JP6409647B2 - High-strength steel sheet with excellent delayed fracture resistance and corrosion resistance - Google Patents
High-strength steel sheet with excellent delayed fracture resistance and corrosion resistance Download PDFInfo
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本発明は、耐遅れ破壊性と耐食性に優れた高強度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、耐遅れ破壊性と耐食性に優れ、好ましくは引張強度１１８０ＭＰａ以上を有する高強度鋼板に関するものである。 The present invention relates to a high-strength steel sheet excellent in delayed fracture resistance and corrosion resistance, and more specifically, a steel sheet suitable mainly for strength members for automobiles and building materials, having excellent delayed fracture resistance and corrosion resistance, Preferably, it relates to a high-strength steel sheet having a tensile strength of 1180 MPa or more.
従来、自動車用鋼板としては、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、特に引張強度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。
Conventionally, as a steel sheet for automobiles, cold-rolled steel sheets are used because of demands on sheet thickness accuracy and flatness, but in recent years, from the viewpoint of reducing CO 2 emissions and ensuring safety of automobiles, High strength is achieved.
However, it is known that the phenomenon of delayed fracture is likely to occur when the strength of the steel material is increased, and this delayed fracture becomes more severe as the strength of the steel material increases, and is particularly noticeable in high-strength steel having a tensile strength of 1180 MPa or more. Become. Note that delayed fracture is a condition in which a high-strength steel material is subjected to static load stress (load stress less than the tensile strength) and suddenly appears to have almost no plastic deformation after a certain period of time. 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。この水素脆性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。また、特許文献２では、遅れ破壊を防止する高強度合金化溶融亜鉛めっき鋼板に関する検討がなされている。
In the case of a steel sheet, this delayed fracture is known to be caused by residual stress when formed into a predetermined shape by press work and hydrogen embrittlement of the steel at the stress concentration part. The hydrogen that causes this hydrogen embrittlement is considered to be hydrogen that penetrates and diffuses into the steel from the external environment in most cases. Typically, the hydrogen generated during corrosion of the steel sheet is in the steel. Invading and spreading.
In order to prevent such delayed fracture in a high-strength steel plate, for example, in
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。また、特許文献２の手法では、めっき中のＦｅ濃度は十数％程度であり、耐食性は得られるものの、優れた耐遅れ破壊特性は期待できない。
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高強度鋼板であって、耐遅れ破壊性と耐食性に優れた高強度鋼板を提供することにある。
However, in the method of
Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art, and is a high-strength steel plate suitable mainly for strength members for automobiles and building materials, and a high-strength steel plate excellent in delayed fracture resistance and corrosion resistance. It is to provide.
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、鋼板表面を、特定の被覆量とＦｅ濃度を有するＦｅ−Ｚｎ合金層で被覆することにより、鋼板内部への水素侵入を大幅に抑制して鋼板の遅れ破壊を防止することができ、しかも優れた耐食性も得られることを見出した。 In order to solve the above-mentioned problems, the present inventors have conducted intensive studies and studies on means for preventing delayed fracture by suppressing hydrogen entering the steel sheet. As a result, by covering the steel sheet surface with a Fe-Zn alloy layer having a specific coating amount and Fe concentration, hydrogen penetration into the steel sheet can be greatly suppressed, and delayed fracture of the steel sheet can be prevented, In addition, it has been found that excellent corrosion resistance can also be obtained.
本発明はこのような知見に基づきなされたもので、以下を要旨とするものである。
［1］鋼板表面がＦｅ−Ｚｎ合金層で被覆され、該Ｆｅ−Ｚｎ合金層は、被覆量が０．１ｇ／ｍ２以上９０ｇ／ｍ２未満、Ｆｅ濃度が４０質量％以上８０質量％未満あることを特徴とする耐遅れ破壊性と耐食性に優れた高強度鋼板。
［2］上記［1］の高強度鋼板において、Ｆｅ−Ｚｎ合金層の被覆量が２０ｇ／ｍ２以上９０ｇ／ｍ２未満であることを特徴とする耐遅れ破壊性と耐食性に優れた高強度鋼板。
The present invention has been made on the basis of such findings and has the following gist.
