JP2001107182A - Free-cutting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide free-cutting steel excellent in machinability in a state as-rolled without using lead and furthermore free from any problem from the point of environmental hygiene in a producing process. SOLUTION: The componential composition of steel is controlled to the one containing, by weight, <0.05% C, <=2.5% Si, 0.1 to 4.0% Mn, >0.2 to 0.5% S, <0.5% Cr, 0.003 to 0.3% Ti and 0.0003 to 0.004% B, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to free-cutting steel,
In particular, the present invention aims to extend the tool life and improve the chip disposability in cutting.

[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 extremely harmful to the human body, large exhaust equipment is required not only in the process of manufacturing steel products but also in the process of manufacturing mechanical parts using it, and there were also problems in the recycling 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, in this method, heat treatment is indispensable as a pre-treatment because carbon in steel is required to be graphitized, and there is a problem that the method is not always economical.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been developed in view of the above-mentioned circumstances, and has excellent machinability as it is rolled without using lead, and has no environmental health problems and is economical. It is an object of the present invention to propose a free-cutting steel that can be manufactured in a flexible manner.

[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 level of machinability as lead-added steel as rolled without adding lead. Obtained knowledge. a) By reducing cementite in steel, tool wear is reduced and tool life is improved. Here, the effect of reducing tool wear becomes particularly significant when the C content is reduced to less than 0.05 wt%.

B) When the C content is reduced to less than 0.05% by weight,
On the other hand, the generated chips are difficult to break, and the chip processing performance is reduced. To solve this, (1) B is 0.0003
0.004 wt% and (2) 0.003 to 0.3 wt% Ti
It is particularly effective to take the two measures of simultaneously adding more than 0.2% by weight of S and more than 0.2% by weight.

The reason is as follows. The addition of Ti and S produces TiS and (Mn, Ti) S in the steel, which acts as a stress concentration source in the chips generated during cutting. At this time, by adding an appropriate amount of B, B segregates around TiS and (Mn, Ti) S, and TiS
In addition, plastic deformation of (Mn, Ti) S is suppressed, and crack generation due to stress concentration is promoted. Further, B has a property of easily segregating on dislocations in the structure, and TiS and (M
The stress concentration on (n, Ti) S causes segregation on dislocations generated in the surrounding matrix, embrittles the matrix and facilitates propagation of the generated cracks. 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 wt%, fine chips having a chip length of 5 mm or less are generated, The chip controllability is significantly improved.

C) In addition to a), to improve the tool life,
The addition of appropriate amounts of Mn, Cr and B is effective. The reason is that the addition of Mn, Cr, and B results in the formation of a bainite structure in the structure. The bainite structure is harder than ferrite, and the carbides in the bainite structure form cementite on a flat plate having a random orientation. Due to the integrated structure, it is difficult to deform by 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, so chip generation is easy This is 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.

D) In addition to a) and c), it is necessary to regulate the amount of Cr added to improve the tool life. Cr improves machinability by improving hardenability and forming a bainite structure.On the other hand, Cr has the property of forming a solid solution in cementite and increasing the hardness of cementite. Depending on the amount, tool wear at the time of cutting is promoted and the tool life is shortened. In particular, when the C content is as small as less than 0.05 wt%, the amount of cementite is also small, so that the addition of Cr tends to increase the Cr concentration in the cementite, and the hardness of cementite is significantly increased. In this regard, if the Cr content is less than 0.5 wt%, the increase in hardness of cementite is suppressed, tool wear is reduced, and tool life is improved.

The present invention is based on the above findings. That is, the gist configuration of the present invention is as follows. 1. C: less than 0.05 wt%, Si: 2.5 wt% or less, Mn: 0.1 to
4.0 wt%, S: more than 0.2 to 0.5 wt%, Cr: less than 0.5 wt%,
Free-cutting steel containing 0.003 to 0.3 wt% of Ti and 0.0003 to 0.004 wt% of B, with the balance being Fe and unavoidable impurities.

2. In the above item 1, further Cu: 2.0 wt%
Hereinafter, Ni: 2.0 wt% or less, Mo: 2.0 wt% or less, and Nb:
A free-cutting steel comprising one or more selected from 0.10 wt% or less.

3. In the above 1 or 2, further, W:
Free cutting steel characterized by containing one or two kinds selected from 0.1 wt% or less and V: 0.5 wt% or less.

4. In the above 1, 2 or 3, P: 0.2 wt% or less, Te: 0.2 wt% or less, Se: 0.2 wt% or less, Ca: 0.02 wt% or less, REM: 0.02 wt% or less, Zr: 0.2 w
t% or less, Bi: 0.3 wt% or less, Sn: 0.3 wt% or less, Sb:
A free-cutting steel containing at least one selected from the group consisting of 0.2 wt% or less and Co: 0.1 wt% or less.

[0015] 5. In the above 1, 2, 3 or 4, further, Al: 1.0 wt% or less, Mg: 0.02 wt% or less, and Hf: 0.
A free-cutting steel characterized by containing one or more selected from 1 wt% or less.

