JP3939582B2

JP3939582B2 - Molding simulation method and apparent friction coefficient determination method applied to the method

Info

Publication number: JP3939582B2
Application number: JP2002118769A
Authority: JP
Inventors: 恭臣森川; 淳加藤; 貢基池田
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2002-04-22
Filing date: 2002-04-22
Publication date: 2007-07-04
Anticipated expiration: 2022-04-22
Also published as: JP2003311338A

Description

【０００１】
【発明が属する技術分野】
本発明は、自動車車体パネル等のプレス成形において、被加工板の成形状態を予測計算する成形シミュレーション法および同法に適用する見かけの摩擦係数の決定方法に関する。
【０００２】
【従来の技術】
近年、自動車車体パネルなどのプレス成形部品の成形過程において、破断やしわの発生などの不良現象の発生の有無を予測計算するとともに、成形不良が生じないように前記予測計算結果を成形金型の製作に反映させるため、主として有限要素法による３次元成形シミュレーションが行われるようになってきた。
【０００３】
一方、一般的に、材料流入量やダイキャビティ内における被加工板に働く張力を制御するため、成形金型の板押さえ部に、被加工板を波形状に変形させながら通過させるビード部が設けられる。
プレス成形加工の実プロセスにおいて、前記ビード部における被加工板の流入制御は極めて重要であり、流入量を制限し過ぎると破断の原因に、また流入量が過剰になるとしわの原因となる。また、成形品の形状によっても流入量を微妙に制御することが要求される場合がある。これらの制御を精度良く行うためには、成形シミュレーションにおいて、ビード部における被加工板に作用する押付力に対する引抜力を正確に算出することが重要である。
【０００４】
従来の成形シミュレーションでは、ビード部通過時の板変形をそのまま計算すると計算時間が膨大になるため、被加工板がビード部を通過する際の摩擦係数を大きめの値（この大きめの値に設定した摩擦係数を「見かけの摩擦係数」と呼ぶ。）を用いることとし、ビード部を通過するのに要する引抜力を被加工板の押付力に見かけの摩擦係数を掛けることによって簡易的に算出している。すなわち、被加工板がビード部を通過するのに要する引抜力は、ビード部と被加工板の接触に伴う摩擦力と、ビード部の形状に沿って変形（曲げ曲げ戻し変形）するのに要する力とによって構成されるが、後者の変形力の計算には多大な時間を要するため、変形に要する力を見かけの摩擦係数に含めて計算することとしている。この見かけの摩擦係数は、物性値の異なる被加工板ごとに推定されたり、簡便な試験によって実測されたりするが、通常、成形シミュレーションの際には２乃至３水準の少数の定数が使用される。
【０００５】
一方、特開平１０−１４６６９７号公報には、シミュレーションに先立って加工実験を行い、この加工実験から製品の板厚分布を測定しておき、金型と材料との摩擦係数を０．３より小さい範囲で金型の部位ごとに連続的に変化させて第１回目のシミュレーションを行い、実験による板厚の測定値とシミュレーションによる板厚の測定値との差が最小となる摩擦係数を求め、次のシミュレーションにはこの摩擦係数を適用する成形シミュレーション法が記載されている。
【０００６】
【発明が解決しようとする課題】
上記のように従来の手法では、ビード部を通過する際の抵抗は、非常に曖昧なものとして扱われており、成形シミュレーションの予測精度を劣化させる原因となっている。すなわち、成形シミュレーションで使用される見かけの摩擦係数には、摩擦係数に影響を与える接触面圧、被加工板の材質が考慮されていないため、成形シミュレーションの結果と実プレス加工における不良発生状況とが大きく異なるようになる。このため、シミュレーション結果を基にして行う金型設計、金型の製作から実プレス試験の結果による型調整などの工程・手間が増え、結果として成形金型の製作コスト高を招来し、さらに得られた成形金型の成形精度についてもばらつきが生じる。
【０００７】
また、前記公報に記載の成形シミュレーション法では、摩擦係数が接触面圧や降伏応力などの材料（素板）の物性に依存して変化することが考慮されておらず、シミュレーションの前に予め行われる加工実験の結果とシミュレーションの計算結果との誤差が最小となるように計算で用いる材料定数の合わせ込みを行っているだけである。このため、この成形シミュレーション法では、材料物性や成形金型の形状が異なる度に実際に製作した成形金型を用いて加工実験を行わなければならず、結局、成形金型の製作に成形シミュレーション結果を反映させることができない。
【０００８】
