JP4023196B2 - Machine structural steel with excellent machinability - Google Patents

Description

【００５４】
表２ Ｓ４５Ｃ系 比較例

【００５５】
表３ Ｓ４５Ｃ系 実施例

【００５６】
表４ Ｓ４５Ｃ系 比較例

【００５７】
［実施例２］
Ｓ１５Ｃ系の快削鋼について、実施例１と同様に、合金の溶製および被削性の試験を行なった。合金組成を表５（実施例）および表６（比較例）に、試験結果を表７（実施例）および表８（比較例）に、それぞれ示す。
【００５８】
表５ Ｓ１５Ｃ系 実施例

【００５９】
表６ Ｓ１５Ｃ系 比較例

【００６０】
表７ Ｓ１５Ｃ系 実施例

【００６１】
表８ Ｓ１５Ｃ系 比較例

【００６２】
［実施例３］
Ｓ５５Ｃ系快削鋼について、実施例１と同様に、合金の溶製および被削性の試験を行なった。合金組成を表９（実施例）および表１０（比較例）に、試験結果を表１１（実施例）および表１２（比較例）に、それぞれ示す。
【００６３】
表９ Ｓ５５Ｃ系 実施例

【００６４】
表１０ Ｓ５５Ｃ系 比較例

【００６５】
表１１ Ｓ５５Ｃ系 実施例

【００６６】
表１２ Ｓ５５Ｃ系 比較例

【００６７】
［実施例４］
ＳＣｒ４１５系快削鋼について、実施例１と同様に合金の溶製および被削性の試験を行なった。合金組成を表１３（実施例）および表１４（比較例）に、試験結果を表１５（実施例）および表１６（比較例）に、それぞれ示す。
【００６８】
表１３ ＳＣｒ４１５系 実施例

【００６９】
表１４ ＳＣｒ４１５系 比較例

【００７０】
表１５ ＳＣｒ４１５系 実施例

【００７１】
表１６ ＳＣｒ４１５系 比較例

【００７２】
［実施例５］
ＳＣＭ４４０系快削鋼について、実施例１と同様に，合金の溶製および被削性の試験を行なった。合金組成を表１７（実施例）および表１８（比較例）に、試験結果を表１９（実施例）および表２０（比較例）に、それぞれ示す。
【００７３】
表１７ ＳＣＭ４４０系 実施例

