JP3460721B2 - Machine structural steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for machine structural use, which is subjected to cutting work such as parts of industrial machines and automobiles. The present invention relates to a steel for machine structural use which has particularly excellent chip disposability and has an effect of extending the life of a cutting tool (hereinafter referred to as “improvement of tool life”).

[0002]

2. Description of the Related Art Mechanical structural steel used for parts such as industrial machines and automobiles is JIS-G-405 specified in JIS standard.
1 Carbon steel for machine structure, JIS-G-4102 nickel chrome steel as alloy steel, JIS-G-4103 nickel chrome molybdenum steel, JIS-G-4104 chrome steel, JIS-G-4106 machine structure Manganese steel and manganese chrome steel for use. In addition, steels with slightly modified specified components of these steels, further added B (boron), etc. to improve hardenability, and Ti, Nb, V, etc. added to improve the structure are also used. Has been done.

In many cases, these steels are produced by rolling, as they are, or after further forging, etc., cut into a predetermined shape and heat-treated according to the required characteristics. To make the final part. In order to improve the production efficiency in this cutting process, it is strongly desired that the steel material has excellent machinability. The excellent machinability means that it takes a long time to replace the tool used during cutting due to wear, that is, the tool life is long,
It means that the chips discharged during cutting are finely divided, that the cutting resistance is low, or that the cutting surface and the grinding surface have a good finish.

When the unmanning and automation of the cutting work progresses, the property of cutting chips in addition to the tool life, that is, "chip disposability" becomes extremely important. The tool life is influenced not only by the characteristics of the steel material but also by the performance of the tool, so the selection of the tool is also important. On the other hand, the excellent chip disposability means that the chips generated during cutting are finely divided and do not cling to the tool, which is largely controlled by the characteristics of the steel material itself. Improving the chip disposability is particularly important for improving the machinability of steel materials.

The machinability of steel can be improved by adding Pb. However, addition of Pb is not only accompanied by a rise in steel prices, but there is a risk of environmental pollution. Therefore, research on a technique for improving the machinability of steel without adding Pb has been advanced. A typical example of this is a technique for improving machinability by utilizing MnS inclusions, and many studies have been made on this technique and its practical application has been carried out.

For example, the steel disclosed in Patent Document 1 (Japanese Examined Patent Publication No. 5-15777) is Mn containing 3 to 55% of Ca.
-Ca-S inclusions are uniformly dispersed in the steel, and the size of these inclusions is such that the major axis L is 20 μm or less and the minor axis W ratio (L / W).
Is steel of 3 or less. However, in this steel, individual sulfides become coarse, and the number of sulfides decreases at the same S concentration.
Therefore, improvement of chip disposability is not always sufficient. Further, since it is premised on Al-killed steel, the oxide-based inclusions are CaO-Al ₂ O ₃ -based even if Ca treatment is performed, and the machinability improving effects such as tool life cannot be sufficiently obtained. Further, if a large number of sulfides containing a high concentration of CaS are to be dispersed in a high S concentration, it is necessary to add a large amount of Ca, which causes a problem that the cost increases.

[0007] Patent Document 2 (Japanese Patent Laid-Open No. 2001-131684) discloses a mechanical structural steel having an average oxygen content of 10% or less in Mn sulfide-based inclusions. The main composition of the steel is
Mass%, C: 0.05 to 0.7%, Si: 2.5% or less, Mn: 0.1 to
3.0%, Al: 0.1% or less, S: 0.003 to 0.2%, N: 0.002 to
0.025%, O (oxygen): 0.003% or less, the balance being Fe.
In addition to these components, one or more selected from the group consisting of rare earth elements, Ca and Mg may be contained in a total amount of 0.01% or less.

However, in the steels of the invention disclosed in the above patent documents, looking at the examples, the average oxygen concentration in the sulfide was set to 10 in order to obtain a sulfide form effective for improving the chip disposability. %, Used as a deoxidizing element
Contains 0.018% or more of Al. In such a case, the oxides present in the steel are mainly hard Al ₂ O ₃ -based oxides, and the tool life is not sufficiently improved. That is,
The invention of the above publication is not an invention intended to improve the chip handling property and the tool life at the same time.

Patent Document 3 (Japanese Patent Laid-Open No. 2000-34538) discloses a steel for machine structural use which contains a predetermined amount of C, Si, Mn, P, S, Al, Ca and N and has excellent turning property. Has been done.
This steel has the following characteristics. That is, the area ratio to the total area of the field of view for sulfides with Ca content exceeding 40% is A, Ca
If the area ratio of the sulfide having a content of 0.3 to 40% to the entire survey field of view is B, and the area ratio of the sulfide having a Ca content of less than 0.3% to the entire survey field is C, then A / (A + B
+ C) ≦ 0.3 and B / (A + B + C) ≧ 0.1. The invention of this patent document is characterized in that the proportion of sulfides containing 0.3 to 40% of Ca is increased. However, if the proportion of Ca-rich sulfides is increased, the individual sulfides become coarse and the number of sulfides decreases, making it difficult to obtain good chip disposability.

Patent Document 4 (Japanese Patent Laid-Open No. 2000-282169) contains C, Si, Mn, P and S, and further contains Zr, Te and Ca.
And containing one or more of Mg,
Steels with Al ≦ 0.01%, total-O ≦ 0.2%, total-N ≦ 0.02% are disclosed. This steel has excellent forgeability by spheroidizing sulfide and has good machinability. In other words, if Ca is added, Ca
It is a steel intended to form a spherical solution by dissolving in S to reduce its deformability. However, in this case, individual sulfides are coarsened, and a sulfide form for obtaining good chip disposability cannot be obtained. That is, the chip disposability is not sufficiently improved.

[0011]

[Patent Document 1] Japanese Patent Publication No. 5-15777

[Patent Document 2] Japanese Patent Laid-Open No. 2001-131684

[Patent Document 3] Japanese Patent Laid-Open No. 2000-34538

[Patent Document 4] Japanese Unexamined Patent Publication No. 2000-282169 All the steels disclosed in the above publications may contain Ca and are mainly steels having improved machinability. However, the amount of Ca added, the timing of addition, and the amount of dissolved oxygen in the steel during the steelmaking process have not been fully considered. Therefore, it does not necessarily improve both chip disposability and tool life.

