JPH07173574A - Low carbon sulfur-based free cutting steel excellent in machinability - Google Patents

Abstract

PURPOSE:To produce a low carbon sulfur-based free cutting steel excellent in machinability and the homogeneity of the material. CONSTITUTION:This low carbon sulfur-based free cutting steel is a one having a compsn. contg., as fundamental conpopnents, by mass, 0.05 to O.15% C, 0.5 to 2.0% Mn, 0.1 to 0.4% S, 0.05 to 0.10% P and 0.005 to 0.040% O, in which the content of Si is limited to <=0.1% and Al to <=0.009%, contg. N in the range of 20 to l50ppm, and the balance substantial Fe, and in which, among inclusions of one or more kinds among Al2O3, SiOz2and MnO and/or inclusions in which >= two kinds among Al2O3, SiO2 and Mn0 are bonded, the diameter of the inclusions including one or more kinds of oxides by >= 50% in total by mass is all regulated to <53mum, or among the inclusions, the number of the ones having >=53mum diameter is regulated to <=100 pieces per kg steel. Moreover, Pb, Bi and Te are incorporated therein by 0.01 to 0.40wt.% in total.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-carbon sulfur-based free-cutting steel excellent in machinability.

[0002]

2. Description of the Related Art Conventionally, the following methods have been proposed as a method for improving machinability when a sulfur-based free-cutting steel is manufactured by a continuous casting method or the like. In Japanese Patent Publication No. 59-19182,
% [S] /% [C]% when manufacturing by continuous casting method
A method has been proposed in which the addition of a deoxidizing agent such as Al or Si, which does not affect the machinability and which produces a deoxidizing product harmful to machinability, or degassing treatment is adopted, by limiting the [O] ratio to suppress the generation of blowholes. There is.

In Japanese Patent Application Laid-Open No. 59-205453, MnS in which Te, Pb and Bi are added to S in combination and the minor axis and major axis have a certain value or more and the major axis / minor axis ratio is 5 or less.
Has been proposed for a free-cutting steel manufacturing method that accounts for 50% or more of all MnS. Japanese Patent Laid-Open No. 62-23970 discloses C-Mn-P-S-Pb, which is a sulfur-lead free-cutting steel produced by a continuous casting method.
It proposes a free-cutting steel that defines the concentration range of O, Si, and Al, and defines the average size of Mn sulfide-based inclusions and the proportion of sulfide-based inclusions that are not bound to oxides.

According to the experience of the present inventors, the minor axis, major axis and major axis / minor axis ratio of MnS, the average size of Mn sulfide inclusions, and the ratio of sulfide inclusions that are not bonded to oxides are specified. The machinability is not good even if the conditions described in JP-A-59-205453 and JP-A-62-23970 are satisfied, or conversely, the machinability is good even if the conditions are not satisfied. There was a case.

The inventions disclosed in Japanese Patent Laid-Open Nos. 62-207547 and 62-207548 limit the specific water content in the continuous casting method or heat and heat the slab in a continuous casting machine. In order to reduce the cooling rate of the slab, MnS
We are trying to improve machinability by increasing the size of the grains. Further, in Japanese Patent Application Laid-Open No. 2-155548, the machinability is improved by controlling the heating rate of the molten steel in the tundish during continuous casting to 10 ° C. or higher and controlling the cooling rate at a specific position in the cross section of the slab to a specific value or less. The method is shown.

JP-A-62-207547 and JP-A-62.
The inventions disclosed in JP-A-207548 and JP-A-2-155548 cannot install a heat retaining zone or a heating zone due to facility restrictions in an existing continuous casting machine, and warming / cooling promotes bulging depending on the shape of a slab. It may not be applicable because it may cause internal cracking.

[0007]

When low-carbon sulfur-based free-cutting steel is produced by the continuous casting method, it is easy to obtain uniform steel characteristics including machinability because the concentration of each component is uniform. Generally, the cross-sectional size of the slab is smaller than that of the steel ingot, and the size of the MnS-based inclusions is reduced due to this, resulting in a decrease in machinability. The smaller the cross-sectional size of the slab, the more difficult it is to secure machinability.

