KR20050016017A - Low-carbon free cutting steel - Google Patents

Abstract

PURPOSE: To provide a low-carbon free cutting steel which is excellent in machinability, hot workability and surface characteristics of the tools after cutting and is manufactured inexpensively particularly in case of machining using cemented carbide tools, and a low-carbon free cutting steel with superior carburizing characteristics in addition to the above characteristics. CONSTITUTION: A low-carbon free cutting steel contains 0.05 mass% to less than 0.20 mass% of C, 0.4 to 2.0 mass% of Mn, 0.21 to 1.0 mass% of S, 0.002 to 0.10 mass% of Ti, 0.001 to 0.30 mass% of P, 0.2 mass% or less of Al, 0.001 to 0.03 mass% of O(oxygen), 0.0005 to 0.02 mass% of N and the balance being Fe and impurities, wherein impurities contained steel satisfy the following expressions (i) and (ii): £Expression (i)| (A+B)/C>=0.8 £Expression (ii)| NA>=5 wherein meanings of A, B, C and NA are as follows: A: the total area occupied by substantial MnS inclusions containing Ti carbide and/or Ti carbonitride among inclusions having 1 μm or more of circle equivalent diameter per 1 mm¬2 of a cross section parallel to a rolling direction, B: the total area occupied by substantial MnS inclusions containing Ti carbide and Ti carbonitride among inclusions having 1 μm or more of circle equivalent diameter per 1 mm¬2 of a cross section parallel to a rolling direction, C: the total area occupied by all inclusions having 1 μm or more of circle equivalent diameter per 1 mm¬2 of a cross section parallel to a rolling direction, and NA: the number of substantial MnS inclusions containing Ti carbide and/or Ti carbonitride among inclusions having 1 μm or more of circle equivalent diameter per 1 mm¬2 of a cross section parallel to a rolling direction.

Description

LOW-CARBON FREE CUTTING STEEL}

This invention relates to the low carbon free cutting steel which does not contain lead (Pb). In particular, even though it does not contain lead, it has excellent machinability even when cutting using a cemented carbide tool, compared to conventional lead free cutting steels and composite free cutting steels in which lead and other free cutting elements are used. It is related with the low-carbon free cutting steel which is excellent also in the property of the after finishing surface, and can be manufactured at low cost. The present invention also relates to a low carbon free cutting steel excellent in carburizing properties in addition to the various properties described above.

Background Art Conventionally, steel materials excellent in machinability, so-called free-cutting steel, have been used for soft small parts that do not require much strength in order to improve productivity. The most well-known free-cutting steel is a sulfur free-cutting steel which added a large amount of S and improved machinability by MnS, a lead free-cutting steel added with Pb, and a composite free-cutting steel including both S and Pb. In particular, free-cutting steel containing Pb has the characteristics of prolonging tool life, excellent cutting chip segmentation, and excellent finish surface roughness of the steel surface after machining. There is also a free-cutting steel containing Te (tellur), Bi (bismuth) or the like for the purpose of improving machinability. These are used in large quantities for various mechanical parts such as electric machine parts and molds, as well as small parts such as brake parts for automobiles and personal computer peripheral parts.

On the other hand, in recent years, cutting at high speeds is possible in accordance with the improvement of the performance of a cutting machine, and accordingly, the improvement of machinability at the time of high speed cutting is strongly desired also for steel materials which are the raw materials of the above components.

In addition, after the parts are finished in a predetermined shape by cutting, carburization may be performed to secure the strength of the surface. Therefore, there are cases where steel materials used for these parts have high machinability and excellent carburizing characteristics.

Steel materials that can be used as materials for the above components are required to have excellent machinability, but the machinability not only prolongs tool life, but also allows cutting chips to be finely divided, that is, excellent in cutting chip processing properties. It is important. The cutting processability is indispensable to the automation of the processing line, and is essential for improving the productivity. In addition to tool life and cutting chip processing properties, it is desirable to have excellent finish properties on the steel surface after cutting, that is, a small finish surface roughness, from the viewpoint of machining accuracy. Among the above free cutting steels, the composite free cutting steels containing lead free cutting steels and other high machinability improving elements together with Pb are excellent in various properties thereof, and among the existing steel materials, the machinability is the most excellent in machinability.

In recent years, with increasing interest in environmental problems, free cutting steels containing no Pb have been strongly desired. This is because steel containing Pb that is harmful to the human body and the global environment not only requires large-scale exhaust facilities in its manufacturing process, but also has increased movement to suppress the use of Pb from the viewpoint of environmental conservation.

In order to meet the above requirements, various proposals have been made regarding low carbon sulfur free cutting steel containing no Pb by replacing lead free cutting steel. However, free cutting steels which contribute to the extension of tool life, excellent cutting chip processing properties, and satisfying all of the characteristics of free cutting steel containing Pb, which is said to have a small finish surface roughness, have not yet been developed.

Patent Document 1 (Japanese Patent Laid-Open No. 2003-49240) discloses a free cutting steel in which Ti or / and Zr carbon sulfide inclusions are present to improve machinability. In this free cutting steel, Ti carbide sulfide or Zr carbide sulfide is dispersed in the steel together with MnS, so that similar lubricating effect of MnS is hardly obtained, and the friction force between the tool and the workpiece increases. As a result, the cutting resistance rises and the construction edge is easily generated in the tool edge. If a construction edge is produced, the finish surface roughness after finishing cutting will become large, and the machining precision in a component will be impaired.

In patent document 1, the Example whose Ti content is 0.1% or less is not found. This indicates that the invention of Patent Literature 1 aims to produce Ti carbohydrates by containing a large amount of Ti. In fact, Ti carbohydrate inclusions are dispersed in a matrix in a matrix with MnS. It is described. In this case, the performance required for steel materials which can be used for the above-mentioned parts such as tool life, cutting chip processability, and finish surface roughness cannot be satisfied.

Patent Document 2 (Japanese Patent Laid-Open No. 2003-49241) is a free-cutting steel containing Ti or / and Zr in the range of (Ti + 0.52Zr) / S <2, and containing a carbon sulfide of Ti or Zr as an inclusion, Free cutting steel which improves the tool life in turning and drill processing is disclosed. In the invention of Patent Document 2, the tool life during turning is improved by generating Ti carbon sulfide in steel. In this technique, the tool life can be reliably improved to a certain extent, but the presence of Ti or Zr carbide sulfide makes it difficult to obtain the lubricating effect of MnS, and the friction force between the tool and the workpiece increases. As a result, the cutting resistance is increased, and the construction edge is easily generated in the tool edge. If the edge is formed, the roughness of the finished surface after cutting increases, resulting in deterioration of machining accuracy.

In addition, Patent Document 2 contains S as defined in the present invention to be described later in a range of 0.21% or more, and an example of a free-cutting steel having a Ti content of 0.1% or less cannot be seen. In this regard, the invention of Patent Document 2 is finished. Obviously, there is no invention in the aim of improving surface roughness and cutting chip processing performance. In short, in the invention of Patent Literature 2, since Ti or Zr carbide sulfide is dispersed in a matrix together with MnS, desired finish surface roughness and cutting chip treatability cannot be obtained.

Patent Literature 3 (Japanese Patent Laid-Open No. 2000-319753) discloses a low carbon sulfur free-cutting steel containing S exceeding 0.4% and not adding Pb with an increased MnS. However, in the steel, the effect of improving the life of the carbide tool is small. In addition, this steel does not improve the cutting chip processability, which is important with tool life, and does not significantly improve the performance of conventional sulfur free cutting steel.

