JP3918787B2 - Low carbon free cutting steel - Google Patents

Description

  The present invention relates to a low carbon free cutting steel containing no lead (Pb). In particular, although it does not contain lead, it is also superior in the case of cutting with carbide tools compared to conventional free-cutting steel and composite free-cutting steel that uses lead and other free-machining improvement elements in combination. The present invention relates to a low-carbon free-cutting steel that has machinability, is excellent in hot workability and finished surface properties after cutting, and can be manufactured at low cost. The present invention also relates to a low-carbon free-cutting steel excellent in carburizing characteristics as well as the above-mentioned various characteristics.

  Conventionally, steel parts having excellent 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 steels are sulfur free-cutting steel with a large amount of S added to improve machinability with MnS, lead free-cutting steel with Pb added, and composite free-cutting steel containing both S and Pb. Cutting steel. In particular, free-cutting steel containing Pb has characteristics such as extending the tool life, excellent chip separation, and excellent finished surface roughness of the steel material after processing. There are also free-cutting steels containing Te (tellurium), Bi (bismuth), etc. for the purpose of improving machinability. These are used in large quantities in various machine parts such as electric equipment parts and dies, as well as small parts such as brake parts for automobiles and PC peripheral equipment parts.

  On the other hand, the recent improvement in performance of cutting machines has enabled high-speed cutting, and accordingly, steel materials that are the materials of the above parts are also strongly desired to improve machinability during high-speed cutting. Yes.

  Further, the above parts may be carburized in order to ensure the strength of the surface after being finished into a predetermined shape by cutting. Therefore, it may be desired that the steel materials used for these parts have high machinability and excellent carburization characteristics.

  The steel material used as the material for the above parts is required to have excellent machinability, but the machinability not only extends the tool life, but also has the property that chips are finely divided, that is, `` It is important to have excellent chip disposal. This chip disposal is indispensable for the automation of the processing line, and is essential for improving the productivity. In addition to tool life and chip disposal, it is desired that the finished surface on the steel surface after cutting is excellent in properties, that is, the finished surface roughness is small, from the viewpoint of machining accuracy. Among the above-mentioned free-cutting steels, lead free-cutting steel and composite free-cutting steel containing Pb and other free-machining improving elements are excellent in these properties, and most excellent in machinability among existing steel materials. Has been.

  In recent years, free-cutting steel containing no Pb has been strongly demanded with increasing interest in environmental problems. This is because steel materials containing Pb, which is harmful to the human body and the global environment, not only require extensive exhaust equipment in the manufacturing process, but also have been increasingly used to suppress the use of Pb from the viewpoint of environmental conservation. Because.

  In order to meet the above requirements, various proposals have been made regarding low-carbon sulfur free-cutting steel that does not contain Pb as an alternative to lead-free-cutting steel. However, free-cutting steels that satisfy all the characteristics of free-cutting steels containing Pb that contribute to the extension of tool life, excellent chip controllability, and low finished surface roughness have not yet been developed.

  Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-49240) discloses a free-cutting steel in which a carburide-based inclusion of Ti or / and Zr is present to improve machinability. In this free-cutting steel, Ti carbon sulfide or Zr carbon sulfide is dispersed in the steel together with MnS, so it is difficult to obtain a pseudo lubricating effect of MnS, and the frictional force between the tool and the work material is low. To rise. As a result, the cutting resistance increases, and the component cutting edge is easily generated at the tool cutting edge. If the component cutting edge is generated, the finished surface roughness after finish cutting increases, and the machining accuracy of the part is impaired.

  In Patent Document 1, there is no example in which the Ti content is 0.1% or less. This indicates that the invention of Patent Document 1 aims to produce Ti carbosulfide by containing a large amount of Ti. Actually, Ti carbosulfide inclusions together with MnS have a grain shape. In the matrix. In this case, the performance required for the steel materials used for the parts, such as tool life, chip disposal, and finished surface roughness, cannot be satisfied.

  Patent Document 2 (Japanese Patent Laid-Open No. 2003-49241) contains Ti or / and Zr in the range of (Ti + 0.52Zr) / S <2, and contains Ti or Zr carbon sulfide as an inclusion. Free-cutting steel is disclosed that has improved tool life in turning and drilling. In the invention of Patent Document 2, Ti carbon sulfide is generated in steel to improve the tool life during turning. This technology can certainly improve the tool life to some extent, but the presence of Ti or Zr carbosulfides makes it difficult to obtain the lubrication effect of MnS, and the frictional force between the tool and the work material increases. . As a result, the cutting resistance increases, and the component cutting edge is easily generated on the tool cutting edge. If the constituent cutting edges are generated, the finished surface roughness after cutting increases, and as a result, the machining accuracy deteriorates.

  In Patent Document 2, there is no example of free cutting steel containing S in the range of 0.21% or more and Ti content of 0.1% or less as specified in the present invention described later. From this, it is clear that the invention of Patent Document 2 is not an invention aimed at improving the finished surface roughness and chip disposal. That is, in the invention of Patent Document 2, Ti or Zr carbon sulfide is dispersed in the matrix together with MnS, so that it is not possible to obtain desired finished surface roughness and chip disposal.

  Patent Document 3 (Japanese Patent Application Laid-Open No. 2000-319753) discloses a low-carbon sulfur free-cutting steel not containing Pb, which contains more than 0.4% S and increases MnS. However, this steel has a small effect of improving the carbide tool life. Further, this steel does not improve the chip disposability considered important with the tool life, and does not greatly improve the performance of conventional sulfur free-cutting steel.

  In Patent Document 4 (Japanese Patent Laid-Open No. 09-53147), C: 0.01 to 0.2%, Si: 0.10 to 0.60%, Mn: 0.5 to 1.75%, P: 0.005 to 0.15%, S: 0.15 to 0.40%, An invention of free-cutting steel containing O (oxygen): 0.001 to 0.010%, Ti: 0.0005 to 0.020%, N 0.003 to 0.03% and excellent in machinability for carbide tools, particularly excellent tool life is disclosed. . In the present invention, it is essential to contain 0.1 to 0.6% of Si together with Ti to improve the life of the carbide tool. Further, according to the present invention, not only the tool life but also the tool life can be obtained by allowing “substantially MnS in which Ti carbide or / and Ti carbonitride is present” to exist in the steel material without containing Si as in the present invention. It is not intended to improve chip disposability and finish surface roughness.

  Patent Document 5 (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%, Zr: 0.1 to 0.6%, In addition, an invention of a low carbon sulfur free-cutting steel with improved mechanical anisotropy satisfying Ti + Zr: 0.3 to 0.6% and (Ti + Zr) / S ratio: 1.1 to 1.5 is disclosed. According to the present invention, Ti and Zr sulfides having high deformation resistance in the hot state are produced by using the above composition, and the mechanical anisotropy and machinability of the steel material are improved. However, since these sulfides having high deformation resistance are difficult to obtain a lubrication effect due to sulfide during cutting, the cutting resistance is increased, and the tool life and the roughness of the finished surface are deteriorated.

Japanese Patent Laid-Open No. 2003-49240

Japanese Patent Laid-Open No. 2003-49241 JP 2000-319753 JP 09-53147 A Japanese Patent No. 3390988

  The problem of the present invention is that, in particular, cemented carbide tools are used as compared with conventional Pb free-cutting steel and composite free-cutting steel using Pb in combination with other free-cutting property imparting elements without containing Pb harmful to the environment. An object of the present invention is to provide a low-carbon free-cutting steel that exhibits excellent machinability in the case of the cutting used, is excellent in hot workability, has excellent surface properties after cutting, and can be manufactured at low cost. It is also an object of the present invention to provide a low carbon free-cutting steel having excellent carburizability in addition to the above various characteristics.

