JP2005350702A - Steel having superior machinability for machine structural use - Google Patents

Steel having superior machinability for machine structural use Download PDF

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JP2005350702A
JP2005350702A JP2004170526A JP2004170526A JP2005350702A JP 2005350702 A JP2005350702 A JP 2005350702A JP 2004170526 A JP2004170526 A JP 2004170526A JP 2004170526 A JP2004170526 A JP 2004170526A JP 2005350702 A JP2005350702 A JP 2005350702A
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steel
ratio
machinability
oxide
sulfide
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JP4906245B2 (en
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Takeshi Fujimatsu
威史 藤松
Norimasa Tokokage
典正 常陰
Kazuhiko Hiraoka
和彦 平岡
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Sanyo Special Steel Co Ltd
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Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for machine structural use having stable and superior machinability without containing Pb which is a substance of giving a load to the environment. <P>SOLUTION: The steel having superior machinability for machine structural use includes, by mass%, 0.10-0.60% C, 0.05-2.0% Si, 0.3-2.5% Mn, 0.02-0.25% S, 0.002-0.030% Al, 0.0005-0.01% Ca, 0.0005-0.008% O, 0.02% or less N and the balance Fe with unavoidable impurities, so that the mass ratio Ca/Al of Ca to Al can be 0.1 to 1.0; and includes a complex sulfide that contains an oxide therein or is adjacent to the oxide, in which the area ratio of oxides to the all complex sulfides contained in the steel is 3 to 90% in the cross-section, and which has the ratio of the pieces of the complex oxides to the pieces of the total sulfides per unit area in an amount of 4 to 80%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

例えば、自動車用部品であるギヤ、シャフトなどの駆動系部品などの用途に用いられる鋼で、熱間圧延ままあるいは熱間鍛造した素材に各種切削加工を施して所定の形状に加工した後、その後、浸炭などの表面硬化処理もしくは焼入れ焼戻しなどを施して使用される被削性に優れた機械構造用快削鋼に関する。   For example, steel used for driving parts such as gears and shafts that are parts for automobiles, etc., after hot-rolled or hot-forged material is subjected to various cutting processes and processed into a predetermined shape, then Further, the present invention relates to a free-cutting steel for machine structure having excellent machinability, which is used after being subjected to surface hardening treatment such as carburizing or quenching and tempering.

主として肌焼鋼が多く用いられる自動車のギヤ、シャフトなどの駆動系部品用途に対して、Ｐｂ快削鋼が被削性重視の観点から少なからず使用されている。しかしながら、近年の環境負荷物質低減の時流から、Ｐｂ快削鋼の使用量削減が求められている。そこでＰｂ快削鋼に代わる非Ｐｂ快削鋼として、鋼に硫黄（Ｓ）を添加して鋼中にマンガン硫化物（ＭｎＳ）を生成させることにより、これを切削時の応力集中源として作用させることで被削性を改善させるＳ系快削鋼が有望視されている。そして、さらにＳ系快削鋼の超硬工具加工性を改善するためＣａを添加する発明が開示されている（例えば、特許文献１参照。）。一方、本願発明者らは、Ｃａを添加したＳ系快削鋼において、鋼中のＣａ含有量やＳ含有量がほぼ同等であるにも関わらず、工具寿命がばらつき、安定した切削加工性が得られないということを見出している。   Pb free-cutting steel is used in many respects from the viewpoint of machinability, for use in drive system components such as gears and shafts of automobiles, which mainly use case-hardened steel. However, due to the recent trend of reducing environmentally hazardous substances, reduction of the amount of Pb free-cutting steel is required. Therefore, as a non-Pb free-cutting steel that replaces Pb free-cutting steel, sulfur (S) is added to the steel to produce manganese sulfide (MnS) in the steel, which acts as a stress concentration source during cutting. Therefore, S-based free-cutting steel that improves machinability is promising. And invention which adds Ca in order to improve the carbide tool workability of S system free-cutting steel is indicated (for example, refer to patent documents 1). On the other hand, the inventors of the present application, in the S-based free-cutting steel to which Ca is added, have varying tool life and stable cutting workability despite the Ca content and S content in the steel being substantially equal. I find that I can not get it.