[0012]

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a mechanical structural steel which does not contain Pb, has improved machinability, particularly chip disposability, and can have a long tool life. Especially.

[0013]

It is well known that the machinability of steel is greatly affected by the state of inclusions of sulfides and oxides. The present inventors have investigated the relationship between the formability and distribution of inclusions in steel and the machinability of the steel for machine structural steels not containing Pb in order to improve the machinability, and the results of the investigation It was investigated. Especially, focusing on the effect of Ca and Ti, the steelmaking conditions were also investigated. In the process, I was able to clarify the following interesting facts.

Ca strongly binds to S to change the form of sulfide mainly composed of MnS and also forms a stable oxide having a strong binding force with oxygen.

If Ca is added without considering the steelmaking conditions,
CaS and Ca-based oxides formed in molten steel serve as MnS-producing nuclei, and the number of sulfides containing 1% or more of Ca increases. However, when adding Ca, if the steelmaking conditions such as the addition amount and the dissolved oxygen amount of steel and the timing of addition of Ca are appropriately selected,
It was found that a large amount of sulfide-based inclusions mainly composed of Ca-free MnS were formed. Then, it became clear that the chip disposability of the steel was improved only in this case.

There are sulfide-based inclusions and oxide-based inclusions, but since fine inclusions such as precipitates have no effect on improving machinability, the area occupied by the observation surface is The size was evaluated by the diameter when it was replaced with the shape, and the investigation was carried out for those with a diameter larger than a certain extent.

As a result, of the total number of sulfide inclusions, when the percentage of sulfides containing almost no Ca exceeds 90%, in other words, the number of MnS inclusions containing Ca is less than 10%. In the case of, it was found that particularly excellent chip disposability was obtained.

If the S content of the steel is the same, the steel having a large number of small sulfides is superior in chip disposability to the steel having a small number of coarse sulfides. When a large amount of sulfides that form a solid solution of Ca are formed in molten steel or from the beginning of solidification, they become crystallization nuclei of MnS and become coarse sulfides.
Therefore, at the same S concentration, the number of dispersed particles is small, and it is difficult to form fine sulfides. On the other hand, if the amount of sulfides that form a solid solution with Ca is reduced, most of the sulfides will be produced as fine sulfides.

The fragmentation of the chips discharged during cutting is caused by the concentration of stress on the inclusions of the deformed chips in the steel and the generation and propagation of cracks. MnS-based inclusions that do not contain Ca are easily deformed in the processing direction such as rolling, and are often stretched. The presence of large stretched inclusions increases the anisotropy of the mechanical properties of the steel, and
It becomes difficult to obtain good chip disposability because the number of inclusions that serve as a stress concentration source as a starting point of the notch and improve chip disposability decreases. However, if there are many small inclusions,
In the chips that are deformed during cutting, the starting points of cracks increase, and stress concentrates on inclusions, which facilitates crack propagation. It is presumed that this is the cause of improvement in chip disposability.

The tool life is greatly affected by the composition of oxides contained in steel. If the oxide is made into a low melting point oxide by adding Ca, the tool life will be significantly extended. Therefore, it is indispensable to perform Ca treatment, and in order to achieve both the above sulfide control and oxide control, the steelmaking conditions including before and after Ca treatment were examined in detail. as a result,
By limiting the content of components such as C, Si, and Mn that have a large interaction with oxygen in steel, containing a specific amount of S, and reducing Al as much as possible, appropriate amounts of Ti and Ca are appropriately added. By adding it at a timing and adjusting the amount of dissolved oxygen, CaO-Al ₂ suitable for improving the tool life is obtained within the same composition range.
It has been clarified that O ₃ —SiO ₂ —TiO ₂ can be controlled as a main constituent. This oxide-based inclusion has a low melting point and is soft, and it is considered that not only the Ca and Ti contained in the oxide-based inclusion help improve the tool life, but also have the effect of assisting the starting point of crack generation in chips and crack propagation.

When the sulfide-based inclusions and oxide-based inclusions are formed in such a form, chip disposability and tool life are improved. In addition to the composition of C, Si, Mn, etc., the mechanical structure The effect of the inclusion of Cr, Ni, Mo, Nb, V and other elements added for the purpose of improving the strength, improving hardenability, improving the structure of the steel for use was investigated. As a result, these elements may improve mechanical properties such as hardness and strength of steel, hardenability, etc., but the same effect of the present invention that machinability is improved with the same composition may be obtained. It could be confirmed.

Then, the limits of the chemical composition and the state of inclusions were further confirmed, and the present invention was completed. The gist of the present invention is as follows.

(1) In mass%, C: 0.1 to 0.6%, Si: 0.
01 to 2.0%, Mn: 0.2 to 2.0%, P: 0.1% or less, S: 0.005
~ 0.2%, Al: 0.009% or less, Ti: 0.001% or more, 0.04%
Less than, Ca: 0.0001 to 0.01%, O (oxygen): 0.001 to 0.01
%, N: 0.02% or less, the balance consisting of Fe and impurities, and the inclusions present in the steel satisfy the following expressions (1) to (4).

[0024] _{n 0 / S (%) ≧} 2500 ···· n 1 / n 0 ≦ 0.1 ······· n 2 ≧ 10 ········ where, n _0, n ₁ , N ₂ is as follows.

