JP5988732B2

JP5988732B2 - Cold work tool steel with high hardness and toughness

Info

Publication number: JP5988732B2
Application number: JP2012148764A
Authority: JP
Inventors: 前田　雅人; 雅人前田
Original assignee: Sanyo Special Steel Co Ltd
Current assignee: Sanyo Special Steel Co Ltd
Priority date: 2012-07-02
Filing date: 2012-07-02
Publication date: 2016-09-07
Anticipated expiration: 2032-07-02
Also published as: JP2014009396A

Description

本発明は、鍛造金型、フォーミングロールあるいは転造ダイスなどの使用条件が特に過酷な冷間加工用として好適な高硬度、高靱性冷間工具鋼およびこの冷間工具鋼からなる金型並びに工具に関する。 The present invention relates to a high-hardness, high-toughness cold tool steel suitable for cold working where the use conditions such as a forging die, a forming roll or a rolling die are particularly severe, and a die and a tool comprising the cold tool steel About.

近年、冷間加工技術の発展に伴って、より高硬度の被加工材を加工したり、その加工量を増大したりするなどにより、冷間加工条件が過酷化している。そのために、冷間工具鋼としては、６３ＨＲＣ以上の硬さがあり、かつ、靭性の高い材料が求められている。更に、経済性の観点から、安価に製造でき、また汎用ダイス鋼と同様の熱処理を行うことで高硬度の工具を得ることが求められている。 In recent years, with the development of cold working technology, the cold working conditions have become severe due to processing a harder workpiece or increasing the amount of processing. Therefore, as cold tool steel, a material having a hardness of 63 HRC or more and high toughness is required. Furthermore, from the viewpoint of economy, it is required to obtain a tool with high hardness by performing a heat treatment similar to that of general-purpose die steel that can be manufactured at low cost.

６３ＨＲＣを超える硬さの材料を得るために、例えばＪＩＳで規定される高速度工具鋼ＳＫＨや、Ｃ、Ｍｏ、Ｗ、Ｖ等の合金元素を多量に加えて多量の硬質炭化物を析出させることによって高硬度を得ている鋼が提案されている（例えば、特許文献１参照。）。しかしながら、これらの鋼には粗大な１次炭化物が多く含有されており、そのために靭性および疲労強度は低い。また、高硬度を得るために、通常ダイス鋼で行われる焼入温度よりも高い１０５０℃以上の温度で焼入れを行い、１次炭化物を固溶させ２次硬化に寄与する炭化物を多量に析出させることを狙いとした鋼種もある（例えば、特許文献２参照。）しかしこの鋼種は、特殊な加熱炉および加熱温度で処理するので、金型の製造コストが高くなる問題がある。 In order to obtain a material with a hardness exceeding 63HRC, for example, by adding a large amount of high-speed tool steel SKH prescribed by JIS and alloy elements such as C, Mo, W, V, etc., to precipitate a large amount of hard carbide Steels having high hardness have been proposed (see, for example, Patent Document 1). However, these steels contain a large amount of coarse primary carbides, and therefore have low toughness and fatigue strength. Moreover, in order to obtain high hardness, quenching is performed at a temperature of 1050 ° C. or higher, which is higher than the quenching temperature normally performed in die steel, and primary carbides are dissolved to precipitate a large amount of carbides that contribute to secondary hardening. There is also a steel type aimed at this (for example, see Patent Document 2). However, since this steel type is processed in a special heating furnace and heating temperature, there is a problem that the manufacturing cost of the mold becomes high.

また、１次炭化物の微細化を図って靭性を向上させた粉末冶金法で製造した粉末高速度工具鋼も開発されている（例えば、特許文献３参照。）。しかし、この粉末高速度工具鋼（すなわち、粉末ハイス）は値段が高く、また焼入温度が高いために金型の製造コストが高いという問題があった。 In addition, powder high-speed tool steel manufactured by a powder metallurgy method in which primary carbide is refined to improve toughness has been developed (see, for example, Patent Document 3). However, this powder high-speed tool steel (that is, powder high speed steel) has a problem that the price is high and the manufacturing cost of the mold is high due to the high quenching temperature.

