JP3875818B2 - Method for producing high-strength steel sheet with excellent fatigue resistance and chemical conversion treatment - Google Patents
Method for producing high-strength steel sheet with excellent fatigue resistance and chemical conversion treatment Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、耐疲労性、化成処理性に優れた高強度鋼板の製造方法に関するものである。
【0002】
【従来の技術】
近年、自動車の軽量化のために高強度鋼板の適用が拡大している。
鋼板の高強度化のために採られる手段の一つとしては、Ni、CrやMo等の焼入れ性を向上させる合金元素を添加して、変態後に生成する金属組織を微細化する、或いは金属組織中に占めるベイナイトやマルテンサイト等の低温変態相の割合を高くして高強度化を図る手段がある。しかしながら、こうした焼入れ性を向上させる合金元素は、強度は高くなるものの、それに伴い伸び等の延性の低下が大きくなって、加工性を低下させるので、自ずとその使用量にも限界があるものであった。
また、NbやV等の析出硬化元素を鋼に多量添加して、熱間圧延後の冷却中にNbやVの微細な炭化物、窒化物、或いは炭窒化物を析出させて硬化する方法もあるが、やはり強度の増加に伴い、伸びの低下を招くものであった。
また上記した焼入れ性を向上させる元素、析出硬化元素は高価なものが多く、多量の使用はコストの増大を招くものでもあった。
【0003】
上記のような強度の増加に伴い伸びの低下を招き、且つ高価である合金元素に対し、フェライト形成元素であるSiは、フェライトに固溶してその固溶硬化によって鋼を高強度化するものであるが、添加量の増大によってフェライトを安定化させるために、強度が増加しても金属組織を微細化したり、低温変態相の占める割合を増大したりする効果が小さい。従って、伸び等の延性特性を大きく低下させずに強度を高めることのできる、所謂強度/延性バランスに優れており、且つ比較的安価な元素である。
しかしながら、従来のSi添加を行った高強度鋼板は、疲労特性向上への対策が十分でなかったために、疲労限応力を引張強さで除した値、即ち疲労限度比が十分高くなく、またばらつきも大きいものであった。
また、鋼板の自動車ユーザーにおける加工工程の概略は、鋼板を油で洗浄する工程、プレス工程、脱脂工程、化成処理工程、塗装工程からなっているが、従来のSi添加高強度鋼板は、化成処理対策も十分になされていなかったために、化成処理工程で十分に化成処理皮膜が形成せず、化成処理性に劣るものであった。このため塗装剤との相性が良くなく、塗装後の耐食性が十分優れていなかったために、使用者側における用途の拡大に対して限界のあるものであった。
【0004】
【発明が解決しようとする課題】
本発明者らは、上記したような従来のSi添加高強度鋼板における疲労限度比、および化成処理性の劣る原因を追求して、問題を解決すべく鋭意研究を重ねた結果、鋼板表面に元の結晶粒界に沿って腐食された粒界腐食層が存在しており、これが疲労亀裂発生の起点となって疲労特性を劣化させていること、また粒界腐食層には冷間圧延時に侵入した圧延油、防錆油等の炭素分が残留しているために化成処理性を落としていることを突き止めた。
そして、このような粒界腐食層は、熱間圧延後の冷却途中において、地鉄が酸化してスケールが生成するに伴い酸素が表面から粒界に進入して、粒界付近のSi,Mn等の酸化性の強い元素を選択的に酸化して形成された粒界酸化層が、酸洗によって浸食されて形成されたものであることを突き止めた。
【0005】
即ち、本発明は上記した従来のSi添加高強度鋼板において存在した粒界腐食層に起因する問題を解決するものであって、耐疲労性、化成処理性に優れた高強度鋼板の製造方法を提供することを課題とする。
【0006】
【課題を解決するための手段】
