JP4692519B2 - High-strength hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents
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本発明は、高強度溶融亜鉛メッキ鋼板およびその製造方法に関するものである。 The present invention relates to a high-strength hot-dip galvanized steel sheet and a method for producing the same.
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、建設部材、機械構造用部品、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、高強度溶融亜鉛メッキ鋼板に係る発明は非常に多く開示されている。特に、適用範囲が拡大する中で加工性に対する要求特性が高まっているため、例えば、特許文献１や特許文献２などのように、加工性に優れた高強度溶融亜鉛メッキ鋼板に関する技術が数多く開示されている。
High-strength hot-dip galvanized steel sheets with a tensile strength exceeding 440 MPa have the advantage of excellent rust prevention and high proof stress, and are widely applied to construction members, machine structural parts, automotive structural parts, and the like. For this reason, very many inventions related to high-strength hot-dip galvanized steel sheets have been disclosed. In particular, since the required characteristics for workability are increasing as the application range is expanded, for example, as disclosed in
特許文献１に記載の技術は、Si-Mn-P系の熱延鋼板を酸洗し、連続溶融亜鉛メッキラインにてAc1点以上に加熱してからMs点以下に急冷して、部分的あるいは全部分マルテンサイトを生成させている。その後、溶融亜鉛浴及び合金化の際の温度により、マルテンサイトを焼戻すというものである。
The technology described in
特許文献２に記載の技術は、Mn-P-Nb(-Ti)系の熱延鋼板を、熱延後急冷して低温巻取りして溶融亜鉛メッキを行う。金属組織は、微細なフェライトマトリックスにパーライト又はセメンタイトが微細に分散した組織であり、比較的軟質なパーライトを微細に分散させることによって、伸びフランジ性を向上させるというものである。
The technique described in
しかし、製造ままの鋼板の加工性に対する要求特性が高まる一方で、適用技術の拡大に伴い、テーラードブランク材などのように、溶接部を含んだ状態で加工されることも多くなってきている。これは、自動車の構造用部品などに適用され、特に異種強度あるいは異板厚の材料を、レーザー溶接あるいはマッシュシーム溶接などの溶接法により接合して成形するもので、この接合素材はTWB（Tailored Welded Blank）と呼ばれている。 However, while the required characteristics with respect to the workability of the as-manufactured steel sheet are increasing, as the applied technology expands, it is often processed in a state including a welded portion such as a tailored blank material. This is applied to structural parts of automobiles, and is formed by joining materials of different strengths or different thicknesses by welding methods such as laser welding or mash seam welding. This joining material is TWB (Tailored Welded Blank).
このように、テーラードブランク材（TWB）自体、あるいは、溶接部を含んだ構造部材の高速変形挙動に対する要求特性が厳しくなるなど、溶接部の特性が加工用素材に対する要求特性として着目されつつある。
しかしながら、上記の従来の加工性に優れた高強度溶融亜鉛メッキ鋼板は、一般にその主たる強化機構がオーステナイト相の急冷により得られるマルテンサイトやベイナイトといった低温変態相を利用しているため、溶接時にHAZ(熱影響部)が軟化してしまうという大きな弱点が存在する。特許文献１および特許文献２等の従来技術においては、溶接時のHAZ軟化については触れられていない。
However, the above-described conventional high-strength hot-dip galvanized steel sheet with excellent workability generally uses a low-temperature transformation phase such as martensite or bainite obtained by quenching the austenite phase as its main strengthening mechanism. There is a big weakness that the (heat-affected zone) softens. In the prior arts such as
このような溶接時のHAZ軟化は、例えば、テーラードブランク材では成形性が劣化するばかりか、変形強度、破断強度、高速変形強度など構造部材としての性能をも劣化させる原因ともなる。構造部材の使用時における性能は、自動車等の安全性を左右しかねない要因ともなるものであり、構造部材用の材料に対しては、使用時の性能の向上が、車体の軽量化による燃費向上と共に、要請されている。 Such softening of HAZ at the time of welding, for example, not only deteriorates formability in a tailored blank material, but also causes deterioration in performance as a structural member such as deformation strength, breaking strength, and high-speed deformation strength. The performance of structural members during use may be a factor that may affect the safety of automobiles, etc.For materials for structural members, improved performance during use is due to the lighter weight of the vehicle body. With improvement, it is requested.
本発明は、上記社会ニーズを鑑み、TWB素材として使用され、成形時に溶接部のHAZ部で破断することなく、単板と同じ成形性を有する高強度溶融亜鉛メッキ鋼板を提供することを目的とする。 In view of the above-mentioned social needs, the present invention aims to provide a high-strength hot-dip galvanized steel sheet that is used as a TWB material and has the same formability as a single sheet without breaking at the HAZ part of the weld during molding. To do.