【００７４】
表１８ ＳＣＭ４４０系 比較例

【００７５】
表１９ ＳＣＭ４４０系 実施例

【００７６】
表２０ ＳＣＭ４４０系 比較例

【００７７】
【発明の効果】
本発明の機械構造用の快削鋼においても、さきに開示した快削鋼と同じ被削性が実現している。すなわち、高い被削性をもたらす介在物である二重構造介在物が最適の形態で存在するから、切削とくに超硬工具旋削において、在来のイオウ快削鋼に対して工具寿命が５倍以上という目標を容易に達成することができる。
【００７８】
さらに本発明の機械構造用の快削鋼では、微量のＴｉおよび（または）Ｚｒを添加することにより、微細に分散したＭｎＳ介在物を有し、それによって切屑破砕性が高い快削鋼となっているから、旋削にとってとりわけ好都合である。鋼中に微細なＴｉ(ＣＮ)および（または）Ｚｒ(ＣＮ)を生成させた製品は、熱間鍛造時の旧オーステナイト結晶粒の成長を抑制することができるから、被削性および切屑破砕性に加えて、疲労強度や曲げ矯正性が高く、このような性質が要求される用途にとって有用である。
【００７９】
本発明の製造方法は、上記のような機械構造用の快削鋼を確実に製造できる方法であって、調整された脱酸を行なうことにより二重構造介在物を有利に形成させ、さらに適切なタイミングで、つまり調整脱酸により二重構造介在物が形成させたのちに、適量のＴｉおよび（または）Ｚｒを添加することにより、ＭｎＳ介在物が微細に分散することにより切屑破砕性のよい快削鋼を得ることができる。Ｔｉ量および（または）Ｚｒ量とともにＯ量およびＮ量を適切にえらんでこの製造方法を実施すれば、鋼中に微細なＴｉ(ＣＮ)および（または）Ｚｒ(ＣＮ)が生成し、疲労強度や曲げ矯正性が改善された機械構造用の快削鋼が製造できる。
【図面の簡単な説明】
【図１】 本発明にしたがう機械構造用の快削鋼中の、介在物の形状を示す顕微鏡写真。
【図２】 在来のイオウ快削鋼中の、介在物の形状を示す顕微鏡写真。
【図３】 機械構造用の快削鋼中において、「二重構造介在物」が占める面積と工具寿命比との関係を示すグラフ。
【図４】 機械構造用の快削鋼における、Ａｌ含有量と工具寿命比との関係をプロットしたグラフ。
【図５】 種々のＳ含有量およびＯ含有量をもつ機械構造用の快削鋼において、工具寿命比５以上の目標が達成できたか否かを示したグラフ。
【図６】 種々のＳ含有量およびＣａ含有量をもつ機械構造用の快削鋼において、工具寿命比５以上の目標が達成できたか否かを示したグラフ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a machine structural steel that has excellent machinability when cutting with a carbide tool and has good chip crushability, and a method for producing the same. The present invention also relates to a mechanical structural steel that exhibits excellent fatigue strength and bending straightness in addition to machinability and chip crushability, and a method for producing the same.
[0002]
In the present invention, the term “double structure inclusion” refers to an inclusion having a structure in which an inclusion mainly composed of an oxide serves as a core and surrounds an inclusion mainly composed of a sulfide. . The terms "tool life ratio" and "life ratio" refer to the tool life of conventional sulfur free-cutting steel having the same S content and the tool of the free-cutting steel according to the present invention in cutting with carbide tools, especially turning. It means the ratio with life. With respect to MnS inclusions, “finely dispersed” means finer grains than MnS inclusions in conventional steel, and is distributed in the steel on average without agglomeration or concentration. Means state.
[0003]
[Prior art]
Research on machine structural steel with high machinability has been conducted for many years, and the applicant has made many proposals so far. As a recent example, Japanese Patent Application Laid-Open No. 10-287953 “Mechanical structural steel excellent in mechanical properties and drilling workability” is one representative. This free-cutting steel is a spindle-type calcium alloy containing 1% or more of Ca and containing a calcium aluminate oxide inclusion containing 8 to 62% of CaO and having a major axis / minor axis ratio of 5 or less. It contains manganese sulfide inclusions. Although the present invention has achieved excellent machinability, it has been experienced that, in practice, the machinability sometimes varies. This is understood to be because there are various forms of calcium / manganese sulfide inclusions.
[0004]
Subsequently, the applicant categorized the Ca-containing sulfide into three according to the Ca content in Japanese Patent Application Laid-Open No. 2000-34538 “Machining steel with excellent turning workability”, A / (A + B + C) ≦ 0.3, and B / (A + B + C) where A is the content exceeding 40%, B is 0.3-40%, and C is less than 0.3%. ) Disclosed that the turning tool life is significantly extended when the condition of ≧ 0.1 is satisfied.
[0005]
Further, the applicant who has advanced the research has reached JP-A-2000-219936 “Free-cutting steel” and has succeeded in reducing the variation in machinability by clarifying the number of inclusions. The steel of the present invention is characterized by containing 5 or more sulfides having an equivalent circle diameter of 5 μm or more and containing 0.1 to 1% of Ca per 3.3 mm ² . However, there was still room for improvement in terms of variation in machinability.
[0006]
Therefore, the applicant applied to machine structural steel with improved machinability variation, especially free-cutting steel with high machinability of carbide tools and achieving machinability of 5 times or more in the above-mentioned tool life ratio. This was also proposed (Japanese Patent Application No. 2001-174606 “Free-cutting steel for machine structure with excellent carbide tool turning ability”). This free-cutting steel is characterized by the presence of inclusions, and contains "1.0 wt% or more of Ca present in contact with oxide inclusions having a CaO content of 8 to 62 wt%. That is, the area occupied by the sulfide inclusions is 2.0 × 10 ⁻⁴ mm ² or more per 3.5 mm ² visual field area ”. A representative example of the form of such a double structure inclusion is shown in the above patent application.
[0007]
The production method capable of reliably generating the double structure inclusion as described above is to perform an operation satisfying the following conditions when melting steel, which is also disclosed in the above patent application.
[S] / [O]: 8 to 40
[Ca] × [S]: 1 × 10 ⁻⁵ to 1 × 10 ⁻³
[Ca] / [S]: 0.01 to 20 and [Al]: 0.001 to 0.020%
[0008]
[Problems to be solved by the invention]
