JPH07188851A - 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 mechinability-improving element by adding small amounts of B to a carbon steel. CONSTITUTION:This steel is a carbon steel containing, by weight, 0.1-1.5% C, <0.5% Si, 0.1-2.0% Mn, 0.0003-0.0150% B, 0.0015-0.0150% N, <0.0030% O, and one or >=2 kinds among 0.1-3.0% Ni, 0.1-3.0% Co, and 0.1-3.O% Cu or further containing 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. BN is precipitated by adding and incorporating B into a low Si type carbon steel, and further, this BN acts as the nucleus of graphitization at the time when ferrite and graphite are formed by the decomposition of cementite and accelerates the graphitization of C by the decomposition of cementite. Because the structure of the carbon steel is composed of soft ferrite and graphite, the graphite free-cutting steel excellent in machinability 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 particularly intended to improve its machinability.

[0002]

2. Description of the Related Art In general, among steel materials used for industrial machines, automobile parts, etc., a steel material having a desired shape by cutting is required to have excellent machinability. For such steel materials, we have used P
This has been dealt with by adding machinable elements such as b, Te, Bi, P, Ca and S individually or in combination. Above all, Pb
Is an element that has been especially prized as a free-cutting element because it does not cause deterioration of the mechanical properties of steel materials and is more economical than Te, Bi, etc. is there. 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, conventionally, there has been a strong demand for development of a steel material having machinability equivalent to that of Pb-added steel without adding Pb.

In order to meet the above-mentioned demand, C in steel is conventionally present as graphite in JP-A-49-67816, JP-A-49-103817 and JP-A-50-96416. Then, a method of improving the machinability without using Pb has been proposed by utilizing the notch and the lubricating effect of this graphite. However, in any of these methods, quenching is indispensable as a pretreatment in order to graphitize C in steel, and thus it cannot always be said that it is an economical method.

[0004]

The present invention has machinability equal to or higher than that of conventional Pb composite-added free-cutting steel without using Pb at all, 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 when B is added to a low Si steel material, BN precipitates and
It was newly found that N serves 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: less than 0.5%, Mn: 0.1 to 2.0%, B: 0.0003
-0.0150%, N: 0.0015-0.0150%, O: 0.0030% or less, and one or two selected from Ni: 0.1-3.0%, Co: 0.1-3.0% and Cu: 0.1-3.0%. A graphite free-cutting steel containing the above and having a balance of Fe and inevitable impurities and having a metal structure mainly composed of ferrite and graphite and having excellent machinability (first invention).

(2) In the first invention, Cr: 0.05 to
1.0%, Mo: 0.05 to 0.5%, V: 0.05 to 0.5% and Nb: 0.005 to 0.05% One or more selected from graphite free-cutting steel excellent in machinability ( Second invention).

[0008]

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 not essential. 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, but when MnS is formed by the addition of S,
It was also found that the formation of BN precipitates was promoted rather than the decrease in the diffusion rate of C in the ferrite being compensated, and the formation of graphite proceeded easily.

This effect 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 component 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: less than 0.5% 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. But the content is 0.5
%, The ductility and toughness of the steel material will be deteriorated.
The upper limit was less than 0.5%.

Mn: 0.1-2.0% Mn is not only necessary for ensuring the strength of steel, 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 enhances hardenability. Therefore, B is positively added.
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-based 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.

Further, in the present invention, as a graphitization promoting element,
At least one selected from Ni, Co and Cu
Further, at least one selected from Cr, Mo, V and Nb can be added as a hardenability improving element. 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 steel. Therefore, 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%.

Co: 0.1-3.0% Co is effective not only for promoting graphitization but also for enhancing the hardenability of steel and for increasing strength. However, if the addition amount is less than 0.1%, the effect is small, while if the addition amount exceeds 3.0%, the effect is saturated and further the toughness of the steel material is deteriorated. Therefore, the content is limited to the range of 0.1 to 3.0%.