The present invention provides a free-cutting steel excellent in machinability even when it is intended to reduce variations in steel properties such as machinability within a lot by a continuous casting method or the like.

[0009]

[Means for Solving the Problems] The present inventors investigated the relationship between the amount of large oxide inclusions (hereinafter referred to as slime inclusions) extracted by the slime method and machinability, It was found that the amount of slime inclusions affects the machinability such as surface roughness and drill life, and the machinability deteriorates when the amount of inclusions increases. Further, in addition to Al ₂ O ₃ based and SiO ₂ based as has been conventionally pointed out, large oxides of Al ₂ O ₃ —SiO ₂ based, Al ₂ O ₃ —MnO based and MnO—SiO ₂ based are available. It has also been found that complex large inclusions are also detrimental to machinability.

Machinability of such large oxide inclusions
Conventional free-cutting steel based on the knowledge that
Improvements have been made to improve the finished surface roughness of the cutting surface and drill life.
Has developed free-cutting steel with excellent machinability. That is, continuous casting method
Included in steel in low-carbon sulfur-based free-cutting steel manufactured by
Limiting the amount of large oxide inclusions harmful to machinability.
Suppresses tool wear, prolongs tool life and increases tool wear.
Trying to prevent an increase in the roughness of the finished surface due to wear
The main points are as follows.
It (1) C: 0.05 to 0.15% by mass%, Mn: 0.
5 to 2.0%, S: 0.1 to 0.4%, P: 0.05 to
0.10%, O: 0.005-0.040% as a basic component
And Si 0.1% or less, Al 0.009%
Limit to the following, N in the range of 20-150ppm,
Steel that is qualitatively made of Fe,₂O ₃, Si
O₂, MnO, and / or Al₂O
₃, SiO₂Of MnO and MnO
Including at least 50% in total of one or more oxides
10 inclusions / kg of steel with a diameter of 53 μm or more
Low-carbon sulfur free-cutting steel characterized by being 0 or less.

In this steel, the number of inclusions having a diameter of less than 53 μm is not limited, and it goes without saying that all the inclusions have a diameter of less than 53 μm. (2) At least one of Pb, Bi, and Te is added to the above free-cutting steel in a total mass of 0.01 to 0.40.
% Low-carbon sulfur-based composite free-cutting steel.

[0012]

The reason for limiting the components of the present invention will be described below. C: C needs to be 0.05% or more in order to secure the finished surface roughness. If it is 0.15% or more, the pearlite structure increases, and as a result, machinability deteriorates. Therefore, C: 0.05-
It was limited to 0.15%. Mn: Mn is 0.5 in order to prevent surface cracking due to deterioration of hot workability due to liquid film embrittlement of FeS during hot rolling.
% Or more is required. If it is 2.0% or more, the amount of solute Mn in the matrix increases and the matrix hardens, so that machinability deteriorates. Therefore, Mn is limited to 0.5 to 2.0%. P: P needs to be 0.05% or more to improve the finished surface roughness. If it is 0.10% or more, mechanical properties and cold workability deteriorate. Therefore, P is limited to 0.05 to 0.10%. S: S is required to be 0.1% or more in order to generate MnS in steel and improve the finished surface roughness. On the other hand, since it must be 0.4% or less to secure cold workability,
S: limited to 0.1 to 0.4%. Pb, Bi, Te: These elements improve the crushability of chips and improve the roughness of the finished surface, so that machinability is improved by adding 0.05% or more of the total weight. However, if the total mass exceeds 0.4%, the hot workability and surface fatigue properties deteriorate. Therefore, the total mass is limited to 0.01 to 0.40%. O: If O is 0.005% or less, MnS is small and machinability is greatly deteriorated, so 0.005% or more is necessary. However, if 0.040% or more, the melting loss of the refractory is severe, and if the molten refractory is mixed in the steel, the machinability decreases, and the sudden boiling phenomenon due to the rapid generation of CO bubbles causes Casting makes casting difficult. Therefore, O: 0.00
It was limited to 5 to 0.040%. Si, Al: When Si exceeds 0.1% and Al exceeds 0.009%, SiO ₂ and A which are hard oxides harmful to machinability
Since l ₂ O ₃ significantly increases and impairs machinability, it is necessary to limit Si to 0.1% or less and Al to 0.009% or less. In the range of N: N of 20 to 200 ppm, the finished surface roughness is improved without significantly reducing the tool life, so the range is limited to this range.

