JP2000034538A - Steel for machine structure excellent in machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a sulfide protective film on the surface and to improve the service life of a tool without deteriorating the quality of the product by specifying the compsn. in steel and controlling the form of Ca in sulfide inclusions. SOLUTION: A steel compsn. is composed of 0.05 to 0.8% C, 0.01 to 2.5% Si, 0.1 to 3.5% Mn, 0.001 to 0.2% P, 0.005 to 0.4% S, 0.001 to 0.1% Al, 0.0005 to 0.12% Ca, 0.005 to 0.01% O, 0.001 to 0.04% N, and the balance Fe. Moreover, in the case the area ratio of the sulfides in which the Ca content exceeds 40% to the area of the whole of the examined and observed visual field is defined as A, the area ratio of the sulfides in which the Ca content is 0.3 to 40% to the area of the whole of the examined and observed visual field as B, and the area ratio of the sulfides in which the Ca content is less than 0.3% to the area of the whole of the examined and observed visual field as C, A/(A+B+C)<=0.3 and B/(A+B+C)>=0.1 are satisfied.

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 excellent in turning workability.

[0002]

2. Description of the Related Art Generally, steel mechanical structural parts used in the automobile industry and the like are roughly processed by plastic working such as forging and then finished to a desired final shape by cutting. For the purpose of reducing the cost of cutting, there is always a great demand for free-cutting steel having excellent machinability.

[0003] Among them, the turning process is a process applied to most parts, and a calcium free-cutting steel using Ca-based oxide inclusions has been developed (for example, Japanese Patent Application Laid-Open No. 5815), which is practically used.
The present inventor also relates to Ca free-cutting steel, as disclosed in Japanese Patent Application No. 9-163.
180 proposed. This aims at improving the machinability.

[0004]

It has been found that there is a large variation in the machinability even with the above two prior arts. In addition, it has not been able to respond to the demand for turning workability that has been increasing more and more in recent years, and there has been a demand for a steel for machine structural use which is more excellent in turning workability and has less variation than conventional materials.

[0005]

As a result of various studies, the present inventor has developed a free-cutting steel having excellent turning workability and little variation by controlling the form of Ca in a sulfide. .

That is, in a steel for machine structural use, a sulfide-based tool protective film is formed on the surface of a turning tool by adjusting the amount of Ca in a sulfide-based inclusion in the steel, and the tool life is greatly improved. The summary is as follows (1) to (3).

(1) C: 0.05-0.8%, Si:
0.01 to 2.5%, Mn: 0.1 to 3.5%, P:
0.001-0.2%, S: 0.005-0.4%, A
l: 0.001 to 0.1%, Ca: 0.0005 to 0.5%
02%, O: 0.0005 to 0.01%, N: 0.00
The content of the sulfide containing 1 to 0.04%, the balance being Fe and unavoidable impurities, and the Ca content exceeding 40% is A, the Ca content is 0.3, When the area ratio of the sulfide of 観 察 40% to the area of the entire observation observation field is B, and the area ratio of the sulfide having a Ca content of less than 0.3% to the entire area of the observation observation field is C, A / ( A + B + C) ≦ 0.3 and B / (A + B)
+ C) ≧ 10.1, a machine structural steel excellent in turning workability.

(2) In addition to the alloy components described in the above (1), Cr: ≦ 3.5%, Mo: ≦ 2.0%, Cu:
≦ 2.0%, Ni: ≦ 4.0%, B: 0.0003-
0.01% or more, and if necessary, Nb: ≦ 0.2%, Ti: ≦ 0.2%,
V: ≦ 0.5%, Ta: ≦ 0.5%, Zr: ≦ 0.5%
Of the sulfides containing one or more of the following, the balance being Fe and unavoidable impurities, and having a Ca content exceeding 40, the area ratio of the sulfides to the total area of the observation field of view being A, C
a represents the area ratio of the sulfide having a content of 0.3 to 40% with respect to the entire area of the survey observation field of view of B, and the area ratio C of the sulfide having a Ca content of less than 0.3% to the area of the entire field of study observation and C, A / (A + B + C) ≦ 0.3 and B /
(A + B + C) ≧ 0.1, a machine structural steel excellent in turning workability.

(3) In addition to the alloy components described in (1) above, Pb: ≦ 0.4%, Bi: ≦ 0.4%, Se:
≦ 0.5%, Te: contains one or more of 0.1%, the balance being Fe and unavoidable impurities,
And the area ratio of the sulfide with the Ca content of more than 40% to the entire area of the observation observation field is A, and the Ca content is 0.3 to
When the area ratio of the sulfide of 40% to the area of the entire observation observation field is B, and the area ratio of the sulfide having a Ca content of less than 0.3% to the area of the entire observation observation field is C, A /
(A + B + C) ≦ 0.3 and B / (A + B + C) ≧
A steel for machine structural use excellent in turning workability, characterized by being 0.1.

