WO2016199843A1 - Free-cutting steel - Google Patents

Abstract

Provided is a free-cutting steel having excellent machinability (surface roughness, tool service life, chip processability) and rusting characteristics. The free-cutting steel according to the present invention has a chemical composition that satisfies formula (1) and contains, in mass%, C: 0.005 to 0.150%, Si: less than 0.010%, Mn: 1.02 to 2.00%, P: 0.010 to 0.200%, S:0.350 to 0.600%, Pb: 0.010 to 0.100%, N: 0.004 to 0.015%, O: 0.0080 to 0.0250%, Al: 0 to 0.003%, one or more species selected from the group comprising Ca, Mg, and Zr: total of 0 to 0.0005%, and B: 0 to 0.0200%, with the remainder being composed of Fe and impurities. (1) Mn/S ≥ 2.90 where the element symbols in formula (1) are assigned the content (mass%) of the corresponding element.

Description

Free-cutting steel

The present invention relates to free-cutting steel, and more particularly to free-cutting steel manufactured into parts by cutting.

Surface quality is important for parts used as small hydraulic parts, OA equipment shafts, hinge shafts, etc. These parts are usually subjected to cutting in order to improve shape accuracy and surface properties, and then surface finishing treatment such as polishing or plating treatment is performed as necessary. Free cutting steel is used for parts for such applications (hereinafter referred to as cutting parts) in order to ensure surface quality. Among free-cutting steels, free-cutting steels based on low carbon steel are defined in, for example, Japan Industrial Standards Committee, Standard Number: JIS G4804 (2008) (Non-patent Document 1). For the cutting parts that require the above-mentioned surface quality, among the free cutting steels specified in JIS G4804 (2008), free cutting steels containing a large amount of Pb, represented by SUM24L, are used.

Free-cutting steel containing a large amount of Pb, such as SUM24L, is said to be excellent in ease of cutting of steel, that is, machinability. In particular, cutting surface properties (surface roughness) among machinability are often determined by a combination of steel and a tool, and steel materials containing a large amount of Pb are considered to have excellent surface roughness.

However, Pb has a high possibility of being designated as an environmentally hazardous substance because it affects the environment. When Pb is designated as an environmentally hazardous substance, its use is restricted. In this case, there is a possibility that free-cutting steel containing a large amount of Pb cannot be used.

A free-cutting steel that replaces a free-cutting steel containing a large amount of Pb has been proposed in JP-A-2004-27297 (Patent Document 1). The free-cutting steel disclosed in Patent Document 1 suppresses the Pb content, contains a large amount of S instead of Pb, and controls the shape of inclusions such as MnS. Thereby, the machinability superior to the conventional lead free-cutting steel is ensured.

JP 2004-27297 A

In the manufacture of cutting parts, the surface properties of the cutting surface such as surface roughness are important, and if it can be finished only by cutting, it is preferable in terms of both performance and manufacturing cost. Even if it is necessary to carry out a polishing process following the cutting process, if the surface roughness of the cutting surface is large or wrinkles remain, it is difficult to finish the polishing process. Therefore, it is required to keep the surface roughness of the cutting surface small.

Furthermore, when manufacturing a large number of cutting parts with an automated manufacturing facility such as 1000 or more per day, excellent chip disposal is required. It is preferable that the chips discharged with the cutting are divided into small pieces and discharged. When the chips remain connected for a long time, the chips are entangled with the cutting part, and wrinkles are likely to occur on the surface of the cutting part. When the chips are entangled with the cutting part, it is further necessary to temporarily stop the production line in order to remove the entangled chips. In this case, unmanned manufacturing becomes difficult, and personnel assignment for monitoring is required. As described above, the chip disposability affects both the quality of the cut part and the manufacturing cost.

For cutting parts used in various precision equipment such as electronic parts, hydraulic parts and sliding parts, the outermost surface of the parts is plated for the purpose of suppressing rust generation during use and improving wear resistance. In some cases, the surface of the component may be subjected to a treatment for increasing the affinity with the resin.

The part after cutting (cutting part) may wait for a long time in the bucket or the like until the next process after cutting. For example, when cutting in Japan and the next process is processed in another factory in another country, a period of several days to several months may elapse after cutting until the next process is performed. In this case, suppression of the occurrence of cutting parts is required.

An object of the present invention is to provide a free-cutting steel excellent in machinability (surface roughness, tool life, chip disposal) and cracking characteristics.

The free-cutting steel according to the present embodiment is, in mass%, C: 0.005 to 0.150%, Si: less than 0.010%, Mn: 1.02 to 2.00%, P: 0.010 to 0.200%, S: 0.350 to 0.600%, Pb: 0.010 to 0.100%, N: 0.004 to 0.015%, O: 0.0080 to 0.0250%, Al : 0 to 0.003%, one or more selected from the group consisting of Ca, Mg and Zr: 0 to 0.0005% in total, and B: 0 to 0.0200%, the balance being Fe And a chemical composition satisfying the formula (1).
Mn / S ≧ 2.90 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

The free-cutting steel according to the present invention is excellent in machinability (surface roughness, tool life, chip disposal) and cracking characteristics.

