JP4662175B2 - Hot-dip galvanized steel sheet based on cold-rolled steel sheet with excellent workability - Google Patents
Hot-dip galvanized steel sheet based on cold-rolled steel sheet with excellent workability Download PDFInfo
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本発明は、加工性に優れた冷延焼鈍鋼板を母材とする溶融亜鉛めっき鋼板（合金化溶融亜鉛めっき鋼板を含む。）並びにその製造方法に関する。 The present invention, galvanized steel sheet to a cold-rolled annealed steel plate with excellent workability matrix (. Including galvannealed steel sheet) and a manufacturing method thereof.
自動車用、家電用鋼板等として、プレス加工性を備えた冷延鋼板やその鋼板を母材とする溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板（以下、特に区別することなく単に溶融亜鉛めっき鋼板という場合は、合金化溶融亜鉛めっき鋼板をも含むものとする。）が用いられている。溶融亜鉛めっきを施すのは、鋼板の耐食性を向上させるためであり、合金化処理を施すのは亜鉛めっき層の密着性を向上させるためである。 Cold rolled steel sheet with press workability, hot dip galvanized steel sheet, alloyed hot dip galvanized steel sheet (hereinafter referred to simply as hot dip galvanized steel sheet without particular distinction) In this case, it is assumed that an alloyed hot-dip galvanized steel sheet is also included. The hot dip galvanization is performed to improve the corrosion resistance of the steel sheet, and the alloying treatment is performed to improve the adhesion of the galvanized layer.
前記冷延鋼板（製品としての冷延鋼板）は、熱延板を冷間加工した後、その鋼板（焼鈍前の冷延鋼板）に再結晶焼鈍を施したものであり、プレス加工性を確保するため、必須組織としてフェライトを有し、第２相としてパーライト、あるいは高強度化を目的として低温変態生成物（マルテンサイト、ベイナイト）を有する複合組織とされる。例えば、特開昭５８−３９７７０号公報には、フェライト＋マルテンサイト＋ベイナイトからなる３相組織の冷延鋼板が、また特開昭５５−１２２８２１号公報にはフェライト＋マルテンサイトからなる２相組織の冷延鋼板を母材とする溶融亜鉛めっき鋼板が記載されている。
近年、製品形状の複雑化に伴い、ますます良好なプレス加工性、特に局部延性が要求されるようになっている。しかしながら、従来の冷延鋼板では、硬い第２相（パーライト、あるいはマルテンサイト、ベイナイト）が破壊の起点になるため、単一組織鋼板に比して局部延性に劣り、この特性を反映する強度−延性バランスが低いという問題がある。 In recent years, with the complication of product shapes, more and more good press workability, particularly local ductility has been required. However, in the conventional cold-rolled steel sheet, since the hard second phase (pearlite, martensite, bainite) becomes the starting point of fracture, the local ductility is inferior to that of a single-structure steel sheet, and the strength that reflects this characteristic − There is a problem that the ductility balance is low.
本発明はかかる問題に鑑みなされたもので、延性、特に強度−延性バランスに優れた冷延鋼板を母材とする溶融亜鉛めっき鋼板を提供することを目的とする。 The present invention has been accomplished in view of such issues, ductility, in particular strength - an object that excellent cold-rolled steel plate in ductility balance to provide a galvanized steel sheet as a base material.
本発明者は、フェライトを含む複合組織を有する冷延鋼板（焼鈍後の冷延鋼板）の延性について鋭意研究した結果、マトリックスであるフェライトを微細化することにより、結晶粒が隣接して形成される３重点の数が増え、その結果としてそこに生成する第２相が微細分散され、このため第２相が破壊の起点として作用し難くなり、局部延性が向上するとの知見を得て本発明を完成するに至った。 As a result of diligent research on the ductility of a cold-rolled steel sheet having a composite structure containing ferrite (cold-rolled steel sheet after annealing), the present inventor has formed crystal grains adjacent to each other by refining the ferrite as a matrix. As a result, the second phase generated therein is finely dispersed, so that the second phase is difficult to act as a starting point of fracture, and the knowledge that the local ductility is improved is obtained. It came to complete.
