JP4580360B2 - Evaluation method of work hardening characteristics of ferritic steel sheet - Google Patents
Evaluation method of work hardening characteristics of ferritic steel sheet Download PDFInfo
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本発明は、フェライト鋼板の塑性変形後の変形組織からその鋼板が有する加工硬化特性を評価する方法に関するものである。 The present invention relates to a method for evaluating work hardening characteristics of a steel sheet from a deformed structure after plastic deformation of a ferritic steel sheet.
フェライト鋼板は、自動車のみならず、家電や建築資材として広範に利用されている。このフェライト鋼板の加工性は、その鋼板を使用して製造される製品や構造物を設計する上で、非常に重要な製品指標である。 Ferritic steel sheets are widely used not only for automobiles but also for home appliances and building materials. The workability of this ferritic steel sheet is a very important product index in designing products and structures manufactured using the steel sheet.
成形後のフェライト鋼板の加工状態は、鋼板の成形性の良否のみならず、使用する鋼板の材質や加工方法の最適化など、鋼板の成形性を検討する上で非常に重要な因子である。 The processing state of the ferritic steel sheet after forming is a very important factor in examining the formability of the steel sheet, such as not only whether the formability of the steel sheet is good but also optimization of the material and processing method of the steel sheet used.
これまで、フェライト鋼板の加工性は、一般的に、引張や圧縮試験などのいわゆる機械的な試験によって評価されてきた。特に、国内では、日本工業規格（ＪＩＳ）などにより、試験方法も規格化されており、これらの試験を通じて、加工性を検討するうえで重要となる応力−歪み曲線を得ることができる。 Until now, the workability of ferritic steel sheets has been generally evaluated by so-called mechanical tests such as tensile and compression tests. In particular, in Japan, test methods are also standardized by Japanese Industrial Standards (JIS) and the like, and stress-strain curves that are important for studying workability can be obtained through these tests.
ところが、最近、開発されつつある高強度フェライト鋼板では、これらの機械的な試験だけでは予測できない加工性に関わる問題が顕在化するようになった。 However, recently, high strength ferritic steel sheets that are being developed have come to manifest problems related to workability that cannot be predicted by these mechanical tests alone.
例えば、自動車用の高強度フェライト鋼板では、その鋼板の高強度化に伴い、一様伸びの減少や、プレス加工時における成形性の不良問題など、同じ組織の鋼板でありながら、加工性に関わる特性が異なるという問題が顕在化してきた。 For example, high strength ferritic steel sheets for automobiles are related to workability even though they are steel sheets of the same structure, such as reduction of uniform elongation and problems of formability during press working as the strength of the steel sheets increases. The problem of different characteristics has become apparent.
例えば、従来から鋼板の加工性を検討するうえで、加工硬化特性は、非常に重要な指標であり、高強度フェライト鋼板に対しても、ｎ値により鋼板の加工性を評価してきた。このｎ値は、応力−歪み曲線を基に、σ：真応力、ε：真歪み、Ｆ：硬化率とする場合に、σ＝Ｆεｎの関係式により求められる鋼板の加工硬化特性を示す指数ｎである。 For example, when studying the workability of a steel sheet, work hardening characteristics are a very important index, and the workability of a steel sheet has been evaluated based on the n value even for high-strength ferritic steel sheets. This n value is an index indicating the work hardening characteristics of a steel sheet obtained from the relational expression of σ = Fε n when σ: true stress, ε: true strain, and F: hardening rate based on the stress-strain curve. n.
すなわち、ｎ値は、加工における硬化の程度を示す材料特性値であり、この値が大きいほど、鋼板の成形限界が向上する。しかし、このｎ値は、引張強度が４００ＭＰａ以下の比較的強度の小さいフェライト鋼板については、そのｎ値と、鋼板が有する加工硬化特性との間に比較的良い一致がみられていたが、４００ＭＰａを超える強度を有する高強度鋼板については、加工硬化特性との対応が必ずしも得られず、高強度鋼板の開発を進めるうえで大きな問題となっていた。 That is, the n value is a material characteristic value indicating the degree of hardening in processing, and the larger this value, the better the forming limit of the steel sheet. However, this n value has a relatively good agreement between the n value and the work-hardening properties of the steel plate for a ferrite steel plate having a relatively low tensile strength with a tensile strength of 400 MPa or less. For high-strength steel sheets having strengths exceeding 1, it is not always possible to achieve work hardening characteristics, which has been a major problem in the development of high-strength steel sheets.
