JP4001787B2 - Cold tool steel with excellent fatigue life and heat treatment method thereof - Google Patents
Cold tool steel with excellent fatigue life and heat treatment method thereof Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、疲労寿命に優れた冷間工具鋼およびその熱処理方法に関するものである。
【０００２】
【従来の技術】
従来、冷間工具鋼は、６０ＨＲＣ前後の高硬度で使用し、多量の炭化物を分散させることによって、耐摩耗性に優れることを特徴としている。しかし、最近、被加工材の高強度化に伴い、耐疲労性の改善が必要となっており、耐摩耗性と耐疲労性を両立させた冷間工具鋼の開発が望まれている。これらのことから、残留オーステナイトを安定させることで経年変化を解消すると共に、高速の付加がかかった時の割れに対する感受性を改善した冷間工具鋼の製造方法として、例えば特開２００１−１３１６３４号公報が開示されている。
【０００３】
また、上記特許と同様に、特開平１１−３１０８２０号公報においても、冷間工具鋼の硬さを維持したまま、鋼に含まれている残留オーステナイトを安定させて、マルテンサイトへの変態の進行を伴う寸法・形状の経年変化の問題を解決した冷間工具鋼が提案されている。さらに、特開２０００−２３４１４８号公報においては、鋼中にＳを添加しＭｎＳを分散形成させることにより、Ｓ無添加鋼に比べ疲労限を向上させ、一方、被削性も向上させた高サイクル疲労寿命および被削性に優れた冷間工具鋼を提案している。
【０００４】
【発明が解決しようとする課題】
上述した特開２００１−１３１６３４号公報は、残留オーステナイトを安定化する手段として、残留オーステナイトをいったん積極的に不安定にしておいて、安定化するという点が特徴である。しかし、この方法では十分な効果が得られず、また、疲労寿命を向上させる観点から、残留オーステナイトの形態をどうように限定すればよいかの点は全く開示されていない。
また、特開平１１−３１０８２０号公報および特開２０００−２３４１４８号公報においても上記同様に、この方法では耐疲労性が十分に得られず、しかも、疲労寿命を向上させる観点から、残留オーステナイトの形態をどうように限定すればよいかの点は全く開示されていない。
【０００５】
これに対し、近年の塑性加工技術の進歩や被加工材の高強度化に伴い、工具の耐摩耗性向上を目的に、さらに耐疲労性を兼ね供えた金型に適した工具鋼が必要とされることから、本発明は、最適な化学成分に限定し、残留オーステナイトを積極的に活用することによって、疲労特性の向上に、硬度と体積％で５〜３５％の残留オーステナイトを平均粒径０．０１〜２μｍに微細分散させることにより、十分な耐疲労性を確保し、耐摩耗性が劣化しない強度の優れた高寿命が得られる冷間工具鋼を提供することを目的とするものである。
【０００６】
【課題を解決するための手段】
その発明の要旨とするところは、
（１）質量％で、Ｃ：０．６０〜１．５０％、Ｓｉ：２．０％以下、Ｍｎ：０．１〜１．５％、Ｃｒ：５．０〜１３．０％、ＭｏまたはＷの１種または２種をＭｏ当量（Ｍｏ＋１／２Ｗ）：０．１〜３．０、ＶまたはＮｂの１種または２種を合計で０．０５〜２．０％含み、残部Ｆｅおよび不可避的不純物からなる鋼を、焼入れ後、３００〜５００℃の焼戻温度で少なくとも１回以上の焼戻しを行い、硬さ５５〜６５ＨＲＣで、５〜３５体積％の残留オーステナイトを平均粒径０．０１〜２μｍに微細分散させたことを特徴とする疲労寿命に優れた冷間工具鋼。
