JP2991869B2 - Machine structural steel with excellent 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 structures used as a material for industrial machines and automobile parts, and more particularly to a steel material for machine structures excellent in cold forgeability.

[0002]

2. Description of the Related Art Generally, cold forging is a processing method widely used in the manufacture of various machine parts such as bolts and nuts because of its high production efficiency, high material yield, and excellent finish dimensional accuracy. . Conventionally, as a steel material applied to this processing method, that is, as a steel for cold forging, C is 0.
Usually, JIS S45C of not more than 45 wt% is used, and it is usually subjected to cold forging after softening by spheroidizing annealing.

In recent years, as a steel material for cold forging, C is 0.45 wt.
% Is increasing. This is because the required level of the surface hardness after quenching and tempering has been further increased in order to further function as a mechanical part. However, an increase in C% in steel not only increases the deformation resistance during cold forging and extremely shortens the life of the cold forging die, but also increases the forging load and reduces the performance of the forging machine. Insufficiency has occurred, and a new problem has arisen that processing is difficult with a so-called conventional forging machine.

[0004] In order to solve such a problem, Japanese Patent Application Laid-Open No. 61-113744 discloses that Si, Mn, Cr and / or Si, Mn, Cr are presumed to be subjected to spheroidizing annealing of carbide before cold rolling. By adjusting the chemical composition of P, N and O, a technique for reducing deformation resistance and improving deformability has been proposed. However, according to the method that the present inventors have performed additional tests, even if the chemical composition is adjusted, at least for the spheroidized annealed structure, there is a limit in reducing the deformation resistance, and it is still high. There was a problem.

[0005] The present inventors have previously found that when carbides in steel are graphitized instead of cementite, the deformation resistance and deformability during cold forging are significantly improved even with the same carbon content. (JP-A-3-146618).
Reference). In addition, the concept of graphitizing carbides in steel is described in, for example, JP-A-49-67816, JP-A-49-67817, JP-A-49-103817, and JP-A-49-103817. Although there are disclosures in -191 and the like,
According to the study of the present inventors, the steels proposed by these known techniques have poor cold deformability and require a remarkably long time to be graphitized, and therefore cannot be implemented on an industrial scale. Have difficulty.

[0006]

The object of the present invention is to
Each of the above is to propose a steel for cold forging that does not have the problems that the conventional technologies have in common, so that the so-called deformation resistance during cold forging is small, and a steel material with good deformability is produced on an industrial scale. It is to establish the technology that can be manufactured.

[0007]

As a result of many experiments and studies aimed at realizing the above-mentioned object, the inventors have found that the production of this type of steel is of such an extent that it can be carried out on an industrially provided scale. As a result of studying the composition of components for improving the graphitization rate, the following findings have been obtained.

The graphitization of cementite in steel is carried out through the processes of decomposition of cementite, diffusion of decomposed C, and crystallization of graphite. Therefore, in order to promote the graphitization, it is necessary to select an element which promotes the action of each of these steps. In this regard, the inventors have found that BN and AlN are extremely effective as nuclei for graphitization crystallization. That is, in the steel material formed with BN and AlN, the progress of graphitization is extremely fast, and the cementite in the steel becomes graphite in a short time. Although the reason for this is not necessarily clear, it is thought that the crystal structures of graphite and these precipitates are similar. However,
BN and AlN alone have a limit in promoting graphitization,
It is necessary that BN and AlN coexist. In order to promote the graphitization, it is naturally necessary to select an element that makes cementite unstable and facilitates decomposition.
In particular, it was found that the effect was remarkable when the amount of Si exceeded a certain value.

In addition, by optimizing the composition of the steel material, the graphite particles become fine particles of about 5 to 10 μm, and as a result, the cold forgeability is greatly improved.