特開昭５７−１４０８５３号公報JP-A-57-140853

本発明が解決しようとする課題は、環境負荷物質であるＰｂを含有することなくＣａを含有するＳ系快削鋼において、特に被削性に優れる化学成分および介在物組成を含有する機械構造用鋼を提供することである。   The problem to be solved by the present invention is an S-based free-cutting steel containing Ca without containing Pb, which is an environmentally hazardous substance, especially for a mechanical structure containing chemical components and inclusion compositions excellent in machinability. Is to provide steel.

本願発明者らの鋭意研究の結果、Ｃａ含有量がほぼ同等の鋼においても、鋼中のＣａとＡｌを所定の質量％比に制御することで酸化物組成がＡｌ2Ｏ3に富む高融点介在物ではなく、ＣａＯ−Ａｌ2Ｏ3やＣａＯ−Ａｌ2Ｏ3−ＳｉＯ2といった若干低融点の化学組成に改質されることを見出し、さらに鋼中に含有される介在物の面積および個数比率を所定の範囲に限定することによって安定した良好な被削性を発揮できることが分かった。 As a result of the intensive studies by the inventors of the present application, even in a steel having approximately the same Ca content, a high melting point in which the oxide composition is rich in Al 2 O 3 by controlling Ca and Al in the steel to a predetermined mass% ratio. It was found that it was modified to a slightly lower melting point chemical composition such as CaO—Al 2 O 3 or CaO—Al 2 O 3 —SiO 2 instead of inclusions, and the area and number of inclusions contained in the steel It was found that stable good machinability can be exhibited by limiting the ratio to a predetermined range.

すなわち、Ｃａによる被削性改善効果を得るには、質量％比でＣａ／Ａｌを０．１以上とする必要がある。一方、質量％比でＣａ／Ａｌが１．０を超えると超硬工具被覆保護効果が飽和するとともに、硫化物が硬くなりすぎるためにドリル寿命の低下を招くことを見出したものである。   That is, in order to obtain the machinability improving effect by Ca, it is necessary to make Ca / Al 0.1 or more by mass% ratio. On the other hand, it was found that when Ca / Al exceeds 1.0 by mass ratio, the carbide tool covering protective effect is saturated and the sulfide is too hard, leading to a reduction in drill life.

さらに、上記のＣａ／Ａｌ比の範囲に制御された本発明の機械構造用鋼において鋼中に存在する不可避不純物と共に含有される酸化物を内包及び隣接する硫化物である複合硫化物について、酸化物を内包若しくは隣接する硫化物の断面積をＳSとし、複合硫化物中の酸化物の断面積をＳOとするとき、この複合硫化物の断面積（ＳS＋ＳO）に占める酸化物の断面積ＳOの割合である面積率が３〜９０％であり、さらに単位面積中の全硫化物の個数に対する酸化物を内包若しくは隣接する上記複合硫化物の個数割合である個数率が４〜８０％であるものとするとき、安定した良好な被削性を有することを見出したものである。 Further, in the steel for machine structural use of the present invention controlled to the above range of Ca / Al ratio, the oxide containing oxides contained together with the inevitable impurities present in the steel is included and the composite sulfide which is an adjacent sulfide is oxidized. the cross-sectional area of the enclosed or adjacent sulfide objects and S S, when the cross-sectional area of the oxide in the complex sulfide and S O, the oxide occupied in the cross-sectional area of the composite sulfide (S S + S O) The area ratio, which is the ratio of the cross-sectional area S 2 O , is 3 to 90%, and the number ratio, which is the number ratio of the composite sulfide containing or adjacent to the oxide to the total number of sulfides in the unit area, is 4 It has been found that when it is -80%, it has a stable and good machinability.

すなわち、上記の課題を解決するための本発明の手段は、請求項１の手段の発明では、質量％で、Ｃ：０．１０〜０．６０％、Ｓｉ：０．０５〜２．０％、Ｍｎ：０．３〜２．５％、Ｓ：０．０２〜０．２５％、Ａｌ：０．００２〜０．０３０％、Ｃａ：０．０００５〜０．０１％、Ｏ：０．０００５〜０．００８％、Ｎ：０．０２％以下、残部Ｆｅおよび不可避不純物からなり、ＣａとＡｌの質量比がＣａ／Ａｌ：０．１〜１．０で、かつ硫化物を含有する鋼であり、この鋼中に含有の硫化物に酸化物を内包若しくは隣接する複合硫化物の占める断面積に対する酸化物の占める断面積の割合である面積率が３〜９０％であり、さらに単位面積中の全硫化物の個数に対する該複合酸化物の個数の割合である個数率が４〜８０％であることを特徴とする被削性に優れた機械構造用鋼である。   That is, the means of the present invention for solving the above-mentioned problems is the mass% in the invention of the means of claim 1, C: 0.10 to 0.60%, Si: 0.05 to 2.0%. , Mn: 0.3-2.5%, S: 0.02-0.25%, Al: 0.002-0.030%, Ca: 0.0005-0.01%, O: 0.0005 -0.008%, N: 0.02% or less, balance Fe and inevitable impurities, Ca / Al mass ratio Ca / Al: 0.1-1.0, and steel containing sulfide Yes, the area ratio, which is the ratio of the cross-sectional area occupied by the oxide to the cross-sectional area occupied by the composite sulfide containing the oxide in the sulfide contained in this steel, is 3 to 90%, and in the unit area The number ratio, which is the ratio of the number of composite oxides to the total number of sulfides, is 4 to 80%. An excellent mechanical structural steel in machinability to.