本発明はかかる問題に鑑みなされたもので、成形シミュレーションにおいて被加工材がビード部を通過するのに要する引抜力を押付力から正確に計算することができる、見かけの摩擦係数の決定方法を提供することを目的とする。また、他の目的として、前記決定方法によって決定された見かけの摩擦係数を用いることによって、実際に成形金型を製作することなく、精度の高い成形状態を予測することができる成形シミュレーション法を提供する。
【０００９】
【課題を解決するための手段】
本発明の見かけの摩擦係数決定方法は、被加工板のダイキャビティ内への流入を制御するビード部が板押さえ部に形成された成形金型を用いて前記板押さえ部に押付力が付加された状態で被加工板をプレス成形する際の、前記被加工板の弾塑性変形量を前記ビード部を通過する被加工板の引抜力に基づいて計算する成形シミュレーション法における、前記引抜力を前記押付力から算出する際に用いられる見かけの摩擦係数の決定方法であって、前記被加工板における接触面圧と摩擦係数との関係を予め実測により求め、前記成形金型のビード部の、被加工板の成形時の流れ方向とプレス方向とを含む横断平面における断面形状と同様の断面形状を有するビード部が形成されたビード金型を想定し、このビード金型に付加された押付力に対してそのビード部を通過する被加工板の各部に生じる接触面圧を算出し、算出された各部の接触面圧に対して実測により求められた摩擦係数を適用して前記ビード金型のビード部を通過するのに要する被加工板の引抜力を算出し、前記引抜力を前記押付力で除して見かけの摩擦係数を決定する。
この見かけの摩擦係数決定方法において、実測により求められた接触面圧と摩擦係数との関係から両者の関係式を求め、ビード金型のビード部を通過する被加工板の引抜力を算出する際に、被加工板の各部に生じる接触面圧に対して前記関係式から算出された摩擦係数を適用することができる。
【００１０】
また、本発明の成形シミュレーション法は、被加工板のダイキャビティ内への流入を制御するビード部が板押さえ部に形成された成形金型を用いて前記板押さえ部に押付力が付加された状態で被加工板をプレス成形する際の、前記被加工板の弾塑性変形量を前記ビード部を通過する被加工板の引抜力に基づいて計算する成形シミュレーション法であって、前記引抜力は前記押付力に見かけの摩擦係数を掛けて算出され、前記見かけの摩擦係数として前記見かけの摩擦係数決定方法によって決定された見かけの摩擦係数を適用する。
この成形シミュレーション法において、成形金型にビード部の断面形状あるいは押付力が異なる複数の板押さえ部を備える場合、各板押さえ部ごとにビード部に対する見かけの摩擦係数として前記見かけの摩擦係数決定方法によって決定した見かけの摩擦係数を適用することができる。
【００１１】
【発明の実施の形態】
図７に示したＵ形部材５１のプレス成形に即して本発明を説明する。
前記Ｕ形部材５１は端部にフランジ５２，５２が形成されたＵ字形状の横断面を有しており、図４に示す成形金型１５によってプレス成形される。この成形金型１５は、ダイ１３と、パンチ１１と、ダイ１３の板押さえ面に付勢するように設けられた押さえ板１７とを有し、前記ダイ１３の板押さえ面にはビード部１６の一方を構成する凹部１４が、押さえ板１７の板押さえ面にはビード部１６の他方を構成する凸部１２が形成されている。前記凹部１４と凸部１２とによってビード部１６が構成される。なお、ダイ１３の板押さえ面、押さえ板１７の板押さえ面およびビード部によって板押さえ部が構成される。
【００１２】
図６は本発明の成形シミュレーション法を実施するための手順を示す主フローチャートであり、摩擦係数の接触面圧に対する依存性を実測により求める事前測定と、見かけの摩擦係数を決定する見かけの摩擦係数決定計算と、その計算結果を用いて成形シミュレーション法を実行する成形シミュレーション計算とに大別される。なお、事前測定データおよび各計算を実行するプログラムはコンピュータの記憶装置に記憶され、これを実行することによって本発明は実施される。
【００１３】
本発明を実施するには、先ず、事前測定（Ｓ１）として摩擦係数の接触面圧に対する依存性を実測により求める。すなわち、被加工板に付加される押付力によって生じる接触面圧と摩擦係数との関係を予め実験的に求める。ここで前記依存性を求めるに至った経緯を説明する。
【００１４】
本発明者は成形金型のビード部を通過する被加工板の各部の面圧は一定でなく、さらに摩擦係数も接触面圧によって変化することに着目した。接触面圧によって摩擦係数が変化する理由は以下のように説明される。被加工板や成形金型の表面には微小な凹凸が形成されており、その凹部には潤滑油が溜まる。この潤滑油が溜まった凹部は「ミクロプール」と呼ばれる。ミクロプールでは、高面圧で被加工板と金型とが接触した際に、潤滑油に静水圧効果が生じ、摩擦係数を低減させる。被加工板の降伏強度が低い場合、表面の凹凸が塑性変形を起こし易く、ミクロプールに静水圧効果が生じ易い。このため、特に被加工板の降伏強度は、接触面圧による摩擦係数の変化を左右する重要な要因となる。従って、プレス成形に用いられる被加工板に対して、その板材の面圧と摩擦係数との関係を正確に把握しておくことが、成形シミュレーションにおけるビード部での予測計算に使用される見かけの摩擦係数を正確に決定する際の基礎データとなる。
【００１５】
