JP6663376B2 - Equipment with bathtub for molten zinc bath - Google Patents
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Description
本発明は、溶融亜鉛浴用浴槽を備えた設備に関するものである。 The present invention relates to equipment which includes a tub for molten zinc.
従来より、鉄鋼材料に施す経済的な防錆処理方法として、一般に溶融亜鉛めっき法が広く用いられている。このような溶融亜鉛めっき法は、溶融亜鉛が入れられた鋼製の浴槽に、めっきを施す鋼材を浸漬して行なわれる。 2. Description of the Related Art Conventionally, a hot-dip galvanizing method has been widely used as an economical rust prevention treatment method applied to steel materials. Such a hot-dip galvanizing method is performed by immersing a steel material to be plated in a steel bath containing molten zinc.
上述のような鋼製の浴槽は、溶融亜鉛と接触する界面において、溶融亜鉛と浴槽の鉄とが反応し、鉄−亜鉛合金層を形成するため、腐食が進行してしまう。この際の腐食速度は、溶融亜鉛の温度が５００℃近傍である場合に非常に大きくなるので、浴槽の温度管理が適切でないと、短期間で減肉による破損が生じたり、穴あきが生じたりすることにより、浴槽が使用できなくなるという問題がある。 In the steel bathtub as described above, molten zinc reacts with iron in the bathtub at an interface in contact with the molten zinc to form an iron-zinc alloy layer, so that corrosion proceeds. Since the corrosion rate at this time becomes extremely large when the temperature of the molten zinc is around 500 ° C., if the temperature of the bathtub is not properly controlled, breakage due to wall thinning in a short period of time or perforation may occur. This causes a problem that the bathtub cannot be used.
上述のような腐食の問題に対し、浴槽をなす鋼の化学成分組成において、Ｃの含有量を高め、且つ、Ｓｉ及びＰの含有量を抑えることにより、溶融亜鉛の温度が５００℃近傍である場合の腐食速度を低減させた鋼材が提案されている（例えば、特許文献１〜４を参照）。特許文献１〜４に記載の鋼材によれば、鋼成分を上記組成とすることにより、溶融亜鉛による腐食作用に対して優れた耐食性を有し、浴槽の減肉や穴あき等を抑制することが可能となる。 With respect to the corrosion problem as described above, the temperature of the molten zinc is around 500 ° C. by increasing the content of C and suppressing the content of Si and P in the chemical composition of the steel forming the bathtub. Steel materials with reduced corrosion rates in such cases have been proposed (for example, see Patent Documents 1 to 4). According to the steel materials described in Patent Literatures 1 to 4, the steel component having the above composition has excellent corrosion resistance against the corrosive action of molten zinc, and suppresses wall thinning and perforation of the bathtub. Becomes possible.
しかしながら、特許文献１〜４に記載の鋼材では、浴槽の減肉や穴あき等を抑制できる一方で、亜鉛の作用による割れが生じ、浴槽の寿命が短くなる場合があった。本発明はこのような実情に鑑みてなされたものであり、溶融亜鉛浴用浴槽を備えた設備を提供することを目的とする。 However, in the steel materials described in Patent Literatures 1 to 4, while thinning and perforation of the bathtub can be suppressed, cracks occur due to the action of zinc, and the life of the bathtub may be shortened. The present invention has been made in view of such circumstances, and an object thereof is to provide a facility which includes a tub for molten zinc.
本発明者等は、溶融亜鉛腐食性に優れる化学成分の鋼を用いて、亜鉛割れ性に及ぼす金属組織の影響について鋭意検討を行なった。その結果、金属組織の７０％以上（面積率）をベイナイト組織にすることにより、耐溶融亜鉛腐食性のみならず、耐亜鉛割れ性にも優れた鋼材が得られることを見出した。本発明は、かかる知見に基づいて完成されたものであり、その要旨とするところは以下の通りである。 The present inventors have conducted intensive studies on the effect of the metal structure on zinc cracking using steel having a chemical composition excellent in molten zinc corrosion resistance. As a result, it has been found that a steel material excellent in not only the corrosion resistance to molten zinc but also the resistance to zinc cracking can be obtained by setting the bainite structure to 70% or more (area ratio) of the metal structure. The present invention has been completed based on such knowledge, and the gist thereof is as follows.
［１］ 質量％でＣ：０．１２超〜０．３０％、Ｓｉ：０．０５％以下、Ｍｎ：０．２０〜２．０％を含有し、Ｐ：０．０１５％以下、Ｓ：０．０３０％以下、Ａｌ：０．０７０％以下に制限し、残部がＦｅおよび不可避的不純物からなり、圧延方向に平行、且つ、板面に垂直な板厚断面において、板厚方向で板面からｔ／４部までの表層の金属組織のうち、面積率で７０％以上がベイナイト組織であり、残部がフェライト組織である鋼板を用いて
構成される溶融亜鉛浴用浴槽を備えた設備。
［２］ 前記鋼板が、さらに、質量％で、Ｎｂ：０．００３〜０．０５０％、Ｖ：０．０１〜０．１０％、Ｔｉ：０．００５〜０．０５０％のうちの１種または２種以上を含有する上記［１］に記載の溶融亜鉛浴用浴槽を備えた設備。
［３］ 前記鋼板が、さらに、質量％で、Ｃｕ：０．１〜０．５％、Ｎｉ：０．１〜２．０％、Ｃｒ：０．１〜２．０％、Ｍｏ：０．０２〜１．０％のうちの１種または２種以上を含有する上記［１］または［２］に記載の溶融亜鉛浴用浴槽を備えた設備。
［４］ 前記鋼板が、さらに、質量％で、Ｃａ：０．０００２〜０．００３０％、Ｍｇ：０．０００２〜０．００３０％、ＲＥＭ：０．０００２〜０．００３０％のうちの１種または２種以上を含有する上記［１］から［３］のいずれか一項に記載の溶融亜鉛浴用浴槽を備えた設備。
［５］ 前記鋼板が、さらに、質量％で、Ｂ：０．０００２〜０．００１０％を含有する上記［１］から［４］のいずれか一項に記載の溶融亜鉛浴用浴槽を備えた設備。
[1] C: more than 0.12 to 0.30%, Si: 0.05% or less, Mn: 0.20 to 2.0% by mass%, P: 0.015% or less, S: 0.030% or less, Al: limited to 0.070% or less, the balance being Fe and unavoidable impurities, and in a sheet thickness section parallel to the rolling direction and perpendicular to the sheet surface, the sheet surface in the sheet thickness direction. of the surface of the metal structure of up to t / 4 parts from more than 70% in the area ratio is bainitic structure, the balance comprising a tub molten zinc constructed using steel is ferrite structure facilities.