FIG. 1A is a schematic diagram showing an S distribution in an observation surface obtained by EPMA analysis. FIG. 1B is a schematic diagram showing a Pb distribution in the same observation surface as that of FIG. 1A obtained by EPMA analysis. FIG. 1C is a schematic diagram of an image obtained by combining FIGS. 1A and 1B. FIG. 2 is a schematic diagram for explaining a criterion for determining whether or not adjacent inclusions are regarded as one inclusion. FIG. 3 is a cross-sectional view of the cast material. FIG. 4 is a schematic diagram of a cutting test machine for explaining a cutting test. FIG. 5A is a perspective view of chips. FIG. 5B is a plan photograph of chips.

The present inventors investigated and examined the machinability and cracking characteristics of free-cutting steel, and obtained the following knowledge.

Mn and Pb in steel form MnS inclusions, Pb inclusions, and composite inclusions containing MnS inclusions and Pb. In this specification, MnS inclusions contain Mn and S, and the balance means inclusions composed of impurities. The Pb inclusion means an inclusion composed of Pb and impurities. The composite inclusions contain MnS and Pb, and the balance means inclusions made of impurities. MnS and Pb may be adjacent to each other to form composite inclusions, or Pb may be dissolved in MnS to form composite inclusions. In the following description, the generic name of MnS inclusions, Pb inclusions, and composite inclusions is referred to as “specific inclusions”.

MnS inclusions are conventionally known as inclusions that enhance machinability. On the other hand, the melting point of the Pb inclusion is lower than the melting point of the MnS inclusion. Therefore, the Pb inclusion exhibits a lubricating action during cutting, and as a result, machinability is increased.

Furthermore, it is considered that the composite inclusions have higher machinability than the MnS inclusions and the Pb inclusions alone. When a crack occurs around the composite inclusion, liquefied Pb enters the opened crack. Thereby, progress of a crack is accelerated | stimulated and machinability increases. Therefore, if specific inclusions including composite inclusions are generated, machinability is enhanced.

Pb is more mobile in the liquid phase than in the solid phase. Therefore, composite inclusions cannot be generated from MnS that precipitates after solidification, but are generated when Pb adheres to MnS crystallized in molten steel. Therefore, in order to produce many composite inclusions, it is preferable that MnS can be produced by crystallization.

On the other hand, these specific inclusions are the starting point of the trap. The occurrence of wrinkles does not depend on the size of specific inclusions, but depends on the number of specific inclusions. As described above, MnS inclusions may crystallize in molten steel or may precipitate after solidification. The size of MnS inclusions due to crystallization is larger than that of MnS inclusions due to precipitation. Therefore, when MnS inclusions are generated by precipitation, the number of MnS inclusions is significantly greater than when MnS inclusions are generated by crystallization. Therefore, in order to reduce the starting point of wrinkles while obtaining machinability, it is preferable to generate MnS inclusions by crystallization as much as possible.

In order to produce a large amount of MnS inclusions by crystallization, a sufficient Mn content relative to the S content in the steel is required. Specifically, the Mn content and the S content satisfy the following formula (1).
Mn / S ≧ 2.90 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

Defined as F1 = Mn / S. If the Mn content is small relative to the S content, MnS is difficult to crystallize in the molten steel. If F1 is 2.90 or more, the Mn content is sufficiently larger than the S content. In this case, MnS is easily crystallized in the liquid phase. Therefore, a sufficient amount of complex inclusions for obtaining machinability are generated, and a sufficient amount of specific inclusions including the complex inclusions can be obtained. As a result, excellent machinability is obtained. Furthermore, since it can suppress that the number of fine MnS inclusions increases excessively, the starting point of soot generation can be suppressed. As a result, the firing characteristics can be improved.

The free-cutting steel according to the present invention completed based on the above knowledge is, in mass%, C: 0.005 to 0.150%, Si: less than 0.010%, Mn: 1.02 to 2.00%, P: 0.010 to 0.200%, S: 0.350 to 0.600%, Pb: 0.010 to 0.100%, N: 0.004 to 0.015%, O: 0.0080 to 0.0250%, Al: 0 to 0.003%, one or more selected from the group consisting of Ca, Mg and Zr: 0 to 0.0005% in total, and B: 0 to 0.0200% And the balance is composed of Fe and impurities and has a chemical composition satisfying the formula (1).
Mn / S ≧ 2.90 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

The chemical composition of the above free-cutting steel may contain Al: 0.001 to 0.003%. The chemical composition of the free-cutting steel may include one or more selected from the group consisting of Ca, Mg, and Zr in a total content of 0.0001 to 0.0005%. The chemical composition of the free-cutting steel may include B: 0.0005 to 0.0200%.

Preferably, in the above-described free-cutting steel, the ratio of the number of Pb inclusions to the total number of specific inclusions is 17% or less.

In this case, the starting characteristics are further enhanced.

Hereinafter, the free-cutting steel according to the present invention will be described in detail. The chemical composition of the free-cutting steel according to the invention contains the following elements: Unless otherwise indicated,% regarding an element means the mass%.

[Chemical composition]
The chemical composition of the steel for machine structural use of this embodiment contains the following elements.