上記の課題は次の発明により解決される。
［１］フェライトと低温変態相の複合組織またはフェライトと低温変態相および残部組織として体積率１０％以下のパーライト、体積率１０％以下の残留オーステナイトから選ばれる1種以上を有する複合組織からなる高強度溶融亜鉛メッキ鋼板において、化学成分として、mass％で、C:0.04%〜0.25%、Si:0.7%以下、Mn:1.4〜3.5%、Cr:0.05〜1%、P:0.05%以下、S:0.01%以下、Nb:0.005〜0.1%を含有し、残部Ｆｅおよび不可避的不純物からなり、かつ複合組織を構成するフェライトと低温変態相の平均粒径が10μm以下であることを特徴とする高強度溶融亜鉛メッキ鋼板。
［２］前記［１］において、HAZ軟化特性に優れていることを特徴とする高強度溶融亜鉛メッキ鋼板。
［３］前記［１］又は［２］記載の高強度溶融亜鉛メッキ鋼板において、化学成分としてさらに、mass％で、Mo:0.05〜1%、V:0.02〜0.5%、Ti:0.005〜0.05%、B:0.0002〜0.002%から選ばれる1種以上を含有することを特徴とする高強度溶融亜鉛メッキ鋼板。
［４］フェライトと低温変態相の複合組織またはフェライトと低温変態相および残部組織として体積率１０％以下のパーライト、体積率１０％以下の残留オーステナイトから選ばれる1種以上を有する複合組織からなる高強度溶融亜鉛メッキ鋼板の製造方法において、前記［１］ないし［３］のいずれかに記載された化学成分の鋼を、鋳造し、Ar3点以上の温度で仕上げ圧延後、800〜700℃の温度域を5℃/sec以上で冷却して450〜700℃で巻き取り、酸洗後の熱延鋼帯を、あるいはさらに20%以上の圧下率で冷間圧延した冷延鋼帯を、連続溶融亜鉛メッキラインにおいて、760〜880℃で均熱した後、1℃/sec以上の冷却速度で600℃以下の温度域まで冷却し、亜鉛メッキ、あるいはさらに合金化処理を行う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。
The above problems are solved by the following invention.
[1] A high structure composed of a composite structure of ferrite and a low temperature transformation phase or a composite structure having at least one selected from ferrite, a low temperature transformation phase and pearlite having a volume ratio of 10% or less and residual austenite having a volume ratio of 10% or less as the remaining structure. In the strength hot-dip galvanized steel sheet, as chemical components, mass%, C: 0.04% to 0.25%, Si: 0.7% or less, Mn: 1.4 to 3.5%, Cr: 0.05 to 1%, P: 0.05% or less, S : 0.01% or less, Nb: 0.005 to 0.1%, comprising the balance Fe and inevitable impurities, and the ferrite and the low-temperature transformation phase constituting the composite structure have an average particle size of 10 μm or less Strength hot dip galvanized steel sheet.
[2] The high-strength hot-dip galvanized steel sheet according to [1], which is excellent in HAZ softening characteristics.
[3] In the high-strength hot-dip galvanized steel sheet according to the above [1] or [2], as a chemical component , mass : Mo: 0.05 to 1%, V: 0.02 to 0.5%, Ti: 0.005 to 0.05% B: A high-strength hot-dip galvanized steel sheet containing at least one selected from 0.0002 to 0.002%.
[4] A high structure comprising a composite structure of ferrite and a low-temperature transformation phase, or a composite structure having at least one selected from ferrite, a low-temperature transformation phase, and pearlite having a volume ratio of 10% or less and residual austenite having a volume ratio of 10% or less as the remaining structure. In the method for producing a high strength galvanized steel sheet, the steel having the chemical composition described in any one of [1] to [3] above is cast, and after finish rolling at a temperature of Ar 3 or higher, 800 to 700 ° C. Cooling at a temperature range of 5 ° C / sec or more and winding at 450 to 700 ° C, continuously hot-rolled steel strip after pickling, or cold-rolled steel strip cold-rolled at a reduction rate of 20% or more In the hot dip galvanizing line, after soaking at 760 to 880 ° C, it is cooled to a temperature range of 600 ° C or lower at a cooling rate of 1 ° C / sec or more, and galvanized or further alloyed. Manufacturing method of high-strength hot-dip galvanized steel sheet
これらの発明は、上記課題を解決するため、鋼成分と溶接結合材の成形性について鋭意検討の結果なされた。検討の結果、ある限定されたC、Si、Mn等の基本成分に適量のNbとCrを複合添加させた鋼を、適切な製造条件下において、平均粒径10μｍ以下のフェライトと低温変態相を主体とする組織に制御することにより、優れた溶接結合材の成形性が得られることが明らかとなった。本発明のポイントは、溶接時のHAZ軟化を抑制すると同時に、組織を均一微細化することにより、成形時に非常に硬質な溶接線に隣接したHAZ部分に応力集中しても、十分な成形性が確保できる点である。 In order to solve the above-mentioned problems, these inventions have been made as a result of intensive studies on the formability of steel components and welded joints. As a result of study, a steel with a certain amount of Nb and Cr added to a limited amount of basic components such as C, Si, Mn, etc., under appropriate manufacturing conditions, ferrite with an average grain size of 10 μm or less and a low-temperature transformation phase It became clear that excellent formability of the welded joint material can be obtained by controlling the main structure. The point of the present invention is that the HAZ softening at the time of welding is suppressed and at the same time the structure is made uniform and fine, so that sufficient formability can be obtained even when stress is concentrated on the HAZ part adjacent to the very hard weld line during molding. It is a point that can be secured.
