JPH07188850A - Graphite free-cutting steel excellent in machinability - Google Patents

Abstract

PURPOSE:To obtain a graphite free-cutting steel excellent in machinability without using Pb, as a machinability-improving element, by adding small amounts of B to a carbon steel. CONSTITUTION:B is added by 0.0003-0.0150%, by weight, to a carbon steel containing 0.1-1.5% C, 0.5-2.0% Si, 0.1-2.0% Mn, 0.0015-0.0150% N, and <0.0030% O, or further, one or >=2 kinds among 0.1-3.0% Ni, 0.1-3.0% Cu, and 0.005-0.10% Ti and one or >=2 kinds among 0.05-1.0% Cr, 0.05-0.5% Mo, 0.05-0.5% V, and 0.005-0.05% Nb are further incorporated, independently or in combination. B and N in this carbon steel form BN, which becomes the nucleus of graphitization at the time of annealing of this steel and acts as an element for accelerating the graphitization of C by the decomposition of cementite. As a result, the carbon steel excellent in machinability by graphite can be obtained without using machinability-improving elements such as Pb.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a graphite free-cutting steel used as a raw material for industrial machines and automobile parts, and is particularly intended to improve its machinability.

[0002]

2. Description of the Related Art Among steel materials used for industrial machines and automobile parts, those having a desired shape by cutting are required to have excellent machinability. For such steels, carbon steels for machine structure have been used for Pb, Te, Bi,
The machinability has been improved by adding machinability elements such as P, Ca and S individually or in combination. Above all, Pb is widely used as a machinability element because it does not deteriorate the mechanical properties of steel by its addition and is more economical than Te and Bi. However, since Pb is extremely harmful to the human body, not only is a large-scale exhaust facility required in the manufacturing process of steel products, and also in the manufacturing process of mechanical parts using it, it is also very important in terms of recycling steel products. There was a problem. Therefore, it has been conventionally desired to develop a steel material having the same machinability as Pb-added steel without adding Pb.

In order to meet the above demands, for example, JP-A-49-67816, JP-A-49-103817 and JP-A-50-9.
Japanese Patent No. 6416 proposes a method of improving machinability without using Pb by allowing C in steel to exist as graphite and utilizing the notch lubrication effect of this graphite. However, in all of these methods, quenching is indispensable as a pretreatment for graphitizing C in steel,
It was not always an economical method.

[0004]

DISCLOSURE OF THE INVENTION The present invention has machinability equal to or higher than that of conventional Pb composite-added free-cutting steel without using Pb, and can be easily graphitized. Moreover, it is an object of the present invention to propose a steel for machine structure, which can easily secure the strength after cutting.

[0005]

Means for Solving the Problems Now, as a result of intensive studies to solve the above-mentioned problems, the inventors have found that B
BN is precipitated by the addition of
It has been newly found that is used as a nucleus of graphitization during annealing to promote graphitization and thus improve 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 wt% (the following is simply indicated as "%"),
Si: 0.5-2.0%, Mn: 0.1-2.0%, B: 0.00
03-0.0150%, N: 0.0015-0.0150%, O: 0.0030%
A graphite free-cutting steel (1st invention) containing the following, the balance being Fe and inevitable impurities, and having a metal structure mainly composed of ferrite and graphite and having excellent machinability.

(2) In the first invention, Ni: 0.1-
3.0%, Cu: 0.1-3.0%, Ti: 0.005-0.10%
Graphite free-cutting steel excellent in machinability containing one or more selected from the above (second invention).

(3) In the first invention, Cr: 0.05 to
1.0%, Mo: 0.05-0.5%, V: 0.05-0.5%,
Nb: Graphite free-cutting steel that contains one or more selected from 0.005 to 0.05% and has excellent machinability (3rd
invention).

(4) In the first invention, Ni: 0.1-
3.0%, Cu: 0.1-3.0%, Ti: 0.005-0.10%
One or more selected from the above, and Cr: 0.05 to 1.
0%, Mo: 0.05 to 0.5%, V: 0.05 to 0.5%,
Nb: 1 or 2 selected from 0.005 to 0.05%
Graphite free-cutting steel containing at least one kind and having excellent machinability (4th
invention).