【０００７】
（２）前記（１）に記載の鋼に、Ｎｉ：２．０％以下、Ｃｏ：３．０％以下、Ｓ：０．２％以下の１種または２種以上添加することを特徴とする疲労寿命に優れた冷間工具鋼。
（３）前記（１）または（２）に記載の鋼を、焼戻温度以下で窒化処理することを特徴とする疲労寿命に優れた冷間工具鋼の熱処理方法にある。
【０００８】
【発明の実施の形態】
以下に、本発明鋼の各化学成分の作用およびその限定理由を説明する。
Ｃ：０．６０〜１．５０％
Ｃは、焼入焼戻により、十分なマトリックス硬さを与えると共に、Ｃｒ，Ｍｏ，Ｖ，Ｎｂなどと結合して炭化物を形成し、強度および耐摩耗性を与える元素である。しかし、添加量が多過ぎると、凝固時に粗大炭化物が析出し、靱性を低下させる。また、残留オーステナイトが多くなりすぎ、逆に硬さが低下する。従って、その上限を１．５０％とした。一方、０．６０％未満では、その効果が十分でないので、その下限を０．６０％とした。
【０００９】
Ｓｉ：２．０％以下
Ｓｉは、主に脱酸剤として添加されると共に、耐酸化性および焼入性に有効な元素であると共に、マトリックス硬さを与える元素である。しかし、２．０％を超えて添加すると、靱性を低下させるので、その上限を２．０％とした。
Ｍｎ：０．１〜１．５％
Ｍｎは、Ｓｉと同様に脱酸剤として添加し鋼の清浄度を高めると共に焼入れ性を高める元素である。しかしながら、１．５％を超えて添加すると、靱性を低下させるので、その上限を１．５％とした。
【００１０】
Ｃｒ：５．０〜１３．０％
Ｃｒは、焼入れ性を高めると共に、炭化物を形成し、耐摩耗性に寄与する元素である。この効果を満足するためには、少なくとも５．０％以上必要である。従って、その下限を５．０％とした。一方、Ｃｒは、凝固時にＣと結合して粗大炭化物が析出し、靱性を低下させるため、その上限を１３．０％とした。
【００１１】
ＭｏまたはＷのいずれか１種または２種をＭｏ当量（Ｍｏ＋１／２Ｗ）：０．１〜３．０％、
ＭｏおよびＷは、共に微細な炭化物を形成し、二次硬化に寄与する重要な元素であると共に、耐軟化抵抗性を改善する元素である。ただし、その効果はＭｏの方がＷよりも２倍強く、同じ効果を得るのに、ＷはＭｏの２倍必要である。この両元素の効果は、Ｍｏ当量（Ｍｏ＋１／２Ｗ）で表すことができる。本発明成分系においては、Ｍｏ当量で少なくとも０．１％以上が必要である。逆に、Ｍｏ当量の過剰添加は、靱性の低下を招くので、その上限を３．０％とした。
【００１２】
ＶまたはＮｂのいずれか１種または２種をＶ当量（Ｖ＋１／２Ｎｂ）：０．０５〜２．０％
Ｖ、Ｎｂは、共に微細な炭化物を形成し二次硬化に寄与する元素であると共に、結晶粒微細化し、耐摩耗性を向上させる元素である。ただし、その効果はＶの方がＮｂよりも２倍強く、同じ効果を得るのに、ＮｂはＶの２倍必要である。この両元素の効果はＶ当量（Ｖ＋１／２Ｎｂ）で表すことができる。本発明成分系においては、Ｖ当量で少なくとも０．０５％以上が必要である。過剰な添加は靱性を低下させるため、その上限を２．０％とした。
【００１３】
Ｎｉ：２．０％以下
Ｎｉは、焼入れ性および靱性の向上に役立つ元素である。しかし、２．０％を超えると残留オーステナイトが多くなりすぎ、逆に硬さが低下する。従って、その上限を２．０％とした。
Ｃｏ：３．０％以下
Ｃｏは、焼戻硬さの向上に役立つ元素である。しかし、３．０％を超えると靱性を低下させる。従って、その上限を３．０％とした。
【００１４】
Ｓ：０．２％以下
Ｓは快削性を確保するために必要な元素である。しかし、過剰な添加は靱性を低下させるため、その上限を０．２％とした。
また、硬さ５５〜６５ＨＲＣ
