JP2007119842A - Method for producing high-strength galvanized steel sheet excellent in stretch-flanging property - Google Patents
Method for producing high-strength galvanized steel sheet excellent in stretch-flanging property Download PDFInfo
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- 229910001335 Galvanized steel Inorganic materials 0.000 title claims abstract description 16
- 239000008397 galvanized steel Substances 0.000 title claims abstract description 16
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 13
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 39
- 239000010959 steel Substances 0.000 claims abstract description 39
- 238000005098 hot rolling Methods 0.000 claims abstract description 28
- 229910052758 niobium Inorganic materials 0.000 claims abstract description 11
- 229910052710 silicon Inorganic materials 0.000 claims abstract description 11
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 9
- 238000005246 galvanizing Methods 0.000 claims abstract description 9
- 238000005275 alloying Methods 0.000 claims abstract description 4
- 239000012535 impurity Substances 0.000 claims abstract description 3
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 3
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 3
- 238000001816 cooling Methods 0.000 claims description 12
- 238000002791 soaking Methods 0.000 claims description 8
- 238000004804 winding Methods 0.000 claims description 6
- 229910052750 molybdenum Inorganic materials 0.000 claims description 5
- 229910052720 vanadium Inorganic materials 0.000 claims description 5
- 238000005097 cold rolling Methods 0.000 claims 1
- 238000005554 pickling Methods 0.000 claims 1
- 238000005266 casting Methods 0.000 abstract description 6
- 230000009977 dual effect Effects 0.000 abstract description 2
- 230000000694 effects Effects 0.000 description 12
- 229910000734 martensite Inorganic materials 0.000 description 8
- 238000001556 precipitation Methods 0.000 description 5
- 229910000859 α-Fe Inorganic materials 0.000 description 5
- 239000000463 material Substances 0.000 description 4
- 238000000034 method Methods 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 230000000052 comparative effect Effects 0.000 description 3
- 238000009749 continuous casting Methods 0.000 description 3
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 3
- 238000000465 moulding Methods 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 229920006395 saturated elastomer Polymers 0.000 description 3
- 238000005204 segregation Methods 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- 238000000137 annealing Methods 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 229910052804 chromium Inorganic materials 0.000 description 2
- 238000010276 construction Methods 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 238000009825 accumulation Methods 0.000 description 1
- 229910001566 austenite Inorganic materials 0.000 description 1
- 229910001563 bainite Inorganic materials 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- 230000006698 induction Effects 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 238000009828 non-uniform distribution Methods 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 230000002265 prevention Effects 0.000 description 1
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- 230000004044 response Effects 0.000 description 1
- 238000005096 rolling process Methods 0.000 description 1
- 238000010583 slow cooling Methods 0.000 description 1
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Abstract
Description
本発明は、建設部材、機械構造用部品、自動車の構造用部品などに適した伸びフランジ成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法に関する。 The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet excellent in stretch flangeability suitable for construction members, machine structural parts, automobile structural parts, and the like.
引張強度が440MPaを超える高強度溶融亜鉛めっき鋼板は、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。特に、自動車産業においては、CO2削減を目的とした燃費向上および乗員の安全性確保の観点より、高強度鋼板の積極的な活用による車体剛性の向上と軽量化の両立を推進している。 High-strength hot-dip galvanized steel sheets having a tensile strength exceeding 440 MPa have advantages of excellent rust prevention and high yield strength, and are widely applied to construction members, machine structural parts, automobile structural parts, and the like. In particular, in the automobile industry, from the viewpoint of improving fuel efficiency for the purpose of reducing CO 2 and ensuring the safety of passengers, both the improvement of vehicle body rigidity and weight reduction are actively promoted by actively utilizing high-strength steel sheets.
このような産業のニーズに対して、素材メーカーでは、より高強度でより高加工性のある材料開発に取組んでいる。この中でも、引張強度が590MPaを超える高強度溶融亜鉛めっき鋼板は、軽量化効果が大きく、延性面でも有利なDP(Dual Phase)鋼をベースとする溶融亜鉛めっき鋼板に関して多数の提案がなされている。 In response to such industrial needs, material manufacturers are working on the development of materials with higher strength and higher workability. Among them, a high strength hot dip galvanized steel sheet having a tensile strength exceeding 590 MPa has many proposals regarding a hot dip galvanized steel sheet based on DP (Dual Phase) steel, which has a large lightening effect and is advantageous in terms of ductility. .