上記の課題を達成するためになされた請求項1に記載の発明は、C:0.03〜0.40%、Si:0.50〜3.0%、Mn:0.30〜3.0%、sol.Al:0.01〜0.10%を含み、残部鉄および不可避不純物からなる化学成分組成を有する鋼板を、熱間における仕上げ圧延を行った後、巻き取り温度を523〜650℃として巻き取ることによって、鋼板表面に生成する粒界腐食層の深さを20μm以下とすることを特徴とする耐疲労性、および化成処理性に優れた高強度鋼板の製造方法を要旨とするものである。
請求項2に記載の発明は、請求項1に記載の発明において、巻き取り温度を523〜600℃として、粒界腐食層の深さを10μm以下とすることを特徴とするものである。
【0007】
【発明の実施の形態】
以下に本発明の限定理由について説明する。
(1)C:0.03〜0.40%、
Cは、炭化物を形成して鋼を強化し、疲労強度を高めるに必須な元素であって、その量が0.03%未満では、所望の強度を得ることが困難になる。しかしCの量が0.40%を越えると、延性の低下が大きく、鋼板の加工性が低下する。従って、Cの範囲は、0.03〜0.40%の間に限定する。
【0008】
(2)Si:0.50〜3.0%、
Siは、フェライト形成元素であって、鋼に添加したときにはフェライト中に固溶して、所謂固溶硬化により鋼を強化するとともに、添加量の増大によってフェライトを安定化させるために、強度が増加してもベイナイト等の金属組織を微細化したり、またベイナイト、マルテンサイト等の低温変態相の占める割合を増大したりする効果が小さい。従って強度の増加に伴う伸びの低下を小さく抑えることのできる元素であり、且つ比較的安価な元素である。しかしながらその量が0.50%未満では、鋼を強化する効果が小さく、強度を十分高めることが出来ない。一方、3.0%を越えると延性の低下が大きくなって、熱間圧延後の鋼板表面に割れや表面疵の発生が多くなる、鋼板の加工性を低下さす等の弊害をもたらす。従って、Siの範囲は、0.50〜3.0%の間に限定する。
【0009】
(3)Mn:0.30〜3.0%、
Mnは、鋼中のSと結合してMnSを形成し、Sによる悪影響、例えばFeSの形成による熱間延性の低下を大幅に改善するとともに、鋼を強化するのに重要な元素である。しかしながら、その量が0.30%未満では延性改善の効果、鋼を強化する効果ともに小さい。一方、3.0%を越えると金属組織中の低温変態層の占める割合を高くして延性を大きく低下させる。従って、Mnの量は、0.30〜3.0%の間に限定する。
【0010】
(4)sol.Al:0.01〜0.10%、
鋼板の結晶粒度は疲労特性に大きな影響を及ぼし、sol.Al(酸可溶Al)が、0.01%未満では、粒度番号5番以上の微細な整粒を得ることができなかったり、大きさの不揃いな混粒となったりして、疲労強度の低下、異方性、硬度のばらつきを招くことになる。一方sol.Alが0.10%を越えるとAl2O3の生成量が多くなって鋼の清浄性が低下し、これが疲労クラックの発生起点となって疲労強度を低下させる。従って、sol.Alの範囲は、0.01〜0.10%の間に限定する。
【0011】
(5)Nb:0.01〜0.05%、
Nbは、微細な炭窒化物を析出し、鋼を微細化するのに有効な元素であるが、0.01%未満ではその効果は小さい。一方0.05%を越えて添加してもその効果は飽和してくるので0.05%までの添加で十分である。従って、Nbを細粒化元素として補足的に用いる場合には、その範囲は0.01〜0.05%の間とするのが望ましい。
【0012】
(6)巻き取り温度:650℃以下
4重式圧延機もしくは6重式圧延機により所定の厚みに、900℃前後の温度で仕上げ圧延された鋼板は、ランナウトテーブルを通ってコイラーで巻き取られるが、ランナウトテーブルの上下にはセクション毎にラミナーフロー方式等の水冷装置が設置されており、鋼板の強度、延性等の機械的性質が要求された性質に合うように、所定の巻き取り温度に制御されて巻き取られる。