The object of the present invention is to improve the machinability by utilizing the form of the double structure inclusion described above, and to improve the machinability by 5 times or more in terms of the tool life ratio. In the free-cutting steel, further improvement is to provide a steel suitable for machining, particularly turning, with good chipping property. It is also included in the object of the present invention to provide a free-cutting steel for a machine structure that exhibits excellent fatigue strength and bending straightness in addition to machinability and chip crushability.
[0009]
[Means for Solving the Problems]
The free-cutting steel for machine structures that achieves the above object and has excellent machinability and good chip crushability according to the present invention has a basic alloy composition of C: 0.05 to 0.8 in weight%. %, Si: 0.01 to 2.5%, Mn: 0.1 to 3.5%, S: 0.01 to 0.2%, Al: 0.001 to 0.020%, Ca: 0.0. In addition to 0005-0.02%, O: 0.0005-0.01% and N: 0.001-0.04%, Ti: 0.002-0.020% and Zr: 0.002-0. 040% of one or two kinds, the balance has an alloy composition consisting of inevitable impurities and Fe, and exists in contact with oxide inclusions having a CaO content of 0.2 to 62% by weight, the area occupied by the sulfide inclusions containing 1.0 wt% or more of Ca is, the viewing area 3.5 mm ² per 2.0 × 10 ^-4 mm ² or more There, MnS inclusions, the number average particle diameter 1.0μm or more things a machine structural steel are dispersed such that S contains five weight per 0.01% / mm ² or more.
[0010]
The method of the present invention for producing a free-cutting steel for machine structures having excellent machinability and good chip crushability is as follows:% by weight, C: 0.05 to 0.8%, Si: 0.01 to 2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Al: 0.001-0.020%, Ca: 0.0005-0.02%, O : 0.0005 to 0.01% and N: 0.001 to 0.04%, the remainder is an inevitable impurity and an alloy composed of Fe is melted. To perform deoxidation adjusted by performing
[S] / [O]: 8 to 40
[Ca] × [S]: 1 × 10 ⁻⁵ to 1 × 10 ⁻³
[Ca] / [S]: 0.01 to 20 and [Al]: 0.001 to 0.020%
The occupied area of sulfide inclusions containing Ca of 1.0 wt% or more present in contact with oxide inclusions having a CaO content of 0.2 to 62 wt% has a viewing area of 3.5 mm ^2. After making it become 2.0 × 10 ⁻⁴ mm ² or more per one, adding one or two of Ti: 0.002 to 0.020% and Zr: 0.002 to 0.040%, Fine Ti oxides and / or Zr oxides are formed by the reaction of O in the adjusted deoxidized steel with Ti and / or Zr, and MnS intervenes with complex oxides containing these as cores By precipitating the product, the MnS inclusions are dispersed so that the number of those having an average particle size of 1.0 μm or more is 5 / mm ² or more per S content of 0.01%. It is a manufacturing method .
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In the steel for machine structure of the present invention, the reason why the composition of the steel having the basic alloy composition is limited as described above is as follows.
[0012]
C: 0.05 to 0.8%
C is a component necessary for ensuring the strength. If the content is less than 0.05%, the strength as steel for machine structural use is insufficient. On the other hand, since C increases the activity of S, when it becomes large, the amount of [S] / [O], [Ca] × [S], [Ca] / [S], and a specific amount of [Al] is increased. Under balance, it becomes difficult to obtain a double structure inclusion. If C is increased, the toughness and machinability are also lowered, so an upper limit of 0.8% was set.
[0013]
Si: 0.01 to 2.5%
Si becomes a component of steel as a deoxidizer at the time of melting, and also has a function of improving hardenability. This effect cannot be expected with a small amount not reaching 0.01%. Since Si also increases the activity of S, the presence of a large amount of Si causes the same problems as a large amount of C, which may hinder the formation of double structure inclusions. A large amount of Si also impairs ductility, and cracks are likely to occur during plastic working, so 2.5% is the upper limit of the amount added.
[0014]
Mn: 0.1 to 3.5%
Mn is an important element that generates sulfides. If the amount is less than 0.1%, the amount of inclusions is insufficient, but if the amount exceeds 3.5%, the steel is hardened and the machinability is lowered.
[0015]
S: 0.01 to 0.2%
S is an essential component rather than useful for improving machinability, and is present in an amount of 0.01% or more. In order to achieve the tool life ratio of 5 or more which is the target of the present invention, S of 0.01% or more is required. When the amount of S exceeds 0.2%, not only the toughness and ductility are deteriorated, but also CaS is produced together with Ca. Since CaS has a high melting point, it becomes an obstacle to the casting process.
[0016]
Al: 0.001 to 0.020%
Necessary for appropriately adjusting the composition of oxide inclusions, at least 0.001% is added. If it exceeds 0.20%, a hard alumina cluster is formed, which impairs the machinability of the steel.
[0017]
Ca: 0.0005 to 0.02%
Ca is a very important component for the steel of the present invention. In order to contain Ca in the sulfide, the addition of 0.0005% or more is essential. On the other hand, the addition of excess Ca exceeding 0.02% leads to the formation of the high melting point CaS, which becomes an obstacle to casting.
[0018]
O: 0.0005 to 0.01%
