JP2009068034A - Ferritic stainless steel sheet superior in formability for extension flange, and manufacturing method therefor - Google Patents
Ferritic stainless steel sheet superior in formability for extension flange, and manufacturing method therefor Download PDFInfo
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- 229910001220 stainless steel Inorganic materials 0.000 title claims abstract description 60
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 11
- 238000000137 annealing Methods 0.000 claims abstract description 32
- 238000010438 heat treatment Methods 0.000 claims abstract description 27
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 14
- 239000010959 steel Substances 0.000 claims abstract description 14
- 239000012535 impurity Substances 0.000 claims abstract description 8
- 229910000859 α-Fe Inorganic materials 0.000 claims description 34
- 239000013078 crystal Substances 0.000 claims description 33
- 239000002245 particle Substances 0.000 claims description 15
- 229910052757 nitrogen Inorganic materials 0.000 claims description 9
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 229910052804 chromium Inorganic materials 0.000 claims description 4
- 229910052698 phosphorus Inorganic materials 0.000 claims description 4
- 239000010935 stainless steel Substances 0.000 claims description 4
- 229910052742 iron Inorganic materials 0.000 claims description 2
- 230000007797 corrosion Effects 0.000 abstract description 15
- 238000005260 corrosion Methods 0.000 abstract description 15
- 238000005098 hot rolling Methods 0.000 abstract description 10
- 238000005097 cold rolling Methods 0.000 abstract description 9
- 229910052799 carbon Inorganic materials 0.000 abstract description 6
- 239000010960 cold rolled steel Substances 0.000 abstract description 6
- 238000004804 winding Methods 0.000 abstract description 4
- 238000005554 pickling Methods 0.000 abstract description 3
- 238000005096 rolling process Methods 0.000 description 20
- 238000012545 processing Methods 0.000 description 13
- 230000000694 effects Effects 0.000 description 6
- 238000000034 method Methods 0.000 description 6
- 239000002244 precipitate Substances 0.000 description 6
- 238000001953 recrystallisation Methods 0.000 description 5
- 238000012360 testing method Methods 0.000 description 5
- 230000008018 melting Effects 0.000 description 4
- 238000002844 melting Methods 0.000 description 4
- 150000004767 nitrides Chemical class 0.000 description 4
- 239000006104 solid solution Substances 0.000 description 4
- 238000005516 engineering process Methods 0.000 description 3
- 230000001771 impaired effect Effects 0.000 description 3
- QZPSXPBJTPJTSZ-UHFFFAOYSA-N aqua regia Chemical compound Cl.O[N+]([O-])=O QZPSXPBJTPJTSZ-UHFFFAOYSA-N 0.000 description 2
- 230000005540 biological transmission Effects 0.000 description 2
- 238000005336 cracking Methods 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 230000002542 deteriorative effect Effects 0.000 description 2
- 238000000605 extraction Methods 0.000 description 2
- 229910052758 niobium Inorganic materials 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 238000009864 tensile test Methods 0.000 description 2
- 239000010409 thin film Substances 0.000 description 2
- 206010070834 Sensitisation Diseases 0.000 description 1
- 239000004566 building material Substances 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 239000012141 concentrate Substances 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 239000010408 film Substances 0.000 description 1
- 238000007710 freezing Methods 0.000 description 1
- 230000008014 freezing Effects 0.000 description 1
- 230000000977 initiatory effect Effects 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 238000003672 processing method Methods 0.000 description 1
- 238000004080 punching Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 230000008313 sensitization Effects 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
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Abstract
Description
本発明は、優れた耐食性を有するフェライト系ステンレス鋼板とその製造方法に関し、特にプレス成形の際の伸びフランジ加工性に優れたフェライト系ステンレス鋼板とその製造方法に関するものである。 The present invention relates to a ferritic stainless steel sheet having excellent corrosion resistance and a method for producing the same, and more particularly to a ferritic stainless steel sheet having excellent stretch flangeability during press forming and a method for producing the same.
フェライト系ステンレス鋼板は、耐食性に優れており、建築材料，輸送機器，家庭電化製品，厨房機器等の様々な用途に使用されている。従来からフェライト系ステンレス鋼板は、特有の表面光沢が重要視される用途に使用されており、様々な形状に成形する際の特性を向上する技術の開発は十分に進展していない。
しかし、近年、フェライト系ステンレス鋼板をプレス成形して、複雑な形状の部材を製造する必要性が高まっている。そのため、フェライト系ステンレス鋼板のプレス成形性を向上させる技術が検討されている。
Ferritic stainless steel sheets are excellent in corrosion resistance and are used in various applications such as building materials, transportation equipment, home appliances, and kitchen equipment. Conventionally, ferritic stainless steel sheets have been used for applications in which specific surface gloss is regarded as important, and the development of technology for improving the characteristics when forming into various shapes has not progressed sufficiently.
However, in recent years, there is an increasing need to press-form ferritic stainless steel plates to manufacture members having complicated shapes. Therefore, a technique for improving the press formability of the ferritic stainless steel sheet has been studied.
たとえば特許文献１には、Ｃ，Ｎを低減するとともにCrを８〜50質量％含有し、圧下率を高めて冷間圧延を行なうことによって面内異方性を軽減したフェライト系ステンレス鋼板が開示されている。この技術では、フェライト系ステンレス鋼板のスプリングバック量を減少させて形状凍結性を向上できるが、伸びフランジ加工性の大幅な向上は期待できない。 For example, Patent Document 1 discloses a ferritic stainless steel sheet that reduces C and N, contains 8 to 50% by mass of Cr, and reduces the in-plane anisotropy by increasing the rolling reduction and performing cold rolling. Has been. With this technology, the amount of springback of the ferritic stainless steel sheet can be reduced to improve the shape freezing property, but a significant improvement in stretch flangeability cannot be expected.
