JPWO2002077309A1 - Cast steel and casting mold - Google Patents

Abstract

By limiting the steel composition for the specific components and adjusting the Ti, Zr, S and N contents in the steel to an appropriate range, fine sulfides of Ti and Zr are formed, and the internal quality after casting and It is a cast steel having excellent machinability. By casting this cast steel, a mold having excellent machinability such that the internal quality in a cast state is comparable to that of a forged steel product can be manufactured. Accordingly, the present invention can be applied to deep engraving in which a processed surface extends inside a cast material, or to use in which a good surface property is strictly required for finishing, which could not be applied conventionally.

Description

▲２▼ 前記▲１▼に記載の鋳鋼において、Ｆｅの一部に代えて、質量％で、Ｃｒ：０．２〜９．０％、Ｎｉ：０．２〜２．０％、Ｍｏ：０．０５〜２．０％およびＶ：０．０１〜１．５％のうちから選ばれた１種または２種以上を含有させてもよい。
▲３▼ 前記▲１▼または▲２▼に記載の鋳鋼において、Ｆｅの一部に代えて、質量％で、Ｃｕ：０．１〜３．０％およびＯ（酸素）：０．００５〜０．０２０％のうちの１種または２種を含有させてもよい。
▲４▼ 前記▲１▼から▲３▼のいずれかに記載の鋳鋼を鋳造してなる金型である。
発明を実施するための最良の形態
本発明に係る鋳鋼の組織および化学組成について、以下で詳細に説明する。なお、以下の説明において、各元素の含有量としての「％」は、特に断らない限り、「質量％」を意味するものとする。
図１は、ＴｉおよびＺｒのいずれをも含有しない鋳鋼による鋳塊のマクロ組織写真である。後述する実施例における鋼番号１６の鋳鋼により直径１４３ｍｍの鋳塊を製造し、そのマクロ組織写真を撮影したものである。中心部に割れや極端なマクロ偏析は認められないものの、組織は粗大で、表面から半径方向に半径の約３０％の範囲が柱状晶からなっている。
図２は、Ｚｒを含有する鋳鋼による鋳塊のマクロ組織写真である。実施例における鋼番号２の鋳鋼により直径１４３ｍｍの鋳塊を製造し、そのマクロ組織写真を撮影したものである。組織はきわめて微細で、柱状晶も認められず熱間鍛造後のマクロ組織に匹敵する。
図３は、Ｔｉを含有する鋳鋼（実施例の鋼番号１）の鋳塊中心部の介在物のミクロ写真であり、図４は、Ｚｒを含有する鋳鋼（実施例の鋼番号２）の鋳塊中心部の介在物のミクロ写真である。
介在物のＥＰＭＡによる点分析で得た硫化物の組成分析結果から、図３の介在物は、大部分がＴｉの硫化物を含有する介在物であり、図４の介在物は、大部分がＺｒの硫化物を含有する介在物であることが判明した。
この現象は鋳鋼中の元素成分のうち、Ｔｉ、Ｚｒ、ＳおよびＮによって支配される。すなわち、これらの元素含有量により前記式（１）で与えられる有効Ｔｉ等量が前記式（２）を満足すると、凝固時に、まず少量のＴｉやＺｒはＮと反応して窒化物を形成し、残りの大部分のＴｉやＺｒは硫化物を生成する。この硫化物が、凝固形態を変化させる最終凝固部で、Ｓ、Ｐ等の大きな濃縮をともなうことなく一挙に凝固を促進させるため、偏析が改善されるとともに、凝固収縮にともなうポロシティー等の鋳造欠陥が改善される。
前記式（１）は硫化物として反応可能なＴｉおよびＺｒの量であり、前記式（２）で与えられる関係を満足すると、化学量論的にはＳは全てＴｉやＺｒの硫化物になる。実際には凝固した後に、少量のＭｎＳが観察されることはあるが、問題にならない程度の量である。式（２）の左辺の値が１．５未満になるとＭｎＳが多量に観察され、凝固組織の改善やこれに関連した欠陥の改善が認められなくなる。
なお、Ｔｉ含有量やＺｒ含有量がＳ含有量に対して多過ぎると、ＴｉやＺｒの炭化物の生成量が多くなり、靭性や被削性が劣化することから、前記式（２）の左辺の値は１０以下であることが好ましい。
以下に、本発明で規定する各元素の含有量の範囲を前記のとおり定めた理由を説明する。
Ｃ：
Ｃは、安価に強度を高めるために有効な元素である。プラスチック金型等では被削性が重視されるため、Ｃ含有量はできるだけ低くし、鋼材の硬度を低くして工具摩耗を低減することが必要である。しかし、０．０２％未満になると硬度低下が著しく、切削加工中にむしれが生じ、かえって被削性が劣化する。また、含有量が０．６０％を超えると硬度が高くなりすぎ、被削性や靭性が劣化する。そこで、Ｃ含有量の範囲を０．０２〜０．４５％とした。好ましい範囲は、０．０８〜０．４５％である。
Ｓｉ：
Ｓｉは、脱酸および被削性改善に有効な元素である。含有量が０．１％未満ではこの効果が十分に得られない。さらに、鋳造時の湯流れが阻害され鋳型内での溶鋼の充満性が不十分となる。一方、含有量が２．５％を超えて高すぎると被削性改善効果や靭性が低下する。そこで、Ｓｉ含有量の範囲を０．１〜２．５％とした。好ましい範囲は、０．５〜２．０％である。
Ｍｎ：
Ｍｎは、脱酸剤として有効な元素であるが、多量に含有すると被削性を劣化させる。含有量が０．１％未満では脱酸効果が得られない。一方、含有量が２．５％を超えると被削性を劣化させる。そこで、Ｍｎ含有量の範囲を０．１〜２．５％とした。好ましい範囲は、０．１〜１．５％である。
Ｐ：
Ｐは、凝固時の偏析を助長し、靭性を劣化させるが、一方では、含有されることにより被削性を改善する作用を有する。Ｐは鋼の使用目的により、不純物として含有されていても良く、また、積極的に含有させても良い。例えば、靭性を重視する場合には、Ｐ含有量は０．０２％以下とすることが好ましいし、被削性を重視する場合には０．０５％以上を含有させることが好ましい。しかし、含有量が０．１０％を超えると靭性の劣化が著しくなるため、Ｐ含有量は０．１０％以下とした。