O is an element necessary for the formation of oxides. In excessively deoxidized steel, a large amount of high melting point CaS is formed, which hinders casting, so O is required to be at least 0.0005%, preferably more than 0.0015%. On the other hand, O exceeding 0.01% results in a large amount of hard oxide, resulting in poor machinability and difficult formation of the desired calcium sulfide. When composite deoxidation is performed using Ca and Al, a CaO.Al ₂ O ₃ -based composite oxide is produced, which is an inclusion with a low melting point and is preferable for machinability.
[0019]
In addition to the formation of the composite oxide, as described below, O forms fine oxides with Ti and / or Zr, which become MnS precipitation nuclei, thereby miniaturizing MnS. In order to expect this effect, a certain minimum amount of Ti oxide, Zr oxide or (Ti + Zr) oxide must be produced,
[O] / [N]: It is necessary to give a condition of 0.06 or more.
[0020]
N: 0.001 to 0.04%
N is an effective component for preventing coarsening of crystal grains, and is important for forming TiN by combining with Ti. From this point of view, 0.001% or more of N is required. An excessive amount of N causes casting defects, so 0.04% was made the upper limit.
[0021]
One or two trace amounts of Ti or Zr of Ti: 0.002 to 0.020% and Zr: 0.002 to 0.040% are combined with O in the steel deoxidized with Ca and Al. To form fine oxides. This serves as a nucleus for the precipitation of MnS, which helps to finely disperse MnS. Ti and Zr are effective even when used alone, but the combination of two types (for example, Ti: 0.005% + Zr: 0.015%) is advantageous in that the effect of refining MnS is high. In order to produce an appropriate amount of Ti oxide or Zr oxide without affecting the formation of double structure inclusions and other oxides, the amounts of Ti and Zr are respectively 0.002 to 0 described above. It is necessary to adjust to the range of .020% and 0.002 to 0.040%. In order to reliably form the double structure inclusions essential in the structural steel of the present invention, it is important to add Ti and / or Zr after carrying out the controlled deoxidation.
[0022]
Ti and Zr also have the effect of suppressing the growth of the prior austenite grain size during hot forging when producing fine Ti (CN) or Zr (CN). To expect this, the above lower limit amount of 0.002% or more of Ti or 0.002% or more of Zr,
[Ti + Zr] × [N ]: 5 × 10 - 6 ~2 × 10 - 3
It is necessary to give The steel of the present invention that achieves this balance exhibits excellent fatigue strength and bend straightness in addition to machinability and chip crushability, and is suitable as a material for crankshafts and connecting rods that require this property. is there.
[0023]
Speaking of P, which is inevitable as an impurity, it is a harmful component for toughness and cannot be present in excess of 0.2%, while P improves the machinability, particularly the finish surface properties. It is also an ingredient. This effect is recognized in the presence of 0.001% or more.
[0024]
In addition to the basic alloy composition described above, the free-cutting steel for machine structure according to the present invention includes one or more elements belonging to the following groups, as required by the application of the steel. It can be additionally contained within the specified composition range. In these modified embodiments, the action of each alloy component that can be arbitrarily added and the reason for limiting the composition range will be described below.
[0025]
Cr: 3.5% or less, Mo: 2.0% or less Cr and Mo increase the hardenability, so an appropriate amount is preferably added. However, if added in a large amount, hot workability is impaired and cracking is caused. In consideration of cost, the upper limit of each was set to 3.5% for Cr and 2.0% for Mo.
[0026]
Cu: 2.0% or less Cu makes the structure dense and increases the strength. Addition of a large amount is not preferable for hot workability and machinability, so the addition is limited to 2.0% or less.
[0027]
Ni: 4.0% or less Ni also enhances hardenability like Cr and Mo, but has a negative effect on machinability. And considering the cost, 4.0% was made the upper limit.
[0028]
B: 0.0005 to 0.01%
B enhances hardenability by adding a small amount. To obtain this effect, 0.0005% or more must be added. Addition exceeding 0.01% is detrimental to hot workability.
[0031]
Both Pb: 0.4% or less and Bi: 0.4% or less are machinability improving elements. Pb exists alone or in the form of adhering to the outer periphery of the sulfide, and itself improves machinability. The upper limit of 0.4% was set for the reason that even if Pb more than this is added, it does not dissolve in the steel, but agglomerates and precipitates, resulting in a steel defect. The same applies to Bi.
[0032]
Se: 0.4% or less, Te: 0.2% or less These are also machinability improving elements. The respective upper limits of 0.4%, 0.2%, 0.1% and 0.05% were determined in consideration of adverse effects on hot workability.
[0033]
Inclusions present in the free-cutting steel for machine structure according to the present invention are double structure inclusions and MnS inclusions as seen in FIG. According to EPMA analysis, the double structure inclusions are oxides of Ca, Mg, Si and Al in the core, and MnS containing CaS surrounds the periphery. MnS inclusions are finely dispersed. On the other hand, MnS in the conventional free-cutting steel in which the effect of improving the machinability of MnS is simply obtained is large as shown in FIG. 2 (MnS in FIG. 2 is stretched by rolling).
