KR20090065784A - Free cutting steel containing sulfur and refining method for manufacturing it - Google Patents

Abstract

Sulfur-containing free-cutting steel and a refining method for manufacturing the same are provided to improve machinability of free-cutting steel by controlling the shape and size of MnS and crystallizing a large amount of MnS inclusion. Free-cutting steel contains C: 0.06~0.12%, Mn: 0.8~1.2%, P: with 0.07~0.11%, S: 0.20~0.34%, N: 0.005~0.013%, O: 0.015~0.025%, the remnant Fe and inevitable impurity. The contents of Mn and S satisfy the equation, Mn/S >= 3.5. The average length(L) and width(W) of the residual non-metal material in the steel satisfy the equation, L/W <= 6.0.

Description

Free Cutting Steel Containing Sulfur and Refining Method for Manufacturing It}

The present invention relates to sulfur-containing free cutting steel (hereinafter referred to as 'S free cutting steel') and a refining method for manufacturing such free cutting steel, and more specifically, OA (OA, office automation equipment) shafts, such as precision hydraulic parts and printers of automobiles. The present invention relates to a refining method for manufacturing S free-cutting steel in a steelmaking process in order to significantly improve the machinability of S free-cutting steel widely used for materials such as shafts and cutting parts.

Free-cutting steels are steels that have a high degree of machinability in steel and should generally have good machinability. The excellent machinability of free-cutting steel is obtained by the shape, particle size, and distribution of MnS, which is a non-metallic inclusion in the steel, especially when cutting steel using a machining device such as a lathe. Non-metallic inclusions, such as MnS, act as stress concentration sources at the point where the tip and steel contact, creating voids at the base interface with these nonmetallic inclusions, and cracks at these voids. It is a principle that promotes growth and reduces the force required for cutting.

Therefore, in order to increase the machinability of the S free cutting steel, a large amount of MnS must be present and randomly distributed. It is also advantageous that the larger the MnS size, the more spherical the shape.

The shape of MnS present in S free cutting steel varies greatly depending on the content of Tundish T. [O] and can be classified into three types, namely spherical (Type I), dendritic (Type II) and irregular shape (Type III). Among these, spherical (Type I) MnS is known to be an inclusion type to improve the machinability of S free-cutting steel, and when the Tundish T. [O] content is increased to about several hundred ppm, it coagulates with the deoxidation process while solidifying in the high-temperature molten steel of free-cutting steel. It is crystallized by complex sulfides, such as Mn (O, S).

On the other hand, the dendrite (Type II) structure is precipitated along the primary grain boundary when the Td [O] content is relatively low, such as several tens of ppm, and then easily stretched along the rolling direction during hot rolling of the steel. Greatly deteriorates the anisotropy of the material. The dendritic structure is a form that is produced during solidification of general steel except free cutting steel, and the mechanical properties of the steel are greatly deteriorated. Therefore, in the refining process, the S content is reduced to about several ppm in order to suppress MnS precipitation.

The irregular type (Type III) MnS is characterized by the low content of Tundish T. [O] in molten steel as low as several ppm and the high content of molten aluminum. It is known to exist in a square shape.

The prior art for free-cutting steel relates to a method for producing Bi-S-based free-cutting steel, which relates to Bi-S-based free-cutting steel, and has not provided a method for controlling the MnS shape. In addition, there is related to a sulfur-based continuous casting free-cutting steel having a machinability equivalent to that of a free-cut steel manufactured by the conventional ingot method, and the prior art appropriately includes C, Mn, P, S, N, O ₂ , and the like. It is characterized by setting the average size of inclusions to 50 micrometer <^2> or less. However, the prior art proposes only MnS size, and does not explain the effect of MnS shape on machinability.

The present invention solves the above problems and controls the shape and size of MnS in manufacturing S free-cutting steel and extracts a large amount of MnS inclusions, while greatly improving the machinability of the free-cutting steel while maintaining the hot rollability of the steel sheet to the level of the existing steel or more. It is an object of the present invention to provide an S free cutting steel and its refining method for the purpose of increasing.

