JP2002003991A - Free cutting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide free cutting steel excellent in machinability in an as-rolled state without addition of lead. SOLUTION: The chemical composition of the steel is regulated so that it consists of, by mass, <0.05% C, <=2.5% Si, 0.10-4.0% Mn, >0.20-0.8% S, >0.004-<0.08% B, >0.008-0.050% O, <0.050% N and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to free-cutting steel,
Particularly, in a cutting process, an attempt is made to advantageously improve machinability such as tool life and chip disposability.

[0002]

2. Description of the Related Art Conventionally, free-cutting steels having excellent machinability, such as tool life and chip disposability during cutting, include sulfur free-cutting steel and lead free-cutting steel specified in JIS, and other materials. Various steel materials such as calcium free cutting steel, tellurium free cutting steel, selenium free cutting steel, and bismuth free cutting steel have been developed.

[0003] Above all, lead free-cutting steels are widely used as free-cutting steels because they have excellent machinability and are more economical than tellurium or bismuth. However, since lead is harmful to the human body, there were problems not only in the manufacturing process of steel products but also in the manufacturing process of mechanical parts using it, requiring environmental measures and limiting the recycling destination of steel products. . For this reason, conventionally, there has been a demand for the development of a free-cutting steel having the same machinability as lead-added steel without adding lead.

In order to meet the above demand, for example, Japanese Patent Application Laid-Open No. 50-96416 discloses a method in which carbon in steel is present as graphite, and lead is used by utilizing the notch lubrication effect of the graphite. There has been proposed a method for improving machinability without the use of such a material. However, this method has a problem that it is not necessarily an economical method because it is necessary to graphitize C in steel and heat treatment is indispensable as a pretreatment thereof.

[0005]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above circumstances, and has as its object to propose a free-cutting steel which is excellent in machinability without using lead and as-rolled. .

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on the composition of a steel material having the same machinability as lead-added steel as rolled without adding lead. I got 1) By reducing cementite in steel, tool wear is reduced and tool life is improved. This effect is particularly remarkable when the C content is reduced to less than 0.05 mass%.

2) If the C content is reduced to less than 0.05 mass%, on the other hand, the generated chips are difficult to break, and the chip disposability deteriorates. To solve this, (a) B is more than 0.004 mass%, less than 0.08 mass%, and N is 0.050 mass%.
(b) Mn is 0.10 mass% or more, S is more than 0.20 mass% and O is added.
It is particularly effective to take the two measures of adding more than 0.008 mass% at the same time.

The reason is as follows. By adjusting the composition within the composition range specified in the present invention, O in steel
Combines with Mn to produce MnO. As a result, MnS on MnO
Are generated, and a MnO-MnS composite inclusion is formed. MnO-
Since the MnS composite inclusions are hardly elongated by rolling and exist in a relatively spherical shape, they act as a stress concentration source during cutting. At this time, by adding B in the above range, B segregates at the interface between the MnO-MnS composite inclusion and the matrix, and further suppresses plastic deformation of the MnO-MnS composite inclusion, It promotes the formation of cracks due to stress concentration.
In addition, B and N have a property of easily segregating on dislocations in the structure, and segregate on dislocations generated in the surrounding matrix by stress concentration on the MnO-MnS composite inclusions, thereby embrittle the matrix. Facilitates the propagation of the generated cracks. Further, BN generated by the combination of B and N has a lubricating action and acts as a stress concentration source, as is well known, thereby effectively contributing to an improvement in tool life and an improvement in chip breakage. As a result of these actions, the breakability of the chips is significantly improved. As a result, even in a low C steel having a C content of less than 0.05 mass%, fine chips having a chip length of 5 mm or less are generated, The chip controllability is significantly improved.

3) In addition to the above 1), addition of appropriate amounts of Mn and B is effective for improving the tool life. The reason is that the addition of Mn and B forms a bainite structure in the structure. The bainite structure is harder than ferrite, and the carbides in the bainite structure accumulate cementite on a flat plate having a random orientation. Due to the structure, it is difficult to deform itself, so it becomes a source of stress concentration during cutting, and when stress concentration occurs, it has a notch effect on the surrounding ferrite, making chip generation easy Because. When the bainite structure is mixed in the structure, the cutting resistance during cutting is reduced and the tool life is improved by the above-described action. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. By mass percentage, C: less than 0.05%, Si: 2.5% or less, Mn: 0.10% or more, 4.
0% or less, S: more than 0.20%, 0.8% or less, B: 0.004%
A free-cutting steel characterized by containing more than 0.08%, O: more than 0.008%, 0.050% or less and N: less than 0.050%, with the balance being Fe and inevitable impurities.

