JP5645218B2 - Mold steel for plastic molding with excellent toughness, corrosion resistance and specularity - Google Patents
Mold steel for plastic molding with excellent toughness, corrosion resistance and specularity Download PDFInfo
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この発明は、鏡面性及び耐食性に優れ、かつ高靭性を備えたプリハードンタイプの高清浄度のプラスチック成形金型鋼に関するものである。 The present invention relates to a pre-hardened plastic mold metal having a high cleanliness and excellent in specularity and corrosion resistance and having high toughness.
プラスチック成形では、プラスチックが充填される金型の意匠面がプラスチックに転写されることから、金型の表面肌を管理する必要があり、鏡面磨き性は金型鋼として重要な特性の一つである。金型の鏡面磨き性に作用する因子の一つとして金型鋼の硬さが挙げられ、一般的に金型鋼の硬さが高いほど鏡面性は良くなる。これは、素地と、工業的に鋼を製造する上で不可避に生成する硬質な非金属介在物などとの硬さの差異が、小さくなるためなどとして解釈されている。 In plastic molding, the design surface of the mold filled with plastic is transferred to the plastic, so it is necessary to manage the surface of the mold, and mirror polishability is one of the important characteristics of mold steel. . One factor affecting the mirror polishability of the mold is the hardness of the mold steel. Generally, the higher the hardness of the mold steel, the better the mirror finish. This is interpreted as the difference in hardness between the substrate and the hard non-metallic inclusions inevitably generated when industrially manufacturing steel is reduced.
プラスチック製品の種々の鏡面性の要求に応えるために、様々な硬さを有する鋼種が金型鋼として製品化されている。たとえば、研磨材の粒度が＃２０００程度の比較的低い鏡面性で足りる場合には、ショア硬さが３０ＨＳ程度の炭素鋼が、また、＃１００００以上の高い鏡面性が必要な場合には、５０ＨＳを超えるマルテンサイト系ステンレス鋼などが使われている。
また、マルテンサイトもしくはベイナイトの素地に、鋼の製造工程において負荷される熱処理によってＮｉ−Ａｌ系金属間化合物及びＣｕなどを時効析出させ、硬さをロックウェル硬さで４０ＨＲＣ（５４ＨＳ）程度に調整して出荷されるプリハードンタイプの金型鋼がある。
In order to meet the various specularity requirements of plastic products, steel types having various hardness have been commercialized as mold steel. For example, when a relatively low specularity with an abrasive grain size of about # 2000 is sufficient, carbon steel with a Shore hardness of about 30 HS is needed, and when a high specularity of # 10000 or more is required, 50HS is used. More than martensitic stainless steel is used.
Also, Ni-Al intermetallic compounds and Cu are aged on the martensite or bainite substrate by a heat treatment applied in the steel manufacturing process, and the hardness is adjusted to about 40 HRC (54 HS) in terms of Rockwell hardness. And pre-hardened mold steels.
特許文献１、特許文献２、特許文献３及び特許文献４には、前記した析出硬化タイプの金型鋼の耐食性を改善するために、一定量のＣｒが添加された鋼種が提案されている。耐食性は、塩化ビニルにおける塩素ガスなどのように、高腐食性のガスが発生する樹脂を用いる場合に必要である。また、金型内の溶融樹脂の合流部分において発生するウェルドラインを防止するために、近年になって金型の意匠面を急速加熱・冷却するウェルドレス成形法が開発された。この方法において、意匠面近傍に設けた配管内に蒸気や液体を通して加熱や冷却を行う場合、配管内表面に錆が生じれば金型への伝熱効率が低下するので、この点からも耐食性が必要である。
Patent Document 1,
一方で、金型形状の複雑化に伴って型に薄肉部が多く発生するようになり、これらの部位の加工時に割れが生じるケースがある。これを防ぐために金型鋼への靭性要求が高まっており、特許文献５や特許文献６には、前記した析出硬化タイプの金型鋼の靭性を改善した鋼種が提案されている。 On the other hand, as the mold shape becomes more complicated, there are cases in which a lot of thin portions are generated in the mold, and cracks occur when these parts are processed. In order to prevent this, the demand for toughness of mold steel is increasing, and Patent Documents 5 and 6 propose steel types that improve the toughness of the precipitation hardening mold steel described above.