硬さは、耐疲労特性の向上に役立ち、少なくとも５５ＨＲＣ以上の硬さが必要である。しかし、逆に６５ＨＲＣを超えると耐疲労特性を低下させることから、その上限を６５ＨＲＣとした。
【００１５】
残留オーステナイト５〜３５体積％
体積％で５〜３５％の残留オーステナイト組織を残留させたことにより、耐疲労特性の向上を図る。すなわち、安定化した残留オーステナイトは応力集中個所およびき裂先端において衝撃材の役割を果たし、疲労特性の向上を図る。しかし、３５体積％を超えると硬さが低下することから、その上限を３５体積％とした。
平均粒径０．０１〜２μｍ
残留オーステナイトを平均粒径０．０１〜２μｍに微細分散させることにより耐疲労特性の向上を図る。しかし、２μｍを超えると残留オーステナイトが多くなり過ぎて、逆に硬さが低下することから、その範囲を０．０１〜２μｍとした。
【００１６】
焼戻温度：３００〜５００℃
焼戻温度を３００〜５００℃としたのは、残留オーステナイト中にＣを固溶させるのに必要な温度であり、残留オーステナイトの安定化を図る。この安定化した残留オーステナイトは、繰返し応力による歪誘起変態を生じ難く、寿命向上に寄与する。しかし、５００℃を超えると残留オーステナイトが分解することから、その上限を５００℃とした。
窒化処理：焼戻温度以下
窒化処理は、残留オーステナイト中にＮを固溶させ、残留オーステナイトの安定化を高める。しかし、焼戻温度を超えると残留オーステナイトが分解することから、その上限を５００℃とした。
【００１７】
【実施例】
以下に、本発明を実施例に基づいて具体的に説明する。
表１に示す組成の鋼１００ｋｇを真空誘導溶解炉にて溶製し、インゴットに鋳造し、１１００℃に加熱、角１００ｍｍ×１００ｍｍ×１００ｍｍに鍛伸後焼なましを行い供試材とした。各試験片は１０３０℃に３０分保持後、空冷し、３００〜５００℃で６０分保持後空冷処理を２回施し、シャルピー衝撃試験片、回転曲げ疲労試験片を作製した。その結果を表２に示す。
また、シャルピー衝撃試験は、１０Ｒ２ｍｍＣノッチ試験片を用い、常温で試験を実施した。回転曲げ疲労試験は、平行部φ８×１７Ｌの試験片を用い、応力振幅１０５０ＭＰａ、常温で試験を実施した。さらに、被削性については、各種の焼なまし材を、ＳＫＨ５１製、φ５ｍｍのドリルで１０ｍｍ穿孔するのに要する時間で示した。なお、窒化処理は、イオン窒化装置を用い、焼戻温度以下で１〜５時間保持して評価した。
【００１８】
【表１】
【表２】
表２に示すように、本発明鋼Ａ〜Ｆはいずれも残留オーステナイト量が５〜３５％の範囲内であり、かつ、残留オーステナイト平均粒径が０．０１〜２μｍの範囲である。また、５５〜６５ＨＲＣの硬さを維持した上で、耐摩耗性の向上、疲労強度、金型寿命延長を図ることが出来た。これに対し、比較例であるＧ〜Ｉは成分組成ないし残留オーステナイト量の範囲、残留オーステナイトの平均粒径、硬さおよび焼戻温度の条件が外れているために、シャルピー衝撃値が低く、また、疲労寿命が短く、金型寿命および被削性が本発明より劣ることが判る。
【００２１】
【発明の効果】
以上述べたように、本発明鋼は、冷間工具鋼としての残留オーステナイトを積極的に活用することにより、疲労特性を向上させ、極めて優れた型寿命を確保することが可能となり、金型用工具鋼として従来のものに比べて経済的で極めて有利なものとなる優れた効果を奏するものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a cold tool steel having an excellent fatigue life and a heat treatment method thereof.