本発明の高強度溶融亜鉛メッキ鋼板は、C、Si、Mn等の基本成分に適量のNbとCrを複合添加させた鋼を、平均粒径10μｍ以下のフェライトと低温変態相を主体とする組織に制御することにより、HAZ軟化を抑制すると同時に十分な成形性の確保が可能である。その結果、本発明によれば、テーラードブランク材での成形性の劣化が小さい高強度溶融亜鉛めっき鋼板を、特に自動車メーカーに提供することができるので、工業的価値は極めて高い。 The high-strength hot-dip galvanized steel sheet according to the present invention is a steel in which a suitable amount of Nb and Cr is added to basic components such as C, Si, Mn, etc., and has a structure mainly composed of ferrite having an average particle size of 10 μm or less and a low-temperature transformation phase By controlling to, the HAZ softening can be suppressed and sufficient moldability can be secured. As a result, according to the present invention, a high-strength hot-dip galvanized steel sheet having a small formability deterioration with a tailored blank material can be provided to automobile manufacturers in particular, so that the industrial value is extremely high.
以下に、具体的な化学成分、組織限定理由と本鋼板を得るための製造方法について説明する。
まず、化学成分の限定理由について述べる。
C:0.04%〜0.25%
Cは、所望の強度を確保するために必須の元素であり、そのためには0.04%以上必要である。一方、Cを0.25%を超えて添加すると低温変態相の体積率が増加しすぎて、低温変態相の結晶粒同士が連結しやすくなり、組織の微細分散が困難となる。従って、Cは、下限は強度を確保するため、上限は組織の微細分散を確保するため、0.04%〜0.25%の範囲内とする。
Below, a specific chemical component, a structure limitation reason, and the manufacturing method for obtaining this steel plate are demonstrated.
First, the reasons for limiting chemical components will be described.
C: 0.04% ~ 0.25%
C is an essential element for securing a desired strength, and for that purpose, 0.04% or more is necessary. On the other hand, if C is added in excess of 0.25%, the volume fraction of the low-temperature transformation phase increases too much, and the crystal grains of the low-temperature transformation phase are easily connected to each other, making it difficult to finely disperse the structure. Therefore, C is set within a range of 0.04% to 0.25% in order to ensure strength at the lower limit and to ensure fine dispersion of the structure at the upper limit.
Si:0.7%以下
Siは、フェライト+マルテンサイト2相組織を安定して得るためには有効な添加元素であるが、添加量が0.7%を超えると亜鉛メッキの密着性や表面外観が著しく劣化する。従って、Siを0.7%以下とする。
Si: 0.7% or less
Si is an effective additive element for stably obtaining a ferrite + martensite two-phase structure. However, if the addition amount exceeds 0.7%, the adhesion and surface appearance of galvanization are significantly deteriorated. Therefore, Si is made 0.7% or less.
Mn:1.4〜3.5%
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1.4%が下限として必要であるが、3.5%を超えて過剰に添加するとオーステナイトが安定化しすぎて、Cの過剰添加同様、低温変態相が微細分散されにくくなり、所望の効果が得られなくなる。従って、Mnを1.4〜3.5%の範囲内とする。
Mn: 1.4-3.5%
Like C, Mn is an essential element for ensuring a desired strength. In order to obtain the desired strength, 1.4% is necessary as a lower limit.However, if it is added excessively over 3.5%, austenite is overstabilized, and the low-temperature transformation phase is less likely to be finely dispersed as in the case of excessive addition of C. The effect cannot be obtained. Therefore, Mn is set within the range of 1.4 to 3.5%.
Cr:0.05〜1%
Crは、HAZ部の硬度低下を抑制するために必要な元素であり、少なくとも0.05%以上の添加が必要である。一方、Crを1%を超えて添加すると表面性状が劣化する。従って、Crを0.05〜1%の範囲内とする。
Cr: 0.05-1%
Cr is an element necessary for suppressing a decrease in the hardness of the HAZ part, and it is necessary to add at least 0.05% or more. On the other hand, if the Cr content exceeds 1%, the surface properties deteriorate. Therefore, Cr is set within a range of 0.05 to 1%.
P:0.05%以下
PはSiと同様に、フェライト+マルテンサイト2相組織を安定して得るためには有効な添加元素であるが、添加量が0.05%を超えると溶接部の靭性が劣化する。従って、Pを0.05%以下とする。
P: 0.05% or less
Like Si, P is an effective additive element for stably obtaining a ferrite + martensite two-phase structure. However, if the addition amount exceeds 0.05%, the toughness of the weld deteriorates. Therefore, P is made 0.05% or less.