Ｓ：
Ｓは、被削性を改善するために含有させる元素である。含有量が０．０２％未満では、ＴｉやＺｒの硫化物の生成が不十分であり、十分な効果が得られない。一方、含有量が０．６０％を超えると、ＦｅＳやＭｎＳが多量に生成し、凝固時の偏析やボイド、あるいは割れを助長する。そこで、Ｓ含有量の範囲は０．０２〜０．６０％とした。好ましい範囲は０．０５〜０．４０％であり、より好ましい範囲は０．０５〜０．２０％である。
ＴｉおよびＺｒ：
ＴｉおよびＺｒは、硫化物を生成させる効果を有する元素であり、本発明においては、これらの元素のうちのいずれか１種または２種を含有させる。いずれもその含有量が０．０５％未満では十分な硫化物を生成することが出来ないため、ＭｎＳが多量に生成し、凝固組織の改善やこれに関連した欠陥の改善が認められなくなる。一方、Ｔｉについては１．０％を超えると、また、Ｚｒについては０．５０％を超えると、硫化物以外に炭化物の生成量が増加し、被削性や靭性を劣化させる。
そこで、Ｔｉ含有量の範囲は０．０５〜０．２５％とした。好ましい範囲は０．１０〜０．２５％である。
また、Ｚｒ含有量の範囲は０．０５〜０．５０％とした。好ましい範囲は０．０５〜０．２０％であり、より好ましい範囲は０．１０〜０．２０％である。
なお、これらの元素は、前記式（１）で与えられる有効Ｔｉ等量が前記式（２）を満足するように含有させる必要がある。
Ｎ：
Ｎは、ＴｉやＺｒの硫化物を生成する前に窒化物を生成するため、ＴｉやＺｒの硫化物の生成を抑制する元素である。さらに生成した窒化物は硬質で、工具刃先を痛め、工具寿命を低下させる。このため、上限は０．０２０％とした。含有量は低ければ低いほどよく、０．００２％未満では、Ｎによる悪影響の原因となる窒化物の生成は回避でき、好ましい。
Ａｌ：
Ａｌは、強力な脱酸元素であり、靭性を改善する効果も有する。０．００１％未満ではその効果が得られず、一方、０．０３％を超えて含有させると脱酸効果が飽和するとともに、被削性が低下する。そこで、Ａｌ含有量の範囲を０．００１〜０．０３％とした。
なお、本発明において、Ａｌ含有量とは、酸可溶Ａｌ含有量（ｓｏｌ．Ａｌ含有量）をいう。
Ｃｒ：
Ｃｒは、強度、靭性および耐熱性の向上に効果を有する元素である。含有してもしなくてもよいが、強度、靭性および耐熱性の向上を要求される場合には、０．２％以上を含有させることにより、これらの効果が得られる。特に、熱間鍛造用金型ではＣｒを３．０％以上含有させることが好ましい。一方、９．０％を超えて含有させると、靭性が劣化する。そこで、含有させる場合の含有量の範囲を０．２〜９．０％とした。含有量の好ましい範囲は０．５〜５．０％である。
Ｍｏ：
Ｍｏは、強度、靭性および高温強度の改善に効果を有する元素である。含有してもしなくてもよいが、強度、靭性および高温強度の改善を要求される場合には、０．０５％以上を含有させることにより、これらの効果が得られる。一方、２．０％を超えて含有させても高温強度の改善効果は飽和し、また、靭性が劣化する。そこで、含有させる場合の含有量の範囲を０．０５〜２．０％とした。含有量の好ましい範囲は０．０５〜１．０％である。
Ｎｉ：
Ｎｉは、強度および靭性の改善に効果を有する元素である。含有してもしなくてもよいが、靭性の改善を要求される場合は、０．２％以上を含有させることにより、その効果が得られる。一方、２．０％を超えて含有させると、靭性の改善効果は得られるものの、Ｎｉ元素は高価なことから、経済性を損なうこととなる。そこで、含有させる場合の含有量の範囲を０．２〜２．０％とした。含有量の好ましい範囲は０．２〜１．５％である。
Ｖ：
Ｖは、強度改善に効果を有し、かつ、含有による靭性劣化が少ない元素である。含有してもしなくてもよいが、強度改善を要求される場合は、０．０１％以上を含有させることにより、その効果が得られる。一方、１．５％を超えて含有させると、靭性が劣化する。そこで、含有させる場合の含有量の範囲を０．０１〜１．５％とした。含有量の好ましい範囲は０．０５〜０．５％である。
Ｃｕ：
Ｃｕは、被削性、強度および靭性の改善に効果を有する元素である。含有してもしなくてもよいが、被削性、強度および靭性の改善を要求される場合は、０．１％以上を含有させることにより、それらの効果が得られる。一方、３．０％を超えて含有させると、靭性が劣化する。そこで、含有させる場合の含有量の範囲を０．１〜３．０％とした。含有量の好ましい範囲は０．２〜１．０％である。
Ｏ：
Ｏ（酸素）は、適度に含有させることにより、鋼中にて低融点の酸化物を形成し、被削性を改善する効果を有する。含有してもしなくてもよいが、酸化物の形成による被削性の改善を要求される場合は、０．００５％以上を含有させることにより、その効果が得られる。一方、０．０２０％を超えて含有させると、鋼中に巨大介在物が形成され、金型表面の仕上げ精度が低下し、靭性も劣化する。そこで、含有させる場合の含有量の範囲を０．００５〜０．０２０％とした。含有量の好ましい範囲は０．０１０〜０．０２０％である。
ここで、Ｏ（酸素）とは、鋼中の全酸素をいう。
熱処理：
本発明にて得られた鋳塊は、その後の熱処理によってさらにミクロ組織や硬度、靭性を改善することができる。また、被削性を改善するために、必要な硬度にまで軟化することもできる。この熱処理は、８５０〜１０５０℃に加熱後、焼準または焼入れを行い、その後、７００℃以下での焼戻しによる軟化処理を行うか、または歪み取り焼鈍を行うか、または焼戻しおよび歪み取り焼鈍を行うことにより、実施することができる。
（実施例）
次頁の表１に示す化学組成を有する試験鋼を溶製し、直径１４３ｍｍの円柱状の鋳塊を作製した。
鋳造のままでマクロ試験を実施した後、さらに、以下の試験を実施した。
〔硬度〕マクロ試験に用いた試験板を用い、ＪＩＳ Ｚ ２２４３に規定されたブリネル硬さ試験方法に準じて、前記鋳塊中心部位置、半径の１／２位置および表面近傍位置において、硬度を測定し、これらの平均値にて評価した。
〔靭性〕ＪＩＳ Ｚ ２２４２に規定されたシャルピー衝撃試験方法に準じて、鋳塊の円柱中心軸に平行方向で且つ半径の１／２の位置から、Ｕノッチ試験片（ＪＩＳ Ｚ ２２０２に規定された幅が１０ｍｍのフルサイズ試験片）を採取し、室温にて衝撃値を測定した。
〔フライス加工性〕硬さ試験および衝撃試験用の各試験片を採取後、鋳塊を直径を含む円筒軸方向に切断した後、フライス加工を実施し、工具の摩耗量でフライス加工性を評価した。

