JP2011026693A - High hardness steel having excellent softening resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high hardness steel having excellent softening resistance, which maintains high hardness even in the case of a temperature rise to the vicinity of 500°C, by the optimum balance of its composition even if a Cr content is reduced. <P>SOLUTION: The high hardness steel having excellent softening resistance has a composition comprising, by mass, >0.6 to 0.7% C, >0.25 to 2.0% Si, >0.25 to 1.0% Mn, 8.0 to <11.0% Cr, Mo+1/2W: >1.5 to 5.0% and >0.02 to 0.06% N, and the balance Fe with impurities, wherein the relation of C/Cr: >0.056 to 0.085 and C+N: >0.63 to 0.75 is satisfied. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

本発明は、５００℃付近までの温度上昇にも高硬度を維持できる軟化抵抗に優れた高硬度鋼に関するものである。   The present invention relates to a high-hardness steel excellent in softening resistance that can maintain high hardness even when the temperature rises to around 500 ° C.

従来から、比較的高い温度で使用が可能な軸受鋼として、ＪＩＳ ＳＵＳ４４０Ｃ鋼、ＢＡＳ ４４０Ｍ鋼（１．１Ｃ−１４．５Ｃｒ−４Ｍｏ−Ｆｅ），０．７Ｃ−１２Ｃｒ鋼等が用いられている。これらのマルテンサイトステンレス鋼は、耐熱性、耐食性、耐摩耗性の点でＪＩＳ Ｇ４８０５に規定されている高炭素クロム軸受鋼（ＳＵＪ）より優れているが、転動疲労特性が劣る欠点がある。
これに対して、本願出願人は特許第３２４１９２１号として、転動疲労特性に優れた耐摩耐食軸受鋼を提案した。 Conventionally, JIS SUS440C steel, BAS 440M steel (1.1C-14.5Cr-4Mo-Fe), 0.7C-12Cr steel, and the like have been used as bearing steel that can be used at relatively high temperatures. These martensitic stainless steels are superior to the high carbon chromium bearing steel (SUJ) defined in JIS G4805 in terms of heat resistance, corrosion resistance, and wear resistance, but have a drawback of poor rolling fatigue characteristics.
On the other hand, the applicant of the present application has proposed a wear-resistant and corrosion-resistant bearing steel excellent in rolling fatigue characteristics as Japanese Patent No. 3224121.

特許第３２４１９２１号公報Japanese Patent No. 3224121

上述した特許第３２４１９２１号は、Ｃ、Ｎ、Ｃｒを高めた軸受鋼である。この特許第３２４１９２１号で提案した軸受鋼を更に検討した結果、Ｃを高めると一次炭化物が晶出し、疲労強度が低下する危険性がある。また、Ｎ含有量が比較的高いため、添加が困難となり、製造性に問題があった。
Ｎ含有量を高めるためには、Ｃｒも高めることが有効であり、特許第３２４１９２１号では、１２．４％以上の範囲の軸受鋼が提案されている。しかしながら、最近では、合金の低コスト化が求められており、必要最小限のＣｒ添加量とする必要がある。
本発明の目的は、Ｃｒ含有量を低減しても組成の最適バランスにより、５００℃付近までの温度上昇にも高硬度を維持できる軟化抵抗に優れた高硬度鋼を提供することである。 The above-mentioned Japanese Patent No. 3224121 is a bearing steel in which C, N, and Cr are increased. As a result of further examination of the bearing steel proposed in Japanese Patent No. 3241421, when C is increased, there is a risk that primary carbides crystallize and fatigue strength decreases. Moreover, since N content is comparatively high, addition became difficult and there existed a problem in manufacturability.
In order to increase the N content, it is effective to increase Cr, and Japanese Patent No. 3241922 proposes a bearing steel in a range of 12.4% or more. However, recently, cost reduction of the alloy has been demanded, and it is necessary to make the necessary minimum Cr addition amount.
An object of the present invention is to provide a high-hardness steel excellent in softening resistance that can maintain a high hardness even with a temperature increase up to about 500 ° C. by an optimal balance of the composition even if the Cr content is reduced.

本発明は、上述した課題に鑑みてなされたものである。
すなわち本発明は、質量％で、Ｃ：０．６％を超え〜０．７％、Ｓｉ：０．２５％を超え〜２．０％以下、Ｍｎ：０．２５％を超え〜１．０％以下、Ｃｒ：８．０〜１１．０％未満、Ｍｏ＋１／２Ｗ：１．５％を超え〜５．０％、Ｎ：０．０２％を超えて０．０６％以下を含有し、残部はＦｅ及び不純物でなり、且つ、Ｃ／Ｃｒ：０．０５６を超え〜０．０８５、Ｃ＋Ｎ：０．６３％を超え０．７５％の関係を満たす軟化抵抗に優れた高硬度鋼である。
また本発明では、更に質量％で、Ｓ：０．１％以下、Ｃａ：０．１％以下、Ｍｇ：０．０３％以下の一種以上を含有させ、優れた被削性を付与することができる。 The present invention has been made in view of the above-described problems.
That is, the present invention is, by mass%, C: more than 0.6% to 0.7%, Si: more than 0.25% to 2.0% or less, Mn: more than 0.25% to 1.0% %: Cr: 8.0 to less than 11.0%, Mo + 1 / 2W: more than 1.5% to 5.0%, N: more than 0.02% and 0.06% or less, the balance Is a high-hardness steel excellent in softening resistance, which is composed of Fe and impurities and satisfies the relationship of C / Cr: more than 0.056 to 0.085, C + N: more than 0.63% and 0.75%.
Further, in the present invention, it is possible to further contain one or more of S: 0.1% or less, Ca: 0.1% or less, and Mg: 0.03% or less in mass% to give excellent machinability. it can.

