JP3911972B2 - Manufacturing method of high strength hot-dip galvanized steel sheet - Google Patents
Manufacturing method of high strength hot-dip galvanized steel sheet Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は、自動車の構造用部品などに適用される、優れた延性を付与することが可能な高強度溶融亜鉛メッキ(めっき)鋼板の製造方法に関する。
【0002】
【従来技術】
引張強度が440MPaを超える高強度溶融亜鉛メッキ鋼板は、その優れた防錆性と高い耐力を利点とし、とくに自動車の構造用部品などに広く適用されている。このため、自動車用高強度溶融亜鉛メッキ鋼板に係る発明が多く開示されている。
【0003】
高強度溶融亜鉛めっき鋼板は、その用途や要求特性に応じて、析出強化、固溶強化やマルテンサイト、ベイナイトといった低温変態相を利用した変態組織強化など、様々の成分系や組織で製造されている。
一般に、延性を向上させるには、鋼成分においてはSiを添加したり、あるいは組織においてはフェライト+マルテンサイトに制御するなどの手法が用いられる。
【0004】
例えば、特開平7-54051号公報には、延性、伸びフランジ性に優れた高強度溶融Znめっき鋼板を得る方法が提案されている。この技術は、1.0〜2.0%Mn-Nb添加鋼(更にTi添加)のスラブを素材とし、1000〜850℃の仕上げ圧延温度で熱間圧延した後、600℃までを40℃/sec以上の平均冷却速度で冷却し、600℃以下を30℃/sec以下の平均冷却速度で冷却し、550〜400℃の温度範囲で巻き取り、次いで溶融Znめっきを施すというものである。
【0005】
【発明が解決しようとする課題】
しかし、溶融亜鉛めっき鋼板ではめっき密着性の観点から延性向上に有効なSi添加量に大きな制約があり、延性向上には限界が生じる。また、高降伏比が必要な用途では、延性面で有利なフェライト+マルテンサイト鋼は適用できず、延性面で不利な析出強化によるフェライト+パーライト鋼が使用されている。すなわち、溶融亜鉛めっき鋼板については、これらの制約を受けずに延性向上を図る手段は見出されていないのが実状である。
【0006】
本発明は、上記の問題を解決し、優れた延性を付与することが可能な高強度溶融亜鉛めっき鋼板の製造方法を提供することを目的とする。
【0007】
【課題を解決するための手段】
上記課題は、次の発明により解決される。その発明は、mass%で、C:0.01〜0.3%,Si:0.7%以下,Mn:1〜3%,P:0.08%以下,S:0.01%以下, sol.Al: 0.08%以下,N: 0.007%以下を含有し、残部 Fe および不可避的不純物からなる鋼を、Ar3点以上で仕上げ熱延し、2.5秒以内に平均冷却速度100℃/sec以上の急速冷却を開始し、700℃以下500℃超えの温度まで冷却した後巻き取り、その後、酸洗あるいはさらに冷間圧延して、連続溶融亜鉛めっきを行うとともに720℃以上で焼鈍することを特徴とする高強度溶融亜鉛メッキ鋼板の製造方法である。
【0008】
本発明は、上記問題点を鑑み、溶融亜鉛めっき後の延性を向上させるため、鋼成分以外でいかに製造条件で制御すべきかを課題として検討する中でなされた。特に、上記課題を解決するため、高強度溶融亜鉛めっき鋼板の延性に及ぼす熱延条件や溶融亜鉛めっき時の熱サイクルの影響について鋭意検討された。この結果、熱延板段階においてパーライトが層状に分布したいわゆるバンド状組織の形成を抑制して、組織を均一微細化し、しかも、パーライト粒内のフェライトとセメンタイトの層状組織を微細とすること(パーライトのラメラ間隔の微細化)が、高強度溶融亜鉛めっき鋼板の延性の向上に有効であることが明らかとなった。
【0009】
以上の技術は、化学成分の制御に加え、熱延ラインでは仕上げ圧延後の冷却パターンの制御が必要であり、以下に、それぞれの限定理由を説明する。
まず、成分限定理由について述べる。
【0010】
C:0.01%〜0.3%