なお、伸びフランジ加工は、プレス成形によって生じる打抜き部をフランジとして伸ばす過酷な加工法である。
特許文献２には、Ｃ，Ｎを低減するとともにCrを10〜35質量％含有し、微細なNb炭窒化物を分布させることによって再結晶を促進させたフェライト系ステンレス鋼板が開示されている。この技術では、フェライト系ステンレス鋼板の張出し性と深絞り性の改善は可能であるが、伸びフランジ加工性の大幅な向上は期待できない。
The stretch flange processing is a severe processing method in which a punched portion generated by press molding is extended as a flange.
Patent Document 2 discloses a ferritic stainless steel sheet that reduces C and N, contains 10 to 35% by mass of Cr, and promotes recrystallization by distributing fine Nb carbonitride. With this technology, it is possible to improve the stretchability and deep drawability of the ferritic stainless steel sheet, but a significant improvement in stretch flangeability cannot be expected.
特許文献３には、Ｃ，Ｎを低減するとともにCrを８〜35質量％含有するフェライト系ステンレス鋼板が開示されている。この技術は、フェライト系ステンレス鋼板の厚さと使用する温度とを規定することによって耐水蒸気酸化性を向上させるものである。しかし伸びフランジ加工を行なうと、フェライト系ステンレス鋼板に割れが生じる。
伸びフランジ加工による割れは、MnＳが起点になる。したがってMnＳの析出を抑制すれば割れの発生頻度を下げることができ、フェライト系ステンレス鋼板の伸びフランジ加工性が向上する可能性がある。ただしMnＳの低減はそれを起点とする割れを防止する技術であり、伸びフランジ加工性を阻害するその他の要因に対する研究は未だ十分になされていない。
MnS is the starting point for cracks due to stretch flange processing. Therefore, if the precipitation of MnS is suppressed, the occurrence frequency of cracks can be lowered, and the stretch flangeability of the ferritic stainless steel sheet may be improved. However, the reduction of MnS is a technique for preventing cracks originating from it, and research on other factors that hinder stretch flangeability has not been sufficiently conducted.
本発明は、優れた耐食性を有し、かつ優れた伸びフランジ加工性を有するフェライト系ステンレス鋼板とその製造方法を提供することを目的とする。 An object of the present invention is to provide a ferritic stainless steel sheet having excellent corrosion resistance and excellent stretch flangeability and a method for producing the same.
発明者らは、フェライト系ステンレス鋼板の伸びフランジ加工を行なって割れを発生させ、その割れを調査研究した。その結果、
(a)割れの起点はフェライト結晶粒の粒界に析出したMnＳであり、その他にCuＳが析出している、
(b)割れの起点ではCuＳとMnＳが集団を形成しており、その集団の中にCuＳおよび／またはMnＳが10個以上凝集している、
(c)凝集したCuＳ，MnＳの粒径はいずれも１μｍ以下である、
(d)圧延方向に展伸したフェライト結晶粒のアスペクト比が２を超えると、圧延方向に割れが発生し易くなる
という知見を得た。なおアスペクト比は、フェライト結晶粒の長径を短径で除した値である。
The inventors performed stretch flange processing of a ferritic stainless steel plate to generate a crack, and investigated and researched the crack. as a result,
(a) The starting point of the crack is MnS precipitated at the grain boundaries of the ferrite crystal grains, and CuS is also precipitated.
(b) CuS and MnS form a group at the origin of cracking, and 10 or more CuS and / or MnS are aggregated in the group.
(c) The particle sizes of the agglomerated CuS and MnS are both 1 μm or less.
(d) It has been found that when the ferrite crystal grains expanded in the rolling direction have an aspect ratio of more than 2, cracks are likely to occur in the rolling direction. The aspect ratio is a value obtained by dividing the major axis of the ferrite crystal grains by the minor axis.
そこで、伸びフランジ加工による割れとCuＳとの関係について詳細に調査したところ、CuＳがMnＳに付着するように析出してMnＳの周囲に歪みを生じさせ、その歪みが割れの伝播の媒体となり、伸びフランジ加工性の劣化を招くことが分かった。
本発明は、これらの知見に基づいてなされたものである。
すなわち本発明は、Ｃ：0.015質量％以下，Si：0.3質量％以下，Mn：0.10〜0.40質量％，Ｐ：0.04質量％以下，Ｓ：0.008質量％以下，Al：0.08質量％以下，Ｎ：0.015質量％以下，Cr：20.5〜23.5質量％，Cu：0.3〜0.7質量％，Ni：0.5質量％以下，Nb：0.25〜0.55質量％を含有し、残部がFeおよび不可避的不純物からなる組成と、粒径が１μｍ以下のCuＳおよび／またはMnＳの粒子10個以下がフェライト結晶粒の粒界に凝集体を形成し、かつフェライト結晶粒のアスペクト比が２以下である組織と、を有するフェライト系ステンレス鋼板である。
Therefore, when the relationship between cracks due to stretch flange processing and CuS was investigated in detail, CuS was deposited so as to adhere to MnS, causing distortion around MnS, and that strain became a medium for propagation of cracks. It was found that flange workability was degraded.
The present invention has been made based on these findings.
That is, the present invention includes C: 0.015 mass% or less, Si: 0.3 mass% or less, Mn: 0.10 to 0.40 mass%, P: 0.04 mass% or less, S: 0.008 mass% or less, Al: 0.08 mass% or less, N: 0.015% by mass or less, Cr: 20.5-23.5% by mass, Cu: 0.3-0.7% by mass, Ni: 0.5% by mass or less, Nb: 0.25-0.55% by mass, with the balance consisting of Fe and inevitable impurities And a structure in which not more than 10 CuS and / or MnS particles having a particle size of 1 μm or less form aggregates at the grain boundaries of the ferrite crystal grains, and the aspect ratio of the ferrite crystal grains is 2 or less. Stainless steel sheet.
