method according to claim 1,
The sulfide,
Steel of 500 pieces / ㎟ or more. The method according to claim 3 or 4,
Wherein the Mn and the Cu satisfy the following equations.
0.4? M? 0.6
(Where M is Cu (wt%) / (Mn (wt%) + Cu (wt%) x 100) The process of preparing a charter;
Refining the molten iron to produce molten steel containing sulfides containing Cu and Mn;
Casting the casting using the molten steel; And
And rolling the casting. The method of claim 6,
And adding scrap containing copper into the molten steel during at least one of a process of preparing the molten iron and a process of manufacturing the molten steel. The method of claim 7,
And a step of preparing the molten steel immediately after the step of preparing the molten iron. The method of claim 8,
The process for producing the molten steel includes:
A primary refining process of blowing oxygen and refining the charcoal; And
And a secondary refining process of adjusting the components of the molten steel for which the primary refining process is completed,
In the primary refining process, the dissolved oxygen in the molten steel is adjusted to 300 ppm or less,
Wherein the secondary refining process regulates dissolved oxygen in the molten steel to 20 to 40 ppm. The method of claim 6 or claim 9,
The process for producing the molten steel includes:
C: 0.08 to 0.13 wt%, Si: 0.20 wt% or less, Mn: 0.85 to 1.1 wt%, P: 0.08 to 0.11 wt%, S: 0.24 to 0.3 wt% 0.06 wt.%, T. [O]: 0.005 to 0.008 wt.%, The balance Fe and unavoidable impurities. The method of claim 6,
The step of rolling the casting comprises:
Treating the cast product at a temperature of 1100 to 1300 캜 for 3 hours to 4 hours, and then rolling the steel product. The method of claim 10,
The step of rolling the casting comprises:
Treating the cast product at a temperature of 1100 to 1200 ° C for 2 to 4 hours, and then rolling the cast product.

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