本発明の軟化抵抗に優れた高硬度鋼は、合金元素の添加量を低減しても、例えば、４００〜５００℃に加熱しても使用時に軟化し難い良好な硬さを維持できることから、樹脂などの成形型、軸受、各種工具用の合金として最適である。   The high hardness steel excellent in softening resistance of the present invention can maintain a good hardness that is difficult to soften during use even if the amount of alloying elements is reduced, for example, even when heated to 400 to 500 ° C. It is most suitable as an alloy for molds, bearings, and various tools.

塩水噴霧試験の結果を示す外観写真である。It is an external appearance photograph which shows the result of a salt spray test. 本発明の高硬度鋼断面の光学顕微鏡写真である。It is an optical microscope photograph of the high hardness steel cross section of this invention. 疲労試験結果を示す図である。It is a figure which shows a fatigue test result.

上述したように、本発明の重要な特徴はＣｒ含有量を低減しても、組成の最適バランスにより、４００〜５００℃の高温に加熱した場合でも５８ＨＲＣ近辺の良好な硬さを維持できる合金組成にある。以下に詳しく本発明を説明する。なお、特に記載の無い限り、各元素の含有量は質量％で記す。
Ｃ：０．６％を超え〜０．７％
Ｃは、基地の硬さを高め、高温焼戻しによってＣｒやＭｏとの炭化物を形成して高硬度鋼の耐磨耗性を確保するための重要な元素であるため必須で含有する。
しかし、Ｃが０．７％を超えると一次炭化物が晶出し易くなり、疲労強度が低下する。
また、Ｃが０．５％未満であると、上述のＣの添加効果が得られない。そのため、Ｃの範囲は０．６を超え〜０．７％に限定する。上述の効果を確実に得るための好ましいＣの下限は０．６２％、上限は０．６８％である。 As described above, an important feature of the present invention is that the alloy composition can maintain a good hardness in the vicinity of 58HRC even when it is heated to a high temperature of 400 to 500 ° C., even if the Cr content is reduced, due to the optimum balance of the composition. It is in. The present invention is described in detail below. Unless otherwise specified, the content of each element is expressed in mass%.
C: Over 0.6% to 0.7%
C is an essential element for increasing the hardness of the base and forming carbides with Cr and Mo by high-temperature tempering to ensure the wear resistance of the high-hardness steel.
However, when C exceeds 0.7%, the primary carbide is easily crystallized and the fatigue strength is lowered.
Further, if C is less than 0.5%, the above-mentioned effect of adding C cannot be obtained. Therefore, the range of C exceeds 0.6 and is limited to 0.7%. The preferable lower limit of C for surely obtaining the above effect is 0.62%, and the upper limit is 0.68%.

Ｓｉ：０．２５％を超え〜２．０％以下
Ｓｉは、脱酸元素として添加する他、本発明においては高温焼戻しの硬さを高める効果があるので０．２５％を超えて２．０％以下の範囲において必須で添加する。Ｓｉが０．２５％以下の範囲では、前記のＳｉ添加効果が望めず、また、Ｓｉを２．０％を超えて添加しても、Ｓｉの高温焼戻しの硬さを高める効果の向上は望めず、かえって靭性や熱間加工性を阻害する。そのため、Ｓｉは０．２５％を超え〜２．０％以下の範囲に規定する。
なお、Ｓｉの高温焼戻しの硬さを高める効果を確実に実現できる好ましい下限は０．３％、更に好ましくは０．５％とすれば良く、好ましい上限は１．３％、更に好ましくは１．１％である。 Si: more than 0.25% to 2.0% or less Si is added as a deoxidizing element, and in the present invention, it has an effect of increasing the hardness of high-temperature tempering, so it exceeds 0.25% and exceeds 2.0. It is essential and added in the range of% or less. If Si is in the range of 0.25% or less, the above-mentioned effect of adding Si cannot be expected, and even if Si is added in excess of 2.0%, an improvement in the effect of increasing the hardness of high-temperature tempering of Si can be expected. Instead, it impedes toughness and hot workability. Therefore, Si is specified in the range of more than 0.25% and not more than 2.0%.
In addition, the preferable lower limit which can implement | achieve the effect which raises the hardness of high temperature tempering of Si reliably is 0.3%, More preferably, what is necessary is just 0.5%, and a preferable upper limit is 1.3%, More preferably, 1. 1%.