[1] The steel sheet surface is coated with an Fe—Zn alloy layer, and the Fe—Zn alloy layer has a coating amount of 0.1 g / m 2 or more and less than 90 g / m 2 and an Fe concentration of 40% by mass or more and less than 80% by mass. A high-strength steel sheet with excellent delayed fracture resistance and corrosion resistance.
[2] In the high-strength steel sheet of [1], the coating amount of the Fe—Zn alloy layer is 20 g / m 2 or more and less than 90 g / m 2 , and the high strength excellent in delayed fracture resistance and corrosion resistance steel sheet.
本発明の高強度鋼板は、遅れ破壊が効果的に抑制される優れた耐遅れ破壊特性を有するとともに、優れた耐食性を有する。 The high-strength steel sheet of the present invention has excellent delayed fracture resistance that effectively suppresses delayed fracture, and also has excellent corrosion resistance.
本発明の高強度鋼板の基質となる鋼板（素材鋼板）は、引張強度が１１８０ＭＰａ以上の鋼板であることが好ましく、１３４０ＭＰａ以上であることがより好ましい。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１３４０ＭＰａ以上の鋼板でより顕著に発現されるためである。 The steel plate (material steel plate) serving as the substrate of the high-strength steel plate of the present invention is preferably a steel plate having a tensile strength of 1180 MPa or more, and more preferably 1340 MPa or more. A steel sheet with low tensile strength is essentially less susceptible to delayed fracture. This is because the effect of the present invention is manifested even in a steel sheet having a low tensile strength, but is remarkably exhibited in a steel sheet having a tensile strength of 1180 MPa or more, and more prominently exhibited in a steel sheet having a tensile strength of 1340 MPa or more.
鋼板の化学組成および鋼組織は、特に限定されない。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。ただし、これらのうち、自動車分野や建材分野などにおいて用いられる、特に自動車分野などにおいて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１３４０ＭＰａ以上の高強度冷延鋼板がさらに好ましい。 The chemical composition and steel structure of the steel plate are not particularly limited. Moreover, it does not specifically limit about the rolling method etc. Any of a hot-rolled steel plate and a cold-rolled steel plate may be sufficient. However, among these, a high-strength cold-rolled steel sheet having a tensile strength of 1180 MPa or more, which is often used in the automobile field, the building material field, or the like, particularly in the automobile field, is preferable, and a high-strength cold-rolled steel sheet having a tensile strength of 1340 MPa or more. Further preferred.
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や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といった化学組成的改質、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. In order 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, Cr, precipitation strengthening by charcoal / nitrides such as Ti, Nb, V, Al, W, Zr, Chemical composition modification such as addition of strengthening elements such as Hf, Co, B, Cu, rare earth elements, strengthening by recovery annealing at a temperature at which recrystallization does not occur, or leaving an unrecrystallized region without complete recrystallization Partial recrystallization strengthening, strengthening by transformation structure such as bainite or martensite single phase or composite structure of ferrite and these transformation structures, Hall-Pe when ferrite grain size is d Systematic or structural modifications such as grain refinement strengthening represented by tch formula: σ = σ 0 + kd −1/2 (where σ: stress, σ 0 , k: material constant) and work strengthening by rolling, etc. The qualities can be performed singly or in combination.
このような高強度冷延鋼板の組成として、例えば、質量％で、Ｃ：０．１〜０．４％、Ｓｉ：０〜３．０％、Ｍｎ：１〜１０％、Ｐ：０〜０．０５％、Ｓ：０〜０．００５％、残部がＦｅおよび不可避的不純物であるもの、これにＣｕ、Ｔｉ、Ｖ、Ａｌ、Ｃｒ、Ｎｉなどの１種又は２種以上を添加したもの、などを例示することができる。 As a composition of such a high-strength cold-rolled steel sheet, for example, in mass%, C: 0.1 to 0.4%, Si: 0 to 3.0%, Mn: 1 to 10%, P: 0 to 0 .05%, S: 0 to 0.005%, the balance being Fe and inevitable impurities, and one or more of Cu, Ti, V, Al, Cr, Ni, etc. added thereto, Etc. can be illustrated.