[0010]

According to the present invention, it is possible to perform the graphitization treatment as it is after the hot rolling, and the quenching treatment for promoting the graphitization is unnecessary. It is known that the graphitization of cementite generally consists of the processes of decomposition of cementite, diffusion of C atoms in ferrite, crystallization of graphite and diffusion of Fe atoms. It has been thought that when solid solution occurs, the diffusion rate of C atoms slows down. However, when MnS is formed by the addition of S, C
It was also found that the formation of BN precipitates was promoted rather than the decrease in the diffusion rate in the ferrite of No. 1 was compensated, and that the formation of graphite was facilitated.

The effect of graphite on machinability is remarkable when cutting at a relatively low speed with high speed tool steel (high speed steel).
It is presumed that this is because graphite acts as a chip breaker and the temperature rise of the tool during cutting is suppressed by the lubricating action of graphite.

The reason why the composition of the steel material is limited to the above range in the present invention will be described below. C: 0.1 to 1.5% C is an element which is not only essential for forming a graphite phase but also important for securing the strength of mechanical parts. However, if the content is less than 0.1%, its effect is not obtained. Small, while 1.5%
The effect reaches saturation even if added over 0.1%, so 0.1
Limited to ~ 1.5%.

Si: 0.5 to 2.0% Si is an element necessary for deoxidation, and is also useful as an element for destabilizing cementite in steel and promoting graphitization, so it is positively added. However, the content is 0.5
If the content is less than 2.0%, the effect is poor. On the other hand, if the content exceeds 2.0%, the effect of promoting graphitization reaches saturation, and the temperature range in which the liquid phase is generated decreases and the appropriate temperature range during hot rolling narrows. Therefore, the range is 0.5 to 2.0%.

Mn: 0.1-2.0% Mn is not only necessary for securing the strength of the steel material, but also S
It combines with MnS to form MnS, which acts as a precipitation nucleus of BN and contributes effectively to the promotion of graphitization. Therefore, it is positively utilized. However, if it is added less than 0.1%, it contributes to the strength and the formation of MnS. However, if added in excess of 2.0%, toughness will be deteriorated, so the content was limited to 0.1-2.0%.

B: 0.0003 to 0.0150% B forms BN and serves as a nucleus for crystallization of graphite, and at the same time, it is positively added because it enhances hardenability.
If it is less than 0003%, its effect is poor, while 0.0150
If it is contained in excess of 0.1%, not only the effect reaches saturation but also the hot workability deteriorates, so the content was limited to 0.0003 to 0.0150%.

N: 0.0015 to 0.0150% N is positively used because it combines with B to form BN, and crystallization of graphite proceeds with this as a nucleus. B
In order to form N, stoichiometrically, it suffices to add approximately the same amount as B, but in order to uniformly disperse BN, N should be slightly over the stoichiometric value. It is preferable to set to. Further, N facilitates generation of chips during cutting due to dynamic strain aging, and therefore, in that sense, it is better to add an amount that is more than stoichiometric equivalent to B. Further, as will be described later, it is preferable to set N on the excess side with respect to B also in order to secure mechanical strength by precipitation hardening of V and Nb. Considering the above points, if the content is less than 0.0015%, its effect is small, while
Even if the content exceeds 0.0150%, the effect reaches saturation, so the content was made 0.0015 to 0.0150%.

O: 0.0030% or less O forms a hard oxide type non-metallic inclusion, which damages the cutting tool at the time of cutting and reduces the machinability, so it is desirable to reduce it as much as possible. Content up to 0.0030% is acceptable.

The basic component composition of the present invention has been described above. However, in the present invention, at least one selected from Ni, Cu, and Ti as a graphitization-promoting element, and as a hardenability-improving element are further added as necessary. At least one selected from Cr, Mo, V, and Nb can be added. The reasons for limiting each element will be described below.

Ni: 0.1-3.0% Ni is an element that promotes graphitization and at the same time improves the hardenability of the steel, so it is used to secure higher strength by quenching and tempering treatment, but less than 0.1% Since its addition effect is small and it is an expensive element,
Addition of more than 3.0% causes economic disadvantage, so 0.1 to 3.
The range was limited to 0%.

Cu: 0.1-3.0% Cu is not only effective in promoting graphitization, but also enhances the hardenability of steel and contributes to the increase in strength by precipitation strengthening, but if less than 0.1%, The effect of addition is small, while the effect reaches saturation even if added over 3.0%, so the range was limited to 0.1-3.0%.

Ti: 0.005 to 0.10% Ti has an action of promoting graphitization together with Si. To obtain this effect, at least 0.005% must be added, but if it exceeds 0.10%, coarse TiN is formed. However, the tool life is reduced, so the range is limited to 0.005 to 0.10%.

