JP3249646B2 - Machine structural steel 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 a carbon steel for machine structural use used as a material for machine parts such as industrial machines and automobiles, and more particularly, to improve 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 of 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 performed, contrary to the case of 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, the following problems remain. That is, in the above method, since the Si content is as high as 1.9 to 3.0 mass%, the cementite in the steel can be graphitized relatively quickly, but the Si forms a solid solution in the ferrite phase to reduce the deformability of the ferrite. Because of this, the deformability during cold forging decreases, and the deformation resistance during cold forging is also high due to the solid solution strengthening effect of Si. Also,
In this method, since the graphite particle size after graphitization is large, the improvement in deformability and machinability in cold forging remains relatively low. Further, considering production on an industrial scale, a long-time annealing treatment is required for graphitization, and the heat treatment cost is high.

[0005]

SUMMARY OF THE INVENTION The present invention advantageously overcomes the above-mentioned problems of the conventional method. Even if the Si content is reduced, not only the graphitization time can be shortened, but also the graphitization can be shortened. An object of the present invention is to propose a carbon steel for machine structural use which enables finer graphite particles to be refined later, and which 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. That is, the graphitization of cementite proceeds in the process of decomposition of cementite → diffusion of C in ferrite → crystallization of graphite. For the decomposition of cementite, Si,
It is effective to add elements such as Ni, Cu, and Co that dissolve in ferrite rather than cementite. For crystallization of graphite, nitrides such as ALN and BN are effective, and using these as nuclei promotes graphitization. If a large number of nitrides serving as nuclei for crystallization of such graphite are formed, graphitization is remarkably promoted even if alloy elements such as Si that promote the decomposition of cementite are reduced. The reason that these nitrides act as nuclei for crystallization of graphite is that
Although it has not been clarified yet, it is presumed that the crystal structure is similar to graphite.

Further, it has been found that, by forming such a nitride in advance, not only the graphitization is promoted, but also the graphite particle size after the graphitization is remarkably reduced. Was. Moreover, when the relationship between the particle size of graphite and the cold forgeability and machinability was examined, it was found that the finer the graphite particle size, the better both the 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%, Mn: 0.1 to 2.0 mass%, Al: 0.01 to 0.5 mass%, B: 0.0003 to 0.0150 mass%, N: 0.0015 to 0.0150 mass%, O : 0.0030 mass% or less, and one or more selected from Ni: 0.1 to 3.0 mass%, Cu: 0.1 to 3.0 mass%, Co: 0.1 to 3.0 mass%, with the balance being substantially to become a composition of Fe, moreover metallographic gaff ferrite and machinability, characterized in that consists of graphite and cold forging excellent in mechanical structural steel (first invention).

2) In the above-mentioned first invention, Cr:
A steel for machine structural use having excellent machinability and cold forgeability containing one or two selected from 0.05 to 1.0 mass% and Mo: 0.05 to 0.5 mass% (second invention).

3) In the above first invention, V:
0.05-0.5 mass%, Nb: 0.005-0.05mass% Ti: 0.
Steel for machine structural use containing at least one selected from 005 to 0.05 mass% and having excellent machinability and cold forgeability (third invention).

4) In the above-mentioned first invention, Cr:
0.05 to 1.0 mass%, Mo: one or two selected from 0.05 to 0.5 mass%, V: 0.05 to 0.5 mass%,
Nb: 0.005 to 0.05 mass% Ti: at least one selected from 0.005 to 0.05 mass%, which is excellent in machinability and cold forgeability (fourth invention).

5) The steel according to any one of the first to fourth inventions is further provided with S: 0.005 to 0.25 mass%,
P: 0.005 to 0.15 mass%, Se: 0.003 to 0.10 mass%, C
a: 0.0002 to 0.30 mass%, Te: 0.002 to 0.5 mass%, P
b: A machine excellent in machinability and cold forgeability containing one or more machinability improving elements selected from 0.03 to 0.30 mass% and Bi: 0.01 to 0.3 mass%. Structural steel (fifth invention).

[0013]

[Table 1]

Si: less than 0.5 mass% Si is an element that promotes the graphitization of cementite and is also effective as a deoxidizing material, 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 less than 0.1 mass% is added, a satisfactory strength cannot be obtained. %, The deformation resistance after graphitization increases, so the range was limited to 0.1 to 2.0 mass%.