Patent Document 4 (Japanese Patent Laid-Open No. 09-53147) discloses C: 0.01 to 0.2%, Si: 0.10 to 0.60%, Mn: 0.5 to 1.75%, and P: 0.005 to 0.15%. : 0.15-0.40%, O (oxygen): 0.001-0.010%, Ti: 0.005-0.020%, N: 0.03-0.03%, The machinability to a carbide tool, In particular, the invention of free cutting steel excellent in tool life is disclosed. In the above invention, it is essential to improve the lifespan of the cemented carbide tool as essential as containing 0.1 to 0.6% of Si together with Ti. In addition, in the present invention, not only the tool life, but also the cutting life of the chip and the finish surface are provided by the existence of "substantial MnS inherent in Ti carbide and / or Ti carbonitride" even in the steel material even without containing Si as in the present invention. It is not intended to improve the roughness.

Patent Document 5 (Japanese Patent No. 3390988) contains C: 0.02 to 0.15%, Mn: 0.3 to 1.8%, S: 0.225 to 0.5%, Ti: 0.1 to 0.6, and Zr: 0.1 to 0.6%. Moreover, invention of the low carbon sulfur free-cutting steel which satisfy | filled Ti + Zr: 0.3 to 0.6% and (Ti + Zr) / S ratio: 1.1 to 1.5 and improved the mechanical anisotropy is disclosed. This invention improves the mechanical anisotropy and machinability of steel materials by making the said composition produce the sulfide of Ti and Zr with high deformation resistance in hot. However, these sulfides having high deformation resistance hardly obtain a lubrication effect by sulfides during cutting, resulting in high cutting resistance, deterioration of tool life and deterioration of finish surface roughness.

The problem of the present invention is superior to conventional lead free cutting steel and composite free cutting steel in which other free cutting elements are used together with Pb without harmful to the environment, particularly in the case of cutting using cemented carbide tools. It is to provide a low-carbon free-cutting steel which exhibits machinability, is excellent in hot workability, is also excellent in surface properties after cutting, and can be manufactured at low cost. Moreover, it is also a subject of this invention to provide the low carbon free cutting steel which also has the outstanding carburism property in addition to the said various characteristics.

It is well known that inclusion states such as sulfides greatly influence the machinability of steel. Incidentally, there are various kinds of inclusions observed in the steel containing C, Ti, S, N, and 0. For example, Ti sulfide, Ti carbide sulfide, Ti carbide, Ti carbonitride, Ti nitride, Ti oxide. Furthermore, when Mn is contained, Mn sulfide represented by the chemical formula of "MnS" also exists. In addition to these, when Al and Si are contained, these oxides also exist. These forms of existence are various, and the composition and the form of these inclusions greatly influence the machinability and other mechanical properties of the steel.

MEANS TO SOLVE THE PROBLEM The present inventors first applied for the invention regarding the low carbon sulfur free cutting steel which does not contain P by Japanese Patent Application No. 2002-26368. The said free cutting steel contains C, Mn, S, Ti, Si, P, Al, 0, and N, The content of Ti and S satisfy | fills following formula (A), and the atomic ratio of Mn and S is A low carbon sulfur free-cutting steel which satisfies the formula (B) and contains MnS inherent in Ti sulfide or / and Ti carbon sulfide.

                       Ti (mass%) / s (mass%) <1 ... (A)

                       Mn /S≥1...(B)

The steel of the above-mentioned invention has much better tool life compared to lead free cutting steel and has excellent cutting chip processing properties. However, in the steel, there are some difficulties in the surface property after cutting. That is, when finish cutting was performed, the problem that the finish surface roughness may become large was discovered.

The presence of substantial Ti sulfide or / and Ti carbide sulfide in the matrix makes it difficult to obtain a similar lubricating effect of MnS, which leads to an increase in cutting resistance and formation of constituent edges in the tool edge, resulting in gloss on the surface of the steel after cutting. It is thought that the finish surface roughness is deteriorated without giving a drawing. The term &quot; substantial Ti sulfide or / and Ti carbon sulfide &quot; means inclusions having a total of 50% or more of the area ratio occupied by Ti sulfide and Ti carbon sulfide in one inclusion, and described later in FIG. Some of them are shown in).

Therefore, as a result of examining to solve the above problems, new facts as described below were obtained.

(1) When the "substantial Ti sulfide or / and Ti carbon sulfide" cuts the steel which exists in a matrix, a constituent edge is produced in a tool edge and the roughness of a finish surface deteriorates.

(2) By suppressing the generation of "substantial Ti sulfide or / and Ti carbon sulfide" as much as possible, and having a lot of MnS in steel, generation | occurrence | production of a constituent edge is suppressed and favorable finish surface roughness can be obtained.

(3) However, in steels containing only MnS without containing Ti, the carbide tool life is deteriorated. In order to improve the life of the cemented carbide tool, it is necessary to add Ti and to have "substantial MnS inherent in Ti carbide and / or Ti carbonitride".

(4) "Substantial MnS inherent in Ti carbide and / or Ti carbonitride" improves tool life and does not impair the similar lubricating effect of MnS.

Based on the above findings, the relationship between chemical composition and inclusion type was examined in detail. As a result, it came to invent the low carbon free cutting steel shown below. The low carbon free cutting steel has a machinability equal to or higher than that of lead free cutting steel or composite free cutting steel. In addition,% regarding component content is the mass%.

C: 0.05% to less than 0.20%, Mn: 0.4% to 2.0%, S: 0.21 to 1.O%, Ti: 0.002 to 0.10%, P: 0.001 to 0.30%, Al: 0.2% or less, 0 (oxygen) ): O.001 to 0.03%, N: 0.OOO5 to 0.02%, the remainder is a steel made of Fe and impurities, and the inclusions contained in the steel satisfy the following formulas (i) and (ii): Low carbon sulfur free cutting steel, characterized in that.

           (A + B) / C ≥ 0.8 .... (i)

N _A ≥ 5 .... ii)

Here, the meanings of A, B, C and N _A are as follows.

A; The total area occupied by substantial MnS inherent in Ti carbide and / or Ti carbonitride among inclusions of 1 µm or more in equivalent circular diameter in 1 mm 2 of a cross section parallel to the rolling direction.

B; The total area occupied by substantial MnS in which Ti carbide and Ti carbonitride are not inherent among inclusions having a diameter of 1 µm or more in a circular equivalent in mm 2 of a cross section parallel to the rolling direction.

C; The total area occupied by the development material of 1 micrometer or more in diameter per circle in 1 mm <2> of cross section parallel to a rolling direction.

N _A ; The substantial number of MnS inherent in Ti carbide and / or Ti carbonitride among the inclusions of 1 micrometer or more in circular equivalent diameters in 1 mm <2> of a cross section parallel to a rolling direction.

Said low carbon free cutting steel can contain 1 or more types of components chosen from at least 1 group among the following 1st group to 3rd group.

First group

Se: 0.005 to 0.10%, Te: 0.05 to 0.10%, Bi: 0.01 to 0.3%, Sn: 0.01 to 0.3%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.90%, B: 0.0002% to 0.02% and rare earth elements: 0.05% to 0.02%.

2nd group

Cu: 0.01-1. 0, Ni: 0.01% to 2.0%, Mo: 0.01% to 0.5%, V: 0.005% to 0.5%, and Nb: 0.005% to 0.5%.

3rd group

Si: 0.1 to 2.0% and Cr: 0.03 to 1.O%.

Here, the term "substantially MnS inherent in Ti carbide and / or Ti carbonitride" means that the area ratio of MnS in one inclusion is 50% or more, and Ti carbide or / and Ti carbonitride is inherent. The term "substantial MnS in which Ti carbide and Ti carbonitride are not inherent" means that the area ratio of MnS in one inclusion is 50% or more, and that Ti carbide and Ti carbonitride are inherent (coexistence). We say inclusion not doing. In addition, these "substantial MnS inherent in Ti carbide and / or Ti carbonitride" and "substantial MnS in which Ti carbide and Ti carbonitride are inherent" are the sulfides other than Ti carbide and Ti carbonitride, Carbide sulfide, carbide, nitride, etc. may be inherent.