N ₀ : Total number of sulfides having a circle equivalent diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction (pieces / m
m ² ), n ₁ : The number of MnS containing 1.0% or more of Ca with a circle equivalent diameter of 1 μm or more in 1 mm ^{2 of} a cross section parallel to the rolling direction (pieces / m
m ² ), n ₂ : Ca included in the oxide-based inclusions
The total of O, Al ₂ O ₃ , SiO ₂ and TiO ₂ accounts for 80 mass% or more.
CaO: 5 to 60%, Al , which is an oxide-based inclusion, which is calculated as the total of these four oxides is 100% by mass.
₂ O ₃ : 5-60%, SiO ₂ : 10-80%, TiO ₂ : 0.1-40% CaO
-Al ₂ O ₃ -SiO ₂ -TiO ₂ and the circle equivalent diameter of 1 μm or more, but the number of cross sections parallel to the rolling direction in 1 mm ² (pieces / mm ² ).

(2) In addition to the components described in (1) above, a mechanical structure containing one or more components selected from the following first group and / or second group and satisfying the above formulas, formulas and formulas. For steel.

First group Cr: 0.02 to 2.5%, V: 0.05 to 0.5%, Mo: 0.05 to 1.0%,
Nb: 0.005-0.1%, Cu: 0.02-1.0% and Ni: 0.05-
2.0% Second group Se: 0.0005 to 0.01%, Te: 0.0005 to 0.01%, Bi: 0.05 to
0.3%, Mg: 0.0001 to 0.0020% and rare earth element: 0.000
1 to 0.0020%

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the form of the steel material of the present invention is limited, such as the distribution and composition of inclusions, will be described below. In addition, in the following description,% regarding the composition of steel means "mass%".

When the size of the target inclusion is replaced by a circular shape when the shape observed in the cross section parallel to the rolling direction is replaced with a circular shape, the diameter is limited to 1 μm or more. This is because the inclusions have almost no effect on the tool life or chip disposability. Inclusions with a diameter of more than 10 μm when replaced with this circular shape impair the properties such as strength of steel and prevent the uniform dispersion of inclusions, and are effective in improving machinability, especially chip disposability. It is not preferable because it is not available.

Most of the inclusions observed in a cross section parallel to the working direction are elongated in the working direction or have an unspecified shape. For the shape investigation, the cross section of the steel sample was mirror-polished.
Take a photo with an optical microscope observation of about 400 times,
The area is calculated by the image analysis method, and when the area is converted into a circle, inclusions with a diameter of 1 μm or more are targeted. At this time, inclusions having the same composition, which can be clearly judged to have been divided by rolling, may be treated as one inclusion. The composition of the inclusions is analyzed by, for example, an apparatus capable of analyzing EPMA or a minute portion equivalent thereto.

Of these inclusions, when the total number of sulfide-based inclusions containing MnS per mm ² is n ₀ and the analysis value of S is S (%), n ₀ / S (%) ≧ 2500 ・ ・ ・When n ₀ / S (%) is less than 2500, S
When the steels having the same content are compared, the number of inclusions is small, and not only the characteristics as a steel material are inferior, but also the chip disposability is inferior. The reason why the number of inclusions decreases with the same S content is that each sulfide is coarsened. Good chip disposability can be obtained within the range satisfying the formula, but in order to obtain more stable and good chip disposability, n ₀ / S (%) is preferably 3500 or more. n
₀ may be large as long as the formula is satisfied, but if it becomes too large, it becomes difficult to obtain mechanical properties such as tensile strength and fatigue strength as steel for machine structural use, so it is preferably 2000 or less, It is more preferably 1000 or less.

Among the sulfide-based inclusions, the inclusions are Ca
Although the contains more than 1.0 wt% the number per 1 mm ²
When n _1, it is assumed that n ₁ / n ₀ ≦ 0.1 ... This is because when the ratio of sulfide containing 1.0% by mass or more of Ca to the total number of sulfides exceeds 0.1, the individual inclusions tend to be coarse and the chip disposability decreases. Within the range of the above formula, it becomes possible to reduce inclusions in the steel. This is Ca 1.0
This leads to an increase in the number of sulfide-based inclusions that do not contain mass% or more, and good chip disposability can be obtained. In addition,
In order to obtain more stable and good chip disposability, n ₁ /
It is desirable that n ₀ is 0.08 or less. The smaller n ₁ is, the better, and it may be 0.

Among oxide-based inclusions, oxide-based inclusions in which the total of CaO, Al ₂ O ₃ , SiO ₂ and TiO ₂ contained in the inclusions accounts for 80% by mass or more. When the total amount of the oxides of the species is 100% by mass, the respective content ranges are CaO: 5-60%, Al ₂ O ₃ : 5-60%, SiO ₂ : 10-80%,
TiO ₂ : 0.1 to 40%, but the number n per 1 mm ²
₂ is n ₂ ≧ 10 ...

The above-mentioned oxide inclusions are not less than 10 in the case of less than 10 in addition to the oxide having a low melting point composition formed by addition of Ca and Ti, as well as Al ₂ O ₃ This is because a hard oxide is formed with a high melting point composition such as, and the effect of extending the tool life cannot be obtained.

The range of the content of each oxide in the inclusion is limited because the oxide in this composition range has a low melting point. As long as it is limited to this composition range, the oxide softens as the cutting temperature rises, so that the oxide itself does not accelerate the wear of the tool and contributes to the extension of the tool life. If it deviates from this composition range, the melting point of the oxide is increased and the hardness is increased, and the oxide itself accelerates the wear of the tool, so that the tool life is shortened.

In order to obtain the mechanical properties and machinability necessary for the steel for machine structure as well as the morphology of the inclusions in the steel as described above, the components contained in the steel are limited as follows. Must.

The C content is 0.1-0.6%. C is an important element that controls the properties relating to the strength of steel, and its content is usually determined in consideration of mechanical properties. C
If less than 0.1%, the mechanical properties as crankshafts and other mechanical parts for automobiles cannot be obtained. On the other hand, 0.
If it exceeds 6%, the tool life will be significantly reduced and desired machinability cannot be obtained. In order to stably obtain the mechanical properties, hardness, toughness, fatigue strength and machinability of crankshafts and other mechanical parts for automobiles, the C content is preferably 0.30 to 0.55%.