鋼板は上記したように、900℃前後の高温で仕上げられるために、その後のコイラーで巻き取られるまでの間に、酸化が進行して酸化スケールを生成するので、巻き取り温度は、酸化スケールの厚みや性状に大きな影響を及ぼす。酸化スケールの生成は、鋼板表面の地鉄が酸素と結合して、FeO(ウスタイト)やFe3O4(マグネタイト)を生成しながら進行するものであるが、酸化の進行とともに表面直下の結晶粒界にも酸素が進入して、粒界付近のSi,Mn等の酸化性の強い元素が酸素と結合して粒界酸化層を形成する。SiはMnよりも酸化性が強く、従って高Si鋼に形成される粒界酸化層は低Si鋼に比べると深くなる。粒界酸化層は酸洗により浸食されて、粒界腐食層を形成する。
粒界酸化層の深さは仕上げから巻き取りに至るまでの間の冷却速度等の熱履歴に左右されるが、巻き取り温度が650℃より高くなると、酸洗後の熱延鋼板、冷延鋼板に形成される粒界腐食層の深さを20μm以下とすることが困難になるので、巻き取り温度は650℃以下に限定する。粒界腐食層の深さをさらに浅くするには巻き取り温度は600℃以下にするのが望ましい。
【0013】
(7)粒界腐食層の深さ:20μm以下、
粒界酸化層は図4に示すように、鋼の表面の結晶粒界20に楔状に形成された酸化スケールである。そして、粒界酸化層は図5に示すように、酸洗によって浸食され、表面の粗度を低下させるとともに、粒界腐食層23を形成する。粒界腐食層は、起点となって容易に疲労亀裂を発生し疲労強度を低下させる。また表面の凹凸は化成処理剤の均一な付着を阻害するとともに、粒界の隙間に入り込んだ潤滑油や防錆油は脱脂によって除去することが困難であるために、化成処理性を低下し、塗装剤との相性を悪くする。
鋼板に形成される粒界腐食層の深さが20μmを越えると、疲労強度の低下、化成処理性の低下の程度が大きくなるので、粒界腐食層の深さは20μm以下に限定する。なおより優れた耐疲労性、化成処理性を有する鋼板を得るには、粒界腐食層の深さは10μm以下であることが望ましい。粒界腐食層の深さを10μm以下とすることによって、疲労限度比0.45以上の優れた耐疲労性と、より化成処理性の良い鋼板を得ることが可能になる。
【0014】
図1に本発明に係る鋼板の製造・加工工程の概略を示す。
鋼板製造者側においては、連続鋳造により製造されたスラブは、スラブを加熱する加熱炉1、中間厚みまでの圧延および幅圧下を行う粗圧延機2を経て、仕上げ圧延機3により900℃前後の仕上げ温度で所定厚みまで圧延される。仕上げ圧延機3を出た鋼板はランナウトテーブル4を通ってコイラー6で巻き取られるが、ランナウトテーブル4の上下にはセクション毎にラミナーフロー方式の水冷装置5が設置されており、これにより巻き取り温度を523〜650℃になるように制御して、金属組織、機械的性質を調整するとともに、表面の粒界酸化の進行を小さく抑制する。巻き取り温度の制御は仕上げ圧延機3出側の温度計測定値によるフィードフォワード方式およびコイラー直前の温度計測定値によるフィードバック方式により高精度に温度が制御される。製造されたコイル状の鋼板は表面のスケール除去のための酸洗槽7を経てそのまま熱延鋼板として出荷されるか、冷間圧延機8にて常温で所定の厚みにまで圧延され、その後連続焼鈍、過時効処理装置9により連続焼鈍、過時効処理されて冷延鋼板として出荷される。冷間圧延においては通常は潤滑のため圧延油が使用される。
【0015】
自動車メーカー等の鋼板加工者側においては、鋼板は、油で洗浄する洗浄装置11、成形するプレス12、表面の油分を洗浄する脱脂装置13、燐酸亜鉛の結晶を鋼板表面に析出させる化成処理装置14、ロール塗装等の方法により塗装する塗装装置15からなる工程により処理されて、自動車ボデー等の各種の製品16に加工される。
【0016】
【実施例】
以下に本発明を実施例により説明する。
転炉溶製、連続鋳造の工程により製造した表1に示す化学成分を有するスラブを加熱炉にて1200℃に加熱し、粗圧延を経て、880〜915℃の間の温度で仕上げて2.2mmの鋼板に熱間圧延した後、ランナウトテーブルに送出した。ランナウトテーブルを送出中の鋼板の上下からラミナーフロー方式によって鋼板を冷却し、温度計測定値によるフィードバック、フィードフォワード方式により水量を調整して巻き取り温度を調整した後、コイラーにより巻き取って熱延鋼板を製造した。一部の試験においては、熱延鋼板を50%の圧下で冷間圧延し、その後連続焼鈍、過時効処理を行った。連続焼鈍の条件は、焼鈍温度850℃、急冷開始温度670℃、急冷終了温度250℃、冷却速度150℃/secであり、過時効処理は280℃×400secとした。
