JP4452816B2

JP4452816B2 - Semi-melt forming method for austenitic stainless steel

Info

Publication number: JP4452816B2
Application number: JP2000005000A
Authority: JP
Inventors: 聡川村; 謙治三輪
Original assignee: National Institute of Advanced Industrial Science and Technology AIST
Current assignee: National Institute of Advanced Industrial Science and Technology AIST
Priority date: 2000-01-13
Filing date: 2000-01-13
Publication date: 2010-04-21
Anticipated expiration: 2020-01-13
Also published as: JP2001198661A

Description

【０００１】
【発明の属する技術分野】
本発明は、オーステナイト系ステンレス鋼の成形方法に関わり、固体材料を固相と液相が共存する領域まで加熱し、加圧成形して、最終製品に近い形状のものを製造する半溶融成形方法に関するものである。
【０００２】
【従来の技術】
一般的な金属材料の成形法として、固相状態の材料を成形する冷間、熱間の塑性加工と、液相状態の材料を成形する鋳造加工が行われている。
鋳造加工の場合は、固体の金属材料を加熱して一旦液体の状態にした後、鋳型に流し込む方法なので、製品の形状に対する自由度が大きく、最終製品に近い形状のものを容易に製造することができるが、通常、製品には、凝固収縮による引け巣や割れ、溶存ガスによるピンホールなどの各種欠陥が発生する。また、製品の各部では冷却速度が異なるので、成分の偏析や、結晶粒の大きさの不均一が起こる。
【０００３】
このような鋳造加工の問題点を解決し、製品の品質を改善する目的で、金属の固液共存状態からの成形が行われている。すなわち、欠陥発生の原因となる液相の存在割合が少ないほど、凝固時の収縮量、ピンホールの発生、金属組織的な不均一さの発生などが少なく、製品は高品質になるからである。
固液共存状態からの成形法には、液体状態から温度を下げてくる半凝固法と、固体状態から温度を上げてくる半溶融法がある。
ここで、半凝固法の場合は、溶融した材料を攪拌しながら冷却し、一部を凝固させて固相を生じせしめ、その状態で鋳型内に加圧注入する方法が一般的であるのに対して、半溶融法の場合は、固体材料を加熱し、一部を溶融させて液相を生じせしめ、その状態で鋳型内に加圧注入するのが一般的である。したがって、半凝固法に比べて成形装置が簡単であるとともに、出発素材である固体材料の一部がそのまま残り、もとの固体材料の物性を維持できるという利点がある。
【０００４】
いずれの方法においても、成形の可否を決めるポイントは固相率である。固相率とは、固液共存状態にある材料における固相の占める体積の割合であり、固相率が高い場合は成形性が悪く、逆に固相率が低い場合は成形性は良いものの、液相が多いので、鋳造加工の場合のような欠陥の発生が起こりやすくなる。したがって、半凝固、半溶融成形法においては、成形に最適な固相率の範囲を見出すことが非常に重要である。
【０００５】
また、材料が異なれば、その液相線温度、固相線温度、高温での材料物性などのすべてが異なるので、材料毎に成形に最適な固相率範囲を見出す必要がある。成形が比較的容易な低融点金属材料については、これまでに、成形法に関する検討が多くなされており、Ａｌ合金では、固相と液相の存在比率に関する提案がなされた例がある。
しかし、高融点金属材料であるオーステナイト系ステンレス鋼については殆ど検討された例がない。ボロン化合物を微細分散化した含ボロンオーステナイト系ステンレス鋼について、鋼塊や連鋳スラブを製造する場合の半凝固スラリーの固相率を定めた例はあるものの、一般的なオーステナイト系ステンレス鋼を素材として半溶融成形による部品製造の例は見当たらない。
【０００６】
