KR20100018347A - Free-cutting steel with excellent machinability and manufacturing method thereof - Google Patents

Abstract

PURPOSE: Free-cutting steel with excellent machinability and a manufacturing method thereof are provided to facilitate production of free-cutting steel, improve machinability, and improve hot rolling property of the free-cutting steel. CONSTITUTION: Free-cutting steel with excellent machinability comprises the following steps of: refining a converter by hypersonically injecting oxygen into molten metal in the converter and stopping the injection of the oxygen when free oxygen is 400~1000ppm; tapping the molten metal to a teeming ladle; transferring the teeming ladle to LF and refining the LF until the free oxygen content of the molten metal becomes a range of 100~200ppm; continuously casting the molten steel into billet; and press-fitting the billet into a wire rod by maintaining the billet in a heating furnace at 1200~1350°C for 2~5 hours.

Description

Free-Cutting Steel with Excellent Machinability and Manufacturing Method Thereof}

The present invention relates to an environment-friendly lead-free free-cutting steel excellent in machinability and a manufacturing method thereof, and more specifically, ① by adding an appropriate amount of Ti, Cr, N, etc. ② control the ratio of Mn / S of the components to 3.5 or more, ③ Computational small amount (T. [O]), while a limit to less than 300ppm by ④ MnS 5㎛ area of ^two or more the number of the rolling direction cross-section of the MnS inclusions is adapted to present in a range of 300-1000 per mm ² machinability and hot rolling It relates to an environment-friendly lead-free free-cutting steel with improved properties and a method of manufacturing the same.

Free-cutting steel refers to a steel that has been highly improved in machinability of steel, commonly referred to as machinability. Such free-cutting steel is widely used as a material for shafts and cutting parts of office automation devices that can be used in automobile hydraulic parts, printers, etc., and its use or demand is gradually increasing.

Free-cutting steels must basically have good machinability, for which they contain unique ingredients or inclusions. As a mechanism for improving such machinability, a non-metallic inclusion present in the steel, in particular, may be used, and a representative of such nonmetallic inclusions is MnS. The machinability of free-cutting steel can be obtained by controlling the size, shape, distribution, etc. of MnS. More specifically, when cutting steel using a machining device such as a lathe, MnS is in contact with the tip of the tool. Non-metallic inclusions act as a stress concentration source, and voids are generated at the matrix interface with these nonmetallic inclusions, thereby promoting crack growth in the voids, thereby reducing the force required for cutting. It works as a reducing principle.

Therefore, in order to improve the machinability of free-cutting steel, ① MnS must remain large, ② must be distributed randomly, ③ MnS size is large, especially the shape is closer to spherical shape.

The shape of MnS present in free-cutting steel varies greatly depending on the oxygen content of the playing tundish, and these shapes are largely divided into three types: spherical (Type I), dendritic (Type II), and irregular (Type III). ) And the like.

The spherical (Type I) is known to be useful for improving machinability of free-cutting steel. When the Tundish oxygen content (T. [O]) is high, about several hundred ppm, Mn (O) is solidified in hot molten steel in parallel with deoxidation. And S). On the other hand, the dendrite (Type II) structure is not crystallized in molten steel during solidification when the content of Tundish T. [O] is relatively low, such as several tens of ppm, but precipitates along the primary grain boundary, and then in the hot rolling process of the steel. It is easily stretched along the rolling direction, greatly deteriorating the anisotropy of the material. The dendritic structure is a form that is produced during the solidification of the general steel except the free-cutting steel, and since the mechanical properties of the steel is greatly deteriorated, in the refining process, much effort has been made to reduce the S content to about several ppm in order to suppress MnS precipitation. Finally, irregular type (Type III) MnS is characterized by low Tundish T. [O] content in molten steel, which is produced as MnS single inclusion at high temperature when molten aluminum is high. It is known to exist in square form.

As a prior art for such a free cutting steel, Japanese Patent Laid-Open No. 2004-027333 has been proposed. The prior art discloses a technique of limiting elements such as C, Si, Mn, S, P, Nb, and O to a specific range, and simultaneously limiting the area ratio of polygonal ferrite to 5% or more as a microstructure. . However, the present invention does not clearly show the role of these ferroalloys in free cutting steel while adding a large amount of expensive alloying elements such as Nb, Mo, and Zr. In addition, while the area ratio of polygonal ferrite is limited to a desired range, there is a problem in that the measurement method is not presented in detail.

