JPH06336644A - Steel for machine structure excellent in machinability - Google Patents

Abstract

PURPOSE:To eliminate pretreatment before graphitization annealing in steel and to improve its machinability by specifying C, Si, Mn, Cr, Ti, B, N, O, Ni or the like and forming its structure of ferrite and graphite. CONSTITUTION:This steel for machine structures is formed of a compsn. contg., by weight, 0.1 to 1.5% C, 0.5 to 2% Si, 0.1 to 2% Mn, <=0.1% Cr, 0.005 to 0.1% Ti, 0.0003 to 0.015% B, 0.0015 to 0.015% N and <=0.003% O, and the balance Fe or furthermore contg. one or more kinds selected from 0.1 to 3% Ni, 0.1 to 3% Cu, 0.005 to 0.5% V and 0.005 to 0.05% Nb. Then, its structure is constituted of ferrite and graphite, hardening as pretreatment is eliminated, and its machinability is made excellent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for machine structural use used as a material for industrial machines and automobile parts, that is, a graphite composite free-cutting steel, and is developed especially with the aim of improving its machinability. is there.

[0002]

2. Description of the Related Art Among steel materials used for industrial machines, automobile parts, etc., those having a predetermined shape by cutting are required to have excellent machinability. Conventionally, such steel materials have been improved in machinability by adding Pb, Te, Bi, P, Ca and S, etc., singly or in combination, to carbon steel for machine structural use. Above all, Pb is often used as a free-machining element because its addition does not cause deterioration of the mechanical properties of steel materials and is economical compared to Te, Bi and the like.

However, since Pb is extremely harmful to the human body, a large-scale exhaust facility is required not only in the manufacturing process of steel products but also in the manufacturing process of machine parts using it, and it is necessary to recycle steel products. There were many problems too. Therefore, it has been desired to develop a steel material having machinability equivalent to that of Pb-added steel without adding Pb, but recently, graphite steel has been attracting attention as an alternative to Pb-added free-cutting steel. Has been done.

As free-cutting steel using graphite, for example, JP-A-49-67816, JP-A-49-103817 and JP-A-50-964 are known.
The steel disclosed in each publication of No. 16 is known. These steel materials have the machinability improved without using Pb by making C in the steel exist as graphite and utilizing the notch and the lubricating effect. However, these steel materials
In order to graphitize C in steel, quenching as a pretreatment was indispensable in all cases, and it was not always economical.

[0005]

SUMMARY OF THE INVENTION The present invention is aimed at developing a technique capable of advantageously overcoming the above-mentioned problems of the prior art, and does not require quenching as a pretreatment during graphitization, and is not The purpose is to propose a machine structural steel having excellent machinability, that is, a graphite composite free-cutting steel.

[0006]

[Means for Solving the Problems] As a result of intensive studies to solve the above problems, the inventors have added Ti and B in the steel together and suppressed the amounts of Cr and O in the steel. By doing so, it was found that it is possible to perform graphitization as hot rolling without performing quenching as a pretreatment, and further, machinability is significantly improved. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: 0.1-1.5 wt%, Si: 0.5-2.0 wt%, M
n: 0.1-2.0 wt%, Cr: 0.10 wt% or less, Ti: 0.005
~ 0.1 wt%, B: 0.0003 ~ 0.0150 wt%, N: 0.0015 ~
0.0150wt% and O: 0.0030wt% or less, the balance is
A steel for machine structural use (first invention), which has a composition of Fe and unavoidable impurities and is mainly composed of ferrite and graphite, and is excellent in machinability.

2. C: 0.1-1.5 wt%, Si: 0.5-
2.0 wt%, Mn: 0.1 to 2.0 wt%, Cr: 0.10 wt% or less, Ti: 0.005 to 0.1 wt%, B: 0.0003 to 0.0150 wt%,
N: 0.0015 to 0.0150 wt% and O: 0.0030 wt% or less, Ni: 0.1 to 3.0 wt%, Cu: 0.1 to 3.0 wt%
%, V: 0.005-0.50 wt% and Nb: 0.005-0.05 wt%
Contains one or more selected from
A mechanical structural steel excellent in machinability, characterized in that it has a composition of Fe and inevitable impurities, and its structure is mainly composed of ferrite and graphite (second invention).