これまでにオーステナイト系ステンレス鋼の半溶融成形の例がない理由は、成形の困難さにあると考えられる。オーステナイト系ステンレス鋼は融点が高く（液相線温度が約１４７０℃）、固液共存範囲（液相線温度と固相線温度の差）が狭い（約５０〜６０℃）。したがって、半溶融成形やダイキャストの分野で既に実績のあるＡｌ合金、Ｍｇ合金などの低融点金属材料に比べて、金型（鋳型）に押し込まれた場合の温度降下が大きい。すなわち、金型による冷却速度が大きいので、半溶融材料は急速に流動性を失い凝固してしまい、金型に完全に充填されない事態も生じうる。ここで、金型温度を上げれば、凝固までの時間を遅らせることができるが、金型材質として一般に用いられている工具鋼の場合、高温強度上の問題から、その温度を３００℃程度までしか上げることができない。
【０００７】
成形時の加圧力を大きくすれば半溶融材料の変形量が大きくなるので、金型への充填性は向上するが、金型の塑性変形や治具の損傷などを起こす可能性があるとともに、大きな型締め力を必要とするので、成形圧力を数(1/10)MPa（数kgf/cm^２）より高くするのは好ましくない。
このような状況の下で、オーステナイト系ステンレス鋼の半溶融成形を可能にする最適固相率範囲を提案することは大きな意義がある。
【０００８】
【発明が解決しようとする課題】
本発明では、固液共存状態のオーステナイト系ステンレス鋼材料を合金などの鋳型内へ注入して製品形状に半溶融成形する際に、欠陥の発生のない、高品質な製品を得るための最適固相率範囲を提供することができるオーステナイト系ステンレス鋼の半溶融成形方法を提供することを課題とする。
【０００９】
【課題を解決するための手段】
本発明の方法は、オーステナイト系ステンレス鋼材料の半溶融成形加工において、欠陥のない健全な製品を得るためには、固相率が重要なパラメータであるとの見地の下に見出されたものである。本件発明者らは、半溶融材料の加熱温度によって決まる固相率と金型への湯廻り性、および固相率と成形体の内部欠陥との関係について、実験による詳細な検討を行い、半溶融成形に最適な固相率範囲を見出した。
【００１０】
本発明では、固相率と温度の関係が線形であると仮定して導かれる一般的な下記の数式で、固相率を定義する。
ｆ_Ｓ＝（Ｔ_Ｌ−Ｔ）÷（Ｔ_Ｌ−Ｔ_Ｓ）×１００
ここで、Ｔ_Ｓ≦Ｔ≦Ｔ_Ｌであり、Ｔ≦Ｔ_Ｓではｆ_Ｓ＝１、Ｔ_Ｌ≦Ｔではｆ_Ｓ＝０である。ｆ_Ｓは固相率を表す。Ｔ_Ｌは供試オーステナイト系ステンレス鋼材料の液相線温度、Ｔ_Ｓは供試オーステナイト系ステンレス鋼材料の固相線温度であり、いずれも予め熱分析などの手段によって求められる定数である。Ｔは成形直前の半溶融材料の温度であり、熱電対などを用いて、成形の工程において測定される値である。
【００１１】
本発明は、オーステナイト系ステンレス鋼の固体材料を加熱して固相と液相とが共存した半溶融材料とし、これを金型などの鋳型内に押出して成形する方法において、成形直前の半溶融材料の温度を測定し、この温度から求められる前記固相率が７０％以下、望ましくは５０％以下の状態で半溶融成形することを特徴とする方法である。
【００１２】
本件発明者は、実験による詳細な検討の結果、以下の知見を見出したものである。
▲１▼固相率が７０％以上の場合は、半溶融材料の成形性が悪く、金型に完全に充填されない。
▲２▼固相率が７０％以下の場合は、数(1/10)MPa（数kgf/cm^２）以下の低い成形圧力で半溶融材料が変形し、金型に完全に充填される。
▲３▼固相率が５０％以上の場合は、出発材料である固相がそのまま塊状になって成形体内部に残留し、組織が不均一であり、機械的特性に悪影響を及ぼす可能性がある。
▲４▼固相率が５０％以下の場合は、成形性が極めて良好であり、数(1/10)MPa（数kgf/cm^２）以下の低い成形圧力で半溶融材料が金型の隅部まで完全に充填され、かつ、均一な内部組織を有する成形体が得られる。
▲５▼固相率が０％で、材料全体が液相状態にある場合は、鋳造加工の場合と同様に、凝固収縮による引け巣や割れ、溶存ガスによるピンホールなどの各種欠陥が発生する。
以上のように、オーステナイト系ステンレス鋼材料の半溶融成形加工において、湯廻り性が良く、欠陥の発生が少なく、また均一な組織を得ることができる固相率範囲は７０％以下、望ましくは５０％以下である。