Ｍｎ：０．２５％を超え〜１．０％以下
Ｍｎは、添加すると靭性を劣化させることなく鋼の強度を増すことができ、高温焼戻しの硬さも改善される。但し、過度なＭｎは、加工性、低温靭性を低下させる。Ｍｎが０．２５％以下の範囲では、前記のＭｎ添加効果が望めず、また、Ｍｎが１．０％を超えて添加しても、加工硬化し易くなり、加工時に材料の弾性限界点、降伏点、引張り強さ、疲労限界等が増加し、伸び、絞りが減少する。更に１．０％を超える範囲では焼戻し時に脆性が発生するので、Ｍｎの上限を１．０％以下に規定する。鋼の強度を確保して、加工性の低下や焼戻し時の脆化をより確実に抑制できる好ましいＭｎの下限は０．３％、さらに好ましくは０．６％である。好ましい上限は０．８％である。 Mn: more than 0.25% to 1.0% or less When Mn is added, the strength of the steel can be increased without deteriorating the toughness, and the hardness of high-temperature tempering can be improved. However, excessive Mn reduces workability and low temperature toughness. When Mn is in the range of 0.25% or less, the above Mn addition effect cannot be expected, and even if Mn exceeds 1.0%, it becomes easy to work and harden, and the elastic limit point of the material during processing, Yield point, tensile strength, fatigue limit, etc. increase, elongation and drawing decrease. Furthermore, since brittleness occurs during tempering in the range exceeding 1.0%, the upper limit of Mn is specified to be 1.0% or less. The preferable lower limit of Mn that can ensure the strength of the steel and more reliably suppress deterioration of workability and embrittlement during tempering is 0.3%, more preferably 0.6%. A preferred upper limit is 0.8%.

Ｃｒ：８．０〜１１．０％未満
Ｃｒは、焼入れ性を向上させ、高温焼戻しの硬さを高める効果があるため、８．０％以上を必須で添加する。また、Ｃｒは、耐食性を向上させる効果もある一方で、過度のＣｒ添加は加工性、低温靭性に悪影響を及ぼすので、１１％未満を上限と規定する。また、Ｃｒの増加と共に高温焼戻しの硬さが上昇するとは限らず、適当なＣｒ量において硬さが最高となる。この効果を確実に得るにはＣｒの下限を９．０％、さらに好ましくは９．５％、上限を１０．５％とするのが好ましい。 Cr: less than 8.0 to 11.0% Cr has an effect of improving hardenability and increasing the hardness of high-temperature tempering, so 8.0% or more is essential. Cr also has the effect of improving corrosion resistance, but excessive addition of Cr adversely affects workability and low-temperature toughness, so less than 11% is defined as the upper limit. Moreover, the hardness of high-temperature tempering does not necessarily increase with the increase of Cr, and the hardness becomes the highest at an appropriate amount of Cr. In order to reliably obtain this effect, the lower limit of Cr is preferably 9.0%, more preferably 9.5%, and the upper limit is preferably 10.5%.

Ｍｏ＋１／２Ｗ：１．５を超え〜５．０％
Ｍｏ及びＷは、固溶強化および／または炭化物の析出硬化により高温焼戻し後の軟化抵抗を向上させ、耐摩耗性、耐熱疲労性を改善するために単独または複合で添加できる。更に、硬質な炭化物を作り、硬さを向上させる。
ＷはＭｏの約２倍の原子量であることからＭｏ＋１／２Ｗで規定する（当然、何れか一方のみの添加としても良いし、双方を共添加することもできる）。Ｍｏ、Ｗが少ないと高温焼戻しの硬さの改善が得られなくなるため、下限を１．５％を超える範囲に規定する。但し、添加量が５．０％を超えても上記の効果の向上はあまり望めない。そのため、Ｍｏ＋１／２Ｗの上限を５．０％に規定する。好ましくはＭｏ＋１／２Ｗの下限を１．７％、上限を４．０％、さらに好ましくは２．５％未満とすれば良い。 Mo + 1 / 2W: more than 1.5 to 5.0%
Mo and W can be added singly or in combination to improve softening resistance after high-temperature tempering by solid solution strengthening and / or precipitation hardening of carbides, and to improve wear resistance and heat fatigue resistance. Furthermore, a hard carbide is made and the hardness is improved.
Since W has an atomic weight approximately twice that of Mo, it is defined by Mo + 1 / 2W (of course, either one may be added, or both may be added together). When there is little Mo and W, since the improvement of the hardness of high temperature tempering will not be obtained, a minimum is prescribed | regulated to the range exceeding 1.5%. However, even if the addition amount exceeds 5.0%, the improvement of the above effect cannot be expected. Therefore, the upper limit of Mo + 1 / 2W is specified as 5.0%. Preferably, the lower limit of Mo + 1 / 2W is 1.7%, the upper limit is 4.0%, more preferably less than 2.5%.

Ｎ：０．０２％を超えて０．０６％以下
Ｎは、添加により固溶強化、析出硬化および／または、炭化物の結晶粒を微細化させ硬さを向上させる他、高温焼戻しの硬さやクリープ特性の改善に有効な成分であるが、過度な添加は加工性、低温靭性を低下させるので、上限を０．０６％に規定する。一方、Ｎが過度に少なくなると、固溶強化、析出硬化および／または、結晶粒を微細化させ硬さを向上させる効果が出ないため、Ｎの下限は、０．０２％を超えた範囲に規定する。高温焼戻しの硬さを確保するために、好ましくはＮの下限を０．０４％、上限を０．０５％未満とすれば良い。 N: more than 0.02% and not more than 0.06% N is added to improve the hardness by solid solution strengthening, precipitation hardening and / or carbide grains, and to improve hardness and creep of high temperature tempering Although it is an effective component for improving the characteristics, excessive addition reduces workability and low temperature toughness, so the upper limit is specified to 0.06%. On the other hand, if N is excessively reduced, there is no effect of solid solution strengthening, precipitation hardening and / or improvement of hardness by refining crystal grains, so the lower limit of N is in a range exceeding 0.02%. Stipulate. In order to ensure the hardness of high temperature tempering, the lower limit of N is preferably 0.04% and the upper limit is less than 0.05%.