Cu: 0.1 to 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%.

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% Mo is an element that improves the hardenability of steel, so 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 at the same time precipitates fine V carbonitrides and strengthens the strength of the steel by quenching and tempering by strengthening the precipitation 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 by quenching and tempering of the steel by strengthening the precipitation, so if necessary, 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 steel 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 the 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 during cutting to improve machinability, and at the same time promotes graphitization by becoming the 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%.

Further, in the present invention, not only the composition range of the components but also the metal structure is important, and the structures are 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 ° C to Ac ₁ . In this graphitization treatment, quenching as a pretreatment is not necessary.

[0033]

[Examples] Steels having the chemical composition shown in Table 1 were melted in a converter, made into blooms by continuous casting, and then billet-rolled to obtain bar steels having a diameter of 52 mm. Then, a soaking process was performed at 700 ° C. for 15 hours, and then air-cooling was performed for graphitization to measure the graphitization rate and hardness after graphitization annealing, and a machinability test using a high speed tool steel was performed. Hardness is 870 ℃, 30 min
Hardness after heating, and then after tempering at 550 ° C. for 1 h. For machinability test, high speed tool steel SK
Using H4, cutting: 2 mm, feed: 0.25 mm / rev. Cutting speed: 70 m / min, and the time until cutting becomes impossible was evaluated as the tool life. Further, hardness after heating at 870 ° C. for 30 minutes, quenching, and tempering at 550 ° C. for 1 hour was measured.
The test results obtained are shown in Table 2.

[0034]

[Table 1]

[0035]

[Table 2]

Specimen Nos. 1 to 13 are steels of the present invention. Also,
No. 14 to 17 are at least C, Si, Mn, N, B, Ni, Co and Cu.
This is the case where one component is outside the scope of the present invention. Also, No.1
8 and 19 are free-cutting steels containing JIS S25C and S53C with Pb added. Each of the steels of the present invention has a graphitization rate of 75% or more, which corresponds to the graphitization rate, and the tool life is equal to or more than that of the conventional steel. On the other hand, the comparative steel has a lower graphitization rate and a shorter tool life than the Pb free-cutting steel. Specimen Nos. 1 to 13 are Cu, Co,
Despite the high graphitization ratio due to the precipitation strengthening of Cr, Mo, V, and Nb, the hardness is increasing and it is effective when it is necessary to increase the strength of machine parts regardless of quenching and tempering. It is shown. Looking at the hardness after quenching and tempering, the steel with Ni, Cu, Co, Cr, Mo, V and Nb added showed a higher hardness after quenching and tempering than No. 15 steel with the same C content. This shows that it is effective when it is necessary to secure the strength as the final part by the QT process. On the other hand, in Nos. 14 to 17 which are out of the scope of the present invention, graphitization does not proceed sufficiently, and thus machinability is poor.

[0037]

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 (2)

[Claims] 1. C: 0.1 to 1.5 wt%, Si: less than 0.5 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 Including,
And Ni: 0.1-3.0 wt%, Co: 0.1-3.0 wt% and
Cu: One or more selected from 0.1 to 3.0 wt% is contained, and the balance is composed of Fe and inevitable impurities, and the metal structure is mainly composed of ferrite and graphite. Graphite free-cutting steel with excellent machinability. 2. C: 0.1 to 1.5 wt%, Si: less than 0.5 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 Including,
And Ni: 0.1-3.0 wt%, Co: 0.1-3.0 wt% and
Contains one or more selected from Cu: 0.1-3.0 wt%, Cr: 0.05-1.0 wt%, Mo: 0.05
~ 0.5 wt%, V: 0.05 ~ 0.5 wt% and Nb: 0.005 ~ 0.05 wt% selected from the group consisting of 1 or 2 or more, with the balance being Fe and inevitable impurities, and having a metallic structure. Graphite free-cutting steel with excellent machinability, characterized in that is mainly composed of ferrite and graphite.