JP2008019502A - High-strength galvanized steel sheet excellent in workability, paint bake hardenability and resistance to natural aging and its production method - Google Patents
High-strength galvanized steel sheet excellent in workability, paint bake hardenability and resistance to natural aging and its production method Download PDFInfo
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本発明は、自動車のパネル部材用途、特に外板パネル用途に好適な加工性、塗装焼付硬化性及び常温非時効性に優れた亜鉛めっき鋼板並びにその製造方法に関する。 The present invention relates to a galvanized steel sheet excellent in workability, paint bake hardenability, and room temperature non-aging properties suitable for use in automobile panel members, particularly outer panel applications, and a method for producing the same.
鋼材使用量の減少による自動車の軽量化を目的とした自動車用鋼板の高強度化が進められており、パネル部材用途、特に外板パネルには、引張強度（Ｔｅｎｓｉｌｅ Ｓｔｒｅｓｓ、ＴＳともいう）が３４０ＭＰａ級の鋼板が実用化されている。更に、最近では３９０〜５００ＭＰａという高強度を有し、かつ優れた成形加工性を確保することが必要とされ、耐食性の観点から、亜鉛めっきが施された亜鉛めっき鋼板が要求されている。一般に、引張強度が増加すると、降伏強度も増加して加工性、特に成形性が損なわれるが、主相フェライトと硬質第２相（マルテンサイト／ベイナイト／残留γ）からなる複合組織とすることによって引張強度（ＴＳ）を高め、降伏強度（Ｙｉｅｌｄ Ｓｔｒｅｓｓ、ＹＳともいう）を低下させた亜鉛めっき鋼板が提案されている（例えば、特許文献１〜４）。
Steel sheets for automobiles are being made stronger for the purpose of reducing the weight of automobiles by reducing the amount of steel used. For panel member applications, particularly outer panel panels, tensile strength (also referred to as Tensile Stress, TS) is 340 MPa. Grade steel plates are in practical use. Furthermore, recently, it has been required to have a high strength of 390 to 500 MPa and to ensure excellent formability, and from the viewpoint of corrosion resistance, a galvanized steel sheet to which galvanization has been applied is required. Generally, when the tensile strength increases, the yield strength also increases and the workability, particularly the formability, is impaired. However, by forming a composite structure composed of main phase ferrite and hard second phase (martensite / bainite / residual γ). There have been proposed galvanized steel sheets that have increased tensile strength (TS) and reduced yield strength (also referred to as Yield Stress, YS) (for example,
特許文献１〜４において提案されている亜鉛めっき鋼板は、ＹＳ／ＴＳで表される降伏比（Ｙｉｅｌｄ Ｒａｔｉｏ、ＹＲともいう）も低く、成形性に優れる高強度亜鉛めっき鋼板である。また、特許文献１に記載されているように、降伏強度の低下によって、プレス成形後の弾性回復に起因する面歪み模様、いわゆる面歪みの発生を抑制することができる。しかし、特許文献１において提案されている亜鉛めっき鋼板は、強度が高いものの、延性が若干低いため、ＴＳと伸び（Ｅｌｏｎｇａｔｉｏｎ、Ｅｌともいう）との積、即ち強度−延性バランスＴＳ×Ｅｌが不十分である。そのため、成形条件が厳しい加工が行われる部位で破断する可能性があり、成形性が必ずしも十分ではない。また、特許文献２には、亜鉛めっき鋼板の伸びは示されていない。これに対して、特許文献３及び４において提案されている亜鉛めっき鋼板は、降伏比が低いだけでなく、延性も高く、強度−延性バランスＴＳ×Ｅｌにも優れている。しかし、引用文献１〜４において提案されている亜鉛めっき鋼板は、何れもプレス成形、塗装焼付処理を行った後の耐デント性を考慮したものではない。
The galvanized steel sheets proposed in
プレス成形後の塗装焼付処理による部材の降伏強度の増加は、耐デント性の向上に有効である。そのため、成形性と耐デント性の両立には、塗装焼付硬化性（Ｂａｋｅ Ｈａｒｄｎａｂｉｌｉｔｙ、ＢＨ性ともいう）の向上が要求される。ＢＨ性は、塗装焼付処理のような低温での熱処理によって、成形時に導入された転位へのＣ、Ｎ等の固着、炭窒化物の析出が生じるという、いわゆる歪み時効現象によって発現される特性である。このようなＢＨ性と成形性を両立させた高強度の亜鉛めっき鋼板が提案されているものの、常温非時効性を考慮したものではない（例えば、特許文献５）。 An increase in the yield strength of the member by the paint baking process after press molding is effective for improving the dent resistance. Therefore, in order to achieve both formability and dent resistance, improvement in paint bake hardenability (also called Bake Hardability, BH property) is required. The BH property is a characteristic expressed by a so-called strain aging phenomenon in which C, N, etc. adhere to dislocations introduced during molding and carbonitride precipitation occurs by heat treatment at a low temperature such as paint baking treatment. is there. Although a high-strength galvanized steel sheet having both BH properties and formability has been proposed, it does not take into account non-aging properties at room temperature (for example, Patent Document 5).
ＢＨ性に優れた鋼板は、常温で数ヶ月保持すると常温時効によって降伏点伸びが生じるようになる。そのため、降伏点伸びに起因するストレッチャーストレインと呼ばれる模様がプレス成形後に発生し、表面外観が劣化する。このような問題に対して、常温で数ヶ月保持しても降伏点伸びを生じない特性、即ち常温非時効性に優れた、ＢＨ性を発現する鋼板（ＢＨ鋼板ともいう）が提案されている（例えば、特許文献６、７）。しかし、特許文献６において提案されている亜鉛めっき鋼板は降伏比が高く、特許文献７において提案されている亜鉛めっき鋼板は、強度−延性バランスＴＳ×Ｅｌが低いという問題がある。 A steel sheet with excellent BH properties will have yield point elongation due to aging at room temperature when held for several months at room temperature. For this reason, a pattern called stretcher strain resulting from the elongation at yield point occurs after press molding, and the surface appearance deteriorates. In order to solve such a problem, a steel sheet (also referred to as a BH steel sheet) exhibiting the BH property, which does not cause yield point elongation even when kept at room temperature for several months, that is, has excellent room temperature non-aging properties, has been proposed. (For example, Patent Documents 6 and 7). However, the galvanized steel sheet proposed in Patent Document 6 has a high yield ratio, and the galvanized steel sheet proposed in Patent Document 7 has a problem that the strength-ductility balance TS × El is low.
以上のように、自動車のパネル部材用途、特に外板パネルに使用される鋼板に要求されるような、低い降伏強度及び降伏比、高い強度−延性バランス及びＢＨ性を兼備し、常温非時効性を同時に満足することは極めて困難であり、このような高強度亜鉛めっき鋼板は、従来、存在しなかった。即ち、特許文献１〜４において提案されている亜鉛めっき鋼板は、ＢＨ性及び常温非時効性について考慮されたものではなく、更に、特許文献１において提案されている亜鉛めっき鋼板は、強度−延性バランスが不十分であり、特許文献２に提案されている亜鉛めっき鋼板の伸びは不明である。また、特許文献５において提案されている亜鉛めっき鋼板は、常温非時効性を考慮したものではなく、特許文献６において提案されている亜鉛めっき鋼板は降伏比が高く、特許文献７において提案されている亜鉛めっき鋼板は、強度−延性バランスＴＳ×Ｅｌが低い。即ち、特許文献１〜７において提案されている亜鉛めっき鋼板は、降伏強度、降伏比、強度−延性バランス、ＢＨ性、常温非時効性の何れかが不十分であるという問題を有している。
As mentioned above, it has low yield strength and yield ratio, high strength-ductility balance and BH property, as required for steel panel used in automobile panel parts, especially outer panel, and is non-aging at room temperature. It is extremely difficult to satisfy the requirements at the same time, and such a high-strength galvanized steel sheet has not existed conventionally. That is, the galvanized steel sheets proposed in
本発明は上述した実状に鑑みてなされたものであって、優れた加工性、特に成形性及び耐デント性を有し、面歪み及びストレッチャーストレインによる表面性状の劣化がないことを要求される自動車用のパネル部材、特に表面品位が要求される外板パネル用途に好適な、引張強度が３９０〜６００ＭＰａであり、特に上限が５００ＭＰａ以下であっても、降伏強度及び降伏比が低く、強度−延性バランスに優れ、ＢＨ性及び常温非時効性を兼備した亜鉛めっき鋼板及びその製造方法を安価に提供することを目的とする。 The present invention has been made in view of the above-described actual conditions, and has excellent workability, particularly moldability and dent resistance, and is required to have no surface distortion and deterioration of surface properties due to stretcher strain. Suitable for automotive panel members, particularly for outer panel applications requiring surface quality, the tensile strength is 390 to 600 MPa, and even when the upper limit is 500 MPa or less, the yield strength and yield ratio are low, and the strength- An object of the present invention is to provide a galvanized steel sheet excellent in ductility balance, having both BH properties and room temperature non-aging properties, and a method for producing the same.
本発明は、ＢＨ性及び常温非時効性を兼備し、引張強度が３９０〜６００ＭＰａである亜鉛めっき鋼板の加工性、特に成形性の向上、即ち、降伏強度及び降伏比の低下並びに強度−延性バランスの向上という課題を解決するために行った検討によって得られた知見、即ち、合金元素量、特にＡｌ及びＣｒの添加量の適正化によってミクロ組織を適正化し、従来に比べて優れた成形加工性を有し、さらに耐デント性、常温非時効性を兼備する鋼板が安価に製造できるという知見に基づいてなされたものであり、その要旨は以下のとおりである。
（１） 質量％で、Ｃ：０．０２〜０．０８％、Ｓｉ：０．５％以下、Ｍｎ：１．０〜２．５％、Ｐ：０．０５％以下、Ｓ：０．０２％以下、Ａｌ：０．０００５〜０．０１４％、Ｃｒ：０．２％超、１．５％以下、Ｎ：０．００１〜０．００８％を含み、残部がＦｅ及び不可避的不純物からなり、Ｃｒ／Ａｌ：３０以上を満足し、金属組織におけるマルテンサイトの面積率が３〜２０％であり、フェライトの面積率が８０％以上であることを特徴とする加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（２） さらに、Ｂ：０．０００３〜０．００３質量％を含有することを特徴とする上記（１）記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（３） さらに、Ｍｏ、Ｗの一方又は双方を合計で０．０１〜１．０質量％含有することを特徴とする上記（１）又は（２）記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（４） さらに、Ｎｂ、Ｔｉ、Ｖの１種又は２種以上を合計で０．００２〜０．０４質量％含有することを特徴とする上記（１）〜（３）の何れか１項に記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（５） さらに、Ｃｕ、Ｎｉの一方又は双方を合計で０．０２〜０．３質量％含有することを特徴とする上記（１）〜（４）の何れか１項に記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（６） さらに、Ｃａ、Ｍｇ、Ｚｒ、Ｌａ、Ｃｅの１種又は２種以上を合計で０．０００３〜０．０１質量％含有することを特徴とする上記（１）〜（５）の何れか１項に記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（７） フェライトのうち未再結晶フェライトの分率が面積率で１０％以下であることを特徴とする上記（１）〜（６）の何れか１項に記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（８） フェライト粒径が５〜２０μｍであることを特徴とする上記（１）〜（７）の何れか１項に記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（９） 降伏強度が２７０ＭＰａ以下、降伏比が０．５５以下、２％引張予歪付加後に１７０℃で１２００ｓ保持する熱処理を行った後の歪み時効降伏強度が３３０ＭＰａ以上であることを特徴とする上記（１）〜（８）の何れか１項に記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。
（１０） 上記（１）〜（６）の何れか１項に記載の成分組成からなる鋼片を熱間圧延し、冷間圧延を施し、最高到達温度が７２０〜８５０℃、７２０〜８５０℃の温度範囲に１０ｓ以上保持する焼鈍を行った後、７００℃から５００℃までの間の冷却速度を３℃／ｓ以上として冷却し、亜鉛めっきを施すこと特徴とする加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板の製造方法。
（１１） 上記（１０）記載の熱間圧延において、仕上圧延後、冷却し、３５０℃以上で巻き取ること特徴とする加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板の製造方法。
（１２） 溶融亜鉛めっきを５５０℃以下の温度で施すことを特徴とする上記（１０）又は（１１）記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板の製造方法。
（１３） 上記（１０）〜（１２）の何れか１項に記載の方法によって製造した加工性及び時効特性に優れた亜鉛めっき鋼板に、５５０℃以下で合金化処理を行うことを特徴とする加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板の製造方法。
（１４） 焼鈍と溶融亜鉛めっきを連続焼鈍−亜鉛めっきラインによって行うこと特徴とする上記（１０）〜（１３）の何れか１項に記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板の製造方法。
（１５） 上記（１０）〜（１４）の何れか１項に記載の方法によって製造した加工性及び時効特性に優れた亜鉛めっき鋼板に、伸び率が２％以下である調質圧延を施すことを特徴とする加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板の製造方法。
The present invention combines BH properties and room temperature non-aging properties, and improves the workability of galvanized steel sheets having a tensile strength of 390 to 600 MPa, particularly formability, that is, yield strength and yield ratio decrease, and strength-ductility balance. Knowledge obtained through studies conducted to solve the problem of improvement of the alloy, that is, by optimizing the microstructure by optimizing the amount of alloying elements, especially the addition amount of Al and Cr, and superior molding processability compared to the past In addition, the present invention was made based on the knowledge that a steel sheet having both dent resistance and room temperature non-aging properties can be produced at low cost, and the gist thereof is as follows.