鋼板よりJIS1号引張試験片、および試験部の幅20mm、ノッチ、30Rのシェンク型疲労試験片を採取して、引張試験、およびシェンク型疲労試験を行った。疲労試験は回転数1800回/分で行い、繰り返し曲げ回数が107回でも破断しない疲労限を求め、引張強さで除して疲労限度比とした。
また、塩酸により酸洗後の熱延鋼板、あるいは酸洗−冷間圧延−連続焼鈍−過時効処理した冷延鋼板に生成した粒界腐食層の深さを顕微鏡により測定した。さらに、熱延鋼板、冷延鋼板に燐酸亜鉛系の処理液を用いて化成処理を施した。化成処理皮膜の判定は、走査電子顕微鏡により行い、全面に渡り極めて均一に皮膜が形成されているものを特に良好(◎)、支障の無い程度の均一さで皮膜が形成されているものを良好(○)、部分的に皮膜が形成されていたり、皮膜が形成されていないものは不良(×)と判定した。
【0017】
【表1】
【0018】
表1において試験No.4〜6、および9は冷延鋼板についてのものであり、その他は熱延鋼板である。
試験No.1〜6の実施例において、鋼は本発明の範囲内の化学成分を有し、巻き取り温度は523〜650℃に制御されている。降伏応力、引張強さは、化学成分に応じて低い領域から高い領域に渉っているが、高Siとしたことにより1000 N/mm2を越える引張強さでも10%を越える高い伸びを有している。
巻き取り温度を低めたことにより粒界腐食層深さは20μm以下と浅く、従って疲労限度比はいずれも0.44以上で優れた耐疲労性を有している。
また、粒界腐食層の深さが浅くなって表面の凹凸、残留油分が減少したことにより化成剤はむら無く均一に付着しており、化成処理性は良好なものであった。
実施例のなかでも、試験No.1〜2、5〜6の実施例は、巻き取り温度を600℃以下として低温で巻き取ったことにより、生成した粒界腐食層深さは10μm以下と浅く、従って、疲労限度比は0.45以上と高く、また化成処理性も特に良好なものであった。
【0019】
これに対し、試験No.7〜10の比較例においては、いずれも巻き取り温度が650℃より高かったために酸化が進行し、結果として20μmを越える粒界腐食層が生成してしまった。このため、粒界腐食層を起点に疲労亀裂が発生し疲労の進行が速く、疲労限度比は0.41以下と低いものであり、また表面の凹凸が大きく、残留油部も多いために、化成処理皮膜が均一に形成されず、化成処理性の劣るものしか得られていない。
【0020】
【発明の効果】
以上に説明したとおり本発明によれば、従来のSi添加高強度鋼板に存在した粒界腐食層の深さを浅くすることが可能であって、粒界腐食層を起点に発生する疲労亀裂を抑制することによって耐疲労性を改善するとともに、表面の凹凸を減少することによって均一な化成処理皮膜を付着させることができる。
従って、本発明は耐疲労性、および化成処理性に優れ、且つ安価な高強度鋼板の製造方法を提供するものであって、産業界の発展に寄与するところ大なるものである。
【図面の簡単な説明】
【図1】 鋼板の製造・加工工程フローを示す図。
【図2】 粒界腐食層深さと疲労限度比の関係を示す図。
【図3】 粒界腐食層深さと化成処理性の関係を示す図。
【図4】 スケールを有する鋼板の表面を説明する図。
【図5】 化成処理前の鋼板の表面を説明する図。
【符号の説明】
1 加熱炉
2 粗圧延機
3 仕上げ圧延機
4 ランナウトテーブル
5 水冷装置
6 コイラー
7 酸洗槽
8 冷間圧延機
9 連続焼鈍、過時効処理装置
10 鋼板製造者側工程
11 洗浄装置
12 プレス
13 脱脂装置
14 化成処理装置
15 塗装装置
16 製品
17 鋼板加工者側工程
18 酸化粒界
19 地鉄
20 結晶粒界
21 粒界酸化層
22 残留油分
23 粒界腐食層
24 酸化スケール[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength steel sheet excellent in fatigue resistance and chemical conversion treatment.
