JP2019174124A - Delayed fracture property evaluation method of high strength steel plate - Google Patents
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Abstract
Description
本発明は、製品形状にプレス成形される高張力鋼板の遅れ破壊特性の評価方法に関する技術である。特に、自動車のセンターピラーやＡピラーロアなどの、プレス成形された車体構造部材におけるフランジ端部などにおける遅れ破壊特性の評価方法に好適な技術である。 The present invention is a technique related to a method for evaluating delayed fracture characteristics of a high-tensile steel sheet press-formed into a product shape. In particular, it is a technique suitable for a method for evaluating delayed fracture characteristics at a flange end portion of a press-formed vehicle body structural member such as an automobile center pillar or A pillar lower.
近年、ＣＯ２排出量などの環境規制の厳格化を受け、自動車は燃費向上を目的とした車体の軽量化が求められている。同時に、衝突時に人や車にダメージが少ない、安全性の高い車体が求められている。このニーズに対して、車体構造部材では１ＧＰａ以上の引張強度を持つ高張力鋼板の適用が進んでいる。
自動車の車体構造部材は、一般にプレス成形によって製造されるが、例えば引張強度で９８０ＭＰａを超える高強度の部材では、プレス成形後の残留応力と使用中の環境から侵入する水素に起因した遅れ破壊が懸念される。そのため、高張力鋼板を上述のような車体構造部材として適用するためには、その高張力鋼板が遅れ破壊特性に優れていることの評価が必要となる。
In recent years, in response to stricter environmental regulations such as CO 2 emissions, automobiles are required to be lighter in weight to improve fuel efficiency. At the same time, there is a need for a highly safe car body that causes less damage to people and vehicles in the event of a collision. In response to this need, high-strength steel sheets having a tensile strength of 1 GPa or more are being applied to body structural members.
Automobile body structural members are generally manufactured by press molding. For example, in a high-strength member having a tensile strength exceeding 980 MPa, delayed fracture due to residual stress after press molding and hydrogen entering from the environment in use is caused. Concerned. Therefore, in order to apply a high-tensile steel plate as a vehicle body structural member as described above, it is necessary to evaluate that the high-tensile steel plate is excellent in delayed fracture characteristics.
従来から高強度化の検討が進められてきたボルトやＰＣ鋼棒、ラインパイプなどに使用される鋼材の遅れ破壊特性は、非特許文献１に述べられているように、公的規格を含む種々の手法が確立されている。しかし、薄鋼板、特に高張力鋼板の遅れ破壊特性評価の方法については、いまだ確立されていないのが現状である。
その理由としては、薄鋼板をプレス加工によって種々の形状に成形して使用することが一因として挙げられる。すなわち、薄鋼板は、プレス加工によるひずみや部品として使用される際の組み付けなどによる残留応力など、ボルト部材などの使用条件下では考慮しなくても良い遅れ破壊特性に影響を及ぼす因子がある。従って、ボルト部材などの遅れ破壊特性評価方法を薄鋼板にそのまま適用しても、十分に正しい評価が行えるとは言えない。
The delayed fracture characteristics of steel materials used for bolts, PC steel rods, line pipes, etc., for which high strength has been studied, are various, including public standards, as described in Non-Patent Document 1. A method has been established. However, the method for evaluating delayed fracture characteristics of thin steel sheets, particularly high-tensile steel sheets, has not yet been established.
One reason for this is that thin steel plates are formed into various shapes by press working. That is, the thin steel sheet has factors that affect delayed fracture characteristics that do not need to be taken into account under use conditions such as bolt members, such as strain due to press working and residual stress due to assembly when used as a part. Therefore, even if the delayed fracture characteristic evaluation method for bolt members or the like is applied to a thin steel plate as it is, it cannot be said that a sufficiently correct evaluation can be performed.
このように、より正確に薄鋼板の遅れ破壊特性を評価するためには、加工によるひずみの影響や組み付けなどによって発生が予想される残留応力の影響、さらには使用環境からの材料への水素侵入量の影響等を適切に反映できる試験方法が必要となる。
このような高張力鋼板の遅れ破壊特性評価方法として、特許文献１では、高張力鋼板をＵ字形状に曲げ加工して薄鋼板にひずみを導入し、この曲げ加工後に発生するスプリングバックした鋼板をボルトで締め込むことで応力を付加した試験片を作成し、電気チャージ法によって試験片中に水素を導入し、破壊が生じるまでの時間を測定する手法を提案している。
Thus, in order to more accurately evaluate the delayed fracture characteristics of thin steel sheets, the effects of strain due to processing and the effects of residual stress expected to occur due to assembly, etc., as well as hydrogen intrusion into the material from the usage environment A test method that can appropriately reflect the effect of the amount is required.