なお、この場合、該複合硫化物の断面積に対する酸化物の占める断面積の割合である面積率は鋼材の圧鍛方向に平行な被検面積１ｍｍ2（縦横１ｍｍ）中の任意の複合硫化物２０個について酸化物の面積率（％）を求め、その平均値を用いるものとする。また、単位面積中の全硫化物の個数に対する酸化物を内包または隣接する複合硫化物の個数の割合である個数率は鋼材の圧延方向に平行な被検面積１ｍｍ2（縦横１ｍｍ）中の複合硫化物個数を全硫化物個数で除したものである。 In this case, the area ratio, which is the ratio of the cross-sectional area occupied by the oxide to the cross-sectional area of the composite sulfide, is an arbitrary composite sulfide in a test area 1 mm 2 (longitudinal and lateral 1 mm) parallel to the forging direction of the steel material. The area ratio (%) of the oxide is obtained for 20 pieces, and the average value is used. In addition, the number ratio, which is the ratio of the number of composite sulfides containing or adjacent to the oxide to the total number of sulfides in a unit area, is a composite in a test area 1 mm 2 (longitudinal and lateral 1 mm) parallel to the rolling direction of the steel material. The number of sulfides divided by the total number of sulfides.

さらに、上記手段に加えて、鋼成分は質量％で、Ｎｉ：０．１〜２．５％、Ｃｒ：０．１〜２．５％、Ｍｏ：０．０５〜１．５０％、Ｖ：０．０１〜０．５０％、Ｔｉ：０．０１〜０．５０％、Ｎｂ：０．００１〜０．３０％、Ｂ：０．０００３〜０．００５％から選択した１種または２種以上を含有することを特徴とする請求項１の手段の被削性に優れた機械構造用鋼である。   Further, in addition to the above means, the steel components are in mass%, Ni: 0.1 to 2.5%, Cr: 0.1 to 2.5%, Mo: 0.05 to 1.50%, V: One or more selected from 0.01 to 0.50%, Ti: 0.01 to 0.50%, Nb: 0.001 to 0.30%, B: 0.0003 to 0.005% The steel for machine structural use according to claim 1, which is excellent in machinability.

上記の機械構造用鋼とすることで、疲労強度と強度異方性を改善し、安定した被削性を発揮する機械構造用鋼が得られる。特に上記において、Ｃａ／Ａｌ比を０．１〜１．０とするが、これはＣａを被削性に有効な介在物組成制御に作用させるために必要であり、さらに酸化物を内包若しくは隣接する硫化物である複合硫化物において、複合硫化物の占める面積に対する酸化物の占める面積の割合の面積率が３〜９０％であり、さらに単位面積中の全硫化物の個数に対する酸化物を内包若しくは隣接する複合硫化物の個数の割合である個数率が４〜８０％であるものとする。このことにより、安定した被削性を発揮する機械構造用鋼が得られるのである。   By using the above steel for machine structure, a steel for machine structure that improves fatigue strength and strength anisotropy and exhibits stable machinability can be obtained. In particular, in the above, the Ca / Al ratio is set to 0.1 to 1.0, which is necessary to cause Ca to act on the inclusion composition control effective for machinability, and further includes or includes an oxide. In the composite sulfide, which is a sulfide to be produced, the area ratio of the area occupied by the oxide to the area occupied by the composite sulfide is 3 to 90%, and the oxide is included with respect to the total number of sulfides in the unit area. Alternatively, the number ratio, which is the ratio of the number of adjacent composite sulfides, is 4 to 80%. As a result, a machine structural steel that exhibits stable machinability is obtained.

