JP4034700B2 - Manufacturing method of high S free cutting steel with excellent machinability and high S free cutting steel - Google Patents

Description

【００３１】
表１から明らかなように、Ｓが少なすぎる場合、Ｐｂ複合快削鋼に匹敵するような切屑処理性が得られない（Ｎｏ．１）。またＳを多くしても、Ｍｎ／Ｓ比が小さいと表面疵が発生する（Ｎｏ．２）。さらにＳを多くしてＭｎ／Ｓ比も大きくしても、取鍋でのフリー酸素が少なくなっていると、ＭｎＳサイズが小さくなるため、Ｐｂ複合快削鋼に匹敵するような仕上げ面粗さが得られない（Ｎｏ．３〜５）。なおＮｏ．２は鋼中のＳｉ量が少なくなっているが、これは脱酸剤として添加したＳｉがスラグ（ＳｉＯ₂）として十分に除去されているためである。さらにＮが少なすぎる場合にもＰｂ複合快削鋼に匹敵するような仕上げ面粗さが得られない（Ｎｏ．６）。
【００３２】
これらに対して化学成分を適切とし、かつ取鍋でのフリー酸素を十分に確保した場合には、ＭｎＳサイズを大きくできるため、表面疵を発生させることなくＰｂ複合快削鋼に匹敵するような被削性（切屑処理性、仕上げ面粗さ）を達成できる（Ｎｏ．７〜１７）。特に造塊法によって製造すると、連続鋳造法による場合よりも仕上げ面粗さを高めることができる（Ｎｏ．１８）。
【００３３】
【発明の効果】
本発明によれば被削性（特に切屑処理性）を高めた高Ｓ快削鋼において、Ｍｎ／Ｓ比が高められているために表面疵が抑制されている。さらにはＭｎ／Ｓ比を高めると溶鋼中のフリー酸素濃度が下がって仕上げ面粗さが悪化し易くなるにも拘わらず、本発明では脱酸工程を工夫することによって溶鋼中のフリー酸素濃度の低減を抑制してＭｎＳ介在物を大きくしているため、仕上げ面粗さも改善できる。そのため表面疵の発生防止、仕上げ面粗さ（及び切屑処理性）の点でＰｂ快削鋼に匹敵するＰｂフリー快削鋼を得ることができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technology of free-cutting steel (Pb-free free-cutting steel) that does not use Pb, which is harmful to human bodies and difficult to recycle.
[0002]
[Prior art]
Pb contained in free-cutting steel is an extremely effective element for improving machinability. However, there is a risk of melting from the landfilled scrap, and Pb gas harmful to the human body is generated when dissolved, which has been an obstacle to recycling. For this reason, the abolition of Pb use to steel is called for. When Pb is removed, the machinability deteriorates, so it is necessary to add an alternative free-cutting element.
[0003]
In order to improve machinability (especially chip disposability), an increase in S (high S free-cutting steel) has been proposed. For example, Patent Document 1 proposes that S be more than 0.4%. Yes. However, as this patent document 1 points out, when S content rate increases, hot workability will fall, for example, the surface flaw of a rolling material will increase. Therefore, in Patent Document 1, it is recommended to increase the Mn / S ratio to 5.0 at the maximum by increasing the Mn content. Furthermore, focusing on the fact that if the oxygen content in the steel is low, the MnS is reduced in size and machinability is reduced, and it has also been proposed to increase the oxygen concentration in the steel to about 0.005 to 0.040%. . However, as described in the prior art column of Patent Document 2 described later, even if the method of Patent Document 1 (improvement of Mn / S ratio, improvement of oxygen concentration in steel) is used, the finished surface roughness still remains. Is insufficient.
[0004]
There exists patent document 2 as steel which could improve finishing surface roughness. The invention of Patent Document 2 is a high-S free-cutting steel in which S is increased to 0.4% or more, the Mn / S ratio is increased to a maximum of 5.0, and the oxygen concentration in the steel is 0.005. Although it is common to the above-mentioned patent document 1 in that it is increased to ~ 0.040%, it is improved in that 0.0015 to 0.60% of Sn is added to improve poor finished surface roughness. . If Sn is used, the constituent cutting edge formed at the time of cutting can be made fragile, and the fragment of the constituent cutting edge adhering to the cutting surface is reduced. However, since Sn degrades hot workability, the hot workability improvement effect (surface flaw reduction effect) due to the improvement of the Mn / S ratio is negated. Therefore, even by the method of Patent Document 2, it is difficult to achieve both reduction of surface wrinkles and finished surface roughness.
