JP3507723B2 - Bi free cutting steel - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a Bi free-cutting steel excellent in machinability, particularly chip disposability.

[0002]

As a means for improving the machinability of steel materials, the addition of machinability improving elements such as lead and sulfur has been widely put into practical use. Among them, the lead free-cutting steel containing lead is excellent in machinability, particularly chip disposability, and has greatly contributed to the automation of the cutting process in recent years. Moreover, since lead is granular and finely dispersed in the steel, mechanical properties are less deteriorated than those without lead added, and thus it has been widely used in practice.

However, in recent years, as the problem of environmental pollution has been raised in the production and use of lead free-cutting steel containing harmful lead, its generalization has been reviewed, and the use of lead free-cutting steel is rapidly reduced. It is supposed to be done.

On the other hand, in the sulfur free-cutting steel added with sulfur, if the sulfur is excessively contained due to the influence of the sulfide-based inclusions stretched in the rolling direction, the vertical direction to the rolling direction (hereinafter referred to as "cross grain") Mechanical properties are adversely affected, the required properties as mechanical parts cannot be satisfied, or sulfide-based inclusions stretched during hot forging or cold forging tend to be the starting points for cracking. There is also a problem that cracks are likely to occur even when induction hardening is performed after the parts are molded. Therefore, the amount of sulfur added is limited, and the machinability is insufficient as compared with lead free-cutting steel.

Bi free-cutting steel is known as a machinability improving means other than the above, and is disclosed in, for example, JP-A-56-35758.
No. 56-38453, No. 60-152653, etc. have been proposed, but it can be said that sufficient research has been conducted on how the existence form of Bi inclusions affects machinability. Absent.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances. In particular, attention is paid to Bi free-cutting steel, and the amount of Bi added is the same level as that of conventional Bi free-cutting steel. Even so, it is to provide a Bi free-cutting steel having dramatically improved machinability, particularly chip disposability which is most important in automation of a cutting process.

[0007]

The Bi free-cutting steel according to the present invention, which has been able to solve the above-mentioned problems, contains Pb substantially.
Machine structural steel not containing Bi content of 0.01
˜0.3% (meaning mass%, the same applies hereinafter) , and an area of 1 μm ² observed in the field of view of a cross section of 1 mm ^{2 in} the rolling direction.
The ratio of the number of the metal Bi and the Bi content expressed by the following formula (1) is 50 or more, more preferably 70 or more,
In particular, the point is that the chip disposability is remarkably improved. [ Metallic Bi number (pieces / mm ² )] / [Bi content rate (mass% × 10)] (1) In the Bi free-cutting steel according to the present invention, the characteristics are most effectively exhibited. The base composition of steel is C:
0.06 to 0.70%, Si: 2.5% or less, Mn:
Including 0.1 to 3%, S: 0.15% or less, O: 0.0
It is a steel for machine structural use that is suppressed to 03% or less.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Under the above-mentioned problems, the inventors of the present invention specifically target Bi free-cutting steels with a Bi content at the same level as that of conventional Bi free-cutting steels, and in particular machinability, especially chips. We have been conducting intensive research with the aim of further improving processability. as a result,
A steel having a ratio of the Bi content of 50 or more and the number of the metal Bi having an area of 1 μm ² or more observed in the field of view of the cross section of 1 mm ^{2 in} the rolling direction of the steel material represented by the formula (1) is stably high The inventors of the present invention have come to the present invention by knowing that the above chip disposability is exhibited.

Like lead, Bi has a low melting point, and it is considered that molten metal embrittles when the temperature of the steel material rises during cutting and improves the chip disposability by improving the lubricating action on the tool surface. However, at present, no research has been done from the viewpoint of in what form of existence the effect is more effectively exerted.

Therefore, how the effect of improving the chip disposability by adding Bi changes depending on the existence form of Bi ,
As a result of further research from the viewpoint of how to adjust the existence form to more effectively utilize the above-mentioned addition effect of Bi, the ratio represented by the formula (1) is 50 or more, and more preferably 70 or more. It was found that the finely dispersed metal Bi as described above more effectively exerts the molten metal embrittlement and the lubricating action due to the above-mentioned Bi, and reliably and effectively exhibits the chip disposability. Of.

Here, the area observed on the surface to be inspected is 1 μm.
The number of metal Bi of ² or more was selected as the evaluation target of the chip disposability . Among the metal Bi finely dispersed in steel, the influence of the molten metal embrittlement and the chip disposability due to the lubricating action at the time of machining is affected. The metal Bi that exerts an area of 1 μm ²
This is because the metal Bi less than this value hardly contributes to the improvement of chip disposability. In addition, the above formula (1)
The reason that the ratio shown by is defined to be 50 or more is that the chip disposability of the level intended by the present invention cannot be secured when the ratio is less than 50, as will be apparent from the examples described later.