特開２０００−７３１４２号公報JP 2000-73142 A 特開平０５−１５６４０７号公報JP 05-156407 A 特開平０５−１６３５５１号公報Japanese Patent Laid-Open No. 05-163551

本発明が解決しようとする課題は、上述したＪＩＳ規定の高速度工具鋼ＳＫＨや特許文献１のように靱性や疲労強度が低いという問題がなく、さらに粉末高速度工具鋼のような材料自体の価格や金型の熱処理価格が高くない、溶製による新規な冷間工具鋼の鋼材の提供することである。 The problem to be solved by the present invention is that there is no problem that the toughness and fatigue strength are low as in the high speed tool steel SKH specified in the above-mentioned JIS and Patent Document 1, and further, the material itself such as powder high speed tool steel. It is to provide a new cold-work tool steel by melting, which is not expensive or heat treatment for molds.

上述した様な問題を解消するために、発明者は鋭意開発を進めた結果、請求項に示す合金成分式、および製造条件とすることで、１０２０〜１０５０℃未満で焼入処理を行い、焼戻し後の硬さが６３ＨＲＣ以上となる高硬度で高靭性の鋼が得られることを見出した。 In order to solve the problems as described above, the inventor has intensively developed, and as a result, the alloy composition formula and the manufacturing conditions shown in the claims are used, and the quenching treatment is performed at less than 1020 to 1050 ° C. It was found that a steel having high hardness and high toughness with a later hardness of 63 HRC or more can be obtained.

課題を解決するための本発明の手段は、質量％で、Ｃ：０．６〜０．９％未満、Ｓｉ：０．６〜１．０％、Ｍｎ：≦０．６％、Ｃｒ：５．０〜１０．０％、Ｍｏ＋Ｗ／２：２．７〜５．０％、Ｖ：０．０５〜０．３５％未満、Ｎ：＜２５０ｐｐｍを含有し、かつ、Ｋ＝−０．７４Ｃ＋０．２４Ｃｒ＋０．７３（Ｍｏ＋Ｗ／２）＋２．１２Ｖ…（１）とするとき、Ｋ：３．０〜５．５％を満足し、残部Ｆｅおよび不可避不純物からなる鋼を、１０２０〜１０５０℃未満で焼入れを行ない１次炭化物の面積率を５％以下とし、ついで５００〜６００℃とする焼戻し工程を少なくとも２回繰り返して硬さを６３ＨＲＣ以上とすることを特徴とする冷間加工用として好適な高硬度、高靱性の冷間工具鋼の製造方法である。 The means of the present invention for solving the problem is, in mass%, C: 0.6 to less than 0.9%, Si: 0.6 to 1.0%, Mn: ≦ 0.6%, Cr: 5 0.0 to 10.0%, Mo + W / 2: 2.7 to 5.0%, V: 0.05 to less than 0.35%, N: <250 ppm, and K = −0.74C + 0. 24Cr + 0.73 (Mo + W / 2) + 2.12V (1) When K: 3.0 to 5.5% is satisfied, the steel composed of the balance Fe and inevitable impurities is quenched at less than 1020 to 1050 ° C. High hardness suitable for cold working, characterized in that the area ratio of the primary carbide is 5% or less, and then the tempering step of 500 to 600 ° C. is repeated at least twice to make the hardness 63 HRC or more. This is a method for producing a cold tough steel with high toughness .

ここで、本発明の上記の手段における冷間工具鋼の化学成分の限定理由、（１）式であるＫ値、焼入れ温度および鋼の硬さについて、以下に説明する。なお、以下における％は質量％を示す。 Here, the reasons for limiting the chemical components of the cold tool steel in the above-described means of the present invention, the K value, the quenching temperature, and the hardness of the steel, which are represented by the formula (1), are described below. In addition,% in the following shows the mass%.

Ｃ：０．６〜０．９％未満
Ｃは、硬質炭化物を形成し、鋼の硬さおよび耐摩耗性を向上させるとともに、焼入性を向上させる元素である。しかし、Ｃが０．６％未満であるとこれらの効果は得られない。一方、Ｃが０．９％以上であると、鋼中に粗大な炭化物を形成し、靱性を悪化する。そこで、Ｃは、０．６〜０．９％未満とし、望ましくは０．７〜０．８４％とする。 C: Less than 0.6 to 0.9% C is an element that forms hard carbides, improves the hardness and wear resistance of steel, and improves hardenability. However, if C is less than 0.6%, these effects cannot be obtained. On the other hand, if C is 0.9% or more, coarse carbides are formed in the steel and the toughness is deteriorated. Therefore, C is set to 0.6 to less than 0.9%, preferably 0.7 to 0.84%.

