JP3354256B2 - Carbon steel for machine structure with excellent machinability and cold forgeability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to carbon steel for machine structural use used as a material for machine parts such as industrial machines and automobiles, and more particularly, to improving its machinability and cold forgeability.

[0002]

2. Description of the Related Art Machine parts of industrial machines and automobiles are
Generally, carbon steel or alloy steel for machine structure is used as a material, and after being formed into a predetermined shape through cold forging or cutting process, it is quenched and tempered to secure the strength as a machine part, and the product and Is done. Therefore, this type of steel material is required to have not only machinability but also cold forgeability.

[0003] As a method for improving the machinability of steel, a free-machining element such as Pb, S, Bi, Te and Ca is added to steel.
Methods for forming non-metallic inclusions in steel are known. On the other hand, as a means for improving cold forgeability, particularly deformability during cold forging, reduction of non-metallic inclusions in steel is recommended as opposed to machinability. Therefore, machinability and cold forgeability are properties that are always required for steel materials for machine structures such as industrial machines and automobile parts,
It is extremely difficult to achieve both, and there is a problem that one of the characteristics must be sacrificed.

As a solution to the above problem, Japanese Patent Laid-Open Publication No. 51-576
No. 21 proposes a steel material in which cold forgeability and machinability are simultaneously improved by graphitizing cementite in steel. However, according to the study of the present inventors, various problems described below remain. That is, in the above method, since the Si content is as high as 1.9 to 3.0 mass%, cementite in the steel is destabilized and graphitization is completed relatively quickly, but Si itself forms a solid solution in the ferrite phase. In order to reduce the deformability of ferrite, the deformability during cold forging decreases, and the deformation resistance during cold forging increases due to the solid solution strengthening action of Si. Further, in this method, since the graphite particle size after graphitization is large, the improvement in deformability and machinability in cold forging is relatively low. Furthermore, the graphitization rate is not sufficient, and a long-time annealing treatment is required for graphitization, so that the heat treatment cost is high.

[0005]

SUMMARY OF THE INVENTION The present invention advantageously overcomes the problems of the prior art as described above.
Even if the content of is reduced, not only can the graphitization time be shortened,
It is an object of the present invention to propose a carbon steel for machine structural use which enables finer graphite grains after graphitization and has both excellent machinability and cold forgeability.

[0006]

Means for Solving the Problems The inventors of the present invention have studied the graphitization behavior of cementite in steel in order to solve the above problems, and have obtained the following knowledge. (1) Graphitization of cementite proceeds in the process of decomposition of cementite → diffusion of C in ferrite → crystallization of graphite. (2) For decomposition of cementite, Si, Ni, Cu and
It is effective to add an element such as Co which dissolves in ferrite rather than cementite. (3) For the crystallization of graphite, the presence of precipitates in steel such as various nitrides and sulfides is effective, and graphite is nucleated using these as nuclei. (4) If a large number of such precipitates as nuclei for crystallization of graphite are formed, the decomposition of cementite is promoted.
Even if alloying elements such as Si are reduced, graphitization is remarkably promoted. Here, although the reason why these precipitates act as nuclei for crystallization of graphite has not been elucidated yet,
It is presumed that the crystal structure is similar to graphite.

Further, it has been found that by forming such precipitates in advance, not only the graphitization is promoted but also the graphite particle size after the graphitization is remarkably reduced. Was. Furthermore, when the relationship between the particle size of graphite and cold forgeability and machinability was examined, it was found that the finer the graphite particle size, the better both cold forgeability and machinability. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: 0.1 to 1.5 mass%, Si: less than 0.5 mass%, M
n: 0.1 to 2.0 mass%, V: 0.05 to 0.5 mass%, N:
0.0015 to 0.0150 mass%, O: 0.0030 mass% or less,
And Ni: 0.1 to 3.0 mass%, Cu: 0.1 to 3.0 mass%,
Co: contains at least one selected from 0.1 to 3.0 mass%, with the balance being substantially composed of Fe and having a metal structure mainly composed of ferrite and graphite. Carbon steel for machine structure with excellent hot forgeability (first invention).