(1) By mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.02 %: Al: 0.0005 to 0.014%, Cr: more than 0.2%, 1.5% or less, N: 0.001 to 0.008%, with the balance being Fe and inevitable impurities Cr / Al: satisfying 30 or more, martensite area ratio in metal structure is 3 to 20%, ferrite area ratio is 80% or more, workability, paint bake hardenability and High-strength galvanized steel sheet with excellent non-aging at room temperature.
(2) Further, B: 0.0003 to 0.003% by mass High-strength galvanized steel sheet excellent in workability, paint bake hardenability and room temperature non-aging properties as described in (1) above .
(3) Further, one or both of Mo and W are contained in a total of 0.01 to 1.0% by mass, the workability, paint bake hardenability and normal temperature described in (1) or (2) above High strength galvanized steel sheet with excellent non-aging properties.
(4) Further, in any one of the above (1) to (3), one or more of Nb, Ti, and V are contained in a total amount of 0.002 to 0.04% by mass. High-strength galvanized steel sheet with excellent workability, paint bake hardenability and non-aging at room temperature.
(5) Furthermore, the workability according to any one of (1) to (4) above, wherein one or both of Cu and Ni are contained in a total amount of 0.02 to 0.3 mass%. High-strength galvanized steel sheet with excellent paint bake hardenability and non-aging at room temperature.
(6) Further, any one of the above (1) to (5), characterized in that one or more of Ca, Mg, Zr, La and Ce are contained in a total amount of 0.0003 to 0.01% by mass. A high-strength galvanized steel sheet excellent in workability, paint bake hardenability and non-aging at room temperature.
(7) Workability and paint bake hardenability described in any one of (1) to (6) above, wherein the fraction of unrecrystallized ferrite in the ferrite is 10% or less in terms of area ratio And high-strength galvanized steel sheet with excellent non-aging properties at room temperature.
(8) High strength excellent in workability, paint bake hardenability and non-aging property at room temperature according to any one of the above (1) to (7), wherein the ferrite particle size is 5 to 20 μm Galvanized steel sheet.
(9) The yield strength is 270 MPa or less, the yield ratio is 0.55 or less, and the strain aging yield strength after heat treatment is maintained at 170 ° C. for 1200 s after the addition of 2% tensile pre-strain is 330 MPa or more. A high-strength galvanized steel sheet excellent in workability, paint bake hardenability and non-aging property at room temperature according to any one of (1) to (8) above.
(10) The steel piece which consists of a component composition of any one of said (1)-(6) is hot-rolled, cold-rolled, and the highest ultimate temperature is 720-850 degreeC, 720-850 degreeC. Workability, paint bake hardenability, characterized in that after annealing at a temperature range of 10 s or more, cooling is performed at a cooling rate between 700 ° C. and 500 ° C. at 3 ° C./s or more, and galvanization is performed. And the manufacturing method of the high intensity | strength galvanized steel plate excellent in non-aging property at normal temperature.
(11) In the hot rolling as described in (10) above, after high-strength galvanization excellent in workability, paint bake hardenability and non-aging at room temperature, which is cooled and wound at 350 ° C. or higher after finish rolling. A method of manufacturing a steel sheet.
(12) A high-strength galvanized steel sheet excellent in workability, paint bake hardenability and non-aging property at room temperature as described in (10) or (11) above, wherein hot dip galvanizing is performed at a temperature of 550 ° C. or less Production method.
(13) A galvanized steel sheet excellent in workability and aging characteristics produced by the method according to any one of (10) to (12) above is subjected to an alloying treatment at 550 ° C. or less. A method for producing a high-strength galvanized steel sheet with excellent workability, paint bake hardenability and non-aging at room temperature.
(14) Annealing and hot dip galvanizing are performed by a continuous annealing-zinc plating line, and the workability, paint bake hardenability and non-aging at room temperature described in any one of (10) to (13) above An excellent method for producing high-strength galvanized steel sheets.
(15) Apply temper rolling with an elongation of 2% or less to a galvanized steel sheet produced by the method according to any one of (10) to (14) and excellent in workability and aging characteristics. A process for producing a high-strength galvanized steel sheet excellent in processability, paint bake hardenability and non-aging at room temperature.
本発明によれば、引張強度が３９０〜６００ＭＰａであり、特に上限が５００ＭＰａ以下であっても、降伏強度及び降伏比が低く、強度−延性バランスに優れ、ＢＨ性及び常温非時効性を兼備した亜鉛めっき鋼板及びその製造方法を安価に提供することが可能になり、産業上の貢献が極めて顕著である。更に、本発明は、優れた加工性、特に成形性、耐デント性及び表面品位が要求される自動車用のパネル部材、特に外板パネル部材の板厚を減少させることを可能にするものであり、自動車車体の軽量化等に対する貢献が大きいという極めて顕著な効果を奏するものである。 According to the present invention, the tensile strength is 390 to 600 MPa, and even when the upper limit is 500 MPa or less, the yield strength and yield ratio are low, the strength-ductility balance is excellent, and BH properties and room temperature non-aging properties are combined. The galvanized steel sheet and the manufacturing method thereof can be provided at low cost, and the industrial contribution is extremely remarkable. Furthermore, the present invention makes it possible to reduce the thickness of panel members for automobiles, particularly outer panel members, which require excellent workability, particularly moldability, dent resistance and surface quality. It has a very remarkable effect that the contribution to the weight reduction of the automobile body is great.
本発明者らは、ＢＨ性及び常温非時効性を兼備する高強度亜鉛めっき鋼板の降伏比の低下と強度−延性バランスの向上を目的とし、鋼板の成分組成、ミクロ組織、更に鋼板の製造方法について検討を行った。まず、ＣとＭｎを適正量含有した成分系を基本組成とし、ＴＳが３９０〜５００ＭＰａになるように、金属組織をフェライトと硬質第２相からなる複相組織とし、焼入れ性を高める元素であるＣｒと、窒化物を形成する元素であるＡｌに注目し、引張特性に対するＣｒ量及びＡｌ量の効果について検討した。ここで、硬質第２相とは、マルテンサイト、ベイナイト及び残留オーステナイトである。 The present inventors aim to reduce the yield ratio and improve the strength-ductility balance of a high-strength galvanized steel sheet having both BH properties and room temperature non-aging properties. Was examined. First, a component system containing appropriate amounts of C and Mn is a basic composition, and the metal structure is a multiphase structure composed of ferrite and a hard second phase so that TS becomes 390 to 500 MPa, and is an element that enhances hardenability. Focusing on Cr and Al, which is an element forming a nitride, the effects of Cr amount and Al amount on tensile properties were examined. Here, the hard second phase is martensite, bainite, and retained austenite.
図１は、Ｃｒ量及びＡｌ量に対して、降伏比ＹＲ［−］の数値を示したものであり、更に、強度−延性バランスＴＳ×Ｅｌ［ＭＰａ・％］が１７０００を超えるものを※印で示している。なお、引張特性は、ＪＩＳ Ｚ ２２４１に準拠した引張試験によって測定したものであり、Ｅｌ［％］は破断伸びである。また、引張試験において、降伏現象が見られた場合は上降伏点を降伏強度として評価し、降伏現象が見られない場合には０．２％耐力を降伏強度として評価した。 FIG. 1 shows the numerical value of the yield ratio YR [−] with respect to the Cr content and the Al content, and further, the strength-ductility balance TS × El [MPa ·%] exceeds 17,000. Is shown. In addition, a tensile characteristic is measured by the tension test based on JISZ2241, and El [%] is elongation at break. In the tensile test, when a yield phenomenon was observed, the upper yield point was evaluated as the yield strength. When no yield phenomenon was observed, the 0.2% proof stress was evaluated as the yield strength.
図１から、Ｃｒ量、Ａｌ量、Ｃｒ／Ａｌ比を適正な範囲とすることが降伏比の低減及び強度−延性バランスの向上に極めて重要であることがわかる。Ｃｒ含有鋼においてＡｌ量を減ずることにより加工性が大きく向上する原因については定かではないが、焼鈍後冷却時のフェライト変態の抑制によるマルテンサイト量の増加と鋼中に存在するＡｌ系析出物の減少によって降伏強度が低下している可能性がある。なお、Ａｌ系析出物の減少による降伏強度の低下は、結晶粒径の微細化が抑制された効果であると考えられる。更に、Ａｌ系析出物の減少は、延性の向上によるＴＳ×Ｅｌの増加し、即ち強度−延性バランスの向上にも寄与している可能性がある。 From FIG. 1, it can be seen that making the Cr amount, Al amount, and Cr / Al ratio within appropriate ranges is extremely important for reducing the yield ratio and improving the strength-ductility balance. The reason why workability is greatly improved by reducing the amount of Al in Cr-containing steels is not clear, but the increase in martensite due to suppression of ferrite transformation during cooling after annealing and the presence of Al-based precipitates present in the steel The yield strength may have decreased due to the decrease. Note that the decrease in yield strength due to the decrease in Al-based precipitates is considered to be an effect of suppressing the refinement of the crystal grain size. Furthermore, the decrease in Al-based precipitates may contribute to an increase in TS × El due to an improvement in ductility, that is, an improvement in the strength-ductility balance.
また、ＡｌＮの析出が抑制されると固溶Ｎ量が増加し、これによりＢＨ性が確保できる。一方、通常、固溶Ｎ量の増加によって常温非時効性は劣化するが、適正量のＣｒを添加することにより常温非時効性が確保できる。この原因については定かではないが、Ｃｒ−Ｎ原子間の相互作用により、常温でのＮの拡散が遅延しているものと考えられる。以上のように、Ａｌ添加量の低減とＣｒの添加は、高ＢＨ性と常温非時効性の両立にも有効であることがわかった。また、鋼中に存在するＡｌ系析出物の減少により、従来鋼に比べて表面欠陥発生率が減少するという、極めて重要な効果も得られることも見出した。
なお、本発明者らは、引張強度が３００〜６００ＭＰａの鋼板についても、Ｃｒ量、Ａｌ量、Ｃｒ／Ａｌ比の最適化が、降伏比の低減及び強度−延性バランスの向上に極めて有効であること、Ａｌ添加量の低減とＣｒの添加が、高ＢＨ性と常温非時効性の両立にも有効であることを確認した。
In addition, when the precipitation of AlN is suppressed, the amount of solute N increases, thereby ensuring BH properties. On the other hand, normally, the normal temperature non-aging property deteriorates with an increase in the amount of dissolved N, but the normal temperature non-aging property can be secured by adding an appropriate amount of Cr. Although the cause of this is not clear, it is considered that the diffusion of N at room temperature is delayed due to the interaction between Cr—N atoms. As described above, it has been found that the reduction of the Al addition amount and the addition of Cr are effective in achieving both high BH properties and room temperature non-aging properties. It has also been found that the reduction of Al-based precipitates present in the steel also provides a very important effect that the surface defect generation rate is reduced as compared with the conventional steel.
