JP5461929B2 - BN free-cutting steel with excellent chip disposal - Google Patents

Description

  The present invention relates to BN free-cutting steel, and particularly to a machinability that is excellent in chip disposal.

  Free-cutting steel is a steel whose machinability is improved by containing a low-melting-point metal or a free-cutting compound in the steel. Pb and Bi are low-melting-point metals, sulfides (sulfide) as free-cutting compounds. ), Oxides (calcium-based), and those using BN compounds have been developed.

  Among these free-cutting steels, Pb free-cutting steel has been used widely because it is inexpensive and has excellent performance. However, the application of Pb has been restricted from the viewpoint of global environmental protection, and new lead-free free-cutting steel has been newly developed. Interest in BN free-cutting steel, which is a type of steel, is increasing.

  Patent Document 1 relates to BN free-cutting steel excellent in hot ductility and improved in mechanical properties and machinability without deteriorating hot ductility when BN is contained in steel. And those suitable as automobile parts such as undercarriage parts.

  Patent Document 2 relates to high-strength, high-toughness free-cutting steel, in order to improve fatigue strength and impact resistance characteristics in a certain angle direction with BN expanding in the working direction by rolling or casting, and finely sulfide (sulfide). It is described that the adverse effect of BN having sulfide (sulfide) as a precipitation nucleus is reduced.

  Patent Document 3 relates to a non-tempered free-cutting steel that is finished by performing a finishing process such as a cutting process without performing a tempering treatment, so that excellent machinability can be obtained without reducing the strength. BN free-cutting steel containing a specific amount of is described.

  Patent Document 4 relates to a free-cutting steel for carburizing and quenching that exhibits excellent machinability when carburizing and quenching of a surface layer that has been finished by conventional grinding after carburizing and quenching is finished by cutting with a ceramic tool.

  By separately adding B and N into the molten steel, the problem of difference in specific gravity when the BN compound is directly added to the molten steel is solved, and the strength and machinability are ensured with the N / B ratio as a specific range. Furthermore, BN free-cutting steel is described in which generation of a grain boundary oxide layer harmful to the tool life is suppressed by reducing the amount of Si, and tool wear in cutting of the carburized and quenched portion is greatly reduced.

  Patent Document 5 is a steel for induction-hardened gears that enables surface hardening treatment by induction hardening instead of carburizing and quenching performed after machining such as gear cutting, and becomes a fine particle in the material structure. BN free-cutting steel with improved machinability due to the lubrication effect of uniformly distributed BN is described.

Japanese Patent No. 2733898 JP-A-6-145890 JP-A-1-219148 Japanese Patent Laid-Open No. 3-10047 Japanese Patent No. 3239432

  As described above, various proposals have been made regarding BN free-cutting steel. Generally, BN free-cutting steel is difficult to sufficiently improve machinability because BN compounds are fine precipitates. Improvements are desired.

  Although machinability is a concept that includes tool life, chip disposability, surface roughness, etc., the study of machinability in BN free-cutting steel is made mainly from the viewpoint of tool life, and the continuity of chips generated by cutting. There are few studies from the viewpoint of chip disposal that is evaluated by the properties.

  Then, an object of this invention is to provide BN free cutting steel excellent in machinability, especially chip disposal.

There are four main mechanisms for improving the machinability of free-cutting steel.
1. Chip generation is facilitated by the notch effect.
2. It embrittles the material and facilitates chip generation in the shear region.
3. Provide lubricity at the contact surface between the tool and chips or work material.
4). Prevent diffusion reaction between tool and work material.

  Among these, the present inventors proceeded with studies from the viewpoint of the mechanisms 1 and 2 that are particularly important for improving chip disposal. In the case of BN free-cutting steel, the B content, N content, Si content in the steel When the amount is a specific amount and is added so as to satisfy a specific balance, the fracture strain in the shear region of the chip is small, and the crystallization of BN is promoted, and the chip disposability is greatly improved. I found it.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. In mass%, C: 0.10 to less than 0.50%, Si: more than 0.35 to 0.79 %, Mn: 0.30 to 2.0%, S: more than 0.015 to 0.2% , Al: 0.010 to 0.10%, B: 0.0050 to 0.0150%, N: 0.0100 to 0.0200%, comprising the remainder Fe and unavoidable impurities, (1) BN free-cutting steel excellent in chip disposal, characterized by satisfying the formula.
Serial 5.1 ≦ [Si] - [N ] + 3 × 10 -6 [B] 3 -8 [B] 2 +900 [B] ≦ 13.9 ···· (1) where each description in [] Element content (% by mass)
2. Furthermore, BN pleasure excellent in chip treatability according to 1, which contains one or more selected from Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less Cutting steel.