The Si content is 0.01 to 2.0%. Si is an essential element for obtaining the oxide composition of the present invention, and is also included for the purpose of deoxidizing molten steel. If the content is less than 0.01%, the desired oxide composition cannot be obtained, and if it exceeds 2.0%, not only the effect is saturated, but also the toughness of the steel decreases. Therefore, the Si content is set to 0.01 to 2.0%. In addition, as a range that stably obtains a desired oxide composition and does not deteriorate mechanical properties, a more preferable Si content range is 0.15 to 1.
It is 0%.

The Mn content is 0.2 to 2.0%. Mn is an important element for forming sulfide-based inclusions that have a great effect on improving machinability, and also has a deoxidizing effect on molten steel.
In addition, when S is added for the purpose of improving machinability, it has an effect of suppressing deterioration of hot workability of steel material, but for that purpose, the content of 0.2% or more is essential. But 2.0
%, The cutting resistance increases, so 0.2 to 2.0% is used. In the case of steel used after heat treatment, Mn is an element that greatly contributes to hardenability, and the content for this purpose is appropriately selected within the above range. At this time, the more preferable range of the Mn content is 0.4 to 1.70%.

The S content is 0.005 to 0.2%. S is necessary for improving the machinability, and is combined with Mn or the like to be present in the form of a sulfide-based inclusion. It is a sulfide-based inclusion
Since the morphology of MnS is likely to change due to the addition of Ca or Ti during the solidification process of steel, the morphology of this MnS-based sulfide is simultaneously defined in the present invention. If the content is less than 0.005%, the effect of improving the machinability cannot be obtained, and if the content is too large, the hot workability deteriorates and the toughness of the steel deteriorates.
% To 0.2%. A preferable range for achieving both machinability and mechanical properties as a steel for machine structural use is 0.
It is from 01 to 0.18%. Good machinability and mechanical properties can be obtained within this range, but in order to achieve suitable mechanical properties and machinability both as a heat-treated steel and machine structural steel, S is 0.03 to 0.12% is desirable.

The content of Al (sol.Al, that is, acid-soluble Al) is
0.009% or less. Al has a great effect of deoxidizing molten steel,
Add for acid adjustment. However, as a result of deoxidation
Al ₂O₃Is hard and reduces tool life, so this is often
The upper limit of Al is set to 0.009% so that it does not become complicated.

Within this range, it is possible to reduce the frequency with which a single oxide of Al ₂ O ₃ and Al ₂ O ₃ is mainly formed. Most of the Al content of a small amount of deoxidizer used for rapid oxygen reduction in the early stage of steelmaking or the Al content that inevitably enters from ferroalloys etc. is CaO-Al ₂ O ₃ -SiO ₂
-Since it is used to form TiO ₂ oxide, it is not a problem.
Therefore, the Al content is 0.009% or less, and the lower limit value is not particularly set. Further, the Al content desirable for forming the above oxide more stably is 0.005% or less.

The content of Ti is 0.001% or more and less than 0.04%.
To do. CaO-Al for Ti₂O₃-SiO₂-TiO ₂An oxide of
Has the effect of stable generation and the effect of miniaturization
Since it has fruit, it is an essential element in the steel of the present invention. Contains Ti
Not CaO-Al₂O₃-SiO₂Also good for machinability in the system
It is possible to form low melting point oxides, but₂
The effect is further enhanced by compounding the above. Ti's
If the content is less than 0.001%, the effect does not appear.
When contained above, the effect is not only saturated, but also hard
Precipitation of TiN increases, which shortens the tool life. Work
Better to stably form oxides suitable for
The Ti content is 0.005 to 0.025%.

The Ca content is 0.0001% to 0.01%. Ca
Has the effect of improving the tool life and is necessary for the formation of an oxide of CaO—Al ₂ O ₃ —SiO ₂ —TiO ₂ that is effective in improving machinability. If it is less than 0.0001%, such effect is not sufficient. On the other hand, if it exceeds 0.01%, not only the above oxide cannot be formed, but also the production yield increases due to the low Ca addition yield. In addition, MnS that dissolves Ca in solid solution increases and MnS coarsens. In other words, the number of MnS is reduced, and desired effects such as improved chip disposability cannot be obtained. A more preferable Ca content for more stably obtaining the inclusion morphology defined in the present invention is 0.0005 to 0.005%. In addition, in order to obtain an inclusion form suitable for improving the machinability defined in the present invention, it is necessary to consider the steelmaking conditions before and after the addition of Ca.

The content of O (oxygen) is 0.001% to 0.01%. Oxygen is preferable for improving machinability CaO-Al ₂ O ₃ -SiO _2-
It is an important element for producing a TiO ₂ oxide and obtaining a morphology and number of sulfide inclusions which are preferable for improving machinability. If the content is less than 0.001%, such an effect is not sufficient, and it becomes difficult to obtain a morphology of oxide inclusions preferable for improving machinability. On the other hand, if the content exceeds 0.01%
Sulfide-based inclusions containing MnS, etc. become coarse, and the amount of oxide-based inclusions increases, which not only reduces machinability but also deteriorates the properties of steel such as toughness. In order to more reliably and stably obtain the morphology and composition of the inclusions defined in the present invention, the oxygen content is preferably 0.005% or less.

The control of the steelmaking process is important because the morphology of sulfide-based inclusions and the composition of oxide-based inclusions that improve chip disposability and prolong tool life are controlled in the steelmaking process.

The melting procedure for obtaining the morphology and composition of the inclusions specified in the present invention will be illustrated and described. The manufacturing method of the steel of the present invention is not limited to the manufacturing method according to the following procedure.

First, excess oxygen is adjusted by subjecting molten steel containing a small amount of carbon to vacuum treatment in a state where the Al content is small. After that, main C, Si, Mn, S and other elements are adjusted to the target contents, and then the dissolved oxygen amount is preliminarily adjusted. At this time, Al may be added to adjust the amount of dissolved oxygen, if necessary. However, the Al content contained at this time is 0.009% or less, preferably 0.005% or less, as described above. Then, Ti is added, and finally, it is treated with Ca and cast into an ingot or a slab.

