JP6798384B2 - High-strength, high-ductility steel sheet and its manufacturing method - Google Patents
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Description
本発明は、強度と延性に優れた高強度高延性鋼板およびその製造方法に関するものであり、残留オーステナイトの三次元分布形態を制御した高強度高延性鋼板およびその製造方法に関するものである。 The present invention relates to a high-strength high-ductility steel sheet having excellent strength and ductility and a method for producing the same, and relates to a high-strength high-ductility steel sheet having a controlled three-dimensional distribution form of retained austenite and a method for producing the same.
近年、自動車材料の軽量化のために、引張強度が１２７０ＭＰａ級の極めて高い強度を有する高強度鋼板の適用が要求されている。この高強度材料の一つとして、ＴＲＩＰ（Ｔｒａｎｓｆｏｒｍａｔｉｏｎ Ｉｎｄｕｃｅｄ Ｐｌａｓｔｉｃｉｔｙ）鋼の適用が検討されている。ＴＲＩＰ鋼は、残留オーステナイトが歪によりマルテンサイトへ変態する変態誘起塑性効果を利用した、延性および成形性に優れた鋼板である。 In recent years, in order to reduce the weight of automobile materials, it has been required to apply a high-strength steel sheet having an extremely high tensile strength of 1270 MPa class. As one of the high-strength materials, the application of TRIP (Transformation Induced Plasticity) steel is being studied. TRIP steel is a steel sheet having excellent ductility and formability by utilizing a transformation-induced plastic effect in which retained austenite is transformed into martensite by strain.
ＴＲＩＰ鋼は残留オーステナイトの加工によるマルテンサイトへの誘起変態を利用しているため、その特性は、マルテンサイト変態しにくい残留オーステナイトに大きく影響される。残留オーステナイトの安定性が低いと、変形時に容易にマルテンサイト変態するために、材料の硬化が早く伸びが低減する。このため、変形に対してマルテンサイト変態しにくい残留オーステナイトが鋼材中に存在することが必要である。したがって、より一層の材料特性の向上と制御のためには、変態しにくい残留オーステナイトの評価や解析が重要である。また、ＴＲＩＰ鋼の機械特性は、三次元的な変形挙動によって残留オーステナイトに加わる応力の影響を受けるため、残留オーステナイトの体積率だけでなく、ミクロ組織中での三次元形状やサイズ等といった形態評価がより一層重要になっている。 Since TRIP steel utilizes the induced transformation to martensite by processing retained austenite, its properties are greatly affected by retained austenite, which is resistant to martensitic transformation. When the stability of retained austenite is low, the material is cured quickly and its elongation is reduced because it is easily transformed into martensite during deformation. Therefore, it is necessary that retained austenite, which is resistant to martensitic transformation with respect to deformation, is present in the steel material. Therefore, in order to further improve and control the material properties, it is important to evaluate and analyze retained austenite, which is difficult to metamorphose. In addition, since the mechanical properties of TRIP steel are affected by the stress applied to retained austenite due to the three-dimensional deformation behavior, not only the volume fraction of retained austenite but also the morphological evaluation such as the three-dimensional shape and size in the microstructure are evaluated. Is becoming even more important.
例えば、特許文献１には集束イオンビーム（ＦＩＢ）加工とＳＥＭ像取得を連続的に繰り返し、鋼材表層部のミクロ組織を三次元観察する方法が開示されている。また、非特許文献１には、ＦＩＢ加工とＳＥＭ像取得の後に、さらに電子線後方散乱回折法（ＥＢＳＤ）像の測定と解析を連続的に繰り返し、共析鋼中のセメンタイトの三次元構造解析に適用した例が開示されている。 For example, Patent Document 1 discloses a method of three-dimensionally observing the microstructure of the surface layer of a steel material by continuously repeating focused ion beam (FIB) processing and SEM image acquisition. Further, in Non-Patent Document 1, after FIB processing and SEM image acquisition, measurement and analysis of electron backscatter diffraction (EBSD) image are continuously repeated to analyze the three-dimensional structure of cementite in eutectoid steel. An example applied to is disclosed.
しかしながら、特許文献１では、残留オーステナイトの安定性を考慮しておらず、また、残留オーステナイトの三次元分布形態のＴＲＩＰ鋼板の機械特性への影響について検討されていない。 However, Patent Document 1 does not consider the stability of retained austenite, and does not study the influence of the three-dimensional distribution form of retained austenite on the mechanical properties of the TRIP steel sheet.
そこで本発明は、上記実情に鑑み、残留オーステナイトを制御し、強度と延性に優れた高強度高延性鋼板およびその製造方法を提供することを目的とする。 Therefore, in view of the above circumstances, it is an object of the present invention to provide a high-strength, high-ductility steel sheet which controls retained austenite and has excellent strength and ductility, and a method for producing the same.
本発明者らは鋭意検討した結果、ミクロ組織が、残留オーステナイトを１０〜３０ｖｏｌ％以上含み、この残留オーステナイトのうち、三次元構造における最長のフェレ径に対する最短のフェレ径の比（アスペクト比）が２以上の残留オーステナイトの割合が６０％以上であることにより、強度と延性に優れた高強度高延性鋼板を得られることを見出した。さらに、本発明の高強度高延性鋼板は、所定のＦＩＢ条件でイオン照射を行った際、イオン照射前の残留オーステナイトと比べて面積率で４０％以上の残留オーステナイトがイオン照射後に残存することもわかった。 As a result of diligent studies by the present inventors, the microstructure contains 10 to 30 vol% or more of retained austenite, and the ratio (aspect ratio) of the shortest ferret diameter to the longest ferret diameter in the three-dimensional structure of the retained austenite is It has been found that a high-strength, high-ductility steel plate having excellent strength and ductility can be obtained when the ratio of the retained austenite of 2 or more is 60% or more. Further, in the high-strength and high-ductility steel sheet of the present invention, when ion irradiation is performed under predetermined FIB conditions, residual austenite having an area ratio of 40% or more may remain after ion irradiation as compared with retained austenite before ion irradiation. all right.
本発明は、かかる知見に基づき、さらに検討を加えて完成されたものである。本発明の要旨は次のとおりである。
［１］質量％で、Ｃ：０．２０〜０．４０％、Ｓｉ：１．０〜２．０％、Ｍｎ：１．５〜２．５％、Ｐ：０．０２５％以下、Ｓ：０．００４％以下、Ａｌ：０．１％以下、Ｎ：０．０１％以下を含有し、残部Ｆｅおよび不可避的不純物からなる成分組成であり、残留オーステナイトを１０〜３０ｖｏｌ％含み、前記残留オーステナイトのうち、アスペクト比が２以上の残留オーステナイトの割合が６０％以上であり、さらに、イオンの加速電圧：８ｋＶ以下、下記式（１）を満たすイオンｄｏｓｅ量のイオンビームを鋼板表面に対して垂直方向にイオン照射を行ったとき、イオン照射前の残留オーステナイトの面積率と比べて４０％以上の残留オーステナイトがイオン照射後に残存することを特徴とする高強度高延性鋼板。
なお、アスペクト比とは、三次元構造における最長のフェレ径に対する最短のフェレ径の比である。
ｂ＝３０−１０×ｌｎ（ａ）・・・（１）
ａ：イオンの加速電圧（ｋＶ）
ｂ：イオンｄｏｓｅ量（ｐＣ／（μｍ）２）
［２］前記アスペクト比が２以上の残留オーステナイトにおいて、最長のフェレ径が５００ｎｍ以下である残留オーステナイトが４０％以上であることを特徴とする［１］に記載の高強度高延性鋼板。
［３］［１］に記載の成分組成を有する鋼素材を加熱した後、熱間圧延し、前記熱間圧延終了後冷却して巻取り、次いで冷間圧延を施した後、
８００℃〜９５０℃に加熱し、直ちに冷却速度１０℃／ｓ以上で３００〜５００℃まで冷却し、１００〜１０００ｓ保持する第一の熱処理と、
７００〜８５０℃に加熱し、直ちに冷却速度１０℃／ｓ以上で３００〜５００℃まで冷却し、１００〜１０００ｓ保持する第二の熱処理
からなる焼鈍を施すことを特徴とする高強度高延性鋼板の製造方法。
［４］前記第二の熱処理は、複数回行うことを特徴とする［３］に記載の高強度高延性鋼板の製造方法。
The present invention has been completed with further studies based on such findings. The gist of the present invention is as follows.
[1] By mass%, C: 0.25 to 0.40%, Si: 1.0 to 2.0%, Mn: 1.5 to 2.5%, P: 0.025% or less, S: It is a component composition containing 0.004% or less, Al: 0.1% or less, N: 0.01% or less, the balance Fe and unavoidable impurities, and contains 10 to 30 vol% of retained austenite. Among them, the ratio of retained austenite having an aspect ratio of 2 or more is 60% or more, and an ion beam having an ion acceleration voltage of 8 kV or less and an ion dose amount satisfying the following formula (1) is perpendicular to the surface of the steel plate. A high-strength, high-ductility steel plate characterized in that when ion irradiation is performed in a direction, 40% or more of retained austenite remains after ion irradiation as compared with the area ratio of retained austenite before ion irradiation.