3. In addition to the steel of the first invention or the second invention, Pb: 0.03 to 0.30 wt%, Te: 0.002 to 0.50 wt%,
P: 0.010 to 0.15 wt%, Ca: 0.0002 to 0.30 wt%, Bi:
0.01-0.30wt%, Se: 0.003-0.10wt% and S: 0.
It contains one or more selected from 010 to 0.25 wt%, and the balance is Fe and inevitable impurities.
A steel for machine structural use with excellent machinability, whose structure is mainly composed of ferrite and graphite. (Third invention)

[0010]

According to the present invention, by adding Ti and B in combination and suppressing the amounts of Cr and O mixed in the steel, graphitization proceeds without quenching as a pretreatment. It is thought to be as follows. (1) By controlling the amount of O in steel to be low,
Si content increases and promotes graphitization. (2) Graphitization of iron carbide is composed of decomposition of iron carbide, diffusion of C atom in ferrite, crystallization of graphite and diffusion of Fe atom which is a constituent of decomposed iron carbide. Cr has the adverse effect of stabilizing the carbide and delaying its decomposition, but by controlling the Cr content to be low, the decomposition of iron carbide during annealing is accelerated, and graphitization proceeds. (3) B and Ti combine with N to form a nitride,
Due to the formation of this nitride, the amount of solid solution N, which is a strong iron carbide stabilizing element, is decreased, so that the iron carbide is indirectly unstable, and as a result, the decomposition of the iron carbide is accelerated during annealing, and at the same time, the nitriding The substance also acts as a nucleus for crystallization of graphite.

As described above, according to the present invention, it is possible to perform the graphitization treatment as it is after the hot rolling, and it is possible to perform the quenching as the pretreatment which has been indispensable for promoting the graphitization conventionally. , Has machinability equal to or higher than that of conventional Pb-added free-cutting steel, while if a small amount of free-cutting element such as Pb is added, machine structural steel with extremely high machinability is obtained. Be done. In the present invention, from the viewpoint of free-cutting property based on the lubricating action, it is desirable that the structure contains at least 0.6% by volume of the graphite phase.

In the present invention, the reason why the component composition is limited to the above range is shown below. C: 0.1-1.5 wt% C is an essential element not only for forming the graphite phase but also for ensuring the strength of mechanical parts, but if the content is less than 0.1 wt%, its effect is The effect is saturated even if it is small and contained in a large amount exceeding 1.5 wt%.
It was limited to the range of 0.1 to 1.5 wt%.

Si: 0.5-2.0 wt% Si not only acts as a deoxidizer during the melting of steel,
Since it also has the function of destabilizing iron carbide in steel and promoting graphitization, it is positively added. However, if the content is
If less than 0.5 wt%, the effect is poor, and if more than 2.0 wt% is contained, the effect reaches saturation,
Since the liquidus generation temperature decreases and the appropriate temperature range during hot rolling narrows, the content was made to fall within the range of 0.5 to 2.0 wt%.

Mn: 0.1-2.0 wt% Mn is an element that not only contributes effectively as a deoxidizer during the melting of steel, but is also useful for ensuring the strength of the steel material, but its content is 0.1 wt. If it is less than%, the effect is poor, and
If added in excess of 2.0 wt%, the toughness will deteriorate, so
It was limited to the range of 0.1-2.0 wt%.

Cr: 0.10 wt% or less Cr is an element that stabilizes iron carbide and inhibits graphitization, so it is desirable to reduce it as much as possible, but 0.10 wt% is acceptable.