本発明において、上述した各元素の含有量を調整した上で、Ｃ，Ｎ，Ｃｒのバランスを適正化する必要がある。
Ｃ／Ｃｒ：０．０５６を超え〜０．０８５
Ｃ含有量に対し、Ｃｒ含有量が多くなって、Ｃ／Ｃｒが０．０５６以下となるとＨＲＣ５８以上の高硬度とし難くなる。一方、Ｃｒ含有量に対して、Ｃ含有量が多くなってＣ／Ｃｒが０．０８５を超えると、一次炭化物が晶出し易くなったり、炭化物が粗大になり易くなって疲労強度を低下させる心配がある。そのため、ＣとＣｒのバランスを、Ｃ／Ｃｒで０．０５６を超えて０．０８５の範囲に限定する。
Ｃ＋Ｎ：０．６３％を超え０．７５％
ＣとＮの含有量の総和が０．６３％未満であると、ＨＲＣ５８以上の高硬度とし難くなる。一方、ＣとＮの含有量の総和が０．７５％を超えると、粗大な炭化物が形成し易くなり、疲労強度を低下させたり、残留オーステナイトが増加し易くなって、経年変化の心配がある。そのため、ＣとＮの総和は０．６３％を超え０．７５％の範囲に限定する。 In the present invention, it is necessary to optimize the balance of C, N, and Cr after adjusting the content of each element described above.
C / Cr: more than 0.056 to 0.085
When the Cr content increases with respect to the C content and the C / Cr is 0.056 or less, it becomes difficult to achieve a high hardness of HRC58 or higher. On the other hand, if the C content increases with respect to the Cr content and the C / Cr exceeds 0.085, the primary carbide is likely to be crystallized or the carbide tends to become coarse and the fatigue strength may be reduced. There is. Therefore, the balance of C and Cr is limited to a range of 0.085 exceeding 0.056 in C / Cr.
C + N: more than 0.63% and 0.75%
When the total content of C and N is less than 0.63%, it becomes difficult to achieve a high hardness of HRC58 or higher. On the other hand, if the total content of C and N exceeds 0.75%, coarse carbides are likely to be formed, fatigue strength is reduced, and retained austenite is likely to increase, which may cause aging. . Therefore, the sum total of C and N is limited to a range exceeding 0.63% and 0.75%.

本発明では、上記の必須元素に加えて、更に質量％で、Ｓ：０．１％以下、Ｃａ：０．１％以下、Ｍｇ：０．０３％以下の一種以上を含有させても良い。
必須で添加するＭｎ等との硫化物を形成し、被削性を改善するＳは、０．１％以下の範囲で含有しても良く、特にＳは、微量添加すると、Ｍｎ等との硫化物を形成し、被削性を改善するため、必要に応じて添加できる。一方でＳを多量に添加すると、熱間加工性、耐溶接高温割れ性、耐食性に悪影響を及ぼすため、０．１％以下と規定する。被削性の改善に好ましくは、Ｓの下限を０．０３％、上限を０．０８％とすれば良い。
また、Ｓと同様に、被削性を改善するＣａ及びＭｇについても、Ｃａ：０．１％以下、Ｍｇ：０．０３％以下の範囲で添加することができる。
Ｓ，Ｃａ，Ｍｇについては、複合添加も可能である。
本発明では、上述した元素の他は、Ｆｅ及び製造上不可避的に混入する元素は当然含有する。 In the present invention, in addition to the above essential elements, one or more of S: 0.1% or less, Ca: 0.1% or less, and Mg: 0.03% or less may be further contained in mass%.
S that improves the machinability by forming a sulfide with Mn or the like that is essential may be contained in a range of 0.1% or less. In particular, when S is added in a small amount, it is sulfided with Mn and the like. In order to form an object and improve machinability, it can be added as necessary. On the other hand, when a large amount of S is added, it adversely affects hot workability, weld hot cracking resistance, and corrosion resistance. For improving machinability, the lower limit of S is preferably 0.03% and the upper limit is 0.08%.
Similarly to S, Ca and Mg that improve machinability can also be added within a range of Ca: 0.1% or less and Mg: 0.03% or less.
For S, Ca and Mg, combined addition is also possible.
In the present invention, in addition to the above-described elements, naturally, Fe and elements inevitably mixed in production are contained.