Al: 0.01 to 0.5 mass% Al is a strong deoxidizing material and, at the same time, combines with N to form AlN.
Is formed, which is used effectively as a nucleus for crystallization of graphite.However, if the content is less than 0.01 mass%, its effect is poor. On the other hand, if it exceeds 0.5 mass%, its effect reaches saturation. , 0.01 to 0.5 mass%.

B: 0.0003 to 0.0150 mass% B is added positively because it combines with N to form BN, which becomes a nucleus for graphite crystallization and promotes graphitization. B is an element that also contributes to the improvement of hardenability, and is also useful for securing the strength as a mechanical part by quenching and tempering. However, if the content is less than 0.0003 mass%, the effect of the addition is poor.
Addition exceeding 0.0150 mass% promotes cracking of the slab during continuous forging, so the content was limited to the range of 0.0003 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, the following elements can be further added in the present invention. Cr: 0.05-1.0 mass%, Mo: 0.05-0.5 mass% Cr and Mo are uniform as hardenability improving elements,
This is useful when securing the strength as a mechanical part by quenching and tempering. However, these elements also function to stabilize cementite and delay graphitization. Therefore, when these elements are added, they are effective in improving the hardenability, but on the other hand, they need to be added in a range that does not inhibit graphitization.
It was limited to the range of 05 to 1.0 mass% and Mo: 0.05 to 0.5 mass%.

V: 0.05-0.5 mass%, Nb: 0.005-0.05
mass%, Ti: 0.005 to 0.05mass% V, Nb, and Ti all combine with N to form nitrides, which act as nuclei for graphite crystallization during graphitization to promote graphitization. At the same time, it has the effect of promoting the miniaturization of graphite. In addition, these elements form fine carbides, and contribute to improvement in strength by precipitation strengthening. Therefore, these elements, when it is necessary to increase the graphitization rate, when it is necessary to make the graphite grains finer, or when securing the strength as a mechanical part without quenching and tempering, Alternatively, when securing the strength required for large mechanical parts by QT, it is used when it is necessary to further increase the strength of the central part of the member that is hard to cure. The contents are respectively V: 0.05 to 0.5 mass%, N
b: 0.005 to 0.05 mass%, Ti: 0.005 to 0.05 mass%, the lower limit of each element is defined by the minimum amount necessary to obtain the above-mentioned effects, while the upper limit is determined by adding As a result, the amount of graphite formed in the steel decreases, and the amount is defined by an allowable upper limit that does not cause a decrease in machinability.

In the present invention, S, P, Se, Ca, Te, Pb, etc., which are generally known as machinability improving elements, can be further added. P: 0.005 to 0.15 mass% P is a useful element that improves machinability by hardening the ferrite layer, but is also an element that inhibits graphitization. To improve machinability, at least 0.00
Addition of 5 mass% is required, while addition of more than 0.15 mass% inhibits graphitization and consequently lowers machinability. Therefore, the content is limited to less than 0.15 mass%.

S: 0.005 to 0.25 mass% S forms MnS, which acts as a chip breaker at the time of cutting to improve machinability and promotes graphitization by becoming a core of graphitization. As a result, it is added positively to further improve machinability, but 0.005 mass
%, The effect is poor. On the other hand, if the content exceeds 0.25 mass%, the effect is saturated.
It was limited to the range of mass%.

Se: 0.003 to 0.10 mass% Se combines with Mn to form MnSe, which acts as a chip breaker to improve machinability, and at the same time, MnSe becomes a nucleus for graphitization and becomes graphitized. Since the machinability is further improved by accelerating, it is added positively. However, if the addition amount is less than 0.003 mass%, the above effect is small, while if it exceeds 0.10 mass%, the effect is saturated. Therefore, the content is limited to the range of 0.003 to 0.10 mass%.

Ca: 0.0002-0.30 mass% Ca forms Ca-based oxides, which act as nuclei for graphitization and promote graphitization. In addition, since MnS is combined with MnS and the form of precipitation of MnS is made into a spindle shape, which contributes to improvement of machinability, it is positively added. However, its addition amount is 0.
If it is less than 0002 mass%, the effect of the addition is poor.
When added in excess of mass%, a large amount of oxide-based nonmetallic material is formed, which lowers the fatigue strength as a mechanical part. Therefore, the content was limited to the range of 0.0003 to 0.30 mass%.