すなわち、本発明の溶融亜鉛めっき鋼板は、冷延鋼板は、mass％（以下、単に「％」と表示する。）で
Ｃ ：０．０４〜０．１６％
Ｓｉ：０．５％以下、
Ｍｎ：０．５〜１．５％、
Ｐ ：０．２０％以下、
Ｓ ：０．０１％以下、
Ａｌ：０．００５〜０．１０％、
Ｎ ：０．００５％以下、
を含み、残部Ｆｅ及び不可避的不純物からなり、組織がフェライトと、パーライト及び面積率で３％未満（０％を含む）のベイナイトで形成された第２相からなり、前記フェライトの平均粒径が相当円直径で２〜６μm とされた加工性に優れた冷延鋼板を母材とし、その表面に溶融亜鉛めっき層あるいは合金化溶融亜鉛めっき層が形成されたものである。
That is, in the hot-dip galvanized steel sheet of the present invention , the cold-rolled steel sheet is mass% (hereinafter simply referred to as “%”) C: 0.04 to 0.16%
Si: 0.5% or less,
Mn: 0.5 to 1.5%
P: 0.20% or less,
S: 0.01% or less,
Al: 0.005 to 0.10%,
N: 0.005% or less,
And the balance is composed of Fe and inevitable impurities, and the structure is composed of ferrite and a second phase formed of pearlite and bainite having an area ratio of less than 3% (including 0%), and the average particle diameter of the ferrite is A cold-rolled steel sheet having an equivalent circular diameter of 2 to 6 μm and excellent workability is used as a base material, and a hot-dip galvanized layer or an alloyed hot-dip galvanized layer is formed on the surface thereof .
上記冷延鋼板によると、同強度レベルの冷延鋼板に比して、フェライトの平均粒径が２〜６μm であるので、結晶粒の３重点が増え、そこに生成する第２相を微細化することができ、このため局部延性、強度−延性バランス（引張強さＴＳ×伸びＥｌ値）が向上し、プレス加工性に優れる。また、上記組成により、容易にＴＳを３００〜５００ＭＰａ程度とすることができ、この種の強度レベルの鋼板において優れた強度−延性バランスを得ることができる。 According to the cold-rolled steel sheet, the average grain size of ferrite is 2 to 6 μm compared to the cold-rolled steel sheet of the same strength level, so the triple point of crystal grains is increased and the second phase generated there is refined. Therefore, local ductility and strength-ductility balance (tensile strength TS × elongation El value) are improved, and press workability is excellent. Moreover, TS can be easily made into about 300-500 MPa by the said composition, and the strength-ductility balance excellent in the steel plate of this kind of strength level can be obtained.
前記冷延鋼板を母材として、これに溶融亜鉛めっき層、合金化溶融亜鉛めっき層を形成することによって、プレス加工性のみならず、耐食性にも優れた溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板が得られる。 Using the cold-rolled steel sheet as a base material, a hot-dip galvanized layer and an alloyed hot-dip galvanized layer are formed on the cold-rolled steel sheet, thereby providing not only press workability but also excellent corrosion resistance. A steel plate is obtained.
前記冷延鋼板は、前記成分の冷延鋼板（焼鈍前の冷延板）をフェライト＋オーステナイトの２相共存温度域内の焼鈍温度まで、５００〜７００℃の温度範囲を１０℃/sec以上の昇温速度の下で加熱して再結晶焼鈍した後、冷却することによって好適に製造することができる。
５００〜７００℃の温度範囲を１０℃/sec以上の昇温速度で加熱することによって、この温度範囲におけるフェライトの加工組織の回復を抑制して、焼鈍温度においてフェライト結晶粒を加工組織から一気に再結晶させることができ、これによってフェライト結晶粒を２〜６μm 程度に微細化することができ、このため第２相の核生成の発生源となる結晶粒の３重点を多数形成することができ、第２相を微細化することができる。
The cold-rolled steel sheet has a temperature range of 500 to 700 ° C. increased to 10 ° C./sec or more up to an annealing temperature within the two-phase coexisting temperature range of ferrite and austenite. It can manufacture suitably by heating under a temperature rate and performing recrystallization annealing and then cooling.