上記した耐食性向上型の析出硬化タイプの金型鋼では、いずれも靭性があまり高くないという課題がある。特許文献３には、ＪＩＳ Ｚ ２２４２で規定されているノッチ深さ２ｍｍのＵノッチ試験片で実施した室温でのシャルピー衝撃試験結果が開示されているが、発明鋼中のほとんどの衝撃値は１０〜３０Ｊ／ｃｍ２であり、前述した型の薄肉部加工時における欠けを防止する目的としては十分な値ではない。また、特許文献２や特許文献４では、靭性に関する試験値が開示されていない。
The above-described precipitation-hardening type die steels with improved corrosion resistance have a problem that the toughness is not so high. Patent Document 3 discloses a Charpy impact test result at room temperature performed with a U-notch test piece having a notch depth of 2 mm as defined in JIS Z 2242. Most impact values in invention steel are 10 It is ˜30 J / cm 2 , and is not a sufficient value for the purpose of preventing chipping during the processing of the thin portion of the mold described above. Moreover, in
一方、上記した靭性向上型の析出硬化タイプの金型鋼では、いずれも耐食性が十分ではないという課題がある。鋼中にＣｒが存在するとＣｒが環境中の酸素と結びついて酸化物を形成し、これが保護作用を示すために、Ｃｒ添加量が増加すると耐食性は向上する。特許文献５や特許文献６では、Ｃｒ量の上限を０．６０質量％と規定しており、これでは十分な耐食性を得ることができない。 On the other hand, any of the above-described toughness-enhanced precipitation hardening type mold steels has a problem that the corrosion resistance is not sufficient. When Cr is present in the steel, Cr is combined with oxygen in the environment to form an oxide, which exhibits a protective action. Therefore, the corrosion resistance is improved when the amount of Cr added is increased. In Patent Document 5 and Patent Document 6, the upper limit of the Cr amount is defined as 0.60% by mass, and sufficient corrosion resistance cannot be obtained with this.
本発明は上記の課題を解決するためになされたもので、靭性と耐食性を同時に満たしつつ、かつ鏡面性に優れた金型鋼を提供することを目的とする。 The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a mold steel excellent in specularity while simultaneously satisfying toughness and corrosion resistance.
すなわち、本発明の靭性と耐食性、及び鏡面性に優れたプラスチック成形用金型鋼のうち、第１の本発明は、質量％で、Ｃ：０．０１〜０．１５％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．３〜２．０％、Ｃｒ：２．０〜６．０％、Ｎｉ：２．０〜５．０％、Ａｌ：０．０５〜０．５％、Ｂ：０．００１〜０．０１％、ＭｏとＷを単独もしくは複合でＭｏ＋１／２Ｗ：０．４〜１．３％含有し、残部がＦｅおよび不可避不純物からなり、かつ不可避不純物中でＣｕ：０．１％以下、Ｓ：０．００２％以下、Ｏ：０．００１５％以下、Ｎ：０．０１％以下に規制したことを特徴とする。 That is, among plastic mold steels excellent in toughness, corrosion resistance, and specularity of the present invention, the first present invention is in mass%, C: 0.01 to 0.15%, Si: 0.5 -2.0%, Mn: 0.3-2.0%, Cr: 2.0-6.0%, Ni: 2.0-5.0%, Al: 0.05-0.5%, B: 0.001 to 0.01%, Mo and W alone or in combination containing Mo + 1 / 2W: 0.4 to 1.3%, the balance being Fe and inevitable impurities, and Cu: It is characterized by being restricted to 0.1% or less , S: 0.002% or less, O: 0.0015% or less, and N: 0.01% or less.
第２の本発明の靭性と耐食性、及び鏡面性に優れたプラスチック成形用金型鋼は、前記第１の本発明において、質量％で、さらにＶを０．３％以下含有することを特徴とする。 The mold steel for plastic molding excellent in toughness, corrosion resistance, and specularity according to the second aspect of the present invention is characterized in that, in the first aspect of the present invention, the mold steel further contains V in an amount of 0.3% or less. .