上述のように、引張強度が４００Ｍｐａ以上の高強度フェライト鋼板の加工硬化特性を評価するうえで、その特性を正確に評価する手法の開発が求められていた。 As described above, when evaluating the work hardening characteristics of a high strength ferritic steel sheet having a tensile strength of 400 Mpa or more, development of a method for accurately evaluating the characteristics has been required.
本発明は、上記従来技術の現状を踏まえ、引張強度が４００Ｍｐａ以上の高強度フェライト鋼板について、その加工硬化特性を正確に評価することが可能な方法を提供することを目的とする。 An object of the present invention is to provide a method capable of accurately evaluating the work-hardening characteristics of a high-strength ferritic steel sheet having a tensile strength of 400 Mpa or more based on the current state of the prior art.
フェライト鋼板を成形加工するためには、ある一定以上の応力を負荷して、塑性変形させる必要がある。この塑性変形に伴い、フェライト結晶粒内には転位が形成され、歪みが導入されることとなる。塑性変形初期では、この転位は、粒内に広く分布するが、変形量が多くなると、転位は増殖するとともに、さらに転位同士が絡み合い堆積する。 In order to form a ferritic steel sheet, it is necessary to apply a certain stress or more to cause plastic deformation. With this plastic deformation, dislocations are formed in the ferrite crystal grains, and strain is introduced. In the early stage of plastic deformation, the dislocations are widely distributed in the grains, but when the amount of deformation increases, the dislocations proliferate and the dislocations are further entangled and deposited.
この転位同士の反応に伴い、フェライト結晶粒内に、転位が集積した領域と転位が殆んど存在しない領域が形成され、これは、一般に転位セル構造として理解されている。そして、この転位セル構造の形成が、鋼板のマクロな加工硬化挙動に関わる大きな支配要因のひとつとされていた。 Along with the reaction between the dislocations, a region where dislocations are accumulated and a region where almost no dislocations exist are formed in the ferrite crystal grains, and this is generally understood as a dislocation cell structure. The formation of the dislocation cell structure was regarded as one of the major controlling factors related to the macro work hardening behavior of the steel sheet.
本発明者を含む研究グループは、塑性変形後のフェライト鋼板のミクロ組織について鋭意研究を進めたところ、成形したフェライト鋼板内部のフェライト結晶粒に形成される転位セル構造は、方向性を有する転位セルが並んだ形態の転位セル構造となることが解った。 The research group including the present inventor has conducted earnest research on the microstructure of the ferrite steel sheet after plastic deformation, and the dislocation cell structure formed in the ferrite crystal grains inside the formed ferrite steel sheet is a dislocation cell having directionality. It has been found that a dislocation cell structure in a form in which is arranged.
さらに、これらの転位セル構造の形態と加工硬化特性について調査を進めたところ、塑性変形を受けた部位において観察される転位セル構造の中で、方向性を有する転位セルが並んだ転位セル構造からなるフェライト結晶粒の存在割合と加工硬化特性、特に、一様伸びとの間に良い相関関係があることが解った。 Furthermore, investigations were made on the morphology and work hardening characteristics of these dislocation cell structures. From the dislocation cell structure in which dislocation cells having directionality are arranged among the dislocation cell structures observed at the site subjected to plastic deformation. It has been found that there is a good correlation between the abundance of ferrite crystal grains and work hardening characteristics, particularly uniform elongation.
すなわち、塑性変形を受けた部位において方向性を有する転位セルが並んだ転位セル構造からなるフェライト結晶粒の存在割合が大きくなるほど、一様伸び特性が優れることが解った。さらに、その割合割合を算出することで、成形されたフェライト鋼板の一様伸びを、正確に評価する方法を発明するに至った。 That is, it has been found that the uniform elongation characteristics are more excellent as the proportion of ferrite crystal grains having a dislocation cell structure in which dislocation cells having directionality are arranged in a portion subjected to plastic deformation increases. Furthermore, by calculating the ratio, the inventors have invented a method for accurately evaluating the uniform elongation of the formed ferritic steel sheet.
本発明は、前述の知見に基づいて構成されており、その主旨とするところは、以下の通りである。 The present invention is configured based on the above-mentioned knowledge, and the main points thereof are as follows.
