JP6638694B2 - Steel plate with excellent delayed fracture resistance with tensile strength of 1180 MPa or more - Google Patents
Steel plate with excellent delayed fracture resistance with tensile strength of 1180 MPa or more Download PDFInfo
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Description
本発明は、耐遅れ破壊性に優れた鋼板に関するものであり、詳細には、主として自動車、建材用の強度部材に好適な鋼板であって、耐遅れ破壊性に優れた引張強度１１８０ＭＰａ以上の高強度鋼板に関するものである。 The present invention relates to a steel sheet having excellent delayed fracture resistance, and more particularly, to a steel sheet suitable mainly for automobiles and strength members for building materials, and a high tensile strength of 1180 MPa or more excellent in delayed fracture resistance. It relates to a high strength steel sheet.
自動車用鋼板には、板厚精度や平担度に関する要求から冷延鋼板が用いられているが、近年、自動車のＣＯ２排出量の低減および安全性確保の観点から、自動車用鋼板の高強度化が図られている。
しかしながら、鋼材の強度を高めていくと、遅れ破壊という現象が生じやすくなることが知られており、この遅れ破壊は鋼材強度の増大とともに激しくなり、特に引張り強さ１１８０ＭＰａ以上の高強度鋼で顕著となる。なお、遅れ破壊とは、高強度鋼材が静的な負荷応力（引張り強さ以下の負荷応力）を受けた状態で、ある時間が経過したとき、外見上はほとんど塑性変形を伴うことなく、突然脆性的な破壊が生じる現象である。
The automotive steel sheet, although cold-rolled steel sheet is used from the request for accuracy of plate thickness and a flat担度, recently, in view of the reduction and safety CO 2 emissions of the automobile, the high strength of automotive steel sheets Is being planned.
However, it is known that when the strength of steel is increased, the phenomenon of delayed fracture is likely to occur. This delayed fracture becomes more severe with the increase in steel strength, and is particularly remarkable in high-strength steel having a tensile strength of 1180 MPa or more. Becomes Delayed fracture is a condition in which a high-strength steel material is subjected to a static load stress (a load stress equal to or less than the tensile strength). This is a phenomenon in which brittle fracture occurs.
この遅れ破壊は、鋼板の場合、プレス加工により所定の形状に成形したときの残留応力と、応力集中部における鋼の水素脆性により生じるものであることが知られている。この遅れ破壊の原因となる水素は、ほとんどの場合、外部環境から鋼中に侵入、拡散した水素であると考えられており、代表的には、鋼板の腐食の際に発生した水素が鋼中に侵入、拡散したものである。
高強度鋼板におけるこのような遅れ破壊を防止するために、例えば、特許文献１では、鋼板の組織や成分を調整することにより、遅れ破壊感受性を弱める検討がなされている。
In the case of a steel sheet, it is known that the delayed fracture is caused by residual stress when the steel sheet is formed into a predetermined shape by press working and hydrogen embrittlement of the steel in a stress concentrated portion. In most cases, the hydrogen that causes this delayed fracture is considered to be hydrogen that has invaded and diffused into the steel from the external environment.Hydrogen generated during the corrosion of steel sheets is typically Invaded and spread into
In order to prevent such delayed fracture in a high-strength steel sheet, for example, Patent Literature 1 discusses reducing the delayed fracture sensitivity by adjusting the structure and components of the steel sheet.
しかし、特許文献１の手法では、外部環境から鋼板内部に侵入する水素量は変化しないため、遅れ破壊の発生を遅らせることは可能であるが、遅れ破壊自体を防止することはできない。
したがって本発明の目的は、以上のような従来技術の課題を解決し、主として自動車、建材用の強度部材に好適な鋼板であって、引張強度１１８０ＭＰａ以上を有する耐遅れ破壊性に優れた鋼板を提供することにある。
また、本発明の他の目的は、耐遅れ破壊性に優れるとともに、耐食性および溶接性にも優れた鋼板を提供することにある。
However, in the method of Patent Document 1, since the amount of hydrogen entering the inside of the steel sheet from the external environment does not change, it is possible to delay the occurrence of delayed fracture, but it is not possible to prevent delayed fracture itself.
Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, and mainly provide a steel sheet suitable for a strength member for automobiles and building materials, and a steel sheet excellent in delayed fracture resistance having a tensile strength of 1180 MPa or more. To provide.
Another object of the present invention is to provide a steel sheet having excellent delayed fracture resistance and also excellent corrosion resistance and weldability.
本発明者らは、上記の課題を解決すべく、鋼板内部に侵入する水素を抑制することにより遅れ破壊を防止する手段について、鋭意検討および研究を重ねた。その結果、大気環境で使用される鋼板への水素侵入は腐食過程で形成される水膜のｐＨ低下が主因であることから、鋼板表面にｐＨ緩衝性を有する成分を含有した皮膜を形成することにより、鋼板内部への水素侵入を大幅に抑制し、鋼板の遅れ破壊を効果的に抑制できることを見出した。 Means for Solving the Problems In order to solve the above-described problems, the present inventors have intensively studied and studied a means for preventing delayed fracture by suppressing hydrogen entering into the inside of a steel sheet. As a result, since hydrogen intrusion into steel sheets used in the atmospheric environment is mainly caused by a decrease in the pH of the water film formed during the corrosion process, it is necessary to form a film containing a pH buffering component on the steel sheet surface. It has been found that by this, hydrogen intrusion into the inside of the steel sheet can be largely suppressed, and delayed fracture of the steel sheet can be effectively suppressed.
また、自動車用鋼板として使用するためには、優れた耐食性とともに、所定の形状に加工された後に組み付けるための溶接性（導電性）が必要とされる。そこで、耐遅れ破壊性とともに、優れた耐食性および溶接性を得るために検討を行った結果、皮膜中のｐＨ緩衝性を有する成分の付着量や皮膜厚を最適化することにより、上述したような優れた耐遅れ破壊性とともに、優れた耐食性と溶接性が得られることを見出した。 Moreover, in order to use it as a steel plate for automobiles, it is necessary to have not only excellent corrosion resistance but also weldability (conductivity) for assembling after processing into a predetermined shape. Therefore, as a result of studying to obtain excellent corrosion resistance and weldability together with delayed fracture resistance, by optimizing the adhesion amount and the film thickness of the component having pH buffering property in the film, It has been found that excellent corrosion resistance and weldability can be obtained together with excellent delayed fracture resistance.
本発明は、以上のような知見に基づきなされたものであり、その要旨は以下のとおりである。
［1］引張強度が１１８０ＭＰａ以上の鋼板の表面に、０．１ＮのＮａ塩の場合にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）を含有する皮膜を有し、該皮膜中のアニオン化合物（x）の付着量が前記Ｎａ塩換算で１００ｍｇ／ｍ２以上であることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
［2］引張強度が１１８０ＭＰａ以上の鋼板の表面に、０．１ＮのＮａ塩の場合にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を有し、該有機樹脂層中のアニオン化合物（x）の付着量が前記Ｎａ塩換算で１００ｍｇ／ｍ２以上であることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
The present invention has been made based on the above findings, and the gist is as follows.
[1] On the surface of a steel sheet having a tensile strength of 1180 MPa or more, a film containing an anion compound (x) having a pH buffering property in the range of pH 3 to 6.5 in the case of 0.1 N Na salt, A steel sheet excellent in delayed fracture resistance having a tensile strength of 1180 MPa or more, characterized in that the amount of anion compound (x) attached to the film is 100 mg / m 2 or more in terms of the Na salt.
[2] On the surface of a steel plate having a tensile strength of 1180 MPa or more, an organic resin layer containing an anion compound (x) having a pH buffering property in the range of pH 3 to 6.5 in the case of 0.1 N Na salt is provided. A steel sheet excellent in delayed fracture resistance having a tensile strength of 1180 MPa or more, wherein the amount of the anion compound (x) in the organic resin layer is 100 mg / m 2 or more in terms of the Na salt.
［3］引張強度が１１８０ＭＰａ以上の鋼板の表面に、０．１ＮのＮａ塩の場合にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）を含有する下層皮膜を有し、該下層皮膜の上層に有機樹脂層を有し、前記下層皮膜中のアニオン化合物（x）の付着量が前記Ｎａ塩換算で１００ｍｇ／ｍ２以上であることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
［4］引張強度が１１８０ＭＰａ以上の鋼板の表面に、０．１ＮのＮａ塩の場合にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）を含有する下層皮膜を有し、該下層皮膜の上層に、０．１ＮのＮａ塩の場合にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を有し、前記下層皮膜及び有機樹脂層中のアニオン化合物（x）の合計付着量が前記Ｎａ塩換算で１００ｍｇ／ｍ２以上であることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
[3] On the surface of a steel plate having a tensile strength of 1180 MPa or more, an underlayer film containing an anion compound (x) having a pH buffering property in the range of pH 3 to 6.5 in the case of 0.1 N Na salt, An organic resin layer is provided as an upper layer on the lower film, and the amount of the anion compound (x) in the lower film is 100 mg / m 2 or more in terms of the Na salt, and the tensile strength is 1180 MPa or more. A steel sheet with excellent delayed fracture resistance.
[4] On the surface of a steel sheet having a tensile strength of 1180 MPa or more, an underlayer film containing an anion compound (x) having a pH buffering property in the range of pH 3 to 6.5 in the case of 0.1 N Na salt, An organic resin layer containing an anion compound (x) having a pH buffering property in the range of pH 3 to 6.5 in the case of 0.1 N Na salt on the upper layer of the lower layer film, wherein the lower layer film and the organic resin A steel sheet excellent in delayed fracture resistance having a tensile strength of 1180 MPa or more, characterized in that the total adhesion amount of the anion compound (x) in the layer is 100 mg / m 2 or more in terms of the Na salt.