【００１３】
【発明の実施の形態】
（供試材）
代表的なオーステナイト系ステンレス鋼材料である市販のＳＵＳ３０４を半溶融成形に使用した。表１に本材料の組成と示差熱分析によって測定した液相線温度、固相線温度を示す。本材料から、直径３０mm、長さ２０mmの円柱状素材を加工し、半溶融成形に供した。
【表１】

【００１４】
（成形装置）
図１に半溶融成形に使用した装置の概略を示す。円柱状の半溶融成形用材料１は窒化珪素製ダイス２の上に設置され、これらは窒化珪素製筒状ホルダ３内に収納される。ここで、ダイス２は筒状ホルダ３に自在に摺動するように嵌合される構造になっており、ダイス２は下部加圧軸５に接続され、加圧プランジャの役目をする。筒状ホルダ３は、固相率が低くなった時の半溶融成形用材料１の自立を保持する役目と加圧成形時のシリンダの役目をする。熱電対６の先端は半溶融成形材料１の底面中心部にスポット溶接され、ダイス２の中心軸部に設けた穴７を通って配線される。熱源には２０kw、４kHz の高周波誘導加熱装置を使用する。筒状ホルダ３の外周部に、加熱用コイル８が配置され、前記熱電対６で測定した温度を基に高周波出力を調整し、半溶融成形用材料１を加熱する。ＳＫＤ６１製の二つ割金型９が筒状ホルダ３の上に設置され、上部加圧軸１０によって筒状ホルダ３に押し付けられている。
半溶融成形用材料１を熱電対６の信号を基に加熱用コイル８で加熱し、成形が可能な所定の温度に到達、保持後、下部加圧軸５を作動させ、半溶融成形用材料１を金型９のキャビティ１１内に押出すことで成形が行われる。
【００１５】
（固相率の求め方）
半溶融成形実験のパラメータとなる半溶融材料１の固相率は熱電対６で測定した温度から求めた。ただし、本実施例のような方法で金属材料を加熱した場合、通常、材料全体を均一な温度にするのは困難であり、材料自体に温度分布が生じる。すなわち、図１の熱電対６で測定した温度は必ずしも半溶融成形用材料１を代表する温度にはならない可能性がある。
そこで、図１の熱電対６をスポット溶接して測定した底面中心部の位置の温度と材料全体の平均温度との関係を求めた。予め図２に示すように、ダミーの加熱用材料に対して、図１の熱電対６をスポット溶接した位置ａおよびその他の代表的位置ｂ〜ｆに熱電対を配置した。このダミーを図１の半溶融成形用材料１の代わりに装置内に設置し、位置ａの温度が所定の一定温度（以後、設定温度とよぶ）に保持するように高周波加熱した。この際の各位置の温度を測定し、測定値を次式に代入して材料全体の平均温度Ｔを算出した。
Ｔ＝｛(Ｔa＋Ｔb＋Ｔd＋Ｔe)÷４＋(Ｔb＋Ｔc＋Ｔe＋Ｔf)÷４｝÷２
ここで、Ｔa、Ｔb、Ｔc、Ｔd、Ｔe、Ｔf はそれぞれ、位置ａ、ｂ、ｃ、ｄ、ｅ、ｆの温度である。
設定温度すなわち、位置ａ（図１の熱電対６をスポット溶接した位置）の温度Ｔａをパラメータとして変えた温度測定実験を行い、図３に示す設定温度Ｔａと材料全体の平均温度Ｔとの関係を得た。
【００１６】
本図から、材料全体の平均温度Ｔは設定温度Ｔa の関数として最小自乗法で線形近似すると以下のように表されることがわかった。
供試材の液相線温度である１４６９℃以下では、
Ｔ＝７１６＋０．５１２６Ｔａ
供試材の液相線温度である１４６９℃以上では、
Ｔ＝Ｔa
次に、材料全体の平均温度Ｔを次式に代入して、その温度に対応する固相率を求めた。この固相率は材料全体の平均固相率と見なしてよい。
ｆs＝（Ｔ_Ｌ−Ｔ）÷（Ｔ_Ｌ−Ｔs）×１００
ここで、Ｔs≦Ｔ≦Ｔ_Ｌであり、Ｔ≦Ｔsではｆs＝１、Ｔ_Ｌ≦Ｔではｆs＝０である。ｆsは固相率を表す。Ｔ_Ｌは供試ＳＵＳ３０４の液相線温度１４６９℃、Ｔsは固相線温度１４１５℃である。ＴはＴaの関数であるので、Ｔaが決まれば、ｆ_Ｓが決まることになる。
【００１７】
以上の方法によって、図１の熱電対６をスポット溶接した位置（図２のａの位置）の温度と材料全体の平均の固相率の関係が明らかになった。すなわち、図１の熱電対６をスポット溶接した位置（図２のａの位置）の温度を測定し、この温度がある一定の設定温度になるような加熱制御を行うことによって、半溶融成形用材料１を所望の固相率の状態にすることができる。
【００１８】
（実施例１）