Ti: 0.005-0.1 wt% Ti is a useful element that combines with N to form TiN, prevents the stabilization of iron carbides, and at the same time acts as a nucleus in the crystallization of graphite. It also contributes effectively as an agent. Therefore, Ti is positively added, but the content is 0.005wt
If it is less than 0.1%, its effect is poor, while if it exceeds 0.1% by weight, graphitization is adversely affected, so 0.005 to 0.
The content was set to be 1 wt%.

B: 0.0003 to 0.0150 wt% B combines with N to form BN, inhibits the stabilization of iron carbides, and at the same time acts as a nucleus in the crystallization of graphite and promotes graphitization, so it is positive. 0.0003
If it is less than wt%, its effect is small, and if it is added in excess of 0.0150 wt%, graphitization is adversely affected, so 0.0003 to 0.
The content was set to be in the range of 0150 wt%.

N: 0.0015 to 0.0150 wt% N forms a nitride with Ti and B and promotes crystallization of graphite by using this as a nucleus, so it is positively added, but if the content is less than 0.0015 wt%. Its effect is small, while 0.0150
If added in excess of wt%, the graphitization will be adversely affected.
The content was set to 0.0015 to 0.0150 wt%.

O: 0.0030 wt% or less O combines with Si to form SiO ₂ and impairs graphitization by Si, so it is preferable to reduce it as much as possible.
Up to 0.0030wt% is acceptable.

Although the basic components have been described above, in the present invention, Ni, Cu, V and Nb are further added as strength improving elements.
One or more selected from the above can be contained in the following range. Ni: 0.1 to 3.0 wt% Ni not only enhances the hardenability of steel and is not only useful for improving the strength by quenching and tempering treatment, but also contributes effectively to the promotion of graphitization. Since the effect of addition is poor, and Ni is an expensive element, addition of more than 3.0 wt% causes an increase in cost, so the range was limited to 0.1-3.0 wt%.

Cu: 0.1 to 3.0 wt% Cu is an element that not only enhances the hardenability of steel, but also contributes effectively to the improvement of the strength after quenching and tempering by precipitation strengthening and is also useful for promoting graphitization. However, if the content is
If less than 0.1 wt%, the effect of addition is poor, while 3.0
Even if added in excess of wt%, the effect reaches saturation, so 0.1
Limited to ~ 3.0 wt%.

V: 0.005 to 0.50 wt% V is a useful element that enhances the hardenability of steel, forms fine V carbonitrides, and enhances the strength after quenching and tempering by precipitation strengthening thereof. Content 0.005
If it is less than wt%, its effect is poor, while 0.50 wt%
Since the effect will be saturated even if added over 0.005%,
It was limited to the range of 0.50 wt%.

Nb: 0.005 to 0.05 wt% Nb, like V, enhances the hardenability of the steel and,
It is a useful element that forms Nb carbonitride and improves its strength after quenching and tempering by strengthening its precipitation. If the content is less than 0.005 wt%, the effect of addition is poor, while if added over 0.05 wt%, the effect saturates.
It was limited to the range of 0.005 to 0.05 wt%.

Furthermore, in the present invention, in addition to the above components, any one or more selected from the following component elements can be contained. The inclusion of these elements, together with the effect of improving the machinability due to graphitization of C in the steel, further improves the machinability of the steel.

Pb: 0.03 to 0.30 wt% 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. On the contrary, since it reduces machinability, it is added in the range of 0.03 to 0.30 wt% in order to satisfy both characteristics.

Te: 0.002 to 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, as a result of studying within a range where contribution to improvement of machinability is recognized and inhibition to graphitization is not significantly observed, the appropriate amount is 0.002 to 0.
It is 50 wt%.

P: 0.010 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. At least 0.01 to improve machinability
It is necessary to add 0 wt% or more. 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% This Ca forms Ca-based oxides, which act as nuclei for graphitization and promote graphitization, thereby further improving the MnS content.
The machinability is improved by making the form of the spindle shape. 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 like 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% This 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
If it is less than 0.003 wt%, its effect is small. On the other hand, if it is added in excess of 0.10 wt%, the effect is saturated, so 0.003 to 0.10
Add in the range of wt%.