The reason for limiting the non-metallic inclusions is that Al ₂ O ₃ ,
_{SiO 2, Al 2 O 3 -SiO} 2, Al 2 O 3 -Mn
O, MnO-SiO ₂ or MnO-SiO ₂ -Al ₂
Oxide inclusions containing 50% by mass or more of O ₃ are high hardness inclusions, and when large oxides having a diameter of 53 μm or more among these inclusions increase tool wear, the tool wear becomes remarkable. This causes a decrease in life and an increase in finished surface roughness.

Described in Japanese Patent Publication No. 3-37822
As you can see, Al₂O₃System and SiO ₂Machining of system inclusions
However, the present inventors have found that
Of machinability of composite free-cutting steel containing carbon-based sulfur and Pb
As a result of investigating the cause of variation, Al
₂O₃System and SiO₂In addition to system inclusions, Al₂O₃-Si
O₂, Al₂O₃-MnO, MnO-SiO₂Or M
nO-SiO₂-Al ₂O₃Oxide system containing a large amount of
It has been found that existing objects also promote tool wear. Al₂O₃
-SiO₂, Al₂O₃-MnO or MnO-SiO
₂-Al₂O₃Oxide inclusions containing a large amount of pure
Al₂O₃And SiO₂Harder than MnS
Has a hardness of several times to several tens of times, and therefore wears tools.
It is thought to be a cause of this.

Further, the machinability also depends on the size of inclusions having a composition harmful to the machinability, and as long as the inclusion diameters are finely dispersed in the micron or submicron order even in the above composition, they are Has little effect on the machinability, and even if the amount of inclusions increases, the machinability does not deteriorate much. The present inventor investigated the oxide inclusions by an optical microscope or a slime method, and found that the diameter of the inclusions was 10 μm.
The machinability is affected as the number of the above increases, and in particular, the diameter is 5 among the above inclusions that can be extracted by the slime method.
The machinability deteriorated remarkably when the diameter of 3 μm or more increased.

In FIG. 1, among the oxides harmful to the machinability of the present invention steel and the comparative steel described later, the diameter thereof is 53 μm.
The relationship between the amount of oxide inclusions and the surface roughness of plunge cut is shown below. In both the free-cutting steels containing sulfur and Pb, the finished surface roughness (Rz) increases as the size of large oxides with a diameter of 53 μm or more increases.
When the amount of slime inclusions contained per hit exceeds 100, Rz is significantly deteriorated. On the contrary, the surface roughness can be improved by reducing the amount of inclusions contained in the steel material to 100 (pieces / kg) or less. Note that when large oxides having a diameter of 53 μm or more are reduced, oxides less than 53 μm also tend to be reduced, and it is considered that those reductions also contribute to the improvement of machinability.

In order to reduce the above-mentioned large-sized oxide inclusions which are harmful to the machinability as described above, the Al and Si concentrations are controlled within the above ranges to suppress the formation of the inclusions. Is not sufficient, for example, in the case of producing the present free-cutting steel by the continuous casting method, increasing the cross-sectional size of the continuous cast slab,
Positively intervene by decreasing the casting speed, using a large capacity tundish, using a tundish induction heating device to increase the degree of superheat of molten steel during casting, or using a continuous casting machine with a vertical section. It is necessary to promote the floating separation of objects.

Further detailed description will be given below based on embodiments of the present invention.