Te: 0.002 to 0.5 mass% Te forms MnTe, which acts as a chip breaker and improves the machinability. Therefore, Te is actively used. However, since Te is an element that inhibits graphitization, When added in a large amount, machinability is adversely deteriorated. Therefore, the content is set in the range of 0.002 to 0.5 mass%, which contributes to the improvement of machinability and at the same time, does not significantly inhibit the graphitization.

Pb: 0.03 to 0.3 mass% Pb is an element that has a low melting point and is melted by the heat generated by the steel material during cutting to improve machinability by a liquid lubrication effect, but impairs graphitization. Conversely, since it also has the effect of reducing machinability, the range is limited to 0.03 to 0.30 mass% in consideration of both characteristics.

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 has the effect of improving the machinability by the liquid lubrication effect. In view of both properties, the range is limited to 0.01 to 0.30 mass%, since the formation of the material is hindered and the machinability is reduced.

Further, 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. This is because it is an essential condition of the present invention to suppress the temperature rise of the cutting tool during cutting by the lubricating action of graphite and thereby to improve the life of the cutting tool. Here, the preferred content of graphite in the steel is 0.1 to 1.2%. For this purpose, it is necessary to carry out a graphitization treatment in a temperature range of 600 to 750 ° C for 5 to 20 hours. In the graphitizing treatment, quenching as a pretreatment is not necessary.

When the machinability improving element is added, the hot workability deteriorates. Therefore, it is preferable to perform heating at about 1000 ° C. or more and hot rolling at 850 ° C. or more. As a heat treatment for graphitization, it is possible to sufficiently graphitize only by holding in a temperature range of less than Ac ₁ point for about 5 to 30 hours. As such machinability improving elements, particularly, Te, P When elements which inhibit graphitization, such as Bi, Pb, and Pb, are added alone, it is preferable to extend the treatment time within the above range.

[0031]

【Example】

Example 1 A steel material having the composition shown in Tables 1 and 2 was melted in a converter,
After blooming by continuous casting, 35 mm
mmφ steel bars. Then, it was graphitized at 700 ° C. for 19 hours. The hardness, graphite content and graphite particle size of the steel material thus obtained were measured by an image analyzer. The graphitization ratio was defined as the ratio between the measured amount of graphite and 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 is
Using high-speed tool steel SKH4, it was carried out under the conditions of outer periphery turning. The depth of cut and feed rate during cutting are 2.0 respectively
mm, 0.25 mm / rev., and the time until cutting became impossible was defined as tool life. The cold forging test is 15mmφ × 2
The test was performed by a compression test using a 2.5 mm 1 cylindrical test piece. While calculating the deformation resistance from the deformation load during compression,
Cracks generated on the side surfaces of the test pieces after the test were visually confirmed, and the compression ratio at which cracks occurred in half of the test pieces was defined as the critical compression ratio. Table 3 shows the obtained test results.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

In the table, Nos. 1 to 12 are the first to fourth invention steels. Nos. 14 to 26 are steels in which Si is increased from Nos. 1 to 12 outside the scope of the present invention. Further, Nos. 26 to 31 are steels in which components other than Si deviated from the proper range of the present invention. Nos. 32 and 33 are free cutting steels with Pb added to JIS S25C and JIS S45C. As is clear from Table 3, the machinability of each of Nos. 1 to 26 is superior to that of Pb free-cutting steel, but the deformation resistance is higher when comparing No. 1 to 12 and No. 14 to 26. No.
14 to 26 is higher, and is inferior in cold forgeability to No. 1 to 12 which are inventive steels. Nos. 27 to 31 have a low graphitization rate, and therefore have extremely high deformation resistance. Machinability is superior to Pb free-cutting steel, but not as good as the steel of the present invention.

Example 2 In the same manner as in Example 1, a steel bar having a chemical composition shown in Tables 4 and 5 was used as a 52 mmφ steel bar. Table 6 summarizes the results of a study on the graphitization rate, graphite particle size, hardness, and tool life of the obtained steel material. Test steel No.1 ~
12 (No. 4 missing number) is an example of the fifth invention steel. No.
14 to 26 are Pb, Bi, Te and
It excludes machinability improving elements such as Ca. No. 27-28
In the present invention, the element required for graphitization is out of the range. In addition, Nos. 29 and 30 are
Sb is a Pb free-cutting steel with Pb, Bi, and Te added to carbon steel for machine structural use. Comparing the steel of the present invention with comparative steels Nos. 14 to 26, the steel of the present invention not only has a longer tool life, but also has a much longer tool life than the conventional steel. Furthermore, the graphitization rate of the steel of the present invention is lower than that of the comparative steels Nos. 14 to 26 as a whole, because a relatively short time treatment such as 700 ° C. × 10 h was performed. As described above, if the processing time is extended to about 30 hours, the service life can be further improved. Comparative steel Nos. 27 and 28 have a very short tool life and poor machinability due to a low graphitization rate.

