JP2001214239A - Steel for machine structural use excellent in partibility of chip and producing method thetrfor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for machine structural use free from the anxiety about the occurrence of the problem of environmental contamination, moreover widely applicable without being provided with machining conditions and using limitation and particularly excellent in the partibility of chips extremely important for automating machining operation. SOLUTION: As to this steel for machine structural use in steel for machine structures containing, by mass, C of 0.1 to 0.6%, Si of 2.5% or less (not inclusive of 0%), Mn of 0.2 to 3.0%, S of <0.150% (not inclusive of 0%) and O of 0.001 to 0.015%, the number of inclusions with a length of 1 μm or more appearing in the longitudinal section is 50 to 1,000 pieces per mm2 of the cross section, the ratio of the number of oxides occupying in the total content if the inclusions is 10% or more, and also, in the oxides, the ratio of the number of the oxide of at least one kind of element selected from the group consisting of Na, Li and B or the multiple oxide of at least two kinds of elements selected from the group consisting of Na, Li, B and Si is 5% or more, and the method for producing the same is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for machine structural use having excellent chip breaking property and a method for producing the same. More specifically, the present invention relates to a steel material containing a specific metal oxide in a specific amount.
The present invention relates to steel for machine structural use with improved chip breaking properties and a method for producing the same.

[0002]

2. Description of the Related Art Many free-cutting steels have been developed since steel materials are often cut to form various product shapes. Among the cutting properties, the chip breaking property in which the cut piece is quickly cut and separated from the tool is regarded as an important property in order to smoothly perform the automation of the cutting processing without any trouble.

[0003] By the way, lead free-cutting steel, which has been most frequently used in the past, exhibits the effect of increasing the chip breaking property by melting low-melting-point lead added to the steel by raising the temperature of the work material during cutting. And the deterioration of mechanical properties is relatively small,
It has been widely used as steel for machine structural use. However, in recent years,
With increasing awareness of environmental issues, lead free-cutting steels containing lead, which is a harmful heavy metal, as a machinable component tend to be repelled.

[0004] On the other hand, sulfur free-cutting steel does not cause the pollution problem pointed out by lead free-cutting steel, and is excellent in tool life and chip breaking performance. Demand for free-cutting steel is increasing. However, in free-cutting sulfur steel, sulfides in the steel tend to elongate in the rolling direction during the rolling process, and the sulphide in the direction perpendicular to the rolling direction (hereinafter, referred to as
In addition to adversely affecting the mechanical properties (abbreviated as the C direction), there is a problem that sulfides become a starting point during hot forging and cracks easily occur, so the amount of S added is limited. The scope of application is naturally limited.

[0005] Further, Ca free-cutting steel forms a protective film called Belague on the surface of a cutting tool by controlling a complex oxide of Si, A1, and Ca. These oxides have a melting point of 1400 ° C. It is very high before and after, and is effective for improving tool life in high cutting speed region using carbide tools, but does not show satisfactory machinability improvement effect in low cutting speed region, and chip breaking performance Is not good either.

[0006]

As described above, the conventional free-cutting steel is required to be further improved, for example, the use thereof is evaded due to environmental problems, or the cutting conditions are limited.
The present invention has been made in view of such circumstances, has no risk of causing environmental pollution, and can be widely applied without being subjected to cutting conditions and application restrictions. Another object of the present invention is to provide a steel for machine structural use which is extremely important in chip breaking.

[0007]

Means for Solving the Problems The steel for machine structural use according to the present invention, which can solve the above problems and has excellent chip breaking property, is C: 0.1 to 0.6% by mass% Si: 2 0.5% or less (excluding 0%) Mn: 0.2 to 3.0% S: less than 0.150% (excluding 0%) O: 0.001 to 0.015% In steel, major axis 1 appearing in a longitudinal section
Inclusions of μm or more are 50 to 150 per 1 mm ^{2 in} cross-sectional area.
0, the proportion of the number of oxides in the total amount of the inclusions is 10% or more, and among the oxides, at least one element selected from the group consisting of Na, Li and B The number of oxides is 5% or more, or the number ratio of composite oxides composed of oxides of at least two elements selected from the group consisting of Na, Li, B and Si is 5%. % Has a gist where it is.