図１は、降伏強度（ＹＰ）の異なる鋼板を用いて高面圧摺動試験により実測された接触面圧と摩擦係数との関係を示す図である。同図から明らかなように、降伏強度、接触面圧によって摩擦係数は大きく相違し、低降伏強度材では１５０ＭＰａ当たりで急激に摩擦係数が低下する。これは、先に述べたように、鋼板の表面に塑性変形が生じてミクロプールが静水圧効果を奏するようになるからである。さらに面圧が増加するとミクロプールが圧壊され、静水圧効果が消失するようになるため、摩擦係数も上昇する。
【００１６】
次に、実測により求められた接触面圧と摩擦係数との関係に基づいて、材質（降伏強度）毎に接触面圧をパラメータとする摩擦係数の近似式を導出する（Ｓ２）。例えば、図１に示す降伏強度の各鋼板に対して、各々摩擦係数μを接触面圧ｐの関数として表すことができるが、ここでは、下記式(1) に示すようにμをｐ（ＭＰａ）および降伏強度ＹＰ（ＭＰａ）の関数（近似式）μ＝ｆ（ｐ，ＹＰ）として表した。実測したデータをかかる関係式に整理しておくことで、後述するビード金型のビード部を通過する際の引抜力を容易に計算することができる。図１は降伏強度の異なる２種の鋼板について示すものであるが、より多くの降伏強度の異なる板材についてμ＝ｆ（ｐ，ＹＰ）の近似式を求めることができる。なお、下記式(1) 中のＡは実測値に適合させるための定数であり、通常、０．８〜１．２の値の数値が選定される。Ａ＝１として計算した結果を図２に示す。

【００１７】
次ぎに、設計、計画する成形金型１５のビード部１６と、被加工板Ｗの流れ方向（ダイキャビティへの流入方向）と加圧方向とを含む平面（「横断平面」と呼ぶ。）における断面（「横断面」と呼ぶ。）形状が同一のビード部が形成されたビード金型を想定する（Ｓ３）。すなわち、図３に示すように、被加工板Ｗの流れ方向と垂直な方向に沿って凸部２が板押さえ面に直線状に形成された下型１と、前記凸部２に対応してこの凸部２を噛み合わせ状に装入することができる凹部４が板押さえ面に直線状に形成された上型３とからなるビード金型５を想定する。前記凸部２と凹部４とによってビード部６が構成され、凸部２と凹部４との間に被加工板Ｗが部分的に接触した状態で挟持される。この場合、成形金型１５における被加工板Ｗのダイキャビティへの流入方向は、図３のビード金型５における被加工板Ｗの引抜方向に対応し、被加工板のビード部における接触条件は成形金型とビード金型とで同じになる。
【００１８】
次ぎに、計画中の成形金型１５の金型条件（ビード長さ）、被加工板の形状・材質条件（板厚、応力・歪線図）、プレス条件（板押さえ部の押付力Ｐ）等のデータに基づき、ビード金型５のビード部６に挟持された被加工板Ｗを押付力Ｐで加圧した状態で、被加工板Ｗをビード部６から引き出すのに要する引抜力Ｆを算出し、見かけの摩擦係数＝Ｆ／Ｐを求める（Ｓ４）。この引抜力Ｆの算出は、有限要素法によって簡単に行うことができる。概念的には、有限要素法によりビード金型５のビード部６に当接する被加工板Ｗの各部に生じる接触面圧を算出し、この算出された面圧に対応して前記関係式から摩擦係数を算出し、各部における接触面圧と対応する摩擦係数との積、すなわち被加工板Ｗを単位長さのビード部６を通過させる際に生じる単位摩擦力を算出するとともにビード部６を通過する際の単位長さ当たりの曲げ曲げ戻し変形に要する単位変形力を求め、この単位摩擦力および単位変形力のビード長さにおける総和を求めることによって引抜力Ｆが算出される。なお、上記説明では、接触面圧に対応する摩擦係数を関係式から求めるようにしたが、実測された接触面圧と摩擦係数との対応関係から、所期の面圧に対応する摩擦係数を選択して、あるいは実測値を補間して求めるようにしてもよい。
【００１９】
図５は、ビード金型５に付加された押付力に対して算出された被加工板（降伏強度：１４３ＭＰａ）の引抜力（●）と、想定したビード金型を実際に製作し、これを用いて押付力に対して実測された引抜力（■）との関係をを示す図であり、両者は良好な一致を示している。なお、図５には一定値の摩擦係数を用いて算出した引抜力（▲）も併記した。
【００２０】
上記の説明は、成形品が比較的簡単な形状をしており、成形金型のビード部も１種類の単純な直線形状をしているが、複雑形状の成形品では成形形状を制御するために、ビード部の横断面形状あるいは押付力が異なる複数の板押さえ部が計画される場合がある。このような場合には、成形金型の各々の板押さえ部に対して、同部に設けられたビード部と同形状のビード部を有するビード金型を想定し、各押さえ部のビード部ごとに付加される押付力に基づいて見かけの摩擦係数を求めればよい（Ｓ５）。
【００２１】