[2] The steel sheet is further one of mass%, Nb: 0.003 to 0.050%, V: 0.01 to 0.10%, Ti: 0.005 to 0.050%. or equipment having a tub molten zinc as described in [1] containing two or more.
[3] The steel sheet further contains, by mass%, Cu: 0.1 to 0.5%, Ni: 0.1 to 2.0%, Cr: 0.1 to 2.0%, and Mo: 0.1 to 2.0%. above containing one or more of the 02 to 1.0% [1] or [2] installation comprising a tub molten zinc bath according to.
[4] The steel sheet further includes one of Ca: 0.0002 to 0.0030%, Mg: 0.0002 to 0.0030%, and REM: 0.0002 to 0.0030% by mass%. or equipment having a tub molten zinc bath according the above [1] containing two or more in any one of [3].
[5] The steel plate further contains, by mass%, B: with a tub molten zinc bath according to any one of [4] [1] containing from 0.0002 to 0.0010% Facility.
本発明の耐溶融亜鉛腐食性および耐亜鉛割れ性に優れた溶融亜鉛浴設備用鋼板を用いて溶融亜鉛浴用釜（浴槽）を構成することにより、腐食や割れ等が生じにくく、長寿命の溶融亜鉛浴用釜が得られるので、産業上の効果は極めて大きく、また、構造物の安全性の観点から社会に対する貢献も計り知れない。 By using a steel plate for hot-dip zinc bath equipment with excellent hot-dip zinc corrosion resistance and hot-dip galvanizing resistance of the present invention, a hot-dip bath (bath) for hot-dip zinc bath is less likely to be corroded or cracked and has a long service life. Since a zinc bath is obtained, the industrial effect is extremely large, and the contribution to society is immeasurable from the viewpoint of structural safety.
以下、本発明の耐溶融亜鉛腐食性および耐亜鉛割れ性に優れた溶融亜鉛浴設備用鋼板（以下、単に溶融亜鉛浴設備用鋼板と略称することがある）とその製造方法の実施の形態について、図面を適宜参照しながら説明する。
なお、この実施形態は、発明の趣旨をより良く理解させるために詳細に説明するものであるから、特に指定の無い限り、本発明を限定するものではない。
また、以下の説明において、化学成分組成における各成分の含有量を示す「％」は、特に指定の無い限り「質量％」を示す。
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an embodiment of a steel sheet for molten zinc bath equipment (hereinafter may be simply abbreviated as a steel sheet for molten zinc bath equipment) and a method of manufacturing the same, which are excellent in hot-dip zinc corrosion resistance and zinc cracking resistance of the present invention. This will be described with reference to the drawings as appropriate.
This embodiment is described in detail for better understanding of the gist of the invention, and thus does not limit the invention unless otherwise specified.
In the following description, “%” indicating the content of each component in the chemical component composition indicates “% by mass” unless otherwise specified.
［溶融亜鉛浴設備用鋼板］
本発明の耐溶融亜鉛腐食性および耐亜鉛割れ性に優れた溶融亜鉛浴設備用鋼板は、質量％でＣ：０．１０〜０．３０％、Ｓｉ：０．０５％以下、Ｍｎ：０．２０〜２．０％を含有し、Ｐ：０．０１５％以下、Ｓ：０．０３０％以下、Ａｌ：０．０７０％以下に制限し、残部がＦｅおよび不可避的不純物からなり、圧延方向に平行、且つ、板面に垂直な板厚断面において、板厚方向で板面からｔ／４部までの表層の金属組織のうち、面積率で７０％以上がベイナイト組織とされ、概略構成される。
[Steel for molten zinc bath equipment]
The steel sheet for molten zinc bath equipment having excellent molten zinc corrosion resistance and zinc cracking resistance according to the present invention has C: 0.10 to 0.30%, Si: 0.05% or less, and Mn: 0. 20 to 2.0%, P: 0.015% or less, S: 0.030% or less, Al: 0.070% or less, the balance being Fe and unavoidable impurities, and in the rolling direction. In a plate thickness section parallel to and perpendicular to the plate surface, of the surface layer metal structure from the plate surface to t / 4 part in the plate thickness direction, 70% or more in area ratio is a bainite structure and is roughly configured. .
＜化学成分組成＞
本発明の溶融亜鉛浴設備用鋼板は、Ｃ：０．１０〜０．３０％、Ｓｉ：０．０５％以下、Ｍｎ：０．２〜２．０％、Ｐ：０．０１５％以下、Ｓ：０．０３０％以下、Ａｌ：０．０７０％以下の各成分を必須元素（あるいは不可避的元素）として含有し、残部がＦｅおよび不可避的不純物からなる。
また、本発明の溶融亜鉛浴設備用鋼板では、さらに、質量％で、Ｎｂ：０．００３〜０．０５０％、Ｖ：０．０１〜０．１０％、Ｔｉ：０．００５〜０．０５０％、Ｃｕ：０．１〜０．５％、Ｎｉ：０．１〜２．０％、Ｃｒ：０．１〜２．０％、Ｍｏ：０．０２〜１．０％、Ｃａ：０．０００２〜０．００３０％、Ｍｇ：０．０００２〜０．００３０％、ＲＥＭ：０．０００２〜０．００３０％、Ｂ：０．０００２〜０．００１０％のうちの１種または２種以上を、適宜選択して含有する構成とすることができる。
以下に、本発明における鋼材の化学成分組成の限定理由を説明する。
<Chemical composition>
The steel sheet for molten zinc bath equipment according to the present invention has C: 0.10 to 0.30%, Si: 0.05% or less, Mn: 0.2 to 2.0%, P: 0.015% or less, S: : 0.030% or less and Al: 0.070% or less as essential elements (or unavoidable elements), with the balance being Fe and unavoidable impurities.
Further, in the steel sheet for a molten zinc bath facility of the present invention, Nb: 0.003 to 0.050%, V: 0.01 to 0.10%, Ti: 0.005 to 0.050 by mass%. %, Cu: 0.1 to 0.5%, Ni: 0.1 to 2.0%, Cr: 0.1 to 2.0%, Mo: 0.02 to 1.0%, Ca: 0. 0002-0.0030%, Mg: 0.0002-0.0030%, REM: 0.0002-0.0030%, B: 0.0002-0.0010%, one or more of: The composition may be appropriately selected and contained.
The reasons for limiting the chemical composition of the steel material in the present invention will be described below.