[0002]
[Prior art]
In recent years, the application of high-strength steel sheets has been expanded to reduce the weight of automobiles.
One of the measures taken to increase the strength of steel sheets is to add alloy elements that improve the hardenability such as Ni, Cr and Mo, and to refine the metal structure generated after transformation, or the metal structure There is a means for increasing the strength by increasing the proportion of low-temperature transformation phases such as bainite and martensite. However, although such an alloy element that improves hardenability increases in strength, it causes a decrease in ductility such as elongation, which decreases workability, so that its use amount is naturally limited. It was.
There is also a method in which a large amount of precipitation hardening elements such as Nb and V are added to the steel to precipitate and harden fine carbides, nitrides, or carbonitrides of Nb and V during cooling after hot rolling. However, as the strength increased, the elongation decreased.
In addition, the elements for improving the hardenability and the precipitation hardening elements described above are often expensive, and the use of a large amount thereof causes an increase in cost.
[0003]
Si, which is a ferrite-forming element, increases the strength of steel by its solid solution hardening, in contrast to expensive alloy elements that cause a decrease in elongation with the increase in strength as described above. However, in order to stabilize the ferrite by increasing the addition amount, even if the strength is increased, the effect of miniaturizing the metal structure or increasing the proportion of the low-temperature transformation phase is small. Therefore, it is an element that is excellent in a so-called strength / ductility balance that can increase strength without greatly reducing ductility characteristics such as elongation and is relatively inexpensive.
However, conventional high-strength steel sheets to which Si has been added did not have sufficient measures to improve fatigue characteristics, so the value obtained by dividing the fatigue limit stress by the tensile strength, that is, the fatigue limit ratio is not sufficiently high, and there are variations. Was also big.
In addition, the outline of the machining process for automobile users of steel sheets consists of a process of washing the steel sheet with oil, a pressing process, a degreasing process, a chemical conversion treatment process, and a painting process. Since sufficient countermeasures have not been taken, a chemical conversion treatment film is not sufficiently formed in the chemical conversion treatment step, and the chemical conversion property is poor. For this reason, the compatibility with the coating agent is not good, and the corrosion resistance after painting is not sufficiently excellent, so there is a limit to the expansion of applications on the user side.
[0004]
[Problems to be solved by the invention]
The present inventors pursued the fatigue limit ratio in the conventional Si-added high-strength steel sheet as described above and the cause of inferior chemical conversion treatment, and as a result of earnest research to solve the problem, There is a grain boundary corroded layer that corroded along the grain boundary of the steel, and this is the origin of fatigue cracks and deteriorates the fatigue characteristics, and the grain boundary corroded layer penetrates during cold rolling. As a result, it was found that the chemical conversion processability was lowered due to the remaining carbon such as rolled oil and rust preventive oil.
And such a grain boundary corrosion layer is in the middle of cooling after hot rolling, and oxygen enters the grain boundary from the surface as the base iron is oxidized and scale is generated, and Si, Mn near the grain boundary It was found that the grain boundary oxide layer formed by selectively oxidizing such highly oxidative elements such as those formed by erosion by pickling.
[0005]
That is, the present invention solves the problem caused by the intergranular corrosion layer existing in the above-described conventional Si-added high-strength steel sheet, and provides a method for producing a high-strength steel sheet having excellent fatigue resistance and chemical conversion treatment. The issue is to provide.