C: 0.005 to 0.150%
Carbon (C) increases the basic strength of steel. Free-cutting steel manufactured for cutting parts is usually cut after being subjected to processing such as wire drawing. If the C content is 0.005% or more, the strength of the free-cutting steel will be suitable for the tool life and surface roughness of cutting after wire drawing. If the C content is less than 0.005%, the steel becomes soft and wire drawing and cutting become difficult. In particular, flaking is likely to occur in cutting. On the other hand, if the C content exceeds 0.150%, the steel hardens and cold workability deteriorates, and further, tool wear becomes severe in cutting after wire drawing. Therefore, the C content is 0.005 to 0.150%. A preferable lower limit of the C content is 0.06%. The upper limit with preferable C content is 0.120%. If the C content is 0.06 to 0.120%, the rate of occurrence of rolling wrinkles decreases.

Si: less than 0.010% Silicon (Si) usually deoxidizes steel. However, in the present invention, in order to control the shape of MnS, it is necessary to leave some oxygen (O) in the steel. If the Si content is too high, the O concentration in the steel will be too low. If the Si content is too high, a hard oxide such as SiO ₂ will remain in the steel and the machinability will be reduced. Furthermore, the amount of oxygen in MnS is reduced by the formation of hard oxide. In this case, MnS is stretched by rolling and wire drawing, and MnS having a large aspect ratio is generated. MnS having a large aspect ratio decreases the surface roughness (surface properties). Accordingly, the Si content is less than 0.010%.

Mn: 1.02 to 2.00%
Manganese (Mn) deoxidizes steel, but its deoxidizing power is weaker than other deoxidizing elements such as Si and Al, so that a large amount is allowed. Mn further increases the strength of the steel. Further, Mn combines with S in the steel to form MnS and enhances machinability. In free-cutting steel containing a large amount of S, MnS does not crystallize unless Mn is also contained in a large amount.

In the present invention, MnS is crystallized in the steel in the steel making process, and therefore sufficient Mn is contained with respect to the S content. When the Mn content is less than 1.02%, the Mn content is not sufficient with respect to the S content, so that crystallization of MnS is delayed. In this case, MnS precipitated after solidification increases. When MnS is crystallized, the compounding ratio with Pb is higher than when it is precipitated, and the machinability is improved. Therefore, when the Mn content is less than 1.02%, the machinability is lowered. On the other hand, if the Mn content exceeds 2.00%, the hardenability of the steel becomes too high and the steel becomes brittle. Therefore, surface flaws are likely to occur due to rolling or wire drawing. Therefore, the Mn content is 1.02 to 2.00%. The minimum with preferable Mn content is 1.10%, More preferably, it is 1.30%.

P: 0.010 to 0.200%
Phosphorus (P) embrittles steel and improves machinability. In the present invention, since the C content for increasing the strength is low, P further increases the strength of the steel together with Mn. If the P content is less than 0.010%, the machinability is low and the surface roughness is poor. Furthermore, the strength of the steel becomes insufficient. On the other hand, if the P content exceeds 0.200%, the hot ductility of the steel is lowered, and rolling defects are liable to occur, which impairs the production stability. Therefore, the P content is 0.010 to 0.200%. A preferable lower limit of the P content is 0.050%. The upper limit with preferable P content is 0.100%.

S: 0.350 to 0.600%
Sulfur (S) forms MnS in steel and improves machinability. If the S content is less than 0.350%, sufficient machinability cannot be obtained. On the other hand, if the S content exceeds 0.600%, grain boundary embrittlement tends to occur due to grain boundary segregation. Therefore, the S content is 0.350 to 0.600%. A preferable lower limit of the S content is 0.400%. The upper limit with preferable S content is 0.550%. When giving priority to machinability, the preferable range of the S content is 0.450 to 0.500%. When giving priority to manufacturability, the preferable range of the S content is 0.400 to 0.450%.

Pb: 0.010 to 0.100%
Lead (Pb) increases the machinability of steel. If the Pb content is less than 0.010%, the machinability is insufficient. On the other hand, the Pb content is. If it exceeds 0.100%, the steel becomes brittle, the productivity is lowered, and rolling defects are likely to occur. If the Pb content exceeds 0.100%, more Pb inclusions (lead grains) exist alone, and the cracking characteristics are deteriorated. Therefore, the Pb content is 0.010 to 0.100%. A preferable lower limit of the Pb content is 0.020%. The upper limit with preferable Pb content is 0.040%. If the Pb content is 0.020 to 0.040%, the deterioration of the wrinkle characteristics is suppressed, and further, machinability is further enhanced by combining Pb with MnS.

N: 0.004 to 0.015%
Nitrogen (N) increases the strength of the steel and increases the cutting resistance in cutting, but improves the surface roughness. If the N content is 0.004% or more, the above effect can be obtained. If the N content is less than 0.004%, the manufacturing cost becomes too high, making industrial production difficult. On the other hand, if the N content exceeds 0.015%, the steel material becomes brittle and surface flaws are likely to occur during rolling and wire drawing. Therefore, the N content is 0.004 to 0.015%. The preferable lower limit of the N content when priority is given to machinability is 0.007%, and the preferable upper limit of the N content when priority is given to manufacturability is 0.012%. The N content here means the total N (TN) content.