以下の実施例で本発明を更に詳しく説明する。
本発明と比較例の１０ｋｇ鋼塊を真空溶解で作製し、高硬度鋼を得た。作製した高硬度鋼の化学組成を表１に示す。
表中のＮｏ．１〜１１は本発明の高硬度鋼、Ｎｏ．２１〜２７は比較鋼であり、比較鋼のうち、Ｎｏ．２３は代表的な一般軸受鋼のＳＵＪ２相当鋼、Ｎｏ．２４は高温軸受に用いられるＭ５０相当鋼、Ｎｏ．２５は耐食用軸受に用いられるＳＵＳ４４０Ｃ相当鋼、Ｎｏ．２６は代表的高硬度・高耐磨耗材料のＳＫＤ１１相当鋼、Ｎｏ．２７は代表的高速度工具鋼のＳＫＨ５１相当鋼である。
また、併せて、本発明の高硬度鋼（Ｎｏ．１２）の量産規模の溶解を行った。溶解は、３ｔｏｎ溶解（大気溶解）で作製した。 The following examples further illustrate the present invention.
The 10 kg steel ingots of the present invention and comparative examples were produced by vacuum melting to obtain high hardness steel. Table 1 shows the chemical composition of the produced high hardness steel.
No. in the table. 1 to 11 are high hardness steels of the present invention, No. 1 to No. 11. Nos. 21 to 27 are comparative steels. No. 23 is a typical general bearing steel SUJ2 equivalent steel, No. 23. No. 24 is M50 equivalent steel used for high temperature bearings, No. 24. No. 25 is a SUS440C equivalent steel used for corrosion resistant bearings, No. 25. No. 26 is a typical high hardness and high wear resistant material SKD11 equivalent steel, No. 26. 27 is a typical high-speed tool steel SKH51 equivalent steel.
In addition, mass production scale melting of the high hardness steel (No. 12) of the present invention was performed. Dissolution was made by 3ton dissolution (atmospheric dissolution).

作製した高硬度鋼のうち、本発明鋼Ｎｏ．１〜１１及び比較鋼Ｎｏ．２１〜２７を均質化焼鈍した後、１１５０℃で熱間鍛造し、１５ｍｍ（Ｔ）×１５ｍｍ（Ｗ）×１０００ｍｍ（Ｌ）の高硬度鋼の鍛造材を得た。
その後、７８０℃で３時間の焼鈍を行った。更に、焼鈍した高硬度鋼から１５ｍｍ（Ｔ）×１５ｍｍ（Ｗ）×１５ｍｍ（Ｌ）の硬度測定用試験片を作製した。
この試験片を大気炉内で表２に示した焼入れ条件で加熱保持し、油冷または空冷にて焼入れを行った。一部の本発明鋼と比較鋼は、−８０℃のエタノール中に１時間冷却保持するサブゼロ処理を焼入れと同時に行った。焼入れまたはサブゼロ処理後に試験片の両面を平行研磨し、５００℃付近で焼戻しを行なった。焼戻し条件は表２に示し、焼戻し後に常温環境下でロックウェル硬さＣスケールにて硬さを測定した。その結果を表２に示す。 Among the produced high hardness steels, the present invention steel No. 1-11 and comparative steel No.1. After homogenizing and annealing Nos. 21 to 27, hot forging was performed at 1150 ° C. to obtain a forged material of high hardness steel of 15 mm (T) × 15 mm (W) × 1000 mm (L).
Thereafter, annealing was performed at 780 ° C. for 3 hours. Further, a test piece for hardness measurement of 15 mm (T) × 15 mm (W) × 15 mm (L) was produced from the annealed high hardness steel.
This test piece was heated and held in the atmospheric furnace under the quenching conditions shown in Table 2, and quenched by oil cooling or air cooling. Some steels of the present invention and comparative steels were subjected to sub-zero treatment that was cooled and held in ethanol at −80 ° C. for 1 hour simultaneously with quenching. After quenching or sub-zero treatment, both surfaces of the test piece were parallel polished and tempered at around 500 ° C. Tempering conditions are shown in Table 2, and after tempering, the hardness was measured on a Rockwell hardness C scale in a normal temperature environment. The results are shown in Table 2.

量産規模溶解を実施したＮｏ．１２については、均質化焼鈍した後、１２００〜１２５０℃で熱間鍛造を行い、続いて１１００〜１１５０℃で熱間圧延を行い、φ１４ｍｍの高硬度鋼の圧延材を得た。
その後、７８０℃で３時間の焼鈍を行った。更に、焼鈍した高硬度鋼からφ１４ｍｍ×１５ｍｍ（Ｌ）の硬度測定用試験片を作製した。
この試験片を大気炉内で表２に示した焼入れ条件で加熱保持し、空冷にて焼入れを行った。一部は、−８０℃のエタノール中に１時間冷却保持するサブゼロ処理を焼入れと同時に行った。焼入れまたはサブゼロ処理後に試験片の両面を平行研磨し、高温の軟化抵抗を評価するため、５００℃付近で高温焼戻しを行なった。焼戻し条件は表２に示し、焼戻し後に常温環境下でロックウェル硬さＣスケールにて硬さを測定した。その結果を表２に併せて示す。 No. which carried out mass production scale dissolution. No. 12, after homogenization annealing, hot forging was performed at 1200 to 1250 ° C., followed by hot rolling at 1100 to 1150 ° C. to obtain a rolled material of φ14 mm high-hardness steel.
Thereafter, annealing was performed at 780 ° C. for 3 hours. Furthermore, a test piece for hardness measurement of φ14 mm × 15 mm (L) was produced from the annealed high hardness steel.
This test piece was heated and held in an atmospheric furnace under the quenching conditions shown in Table 2, and quenched by air cooling. A part was subjected to sub-zero treatment in -80 ° C ethanol for 1 hour while being quenched. In order to evaluate high-temperature softening resistance, both surfaces of the test piece were subjected to high-temperature tempering at around 500 ° C. after quenching or sub-zero treatment. Tempering conditions are shown in Table 2, and after tempering, the hardness was measured on a Rockwell hardness C scale in a normal temperature environment. The results are also shown in Table 2.