[0002]
[Prior art]
Conventionally, cold tool steel is characterized by excellent wear resistance by using a high hardness of around 60 HRC and dispersing a large amount of carbide. However, with the recent increase in the strength of workpieces, it is necessary to improve the fatigue resistance, and the development of cold tool steel that achieves both wear resistance and fatigue resistance is desired. From these facts, as a method for producing a cold tool steel that eliminates secular change by stabilizing retained austenite and has improved sensitivity to cracking when high-speed addition is applied, for example, Japanese Patent Application Laid-Open No. 2001-131634 Is disclosed.
[0003]
Similarly to the above patent, in JP-A-11-310820, the retained austenite contained in the steel is stabilized while maintaining the hardness of the cold tool steel, and the transformation to martensite proceeds. Cold work tool steels have been proposed that solve the problem of aging in dimensions and shape. Furthermore, in Japanese Patent Application Laid-Open No. 2000-234148, by adding S to steel and forming MnS in a dispersed manner, the fatigue limit is improved as compared with steel without S, and on the other hand, machinability is also improved. We have proposed cold work tool steel with excellent fatigue life and machinability.
[0004]
[Problems to be solved by the invention]
Japanese Patent Laid-Open No. 2001-131634 described above is characterized in that, as a means for stabilizing retained austenite, the retained austenite is once made positively unstable and stabilized. However, this method does not provide a sufficient effect, and there is no disclosure of how to limit the form of retained austenite from the viewpoint of improving fatigue life.
Also in JP-A-11-310820 and JP-A-2000-234148, as described above, this method does not provide sufficient fatigue resistance, and from the viewpoint of improving fatigue life, the form of retained austenite There is no disclosure of how to limit this.
[0005]
On the other hand, with the recent progress in plastic working technology and the increased strength of workpieces, tool steel suitable for molds that also have fatigue resistance is required for the purpose of improving wear resistance of tools. Therefore, the present invention is limited to the optimum chemical component, and by actively utilizing the retained austenite, the average particle size of the retained austenite of 5 to 35% in hardness and volume% is improved in fatigue characteristics. It is intended to provide a cold work tool steel that ensures sufficient fatigue resistance by fine dispersion to 0.01 to 2 μm, and can provide a long life with excellent strength without deterioration of wear resistance. is there.
[0006]
[Means for Solving the Problems]
The gist of the invention is that
(1) By mass%, C: 0.60 to 1.50%, Si: 2.0% or less, Mn: 0.1 to 1.5%, Cr: 5.0 to 13.0%, Mo or 1 type or 2 types of W are equivalent to Mo (Mo + 1 / 2W): 0.1 to 3.0, 1 type or 2 types of V or Nb are included in total of 0.05 to 2.0%, the remainder being Fe and inevitable After quenching the steel composed of mechanical impurities , tempering is performed at least once at a tempering temperature of 300 to 500 ° C., and a residual austenite of 5 to 35% by volume with a hardness of 55 to 65 HRC is set to an average particle size of 0.01. Cold tool steel with excellent fatigue life, characterized by being finely dispersed to ˜2 μm.
[0007]
(2) One or more of Ni: 2.0% or less, Co: 3.0% or less, and S: 0.2% or less are added to the steel described in (1). Cold tool steel with excellent fatigue life.
(3) A method of heat treating cold tool steel having excellent fatigue life, characterized by nitriding the steel described in (1) or (2) at a temperature lower than the tempering temperature.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Below, the effect | action of each chemical component of this invention steel and its reason for limitation are demonstrated.
C: 0.60 to 1.50%
C is an element that gives sufficient matrix hardness by quenching and tempering, and forms carbides by combining with Cr, Mo, V, Nb, etc., and gives strength and wear resistance. However, if the amount added is too large, coarse carbides precipitate during solidification and lower toughness. Moreover, residual austenite increases too much and conversely the hardness decreases. Therefore, the upper limit was made 1.50%. On the other hand, if it is less than 0.60%, the effect is not sufficient, so the lower limit was made 0.60%.