In the case of containing B described later, N combines with B to form BN, and the machinability of steel is further enhanced. In that case, the N content is preferably equal to the B content.

O: 0.0080 to 0.0250%
Oxygen (O) generates an oxide. O is also contained in sulfides and controls the form of sulfides. Specifically, O is contained in MnS and suppresses stretching of MnS during rolling and stretching. If the O content is less than 0.0080%, the oxygen content in MnS is low, so MnS is stretched during rolling and wire drawing, and the aspect ratio is increased. On the other hand, if the O content exceeds 0.0250%, defects are likely to occur in the cast structure. Furthermore, a large amount of oxide is generated, and the machinability of the steel is lowered. The generation of a large amount of oxide further promotes the melting loss of the refractories in the production line and reduces the production stability. Therefore, the O content is 0.0080 to 0.0250%. The minimum with preferable O content is 0.0120%, More preferably, it is 0.0150%. The upper limit with preferable O content is 0.0200%, More preferably, it is 0.0180%. Here, the O content means the total O (TO) content.

The balance of the chemical composition of the free-cutting steel according to the present invention consists of Fe and impurities. Here, an impurity means what is mixed from the ore as a raw material, scrap, or a manufacturing environment when manufacturing steel materials industrially.

[Arbitrary elements]
The chemical composition of the free-cutting steel according to the invention may further contain Al.

Al: 0 to 0.003%
Aluminum (Al) is an optional element. Al is a deoxidizing element of steel. However, in the present invention, oxygen needs to remain to some extent in the steel in order to control the shape of MnS. If the Al content exceeds 0.003%, the amount of oxygen in MnS becomes too low, MnS is stretched by rolling and wire drawing, the aspect ratio is increased, and the surface roughness is lowered. If the Al content exceeds 0.003%, further, alumina-based inclusions (hard oxides) remain in the steel, thereby reducing the machinability of the steel. Therefore, the Al content is 0 to 0.003%. A preferred lower limit of the Al content is 0.001%. The Al content here means the total Al (t-Al) content.

The chemical composition of the free-cutting steel according to the present invention may further contain one or more selected from the group consisting of Ca, Mg and Zr. These elements are arbitrary elements.

One or more selected from the group consisting of Ca, Mg and Zr: 0 to 0.0005% in total
Calcium (Ca) magnesium (Mg) and zirconium (Zr) both increase the workability of steel by controlling the form of inclusions and precipitates. Hereinafter, each element will be described.

Ca produces composite sulfide (Mn, Ca) S with CaS and MnS, and suppresses the stretching of MnS during rolling and wire drawing. This increases the workability and machinability of the steel. However, if the Ca content exceeds 0.0005%, the oxygen content in MnS decreases, and MnS having a large aspect ratio is formed by rolling and wire drawing. Therefore, the upper limit of the Ca content is 0.0005%, more preferably 0.0003%. A preferable lower limit of the Ca content is 0.0001%.

Magnesium (Mg) generates composite sulfide (Mn, Mg) S with MgS and MnS, and suppresses the extension of MnS. This increases the workability and machinability of the steel. Mg further generates fine Mg oxides and becomes a nucleus for the formation of sulfides such as MnS. Thereby, the number of large MnS inclusions can be increased. On the other hand, if the Mg content exceeds 0.0005%, the oxygen content in MnS decreases and MnS is easily stretched. Further, a large amount of oxide is generated in the molten steel and adheres to the refractory or adheres to the nozzle and causes nozzle clogging. Therefore, the upper limit of Mg content is 0.0005%. A preferable lower limit of the Mg content is 0.0001%.

Zirconium (Zr) generates oxides, sulfides and nitrides, and controls the form of inclusions and precipitates. The preferable lower limit of the Zr content for controlling the form of inclusions to improve the workability and machinability of the steel is 0.0002%. On the other hand, if the Zr content is too high, the oxygen content in MnS decreases, and MnS is easily stretched. Further, a large amount of hard oxide is generated, and machinability is lowered. Therefore, the upper limit of the Zr content is 0.0005%, more preferably 0.0003%. A preferable lower limit of the Zr content is 0.0001%.

In the present invention, the total of one or more selected from the group consisting of Ca, Mg and Zr is 0 to 0.0005%. In this case, as described above, the form of precipitates and inclusions is controlled to improve the workability of steel. Preferably, 2 or more types selected from the group which consists of Ca, Mg, and Zr are contained. In this case, many spherical sulfides can be dispersed, and the workability of steel is further enhanced. A more preferable upper limit of the total content of these elements is 0.0003%. A preferable lower limit of the total content of these elements is 0.0001%.

The free-cutting steel of this embodiment may further contain B. B is an optional element.