上記の引張強度を有する高強度冷延鋼板として商業的に入手可能なものとして、例えば、ＪＦＥ−ＣＡ１１８０、ＪＦＥ−ＣＡ１３７０、ＪＦＥ−ＣＡ１４７０、ＪＦＥ−ＣＡ１１８０ＳＦ、ＪＦＥ−ＣＡ１１８０Ｙ１、ＪＦＥ−ＣＡ１１８０Ｙ２（以上、ＪＦＥスチール株式会社製）などが例示できる。
本発明において基質となる鋼板（素材鋼板）の厚さは、特に限定されるものではないが、０．８〜２．５ｍｍ程度が好ましく、１．２〜２．０ｍｍ程度がより好ましい。
Examples of commercially available high-strength cold-rolled steel sheets having the above-described tensile strength include, for example, JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, JFE-CA1180Y2 (and above, JFE Steel) and the like can be exemplified.
In the present invention, the thickness of the steel plate (material steel plate) serving as a substrate is not particularly limited, but is preferably about 0.8 to 2.5 mm, and more preferably about 1.2 to 2.0 mm.
本発明の高強度鋼板は、上記したような鋼板（素材鋼板）の表面が特定のＦｅ−Ｚｎ合金層、すなわち、被覆量が０．１ｇ／ｍ２以上９０ｇ／ｍ２未満、Ｆｅ濃度が４０質量％以上８０質量％未満のＦｅ−Ｚｎ合金層で被覆されたものである。
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、湿潤環境下におけるＦｅ錆の酸化還元反応が大きく寄与していると考えられる。すなわち、水素侵入を抑制するためには、Ｆｅ錆を変化しにくい状態にするいわゆる「安定錆」を形成することが重要であることが判った。この理由は必ずしも明らかではないが、Ｚｎの腐食の過程で生成する水酸化亜鉛［Ｚｎ（ＯＨ）２］は保護性の腐食生成物であるといわれているが、高Ｆｅ濃度のＦｅ−Ｚｎ層の形成により、この保護性が高い腐食生成物を含む安定錆が形成され、この安定錆が水素侵入を抑制するものと推定される。
In the high-strength steel plate of the present invention, the surface of the steel plate (raw steel plate) as described above is a specific Fe—Zn alloy layer, that is, the coating amount is 0.1 g / m 2 or more and less than 90 g / m 2 , and the Fe concentration is 40. It is coated with an Fe—Zn alloy layer of not less than 80% by mass.
According to the research and examination results of the present inventors, it is considered that the hydrogen intrusion into the steel sheet during the corrosion process greatly contributes to the oxidation / reduction reaction of Fe rust in a wet environment. That is, in order to suppress hydrogen intrusion, it has been found that it is important to form so-called “stable rust” that makes Fe rust difficult to change. The reason for this is not necessarily clear, but zinc hydroxide [Zn (OH) 2 ] produced during the corrosion of Zn is said to be a protective corrosion product, but a Fe-Zn layer having a high Fe concentration. It is estimated that stable rust containing corrosion products with high protection is formed, and this stable rust suppresses hydrogen intrusion.
また、Ｆｅ−Ｚｎ合金層（例えば、めっき皮膜）は、加工後に地鉄に達するクラックが無数に入り、鋼板表面を完全には被覆していない状態となるが、Ｆｅ濃度が４０質量％以上８０質量％未満のＦｅ−Ｚｎ合金層の場合には、上述のように腐食環境下で保護性の高いＺｎの腐食生成物を含む安定錆が形成され、その腐食生成物がクラック部分を埋めることで水素侵入を抑制するものと考えられる。 In addition, the Fe—Zn alloy layer (for example, a plating film) has innumerable cracks reaching the base iron after processing and does not completely cover the steel sheet surface, but the Fe concentration is 40% by mass or more and 80%. In the case of an Fe—Zn alloy layer of less than mass%, stable rust containing a corrosion product of Zn having high protection in a corrosive environment is formed as described above, and the corrosion product fills the crack portion. It is thought to suppress hydrogen intrusion.
