CN1580312A - Low-carbon free cutting steel - Google Patents

Abstract

A low-carbon free-cutting steel containing C: less than 0.05%-0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, and P: 0.001-0.30% in mass % , Al: 0.2% or less, O (oxygen): 0.001-0.03%, N: 0.0005-0.02%, the balance is Fe and unavoidable impurities, and the inclusions in steel satisfy the following formulas (1) and (2) , (A+B)/C≥0.8...(1); N _A ≥5...(2) In the formula, the meanings of A, B, C and N _A are as follows: 1mm on the section parallel to the rolling direction ² In the inclusions with an equivalent circle diameter of 1 μm or more, A: the total area occupied by the substantial MnS with Ti carbides or/and Ti carbonitrides inside; B: no Ti carbides and Ti carbonitrides inside C: the total area occupied by all inclusions; N _A : the number of substantial MnS with Ti carbides or/and Ti carbonitrides inside.

Description

Low carbon free cutting steel

technical field

The present invention relates to a low-carbon free-cutting steel that does not contain lead (Pb), and particularly relates to a composite free-cutting steel that does not contain lead but is compatible with conventional lead free-cutting steels and that contains both lead and other elements that improve cutting performance Compared with low-carbon free-cutting steel with good cutting performance, hot working performance and good surface properties after cutting and low production cost when using cemented carbide tools for cutting. In addition, the present invention also relates to a low-carbon free-cutting steel having good carburizing properties in addition to the various properties described above.

Background technique

In the past, for small soft parts that do not particularly require strength, steels with good machinability, so-called free-cutting steels, were often used in order to increase productivity. The most familiar free-cutting steels are: sulfur free-cutting steel containing a large amount of S and using MnS to improve cutting performance; lead free-cutting steel with Pb added; and composite free-cutting steel containing both S and Pb. In particular, free-cutting steel containing Pb has the characteristics of prolonging the life of the tool, having good breakability of chips, and having good machined surface roughness of the machined steel surface. In addition, there are free-cutting steels containing Te (tellurium), Bi (bismuth), etc. in order to improve cutting performance. These steels are widely used in small parts such as brake parts of automobiles, electronic equipment parts such as peripheral parts of personal computers, and various mechanical parts such as molds.

On the other hand, in recent years, due to the improvement of the performance of cutting machines, high-speed cutting has become possible. With this trend, it is urgently desired to improve the cutting performance during high-speed cutting of the steel materials that constitute the raw materials of the above-mentioned parts.

In addition, after the above-mentioned components are cut into a predetermined shape, carburizing treatment may be required in order to ensure the strength of the surface. Therefore, for the steel materials used for these parts, in addition to high cutting performance, good carburizing performance is also desired.

The steel materials used as raw materials for the above-mentioned components are required to have good cutting performance. The above-mentioned cutting performance not only extends the life of the tool, but also particularly emphasizes the property of finely breaking and breaking chips, that is, "chip disposability". This kind of swarf handling is an indispensable condition for realizing the automation of the processing line, and it is also necessary for improving productivity. In addition to tool life and chip disposability, from the viewpoint of machining accuracy, it is also desirable that the machined surface of the steel material after cutting is in good condition, that is, the roughness of the machined surface is relatively small. Among the above-mentioned free-cutting steels, lead free-cutting steels and composite free-cutting steels containing Pb and other elements for improving cutting performance are excellent in these properties and have the best cutting performance among existing steel materials.

In recent years, as people pay more and more attention to environmental issues, it is urgent to develop free-cutting steels that do not contain Pb. This is because steel materials containing Pb, which is harmful to the human body and the global environment, not only require large-scale exhaust equipment in the manufacturing process, but also from the viewpoint of environmental protection, there are increasing calls to restrict the use of Pb.

In order to meet the above requirements, as a substitute product of lead free cutting steel, many technical proposals about Pb-free low carbon sulfur free cutting steel have been proposed. However, no free-cutting steel has been developed so far that can fully meet the performance requirements of free-cutting steels containing Pb, that is, prolong tool life, good chip handling, and low surface roughness.

Patent Document 1 (Japanese Unexamined Patent Publication No. 2003-49240) discloses a free-cutting steel in which carbosulfide-based inclusions of Ti and/or Zr are present to improve cutting performance. In this free-cutting steel, since Ti carbosulfide or Zr carbosulfide is dispersed together with MnS, it is difficult to obtain the pseudo-lubricating effect of MnS, and the friction force between the tool and the material to be cut increases. As a result, the cutting resistance increases, and knives tend to form on the cutting edge of the tool. Once the knives are formed, the roughness of the machined surface after finishing cutting increases, and the machining accuracy of the parts is damaged.

In Patent Document 1, there are no examples where the Ti content is 0.1% or less. This shows that the object of the invention of Patent Document 1 is to generate Ti carbosulfides by containing a large amount of Ti. Actually, this document describes that granular Ti carbosulfide-based inclusions are dispersed in the matrix together with MnS. In this case, the properties such as tool life, chip disposability, and machined surface roughness required for the steel used for the above-mentioned parts cannot be satisfied.

Patent Document 2 (Japanese Unexamined Patent Publication No. 2003-49241) discloses a free-cutting steel that contains Ti or/and Zr, (Ti+0.52Zr)/S<2, and contains Ti or Zr carbosulfide as inclusions. , Improved tool life during turning and drilling. In the invention of this Patent Document 2, Ti carbosulfides are formed in steel to improve tool life during turning. Using this technology can indeed improve the life of the tool to a certain extent, but due to the presence of Ti or Zr carbosulfides, it is difficult to obtain the lubrication effect of MnS, and the friction between the tool and the material to be cut increases. As a result, the cutting resistance increases, and knives tend to form on the cutting edge of the tool. Once the knives are formed, the roughness of the machined surface after cutting increases, resulting in deterioration of machining accuracy.

In Patent Document 2, there is no example of a free-cutting steel containing S0.21% or more and Ti0.1% or less specified in the present invention described below. From this, it can be seen that the invention of Patent Document 2 is not an invention aimed at improving the roughness of the machined surface and improving chip disposability. That is, in the invention of Patent Document 2, Ti or Zr carbosulfides are dispersed together with MnS in the matrix, so that desired machined surface roughness and chip treatability cannot be obtained.

Patent Document 3 (Japanese Patent Publication No. Hei 2000-319753) discloses a Pb-free low-carbon sulfur free-cutting steel containing more than 0.4% of S and increasing the MnS content. However, this steel has little effect on improving the life of carbide tools. In addition, this steel does not improve the chip disposability, which is as important as tool life, and does not significantly improve the performance of conventional sulfur free-cutting steels.

Patent Document 4 (JP-A-09-53147) discloses an invention on a free-cutting steel containing C: 0.01-0.2%, Si: 0.10-0.60%, Mn: 0.5-1.75%, P: 0.005-0.15%, S; 0.15-0.40%, O (oxygen): 0.001-0.010%, Ti: 0.0005-0.020%, N: 0.003-0.03%, good cutting performance for carbide tools, especially tool life . In this invention, the steel must contain Ti and 0.1-0.6% Si to improve the life of the cemented carbide tool. In addition, this invention does not contain silicon like the present invention, and by adding "substantial MnS with Ti carbide or/and Ti carbonitride inside" to the steel material, not only the life of the tool is improved, but also the chip handling property is improved. Machining surface roughness.

Patent Document 5 (Patent No. 3390988) discloses an invention of a low-carbon sulfur free-cutting steel, which contains C: 0.02-0.15%, Mn: 0.3-1.8%, S: 0.225-0.5%, Ti: 0.1-0.6%, Zr: 0.1-0.6%, and satisfy Ti+Zr: 0.3-0.6%, (Ti+Zr)/S ratio: 1.1-1.5, improving the anisotropy of mechanical properties. The invention generates Ti and Zr sulfides with high deformation resistance during hot working through the above composition, and improves the mechanical anisotropy and cutting performance of the steel. However, due to the presence of these sulfides with high deformation resistance, it is difficult to obtain the lubricating effect of the sulfides during cutting, so the cutting resistance increases, the life of the tool deteriorates, and the roughness of the machined surface also deteriorates.

[Patent Document 1] JP-A-2003-49240

[Patent Document 2] JP-A-2003-49241

[Patent Document 3] JP-A-2000-319753

[Patent Document 4] Japanese Unexamined Patent Publication No. 09-53147

[Patent Document 5] Patent No. 3390988

Contents of the invention

The task of the present invention is to provide a low-carbon free-cutting steel that does not contain Pb that is harmful to the environment. In the case of cutting with a cemented carbide tool, it shows excellent cutting performance, good thermal processing performance, good surface properties after cutting, and can be manufactured at low production costs. Another object of the present invention is to provide a low-carbon free-cutting steel having good carburizing performance in addition to the above-mentioned various properties.