Ｓｉ：０．６〜１．０％
Ｓｉは、脱酸剤および鋼の基地の硬さを得るために必要な元素である。しかし、Ｓｉが０．６％未満では、これらの効果は得られない。一方、Ｓｉが１．０％を超えると、鋼の靱性や加工性が悪化する。そこで、Ｓｉは、０．６〜１．０％とし、望ましくは０．７〜０．９％とする。 Si: 0.6 to 1.0%
Si is an element necessary for obtaining hardness of a deoxidizer and a steel base. However, if Si is less than 0.6%, these effects cannot be obtained. On the other hand, if Si exceeds 1.0%, the toughness and workability of steel deteriorate. Therefore, Si is set to 0.6 to 1.0%, preferably 0.7 to 0.9%.

Ｍｎ：≦０．６％
Ｍｎは、脱酸剤および鋼の焼入性を得るために必要な元素である。しかし、Ｍｎが０．１％未満では、これらの効果は得られない。一方、Ｍｎが０．６％を超えると、鋼のマトリックスを脆化させ、靱性が悪化する。そこで、Ｍｎは、０．６％以下とし、望ましくは０．１〜０．６％とする。 Mn: ≦ 0.6%
Mn is an element necessary for obtaining the hardenability of the deoxidizer and steel. However, if Mn is less than 0.1%, these effects cannot be obtained. On the other hand, if Mn exceeds 0.6%, the steel matrix becomes brittle and the toughness deteriorates. Therefore, Mn is 0.6% or less, preferably 0.1 to 0.6%.

Ｃｒ：５．０〜１０．０％
Ｃｒは、硬質炭化物を形成し、鋼の硬さおよび耐摩耗性を向上させるとともに焼入性を高める元素である。しかし、Ｃｒが５．０％未満であるとこれらの効果は得られない。一方、Ｃｒが１０．０％を超えると、鋼中に粗大な炭化物を形成し、鋼の靱性を悪化し、更に軟化抵抗性を悪化する。そこで、Ｃｒは、５．０〜１０．０％とする。 Cr: 5.0 to 10.0%
Cr is an element that forms hard carbides, improves the hardness and wear resistance of the steel, and improves hardenability. However, if Cr is less than 5.0%, these effects cannot be obtained. On the other hand, when Cr exceeds 10.0%, coarse carbides are formed in the steel, the toughness of the steel is deteriorated, and the softening resistance is further deteriorated. Therefore, Cr is set to 5.0 to 10.0%.

Ｍｏ＋Ｗ／２：２．７〜５．０％
ＭｏおよびＷは、硬質炭化物を形成し、鋼の硬さおよび耐摩耗性を向上させるとともに焼入性および焼戻軟化抵抗性を高める元素である。これらの効果を得るためには、Ｍｏ＋Ｗ／２が２．７以上必要である。一方、Ｍｏ＋Ｗ／２が５．０％を超えると粗大な炭化物を形成し、靱性を悪化する。そこで、Ｍｏ＋Ｗ／２は、２．７〜５．０％とし、望ましくは２．７〜４．０％とする。 Mo + W / 2: 2.7 to 5.0%
Mo and W are elements that form hard carbides, improve the hardness and wear resistance of the steel, and increase the hardenability and temper softening resistance. In order to obtain these effects, Mo + W / 2 needs to be 2.7 or more . On the other hand, when Mo + W / 2 exceeds 5.0%, coarse carbides are formed and the toughness is deteriorated. Therefore, Mo + W / 2 is set to 2.7 to 5.0%, preferably 2.7 to 4.0%.