The main characteristics of the low carbon free cutting steel of the present invention are as follows.

(1) C is made into 0.05 to less than 0.20%, S is contained in 0.21 to 1.0% of range, and content of Ti shall be 0.002 to 0.1%.

(2) Ti combines with C, S, N and O to form sulfides, carbon sulfides, carbides, carbonitrides and oxides. Ti is more susceptible to sulfide formation than Mn, and thus Ti sulfide and Ti carbonitride are easily formed. However, considering carefully the content balance of Mn, Ti, S, and N, "substantially MnS inherent in Ti carbide or / and Ti carbonitride" is produced without generating much "substantially Ti carbonitride or / and Ti sulfide." And "substantial MnS in which Ti carbide and Ti carbon nitride are not inherent" can be present.

(3) When the chemical composition shown in (1) above is used, and the inclusion type as shown in (2) above is obtained, the inclusions present in the matrix are softened during cutting to exhibit a lubricating effect. Substantial MnS ”occupies most of the development material, and sulfides other than the“ real MnS ”, that is,“ substantially Ti sulfide or / and Ti carbon sulfide ”are almost in a state. At this time, in order to obtain good finish surface roughness, the production amount of "substantial MnS" must take up most of the production amount of the development material. Specifically, the total area of "substantial MnS" having a circular equivalent diameter of 1 µm or more in the observation surface 1 mm2 of the rolling direction cross section must occupy 80% or more of the total area of the developing material having a circular equivalent diameter of 1 µm or more. . Only in this case, generation of a constituent edge to the tool edge caused by the presence of "substantial Ti sulfide or / and Ti carbon sulfide" is suppressed, and good finish surface roughness can be obtained.

The above-mentioned "substantial MnS" is an inclusion having an area ratio of 50% or more of MnS in one inclusion, and "substantial MnS in which Ti carbide and / or Ti carbonitride is inherent" and "Ti carbide and Ti carbonitride are not inherent. Substantial MnS ”.

As shown by the above formula (i), "to occupy 80% or more" is "A + B" of formula (i). In addition, the definition of A and B is the area occupied by "substantial MnS inherent in Ti carbide and / or Ti carbonitride" among sulfides whose circular equivalent diameter is 1 micrometer or more in 1 mm <2> of the cross section parallel to a rolling direction, and It is an area which "the substantial MnS which Ti carbide and Ti carbon nitride do not have inherent" occupies.

And &quot; substantial MnS inherent in Ti carbide and / or Ti carbonitride &quot;,&quot; substantial MnS in which Ti carbide and Ti carbonitride are not inherent &quot;,&quot; substantial Ti sulfide or / and Ti carbohydrate &quot; and other sulfides, a carbosulfide, carbide, nitride, oxide, Al ₂ 0 _3, SiO ₂ and so on that (i) expression of the C a total of the total area.

(4) Even if steel materials containing inclusions as shown in the above (3), that is, almost no "substantial Ti sulfide or / and Ti carbon sulfide" are present, and most of the inclusions contained in steel are "substantial MnS", If "substantially MnS incorporating Ti carbide and / or Ti carbonitride" exists, when cutting in the high speed area | region where cutting temperature becomes high, a hard TiN film will be formed in a tool surface, and a tool life can be obtained by protecting a tool. have.

(5) In steel having substantial MnS inherent in Ti carbide and / or carbonitride, the "substantial MnS" is finer than the MnS contained in the conventional free cutting steel of JIS SUM22L-24L, and the number is Increases. In this case, since these fine "substantial MnS" become a starting point of the stress concentration in cutting, and it is easy to promote crack propagation, the cutting chip processability more than the comparable free cutting steel can be acquired.

(6) `` Steel having a substantial MnS inherent in Ti carbide and / or Ti carbonitride has no problem in hot workability. Therefore, it is possible to increase the S content effective for improving machinability, and even in this case, continuous It does not cause any trouble in manufacturing by casting facilities or the like. Moreover, since the amount of Ti added also exhibits a sufficient effect in a small amount, it is possible to complete the cost required for manufacturing less and to apply as an inexpensive steel material.

As mentioned above, when the range of an alloy component is limited and the shape of an interference | inclusion is adjusted, the outstanding machinability can be obtained. However, steel materials that can be used for automobile parts are sometimes desired to have excellent carburizing characteristics in addition to machinability. From there, the influence on the characteristics of the Si and Cr steels was examined, and the adjustment of the amount of Si and the amount of Cr did not impair the form of the electrical inclusions, and thus, the carburizing characteristics were not deteriorated. It became clear that it could be improved.

Si and Cr are dissolved in austenite and the hardenability of the steel is increased to increase the carburizing depth and the hardness of the carburizing layer in the carburizing treatment. In addition to Si and Cr, there are Mn, Mo, P, and the like to improve hardenability. However, it is necessary to contain Mn in sufficient quantity with respect to S amount from a machinability or hot workability viewpoint, and it requires a large amount addition. In this case, in order to improve hardenability, the addition of Mn again only increases the cost. In addition, Mo is also effective in increasing the hardenability of steel, but since Mo is more than Si and Cr, the production cost is increased by adding a considerable amount of Mo, which can achieve an equivalent effect. P also has the same effect, but when added, the hardness of steel is rapidly increased, which leads to deterioration of machinability. However, these elements may be added in a range that does not impair machinability or mechanical properties when not concerned with material cost. However, Si and Cr are preferred as components for improving carburizing properties when it is desired to be manufactured at low cost without compromising machinability.

Best Mode for Carrying Out the Invention

1. `` Substantial MnS inherent in Ti carbide and / or Ti carbonitride ''

Ti is combined with S, C, N, O, Ti sulfide or Ti carbohydrate represented by the formula of TiS and Ti ₄ C ₂ S ₂ , Ti carbide represented by the formula of TiC or Ti (CN), TiN, TiO, Ti inclusions such as Ti carbonitride, Ti nitride, and Ti oxide are formed. In addition, although Ti may exist in solid solution in MnS and exists as (Mn, Ti) S, since the amount of Ti dissolved in the MnS is very small, this sulfide is substantially MnS.

On the other hand, Ti is not dissolved in MnS, and may be present in phase separation with MnS in one inclusion. Ti is in the form of TiC or / and Ti (C, N), that is, MnS is clearly present in a different form, and its presence is in the vicinity of one sulfide or in MnS. They exist in a variety of forms, including surroundings.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows typically the inclusion which exists in the free cutting steel containing Ti, (a) is the free cutting steel of a comparative example, and (b) is the free cutting steel of this invention. In the steel shown in Fig. 1 (a), even when co-existing with MnS among Ti sulfides, carbon sulfides, or single inclusions, the area ratio of Ti sulfides and carbon sulfides is 50% or more, and substantially Therefore, there are many inclusions which can be regarded as sulfides or carbon sulfides of Ti, that is, "substantial Ti sulfides and / or Ti carbon sulfides". On the other hand, in the steel of the present invention shown in FIG. 1 (b), inclusions in which Ti carbide or / and Ti carbonitride is enclosed in the outer circumference or inside of MnS, that is, "Ti carbide or / and Ti carbonitride are inherent Substantial MnS &quot; and &quot; substantial MnS in which Ti carbide and Ti carbon nitride are not inherent &quot;

In the steel shown in FIG. 1 (b), even when Ti sulfide, Ti carbide sulfide, Ti carbide, Ti carbonitride, Ti nitride, Ti oxide, and other inclusions are apparently phase separated from MnS, The fact that the area ratio which MnS occupies is 50% or more judges that it is substantially one MnS, ie, "real MnS." Conversely, inclusions having an area ratio of 50% or more of oxides, nitrides, carbides, and the like formed from these Ti inclusions or other components among the inclusions are not "substantial MnS" but substantially one Ti inclusion or its inclusions. It is judged that it is an oxide, nitride, carbide, etc. which consist of an external component.