As a method for evaluating delayed fracture characteristics of such a high-tensile steel sheet, in Patent Document 1, a high-strength steel sheet is bent into a U-shape, strain is introduced into the thin steel sheet, and a spring-backed steel sheet generated after this bending process is used. A method is proposed in which a test piece to which stress is applied by bolting is created, hydrogen is introduced into the test piece by an electric charging method, and the time until failure occurs is measured.
特許文献２では、引張強度１１８０ＭＰａ以上の高張力鋼板において、鋼板の伸び量の２０〜８０％の引張予ひずみを付与した後に、曲げ部の角度が３０〜９０度となるＶ字形状に曲げ加工した試験片を用意し、その試験片の両端部をボルトで締め付けることによって、試験片に残留応力を付加させた状態での遅れ破壊特性の評価を提案している。
さらに特許文献３では、Ｖ曲げ成形した試験片にボルトによる締め込みを行う際に、ボルト周辺のたわみによって評価したい曲げ部の応力に影響を及ぼさないように、試験片の対向する板面に面接触する傾斜面を設けた治具を提案し、車体構造部材の遅れ破壊特性の評価を行うことが記載されている。
In Patent Document 2, in a high-tensile steel sheet having a tensile strength of 1180 MPa or more, after applying a tensile pre-strain of 20 to 80% of the elongation of the steel sheet, the bending portion is bent into a V-shape with an angle of 30 to 90 degrees. Proposed evaluation of delayed fracture characteristics in a state in which residual stress is applied to the test piece by preparing the test piece and tightening both ends of the test piece with bolts.
Further, in Patent Document 3, when a V-bending test piece is tightened with a bolt, the surface of the opposing plate surface of the test piece is not affected so as not to affect the stress of the bending portion to be evaluated by the deflection around the bolt. It describes that a jig provided with a contact inclined surface is proposed, and the delayed fracture characteristics of a vehicle body structural member are evaluated.
上記の特許文献１および３に記載の評価方法は、曲げ加工部に発生する遅れ破壊の評価方法である。実際の部品は絞り加工で製造されることもあり、絞り加工では成形中にフランジ端部が縮み変形を受け、成形後にフランジ端部（ブランクの端部）に高い引張応力が残留する場合がある。
また、フランジ端部などのブランク端部は、通常、せん断加工を受けており、せん断加工は材料に大きな塑性ひずみ（ダメージ）が入る。そのためフランジ端部に引張残留応力が発生する箇所は遅れ破壊の危険性が非常に高く、この遅れ破壊の形態は曲げ部の評価方法である上記の特許文献１から３では評価することができない。
The evaluation methods described in Patent Documents 1 and 3 are evaluation methods for delayed fracture occurring in a bent portion. Actual parts may be manufactured by drawing. In drawing, the flange end may undergo shrinkage and deformation during molding, and high tensile stress may remain at the flange end (blank end) after molding. .
Also, blank ends such as flange ends are usually subjected to a shearing process, and the shearing process causes a large plastic strain (damage) in the material. Therefore, the location where the tensile residual stress is generated at the flange end portion has a very high risk of delayed fracture, and this delayed fracture mode cannot be evaluated in the above-mentioned Patent Documents 1 to 3 which are methods for evaluating a bent portion.
本発明は、上記のような点に鑑みてなされたもので、高張力鋼板を絞り加工などのプレス加工した際に、フランジ部などの材料端部に発生する遅れ破壊特性を評価することを目的としている。 The present invention has been made in view of the above points, and an object of the present invention is to evaluate delayed fracture characteristics generated at a material end portion such as a flange portion when a high-strength steel sheet is subjected to press working such as drawing. It is said.
課題を解決するために、本発明の一態様である高張力鋼板の遅れ破壊特性評価方法では、正四角形形状以上の角数の正多角形形状の高張力鋼板の試験片に対し、断面正多角形形状のパンチを備えた金型で深絞り成形を施し、成形後の試験片を水素侵入環境下（水素侵入雰囲気）に置いて当該試験片の亀裂の発生状況によって高張力鋼板の遅れ破壊性を評価し、
上記深絞り成形における、成形高さ及び絞り比（試験片の対角線長さ／パンチの対角線長さ）と、試験片端部に発生する最大引張残留応力との関係を予め求め、その求めた関係に基づき適切な深絞り加工条件を検定する。
In order to solve the problem, in the method for evaluating delayed fracture characteristics of a high-tensile steel sheet according to one aspect of the present invention, the cross-sectional regularity of a test piece of a regular polygonal high-tensile steel sheet having a number of corners equal to or greater than a regular tetragonal shape is provided. Deep drawing is performed with a die equipped with a square punch, and the molded specimen is placed in a hydrogen intrusion environment (hydrogen intrusion atmosphere). Evaluate and
In the above deep drawing, the relationship between the forming height and drawing ratio (diagonal length of the test piece / diagonal length of the punch) and the maximum tensile residual stress generated at the end of the test piece is obtained in advance, and the obtained relationship Appropriate deep drawing processing conditions are verified based on this.