本願発明の鋼成分の限定理由を以下に説明する。なお、それぞれの％は質量％を示すものとする。   The reasons for limiting the steel components of the present invention will be described below. In addition, each% shall show the mass%.

Ｃ：０．１０〜０．６０％
Ｃは、強度確保に必要な元素で、このためには０．１０％以上が必要である。しかし０．６０％を超えると被削性を低下する。そこでＣは０．１０〜０．６０％とする。 C: 0.10 to 0.60%
C is an element necessary for securing the strength, and for this purpose, 0.10% or more is necessary. However, if it exceeds 0.60%, the machinability deteriorates. Therefore, C is set to 0.10 to 0.60%.

Ｓｉ：０．０５〜２．０％
Ｓｉは、脱酸剤として必要な元素で、このためには０．０５％以上必要である。しかし、２．０％を超えると硬質酸化物増大による被削性の低下を招き、さらに、浸炭用途では、浸炭層表面の粒界酸化層深さが増大し、疲労寿命を低下する。そこでＳｉは０．０５〜２．０％とし、望ましくは０．０５〜０．５％とする。 Si: 0.05-2.0%
Si is an element necessary as a deoxidizer, and 0.05% or more is necessary for this purpose. However, if it exceeds 2.0%, the machinability is reduced due to an increase in the hard oxide. Further, in carburizing applications, the grain boundary oxide layer depth on the surface of the carburized layer is increased, and the fatigue life is reduced. Therefore, Si is set to 0.05 to 2.0%, preferably 0.05 to 0.5%.

Ｍｎ：０．３〜２．５％
Ｍｎは、焼入性確保に必要な元素で、またＭｎＳ生成に必要な元素であり、このためには０．３％以上必要である。しかし、２．５％より多過ぎると被削性低下を招き、さらに浸炭用途では過剰Ｍｎが浸炭層表面の浸炭異常層深さを増大し、疲労寿命を低下させる。そこでＭｎは０．３〜２．５％とし、望ましくは０．４〜１．５％とする。 Mn: 0.3 to 2.5%
Mn is an element necessary for ensuring hardenability and an element necessary for generating MnS. For this purpose, 0.3% or more is necessary. However, if it is more than 2.5%, the machinability is lowered, and in the case of carburizing, excessive Mn increases the carburizing abnormal layer depth on the surface of the carburizing layer and reduces the fatigue life. Therefore, Mn is set to 0.3 to 2.5%, preferably 0.4 to 1.5%.

Ｓ：０．０２〜０．２５％
Ｓは、ドリル加工や歯切り加工等の際の被削性確保に必要な元素で、また切り屑処理性確保に必要な元素であり、このためには０．０２％以上必要である。しかし、０．２５％より多過ぎると静的強度、疲労強度などの強度特性を低下し、さらに熱間加工性を低下する。そこでＳは０．０２〜０．２５％とし、望ましくは０．０３〜０．１６％とする。 S: 0.02-0.25%
S is an element necessary for ensuring machinability at the time of drilling, gear cutting, and the like, and is an element necessary for ensuring chip disposal, and for this purpose, 0.02% or more is necessary. However, if it is more than 0.25%, strength properties such as static strength and fatigue strength are lowered, and hot workability is further lowered. Therefore, S is 0.02 to 0.25%, preferably 0.03 to 0.16%.

Ａｌ：０．００２〜０．０３０％
Ａｌは、窒化物を形成して浸炭時の結晶粒粗大化抑制に効果のある元素で、このためには０．００２％以上必要である。しかし、被削性および疲労寿命に有害なＡｌ2Ｏ3を低減する必要があるので、上限を０．０３０％とする。そこでＡｌは０．００２〜０．０３０％とし、望ましくは０．００２〜０．０２５％とする。 Al: 0.002 to 0.030%
Al is an element that forms nitrides and is effective in suppressing grain coarsening during carburization. For this purpose, 0.002% or more is necessary. However, since it is necessary to reduce Al 2 O 3 which is harmful to machinability and fatigue life, the upper limit is made 0.030%. Therefore, Al is made 0.002 to 0.030%, preferably 0.002 to 0.025%.