  It is well known that the state of inclusions such as sulfides greatly affects the machinability of steel. There are various inclusions observed in steel containing C, Ti, S, N, and O. For example, Ti sulfide, Ti carbon sulfide, Ti carbide, Ti carbonitride, Ti nitride, and Ti oxide. Further, if Mn is contained, Mn sulfide represented by the chemical formula “MnS” is also present. In addition to these, if Al or Si is contained, these oxides also exist. These forms exist in various ways, and the composition and form of these inclusions greatly affect the machinability and other mechanical properties of steel.

The present inventors previously filed a patent application for an invention related to low-carbon sulfur free-cutting steel containing no Pb by Japanese Patent Application No. 2002-26368. The free-cutting steel contains C, Mn, S, Ti, Si, P, Al, O, and N in specified amounts, the contents of Ti and S satisfy the following formula (A), and atoms of Mn and S ratio satisfies the formula (B) below, a low carbon sulfur free cutting steel which is characterized by and containing MnS to Ti sulfide or / and Ti carbosulfide are inherent.

Ti (wt%) / S (wt%) <1 ··· (A) ,
Mn / S ≧ 1 (B) .
The steel of the prior invention has much longer tool life than Pb free-cutting steel and has excellent chip disposal. However, this steel has some difficulties in surface properties after cutting. That is, when finishing cutting was performed, the problem that the finished surface roughness sometimes increased became clear.

When substantial Ti sulfide and / or Ti carbosulfide exist in Mato Riku scan, for pseudo lubricating effect of MnS becomes difficult to obtain, the cutting resistance increases, the built-up edge on the tool edge generation Therefore, it is considered that the finished surface roughness is deteriorated without giving gloss to the surface of the steel material after cutting. The above "substantial Ti sulfide or / and Ti carbosulfide" means that the total area ratio occupied by Ti sulfide and Ti carbosulfide in one inclusion is 50% or more. This means inclusions, some of which are shown in FIG.

  As a result of studies to solve the above problems, the following new findings were obtained.

  (1) When cutting steel in which “substantially Ti sulfide or / and Ti carbosulfide” is present in the matrix, the component cutting edge is generated in the tool cutting edge and the roughness of the finished surface is deteriorated.

  (2) Suppressing the generation of “substantial Ti sulfide and / or Ti carbosulfide” as much as possible, and by making MnS present in the steel as much as possible, the generation of the component cutting edge is suppressed, and a good finished surface Roughness can be obtained.

(3) However, the tool life of cemented carbide deteriorates in steel that does not contain Ti and contains only MnS. To improve the carbide tool life, the addition of Ti, it is necessary to present the one 且 "substantial MnS which Ti carbide and / or Ti carbonitride included therein".

  (4) “Substantially MnS containing Ti carbide and / or Ti carbonitride” improves the tool life while not impairing the pseudo lubricating effect of MnS.

Based on the above findings, the relationship between chemical composition and inclusion morphology was further examined in detail. As a result, the inventors have invented the low carbon free cutting steels shown in the following [1] to [5] . Low carbon free cutting steel of these has a Pb free-cutting steel and composite free cutting steels least equivalent machinability. In addition,% regarding component content is the mass%. In the following description, the inventions related to the low carbon free cutting steels [1] to [5] below are referred to as “present invention [1]” to “present invention [5]”, respectively. Also, it may be collectively referred to as “the present invention”.

[1] C: 0.05% to less than 0.20%, Mn: 0.4 to 2.0%, S: 0.21 to 1.0%, Ti: 0.002 to 0.10%, P: 0.001 to 0.30%, Al: 0.2% or less, O (oxygen) : 0.001 to 0.03% and N: 0.0005 to 0.02%, the balance being Fe and impurities, and the inclusions contained in the steel satisfy the following formulas ( A ) and (B): low-carbon Motokokoroyo cutting steel to.

[2] C: 0.05% to less than 0.20%, Mn: 0.4 to 2.0%, S: 0.21 to 1.0%, Ti: 0.002 to 0.10%, P: 0.001 to 0.30%, Al: 0.2% or less, O (oxygen) : 0.001-0.03% and N: 0.0005-0.02%, and Se: 0.0005-0.10%, Te: 0.0005-0.10%, Sn: 0.01-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.005 %, B: 0.0002 to 0.02% and rare earth element: 0.0005 to 0.02%, one or more selected from the group consisting of Fe and impurities, and the inclusions contained in the steel are A low-carbon free-cutting steel characterized by satisfying the following formulas A and B:

[3] C: 0.05% to less than 0.20%, Mn: 0.4 to 2.0%, S: 0.21 to 1.0%, Ti: 0.002 to 0.10%, P: 0.001 to 0.30%, Al: 0.2% or less, O (oxygen) : 0.001-0.03% and N: 0.0005-0.02%, and Cu: 0.01-1.0%, Ni: 0.01-2.0%, Mo: 0.01-0.5%, V: 0.005-0.5% and Nb: 0.005-0.5 The steel is composed of one or more selected from the group consisting of% and the balance is Fe and impurities, and the inclusions contained in the steel satisfy the following formulas (A) and (B). Carbon free-cutting steel.

[4] C: 0.05% to less than 0.20%, Mn: 0.4 to 2.0%, S: 0.21 to 1.0%, Ti: 0.002 to 0.10%, P: 0.001 to 0.30%, Al: 0.2% or less, O (oxygen) : 0.001 to 0.03% and N: 0.0005 to 0.02%, and 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: 0.0002 to 0.02% and rare earth elements: 0.0005 to 0.02%, Cu: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Mo: Steel containing at least one selected from the group consisting of 0.01 to 0.5%, V: 0.005 to 0.5% and Nb: 0.005 to 0.5%, the balance being Fe and impurities, and the inclusions contained in the steel A low-carbon free-cutting steel characterized in that the material satisfies the following formulas A and B:
[5] The low-carbon free-cutting steel according to any one of [1] to [4], containing one or two of Si: 0.1 to 2.0% and Cr: 0.03 to 1.0%.

(A + B) /C≧0.8...
N _A ≧ 5 ... b,
Where A, B, C and N _A The meaning of is as follows.

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

B: 1 mm in cross section parallel to the rolling direction ² Of the inclusions with an equivalent circle diameter of 1 μm or more, the total area occupied by substantial MnS without Ti carbide and Ti carbonitride.

C: 1 mm in cross section parallel to the rolling direction ² The total area occupied by all inclusions with a circle equivalent diameter of 1 μm or more.

N _A : 1mm of cross section parallel to rolling direction ² Among the inclusions having a circle-equivalent diameter of 1 μm or more in the middle, the number of substantial MnS containing Ti carbide and / or Ti carbonitride.

Hereinafter, “Se: 0.0005 to 0.10%, Te: 0.0005 to 0.10%, Sn: 0.01 to 0.3%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.005%, B: 0.0002 to 0.02%, and rare earth elements: 0.0005 to The element of “group consisting of 0.02%” is the element of “Group 1A”, “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: 0.0002 to 0.02% and rare earth element: 0.0005 to 0.02% "element" group 1B element, "Cu: 0.01 to 1.0%, Ni: 0.01 -2.0%, Mo: 0.01-0.5%, V: 0.005-0.5% and Nb: 0.005-0.5% group "element" second group element, "Si: 0.1-2.0% and Cr: 0.03" The element of “group consisting of ˜1.0%” is referred to as the “third group” element.

  Here, “substantially MnS in which Ti carbide or / and Ti carbonitride is inherent” means, as will be described in detail later, the area ratio occupied by MnS in one inclusion is 50% or more, An inclusion in which Ti carbide and / or Ti carbonitride is present (coexist). On the other hand, “substantially MnS free of Ti carbide and Ti carbonitride” means that the area ratio occupied by MnS in one inclusion is 50% or more, and Ti carbide and Ti carbonitride are inherent. An inclusion that does not coexist. These “substantially MnS containing Ti carbide and / or Ti carbonitride” and “substantially MnS not containing Ti carbide and Ti carbonitride” are both included in Ti carbide and Ti carbide. It may contain sulfides other than carbonitrides, carbonitrides, carbides, nitrides and the like.