[0008] The manufacturing method according to the present invention is a method for manufacturing a steel for machine structural use having excellent chip breaking property.
100 ppm or more of an oxide of at least one element selected from the group consisting of Na, Li and Si and having a melting point of 1000 ° C. or less is added to the molten steel, or is composed of Na, Li, B and Si. A melting point of at least two oxides selected from the group consisting of oxides of 1000
It is characterized by adding a composite oxide of 100 ° C. or less to molten steel at 100 ppm or more. These melting points are 1000
The above-mentioned oxide having a temperature of not more than ° C. can be uniformly mixed and dispersed in steel by adding it to molten steel in at least one of a ladle, a tundish and a mold.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Under the above-mentioned problems, the present inventors have sought a free-cutting component to replace conventional lead, sulfur, Ca oxide and the like, and have particularly sought to improve the chip breaking performance. Research has been advanced. As a result, an oxide of at least one element selected from the group consisting of Na, Li and Si, or at least two types of oxides selected from the group consisting of Na, Li, B and Si in the steel for machine structural use Those containing an appropriate amount of the compound oxide of the element were confirmed to exhibit excellent and excellent chip breaking properties, and the present invention was conceived.

[0010] In order to enhance the chip breaking property, it is needless to say that a starting point for cutting the chips is required. The starting point is the metal melting embrittlement by lead dispersed in steel in lead free-cutting steel, and since lead has a low melting point (about 327 ° C),
It is considered that the chips are embrittled by the melting of lead accompanying the temperature rise of the work material during the cutting process, and the chip breaking property is improved.

On the other hand, in the case of sulfur free-cutting steel, particularly, the expanded MnS
Is the starting point of chip breaking, but it is generally considered that the chip breaking property is also improved because the amount of MnS increases with an increase in the amount of sulfur.

However, as described above, the use of lead tends to be avoided in the future from the viewpoint of environmental pollution, and the amount of sulfur used is limited from the viewpoint of deterioration of the mechanical properties in the C direction. Further improvement in machinability is required.

Therefore, the present inventors have found a component for improving the chip breaking property in place of lead and sulfur, and in particular, for a component for increasing the chip breaking property by embrittlement of inclusions due to a rise in the temperature of the tool edge during cutting. I sought. As a result, as described above, an oxide of at least one element selected from the group consisting of Na, Li and Si, or Na, L
It has been confirmed that a composite oxide of at least two elements selected from the group consisting of i, B and Si has an effect of significantly increasing the chip breaking property of steel for machine structural use.

[0014] Metal oxides are generally hard and have been considered to deteriorate machinability. It is unlikely that metal oxides will be actively used as a machinability improving component. Was. However, oxides of at least one element selected from the group consisting of Na, Li, and Si, or Na, Li,
Composite oxide of at least two elements selected from the group consisting of B and Si [hereinafter collectively referred to as (composite)
Oxide may be contained in an appropriate amount, these (composite) oxides melt and soften in the temperature range of the cutting edge during high-speed cutting, and the resulting melt embrittlement significantly improves chip breaking. Was found.

The present invention is characterized in that a suitable amount of the above (composite) oxide is contained in the steel for machine structural use as a chip breaking improving component as described above. It is necessary that 50 to 1500, more preferably 150 to 1000, inclusions having a major axis of 1 to 15 μm appearing on the surface are dispersed per 1 mm ^{2 of} cross-sectional area. Incidentally, the inclusions that may be present in steel include oxides, sulfides, nitrides, and the like. Among these inclusions, fine inclusions having a major axis of less than 1 μm may have poor chip breaking properties and mechanical properties. The effect is small. On the other hand, if the major axis is 1
If a large number of inclusions exceeding 5 μm are present in the steel, the mechanical properties of the steel for machine structural use, particularly toughness and ductility, may be significantly adversely affected. There are few inclusions.