By heating the temperature range of 500 to 700 ° C. at a temperature increase rate of 10 ° C./sec or more, recovery of the ferrite processed structure in this temperature range is suppressed, and the ferrite crystal grains are re-started from the processed structure at the annealing temperature all at once. The ferrite crystal grains can be refined to about 2 to 6 μm, so that many triple points of the crystal grains that are the source of nucleation of the second phase can be formed. The second phase can be refined.
溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板を製造する場合には、連続焼鈍ラインにて前記昇温速度条件に従って２相共存域に加熱し、再結晶焼鈍を行えばよい。 When producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, recrystallization annealing may be performed by heating to a two-phase coexistence region according to the temperature increase rate condition in a continuous annealing line.
本発明の溶融亜鉛めっき鋼板によれば、その母材の冷延鋼板が特に所定の成分の下、フェライトの平均粒径を２〜６μm と微細化したので、これに伴ってフェライト結晶粒の３重点に生成する第２相を微細に分散することができ、同成分系の冷延鋼板であれば強度−延性バランスが向上し、局部延性の向上により優れたプレス加工性を得ることができる。このため、耐食性のみならず、プレス加工性にも優れた溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板を提供することができる。 According to the hot-dip galvanized steel sheet of the present invention, since the base cold-rolled steel sheet is refined to an average ferrite grain size of 2 to 6 μm, particularly under a predetermined component, the ferrite crystal grains 3 The second phase generated in the emphasis can be finely dispersed, and if it is a cold-rolled steel sheet of the same component system, the strength-ductility balance is improved, and excellent press workability can be obtained by improving the local ductility. For this reason, not only corrosion resistance but also hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet that are excellent in press workability can be provided .
本発明の溶融亜鉛めっき鋼板の母材に係る冷延鋼板の化学組成は以下のとおりであり、各成分の限定理由について説明する。
Ｃ ：０．０４〜０．１６％、
Ｓｉ：０．５％以下、
Ｍｎ：０．５〜１．５％、
Ｐ ：０．２０％以下、
Ｓ ：０．０１％以下、
Ａｌ：０．００５〜０．１０％、
Ｎ ：０．００５％以下、
を含み、残部がＦｅおよび不可避的不純物からなる。
The chemical composition of the cold-rolled steel sheet according to the base material of the hot-dip galvanized steel sheet of the present invention is as follows, and the reasons for limiting each component will be described.
C: 0.04 to 0.16%,
Si: 0.5% or less,
Mn: 0.5 to 1.5%
P: 0.20% or less,
S: 0.01% or less,
Al: 0.005 to 0.10%,
N: 0.005% or less,
The balance consists of Fe and inevitable impurities.
Ｃ：０．０４〜０．１６％
プレス加工性を向上させるにはＣ量は少ないほどよいが、０．０４％未満では強度低下が過大であり、強度−延性バランスが低下する。一方、０．１６％を超えると、後述のＭｎ量ではパーライト量が過多になり、またパーライトのラメラ間隔が広くなって、延性が劣化するようになる。このため、本発明ではＣ量の下限を０．０４％、好ましくは０．０８％、より好ましくは０．０９％とし、上限を０．１６％、好ましくは０．１５％、より好ましくは０．１３％とする。
C: 0.04 to 0.16%
In order to improve the press workability, the smaller the amount of C, the better. However, if it is less than 0.04%, the strength is excessively lowered and the strength-ductility balance is lowered. On the other hand, if it exceeds 0.16%, the amount of pearlite becomes excessive with the amount of Mn described later, the lamella spacing of pearlite becomes wide, and the ductility deteriorates. Therefore, in the present invention, the lower limit of the C amount is 0.04%, preferably 0.08%, more preferably 0.09%, and the upper limit is 0.16%, preferably 0.15%, more preferably 0. .13%.