また本発明は、Ｃ：0.015質量％以下，Si：0.3質量％以下，Mn：0.10〜0.40質量％，Ｐ：0.04質量％以下，Ｓ：0.008質量％以下，Al：0.08質量％以下，Ｎ：0.015質量％以下，Cr：20.5〜23.5質量％，Cu：0.3〜0.7質量％，Ni：0.5質量％以下，Nb：0.25〜0.55質量％を含有し、残部がFeおよび不可避的不純物からなるスラブを製造し、スラブを1000℃以上に加熱した後、仕上げ温度を800℃以上1000℃未満とし巻取り温度を500℃以下として熱間圧延を行ない、得られた熱延鋼板に加熱温度900℃以上かつ加熱時間500秒以下で熱延板焼鈍を施し、さらに酸洗を施し、次いで冷間圧延を行ない、得られた冷延鋼板に加熱温度850℃以上で冷延板焼鈍を施すフェライト系ステンレス鋼板の製造方法である。 Moreover, this invention is C: 0.015 mass% or less, Si: 0.3 mass% or less, Mn: 0.10-0.40 mass%, P: 0.04 mass% or less, S: 0.008 mass% or less, Al: 0.08 mass% or less, N: A slab containing 0.015% by mass or less, Cr: 20.5 to 23.5% by mass, Cu: 0.3 to 0.7% by mass, Ni: 0.5% by mass or less, Nb: 0.25 to 0.55% by mass, the balance being Fe and inevitable impurities After manufacturing and heating the slab to 1000 ° C or higher, hot rolling is performed with a finishing temperature of 800 ° C or higher and lower than 1000 ° C and a coiling temperature of 500 ° C or lower, and the resulting hot rolled steel sheet has a heating temperature of 900 ° C or higher and A ferritic stainless steel sheet that is subjected to hot-rolled sheet annealing at a heating time of 500 seconds or less, further pickled, then cold-rolled, and subjected to cold-rolled sheet annealing at a heating temperature of 850 ° C. or higher on the resulting cold-rolled steel sheet. It is a manufacturing method.
本発明によれば、優れた耐食性を有し、かつ優れた伸びフランジ加工性を有するフェライト系ステンレス鋼板を得ることができる。 According to the present invention, a ferritic stainless steel sheet having excellent corrosion resistance and excellent stretch flangeability can be obtained.
まず、本発明のフェライト系ステンレス鋼板の成分の限定理由を説明する。
Ｃ：0.015質量％以下
Ｃは、フェライト系ステンレス鋼板の強度を高める作用を有する元素である。しかしＣは、後述するCrと結合することによってCr炭化物を析出して固溶Crを減少させ、フェライト系ステンレス鋼板の耐食性を劣化させる。Ｃ含有量が0.015質量％を超えると、Cr炭化物が析出して十分な固溶Crを維持できないので、フェライト系ステンレス鋼板の耐食性が劣化するとともに、析出したCr炭化物が起点となって割れを発生させる惧れがある。したがって、Ｃは0.015質量％以下とする。
First, the reasons for limiting the components of the ferritic stainless steel sheet of the present invention will be described.
C: 0.015 mass% or less C is an element having an effect of increasing the strength of the ferritic stainless steel sheet. However, C combines with Cr, which will be described later, to precipitate Cr carbide and reduce solute Cr, thereby deteriorating the corrosion resistance of the ferritic stainless steel sheet. If the C content exceeds 0.015% by mass, Cr carbide precipitates and sufficient solid solution Cr cannot be maintained, so the corrosion resistance of the ferritic stainless steel sheet deteriorates and cracks occur due to the precipitated Cr carbide. There is a fear. Therefore, C is 0.015 mass% or less.
Si：0.3質量％以下
Siは、フェライト系ステンレス鋼の溶製段階で脱酸剤として用いられる。しかし固溶強化によってフェライト系ステンレス鋼板を硬質化する作用を有する。Si含有量が0.3質量％を超えると、フェライト系ステンレス鋼板が著しく硬質化して延性の低下を招く。したがって、Siは0.3質量％以下とする。
Si: 0.3% by mass or less
Si is used as a deoxidizer in the melting stage of ferritic stainless steel. However, it has the effect of hardening the ferritic stainless steel sheet by solid solution strengthening. When the Si content exceeds 0.3% by mass, the ferritic stainless steel sheet becomes extremely hard and the ductility is lowered. Therefore, Si is 0.3 mass% or less.
Mn：0.10〜0.40質量％
Mnは、フェライト系ステンレス鋼の溶製段階で脱酸剤として用いられる。Mn含有量が0.10質量％未満では、MnＳが減少し、後述するCuＳが増加するので、割れの伝播が促進されて伸びフランジ加工性が劣化する。一方、0.40質量％を超えると、MnＳの析出が促進されて、割れの起点の発生が助長されるので、伸びフランジ加工性が劣化する。したがって、Mnは0.10〜0.40質量％の範囲内とする。
Mn: 0.10 to 0.40 mass%
Mn is used as a deoxidizer in the melting stage of ferritic stainless steel. If the Mn content is less than 0.10% by mass, MnS decreases and CuS described later increases, so that propagation of cracks is promoted and stretch flangeability deteriorates. On the other hand, if it exceeds 0.40% by mass, the precipitation of MnS is promoted and the generation of crack starting points is promoted, so that stretch flangeability is deteriorated. Therefore, Mn is in the range of 0.10 to 0.40 mass%.
Ｐ：0.04質量％以下
Ｐは、フェライト結晶粒の粒界に偏析して脆性破壊を誘起する。Ｐ含有量が0.04質量％を超えると、粒界における偏析の問題に加えて、固溶強化によってフェライト系ステンレス鋼板が硬質化する。したがって、Ｐは0.04質量％以下とする。
Ｓ：0.008質量％以下
Ｓは、Mn，Cuと結合してMnＳ，CuＳを析出させる元素である。MnＳとCuＳの凝集体は伸びフランジ加工による割れの起点や亀裂伝播の原因となる。Ｓ含有量が0.008質量％を超えると、MnＳとCuＳの凝集体が多量に析出して、フェライト系ステンレス鋼板の伸びフランジ加工性の劣化を招く。したがって、Ｓは0.008質量％以下とする。
P: 0.04 mass% or less P segregates at the grain boundary of ferrite crystal grains to induce brittle fracture. When the P content exceeds 0.04% by mass, in addition to the problem of segregation at the grain boundaries, the ferritic stainless steel sheet is hardened by solid solution strengthening. Therefore, P is 0.04 mass% or less.