Ｖ：０．０５〜０．３５％未満
Ｖは、硬質炭化物を形成し、鋼の硬さおよび耐摩耗性を向上させるとともに、焼入れ時の結晶粒の粗大化を抑制する効果があり、靱性の向上に寄与する元素である。しかし、Ｖが０．０５％未満ではこれらの効果は得られない。一方、Ｖが０．３５％以上では粗大な炭化物を形成し、鋼の靱性を悪化する。そこで、Ｖは、０．０５〜０．３５％未満とし、望ましくは０．１〜０．３４％とする。 V: 0.05 to less than 0.35% V forms a hard carbide, improves the hardness and wear resistance of the steel, and has the effect of suppressing the coarsening of crystal grains during quenching. It is an element that contributes to improvement. However, if V is less than 0.05%, these effects cannot be obtained. On the other hand, if V is 0.35% or more, coarse carbides are formed and the toughness of the steel is deteriorated. Therefore, V is set to 0.05 to less than 0.35%, preferably 0.1 to 0.34%.

Ｎ：２５０ｐｐｍ未満
Ｎは、２５０ｐｐｍ以上含有されると、鋼中に粗大な窒化物を形成して、靱性を悪化する。そこで、Ｎは、２５０ｐｐｍ未満とする。 N: Less than 250 ppm When N is contained in an amount of 250 ppm or more, coarse nitrides are formed in the steel and the toughness is deteriorated. Therefore, N is set to less than 250 ppm.

Ｋ＝−０．７４Ｃ＋０．２４Ｃｒ＋０．７３（Ｍｏ＋Ｗ／２）＋２．１２Ｖ…（１）とするとき、Ｋ：３．０〜５．５％
Ｋの値は、２次硬化に寄与する２次炭化物を形成する元素の、焼入処理後における固溶状態を表す値である。各化学成分の限定範囲内において、２次炭化物を形成するＣｒ、Ｍｏ、Ｗ、Ｖの量に対してＣ量が増えＫの値が低くなり過ぎると、Ｃｒ、Ｍｏ、Ｗ、Ｖが１次炭化物として析出してしまい、焼戻後の２次炭化物量が減る。そのため２次硬化量が減少し、鋼の目的とする硬さが得られなくなる。そこで、鋼の必要な硬さを得るためには、（１）式であるＫの値を３．０％以上とする必要がある。しかし、Ｃ量に対してＣｒ、Ｍｏ、Ｗ、Ｖの量が増えＫの値が５．５％より高くなると、固溶するＣｒ、Ｍｏ、Ｗ、Ｖの元素が増えることで硬さの低い残留オーステナイトが焼入れ時に過剰に生じ、焼戻し後も分解せずに残存する量が増えるため、鋼の十分な硬さが得られなくなり、また焼戻し後の組織が基地組織と残留オーステナイトが不均一に混ざった組織状態となるため、靭性が低下する。そこで、（１）式であるＫの値は、３．０〜５．５％とし、望ましくは３．５〜５．０％とする。 K = −0.74C + 0.24Cr + 0.73 (Mo + W / 2) + 2.12V (1), K: 3.0 to 5.5%
The value of K is a value representing the solid solution state of the element forming the secondary carbide that contributes to the secondary hardening after the quenching treatment. Within the limited range of each chemical component, if the amount of C increases and the value of K becomes too low relative to the amount of Cr, Mo, W, V forming the secondary carbide, Cr, Mo, W, V becomes primary. It precipitates as carbide and the amount of secondary carbide after tempering is reduced. Therefore, the amount of secondary hardening decreases and the intended hardness of steel cannot be obtained. Therefore, in order to obtain the necessary hardness of the steel, the value of K in the formula (1) needs to be 3.0% or more. However, if the amount of Cr, Mo, W, V increases with respect to the amount of C, and the value of K is higher than 5.5%, the hardness of Cr, Mo, W, V is increased due to the increase in the elements of solid solution, and the hardness is low Residual austenite is excessively generated during quenching, and the amount remaining without being decomposed even after tempering increases, so that sufficient hardness of the steel cannot be obtained. As a result, the toughness is reduced. Therefore, the value of K in the formula (1) is set to 3.0 to 5.5%, preferably 3.5 to 5.0%.