［5］上記［3］または［4］の鋼板において、下層皮膜が、さらに、Ａｌ、Ｍｇ、Ｃａ、Ｚｎ、Ｖ、Ｍｏの中から選ばれる金属の１種以上を含有し、該金属の合計付着量が１０〜２０００ｍｇ／ｍ２であることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
［6］引張強度が１１８０ＭＰａ以上の鋼板の表面に、Ａｌ、Ｍｇ、Ｃａ、Ｚｎ、Ｖ、Ｍｏの中から選ばれる金属の１種以上を含有し、該金属の合計付着量が１０〜２０００ｍｇ／ｍ２である下層皮膜を有し、該下層皮膜の上層に、０．１ＮのＮａ塩の場合にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を有し、該有機樹脂層中のアニオン化合物（x）の付着量が前記Ｎａ塩換算で１００ｍｇ／ｍ２以上であることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
[5] In the steel sheet according to the above [3] or [4], the lower layer coating further contains at least one metal selected from Al, Mg, Ca, Zn, V, and Mo, and A steel sheet excellent in delayed fracture resistance and having a tensile strength of 1180 MPa or more, wherein the steel sheet has an adhesion amount of 10 to 2000 mg / m 2 .
[6] The surface of a steel sheet having a tensile strength of 1180 MPa or more contains one or more metals selected from Al, Mg, Ca, Zn, V, and Mo, and has a total adhesion amount of the metal of 10 to 2000 mg / It has a lower film which m 2, and the upper layer of the lower layer film, an organic resin layer containing an anionic compound having a pH-buffering in the range of pH3~6.5 the case of 0.1N of Na salt (x) Wherein the amount of the anion compound (x) in the organic resin layer is 100 mg / m 2 or more in terms of the Na salt, and the tensile strength is 1180 MPa or more. steel sheet.
［7］上記［2］〜［6］のいずれかの鋼板において、有機樹脂層の膜厚が０．３〜４．０μｍであることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
［8］上記［2］〜［7］のいずれかの鋼板において、有機樹脂層の有機樹脂がエポキシ系樹脂、アクリル系樹脂、エチレン樹脂の中から選ばれる１種以上からなることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
［9］上記［1］〜［8］のいずれかの鋼板において、アニオン化合物（x）の付着量が前記Ｎａ塩換算で５０００ｍｇ／ｍ２以下であることを特徴とする引張強度が１１８０ＭＰａ以上である耐遅れ破壊性に優れた鋼板。
[7] The steel sheet according to any of [2] to [6], wherein the organic resin layer has a thickness of 0.3 to 4.0 μm, and the tensile strength is 1180 MPa or more. Excellent steel plate.
[8] The steel sheet according to any one of [2] to [7], wherein the organic resin of the organic resin layer is at least one selected from an epoxy resin, an acrylic resin, and an ethylene resin. A steel plate having a tensile strength of 1180 MPa or more and excellent in delayed fracture resistance.
[9] The steel sheet according to any one of [1] to [8], wherein the attached amount of the anion compound (x) is 5,000 mg / m 2 or less in terms of the Na salt, and the tensile strength is 1180 MPa or more. A steel sheet with excellent delayed fracture resistance.
本発明の高強度鋼板は、鋼板内部への水素の侵入が抑制され、遅れ破壊が効果的に抑制される優れた耐遅れ破壊性を有する。また、鋼板の腐食しろの削減により鋼板の板厚も小さくすることができるので、自動車分野、建材分野に適用する強度部材の重量削減が可能となる。
また、本発明の高強度鋼板のなかで、皮膜中のｐＨ緩衝性を有する成分の付着量や皮膜厚を最適化したものは、上述した優れた耐遅れ破壊性とともに、優れた耐食性と溶接性を有する。
The high-strength steel sheet of the present invention has excellent delayed fracture resistance in which hydrogen is prevented from entering the inside of the steel sheet and delayed fracture is effectively suppressed. Further, since the thickness of the steel sheet can be reduced by reducing the margin of corrosion of the steel sheet, it is possible to reduce the weight of the strength member applied to the automotive field and the building material field.
Among the high-strength steel sheets of the present invention, those in which the amount of the component having pH buffering properties in the coating and the coating thickness are optimized are excellent corrosion resistance and weldability together with the above-mentioned excellent delayed fracture resistance. Having.
本発明の耐遅れ破壊性に優れた鋼板の基材となる鋼板は、引張強度が１１８０ＭＰａ以上の鋼板である。引張強度が１１８０ＭＰａ以上であれば、その化学組成や鋼組織は特に限定されず、また、圧延方法などについても特に限定されず、熱延鋼板、冷延鋼板のいずれでもよい。しかしながら、このうち、自動車分野や建材分野などで用いられる、特に自動車分野などで多く用いられる引張強度が１１８０ＭＰａ以上の高強度冷延鋼板が好ましく、引張強度が１３２０ＭＰａ以上の高強度冷延鋼板がさらに好ましい。引張強度が低い鋼板は、本質的に遅れ破壊が生じにくい。本発明の効果は、引張強度が低い鋼板でも発現されるが、引張強度が１１８０ＭＰａ以上の鋼板で顕著に発現され、引張強度が１３２０ＭＰａ以上の鋼板でより顕著に発現されるためである。 The steel sheet serving as the base material of the steel sheet having excellent delayed fracture resistance according to the present invention is a steel sheet having a tensile strength of 1180 MPa or more. If the tensile strength is 1180 MPa or more, the chemical composition and the steel structure are not particularly limited, and the rolling method and the like are not particularly limited, and either a hot-rolled steel sheet or a cold-rolled steel sheet may be used. However, among these, a high-strength cold-rolled steel sheet having a tensile strength of 1180 MPa or more, which is used in the field of automobiles and building materials, and is particularly often used in the field of automobiles, is preferred. preferable. A steel plate having a low tensile strength is essentially resistant to delayed fracture. This is because the effect of the present invention is exhibited even in a steel sheet having a low tensile strength, but is significantly exhibited in a steel sheet having a tensile strength of 1180 MPa or more, and is more significantly exhibited in a steel sheet having a tensile strength of 1320 MPa or more.
本発明において好ましく用いられる高強度冷延鋼板は、所望の引張強度を有するものであれば、いかなる組成および組織を有するものでもよく、機械特性などの諸特性を向上させるために、例えば、Ｃ、Ｎなどの侵入型固溶元素およびＳｉ、Ｍｎ、Ｐ、Ｃｒなどの置換型固溶元素の添加による固溶体強化、Ｔｉ、Ｎｂ、Ｖ、Ａｌなどの炭・窒化物による析出強化、Ｗ、Ｚｒ、Ｈｆ、Ｃｏ、Ｂ、Ｃｕ、希土類元素などの強化元素の添加などの化学組成的改質、再結晶の起こらない温度で回復焼きなましすることによる強化あるいは完全に再結晶させずに未再結晶領域を残す部分再結晶強化、ベイナイトやマルテンサイト単相化あるいはフェライトとこれら変態組織の複合組織化といった変態組織による強化、フェライト粒径をｄとしたときのHall-Petchの式：σ＝σ０+ｋｄ-1/2（式中σ：応力、σ０，ｋ：材料定数）で表される細粒化強化、圧延などによる加工強化といった組織的ないし構造的改質を単独でまたは複数を組み合わせて行うことができる。 The high-strength cold-rolled steel sheet preferably used in the present invention may have any composition and structure as long as it has a desired tensile strength.To improve various properties such as mechanical properties, for example, C, Solid solution strengthening by the addition of interstitial solid solution elements such as N and substitutional solid solution elements such as Si, Mn, P, and Cr; precipitation strengthening by carbon and nitride such as Ti, Nb, V, and Al; Chemical compositional reforming such as addition of strengthening elements such as Hf, Co, B, Cu, and rare earth elements, strengthening by recovery annealing at a temperature at which recrystallization does not occur, or unrecrystallized region without complete recrystallization. Partial recrystallization strengthening, strengthening by transformation structure such as bainite or martensite single phase or complex structure of ferrite and these transformation structures, Hall when the ferrite grain size is d -Petch formula: σ = σ 0 + kd -1/2 (where σ: stress, σ 0 , k: material constant) expressed in terms of organization or structure, such as grain refinement strengthening and work strengthening by rolling, etc. Modifications can be performed alone or in combination.
このような高強度冷延鋼板の組成としては、例えば、Ｃ：０．１〜０．４質量％、Ｓｉ:０〜２．５質量％、Ｍｎ：１〜３質量％、Ｐ：０〜０．０５質量％、Ｓ：０〜０．００５質量％、残部がＦｅおよび不可避的不純物であるもの、さらに、これにＣｕ、Ｔｉ、Ｖ、Ａｌ、Ｃｒなどの１種または２種以上を添加したもの、などを例示できる。
また、高強度冷延鋼板として商業的に入手可能なものとしては、例えば、ＪＦＥ−ＣＡ１１８０、ＪＦＥ−ＣＡ１３７０、ＪＦＥ−ＣＡ１４７０、ＪＦＥ−ＣＡ１１８０ＳＦ、ＪＦＥ−ＣＡ１１８０Ｙ１、ＪＦＥ−ＣＡ１１８０Ｙ２(以上、ＪＦＥスチール（株）製）、ＳＡＦＣ１１８０Ｄ（新日鐵住金（株）製）などが非限定的に例示できる。
As the composition of such a high-strength cold-rolled steel sheet, for example, C: 0.1 to 0.4% by mass, Si: 0 to 2.5% by mass, Mn: 1 to 3% by mass, P: 0 to 0% 0.05% by mass, S: 0 to 0.005% by mass, the balance being Fe and unavoidable impurities, and one or more of Cu, Ti, V, Al, Cr and the like were added thereto. And the like.
Examples of commercially available high-strength cold-rolled steel sheets include, for example, JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, JFE-CA1180Y2 (JFE-CA1180Y2). )) And SAFC1180D (manufactured by Nippon Steel & Sumitomo Metal Corporation).