（１）成形されたフェライト鋼板の塑性変形を受けた部位において、同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト結晶粒の存在割合を測定した後、該フェライト結晶粒の存在割合に基づいて、前記フェライト鋼板の加工硬化特性を評価することを特徴とするフェライト鋼板の加工硬化特性の評価方法。 (1) After measuring the abundance ratio of ferrite crystal grains having a dislocation cell structure in which dislocation cells having the same direction are arranged in a portion subjected to plastic deformation of the formed ferrite steel sheet, the abundance ratio of the ferrite crystal grains Based on the above, the work hardening characteristics of the ferritic steel sheet are evaluated.
（２）前記同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト結晶粒の存在割合を電子顕微鏡を用いた観察技術により測定することを特徴とする上記（１）に記載されたフェライト鋼板の加工硬化特性の評価方法。 (2) The ferrite described in (1) above, wherein the proportion of ferrite crystal grains having a dislocation cell structure in which dislocation cells having the same directionality are arranged is measured by an observation technique using an electron microscope. Evaluation method of work hardening characteristics of steel sheet.
本発明によれば、これまでに困難とされてきた、特に、引張強度が４００ＭＰａ以上の高強度フェライト鋼板の加工硬化特性を正確に評価することが可能であり、本発明は、高強度フェライト鋼板の加工性向上のための技術開発における評価方法として寄与するところが大きいものである。 According to the present invention, it is possible to accurately evaluate the work-hardening characteristics of a high-strength ferritic steel sheet that has been considered difficult so far, in particular, a tensile strength of 400 MPa or more. This greatly contributes as an evaluation method in technology development for improving the workability of steel.
以下に、本発明の内容を詳細に説明する。まず、塑性変形に伴いフェライト結晶粒内に形成される転位セル構造について説明する。 The contents of the present invention will be described in detail below. First, a dislocation cell structure formed in a ferrite crystal grain with plastic deformation will be described.
図１に、成形されたフェライト鋼板内部で観察される典型的な転位セル構造を模式的に示す。図１中でフェライト結晶１の粒界２内に示した直線部分が転位セル壁３であり、塑性変形によって転位が高密度に堆積し壁状に存在する領域である。 FIG. 1 schematically shows a typical dislocation cell structure observed inside the formed ferritic steel sheet. In FIG. 1, the straight line portion shown in the grain boundary 2 of the ferrite crystal 1 is the dislocation cell wall 3, which is a region where dislocations are deposited with high density by plastic deformation and exist in a wall shape.
この転位セル壁３は、結晶面である｛１１０｝面、｛１１２｝面、又は、｛１２３｝面のうちひとつの結晶面に沿って存在することが多い。これら転位セル壁３が沿う結晶面は、透過電子顕微鏡で得られる電子回折図形を解析することにより、決定することが可能である。 This dislocation cell wall 3 often exists along one crystal plane of {110} plane, {112} plane, or {123} plane which is a crystal plane. The crystal plane along which these dislocation cell walls 3 are aligned can be determined by analyzing an electron diffraction pattern obtained with a transmission electron microscope.
本発明では、一対の転位セル壁で仕切られた領域を転位セルとし、図１に示すような直線状に観察される転位セル壁３からなるものを、特に、方向性を有する転位セルと定義する。 In the present invention, a region divided by a pair of dislocation cell walls is defined as a dislocation cell, and a dislocation cell wall 3 observed in a straight line as shown in FIG. 1 is defined as a dislocation cell having directionality. To do.
塑性変形に伴い、フェライト結晶粒内に転位セルが形成されるが、各フェライト結晶粒にかかる応力の方向とその粒が持つ結晶方位との関係、さらには、フェライト結晶１内で活動するすべり系などに関係して、フェライト結晶１内に形成される転位セル構造の形態が異なることが知られている。 With plastic deformation, dislocation cells are formed in the ferrite crystal grains. The relationship between the direction of stress applied to each ferrite crystal grain and the crystal orientation of the grain, and the slip system that is active in the ferrite crystal 1 It is known that the form of the dislocation cell structure formed in the ferrite crystal 1 is different.
但し、その詳細メカニズムは明らかとなっておらず、今後の研究が待たれるところである。 However, the detailed mechanism has not been clarified, and future research is awaited.