前頁の表２に、各試験鋼についての前記式（１）および式（２）による計算値、前記の試験結果およびマクロ組織の調査結果を示した。
表１に示された試験鋼のうちで、鋼番号１〜１５の鋼は本発明鋼であり、いずれも鋼成分は本発明で規定する化学組成の範囲を満足している。また、表２に示されるとおり、鋼の化学組成に基づき計算される式（１）の値は、式（２）により与えられる関係を満足している。
試験鋼のマクロ組織は、以下のように評価した。
図１に示される鋼番号１６の鋼（比較鋼）のマクロ組織を粗大で望ましくない組織として×印で表し、図２に示される鋼番号２の鋼（本発明鋼）のマクロ組織を微細で良好な組織として○印で表し、それと同等またはそれ以上の組織も○印で表した。また、×印と○印との中間の組織を△で表した。
本発明鋼のマクロ組織は、概ね評価○となっている。一部の試験鋼（鋼番号７、８、１４および１５）では評価△のものもあったが、これはＳ含有量が比較的少なく、凝固組織の改善に寄与するＴｉやＺｒの硫化物の生成量が少なかったためである。
鋼番号１４および１５の鋼は、鋳造のままでは硬度が高すぎるため、焼準後、焼戻し処理を行い、軟化させることにより、鋳造後の切削加工を容易にしている。特に鋼番号１５の鋼は、熱間鍛造用金型に使用されるため、切削加工後、改めて焼入れおよび焼戻しの熱処理が施される。
衝撃値は、Ｓ含有量が高いレベルにあるにも拘わらず、鋼番号１５の鋼を除き、いずれの本発明鋼の場合も良好である。
また、いずれの本発明鋼についても、フライス加工における工具摩耗量は少なく、良好な被削性を有している。
鋼番号７の鋼はＣｒおよびＣｕを、また、鋼番号１３の鋼はＣｒおよびＶを含有し、硬度は上昇しているが、靭性が改善されている。
鋼番号９および１０の鋼はＮｉを含有し、靭性改善の効果が認められる。鋼番号３の鋼はＣｒおよびＭｏを含有し、強度および靭性の改善効果が認められる。
鋼番号１１および１２の鋼はＶを含有し、強度および靭性が大きく改善されている。鋼番号５の鋼はＣｕを含有し、強度が高められているが、被削性も改善されている。
鋼番号６の鋼はＣｕおよびＯ（酸素）を含有し、被削性が改善されている。鋼番号７および８の鋼はＯ（酸素）を含有し、被削性が改善されている。
これらに対して、鋼番号１６〜２２の鋼は比較鋼であり、鋼成分または式（２）により与えられる関係の少なくとも１つが、本発明で規定される範囲から外れている。
このうちで、鋼番号１６、１７、２０、２１および２２の鋼は、式（２）により与えられる関係を満足せず、マクロ組織の評価は×または××である。また、鋼番号１８および１９の鋼は、式（２）により与えられる関係を満足するが、それぞれ、ＰおよびＴｉ含有量が過多となっていることから、ＰやＴｉの偏析が激しく、鋼塊中心部に割れを伴う顕著な偏析が観察された。
鋼番号１６の鋼は、柱状晶が著しく成長したマクロ組織となり、その結果、鋼塊中心部にＰ、Ｓ等の顕著な偏析組織が認められた。さらに、その偏析組織の存在する位置でポロシティーおよび割れも観察された。また、鋼番号２０の鋼も同様である。
鋼番号２２の鋼においては、さらに、低い衝撃値にみられるとおり、靭性に劣っていた。
鋼番号１７の鋼はＭｎおよびＡｌ含有量が高過ぎることから、また、鋼番号１８の鋼はＮ含有量が高過ぎることから、フライス加工性に劣っていた。鋼番号１９の鋼はＴｉ含有量が高過ぎるために大量のＴｉＣが生成し、しかも硬度が高いことから、また、鋼番号２１の鋼はＣ含有量が高過ぎ、しかもＳ含有量が不足していることから、フライス加工性に劣っていた。
以上の結果から明らかなように、鋼中のＴｉ、Ｚｒ、ＳおよびＮ含有量を適切な範囲に調整することにより、ＴｉやＺｒの微細な硫化物を形成させ、鋳造後の内部品質および被削性に優れた鋳鋼ならびに、鋳造状態での内部品質が鍛鋼製品に匹敵するような優れた被削性を有する金型を製造することが可能である。
産業上の利用の可能性
本発明の鋳鋼および鋳造金型によれば、特定成分について鋼組成を制限するとともに、鋼中のＴｉ、Ｚｒ、ＳおよびＮ含有量を適切な範囲に調整することにより、ＴｉやＺｒの微細な硫化物を形成させ、鋳造後の内部品質および被削性に優れた特性が確保でき、この鋳鋼を鋳造することによって、鋳造状態での内部品質が鍛鋼製品に匹敵するような優れた被削性を有する金型を製造することができる。これにより、従来では適用できなかった、鋳造材で加工表面が素形材内部におよぶ深彫用や、仕上げ用で表面性状の良さが厳しく要求される用途にも、広く適用することができる。
【図面の簡単な説明】
図１は、ＴｉおよびＺｒのいずれも含有しない鋳鋼についての鋳塊のマクロ組織写真を示す図である。
図２は、Ｚｒを含有する鋳鋼についての鋳塊のマクロ組織写真を示す図である。
図３は、Ｔｉを含有する鋳鋼についての鋳塊中心部の介在物のミクロ写真を示す図である。
図４は、Ｚｒを含有する鋳鋼についての鋳塊中心部の介在物のミクロ写真を示す図である。TECHNICAL FIELD The present invention relates to cast steel excellent in internal quality and machinability after casting, and a cast steel product obtained by casting the cast steel, especially a mold, and in particular, suppresses various defects in a cast state, The present invention relates to a cast steel having an internal quality comparable to a forged steel product and excellent machinability, and a mold using the same.
BACKGROUND ART A mold is an indispensable processing member (tool) for molding various materials including metal and plastic. For this, various steel materials are generally used.