また、基材である鋼板の板厚も特に限定されないが、０．８〜２．５ｍｍ程度、より好ましくは１．２〜２．０ｍｍ程度が適当である。
一般に自動車や建材用の高強度部材には、耐食性を高める目的から亜鉛系めっき鋼板が採用されるが、亜鉛系めっきは腐食過程において多くの水素を発生させるため、遅れ破壊特性に悪影響を与える。このため、亜鉛系めっき鋼板は本発明の基材鋼板として好ましくない。
The thickness of the steel sheet as the base material is not particularly limited, but is suitably about 0.8 to 2.5 mm, more preferably about 1.2 to 2.0 mm.
Generally, zinc-plated steel sheets are used for high-strength members for automobiles and building materials for the purpose of improving corrosion resistance. However, zinc-based plating generates a large amount of hydrogen in a corrosion process, and thus has an adverse effect on delayed fracture characteristics. For this reason, galvanized steel sheets are not preferred as the base steel sheet of the present invention.
本発明者らの研究および検討結果によれば、腐食過程における鋼板内部への水素侵入は、乾燥湿潤が繰り返される腐食環境下において、鋼板の乾燥過程におけるｐＨの低下が大きく寄与していると考えられる。大気環境で鋼板に付着する塩分としては、海水からの飛来塩分や路面の凍結防止剤として散布される融雪塩などが挙げられる。これらの塩分が湿度上昇や路面水の付着により吸水することで水膜を形成し、鋼板を腐食させる。初期の塩水は通常の環境においては中性であるが、腐食反応に伴いｐＨは約９まで上昇する。その後、湿度低下に伴い水膜が減少し、塩化物濃度が高くなることで溶液のｐＨは低下し、鋼板への水素侵入量が増加する。すなわち、水素侵入を抑制するためには、腐食過程でのｐＨ低下を抑制することが重要である。 According to the research and examination results of the present inventors, it is considered that hydrogen intrusion into the inside of a steel sheet in a corrosion process is largely contributed by a decrease in pH in a drying process of a steel sheet in a corrosive environment where dry and wet are repeated. Can be Examples of the salt attached to the steel sheet in the air environment include salt coming from seawater and snow-melting salt sprayed as a deicing agent on the road surface. These salts absorb water due to an increase in humidity or adhesion of road surface water to form a water film and corrode the steel sheet. The initial salt water is neutral in a normal environment, but the pH rises to about 9 with a corrosive reaction. Thereafter, as the humidity decreases, the water film decreases, and the chloride concentration increases, thereby lowering the pH of the solution and increasing the amount of hydrogen entering the steel sheet. That is, in order to suppress hydrogen intrusion, it is important to suppress a decrease in pH during the corrosion process.
このため本発明の鋼板は、引張強度が１１８０ＭＰａ以上である鋼板表面に、ｐＨ緩衝性を有するアニオン化合物を含有する皮膜を形成することで、腐食過程でのｐＨ低下を抑制し、遅れ破壊を抑制できる特性を具備させるようにするものである。ここで、ｐＨ緩衝性は物質の種類により大きく異なるが、本発明者らによる検討の結果、０．１ＮのＮａ塩の場合（すなわち、アニオン化合物がＮａ塩のアニオンであると仮定して、その０．１ＮのＮａ塩水溶液の場合）にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）を皮膜中に含有させる必要があることが判った。これは腐食過程において鋼板に付着した水膜がｐＨ９〜ｐＨ２の範囲で変化するためであり、ｐＨ３未満でｐＨ緩衝性を示す物質の場合には、たとえ腐食過程での水膜がｐＨ緩衝性を示したとしても、割れ発生に十分な量の水素が侵入するため好ましくない。一方、ｐＨ６．５より大きい範囲は、水素侵入する量が少ない環境であるため、ｐＨ６．５より大きい範囲でｐＨ緩衝性を示す物質の場合は水素侵入の抑制効果が小さいため好ましくない。 For this reason, the steel sheet of the present invention suppresses a decrease in pH during the corrosion process and suppresses delayed fracture by forming a film containing an anion compound having a pH buffering property on the surface of the steel sheet having a tensile strength of 1180 MPa or more. It is intended to provide the characteristics that can be achieved. Here, the pH buffering property varies greatly depending on the type of the substance, but as a result of the study by the present inventors, in the case of a 0.1 N Na salt (that is, assuming that the anion compound is an anion of the Na salt, It was found that it was necessary to include an anion compound (x) having a pH buffering property in the range of pH 3 to 6.5 in the case of a 0.1N Na salt aqueous solution) in the film. This is because the water film adhered to the steel plate changes in the range of pH 9 to pH 2 during the corrosion process. In the case of a substance exhibiting a pH buffering property below pH 3, even if the water film in the corrosion process has a pH buffering property. Even if it is shown, it is not preferable because a sufficient amount of hydrogen enters to generate cracks. On the other hand, a range higher than pH 6.5 is an environment in which the amount of hydrogen entering is small, and a substance exhibiting a pH buffering property in a range higher than pH 6.5 is not preferable because the effect of suppressing hydrogen penetration is small.
このため本発明の高強度鋼板は、引張強度が１１８０ＭＰａ以上の鋼板の表面に、０．１ＮのＮａ塩の場合にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）を含有する皮膜を形成するものである。
ここで、アニオン化合物（x）が有するｐＨ緩衝性は、０．１ｍｏｌ／Ｌのアニオン化合物のＮａ塩（水溶液）に０．１ｍｏｌ／Ｌの塩酸を滴下し、ｐＨが３となる塩酸滴下量が、純水に上記０．１ｍｏｌ／Ｌの塩酸を滴下した場合の塩酸滴下量に比べて３０倍以上であるようなｐＨ緩衝性能であることが好ましい。
なお、本発明において、ｐＨ３〜６．５の範囲でｐＨ緩衝性を有するとは、ｐＨ３〜６．５の範囲内の任意のｐＨ領域（範囲）においてｐＨ緩衝性を有すればよいことを意味し、ｐＨ３〜６．５の全ての領域でｐＨ緩衝性を有する必要はない。
Therefore, the high-strength steel sheet of the present invention contains, on the surface of a steel sheet having a tensile strength of 1180 MPa or more, an anion compound (x) having a pH buffering property in the range of pH 3 to 6.5 in the case of 0.1N Na salt. It forms a film that does.
Here, the pH buffering property of the anion compound (x) is such that 0.1 mol / L of hydrochloric acid is dropped into 0.1 mol / L of the Na salt (aqueous solution) of the anion compound, and the drop amount of hydrochloric acid at which the pH becomes 3 is reduced. It is preferable that the pH buffer performance is 30 times or more the amount of hydrochloric acid dropped when pure hydrochloric acid of 0.1 mol / L is dropped into pure water.
In the present invention, having a pH buffering property in the range of pH 3 to 6.5 means that it is only necessary to have a pH buffering property in any pH range (range) within the range of pH 3 to 6.5. However, it is not necessary to have a pH buffering property in all the range of pH 3 to 6.5.
０．１ＮのＮａ塩の場合にｐＨ３〜６．５の範囲でｐＨ緩衝性を有するアニオン化合物（x）（以下、説明の便宜上、単に「ｐＨ緩衝性を有するアニオン化合物（x）」という）の具体例としては、例えば、モリブデン酸、タングステン酸、メタクリル酸、サリチル酸、マレイン酸、コハク酸、リンゴ酸、クエン酸、酢酸などが挙げられ、これらの１種以上を用いることができる。アニオン化合物がｐＨ３〜６．５の範囲でｐＨ緩衝性を示すかどうかは、当該アニオン化合物を溶解させ、例えば０．１Ｎの水酸化ナトリウムでｐＨを上昇させた場合の添加量に対する停滞域で評価できるが、酸解離定数pKaの±１でｐＨ緩衝性が高くなることが知られているため、pKaが３〜６．５の間にあるアニオン化合物のＮａ塩として判断することもできる。例えば、上述したアニオン化合物のpKaは、それぞれモリブデン酸が［３．６、３．９］、タングステン酸が［４．６、３．５］、メタクリル酸が［４．７］、サリチル酸が［３．０］、マレイン酸が［１．９、６．３］、コハク酸が［４．０、５．２］、リンゴ酸が［３．４、５．１］、クエン酸が［３．１、４．８、６．４］、酢酸が［４．６］であり、多段解離する場合はいずれかのpKaが３〜６．５の範囲であればよい。一方、リン酸のpKaは［２．２、７．２、１２．７］であり、いずれも３〜６．５の範囲外である。通常、これらのアニオン化合物（x）は、当該アニオンの金属塩（例えば、Ｎａ、Ｃａ、Ｆｅなどをカチオンとする金属塩）を皮膜形成用の処理液中に添加することにより、皮膜中に含有させることができる。 In the case of a 0.1 N Na salt, an anion compound (x) having a pH buffering property in the range of pH 3 to 6.5 (hereinafter, simply referred to as “anion compound having a pH buffering property (x)” for convenience of explanation) Specific examples include, for example, molybdic acid, tungstic acid, methacrylic acid, salicylic acid, maleic acid, succinic acid, malic acid, citric acid, and acetic acid, and one or more of these can be used. Whether the anionic compound exhibits pH buffering in the pH range of 3 to 6.5 is evaluated by dissolving the anionic compound and increasing the pH with 0.1 N sodium hydroxide. However, since it is known that the pH buffering property increases when the acid dissociation constant pKa is ± 1, it can also be determined as an Na salt of an anion compound having a pKa between 3 and 6.5. For example, the pKa of the above-mentioned anion compound is [3.6, 3.9] for molybdic acid, [4.6, 3.5] for tungstic acid, [4.7] for methacrylic acid, and [3] for salicylic acid. 0.0], maleic acid [1.9, 6.3], succinic acid [4.0, 5.2], malic acid [3.4, 5.1], and citric acid [3.1] 4.8, 6.4] and acetic acid is [4.6], and in the case of multi-stage dissociation, any of pKa may be in the range of 3 to 6.5. On the other hand, the pKa of phosphoric acid is [2.2, 7.2, 12.7], which is out of the range of 3-6.5. Usually, these anion compounds (x) are contained in a film by adding a metal salt of the anion (for example, a metal salt having a cation of Na, Ca, Fe, or the like) to a treatment solution for film formation. Can be done.