本発明の一態様によれば、正多角形形状の試験片を適正な条件で深絞り加工することにより、高張力鋼板のフランジ部等、ブランク端部（プレス成形品端部）での遅れ破壊特性を精度よく評価することが可能となる。
ここで、せん断加工によって試験片を所定の輪郭形状に成形した場合には、上記最大引張残留応力として、フランジ端部における稜線方向（周方向）に発生する最大引張残留応力を採用すればよい。
なお、本発明の一態様によれば、深絞り加工によって試験片端部（フランジ部）に残留応力を付与するが、対象とする製品形状へのプレス成形は、深絞り加工に限定されるものではない。本発明では、試験片端部（フランジ部）に目的の残留応力を付与する手段として深絞り加工を採用している。
According to one aspect of the present invention, delayed fracture at a blank edge (press-molded product edge), such as a flange portion of a high-tensile steel plate, by deep drawing a regular polygonal test piece under appropriate conditions. It becomes possible to accurately evaluate the characteristics.
Here, when the test piece is formed into a predetermined contour shape by shearing, the maximum tensile residual stress generated in the ridge line direction (circumferential direction) at the flange end may be adopted as the maximum tensile residual stress.
In addition, according to one aspect of the present invention, residual stress is applied to the end portion (flange portion) of the test piece by deep drawing, but press forming to the target product shape is not limited to deep drawing. Absent. In the present invention, deep drawing is employed as means for imparting a desired residual stress to the end portion (flange portion) of the test piece.
次に、本発明の実施形態について図面を参照して説明する。
まず、本実施形態の前提とする、高張力鋼板の遅れ破壊特性評価方法は、図１に示すように、せん断加工による試験片作製の工程Ａ、深絞り成形の工程Ｂ、及び水素侵入環境下設置の工程Ｃを有する。そして、水素侵入環境下に置かれた試験片のフランジ端部（試験片端部）における亀裂の発生状況を評価することで、高張力鋼板の遅れ破壊特性を評価する。亀裂発生状況の評価方法自体は、従来の方法と同様に評価を行い、例えば、水素侵入環境下で予め設定した以上の亀裂が発生するまでの時間で評価する。
ここで、本実施形態は、高張力鋼板が、引張強度が９８０ＭＰａ以上の高張力鋼板の場合に特に好適である。
Next, embodiments of the present invention will be described with reference to the drawings.
First, as shown in FIG. 1, a method for evaluating delayed fracture characteristics of a high-strength steel sheet, which is a premise of the present embodiment, is a test piece preparation process A by shearing, a deep drawing process B, and a hydrogen intrusion environment. It has installation process C. Then, the delayed fracture characteristics of the high-tensile steel sheet are evaluated by evaluating the occurrence of cracks at the flange end (test piece end) of the test piece placed in a hydrogen intrusion environment. The evaluation method of the crack occurrence status itself is evaluated in the same manner as the conventional method, for example, by the time until cracks more than preset in the hydrogen intrusion environment occur.
Here, this embodiment is particularly suitable when the high-tensile steel plate is a high-tensile steel plate having a tensile strength of 980 MPa or more.
＜試験片作製の工程Ａ＞
工程Ａは、作製する製品と同じ材料及び厚さからなる鋼板、すなわち製品に使用する鋼板と同じ鋼板を用意し、その鋼板をせん断加工して、所定形状の試験片１を作製する工程である。試験片１の所定形状は、外周の輪郭形状が、辺（稜線）が４本以上の正多角形形状、つまり正四角形形状以上の角数の正多角形形状に設定する。
試験片１の形状の例を図２に示す。図２（ａ）は正八角形形状の試験片１の場合を例示し、図２（ｂ）は正方形形状の試験片１の場合を例示している。
角数（稜線）を４以上としているのは、角数が３の正三角形形状の試験片１を採用した場合、深絞り加工時のしわ抑えが不十分となる領域が発生しやすいためである。フランジ部にしわが発生すると、本発明の評価に適さない場合が多い。
せん断加工によって試験片１を作製した場合には、製品形状に成形される材料端部にせん断加工で負荷される塑性ひずみ（ダメージ）と同様な塑性ひずみを、試験片１の端部に付与出来る。
<Process A for Specimen Production>
Step A is a step of preparing a test piece 1 having a predetermined shape by preparing a steel plate made of the same material and thickness as the product to be produced, that is, the same steel plate as that used for the product, and shearing the steel plate. . The predetermined shape of the test piece 1 is set such that the outer peripheral contour shape is a regular polygon shape having four or more sides (ridge lines), that is, a regular polygon shape having a number of corners greater than or equal to a regular square shape.