「Ｃ：炭素」０．１０〜０．３０質量％
Ｃは、鋼板の強度向上のために重要な元素であり、ベイナイトの生成にも寄与する。Ｃの含有量が０．１０質量％未満になると、鋼板の耐溶融亜鉛腐食性が大きく低下し、ベイナイトの生成が不十分になって耐溶融亜鉛割れ性も低下することから、０．１０質量％以上のＣを添加する必要がある。また、良好な焼入れ性を確保する観点から、０．１２質量％超の添加がより好ましい。但し、Ｃを０．３０質量％以上添加すると、鋼板の溶接性が劣化するため、０．３０質量％を上限とした。
"C: carbon" 0.10 to 0.30 mass%
C is an important element for improving the strength of the steel sheet, and also contributes to the formation of bainite. When the content of C is less than 0.10% by mass, the molten zinc corrosion resistance of the steel sheet is greatly reduced, the formation of bainite becomes insufficient, and the molten zinc cracking resistance is also reduced. % Or more of C must be added. From the viewpoint of securing good hardenability, addition of more than 0.12% by mass is more preferable. However, if C is added in an amount of 0.30% by mass or more, the weldability of the steel sheet deteriorates, so the upper limit was set to 0.30% by mass.
「Ｓｉ：ケイ素」０．０５質量％以下
Ｓｉは、脱酸作用を有するが、強力な脱酸元素であるＡｌが十分に添加されている場合には不要である。Ｓｉは、母材を強化する作用もあるが、他の元素に比べるとその効果は相対的に小さい。また、Ｓｉは、耐溶融亜鉛腐食性を大きく低下させることから、その含有量が少ない方が好ましく、操業上安定して低減可能な０．０５質量％を上限とする。また、製鋼上の制限もあるが、Ｓｉの含有量を０．０２質量％以下とすることがより好ましい。
"Si: silicon" 0.05% by mass or less Si has a deoxidizing effect, but is unnecessary when Al, which is a strong deoxidizing element, is sufficiently added. Si also has the effect of strengthening the base material, but its effect is relatively small as compared with other elements. Further, since Si greatly reduces the corrosion resistance to molten zinc, the content of Si is preferably small, and the upper limit is 0.05% by mass, which can be stably reduced in operation. In addition, although there are restrictions on steelmaking, it is more preferable to set the Si content to 0.02% by mass or less.
「Ｍｎ：マンガン」０．２０〜２．０質量％
Ｍｎは、母材強度およびベイナイトを確保する観点から添加する元素であり、母材強度および耐溶融亜鉛割れ性に寄与するためには０．２０質量％以上の添加が必要である。また、同時に添加するＣの添加量が０．１２質量％以下と低い場合には、目標となる組織を安定的に得るため、Ｍｎを０．５質量％超で添加することが好ましい。但し、２．０質量％以上のＭｎの添加は、溶接性を大きく劣化させることから、Ｍｎの添加範囲を０．２０〜２．０質量％とした。
"Mn: manganese" 0.20 to 2.0 mass%
Mn is an element added from the viewpoint of securing the base material strength and bainite. In order to contribute to the base material strength and the resistance to molten zinc cracking, Mn must be added in an amount of 0.20% by mass or more. When the amount of C added simultaneously is as low as 0.12% by mass or less, it is preferable to add Mn at more than 0.5% by mass in order to stably obtain a target structure. However, since addition of Mn of 2.0% by mass or more significantly deteriorates weldability, the range of Mn addition was set to 0.20 to 2.0% by mass.
「Ｐ：リン」０．０１５質量％以下
Ｐは、不純物元素であり、不可避的に鋼板中に含有されるが、Ｓｉと同様に耐溶融亜鉛腐食性を大きく低下させ、溶接性にも悪影響を及ぼすことから、その含有量は少ない方が好ましく、操業上安定して低減可能な０．０１５質量％を上限とした。また、製鋼上の制限もあるが、Ｐの含有量は０．００８質量％以下とすることがより好ましい。
"P: phosphorus" 0.015% by mass or less P is an impurity element and is inevitably contained in the steel sheet. However, similarly to Si, it greatly reduces the molten zinc corrosion resistance and adversely affects weldability. Therefore, the content is preferably small, and the upper limit is set to 0.015% by mass, which can be reduced stably in operation. In addition, although there are restrictions on steelmaking, the content of P is more preferably 0.008% by mass or less.
「Ｓ：硫黄」０．０３０質量％以下
Ｓも、上記Ｐと同様、鋼板中に不可避的に含有される元素であるが、Ｓは母材靭性や溶接性を低下させるため、少ない方が好ましいことから、操業上安定して低減可能な０．０３０質量％を上限とした。
"S: Sulfur" 0.030% by mass or less S is an element inevitably contained in a steel sheet as in the case of the above P, but a smaller amount of S is preferable because it lowers base material toughness and weldability. Therefore, the upper limit was set to 0.030% by mass, which can be stably reduced in operation.
「Ａｌ：アルミニウム」０．０７０質量％以下
Ａｌは、脱酸に用いられる元素であり、その脱酸効果を得るためには０．０１５質量％以上の添加が好ましい。しかしながら、０．０７０質量％以上のＡｌの添加は、鋼中に粗大な介在物を多く存在させ、靭性を低下させることから、その上限を０．０７０質量％とした。
"Al: aluminum" 0.070% by mass or less Al is an element used for deoxidation, and it is preferable to add 0.015% by mass or more in order to obtain the deoxidizing effect. However, the addition of 0.070% by mass or more of Al causes a large amount of coarse inclusions in the steel and lowers the toughness. Therefore, the upper limit is set to 0.070% by mass.
「Ｎｂ：ニオブ」０．００３〜０．０５０質量％
「Ｖ：バナジウム」０．０１〜０．１０質量％
「Ｔｉ：チタン」０．００５〜０．０５０質量％
Ｎｂ、Ｖ、Ｔｉは、本発明における選択的元素であり、母材強度を確保させるために必要に応じて添加するが、ともに多く添加すると母材靭性や溶接性を劣化させることから、Ｎｂを０．００３〜０．０５０質量％、Ｖを０．０１〜０．１０質量％、Ｔｉを０．００５〜０．０５０質量％の添加量とした。また、これらの各元素は、母材強度確保の観点から、それぞれ単独で添加しても、複合添加しても良い。
"Nb: niobium" 0.003 to 0.050 mass%
"V: vanadium" 0.01 to 0.10 mass%
"Ti: titanium" 0.005 to 0.050 mass%
Nb, V, and Ti are selective elements in the present invention, and are added as needed in order to secure the base metal strength. However, when added in large amounts, the base metal toughness and weldability are deteriorated. 0.003 to 0.050 mass%, V was 0.01 to 0.10 mass%, and Ti was 0.005 to 0.050 mass%. Each of these elements may be added alone or in combination from the viewpoint of securing the base material strength.