[0009] 2. In the first invention steel, Cr:
Carbon steel for machine structural use comprising a composition containing one or two selected from 0.05 to 1.0 mass% and Mo: 0.05 to 0.5 mass% and having excellent machinability and cold forgeability (second invention) ).

[0010] 3. In the first invention steel, Nb:
Carbon steel for machine structural use having excellent machinability and cold forgeability comprising a composition containing one or two selected from 0.005 to 0.05 mass% and Al: 0.01 to 0.5 mass% (third invention) ).

4. In the first invention steel, Cr:
0.05 to 1.0 mass%, Mo: one or two selected from 0.05 to 0.5 mass%, Nb: 0.005 to 0.05 mass%,
Al: Carbon steel for machine structural use having a composition containing one or two selected from 0.01 to 0.5 mass% and having excellent machinability and cold forgeability (a fourth invention).

5. In each of the first, second, third or fourth invention steels, Pb: 0.03 to 0.30 mass%, Te:
0.002-0.50 mass%, P: 0.030-0.15mass%, Ca: 0.
0002 to 0.30 mass%, Bi: 0.01 to 0.30 mass%, Se: 0.00
3 to 0.10 mass%, S: 0.030 to 0.25 mass% A carbon steel for machine structural use (No. 5) which is excellent in machinability and cold forgeability and has a composition containing one or more selected from among 0.030 to 0.25 mass%. invention).

[0013]

The reason why the composition of steel in the present invention is limited to the above range will be described below. C: 0.1 to 1.5 mass% C is not only indispensable for forming a graphite phase, but also an essential component for securing strength as a mechanical part. However, if the content is less than 0.1 mass%, a sufficient graphite phase is not formed to improve machinability, while
If the content exceeds 0.1%, the cold forgeability decreases.
Limited to the range of ~ 1.5 mass%.

Si: less than 0.5 mass% Si is an element that promotes the graphitization of cementite and is also effective as a deoxidizing agent, but on the other hand, it reduces the ductility of the ferrite phase after graphitization, and Since there is a disadvantage of lowering the forgeability, it is not preferable to add an excessively large amount from the viewpoint of improving the cold forgeability. It was to be contained.

Mn: 0.1 to 2.0 mass% Mn is an effective component for securing the strength as a mechanical part. However, if it is less than 0.1 mass%, a satisfactory strength cannot be obtained. %, The deformation resistance after graphitization increases, so the range was limited to 0.1 to 2.0 mass%.

V: 0.05-0.5 mass% V suppresses the growth of γ grains in the heating step of hot rolling to refine the structure after hot rolling, thereby effectively promoting graphitization. . At the same time, quenching properties are improved, and at the same time, V carbonitrides are formed, and the precipitation strengthening contributes to an improvement in strength. However, the content is 0.05ma
If less than ss%, the effect of the addition is poor, while 0.5 mass
%, The effect reaches saturation.
Limited to the range of 05 to 0.5 mass%.

N: 0.0015 to 0.0150 mass% N is not only a nitride or carbonitride forming element serving as a nucleus for crystallization of graphite, but also solute N is an element of machinability due to dynamic strain aging. It also effectively contributes to improvement. However, if the content is less than 0.0015 mass%, the absolute amount of the precipitate which becomes the nucleus of crystallization of graphite is insufficient, while if added in more than 0.0150 mass%, the hot workability is reduced, and the Since cracks and flaws easily occur, the range is limited to the range of 0.0015 to 0.0150 mass%.

O: 0.0030 mass% or less O forms hard nonmetallic inclusions in steel and deteriorates cold forgeability and machinability. Therefore, it is desirable to reduce O as much as possible. Permissible.