An example of the shape of the test piece 1 is shown in FIG. 2A illustrates the case of the regular octagonal test piece 1, and FIG. 2B illustrates the case of the square test piece 1.
The reason why the number of corners (ridgeline) is 4 or more is that when the equilateral triangle-shaped test piece 1 having three corners is adopted, a region where wrinkle suppression at the time of deep drawing is insufficient is likely to occur. . If wrinkles occur in the flange portion, it is often not suitable for the evaluation of the present invention.
When the test piece 1 is produced by shearing, the end of the test piece 1 can be imparted with plastic strain similar to the plastic strain (damage) applied to the end of the material molded into the product shape by shearing. .
＜深絞り成形の工程Ｂ＞
工程Ｂは、工程Ａで作製した試験片１に対し、断面正多角形状のパンチ１０、ダイ１１（上型）及びブランクホルダー１２（シワ押さえ）を備えた金型（図４参照）によって深絞り成形を施す工程である。金型は、一般的な深絞り成形で使用される金型を使用すれば良く、パンチ１０は、例えば図３のような断面正多角形状（図３は正１２角形形状の例である）の柱形状となっている。ここで、ブランクホルダー１２に、材料の流入を制御するビードを付与しても良いが、本発明では材料端部（フランジ部１ａ）に応力を発生させることが重要なため、ブランクホルダー１２にビードを付与しない方が好ましい。
<Deep-drawing process B>
Step B is a deep drawing of the test piece 1 produced in Step A by a die (see FIG. 4) provided with a punch 10 having a regular polygonal cross section, a die 11 (upper die), and a blank holder 12 (wrinkle presser). This is a process of forming. The die may be a die used in general deep drawing, and the punch 10 has, for example, a regular polygonal cross section as shown in FIG. 3 (FIG. 3 is an example of a regular dodecagon). It has a pillar shape. Here, the blank holder 12 may be provided with a bead for controlling the inflow of the material. However, in the present invention, it is important to generate a stress at the material end (flange portion 1a). It is preferable not to give.
ここで、試験片１の正多角形形状の角数と、パンチ１０の正多角形状の角数を一致させる必要はないが、パンチ１０の正多角形状の角数は、好ましくは試験片１の正多角形形状の角数以上、より好ましくは試験片１の正多角形状の角数より大きいことである。
深絞り成形の工程Ｂを、模式図である図４を参照して説明する。深絞り加工は、一般的な深絞り成形を採用すればよい。まず図４（ａ）のように、試験片１の外周であるフランジ部１ａを、上型であるダイ１１とブランクホルダー１２とで、所定のシワ押さえ力で挟持する。その状態で、パンチ１０をプレス方向に移動させて、図４（ｂ）のように、試験片１を所定の成形高さＨに深絞りする。
Here, it is not necessary to match the number of corners of the regular polygon of the test piece 1 and the number of corners of the regular polygon of the punch 10, but the number of corners of the regular polygon of the punch 10 is preferably that of the specimen 1. More than the number of corners of the regular polygonal shape, more preferably larger than the number of corners of the regular polygonal shape of the test piece 1.
The deep drawing process B will be described with reference to FIG. For the deep drawing process, general deep drawing may be employed. First, as shown in FIG. 4A, the flange portion 1 a that is the outer periphery of the test piece 1 is sandwiched between the upper die 11 and the blank holder 12 with a predetermined wrinkle pressing force. In this state, the punch 10 is moved in the pressing direction, and the test piece 1 is deep-drawn to a predetermined molding height H as shown in FIG.
成形はフランジ部１ａを残した状態で終了し、成形高さＨを調整してフランジ端部に発生する応力を調整する。すなわち、所定のシワ押さえ力は、深絞り成形中にフランジ部１ａにシワが発生しないシワ押さえ力に設定する。
また、パンチ肩で割れが発生すると、フランジ端部の応力状態に変化を及ぼす可能性があるため、材料の延性に応じて割れが発生しないパンチ１０の肩Ｒを選定する。
また、図３及び図４では、断面正多角形状のパンチ１０であって、縦壁１０ａが直壁（プレス方向に対する角度が０度）である場合を例示しているが、縦壁１０ａが斜壁であっても問題ない。縦壁１０ａに対し、鉛直方向（プレス方向）から角度（傾斜）を付けるほど、成形高さＨを増加させることが難しくなるため、パンチ１０の縦壁１０ａの傾斜角は、プレス方向に対して０度（直壁）以上４５度以下が好ましい。
Molding ends with the flange portion 1a left, and the molding height H is adjusted to adjust the stress generated at the flange end. That is, the predetermined wrinkle pressing force is set to a wrinkle pressing force that does not generate wrinkles in the flange portion 1a during deep drawing.