「Ｃｕ：銅」０．１〜０．５質量％
「Ｎｉ：ニッケル」０．１〜２．０質量％
「Ｃｒ：クロム」０．１〜２．０質量％
「Ｍｏ：モリブデン」０．０２〜１．０質量％
Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏも、本発明における選択的元素であり、上述したＮｂ、Ｖ、Ｔｉと同様に、母材強度を確保させるために必要に応じて添加するが、ともに多く添加すると母材靭性や溶接性を劣化させることから、Ｃｕを０．１〜０．５質量％、Ｎｉを０．１〜２．０質量％、Ｃｒを０．１〜２．０質量％、Ｍｏを０．０２〜１．０質量％の添加量とした。また、これらの各元素は、母材強度確保の観点から、それぞれ単独で添加しても、複合添加しても良い。
"Cu: copper" 0.1 to 0.5% by mass
"Ni: nickel" 0.1 to 2.0 mass%
"Cr: chromium" 0.1 to 2.0% by mass
"Mo: molybdenum" 0.02 to 1.0% by mass
Cu, Ni, Cr, and Mo are also selective elements in the present invention. Like Nb, V, and Ti described above, Cu is added as necessary to secure the base material strength. Since the material toughness and the weldability are deteriorated, Cu is 0.1 to 0.5% by mass, Ni is 0.1 to 2.0% by mass, Cr is 0.1 to 2.0% by mass, and Mo is 0%. 0.02 to 1.0% by mass. Each of these elements may be added alone or in combination from the viewpoint of securing the base material strength.
「Ｃａ：カルシウム」０．０００２〜０．００３０質量％
「Ｍｇ：マグネシウム」０．０００２〜０．００３０質量％
「ＲＥＭ：希土類元素（ランタノイド系元素）」０．０００２〜０．００３０質量％
Ｃａ、Ｍｇ、ＲＥＭも、本発明における選択的元素であり、大入熱溶接熱影響部の靭性を確保するために必要に応じて添加するが、大量に添加すると鋼中に粗大介在物が残留して母材や溶接熱影響部の靭性を低下させることから、Ｃａを０．０００２〜０．００３０質量％、Ｍｇを０．０００２〜０．００３０質量％、ＲＥＭを０．０００２〜０．００３０質量％の添加量とした。また、これらの各元素は、母材強度確保の観点から、それぞれ単独で添加しても、複合添加しても良い。
"Ca: calcium" 0.0002 to 0.0030 mass%
"Mg: magnesium" 0.0002 to 0.0030 mass%
"REM: rare earth element (lanthanoid element)" 0.0002 to 0.0030 mass%
Ca, Mg, and REM are also selective elements in the present invention, and are added as necessary in order to secure the toughness of the heat-affected zone with large heat input welding, but if large amounts are added, coarse inclusions remain in the steel. To reduce the toughness of the base metal and the heat affected zone by welding, so that Ca is 0.0002 to 0.0030% by mass, Mg is 0.0002 to 0.0030% by mass, and REM is 0.0002 to 0.0030%. The amount added was% by mass. Each of these elements may be added alone or in combination from the viewpoint of securing the base material strength.
「Ｂ：ボロン（ホウ素）」０．０００２〜０．００１０質量％
Ｂも、本発明における選択的元素であり、鋼板の母材強度を確保するために必要に応じて添加する。その母材強度効果は、Ｂの０．０００２％以上の添加から発現するが、大量に添加すると母材靭性や溶接性を劣化させることがあることから、０．０００２〜０．００１０質量％の添加量とした。
"B: boron (boron)" 0.0002 to 0.0010 mass%
B is also a selective element in the present invention, and is added as necessary to secure the base material strength of the steel sheet. The base metal strength effect is manifested when 0.0002% or more of B is added. However, if added in a large amount, the base metal toughness and weldability may be deteriorated. The addition amount was used.
＜金属組織＞
本発明の溶融亜鉛浴設備用鋼板は、圧延方向に平行、且つ、板面に垂直な板厚断面において、板厚方向で板面からｔ／４部までの表層の金属組織のうち、面積率で７０％以上がベイナイト組織として構成されている。なお、本発明では、光学顕微鏡によって金属組織を観察し、ラス状の組織をベイナイト組織（ベイナイト相とも称する）と判定する。ベイナイト組織の残部は、フェライト組織（フェライト相とも称する）である。
<Metal structure>
In the steel sheet for molten zinc bath equipment of the present invention, in a sheet thickness section parallel to the rolling direction and perpendicular to the sheet surface, the area ratio of the metal structure of the surface layer from the sheet surface to t / 4 part in the sheet thickness direction. , 70% or more is constituted as a bainite structure. In the present invention, a metal structure is observed with an optical microscope, and a lath-like structure is determined to be a bainite structure (also referred to as a bainite phase). The remainder of the bainite structure is a ferrite structure (also referred to as a ferrite phase).
上述のような化学成分組成を有する鋼は、例えば、上記鋼成分とされた鋼塊を真空溶解炉で溶製することによって得られる。また、金属組織は、詳細を後述する製造方法において、鋼塊から鋼板を製造する際の圧延温度と冷却条件を変えることで変化させることができる。これにより、表層の金属組織が、ベイナイト相単独か、または、フェライト相とベイナイト相とが混合するものとなる鋼板を製造することができる。 The steel having the above-mentioned chemical composition is obtained, for example, by melting a steel ingot having the above-mentioned steel composition in a vacuum melting furnace. Further, the metal structure can be changed by changing a rolling temperature and a cooling condition when a steel sheet is manufactured from a steel ingot in a manufacturing method described in detail below. This makes it possible to manufacture a steel sheet having a bainite phase alone or a mixture of a ferrite phase and a bainite phase in the metal structure of the surface layer.
本発明者等は、上記組成とされた溶融亜鉛浴設備用鋼板を用いて、溶融亜鉛腐食と亜鉛割れ試験を行い、この結果を図１及び図２のグラフに示した。ここで、溶融亜鉛腐食は、鋼板から採取した試験片を溶融した純亜鉛に浸漬し、単位面積当たりの重量の変化で耐溶融亜鉛腐食性を評価する試験である。また、亜鉛割れ試験は、切欠付き丸棒引張り試験の切欠きに溶融亜鉛を付着させ、高温で荷重を負荷する試験である。
図１は、金属組織形態と溶融亜鉛腐食量との関係を示すグラフであり、金属組織形態の違いによる腐食速度の変化は認められなかった。
図２は、金属組織形態とＳ ＬＭ−４００ 値（％）の関係を示すグラフである。Ｓ ＬＭ−４００ 値は耐溶融亜鉛割れ性の指標で、Ｓ ＬＭ とは、試験片に溶融亜鉛めっきを施した場合の、ＮＢＴ試験（切欠付き丸棒引張り試験）を試験温度５００℃で行い、破断強度を、めっきを施さない場合の破断強度で除した値であり、Ｓ ＬＭ−４００ 値は、破断時間が４００秒の際のＳ ＬＭ を表し、この数値が大きいほど耐亜鉛割れ性が高いことを示す。
The present inventors conducted a molten zinc corrosion and zinc cracking test using the steel sheet for a molten zinc bath facility having the above composition, and the results are shown in the graphs of FIG. 1 and FIG. Here, the molten zinc corrosion is a test in which a test piece collected from a steel plate is immersed in molten pure zinc, and the change in weight per unit area is evaluated to evaluate the molten zinc corrosion resistance. The zinc cracking test is a test in which molten zinc is adhered to the notch of a notched round bar tensile test and a load is applied at a high temperature.