  According to the present invention, among machinability, BN free-cutting steel that is particularly excellent in chip disposal is obtained, cutting efficiency is improved, and it is extremely useful industrially.

The reasons for limiting the chemical components in the present invention will be described below. In the description,% is mass%.
C: 0.10 to less than 0.50% C is an element necessary for ensuring strength and, consequently, machinability. If the content is less than 0.10%, the structure is mainly composed of ferrite and the ductility is excessively increased, so that machinability cannot be ensured. On the other hand, if it is 0.50% or more, it becomes a pearlite-based structure, and the machinability, particularly the surface roughness, decreases. Therefore, the content is 0.10 to less than 0.50%.

Si: more than 0.35 to 1.50%
Si is an important element related to the basis of the present invention, which dissolves in the ferrite structure and raises the strength and at the same time embrittles the material. Chip disposability is improved by reducing the breaking strain in the shear region of the chip. However, since the material itself becomes brittle by adding Si, the fracture strain in the shear region of the chip is also reduced, contributing to improvement of the chip disposability. .

  Moreover, since Si functions effectively as a Si oxide as a nucleus for crystallization of BN inclusions, by adding an appropriate amount of Si balanced with the B and N amounts, the chip disposal performance is dramatically improved. Will improve. If the content is 0.35% or less, sufficient effects cannot be obtained. On the other hand, if it exceeds 1.50%, the effects are saturated and the strength is excessively increased, and in particular, the tool life in the machinability. Deteriorate. Therefore, the content exceeds 0.35 to 1.50%.

Mn: 0.30 to 2.0%
Mn constitutes MnS sulfide, which is a free-cutting compound, and is an element necessary for ensuring strength and hot workability. If the content is less than 0.30%, the amount of MnS sulfide is small, and therefore machinability is deteriorated. Further, the formation of low melting point FeS sulfide reduces the hot workability, and surface flaws are likely to occur during hot rolling or hot forging. On the other hand, if it exceeds 2.0%, the increase in strength becomes excessive and the machinability deteriorates. Therefore, the content is made 0.30 to 2.0%.

S: more than 0.015 to 0.2%
S is an important element constituting MnS sulfide which is a free-cutting compound. When the content is 0.015% or less, since the amount of MnS sulfide is small, the effect of improving machinability is small. On the other hand, if it exceeds 0.2%, the hot workability is lowered, and surface flaws are likely to occur during hot rolling or hot forging. Therefore, the content exceeds 0.015 to 0.2%.

Al: 0.010 to 0.10%
Al is an element necessary for deoxidation, and in the case of BN free-cutting steel, an AlN film is formed on the tool surface to suppress diffusion wear of the tool during cutting, so that the tool life of BN free-cutting steel is improved. Is an important element. If less than 0.010%, the effect cannot be obtained, so 0.010% or more. On the other hand, if the content exceeds 0.10%, the effect is saturated and hard alumina-based oxides increase, and the machinability, particularly the tool life, is reduced. Therefore, the content is made 0.010 to 0.10%. .

B: 0.0050 to 0.0150%
B is an element necessary for generating BN inclusions that improve machinability, and is an important element related to the basis of the present invention. Of the machinability, it is effective to improve the chip disposal by the notch effect, and the chip disposal is dramatically improved by coexisting with an appropriate amount of Si balanced with the B amount and the N amount. .

  If the content is less than 0.0050%, it is not possible to generate a sufficient amount of BN inclusions to improve machinability including chip disposal. On the other hand, if it exceeds 0.0150%, the hot workability is lowered, and surface flaws are likely to occur during hot rolling or hot forging. Therefore, the content is made 0.0050 to 0.0150%.