In addition, the inventors of the present invention are extremely effective in reducing the yield ratio and improving the strength-ductility balance even when the steel has a tensile strength of 300 to 600 MPa. In addition, it was confirmed that the reduction in the amount of Al added and the addition of Cr are effective in achieving both high BH properties and non-aging at room temperature.
以下、本発明について詳細に説明する。 Hereinafter, the present invention will be described in detail.
まず成分の限定理由について説明する。成分含有量は質量％である。 First, the reasons for limiting the components will be described. The component content is% by mass.
Ａｌは、本発明において最も重要な元素の一つであり、適正量の添加によって降伏比が低減し、強度−延性バランスが向上する。Ａｌ量の上限は、フェライト変態の促進によるマルテンサイトの減少の観点から０．０１４％以下とし、これにより降伏比を低下させることができる。一方、製鋼コストの観点からＡｌ量の下限を０．０００５％以上とした。脱酸剤としてＡｌを使用する場合は、含有量を０．００１％以上にすることが好ましい。また、脱酸不良に起因するめっき表面品位の劣化を確実に防ぐためには、Ａｌ量を０．００２％以上とすることが好ましい。なお、Ａｌ量を０．００８％以下に減少させると、Ａｌ系析出物の生成が抑制されて延性が向上し、強度−延性バランスが向上するため好ましい。更に、Ａｌ量の上限を０．００５％以下とすることが好ましく、これにより、降伏強度が低下して面歪みの発生が抑制され、Ａｌ系析出物が減少して介在物に起因する表面疵の発生が抑制され、優れた表面品質を得ることができる。 Al is one of the most important elements in the present invention, and by adding an appropriate amount, the yield ratio is reduced and the strength-ductility balance is improved. The upper limit of the amount of Al is 0.014% or less from the viewpoint of martensite reduction due to the promotion of ferrite transformation, whereby the yield ratio can be lowered. On the other hand, the lower limit of the Al content is set to 0.0005% or more from the viewpoint of steelmaking cost. When using Al as a deoxidizer, the content is preferably 0.001% or more. Moreover, in order to prevent reliably the deterioration of the plating surface quality resulting from poor deoxidation, the Al content is preferably 0.002% or more. Note that it is preferable to reduce the Al content to 0.008% or less because the formation of Al-based precipitates is suppressed, the ductility is improved, and the strength-ductility balance is improved. Furthermore, it is preferable that the upper limit of the Al amount is 0.005% or less, whereby the yield strength is reduced and the occurrence of surface distortion is suppressed, and Al-based precipitates are reduced to reduce surface defects caused by inclusions. Is suppressed, and excellent surface quality can be obtained.
Ｃｒも本発明において重要な元素の一つであり、適正な量のマルテンサイトを生成させて、引張強度の向上と、降伏強度の低下を両立させるために添加される。またＣｒは常温非時効性を向上させるために欠かせない元素である。Ｃｒ添加量が０．２％以下であるとこれらの効果が不十分であり、一方１．５％を超えると引張強度が高くなりすぎ、５００ＭＰａを超えて、成形性を損なう。そのためＣｒ添加量を０．２％超、１．５％以下の範囲とした。降伏比を更に低減させるためには、０．３％以上の添加が好ましい。 Cr is also an important element in the present invention, and is added in order to generate an appropriate amount of martensite and achieve both improvement in tensile strength and reduction in yield strength. Cr is an element indispensable for improving the non-aging property at room temperature. If the Cr addition amount is 0.2% or less, these effects are insufficient. On the other hand, if it exceeds 1.5%, the tensile strength becomes too high, and if it exceeds 500 MPa, the formability is impaired. Therefore, the Cr addition amount is set to a range of more than 0.2% and 1.5% or less. In order to further reduce the yield ratio, addition of 0.3% or more is preferable.
Ｃｒ／Ａｌは、本発明において重要なパラメータである。理由は定かではないが、Ｃｒ／Ａｌが３０未満であると降伏比が十分に低下しないため、その下限を３０以上とした。強度−延性バランスの観点から、より好ましくは５０以上である。なお、Ｃｒ／Ａｌの上限はＣｒ量の上限が１．５％であり、Ａｌ量の下限が０．０００５％であることから、３０００である。 Cr / Al is an important parameter in the present invention. The reason is not clear, but if Cr / Al is less than 30, the yield ratio does not decrease sufficiently, so the lower limit was made 30 or more. From the viewpoint of strength-ductility balance, it is more preferably 50 or more. The upper limit of Cr / Al is 3000 because the upper limit of Cr content is 1.5% and the lower limit of Al content is 0.0005%.
Ｃは、マルテンサイト量を適正な範囲に制御して降伏強度を低下させるため、０．０２〜０．０８％を添加する。Ｃ量が０．０２％未満であるとマルテンサイト量が減少して降伏強度が増加し、一方、Ｃ量が０．０８％を超えると引張強度の増加とともに降伏強度も増加し、何れの場合にも加工性、特に成形性が低下し、面歪みを生じて表面品質を損なう。降伏強度をより低下させ、幅広い成形条件でも面歪みを発生させないためにはＣ量の上限を０．０６％以下とすることが好ましい。 C is added in an amount of 0.02 to 0.08% in order to control the martensite amount to an appropriate range and lower the yield strength. If the C content is less than 0.02%, the martensite content decreases and the yield strength increases. On the other hand, if the C content exceeds 0.08%, the yield strength increases as the tensile strength increases. In addition, the workability, particularly the moldability, is reduced, and surface distortion occurs and the surface quality is impaired. In order to further reduce the yield strength and prevent surface distortion even under a wide range of molding conditions, the upper limit of the C content is preferably set to 0.06% or less.
Ｓｉは脱酸元素であるが、０．５％を超えて含有させると亜鉛めっきの密着性が悪くなり表面品質を損なう。亜鉛めっき鋼板の表面品質の観点から、０．３％以下がより好ましいＳｉ量の上限である。鋼の脱酸は、Ａｌ及びＴｉの添加によっても可能であるため、Ｓｉ量の下限は限定しないが、通常、０．００１％以上を含有する。また、Ｓｉを鋼板の金属組織及び強度の調整に活用する場合には、０．０１％以上の添加が好ましい。 Si is a deoxidizing element, but if it exceeds 0.5%, the adhesion of galvanizing deteriorates and the surface quality is impaired. From the viewpoint of the surface quality of the galvanized steel sheet, 0.3% or less is a more preferable upper limit of the Si amount. Since deoxidation of steel is possible by addition of Al and Ti, the lower limit of the amount of Si is not limited, but usually contains 0.001% or more. Moreover, when utilizing Si for the metal structure and intensity | strength adjustment of a steel plate, 0.01% or more of addition is preferable.
Ｍｎは、適正な量のマルテンサイトを生成させて、降伏強度及び降伏比を低下させるために、１．０％以上含有させることが必要である。一方、２．５％超のＭｎを添加すると鋼板の降伏強度が増加し、成形性及び耐面歪み性が劣化する。したがって、Ｍｎ含有量を１．０〜２．５％の範囲に制限した。降伏強度をより低下させて加工性、特に成形性を更に高めるには、上限を２．２％以下とすることが好ましい。 Mn is required to be contained in an amount of 1.0% or more in order to generate an appropriate amount of martensite and lower the yield strength and yield ratio. On the other hand, when 2.5% or more of Mn is added, the yield strength of the steel sheet increases, and the formability and surface strain resistance deteriorate. Therefore, the Mn content is limited to a range of 1.0 to 2.5%. In order to further reduce the yield strength and further improve the workability, particularly the formability, the upper limit is preferably made 2.2% or less.
Ｐは不純物であり、含有量が０．０５％を超えると２次加工性が劣化することがあり、連続ラインで焼鈍と溶融亜鉛めっきを行い、更に合金化させる際には、合金化反応を遅延させて生産性を損なうことがある。したがって、Ｐの含有量の上限を０．０５％以下に制限した。Ｐ量の下限は限定しないが、通常、０．００１％以上を含有する。また、Ｐは強力な固溶強化元素であることから、鋼板の強度の調整に用いる場合には、０．００５％以上を含有させることが好ましい。 P is an impurity, and when the content exceeds 0.05%, the secondary workability may be deteriorated. When the alloy is annealed and galvanized in a continuous line and further alloyed, an alloying reaction is performed. Delays can impair productivity. Therefore, the upper limit of the P content is limited to 0.05% or less. Although the minimum of P amount is not limited, Usually, 0.001% or more is contained. Further, since P is a strong solid solution strengthening element, when used for adjusting the strength of the steel sheet, it is preferable to contain 0.005% or more.
Ｓも不純物であり、０．０２％を超えると熱間脆性によって表面品質が劣化する可能性があるため、上限を０．０２％以下に限定した。下限は限定しないが、０．００１％未満とするには製鋼コストが上昇する。鋼中に微細な硫化物を存在させ、結晶粒径を制御するには、Ｓを０．００２％以上含有させることが好ましい。一方、０．０１２％超のＳを含有させると、鋼の結晶粒径が微細になりすぎて降伏強度が上昇し、加工性、特に成形性及び表面品位を損なうことがあるため、上限を０．０１２％以下とすることが好ましい。 S is also an impurity, and if it exceeds 0.02%, the surface quality may deteriorate due to hot brittleness, so the upper limit was limited to 0.02% or less. The lower limit is not limited, but to make it less than 0.001%, the steelmaking cost increases. In order to allow fine sulfides to be present in the steel and control the crystal grain size, it is preferable to contain 0.002% or more of S. On the other hand, if more than 0.012% S is contained, the crystal grain size of the steel becomes too fine and the yield strength increases, which may impair the workability, particularly the formability and surface quality. 0.012% or less is preferable.
Ｎは、成形後、塗装焼付処理によって降伏強度を上昇させ、パネル部材の耐デント性を高めるための重要な添加元素である。Ｎの添加量が０．００１％未満であると耐デント性の効果が十分には得られず、一方０．００８％を超えると降伏比が増加し、加工性が劣化すると共に、常温非時効性を確保することが困難になる。したがって、Ｎ含有量の範囲を０．００１〜０．００８％に限定した。より高い加工性を確保する観点から、Ｎ量の好ましい上限は０．００６％以下である。 N is an important additive element for improving the dent resistance of the panel member by increasing the yield strength by paint baking after forming. If the addition amount of N is less than 0.001%, the effect of dent resistance cannot be sufficiently obtained. On the other hand, if it exceeds 0.008%, the yield ratio increases, workability deteriorates, and non-aging at room temperature. It becomes difficult to ensure the property. Therefore, the range of N content is limited to 0.001 to 0.008%. From the viewpoint of securing higher workability, the preferable upper limit of the N amount is 0.006% or less.
更に、Ｂ、Ｍｏ、Ｗ、Ｎｂ、Ｔｉ、Ｖ、Ｃｕ、Ｎｉ、Ｃａ、Ｍｇ、Ｚｒ、Ｌａ、Ｃｅの１種又は２種以上を含有させても良い。 Furthermore, you may contain 1 type, or 2 or more types of B, Mo, W, Nb, Ti, V, Cu, Ni, Ca, Mg, Zr, La, and Ce.
Ｂは、鋼板のマルテンサイトの量を増加させて、降伏比を低下させる元素であり、また、Ｐによる２次加工脆性の抑制にも有効である。Ｂの含有量が０．０００３％未満であるとその効果が十分に得られないことがあり、一方、０．００３％を超えると強度−延性バランスが劣化し、加工性が低下することがある。したがって、Ｂの添加量を０．０００３〜０．００３％の範囲とすることが好ましい。 B is an element that decreases the yield ratio by increasing the amount of martensite in the steel sheet, and is also effective in suppressing secondary work brittleness due to P. If the content of B is less than 0.0003%, the effect may not be sufficiently obtained. On the other hand, if it exceeds 0.003%, the strength-ductility balance may be deteriorated and workability may be deteriorated. . Therefore, it is preferable that the addition amount of B is in the range of 0.0003 to 0.003%.