2. In the above item 1, the composition contains one or more selected from among Cu ≦ 2.0%, Ni ≦ 2.0%, Cr ≦ 3.0%, Mo ≦ 2.0% and Nb ≦ 0.10% by mass percentage. Free-cutting steel characterized by the following.

3. A free-cutting steel according to the above item 1 or 2, characterized in that the composition further comprises one or two selected from W ≦ 0.1% and V ≦ 0.5% by mass percentage.

4. In the above 1, 2 or 3, P ≦ 0.2%, Te ≦ 0.2%, Se ≦ 0.2%, Ca ≦ 0.02%, RE
M ≤ 0.02%, Zr ≤ 0.2%, Bi ≤ 0.3%, Sn ≤ 0.3%, Sb
A free-cutting steel having a composition containing one or more selected from among ≦ 0.2%, Co ≦ 0.1% and Ti ≦ 0.3%.

5. In the above 1, 2, 3 or 4, the composition is characterized in that the composition contains one or more selected from among Mg ≦ 0.02%, Hf ≦ 0.1% and Al ≦ 1.0% by mass percentage. Free cutting steel.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the reason why the composition of a steel material in the present invention is limited to the above range will be described. C: less than 0.05 mass% (hereinafter simply referred to as%) C is added to ensure strength. However, 0.05
%, The wear of the tool during cutting increases and the machinability decreases. Therefore, C was limited to less than 0.05%. In addition, it is preferably 0.04% or less.

Si: 2.5% or less Si is effective in securing strength by solid solution strengthening and is also effective as a deoxidizing element. However, if added in excess of 2.5%, the tool life is shortened, so that 2.5% or less. Restricted to. Preferably it is 0.6% or less.

Mn: 0.10% or more and 4.0% or less Mn has a function of improving hardenability, promoting formation of a bainite structure, and improving machinability. It is also effective in securing strength. Further, it has the effect of forming MnO-MnS composite inclusions by combining with S and MnS or with O and S, thereby improving machinability. To obtain these effects, the content of at least 0.10% is necessary. However, if it exceeds 4.0%, the strength increases and the machinability decreases, so Mn was limited to the range of 0.10 to 4.0%. It is more preferably in the range of 0.5 to 2.5%.

S: more than 0.20%, not more than 0.8% S combines with Mn in steel and becomes MnS, which is a source of stress concentration during cutting, facilitates cutting of chips, and improves machinability. It is a useful element. However, the content is 0.20
%, The effect of the addition is poor. On the other hand, if it exceeds 0.8%, the hot workability is reduced.
Exceeded to a range of 0.8% or less.

B: more than 0.004% and less than 0.08% B segregates at the interface between the MnO—MnS composite inclusion and the matrix,
The present invention suppresses plastic deformation of the MnO-MnS composite inclusion during chip generation, promotes generation of cracks due to stress concentration, and improves chip processing. Further, BN generated by bonding with N has a lubricating action and an action as a stress concentration source, so that the tool life and the chip breaking property are improved. Further, since it has an effect of improving hardenability, generating a bainite structure and improving tool life, it is positively added. However, when the content is 0.004% or less, the effect is small. On the other hand, when the content is 0.08% or more, the effect reaches saturation and rather increases the component cost.
It is limited to the range of more than 0.004% and less than 0.08%. In addition, it is preferably 0.015% or less.

O: more than 0.008%, not more than 0.050% O combines with Mn to form MnO. Thereby, MnS is generated on MnO, and a MnO-MnS composite inclusion is formed.
Since the MnO-MnS composite inclusion is hard to be elongated by rolling and exists in a relatively spherical shape, it acts as a stress concentration source during cutting. For this reason, O is added positively, but O.
At 008% or less, the effect is small, while 0.050%
If O is added in excess of 0%, internal defects occur in the steel ingot, so O was limited to a range of more than 0.008% and 0.050% or less. Incidentally, the content is preferably 0.030% or less.

N: less than 0.050% N tends to segregate on dislocations in the structure, and segregates on dislocations formed in the surrounding matrix due to stress concentration on the MnO-MnS composite inclusion during cutting. In this way, the base material is embrittled to facilitate propagation of generated cracks, thereby improving chip breakability. Further, BN is formed by bonding with B, and is positively added because it has a function of improving tool life and chip breaking property by its lubricating action and stress concentration action. However, when the content is 0.05% or more, internal defects and surface defects of the steel ingot occur, so N was limited to less than 0.05%.

Although the basic components have been described above, the present invention can further improve machinability and strength by adding the following components in addition to the basic components described above. First, one or more selected from Cu, Ni, Cr, Mo and Nb to improve hardenability, generate a bainite structure to improve machinability, and increase strength. Can be added.