S:0.01%以下
Sは不純物であり、含有量が高いとPと同様に溶接部の靭性が劣化する。このためSを0.01%以下とする。
S: 0.01% or less
S is an impurity, and if the content is high, the toughness of the welded portion deteriorates as in the case of P. For this reason, S is made 0.01% or less.
sol.Al:0.05%以下
sol.Alは、通常の鋼に含有される量0.05%以下であれば本発明の効果を損なわない。従って、sol.Alを0.05%以下とする。
sol.Al: 0.05% or less
If the amount of sol.Al contained in ordinary steel is 0.05% or less, the effect of the present invention is not impaired. Therefore, sol.Al is made 0.05% or less.
Nb:0.005〜0.1%
Nbは、本発明の特徴であるフェライト粒の微細化に必要な元素であり、そのためには少なくとも0.005%の添加が必要である。一方、0.1%を超えて過剰に添加しても、その効果が飽和するばかりか、かえって加工性を劣化させる。従って、Nbを0.005〜0.1%の範囲内とする。
Nb: 0.005-0.1%
Nb is an element necessary for refinement of ferrite grains, which is a feature of the present invention, and for that purpose, addition of at least 0.005% is necessary. On the other hand, adding excessively exceeding 0.1% not only saturates the effect, but also degrades the workability. Therefore, Nb is set in the range of 0.005 to 0.1%.
N:0.007%以下
Nは、通常の鋼に含有される量0.007%以下であれば本発明の効果を損なわない。従って、Nを0.007%以下とする。
N: 0.007% or less
If N is 0.007% or less contained in ordinary steel, the effect of the present invention is not impaired. Therefore, N is set to 0.007% or less.
本発明では、上記元素の他に、さらにMo、V、Ti、Bから選ばれる1種以上の元素を含有させることができる。 In the present invention, in addition to the above elements, one or more elements selected from Mo, V, Ti, and B can be further contained.
Mo:0.05〜1%、V:0.02〜0.5%、Ti:0.005〜0.05%、B:0.0002〜0.002%
これらの元素は、いずれもフェライト粒を微細化させて、本発明の効果を補助的に高めることができる。さらに、Mo、Vは鋼板の焼き入れ性を上昇させ、Tiは補助的に組織を微細化させ、また、Bはフェライトの析出を抑制して強度を上昇させる効果がある。それぞれの元素の下限は、所望の効果が得られる最低限の量であり、また、上限は、効果が飽和する量である。
Mo: 0.05-1%, V: 0.02-0.5%, Ti: 0.005-0.05%, B: 0.0002-0.002%
Any of these elements can make the ferrite grains finer and supplementarily enhance the effect of the present invention. Furthermore, Mo and V increase the hardenability of the steel sheet, Ti supplementarily refines the structure, and B has the effect of suppressing the precipitation of ferrite and increasing the strength. The lower limit of each element is the minimum amount at which the desired effect is obtained, and the upper limit is the amount at which the effect is saturated.
上記元素以外の残部はFeおよび不可避不純物とする。 The balance other than the above elements is Fe and inevitable impurities.
フェライトおよび低温変態相の平均粒径：10μm以下
フェライトおよび低温変態相の平均粒径を10μm以下に微細にすることにより、良好な成形性が得られる。従って、フェライトおよび低温変態相の平均粒径を10μm以下とする。
Average particle size of ferrite and low-temperature transformation phase: 10 μm or less Fine formability can be obtained by reducing the average particle size of ferrite and low-temperature transformation phase to 10 μm or less. Accordingly, the average particle size of the ferrite and the low-temperature transformation phase is set to 10 μm or less.
製造方法の発明は、フェライトと低温変態相の複合組織またはフェライトと低温変態相および残部組織として体積率１０％以下のパーライト、体積率１０％以下の残留オーステナイトから選ばれる1種以上を有する複合組織からなる高強度溶融亜鉛メッキ鋼板の製造方法において、前述の発明の化学成分の鋼を、鋳造後、Ar3点以上の温度で仕上げ圧延後、800〜700℃の温度域を5℃/sec以上で冷却して450〜700℃で巻き取り熱延鋼帯とし、酸洗後、あるいはさらに20%以上の圧下率で冷間圧延した後、連続溶融亜鉛メッキラインにおいて、760〜880℃で均熱した後、1℃/sec以上の冷却速度で600℃以下の温度域まで冷却し、亜鉛メッキ、あるいはさらに合金化処理を行う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。 The invention of the manufacturing method includes a composite structure of ferrite and a low temperature transformation phase or a composite structure having at least one selected from ferrite, a low temperature transformation phase and pearlite having a volume ratio of 10% or less and a retained austenite having a volume ratio of 10% or less as the remaining structure. In the method for producing a high-strength hot-dip galvanized steel sheet, the steel of the above-described chemical composition is cast, and after finish rolling at a temperature of 3 or more points of Ar, a temperature range of 800 to 700 ° C. is 5 ° C./sec or more. After cooling at 450 to 700 ° C to form a hot rolled steel strip, pickling or cold rolling at a reduction rate of 20% or more, and soaking at 760 to 880 ° C in a continuous hot dip galvanizing line Then, it is cooled to a temperature range of 600 ° C. or lower at a cooling rate of 1 ° C./sec or higher, and galvanized or further alloyed, and is a method for producing a high-strength hot-dip galvanized steel sheet.