以上の推定メカニズムから、水素侵入を抑制するために有効な腐食生成物とするためには、Ｆｅ−Ｚｎ合金層の被覆量が０．１ｇ／ｍ２以上９０ｇ／ｍ２未満、Ｆｅ−Ｚｎ合金層のＦｅ濃度が４０質量％以上８０質量％未満であることが必要である。また、特に腐食に厳しい部材に適用する場合には、Ｆｅ−Ｚｎ合金層の被覆量を２０ｇ／ｍ２以上９０ｇ／ｍ２未満とすることが好ましい。 From the above estimation mechanism, in order to obtain an effective corrosion product for suppressing hydrogen intrusion, the coating amount of the Fe—Zn alloy layer is 0.1 g / m 2 or more and less than 90 g / m 2 , Fe—Zn alloy. It is necessary that the Fe concentration of the layer is 40% by mass or more and less than 80% by mass. In addition, when applied to a member particularly severe to corrosion, the coating amount of the Fe—Zn alloy layer is preferably 20 g / m 2 or more and less than 90 g / m 2 .
Ｆｅ−Ｚｎ合金層の被覆量が０．１ｇ／ｍ２未満では、十分な水素侵入抑制効果と耐食性が得られない。一方、９０ｇ／ｍ２以上では、鋼板の腐食抑制に寄与するＺｎ量が多く耐食性は向上するが、Ｚｎの腐食に伴い発生する水素量が増加するため、水素の侵入を助長してしまう。
また、Ｆｅ濃度が４０質量％未満では、鋼板の腐食抑制に寄与するＺｎ量が多く耐食性は向上するが、Ｚｎの腐食に伴い発生する水素量が増加するため、水素の侵入を助長してしまう。一方、８０質量％以上では、鋼板の腐食抑制に寄与するＺｎ量が少なく耐食性が低下してしまう。
When the coating amount of the Fe—Zn alloy layer is less than 0.1 g / m 2 , sufficient hydrogen penetration inhibiting effect and corrosion resistance cannot be obtained. On the other hand, if it is 90 g / m 2 or more, the amount of Zn contributing to the corrosion inhibition of the steel sheet is large and the corrosion resistance is improved, but the amount of hydrogen generated due to the corrosion of Zn increases, so the penetration of hydrogen is promoted.
Further, when the Fe concentration is less than 40% by mass, the amount of Zn contributing to the corrosion inhibition of the steel sheet is large and the corrosion resistance is improved. However, the amount of hydrogen generated due to the corrosion of Zn increases, which promotes the penetration of hydrogen. . On the other hand, if it is 80% by mass or more, the amount of Zn contributing to the corrosion inhibition of the steel sheet is small and the corrosion resistance is lowered.
鋼板表面をＦｅ−Ｚｎ合金層で被覆する方法については、特別な制限はなく、公知の方法を適用することが可能であるが、例えば、電気めっき法（Ｆｅ−Ｚｎ合金電気めっき法）、無電解めっき法、蒸着法等を用いることができる。一方、溶融めっき法（合金化溶融亜鉛めっき）では、めっき中Ｆｅ濃度を４０質量％以上８０質量％未満とするためには、高温かつ長時間の加熱が必要となり、鋼板の引張強度の制御が困難となってしまう。 There is no particular limitation on the method of coating the steel sheet surface with the Fe—Zn alloy layer, and a known method can be applied. For example, electroplating (Fe—Zn alloy electroplating), no An electrolytic plating method, a vapor deposition method, or the like can be used. On the other hand, in the hot dipping method (alloyed hot dip galvanizing), in order to make the Fe concentration during plating 40% by mass or more and less than 80% by mass, heating at a high temperature for a long time is required, and the tensile strength of the steel sheet is controlled. It becomes difficult.