焼入れ温度は１０２０〜１０５０℃未満
焼入温度が低すぎると、たとえＡｃ3点よりも高い温度であっても、１次炭化物が十分固溶せず２次硬化を起こす元素の固溶量が少なくなるため、焼戻ししても６３ＨＲＣ以上の硬さが得られないため、焼入温度下限を１０２０℃とする。しかし、１０５０℃以上では、利便性が悪化するとともに、熱処理にコストが嵩む。そこで、焼入温度は１０２０〜１０５０℃未満とする。 The quenching temperature is less than 1020-1050 ° C. If the quenching temperature is too low, even if the temperature is higher than the Ac 3 point, the primary carbide does not sufficiently dissolve, and the amount of elements that cause secondary hardening decreases. Therefore, the hardness lower than 63HRC cannot be obtained even after tempering, so the lower limit of quenching temperature is 1020 ° C. However, if it is 1050 degreeC or more, while the convenience will deteriorate, the cost for heat processing will increase. Therefore, the quenching temperature is set to less than 1020 to 1050 ° C.

冷間工具鋼の硬さは６３ＨＲＣ以上
高硬度の鋼材を加工するためには、冷間工具鋼の硬さは少なくとも６３ＨＲＣが必要である。そこで、冷間工具鋼の硬さは６３ＨＲＣ以上とする。 The hardness of the cold tool steel is 63 HRC or more In order to process a steel material having a high hardness, the hardness of the cold tool steel needs to be at least 63 HRC. Therefore, the hardness of the cold tool steel is set to 63 HRC or more.

焼戻温度を５００℃〜６００℃とする焼戻工程を少なくとも２回以上
特に耐摩耗性が要求される用途においては、焼入焼戻し処理後、処理温度が４００℃以上の表面硬化処理が施される場合があるため、表面硬化処理時に組織変化を起こさないよう、４００℃以上の高温焼戻しにおいて、所要の硬さを得ることが必要であり、２次硬化が起こる５００℃〜６００℃を焼戻温度で焼戻しを行うこととした。また、焼戻回数が１回のみだと、十分に２次硬化が起こらないため、焼戻し回数は２回以上とした。 In applications where wear resistance is required at least two times for the tempering process at a tempering temperature of 500 ° C. to 600 ° C., a surface hardening treatment at a treatment temperature of 400 ° C. or higher is performed after the quenching and tempering treatment. Therefore, it is necessary to obtain the required hardness in high-temperature tempering at 400 ° C. or higher so as not to cause a structural change during the surface hardening process, and tempering is performed at 500 ° C. to 600 ° C. in which secondary hardening occurs. Tempering was performed at temperature. Further, if the number of tempering is only once, secondary curing does not occur sufficiently, so the number of tempering was set to 2 or more.

また、本発明では、冷間度工具鋼を材料コストのかかる粉末冶金により製造するのではなく、コストの安価な溶解法により製造することでコストを下げるものである。 Further, in the present invention, the cold tool steel is not manufactured by powder metallurgy, which requires high material cost, but is manufactured by an inexpensive melting method, thereby reducing the cost.

本発明は上記の手段における化学成分の範囲を、化学成分のうちＣ、Ｃｒ、Ｍｏ＋Ｗ／２、およびＶの関係からなる（１）式のＫの値を３．０〜５．５％の範囲を満足するものとし、かつ、溶解法により、鋼を得て、さらにこの鋼を１０２０〜１０５０℃未満から焼入れし、５００〜６００℃で２回以上焼戻しすることで、高速工具鋼として必要な６３ＨＲＣ以上の硬さと靱性を得ることができ、鍛造金型、フォーミングロールあるいは転造ダイスなどの使用条件の特に過酷な冷間加工用として好適な、高硬度、高靱性の冷間工具鋼およびその鋼材からなる金型並びに工具を得ることができる。 In the present invention, the range of the chemical component in the above means is the range of 3.0 to 5.5% of the value of K in the formula (1) comprising the relationship of C, Cr, Mo + W / 2 and V among the chemical components. 63HRC required as high-speed tool steel by obtaining steel by a melting method, quenching this steel from less than 1020 to 1050 ° C., and tempering at 500 to 600 ° C. twice or more. High hardness and toughness cold work tool steel and its steel material that can obtain the above hardness and toughness, and are suitable for particularly severe cold working conditions such as forging dies, forming rolls or rolling dies. A mold and a tool can be obtained.

本発明の実施の形態について、以下に表を参照して説明する。 Embodiments of the present invention will be described below with reference to the tables.