N: 0.0100 to 0.0200%
N is an element necessary for generating BN inclusions that improve machinability, and is an important element related to the basis of the present invention. Of the machinability, it is effective to improve the chip disposal by the notch effect, and the chip disposal is dramatically improved by coexisting with an appropriate amount of Si balanced with the B amount and the N amount. . If the content is less than 0.0100%, a sufficient amount of BN inclusions cannot be generated to improve machinability including chip disposal. On the other hand, if it exceeds 0.0200%, the hot workability is lowered, and surface flaws are likely to occur during hot rolling or hot forging. Therefore, the content is set to 0.0100 to 0.0200%.

5.1 ≦ [Si] − [N] + 3 × 10 ⁻⁶ [B] ³ −8 [B] ² +900 [B] ≦ 13.9
[Si] - [N] + 3 × 10 -6 [B] 3 -8 [B] 2 +900 [B] is involved in the basis of the present invention that defines the balance of the Si content in steel, B amount and N amount It is an important index. When this value is less than 5.1 or exceeds 13.9, a stable AlN coating cannot be formed on the tool surface. Moreover, since Si oxide does not function effectively as a nucleus at the time of crystallization of BN inclusions, chip disposability is not improved, so this index is set to 5.1 or more and 13.9 or less. In addition, the value in [] is a numerical value expressed in mass% of each element described.

  In order to satisfy this index, by adding Si amount, B amount and N amount within the above upper and lower limits, an AlN film is formed on the tool surface, and the BN inclusions are notched. Since the effect is improved and the material becomes brittle, the chip disposability is dramatically improved.

  The BN free-cutting steel according to the present invention has a notch effect of BN inclusions in a state where the material is embrittled due to the addition of Si, and cracks are generated starting from the BN inclusions. The processability is greatly improved. In addition, since Si functions as a Si oxide and BN inclusions effectively function as nuclei at the time of crystallization, the amount of BN inclusions as a starting point of crack generation is increased. The above is the basic component composition of the present invention, but in order to further improve the characteristics, it is possible to contain one or more of Ni, Cr and Mo.

One, two or more of Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less improves hardenability and contributes to an increase in strength. Addition exceeding Ni: 2.0%, Cr: 2.0%, Mo: 2.0% results in excessive increase in strength, which deteriorates machinability and is economically disadvantageous. Therefore, when adding, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less. Hereinafter, the present invention will be described in detail according to examples.

<Carbon steel for machine structure (component composition: SC)>
Steel having chemical composition within the scope of the present invention shown in Table 1 (hereinafter referred to as the present invention example) No. No. 1-10 and steel having a chemical composition outside the scope of the present invention (hereinafter referred to as Comparative Example) No. 11 to 26 were melted and cast into a steel ingot having a cast cross section of 400 × 310 (mm), and then hot rolled to a steel bar having a diameter of 90 mm, and a machinability test, a tensile test, and a surface flaw test were performed.

  The machinability test was performed under the conditions shown in Table 2, and the tool life, chip disposal, and finished surface roughness were evaluated. The test was started after first cutting the outer periphery by 1 mm to remove the scale and decarburized layer. When the diameter of the round bar reached 44 mm, the use of the sample was finished (the range of 1 mm to 23 mm below the surface was used in the radial direction of the round bar with a diameter of 90 mm).

  The tensile test was conducted by collecting a JIS No. 4 tensile test piece from the area where the machinability test was conducted (12 mm below the surface coincides with the central axis in the longitudinal direction of the tensile test piece), and the tensile strength (strength), drawing (Ductility value) was measured. Moreover, the surface wrinkle test pickled the round bar cut | disconnected to 300 mm length, and measured the number of surface wrinkles visually. The surface flaw test investigates hot ductility.

Table 3 shows the test results. The scope of the present invention was as follows: outer life: 20 min or more, chip rating: 20 points or less, surface roughness (Rz): 8 μm or less, and number of surface defects: 5 or less. Examples of the present invention (Nos . 1 to 10 , except for No. 4 ) have a tensile strength (strength), drawing (ductility value), surface properties, or a plurality of characteristics. ) Better. Each comparative example is described below.