The aspect ratio is the ratio of the shortest ferret diameter to the longest ferret diameter in the three-dimensional structure.
b = 30-10 × ln (a) ... (1)
a: Ion accelerating voltage (kV)
b: Amount of ion dose (pC / (μm) 2 )
[2] The high-strength, high-ductility steel sheet according to [1], wherein the retained austenite having the longest ferret diameter of 500 nm or less is 40% or more in the retained austenite having an aspect ratio of 2 or more.
[3] The steel material having the component composition according to [1] is heated, then hot-rolled, cooled after the hot-rolling is completed, wound up, and then cold-rolled.
The first heat treatment of heating to 800 ° C. to 950 ° C., immediately cooling to 300 to 500 ° C. at a cooling rate of 10 ° C./s or higher, and holding for 100 to 1000 s,
A high-strength, high-ductility steel sheet that is heated to 700 to 850 ° C., immediately cooled to 300 to 500 ° C. at a cooling rate of 10 ° C./s or higher, and annealed by a second heat treatment that holds 100 to 1000 s. Production method.
[4] The method for producing a high-strength, high-ductility steel sheet according to [3], wherein the second heat treatment is performed a plurality of times.
本発明によれば、強度と延性に優れた高強度高延性鋼板が得られる。 According to the present invention, a high-strength, high-ductility steel sheet having excellent strength and ductility can be obtained.
なお、本発明における高強度高延性とは、引張強度（ＴＳ）が１２００ＭＰａ以上であり、かつＴＳと全伸び（Ｅｌ）との積であるＴＳ（ＭＰａ）×Ｅｌ（％）が、２４０００ＭＰａ・％以上であることをいう。 The high strength and high ductility in the present invention means that the tensile strength (TS) is 1200 MPa or more and the product of TS and total elongation (El) is TS (MPa) × El (%) of 24000 MPa ·%. That is all.
まず、本発明の高強度高延性鋼板の組成限定理由について説明する。なお、組成における質量％は、とくに断らない限り、単に％で記す。 First, the reason for limiting the composition of the high-strength, high-ductility steel sheet of the present invention will be described. Unless otherwise specified, the mass% in the composition is simply expressed as%.
Ｃ：０．２０〜０．４０％
炭素は材料の強度とともに、変形に対してマルテンサイト変態しにくい残留オーステナイトの存在（残留オーステナイトの安定化）のために必要な元素である。Ｃが低いと引張強度１２００ＭＰａ以上の実現や安定した残留オーステナイトの形成が出来ない。このため、Ｃは０．２０％以上とする。また、Ｃが０．４０％を超えると、実材料として溶接性に問題がある。したがって、Ｃは０．２０〜０．４０％とする。好ましくは、０．２５〜０．３４％以下である。
C: 0.20 to 0.40%
Carbon is an element necessary for the strength of the material and the presence of retained austenite (stabilization of retained austenite) that is resistant to martensitic transformation with respect to deformation. If C is low, it is not possible to achieve a tensile strength of 1200 MPa or more and to form stable retained austenite. Therefore, C is set to 0.20% or more. Further, when C exceeds 0.40%, there is a problem in weldability as an actual material. Therefore, C is set to 0.25 to 0.40%. Preferably, it is 0.25 to 0.34% or less.
Ｓｉ：１．０〜２．０％
Ｓｉは、フェライト中で高い固溶強化能を有し、鋼板強度の増加に寄与するとともに、炭化物（セメンタイト）の生成を抑制し、残留オーステナイトの安定化に寄与するため必要である。また、Ｓｉは、フェライト中の固溶Ｃを残留オーステナイトへ排出させ、フェライトを清浄化し、鋼板延性の向上に寄与する作用を有する。このような効果を得るためには、Ｓｉは１．０％以上の含有を必要とする。一方、Ｓｉが２．０％を超えると、残留オーステナイトの生成が阻害される。したがって、Ｓｉは１．０〜２．０％とする。好ましくは、１．２〜１．８％である。
Si: 1.0-2.0%
Si is necessary because it has a high solid solution strengthening ability in ferrite, contributes to an increase in steel sheet strength, suppresses the formation of carbides (cementite), and contributes to stabilization of retained austenite. Further, Si has an action of discharging the solid solution C in the ferrite to the retained austenite, purifying the ferrite and contributing to the improvement of the ductility of the steel sheet. In order to obtain such an effect, Si needs to be contained in an amount of 1.0% or more. On the other hand, when Si exceeds 2.0%, the production of retained austenite is inhibited. Therefore, Si is set to 1.0 to 2.0%. It is preferably 1.2 to 1.8%.
Ｍｎ：１．５〜２．５％
Ｍｎは、固溶強化あるいは焼入れ性向上を介して鋼板の強度増加に有効に寄与するとともに、オーステナイト安定化元素であり、所望の残留オーステナイト量の確保に必要不可欠な元素である。このような効果を得るために、Ｍｎは１．５％以上の含有を必要とする。一方、２．５％を超えて過剰に含有すると、所望の残留オーステナイト量を得ることが困難になる。したがって、Ｍｎは１．５〜２．５％とする。なお、好ましくは１．８〜２．２％である。
Mn: 1.5-2.5%
Mn is an austenite-stabilizing element that effectively contributes to increasing the strength of the steel sheet through solid solution strengthening or hardenability improvement, and is an essential element for securing a desired residual austenite amount. In order to obtain such an effect, Mn needs to be contained in an amount of 1.5% or more. On the other hand, if it is excessively contained in excess of 2.5%, it becomes difficult to obtain a desired residual austenite amount. Therefore, Mn is set to 1.5 to 2.5%. It is preferably 1.8 to 2.2%.
上記した成分が基本の成分である。本発明では、基本の組成に加えてさらに、以下の成分を含有してもよい。 The above-mentioned components are the basic components. In the present invention, the following components may be further contained in addition to the basic composition.
Ｐ：０．０２５％以下
Ｐは、固溶強化により強度の上昇に寄与できるが、溶接性に悪影響を及ぼすため０．０２５％以下とする。好ましくは０．０１％以下である。
P: 0.025% or less P can contribute to the increase in strength by strengthening the solid solution, but it has an adverse effect on weldability, so it should be 0.025% or less. It is preferably 0.01% or less.
Ｓ：０．００４％以下
Ｓは、Ｍｎと結合することによりＭｎＳ を形成し介在物割れの起点となるため、Ｓ量は極力少ないほうが好ましい。そこで０．００４％以下とする。好ましくは０．００２％以下である。
S: 0.004% or less Since S forms MnS by combining with Mn and becomes the starting point of inclusion cracking, it is preferable that the amount of S is as small as possible. Therefore, it is set to 0.004% or less. It is preferably 0.002% or less.
Ｎ：０．０１％以下
Ｎは、時効性に影響を及ぼす元素であり、時効効果により伸びが低下する。そのためＮ量は低いほうが好ましく、０．０１％以下とする。好ましくは０．００５％以下である。
N: 0.01% or less N is an element that affects the aging effect, and the elongation decreases due to the aging effect. Therefore, the amount of N is preferably low, and is 0.01% or less. It is preferably 0.005% or less.
Ａｌ：０．１％以下
Ａｌは、フェライト生成元素であり、強度と延性のバランス（強度延性バランス）を向上させる元素である。しかし、０．１％を超えて添加すると表層部の介在物が増加し延性が低下する。このため、Ａｌは０．１％以下とする。なお、鋼の脱酸剤の点から、０．０１％以上含有することが好ましい。
Al: 0.1% or less Al is a ferrite-forming element and is an element that improves the balance between strength and ductility (strength ductility balance). However, if it is added in excess of 0.1%, inclusions on the surface layer increase and ductility decreases. Therefore, Al is set to 0.1% or less. From the viewpoint of a steel deoxidizer, it is preferably contained in an amount of 0.01% or more.
上記した成分以外の残部は、Ｆｅおよび不可避的不純物からなる。なお、不可避的不純物としては、Ｔｉ：０．１％以下、Ｎｂ：０．１％以下が許容できる。 The rest other than the above components consist of Fe and unavoidable impurities. As the unavoidable impurities, Ti: 0.1% or less and Nb: 0.1% or less are acceptable.
次に、本発明のミクロ組織について説明する。本発明はＴＲＩＰ効果によって高強度高延性を実現するものである。そのためには、残留オーステナイトの形態を制御することが重要である。本発明では、以下のように残留オーステナイトを規定する。 Next, the microstructure of the present invention will be described. The present invention realizes high strength and high ductility by the TRIP effect. For that purpose, it is important to control the morphology of retained austenite. In the present invention, retained austenite is defined as follows.