Cは、所望の強度を確保するために必須の元素であり、440MPa以上の強度を得るためには0.01%以上とする必要がある。一方0.03%を超える多量の添加を行うと、熱延板でのパーライト分率が増加して、仕上げ熱延後の大冷却を活用しても、層状組織の低減が十分でなくなる。
【0011】
Si:0.7%以下
Siは、鋼の延性を向上させるために有効な元素であるが、添加量が多くなると亜鉛メッキの密着性や表面外観が著しく劣化するので、上限を0.7%に規定する。
【0012】
Mn:1〜3%
MnはC同様、所望の強度を確保するために必須の元素である。所望の強度を得るため1%が下限として必要であるが、過剰に添加するとCの過剰添加と同様、仕上げ熱延後の大冷却を活用しても、層状組織の低減が十分でなくなる。このため、上限を3%に規定する。
【0013】
P:0.08%以下
Pは、固溶強化能の大きい元素で、高強度鋼板の製造には必須の元素であるが、添加量が多くなるとSi同様、めっき品質が劣化するので、上限を0.08%に規定する。
【0014】
S:0.01%以下
Sは不純物であり、含有量が高いと鋼中の介在物が増加して、加工性が劣化する。このため上限を0.01%に規定する。
【0015】
sol.Al: 0.08%以下
sol.Alは、通常の鋼に含有される量であれば本発明の効果を損なわず、0.08%以下であれば良い。
【0016】
N: 0.007%以下
Nは、通常の鋼に含有される量であれば本発明の効果を損なわず、0.007%以下であれば良い。
【0017】
その他、本発明では、さらにNb,Ti:0.005〜0.5%,B:0.0002〜0.005%から選ばれる1種または2種以上、あるいはさらに、V,Cr,Mo:0.01〜1%から選ばれる1種または2種以上を含有する鋼を用いることができる。
【0018】
Nb,Ti: 1種以上それぞれ0.005〜0.5%
Nb,Tiは組織の微細化や析出強化により高強度化する目的で添加する。それぞれの下限は所望の効果が得られる最低限量であり、また、上限は、これ以上添加しても効果が飽和するばかりか、延性が劣化するため規定した。
【0019】
B:0.0002〜0.005%
Bはフェライトの析出抑制により低温変態相体積率を向上させて高強度化する目的で添加する。下限は所望の効果が得られる最低限量であり、また、上限は、これ以上添加しても効果が飽和するばかりか、延性が劣化するため規定した。
【0020】
V,Cr,Mo: 1種以上それぞれ0.01〜1%
V,Cr,Moは、鋼の焼き入れ性を向上させて高強度化する目的で添加するが、それぞれの下限は所望の効果が得られる最低限量であり、また、上限は、これ以上添加しても効果が飽和するため規定した。
なお、Nb,Tiの1種以上とV,Cr,Moの1種以上とを、ともに添加しても良い。
【0021】
次に、本発明における製造条件の限定理由に関して説明する。
本発明の製造法においては、仕上げ熱延後の冷却速度を100℃/secを超える冷却速度とする。これは検討の結果、後述のようにこの冷却速度域において、明瞭な延性向上効果があることが明らかとなったことによる。冷却速度の下限の100℃/secは、これ以下では所望の効果が得られなくなるため規定されている。
【0022】
この大冷却(急速冷却)は、500℃超え700℃以下の温度範囲で停止させる必要がある。これは、極端に低い500℃以下の温度まで急速冷却すると、ベイナイトやマルテンサイトなどの低温変態組織が形成され、これを連続溶融亜鉛めっきラインにおいて焼鈍すると、針状のフェライトが形成され、延性が劣化することを防止するためである。
【0023】
また、急速冷却の停止温度が700℃を超える(700℃より高い)と、Cの拡散速度が十分大きいためバンド状になりやすく、バンド組織の抑制が十分でない。あるいは、パーライトが高温で生成するためパーライトの層状組織の微細化が十分でなく、やはり、所望の効果が得られなくなってしまう。従って、100℃/sec を超える大冷却は、500℃超え700℃以下の温度範囲で停止させることに規定する。
【0024】
また、熱延終了後の急冷開始までの時間が極端に長くなると、粒成長により組織が粗大化するばかりか、粗大なパーライトの析出も開始してしまうので、できるだけ短いほうが本発明の効果を得やすく、熱延終了後は2.5秒以内に100℃/secを超える急冷を開始する必要がある。