電気めっき法（Ｆｅ−Ｚｎ合金電気めっき法）の場合には、めっき浴に含まれるＺｎ、Ｆｅの濃度を調整することでＦｅ−Ｚｎ合金層のＦｅ濃度を変えることができ、また、電解時間を調整することでＦｅ−Ｚｎ合金層の被覆量を変えることができる。また、無電解めっき法の場合には、めっき浴に含まれるＺｎ、Ｆｅの濃度を調整することでＦｅ−Ｚｎ合金層のＦｅ濃度を変えることができる。また、蒸着法の場合には、ターゲットとしてＦｅ及びＺｎを用いてＦｅ濃度を調整する。 In the case of electroplating (Fe—Zn alloy electroplating), the Fe concentration of the Fe—Zn alloy layer can be changed by adjusting the concentrations of Zn and Fe contained in the plating bath, and the electrolysis time It is possible to change the coating amount of the Fe—Zn alloy layer by adjusting. In the case of the electroless plating method, the Fe concentration of the Fe—Zn alloy layer can be changed by adjusting the concentrations of Zn and Fe contained in the plating bath. In the case of vapor deposition, Fe concentration is adjusted using Fe and Zn as targets.
本発明の高強度鋼板は、上述したＦｅ−Ｚｎ合金層を鋼板片面に被覆したものでもよいし、鋼板両面に被覆したものでもよい。
なお、Ｆｅ−Ｚｎ合金層は、基本的にＦｅとＺｎからなるものであるが、例えば、Ｖ、Ｍｏ、Ｗなどのような合金元素の１種以上を少量（例えば１０mass％以下）添加してもよい。
The high-strength steel plate of the present invention may be one in which the above-described Fe—Zn alloy layer is coated on one side of the steel plate or may be coated on both sides of the steel plate.
The Fe—Zn alloy layer is basically composed of Fe and Zn. For example, a small amount (for example, 10 mass% or less) of one or more alloy elements such as V, Mo, and W is added. Also good.
基質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、冷延鋼板の表面をＦｅ−Ｚｎ合金層で被覆する場合における、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、基質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。 The manufacturing method of the steel plate used as a substrate is not particularly limited. In order to facilitate understanding of the present invention, a series of processes from steelmaking in the case where the surface of a cold-rolled steel sheet is coated with an Fe—Zn alloy layer will be briefly described with an example. However, as a manufacturing process of the steel plate used as a substrate, it is not limited to the following illustrations.
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、７５０〜９００℃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、常法に従い伸長率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、このようにして得られた冷延鋼板表面に、電気めっき法、無電解めっき法、蒸着法等の方法にて、Ｆｅ−Ｚｎ合金層を被覆量が０．１ｇ／ｍ２以上９０ｇ／ｍ２未満で、Ｆｅ濃度が４０質量％以上８０質量％未満となるように被覆する。これにより本発明の高強度冷延鋼板を得ることができる。 A steel having a predetermined component composition is melted and slab is formed by continuous casting 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 up at 500 to 650 ° C. Subsequently, after pickling, cold rolling is performed at a rolling reduction of 30 to 70%. Thereafter, if necessary, purifying treatment is performed by washing with an alkali or a mixed solution of an alkali and a surfactant and a chelating agent, electrolytic washing, warm water washing, and drying according to a conventional method, and then at 750 to 900 ° C. Heat treatment, rapid cooling, and adjustment of the tensile strength of the steel sheet. Furthermore, if necessary, a cold-rolled steel sheet having a desired tensile strength is obtained by performing temper rolling with an elongation ratio of about 0.01 to 0.5% according to a conventional method. The Fe-Zn alloy layer is coated on the surface by an electroplating method, an electroless plating method, a vapor deposition method, or the like, and the coating amount is 0.1 g / m 2 or more and less than 90 g / m 2 , and the Fe concentration is 40% by mass or more. It coat | covers so that it may become less than 80 mass%. Thereby, the high-strength cold-rolled steel sheet of the present invention can be obtained.