In order to achieve the above object, the present invention is in weight%, C: 0.06 to 0.12%, Mn: 0.8 to 1.2%, P: 0.07 to 0.11%, S: 0.20 to 0.34%, N: 0.005 to 0.013%, O (Means Tundish T. [O]): 0.015 to 0.025%, balance Fe and other unavoidable impurities, the ratio of the content of Mn and S being Mn / S ≥ 3.5, the average of residual nonmetallic inclusions in steel A sulfur-containing free cutting steel is provided wherein the length (L) / width (W) ratio is L / W &lt; 6.0. In this case, the free cutting steel may have 50% or more of the residual nonmetallic inclusions satisfying L / W ≤ 6.0 based on the total residual nonmetallic inclusions, and most of the residual nonmetallic inclusions may be MnS.

Furthermore, in order to provide such sulfur-containing free-cutting steel, the present invention includes at least 75% of molten iron and the balance is 40 to 60 Nm per ton of oxygen at a supersonic speed of oxygen in the molten metal while firstly refining the molten metal consisting of scrap metal in a converter or an electric furnace. Primary refining in a converter or electric furnace blown by ³ , a molten steel tapping step of tapping the completed molten steel into teaming ladles, and 2 to transfer and refine the teaming laminations after finishing taping to a ladle refining furnace Provides a method for refining sulfur-containing free cutting steel, characterized in that it comprises a continuous casting step of transferring the molten steel after the secondary refining step and the secondary refining is continuous casting tundish (Tundish) for continuous casting.

In this case, the molten steel tapping step, the step of inputting the ferroalloy containing carbon powder, Fe-Mn, Fe-P, Fe-S at the time of 50-80% of the total tapping time during tapping and before the tapping is finished A slag deoxidizer may be added to the upper portion of the teaming ladle to further perform deoxidation of the ladle slag.

Furthermore, the secondary refining step performed in the ladle refining furnace may further include blowing N in gas or solid state at a flow rate of 0.5 to 1.5 Nm ³ / min by a high or low wicking method. In the step, using the heat of oxidation reaction of a metal such as electric arc (Arc) or Al may further include a step of increasing the total temperature rise temperature of the molten steel by 50 ~ 70 ° for less than 40 minutes.

According to the present invention, it is possible to easily manufacture sulfur-containing free-cutting steel in steelmaking and refining processes, and by refining a large amount of spherical MnS in the steelmaking step, it is possible to maintain the hot rollability of the steel sheet equal to or higher than that of existing materials while greatly improving machinability. Excellent sulfur-containing free cutting steel can be easily provided.

Hereinafter, the component system which comprises S free cutting steel of this invention is demonstrated in detail.

C: 0.06 to 0.12%

C exists as some pearlite in steel and acts as an element that suppresses the build-up edges in the cutting tool when cutting steel. When the content of C is less than 0.06%, it is difficult to expect the effect of suppressing the constituent edges, so in the present invention it should contain at least 0.06% of the carbon component. On the other hand, if the content of C exceeds 0.12%, the hardness of the free-cutting steel is excessively increased so that the tool life can be greatly shortened. Therefore, the content of C is limited to 0.06 to 0.12%.

Mn: 0.8-1.2%

Mn is an element necessary for forming MnS inclusions, and a part of the added Mn acts as a deoxidizer to form MnO, which can act as a nucleus for formation of MnS inclusions to promote the formation of spherical MnS inclusions. Add 0.8% or more. If the content is less than 0.8%, this effect is insignificant, it is difficult to effectively sort out the MnS inclusions, and the surface defects of the steel sheet may be increased during hot rolling. In addition, when the content of Mn exceeds 1.2%, the hardness of the steel is excessively increased, so that the tool life may be reduced, so Mn is added in an amount of 0.8 to 1.2%.

P: 0.07 to 0.11%

P acts as an element for suppressing the construction edge which is easily formed at the tip of the cutting tool. If the P content is less than 0.07%, such an effect is difficult to expect. On the other hand, if the P content exceeds 0.11%, the effect of suppressing the construction edge is excellent, but the tool life is shortened due to the increase of the hardness of the steel, so the P range is 0.07 to 0.11%. It is limited to.