前記市販ＳＵＳ３０４材料から加工した直径３０mm、長さ２０mmの素材を半溶融成形用材料１として筒状ホルダ内３にセットし、固相率が２９％、すなわち、半溶融成形用材料１の平均温度が１４５３℃になるように、熱電対６の設定温度で１４３８℃になるまで加熱し、２分間温度保持した。
この後、下部加圧軸５を作動させ、圧力９８００Ｎ、速度１０mm/sの条件で半溶融成形用材料１を金型９のキャビティ１１内に押出して半溶融成形を行った。
冷却後、金型を分解して、成形体の外観を調査したところ、湯廻り不良、湯じわなどの表面欠陥発生は見られなかった。また、成形体を縦方向中心面で切断し、断面を研磨して、内部の状況を調査したところ、引け巣、ピンホールなどの内部欠陥の発生は見られなかった。
【００１９】
（その他の実施例および比較例）
表２に固相率（具体的には、熱電対６で測定される温度が一定になるように設定した温度）を変えて同様の実験を行った結果を示す。欠陥の発生したものについては×印、欠陥の発生しなかったものにはついては○印で表す。また、表２中には実施例１についても併せて記載した。
ここで、比較例１と実施例１〜４は成形前の素材が固相と液相の共存状態にあったものであり、比較例２，３は成形前の素材が液相の状態にあったものである。
【表２】

【００２０】
固相率８４％の比較例１では、隅部に湯廻り不良を生じ、金型に完全に充填されなかった。これは、固相率８４％の半溶融材料の塑性変形能が小さいためであると考えられる。ただし、実施例１の場合と同様に、その縦方向断面を調査したところ、成形体内部に欠陥は発生していなかった。
固相率７０％以下の固液共存領域にある半溶融材料を成形した実施例２〜４では、実施例１と同様に、表面欠陥、内部欠陥ともに発生は見られなかった。しかし、固相率５０％以上７０％以下の実施例３、実施例４では、出発材料である固相がそのまま数mm程度の大きさで塊状になって成形体内部に残留していた。このような組織の不均一は、成形体の機械的特性に悪影響を及ぼす可能性があると考えられる。
【００２１】
一方、全て液相状態で固相が存在しない材料を成形した比較例２，３では、金型に完全に充填し、良好な表面が得られたものの、その内部には、一般の鋳造品に発生すると同様の引け巣やピンホールが発生した。
以上、本発明を実施例に基づき説明したが、本発明は実施例の方法に何ら制約を受けることがない。半溶融材料の平均の固相率を求める場合、加熱方法および治具の材質や構造の変更によって均熱性が向上し、半溶融材料の各位置での温度差が殆どない状態が得られるのであれば、代表的な１箇所のみを温度測定して、その結果から固相率を算出すれば良い。また、温度測定の方法としては、熱電対の代わりに放射温度計を使用してもよい。
【００２２】
【発明の効果】
以上説明したように、本発明によれば、固液共存状態のオーステナイト系ステンレス鋼材料を金型などの鋳型内へ注入して製品形状に半溶融成形する際に、本発明の固相率範囲にある固液共存状態のオーステナイト系ステンレス鋼材料は、流動性に優れるので、型内へ注入する際に、湯廻り不良、湯じわなどの欠陥を生じることがない。また、液相線を超えない温度領域にあるため、引け巣やピンホールなどの内部欠陥が殆ど生じない。
【図面の簡単な説明】
【図１】半溶融成形装置の一例を示す縦断側面図である。
【図２】温度測定位置を示す模式図である。
【図３】設定温度Ｔａと材料全体の平均温度Ｔとの関係を示す図である。
【符号の説明】
１半溶融成形用材料
２ダイス
３筒状ホルダ
４台
５下部加圧軸
６熱電対
７熱電対配線穴
８加熱用コイル
９金型
１０上部加圧軸
１１キャビティ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming austenitic stainless steel, and heats a solid material to a region where a solid phase and a liquid phase coexist, and press-molds to produce a semi-molten molding method having a shape close to the final product It is about.