It is well known that the state of inclusions such as sulfide has a great influence on the machinability of steel. There are various inclusions observed in steel containing C, Ti, S, N, O, such as Ti sulfide, Ti carbosulfide, Ti carbide, Ti carbonitride, Ti nitride, Ti oxide things. In addition, Mn sulfide represented by the chemical formula "MnS" also exists in the steel if Mn is also contained. In addition to these inclusions, if the steel contains Al and Si, oxides of these elements are also present. These inclusions exist in various forms, and the composition and existence form of inclusions have a great influence on the cutting performance and other mechanical properties of steel.

Prior to this, the present inventors had applied for a patent on a low-carbon sulfur free-cutting steel that does not contain Pb (patent application number: No. 2002-26368). The free cutting steel is characterized by containing specified amounts of C, Mn, S, Ti, Si, P, Al, O and N, the content of Ti and S satisfying the following formula (A), and the atomic ratio of Mn and S satisfying the following Formula (B), and contains MnS in which Ti sulfide or/and Ti carbosulfide exists.

Ti(mass%)/S(mass%)＜1...(A)

Mn/S≥1...(B)

Compared with Pb free cutting steel, this steel has much better tool life and also has good chip handling. However, such steels have some problems in terms of surface properties after cutting, that is, after finish cutting, the roughness of the machined surface may sometimes increase significantly.

When there are substantial Ti sulfides or/and Ti carbosulfides in the matrix, it is difficult to obtain the pseudo-lubricating effect of MnS, so the cutting resistance increases, and it is easy to form knives at the edge of the tool, so the surface roughness of the steel after cutting Poor, the roughness of the machined surface deteriorates. The "substantial Ti sulfide or/and Ti carbon sulfide" mentioned above refers to the inclusion in which the total area ratio of Ti sulfide and Ti carbon sulfide in one inclusion is more than 50%. Several such inclusions are shown in Figure 1(a).

In order to solve the above-mentioned problems, the inventors of the present invention conducted studies, and as a result, obtained new findings described below.

(1) When cutting steel in which "substantial Ti sulfide or/and Ti carbosulfide" exists in the matrix, a burr is formed on the cutting edge of the tool, deteriorating the roughness of the machined surface.

(2) Suppress the formation of "substantial Ti sulfide or/and Ti carbosulfide" as much as possible and make a lot of MnS in the steel, which can suppress the formation of knife burrs and obtain good surface roughness.

(3) However, with steel containing no Ti and only MnS, the life of the cemented carbide tool deteriorates. In order to improve the life of the cemented carbide tool, Ti must be added, and the steel contains "substantial MnS with Ti carbide or/and Ti carbonitride inside".

(4) "substantial MnS with Ti carbide or/and Ti carbonitride inside" improves the life of the tool without impairing the quasi-lubricating effect of MnS.

Based on the above knowledge, the present inventors further studied the relationship between the chemical composition and the shape of the inclusions, and as a result, invented the low-carbon free-cutting steel described below. The low-carbon free-cutting steel has cutting performance equal to or higher than that of Pb free-cutting steel and composite free-cutting steel. In addition, % concerning the content of a component below is mass %.

A low-carbon sulfur free-cutting steel, characterized in that C: less than 0.05% to 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%, N; 0.0005-0.02%, the balance is Fe and unavoidable impurities, and the inclusions contained in the steel satisfy the following (1) formula and (2 )Mode,

(A+B)/C≥0.8....(1)

N _A ≥5....(2)

In the formula, the meanings of A, B, C and N _A are as follows:

A: The total area occupied by the substantial MnS with Ti carbide or/and Ti carbonitride inside the inclusions within 1 ^mm2 of the cross section parallel to the rolling direction and with an equivalent circle diameter of 1 μm or more;

B: The total area occupied by substantial MnS without Ti carbides and Ti carbonitrides within ^1mm2 of the cross section parallel to the rolling direction, among the inclusions with an equivalent circle diameter of 1μm or more;

C: The total area occupied by all inclusions with an equivalent circle diameter of 1 μm or more within ^1mm2 of the section parallel to the rolling direction;

N _A : The number of substantive MnS in which Ti carbides or/and Ti carbonitrides are present in inclusions within 1 mm ² of a section parallel to the rolling direction and having a circle-equivalent diameter of 1 μm or more.

The low-carbon free-cutting steel may contain one or more components selected from at least one of the following Groups 1 to 3.

Group 1:

Se: 0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn: 0.01-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02%, and rare earth Elements: 0.0005-0.02%.

Group 2:

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%.

Group 3:

Si: 0.1-2.0% and Cr: 0.03-1.0%.

The term "substantial MnS containing Ti carbides or/and Ti carbonitrides inside", as described below, means that the area ratio of MnS in one inclusion is 50% or more, and there is Ti inside. Inclusions where carbides and/or Ti carbonitrides exist (coexist). In addition, the term "substantial MnS without Ti carbides and Ti carbonitrides inside" means that the area ratio occupied by MnS in one inclusion is 50% or more, and there are no Ti carbides and Ti carbonitrides inside. Inclusions where compounds exist (coexist). In addition, these "substantial MnS with Ti carbide or/and Ti carbonitride inside" and "substantial MnS without Ti carbide and Ti carbonitride inside" can have Ti carbide and Ti Sulfides, carbon sulfides, carbides, nitrides, etc. other than carbonitrides.

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

(1) Contain C: less than 0.05% to 0.20%, S: 0.21%-1.0%, and Ti content is 0.002-0.1%.

(2) Ti combines with C, S, N and O to form sulfides, carbon sulfides, carbides, carbonitrides and oxides. Compared with Mn, Ti has a stronger tendency to form sulfide, so it is easy to form Ti sulfide and Ti carbon sulfide. However, as long as the content balance of Mn, Ti, S and N is carefully considered, many "substantial Ti carbon sulfides or/and Ti sulfides" will not be generated, and many "internal Ti carbides or/and Ti sulfides" may exist. Substantial MnS of carbonitrides" and "substantial MnS without Ti carbides and Ti carbonitrides inside".

(3) When the chemical composition described in the above (1) is adopted and the form of inclusions described in the above (2) is obtained, the inclusions existing in the matrix soften during the cutting process and exert a lubricating effect " Substantial MnS" accounts for more than half of all inclusions, and sulfides other than this "substantial MnS", that is, "substantial Ti sulfide or/and Ti carbon sulfide" basically do not exist. At this time, in order to obtain a good machined surface roughness, the formation amount of "substantial MnS" must account for most of the formation amount of all inclusions. Specifically, the total area of "substantial MnS" with a circle-equivalent diameter of 1 μm or more must account for 80% of the total area of all inclusions with a circle-equivalent diameter of 1 μm or more within 1 mm2 of the viewing surface of the ^section in the rolling direction above. Only in this case can the generation of knives at the edge of the tool due to the existence of "substantial Ti sulfide or/and Ti carbosulfide" be suppressed, and a good machined surface roughness can be obtained.

The "substantial MnS" mentioned above is an inclusion in which MnS accounts for more than 50% of the area in one inclusion, and it consists of "substantial MnS with Ti carbide or/and Ti carbonitride inside". and "substantial MnS without Ti carbides and Ti carbonitrides inside".

As shown in the formula (1) above, "A+B" in the formula (1) accounts for more than 80%. The definition of A and B is, within ^1mm2 of the section parallel to the rolling direction, in the sulfide with an equivalent circle diameter of 1μm or more, "substantial MnS with Ti carbide or/and Ti carbonitride inside" and the area occupied by "substantial MnS without Ti carbides and Ti carbonitrides inside".

In addition, "substantial MnS with Ti carbide or/and Ti carbonitride inside", "substantial MnS without Ti carbide and Ti carbonitride inside", "substantial Ti sulfide or/and Ti carbonitride The total area of "sulfide" and other sulfides, carbon sulfides, carbides, nitrides, oxides, Al ₂ O ₃ , SiO _{2 ,} etc. is C in the formula (1).

(4) Even if it is a steel containing the inclusions mentioned in (3) above, that is, there is basically no "substantial Ti sulfide or/and Ti carbosulfide", most of the inclusions contained in the steel are "Substantial MnS", but as long as there is "substantial MnS with Ti carbide or/and Ti carbonitride inside", when cutting in the high-speed region where the cutting temperature rises, due to the formation of a hard TiN film on the surface of the tool , to protect the tool, you can get a good tool life.

(5) In steels with "substantial MnS containing Ti carbides and/or carbonitrides inside", this "substantial MnS" is different from the MnS contained in the conventional JIS SUM22L-24L composite free-cutting steel Compared with them, they are smaller and more in number. In this case, these fine "substantial MnS" form the starting point of stress concentration during cutting, which contributes to crack propagation, so that chip handling properties equal to or higher than those of composite free-cutting steel can be obtained.

(6) For steel containing "substantial MnS with Ti carbide or/and Ti carbonitride inside", there is no problem with hot workability at all, so the S content that can effectively improve cutting performance can be increased. In this case, for There are no obstacles to manufacture using continuous casting equipment. In addition, since the effect can be fully exhibited by adding a small amount of Ti, the production cost is reduced, and it is suitable for production of inexpensive steel materials.