Cr: 0.05 to 1.0% Cr is an element that improves the hardenability of steel, so it is used when it is necessary to secure the strength by quenching and tempering. However, on the other hand, addition of a large amount is not preferable because it is an element that inhibits graphitization. If the content is less than 0.05%, the hardenability improvement effect is poor, while if it exceeds 1.0%, the iron carbide in the steel is significantly stabilized and the graphitization is delayed, so the content should be within the range of 0.05 to 1.0%. Limited

Mo: 0.05 to 0.5% Since Mo is an element that improves the hardenability of steel, it is used when it is necessary to increase the strength by quenching and tempering. On the other hand, it is also an element that inhibits graphitization, so addition of a large amount is not preferable. The content here is 0.
If it is less than 05%, the effect of improving hardenability is small, while it is 0.5%.
If it is contained in excess of 0.05%, the graphitization rate will decrease.
The range is limited to 0.5%.

V: 0.05 to 0.5% V improves the hardenability of the steel and also precipitates fine V carbonitrides to increase the strength of the steel by quenching and tempering due to the precipitation strengthening thereof. Added.
Further, it is effective not only for quenching and tempering but also for increasing the strength of the steel, but it is also an element that hinders graphitization, so addition of a large amount is not preferable. If the content is less than 0.05%, the effect is small. On the other hand, if the content exceeds 0.5%, graphitization is delayed, so the content is limited to 0.05 to 0.5%.

Nb: 0.005 to 0.05% Nb improves the hardenability of the steel and also precipitates fine Nb carbonitrides to increase the strength of the steel by quenching and tempering by precipitation strengthening. Added.
Further, similar to V, it is effective not only for quenching and tempering but also for increasing the strength of steel, but it is also an element that hinders graphitization, so addition of a large amount is not preferable. Here the content is 0.0
The effect was small when the content was less than 05%, while the graphitization was delayed when the content was more than 0.05%, so the range was limited to 0.005 to 0.05%.

If desired, the following free-cutting elements may be added to the above steels of the present invention. Pb: 0.03 to 0.50% Pb has a low melting point, so it is an element that melts due to the heat generation of the steel material during cutting and significantly improves machinability due to the liquid lubrication effect, but on the other hand, it inhibits graphitization and conversely Since it reduces the machinability, it is added within the range of 0.03 to 0.50 wt% in order to satisfy both characteristics.

Te: 0.002-0.50 wt% Te forms MnTe, which acts as a chip breaker to improve machinability. On the other hand, since it is also an element that inhibits graphitization, if added in a large amount, machinability rather deteriorates. Therefore, the proper amount is 0.002-
It is 0.50 wt%.

P: 0.03 to 0.15 wt% P is an element that improves the machinability by hardening the ferrite phase, but is also an element that inhibits graphitization. To improve machinability, at least 0.03
It is necessary to add more than wt%. However, if added in excess of 0.15 wt%, graphitization will be hindered and machinability will be adversely reduced, so 0.15 wt% is added as the upper limit.

Ca: 0.0002 to 0.30 wt% Ca forms a Ca-Al type oxide, which acts as a nucleus of graphitization and promotes graphitization, thereby improving machinability. Such an effect does not clearly appear when added in an amount of less than 0.0002 wt%, whereas when added in excess of 0.30 wt%, oxide-based non-metallic substances increase, which reduces the fatigue strength of machine parts. The amount of Ca added is in the range of 0.0002 to 0.30 wt%.

Bi: 0.01 to 0.30 wt% Bi has a low melting point similar to Pb, so it is an element that melts due to the heat generation of the steel material during cutting and significantly improves machinability due to the liquid lubrication effect. It inhibits graphitization and conversely reduces machinability.
Add within the range of 30wt%.

Se: 0.003 to 0.10 wt% Se combines with Mn to form MnSe, which acts as a chip breaker to improve machinability. At the same time, this MnSe serves as a nucleus for graphitization and promotes graphitization, thereby improving machinability. This effect is 0.00
If it is less than 3 wt%, its effect is small, while if it is added in excess of 0.10 wt%, its effect is saturated, so 0.003 to 0.10 wt%
Add within the range.

S: 0.03 to 0.25 wt% 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 has the effect of improving machinability. If the addition amount is less than 0.03 wt%, the effect is poor, so 0.030 wt% or more is added. On the other hand, if the addition amount exceeds 0.25 wt%, the effect is saturated, so the upper limit should be 0.25 wt%. Is good.