[0005]
As an example of improving the finished surface roughness without using Sn, there is Patent Document 3. This Patent Document 3 is common to the above Patent Documents 1 and _{2 in} that S is increased to about 0.5% and O ₂ in the steel is increased to 100 to 300 ppm. It is shown using a specific formula that the finished surface roughness becomes better as O ₂ increases and the MnS size increases. However, in Patent Document 3, no consideration is given to the Mn / S ratio, and when referring to the column of Examples, the Mn / S ratio is 3.3 at the maximum. Therefore, the effect of improving surface defects is insufficient, and it is difficult to achieve both reduction of surface defects and finished surface roughness even by the method of Patent Document 3.
[0006]
[Patent Document 1]
JP 2000-319753 A [Patent Document 2]
Japanese Patent Laid-Open No. 2002-249848 [Patent Document 3]
JP-A-5-345951
[Problems to be solved by the invention]
The present invention has been made paying attention to the circumstances as described above, and its purpose is to prevent the occurrence of surface flaws and to achieve a finished surface in high-S free-cutting steel with improved machinability (particularly chip disposal). The purpose is to establish a technique capable of achieving both improvement of roughness.
[0008]
[Means for Solving the Problems]
In order to achieve both the prevention of surface flaws and the improvement of the finished surface roughness, it is assumed in Patent Document 3 that the Mn / S ratio is improved. However, when the Mn / S ratio is increased, the MnS size is reduced, and the effect of improving the finished surface roughness cannot be sufficiently obtained. Moreover, the decrease in MnS size was the same even when the oxygen concentration in the steel was controlled to 0.005 to 0.040%, as taught in Patent Documents 1 to 3.
[0009]
Therefore, as a result of intensive studies to solve the above problems, the present inventors discovered that the oxygen concentration in the steel has no direct relationship with the MnS size. The immediately preceding free oxygen concentration has a large effect on the MnS size. Reducing the free oxygen concentration can generate MnS with a large size while increasing the Mn / S ratio. The present invention has been completed by finding out that both improvement of roughness can be achieved.
[0010]
That is, the manufacturing method of the high S free-cutting steel excellent in machinability according to the present invention is: C: 0.01 to 0.25% (meaning mass%, hereinafter the same), Si: 0.01% or less. (Not including 0%), Mn: 1.3 to 3.5%, P: not more than 0.2% (not including 0%), S: 0.38 to 0.8%, Al: 0.01 % Or less (excluding 0%), N: 0.007% or more, O: 0.008% or more, the balance is made of Fe and inevitable impurities, and the Mn / S ratio (mass ratio) is 3. The high-S free-cutting steel manufacturing method of 5 or more has a gist in that the free oxygen concentration of the molten steel immediately before casting is 0.004% or more. In order to make the free oxygen concentration of molten steel 0.004% or more, it is effective not to add Si and Al as deoxidizers. In the high S free-cutting steel obtained in this way, the average area of MnS is 10 μm ² or more, and the finished surface roughness is improved. The high-S free-cutting steel of the present invention is produced by a continuous casting method or an ingot-making method, and the ingot-making method is advantageous for increasing the MnS size. A steel part can be manufactured by cutting this high-S free-cutting steel.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the components of the molten steel are adjusted in accordance with a conventional method except for the deoxidation operation described below, and then, if necessary, after rolling in pieces, by hot rolling, C: 0.01 to 0.2% (mass%) The same applies hereinafter), Si: 0.01% or less, Mn: 1.3 to 3%, P: 0.2% or less, S: 0.38 to 0.8%, Al: 0.01% or less , N: 0.007% or more, O: 0.008% or more, the balance being Fe and inevitable impurities, Mn / S ratio (mass ratio) of 3.5 or more high S free cutting steel Is manufacturing. The reasons for limiting each component are as follows.
[0012]
C: 0.01 to 0.25%
C is an element indispensable for determining the basic strength of steel, and has the effect of improving the finished surface roughness by adding a predetermined amount or more. However, if it is added excessively, the tool life is reduced. The amount of C is set to an appropriate range according to other constituent requirements of steel (other components, MnS size described later, etc.), but in the present invention, 0.01% or more (preferably 0.03% or more, More preferably, it exceeds 0.05%) and 0.25% or less (preferably 0.20% or less, more preferably 0.17% or less).