As mentioned above, as long as the range of alloy composition is limited and the morphology of inclusions is adjusted, good cutting performance can be obtained. However, steels used for automobile parts are sometimes desired to have good carburizing properties in addition to machinability. Therefore, the present inventors investigated the influence of Si and Cr on the performance of steel, and found that by adjusting the amount of Si and Cr, the form of the above-mentioned inclusions will not be damaged, so the machinability of the steel material will not be deteriorated, and it can be improved. Carburizing properties.

Si and Cr dissolve in austenite, improve the hardenability of steel, and increase the carburization depth and hardness of the carburized layer during carburizing treatment. In addition to Si and Cr, elements that improve hardenability include Mn, Mo, P, etc. However, from the standpoint of cutting performance or hot workability, Mn must be contained in a sufficient amount relative to the amount of S, and a large amount of addition is required. In such a case, in order to improve hardenability, further addition of Mn leads to an increase in manufacturing cost. In addition, Mo can also effectively improve the hardenability of steel, but the price of Mo is higher than that of Si and Cr. Therefore, if a considerable amount of Mo is added to obtain the same effect, the manufacturing cost will increase. P also has the same effect, but when P is added, the hardness of the steel material itself increases sharply, resulting in deterioration of machinability. In addition, these elements can also be added within the range that does not impair the cutting performance and mechanical performance when the material cost is not limited. However, when it is desired to manufacture at low production cost without impairing machinability, Si and Cr are preferably used as components for improving carburizing performance.

Description of drawings

Fig. 1 is a schematic view showing the morphology of inclusions in steel of the present invention and comparative steel.

Fig. 2 is a graph showing the relationship between (A+B)/C and the average processed surface roughness of steels of the present invention and comparative steels.

Fig. 3 is a graph showing the relationship between the average machined surface roughness and the tool life of steels of the present invention and comparative steels.

Fig. 4 is a graph showing the relationship between the chip disposability and the tool life of steels of the present invention and comparative steels.

Detailed ways

1. About "substantial MnS with Ti carbide or/and Ti carbonitride inside"

Ti combines with S, C, N, O to form Ti sulfide and Ti carbon sulfide represented by the chemical formula TiS and Ti ₄ C ₂ S ₂ , and Ti carbide represented by the chemical formula TiC and Ti(CN), TiN, TiO , Ti carbonitride, Ti nitride, Ti oxide and other Ti-based inclusions. In addition, sometimes, Ti is dissolved in MnS and exists in the form of (Mn, Ti) S, but this Ti dissolved in MnS is extremely small, so this sulfide is essentially MnS.

In other cases, Ti is not solid-soluble in MnS, and exists in an inclusion clearly phase-separated from MnS. The Ti exists in the form of TiC or/and Ti(C,N), that is, exists in a form that is significantly different from the composition of MnS, and exists in various forms, such as existing around a sulfide, Or exist in the form of being surrounded in MnS and so on.

Fig. 1 is a schematic diagram of inclusions present in a Ti-containing free-cutting steel, wherein (a) is a free-cutting steel of a comparative example, and (b) is a free-cutting steel of the present invention. In the steel shown in Figure 1(a), there are many Ti sulfides and carbon sulfides that exist alone, or the area ratio of Ti sulfides and carbon sulfides when they coexist with MnS in one inclusion It is more than 50%, which can be regarded as the inclusion of Ti sulfide and carbon sulfide in essence, that is, the aforementioned "substantial Ti sulfide or/and Ti carbon sulfide". On the other hand, in the steel of the present invention shown in Fig. 1(b), there are many Ti carbides or/and Ti carbonitrides that are incorporated into the outer peripheral portion of MnS or inclusions that exist inside, that is, "there are Ti carbides inside". or/and substantial MnS of Ti carbonitrides" and "substantial MnS without Ti carbides and Ti carbonitrides inside".

In the steel shown in Fig. 1(b) above, when Ti sulfide and Ti carbosulfide or Ti carbide and Ti carbonitride, Ti nitride, Ti oxide, and other inclusions are clearly phase-separated from MnS, The area ratio occupied by MnS is 50% or more, which is considered to be substantially one MnS, that is, "substantial MnS". Conversely, in an inclusion, these Ti-based inclusions or inclusions composed of oxides, nitrides, carbides, etc. composed of other components account for more than 50% of the area ratio, not "substantial MnS", in fact It is considered to be a Ti-based inclusion and oxides, nitrides, carbides, etc. composed of other components.

Among the above-mentioned MnS, inclusions in which Ti carbide or/and Ti carbonitride and MnS are clearly phase-separated and MnS occupy an area ratio of 50% or more are defined as "the presence of Ti carbide or/and Substantial MnS of Ti carbonitrides". On the other hand, "substantial MnS without Ti carbides and Ti carbonitrides inside" refers to the above-mentioned Ti-based inclusions other than Ti carbides and Ti carbonitrides, or oxides, nitrogen, etc. composed of other components. Inclusions such as carbides and carbides are clearly phase-separated from MnS in one inclusion, and MnS occupies an area ratio of 50% or more, and MnS substantially plays the role of MnS, and there is no such Ti-based inclusion at all. MnS with inclusions such as oxides, nitrides, and carbides composed of other components. That is, the sum of "substantial MnS with Ti carbides and/or Ti carbonitrides inside" and "substantial MnS with no Ti carbides and Ti carbonitrides inside" represents inclusions that are substantially regarded as MnS (the above-mentioned "substantial MnS"), the other inclusions are Ti-based inclusions such as Ti sulfide, Ti carbon sulfide, Ti carbide, Ti carbonitride, Ti nitride, Ti oxide, etc. and oxides, carbides, and nitrides composed of other elements.

The above-mentioned area ratio occupied by MnS or Ti-based inclusions in one inclusion can be calculated by using EPMA (Electron Probe Microanalyzer) and EDX (Energy Dispersive X-ray Analyzer), etc. The microscopic test pieces cut from the round bar used in the test are determined by surface analysis and quantitative analysis. In addition, "substantial MnS with Ti carbide or/and Ti carbonitride inside" and "substantial MnS without Ti carbide and Ti carbonitride inside" and other inclusions in steel can also be used in the same way. Confirmed by the method, the total area and number can also be measured by image analysis and other methods. In this case, measurements can be made in many fields of view so that the total area of the observation fields exceeds 1 mm ² , and then the total area and number of inclusions are converted into the average total area and average number per 1 mm ² .

2. The basis for specifying (A+B)/C≥0.8

In the above formula (1), A is the substance that "there are Ti carbides or/and Ti carbonitrides inside the inclusions with an equivalent circle diameter of 1 μm or more within 1 ^mm2 of the cross section parallel to the rolling direction. The total area occupied by MnS", B is the inclusions with an equivalent circle diameter of 1 μm or more within ^1mm2 on the cross section parallel to the rolling direction, "substantial MnS without Ti carbides and Ti carbonitrides inside" the total area occupied. The "circle-equivalent diameter" mentioned here refers to the diameter when the area of one inclusion is calculated by the above-mentioned image analysis method and converted into a circle having the same area. The "equivalent circle diameter is 1 μm or more" is defined because inclusions smaller than 1 μm have substantially no influence on cutting performance.

The above formula (1) indicates that the total amount of A and B must be 80% or more of the total area occupied by all inclusions with an equivalent circle diameter of 1 μm or more. As long as it is within this range, good cutting performance can be obtained, and the more preferable range is 90% or more. In addition, as mentioned above, inclusions other than those indicated by A and B refer to nitrides, carbides, oxides, "substantial Ti sulfides or/and Ti carbosulfides", etc. existing alone. That is, formula (1) indicates that the total area of those inclusions other than "substantial MnS" is less than 20% of the total area (C in formula (1)) occupied by all inclusions, and it is more preferable that the total area less than 10%.

If Ti is added to steel containing a large amount of S in order to improve cutting performance, since Ti has a stronger tendency to form sulfide than Mn, it is easy to form Ti sulfide and Ti carbosulfide. However, although the formula (1) defined in the present invention presupposes the addition of Ti, it intends to suppress the formation of Ti sulfide and Ti carbosulfide. This is because Ti sulfide and Ti carbosulfide hinder the quasi-lubricating effect of MnS during cutting. Once the quasi-lubricating effect of MnS is damaged, the friction between the tool and the material to be cut will increase, forming a burr on the cutting edge of the tool, resulting in deterioration of the machined surface roughness. Therefore, it is necessary to suppress the formation of Ti sulfide and Ti carbon sulfide. That is, as specified in formula (1), there is basically no "substantial Ti sulfide or/and Ti carbosulfide" existing alone in the steel, and more than 80% of the inclusions contained in the steel are "substantial MnS", so that the quasi-lubrication effect during cutting can be obtained.

In this way, when the steel composition range specified in the present invention satisfies the formula (1), good machined surface roughness equal to or better than that of conventional Pb free cutting steels and composite free cutting steels can be obtained during finish cutting. On the other hand, even within the range of the chemical composition specified in the present invention, if the formula (1) is not satisfied, good cutting performance cannot be obtained.