Ｍｏ及びＷは、何れも鋼板のマルテンサイトの量を増加させて、降伏比を低下させる元素である。Ｍｏ、Ｗの一方又は双方の合計量が０．０１％未満であるとその効果が十分でないことがあり、一方、１．０％を超えると強度−延性バランスが劣化し、加工性が低下することがある。したがって、Ｍｏ、Ｗの一方又は双方の合計量を０．０１〜１．０％の範囲とすることが好ましい。合金コストの観点からはＭｏ、Ｗの一方又は双方の合計量を０．３％以下とすることがより好ましい。 Mo and W are both elements that increase the amount of martensite in the steel sheet and lower the yield ratio. If the total amount of one or both of Mo and W is less than 0.01%, the effect may not be sufficient. On the other hand, if it exceeds 1.0%, the strength-ductility balance deteriorates and the workability decreases. Sometimes. Therefore, the total amount of one or both of Mo and W is preferably in the range of 0.01 to 1.0%. From the viewpoint of alloy cost, it is more preferable to set the total amount of one or both of Mo and W to 0.3% or less.
Ｎｂ、Ｔｉ、Ｖは炭化物、窒化物、炭窒化物を形成し、鋼板の結晶粒径、降伏強度及び加工性、特に成形性の制御に有効である。更に炭化物、窒化物、炭窒化物の形成は、炭素及び窒素の固溶量にも影響するため、Ｎｂ、Ｔｉ、Ｖは、ＢＨ量、歪み時効降伏強度及び常温非時効性の制御にも有効である。Ｎｂ、Ｔｉ、Ｖの１種又は２種以上の合計量が０．００２％未満である場合には、この効果が十分に得られないことがあり、一方、０．０４％を超えると成形性が劣化することがある。したがって、Ｎｂ、Ｔｉ、Ｖの１種又は２種以上の合計量を０．００２〜０．０４％の範囲とすることが好ましい。また、Ｎｂ、Ｔｉ、Ｖの１種又は２種以上の合計量を０．０３％超とすると、微細な析出物の形成によって伸びが低下し、ＴＳ×Ｅｌが小さくなることがあるため、好ましい上限は０．０３％以下である。 Nb, Ti, and V form carbides, nitrides, and carbonitrides, and are effective in controlling the crystal grain size, yield strength, and workability of the steel sheet, particularly formability. Furthermore, since the formation of carbides, nitrides, and carbonitrides also affects the solid solution amount of carbon and nitrogen, Nb, Ti, and V are also effective in controlling the BH amount, strain aging yield strength, and room temperature non-aging properties. It is. If the total amount of one or more of Nb, Ti, and V is less than 0.002%, this effect may not be sufficiently obtained. May deteriorate. Therefore, it is preferable that the total amount of one or more of Nb, Ti, and V is in the range of 0.002 to 0.04%. Further, if the total amount of one or more of Nb, Ti, and V exceeds 0.03%, it is preferable because elongation may decrease due to formation of fine precipitates, and TS × El may be reduced. The upper limit is 0.03% or less.
Ｃｕ及びＮｉは、鋼板の強度調整に有用な固溶強化元素であり、Ｃｕ、Ｎｉの一方又は双方を合計量で０．０２％以上添加することが好ましい。一方、Ｃｕ、Ｎｉの一方又は双方の合計量が０．３％を超えると、表面品位が低下することがあるため、上限を０．３％以下とすることが好ましい。 Cu and Ni are solid solution strengthening elements useful for adjusting the strength of the steel sheet, and it is preferable to add one or both of Cu and Ni in a total amount of 0.02% or more. On the other hand, if the total amount of one or both of Cu and Ni exceeds 0.3%, the surface quality may be lowered, so the upper limit is preferably made 0.3% or less.
Ｃａ、Ｍｇ、Ｚｒ、Ｌａ及びＣｅは、介在物の形態及び分布の制御に有用な元素であり、１種又は２種以上を合計で０．０００３％以上含有することが好ましい。一方、Ｃａ、Ｍｇ、Ｚｒ、Ｌａ及びＣｅの１種又は２種以上の合計が０．０１％を超えると、表面品質が低下することがあるため、上限を０．０１％以下とすることが好ましい。 Ca, Mg, Zr, La, and Ce are elements useful for controlling the form and distribution of inclusions, and preferably contain one or two or more in total of 0.0003% or more. On the other hand, if the total of one or more of Ca, Mg, Zr, La, and Ce exceeds 0.01%, the surface quality may deteriorate, so the upper limit may be made 0.01% or less. preferable.
なお、不可避不純物として含有されるＯ量も制限することが好ましい。Ｏ量は脱酸の方法によりその残留量が大きく変化するが、不可避的に０．０００５％以上を含有することが多い。なお、本発明の鋼のようにＡｌの含有量が低い場合、Ｏ量の上限が０．０１％以下になることがある。 It is preferable to limit the amount of O contained as an inevitable impurity. The amount of O varies greatly depending on the deoxidation method, but it inevitably contains 0.0005% or more. When the Al content is low as in the steel of the present invention, the upper limit of the O amount may be 0.01% or less.
次に金属組織について説明する。 Next, the metal structure will be described.
本発明の鋼は、フェライトと硬質第２相からなり、硬質第２相はマルテンサイト、ベイナイト、残留オーステナイトからなる。本発明において、鋼板の金属組織の観察は、ＪＩＳ Ｇ ０５５１に準拠して、光学顕微鏡によって行えば良い。組織観察用の試料は、圧延方向に平行な板厚断面（Ｌ断面という。）を観察面として採取することが好ましく、観察面を研磨、ナイタールエッチ、必要に応じてレペラー腐食法エッチすれば良い。マルテンサイト及びフェライトの面積率は、光学顕微鏡によって撮影した組織写真を用いて、ポイントカウント法又は画像解析によって測定することができる。また、フェライト粒径の測定もＪＩＳ Ｇ ０５５１に準拠し、切断法、比較法によって行えば良く、光学顕微鏡によって撮影した組織写真を画像解析して求めることもできる。 The steel of the present invention is composed of ferrite and a hard second phase, and the hard second phase is composed of martensite, bainite, and retained austenite. In the present invention, the observation of the metal structure of the steel sheet may be performed with an optical microscope in accordance with JIS G 0551. A sample for observing the structure is preferably obtained by taking a plate thickness cross section (referred to as an L cross section) parallel to the rolling direction as an observation surface, and if the observation surface is polished, nital etched, or if necessary, a repeller corrosion method is etched. good. The area ratio of martensite and ferrite can be measured by a point count method or image analysis using a structure photograph taken with an optical microscope. Further, the ferrite particle size may be measured by a cutting method or a comparison method in accordance with JIS G 0551, and a structure photograph taken with an optical microscope can be obtained by image analysis.
マルテンサイトの面積率は極めて重要であり、これを３〜２０％の範囲とすることによって、強度、降伏強度、降伏比、強度−延性バランスの全てを良好な範囲とすることが可能である。マルテンサイトの面積率が３％未満であると、降伏比を０．５５以下に低減させることが困難になり、一方、２０％を超えると降伏比は低くなるものの、降伏強度は上昇して加工性、特に成形性及び耐面歪み性が損なわれる。より優れた成形性と耐面歪み性を得るためには、マルテンサイトの面積率の上限を１２％以下とすることが好ましい。また、マルテンサイトの面積率が低下すると、常温非時効性が劣化することがある。 The area ratio of martensite is extremely important, and by setting this in the range of 3 to 20%, it is possible to make all of the strength, yield strength, yield ratio, and strength-ductility balance good. When the area ratio of martensite is less than 3%, it is difficult to reduce the yield ratio to 0.55 or less. On the other hand, when it exceeds 20%, the yield ratio is lowered, but the yield strength is increased and the processing is increased. Properties, particularly moldability and surface distortion resistance are impaired. In order to obtain more excellent moldability and surface distortion resistance, it is preferable that the upper limit of the martensite area ratio is 12% or less. Moreover, when the area ratio of martensite falls, the non-aging property at room temperature may deteriorate.
マルテンサイトは、レペラー腐食法により、光学顕微鏡により明部として観察される。レペラー腐食法ではマルテンサイトだけでなく、残留オーステナイト相も明部として観察される。残留オーステナイトの分率は、Ｘ線回折法によって測定できるので、レペラー腐食法により、光学顕微鏡を用いてマルテンサイトの面積率を求める場合は、ポイントカウント法によって求めた明部の面積率から、Ｘ線回折法によって測定した残留オーステナイト相の分率を差し引けば良い。 Martensite is observed as a bright portion by an optical microscope by the Repeller corrosion method. In the repeller corrosion method, not only martensite but also the retained austenite phase is observed as bright parts. Since the fraction of retained austenite can be measured by the X-ray diffraction method, when determining the martensite area ratio using an optical microscope by the Repeller corrosion method, from the area ratio of the bright part determined by the point count method, X What is necessary is to subtract the fraction of retained austenite phase measured by the line diffraction method.
フェライトの面積率は、８０％未満であると、硬質第２相が増加して降伏強度及び降伏比が大きくなり、加工性、特に成形性及び耐面歪み性が劣化する。そのためフェライトの面積率の下限８０％以上に制限した。耐面歪み性の観点から、フェライトの面積率の下限を８５％以上とすることが好ましい。 If the area ratio of the ferrite is less than 80%, the hard second phase is increased, the yield strength and the yield ratio are increased, and workability, particularly formability and surface strain resistance are deteriorated. Therefore, the lower limit of the ferrite area ratio is limited to 80% or more. From the viewpoint of surface distortion resistance, the lower limit of the area ratio of ferrite is preferably 85% or more.
なお、硬質第２相は、マルテンサイト以外に、ベイナイト、残留オーステナイトの何れか一方又は双方を含むことがある。ベイナイト、残留オーステナイトの一方又は双方の面積率の合計については特に規定しないが、下限は０％であり、上限は、フェライトの面積率の下限が８０％であり、マルテンサイトの面積率の下限が３％であることから、１７％以下である。 The hard second phase may contain one or both of bainite and retained austenite in addition to martensite. Although the total area ratio of one or both of bainite and retained austenite is not particularly specified, the lower limit is 0%, and the upper limit is 80% of the lower area ratio of ferrite, and the lower limit of the area ratio of martensite is Since it is 3%, it is 17% or less.
フェライトは（１）焼鈍中に再結晶したフェライト（再結晶フェライトという。）、（２）焼鈍中に再結晶が完了せずに回復し、フェライト粒内に転位セル構造が残存したフェライト（未再結晶フェライトという。）、（３）焼鈍中にオーステテナイト相から変態によって生じたポリゴナル状のフェライト（変態フェライトという。）、の３種のフェライトからなる。フェライトのうち、未再結晶フェライトの分率が面積％で１０％を超えると降伏強度及び降伏比が増大し、成形性及び耐面歪み性が悪化することがある。そのため、未再結晶フェライトの分率の上限を１０％以下とすることが好ましい。より優れた成形性と耐面歪み性を得るためには、未再結晶フェライトの分率を５％以下とすることがより好ましい。 Ferrites are (1) ferrite recrystallized during annealing (referred to as recrystallized ferrite), and (2) ferrite that recovered without being recrystallized during annealing, and the dislocation cell structure remained in the ferrite grains (unrecrystallized). (3) Three types of ferrite, namely, a polygonal ferrite (referred to as transformation ferrite) generated by transformation from the austenite phase during annealing. Among the ferrites, if the fraction of unrecrystallized ferrite exceeds 10% in area%, the yield strength and yield ratio increase, and the formability and surface strain resistance may deteriorate. Therefore, it is preferable that the upper limit of the fraction of non-recrystallized ferrite is 10% or less. In order to obtain more excellent moldability and surface distortion resistance, it is more preferable to set the fraction of non-recrystallized ferrite to 5% or less.