第３の本発明の靭性と耐食性、及び鏡面性に優れたプラスチック成形用金型鋼は、前記第１または第２の本発明において、硬さが３７ＨＲＣ以上で、かつ室温におけるＵノッチ試験片のシャルピー衝撃値が４０Ｊ／ｃｍ２以上であることを特徴とする。 The plastic mold steel excellent in toughness, corrosion resistance, and specularity of the third aspect of the present invention is the Charpy of the U-notch specimen at room temperature having a hardness of 37 HRC or more in the first or second aspect of the present invention. The impact value is 40 J / cm 2 or more.
すなわち、本発明では、２．０〜６．０％のＣｒを添加することにより、優れた耐食性を具備させている。さらに、従来の耐食性向上型の析出硬化タイプの金型鋼とは異なり、Ｃｕ量を不純物レベルに抑えた上でＡｌを０．０５〜０．５％の範囲に限定することにより、優れた靭性と鏡面性が得られることを見出した。また、Ｖを０．３％以下の範囲で添加しても、優れた靭性と鏡面性が得られることを見出した。 That is, in the present invention, excellent corrosion resistance is achieved by adding 2.0 to 6.0% Cr. Furthermore, unlike conventional precipitation hardening type mold steels with improved corrosion resistance, by limiting the amount of Cu to an impurity level and limiting Al to a range of 0.05 to 0.5%, excellent toughness and It was found that specularity can be obtained. It was also found that excellent toughness and specularity can be obtained even when V is added in a range of 0.3% or less.
次に、本発明で成分範囲を限定した理由を以下に説明する。
Ｃ：０．０１〜０．１５％
Ｃは焼入れ性を向上させる元素であり、また目的の硬さに調整するためにも０．０１％以上の含有が必要である。一方、多量に含有した場合にはＣｒと結合して炭化物を形成し、素地のＣｒ濃度低下に伴って耐食性が低下するとともに、溶接性も劣化することから、その上限を０．１５％とする。上記と同様の理由により、より好ましくは、その含有量を下限で０．０３％、上限で０．１％とする。
Next, the reason for limiting the component range in the present invention will be described below.
C: 0.01 to 0.15%
C is an element that improves the hardenability and needs to be contained in an amount of 0.01% or more in order to adjust to the desired hardness. On the other hand, when it is contained in a large amount, it forms a carbide by combining with Cr, and the corrosion resistance is lowered and the weldability is deteriorated as the Cr concentration of the base is lowered, so the upper limit is made 0.15%. . For the same reason as described above, the content is more preferably 0.03% at the lower limit and 0.1% at the upper limit.
Ｓｉ：０．５〜２．０％
Ｓｉは溶製時に脱酸剤として作用するとともに、被削性を向上させる効果も有する。 そのためには、０．５％以上の含有を要する。一方、含有量が多い場合は、成分偏析が生じて鏡面性を劣化させるとともに、過度の靭性低下を招くので、その含有量の上限を２．０％とする。上記と同様の理由により、より好ましくはその上限を１．５％とする。
Si: 0.5 to 2.0%
Si acts as a deoxidizer during melting and has an effect of improving machinability. For that purpose, 0.5% or more of content is required. On the other hand, when the content is large, component segregation occurs and the specularity is deteriorated and excessive toughness is lowered. Therefore, the upper limit of the content is set to 2.0%. For the same reason as described above, the upper limit is more preferably set to 1.5%.
Ｍｎ：０．３〜２．０％
Ｍｎは焼入れ性向上に効果的な元素であり、添加により良好な機械的性質を得ることができる。その効果を得るためには、０．３％以上の含有が必要である。ただし、過度の含有は靭性の低下を招くので、上限を２．０％とした。上記と同様の理由により、より好ましくは下限を０．３％、上限を１．５％とする。
Mn: 0.3 to 2.0%
Mn is an element effective for improving the hardenability, and good mechanical properties can be obtained by addition. In order to acquire the effect, containing 0.3% or more is required. However, excessive content causes a decrease in toughness, so the upper limit was made 2.0%. For the same reason as described above, the lower limit is more preferably 0.3% and the upper limit is 1.5%.