For example, a carbon steel containing 0.10 to 0.50% by mass of C or a steel material added with Cr, Mo, Cu, Ni, V, or the like is used for a plastic molding die. The hot forging die mainly includes JIS SKD61, Cr: 1 to 9% by mass, V: 0.2 to 1.2% by mass, Mo: 0.2 to 2.0% by mass. A steel material as a component is used. Further, a steel material containing, for example, about 1% C-3.5% Cr by mass% is used for the cold forming mold.
In many cases, these molds are mainly hot-worked from steel materials to form shaped materials, and cut as needed, and then cut into predetermined mold shapes by users in many cases. However, in the method of cutting after hot working, it takes many days to obtain a shaped material and finish it by cutting, and there is a limit in terms of shortening the delivery time. Furthermore, due to the hot working finish, it is difficult to machine into a shape close to the required shape, so-called near net shape. It requires time and large tool costs.
On the other hand, cast materials can be cast into shapes close to the final finished shape, including various cooling holes, but have the following problems, and their application has been greatly restricted.
For example, Japanese Patent Application Laid-Open No. H11-279673 proposes using a Zn casting alloy having a low melting point and easy handling. That is, in weight%, Mg: 1.5 to 2.5%, Al: 3 to 5%, and Cu: 2 to 4%, the balance being substantially composed of Zn, and a solidification start temperature of 390 ° C or lower. A zinc alloy having a Vickers hardness of 150 or more and in which generation of porosity is suppressed.
Japanese Patent Application Laid-Open No. H10-147840 proposes manufacturing a mold from cast steel. That is, in weight%, C: 0.5 to 1.0%, Si: 0.25 to 1.5%, Mn: 1.0 to 1.85%, Cr: 0.6 to 5.0%, One or two types of Mo and W are contained in (Mo + W / 2) in an amount of 0.06 to 0.80%, S: 0.10 to 0.40%, and the balance substantially consists of Fe; This is the manufacture of a mold using free-cutting cast steel in which system inclusions are dispersed in a matrix in a granular manner.
However, since the former is an alloy mainly composed of Zn, it has excellent machinability, but has a limit in hardness required to improve the life as a mold. Further, Zn alloys are significantly more expensive than steel materials.
In the latter, since the same steel composition as the conventional one is used, sufficient hardness can be obtained. However, when S is positively added to improve machinability and the amount of MnS is increased, the cast steel is used. It promotes casting defects such as segregation of S and S or cracks caused by these. Furthermore, in cast steel, the porosity is not pressed after hot working is not applied thereafter, resulting in surface defects after cutting.
Therefore, the conventional technique cannot be applied to the use of a cast material for deep engraving in which the processed surface extends inside the cast material, or the use for finishing, which requires strict surface properties.