表１に高速度工具鋼の供試鋼である発明鋼のＡ〜Ｃ、Ｅ〜ＨおよびＪの７区分と比較鋼のＫ〜Ｘの１４区分のＦｅおよび不可避不純物以外の化学成分およびＫ式の値をそれぞれ示す。これらの表１に示す化学成分およびＫ値を有する鋼成分の７区分の発明鋼および１４区分の比較鋼について、本発明手段の溶解法として、それぞれ１００ｋｇの溶鋼を真空誘導溶解炉にて溶製してそれぞれの鋼を得た。これらの鋼を断面５０ｍｍ²の角材に鍛伸した。次いで、これらの角材を加熱して１０３０℃で３０分保持後、空冷して焼入れし、その後、５００〜６００℃で１時間保持して空冷する焼戻工程を２回繰り返して、焼入焼戻処理を実施した。 Table 1 shows chemical components other than Fe and unavoidable impurities and K formulas of 7 sections of invention steels A to C, E to H, and J of 14 steels of comparative steels, which are test steels of high-speed tool steel. Each value is shown. With regard to these 7 chemicals and 14 comparative steels having chemical values and K values shown in Table 1, 100 kg of molten steel is melted in a vacuum induction melting furnace as a melting method of the means of the present invention. And obtained each steel. These steels were forged into square bars having a cross section of 50 mm ² . Next, these squares are heated and held at 1030 ° C. for 30 minutes, then air-cooled and quenched, and then kept at 500 to 600 ° C. for 1 hour and air-cooled twice to quench and temper. Processing was carried out.

表２に、上記の溶解法により得られた供試鋼である発明鋼のＡ〜Ｃ、Ｅ〜ＨおよびＪの７区分と比較鋼のＫ〜Ｘの１４区分の冷間工具鋼について、焼入れ後の残留オーステナイト量、焼入れ後の１次炭化物の面積率、焼戻硬さおよびシャルピー衝撃試験の結果をそれぞれ示す。この表２において、焼入れ後の残留オーステナイト量の測定については、焼入れ後の試料を用いて、Ｘ線回析法により定量した。１次炭化物の面積率の測定は、焼入れ後の試料の切断面を研磨後、１０％ナイタール液で腐食し、２００倍の顕微鏡にてその切断面２ｍｍ²の範囲の画像をコンピューターに取り込み、画像解析ソフトを用いて２０μｍ²以上の炭化物の面積率を求めた。焼戻硬さは５００〜６００℃の焼戻処理の温度範囲で最も高い硬さで評価しており、硬さ測定は、常温でのロックウェルＣスケールで測定した。さらに、シャルピー衝撃試験は、上記の焼入焼戻試料から断面が１０ｍｍ²で長さ５５ｍｍのノッチ幅２ｍｍのＵノッチであるＪＩＳのＺ２２４２：２００５におけるシャルピー試験片を割出し、冷間で衝撃試験を行い、その衝撃値の測定を行った。粉末冶金により製造されたＪＩＳ規格の鋼種であるＳＫＨ５１は６３ＨＲＣで、２５Ｊ／ｃｍ²の衝撃値が得られる。そこで、この２５Ｊ／ｃｍ²の値を基準とし、これと同じかこれより高い衝撃値が得られれば、靱性が良いを示す○で評価し、これより低い衝撃値が得られれば、靱性が悪いを示す×で評価し、表２にそれぞれ示した。 Table 2 shows quenching of cold work tool steels of 7 sections of inventive steels A to C, E to H and J and 14 sections of comparative steels K to X which are test steels obtained by the above melting method. The amount of retained austenite after, the area ratio of primary carbide after quenching, the tempering hardness, and the Charpy impact test results are shown respectively. In Table 2, the amount of retained austenite after quenching was quantified by an X-ray diffraction method using a sample after quenching. The area ratio of the primary carbide is measured by polishing the cut surface of the sample after quenching, corroding with 10% nital liquid, and capturing an image in the range of 2 mm ² of the cut surface with a 200 × microscope. The area ratio of carbides of 20 μm ² or more was determined using analysis software. The tempering hardness was evaluated with the highest hardness in the temperature range of tempering treatment of 500 to 600 ° C., and the hardness was measured on the Rockwell C scale at room temperature. Further, the Charpy impact test was performed by indexing a Charpy test piece in JIS Z2242: 2005, which is a U-notch having a cross-section of 10 mm ² and a length of 55 mm and a notch width of 2 mm from the above-mentioned quenching and tempering sample, and performing a cold impact test. The impact value was measured. SKH51, which is a JIS standard steel produced by powder metallurgy, is 63HRC, and an impact value of 25 J / cm ² is obtained. Therefore, with the value of 25 J / cm ² as a reference, if an impact value equal to or higher than this is obtained, it is evaluated with ○ indicating that the toughness is good, and if an impact value lower than this is obtained, the toughness is poor. The results are shown in Table 2.