腐食過程で生成する水素によって低下するｐＨを抑制する効果を発現するためには、皮膜中でのアニオン化合物（x）の付着量がＮａ塩換算（当該アニオン化合物（x）がＮａ塩のアニオンであると仮定した場合のＮａ塩換算の付着量。以下同様）で１００ｍｇ／ｍ２以上であることが必要である。一方、遅れ破壊を抑制するという観点からは付着量に上限はないが、付着量が多くなり過ぎると自動車などの塗装工程において密着性が劣化するため、自動車用途などに適用する鋼板の場合には、５０００ｍｇ／ｍ２以下の付着量とすることが好ましい。 In order to exhibit the effect of suppressing the pH lowered by the hydrogen generated in the corrosion process, the amount of the anion compound (x) attached to the film is calculated in terms of Na salt (where the anion compound (x) is an anion of the Na salt). It is necessary to be 100 mg / m 2 or more in terms of the amount of Na salt conversion assuming that the amount is present. On the other hand, from the viewpoint of suppressing delayed fracture, there is no upper limit on the amount of adhesion, but if the amount of adhesion is too large, the adhesion deteriorates in the coating process of automobiles and the like. It is preferable that the adhesion amount is 5000 mg / m 2 or less.
ｐＨ緩衝性を有するアニオン化合物（x）を含有する皮膜を鋼板表面に形成する形態に制限はなく、ｐＨ緩衝性を有するアニオン化合物（x）を単独で或いは他の無機成分などとともに含有する皮膜を形成してもよいし、ｐＨ緩衝性を有するアニオン化合物（x）を含有する有機皮膜（有機樹脂層）を形成してもよいが、後者の場合には、鋼板が腐食環境に曝されたときに有機樹脂層が腐食環境に対するバリア層として機能するため、特に優れた耐食性が得られるので、より好ましい。 There is no limitation on the form in which the film containing the pH buffering anion compound (x) is formed on the surface of the steel sheet, and the film containing the pH buffering anion compound (x) alone or together with other inorganic components is used. It may be formed, or an organic film (organic resin layer) containing an anion compound (x) having a pH buffering property may be formed, but in the latter case, when the steel sheet is exposed to a corrosive environment. Since the organic resin layer functions as a barrier layer against a corrosive environment, particularly excellent corrosion resistance can be obtained.
アニオン化合物（x）を含有する有機樹脂層の場合には、有機樹脂層の膜厚が小さいと腐食環境に対するバリア層としての機能が低下するため、膜厚は０．３μｍ以上とすることが好ましい。一方、自動車用鋼板の場合、プレス加工により所定の形状に加工された後に、スポット溶接により鋼板どうしを組み付ける工程がある。このとき、有機樹脂層が厚すぎると溶接時の電流が流れず溶接不良となる場合があるため、鋼板の接合にスポット溶接を用いるような用途の場合は、有機樹脂層の膜厚は４．０μｍ以下とすることが好ましい。
ここで、有機樹脂層の膜厚は、皮膜断面を観察し、任意視野の複数箇所（例えば３箇所）で有機樹脂層の厚さ（基材鋼板面から有機樹脂層の表面までの厚さ）を測定し、それらの平均値をもって膜厚とする。断面加工の方法は特に限定されないが、例えばＦＩＢ加工などが挙げられる。
In the case of an organic resin layer containing an anion compound (x), the function as a barrier layer against a corrosive environment is reduced if the film thickness of the organic resin layer is small, so that the film thickness is preferably 0.3 μm or more. . On the other hand, in the case of a steel plate for an automobile, there is a process of assembling the steel plates by spot welding after being processed into a predetermined shape by press working. At this time, if the organic resin layer is too thick, the current during welding may not flow, resulting in poor welding. In the case of using spot welding for joining steel sheets, the thickness of the organic resin layer is set to 3. It is preferable that the thickness be 0 μm or less.
Here, the thickness of the organic resin layer is determined by observing the cross section of the film and measuring the thickness of the organic resin layer (thickness from the surface of the base steel plate to the surface of the organic resin layer) at a plurality of locations (for example, three locations) in an arbitrary field of view. Is measured, and the average value thereof is defined as the film thickness. The method for processing the cross section is not particularly limited, and examples thereof include FIB processing.
有機樹脂層に用いる有機樹脂の種類に特に制限はなく、例えば、エポキシ系樹脂（エポキシ樹脂、変性エポキシ樹脂など）、ウレタン樹脂、アルキド樹脂、アクリル系樹脂、エチレン樹脂（ポリオレフィン樹脂）、ポリエステル樹脂、ポリブタジエン樹脂、アミノ樹脂、フェノール樹脂、フッ素樹脂、シリコン樹脂などが挙げられ、これらの１種以上を用いることができる。また、これらのなかで、エポキシ系樹脂、アクリル系樹脂、エチレン系樹脂が、腐食因子である水分や塩化物をバリアする効果が高いため特に好ましい。 There is no particular limitation on the type of organic resin used for the organic resin layer. For example, epoxy resins (epoxy resins, modified epoxy resins, etc.), urethane resins, alkyd resins, acrylic resins, ethylene resins (polyolefin resins), polyester resins, Examples thereof include a polybutadiene resin, an amino resin, a phenol resin, a fluorine resin, and a silicon resin, and one or more of these can be used. Of these, epoxy resins, acrylic resins, and ethylene resins are particularly preferable because of their high barrier effect against moisture and chloride as corrosion factors.
また、本発明の高強度鋼板の他の形態としては、以下のようなものが挙げられる。
（i）鋼板の表面に、ｐＨ緩衝性を有するアニオン化合物（x）を含有する下層皮膜を有し、その上層に有機樹脂層を有する鋼板。
（ii）鋼板の表面に、ｐＨ緩衝性を有するアニオン化合物（x）を含有する下層皮膜を有し、その上層にｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を有する鋼板。
（iii）鋼板の表面に、特定の下層皮膜（例えば無機皮膜）を有し、その上層にｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を有する鋼板。
Other forms of the high-strength steel sheet of the present invention include the following.
(I) A steel sheet having a lower layer film containing an anion compound (x) having a pH buffering property on the surface of a steel sheet, and having an organic resin layer as an upper layer.
(Ii) A steel sheet having, on the surface of a steel sheet, a lower layer film containing an anion compound (x) having a pH buffering property, and an organic resin layer containing an anion compound (x) having a pH buffering property on an upper layer.
(Iii) A steel sheet having a specific lower layer film (for example, an inorganic film) on the surface of the steel sheet, and having an organic resin layer containing an anion compound (x) having a pH buffering property as an upper layer.
上記（i）の鋼板では、下層皮膜中でのｐＨ緩衝性を有するアニオン化合物（x）の付着量はＮａ塩換算で１００ｍｇ／ｍ２以上であり、好ましくは５０００ｍｇ／ｍ２以下である。また、上記（ii）の鋼板では、下層皮膜と有機樹脂層中でのｐＨ緩衝性を有するアニオン化合物（x）の合計付着量はＮａ塩換算で１００ｍｇ／ｍ２以上であり、好ましくは５０００ｍｇ／ｍ２以下である。また、上記（iii）の鋼板では、有機樹脂層中でのｐＨ緩衝性を有するアニオン化合物（x）の付着量はＮａ塩換算で１００ｍｇ／ｍ２以上であり、好ましくは５０００ｍｇ／ｍ２以下である。これらの理由は、上述した通りである。 In the steel sheet (i), the adhesion amount of the anion compound (x) having a pH buffering property in the lower layer coating is 100 mg / m 2 or more, preferably 5000 mg / m 2 or less in terms of Na salt. In addition, in the steel sheet (ii), the total adhesion amount of the anion compound (x) having a pH buffering property in the lower film and the organic resin layer is 100 mg / m 2 or more in terms of Na salt, and preferably 5000 mg / m 2. m 2 or less. Further, in the steel sheet of the above (iii), the adhesion amount of the anion compound (x) having a pH buffering property in the organic resin layer is 100 mg / m 2 or more as Na salt conversion, preferably 5000 mg / m 2 or less. is there. These reasons are as described above.
上述した（i）、（ii）の形態の高強度鋼板において、ｐＨ緩衝性を有するアニオン化合物（x）を含有する下層皮膜は、さらにＡｌ、Ｍｇ、Ｃａ、Ｚｎ、Ｖ、Ｍｏの中から選ばれる金属の１種以上を含有することができ、これにより、本発明の効果をより高めることができる。その理由は必ずしも明らかではないが、Ｆｅ、Ａｌ、Ｍｇ、Ｃａ、Ｚｎ、Ｖ、Ｍｏのカチオン種自体が遅れ破壊を抑制する効果を発現することに加えて、下層皮膜中に含有させたｐＨ緩衝性を示すアニオン化合物（x）とＡｌ、Ｍｇ、Ｃａ、Ｚｎ、Ｖ、Ｍｏの１種以上のカチオン種が結合することで不溶性物質となり、腐食自体を抑制する効果を発現すること、などによるものと考えられる。
このような効果を発現させるためには、当該金属の合計付着量を１０ｍｇ／ｍ２以上とする必要がある。一方、付着量が２０００ｍｇ／ｍ２を超えると、上層側の皮膜との界面の密着性が劣化し、プレス加工時に剥がれを生じる恐れがあるため、付着量は２０００ｍｇ／ｍ２以下とする必要がある。
In the high-strength steel sheets of the above-mentioned forms (i) and (ii), the undercoat containing the anion compound (x) having a pH buffering property is further selected from Al, Mg, Ca, Zn, V and Mo. Of the present invention, and thereby the effect of the present invention can be further enhanced. Although the reason is not always clear, the cation species of Fe, Al, Mg, Ca, Zn, V, and Mo themselves exhibit the effect of suppressing delayed destruction, and the pH buffer contained in the underlayer coating. By combining an anionic compound (x) exhibiting a property with one or more cationic species of Al, Mg, Ca, Zn, V, and Mo to form an insoluble substance and exhibit an effect of suppressing corrosion itself. it is conceivable that.