At this time, if the absolute amount of Bi content in the steel is insufficient, it becomes difficult to satisfy the ratio represented by the above formula (1), so that the Bi content in the steel is 0.01% or more. However, it is preferable that the content be 0.03% or more. However, when the Bi content exceeds 0.3%, the chip disposability reaches a saturated state, the machinability improving effect is not further exhibited, and the toughness of the steel material is significantly impaired. It should be suppressed below, and more preferably 0.15% or less.

The size of the metal Bi that is finely dispersed in the steel can be easily adjusted by changing the size of the Bi source added to the molten steel at the melting stage. That is, B
i has a freezing point of 273 ° C. and a freezing point (150
Since it is much lower than that of (0 ° C or higher), it exists as a liquid state in the steel from the melting to the rolling stage, and the rolled steel material solidifies only when the temperature is lowered to 273 ° C or lower. Bi is
Low affinity for molten steel and density of 9.8 g / cm
³ , there is no big difference with the density of iron (7.8 g / cm ³ ),
Further, it does not form a solid solution in the steel and is easily dispersed in the steel material in the form of Bi particles. Therefore, if Bi is added to the molten steel in the form of fine powder and the molten steel is sufficiently stirred by electromagnetic stirring, gas bubbling, etc., and if the solidification rate during casting is well controlled, the steel material will be added according to the size of the added Bi. The metal Bi can be evenly distributed in a finely dispersed state.

As a Bi source to be added to steel, metallic Bi is used.
May be used as it is, but may be added as a Bi oxide. However, the Bi oxide is easily reduced under high temperature conditions, and when it comes into contact with molten steel at high temperature, it is rapidly reduced by carbon and aluminum in the molten steel to form Bi. Further, since bismuth oxide has a melting point of 820 ° C., which is considerably higher than the melting point of metal bismuth, it is more preferable to add bismuth as an oxide than to add bismuth as a metal, because the growth of adhesion between Bi raw material particles during addition is suppressed. It is considered that fine dispersion in steel is promoted.

The Bi raw material may be added in a powder form by blowing it into a molten steel together with a carrier gas such as an inert gas, or in the form of a wire filled with a Bi raw material powder in an iron casing and drawn out. A method such as supplying from the surface of the molten metal may be adopted.

In the present invention, gold finely dispersed in the steel material
The genus Bi was measured as follows. That is, a cross section parallel to the rolling direction of the steel material to which Bi is added is polished in the melting stage, and a backscattered electron image is taken by observing the cross section with an SEM (magnification: 200 times). This photograph was placed on an automatic measuring device and the area observed in a visual field of 1 mm ² was 1 μm ^2.
The number of the above metal Bi is measured. At this time, the same measurement was performed at five arbitrarily selected points in the cross section, and the average value was taken as the number of metal Bi .

The present invention resides in that the Bi addition amount to the steel is specified as described above, and the ratio between the number of Bi of a specific size finely dispersed in the steel and the Bi addition amount is specified. It is characterized by more effectively exerting the molten metal embrittlement and the lubricating action during the grinding process by the finely dispersed Bi, and the component composition of the steel material itself is not particularly limited, but it is a preferable base from the viewpoint of machinability. As a steel material,
C: 0.06 to 0.70%, Si: 2.5% or less, M
n: 0.1 to 3% included, S: 0.15% or less, O:
Mechanical structural steel suppressed to 0.003% or less, more specifically, for example, mechanical structural carbon steel specified in JIS G4051, nickel-chromium steel specified in JIS G4102, and JIS G4103. Nickel / chromium / molybdenum steel, chrome steel specified in JIS G4104, chrome molybdenum steel specified in JIS G4105, manganese steel for machine structure specified in JIS G4106, manganese chrome steel, etc. A theory that defines the preferred chemical composition of steel
The reasons are as follows.

C: 0.06 to 0.70% C is an important element for securing the strength of the final product, and is 0.06% or more for securing the strength required as steel for machine structural use. , And more preferably 0.08% or more is desirable. However, if it is too large, the toughness is reduced and the tool life and machinability are reduced, so 0.70% or less, more preferably 0.60% or less is desirable.

Si: 2.5% or less Si acts effectively as a deoxidizing element and also contributes to the strength improvement of steel by solid solution strengthening. However, if too large, the machinability is adversely affected, so it is preferable to use about 2.5% or less, more preferably 1.5% or less.