(4) In addition to the alloy components described in (2) above, Pb: ≦ 0.4%, Bi: ≦ 0.4%, Se:
≦ 0.5%, Te: ≦ 0.1%, one or more of which contain Ca, and the Ca content exceeds 40%. The area ratio of the sulfide having a Ca content of 0.3 to 40% to the entire area of the observation observation field of view is B, and the area ratio of the sulfide having a Ca content of less than 0.3% to the entire area of the inspection observation field is C.
Where A / (A + B + C) ≦ 0.3 and B / (A
+ B + C) ≧ 0.1, a steel for machine structural use excellent in turning workability.

The steel of the present invention is characterized by the above composition and inclusion form, and is effective for both ingot ingot ingot ingot casting and continuous casting, regardless of the refining process and the method of adding Ca.

[0012]

The reasons for limiting the chemical composition of steel and the form of inclusions in the present invention will be described below.

C: 0.05-0.8%. C is an element necessary for securing strength, and is 0.05
%, The toughness and machinability are degraded.

Si: 0.01 to 2.5%. Si is an element that is contained as a deoxidizing agent at the time of smelting and improves the hardenability. If it is less than 0.01%, the desired effect cannot be obtained, and if it is added in a large amount exceeding 2.5%, the ductility is reduced, and cracks tend to occur during plastic working.

Mn: 0.1 to 3.5%. Mn is a sulfide-forming element. If it is less than 0.1%, a desired effect cannot be obtained. If it exceeds 3.5%, the hardness of steel is increased and the machinability is reduced.

P: 0.001 to 0.2%. P is added for improving the machinability, particularly the surface properties.
If it is less than 0.001%, the effect cannot be obtained, and if it exceeds 0.2%, the toughness is significantly deteriorated.

S: 0.005 to 0.4%. S is an element effective for improving machinability. If it is less than 0.005%, the desired effect cannot be obtained. If it exceeds 0.4%, not only the toughness and ductility are deteriorated, but also Ca and C having a high melting point.
The formation of aS causes a great obstacle to the casting process.

Al: 0.001 to 0.1%. Al is an element necessary for deoxidation, and 0.001% or more is required to obtain the effect. On the other hand, if it exceeds 0.1%, hard alumina clusters are formed, and the machinability of steel is deteriorated.

Ca: 0.0005-0.02%. Ca is an element having a very important meaning in the present invention. In order to contain Ca in the sulfide, Ca is added in an amount of 0.
It is necessary to contain 0005% or more. On the other hand, 0.0
If it exceeds 2%, excess Ca forms CaS having a high melting point, causing a great obstacle to the casting process.

O: 0.0005 to 0.01%. O is an element necessary for forming an oxide. An excessively small amount of O generates a large amount of high-melting-point Ca sulfide and deteriorates castability. Therefore, 0.0005% or more of O is required, and desirably, 0.0015% or more of O is required. On the other hand, if the content exceeds 0.01%, the machinability is deteriorated by a large amount of hard oxide, and the generation of Ca sulfide becomes difficult.

N: 0.001 to 0.04%. N is an element effective for preventing the crystal grains from becoming coarse, and 0.00
1% or more is required. On the other hand, when the content exceeds 0.04%, it causes a great obstacle to the casting process.

The steel for machine structural use having excellent machinability according to the present invention further comprises Cr, Mo, Cu, Ni,
One or two of B, Nb, Ti, V, Ta, and Zr
It may contain more than one species. Furthermore, in addition to these, P
One, two or more of b, Bi, Se, and Te may be included. The effect of these alloy elements and the reason for limiting the content will be described.

Cr: ≦ 3.5%. Cr is an element effective for improving the hardenability, but if it exceeds 3.5%, it is disadvantageous in terms of cost, and furthermore, the steel frequently cracks during hot working.

Mo: ≦ 2.0%. Mo is an element effective for improving hardenability like Cr, but if it exceeds 2.0%, it is disadvantageous in terms of cost.
Further, the machinability is deteriorated and the steel is frequently cracked during hot working.

Cu: ≤2.0%. Cu densifies the structure and improves the strength. Meanwhile, 2.
If it exceeds 0%, hot workability is deteriorated and machinability is also reduced.

Ni: ≦ 4.0%. Ni is an element effective for improving hardenability like Cr, but if it exceeds 4.0%, it is disadvantageous in terms of cost.
Further, the machinability is reduced.

