JPH06279849A - Production of steel for machine structure excellent in machinability - Google Patents

Abstract

PURPOSE:To obtain steel products having machinability equal to or above that of the conventional Pb composite free cutting steel with a high productivity. CONSTITUTION:Steel contg., by weight, 0.1 to 1.5% C, 0.5 to 2.0% Si, 0.1 to 2.0% Mn, 0.01 to 0.5% Al, 0.0015 to 0.0l50% N and <=0.0030% O and contg., at need, one or >=two kinds selected from the group of 0.1 to 3.0% Ni, 0.1 to 3.0% Cu, 0.05 to 1.0% Cr, 0.05 to 0.5% Mo, 0.005 to 0.05%. Nb, 0.005 to 0.05% Ti and 0.001 to 0.O5% rare earth metals and/or the group of 0.03 to 0.30% Pb, 0.002 to 0.50% Te, 0.010 to 0.15% P, 0.0002 to 0.30% Ca, 0.01 to 0.30%. Bi, 0.003 to 0.10% Se and 0.010 to O.25% S is heated to the AlN solid solution temp., is subjected to hot rolling and is thereafter annealed to form its metallic structure into a one essentially consisting of ferrite and graphite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel for machine structural use which is used as a material for industrial machines and automobile parts and which has excellent machinability.

[0002]

2. Description of the Related Art Generally, 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. Up to now, the improvement of the machinability of the steel for machine structure has been dealt with by the method of adding the machinability elements such as Pb, Te, Bi, P, Ca or S alone or in combination. Above all, Pb
Does not lead to deterioration of the mechanical properties of steel even if added, and is cheaper than Te, Bi, etc.
Is one. However, since this Pb is extremely harmful to the human body, it not only requires a large-scale exhaust facility in the manufacturing process of steel products, and also in the manufacturing process of mechanical parts using it, it is also necessary to recycle steel products. Also had a problem. Therefore, it has been strongly desired to develop a steel material having machinability equivalent to that of Pb-added steel without adding Pb.

In order to meet the above-mentioned demands, in the prior art, for example, Japanese Patent Laid-Open Nos. 49-67816, 49-103817 and 50-96416, C in steel is present as graphite. Then, a method for improving machinability without using Pb is proposed by utilizing the notch and the lubricating effect of this graphite. However, these conventional methods have not necessarily been economical methods because it is necessary to graphitize C in steel and quenching is indispensable as a pretreatment.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 2-111842 discloses a method of making graphitization possible only by graphitization without requiring quenching (pretreatment) as it is rolled.
That is, in this method, it is indispensable to add B in an appropriate amount, so that the iron carbide is destabilized, and it is attempted to promote graphitization by forming a nitride to provide a nucleus for graphitization. It is a thing. However, the addition of B to this type of steel material has a problem that cracks are likely to occur during solidification.

[0005]

DISCLOSURE OF THE INVENTION The present invention is intended to advantageously solve the above-mentioned problems of the prior art, in which so-called hot-rolling can be performed without graphitization. It is easy and has machinability equal to or higher than that of conventional Pb-added free-cutting steel without relying on addition of Pb, while extremely high if a small amount of free-cutting element such as Pb is added. It is an object of the present invention to propose a method for advantageously manufacturing a machine structural steel exhibiting free-cutting properties.

[0006]

[Means for Solving the Problems] As a result of intensive studies to solve the above problems, the inventors of the present invention have found that carbon steel has Al
Is added and the O in the steel is regulated to a low level, and heating is performed to a temperature at which Al sufficiently forms a solid solution prior to hot rolling. The present invention was conceived based on the finding that the graphitization can be sufficiently promoted with the hot rolling as it is, and thus the machinability of the carbon steel for machine structural use can be remarkably improved.

