JP2001152281A - Free cutting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a free cutting steel excellent in machinability as-rolled and moreover free from any problems from the viewpoint of environmental hygiene in the producing process without using lead. SOLUTION: This steel has a componential composition containing, by weight, <0.05% C, <0.05% Si, 0.1 to 4.0% Mn, >0.15 to 0.5% S, 0.003 to 0.3% Ti, 0.0003 to <0.0040% 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 is intended to advantageously improve the tool life and 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, 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]

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 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. Here, the effect of reducing tool wear becomes particularly remarkable when the C content is reduced to less than 0.05 wt%.

2) 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, (a) B is 0.0003
(b) added in the range of not less than 0.0040 wt% and wt%
It is particularly effective to take the two measures of adding Ti in an amount exceeding 0.003 to 0.3 wt% and S in excess of 0.15 wt%.

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 generation of cracks 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.

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, which is harder than ferrite, and in which the carbides in the bainite structure have accumulated 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.

4) In addition to the above 1) and 3), the reduction of Si has a remarkable effect on the improvement of tool life. I mean,
Si is an oxide-forming element and generates SiO ₂ , but the reduction of Si causes O in the steel to combine with Mn to generate MnO. Thereby, MnS is generated on MnO, and a MnO-MnS composite inclusion is formed. Since the MnO-MnS composite inclusions are difficult to elongate by rolling and exist in a relatively spherical shape,
The tool life is improved by acting as a stress concentration source during cutting and reducing cutting resistance. Therefore, the amount of Si is 0.
By limiting the content to less than 05 wt%, MnO-MnS composite inclusions are generated and the cutting resistance during cutting is reduced,
Tool life is improved. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: less than 0.05 wt%, Si: less than 0.05 wt%, Mn: 0.1 to 4.0 wt%, S: more than 0.15 wt%, 0.5 wt% or less, Ti: 0.003 to 0.3 wt% and B: 0.0003 wt% or more, 0.0040 A free-cutting steel containing less than wt%, with the balance being Fe and inevitable impurities.

2. In the above item 1, the steel further comprises one selected from the group consisting of Cu: 2.0 wt% or less, Ni: 2.0 wt% or less, Cr: 3.0 wt% or less, Mo: 2.0 wt% or less, and Nb: 0.10 wt% or less. A free-cutting steel characterized by having a composition containing two or more types.

3. The free-cutting steel according to 1 or 2, wherein the steel further has a composition containing one or two selected from W: 0.1 wt% or less and V: 0.5 wt% or less.

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

[0015] 5. In the above 1, 2, 3, or 4, the steel further has a composition containing one or more kinds selected from Mg: 0.02 wt% or less, Hf: 0.1 wt% or less, and Al: 1.0 wt% or less. Free-cutting steel characterized by becoming.

[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: less than 0.05 wt% Si is an effective element for deoxidation. However, if the content is 0.05 wt% or more, a sufficient reduction in cutting resistance cannot be achieved, leading to a reduction in tool life. Si was limited to less than 0.05 wt%.

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. The preferred range is 0.5 to 2.5 wt%.

S: more than 0.15 wt% and not more than 0.5 wt% S combines with Mn, Ti, etc. in steel and becomes MnS, (Mn, Ti) S and TiS, and becomes a source of stress concentration during cutting. Is a useful element that facilitates cutting of chips and improves machinability. However, if the content is less than 0.15 wt%, the effect of the addition is poor. On the other hand, if the content exceeds 0.5 wt%, the hot workability is reduced, so that S exceeds 0.15 wt% and S exceeds 0.5 wt%.
It was limited to the following range. The preferred range is 0.2 to 0.4 wt.
%.

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. Other important effects of Ti addition include TiS and (Mn, Ti)
S is generated and becomes a source of stress concentration in the chips, and has an effect of improving the chip disposability. However, the content
If the amount is less than 0.003 wt%, the effect of the addition is poor.
When added in excess of 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 wt% or more and less than 0.0040 wt% B segregates around TiS and (Mn, Ti) S, and suppresses plastic deformation of TiS and (Mn, Ti) S at the time of chip formation. Thus, the generation of cracks due to stress concentration is promoted, and the chip disposability is improved. 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, while if the content is 0.004 wt% or more, the effect reaches saturation, and the cost of the component is rather increased. The range was limited to less than wt%.