【0025】
ただし、圧延終了後、極端に短い時間で急速冷却を開始すると、圧延組織が完全に再結晶しない状態となり、組織が不均一になりやすくなる。その結果、コイル長手方向と幅方向の材質のバラツキが大きくなる傾向があるため、急冷開始は熱延終了後0.5秒を超えることが好ましい。
【0026】
熱延仕上げ温度は、加工組織が残留して延性が劣化しなようにAr3点以上とする。
【0027】
以上のように熱延条件を制御した後、酸洗後、あるいは、さらに冷間圧延した後、連続溶融亜鉛めっきラインにて焼鈍とめっきを行うが、焼鈍温度は720℃以上として、熱延段階で形成されたコロニーの大きなパーライトあるいは冷延過程で粉砕されたパーライトの再固溶を促進させる必要がある。このように焼鈍温度を制御することで、パーライトやセメンタイトにより強化する場合や、さらに析出強化で強化する場合においては、コロニーの大きなパーライトや粉砕されたパーライトの消失により、延性が向上する。
【0028】
また、ベイナイトやマルテンサイトなど低温変態相で強化する場合においては、パーライトの再固溶が促進されることで、焼鈍時にパーライトの溶け残りもなく、逆変態オーステナイトが安定して得られるため、延性が向上する。
【0029】
実施例でも示すが、後者のような低温変態相で強化する場合において、このような延性向上効果は、やや大きい。これは、前者における、コロニーの大きなパーライトや粉砕されたパーライトの消失による延性向上効果よりも、後者における、パーライトの溶け残り解消による延性向上効果のほうが大きいためと考えられる。
【0030】
焼鈍後、亜鉛めっきされるが、このあとさらに合金化されても、本発明の効果は損なわれない。
【0031】
【発明の実施の形態】
発明の実施に当たっては、通常の製鋼プロセスにより化学成分を上述の範囲内に調節し、熱延条件、とりわけ仕上圧延終了後の冷却条件を制御して熱延鋼板を製造する。その後は、溶融亜鉛めっきを行うことで、本発明の溶融亜鉛めっき鋼板を製造することができる。その他とくに言及していないが、造塊あるいは連続鋳造によるスラブ製造法や、熱延での粗熱延バー接続による連続熱延、また、熱延過程でのインダクションヒーターを利用した200℃以内の昇温などは、本発明の効果に対して影響を及ぼさない。
【0032】
本発明の製造法においては、仕上げ熱延後の冷却速度が最も重要となる。これまで開示されている特許では、例えば特開平7-54051におけるように高々100℃/secまでの冷却速度の検討であり、延性に及ぼす冷却速度の影響が明確になっていなかった。発明の検討に用いた、最大1000℃/secの大冷却が可能な実験設備を用いた結果、100℃/secを超える冷却速度域において、明瞭な延性向上効果があることが突き止められた。
【0033】
この延性向上効果が得られる理由については、次のように考えられる。上述したように、100℃/secを超える大冷却速度を仕上げ熱延後の鋼帯に適用することで、熱延板段階においてパーライトが層状に分布したいわゆるバンド状組織の形成を抑制して、組織を均一微細化することができる。これに加えて、パーライト粒内のフェライトとセメンタイトの層状組織が微細化するため、その後の連続溶融亜鉛めっきでの均熱時において、熱延段階で形成されたコロニーの大きなパーライトあるいは冷延過程で粉砕されたパーライトの再固溶が促進される。その結果、塑性変形を受けた時のクラックの起点が少なくなるため、延性が向上するものと考えられる。
【0034】
なお、急速冷却後は、そのまま冷却を停止して巻き取っても、あるいは、さらに、通常の30℃/sec以下の冷却速度で所定の温度に制御して巻き取ってもかまわない。
【0035】
また、化学成分については、材質上の観点から次のようにすると良い。
【0036】
まずCについては、強度確保のし易さからは0.05%以上、延性の観点からは0.2%以下とすることが好ましい。Mnについては、低温変態相を利用しない場合は2%以下でよい。また、Nb,Tiについては、延性の観点からは0.1%以下とすることが好ましい。
【0037】
また、冷却速度については、延性向上効果を確実に確保するため下限を110℃/secとすることが好ましい。
【0038】
【実施例】
