JP4964494B2 - High-strength steel sheet excellent in hole expansibility and formability and method for producing the same - Google Patents
High-strength steel sheet excellent in hole expansibility and formability and method for producing the same Download PDFInfo
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Description
本発明は、最大引張強度（ＴＳ）が５９０ＭＰａ以上で、穴拡げ性に優れた自動車用の構造用部材、補強用部材、足廻り用部材などに特に適した高張力冷延鋼板、高強度溶融亜鉛めっき鋼板、高強度合金化溶融亜鉛めっき鋼板などの高強度鋼板及びその製造方法に関するものである。
なお、本発明における高強度鋼板とは、通常の冷延鋼板のほか、亜鉛めっき鋼板やＡｌめっき鋼板、電気めっき鋼板に代表される各種めっき鋼板を含む。また、めっき層中には、純亜鉛に加え、Ｆｅ、Ａｌ、Ｍｇ、Ｍｎ、Ｓｉ、Ｃｒなどを含有しても構わない。
The present invention has a maximum tensile strength (TS) of 590 MPa or more, a high-tensile cold-rolled steel sheet particularly suitable for automotive structural members, reinforcing members, suspension members, etc., which has excellent hole expandability, high-strength melting The present invention relates to a high-strength steel sheet such as a galvanized steel sheet and a high-strength galvannealed steel sheet and a method for producing the same.
In addition, the high-strength steel plate in this invention contains the various cold-rolled steel plates and the various plated steel plates represented by the galvanized steel plate, the Al-plated steel plate, and the electroplated steel plate. In addition to pure zinc, the plating layer may contain Fe, Al, Mg, Mn, Si, Cr, and the like.
自動車のクロスメンバーやサイドメンバー等の部材は、近年の燃費軽量化の動向に対応すべく軽量化が検討されており、材料面では、薄肉化しても強度及び衝突安全性が確保されるという観点から鋼板の高強度化が進められている。しかしながら、材料の成形性は強度が上昇するのに伴って劣化するので、上記部材の軽量化を実現するには、プレス成形性と高強度の両方を満足する鋼板を製造する必要がある。主に自動車の構造用部材や補強用部材に使用される高強度鋼板は、曲げ性、穴拡げ性、延性に優れることが要求される。特に、複雑形状を有する部品の成形には、伸びや穴拡げといった個別の特性が優れているだけでなく、その両方が優れていることが求められる。 Lightweight parts of automobile cross members and side members are being studied in order to respond to recent trends in lighter fuel consumption, and in terms of materials, the strength and collision safety can be ensured even if they are made thinner. Since then, the strength of steel sheets has been increasing. However, since the formability of the material deteriorates as the strength increases, it is necessary to manufacture a steel sheet that satisfies both the press formability and high strength in order to realize the weight reduction of the member. High-strength steel sheets used mainly for automobile structural members and reinforcing members are required to be excellent in bendability, hole expansibility, and ductility. In particular, molding of a part having a complicated shape requires not only excellent individual characteristics such as elongation and hole expansion, but also excellent both.
一般的に、少ない合金含有量で鋼板強度を効率的に向上させる手法としては、延性に富んだ軟質なフェライト組織中に、硬質なマルテンサイトや残留オーステナイトを分散させ、強化させたＤＰ鋼やＴＲＩＰ鋼がある。しかしながら、第２相にマルテンサイトや残留オーステナイトを活用した場合、穴拡げ性が著しく低下してしまうという問題がある（例えば、非特許文献１を参照）。このように強度と穴拡げ性の両立は難しい。 In general, as a technique for efficiently improving the steel sheet strength with a small alloy content, hard martensite and retained austenite are dispersed in a soft ferrite structure rich in ductility, and strengthened DP steel or TRIP. There is steel. However, when martensite or retained austenite is used for the second phase, there is a problem that the hole expandability is significantly lowered (see, for example, Non-Patent Document 1). Thus, it is difficult to achieve both strength and hole expandability.
このような課題に対し、強度と穴拡げ性を両立させる手法としては、主相をベイナイト組織とすることで、優れた穴拡げ性及び強度を具備する手法が、下記特許文献１に開示されている。しかしながら、この方法では主相がベイナイト組織であるために、伸びも低いことから成形性に劣るという問題を有していた。加えて、７８０ＭＰａ級の強度確保を考えた場合、ＣやＭｎの添加量を抑える必要があることから、焼入れ性が劣化し、主相をベイナイト組織とすることが難しいという問題を有している。 For such a problem, as a technique for achieving both strength and hole expansibility, a technique having excellent hole expansibility and strength by making the main phase a bainite structure is disclosed in Patent Document 1 below. Yes. However, this method has a problem that since the main phase is a bainite structure and the elongation is low, the formability is poor. In addition, when considering securing the strength of the 780 MPa class, it is necessary to suppress the addition amount of C and Mn, so that the hardenability deteriorates and it is difficult to make the main phase into a bainite structure. .
これに対し、主相であるフェライトと硬質組織であるマルテンサイトの硬度差を低減させる手法が、下記非特許文献１に開示されている。マルテンサイトとフェライトの硬度差低減には、マルテンサイトを形成させるための焼き入れと、焼き戻しが必要であることから、低温への急冷と再加熱が必須である。しかしながら、高温からの焼入れを行うと、フェライト中には多量の固溶Ｃを含むことになり、フェライトを強化し、フェライトの特徴である優れた延性を損なうことになり、穴拡げ性には優れるもののその特徴である優れた成形性を減じてしまうという課題を有している。 On the other hand, Non-Patent Document 1 below discloses a technique for reducing the hardness difference between ferrite as a main phase and martensite as a hard structure. In order to reduce the hardness difference between martensite and ferrite, quenching and tempering to form martensite are necessary, and thus rapid cooling to low temperature and reheating are essential. However, when quenching from a high temperature, the ferrite contains a large amount of solid solution C, strengthens the ferrite, impairs the excellent ductility that is characteristic of ferrite, and excels in hole expansibility. However, there is a problem that the excellent moldability that is a characteristic of the material is reduced.
あるいは、水焼入れを行わずに、Ｍｓ（マルテンサイト変態開始温度）以下へと冷却した後、２００〜４００℃に再加熱する手法が下記特許文献２示されている。しかしながら、Ｍｓ以下に焼き入れたとしても、マルテンサイト変態は完了していないことから、残ったオーステナイトは、その後の熱処理にても焼き戻されずに、熱処理後再度室温まで冷却される過程で初めてマルテンサイトへと変態する。これら焼き戻し後に形成したマルテンサイトは硬く、穴拡げ性に悪影響を及ぼすことから、穴拡げ性の更なる向上には課題があった。また、フェライト中に固溶Ｃの増大に伴う延性劣化の問題を回避できない。加えて、組織をフェライト及びマルテンサイトよりなる組織とするため、高温からの急冷が必須であり、大幅な設備投資が必要であるという問題を有していた。その結果、めっき鋼板への適用は限られていた。 Alternatively, a method of reheating to 200 to 400 ° C. after cooling to Ms (martensitic transformation start temperature) or less without performing water quenching is shown in Patent Document 2 below. However, even if quenched below Ms, the martensitic transformation has not been completed, so that the remaining austenite is not tempered in the subsequent heat treatment and is only martensified in the process of being cooled to room temperature after the heat treatment. Transform into a site. Since the martensite formed after tempering is hard and adversely affects the hole expandability, there has been a problem in further improving the hole expandability. Moreover, the problem of ductility deterioration accompanying the increase of the solid solution C in the ferrite cannot be avoided. In addition, since the structure is made of ferrite and martensite, rapid cooling from a high temperature is essential, and a large capital investment is required. As a result, application to plated steel sheets has been limited.
これに対し、硬質組織の硬度を低減させるのではなく、Ｓｉの固溶強化を用いて、軟質組織の硬度を増加させることで穴拡げ性を向上させる手法（例えば、非特許文献１を参照）、あるいは、ＴｉやＮｂを単独又は複合添加することで、フェライト組織を析出強化し穴拡げ性を向上させる手法（例えば、非特許文献２を参照）がある。しかしながら、Ｓｉの固溶強化能は限られていることから、Ｓｉの固溶強化のみで強度確保を行う場合、５９０ＭＰａ以上の強度確保のためには、多量のＳｉ添加が必要であり、化成性、めっき性及び溶接性を低下させるという問題を有している。 On the other hand, instead of reducing the hardness of the hard tissue, a technique for improving hole expansibility by increasing the hardness of the soft tissue using solid solution strengthening of Si (see, for example, Non-Patent Document 1). Alternatively, there is a method (for example, see Non-Patent Document 2) in which, by adding Ti or Nb alone or in combination, the ferrite structure is precipitation strengthened and the hole expandability is improved. However, since the solid solution strengthening ability of Si is limited, when securing strength only by solid solution strengthening of Si, a large amount of Si must be added to secure strength of 590 MPa or more. , Have the problem of reducing the plating and weldability.
一方、ＴｉやＮｂ添加による穴拡げ性の向上は、ＴｉやＮｂの炭窒化物の析出が熱延時に起こることから、熱延鋼板にての活用は可能なものの、その後、冷延−熱処理を行う冷延鋼板やめっき鋼板では、その析出強化能が低下することから多量のＴｉやＮｂ添加が必要であり、経済性に劣る。あるいは、析出強化は、軟質なフェライトを硬化させることから、延性の劣化が大きいという問題を有していた。
そこで、本発明は、このような従来の事情に鑑みて提案されたものであり、最大引張強度（ＴＳ）が５９０ＭＰａ以上で、穴拡げ性に優れた高張力冷延鋼板、高強度溶融亜鉛めっき鋼板、高強度合金化溶融亜鉛めっき鋼板などの高強度鋼板高強度鋼板及びその製造方法を提供することを目的とする。 Therefore, the present invention has been proposed in view of such conventional circumstances, and has a maximum tensile strength (TS) of 590 MPa or more, a high-tensile cold-rolled steel sheet excellent in hole expansibility, and a high-strength hot-dip galvanizing. An object is to provide a high-strength steel plate such as a steel plate, a high-strength galvannealed steel plate, a high-strength steel plate, and a method for producing the same.
本発明者らは、上記の目的を達成すべく鋭意検討を重ねた結果、穴拡げ性と成形性を具備するためには、焼入れと焼入れ後のマルテンサイトの焼き戻しに加え、焼き入れ前でのフェライト中の固溶Ｃ量の制御が重要であることを見出した。従来は、より高温から急冷を行うことで組織制御を行ってきたが、従来とは逆に、７５０〜６２０℃間をゆっくりと冷却することで、フェライト中にＣを濃化させ、焼き戻し時にこのＣを炭化物として析出させることで、延性を向上させることができることを見出した。同時に、鋼板成分を制御することで、低冷速でもフェライト及びマルテンサイトよりなる組織にできる。この結果、フェライトの優れた延性と、焼き戻しマルテンサイトを用いた強度と穴拡げ性の両立が容易に達成可能となった。 As a result of intensive studies to achieve the above-mentioned object, the present inventors, in order to have hole expandability and formability, in addition to quenching and tempering of martensite after quenching, before quenching It was found that it is important to control the amount of dissolved C in ferrite. Conventionally, the structure has been controlled by quenching from a higher temperature, but conversely, by slowly cooling between 750 and 620 ° C., C is concentrated in the ferrite and tempered. It has been found that the ductility can be improved by precipitating C as a carbide. At the same time, by controlling the steel plate components, a structure composed of ferrite and martensite can be obtained even at a low cooling speed. As a result, it was possible to easily achieve both excellent ductility of ferrite and strength and hole expansibility using tempered martensite.