未再結晶フェライトの分率とは、未再結晶フェライト、再結晶フェライト、変態フェライトの分率の合計を面積率で１００％とした場合の未再結晶フェライトの分率である。未再結晶フェライトとその他のフェライト、即ち、再結晶フェライト及び変態フェライトのとの判別は、ＥＢＳＰによって行えば良く、光学顕微鏡の組織写真と照合して、未再結晶フェライトの面積を求め、未再結晶フェライトの面積をフェライトの面積の合計で除し、百分率として、未再結晶フェライトの面積率を表せば良い。フェライトの面積の合計とは、再結晶フェライトの面積、未再結晶フェライト面積、変態フェライトの面積の合計であり、これらは、光学顕微鏡の組織写真によって硬質第２相と判別することができる。 The fraction of non-recrystallized ferrite is the fraction of non-recrystallized ferrite when the sum of the fractions of non-recrystallized ferrite, recrystallized ferrite, and transformed ferrite is 100% in area ratio. Discrimination between unrecrystallized ferrite and other ferrites, that is, recrystallized ferrite and transformed ferrite, may be performed by EBSP. The area of the unrecrystallized ferrite is obtained by comparing with the microstructure photograph of the optical microscope. The area ratio of non-recrystallized ferrite may be expressed as a percentage by dividing the area of crystalline ferrite by the total area of ferrite. The total area of ferrite is the total area of recrystallized ferrite, non-recrystallized ferrite, and area of transformed ferrite, and these can be distinguished from the hard second phase by a micrograph of an optical microscope.
未再結晶フェライトとその他のフェライト、即ち再結晶フェライト及び変態フェライトとの判別は、Ｅｌｅｃｔｒｏｎ Ｂａｃｋ Ｓｃａｔｔｅｒ ｄｉｆｆｒａｃｔｉｏｎ Ｐａｔｔｅｒｎ（ＥＢＳＰという）の結晶方位測定データをＫｅｒｎｅｌ Ａｖｅｒａｇｅ Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ法（ＫＡＭ法という）で解析することによって行えば良い。 Discrimination between unrecrystallized ferrite and other ferrites, that is, recrystallized ferrite and transformed ferrite, is made by analyzing crystal orientation measurement data of Electron Back Scatter diffraction Pattern (referred to as EBSP) by the Kernel Average Misoration method (referred to as KAM method) Just do it.
ＥＢＳＰの結晶方位測定に供する試料は、機械研磨などによって鋼板を所定の板厚まで減厚し、次いで電解研磨などによって歪みを除去すると同時に板厚１／４面が測定面となるように作製すれば良い。ＥＢＳＰによる結晶方位の測定は、測定間隔を０．２μｍとし、任意の板断面の板厚方向の１／４厚の位置で１００×１００μｍの範囲において行えば良く、測定点はピクセルとして出力される。 Samples to be used for EBSP crystal orientation measurement are prepared so that the steel plate is reduced to a predetermined thickness by mechanical polishing, etc., and then the strain is removed by electrolytic polishing, etc., and at the same time, the 1/4 thickness is the measurement surface. It ’s fine. The crystal orientation measurement by EBSP may be performed in a range of 100 × 100 μm at a position of ¼ thickness in the thickness direction of any plate cross section with a measurement interval of 0.2 μm, and the measurement points are output as pixels. .
ＥＢＳＰによって結晶粒毎の結晶方位を測定すると、未再結晶フェライトでは、比較的連続的な結晶方位変化が粒内に存在し、再結晶フェライト及び変態フェライトの粒内では結晶方位の変化が極めて小さい。ＫＡＭ法では、隣接したピクセル（隣接測定点という。）との結晶方位差を定量的に示すことが可能であり、隣接測定点との平均結晶方位差が１°超である領域を未再結晶フェライトと定義する。 When the crystal orientation for each crystal grain is measured by EBSP, a relatively continuous crystal orientation change exists in the grains in the non-recrystallized ferrite, and the crystal orientation changes are extremely small in the grains of the recrystallized ferrite and the transformed ferrite. . In the KAM method, it is possible to quantitatively indicate the crystal orientation difference between adjacent pixels (referred to as adjacent measurement points), and an area where the average crystal orientation difference from the adjacent measurement points exceeds 1 ° is not recrystallized. Defined as ferrite.
フェライト粒径は、再結晶フェライト、未再結晶フェライト、変態フェライトを区別することなく測定されたフェライトの結晶粒径であり、５μｍ未満であると、降伏比が増加して耐面歪み性が悪化し、一方、２０μｍを超えると成形後の表面外観が劣化し、外板パネル用としては好ましくない。そのため、フェライト粒径は５〜２０μｍの範囲とすることが好ましい。 The ferrite grain size is the ferrite grain size measured without distinguishing between recrystallized ferrite, non-recrystallized ferrite, and transformed ferrite. If it is less than 5 μm, the yield ratio increases and the surface strain resistance deteriorates. On the other hand, if it exceeds 20 μm, the surface appearance after molding deteriorates, which is not preferable for an outer panel. Therefore, the ferrite particle size is preferably in the range of 5 to 20 μm.
降伏強度及び降伏比は、加工性、特に成形性と耐面歪み性の相関が強く、降伏強度が２７０ＭＰａを超え、降伏比が０．５５を超える鋼板は加工性が十分ではなく、また成形後に面歪みが発生しやすい。そのため、降伏強度及び降伏比の上限を、それぞれ、２７０ＭＰａ以下及び０．５５以下とすることが好ましい。広範な成形条件でもより優れた加工性が確保でき、面歪が発生しない、降伏強度及び降伏比の更に好ましい上限は、それぞれ、２５０ＭＰａ以下及び０．５３以下である。一方、降伏強度及び降伏比の下限を、それぞれ、１８０ＭＰａ未満及び０．４未満とすることは現状の技術では困難である。 Yield strength and yield ratio have a strong correlation between workability, particularly formability and surface strain resistance. Steel plates with yield strength exceeding 270 MPa and yield ratio exceeding 0.55 are not sufficiently workable, and after forming Surface distortion is likely to occur. Therefore, it is preferable that the upper limits of the yield strength and the yield ratio are 270 MPa or less and 0.55 or less, respectively. Further preferable upper limits of yield strength and yield ratio at which excellent workability can be secured even under a wide range of molding conditions and surface distortion does not occur are 250 MPa or less and 0.53 or less, respectively. On the other hand, it is difficult with the current technology to set the lower limits of the yield strength and the yield ratio to less than 180 MPa and less than 0.4, respectively.
２％引張予歪付加後に１７０℃×２０分時効した後の再引張り時における降伏強度（以下、単に歪み時効降伏強度ともいう。）及びＢＨ量は、成形を行い、さらに塗装焼付け処理を行った部品の耐デント性との正の相関を有するものであり、ＪＩＳ Ｇ ３１３５の附属書に記載された塗装焼付硬化試験方法に準拠して測定される歪み時効降伏荷重及びＢＨ量である。歪み時効降伏強度が３３０ＭＰａ未満である場合、ＢＨ量が５０ＭＰａ未満である場合は、耐デント性の観点から、鋼板を十分に薄肉化できないことがある。そのため、歪み時効降伏強度の下限及びＢＨ量の下限は、それぞれ、３３０ＭＰａ以上及び５０ＭＰａ以上であることが好ましい。衝撃エネルギー吸収特性を高める観点から、歪み時効降伏強度の好ましい下限は３５０ＭＰａ以上である。なお、ＢＨ量が１００ＭＰａを超えると、常温非時効性を損なうことがある。また、ＴＳの上限が５００ＭＰａであるため、歪み時効降伏強度を４５０ＭＰａ超とすることは困難である。 The yield strength at the time of re-tensioning after aging at 170 ° C. for 20 minutes after the addition of 2% tensile pre-strain (hereinafter also simply referred to as strain aging yield strength) and the amount of BH were molded and further subjected to paint baking treatment. It has a positive correlation with the dent resistance of the part, and is a strain aging yield load and BH amount measured according to the paint bake hardening test method described in the appendix of JIS G 3135. When the strain aging yield strength is less than 330 MPa and the BH content is less than 50 MPa, the steel sheet may not be sufficiently thinned from the viewpoint of dent resistance. Therefore, the lower limit of the strain aging yield strength and the lower limit of the BH amount are preferably 330 MPa or more and 50 MPa or more, respectively. From the viewpoint of improving impact energy absorption characteristics, a preferred lower limit of the strain aging yield strength is 350 MPa or more. In addition, when the amount of BH exceeds 100 MPa, normal temperature non-aging property may be impaired. Moreover, since the upper limit of TS is 500 MPa, it is difficult to make the strain aging yield strength more than 450 MPa.
強度−成形性バランスＴＳ×Ｅｌ［ＭＰａ・％］は、成形性の指標であり、１６０００未満であると成形時に破断する場合があるので、１６０００以上であることが好ましい。より厳しい成形条件でも破断しない条件として、ＴＳ×Ｅｌ［ＭＰａ・％］が１７０００以上であることがより好ましい。強度−成形性バランスは、Ａｌ量の低減、Ｃｒ添加の最適化、Ｃｒ／Ａｌ比の増加によって向上する。Ａｌ量の低減による効果は、Ａｌ系析出物量の減少による延性の向上である。また、Ｃｒ添加の最適化、Ｃｒ／Ａｌ比の増加は、マルテンサイト量の増加による降伏比の低下と相関があると考えられる。ＴＳ×Ｅｌ［ＭＰａ・％］の上限については、２００００［ＭＰａ・％］を超えることは困難である。 The strength-formability balance TS × El [MPa ·%] is an index of moldability, and if it is less than 16000, it may break at the time of molding, so it is preferably 16000 or more. It is more preferable that TS × El [MPa ·%] is 17000 or more as a condition that does not break even under more severe molding conditions. The strength-formability balance is improved by reducing the Al content, optimizing the Cr addition, and increasing the Cr / Al ratio. The effect of reducing the amount of Al is improving ductility by reducing the amount of Al-based precipitates. Further, optimization of Cr addition and increase in Cr / Al ratio are considered to correlate with a decrease in yield ratio due to an increase in the amount of martensite. About the upper limit of TS × El [MPa ·%], it is difficult to exceed 20000 [MPa ·%].
次に、製造方法の限定理由について説明する。 Next, the reason for limiting the manufacturing method will be described.
鋼の溶製は、常法によって行えば良く、熱間圧延に供する鋼片は特に限定しない。即ち、連続鋳造スラブや薄スラブキャスターなどで製造したものであれば良い。また、鋳造後、冷却された鋼片を再加熱しても良く、鋳造後、Ａｒ3温度未満に冷却することなく、熱間圧延を行う連続鋳造−直接圧延（ＣＣ−ＤＲ）のようなプロセスにも適合する。熱間圧延の温度範囲及び圧下率は特に条件を限定する必要はなく、常法によって行えば良いが、焼鈍後の結晶粒径を適正に制御するためには、仕上圧延温度はＡｒ3温度以上であることが好ましい。 The melting of steel may be performed by a conventional method, and the steel slab used for hot rolling is not particularly limited. That is, what was manufactured with the continuous casting slab, the thin slab caster, etc. should just be used. Moreover, after casting, the cooled steel slab may be reheated, and after casting, it is subjected to a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed without cooling below the Ar3 temperature. Is also suitable. The temperature range and rolling reduction of the hot rolling do not need to be particularly limited, and may be performed by a conventional method. However, in order to properly control the crystal grain size after annealing, the finish rolling temperature is Ar3 temperature or higher. Preferably there is.
熱間圧延において、仕上圧延の後、冷却し、巻き取る際の温度、即ち巻取温度の下限は３５０℃以上に制限することが好ましい。これは、巻取温度が３５０℃未満であると、冷間圧延後、焼鈍し、亜鉛めっきを施した鋼板の降伏強度が増加し、加工性と耐面歪み性が劣化することがあるためである。 In hot rolling, it is preferable to limit the temperature when cooling and winding after finish rolling, that is, the lower limit of the winding temperature, to 350 ° C. or higher. This is because if the coiling temperature is less than 350 ° C., the yield strength of the steel sheet that has been annealed and galvanized after cold rolling increases, and the workability and surface strain resistance may deteriorate. is there.
熱間圧延後、巻き取り、冷却して、酸洗し、冷間圧延を行う。冷間圧延率の条件は特に定めないが、圧下率が９０％を超えると製品の機械的性質の異方性が大きくなるので、上限を９０％以下とすることが好ましい。冷間圧延の圧下率の下限については、生産性の観点から５０％以上であることが好ましい。 After hot rolling, it is wound, cooled, pickled, and cold rolled. The conditions for the cold rolling rate are not particularly defined, but if the rolling reduction exceeds 90%, the anisotropy of the mechanical properties of the product increases, so the upper limit is preferably 90% or less. The lower limit of the cold rolling reduction is preferably 50% or more from the viewpoint of productivity.