[0013]
Si: 0.01% or less (excluding 0%)
As described later, in the present invention, Si as a deoxidizer is basically not added. However, Si is contained in the raw material (iron ore, scrap, etc.) and is mixed in as SiO ₂ even after the Si removal treatment. Also since the case of producing the other steels are frequently used as a deoxidizer, coming mixed as SiO ₂ from manufacturing facilities. If it is mixed as SiO ₂ , the function as a deoxidizer is lost, and the amount of Si is not particularly limited, but usually Si is 0.01% or less (eg, 0.009% or less, particularly 0.007%). (In practice, it is never 0%).
[0014]
Mn: 1.3 to 3.5%
Mn is an indispensable element in the present invention because it forms MnS and improves machinability. Moreover, in order to suppress generation | occurrence | production of the liquid phase during rolling by the production | generation of FeS, it is effective also at the point which reduces surface flaws. However, when Mn is excessive, the tool life is reduced. Mn content is 1.3% or more (preferably 1.5% or more, more preferably 2.0% or more), 3.5% or less (preferably 3.2% or less, more preferably 3.0% or less) )
[0015]
P: 0.2% or less (excluding 0%)
If P is excessive, it is recommended to reduce it because the tool life is reduced. Therefore, P is 0.2% or less, preferably 0.15% or less, more preferably 0.10% or less. In practice, it is difficult to set P to 0%, and P is also effective in improving machinability. Therefore, P is preferably 0.01% or more, more preferably 0.05% or more.
[0016]
S: 0.38 to 0.8%
S is an indispensable element in the present invention because it forms MnS to enhance machinability. Moreover, in order to obtain machinability equivalent to Pb free-cutting steel, it is necessary to sufficiently increase the amount of S. Therefore, in the present invention, the S content is 0.38% or more, preferably 0.40% or more, and more preferably 0.45% or more. However, when S is excessive, generation of surface flaws is recognized. Therefore, the S content is 0.8% or less, preferably 0.7% or less, and more preferably 0.65% or less.
[0017]
Al: 0.01% or less (excluding 0%)
As described later, in the present invention, basically, Al as a deoxidizer is not added. However, since Al is frequently used as a deoxidizer in the production of other steels, Al ₂ O ₃ is mixed from the production facility. If it is mixed as Al ₂ O ₃ , the function as a deoxidizer is lost, and the amount of Al is not particularly limited, but usually Al is 0.01% or less (for example, 0.009% or less, in particular, 0.1%). 006% or less) (in practice, it is never 0%).
[0018]
N: Finished surface roughness is improved as N increases by 0.007% or more . N is 0.007% or more, preferably 0.008% or more, and more preferably N is 0.009% or more. Although the upper limit of N is not limited from the viewpoint of surface defects and finished surface roughness, it is desirable to set the upper limit because blow holes are generated when N increases. Preferably N is 0.02% or less, more preferably 0.015% or less, particularly 0.013% or less.
[0019]
O: 0.008% or more As described later, in the present invention, it is important to increase the free oxygen concentration in the molten steel immediately before casting, and the oxygen concentration in the steel is not particularly set. 0.008% or more, preferably 0.010% or more, and more preferably about 0.013% or more. Although there is no direct relationship between surface defects and finished surface roughness and oxygen concentration in steel, the higher the oxygen concentration in steel, the more the oxides (SiO ₂ , Al ₂ O _3, etc.) derived from raw materials and production equipment. There is a tendency to increase, and the tool life tends to decrease. Therefore, it is recommended that the oxygen in the steel is preferably 0.03% or less, more preferably 0.02% or less, particularly 0.018% or less.
[0020]
Moreover, in this invention, ratio (Mn / S; mass reference | standard) of Mn and S is set to 3.5 or more. If Mn is too small compared to S, generation of surface defects cannot be prevented. A preferable Mn / S ratio (mass ratio) is 3.8 or more, more preferably 4.0 or more.