In order to secure the excellent chip breaking property intended in the present invention, the number of oxides occupying in all the inclusions having a major axis of 1 μm or more is required under the condition that the size and the number of the inclusions are satisfied. Is 10% or more, and
It is essential that the ratio of the number of the (composite) oxide in the oxide is 5% or more.

That is, in the present invention, basically, in addition to specifying the number of oxides among the inclusions dispersed in the steel material, the ratio of the number occupied by the (composite) oxide in the oxides By specifying the above, it has succeeded in securing excellent chip breaking performance which is not inferior to conventional free-cutting steel.

Incidentally, all inclusions appearing in the steel cross section include oxides, sulfides, nitrides, carbides, and composites thereof, and among oxides, alumina,
Although silica, manganese oxide, chromium oxide, and composites thereof are included, oxides having a major axis of 1 μm or more occupying in all the inclusions are required to secure the excellent level of chip breaking intended in the present invention. It is essential that the ratio of the number is 10% or more, and the ratio of the number occupied by the (composite) oxide in the oxide is 5% or more.

If the number ratio of the oxide in the total inclusions is less than 10% and the number ratio of the (composite) oxide in the oxide is less than 5%, the specific (composite) oxide ) The chip breaking improvement effect expected from oxides is not effectively exhibited. That is, these specific (composite) oxides have a melting point of 1000 ° C. or less, and preferably those (composite) oxides added to molten steel by a method described later.
If the component composition of the oxide is adjusted to have a melting point of 800 ° C. or less and the number of the specific (composite) oxides having a major axis of 1 μm or more is controlled so as to satisfy the above requirements, the heat generated during the cutting process can be reduced. Owing to the melting embrittlement action of the specific (composite) oxide, the chip breaking property is greatly improved.

Moreover, since most of the above (composite) oxides are spherical, the mechanical properties do not deteriorate, and rather contribute to the improvement of the mechanical properties depending on the form of the oxide.

The chip breaking improvement effect of the above oxides is effectively exerted independently on each of Na, Li and B oxides, and also effective as two or three kinds of composite oxides. However, with respect to the oxide of Si, even if it is present as a single oxide in a predetermined amount, it is not possible to obtain the chip breaking property as intended by the present invention,
It is essential that the compound is present as a composite oxide with at least one oxide of a, Li, and B. This is because the single oxide of Si has a higher melting point than the Na oxide, the Li oxide, and the B oxide, so that the melt embrittlement effect due to the temperature rise during high-speed cutting is not effectively exhibited.

The above-mentioned N, which contributes to the improvement of the chip breaking property,
Na, Li, and B, which are oxide sources of a, Li, and B, are hardly contained in ordinary molten steel. Therefore, in order to achieve the above object of the present invention, Na oxide, Li oxide,
It is necessary to add a B oxide or a composite oxide thereof. Specifically, at least one of a ladle, a tundish and a mold, the oxide or the composite oxide with Si in molten steel. Is added and these are contained in molten steel.

At this time, it is desirable to adjust the components of the oxide or composite oxide to be added so that the melting point before the addition is 1000 ° C. or less, more preferably 800 ° C. or less. The above oxides and composite oxides added in the molten steel cause a morphological change in the molten steel at a high temperature. However, if the method of adding at the above-mentioned position is adopted, the melting temperature of 1200 may be obtained even in the finally obtained steel material. It can be present as a (composite) oxide having a temperature of about ° C or lower.

The amount of the oxide or composite oxide added is 100 ppm or more, more preferably 3 ppm, based on the molten steel.
The content should be at least 00 ppm, and if it is less than 100 ppm, a sufficient size (1 μm in major axis) can be obtained in the finally obtained steel.
m or more) and the amount (number) of (composite) oxides cannot be present, making it difficult to obtain the excellent chip disposability intended in the present invention. Even if the above oxides and composite oxides are added in an excessive amount, most of the oxides and slag are floated and separated on the molten metal surface without yield in the molten steel, so that no actual harm is caused, but the effect is about 2000 ppm. And further addition is economically useless, preferably at 1000 pp.
m or less is sufficient. In addition, it is advantageous to add these oxides to the molten steel as a composite oxide rather than as a single oxide, since the yield in the steel is easy. The melting point of the oxide to be added may be determined in advance based on the values in the phase diagram.