As another prior art for free cutting steel, Japanese Patent Laid-Open No. 2000-265243 is known. Here, alloy elements C, Si, Mn, S, O, Bi and the like is added in a certain amount, characterized in that the ratio of the number of Bi inclusions and the Bi content per mm ^{2 in the} cross section of the rolling direction to a certain value or more. On the other hand, the present invention limits the ratio of the number of Bi inclusions and Bi content, it is difficult to actually control the ratio in the manufacturing process of free cutting steel. Furthermore, this invention is characterized by adding oxygen at 0.003% by weight or less, and it is difficult to provide high oxygen free cutting steel having excellent machinability for controlling the MnS shape to Type I, that is, spherical shape.

Another conventional technique for producing free cutting steel is Japanese Patent Laid-Open No. 1993-345951. The present invention relates to sulfur-based continuous casting free cutting steel having the same level of machinability as the free cutting steel manufactured by the conventional ingot method, and includes carbon (C), manganese (Mn), phosphorus (P), sulfur (S), and nitrogen (N). ) And oxygen (O ₂ ), and the average size of the MnS inclusions is 50 μm ² or less. However, the present invention discloses the contents of MnS, but only suggests the particle size, and does not explain the effect of MnS shape on machinability.

Another invention for free cutting steel is Japanese Patent Laid-Open No. 1993-173593. The proposal is based on carbon (C), manganese (Mn), phosphorus (P), sulfur (S), nitrogen (N) and oxygen (O ₂ ) as the basic components, 0.1 wt% or less Si, 0.009 wt% Al. It is limited to% or less, and the total mass of N and oxide inclusions in the range of 20 to 150 ppm is 50% or more. However, in view of the fact that it is practically difficult to measure the mass of oxide-based inclusions in free-cutting steels, it is considered that the present invention, which controls a value that is difficult to measure to a certain range, is problematic in its effectiveness and practicality.

Another prior art for free cutting steel is Korean Patent Publication No. 1999-0047911. The present invention relates to a method for producing Bi-S-based free cutting steel, characterized in that the high-temperature ductility is increased by adjusting the free-cutting steel and the austenite grain size of excellent physical properties to a certain size. That is, by weight, carbon: 0.05-0.15%, manganese: 0.5-2.0%, sulfur: 0.15-0.40%, phosphorus: 0.01-0.10%, oxygen: 0.003-0.020%, bismuth: 0.03-0.30%, silicon: 0.01% or less, aluminum less than 0.0009%, the remainder is made of iron and inevitable impurities, the shear fraction of MnS inclusions adsorbed with MnS and bismuth is 0.5% -2.0%, and the shear fraction of bismuth is 0.030% It proposes a Bi-S-based free cutting steel of -0.30%, which relates to a Bi-S-based free cutting steel, and does not provide a method for controlling the MnS shape as in the present invention.

SUMMARY OF THE INVENTION The present invention proposes a feature of free cutting steel that has not been clearly pointed out by the above-described problems or prior arts, and also provides an unleaded free cutting steel that is suitable for environmental regulation standards and has excellent characteristics such as cutting property and hot rolling property. .

To this end, in the present invention, in weight%, C: 0.03 ~ 0.13%, Si: 0.1% or less, Mn: 0.7 ~ 2.0%, P: 0.05 ~ 0.15%, S: 0.2 ~ 0.5%, B: 0.001 ~ 0.01% , Cr: 0.1 to 0.5%, Ti: 0.05 to 0.4%, N: 0.005 to 0.015%, O: 0.03% or less, providing a lead-free free-cutting steel containing residual Fe and other unavoidable impurities, the wire rolling direction of the lead-free free-cutting steel MnS inclusions having a particle size of 5 μm ² or larger in cross section can be present in the range of 300-1000 per mm ² of material. In this case, the weight ratio between Mn and S may be Mn / S ≧ 3.5.