In order to exhibit such an effect, the total amount of the metal to be deposited needs to be 10 mg / m 2 or more. On the other hand, if the adhesion amount exceeds 2000 mg / m 2 , the adhesion at the interface with the upper layer film is deteriorated, and there is a possibility of peeling during press working. Therefore, the adhesion amount needs to be 2000 mg / m 2 or less. is there.
また、上述した（iii）の形態の高強度鋼板において、下層皮膜は、Ａｌ、Ｍｇ、Ｃａ、Ｚｎ、Ｖ、Ｍｏの中から選ばれる金属の１種以上を含有することができ、これにより、本発明の効果をより高めることができる。その理由は、Ｆｅ、Ａｌ、Ｍｇ、Ｃａ、Ｚｎ、Ｖ、Ｍｏのカチオン種自体が遅れ破壊を抑制する効果を発現することに加えて、上層の有機皮膜中に含有させたｐＨ緩衝性を示すアニオン化合物（x）とＡｌ、Ｍｇ、Ｃａ、Ｚｎ、Ｖ、Ｍｏの１種以上のカチオン種が結合することで不溶性物質となり、腐食自体を抑制する効果を発現すること、などによるものと考えられる。
このような効果を発現させるためには、当該金属の合計付着量を１０ｍｇ／ｍ２以上とする必要がある。一方、付着量が２０００ｍｇ／ｍ２を超えると、上層側の皮膜との界面の密着性が劣化し、プレス加工時に剥がれを生じる恐れがあるため、付着量は２０００ｍｇ／ｍ２以下とする必要がある。
上述したＡｌ、Ｍｇ、Ｃａなどの金属は、通常、当該金属を含む金属塩を皮膜形成用の処理液中に添加することにより、皮膜中に含有させることができる。
Further, in the high-strength steel sheet of the above-mentioned form (iii), the lower layer coating can contain at least one metal selected from Al, Mg, Ca, Zn, V, and Mo. The effects of the present invention can be further enhanced. The reason is that, in addition to the cation species of Fe, Al, Mg, Ca, Zn, V, and Mo themselves exhibiting the effect of suppressing delayed destruction, they exhibit pH buffering properties contained in the upper organic film. It is considered that the anion compound (x) and at least one kind of cations of Al, Mg, Ca, Zn, V, and Mo combine to form an insoluble substance and exhibit an effect of suppressing corrosion itself. .
In order to exhibit such an effect, the total amount of the metal to be deposited needs to be 10 mg / m 2 or more. On the other hand, if the adhesion amount exceeds 2000 mg / m 2 , the adhesion at the interface with the upper layer film is deteriorated, and there is a possibility of peeling during press working. Therefore, the adhesion amount needs to be 2000 mg / m 2 or less. is there.
The above-mentioned metals such as Al, Mg, Ca and the like can usually be contained in the film by adding a metal salt containing the metal to the treatment liquid for film formation.
鋼板表面に、ｐＨ緩衝性を有するアニオン化合物（x）を含有する皮膜を形成するには、ｐＨ緩衝性を有するアニオン化合物（x）を含有する処理液（水溶液）を鋼板表面にコーティングした後、加熱乾燥させる方法が採られる。
鋼板表面又は下層皮膜表面に、ｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を形成するには、有機樹脂を溶媒（水および／または有機溶剤）に溶解および／または分散させ、さらにｐＨ緩衝性を有するアニオン化合物（x）を添加した処理液（樹脂溶液）を鋼板表面又は下層皮膜表面にコーティングした後、加熱乾燥させる方法が採られる。
また、処理液にｐＨ緩衝性を有するアニオン化合物（x）を添加するには、当該アニオンの金属塩を処理液中に添加すればよい。
In order to form a film containing an anion compound having pH buffering properties (x) on the surface of a steel sheet, a coating solution (aqueous solution) containing an anion compound having pH buffering properties (x) is coated on the steel sheet surface, A method of heating and drying is employed.
In order to form an organic resin layer containing an anion compound (x) having a pH buffering property on the surface of a steel sheet or the surface of a lower layer film, an organic resin is dissolved and / or dispersed in a solvent (water and / or an organic solvent), Furthermore, a method is adopted in which a treatment liquid (resin solution) to which an anion compound (x) having a pH buffering property is added is coated on the surface of the steel sheet or the surface of the lower layer film, and then dried by heating.
Further, to add an anion compound (x) having a pH buffering property to the treatment liquid, a metal salt of the anion may be added to the treatment liquid.
ｐＨ緩衝性を有するアニオン化合物（x）を含有する処理液（水溶液又は樹脂溶液）をコーティングする方法に特別な制限はなく、公知の方法、例えば、塗布方式、浸漬方式、スプレー方式のいずれでもよい。塗布方式では、ロールコーター（３ロール方式、２ロール方式など）、スクイズコーター、ダイコーターなどのいずれの塗布手段を用いてもよい。また、スクイズコーターなどによる塗布処理、浸漬処理、スプレー処理の後に、エアナイフ法やロール絞り法により塗布量の調整、外観の均一化、膜厚の均一化を行うことも可能である。コーティングした処理液を加熱乾燥する方法は任意であり、例えば、ドライヤー、熱風炉、高周波誘導加熱炉、赤外線炉等の手段を用いることができる。 There is no particular limitation on the method of coating the treatment liquid (aqueous solution or resin solution) containing the pH buffering anion compound (x), and any known method, for example, any of an application method, an immersion method, and a spray method may be used. . In the coating method, any coating means such as a roll coater (such as a three-roll method or a two-roll method), a squeeze coater, or a die coater may be used. Further, after the coating, dipping, and spraying with a squeeze coater or the like, adjustment of the coating amount, uniform appearance, and uniform film thickness can be performed by an air knife method or a roll drawing method. The method of heating and drying the coated treatment liquid is arbitrary, and for example, a means such as a dryer, a hot air oven, a high-frequency induction heating oven, an infrared oven, or the like can be used.
皮膜中のｐＨ緩衝性を有するアニオン化合物（x）の付着量は、例えば、（i）アニオン化合物を構成する元素やカチオン種を、蛍光Ｘ線を用いて既知の元素量を検量板として算出する方法、（ii）皮膜を塩酸などに溶解させ、ＩＣＰにより定量化する方法、などで測定することができる。上記（i）の方法では、皮膜を付与していない鋼板自体の元素量を予め測定しておき、皮膜を付与した後に当該元素を測定することで皮膜量として算出することができる。
また、インヒビターを入れた塩酸に浸漬し、所定時間ごとに質量変化を測定すると、皮膜の溶解と下地鋼板の溶解速度が大きく異なるため、溶解速度の変曲点の質量から皮膜量を算出することができる。
以上の測定法によれば、無機皮膜、有機皮膜にかかわりなく、皮膜中のｐＨ緩衝性を有するアニオン化合物（x）の付着量を測定できる。
The adhesion amount of the anion compound (x) having a pH buffering property in the film is calculated, for example, using (i) the element and cation species constituting the anion compound using a known element amount using a fluorescent X-ray as a calibration plate. And (ii) a method in which the film is dissolved in hydrochloric acid or the like and quantified by ICP. In the method (i), the amount of the element of the steel sheet itself to which the film is not applied is measured in advance, and the amount of the film can be calculated by measuring the element after applying the film.
Also, when immersed in hydrochloric acid containing an inhibitor and the mass change is measured at predetermined time intervals, the dissolution rate of the coating and the dissolution rate of the base steel sheet are significantly different. Can be.
According to the above measurement method, the amount of the anion compound (x) having a pH buffering property in the film can be measured irrespective of the inorganic film or the organic film.
本発明において基材として使用される鋼板の製造方法は特に限定されない。本発明の理解を容易にするために、例えば、冷延鋼板の表面にｐＨ緩衝性を有するアニオン化合物（x）を含有する皮膜（有機樹脂層である場合を含む。）を形成する場合における、製鋼からの一連のプロセスについて、一例を挙げて簡単に説明する。但し、基材となる鋼板の製造工程としては、以下の例示に限定されるものではない。
所定の成分組成の鋼を溶製し、常法に従い連続鋳造でスラブとする。次いで、得られたスラブを加熱炉中で１１００〜１３００℃の温度で加熱し、７５０〜９５０℃の仕上げ温度で熱間圧延を行い、５００〜６５０℃にて巻き取る。これに続いて酸洗後、圧下率３０〜７０％の冷間圧延を行う。その後、必要に応じて、常法に従い、アルカリまたはアルカリと界面活性剤およびキレート剤との混合溶液による洗浄、電解洗浄、温水洗浄、乾燥を行う清浄化処理を行った後、６５０〜９００℃にて加熱処理し、急速冷却を行い、鋼板の引張強度の調整を行う。さらに必要に応じて、常法に従い０．０１〜０．５％程度の調質圧延を行うことで所望の引張強度を有する冷延鋼板を得る。
このようにして得られた冷延鋼板表面に、さきに述べた方法で処理液（水溶液又は樹脂溶液）をコーティングした後、加熱乾燥することにより皮膜を形成する。以上により、本発明の耐遅れ破壊性に優れた高強度鋼板を得ることができる。
The method for producing the steel sheet used as the substrate in the present invention is not particularly limited. In order to facilitate the understanding of the present invention, for example, in the case of forming a film (including an organic resin layer) containing an anion compound (x) having a pH buffering property on the surface of a cold-rolled steel sheet, A series of processes from steelmaking will be briefly described with an example. However, the manufacturing process of the steel sheet serving as the base material is not limited to the following examples.