Among the MnS, in particular Ti carbides and / or Ti carbonitrides are inherently phase-separated from MnS, and inclusions having an area ratio of 50% or more of MnS include "substantially MnSs in which Ti carbides and / or Ti carbonitrides are inherent. ”. On the other hand, "substantial MnS in which Ti carbide and Ti carbonitride are not inherent" means oxides, nitrides, carbides, and the like of Ti inclusions or other components other than Ti carbide and Ti carbonitride and one MnS. Among the inclusions, MnS, which is apparently present in phase separation and occupies 50% or more of the area ratio of MnS, substantially includes Mns, and oxides, nitrides, carbides, and the like formed from the Ti-based inclusions and other components. MnS does not exist. In short, the sum of "substantial MnS inherent in Ti carbide and / or Ti carbonitride" and "substantial MnS in which Ti carbide and Ti carbonitride are not inherent" include inclusions (the "substantial MnS" described above). ), And the other inclusions are Ti sulfides, Ti carbide sulfides, Ti carbides, Ti carbon nitrides, Ti nitrides, oxides composed of Ti-based inclusions and other elements such as Ti oxides, carbides and nitrides. .

The area ratio of the MnS or Ti-based inclusions in one of the inclusions described above is analyzed by surface analysis and the like by means of an EPMA (electron beam microanalyzer) or EDX (energy dispersive X-ray analyzer) on a micro test piece cut out from a round bar provided for a cutting test. Can be grasped by quantitative analysis. In addition, "substantial MnS inherent in Ti carbide and / or Ti carbonitride", "substantial MnS in which Ti carbide and Ti carbonitride are inherent" in steel, and other inclusions can be confirmed in the same manner, and the total area thereof is also found. The number and the number can also be measured by techniques such as image analysis. In that case, when it measures by the some visual field which the sum total of a viewing field area exceeds 1 mm <2>, what is necessary is just to convert the total area and number of each interference | inclusion into the average total area per lmm <2>, average number.

2. Reason for setting (A + B) /C≥0.8

A in the formula (i) is a total area occupied by "substantial MnS inherent in Ti carbide and / or Ti carbonitride" among inclusions of 1 µm or more in circular equivalent diameter in 1 mm 2 of a cross section parallel to the rolling direction, B is a total area occupied by "substantial MnS in which Ti carbide and Ti carbonitride are not inherent" among inclusions having a circular equivalent diameter or more in 1 mm 2 of a cross section parallel to the rolling direction. Here, the "circular equivalent diameter" refers to the diameter when the area of one inclusion is obtained by a technique such as previous image analysis and converted into a circle having the same area. Is limited because the inclusions of less than 1 µm have little effect on machinability.

The former formula (i) indicates that the total of the A and B is required at least 80% of the total area occupied by the development materials having a diameter of 1 µm or more per circular equivalent. Good machinability can be obtained as long as it is the said range, More preferably, it is 90% or more. In addition, as mentioned above, the present invention refers to nitrides, carbides, oxides, and &quot; substantially Ti sulfides and / or Ti carbon sulfides &quot; which are present alone with inclusions other than those represented by A and B. In other words, the expression (i) indicates that the total area of these inclusions other than "substantial MnS" is less than 20% of the total area (C of formula (i)) occupied by the development object. It is still more preferable if this total area is less than 10%.

When Ti is added to steel containing a large amount of S due to the improved machinability, Ti tends to form sulfides more strongly than Mn, and thus it is easy to form Ti sulfides and Ti carbon sulfides. However, the formula (i) Assuming that Ti is added, it is intended to suppress the formation of Ti sulfide and Ti carbon sulfide. This is because Ti sulfide and Ti carbon sulfide inhibit the similar lubricating effect of MnS during cutting. When the similar lubricating effect of MnS is impaired, the frictional force between the tool and the workpiece increases, and it is considered that the finish surface roughness deteriorates in order to produce the component edge on the tool edge. Therefore, production of Ti sulfide and Ti carbon sulfide must be suppressed. In short, as defined by the formula (i), when "substantially Ti sulfide or / and Ti carbon sulfide" which is present alone is hardly present in the steel, and 80% or more of the inclusions contained in the steel are referred to as "substantial MnS" Similar lubrication effect can be obtained during cutting.

Thus, when it is limited to the composition range of the steel prescribed | regulated by this invention and satisfy | fills (i) Formula, in finish cutting, favorable finish surface roughness equivalent to or more than the conventional lead free cutting steel and composite free cutting steel can be obtained. On the other hand, even within the chemical composition specified in the present invention, good machinability cannot be obtained unless the formula (i) is satisfied.

3. Reason for N _A ≥ 5

N _A of the above (ii) is an expression "of rolling circle-equivalent diameter of the inclusions over 1㎛ in the 1㎟ of a cross section parallel to the direction, the Ti carbide and / or Ti carbonitride inherent number of substantial MnS to". Said "substantial MnS inherent in Ti carbide and / or Ti carbonitride" points out that the area ratio of MnS in one inclusion is 50% or more as mentioned above. In addition, since "the substantial MnS in which Ti carbide and / or Ti carbonitride is inherent" does not substantially impair similar lubricating effect, a constituent edge is hardly formed and the finish surface roughness of the workpiece is not deteriorated.

In addition, when cutting steel in which "substantially MnS inherent in Ti carbide and / or Ti carbonitride" exists in a high speed area exceeding 100 m / min using a carbide tool, the surface of the tool is observed in detail. It was found that TiN was formed on the tool surface. In the tool surface which contacts the workpiece during cutting, it is considered that the Ti-based inclusions react and deteriorate as the temperature rises due to friction, thereby forming hard TiN having a layer thickness of several micrometers to several tens of micrometers. Its presence is determined by surface analysis and point analysis by AES (auger electron spectroscopy) or EPMA (electron beam microanalyzer) on the tool surface after removal of carbon-based contamination (oil component, etc.) from the tool surface by Ar sputtering after cutting. I could confirm it. According to this, the surface area of TiN adhering to the tool was 10 to 80% of the contact area between the workpiece and the tool, and the rest was MnS or Fe attached to the cutting process or the tool base with no deposit. As the hard TiN is formed on the tool surface, mechanical diffusion due to thermal diffusive wear of the tool and hard inclusions is suppressed, and the tool life can be particularly excellent compared with conventional S free cutting steel or composite free cutting steel with Pb. I think there is.

In order to obtain the above effects, "substantial MnS inherent in Ti carbide and / or Ti carbonitride" may be present in five or more, preferably 10 or more, in the observation surface 1 mm2 in the rolling direction cross section. .

On the other hand, even within the chemical composition specified in the present invention, good machinability cannot be obtained unless the formula (ii) is satisfied.

MnS exists very finely in the steel which added Ti and made into the form of the inclusion which satisfy | fills Formula (i) and (ii). That is, the number of MnS is remarkably large. The fine MnS acts as a stress concentration point of the cutting chip generated at the time of cutting and improves the cutting chip treatability for promoting crack propagation in the cutting chip.

As a result, "substantially MnS inherent in Ti carbide and / or Ti carbonitride" exists in steel stably, and it is made into 5 or more in 1 mm <2> of observation surfaces of a rolling direction cross section, and 1 mm <2> of observation surfaces of a rolling direction cross section. If the sum of the total area of "substantial MnS inherent in Ti carbide and / or Ti carbonitride" and "substantial MnS in which Ti carbide and Ti carbonitride is inherent" is more than 80% of the total area of the development material, As described above, tool life, finish surface roughness, and cutting chip processing properties equivalent to those of lead free cutting steel or composite free cutting steel can be obtained. In order to realize this type of inclusion more stably and inexpensively produce steel having excellent machinability by the continuous casting method or the like, it is necessary to consider the balance of the contents of Mn, Ti, S and N. Specifically, it may be carried out as follows.