Ｃａ：０．０００５〜０．０１％
Ｃａは、超硬旋削加工時の工具保護に有効に作用して切削加工性を改善する元素であり、また、硫化物形態制御による疲労強度や静的強度改善にも有効である。そのためには０．０００５％以上必要である。しかし、０．０１％を超えると疲労寿命や静的強度を低下させる大型介在物が生成し、また製造性の悪化を招くとともに製造コストの増大をもたらす。そこでＣａは０．０００５〜０．０１％とし、望ましくは０．０００５〜０．００５０％とする。 Ca: 0.0005 to 0.01%
Ca is an element that effectively acts on tool protection during carbide turning to improve machinability, and is also effective in improving fatigue strength and static strength by sulfide form control. For that purpose, 0.0005% or more is necessary. However, if it exceeds 0.01%, large inclusions that reduce the fatigue life and static strength are generated, and the productivity is deteriorated and the manufacturing cost is increased. Therefore, Ca is 0.0005 to 0.01%, preferably 0.0005 to 0.0050%.

Ｏ：０．０００５〜０．００８％
Ｏは、工具被覆保護に有効なＣａ系酸化物生成に必要な元素で、このためには０．０００５％以上必要である。しかし、０．００８％より多過ぎると、酸化物系介在物が多量に生成して疲労強度を低下する。そこでＯは０．０００５〜０．００８％とし、望ましくは０．０００５〜０．００５％とする。 O: 0.0005 to 0.008%
O is an element necessary for the production of Ca-based oxides effective for tool coating protection, and for this purpose, 0.0005% or more is necessary. However, if it is more than 0.008%, a large amount of oxide inclusions are generated and the fatigue strength is lowered. Therefore, O is 0.0005 to 0.008%, preferably 0.0005 to 0.005%.

Ｎ：０．０２％以下
Ｎは、Ａｌと窒化物を形成し、結晶粒粗大化の抑制に効果のある元素である。しかし、０．０２％より多くても結晶粒粗大化の抑制効果が飽和する。そこでＮは０．０２％以下とする。 N: 0.02% or less N is an element that forms nitrides with Al and is effective in suppressing grain coarsening. However, even if it exceeds 0.02%, the effect of suppressing the coarsening of crystal grains is saturated. Therefore, N is set to 0.02% or less.

質量％比でＣａ／Ａｌ：０．１〜１．０
Ｃａ／Ａｌにおいて、下限の０．１はＣａによる酸化物および硫化物の制御に必要であり、上限の１．０を超えると、工具被覆保護効果が飽和し、さらに硫化物が硬くなり過ぎるためにドリル寿命の低下を招く。そこでＣａ／Ａｌは０．１〜１．０とし、望ましくは０．１〜０．８とする。さらに望ましくは０．１〜０．６とする。 Ca / Al by mass% ratio: 0.1 to 1.0
In Ca / Al, the lower limit of 0.1 is necessary for the control of oxides and sulfides by Ca. If the upper limit of 1.0 is exceeded, the tool covering protection effect is saturated and the sulfide becomes too hard. This will reduce the drill life. Therefore, Ca / Al is 0.1 to 1.0, preferably 0.1 to 0.8. More desirably, it is 0.1 to 0.6.

｛ＳO／（ＳS＋ＳO）｝×１００＝３〜９０（％）……（１）
（酸化物を内包若しくは隣接する硫化物である複合硫化物の個数／全硫化物の個数）×１００＝４〜８０（％）……（２）
酸化物を内包または隣接して有する硫化物の断面積をＳSとし該酸化物の断面積をＳOとするとき、この複合硫化物の断面積（ＳS＋ＳO）に対する酸化物の占める断面積ＳOの割合である面積率が３〜９０％であり、さらに単位面積すなわち１平方ｍｍ中の全硫化物の個数に対する複合硫化物の個数の割合である個数比率が４〜８０％であるものとするものである。すなわち、式（１）が３％未満では超硬工具摩耗抑制に効果が無く、９０％を超えるとドリル加工時の工具と酸化物の接触頻度が高くなり、工具摩耗量が増大する。さらに全硫化物個数に対する複合硫化物の個数の比率である（２）式が４％未満では超硬工具摩耗抑制効果が不足し、８０％を超えると通常のＭｎＳより硬質の複合硫化物が大多数を占めるため、ドリル加工における工具摩耗量の増大を招く。そこで、上記の（１）式および（２）式とする。 {S O / (S S + S O )} × 100 = 3 to 90 (%) (1)
(Number of complex sulfides containing oxides or adjacent sulfides / number of total sulfides) × 100 = 4 to 80 (%) (2)
When the cross-sectional area of the oxide of the cross-sectional area as S S sulfide having encapsulated or adjacent to the oxide and S O, the cross-sectional occupied by the oxide to the cross-sectional area of the composite sulfide (S S + S O) The area ratio that is the ratio of the area S 2 O is 3 to 90%, and the number ratio that is the ratio of the number of composite sulfides to the number of total sulfides in a unit area, that is, 1 mm 2 is 4 to 80%. It is intended. That is, if the formula (1) is less than 3%, there is no effect in suppressing carbide tool wear, and if it exceeds 90%, the contact frequency between the tool and the oxide during drilling increases and the amount of tool wear increases. Furthermore, if the formula (2), which is the ratio of the number of composite sulfides to the total number of sulfides, is less than 4%, the effect of suppressing carbide tool wear is insufficient, and if it exceeds 80%, composite sulfides harder than ordinary MnS are larger. Since it occupies a large number, the amount of tool wear in drilling increases. Therefore, the above equations (1) and (2) are used.