次に、上記見かけの摩擦係数計算によって算出された見かけの摩擦係数を三次元プレス成形加工における成形シミュレーション計算に適用する（Ｓ６）。すなわち、有限要素法を用いた成形シミュレーションにおいて、板押さえ面をビード部の無い平坦面と仮定し、この面における摩擦係数として前記見かけの摩擦係数を適用し、被加工材がビード部を通過する際の引抜力を、見かけの摩擦係数を有する前記平坦板押さえ面を通過する際の引抜力として求める。もちろん、成形シミュレーション計算を行うには、計算機に摩擦係数（本発明ではこの摩擦係数として前記見かけの摩擦係数が適用される。）のほか、金型条件（成形部の形状、ビード部長さ）、被加工板の形状・材質条件（サイズ、板厚、応力・歪線図）、プレス条件（板押さえ部の押付力、成形速度）等のデータを入力し、これらのデータに基づいて、プレス成形の開始から完了まで単位時間毎の引抜力を求め、逐次算出される引抜力に基づいて被加工板の成形状態、すなわち被加工板の形状、歪分布、応力分布などの被加工板各部の物理的特性が逐次算出される。なお、板押さえ部に横断面形状が異なるビード部がある場合、あるいはビード部の横断面形状が同じでも押付力が異なる板押さえ部がある場合、先に述べたように各板押さえ部ごとに見かけの摩擦係数を求め、各板押さえ部にその値を適用する。
【００２２】
上記のようにビード金型５のビード部６を通過する被加工板の各部に生じる接触面圧に基づいて引抜力Ｆを算出し、この引抜力Ｆを押付力Ｐで除すことによって算出された見かけの摩擦係数（Ｆ／Ｐ）は、実測によって得た見かけの摩擦係数と非常に近似した値を取る。従って、前記算出された見かけの摩擦係数を成形シミュレーションに適用することにより、成形シミュレーションの予測精度を向上させることができ、成形シミュレーションの予測精度を向上させることができる。
【００２３】
この成形シミュレーション法による計算結果は、プレス成形に供される成形金型の設計並びに被加工板や成形条件の選定、設定に利用される。成形金型の設計に利用する場合、成形金型を設計、計画した後、金型を実際に製作する前に、成形シミュレーションを実行し、その計算結果から成形状態に問題のある部位が認められたとき、計画された成形金型の形状を修正し、再度、成形シミュレーションを実行する。かかる修正作業を成形状態に不具合が無くなるまで繰り返して行い、成形不具合が生じない成形金型を設計する。このようにして設計、製作された成形金型は、ほとんど修正なしで、あるいは軽微な修正を金型に施す程度で製品の成形に使用することができる。このため、従来のように、設計した成形金型を製作し、これを用いて成形実験を行い、その結果によって金型を大幅に修正する、場合によっては再加工するというような、製作調整期間の長期化や製作コスト高を防止することができる。
【００２４】
【発明の効果】
以上説明したように、本発明によって決定された見かけの摩擦係数を、成形シミュレーション法における、被加工板が成形金型のビード部を通過するのに要する引抜力の計算に適用することによって、引抜力を高精度に予測計算することができ、このため成形金型による成形実験を行うことなく、成形シミュレーションの予測精度を向上させることができる。また、かかる成形シミュレーション結果を利用することによってプレス成形に供する成形金型の調整期間の短縮、製作コストの低減、成形精度の向上を図ることができる。
【図面の簡単な説明】
【図１】実測による摩擦係数と接触面圧との関係を示すグラフである。
【図２】実測データに基づいて得られた関係式により算出した摩擦係数と接触面圧との関係を示すグラフである。
【図３】ビード金型の斜視図である。
【図４】Ｕ形部材の成形金型の一例を示す要部断面模式図である。
【図５】ビード金型に付加した押付力と、被加工材がビード部を通過するのに要する引抜力との関係を示すグラフである。
【図６】本発明にかかる見かけの摩擦係数決定方法を含む成形シミュレーション法の実行手順を示す主フローチャートである。
【図７】実施形態における成形対象のＵ形部材を示す斜視図である。
【符号の説明】
５ビード金型
６ビード部
Ｐ押付力
Ｆ引抜力
Ｗ被加工板[0001]
[Technical field to which the invention belongs]
The present invention relates to a forming simulation method for predicting and calculating a forming state of a processed plate in press forming of an automobile body panel or the like, and an apparent friction coefficient determining method applied to the method.
[0002]
[Prior art]
In recent years, in the molding process of press-molded parts such as automobile body panels, the presence or absence of failure phenomena such as breakage and wrinkles has been predicted and calculated, and the predicted calculation results of molding dies are used to prevent molding failures. In order to reflect this in production, a three-dimensional forming simulation by the finite element method has been mainly performed.