Ni: 0.1 to 3.0 mass%, Cu: 0.1 to 3.0 ma
ss%, Co: 0.1 to 3.0 mass% Ni, Cu and Co not only contribute to the promotion of graphitization by promoting the decomposition of cementite, but also contribute to the formation of Si in the ferrite phase. As described above, since it has the advantage that the ductility of the ferrite phase is not hindered and the solid solution strengthening action is weaker than that of Si, it does not impair the cold forgeability. However, if the content is less than 0.1 mass%, the effect of the addition is poor. On the other hand, even if the content exceeds 3.0 mass%, the effect reaches saturation, so that each content is in the range of 0.1 to 3.0 mass%. It was taken.

Although the basic components have been described above, in the present invention, the following elements can be further added for the purpose of further promoting graphitization or further improving the strength. Cr: 0.05-1.0 mass%, Mo: 0.05-0.5 mass%
Cr and Mo are equivalent as hardenability improving elements,
It is useful when securing the strength as a machine part by quenching and tempering. However, these elements also have the effect of stabilizing cementite when it penetrates into cementite and delaying graphitization. Therefore, when adding these elements, it is necessary to add them within a range that does not hinder graphitization, while being effective in improving hardenability. From such a viewpoint, Cr: 0.05 to 1.0 mass% and Mo: 0.05
The range was limited to 0.5 mass%.

Nb: 0.005 to 0.05 mass% Nb refines the structure after hot rolling by suppressing the growth of γ grains in the heating step of hot rolling, thereby contributing to the promotion of graphitization. In addition, Nb carbonitride is formed at the same time as the hardenability is improved, and the precipitation strengthening contributes to the improvement of the strength. Therefore, these elements are used when the strength required as a mechanical component is secured by quenching / tempering or precipitation. However, if the content is less than 0.005 mass%, the effect of the addition is poor. On the other hand, if the content exceeds 0.05 mass%, the effect reaches saturation, so the content should be 0.005 to 0.05 mass%. did.

Al: 0.01 to 0.5 mass% Al combines with N to form AlN, which acts as a nucleus for graphite nucleation, thereby not only effectively contributing to the promotion of graphitization but also deoxidizing. As it is also useful as an agent, it is added as needed, but if its content is less than 0.01 mass%, its effect is small, while if it exceeds 0.5 mass%, the graphitization promoting effect only reaches saturation. In addition, Al significantly contained in the range of 0.01 to 0.5 mass% because hot deformability was significantly reduced.

Further, in the present invention, if the following free-machining elements are added in addition to the above-mentioned components, the machinability is further improved by combining with the effect of improving the machinability due to the graphitization of C in steel. Can be done. S: 0.030 to 0.25mass% S forms MnS, which acts as a chip breaker during cutting to improve machinability, and at the same time, promotes graphitization by becoming the core of graphitization. Also contributes to improving machinability. However, the content is 0.030
If less than mass%, the effect is poor, while 0.25mass%
The effect saturates even if it exceeds 0.030 to 0.25 mass
%.

P: 0.030 to 0.15 mass% P is an element that improves the machinability by hardening the ferrite layer and also inhibits the graphitization. Here, in order to improve machinability, addition of at least 0.030 mass% is necessary. However, if added in excess of 0.15 mass%, graphitization is inhibited, and consequently machinability deteriorates.
It was limited to the range of 0.030 to 0.15 mass%.

Se: 0.003 to 0.10 mass% Se combines with Mn, and the formed MnSe acts as a chip breaker during cutting to improve machinability,
By becoming the core of graphitization, graphitization is promoted, and this also contributes to improvement in machinability. However, if the content is less than 0.003 mass%, the effect is poor, while on the other hand, the content is less than 0.
Since the effect reaches saturation even if it is added in excess of 10 mass%, it is limited to the range of 0.003 to 0.10 mass%.

Ca: 0.0002 to 0.30 mass% Ca forms a Ca-based oxide, which acts as a nucleus for graphitization to promote graphitization. Further, it has an effect of improving machinability by binding to MnS and making the form of MnS into a spindle shape. However, the content is 0.00
If it is less than 02 mass%, its effect is poor, while 0.30%
If added in excess of mass%, oxide-based nonmetallic inclusions increase and the fatigue strength of mechanical parts decreases, so 0.00
Limited to the range of 02 to 0.30 mass%.