[0002]
[Prior art]
As a general method for forming a metal material, cold and hot plastic processing for forming a solid phase material and casting processing for forming a liquid phase material are performed.
In the case of casting, it is a method in which a solid metal material is heated to a liquid state and then poured into a mold. Therefore, there is a large degree of freedom in the shape of the product, and it is easy to manufacture a shape close to the final product. However, in general, various defects such as shrinkage cavities and cracks due to solidification shrinkage and pinholes due to dissolved gas occur in the product. In addition, since each part of the product has a different cooling rate, segregation of components and non-uniform crystal grain size occur.
[0003]
For the purpose of solving such problems of casting and improving the quality of products, molding is performed from a solid-liquid coexistence state. That is, the smaller the proportion of the liquid phase that causes defects, the lower the amount of shrinkage during solidification, the occurrence of pinholes, the occurrence of non-uniform metal structure, and the higher the quality of the product. .
The molding method from the solid-liquid coexistence state includes a semi-solidification method in which the temperature is lowered from the liquid state and a semi-melting method in which the temperature is raised from the solid state.
Here, in the case of the semi-solidification method, a method is generally used in which the molten material is cooled while stirring, and a part of the material is solidified to form a solid phase, and in that state, pressure is injected into the mold. On the other hand, in the case of the semi-melting method, it is common to heat a solid material, melt a part thereof to form a liquid phase, and in that state, pressurize it into a mold. Therefore, there are advantages that the molding apparatus is simpler than the semi-solidification method, and a part of the solid material as the starting material remains as it is, and the physical properties of the original solid material can be maintained.
[0004]
In either method, the point that determines whether or not molding is possible is the solid phase ratio. The solid phase ratio is a ratio of the volume occupied by the solid phase in the material in the solid-liquid coexistence state. When the solid phase ratio is high, the moldability is poor, and conversely, when the solid phase ratio is low, the moldability is good. Since there are many liquid phases, defects are likely to occur as in the case of casting. Therefore, in the semi-solidification and semi-melt molding methods, it is very important to find a range of the solid phase ratio optimum for molding.
[0005]
In addition, if the materials are different, all of the liquidus temperature, the solidus temperature, the physical properties of the material at a high temperature, and the like are different, so it is necessary to find the optimum solid phase ratio range for molding for each material. For the low melting point metal material that is relatively easy to form, many studies have been made on the forming method so far, and in the case of the Al alloy, there is an example in which a proposal regarding the abundance ratio of the solid phase and the liquid phase has been made.
However, there has been almost no study on austenitic stainless steel, which is a refractory metal material. For boron-containing austenitic stainless steel with finely dispersed boron compound, although there is an example of determining the solid phase ratio of semi-solid slurry when producing steel ingots and continuous cast slabs, general austenitic stainless steel is used as the material There are no examples of manufacturing parts by semi-melt molding.
[0006]
The reason why there is no example of semi-melt molding of austenitic stainless steel so far is considered to be difficulty in molding. Austenitic stainless steel has a high melting point (liquidus temperature is about 1470 ° C.) and a solid-liquid coexistence range (difference between liquidus temperature and solidus temperature) is narrow (about 50 to 60 ° C.). Therefore, compared with low melting point metal materials such as Al alloys and Mg alloys that have already been proven in the fields of semi-melt molding and die casting, the temperature drop when pressed into a mold (mold) is large. That is, since the cooling rate by the mold is high, the semi-molten material rapidly loses fluidity and solidifies, and there may occur a situation where the mold is not completely filled. Here, if the mold temperature is raised, the time to solidification can be delayed. However, in the case of tool steel generally used as a mold material, the temperature is limited to about 300 ° C. due to the problem of high-temperature strength. I can't raise it.
[0007]
Increasing the pressing force during molding increases the amount of deformation of the semi-molten material, which improves the filling ability of the mold, but may cause plastic deformation of the mold and damage to the jig. Since a large clamping force is required, it is not preferable to make the molding pressure higher than several (1/10) MPa (several kgf / cm ² ).