Mn: 0.1 to 3% Mn not only enhances the hardenability of the steel material and contributes to the improvement of the strength, but also forms the sulfide inclusions to enhance the machinability. Although such an effect is effectively exhibited by containing 0.1% or more, if too much is present, the machinability tends to deteriorate rather. Therefore, the content should be about 3% or less, and more preferably 2.5% or less. It is good to suppress.

S: 0.15% or less S has the action of forming sulfide-based inclusions such as MnS to enhance the machinability, but if it is too large, it cracks during hot forging or cold forging. Since it becomes the starting point of generation and reduces the deformability, it is about 0.15% or less, more preferably 0.12%.
The following should be suppressed.

Al: 0.1% or less Al is important as a deoxidizing element when steel is melted,
It also has a function of forming a nitride and refining austenite crystal grains to enhance toughness. However, if it is too large, the crystal grains will become coarser and adversely affect the toughness.
It should be suppressed to about 0.1% or less, more preferably 0.06% or less.

Ni: 3% or less, Cr: 5% or less, Mo:
One or more of these elements such as 1.2% or less and Cu: 1% or less are all useful elements for improving the strength of the steel material, but if they are too much, obstacles such as decrease in machinability appear. Therefore, an appropriate amount can be selected from the above range by appropriately selecting according to the application.

Ca: 0.05% or less, Zr: 0.2% or less, REM: 0.3% or less, one or more of these elements, MnS produced by the above-mentioned reaction of Mn and S is granulated, In addition to having the effect of improving the anisotropy, it also effectively acts on the improvement of machinability. However, since their effects are saturated within the above range, any further addition is useless.

V: 0.5% or less, Ti: 0.3% or less,
Nb: One or more of 0.3% or less These elements effectively refine the steel braid after conditioning to improve the strength / toughness balance. Further, even in the case of non-heat treated steel, it has the effect of significantly increasing the strength, and also finely assembles the combined fibers to enhance the toughness. However, since their effects saturate within the above range, any further addition is useless.

B: 0.0003 to 0.01% B is added in an extremely small amount to enhance the hardenability and contributes to the improvement of strength. However, the effect is saturated at about 0.01%, so any further addition is useless. Is. The more preferable addition amount of B is 0.0
It is in the range of 005 to 0.005%.

O: 0.003% or less O is present in the steel as a hard non-metallic inclusion such as A1 ₂ O ₃ and accelerates wear of the cutting tool.
% Or less, and more preferably 0.002% or less.

N: 0.003 to 0.03% N combines with Al, Ti or the like to form a nitride, which makes the austenite crystal grains finer and effectively acts to improve toughness and fatigue strength. However, if it is too large, the toughness is adversely deteriorated, so 0.003 to 0.03%, and more preferably 0.
It is desirable to contain it in the range of 003 to 0.02%.

Bi: 0.01 to 0.3% Bi is dispersed alone in the steel as shown above, and is effective in enhancing the machinability, especially the chip disposability by the molten metal embrittlement action and the lubricating action. Is an element that is 50% in the ratio of the above formula (1).
In order to secure the above, 0.01% or more must be contained. However, the action of Bi is saturated at about 0.3%, and if added in excess of this amount, the toughness will have a marked adverse effect, so it should be suppressed to 0.3% or less.

[0030]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and may be appropriately applied within a range compatible with the gist of the preceding and the following. Modifications can be made and implemented, and all of them are included in the technical scope of the present invention.

Example Using carbon steel S45C for machine structure as a base steel material,
When the converter was melted, Bi was 0.03%, 0.05% and 0.
Aiming at 3 levels of 1%, the mixture was thoroughly stirred by magnetic stirring. After casting using this molten steel, it is rolled to a diameter of 5
A 0 mm steel bar was obtained.

The Bi raw material to be added at the time of melting is ultrafine bismuth oxide powder having a particle size of 10 mm or less, and a particle size of 1
By using 4 kinds of metal bismuth ultrafine powder of 5 μm or less, metal bismuth powder of 0.5 mm or less in particle diameter, metal bismuth shot of 3 to 8 mm in particle diameter, and adjusting the compounding ratio thereof, The metal Bi size and number were controlled. Each of these Bi raw materials was charged in a thin iron casing and added to the molten steel in the form of a wire having a width of 15 mm and a thickness of 6 mm.

The use ratio of the powder used as the Bi raw material and the chemical composition of each Bi free-cutting steel obtained are shown in Table 1, and the number of metal Bi in each Bi free-cutting steel and the ratio of the above formula (1), Table 2 collectively shows chip disposability of each free-cutting steel.