S: 0.20 to 0.34%

In free-cutting steel, S forms MnS inclusions during solidification, which adds 0.20% by weight or more because it improves the machinability of the steel, thereby reducing the wear of the cutting tool and improving the surface roughness of the workpiece. However, excessively increasing the content of S produces a relatively low melting point FeS in the steel, such FeS decreases the ductility at high temperature, hot rolling can be significantly worse. In addition, if the content of S increases more than necessary, the surface defects of the steel may increase and the toughness and ductility of the steel may be significantly reduced, so the upper limit of the amount of S added is controlled to 0.34%.

N: 0.005 to 0.013%

Since N acts as an element that affects the formation of the edging edges and the surface roughness of the cutting parts, N is added at least 0.005%. If the content is less than 0.005%, the amount of constituent edges may be increased and the surface roughness may be reduced.On the other hand, if the content of N is more than 0.013%, the surface defects of the finished free-cutting steel strip may be greatly increased. It is limited to%.

O: 0.015 to 0.025% (O means Tundish T. [O] in the present invention)

O forms initial MgO of solidified steel in the mold during continuous casting of free-cutting steel, and the MnO acts as a nucleation site for crystallizing MnS. In addition, as described above, when the content of Turnedish T. [O] is low, MnS of Type II or Type III may be precipitated during solidification of the molten steel and the machinability of the free cutting steel may be deteriorated, so the lower limit thereof is limited to 0.015%. However, if the oxygen content exceeds 0.025%, surface defects may increase during continuous casting, and MnO oxide is excessively generated during molten steel coagulation, which may act as a cause of internal defects in the steel sheet, so that O is controlled to 0.015 to 0.025%.

Weight ratio of S and Mn: 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, in the present invention, Mn / S is controlled to 3.5 or more based on the weight% of Mn and S. This is to suppress the hot brittleness by S by combining Mn with S, and to form MnS to help improve the machinability. In order to obtain such an effect, the ratio of Mn / S should be 3.5 or more, and in less than 3.5, the effect is insignificant and it is difficult to obtain desired machinability.

Average length (L) / width (W) ratio of nonmetallic inclusions: L / W ≤ 6.0

The nonmetallic inclusions remaining in the free cutting steel vary greatly in the machinability of the steel depending on the length (L) / width (W) ratio. That is, as shown in FIG. 2, the L / W ratio of the nonmetallic inclusions, particularly MnS, remaining in the sulfur free cutting steel gradually becomes spherical (Type I) as the value thereof decreases, and the machinability increases as described above. Non-metallic inclusions included in the free cutting steel may be various, such as alumina, MnS, etc. In the present invention, mainly MnS is present as nonmetallic inclusions. In the present invention, the occupancy ratio of inclusions having an L / W ratio of 6.0 or less based on the mass of the total nonmetallic inclusions should be 50% or more in the steel, because when the presence is less than 50%, the machinability is drastically reduced.

Hereinafter, a method for refining molten steel for easily manufacturing the S free cutting steel of the present invention will be described in detail.

First, the molten metal charged into the converter or the electric furnace is refined through the first refining step. Oxygen is blown into the molten metal by 40 to 60 standard cubic meters (Nm ³ ) per ton of molten metal at supersonic speed. In this case, if the flow rate of oxygen is 180 to 230 Nm ³ / min, the desired amount of oxygen can be blown within an appropriate operating time. The molten metal formed in the converter is a mixture of molten iron transferred from the blast furnace, that is, molten iron of 4.0 to 4.4% by weight of carbon and the balance of scrap metal. At this time, the molten iron is preferably 75% by weight or more, and the remainder is preferably composed of scrap iron. If the molten iron is less than 75% by weight, the heat source is insufficient in the converter, and an additional expensive heat source agent must be added. In addition, iron (Fe) is oxidized during oxygen injection to increase T.Fe in the final refining slag.

When oxygen is blown into the molten metal, C, Si, Mn, P, etc. in the molten metal cause oxidation reaction with the blown oxygen, thereby increasing the melt temperature. Accordingly, the gas streams such as CO and CO ₂ generated by the oxidation reaction move to the hood installed in the upper part of the converter, and the solid or liquid phases such as SiO ₂ , MnO, and P ₂ O ₅ move to the slag layer formed on the upper side of the molten metal. At this time, the oxygen injection amount per ton of molten metal is maintained at about 40 ~ 60Nm ³ when the oxygen injection, but when the oxygen injection amount is less than 40Nm ³ C content becomes high at the end of refining, so it is difficult to secure the appropriate C content 0.06 ~ 0.12% by weight range On the other hand, if it exceeds 60 Nm ³ , the refining process after the converter may be complicated by increasing T.Fe in the slag. During the oxygen injection, CaO is added periodically to adjust the slag basicity, that is, the CaO / SiO ₂ ratio to an appropriate range, and when the oxygen injection is completed, the converter refining is terminated.