表１および表２を考察すると、発明鋼ではＮｏ．Ａ〜Ｃ、Ｅ〜ＨおよびＪのいずれも焼戻硬さは６３ＨＲＣ以上であり、かつ、シャルピー衝撃試験の結果は○であり、したがって衝撃値は２５Ｊ／ｃｍ²以上の高い値であった。これに対して、比較鋼ではＣ、Ｓｉ、Ｃｒ、Ｍｏ＋Ｗ／２のいずれかの含有量が請求項の範囲より低いものでは、焼戻硬さが６３ＨＲＣより低く、一方、Ｃ、Ｓｉ、Ｍｎ、Ｍｏ＋Ｗ／２、Ｖのいずれかの含有量が請求項の範囲より高いものでは、発明鋼に比べ１次炭化物が多くなっており、シャルピー衝撃試験の結果は×であり、したがって衝撃値は２５Ｊ／ｃｍ²より低い値で靭性が低いものであった。さらにＫ値が３．０未満の比較鋼では硬さが低く、またＫ値が５．５越の比較鋼では焼戻硬さは６３ＨＲＣより低く、かつ、靱性も低いことがわかる。 When Table 1 and Table 2 are considered, in the invention steel, No. All of A to C, E to H, and J had a tempering hardness of 63 HRC or more, and the result of the Charpy impact test was ◯. Therefore, the impact value was a high value of 25 J / cm ² or more. On the other hand, in the comparative steel, if the content of any one of C, Si, Cr, and Mo + W / 2 is lower than the scope of the claims, the tempering hardness is lower than 63 HRC, while C, Si, Mn, In the case where the content of either Mo + W / 2 or V is higher than the range of the claims, the amount of primary carbide is larger than that of the steel according to the invention, and the result of the Charpy impact test is x, so the impact value is 25 J / The toughness was low at a value lower than cm ² . Further, it can be seen that the comparative steel having a K value of less than 3.0 has a low hardness, and the comparative steel having a K value of over 5.5 has a tempering hardness lower than 63 HRC and a low toughness.