[0003]
On the other hand, in general, in order to control the amount of material inflow and the tension acting on the work plate in the die cavity, a bead portion that allows the work plate to pass while being deformed into a wave shape is provided in the plate pressing portion of the molding die. It is done.
In the actual process of press forming, the inflow control of the processed plate in the bead portion is extremely important. If the inflow amount is limited too much, it causes breakage, and if the inflow amount becomes excessive, it causes wrinkles. In some cases, it is required to finely control the amount of inflow depending on the shape of the molded product. In order to perform these controls with high accuracy, it is important to accurately calculate the pulling force with respect to the pressing force acting on the work plate in the bead portion in the forming simulation.
[0004]
In the conventional molding simulation, if the plate deformation at the time of passing the bead part is calculated as it is, the calculation time becomes enormous, so the friction coefficient when the work plate passes through the bead part is set to a larger value (this larger value is set). The friction coefficient is called “apparent friction coefficient”), and the pulling force required to pass through the bead portion is simply calculated by multiplying the pressing force of the work plate by the apparent friction coefficient. Yes. That is, the pulling force required for the work plate to pass through the bead portion is required for deformation (bending and bending back deformation) along the frictional force accompanying the contact between the bead portion and the work plate and the shape of the bead portion. Although the calculation of the latter deformation force requires a lot of time, the force required for the deformation is included in the apparent friction coefficient. This apparent coefficient of friction is estimated for each processed plate having different physical property values or is actually measured by a simple test. Usually, a small number of constants of 2 to 3 levels are used for forming simulation. .
[0005]
On the other hand, in Japanese Patent Laid-Open No. 10-146697, a machining experiment is performed prior to the simulation, and the product thickness distribution is measured from the machining experiment, and the coefficient of friction between the mold and the material is smaller than 0.3. The first simulation is performed by continuously changing the mold part in the range, and the friction coefficient that minimizes the difference between the experimental thickness measurement value and the simulation thickness measurement value is obtained. In this simulation, a forming simulation method using this friction coefficient is described.
[0006]
[Problems to be solved by the invention]
As described above, in the conventional method, the resistance at the time of passing through the bead portion is treated as very ambiguous, which causes the prediction accuracy of the molding simulation to deteriorate. In other words, the apparent friction coefficient used in the molding simulation does not take into consideration the contact surface pressure that affects the friction coefficient and the material of the work plate. Will be very different. For this reason, the process and labor from mold design and mold production based on simulation results to mold adjustment based on the result of actual press test increase, resulting in higher production cost of the mold. Variations also occur in the molding accuracy of the molding dies.
[0007]
Further, in the forming simulation method described in the above publication, it is not considered that the friction coefficient changes depending on the physical properties of the material (material plate) such as contact surface pressure and yield stress. The material constants used in the calculation are merely adjusted so that the error between the result of the machining experiment and the calculation result of the simulation is minimized. For this reason, in this molding simulation method, a machining experiment must be performed using a molding die that is actually manufactured every time the material properties and the shape of the molding die are different. The result cannot be reflected.
[0008]
The present invention has been made in view of such a problem, and provides an apparent friction coefficient determination method capable of accurately calculating a pulling force required for a workpiece to pass through a bead portion in a forming simulation from a pressing force. The purpose is to do. Further, as another object, a molding simulation method capable of predicting a highly accurate molding state without actually manufacturing a molding die by using the apparent friction coefficient determined by the determination method is provided. To do.
[0009]
[Means for Solving the Problems]
In the apparent friction coefficient determination method of the present invention, a pressing force is applied to the plate pressing portion using a molding die in which a bead portion for controlling the inflow of the workpiece plate into the die cavity is formed on the plate pressing portion. In the forming simulation method for calculating the amount of elastic-plastic deformation of the work plate when the work plate is press-molded in a state where the work plate passes through the bead portion, the pulling force in the forming simulation method is calculated. A method for determining an apparent friction coefficient used when calculating from the pressing force, wherein a relationship between a contact surface pressure and a friction coefficient in the workpiece plate is obtained in advance by actual measurement, and a bead portion of the bead portion of the molding die is measured. Assuming a bead mold in which a bead portion having a cross-sectional shape similar to the cross-sectional shape in the transverse plane including the flow direction and the pressing direction at the time of forming the processed plate is assumed, the pressing force applied to the bead die is for The contact surface pressure generated in each part of the processed plate that passes through the bead part of the bead mold, and the bead part of the bead mold is applied by applying a friction coefficient obtained by actual measurement to the calculated contact surface pressure of each part. The pulling force of the work plate required for passing is calculated, and the apparent friction coefficient is determined by dividing the pulling force by the pressing force.
In this apparent friction coefficient determination method, when the relational expression between the contact surface pressure and the friction coefficient obtained by actual measurement is obtained, the drawing force of the work plate passing through the bead part of the bead mold is calculated. In addition, the friction coefficient calculated from the relational expression can be applied to the contact surface pressure generated in each part of the processed plate.