この発明は、前述の発明の高強度溶融亜鉛メッキ鋼板を製造するための方法である。以下、個々の製造工程について説明する。 This invention is a method for producing the high-strength hot-dip galvanized steel sheet of the above-mentioned invention. Hereinafter, each manufacturing process will be described.
発明の実施に当たっては、造塊あるいは連続鋳造等のスラブ製造方法によりスラブを鋳造後、通常の熱延方法、あるいは粗熱延バー接続による連続熱延、インダクションヒーターによる昇温等を含む種々の方法で熱延を行う。これらの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℃以内の昇温なども、本発明の効果に対して影響を及ぼさない。
仕上圧延温度：Ar3点以上
仕上圧延温度がAr3点未満になると、フェライトが生成し、その加工歪による粗大化等により、組織が不均一となる。従って、仕上圧延温度をAr3点以上とする。その他特に規定していないが、熱延終了後に1秒以内に100〜300℃/secといった大冷却を活用することは、組織微細化の上で好ましい。これにさらに仕上げ熱延大圧下を組み合わせるなど、熱延板粒径を小さくする限りは、本発明の効果を阻害しない。
In carrying out the invention, after casting a slab by a slab manufacturing method such as ingot casting or continuous casting, various methods including a normal hot rolling method, continuous hot rolling by rough hot rolling bar connection, temperature rise by an induction heater, etc. Perform hot rolling. These slab manufacturing methods by ingot or continuous casting, continuous hot rolling by rough hot rolling bar connection in hot rolling, and temperature rise within 200 ° C. using an induction heater in the hot rolling process are also included in the present invention. Does not affect the effects of
Finish rolling temperature: Ar 3 points or more When the finish rolling temperature is less than Ar 3 points, ferrite is generated, and the structure becomes non-uniform due to coarsening due to the processing strain. Therefore, the finish rolling temperature is set to Ar 3 or higher. Although not otherwise specified, it is preferable to use large cooling such as 100 to 300 ° C./sec within 1 second after the end of hot rolling in order to refine the structure. The effect of the present invention is not hindered as long as the hot-rolled plate particle size is reduced, for example, by further combining with hot rolling under large rolling.
圧延後の冷却条件：800〜700℃の温度域を冷却速度5℃／sec以上
本発明の化学成分の鋼では、この温度域においてフェライトが析出するが、特に800〜700℃の温度域で5℃／sec未満の冷却では、フェライトが粗大に析出して組織が不均一となる。従って、圧延後の冷却条件については、800〜700℃の温度域を5℃／sec以上の冷却速度とする。
Cooling conditions after rolling: In a steel having the chemical composition of the present invention, a temperature range of 800 to 700 ° C. at a cooling rate of 5 ° C./sec or more, ferrite precipitates in this temperature range, but particularly in the temperature range of 800 to 700 ° C. When the cooling is less than ° C./sec, ferrite precipitates coarsely and the structure becomes non-uniform. Therefore, regarding the cooling conditions after rolling, the temperature range of 800 to 700 ° C. is set to a cooling rate of 5 ° C./sec or more.
巻き取り温度：450〜700℃
その後、450〜700℃で巻き取り熱延鋼帯とする。巻き取り温度は、NbCの析出に著しく影響を及ぼすため、確実に制御し、熱延板段階でNbCを微細に分散析出させておく必要がある。巻き取り温度が450℃未満では、NbCの析出が不十分となる。一方、巻き取り温度が700℃超では、NbCが粗大に析出して、熱延板段階でNbCを微細に分散析出させることができなくなる。従って、巻き取り温度を450〜700℃の範囲内とする。
Winding temperature: 450-700 ° C
Then, it is set as a hot-rolled steel strip wound at 450 to 700 ° C. Since the coiling temperature significantly affects the precipitation of NbC, it is necessary to control it reliably and to finely disperse and precipitate NbC at the hot rolling stage. When the coiling temperature is less than 450 ° C., the precipitation of NbC becomes insufficient. On the other hand, when the coiling temperature exceeds 700 ° C., NbC precipitates coarsely, and NbC cannot be finely dispersed and precipitated in the hot-rolled sheet stage. Accordingly, the winding temperature is set within the range of 450 to 700 ° C.
この後、酸洗して、あるいは、さらに冷間圧延した後、連続溶融亜鉛メッキラインにおいてめっきを施す。 Then, after pickling or further cold rolling, plating is performed in a continuous hot dip galvanizing line.