The present invention is characterized in that the size and amount of the (composite) oxide contained in the steel for machine structural use are specified as described above, and the type of the steel material is not particularly limited. Carbon steel for machine structure specified in G4051; nickel-chromium steel specified in JIS G4102; nickel-chromium-molybdenum steel specified in JIS G4103; manganese steel for machine structure applied in JIS G4106; It can be widely applied to chrome steel, etc., and can be applied to all mechanical structural steels containing other elements as appropriate, but the standard chemical of mechanical structural steel in which the features of the present invention are most effectively exhibited. Examples of the components are as follows.

C: 0.1-0.6% C is an important element as an element for improving the strength of steel, but on the other hand, it is also an element that reduces ductility, and in a low-carbon steel region where its content is extremely low, It has the effect of appropriately reducing the ductility of steel and increasing the chip breaking performance. Therefore, the C content should be 0.1% or more, more preferably 0.2% or more for use as steel for machine structural use. However, if the C content is too large, the steel becomes high quality and the tool life is shortened. Since it causes a reduction, the content is preferably suppressed to 0.6% or less, more preferably 0.5% or less. Therefore, it was set to 0.1 to 0.6%.

Si: 2.5% or less (excluding 0%) Si is usually mixed in as a deoxidizing agent used during melting, but Si has a very high reactivity with oxygen. If it exceeds 5%, it becomes difficult not only to secure the required oxygen content in the steel, but also to deprive the oxygen of Na, Li, and B oxides to exert the chip breaking property improving effect. So
It is desirable to keep it at 2.5% or less, more preferably at 1.5% or less.

Mn: 0.2-3.0% Mn is an element which effectively produces MnS which is effective for improving the chip breaking property and also effectively acts for improving the hot workability. It is desirable that the content be 0.2% or more, more preferably 0.5% or more, in order to exert the effect effectively. However, if the Mn content is too large, the work hardening of the steel material becomes remarkable, and the tool life is shortened. Therefore, it is desirable to suppress the content to 3.0% or less, more preferably 2.5% or less.

S: less than 0.150% (excluding 0%) S is an effective M for improving the overall machinability including the chip breaking property.
Although it is an element forming nS, if it exceeds 0.150%, hot workability and ductility are significantly deteriorated. Therefore, the S content is desirably suppressed to less than 0.150%. In particular, when the S content is applied as a component for a mechanical structure in which mechanical characteristics are emphasized, the S content is 0.12% or less, more preferably 0.1% or less. 08
% Is desired.

O: 0.001 to 0.015% O is an important element that affects the form of the (composite) oxide deposited. When the total oxygen content is less than 0.001%,
The (composite) oxide of the size and amount defined in the present invention cannot be present, and O of 0.001% or more is essential to secure the level of chip breaking intended in the present invention. However, if the amount of O is too large, the production of iron oxide is promoted during smelting, which damages the refractory lining of the smelting furnace and shortens the life of the furnace. Therefore, the content of O is 0.01% from the viewpoint of furnace life.
It is desirable to keep the content to 5% or less, more preferably 0.010% or less.

The preferred elements contained in the steel for machine structural use excellent in chip breaking property used in the present invention are as described above, and the balance is substantially Fe, but a trace amount is contained in the structural steel. Needless to say that the inclusion of unavoidable impurities is allowed,
It is also possible to use steel in which other elements are positively contained as long as the effect of the present invention is not adversely affected. Examples of other elements that can be positively added include Pb, Bi, Te, and the like, which have an effect of improving the chip breaking property, and they can contain one or more kinds, but they are usually used. It is desirable to suppress the total to about 0.5% or less.