Cu: 2.0% or less Cu can be added to improve the hardenability, to improve the machinability due to the formation of bainite structure, and to secure the strength. However, if the content exceeds 2.0%, the strength is increased, the machinability is reduced, and the cost is increased. Therefore, Cu is contained at 2.0% or less. Especially preferably, it is 1.0% or less.

Ni: 2.0% or less Ni can be added in order to improve machinability by forming a bainite structure by improving hardenability and to secure strength. However, excessive addition not only increases the cost, but also increases the strength and lowers the machinability.
% Or less. Preferably 1.0%
It is as follows.

Cr: 3.0% or less Cr is a useful element that promotes the formation of a bainite structure by improving hardenability, and thereby improves machinability and strength. However, when added above 3.0%,
Since not only the strength is increased and the machinability is reduced, but also the component cost is increased, the content of Cr is set to 3.0% or less. Incidentally, the content is preferably 1.5% or less.

Mo: 2.0% or less Mo can be added in order to improve machinability by forming a bainite structure by improving hardenability and secure strength. However, excessive addition not only increases the cost, but also increases the strength and reduces the machinability.
It was made to be contained below. It is more preferably at most 1.0%.

Nb: 0.10% or less Nb can be added in order to improve hardenability, improve machinability due to formation of bainite structure, and secure strength. However, if added in excess, not only increases the component cost, but also increases the strength and lowers the machinability. Therefore, Nb is contained at 0.10% or less.

In order to improve the strength, W and V
One or two selected from the above can be added. W: 0.1% or less W has an effect of improving the strength by solid solution, but if added in excess of 0.1%, the machinability is reduced. Therefore, W is contained at 0.1% or less.

V: not more than 0.5% V is a useful element for improving the strength by precipitation strengthening by V (C, N). However, when added in excess of 0.5%, the machinability is reduced. It was made to be contained below.

Further, in order to further improve the machinability, P, Te, Se, Ca, REM, Zr, Bi, Sn, Sb, Co and Ti
One or two or more selected from the above can be contained. P: 0.2% or less P has the effect of significantly improving the chip disposability by facilitating the propagation of cracks in the generated chips, but when added in excess of 0.2%, the hot workability is reduced. To reduce the amount, the P content was limited to 0.2% or less.

Te: 0.2% or less, Se: 0.2% or less Te and Se combine with Mn to form MnTe and MnSe, which act as chip breakers to improve machinability. However, the addition of more than 0.2% saturates the effect and increases the cost of the components.

Ca: 0.02% or less, REM: 0.02% or less, Zr:
0.2% or less All of Ca, REM and Zr form sulfides with MnS, which improves the machinability by acting as a chip breaker. However, Ca: 0.02%, REM:
Addition of more than 0.02% and Zr: 0.2% saturates the effect and increases the cost of the components. Therefore, both of them are contained in the above ranges.

Bi: 0.3% or less Bi can be added for this purpose because it improves the machinability by the action of melting, lubrication and embrittlement during cutting. However, adding more than 0.3% not only saturates the effect, but also increases the component cost.
% Or less.

Sn: 0.3% or less, Sb: 0.2% or less, Co: 0.
1% or less Sn, Sb and Co are all elements that improve machinability by embrittlement. However, Sn: 0.3%, Sb:
The addition of more than 0.2% and Co: 0.1% saturates the effect, increases the cost and is disadvantageous economically.
All were contained in the above range.

Ti: not more than 0.3% Ti generates TiS and (Mn, Ti) S and becomes a source of stress concentration in chips, and has an effect of improving chip disposability.
However, if added in excess of 0.3%, coarse TiN precipitates, increasing tool wear during cutting and reducing machinability. Therefore, Ti was limited to a range of 0.3% or less. In addition, it is preferably 0.01% or less.

In addition, the following elements can be added. Mg: 0.02% or less and Hf: 0.1% or less Mg and Hf are not only deoxidizing elements, but also have the effect of improving the machinability as a stress concentration source. Can be. However, if added excessively, the effect is saturated and the component cost is increased. Therefore, the added amount is limited to the above range.

Al: 1.0% or less Al is an element effective for securing strength by solid solution strengthening and deoxidizing. Al ₂ O ₃ which is an oxide acts as a stress concentration source during cutting, and is not affected. It also has the effect of improving machinability. On the other hand, since the oxide is hard, it also has the effect of promoting tool wear. Therefore, it can be added when utilizing the effects of solid solution strengthening and stress concentration. However, if the addition exceeds 1.0%, not only the effect of promoting hard tool wear by the hard oxide becomes remarkable, but also the cost increases, so the content was limited to 1.0% or less. Incidentally, preferably 0.
Less than 02%.

In the present invention, Pb is basically not added for the purpose thereof, but this does not mean that Pb cannot be technically added. That is, as long as it is only necessary to consider only the free-cutting property, it does not hinder the addition. However, even in this case, the amount of addition is preferably suppressed to about 0.2% or less from the viewpoint of environmental hygiene.