残留オーステナイト：１０〜３０ｖｏｌ％
ＴＲＩＰ効果を実現するために、残留オーステナイトの体積率は１０ｖｏｌ％以上が必要である。体積率が１０ｖｏｌ％未満では、十分な延性が得られず、ＴＲＩＰ効果も小さい。一方で、体積率が３０ｖｏｌ％を超えると、十分な強度が得られない。
Residual austenite: 10-30 vol%
In order to realize the TRIP effect, the volume fraction of retained austenite needs to be 10 vol% or more. If the volume ratio is less than 10 vol%, sufficient ductility cannot be obtained and the TRIP effect is small. On the other hand, if the volume fraction exceeds 30 vol%, sufficient strength cannot be obtained.
なお、残留オーステナイト以外の残部は、マルテンサイト、ベイナイト、フェライトからなる。ここでいう「マルテンサイト」は、フレッシュマルテンサイト、焼戻マルテンサイト相を含むもので、「ベイナイト」は、上部ベイナイト、下部ベイナイト、ベイニティックフェライトを含むものである。これら相の規定は特に限定しないが、残部の主相はフェライトとベイナイトであることが好適である。初期段階でマルテンサイトが全組織分率で１５ｖｏｌ％を超えると、材料の延性が安定化せず、高延性を実現できないために、マルテンサイトは１５ｖｏｌ％以下であることが望ましい。また、組織全量に対する体積率で１０ｖｏｌ％以下（０％を含む）であれば、パーライトおよび／または炭化物を含んでもよい。なお、炭化物には、セメンタイト、Ｔｉ系炭化物、Ｎｂ系炭化物が含まれる。 The rest other than retained austenite is composed of martensite, bainite, and ferrite. The term "martensite" as used herein includes fresh martensite and tempered martensite phases, and "bainite" includes upper bainite, lower bainite, and bainitic ferrite. The specifications of these phases are not particularly limited, but it is preferable that the remaining main phases are ferrite and bainite. If martensite exceeds 15 vol% in the total structure fraction at the initial stage, the ductility of the material is not stabilized and high ductility cannot be achieved. Therefore, it is desirable that martensite is 15 vol% or less. Further, pearlite and / or carbide may be contained as long as the volume ratio to the total amount of the tissue is 10 vol% or less (including 0%). The carbides include cementite, Ti-based carbides, and Nb-based carbides.
また、本発明における残留オーステナイトの体積率の求め方については、後述する三次元分布に基づいて残留オーステナイトの体積率を求めてもよく、また、ＸＲＤ、飽和磁化法といった、従来の手法により残留オーステナイトの体積率を求めてもよい。 Further, as for the method of obtaining the volume fraction of retained austenite in the present invention, the volume fraction of retained austenite may be obtained based on the three-dimensional distribution described later, and the retained austenite may be obtained by a conventional method such as XRD or saturation magnetization method. The volume fraction of may be obtained.
アスペクト比が２以上の残留オーステナイトの割合：６０％以上
残留オーステナイトの形状は、ＴＲＩＰ効果に影響を及ぼす。本発明の残留オーステナイトのうち、アスペクト比が２以上の残留オーステナイトの割合が６０%以上であるとき、優れた高強度高延性を示す。ここでアスペクト比とは、三次元構造における最長のフェレ径に対する最短のフェレ径の比である。アスペクト比が２以上の残留オーステナイトの割合が６０％に満たない場合、変形開始直後にマルテンサイト変態が生じてしまい、所定の延性を得ることができない。この理由は必ずしも明確ではないが、残留オーステナイトの結晶粒が等方的形状に近いほど、変形に対する安定性が低く容易にマルテンサイト変態することが一因であると考えられる。
Percentage of retained austenite with an aspect ratio of 2 or more: 60% or more The shape of retained austenite affects the TRIP effect. Among the retained austenites of the present invention, when the proportion of retained austenite having an aspect ratio of 2 or more is 60% or more, excellent high strength and high ductility are exhibited. Here, the aspect ratio is the ratio of the shortest ferret diameter to the longest ferret diameter in the three-dimensional structure. If the proportion of retained austenite having an aspect ratio of 2 or more is less than 60%, martensitic transformation occurs immediately after the start of deformation, and a predetermined ductility cannot be obtained. The reason for this is not always clear, but it is considered that the closer the crystal grains of retained austenite are to the isotropic shape, the lower the stability to deformation and the easier the martensitic transformation.
さらに、本発明では、アスペクト比が２以上の残留オーステナイトにおいて、三次元構造における最長のフェレ径５００ｎｍ以下の割合が４０％以上であることが好ましい。残留オーステナイトの結晶粒径も、ＴＲＩＰ効果に影響を及ぼす。アスペクト比が２以上の残留オーステナイトにおいて、最長のフェレ径が５００ｎｍ以下の割合が４０%以上であることにより、より優れた高強度高延性を得ることができる。アスペクト比が２以上の残留オーステナイトにおいて、最長のフェレ径が５００ｎｍ以下の割合が４０％に満たない場合、変形開始直後にマルテンサイト変態を生じやすく、延性向上が抑制される。 Further, in the present invention, in retained austenite having an aspect ratio of 2 or more, the ratio of the longest ferret diameter of 500 nm or less in the three-dimensional structure is preferably 40% or more. The grain size of retained austenite also affects the TRIP effect. In the retained austenite having an aspect ratio of 2 or more, when the ratio of the longest ferret diameter of 500 nm or less is 40% or more, more excellent high strength and high ductility can be obtained. In retained austenite having an aspect ratio of 2 or more, when the ratio of the longest ferret diameter of 500 nm or less is less than 40%, martensitic transformation is likely to occur immediately after the start of deformation, and improvement in ductility is suppressed.
なお、本発明におけるアスペクト比、すなわち三次元構造における最長のフェレ径に対する最短のフェレ径の比は、例えば、加速電圧が８ｋＶ以下、一次イオンビーム電流が１．０ｎＡ以下の集束イオンビームにより、加工後の表面が前記集束イオンビームに対して平行になるように該鋼材を平面に加工処理する工程と、走査電子顕微鏡（ＳＥＭ）により加工後の表面を観察しＳＥＭ像を取得する工程と、を繰り返し行い、得られた前記ＳＥＭ像を連続的に繋いで残留オーステナイトの三次元分布を構築し、この構築した三次元分布に基づいて、求めることができる。 The aspect ratio in the present invention, that is, the ratio of the shortest ferret diameter to the longest ferret diameter in the three-dimensional structure is processed by, for example, a focused ion beam having an acceleration voltage of 8 kV or less and a primary ion beam current of 1.0 nA or less. A step of processing the steel material into a flat surface so that the subsequent surface is parallel to the focused ion beam, and a step of observing the processed surface with a scanning electron microscope (SEM) and acquiring an SEM image. It can be repeated to construct a three-dimensional distribution of retained austenite by continuously connecting the obtained SEM images, and can be obtained based on the constructed three-dimensional distribution.
集束イオンビームで鋼材試料を加工処理する工程では、集束イオンビーム、すなわちＦＩＢ（一次イオンビーム）であらかじめ決定した領域を真空中でスパッタ加工することにより、平滑な面を露出させる。一次イオンビームは加工後の表面が前記集束イオンビームに対して平行になるように試料に照射すればよい。 In the step of processing a steel sample with a focused ion beam, a smooth surface is exposed by sputtering a region predetermined by a focused ion beam, that is, a FIB (primary ion beam) in a vacuum. The primary ion beam may irradiate the sample so that the processed surface is parallel to the focused ion beam.
ＦＩＢ加工条件は、加速電圧が８ｋＶ以下、かつ一次イオンビーム電流が１．０ｎＡ以下とする。加速電圧が８ｋＶより高いと、一次イオンが試料表面に衝突するエネルギーが高すぎるため、電流量を少なくしても格子欠陥が発生し、残留オーステナイトがマルテンサイトに変態する。電流量が１．０ｎＡより多いと、照射フラックス（一次イオンが単位面積当たり単位時間に試料にドーズされる量）の影響のため、低加速電圧でも残留オーステナイトがマルテンサイトに変態する。したがって、正確な組織評価や相分率の定量を行うためには、８ｋＶ以下かつ１．０ｎＡ以下とすることが好ましい。一次イオンビーム条件の下限値は特にないが、作業効率の観点から、３ｋＶ以上かつ０．１ｎＡ以上が好ましい。なお、一次イオンビームのイオン種は、試料と化学反応しないイオンであれば特に限定しないが、スパッタレートの比較的大きいＧａイオン（Ｇａ＋）が好適である。 The FIB processing conditions are an accelerating voltage of 8 kV or less and a primary ion beam current of 1.0 nA or less. If the accelerating voltage is higher than 8 kV, the energy at which the primary ions collide with the sample surface is too high, so that lattice defects occur even if the amount of current is reduced, and retained austenite is transformed into martensite. When the amount of current is more than 1.0 nA, residual austenite is transformed into martensite even at a low accelerating voltage due to the influence of irradiation flux (the amount of primary ions dosed to the sample per unit area per unit time). Therefore, in order to perform accurate tissue evaluation and quantification of the phase fraction, it is preferable to set it to 8 kV or less and 1.0 nA or less. There is no particular lower limit for the primary ion beam conditions, but from the viewpoint of work efficiency, 3 kV or more and 0.1 nA or more are preferable. The ion species of the primary ion beam is not particularly limited as long as it is an ion that does not chemically react with the sample, but Ga ion (Ga + ) having a relatively large sputtering rate is preferable.