表２に示すように、本発明の高硬度鋼は、５００℃程度での焼戻し後の硬さが５８ＨＲＣ以上になっていることが分かる。
一方、比較鋼Ｎｏ．２１及びＮｏ．２２は５８ＨＲＣ以上の硬さを得ることができず、ＳＵＪ２相当鋼のＮｏ．２３では、５００℃の焼戻しにより５８ＨＲＣを大きく下回っていることが分かる。なお、ＳＵＪ２相当鋼のＮｏ．２３においては、１８０℃の焼戻しにより、６３ＨＲＣが得られたが、５００℃の高温焼戻しでは４７ＨＲＣ以下に硬さが低下していることから、ＳＵＪ２相当鋼は高温での適用には不向きであることが分かる。
以上のことから、本発明の高硬度鋼は、高い軟化抵抗を有することが分かる。 As shown in Table 2, it can be seen that the high hardness steel of the present invention has a hardness after tempering at about 500 ° C. of 58 HRC or more.
On the other hand, Comparative Steel No. 21 and no. No. 22 could not obtain a hardness of 58HRC or higher, and No. 22 of SUJ2 equivalent steel. It can be seen that No. 23 is much lower than 58HRC by tempering at 500 ° C. In addition, SUJ2 equivalent steel No. In No. 23, 63 HRC was obtained by tempering at 180 ° C., but the hardness decreased to 47 HRC or less at high temperature tempering at 500 ° C., so that SUJ2 equivalent steel is not suitable for application at high temperature. I understand.
From the above, it can be seen that the high hardness steel of the present invention has a high softening resistance.

次に、被削性改善元素を添加した本発明の高硬度鋼のうち、Ｎｏ．３〜８の高硬度鋼について、被削性試験を行なった。被削性改善効果を確認するため、被削性改善元素を無添加としたＮｏ．９についても、同様に被削性試験を行なった。
被削性試験は、焼鈍ままの素材から１５ｍｍ（Ｔ）×１５ｍｍ（Ｗ）×２２ｍｍ（Ｌ）の寸法の試験片を切り出し、切り出した被削性試験片にドリルで孔開け加工を行って、ドリルの磨耗量にて評価を実施した。評価は、ドリルの最外周部をＡと表記し、４／Ｄ部をＢと表記して、試験結果を表４に示す。なお、試験条件は以下のとおりである。
ドリル径：φ４．０ｍｍストレートシャンクドリル
切削深さ：２０ｍｍ
切削速度：３０ｍ／ｍｉｎ
送り速度：０．０５ｍｍ／ｒｅｖ
ステップフィード：１０ｍｍ
切削液：水溶性 Next, among the high hardness steels of the present invention to which the machinability improving element is added, No. A machinability test was performed on 3 to 8 high hardness steels. In order to confirm the machinability improving effect, no. Similarly, the machinability test was conducted for No. 9 as well.
In the machinability test, a test piece having a size of 15 mm (T) × 15 mm (W) × 22 mm (L) was cut out from the raw material as annealed, and the cut machinability test piece was drilled with a drill, Evaluation was performed based on the amount of wear of the drill. In the evaluation, the outermost peripheral part of the drill is expressed as A, the 4 / D part is expressed as B, and the test results are shown in Table 4. The test conditions are as follows.
Drill diameter: φ4.0mm straight shank drill Cutting depth: 20mm
Cutting speed: 30 m / min
Feeding speed: 0.05mm / rev
Step feed: 10mm
Cutting fluid: Water-soluble

表３に示すように、被削性改善元素を添加した本発明の高硬度鋼は、被削性改善元素を無添加としたＮｏ．９の高硬度鋼よりも被削性が向上しているのがわかる。中でも適量のＳとＣａを複合添加したＮｏ．７の高硬度鋼が最も被削性が良いことが分かる。   As shown in Table 3, the high-hardness steel of the present invention to which the machinability improving element was added is No. 1 with no machinability improving element added. It can be seen that the machinability is improved as compared with 9 high hardness steel. Among them, No. 1 in which an appropriate amount of S and Ca was added in combination. It can be seen that the 7 hard steel has the best machinability.