S: 0.008% by mass or less S is an element that bonds with Mn and Cu to precipitate MnS and CuS. Aggregates of MnS and CuS cause crack initiation and crack propagation due to stretch flange processing. When the S content exceeds 0.008% by mass, a large amount of MnS and CuS aggregates are precipitated, leading to deterioration of stretch flangeability of the ferritic stainless steel sheet. Therefore, S is 0.008 mass% or less.
Al：0.08質量％以下
Alは、フェライト系ステンレス鋼の溶製段階で脱酸剤として用いられる。Al含有量が0.08質量％を超えると、後述するＮと結合してAlＮを析出する。このAlＮは、熱間圧延の後の焼鈍（以下、熱延板焼鈍という）や冷間圧延の後の焼鈍（以下、冷延板焼鈍という）におけるフェライト結晶粒の再結晶を阻害する。そのためフェライト結晶粒が圧延方向に展伸して、割れが発生し易くなる。したがって、Alは0.08質量％以下とする。好ましくは0.05質量％以下である。ただし、Al含有量が0.02質量％未満では、脱酸効果が得られない。したがって、同じ脱酸剤のSiの含有量が0.15質量％以下のときは、Alは0.02〜0.08質量％の範囲内が好ましい。
Al: 0.08 mass% or less
Al is used as a deoxidizer in the melting stage of ferritic stainless steel. When Al content exceeds 0.08 mass%, it couple | bonds with N mentioned later and precipitates AlN. This AlN inhibits recrystallization of ferrite crystal grains in annealing after hot rolling (hereinafter referred to as hot-rolled sheet annealing) and annealing after cold rolling (hereinafter referred to as cold-rolled sheet annealing). Therefore, the ferrite crystal grains are expanded in the rolling direction, and cracks are likely to occur. Therefore, Al is 0.08 mass% or less. Preferably it is 0.05 mass% or less. However, if the Al content is less than 0.02% by mass, the deoxidation effect cannot be obtained. Therefore, when the content of Si in the same deoxidizer is 0.15% by mass or less, Al is preferably in the range of 0.02 to 0.08% by mass.
Ｎ：0.015質量％以下
Ｎは、後述するCrと結合することによってCr窒化物を析出して固溶Crを減少させ、フェライト系ステンレス鋼板の耐食性を劣化させる。Ｎ含有量が0.015質量％を超えると、Cr窒化物が析出して十分な固溶Crを維持できないので、フェライト系ステンレス鋼板の耐食性が劣化するとともに、析出したCr窒化物が起点となって割れを発生させる惧れがある。したがって、Ｎは0.015質量％以下とする。
N: 0.015% by mass or less N combines with Cr, which will be described later, to precipitate Cr nitride and reduce solute Cr, thereby deteriorating the corrosion resistance of the ferritic stainless steel sheet. If the N content exceeds 0.015% by mass, Cr nitride precipitates and sufficient solid solution Cr cannot be maintained, so the corrosion resistance of the ferritic stainless steel sheet deteriorates and cracks originate from the precipitated Cr nitride. Is likely to occur. Therefore, N is 0.015 mass% or less.
Cr：20.5〜23.5質量％
Crは、フェライト系ステンレス鋼板の表面に不動態皮膜を形成して耐食性を高める元素である。一般的なフェライト系ステンレス鋼（たとえばSUS430等）はCrを18質量％程度含有するが、本発明はSUS430を凌ぐ耐食性を有するフェライト系ステンレス鋼板の伸びフランジ加工性の向上を課題としており、Cr含有量が20.5質量％以下では、十分な耐食性が得られない。一方、23.5質量％を超えると、熱延板焼鈍や冷延板焼鈍における再結晶が阻害され、粗大なフェライト結晶粒が圧延方向に展伸し易くなる。またNbとCrを含有する析出相が生じて、硬質化して伸びフランジ加工が難しくなる。したがって、Crは20.5〜23.5質量％の範囲内とする。
Cr: 20.5-23.5 mass%
Cr is an element that forms a passive film on the surface of a ferritic stainless steel sheet to enhance corrosion resistance. Common ferritic stainless steels (such as SUS430) contain approximately 18% by mass of Cr. However, the present invention has an object to improve the stretch flangeability of ferritic stainless steel sheets having corrosion resistance exceeding that of SUS430. If the amount is 20.5% by mass or less, sufficient corrosion resistance cannot be obtained. On the other hand, if it exceeds 23.5% by mass, recrystallization in hot-rolled sheet annealing and cold-rolled sheet annealing is hindered, and coarse ferrite crystal grains are easily expanded in the rolling direction. In addition, a precipitate phase containing Nb and Cr is generated and becomes hard and stretch flange processing becomes difficult. Therefore, Cr is in the range of 20.5 to 23.5 mass%.
Cu：0.3〜0.7質量％
Cuは、アノード反応による地鉄の溶解を低減する作用を有する。Cu含有量が0.3質量％未満では、この効果は得られない。一方、0.7質量％を超えると、MnＳの周囲にCuＳが析出し易くなり、伸びフランジ加工の際にCuＳとMnＳの凝集体を起点として割れが発生する。また、Cuが析出して硬質低延性化してしまう。したがって、Cuは0.3〜0.7質量％の範囲内とする。
Cu: 0.3-0.7 mass%
Cu has the effect of reducing the dissolution of the steel from the anode reaction. If the Cu content is less than 0.3% by mass, this effect cannot be obtained. On the other hand, if it exceeds 0.7% by mass, CuS is likely to precipitate around MnS, and cracks are generated starting from the aggregate of CuS and MnS during stretch flange processing. Also, Cu precipitates and hard and ductile. Therefore, Cu is within the range of 0.3 to 0.7 mass%.