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

(1) C is 0.05% to less than 0.20%, S is contained in the range of 0.21 to 1.0% , and the Ti content is 0.002 to 0.10% .

  (2) Ti combines with C, S, N, and O to form sulfide, carbon sulfide, carbide, carbonitride, and oxide. Ti has a tendency to form sulfides more easily than Mn, so Ti sulfide and Ti carbon sulfide are easily formed. However, if careful consideration is given to the content balance of Mn, Ti, S and N, a large amount of “substantial Ti carbosulfide or / and Ti sulfide” is not produced, and “Ti carbide or / and Ti There can be many "substantially MnS in which carbonitride is inherent" and "substantially MnS in which Ti carbide and Ti carbonitride are not present".

(3) C, S content and Ti to satisfy the conditions shown in (1), and when the (2) inclusions form as shown in is obtained, present in Mato Riku scan mediated Most of the inclusions are "substantial MnS", which softens during cutting and exerts a lubricating effect, and sulfides other than this "substantial MnS", that is, "substantial Ti sulfide" Or / and Ti carbosulfide "is almost absent. At this time, in order to obtain a good finished surface roughness, the production amount of “substantially MnS” must occupy most of the production amount of all inclusions. Specifically, the total area of “substantially MnS” with an equivalent circle diameter of 1 μm or more on the observation surface 1 mm ² in the cross section in the rolling direction is 80% or more of the total area of all inclusions with an equivalent circle diameter of 1 μm or more. Need to occupy. Only in this case, the generation of the component cutting edge on the tool cutting edge caused by the presence of “substantial Ti sulfide or / and Ti carbon sulfide” is suppressed, and good finished surface roughness can be obtained.

  The above “substantially MnS” is an inclusion whose area ratio of MnS in one inclusion is 50% or more, and “substantially MnS in which Ti carbide or / and Ti carbonitride is contained” It consists of “substantially MnS free of Ti carbide and Ti carbonitride”.

As shown in the above equation (a), “occupies 80% or more” is the equation (A + B). The definition of A and B is “substantially MnS in which Ti carbide and / or Ti carbonitride is contained” among sulfides having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction. And the area occupied by “substantially MnS free of Ti carbide and Ti carbonitride”.

And “substantially MnS containing Ti carbide and / or Ti carbonitride”, “substantially MnS not containing Ti carbide and Ti carbonitride”, “substantial Ti sulfide and / or Ti charcoal” The sum of the total areas of “sulfides” and other sulfides, carbon sulfides, carbides, nitrides, oxides, Al ₂ O ₃ , SiO _2, etc. is C in formula (a).

  (4) Steel materials containing inclusions as shown in (3) above, that is, “substantial Ti sulfide or / and Ti carbon sulfide” hardly exist, and most of the inclusions contained in the steel Is “substantially MnS”, but if “substantially MnS containing Ti carbide and / or Ti carbonitride” exists, when cutting at a high speed range where the cutting temperature is high, A hard TiN film is formed and an excellent tool life can be obtained by protecting the tool.

  (5) In steels with “substantially MnS containing Ti carbide and / or carbonitride”, the “substantial MnS” is included in MnS included in conventional free-cutting steels of JIS SUM22L to 24L. Compared with it, it is fine and the number increases. In this case, since these fine “substantially MnS” is a starting point of stress concentration during cutting and facilitates crack propagation, chip treatability equal to or higher than that of the composite free-cutting steel can be obtained.

  (6) Steel with “substantially MnS containing Ti carbide and / or Ti carbonitride” has no problem in hot workability. It is possible to increase the number, and even in that case, there is no problem in manufacturing with a continuous casting facility or the like. In addition, since a sufficient amount of Ti is added even if the amount of Ti added is small, the manufacturing cost can be reduced and the steel can be applied as an inexpensive steel material.

  As described above, excellent machinability can be obtained by limiting the range of alloy components and adjusting the form of inclusions. However, in some cases, it is desired that steel materials used for automobile parts have excellent carburization characteristics in addition to machinability. Therefore, as a result of investigating the influence of Si and Cr on the steel properties, the carburization was performed without deteriorating the form of the inclusions by adjusting the Si content and Cr content, and therefore without degrading the machinability of the steel material. It became clear that the characteristics could be improved.

Si and Cr are dissolved in austenite to increase the hardenability of the steel, thereby increasing the carburization depth and the hardness of the carburized layer in the carburizing process. In addition to Si and Cr, elements that enhance hardenability include Mn, Mo, and P. However, Mn is one have machinability is required to contain an amount sufficient to the S amount in view of hot workability, and requires a large amount of additives. In this case, in order to improve the hardenability, the addition of Mn further increases the cost. Mo is also effective in enhancing the hardenability of steel, but Mo is more expensive than Si and Cr, so if a considerable amount of Mo that can obtain the same effect is added, the manufacturing cost increases. P has the same effect, but if added, the hardness of the steel material itself is rapidly increased, so that the machinability is deteriorated. However, these elements may be added within a range that does not impair the machinability and mechanical properties when not limited by the material cost. However, Si and Cr are desirable as components for improving carburization characteristics when it is desired to manufacture at low cost without impairing the machinability.

1. About "substantially MnS containing Ti carbide and / or Ti carbonitride"
Ti combines with S, C, N, O and is represented by the chemical formula of Ti sulfide and Ti carbon sulfide, TiC, Ti (CN), TiN, and TiO represented by the chemical formula of TiS and Ti ₄ C ₂ S ₂ Ti-based inclusions such as Ti carbide, Ti carbonitride, Ti nitride, and Ti oxide are formed. Ti may be dissolved in MnS and exist as (Mn, Ti) S, but the amount of Ti dissolved in MnS is very small, so this sulfide is substantially MnS. .

  On the other hand, Ti may not exist as a solid solution in MnS, but may exist in a single inclusion in an apparent phase separation from MnS. The Ti exists in the form of TiC or / and Ti (C, N), that is, the composition clearly differs from that of MnS, and the existence form exists in the vicinity of one sulfide or MnS. There are various types such as being surrounded by the inside.

  FIG. 1 is a diagram schematically showing inclusions present in free-cutting steel containing Ti. (A) is a free-cutting steel of a comparative example, and (b) is a free-cutting steel of the present invention. The steel shown in FIG. 1 (a) occupies Ti sulfide and carbon sulfide even when coexisting with MnS in a single Ti sulfide or carbon sulfide, or in one inclusion. Inclusions that have an area ratio of 50% or more and can be regarded as substantially Ti sulfide or carbon sulfide, that is, the above-mentioned "substantial Ti sulfide or / and Ti carbon sulfide" There are many. On the other hand, in the steel of the present invention shown in FIG. 1 (b), Ti carbides and / or Ti carbonitrides are included in the outer peripheral portion or inside of MnS, that is, “Ti carbide or / and Ti charcoal”. There are many "substantially MnS in which nitride is inherent" and "substantially MnS in which Ti carbide and Ti carbonitride are not present".

  In the steel shown in FIG. 1 (b), Ti sulfide, Ti carbon sulfide, Ti carbide, Ti carbonitride, Ti nitride, Ti oxide and other inclusions are clearly MnS. Even in the case where the phase ratio is inherent in the phase separation, if the area ratio occupied by MnS is 50% or more, it is determined as substantially one MnS, that is, “substantially MnS”. On the contrary, inclusions whose area ratio occupied by oxides, nitrides, carbides, etc. composed of these Ti-based inclusions or other components in one inclusion is 50% or more is “substantially MnS”. Rather, it is determined as an oxide, nitride, carbide, or the like substantially consisting of one Ti-based inclusion or other component.