DISCLOSURE OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems in the prior art, and an object thereof is to provide a cast steel having excellent internal quality and machinability after casting, and a cast steel obtained by casting the cast steel. It is an object of the present invention to provide a mold that suppresses various defects in a cast state and has internal quality comparable to forged steel products and excellent machinability.
The present inventors have developed a cast steel and a mold that can simultaneously satisfy the following goals in order to solve the above-mentioned problems.
1) A mold material is manufactured by casting, the amount of cutting for finishing the mold is greatly reduced, and the machining time and machining cost are greatly reduced.
2) Develop cast steel from which a mold with sufficient hardness can be obtained by casting.
3) In order to simultaneously solve the problems of machinability and casting defects that are problematic in cast steel dies, a high S content component system is used to achieve the same internal quality as forged steel as cast.
The internal quality here refers not only to the internal quality of the casting, but also to the quality of the surface portion that becomes a new surface by cutting.
In addition, if the above problems can be solved, the above-mentioned cast steel can of course be applied to cast steel for general mechanical parts and the like.
As a result of the above development, the following findings were obtained.
a) When a large amount of S is contained in steel for the purpose of improving machinability and a necessary amount of Ti or Zr is added, a sulfide containing Ti or Zr is formed, and MnS is completely eliminated or formed. Is significantly suppressed, and as a result, sulfides containing Ti and Zr are finely dispersed in the cast steel.
b) As a result of the above a), not only the machinability is improved, but also a solidification morphology different from the conventional one is obtained, and segregation of S is greatly improved as cast even if the S content is high. At the same time, the columnar crystals decrease or disappear, and not only defects such as porosity and segregation but also irregularities on the cutting surface caused by the coarse columnar crystals are improved, and an epoch-making internal quality comparable to forged steel is obtained.
The gist of the present invention completed on the basis of the above findings is as follows.
{Circle around (1)} By mass%, C: 0.02 to 0.45%, Si: 0.1 to 2.5%, Mn: 0.1 to 2.5%, P: 0.10% or less, S: 0.02 to 0.60%, N: 0.020% or less, Al: 0.001 to 0.03%, Ti: 0.05 to 0.25%, and Zr: 0.05 to 0 .50% of one or two kinds, the balance being Fe and impurities, and the effective Ti equivalent (Ti *) given by the following equation (1) satisfies the relationship given by the following equation (2); A cast steel excellent in internal quality and machinability after casting, characterized in that the steel contains a sulfide containing at least one of Ti and Zr.

(2) In the cast steel according to (1), instead of part of Fe, Cr: 0.2 to 9.0%, Ni: 0.2 to 2.0%, Mo: 0 0.05 to 2.0% and V: 0.01 to 1.5%.
(3) In the cast steel according to (1) or (2), instead of a part of Fe, Cu: 0.1 to 3.0% and O (oxygen): 0.005 to 0 by mass%. One or two of 0.020% may be contained.
(4) A mold formed by casting the cast steel according to any of (1) to (3).
BEST MODE FOR CARRYING OUT THE INVENTION The structure and chemical composition of a cast steel according to the present invention will be described in detail below. In the following description, “%” as the content of each element means “% by mass” unless otherwise specified.
FIG. 1 is a photograph of a macro structure of an ingot made of cast steel containing neither Ti nor Zr. The ingot having a diameter of 143 mm was manufactured from cast steel of steel No. 16 in an example described later, and a macrostructure photograph thereof was taken. Although no cracks or extreme macro-segregation are observed at the center, the structure is coarse and columnar crystals are formed in a range of about 30% of the radius in the radial direction from the surface.
FIG. 2 is a photograph of a macro structure of an ingot made of cast steel containing Zr. The ingot having a diameter of 143 mm was manufactured from the cast steel of steel No. 2 in the example, and a macrostructure photograph thereof was taken. The structure is extremely fine, and no columnar crystals are observed, which is comparable to the macrostructure after hot forging.
FIG. 3 is a microphotograph of inclusions at the center of the ingot of a cast steel containing Ti (Steel No. 1 of Example), and FIG. 4 is a cast of a cast steel containing Zr (Steel No. 2 of Example). It is a microphotograph of the inclusion of a lump center part.
From the results of sulfide composition analysis obtained by point analysis of inclusions by EPMA, the inclusions in FIG. 3 are mostly inclusions containing Ti sulfides, and the inclusions in FIG. The inclusion was found to be an inclusion containing a sulfide of Zr.
This phenomenon is governed by Ti, Zr, S and N among the elemental components in the cast steel. That is, when the effective Ti equivalent given by the above formula (1) satisfies the above formula (2) by the content of these elements, at the time of solidification, a small amount of Ti or Zr first reacts with N to form nitride. Most of the remaining Ti and Zr form sulfides. This sulfide promotes solidification at once in the final solidification part that changes the solidification morphology without significant concentration of S, P, etc., so segregation is improved and casting such as porosity accompanying solidification shrinkage is improved. Defects are improved.
The above formula (1) is the amount of Ti and Zr that can react as sulfides, and if the relationship given by the above formula (2) is satisfied, all of S becomes sulfides of Ti or Zr stoichiometrically. . Actually, a small amount of MnS may be observed after solidification, but it is an amount that does not cause a problem. When the value on the left side of the equation (2) is less than 1.5, a large amount of MnS is observed, and no improvement in the solidification structure and no improvement in defects related thereto are recognized.
If the Ti content or the Zr content is too large with respect to the S content, the amount of carbides of Ti or Zr increases, and the toughness and machinability deteriorate. Therefore, the left side of the above equation (2) Is preferably 10 or less.