The present invention has been made based on the above findings, and the gist thereof is as follows. C: 0.4 to 1.1 wt%, Si: more than 1.5 to 2.0 wt%, Mn: 0.10 to 0.9 wt%, P ≦ 0.030 wt%, S: 0.001 to 0.03 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.01 ~0.5 wt%, N: 0.0015~0.0070wt% , containing O ≦ 0.0030 wt% and Cr ≦ 0.10 wt%, the balance consists of Fe and unavoidable impurities, moreover, the tissue is ferrite and graphite or even
A steel for machine structural use (first invention steel) excellent in cold forgeability , characterized by being made of cementite .

[0011] The steel of the first invention further contains any one or two of Cu: 0.10 to 1.0 wt% and Ni: 0.10 to 2.0 wt%, and the balance consists of Fe and unavoidable impurities. , In addition, the organization ferrite
Steel for machine structural use (second invention steel) having excellent cold forgeability , characterized by being made of graphite and graphite or further cementite .

[0012] The steel of the first invention further contains one or two of Nb: 0.001 to 0.05 wt% and Ti: 0.001 to 0.05 wt%, and the balance consists of Fe and unavoidable impurities. , In addition, the organization ferrite
Steel for machine structural use (third invention steel) excellent in cold forgeability , characterized by being made of graphite and graphite or further cementite .

In the steel of the first invention, one or more of Cu: 0.10 to 1.0 wt% and Ni: 0.10 to 2.0 wt%, Nb: 0.001 to 0.05 wt%, and Ti: 0.001 to 0.05 containing the wt% of any one or two, the balance consists of Fe and unavoidable impurities, moreover, tissue Blow
A steel for machine structural use excellent in cold forgeability, which is made of graphite and graphite or further cementite (fourth invention steel).

[0014]

The reason why the composition of steel in the present invention is limited to the above range will be described below.

C: 0.4-1.1 wt% C is an element indispensable for the formation of a graphite phase, and is also an important element for securing the strength as a mechanical part. If the content is less than 0.4 wt%, the desired effects of the present invention cannot be obtained. On the other hand, if the content exceeds 1.1 wt%, the deformation resistance during hot rolling increases and hot rolling becomes difficult, so the content is limited to the range of 0.4 to 1.1 wt%.

Si: more than 1.5 to 2.0 wt% Si is used positively because it is an element that destabilizes cementite in steel to promote graphitization and is also useful for deoxidation. When this content exceeds 1.5 wt%, A _c1
The transformation point rises, and the graphitization can be completed in a short time during tempering. That is, if the addition is 1.5 wt% or less, a long time is required for graphitization. On the other hand, 2.0
If it is added in excess of wt%, decarburization during hot rolling becomes remarkable, and mechanical properties after quenching and tempering deteriorate.
The range was from super to 2.0 wt%.

Mn: 0.10-0.9 wt% Mn is a useful element for securing hardenability, so it is actively used. However, if less than 0.1 wt%, its effect is small, while 0.9 wt% is reduced. If added in excess, graphitization will be impaired, so the upper limit should be 0.9 wt%.

P ≦ 0.030 wt% Since P deteriorates cold forgeability, it is desirable to reduce P as much as possible, but it is acceptable if its content is up to 0.03 wt%.

S: 0.001 to 0.03 wt% Since S is an element for improving machinability, S is actively added, but if it is less than 0.001 wt%, its effect is poor, and
It is necessary to add more than wt%, but if it exceeds 0.03wt%, the deformability of steel material is remarkably reduced.
The upper limit is set to 0.03 wt%.

B: 0.0003 to 0.0150 wt% B is added positively because addition of a small amount of B improves the hardenability and at the same time remarkably increases the graphitization rate by the formation of BN which is the nucleus of crystallization of graphite. , 0.0003wt%
If the amount is less than 0.0150 wt%, the effect is saturated.
The range was limited to 0.0150 wt%. Preferably, more than 0.0050
It is better to add in the range of 0.0150 wt%.