A steel having a predetermined composition is melted and continuously cast into a slab according to a conventional method. Next, the obtained slab is heated at a temperature of 1100 to 1300 ° C in a heating furnace, hot-rolled at a finishing temperature of 750 to 950 ° C, and wound at 500 to 650 ° C. Subsequently, after pickling, cold rolling is performed at a rolling reduction of 30 to 70%. Thereafter, if necessary, according to a conventional method, after performing a cleaning treatment of washing with an alkali or a mixed solution of an alkali and a surfactant and a chelating agent, electrolytic washing, hot water washing, and drying, and then performing heating at 650 to 900 ° C. Heat treatment and rapid cooling to adjust the tensile strength of the steel sheet. Further, if necessary, a cold-rolled steel sheet having a desired tensile strength is obtained by performing temper rolling of about 0.01 to 0.5% according to a conventional method.
The surface of the cold-rolled steel sheet thus obtained is coated with a treatment liquid (aqueous solution or resin solution) by the method described above, and then dried by heating to form a film. As described above, a high-strength steel sheet having excellent delayed fracture resistance according to the present invention can be obtained.
素材鋼板として、Ｃ：０．１９質量％、Ｓｉ：０．４質量％、Ｍｎ：１．５３質量％、Ｐ：０．０１１質量％、Ｓ：０．００１質量％、残部Ｆｅおよび不可避的不純物からなる成分を有し、引張強度が１４８０ＭＰａ、板厚が１．６ｍｍの冷延鋼板（冷間圧延ままの鋼板）を用いた。この冷延鋼板をトルエンに浸漬して５分間超音波洗浄を行った後、単層皮膜又は下層皮膜と有機樹脂層からなる複層皮膜を形成した。 As a material steel sheet, C: 0.19% by mass, Si: 0.4% by mass, Mn: 1.53% by mass, P: 0.011% by mass, S: 0.001% by mass, balance Fe and inevitable impurities A cold-rolled steel sheet (steel sheet as cold-rolled) having a tensile strength of 1480 MPa and a sheet thickness of 1.6 mm was used. After the cold-rolled steel sheet was immersed in toluene and subjected to ultrasonic cleaning for 5 minutes, a single-layer film or a multilayer film composed of a lower layer film and an organic resin layer was formed.
ｐＨ緩衝性を有するアニオン化合物（x）を含有する皮膜は、アニオン化合物（x）を含有するＮａ塩水溶液を鋼板に塗布した後、ＩＨ加熱炉で到達板温が１４０℃となるように加熱乾燥して成膜した。このとき、溶液の濃度を変化させることにより成膜後のアニオン化合物（x）の付着量を変化させた。
また、ｐＨ緩衝性を有するアニオン化合物（x）を含有しない無機皮膜は、本発明例にあっては、前記アニオン化合物（x）を含有する皮膜の成膜前又は後に硝酸塩水溶液を鋼板に塗布した後、ＩＨ加熱炉で到達板温が１４０℃となるように加熱乾燥して成膜した。また、アニオン化合物（x）を含有しない無機皮膜を有する比較例にあっては、硝酸塩水溶液を鋼板に塗布した後、ＩＨ加熱炉で到達板温が１４０℃となるように加熱乾燥して成膜した。
The coating containing the anion compound (x) having a pH buffering property is applied to a steel sheet by applying an aqueous solution of a Na salt containing the anion compound (x), and then heated and dried in an IH heating furnace so that the reached plate temperature becomes 140 ° C. The film was formed. At this time, the attached amount of the anion compound (x) after film formation was changed by changing the concentration of the solution.
In the present invention, the inorganic film not containing the pH buffering anion compound (x) was coated with a nitrate aqueous solution on the steel sheet before or after the film containing the anion compound (x) was formed. Thereafter, the film was heated and dried in an IH heating furnace so that the reached plate temperature became 140 ° C. to form a film. In the comparative example having an inorganic film not containing the anion compound (x), a nitrate aqueous solution was applied to a steel sheet, and then heated and dried in an IH heating furnace so that the reached sheet temperature was 140 ° C., to form a film. did.
有機樹脂層用には下記Ａ１〜Ａ４の有機樹脂を用い、いずれかの有機樹脂を含む処理液をロール方式による塗布法で塗布した後、到達板温が１２０℃となるようにＩＨ加熱炉で加熱することで有機樹脂層を形成した。ｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を形成する場合には、有機樹脂を含む処理液に当該アニオンの金属塩を添加することで調製された、ｐＨ緩衝性を有するアニオン化合物（x）を含有する処理液を用いた。
Ａ１：フッ素樹脂（旭硝子（株）製、商品名：ルミフロン ＬＦ５５２）
Ａ２：ポリオレフィン樹脂（東邦化学工業（株）製、商品名：ＨＹＴＥＣ Ｓ−３１２１）
Ａ３：エポキシ系樹脂（ジャパンエポキシレジン（株）製、商品名：ｊＥＲ１００９）
Ａ４：エチレンアクリル樹脂（日本パーカライジング社製）
For the organic resin layer, an organic resin of the following A1 to A4 is used, and after applying a treatment liquid containing any of the organic resins by a coating method using a roll method, an IH heating furnace is used so that the reached plate temperature becomes 120 ° C. The organic resin layer was formed by heating. When forming an organic resin layer containing an anion compound (x) having a pH buffering property, an anion having a pH buffering property prepared by adding a metal salt of the anion to a treatment solution containing the organic resin. A treatment solution containing the compound (x) was used.
A1: Fluororesin (made by Asahi Glass Co., Ltd., trade name: Lumiflon LF552)
A2: polyolefin resin (manufactured by Toho Chemical Industry Co., Ltd., trade name: HYTEC S-3121)
A3: Epoxy resin (trade name: jER1009, manufactured by Japan Epoxy Resin Co., Ltd.)
A4: Ethylene acrylic resin (manufactured by Nippon Parkerizing Co., Ltd.)
以上のようにして得られた各鋼板について、以下の特性を評価した。その結果を、皮膜構成とともに、表１〜表４に示す。
皮膜中でのｐＨ緩衝性を有するアニオン化合物（x）の付着量の測定は、蛍光X線を用い、付与前後のＮａ量の差異から算出した。また、皮膜中でのＡｌ、Ｃａなどの金属の付着量の測定も同様に蛍光X線を用い、付与前後の金属量の差異から算出した。
有機樹脂層の膜厚の測定は、ＦＩＢ加工により得られた断面をＳＥＭ観察し、任意視野の３箇所で有機樹脂層の厚さ（基材鋼板面から有機樹脂層の表面までの厚さ）を測定し、それらの平均値を膜厚とした。
The following characteristics were evaluated for each steel sheet obtained as described above. The results are shown in Tables 1 to 4 together with the film configuration.
The adhesion amount of the anion compound having pH buffering property (x) in the film was measured using fluorescent X-rays, and was calculated from the difference in the amount of Na before and after the application. In addition, the measurement of the adhesion amount of metal such as Al and Ca in the film was similarly performed using fluorescent X-rays, and was calculated from the difference in the amount of metal before and after application.
The thickness of the organic resin layer is measured by observing a cross section obtained by FIB processing with an SEM and measuring the thickness of the organic resin layer at three points in an arbitrary field of view (thickness from the base steel plate surface to the surface of the organic resin layer). Was measured, and their average value was defined as the film thickness.
（１）加工性の評価
発明例および比較例の鋼板をそれぞれ幅３５ｍｍ×長さ１００ｍｍにせん断した後、せん断時の残留応力を除去するために幅が３０ｍｍとなるまで研削加工を施し、試験片を作製した。この試験片に対して、３点曲げ試験機を用いて１８０°曲げ加工を施し、加工性を評価した。この１８０°曲げ加工での曲げの曲率半径は４ｍｍＲとした。加工性の評価は、曲げ加工後の加工部にダンプロンテープ（「ダンプロン」は登録商標）を接着・剥離し、そのテープを銅板に接着させた後に蛍光Ｘ線を用いて前記皮膜量の計測に採用した成分の強度変化を測定し、その強度変化から付与皮膜の剥れ量を求め、以下の基準により評価した。この評価では、○，△を良好とし、×はプレス欠陥となるため不良とした。なお、下記の皮膜量とは単層皮膜、複層皮膜（下層皮膜＋有機樹脂層）を問わず皮膜全体の付着量のことである。
〇：皮膜の剥れなし
△：皮膜の剥れ量が皮膜量の５％未満
×：皮膜の剥れ量が皮膜量の５％以上
(1) Evaluation of workability After the steel sheets of the invention example and the comparative example were each sheared to a width of 35 mm x a length of 100 mm, grinding was performed until the width became 30 mm in order to remove residual stress at the time of shearing. Was prepared. This test piece was subjected to 180 ° bending using a three-point bending tester, and the workability was evaluated. The radius of curvature of this 180 ° bending was 4 mmR. The workability was evaluated by bonding and peeling a damptron tape (“Damptron” is a registered trademark) to the processed portion after bending, bonding the tape to a copper plate, and measuring the amount of the film using fluorescent X-rays. The change in the strength of the components used in Example 1 was measured, and the amount of peeling of the applied film was determined from the change in the strength, and evaluated according to the following criteria. In this evaluation, ○ and △ were regarded as good, and × as a defect due to a press defect. In addition, the following coating amount is the adhesion amount of the whole coating regardless of a single-layer coating or a multi-layer coating (lower coating + organic resin layer).