Ni：0.5質量％以下
Niは、耐食性を向上する作用を有する。しかしNi含有量が0.5質量％を超えると、フェライト系ステンレス鋼板が硬質化して延性が損なわれる。したがって、Niは0.5質量％以下とする。
Nb：0.25〜0.55質量％
Nbは、Ｃ，Ｎと結合してNb炭化物やNb窒化物を形成することによってＣ，Ｎを固定し、Cr炭窒化物による鋭敏化を防止する作用を有する。Nb含有量が0.25質量％未満では、この効果は得られない。一方、0.55質量％を超えると、熱延板焼鈍や冷延板焼鈍における再結晶を阻害する。またCrとNbを含有する相が析出して、硬質低延性化して伸びフランジ加工が難しくなる。したがって、Nbは0.25〜0.55質量％の範囲内とする。
Ni: 0.5% by mass or less
Ni has an action of improving corrosion resistance. However, if the Ni content exceeds 0.5% by mass, the ferritic stainless steel sheet becomes hard and ductility is impaired. Therefore, Ni is 0.5 mass% or less.
Nb: 0.25 to 0.55 mass%
Nb binds to C and N to form Nb carbide and Nb nitride, thereby fixing C and N and preventing sensitization by Cr carbonitride. If the Nb content is less than 0.25% by mass, this effect cannot be obtained. On the other hand, when it exceeds 0.55 mass%, recrystallization in hot-rolled sheet annealing and cold-rolled sheet annealing is inhibited. In addition, a phase containing Cr and Nb is precipitated, resulting in hard and low ductility, making stretch flange processing difficult. Therefore, Nb is in the range of 0.25 to 0.55 mass%.
上記した成分以外の残部は、Feおよび不可避的不純物である。不可避的不純物は可能な限り低減することが好ましい。たとえば、Ｂ：0.001％質量以下，Mo：0.1質量％以下，Ｖ：0.05質量％以下，Mg：0.01質量％以下，Ca：0.01質量％以下が許容できる。ただし不可避的不純物は、これらの元素に限定するものではない。
次に、本発明の高耐食性フェライト系ステンレス鋼板の組織を説明する。
The balance other than the above components is Fe and inevitable impurities. Inevitable impurities are preferably reduced as much as possible. For example, B: 0.001% or less, Mo: 0.1% or less, V: 0.05% or less, Mg: 0.01% or less, Ca: 0.01% or less are acceptable. However, inevitable impurities are not limited to these elements.
Next, the structure of the high corrosion resistance ferritic stainless steel sheet of the present invention will be described.
フェライト結晶粒の粒界に形成される凝集体の構成：粒径が１μｍ以下のCuＳおよび／またはMnＳの粒子が10個以下
CuＳ，MnＳは、Feとの密着性が低いので、その粒径が１μｍを超えると、伸びフランジ加工によって容易に割れが発生する。したがって本発明では、凝集体を構成するCuＳ，MnＳの粒径を１μｍ以下として割れの発生と伝播を抑制する。また、粒界に凝集するCuＳ，MnＳの粒子が10個を超えると、Feとの密着性が低いために、伸びフランジ加工によって粒子の間で割れが発生し易くなる。したがって、凝集体を構成するCuＳ，MnＳの粒子は10個以下とする。なお粒子の粒径と個数は、ステンレス鋼板の表面から0.4mm研削し、その面（すなわち厚さ方向中央の面）から抽出レプリカ法で薄膜を作製し、透過型電子顕微鏡でCuＳ，MnＳの粒径と凝集している個数を測定する。
Aggregate structure formed at the grain boundaries of ferrite crystal grains: 10 or less CuS and / or MnS particles with a particle size of 1 μm or less
Since CuS and MnS have low adhesion with Fe, if the particle size exceeds 1 μm, cracks are easily generated by stretch flange processing. Therefore, in the present invention, the generation and propagation of cracks are suppressed by setting the particle size of CuS and MnS constituting the aggregate to 1 μm or less. Further, when the number of CuS and MnS particles aggregated at the grain boundary exceeds 10, the adhesion with Fe is low, so that cracking tends to occur between the particles due to stretch flange processing. Therefore, the number of CuS and MnS particles constituting the aggregate is 10 or less. The particle size and number of particles were 0.4mm ground from the surface of the stainless steel plate, a thin film was prepared from the surface (ie, the central surface in the thickness direction) by the extraction replica method, and the CuS and MnS particles were obtained using a transmission electron microscope. Measure the diameter and the number of aggregates.
フェライト結晶粒のアスペクト比：２以下
アスペクト比はフェライト結晶粒の長径を短径で除した値である。フェライト系ステンレス鋼板では、熱間圧延や冷間圧延によってフェライト結晶粒が圧延方向に長くなる。そのため、フェライト結晶粒の長径は圧延方向の粒径，短径は厚さ方向の粒径にほぼ一致する。つまりアスペクト比が２を超えるフェライト結晶粒は、圧延方向の粒径が厚さ方向の粒径の２倍を超えることを意味する。このような圧延方向に展伸したフェライト結晶粒は、伸びフランジ加工の際に、圧延方向に平行な粒界に応力が集中する。そのため、圧延方向に平行な割れが発生し易くなり、伸びフランジ加工性が劣化する。したがって、フェライト結晶粒のアスペクト比は２以下とする。なおアスペクト比は、ステンレス鋼板の圧延方向に平行な断面の厚さ方向中央部を鏡面研磨し、王水でエッチングし、その面を顕微鏡で観察して、フェライト結晶粒のアスペクト比を求める。
Aspect ratio of ferrite crystal grains: 2 or less The aspect ratio is a value obtained by dividing the major axis of the ferrite crystal grains by the minor axis. In a ferritic stainless steel sheet, ferrite crystal grains are elongated in the rolling direction by hot rolling or cold rolling. For this reason, the major axis of the ferrite crystal grains substantially coincides with the grain diameter in the rolling direction, and the minor axis substantially coincides with the grain diameter in the thickness direction. That is, a ferrite crystal grain having an aspect ratio exceeding 2 means that the grain size in the rolling direction exceeds twice the grain size in the thickness direction. In the ferrite crystal grains expanded in the rolling direction, stress concentrates on the grain boundaries parallel to the rolling direction at the time of stretch flange processing. Therefore, it becomes easy to generate | occur | produce a crack parallel to a rolling direction, and stretch flange workability deteriorates. Therefore, the aspect ratio of the ferrite crystal grains is 2 or less. The aspect ratio is determined by mirror-polishing the central portion in the thickness direction of the cross section parallel to the rolling direction of the stainless steel plate, etching with aqua regia, and observing the surface with a microscope to obtain the aspect ratio of the ferrite crystal grains.