以下に本発明による効果を具体的に示す。
まず、表1に成分を示す本発明成分鋼A〜Eを溶製した後、圧延素材とした。これらを表2に示す熱延条件で熱延鋼帯とし、酸洗し、あるいは一部は冷延率62%で冷間圧延して、焼鈍下地を準備した。
【0039】
【表1】
【0040】
【表2】
【0041】
続いて、実験室の熱処理シミュレーターにて、連続溶融亜鉛メッキライン相当の熱処理条件で焼鈍した。表3に、この熱処理条件を連続溶融亜鉛メッキ条件として、主な圧延条件(表2に同じ)と共に示す。これらの鋼板の組織と、圧延方向と直角に試験片どりしたJIS5号試験片を用いた引張強度(TS)と延性(El)を、表3に併せて記載した。
【0042】
【表3】
【0043】
図1は、鋼AとBの延性に及ぼす、熱延終了後の一次冷却速度の影響を示す。この図より、本発明が開示する一次冷却速度の範囲に制御することで、延性が向上していることがわかる。とくに急冷開始時間を0.5秒を超えて2.5秒以内に制御することで、上記の効果は顕著となる。また、前述したとおり、フェライト+マルテンサイト鋼である鋼Aでは、フェライト+パーライト(+セメンタイト)組織をベースに析出強化により強化した鋼Bに対して、延性向上しろが1%程度大きいことがわかる。
【0044】
【発明の効果】
本発明は、化学成分の制御に加え、熱延ラインでは仕上げ圧延後の冷却パターンの制御により、熱延板段階においていわゆるバンド状組織の形成を抑制して、組織を均一微細化している。その結果、本発明によれば、鋼成分や組織の制約によらず、延性に優れる高強度溶融亜鉛メッキ鋼板が得られるので、工業的価値は極めて高い。
【図面の簡単な説明】
【図1】延性に及ぼす熱延終了後の一次冷却速度の影響を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-strength hot-dip galvanized (plated) steel sheet capable of imparting excellent ductility, which is applied to structural parts of automobiles.
[0002]
[Prior art]
High-strength hot-dip galvanized steel sheets with a tensile strength exceeding 440 MPa have the advantage of excellent rust prevention and high yield strength, and are widely applied especially to structural parts of automobiles. For this reason, many inventions related to high strength hot dip galvanized steel sheets for automobiles have been disclosed.
[0003]
High-strength hot-dip galvanized steel sheets are manufactured in various component systems and structures, such as precipitation strengthening, solid solution strengthening, and transformation structure strengthening using low-temperature transformation phases such as martensite and bainite, depending on the application and required characteristics. Yes.
In general, in order to improve ductility, a technique such as adding Si to the steel component or controlling to ferrite + martensite in the structure is used.