3. The basis for specifying N _A ≥ 5

N _A in the above formula (2) is the essence of "the presence of Ti carbides or/and Ti carbonitrides in the inclusions with an equivalent circle diameter of 1 μm or more within 1 ^mm2 of the cross section parallel to the rolling direction. The number of MnS'. As described above, "substantial MnS having Ti carbides and/or Ti carbonitrides inside" means that the area ratio of MnS in one inclusion is 50% or more. This "substantial MnS with Ti carbide or/and Ti carbonitride inside" basically does not impair the pseudo-lubricating effect, so it is not easy to form a knife edge, and the machined surface roughness of the material to be cut will not deteriorate.

In addition, when using a cemented carbide tool to cut steel containing "substantial MnS with Ti carbide or/and Ti carbonitride inside" in a high-speed area exceeding 100 m/min, and then carefully observing the surface of the tool, it was found that the TiN is formed on the surface. It is believed that during the cutting process, on the surface of the tool in contact with the material to be cut, the temperature rises due to friction, and as the temperature rises, the Ti-based inclusions react and deteriorate, forming a layered layer with a thickness of several μm to tens of μm. Hard TiN. The presence of TiN can be confirmed by the following method, that is, after cutting, use Ar sputtering to remove carbon pollution (oil, etc.) instrument) for surface analysis and point analysis on the tool surface. According to these analysis results, the surface area of TiN attached to the tool is 10-80% of the contact surface between the material to be cut and the tool, and the rest is MnS and Fe attached during cutting or the tool matrix without attachment. Since this hard TiN is formed on the surface of the tool, the thermal diffusion wear of the tool and the mechanical wear caused by hard inclusions are suppressed. Compared with the previous sulfur free cutting steel and the composite free cutting steel of sulfur and lead, Excellent tool life can be achieved.

In order to obtain the above effects, it is only necessary ^to have 5 or more, preferably 10 or more "substantial MnS with Ti carbide or/and Ti carbonitride inside" within 1 mm of the cross-sectional observation plane in the rolling direction.

On the other hand, even within the range of the chemical composition specified in the present invention, if the condition of the formula (2) is not satisfied, it is impossible to obtain good cutting performance.

In steel to which Ti is added to form inclusions satisfying the formulas (1) and (2), MnS exists very finely. That is, the number of objects of MnS is significantly increased. Such minute MnS acts as a stress concentration point for chips generated during cutting, and contributes to the growth of cracks in chips, thereby improving chip handling properties.

In summary, as long as the steel stably contains "substantial MnS with Ti carbides or/and Ti carbonitrides inside", there are more than 5 within ^1mm2 of the observation surface of the section in the rolling direction, and the rolling The total of "substantial MnS with Ti carbides or/and Ti carbonitrides inside" and "substantial MnS without Ti carbides and Ti carbonitrides" within ^1mm2 of the viewing surface of the cross-section in the drawing direction The area is more than 80% of the total area of all inclusions. As mentioned above, it is possible to obtain tool life, machined surface roughness, and chip handling properties equal to or higher than lead free cutting steel and composite free cutting steel. In order to realize such inclusion morphology more stably and to manufacture steel with good machinability at low production cost by continuous casting method, the balance of Mn, Ti, S and N and content must be considered. Specifically, just follow the instructions below.

(a)Ti(%)/S(%)≤0.25

When a large amount of Ti is added relative to the amount of S, that is, when Ti/S exceeds 0.25 in terms of mass % ratio, many Ti sulfides and Ti carbosulfides are present in the steel. As a result, the expression (1) cannot be satisfied, and the pseudo-lubricating effect by MnS is impaired. At this time, the cutting resistance increases, and a knives tend to form easily on the cutting edge of the tool. As a result, the surface roughness deteriorates during finish cutting, and the machining accuracy deteriorates.

Conversely, when a small amount of Ti is added relative to the amount of S, that is, when the mass ratio of the two Ti/S is 0.25 or less, Ti forms Ti carbide or Ti carbonitride, and there is basically no "substantial Ti sulfide or/and Ti carbosulfide".

Ti carbides and Ti carbonitrides are precipitated in various forms, and they sometimes exist inside a single MnS. When using cemented carbide tools for high-speed cutting of steel containing "substantial MnS with Ti carbides or/and Ti carbonitrides inside", good tool life can be obtained, that is, to suppress the "substantial Ti sulfidation that exists alone Ti (%)/S (%) can be adjusted to 0.25 or less for the formation of Ti carbon sulfides and/or Ti carbon sulfides.

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

S is an element that causes cracks during hot working. However, as long as the appropriate composition is maintained, [Mn]/[S]≥1 in terms of the atomic ratio, that is, the ratio of the number of atoms (moles), Mn will form MnS crystals and precipitate, even if Ti(%)/S(% )≤0.25, there will be no problem with hot workability. In addition, as long as it is within this range, for example, under the premise of continuous casting, there is no problem with hot workability, so more S can be added to increase MnS that can effectively improve cutting performance, and even a higher The S content does not impair the shape of inclusions represented by the formulas (1) and (2).

In the case of [Mn]/[S]<1, if the amount of Ti added exceeds the amount of S, a large amount of sulfides of FeS dissolved in MnS and TiS will become the main body, and the hot workability cannot be improved. In addition, even if [Mn]/[S]<1, if the amount of Ti added exceeds the amount of S, hot workability can be improved. However, in this case, since Ti has a greater tendency to form sulfide than Mn, the main sulfide formed is not MnS, but Ti sulfide or Ti carbosulfide, which is harder than MnS, is the main one. In this case, as described above, the pseudo-lubrication effect due to the soft sulfide cannot be obtained between the tool and the workpiece during cutting, the cutting resistance increases, and the machined surface roughness deteriorates. That is, the amount of Mn and S is limited to [Mn]/[S]≥1 in terms of atomic ratio, which has the effect of improving machinability due to MnS and can also obtain good hot ductility, so it is a desirable condition .

(c) Ti(%)/N(%)≥1.35

The main feature of the free cutting steel of the present invention is that it contains "substantial MnS in which Ti carbides or/and Ti carbonitrides exist". If Ti(%)/N(%)<1.35, "substantial MnS with Ti carbide or/and Ti carbonitride inside" cannot be obtained sufficiently in some cases. In this case, most of the added Ti forms TiN crystals and precipitates at the initial stage of solidification, so sufficient Ti cannot be ensured to form "substantial MnS with Ti carbides or/and Ti carbonitrides inside". Therefore, Ti(%)/N(%) is preferably 1.35 or more. In order to obtain "substantial MnS with Ti carbide or/and Ti carbonitride inside" more stably, Ti(%)/N(%) Just above 1.5.

4. The basis for the limitation of chemical composition

The basis for defining the chemical composition in the present invention and the actions and effects of the various components will be described below.

C: less than 0.05% to 0.20%

C is an important element that greatly affects cutting performance. When the C content is 0.20% or more, the strength of the steel increases and the machinability deteriorates, so it is not suitable for applications in which machinability is particularly required. However, when the C content is less than 0.05%, the steel material becomes too soft, gnawing occurs during cutting, the wear of the tool is accelerated, and the roughness of the machined surface increases. Therefore, the appropriate C content is less than 0.05% to 0.20%. In order to obtain better cutting performance, the preferred range of C content is 0.07-0.18%.

Mn: 0.4-2.0%

Mn and S form sulfide-type inclusions and are important elements that greatly affect cutting performance. When its content is less than 0.4%, the absolute amount of sulfide formed is insufficient, and satisfactory cutting performance cannot be obtained. In addition, Mn is an element that improves the hardenability of steel, so when it is desired to obtain good carburizing performance, its content can be increased. However, in order for Mn and S to form MnS, a large amount of Mn must be contained in the steel of the present invention containing a large amount of S. If Mn is added to improve carburizing performance, the content of Mn increases, which is disadvantageous from the viewpoint of production cost. Therefore, the upper limit of the Mn content is made 2.0%. When it exceeds 2.0%, the strength of the steel material increases, the cutting resistance increases, and the life of the tool decreases. In order to reduce cutting resistance, increase tool life, improve chip disposal, increase machined surface roughness, and improve hot workability, the relationship between the content of Mn and the content of S is important. In order to surely obtain these properties, the content of Mn is preferably 0.6-1.8%.

S: 0.21-1.0%

S forms sulfides with Mn and is an element effective in improving cutting performance. The effect of improving machinability by MnS increases with the amount of MnS produced, so selection of the S content is important. When its content is less than 0.21%, sufficient amount of sulfide inclusions cannot be obtained, and satisfactory cutting performance cannot be obtained; on the contrary, when the S content exceeds 0.35%, the hot working performance deteriorates, which promotes S segregation in the central part of the steel ingot, Cracks are initiated during forging, but this can be increased up to 1.0% as long as the proper composition is maintained. In order to improve the machinability by using MnS, it is preferable to add more S, more preferably 0.35% or more, most preferably more than 0.40%. However, if it is added too much, the recovery rate will deteriorate and the production cost will increase, so the preferable upper limit of the S content is 0.70%.