  Among the above MnS, in particular, Ti carbides and / or Ti carbonitrides are clearly phase-separated from MnS and include inclusions whose area ratio occupied by MnS is 50% or more, “Ti carbides and / or Ti charcoal. It is defined as “substantially MnS in which nitride is inherent”. On the other hand, “substantially MnS without Ti carbide and Ti carbonitride” means that the above Ti-based inclusions excluding Ti carbide and Ti carbonitride, or oxides, nitrides and carbides composed of other components MnS and MnS, which are clearly separated from each other in one inclusion, have an area ratio of 50% or more occupied by MnS, and substantially play a role as MnS, and the above Ti-based inclusions MnS is free from inclusions such as oxides, nitrides and carbides composed of other components. That is, the sum of “substantially MnS containing Ti carbide and / or Ti carbonitride” and “substantially MnS not containing Ti carbide and Ti carbonitride” is considered to be substantially regarded as MnS. (Increased “substantially MnS”), and other inclusions include Ti sulfide, Ti carbon sulfide, Ti carbide, Ti carbonitride, Ti nitride, and Ti oxide. Oxides, carbides, nitrides, and the like composed of system inclusions and other elements.

The area ratio of MnS and Ti inclusions in one inclusion mentioned above is the EPMA (electron beam microanalyzer) or EDX (energy dispersive X) compared to the micro specimen cut out from the round bar subjected to the cutting test. It can be grasped by performing surface analysis and quantitative analysis by line analysis apparatus) or the like. In addition, “substantially MnS containing Ti carbide and / or Ti carbonitride” or “substantially MnS not containing Ti carbide and Ti carbonitride” in steel, and other inclusions are similarly processed. The total area and number can be measured by a technique such as image analysis. At that time, if the total of the observation field area was measured at a plurality of visual field to exceed 1 mm ^2, the average total area per 1 mm ² total area and the number of each of inclusions, when converted to the average number Good.

2. (A + B) /C≧0.8 Reason A in the formula (A) is “Ti carbide or / and Ti carbonitriding among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction. The total area occupied by “substantially MnS” is B, and “B” is the inclusion of “Ti carbide and Ti carbonitride among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction. It is the total area occupied by “substantially invisible MnS”. Here, “circle equivalent diameter” refers to a diameter when the area of one inclusion is obtained by the above-described technique such as image analysis and converted into a circle having the same area. The reason why the circle-equivalent diameter is limited to 1 μm or more is that inclusions less than 1 μm have almost no effect on machinability.

The above-mentioned formula (A) shows that the sum of A and B needs to be 80% or more of the total area occupied by all the inclusions having an equivalent circle diameter of 1 μm or more. Within this range it is possible to obtain good machinability, but preferred for more is 90% or more. In addition, as described above, inclusions other than those represented by A and B indicate nitrides, carbides, oxides, “substantial Ti sulfides and / or Ti carbon sulfides” and the like that exist alone. . In other words, the formula (A) indicates that the total area of these inclusions other than “substantial MnS” is less than 20% of the total area occupied by all the inclusions (C in the formula (C)). The total area is preferably in the further if less than 10%.

  When Ti is added to steel containing a large amount of S to improve machinability, Ti has a tendency to form sulfides more than Mn, so Ti sulfide and Ti carbon sulfide are easily formed. However, the formula (a) defined in the present invention is intended to suppress the formation of Ti sulfide and Ti carbon sulfide while assuming the addition of Ti. This is because Ti sulfide or Ti carbon sulfide inhibits the pseudo lubricating effect of MnS during cutting. When the pseudo lubrication effect of MnS is impaired, it is considered that the frictional force between the tool and the work material is increased, and the finished surface roughness is deteriorated because the constituent cutting edge is generated at the tool cutting edge. Therefore, generation of Ti sulfide and Ti carbosulfide must be suppressed. That is, as defined by the formula (1), “substantial Ti sulfide or / and Ti carbon sulfide” present alone is hardly present in the steel, and 80% of the inclusions contained in the steel. When the above is “substantially MnS”, a pseudo lubricating effect during cutting can be obtained.

As described above, when the composition range of the steel specified in the present invention is limited and the formula (1) is satisfied, the finish surface roughness is excellent in surface finish, which is equal to or better than that of the conventional Pb free-cutting steel or composite free-cutting steel. You can get it. On the other hand, be in the range of chemical composition specified in the present onset bright, if not met Lee type, it is impossible to obtain good machinability.

3. N N _A reason the Russian expression that _A ≧ 5 is Chi sales of a circle equivalent diameter of 1μm or more inclusions in 1mm ² in cross section parallel to the "rolling direction, Ti carbide and / or Ti carbonitride inherent The actual number of MnS ”. This “substantially MnS in which Ti carbide or / and Ti carbonitride is inherent” indicates that the area ratio of MnS in one inclusion is 50% or more as described above. This “substantially MnS containing Ti carbide and / or Ti carbonitride” does not substantially impair the pseudo-lubricating effect. Roughness does not deteriorate.

  In addition, after cutting steel with “substantially MnS containing Ti carbide and / or Ti carbonitride” at a high speed range exceeding 100 m / min using a carbide tool, the tool surface is detailed. When observed, TiN was found to be formed on the tool surface. On the tool surface that comes into contact with the work material during cutting, Ti-based inclusions react and change as the temperature rises due to friction, resulting in the formation of a hard TiN with a thickness of several μm to several tens of μm. it is conceivable that. Its existence is confirmed by surface analysis and point analysis using AES (Auger Electron Spectroscopy) or EPMA (Electron Beam Microanalyzer) on the tool surface after carbon-based contamination (oil content, etc.) has been removed by Ar sputtering after the end of cutting. We were able to. According to it, the surface area of TiN adhering to the tool was 10 to 80% of the contact area between the work material and the tool, and the rest was a tool base without MnS or Fe adhering to the tool or adhering during machining. . By forming this hard TiN on the tool surface, thermal diffusion wear of the tool and mechanical wear due to hard inclusions are suppressed, compared to conventional S free cutting steel and composite free cutting steel with Pb. However, it is considered that a much superior tool life can be obtained.

In order to obtain the effect as described above, “substantially MnS containing Ti carbide and / or Ti carbonitride” is 5 or more, preferably 10 or more in 1 mm ² of the observation surface in the cross section in the rolling direction. It only has to exist.

On the other hand, be in the range of chemical composition specified in the present onset bright, it is impossible to obtain good machinability does not satisfy the B type.

  MnS is very finely present in steels with inclusions satisfying A and B by adding Ti. That is, the number of MnS is remarkably large. This fine MnS acts as a stress concentration point of chips generated during cutting, and promotes crack propagation in the chips, thereby improving chip disposal.

Or, in short, a "Ti carbide and / or Ti substantial MnS which carbonitride included therein" be present stably in the steel, and 5 or more in the observation plane 1 mm ² in the rolling direction cross-rolling direction The total area of “substantially MnS containing Ti carbide and / or Ti carbonitride” and “substantially MnS not containing Ti carbide and Ti carbonitride” in 1 mm ^{2 of the} cross-sectional observation surface is all if more than 80% of the inclusions total area, as indicated above, it is possible to obtain Pb free-cutting steel and composite free cutting steels and equal or tool life, finished surface roughness and chip disposability . Considering the balance of the contents of Mn, Ti, S and N in order to realize such inclusion form more stably and to produce steel materials with excellent machinability at low cost by continuous casting method etc. There is a need to. Specifically, the following may be performed.

(A) Ti (%) / S (%) ≦ 0.25
When added more of Ti to the S amount, i.e., to include a lot of Ti to the S amount, if Ti / S at mass ratio exceeds 0.25, Ti sulfide and Ti carbosulfide are abundant Will do. As a result, the equation (1) is not satisfied, and the pseudo lubricating effect by MnS is impaired. At this time, the cutting resistance is increased, and the component cutting edge tends to be easily formed on the tool cutting edge. As a result, the surface roughness at the time of finish cutting is deteriorated, and the processing accuracy is deteriorated.