Under such circumstances, it is significant to propose an optimum solid phase ratio range that enables semi-melt forming of austenitic stainless steel.
[0008]
[Problems to be solved by the invention]
In the present invention, when an austenitic stainless steel material coexisting with a solid and liquid is poured into a mold such as an alloy and semi-melt molded into a product shape, an optimum solid for obtaining a high-quality product free from defects is obtained. It is an object of the present invention to provide a semi-molten forming method of austenitic stainless steel that can provide a phase range.
[0009]
[Means for Solving the Problems]
The method of the present invention was found from the viewpoint that the solid phase ratio is an important parameter for obtaining a sound product with no defects in the semi-melt forming process of austenitic stainless steel materials. It is. The inventors of the present invention conducted detailed examinations by experiments on the solid phase ratio determined by the heating temperature of the semi-molten material and the ability to pour into the mold, and the relationship between the solid phase ratio and the internal defects of the molded body. The optimum solid phase ratio range was found for melt molding.
[0010]
In the present invention, the solid phase ratio is defined by the following general formula derived on the assumption that the relationship between the solid phase ratio and the temperature is linear.
f _S = (T _L −T) ÷ (T _L −T _S ) × 100
Here, a _{_T S} ≦ _T ≦ _T _L, a T ≦ _T In _{_{_{S f S = 1, T L}}} ≦ T in _f S = 0. f _S represents the solid fraction. T _L is tested austenitic stainless steel material of the liquidus temperature, the T _S is the solidus temperature of the test austenitic stainless steel material, both a constant determined by such means as previously thermal analysis. T is the temperature of the semi-molten material immediately before molding, and is a value measured in the molding process using a thermocouple or the like.
[0011]
The present invention provides a semi-molten material in which a solid phase and a liquid phase coexist by heating a solid material of austenitic stainless steel and extruding it into a mold such as a mold. The method is characterized in that the temperature of the material is measured, and semi-melt molding is performed in a state where the solid phase ratio determined from this temperature is 70% or less, preferably 50% or less.
[0012]
The present inventor has found the following findings as a result of detailed examination by experiment.
(1) When the solid phase ratio is 70% or more, the moldability of the semi-molten material is poor and the mold is not completely filled.
( ² ) When the solid phase ratio is 70% or less, the semi-molten material is deformed at a low molding pressure of several (1/10) MPa (several kgf / cm ² ) or less, and the mold is completely filled.
(3) If the solid phase ratio is 50% or more, the solid phase that is the starting material will remain in a lump and remain inside the molded body, and the structure will be uneven, which may adversely affect the mechanical properties. is there.
(4) When the solid phase ratio is 50% or less, the moldability is very good, and the semi-molten material is at the corner of the mold at a low molding pressure of several (1/10) MPa (several kgf / cm ² ) or less. A molded body that is completely filled up to a portion and has a uniform internal structure is obtained.
(5) When the solid phase ratio is 0% and the entire material is in a liquid phase, various defects such as shrinkage cavities and cracks due to solidification shrinkage and pinholes due to dissolved gas occur as in the case of casting. .
As described above, in the semi-melt molding process of austenitic stainless steel material, the range of the solid phase ratio in which hot meltability is good, the occurrence of defects is small, and a uniform structure can be obtained is 70% or less, preferably 50 % Or less.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
(Sample material)
A commercially available SUS304, which is a typical austenitic stainless steel material, was used for semi-melt molding. Table 1 shows the composition of this material and the liquidus temperature and solidus temperature measured by differential thermal analysis. From this material, a cylindrical material having a diameter of 30 mm and a length of 20 mm was processed and subjected to semi-melt molding.
[Table 1]

[0014]
(Molding equipment)
FIG. 1 shows an outline of an apparatus used for semi-melt molding. A cylindrical semi-melt molding material 1 is placed on a silicon nitride die 2, and these are accommodated in a silicon nitride cylindrical holder 3. Here, the die 2 is configured to be fitted to the cylindrical holder 3 so as to freely slide. The die 2 is connected to the lower pressure shaft 5 and serves as a pressure plunger. The cylindrical holder 3 plays a role of holding the self-supporting of the semi-molten molding material 1 when the solid phase ratio becomes low and a role of a cylinder at the time of pressure molding. The tip of the thermocouple 6 is spot welded to the center of the bottom surface of the semi-molten molding material 1 and wired through a hole 7 provided in the center shaft of the die 2. A high-frequency induction heating device of 20 kw and 4 kHz is used as the heat source. A heating coil 8 is disposed on the outer periphery of the cylindrical holder 3, and the high frequency output is adjusted based on the temperature measured by the thermocouple 6 to heat the semi-molten molding material 1. A two-part die 9 made of SKD61 is installed on the cylindrical holder 3 and pressed against the cylindrical holder 3 by the upper pressure shaft 10.