Al: 0.01-0.5 wt% Al is an element that is effective for deoxidation and effective for improving the graphitization rate by forming AlN.
Solid solution Al is positively added because it facilitates the decomposition of cementite and improves the graphitization rate. If the amount is less than 0.01 wt%, the effect is poor.
If it is added in excess of 0.5 wt%, melting becomes difficult.
The range was 01 to 0.5 wt%.

N: 0.0015 to 0.0070 wt% N promotes graphitization by forming AlN and BN, and is an essential adjusting component in the present invention.
If the content is less than 0.0015 wt%, the formation of precipitates which are nuclei for crystallization of these graphites is not sufficiently performed.
% Or more is required. However, if added in excess of 0.0070 wt%, the cold forgeability is impaired, so that the content should be less than 0.0070 wt%.

O: 0.0030 wt% or less O forms oxide-based nonmetallic inclusions and deteriorates the deformability during cold forging. Therefore, it is desirable to reduce O as much as possible. Is acceptable.

Cr: 0.10 wt% or less Since Cr stabilizes cementite by partitioning into cementite and significantly reduces graphitization, it should be reduced as much as possible. Therefore, its amount must be suppressed to 0.10 wt% or less.

Cu: 0.10 to 1.0 wt% Cu accelerates graphitization by destabilizing cementite. Also, it may be added to improve the hardenability, but if it is less than 0.1 wt%, its effect is poor, and if it exceeds 1.0 wt%, the hot deformability deteriorates. % Range.

Ni: 0.10 to 2.0 wt% Ni, like Cu, is an element which makes cementite unstable and promotes graphitization and at the same time improves the hardenability of steel. Therefore, its addition is effective. However, if it is less than 0.1 wt%, the effect of addition is poor, and since it is an expensive element, if it exceeds 2.0 wt%, the industrial utility value obtained by the present invention is lost. %
It was limited to the following range.

Nb: 0.001 to 0.05 wt% Nb forms carbonitrides and suppresses the growth of γ grains in the heating process of hot rolling to refine the structure after hot rolling. As a result, the ferrite after graphitization becomes finer and the cold forgeability is improved, so that its addition is effective. In this case, it is necessary to add at least 0.001 wt% or more. However, if the addition exceeds 0.05 wt%, the effect is saturated, and at the same time, the cold forgeability is rather lowered by precipitation strengthening. Therefore, the addition is performed in the range of 0.001 to 0.05 wt%.

Ti: 0.001 to 0.05 wt% Ti forms carbonitride, and in the process of hot rolling, γ
The structure after hot rolling is refined by suppressing the growth of grains. As a result, the ferrite after graphitization becomes finer and the cold forgeability is improved, so that its addition is effective. In this case, it is necessary to add at least 0.001 wt% or more. However, if the addition exceeds 0.05 wt%, the effect is saturated, and at the same time, the cold forgeability is rather lowered by precipitation strengthening. Therefore, the addition is performed in the range of 0.001 to 0.05 wt%.

The steel for cold forging according to the present invention is required to have a structure mainly composed of graphite and ferrite, but may contain a small amount of cementite. As the heat treatment for graphitization, graphitization is possible only by maintaining the temperature in the temperature range below the _Ac1 point for about 5 to 30 hours, and quenching, cold working, and the like as pretreatment are unnecessary.

[0030]

EXAMPLES Steel having the composition shown in Table 1 was melted in a 180-ton converter, then subjected to vacuum degassing, continuously cast, and then hot-rolled into 150 mm square billets. . Furthermore, these billets are hot-rolled to 50 mm
φ steel bar. Next, these bars were given 700 ° C x 7h →
An air-cooled graphitization treatment was performed, and then a cylindrical test piece of 15 mmφ × 22.5 mmH was prepared. A compression test was performed under the condition of complete constraint of the end face, and the cold deformation resistance and the critical compression rate during processing were determined. Here, the critical compression ratio was a compression ratio at which cracks began to occur in the test piece. Further, the microstructure after the treatment was observed, the number of graphite particles and the number of carbide particles were counted, and the number of graphite particles /
(Number of graphite particles + number of carbide particles) x 100 (%) was quantified as the graphitization rate. The results are shown in Table 1. N in Table 1
o. 1 to 12 are steels of the present invention, and the graphitization ratio is 98% or more despite short-time treatment. As a result, the deformation resistance and the critical compression ratio during cold forging are high. In addition, No. 13 to 20 are cases where the chemical composition is out of the range of the present invention, the graphitization rate is low, and as a result,
The deformation resistance and the critical compression ratio during cold forging are deteriorated as compared with the steel of the present invention.