Conversely, when adding a small amount of Ti with respect to the S amount, that is , when the Ti content is very small with respect to the S content , when Ti / S is 0.25 or less by mass ratio, Ti is Ti carbides some have forms Ti carbonitride, "substantial Ti sulfide and / or Ti carbosulfide" are almost not exist in isolation.

  Ti carbide and Ti carbonitride precipitate in various forms, but may exist in a form inherent in one MnS. When a steel containing “substantially MnS containing Ti carbide and / or Ti carbonitride” is cut in a high speed region using a carbide tool, an excellent tool life can be obtained. That is, in order to suppress the generation of “substantial Ti sulfide or / and Ti carbon sulfide” existing alone, Ti (%) / S (%) may be adjusted to 0.25 or less.

(B) The amount of Mn and S is atomic ratio [Mn] / [S] ≧ 1
S is an element that induces cracking during hot working. However, since Mn crystallizes as MnS if the appropriate composition of [Mn] / [S] ≧ 1 is maintained in the atomic ratio, that is, the ratio of the number of atoms (number of moles), even if Ti (%) / Even if S (%) ≦ 0.25, there is no problem in hot workability. Also, within this range, for example, even with the premise of production by continuous casting, since there is no problem in hot workability, it is possible to increase MnS effective for improving machinability by adding a large amount of S. In addition, even if the S content is high, the inclusion forms represented by the formulas (a) and (b) are not impaired.

When [Mn] / [S] <1, if not added so that Ti content exceeds S content, FeS is mainly composed of sulfides in MnS and TiS, improving hot workability. I can't do it. Even when [Mn] / [S] <1, the hot workability can be improved by adding Ti in a range exceeding the amount of S. However, the major product sulfides for sulfide generation tendency of Ti is greater than Mn, if its is not MnS, some have hard Ti sulfide than MnS becomes principal Ti carbosulfide. In this case, as described above, a pseudo lubrication effect due to the soft sulfide is not obtained between the tool and the work material during cutting, and the cutting resistance increases and the finished surface roughness deteriorates. In other words, the amount of Mn and S in terms of atomic ratio [Mn] / [S] ≧ 1 is accompanied by the effect of improving machinability by MnS and at the same time desired conditions for obtaining good hot ductility It is.

(C) Ti (%) / N (%) ≧ 1.35
A major feature of the free-cutting steel of the present invention is that it contains “substantially MnS containing Ti carbide and / or Ti carbonitride”. If Ti (%) / N (%) <1.35, this “substantially MnS containing Ti carbide and / or Ti carbonitride” may not be sufficiently obtained. In this case, most of the added Ti crystallizes as TiN at an early stage of solidification, so that sufficient Ti is formed to form “substantially MnS in which Ti carbide and / or Ti carbonitride is inherent”. This is considered to be impossible to secure. Therefore, it is desirable that Ti (%) / N (%) is 1.35 or more. To obtain “substantially MnS containing Ti carbide or / and Ti carbonitride” more stably, Ti (% ) / N (%) may be 1.5 or more.

2. Reasons for limiting chemical composition Hereinafter, the reasons for limiting the chemical composition in the present invention will be described together with the effects of the respective components.

C: 0.05% to less than 0.20% C is an important element that greatly affects the machinability. When the C content is 0.20% or more, the strength of the steel material is increased and the machinability is deteriorated, so that it is unsuitable as a steel material for use in which machinability is regarded as important. However, if it is less than 0.05%, the steel material becomes too soft, causing flaking during cutting, which in turn promotes tool wear and increases the finished surface roughness. Therefore, the proper content of C is 0.05% to less than 0.20%. In addition, the more suitable range of C amount for obtaining further better machinability is 0.07 to 0.18%.

Mn: 0.4-2.0%
Mn is an important element that forms sulfide inclusions with S and has a great influence on machinability. If it is less than 0.4%, the absolute amount as a sulfide is insufficient and satisfactory machinability cannot be obtained. Further, since Mn is an element that enhances the hardenability of steel, the content may be increased in order to obtain excellent carburizing characteristics. However, since Mn forms MnS together with S, the steel of the present invention containing a large amount of S needs to contain a large amount of Mn. If Mn is added to enhance the carburizing characteristics, the Mn content is added, which is not preferable from the viewpoint of manufacturing cost. Therefore, the upper limit of Mn content was set to 2.0%. If it exceeds 2.0%, the strength of the steel material increases and the cutting resistance increases, and the tool life is reduced. Furthermore, the relationship with the amount of S is important in order to reduce cutting resistance, improve tool life, improve chip disposal, improve finished surface roughness, and improve hot workability. In order to ensure these performances, the Mn content is preferably 0.6 to 1.8%.

S: 0.21 to 1.0%
S is an element effective for improving machinability by forming a sulfide together with Mn. Since the machinability improving effect by MnS is improved according to the amount of production, selection of the 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 exceeds 0.35%, hot workability is deteriorated and S segregation in the central part of the steel ingot is promoted, and cracking is induced during forging. Can be increased to 1.0%. In order to improve machinability by MnS, addition of further S is preferable, and 0.35% or more is more preferable. More to further improve preferred further content of more than 0.40%. However, excessive addition leads to cost increase due to deterioration of yield, so the preferable upper limit of S content is 0.70%.

Ti: 0.002-0.10%
Ti forms Ti carbide and / or Ti carbonitride together with N and C, and these are essential elements that are important for the presence of MnS in the steel. As described above, when “substantially MnS containing Ti carbide and / or Ti carbonitride” is present in the steel material, the tool life in high-speed cutting using a cemented carbide tool is dramatically improved. In order to make such MnS exist, the content of 0.002% or more is necessary. In order to stably disperse this in steel and obtain a good tool life without deteriorating the finished surface roughness. Therefore, it is necessary to consider the balance between the Ti content and the S or N content. On the other hand, if the Ti content exceeds 0.10%, “substantial Ti sulfide or / and Ti carbon sulfide” is present in the steel, so that the finished surface roughness in finish cutting is deteriorated. Therefore, the upper limit of Ti is set to 0.10%. In order to obtain a more stable and excellent finished surface roughness, the Ti content is preferably 0.08% or less. Furthermore, it is more preferable if it is less than 0.03%.

  On the other hand, when the Ti content is less than 0.002%, it is not possible to produce “substantially MnS containing Ti carbide and / or Ti carbonitride” sufficient to improve the tool life. In order to more reliably generate this MnS and improve the life of the carbide tool, it is desirable that the Ti content exceeds 0.01%.

P: 0.001 ~ 0.30%
P increases the hardenability of the steel and increases the strength. In order to obtain the effect, 0.001% or more may be contained. Further, if 0.30% or less, without deteriorating the machinability, only while the hardenability and strength can be ensured, thereby deteriorating the machinability becomes too high strength when the content exceeds 0.30% is Instead, it promotes segregation of the steel ingot and degrades hot workability. Therefore, the content of P is set to 0.001 to 0.30%. A more preferable content is 0.005 to 0.13% for stably maintaining good machinability and strength.

Al: 0.2% or less (may not be added)
Al is used as a powerful deoxidizing element, and may be contained as long as it is 0.2% or less. However, the oxide produced by deoxidation is hard, and if the content exceeds 0.2%, a large amount of hard oxide is produced, and the machinability is deteriorated. Therefore, it is more preferable to make it 0.1% or less. Moreover, when sufficient deoxidation is possible by addition of C or Mn, Al may not be added, and the content thereof may be an impurity level of 0.002% or less.