The semi-molten molding material 1 is heated by the heating coil 8 based on the signal of the thermocouple 6, reaches a predetermined temperature at which molding is possible, and after holding, the lower pressure shaft 5 is operated, and the semi-melt molding material Molding is performed by extruding 1 into the cavity 11 of the mold 9.
[0015]
(How to find the solid fraction)
The solid phase ratio of the semi-molten material 1 that is a parameter of the semi-melt molding experiment was obtained from the temperature measured by the thermocouple 6. However, when the metal material is heated by the method as in this embodiment, it is usually difficult to bring the entire material to a uniform temperature, and a temperature distribution occurs in the material itself. That is, the temperature measured by the thermocouple 6 in FIG. 1 may not necessarily be a temperature representative of the semi-melt molding material 1.
Therefore, the relationship between the temperature at the center of the bottom surface measured by spot welding the thermocouple 6 of FIG. 1 and the average temperature of the entire material was determined. As shown in FIG. 2 in advance, thermocouples were arranged at the position a where the thermocouple 6 of FIG. 1 was spot welded and other representative positions b to f with respect to the dummy heating material. This dummy was placed in the apparatus instead of the semi-molten molding material 1 in FIG. 1, and heated at a high frequency so that the temperature at the position a was kept at a predetermined constant temperature (hereinafter referred to as a set temperature). The temperature at each position at this time was measured, and the average value T of the entire material was calculated by substituting the measured value into the following equation.
T = {(Ta + Tb + Td + Te) ÷ 4 + (Tb + Tc + Te + Tf) ÷ 4} / 2
Here, Ta, Tb, Tc, Td, Te, and Tf are the temperatures of the positions a, b, c, d, e, and f, respectively.
A temperature measurement experiment was performed by changing the temperature Ta at the set temperature, that is, the position a (the position where the thermocouple 6 in FIG. 1 was spot welded) as a parameter, and the relationship between the set temperature Ta and the average temperature T of the entire material shown in FIG. Got.
[0016]
From this figure, it was found that the average temperature T of the entire material is expressed as follows when linearly approximated by the method of least squares as a function of the set temperature Ta.
Below 1469 ° C, which is the liquidus temperature of the test material,
T = 716 + 0.5126Ta
At 1469 ° C or higher, which is the liquidus temperature of the test material,
T = Ta
Next, the average temperature T of the entire material was substituted into the following equation, and the solid phase ratio corresponding to that temperature was determined. This solid phase ratio may be regarded as the average solid phase ratio of the entire material.
fs = ( _TL− T) ÷ ( _TL− Ts) × 100
Here, Ts ≦ T ≦ _TL , fs = 1 when T ≦ Ts, and fs = 0 when T _L ≦ T. fs represents a solid phase ratio. _TL is the liquidus temperature 1469 ° C. of the test SUS304, and Ts is the solidus temperature 1415 ° C. Since T is a function of Ta, f _S is determined when Ta is determined.
[0017]
By the above method, the relationship between the temperature at the position where the thermocouple 6 in FIG. 1 is spot-welded (position a in FIG. 2) and the average solid phase ratio of the entire material has been clarified. That is, by measuring the temperature at the position where the thermocouple 6 in FIG. 1 is spot-welded (position a in FIG. 2) and performing heating control so that this temperature is a certain set temperature, The material 1 can be brought into a desired solid phase ratio state.
[0018]
Example 1
A material 30 mm in diameter and 20 mm in length processed from the commercially available SUS304 material is set as a semi-melt molding material 1 in a cylindrical holder 3 and has a solid phase ratio of 29%, that is, the average temperature of the semi-melt molding material 1 Was set to 1438 ° C. at the set temperature of the thermocouple 6 so that the temperature would be 1453 ° C., and the temperature was maintained for 2 minutes.