Ｃｒ：２．０〜６．０％
Ｃｒは耐食性向上に最も有効な元素であり、また焼入れ性の向上という作用も同時にもたらす。含有量を増加させるほど耐食性は向上するが、一方で過度の含有は熱伝導率の低下をもたらすことから、含有量を２．０〜６．０％に調整する必要がある。上記と同様の理由により、より好ましくはその含有量を下限で３．０％、上限で５．０％とするのが良い。
Cr: 2.0-6.0%
Cr is the most effective element for improving the corrosion resistance, and also brings about the effect of improving the hardenability. As the content is increased, the corrosion resistance is improved. On the other hand, excessive content causes a decrease in thermal conductivity, so the content needs to be adjusted to 2.0 to 6.0%. For the same reason as described above, the content is more preferably 3.0% at the lower limit and 5.0% at the upper limit.
Ｎｉ：２．０〜５．０％
ＮｉはＭｎと同様に焼入れ性を向上させ、良好な強度及び靭性を実現するために有効な元素である。また、Ａｌと結合して、Ｎｉ−Ａｌ系金属間化合物を形成することで析出硬化をもたらす。その作用を得るためには、２．０％以上の含有が必要である。一方、多量に添加しても含有量の割には上記効果が顕著には現れないことに加えて、熱伝導率が低下してプラスチックの成形時間の長時間化を招くことから、上限を５．０％とする。上記と同様の理由により、より好ましくは下限を２．５％、上限を４．５％とする。
Ni: 2.0-5.0%
Ni, like Mn, is an effective element for improving hardenability and realizing good strength and toughness. Moreover, it couple | bonds with Al and brings about precipitation hardening by forming a Ni-Al type intermetallic compound. In order to acquire the effect | action, containing 2.0% or more is required. On the other hand, even if it is added in a large amount, the above effect does not appear remarkably with respect to the content. In addition, since the thermal conductivity is lowered and the molding time of the plastic is increased, the upper limit is set to 5%. 0.0%. For the same reason as above, the lower limit is more preferably 2.5% and the upper limit is 4.5%.
Ａｌ：０．０５〜０．５％
ＡｌはＳｉと同様に鋼塊溶製時に脱酸剤として用いられ、また、時効処理によりＮｉと結合して金属間化合物を形成し、析出硬化をもたらすので、これらの作用を得るために０．０５％以上の含有を要する。一方で多量の含有は、金属間化合物の過度の増加をもたらして靭性を劣化させることから、その上限を０．５％とする。上記と同様の理由により、より好ましくは下限を０．１％、上限を０．４％とする。
Al: 0.05-0.5%
Al, like Si, is used as a deoxidizing agent when steel ingots are melted, and is bonded to Ni by aging treatment to form an intermetallic compound, resulting in precipitation hardening. Content of 05% or more is required. On the other hand, since a large amount causes an excessive increase in intermetallic compounds and deteriorates toughness, the upper limit is made 0.5%. For the same reason as described above, the lower limit is more preferably 0.1% and the upper limit is 0.4%.
Ｂ：０．００１〜０．０１％
Ｂは焼入れ性の向上効果を有するに加えて、被削性を付与させる作用もあるため、０．００１％以上の含有が必要である。一方で過度に含有した場合は、熱間加工性を阻害することに加えて溶接時の割れ感受性を高めるために、その上限を０．０１％とする。また、同様の理由で上限を０．００５％にするのが望ましい。
B: 0.001 to 0.01%
In addition to improving the hardenability, B also has an effect of imparting machinability, so it is necessary to contain 0.001% or more. On the other hand, when it contains excessively, in order to increase the cracking sensitivity at the time of welding in addition to inhibiting hot workability, the upper limit is made 0.01%. For the same reason, it is desirable to set the upper limit to 0.005%.
Ｃｕ：０．１％以下
Ｃｕは時効処理によって析出し、素材を硬化させる作用を有するものの、靭性を著しく劣化させる。また、Ｃｕ添加鋼を製造した場合、鋼塊製造用の設備がＣｕで汚染されて、同一設備を使って製造するその後の製品にＣｕが混入する可能性がある。Ｃｕは熱間加工性の著しい低下をもたらすので、Ｃｕ添加鋼を製造した後に、比較的Ｃｕ感受性が低い鋼を釜洗いの目的で製造するなどの制約が生じる。したがって、Ｃｕ含有量は極力低減させる必要があり、上限を０．１％に規制する。
Cu: 0.1% or less Cu is precipitated by aging treatment and has the effect of hardening the material, but significantly deteriorates toughness. Moreover, when manufacturing Cu addition steel, the equipment for steel ingot manufacture is contaminated with Cu, and Cu may mix in the subsequent product manufactured using the same equipment. Since Cu causes a significant decrease in hot workability, there is a restriction that, for example, a steel having a relatively low Cu sensitivity is produced for the purpose of washing in a kettle after the Cu-added steel is produced. Therefore, the Cu content needs to be reduced as much as possible, and the upper limit is restricted to 0.1% .