O (oxygen): 0.001 to 0.03%
The effect of oxygen in the steel of the present invention is not impaired by the deoxidized state, but by containing an appropriate amount of oxygen, the solution of MnS by solid solution in MnS is prevented, and mechanical properties differ. Improve directionality. Furthermore, it is effective for improving machinability, hot workability, and S segregation. However, if it exceeds 0.03%, problems such as inferior deterioration of the refractory during melting will occur. Therefore, the range of oxygen content was made 0.001 to 0.03%. A more preferable range for appropriately obtaining the above effect is 0.0015 to 0.01%.

N: 0.0005-0.02%
N tends to form a hard nitride together with Al and Ti. These nitrides have the effect of refining crystal grains. However, the presence of a large amount of these nitrides facilitates tool wear and degrades machinability. Since Ti is added as an essential component to the steel of the present invention, the smaller the N content, the better. However, in order to obtain the above effect, 0.0005% or more is included. On the other hand, if the N content is excessive, coarse TiN is generated, which may impair the machinability, so the upper limit of N was set to 0.02%. In order to ensure better machinability, the upper limit of the N content is preferably 0.015%. In the present invention, since the machinability is improved by the presence of “substantially MnS containing Ti carbide and / or Ti carbonitride”, this MnS is stably present in the steel. It is preferable that Ti and N satisfy Ti (%) / N (%) ≧ 1.35. As described above, when Ti (%) / N (%) <1.35, most of the added Ti is generated as TiN at an early stage of solidification, and “Ti carbide or / and This is because the substantial MnS in which Ti carbonitride is inherent cannot be obtained stably.

  As described above, the chemical composition composed of the elements whose contents are adjusted respectively, and the inclusion form defined by the formulas (a) and (b) have excellent machinability, hot workability, and finished surface properties. Carbon free-cutting steel can be obtained.

For the above reason, the chemical composition of the low carbon free cutting steel of the present invention [1] contains C, Mn, S, Ti, P, Al, O (oxygen) and N in the above-mentioned range, with the balance being Fe. And that it consists of impurities. Low carbon free cutting steel of the present invention, further the "first 1A group", "the 1B group", one or more components selected from at least one group of "second group" and the "third group" Can be included.

(1) elements of "a 1 A group" and "second group 1B"
Elements of "a 1 A group" and "second group 1B" is without prejudice to the effect obtained by the present invention (1) is an element to further improve the machinability of the steel. Therefore, in order to obtain more excellent machinability, one or more kinds may be contained. For this reason, the chemical composition of the low carbon free-cutting steel according to the present invention [2] is defined as containing at least one element selected from the elements of “Group 1A”, and according to the present invention [4]. The chemical composition of the low carbon free-cutting steel is defined as containing one or more selected from the elements of “Group 1B”.

Se: 0.0005-0.10%, Te: 0.0005% -0.10%
Se and Te generate Mn (S, Se) and Mn (S, Te) together with Mn. These elements are effective elements for improving the machinability since they play a pseudo lubricating effect during cutting, like MnS, and may be contained within the above range for further improving machinability. However, if each content is less than 0.0005%, the effect is poor. On the other hand, when the content of both Se and Te exceeds 0.10%, not only is the effect saturated, but also the workability is deteriorated and the hot workability is deteriorated. In order to achieve both a more stable and excellent hot workability and machinability, it is preferable that each 0.0010 to 0.05%.

Bi: 0.01 to 0.3%, Sn: 0.01 to 0.3%
Bi and Sn have the effect of improving the machinability of steel. This is presumably because, like Pb, it exhibits a lubricating effect during cutting as a low melting point metal inclusion. In order to surely obtain the effect, each content is preferably 0.01% or more. However, when the content exceeds 0.3%, not only the effect is saturated but also hot workability is deteriorated. In order to achieve both a more stable and excellent hot workability and machinability, it is preferable that each from 0.03 to 0.1%. The present invention [2] containing one or more elements selected from the elements of “Group 1A” is a low-carbon free-cutting steel containing no Bi.

Ca: 0.0001-0.01%
Since Ca has a large affinity for S and O (oxygen), it forms sulfides and oxides in steel. In addition, Ca forms a solid solution in MnS to form (Mn, Ca) S. However, since the amount of Ca that forms a solid solution is very small, the effect as MnS is not impaired. Further, the oxide formed by Ca is a low melting point oxide, and is an effective additive element for further improving the machinability in the steel of the present invention. In order to reliably obtain the effect of improving machinability by the addition of Ca, the lower limit of the Ca content is preferably 0.0001%. However, since Ca has a poor addition yield, a large amount of Ca is required to increase the Ca content, which is not preferable from the viewpoint of manufacturing cost. Therefore, the upper limit of the Ca content is set to 0.01%. A more preferred upper limit is 0.005%.

Mg: 0.0001-0.005%
Since Mg also has a large affinity for S and O (oxygen) in steel, it forms sulfides or oxides. A sulfide or oxide containing Mg functions as a crystallization nucleus of MnS and has an effect of suppressing the stretching of MnS. When such an effect is desired, it may be added. In order to sufficiently obtain the effect, the lower limit of the Mg amount is preferably 0.0001% or more. However, since the oxide formed by Mg is hard, if there is too much Mg content, it will become a factor which degrades machinability. Therefore, the upper limit of Mg is set to 0.005%. In addition, the preferred upper limit in order to achieve both excellent machinability and the effect of suppressing the stretching of MnS is 0.002%.

B: 0.0002-0.02%
B is, O (oxygen) some have binds with N, form an oxide or nitride, it may be added as needed because it has an effect of improving the machinability. In order to obtain the effect, 0.0002% or more may be contained. In order to obtain the effect more reliably, 0.0010% or more is desirable. However, if the B content exceeds 0.02%, not only the effect is saturated, but also hot workability is deteriorated.

Rare earth elements: 0.0005-0.02%
Rare earth elements are a group of elements classified as lanthanoids. When this is added, misch metal or the like mainly containing these is usually used. The rare earth element content in the present invention is represented by the total amount of one or more elements in the rare earth elements. The rare earth element forms an oxide together with oxygen and combines with S to form a sulfide, thereby improving machinability. In order to ensure the effect, 0.0005% or more may be contained. However, when the content exceeds 0.02%, the effect is saturated. Moreover, since rare earth elements have a low addition yield, it is not economical to contain a large amount.

(2) Element 2 Group 2 All elements of Group 2 have the effect of increasing the strength of steel. If necessary, one or more of them can be contained. For this reason, the chemical composition of the low carbon free-cutting steel according to the present invention [3] and the present invention [4] is specified to contain at least one selected from the elements of the “second group”.

Cu: 0.01-1.0%
Cu has the effect of improving the strength of steel by precipitation strengthening. In order to obtain this effect, the content must be 0.01% or more. To obtain a more reliable effect thereof is preferably set to the 0.1% or more. However, if the content exceeds 1.0%, not only the hot workability is deteriorated but also the above effect is saturated by the coarsening of the Cu precipitates, and the machinability is also lowered.

Ni: 0.01-2.0%
Ni has the effect of improving the strength of the steel by solid solution strengthening. In order to reliably obtain this effect, the content is preferably set to 0.01% or more. However, if it exceeds 2.0%, machinability is deteriorated and hot workability is also deteriorated.

Mo: 0.01-0.5%
Mo is an element that can enhance the hardenability, but it is more expensive than Si or Cr if a significant amount of Mo is added to obtain the same effect as the addition of Si or Cr in order to improve carburizing characteristics. However, there is a drawback that the manufacturing cost increases. However, Mo has the effect of refining the structure and improving toughness. If these effects are desired, they may be added. In order to reliably obtain the effect, the content is desirably 0.01% or more. However, if it exceeds 0.5%, not only will the effect be saturated, but the manufacturing cost of steel will increase.