Ti: 0.002-0.10%

Ti forms Ti carbides or/and Ti carbonitrides with N and C, and is an important element essential for the presence of MnS containing these compounds in steel. As mentioned above, if the steel contains "substantial MnS with Ti carbides or/and Ti carbonitrides inside", the life of the tool will be greatly improved when using a cemented carbide tool for high-speed cutting. In order for such MnS to exist, its content must be above 0.002%. However, in order to stably disperse it in the steel, not to deteriorate the surface roughness of the machined surface, and to obtain a good tool life, the content of Ti and the content of S and N must be considered. balance between content. In addition, when the Ti content exceeds 0.10%, due to the existence of "substantial Ti sulfide or/and Ti carbosulfide" in the steel, the surface roughness of the finish cutting is deteriorated. Therefore, the upper limit of the Ti content is limited to 0.10%. In order to obtain a good surface roughness more stably, the Ti content is preferably below 0.08%, more preferably below 0.03%.

On the other hand, when the Ti content is less than 0.002%, a sufficient amount of "substantial MnS containing Ti carbides and/or Ti carbonitrides inside" required to improve the tool life cannot be produced. In order to reliably generate this kind of MnS and improve the life of the cemented carbide tool, it is desirable that the content of Ti exceeds 0.01%.

P: 0.001-0.30%

P increases the hardenability of steel and also increases the strength of steel. In order to obtain this effect, its content should be 0.001% or more. In addition, as long as its content is less than 0.30%, the machinability will not deteriorate, and hardenability and strength can be ensured. However, when the content exceeds 0.30%, the strength of the steel will be too high, not only the machinability will be deteriorated, but also the ingot will be accelerated. segregation, resulting in deterioration of hot workability. Therefore, the content of P is specified to be 0.001-0.30%. For stably maintaining good cutting performance and strength, the more preferable content range is 0.005-0.13%.

Al: 0.2% or less (may not be added)

Al is used as a strong deoxidizer, and its content is ok below 0.2%. However, the oxides generated by deoxidation are hard. If the content exceeds 0.2%, a large amount of hard oxides will be generated, resulting in deterioration of cutting performance. Therefore, it is preferably 0.1% or less. In addition, when sufficient deoxidation can be achieved by adding C and Mn, Al may not be added, and its content may be an impurity content level of 0.002% or less.

O (oxygen): 0.001-0.03%

The effect of oxygen in the steel of the present invention is not damaged due to the deoxidation state. It contains an appropriate amount of oxygen and is dissolved in MnS, which can prevent the extension of MnS caused by rolling and improve the anisotropy of mechanical properties. In addition, oxygen is also effective for the improvement of cutting performance, hot workability and S segregation. However, if its content exceeds 0.03%, it will cause deterioration of the refractory during smelting. Therefore, the range of the oxygen content is specified as 0.001-0.03%. In order to properly obtain the above effects, a more preferable content range is 0.0015-0.01%.

N: 0.0005-0.02%

N easily forms hard nitrides with Al and Ti, and these nitrides have the effect of refining grains. However, if these nitrides are present in a large amount, the wear of the tool tends to be accelerated, resulting in deterioration of the cutting performance. In the steel of the present invention, since Ti is an essential component to be added, the smaller the N content, the better. However, in order to obtain the above effects, it is specified to contain 0.0005% or more. On the other hand, if the N content is too high, coarse TiN may be formed and the machinability may be impaired, so the upper limit of the N content is made 0.02%. In order to ensure better cutting performance, the upper limit of the N content is preferably 0.015%. In addition, in the present invention, the existence of "substantial MnS with Ti carbides or/and Ti carbonitrides inside" is used to improve the cutting performance. In order to make such MnS stably exist in the steel, Ti and N should meet the requirements of Ti (%)/N(%)≥1.35. This is because, as mentioned above, when Ti(%)/N(%)<1.35, most of the added Ti will form TiN in the initial stage of solidification, and it is impossible to stably obtain the "internal presence of Ti carbide or/and Ti carbonitride". Substantial MnS of compounds".

By adjusting the chemical composition of each element in the content and the inclusion morphology specified in formula (1) and formula (2) as described above, low-carbon steel with good cutting performance, hot workability and machined surface properties can be obtained. Free cutting steel.

The low-carbon free-cutting steel of the present invention may further contain one or more components selected from at least one of the following groups 1 to 3.

(1) Group 1 element

Elements in the first group are elements that further improve the machinability of steel without impairing the effect of the present invention, in addition to the above-mentioned main components. Therefore, in order to obtain better cutting performance, one or more of these elements may be contained.

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

Se and Te, and Mn generate Mn(S, Se) and Mn(S, Te). Like MnS, these inclusions act as pseudo-lubricants during cutting and are elements effective in improving cutting performance. These inclusions may be contained within the above-mentioned range in order to further improve cutting performance. However, when the content of each of them is less than 0.0005%, the effect is insufficient, and conversely, when the content of Se and Te exceeds 0.10%, the effect becomes saturated, which is not only uneconomical, but also deteriorates the hot workability. In order to have both good hot workability and cutting performance more stably, their respective contents are preferably 0.0010-0.05%.

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

Bi and Sn have the effect of improving the machinability of steel. This is considered to be because, like Pb, they form low-melting-point metal inclusions and play a lubricating role during cutting. In order to reliably obtain this effect, their respective contents are preferably 0.01% or more. However, when the contents thereof exceed 0.3%, respectively, the above effects are saturated, and hot workability is deteriorated. In order to have both good hot workability and cutting performance more stably, their content is preferably 0.03-0.1% respectively.

Ca: 0.0001-0.01%

Ca has a high affinity for S and O (oxygen), and forms sulfides and oxides in steel. In addition, Ca is solid-dissolved in MnS to form (Mn, Ca)S, but the amount of Ca solid-dissolved therein is small, so the effect of MnS will not be impaired. In addition, oxides formed of Ca are low-melting oxides, and are effective additive elements for further improving machinability in the steel of the present invention. In order to reliably obtain the effect of improving machinability by adding Ca, the lower limit of the Ca content should be 0.0001%. However, since the effective utilization rate of Ca addition is very poor, in order to increase the Ca content, a large amount of Ca must be added, which is unfavorable from the viewpoint of production cost. Therefore, the upper limit of the Ca content is made 0.01%, and the preferable upper limit is 0.005%.

Mg: 0.0001-0.005%

Mg also has a high affinity for S and O (oxygen) in steel, and forms sulfides or oxides. Sulfides and oxides containing Mg function as crystal nuclei of MnS and have the effect of inhibiting the elongation of MnS. It can also be added when this effect is required. In order to sufficiently obtain this effect, the lower limit of the Mg content is preferably 0.0001% or more. However, the oxide formed by Mg is hard, and if the Mg content is too large, the machinability will deteriorate. Therefore, the upper limit of the content of Mg is made 0.005%. In order to achieve both the effect of suppressing the elongation of MnS and good cutting performance, the preferable upper limit is 0.002%.

B: 0.0002-0.02%

B combines with O (oxygen) or N to form oxides or nitrides, which has the effect of improving cutting performance and can be added as needed. In order to obtain this effect, its content should be more than 0.0002%. In order to obtain this effect more reliably, its content is desirably 0.0010% or more. However, when the B content exceeds 0.02%, not only the above effects are saturated, but also the hot workability is deteriorated.

Rare earth elements: 0.0005-0.02%

Rare earth elements are a group of elements classified as lanthanides. When rare earth elements are added, a misch metal alloy or the like mainly composed of these elements is generally used. The content of rare earth elements in the present invention is represented by the total amount of one or more elements of rare earth elements. Rare earth elements form oxides with oxygen, and combine with S to form sulfides to improve cutting performance. In order to reliably obtain this effect, its content should be 0.0005% or more. However, when the content thereof exceeds 0.02%, the above-mentioned effects are saturated. In addition, the effective utilization rate of addition of rare earth elements is low, so it is not economical to contain a large amount of rare earth elements.

(2) Group 2 elements

All elements in the second group are elements that have an effect of increasing the strength of steel. One or more of these elements may be contained as needed.

Cu: 0.01-1.0%

Cu has the effect of increasing the strength of steel by precipitation strengthening. In order to obtain this effect, its content must be above 0.01%. In order to obtain such an effect more reliably, it is desirable to add 0.1% or more. However, when the content thereof exceeds 1.0%, not only the hot workability is deteriorated, or the above effect is saturated due to the coarsening of Cu precipitates, but also the machinability is reduced.

Ni: 0.01-2.0%

Ni has the effect of increasing the strength of steel by solid solution strengthening. In order to securely obtain this effect, its content is preferably 0.01% or more. However, when the content exceeds 2.0%, the machinability deteriorates and the hot workability also deteriorates.