(a) T (%) / S (%) ≤ 0.25

In the case where a large amount of Ti is added with respect to the amount of S, that is, when Ti / S exceeds 0.25 as the mass% ratio, a large amount of Ti sulfide and Ti carbon sulfide are present. As a result, the expression (i) is not satisfied and the similar lubricating effect by MnS is impaired. At this time, the cutting resistance tends to rise and tend to form a constituent edge on the tool edge. As a result, the surface roughness at the time of finishing cutting deteriorates, and the machining precision becomes poor.

On the contrary, when Ti is added in a small amount with respect to the amount of S, when Ti / s is 0.25 or less at each mass ratio, Ti forms Ti carbide or Ti carbonitride, and the "substantial Ti sulfide or / and Ti carbide sulfide" Silver rarely exists alone.

Ti carbide and Ti carbonitride are precipitated in various forms, but may exist in a form inherent in one MnS. When the steel including "substantial MnS inherent in Ti carbide and / or Ti carbonitride" is cut in a high speed region using a carbide tool, excellent tool life can be obtained. That is, in order to suppress generation | occurrence | production of "the real Ti sulfide or / and Ti carbon sulfide" which exists independently, Ti (%) / S (%) may be adjusted to 0.25 or less.

(b) The amount of Mn and S is [Mn] / [S] ≥1 in atomic ratio

S is an element causing cracks during hot working. However, if the proper composition of [Mn] / [S] ≥1 is maintained at the atomic ratio, that is, the ratio of the number of atoms (moles), Mn is crystallized as MnS. Thus, for example, Ti (%) / S (% ) ≤0.25, there is no problem in hot workability. In this range, even if it is manufactured under continuous casting, for example, since there is no problem in hot workability, it is possible to add much S to increase MnS effective for improving machinability, and at the same time to have high S content. There is no damage to the form of the inclusions shown in the formulas (i) and (ii).

In the case of [Mn] / [S] <1, sulfides in which FeS is largely dissolved in MnS and TiS are mainly used unless the amount of Ti exceeds S amount, and the hot workability cannot be improved. Even in the case of Mn] / [S] <1, hot workability can be improved by adding Ti in the range exceeding S amount. However, in this case, since the tendency of sulfide formation of Ti is larger than Mn, the main sulfide is not MnS, but mainly Ti sulfide or Ti carbon sulfide that is harder than MnS. In this case, as described above, similar lubrication effects due to soft sulfides cannot be obtained between the tool and the workpiece during cutting, the cutting resistance increases, and the finish surface roughness deteriorates. In other words, [Mn] / [S] ≧ 1 in terms of the atomic ratio of Mn and S is a condition required for achieving the effect of improving the machinability by MnS and obtaining good hot ductility.

(c) Ti (%) / N (%) ≥ 1.35

The great feature of the free cutting steel of this invention is that it contains "the substantial MnS inherent in Ti carbide and / or Ti carbonitride." For example, when Ti (%) / N (%) <1.35, the above "substantial MnS inherent in Ti carbide and / or Ti carbonitride" may not be sufficiently obtained. In this case, since most of the added Ti is crystallized as TiN at a rapid solidification stage, it is considered that sufficient Ti cannot be secured to form "substantial MnS inherent in Ti carbide and / or Ti carbonitride". Therefore, it is preferable that Ti (%) / N (%) is 1.35 or more, and Ti (%) / N (%) is added in order to more stably obtain "the substantial MnS inherent in Ti carbide and / or Ti carbonitride." It is good to say more than 1.5.

4. Reasons for Limiting Chemical Composition

Hereinafter, the reason for limiting the chemical composition in the present invention will be described together with the effect of each component.

C: 0.05% to less than 20%

C is an important element which greatly affects machinability. When the C content is 0.2% or more, the strength of the steel is increased to deteriorate the machinability. Therefore, the C content is not suitable as a steel material for applications in which machinability is important. However, at less than 0.05%, the steel is too soft, causing tearing during cutting, which promotes tool wear, and at the same time, increases the finish surface roughness. Therefore, the appropriate content of C is from 0.05% to less than 0.20%. In addition, the more suitable range of C amount for obtaining better machinability is 0.07 to 0.18%.

Mn: 0.4-2.0%

Mn forms an sulfide-based inclusion with S and is an important element having a great influence on machinability. If the amount is less than 0.4%, the absolute amount as a sulfide is insufficient and satisfactory machinability cannot be obtained. In addition, since Mn is an element which raises hardenability of steel, when it is desired to obtain excellent carburizing characteristics, it is good to increase the content. However, since Mn forms MnS together with S, it is necessary to contain a large amount of Mn in the steel of the present invention containing a large amount of S. When Mn is added in order to increase carburizing characteristics, Mn content is added, which is not preferable from the viewpoint of manufacturing cost. Therefore, the upper limit of Mn amount was made into 2.O%. When it exceeds 2.0%, the strength of the steel increases and the cutting resistance increases, thereby decreasing the tool life. In addition, the relationship with the amount of S is important in order to reduce cutting resistance, improve tool life, improve cutting performance of limited cutting chips, improve finish surface roughness, and improve hot workability. In addition, in order to ensure these performances reliably, it is preferable to make Mn content into 0.6 to 1.8%.

S: 0.21 to 1.0%

S is an element effective in forming a sulfide together with Mn to improve machinability. Since the machinability improvement effect by MnS improves according to the generation amount, selection of S content is important. If it is less than 0.21%, a sufficient amount of sulfide inclusions cannot be obtained, and satisfactory machinability cannot be expected. On the other hand, if the amount of S is usually more than 0.35%, the hot workability is deteriorated, the segregation of the central portion of the steel ingot is promoted, and cracking is caused during forging, but the upper limit can be increased to 1.0% if the proper composition is maintained. In order to improve the machinability by MnS, addition of S is more preferable, and it is more preferable if it is 0.35% or more. Furthermore, content exceeding 0.40% is more preferable for further improvement. However, since excessive addition leads to the cost increase by deterioration of a product yield, the upper limit with preferable S content is 0.70%.

Ti: 0.90 to 0.1%

Ti together with N or C forms Ti carbides and / or Ti carbonitrides and is an essential element for the presence of Mns inherent in the steel. As described above, when "substantially MnS in which Ti carbide and / or Ti carbonitride is inherent" exists in steel, the tool life in the high speed cutting using a carbide tool improves remarkably. In order to exist such MnS, the content of 0.002% or more is required, but in order to stably disperse it in steel and not to deteriorate the finish surface roughness and to obtain a good tool life, the content of Ti balances the content of S and N. It is necessary to consider. When the Ti content is more than 0.10%, since "substantially Ti sulfide or / and Ti carbon sulfide" exists in steel, the roughness of the finish surface in finishing cutting is deteriorated. Therefore, the upper limit of Ti is 0.1% It was set as. In order to obtain more stable and excellent finish surface roughness, it is preferable that Ti content is 0.08% or less. Furthermore, it is more preferable if it is less than 0.03%.

On the other hand, when the content of Ti is less than 0.02%, it is not possible to produce "substantially MnS inherent in Ti carbide and / or Ti carbonitride" in an amount sufficient to improve tool life. In order to produce the MnS more reliably and to improve the life of the carbide tool, the content of Ti of more than 0.01% is preferable.