一方で、自動車部品では、部品組立て上、フランジ成形は省略することが不可能であるため、いかに材料を高強度化しても、伸びフランジ成形性は自動車用材料として必須である。 On the other hand, in the case of automobile parts, flange molding cannot be omitted in assembling the parts. Therefore, stretch flange formability is indispensable as a material for automobiles, no matter how high the material strength is.
ところが、DP鋼は、軟質なフェライトと硬質なマルテンサイトの複合組織で形成されているため、一般的に伸びフランジ成形性に乏しいことが課題である。
このような課題を解決して、伸びフランジ成形性の優れた高強度溶融亜鉛めっき鋼板を得る技術として、例えば特許文献1や特許文献2などが提案されている。
However, since DP steel is formed of a composite structure of soft ferrite and hard martensite, it generally has a problem that stretch flange formability is poor.
As a technique for solving such problems and obtaining a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability, for example, Patent Document 1 and Patent Document 2 have been proposed.
特許文献1に開示された技術は、DP鋼において成分組成を制御して、伸びフランジ成形性を向上させようとするものであるが、引張張力、伸びフランジ成形性の指標である穴広げ率などにおいて、至近のニーズに対して必ずしも十分なレベルの特性が得られていない。 The technique disclosed in Patent Document 1 attempts to improve stretch flange formability by controlling the composition of components in DP steel, but includes tensile tension, hole expansion ratio that is an index of stretch flange formability, etc. However, a sufficient level of characteristics is not always obtained for the immediate needs.
また、特許文献2に開示された技術では、鋼板組織中第2相をベイナイト主体としているため、伸びフランジ成形性は良好なものの、延性が犠牲になっており、汎用性の高い鋼板は得られない。
本発明はかかる事情に鑑みてなされたものであって、延性には有利ではあるが通常は伸びフランジ成形性が劣るDP組織を有しながら、伸びフランジ成形性の優れた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。 The present invention has been made in view of such circumstances, and is a high-strength hot-dip galvanized steel sheet that has a DP structure that is advantageous in ductility but usually inferior in stretch flange formability and has excellent stretch flange formability. It aims at providing the manufacturing method of.
本発明者らは、上記課題を解決すべく連続溶融亜鉛めっきラインでめっきが施されるDP鋼について鋼の成分組成および熱延条件、さらにこれらによって得られる鋼組織が及ぼす伸びフランジ成形性への影響について鋭意検討した結果、伸びフランジ割れの起点であるサルファイドの体積率およびその形態を制御しつつ、組織を微細化し、かつ層状組織を低減することにより、DP鋼においても極めて伸びフランジ成形性に優れた鋼板が得られることを見出した。
具体的には、これまでも種々提案されているように、Sレベルを低減させることにより伸びフランジ性をある程度向上させることができるが、詳細に検討した結果、590MPa級を超えるDP鋼においては、単純にS含有率を低減するだけでは、十分な伸びフランジ成形性の向上を達成することは困難であり、Sの低減に加えて組織を微細化することが必須である。組織を微細化するためにはNbを添加することが有効であるが、単にNbを添加して組織を微細化しても熱延条件が不適切な場合には板厚方向に層状に分布した不均一組織が発達して、やはり所望の伸びフランジ性が得られず、熱延条件を適切に規定する必要がある。
In order to solve the above-mentioned problems, the present inventors have found that DP steel plated by a continuous hot dip galvanizing line has a compositional composition and hot-rolling conditions of steel, and further to stretch flangeability exerted by the steel structure obtained thereby. As a result of diligent examination of the influence, the volume ratio and form of sulfide, which is the starting point of stretch flange cracking, are controlled, and the structure is refined and the layered structure is reduced, so that even DP steel has extremely stretch flange formability. It has been found that an excellent steel sheet can be obtained.
Specifically, as various proposals have been made so far, the stretch flangeability can be improved to some extent by reducing the S level. However, as a result of detailed examination, in DP steel exceeding 590 MPa class, It is difficult to achieve sufficient improvement in stretch flangeability by simply reducing the S content, and it is essential to refine the structure in addition to reducing S. In order to refine the structure, it is effective to add Nb. However, if the hot rolling conditions are inappropriate even if the structure is refined simply by adding Nb, the non-uniform distribution distributed in the thickness direction of the plate is not possible. A uniform structure develops, and also the desired stretch flangeability cannot be obtained, and it is necessary to appropriately define the hot rolling conditions.