The reason why the manufacturing method of the above procedure is desirable is
It is as follows.

Oxides produced by deoxidation by Mn and Si added when adjusting the main components by removing excess oxygen of molten steel containing a small amount of carbon are Al ₂ produced when Al is added. O ₃ is no longer an oxide with an excessive composition. When adjusting the components, it is necessary to adjust so that the amount of dissolved oxygen does not become too low due to the deoxidation reaction caused by C, Mn and Si. To adjust the amount of dissolved oxygen, add Ca before casting to generate Ca as an oxide and dissolve Ca in solid solution.
The purpose is to prevent the formation of Ca-based sulfide, which causes MnS. Next, in order to adjust the dissolved oxygen amount, Al may be added as necessary, but since it is the minimum necessary addition after the oxygen concentration and the composition of the oxide-based inclusions are already adjusted, Excessive Al ₂ O ₃ based oxide is not formed. Since the presence of Al ₂ O ₃ still shortens the tool life, the Al content at this stage needs to be 0.009% or less, and more preferably 0.005% or less.

Then, by adding Ti, the deoxidation proceeds further, but the Ti oxide formed at that time becomes a thermodynamically more stable form by combining with the existing oxide, and suppresses the formation of large inclusions. Then, it has the effect of uniformly dispersing inclusions of about 1 to 10 μm. Subsequent treatment of Ca is added in the form of alloys such as calcium silicon or alloy iron,
Ca hardly dissolves in the molten steel, and reacts with oxygen in the molten steel and the dispersed oxide to form CaO—Al ₂ O ₃ —SiO ₂ —TiO ₂ oxide.

One of the mechanical structural steels of the present invention has the above-mentioned components, and the balance is Fe and impurities. Of the impurities, the contents of P and N are set to the following upper limit values or less.

P: 0.1% or less P is an element mixed in steel as an impurity. Although it has a solid solution strengthening effect and an effect of improving hardenability, it deteriorates the toughness of steel, so the range is set to 0.1% or less so that the adverse effect is not significant. 0.05% or less is desirable,
The less, the better.

N: 0.02% or less N coexists with Al to form fine nitrides, which has the effect of refining the crystal grains of steel. However, in the present invention, since the Al content of steel is limited to a low level, such an effect cannot be expected, and rather than that, TiN may be combined with Ti to form TiN, which may deteriorate the tool life. Therefore,
The smaller the content, the better. If 0.02% or less, the adverse effect is not large, so the allowable upper limit is 0.02%. More preferably, it is 0.015% or less.

Another one of the steels for machine structural use of the present invention is the following first group or / and second group in addition to the above components.
A steel containing one or more components selected from the group of components.

First group Cr: 0.02 to 2.5%, V: 0.05 to 0.5%, Mo: 0.05 to 1.0%,
Nb: 0.005-0.1%, Cu: 0.02-1.0% and Ni: 0.05-
2.0%.

Second group Se: 0.0005 to 0.01%, Te: 0.0005 to 0.01%, Bi: 0.05 to
0.3%, Mg: 0.0001 to 0.0020% and rare earth element: 0.000
1 to 0.0020%.

All of the above-mentioned components belonging to the first group contribute to improving the strength of steel. Further, the components belonging to the second group contribute to the improvement of the machinability of steel. The reasons for controlling the contents of these elements are as follows.

Cr: 0.02 to 2.5% or less Cr has the effect of improving the hardenability of steel, and is preferably added to alloy steels for machine structures. For the purpose of improving the hardenability, the content is preferably 0.02% or more, but if it exceeds 2.5%, the hardenability becomes too high, which not only lowers the durability ratio and the yield ratio but also deteriorates the machinability. Therefore, the Cr content is 0.02 to 2.5%.

Mo: 0.05 to 1.0% Mo has the effect of refining the ferrite / pearlite structure, and has the effect of improving hardenability and toughness when tempering. In order to reliably obtain the effect, the content is preferably 0.05% or more. However
If it exceeds 1.0%, the effect is saturated, which may rather reduce the fatigue strength and increase the cost. Therefore Mo
Content of 0.05 to 1.0%.

Ni: 0.05 to 2.0% Ni has the effect of improving the strength of steel by solid solution strengthening, and also has the effect of improving hardenability and toughness. In order to reliably obtain this effect, its content is preferably 0.05% or more. However, if it exceeds 2.0%, not only the above effect is saturated, but also the hot workability is deteriorated. Therefore, the proper content of Ni is 0.05 to 2.0%.

Cu: 0.02 to 1.0% Cu has an effect of improving the hardenability of steel, and if it is desired to obtain the effect, it is preferable to contain 0.02% or more. Furthermore, since precipitation strengthening has the effect of improving the strength of the steel,
In order to obtain this effect, the content is preferably 0.1% or more. However, if the content exceeds 1.0%, the hot workability is deteriorated, and the above effects are lost due to the coarsening of Cu precipitates. Therefore, the Cu content is 0.02
~ 1.0%

V: 0.05 to 0.5% Nb: 0.005 to 0.1% V and Nb are precipitated as fine nitrides and carbonitrides,
Improve the strength of steel. In order to surely obtain the effect, it is preferable that the V content is 0.05% or more and the Nb content is 0.005% or more. However, when V exceeds 0.5% and Nb exceeds 0.1%, not only the above effect is saturated, but also a large amount of nitrides and carbides are generated, which deteriorates the machinability of the steel and lowers the toughness. . Therefore, the V content is 0.05-0.
The content of 5% and Nb is 0.005 to 0.1%.

Se: 0.0005 to 0.01%, Te: 0.0005 to 0.01% Se and Te form MnSe or MnTe together with Mn to improve the machinability of steel. To obtain this effect, Se and Te
It is desirable that the content of each be 0.0005% or more. However, if the content of both Se and Te exceeds 0.01%, not only the effect is saturated but also the hot workability is deteriorated. Therefore, the proper content of Se and Te is 0.0005 to 0.01%, respectively.