続いて、本発明の高硬度鋼と比較鋼について耐食性を調査した。
耐食性の調査は、本発明の高硬度鋼Ｎｏ．１と比較鋼Ｎｏ．２５（ＳＵＳ４４０Ｃ相当鋼）、Ｎｏ．２６（ＳＫＤ１１相当鋼）、Ｎｏ．２７（ＳＫＤ５１相当鋼）について行なった。試験片はそれぞれ、Ｎｏ．１が１０５０℃×３０分、空冷の焼入れと、５２０℃×１時間の焼戻しを２回行い、Ｎｏ．２５が１０５０℃×３０分、空冷の焼入れと−８０℃のサブゼロ処理、１８０℃×１時間の焼戻しを行い、Ｎｏ．２６が１０２５℃×３０分、空冷の焼入れと、５２０℃×１時間の焼戻しを行い、Ｎｏ．２７が１２００℃×３０分、空冷の焼入れと、５６０℃×１時間の焼戻しを２回行った。その後、１ｃｍの立方体に加工したものを用いた。
試験は、ＪＩＳ Ｚ ２３７１号に規定された塩水噴霧試験により耐食性の比較を行った。試験条件は、試験温度３５℃、試験湿度９５％〜９８％、５％塩水を使用し、噴霧圧力１ｋｇｆ／ｃｍ^２、塩水噴霧量１ｍｌ／ｈｒで１０時間試験を継続した。試験後の結果を図１に示す。図１の結果から、本発明の高硬度鋼は、耐食性軸受に用いられるＳＵＳ４４０Ｃ相当鋼（Ｎｏ．２５）に近い耐食性を有することが分かる。 Subsequently, the corrosion resistance of the high hardness steel and the comparative steel of the present invention was investigated.
The investigation of the corrosion resistance was conducted according to the high hardness steel No. 1 of the present invention. 1 and comparative steel no. 25 (SUS440C equivalent steel), no. 26 (SKD11 equivalent steel), No. 27 (SKD51 equivalent steel). Each test piece is No. No. 1 performed 1050 ° C. × 30 minutes, air-cooled quenching and 520 ° C. × 1 hour tempering twice. No. 25 is 1050 ° C. × 30 minutes, air-cooled quenching and −80 ° C. subzero treatment, 180 ° C. × 1 hour tempering, No. 26 performs 1025 ° C. × 30 minutes, air-cooled quenching and 520 ° C. × 1 hour tempering. 27, 1200 ° C. × 30 minutes, air-cooled quenching and 560 ° C. × 1 hour tempering were performed twice. Then, what was processed into a 1 cm cube was used.
In the test, the corrosion resistance was compared by a salt spray test specified in JIS Z 2371. The test conditions were a test temperature of 35 ° C., a test humidity of 95% to 98%, and 5% salt water, and the test was continued for 10 hours at a spray pressure of 1 kgf / cm ² and a salt spray amount of 1 ml / hr. The result after the test is shown in FIG. From the results of FIG. 1, it can be seen that the high hardness steel of the present invention has corrosion resistance close to that of SUS440C equivalent steel (No. 25) used for corrosion resistant bearings.

次に、金属組織観察と疲労強度の測定を行なった。
また、本発明Ｎｏ．１２の焼鈍材から、φ１４ｍｍ×１０ｍｍ（Ｌ）の金属組織観察用試験片を切り出して焼入れと焼戻しを行なった。熱処理条件は、表２中に示した、１０５０℃×３０分、空冷の焼入れを行い、さらに−８０℃のサブゼロ処理の後、５００℃×１時間の焼戻しの処理を行った。
焼戻し後の金属組織観察用試験片を適当な大きさに切り出し、フェノール樹脂に埋込んで素材表面を鏡面に仕上げ、金属組織観察面をピクリン酸にて腐食を行い、光学顕微鏡にて組織観察を行なった。図２に本発明のＮｏ．１２のミクロ写真を示す。
図２より、粗大な１次炭化物が観察されず、高い疲労強度が期待できる金属組織となっていることを確認したため、回転曲げ疲労試験を行い、疲労強度を測定した。
疲労強度測定は、上記と同様、比較鋼Ｎｏ．２５（ＳＵＳ４４０Ｃ相当鋼）、Ｎｏ．２６（ＳＫＤ１１相当鋼）、Ｎｏ．２７（ＳＫＤ５１相当鋼）についても行なった。図３に疲労強度を示す。なお、図中に示した矢印は破断しなかったことを示す。
図２及び図３から、本発明の高硬度鋼は、１次炭化物が少ない均一な金属組織を有し、高い疲労強度を有するものであることを確認した。 Next, the metal structure was observed and the fatigue strength was measured.
In addition, the present invention No. From the 12 annealed materials, φ14 mm × 10 mm (L) specimens for metallographic observation were cut out and quenched and tempered. As for the heat treatment conditions, 1050 ° C. × 30 minutes air-cooled quenching shown in Table 2 was performed, and after -80 ° C. sub-zero treatment, 500 ° C. × 1 hour tempering treatment was performed.
Cut the specimen for metallographic observation after tempering to an appropriate size, embed it in phenolic resin, finish the material surface to a mirror surface, corrode the metallographic observation surface with picric acid, and observe the structure with an optical microscope I did it. FIG. 12 micrographs are shown.
Since it was confirmed from FIG. 2 that a coarse primary carbide was not observed and the metal structure was expected to have high fatigue strength, a rotating bending fatigue test was performed to measure the fatigue strength.
In the same manner as described above, the fatigue strength measurement was performed using the comparative steel No. 25 (SUS440C equivalent steel), no. 26 (SKD11 equivalent steel), No. 27 (steel equivalent to SKD51) was also performed. FIG. 3 shows the fatigue strength. In addition, the arrow shown in the figure shows that it did not fracture.
2 and 3, it was confirmed that the high hardness steel of the present invention has a uniform metal structure with a small amount of primary carbides and has a high fatigue strength.