P: 0.001 to 0.30%

P increases the hardenability of the steel and also increases the strength. In order to acquire the effect, it is good to contain 0.OO1% or more. If the content is 0.30% or less, hardenability and strength can be ensured without degrading the machinability, but if the content exceeds 0.30%, the strength becomes too high to degrade the machinability, and also promotes segregation of the steel ingots and hot. Deteriorates workability. Therefore, content of P was made into 0.001 to 0.30%. Moreover, in order to keep favorable machinability and strength stably, more preferable content is 0.005 to 0.13%.

Al: 0.2% or less (not added)

A1 can be used as a strong deoxidation element and may be contained as it is 0.2% or less. However, the oxide produced by deoxidation is hard, and when content exceeds 0.2%, hard oxide will produce | generate in large quantity and deteriorate machinability. Therefore, more preferably, 0.1% or less. In addition, when sufficient deoxidation is possible by addition of C or Mn, Al may not be added and its content may be an impurity level of 0.002% or less.

0 (oxygen): 0.OO1-0.03%

Although the effect of oxygen in the steel of the present invention is not impaired by the deoxidation state, by containing an appropriate amount of oxygen, it is dissolved in MnS to prevent the stretching of MnS by rolling, thereby improving the anisotropy of mechanical properties. Moreover, it is also effective for improving machinability, hot workability and S segregation. However, when it exceeds 0.03%, it will cause trouble, such as a heat loss of the refractory at the time of a solvent. Therefore, the oxygen content was in the range of 0.001% to 0.03%. In order to acquire the said effect suitably, the more preferable range is 0.0015 to 0.01%.

N: 0.005% to 0.02%

N tends to form a hard nitride with Al and Ti. These nitrides have an effect of miniaturizing crystal grains. However, the presence of these nitrides in large quantities tends to promote tool wear and deteriorate machinability. Since Ti is added to the steel of this invention as an essential component, the smaller the content of N, the more preferable. However, in order to acquire the effect of electricity, it was made to contain 0.0005% or more. On the other hand, when N content becomes excess, rough TiN will produce | generate and there exists a possibility that the machinability may be impaired, The upper limit of N was made into 0.02%. In order to ensure better machinability, the upper limit of the amount of N is preferably 0.015%. In addition, in the present invention, since the machinability is improved by the presence of "substantially MnS inherent in Ti carbide and / or Ti carbonitride," Ti and N are Ti in order to stably exist in steel. It is preferable to satisfy (%) / N (%) ≥1.35. As described above, when Ti (%) / N (%) <1.35, most of the added Ti is generated as TiN at a stage of rapid solidification, and "Ti carbide or / and Ti carbonitride is inherent. It is because substantial MnS "cannot be obtained stably.

As described above, the low carbon free-cutting steel having excellent machinability, hot workability, and finish surface properties can be obtained by the chemical composition composed of elements whose contents are adjusted respectively, and the inclusion forms specified in formulas (i) and (ii).

The low carbon free cutting steel of the present invention may further include at least one component selected from at least one group of the third group to the third group.

(1) Element of group 1

The elements of the first group are elements which further improve the machinability of the steel, without impairing the effect obtainable by the present invention in addition to the above-described main composition. Therefore, in order to obtain more machinability, you may contain 1 or more types.

Se: 0.0005-10.10%, Te: 0.95-0.10%

Se and Te produce Mn (S, Se) and Mn (S, Te) with Mn. These are effective elements for improving machinability because they play a similar lubricating effect during cutting, like MnS, and may be contained within the above ranges for further machinability. However, the effect is insufficient when each content is less than 0.5%. On the other hand, if the content of Se and Te exceeds 0.10%, the effect is not only saturated, but also uneconomical and deteriorates hot workability. In order to make more stable and excellent hot workability and machinability compatible, it is preferable that they are respectively 0.0010 to 0.05%.

Bi: 0.01-0.3%, Sn: 0.01-0.3%

Bi and Sn have an effect of improving the machinability of the steel. It is thought that this is because, as with Pb, a low melting point metal inclusion exhibits a lubricating effect during cutting. In order to reliably obtain the effect, it is preferable to make each content 0.01% or more. However, when the content exceeds 0.3% each, the effect is not only saturated but also deteriorates hot workability. In order to make more stable and excellent hot workability and machinability compatible, it is preferable that they are 0.03 to 0.1%, respectively.

Ca: 0.0001 to 0.01%

Ca has a large affinity for S or 0 (oxygen), and therefore forms sulfides and oxides in the steel. In addition, Ca is dissolved in MnS to form (Mn, Ca) S, but since Ca is dissolved in a small amount, the effect as MnS is not impaired. In addition, the oxide formed by Ca is a low melting oxide, and is also an effective element for improving the machinability even in the steel of the present invention. When it is going to acquire the effect of the machinability improvement by addition of Ca reliably, it is preferable that the minimum of Ca amount is 0.0001%. However, since the yield of additives is poor, Ca requires a large amount of Ca to increase the content of Ca, which is not preferable from the viewpoint of production cost. Therefore, the upper limit of Ca content was made into 0.01%. More preferably, the upper limit is 0.95%.

Mg: 0.0001 to 0.005%

Mg also has a large affinity for S and O (oxygen) in the steel, thus forming sulfides or oxides. Sulfides and oxides containing Mg function as crystallization nuclei of MnS and have an effect of suppressing elongation of MnS. When you want to acquire such an effect, you may add. In order to fully acquire the effect, it is preferable that the minimum of Mg amount is 0.0001% or more. However, since oxide formed by Mg is hard, when there is too much content of Mg, it will become a factor which degrades machinability. Therefore, the upper limit of Mg was 0.15%. Moreover, in order to make the effect of suppressing elongation of MnS and excellent machinability compatible, a preferable upper limit is 0.002%.

B: 0.0002-0.02%

B combines with 0 (oxygen) or N, forms an oxide and a nitride, and has the effect of improving machinability, and may be added as necessary. In order to acquire the effect, it is good to contain O.0002% or more. In order to acquire the effect more reliably, 0.0010% or more is preferable. However, when the content of B exceeds 0.02%, the effect is not only saturated but also deteriorates hot workability.

Rare Earth Element: 0.0005 to 0.02%

Rare earth elements are an elemental group classified as lanthanoids. When adding this, the Misch metal etc. which use these as a main component are used normally. Content of the rare earth element in this invention is represented by the total amount of 1 type, or 2 or more types of elements in a rare earth element. The rare earth element forms an oxide together with oxygen, combines with S to form a sulfide, and improves machinability. In order to reliably obtain the effect, the rare earth element may be contained at 0.0005% or more. However, if the content exceeds 0.02%, the effect is saturated. In addition, since rare earth elements have a low yield of added products, it is not economical to contain a large amount.

(2) Elements of the second group

All of the elements of the second group have the effect of increasing the strength of the steel. If necessary, one or more of them can be contained.

Cu: 0.01% to 1.0%

Cu has the effect of improving the strength of the steel by precipitation strengthening. In order to acquire the said effect, it is necessary to make the content into 0.01% or more. In order to acquire the said effect more reliably, it is preferable to add 0.1% or more. However, when the content is more than 1.0%, the hot workability is deteriorated, or the coagulation of Cu precipitates not only saturates the electric effect but also lowers the machinability.

Ni: 0.01-2.O%

Ni has the effect of improving the strength of the steel by solid solution strengthening. In order to reliably obtain the said effect, it is preferable to make the content into 0.01% or more. However, if it exceeds 2.0%, the machinability deteriorates and the hot workability also deteriorates.

Mo: 0.01% to 0.5%

Mo is an element capable of increasing hardenability, but in order to improve the carburizing property, when a considerable amount of Mo, which is equivalent to the addition of Si or Cr, is added, it is more expensive than Si or Cr. have. However, Mo also has the effect of miniaturizing the tissue and improving toughness. When you want to acquire these effects, you may add. In order to reliably acquire the said effect, it is preferable to make content into 0.01% or more. However, if the content exceeds 0.5%, not only the effect is saturated but also the manufacturing cost of the steel increases.