That is, the gist of the present invention is as follows. (1) C: 0.1 to 1.5 wt%, Si: 0.5 to 2.0 wt% Mn: 0.1 to 2.0 wt%, Al: 0.01 to 0.5 wt%, N: 0.
0015-0.0150wt%, O: ≤ 0.0030wt%, the balance is
A steel slab composed of Fe and unavoidable impurities is heated to AlN solid solution temperature, hot-rolled, and then annealed to form a metal structure mainly composed of ferrite and graphite. A method for producing machine structural steel having excellent machinability (first invention). (2) C: 0.1-1.5 wt%, Si: 0.5-2.0 wt%, M
n: 0.1 to 2.0 wt%, Al: 0.01 to 0.5 wt%, N: 0.0
015-0.0150wt%, O: ≤ 0.0030wt%, and Ni:
0.1 to 3.0 wt%, Cu: 0.1 to 3.0 wt%, Cr: 0.05 to
1.0 wt%, Mo: 0.05-0.5 wt%, Nb: 0.005-0.05
wt%, Ti: 0.005 to 0.05 wt%, and REM: 0.001
~ 0.05wt% containing one or more selected from the rest, the balance
A steel slab having a composition of Fe and unavoidable impurities is heated to an AlN solution temperature, hot-rolled, and then annealed to form a metal structure mainly composed of ferrite and graphite. A method for producing machine structural steel having excellent machinability (second invention). (3) In addition to the steel composition 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 0.15wt%, Ca: 0.0002 to 0.30wt%, Bi: 0.01
~ 0.30wt%, Se: 0.003 ~ 0.10wt% and S: 0.01
By heating a steel slab having a composition containing one or more selected from 0 to 0.25 wt% to an AlN solid solution temperature, performing hot rolling, and then performing annealing, mainly A method for producing a steel for machine structural use, which is excellent in machinability, characterized by having a metal structure composed of ferrite and graphite (third invention).

[0008]

According to the above-mentioned constitution of the present invention, the mechanism by which the promotion of graphitization is effectively realized cannot be said to have been completely elucidated, but it is presumed as follows. First, it regulates the O content in steel to a low level (≤0.00
30 wt%) reduces the amount of Al ₂ O _3, which is an oxide-based non-metallic inclusion harmful to machinability, and increases the amount of solid solution Al. Then, this solid solution Al combines with N during annealing to facilitate the formation of AlN. Generally, the graphitization mechanism of iron carbide is as follows: a. Decomposition of iron carbide (cementite), b. Diffusion of C atom into ferrite, c. Crystallization of graphite, d. It is known to consist of the diffusion process of Fe atoms. In such a graphitization mechanism, when the annealing treatment is performed, AlN formed at this time reduces the solid solution N which is a strong cementite-stable element, whereby the cementite becomes in an unstable state and annealed. It accelerates the decomposition of cementite in the worst case and at the same time provides the nucleus for crystallization of graphite. As a result, graphitization is promoted. Therefore, when annealing
In order to form AlN, it is necessary to heat to a high temperature as described later during the hot rolling.

The reason why the composition of the steel in the present invention is limited to the above range will be described below. C: 0.1 to 1.5 wt% C is not only an essential component for forming a graphite phase, but also an element necessary for securing the strength of mechanical parts, but if it is less than 0.1 wt%, its effect is small. , While 1.5 wt%
Even if added in excess of 0.1, the effect will saturate, so 0.1-1.
It was limited to the range of 5 wt%.

Si: 0.5 to 2.0 wt% Si is necessary as a deoxidizing agent during melting, and is also effective as an element for destabilizing iron carbide (cementite) in steel and promoting graphitization. Moreover, since it is also a strength increasing component, it is positively added. Its content is 0.5w%
If less than, these effects are poor. On the other hand, 2.0 wt% of this Si
%, Not only the effect of promoting graphitization reaches saturation, but also the temperature range in which the liquid phase occurs decreases and the suitable temperature range during hot rolling narrows, so the range of 0.5 to 2.0 wt% Limited to.

Mn: 0.1-2.0 wt% Mn is an element which is effective not only as a deoxidizing agent during melting but also for ensuring the strength of the steel material. However, if the content is less than 0.1 wt%, the contribution to the strength is small, while if it exceeds 2.0 wt%, the toughness is deteriorated, so the range is limited to 0.1 to 2.0 wt%.

Al: 0.01 to 0.5 wt% Al not only has the function of assisting deoxidation, but also promotes the decomposition of cementite during annealing and, at the same time, acts to prevent the stabilization of cementite by forming AlN by combining with N. Yes, and promotes graphitization by acting as nuclei in the crystallization of graphite. Therefore, in the present invention, this Al is positively added, but if its content is less than 0.01%, its effect is poor, and if it exceeds 0.5%, the graphitization promoting action reaches saturation. Since the hot deformability is remarkably reduced, the content is set within the range of 0.01 to 0.5 wt%.