ＦＩＢ加工で平滑面出しを行った後、真空中で直ちに該加工面をＳＥＭ観察し、ＳＥＭ像を取得する。ＳＥＭ像は二次電子像、反射電子像のいずれでもよいが、残留オーステナイトは母相に比べて炭素濃度が高いため、この組成差をコントラストとして識別できる反射電子像を取得することが好ましい。この場合、残留オーステナイトは母相よりやや暗いコントラストとして識別される。ＳＥＭ観察条件は特に限定しないが、後述するＥＢＳＤ測定を行う場合は、ＥＢＳＤ像取得条件と同一とすることが作業性の点で好ましい。なお、組成を確認する目的で、特性Ｘ線分光分析を行ってもよい。 After smoothing the surface by FIB processing, the processed surface is immediately observed by SEM in vacuum to obtain an SEM image. The SEM image may be either a secondary electron image or a backscattered electron image, but since retained austenite has a higher carbon concentration than the parent phase, it is preferable to obtain a backscattered electron image in which this composition difference can be identified as contrast. In this case, retained austenite is identified as a slightly darker contrast than the parent phase. The SEM observation conditions are not particularly limited, but when the EBSD measurement described later is performed, it is preferable that the SEM observation conditions are the same as the EBSD image acquisition conditions from the viewpoint of workability. In addition, characteristic X-ray spectroscopic analysis may be performed for the purpose of confirming the composition.
こうして設定したＦＩＢの加工条件およびＳＥＭの走査電子像取得条件の下で、ＦＩＢにより観察面を加工する処理とＳＥＭにより加工後の観察面のＳＥＭ像を取得する処理とを行い、観察面の画像を保存する。次にで、ＦＩＢにより加工後の表面が直前の観察面（前回の加工面）に対して平行となるように加工を行い、加工後の観察面のＳＥＭ像を取得し、観察面の画像を保存する。これらの処理は、所定の回数だけ繰り返し行う。例えば、ＦＩＢによる加工ステップ（スライス幅）を１００ｎｍとし、ＦＩＢによる加工とＳＥＭによる観察を１００回繰り返す。これにより、１０μｍ３程度の領域を観察することになり、保存した画像を加工ステップ順に繋ぎ合わせることにより、この領域におけるミクロ組織（残留オーステナイト）の三次元分布を求めることができる。なお、本発明において、ＦＩＢによるスライス幅は大きいほど測定時間が短くなるが、構築した３次元像の分解能は低下する。これらを考慮すると、スライス幅は１５０ｎｍ以下とすることが好適であり、５０ｎｍ以下であればさらに好適である。また、三次元分布を求める組織の領域（加工領域）は、残留オーステナイトが分布した鋼材組織の均一性を考慮して、５μｍ×５μｍ×５μｍ以上であればよく、１０μｍ×１０μｍ×１０μｍ以上であればさらに好適である。上限は特にないが、加工時間やスパッタによる試料ダメージの観点から、３０μｍ×３０μｍ×３０μｍである。 Under the FIB processing conditions and SEM scanning electron image acquisition conditions set in this way, the process of processing the observation surface by FIB and the process of acquiring the SEM image of the processed observation surface by SEM are performed, and the image of the observation surface is performed. To save. Next, the processed surface is processed by FIB so as to be parallel to the immediately preceding observation surface (previously processed surface), an SEM image of the processed observation surface is acquired, and an image of the observation surface is obtained. save. These processes are repeated a predetermined number of times. For example, the processing step (slice width) by FIB is set to 100 nm, and processing by FIB and observation by SEM are repeated 100 times. As a result, a region of about 10 μm 3 is observed, and the three-dimensional distribution of the microstructure (residual austenite) in this region can be obtained by connecting the stored images in the order of processing steps. In the present invention, the larger the slice width by FIB, the shorter the measurement time, but the lower the resolution of the constructed three-dimensional image. Considering these, the slice width is preferably 150 nm or less, and more preferably 50 nm or less. Further, the region (processed region) of the structure for which the three-dimensional distribution is to be obtained may be 5 μm × 5 μm × 5 μm or more in consideration of the uniformity of the steel structure in which the retained austenite is distributed, and may be 10 μm × 10 μm × 10 μm or more. Is even more suitable. There is no particular upper limit, but it is 30 μm × 30 μm × 30 μm from the viewpoint of processing time and sample damage due to sputtering.
前記工程で取得したＳＥＭ像を取得順に連続的に繋ぎ合わせて、三次元のＳＥＭ像を構築する。この段階で、所定の枚数の観察面の画像が得られている。例えば、上記した加工ステップ：１００ｎｍ、繰り返し回数：１００回の場合には、１００枚の画像が得られている。ここでは、これらの画像を加工ステップ順に繋ぎ合わせて１つに統合し、残留オーステナイトの三次元分布を得る。これは、適切なソフトウェア（例えば、ＦＥＩ製のＡｖｉｚｏやＡｍｉｒａ等の三次元画像再構築・解析ソフト）を用いて行うことができ、各観察面における残留オーステナイトの二次元的な分布を三次元的に可視化することができる。その際、必要に応じて、画像間の位置ずれの補正やコントラストの調整を自動あるいは手動で行うことができる。 The SEM images acquired in the above step are continuously connected in the order of acquisition to construct a three-dimensional SEM image. At this stage, a predetermined number of images of the observation surface are obtained. For example, when the above-mentioned processing step: 100 nm and the number of repetitions: 100 times, 100 images are obtained. Here, these images are joined in the order of processing steps and integrated into one to obtain a three-dimensional distribution of retained austenite. This can be done using appropriate software (eg, 3D image reconstruction / analysis software such as FEI's Avizo or Amira), and the two-dimensional distribution of retained austenite on each observation surface is three-dimensional. Can be visualized in. At that time, if necessary, it is possible to automatically or manually adjust the positional deviation between the images and the contrast.
こうして最終的に得られた残留オーステナイトの三次元分布から、残留オーステナイトの三次元形状評価を行う。形状評価については、三次元画像解析ソフトを用いて、最長および最短のフェレ径を求め、その比率からアスペクト比を求める。 From the three-dimensional distribution of retained austenite finally obtained in this way, the three-dimensional shape evaluation of retained austenite is performed. For shape evaluation, the longest and shortest ferret diameters are obtained using three-dimensional image analysis software, and the aspect ratio is obtained from the ratio.
また、上記のＳＥＭ像取得において、各組織の識別を確実にする目的で、ＳＥＭ像取得工程に引き続き、電子線をステップ走査しながらＳＥＭ像を取得した観察面の後方散乱電子回折像（ＥＢＳＤ像）を測定して、ＥＢＳＤ像を解析する工程をさらに備えていてもよい。 Further, in the above-mentioned SEM image acquisition, for the purpose of ensuring the identification of each tissue, the backscattered electron diffraction image (EBSD image) of the observation surface obtained by step-scanning the electron beam following the SEM image acquisition step. ) May be further provided to analyze the EBSD image.
ＥＢＳＤ像の測定条件としては、入射電子線の加速電圧は５〜１５ｋＶが好適である。この範囲内であれば、正しいバックグランド除去およびＥＢＳＤ像の解析が行える。入射電子線の加速電圧が５ｋＶより低いと、ＥＢＳＤ像のＳ／Ｎ比が悪くなり像解析が困難となる。入射電子線の加速電圧が１５ｋＶより高いと、後方散乱電子の発生領域が大きくなりＥＢＳＤ像の空間分解能が下がるため、微細な残留オーステナイトの識別が困難となる。測定ステップは、微細な残留オーステナイトの認識が重要となるため、５０ｎｍ以下で行うことが好ましい。測定時間は、検出器の感度と処理速度に依存するために一概に決定できないが、工業的に有効に利用するには、例えば１０μｍ×１０μｍ×１０μｍの加工領域を１５０ｎｍ以下のスライス幅でＦＩＢ加工する場合に、ＦＩＢ加工、ＳＥＭ観察、ＥＢＳＤ測定〜解析を含め一連の工程を３０時間以内で行えるように、測定ステップと１ステップ当たりの測定時間を設定することが望ましい。上記のＳＥＭ像に替えて、ＥＢＳＤ解析結果を用いて、残留オーステナイトの三次元分布を取得する場合、後方散乱電子回折像の解析結果から相マップを作成し、該相マップを連続的に繋いで残留オーステナイトの三次元分布を構築すればよい。三次元分布の構築には、上述した市販の三次元画像再構築・解析ソフトを用いて、１スライスごと相マップを加工ステップ順に繋ぎ合わせて１つに統合すればよい。このようにして、残留オーステナイトの二次元的な分布を三次元的に可視化できる。 As the measurement condition of the EBSD image, the accelerating voltage of the incident electron beam is preferably 5 to 15 kV. Within this range, correct background removal and EBSD image analysis can be performed. If the accelerating voltage of the incident electron beam is lower than 5 kV, the S / N ratio of the EBSD image becomes poor and image analysis becomes difficult. When the accelerating voltage of the incident electron beam is higher than 15 kV, the region where backscattered electrons are generated becomes large and the spatial resolution of the EBSD image decreases, so that it becomes difficult to identify fine retained austenite. Since it is important to recognize fine retained austenite, the measurement step is preferably performed at 50 nm or less. The measurement time cannot be unconditionally determined because it depends on the sensitivity and processing speed of the detector, but in order to use it industrially effectively, for example, a processing area of 10 μm × 10 μm × 10 μm is processed by FIB with a slice width of 150 nm or less. In this case, it is desirable to set the measurement step and the measurement time per step so that a series of steps including FIB processing, SEM observation, EBSD measurement and analysis can be performed within 30 hours. When acquiring the three-dimensional distribution of retained austenite using the EBSD analysis result instead of the above SEM image, a phase map is created from the analysis result of the backscattered electron diffraction image, and the phase maps are continuously connected. A three-dimensional distribution of retained austenite may be constructed. To construct the three-dimensional distribution, the above-mentioned commercially available three-dimensional image reconstruction / analysis software may be used to connect the phase maps for each slice in the order of processing steps and integrate them into one. In this way, the two-dimensional distribution of retained austenite can be visualized three-dimensionally.