Further, Sn may be added to improve machinability. However, Sn is an extremely strong graphitization-inhibiting element, so its addition amount must be limited to less than 0.5 wt%.

In the present invention, not only the composition range of the components but also the metal structure is important, and the structure is mainly ferrite and graphite. This is because it is an essential condition for the present invention to suppress the temperature rise of the cutting tool during cutting due to the lubricating action of graphite and thereby to improve the life of the cutting tool. Here, the preferred content of graphite in steel is
90 to 100%. For that purpose, as a graphitization treatment,
It is necessary to carry out a treatment for holding for 5 hours or more in the temperature range of 00 to Ac ₁ ° C. In this graphitization treatment, quenching as a pretreatment is not necessary.

[0035]

[Examples] The steel materials shown in Table 1 were melted by a converter and continuously cast into blooms. A 52 mmφ steel bar was obtained through billet rolling. These were subjected to graphitization treatment at 700 ° C. for 15 hours and then air cooling, and a graphitization rate after graphitization annealing, hardness and machinability test with high speed tool steel were carried out. For the machinability test, high-speed tool steel SKH was used, Q was carried out under the conditions of depth of cut: 2 mm, feed: 0.25 mm / rev. Cutting speed: 70 m / min, and the time until cutting becomes impossible is evaluated as the tool life. did. The test results obtained are shown in Table 2.

[0036]

[Table 1]

[0037]

[Table 2]

Specimen steel Nos. 1 to 21 are steels of the present invention. Also,
Nos. 22 to 25 are cases where either Si or B is outside the scope of the present invention. No. 26 and 27 are SAE 12L14 and
It is a free-cutting steel in which Pb is added to JIS S45C. The steel of the present invention is
Corresponding to the graphitization rate of 75% or more, the tool life is equivalent to or higher than that of conventional steel. On the other hand, the comparative steels have low graphitization rates and both have shorter tool life than the SAE 12L14 equivalent steels. That is, by using the present invention, it is possible to obtain machinability equivalent to that of the conventional Pb composite free-cutting steel without using Pb, and to industrially manufacture mechanical parts without adversely affecting the environment. Is possible.

[0039]

As described above, according to the present invention, it is possible to obtain the same machinability as that of the conventional Pb composite-added free-cutting steel without using Pb, and therefore, it does not adversely affect the environment. In addition, industrially excellent mechanical parts can be manufactured.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Division (72) Inventor Akihiro Matsuzaki 1 Kawasaki-cho, Chuo-ku, Chiba Address: Kawasaki Steel Corporation Technical Research Division

Claims (4)

[Claims] 1. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, N: 0.0015 to 0.0150 wt%, O: 0.0030 wt% or less, and the balance is Fe and inevitable impurities. Graphite free-cutting steel with excellent machinability, characterized in that the metal structure mainly consists of ferrite and graphite. 2. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, N: 0.0015 to 0.0150 wt%, O: 0.0030 wt% or less,
Also, it contains one or more selected from Ni: 0.1 to 3.0 wt%, Cu: 0.1 to 3.0 wt%, Ti: 0.005 to 0.10 wt%, and the balance is Fe and inevitable impurities. Moreover, graphite free-cutting steel with excellent machinability, characterized in that the metal structure is mainly composed of ferrite and graphite. 3. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, N: 0.0015 to 0.0150 wt%, O: 0.0030 wt% or less,
In addition, Cr: 0.05 to 1.0 wt%, Mo: 0.05 to 0.5 wt%, V: 0.05 to 0.5 wt%, Nb: 0.005 to 0.05 wt% and one or more selected from the rest, with the balance being Fe. A graphite free-cutting steel excellent in machinability, characterized by having an unavoidable impurity composition and having a metal structure mainly composed of ferrite and graphite. 4. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, B: 0.0003 to 0.0150 wt%, N: 0.0015 to 0.0150 wt%, O: 0.0030 wt% or less,
In addition, it contains one or more selected from Ni: 0.1 to 3.0 wt%, Cu: 0.1 to 3.0 wt%, Ti: 0.005 to 0.10 wt%, and Cr: 0.05 to 1.0 wt%, Mo: 0.
05 to 0.5 wt%, V: 0.05 to 0.5 wt%, Nb: 0.005 to 0.05 wt%, and one or more selected from the rest, with the balance being Fe and inevitable impurities, and metal. Graphite free-cutting steel with excellent machinability, whose structure is mainly composed of ferrite and graphite.