次に、本発明のフェライト系ステンレス鋼板の製造方法を説明する。
所定の成分を有するフェライト系ステンレス鋼を溶製し、さらにスラブとした後、1000℃以上に加熱して熱間圧延（仕上げ温度：800℃以上1000℃未満，巻取り温度：500℃以下）を行ない、熱延鋼板とする。
加熱温度：1000℃以上
スラブの加熱温度が1000℃未満では、熱間圧延中にスラブの温度が低下し、フェライト結晶粒が圧延方向に展伸し易くなる。粗大なMnＳも溶解しないので、１μｍ以上の大きさになり易い。したがって、加熱温度は1000℃以上とする。
Next, the manufacturing method of the ferritic stainless steel sheet of this invention is demonstrated.
After melting ferritic stainless steel with the prescribed components and making it into a slab, heat it to 1000 ° C or higher and perform hot rolling (finishing temperature: 800 ° C or higher and lower than 1000 ° C, winding temperature: 500 ° C or lower) Perform hot-rolled steel sheet.
Heating temperature: 1000 ° C. or more When the slab heating temperature is less than 1000 ° C., the temperature of the slab decreases during hot rolling, and the ferrite crystal grains easily spread in the rolling direction. Since coarse MnS does not dissolve, it tends to have a size of 1 μm or more. Therefore, the heating temperature is 1000 ° C. or higher.
仕上げ温度：800℃以上1000℃未満
熱間圧延の仕上げ温度が800℃未満では、圧延荷重が上昇して圧延ロールに表面疵が発生し易くなる。その表面疵が熱延鋼板に転写されて、フェライト系ステンレス鋼板の表面性状が損なわれ、フェライト系ステンレス鋼板の歩留り低下を招く。また、その表面に生じた凹凸が伸びフランジ加工の際にフェライト系ステンレス鋼板とダイスとの摩擦抵抗を増大させるので、伸びフランジ加工性が劣化する。一方、1000℃以上では、CuＳが粗大化して、伸びフランジ加工性が劣化し易くなる。したがって、仕上げ温度は800℃以上1000℃未満とする。
Finishing temperature: 800 ° C. or more and less than 1000 ° C. When the finishing temperature of hot rolling is less than 800 ° C., the rolling load increases and surface flaws are likely to occur on the rolling roll. The surface defects are transferred to the hot-rolled steel sheet, the surface properties of the ferritic stainless steel sheet are impaired, and the yield of the ferritic stainless steel sheet is reduced. Moreover, since the unevenness generated on the surface increases the frictional resistance between the ferritic stainless steel plate and the die during the stretch flange processing, stretch flange workability deteriorates. On the other hand, at 1000 ° C. or higher, CuS becomes coarse and stretch flangeability tends to deteriorate. Therefore, the finishing temperature is 800 ° C. or higher and lower than 1000 ° C.
巻取り温度：500℃以下
熱延鋼板の巻取り温度が500℃を超えると、フェライト結晶粒の粒界に粗大なCuＳが析出して、フェライト系ステンレス鋼板の伸びフランジ加工性が劣化する。したがって、熱延鋼板の巻取り温度は500℃以下とする。なお、巻取り温度が350℃未満では、熱延鋼板の形状が損なわれる。したがって、巻取り温度は350〜500℃の範囲内が好ましい。
Winding temperature: 500 ° C. or less When the winding temperature of a hot-rolled steel sheet exceeds 500 ° C., coarse CuS precipitates at the grain boundaries of the ferrite crystal grains, and the stretch flangeability of the ferritic stainless steel sheet deteriorates. Therefore, the coiling temperature of the hot rolled steel sheet is set to 500 ° C. or less. Note that when the coiling temperature is less than 350 ° C., the shape of the hot-rolled steel sheet is impaired. Therefore, the coiling temperature is preferably in the range of 350 to 500 ° C.
このようにして得られた熱延鋼板に熱延板焼鈍を施し、さらに酸洗を施す。熱延板焼鈍は、加熱温度：900℃以上，加熱時間：500秒以下で行なう。
熱延板焼鈍の加熱温度：900℃以上，加熱時間：500秒以下
熱延板焼鈍の加熱温度が900℃未満では、熱間圧延によって圧延方向に展伸したフェライト結晶粒が再結晶し難い。そのため、後述する冷間圧延によってフェライト結晶粒の展伸がさらに助長されるので、フェライト系ステンレス鋼板の伸びフランジ加工性が劣化する。したがって、熱延板焼鈍の加熱温度は900℃以上とする。加熱時間が500秒を超えると、析出したCuＳ，MnＳが粗大化もしくは10個以上凝集し易くなる。そのため、冷間圧延によってフェライト結晶粒が著しく展伸するので、フェライト系ステンレス鋼板の伸びフランジ加工性が劣化する。したがって、熱延板焼鈍の加熱時間は500秒以下とする。
The hot-rolled steel sheet thus obtained is subjected to hot-rolled sheet annealing and further pickled. Hot-rolled sheet annealing is performed at a heating temperature of 900 ° C. or more and a heating time of 500 seconds or less.