[0009]
Si: 2.0% or less Si is mainly added as a deoxidizer, is an element effective for oxidation resistance and hardenability, and is an element that gives matrix hardness. However, if added over 2.0%, the toughness is lowered, so the upper limit was made 2.0%.
Mn: 0.1 to 1.5%
Mn is an element that is added as a deoxidizer in the same way as Si to increase the cleanliness of steel and enhance the hardenability. However, if added over 1.5%, the toughness is lowered, so the upper limit was made 1.5%.
[0010]
Cr: 5.0 to 13.0%
Cr is an element that enhances hardenability, forms carbides, and contributes to wear resistance. In order to satisfy this effect, at least 5.0% or more is necessary. Therefore, the lower limit was made 5.0%. On the other hand, Cr combines with C during solidification to precipitate coarse carbides and reduce toughness, so the upper limit was made 13.0%.
[0011]
Any one or two of Mo or W is equivalent to Mo equivalent (Mo + 1 / 2W): 0.1 to 3.0%,
Mo and W are both elements that form fine carbides and contribute to secondary hardening, and are elements that improve resistance to softening. However, the effect of Mo is twice as strong as that of W. To obtain the same effect, W needs to be twice that of Mo. The effect of both elements can be expressed by Mo equivalent (Mo + 1 / 2W). In the component system of the present invention, the Mo equivalent must be at least 0.1% or more. On the contrary, excessive addition of Mo equivalent causes a decrease in toughness, so the upper limit was made 3.0%.
[0012]
One or two of V or Nb is V equivalent (V + 1 / 2Nb): 0.05 to 2.0%
V and Nb are both elements that form fine carbides and contribute to secondary hardening, as well as elements that refine crystal grains and improve wear resistance. However, the effect of V is twice as strong as that of Nb, and Nb needs to be twice that of V to obtain the same effect. The effect of both elements can be expressed in terms of V equivalent (V + 1 / 2Nb). In the component system of the present invention, the V equivalent must be at least 0.05% or more. Since excessive addition reduces toughness, the upper limit was made 2.0%.
[0013]
Ni: 2.0% or less Ni is an element useful for improving hardenability and toughness. However, if it exceeds 2.0%, the amount of retained austenite increases too much, and conversely the hardness decreases. Therefore, the upper limit was made 2.0%.
Co: 3.0% or less Co is an element useful for improving the tempering hardness. However, if it exceeds 3.0%, the toughness is lowered. Therefore, the upper limit was made 3.0%.
[0014]
S: 0.2% or less S is an element necessary for ensuring free-cutting properties. However, excessive addition reduces toughness, so the upper limit was made 0.2%.
Also, hardness 55-65HRC
The hardness is useful for improving the fatigue resistance and needs to be at least 55 HRC or more. However, on the contrary, if it exceeds 65 HRC, the fatigue resistance is lowered, so the upper limit was set to 65 HRC.
[0015]
Residual austenite 5-35% by volume
Fatigue resistance is improved by leaving a residual austenite structure of 5 to 35% by volume. In other words, the stabilized retained austenite plays the role of an impact material at the stress concentration site and the crack tip, thereby improving the fatigue characteristics. However, since hardness will fall when it exceeds 35 volume%, the upper limit was made into 35 volume%.
Average particle size 0.01-2 μm
Fatigue resistance is improved by finely dispersing residual austenite to an average particle size of 0.01 to 2 μm. However, if it exceeds 2 μm, the amount of retained austenite increases too much, and conversely the hardness decreases, so the range was set to 0.01 to 2 μm.
[0016]
Tempering temperature: 300-500 ° C
The tempering temperature of 300 to 500 ° C. is a temperature necessary for dissolving C in the retained austenite, and stabilizes the retained austenite. This stabilized retained austenite hardly causes strain-induced transformation due to repeated stress, and contributes to the improvement of the life. However, when the temperature exceeds 500 ° C., the retained austenite decomposes, so the upper limit was set to 500 ° C.
Nitriding treatment: tempering temperature or lower nitriding treatment makes N dissolve in the retained austenite and improves the stabilization of the retained austenite. However, since the retained austenite decomposes when the tempering temperature is exceeded, the upper limit was set to 500 ° C.