Ｓ：０．００２％以下、Ｏ：０．００１５％以下、Ｎ：０．０１％以下
ＳはＭｎ、ＯはＳｉやＡｌなど、ＮはＡｌなどと結合して非金属介在物を形成する。これらは、鏡面研磨時に脱落してピンホール欠陥の原因になりうるため、鏡面性を高める上での障害となる。また、腐食環境下での錆の起点ともなりうる。これらの理由から、上記した非金属介在物はできるだけ少なくするのが望ましく、そのためには、Ｓ、Ｏ、Ｎの含有量を極力低減させることが必要である。このため、Ｓ、Ｏ、Ｎの上限は、それぞれ０、００２％、０．００１５％、０．０１％とする。また、望ましくは、上限をさらにＳで０．００１％、Ｏで０．００１％、Ｎで０．００８％に規制する。
S: 0.002% or less, O: 0.0015% or less, N: 0.01% or less S is Mn, O is Si or Al, N is bonded to Al or the like to form non-metallic inclusions. Since these may fall off during mirror polishing and cause pinhole defects, they become an obstacle to improving the mirror surface. It can also be a starting point for rust in corrosive environments. For these reasons, it is desirable to reduce the above-described nonmetallic inclusions as much as possible. To that end, it is necessary to reduce the contents of S, O, and N as much as possible. For this reason, the upper limits of S, O, and N are set to 0, 002%, 0.0015%, and 0.01%, respectively. Desirably, the upper limit is further restricted to 0.001% for S, 0.001% for O, and 0.008% for N.
Ｍｏ＋１／２Ｗ：０．４〜１．５％
ＭｏとＷは、溶体化処理後の冷却時あるいは時効処理時に微細な炭化物を形成し、硬さ向上の役割を果たすが、過剰に添加すると靭性の低下をもたらすことから、上限及び下限を定めることが必要である。ここでＷは、Ｍｏに対して質量％でほぼ倍の量で同様の効果が認められることから、Ｍｏ＋１／２Ｗの計算式で、下限を０．４％、上限を１．５％に規制する。なお、ＭｏはＣｒと同様に耐食性向上効果も有することから、その下限を０．３％にするのが望ましい。
Mo + 1 / 2W: 0.4-1.5%
Mo and W form fine carbides at the time of cooling or aging treatment after solution treatment, and play a role in improving hardness, but if added excessively, the toughness is reduced, so the upper and lower limits are determined. is necessary. Here, since W has the same effect as Mo in mass% with respect to Mo, the same effect is recognized, so the lower limit is regulated to 0.4% and the upper limit is regulated to 1.5% in the formula of Mo + 1 / 2W. . Since Mo has an effect of improving corrosion resistance like Cr, the lower limit is desirably set to 0.3%.
Ｖ：０．３％以下
Ｖは焼戻し軟化抵抗性を高めると共に、硬質の炭化物を微細に形成して耐摩耗性を向上させる効果があるので所望により含有させる。ただし、多すぎると金型加工時の工具の摩耗を増加させるとともに、多量の炭化物の析出による靭性低下を招くので、０．３％以下とする。なお、上記効果を得るために、０．０５％以上含有するのが望ましい。
V: 0.3% or less V increases the resistance to temper softening and has the effect of improving the wear resistance by finely forming hard carbides. However, if the amount is too large, the wear of the tool during die machining is increased and the toughness is reduced due to the precipitation of a large amount of carbides. In addition, in order to acquire the said effect, containing 0.05% or more is desirable.
以上説明したように、本発明によれば、Ｃｒ含有によって優れた耐食性を具備し、さらにＣｕ量を不純物レベルに抑えた上でＡｌを適量含有することにより優れた靭性と鏡面性が得られる効果がある。 As described above, according to the present invention, the effect of obtaining excellent toughness and specularity can be obtained by including an appropriate amount of Al after suppressing the amount of Cu to an impurity level while having excellent corrosion resistance by containing Cr. There is.