Furthermore, the present invention, the converter refining step of blowing oxygen to the molten metal in the converter at supersonic speed to terminate the oxygen injection when the free oxygen is 400 ~ 1000ppm, tapping step of tapping the molten oxygen is completed to the teaming ladle in the non-deoxidation state After the transfer of the Timing Rays to the LF, the heating step of LF refining of the molten steel until the free oxygen concentration of the molten metal is in the range of 100 to 200 ppm, and the concentration of the free oxygen is 50 to 150 ppm at 10 to 50% of the casting time. It provides a continuous casting step of casting molten steel into a billet, and a wire-rolling step of rolling the wire billet while maintaining the billet at 1200 ~ 1350 ℃ temperature in a heating furnace for 2 to 5 hours. In this case, in the continuous casting step, the molten steel may be manufactured as a bloom, and then may be manufactured as a billet by rolling a steel sheet. At this time, the bloom is maintained at a heating furnace temperature of 1250 ° C. or higher for 4 to 10 hours and the bloom is billed. It may further comprise a rolling step rolling step.

In addition, the lead-free free-cutting steel in weight%, C: 0.03 ~ 0.13%, Si: 0.1% or less, Mn: 0.7 ~ 2.0%, P: 0.05 ~ 0.15%, S: 0.2 ~ 0.5%, B: 0.001 ~ 0.01% , Cr: 0.1 to 0.5%, Ti: 0.05 to 0.4%, N: 0.005 to 0.015%, O: 0.03% or less, residual Fe and other unavoidable impurities, particle size 5㎛ ² The above MnS inclusions may be present in the range of 300-1000 per mm ² of material. In particular, the continuous casting step may use a mold electron stirring device, a soft reduction device or a mold electron stirring device and a soft reduction device, and the weight ratio between Mn and S is Mn / S≥ May be 3.5.

As described above, according to the present invention, an S-free cutting steel can be easily manufactured in steelmaking and a performing process, and a large amount of spherical MnS is crystallized in the steelmaking step, thereby greatly improving machinability and having an excellent hot rolling property. Can provide.

In order to achieve the above object, the present invention adds ① Ti, Cr, N, etc. in an appropriate amount, ② controls the ratio of Mn / S in the component to 3.5 or more, and ③ the oxygen content (T. [O]) of 300 ppm or less. The main content is to control the number of MnS inclusions so that MnS with an area of 5㎛ ² or more in the rolling direction cross section exists in the range of 300 ~ 1000 per mm ² .

The MnS inclusions are present in the free-cutting steel in the shape as shown in FIG. 1, and especially when Ti, Cr, and N are appropriately limited by the above-described technical configuration, the (Cr, Ti) S-based or As shown in FIG. 2, (Cr, Ti) N-based fine precipitates are precipitated in a large amount at the grain boundary, thus improving the machinability by preventing work hardening during machining of parts. By improving the toughness, it is possible to suppress the production of build-up edges (BUE), improve the chip segmentability, and improve the machinability of free-cutting steel. This can have the effect of increasing the tool life and the excellent surface roughness of the steel.

Hereinafter, the component system constituting the present invention will be described in more detail.

Carbon (C): 0.03 to 0.13 wt%

C is an element that forms carbide to increase the strength and hardness of the material. In free cutting steel, C exists as a part of pearlite, and when a steel is cut, a build-up edge (hereinafter referred to as BUE) is generated in the tool. It plays a role in suppressing becoming. If the C content is less than 0.03%, it is difficult to increase the hardness of the raw material in the desired range, and there is no effect of suppressing the construction edge. On the other hand, if the C content exceeds 0.13%, the hardness of the material is excessively increased, so that the tool life is greatly shortened. In the present invention, the C content is limited to the range of 0.03 to 0.13%.

Silicon (Si): 0.1 wt% or less

Si is an element remaining in the raw material due to pig iron or a deoxidizer, and since it is mostly dissolved in ferrite unless it forms an oxide, that is, SiO ₂ , it is known that it does not affect the mechanical properties of general free cutting steel. However, according to the experiments of the present inventors, in the high oxygen free cutting steel, when the Si content exceeds 0.1%, SiO ₂ is generated, and the tool life is greatly reduced during machining of the free cutting steel. Therefore, in the present invention, Si is not added. It is done. However, Si may inevitably flow from ferroalloy, refractory, etc. in the steelmaking process, so that the Si content present in the free cutting steel of the present invention is controlled to 0.1% or less.