S: 0.010 to 0.25 wt% S forms MnS, which acts as a chip breaker at the time of cutting to improve the machinability and at the same time serves as a core of graphitization, thereby promoting graphitization. Has the effect of improving machinability. If the addition amount is less than 0.010 wt%, the effect is poor, so 0.010 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 was made 0.25 wt%.

Although the action and effect of each component and the reason for limitation have been described above, in the present invention, further, in order to secure the strength of the mechanical part by quenching / tempering treatment or precipitation strengthening of the ferrite phase, the above-mentioned is given. Effective component elements other than the component elements may be added appropriately.

In the present invention, although not particularly specified, Sn may be further added to improve machinability. However, Sn is an extremely strong graphitization-inhibiting element, so the addition amount must be limited to less than 0.5 wt%.

In terms of metallographic structure, the structure of the present invention is
It is necessary to make the structure mainly composed of graphite and ferrite, but it may be present as cementite up to about 50% of the added C amount.

By adjusting the compositional range of the basic components described above, it is possible to obtain a steel material having a metal structure mainly composed of ferrite and graphite and having excellent machinability, without the need for quenching as a pretreatment. Here, in order to obtain 10% or more of the graphite phase necessary for securing the machinability due to the self-lubricating action described above,
The graphitization treatment is preferably performed in the temperature range of 600 ° C. to Ac ₁ for 5 to 30 hours.

Next, the manufacturing method of the present invention will be described. First, regarding the production of the raw material, it is melted in a conventionally known converter, electric furnace, or the like, and then formed into a slab or a bloom by a continuous casting method or an ingot-agglomeration method. Then, after hot rolling into a predetermined shape, graphitization annealing is performed under the above conditions to precipitate a predetermined amount of graphite phase in the metal structure. Here, hot rolling in the case of containing free-cutting elements is 1000 ° C.
The above heating and rolling at 850 ° C or higher are desirable. Then, after forming into a predetermined part shape, a machine part is formed. The product may be subjected to a nitriding treatment.

[0037]

【Example】

Example 1 Steels having the chemical compositions shown in Table 1 were melted in a converter, bloomed by continuous casting, billet-rolled, and 52 mmφ.
Of steel bar. Then, after heating at 700 ° C. for 10 hours, a graphitization annealing treatment by air cooling was performed, and then the graphitization rate, hardness and machinability were investigated. For the machinability test, using high-speed tool steel SKH4, depth of cut: 2 mm, feed rate: 0.25 mm
/ rev. and cutting speed: It was carried out under the condition of 70 m / min, and evaluated by the time until cutting became impossible (tool life). Also,
As for hardness, a test piece of 35 mmφ × 100 mml was prepared, heated at 870 ° C for 30 min, quenched, then quenched at 550
The hardness was evaluated after the tempering treatment for 1 hour at ℃. Table 2 shows the obtained investigation results.

[0038]

[Table 1]

[0039]

[Table 2]

In Table 1, Nos. 1 to 12 are invention steels, Nos. 13 to 18 are comparative steels, and Nos. 19 and 20 are Pb free-cutting steels obtained by adding Pb to JIS standard S53C. As is clear from Table 2, in each of the invention steels, C in the steel is graphitized by 70% or more after the graphitization treatment.
Therefore, it has excellent machinability and has a tool life equivalent to or better than that of No. 20 Pb free-cutting steel. On the other hand, Si, Cr, Ti,
In each of the comparative steels in which B, O and N deviated from the proper ranges of the present invention, graphitization did not proceed sufficiently,
Machinability is inferior to Nos. 19 and 20. Also,
Looking at the hardness after quenching and tempering, Cu, Ni, Nb and V
No. 6-12 which added one or more of
The hardness after quenching and tempering is higher than that of No. 3 with the same C content, indicating that it is effective when it is necessary to secure the strength after QT treatment.