さらに、本発明の高強度高延性鋼板は、下記のＦＩＢ条件でイオン照射を行った際、イオン照射前の残留オーステナイトと比べて面積率で４０％以上の残留オーステナイトがイオン照射後に残存することを特徴とする。不安定な残留オーステナイトは、変形時に容易にマルテンサイト変態するために、材料の硬化が早く伸びが低減する。このため、変形に対してマルテンサイト変態しにくい残留オーステナイトが鋼材中に存在することが必要である。ＦＩＢを用いたイオン照射を行うことで、変形に対して変態しやすい残留オーステナイトが、オーステナイト−マルテンサイト変態を引き起こすという知見に基づき、所定のＦＩＢ条件でイオン照射した際のイオン照射後の残留オーステナイトの残存量が４０％以上であれば、強度および延性に優れた鋼板を得ることができる。 Furthermore, in the high-strength, high-ductility steel sheet of the present invention, when ion irradiation is performed under the following FIB conditions, residual austenite having an area ratio of 40% or more remains after ion irradiation as compared with retained austenite before ion irradiation. It is a feature. Unstable retained austenite easily undergoes martensitic transformation during deformation, so that the material cures quickly and elongation is reduced. Therefore, it is necessary that retained austenite, which is resistant to martensitic transformation with respect to deformation, is present in the steel material. Based on the finding that retained austenite, which is easily transformed by deformation, causes austenite-martensite transformation by ion irradiation using FIB, retained austenite after ion irradiation when ion-irradiated under predetermined FIB conditions When the residual amount of is 40% or more, a steel plate having excellent strength and ductility can be obtained.
イオン照射については、ＦＩＢを用いて行えばよい。イオン照射の条件は、イオンの加速電圧は８ｋＶ以下とし、下記式（１）を満たすイオンｄｏｓｅ量のイオンビームを鋼板表面に対して垂直方向に照射する。
ｂ＝３０−１０×ｌｎ（ａ）・・・（１）
ａ：加速電圧（ｋＶ）
ｂ：イオンｄｏｓｅ量（ｐＣ／（μｍ）２）
イオンの加速電圧が８ｋＶより高いと、イオンｄｏｓｅ量を式（１）を満たすように照射しても、歪に対してマルテンサイト変態しにくい残留オーステナイトがマルテンサイト変態を起こしてしまい、残留オーステナイトの歪に対する安定性を正確に評価することが困難になる。このため、イオンの加速電圧は８ｋＶ以下とする。なお、下限値については、３ｋＶ以上が好ましい。
Ion irradiation may be performed using FIB. The conditions for ion irradiation are such that the accelerating voltage of the ions is 8 kV or less, and an ion beam having an ion dose amount satisfying the following formula (1) is irradiated in the direction perpendicular to the surface of the steel sheet.
b = 30-10 × ln (a) ... (1)
a: Acceleration voltage (kV)
b: Amount of ion dose (pC / (μm) 2 )
When the accelerating voltage of ions is higher than 8 kV, even if the amount of ion dose is irradiated so as to satisfy the equation (1), the retained austenite, which is difficult to undergo martensitic transformation with respect to strain, undergoes martensitic transformation, and the retained austenite It becomes difficult to accurately evaluate the stability against strain. Therefore, the accelerating voltage of ions is set to 8 kV or less. The lower limit is preferably 3 kV or higher.
一次イオンビーム電流は高過ぎると照射フラックスによる影響のため、低加速電圧でも変態しにくい残留オーステナイトであってもマルテンサイト変態する。このため、一次イオンビーム電流は、１．０ｎＡ以下であることが好ましい。一次イオンビーム電流の下限値は０．０３ｎＡ以上であることが好ましい。 If the primary ion beam current is too high, it will be affected by the irradiation flux, so even retained austenite, which is difficult to transform even at a low accelerating voltage, will undergo martensitic transformation. Therefore, the primary ion beam current is preferably 1.0 nA or less. The lower limit of the primary ion beam current is preferably 0.03 nA or more.
また、一次イオンビームのイオン種は、試料と化学反応しないイオンであれば特に限定しないが、スパッタレートの比較的大きいＧａイオン（Ｇａ＋）が好適である。 The ion species of the primary ion beam is not particularly limited as long as it does not chemically react with the sample, but Ga ion (Ga + ) having a relatively large sputtering rate is preferable.
なお、イオン照射を行う試料については、通常の機械研磨を行い、試料表面を鏡面にした後、電解研磨を行う。その後、対象領域についてＳＥＭ観察を行って観察視野を決定すればよい。イオン照射の照射領域は、１０×１０μｍ以上が好ましく、２０×２０μｍ以上がより好ましい。 The sample to be irradiated with ions is subjected to normal mechanical polishing, the surface of the sample is made a mirror surface, and then electrolytic polishing is performed. After that, the observation field of view may be determined by performing SEM observation on the target area. The irradiation region of ion irradiation is preferably 10 × 10 μm or more, and more preferably 20 × 20 μm or more.
また、イオン照射前後の残留オーステナイト量の測定は、観察視野において、ＥＢＳＤを用いて相マップを作成し、オーステナイトの面積を求めればよい。この時、残留オーステナイトを認識するために、正確なバックグラウンド除去およびＥＢＳＤ像の解析ができる測定時間で行えばよく、また、微細な残留オーステナイトの認識が重要となるため、ステップサイズは５０ｎｍ以下が好ましい。ＥＢＳＤ測定における加速電圧は５ｋＶ以上で行うことが好ましく、１５ｋＶ以上であることがより好ましい。電流量は１．０ｎＡ以上が好ましい。 Further, for the measurement of the amount of retained austenite before and after ion irradiation, a phase map may be created using EBSD in the observation field of view, and the area of austenite may be obtained. At this time, in order to recognize the retained austenite, the measurement time may be such that accurate background removal and EBSD image analysis can be performed, and since recognition of fine retained austenite is important, the step size should be 50 nm or less. preferable. The acceleration voltage in the EBSD measurement is preferably 5 kV or more, and more preferably 15 kV or more. The amount of current is preferably 1.0 nA or more.
本発明の高強度高延性鋼板は、耐食性向上のために、めっき層を形成してもよい。めっき種は特に限定はなく、Ｚｎ、Ｚｎ−Ｆｅ合金、Ｚｎ−Ｎｉ合金、Ｚｎ−Ａｌ合金、Ｚｎ−Ａｌ−Ｍｇ合金、Ａｌ−Ｓｉ合金等のいずれも適用可能である。 The high-strength, high-ductility steel sheet of the present invention may form a plating layer in order to improve corrosion resistance. The type of plating is not particularly limited, and any of Zn, Zn-Fe alloy, Zn-Ni alloy, Zn-Al alloy, Zn-Al-Mg alloy, Al-Si alloy and the like can be applied.
次に本発明の高強度高延性鋼板の製造方法について説明する。 Next, a method for producing a high-strength, high-ductility steel sheet of the present invention will be described.
本発明では、上記した組成の鋼素材に、熱間圧延工程と、酸洗工程と、冷間圧延工程と、焼鈍工程とを順次施して、高強度高延性鋼板を得ることができる。 In the present invention, a steel material having the above composition is sequentially subjected to a hot rolling step, a pickling step, a cold rolling step, and an annealing step to obtain a high-strength, high-development steel sheet.
鋼素材の製造方法については、とくに限定はなく、転炉等の常用の溶製方法で上記した組成の溶鋼を溶製し、常用の連続鋳造法で所定寸法のスラブ等の鋳片（鋼素材）とすることができる。なお、造塊−分塊圧延により鋼片（鋼素材）としてもよい。 The method for manufacturing the steel material is not particularly limited, and the molten steel having the above composition is melted by a common melting method such as a converter, and a slab or other slab (steel material) having a predetermined size is melted by a normal continuous casting method. ) Can be. In addition, a steel piece (steel material) may be obtained by ingot-block rolling.