Bi: 0.05-0.3% Bi improves the machinability of steel. It is considered that this is because, like Pb, it exhibits a lubricating effect during cutting as a low melting point inclusion. To ensure that effect, the content should be 0.05%.
The above is preferable. However, if the content exceeds 0.3%, not only the effect is saturated, but also the hot workability of steel deteriorates. Therefore, the proper content of Bi is 0.05-0.
3%.

Mg: 0.0001 to 0.0020% Mg has a strong affinity with S and O (oxygen) and forms sulfides and oxides. If the content is 0.0020% or less, no harmful oxide is formed, and sulfides are mainly formed to improve machinability of steel. To obtain this effect, a content of 0.0001% or more is required. 0.0005% or more is more desirable. However,
If it exceeds 0.0020%, the oxide becomes hard and has a high melting point, which deteriorates machinability. Therefore, the proper content when adding Mg is 0.0001 to 0.0020%, and more preferably 0.0005 to 0.000.
It is 0020%.

Rare earth element: 0.0001 to 0.0020% When a rare earth element is contained, inclusions containing sulfides are formed and the number of sulfides is increased, so that machinability is improved. Rare earth elements include La, Ce, Nd, etc., and are abbreviated as REM. A misch metal may be used to add the rare earth element. The total of one or more rare earth elements is 0.
If it is 0001% or more, the effect will appear. To obtain the effect more reliably, it is desirable to contain 0.0005% or more. However, if it exceeds 0.0020%, the ratio of oxides or sulfides containing rare earth elements increases, and the desired inclusion morphology cannot be obtained, so machinability cannot be improved. Therefore, the proper content of rare earth elements is 0.0001 to 0.0020%.

[0068]

EXAMPLE Steels having the chemical compositions shown in Tables 1 and 2 were melted by the following procedure to obtain a steel ingot of 150 kg. However, some of the steels in Table 2 were melted by the procedure described below. The steels Nos. 74 and 75 in Table 2 are steels containing Pb.

The molten steel was vacuum-treated in a state of containing a small amount of carbon to adjust excess oxygen in a state of containing a small amount of Al.

Then, after adjusting the atmosphere in the furnace to an argon atmosphere, first, C, Si, Mn, S, which are main components, and other elements are adjusted to predetermined amounts, and the dissolved oxygen amount is adjusted as necessary. Iron oxide was added.
Then, if it was necessary to further adjust the dissolved oxygen amount, Al was added.

Then, Ti was added, and finally treated with Ca, and then cast to obtain an ingot or a slab.

Each of the steels in Table 1 is a steel having the composition range specified in the present invention. The steels in Table 2 are out of the composition range defined in the present invention.

Among the steels shown in Table 2, steels having different inclusion morphologies from those defined in the present invention even within the same composition range were melted by the following method. That is, in the melting process with a high oxygen concentration, the vacuum treatment in a state containing a small amount of C was not performed, or the iron oxide was excessively added for adjusting the dissolved oxygen concentration in the middle stage. In the melting process with a high Al concentration, Al is added at the stage of adjusting the main components, and if it is sufficiently deoxidized, look at the analysis results, etc., and usually add Al immediately before adding Ca to deoxidize it. I decided to do it. For steel that has not been further deoxidized with Al,
After deoxidation with C, Si and Mn, Ti and Ca were added immediately before casting without adjusting the amount of dissolved oxygen by adding iron oxide or the like. Excess Ca, which does not contribute to the deoxidation reaction in this melting method, has a strong affinity with S and therefore Ca in the molten steel stage.
It forms S and becomes a nuclei for forming MnS that crystallizes later. As a result, in a state where deoxidation is sufficiently performed, when excess Ca that does not contribute to the deoxidation reaction is contained, CaS is generated in the molten steel stage,
Since MnS crystallizes using it as a production nucleus, the number of MnS (n ₁ ) that dissolves Ca by 1% or more increases.
₁ / n ₀ ”exceeds 0.1. As a result, sulfides are coarsened and the total number of inclusions (n ₀ ) is reduced, so that the formula, that is, “n ₀ / S (%) ≧ 2500” is not satisfied, and the desired chip disposability is not achieved. You won't get it.

Each steel ingot was heated to 1250 ° C. and then hot forged at a temperature up to 1000 ° C. to finish it into a round bar with a diameter of 70 mm,
After forging, it was air-cooled to room temperature. From the surface of the obtained round bar
A cross section parallel to the stretching direction was mirror-polished with a test piece taken at a position at a depth of 17.5 mm, that is, at a position 1/2 the radius of the round bar, and one sample was prepared using EPMA at a magnification of 400 times. The number of sulfides and oxides having a diameter converted to a circle (diameter equivalent to a circle) of 1 μm or more was counted by observing 20 fields or more. Then, in each field of view, selected arbitrarily
Ten or more of the above sulfides and oxides were quantitatively analyzed to determine their composition.

[0075]

[Table 1]

[0076]

[Table 2]

Sample unit of inclusions observed in this way
Area 1 mm²Total number of sulfides (n ₀) And steel S analysis
From the result and "n₀/ S (%) "was calculated. Next, sulfide
Find the number of existing substances that contain 1.0% by mass or more of Ca.
`` N₁/ N₀Was calculated.

Regarding the oxides analyzed above,
When the total of CaO, Al ₂ O ₃ , SiO ₂ and TiO ₂ of the composition occupies 80% by mass or more and the total amount of these four components is 100% by mass, CaO is 5 to 60%, Al ₂ O ₃ is 5-60%, SiO ₂ is 10-
Number of oxide inclusions (n = 80%, TiO ₂ 0.1-40%)
₂ ) asked. The investigation results of these inclusions are shown in Tables 3 and 4. In addition, the value with * in Table 4 indicates a value that does not satisfy the conditions defined in the present invention or a value that does not reach the target performance.

[0079]

[Table 3]

[0080]

[Table 4]

The machinability is the diameter produced as described above.
From a 70 mm round bar, a cylindrical test piece was cut into a length of 60 mm, and a drilling test was conducted in a direction perpendicular to the cross section to evaluate. Drilling conditions are made of high-speed steel with a diameter of 6
Using a mm straight shank drill, a water-soluble cutting oil (emulsion type) was used to feed 0.15 mm / rev, the rotation speed was 980 rpm, and the hole depth was 50 mm.