[0010]
Further, in the molding simulation method of the present invention, a pressing force is applied to the plate pressing portion using a molding die in which a bead portion for controlling the flow of the workpiece plate into the die cavity is formed on the plate pressing portion. A molding simulation method for calculating the amount of elastoplastic deformation of the work plate when the work plate is press-formed in a state based on the pulling force of the work plate passing through the bead portion, wherein the pulling force is The apparent friction coefficient calculated by multiplying the pressing force by the apparent friction coefficient and determined by the apparent friction coefficient determination method is applied as the apparent friction coefficient.
In this molding simulation method, when the molding die includes a plurality of plate pressing portions having different cross-sectional shapes or pressing forces of the bead portions, the apparent friction coefficient determination method as an apparent friction coefficient for the bead portions for each plate pressing portion. The apparent coefficient of friction determined by can be applied.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in line with the press molding of the U-shaped member 51 shown in FIG.
The U-shaped member 51 has a U-shaped cross section in which

flanges

52, 52 are formed at the ends, and is press-molded by a molding die 15 shown in FIG. The molding die 15 includes a die 13, a punch 11, and a pressing plate 17 provided so as to be urged against a plate pressing surface of the die 13, and a bead portion 16 is provided on the plate pressing surface of the die 13. The concave portion 14 constituting one of the convex portions 12 is formed on the plate pressing surface of the pressing plate 17 and the convex portion 12 constituting the other of the bead portions 16 is formed. The concave portion 14 and the convex portion 12 constitute a bead portion 16. The plate pressing portion is constituted by the plate pressing surface of the die 13, the plate pressing surface of the pressing plate 17, and the bead portion.
[0012]
FIG. 6 is a main flow chart showing the procedure for carrying out the molding simulation method of the present invention. The prior measurement for determining the dependence of the friction coefficient on the contact surface pressure by actual measurement and the apparent friction coefficient for determining the apparent friction coefficient. The calculation is roughly divided into a determination calculation and a molding simulation calculation that executes a molding simulation method using the calculation result. The prior measurement data and the program for executing each calculation are stored in a storage device of a computer, and the present invention is implemented by executing this.
[0013]
To implement the present invention, first, as a pre-measurement (S1), the dependence of the friction coefficient on the contact surface pressure is obtained by actual measurement. That is, the relationship between the contact surface pressure generated by the pressing force applied to the work plate and the friction coefficient is experimentally obtained in advance. Here, the background for obtaining the dependency will be described.
[0014]
The inventor has paid attention to the fact that the surface pressure of each part of the processed plate passing through the bead portion of the molding die is not constant, and the friction coefficient also changes depending on the contact surface pressure. The reason why the coefficient of friction changes depending on the contact surface pressure is explained as follows. Minute irregularities are formed on the surface of the work plate and the molding die, and lubricating oil accumulates in the concave portions. The concave portion in which the lubricating oil is accumulated is called a “micropool”. In the micro pool, when the work plate and the mold come into contact with each other at a high surface pressure, a hydrostatic pressure effect is generated in the lubricating oil, and the friction coefficient is reduced. When the yield strength of the work plate is low, the unevenness on the surface tends to cause plastic deformation, and the hydrostatic pressure effect tends to occur in the micropool. For this reason, in particular, the yield strength of the plate to be processed is an important factor that affects the change in the friction coefficient due to the contact surface pressure. Therefore, it is apparent that the relationship between the surface pressure of the plate material and the coefficient of friction of the work plate used for press forming is used for predictive calculation at the bead part in forming simulation. This is the basic data for accurately determining the coefficient of friction.
[0015]
FIG. 1 is a diagram showing a relationship between a contact surface pressure and a friction coefficient measured by a high surface pressure sliding test using steel plates having different yield strengths (YP). As is clear from the figure, the coefficient of friction varies greatly depending on the yield strength and contact surface pressure, and the coefficient of friction decreases sharply per 150 MPa for low yield strength materials. This is because, as described above, plastic deformation occurs on the surface of the steel sheet, and the micropool has a hydrostatic pressure effect. Further, when the surface pressure increases, the micropool is collapsed and the hydrostatic pressure effect disappears, so that the friction coefficient also increases.
[0016]
Next, based on the relationship between the contact surface pressure and the friction coefficient obtained by actual measurement, an approximate expression of the friction coefficient using the contact surface pressure as a parameter for each material (yield strength) is derived (S2). For example, for each steel plate having the yield strength shown in FIG. 1, the friction coefficient μ can be expressed as a function of the contact surface pressure p. Here, μ is expressed as p (MPa) as shown in the following formula (1). ) And a yield strength YP (MPa) function (approximate expression) μ = f (p, YP). By arranging the actually measured data in such a relational expression, the pulling force when passing through a bead portion of a bead mold to be described later can be easily calculated. FIG. 1 shows two types of steel plates having different yield strengths, but an approximate expression of μ = f (p, YP) can be obtained for more plate materials having different yield strengths. In the following formula (1), A is a constant for adapting to the actual measurement value, and a numerical value of 0.8 to 1.2 is usually selected. FIG. 2 shows the result calculated with A = 1.