Further, if a crack occurs in the punch shoulder, there is a possibility of changing the stress state at the flange end, and therefore the shoulder R of the punch 10 that does not generate a crack is selected according to the ductility of the material.
3 and 4 exemplify the case where the punch 10 has a regular polygonal cross section, and the vertical wall 10a is a straight wall (the angle with respect to the pressing direction is 0 degree), the vertical wall 10a is oblique. There is no problem even if it is a wall. As the vertical wall 10a is angled (inclined) from the vertical direction (press direction), it becomes more difficult to increase the forming height H. Therefore, the inclination angle of the vertical wall 10a of the punch 10 is relative to the press direction. It is preferably 0 degree (straight wall) or more and 45 degrees or less.
また、試験片１の全周にわたり均等な成形状態に近づけるために、図５に示すように、試験片１の中心とパンチ１０の中心とを合わせるように、試験片１を金型にセッティングして成形を実施する。
ここで、成形後のフランジ端部に発生する引張応力は、試験片１の対角線長さＤ１とパンチ１０の対角線長さＤ０との比（Ｄ１／Ｄ０）である絞り比δによっても変化させることが可能である。絞り比δが小さいと成形終了時にフランジを残すことが困難となるおそれがあり、絞り比δが大きいと材料が流入しないため成形直後にパンチ肩で割れが発生する。よって、絞り比δは１．２以上２．０以下の範囲が好ましい。
In addition, in order to approach a uniform molding state over the entire circumference of the test piece 1, the test piece 1 is set in the mold so that the center of the test piece 1 and the center of the punch 10 are aligned as shown in FIG. To perform molding.
Here, the tensile stress generated at the flange end after molding is also changed by the drawing ratio δ which is the ratio (D1 / D0) of the diagonal length D1 of the test piece 1 and the diagonal length D0 of the punch 10. Is possible. If the drawing ratio δ is small, it may be difficult to leave the flange at the end of molding, and if the drawing ratio δ is large, the material does not flow in, so a crack occurs at the punch shoulder immediately after molding. Therefore, the aperture ratio δ is preferably in the range of 1.2 to 2.0.
＜水素侵入環境下設置の工程Ｃ＞
工程Ｃでは、工程Ｂで深絞り成形し離型した試験片１を水素侵入環境下（水素侵入雰囲気）に設置して、当該試験片１のフランジ端部における亀裂の発生状況（例えば発生までの時間）によって、上記高張力鋼板の遅れ破壊性を評価する。
試験片１の水素侵入環境下への設置は、例えば、図６（ｂ）のように、塩酸やＮＨ４ＳＣＮ水溶液などの酸液２０を収容した浴槽２１内に、成形後の試験片１（図６（ａ）参照）を浸漬することで実施する。
ここで、本実施形態における高張力鋼板の遅れ破壊特性評価方法では、上記の各工程Ａ〜Ｃに加えて、必要なフランジ端部の最大引張応力を付与させることができる深絞り成形時の絞り比δや成形高さＨを決定する成形条件決定工程Ｄを有する（図１参照）。
次に、その成形条件決定工程Ｄについて説明する。
<Process C in a hydrogen intrusion environment>
In step C, the test piece 1 that has been deep-drawn and released in step B is placed in a hydrogen intrusion environment (hydrogen intrusion atmosphere), and cracks occur at the flange end of the test piece 1 (for example, up to the occurrence). The delayed fracture property of the high-tensile steel sheet is evaluated according to time.
For example, as shown in FIG. 6B, the test piece 1 is placed in a hydrogen intrusion environment in a bathtub 21 containing an acid solution 20 such as hydrochloric acid or an NH 4 SCN aqueous solution, as shown in FIG. It implements by immersing FIG. 6 (a).
Here, in the method for evaluating delayed fracture characteristics of a high-tensile steel plate according to the present embodiment, in addition to the above-described steps A to C, drawing at the time of deep drawing that can give the required maximum tensile stress at the flange end. A molding condition determining step D for determining the ratio δ and the molding height H is included (see FIG. 1).
Next, the molding condition determining step D will be described.
＜成形条件決定工程Ｄ＞
工程Ｄでは、遅れ破壊用の試験片１を作製するための深絞り成形の成形条件を決定する。
遅れ破壊用の試験片１を作製するための、工程Ｂにおける深絞り成形の成形条件は、以下の手順で決定する。
まず、成形高さＨおよび絞り比δと、金型から離型した試験片１のフランジ端部に発生する稜線方向の最大引張残留応力との関係を、使用する金型構成、深絞りした試験片１の形状、試験片１の材料に応じて予め求める。なお、金型からの離型によって、試験片１にはスプリングバックが発生する。
<Molding condition determination step D>
In step D, the molding conditions for deep drawing for producing the test piece 1 for delayed fracture are determined.