[0021]
When producing steel as described above, the present invention is characterized by devising a deoxidation operation immediately before casting (for example, a deoxidation operation by adjusting slag during ladle refining). In ladle refining, deoxidation is usually performed by adding a deoxidizer such as Si or Al. However, when the Mn / S ratio is increased as in the present invention, since Mn functions as a deoxidizer, the free oxygen concentration in the molten steel has already been reduced. If Si or Al is added according to a conventional method, the free oxygen concentration in the molten steel is remarkably reduced, MnS precipitated during casting is refined, and the finished surface roughness is deteriorated. is there. Further, in the conventional method, it has been considered to secure a predetermined amount or more of oxygen in the steel. However, such an idea makes it possible to sufficiently raise the oxide without causing SiO ₂ or Al ₂ O ₃ to be brought into the steel. It was only meant to be cast after that, and free oxygen in the molten steel was not controlled at all. Rather, Si and Al have been used as a deoxidizer, so free oxygen in the molten steel Was extremely small. On the other hand, in the present invention, the free oxygen concentration is not reduced excessively, and basically, MnS is prevented from being refined by not adding Si or Al as a deoxidizer. By preventing the miniaturization of MnS, the degradation of the finished surface roughness can be prevented.
[0022]
The free oxygen concentration in the molten steel immediately before casting is 0.004% or more, preferably 0.0043% or more, and more preferably 0.0045% or more. As long as such a free oxygen concentration can be secured, Si or Al may be added if necessary. If free oxygen is not excessive, the MnS size can be prevented from becoming too large, and the generation of surface flaws can be further suppressed. Accordingly, the free oxygen concentration is preferably 0.01% or less, more preferably 0.008% or less, and particularly about 0.006% or less.
[0023]
When the free oxygen concentration in the molten steel is controlled as described above, it is possible to prevent MnS from being refined even though the Mn / S ratio is increased. Therefore, generation | occurrence | production of a surface flaw can be prevented and finishing surface roughness can also be improved. The average area of MnS inclusions in the steel of the present invention obtained as described above is 10 μm ² or more, preferably 20 μm ² or more, more preferably 50 μm ² or more. The average area of MnS inclusions is preferably 300 μm ² or less, more preferably 200 μm ² or less, particularly 150 μm ² or less.
[0024]
The casting method is not particularly limited, and may be a continuous casting method or an ingot-making method. According to the continuous casting method, productivity can be increased. Also, the ingot forming method can slow down the cooling rate at the time of casting and easily increases the MnS size, which is more effective for improving the finished surface roughness. In the case of the ingot-making method, it is recommended that the size of one slab is, for example, 5 tons or more.
[0025]
The steel of the present invention obtained as described above is a high-S free-cutting steel, so that machinability (particularly chip disposal) is improved, and since the Mn / S ratio is increased, surface flaws are present. Further, since the MnS inclusions are increased by devising the deoxidation step, the finished surface roughness is also improved. Therefore, it is extremely useful as free-cutting steel, and various steel parts [for example, hydraulic parts (for example, hydraulic control parts such as anti-lock brake system (ABS), automatic transmission (AT), etc.), etc.] easily and accurately. Can be manufactured well.
[0026]
【Example】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.
[0027]
Experimental example 1
Table 1 shows the steel produced from the blast furnace after hot metal pretreatment and converter refining to adjust the amount of C, Si and P, and then refining with addition of alloying elements (Mn) in a ladle. Chemical component steel was produced and immediately cast by continuous casting (slab size: 300 mm × 430 mm) or ingot (size: 8 tons). Subsequently, after making into 155 mm x 155 mm x 10 m by partial rolling, the wire rod of 9.5 mm in diameter was obtained by hot rolling. In the ladle, no. Si or Al was added only in the examples of 3 to 5, and Si and Al were not added in the other examples. The wire was drawn to give a polishing rod with a diameter of 8 mm and a length of 3 m.
[0028]
The characteristics of the obtained polishing bar were examined as follows.
[Surface]
The presence or absence of surface wrinkles was examined for all polishing rods obtained from one charge for melting. A product having a wrinkle with a depth of 0.2 mm or more was regarded as a defective product and evaluated according to the following criteria.
○: Defect product occurrence rate 5% or less ×: Defect product occurrence rate over 5% [MnS size]
The polishing rod was cut in the rolling direction, and an optical microscope image (magnification 30 times) of the cross section was received by a solid state camera. The area of each MnS was measured by image analysis, the average value of those having a width exceeding 1 μm was calculated, and evaluated according to the following criteria.
○: MnS area (average) is 10 μm ² or more ×: MnS area (average) is less than 10 μm ² [Machinability]
Cutting a wire to produce 3000 test materials, and forming a groove by feeding a forming bit (His SKH4A) in the radial direction of the test material while rotating the test material around its axis (cutting speed: 90 m / mm) Minute, feed amount 0.03 mm / rev, cut amount: 1.0 mm), and the finished surface roughness (average) and chip treatability (average) at the bottom of the groove were examined.