Manganese (Mn): 0.7 to 2.0 wt%

Mn is an essential alloy element in the present invention in order to form a non-metallic inclusion MnS for imparting machinability of the steel, MnS inclusions can be effectively determined by adding more than 0.7%, further increasing the surface defect of the steel sheet during hot rolling You can get the effect. However, if the Mn content is excessively exceeding 2.0%, the hardness of the steel may be increased, thereby decreasing the tool life. In addition, in the Mn content ranges from 0.7 to 2.0%, some Mn combines with oxygen to produce MnO, which acts as a nucleus for MnS during the coagulation process, thereby promoting the formation of spherical MnS inclusions.

Phosphorus (P): 0.05-0.15 wt%

P is an element for suppressing the BUE that is easily formed at the tip of the cutting tool, and when the P content is less than 0.05%, it is difficult to expect the effect of suppressing the production of BUE, whereas when the P content exceeds 0.15%, the BUE suppression effect is excellent but the hardness of the steel In this invention, the P content is limited to the range of 0.05 to 0.15% because of the increase in the cutting tool life.

Sulfur (S): 0.2-0.5 wt%

In free cutting steel, S is used to form MnS inclusions upon solidification. MnS is very important in the present invention because it serves to improve the machinability of the steel to reduce the wear of the cutting tool and to improve the surface roughness of the workpiece. S is added at least 0.2% for this purpose. However, according to the experiments of the present inventors, an excessively large amount of S addition promotes the precipitation of reticulated FeS at grain boundaries, and hot rolling can be greatly reduced because such FeS is very fragile and has a low melting point. In addition, if the S content is increased more than necessary, the surface defects of the steel increase and at the same time the steel toughness and ductility are significantly reduced, the content of S should not exceed 0.5%.

Boron (B): 0.001 to 0.01% by weight

B serves to increase the hardenability in the steel, the present invention is added to 10 ~ 100ppm for this purpose. When B is added less than 10ppm, it is difficult to obtain an adequate effect of increasing the hardenability. According to the experiments of the present inventors, when the amount of B exceeds 100ppm, the hardenability can be sufficiently obtained, but the hot ductility is lowered, resulting in hot rolling. It is difficult to limit the range.

Chromium (Cr): 0.1 to 0.5 wt%

Cr is an element that serves to expand the austenite region in carbon steel, and is an important and universal alloying element having a low cost and a property of forming carbides which do not cause embrittlement even when a large amount is added. In the present invention, Cr is added for the purpose of improving machinability, and according to the experiments of the present inventors, there is almost no machinability improvement effect when Cr is added at less than 0.1%, while machinability is limited when it exceeds 0.5%. When the value was reached and no longer improved, the amount of Cr added was limited to 0.1 to 0.5%.

Titanium (Ti): 0.05-0.4 wt%

Ti exhibits a strong affinity with any element such as O, N, C, S and H, and is particularly used for deoxidation, denitrification, deflow reaction and the like. In addition, Ti easily forms carbides and serves to refine the grains. Also in the experiment concerning this invention, when 0.05% or more of Ti was added, it turned out that cutting property is greatly improved by refinement | miniaturization of a grain. However, when added in excess of 0.4%, the effect of improving the machinability reaches a limit, but rather shows a problem that the cutting tool life is shortened due to TiO ₂ formed in the material, the content of Ti is limited to 0.05 ~ 0.4%.

Nitrogen (N): 0.005 to 0.015 wt%

N is an element that affects the formation of the edge of the cutting tool and the surface roughness of the cutting parts. If the nitrogen content is less than 0.005% by weight, the production of BUE increases and the surface roughness is poor. In addition, as the nitrogen content increases, the production of BUE decreases, but when the amount of nitrogen exceeds 0.015%, the surface defects of the finished free-cut steel sheet may increase, which may be a problem. In the present invention, the N content is 0.005 to 0.015%. It is limited.