V: 0.005-0.5%

V precipitates as fine nitrides or carbonitrides and improves the strength of the steel. Although the said effect can be obtained by containing 0.05% or more, when it is desired to acquire an effect more reliably, containing 0.01% or more is preferable. However, if the content exceeds 0.5%, not only the above-mentioned effect is saturated but also excessive generation of nitrides and carbides results in deterioration of machinability.

Nb: 0.005-0.5%

Nb precipitates as fine nitrides or carbonitrides and improves the strength of the steel. Although the said effect can be obtained by containing 0.05% or more, when it is desired to acquire an effect more reliably, containing 0.01% or more is preferable. However, if the content exceeds 0.5%, the above-mentioned effect is not only saturated, and since nitrides and carbides are excessively produced, not only does the machinability deteriorate but also it is not economical.

(3) Elements of the third group

The element of the 3rd group is an element which can contain the said one or both within the following range, when it wants to improve the carburization characteristic of steel.

Si: 0.1 to 2.0%

In the free cutting steel of the invention according to claims 1 to 4, Si is not actively added. Therefore, Si is one of impurities, and its content is less than 0.1%. In addition, even in the free-cutting steel of the invention according to claims 1 to 4, Si may be added as a deoxidation element in order to make an appropriate amount of oxygen in the steel, but in this case, it is not necessary to actively remain, and Si remaining in the steel Impurity, less than 0.1%.

In addition, Si has an effect of increasing the strength of the steel and enhancing the hardenability of the steel by solid solution in ferrite.

By increasing the hardenability of the steel, it is possible to improve the carburizing characteristics desired for automobile parts. Only in that case, it is good to contain Si 0.1% or more. When it is desired to improve the carburization characteristic more reliably, the content of more than 0.6% is preferable. If it exceeds 2.0%, the machinability is adversely affected, such as deterioration of hot workability or high cutting resistance in order to solidify the ferrite phase. In addition, even at the level of less than 0.1% of impurities, the amount of oxygen in the steel can be in an appropriate range by appropriate addition of C, Mn, and Al.

Cr: 0.03 to 1.O%

Cr is an element that can improve carburization characteristics by adding a small amount in order to increase the hardenability of steel. In the steel containing Cr, the carburizing property is improved, the carburized layer hardness after carburizing treatment is high, and the effective hardening depth can also be increased. In order to acquire the effect, you may contain 0.03% or more of Cr. In addition, when it is desired to improve the carburizing characteristic more reliably, the content of more than 0.05% is preferable. However, when the Cr content exceeds 1.0%, not only the machinability is deteriorated, but also the manufacturing cost is high.

When Si or / and Cr is contained, steel having excellent carburization characteristics can be obtained in addition to excellent machinability and hot workability.

EXAMPLE

1. Preparation of Sample

Using a high frequency heating induction furnace, a 150 kg ingot (about 220 mm in diameter) having various compositions shown in Tables 1 and 2 was produced. What is shown in Table 1 is steel of this invention, and what is shown in Table 2 is a conventional steel or a comparative steel. In order to stably produce substantial MnS containing Ti carbide and / or Ti carbonitride, the cast piece was heated to a high temperature of 1250 ° C. and then held at that temperature for 2 hours or more. Then, assuming a rolling process, forging was performed at a finishing temperature of 1000 ° C. or higher, followed by air cooling to obtain a round bar having a diameter of 65 mm. Then, the said short-seat material was heated to 950 degreeC, the normalizing process which air-cooled was performed after hold | maintaining for 1 hour. In addition, steel No. 51-53 of a comparative example is inferior to hot work property, and the crack generate | occur | produced at the time of forging, and since it was not able to make it as a short material, the following irradiation is not performed.

2. Investigation in the form of inclusions

Inclusions observed in the cross section parallel to the rolling direction are those which are extended in the processing direction and are of unspecified shape. In the investigation of the number and area of the inclusions, the micro-observing test piece is cut out from the longitudinal cross-sectional direction of the Df / 4 (Df; diameter of the shorting material) portion of the shorting material, and embedded in resin, followed by mirror polishing. Photographs were taken at 400-fold optical microscope observation, and the number and area of the inclusions were determined by methods such as image analysis. In that case, the diameter and circle equivalent diameter when it converted into the circle which has the same area were made into 1 micrometer or more. The original equivalent diameter is limited to 1 µm or more because, as described above, inclusions of less than 1 µm have little effect on machinability.

In addition, the composition of these inclusions was confirmed as follows. That is, as described above, the micro test piece cut out from the longitudinal cross-sectional direction of the Df / 4 (Df; diameter of the short material) portion of the short material is buried in resin and subjected to mirror polishing, followed by EPMA (electron beam micro analyzer). Surface analysis and quantitative analysis were performed by EDX (Energy Dispersive X-ray Analyzer), etc. The observation magnification at this time may be selected not to exceed 10000 times, and MnS and Ti carbide in one inclusion at the observation magnification. Or / and an inclusion in which Ti carbonitride is clearly observed by phase separation and that the area ratio of MnS is 50% or more is "substantial MnS inherent in Ti carbide and / or Ti carbonitride". From the results observed in this way, the areas of the respective "substantial MnS in which Ti carbides and / or Ti carbonitrides are inherent" and "substantial MnS in which Ti carbides and Ti carbonitrides are not contained" are obtained. (A + B) / C was calculated after calculating the total of the area of these inclusions in the rolling direction cross section 1 mm 2, and calculating the total of the area occupied by the developing material in the rolling direction cross section 1 mm 2.

From the above results, the number of "substantial MnS inherent in Ti carbide and / or Ti carbonitride" was measured, and it was set as "(circle)" in the steel in which five or more average numbers per 1 mm <2> of rolling direction existed. On the contrary, "x" was made about the steel which "substantial MnS in which Ti carbide and / or Ti carbonitride is inherent" does not satisfy five. In addition, steel No. 35-37 of the comparative example shown in Table 2 is a lead free cutting steel and S free cutting steel which does not contain Ti, and since substantially "the substantial MnS in which Ti carbide or / and Ti carbonitride is inherent" did not exist, These calculations are not performed.

3. Machinability test

In the machinability test, a test was carried out to investigate the tool life and the finish surface roughness using a round bar whose outer diameter was cut to 60 mm in diameter using the short material. Tool life test uses P20 cemented carbide tools specified in JIS without coating treatment, cutting speed; 150 m / min, feed; Turning at dry conditions was performed at 0.10 mm / rev and a cutting depth of 2.Omm, and the average amount of relief surface wear after 30 minutes from the start of cutting was measured. In addition, for the material (hereinafter referred to as test material) provided for the test in which the average relief surface wear amount reached 200 µm or more within 30 minutes, the arrival time and average relief surface wear amount (VB) at that time were determined. Measured.

Evaluation performed the time which the average relief surface wear amount VB reaches 100 micrometers as a target of tool life. In addition, since the wear resistance was excellent and the progress of the wear was very small during the test, for the lack of test materials, the time required for the average relief surface wear amount to reach 100 µm from the turning time-tool wear curve was calculated by regression. . In addition, the cutting chip processability was evaluated by extracting 200 representative ones of the cutting chips discharged at this time, measuring the weight, and calculating the number per unit weight.

Since the finish surface roughness is evaluated by the surface roughness after finishing cutting, the surface of the workpiece | work after cutting under the following conditions was evaluated using the stylus roughness meter. Cutting conditions include cutting speed using JIS K carbide tools subjected to TiAlN multilayer coating; 100 m / min, feed; 0.05 mm / rev, depth of cut; 0.5 mm was used for turning under wet conditions using a water-soluble emulsion lubricant.