Example 2 Steels having chemical compositions shown in Tables 3 and 4 were melted in a converter,
After vacuum degassing and melting, continuous casting was performed to form blooms, then 150 mm square billets were formed, and then hot rolling was performed to 52 mm.
φ steel bar. These steel bars were subjected to a graphitization treatment of 700 ° C. × 10 h → air cooling, and a graphitization rate after graphitization annealing, hardness, and a machinability test with a high speed tool steel were carried out. For the machinability test, high-speed tool steel SKH4 was used and the cut: 2 mm,
It was carried out under the conditions of feed: 0.25 mm / rev. And cutting speed: 70 m / min, and the time until cutting became impossible was evaluated as the tool life. The results obtained are shown in Table 5.

[0042]

[Table 3]

[0043]

[Table 4]

[0044]

[Table 5]

Specimen steel Nos. 1 to 11 are the third invention steels. Further, Nos. 17 to 22 are second invention steels, and Nos. 12 to 16 are first invention steels. Nos. 23 to 28 are those in which the elements necessary for graphitization are outside the scope of the invention. Furthermore, No. 29 and 30 are
It is a Pb free-cutting steel in which Pb is added to JIS mechanical carbon steel. Comparing No. 12 to 22 of the third invention steel and the first to second invention steels, when the amount of C is the same, the third invention is not only longer in tool life but also compared with the conventional steel. Even so, the tool life has been greatly improved. Comparative Steel Nos. 23 to 28 have a very short tool life due to a low graphitization rate and are inferior in machinability.

[0046]

As described above, according to the present invention, the machinability equivalent to that of the conventional Pb composite free-cutting steel can be obtained without using Pb, so that the mechanical parts industry can be manufactured without adversely affecting the environment. Manufacturing becomes possible. Further, if a free-cutting element is added as required, it is possible to obtain a steel for machine structural use which is further excellent in machinability together with the effect of improving the machinability by graphitizing C in the steel. Further, the steel of the present invention does not necessarily require a pretreatment before graphitization annealing and can be graphitized as it is rolled, and therefore has excellent productivity.

Front page continuation (72) Inventor Akihiro Matsuzaki 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture, Kawasaki Steel Corporation Technical Research Division (72) Inventor Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Iron & Steel Co., Ltd.

Claims (3)

[Claims] 1. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1-2.0 wt%, Cr: 0.10 wt% or less, Ti: 0.005-0.1 wt%, B: 0.0003-0.0150 wt%, N: 0.0015-0.0150 wt% and O: 0.0030 wt% or less However, the balance becomes the composition of Fe and unavoidable impurities,
A steel for machine structural use with excellent machinability, whose structure is mainly composed of ferrite and graphite. 2. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1-2.0 wt%, Cr: 0.10 wt% or less, Ti: 0.005-0.1 wt%, B: 0.0003-0.0150 wt%, N: 0.0015-0.0150 wt%, O: 0.0030 wt% or less And Ni: 0.1 to 3.0 wt%, Cu: 0.1 to 3.0 wt%, V: 0.005 to 0.50 wt% and Nb: 0.005 to 0.05 wt%.
A steel for machine structural use, which has a composition of Fe and inevitable impurities, and whose structure is mainly composed of ferrite and graphite, and which has excellent machinability. 3. The steel according to claim 1, further comprising Pb: 0.03 to 0.30 wt%, Te: 0.002 to 0.50 wt%, P: 0.010 to 0.15 wt%, Ca: 0.0002 to 0.30 wt%, Bi. : 0.01 to 0.30 wt%, Se: 0.003 to 0.10 wt% and S: 0.010 to 0.25 wt% One or more selected from the rest, and the balance
A steel for machine structural use, which has a composition of Fe and unavoidable impurities and is mainly composed of ferrite and graphite and has excellent machinability.