In this test, the tool life was judged by the number of holes when it became impossible to drill due to the wear of the cutting edge. Also,
For the evaluation of chip disposability, the number of chips discharged per unit mass was measured, and the value obtained by dividing the number of chips by the S content (mass%) of the steel was defined as the chip disposability index (f). It is known that the number of chips per unit mass increases as the amount of S contained in steel increases, and for the same S content, the greater the number of chips per unit mass, the better the chip treatability. . The results of these machinability evaluations are also shown in Tables 3 and 4.

As can be seen from the results of measurement of the number of inclusions and machinability shown in Tables 3 and 4, the sulfide-based and oxide-based inclusions have the chemical composition defined by the present invention. The steels satisfying the conditions defined in the present invention, that is, the steels shown in Table 1 are excellent in both chip disposability and tool life as compared with any of the steels in Table 2 except No. 78 and 79 steels. Is shown. The steels in Table 1 are N shown as a reference example.
It is clear that it shows machinability equivalent to or better than the steels with Pb added at o.78 and 79.

FIG. 1 shows the relationship between the chip disposal index and the S content shown in Tables 3 and 4. In addition,
In particular, the data for steels No. 66 to 77, which have poor tool life, are excluded. From this figure, it can be seen that the steel of the present invention has excellent chip disposability as long as the level of S content is the same.

FIG. 2 is a diagram showing the relationship between the chip disposal index and “n ₁ / n ₀ ” shown in Tables 3 and 4. However,
In particular, the data for steels No. 66 to No. 77, which have poor tool life, are excluded. From this figure, it can be seen that the steel of the present invention satisfying “n ₁ / n ₀ ≦ 0.1” has excellent chip disposability.

FIG. 3 shows the relationship between the chip disposal index shown in Tables 3 and 4 and "n ₀ / S (%)". The data for steels No. 66 to 77, which have particularly poor tool life, were excluded. From FIG. 3, “n ₀ / S (%) ≧ 250
It can be seen that the steel of the present invention satisfying “0” has excellent chip disposability.

FIG. 4 is a diagram showing the relationship between the tool life and the S content shown in Tables 3 and 4. The data for steels No. 43 to No. 65, which have particularly poor chip disposability, were excluded. This figure also shows that the tool life of the steel of the present invention is excellent as long as the S content level is the same.

FIG. 5 shows the relationship between the tool life and n ₂ shown in Tables 3 and 4. In this figure, except for the data of steels No. 43 to No. 65, which are particularly inferior in chip disposability, in order to compare at the same S content level, 0.
The steel of the present invention containing S in the range of 074 to 0.119% (No. 8 to 1)
1,17-18,21,23,27-28,31-32,34,40 steel),
The data of the steels of Nos. 70 and 72 to 76 which are comparative examples are shown. From FIG. 5, when comparing at the same S content level, “n ₂ ≧
It is clear that the steel according to the present invention satisfying 10 ”has an excellent tool life.

[0089]

EFFECTS OF THE INVENTION The steel material for machine structure of the present invention is excellent in machinability, particularly chip disposability, although it does not contain Pb.
It is also effective in extending the tool life. By using this steel as a material for a part that requires cutting, the manufacturing cost of that part can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a chip disposal index of steel and an S content.

FIG. 2 is a diagram showing a relationship between a chip disposal index of steel and “n ₁ / n ₀ ”.

FIG. 3 is a diagram showing a relationship between a chip disposal index of steel and “n ₀ / S (%)”.

FIG. 4 is a diagram showing the relationship between tool life and S content.

FIG. 5 is a diagram showing a relationship between tool life and n ₂ .

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toru Kato 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Hitoshi Matsumoto, 1 Konomi-cho, Kitakyushu-ku, Fukuoka Address Sumitomo Metal Kokurauchi (72) Inventor Hiroaki Tahira 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (56) Reference JP 2001-288531 (JP, A) JP 2001-234279 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 301 C22C 38/14 C22C 38/60

Claims (4)