  Comparative Example No. No. 11 has a C content of not more than the lower limit of the claims of the present invention, and thus becomes a structure mainly composed of ferrite, and the ductility is excessively increased, so that machinability is inferior to that of the present invention example. Comparative Example No. No. 12 has a pearlite-based structure because the C amount is equal to or greater than the upper limit of the claims of the present invention, and therefore machinability, particularly surface roughness, is inferior to that of the present invention example.

  Comparative Example No. In No. 13, since the amount of Si is not more than the lower limit value of the claims of the present invention, among the machinability, chip treatability is inferior to that of the present invention example. Comparative Example No. In No. 14, since the Si amount is not less than the upper limit value of the claimed range of the present invention, the strength becomes too high, so that the tool life is inferior and the machinability is inferior compared to the inventive example.

  Comparative Example No. In No. 15, since the amount of Mn is less than the lower limit of the claims of the present invention, the machinability and the number of surface defects are large, and the hot ductility is inferior to that of the present invention example. Comparative Example No. No. 16, since the amount of Mn is not less than the upper limit value of the claims of the present invention, the strength becomes too high, so that the tool life and chip disposal are inferior, and the machinability is inferior compared to the inventive examples. Yes.

  Comparative Example No. No. 17, because the amount of S is less than the lower limit of the claims of the present invention, and the amount of sulfide that improves the machinability is small, the tool life and chip machinability are inferior to those of the present invention example. ing. Comparative Example No. No. 18, since the amount of S is equal to or greater than the upper limit of the claims of the present invention, the machinability is improved by increasing the amount of sulfide that improves the machinability, but the hot ductility deteriorates because the amount of S is large. The surface properties are poor. If the surface properties are poor, the surface roughness is adversely affected and the tool life is deteriorated, so that the chip disposability is deteriorated and the machinability is also adversely affected.

  Comparative Example No. In No. 19, since the amount of Al is not more than the lower limit of the claims of the present invention, a stable AlN film is not formed on the tool surface, so that the machinability is inferior to that of the present invention example. Comparative Example No. In No. 20, since the amount of Al is not less than the upper limit of the claims of the present invention, hard alumina-based oxides increase and the machinability is inferior to that of the present invention examples.

  Comparative Example No. No. 21, because the amount of B is less than the lower limit value of the claims of the present invention, a stable AlN film and a sufficient amount of BN inclusions are not generated on the tool surface, so that machinability becomes an example of the present invention. It is inferior in comparison. Comparative Example No. No. 22, because the amount of B is not less than the upper limit of the claims of the present invention, the machinability is improved, but the hot ductility is lowered and the surface properties are poor. If the surface properties are poor, the surface roughness is adversely affected and the tool life is deteriorated, so that the chip disposability is deteriorated and the machinability is also adversely affected.

  Comparative Example No. 23, since the N amount is less than or equal to the lower limit value of the claims of the present invention, a stable AlN film and a sufficient amount of BN inclusions are not generated on the tool surface, so that the machining including the chip processing property is included. The properties are inferior to those of the examples of the present invention. Comparative Example No. In No. 24, the N amount is equal to or greater than the upper limit of the claims of the present invention, so that the machinability is equivalent to that of the present invention example, but the hot ductility is lowered and the surface properties are poor. If the surface properties are poor, the surface roughness is adversely affected and the tool life is deteriorated, so that the chip disposability is deteriorated and the machinability is also adversely affected.

Comparative Example No. 25, [Si] - [N] + 3 × 10 -6 [B] 3 -8 [B] 2 +900 [B] for the following lower limit of the claimed range of expression values present invention, stable on the tool surface Since no AlN film is formed, the cutting tool life is inferior to that of the example of the present invention, and the chip disposability is not improved. Therefore, the machinability is inferior to that of the example of the present invention. Comparative Example No. 26, [Si] - [N] + 3 × 10 -6 [B] 3 -8 [B] 2 +900 [B] value of expression for more than the upper limit of the claims of the present invention, stable on the tool surface Since no AlN film is formed, the cutting tool life is inferior to that of the present invention example, and the chip disposability is not improved, so that the machinability is inferior to that of the present invention example.