本発明はこれらの知見に基づいて完成されたものであり、以下の(1)〜(3)を提供する。
(1)質量%で、C:0.03〜0.15%、Si:2%以下、Mn:1.7〜3%、P:0.05%以下、S:0.001%以下、Nb:0.005〜0.1%を含有し、残部がFeおよび不可避不純物からなる鋼を鋳造後、熱延仕上げ温度FTを(1)式で定まる温度FTmin以上に制御して熱間圧延し、450〜650℃で巻取って熱延鋼帯とし、酸洗後、冷間圧延し、連続溶融亜鉛めっきラインにおいて、800〜900℃で均熱した後、3℃/sec以上の冷却速度で600℃以下の温度域まで冷却し、溶融亜鉛めっき、あるいはさらに合金化処理を施すことを特徴とする伸びフランジ成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法を提供する。
FTmin(℃)=500×Nb+20×Si+50×C+860 (1)
(ただし、Nb、Si、Cは各成分の質量%を表す)
(2)質量%で、さらに、Cr:0.02〜0.5%、Mo:0.02〜0.5%、V:0.02〜0.5%から選択される1種または2種以上を含有することを特徴とする上記(1)に記載の伸びフランジ成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法を提供する。
(3)質量%で、さらに、Ti:0.005〜0.05%、B:0.0002〜0.002%を含有することを特徴とする上記(1)または(2)に記載の伸びフランジ成形性に優れた高強度溶融亜鉛めっき鋼板の製造方法を提供する。
The present invention has been completed based on these findings and provides the following (1) to (3).
(1) By mass%, C: 0.03 to 0.15%, Si: 2% or less, Mn: 1.7 to 3%, P: 0.05% or less, S: 0.001% or less, Nb : After the steel containing 0.005 to 0.1% and the balance of Fe and inevitable impurities is cast, hot rolling is performed by controlling the hot rolling finishing temperature FT to be equal to or higher than the temperature FT min determined by the equation (1) , Rolled at 450 to 650 ° C. to form a hot rolled steel strip, pickled, cold rolled, soaked at 800 to 900 ° C. in a continuous hot dip galvanizing line, and cooled at a cooling rate of 3 ° C./sec or more. Provided is a method for producing a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability, which is cooled to a temperature range of 600 ° C. or lower and subjected to hot-dip galvanizing or further alloying treatment.
FT min (° C.) = 500 × Nb + 20 × Si + 50 × C + 860 (1)
(However, Nb, Si, and C represent mass% of each component)
(2) 1% or 2 types selected from Cr: 0.02-0.5%, Mo: 0.02-0.5%, V: 0.02-0.5% The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the stretch flangeability as described in said (1) characterized by containing the above is provided.
(3) Elongation as described in (1) or (2) above, further comprising Ti: 0.005 to 0.05% and B: 0.0002 to 0.002% by mass% A method for producing a high-strength hot-dip galvanized steel sheet excellent in flange formability is provided.
本発明によれば、鋼の成分組成を適切な範囲に規定し、鋼成分中のNb、Si、Cの含有量により定まる特定の温度以上で熱延することにより、DP鋼でありながら優れた伸びフランジ成形性を有する高強度溶融亜鉛めっき鋼板を得ることができ、工業的価値は極めて高い。 According to the present invention, the steel component composition is defined within an appropriate range, and it is excellent in spite of being DP steel by hot rolling above a specific temperature determined by the content of Nb, Si, and C in the steel component. A high-strength hot-dip galvanized steel sheet having stretch flange formability can be obtained, and the industrial value is extremely high.
以下、本発明について、化学成分、製造方法に分けて具体的に説明する。 Hereinafter, the present invention will be specifically described by dividing it into chemical components and production methods.
[化学成分]
まず、化学成分について説明する。以下の説明において%は質量%である。
C:0.03〜0.15%
Cはマルテンサイト体積率を確保して所望の強度を確保するために必須の元素であり、その効果を得るためには0.03%以上必要である。しかし、Cを0.15%を超えて過剰に含有するとマルテンサイト相の硬度が上昇し、フェライト相との硬度差が増大しすぎて伸びフランジ成形性が劣化してしまう。このため、C含有量を0.03〜0.15%とする。
[Chemical composition]
First, chemical components will be described. In the following description, “%” means “% by mass”.