FIG. 1 is a graph showing the relationship between the metallographic structure and the amount of molten zinc corrosion. No change in the corrosion rate due to the difference in the metallographic structure was observed.
FIG. 2 is a graph showing the relationship between the metal structure morphology and the SLM-400 value (%). In S LM-400 value is an index of resistance to molten zinc cracking, the S LM, in the case of galvanized the specimen, performed NBT test (notched round bar tensile test) at a test temperature 500 ° C., The breaking strength is a value obtained by dividing the breaking strength when no plating is applied, and the SLM-400 value represents SLM when the breaking time is 400 seconds, and the larger the value, the higher the zinc cracking resistance. Indicates that
本発明者等は、図２のグラフに示すように、鋼板の耐亜鉛割れ性は、金属組織中にベイナイト組織を含む方が高くなることを見出した。また、本発明者等は、耐亜鉛割れ性は、粒状のフェライトが少ない方が高く、ベイナイト組織を７０％以上含むことで耐亜鉛割れ性が向上することを見出した。さらに、ベイナイト組織を８０％以上含む金属組織とすることで、耐亜鉛割れ性が大きく向上することを見出した。本発明の溶融亜鉛浴用浴槽を備えた設備は、上述のような知見に基づいてなされたものであり、非常に優れた耐溶融亜鉛腐食性および耐亜鉛割れ性を同時に実現できるものである。 The present inventors have found that as shown in the graph of FIG. 2, the zinc cracking resistance of the steel sheet is higher when the bainite structure is included in the metal structure. In addition, the present inventors have found that the zinc cracking resistance is higher when there is less granular ferrite, and that the zinc cracking resistance is improved by including a bainite structure of 70% or more. Furthermore, it has been found that by using a metal structure containing a bainite structure of 80% or more, zinc cracking resistance is greatly improved. Equipment equipped with a tub molten zinc bath of the present invention has been made based on the findings as described above, in which a very excellent molten zinc corrosion resistance and zinc cracking resistance at the same time can be realized.
以上説明したように、本発明の耐溶融亜鉛腐食性および耐亜鉛割れ性に優れた溶融亜鉛浴設備用鋼板を用いて溶融亜鉛浴用釜（浴槽）を構成することにより、腐食や割れ等が生じにくく、長寿命の溶融亜鉛浴用釜が得られるので、産業上の効果は極めて大きく、また、構造物の安全性の観点から社会に対する貢献も計り知れない。 As described above, corrosion and cracks and the like are caused by forming a molten zinc bath pot (bath tub) using the steel sheet for molten zinc bath equipment having excellent resistance to molten zinc corrosion and zinc cracking resistance of the present invention. Since it is difficult to obtain a long-life hot-dip bath for molten zinc, the industrial effect is extremely large, and the contribution to society is immeasurable from the viewpoint of structural safety.
［溶融亜鉛浴設備用鋼板の製造方法］
本発明の耐溶融亜鉛腐食性および耐亜鉛割れ性に優れた溶融亜鉛浴設備用鋼板の製造方法は、上述した化学成分組成を有する鋼を鋳造してスラブとし、該スラブをそのままか、あるいは、冷片とした後、９５０℃以上に加熱し、板表面温度が、Ａｒ３点＋１５０℃以下、且つ、Ａｒ３点以上の温度域で仕上げ圧延を行い、直ちに、水冷する方法である。
Ａｒ３（℃）＝９１０−３１０Ｃ−８０Ｍｎ−２０Ｃｕ−１５Ｃｒ−５５Ｎｉ−８０Ｍｏ−０．３５（ｔ−８） ・・・・・ （１）
但し、上記（１）式において、ｔは仕上げ圧延後の板厚（ｍｍ）であり、各成分は質量％である。
以下に、本発明の溶融亜鉛浴設備用鋼板の製造方法における限定理由について説明する。
[Method of manufacturing steel sheet for molten zinc bath equipment]
The method for producing a steel sheet for molten zinc bath equipment excellent in molten zinc corrosion resistance and zinc cracking resistance of the present invention is to cast a steel having the above-described chemical composition into a slab, or the slab as it is, or This is a method of heating to 950 ° C. or higher after cold flakes, performing finish rolling in a temperature range of a plate surface temperature of Ar 3 points + 150 ° C. or lower and Ar 3 points or higher, and immediately cooling with water.
Ar 3 (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo-0.35 (t-8) (1)
However, in the above formula (1), t is the thickness (mm) after finish rolling, and each component is% by mass.
Hereinafter, the reasons for limitation in the method for producing a steel sheet for a molten zinc bath facility of the present invention will be described.
「圧延前の鋳片の加熱温度」
本発明の製造方法においては、圧延前の鋳片の加熱温度を９５０℃以上とする。本発明では、規定の化学成分組成を有した鋼を鋳造して鋳片（スラブ）とした後、この鋳片をそのままか、あるいは冷片とした後、９５０℃以上に加熱し、圧延前の鋳片全体が９５０℃以上となるように、炉内に保持する。
このような圧延前の鋳片の加熱温度が低くなり過ぎると、鋳片の圧延時に、圧延機にかかる負荷が大きくなることから、下限温度を９５０℃とした。また、この加熱温度は、好ましくは１０００℃以上、より好ましくは１１００℃以上とする。加熱温度の上限は、特に制限しないが、生産性を考慮すると、１２５０℃が好ましい。金属組織の粗大化を抑制するために、より好ましくは加熱温度の上限を１１５０℃とする。
"Heating temperature of slab before rolling"
In the production method of the present invention, the heating temperature of the slab before rolling is 950 ° C. or higher. In the present invention, steel having a specified chemical composition is cast to form a cast piece (slab), and then the cast piece is used as it is or as a cold piece, and then heated to 950 ° C. or higher, before rolling. The slab is kept in a furnace so that the temperature is 950 ° C. or higher.