[0006]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the invention according to claim 1 is: C: 0.03-0.40%, Si: 0.50-3.0%, Mn: 0.30-3.0 %, Sol. A steel sheet containing Al: 0.01 to 0.10% and having a chemical composition composed of the remaining iron and inevitable impurities is subjected to hot finish rolling and then wound at a winding temperature of 523 to 650 ° C. Thus, the gist of the method for producing a high-strength steel sheet excellent in fatigue resistance and chemical conversion treatment is characterized in that the depth of the intergranular corrosion layer formed on the steel sheet surface is 20 μm or less.
The invention according to claim 2 is characterized in that, in the invention according to claim 1, the coiling temperature is set to 523 to 600 ° C., and the depth of the intergranular corrosion layer is set to 10 μm or less .
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The reason for limitation of the present invention will be described below.
(1) C: 0.03 to 0.40%,
C is an element essential for strengthening steel by forming carbides and increasing fatigue strength. If the amount is less than 0.03%, it is difficult to obtain a desired strength. However, if the amount of C exceeds 0.40%, the ductility is greatly lowered, and the workability of the steel sheet is lowered. Therefore, the range of C is limited to 0.03 to 0.40%.
[0008]
(2) Si: 0.50 to 3.0%,
Si is a ferrite-forming element, and when added to steel, it dissolves in the ferrite, strengthens the steel by so-called solid solution hardening, and increases the strength to stabilize the ferrite by increasing the amount of addition. Even so, the effect of miniaturizing the metal structure such as bainite or increasing the proportion of the low-temperature transformation phase such as bainite or martensite is small. Therefore, it is an element that can suppress a decrease in elongation accompanying an increase in strength, and is a relatively inexpensive element. However, if the amount is less than 0.50 %, the effect of strengthening the steel is small and the strength cannot be sufficiently increased. On the other hand, if the content exceeds 3.0%, the ductility is greatly reduced, and the occurrence of cracks and surface flaws on the surface of the steel sheet after hot rolling is increased, and the workability of the steel sheet is reduced. Therefore, the range of Si is limited to 0.50 to 3.0%.
[0009]
(3) Mn: 0.30 to 3.0%,
Mn combines with S in the steel to form MnS, and is an important element for strengthening the steel while greatly improving the adverse effects due to S, for example, the decrease in hot ductility due to the formation of FeS. However, if the amount is less than 0.30%, both the effect of improving ductility and the effect of strengthening steel are small. On the other hand, if it exceeds 3.0%, the proportion of the low-temperature transformation layer in the metal structure is increased to greatly reduce the ductility. Therefore, the amount of Mn is limited to 0.30 to 3.0%.
[0010]
(4) sol. Al: 0.01 to 0.10%,
The crystal grain size of the steel sheet has a great influence on the fatigue characteristics. If Al (acid-soluble Al) is less than 0.01%, fine sized particles having a particle size number of 5 or more cannot be obtained, or mixed grains having irregular sizes may be obtained. This leads to a decrease in anisotropy and hardness. On the other hand, sol. If Al exceeds 0.10%, the amount of Al 2 O 3 produced increases and the cleanliness of the steel decreases, which becomes the starting point of fatigue cracks and decreases the fatigue strength. Therefore, sol. The range of Al is limited to 0.01 to 0.10%.
[0011]
(5) Nb: 0.01 to 0.05%,
Nb is an element effective for precipitating fine carbonitrides and making steel finer, but if less than 0.01%, the effect is small. On the other hand, even if added over 0.05%, the effect is saturated, so addition up to 0.05% is sufficient. Therefore, when Nb is supplementarily used as the atomizing element, the range is preferably set between 0.01 and 0.05%.
[0012]
(6) Winding temperature: 650 ° C. or less A steel sheet that has been finish-rolled to a predetermined thickness at a temperature of about 900 ° C. by a quadruple rolling mill or a six-fold rolling mill is wound by a coiler through a runout table. However, a water-cooling device such as a laminar flow method is installed at the top and bottom of the run-out table for each section, so that the mechanical properties such as the strength and ductility of the steel sheet meet the required properties, and the specified winding temperature is maintained. It is controlled and wound up.