The molding conditions for deep drawing in step B for producing the test piece 1 for delayed fracture are determined by the following procedure.
First, the relationship between the molding height H and the drawing ratio δ and the maximum tensile residual stress in the ridge line direction generated at the flange end of the test piece 1 released from the die, the die configuration to be used, and the deep drawing test It calculates | requires previously according to the shape of the piece 1, and the material of the test piece 1. FIG. Note that springback occurs in the test piece 1 by releasing from the mold.
次に、必要な最大引張応力から、上記の関係を用いて深絞り成形条件を決定する。
上記の成形高さＨおよび絞り比δと最大引張残留応力との関係は、例えば図７に示す関係となっている。
図７に示すような、成形高さＨおよび絞り比δと最大引張残留応力との関係は、例えばコンピュータによるシミュレーション解析を用いることが出来る。
ここで、図７に示す関係は、正八角形の試験片１（図２（ａ）参照）と、柱状の断面正多角形状のパンチ１０（図３参照）を用いて、図４に示すような金型構成で深絞り成形した際の上記関係であって、シミュレーション解析で求めた例である。このとき、パンチ１０の対角線長さＤ０を５０ｍｍに設定し、試験片１を、引張強度１４７０ＭＰａ級の冷延鋼板（板厚１．４ｍｍ）から作製して、絞り比δ１．６では成形高さを８．２ｍｍ、絞り比１．８では成形高さを１０．５ｍｍに設定して、成形高さＨとフランジ端部における稜線方向に発生する最大引張残留応力との関係を求めた。なお、シミュレーション解析は２次元のシェル要素を用いて実施した。
Next, the deep drawing molding conditions are determined from the necessary maximum tensile stress using the above relationship.
The relationship between the molding height H and the drawing ratio δ and the maximum tensile residual stress is, for example, the relationship shown in FIG.
As shown in FIG. 7, for example, a computer simulation analysis can be used for the relationship between the molding height H and the drawing ratio δ and the maximum tensile residual stress.
Here, the relationship shown in FIG. 7 is as shown in FIG. 4 using a regular octagonal test piece 1 (see FIG. 2 (a)) and a columnar punch 10 having a regular polygonal cross section (see FIG. 3). This is the above relationship when deep drawing is performed with a mold configuration, and is an example obtained by simulation analysis. At this time, the diagonal length D0 of the punch 10 is set to 50 mm, and the test piece 1 is made from a cold-rolled steel sheet (sheet thickness: 1.4 mm) having a tensile strength of 1470 MPa, and the forming height is as high as the drawing ratio δ1.6. Was set to 8.2 mm and the draw ratio was set to 10.5 mm, and the relationship between the molding height H and the maximum tensile residual stress generated in the ridgeline direction at the flange end was determined. The simulation analysis was performed using a two-dimensional shell element.
上記シミュレーション解析は３次元ソリッドモデルを用いても構わない。また、深絞り成形実験後の試験片１を用いて最大引張残留応力を実際に計測することで、上記関係を求めても良い。
そして、プレス加工した製品において、例えば、最大引張残留応力６００ＭＰａが必要な場合、図７の関係から、絞り比δ＝１．６では成形高さＨ＝８．２ｍｍ、絞り比δ＝１．８では成形高さＨ＝１０．５ｍｍと決定することができる。
このように成形条件決定工程Ｄで求めた成形条件で、上記の深絞り成形によって試験片１を成形し（工程Ｂ）、その所定の最大引張残留応力が付与された試験片１を、水素侵入環境下に置いて当該試験片１の亀裂の発生状況によって高張力鋼板の遅れ破壊性を評価する（工程Ｃ）。ここで、自動車のセンターピラーなどの車体構造部材は、素材の０．２％耐力の５０％〜１５０％の範囲の残留応力が付加されている状態で組み付けられることが多いことから、成形条件決定工程Ｄにおいて、深絞り試験片１の最大引張残留応力を上記の応力範囲内に設定することで、実際の自動車部品に近い状態で遅れ破壊特性の評価を行うことができて、評価精度が向上する。
The simulation analysis may use a three-dimensional solid model. Moreover, you may obtain | require the said relationship by actually measuring the maximum tensile residual stress using the test piece 1 after a deep drawing experiment.
For example, if the maximum tensile residual stress of 600 MPa is required in the pressed product, the molding height H = 8.2 mm and the drawing ratio δ = 1.8 from the relationship shown in FIG. Then, it can be determined that the forming height H = 10.5 mm.