If the heating temperature of the slab before rolling becomes too low, the load applied to the rolling mill during the rolling of the slab increases, so the lower limit temperature was set to 950 ° C. The heating temperature is preferably at least 1000 ° C, more preferably at least 1100 ° C. The upper limit of the heating temperature is not particularly limited, but is preferably 1250 ° C. in consideration of productivity. In order to suppress the coarsening of the metal structure, the upper limit of the heating temperature is more preferably set to 1150 ° C.
「仕上げ圧延温度」
本発明の製造方法においては、上述したように、仕上げ圧延温度について、鋼板表面温度が、Ａｒ３点＋１５０℃以下、且つ、Ａｒ３点以上の温度域としている。仕上げ圧延を行なう際の温度域をこの範囲とし、直ちに水冷することで、鋼板の表層、つまり、板面からｔ／４部までの層域における金属組織の７０％以上（面積率）をベイナイト組織とすることが可能となる。
なお、鋼板表面温度は、放射温度計で測定できる。
"Finish rolling temperature"
In the manufacturing method of the present invention, as described above, the finish rolling temperature is set so that the surface temperature of the steel sheet is equal to or lower than Ar 3 points + 150 ° C. and equal to or higher than Ar 3 points. By setting the temperature range at the time of performing the finish rolling to this range and immediately cooling with water, the bainite structure is used to reduce 70% or more (area ratio) of the metal structure in the surface layer of the steel plate, that is, the layer region from the plate surface to t / 4 part. It becomes possible.
The steel sheet surface temperature can be measured with a radiation thermometer.
本発明の製造方法では、鋼板の金属組織に占めるベイナイト組織の割合を上記範囲とするため、オーステナイト域の温度で仕上げ圧延を終え、水冷する必要があることから、仕上げ圧延温度の下限を、上記（１）式によって規定されるＡｒ３点の温度とした。また、仕上げ圧延温度が高過ぎると、後工程の水冷の際に冷却不足や冷却むらが生じ、不均一な組織になることから、仕上げ圧延温度の上限をＡｒ３点＋１５０℃とした。
なお、Ａｒ３点を求めるための上記（１）式としては、参考文献｛「制御圧延・制御冷却」小指軍夫著、地人書館（１９９７）｝ｐ．２６に記載の式（２−３）を用いた。
In the production method of the present invention, in order to set the proportion of the bainite structure in the metal structure of the steel sheet to the above range, the finish rolling at the temperature in the austenite region is completed, and it is necessary to perform water cooling. The temperature at three points of Ar defined by the equation (1) was used. In addition, if the finish rolling temperature is too high, insufficient cooling or uneven cooling occurs during water cooling in the subsequent step, resulting in an uneven structure. Therefore, the upper limit of the finish rolling temperature was set to three points of Ar + 150 ° C.
The above equation (1) for obtaining the three Ar points is described in reference document “Controlled Rolling / Controlled Cooling” by Gunao Kodashi, Chijin Shokan (1997) {p. Formula (2-3) described in No. 26 was used.
また、仕上げ圧延後の板厚は、鋼板を溶融亜鉛浴用釜に用いる場合、通常、例えば、６ｍｍ〜７０ｍｍ程度の板厚とすることができ、また、この際の総圧延率は、８０〜９８％程度とすれば、鋼板特性を損なうことなく、且つ、高い生産性で溶融亜鉛浴設備用鋼板を製造することが可能となり、望ましい。 When the steel sheet is used for a hot-dip zinc bath, the sheet thickness after finish rolling can be, for example, usually about 6 mm to 70 mm, and the total rolling ratio at this time is 80 to 98 mm. %, It is possible to manufacture a steel sheet for molten zinc bath equipment without impairing the properties of the steel sheet and with high productivity, which is desirable.
「水冷条件」
本発明の製造方法においては、上記温度領域において仕上げ圧延を行なった後、直ちに鋼板を水冷手段によって冷却することが好ましい。このように、仕上げ圧延後、直ちに水冷を開始する目的は、フェライト組織の生成を抑制し、ベイナイト組織の面積率を７０％以上にすることであり、圧延機から水冷装置まで搬送する間の空冷は許容される。また、水冷の停止温度は、特に制限しないが、過剰な硬化を抑制して靱性を確保する必要がある場合は、４５０〜７００℃が好ましい。なお、冷却速度は板厚によって変化するため、予備試験等を行い、所望の金属組織が得られるように、水冷の水量密度を制御すればよい。
"Water cooling conditions"
In the production method of the present invention, it is preferable that the steel sheet is immediately cooled by water cooling means after the finish rolling in the above temperature range. Thus, the purpose of starting the water cooling immediately after the finish rolling is to suppress the formation of the ferrite structure and to increase the area ratio of the bainite structure to 70% or more, and to perform the air cooling during transport from the rolling mill to the water cooling device. Is acceptable. The temperature at which water cooling is stopped is not particularly limited, but is preferably 450 to 700 ° C. when it is necessary to suppress excessive hardening and ensure toughness. Since the cooling rate changes depending on the sheet thickness, a preliminary test or the like may be performed to control the water density of the water cooling so as to obtain a desired metal structure.
以下、本発明に係る耐溶融亜鉛腐食性および耐亜鉛割れ性に優れた溶融亜鉛浴設備用鋼板およびその製造方法の実施例を挙げ、本発明をより具体的に説明するが、本発明は、もとより下記実施例に限定されるものではなく、前、後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれるものである。 Hereinafter, the present invention will be more specifically described with reference to Examples of a molten zinc bath facility steel sheet and a method for producing the same, which are excellent in molten zinc corrosion resistance and zinc cracking resistance according to the present invention. Naturally, the present invention is not limited to the following Examples, and it is also possible to carry out the present invention by appropriately modifying it within a range that can be adapted to the gist of the following and the following.All of them are included in the technical scope of the present invention. Things.
［サンプルの製造］
製鋼工程において溶鋼の脱酸・脱硫と化学成分を制御し、連続鋳造によって下記表１に示す化学成分のスラブを製造した。この際、加熱温度は、下記表２に示す温度とした。次いで、下記表２及び表３に示す製造条件で前記スラブを再加熱（仕上げ圧延温度）し、仕上げ圧延することで板厚３２〜７０ｍｍに仕上げ、直ちに水冷による加速冷却を行い、溶融亜鉛浴設備用鋼板を製造した。なお、一部の鋼板は、比較のために、仕上げ圧延後、空冷した（下記表２中における圧延条件Ｂ１「圧延後水冷：無し」）。
[Production of sample]
In the steelmaking process, the deoxidation and desulfurization of the molten steel and the chemical components were controlled, and slabs having the chemical components shown in Table 1 below were produced by continuous casting. At this time, the heating temperature was a temperature shown in Table 2 below. Next, the slab was reheated (finish rolling temperature) under the manufacturing conditions shown in Tables 2 and 3 below, and finished to a plate thickness of 32 to 70 mm by finish rolling. Immediately, accelerated cooling by water cooling was performed. Steel sheets for automobiles. For comparison, some steel sheets were air-cooled after finish rolling (rolling condition B1 in Table 2 below: "water-cooled after rolling: none").