すなわち、本発明は穴拡げ性と成形性に優れた高強度鋼板及びその製造方法であって、その要旨は以下の通りである。
（１） 質量％で、
Ｃ：０．０５〜０．２５％未満、
Ｓｉ：２．０％以下、
Ｍｎ：１．１〜３．０％、
Ｐ：０．０４％以下、
Ｓ：０．０１％以下、
Ａｌ：２．０％以下、
Ｎ：０．０１％以下、
Ｏ：０．０１％以下
を含有し、残部がＦｅ及び不可避不純物からなり、鋼板のミクロ組織が体積分率で、４０％以上のフェライトと、５％以上の焼き戻しマルテンサイトとを含有し、ダイナミック硬度計を用いて測定したフェライトの硬度（ＤＨＴＦ）とダイナミック硬度計を用いて測定した焼き戻しマルテンサイトの硬度（ＤＨＴＭ）の比（ＤＨＴＭ／ＤＨＴＦ）が１．５〜３．０であり、残部組織がベイナイト組織よりなることを特徴とする引張最大強度が５９０ＭＰａ以上の穴拡げ性と成形性に優れた高強度鋼板。
（２） 前記フェライト組織中に含まれるＣ量が０．００４％以上であることを特徴とする（１）に記載の穴拡げ性と成形性に優れた高強度鋼板。
（３） さらに、鋼中に質量％で、
Ｍｏ：０．０５〜３％、
Ｃｒ：０．０５〜３％、
Ｎｉ：０．０５〜３％、
Ｃｕ：０．０５〜３％、
Ｗ：０．０５〜３％
の１種又は２種以上を含有することを特徴とする（１）又は（２）に記載の穴拡げ性と成形性に優れた高強度鋼板。
（４） さらに、鋼中に質量％で、
Ｔｉ：０．００５〜０．３％、
Ｎｂ：０．００５〜０．３％、
Ｖ：０．００５〜０．３％
の１種又は２種以上を含有することを特徴とする（１）乃至（３）の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板。
（５） さらに、鋼中に質量％で、
Ｂ：０．０００１〜０．１％
を含有することを特徴とする（１）乃至（４）の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板。
（６） さらに、鋼中に質量％で、
Ｃａ：０．０００５〜０．０１％、
Ｍｇ：０．０００５〜０．０１％、
Ｚｒ：０．０００５〜０．０１％、
ＲＥＭ：０．０００５〜０．０１％、
Ｙ：０．０００５〜０．０１％
の１種又は２種以上を含有することを特徴とする（１）乃至（５）の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板。
（７） （１）乃至（６）の何れかに記載の化学成分からなる鋳造スラブを直接又は一旦冷却した後１１００℃以上に加熱し、Ａｒ３変態点以上で熱間圧延を完了し、６３０℃以下の温度域にて巻き取り、酸洗後、圧下率４０〜７０％の冷延を施し、連続焼鈍ラインを通板するに際して、７５０〜９００℃で焼鈍した後、平均冷却速度を１２℃／秒以下で焼鈍温度〜６２０℃間を冷却し、６２０℃〜５００℃間を平均冷却速度４〜５０℃／秒で（Ｍｓ−５０）℃以下まで冷却し、その後、２００〜６００℃で１０秒以上焼鈍することを特徴とする（１）乃至（６）の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板の製造方法。
（８） （１）乃至（６）の何れかに記載の化学成分からなる鋳造スラブを直接又は一旦冷却した後１１００℃以上に加熱し、Ａｒ３変態点以上で熱間圧延を完了し、６３０℃以下の温度域にて巻き取り、酸洗後、圧下率４０〜７０％の冷延を施し、連続溶融亜鉛めっきラインを通板するに際して、７５０〜９００℃で焼鈍し、平均冷却速度を１２℃／秒以下で焼鈍温度〜６２０℃間を冷却し、６２０℃〜５００℃間を平均冷却速度４〜５０℃／秒で（亜鉛めっき浴温度−４０）℃〜（亜鉛めっき浴温度＋５０）℃に冷却後、亜鉛めっき浴に浸漬し、（Ｍｓ−５０）℃以下まで冷却し、その後、２００〜６００℃で１０秒以上焼鈍することを（１）乃至（６）の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板の製造方法。
（９） （１）乃至（６）の何れかに記載の穴拡げ性と成形性に優れた高強度冷延鋼板に溶融亜鉛めっき処理を施した高強度溶融亜鉛めっき鋼板の製造方法であって、（１）乃至（６）の何れかに記載の化学成分からなる鋳造スラブを直接又は一旦冷却した後１１００℃以上に加熱し、Ａｒ３変態点以上で熱間圧延を完了し、６３０℃以下の温度域にて巻き取り、酸洗後、圧下率４０〜７０％の冷延を施し、連続溶融亜鉛めっきラインを通板するに際して、７５０〜９００℃で焼鈍し、平均冷却速度を１２℃／秒以下で焼鈍温度〜６２０℃間を冷却し、６２０℃〜５００℃間を平均冷却速度４〜５０℃／秒で（亜鉛めっき浴温度−４０）℃〜（亜鉛めっき浴温度＋５０）℃に冷却後、亜鉛めっき浴に浸漬し、必要に応じて４６０℃以上の温度で合金化処理を施し、（Ｍｓ−５０）℃以下まで冷却し、その後、２００〜６００℃で１０秒以上焼鈍することを特徴とする（１）乃至（６）の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板の製造方法。
That is, the present invention is a high-strength steel plate excellent in hole expansibility and formability and a method for producing the same, and the gist thereof is as follows.
(1) In mass%,
C: 0.05 to less than 0.25%,
Si: 2.0% or less,
Mn: 1.1 to 3.0%
P: 0.04% or less,
S: 0.01% or less,
Al: 2.0% or less,
N: 0.01% or less,
O: 0.01% or less, the balance is Fe and inevitable impurities, the microstructure of the steel sheet is 40% or more ferrite and 5% or more tempered martensite in volume fraction, the ratio of the dynamic hardness meter ferrite was measured using a hardness (DHTF) and the hardness of the dynamic hardness meter tempered martensite was measured using (DHTM) (DHTM / DHTF) is 1.5 to 3.0, A high-strength steel sheet excellent in hole expansibility and formability having a maximum tensile strength of 590 MPa or more, wherein the remaining structure is a bainite structure .
(2) The high-strength steel sheet excellent in hole expansibility and formability according to (1), wherein the amount of C contained in the ferrite structure is 0.004% or more.
(3) Furthermore, in steel,
Mo: 0.05-3%,
Cr: 0.05-3%,
Ni: 0.05-3%,
Cu: 0.05-3%,
W: 0.05-3%
A high-strength steel sheet excellent in hole expansibility and formability according to (1) or (2), characterized by containing one or more of the above.
(4) Furthermore, in steel,
Ti: 0.005 to 0.3%,
Nb: 0.005-0.3%
V: 0.005-0.3%
A high-strength steel sheet excellent in hole expansibility and formability according to any one of (1) to (3), characterized by containing one or more of the above.
(5) Furthermore, in mass% in steel,
B: 0.0001 to 0.1%
A high-strength steel sheet excellent in hole expansibility and formability as described in any one of (1) to (4).
(6) Furthermore, in the steel by mass%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
Zr: 0.0005 to 0.01%,
REM: 0.0005 to 0.01%,
Y: 0.0005 to 0.01%
The high-strength steel sheet excellent in hole expansibility and formability according to any one of (1) to (5), characterized by containing one or more of the following.
(7) The cast slab made of the chemical component according to any one of (1) to (6) is directly or once cooled and then heated to 1100 ° C. or higher, and the hot rolling is completed at or above the Ar3 transformation point, to 630 ° C. Winding in the following temperature range, pickling, cold rolling with a rolling reduction of 40 to 70%, passing through a continuous annealing line, annealing at 750 to 900 ° C, then the average cooling rate of 12 ° C / The annealing temperature is cooled to 620 ° C. in less than a second, the temperature is cooled to 620 ° C. to 500 ° C. at an average cooling rate of 4 to 50 ° C./second to (Ms-50) ° C. or less, and then 200 to 600 ° C. The method for producing a high-strength steel sheet excellent in hole expansibility and formability according to any one of (1) to (6) , characterized by annealing as described above .
(8) The cast slab composed of the chemical component according to any one of (1) to (6) is directly or once cooled and then heated to 1100 ° C. or higher, and the hot rolling is completed at the Ar3 transformation point or higher, and 630 ° C. Winding at the following temperature range, pickling, cold rolling with a rolling reduction of 40 to 70%, and passing through a continuous hot dip galvanizing line, annealing at 750 to 900 ° C, and an average cooling rate of 12 ° C / Sec or less, the annealing temperature is cooled to 620 ° C., and the temperature between 620 ° C. and 500 ° C. is reduced to an average cooling rate of 4 to 50 ° C./second (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. After cooling, immersing in a galvanizing bath, cooling to (Ms-50) ° C. or lower, and then annealing at 200 to 600 ° C. for 10 seconds or longer , the hole expansion according to any one of (1) to (6) Of high-strength steel sheet with excellent properties and formability.
(9) A method for producing a high-strength hot-dip galvanized steel sheet obtained by subjecting a high-strength cold-rolled steel sheet excellent in hole expansibility and formability according to any one of (1) to (6) to hot-dip galvanizing treatment. The casting slab comprising the chemical component according to any one of (1) to (6) is directly or once cooled and then heated to 1100 ° C. or higher, and the hot rolling is completed at the Ar3 transformation point or higher, and the casting slab is 630 ° C. or lower. Winding in a temperature range, pickling, cold rolling with a rolling reduction of 40 to 70%, and passing through a continuous hot dip galvanizing line, annealing at 750 to 900 ° C., and an average cooling rate of 12 ° C./second After cooling between annealing temperature and 620 ° C., after cooling between 620 ° C. and 500 ° C. at an average cooling rate of 4 to 50 ° C./sec to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. Soak in a galvanizing bath and, if necessary, a temperature of 460 ° C or higher Subjected to alloying treatment, (Ms-50) ° C. and cooled to below, then, hole expandability according to any one of, characterized in that annealing for 10 seconds or more at 200 to 600 ° C. (1) to (6) And manufacturing method of high strength steel sheet with excellent formability.
以上のように、本発明によれば、自動車用の構造用部材、補強用部材、足廻り用部材などに好適な最大引張強度（ＴＳ）が５９０ＭＰａ以上の高強度と、穴拡げ性と成形性に優れた成形加工性及び溶接性を兼備する高張力冷延鋼板、高強度溶融亜鉛めっき鋼板、高強度合金化溶融亜鉛めっき鋼板などの高強度鋼板及びその製造方法を安価に提供することができる。 As described above, according to the present invention, the maximum tensile strength (TS) suitable for automobile structural members, reinforcing members, suspension members and the like is high strength of 590 MPa or more, hole expansibility and formability. High strength steel sheets such as high-tensile cold-rolled steel sheets, high-strength hot-dip galvanized steel sheets, high-strength galvannealed steel sheets, and methods for producing the same can be provided at low cost. .
以下、本発明を適用した穴拡げ性と成形性に優れた高強度鋼板及びその製造方法について詳細に説明する。
本発明者らは、上記課題を解決すべく鋭意検討を行った。まず、５９０ＭＰａ以上の強度と優れた穴拡げ性及び延性を両立させるには、ミクロ組織にて、フェライトを４０％以上、焼き戻しマルテンサイトを５％以上含有する必要がある。特に、フェライトの優れた延性を確保するためには、フェライト中に含まれるＣ量を０．００４％以上とすることが重要である。また、マルテンサイトの焼き戻しを行うにあたっては、一旦、鋼板組織をフェライトとマルテンサイトよりなる組織とした後、２００〜６００℃の温度域で焼き戻しを行い、フェライトの硬度（ＤＨＴＦ）とマルテンサイト硬度（ＤＨＴＭ）の硬度比（ＤＨＴＭ／ＤＨＴＦ）を１．５〜３．０の範囲に制御することで行われる。ここで、ＤＨＴＦ及びＤＨＴＭは、ダイナミック硬度計を用いて測定される値であり、硬度測定時の圧子の押し込み深さより求められる値である。
Hereinafter, a high-strength steel sheet excellent in hole expansibility and formability to which the present invention is applied and a manufacturing method thereof will be described in detail.