Ｎｉ：０．１〜２．５％
Ｎｉは、焼入性および靱性の確保に必要な元素であり、このためには０．１％以上必要である。しかし、２．５％より多過ぎると切削性を低下させ、また高価な元素であるためコストアップを招く。そこでＮｉは０．１〜２．５％とする。 Ni: 0.1 to 2.5%
Ni is an element necessary for ensuring hardenability and toughness, and for this purpose, 0.1% or more is necessary. However, if it is more than 2.5%, the machinability is lowered, and the cost is increased because it is an expensive element. Therefore, Ni is set to 0.1 to 2.5%.

Ｃｒ：０．１〜２．５％
Ｃｒは、基地の焼入性の確保に必要な元素で、このためには０．１％以上必要である。しかし、２．５％より多過ぎると切削性の低下を招く。そこでＣｒは０．１〜２．５％とし、望ましくは０．６〜１．５％とする。 Cr: 0.1 to 2.5%
Cr is an element necessary for ensuring the hardenability of the base, and for this purpose, 0.1% or more is necessary. However, if it is more than 2.5%, the machinability is lowered. Therefore, Cr is set to 0.1 to 2.5%, preferably 0.6 to 1.5%.

Ｍｏ：０．０５〜１．５０％
Ｍｏは、基地の焼入性確保に必要な元素で、このためには０．０５％以上必要である。しかし、１．５０％より多過ぎると切削性を低下し、また製造コストを増大させる。そこでＭｏは０．０５〜１．５０％とする。 Mo: 0.05 to 1.50%
Mo is an element necessary for ensuring the hardenability of the base, and 0.05% or more is necessary for this purpose. However, if it is more than 1.50%, the machinability is lowered and the manufacturing cost is increased. Therefore, Mo is set to 0.05 to 1.50%.

Ｖ：０．０１〜０．５０％
Ｖは、基地の焼入性の確保に必要な元素で、このためには０．０１％以上必要である。しかし、０．５０％より多過ぎると切削性を低下し、また製造コストを増大させる。そこでＶは０．０１〜０．５０％とする。 V: 0.01 to 0.50%
V is an element necessary for ensuring the hardenability of the base, and for this purpose, it is necessary to be 0.01% or more. However, if it is more than 0.50%, the machinability is lowered and the production cost is increased. Therefore, V is set to 0.01 to 0.50%.

Ｔｉ：０．０１〜０．５０％
Ｔｉは、浸炭時の結晶粒粗大化抑制に効果のある元素であり、このためには０．０１％以上必要である。しかし、０．５０％より多過ぎると熱間加工性および切削性を損なうＴｉＳの生成を抑制することができなくなる。そこでＴｉは０．０１〜０．５０％とする。 Ti: 0.01 to 0.50%
Ti is an element effective in suppressing grain coarsening during carburization, and for this purpose, 0.01% or more is necessary. However, if it exceeds 0.50%, it becomes impossible to suppress the production of TiS that impairs hot workability and machinability. Therefore, Ti is set to 0.01 to 0.50%.