Next, a method for analyzing the (composite) oxide will be described. The test material was prepared by forging a 155 mm x 155 mm slab into a 50 mm diameter round bar at 1200 ° C.
A quenching and tempering treatment was performed at 850 ° C. × 1 hr oil cooling → 500 ° C. × 2 hr water cooling, and the Vickers hardness was adjusted to 270 ± 15. After polishing the D / 4 position of the cross section parallel to the forging direction of the forged material, a load of 5% was applied to specify the observation position of the test material.
Indentations were made with kg.

Then, the number of inclusions was measured using an optical microscope in the vicinity of the indentation at a magnification of 100 × 0.5 m per visual field.
Observe the area of mx 0.5mm by 4 visual fields, and the major axis is 1μ.
Image analysis was performed for inclusions of m or more. Next, the composition of the inclusions was identified by observing four visual fields at an accelerating voltage of 15 kV and a magnification of 500 times using EPMA of “JXA-8800RL” manufactured by JEOL Ltd. In this observation, Na, Li, B, S
The presence of each element was confirmed by mapping i, O and Mn, S, Cr. However, in EPMA, Li and B
Since it is difficult to detect such light elements, the same region as in EPMA observation was observed in four fields at a magnification of 500 times under SIMS of CAMECA-imsf5f under primary ion conditions of O ₂ ⁺ -8 keV-1 nA. However, in SIMS, O ₂ ⁺ is used as a primary ion, and although the existence of O is not clear,
Those in which O was observed in EPMA were judged to be oxides. E
Oxides of which at least one selected from Na, Li, B, and Si and the presence of oxygen O were confirmed by PMA and SIMS observations were defined as (composite) oxides. In this observation, the major axis was 1 μm
Oxides and (composite) observed among all the above inclusions
Their proportion was determined from the number of oxides. Oxide and MnS
The inclusion consisting of was counted as oxide.

Next, the evaluation method will be described. The evaluation items were two, namely, chip breaking property and transverse grain toughness value, and the chip breaking property was evaluated by a carbide turning test. As shown in Table 1, the test conditions were a cutting speed of 150 (m / min) and a feed of 0.0.
Twelve conditions for each steel type by changing four levels of 5, 0.5, 0.2, 0.3 (mm / rev) and three levels of notch to 0.5, 1.0, 2.0 (mm) Performed in Then, the chips under each cutting condition were distributed based on the evaluation points shown in FIG. 1, and the standard evaluation points were multiplied by the distribution ratio of the chips. The total of the 12 conditions was defined as the chip disposability index. Assuming that the chip is in the finest state at the right end of Table 2 under all conditions, the chip disposability index is 100 (accurately, 99.96).

[0035]

[Table 1]

The transverse grain toughness was measured by an impact test according to JIS No. 3. The impact test piece was cut out from a direction perpendicular to the forging direction and used for the test.

[0037]

EXAMPLES Hereinafter, the structure and operation and effect of 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 the present invention is applicable to the above and following points. It is also possible to carry out the present invention with appropriate modifications as far as possible, and all of them are included in the technical scope of the present invention.

Example The steel of the present invention was prepared by adding a composition-adjusted (composite) oxide to a low-carbon steel melted in a 500 kg high-frequency melting furnace.
m. Table 2 shows the main components of the steels of the present invention (Nos. 1 to 11) and comparative steels (Nos. 12 to 20), the compositions of the added oxides, the added amounts, and the melting points. In addition, No. Nos. 12 and 13 are current lead free-cutting steels. No. 14 is a base steel. 15,
Numeral 16 indicates the amount of S added.