Mo: 0.01-0.5%

Mo is an element that can improve hardenability. However, if a considerable amount of Mo is added to obtain the same effect as the addition of Si and Cr in order to improve carburization performance, because its price is higher than that of Si and Cr, it is very expensive. The disadvantage of increased cost. However, Mo also has the effect of refining the structure and improving the toughness, and it may be added when these effects are desired. In order to surely obtain this effect, its content is desirably 0.01% or more. However, if it exceeds 0.5%, not only the above effects are saturated, but also the production cost of steel increases.

V: 0.005-0.5%

V forms fine nitride and carbonitride precipitates, increasing the strength of steel. This effect can be obtained when its content is 0.005% or more, and it is preferable to contain 0.01% or more when it is desired to obtain this effect more reliably. However, when its content exceeds 0.5%, not only the above-mentioned effects are saturated, but also excessive nitrides and carbides are formed, resulting in reduced cutting performance.

Nb: 0.005-0.5%

Nb forms fine nitrides and carbonitrides to precipitate and increases the strength of the steel. This effect can be obtained when its content is 0.005% or more, and it is preferable to contain 0.01% or more when it is desired to obtain this effect more reliably. However, when its content exceeds 0.5%, not only the above-mentioned effects are saturated, but excessive nitrides and carbides are formed, resulting in a decrease in cutting performance, and it is also uneconomical.

(3) Group 3 elements

Group 3 elements are elements that may contain one or both of them within the following ranges when it is desired to improve the carburizing performance of steel.

Si: 0.1-2.0%

In the free-cutting steel of the present invention described in claims 1-4, Si is not intentionally added. Therefore, Si is an impurity whose content is less than 0.1%. In addition, in the free-cutting steel of the present invention described in claims 1-4, Si may also be added as a deoxidizing element in order to make an appropriate amount of oxygen in the steel exist, but in this case, it is not necessary to intentionally remain, and the steel The remaining Si in the alloy is an impurity, and its content is less than 0.1%.

In addition, Si dissolves in ferrite to increase the strength of the steel and also has the effect of improving the hardenability of the steel. By improving the hardenability of steel, the purpose of improving carburizing performance required as an automobile part can be achieved. Only in this case, 0.1% or more of Si may be contained. When it is desired to improve the carburizing performance more reliably, the content is expected to exceed 0.6%, but when the content exceeds 2.0%, the hot workability will deteriorate, or the cutting resistance will increase due to the solid solution strengthening of the ferrite phase, which will affect the cutting performance. produce adverse effects. In addition, even at an impurity content level of less than 0.1%, the oxygen content in the steel can be brought into an appropriate range by appropriately adding C, Mn, and Al.

Cr: 0.03-1.0%

Cr improves the hardenability of steel, and a small amount of addition can improve carburizing performance. The carburizing performance of steel containing Cr is improved, the hardness of the carburized layer after carburizing treatment is higher, and the effective hardening depth can also be increased. In order to obtain this effect, the content of Cr should be 0.03% or more. In addition, in order to improve carburizing performance more reliably, its content is desirably higher than 0.05%. However, when the Cr content exceeds 1.0%, not only the machinability deteriorates, but also the production cost increases.

When the above-mentioned Si and/or Cr are contained, steel having good machinability and hot workability and good carburizing performance can be obtained.

【Example】

1. Preparation of samples

150 kg of steel ingots (diameter: about 220 mm) having various compositions shown in Table 1 and Table 2 were produced using a high-frequency heating induction furnace. Table 1 shows steels of the present invention, and Table 2 shows conventional steels or comparative steels. These ingots were heated to a high temperature of 1250°C in order to stably produce "substantial MnS with Ti carbides or/and Ti carbonitrides inside", and then kept at this temperature for more than 2 hours, and then in order to simulate the rolling The process is precision forging above 1000°C, and a round bar with a diameter of 65mm is obtained after air cooling. Then, the stretched forged material was heated to 950° C., maintained for 1 hour, and then air-cooled to perform normalizing treatment. In addition, the steel No. 51-53 of the comparative example was poor in hot workability, cracks occurred during forging, and it could not be made into a stretch forging material, so the following investigation was not carried out.

Table 1 Steel No. Chemical composition (mass%, balance: Fe and impurities) [Mn]/[S] (atomic ratio) Ti/S (mass% ratio) Ti/N (mass% ratio) (A+B)/C N _A ≥ 5 C Si mn P S Al Ti Cr N o other 1 0.05 <0.01 0.89 0.015 0.42 <0.001 0.092 - 0.0012 0.0096 - 1.11 0.22 17.00 0.95 ○ 2 0.06 <0.01 1.13 0.019 0.41 <0.001 0.060 - 0.0085 0.0089 - 1.61 0.15 7.02 0.96 ○ 3 0.09 0.06 0.97 0.032 0.44 0.002 0.009 - 0.0028 0.0018 - 1.28 0.02 3.21 0.99 ○ 4 0.10 0.04 1.49 0.029 0.55 0.001 0.061 - 0.0047 0.0033 - 1.58 0.11 12.98 0.98 ○ 5 0.10 0.05 1.58 0.032 0.49 <0.001 0.018 - 0.0047 0.0027 - 1.88 0.04 3.83 0.97 ○ 6 0.10 <0.01 0.96 0.016 0.36 <0.001 0.087 - 0.0063 0.0046 - 1.56 0.24 13.79 0.88 ○ 7 0.10 <0.01 0.85 0.018 0.35 <0.001 0.080 - 0.0085 0.0079 - 1.42 0.23 9.41 0.94 ○ 8 0.12 0.04 1.10 0.030 0.38 <0.001 0.026 - 0.0125 0.0018 - 1.69 0.07 2.08 0.96 ○ 9 0.13 <0.01 1.59 0.032 0.48 <0.001 0.024 - 0.0050 0.0043 - 1.95 0.05 4.80 0.98 ○ 10 0.15 0.07 1.49 0.030 0.49 0.025 0.014 - 0.0047 0.0020 - 1.77 0.03 2.98 0.99 ○ 11 0.18 0.03 1.52 0.058 0.47 0.002 0.021 - 0.0130 0.0036 - 1.89 0.04 1.62 0.96 ○ 12 0.09 0.02 1.17 0.022 0.40 0.005 0.053 0.20 0.0054 0.0035 - 1.71 0.13 9.81 0.95 ○ 13 0.11 0.46 1.60 0.020 0.60 <0.001 0.014 - 0.0101 0.0026 - 1.56 0.02 1.39 0.98 ○ 14 0.12 1.28 1.46 0.015 0.44 0.002 0.024 - 0.0058 0.0027 - 1.93 0.05 4.14 0.98 ○ 15 0.15 0.04 1.12 0.026 0.40 <0.001 0.027 0.50 0.0125 0.0038 - 1.63 0.07 2.16 0.96 ○ 16 0.18 0.85 1.36 0.040 0.39 <0.001 0.021 - 0.0134 0.0031 - 2.04 0.05 1.57 0.97 ○ 17 0.18 0.01 1.46 0.028 0.46 0.002 0.025 0.15 0.0048 0.0064 - 1.85 0.05 5.21 0.98 ○ 18 0.14 0.06 1.65 0.028 0.45 0.002 0.018 - 0.0049 0.0024 Se: 0.010 2.14 0.04 3.67 0.99 ○ 19 0.08 0.13 1.60 0.030 0.42 <0.001 0.014 - 0.0095 0.0028 Te: 0.015 2.22 0.03 1.47 0.98 ○ 20 0.12 0.03 1.50 0.031 0.45 0.002 0.022 - 0.0120 0.0032 Bi: 0.05 1.95 0.05 1.83 0.98 ○ twenty one 0.16 0.01 1.35 0.025 0.42 0.002 0.020 - 0.0059 0.0055 Sn: 0.04 1.88 0.07 4.75 0.98 ○ twenty two 0.06 0.01 0.88 0.017 0.49 0.001 0.082 - 0.0062 0.0073 Ca: 0.0029 1.05 0.17 13.27 0.99 ○ twenty three 0.10 0.05 1.02 0.020 0.46 0.004 0.025 - 0.0037 0.0026 Mg: 0.0015 1.29 0.05 6.76 0.97 ○ twenty four 0.10 0.01 1.48 0.027 0.47 0.002 0.021 - 0.0095 0.0048 B: 0.0025 1.84 0.04 2.21 0.97 ○ 25 0.11 0.01 1.22 0.029 0.37 0.002 0.022 - 0.0124 0.0025 Cu: 0.10 1.92 0.06 1.77 0.98 ○ 26 0.15 0.02 1.38 0.035 0.43 0.002 0.025 - 0.0045 0.0056 V: 0.05 1.87 0.06 5.56 0.98 ○ 27 0.12 <0.01 1.55 0.030 0.46 <0.001 0.026 - 0.0047 0.0050 Nb: 0.12 1.97 0.06 5.53 0.98 ○ 28 0.16 0.07 1.00 0.025 0.25 0.002 0.060 - 0.0101 0.0092 Ni: 0.10 2.33 0.24 5.93 0.89 ○ 29 0.10 0.03 1.40 0.038 0.41 0.002 0.029 - 0.0052 0.0064 Mo: 0.10 1.99 0.07 5.58 0.97 ○ 30 0.11 0.01 1.25 0.035 0.46 <0.001 0.025 - 0.0130 0.0048 Ca: 0.0015, Mg: 0.0018 1.59 0.05 1.92 0.97 ○ 31 0.09 0.05 1.44 0.027 0.43 0.003 0.022 - 0.0112 0.0030 Bi: 0.07, Nb: 0.10 1.95 0.05 1.96 0.98 ○ 32 0.10 0.02 1.46 0.026 0.42 0.003 0.022 - 0.0132 0.0024 Sn: 0.07, V: 0.10 2.03 0.05 1.67 0.98 ○ 33 0.12 0.05 0.99 0.031 0.40 0.001 0.029 - 0.0085 0.0030 Ca: 0.001, Mo: 0.12 1.44 0.07 3.41 0.97 ○ 34 0.10 0.03 1.52 0.032 0.45 <0.001 0.019 - 0.0090 0.0035 Te: 0.012, Ni: 0.11 1.97 0.04 2.11 0.98 ○