[0034]
The form of the double structure inclusion is necessary to achieve the machinability of the tool life ratio of 5, which is a target of the present invention, through the mechanism to be discussed later, and realizes such an inclusion form. This is also the operating condition described above. The significance of these conditions has already been explained with the data in the disclosure of the previous invention, but since it is also important for the present invention, the explanation will be repeated below.
[0035]
The occupied area of sulfide inclusions containing Ca of 1.0 wt% or more present in contact with oxide inclusions having a CaO content of 0.2 to 62 wt% has a viewing area of 3.5 mm ^2. and the area occupied by the 2.0 × 10 ^-4 mm ² or more satisfy the above conditions inclusions per the tool life obtained when performing turning according carbide tool, showing the same S content of sulfur free-cutting steel The correlation with the ratio to tool life is shown in the graph of FIG. This data was obtained by turning the S45C free cutting steel according to the present invention, and the result of the tool life ratio of 5 or more is that the double structure inclusion is 2.0 × 10 ⁻⁴ mm. It shows what can be achieved when it occupies ² or more.
[0036]
Al: 0.001 to 0.020%
FIG. 4 is a graph obtained by plotting the correlation between the Al content and the tool life ratio. This graph shows the necessity for the Al content to be within the above-mentioned range in order to achieve the target with a tool life ratio of 5 or more.
[0037]
[S] / [O]: 8 to 40
In the free-cutting steel for machine structures having various S contents and O contents, whether or not the tool life ratio of 5 or more can be achieved is distinguished by different plots in the graph of FIG. Those that achieved the goal (● plot) are within the triangular region between the straight line [S] / [O] = 8 and the straight line [S] / [O] = 40, and those that are not ( It can be seen that the x plot is outside the region.
[0038]
[Ca] / [S]: 0.01-20
[Ca] × [S]: 1 × 10 ⁻⁵ to 1 × 10 ⁻³
Similarly to the above, in the free-cutting steel for machine structures having various S contents and Ca contents, the graph of FIG. 5 shows whether or not the tool life ratio of 5 or more can be achieved. . The target (● plot) is sandwiched between a straight line with [Ca] / [S] of 0.01 and a straight line with 0.20, and [Ca] × [S] is 1 × 10. it can be seen that are concentrated in quadrilateral region sandwiched between a straight line is a straight line and 1 × 10 ^{-3 -5.} All of the above conditions satisfying the conditions [S] / [O], [Ca] / [S] and [Ca] × [S] all achieve the tool life ratio of 5 or more.
[0039]
The inventors believe that the mechanical structural steel of the present invention exhibits excellent machinability is a mechanism of better protection and lubrication of the tool surface by the double structure inclusions. This is also included in the disclosure of the present invention, but will be described again.
[0040]
The double structure inclusion is a CaO.Al ₂ O ₃ based complex oxide, and a (Ca, Mn) S complex sulfide surrounds the core. This oxide has a low melting point in the CaO—Al ₂ O ₃ system, while the composite sulfide has a higher melting point than simple sulfide MnS. This double structure inclusion ensures that the sulfide surrounds the oxide by making the oxide have a low melting point of CaO—Al ₂ O ₃ system. The action of the sulfide inclusions softening to cut and cover and protect the tool surface during cutting is well known, but if only sulfide is present, the formation and maintenance of this coating is not stable. As the inventors of the present invention have found, when a CaO—Al ₂ O ₃ low melting point oxide coexists with sulfide inclusions, a coating is stably formed, and a (Ca, Mn) S system is formed. This composite sulfide has higher lubricating performance than simple MnS.
[0041]
The significance that the (Ca, Mn) S-based composite sulfide forms a film on the tool surface is to suppress the wear of the carbide tool called “thermal diffusion wear”. In thermal diffusion wear, when a tool comes into contact with chips generated from a cutting object at a high temperature, carbides represented by tungsten carbide WC constituting the tool material are thermally decomposed, and C diffuses into the chip metal and is lost. As a result, the tool becomes brittle and wears. When a highly lubricious coating is formed on the tool surface, the temperature rise of the tool is prevented and C diffusion is suppressed.
[0042]
If the viewpoint is changed, the double structure inclusion CaO-Al ₂ O ₃ / (Ca, Mn) S of the free-cutting steel of the present invention is the same as the conventional sulfur free-cutting steel MnS, It can be said that it has both advantages with anorcite CaO.Al ₂ O ₃ .2SiO ₂ which is an inclusion of calcium free-cutting steel. Although MnS exhibits lubricity on the tool surface, the stability of the coating is not so good and is ineffective against thermal diffusion wear. On the other hand, CaO.Al ₂ O ₃ .2SiO ₂ forms a stable film to prevent thermal diffusion wear, but has poor lubricity. On the other hand, the double structure inclusion of the present invention forms a stable film to effectively prevent thermal diffusion wear and exhibits better lubricity.