Heating temperature of hot-rolled sheet annealing: 900 ° C or higher, heating time: 500 seconds or less If the heating temperature of hot-rolled sheet annealing is less than 900 ° C, the ferrite crystal grains stretched in the rolling direction by hot rolling are difficult to recrystallize. Therefore, since the extension of ferrite crystal grains is further promoted by cold rolling described later, the stretch flangeability of the ferritic stainless steel sheet is deteriorated. Accordingly, the heating temperature for hot-rolled sheet annealing is set to 900 ° C. or higher. When the heating time exceeds 500 seconds, the precipitated CuS and MnS are likely to become coarse or aggregate 10 or more. Therefore, since the ferrite crystal grains remarkably expand by cold rolling, the stretch flangeability of the ferritic stainless steel sheet is deteriorated. Accordingly, the heating time for hot-rolled sheet annealing is set to 500 seconds or less.
酸洗は、特に条件を限定せず、従来から知られている方法で操業する。
次いで、冷間圧延を行ない、冷延鋼板とする。得られた冷延鋼板に冷延板焼鈍を施して、フェライト系ステンレス鋼板とする。冷延板焼鈍は、加熱温度：850℃以上で行なうことが好ましい。
冷延板焼鈍の加熱温度：850℃以上
冷延板焼鈍の加熱温度が850℃未満では、フェライト系ステンレス鋼板のフェライト結晶粒が圧延方向に展伸したままであり、伸びフランジ加工性が劣化する。また、フェライト結晶粒の再結晶が阻害されるので、フェライト系ステンレス鋼板の延性が低下する。したがって、冷延板焼鈍の加熱温度は850℃以上とする。
The pickling is not particularly limited, and is performed by a conventionally known method.
Next, cold rolling is performed to obtain a cold rolled steel sheet. The obtained cold-rolled steel sheet is subjected to cold-rolled sheet annealing to obtain a ferritic stainless steel sheet. Cold-rolled sheet annealing is preferably performed at a heating temperature of 850 ° C. or higher.
Heating temperature for cold-rolled sheet annealing: 850 ° C or more If the heating temperature for cold-rolled sheet annealing is less than 850 ° C, the ferrite crystal grains of the ferritic stainless steel sheet remain stretched in the rolling direction and the stretch flangeability deteriorates. . Further, since recrystallization of ferrite crystal grains is hindered, the ductility of the ferritic stainless steel sheet is lowered. Accordingly, the heating temperature for cold-rolled sheet annealing is set to 850 ° C. or higher.
冷延鋼板に調質圧延を施しても良い、調質圧延の圧下率は、0.5〜1.5％の範囲内が好ましい。 The cold rolling steel sheet may be subjected to temper rolling. The temper rolling reduction is preferably in the range of 0.5 to 1.5%.
表１に示す成分のフェライト系ステンレス鋼を溶製し、さらにスラブとした後、そのスラブを1150℃に加熱して熱間圧延を行ない、板厚４mmの熱延鋼板とした。熱間圧延の条件は表２に示す通りである。得られた熱延鋼板に熱延板焼鈍を施し、さらに酸洗を施した。熱延板焼鈍の条件は表２に示す通りである。次いで冷間圧延を行ない厚さ0.8mmの冷延鋼板とした。 A ferritic stainless steel having the components shown in Table 1 was melted to form a slab, and the slab was heated to 1150 ° C. and hot-rolled to obtain a hot-rolled steel sheet having a thickness of 4 mm. The conditions for hot rolling are as shown in Table 2. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing and further subjected to pickling. The conditions for hot-rolled sheet annealing are as shown in Table 2. Next, cold rolling was performed to obtain a cold-rolled steel sheet having a thickness of 0.8 mm.
得られた冷延鋼板に冷延板焼鈍を施し、さらに酸洗を施した。冷延板焼鈍の条件は表２に示す通りである。
このようにして製造したフェライト系ステンレス鋼板の圧延方向に平行な断面の厚さ方向中央部を鏡面研磨し、王水でエッチングし、その面を顕微鏡で観察して、フェライト結晶粒のアスペクト比を求めた。その結果を表２に示す。
The obtained cold-rolled steel sheet was subjected to cold-rolled sheet annealing and further pickled. The conditions for cold-rolled sheet annealing are as shown in Table 2.
The central part in the thickness direction of the cross section parallel to the rolling direction of the ferritic stainless steel sheet thus manufactured is mirror-polished, etched with aqua regia, and the surface is observed with a microscope to determine the aspect ratio of the ferrite crystal grains. Asked. The results are shown in Table 2.
次に、フェライト系ステンレス鋼板の表面から0.4mm研削し、その面（すなわち厚さ方向中央の面）から抽出レプリカ法で薄膜を作製し、透過型電子顕微鏡でCuＳ，MnＳの粒径と凝集している個数を測定した。その結果を表２に示す。
また、フェライト系ステンレス鋼板からＪＩＳ−13号Ｂ引張試験片を採取し、引張試験を行なった。その結果を表２に示す。
Next, 0.4mm is ground from the surface of the ferritic stainless steel plate, a thin film is produced from the surface (ie, the central surface in the thickness direction) by the extraction replica method, and agglomerates with the particle size of CuS and MnS using a transmission electron microscope. The number is measured. The results are shown in Table 2.
Further, a JIS-13B tensile test piece was sampled from a ferritic stainless steel plate and subjected to a tensile test. The results are shown in Table 2.