Oxygen (O): 0.03% by weight or less

Oxygen (O) forms fine MnO in the early stage of solidification of molten steel in the mold during free-cutting steel casting, and the MnO acts as a nucleation site for crystallizing MnS. Here, the oxygen refers to the total oxygen amount (T. [O], total oxygen) of the cast (or steel) cast is completed. As described above, when oxygen is several tens of ppm or less, MnS of Type II or Type III precipitates during solidification of molten steel, and this shape of MnS deteriorates the machinability of free-cutting steel. In order to maximize the machinability, the present invention aims at the determination of Type I, that is, the spherical MnS. Experimental results show that the higher the oxygen content, the more spherical MnS tends to be determined. If the excess exceeds 0.03%, surface defects such as pin holes and blow holes may increase greatly in the solidified slab, so the upper limit thereof is limited.

Weight ratio of manganese (Mn) to sulfur (S): Mn / S ≥ 3.5

In addition to the above component content regulation, in order to provide a free cutting steel having excellent ductility at high temperature according to the present invention, the relationship between the Mn and S is controlled so that the Mn / S ratio is 3.5 or more based on the weight%. This is because it is important to combine Mn with S to avoid hot brittleness due to FeS, and to secure a Mn amount of a predetermined amount or more. Especially when the ratio of Mn / S is less than 3.5, hot rolling property falls and it is difficult to manufacture the free cutting steel pursued by this invention.

Number of MnS: 300 ~ 1000 MnS per mm ^{2 of} 5㎛ ² or more in rolling direction cross section

Eco-friendly lead-free free-cut steel of the present invention greatly varies the machinability depending on the size and distribution of the non-metallic inclusions MnS remaining in the steel. In general, the larger the MnS size and the larger the number, the better the machinability of the steel. According to the optical microscope observation and the machinability evaluation results of the present inventors, MnS exceeding the size of 5 μm ² in the rolling direction, that is, the cross section in the L direction, is mm The best machinability of steels was found at 300 to 1000 per ^two . If the MnS is less than 300, the tool life is reduced due to the decrease in machinability, and the surface roughness of the machined part is also inferior. On the other hand, if the number exceeds 1000, the tool life may be increased, but chip throughput is deteriorated, so the number of MnS is preferably controlled to 300 to 1000.

Hereinafter, the manufacturing method of the high-cutting steel which contains the above-mentioned alloy component and can use an alloy component efficiently for the purpose of invention is demonstrated in detail (refer FIG. 3).

Converter refining stage

First, oxygen is blown into the molten metal in the converter at supersonic speed to remove impurities C, Si, Mn, and P contained in the molten metal by air or slag. In the converter refining, the oxygen injection is preferably completed when the free oxygen of the molten metal in the converter is in the range of 400 to 1000 ppm. If the oxygen content is less than 400ppm, since the carbon content of the molten metal exceeds the composition range of the present invention, it is difficult to control the carbon component, whereas if it exceeds 1000ppm, it may be disadvantageous because it may cause excessive erosion of a refractory such as a converter and teaming ladles.

Tapping stage

Subsequently, the molten oxygen-blended molten metal is subjected to a step of tapping the teaming ladle in a non-deoxidized state, that is, in a non-deoxidized state, and if necessary, an additional raw material such as ferroalloy may be added during the tapping step. The addition of ferroalloy and subsidiary materials is intended to form molten steel and slag in an appropriate range.

Heat rise stage of molten metal

When the tapping is finished, the teaming ladles are transferred to the LF and the molten steel is heated. The heating of the molten metal increases the temperature of the molten steel by supplying an electric arc to the molten steel through a carbon electrode rod previously formed in the LF. During the heating process, ferroalloy or subsidiary materials may be added if necessary. In some cases, the sample of molten steel and the oxygen concentration of the molten steel may be measured. In the process of heating up the molten steel, oxygen concentration in the slag is introduced into the molten metal by decomposing oxygen compounds in the slag or oxygen in the atmosphere by an electric arc. In LF, it is preferable to finish LF refining in the range of 100-200 ppm of free oxygen concentration of a molten metal. When LF refining is terminated at less than 100ppm of free oxygen, it is difficult to form a desired MnS. On the other hand, when LF refining is terminated in a situation exceeding 200ppm, it is difficult to predict fluctuation of molten steel component in a subsequent process, and thus LF refining is difficult. The free oxygen concentration at the end of this time is controlled in the range of 100 to 200 ppm.