冷間圧延の圧下率：冷間圧延する場合20%以上
連続溶融亜鉛メッキラインにおいてめっきを施す前に冷間圧延する場合は、圧下率が20%未満では、焼鈍の際に歪粒成長が起こり、かえって組織が粗大となる。従って、冷間圧延する場合は圧下率を20%以上とする。また、めっき前のNi等のプレメッキ、表面研削など、鋼板材質を変化させない処理を施すことは、本発明の効果を何ら損なわない。
Cold rolling reduction ratio: 20% or more when cold rolling When cold rolling before plating in a continuous hot dip galvanizing line, strain grain growth occurs during annealing if the rolling reduction is less than 20% On the contrary, the organization becomes coarse. Therefore, when cold rolling, the rolling reduction is 20% or more. Moreover, applying the treatment that does not change the steel plate material, such as pre-plating of Ni before plating, surface grinding, etc., does not impair the effects of the present invention.
均熱温度（加熱温度）：760〜880℃
連続溶融亜鉛メッキラインにおける均熱温度が760℃未満では、十分な体積率のオーステナイト相が得られず、所望の効果（組織）が得られなくなる。一方、880℃を超えて均熱すると組織が粗大化するので、やはり所望の効果（組織）が得られなくなる。従って、連続溶融亜鉛メッキラインにおける均熱温度（加熱温度）を760〜880℃の範囲内とする。
Soaking temperature (heating temperature): 760-880 ℃
When the soaking temperature in the continuous hot dip galvanizing line is less than 760 ° C., a sufficient austenite phase cannot be obtained, and the desired effect (structure) cannot be obtained. On the other hand, since the structure becomes coarse when soaking above 880 ° C., the desired effect (structure) cannot be obtained. Therefore, the soaking temperature (heating temperature) in the continuous hot dip galvanizing line is set within the range of 760 to 880 ° C.
均熱後の冷却条件：600℃以下の温度まで冷却速度1℃/sec以上
均熱後の冷却速度が1℃/sec未満では、フェライトが粗大に析出し、また、低温変態相が生成しなくなり、所望の効果（組織）が得られなくなる。また、冷却速度が1℃/sec以上であっても、600℃まで到達する前に、冷却速度が1℃/sec未満となると、やはりフェライトの粗大析出や低温変態相の減少・消滅が起こり、所望の効果（組織）が得られなくなる。従って、均熱後の冷却条件を、600℃以下の温度まで1℃/sec以上の冷却速度とする。
Cooling condition after soaking: Cooling rate of 1 ° C / sec or more to a temperature of 600 ° C or less If the cooling rate after soaking is less than 1 ° C / sec, ferrite precipitates coarsely and no low-temperature transformation phase is generated. The desired effect (tissue) cannot be obtained. In addition, even if the cooling rate is 1 ° C / sec or more, before reaching 600 ° C, if the cooling rate becomes less than 1 ° C / sec, the coarse precipitation of ferrite and the decrease / annihilation of the low-temperature transformation phase occur, The desired effect (structure) cannot be obtained. Therefore, the cooling condition after soaking is set to a cooling rate of 1 ° C./sec or more to a temperature of 600 ° C. or less.
このようにして、600℃以下の温度域まで冷却した後は、亜鉛メッキ、あるいはさらに合金化処理を行う。溶融亜鉛めっき後についても、表層電気めっきや化成皮膜の塗布など、鋼板材質を変化させない処理を施すことは、本発明の効果を何ら損なわない。 In this way, after cooling to a temperature range of 600 ° C. or lower, galvanization or further alloying treatment is performed. Even after hot dip galvanization, applying the treatment that does not change the steel plate material, such as surface electroplating or chemical coating, does not impair the effects of the present invention.
このようにして製造された本発明の溶融亜鉛めっき鋼板において、組織が均一微細化されるのは、上述したNbの添加が、同時に組織の微細化にも有効となるためと考えられる。ただし、仕上げ熱延後のフェライト析出温度域の冷却速度を十分にとってフェライトの粗大析出を抑制し、かつ、巻き取り温度をNbCが析出するよう適正化する必要がある。 The reason why the structure is uniformly refined in the hot-dip galvanized steel sheet of the present invention thus manufactured is considered that the above-described addition of Nb is effective for the refinement of the structure at the same time. However, the cooling rate in the ferrite precipitation temperature range after finish hot rolling must be sufficiently increased to suppress coarse ferrite precipitation and to optimize the winding temperature so that NbC precipitates.
次に、組織の限定理由について若干の補足を加える。本発明では、組織を平均粒径10μｍ以下のフェライトと低温変態相を主体として構成させる。フェライトおよび低温変態相の平均粒径を10μｍ以下とさせるのは、前述したように、組織を均一微細化することにより、成形時に非常に硬質な溶接線に隣接したHAZ部分に応力集中しても、十分な成形性を確保させるためである。 Next, a few supplements are added regarding the reasons for limiting the organization. In the present invention, the structure is mainly composed of ferrite having an average particle size of 10 μm or less and a low-temperature transformation phase. The average grain size of the ferrite and low-temperature transformation phase is set to 10 μm or less, as described above, even if stress is concentrated on the HAZ part adjacent to the very hard weld line by forming a uniform structure. This is to ensure sufficient formability.