The present inventors have intensively studied to solve the above problems. First, in order to achieve both a strength of 590 MPa or more and excellent hole expandability and ductility, it is necessary to contain 40% or more of ferrite and 5% or more of tempered martensite in the microstructure. In particular, in order to ensure excellent ductility of ferrite, it is important that the amount of C contained in the ferrite is 0.004% or more. When tempering martensite, the steel sheet structure is once made of ferrite and martensite, and then tempered at a temperature range of 200 to 600 ° C. to determine the ferrite hardness (DHTF) and martensite. It is performed by controlling the hardness ratio (DHTM / DHTF) of the hardness (DHTM) to a range of 1.5 to 3.0. Here, DHTF and DHTM are values measured using a dynamic hardness meter, and are values obtained from the indentation depth of the indenter at the time of hardness measurement.
次に、引張最大強度の限定理由について説明する。
引張最大強度を５９０ＭＰａ以上としたのは、５９０ＭＰａ以上のハイテンにて、穴拡げ性や成形性（伸びと穴拡げ性バランス）の劣化が顕著となるためである。特に、５９０ＭＰａ以上のハイテンにおいては、マルテンサイトや残留オーステナイトを用いた組織強化が行われる場合が多く、特に、穴拡げ性の劣化が顕著である。そこで、引張最大強度の限定理由を５９０ＭＰａ以上とした。
Next, the reason for limiting the maximum tensile strength will be described.
The reason why the maximum tensile strength is set to 590 MPa or more is that hole expandability and moldability (elongation and hole expandability balance) are significantly deteriorated with high tensile strength of 590 MPa or more. In particular, high tensile strength of 590 MPa or more often involves strengthening the structure using martensite or retained austenite, and the deterioration of hole expansibility is particularly significant. Therefore, the reason for limiting the maximum tensile strength is set to 590 MPa or more.
次に、ミクロ組織の限定理由について述べる。
フェライトの体積分率を４０％以上としたのは、軟質なフェライトを用いて伸びを確保するためである。体積分率が４０％未満では、優れた延性の確保が行えないことから、その下限を４０％とした。一方で、体積分率が９５％超となると、強度確保に活用している焼き戻しマルテンサイトの体積分率を５％以上とすることができないことからその上限は９５％である。
Next, the reason for limiting the microstructure will be described.
The reason why the volume fraction of ferrite is set to 40% or more is to ensure elongation by using soft ferrite. If the volume fraction is less than 40%, excellent ductility cannot be ensured, so the lower limit was made 40%. On the other hand, if the volume fraction exceeds 95%, the upper limit is 95% because the volume fraction of tempered martensite used for securing the strength cannot be 5% or more.
焼き戻しマルテンサイトの体積分率を５％以上としたのは、焼き戻しマルテンサイトの体積率が５％未満であると、５９０ＭＰａ以上の強度確保が難しいためである。一方では、マルテンサイトは焼き戻しを行ったとしても、転位を多く含むことから延性に乏しく、過剰な体積率の増加は、延性の低下をもたらすことから好ましくない。 The reason why the volume fraction of tempered martensite is 5% or more is that it is difficult to ensure a strength of 590 MPa or more when the volume fraction of tempered martensite is less than 5%. On the other hand, even if tempering is performed, martensite is poor in ductility because it contains many dislocations, and an excessive increase in volume fraction is not preferable because it causes a decrease in ductility.
鋼板中に含まれる主相であるフェライトと焼き戻しマルテンサイト以外の各組織の分率は特に定めることなく、本発明の効果は発揮される。なお、フェライト相の形態としてはポリゴナルフェライトの他に、アシキラーフェライト、回復した未再結晶フェライトを含むものとする。鋼板の高強度化を目的に、ベイナイト組織を含有してもよい。一方で、鋼板中にオーステナイトが残留すると、切断あるいは打抜き加工等の強加工を受けた場合、硬質なマルテンサイトへと変態し、穴拡げ性を劣化させる原因となることから、体積分率を３％未満とすることが望ましい。 The effect of the present invention is exhibited without particularly determining the fraction of each structure other than ferrite and tempered martensite, which are the main phases contained in the steel sheet. In addition, as a form of the ferrite phase, in addition to polygonal ferrite, it is assumed that it includes acicular ferrite and recovered non-recrystallized ferrite. For the purpose of increasing the strength of the steel sheet, it may contain a bainite structure. On the other hand, if austenite remains in the steel sheet, it undergoes transformation to hard martensite when subjected to strong processing such as cutting or punching, which causes deterioration of hole expandability. It is desirable to make it less than%.
なお、上記ミクロ組織、フェライト、マルテンサイト、ベイナイト、オーステナイト及び残部組織の同定、存在位置の観察及び面積率の測定は、ナイタール試薬及び特開５９−２１９４７３号公報に開示された試薬により鋼板圧延方向断面または圧延方向直角方向断面を腐食して、１０００倍の光学顕微鏡観察及び１０００〜１０００００倍の走査型及び透過型電子顕微鏡により定量化が可能である。加えて、焼き戻しマルテンサイトの同定に関しては、走査型及び透過型電子顕微鏡により観察を行い内部にＦｅを多く含有する炭化物を含むものを焼き戻しマルテンサイト、炭化物をほとんど含まないものをマルテンサイトとした。Ｆｅを多く含む炭化物としては、様々な結晶構造を有するものが報告されているが、本焼き戻しの目的がマルテンサイトの軟化にあることから、何れのＦｅ系炭化物を含有しても構わない。また、焼き戻し条件によっては、複数種のＦｅ系炭化物が存在する場合があった。 The microstructure, ferrite, martensite, bainite, austenite, and the remaining structure are identified, the position is observed, and the area ratio is measured in the rolling direction of the steel sheet using the Nital reagent and the reagent disclosed in JP-A-59-219473. The cross section or the cross section in the direction perpendicular to the rolling direction is corroded, and quantification is possible by observation with an optical microscope of 1000 times and scanning and transmission electron microscopes of 1000 to 100,000 times. In addition, with regard to the identification of tempered martensite, what was observed with a scanning and transmission electron microscope and contained carbide containing a large amount of Fe inside was tempered martensite, and what contained almost no carbide was martensite. did. As carbides containing a large amount of Fe, those having various crystal structures have been reported. However, since the purpose of this tempering is to soften martensite, any Fe-based carbide may be contained. Further, depending on the tempering conditions, there are cases where plural types of Fe-based carbides exist.
フェライト中のＣ含有率を０．００４％以上としたのは、Ｃの固溶強化やＦｅ系の炭化物析出によるフェライトの延性の劣化を抑制するためである。フェライト中のＣ含有量とは、一つのフェライト粒内に含まれる固溶ＣとセメンタイトをはじめとするＦｅ系炭化物中に含まれるＣの合計質量％を意味する。固溶Ｃの増加はフェライトの延性を低下させることから、フェライト中の固溶Ｃ量を低下させることで、延性を確保することが必要である。本鋼は、焼鈍後の冷却条件を制御することで、焼き入れ前のフェライト中のＣ量を増加させ、引き続いて行われる焼き戻し処理時に炭化物として析出させることで、固溶Ｃを減少させている。フェライト中のＣ量が少ないと、炭化物析出に長時間を要することから好ましくない。そこで、下限値を０．００４％以上とした。また、フェライト中に含まれる固溶Ｃは時効劣化を通じて延性を劣化させることからも望ましくない。ただし、これら炭化物の一部は、フェライトとマルテンサイトの界面に存在する場合があるが、これらはマルテンサイト側に析出しているものとみなした。 The reason why the C content in the ferrite is set to 0.004% or more is to suppress deterioration of the ductility of the ferrite due to solid solution strengthening of C and precipitation of Fe-based carbides. The C content in ferrite means the total mass% of C contained in Fe-based carbides including solid solution C and cementite contained in one ferrite grain. Since the increase in the solid solution C decreases the ductility of the ferrite, it is necessary to ensure the ductility by reducing the amount of the solid solution C in the ferrite. This steel increases the amount of C in the ferrite before quenching by controlling the cooling conditions after annealing, and reduces solid solution C by precipitating as carbide during the subsequent tempering treatment. Yes. If the amount of C in the ferrite is small, it is not preferable because it takes a long time for carbide precipitation. Therefore, the lower limit is set to 0.004% or more. Further, solute C contained in ferrite is not desirable because it deteriorates ductility through aging deterioration. However, some of these carbides may exist at the interface between ferrite and martensite, but these were considered to be precipitated on the martensite side.
なお、上記フェライト粒内に含まれるＣ含有量は、３次元アトムプローブによる測定や、透過型電子顕微鏡とエネルギー分散型Ｘ線分析装置（ＥＤＳ）を行うことで、フェライト粒内のＣ量の分析はできる。特に、３次元アトムプローブを用いた測定は、原子の存在位置の同定も可能であることから、炭化物として存在しているか、あるいは、固溶しているかの判別も可能である。 In addition, the C content contained in the ferrite grain is analyzed by measuring with a three-dimensional atom probe or by performing a transmission electron microscope and an energy dispersive X-ray analyzer (EDS). I can. In particular, since measurement using a three-dimensional atom probe can also identify the location of an atom, it can also be determined whether it is present as a carbide or solid solution.
マルテンサイトの硬度（ＤＨＴＭ）とフェライトの硬度（ＤＨＴＦ）の比である（ＤＨＴＭ／ＤＨＴＦ）の上限を３．０としたのは、両組織の間の硬度差が大きいと界面に変形が集中し、マイクロボイド形成の起点となり、穴拡げ性を大幅に劣化させてしまう。一方では、硬度差の比が１．５を下回ると、５９０ＭＰａ以上の強度確保が難しい。このことかららその下限を１．５と定めた。 The upper limit of (DHTM / DHTF), which is the ratio of the hardness of martensite (DHTM) to the hardness of ferrite (DHTF), is set to 3.0. If the hardness difference between the two structures is large, deformation is concentrated on the interface. As a starting point of microvoid formation, the hole expandability is greatly deteriorated. On the other hand, when the hardness difference ratio is less than 1.5, it is difficult to secure a strength of 590 MPa or more. From this, the lower limit was set to 1.5.
フェライトの結晶粒径については特に限定しないが、強度と伸びのバランスの観点から公称粒径で７μｍ以下であることが望ましい。 The crystal grain size of ferrite is not particularly limited, but the nominal grain size is preferably 7 μm or less from the viewpoint of the balance between strength and elongation.
次に、成分の限定理由について説明する。なお、％は質量％を意味する。
（Ｃ：０．０５〜０．２５％未満）
Ｃは、鋼板の強度を上昇できる元素である。しかし、０．０５％未満であると体積率５％以上の焼き戻しマルテンサイト体積率を確保することができず、５９０ＭＰａ以上の引張強度の確保が難しくなる。一方、０．２５％以上となるとスポット溶接性の確保が困難となる。このため、その範囲を０．０５〜０．２５％未満に限定した。しかしながら、部材の接合を、ボルト等を用いた機械的な接合を行うのであれば、０．２５％以上含有したとしても問題はない。
Next, the reasons for limiting the components will be described. In addition,% means the mass%.
(C: 0.05 to less than 0.25%)
C is an element that can increase the strength of the steel sheet. However, if it is less than 0.05%, a tempered martensite volume ratio of 5% or more cannot be ensured, and it becomes difficult to ensure a tensile strength of 590 MPa or more. On the other hand, if it is 0.25% or more, it is difficult to ensure spot weldability. For this reason, the range was limited to 0.05 to less than 0.25%. However, if the members are joined mechanically using bolts or the like, there is no problem even if the content is 0.25% or more.
（Ｓｉ：２．０％以下）
Ｓｉは、強化元素であり、鋼板の強度を上昇させることから添加してもよい。しかしながら、２．０％超の添加は、成形性が低下することからその上限を２．０％とした。下限は、特に限定しないが、０．０００５％以下とするのは困難であるのでこれが実質的な下限である。
(Si: 2.0% or less)
Si is a strengthening element and may be added because it increases the strength of the steel sheet. However, addition exceeding 2.0% lowers the moldability, so the upper limit was made 2.0%. The lower limit is not particularly limited, but it is difficult to set the lower limit to 0.0005% or less, which is a practical lower limit.