なお、冷延鋼板表面をＦｅ−Ｚｎ合金層で被覆するのにめっき法、特に電気めっき法を用いた場合において、めっき処理時に鋼板およびＦｅ−Ｚｎ合金層中に水素が侵入するおそれがあるときは、必要に応じて、めっき処理後に１００〜３００℃程度の温度でベーキング処理を施し、鋼板およびＦｅ−Ｚｎ合金層中に侵入した水素を除去する処理を施してもよい。 In the case of using a plating method, particularly an electroplating method, to coat the surface of the cold-rolled steel sheet with the Fe—Zn alloy layer, when hydrogen may enter the steel sheet and the Fe—Zn alloy layer during the plating process. If necessary, after the plating treatment, a baking treatment may be performed at a temperature of about 100 to 300 ° C. to remove hydrogen that has entered the steel plate and the Fe—Zn alloy layer.
素材鋼板として、表１の成分組成を有する板厚１．５ｍｍの冷延鋼板（引張強度１４８０ＭＰａ）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い、防錆油を除去した後、Ｆｅ−Ｚｎ合金電気めっきを施し、鋼板表面にＦｅ−Ｚｎ合金層（めっき層）を形成した。 As the material steel plate, a cold-rolled steel plate (tensile strength 1480 MPa) having a thickness of 1.5 mm and having the composition shown in Table 1 was used. This cold-rolled steel sheet was immersed in toluene and subjected to ultrasonic cleaning for 5 minutes to remove rust preventive oil, and then Fe-Zn alloy electroplating was performed to form a Fe-Zn alloy layer (plating layer) on the steel sheet surface. .
電気めっき液としては、二価の鉄イオン濃度：１５０ｇ／Ｌ、二価の亜鉛イオン濃度：４０ｇ／Ｌを硫酸塩として添加し、硫酸によりｐＨ２．０に調整したものを用いた。電流密度を１０〜８０Ａ／ｄｍ２の範囲で調整することでＦｅ−Ｚｎ合金層のＦｅ濃度を変化させ、また、電解時間を調整することでＦｅ−Ｚｎ合金層の被覆量を変化させた。
なお、以上のめっき処理を行わない鋼板を比較例の１つとした。
As the electroplating solution, a divalent iron ion concentration: 150 g / L and a divalent zinc ion concentration: 40 g / L were added as sulfates and adjusted to pH 2.0 with sulfuric acid. The Fe concentration of the Fe—Zn alloy layer was changed by adjusting the current density in the range of 10 to 80 A / dm 2 , and the coating amount of the Fe—Zn alloy layer was changed by adjusting the electrolysis time.
In addition, the steel plate which does not perform the above plating process was made into one of the comparative examples.
Ｆｅ−Ｚｎ合金層の被覆量は、鋼板を塩酸に浸漬してＦｅ−Ｚｎ合金層を溶解させ、溶解前後の質量差から求めた。また、Ｆｅ含有率はＩＣＰ分析法にてＦｅ含有量を測定した。
Ｆｅ−Ｚｎ合金層のＦｅ濃度は、塩酸で合金層を溶解させて得られた溶液から、ＩＣＰ発光分光分析法によりＦｅ濃度を測定した。
The coating amount of the Fe—Zn alloy layer was determined from the difference in mass before and after dissolution by immersing the steel sheet in hydrochloric acid to dissolve the Fe—Zn alloy layer. The Fe content was measured by ICP analysis.
The Fe concentration of the Fe—Zn alloy layer was measured by ICP emission spectroscopic analysis from a solution obtained by dissolving the alloy layer with hydrochloric acid.