本発明者らの詳細な検討の結果、種々の転位セル構造の形態で、図１に示すような転位セル壁３が直線状である、同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト粒４の存在割合は、鋼板の加工硬化特性を示す指標の中で、特に、一様伸びとの間に良い相関関係があることを確認した。 As a result of detailed investigations by the present inventors, from the dislocation cell structure in which dislocation cell walls 3 as shown in FIG. 1 are linear and dislocation cells having the same direction are arranged in various dislocation cell structure forms. It was confirmed that the abundance ratio of the ferrite grains 4 has a good correlation with the uniform elongation among the indexes indicating the work hardening characteristics of the steel sheet.
なお、転位セル構造の形態として、２種以上の異なる方向性を有する転位セルが並んだ転位セル構造５、又は、転位セル壁が曲線状などである方向性を有しない転位セルで構成された転位セル構造は、鋼板の一様伸びとの対応関係が乏しい。 In addition, as a form of the dislocation cell structure, the dislocation cell structure 5 in which dislocation cells having two or more different directions are arranged, or a dislocation cell having a direction in which the dislocation cell wall has a curved shape is formed. The dislocation cell structure has poor correspondence with the uniform elongation of the steel sheet.
本発明は、上記知見を基になされたものであり、成形されたフェライト鋼板の塑性変形を受けた部位において観察される転位セル構造の中で、同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト粒４の存在割合を測定し、その存在割合から、このフェライト鋼板の一様伸びを評価することを特徴とする。 The present invention has been made on the basis of the above knowledge, and among the dislocation cell structures observed in the part of the formed ferritic steel sheet that has undergone plastic deformation, the dislocation cells are arranged with dislocation cells having the same direction. The present invention is characterized in that the existence ratio of the ferrite grains 4 having a structure is measured, and the uniform elongation of the ferrite steel sheet is evaluated from the existence ratio.
本発明の評価法によれば、成形されたフェライト鋼板の塑性変形を受けた部位において、同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト粒４の存在割合が大きい場合に、そのフェライト鋼板の一様伸びも大きな値を示す。 According to the evaluation method of the present invention, when the proportion of ferrite grains 4 having a dislocation cell structure in which dislocation cells having the same direction are arranged is large in a portion subjected to plastic deformation of a formed ferritic steel sheet, The uniform elongation of the ferritic steel sheet also shows a large value.
成形されたフェライト鋼板の一様伸びを評価する際には、予め、既知の試験材の引張加工試験を行い、上記転位セル構造からなるフェライト粒４の存在割合と、この鋼板の一様伸びとの対応関係を求めておき、この関係を利用して、実際の鋼板を成形した後に、この鋼板の一様伸びを評価するのが好ましい。 When evaluating the uniform elongation of the formed ferritic steel sheet, a tensile test of a known test material is performed in advance, the abundance ratio of the ferrite grains 4 composed of the dislocation cell structure, and the uniform elongation of the steel sheet It is preferable to evaluate the uniform elongation of the steel plate after the actual steel plate is formed using this relationship.
この同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト粒４の存在割合の測定は、透過電子顕微鏡を用いる転位セル構造の観察により行うことができる。 Measurement of the abundance ratio of ferrite grains 4 having a dislocation cell structure in which dislocation cells having the same directionality are arranged can be performed by observing the dislocation cell structure using a transmission electron microscope.
この透過電子顕微鏡を用いて、成形したフェライト鋼板のミクロ組織における転位セル構造を観察する方法について説明する。 A method for observing the dislocation cell structure in the microstructure of the formed ferritic steel sheet using this transmission electron microscope will be described.
まず、厚み０．１μｍ以下の観察用の薄片試料を準備し、その薄片試料を透過電子顕微鏡の鏡筒内に装入し、薄片試料に電子線を照射する。透過電子顕微鏡法は、照射された電子線の中で、薄片試料中を透過した電子を利用して結像し、鋼板内部に形成された三次元のミクロ組織を二次元画像として投影、観察する手法である。 First, a thin sample for observation having a thickness of 0.1 μm or less is prepared, the thin sample is inserted into a lens barrel of a transmission electron microscope, and the thin sample is irradiated with an electron beam. Transmission electron microscopy uses an electron beam that has been transmitted through a thin sample in an irradiated electron beam to form an image, and the three-dimensional microstructure formed inside the steel sheet is projected and observed as a two-dimensional image. It is a technique.
まず、透過電子顕微鏡用の観察試料の作製方法について、図２を用いて説明する。 First, a method for producing an observation sample for a transmission electron microscope will be described with reference to FIG.