[0017]
【Example】
The present invention will be specifically described below based on examples.
100 kg of steel having the composition shown in Table 1 was melted in a vacuum induction melting furnace, cast into an ingot, heated to 1100 ° C., forged to 100 mm × 100 mm × 100 mm and annealed to obtain a test material. Each test piece was held at 1030 ° C. for 30 minutes and then air-cooled, held at 300 to 500 ° C. for 60 minutes and then subjected to air cooling twice to prepare a Charpy impact test piece and a rotating bending fatigue test piece. The results are shown in Table 2.
The Charpy impact test was conducted at room temperature using a 10R2 mmC notch test piece. In the rotating bending fatigue test, a test piece having a parallel part φ8 × 17 L was used, and the test was performed at a stress amplitude of 1050 MPa and at room temperature. Further, the machinability is indicated by the time required for drilling 10 mm of various annealed materials with a Ø5 mm drill made of SKH51. The nitriding treatment was evaluated by using an ion nitriding apparatus and maintaining it at a temperature lower than the tempering temperature for 1 to 5 hours.
[0018]
[Table 1]
[Table 2]
As shown in Table 2, all of the inventive steels A to F have a retained austenite amount in the range of 5 to 35% and a retained austenite average particle diameter in the range of 0.01 to 2 μm. Further, while maintaining the hardness of 55 to 65 HRC, it was possible to improve wear resistance, fatigue strength, and extend the mold life. On the other hand, Comparative Examples G to I have low Charpy impact values because the conditions of the component composition or the amount of retained austenite, the average particle size of retained austenite, the hardness, and the tempering temperature are not satisfied. It can be seen that the fatigue life is short and the mold life and machinability are inferior to those of the present invention.
[0021]
【The invention's effect】
As described above, the steel of the present invention can improve the fatigue characteristics by actively utilizing the retained austenite as a cold tool steel, and can ensure an extremely excellent mold life. As a tool steel, an excellent effect is obtained which is economical and extremely advantageous as compared with the conventional tool steel.
Claims (3)
Ｃ：０．６０〜１．５０％、
Ｓｉ：２．０％以下、
Ｍｎ：０．１〜１．５％、
Ｃｒ：５．０〜１３．０％、
ＭｏまたはＷの１種または２種をＭｏ当量（Ｍｏ＋１／２Ｗ）：０．１〜３．０、ＶまたはＮｂの１種または２種を合計で０．０５〜２．０％含み、残部Ｆｅおよび不可避的不純物からなる鋼を、焼入れ後、３００〜５００℃の焼戻温度で少なくとも１回以上の焼戻しを行い、硬さ５５〜６５ＨＲＣで、５〜３５体積％の残留オーステナイトを平均粒径０．０１〜２μｍに微細分散させたことを特徴とする疲労寿命に優れた冷間工具鋼。% By mass
C: 0.60 to 1.50%,
Si: 2.0% or less,
Mn: 0.1 to 1.5%
Cr: 5.0 to 13.0%,
1 type or 2 types of Mo or W, Mo equivalent (Mo + 1 / 2W): 0.1-3.0, 1 type or 2 types of V or Nb are included in total in 0.05-2.0%, and the balance Fe And after tempering the steel consisting of inevitable impurities, it is tempered at least once at a tempering temperature of 300 to 500 ° C., and has a hardness of 55 to 65 HRC and 5 to 35 vol% residual austenite having an average particle size of 0. Cold tool steel with excellent fatigue life, characterized by being finely dispersed in 0.01-2 μm.
Ｎｉ：２．０％以下、
Ｃｏ：３．０％以下、
Ｓ：０．２％以下
の１種または２種以上添加することを特徴とする疲労寿命に優れた冷間工具鋼。The steel according to claim 1,
Ni: 2.0% or less,
Co: 3.0% or less,
S: Cold tool steel with excellent fatigue life, characterized by adding one or more of 0.2% or less.
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