Hereinafter, the reason why the range of the content of each element specified in the present invention is determined as described above will be described.
C:
C is an element effective for increasing the strength at low cost. In a plastic mold or the like, since machinability is emphasized, it is necessary to reduce the C content as much as possible and reduce the hardness of the steel material to reduce tool wear. However, when the content is less than 0.02%, the hardness is remarkably reduced, and peeling occurs during the cutting, and the machinability is rather deteriorated. On the other hand, if the content exceeds 0.60%, the hardness becomes too high, and the machinability and toughness deteriorate. Therefore, the range of the C content is set to 0.02 to 0.45%. A preferred range is from 0.08 to 0.45%.
Si:
Si is an element effective for deoxidation and improvement of machinability. If the content is less than 0.1%, this effect cannot be sufficiently obtained. Furthermore, the flow of molten metal during casting is impeded, and the filling of molten steel in the mold becomes insufficient. On the other hand, if the content exceeds 2.5% and is too high, the effect of improving machinability and the toughness decrease. Therefore, the range of the Si content is set to 0.1 to 2.5%. A preferred range is 0.5-2.0%.
Mn:
Mn is an effective element as a deoxidizing agent, but when contained in a large amount, it deteriorates machinability. If the content is less than 0.1%, a deoxidizing effect cannot be obtained. On the other hand, if the content exceeds 2.5%, the machinability deteriorates. Therefore, the range of the Mn content is set to 0.1 to 2.5%. A preferred range is 0.1-1.5%.
P:
P promotes segregation during solidification and degrades toughness, but on the other hand, has the effect of improving machinability by being contained. P may be contained as an impurity or may be positively contained depending on the purpose of use of the steel. For example, when importance is placed on toughness, the P content is preferably set to 0.02% or less, and when importance is attached to machinability, 0.05% or more is preferably contained. However, when the content exceeds 0.10%, the toughness deteriorates remarkably, so the P content is set to 0.10% or less.
S:
S is an element contained to improve machinability. If the content is less than 0.02%, the formation of sulfides of Ti and Zr is insufficient, and a sufficient effect cannot be obtained. On the other hand, if the content exceeds 0.60%, a large amount of FeS or MnS is generated, which promotes segregation, voids, or cracks during solidification. Therefore, the range of the S content is set to 0.02 to 0.60%. A preferred range is 0.05 to 0.40%, and a more preferred range is 0.05 to 0.20%.
Ti and Zr:
Ti and Zr are elements having an effect of generating a sulfide, and in the present invention, any one or two of these elements are contained. In any case, if the content is less than 0.05%, a sufficient sulfide cannot be generated, so that a large amount of MnS is generated, and no improvement in the solidification structure and no improvement in defects related thereto are recognized. On the other hand, if the content of Ti exceeds 1.0% and the content of Zr exceeds 0.50%, the amount of carbides other than sulfides increases, thereby deteriorating machinability and toughness.
Therefore, the range of the Ti content is set to 0.05 to 0.25%. The preferred range is 0.10 to 0.25%.
The range of the Zr content was set to 0.05 to 0.50%. A preferred range is 0.05 to 0.20%, and a more preferred range is 0.10 to 0.20%.
These elements must be contained so that the effective Ti equivalent given by the formula (1) satisfies the formula (2).
N:
N is an element that suppresses the generation of sulfides of Ti and Zr because it generates nitrides before generating sulfides of Ti and Zr. Furthermore, the formed nitride is hard and damages the cutting edge of the tool and shortens the tool life. Therefore, the upper limit is set to 0.020%. The lower the content, the better. If the content is less than 0.002%, the generation of a nitride which causes an adverse effect of N can be avoided, which is preferable.
Al:
Al is a strong deoxidizing element and has an effect of improving toughness. When the content is less than 0.001%, the effect cannot be obtained. On the other hand, when the content exceeds 0.03%, the deoxidizing effect is saturated and the machinability is reduced. Therefore, the range of the Al content is set to 0.001 to 0.03%.
In the present invention, the Al content refers to an acid-soluble Al content (sol. Al content).
Cr:
Cr is an element that has an effect on improving strength, toughness, and heat resistance. It may or may not be contained, but when improvement in strength, toughness and heat resistance is required, these effects can be obtained by containing 0.2% or more. In particular, it is preferable that the hot forging die contain Cr at 3.0% or more. On the other hand, if the content exceeds 9.0%, the toughness deteriorates. Therefore, the range of the content in the case of being contained is set to 0.2 to 9.0%. A preferable range of the content is 0.5 to 5.0%.
Mo:
Mo is an element having an effect on improving strength, toughness and high-temperature strength. It may or may not be contained, but when improvement in strength, toughness and high-temperature strength is required, these effects can be obtained by containing 0.05% or more. On the other hand, even if the content exceeds 2.0%, the effect of improving the high-temperature strength is saturated, and the toughness is deteriorated. Therefore, the range of the content in the case of being contained is set to 0.05 to 2.0%. A preferable range of the content is 0.05 to 1.0%.
Ni:
Ni is an element having an effect on improving strength and toughness. It may or may not be contained, but when improvement in toughness is required, the effect can be obtained by containing 0.2% or more. On the other hand, if the content exceeds 2.0%, the effect of improving toughness can be obtained, but the cost of Ni element is high, which impairs economic efficiency. Therefore, the range of the content when it is contained is set to 0.2 to 2.0%. A preferable range of the content is 0.2 to 1.5%.