[0037]

[Table 4]

[0038]

[Table 5]

[0039]

[Table 6]

[0040]

As described above, according to the present invention, it is possible to easily obtain a steel material having low deformation resistance at the time of cold forging and excellent machinability, which greatly contributes to the manufacture of machine parts.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (56) References JP-A-2-107742 (JP, A) JP-A-64-25946 (JP, A) JP-A-1-132739 (JP, A) JP-A-2-111842 (JP, A) JP-A-61-261464 (JP, A) JP-A-7-3390 (JP, A) JP, A) JP-A-6-212351 (JP, A) JP-A-6-248387 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (5)

(57) [Claims] 1. C: 0.1 to 1.5 mass%, Si: less than 0.5 mass%, Mn: 0.1 to 2.0 mass%, Al: more than 0.01 to 0.5 mass%, B: 0.0003 to 0.0150 mass%, N: 0.0015 to 0.0150 mass%, O: 0.0030 mass% or less, Ni: 0.1 to 3.0 mass%, Cu: 0.1 to 3.0 mass%, Co: 0.1 to 3.0 mass% And the rest
Fe and becomes compositions consisting of unavoidable impurities, moreover metallographic gaff ferrite and graphite or ferrite, Se
A steel for machine structural use that is excellent in machinability and cold forgeability , characterized by being made of mentite and graphite . 2. C: 0.1 to 1.5 mass%, Si: less than 0.5 mass%, Mn: 0.1 to 2.0 mass%, Al: more than 0.01 to 0.5 mass%, B: 0.0003 to 0.0150 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: 0.1 to 3.0 mass% Further, one or two selected from Cr: 0.05 to 1.0 mass% and Mo: 0.05 to 0.5 mass% are contained, and the balance is Fe and
And becomes a composition consisting of incidental impurities, moreover metallographic gaff ferrite and graphite or ferrite, cement
A machine structural steel with excellent machinability and cold forgeability characterized by being made of tight and graphite . 3. C: 0.1 to 1.5 mass%, Si: less than 0.5 mass%, Mn: 0.1 to 2.0 mass%, Al: more than 0.01 to 0.5 mass%, B: 0.0003 to 0.0150 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: 0.1 to 3.0 mass% V: 0.05 to 0.5 mass%, Nb: 0.005 to 0.05 mass%, Ti: 0.005 to 0.05 mass%, and the balance is composed of Fe and unavoidable impurities. organization Gaff ferrite and graphite or ferrite, cement
A machine structural steel with excellent machinability and cold forgeability characterized by being made of tight and graphite . 4. C: 0.1 to 1.5 mass%, Si: less than 0.5 mass%, Mn: 0.1 to 2.0 mass%, Al: more than 0.01 to 0.5 mass%, B: 0.0003 to 0.0150 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: 0.1 to 3.0 mass% Further, one or two selected from Cr: 0.05 to 1.0 mass% and Mo: 0.05 to 0.5 mass%, V: 0.05 to 0.5 mass%, Nb: 0.005 to 0.05 mass% Ti: 0.005 to 0.05 mass% At least one selected from the group consisting of
And becomes the composition consisting of incidental impurities, moreover metallographic gaff ferrite and graphite or ferrite, cementite
A steel for machine structural use that is excellent in machinability and cold forgeability , characterized by being made of graphite and graphite . 5. The steel according to claim 1, further comprising: P: 0.005 to 0.15 mass%, S: 0.005 to 0.25 mass%, Se: 0.003 to 0.10 mass%, Ca : One or two or more machinability improving elements selected from 0.0002 to 0.30 mass%, Te: 0.002 to 0.5 mass%, Pb: 0.03 to 0.30 mass%, and Bi: 0.01 to 0.3 mass% Machine structural steel with excellent machinability and cold forgeability.