Thereafter, the lower pressure shaft 5 was operated, and the semi-molten molding material 1 was extruded into the cavity 11 of the mold 9 under the conditions of a pressure of 9800 N and a speed of 10 mm / s to perform semi-molten molding.
After cooling, the mold was disassembled and the appearance of the molded body was examined. As a result, surface defects such as poor hot water and hot water wrinkles were not found. Further, when the molded body was cut at the center plane in the longitudinal direction, the cross section was polished, and the internal situation was investigated, the occurrence of internal defects such as shrinkage nests and pinholes was not observed.
[0019]
(Other examples and comparative examples)
Table 2 shows the results of a similar experiment performed by changing the solid phase ratio (specifically, the temperature set so that the temperature measured by the thermocouple 6 is constant). Those with defects are indicated by x, and those without defects are indicated by ◯. In Table 2, Example 1 is also described.
In Comparative Example 1 and Examples 1 to 4, the material before molding was in the coexistence state of the solid phase and the liquid phase, and in Comparative Examples 2 and 3, the material before molding was in the liquid phase state. It is a thing.
[Table 2]

[0020]
In Comparative Example 1 having a solid phase ratio of 84%, poor hot water was produced at the corners, and the mold was not completely filled. This is considered to be because the plastic deformability of the semi-molten material having a solid phase ratio of 84% is small. However, as in the case of Example 1, when the longitudinal cross section was examined, no defects were generated inside the molded body.
In Examples 2 to 4 in which a semi-molten material in a solid-liquid coexistence region having a solid phase ratio of 70% or less was formed, neither surface defects nor internal defects were observed as in Example 1. However, in Example 3 and Example 4 where the solid phase ratio was 50% or more and 70% or less, the solid phase as the starting material remained in the shape of a few millimeters as it was and remained in the compact. Such unevenness of the structure is considered to have an adverse effect on the mechanical properties of the molded body.
[0021]
On the other hand, in Comparative Examples 2 and 3 in which a material having no solid phase in the liquid phase state was formed, the mold was completely filled and a good surface was obtained. When it occurred, the same shrinkage nest and pinhole were generated.
As mentioned above, although this invention was demonstrated based on the Example, this invention does not receive a restriction | limiting at all in the method of an Example. When determining the average solid fraction of the semi-molten material, it is possible to improve the thermal uniformity by changing the heating method and the material and structure of the jig, and obtain a state where there is almost no temperature difference at each position of the semi-molten material. For example, it is only necessary to measure the temperature at only one representative location and calculate the solid phase ratio from the result. Further, as a temperature measurement method, a radiation thermometer may be used instead of the thermocouple.
[0022]
【The invention's effect】
As described above, according to the present invention, when the austenitic stainless steel material coexisting with solid and liquid is injected into a mold such as a mold and semi-melt molded into a product shape, the solid phase ratio range of the present invention The austenitic stainless steel material in the solid-liquid coexisting state is excellent in fluidity, and therefore does not cause defects such as poor hot water and hot water when injected into the mold. Moreover, since it exists in the temperature range which does not exceed a liquidus line, internal defects, such as a shrinkage nest and a pinhole, hardly arise.
[Brief description of the drawings]
FIG. 1 is a longitudinal side view showing an example of a semi-melt molding apparatus.
FIG. 2 is a schematic diagram showing a temperature measurement position.
FIG. 3 is a diagram showing a relationship between a set temperature Ta and an average temperature T of the whole material.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Semi-melt molding material 2 Dies 3 Cylindrical holder 4 units 5 Lower pressurizing shaft 6 Thermocouple 7 Thermocouple wiring hole 8 Heating coil 9 Mold 10 Upper pressurizing shaft 11 Cavity

Claims

In a semi-molten molding method in which a solid material of austenitic stainless steel is heated to form a semi-molten material in which a solid phase and a liquid phase coexist, and this is extruded into a mold such as a mold.
The temperature at a plurality of locations including the specific location of the semi-molten material is measured, the average temperature (T) of the semi-molten material is calculated, the temperature (Ta) at the specific location, the average temperature (T) and the semi-melt By determining the solid phase ratio (fs) of the material, the solid phase ratio (fs) of the semi-molten material is controlled by controlling the temperature (Ta) at the specific location,
The solid phase ratio (fs) of the semi-molten material is 29% or less and 4% or more.