[0004]
For example, Japanese Patent Application Laid-Open No. 7-54051 proposes a method for obtaining a high-strength hot-dip Zn-plated steel sheet having excellent ductility and stretch flangeability. This technology uses 1.0 to 2.0% Mn-Nb-added steel (and Ti addition) slab as a raw material, and after hot rolling at a finish rolling temperature of 1000 to 850 ° C, an average of over 40 ° C / sec up to 600 ° C Cooling is performed at a cooling rate, cooling is performed at 600 ° C. or less at an average cooling rate of 30 ° C./sec or less, winding is performed in a temperature range of 550 to 400 ° C., and then hot-dip Zn plating is performed.
[0005]
[Problems to be solved by the invention]
However, in the hot dip galvanized steel sheet, there is a great restriction on the amount of Si added effective in improving ductility from the viewpoint of plating adhesion, and there is a limit in improving ductility. In applications where a high yield ratio is required, ferrite + martensitic steel, which is advantageous in terms of ductility, cannot be applied, and ferrite + pearlite steel, which is disadvantageous in terms of ductility, is used. That is, as for the hot-dip galvanized steel sheet, no means has been found to improve ductility without receiving these restrictions.
[0006]
An object of this invention is to provide the manufacturing method of the high intensity | strength hot-dip galvanized steel plate which can solve said problem and can provide the outstanding ductility.
[0007]
[Means for Solving the Problems]
The above problem is solved by the following invention. The invention is mass%, C: 0.01 to 0.3%, Si: 0.7% or less, Mn: 1 to 3%, P: 0.08% or less, S: 0.01% or less, sol.Al: 0.08% or less, N: Finishing hot rolling of steel containing 0.007% or less and the balance Fe and inevitable impurities at Ar 3 points or more, and starting rapid cooling with an average cooling rate of 100 ° C / sec or more within 2.5 seconds, 700 ° C or less Winding after cooling to a temperature exceeding 500 ° C, then pickling or further cold rolling to perform continuous hot dip galvanizing and annealing at 720 ° C or higher manufacture of high strength hot dip galvanized steel sheet Is the method.
[0008]
In view of the above-mentioned problems, the present invention was made in consideration of how to control under manufacturing conditions other than steel components in order to improve ductility after hot dip galvanization. In particular, in order to solve the above-mentioned problems, intensive studies were conducted on the effects of hot rolling conditions and thermal cycles during hot dip galvanizing on the ductility of high strength hot dip galvanized steel sheets. As a result, the formation of a so-called band-like structure in which pearlite is distributed in layers in the hot-rolled sheet stage is suppressed, the structure is made uniform and fine, and the layered structure of ferrite and cementite in the pearlite grains is made fine (perlite). It has been clarified that the refinement of the lamellar spacing in the above is effective in improving the ductility of the high-strength hot-dip galvanized steel sheet.
[0009]
The above technique requires control of the cooling pattern after finish rolling in the hot rolling line in addition to control of chemical components, and the reasons for limitation will be described below.
First, the reason for component limitation will be described.
[0010]
C: 0.01% to 0.3%
C is an essential element for securing a desired strength, and needs to be 0.01% or more in order to obtain a strength of 440 MPa or more. On the other hand, when a large amount exceeding 0.03% is added, the pearlite fraction in the hot-rolled sheet increases, and even if the large cooling after finishing hot rolling is used, the reduction of the layered structure becomes insufficient.
[0011]
Si: 0.7% or less
Si is an effective element for improving the ductility of steel. However, since the adhesiveness and surface appearance of galvanizing are significantly deteriorated when the addition amount is increased, the upper limit is defined as 0.7%.
[0012]
Mn: 1-3%
Like C, Mn is an essential element for ensuring a desired strength. In order to obtain a desired strength, 1% is necessary as the lower limit. However, when excessively added, as in the case of excessive addition of C, even if large cooling after finishing hot rolling is used, the reduction of the layered structure becomes insufficient. For this reason, the upper limit is defined as 3%.
[0013]
P: 0.08% or less
P is an element having a large solid solution strengthening ability and an essential element for the production of high-strength steel sheets. However, as the addition amount increases, the plating quality deteriorates like Si, so the upper limit is specified as 0.08%.