［評価試験］
上記方法によって製造した溶融亜鉛浴設備用鋼板について、以下のような評価試験を行った。
金属組織の評価については、鋼板の板面からｔ／４部の表層を顕微鏡で観察し、１００倍で撮影した写真から、表層の金属組織においてベイナイト組織が占めている割合を画像解析にて調査した。そして、鋼板表層の金属組織におけるベイナイト組織が７０％以上のものを「○」とし、７０％未満のものを「×」として評価した。
[Evaluation test]
The following evaluation tests were performed on the steel sheets for molten zinc bath equipment manufactured by the above method.
Regarding the evaluation of the metallographic structure, the surface layer of the t / 4 part was observed with a microscope from the plate surface of the steel sheet, and the ratio of bainite structure occupying the metallographic structure of the surface layer was examined by image analysis from a photograph taken at 100 ×. did. Then, those having a bainite structure of 70% or more in the metal structure of the steel sheet surface layer were evaluated as “○”, and those having a bainite structure of less than 70% were evaluated as “X”.
また、耐溶融亜鉛腐食性の評価については、鋼板の表層から採取した４０ｍｍ×２５ｍｍ×４ｍｍの試験片を、温度が５００℃とされた純度９９．９９％の亜鉛中に２４時間浸漬させた。そして、２４時間浸漬後の腐食が２００ｍｇ／ｃｍ２以下であるものを「○」とし、２００ｍｇ／ｃｍ２超であるものを「×」として評価した。
また、耐亜鉛割れ性の評価については、ＮＢＴ試験（切欠付き丸棒引張り試験）で評価した（新日鉄技報３４８号、１９９３年、ｐ．６３−７０を参照）。そして、試験片の切欠に亜鉛線材を巻き付けて加熱し、溶融亜鉛を付着させ、試験温度を５００℃として、破断時間４００秒におけるＳ ＬＭ 値（Ｓ ＬＭ−４００ 値）が８０％以上であるものを「○」とし、８０％未満のものを「×」として評価した。
For evaluation of the molten zinc corrosion resistance, a 40 mm × 25 mm × 4 mm test piece taken from the surface layer of a steel sheet was immersed in zinc having a temperature of 500 ° C. and a purity of 99.99% for 24 hours. Then, those corrosion after immersion for 24 hours is 200 mg / cm 2 or less as "○", was evaluated as a 200 mg / cm 2 than as "×".
The zinc cracking resistance was evaluated by an NBT test (notched round bar tensile test) (see Nippon Steel Technical Report No. 348, 1993, pp. 63-70). Then, a zinc wire rod is wound around the notch of the test piece and heated, and molten zinc is adhered. The test temperature is set to 500 ° C., and the SLM value ( SLM-400 value) at a rupture time of 400 seconds is 80% or more. Was evaluated as “○”, and one having less than 80% was evaluated as “X”.
また、溶接性の試験として、ｙ型溶接割れ試験（ＪＩＳ Ｚ３１５８）により断面割れの発生の有無で評価した。すなわち、それぞれの最終板厚のもので、ルート間隔１．０ｍｍとして、開先を用意し、溶接材料としてＹＧＷ１５（ＪＩＳ Ｚ３３１２）規格のソリッドワイヤ材を用いて、入熱量３０〜３６ｋＪ／ｃｍで、ＭＡＧ溶接による多層盛溶接を行って、ｙ型溶接試験片を作製し、ｙ型溶接部の各５断面の鏡研磨面において、断面割れの有無を肉眼で確認した。各５断面で断面割れが、肉眼で全く観察されなかったものを「○」とし、１断面以上で断面割れが肉眼で確認されたものは「×」として評価した。 In addition, as a weldability test, evaluation was made by the presence or absence of occurrence of a cross-sectional crack by a y-type weld crack test (JIS Z3158). That is, a groove having a final thickness of 1.0 mm, a root interval of 1.0 mm, a groove prepared, and a YGW15 (JIS Z3312) standard solid wire material as a welding material, with a heat input of 30 to 36 kJ / cm, Multi-layer welding by MAG welding was performed to prepare a y-type welded test piece, and the presence or absence of cross-sectional cracks was visually confirmed on each of the five mirror-polished surfaces of the y-type weld. In each of the five sections, a section in which no cross-sectional crack was observed with the naked eye was evaluated as “○”, and a section in which one or more cross sections were confirmed with the naked eye was evaluated as “x”.
本実施例における鋼板（本発明鋼および比較鋼）の化学成分組成の一覧を下記表１に示すとともに、仕上げ圧延前の加熱温度、仕上げ圧延温度および水冷の有無の圧延条件一覧を下記表２に示す。また、下記表１に示す化学成分組成を有する鋼の各々と製造条件の組み合わせの一覧、及び、各評価結果の一覧を下記表３に示す。 Table 1 below shows a list of the chemical composition of the steel sheet (inventive steel and comparative steel) in this example, and Table 2 below shows a heating temperature before finish rolling, a finish rolling temperature, and a rolling condition list with or without water cooling. Show. Table 3 below shows a list of combinations of the steels having the chemical composition shown in Table 1 and the manufacturing conditions, and a list of the evaluation results.
［評価結果］
表１〜表３に示すように、本発明で規定する化学成分組成を有し、本発明で規定する製造条件によって作製した溶融亜鉛浴設備用鋼板（本発明鋼）は、鋼板の表層における金属組織の７０％以上をベイナイト組織が占めており、また、耐溶融亜鉛腐食性、耐亜鉛割れ性及び耐溶接割れ性の評価が何れも○の評価であり、耐溶融亜鉛腐食性および耐亜鉛割れ性に優れていることが明らかとなった。
[Evaluation results]
As shown in Tables 1 to 3, the steel sheet for molten zinc bath equipment (the steel of the present invention) having the chemical component composition specified in the present invention and manufactured under the manufacturing conditions specified in the present invention is a metal in the surface layer of the steel sheet. The bainite structure accounts for 70% or more of the structure, and the evaluation of molten zinc corrosion resistance, zinc cracking resistance and weld cracking resistance is all ○. It was found that the film had excellent properties.