（Ｍｎ：１．１〜３．０％）
Ｍｎは、強化元素であり、鋼板の強度を上昇させることに有効である。しかしながら、３．０％超となると鋼板の成形性が低下することからその上限を３．０％とした。１．１％未満では、冷却時にパーライト組織や炭化物が出てしまい焼き戻しマルテンサイトの体積率を５％以上とすることができない。このことから、下限を５％とした。特に、本鋼は、フェライト中に含まれるＣを低減させるため、７５０〜６２０℃間を比較的低い冷却速度にて冷却を行うことから、高温域から急冷を行う鋼に比較して、かなりパーライト変態や炭化物の析出が起こりやすい。このことから、Ｍｎの添加は特に重要である。
(Mn: 1.1-3.0%)
Mn is a strengthening element and is effective in increasing the strength of the steel sheet. However, if it exceeds 3.0%, the formability of the steel sheet decreases, so the upper limit was made 3.0%. If it is less than 1.1%, a pearlite structure or carbide is produced during cooling, and the volume ratio of tempered martensite cannot be made 5% or more. For this reason, the lower limit was made 5%. In particular, since this steel cools between 750 and 620 ° C. at a relatively low cooling rate in order to reduce C contained in ferrite, it is considerably more pearlite than steel that is rapidly cooled from a high temperature range. Transformation and carbide precipitation are likely to occur. For this reason, the addition of Mn is particularly important.
（Ａｌ：２．０％以下）
Ａｌは、フェライト形成を促進し、延性を向上させるので添加してもよい。しかしながら、過剰な添加は成形性を劣化させることから、その上限を２．０％とした。下限は、特に限定しないが、０．０００５％以下とするのは困難であるのでこれが実質的な下限である。また、脱酸材としても活用可能である。
(Al: 2.0% or less)
Al may be added because it promotes ferrite formation and improves ductility. However, excessive addition deteriorates moldability, so the upper limit was made 2.0%. The lower limit is not particularly limited, but it is difficult to set the lower limit to 0.0005% or less, which is a practical lower limit. It can also be used as a deoxidizer.
（Ｐ：０．０４％以下）
Ｐは、鋼板の板厚中央部に偏析する傾向があり、溶接部を脆化させる。０．０４％を超えると溶接部の脆化が顕著になるため、その適正範囲を０．０４％以下に限定した。Ｐの下限値は特に定めないが、０．０００１％未満とすることは、経済的に不利であることからこの値を下限値とすることが好ましい。
(P: 0.04% or less)
P tends to segregate in the central part of the plate thickness of the steel sheet, causing the weld to become brittle. When the content exceeds 0.04%, the weld becomes brittle, so the appropriate range is limited to 0.04% or less. Although the lower limit value of P is not particularly defined, it is preferable to set this value as the lower limit value because it is economically disadvantageous to set it to less than 0.0001%.
（Ｓ：０．０１％以下）
Ｓは、溶接性並びに鋳造時及び熱延時の製造性に悪影響を及ぼす。このことから、その上限値を０．０１％以下とした。Ｓの下限値は特に定めないが、０．０００１％未満とすることは、経済的に不利であることからこの値を下限値とすることが好ましい。また、ＳはＭｎと結びついて粗大なＭｎＳを形成することから、穴拡げ性を低下させる。このことから、穴拡げ性向上のためには、できるだけ少なくする必要がある。
(S: 0.01% or less)
S adversely affects weldability and manufacturability during casting and hot rolling. Therefore, the upper limit is set to 0.01% or less. Although the lower limit of S is not particularly defined, it is preferable to set this value as the lower limit because it is economically disadvantageous to make it less than 0.0001%. In addition, since S is combined with Mn to form coarse MnS, the hole expandability is lowered. For this reason, it is necessary to reduce as much as possible in order to improve hole expansibility.
（Ｎ：０．０１％以下）
Ｎは、粗大な窒化物を形成し、曲げ性や穴拡げ性を劣化させることから、添加量を抑える必要がある。これは、Ｎが０．０１％を超えると、この傾向が顕著となることから、Ｎ含有量の範囲を０．０１％以下とした。加えて、溶接時のブローホール発生の原因になることから少ない方がよい。下限は、特に定めることなく本発明の効果は発揮されるが、Ｎ含有量を０．０００５％未満とすることは、製造コストの大幅な増加を招くことから、これが実質的な下限である。
(N: 0.01% or less)
N forms coarse nitrides and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. This is because when N exceeds 0.01%, this tendency becomes remarkable. Therefore, the range of N content is set to 0.01% or less. In addition, it is better to reduce the number of blowholes during welding. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, if the N content is less than 0.0005%, the manufacturing cost is significantly increased, and this is a substantial lower limit.
（Ｏ：０．０１％以下）
Ｏは、酸化物を形成し、成形性を劣化させることから、添加量を抑える必要がある。特に、Ｏが０．０１％を超えると、この傾向が顕著となることから、Ｏ含有量の上限を０．０１％以下とした。０．００１％未満とすることは、過度のコスト高を招き経済的に好ましくないことから、これを下限とした。
(O: 0.01% or less)
O forms an oxide and degrades the moldability, so the amount added must be suppressed. In particular, when O exceeds 0.01%, this tendency becomes remarkable, so the upper limit of the O content is set to 0.01% or less. Setting it to less than 0.001% invites excessive cost and is not economically preferable, so this was made the lower limit.
（Ｃｒ：０．０５〜３％）
Ｃｒは、強化元素であるとともに焼入れ性の向上に重要である。しかし、０．０５％未満ではこれらの効果が得られないため下限値を０．０５％とした。逆に、３％超含有すると製造時及び熱延時の製造性に悪影響を及ぼすため、上限値を３％とした。
(Cr: 0.05-3%)
Cr is a strengthening element and is important for improving hardenability. However, if it is less than 0.05%, these effects cannot be obtained, so the lower limit was made 0.05%. On the other hand, if the content exceeds 3%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was made 3%.
（Ｎｉ：０．０５〜３％）
Ｎｉは、強化元素であるとともに焼入れ性の向上に重要である。しかし、０．０５％未満ではこれらの効果が得られないため下限値を０．０５％とした。逆に、３％超含有すると製造時及び熱延時の製造性に悪影響を及ぼすため、上限値を３％とした。
(Ni: 0.05-3%)
Ni is a strengthening element and is important for improving hardenability. However, if it is less than 0.05%, these effects cannot be obtained, so the lower limit was made 0.05%. On the other hand, if the content exceeds 3%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was made 3%.
（Ｃｕ：：０．０５〜３％）
Ｃｕは、強化元素であるとともに焼入れ性の向上に重要である。しかし、０．０５％未満ではこれらの効果が得られないため下限値を０．０５％とした。逆に、３％超含有すると製造時及び熱延時の製造性に悪影響を及ぼすため、上限値を１％とした。
(Cu :: 0.05-3%)
Cu is a strengthening element and is important for improving hardenability. However, if it is less than 0.05%, these effects cannot be obtained, so the lower limit was made 0.05%. Conversely, if the content exceeds 3%, the manufacturability at the time of production and hot rolling is adversely affected, so the upper limit was made 1%.
（Ｍｏ：０．０５〜３％）
Ｍｏは、強化元素であるとともに焼入れ性の向上に重要である。しかし、０．０５％未満ではこれらの効果が得られないため下限値を０．０５％とした。逆に、３％超含有すると製造時及び熱延時の製造性に悪影響を及ぼすため、上限値を３％とした。
(Mo: 0.05-3%)
Mo is a strengthening element and is important for improving hardenability. However, if it is less than 0.05%, these effects cannot be obtained, so the lower limit was made 0.05%. On the other hand, if the content exceeds 3%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was made 3%.
（Ｗ：０．０５〜３％）
Ｗは、強化元素であるとともに焼入れ性の向上に重要である。しかし、０．０５％未満ではこれらの効果が得られないため下限値を０．０５％とした。逆に、３％超含有すると製造時及び熱延時の製造性に悪影響を及ぼすため、上限値を３％とした。
(W: 0.05-3%)
W is a strengthening element and is important for improving hardenability. However, if it is less than 0.05%, these effects cannot be obtained, so the lower limit was made 0.05%. On the other hand, if the content exceeds 3%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was made 3%.
（Ｂ：０．０００１〜０．１％）
Ｂもまた必要に応じて添加できる。Ｂは、０．０００１質量％以上の添加で粒界の強化や鋼材の強度化をもたらすことから有効であるが、その添加量が０．１質量％を超えると、その効果が飽和するばかりでなく、熱延時の製造製を低下させることから、その上限を０．１質量％とした。
(B: 0.0001 to 0.1%)
B can also be added as needed. B is effective because the addition of 0.0001% by mass or more brings about strengthening of grain boundaries and strengthening of the steel material. However, when the addition amount exceeds 0.1% by mass, the effect is not only saturated. However, since the production at the time of hot rolling is lowered, the upper limit was made 0.1 mass%.
（Ｎｂ：０．００５〜０．３％）
Ｎｂは、強化元素である。析出物強化、フェライト結晶粒の成長抑制による細粒強化及び再結晶の抑制を通じた転位強化にて、鋼板の強度上昇に寄与することから、添加してもよい。これらの効果は、０．００５％未満ではこれらの効果が得られないため、下限値を０．００５％とした。０．３％超含有すると、炭窒化物の析出が多くなり成形性が劣化するため、上限値を０．３％とした。また、析出強化は、フェライト、もしくは、フェライト及びマルテンサイトの両方を強化する必要があることから、熱延段階での析出、あるいは、焼鈍時であれば、加熱中あるいは焼鈍中に析出させることが望ましい。
(Nb: 0.005-0.3%)
Nb is a strengthening element. It may be added because it contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. Since these effects cannot be obtained if these effects are less than 0.005%, the lower limit is set to 0.005%. If the content exceeds 0.3%, carbonitride precipitation increases and the formability deteriorates, so the upper limit was made 0.3%. In addition, since precipitation strengthening requires strengthening ferrite, or both ferrite and martensite, precipitation during the hot rolling stage, or during annealing, may be precipitated during heating or annealing. desirable.
（Ｔｉ：０．００５〜０．３％）
Ｔｉは、強化元素である。析出物強化、フェライト結晶粒の成長抑制による細粒強化及び再結晶の抑制を通じた転位強化にて、鋼板の強度上昇に寄与することから、添加してもよい。これらの効果は、０．００５％未満ではこれらの効果が得られないため、下限値を０．００５％とした。０．３％超含有すると、炭窒化物の析出が多くなり成形性が劣化するため、上限値を０．３％とした。また、析出強化は、フェライト、もしくは、フェライト及びマルテンサイトの両方を強化する必要があることから、熱延段階での析出、あるいは、焼鈍時であれば、加熱中あるいは焼鈍中に析出させることが望ましい。加えて、Ｔｉは強力な窒化物形成元素であることから、Ａｌに比較して、Ｎと優先的に結び付くことで、微細な窒化物を形成し、粗大なＡｌＮの形成を抑制し、曲げ性の劣化を抑制することから、添加してもよい。
(Ti: 0.005-0.3%)
Ti is a strengthening element. It may be added because it contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. Since these effects cannot be obtained if these effects are less than 0.005%, the lower limit is set to 0.005%. If the content exceeds 0.3%, carbonitride precipitation increases and the formability deteriorates, so the upper limit was made 0.3%. In addition, since precipitation strengthening requires strengthening ferrite, or both ferrite and martensite, precipitation during the hot rolling stage, or during annealing, may be precipitated during heating or annealing. desirable. In addition, since Ti is a strong nitride-forming element, it binds preferentially to N compared to Al, thereby forming fine nitrides, suppressing the formation of coarse AlN, and bending properties. It may be added in order to suppress the deterioration.