C: 0.03-0.15%
C is an essential element for securing the martensite volume fraction and securing a desired strength, and 0.03% or more is necessary for obtaining the effect. However, when C is contained excessively exceeding 0.15%, the hardness of the martensite phase increases, the hardness difference from the ferrite phase increases excessively, and the stretch flangeability deteriorates. For this reason, C content is made into 0.03 to 0.15%.
Si:2%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有効な元素であり、また含有量が増加すると延性が向上する。しかし、2%を超えて多量に含有されるとフェライトの高温域での析出が促進され組織が粗大になるため好ましくない。このため、Si含有量を2%以下とする。
Si: 2% or less Si is an effective element for stably obtaining a ferrite + martensite two-phase structure, and the ductility improves as the content increases. However, if it is contained in a large amount exceeding 2%, precipitation of ferrite in a high temperature range is promoted and the structure becomes coarse, which is not preferable. For this reason, Si content shall be 2% or less.
Mn:1.7〜3%
MnもCと同様、強度確保に必須の元素であり、高張力鋼を得るためには不可欠である。加えて、本発明では、S含有量を下げてMnSの体積率を低減させるとともに、MnSをできるだけ高温で析出させて比較的粗大化させることにより伸びフランジ成形におけるクラック起点を抑制させるため、Mnは1.7%以上必要である。しかし、Mnを3%を超えて過剰に含有すると鋳造時の偏析に起因した板厚方向の層状組織が発達して伸びフランジ成形性が劣化し、望ましくない。このため、Mnの含有量を1.7〜3%とする。
Mn: 1.7-3%
Mn, like C, is an essential element for securing strength, and is indispensable for obtaining high-tensile steel. In addition, in the present invention, the S content is reduced to reduce the volume fraction of MnS, and MnS is precipitated at a high temperature as much as possible to relatively coarsen, thereby suppressing the crack starting point in stretch flange molding. 1.7% or more is necessary. However, if Mn is contained excessively in excess of 3%, a lamellar structure in the thickness direction due to segregation during casting develops and stretch flange formability deteriorates, which is not desirable. For this reason, content of Mn shall be 1.7 to 3%.
P:0.05%以下
Pは、Siと同様、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必須の元素である。しかし、その含有量が0.05%を超えると溶接性が劣化する。このため、Pの含有量を0.05%以下とする。
P: 0.05% or less P, like Si, is an essential element for stably obtaining a ferrite + martensite two-phase structure. However, if the content exceeds 0.05%, the weldability deteriorates. For this reason, content of P shall be 0.05% or less.
S:0.001%以下
本発明では、Sを低減することが伸びフランジ成形性を向上させる上で極めて重要となる。Sの含有量が高いと、上述のように伸びフランジ成形において起点となるMnSの体積率が増大して所望の伸びフランジ成形性が得られない。このため、Sの含有量を0.001%以下とする。
S: 0.001% or less In the present invention, reducing S is extremely important in improving stretch flange formability. If the S content is high, the volume fraction of MnS that is the starting point in stretch flange molding as described above increases, and the desired stretch flange formability cannot be obtained. For this reason, content of S shall be 0.001% or less.
なお、AlとNは、不可避的に含有されるが、通常の鋼に含有されている量であれば本発明の効果を損なわず、Alはsol.Alとして0.05%以下、Nは0.007%以下であれば問題がない。 Al and N are inevitably contained, but the amount of the present invention is not impaired as long as it is contained in ordinary steel. If Al is 0.05% or less and N is 0.007% or less, there is no problem.
Nb:0.005〜0.1%
Nbは組織を微細化させる点において、本発明では必須の含有元素である。十分な微細効果を得るためには0.005%以上必要である。一方、0.1%を超えるとNbCの析出が不均一となり、逆に層状組織の発達を助長して伸びフランジ成形性が劣化する。このため、Nb含有量を0.005〜0.1%とする。
Nb: 0.005 to 0.1%
Nb is an essential element in the present invention in that the structure is refined. In order to obtain a sufficient fine effect, 0.005% or more is necessary. On the other hand, if it exceeds 0.1%, the precipitation of NbC becomes non-uniform, and conversely, the development of the lamellar structure is promoted and the stretch flangeability deteriorates. For this reason, Nb content shall be 0.005-0.1%.