Since the steel sheet is finished at a high temperature of about 900 ° C. as described above, oxidation proceeds to generate an oxide scale until it is wound by a subsequent coiler. Greatly affects thickness and properties. Oxide scale is generated while the iron on the steel sheet surface combines with oxygen to produce FeO (wustite) and Fe 3 O 4 (magnetite). Oxygen also enters the boundary, and a highly oxidizable element such as Si or Mn near the grain boundary combines with oxygen to form a grain boundary oxide layer. Si is more oxidizable than Mn, so the grain boundary oxide layer formed in high-Si steel is deeper than in low-Si steel. The grain boundary oxide layer is eroded by pickling to form a grain boundary corrosion layer.
The depth of the grain boundary oxide layer depends on the heat history such as the cooling rate from finishing to winding, but when the winding temperature is higher than 650 ° C., the hot-rolled steel sheet after pickling, cold-rolling Since it becomes difficult to set the depth of the intergranular corrosion layer formed on the steel sheet to 20 μm or less, the winding temperature is limited to 650 ° C. or less. In order to further reduce the depth of the intergranular corrosion layer, it is desirable that the winding temperature be 600 ° C. or lower.
[0013]
(7) Depth of intergranular corrosion layer: 20 μm or less,
As shown in FIG. 4, the grain boundary oxide layer is an oxide scale formed in a wedge shape at the
If the depth of the intergranular corrosion layer formed on the steel sheet exceeds 20 μm, the degree of reduction in fatigue strength and chemical conversion treatment becomes large, so the depth of the intergranular corrosion layer is limited to 20 μm or less. In order to obtain a steel sheet having better fatigue resistance and chemical conversion treatment, the depth of the intergranular corrosion layer is desirably 10 μm or less. By setting the depth of the intergranular corrosion layer to 10 μm or less, it is possible to obtain a steel sheet having excellent fatigue resistance with a fatigue limit ratio of 0.45 or more and better chemical conversion treatment.
[0014]
FIG. 1 shows an outline of manufacturing and processing steps of a steel sheet according to the present invention.
On the steel sheet manufacturer side, the slab produced by continuous casting is heated to about 900 ° C. by a
[0015]
On the side of a steel plate processor such as an automobile manufacturer, the steel plate is a
[0016]
【Example】
Hereinafter, the present invention will be described by way of examples.
1. A slab having chemical components shown in Table 1 manufactured by converter melting and continuous casting is heated to 1200 ° C. in a heating furnace, subjected to rough rolling, and finished at a temperature between 880 and 915 ° C. After hot-rolling to a 2 mm steel plate, it was sent to a run-out table. The steel plate is cooled from the top and bottom of the steel plate being sent out by the laminar flow method, the temperature is adjusted by feedback by the thermometer, the water amount is adjusted by the feed forward method, and then the hot rolled steel plate is wound by a coiler. Manufactured. In some tests, the hot-rolled steel sheet was cold-rolled under 50% reduction, followed by continuous annealing and overaging treatment. The conditions for continuous annealing were an annealing temperature of 850 ° C., a rapid cooling start temperature of 670 ° C., a rapid cooling end temperature of 250 ° C., a cooling rate of 150 ° C./sec, and an overaging treatment of 280 ° C. × 400 sec.
A JIS No. 1 tensile test piece and a Schenk type fatigue test piece having a width of 20 mm, a notch, and a 30R of the test part were collected from the steel sheet and subjected to a tensile test and a Schenk type fatigue test. Fatigue test was performed at a rotation number 1800 times / min, also determine the fatigue limit is not broken at repetitive bending times is 10 7 times, and the fatigue limit ratio by dividing the tensile strength.
Moreover, the depth of the intergranular corrosion layer produced | generated in the hot-rolled steel plate after pickling with hydrochloric acid, or the cold-rolled steel plate pickled-cold-rolled-continuously annealed-over-aged was measured with the microscope. Furthermore, the hot-rolled steel sheet and the cold-rolled steel sheet were subjected to chemical conversion treatment using a zinc phosphate-based treatment liquid. The chemical conversion coating is judged by a scanning electron microscope, particularly good (◎) for coatings that are extremely uniform over the entire surface, and good for coatings that are formed to a uniform degree without any hindrance. (Circle), the thing in which the membrane | film | coat was partially formed or the membrane | film | coat was not formed was determined to be inferior (x).