V: 0.005-0.5%
V precipitates as fine nitrides and carbonitrides and improves the strength of the steel. The effect can be obtained with a content of 0.005% or more. However, when the effect is desired to be obtained more reliably, the content is preferably 0.01% or more. However, if it exceeds 0.5%, not only the above effects are saturated, but also nitrides and carbides are generated excessively, leading to a decrease in machinability.

Nb: 0.005-0.5%
Nb precipitates as fine nitrides and carbonitrides and improves the strength of the steel. The effect can be obtained with a content of 0.005% or more. However, when the effect is desired to be obtained more reliably, the content is preferably 0.01% or more. However, if it exceeds 0.5%, not only the above effects are saturated, but also nitrides and carbides are excessively generated, which leads to a decrease in machinability and is not economical.

(3) Group 3 element The group 3 element is an element that may contain one or both of the elements within the following range when it is desired to enhance the carburizing characteristics of the steel. For this reason, the chemical composition of the low-carbon free-cutting steel according to the present invention [5] is defined as containing one or two selected from the elements of “Group 3”.

Si: 0.1-2.0%
In the low carbon free cutting steel according to the present invention [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 low carbon free cutting steel according to the present invention [1] to [4] , Si may be added as a deoxidizing element in order to make oxygen in the steel an appropriate amount. There is no need to leave it, and Si remaining in the steel is an impurity and is less than 0.1%.

  Moreover, Si has the effect of improving the hardenability of the steel while improving the strength of the steel by dissolving in the ferrite. By improving the hardenability of steel, it is possible to improve the carburizing characteristics desired for automobile parts. Only in that case, Si may be contained by 0.1% or more. When it is desired to improve the carburizing characteristics more reliably, a content exceeding 0.6% is desirable. However, if it exceeds 2.0%, the hot workability deteriorates and the machinability is adversely affected, for example, the cutting resistance increases because the ferrite phase is solid-solution strengthened. Even if the impurity level is less than 0.1%, the amount of oxygen in the steel can be in an appropriate range by appropriate addition of C, Mn, and Al.

Cr: 0.03-1.0%
Cr is an element that can improve carburizing characteristics by adding a small amount in order to enhance the hardenability of steel. Steel containing Cr has improved carburizing characteristics, has a high carburized layer hardness after carburizing treatment, and can increase the effective hardening depth. In order to obtain the effect, 0.03% or more of Cr may be contained. Moreover, when it is desired to improve the carburizing characteristics more reliably, a content exceeding 0.05% is desirable. However, if the Cr content exceeds 1.0%, not only the machinability is deteriorated, but also the production cost increases.

  When the Si or / and Cr are contained, a steel having excellent carburizing characteristics in addition to excellent machinability and hot workability can be obtained.

1. Preparation of Samples 150 kg steel ingots (diameter: about 220 mm) having various compositions shown in Tables 1 and 2 were prepared using a high-frequency heating induction furnace. Table 1 shows the steel of the present invention, and Table 2 shows the conventional steel or the comparative steel. In order to stably produce these steel ingots , “substantially MnS containing Ti carbide and / or Ti carbonitride”, after heating to a high temperature of 1250 ° C, hold for 2 hours or more and 1000 ° C or more The forging was finished with, and a round bar with a diameter of 65mm was obtained by air cooling. Thereafter, this forged material was heated to 950 ° C., held for 1 hour, and then subjected to a normalizing process in which it was air-cooled. In addition, the steel No. 51 to 53 of the comparative example is inferior in hot workability, cracks occurred during forging and could not be a forged material, so the following investigation was not conducted .

2. Investigation of Inclusion Forms Inclusions observed in a cross section parallel to the rolling direction are often elongated in the processing direction or unspecified shapes. When investigating the number and area of inclusions, a specimen for micro observation was cut out from the longitudinal section of the Df / 4 (Df : diameter of forged material) part of the forged material, embedded in resin, and then mirror polished. The photograph was taken with an optical microscope of 400 times, and the number and area of the inclusions were obtained by a technique such as image analysis. At that time, the diameter when converted into a circle having the same area, that is, a circle equivalent diameter of 1 μm or more was used. The reason why the equivalent circle diameter is limited to 1 μm or more is that, as described above, inclusions less than 1 μm have little effect on machinability.

The composition of these inclusions was confirmed as follows. That is, as described above, a micro test piece cut out from the longitudinal cross-sectional direction of the Df / 4 (Df : diameter of forged material) part of the forged material is embedded in a resin, and then mirror-polished to perform EPMA (electron beam). Surface analysis and quantitative analysis were performed using a microanalyzer) or EDX (energy dispersive X-ray analyzer). The observation magnification at this time may be selected within a range not exceeding 10000 times, and at this observation magnification, MnS and Ti carbide or / and Ti carbonitride are clearly phase-separated and observed in one inclusion, and MnS The inclusion that was confirmed to have an area ratio of 50% or more is “substantially MnS containing Ti carbide and / or Ti carbonitride”. From the results observed in this manner, individual “substantially free MnS containing Ti carbide and / or Ti carbonitride” having an equivalent circle diameter of 1 μm or more and “substantially free of Ti carbide and Ti carbonitride” after having determined the area of MnS ", it calculates the sum of the areas of these inclusions in the rolling direction cross-section 1 mm ^2, further, in order to calculate the total area occupied by all the inclusions in the rolling direction cross-section 1 mm ² ( A + B) / C was determined.

Based on the above results, the number of “substantially MnS containing Ti carbide and / or Ti carbonitride” was measured, and “○” was indicated for steels with an average number of 5 or more per 1 mm ² in the rolling direction. did. On the other hand, “x” was assigned to the steel in which “substantially MnS containing Ti carbide and / or Ti carbonitride” was less than five. In addition, steel Nos. 35 to 37 of comparative examples shown in Table 2 are Pb free cutting steel and S free cutting steel not containing Ti, and substantially “substantially containing Ti carbide and / or Ti carbonitride. Since there was no “MnS”, these calculations were not performed.

3. Machinability test In the machinability test, a test for investigating the tool life and the finished surface roughness was performed using a round bar obtained by cutting the forged material to a diameter of 60 mm. Tool life test using a P20 or a carbide tool coated are specified in JIS not subjected, Cutting speed: 150 meters / min, Feed: 0.10 mm / rev, Depth of cut: at 2.0 mm, a dry condition Turning was performed, and the average flank wear was measured 30 minutes after the start of cutting. For the specimens whose average flank wear amount reached 200 μm or more within 30 minutes, the arrival time and the average flank wear amount (VB) at that time were measured.

The evaluation was performed using the time for the average flank wear (VB) to reach 100 μm as a guide for the tool life. For samples with insufficient wear due to excellent wear resistance during the test and extremely low wear progression rate, the time required for the average flank wear to reach 100 μm was calculated by regression from the turning time-tool wear curve. did. Also, chip disposability is typical was 200 taken out at this time discharged chips were evaluated by calculating the number per unit Mass on having measured its mass.

Since the finished surface roughness is evaluated by the surface roughness after finish cutting, the surface of the work material after cutting under the following conditions was evaluated using a stylus roughness meter. Cutting conditions are JIS K class carbide tools with TiAlN multilayer coating, cutting speed : 100m / min, feed : 0.05mm / rev, cutting depth : 0.5mm, and water-soluble emulsion type lubricant is used. Turning was performed under wet conditions. For the test piece after cutting the test steel for 1 minute under these conditions, move the stylus in the axial direction with a stylus roughness meter to measure the average finished surface roughness (Ra). Was evaluated.

4). Hot workability test In order to simulate the manufacturing conditions with continuous casting equipment, hot workability is a Di / 8 (Di : diameter of steel ingot) close to the surface of a 150 kg steel ingot produced by the same method as described above. A high temperature tensile test piece with a diameter of 10 mm and a length of 130 mm from the height of the steel ingot centered on the position, heated to 1250 ° C by direct energization with a fixed interval of 110 mm, held for 5 minutes, then 10 ° C / after holding for 10 seconds and cooled to 1100 ° C. at a sec cooling rate, strain rate: tensile test was carried out at 10 ^-3 / s. At that time, the hot workability was evaluated by measuring the squeezing of the fractured portion.