引張試験により引張方向９に塑性変形を受けた引張試験片６の中心部から、精密切断機などを用いて、適当なサイズ（Ｌ：１０ｍｍ×Ｗ：１０ｍｍ）を有する板状の微小試料片７を取り出す。 A plate-like micro sample piece 7 having an appropriate size (L: 10 mm × W: 10 mm) is used from the center of the tensile test piece 6 that has undergone plastic deformation in the tensile direction 9 by a tensile test using a precision cutting machine or the like. Take out.
次に、切断された微小試料片７から、エメリー紙などの研磨紙を用いる機械研磨により、微小試料片７の板厚中心１／２ｔの領域から、厚みＴ：１００μｍ前後の試料片８を作製する。 Next, a sample piece 8 having a thickness T of about 100 μm is produced from the cut fine sample piece 7 by mechanical polishing using polishing paper such as emery paper from the region of the thickness center 1 / 2t of the fine sample piece 7. To do.
この箔状の試料片８から専用のパンチを用いて、直径Ｄ：３ｍｍφのディスク状のサンプルを準備する。続いて、電解液を用いた電解研磨法又はイオンミリング法により、透過電子顕微鏡観察用の薄膜試料を作製する。 A disk-shaped sample having a diameter D of 3 mmφ is prepared from the foil-shaped sample piece 8 using a dedicated punch. Subsequently, a thin film sample for observation with a transmission electron microscope is prepared by an electrolytic polishing method using an electrolytic solution or an ion milling method.
次に、透過電子顕微鏡によりフェライト結晶粒内の転位セル構造を観察する方法について説明する。上記作製手順で準備した薄片試料を、透過電子顕微鏡専用の試料ホルダーの先端に固定し、顕微鏡本体の試料室に装入する。装入後、電子線を発生させ、薄膜試料に電子線を照射する。 Next, a method for observing the dislocation cell structure in the ferrite crystal grains with a transmission electron microscope will be described. The thin sample prepared in the above production procedure is fixed to the tip of a sample holder dedicated to a transmission electron microscope, and is loaded into the sample chamber of the microscope main body. After charging, an electron beam is generated and the thin film sample is irradiated with the electron beam.
観察室内の蛍光板上に投影された電子顕微鏡像を観察することより、フェライト結晶粒内の転位セル構造を観察する。転位セル構造において、転位セル壁の周辺は、フェライトの結晶構造が微視的に乱れた領域となる。このため、転位セル構造が存在する領域では、直進する電子線が散乱することとなり、顕微鏡に具備された対物絞りを用いて透過波のみを結像すれば、特に、転位セル壁は、黒いコントラストとして観察することができる。 By observing an electron microscope image projected on the fluorescent screen in the observation chamber, the dislocation cell structure in the ferrite crystal grains is observed. In the dislocation cell structure, the periphery of the dislocation cell wall is a region in which the crystal structure of ferrite is microscopically disturbed. For this reason, in the region where the dislocation cell structure exists, the electron beam traveling straight is scattered, and if only the transmitted wave is imaged using the objective aperture provided in the microscope, the dislocation cell wall has a black contrast. Can be observed.
必要に応じて、観察された転位セル構造を電子顕微鏡専用のフィルムへ撮影、又は、ＣＣＤカメラを用いてデジタル画像として保存する。なお、転位セル構造を観察する装置として、透過電子顕微鏡以外に、電子線を数ｎｍ程度のビーム径に絞って薄片試料に走査しながら顕微鏡像を観察する走査型透過電子顕微鏡を使用してもよい。 If necessary, the observed dislocation cell structure is photographed on a film dedicated to an electron microscope, or stored as a digital image using a CCD camera. As a device for observing the dislocation cell structure, in addition to the transmission electron microscope, a scanning transmission electron microscope that observes a microscopic image while scanning a thin piece sample with a beam diameter of about several nanometers can be used. Good.
転位セル構造の形態調査における像倍率は、観察を行うフェライト結晶粒のサイズを考慮し、５０００〜２００００倍の範囲が好ましい。転位セル構造の形態調査では、転位セル構造が観察されるフェライト結晶粒の粒数をカウントするとともに、一方向に並んだ転位セル構造が観察されるフェライト結晶粒の粒数をカウントし、その割合を算出すればよい。 The image magnification in the dislocation cell structure morphology investigation is preferably in the range of 5000 to 20000 times in consideration of the size of ferrite crystal grains to be observed. In the morphology investigation of the dislocation cell structure, the number of ferrite crystal grains in which the dislocation cell structure is observed is counted, and the number of ferrite crystal grains in which the dislocation cell structure aligned in one direction is counted, and the ratio May be calculated.