Table 2 Steel No. Chemical composition (mass%, balance: Fe and impurities) [Mn]/[S] (atomic ratio) Ti/S (mass% ratio) Ti/N (mass% ratio) (A+B)/C N _A ≥ 5 C Si mn P S Al Ti Cr N o other 35 0.07 <0.01 1.02 0.070 0.32 0.002 - - 0.0052 0.0160 *Pb: 0.31 1.86 - - - - 36 0.08 0.01 1.12 0.060 0.31 0.002 - - 0.0084 0.0145 *Pb: 0.18 2.11 - - - - 37 0.08 0.01 1.02 0.067 0.33 0.002 - - 0.0066 0.0150 - 1.80 - - - - 38 0.06 0.06 0.86 0.020 0.42 0.001 *0.280 - 0.0050 0.0068 - 1.20 *0.67 56.0 *0.75 ○ 39 0.08 0.02 1.15 0.033 0.35 0.001 *0.250 - 0.0079 0.0052 - 1.92 *0.71 31.6 *0.70 ○ 40 0.10 0.01 1.12 0.028 0.36 <0.001 *0.330 - 0.0085 0.0053 - 1.82 *0.92 38.8 *0.65 ○ 41 0.18 0.03 0.47 0.016 0.29 0.003 *0.420 - 0.0059 0.0019 - *0.95 *1.45 71.2 *0.20 ○ 42 0.10 0.01 1.05 0.013 0.39 0.001 0.006 - 0.0099 0.0035 - 1.57 0.02 *0.6 0.99 ×* 43 0.09 0.35 1.21 0.025 0.30 0.001 0.009 - 0.0138 0.0084 - 2.35 0.03 *0.7 0.98 ×* 44 *0.52 0.17 0.52 0.016 0.21 <0.001 0.050 - 0.0079 0.0180 - 1.45 0.24 6.4 0.96 ○ 45 *0.45 <0.01 0.85 0.019 0.32 0.002 0.078 - 0.0046 0.0058 - 1.55 0.24 17.0 0.95 ○ 46 *0.01 <0.01 0.98 0.016 0.33 0.002 0.067 - 0.0048 0.0048 - 1.73 0.20 14.0 0.96 ○ 47 0.10 0.02 1.45 0.016 0.49 *0.35 0.028 - 0.0057 0.0017 - 1.73 0.06 4.9 0.97 ○ 48 0.07 0.01 0.92 0.015 0.47 <0.001 0.056 *2.50 0.0075 0.0036 - 1.14 0.12 7.5 0.99 ○ 49 0.09 *2.55 1.56 0.020 0.45 0.001 0.065 - 0.0058 0.0024 - 2.02 0.14 11.2 0.98 ○ 50 0.05 <0.01 0.48 0.015 *0.15 <0.001 0.022 - 0.0095 0.0193 - 1.87 0.15 2.3 0.92 ○ 51 0.09 0.01 1.85 0.018 *1.14 0.001 0.102 - 0.0078 0.0128 - *0.95 0.09 13.1 - - 52 0.06 <0.01 *0.21 0.016 0.33 0.001 0.074 - 0.0073 0.0078 - *0.37 0.22 10.1 - - 53 0.08 <0.01 1.25 0.025 0.44 0.002 0.072 - 0.0050 0.0049 *Te: 0.12 1.66 0.16 14.4 - - 54 0.06 <0.01 1.03 0.016 0.46 <0.001 0.045 - 0.0059 0.0085 *V: 2.0 1.31 0.10 7.6 0.99 ○ 55 0.15 <0.01 1.11 0.015 0.46 0.002 0.080 - 0.0078 0.0063 *Mo: 1.50 1.41 0.17 10.3 0.98 ○

          * means that the conditions stipulated in the present invention are not met

2. Investigation of the shape of inclusions

The inclusions observed on the section parallel to the rolling direction are mostly elongated in the processing direction and of indeterminate shape. When investigating the number and area of inclusions, cut out a test piece for observing the microstructure from the longitudinal section direction of the Df/4 (Df: diameter of the stretched forging material) part of the stretched forging material, and embed it in resin After that, mirror polishing is carried out, and the number and area of inclusions are obtained by using a 400-fold optical microscope to observe and take pictures, and image analysis and other methods are used. In this case, the target is an inclusion whose diameter (circle-equivalent diameter) when converted to a circle having the same area is 1 μm or more. As described above, the circle-equivalent diameter is limited to 1 μm or more because inclusions smaller than 1 μm have little influence on cutting performance.

In addition, confirm the composition of these inclusions as described below. That is, as described above, a microscopic test piece was cut out in the direction of the longitudinal section of the Df/4 (Df: diameter of the drawn forging material) portion of the stretched forged material, embedded in resin, and then mirror-polished. EPMA (Electron Probe Microanalyzer) and EDX (Energy Dispersive X-ray Analyzer), etc. perform surface analysis and timing analysis. At this time, the observation magnification can be selected within the range of no more than 10000 times. Under this observation magnification, it is observed that MnS is clearly phase-separated from Ti carbide or/and Ti carbonitride in one inclusion and the area ratio of MnS Inclusions of 50% or more are "substantial MnS in which Ti carbides or/and Ti carbonitrides exist". Based on the results of such observations, each of the "substantial MnS with Ti carbides or/and Ti carbonitrides inside" and "substantial MnS with no Ti carbides and Ti carbonitrides inside" with an equivalent circle diameter of 1 μm or more was determined. ", calculate the total area of these inclusions within 1mm ² on the rolling direction section, and then calculate the total area occupied by all inclusions within 1mm ² on the rolling direction section, and calculate (A +B)/C.

Based on the above results, the number of "substantial MnS with Ti carbides or/and Ti carbonitrides inside" was measured, and for steels with an average number of 5 or more per ¹ mm2 on the cross-section in the rolling direction, it is marked as "○". On the contrary, "×" is marked for steels with less than 5 "substantial MnS having Ti carbides and/or Ti carbonitrides inside". In addition, the steel No. 35-37 of the comparative example shown in Table 2 is a Pb free cutting steel and an S free cutting steel that do not contain Ti, and substantially does not contain the essence of "the presence of Ti carbides or/and Ti carbonitrides inside". MnS", so these calculations were not performed.

3. Cutting performance test

In the machinability test, a test for investigating tool life and machined surface roughness was performed using a round bar in which the outer surface of the elongated forged material was cut to a diameter of 60 mm. The tool life test uses uncoated P20 cemented carbide tools specified in JIS under the conditions of cutting speed: 150 m/min, feed rate: 0.10 mm/rev, depth of cut: 2.0 mm, and dry cutting Turning was carried out on the ground surface, and the average flank wear amount was measured 30 minutes after the start of cutting. In addition, for the test material 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 until the average burr wear (VB) reached 100 μm as an index of tool life. For the case where the test material is insufficient due to good wear resistance and extremely low wear rate during the test, the time when the average thick gap surface wear reaches 100 μm is calculated by regression analysis according to the turning time-tool wear curve. In addition, 200 representative chips were selected from the discharged chips, their weight was measured, and the number of chips per unit weight was calculated to evaluate the chip handling properties.

The processed surface roughness was evaluated by the surface roughness after finish cutting, and the surface of the workpiece after cutting under the following conditions was evaluated using a stylus type surface roughness meter. The cutting conditions described are, using JISK cemented carbide tools coated with TiALN multilayer, cutting speed: 100 m/min, feed rate: 0.05 mm/rev, depth of cut: 0.5 mm, using water-soluble emulsion type Lubricating oil for turning under wet conditions. For the test piece after cutting the steel for the test under the above conditions for 1 minute, use a stylus type surface roughness measuring instrument to move the stylus along the axial direction of the test piece to measure the average machined surface roughness (Ra), Evaluate the machined surface roughness.