Ｓｉ：０．５％以下
Ｓｉは固溶強化元素として鋼板の強度向上に寄与するが、その一方で延性を低下させる。また、過多に添加すると溶融亜鉛めっき付着性を著しく劣化させる。このため、本発明では上限を０．５％、好ましくは０．２％とする。
Si: 0.5% or less Si contributes to improving the strength of the steel sheet as a solid solution strengthening element, but reduces ductility. Moreover, when it adds excessively, hot-dip galvanization adhesiveness will deteriorate remarkably. Therefore, in the present invention, the upper limit is set to 0.5%, preferably 0.2%.
Ｍｎ：０．５〜１．５％
Ｍｎは固溶強化元素として鋼の強度を向上させる。０．５％未満では強度が不足して強度−延性バランスが低下する。一方、１．５％を超えると、パーライト＋ベイナイトを主体とした組織になり、延性が低下するようになる。このため、Ｍｎ量の下限を０．５％、好ましくは０．８％、より好ましくは０．９％とし、上限を１．５％、好ましくは１．３％、より好ましくは１．１％とする。
Mn: 0.5 to 1.5%
Mn improves the strength of steel as a solid solution strengthening element. If it is less than 0.5%, the strength is insufficient and the strength-ductility balance is lowered. On the other hand, if it exceeds 1.5%, it becomes a structure mainly composed of pearlite + bainite, and the ductility is lowered. Therefore, the lower limit of the amount of Mn is 0.5%, preferably 0.8%, more preferably 0.9%, and the upper limit is 1.5%, preferably 1.3%, more preferably 1.1%. And
Ｐ：０．２０％以下
Ｐは安価な固溶強化元素であり、鋼を強化するには有用な元素であるが、本発明では延性の向上を重視するため、少ないほどよく、０．２０％以下に止める。好ましくは０．１０％とするのがよい。
P: 0.20% or less P is an inexpensive solid solution strengthening element and is an element useful for strengthening steel. However, in the present invention, importance is placed on improving ductility. Stop below. Preferably it is 0.10%.
Ｓ：０．０１％以下
ＳはＳ系析出物（主にＭｎＳ）を生成し、延性を劣化させるので、少ないほどよく、本発明では０．０１％以下、好ましくは０．００６％以下に止める。
S: 0.01% or less Since S produces S-based precipitates (mainly MnS) and deteriorates ductility, the smaller the better, in the present invention, 0.01% or less, preferably 0.006% or less. .
Ａｌ：０．００５〜０．１０％
Ａｌは主に脱酸剤として作用し、少なくとも０．００５％添加する必要がある。しかし、過多に添加すると脱酸効果が飽和するだけでなく、アルミナ系介在物の生成により延性劣化、連鋳ノズル詰まりによる生産性の劣化等の問題を引き起こすので、上限を０．１０％とする。
Al: 0.005-0.10%
Al mainly acts as a deoxidizer and should be added at least 0.005%. However, if it is added excessively, not only the deoxidation effect is saturated, but also the production of alumina inclusions causes problems such as ductility deterioration and productivity deterioration due to continuous nozzle clogging, so the upper limit is made 0.10% .
Ｎ：０．００５％以下
Ｎはその含有量が多いほど、Ｎを固定するのに要する窒化物形成元素添加量が増えて製造コスト高を招き、また延性を阻害するようになるので、本発明では少ないほどよく、Ｎ量の上限を０．００５％、好ましくは０．００３％とする。
N: 0.005% or less As the content of N increases, the amount of nitride-forming element added to fix N increases, resulting in an increase in production cost and impairing ductility. Then, the smaller the content, the better. The upper limit of the N amount is 0.005%, preferably 0.003%.
本発明に係る冷延鋼板の化学組成は、以上の基本成分のほか、Ｆｅおよび残部不可避的不純物からなるが、基本成分の各作用を妨げない元素や、材質特性を向上させる元素を添加することができ、例えば、下記のＣｒ、Ｂの１種または２種を添加することができる。 The chemical composition of the cold-rolled steel sheet according to the present invention is composed of Fe and the balance inevitable impurities in addition to the basic components described above, but elements that do not interfere with the functions of the basic components and elements that improve material properties are added. For example, one or two of the following Cr and B can be added.