HAZ軟化抑制については次のように考えられる。すなわち、転位密度の高いマルテンサイトあるいはベイナイトを硬質相とし、Crによる2次析出強化とNbCの微細析出による転位回復の抑制を利用することで、短時間での昇温でも硬質相の強度低下が下げられる。この結果、HAZ部の硬度低下を抑制できる。 HAZ softening suppression is considered as follows. In other words, martensite or bainite with a high dislocation density is used as the hard phase, and the strength of the hard phase is reduced even when the temperature is increased in a short time by utilizing secondary precipitation strengthening due to Cr and suppression of dislocation recovery due to fine precipitation of NbC. Be lowered. As a result, a decrease in the hardness of the HAZ portion can be suppressed.
また、低温変態相を含有させるのは、転位密度の高いマルテンサイトあるいはベイナイトを硬質相とすることで、Crによる2次析出強化とNbCの微細析出による転位回復の抑制を利用して、短時間での昇温でも硬質相の強度低下が下げられるためである。 In addition, the low temperature transformation phase is contained by using martensite or bainite with a high dislocation density as a hard phase, making use of secondary precipitation strengthening due to Cr and suppression of dislocation recovery due to fine precipitation of NbC. This is because a decrease in the strength of the hard phase can be reduced even at a high temperature.
本発明の高強度溶融亜鉛メッキ鋼板は、フェライトと低温変態相等の複合組織からなり、フェライトと低温変態相を主体としている。従って、低温変態相以外の組織が多少含まれていてもよいが、パーライトについては、体積率で10%以上を超えて多量に析出した組織だと、上記のHAZ軟化防止の原理が活用できないばかりか、成形性自体が劣化するので好ましくない。 The high-strength hot-dip galvanized steel sheet of the present invention is composed of a composite structure such as ferrite and a low-temperature transformation phase, and is mainly composed of ferrite and a low-temperature transformation phase. Therefore, a structure other than the low-temperature transformation phase may be included to some extent, but for pearlite, the above-described principle of preventing HAZ softening cannot be utilized if the structure is a large amount of precipitate exceeding 10% by volume. Or, the moldability itself deteriorates, which is not preferable.
また、残留オーステナイトについても、含有率が大きくなると溶接時の熱影響によりフェライトと炭化物に分解して軟化の原因になってしまう。このため、残留オーステナイトの含有率は10%以下とすることが望ましい。 In addition, residual austenite also becomes soft due to decomposition into ferrite and carbides due to the thermal effect during welding when the content is increased. For this reason, the content of retained austenite is preferably 10% or less.
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〜Rと比較成分鋼a〜kを転炉で出鋼し、連続鋳造によりスラブとした。これらのスラブを表２に示す条件で熱延鋼帯とし、酸洗後、冷延率65%で冷間圧延して、メッキ下地を準備した。続いて、連続溶融亜鉛メッキラインにて、表２に示す条件で溶融亜鉛メッキもしくは合金化溶融亜鉛メッキ鋼板を製造した。なお、表２に示した以外の製造条件についても、いずれも本発明の製造条件の範囲内である。
The effect by this invention is shown concretely below.
First, the present invention component steels A to R and comparative component steels a to k, whose components are shown in Table 1, were produced in a converter and made into a slab by continuous casting. These slabs were made into hot-rolled steel strips under the conditions shown in Table 2, and after pickling, they were cold-rolled at a cold rolling rate of 65% to prepare a plating base. Subsequently, a hot dip galvanized or alloyed hot dip galvanized steel sheet was produced under the conditions shown in Table 2 in a continuous hot dip galvanizing line. In addition, also about manufacturing conditions other than having shown in Table 2, all are in the range of the manufacturing conditions of this invention.
これらの鋼板の組織を解析した結果と、特性についても、表２に併せて示している。残留オーステナイトはX線により定量化している。TWB特性については、レーザーで突合わせ溶接した材料をエリクセン試験して、溶接しない場合の成形高さと溶接材料の成形高さとの差、および破断位置により評価した。溶接は、下記に示す条件で行った。 Table 2 also shows the results of analyzing the structure of these steel plates and the characteristics. Residual austenite is quantified by X-ray. The TWB characteristics were evaluated by conducting an Erichsen test on the material butt-welded with a laser, and the difference between the molding height when not welding and the molding height of the welding material, and the fracture position. Welding was performed under the following conditions.
レーザ機種：炭酸ガスレーザ
波長：１０．６μm
ビームモード：リングモードＭ＝２
レーザ集光系：ＺｎＳｅ製レンズ
焦点距離：２５４ｍｍ出力：４ｋＷ
溶接速度：４ｍ／ｍｉｎ
シールドガス：アルゴン２０リットル／ｍｉｎ。
Laser model: Carbon dioxide laser wavelength: 10.6 μm
Beam mode: Ring mode M = 2
Laser focusing system: ZnSe lens Focal length: 254 mm Output: 4 kW
Welding speed: 4m / min
Shielding gas:
次に、本発明成分鋼C,I,JおよびQと比較成分鋼dを転炉で出鋼し、種々の製造条件、即ち本発明法およびこれらからはずれる製造条件で製造した。表３に、これらの製造条件と、得られた鋼板について上記の試験を行った結果をまとめて示す。 Next, the component steels C, I, J and Q of the present invention and the comparative component steel d were produced in a converter and manufactured under various production conditions, that is, the method of the present invention and production conditions deviating from these. Table 3 summarizes these manufacturing conditions and the results of the above tests performed on the obtained steel sheet.