Continuous casting step

Continuous casting is performed by transferring molten steel after LF refining to continuous casting machine. After continuous casting is started, the free oxygen concentration of molten steel is measured to determine in advance whether the machinability of the free cutting steel to be cast is good. The free oxygen concentration is measured at 10-50% of the total casting time, and the free oxygen concentration is sufficient in the range of 50-150 ppm. When free oxygen is measured when the casting time is less than 10%, it is difficult to obtain accurate free oxygen concentration due to the influence of tundish refractory material or tundish insulation material, and when it is measured over 50%, the opportunity to control the oxygen concentration is lost. It is disadvantageous. In addition, when the measured free oxygen concentration is less than 50ppm, according to the experiment, the machinability was relatively poor, whereas if it exceeds 150ppm, the pinhole and blowhole increased slightly in the cast steel, which is not good. In casting the free-cutting steel, the mold casting system, that is, the mold EMS and the soft reduction device, can be operated to obtain a better cast. If the mold EMS device is operated, the spherical and large MnS inclusions are obtained. Advantageously, the low pressure device is very advantageous in reducing center segregation of the cast steel and reducing surface defects such as pinholes and blowholes on the surface of the cast steel. Continuous casting of free-cutting steel is basically possible even if continuous casting is performed with blooms such as 300mm x 400mm and 400mm x 500mm or billets such as 120mm x 120mm and 160mm x 160mm. However, if casting in bloom, it is enough to go through the steel rolling process, that is, the manufacturing process of the billet, and if rolling into the billet, it is sufficient to omit the steel rolling process and perform wire rod rolling.

Rolling Step

When the bloom casting is performed instead of the billet casting in the continuous casting step, a process of subsequently rolling the bloom into the billet is additionally included. Rolling into 120 mm × 120 mm or 160 mm × 160 mm billets using 300 mm × 400 mm or 400 mm × 500 mm blooms is commonly referred to as slab rolling or billetizing, the most important of which is the temperature of the slabs and the furnace holding time. When the rolling is performed in a state where the temperature of the steel sheet is low, the surface of the manufactured billet is severely exploded. In the present invention, the heating temperature of 1250 ° C. or more is limited to maintaining the temperature of 4 to 10 hours. If the slab temperature is less than 1250 ℃, it is experimentally confirmed that the surface quality of the manufactured billet is poor no matter how long it is maintained in the furnace, while the holding time in the furnace is 4 even if the bloom temperature is kept above 1250 ℃. Less than time, the billet surface quality was similarly poor. In addition, if the temperature of the bloom is maintained for more than 10 hours in the furnace while maintaining the temperature of more than 1250 ℃, while the productivity is greatly reduced, the billet surface quality is only equivalent to that of 4 to 10 hours, so that the furnace maintenance time It is limited to 4 to 10 hours.

Wire Rolling Step

If the billet is produced by casting the free-cutting steel into the billet or by casting the bloom and rolling the steel sheet, the billet is then rolled into the wire rod. The most important factor in manufacturing the wire rod from the free-cutting steel billet by this step is The temperature and heating time of the billet furnace. In order to obtain a free-cutting steel wire having excellent surface quality, it is preferable to maintain the billet temperature in a heating furnace for 2 to 5 hours in the range of 1200 to 1350 ° C. If the billet temperature is less than 1200 ℃, even if the holding time of the heating furnace is long, it is difficult to obtain a good wire surface quality, if it exceeds 1350 ℃ it was difficult to obtain a relatively better wire surface quality than 1200 ~ 1350 ℃. In addition, it was difficult to obtain a good surface quality of the wire rod in the above temperature range when the holding time of the heating furnace is less than 2 hours, it was concluded that better wire surface quality is difficult to be obtained compared to 2 to 5 hours even if it exceeds 5 hours.

The present invention will be described in more detail with reference to the following examples.

Example 1

In this example, the steel pieces of the composition of the Examples and Comparative Examples (SUM24L) shown in Table 1 below were prepared in a 200 kg class high frequency air induction melting furnace. Here, the comparative example is Pb free-cutting steel, which is currently the most widely used, that is, SAE12L14, and the experimental example and the comparative example were manufactured through the same experimental equipment and manufacturing process, and the steel sheet size was 230 mm X 230 mm X 350 mm.