Cr:0.02〜0.5%、Mo:0.02〜0.5%、V:0.02〜0.5%から選択される1種または2種以上
本発明では、Mnのほかに焼入れ性を高めてマルテンサイトを安定的に得るために必要に応じてCr、Mo、Vの1種または2種以上の元素を添加してもよい。上記効果を得るためには、いずれの元素も0.02%以上とすることが必要である。また、0.5%を超えると、上述の効果が飽和するばかりか、鋳造時の偏析に起因した板厚方向の層状組織が発達して伸びフランジ成形性が劣化し、好ましくない。このため、Cr、Mo、Vを添加する場合には、これらの含有量をそれぞれ0.02〜0.5%とする。
One or more selected from Cr: 0.02-0.5%, Mo: 0.02-0.5%, V: 0.02-0.5% In the present invention, in addition to Mn In order to improve the hardenability and stably obtain martensite, one or more elements of Cr, Mo, V may be added as necessary. In order to obtain the above effect, it is necessary that both elements be 0.02% or more. On the other hand, if it exceeds 0.5%, not only is the above effect saturated, but also a lamellar structure in the plate thickness direction due to segregation during casting develops and stretch flangeability deteriorates, which is not preferable. For this reason, when adding Cr, Mo, and V, these content shall be 0.02 to 0.5%, respectively.
Ti:0.005〜0.05%、B:0.0002〜0.002%
本発明では、固溶Bを鋼中に含有させることによりマルテンサイトの体積率を高めて高強度化する目的、あるいは、フェライトの析出温度を低下させて組織を微細化する目的で、必要に応じてTiとBを複合して添加してもよい。TiとBを複合で添加する理由は、NをTiで固着してBNの生成を抑制し、固溶Bを確保するためである。したがって、Ti含有量の下限はNを固着するため必要な最低限である0.005%とし、上限はこれ以上含有しても効果が飽和する0.05%とする。また、B含有量が0.0002%より少ない値では高強度化あるいは組織微細化効果が得られず、B含有量が0.002%を超えると効果が飽和する。このため、TiおよびBを添加する場合には、Ti含有量を0.005〜0.05%、B含有量を0.0002〜0.002%とする。
Ti: 0.005-0.05%, B: 0.0002-0.002%
In the present invention, the solid solution B is contained in the steel for the purpose of increasing the volume ratio of martensite and increasing the strength, or for the purpose of reducing the precipitation temperature of ferrite and refining the structure as necessary. Ti and B may be added in combination. The reason why Ti and B are added in combination is that N is fixed with Ti to suppress the formation of BN and to secure solid solution B. Therefore, the lower limit of the Ti content is set to 0.005% which is the minimum necessary for fixing N, and the upper limit is set to 0.05% at which the effect is saturated even if it is contained more than this. Further, when the B content is less than 0.0002%, the effect of increasing the strength or refining the structure cannot be obtained, and when the B content exceeds 0.002%, the effect is saturated. For this reason, when adding Ti and B, the Ti content is set to 0.005 to 0.05% and the B content is set to 0.0002 to 0.002%.
[製造方法]
本発明では、鋳造後、熱延仕上げ温度FTを(1)式で定まる温度FTmin以上に制御して熱間圧延を行う。
FTmin(℃)=500×Nb+20×Si+50×C+860 (1)
(ただし、Nb、Si、Cは各成分の質量%を表す)
FTminは層状組織形成に及ぼす熱延条件の影響について本発明者らが鋭意検討した結果得られた仕上げ温度の下限値である。このような下限値が存在するのは、Nb、Si、Cといった熱間での変形抵抗を増大させる元素が存在すると、ある臨界の温度以下になると熱間圧延中に圧延方向と平行にNbCの不均一析出が生じやすくなり、焼鈍後の板厚方向の層状組織が顕著になってS含有量の低減あるいは組織微細化による効果が得られなくなり、伸びフランジ成形性が劣化するためと考えられる。そして、このFTminは、Nb、Si、C含有量の増加とともに上昇する。これは、Nb、Si、Cが増加すると、より熱間での変形抵抗が増大して、上記層状組織が形成されやすくなり、熱延仕上げ温度を上昇させる必要があるからである。この場合に、NbCの不均一が生じるのは、変形抵抗の増大とともに熱間圧延中にオーステナイトの変形帯や粒界近傍など歪みの蓄積が大きい部分に優先的にNbCが析出するためと考えられる。すなわち、FTminは、熱延仕上げ温度がこれ以下になると上述の不均一組織が発達しやすくなることを意味し、熱延仕上げ温度をFTmin以上にすることにより、このような不均一組織の発達を抑制して良好な伸びフランジ成形性を得ることが可能となる。なお、加熱温度はNbCが完全に溶解する温度であればよいため、1100〜1300℃の範囲で加熱すれば良い。
[Production method]
In the present invention, after the casting, hot rolling is performed by controlling the hot rolling finishing temperature FT to be equal to or higher than the temperature FT min determined by the equation (1).