[0043]
Since the formation of such a double structure inclusion starts from preparing a low-melting-point composite oxide as described above, the amount of Al is important first, and the presence of at least 0.001% is necessary. . If the Al content is too large, the melting point of the composite oxide becomes high, so the content is made 0.020% or less. Next, in order to adjust the production amount of CaS, [Ca] × [S] and [Ca] / [S] are controlled to the above-described values.
[0044]
The above-mentioned mechanism is not a hypothesis but is based on the fact. In the previous invention, the state of the carbide tool surface after turning the free-cutting steel and the analysis result of the molten inclusions attached to it Was clarified by comparison with the case of turning conventional free-cutting steel.
[0045]
The good chip crushability that characterizes the free-cutting steel for machine structures of the present invention is brought about by the refinement of MnS inclusions as described above. Assuming that the amount of inclusions is constant, refinement means an increase in the number of inclusions. The amount of MnS inclusions in the steel of the present invention is mainly determined by the S content. Since the amount of S varies from 0.01 to 0.2%, the amount of MnS also varies accordingly, and the number of refined inclusions also increases or decreases. Among the steels of the present invention, the MnS inclusions are finer than the conventional MnS inclusions in the steel, but among them, the influence of the presence on the chip crushability is still the average particle size. 1.0 μm or more. (Here, “average particle diameter” refers to the average value of the major axis and the minor axis of the particle cross section appearing in the microscope field of view.)
[0046]
Although the S content is different, the present number of the MnS inclusions having an average particle size of 1.0 μm or more per unit cross-sectional area (mm ² ) of the steel of the present invention having high chip crushability is 400 times magnification As a result of investigation using a microscope, the following number of inclusions was obtained, and it was found that the relationship with the amount of S was also almost constant.
S content of steel MnS inclusion number 0.01% per 0.01% S 5.4 / mm ² 5.4 / mm ²
0.03% 16.2 pieces / mm ² 5.4 pieces / mm ²
0.062% 32.0 pieces / mm ² 5.2 pieces / mm ²
0.125% 32.0 pieces / mm ² 6.2 pieces / mm ²
From this data, it was concluded that good chip crushability could be ensured if the number of MnS inclusions was 5 / mm ² or more per 0.01% S content over various S content ranges.
[0047]
【Example】
In the tables of Examples and Comparative Examples below, those whose numbers begin with an uppercase letter (A, B,...) Are examples, and those that begin with a lowercase letter (a, b,...) Are comparative examples. The composition table is expressed by weight% and the balance is Fe, while O, N, Ca and Mg are expressed in ppm. The melted alloy was cast into an ingot, and a round bar type test piece having a diameter of 72 mm was taken from the ingot and used for the test. The method and evaluation of each test are as follows.
[0048]
[Inclusion Area] When the sulfide containing Ca existing in contact with the oxide containing Al ₂ O ₃ has a dedicated area of 2 × 10 ⁻⁴ mm ² /3.5 mm ² or more (good) ○ mark), otherwise it was judged as defective (× mark).
[0049]
[Machinability] Turning with the following conditions using a carbide tool,
Speed: 200m / min Feeding: 0.2mm / Rotation depth: 2.0mm
With a tool life of sulfur free-cutting steel with S content in the range of 0.01-0.2% as standard, 5 times longer tool life can be achieved. Good machinability (○ mark), 2 times or more When it was less than 5 times, machinability was acceptable (Δ mark), and when it was less than 2 times, machinability was poor (x mark). The tool life was evaluated by the processing time until the average flank wear width became 0.2 mm.
[0050]
[Presence / absence of protective coating] Analyzing the substance attached to the surface of the cutting tool, it is considered that a lubricating protective coating was formed when Ca, Mn and S were detected (circle mark), otherwise formed. Not marked (x mark).
[0051]
[Crushability of chips] Collect chips when cutting under the following conditions,
Speed: 150m / min Feed: 0.025-0.200mm / Rotation depth: 0.3-1.0mm
Tool: DNMG150480-MA
A score of 0 to 4 points was given according to the length, and the total of the points obtained by 30 cutting tests was defined as a “chip chipping index”. A case where the chip crushability index was higher than that of the sulfur free-cutting steel having the same sulfur content was judged as good (marked with ○), and a case where the chip smashability was low or equal was marked as poor (marked with ×).
[0052]
[Example 1]
The present invention was applied to the S45C system. The alloy compositions are shown in Table 1 (Example) and Table 2 (Comparative Example), and the component ratio of each free-cutting steel, that is, [S] / [O], [Ca] · [S] × 10 ⁻⁵ and [Ca]. Table 3 (Examples) and Table 4 (Comparative Examples) collectively show the values of / [S], the form of inclusions, the presence or absence of a protective coating, machinability and chip crushability.
[0053]
Table 1 Examples of S45C system