冷間圧延後は焼鈍、冷却し、亜鉛めっきを施す。亜鉛めっきは溶融亜鉛めっき、電気亜鉛めっきの何れでも良いが、耐食性と生産性の観点から溶融亜鉛めっきが好ましく、更に生産性を向上させるには、連続焼鈍後の冷却の過程でそのまま溶融亜鉛めっきを行う連続焼鈍−溶融亜鉛めっきラインにて行うことが好ましく、そのまま合金化処理を行っても良い。また、焼鈍を連続焼鈍ラインで行い、別工程でめっきを施しても、加工性、ＢＨ性及び常温非時効性には特に影響を及ぼすものではない。 After cold rolling, it is annealed, cooled, and galvanized. Galvanization may be either hot dip galvanization or electrogalvanization, but hot dip galvanization is preferred from the viewpoint of corrosion resistance and productivity. To further improve productivity, hot dip galvanization is performed in the course of cooling after continuous annealing. It is preferable to carry out in a continuous annealing-hot dip galvanizing line, and the alloying treatment may be carried out as it is. Even if annealing is performed on a continuous annealing line and plating is performed in a separate process, workability, BH properties, and non-aging properties at room temperature are not particularly affected.
焼鈍を行う際の最高到達温度は、７２０℃未満では、加熱時に、フェライトからオーステナイトへの変態が十分ではないため、冷却後のマルテンサイトの面積率が減少し、降伏比が高くなる。また、焼鈍の最高到達温度が低いと、未再結晶フェライトが増加し、降伏比が高くなり加工性及び耐面歪み性が悪化することがある。一方、焼鈍の最高到達温度が８５０℃を超えると、オーステナイトへの変態が促進されて硬質第２相が増加し、フェライトの面積率が減少するため、降伏比が高くなる。また、焼鈍を行う際の最高到達温度が高すぎると、結晶粒径が粗大化し、通板中に鋼板が破断し、生産性が大きく低下することがある。更に、固溶Ｃ及び固溶Ｎが増加して、常温非時効性が劣化することもある。したがって焼鈍の最高到達温度の範囲を７２０〜８５０℃に制限した。 If the maximum temperature at which annealing is performed is less than 720 ° C., the transformation from ferrite to austenite is not sufficient during heating, so that the area ratio of martensite after cooling decreases and the yield ratio increases. In addition, when the maximum annealing temperature is low, non-recrystallized ferrite increases, the yield ratio becomes high, and workability and surface strain resistance may deteriorate. On the other hand, when the maximum temperature of annealing exceeds 850 ° C., transformation to austenite is promoted, the hard second phase increases, and the area ratio of ferrite decreases, so the yield ratio increases. In addition, if the maximum temperature reached during annealing is too high, the crystal grain size becomes coarse, the steel sheet breaks during threading, and the productivity may be greatly reduced. Furthermore, solid solution C and solid solution N increase, and the normal temperature non-aging property may deteriorate. Therefore, the range of the highest ultimate temperature of annealing was limited to 720-850 degreeC.
焼鈍の最高到達温度とは、焼鈍における鋼板の表面温度の上限であり、その温度に到達した後、直ちに冷却しても良く、その温度で保持しても良いが、７２０〜８５０℃の範囲内での保持時間が１０ｓ未満であると加熱時のフェライトからオーステナイトへの変態が不十分であるため、冷却後のマルテンサイトの面積率が減少し、降伏比が高くなる。そのため、焼鈍における７２０〜８５０℃の範囲内での保持時間の下限を１０ｓ以上とする。また、７２０〜８５０℃における保持時間が短いと、鋼の再結晶及び粒成長が不十分であり、組織の均一性が悪く、成形後の表面品質が劣化することがある。一方、７２０〜８５０℃の範囲内での保持時間の上限は特に定めないが、長時間であると連続焼鈍ラインにおける生産性が低下するので、３００ｓ以下とすることが好ましい。なお、７２０〜８５０℃における保持時間とは、７２０℃以上に加熱され、７２０℃未満に冷却されるまでに要する時間である。 The maximum temperature of annealing is the upper limit of the surface temperature of the steel sheet during annealing, and after reaching that temperature, it may be cooled immediately or held at that temperature, but within the range of 720-850 ° C. If the holding time is less than 10 s, the transformation from ferrite to austenite during heating is insufficient, so that the area ratio of martensite after cooling decreases and the yield ratio increases. Therefore, the lower limit of the holding time within the range of 720 to 850 ° C. in annealing is set to 10 s or more. Moreover, when the holding time in 720-850 degreeC is short, recrystallization and grain growth of steel are inadequate, the uniformity of a structure | tissue is bad, and the surface quality after shaping | molding may deteriorate. On the other hand, the upper limit of the holding time within a range of 720 to 850 ° C. is not particularly defined, but if it is a long time, productivity in the continuous annealing line is lowered, and therefore it is preferably set to 300 s or less. The holding time at 720 to 850 ° C. is the time required for heating to 720 ° C. or higher and cooling to below 720 ° C.
焼鈍後、７００℃から５００℃までの温度範囲内を３℃／ｓ以上で冷却する。これは、この温度範囲内での冷却速度が３℃／ｓ未満であるとマルテンサイト変態が抑制され、マルテンサイトの面積率が減少して降伏比が増加し、耐面歪み性が劣化するためである。冷却速度は、鋼板の表面温度が７００℃から５００℃までの所要時間によって計算され、水冷、ミスト冷却、強制空冷、ロール冷却等によって適宜制御させれば良い。冷却速度の上限については特に定めないが、鋼板の変形の防止や、鋼帯内の部位による材質変化の抑制のためには、５０℃／ｓ以下であることが好ましい。 After annealing, the temperature range from 700 ° C. to 500 ° C. is cooled at 3 ° C./s or more. This is because if the cooling rate within this temperature range is less than 3 ° C./s, the martensite transformation is suppressed, the area ratio of martensite decreases, the yield ratio increases, and the surface strain resistance deteriorates. It is. The cooling rate is calculated by the time required for the surface temperature of the steel sheet to be 700 ° C. to 500 ° C., and may be appropriately controlled by water cooling, mist cooling, forced air cooling, roll cooling, or the like. The upper limit of the cooling rate is not particularly defined, but it is preferably 50 ° C./s or less in order to prevent deformation of the steel sheet and to suppress material change due to the site in the steel strip.
連続焼鈍ラインで焼鈍を行う場合には、冷却後、過時効帯を通過することがある。この場合、鋼板の温度については特に定めないが、３５０℃を超えると降伏比が低下し、加工性と耐面歪み性が劣化する場合があることから、過時効帯を通板する際の板温度は３５０℃以下であることが好ましい。また、過時効帯の通過に要する時間は、通常、１００〜５００ｓの範囲である。 When annealing is performed in a continuous annealing line, it may pass through an overaging zone after cooling. In this case, the temperature of the steel plate is not particularly defined, but if it exceeds 350 ° C., the yield ratio decreases, and the workability and surface strain resistance may deteriorate. The temperature is preferably 350 ° C. or lower. Moreover, the time required for the passage of the overaging zone is usually in the range of 100 to 500 s.
鋼板を冷却した後、溶融亜鉛めっきを施す場合は、５５０℃以下の温度で行うことが好ましい。これは、溶融亜鉛めっきを施す際に、鋼板が加熱される温度が５５０℃を超えると、マルテンサイト相が焼き戻されて、引張強度が低下し、降伏強度が上昇して降伏比が増加し、加工性と耐面歪み性が劣化することがあるためである。なお、溶融亜鉛めっきの温度とは、めっき槽内の溶融亜鉛（めっき浴ともいう。）に入る際の鋼板の温度であり、めっき浴の温度も５５０℃以下とすることが好ましい。したがって、連続焼鈍後、鋼板は５００℃以下まで冷却されるが、そのまま溶融亜鉛めっきを施す場合、めっき浴の温度を５００℃超から５５０℃以下の範囲の任意の温度とすれば、その温度で溶融亜鉛めっきが施されることになる。 When the hot dip galvanizing is performed after cooling the steel plate, it is preferably performed at a temperature of 550 ° C. or lower. This is because when the temperature at which the steel sheet is heated exceeds 550 ° C. during hot dip galvanizing, the martensite phase is tempered, the tensile strength decreases, the yield strength increases, and the yield ratio increases. This is because workability and surface distortion resistance may deteriorate. In addition, the temperature of hot dip galvanization is the temperature of the steel plate at the time of entering the hot dip zinc (it is also called a plating bath) in a plating tank, and it is preferable that the temperature of a plating bath shall also be 550 degrees C or less. Therefore, after continuous annealing, the steel sheet is cooled to 500 ° C. or lower. However, when hot dip galvanizing is performed as it is, if the temperature of the plating bath is an arbitrary temperature in the range of more than 500 ° C. to 550 ° C. or lower, the temperature is Hot dip galvanization will be performed.
亜鉛めっき後は、耐食性をより向上させるため、合金化処理を施すことが好ましい。また、合金化処理の温度も５５０℃を超えると、マルテンサイト相が焼き戻されて、引張強度が低下し、降伏強度が上昇して降伏比が増加するため、５５０℃以下で合金化処理を施すことが好ましい。なお、生産性の観点から、合金化処理は、連続焼鈍−溶融亜鉛めっきラインに直結した合金化処理炉によって行うことが好ましい。 After galvanization, it is preferable to perform an alloying treatment in order to further improve the corrosion resistance. Also, when the temperature of the alloying treatment exceeds 550 ° C., the martensite phase is tempered, the tensile strength is lowered, the yield strength is increased, and the yield ratio is increased. Therefore, the alloying treatment is performed at 550 ° C. or less. It is preferable to apply. From the viewpoint of productivity, the alloying treatment is preferably performed in an alloying treatment furnace directly connected to the continuous annealing / hot dip galvanizing line.
更に、溶融亜鉛めっき後の冷却中、又は溶融亜鉛めっきに続いて合金化処理を行った後の冷却中にもマルテンサイト変態が起こり得る。そのため、降伏比を下げて加工性及び耐面歪み性を安定的に確保する観点から、溶融亜鉛めっき後又は合金化処理後の冷却は、少なくとも２００℃までの温度を、５℃／ｓ以上の冷却速度で行うことが好ましい。優れた耐面ひずみ性と強度−延性バランスを得るための、より好ましい冷却速度は１０℃／ｓ以上である。 Furthermore, martensitic transformation can also occur during cooling after hot dip galvanizing or during cooling after galvanizing and subsequent alloying treatment. Therefore, from the viewpoint of stably securing the workability and the surface distortion resistance by lowering the yield ratio, the cooling after the hot dip galvanization or the alloying treatment is performed at a temperature of at least 200 ° C. at 5 ° C./s or more. It is preferable to carry out at a cooling rate. A more preferable cooling rate for obtaining excellent surface strain resistance and strength-ductility balance is 10 ° C./s or more.
調質圧延は、形状矯正と表面性状確保のために行い、伸び率２％以下の範囲で行うことが好ましい。これは、伸び率が２％を超えると、ＢＨ量が低下することがあるためである。 The temper rolling is performed to correct the shape and secure the surface properties, and is preferably performed within a range of elongation of 2% or less. This is because if the elongation exceeds 2%, the amount of BH may decrease.
本発明を実施例により詳細に説明する。 The present invention will be described in detail with reference to examples.
表１に示す成分の鋼を溶製し、次いでスラブを１２００℃で再加熱し、仕上げ圧延温度８６０〜９４０℃の範囲内での熱間圧延を行い、表２に示す条件で巻取処理を行った。得られた熱延鋼板を酸洗し、７０〜８０％の冷延率で冷間圧延を行い、板厚０．８ｍｍの鋼板とした。これらの鋼板に、表２に示した条件で、焼鈍、亜鉛めっき、合金化処理を施した。なお、熱間圧延及び焼鈍において、鋼板の表面の温度を、放射温度計によって測定した。 Steel of the components shown in Table 1 is melted, and then the slab is reheated at 1200 ° C., hot rolling is performed within the range of the finish rolling temperature of 860 to 940 ° C., and the winding process is performed under the conditions shown in Table 2. went. The obtained hot-rolled steel sheet was pickled and cold-rolled at a cold rolling rate of 70 to 80% to obtain a steel sheet having a thickness of 0.8 mm. These steel sheets were annealed, galvanized, and alloyed under the conditions shown in Table 2. In the hot rolling and annealing, the surface temperature of the steel sheet was measured with a radiation thermometer.