Ｎｂ：０．００１〜０．３０％
Ｎｂは、浸炭時の結晶粒粗大化抑制に効果のある元素であり、このためには０．００１％以上必要である。しかし、０．３０％より多過ぎるとコストアップをもたらすとともに、切削性を損なうＮｂＣの生成を抑制することができなくなる。そこでＮｂは０．００１〜０．３０％とする。 Nb: 0.001 to 0.30%
Nb is an element effective in suppressing the coarsening of crystal grains during carburization. For this purpose, 0.001% or more is necessary. However, if it is more than 0.30%, the cost is increased, and the production of NbC that impairs the machinability cannot be suppressed. Therefore, Nb is set to 0.001 to 0.30%.

Ｂ：０．０００３〜０．００５％。
Ｂは、焼入性確保および粒界強化に必要な元素であり、このためには０．０００３％以上必要である。しかし、０．００５％を超えるとその効果が飽和する。そこでＢは０．０００３〜０．００５％とする。 B: 0.0003 to 0.005%.
B is an element necessary for ensuring hardenability and strengthening grain boundaries. For this purpose, B is required to be 0.0003% or more. However, when it exceeds 0.005%, the effect is saturated. Therefore, B is set to 0.0003 to 0.005%.

本発明の機械構造用鋼は、快削性成分として環境負荷元素であるＰｂを含有しないにもかかわらず、工具寿命にばらつきのない安定した優れた切削加工性を得ることができ、かつ、環境負荷物質の削減に貢献するなど、優れた効果を奏するものである。   The steel for machine structural use according to the present invention can obtain stable and excellent machinability with no variation in tool life despite the fact that it does not contain Pb, which is an environmental load element, as a free-cutting component. It has excellent effects such as contributing to the reduction of load substances.

本発明を実施するための最良の形態について、以下に表を参照して実施例１により説明する。   The best mode for carrying out the present invention will be described below with reference to a table according to a first embodiment.

表１に示す化学成分の鋼を１００ｋｇ真空誘導炉で溶解し、インゴットに鋳造し、１２００℃に加熱してφ６５ｍｍ材および４０ｍｍ角材に圧鍛し、９００℃で１時間保持して空冷することで焼きならしを行い、下記の試験の供試材とした。   By melting steel of chemical composition shown in Table 1 in a 100 kg vacuum induction furnace, casting into an ingot, heating to 1200 ° C., forging into φ65 mm material and 40 mm square material, holding at 900 ° C. for 1 hour and air cooling. Normalization was performed and used as test materials for the following tests.

表１に示す１〜３１の比較鋼および発明鋼の上記の焼きならしを行ったφ６５ｍｍ材をφ６０ｍｍ材に旋削し、表２に示す評価条件により超硬工具旋削試験に供した。   The φ65 mm material subjected to the above normalization of the comparative steels 1 to 31 shown in Table 1 and the inventive steel was turned into a φ60 mm material and subjected to a carbide tool turning test under the evaluation conditions shown in Table 2.

さらに、表１に示す１〜３１の比較鋼および発明鋼の上記の焼きならしを行った４０ｍｍ角材を３５ｍｍ角材にフライス加工して表３に示すドリル寿命試験に供した。Ｃａ／Ａｌ比、複合硫化物に占める酸化物の面積比、並びに全硫化物の個数に対する複合硫化物個数の個数率が上限を超えるものはドリル加工における工具摩耗が増大し、その特性劣化をドリル寿命（ドリル穿孔不能までの穴数）をもって評価する。   Further, the 40 mm square material subjected to the above normalization of the comparative steels 1 to 31 shown in Table 1 and the inventive steel was milled into a 35 mm square material and subjected to the drill life test shown in Table 3. When the ratio of the number of composite sulfides exceeds the upper limit for the Ca / Al ratio, the oxide area ratio in the composite sulfides, and the number of composite sulfides exceeds the upper limit, the tool wear in drilling increases, and the characteristic deterioration is Evaluation is based on the lifetime (number of holes until drilling is impossible).

これらの１〜３１の比較鋼および発明鋼のＣａ／Ａｌ比、複合硫化物に占める酸化物の面積比、並びに全硫化物の個数に対する複合硫化物個数の個数率（ただし、比較鋼のうちＰｂ含有鋼を除く）、さらにこれらの超硬工具旋削試験およびドリル寿命試験の結果である逃げ面摩耗量およびすくい面摩耗量の超硬工具摩耗並びにドリル寿命について表４に示す。さらに図１に比較鋼２、３、４および発明鋼５、６、７の組織の複合硫化物の形状をＳＥＭ写真で示す。発明鋼５、６、７は被削性に特に優れる。   The Ca / Al ratio of these comparative steels 1 to 31 and invention steel, the area ratio of oxides in the composite sulfide, and the number ratio of the composite sulfide number to the total sulfide number (however, Pb of the comparative steel Table 4 shows the carbide tool wear and the drill life of the flank wear amount and the rake face wear amount, which are the results of these carbide tool turning test and drill life test. Furthermore, the shape of the composite sulfide of the structure | tissue of comparative steel 2, 3, 4 and invention steel 5, 6, 7 is shown by the SEM photograph in FIG. Inventive steels 5, 6, and 7 are particularly excellent in machinability.