[0019]

Examples As examples of the steel of the present invention, the component steels of the present invention and the comparative steels were cast into a medium cross section bloom of 162 mm × 162 mm cross section, and the cast pieces were heated in a heating furnace of a steel bar factory for 120 hours.
The machinability was investigated with 80φ steel bar heated and rolled to 0 ° C. Also, in the production of comparative steel, the casting speed was 2.5 m / min.
However, in the production of the invention steel, the casting speed was 1.5 m / min.
The large inclusions injected into the strand were separated by levitation.

The machinability was evaluated by plunge cutting and drill cutting. The machining test conditions are as follows. Plunge cut conditions: Tool: SKH57, cutting speed: 80 m / min Feed: 0.05 mm / rev, 2 sec cutting / 5 sec non-cutting Finished surface roughness was evaluated by JIS Rz.

Drilling conditions: Tool: SKH9 1
0φ, Cutting speed: 70 to 90 m / min Feed: 0.33 mm / rev, Cutting oil: Yes Drill cutting _performance was evaluated at the maximum possible cutting speed V _l.1000 (m / min) for drilling 1000 mm.

As the oxide inclusions, a sample of 40 mm width × 40 mm thickness × 200 mm length was cut out from the surface portion of the bloom slab, and the oxide inclusions were extracted from the sample by the slime method, and the inclusion composition was determined by EPMA. investigated. The machinability investigation results are summarized in Tables 1 and 2, and the relationship between the amount of large inclusions and the finished surface roughness is plotted in FIG.

[0023]

[Table 1]

[0024]

[Table 2]

The steels of the present invention to which Pb, Bi and Te are not added are listed as No. 2 in Table 2. Nos. 1 to 3 and comparative steels corresponding thereto are No. 11 to 13 steel. Furthermore, Pb, Bi, Te
The steel of the present invention added with No. Nos. 4 to 10 steels and comparative steels in that case are No. 14 to 20 steel.

Steels containing Pb, Bi and Te and steels not containing Al ₂ O ₃ , SiO ₂ , Al ₂ O ₃ --Si
Compared with the comparative steel, the present invention steel has less large-sized oxide-based inclusions containing O ₂ , Al ₂ O ₃ —MnO, or MnO—SiO ₂ as the main component than the comparative steel. Roughness and drill life are remarkably improved, and the steel of the present invention is superior in machinability.

Comparing the steel of the present invention containing no Pb, Bi, and Te with the comparative steel containing them, the machinability of the steel of the present invention in which large oxides are greatly reduced is equal to or higher than that of the steel of the present invention. That is, free-cutting steel having machinability equal to or higher than that of the conventional steel to which free-cutting elements such as Pb, Bi and Te are added can be manufactured without adding them, and the manufacturing cost can be reduced accordingly.

[0028]

As is clear from the above examples of the present invention, when the steel of the present invention is manufactured by the continuous casting method, there is little variation in the steel characteristics such as machinability within the lot and the machinability is excellent. It is possible to provide free-cutting steel, which has an extremely large industrial effect.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of large oxide inclusions and the finished surface roughness.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Toshifumi Ogawa 12 Nakamachi, Muroran-shi, Hokkaido Nippon Steel Corporation Stock Muroran Works

Claims (2)

[Claims] 1. C: 0.05 to 0.15% by mass%, M
n: 0.5 to 2.0%, S: 0.1 to 0.4%, P:
0.05-0.10%, O: 0.005-0.040%
Was the basic component, further 0.1% or less of Si, limiting the Al below 0.009%, the N in the range of 20～150Ppm, a balance substantially of steel consisting of Fe, Al ₂ O
_3, SiO _2, MnO 1 or more of, and / or Al to ₂ O _3, SiO _2, a total of 50% or more of one or more oxides in the mass of inclusions or two or more bound of MnO Of inclusions with a diameter of 53 μm or more, steel is 1 kg
Low charcoal sulfur free-cutting steel characterized by less than 100 pieces per unit. 2. Further, at least one of Pb, Bi, and Te is contained in a total mass% of 0.01 to 0.4.
The low carbon sulfur-based composite free-cutting steel according to claim 1, containing 0%.