B: 0 to 0.0200%
Boron (B) combines with N to form BN and enhances the machinability of the steel. When BN exists at the grain boundary, the machinability is improved by embrittlement of the grain boundary. When BN is present in the vicinity of MnS, the embrittlement effect due to the stress concentration of MnS is promoted by suppressing the deformation of sulfide, and the machinability of steel is increased. However, if the B content exceeds 0.0200%, the B oxide is generated and the machinability of the steel is lowered, or the B oxide reacts with the refractory to promote melting damage. Therefore, the B content is 0 to 0.0200%. A preferable lower limit of the B content for further effectively improving the machinability is 0.0005%, and more preferably 0.0008%. The upper limit with preferable B content is 0.0150%. In order to further increase the machinability of steel by increasing the amount of BN produced, it is preferable to satisfy 0.7 ≦ B / N ≦ 1.8.

[Regarding Formula (1)]
The chemical composition of the above-described free-cutting steel further satisfies the formula (1).
Mn / S ≧ 2.90 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

Defined as F1 = Mn / S. If the Mn content is small relative to the S content, MnS is difficult to crystallize in the molten steel. As a result, the solid solution S after solidification tends to remain at the grain boundaries and often deteriorates the hot ductility. In this case, MnS precipitates after solidification. Precipitated MnS is very fine compared to crystallized MnS. Furthermore, the number of precipitated MnS is extremely large compared to the number of crystallized MnS. MnS can be a starting point for generation of soot. Therefore, the larger the number of MnS, the lower the firing characteristics. Furthermore, Pb tends to adhere to MnS crystallized in the liquid phase, but hardly adheres to MnS precipitated in the solid phase. Therefore, composite inclusions are not easily generated.

If F1 is 2.90 or more, the Mn content is sufficiently larger than the S content. In this case, MnS is easily crystallized in the liquid phase. Therefore, a sufficient amount of specific inclusions for obtaining machinability is obtained, and excellent machinability is obtained. Furthermore, it is possible to suppress an excessive increase in the number density of fine MnS and to suppress the starting point of soot generation. As a result, the firing characteristics can be improved. The minimum with preferable F1 is 3.00, More preferably, it is 3.30. As F1 is higher, MnS can be crystallized in the liquid phase from the initial stage of solidification, and as a result, complex inclusions of MnS and Pb are likely to be generated.

[Number of coarse specific inclusions TN]
Preferably, the free-cutting steel according to the present embodiment further includes any of MnS inclusions, Pb inclusions, and composite inclusions, and the total number of specific inclusions having an equivalent circle diameter of 10 μm or more is 200 to 10,000. / Mm ² or more. In this case, the sprout characteristic is further enhanced. Hereinafter, the specific inclusion having an equivalent circle diameter of 10 μm or more is referred to as “coarse specific inclusion”.

Here, the Pb inclusion means a single Pb grain. The composite inclusion contains MnS and Pb.

粗 Coarse inclusions increase machinability. In particular, MnS inclusions having an equivalent circle diameter of 10 μm or more and composite inclusions of MnS and Pb enhance chip disposal. In the case of a composite inclusion, Pb softened by cutting heat penetrates into the generated crack and promotes the progress of the crack, so that the chip disposal is particularly improved.

[Ratio of the number of Pb inclusions to the number of coarse specific inclusions (Pb ratio) RA]
More preferably, among the coarse specific inclusions, the ratio of the number of Pb inclusions to the total number of coarse specific inclusions (hereinafter referred to as Pb ratio) RA is 17% or less. If the Pb ratio RA is high, the number of Pb inclusions increases. As described above, Pb inclusions reduce the sag characteristics. Therefore, the one where the ratio of the Pb inclusion in a coarse specific inclusion is low is preferable. If the Pb ratio RA is 17% or less, the number of Pb inclusions is sufficiently small. Therefore, the sprout characteristic is further enhanced. A preferable upper limit of the Pb ratio RA is 13%.

[Measurement Method of Number of Coarse Specific Inclusions TN and Pb Ratio RA]
The number TN and the Pb ratio RA are measured by the following method. A sample is taken from free-cutting steel. For example, when the free-cutting steel is a steel bar or a wire rod, a sample is taken from the central portion (hereinafter referred to as R / 2 portion) of the radius R connecting the surface and the central axis in the cross section (surface perpendicular to the axial direction). Collect. 20 fields of view are randomly observed at a magnification of 1000 times using a scanning electron microscope (SEM) on the cross section (surface) of the R / 2 part sample. In each field of view (referred to as an observation surface), specific inclusions (MnS inclusions, Pb inclusions, composite inclusions) are specified. Specific inclusions and other inclusions can be distinguished by contrast. Further, among the specific inclusions, MnS inclusions, Pb inclusions, and composite inclusions are specified by the following methods, respectively.

At each observation surface, an image of the S distribution and the Pb distribution in the observation surface is obtained by a wavelength dispersive X-ray analyzer (EPMA). FIG. 1A is a schematic diagram showing the S distribution in the observation surface obtained by EPMA analysis, and FIG. 1B is a schematic diagram showing the Pb distribution in the same observation surface obtained by EPMA analysis as in FIG. 1A. is there.

Numeral 10 in FIG. 1A is an area where S exists. Since S exists almost as MnS, it can be considered that MnS exists in the code | symbol 10 in FIG. 1A. Reference numeral 20 in FIG. 1B is an area where Pb exists.