Table 3 and FIGS. 2 to 4 show the evaluation results of the test materials. Table 4 shows the results of the chip breaking judgment of the base steel (No. 14). As can be seen from FIG. 2, when the proportion of the (composite) oxide exceeds 5%, the chip breaking property is greatly improved, and is higher than that of lead free cutting steel or sulfur free cutting steel. As shown in FIG.
In order to improve the chip breaking property, the (composite) oxide should be 100
It turns out that it is necessary to add more than ppm. FIG.
As shown in the figure, the steel of the present invention has significantly improved chip breaking performance and grain impact value as compared with base steel, S-reduced or S-increased steel, and has properties equivalent to those of lead free-cutting steel.

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

[Table 4]

[0043]

According to the present invention, the ratio of the number of oxides to all the inclusions having a major axis of 1 μm or more in the longitudinal section is 10% or more, and N of the oxides is N.
oxides of at least one element selected from the group consisting of a, Li and B, or Na, Li, B and S
According to the steel of the present invention in which the ratio of the number of composite oxides of at least two elements selected from the group consisting of i is 5% or more, the chip breaking property is excellent without lowering the mechanical properties in the C direction. Can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing evaluation criteria for chip disposability adopted in an experiment.

FIG. 2 is a graph showing a relationship between a ratio of a (composite) oxide in a test steel and a chip breaking index.

FIG. 3 is a graph showing the relationship between the amount of oxide added to steel and the chip breaking index.

FIG. 4 is a graph showing the transverse grain toughness value and the chip breaking index of a test steel and a steel satisfying the requirements of the present invention.

Continuing on the front page (72) Inventor Hiroshi Ieguchi 1-5-5 Takatsudai, Nishi-ku, Kobe City Inside Kobe Research Institute, Kobe Steel Ltd. (72) Inventor Takehiro Tsuchida 1-5-5, Takatsukadai, Nishi-ku, Kobe City No. Kobe Steel, Ltd.Kobe Research Institute (72) Katsuhiko Ozaki 1-5-5 Takatsukadai, Nishi-ku, Kobe, Japan Kobe Research Institute, Kobe Steel Co., Ltd. (72) Inventor Masami Somegawa Kobe 2 Nadahama-Higashi-cho, Nada-ku Kobe Steel Works, Ltd.Kobe Works

Claims (5)

[Claims] C: 0.1 to 0.6% by mass% Si: 2.5% or less (excluding 0%) Mn: 0.2 to 3.0% S: less than 0.150% ( 0%) O: 0.001 to 0.015% containing steel for machine structural use, inclusions having a major axis of 1 μm or more appearing in a longitudinal section of the steel are 50 to 50 / mm ^2.
When the number of oxides is 1500% or more in the total amount of the inclusions, the number of oxides is 10% or more.
A steel for machine structural use excellent in chip breaking, wherein the number of oxides of at least one element selected from the group consisting of Li and B is 5% or more. 2. C: 0.1 to 0.6% by mass% Si: 2.5% or less (excluding 0%) Mn: 0.2 to 3.0% S: less than 0.150% ( 0%) O: 0.001 to 0.015% containing steel for machine structural use, inclusions having a major axis of 1 μm or more appearing in a longitudinal section of the steel are 50 to 50 / mm ^2.
The number of oxides in the total amount of the inclusions is 1500% or more, and among the oxides,
a for a mechanical structure excellent in chip breaking, characterized in that the number ratio of a composite oxide composed of oxides of at least two elements selected from the group consisting of a, Li, B and Si is 5% or more. steel. 3. The method according to claim 1, wherein the melting point of an oxide of at least one element selected from the group consisting of Na, Li, and Si is 1000 ° C. or less. A method for producing steel for machine structural use having excellent chip breaking properties, characterized by adding 100 ppm or more of the oxide of (1) to molten steel. 4. The method for producing a steel for machine structural use according to claim 2, wherein the melting point of an oxide of at least two elements selected from the group consisting of Na, Li, B and Si is 1000. 100 ° C or less complex oxide in molten steel
A method for producing steel for machine structural use having excellent chip breaking properties, characterized by adding at least ppm. 5. The method according to claim 3, wherein the oxide having a melting point of 1000 ° C. or less is added to molten steel at at least one of a ladle, a tundish and a mold.