FT min (° C.) = 500 × Nb + 20 × Si + 50 × C + 860 (1)
(However, Nb, Si, and C represent mass% of each component)
FT min is a lower limit value of the finishing temperature obtained as a result of intensive studies by the present inventors on the influence of hot rolling conditions on the formation of a layered structure. Such a lower limit exists because when there is an element that increases the deformation resistance during hot, such as Nb, Si, or C, when the temperature falls below a certain critical temperature, the NbC is in parallel with the rolling direction during hot rolling. This is presumably because non-uniform precipitation tends to occur, the layered structure in the thickness direction after annealing becomes prominent, and the effect of reducing the S content or refining the structure cannot be obtained, and stretch flangeability deteriorates. Then, the FT min rises Nb, Si, with increasing C content. This is because when Nb, Si, and C increase, the hot deformation resistance increases, the layered structure tends to be formed, and the hot rolling finishing temperature needs to be raised. In this case, the non-uniformity of NbC occurs because NbC preferentially precipitates at a portion where the accumulation of strain is large, such as the deformation zone of austenite and the vicinity of the grain boundary during hot rolling as the deformation resistance increases. . That is, FT min means that the above-mentioned non-uniform structure is likely to develop when the hot-rolling finishing temperature is lower than this, and by setting the hot-rolling finishing temperature to be equal to or higher than FT min , It is possible to obtain good stretch flangeability by suppressing the development. In addition, since heating temperature should just be a temperature which NbC melt | dissolves completely, what is necessary is just to heat in the range of 1100-1300 degreeC.
・巻取り温度:450〜650℃
以上のような条件で、熱間圧延し、冷却した後、450〜650℃で巻取る。これは、巻取り温度が650℃を超えると、鋳造偏析に起因した層状組織が発達しやすくなり、450℃未満ではNbCの析出が不均一になりやはり焼鈍後の層状組織が発達しやすくなるためである。また、熱間圧延後の冷却速度は特に規定しないが、5℃/sec未満の緩冷却では、層状組織が発達しやすいため5℃/sec以上で冷却することが望ましい。
-Winding temperature: 450-650 ° C
Under the conditions as described above, after hot rolling and cooling, winding is performed at 450 to 650 ° C. This is because if the coiling temperature exceeds 650 ° C., the layered structure due to casting segregation tends to develop, and if it is less than 450 ° C., the precipitation of NbC becomes non-uniform and the layered structure after annealing also tends to develop. It is. In addition, although the cooling rate after hot rolling is not particularly specified, it is desirable to cool at 5 ° C./sec or more because the lamellar structure is likely to develop in the slow cooling of less than 5 ° C./sec.
なお、熱延終了後、1秒以内に100〜300℃/secの高速冷却を活用することや、これに大圧下の仕上げ熱延を組み合わせるなどの熱延板粒径を小さくする工程については、巻取り温度が上記の450〜650℃である限り本発明の効果を阻害せず、許容される。 In addition, about the process of making hot-rolled sheet particle size small, such as utilizing high-speed cooling of 100 to 300 ° C./sec within 1 second after completion of hot rolling, and combining this with finishing hot rolling under large pressure, As long as the winding temperature is 450 to 650 ° C., the effect of the present invention is not hindered and allowed.