（Ｖ：０．００５〜０．３％）
Ｖは、強化元素である。析出物強化、フェライト結晶粒の成長抑制による細粒強化及び再結晶の抑制を通じた転位強化にて、鋼板の強度上昇に寄与することから、添加してもよい。これらの効果は、０．００５％未満ではこれらの効果が得られないため、下限値を０．００５％とした。０．３％超含有すると、炭窒化物の析出が多くなり成形性が劣化するため、上限値を０．３％とした。また、析出強化は、フェライト、もしくは、フェライト及びマルテンサイトの両方を強化する必要があることから、熱延段階での析出、あるいは、焼鈍時であれば、加熱中あるいは焼鈍中に析出させることが望ましい。また、Ｖの炭化物は、水素脆性の抑制をもたらすことから、９８０ＭＰａ以上の高強度鋼板の製造にあたっては、Ｖを添加してもよい。
(V: 0.005-0.3%)
V is a strengthening element. It may be added because it contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. Since these effects cannot be obtained if these effects are less than 0.005%, the lower limit is set to 0.005%. If the content exceeds 0.3%, carbonitride precipitation increases and the formability deteriorates, so the upper limit was made 0.3%. In addition, since precipitation strengthening requires strengthening ferrite, or both ferrite and martensite, precipitation during the hot rolling stage, or during annealing, may be precipitated during heating or annealing. desirable. Further, since V carbides suppress hydrogen embrittlement, V may be added in the production of a high strength steel plate of 980 MPa or more.
（Ｃａ、Ｍｇ、Ｚｒ、ＲＥＭ、Ｙ：０．０００５〜０．０１％）
Ｃａ、Ｍｇ、Ｚｒ、Ｙ、ＲＥＭのうち１種または２種以上を合計で０．０００５〜０．０１％添加できる。Ｃａ、Ｍｇ、Ｚｒ、Ｙ及びＲＥＭは脱酸に用いる元素であり、１種または２種を合計で０．０００５％以上含有することが好ましい。しかしながら、合計の含有量が０．０１％を超えると、成形加工性の悪化の原因となる。そのため、合計量の範囲を０．０００５〜０．０１％とした。
なお、本発明において、ＲＥＭとは、Ｌａ及びランタノイド系列の元素を指すものであり、ミッシュメタルにて添加されることが多く、ＬａやＣｅ等の系列の元素を複合で含有する。ただし、金属ＬａやＣｅを添加したとしても本発明の効果は発揮される。
(Ca, Mg, Zr, REM, Y: 0.0005 to 0.01%)
One or more of Ca, Mg, Zr, Y, and REM can be added in a total amount of 0.0005 to 0.01%. Ca, Mg, Zr, Y, and REM are elements used for deoxidation, and it is preferable that one or two of them are contained in a total amount of 0.0005% or more. However, if the total content exceeds 0.01%, the moldability is deteriorated. Therefore, the total amount range is set to 0.0005 to 0.01%.
In the present invention, REM refers to La and lanthanoid series elements, which are often added by misch metal, and contain a series of elements such as La and Ce. However, the effects of the present invention are exhibited even when metal La or Ce is added.
本発明では、優れた穴拡げ性を有する引張最大強度５９０ＭＰａ以上の高強度鋼板を提供する。具体的には引張最大強度ＴＳ（ＭＰａ）×穴拡げ試験による穴拡げ値λ（％）で４００００（ＭＰａ×％）以上を有するものを優れた穴拡げ性を有するものと定義する。成形性に関しては、引張試験における破断伸びＥｌ（％）が１５０００（ＭＰａ×％）以上を有するものを穴拡げ性と伸びに優れた高強度鋼板と定義する。 In the present invention, a high-strength steel sheet having a maximum tensile strength of 590 MPa or more and having excellent hole expansibility is provided. Specifically, a material having a maximum tensile strength TS (MPa) × a hole expansion value λ (%) by a hole expansion test of 40000 (MPa ×%) or more is defined as having excellent hole expandability. With respect to formability, one having a breaking elongation El (%) of 15000 (MPa ×%) or more in a tensile test is defined as a high-strength steel sheet excellent in hole expansibility and elongation.
熱間圧延に供するスラブは特に限定するものではない。すなわち、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よい。また、鋳造後に直ちに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。 The 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. It is also compatible with processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.
熱延スラブ加熱温度は、鋳造中時に析出した炭窒化物を再溶解させる必要があるので、１１００℃以上にする必要がある。上限は特に定めることなく、本発明の効果は発揮されるが、加熱温度を過度に高温にすることは、経済上好ましくないことから、加熱温度の上限は１３００℃未満とすることが望ましい。 The hot-rolled slab heating temperature needs to be 1100 ° C. or higher because it is necessary to redissolve carbonitride precipitated during casting. Although the upper limit is not particularly defined, the effect of the present invention is exhibited. However, since it is not economically preferable to raise the heating temperature to an excessively high temperature, the upper limit of the heating temperature is preferably less than 1300 ° C.
仕上げ圧延温度はオーステナイト＋フェライトの２相域になると、鋼板内の組織不均一性及び材質の異方性が大きくなり、焼鈍後の成形加工性が劣化するので、Ａｒ３変態点以上の温度が望ましい。 When the finish rolling temperature is in the two-phase region of austenite + ferrite, the structure non-uniformity in the steel sheet and the material anisotropy increase, and the formability after annealing deteriorates. Therefore, a temperature equal to or higher than the Ar3 transformation point is desirable. .
なお、Ａｒ３変態点は次の式により計算する。
Ａｒ３＝９０１−３２５×Ｃ＋３３×Ｓｉ−９２×（Ｍｎ＋Ｎｉ／２＋Ｃｒ／２＋Ｃｕ／２＋Ｍｏ／２）
The Ar3 transformation point is calculated by the following formula.
Ar3 = 901-325 × C + 33 × Si-92 × (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2)
巻き取り温度は６３０℃以下にする必要がある。６３０℃を超えると熱延組織中に粗大なフェライトやパーライト組織が存在するため、焼鈍後の組織不均一性が大きくなり、最終製品の曲げ性や穴拡げ性が劣化する。焼鈍後の組織を微細にして強度延性バランスを向上させる観点からは６００℃以下で巻き取ることがより好ましい。下限については特に定めることなく本発明の効果は発揮されるが、室温以下の温度で巻き取ることは技術的に難しいので、これが実質の下限となる。なお、熱延時に粗圧延板同士を接合して連続的に仕上げ圧延を行ってもよい。また、粗圧延板を一旦巻き取っても構わない。 The winding temperature needs to be 630 ° C. or lower. When the temperature exceeds 630 ° C., coarse ferrite and pearlite structures exist in the hot-rolled structure, so that the structure non-uniformity after annealing increases, and the bendability and hole expandability of the final product deteriorate. From the viewpoint of making the microstructure after annealing fine and improving the strength ductility balance, it is more preferable to wind up at 600 ° C. or lower. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, since it is technically difficult to wind up at a temperature of room temperature or lower, this is the actual lower limit. Note that rough rolling sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, you may wind up a rough rolling board once.
このようにして製造した熱延鋼板に、必要に応じて酸洗を行ってもよい。酸洗は鋼板表面の酸化物の除去が可能であることから、化成処理性及びめっき性向上のためには重要である。酸洗は、インラインで行ってもよく、オフラインで行ってもよい。また、一回の酸洗を行ってもよく、複数回に分けて酸洗を行ってもよい。 The hot-rolled steel sheet thus manufactured may be pickled as necessary. Pickling is important for improving chemical conversion and plating properties because it can remove oxides on the surface of the steel sheet. Pickling may be performed inline or offline. In addition, pickling may be performed once, or pickling may be performed in a plurality of times.
酸洗した熱延鋼板を圧下率４０〜７０％で冷間圧延して、連続焼鈍ライン又は連続溶融亜鉛めっきラインに通板する。圧下率が４０％未満では、形状を平坦に保つことが困難である。また、最終製品の延性が劣悪となるのでこれを下限とする。一方、７０％を越える冷延は、冷延荷重が大きくなりすぎてしまい冷延が困難となることから、これを上限とする。より好ましい範囲は４５〜６５％である。圧延パスの回数、各パス毎の圧下率については特に規定することなく本発明の効果は発揮される。 The pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 40 to 70% and passed through a continuous annealing line or a continuous hot dip galvanizing line. If the rolling reduction is less than 40%, it is difficult to keep the shape flat. Moreover, since the ductility of the final product becomes poor, this is the lower limit. On the other hand, cold rolling exceeding 70% makes the cold rolling difficult because the cold rolling load becomes too large. A more preferable range is 45 to 65%. The effect of the present invention is exhibited without particularly specifying the number of rolling passes and the rolling reduction for each pass.
連続焼鈍ラインを通板する場合の焼鈍温度は特に定めることなく本発明の効果は発揮される。しかしながら、加熱速度が０．４℃／秒未満では、加熱時間が長くなりすぎてしまい、経済性に劣ることから望ましくない。一方、加熱速度を１００℃／秒超とすることは、過度の設備投資を招き、経済的に好ましくないことから、これが実質的な上限である。 The effect of the present invention is exhibited without particularly setting the annealing temperature when passing through the continuous annealing line. However, when the heating rate is less than 0.4 ° C./second, the heating time becomes too long, which is not preferable because it is inferior in economic efficiency. On the other hand, setting the heating rate to more than 100 ° C./second invites excessive capital investment and is not economically preferable, so this is a practical upper limit.
最高加熱温度は、７５０〜９００℃の範囲である。最高加熱温度が７５０℃未満になると、熱延時に形成した炭化物が再固溶するのに時間がかかりすぎてしまい体積率で５％以上の焼き戻しマルテンサイトを確保できないことから、５９０ＭＰａ以上の強度が確保できない。さらには、鋼板中に残存する粗大な炭化物は、穴拡げ性の劣化をもたらす。このことから、７５０℃が最高加熱温度の下限である。一方、過度の高温加熱は、コストの上昇を招くことから経済的に好ましくないばかりでなく、高温通板時の板形状が劣悪になったり、ロールの寿命を低下させたりとトラブルを誘発することから、最高加熱温度の上限を９００℃とする。この温度域での熱処理時間は特に限定しないが、炭化物の溶解のために、１０秒以上の熱処理が望ましい。一方、熱処理時間が６００秒超となると、コストの上昇を招くことから経済的に好ましくない。熱処理についても、最高加熱温度にて等温保持を行ってもよく、傾斜加熱を行い最高加熱温度に到達した後、直ちに、冷却を開始したとしても、本発明の効果は発揮される。 The maximum heating temperature is in the range of 750 to 900 ° C. When the maximum heating temperature is less than 750 ° C., it takes too much time for the carbide formed during hot rolling to re-dissolve, and tempered martensite with a volume ratio of 5% or more cannot be secured. Cannot be secured. Furthermore, coarse carbides remaining in the steel plate cause deterioration of hole expansibility. Therefore, 750 ° C. is the lower limit of the maximum heating temperature. On the other hand, excessively high temperature heating not only is economically undesirable because it leads to an increase in cost, but also induces troubles such as deterioration of the plate shape at the time of hot plate passing and reduction in the life of the roll. Therefore, the upper limit of the maximum heating temperature is set to 900 ° C. The heat treatment time in this temperature range is not particularly limited, but a heat treatment of 10 seconds or more is desirable for dissolving the carbide. On the other hand, if the heat treatment time exceeds 600 seconds, the cost increases, which is not economically preferable. Regarding the heat treatment, isothermal holding may be performed at the maximum heating temperature, and even if cooling is started immediately after the gradient heating is performed and the maximum heating temperature is reached, the effect of the present invention is exhibited.