[0014]
S: 0.01% or less
S is an impurity, and if the content is high, inclusions in the steel increase and workability deteriorates. For this reason, the upper limit is defined as 0.01%.
[0015]
sol.Al: 0.08% or less
The amount of sol.Al may be 0.08% or less without impairing the effects of the present invention as long as it is contained in ordinary steel.
[0016]
N: 0.007% or less
N may be 0.007% or less without impairing the effects of the present invention as long as it is contained in ordinary steel.
[0017]
In addition, in the present invention, one or more selected from Nb, Ti: 0.005 to 0.5%, B: 0.0002 to 0.005%, or one more selected from V, Cr, Mo: 0.01 to 1% Alternatively, steel containing two or more types can be used.
[0018]
Nb, Ti: 1 type or more 0.005-0.5% each
Nb and Ti are added for the purpose of increasing strength by refining the structure and strengthening precipitation. Each lower limit is the minimum amount for obtaining a desired effect, and the upper limit is defined not only because the effect is saturated but also the ductility deteriorates even if it is added more than this.
[0019]
B: 0.0002-0.005%
B is added for the purpose of increasing the volume ratio of the low-temperature transformation phase by suppressing the precipitation of ferrite and increasing the strength. The lower limit is the minimum amount at which a desired effect can be obtained, and the upper limit is specified because not only the effect is saturated but also the ductility deteriorates even if added more.
[0020]
V, Cr, Mo: 1 ~ 1% each 0.01 ~ 1%
V, Cr, and Mo are added for the purpose of improving the hardenability of the steel to increase the strength, but the lower limit of each is the minimum amount that can achieve the desired effect, and the upper limit is added more than this. Even though the effect is saturated, it was specified.
One or more of Nb and Ti and one or more of V, Cr and Mo may be added together.
[0021]
Next, the reason for limiting the manufacturing conditions in the present invention will be described.
In the production method of the present invention, the cooling rate after finish hot rolling is set to a cooling rate exceeding 100 ° C./sec. This is because, as a result of the examination, it became clear that there is a clear ductility improving effect in this cooling rate region as will be described later. The lower limit of the cooling rate, 100 ° C./sec, is specified because the desired effect cannot be obtained below this value.
[0022]
This large cooling (rapid cooling) needs to be stopped in a temperature range of 500 ° C. to 700 ° C. This is because when rapidly cooled to an extremely low temperature of 500 ° C. or lower, low-temperature transformation structures such as bainite and martensite are formed, and when this is annealed in a continuous hot dip galvanizing line, acicular ferrite is formed and ductility is reduced. This is to prevent deterioration.
[0023]
Also, when the rapid cooling stop temperature exceeds 700 ° C. (higher than 700 ° C.), the diffusion rate of C is sufficiently high, and thus it tends to form a band, and the band structure is not sufficiently suppressed. Or, since pearlite is generated at a high temperature, the layered structure of pearlite is not sufficiently refined, and the desired effect cannot be obtained. Therefore, it is specified that large cooling exceeding 100 ° C / sec is stopped in a temperature range exceeding 500 ° C and 700 ° C or less.
[0024]
In addition, if the time from the end of hot rolling to the start of quenching becomes extremely long, not only the structure becomes coarse due to grain growth, but also precipitation of coarse pearlite starts. It is easy to perform rapid cooling exceeding 100 ° C / sec within 2.5 seconds after the end of hot rolling.
[0025]
However, if rapid cooling is started in an extremely short time after the end of rolling, the rolled structure is not completely recrystallized, and the structure tends to be non-uniform. As a result, since the variation in the material in the coil longitudinal direction and the width direction tends to increase, it is preferable that the start of rapid cooling exceeds 0.5 seconds after the end of hot rolling.
[0026]
The hot rolling finishing temperature is set at 3 or more points so that the processed structure remains and ductility does not deteriorate.