B: 0.0003-0.01%. B is an element that improves the hardenability by adding a small amount. If less than 0.0003%, the effect cannot be obtained.
If it exceeds 01%, the crystal grains are coarsened and the steel is frequently cracked during hot working.

Nb: ≦ 0.2%. Nb is an effective element for preventing coarsening of crystal grains at a high temperature, but the effect is saturated even if it is contained in excess of 0.2%. good.

Ti: ≦ 0.2% Ti is an element that combines with N to form TiN and exerts the effect of improving the hardenability of B. However, if it exceeds 0.2%, TiN becomes excessive, and the steel frequently cracks during hot working. .

V: ≤0.5%. V combines with C and N to form carbonitrides and has an effect of making crystal grains fine. If the content exceeds 0.5%, the effect is saturated. Therefore, the content may be added up to 0.5% as needed.

Ta: ≦ 0.5%. Ta is an element effective for refining crystal grains and improving toughness. If the content exceeds 0.5%, the effect is saturated. Therefore, the content may be added up to 0.5% as needed.

Zr: ≦ 0.5% Zr has properties similar to Ta, and is an effective element for refining crystal grains and improving toughness. Even if the content exceeds 0.5%, the effect is saturated.
You may add up to 5%.

Pb: ≦ 0.4% Pb is a well-known element that improves machinability. P
b exists alone or in such a form as to adhere to the outer periphery of the sulfide, and has an effect of improving machinability by itself. 0.
In the case of 4% or more, the solubility of Pb in steel is exceeded, and excess Pb alone agglomerates and precipitates due to its large specific gravity to become defects in the steel. Therefore, the upper limit is made 0.4%. .

Bi: ≦ 0.4%. Bi is an element having properties similar to Pb and improving machinability. If the content is 0.4% or more, the solubility of Bi in steel is exceeded, and excess Bi alone agglomerates and precipitates due to its large specific gravity to become defects in the steel.
4%.

Se: ≦ 0.5%. Se is a well-known element that improves machinability.
If it exceeds 0.5%, the hot workability is deteriorated and cracks are easily generated, so the upper limit is made 0.5%.

Te: ≦ 0.1%. Te is a well-known element that improves machinability.
If it exceeds 0.1%, the hot workability is deteriorated and cracks are likely to occur, so the upper limit is made 0.1%.

Form of Inclusions: A field of view of 0.
As a result of analyzing sulfides in an area of not less than 05 square millimeters, the area ratio of the sulfide having a Ca content of more than 40% to the area of the entire observation visual field was A, and the Ca content was 0.3.
When the area ratio of the sulfide of 硫化 40% to the area of the entire observation observation field is B, and the area ratio of the sulfide having a Ca content of less than 0.3% to the entire area of the observation observation field is C, A
/(A+B+C)≦0.3 and B / (A + B + C) ≧
0.1. In steel having the above chemical composition, sulfide is generally composed mainly of MnS, and a part of Mn is substituted by Ca. However, the nature of the sulfide varies depending on the degree of substitution with Ca. When the area ratio A exceeds 30% of the entire sulfide, the sulfide has an excessive amount of CaS having a high melting point, thereby deteriorating the manufacturability, and has little effect of improving the life of the turning tool and has a large variation. On the other hand, the area ratio B is 10% of the entire sulfide.
If it is less than the range, the degree of substitution of Mn for Ca is too small to obtain a desired tool life improving effect.

[0038]

EXAMPLES The features of the steel of the present invention will be described with reference to examples.
Steels having the chemical compositions shown in Tables 1 to 4 were melted in a 5-ton arc furnace or a 150 kg high-frequency vacuum induction furnace. The obtained steel ingot was rolled or forged into a round bar having a diameter of 90 mm for the steel types in Tables 1 and 3, and Tables 2 and 4
The steel type was rolled or forged into a round bar having a diameter of 50 mm, and each was subjected to an investigation after heat treatment. In Table 1, which corresponds to claim 1, S15C, S
45C and S55C, SCr415, SNCM420 and SCM440 in Table 2 corresponding to claim 2, S45C in Table 3 corresponding to claim 3, and S45C.
In Table 4 corresponding to the above, SCM440 was adopted.

In order to evaluate the manufacturability, the case where the casting nozzle was blocked by the precipitation of the high melting point material during casting and the casting could not be continued with 10% or more of the entire casting amount being regarded as poor castability. Further, when the amount of defects due to surface flaws after rolling or forging exceeded 5% of the total amount of rolling or forging, hot workability was regarded as poor.

In order to evaluate the machinability, the steel types shown in Tables 1 to 4 were subjected to the heat treatments shown in Table 5, respectively.
Turning was performed under the conditions shown in Table 1. About the obtained tool life time, comparative steel a1, b1, c1, d1,
Assuming that the tool life time of e1 and f1 is 1, each tool life time ratio was calculated and set as each tool life ratio.