以上のようにして得られた試験片について、以下の評価を行った。得られた結果を、Ｆｅ−Ｚｎ合金層の構成とともに表２に示す。
（１）耐遅れ破壊性の評価
研削加工を施して作製した試験片（３０ｍｍ×９９．５ｍｍ）を曲率半径４ｍｍＲで１８０°曲げ加工し、図１に示すように、この曲げ試験片１を内側間隔が８ｍｍとなるようにボルト２とナット３で拘束して試験片形状を固定し、遅れ破壊評価用試験片を得た。このようにして作製した遅れ破壊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８０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、割れ発生サイクル数を測定した。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。
〇：３０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
The test piece obtained as described above was evaluated as follows. The obtained results are shown in Table 2 together with the structure of the Fe—Zn alloy layer.
(1) Evaluation of delayed fracture resistance A test piece (30 mm × 99.5 mm) produced by grinding was bent 180 ° with a radius of curvature of 4 mmR, and as shown in FIG. The test piece shape was fixed by restraining with the bolt 2 and the nut 3 so that the interval was 8 mm, and a test piece for delayed fracture evaluation was obtained. For the delayed fracture evaluation test piece produced in this way, a combined cycle corrosion test (see FIG. 2) consisting of drying, wetting, and salt water soaking processes defined in SAE J2334 defined by the American Society of Automotive Engineers, Up to 80 cycles were performed. The presence or absence of cracks was visually examined before the salt water immersion process in each cycle, and the number of crack generation cycles was measured. In addition, this test was performed on three specimens of each steel plate, and the average value was evaluated. Evaluation was performed according to the following criteria from the number of cycles.
○: 30 cycles or more Δ: 10 cycles or more, less than 30 cycles ×: Less than 10 cycles
（２）耐食性の評価
上記耐遅れ破壊性評価を行ったサンプルについて、各サイクルの塩水浸漬工程前に目視により赤錆発生の有無を調査し、赤錆発生サイクル数を測定した。評価はサイクル数から、以下の基準により評価した。
○：５サイクル経過時点で赤錆発生なし
×：５サイクル未満で赤錆発生あり
(2) Evaluation of corrosion resistance About the sample which performed the said delayed fracture resistance evaluation, the presence or absence of red rust generation | occurrence | production was visually observed before the salt water immersion process of each cycle, and the number of red rust generation | occurence | production cycles was measured. Evaluation was performed according to the following criteria from the number of cycles.
○: No red rust occurred after 5 cycles x: Red rust occurred after less than 5 cycles
表２によれば、本発明例の鋼板は、いずれも優れた耐遅れ破壊性と耐食性が得られている。これに対して比較例の鋼板は、耐遅れ破壊性、耐食性のいずれか又は両方が劣っている。 According to Table 2, all the steel plates of the present invention have excellent delayed fracture resistance and corrosion resistance. On the other hand, the steel plate of the comparative example is inferior in delayed fracture resistance and / or corrosion resistance.
１ 試験片
２ ボルト
３ ナット
1 Test piece 2 Bolt 3 Nut
Claims (1)
鋼板表面がＦｅ−Ｚｎ合金層（但し、合金化溶融Ｚｎめっき層、及びＺｎ，Ｆｅ以外の合金元素を１０質量％を超えて含むＦｅ−Ｚｎ合金層を除く。）で被覆され、該Ｆｅ−Ｚｎ合金層は、被覆量が４５ｇ／ｍ ２ 以上９０ｇ／ｍ２未満、Ｆｅ濃度が４０質量％以上６０質量％以下であることを特徴とする耐遅れ破壊性と耐食性に優れた高強度冷延鋼板。 A steel plate having a tensile strength of 1340 MPa or more,
The surface of the steel sheet is coated with an Fe—Zn alloy layer (excluding the alloyed hot-dip Zn plating layer and the Fe—Zn alloy layer containing more than 10 mass% of alloy elements other than Zn and Fe ), and the Fe— The Zn alloy layer has a coating amount of 45 g / m 2 or more and less than 90 g / m 2 and a Fe concentration of 40% by mass or more and 60% by mass or less , and is a high strength cold rolling excellent in delayed fracture resistance and corrosion resistance. Steel sheet .
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