Subsequently, the molten steel in which the primary refining is completed is tapped into the teaming ladles. When tapping, 50 ~ 80% of the total tapping time adds C, Mn, P, and S components by adding the ferroalloy, and adds slag deoxidizer before tapping finishes to deoxidize ladle slag. Conduct. When the addition time of the ferroalloy is out of 50 ~ 80% of the total tapping time, the recovery rate of ferroalloy is greatly reduced, it is important to add according to the addition time. Here, the ferroalloy and slag deoxidizer are stored in a basis weight beforehand in a hopper installed near a converter or an electric furnace and added to the teaming ladle at that point. The ferroalloy used at this time includes carbon powder for C component addition, Fe-Mn for Mn addition, Fe-P for P addition, Fe-S for addition of S component, and the like.

A slag deoxidizer is then added on top of the teaming ladle to reduce T.Fe in slag to 8% by weight or less. In general, the slag subjected to converter refining contains 10-20 wt% of T.Fe, and it is very important to reduce the T.Fe content of the ladle slag to 8 wt% or less by adding a slag deoxidizer. The reason is that the dissolved oxygen in the molten steel can be easily transferred to the slag layer in the subsequent LF process, and if the T.Fe content of the slag exceeds 8% by weight immediately after tapping, the oxygen content of the molten steel is then 0.015 to 0.025% by weight. This is because it is difficult to control properly.

The point of addition of the slag deoxidizer is the time after the tapping is made about 80%. If the slag deoxidizer is added before that, the oxygen content in the molten steel is excessively reduced, which is not good. In addition, the slag deoxidizer composition which can be used in the present invention may be a commercially supplied slag deoxidizer composed of a metal Al content of 35% by weight or more and composed of balance CaO, Al ₂ O ₃ , SiO ₂ , MgO and volatile components.

Subsequently, the teaming ladles having finished tapping are transferred to a ladle refining furnace, and the N gas is blown at a flow rate of 0.5 to 1.5 Nm ³ / min to increase the nitrogen content to a desired range. That is, since the N content of the molten steel arrived at the ladle refining furnace is 0.002 to 0.004% by weight, by blowing N gas to it to control the content of N in the range of 0.005 to 0.013% by weight. As a method of adding the gas N, nitrogen gas is blown through a top blowing method in which a lance formed of a refractory formed from a ladle is immersed in molten steel and blown nitrogen through the lance, and a bottom plug mounted at the bottom of the ladle. Both low odor methods and the like can be used.

The blown N may be a solid state N such as Fe-N, CaCN _2, etc., but it is preferable to add a gaseous N in consideration of recovery rate, cost, and the like. In the addition of substrate N in molten steel, it is important in particular for controlling the intake flow rate of N to 0.5 ~ 1.5Nm ³ / min range, the nitrogen blow flow rate when less than 0.5Nm ³ / min, the longer the addition time of the low recovery rate N N On the other hand, if the N content exceeds 1.5 Nm ³ / min, it is difficult to control the N component due to the rapid increase of the N content, and the operation is dangerous because the overflow of slag and molten steel occurs over the teaming ladle. This is because the overall mistake rate may decrease.

Subsequently, an electric arc is generated or metal additives such as Al and oxygen are blown simultaneously to raise the molten steel temperature to a desired range, and then fine alloying of the molten steel is added by adding iron alloy. At this time, the temperature increase operation is performed up to 40 minutes or less. If the heating operation exceeds 40 minutes, excessive erosion of the slag line inside the teaming ladle may shorten the life of the teaming ladle. If heating is performed using electric arc, the arc heat is generated by energizing three-phase alternating current through a carbon electrode. Also, in heating up the molten steel, the total temperature is controlled to 50 ~ 70 °. When the temperature is completed, the contents of the alloy components such as C, Mn, P, S, etc. are analyzed, and if the desired range is not met, the chemical composition may be controlled by adding an additional trace amount of ferroalloy in the Lazer refining furnace. Here, the alloy component adjustment amount is preferably 10% or less of the target content. For example, since the content range of C of the present invention is 0.06 to 0.12% by weight, the adjustment amount of C is adjusted to 0.006 to 0.012% by weight or less corresponding to 10%.