[0029]
For the finished surface roughness and chip treatability of the test material, the same cutting test was performed using a Pb composite free-cutting steel “12L14A” manufactured by Kobe Steel Co., Ltd. (component standard: C: 0.15% or less, Mn: 0.85) To 1.15, P: 004 to 0.09, S: 0.28 to 0.35, Pb: 0.20 to 0.35), and finished surface of this Pb composite free-cutting steel “12L14A” Evaluation was made by comparing with the results of roughness (10-point average roughness Rz was 10 to 15 μm ² ) and chip disposal.
○: It is equal to or more than “12L14A”. X: Table 1 shows results inferior to “12L14A”.
[0030]
[Table 1]

[0031]
As is apparent from Table 1, when S is too small, chip treatability comparable to Pb composite free-cutting steel cannot be obtained (No. 1). Even if S is increased, surface defects occur when the Mn / S ratio is small (No. 2). Even if S is increased and the Mn / S ratio is increased, if the free oxygen in the ladle is reduced, the MnS size is reduced, so that the finished surface roughness is comparable to Pb composite free-cutting steel. Cannot be obtained (No. 3 to 5). No. No. 2 shows that the amount of Si in the steel is small, because Si added as a deoxidizer is sufficiently removed as slag (SiO ₂ ). Furthermore, even when N is too small, a finished surface roughness comparable to Pb composite free-cutting steel cannot be obtained (No. 6).
[0032]
If the chemical components are appropriate for these and sufficient free oxygen in the ladle is secured, the MnS size can be increased, so that it is comparable to Pb composite free-cutting steel without generating surface flaws. Machinability (chip disposal, finished surface roughness) can be achieved (Nos. 7 to 17). In particular, when manufactured by the ingot-making method, the finished surface roughness can be increased as compared with the case of the continuous casting method (No. 18).
[0033]
【The invention's effect】
According to the present invention, in the high S free-cutting steel having improved machinability (particularly chip disposal), surface flaws are suppressed because the Mn / S ratio is increased. Furthermore, even if the Mn / S ratio is increased, the free oxygen concentration in the molten steel decreases and the finished surface roughness tends to deteriorate, but in the present invention, the deoxidation process is devised to reduce the free oxygen concentration in the molten steel. Since the reduction is suppressed and the MnS inclusions are enlarged, the finished surface roughness can also be improved. Therefore, Pb-free free-cutting steel comparable to Pb free-cutting steel in terms of prevention of surface flaws and finish surface roughness (and chip disposal) can be obtained.

Claims (5)

C: 0.01 to 0.25% (meaning mass%, the same applies hereinafter),
Si: 0.01% or less (excluding 0%),
Mn: 1.3-3.0 %
P: 0.2% or less (excluding 0%),
S: 0.38 to 0.8%,
Al: 0.01% or less (excluding 0%),
N: 0.007% or more and 0.02% or less ,
O: 0.008% or more and 0.03% or less ,
The balance consists of Fe and inevitable impurities,
In the manufacturing method of high S free cutting steel whose Mn / S ratio (mass ratio) is 3.5 or more,
A method for producing high-S free-cutting steel excellent in machinability, characterized in that the free oxygen concentration of molten steel immediately before casting is 0.004% or more and 0.01% or less .   The manufacturing method of the high S free cutting steel of Claim 1 which does not add Si and Al as a deoxidizer.   The manufacturing method of the high S free-cutting steel in any one of Claim 1 or 2 manufactured by the continuous casting method or the ingot-making method. Steel produced without adding Si and Al as deoxidizers,
C: 0.01 to 0.25 %,
Si: 0.01% or less (excluding 0%),
Mn: 1.3-3.0 %
P: 0.2% or less (excluding 0%),
S: 0.38 to 0.8%,
Al: 0.01% or less (excluding 0%),
N: 0.007% or more and 0.02% or less ,
O: 0.008% or more and 0.03% or less ,
The balance consists of Fe and inevitable impurities,
Mn / S ratio (mass ratio) is 3.5 or more,
A high-S free-cutting steel excellent in machinability, characterized in that the average cross-sectional area of MnS having a width exceeding 1 μm in a cross section in the rolling direction is 10 μm ² or more.   A steel part obtained by cutting the high-S free-cutting steel according to claim 4.