[0016]

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 wt% C is added for securing strength.
Addition of wt% or more increases tool wear during cutting,
C was limited to less than 0.05% by weight because of poor machinability.
In addition, it is preferably 0.04 wt% or less.

Si: 2.5 wt% or less Si is an element effective for deoxidation, but if added in excess of 2.5 wt%, the machinability is reduced.

Mn: 0.1 to 4.0 wt% Mn has the function of improving hardenability, promoting formation of a bainite structure, and improving machinability. It is also effective in securing strength. Further, it has an effect of forming MnS by combining with S, thereby improving machinability. In order to obtain these effects, it is necessary to contain at least 0.1 wt%, but if it exceeds 4.0 wt%, the strength increases and the machinability decreases, so that Mn falls within the range of 0.1 to 4.0 wt%. Limited.

S: more than 0.2 to 0.5 wt% S combines with Mn, Ti, etc. in steel to form MnS, (Mn, Ti) S and TiS, which are sources of stress concentration during cutting, and Is a useful element that facilitates the separation of the steel and improves machinability. However, if the content is less than 0.2 wt%, the effect of the addition is poor, while if it exceeds 0.5 wt%, the hot workability is reduced. Therefore, S is limited to the range of more than 0.2 to 0.5 wt%.

Cr: less than 0.5 wt% Cr has the function of improving hardenability and accelerating the formation of a bainite structure to improve machinability. It forms a solid solution in cementite and increases the hardness of cementite to reduce machinability. Therefore,
Cr was limited to less than 0.5 wt%. Incidentally, preferably 0.4 wt
% Or less.

Ti: 0.003 to 0.3 wt% Ti has the property of fixing N in steel as TiN. Utilizing this property, it is possible to prevent B, which is effective for hardenability, from being combined with N to form BN, thereby preventing the effect of improving hardenability from being lost. In addition, TiS and (Mn, T
i) S is generated and becomes a source of stress concentration in chips, and has an effect of improving chip processing. However, if the content is less than 0.003 wt%, the effect of the addition is poor.
If added in excess of 0.3 wt%, coarse TiN precipitates, increasing tool wear during cutting and reducing machinability, so the Ti content was limited to the range of 0.003-0.3 wt%. In addition, a preferable range is 0.01 to 0.1 wt%.

B: 0.0003 to 0.004 wt% B segregates around TiS and (Mn, Ti) S, suppresses plastic deformation of TiS and (Mn, Ti) S when chips are generated, and concentrates stress. Promotes the generation of cracks and improves chip disposal. In addition, it is positively added to improve hardenability, generate a bainite structure, and improve tool life. However, if the content is less than 0.0003 wt%, the effect is small. On the other hand, if the content exceeds 0.004 wt%, the component cost increases, so that B is 0.0003 to 0.004%.
Limited to the wt% range.

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 above-described basic components. First, in order to improve hardenability, generate a bainite structure, improve machinability, and increase strength, one or two selected from Cu, Ni, Mo, and Nb are selected.
More than one species can be added.

Cu: 2.0 wt% or less Cu can be added to improve hardenability, improve machinability due to formation of bainite structure, and secure strength. However, if the content exceeds 2.0 wt%,
Since the strength is increased and the machinability is decreased, and the cost is increased, Cu is contained at 2.0 wt% or less.
Particularly preferably, the content is 1.0% by weight or less.

Ni: 2.0 wt% or less Ni can be added for improving machinability and improving strength by forming bainite structure by improving hardenability. However, excessive addition not only increases the cost, but also increases the strength and lowers the machinability.
It was made to be contained by wt% or less. Particularly preferably 1.0 wt
% Or less.

Mo: 2.0 wt% or less Mo can be added to improve the machinability by forming a bainite structure by improving the hardenability and to secure the strength. However, excessive addition not only increases the cost, but also increases the strength and reduces the machinability.
% Or less. 1.0 wt% is particularly preferred.
It is as follows.

Nb: 0.10 wt% or less Nb can be added to improve hardenability, improve machinability due to formation of bainite structure, and secure strength. However, if added excessively, not only does the component cost increase, but also the strength increases and the machinability deteriorates. Therefore, Nb should be contained at 0.10 wt% or less.

Next, in order to improve the strength, W and V
One or two selected from the above can be added. W: 0.1 wt% or less W has an effect of improving the strength by solid solution, but if added in excess of 0.1 wt%, the machinability decreases, so W is 0.1 wt%.
It was made to be contained below.

V: 0.5 wt% or less V is a useful element for improving the strength by precipitation strengthening by V (C, N). However, when added in excess of 0.5 wt%, the machinability is reduced. The content was 0.5 wt% or less.

Further, in order to further improve the machinability, P, Te, Se, Ca, REM, Zr, Pb, Bi, Sn, Sb and Co
At least one selected from the above can be contained. P: 0.2 wt% or less P has the effect of remarkably improving the chip disposability by facilitating the propagation of cracks in the generated chips, but when added in excess of 0.2 wt%, hot working is performed. Therefore, the amount of P is limited to 0.2 wt% or less, because it deteriorates the properties.