V:
V is an element that has the effect of improving the strength and has little toughness deterioration due to its inclusion. It may or may not be contained, but when strength improvement is required, the effect can be obtained by containing 0.01% or more. On the other hand, if the content exceeds 1.5%, the toughness deteriorates. Therefore, the range of the content in the case where it is contained is set to 0.01 to 1.5%. A preferable range of the content is 0.05 to 0.5%.
Cu:
Cu is an element having an effect on improving machinability, strength and toughness. It may or may not be contained, but when improvement in machinability, strength and toughness is required, these effects can be obtained by containing 0.1% or more. On the other hand, if the content exceeds 3.0%, the toughness deteriorates. Therefore, the range of the content in the case of being contained is set to 0.1 to 3.0%. A preferable range of the content is 0.2 to 1.0%.
O:
O (oxygen) has an effect of forming an oxide having a low melting point in steel and improving the machinability by being appropriately contained. It may or may not be contained, but when improvement in machinability by formation of an oxide is required, the effect can be obtained by containing 0.005% or more. On the other hand, if the content exceeds 0.020%, giant inclusions are formed in the steel, the finishing accuracy of the mold surface is reduced, and the toughness is also deteriorated. Therefore, the content range in the case of being contained is set to 0.005 to 0.020%. A preferable range of the content is 0.010 to 0.020%.
Here, O (oxygen) refers to the total oxygen in the steel.
Heat treatment:
The ingot obtained in the present invention can be further improved in microstructure, hardness and toughness by a subsequent heat treatment. Further, in order to improve the machinability, it can be softened to a required hardness. In this heat treatment, after heating to 850 to 1050 ° C., normalizing or quenching is performed, and thereafter, softening treatment by tempering at 700 ° C. or lower, strain removing annealing, or tempering and strain removing annealing is performed. This can be implemented.
(Example)
Test steel having the chemical composition shown in Table 1 on the next page was melted to produce a cylindrical ingot having a diameter of 143 mm.
After the macro test was carried out as cast, the following test was further carried out.
[Hardness] Using the test plate used in the macro test, the hardness was measured at the center position of the ingot, a half position of the radius, and a position near the surface in accordance with the Brinell hardness test method specified in JIS Z 2243. It measured and evaluated by these average values.
[Toughness] In accordance with the Charpy impact test method specified in JIS Z 2242, a U-notch test piece (specified in JIS Z 2202) was placed in a direction parallel to the central axis of the ingot and at a half radius. A full-size test piece having a width of 10 mm) was collected, and the impact value was measured at room temperature.
[Milling] After collecting each test piece for hardness test and impact test, cut the ingot in the cylindrical axis direction including the diameter, then carry out milling and evaluate the milling workability by the wear amount of the tool did.

Table 2 on the previous page shows the values calculated by the above formulas (1) and (2), the test results, and the macrostructure investigation results for each test steel.
Among the test steels shown in Table 1, the steels of steel numbers 1 to 15 are the steels of the present invention, and all of the steel components satisfy the range of the chemical composition specified in the present invention. Further, as shown in Table 2, the value of equation (1) calculated based on the chemical composition of steel satisfies the relationship given by equation (2).
The macrostructure of the test steel was evaluated as follows.
The macrostructure of steel No. 16 (comparative steel) shown in FIG. 1 is represented by a cross mark as a coarse and undesirable structure, and the macrostructure of steel No. 2 (steel of the present invention) shown in FIG. A good organization was represented by a circle, and a similar or better organization was also represented by a circle. Further, an intermediate structure between the mark x and the mark o was represented by a triangle.
The macrostructure of the steel of the present invention is generally evaluated as “good”. Some of the test steels (Steel Nos. 7, 8, 14, and 15) had a rating of △, but this was due to the relatively low S content and the sulfide of Ti and Zr that contributed to the improvement of the solidification structure. This is because the amount of generation was small.
Since the steels of Nos. 14 and 15 have too high a hardness as cast, they are tempered and softened after normalizing to facilitate cutting after casting. In particular, since the steel of steel number 15 is used for a hot forging die, after the cutting, heat treatment of quenching and tempering is performed again.
The impact values are good for any of the steels of the invention, except for steel No. 15, despite the high levels of S content.
In addition, all of the steels of the present invention have a small amount of tool wear in milling and have good machinability.
Steel No. 7 contains Cr and Cu, and Steel No. 13 contains Cr and V. Although the hardness is increased, the toughness is improved.
The steels of steel numbers 9 and 10 contain Ni, and the effect of improving toughness is recognized. Steel No. 3 contains Cr and Mo, and the effect of improving strength and toughness is recognized.
Steels Nos. 11 and 12 contain V and have significantly improved strength and toughness. Steel No. 5 contains Cu and has increased strength, but also has improved machinability.
Steel No. 6 contains Cu and O (oxygen), and has improved machinability. Steels Nos. 7 and 8 contain O (oxygen) and have improved machinability.
On the other hand, the steels of steel numbers 16 to 22 are comparative steels, and at least one of the steel components or the relation given by the formula (2) is out of the range specified in the present invention.
Among them, the steels of steel numbers 16, 17, 20, 21 and 22 do not satisfy the relationship given by the equation (2), and the evaluation of the macrostructure is x or xx. Further, the steels of steel numbers 18 and 19 satisfy the relationship given by the equation (2). However, since the contents of P and Ti are respectively excessive, segregation of P and Ti is severe, and Remarkable segregation with cracks was observed at the center.
Steel No. 16 had a macrostructure in which columnar crystals were remarkably grown. As a result, a remarkable segregation structure such as P and S was observed in the center of the steel ingot. Further, porosity and cracks were also observed at the position where the segregated structure was present. The same applies to steel with steel number 20.
Steel No. 22 was further inferior in toughness as seen in the lower impact value.