上述のようにして得られた熱延鋼帯を、酸洗後、冷間圧延し、連続溶融亜鉛めっきラインにおいて、800〜900℃で均熱した後、3℃/sec以上の冷却速度で600℃以下の温度域まで冷却し、亜鉛めっきあるいはさらに合金化処理を行う。 The hot-rolled steel strip obtained as described above is pickled, cold-rolled, soaked at 800 to 900 ° C. in a continuous hot-dip galvanizing line, and then 600 ° C. at a cooling rate of 3 ° C./sec or more. Cool to a temperature range of ℃ or less, and perform galvanization or further alloying treatment.
・均熱温度:800〜900℃
連続溶融亜鉛めっきラインの均熱温度は上記のように800℃〜900℃とする。これは、800℃未満では圧延組織に起因した層状組織が残留して伸びフランジ成形性が劣化し、900℃を超えると組織が粗大化して所望の伸びフランジ成形性が得られないためである。
-Soaking temperature: 800-900 ° C
The soaking temperature of the continuous hot dip galvanizing line is set to 800 ° C. to 900 ° C. as described above. This is because if the temperature is less than 800 ° C., the layered structure resulting from the rolled structure remains and stretch flange formability deteriorates, and if it exceeds 900 ° C., the structure becomes coarse and the desired stretch flange formability cannot be obtained.
・冷却速度:3℃/sec以上
均熱後の冷却速度は、3℃/sec未満ではマルテンサイトが得られなくなるため、3℃/sec以上とする。
-Cooling rate: 3 ° C / sec or more The cooling rate after soaking is 3 ° C / sec or more because martensite cannot be obtained if it is less than 3 ° C / sec.
・冷却温度:600℃以下
また、均熱後の冷却温度が、600℃を超えるとパーライトが析出する。このため、均熱後の冷却温度は600℃以下とする。
-Cooling temperature: 600 degrees C or less Moreover, when the cooling temperature after soaking exceeds 600 degreeC, pearlite will precipitate. For this reason, the cooling temperature after soaking shall be 600 degrees C or less.
なお、本発明においては、スラブの製造方法は、造塊、連続鋳造のいずれでも本発明の効果が得られる。また、熱延での粗熱延バー接続による連続熱延、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、本発明の効果に影響を及ぼさず、許容される。 In the present invention, the slab manufacturing method can achieve the effects of the present invention by either ingot casting or continuous casting. In addition, continuous hot rolling by rough hot-rolling bar connection in hot rolling, temperature rise within 200 ° C. using an induction heater in the hot rolling process, etc. are allowed without affecting the effects of the present invention.
以下、本発明の実施例について具体的に説明する。
まず、本発明にいたる基本的検討を行った結果について示す。ここでは、S含有量および熱延条件と伸びフランジ成形性との関係を把握した。表1に成分を示す本発明成分鋼1A〜1Cと比較成分鋼1D〜2E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に示す仕上げ温度(FDT)で熱間圧延し、表2に示す巻取り温度(CT)で巻取って3mm厚の熱延鋼帯とした後、酸洗して1.4mm厚に冷延し、続いて、連続溶融亜鉛めっきラインにて、表2に示す条件で焼鈍(均熱)し、冷却して合金化溶融亜鉛めっき鋼板を製造した。また、機械的性質についても、表2に併せて記載した。伸びフランジ成形性は、鉄鋼連盟規格で定められている試験方法にて測定した穴広げ率λで評価した。
Examples of the present invention will be specifically described below.
First, the results of a basic study leading to the present invention will be described. Here, the relationship between the S content and hot rolling conditions and stretch flange formability was grasped. Inventive component steels 1A to 1C and comparative component steels 1D to 2E, whose components are shown in Table 1, were produced in a converter and made into slabs by continuous casting. These slabs were hot-rolled at the finishing temperature (FDT) shown in Table 2, wound at the winding temperature (CT) shown in Table 2 to form a hot-rolled steel strip having a thickness of 3 mm, and then pickled. It was cold-rolled to a thickness of 4 mm, and subsequently annealed (soaking) under the conditions shown in Table 2 in a continuous hot-dip galvanizing line, and cooled to produce an alloyed hot-dip galvanized steel sheet. The mechanical properties are also shown in Table 2. Stretch flange formability was evaluated by the hole expansion ratio λ measured by the test method defined in the Federation of Iron and Steel.