Te: 0.2 wt% or less, Se: 0.2 wt% or less Te and Se respectively combine with Mn to form MnTe and MnSe, which act as a chip breaker to improve machinability. However, adding more than 0.2 wt% saturates the effect and increases the cost of the components. Therefore, the content of each is set to 0.2 wt% or less.

Ca: 0.02 wt% or less, REM: 0.02 wt% or less,
Zr: 0.2 wt% or less Each of Ca, REM and Zr forms a sulfide together with MnS and acts as a chip breaker to improve machinability. However, Ca: 0.02wt%, RE
Addition of more than M: 0.02 wt% and Zr: 0.2 wt% saturates the effect and raises the cost of the components. Therefore, both of them are contained in the above ranges.

Bi: 0.3 wt% or less Bi, like Pb, improves the machinability by the action of melting, lubrication and embrittlement during cutting, and can be added for this purpose. However, adding more than 0.3 wt% not only saturates the effect, but also increases the component cost. Therefore, Bi is contained at 0.3 wt% or less.

Sn: 0.3 wt% or less, Sb: 0.2 wt% or less, C
o: 0.1 wt% or less Sn, Sb and Co are all elements that improve machinability by embrittlement. However, Sn: 0.3 wt%, S
b: Addition exceeding 0.2 wt% and Co: 0.1 wt%
Since the effects are saturated, the cost is increased, and it is economically disadvantageous, all of them are contained in the above range.

Al: 1.0 wt% or less, Mg: 0.02 wt% or less, H
f: 0.1 wt% or less Al, Mg and Hf not only have a deoxidizing effect but also have an effect of improving the machinability as a source of stress concentration, so that they can be appropriately contained as necessary. . However, if the addition is excessive, the effect is saturated and the component cost increases. Therefore, the addition amount is limited to the above range.

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 wt% 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 a nitriding or carburizing treatment or the like.

[0038]

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 10 kgf 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 evaluated by a peripheral turning test using a high speed tool (SKH4) under the conditions of cutting speed: 100 m / min, feed: 0.25 mm / rev, depth of cut: 2.0 mm, and no lubrication. further,
The tool life judgment was evaluated based on the total cutting time until complete damage.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

As shown in Table 3, all of the steels according to the present invention No. 1 to No. 16 had a tool life of 19.9 to 28.2 min.
And No. 29 of the conventional lead-added non-heat treated steel (JIS SUM 24L)
Compared to 6.5 min, a very good life is obtained. Regarding the chip shape, the length is 5mm
The following fine and fine chips are obtained. On the other hand, among the comparative steels of No. 17 to No. 28, C is out of the proper range in No. 17 and the tool life is reduced to less than half compared to the inventive steel. In No. 18, Si was out of the proper range, and the tool life was shortened. In No. 19, the tool life and chip shape were deteriorated because Ti was less than the lower limit of the present invention.
In No. 20, the tool life was shortened because Ti exceeded the upper limit of the present invention. In No. 21, the tool life was shortened because Mn exceeded the upper limit of the present invention. In No. 22, since Mn was below the lower limit of the present invention, the tool life was shortened and the chip shape was poor. In No. 23, since S was less than the lower limit of the present invention, the tool life was shortened and the chip shape was poor. In No. 24, since S exceeded the upper limit of the present invention, hot cracking occurred during rolling, and rolling was forced to be stopped. No. 25 has a shorter tool life because Cr exceeds the upper limit of the present invention. In No. 26, since B is less than the lower limit of the present invention, the tool life is short and the chip shape is poor.

[0043]

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.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Kenichi Amano 1-chome, Kawasaki-dori, Mizushima, Kurashiki-shi, Okayama Pref.

Claims (5)

[Claims] 1. C: less than 0.05 wt%, Si: 2.5 wt% or less, Mn: 0.1 to 4.0 wt%, S: more than 0.2 to 0.5 wt%, Cr: less than 0.5 wt%, Ti: 0.003 to 0.3 wt% And B: a free-cutting steel containing 0.0003 to 0.004 wt%, with the balance being Fe and inevitable impurities. 2. The method according to claim 1, further comprising one or more selected from Cu: 2.0 wt% or less, Ni: 2.0 wt% or less, Mo: 2.0 wt% or less, and Nb: 0.10 wt% or less. Free-cutting steel characterized by containing. 3. The method according to claim 1, further comprising:
Free cutting steel characterized by containing one or two kinds selected from 0.1 wt% or less and V: 0.5 wt% or less. 4. The method according to claim 1, wherein P: 0.2 wt% or less, Te: 0.2 wt% or less, Se: 0.2 wt% or less, Ca: 0.02 wt% or less, REM: 0.02 wt% or less, Zr: 0.2 wt% or less, Bi: 0.3 wt% or less, Sn: 0.3 wt% or less, Sb: 0.2 wt% or less, Co: 0.1 wt% or less Free cutting steel. 5. The method according to claim 1, 2, 3 or 4, further comprising one or more selected from among Al: 1.0 wt% or less, Mg: 0.02 wt% or less, and Hf: 0.1 wt% or less. Free-cutting steel characterized by the following.