次いで、上記した組成の鋼素材を熱間圧延し、熱延板とする。熱間圧延工程では、上記した組成の鋼素材を加熱炉にて加熱し、熱間圧延により所定厚さの熱延板とすることができればよく、常用の熱間圧延方法がいずれも適用できる。本発明では、上記の鋼素材を１１００〜１２５０℃の範囲の温度に加熱し、熱間仕上げ圧延機出側温度を８５０〜９５０℃とする熱間圧延を施すことが好ましい。 Next, the steel material having the above composition is hot-rolled to obtain a hot-rolled plate. In the hot rolling step, it is sufficient that the steel material having the above composition is heated in a heating furnace and hot rolled to obtain a hot rolled plate having a predetermined thickness, and any of the usual hot rolling methods can be applied. In the present invention, it is preferable to heat the above steel material to a temperature in the range of 1100 to 1250 ° C. and perform hot rolling to set the hot finish rolling mill outlet temperature to 850 to 950 ° C.
熱間圧延終了後は、冷却しコイルに巻き取る。冷却・巻き取り工程は、４５０〜９５０℃の温度域を平均冷却速度４０〜１００℃／ｓで冷却し、巻取温度４５０〜６５０℃で巻き取り、所定寸法形状の熱延コイルとすることが好ましい。 After the hot rolling is completed, it is cooled and wound around a coil. In the cooling / winding process, the temperature range of 450 to 950 ° C. is cooled at an average cooling rate of 40 to 100 ° C./s, and the coil is wound at a winding temperature of 450 to 650 ° C. to obtain a hot-rolled coil having a predetermined size and shape. preferable.
次いで、得られた熱延コイルを巻き戻して酸洗を施すことが好ましい。酸洗工程は、熱延板表面の黒皮を除去し冷間圧延を施すことができる程度に酸洗できればよい。したがって、酸洗条件についてはとくに限定する必要はなく、塩酸、硫酸等を使用する常用の酸洗方法がいずれも適用できる。 Next, it is preferable to rewind the obtained hot-rolled coil and perform pickling. In the pickling step, it is sufficient that the black skin on the surface of the hot-rolled plate can be removed and pickled to the extent that cold rolling can be performed. Therefore, the pickling conditions are not particularly limited, and any of the usual pickling methods using hydrochloric acid, sulfuric acid, or the like can be applied.
酸洗工程を経た熱延板に冷間圧延を施し、所定板厚の冷延鋼板とする。得られた冷延鋼板にさらに以下に説明する焼鈍を施し、高強度高延性鋼板とする。 The hot-rolled sheet that has undergone the pickling process is cold-rolled to obtain a cold-rolled steel sheet having a predetermined thickness. The obtained cold-rolled steel sheet is further annealed as described below to obtain a high-strength, high-ductility steel sheet.
冷間圧延条件については特に限定されず、本発明では、従来既知の方法を用いればよい。また、圧下率についても特に限定されないが、３０％以上が好ましい。 The cold rolling conditions are not particularly limited, and in the present invention, a conventionally known method may be used. The reduction rate is also not particularly limited, but is preferably 30% or more.
本発明における焼鈍工程は、以下に述べるように第一および第二の熱処理からなることを特徴とする。 The annealing step in the present invention is characterized by comprising first and second heat treatments as described below.
第一の熱処理：８００℃〜９５０℃に加熱し、直ちに冷却速度１０℃／ｓ以上で３００〜５００℃まで冷却し、１００〜１０００ｓ間保持
第一の熱処理では、オーステナイトの生成を目的として、冷延鋼板を加熱温度８００℃〜９５０℃に加熱する。次いで、ベイナイト変態とオーステナイトへのＣの拡散（分配）やＳｉ、Ａｌ等合金元素の濃化を目的として、直ちに冷却速度１０℃／ｓ以上で冷却停止温度３００〜５００℃まで冷却し、１００〜１０００ｓ保持する。
First heat treatment: Heat to 800 ° C to 950 ° C, immediately cool to 300 to 500 ° C at a cooling rate of 10 ° C / s or higher, and hold for 100 to 1000 s. In the first heat treatment, cool for the purpose of producing austenite. The rolled steel sheet is heated to a heating temperature of 800 ° C. to 950 ° C. Next, for the purpose of bainite transformation, diffusion (distribution) of C to austenite, and concentration of alloying elements such as Si and Al, the mixture is immediately cooled to a cooling stop temperature of 300 to 500 ° C. at a cooling rate of 10 ° C./s or more, and 100 to 100 to Hold for 1000s.
第一の熱処理における加熱温度が８００℃より低い場合は、オーステナイトの生成が十分でなく、少ないオーステナイトへのＣ分配が進行する。加熱温度が低いため、オーステナイトが微細なまま残存し、Ｃ濃度の高いオーステナイトが母相中に微細に分散した組織が得られる。この結果、加熱後に直ちに行われる冷却および保持工程（３００〜５００℃まで冷却し１００〜１０００ｓ保持する工程）において、Ｃ元素の偏析に起因してマルテンサイト変態するため、所定量の残留オーステナイトを得ることができない。第一の熱処理における加熱温度が９５０℃より高い場合、オーステナイトが粗大化するため、オーステナイトの粒界から生成するフェライトの核生成頻度が減少し、延性が低下する。より好ましくは、第一の熱処理における加熱温度は８４０〜９３０℃である。 When the heating temperature in the first heat treatment is lower than 800 ° C., the formation of austenite is not sufficient, and C distribution to a small amount of austenite proceeds. Since the heating temperature is low, austenite remains finely, and a structure in which austenite having a high C concentration is finely dispersed in the matrix can be obtained. As a result, in the cooling and holding step (step of cooling to 300 to 500 ° C. and holding for 100 to 1000 s) immediately after heating, martensitic transformation occurs due to segregation of element C, so that a predetermined amount of retained austenite is obtained. Can't. When the heating temperature in the first heat treatment is higher than 950 ° C., the austenite becomes coarse, so that the nucleation frequency of ferrite formed from the grain boundaries of austenite decreases, and the ductility decreases. More preferably, the heating temperature in the first heat treatment is 840 to 930 ° C.
冷却速度が１０℃／ｓより低いと、冷却中に過度にフェライト相が生成し、ベイナイト相を得ることが困難になる。なお、冷却速度の上限については特に既定しないが、次に行う第二の熱処理において最適な量の残留オーステナイトを得るために１００℃／ｓとすることが好ましい。 If the cooling rate is lower than 10 ° C./s, an excessive ferrite phase is formed during cooling, making it difficult to obtain a bainite phase. Although the upper limit of the cooling rate is not particularly defined, it is preferably set to 100 ° C./s in order to obtain an optimum amount of retained austenite in the second heat treatment to be performed next.
冷却停止温度が３００℃より低い場合、マルテンサイト変態が生じやすく、一方で、冷却停止温度が５００℃より高い場合は、フェライトやパーライトが過剰に生成し、次に行う第二の熱処理時の加熱中にフェライトやパーライトの粒界や粒界三重点に逆変態オーステナイトが生成してできる塊状組織が増加し、所望のアスペクト比を持った残留オーステナイトが得られない。より好ましくは、３３０〜４５０℃である。 When the cooling stop temperature is lower than 300 ° C, martensitic transformation is likely to occur, while when the cooling stop temperature is higher than 500 ° C, ferrite and pearlite are excessively generated, and heating during the second heat treatment to be performed next is performed. The massive structure formed by the formation of reverse-transformed austenite at the grain boundaries and triple points of grain boundaries of ferrite and pearlite increases, and retained austenite with a desired aspect ratio cannot be obtained. More preferably, it is 330 to 450 ° C.
第一の熱処理において３００〜５００℃で冷却停止した後１００〜１０００ｓ保持するが、３００〜５００℃で保持したベイナイト組織であれば、第二の熱処理でオーステナイトがベイナイトのラス状組織に沿って生成・成長しラス状になるのに対して、保持温度が５００℃より高いと第二の熱処理でオーステナイトが粒界三重点で生成しやすいためラス状ではなく塊状になってしまう。 In the first heat treatment, cooling is stopped at 300 to 500 ° C. and then held for 100 to 1000 s, but if the bainite structure is held at 300 to 500 ° C., austenite is generated along the lath-like structure of bainite in the second heat treatment. -While it grows and becomes lath-shaped, if the holding temperature is higher than 500 ° C., austenite is likely to be formed at the triple point of the grain boundary in the second heat treatment, so that it becomes lumpy instead of lath-shaped.
また、保持時間については、１００ｓより短いとベイナイト変態が不十分となる。一方で、１０００ｓを超えて保持してもさらなる効果は得られず、操業性の観点からも好ましくない。なお、１００〜１０００ｓ保持後の冷却条件については、特に制限はなく、１００〜１０００ｓ保持後、空冷あるいはガス冷却などの常法により室温まで冷却すればよい。 Further, if the holding time is shorter than 100 s, the bainite transformation becomes insufficient. On the other hand, even if it is held for more than 1000 s, no further effect can be obtained, which is not preferable from the viewpoint of operability. The cooling conditions after holding for 100 to 1000 s are not particularly limited, and after holding for 100 to 1000 s, it may be cooled to room temperature by a conventional method such as air cooling or gas cooling.