焼鈍温度から６２０℃までの温度範囲を平均冷却速度１２℃／秒以下にて冷却する。冷却速度の上限を１２℃／秒としたのは、焼き入れ前のフェライト中のＣ含有量を増加させ、引き続いて行われる焼き戻し工程で、炭化物として析出させることで、製品中のフェライトに含まれる固溶Ｃを減少させるためである。平均冷却速度が、１２℃／秒を上回ると、Ｃが十分に濃化しないことから、フェライト中での炭化物析出が起こり難く、延性が劣化することから好ましくない。一方で、下限は特に定めることなく本発明の効果は発揮されるが、０．１℃／秒を下回る過度の緩冷却は生産性に劣ることから好ましくない。
平均加熱速度とは、焼鈍温度から６２０℃での平均の冷却速度を意味し、ガス、ミスト、ロール冷却いずれの方法を用いてもよい。特に、フェライトの平衡のC固溶限は低温側で低くなることから、７５０℃未満での滞留時間を長くすることが成形性を向上させる上で特に重要である。
The temperature range from the annealing temperature to 620 ° C. is cooled at an average cooling rate of 12 ° C./second or less. The upper limit of the cooling rate was set to 12 ° C./second because the C content in the ferrite before quenching was increased and precipitated as carbides in the subsequent tempering step, so that it was included in the ferrite in the product. This is to reduce the dissolved C. When the average cooling rate exceeds 12 ° C./second, C is not sufficiently concentrated, so that carbide precipitation in ferrite hardly occurs and ductility deteriorates, which is not preferable. On the other hand, although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, excessive slow cooling below 0.1 ° C./second is not preferable because productivity is poor.
The average heating rate means an average cooling rate from the annealing temperature to 620 ° C., and any method of gas, mist, and roll cooling may be used. In particular, since the C solid solubility limit of the ferrite equilibrium is lowered on the low temperature side, it is particularly important to increase the residence time at less than 750 ° C. in order to improve the moldability.
上記焼鈍終了後、引き続き６２０℃〜５００℃間を、４〜２００℃／秒にて冷却し、硬質相を形成させる処理を施す。６２０〜５００℃の温度域にてオーステナイトがパーライトへと変態することから、その抑制が必要である。この間の平均冷却速度が４℃／秒未満では、オーステナイトがパーライトへと変態してしまい、十分な量のマルテンサイトやベイナイト組織が得られず、５９０ＭＰａ以上の強度確保が困難になることから、下限値を４℃／秒以上とした。また、２００℃／秒を超えて、冷却したとしても、材質上はなんら問題を生じないが、過度に冷却速度を上げることは、製造コスト高を招くことになるので、上限を２００℃／秒とすることが好ましい。６２０℃と５００℃間の冷却方法については、ロール冷却、空冷、水冷及びこれらを併用したいずれの方法でも構わない。冷却速度を限定する温度範囲を５００〜６２０℃としたのは、この温度域にてパーライト変態が起こるからである。６２０℃を超える温度からの急冷は、フェライト中に過飽和に含まれるＣ量を増加させることになるので好ましくない。 After the completion of the annealing, a process of cooling between 620 ° C. and 500 ° C. at 4 to 200 ° C./second to form a hard phase is performed. Since austenite transforms into pearlite in the temperature range of 620 to 500 ° C., it is necessary to suppress it. If the average cooling rate during this period is less than 4 ° C / second, austenite is transformed into pearlite, and a sufficient amount of martensite or bainite structure cannot be obtained, and it is difficult to ensure a strength of 590 MPa or more. The value was set to 4 ° C./second or more. Further, even if the cooling is performed at a temperature exceeding 200 ° C./second, no problem is caused in terms of the material. However, excessively increasing the cooling rate causes an increase in manufacturing cost, so the upper limit is 200 ° C./second. It is preferable that The cooling method between 620 ° C. and 500 ° C. may be roll cooling, air cooling, water cooling, or any method using these in combination. The reason why the temperature range for limiting the cooling rate is 500 to 620 ° C. is that pearlite transformation occurs in this temperature range. Rapid cooling from a temperature exceeding 620 ° C. is not preferable because it increases the amount of C contained in the supersaturation in the ferrite.
その後、（Ｍｓ−５０）℃以下の温度まで冷却を行う。冷却停止温度を（Ｍｓ−５０）℃としたのは、冷却停止温度が高いと鋼板中に存在しているオーステナイトをマルテンサイトに変態させることができず、引き続いて行われる焼き戻し処理後の冷却過程にて、マルテンサイトへと変態する。このマルテンサイトは焼き戻しされないことから非常に硬く、フェライトとマルテンサイトの硬度比（ＤＨＴＭ／ＤＨＴＭ）が３を超えることから、穴拡げ性を低下させることから、（Ｍｓ−５０）℃とする必要がある。 Then, it cools to the temperature below (Ms-50) degreeC. The reason why the cooling stop temperature is set to (Ms-50) ° C. is that if the cooling stop temperature is high, the austenite present in the steel sheet cannot be transformed into martensite, and the cooling after the tempering process performed subsequently is performed. In the process, it transforms into martensite. This martensite is very hard because it is not tempered, and since the hardness ratio of ferrite to martensite (DHTM / DHTM) exceeds 3, the hole expandability is lowered, so it is necessary to set it to (Ms-50) ° C. There is.
冷却後、２００〜６００℃で１０秒以上の焼き戻し処理を行うことで、マルテンサイト中にＦｅを主体とする炭化物を析出させることで、マルテンサイトとフェライトの強度差を低減できる。下限を２００℃以上としたのは、２００℃未満では炭化物の析出に長時間を要することから好ましくなく、さらには、これら炭化物が微細であることから、マルテンサイト強度があまり低下せず、ＤＨＴＭ／ＤＨＴＦを３．０未満とすることが困難なためである。一方では、６００℃を超える高温での焼き戻しは、マルテンサイト強度を極端に低下させるため、５９０ＭＰａ以上の強度確保が難しいためである。 After cooling, by performing a tempering treatment at 200 to 600 ° C. for 10 seconds or more, carbides mainly composed of Fe are precipitated in martensite, whereby the strength difference between martensite and ferrite can be reduced. The lower limit of 200 ° C. or higher is not preferable because it takes a long time for precipitation of carbides at less than 200 ° C. Furthermore, since these carbides are fine, the martensite strength does not decrease so much and DHTM / This is because it is difficult to make DHTF less than 3.0. On the other hand, tempering at a high temperature exceeding 600 ° C. extremely reduces the martensite strength, and thus it is difficult to secure a strength of 590 MPa or more.
焼き戻し処理の方法に関しては、特に限定することなく本発明の効果は得られるが、過時効帯の温度を高温に保ち輻射熱にての鋼板を加熱する方法、通電加熱、誘導加熱あるいは箱型焼鈍炉を用いたオフラインでの加熱などの何れの方法を用いたとしても構わない。ただし、箱型焼鈍炉を用いた加熱は、徐々に加熱することから、コイル内を均一に加熱できることから、好ましい。 With respect to the method of tempering treatment, the effects of the present invention can be obtained without any particular limitation, but the method of heating the steel sheet with radiant heat while keeping the temperature of the overaging zone high, current heating, induction heating or box annealing Any method such as off-line heating using a furnace may be used. However, heating using a box-type annealing furnace is preferable because the inside of the coil can be uniformly heated because the heating is performed gradually.
熱処理後のスキンパス圧延の圧下率は、０．１〜１．５％の範囲が好ましい。０．１％未満では効果が小さく、制御も困難であることから、これが下限となる。１．５％超えると生産性が著しく低下するのでこれを上限とする。スキンパスは、インラインで行ってもよく、オフラインで行ってもよい。また、一度に目的の圧下率のスキンパスを行ってもよく、数回に分けて行っても構わない。 The reduction ratio of the skin pass rolling after the heat treatment is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. If it exceeds 1.5%, the productivity is remarkably lowered, so this is the upper limit. The skin pass may be performed inline or offline. In addition, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.
冷延後に溶融亜鉛めっきラインを通板する場合の最高加熱温度も連続焼鈍ラインを通板する場合と同様の理由により、７５０〜９００℃とする。焼鈍後の冷却に関しても、連続焼鈍ラインを通板する場合と同様の理由により、７５０℃〜６２０℃間を平均冷却速度１２℃／秒以下で冷却する必要がある。６２０〜５００℃間の冷却速度も、連続焼鈍ラインと同様の理由で、４℃／秒以上の冷却速度にて冷却する必要がある。 The maximum heating temperature when passing through the hot-dip galvanizing line after cold rolling is set to 750 to 900 ° C. for the same reason as when passing through the continuous annealing line. Regarding cooling after annealing, it is necessary to cool between 750 ° C. and 620 ° C. at an average cooling rate of 12 ° C./second or less for the same reason as when passing through a continuous annealing line. The cooling rate between 620 and 500 ° C. needs to be cooled at a cooling rate of 4 ° C./second or more for the same reason as in the continuous annealing line.
めっき浴浸漬板温度は、溶融亜鉛めっき浴温度より４０℃低い温度から溶融亜鉛めっき浴温度より５０℃高い温度までの温度範囲とすることが望ましい。浴浸漬板温度が溶融亜鉛めっき浴温度−４０）℃を下回ると、めっき浴浸漬進入時の抜熱が大きく、溶融亜鉛の一部が凝固してしまいめっき外観を劣化させる場合があることから、下限を（溶融亜鉛めっき浴温度−４０）℃とする。ただし、浸漬前の板温度が（溶融亜鉛めっき浴温度−４０）℃を下回っても、めっき浴浸漬前に再加熱を行い、板温度を（溶融亜鉛めっき浴温度−４０）℃以上としてめっき浴に浸漬させてもよい。また、めっき浴浸漬温度が（溶融亜鉛めっき浴温度＋５０）℃を超えると、めっき浴温度上昇に伴う操業上の問題を誘発する。また、めっき浴は、純亜鉛に加え、Ｆｅ、Ａｌ、Ｍｇ、Ｍｎ、Ｓｉ、Ｃｒなどを含有しても構わない。 The plating bath immersion plate temperature is preferably in a temperature range from a temperature 40 ° C. lower than the hot dip galvanizing bath temperature to a temperature 50 ° C. higher than the hot dip galvanizing bath temperature. If the bath immersion plate temperature is lower than the hot dip galvanizing bath temperature −40) ° C., the heat removal at the time of immersion in the plating bath is large, and part of the molten zinc may solidify and deteriorate the plating appearance. The lower limit is (hot dip galvanizing bath temperature −40) ° C. However, even if the plate temperature before immersion is lower than (hot dip galvanizing bath temperature −40) ° C., reheating is performed before immersion in the plating bath, and the plate temperature is set to (hot dip galvanizing bath temperature −40) ° C. or higher. It may be immersed in. On the other hand, if the plating bath immersion temperature exceeds (hot dip galvanizing bath temperature +50) ° C., operational problems accompanying the rise of the plating bath temperature are induced. Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, etc. in addition to pure zinc.
また、めっき層の合金化を行う場合には、４６０℃以上で行う。合金化処理温度が４６０℃未満であると合金化の進行が遅く、生産性が悪い。上限は特に限定しないが、６００℃を超えると、パーライト変態が起こりマルテンサイト体積率を減少させ、引張強度５９０ＭＰａ以上の確保が難しくなるので、これが実質的な上限である。
冷却停止温度に関しても、連続冷却ラインを通板させた場合と同様の理由により、（Ｍｓ−５０）℃以下とすることが望ましい。焼き戻し温度と時間も、連続冷却ラインを通板させた場合と同様の理由により、２００〜６００℃で、１０秒以上とする必要がある。ただし、めっき及び合金化の後、焼き戻し処理を施すことから、合金化反応が進みめっき中に含まれるＦｅ％が向上する傾向にある。このことから、焼き戻し処理を行う前のめっき層中のＦｅ％は低めに抑えることが望ましい。また、焼き入れ処理をめっき浴浸漬前に行い、その後のめっき浴温度への加熱や合金化処理を用いて焼き戻しを施しても構わない。
Moreover, when alloying a plating layer, it carries out at 460 degreeC or more. When the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. The upper limit is not particularly limited, but if it exceeds 600 ° C., pearlite transformation occurs, the martensite volume fraction is reduced, and it becomes difficult to ensure a tensile strength of 590 MPa or more, so this is a substantial upper limit.