As shown in FIG. 1B, Pb may be divided by rolling or the like and arranged in the rolling direction as shown by reference numeral 20A. The same applies to S. As shown in FIG. 2, in the image obtained by the EPMA analysis, when all the adjacent inclusions IN have an equivalent circle diameter of 5 μm or more, if the interval D between the adjacent inclusions IN is within 10 μm, these The inclusion IN is regarded as one inclusion. The area of the specified inclusion is obtained, and the diameter of a circle having the same area as the area is defined as the equivalent circle diameter (μm). In the inclusion group defined as one inclusion, the equivalent circle diameter is the same circle diameter as the total area of the inclusion group.

FIG. 1C is an image obtained by synthesizing FIG. 1B with FIG. 1A. Referring to FIG. 1C, when Pb inclusion 20 overlaps with MnS inclusion 10, the inclusion is recognized as composite inclusion 30. On the other hand, referring to FIG. 1C, when MnS inclusions 10 and Pb inclusions 20 do not overlap (region A1, region A2, etc. in FIG. 1C), these inclusions are MnS inclusions and Pb inclusions. Identifies it.

By the above method, MnS inclusions, Pb inclusions, and composite inclusions are specified using a scanning microscope and EPMA. The area of each specified inclusion is determined, and the diameter of a circle having the same area is defined as the equivalent circle diameter (μm) of each inclusion.

Among the specific inclusions, coarse specific inclusions having an equivalent circle diameter of 10 μm or more are specified. Obtains the total number of the identified coarse identified inclusions (number at 20 fields), is converted into 1 mm ² number per TN (pieces / mm ^2). The number TN is obtained by the above method. Furthermore, among the specified coarse specific inclusions, the number MN (pieces / mm ² ) of Pb inclusions having an equivalent circle diameter of 10 μm or more is obtained, and based on the following formula (A), the Pb ratio RA (%) Ask for.
RA = MN / TN × 100 (A)

[Production method]
The free-cutting steel of the present invention can be manufactured by a known manufacturing method. In an example of the method for producing free-cutting steel of the present invention, first, molten steel satisfying the above-mentioned chemical composition is made into a slab by a continuous casting method. Alternatively, the molten steel is made into an ingot by an ingot forming method (casting process). And a slab or an ingot is hot-worked 1 or more times, and a free-cutting steel material is manufactured (hot work process). The casting process and the hot working process may be performed by a well-known method. Hereinafter, each process will be described.

[Casting process]
First, molten steel is melted by a known method such as a converter or an electric furnace. And the slab or ingot is manufactured by casting the manufactured molten steel. Hereinafter, slabs and ingots are collectively referred to as materials.

The solidification cooling rate RC at the time of casting may be a well-known rate, and is not particularly limited. The solidification cooling rate RC is, for example, 150 ° C./min or less. In order to generate more complex inclusions of MnS and Pb, it is preferable that MnS crystallizes in the liquid phase and that the Pb existing in the liquid phase adheres to MnS. Therefore, it is preferable that the solidification cooling rate RC is low.

A preferable solidification cooling rate RC is 50 ° C./min or less. In this case, MnS inclusions are sufficiently crystallized and grow in the molten steel. Therefore, coarse specific inclusions are easily generated, and the Pb ratio RA is 17% or less.

Further preferable solidification cooling rate RC is 20 ° C./min or less. In this case, coarse MnS tends to crystallize and grow in the liquid phase. Furthermore, since the time until solidification is long, it is possible to secure a sufficient time for Pb to move in the molten steel and adhere to coarse MnS. Therefore, composite inclusions containing MnS and Pb are easily generated, and the Pb ratio RA is 13% or less.

The solidification cooling rate can be determined from the cast material. FIG. 3 is a cross-sectional view of the cast material. Among the materials of thickness W (mm), the cooling rate from the liquidus temperature to the solidus temperature at the point P1 at the position W / 4 from the surface toward the material center is the solidification cooling rate RC in the casting process. (° C./min). The solidification cooling rate RC can be obtained by the following method. Cut the solidified material in the transverse direction. Of the cross section of the material, the secondary dendrite arm interval λ2 (μm) in the thickness direction of the solidified tissue at the point P1 is measured. Using the measured value λ2, the cooling rate RC (° C./min) is obtained based on the following equation (3).
RC = (λ2 / 770) ^{− (1 / 0.41)} (3)

The secondary dendrite arm interval λ2 depends on the solidification cooling rate. Therefore, the solidification cooling rate RC can be obtained by measuring the secondary dendrite arm interval λ2.

[Hot working process]
In the hot working process, one or more hot workings are usually performed. The material is heated before each hot working. Thereafter, hot working is performed on the material. Hot working is, for example, block rolling or hot forging. The material after hot working is cooled by a known cooling method such as air cooling. Then, if necessary, a second hot working is performed to manufacture a steel material. For example, the material is rolled by a continuous rolling mill to produce a bar steel or a wire rod. Free cutting steel is manufactured by the above manufacturing process.

As described above, the free-cutting steel of the present invention is excellent in machinability improvement (surface roughness, tool life, chip disposal) and cracking characteristics. Therefore, if the free-cutting steel of the present invention is used, complex shaped parts and precision parts manufactured through a plurality of processes can be manufactured with high accuracy and a low defect rate. Therefore, if the free-cutting steel of the present invention is used, it is easy to automate and unmanned in the part manufacturing process, and rust is not easily generated even if the manufactured cutting part is stored for a long period of time.