Steel No. 17 was inferior in milling workability because the Mn and Al contents were too high, and steel No. 18 was too high in N content. Steel No. 19 has too high a Ti content, so that a large amount of TiC is generated and has high hardness. Steel No. 21 has a too high C content and an insufficient S content. Therefore, the milling processability was poor.
As is clear from the above results, fine sulfides of Ti and Zr are formed by adjusting the contents of Ti, Zr, S, and N in the steel to an appropriate range, and the internal quality and coating after casting are formed. It is possible to manufacture a cast steel excellent in machinability and a mold having excellent machinability such that the internal quality in a cast state is comparable to a forged steel product.
INDUSTRIAL APPLICABILITY According to the cast steel and the casting mold of the present invention, while limiting the steel composition for specific components, the content of Ti, Zr, S and N in steel is adjusted to an appropriate range. , And fine sulfides of Ti and Zr are formed, and excellent characteristics of internal quality and machinability after casting can be secured. By casting this cast steel, the internal quality in a cast state is comparable to a forged steel product A mold having such excellent machinability can be manufactured. Accordingly, the present invention can be widely applied to deep engraving in which a processed surface extends inside a cast material, and use for finishing, which requires strict surface properties, which could not be conventionally applied.
[Brief description of the drawings]
FIG. 1 is a diagram showing a macro structure photograph of an ingot of a cast steel containing neither Ti nor Zr.
FIG. 2 is a view showing a macro structure photograph of an ingot of a cast steel containing Zr.
FIG. 3: is a figure which shows the microphotograph of the inclusion of the ingot center part about the cast steel containing Ti.
FIG. 4 is a diagram showing a microphotograph of inclusions at the center of the ingot for Zr-containing cast steel.

▲２▼ 前記▲１▼に記載の鋳鋼において、Ｆｅの一部に代えて、質量％で、Ｃｒ：０．２〜９．０％、Ｎｉ：０．２〜２．０％、Ｍｏ：０．０５〜２．０％およびＶ：０．０１〜１．５％のうちから選ばれた１種または２種以上を含有させてもよい。
▲３▼ 前記▲１▼または▲２▼に記載の鋳鋼において、Ｆｅの一部に代えて、質量％で、Ｃｕ：０．１〜３．０％およびＯ（酸素）：０．００５〜０．０２０％のうちの１種または２種を含有させてもよい。
▲４▼ 前記▲１▼から▲３▼のいずれかに記載の鋳鋼を鋳造してなる金型である。[0004]
Even when the S content is high, segregation of S is greatly improved while casting, and columnar crystals are reduced or disappear, and not only defects such as porosity and segregation, but also coarse columnar The unevenness of the cutting surface caused by the crystal is also improved, and an epoch-making internal quality comparable to forged steel is obtained.
The gist of the present invention completed on the basis of the above findings is as follows.
{Circle around (1)} By mass%, C: 0.02 to 0.45%, Si: 0.1 to 2.5%, Mn: 0.1 to 2.5%, P: 0.10% or less, S: 0.02 to 0.60%, N: 0.020% or less, Al: 0.001 to 0.03%, Ti: 0.05 to 0.25%, and Zr: 0.05 to 0 .50% of one or two kinds, the balance being Fe and impurities, and the effective Ti equivalent (Ti *) given by the following equation (1) satisfies the relationship given by the following equation (2); It is a cast steel which is excellent in internal quality and machinability after casting and which is used as cast, wherein the steel contains a sulfide containing at least one of Ti and Zr.

(2) In the cast steel according to (1), instead of part of Fe, Cr: 0.2 to 9.0%, Ni: 0.2 to 2.0%, Mo: 0 0.05 to 2.0% and V: 0.01 to 1.5%.
(3) In the cast steel according to (1) or (2), instead of a part of Fe, Cu: 0.1 to 3.0% and O (oxygen): 0.005 to 0 by mass%. One or two of 0.020% may be contained.
(4) A mold formed by casting the cast steel according to any of (1) to (3).

Claims (5)

In mass%, C: 0.02 to 0.45%, Si: 0.1 to 2.5%, Mn: 0.1 to 2.5%, P: 0.10% or less, S: 0.02 0.60%, N: 0.020% or less, Al: 0.001 to 0.03%, Ti: 0.05 to 0.25% and Zr: 0.05 to 0.50% And the balance consists of Fe and impurities, and the effective Ti equivalent (Ti *) given by the following formula (1) satisfies the relationship given by the following formula (2). A cast steel excellent in internal quality and machinability after casting, characterized by containing a sulfide containing at least one of Ti and Zr.

Instead of part of Fe, Cr: 0.2 to 9.0%, Ni: 0.2 to 2.0%, Mo: 0.05 to 2.0%, and V: 0.01 by mass%. The cast steel having excellent internal quality and machinability after casting according to claim 1, wherein the cast steel contains one or more selected from among 1.5% to 1.5%. It is characterized in that one or two of Cu: 0.1 to 3.0% and O (oxygen): 0.005 to 0.020% are contained by mass% instead of part of Fe. A cast steel excellent in internal quality and machinability after casting according to claim 1. Instead of part of Fe, Cr: 0.2 to 9.0%, Ni: 0.2 to 2.0%, Mo: 0.05 to 2.0%, and V: 0.01 by mass%. One or two or more selected from among 1.5% to 1.5%, and further, one of Cu: 0.1 to 3.0% and O (oxygen): 0.005 to 0.020%. The cast steel according to claim 1, wherein the cast steel is excellent in internal quality and machinability after casting. A mold formed by casting the cast steel according to claim 1.

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