<Chromium steel (component composition: SCr type)>
Steel having a chemical composition within the scope of the present invention shown in Table 4 (hereinafter referred to as the present invention example) No. No. 27-29, and steel having a chemical composition outside the scope of the present invention (hereinafter referred to as comparative example) No. 30 to 32 were melted and cast into a steel ingot with a casting cross section of 400 × 310 mm, and then hot rolled to a steel bar having a diameter of 90 mm, and machinability test, tensile test and surface flaw test were conducted according to Example 1. It was.

  Table 5 shows the test results. The scope of the present invention was as follows: outer life: 10 min or more, chip rating: 20 points or less, surface roughness (Rz): 8 μm or less, and number of surface defects: 5 or less. Invention Example No. Compared with 27-29, Comparative Example No. In Nos. 30 to 32, the Cr amount is not less than the upper limit value of the claims of the present invention, so the strength is high and the machinability is inferior. No. 30 <No. 31 <No. Although the Cr content increases in the order of 32, the effect of increasing the strength is saturated, and the content exceeding 2.0% is economically disadvantageous.

<Chromium molybdenum steel (component composition: SCM)>
Steel having a chemical composition within the scope of the present invention shown in Table 6 (hereinafter, the present invention example) No. No. 33-35, and steel having a chemical composition outside the scope of the present invention (hereinafter referred to as Comparative Example) No. 36 to 38 were melted and cast into a steel ingot with a cast cross section of 400 × 310 mm, and then hot rolled to a steel bar having a diameter of 90 mm, and machinability test, tensile test and surface flaw test were conducted in accordance with Example 1. It was.

  Table 7 shows the test results. The scope of the present invention was as follows: outer life: 10 min or more, chip rating: 20 points or less, surface roughness (Rz): 8 μm or less, and number of surface defects: 5 or less. Invention Example No. Compared with 33-35, Comparative Example No. In Nos. 36 to 38, the amount of Mo is not less than the upper limit value of the claims of the present invention, so that the strength is high and the machinability is poor. No. 36 <No. 37 <No. The amount of Mo increases in the order of 38, but the effect of increasing the strength is saturated, and the content exceeding 2.0% is economically disadvantageous.

<Nickel chromium molybdenum steel (component composition: SNCM)>
Steel having a chemical composition within the scope of the present invention shown in Table 8 (hereinafter, the present invention example) No. Nos. 39 to 41 and steel having a chemical composition outside the scope of the present invention (hereinafter referred to as Comparative Example) No. 42 to 44 were melted and cast into a steel ingot with a cast cross section of 400 × 310 mm, and then hot-rolled to a steel bar having a diameter of 90 mm. It was.

  Table 9 shows the test results. The scope of the present invention was as follows: outer life: 10 min or more, chip rating: 20 points or less, surface roughness (Rz): 8 μm or less, and number of surface defects: 5 or less. Invention Example No. When compared with 39-41, Comparative Example No. Nos. 42 to 44 are high in strength and inferior in machinability because the amount of Ni is not less than the upper limit value of the claims of the present invention. No. 42 <No. 43 <No. The amount of Ni increases in the order of 44, but the effect of increasing the strength is saturated, and the content exceeding 2.0% is economically disadvantageous.

Claims (2)

In mass%, C: 0.10 to less than 0.50%, Si: more than 0.35 to 0.79 %, Mn: 0.30 to 2.0%, S: more than 0.015 to 0.2% , Al: 0.010 to 0.10%, B: 0.0050 to 0.0150%, N: 0.0100 to 0.0200%, comprising the remainder Fe and unavoidable impurities, (1) BN free-cutting steel excellent in chip disposal, characterized by satisfying the formula.
Serial 5.1 ≦ [Si] - [N ] + 3 × 10 -6 [B] 3 -8 [B] 2 +900 [B] ≦ 13.9 ···· (1) where each description in [] Element content (% by mass)   Furthermore, it was excellent in the chip disposal property of Claim 1 containing 1 type (s) or 2 or more types chosen from Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less BN free-cutting steel.