[0027]
After controlling the hot rolling conditions as described above, after pickling, or further cold rolling, annealing and plating are performed in a continuous hot dip galvanizing line. It is necessary to promote the re-solution of the large pearlite of the colonies formed in step 1 or the pearlite pulverized in the cold rolling process. By controlling the annealing temperature in this way, when strengthening with pearlite or cementite, or when strengthening with precipitation strengthening, ductility is improved by the disappearance of large collite of colonies or pulverized pearlite.
[0028]
Also, when strengthening in low-temperature transformation phase such as bainite and martensite, the re-solid solution of pearlite is promoted, so that pearlite does not remain undissolved during annealing, and reverse transformed austenite can be obtained stably. Will improve.
[0029]
As shown in the examples, in the case of strengthening with the low-temperature transformation phase such as the latter, such a ductility improving effect is somewhat large. This is presumably because the ductility improvement effect by eliminating the dissolution of pearlite in the latter is larger than the ductility improvement effect by disappearance of pearlite having large colonies or disappearance of pulverized pearlite in the former.
[0030]
After annealing, it is galvanized, but even if further alloyed thereafter, the effect of the present invention is not impaired.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
In carrying out the invention, a chemical composition is adjusted within the above range by a normal steelmaking process, and hot-rolled steel sheets are manufactured by controlling hot-rolling conditions, particularly cooling conditions after finishing rolling. Thereafter, the hot dip galvanized steel sheet of the present invention can be manufactured by hot dip galvanizing. Although not specifically mentioned, slab manufacturing method by ingot casting or continuous casting, continuous hot rolling by hot-rolling rough hot-rolling bar connection, and heating within 200 ° C using an induction heater in the hot-rolling process. Temperature and the like do not affect the effect of the present invention.
[0032]
In the production method of the present invention, the cooling rate after finish hot rolling is most important. In the patents disclosed so far, for example, as in Japanese Patent Application Laid-Open No. 7-54051, the cooling rate up to 100 ° C./sec is examined, and the influence of the cooling rate on the ductility has not been clarified. As a result of using the experimental equipment capable of large cooling at a maximum of 1000 ° C./sec used for the examination of the invention, it was found that there is a clear ductility improving effect in a cooling rate region exceeding 100 ° C./sec.
[0033]
The reason why this effect of improving ductility is obtained is considered as follows. As described above, by applying a large cooling rate exceeding 100 ° C./sec to the steel strip after hot rolling, the formation of a so-called band-like structure in which pearlite is distributed in layers in the hot-rolled sheet stage is suppressed, The tissue can be uniformly refined. In addition, since the layered structure of ferrite and cementite in the pearlite grains is refined, during the subsequent soaking in continuous hot dip galvanizing, colonies formed in the hot rolling stage have large pearlite or cold rolling processes. Re-solution of pulverized perlite is promoted. As a result, since the starting point of the crack when subjected to plastic deformation is reduced, it is considered that the ductility is improved.
[0034]
Note that after the rapid cooling, the cooling may be stopped as it is and the winding may be performed, or further, the winding may be performed while being controlled to a predetermined temperature at a normal cooling rate of 30 ° C./sec or less.
[0035]
The chemical components are preferably as follows from the viewpoint of materials.
[0036]
First, C is preferably 0.05% or more for ease of securing the strength and 0.2% or less from the viewpoint of ductility. Mn may be 2% or less when the low temperature transformation phase is not used. Nb and Ti are preferably 0.1% or less from the viewpoint of ductility.
[0037]
As for the cooling rate, the lower limit is preferably set to 110 ° C./sec in order to ensure the effect of improving ductility.
[0038]
【Example】
The effect by this invention is shown concretely below.
First, the present invention component steels A to E whose components are shown in Table 1 were melted, and then rolled. These were made into hot-rolled steel strips under the hot-rolling conditions shown in Table 2, and pickled or partially cold-rolled at a cold rolling rate of 62% to prepare an annealing base.