N: 0.0015 to 0.0150 wt% N combines with Al to form AlN, which acts as nuclei for crystallization of graphite and promotes graphitization, so N is positively added. If its content is less than 0.0015%, its effect of addition is small, while if it exceeds 0.0150%, it will rather hinder graphitization, so it was limited to the range of 0.0015 to 0.0150 wt%.

O: ≦ 0.0030% In the present invention, the role of O is important. O is Al
Hard Al combined with₂O ₃To form. This Al₂O₃Generation of
Is a graphite by inhibiting the effect of Al addition described above.
Inhibits nucleation for crystallization of
It reduces the graphitization effect. Besides, it contains a large amount of O
Al formed by₂O₃Damages the cutting tool during cutting
The damage causes a decrease in machinability. Therefore this O
Should be as low as possible. But Ama
Since a very low O puts an excessive burden on refining, 0.0030wt%
The upper limit is 0.0030, considering that up to 0.0030 is allowed.
Limited to wt%.

In the present invention, the reason for limiting the component composition necessary for promoting graphitization and improving machinability is as described above. In the present invention, by further containing one or more components selected from the following Ni, Cu, Cr, Mo, Nb, Ti and REM in addition to the above main components, if necessary. It is effective to add and improve other properties in addition to promoting the action and effect of each of the main components listed above. The reasons for limiting the composition of these additive components will be described below.

Ni: 0.1-3.0 wt% Ni is effective in destabilizing cementite to promote graphitization, and at the same time, enhances hardenability and steel strength. If the content is less than 0.1 wt%, the effect of this addition is small, while if the content exceeds 3.0 wt%, not only the effect of addition saturates but also it is not economical because it is an expensive element, so 0.1-3.0 wt% Limited to the range.

Cu: 0.1-3.0 wt% Cu not only acts to destabilize cementite and effectively promotes graphitization, but also improves machinability and hardenability, and further strengthens steel by precipitation strengthening. Is a useful element that increases the strength of. If the content is less than 0.1 wt%, the effect of addition is small, while if added over 3.0 wt%, the effect reaches saturation, so the range was limited to 0.1-3.0 wt%.

Cr: 0.05 to 1.0 wt% Cr enhances the hardenability of steel and effectively acts to increase the strength by quenching and tempering. On the other hand, however, Cr is also an element that hinders graphitization, so addition of a large amount is not preferable. If its content is less than 0.05 wt%, the effect of addition to improve hardenability is small, while if it exceeds 1.0 wt%, cementite in steel is stabilized and graphitization is delayed.
It was limited to the range of 0.1-1.0 wt%.

Mo: 0.05 to 0.5 wt% Mo enhances the hardenability of steel and effectively acts to increase the strength by quenching and tempering. However, on the other hand, since Mo is also an element that inhibits graphitization, it is not preferable to add a large amount.
If the content is less than 0.05 wt%, the effect of addition is small, while if it exceeds 0.5 wt%, the graphitization rate is significantly reduced, so the content was limited to the range of 0.05 to 0.5 wt%.

Nb: 0.005 to 0.05% Nb improves the hardenability of steel, and at the same time fine Nb char.
Precipitates nitrides and increases their strength by virtue of their precipitation strengthening. Further, Nb has an effect of increasing the strength of steel even if quenching and tempering is not necessarily performed only by containing it, but it is an element that hinders graphitization, so addition of a large amount is not preferable. If the content is less than 0.005 wt%, the effect of addition is poor, while if it exceeds 0.05%, graphitization is delayed, so the content was limited to the range of 0.005 to 0.05%.

Ti: 0.005 to 0.05 wt% Ti precipitates fine Ti carbon / nitride and increases the strength of steel by precipitation strengthening. In addition, this Ti has the effect of increasing the strength of the steel even if quenching and tempering is not necessarily performed by only containing it. For this purpose, addition of less than 0.005 wt% is necessary, while addition of more than 0.05 wt% delays graphitization, so the range was limited to 0.005 to 0.05 wt%.