これに対し、比較鋼である鋼板記号Ｃ１およびＣ２はＣ量が少ないため、鋼板表層の金属組織に占めるベイナイト組織が少なく、耐溶融亜鉛腐食性および耐亜鉛割れ性の何れもが劣っている。また、Ｃ３およびＣ１０はＳｉ量が多いため、耐溶融亜鉛腐食性が劣っている。また、Ｃ４はＰ量が多いため、耐溶融亜鉛腐食性および溶接性が劣っている。
また、比較鋼である鋼板記号Ｃ５〜Ｃ８は、化学成分組成は本発明で規定する範囲に含まれているものの、何れも仕上げ圧延後の水冷を行なわなかった例であり、製造条件が本発明で規定する範囲に含まれていないため、鋼板表層の金属組織に占めるベイナイト組織が少なく、耐溶融亜鉛腐食性は良好であるものの、耐亜鉛割れ性が劣っている。
On the other hand, since the steel symbols C1 and C2, which are comparative steels, have a small C content, the bainite structure in the metal structure of the steel sheet surface layer is small, and both the zinc corrosion resistance and the zinc cracking resistance are inferior. Further, since C3 and C10 have a large amount of Si, the molten zinc corrosion resistance is inferior. Further, C4 has a large P content, and thus is inferior in molten zinc corrosion resistance and weldability.
Further, the steel sheet symbols C5 to C8, which are comparative steels, are examples in which the chemical composition is included in the range specified in the present invention, but none of the steel sheets is subjected to water cooling after finish rolling, and the production conditions are the same as those of the present invention. , The bainite structure in the metal structure of the steel sheet surface layer is small, and the molten zinc corrosion resistance is good, but the zinc cracking resistance is inferior.
また、比較鋼である鋼板記号Ｃ９はＣ量が多く、Ｃ１１はＭｎ量が多いため、耐溶接割れ性が劣っている。また、Ｃ１２はＭｎ量が少ないため、鋼板表層の金属組織に占めるベイナイト組織が少なく、耐亜鉛割れ性が劣っている。
また、比較鋼である鋼板記号Ｃ１５は、仕上げ圧延温度が低いため、鋼板表層の金属組織に占めるベイナイト組織が少なく、耐亜鉛割れ性の評価が劣っている。
Further, since the steel plate symbol C9, which is a comparative steel, has a large C content and C11 has a large Mn content, the weld crack resistance is inferior. Further, since C12 has a small Mn content, the bainite structure in the metal structure of the steel sheet surface layer is small, and the zinc cracking resistance is poor.
Further, steel plate symbol C15, which is a comparative steel, has a low finish rolling temperature, so that the bainite structure in the metal structure of the steel sheet surface layer is small, and the evaluation of zinc cracking resistance is inferior.
以上説明した実施例の結果より、本発明の耐溶融亜鉛腐食性および耐亜鉛割れ性に優れた溶融亜鉛浴設備用鋼板が、溶融亜鉛による腐食に対して優れた耐食性を有し、且つ、溶融亜鉛に起因した割れが生じにくく、優れた鋼板特性を有していることが明らかである。 From the results of the examples described above, the steel sheet for molten zinc bath equipment having excellent resistance to molten zinc corrosion and zinc cracking of the present invention has excellent corrosion resistance to corrosion by molten zinc, and It is clear that cracks caused by zinc hardly occur and the steel sheet has excellent steel sheet properties.
Claims (5)
Ｃ ：０．１２超〜０．３０％、
Ｓｉ：０．０５％以下、
Ｍｎ：０．２０〜２．０％
を含有し、
Ｐ ：０．０１５％以下、
Ｓ ：０．０３０％以下、
Ａｌ：０．０７０％以下
に制限し、残部がＦｅおよび不可避的不純物からなり、
圧延方向に平行、且つ、板面に垂直な板厚断面において、板厚方向で板面からｔ／４部までの表層の金属組織のうち、面積率で７０％以上がベイナイト組織であり、残部がフェライト組織である鋼板を用いて構成される溶融亜鉛浴用浴槽を備えた設備。 C: by mass%: more than 0.12 to 0.30%,
Si: 0.05% or less,
Mn: 0.20 to 2.0%
Containing
P: 0.015% or less,
S: 0.030% or less,
Al: limited to 0.070% or less, the balance being Fe and unavoidable impurities,
In a sheet thickness section parallel to the rolling direction and perpendicular to the sheet surface, of the surface layer metal structure from the sheet surface to t / 4 part in the sheet thickness direction, 70% or more in area ratio is a bainite structure, and the remainder is equipment but with a tub molten zinc constructed using steel is ferrite structure.
Ｎｂ：０．００３〜０．０５０％、
Ｖ ：０．０１〜０．１０％、
Ｔｉ：０．００５〜０．０５０％
のうちの１種または２種以上を含有する請求項１に記載の溶融亜鉛浴用浴槽を備えた設備。 The steel sheet further comprises, in mass%,
Nb: 0.003 to 0.050%,
V: 0.01 to 0.10%,
Ti: 0.005 to 0.050%
One or equipment having a tub molten zinc bath according to claim 1 containing two or more of.
Ｃｕ：０．１〜０．５％、
Ｎｉ：０．１〜２．０％、
Ｃｒ：０．１〜２．０％、
Ｍｏ：０．０２〜１．０％
のうちの１種または２種以上を含有する請求項１または請求項２に記載の溶融亜鉛浴用浴槽を備えた設備。 The steel sheet further comprises, in mass%,
Cu: 0.1-0.5%,
Ni: 0.1 to 2.0%,
Cr: 0.1 to 2.0%,
Mo: 0.02 to 1.0%
One or equipment having a tub molten zinc bath according to claim 1 or claim 2 containing two or more of.
Ｃａ：０．０００２〜０．００３０％、
Ｍｇ：０．０００２〜０．００３０％、
ＲＥＭ：０．０００２〜０．００３０％
のうちの１種または２種以上を含有する請求項１から請求項３のいずれか一項に記載の溶融亜鉛浴用浴槽を備えた設備。 The steel sheet further comprises, in mass%,
Ca: 0.0002-0.0030%,
Mg: 0.0002-0.0030%,
REM: 0.0002-0.0030%
One or equipment having a tub molten zinc bath according to claims 1 containing two or more in any one of claims 3 of the.
Ｂ：０．０００２〜０．００１０％
を含有する請求項１から請求項４のいずれか一項に記載の溶融亜鉛浴用浴槽を備えた設備。 The steel sheet further comprises, in mass%,
B: 0.0002-0.0010%
Equipment equipped with a tub molten zinc bath according to any one of claims 1 to 4 containing.
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