〇: No peeling of the film △: Less than 5% of the amount of the film ×: More than 5% of the amount of the film
（２）耐遅れ破壊性の評価
上記（１）と同様にして研削加工を施して作製した試験片を曲率半径４ｍｍＲで１８０°曲げ加工して曲げ試験片とし、図１に示すように、この曲げ試験片１を内側間隔が８ｍｍとなるようにしてボルト２とナット３で拘束して試験片形状を固定し、耐遅れ破壊性評価用試験片を得た。このようにして作製した耐遅れ破壊性評価用試験片に対し、米国自動車技術会で定めたＳＡＥ Ｊ２３３４に規定された、乾燥・湿潤・塩水浸漬の工程からなる複合サイクル腐食試験（図２参照）を、最大４０サイクルまで実施した。各サイクルの塩水浸漬の工程前に目視により割れの発生の有無を調査し、割れ発生サイクルを測定した。また、本試験は、各鋼板３検体ずつ実施し、その平均値をもって評価を行った。評価はサイクル数から、以下の基準により評価した。なお、表中の割れサイクル数４０超とは、本実施例の結果では、割れが発生しなかったことを示す。
◎：３０サイクル以上
○：１０サイクル以上３０サイクル未満
×：１０サイクル未満
(2) Evaluation of delayed fracture resistance A test piece prepared by grinding in the same manner as in (1) above was bent at 180 ° with a curvature radius of 4 mmR to obtain a bent test piece, as shown in FIG. The bending test piece 1 was restrained by the bolt 2 and the nut 3 so that the inner interval was 8 mm, and the shape of the test piece was fixed to obtain a test piece for evaluating delayed fracture resistance. The test specimen for evaluating delayed fracture resistance prepared in this manner was subjected to a combined cycle corrosion test comprising a dry / wet / salt water immersion process specified in SAE J2334 stipulated by the American Society of Automotive Engineers (see FIG. 2). Was performed for up to 40 cycles. Before each step of salt water immersion in each cycle, the presence or absence of cracks was visually inspected, and the cycle of crack generation was measured. In addition, this test was performed for three samples of each steel plate, and the average value was used for evaluation. The evaluation was based on the following criteria based on the number of cycles. The number of crack cycles exceeding 40 in the table indicates that no crack occurred in the results of this example.
◎: 30 cycles or more ○: 10 cycles or more and less than 30 cycles ×: less than 10 cycles
（３）導電性の評価
溶接性の指標として導電性を評価した。発明例および比較例の鋼板の試験片について、三菱化学アナリテック（株）製「ロレスタＧＰ ＡＳＰ端子」を用い表面抵抗値を測定し、表面抵抗値が１０−４Ω以下となる割合（％）により、以下の判定基準で評価した。
○：８０％以上
△：６０％以上８０％未満
×：６０％未満
(3) Evaluation of conductivity The conductivity was evaluated as an index of weldability. With respect to the test pieces of the steel sheets of the invention examples and the comparative examples, the surface resistance was measured using “Loresta GP ASP terminal” manufactured by Mitsubishi Chemical Analytech Co., Ltd. The evaluation was made according to the following criteria.
：: 80% or more △: 60% or more and less than 80% ×: less than 60%
（４）耐食性の評価
発明例および比較例の鋼板を１３０ｍｍ×７０ｍｍと４０ｍｍ×１１０ｍｍにせん断して平板試験片とし、この２枚の平板試験片の評価面どうしを重ね合わせてスポット溶接により接合し、図３に示すような耐食性試験用試験片とした。この耐食性試験用試験片に、日本パーカライジング（株）製「パルボンド」を用い、標準条件（３５℃、１２０秒）で浸漬による化成処理を施し、次いで、関西ペイント（株）製の電着塗料「ＧＴ−１０」を用いた電着塗装と焼付処理を行った。電着塗装の塗膜厚は１５μｍとし、市販の電磁膜厚計を用いて膜厚の測定を行った。
(4) Evaluation of Corrosion Resistance The steel sheets of the invention and comparative examples were sheared into 130 mm × 70 mm and 40 mm × 110 mm to form plate test pieces, and the evaluation surfaces of the two plate test pieces were overlapped and joined by spot welding. A test piece for a corrosion resistance test as shown in FIG. This test piece for corrosion resistance test was subjected to a chemical conversion treatment by immersion under standard conditions (35 ° C., 120 seconds) using “Palbond” manufactured by Nippon Parkerizing Co., Ltd., and then an electrodeposition paint “Kansai Paint Co., Ltd.” Electrodeposition coating using GT-10 and baking treatment were performed. The coating thickness of the electrodeposition coating was 15 μm, and the film thickness was measured using a commercially available electromagnetic film thickness meter.
この電着塗装を施した耐食性試験用試験片に対し、米国自動車技術会で定めたＳＡＥ Ｊ２３３４に規定された、乾燥・湿潤・塩水浸漬の工程からなる複合サイクル腐食試験（図２参照）を３０サイクル実施し、下記の手順で耐食性の評価を行った。
（1）スポット溶接部を打ち抜き、合わせ構造部を分解する
（2）塗装の剥離（ネオス社製「デスコート３００」１５分浸漬）
（3）めっき・錆の除去（希薄塩酸浸漬）
（4）合わせ構造部に生じた最大侵食深さをポイントマイクロメーターで測定
A 30-cycle composite cycle corrosion test (see FIG. 2) comprising the steps of drying, wetting, and salt water immersion specified in SAE J2334 stipulated by the American Society of Automotive Engineers was performed on the electrodeposited corrosion resistance test specimen. The cycle was performed, and the corrosion resistance was evaluated according to the following procedure.
(1) Punch out the spot weld and disassemble the joint structure. (2) Peel off the coating (Deocoat 300 manufactured by NEOS for 15 minutes)
(3) Removal of plating and rust (immersion in dilute hydrochloric acid)
(4) Measure the maximum erosion depth generated in the joint structure with a point micrometer
耐食性は、冷延鋼板ままの最大侵食深さを１とした場合の最大侵食深さ比（Ａ）を算出し、以下のように評価した。
◎：Ａ≦０．６
○：０．６＜Ａ≦０．９５
△：０．９５＜Ａ≦１．２
×：１．２＜Ａ
The corrosion resistance was evaluated as follows by calculating the maximum erosion depth ratio (A) when the maximum erosion depth of the cold-rolled steel sheet was set to 1.
：: A ≦ 0.6
：: 0.6 <A ≦ 0.95
Δ: 0.95 <A ≦ 1.2
×: 1.2 <A
表１〜表４において、No.１の鋼板は、ｐＨ緩衝性を有するアニオン化合物（x）を含む皮膜を形成していない比較例（冷延鋼板ままの比較例）であるが、早期に遅れ破壊が発生しており、遅れ破壊特性が低いことが判る。
No.２〜１３の鋼板は、ｐＨ緩衝性を有するアニオン化合物（x）を含有する単層皮膜を形成した例である。このうち、No.２〜９の鋼板は、ｐＨ緩衝性を有するアニオン化合物（x）としてクエン酸を含む皮膜を塗布法により形成したものであるが、発明例であるNo.３〜８の鋼板は、いずれも皮膜剥れがなく、耐遅れ破壊性も良好である。これに対して、ｐＨ緩衝性を有するアニオン化合物（x）の付着量が本発明条件を下回るNo.２の鋼板は、冷延鋼板ままの比較例であるNo.１の鋼板に較べて、耐遅れ破壊性が若干向上しているが、発明例であるNo.３〜８の鋼板に較べて耐遅れ破壊性が劣っている。また、ｐＨ緩衝性を有するアニオン化合物（x）の付着量が本発明の好適条件を超えるNo.９の鋼板は、曲げ加工で皮膜の剥れが認められることから、プレス加工がなされる鋼板には適していないことが判る。
In Tables 1 to 4, the steel sheet No. 1 is a comparative example in which a film containing the anion compound (x) having a pH buffering property was not formed (comparative example of a cold-rolled steel sheet), but was delayed early. It can be seen that the fracture has occurred and the delayed fracture characteristic is low.
The steel sheets of Nos. 2 to 13 are examples in which a single-layer film containing an anion compound (x) having a pH buffering property was formed. Among them, the steel sheets of Nos. 2 to 9 were formed by coating a film containing citric acid as an anion compound (x) having a pH buffering property. Have no peeling of the film and have good delayed fracture resistance. On the other hand, the steel sheet of No. 2 in which the adhesion amount of the anion compound (x) having a pH buffering property is lower than the conditions of the present invention has a higher resistance to resistance than the steel sheet of No. 1 which is a cold rolled steel sheet. Although the delayed fracture property is slightly improved, the delayed fracture resistance is inferior to the steel sheets of Nos. 3 to 8 which are the invention examples. In addition, No. 9 steel sheet having an adhesion amount of the anion compound (x) having a pH buffering property exceeding the preferred conditions of the present invention was subjected to press working since the peeling of the film was observed by bending. Is not suitable.
No.１０〜１３の鋼板は、皮膜中のｐＨ緩衝性を有するアニオン化合物（x）としてクエン酸以外のものを用いた発明例であるが、いずれも良好な耐遅れ破壊性と加工性が得られている。また、No.１４の鋼板は、ｐＨ緩衝性を有するアニオン化合物（x）を含有する単層皮膜を浸漬法で形成した発明例であるが、良好な耐遅れ破壊性と加工性が得られている。
No.１５の鋼板は、ｐＨ３〜６の範囲でｐＨ緩衝性を有するアニオン化合物（x）ではないリン酸を含有する皮膜を形成した比較例であり、また、No.１６〜１８の鋼板は、Ａｌ、Ｍｇ又はＣａ（いずれも硝酸塩として添加）のみを含有する無機皮膜を形成した比較例であるが、いずれも早期に遅れ破壊が発生しており、遅れ破壊特性が低いことが判る。
The steel sheets of Nos. 10 to 13 are invention examples in which a compound other than citric acid was used as the anion compound (x) having a pH buffering property in the film, but all of them exhibited good delayed fracture resistance and workability. Have been. Further, the steel sheet No. 14 is an invention example in which a single-layer film containing an anion compound (x) having a pH buffering property is formed by an immersion method. However, good delayed fracture resistance and workability are obtained. I have.