In this way, the test piece 1 is formed by the above-described deep drawing under the molding conditions obtained in the molding condition determination step D (step B), and the test piece 1 to which the predetermined maximum tensile residual stress is applied is hydrogen-invaded. The delayed fracture property of the high-tensile steel sheet is evaluated according to the state of occurrence of cracks in the test piece 1 under the environment (Step C). Here, body condition members such as automobile center pillars are often assembled with a residual stress in the range of 50% to 150% of the 0.2% proof stress of the material. In step D, by setting the maximum tensile residual stress of the deep-drawn test piece 1 within the above-mentioned stress range, it is possible to evaluate delayed fracture characteristics in a state close to that of an actual automobile part, thereby improving the evaluation accuracy. To do.
次に、本発明に基づく実施例について説明する。
板厚１．４ｍｍの１４７０ＭＰａ級鋼板を供試材とし、一連の評価を実施した。表１に供試材の引張特性を示す。
Next, examples according to the present invention will be described.
A series of evaluations were performed using a 1470 MPa class steel plate having a thickness of 1.4 mm as a test material. Table 1 shows the tensile properties of the test materials.
この供試材を用いて、せん断加工で正八角形の試験片１を作製し、柱形状の断面正多角形状のパンチ１０を用いて、図４に示すような金型構成で深絞り成形を実施した。
パンチ１０の寸法（対角線長さ）Ｄ０は５０ｍｍとし、試験片１の寸法Ｄ１は９０ｍｍとし、絞り比δを１．８に設定した。図５のように、試験片１とパンチ１０の中心が一致するようにして、試験片１を金型に設定して、成形高さＨ＝１５ｍｍまで成形した。
Using this specimen, a regular octagonal test piece 1 is produced by shearing, and deep drawing is performed with a die configuration as shown in FIG. did.
The dimension (diagonal length) D0 of the punch 10 was 50 mm, the dimension D1 of the test piece 1 was 90 mm, and the drawing ratio δ was set to 1.8. As shown in FIG. 5, the test piece 1 was set as a mold so that the center of the test piece 1 and the center of the punch 10 coincided with each other, and was molded to a molding height H = 15 mm.
ここで、図７から、成形高さＨ＝１５ｍｍ時の最大引張残留応力は、０．２％耐力のほぼ１００％と同じである。
図７のような成形高さＨおよび絞り比δと、金型から離型した試験片１のフランジ端部に発生する稜線方向の最大引張残留応力との関係は、上述の通りシミュレーション解析により求めた。このとき、解析には有限要素法ソフトウェアＬＳ−ＤＹＮＡ ｖｅｒ．９７１を用いた。
上記プレス成形で作製した試験片１の遅れ破壊特性を評価するため、水素侵入条件下で試験片１を保持した。水素を試験片１に侵入させる条件としては種々あるが、今回はｐＨ１の塩酸に試験片１を浸漬させ、試験片１に水素が十分侵入して飽和状態となると考えられる１００時間保持した。
表２に実験結果を示す。
Here, from FIG. 7, the maximum tensile residual stress at the molding height H = 15 mm is the same as almost 100% of the 0.2% proof stress.
The relationship between the molding height H and the drawing ratio δ as shown in FIG. 7 and the maximum tensile residual stress in the ridge line direction generated at the flange end of the test piece 1 released from the mold is obtained by simulation analysis as described above. It was. At this time, the finite element method software LS-DYNA ver. 971 was used.
In order to evaluate the delayed fracture characteristics of the test piece 1 produced by the press molding, the test piece 1 was held under hydrogen intrusion conditions. There are various conditions for allowing hydrogen to enter the test piece 1, but this time, the test piece 1 was immersed in hydrochloric acid having a pH of 1, and the test piece 1 was held for 100 hours, which is considered to be sufficiently saturated with hydrogen.
Table 2 shows the experimental results.
実験結果から、試験片１のフランジ部１ａの端部に発生する最大残留応力が本供試材の０．２％耐力の５０％である６００ＭＰａ以下の条件下の評価では、遅れ破壊が発生しないが、６００ＭＰａを超える条件では遅れ破壊が発生した。
以上のことから、本発明によれば、プレス加工で製造される高張力鋼板を適用したセンターピラーやサイドシル等の自動車車体構造部材において、使用中に生じる遅れ破壊の可能性を部材の設計段階で適切に評価することが可能となり、自動車車体構造部材を効率的に設計、開発することができることが分かる。
From the experimental results, delayed fracture does not occur in the evaluation under the condition of 600 MPa or less where the maximum residual stress generated at the end of the flange portion 1a of the test piece 1 is 50% of the 0.2% proof stress of the specimen. However, delayed fracture occurred under conditions exceeding 600 MPa.
From the above, according to the present invention, in automobile body structure members such as center pillars and side sills, to which high-tensile steel plates manufactured by press working are applied, the possibility of delayed fracture that occurs during use is determined at the member design stage. It becomes possible to evaluate appropriately, and it turns out that a vehicle body structural member can be designed and developed efficiently.