第一の熱処理によって、ベイナイトの粒界や粒界三重点に塊状にＣや合金濃度の高い残留オーステナイトが分布した組織が得られる。第一の熱処理を経た鋼板について、引き続き第二の熱処理を施す。 By the first heat treatment, a structure in which C and retained austenite having a high alloy concentration are distributed in a mass at the grain boundaries and triple points of the grain boundaries of bainite can be obtained. The steel sheet that has undergone the first heat treatment is subsequently subjected to the second heat treatment.
第二の熱処理：７００〜８５０℃に加熱し、直ちに冷却速度１０℃／ｓ以上で３００〜５００℃まで冷却し、１００〜１０００ｓ保持
第二の熱処理では、母相ベイナイトのラス境界から逆変態によりオーステナイト相を生成させ、ラス境界に沿って成長させることを目的として、加熱温度７００〜８５０℃に加熱する。
Second heat treatment: Heat to 700 to 850 ° C, immediately cool to 300 to 500 ° C at a cooling rate of 10 ° C / s or higher, and hold for 100 to 1000 s. In the second heat treatment, reverse transformation occurs from the lath boundary of the parental bainite. The austenite phase is heated to a heating temperature of 700-850 ° C. for the purpose of forming and growing along the lath boundary.
次いで、ベイナイト変態を進め、ベイナイトのラス境界に分布するオーステナイトを減肉することでアスペクト比をさらに高くしたり、分断することで微細化させるとともに、オーステナイトへのＣや合金元素の濃化をさらに高めることを目的として、直ちに冷却速度１０℃／ｓ以上で冷却停止温度３００〜５００℃まで冷却し、１００〜１０００ｓ保持する。 Next, the bainite transformation is promoted, and the austenite distributed at the lath boundary of the bainite is thinned to further increase the aspect ratio, and the austenite is divided to make it finer, and the concentration of C and alloying elements to austenite is further increased. For the purpose of increasing the temperature, the mixture is immediately cooled to a cooling stop temperature of 300 to 500 ° C. at a cooling rate of 10 ° C./s or higher and maintained for 100 to 1000 s.
第二の熱処理における加熱温度が７００℃より低い場合は、三次元構造におけるアスペクト比の高い安定したオーステナイトの生成が十分ではなく、所望の残留オーステナイトを含有する最終製品が得られない。加熱温度が８５０℃より高いと、Ｃや合金元素の濃度の低い残留オーステナイトが増加するため、引き続く冷却過程でマルテンサイト変態が生じ、所定量の残留オーステナイトが得られない。より好ましくは、第二の熱処理における加熱温度は７３０〜８１０℃である。 If the heating temperature in the second heat treatment is lower than 700 ° C., stable austenite with a high aspect ratio in the three-dimensional structure is not sufficiently produced, and a final product containing the desired retained austenite cannot be obtained. When the heating temperature is higher than 850 ° C., retained austenite having a low concentration of C and alloying elements increases, so that martensitic transformation occurs in the subsequent cooling process, and a predetermined amount of retained austenite cannot be obtained. More preferably, the heating temperature in the second heat treatment is 730 to 810 ° C.
冷却速度が１０℃／ｓより低いと、過度にフェライト相が増加し、所望量の残留オーステナイトが得られない。冷却停止温度が３００℃より低い場合はマルテンサイト変態が生じやすく、冷却停止温度が５００℃より高い場合は、ベイナイト変態が抑制されオーステナイト相へのＣや合金元素の濃化が十分行われず、所望の残留オーステナイトが得られない。より好ましくは、第二の熱処理における冷却停止温度は３３０〜４５０℃である。 If the cooling rate is lower than 10 ° C./s, the ferrite phase increases excessively and the desired amount of retained austenite cannot be obtained. When the cooling stop temperature is lower than 300 ° C, martensitic transformation is likely to occur, and when the cooling stop temperature is higher than 500 ° C, bainite transformation is suppressed and C and alloying elements are not sufficiently concentrated in the austenite phase, which is desirable. Residual austenite cannot be obtained. More preferably, the cooling stop temperature in the second heat treatment is 330 to 450 ° C.
また、保持時間については、１００ｓより短いとベイナイト変態が不十分である。１０００ｓを超えて保持してもさらなる効果は得られず、操業性の観点からも好ましくない。また、第二の熱処理後の冷却条件については、特に制限はない。 If the retention time is shorter than 100 s, the bainite transformation is insufficient. Even if it is held for more than 1000 s, no further effect can be obtained, which is not preferable from the viewpoint of operability. The cooling conditions after the second heat treatment are not particularly limited.
なお、第二の熱処理については、残留オーステナイトの微細化とＣや合金元素の濃化を目的として、複数回繰り返し行ってもよく、例えば、第二の熱処理を２〜３回行うことが好ましい。 The second heat treatment may be repeated a plurality of times for the purpose of refining the retained austenite and concentrating C and alloying elements. For example, it is preferable to carry out the second heat treatment 2-3 times.
以上より、本発明では、第一の熱処理によって、ベイナイトの粒界や粒界三重点に塊状にＣや合金濃度の高い残留オーステナイトが分布した組織が得られる。第一の熱処理を経た鋼板について、引き続き第二の熱処理を施すことにより、母相ベイナイトのラス境界から逆変態によりオーステナイト相を生成させ、ラス境界に沿って成長させる。その結果、所望のアスペクト比の残留オーステナイトを得ることができる。また、本発明の製造条件によって得られた残留オーステナイトは、変形に対してマルテンサイト変態しにくい残留オーステナイトであるため、前述のイオン照射条件でイオン照射したとき、イオン照射前の残留オーステナイトと比べて４０％以上の残留オーステナイトがイオン照射後にも残存する。 From the above, in the present invention, by the first heat treatment, a structure in which C and retained austenite having a high alloy concentration are distributed in a mass at the grain boundaries and triple points of the grain boundaries of bainite can be obtained. The steel sheet that has undergone the first heat treatment is subsequently subjected to the second heat treatment to generate an austenite phase from the lath boundary of the parent bainite by reverse transformation and grow along the lath boundary. As a result, retained austenite with a desired aspect ratio can be obtained. Further, since the retained austenite obtained under the production conditions of the present invention is a retained austenite that is less likely to undergo martensitic transformation due to deformation, when ion-irradiated under the above-mentioned ion irradiation conditions, it is compared with the retained austenite before ion irradiation. More than 40% of retained austenite remains even after ion irradiation.
以下、実施例に基づき、さらに本発明について説明する。 Hereinafter, the present invention will be further described based on Examples.
表１に示す組成の鋼素材を、１２００℃に加熱し、熱間圧延出側温度９００℃で熱間圧延し、５００℃でコイルに巻き取った。次いで、冷間圧延し１．６ｍｍ厚の冷延鋼板とした。次いで、表２に示す条件で焼鈍を行った。 The steel material having the composition shown in Table 1 was heated to 1200 ° C., hot-rolled at a hot-rolling output side temperature of 900 ° C., and wound around a coil at 500 ° C. Then, it was cold-rolled to obtain a 1.6 mm thick cold-rolled steel sheet. Then, annealing was performed under the conditions shown in Table 2.
得られた各鋼板の引張強度（ＴＳ）、全伸び（Ｅｌ）、ミクロ組織（残留オーステナイト量）、イオン照射前後の残留オーステナイト量について、それぞれ評価を行った。各評価方法は以下のとおりである。 The tensile strength (TS), total elongation (El), microstructure (residual austenite amount), and residual austenite amount before and after ion irradiation of each of the obtained steel sheets were evaluated. Each evaluation method is as follows.
（１）引張試験
引張試験は、得られた鋼板について、１３号Ｂ試験片を作製し、ＪＩＳ Ｚ ２２４１に準拠してＬ方向の引張強度（ＴＳ）、全伸び（Ｅｌ）を測定した。
(1) Tensile test In the tensile test, a No. 13 B test piece was prepared from the obtained steel sheet, and the tensile strength (TS) and total elongation (El) in the L direction were measured in accordance with JIS Z 2241.