The steel sheet No. 15 is a comparative example in which a film containing phosphoric acid that is not an anion compound (x) having a pH buffering property in the range of pH 3 to 6 was formed, and the steel sheets No. 16 to 18 were: Comparative Examples in which an inorganic coating containing only Al, Mg or Ca (all added as nitrate) were formed, all show that delayed fracture occurred early and the delayed fracture characteristics were low.
No.１９〜３０、No.３７〜３９の鋼板は、ｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を形成した実施例である。このうち、No.１９〜２２、No.２４、No.２５、No.２８〜３０の鋼板は、有機樹脂がエポキシ系樹脂であって、ｐＨ緩衝性を有するアニオン化合物（x）としてクエン酸を含有する発明例であり、いずれも良好な耐遅れ破壊性と加工性が得られているが、膜厚が０．３μｍ以上の場合に特に優れた耐食性が得られており、より好ましいことが判る。一方、有機樹脂層の膜厚が４．５μｍであるNo.３０は、導電性が低いためスポット溶接を適用する用途には不向きであることが判る。No.２６、No.２７の鋼板は、有機樹脂層中のｐＨ緩衝性を有するアニオン化合物（x）としてクエン酸以外のものを用いた発明例であるが、いずれも良好な耐遅れ破壊性と加工性が得られている。一方、No.２３の鋼板は、ｐＨ緩衝性を有するアニオン化合物（x）の付着量が本発明条件を下回る比較例であるが、発明例であるNo.２２の鋼板に較べて耐遅れ破壊性が劣っている。 No. 19 to No. 30 and No. 37 to No. 39 are examples in which an organic resin layer containing an anion compound (x) having a pH buffering property was formed. Among them, the steel plates of Nos. 19 to 22, No. 24, No. 25, and No. 28 to 30 have an organic resin of an epoxy resin and a citric acid as an anion compound (x) having a pH buffering property. It is an invention example to contain, and all show good delayed fracture resistance and workability, but particularly excellent corrosion resistance is obtained when the film thickness is 0.3 μm or more, which is more preferable. . On the other hand, it can be seen that No. 30 in which the film thickness of the organic resin layer is 4.5 μm is not suitable for applications to which spot welding is applied due to low conductivity. The steel sheets No. 26 and No. 27 are invention examples using an anion compound (x) having a pH buffering property other than citric acid in the organic resin layer. Workability has been obtained. On the other hand, the steel sheet of No. 23 is a comparative example in which the adhesion amount of the pH buffering anion compound (x) is lower than the conditions of the present invention, but has a delayed fracture resistance as compared with the steel sheet of No. 22 which is an invention example. Is inferior.
No.３７〜３９の鋼板は、エポキシ樹脂以外の有機樹脂を用いた発明例であるが、いずれも良好な耐遅れ破壊性と加工性が得られている。ただし、フッ素樹脂を用いたNo.３９の鋼板の耐食性は、皮膜を付与しない場合に比べ十分向上しているが、エポキシ系樹脂やアクリル系樹脂を用いた他の発明例に比べ若干劣っている。
No.３１〜３６の鋼板は、鋼板表面にＣａ（硝酸塩として添加）を含有する無機皮膜を形成した上で、No.２２の鋼板と同様のｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を形成した発明例であるが、加工性は良好であり、かつ遅れ破壊が全く発生せず、より優れた耐遅れ破壊性が得られることが判る。
The steel sheets Nos. 37 to 39 are examples of the invention using an organic resin other than the epoxy resin, but all have good delayed fracture resistance and workability. However, although the corrosion resistance of the steel sheet No. 39 using a fluororesin is sufficiently improved as compared with the case where no film is provided, it is slightly inferior to other invention examples using an epoxy resin or an acrylic resin. .
The steel sheets of Nos. 31 to 36 form an inorganic film containing Ca (added as nitrate) on the surface of the steel sheet, and then contain an anion compound (x) having the same pH buffering property as the steel sheet of No. 22. Although it is an invention example in which an organic resin layer is formed, it is understood that workability is good, delayed fracture does not occur at all, and more excellent delayed fracture resistance can be obtained.
No.４０〜５６の鋼板は、ｐＨ緩衝性を有するアニオン化合物（x）を含有する下層皮膜を形成し、その上層に有機樹脂層を形成した実施例であるが、発明例であるNo.４０〜４９、No.５１〜５６の鋼板は、いずれも良好な耐遅れ破壊性と加工性が得られている。このうち、No.４４〜４９、No.５１〜５６の鋼板は、鋼板表面にNo.４０の鋼板と同様のｐＨ緩衝性を有するアニオン化合物（x）を含有し、さらにＡｌ、Ｍｇ、Ｃａなど（いずれも硝酸塩として添加）の金属を含有する下層皮膜と有機樹脂層（上層皮膜）を形成した発明例であるが、適量の金属を含有することにより、加工性は良好であり、かつ遅れ破壊が全く発生せず、より優れた耐遅れ破壊性が得られることが判る。No.４６〜４９、No.５１〜５３の鋼板は、Ｃａの付着量を変えた発明例であるが、Ｃａ量が１０〜２０００ｍｇ／ｍ２の場合には遅れ破壊が全く発生せず、より好適な遅れ破壊特性が得られることが判る。これに対して、Ｃａの付着量が２０００ｍｇ/ｍ２を超えるNo.５３の鋼板は、プレス加工時に問題になる量ではないが、Ｃａの付着量が多いため一部剥離が発生し、遅れ破壊特性もNo.５２（Ｃａの付着量２０００ｍｇ/ｍ２）の鋼板に比べて劣っていることが判る。これは、皮膜の一部が脱離することで腐食が発生したためであると考えられる。ただし、実用上問題のない遅れ破壊特性である。一方、No.５０の鋼板は、ｐＨ緩衝性を有するアニオン化合物（x）の付着量が本発明条件を下回る比較例であるが、発明例であるNo.４９の鋼板に較べて耐遅れ破壊性が劣っている。 The steel sheets of Nos. 40 to 56 are examples in which a lower layer film containing an anion compound (x) having a pH buffering property is formed, and an organic resin layer is formed on the lower layer film. -49 and Nos. 51-56 have good delayed fracture resistance and workability. Among them, the steel sheets of Nos. 44 to 49 and Nos. 51 to 56 contain an anion compound (x) having the same pH buffering property as the steel sheet of No. 40 on the steel sheet surface, and further include Al, Mg, Ca, etc. This is an example of the invention in which a lower layer film and an organic resin layer (upper layer film) containing a metal (both are added as nitrates) are formed. However, by containing an appropriate amount of metal, workability is good and delayed fracture is achieved. It can be seen that no cracks were generated and more excellent delayed fracture resistance was obtained. The steel sheets of Nos. 46 to 49 and Nos. 51 to 53 are the invention examples in which the amount of Ca attached was changed, but when the Ca amount was 10 to 2000 mg / m 2 , no delayed fracture occurred, and It can be seen that suitable delayed fracture characteristics can be obtained. On the other hand, the No. 53 steel sheet with the amount of Ca exceeding 2000 mg / m 2 is not an amount that causes a problem during press working, but has a large amount of Ca to be partially peeled off, causing delayed fracture. It can be seen that the properties are also inferior to those of the steel sheet of No. 52 (the amount of Ca attached is 2000 mg / m 2 ). This is considered to be because corrosion occurred due to detachment of a part of the film. However, it is a delayed fracture characteristic that has no practical problem. On the other hand, the steel sheet of No. 50 is a comparative example in which the adhesion amount of the anion compound (x) having a pH buffering property is lower than the conditions of the present invention. Is inferior.
No.５７〜６２の鋼板は、鋼板表面にｐＨ緩衝性を有するアニオン化合物（x）を含有する下層皮膜を形成し、その上層にｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層を形成した発明例であり、いずれも良好な耐遅れ破壊性と加工性が得られている。また、No.６０〜６２の鋼板は、鋼板表面にNo.５７〜５９の鋼板と同様のｐＨ緩衝性を有するアニオン化合物（x）を含有し、さらにＣａ（硝酸塩として添加）を含有する下層皮膜とｐＨ緩衝性を有するアニオン化合物（x）を含有する有機樹脂層（上層皮膜）を形成した発明例であるが、加工性は良好であり、かつ遅れ破壊が全く発生せず、より優れた耐遅れ破壊性が得られることが判る。
また、No.３９と同様に、有機樹脂層にフッ素樹脂を用いたNo.５９、No.６２の鋼板の耐食性は、エポキシ系樹脂やアクリル系樹脂を用いた他の発明例に比べ若干劣っている。
The steel sheets of Nos. 57 to 62 form a lower layer film containing an anion compound having pH buffering property (x) on the surface of the steel sheet, and an organic resin layer containing an anion compound having pH buffering property (x) as an upper layer. In each of the examples, good delayed fracture resistance and workability were obtained. The steel sheets of Nos. 60 to 62 contain an anion compound (x) having the same pH buffering property as the steel sheets of Nos. 57 to 59 on the steel sheet surface, and further contain Ca (added as nitrate). And an organic resin layer (upper layer film) containing an anion compound (x) having pH buffering properties, but the processability is good, and delayed fracture does not occur at all, and more excellent resistance. It can be seen that delayed fracture is obtained.
Also, like No. 39, the corrosion resistance of No. 59 and No. 62 steel sheets using a fluororesin for the organic resin layer was slightly inferior to other invention examples using an epoxy resin or an acrylic resin. I have.
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