REM: 0.001 to 0.05 wt% REM is added for the purpose of improving hot workability of steel.
It is also effective in promoting graphitization. For these effects, the use of La and Ce is particularly useful, but the content is 0.
If it is less than 001 wt%, the effect of addition is poor, while if it is added over 0.05 wt%, the effect reaches saturation. Therefore, 0.00
It was limited to the range of 1 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 a Ca-Al type oxide, which acts as a core 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 0.0002 to 0.30 wt.
The range is%.

Bi: 0.01 to 0.30 wt% Bi has a low melting point like Pb, so it is an element that is melted by 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 improves the machinability by acting as a chip breaker. 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 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 0.010 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%.

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%.

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. Regarding the metallographic structure, the structure of the present invention is required to have a structure mainly composed of graphite and ferrite, but it may be present as cementite up to about 50% of the amount of added C. Further, in the present invention, from the viewpoint of free-cutting property based on the lubricating action, it is desirable that at least 10% of graphite phase is contained in the structure.

Next, the manufacturing method of the present invention will be described. First, regarding the production of the raw material, after smelting in a conventionally known converter, electric furnace, etc., it is made into a slab or a bloom by a continuous casting method or an ingot-casting / agglomeration method. Then, after hot rolling to a predetermined shape, graphitization annealing is performed to precipitate a predetermined amount of graphite phase in the metal structure. The invention steel produced in this manner is then formed into a predetermined part shape into a machine part. The product may be subjected to a nitriding treatment.

In heating in the above hot rolling, what is particularly important is to decompose AlN and sufficiently dissolve it as Al. That is, if the coarse AlN generated during solidification remains and the amount of solid solution is small, not only does it not contribute to graphitization but also the AlN remaining during cutting damages the tool and reduces machinability. Is. In order to obtain such a sufficient solid solution amount, it is desirable that the heating temperature be 1000 ° C or higher. In addition, the cooling rate after hot rolling is
In order to prevent the re-formation of N, it is preferable to set it to 0.005 ° C / sec or more. Here, when a free-cutting element is contained, the hot workability decreases, so it is desirable to raise the heating temperature and roll at a temperature of 850 ° C. or higher.

After hot rolling in this way, graphitization is achieved by annealing only, without quenching. The annealing conditions for obtaining the graphite phase of 10% or more necessary for ensuring the machinability due to the self-lubricating action described above may be maintained at a temperature range of 600 ° C. to Ac ₁ for 5 to 30 hours. When the free-cutting element alone contains an element that inhibits graphitization, such as Te, P, Bi, or Pb, it is desirable to prolong the holding time within the above range.

As explained above, in order to improve the machinability of steel, it is not enough to design the above-mentioned composition of ingredients, but it is extremely important to control the so-called metallographic structure. It is necessary to use 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 improve the life of the cutting tool.

[0037]

EXAMPLES The present invention will be described below based on examples. (1) Steels having the chemical compositions shown in Table 1 were melted in a converter and continuously cast into blooms. Next, after billet rolling, 52 mmφ under the conditions (heating temperature, cooling rate) shown in Table 2
Of steel bar. After that, 700 ° C × 10h to these steel bars
A graphitization annealing treatment of heating and air cooling was performed. Each test of graphitization rate after this treatment, hardness, and machinability (tool life) by high speed tool steel was performed. Of these, for the machinability test, high-speed tool steel SKH4 was used, and the cutting depth was 2
mm, feed: 0.25 mm / rev., cutting speed: 70 m / min, and the tool life was evaluated as the time until cutting became impossible. Regarding the hardness, the hardness was measured after heating at 870 ° C. for 30 min, quenching, then tempering at 550 ° C. for 1 h, and then measuring the hardness. The test results obtained are shown in Table 2.

[0038]

[Table 1]

[0039]

[Table 2]

Specimen Nos. 1 to 18 are examples conforming to the present invention. In addition, the test materials Nos. 24 and 25 were obtained by adding Pb to JIS standard S53C.
Pb free cutting steel. As is clear from Tables 1 and 2, almost 100% of the C in the steel after graphitization is graphitized in all the conforming examples of the present invention, which is equal to or higher than the Pb free-cutting steel of the sample material No. 25. Has a tool life. On the other hand, in Nos. 19 to 23, which used steel materials having a composition in which the contents of Si, Al, O and N were deviated, graphitization did not proceed sufficiently, and compared with the test materials Nos. 24 and 25. Machinability is poor. Looking at the hardness after quenching and tempering, the test materials Nos. 6 to 11 and 13 to 18 containing one or more of Cu, Ni, Cr, Mo, Nb and Ti are The hardness after quenching and tempering is higher than that of the test material No. 3 having almost the same C%, which shows that it is effective when it is necessary to secure the strength after the QT treatment.