The molten steel which has the chemical composition shown in Table 1 was manufactured.

Slab was produced by casting molten steel. The solidification cooling rate RC at the time of casting was as shown in Table 1. The manufactured slab was hot-worked to manufacture a steel bar having a diameter of 10 mm. The solidification cooling rate RC was obtained from the above equation (3) by measuring the secondary dendrite arm interval of the ingot. The steel bar was drawn and straightened to produce a steel bar having a diameter of 8 mm.

[Evaluation test]
[Pb ratio RA]
A specimen for observing the structure was collected from R / 2 part of the steel bar of each test number. Of the surface of the test piece, a cross section parallel to the longitudinal direction of the steel bar (that is, the rolling direction or the stretching direction) was defined as an observation surface. Based on the above method, the Pb ratio RA (%) was determined.

[Machinability]
Machinability evaluated the surface roughness, the tool life characteristic by a normal drill, and the chip disposal property. In any of the evaluations, those that were “x” were judged as “low machinability”, and the others were judged as “good machinability”.

[Cutting test]
A steel bar having a diameter of 8 mm was cut at a predetermined length to obtain a cutting test piece. The outer periphery turning shown in FIG. 2 was implemented with respect to the test piece. Specifically, a K10 type carbide tool was used as the tool 10. The nose R of the tool 10 was 0.4, and the rake angle was 5 °. The peripheral turning was carried out at a cutting speed V1: 80 m / min, a feed speed V2: 0.05 mm / rev, a cutting depth D1: 1 mm, and a cutting width L1: 10 mm per test piece. An insoluble cutting oil was used during turning. A turning test under the above conditions was performed on 1000 test pieces.

[Surface roughness evaluation]
In the above cutting test, after the 1000th specimen was turned, the surface roughness of the specimen was measured. The surface roughness was determined by the ten-point surface roughness (Rz) defined in JIS B0601 (2001). The measurement results are shown in the “surface roughness” column of Table 1. In Table 1, “◎” means that the surface roughness is 10 μm Rz JIS or less. “◯” means that the surface roughness is more than 10 to 15 μm Rz JIS. “Δ” means that the surface roughness is more than 15 to 20 μm Rz JIS. “X” means that the surface roughness exceeded 20 μm Rz JIS. When the surface roughness was 20 μm Rz JIS or less, it was evaluated that excellent surface roughness was obtained. In the table, “◯ to ◎” means that evaluations of ◎ and ○ were obtained by a plurality of measurements.

[Tool life evaluation]
The tool wear amount (mm) of the front flank was measured for the tool 10 after the turning of the 1000th test piece was completed. The measurement results are shown in the “Tool wear” column of Table 1. In Table 1, “◎” means that the amount of tool wear is 150 μm or less. “◯” means that the amount of tool wear is more than 150 to 200 μm. “X” means that the amount of tool wear exceeds 200 μm. When the amount of tool wear was 200 μm or less, it was evaluated that the tool life was excellent.

[Evaluation of chip disposal]
In turning the 1000th test piece, the chips 20 shown in FIGS. 3A and 3B were obtained. Therefore, the length L20 of the chip 20 and the diameter D20 were measured. Based on the measurement results, classification was performed as shown in Table 2.

Referring to Table 2, the chip disposal was evaluated as follows. “◎” means that the chip was a coil shape with a diameter of 30 mm or less and the chip length was 20 mm or less. “O” is a coil shape with a chip having a diameter of 30 mm or less, and the chip length was more than 20 mm, or the chip was not a coil shape with a diameter of 30 mm or less, and the chip length was 20 mm or less. It means that there was. “Δ” means that the chip is not in a coil shape with a diameter of 30 mm or less, and the chip length is more than 20 mm to less than 50 mm. “X” means that the chip was not in a coil shape with a diameter of 30 mm or less, and the chip length was 50 mm or more. When the measurement result of chip was “◎”, “◯” or “Δ”, it was evaluated that the chip disposal was excellent.

[Evaluation test of rust characteristics (corrosion resistance)]
A test piece was prepared by cutting a steel bar having a diameter of 8 mm into a predetermined length. The test piece was turned under the same conditions as the cutting test described above. While spraying tap water on the cut surface, the test piece was stored in an atmosphere of 70% humidity and 20 ° C. for 24 hours. After storage, the cut surface of the test piece was observed and the number of rust points was measured. The measurement results are shown in the “Spring characteristics” column of Table 1. “◎” indicates that the saddle point was less than 10. “◯” indicates that the saddle points were 11 to 15 points. “Δ” indicates that the rust point was 16 to 19 points. “X” indicates that the saddle point was 20 points or more.