[0017]
Next, a bead portion 16 of the molding die 15 to be designed and planned, and a plane (referred to as a “transverse plane”) including the flow direction of the work plate W (inflow direction into the die cavity) and the pressing direction. A bead mold having a bead portion having the same cross-sectional shape (referred to as a “cross section”) is assumed (S3). That is, as shown in FIG. 3, the lower mold 1 in which the convex portions 2 are linearly formed on the plate pressing surface along the direction perpendicular to the flow direction of the processed plate W, and the convex portions 2 correspond to the lower mold 1. Assume a bead mold 5 including an upper mold 3 in which a concave portion 4 in which the convex portion 2 can be inserted in a meshing manner is formed linearly on a plate pressing surface. The convex part 2 and the concave part 4 constitute a bead part 6, and the workpiece plate W is sandwiched between the convex part 2 and the concave part 4 in a partially contacted state. In this case, the inflow direction of the work plate W into the die cavity in the molding die 15 corresponds to the drawing direction of the work plate W in the bead die 5 of FIG. 3, and the contact condition at the bead portion of the work plate is The same is true for the molding die and the bead die.
[0018]
Next, the mold conditions (bead length) of the molding die 15 being planned, the shape and material conditions of the processed plate (plate thickness, stress / strain diagram), and pressing conditions (pressing force P of the plate pressing part) Based on such data, the drawing force F required to pull out the work plate W from the bead portion 6 in a state where the work plate W sandwiched between the bead portions 6 of the bead mold 5 is pressed with the pressing force P. The apparent friction coefficient = F / P is calculated (S4). The extraction force F can be easily calculated by the finite element method. Conceptually, the contact surface pressure generated in each part of the work plate W contacting the bead part 6 of the bead mold 5 is calculated by the finite element method, and the friction is calculated from the relational expression corresponding to the calculated surface pressure. The coefficient is calculated, and the product of the contact surface pressure in each part and the corresponding friction coefficient, that is, the unit friction force generated when the workpiece plate W is passed through the bead part 6 having a unit length, and passes through the bead part 6 is calculated. The pulling force F is calculated by obtaining a unit deformation force required for bending / bending return deformation per unit length at the time, and obtaining the sum of the unit friction force and the unit deformation force in the bead length. In the above description, the friction coefficient corresponding to the contact surface pressure is obtained from the relational expression. However, from the correspondence relationship between the actually measured contact surface pressure and the friction coefficient, the friction coefficient corresponding to the desired surface pressure is calculated. It may be determined by selecting or interpolating actual measurement values.
[0019]
FIG. 5 shows the actual drawing of the pulling force (●) of the processed plate (yield strength: 143 MPa) calculated for the pressing force applied to the bead die 5 and the assumed bead die. It is a figure which shows the relationship with drawing force (■) measured with respect to pressing force using, and both have shown favorable agreement. FIG. 5 also shows the pulling force (算出) calculated using a constant friction coefficient.
[0020]
In the above explanation, the molded product has a relatively simple shape, and the bead portion of the molding die has one simple linear shape. However, in the case of a complex shaped molded product, the molded shape is controlled. In addition, a plurality of plate pressing portions having different cross-sectional shapes or pressing forces of the bead portions may be planned. In such a case, assuming a bead mold having a bead part having the same shape as the bead part provided in the same part for each plate pressing part of the molding die, for each bead part of each pressing part What is necessary is just to obtain | require an apparent friction coefficient based on the pressing force added to (S5).
[0021]
Next, the apparent friction coefficient calculated by the apparent friction coefficient calculation is applied to the forming simulation calculation in the three-dimensional press forming process (S6). That is, in the forming simulation using the finite element method, the plate pressing surface is assumed to be a flat surface without a bead portion, the apparent friction coefficient is applied as the friction coefficient on this surface, and the workpiece passes through the bead portion. The pulling force at the time is determined as the pulling force when passing through the flat plate pressing surface having an apparent friction coefficient. Of course, in order to perform the molding simulation calculation, in addition to the friction coefficient (in the present invention, the apparent friction coefficient is applied as the friction coefficient in the present invention), the mold conditions (the shape of the molding part, the bead part length), Input data such as the shape and material conditions (size, thickness, stress / strain diagram) of the work plate, press conditions (pressing force of the plate pressing part, forming speed), etc., and press forming based on these data The drawing force per unit time is calculated from the start to the completion of the process, and the physical state of each part of the work plate, such as the shape of the work plate, strain distribution, stress distribution, etc., based on the successively calculated drawing force The characteristic is calculated sequentially. In addition, when there is a bead portion with a different cross-sectional shape in the plate pressing portion, or when there is a plate pressing portion with a different pressing force even though the cross-sectional shape of the bead portion is the same, as described above, for each plate pressing portion Find the apparent coefficient of friction and apply that value to each plate retainer.