C Si Mn P S N (ppm) Other ingredients Remarks Experimental Example 1 0.12 0.05 1.2 0.10 0.30 130 Cr 0.3, Ti 0.3 Experimental Example 2 0.09 0.01 1.0 0.08 0.25 98 Cr 0.4, Ti 0.2 Comparative Example 1 0.08 0.03 1.2 0.08 0.33 85 Pb 0.25 SUM24L

The steel strip was heated to 1300 ° C. in a heating furnace, and rolled into a 30 mm thick plate using a pilot rolling mill. Subsequently, the sheet was cut into a square having a size of 30 mm × 30 mm in the rolling direction, and then processed into a round bar having a diameter of 25 mm on a lathe. After that, a cutting test was performed on the 25 mm diameter round bar on a CNC lathe to measure tool life and surface roughness of the cutting surface. The cutting performance evaluation experiment was carried out by adopting the conditions of cutting speed 100m / min, cutting depth 1.0mm and feed rate 0.1mm / rev as the cutting conditions, maintaining a dry condition without cutting oil.

In the above experiment, the tool life was generally measured for flank wear according to the cutting time, surface roughness was derived from the roughness according to the cutting time, and the unit of the flank wear and surface roughness was μm, respectively. Smaller value means better surface property. 4 is a graph showing the tool life of the experimental example and the comparative material of the present invention, the experimental examples of the present invention shows that the tool life of the level equivalent to the comparative steels. Furthermore, in FIG. 5, the surface properties were also equal to or better than those of Pb free cutting steel.

The reason why the present invention can show a level equal to or superior to Pb free-cutting steel not only for tool life but also surface roughness by adding a predetermined amount of Cr, Ti, and N instead of Pb, which is harmful to humans, is S: 0.2-0.5% in high oxygen free cutting steel. When Cr: 0.1 to 0.5%, Ti: 0.05 to 0.4% and N: 0.005 to 0.015% are added to the molten steel, the (Cr, Ti) S type and (Cr) before and after the size of 1 µm (micrometer) , Ti) N-based fine precipitates are precipitated in the grain boundary (Grain Boundary), and these precipitates have the effect of preventing the hardening of the steel during the machining of parts, deterioration of fracture toughness of the steel It appears that this is because suppression of BUE production and improvement of chip segmentation are possible.

Example 2

The 300 × 400 mm free cutting steel bloom of continuous casting was rolled into a billet of 160 × 160 mm size in a steel sheet rolling process, and then the billet was rolled into a wire rod of 25 mm diameter in a wire rod rolling process. At that time, the slabs and billets were heated and cooled under normal free-cut steel rolling conditions. Samples were taken from the finished wire rod, and the amount of oxygen was observed with an N / O analyzer, and the MnS area and shape were observed with an optical microscope. On the other hand, the wire rod is completed by cold drawn wire rod is manufactured to 23mm round bar (CD Bar), and then using a CNC lathe was performed after cutting experiments to measure the tool life. Table 2 summarizes the amount of oxygen (ppm) of the wire rod obtained through the above experiment, the number per mm ^{2 of} MnS having an area of 5 μm ² or more, and the tool life. In Table 4, tool life refers to the number of parts that can be cut with one cutting tool.

division Oxygen content (T. [O]), ppm MnS number per unit area (area of 5㎛ ² or more) Tool life (times) Experimental Example 3 133 316 5,300 Experimental Example 4 138 627 5,500 Experimental Example 5 290 525 6,500 Experimental Example 6 235 467 6,200 Comparative Example 2 116 288 4,400 Comparative Example 3 220 219 3,100 Comparative Example 4 315 527 5,800 Comparative Example 5 380 427 5,700