Te: 0.002 to 0.50 mass% Te combines with Mn, and the formed MnTe acts as a chip breaker during cutting to improve machinability, but is also an element that inhibits graphitization. Therefore, if added in an excessively large amount, on the contrary, the machinability is reduced. Therefore, Te is contained in the range of 0.002 to 0.50 mass%, which contributes to the improvement of machinability and does not noticeably inhibit graphitization.

Pb: 0.03 to 0.30 mass% Since Pb has a low melting point, it is melted by the heat generated by the steel material during cutting, and its machinability is improved by its liquid lubrication effect. However, on the other hand, it has the effect of inhibiting graphitization and conversely reducing machinability.
The content was set in the range of 0.03 to 0.30 mass%.

Bi: 0.01 to 0.30 mass% Since Bi has a low melting point like Pb, it is melted by the heat generated by the steel material during cutting, and its liquid lubrication effect improves machinability while inhibiting graphitization. On the contrary, it is also an element that reduces machinability. Therefore, considering both characteristics, 0.
The content was set in the range of 01 to 0.30 mass%.

Further, in the present invention, Sn can be contained for further improving the machinability. However, since Sn is also an extremely powerful graphitization inhibitor,
If added, it must be limited to less than 0.5 mass%.

In the present invention, not only the component composition but also the metal structure is important, and it is necessary to mainly make the structure of ferrite and graphite. However, up to about 50% of the added C amount exists as cementite. Is also good.

Next, regarding the hot rolling conditions of the steel sheet, when a free-cutting element is contained, the hot workability deteriorates, so that the heating temperature is 1000 ° C. or more, and the hot rolling finish temperature is 850.
The temperature is preferably set to not less than ° C. In such hot rolling step, since carbon nitride as a nucleus of crystallization of the graphite is finely dispersed in a temperature range of Ac ₁ or less as graphitized 5-30
Holding for about an hour is sufficient. In addition, when a graphitization inhibiting element such as Te, P, Bi and Pb alone is contained as a free-cutting element, it is preferable to set a longer treatment time within the above graphitization conditions.

[0033]

EXAMPLE A steel material having the composition shown in Tables 1 and 2 was used.
The converter was melted and made into a bloom by continuous casting, and then was rolled into a steel bar having a diameter of 35 mm. Then, 700 ℃, 19h
Was graphitized. The hardness, graphitization rate and graphite particle size of the steel material thus obtained were measured. Here, the graphitization rate was represented by the ratio of the measured amount of graphite to the amount of graphite when all the added C was graphitized. Further, a machinability test and a cold forging test were performed. Here, the machinability test was performed using high-speed tool steel SKH4 under the conditions of outer periphery turning. The cutting depth and feed amount during cutting are respectively
2.0 mm, 0.25 mm / rev., And the time until cutting became impossible was defined as tool life. In addition, cold forging test is 15mm
A compression test was performed using a cylindrical test piece of φ22.5 mml. Deformation resistance was calculated from the deformation load at the time of compression, and cracks generated on the side surface of the test piece after the test were visually confirmed. Table 3 shows the obtained test results.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

In the table, Nos. 1 to 23 are steels of the present invention. Also,
Nos. 24 to 30 are steels in which the Si of Nos. 1 to 7 are increased outside the scope of the present invention. Further, Nos. 31 to 35 are steels in which components other than Si deviated from the proper range of the present invention. No.36 is JI
S Free-cutting steel with Pb added to S53C, No. 37 is SAE standard 12L
14 equivalent steel. As is clear from Table 3, the machinability of each of the steels of the present invention Nos. 1 to 23 is superior to the conventional Pb free-cutting steel. Also, among the inventive steels, the tool life of Nos. 16 to 23 to which the free-cutting element was added was further extended as compared with Nos. 1 to 15. Nos. 24 to 30 also have better machinability than conventional Pb free-cutting steel, but compared to Nos. 1 to 7, the deformation resistance of Nos. In comparison, it is inferior in cold forgeability. In addition, No. 31 to No. 35 have a low graphitization rate, so that not only the deformation resistance is significantly higher than that of the inventive steel, but also the machinability is significantly reduced.