[0031]

[Table 1]

[0032]

As described above, by applying the present invention, it is possible to economically obtain a cold forging steel having low deformation resistance at the time of cold forging and also having excellent deformability.
It is a great contribution to the manufacture of machine parts by cold forging.

Continuing from the front page (72) Inventor Kawasaki Enrichment 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Kawasaki Steel Engineering Co., Ltd. (72) Inventor Toshio Fujita 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation (56) References JP-A-3-146618 (JP, A) JP-A-3-140411 (JP, A) JP-A-64-25946 (JP, A) (58) Fields studied (Int .Cl. ⁶ , DB name) C22C 38/00 301 C22C 38/18 C22C 38/40 C22C 38/50

Claims (4)

(57) [Claims] (1) C: 0.4 to 1.1 wt%, Si: more than 1.5 to 2.
0 wt%, Mn: 0.10 to 0.9 wt%, P ≦ 0.030 wt%, S: 0.001 to 0.03 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.01 to 0.5 wt%, N: 0.0015 to 0.0070 wt%, O ≦ 0.0030wt% and Cr ≦ 0.10wt%, the balance is composed of Fe and unavoidable impurities, and the structure is ferrite and graphite or
Machine structural steel with excellent cold forgeability characterized by being made of cementite . 2. C: 0.4 to 1.1 wt%, Si: more than 1.5 to 2.
0 wt%, Mn: 0.10 to 0.9 wt%, P ≦ 0.030 wt%, S: 0.001 to 0.03 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.01 to 0.5 wt%, N: 0.0015 to 0.0070 wt%, O ≦ 0.0030 wt% and Cr ≦ 0.10 wt%, and one or two of Cu: 0.10-1.0 wt% and Ni: 0.10-2.0 wt%, the balance being Fe and unavoidable impurities And the organization is ferritic.
A steel for machine structural use having excellent cold forgeability , characterized by being made of graphite and graphite or further cementite . (3) C: 0.4 to 1.1 wt%, Si: more than 1.5 to 2.
0 wt%, Mn: 0.10 to 0.9 wt%, P ≦ 0.030 wt%, S: 0.001 to 0.03 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.01 to 0.5 wt%, N: 0.0015 to 0.0070 wt%, It contains O ≦ 0.0030 wt% and Cr ≦ 0.10 wt%, and contains one or two of Nb: 0.001 to 0.05 wt% and Ti: 0.001 to 0.05 wt%, with the balance being Fe and inevitable impurities And the organization is ferritic.
A steel for machine structural use having excellent cold forgeability , characterized by being made of graphite and graphite or further cementite . (4) C: 0.4 to 1.1 wt%, Si: more than 1.5 to 2.
0 wt%, Mn: 0.10 to 0.9 wt%, P ≦ 0.030 wt%, S: 0.001 to 0.03 wt%, B: 0.0003 to 0.0150 wt%, Al: 0.01 to 0.5 wt%, N: 0.0015 to 0.0070 wt%, O ≦ 0.0030 wt% and Cr ≦ 0.10 wt%, and one or two of Cu: 0.10 to 1.0 wt% and Ni: 0.10 to 2.0 wt%, Nb: 0.001 to 0.05 wt% and Ti : Contains 0.001 to 0.05 wt% of one or two kinds, the balance being composed of Fe and unavoidable impurities, and the structure is ferrous.
A steel for machine structural use excellent in cold forgeability, which is made of graphite and graphite or further cementite .