[0022]
As described above, the drawing force F is calculated based on the contact surface pressure generated in each part of the processed plate passing through the bead portion 6 of the bead die 5, and the drawing force F is divided by the pressing force P. The apparent friction coefficient (F / P) takes a value very close to the apparent friction coefficient obtained by actual measurement. Therefore, by applying the calculated apparent friction coefficient to the molding simulation, the prediction accuracy of the molding simulation can be improved, and the prediction accuracy of the molding simulation can be improved.
[0023]
The calculation result obtained by this molding simulation method is used for designing a molding die used for press molding and for selecting and setting a workpiece plate and molding conditions. When using for molding mold design, after designing and planning the molding mold, and before actually manufacturing the mold, a molding simulation is executed, and from the calculation result, there is a part that has a problem in the molding state. At that time, the shape of the planned molding die is corrected, and a molding simulation is executed again. Such correction work is repeated until there is no defect in the molding state, and a molding die that does not cause a molding defect is designed. The molding die designed and manufactured in this way can be used for the molding of a product with almost no modification or a slight modification to the mold. For this reason, as in the past, a designed molding die is produced, a molding experiment is performed using this, and the die is modified significantly according to the result, and in some cases, the production adjustment period is reworked. It is possible to prevent an increase in manufacturing cost and manufacturing costs.
[0024]
【The invention's effect】
As described above, the apparent friction coefficient determined by the present invention is applied to the calculation of the extraction force required for the work plate to pass through the bead portion of the molding die in the molding simulation method. The force can be predicted and calculated with high accuracy, so that the prediction accuracy of the molding simulation can be improved without performing a molding experiment using a molding die. In addition, by using the molding simulation result, it is possible to shorten the adjustment period of the molding die used for press molding, reduce the manufacturing cost, and improve the molding accuracy.
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship between measured friction coefficient and contact surface pressure.
FIG. 2 is a graph showing a relationship between a friction coefficient calculated by a relational expression obtained based on actual measurement data and a contact surface pressure.
FIG. 3 is a perspective view of a bead mold.
FIG. 4 is a schematic cross-sectional view of an essential part showing an example of a molding die for a U-shaped member.
FIG. 5 is a graph showing the relationship between the pressing force applied to the bead mold and the pulling force required for the workpiece to pass through the bead portion.
FIG. 6 is a main flowchart showing an execution procedure of a forming simulation method including an apparent friction coefficient determination method according to the present invention.
FIG. 7 is a perspective view showing a U-shaped member to be molded in the embodiment.
[Explanation of symbols]
5 Bead mold 6 Bead part P Pushing force F Pulling force W Work plate

Claims

When press-molding a processed plate with a pressing force applied to the plate pressing portion using a molding die in which a bead portion for controlling the flow of the processed plate into the die cavity is formed in the plate pressing portion Apparently used when calculating the pulling force from the pressing force in a molding simulation method for calculating the amount of elastic-plastic deformation of the plate to be processed based on the pulling force of the plate to be processed passing through the bead portion. A method for determining a friction coefficient,
Obtaining the relationship between the contact surface pressure and the friction coefficient in the work plate in advance by actual measurement,
Assuming a bead mold in which a bead portion having a cross-sectional shape similar to the cross-sectional shape in the transverse plane including the flow direction and the pressing direction at the time of forming the processed plate of the bead portion of the molding die is formed. Calculate the contact surface pressure generated in each part of the processed plate that passes through the bead part with respect to the pressing force applied to the bead mold,
Applying the coefficient of friction obtained by actual measurement to the contact surface pressure of each part calculated to calculate the drawing force of the work plate required to pass through the bead part of the bead mold,
An apparent coefficient of friction determination method applied to a forming simulation method, wherein the apparent friction coefficient is determined by dividing the pulling force by the pressing force.

When the relational expression between the contact surface pressure and the friction coefficient obtained by actual measurement is obtained, and the drawing force of the work plate passing through the bead part of the bead mold is calculated, it is generated in each part of the work plate. The apparent friction coefficient determination method according to claim 1, wherein the friction coefficient calculated from the relational expression is applied to the contact surface pressure.

When press-molding a processed plate with a pressing force applied to the plate pressing portion using a molding die in which a bead portion for controlling the flow of the processed plate into the die cavity is formed in the plate pressing portion In the forming simulation method for calculating the amount of elastic-plastic deformation of the work plate based on the pulling force of the work plate passing through the bead portion, the pulling force is calculated by multiplying the pressing force by an apparent friction coefficient. A molding simulation method in which the apparent friction coefficient determined by the apparent friction coefficient determination method according to claim 1 is applied as the apparent friction coefficient.

3. The method for determining an apparent friction coefficient according to claim 1 or 2, wherein the molding die includes a plurality of plate pressing portions having different cross-sectional shapes or pressing forces of the bead portions, and an apparent friction coefficient with respect to the bead portion for each plate pressing portion. The molding simulation method according to claim 3, wherein an apparent friction coefficient determined by the method is applied.