加工硬化特性を正確に評価する際の望ましい変形組織として、明瞭な転位セル構造が観察される必要がある。変形量が１０％以上のフェライト鋼板では、変形組織が発達し、転位セル構造が観察されるが、変形量が１０％未満では、フェライト結晶粒内にランダムに分布する転位が観察されるのみで、明瞭な方向性を有する転位セル構造は観察できない。 A clear dislocation cell structure needs to be observed as a desirable deformation structure in accurately evaluating work hardening characteristics. In a ferrite steel sheet with a deformation amount of 10% or more, a deformation structure develops and a dislocation cell structure is observed. However, when the deformation amount is less than 10%, only dislocations randomly distributed in the ferrite crystal grains are observed. A dislocation cell structure having a clear direction cannot be observed.
このため、変形組織から加工硬化特性を正確に評価するためには、評価する部位の変形量が１０％以上であることが望ましい。 For this reason, in order to accurately evaluate the work hardening characteristics from the deformed tissue, it is desirable that the deformation amount of the part to be evaluated is 10% or more.
また、フェライト単相鋼板のみならず、フェライト結晶粒内に析出物や介在物を含む鋼板についても、今回発明した評価方法により、その加工硬化特性を評価できる。 Further, not only the ferrite single-phase steel sheet but also the steel sheet containing precipitates and inclusions in the ferrite crystal grains can be evaluated for work hardening characteristics by the evaluation method invented this time.
以下、本発明のフェライト鋼板の加工硬化特性の評価方法の一実施例について説明する。 Hereinafter, an example of a method for evaluating work hardening characteristics of a ferritic steel sheet according to the present invention will be described.
表１に示す機械的特性を有する各種フェライト鋼板から、ＪＩＳ１３号Ｂ型の圧延方向の引張試験片を準備した。さらに、これらの試験片について引張試験機を用いて、変形量３０％の引張試験を実施した。公称相当歪み速度は、１０-3／ｓｅｃである。 From various ferritic steel sheets having mechanical properties shown in Table 1, JIS No. 13 B type tensile test pieces in the rolling direction were prepared. Further, a tensile test with a deformation amount of 30% was performed on these test pieces using a tensile tester. The nominal equivalent strain rate is 10 −3 / sec.
次に、引張試験後の試料片から、変形組織観察のための薄片試料を作製した。図２は、その作製手順を示す模式図である。 Next, a thin sample for observing a deformed structure was prepared from the sample piece after the tensile test. FIG. 2 is a schematic diagram showing the manufacturing procedure.
まず、引張試験により引張方向９に塑性変形を受けた引張試験片６の中心部から精密切断機などを用いて、サイズＬ：１０ｍｍ×Ｗ：１０ｍｍの板状の微小試料片７を切り出した。切断された微小試料片７をエメリー紙などの研磨紙を用いて機械研磨し、板厚中心１／２ｔの領域から厚みＴ：１００μｍ前後の試料片８を作製した。 First, a plate-shaped minute sample piece 7 having a size L: 10 mm × W: 10 mm was cut out from the center of the tensile test piece 6 that was plastically deformed in the tensile direction 9 by a tensile test. The cut micro sample piece 7 was mechanically polished using polishing paper such as emery paper, and a sample piece 8 having a thickness T of about 100 μm from the region of the thickness center 1 / 2t was produced.
この箔状の試料片８から、専用のパンチを用いて、直径Ｄ：３ｍｍφのディスク状のサンプルを準備した。続いて、過塩素酸５％−酢酸９５％溶液を用いる電解研磨法により、透過電子顕微鏡観察用の薄片試料を作製した。 A disk-shaped sample having a diameter D of 3 mmφ was prepared from the foil-shaped sample piece 8 using a dedicated punch. Subsequently, a thin sample for observation with a transmission electron microscope was prepared by an electropolishing method using a 5% perchloric acid-95% acetic acid solution.
次に、上記の手順で作製を行った薄片試料を、加速電圧２００ｋＶの透過電子顕微鏡に装入し、フェライト結晶粒内に形成された転位セル構造の形態を、鋼板の板面方向から調査した。転位セル構造の形態調査における像倍率は、５０００〜１００００倍の範囲で実施した。また、対物絞りを用いた明視野法により、転位セル構造に対してコントラストを強調した条件下で像観察を行なった。 Next, the thin piece sample prepared according to the above procedure was inserted into a transmission electron microscope with an acceleration voltage of 200 kV, and the form of the dislocation cell structure formed in the ferrite crystal grains was investigated from the plate surface direction of the steel plate. . The image magnification in the form investigation of the dislocation cell structure was in the range of 5000 to 10,000 times. In addition, image observation was performed under a condition in which contrast was enhanced for the dislocation cell structure by a bright field method using an objective aperture.