亜鉛めっきライン又は連続焼鈍−溶融亜鉛めっきラインの直後に設けた合金化炉により、表２の合金化処理温度欄に示した温度で１５ｓ保持する合金化処理を行った。表２において、合金化処理欄が空欄のものは、合金化処理を行わなかった例である。なお、連続焼鈍工程における過時効帯を通過した際の鋼板の温度は３００℃である。更に、全ての亜鉛めっき鋼板に０．６％の伸び率で調質圧延を施した。 An alloying treatment was performed for 15 seconds at the temperature shown in the alloying treatment temperature column of Table 2 by an alloying furnace provided immediately after the galvanizing line or the continuous annealing-hot galvanizing line. In Table 2, the case where the alloying treatment column is blank is an example in which the alloying treatment was not performed. In addition, the temperature of the steel plate at the time of passing through the overaging zone in the continuous annealing process is 300 ° C. Furthermore, all the galvanized steel sheets were subjected to temper rolling at an elongation rate of 0.6%.
表２の設備欄には、焼鈍を連続焼鈍ラインで行った後、亜鉛めっきを亜鉛めっきラインで行ったものをＣＡ、焼鈍及び亜鉛めっきを連続焼鈍−溶融亜鉛めっきラインで行ったものをＣＧとして示した。表２において、ＣＴ［℃］は熱間圧延における巻取温度、ＳＴ［℃］は焼鈍の最高到達温度、ｔ１［ｓ］は焼鈍における加熱及び冷却の際に、７２０〜８５０℃の温度範囲に保持された時間である。また、ＣＲ１［℃／ｓ］は、焼鈍後、７００℃から５００℃までの冷却速度であり、７００℃と５００℃との温度差、即ち２００℃を、７００℃から５００℃の間の冷却に要した時間によって除して求めた。 In the equipment column of Table 2, after performing annealing on the continuous annealing line, CA was performed by galvanizing on the galvanizing line, and CG was performed by performing annealing and galvanizing on the continuous annealing-hot galvanizing line. Indicated. In Table 2, CT [° C.] is the coiling temperature in hot rolling, ST [° C.] is the highest annealing temperature, and t 1 [s] is in the temperature range of 720 to 850 ° C. during heating and cooling in annealing. It is the held time. CR1 [° C./s] is a cooling rate from 700 ° C. to 500 ° C. after annealing, and a temperature difference between 700 ° C. and 500 ° C., ie, 200 ° C., is cooled between 700 ° C. and 500 ° C. Calculated by dividing by the time required.
また、亜鉛めっき温度［℃］は溶融亜鉛めっき浴の温度であり、合金化処理温度［℃］は、合金化炉の炉内温度である。ＣＲ２［℃／ｓ］は、合金化処理を行わないものは亜鉛めっきの温度から２００℃までの冷却速度であり、合金化処理を行ったものは合金化処理の温度から２００℃までの冷却速度である。ＣＲ２［℃／ｓ］は、亜鉛めっき温度又は合金化処理温度と２００℃との温度を、亜鉛めっき又は合金化処理の終了から２００℃までの冷却に要した時間によって除して求めた。 The galvanizing temperature [° C.] is the temperature of the hot dip galvanizing bath, and the alloying treatment temperature [° C.] is the furnace temperature of the alloying furnace. CR2 [° C./s] is the cooling rate from the temperature of galvanizing to 200 ° C. when not alloyed, and the cooling rate from the temperature of alloying to 200 ° C. when alloyed. It is. CR2 [° C./s] was obtained by dividing the galvanizing temperature or alloying treatment temperature and the temperature of 200 ° C. by the time required for cooling from the end of the galvanizing or alloying treatment to 200 ° C.
これらの亜鉛めっき鋼板の金属組織の観察、面積率及び結晶粒径の測定を、ＪＩＳ Ｇ ０５５１に準拠して行った。マルテンサイト及びフェライトの面積率を測定し、フェライト粒径を測定した。Ｌ断面を観察面とし、鏡面研磨した後にレペラー腐食法により腐食を行い、光学顕微鏡により観察を行った際に明部として観察される部分の面積率をポイントカウント法によって測定した。ポイントカウント法によって求めた明部の面積率から、Ｘ線回折法によって測定した残留オーステナイト相の分率を差し引くことによりマルテンサイトの面積率を計算した。フェライトの面積率は、Ｌ断面を鏡面研磨した後にナイタールにより腐食を行い、光学顕微鏡により観察を行いその分率をポイントカウント法によって測定した。また、ＥＢＳＰにより、未再結晶フェライトと、再結晶フェライト及び変態フェライトとを判別し、未再結晶フェライトの面積率を求めた。また、Ｘ線回折法によって測定した亜鉛めっき鋼板の残留オーステナイトの分率は、すべて３％以下であった。 Observation of the metal structure of these galvanized steel sheets, measurement of the area ratio, and crystal grain size were performed in accordance with JIS G 0551. The area ratio of martensite and ferrite was measured, and the ferrite particle size was measured. The L section was used as an observation surface, and after mirror polishing, corrosion was performed by a repeller corrosion method, and the area ratio of a portion observed as a bright portion when observed by an optical microscope was measured by a point count method. The area ratio of martensite was calculated by subtracting the fraction of retained austenite phase measured by the X-ray diffraction method from the area ratio of the bright part determined by the point count method. The area ratio of ferrite was mirror-polished on the L section, then corroded with nital, observed with an optical microscope, and the fraction was measured by the point count method. Further, EBSP was used to discriminate unrecrystallized ferrite from recrystallized ferrite and transformed ferrite, and the area ratio of unrecrystallized ferrite was determined. Moreover, the fraction of the retained austenite of the galvanized steel sheet measured by the X-ray diffraction method was all 3% or less.
結果を表３に示す。なお、表３には、フェライトの面積率をフェライト面積率［％］、マルテンサイトの面積率をマルテンサイト面積率［％］として示した。これらの合計が１００％にならない鋼の金属組織の残部は、その他の組織として示した。表３において、試料Ｎｏ．５及び１０のその他組織の欄は、フェライトの面積率とマルテンサイトの面積率の合計が１００％であるため、空欄である。 The results are shown in Table 3. In Table 3, the area ratio of ferrite is shown as ferrite area ratio [%], and the area ratio of martensite is shown as martensite area ratio [%]. The remainder of the steel metallographic structure that does not add up to 100% is shown as other structures. In Table 3, sample no. The columns of other structures 5 and 10 are blank because the sum of the area ratio of ferrite and the area ratio of martensite is 100%.
更に、亜鉛めっき鋼板から、ＪＩＳ Ｚ ２２０１の５号試験片を採取し、ＪＩＳ Ｚ ２２４１に準拠して、歪み速度１０-3／ｓの条件で引張試験を行った。なお、亜鉛めっき鋼板の採取から引張試験までの間、常温に保持される期間が１ヶ月以内になるように、試験片の採取から引張試験までの間、−２０℃以下に保持した。常温非時効性は、１００℃に３６００ｓ保持する促進時効を行った後、ＪＩＳ Ｚ ２２４１に準拠して、引張試験を行い、降伏点伸び量により評価した。また、ＪＩＳ Ｇ ３１３５の附属書に記載された塗装焼付硬化試験方法に準拠して歪み時効降伏強度及びＢＨ量を評価した。結果を表４に示す。表４において、ＹＳ［ＭＰａ］、ＴＳ［ＭＰａ］、Ｅｌ[％]は、それぞれ、降伏強度、引張強度、全伸びである。時効後ＹＰＥ[％]は、１００℃に３６００ｓ保持する促進時効を行った後の引張試験によって評価した降伏点伸び量である。また、ＵＢＨ［ＭＰａ］及び歪時効ＹＳ［ＭＰａ］は、ＪＩＳ Ｇ ３１３５の附属書に記載された塗装焼付硬化試験方法に準拠して評価した、歪み時効降伏強度及びＢＨ量である。 Further, a JIS Z 2201 No. 5 test piece was collected from the galvanized steel sheet and subjected to a tensile test under the condition of a strain rate of 10 −3 / s according to JIS Z 2241. In addition, it was hold | maintained at -20 degrees C or less from the collection of a test piece to a tensile test so that the period hold | maintained at normal temperature may be within one month from collection | recovery of a galvanized steel plate to a tensile test. The room temperature non-aging property was evaluated by a yield point elongation after performing a tensile aging in accordance with JIS Z 2241 after performing accelerated aging at 100 ° C. for 3600 s. Further, the strain aging yield strength and the BH amount were evaluated in accordance with the paint bake hardening test method described in the appendix of JIS G 3135. The results are shown in Table 4. In Table 4, YS [MPa], TS [MPa], and El [%] are yield strength, tensile strength, and total elongation, respectively. YPE [%] after aging is the yield point elongation evaluated by the tensile test after performing the accelerated aging which hold | maintains for 3600 s at 100 degreeC. Further, UBH [MPa] and strain aging YS [MPa] are strain aging yield strength and BH amount evaluated according to the paint bake hardening test method described in the appendix of JIS G 3135.
表４において、試料Ｎｏ．１〜３及びＮｏ．８〜２３は、本発明例であり、ＹＳが２５０ＭＰａ以下と降伏強度が低く、ＴＳが３９０〜５００ＭＰａの範囲内と高強度であり、かつ、ＹＲが０．５３以下と低降伏比が低く、ＴＳ×Ｅｌが１７０００ＭＰａ・％以上と強度−延性バランスに優れている。更に、ＢＨ量が５０ＭＰａ以上及び歪み時効降伏強度が３３０ＭＰａ以上と、塗装焼付硬化性にも優れ、１００℃で６０分保持し、常温時効を促進させて評価した時効後の降伏点伸び量が０％であり、常温非時効性も確保されている。なお、試料Ｎｏ．３は熱延の巻取温度が好ましい下限よりも低いため、粒径が若干小さくなり、降伏強度及び降伏比が高めであり、ＴＳ×Ｅｌが１７０００ＭＰａ・％以下になっている。試料Ｎｏ．９は、合金化温度が好ましい上限よりも高いため、降伏比が若干高くなっている。 In Table 4, Sample No. 1-3 and No.1. 8-23 is an example of the present invention, YS is 250 MPa or less and yield strength is low, TS is within a range of 390 to 500 MPa and high strength, and YR is 0.53 or less and low yield ratio is low, TS × El is 17000 MPa ·% or more, which is excellent in strength-ductility balance. Furthermore, the BH amount is 50 MPa or more and the strain aging yield strength is 330 MPa or more, and it is excellent in paint bake hardenability, and is maintained at 100 ° C. for 60 minutes, and the yield point elongation after aging evaluated by promoting normal temperature aging is 0. %, And room temperature non-aging property is also secured. Sample No. In No. 3, since the coiling temperature for hot rolling is lower than the preferred lower limit, the particle size is slightly reduced, the yield strength and the yield ratio are high, and TS × El is 17000 MPa ·% or less. Sample No. No. 9 has a slightly higher yield ratio because the alloying temperature is higher than the preferred upper limit.