If the wire oxygen content of the wire rod from Table 2 is included in the limited range of 300 ppm or less of the present invention, if the number of MnS per unit area is in the range of 300 to 1000 (Experimental Examples 3 to 6), the tool life is 5,000 times required by the free cutting steel component customer. It appeared as above. On the other hand, in the case where the total oxygen content is within the scope of the present invention as in Comparative Examples 2 and 3, when the MnS per unit area is less than 300, the tool life is less than 5,000 times. This is thought to be caused by relatively little crack generation and propagation during cutting due to the lack of MnS, which is relatively large in area. On the other hand, in the case of Comparative Examples 4 and 5, the number of MnS per unit area is included in the scope of the present invention, and the cutting tool life exceeds the reference value of 5,000, but the recovery rate from the cast steel to the CD-bar (Cold-Drawn Bar) is less than 80%. The reason for this is that the total oxygen content is over 300ppm, so the pinholes, blowholes, etc. remain on the surface of the cast steel first, and it can be seen that the steel sheet rolling and wire rolling are no longer improved. Therefore, when the amount of oxygen is excessively high, it is difficult to achieve the object pursued by the present invention due to the large number of surface defects, low recovery rate and increased cost.

1 is a micrograph showing the state of the MnS inclusions

2 is a micrograph showing the appearance of Cr, Ti, N, and S-based precipitates coexist with MnS inclusions in high oxygen free cutting steel.

Figure 3 is a schematic view showing a manufacturing process of environmentally friendly lead-free cutting steel of the present invention

Figure 4 is a graph comparing the tool life of the experimental example and the comparative example satisfying the conditions of the present invention

5 is a graph comparing the surface roughness of the experimental example and the comparative example satisfying the conditions of the present invention

Claims (8)

By weight%, C: 0.03 to 0.13%, Si: 0.1% or less, Mn: 0.7 to 2.0%, P: 0.05 to 0.15%, S: 0.2 to 0.5%, B: 0.001 to 0.01%, Cr: 0.1 to 0.5 Lead-free steel, characterized in that it comprises%, Ti: 0.05 to 0.4%, N: 0.005 to 0.015%, O: 0.03% or less, balance Fe and other unavoidable impurities. The lead-free free cutting steel according to claim 1, wherein the lead-free free-cutting steel has MnS inclusions having a particle size of 5 µm ² or more in the cross section of the wire rod rolling direction in the range of 300 to 1000 pieces per mm ² of material. The lead-free free cutting steel according to claim 1, wherein the weight ratio between Mn and S is Mn / S ≧ 3.5. A converter refining step of blowing oxygen into the molten metal in the converter at a supersonic speed and ending the oxygen injection when the free oxygen is 400 to 1000 ppm; A tapping step of tapping the molten oxygen-completed molten metal to the teaming ladle in a non-deoxidation state; A step of raising the molten steel after transferring the teaming layers to LF and LF refining until the free oxygen concentration of the molten metal is in the range of 100 to 200 ppm; A continuous casting step of casting molten steel into a billet such that the concentration of free oxygen is 50 to 150 ppm at a time of 10 to 50% of the casting time; And Wire rod rolling step of maintaining the billet in a heating furnace for 2 to 5 hours at a temperature of 1200 ~ 1350 ℃ rolling; By weight%, C: 0.03 to 0.13%, Si: 0.1% or less, Mn: 0.7 to 2.0%, P: 0.05 to 0.15%, S: 0.2 to 0.5%, B: 0.001 to 0.01%, Cr: 0.1 to 0.5 %, Ti: 0.05 to 0.4%, N: 0.005 to 0.015%, O: 0.03% or less, residual Fe and other unavoidable impurities. The method of claim 4, wherein the lead-free free cutting steel is a method for producing a lead-free free cutting steel characterized in that the wire rod rolling direction so as to present in cross-section ^two or more particle size 5㎛ MnS inclusions in the material 300 ~ 1000 mm per ^second in the range gae. The method of claim 4, wherein the continuous casting step Casting molten steel into bloom; And A steel strip rolling step of rolling the bloom into billets while maintaining the temperature at a heating temperature of 1250 ° C. for 5 to 10 hours; Method for producing a lead-free free cutting steel comprising a. The lead-free casting of claim 4 or 6, wherein the continuous casting step uses a mold electron stirring device, a soft reduction device or a mold electron stirring device, and a soft reduction device. How to. 5. The method of claim 4, wherein the weight ratio between Mn and S is Mn / S ≧ 3.5.

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