図1に、表2の鋼板について、穴広げ率に及ぼすS含有量の影響を示す。図1より、穴広げ率はS含有量を低減することにより上昇するが、特に、本発明の範囲に熱延条件を限定し、かつS含有量を0.001%以下に低減することにより、穴広げ率が著しく向上することが認められる。これに対して、表2のNo.2,5,7では、鋼成分が全て本発明の範囲内であるにもかかわらず、熱延仕上げ温度がFTmin未満と低すぎるため、S含有量が低いにもかかわらず穴広げ率の顕著な上昇が認められない。また、No.13,14は、S含有量と熱延条件が本発明の範囲内であるものの、Nbが本発明範囲をはずれているため、やはり高い穴広げ率が得られていない。 FIG. 1 shows the influence of the S content on the hole expansion ratio for the steel sheets in Table 2. From FIG. 1, the hole expansion rate increases by reducing the S content, but in particular, by limiting the hot rolling conditions to the scope of the present invention and reducing the S content to 0.001% or less, It can be seen that the hole expansion rate is significantly improved. On the other hand, No. In Nos. 2, 5, and 7, although the steel components are all within the scope of the present invention, the hot rolling finish temperature is too low, less than FT min , so that the hole expansion rate is remarkable despite the low S content. Rise is not recognized. No. In Nos. 13 and 14, although the S content and the hot rolling conditions are within the scope of the present invention, Nb is out of the scope of the present invention, so that a high hole expansion ratio is not obtained.
次に、表3に成分を示す本発明成分鋼No.3〜11と比較成分鋼No.12〜15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4に示す仕上げ温度(FDT)で熱間圧延し、表4に示す巻取り温度(CT)で巻取って3mm厚の熱延鋼帯とした後、酸洗して1.4mm厚に冷延し、続いて、連続溶融亜鉛めっきラインにて、表4に示す条件で焼鈍(均熱)し、冷却して溶融亜鉛めっきもしくは合金化溶融亜鉛めっき鋼板を製造した。また、機械的性質についても、表4に併せて記載した。 Next, this invention component steel No. which shows a component in Table 3 is shown. 3-11 and comparative steel No. 12 to 15 were steeled out in a converter and made into slabs by continuous casting. These slabs were hot-rolled at a finishing temperature (FDT) shown in Table 4 and wound at a winding temperature (CT) shown in Table 4 to form a hot-rolled steel strip having a thickness of 3 mm. It was cold-rolled to a thickness of 4 mm, and subsequently annealed (soaking) under the conditions shown in Table 4 in a continuous hot-dip galvanizing line, and cooled to produce hot-dip galvanized or galvannealed steel sheets. The mechanical properties are also shown in Table 4.
図2は、表2および表4に記載の鋼について、引張強度と穴広げ率の関係を示したものである。この図より、本発明鋼では600〜1200MPaという幅広い強度範囲において、比較鋼に比べて極めて高い穴広げ率を有していることがわかる。 FIG. 2 shows the relationship between the tensile strength and the hole expansion rate for the steels listed in Tables 2 and 4. From this figure, it can be seen that the steel according to the present invention has an extremely high hole expansion rate compared with the comparative steel in a wide strength range of 600 to 1200 MPa.
Claims (3)
FTmin(℃)=500×Nb+20×Si+50×C+860 (1)
(ただし、Nb、Si、Cは各成分の質量%を表す) By mass%, C: 0.03 to 0.15%, Si: 2% or less, Mn: 1.7 to 3%, P: 0.05% or less, S: 0.001% or less, Nb: 0.00. After the steel containing 005 to 0.1% and the balance of Fe and inevitable impurities is cast, the hot rolling finish temperature FT is controlled to be equal to or higher than the temperature FT min determined by the formula (1) and hot rolled. Winding at 650 ° C. to form a hot-rolled steel strip, pickling, cold rolling, soaking at 800 to 900 ° C. in a continuous hot dip galvanizing line, and then cooling at a cooling rate of 3 ° C./sec or less to 600 ° C. or less A method for producing a high-strength hot-dip galvanized steel sheet having excellent stretch flange formability, characterized by being cooled to a temperature range of 5 and subjected to hot dip galvanization or further alloying treatment.
FT min (° C.) = 500 × Nb + 20 × Si + 50 × C + 860 (1)
(However, Nb, Si, and C represent mass% of each component)
The stretch flange formability according to claim 1 or 2, further comprising Ti: 0.005 to 0.05% and B: 0.0002 to 0.002% by mass%. A method for producing an excellent high-strength hot-dip galvanized steel sheet.
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