図３は、同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト結晶粒の典型的な透過電子顕微鏡写真を示す図である。 FIG. 3 is a diagram showing a typical transmission electron micrograph of ferrite crystal grains having a dislocation cell structure in which dislocation cells having the same directionality are arranged.
さらに、転位セル構造が観察されるフェライト結晶粒の粒数をカウントするとともに、同一方向性を有する転位セルが並んだ転位セル構造が観察されるフェライト結晶粒の粒数をカウントした。本実施例では、データの記録を目的に観察されたフェライト結晶粒内の転位セル構造を、透過電子顕微鏡に装着した専用のＣＣＤカメラを用いてデジタル画像として、パーソナルコンピューターに適宜保存した。 Further, the number of ferrite crystal grains in which a dislocation cell structure was observed was counted, and the number of ferrite crystal grains in which a dislocation cell structure in which dislocation cells having the same direction were arranged was counted. In this example, the dislocation cell structure in the ferrite crystal grains observed for the purpose of data recording was appropriately stored in a personal computer as a digital image using a dedicated CCD camera attached to a transmission electron microscope.
以上の方法により、観察されるフェライト結晶粒の粒数に対する同一方向性を有する転位セル構造が観察されるフェライト結晶粒の粒数の割合を百分率で求めた。本実施例では、各種鋼板に対し、それぞれ１００個以上のフェライト結晶粒について調査を行った。得られた調査結果を表２に示す。 By the above method, the ratio of the number of ferrite crystal grains in which the dislocation cell structure having the same directionality with respect to the number of ferrite crystal grains to be observed was observed was obtained as a percentage. In this example, 100 or more ferrite crystal grains were investigated for each steel sheet. The obtained survey results are shown in Table 2.
表２には、各鋼板の加工性として一様伸びの値及びｎ値も示した。さらに、表２をもとに、ｎ値と一様伸びの関係及び同一方向性を有する転位セル構造からなるフェライト結晶粒の存在割合と一様伸びの関係を、それぞれプロットしたグラフを、図４（ａ）及び図４（ｂ）に示す。これらのグラフには、回帰分析によって得られた回帰直線も併せて示した（図中の実線、参照）。 Table 2 also shows the uniform elongation value and the n value as the workability of each steel sheet. Further, based on Table 2, a graph plotting the relationship between the n value and the uniform elongation and the relationship between the proportion of ferrite crystal grains made of dislocation cell structures having the same direction and the uniform elongation, respectively, is shown in FIG. It is shown in (a) and FIG. These graphs also show the regression line obtained by the regression analysis (see solid line in the figure).
図４（ａ）から、引張強さが３８０ＭＰａの比較的低強度の鋼板では、ｎ値は、一様伸びと良く対応しているものの、引張強さが４４０ＭＰａより大きな高強度鋼板では、両者に相関が見られないケースがみられる。なお、直線回帰から得られた相関係数Ｒは、０．９４９であった。 From FIG. 4 (a), in a relatively low strength steel plate with a tensile strength of 380 MPa, the n value corresponds well with uniform elongation, but in a high strength steel plate with a tensile strength greater than 440 MPa, There are cases where there is no correlation. The correlation coefficient R obtained from linear regression was 0.949.
一方、本発明法により、フェライト鋼板の変形組織の観察を通して得られた同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト結晶粒の存在割合は、一様伸びとも良く対応しており（図４（ｂ）、参照）、相関係数Ｒも０．９９１と良い相関を示す結果が得られた。 On the other hand, according to the method of the present invention, the existence ratio of ferrite crystal grains composed of dislocation cell structures in which dislocation cells having the same direction obtained by observing deformation structures of ferritic steel sheets correspond well with uniform elongation. (See FIG. 4B.) The correlation coefficient R was 0.991, indicating a good correlation.