5. Carburization test Carburization test was conducted as follows. That is, a cylindrical steel material having a diameter of 24 mm and a length of 50 mm was used for the test piece. This was taken from the R / 2 position of the normalizing material with a diameter of 65 mm. The test piece was heated to 900 ° C. and carburized, and then diffused at 850 ° C. At this time, the carbon potential (C.P.) value during carburization is 0.8%, the treatment time is 75 minutes, and the C.P. P. The value is 0.7% and the processing time is 20 minutes. The test piece after the carburizing treatment was quenched in oil at 80 ° C. Finally, the test piece was heated to 190 ° C. and kept at this temperature for 60 minutes for tempering treatment. The evaluation method of carburizing property is as follows.

  Measure the Vickers hardness distribution from the surface to the inside at a position 25mm from the end of the carburized and tempered specimen (namely, the center in the length direction), and obtain the effective hardening depth of Hv400. It was judged whether the value was larger or smaller than the conventional lead composite free-cutting steel. The conventional lead composite free-cutting steel is steel No. 35 in Table 2, and this effective hardening depth was 0.25 mm. Carburizability is evaluated in the case where the effective hardening depth is ± 0.05 mm with respect to steel No. 35, that is, 0.20 to 0.30 mm, and is equivalent, and less than 0.20 mm is inferior. When exceeded, it was determined to be excellent. The results are shown in Tables 3 and 4 as ◯, × and ◎. An equivalent case is “◯”, an inferior case is “×”, and an excellent case is “◎”.

  Table 3 and Table 4 collectively show the above test results. FIG. 2 shows the relationship between the formula (A + B) / C and the finished surface roughness, FIG. 3 shows the relationship between the finished surface roughness and the tool life, and FIG. 4 shows the relationship between the chip disposal and the tool life.

Steels No. 35 and 36 in Table 2 are composite free-cutting steels, and steel No. 37 is sulfur free-cutting steels, which have been considered to have the best machinability so far. As is apparent from Tables 3, 4 and 2 and 3, the steel of the present invention is excellent in both tool life and finished surface roughness. Furthermore, the inventive steel Nos. 1 to 19 and 21 to 34 have excellent hot workability, and as shown in Table 3, the drawing by high temperature tensile test simulating practical production by continuous casting equipment etc. It is equivalent to or better than free-cutting steel and sulfur free-cutting steel, and there is no problem.

  Steel Nos. 12 to 17 in Table 1 contain at least one of Si and Cr within a specified range in order to improve carburization. It can be seen that these steels exhibit particularly excellent carburizing characteristics among the steels of the present invention. On the other hand, things such as steel Nos. 35 to 55 that are out of the inclusion state or chemical composition defined in the present invention are tool life, finished surface roughness, chip disposal, hot working At least one of the properties is inferior to the steel of the present invention.

  The free-cutting steel of the present invention has machinability equivalent to or better than conventional Pb free-cutting steel and composite free-cutting steel even though it does not contain Pb, and also has excellent finished surface properties after cutting. . Further, those containing Si or / and Cr also have excellent carburizing characteristics. Furthermore, this steel is excellent in hot workability and can be manufactured at low cost by a continuous casting method. Since Pb is not included, there is no risk of environmental pollution. Therefore, the free-cutting steel of the present invention is a steel material that is extremely suitable as a material for various machine parts.

It is a figure which shows typically the form of the inclusion of this invention steel and comparative steel. It is a figure showing the relationship between (A + B) / C and average finishing surface roughness in this invention steel and comparative steel. It is a figure showing the relationship between the average finishing surface roughness and tool life in this invention steel and a comparison steel. It is a figure showing the relationship between the chip processability in this invention steel and comparative steel, and a tool life.

Claims (5)

In mass%, C: 0.05% to less than 0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, P: 0.001-0.30%, Al: 0.2% or less, O (oxygen ): 0.001 to 0.03% and N: 0.0005 to 0.02%, with the balance being steel composed of Fe and impurities, the inclusions contained in the steel satisfy the following formulas (A) and (B): Low carbon free cutting steel.
(A + B) /C≧0.8
N _A ≧ 5... B Here, the meanings of A, B, C and N _A are as follows.
A : Total area occupied by substantial MnS in which Ti carbide or / and Ti carbonitride is contained in inclusions having an equivalent circle diameter of 1 μm or more 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 present among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction.
C : Total area occupied by all inclusions having an equivalent circle 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 in which Ti carbide and / or Ti carbonitride exists among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction. In mass%, C: 0.05% to less than 0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, P: 0.001-0.30%, Al: 0.2% or less, O (oxygen ): 0.001-0.03% and N: 0.0005-0.02%, and Se: 0.0005-0.10%, Te: 0.0005-0.10% , Sn : 0.01-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001- 0.005% B: from .0002 to 0.02% and rare earth elements: containing one or more kinds selected from the group consisting of 0.0005 to 0.02%, the balance being a steel consisting of Fe and impurities, contained in the steel A low-carbon free-cutting steel characterized in that inclusions satisfy the following formulas A and B:
(A + B) /C≧0.8
N _A ≧ 5... B Here, the meanings of A, B, C and N _A are as follows.
A : Total area occupied by substantial MnS in which Ti carbide or / and Ti carbonitride is contained in inclusions having an equivalent circle diameter of 1 μm or more 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 present among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction.
C : Total area occupied by all inclusions having an equivalent circle 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 in which Ti carbide and / or Ti carbonitride exists among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction. In mass%, C: 0.05% to less than 0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, P: 0.001-0.30%, Al: 0.2% or less, O (oxygen ): 0.001 to 0.03% and N: 0.0005 to 0.02%, and 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 containing one or more kinds selected from the group consisting of 0.5%, the balance being a steel consisting of Fe and impurities, and wherein the inclusions contained in the steel satisfies the following i-type and b-type Low carbon free cutting steel.
(A + B) /C≧0.8
N _A ≧ 5... B Here, the meanings of A, B, C and N _A are as follows.
A : Total area occupied by substantial MnS in which Ti carbide or / and Ti carbonitride is contained in inclusions having an equivalent circle diameter of 1 μm or more 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 present among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction.
C : Total area occupied by all inclusions having an equivalent circle 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 in which Ti carbide and / or Ti carbonitride exists among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction. In mass%, C: 0.05% to less than 0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, P: 0.001-0.30%, Al: 0.2% or less, O (oxygen ): 0.001-0.03% and N: 0.0005-0.02%, and Se: 0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn: 0.01-0.3%, Ca: 0.0001- One or more selected from the group consisting of 0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02% and rare earth elements: 0.0005-0.02%, Cu: 0.01-1.0%, Ni: 0.01-2.0%, Mo : 0.01 to 0.5% V: 0.005 to 0.5% and Nb: containing one or more kinds selected from the group consisting of 0.005 to 0.5%, the balance being a steel consisting of Fe and impurities, included in steel A low-carbon free-cutting steel characterized in that inclusions to be satisfied satisfy the following formulas (a) and (b)
(A + B) /C≧0.8
N _A ≧ 5... B Here, the meanings of A, B, C and N _A are as follows.
A : Total area occupied by substantial MnS in which Ti carbide or / and Ti carbonitride is contained in inclusions having an equivalent circle diameter of 1 μm or more 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 present among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction.
C : Total area occupied by all inclusions having an equivalent circle 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 in which Ti carbide and / or Ti carbonitride exists among inclusions having an equivalent circle diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction.   The low-carbon free-cutting steel according to any one of claims 1 to 4, which contains one or two of Si: 0.1 to 2.0 mass% and Cr: 0.03 to 1.0 mass%.

Images