[Surface evaluation test during hot working]
A round bar test piece having a diameter of 10 mm and a length of 100 mm was produced from the steel bar having a diameter of 10 mm. Both ends of the test piece were threaded, the test piece was attached to a jig for tensile test, and then a hot tensile test was conducted by energization heating. Specifically, the test piece was heated to 1100 ° C. by energization heating and held for 3 minutes. Then, it cooled to 900 degreeC by standing_to_cool. A tensile test was performed when the temperature of the test piece reached 900 ° C., and the ductility (drawing value) at the time of fracture was evaluated. In each test number, the above-described tensile test was performed with three test pieces, and the average of the obtained aperture values was obtained. The results are shown in the “surface flaw” column of Table 1. “◎” means that the average aperture value was 60% or more. “◯” means that the average aperture value was 50 to less than 60%. “Δ” means that the average aperture value was 40 to less than 50%. "X" means that the average aperture value was less than 40%. When the average of the drawing values was 40% or more, it was evaluated that the ductility during hot working was excellent and generation of surface defects was suppressed.

[Test results]
Referring to Table 1, all of the test numbers 1 to 34 and 46 to 48 had the chemical composition within the scope of the present invention and satisfied the formula (1). Therefore, cutting accuracy (surface roughness), tool life, and chip disposal are all excellent, and machinability is excellent. Furthermore, it was also excellent in crease characteristics and ductility. In the test numbers 1 to 34 and 46, the number ratio RA of Pb inclusions was 17% or less.

Among the test numbers 1 to 34, particularly, the test numbers 1 to 5, 10, 12, 17 to 19, and 21 to 34 were in a range in which the chemical composition was preferable. Therefore, in these test numbers, compared with the test numbers 6 to 9, 11, 13 to 16, and 20, the wrinkling characteristics were further excellent.

Among the test numbers 1 to 34, in particular, the test numbers 24 to 34 contained arbitrary elements (Ca, Mg, Zr and B). Therefore, in these test numbers, cutting accuracy (surface roughness) was superior to test numbers 1 to 23.

Among the test numbers 1 to 34 and 46 to 48, in particular, in the test numbers 1 to 34 and 46, the cooling rate was 50 ° C./min or less. Therefore, the Pb ratio RA is 17% or less. As a result, compared with test numbers 47 and 48, the wrinkling characteristics were further excellent.

Among the test numbers 1 to 34 and 46 to 48, in particular, in the test numbers 1 to 34, the cooling rate was 20 ° C./min or less. Therefore, the Pb ratio RA was 13% or less. As a result, compared with the test numbers 46 to 48, the wrinkling characteristics were further excellent.

On the other hand, in the test numbers 35 and 36, the S content was less than the prescribed lower limit of the present invention. Therefore, cutting accuracy and tool life were low, and machinability was low. This is probably because there was little generation of MnS inclusions.

In test numbers 37 and 38, the Pb content exceeded the upper limit prescribed in the present invention. Therefore, the glazing property was low and the ductility was also low. This is probably because there were many Pb inclusions.

Test numbers 39 and 40 did not satisfy the formula (1). Therefore, the glazing property was low and the ductility was also low. This is probably because there were few complex inclusions of MnS and Pb and there were many Pb inclusions.

In test number 41, the Si content exceeded the upper limit prescribed in the present invention, and in test number 42, the Al content exceeded the upper limit prescribed in the present invention. Therefore, machinability was low. This is probably because a large amount of hard oxide was generated.

In test number 43, the O content was less than the lower limit prescribed in the present invention. Therefore, machinability was low. This is probably because MnS was stretched and the aspect ratio was large.

In test number 44, the P content was less than the lower limit prescribed in the present invention. Therefore, the cutting accuracy (surface roughness) was low. It is thought that because the P content was too low, the embrittlement effect of the steel was insufficient and machinability was reduced.

In test number 45, the C content exceeded the upper limit prescribed in the present invention. Therefore, the tool life was low. This is probably because the C content was too high and the strength of the steel was high.

In test numbers 49 and 50, the Pb content was less than the lower limit prescribed in the present invention. Therefore, machinability was low.

The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

10 MnS inclusions 20 Pb inclusions 30 Composite inclusions

Claims (5)

1. % By mass
   C: 0.005 to 0.150%,
   Si: less than 0.010%,
   Mn: 1.02 to 2.00%
   P: 0.010 to 0.200%,
   S: 0.350 to 0.600%,
   Pb: 0.010 to 0.100%,
   N: 0.004 to 0.015%,
   O: 0.0080 to 0.0250%,
   Al: 0 to 0.003%,
   One or more selected from the group consisting of Ca, Mg and Zr: 0 to 0.0005% in total; and
   B: 0 to 0.0200% is contained, the balance consists of Fe and impurities,
   Free-cutting steel having a chemical composition satisfying formula (1).
   Mn / S ≧ 2.90 (1)
   Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).
2. The free-cutting steel according to claim 1,
   The chemical composition is
   Free-cutting steel containing Al: 0.001 to 0.003%.
3. The free-cutting steel according to claim 1 or claim 2,
   The chemical composition is
   Free-cutting steel containing 0.0001 to 0.0005% in total of at least one selected from the group consisting of Ca, Mg and Zr.
4. The free-cutting steel according to any one of claims 1 to 3,
   The chemical composition is
   B: Free-cutting steel containing 0.0005 to 0.0200%.
5. The free-cutting steel according to any one of claims 1 to 4,
   Free-cutting steel in which the ratio of the number of Pb inclusions to the total number of the specific inclusions is 17% or less.

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