この表４において、本発明鋼の５〜７、１１〜１３、１７〜１８、２３〜２５および２９〜３１は、逃げ面摩耗量およびすくい面摩耗量並びにドリル寿命でいずれも優れているが、比較鋼の１〜４、８〜９、１４〜１５、１９〜２０、２２、２６〜２７は超硬工具摩耗の点で本発明に比し劣り、さらに比較鋼の４、１０、１６、２１、２８はドリル寿命の点で本発明に比し劣ることが分かる。   In Table 4, 5-7, 11-13, 17-18, 23-25 and 29-31 of the steel of the present invention are all excellent in the flank wear amount and rake face wear amount and drill life, Comparative steels 1-4, 8-9, 14-15, 19-20, 22, 26-27 are inferior to the present invention in terms of carbide tool wear. Further, comparative steels 4, 10, 16, 21 , 28 is inferior to the present invention in terms of drill life.

比較鋼２、３、４および発明鋼５、６、７の複合硫化物の形状を示す組織のＳＥＭ写真である。It is the SEM photograph of the structure | tissue which shows the shape of the composite sulfide of comparative steel 2, 3, 4 and invention steel 5, 6, 7.

Claims (2)

質量％で、Ｃ：０．１０〜０．６０％、Ｓｉ：０．０５〜２．０％、Ｍｎ：０．３〜２．５％、Ｓ：０．０２〜０．２５％、Ａｌ：０．００２〜０．０３０％、Ｃａ：０．０００５〜０．０１％、Ｏ：０．０００５〜０．００８％、Ｎ：０．０２％以下、残部Ｆｅおよび不可避不純物からなり、ＣａとＡｌの質量比がＣａ／Ａｌ：０．１〜１．０で、かつ硫化物を含有する鋼であり、この鋼中に含有の硫化物に酸化物を内包若しくは隣接する複合硫化物の占める断面積に対する酸化物の占める断面積の割合である面積率が３〜９０％であり、さらに単位面積中の全硫化物の個数に対する該複合酸化物の個数の割合である個数率が４〜８０％であることを特徴とする被削性に優れた機械構造用鋼。 In mass%, C: 0.10 to 0.60%, Si: 0.05 to 2.0%, Mn: 0.3 to 2.5%, S: 0.02 to 0.25%, Al: 0.002 to 0.030%, Ca: 0.0005 to 0.01%, O: 0.0005 to 0.008%, N: 0.02% or less, balance Fe and inevitable impurities, Ca and Al Is a steel containing sulfide in a mass ratio of Ca / Al: 0.1 to 1.0, and the cross-sectional area occupied by the composite sulfide containing the oxide in the sulfide contained in this steel or adjacent to it. The area ratio, which is the ratio of the cross-sectional area occupied by the oxide, is 3 to 90%, and the number ratio, which is the ratio of the number of the composite oxide to the total number of sulfides in the unit area, is 4 to 80%. A machine structural steel with excellent machinability that is characterized by being. 鋼成分は上記鋼成分に加えて、質量％で、Ｎｉ：０．１〜２．５％、Ｃｒ：０．１〜２．５％、Ｍｏ：０．０５〜１．５０％、Ｖ：０．０１〜０．５０％、Ｔｉ：０．０１〜０．５０％、Ｎｂ：０．００１〜０．３０％、Ｂ：０．０００３〜０．００５％から選択した１種または２種以上を含有することを特徴とする請求項１に記載の被削性に優れた機械構造用鋼。 In addition to the above steel components, the steel components are in mass%, Ni: 0.1 to 2.5%, Cr: 0.1 to 2.5%, Mo: 0.05 to 1.50%, V: 0 0.01 to 0.50%, Ti: 0.01 to 0.50%, Nb: 0.001 to 0.30%, B: One or more selected from 0.0003 to 0.005% The steel for machine structure excellent in machinability according to claim 1, which is contained.
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