本発明の金型鋼は常法により溶製することができるが、Ｓ、Ｏ、Ｎを厳格に規制するという点では、工レクトロスラグ再溶解法を採用するのが望ましい。また、エレクトロスラグ再溶解法では、緻密な組織が得られるために、この点からも鏡面性に対して有利である。該溶解法では、任意の組成のエレクトロスラグ再溶解鋳塊が得られるように成分設計した電極を用いて溶製される。
好適には、エレクトロスラグ再溶解法により溶製された鋳塊は、必要に応じて鍛造などの加工を施し、さらに熱処理を行う。鍛造、熱処理は常法により行うことができるが、本発明としては特に処理条件を限定するものではない。
上記した熱処理によりプリハードンされた金型鋼は、良好な鏡面研磨性や耐食性、そして靭性を示す。
Although the mold steel of the present invention can be melted by a conventional method, it is desirable to adopt a remelting method of engineering lectroslag in terms of strictly regulating S, O, and N. In addition, the electroslag remelting method is advantageous in terms of specularity because a dense structure is obtained. In the melting method, the electrode is melted using an electrode whose components are designed so as to obtain an electroslag remelting ingot having an arbitrary composition.
Preferably, the ingot made by the electroslag remelting method is subjected to processing such as forging as necessary, and further subjected to heat treatment. Forging and heat treatment can be performed by conventional methods, but the present invention does not particularly limit the processing conditions.
The mold steel pre-hardened by the heat treatment described above exhibits good mirror polishability, corrosion resistance, and toughness.
本発明の実施例を以下に説明する。
表１には、供試材として用意した本発明の成分範囲になる発明鋼と、本発明の成分範囲を外れた比較鋼の化学成分（残部Ｆｅおよびその他の不可避不純物）を示す。
上記で説明したように極低Ｓ化を実現するため、比較鋼Ｎｏ．１８以外については、鋳塊製造過程において工レクトロスラグ再溶解法を用いた。鋳塊溶製後、鍛造により所定寸法への加工を行い、焼ならし、溶体化、時効処理を行い、硬さを約４０ＨＲＣに調整した。
Examples of the present invention will be described below.
Table 1 shows the chemical components (remainder Fe and other inevitable impurities) of the inventive steel prepared as the test material, which falls within the component range of the present invention, and the comparative steel outside the component range of the present invention.
As described above, in order to achieve extremely low S, the comparative steel No. Except for 18, the lectro-slag remelting method was used in the ingot manufacturing process. After the ingot was melted, it was processed into a predetermined size by forging, normalized, solutionized, and subjected to aging treatment, and the hardness was adjusted to about 40 HRC.
なお、マルテンサイトやベイナイト組織では、溶体化処理における冷却速度の低下に伴って、一般的に、硬さと靭性のバランスが悪化する場合がある。
本実施例では、最もバランスが悪化する可能性がある金型中心部における硬さと靭性のバランスを把握するために、溶体化処理の冷却速度は、厚さが３５０ｍｍの金型鋼を油冷した場合の厚さ方向の中心を想定した速度とした。
In the martensite or bainite structure, generally, the balance between hardness and toughness may deteriorate as the cooling rate in the solution treatment decreases.
In this example, in order to grasp the balance between hardness and toughness in the mold center where the balance is most likely to deteriorate, the cooling rate of the solution treatment is when the mold steel having a thickness of 350 mm is oil-cooled. The speed was assumed assuming the center in the thickness direction.
このように製作した供試材より試験片を採取し、以下に記す各種特性を調査した。
まず、靭性については、ＪＩＳ Ｚ ２２４２で規定されているノッチ深さ２ｍｍのＵノッチ試験片を用い、シャルピー衝撃試験を室温にて実施して衝撃値を求めた。
次に耐食性については、２００℃の塩素ガス雰囲気中にて８時間曝露した試験片を、光学顕微鏡を用いて５０倍にて１０視野観察し、認められたピットの総数を１視野当たりの平均値として表した。
また、鏡面研磨性については、５０ｍｍ四方の試験片に対して＃８０００までの磨きを行い、目視で確認できるピンホール数を測定した。
Test specimens were collected from the specimens thus produced, and various characteristics described below were investigated.