Ｃｒ：０．５％以下、Ｂ：０．００５％以下
ＣｒはＣｒＣを、ＢはＢＮを生成するため、フェライト中の固溶Ｃ、固溶Ｎが減少し、局部伸びが向上する。一方、過多に含有すると、フェライト＋ベイナイト組織となり、強度−延性バランスが低下するようになる。好ましくは、Ｃｒ：０．３％以下、Ｂ：０．００３％以下とするのがよい。
Cr: 0.5% or less, B: 0.005% or less Since Cr produces CrC and B produces BN, solid solution C and solid solution N in ferrite are reduced, and local elongation is improved. On the other hand, when it contains excessively, it will become a ferrite + bainite structure and strength-ductility balance will fall. Preferably, Cr: 0.3% or less, B: 0.003% or less.
上記冷延鋼板の組織は、フェライトと、パーライトを第２相とする２相組織とすることが好ましい。第２相としてパーライトのほか、ベイナイトが生成すると局部延性は向上するが、全伸びが低下し、強度−延性バランスが低下するようになる。また、フェライト中には、微細な炭化物（主にセメンタイト）を生成させることが好ましい。これにより、フェライト中の固溶Ｃが抑制されて延性を向上させることができる。なお、組織はフェライト＋パーライトの２相のみからなるものが好ましいが、ベイナイトが不可避的に生成する場合があり、本発明では面積率で３％未満のベイナイトは許容される。 The structure of the cold-rolled steel sheet is preferably a two-phase structure having ferrite and pearlite as the second phase. When bainite is generated in addition to pearlite as the second phase, the local ductility is improved, but the total elongation is lowered and the strength-ductility balance is lowered. Moreover, it is preferable to produce fine carbides (mainly cementite) in the ferrite. Thereby, the solid solution C in a ferrite is suppressed and ductility can be improved. In addition, although the structure | tissue which consists only of two phases of a ferrite + pearlite is preferable, a bainite may inevitably generate | occur | produce and the bainite below 3% is accept | permitted by an area ratio in this invention.
前記フェライトは、加工性確保のための必須組織であり、フェライトの平均粒径は２〜６μm に制限される。２μm 未満では粒径が微細過ぎて加工硬化指数が低下し、かえって延性が低下する。一方、６μm 超では、粒径が粗過ぎて、第２相が析出する起点となる、隣接する結晶粒の粒界の３重点が少なくなり、第２相の微細分散化が難くなり、局部延性が劣化するようになる。このため、本発明では平均粒径の下限を２μm 、好ましくは３μm とし、その上限を６μm 、好ましくは５μm とする。 The ferrite is an essential structure for ensuring workability, and the average grain size of ferrite is limited to 2 to 6 μm. If it is less than 2 μm, the particle size is too fine and the work hardening index is lowered, and the ductility is lowered. On the other hand, if it exceeds 6 μm, the grain size is too coarse, and the triple point of the grain boundary of the adjacent crystal grain that becomes the starting point for precipitation of the second phase is reduced, making it difficult to finely disperse the second phase and local ductility. Will begin to deteriorate. Therefore, in the present invention, the lower limit of the average particle diameter is 2 μm, preferably 3 μm, and the upper limit is 6 μm, preferably 5 μm.
上記本発明の冷延鋼板は、そのままでプレス加工用鋼板として利用することができるが、溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板の母材としても好適に利用することができる。 Although the cold-rolled steel sheet of the present invention can be used as it is as a steel sheet for press working as it is, it can also be suitably used as a base material for hot-dip galvanized steel sheets and galvannealed steel sheets.
次に、本発明の溶融亜鉛めっき鋼板の製造方法について説明する。
本発明の溶融亜鉛めっき鋼板の母材である冷延鋼板は、前記成分の鋼片を熱間圧延した後、冷間圧延し、所定の加熱条件の下で再結晶焼鈍をすることによって製造される。
Next, the manufacturing method of the hot dip galvanized steel sheet of this invention is demonstrated.