すなわち、比較鋼において、Ｎｏ．ＫはＣの含有量が請求項の範囲より高いので、焼戻硬さは６４ＨＲＣと６３ＨＲＣより高いがシャルピー衝撃値は×で２５Ｊ／ｃｍ²より低かった。
Ｎｏ．ＬはＣｒの含有量およびＫ値が請求項の範囲より高いので、１次炭化物や残留オーステナイトが多く、焼戻硬さは６２ＨＲＣと低くシャルピー衝撃値は×であった。
Ｎｏ．ＭはＶの含有量およびＫ値が請求項の範囲より高いので、１次炭化物や残留オーステナイトが多く、焼戻硬さは６２ＨＲＣと低くシャルピー衝撃値は×であった。
Ｎｏ．ＮはＣの含有量が請求項の範囲より低くいので、焼戻硬さは５８ＨＲＣと低いが、シャルピー衝撃値は○で靱性は高かった。
Ｎｏ．ＯはＭｎの含有量が請求項の範囲より高いので、焼戻硬さは６４ＨＲＣと６３ＨＲＣより高いがシャルピー衝撃値は×で２５Ｊ／ｃｍ²より低かった。
Ｎｏ．ＰはＳｉの含有量が請求項の範囲より低いので、焼戻硬さは６２ＨＲＣと低いが、シャルピー衝撃値は○で靱性は高かった。
Ｎｏ．ＱはＳｉの含有量が請求項の範囲より高いので、シャルピー衝撃値は×で靱性は低かった。
Ｎｏ．ＲはＣｒの含有量が請求項の範囲より低く、Ｋ値が請求項の範囲より低いので、焼戻硬さは６２ＨＲＣと低いが、シャルピー衝撃値は○であった。
Ｎｏ．ＳはＮの含有量が請求項の範囲より高いので、シャルピー衝撃値は×であった。
Ｎｏ．ＴはＣ、Ｓｉ、ＣｒおよびＭｏ＋１／２Ｗの含有量が請求項の範囲より低く、Ｋ値が請求項の範囲より低いので、焼戻硬さは５７ＨＲＣと低いが、シャルピー衝撃値は○であった。
Ｎｏ．ＵはＣおよびＶの含有量が請求項の範囲より高いので、１次炭化物が多く、シャルピー衝撃値は×であった。
Ｎｏ．ＶはＶの含有量が請求項の範囲より高いので、１次炭化物が多く、シャルピー衝撃値は×であった。
Ｎｏ．ＷはＫ値が請求項の範囲より低いので、焼戻硬さは６２ＨＲＣと低いが、シャルピー衝撃値は○であった。
Ｎｏ．ＸはＫ値が請求項の範囲より高いので、残留オーステナイトおよび１次炭化物が多く、焼戻硬さ６１ＨＲＣと低くシャルピー衝撃値は×であった。 That is, in the comparative steel, No. Since K has a C content higher than that in the claims, the tempering hardness is higher than 64 HRC and 63 HRC, but the Charpy impact value is x and lower than 25 J / cm ² .
No. Since L has a Cr content and a K value higher than those in the claims, the amount of primary carbide and retained austenite was large, the tempering hardness was as low as 62 HRC, and the Charpy impact value was x.
No. Since M has a V content and a K value higher than the claims, the amount of primary carbide and retained austenite is large, the tempering hardness is as low as 62 HRC, and the Charpy impact value is x.
No. Since N has a C content lower than that in the claims, the tempering hardness was as low as 58 HRC, but the Charpy impact value was good and the toughness was high.
No. Since O has a Mn content higher than that in the claims, the tempering hardness is higher than 64 HRC and 63 HRC, but the Charpy impact value is x and lower than 25 J / cm ² .
No. Since P has a lower Si content than the claimed range, the tempering hardness is as low as 62 HRC, but the Charpy impact value was good and the toughness was high.
No. Since Q has a Si content higher than that in the claims, the Charpy impact value was x and the toughness was low.
No. R has a Cr content lower than that in the claims and a K value lower than that in the claims, so the tempering hardness was as low as 62 HRC, but the Charpy impact value was o.
No. Since S has a higher N content than the scope of the claims, the Charpy impact value was x.
No. T has lower contents of C, Si, Cr and Mo + 1 / 2W than the scope of claims, and K value is lower than the scope of claims, so the tempering hardness is as low as 57 HRC, but the Charpy impact value is ○. It was.
No. U has a higher content of C and V than the scope of claims, so there are many primary carbides, and the Charpy impact value was x.
No. V had a higher content of V than the scope of the claims, so there were many primary carbides, and the Charpy impact value was x.
No. Since W has a K value lower than that in the claims, the tempering hardness was as low as 62 HRC, but the Charpy impact value was ◯.
No. Since X has a K value higher than the range defined in the claims, the amount of retained austenite and primary carbide is large, the tempering hardness is 61HRC, and the Charpy impact value is x.

Claims

In mass%, C: 0.6 to less than 0.9%, Si: 0.6 to 1.0%, Mn: ≦ 0.6%, Cr: 5.0 to 10.0%, Mo + W / 2: 2.7 to 5.0%, V: 0.05 to less than 0.35%, N: <250 ppm, and K = −0.74C + 0.24Cr + 0.73 (Mo + W / 2) +2.12 V ... (1) to the time, K: satisfies 3.0 to 5.5%, the steel balance of Fe and inevitable impurities ing, the area ratio of the primary carbides performs quenching below 1020-1,050 ° C. 5% A method for producing a cold tool steel having high hardness and high toughness suitable for interworking, characterized in that the tempering step at 500 to 600 ° C. is repeated at least twice to make the hardness 63 HRC or more .