First, regarding toughness, a Charpy impact test was performed at room temperature using a U-notch test piece having a notch depth of 2 mm defined in JIS Z 2242, and an impact value was obtained.
Next, for corrosion resistance, a specimen exposed for 8 hours in a chlorine gas atmosphere at 200 ° C. was observed 10 times at 50 times using an optical microscope, and the total number of recognized pits was averaged per field. Expressed as:
Moreover, about mirror polishability, it polished to # 8000 with respect to the 50-mm square test piece, and measured the number of pinholes which can be confirmed visually.
これらの結果を表２及び図１に示す。
本発明の供試材は靭性、鏡面性、耐食性の全てにおいて優れた特性を示すのに対し、比較鋼ではこれら特性のいずれかが劣っていることが確認された。
まず、比較鋼Ｎｏ．９、１０、１１、１２、１４、１５、１６、１７では、Ｃ、Ｓｉ、Ｍｎ、Ｃｕ、Ａｌ、Ｖ、Ｍｏ＋ｌ／２Ｗのいずれか、あるいは複数が前記した規制値上限を上回ったため、シャルピー衝撃値が低下した。このうち、比較鋼Ｎｏ．９は前述した従来の耐食性向上型の析出硬化タイプ相当鋼であり、Ａｌ量及びＣｕ量が、ともに規制値上限を上回っている。また、比較鋼Ｎｏ．１４はＣｕ量が規制値を上回っており、Ｃｕ量が比較的高い場合には、Ａｌ量を規制範囲内に収まるように比較的低くしても、靱性が向上しないことがわかる。
次に、比較鋼Ｎｏ．１０及び比較鋼Ｎｏ．１３では、塩素ガス曝露後のピット数が多く、耐食性が不良であった。比較鋼Ｎｏ．１０ではＣが本発明範囲の上限を超えて含有されているため、前述したように、ＣがＣｒと結びついて炭化物を形成した結果、素地中のＣｒ濃度が低下したためと推察される。
比較鋼Ｎｏ．１３は、前述した従来の靭性向上型の析出硬化タイプ相当鋼であるが、Ｃｒが本発明範囲の下限を下回っているため、耐食性が不良であったと考えられる。さらに、比較鋼Ｎｏ．１８においては、Ｓが本発明の規制値上限を上回ったために粗大なＭｎＳ数が増加し、研磨時にこれらが脱落して多くのピンホールが現出したために鏡面性が低下した。加えて、これらは腐食環境下において錆の起点となるため、ビット数が増加して耐食性が劣化した。
These results are shown in Table 2 and FIG.
The test material of the present invention showed excellent properties in all of toughness, specularity and corrosion resistance, whereas it was confirmed that any of these properties was inferior in the comparative steel.
First, comparative steel No. In 9, 10, 11, 12, 14, 15, 16, and 17, Charpy impact is caused because one or more of C, Si, Mn, Cu, Al, V, and Mo + 1 / 2W exceeded the upper limit of the regulation value. The value decreased. Among these, comparative steel No. 9 is a conventional precipitation hardening type equivalent steel of the above-described corrosion resistance improvement type, and both the Al content and the Cu content exceed the upper limit of the regulation value. Comparative steel No. No. 14 shows that the amount of Cu exceeds the regulation value, and when the amount of Cu is relatively high, the toughness is not improved even if the amount of Al is relatively low so as to be within the regulation range.
Next, comparative steel No. 10 and comparative steel no. In No. 13, the number of pits after exposure to chlorine gas was large, and the corrosion resistance was poor. Comparative steel No. In No. 10, since C is contained exceeding the upper limit of the range of the present invention, as described above, it is surmised that as a result of C being combined with Cr to form a carbide, the Cr concentration in the substrate was lowered.
Comparative steel No. No. 13 is the conventional toughness improving type precipitation hardening type equivalent steel described above, but it is considered that the corrosion resistance was poor because Cr was below the lower limit of the range of the present invention. Furthermore, comparative steel No. In No. 18, since S exceeded the upper limit of the regulation value of the present invention, the number of coarse MnS increased, and these dropped off during polishing and many pinholes appeared, resulting in a decrease in specularity. In addition, since these are starting points of rust in a corrosive environment, the number of bits increased and the corrosion resistance deteriorated.
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