[0054]
Table 2 Comparative example of S45C system

[0055]
Table 3 Examples of S45C system

[0056]
Table 4 Comparative example of S45C system

[0057]
[Example 2]
The S15C-based free-cutting steel was tested for alloy melting and machinability in the same manner as in Example 1. The alloy compositions are shown in Table 5 (Example) and Table 6 (Comparative Example), and the test results are shown in Table 7 (Example) and Table 8 (Comparative Example), respectively.
[0058]
Table 5 Examples of S15C system

[0059]
Table 6 Comparative example of S15C system

[0060]
Table 7 Examples of S15C system

[0061]
Table 8 Comparative example of S15C system

[0062]
[Example 3]
The S55C free cutting steel was subjected to alloy melting and machinability tests in the same manner as in Example 1. The alloy compositions are shown in Table 9 (Example) and Table 10 (Comparative Example), and the test results are shown in Table 11 (Example) and Table 12 (Comparative Example), respectively.
[0063]
Table 9 Example of S55C system

[0064]
Table 10 Comparative example of S55C system

[0065]
Table 11 Examples of S55C system

[0066]
Table 12 Comparative example of S55C system

[0067]
[Example 4]
The SCr415 free cutting steel was subjected to alloy melting and machinability tests in the same manner as in Example 1. The alloy compositions are shown in Table 13 (Example) and Table 14 (Comparative Example), and the test results are shown in Table 15 (Example) and Table 16 (Comparative Example), respectively.
[0068]
Table 13 SCr415 series Examples

[0069]
Table 14 SCr415 series comparative example

[0070]
Table 15 SCr415 series Examples

[0071]
Table 16 SCr415 series comparative example

[0072]
[Example 5]
The SCM440 free cutting steel was subjected to alloy melting and machinability tests in the same manner as in Example 1. The alloy compositions are shown in Table 17 (Example) and Table 18 (Comparative Example), and the test results are shown in Table 19 (Example) and Table 20 (Comparative Example), respectively.
[0073]
Table 17 SCM440 series Examples