[0039]
[Table 1]
[0040]
[Table 2]
[0041]
Then, it annealed on the heat processing conditions equivalent to a continuous hot dip galvanizing line in the heat processing simulator of a laboratory. Table 3 shows this heat treatment condition as a continuous hot dip galvanizing condition together with main rolling conditions (same as in Table 2). Table 3 shows the structure of these steel sheets and the tensile strength (TS) and ductility (El) using JIS No. 5 test pieces that were cut at right angles to the rolling direction.
[0042]
[Table 3]
[0043]
FIG. 1 shows the influence of the primary cooling rate after the end of hot rolling on the ductility of steels A and B. From this figure, it can be seen that the ductility is improved by controlling to the range of the primary cooling rate disclosed in the present invention. In particular, by controlling the rapid cooling start time from 0.5 seconds to 2.5 seconds, the above effect becomes remarkable. Further, as described above, it is understood that the steel A, which is a ferrite + martensitic steel, has a ductility improvement margin of about 1% larger than that of the steel B strengthened by precipitation strengthening based on the ferrite + pearlite (+ cementite) structure. .
[0044]
【The invention's effect】
In the present invention, in addition to the control of chemical components, the control of the cooling pattern after finish rolling in the hot rolling line suppresses the formation of a so-called band-like structure in the hot-rolled sheet stage, thereby making the structure uniform and fine. As a result, according to the present invention, a high-strength hot-dip galvanized steel sheet having excellent ductility can be obtained regardless of the steel component and structure restrictions, and thus the industrial value is extremely high.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of a primary cooling rate after hot rolling on ductility.
Claims (5)
Priority Applications (11)
Application Number | Priority Date | Filing Date | Title |
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JP2000216316A JP3911972B2 (en) | 2000-07-17 | 2000-07-17 | Manufacturing method of high strength hot-dip galvanized steel sheet |
AT00960974T ATE464402T1 (en) | 1999-09-16 | 2000-09-13 | METHOD FOR PRODUCING A THIN STEEL PLATE WITH HIGH STRENGTH |
EP10150015A EP2166121A1 (en) | 1999-09-16 | 2000-09-13 | High strength steel sheet and method for manufacturing the same |
EP00960974A EP1143022B1 (en) | 1999-09-16 | 2000-09-13 | Method for producing a thin steel plate having high strength |
DE60044180T DE60044180D1 (en) | 1999-09-16 | 2000-09-13 | METHOD FOR PRODUCING A THIN STAINLESS STEEL PLATE WITH HIGH STRENGTH |
KR10-2001-7003487A KR100415718B1 (en) | 1999-09-16 | 2000-09-13 | High strength steel sheet and method for manufacturing the same |
PCT/JP2000/006252 WO2001020051A1 (en) | 1999-09-16 | 2000-09-13 | Steel thin plate having high strength and method for production thereof |
EP10150016A EP2166122A1 (en) | 1999-09-16 | 2000-09-13 | Method of manufacturing high strength steel |
US09/827,597 US6663725B2 (en) | 1999-09-16 | 2001-04-05 | High strength steel sheet and method for manufacturing the same |
US10/625,796 US20040112482A1 (en) | 1999-09-16 | 2003-07-23 | High strength steel sheet and method for manufacturing the same |
US11/271,428 US20060065329A1 (en) | 1999-09-16 | 2005-11-10 | High strength steel sheet and method for manufacturing the same |
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JP2000216316A JP3911972B2 (en) | 2000-07-17 | 2000-07-17 | Manufacturing method of high strength hot-dip galvanized steel sheet |
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JP5187320B2 (en) * | 2010-01-06 | 2013-04-24 | 新日鐵住金株式会社 | Cold rolled steel sheet manufacturing method |
JP5446885B2 (en) * | 2010-01-06 | 2014-03-19 | 新日鐵住金株式会社 | Cold rolled steel sheet manufacturing method |
KR102221452B1 (en) * | 2019-05-03 | 2021-03-02 | 주식회사 포스코 | Ultra-high strength steel sheet having shear workability excellent and method for manufacturing thereof |
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