(57) [Claims] 1. In mass%, C: 0.1-0.6%, Si: 0.01-
2.0%, Mn: 0.2-2.0%, P: 0.1% or less, S: 0.005-0.2
%, Al: 0.009% or less, Ti: 0.001% or more and less than 0.04%,
Ca: 0.0001 to 0.01%, O (oxygen): 0.001 to 0.01%, N:
A steel for machine structural use, wherein the content is 0.02% or less, the balance is Fe and impurities, and the inclusions present in the steel satisfy the following expressions. n ₀ / S (%) ≧ 2500 ··· n ₁ / n ₀ ≦ 0.1 ···· n ₂ ≧ 10 ··· · where n ₀ , n ₁ , n ₂ Is as follows. n ₀ : the total number of sulfides with a circle equivalent diameter of 1 μm or more in 1 mm ² of the cross section parallel to the rolling direction (pieces / mm ² ), n ₁ : the circle equivalent diameter of 1 μm in 1 mm ² of the cross section parallel to the rolling direction As a result, the number of MnS containing 1.0% or more of Ca (pieces / m
m ² ), n ₂ : Ca included in the oxide-based inclusions
The total of O, Al ₂ O ₃ , SiO ₂ and TiO ₂ accounts for 80 mass% or more.
CaO: 5 to 60%, Al based on 100% by mass of the total of these four oxides.
₂ O ₃ : 5-60%, SiO ₂ : 10-80%, TiO ₂ : 0.1-40% CaO
-Al ₂ O ₃ -SiO ₂ -TiO ₂ and the circle equivalent diameter of 1 μm or more, but the number of cross sections parallel to the rolling direction in 1 mm ² (pieces / mm ² ). 2. In mass%, C: 0.1-0.6%, Si: 0.01-
2.0%, Mn: 0.2-2.0%, P: 0.1% or less, S: 0.005-0.2
%, Al: 0.009% or less, Ti: 0.001% or more and less than 0.04%,
Ca: 0.0001 to 0.01%, O (oxygen): 0.001 to 0.01%, N:
It is characterized in that it is 0.02% or less, contains one or more elements selected from the following first group, the balance consists of Fe and impurities, and the inclusions present in the steel satisfy the following formulas. Steel for machine structure. 1st group Cr: 0.02-2.5%, V: 0.05-0.5%, Mo: 0.05-1.0%,
Nb: 0.005-0.1%, Cu: 0.02-1.0% and Ni: 0.05-2.0% n ₀ / S (%) ≧ 2500 ··· n ₁ / n ₀ ≦ 0.1 ···· n ₂ ≧ 10 ・ ・ ・ ・ ・ ・ where n ₀ , n ₁ , and n ₂ are as follows. n ₀ : the total number of sulfides with a circle equivalent diameter of 1 μm or more in 1 mm ² of the cross section parallel to the rolling direction (pieces / mm ² ), n ₁ : the circle equivalent diameter of 1 μm in 1 mm ² of the cross section parallel to the rolling direction As a result, the number of MnS containing 1.0% or more of Ca (pieces / m
m ² ), n ₂ : Ca included in the oxide-based inclusions
The total of O, Al ₂ O ₃ , SiO ₂ and TiO ₂ accounts for 80 mass% or more.
CaO: 5 to 60%, Al based on 100% by mass of the total of these four oxides.
₂ O ₃ : 5-60%, SiO ₂ : 10-80%, TiO ₂ : 0.1-40% CaO
-Al ₂ O ₃ -SiO ₂ -TiO ₂ and the circle equivalent diameter of 1 μm or more, but the number of cross sections parallel to the rolling direction in 1 mm ² (pieces / mm ² ). 3. In mass%, C: 0.1-0.6%, Si: 0.01-
2.0%, Mn: 0.2-2.0%, P: 0.1% or less, S: 0.005-0.2
%, Al: 0.009% or less, Ti: 0.001% or more and less than 0.04%,
Ca: 0.0001 to 0.01%, O (oxygen): 0.001 to 0.01%, N:
It is characterized by containing 0.02% or less of one or more elements selected from the following second group, the balance consisting of Fe and impurities, and the inclusions present in the steel satisfying the following formulas. Steel for machine structure. Second group Se: 0.0005-0.01%, Te: 0.0005-0.01%, Bi: 0.05-
0.3%, Mg: 0.0001 to 0.0020% and rare earth element: 0.000
1 to 0.0020% n ₀ / S (%) ≧ 2500 ··· n ₁ / n ₀ ≦ 0.1 ···· n ₂ ≧ 10 ・ ・ ・ ・ ・ ・ where n ₀ , n ₁ and n ₂ are as follows. n ₀ : the total number of sulfides with a circle equivalent diameter of 1 μm or more in 1 mm ² of the cross section parallel to the rolling direction (pieces / mm ² ), n ₁ : the circle equivalent diameter of 1 μm in 1 mm ² of the cross section parallel to the rolling direction As a result, the number of MnS containing 1.0% or more of Ca (pieces / m
m ² ), n ₂ : Ca included in the oxide-based inclusions
The total of O, Al ₂ O ₃ , SiO ₂ and TiO ₂ accounts for 80 mass% or more.
CaO: 5 to 60%, Al , which is an oxide-based inclusion, which is calculated as the total of these four oxides is 100% by mass.
₂ O ₃ : 5-60%, SiO ₂ : 10-80%, TiO ₂ : 0.1-40% CaO
-Al ₂ O ₃ -SiO ₂ -TiO ₂ and the circle equivalent diameter of 1 μm or more, but the number of cross sections parallel to the rolling direction in 1 mm ² (pieces / mm ² ). 4. In mass%, C: 0.1-0.6%, Si: 0.01-
2.0%, Mn: 0.2-2.0%, P: 0.1% or less, S: 0.005-0.2
%, Al: 0.009% or less, Ti: 0.001% or more and less than 0.04%,
Ca: 0.0001 to 0.01%, O (oxygen): 0.001 to 0.01%, N:
0.02% or less, containing at least one element selected from each of the following first and second groups, the balance consisting of Fe and impurities, and the inclusions present in the steel satisfy the following expressions: A steel for machine structural use characterized by: 1st group Cr: 0.02-2.5%, V: 0.05-0.5%, Mo: 0.05-1.0%,
Nb: 0.005-0.1%, Cu: 0.02-1.0% and Ni: 0.05-2.0% Second group Se: 0.0005-0.01%, Te: 0.0005-0.01%, Bi: 0.05-
0.3%, Mg: 0.0001 to 0.0020% and rare earth element: 0.000
1 to 0.0020% n ₀ / S (%) ≧ 2500 ··· n ₁ / n ₀ ≦ 0.1 ···· n ₂ ≧ 10 ・ ・ ・ ・ ・ ・ where n ₀ , n ₁ and n ₂ are as follows. n ₀ : the total number of sulfides with a circle equivalent diameter of 1 μm or more in 1 mm ² of the cross section parallel to the rolling direction (pieces / mm ² ), n ₁ : the circle equivalent diameter of 1 μm in 1 mm ² of the cross section parallel to the rolling direction As a result, the number of MnS containing 1.0% or more of Ca (pieces / m
m ² ), n ₂ : Ca included in the oxide-based inclusions
The total of O, Al ₂ O ₃ , SiO ₂ and TiO ₂ accounts for 80 mass% or more.
CaO: 5 to 60%, Al , which is an oxide-based inclusion, which is calculated as the total of these four oxides is 100% by mass.
₂ O ₃ : 5-60%, SiO ₂ : 10-80%, TiO ₂ : 0.1-40% CaO
-Al ₂ O ₃ -SiO ₂ -TiO ₂ and the circle equivalent diameter of 1 μm or more, but the number of cross sections parallel to the rolling direction in 1 mm ² (pieces / mm ² ).