The molten steel after the secondary refining is transferred to a continuous casting tundish, and the content of the alloying component and oxygen is measured. In this case, the alloy component is obtained by immersing an immersion sampler commonly used in the steelmaking process in molten steel to collect a molten steel sample, and sending the sample to the analysis chamber through a pneumatic tube, and then analyzing the chemical component. Oxygen content can be read by reading the oxygen content displayed on the instrument by immersing the oxygen measuring sampler, such as TO (Temperature Oxygen) (TO), TOS (Temperature Oxygen Sampling) commonly used in the steelmaking process in the tundish internal molten steel.

Subsequently, S free cutting steel is continuously cast, and free cutting steel is 300mm * 400mm, 400mm * 500mm and other continuous castings, or 120mm * 120mm and 160mm * 160mm continuous castings. It is possible.

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

(Example 1)

In this embodiment, as shown in Figure 1 in the 100-ton steelmaking process, 300mm * 400mm sized free-cutting steel slabs were prepared through the converter refining, tapping and addition of ferroalloy, LF refining, Bloom continuous casting. In this embodiment, first, converter refining in which oxygen is blown from the converter is finished, and alloying elements such as C, Mn, P, and S are added by adding ferroalloy at 80% of the tapping point.

Subsequently, ladle slag deoxidation was performed by injecting slag deoxidizer containing 37.5% by weight of metal Al, which is commercially supplied to the teaming rays, at a tapping time of 95% while tapping with the timings. Finally, the molten steel and slag sample inside the teaming ladle were sampled, and the results of the component analysis were shown in Table 1. Looking at the following Table 1, the alloy components of the molten steel of the present invention and all of the comparative materials satisfy the desired scope of the present invention, T.Fe of the slag does not meet or do not meet the scope of the present invention depending on the amount of slag deoxidizer added. It was confirmed. The oxygen component of the molten steel was measured immediately after the molten steel reached the LF.

division Converter end point oxygen (ppm) Slag Deoxidizer Addition (kg / t-s) Slag T.Fe (% by weight) LF arrival oxygen (% by weight) Comparative Example 1 530 1.5 14.5 0.0415 Comparative Example 2 487 1.0 12.8 0.0438 Inventive Example 1 525 2.3 7.6 0.0312 Comparative Example 3 545 3.5 5.1 0.0152 Inventive Example 2 581 3.5 7.5 0.0259 Inventive Example 3 515 2.1 7.8 0.0308 Comparative Example 4 591 4.5 4.5 0.0098

As shown in Table 1, the slag deoxidizer input amount was adjusted at the time of tapping according to the converter end point oxygen content, but in the case of Comparative Examples 1 and 2, the slag T.Fe was found to exceed the upper limit of 8.0 wt%. Therefore, in the comparative examples, the LF-arrival oxygen content exceeds 400 ppm, so in order to control the target range of 0.015-0.025% by weight, there is no choice but to add molten deoxidizer such as Al and Si, but the addition of molten deoxidizer is Al _2. Nonmetallic inclusions such as O ₃ , SiO _2, etc. are formed, which is disadvantageous because it greatly degrades the hot rollability of the free cutting steel.

On the other hand, in the case of Inventive Examples 1 to 3, it is possible to obtain the T.Fe content, i.e., 8% or less, of the desired slag, and at the same time, the oxygen content of molten steel is about 0.0259 to 0.0312% by weight. If it can be seen that the present invention can easily obtain the target oxygen content. On the contrary, in the case of Comparative Examples 3 and 4, the amount of slag deoxidizer was increased so that the amount of T.Fe was greatly lowered to 5.1 and 4.5% by weight, and thus the oxygen content was significantly lowered to 0.0152 and 0.0098% by weight, respectively. . Therefore, in Comparative Examples 3 and 4, the oxygen content is excessively reduced, making it difficult to satisfy the oxygen limit of the present invention.