No. Nos. 1 to 12 are examples satisfying the requirements of the present invention. Nos. 13 to 18 are comparative examples, and No.
Nos. 1 to 4, 13, and 14 aim at Bi 0.03%. 5-
Nos. 8, 15 and 16 aim at 0.05% Bi. 9-1
2, 17 and 18 are examples of Bi 0.1% aiming.

For the metal Bi , after the sections in the rolling direction of the above-mentioned rolled materials were cut and polished, SEM backscattered electron images at the D / 8 position where the machinability was evaluated were photographed at a magnification of 200, and this photograph was automatically taken. It was determined by applying a measuring device and measuring the number of metal Bi having an area of 1 μm ² or more observed in a visual field of 1 mm ² (average value at 5 observation positions).

The chip disposability is 850 ° C. for each free-cutting steel.
After 1 hour heating → oil cooling → 500 ° C. × 2 hour holding → water cooling quenching / tempering treatment, evaluation was carried out by a carbide turning test. As shown in Table 3, the test conditions are cutting speed 150 m /
Feed at 0.05, 0.1, 0.2, 0.3 mm
4 levels of / rev, 0.5, 1.0, 2.0 m depth of cut
Chips cut out under each condition were sampled under various conditions, and a score was assigned based on the rating shown in FIG. 2. The total of 12 conditions for each test free-cutting steel was evaluated as a chip disposability index. That is, if all the conditions were the minimum chip state shown at the right end of FIG. 2, the chip property index becomes 100.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

From the above Tables 1 and 2, Bi content of 0.03
%, 0.05%, and 0.10%, the relationship between the ratio represented by the formula (1) and the chip disposability index is shown in FIG. 1 at any Bi content ratio. also, chip disposability index above ratio is 50 or more indicates a surge tendency, it can be confirmed that the high chip disposability finger <br/> number is obtained by particularly 70 or more.

[0041]

According to the present invention is constructed as described above, gold
By appropriately controlling the size and abundance of the genus Bi and the Bi content, the machinability, especially the chip disposability, can be significantly enhanced. As a result, it is possible to reliably and stably provide Bi free-cutting steel whose machinability, in particular, chip disposability, is surely improved, which greatly contributes to automation of cutting.
Further, if the requirements are satisfied with the same machinability as the conventional one, the purpose can be achieved with a smaller amount of added Bi, so that the cost of Bi free-cutting steel can be reduced.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the [ metal Bi number (pieces / mm ² )] / [Bi content (mass% × 10)] ratio and the chip disposability index of Bi free-cutting steel obtained in an example. .

FIG. 2 is a diagram showing, in tabular form, an evaluation standard for chip disposability.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshitake Matsushima               Kobe, Nada-ku, Nadahama Higashi-cho 2 God               To Steel Works Inside Kobe Steel Works (72) Inventor Fuku ▲ Saki ▼ Yoshio               Kobe, Nada-ku, Nadahama Higashi-cho 2 God               To Steel Works Inside Kobe Steel Works (72) Inventor Kazuhiro Tsuji               Kobe, Nada-ku, Nadahama Higashi-cho 2 God               To Steel Works Inside Kobe Steel Works (72) Inventor Daisuke Ogura               Kobe, Nada-ku, Nadahama Higashi-cho 2 God               To Steel Works Inside Kobe Steel Works (72) Inventor Takehiro Tsuchida               1-5-5 Takatsukadai, Nishi-ku, Kobe-shi Stock               Kobe Steel Co., Ltd.Kobe Research Institute                (56) Reference JP-A-11-217650 (JP, A)                 JP-A-6-212349 (JP, A)                 JP 62-60815 (JP, A)                 JP-A-2-274838 (JP, A)

Claims (2)

(57) [Claims] 1. A steel for machine structural use which is substantially free of Pb.
And the Bi content is 0.01 to 0.3% (meaning mass%, the same applies below) , and the number of metal Bi having an area of 1 μm ² or more observed in the visual field of the rolling direction cross section of 1 mm ^2. A Bi free-cutting steel excellent in chip disposability, characterized by having a ratio of Bi content expressed by the following formula (1) of 50 or more. [Number of metal Bis (number / mm ² )] / [Bi content rate (mass% × 10)] (1) 2. Steel C: 0.06 to 0.70%, Si:
2.5% or less, including Mn: 0.1 to 3%, S: 0.1
The Bi free-cutting steel according to claim 1, which is a steel for machine structural use in which the content is 5% or less and O: 0.003% or less.