１ 試験片
１ａ フランジ部（試験片端部）
１０ パンチ
１１ ダイ
１２ ブランクホルダー
２０ 酸液
２１ 浴槽
Ａ 試験片作製の工程
Ｂ 深絞り成形の工程
Ｃ 水素侵入環境下設置の工程
Ｄ 成形条件決定工程
Ｈ 成形高さ
δ 絞り比
1 Test piece 1a Flange (test piece end)
10 Punch 11 Die 12 Blank holder 20 Acid solution 21 Bathtub A Test piece production process B Deep drawing process C Installation process under hydrogen intrusion environment D Molding condition determination process H Molding height δ Drawing ratio
Claims (6)
上記深絞り成形における、成形高さ及び絞り比（試験片の対角線長さ／パンチの対角線長さ）と、上記成形後の試験片のフランジ部に発生する最大引張残留応力との関係を求め、その求めた関係に基づき、上記高張力鋼板と同じ材料及び厚さからなる鋼板を目的とする製品形状に成形することでその製品形状の材料端部に発生する残留応力の範囲内に、上記最大引張残留応力が存在する上記成形高さ及び絞り比の範囲を求め、
上記求めた深絞り成形の成形高さ及び絞り比の範囲内となるように上記深絞り成形条件を設定して、上記遅れ破壊性の評価を実施することを特徴とする高張力鋼板の遅れ破壊特性評価方法。 A test piece made of a high-tensile steel plate with a regular polygon shape with a number of squares or more is subjected to deep drawing with a die equipped with a punch, die and blank holder with a regular polygonal cross section. By evaluating the delayed fracture property of the high-tensile steel sheet according to the state of cracks generated in the flange portion of the test piece by installing the test piece in a hydrogen intrusion environment,
In the deep drawing, the relationship between the forming height and the drawing ratio (diagonal length of the test piece / diagonal length of the punch) and the maximum tensile residual stress generated in the flange portion of the test piece after the forming is obtained. Based on the obtained relationship, the maximum stress within the range of residual stress generated at the material end of the product shape by forming a steel plate made of the same material and thickness as the high-tensile steel plate into the target product shape. Obtain the range of the above molding height and drawing ratio where tensile residual stress exists,
Delayed fracture of a high-tensile steel sheet, characterized in that the above-mentioned deep drawing forming conditions are set so as to be within the range of the obtained deep drawing forming height and drawing ratio, and the delayed fracture property is evaluated. Characterization method.
上記深絞り成形における、成形高さ及び絞り比（試験片の対角線長さ／パンチの対角線長さ）と、上記試験片のフランジ部に発生する最大引張残留応力との関係から、上記最大引張残留応力が上記高張力鋼板の０．２％耐力の５０％以上１５０％以下の範囲に収まる上記最大引張残留応力が存在する上記成形高さ及び絞り比の範囲を求め、
上記求めた深絞り成形の成形高さ及び絞り比の範囲内となるように、上記深絞り成形条件を設定して、上記遅れ破壊性の評価を実施することを特徴とする高張力鋼板の遅れ破壊特性評価方法。 A test piece made of a high-tensile steel plate with a regular polygon shape with a number of squares or more is subjected to deep drawing with a die equipped with a punch, die and blank holder with a regular polygonal cross section. By evaluating the delayed fracture property of the high-tensile steel sheet according to the state of cracks generated in the flange portion of the test piece by installing the test piece in a hydrogen intrusion environment,
From the relationship between forming height and drawing ratio (diagonal length of test piece / diagonal length of punch) and maximum tensile residual stress generated in the flange portion of the test piece in deep drawing, the maximum tensile residual is obtained. The range of the forming height and the drawing ratio in which the maximum tensile residual stress exists within which the stress falls within the range of 50% to 150% of the 0.2% proof stress of the high-tensile steel sheet,
The delay of the high-tensile steel sheet is characterized in that the deep-drawing conditions are set and the delayed fracture property is evaluated so as to be within the range of the forming height and drawing ratio of the deep drawing formed above. Destructive property evaluation method.
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| JP2022014784A (en) * | 2020-07-07 | 2022-01-20 | Ｊｆｅスチール株式会社 | Method for manufacturing test piece and method for evaluating delayed fracture characteristics of high tensile strength steel plate |
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| JP2022014784A (en) * | 2020-07-07 | 2022-01-20 | Ｊｆｅスチール株式会社 | Method for manufacturing test piece and method for evaluating delayed fracture characteristics of high tensile strength steel plate |
| JP7318602B2 (en) | 2020-07-07 | 2023-08-01 | Ｊｆｅスチール株式会社 | METHOD FOR MANUFACTURE OF TEST SPECIMEN AND METHOD FOR EVALUATION OF DELAYED FRACTURE CHARACTERISTICS OF HIGH-STRENGTH STEEL STEEL |
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