Regarding the cooling stop temperature, it is desirable to set it to (Ms-50) ° C. or lower for the same reason as when the continuous cooling line is passed. The tempering temperature and time need to be 10 seconds or more at 200 to 600 ° C. for the same reason as when the continuous cooling line is passed. However, since tempering treatment is performed after plating and alloying, the alloying reaction proceeds and the Fe% contained in the plating tends to be improved. Therefore, it is desirable to keep Fe% in the plating layer before tempering treatment low. Further, the quenching process may be performed before immersion in the plating bath, and then tempering may be performed using heating to the plating bath temperature or alloying process.
溶融亜鉛めっき鋼板にスキンパス圧延を施しても構わない。また、めっき密着性をさらに向上させるために、焼鈍前に鋼板に、Ｎｉ、Ｃｕ、Ｃｏ、Ｆｅの単独あるいは複数より成るめっきを施しても本発明を逸脱するものではない。さらには、めっき前の焼鈍については、「脱脂酸洗後、非酸化雰囲気にて加熱し、Ｈ２及びＮ２を含む還元雰囲気にて焼鈍後、めっき浴温度近傍まで冷却し、めっき浴に侵漬」というゼンジマー法、「焼鈍時の雰囲気を調節し、最初、鋼板表面を酸化させた後、その後還元することによりめっき前の清浄化を行った後にめっき浴に侵漬」という全還元炉方式、あるいは、「鋼板を脱脂酸洗した後、塩化アンモニウムなどを用いてフラックス処理を行って、めっき浴に侵漬」というフラックス法等があるが、いずれの条件で処理を行ったとしても本発明の効果は発揮できる。また、めっき前の焼鈍の手法によらず、加熱中の露点を−２０℃以上とすることで、Ｓｉを含有する鋼のめっき濡れ性やめっきの合金化の際の合金化反応に有利に働く。 Skin-pass rolling may be applied to the hot-dip galvanized steel sheet. Further, in order to further improve the plating adhesion, the present invention does not depart from the present invention even if the steel plate is plated with Ni, Cu, Co, or Fe alone or before the annealing. Further, regarding annealing before plating, “after degreasing pickling, heating in a non-oxidizing atmosphere, annealing in a reducing atmosphere containing H 2 and N 2 , cooling to near the plating bath temperature, and invading the plating bath. Zenjimer method called “Kizuke”, an all-reduction furnace method called “immersion in the plating bath after adjusting the atmosphere during annealing, first oxidizing the steel plate surface, and then reducing it before cleaning by plating” Alternatively, there is a flux method such as “after degreasing and pickling a steel plate, and then fluxing it with ammonium chloride and soaking it in a plating bath”, etc. The effect of can be demonstrated. In addition, by setting the dew point during heating to −20 ° C. or higher regardless of the annealing method before plating, it works favorably for the plating wettability of the steel containing Si and the alloying reaction during the alloying of the plating. .
なお、本冷延鋼板を電気めっきしても鋼板の有する引張強度、穴拡げ性及び成形性を何ら損なうことはない。すなわち、本発明鋼板は電気めっき用素材としても好適である。
また、本発明の加工性に優れた高強度高延性溶融亜鉛めっき鋼板の素材は、通常の製鉄工程である精錬、製鋼、鋳造、熱延、冷延工程を経て製造されることを原則とするが、その一部あるいは全部を省略して製造されるものでも、本発明に係わる条件を満足する限り、本発明の効果を得ることができる。
In addition, even if this cold-rolled steel sheet is electroplated, the tensile strength, hole expansibility and formability of the steel sheet are not impaired at all. That is, the steel sheet of the present invention is also suitable as a material for electroplating.
In addition, the material of the high-strength and high-ductility hot-dip galvanized steel sheet having excellent workability according to the present invention is manufactured in principle through refining, steelmaking, casting, hot rolling, and cold rolling processes, which are ordinary steelmaking processes. However, even if manufactured by omitting a part or all of them, the effects of the present invention can be obtained as long as the conditions according to the present invention are satisfied.
次に、本発明を実施例により詳細に説明する。
表１に示す成分（単位：質量％）を有するスラブを、１２２０℃に加熱し、仕上げ熱延温度９００℃にて熱間圧延を行い、水冷帯にて水冷の後、表２に示す温度で巻き取り処理を行った。熱延板を酸洗した後、厚み３ｍｍの熱延板を１．２ｍｍまで冷延を行い、冷延板とした。その後、これらの冷延板に表２に示す条件で焼鈍熱処理を行い、７５０℃−６２０℃間を４〜５０℃／秒で冷却し、引き続き、６２０〜５００℃を１〜７０℃／秒で、各温度に冷却し、各温度にて付加的な熱処理を行い、その後室温まで冷却した。最後に、得られた鋼板について０．３％の圧下率でスキンパス圧延を行った。
Next, the present invention will be described in detail with reference to examples.
A slab having the components shown in Table 1 (unit: mass%) is heated to 1220 ° C., hot-rolled at a final hot rolling temperature of 900 ° C., water-cooled in a water-cooled zone, and then at the temperature shown in Table 2. The winding process was performed. After pickling the hot-rolled sheet, a hot-rolled sheet having a thickness of 3 mm was cold-rolled to 1.2 mm to obtain a cold-rolled sheet. Then, annealing treatment is performed on these cold-rolled sheets under the conditions shown in Table 2, cooling between 750 ° C. and 620 ° C. is performed at 4 to 50 ° C./second, and subsequently, 620 to 500 ° C. is performed at 1 to 70 ° C./second. , Cooled to each temperature, subjected to additional heat treatment at each temperature, and then cooled to room temperature. Finally, skin pass rolling was performed on the obtained steel sheet at a rolling reduction of 0.3%.
一部の鋼板については、上記と同様の手法で冷延まで行い、連続合金化溶融亜鉛めっき設備にて、熱処理と溶融亜鉛めっき処理を施した。溶融亜鉛めっきを施す鋼板については、焼鈍の後、これらの冷延板に表２に示す条件で焼鈍熱処理を行い、７５０℃〜６２０℃間を２〜５０℃／秒で冷却し、引き続き６２０〜５００℃間を４℃／秒以上で冷却し、その後、亜鉛めっき浴に通板し、各温度に冷却し、各温度にて付加的な熱処理を行い、その後室温まで冷却した。最後に、得られた鋼板について０．３％の圧下率でスキンパス圧延を行った。その後、室温まで１０℃／秒の冷却速度で室温まで冷却し、最後に、得られた鋼板について０．３％の圧下率でスキンパス圧延を行った。 About some steel plates, it carried out to cold rolling by the method similar to the above, and performed the heat processing and the hot dip galvanization process in the continuous alloying hot dip galvanization equipment. About the steel plate to which hot dip galvanization is applied, after annealing, these cold-rolled plates are subjected to annealing heat treatment under the conditions shown in Table 2, cooled between 750 ° C. and 620 ° C. at 2-50 ° C./second, and subsequently 620 ° C. The temperature was lowered between 500 ° C. at 4 ° C./second or more, then passed through a galvanizing bath, cooled to each temperature, subjected to additional heat treatment at each temperature, and then cooled to room temperature. Finally, skin pass rolling was performed on the obtained steel sheet at a rolling reduction of 0.3%. Thereafter, the steel sheet was cooled to room temperature at a cooling rate of 10 ° C./second, and finally, the obtained steel sheet was subjected to skin pass rolling at a reduction rate of 0.3%.
一部の鋼板については、めっき処理に引き続き合金化処理を行った。その際の目付け量としては、両面とも約５０ｇ／ｍ２とした。めっき後の鋼板に、０．３％スキンパス圧延を施した。特に、本鋼板は、体積率で４０％以上のフェライトを含むことから、鋼板成分より予想されるＭｓ点よりかなり低い傾向にある。これらＭｓ点は、予め実機相当の熱処理を行うことで、Ｍｓ点を測定し、焼き入れ温度を決定した。Ｍｓは２６０〜３８０℃の範囲にあることから、２００℃以下の温度への冷却が望ましい。 About some steel plates, the alloying process was performed following the plating process. The basis weight at that time was about 50 g / m 2 on both sides. The plated steel sheet was subjected to 0.3% skin pass rolling. In particular, the steel sheet contains 40% or more of ferrite by volume ratio, and therefore tends to be considerably lower than the Ms point expected from the steel sheet component. These Ms points were preliminarily subjected to heat treatment corresponding to the actual machine, so that the Ms points were measured and the quenching temperature was determined. Since Ms is in the range of 260 to 380 ° C., cooling to a temperature of 200 ° C. or lower is desirable.
ミクロ組織の同定に関しては、前記手法にて事前に行った。
ミクロ組織中に含まれるマルテンサイトとフェライトの硬度比の測定に関しては、ベルコビッチタイプの三角すい圧子を有するダイナミック微小硬度計を用いて、押し込み深さ測定法にて、硬度を測定した。押し込み荷重は１ｇ重とした。これは、本鋼中に存在するマルテンサイトサイズは、３μｍ以下と非常に小さく通常のビッカース試験機を用いて硬度を測定した場合、圧痕サイズが大きいため、微細なマルテンサイトのみの硬度測定が行い難い、あるいは、圧痕サイズが小さいすぎることから、正確なサイズを測定できないためである。
The microstructure was identified in advance by the above method.
Regarding the measurement of the hardness ratio of martensite and ferrite contained in the microstructure, the hardness was measured by an indentation depth measurement method using a dynamic microhardness meter having a Belkovic type triangular pan indenter. The indentation load was 1 g weight. This is because the martensite size present in this steel is very small, 3 μm or less, and when the hardness is measured using a normal Vickers tester, the indentation size is large, so the hardness of only fine martensite is measured. This is because it is difficult or the indentation size is too small, so that an accurate size cannot be measured.
本試験条件の硬度測定は、ダイナミック微小硬度計を用いて行った。具体的には、押し込み荷重１ｇ重にて、１０００点の圧痕を打ち、硬度分布を求めた後、フーリエ変換を行い個々の組織の平均硬度を算出し、フェライトに対応する最も低い硬度（ＤＨＴＦ）と、マルテンサイトに相当する最も高い硬度（ＤＨＴＭ）の比（ＤＨＴＭ／ＤＨＴＦ）を算出した。ただし、マルテンサイトと焼き戻しマルテンサイトの両方が存在する場合、マルテンサイトと焼き戻しマルテンサイトに相当する硬度が得られるが、優れた穴拡げ性を発揮するためには、フェライトと最も硬質なマルテンサイト間の硬度比を所定の範囲とすることで、優れた穴拡げ性が得られることから、マルテンサイトと焼き戻しマルテンサイトの両方が存在する場合は、マルテンサイトとフェライトの硬度比をＤＨＴＭ／ＤＨＴＦとした。また、ベイナイト組織の硬度は、マルテンサイトに比較して小さいことから、ベイナイト組織の硬度にかかわらず、フェライトとマルテンサイトの硬度比が所定の範囲にあれば本発明の効果である優れた穴拡げ性と成形性は得られる。 The hardness under this test condition was measured using a dynamic micro hardness tester. Specifically, with an indentation load of 1 g, an indentation of 1000 points was made, and after obtaining the hardness distribution, Fourier transform was performed to calculate the average hardness of each structure, and the lowest hardness (DHTF) corresponding to ferrite The ratio (DHTM / DHTF) of the highest hardness (DHTM) corresponding to martensite was calculated. However, when both martensite and tempered martensite are present, hardness equivalent to martensite and tempered martensite can be obtained, but in order to exhibit excellent hole expandability, ferrite and the hardest martensite are obtained. By setting the hardness ratio between sites within a predetermined range, excellent hole expansibility can be obtained. Therefore, when both martensite and tempered martensite exist, the hardness ratio of martensite and ferrite is set to DHTM / It was set as DHTF. In addition, since the hardness of the bainite structure is smaller than that of martensite, the excellent hole expansion that is the effect of the present invention is provided if the hardness ratio of ferrite and martensite is within a predetermined range regardless of the hardness of the bainite structure. And moldability are obtained.