次に、使用環境を想定して高温硬度を測定した。
高温高度の測定は、本発明の高硬度鋼Ｎｏ．１２と耐熱軸受に用いられる比較鋼Ｎｏ．２４（Ｍ５０相当鋼）について行なった。
焼鈍後の本発明のＮｏ．１２をφ１０ｍｍ×５ｍｍ（Ｌ）の高温硬度測定用試験片に加工し、１０５０℃×３０分、空冷の焼入れを行い、さらに−８０℃のサブゼロ処理を行った。その後、５００℃×１時間の焼戻しを行った。
また、焼鈍後の比較鋼Ｍ５０のＮｏ．２４をφ１０ｍｍ×５ｍｍ（Ｌ）の高温硬度測定用試験片に加工し、１１１５℃×１５分、空冷の焼入れを行い、その後、５４５℃×２時間の焼戻しを３回行った。これら試験片の表面を鏡面に仕上げ、Ａｒ雰囲気内で表４に示した温度で保持し、保持温度でビッカース硬さ測定した。その結果を表４に示す。
比較鋼Ｍ５０のＮｏ．２４は代表的な耐熱軸受鋼であり、本発明の高硬度鋼Ｎｏ．１２は、比較鋼Ｎｏ．２４に匹敵する高温硬度を有していることが分かる。 Next, the high temperature hardness was measured assuming the use environment.
The measurement of the high temperature altitude was performed using the high hardness steel No. 1 of the present invention. No. 12 and comparative steel No. used in heat resistant bearings. 24 (M50 equivalent steel).
No. of the present invention after annealing. 12 was processed into a test piece for measuring high temperature hardness of φ10 mm × 5 mm (L), subjected to air cooling at 1050 ° C. for 30 minutes, and further subjected to subzero treatment at −80 ° C. Thereafter, tempering was performed at 500 ° C. for 1 hour.
Moreover, No. of comparative steel M50 after annealing. 24 was processed into a test piece for measuring high-temperature hardness of φ10 mm × 5 mm (L), subjected to air-cooling quenching at 1115 ° C. × 15 minutes, and then tempered at 545 ° C. × 2 hours three times. The surface of these test pieces was finished to a mirror surface, held at the temperature shown in Table 4 in an Ar atmosphere, and Vickers hardness was measured at the holding temperature. The results are shown in Table 4.
No. of comparative steel M50. Reference numeral 24 is a typical heat-resistant bearing steel. No. 12 is a comparative steel No. 12. It can be seen that it has a high temperature hardness comparable to 24.

以上の結果から、本発明の高硬度鋼は高温焼戻し後の硬さが５８ＨＲＣ以上の高硬度を有し、疲労強度も高く、且つ、ＳＵＳ４４０Ｃ相当の耐食性と、Ｍ５０相当の高温硬度と軟化抵抗を兼備することが分かる。
このことから、高い硬度、高い耐食性、高い疲労強度、高い高温硬度、優れた軟化抵抗が必要とされる用途に最適である。特に軸受鋼として好適である。 From the above results, the high hardness steel of the present invention has a hardness after high temperature tempering of 58 HRC or higher, high fatigue strength, corrosion resistance equivalent to SUS440C, high temperature hardness equivalent to M50 and softening resistance. It turns out that it combines.
This makes it ideal for applications that require high hardness, high corrosion resistance, high fatigue strength, high high temperature hardness, and excellent softening resistance. Particularly suitable as bearing steel.

本発明の軟化抵抗に優れた高硬度鋼は、高温加熱時に高硬度が不可欠な用途に適用できる。同時に、高価な合金の使用量を抑えることができるので、これまで高価な合金を使用していた従来のものに比べて経済的で広い範囲での応用を期待することができる。   The high hardness steel excellent in softening resistance of the present invention can be applied to uses where high hardness is indispensable during high temperature heating. At the same time, since the amount of expensive alloy used can be suppressed, it can be expected to be economical and applicable in a wide range as compared with the conventional one using an expensive alloy.

Claims (2)

質量％で、Ｃ：０．６％を超え〜０．７％、Ｓｉ：０．２５％を超え〜２．０％以下、Ｍｎ：０．２５％を超え〜１．０％以下、Ｃｒ：８．０〜１１．０％未満、Ｍｏ＋１／２Ｗ：１．５％を超え〜５．０％、Ｎ：０．０２％を超えて０．０６％以下を含有し、残部はＦｅ及び不純物でなり、且つ、Ｃ／Ｃｒ：０．０５６を超え〜０．０８５、Ｃ＋Ｎ：０．６３％を超え０．７５％の関係を満たすことを特徴とする軟化抵抗に優れた高硬度鋼。   In mass%, C: more than 0.6% to 0.7%, Si: more than 0.25% to 2.0% or less, Mn: more than 0.25% to 1.0% or less, Cr: 8.0 to less than 11.0%, Mo + 1 / 2W: more than 1.5% to 5.0%, N: more than 0.02% and 0.06% or less, with the balance being Fe and impurities And C / Cr: more than 0.056 to 0.085, and C + N: more than 0.63% and satisfying the relationship of 0.75%, a high hardness steel excellent in softening resistance. 更に質量％で、Ｓ：０．１％以下、Ｃａ：０．１％以下、Ｍｇ：０．０３％以下の一種以上を含有する請求項１に記載の軟化抵抗に優れた高硬度鋼。   The high-hardness steel excellent in softening resistance according to claim 1, further comprising one or more of S: 0.1% or less, Ca: 0.1% or less, and Mg: 0.03% or less in mass%.