The cold-rolled steel sheet, which is the base material of the hot-dip galvanized steel sheet of the present invention , is manufactured by hot-rolling the steel slab of the above components, then cold-rolling, and recrystallization annealing under predetermined heating conditions. The
熱間圧延条件については特に制限されない。一般的には、鋼片加熱温度は１１００〜１２５０℃程度とするのがよく、熱延仕上温度はＡr3点以上とするのがよく、巻取温度はフェライト＋パーライト、あるいはフェライト＋ベイナイト組織が得られるように４００〜７００℃程度とするのがよい。巻取温度を６００℃以上の高めにすると熱延鋼板は概ねフェライト＋パーライト組織に、巻取温度を６００未満にするとフェライト＋ベイナイト組織になる。熱延後、酸洗し、冷延率を４０％程度以上、好ましくは５０％程度以上で冷間圧延を行えばよい。 There are no particular restrictions on the hot rolling conditions. In general, the billet heating temperature is preferably about 1100 to 1250 ° C., the hot rolling finishing temperature is preferably Ar 3 points or more, and the coiling temperature is ferrite + pearlite or ferrite + bainite structure. It is preferable that the temperature is about 400 to 700 ° C. When the coiling temperature is raised to 600 ° C. or higher, the hot-rolled steel sheet generally has a ferrite + pearlite structure, and when the coiling temperature is less than 600, it has a ferrite + bainite structure. After hot rolling, pickling and cold rolling may be performed at a cold rolling rate of about 40% or more, preferably about 50% or more.
冷間圧延後の鋼板は、フェライト＋オーステナイトの２相共存域（Ａc1〜Ａc3）内の焼鈍温度にて再結晶焼鈍される。再結晶焼鈍時間は、通常、連続焼鈍（めっき）ラインでは数秒から十数秒程度である。なお、Ａc1、Ａc3は、実測により求めることができるが、下記式（「鉄鋼材料」日本金属学会）により算出しても差し支えない。
Ａc3＝910−203(C)1/2＋44.7(Si)
Ａc1＝723−10.7(Mn)＋29.1(Si)＋16.9(Cr)
The steel sheet after cold rolling is recrystallized and annealed at an annealing temperature within the two-phase coexistence region (Ac1 to Ac3) of ferrite and austenite. The recrystallization annealing time is usually about several seconds to several tens of seconds in a continuous annealing (plating) line. Ac1 and Ac3 can be obtained by actual measurement, but may be calculated by the following formula (“steel material”, Japan Institute of Metals).
Ac3 = 910−203 (C) 1/2 +44.7 (Si)
Ac1 = 723-10.7 (Mn) +29.1 (Si) +16.9 (Cr)
前記焼鈍温度への昇温段階において、回復が進行しないように５００〜７００℃の温度域を１０℃/sec以上の昇温速度ＨＲで速やかに２相共存温度域に昇温することが必要である。１０℃/sec未満では、オーステナイトが生成しないが、比較的高温の温度域を通過する際にフェライト結晶粒に冷間圧延によって導入された転位が回復し、再結晶焼鈍の際に結晶粒が成長してしまい、本発明の企図する平均粒径２〜６μm の微細フェライト粒が得られず、またフェライト粒界の３重点に析出する第２相を微細化することができないようになる。１０℃/sec以上、好ましくは２０℃/sec以上、より好ましくは５０℃/sec以上の昇温速度で前記温度域を昇温することにより、回復がほとんど生じることなく、２相共存域に加熱されるため、平均粒径２〜６μm の微細なフェライト結晶粒の３重点にオーステナイトを微細に析出させることができる。再結晶焼鈍後の冷却速度は制限されず、放冷でよく、冷却速度を上げる必要はない。 In the temperature raising stage to the annealing temperature, it is necessary to quickly raise the temperature range from 500 to 700 ° C. to the two-phase coexisting temperature range at a temperature raising rate HR of 10 ° C./sec or more so that recovery does not proceed. is there. If it is less than 10 ° C / sec, austenite is not generated, but dislocations introduced by cold rolling are recovered in the ferrite grains when passing through a relatively high temperature range, and the grains grow during recrystallization annealing. As a result, fine ferrite grains having an average particle diameter of 2 to 6 μm intended by the present invention cannot be obtained, and the second phase precipitated at the triple point of the ferrite grain boundary cannot be refined. Heating to the two-phase coexistence region with almost no recovery by raising the temperature range at a rate of temperature increase of 10 ° C / sec or more, preferably 20 ° C / sec or more, more preferably 50 ° C / sec or more. Therefore, austenite can be finely precipitated at the triple points of fine ferrite crystal grains having an average particle diameter of 2 to 6 μm. The cooling rate after recrystallization annealing is not limited and may be allowed to cool, and there is no need to increase the cooling rate.