[0074]
Table 18 SCM440 system comparative example

[0075]
Table 19 SCM440 series Examples

[0076]
Table 20 Comparative example of SCM440 system

[0077]
【The invention's effect】
In the free-cutting steel for machine structure of the present invention, the same machinability as that of the free-cutting steel disclosed above is realized. In other words, the double-structured inclusions, which are inclusions that provide high machinability, exist in an optimal form, so that the tool life is more than five times that of conventional sulfur free-cutting steel in cutting, especially carbide tool turning. This goal can be easily achieved.
[0078]
Furthermore, in the free-cutting steel for machine structure of the present invention, by adding a small amount of Ti and / or Zr, it has a finely dispersed MnS inclusion, thereby becoming a free-cutting steel with high chip crushability. This is particularly advantageous for turning. Products with fine Ti (CN) and / or Zr (CN) produced in steel can suppress the growth of prior austenite grains during hot forging, so machinability and chip crushability In addition, it has high fatigue strength and bending straightness, and is useful for applications that require such properties.
[0079]
The production method of the present invention is a method capable of reliably producing free-cutting steel for machine structures as described above, and advantageously forms double-structure inclusions by performing controlled deoxidation, and more appropriately. At the right timing, that is, after the double structure inclusions are formed by controlled deoxidation, by adding an appropriate amount of Ti and / or Zr, the MnS inclusions are finely dispersed so that the chip crushability is good. Free-cutting steel can be obtained. If this manufacturing method is carried out by appropriately selecting the amount of O and N together with the amount of Ti and / or Zr, fine Ti (CN) and / or Zr (CN) is generated in the steel, and fatigue strength And free-cutting steel for machine structures with improved bend straightening.
[Brief description of the drawings]
FIG. 1 is a photomicrograph showing the shape of inclusions in free-cutting steel for machine structures according to the present invention.
FIG. 2 is a photomicrograph showing the shape of inclusions in conventional sulfur free-cutting steel.
FIG. 3 is a graph showing the relationship between the area occupied by “double structure inclusions” and the tool life ratio in free-cutting steel for machine structures.
FIG. 4 is a graph plotting the relationship between the Al content and the tool life ratio in free-cutting steel for machine structures.
FIG. 5 is a graph showing whether or not a tool life ratio of 5 or more has been achieved in free-cutting steel for machine structures having various S contents and O contents.
FIG. 6 is a graph showing whether or not a tool life ratio of 5 or more has been achieved in free-cutting steel for machine structures having various S contents and Ca contents.

Claims (5)

By weight, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Al: In addition to 0.001-0.020%, Ca: 0.0005-0.02%, O: 0.0005-0.01% and N: 0.001-0.04%, Ti: 0.002 -0.020% and Zr: 0.002 to 0.040% of one or two types, the balance is an inevitable impurity and Fe alloy composition, CaO content is 0.2-62 The occupied area of sulfide inclusions containing 1.0% by weight or more of Ca present in contact with the oxide inclusions by weight% is 2.0 × 10 ⁻⁴ mm per 3.5 mm ² visual field area. ² or more, MnS inclusions, the number is a S content of from 5 per 0.01% / mm ² or more the average particle diameter of 1.0μm or more of By dispersed in earthenware pots, high chip friability with excellent machinability steel for machine structural use.   In addition to the alloy components defined in claim 1, Cr: 3.5% or less, Mo: 2.0% or less, Cu: 2.0% or less, Ni: 4.0% or less, and B: 0.00. Machine structural steel containing one or more of 0005 to 0.01%, having excellent machinability and high chip crushability.   In addition to the alloy components defined in claim 1, one or two of Pb: 0.4% or less, Bi: 0.4% or less, Se: 0.4% or less, and Te: 0.2% or less Machine structural steel that contains more than seeds and has excellent machinability and high chip crushability. A method for producing a steel for machine structural use according to any one of claims 1 to 3 , wherein C: 0.05 to 0.8%, Si: 0.01 to 2.5%, Mn : 0.1-3.5%, S: 0.01-0.2%, Al: 0.001-0.020%, Ca: 0.0005-0.02%, O: 0.0005-0 .01% and N: 0.001 to 0.04%, and the balance is adjusted by performing an operation that satisfies the following conditions by melting an alloy composed of inevitable impurities and Fe. Deoxidation,
[S] / [O]: 8 to 40
[Ca] × [S]: 1 × 10 ⁻⁵ to 1 × 10 ⁻³
[Ca] / [S]: 0.01 to 20 and [Al]: 0.001 to 0.020%
The occupied area of sulfide inclusions containing Ca of 1.0 wt% or more present in contact with oxide inclusions having a CaO content of 0.2 to 62 wt% has a viewing area of 3.5 mm ^2. After making it become 2.0 × 10 ⁻⁴ mm ² or more per one, adding one or two of Ti: 0.002 to 0.020% and Zr: 0.002 to 0.040%, A fine Ti oxide and / or Zr oxide is formed by the reaction of O and Ti and / or Zr in the steel after the adjusted deoxidation, and a MnS inclusion is formed using a composite oxide containing these as a nucleus. To obtain MnS inclusions dispersed so that the number of those having an average particle diameter of 1.0 μm or more is 5 / mm ² or more per S content of 0.01% . Machine structural steel with excellent machinability and high chip crushability Production method. When Ti and / or Zr are added, fine Ti (CN) and / or Zr (CN) is formed by adjusting the amount of N so that the following conditions are satisfied:
[Ti + Zr] × [N ]: 5 × 10 - 6 ~2 × 10 - 3
In addition to precipitating MnS inclusions with these as nuclei and maintaining the fine grain size of the prior austenite during hot forging, in addition to machinability and chip crushability, it exhibits excellent fatigue strength and bending straightening The manufacturing method of Claim 4 which obtains the steel for machine structures.

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