この表３より、フェライト粒径および第2相粒径が本発明の範囲内である本発明例では、エリクセン高さの溶接の有無による差が小さい、即ち高いΔh(母材と溶接材のエリクセン高さの差)を示している。一方、上記粒径が本発明の範囲外の比較例では、Δhが低い。また、比較例においては、破断位置がHAZ部であり、後述のようにHAZ軟化が発生していることが推定される。 From Table 3, in the present invention example in which the ferrite grain size and the second phase grain size are within the scope of the present invention, the difference in Erichsen height depending on the presence or absence of welding is small, that is, high Δh (Erichsen between the base material and the welding material). (Height difference). On the other hand, Δh is low in the comparative example in which the particle size is outside the range of the present invention. In the comparative example, the fracture position is the HAZ portion, and it is estimated that HAZ softening has occurred as will be described later.
図1は、表２と表３に示した鋼のΔh(母材と溶接材のエリクセン高さの差)を、フェライト粒径で整理した図である。本図から明らかなように、本発明成分鋼を適切な条件で製造して、フェライト粒径および低温変態相の粒径を10μm以下とすることで、HAZ部での破断もなくΔhが2mm以下となり、高強度とともに良好なTWB特性が得られていることがわかる。なお、本発明鋼の組織は、SEM像で見ると、粒径3μm程度のフェライトとマルテンサイトが微細に分散した組織となっている。 FIG. 1 is a diagram in which Δh (difference in Erichsen height between the base material and the weld material) of the steels shown in Tables 2 and 3 is arranged by the ferrite grain size. As is apparent from this figure, the component steel of the present invention is manufactured under appropriate conditions, and the ferrite grain size and the grain size of the low temperature transformation phase are 10 μm or less, so that Δh is 2 mm or less without breakage at the HAZ part. Thus, it can be seen that high strength and good TWB characteristics are obtained. Note that the structure of the steel of the present invention is a structure in which ferrite and martensite having a particle size of about 3 μm are finely dispersed when viewed from an SEM image.
一方、化学成分が本発明範囲であっても、組織が適切でない場合、Δhは2mmを超えており、また、破断もHAZで生じてTWB特性が劣化しているのがわかる。成分が適切でない比較成分鋼については、組織を適切化してもTWB特性は改善されていない。
図2は、表２中の本発明鋼板17（鋼種Q）（図ａ）と比較鋼板28（鋼種j）（図ｂ）のレーザー溶接部断面の硬度分布を示している。本図から、鋼成分と組織を本発明範囲に制御することで、HAZ軟化が著しく抑制されていることがわかる。
On the other hand, even if the chemical composition is within the range of the present invention, if the structure is not appropriate, Δh exceeds 2 mm, and it can be seen that fracture also occurs in HAZ and the TWB characteristics deteriorate. For the comparative component steels whose components are not appropriate, the TWB characteristics are not improved even if the structure is optimized.
FIG. 2 shows the hardness distribution of the cross section of the laser welded portion of the steel plate 17 of the present invention (steel type Q) (FIG. A) and the comparative steel plate 28 (steel type j) (FIG. B) in Table 2. This figure shows that HAZ softening is remarkably suppressed by controlling the steel components and the structure within the range of the present invention.
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| JPH11343538A (en) * | 1998-05-29 | 1999-12-14 | Kawasaki Steel Corp | Cold-rolled steel sheet suitable for high-density energy beam welding and its production |
| JP2000109951A (en) * | 1998-08-05 | 2000-04-18 | Kawasaki Steel Corp | High strength hot rolled steel sheet excellent in stretch-flanging property and its production |
| JP2000282175A (en) * | 1999-04-02 | 2000-10-10 | Kawasaki Steel Corp | Superhigh strength hot-rolled steel sheet excellent in workability, and its production |
-
2007
- 2007-06-11 JP JP2007153861A patent/JP4692519B2/en not_active Expired - Fee Related
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH11343538A (en) * | 1998-05-29 | 1999-12-14 | Kawasaki Steel Corp | Cold-rolled steel sheet suitable for high-density energy beam welding and its production |
| JP2000109951A (en) * | 1998-08-05 | 2000-04-18 | Kawasaki Steel Corp | High strength hot rolled steel sheet excellent in stretch-flanging property and its production |
| JP2000282175A (en) * | 1999-04-02 | 2000-10-10 | Kawasaki Steel Corp | Superhigh strength hot-rolled steel sheet excellent in workability, and its production |
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| JP2007277729A (en) | 2007-10-25 |
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