Next, preferred conditions for producing the steel of the present invention will be described. First, in the production of a raw material, after smelting in a conventionally known converter or electric furnace or the like, a slab or a bloom is formed by a continuous casting method or an ingot-bulking method. Next, a predetermined shape is formed by hot rolling according to a conventional method. Then, after forming into a predetermined part shape, it is made into a mechanical part. In some cases, the product is subjected to nitriding or carburizing treatment to obtain a product.

[0040]

EXAMPLE A steel material having the composition shown in Tables 1 and 2 was melted in a converter and made into a bloom by continuous casting.
hot rolled into m-square billet, then heating temperature: 1200 ℃
Finishing temperature: 35mmφ by bar rolling at 950 ° C
Steel bar. Table 3 shows the results obtained by examining the hardness and machinability of the bar steel thus obtained. Here, the hardness was measured at a load of 98.07 N by a Vickers hardness tester using a sample taken from a depth of 1/4 of the diameter of the steel bar. The machinability was measured using a high speed tool (SKH4), cutting speed: 100m / min, feed: 0.25mm / rev., Cutting depth: 2.0m
The evaluation was performed by a peripheral turning test under the condition of m and no lubrication. Further, the tool life judgment was evaluated based on the total cutting time until complete damage.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

As shown in Table 3, the invention examples of Nos. 1 to 16 all had a tool life of 26.8 to 32.9 min and 16.5 min of the No. 27 conventional lead-added non-heat treated steel (JIS SUM24L). It is much improved compared to. Regarding the shape of the chips, fine chips having a length of 5 mm or less were obtained. On the other hand, among the comparative examples of No. 17 to No. 26, N
In o.17, since B is less than the lower limit of the present invention, the chip shape and tool life are deteriorated. In No. 18, the tool life was shortened because Si exceeded the upper limit of the present invention.
In No. 19, since C exceeded the upper limit of the present invention, the tool life was reduced to less than half as compared with the invention example. No.20 is
Since Mn is less than the lower limit of the present invention, the tool life is shortened, and the chip shape is also deteriorated. In No. 21, the tool life was shortened because Mn exceeded the upper limit of the present invention. In No. 22, since S was less than the lower limit of the present invention, the tool life was shortened and the chip shape was also poor. No. 23 could not be evaluated because S exceeded the upper limit of the present invention, so that hot cracking occurred during rolling and rolling had to be stopped. In No. 24, since N is higher than the upper limit of the present invention,
An internal defect occurred in the steel ingot and could not be evaluated. In No. 25, since O is less than the lower limit of the present invention, the tool life is short, and the chip shape is also deteriorated. In No. 26, since O exceeded the upper limit of the present invention, internal defects occurred in the steel ingot, and the steel ingot could not be evaluated.

[0045]

As described above, according to the present invention, a steel material having machinability equal to or higher than that of a lead-added free-cutting steel can be obtained in an as-rolled state at a low cost without adding lead.

Continuing on the front page (72) Inventor Kenichi Amano 1-chome, Kawasaki-dori, Mizushima, Kurashiki-shi, Okayama Pref. Kawasaki Steel Corporation Mizushima Steel Works

Claims (5)

[Claims] (1) In terms of mass percentage, C: less than 0.05%, Si: 2.
5% or less, Mn: 0.10% or more, 4.0% or less, S: 0.20%
Over, 0.8% or less, B: Over 0.004%, less than 0.08%, O: 0.
A free-cutting steel containing more than 008%, less than 0.050% and N: less than 0.050%, with the balance being Fe and inevitable impurities. 2. The method according to claim 1, wherein one or more selected from the group consisting of Cu ≦ 2.0%, Ni ≦ 2.0%, Cr ≦ 3.0%, Mo ≦ 2.0% and Nb ≦ 0.10% by mass percentage. A free-cutting steel characterized by having a composition containing: 3. A free-cutting steel according to claim 1, wherein the composition further comprises one or two selected from W ≦ 0.1% and V ≦ 0.5% by mass percentage. . 4. The method according to claim 1, wherein, in terms of mass percentage, P ≦ 0.2%, Te ≦ 0.2%, Se ≦ 0.2%, Ca ≦ 0.02%, RE
M ≤ 0.02%, Zr ≤ 0.2%, Bi ≤ 0.3%, Sn ≤ 0.3%, Sb
A free-cutting steel having a composition containing one or more selected from among ≦ 0.2%, Co ≦ 0.1% and Ti ≦ 0.3%. 5. The composition according to claim 1, wherein the composition further comprises one or more selected from Mg ≦ 0.02%, Hf ≦ 0.1% and Al ≦ 1.0% by mass percentage. Free-cutting steel characterized by becoming.