硬度比ＤＨＴＭ／ＤＨＴＦについては、下記のように評価した。
△：ＤＨＴＭ／ＤＨＴＦ＜１．５
○：１．５≦ＤＨＴＭ／ＤＨＴＦ≦３．０
×：３．０＜ＤＨＴＭ／ＤＨＴＦ
The hardness ratio DHTM / DHTF was evaluated as follows.
Δ: DHTM / DHTF <1.5
○: 1.5 ≦ DHTM / DHTF ≦ 3.0
X: 3.0 <DHTM / DHTF
得られた冷延焼鈍板あるいは亜鉛めっき板について、引張試験を行い、ＹＳ，ＴＳ，Ｅｌを測定した。なお、降伏応力は０．２％オフセット法により測定した。
引張試験は、１．２ｍｍ厚の板から圧延方向に直角方向にＪＩＳ５号試験片を採取し、引張特性を評価した。
穴拡げ性は、直径１０ｍｍの円形穴を、クリアランスが１２．５％となる条件にて打ち抜き、かえりがダイ側となるようにし、６０°円錐ポンチにて成形し、穴拡がり率λ（％）により評価した。各条件とも、５回の穴拡げ試験を実施し、その平均値を穴拡がり率とし、ＴＳ（ＭＰａ）×λ（％）で４００００（ＭＰａ×％）以上を有するものを優れた穴拡げ性を有するものと定義する。
成形性に関しては、引張試験より得られた破断伸びＥｌ（％）とＴＳ（ＭＰａ）の積であるＴＳ（ＭＰａ）×Ｅｌ（％）で１５０００（ＭＰａ×％）以上有するものを成形性が優れているものとした。
なお、表１，２中の下線は、本発明の範囲外の条件である。また、表２中の＊１について、ＣＲは冷延鋼板、ＧＩは溶融亜鉛めっき鋼板、ＧＡは合金化溶融亜鉛めっき鋼板を示す。また、表２中の＊２について、−は各工程を実地していないことを意味する。
The obtained cold-rolled annealed plate or galvanized plate was subjected to a tensile test to measure YS, TS, and El. The yield stress was measured by the 0.2% offset method.
In the tensile test, a JIS No. 5 test piece was sampled in a direction perpendicular to the rolling direction from a 1.2 mm thick plate, and the tensile properties were evaluated.
Hole expandability is 10mm diameter circular hole punched under the condition that the clearance is 12.5%, the burr is on the die side, molded with a 60 ° conical punch, hole expansion rate λ (%) It was evaluated by. Under each condition, five hole expansion tests were conducted, and the average value was defined as the hole expansion ratio. TS (MPa) x λ (%) and those having 40000 (MPa x%) or more had excellent hole expandability. It is defined as having.
Regarding moldability, the moldability is excellent when TS (MPa) × El (%), which is the product of breaking elongation El (%) and TS (MPa) obtained from the tensile test, is 15000 (MPa ×%) or more. It was supposed to be.
The underline in Tables 1 and 2 is a condition outside the scope of the present invention. Moreover, about * 1 in Table 2, CR shows a cold-rolled steel plate, GI shows a hot-dip galvanized steel plate, GA shows an alloyed hot-dip galvanized steel plate. Moreover, about * 2 in Table 2,-means that each process is not implemented.
表２に示すように、表１の成分からなる各鋼を各条件にて熱処理を行い、異なるミクロ組織と特性とを有する鋼板を作製した。なお、各鋼のＭｓ点に関しては、熱延・冷延を行った板を、予め実験室にて同様の熱処理条件で熱処理を行い、Ｍｓ点を測定した。鋼Ｎｏ．Ａ〜ＨのＭｓ点は、２８０℃〜３４０℃の範囲にある。一方、Ｍｎの含有量が本発明の範囲から外れる鋼Ｎｏ．Ｉは、冷却過程でのパーライト変態を抑制することができず、冷却停止までにオーステナイトがパーライトへと完全に変態してしまうため、マルテンサイト変態が起こらず、冷却停止温度が本発明の範囲を満たさない。Ｃ含有量が少ない鋼Ｎｏ．Ｊは、本熱処理条件の範囲内では、フェライト単相組織であることから、冷却過程でマルテンサイト変態が起こらず、冷却停止温度が本発明の範囲を満たさない。得られた結果をもとに、冷却停止後の焼き戻し温度を種々変化させ、鋼板のミクロ組織と特性とを変化させた。なお、用途に合わせて、溶融亜鉛めっき、合金化溶融亜鉛めっきを行った。 As shown in Table 2, each steel consisting of the components in Table 1 was heat-treated under each condition to produce steel plates having different microstructures and characteristics. In addition, regarding the Ms point of each steel, the hot-rolled and cold-rolled plate was heat-treated in advance under the same heat treatment conditions in a laboratory, and the Ms point was measured. Steel No. The Ms points of A to H are in the range of 280 ° C to 340 ° C. On the other hand, steel No. in which the Mn content deviates from the scope of the present invention. I cannot suppress pearlite transformation in the cooling process, and austenite completely transforms into pearlite before cooling is stopped, so martensite transformation does not occur and the cooling stop temperature is within the scope of the present invention. Do not meet. Steel No. with low C content Since J is a ferrite single phase structure within the range of the present heat treatment conditions, martensitic transformation does not occur in the cooling process, and the cooling stop temperature does not satisfy the range of the present invention. Based on the obtained results, the tempering temperature after stopping cooling was variously changed to change the microstructure and properties of the steel sheet. In addition, hot dip galvanization and alloying hot dip galvanization were performed according to the use.
また、ミクロ組織観察結果、フェライト中のＣ濃度、フェライトとマルテンサイトの硬度比（ＤＨＴＭ／ＤＨＴＦ）、引張特性及び穴拡げ率を表３及び表４に示す。
なお、表３，４中の下線は、本発明の範囲外の条件である。また、表３，４中の＊１について、ＣＲは冷延鋼板、ＧＩは溶融亜鉛めっき鋼板、ＧＡは合金化溶融亜鉛めっき鋼板を示す。また、表３，４中の＊３について、−は、マルテンサイトが存在していないことから、硬度比を測定できなかったことを意味する。また、表３，４中の＊４について、−は、フェライトが存在していないことから、硬度比やフェライトに含まれるＣ濃度を測定できなかったことを意味する。
Tables 3 and 4 show the microstructure observation results, the C concentration in the ferrite, the hardness ratio between ferrite and martensite (DHTM / DHTF), tensile properties, and hole expansion rate.
In addition, the underline in Tables 3 and 4 is a condition outside the scope of the present invention. Moreover, about * 1 in Table 3, 4, CR shows a cold-rolled steel plate, GI shows a hot-dip galvanized steel plate, GA shows an alloyed hot-dip galvanized steel plate. Moreover, about * 3 in Tables 3 and 4,-means that the hardness ratio could not be measured because martensite was not present. Moreover, about * 4 in Table 3, 4,-means that the hardness ratio and the C concentration contained in the ferrite could not be measured because no ferrite was present.
表３及び表４に示すように、成分範囲が本鋼板の範囲にあっても、７５０〜６２０℃間での冷却速度が大きいものは、フェライト中のＣ濃度が高く延性に劣る。焼き入れ焼き戻しの条件が本発明の範囲にないものは、穴拡げ率が低く、強度と穴拡げ性が所定の範囲を満たすことができない。 As shown in Table 3 and Table 4, even if the component range is in the range of the steel sheet, those having a large cooling rate between 750 and 620 ° C. have high C concentration in ferrite and poor ductility. When the quenching and tempering conditions are not within the scope of the present invention, the hole expansion rate is low, and the strength and hole expansion ability cannot satisfy the predetermined range.
本発明は、自動車用の構造用部材、補強用部材、足廻り用部材などに好適な、ＴＳで５９０ＭＰａ以上の高強度と優れた穴拡げ性と成形性を兼備する鋼板を安価に提供するものであり、自動車の軽量化に大きく貢献することが期待でき、産業上の効果は極めて高い。 The present invention provides an inexpensive steel sheet having high strength of 590 MPa or more, excellent hole expansibility, and formability suitable for structural members, reinforcing members, and suspension members for automobiles. Therefore, it can be expected to greatly contribute to the weight reduction of automobiles, and the industrial effect is extremely high.
Claims (9)
Ｃ：０．０５〜０．２５％未満、
Ｓｉ：２．０％以下、
Ｍｎ：１．１〜３．０％、
Ｐ：０．０４％以下、
Ｓ：０．０１％以下、
Ａｌ：２．０％以下、
Ｎ：０．０１％以下、
Ｏ：０．０１％以下
を含有し、残部がＦｅ及び不可避不純物からなり、鋼板のミクロ組織が体積分率で、４０％以上のフェライトと、５％以上の焼き戻しマルテンサイトとを含有し、ダイナミック硬度計を用いて測定したフェライトの硬度（ＤＨＴＦ）とダイナミック硬度計を用いて測定した焼き戻しマルテンサイトの硬度（ＤＨＴＭ）の比（ＤＨＴＭ／ＤＨＴＦ）が１．５〜３．０であり、残部組織がベイナイト組織よりなることを特徴とする引張最大強度が５９０ＭＰａ以上の穴拡げ性と成形性に優れた高強度鋼板。 % By mass
C: 0.05 to less than 0.25%,
Si: 2.0% or less,
Mn: 1.1 to 3.0%
P: 0.04% or less,
S: 0.01% or less,
Al: 2.0% or less,
N: 0.01% or less,
O: 0.01% or less, the balance is Fe and inevitable impurities, the microstructure of the steel sheet is 40% or more ferrite and 5% or more tempered martensite in volume fraction, the ratio of the dynamic hardness meter ferrite was measured using a hardness (DHTF) and the hardness of the dynamic hardness meter tempered martensite was measured using (DHTM) (DHTM / DHTF) is 1.5 to 3.0, A high-strength steel sheet excellent in hole expansibility and formability having a maximum tensile strength of 590 MPa or more, wherein the remaining structure is a bainite structure .
Ｍｏ：０．０５〜３％、
Ｃｒ：０．０５〜３％、
Ｎｉ：０．０５〜３％、
Ｃｕ：０．０５〜３％、
Ｗ：０．０５〜３％
の１種又は２種以上を含有することを特徴とする請求項１又は２に記載の穴拡げ性と成形性に優れた高強度鋼板。 Furthermore, in steel,
Mo: 0.05-3%,
Cr: 0.05-3%,
Ni: 0.05-3%,
Cu: 0.05-3%,
W: 0.05-3%
The high-strength steel sheet excellent in hole expansibility and formability according to claim 1 or 2, characterized by containing one or more of the following.
Ｔｉ：０．００５〜０．３％、
Ｎｂ：０．００５〜０．３％、
Ｖ：０．００５〜０．３％
の１種又は２種以上を含有することを特徴とする請求項１乃至３の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板。 Furthermore, in steel,
Ti: 0.005 to 0.3%,
Nb: 0.005-0.3%
V: 0.005-0.3%
The high-strength steel sheet excellent in hole expansibility and formability according to any one of claims 1 to 3, characterized by containing one or more of the following.
Ｂ：０．０００１〜０．１％
を含有することを特徴とする請求項１乃至４の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板。 Furthermore, in steel,
B: 0.0001 to 0.1%
The high-strength steel sheet excellent in hole expansibility and formability according to any one of claims 1 to 4.
Ｃａ：０．０００５〜０．０１％、
Ｍｇ：０．０００５〜０．０１％、
Ｚｒ：０．０００５〜０．０１％、
ＲＥＭ：０．０００５〜０．０１％、
Ｙ：０．０００５〜０．０１％
の１種又は２種以上を含有することを特徴とする請求項１乃至５の何れかに記載の穴拡げ性と成形性に優れた高強度鋼板。 Furthermore, in steel,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
Zr: 0.0005 to 0.01%,
REM: 0.0005 to 0.01%,
Y: 0.0005 to 0.01%
The high strength steel plate excellent in hole expansibility and formability according to any one of claims 1 to 5, characterized by containing one or more of the following.
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