(2) Next, with regard to an example in which the composition of composition is a steel type conforming to the present invention, the heating temperature and the cooling rate are different from the conditions of the above (1), the test material No. 26 in Table 2 is shown. When manufactured under the conditions as shown in ~ 29, graphitization has not progressed sufficiently and machinability is inferior to those of the test materials Nos. 24 and 25. It can be seen that control of the heating temperature during rolling and the cooling rate after rolling is important for promoting graphitization, that is, omitting quenching for graphitization after rolling.

(3) Steels having the chemical compositions shown in Table 3 were melted in a converter, and bloomed by continuous casting. Then, after billet rolling, a steel bar having a diameter of 52 mm was produced under the conditions (heating temperature and cooling rate) shown in Table 2. Then on these steel bars,
A graphitization annealing treatment was performed in which it was heated at 700 ° C for 10 hours and then cooled in air. Each test of graphitization rate after this treatment, hardness, and machinability (tool life) by high speed tool steel was performed. Of these, for the machinability test, high-speed tool steel SKH4 was used, depth of cut: 2 mm, feed: 0.25 mm / rev., Cutting speed: 70
It was carried out under the condition of m / min, and the time until cutting became impossible was evaluated as the tool life. For hardness, 870 ℃
Hardness was measured after heating for 30 minutes and quenching, and then tempering at 550 ° C. for 1 hour. The test results obtained are shown in Table 4.

[0043]

[Table 3]

[0044]

[Table 4]

Specimen Nos. 1 to 18 are third invention examples. Third
When comparing the invention steels with Nos. 1 to 18 in Table 2 of the first and second invention steels, not only the tool life of the third invention is longer, but also the tool life of the conventional steel is higher. Has improved significantly.

[0046]

As described above, according to the present invention,
Since it is possible to obtain machinability equivalent to or better than that of the conventional Pb composite free-cutting steel without using Pb, it is possible to industrially manufacture mechanical parts without adversely affecting the environment. In addition, if free-cutting elements are added as needed, C in steel
Combined with the machinability improvement effect of graphitization, it is possible to obtain a steel for machine structural use, which is even more excellent in machinability. Further, the steel of the present invention does not necessarily require a pretreatment before graphitization annealing, so that it greatly contributes to the improvement of productivity.

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

Claims (3)

[Claims] 1. C: 0.1-1.5 wt%, Si: 0.5-2.0 wt% Mn: 0.1-2.0 wt%, Al: 0.01-0.5 wt%, N: 0.
A steel slab containing 0.01 to 0.0150 wt%, O: ≤ 0.0030 wt% and the balance being Fe and unavoidable impurities is heated to AlN solid solution temperature, hot-rolled, and then annealed. The method for producing a steel for machine structural use having excellent machinability, characterized in that the metal structure mainly comprises ferrite and graphite. 2. C: 0.1-1.5 wt%, Si: 0.5-2.0
wt%, Mn: 0.1 to 2.0 wt%, Al: 0.01 to 0.5 wt%,
N: 0.0015 to 0.0150 wt%, O: ≤ 0.0030 wt%, Ni: 0.1 to 3.0 wt%, Cu: 0.1 to 3.0 wt%, Cr:
0.05 to 1.0 wt%, Mo: 0.05 to 0.5 wt%, Nb: 0.005
~ 0.05wt%, Ti: 0.005-0.05wt%, and REM:
Contains one or more selected from 0.001 to 0.05 wt%, the balance
A steel slab composed of Fe and unavoidable impurities is heated to AlN solid solution temperature, hot-rolled, and then annealed to form a metal structure mainly composed of ferrite and graphite. A method for manufacturing machine structural steel with excellent machinability. 3. The steel composition according to claim 1, further comprising Pb: 0.03 to 0.30 wt% and 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%. A method for producing a steel for machine structural use, which is excellent in machinability, characterized in that a metal structure mainly composed of ferrite and graphite is obtained by heating to a melting temperature, hot rolling, and then annealing.