さらに、フェライト系ステンレス鋼板から穴広げ試験片（100mm×100mm）を切り出して、穴広げ試験を行なった。穴広げ試験では、打抜き加工で穴広げ試験片の中央に直径Ｄ1 （＝10mm）の円形穴を設け、バリの反対側から頂角60°のポンチを挿入して円形穴を押広げて、穴広げ試験片に板厚を貫通する割れが発生したときの円形穴の直径Ｄ2 を測定して、伸びフランジ加工性を評価した。その結果を穴広げ率（＝100×〔Ｄ2−Ｄ1〕／Ｄ1）として表２に示す。穴広げ率が大きいほど、優れた伸びフランジ加工性を有することを意味する。 Furthermore, a hole expansion test piece (100 mm × 100 mm) was cut out from the ferritic stainless steel plate and subjected to a hole expansion test. In the hole expansion test, a circular hole with a diameter D 1 (= 10mm) is provided in the center of the hole expansion test piece by punching, and a circular hole is expanded by inserting a punch with an apex angle of 60 ° from the opposite side of the burr. by measuring the diameter D 2 of the circular hole when cracks passing through the thickness to the hole expanding test piece occurs, it was evaluated stretch flange formability. The results are shown in Table 2 as the hole expansion ratio (= 100 × [D 2 −D 1 ] / D 1 ). It means that it has the outstanding stretch flange workability, so that a hole expansion rate is large.
表２のNo.１〜５は、Mn含有量を変化させた例である。Mnおよび他の元素の含有量が本発明の範囲を満足するNo.２〜４では、穴広げ率が100％を超えた。Mn含有量が本発明の範囲より低いNo.１ではCuＳ，MnＳが大きくかつ多数凝集しているので、穴広げ率が低い。Mn含有量が本発明の範囲より大きいNo.５もCuＳ，MnＳの凝集数が多く、フェライト結晶粒のアスペクト比も大きく、穴広げ率が低い。 Nos. 1 to 5 in Table 2 are examples in which the Mn content was changed. In Nos. 2 to 4 in which the contents of Mn and other elements satisfy the scope of the present invention, the hole expansion ratio exceeded 100%. In No. 1 in which the Mn content is lower than the range of the present invention, CuS and MnS are large and many agglomerates, so the hole expansion rate is low. No. 5 whose Mn content is larger than the range of the present invention also has a large number of CuS and MnS aggregates, a large aspect ratio of ferrite crystal grains, and a low hole expansion rate.
No.６〜９は、Ｓ含有量を変化させた例である。Ｓおよび他の元素の含有量が本発明の範囲を満足するNo.６〜８では、穴広げ率が100％を超えた。Ｓ含有量が本発明の範囲より高いNo.９では、MnＳが大きくかつ多数凝集しているので、穴広げ率が低い。
No.10〜14は、Cu含有量を変化させた例である。Cuおよび他の元素の含有量が本発明の範囲を満足するNo.10〜13では、穴広げ率が100％を超えた。Cu含有量が本発明の範囲より高いNo.14では、CuＳが大きくかつ多数凝集しているので、穴広げ率が低い。
No.15〜18は、Nb含有量を変化させた例である。Nbおよび他の元素の含有量が本発明の範囲を満足するNo.16〜17では、穴広げ率が100％を超えた。Nb含有量が本発明の範囲より低いNo.15では、Cr炭化物が析出して耐食性が悪かった。また、引張強さが大きく、伸びが小さいので、穴広げ率が低い。Nb含有量が本発明の範囲より高いNo.18では、フェライト結晶粒のアスペクト比が大きく、伸びが小さいので、穴広げ率が低い。
Nos. 6 to 9 are examples in which the S content was changed. In Nos. 6 to 8 in which the contents of S and other elements satisfy the scope of the present invention, the hole expansion ratio exceeded 100%. In No. 9 in which the S content is higher than the range of the present invention, MnS is large and many agglomerates, so the hole expansion rate is low.
Nos. 10 to 14 are examples in which the Cu content was changed. In Nos. 10 to 13 in which the contents of Cu and other elements satisfy the scope of the present invention, the hole expansion ratio exceeded 100%. In No. 14 in which the Cu content is higher than the range of the present invention, CuS is large and many agglomerates, so the hole expansion rate is low.
Nos. 15 to 18 are examples in which the Nb content was changed. In Nos. 16 to 17 in which the contents of Nb and other elements satisfy the scope of the present invention, the hole expansion ratio exceeded 100%. In No. 15 where the Nb content is lower than the range of the present invention, Cr carbide was precipitated and the corrosion resistance was poor. Further, since the tensile strength is large and the elongation is small, the hole expansion rate is low. In No. 18, in which the Nb content is higher than the range of the present invention, the aspect ratio of the ferrite crystal grains is large and the elongation is small, so the hole expansion rate is low.
No.19〜24は、熱間圧延の条件を変化させた例である。本発明の範囲を満足するNo.19〜20では、穴広げ率が100％を超えた。巻取り温度が本発明の範囲より高いNo.21では、CuＳ，MnＳの粒径が大きく、フェライト結晶粒のアスペクト比が大きいので、穴広げ率が低い。熱延板焼鈍の加熱温度が本発明の範囲より低いNo.22では、粗大なCuＳ，MnＳが多数凝集しているので、穴広げ率が低い。熱延板焼鈍の加熱時間が本発明の範囲より長いNo.23では、粗大なCuＳ，MnＳが多数凝集し、かつフェライト結晶粒のアスペクト比が大きいので、穴広げ率が低い。冷延板焼鈍の加熱温度が本発明の範囲より低いNo.24では、フェライト結晶粒のアスペクト比が著しく増大し、穴広げ率が低い。 Nos. 19 to 24 are examples in which the hot rolling conditions were changed. In Nos. 19 to 20 satisfying the scope of the present invention, the hole expansion ratio exceeded 100%. In No. 21, where the coiling temperature is higher than the range of the present invention, the grain size of CuS and MnS is large and the aspect ratio of the ferrite crystal grains is large, so the hole expansion rate is low. In No. 22 where the heating temperature of hot-rolled sheet annealing is lower than the range of the present invention, a large number of coarse CuS and MnS are agglomerated, so the hole expansion rate is low. In No. 23, in which the heating time for hot-rolled sheet annealing is longer than the range of the present invention, a large number of coarse CuS and MnS are aggregated and the aspect ratio of the ferrite crystal grains is large, so the hole expansion rate is low. In No. 24 where the heating temperature for cold-rolled sheet annealing is lower than the range of the present invention, the aspect ratio of the ferrite crystal grains is remarkably increased and the hole expansion rate is low.
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