[0017]
[Table 1]
[0018]
In Table 1, test no. 4 to 6 and 9 are for cold-rolled steel sheets, and the others are hot-rolled steel sheets.
Test No. In Examples 1-6, the steel has a chemical composition within the scope of the present invention and the coiling temperature is controlled at 523-650 ° C. Yield stress, tensile strength, although Wataru' high area from the lower area in accordance with the chemical components, have a high elongation of over 10% at a tensile strength exceeding 1000 N / mm 2 by the high Si is doing.
By lowering the coiling temperature, the intergranular corrosion layer depth is as shallow as 20 μm or less, and therefore the fatigue limit ratio is 0.44 or more and excellent fatigue resistance is achieved.
Further, since the depth of the intergranular corrosion layer was shallow and the surface irregularities and the residual oil content were reduced, the chemical conversion agent was evenly adhered uniformly, and the chemical conversion treatment property was good.
Among the examples, test no. In Examples 1 to 2 and 5 to 6, the coiling temperature was 600 ° C. or less and the coil was wound at a low temperature, so that the depth of the intergranular corrosion layer formed was as shallow as 10 μm or less. It was as high as 45 or more, and the chemical conversion treatment was particularly good.
[0019]
In contrast, test no. In all of Comparative Examples 7 to 10, since the winding temperature was higher than 650 ° C., the oxidation proceeded, and as a result, a grain boundary corrosion layer exceeding 20 μm was formed. For this reason, fatigue cracks are generated starting from the grain boundary corrosion layer, the progress of fatigue is fast, the fatigue limit ratio is as low as 0.41 or less, the surface irregularities are large, and there are many residual oil parts, The chemical conversion treatment film is not uniformly formed, and only those having inferior chemical conversion properties are obtained.
[0020]
【The invention's effect】
As described above, according to the present invention, it is possible to reduce the depth of the intergranular corrosion layer existing in the conventional Si-added high-strength steel sheet, and to prevent fatigue cracks generated from the intergranular corrosion layer. By suppressing the fatigue resistance, a uniform chemical conversion coating can be adhered by reducing the surface irregularities.
Accordingly, the present invention provides a method for producing a high-strength steel sheet that is excellent in fatigue resistance and chemical conversion treatment and is inexpensive, and greatly contributes to the development of the industrial world.
[Brief description of the drawings]
FIG. 1 is a diagram showing a flow of manufacturing and processing steps of a steel plate.
FIG. 2 is a diagram showing the relationship between intergranular corrosion layer depth and fatigue limit ratio.
FIG. 3 is a diagram showing the relationship between intergranular corrosion layer depth and chemical conversion treatment.
FIG. 4 is a view for explaining the surface of a steel plate having a scale.
FIG. 5 is a view for explaining the surface of a steel plate before chemical conversion treatment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Heating furnace 2
Claims (2)
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JP2012219366A (en) * | 2011-04-14 | 2012-11-12 | Jfe Steel Corp | Method for manufacturing high tension hot-rolled steel strip excellent in fatigue resistance characteristic |
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JP4698967B2 (en) * | 2004-03-30 | 2011-06-08 | 株式会社神戸製鋼所 | High-strength cold-rolled steel sheet with excellent coating film adhesion and workability |
JP4698968B2 (en) * | 2004-03-30 | 2011-06-08 | 株式会社神戸製鋼所 | High-strength cold-rolled steel sheet with excellent coating film adhesion and workability |
JP4698971B2 (en) * | 2004-03-31 | 2011-06-08 | 株式会社神戸製鋼所 | High-strength cold-rolled steel sheet with excellent coating film adhesion and workability |
JP4764125B2 (en) * | 2005-09-21 | 2011-08-31 | 株式会社東芝 | Ultrasonic diagnostic apparatus and control program for ultrasonic diagnostic apparatus |
JP5520086B2 (en) * | 2010-03-09 | 2014-06-11 | 株式会社神戸製鋼所 | High Si content steel sheet with excellent surface properties and method for producing the same |
JP5298114B2 (en) * | 2010-12-27 | 2013-09-25 | 株式会社神戸製鋼所 | High-strength cold-rolled steel sheet with excellent coating film adhesion and workability, and method for producing the same |
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