Therefore, according to the present embodiment, in order to manufacture free cutting steel having an oxygen content of 0.015 to 0.025% by weight, it is important to add slag deoxidizer during converter winding, and in particular, it is very important to control the amount of slag deoxidizer added to an appropriate range. Could know.

(Example 2)

In the present Example, the experiment of adding N in LF is performed, and Table 2 summarizes the results. In this experiment, N gas was blown for 20-25 minutes through a low odor plug installed at the bottom of the teaming ladle, and the content of each N was measured for each N gas flow rate. The present embodiment includes all cases in which molten steel temperature is increased or not by electric arc during blowing of N gas, but according to the analysis results of the present inventors, the effect of LF temperature increase on nitrogen content increase is insignificant. It was.

division Nitrogen content before blowing (wt%) Blowing flow rate (Nm3 / min) Nitrogen content after blowing (wt%) Blowing time (min) Remarks Comparative Example 5 0.0027 0.30 0.0041 30 No Comparative Example 6 0.0031 0.40 0.0045 30 No Inventive Example 4 0.0023 0.50 0.0054 30 Yes Inventive Example 5 0.0021 1.30 0.0091 25 Yes Inventive Example 6 0.0035 1.20 0.0103 25 Yes Comparative Example 7 0.0034 1.60 0.0141 25 No

In Comparative Examples 5 and 6 of Table 2, although the blowing time was sufficient as 30 minutes, it can be seen that the amount of increase of N was not so small that the limit of the N component of the present invention was not reached. Of course, if the blowing time of N is further extended, it will reach a limited range, but it can be seen that the flow rate is basically a phenomenon that appears, and the blowing time extension is not preferable because it leads to an extension of the production time.

On the other hand, Inventive Example 4 shows that N is blown at a flow rate of 0.5 Nm ³ / min to easily reach the limit of the present invention. Therefore, according to this embodiment, it can be seen that the lower limit of the blowing flow rate of N gas is 0.5 Nm ³ / min.

If the blowing flow rate is gradually increased, the blowing time can also be shortened, such examples can be seen in Inventive Examples 5 and 6. In Inventive Examples 5 and 6, it was found that the blowing flow rate was increased and the blowing time was shorter than 4 in the invention. However, in Comparative Example 7, while the nitrogen component is sufficiently increased, it can be seen that the blowing flow rate is too large, greatly exceeding 0.013% by weight of the present invention. In this case, the surface defect of the finished steel piece is increased, which is very disadvantageous for the production of free cutting steel.

(Example 3)

In the present embodiment, the 300 * 40mm sized Blum slab after continuous casting was rolled into a billet of 160mm * 160mm size in the rolling process, and then the billet was rolled into a wire rod having a diameter of 27mm in the wire rod rolling process. . The steel sheets and billets were heated and cooled under normal free-cut steel rolling conditions. In the present embodiment, the oxygen concentration was measured in the performance tuned, the specimen of the slab after the continuous casting was taken, and the shape of the MnS inclusions was observed by scanning electron microscopy. , And measured the tool life and the results are shown in Table 3 summarized the tundish oxygen concentration, non-metallic inclusion shape and tool life. In Table 3, the occupancy ratio of the spherical inclusions (Type I) indicates the occupancy ratio of inclusions having a length (L) / width (W) ratio of inclusions remaining in the steel of 6.0 or less. In addition, tool life refers to the number of parts that can cut a wire rod with one cutting tool bite.

division Tundish T. [O], weight percent Percentage of Older Type Inclusions (Type I) Tool life (times) The present invention 7 0.0178 58% 5,300 Present invention 8 0.0211 78% 6,500 Comparative Example 8 0.0117 41% 3,100 Comparative Example 9 0.0086 34% 2,600

In the case of Inventive Examples 7 and 8, wherein the oxygen content in the Tundish is contained in 0.015 to 0.025% by weight from Table 3, 50% or more of the nonmetallic inclusions remaining in the steel sheet are spherical inclusions (Type I), and the present inventors As a result of the evaluation, it can be seen that the tool life exceeds 5,000 times required by the free-cutting steel customer.