（２）ミクロ組織
鋼板の板幅方向の中央部の位置において１０ｍｍ×１５ｍｍの試験片を採取し、裏面から化学研磨で厚み３／４の深さ部分までを減肉し、ＦＩＢ−ＳＥＭ−ＥＢＳＤ複合装置（ＦＥＩ社製Ｓｃｉｏｓ）を用いて、ＳＥＭ−ＥＢＳＤ測定面に対して平行に１次イオンビームを照射し任意の位置をＦＩＢ加工、ＳＥＭによる二次電子像の取得、ＥＢＳＤ測定、を１サイクルとして繰り返し行い、残留オーステナイトの三次元分布を取得した。ＦＩＢ加工、ＳＥＭ観察、ＥＢＳＤ測定条件は下記のとおりである。
イオンビーム加工条件
イオン種：Ｇａ＋
加速電圧：７ｋＶ、一次イオンビーム電流値：０．６６ｎＡ
加工領域：５μｍ×５μｍ×５μｍ、スライス幅：５０ｎｍ
ＳＥＭ観察条件
加速電圧：１５ｋＶ
ＥＢＳＤ測定条件
加速電圧：１５ｋＶ、電流量：３ｎＡ、測定ステップ：５０ｎｍ
得られた一連のＳＥＭ−ＥＢＳＤ測定結果から相マップを計算し、三次元画像再構築・解析ソフト（ＦＥＩ製：Ａｖｉｚｏ）を用いて残留オーステナイトの三次元像を構築し、残留オーステナイト量（ｖｏｌ％）、個々の残留オーステナイト結晶粒のアスペクト比および長径を求めた。
(2) Microstructure A test piece of 10 mm × 15 mm was collected at the center of the steel plate in the plate width direction, and the thickness was reduced from the back surface to a depth of 3/4 of the thickness by chemical polishing, and FIB-SEM-EBSD. Using a composite device (Scios manufactured by FEI), irradiate a primary ion beam parallel to the SEM-EBSD measurement surface, perform FIB processing at an arbitrary position, acquire a secondary electron image by SEM, and perform EBSD measurement. It was repeated as a cycle to obtain a three-dimensional distribution of retained austenite. The FIB processing, SEM observation, and EBSD measurement conditions are as follows.
Ion beam processing conditions Ion species: Ga +
Acceleration voltage: 7kV, primary ion beam current value: 0.66nA
Processing area: 5 μm × 5 μm × 5 μm, slice width: 50 nm
SEM observation conditions Acceleration voltage: 15kV
EBSD measurement conditions Acceleration voltage: 15 kV, current amount: 3 nA, measurement step: 50 nm
A phase map is calculated from the obtained series of SEM-EBSD measurement results, and a three-dimensional image of retained austenite is constructed using three-dimensional image reconstruction / analysis software (manufactured by FEI: Avizo), and the amount of retained austenite (vol%). ), The aspect ratio and major axis of each retained austenite crystal grain were determined.
（３）イオン照射前後の残留オーステナイト量
鋼板の板幅方向の中央部の位置において、１０ｍｍ×１５ｍｍの試験片を採取した。得られた試料について、ＦＩＢによるイオン照射とＳＥＭ−ＥＢＳＤ測定の両方を行うことができる、ＦＩＢ−ＳＥＭ−ＥＢＳＤ複合装置（ＦＥＩ製：Ｓｃｉｏｓ）を用いて、集束一次イオン照射を行うとともに、イオン照射前後の試料について、ＳＥＭ−ＥＢＳＤ測定を行った。
(3) Amount of retained austenite before and after ion irradiation A test piece of 10 mm × 15 mm was collected at the central portion of the steel sheet in the plate width direction. The obtained sample is subjected to focused primary ion irradiation and ion irradiation using a FIB-SEM-EBSD composite device (manufactured by FEI: Scios) capable of performing both ion irradiation by FIB and SEM-EBSD measurement. SEM-EBSD measurement was performed on the samples before and after.
ＥＢＳＤ測定の条件については、加速電圧：１５ｋＶ、電流量：６．４ｎＡ、測定ステップ：５０ｎｍ、測定領域：任意の１５×１５μｍの領域とした。また、ＦＩＢよるイオン照射の条件は、加速電圧：５ｋＶ、電流量：０．０４８ｎＡ、ｄｏｓｅ量：１３．９ｐＣ／（μｍ）２とした。 The conditions for EBSD measurement were an accelerating voltage: 15 kV, a current amount of 6.4 nA, a measurement step: 50 nm, and a measurement region: an arbitrary region of 15 × 15 μm. The conditions for ion irradiation by FIB were an accelerating voltage: 5 kV, a current amount: 0.048 nA, and a dose amount: 13.9 pC / (μm) 2 .
ＥＢＳＤ測定により得られた残留オーステナイトの値について、イオン照射後の残留オーステナイトの残存率（％）を導き出した。
なお、イオン照射後の残留オーステナイトの残存率は、
イオン照射後の残留オーステナイトの残存率＝｛イオン照射後の残留オーステナイト量(面積率)／イオン照射前の残留オーステナイト量(面積率)｝×１００
から計算し求めた。
For the value of retained austenite obtained by EBSD measurement, the residual rate (%) of retained austenite after ion irradiation was derived.
The residual rate of retained austenite after ion irradiation is
Residual rate of retained austenite after ion irradiation = {Amount of retained austenite after ion irradiation (area ratio) / Amount of retained austenite before ion irradiation (area ratio)} x 100
Calculated from.
結果を表３に示す。 The results are shown in Table 3.
表３より、本発明例ではＴＳ×ＥＬが２４０００ＭＰａ・％以上を満足する高強度高延性鋼板が得られていることがわかる。特に、アスペクト比２以上の残留オーステナイト相のうち長径５００ｎｍ以下の割合が４０％以上であるＮｏ．１、Ｎｏ．２およびＮｏ．８は、ＴＳ×ＥＬが２６０００ＭＰａ・％以上ときわめて良好である。また、本発明例はいずれもイオン照射後の残留オーステナイトの残存量が４０％以上となっている。 From Table 3, it can be seen that in the example of the present invention, a high-strength, high-ductility steel sheet having a TS × EL of 24,000 MPa ·% or more is obtained. In particular, among the retained austenite phases having an aspect ratio of 2 or more, the proportion of the major axis of 500 nm or less is 40% or more. 1, No. 2 and No. No. 8 has an extremely good TS × EL of 26000 MPa ·% or more. Further, in each of the examples of the present invention, the residual amount of retained austenite after ion irradiation is 40% or more.
一方、比較例はいずれも強度−延性バランスが悪く、ＴＳ×ＥＬ≧２４０００ＭＰａ・％を満たしていない。また、比較例はいずれもイオン照射後の残留オーステナイトの残存量が４０％未満となっている。 On the other hand, in each of the comparative examples, the strength-ductility balance was poor, and TS × EL ≧ 24000 MPa ·% was not satisfied. Further, in each of the comparative examples, the residual amount of retained austenite after ion irradiation is less than 40%.
Claims (4)
なお、アスペクト比とは、三次元構造における最長のフェレ径に対する最短のフェレ径の比である。
ｂ＝３０−１０×ｌｎ（ａ）・・・（１）
ａ：イオンの加速電圧（ｋＶ）
ｂ：イオンｄｏｓｅ量（ｐＣ／（μｍ）２） By mass%, C: 0.25 to 0.40%, Si: 1.0 to 2.0%, Mn: 1.5 to 2.5%, P: 0.025% or less, S: 0.004 % Or less, Al: 0.1% or less, N: 0.01% or less, a component composition consisting of a balance Fe and unavoidable impurities, containing 10 to 30 vol% of retained austenite, and among the retained austenite. The ratio of retained austenite with an aspect ratio of 2 or more is 60% or more, and the ion acceleration voltage: 8 kV or less, the primary ion beam current: 1.0 nA or less, the ion type of the primary ion beam: Ga + , the following formula ( When an ion beam with an ion dose amount satisfying 1) is irradiated with ions in the direction perpendicular to the surface of the steel plate, 40% or more of the retained austenite remains after the ion irradiation as compared with the area ratio of the retained austenite before the ion irradiation. High strength and high ductivity steel plate.
The aspect ratio is the ratio of the shortest ferret diameter to the longest ferret diameter in the three-dimensional structure.
b = 30-10 × ln (a) ... (1)
a: Ion accelerating voltage (kV)
b: Amount of ion dose (pC / (μm) 2 )
ｂ＝３０−１０×ｌｎ（ａ）・・・（１）
ａ：イオンの加速電圧（ｋＶ）
ｂ：イオンｄｏｓｅ量（ｐＣ／（μｍ）２） The steel material having the component composition according to claim 1 is heated, then hot-rolled, cooled and wound after the completion of the hot-rolling, then cold-rolled, and then heated to 800 ° C. to 950 ° C. Immediately after cooling to 300 to 500 ° C. at a cooling rate of 10 ° C./s or higher, the first heat treatment for holding 100 to 1000 s and heating to 700 to 850 ° C. after cooling to 500 ° C., to facilities annealing consisting of a second heat treatment for 100~1000s holding, retained austenite containing 10 to 30 vol%, of the residual austenite, the aspect ratio is the ratio of 2 or more residual austenite 60 % Or more, ion acceleration voltage: 8 kV or less, primary ion beam current: 1.0 nA or less, primary ion beam ion type: Ga + , and an ion beam having an ion dose amount satisfying the following formula (1). A method for producing a high-strength, high-ductility steel sheet in which 40% or more of retained austenite remains after ion irradiation as compared with the area ratio of retained austenite before ion irradiation when ion irradiation is performed in a direction perpendicular to the surface. The aspect ratio is the ratio of the shortest ferret diameter to the longest ferret diameter in the three-dimensional structure.
b = 30-10 × ln (a) ... (1)
a: Ion accelerating voltage (kV)
b: Amount of ion dose (pC / (μm) 2 )
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