[0038]

As described above, according to the present invention,
A steel material having low deformation resistance during cold forging and thus excellent cold forgeability and excellent machinability can be easily obtained, and is extremely useful as a material for manufacturing machine parts.

Continuing on the front page (72) Inventor Kenichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (72) Inventor Nobuyuki Kondo 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel (56) References JP-A-7-188849 (JP, A) JP-A-5-255803 (JP, A) JP-A-53-56121 (JP, A) JP-A-2-107742 ( JP, A) JP-A-3-146618 (JP, A) JP-A-53-18420 (JP, A) JP-A-63-103049 (JP, A) JP-A-49-67817 (JP, A) Hei 2-111842 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (5)

(57) [Claims] 1. C: 0.1 to 1.5 mass%, Si: 0.5 mass%
Less than, Mn: 0.1-2.0 mass%, V: 0.05-0.5 mass%, N: 0.0015-0.0150 mass%, O: 0.0030 mass% or less, Ni: 0.1-3.0 mass%, Cu: 0.1-3.0 mass
%, Co: at least one selected from 0.1 to 3.0 mass%, with the balance substantially consisting of Fe and a metal structure mainly composed of ferrite and graphite. Carbon steel for machine structural use with excellent cold forgeability. 2. C: 0.1 to 1.5 mass%, Si: 0.5 mass%
Less than, Mn: 0.1-2.0 mass%, V: 0.05-0.5 mass%, N: 0.0015-0.0150 mass%, O: 0.0030 mass% or less, Ni: 0.1-3.0 mass%, Cu: 0.1-3.0 mass
%, Co: at least one selected from 0.1 to 3.0 mass%, Cr: 0.05 to 1.0 mass%, Mo: 0.05 to
Contains one or two selected from 0.5 mass%,
The balance is substantially composed of Fe, and the metal structure is mainly composed of ferrite and graphite. The carbon steel for machine structural use excellent in machinability and cold forgeability. 3. C: 0.1 to 1.5 mass%, Si: 0.5 mass%
Less than, Mn: 0.1-2.0 mass%, V: 0.05-0.5 mass%, N: 0.0015-0.0150 mass%, O: 0.0030 mass% or less, Ni: 0.1-3.0 mass%, Cu: 0.1-3.0 mass
%, Co: at least one selected from 0.1 to 3.0 mass%, Nb: 0.005 to 0.05 mass%, Al: 0.01
Machinability and cold working, characterized in that one or two selected from 0.5% by mass or more are contained, and the balance is substantially composed of Fe, and the metal structure is mainly composed of ferrite and graphite. Carbon steel for machine structure with excellent forgeability. 4. C: 0.1 to 1.5 mass%, Si: 0.5 mass%
Less than, Mn: 0.1-2.0 mass%, V: 0.05-0.5 mass%, N: 0.0015-0.0150 mass%, O: 0.0030 mass% or less, Ni: 0.1-3.0 mass%, Cu: 0.1-3.0 mass
%, Co: at least one selected from 0.1 to 3.0 mass%, Cr: 0.05 to 1.0 mass%, Mo: 0.05 to
One or two selected from 0.5 mass%, one or two selected from Nb: 0.005 to 0.05 mass%, and Al: 0.01 to 0.5 mass%, and the balance is substantially A carbon steel for machine structural use having excellent machinability and cold forgeability, characterized by being composed of Fe and having a metal structure mainly composed of ferrite and graphite. 5. The steel according to claim 1, further comprising: Pb: 0.03 to 0.30 mass%, Te: 0.002 to 0.005%.
0.50 mass%, P: 0.030 to 0.15 mass%, Ca: 0.0002 to 0.30 mass%, Bi: 0.01 to 0.30 mass%, Se: 0.003 to 0.10 mass%, S: 0.030 to 0.25 mass% Or 2
A carbon steel for machine structural use having excellent machinability and cold forgeability comprising a composition containing more than one kind.