なお、従来、２相共存域への昇温は、ラジアントチューブにより加熱されており、昇温速度は２〜６℃/sec程度である。もっとも、高速加熱が可能な直火炉を用いて加熱される場合があるが、６００℃を超えると温度むらが著しく、鋼板が変形して通板が不可能となるので、通常、６００℃以下の加熱に止められている。本発明では、誘導加熱（ＩＨ）炉、あるいは６００℃以下を直火炉で加熱し、それ以上をＩＨ炉で加熱することが推奨される。 Conventionally, the temperature increase to the two-phase coexistence region is heated by a radiant tube, and the temperature increase rate is about 2 to 6 ° C./sec. Of course, it may be heated using a direct-fired furnace capable of high-speed heating, but if it exceeds 600 ° C., the temperature unevenness is remarkable, and the steel plate is deformed so that it cannot be passed through. Stopped by heating. In the present invention, it is recommended to heat an induction heating (IH) furnace or 600 ° C. or less in a direct-fired furnace, and further heat it in an IH furnace.
溶融亜鉛めっき鋼板を製造する場合、連続焼鈍めっきラインにて前記ＨＲで昇温し、２相共存域内の焼鈍温度にて焼鈍処理を行い、再結晶焼鈍後、溶融亜鉛めっきを施し、必要に応じて合金化処理を行い、冷却すればよい。溶融亜鉛めっきは、めっき温度が通常４００〜４８０℃程度の溶融亜鉛めっき浴に浸漬することによって行われる。合金化処理は、溶融亜鉛めっき後、引き続いて５００〜７００℃程度、好ましくは５５０〜６００℃程度の温度で、通常、数秒〜十数秒程度加熱保持することによって行われる。 When manufacturing hot dip galvanized steel sheets, the temperature is increased at the HR in the continuous annealing plating line, the annealing treatment is performed at the annealing temperature within the two-phase coexistence region, the hot dip galvanizing is performed after recrystallization annealing, and if necessary It is sufficient to perform alloying treatment and cool. Hot dip galvanization is performed by immersing in a hot dip galvanizing bath whose plating temperature is usually about 400 to 480 ° C. The alloying treatment is carried out after the hot dip galvanization by heating and holding at a temperature of about 500 to 700 ° C., preferably about 550 to 600 ° C., usually for a few seconds to a few dozen seconds.
以下、実施例により本発明をさらに説明するが、本発明はかかる実施例によって限定的に解釈されるものではない。 EXAMPLES Hereinafter, although an Example demonstrates this invention further, this invention is not interpreted limitedly by this Example.
下記表１に記載した化学成分の鋼を真空誘導溶解にて溶製し、その鋼片を１１５０℃にて加熱し、仕上温度を８５０℃として熱間圧延を行い、５６０〜６８０℃にて巻取り、酸洗後、冷延率６０％で冷間圧延を行い、厚さ１．２mmの冷延鋼板を得た。
この冷延鋼板から切り出したサンプルを赤外線イメージ炉を用いて、表２に示す昇温速度（ＨＲ）にて予熱温度（２００℃程度）から同表に示した焼鈍温度（Tann）まで昇温し、同温度にて６０秒間保持した後、冷却（放冷）した。
The steels having the chemical components listed in Table 1 below are melted by vacuum induction melting, the steel pieces are heated at 1150 ° C, hot rolled at a finishing temperature of 850 ° C, and wound at 560 to 680 ° C. After picking and pickling, cold rolling was performed at a cold rolling rate of 60% to obtain a cold rolled steel sheet having a thickness of 1.2 mm.