4. Thermal processing performance test

In order to simulate the manufacturing conditions using continuous casting equipment, a diameter of 10 mm and a length of 130mm high temperature tensile test piece, with a fixed interval of 110mm, directly energized and heated to 1250°C, kept for 5 minutes, then cooled to 1100°C at a cooling rate of 10°C/s, kept for 10 seconds at a rate of 1 0 ^-3 /s Strain rate for tensile tests. At this time, the reduction of area at the fracture site was measured to evaluate the hot workability.

5. Carburizing test

Carburizing tests were carried out as described below. That is, a cylindrical steel material having a diameter of 24 mm and a length of 50 mm was used as a test piece. This test piece was cut out at the R/2 position of the normalizing material with a diameter of 65 mm. This test piece was heated to 900°C for carburizing treatment, and then diffusion treatment was performed at 850°C. At this time, the carbon potential (C.P.) value at the time of carburizing was 0.8%, the treatment time was 75 minutes, and the C.P. value at the time of diffusion was 0.7%, and the treatment time was 20 minutes. The test piece after the carburizing treatment was put into 80° C. oil to cool and quenched. 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 performance is as follows.

Measure the Vickers hardness distribution from the surface to the inside on the cross-section of the test piece that has been carburized, quenched and tempered at a position 25 mm from the end (that is, the center in the longitudinal direction), and obtain the effective hardening depth of Hv400. It is judged whether this value is larger or smaller than the conventional lead-clad free cutting steel. The conventional lead-clad free-cutting steel is steel No. 35 in Table 2, and its effective hardening depth is 0.25 mm. As an evaluation of carburizing performance, when the effective hardening depth is ±0.05mm relative to steel No. 35, that is, when the effective hardening depth is 0.20-0.30mm, the evaluation is equal; when it is less than 0.20mm, the evaluation is inferior; when it exceeds When it is 0.30mm, it is evaluated as superior. In Table 3 and Table 4, the above results are represented by ◯, × and ⊚. When equal, it is "○", when it is inferior, it is "×", and when it is better, it is "◎".

The above test results are summarized in Table 3 and Table 4. In addition, Fig. 2 shows the relationship between (A+B)/C in the formula (1) and the machined surface roughness, Fig. 3 shows the relationship between the machined surface roughness and the tool life, and Fig. 4 shows the chip disposability relationship with tool life.

table 3 Steel No. rate of reduction in area(%) Tool wear after 30 minutes (μm) VB=100μm time (minutes) Chip disposal (number/g) Average roughness of processed surface (μm) Carburizing Performance Evaluation 1 61.8 45 90 18 0.7 ○ 2 64.3 39 98 14 0.6 ○ 3 64.5 48 85 12 0.2 ○ 4 80.2 32 125 18 0.5 ○ 5 76.8 43 96 twenty one 0.3 ○ 6 63.6 44 91 14 0.7 ○ 7 67.2 38 96 16 0.7 ○ 8 65.2 42 98 17 0.3 ○ 9 81.4 36 121 20 0.4 ○ 10 76.8 45 93 19 0.4 ○ 11 73.4 40 103 16 0.4 ○ 12 71.8 42 100 14 0.5 ◎ 13 64.7 45 93 18 0.2 ◎ 14 82.9 46 90 18 0.2 ◎ 15 65.2 42 98 17 0.4 ◎ 16 72.8 44 95 18 0.4 ◎ 17 80.1 42 95 12 0.4 ◎ 18 71.0 38 110 18 0.3 ○ 19 69.5 40 98 19 0.3 ○ 20 62.9 45 92 twenty four 0.3 ○ twenty one 64.8 46 93 13 0.2 ○ twenty two 60.8 29 120 19 0.5 ○ twenty three 62.8 43 95 twenty two 0.6 ○ twenty four 81.2 38 105 16 0.5 ○ 25 75.9 41 101 20 0.4 ○ 26 78.7 43 97 16 0.3 ○ 27 75.4 40 105 twenty two 0.3 ○ 28 80.5 42 93 14 0.8 ○ 29 78.8 36 115 18 0.3 ○ 30 64.2 37 105 twenty four 0.4 ○ 31 60.9 46 91 20 0.3 ○ 32 61.3 44 90 18 0.3 ○ 33 64.3 twenty four 126 19 0.5 ○ 34 67.2 41 99 17 0.3 ○

Table 4 Steel No. rate of reduction in area(%) Tool wear after 30 minutes (μm) VB=100μm time (minutes) Chip disposal (number/g) Average roughness of processed surface (μm) Carburizing Performance Evaluation 35 49.8 98 36 9 0.4 ○ 36 49.6 99 30 8 0.7 ○ 37 55.4 165 17 6 0.7 ○ 38 74.3 35 113 15 1.3 x 39 73.0 40 98 17 1.4 x 40 68.2 45 92 18 1.5 x 41 64.2 71 68 16 1.7 x 42 67.5 89 38 15 0.5 ○ 43 79.0 85 42 13 0.6 ◎ 44 52.8 93 36 12 1.3 ◎ 45 66.9 88 39 15 1.0 ◎ 46 57.5 103 29 8 1.6 x 47 78.7 101 30 16 0.3 ○ 48 56.0 232 12 12 1.8 ◎ 49 59.8 206 16 14 1.9 ◎ 50 65.3 129 20 9 1.3 ○ 51 6.4 - - - - - 52 4.3 - - - - - 53 3.4 - - - - - 54 59.5 198 17 15 1.8 ○ 55 58.5 264 10 10 1.9 ◎

Steel No. 35 and No. 36 in Figure 2 are composite free-cutting steels, and steel No. 37 is sulfur free-cutting steel. They are the steels with the best cutting performance so far. It can be seen from Table 3, Table 4, Figure 2 and Figure 3 that the tool life and surface roughness of the steel of the present invention are good. In addition, steel No. 1-34 of the present invention has good hot workability, and the reduction of area in the high-temperature tensile test simulating the practical manufacturing conditions using continuous casting equipment is shown in Table 3. Composite free-cutting steel and sulfur free-cutting Steel equal or higher without any issues.

Steel Nos. 12-17 in Table 1 are steels containing at least one of Si and Cr within a specified range in order to improve carburizing performance. Among the steels of the present invention, these steels exhibit particularly excellent carburizing properties. On the other hand, steel No. 35-55 is a steel in which one of the morphology and chemical composition of inclusions falls outside the range specified in the present invention, and its tool life, machined surface roughness, chip disposability, and hot workability are at least There is one property which is inferior to the steel of the present invention.

The free-cutting steel of the present invention does not contain Pb, has cutting performance equal to or higher than that of conventional Pb free-cutting steels and composite free-cutting steels, and has good machined surface properties after cutting. In addition, steel containing Si and/or Cr also has good carburizing properties. This steel also has good hot workability and can be manufactured at low production costs by continuous casting. Since it does not contain Pb, there is no need to worry about environmental pollution. Therefore, the free-machining steel of the present invention is very suitable as a material for various machine parts.

Although some typical specific embodiments of the present invention have been described in detail above, those skilled in the art should understand that some modifications can be made to the specific embodiments within the scope of not departing from the spirit of the present invention. Accordingly, all such modifications are intended to be included within the scope of this invention.

Claims (5)

1. A low-carbon free-cutting steel, characterized in that it contains C: less than 0.05% to 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%, the balance is Fe and unavoidable impurities, and the inclusions contained in the steel satisfy the following ( 1) formula and (2) formula, (A+B)/C≥0.8····(1) N _A ≥ 5... (2) In the formula, the meanings of A, B, C and N _A are as follows: A: The total area occupied by the substantial MnS with Ti carbide or/and Ti carbonitride in the inclusions with an equivalent circle diameter of 1 μm or more within ^1mm2 of the cross section parallel to the rolling direction; B: The total area occupied by substantial MnS without Ti carbides and Ti carbonitrides within 1 ^mm2 of the cross section parallel to the rolling direction, among the inclusions with an equivalent circle diameter of 1 μm or more; C: The total area occupied by all inclusions with an equivalent circle diameter of 1 μm or more within ^1mm2 on the section parallel to the rolling direction; N _A : The number of substantive MnS in which Ti carbides and/or Ti carbonitrides exist inside inclusions with an equivalent circle diameter of 1 μm or more within 1 mm ² of a section parallel to the rolling direction. 2. The low-carbon free-cutting steel according to claim 1, wherein instead of a part of Fe, one or more elements selected from the group below are contained Se: 0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn: 0.01-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02%, and rare earth Elements: 0.0005-0.02%. 3. The low-carbon free-cutting steel according to claim 1, wherein instead of a part of Fe, one or more elements selected from the group below are contained 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%. 4. The low-carbon free-cutting steel according to claim 1, wherein instead of a part of Fe, one or more elements selected from the group below are contained Se: 0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn: 0.01-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02%, and rare earth Elements: 0.0005-0.02%, and contain one or more elements selected from the following group 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%. 5. The low-carbon free-cutting steel according to any one of claims 1-4, characterized in that instead of a part of Fe, one of Si: 0.1-2.0% by mass and Cr: 0.03-1.0% by mass or 2 kinds.

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