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 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 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 lowers the machinability.
It was made to be contained by wt% or less. Particularly preferably 1.0 wt
% Or less.

Cr: 3.0 wt% 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, if added in excess of 3.0 wt%, not only the strength increases and the machinability decreases, but the component cost also increases. Therefore, Cr is contained at 3.0 wt% or less. In order to obtain the above effect, it is desirable that Cr be contained at least 0.5 wt%.

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.

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, at least one selected from P, Te, Se, Ca, REM, Zr, Bi, Sn, Sb and Co 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 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 wt% not only saturates the effect, but also increases the component cost.
The content was 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 may be subjected to nitriding or carburizing treatment, etc.

[0038]

EXAMPLE A steel material having the composition shown in Table 1 was melted in a converter, turned into a bloom by continuous casting, and then hot-rolled into a 150 mm square billet. Then, a heating temperature: 1200 ° C., a finishing temperature: The bar was rolled at 950 ° C. to obtain a bar of 35 mmφ. Table 2 shows the results of investigations on the hardness and machinability of the bar steel thus obtained. Here, the hardness is
Using a sample taken from a depth of 1/4 of the diameter of the steel bar, it was measured at a load of 98.07 N by a Vickers hardness tester. The machinability was measured using a high speed tool (SKH4), cutting speed: 100m / min, feed: 0.25mm / rev., Cutting depth: 2.0mm,
It evaluated by the outer periphery turning test under the condition of no lubrication. Further, the tool life judgment was evaluated based on the total cutting time until complete damage.

[0039]

[Table 1]

[0040]

[Table 2]

As shown in Table 2, all of the conforming examples according to the present invention of Nos. 1 to 16 have a tool life of 18.6 to 27.9 min.
16.5 m of No. 26 conventional lead-free non-heat treated steel (JIS SUM24L)
Very good life is obtained compared to in. Also,
Regarding the chip shape, fine chips with a length of 5 mm or less were obtained. In contrast, No. 17 ~
Of the 25 comparative steels, C is out of the proper range for No. 17 and the tool life is reduced to about half that of the inventive steel. No.18
In the case of Si, Si is out of the appropriate range, and the tool life is shortened. N
In the case of o.19, the tool life was shortened because Ti exceeded the upper limit of the present invention. In No. 20, since the Ti content was below the lower limit of the present invention, the tool life and chip shape were deteriorated.
In No. 21, the tool life was shortened because Mn exceeded the upper limit of the present invention. In No. 22, 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. 23, since S exceeded the upper limit of the present invention, hot cracking occurred during rolling, and the rolling had to be stopped. No.24 is because S is less than the lower limit of the present invention,
The tool life is shortened and the chip shape is deteriorated. No.25
Since B is less than the lower limit of the present invention, the tool life is short and the chip shape is deteriorated.

[0042]

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: less than 0.05 wt%, Mn: 0.1 to 4.0 wt%, S: more than 0.15 wt%, 0.5 wt% or less, Ti: 0.003 to 0.3 wt%, and B: 0.0003 A free-cutting steel containing at least wt% and less than 0.0040 wt%, with the balance being Fe and inevitable impurities. 2. The steel according to claim 1, wherein the steel further comprises Cu: 2.0 wt% or less, Ni: 2.0 wt% or less, Cr: 3.0 wt% or less, Mo: 2.0 wt% or less, and Nb: 0.10 wt% or less. A free-cutting steel having a composition containing one or more selected from the group consisting of: 3. The steel according to claim 1, wherein the steel further comprises one or two selected from the group consisting of W: 0.1 wt% or less and V: 0.5 wt% or less. Free cutting steel. 4. The steel according to claim 1, wherein the steel is
Furthermore, P: 0.2 wt% or less, Te: 0.2 wt% or less, Se: 0.2 wt% or less, Sn: 0.3 wt% or less, Zr: 0.2 wt% or less, Ca: 0.02 wt% or less, REM: 0.02 wt% or less, A free-cutting steel having a composition containing at least one selected from the group consisting of Bi: 0.3 wt% or less, Sb: 0.2 wt% or less, and Co: 0.1 wt% or less. 5. The steel according to claim 1, 2, 3, or 4, wherein the steel further comprises one or more selected from the group consisting of: Mg: 0.02 wt% or less, Hf: 0.1 wt% or less, and Al: 1.0 wt% or less. A free-cutting steel characterized by having a composition containing the above.