However, in Comparative Examples 8 and 9, the oxygen content was 0.0015% by weight or less, and it was confirmed that the proportion of spherical inclusions remaining in the steel slab was less than 50%. In addition, as a result of evaluating the tool life of the comparative example was about 3,100 times and 2,600 times, respectively, it was confirmed that the required life of the customer is insufficient. On the other hand, when the tuned oxygen content exceeds 0.025% by weight, the machinability is expected to be somewhat good, but surface defects such as pin holes, blow holes, etc. remain on the surface of the steel sheet immediately after continuous casting. Increasingly, wire rod rolling was found to be difficult.

Therefore, in the case of the present invention through the above embodiment, it was confirmed that the cutting property, the surface characteristics hot rolling property, and the like are higher than or equal to that of the commonly provided free cutting steel.

1 is a schematic diagram showing a method for refining sulfur-containing free cutting steel of the present invention.

2 is a graph showing the relationship between the length (L) / width (W) ratio of inclusions remaining in sulfur-containing free cutting steel and the machinability of steel materials;

Claims (14)

By weight, C: 0.06 to 0.12%, Mn: 0.8 to 1.2%, P: 0.07 to 0.11%, S: 0.20 to 0.34%, N: 0.005 to 0.013%, meaning O (Tundish T. [O] ): 0.015 to 0.025%, including balance Fe and other unavoidable impurities, The ratio of the contents of Mn and S is Mn / S ≧ 3.5, Sulfur-containing free-cutting steel, characterized in that the average length (L) / width (W) ratio of residual nonmetallic inclusions in the steel is L / W ≤ 6.0. The sulfur-containing free cutting steel according to claim 1, wherein the share of the residual nonmetallic inclusions satisfying L / W ≤ 6.0 is at least 50% based on the total residual nonmetallic inclusions. The sulfur-containing free cutting steel according to claim 1 or 2, wherein the residual nonmetallic inclusion is MnS. A first refining step including at least 75% of molten iron and the balance of primary molten metal molten iron in a converter or an electric furnace while blowing oxygen to the molten metal at a supersonic speed by 40 to 60 Nm ³ ; A molten steel tapping step of tapping the molten steel having completed the first refining with teaming lays; A second refining step of transporting and refining the teaming ladles having finished tapping to a ladle refining furnace; And A continuous casting step of transferring the molten steel after the secondary refining to a continuous casting tundish for continuous casting; Sulfur-containing free cutting steel refining method comprising a. The method for refining sulfur-containing free cutting steel according to claim 4, wherein the oxygen blowing flow rate in the first refining step is 180 to 230 Nm ³ / min. The method of claim 4, wherein the molten steel tapping step, Injecting ferroalloy containing carbon powder, Fe-Mn, Fe-P, Fe-S at 50 to 80% of the total tapping time during the tapping; And Adding a slag deoxidizer to the top of the teaming ladle before tapping is finished to perform deoxidation of the ladle slag; Method for refining sulfur-containing free cutting steel further comprising a. 7. The method for refining sulfur-containing free cutting steel according to claim 6, wherein T.Fe in the slag is reduced to 8% by weight or less by deoxidation of the ladle slag. The method of refining sulfur-containing free cutting steel according to claim 6, wherein the slag deoxidizer is performed when the tapping is performed at 80% or more. The method of claim 6, wherein the slag deoxidizer comprises 35 wt% or more of metal Al, and is a slag deoxidizer including residual CaO, Al ₂ O ₃ , SiO ₂ , MgO, and other volatile components. . The method of claim 4, wherein the secondary refining step further comprises the step of blowing N at a flow rate of 0.5 ~ 1.5Nm ³ / min. The method for refining sulfur-containing free cutting steel according to claim 10, wherein N is N in a solid state or a gaseous state. 11. The method for refining sulfur-containing free cutting steel according to claim 10, wherein blowing of the N is performed by the above-up or low odor method. The method of claim 4, wherein the continuous casting step, further comprising the step of generating an electric arc (Arc) to increase the temperature of the molten steel for a time of 40 minutes or less by a total temperature rise temperature of 50 ~ 70 ° Sulfur-containing free cutting steel refining method. 14. The sulfur-containing free cutting steel of claim 13, wherein the continuous casting step further includes an alloy component adjusting step of adjusting the alloy content by adding each alloy component by 10% or less of a target content after completion of the temperature raising step. Refining method.

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