In order to evaluate sulfide, a microscopic sample was prepared for each of the turning test materials, and a field of view of 0.0
Sulfides in areas of 5 square millimeters or more were analyzed.
The area ratio of the sulfide having a Ca content of more than 40% to the area of the entire observation visual field is A, and the Ca content is 0.3 to 40.
%, The area ratio of the sulfide to the entire area of the observation field of view is B, and the area ratio of the sulfide having a Ca content of less than 0.3% to the area of the entire field of observation is C, A, B, C
The area ratio of each sulfide to the whole was determined.

An impact test was performed to evaluate the toughness. A 13 mm square was cut out from some steel types in Table 1 and JIS No. 3 test pieces were prepared after quenching and tempering.
2242.

For the steels in Table 2, the prior austenite grain size after quenching was measured.

For the steels in Tables 3 and 4, a sweat test was carried out with a round bar having a diameter of 50 mm after rolling or forging.
Those with abnormalities were judged to have internal defects.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

[Table 5]

The steels of the present invention shown in Tables 1 to 4 have at least a turning tool life of at least 6 times that of the comparative steel, and have good manufacturability, impact value and internal quality. In contrast, each comparative steel has a short tool life, a problem with manufacturability, low toughness, or a large number of internal defects.

[0051]

As is clear from the above description, the steel of the present invention
The turning tool life can be greatly improved without deteriorating the manufacturability and product quality, and the industrial advantage is extremely large.

Claims (4)

[Claims] 1. C: 0.05-0.8% (hereinafter referred to as% by weight unless otherwise specified), Si: 0.01-2.5%
%, Mn: 0.1 to 3.5%, P: 0.001 to 0.2
%, S: 0.005 to 0.4%, Al: 0.001 to
0.1%, Ca: 0.0005-0.02%, O: 0.
0005-0.01%, N: 0.001-0.04%, the balance consists of Fe and unavoidable impurities, and C
a The content ratio of the sulfide with respect to the entire area of the observation visual field of the sulfide having a content exceeding 40% is A, and the Ca content is 0.3 to 40%.
When the area ratio of the sulfide to the area of the entire observation observation field is B, and the area ratio of the sulfide having a Ca content of less than 0.3% to the area of the entire observation observation field is C, A / (A
+ B + C) ≦ 0.3 and B / (A + B + C) ≧ 0.1
A steel for machine structural use excellent in turning workability, characterized in that: 2. In addition to the alloy components according to claim 1, Cr: ≦ 3.5%, Mo: ≦ 2.0%, Cu: ≦
2.0%, Ni: ≤4.0%, B: 0.0003-0.
01% or more, and if necessary, Nb: ≤ 0.2%, Ti: ≤ 0.2%, V: ≤
0.5%, Ta: ≦ 0.5%, Zr: ≦ 0.5%, contains one or more of them, the balance being Fe and unavoidable impurities, and the Ca content exceeds 40% The area ratio of the sulfide to the area of the entire observation observation field is A, the area ratio of the sulfide having a Ca content of 0.3 to 40% to the entire area of the observation observation field is B, and the Ca content is 0.3% or more. The area ratio of the small sulfide to the total area of the observation field of view is C
Where A / (A + B + C) ≦ 0.3 and B / (A
+ B + C) ≧ 0.1, a steel for machine structural use excellent in turning workability. 3. The alloy according to claim 1, further comprising: Pb: ≦ 0.4%, Bi: ≦ 0.4%, Se: ≦
0.5%, Te: One or more of 0.1%, the balance is composed of Fe and unavoidable impurities, and the Ca content exceeds 40%. The area ratio to the area of A is A, and the Ca content is 0.3 to 4
When the area ratio of the 0% sulfide to the entire area of the observation field of observation is B, and the area ratio of the sulfide having a Ca content of less than 0.3% to the area of the entire observation field of observation is C, A /
(A + B + C) ≦ 0.3 and B / (A + B + C) ≧
A steel for machine structural use excellent in turning workability, characterized by being 0.1. 4. In addition to the alloy components according to claim 2, Pb: ≦ 0.4%, Bi: ≦ 0.4%, Se: ≦
0.5%, Te: One or more of 0.1% or less, and the area ratio of sulfides containing Ca of more than 40% to the area of the entire observation observation field is A, Ca B is the area ratio of the sulfide having a content of 0.3 to 40% to the entire area of the observation observation field of view, and C is the area ratio of the sulfide having a Ca content of less than 0.3% to the entire area of the observation observation field. A / (A + B + C) ≦ 0.3 and B / (A +
(B + C) ≧ 0.1, a machine structural steel excellent in turning workability.