The sample cut out from this cold-rolled steel sheet was heated from the preheating temperature (about 200 ° C.) to the annealing temperature (Tann) shown in the same table at a heating rate (HR) shown in Table 2 using an infrared image furnace. The mixture was held at the same temperature for 60 seconds and then cooled (cooled).
得られたサンプルから組織観察試料を採取し、光学顕微鏡（４００倍）により組織観察を行い、観察された組織写真を画像解析し、フェライト結晶粒径を相当円直径で近似して平均結晶粒径Ｄｆを測定した。また、ＪＩＳ５号試験片を採取し、ＪＩＳ２２４１に定められた試験法による引張試験を行い、機械的性質（ＴＳ：引張強さ、Ｅｌ：伸び）を調べた。調査結果を表２に併せて示す。 Take a tissue observation sample from the obtained sample, observe the structure with an optical microscope (400 times), analyze the image of the observed structure, approximate the ferrite crystal grain size with the equivalent circle diameter, and average grain size Df was measured. Further, a JIS No. 5 test piece was collected and subjected to a tensile test by a test method defined in JIS 2241 to examine mechanical properties (TS: tensile strength, El: elongation). The survey results are also shown in Table 2.
表２より、Ｃ量が発明範囲下の鋼種Ａを用いた試料No. １では引張強さが３００ＭＰａ未満となり、低すぎる。一方、Ｃ量が発明範囲上の鋼種Ｄを用いた試料No. ９では伸びが３０％未満となり、低すぎるため、加工性に劣り、また溶接性にも劣る。また、Ｃ量が発明範囲内でも、昇温速度、あるいは焼鈍温度が発明範囲外の試料No. ５〜７では、フェライト粒径が大きくなり過ぎて強度−延性バランスに劣る。これらに対して他の発明例は平均粒径が４〜５．４μm のフェライト結晶粒を有し、伸びが３４％以上で、ＴＳ×ＥＬ値も１３０００以上であり、強度−延性バランスが良好である。 From Table 2, the tensile strength is less than 300 MPa in the sample No. 1 using the steel type A whose C amount is below the scope of the invention, which is too low. On the other hand, sample No. 9 using steel type D with the C amount within the scope of the invention has an elongation of less than 30% and is too low, so that the workability is poor and the weldability is also poor. Even when the C amount is within the range of the invention, Sample Nos. 5 to 7 whose heating rate or annealing temperature is outside the range of the invention are inferior in the strength-ductility balance because the ferrite grain size becomes too large. On the other hand, other invention examples have ferrite crystal grains having an average grain size of 4 to 5.4 μm, elongation of 34% or more, TS × EL value of 13000 or more, and a good strength-ductility balance. is there.
Claims (3)
Ｃ ：０．０４〜０．１６％、
Ｓｉ：０．５％以下、
Ｍｎ：０．５〜１．５％、
Ｐ ：０．２０％以下、
Ｓ ：０．０１％以下、
Ａｌ：０．００５〜０．１０％、
Ｎ ：０．００５％以下、
を含み、残部Ｆｅ及び不可避的不純物からなり、組織がフェライトと、パーライト及び面積率で３％未満（０％を含む）のベイナイトで形成された第２相からなり、前記フェライトの平均粒径が相当円直径で２〜６μm である、加工性に優れた冷延鋼板を母材とし、その表面に溶融亜鉛めっき層が形成された、溶融亜鉛めっき鋼板。 mass% C: 0.04 to 0.16%,
Si: 0.5% or less,
Mn: 0.5 to 1.5%
P: 0.20% or less,
S: 0.01% or less,
Al: 0.005 to 0.10%,
N: 0.005% or less,
And the balance is composed of Fe and inevitable impurities, and the structure is composed of ferrite and a second phase formed of pearlite and bainite having an area ratio of less than 3% (including 0%), and the average particle diameter of the ferrite is A hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on the surface of a cold-rolled steel sheet having an equivalent circle diameter of 2 to 6 μm and excellent workability.
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