With respect to the test piece after cutting the test steel for 1 minute under these conditions, the stylus was moved in the axial direction with a stylus roughness meter to measure the average surface roughness Ra and to evaluate the finish surface roughness.

4. Hot workability test

In order to simulate the manufacturing conditions by the continuous casting equipment, hot workability is based on the position of Di / 8 (Di; diameter of the steel ingot) close to the surface portion of the 150 kg steel ingot manufactured by the above-described method. A high temperature tensile test piece with a diameter of 10 mm and a length of 130 mm and a fixed interval of 110 mm was heated to 1250 ° C. by direct energization, held for 5 minutes, and then cooled to 1100 ° C. at a cooling rate of 10 ° C./sec for 10 seconds. After holding, strain rate; Tensile tests were conducted at 10 ⁻³ / s. At that time, the cross-sectional reduction rate of the fractured part was measured to evaluate the hot workability.

5. Carburization Test

Carburization test was performed as follows. That is, the cylindrical specimen of 24 mm in diameter and 50 mm in length was used for the test piece. This was extract | collected from the position of R / 2 of the normalizing material of diameter 65mm mentioned above. The test piece was heated to 900 ° C, carburized, and then diffused at 850 ° C. At this time, the carbon potential (C.P.) value at the time of carburizing was 0.8%, and the processing time was 75 minutes, and the C.P. The value is 0.7% and the processing time is 20 minutes. The test piece which completed carburizing process was quenched by cooling in 80 degreeC oil. Finally, the test piece was heated to 190 ° C, held at this temperature for 60 minutes, and subjected to a tampering treatment. The evaluation method of carburizing property is as follows.

Vickers hardness distribution from the surface to the inside is measured at the cross section of the position of 25 mm (ie, the center in the longitudinal direction) from the end of the carburized quenching and tampering test piece, and the effective hardening depth of Hv400 is obtained. It was determined whether the value was larger or smaller than the conventional lead composite free cutting steel. Conventional Pb composite free cutting steel is steel No. 35 of Table 2, and the effective hardening depth was 0.25 mm. In evaluation of carburizing property, it is said that the effective hardening depth is equivalent to ± 0.05mm with respect to steel No. 35, that is, 0.20 to 0.30mm, and deteriorates when less than 0.20mm, and exceeds 0.30mm. It was judged to be excellent. The results are shown in Tables 3 and 4 by ○, ×, and ◎. The case where it is equivalent is "(circle)", the case where it deteriorates is "x", and the case where it is excellent is "(circle)".

Table 3 and Table 4 summarize the above test results. In addition, Fig. 2 shows the relationship between (A + B) / C of the formula (i) and the finish surface roughness, Fig. 3 shows the relationship between the finish surface roughness and the tool life, and Fig. 4 shows the relationship between the cutting chip processability and the tool life. .

Steel Nos. 35 and 36 in Table 2 are composite free-cutting steels, and steel No. 37 is sulfur free-cutting steels. As is apparent from Tables 3, 4, 2 and 3, the steel of the present invention has excellent tool life and finish surface roughness. In addition, the present invention steel Nos. 1 to 34 have excellent hot workability, and the reduction ratio of the cross section by the high temperature tensile test simulating the practical production by continuous casting equipment and the like is also equal to or higher than that of the composite free cutting steel or sulfur free cutting steel. There is no problem.

Steel No. 12-17 of Table 1 contains at least 1 sort (s) of Si and Cr within a prescribed range for carburizing improvement. It turns out that these steels show especially the carburization characteristic among the steels of this invention. On the other hand, as in steel Nos. 35 to 55, at least one of the tool life, the finish surface roughness, the cutting chip processing property, and the hot workability are found to be out of one of the states of the inclusions and the chemical composition, etc. specified in the present invention. It is inferior to the invention steel.

While only a few preferred embodiments of the present invention have been described in detail above, those skilled in the art will appreciate that various modifications may be made to the embodiments without departing from the spirit and advantages of the invention. Accordingly, all such modifications will be construed as being included within the scope of the present invention.

Although the free cutting steel of this invention does not contain Pb, it has the machinability equivalent to or more than the conventional lead free cutting steel and composite free cutting steel, and is also excellent in the finish surface property after cutting. In addition, containing Si or / and Cr also has excellent carburization characteristics. Furthermore, this steel is excellent in hot workability and can be manufactured inexpensively even by the continuous casting method. It does not contain Pb, so there is no concern for environmental pollution. Therefore, the free cutting steel of this invention is a steel material suitable as a raw material of various mechanical parts.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows typically the form of the inclusion of this invention steel and a comparative steel.

Fig. 2 is a diagram showing the relationship between (A + B) / C and average finish surface roughness in the inventive steel and the comparative steel.

3 is a diagram showing the relationship between the average surface roughness and tool life in the inventive steel and the comparative steel.

4 is a diagram showing a relationship between cutting chip processability and tool life in the inventive steel and the comparative steel.

Claims (5)

In mass%, C: 0.05% to less than 0.20%, Mn: 0.4 to 2.0%, S: 0.21 to 1.O%, Ti: 0.002 to 0.10%, P: 0.001 to 0.30%, Al: 0.2 % Or less, 0 (oxygen): 0.001-0.03% and N: 0.0005-0.02%, remainder is steel which consists of Fe and an impurity, and the inclusion contained in steel is represented by following formula (i) and (ii) Low-carbon free-cutting steel characterized by the following formula.                       (A + B) /C≥0.8 .... (i) N _A ≥ 5 .... (ii) Here, the meanings of A, B, C and N _A are as follows. A; The total area occupied by substantial MnS inherent in Ti carbide and / or Ti carbonitride among inclusions of 1 µm or more in equivalent circular diameter in 1 mm 2 of a cross section parallel to the rolling direction. B; Total area occupied by substantial MnS in which Ti carbide and Ti carbonitride are not inherent among the inclusions of 1 micrometer or more in circular equivalent diameter in 1 mm <2> of a cross section parallel to a rolling direction. C; The total area occupied by all inclusions having a diameter of 1 μm or more in 1 mm 2 of a cross section parallel to the rolling direction. N _A ; The substantial number of MnS which Ti carbide and / or Ti carbonitride is intrinsic in 1 micrometer or more of circular equivalent diameter in 1 mm <2> of cross section parallel to a rolling direction. The method of claim 1, Instead of a part of Fe, Se: 0.00 to 0.10%, Te: 0.0005 to 0.10%, Bi: 0.01 to 0.3%, Sn: 0.01 to 0.3%, Ca: 0.00 to 0.01%, Mg: O. A low carbon free cutting steel comprising at least one member selected from the group consisting of 0001 to 0.005%, B: 0.0002 to 0.02% and rare earth elements: 0.0005 to 0.02%. The method of claim 1, One type selected from the group consisting of Cu: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Mo: 0.01 to 0.5%, V: 0.005 to 0.5% and Nb: 0.005 to 0.5% instead of a part of Fe. The low carbon free cutting steel containing the above. The method of claim 1, Instead of a part of Fe, Se: 0.0005 to 0.10%, Te: 0.0005 to 0.10%, Bi: 0.01 to 0.3%, Sn: 0.01 to 0.3%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.005%, B : At least one member selected from the group consisting of 0.0002 to 0.02% and rare earth elements: 0.05 to 0.02%, Cu: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Mo: 0.01 to 0.5%, A low-carbon free cutting steel comprising at least one member selected from the group consisting of V: 0.95 to 0.5% and Nb: 0.95 to 0.5%. The method according to any one of claims 1 to 4, A low-carbon free cutting steel comprising one or two of Si: 0.1 to 2.0 mass% and Cr: 0.03 to 1.0 mass% instead of a part of Fe.