一方、試料Ｎｏ．４〜７は、製造条件が本発明の範囲外であり、試料Ｎｏ．２４〜２８は、成分が本発明の範囲外であるため、特性が劣化した比較例である。試料Ｎｏ．４は、焼鈍の最高到達温度が本発明の範囲より高く、フェライト面積率が減少し、硬質第２相が増加しており、降伏強度及び降伏比が高くなった例である。また、焼鈍の最高到達温度が高いため、固溶Ｃ及びＮ量が増加し、常温非時効性も劣化している。試料Ｎｏ．５は、焼鈍温度が本発明の範囲より低く、マルテンサイトの面積率が減少し、粒径がやや小さく、未再結晶フェライト分率も増加しているため、降伏強度及び降伏比が高い。試料Ｎｏ．６は、焼鈍の際の７２０〜８５０℃の範囲での保持時間が本発明の範囲より短いため、マルテンサイトの面積率が減少し、降伏比が上昇した例である。試料Ｎｏ．７は、焼鈍後の冷却速度が本発明の範囲より遅いため、マルテンサイトの面積率が減少し、降伏比が上昇した例である。また、試料Ｎｏ．５〜７は、マルテンサイトの面積率が低いために、常温非時効性も低下している。 On the other hand, sample No. Nos. 4 to 7 have production conditions outside the scope of the present invention. 24 to 28 are comparative examples in which the characteristics were deteriorated because the components were outside the scope of the present invention. Sample No. No. 4 is an example in which the maximum annealing temperature is higher than the range of the present invention, the ferrite area ratio is decreased, the hard second phase is increased, and the yield strength and yield ratio are increased. Moreover, since the highest temperature reached by annealing is high, the amount of dissolved C and N increases, and the non-aging property at room temperature is also deteriorated. Sample No. No. 5 has a higher yield strength and yield ratio because the annealing temperature is lower than the range of the present invention, the area ratio of martensite is decreased, the particle size is slightly small, and the fraction of unrecrystallized ferrite is increased. Sample No. No. 6 is an example in which the retention time in the range of 720 to 850 ° C. during annealing is shorter than the range of the present invention, so that the martensite area ratio decreases and the yield ratio increases. Sample No. No. 7 is an example in which the area ratio of martensite decreased and the yield ratio increased because the cooling rate after annealing was slower than the range of the present invention. Sample No. In Nos. 5 to 7, since the area ratio of martensite is low, the non-aging property at room temperature is also lowered.
また、試料Ｎｏ．２４は、Ｃｒ量及びＣｒ／Ａｌ比が本発明の範囲より低く、試料Ｎｏ．２６は、Ａｌ量が本発明の範囲より高く、Ｃｒ／Ａｌ比が本発明の範囲より低いため、マルテンサイトの面積率が低下し、降伏比が高くなった例である。試料Ｎｏ．２５は、Ｍｎ量が本発明の範囲より低く、マルテンサイトの面積率が低下し、降伏比が上昇した例である。試料Ｎｏ．２７は、Ｎ量が本発明の範囲よりも多く、降伏比が上昇し、常温非時効性が劣化した例である。試料Ｎｏ．２８は、Ｃ量が本発明の範囲よりも多く、引張強度の増加とともに降伏強度も増加し、常温非時効性も劣化し、更に、Ａｌ量が本発明の範囲より高く、Ｃｒ／Ａｌ比が本発明の範囲より低いため、強度−延性バランスも低下している。 Sample No. No. 24 has a Cr amount and a Cr / Al ratio lower than the range of the present invention. No. 26 is an example in which the Al ratio is higher than the range of the present invention and the Cr / Al ratio is lower than the range of the present invention, so that the area ratio of martensite is decreased and the yield ratio is increased. Sample No. No. 25 is an example in which the amount of Mn is lower than the range of the present invention, the area ratio of martensite is decreased, and the yield ratio is increased. Sample No. No. 27 is an example in which the N amount is larger than the range of the present invention, the yield ratio is increased, and the non-aging property at room temperature is deteriorated. Sample No. No. 28, the amount of C is larger than the range of the present invention, the yield strength increases with the increase in tensile strength, the non-aging property at room temperature deteriorates, the Al amount is higher than the range of the present invention, and the Cr / Al ratio is Since it is lower than the range of the present invention, the strength-ductility balance is also lowered.
表５に示す成分の鋼を溶製し、巻取処理を表６に示す条件とし、表６に示した条件で、焼鈍、亜鉛めっき、合金化処理を施し、板厚０．８ｍｍの鋼板を製造した。なお、表６に記載のない製造条件は、実施例１と同様である。これらの鋼板のマルテンサイト及びフェライトの面積率、フェライト粒径の測定、残部組織の同定、未再結晶フェライトの面積率の測定を、実施例１と同様にして行い、結果を表７に示した。なお、Ｘ線回折法によって測定した残留オーステナイトの分率は、すべて５％以下であった。 Steels having the components shown in Table 5 were melted, and the winding treatment was performed under the conditions shown in Table 6. Under the conditions shown in Table 6, annealing, galvanizing, and alloying treatment were performed, and a steel plate having a thickness of 0.8 mm was obtained. Manufactured. The manufacturing conditions not listed in Table 6 are the same as those in Example 1. The area ratio of martensite and ferrite of these steel sheets, the measurement of ferrite grain size, the identification of the remaining structure, and the measurement of the area ratio of unrecrystallized ferrite were performed in the same manner as in Example 1, and the results are shown in Table 7. . Note that the fraction of retained austenite measured by the X-ray diffraction method was 5% or less.
更に、引張試験、常温非時効性、歪み時効降伏強度及びＢＨ量の評価も実施例１と同様にして行い、結果を表８に示した。表８において、試料Ｎｏ．１０１、１０２及びＮｏ．１０４〜１１３は、ＴＳが５００〜６００ＭＰａの範囲内の本発明例であり、低降伏比が低く、強度−延性バランス、塗装焼付硬化性にも優れ、常温非時効性も確保されている。一方、試料Ｎｏ．１０３は、焼鈍の最高到達温度が本発明の範囲より高く、フェライト面積率が減少し、降伏比が高くなった例である。
Claims (15)
Ｃ ：０．０２〜０．０８％、
Ｓｉ：０．５％以下、
Ｍｎ：１．０〜２．５％、
Ｐ ：０．０５％以下、
Ｓ ：０．０２％以下、
Ａｌ：０．０００５〜０．０１４％、
Ｃｒ：０．２％超、１．５％以下、
Ｎ ：０．００１〜０．００８％
を含み、残部がＦｅ及び不可避的不純物からなり、
Ｃｒ／Ａｌ：３０以上
を満足し、金属組織におけるマルテンサイトの面積率が３〜２０％であり、フェライトの面積率が８０％以上であることを特徴とする加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。 % By mass
C: 0.02 to 0.08%,
Si: 0.5% or less,
Mn: 1.0 to 2.5%
P: 0.05% or less,
S: 0.02% or less,
Al: 0.0005 to 0.014%,
Cr: more than 0.2%, 1.5% or less,
N: 0.001 to 0.008%
And the balance consists of Fe and inevitable impurities,
Cr / Al: satisfying 30 or more, martensite area ratio in metal structure is 3 to 20%, ferrite area ratio is 80% or more, workability, paint bake hardenability and room temperature High strength galvanized steel sheet with excellent non-aging properties.
Ｂ ：０．０００３〜０．００３質量％
を含有することを特徴とする請求項１記載の加工性、塗装焼付硬化性及び常温非時効性に優れた高強度亜鉛めっき鋼板。 further,
B: 0.0003 to 0.003 mass%
The high-strength galvanized steel sheet excellent in workability, paint bake hardenability and non-aging at room temperature according to claim 1.
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
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| JP2009228079A (en) * | 2008-03-24 | 2009-10-08 | Nisshin Steel Co Ltd | Zn-Al-Mg BASED STEEL SHEET HAVING EXCELLENT HOT DIP METAL EMBRITTLEMENT CRACK RESISTANCE AND METHOD FOR PRODUCING THE SAME |
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| US8888933B2 (en) | 2009-05-27 | 2014-11-18 | Nippon Steel & Sumitomo Metal Corporation | High-strength steel sheet, hot-dipped steel sheet, and alloy hot-dipped steel sheet that have excellent fatigue, elongation, and collision characteristics, and manufacturing method for said steel sheets |
| US8911567B2 (en) | 2009-11-09 | 2014-12-16 | Nippon Steel & Sumitomo Metal Corporation | High-strength steel sheet having excellent processability and paint bake hardenability, and method for producing of high-strength steel sheet |
| WO2016043273A1 (en) * | 2014-09-17 | 2016-03-24 | 新日鐵住金株式会社 | Hot-rolled steel sheet |
| KR20160089316A (en) * | 2016-07-18 | 2016-07-27 | 주식회사 포스코 | High strength hot rolled steel sheet having excellent bake hardenability and low yield ratio and method for manufacturing thereof |
| JP2019512600A (en) * | 2016-03-08 | 2019-05-16 | ポスコＰｏｓｃｏ | Hot-dip galvanized steel sheet excellent in bake hardenability and aging resistance and method for producing the same |
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| JP2009035817A (en) * | 2007-07-11 | 2009-02-19 | Jfe Steel Kk | High-strength hot-dip galvanized steel sheet with low yield strength and with high baking hardenability and process for producing the same |
| JP2009228079A (en) * | 2008-03-24 | 2009-10-08 | Nisshin Steel Co Ltd | Zn-Al-Mg BASED STEEL SHEET HAVING EXCELLENT HOT DIP METAL EMBRITTLEMENT CRACK RESISTANCE AND METHOD FOR PRODUCING THE SAME |
| JP2009228080A (en) * | 2008-03-24 | 2009-10-08 | Nisshin Steel Co Ltd | HIGH-YIELD RATIO TYPE Zn-Al-Mg BASED STEEL SHEET HAVING EXCELLENT HOT DIP METAL EMBRITTLEMENT CRACK RESISTANCE AND METHOD FOR PRODUCING THE SAME |
| JP2009270125A (en) * | 2008-04-08 | 2009-11-19 | Sumitomo Metal Ind Ltd | Zinc-based hot dip plated steel sheet |
| KR101009796B1 (en) | 2008-06-26 | 2011-01-21 | 현대제철 주식회사 | Method for manufacturing high-strength hot-dip galvanized steel sheet |
| US9297060B2 (en) | 2009-02-02 | 2016-03-29 | Jfe Steel Corporation | High strength galvanized steel sheet and method for manufacturing the same |
| WO2010087529A1 (en) | 2009-02-02 | 2010-08-05 | Ｊｆｅスチール株式会社 | High-strength hot-dip galvanized steel sheet and manufacturing method therefor |
| US8636852B2 (en) | 2009-02-02 | 2014-01-28 | Jfe Steel Corporation | High strength galvanized steel sheet and method for manufacturing the same |
| US8888933B2 (en) | 2009-05-27 | 2014-11-18 | Nippon Steel & Sumitomo Metal Corporation | High-strength steel sheet, hot-dipped steel sheet, and alloy hot-dipped steel sheet that have excellent fatigue, elongation, and collision characteristics, and manufacturing method for said steel sheets |
| KR101149117B1 (en) * | 2009-06-26 | 2012-05-25 | 현대제철 주식회사 | Steel sheet having excellent low yield ratio property, and method for producing the same |
| US9255318B2 (en) | 2009-06-26 | 2016-02-09 | Jfe Steel Corporation | High-steel galvanized steel sheet and method for manufacturing the same |
| WO2010150919A1 (en) | 2009-06-26 | 2010-12-29 | Ｊｆｅスチール株式会社 | High-strength molten zinc-plated steel sheet and process for production thereof |
| US8911567B2 (en) | 2009-11-09 | 2014-12-16 | Nippon Steel & Sumitomo Metal Corporation | High-strength steel sheet having excellent processability and paint bake hardenability, and method for producing of high-strength steel sheet |
| JP2012052157A (en) * | 2010-08-31 | 2012-03-15 | Jfe Steel Corp | Material for warm press forming, and method of manufacturing member for panel |
| KR101225246B1 (en) * | 2010-09-29 | 2013-01-22 | 현대하이스코 주식회사 | High strength cold-rolled dual phase steel sheet for automobile with excellent formability and method of manufacturing the cold-rolled multi phase steel sheet |
| WO2016043273A1 (en) * | 2014-09-17 | 2016-03-24 | 新日鐵住金株式会社 | Hot-rolled steel sheet |
| CN106715742A (en) * | 2014-09-17 | 2017-05-24 | 新日铁住金株式会社 | Hot-rolled steel sheet |
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| US10655192B2 (en) | 2014-09-17 | 2020-05-19 | Nippon Steel Corporation | Hot-rolled steel sheet |
| JP2019512600A (en) * | 2016-03-08 | 2019-05-16 | ポスコＰｏｓｃｏ | Hot-dip galvanized steel sheet excellent in bake hardenability and aging resistance and method for producing the same |
| KR20160089316A (en) * | 2016-07-18 | 2016-07-27 | 주식회사 포스코 | High strength hot rolled steel sheet having excellent bake hardenability and low yield ratio and method for manufacturing thereof |
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