以上の結果から、本発明評価法、つまり、成形されたフェライト鋼板の塑性変形を受けた部位における同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト結晶粒の存在割合を基にした鋼板の一様伸びの評価方法は、特に、引張強度が４００ＭＰａ以上の高強度フェライト鋼板の加工硬化特性を精度良く評価するために有効な方法であることは明らかである。 Based on the above results, the evaluation method of the present invention, that is, based on the existence ratio of ferrite crystal grains having a dislocation cell structure in which dislocation cells having the same direction in a portion subjected to plastic deformation of a formed ferritic steel sheet are arranged. It is clear that the method for evaluating the uniform elongation of a steel plate is an effective method for accurately evaluating the work hardening characteristics of a high strength ferritic steel plate having a tensile strength of 400 MPa or more.
前述したように、本発明によれば、引張強度が４００ＭＰａ以上の高強度フェライト鋼板の加工硬化特性を正確に評価することができ、高強度フェライト鋼板の技術開発に寄与するところが大きい。したがって、本発明は、鋼板技術において、利用可能性が高いものである。 As described above, according to the present invention, work hardening characteristics of a high strength ferritic steel sheet having a tensile strength of 400 MPa or more can be accurately evaluated, which greatly contributes to technical development of the high strength ferritic steel sheet. Therefore, the present invention has high applicability in steel sheet technology.
１ フェライト結晶
２ 粒界
３ 転位セル壁
４ 同一方向性を有する転位セルが並んだ転位セル構造からなるフェライト粒
５ ２種以上の異なる方向性を有する転位セルが並んだ転位セル構造からなるフェライト粒
６ 引張試験片
７ 微小試料片
８ 試料片
９ 引張方向
１０ 観察方向
Ｌ 微小試験片の長さ
Ｗ 微小試験片の幅
Ｔ 機械研磨後の試料片の厚み
Ｄ ディスク状のサンプルの直径
DESCRIPTION OF SYMBOLS 1 Ferrite crystal 2 Grain boundary 3 Dislocation cell wall 4 Ferrite grain which consists of a dislocation cell structure in which dislocation cells having the same direction are arranged 5 Ferrite grain which has a dislocation cell structure in which dislocation cells having two or more different orientations are arranged 6 Tensile specimen 7 Micro specimen 8 specimen 9 Tensile direction 10 Observation direction L Length of micro specimen W Width of micro specimen T Thickness of specimen after mechanical polishing D Diameter of disk-shaped sample
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|---|---|---|---|---|
| JPS6091247A (en) * | 1983-10-25 | 1985-05-22 | Nippon Steel Corp | Crystal azimuth analyzing method and its apparatus |
| JPH02268262A (en) * | 1989-04-11 | 1990-11-01 | Nippon Steel Corp | Method and apparatus for analyzing crystal orientation |
| JP2001342544A (en) * | 2000-02-18 | 2001-12-14 | Kobe Steel Ltd | Wire or rod steel suppressed arising of deformation resistance at room temperature and in area of working heat generation |
| JP2003014699A (en) * | 2001-04-27 | 2003-01-15 | Nippon Steel Corp | Method and system for measuring voltage pulse width of barkfausen noise |
| JP2003014697A (en) * | 2001-04-27 | 2003-01-15 | Nippon Steel Corp | Method for diagnosing material of ferromagnetic body and system for measuring voltage pulse width of barkhausen noise |
| JP2007263739A (en) * | 2006-03-28 | 2007-10-11 | Nippon Steel Corp | Evaluation method of amount of distortion in local area of formed ferrite steel plate |
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| JPS6091247A (en) * | 1983-10-25 | 1985-05-22 | Nippon Steel Corp | Crystal azimuth analyzing method and its apparatus |
| JPH02268262A (en) * | 1989-04-11 | 1990-11-01 | Nippon Steel Corp | Method and apparatus for analyzing crystal orientation |
| JP2001342544A (en) * | 2000-02-18 | 2001-12-14 | Kobe Steel Ltd | Wire or rod steel suppressed arising of deformation resistance at room temperature and in area of working heat generation |
| JP2003014699A (en) * | 2001-04-27 | 2003-01-15 | Nippon Steel Corp | Method and system for measuring voltage pulse width of barkfausen noise |
| JP2003014697A (en) * | 2001-04-27 | 2003-01-15 | Nippon Steel Corp | Method for diagnosing material of ferromagnetic body and system for measuring voltage pulse width of barkhausen noise |
| JP2007263739A (en) * | 2006-03-28 | 2007-10-11 | Nippon Steel Corp | Evaluation method of amount of distortion in local area of formed ferrite steel plate |
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