JP4998719B2 - Ferritic stainless steel sheet for water heaters excellent in punching processability and method for producing the same - Google Patents
Ferritic stainless steel sheet for water heaters excellent in punching processability and method for producing the same Download PDFInfo
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- 238000004080 punching Methods 0.000 title claims description 32
- 229910001220 stainless steel Inorganic materials 0.000 title claims description 28
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 title claims description 20
- 238000004519 manufacturing process Methods 0.000 title claims description 8
- 229910000859 α-Fe Inorganic materials 0.000 claims description 35
- 229910052758 niobium Inorganic materials 0.000 claims description 31
- 229910052719 titanium Inorganic materials 0.000 claims description 31
- 229910000831 Steel Inorganic materials 0.000 claims description 27
- 239000010959 steel Substances 0.000 claims description 27
- 229910052799 carbon Inorganic materials 0.000 claims description 26
- 229910052757 nitrogen Inorganic materials 0.000 claims description 24
- 239000002131 composite material Substances 0.000 claims description 21
- 239000000203 mixture Substances 0.000 claims description 20
- 238000000137 annealing Methods 0.000 claims description 18
- 238000000034 method Methods 0.000 claims description 12
- 238000004804 winding Methods 0.000 claims description 7
- 239000012535 impurity Substances 0.000 claims description 6
- 229910052698 phosphorus Inorganic materials 0.000 claims description 6
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 238000005097 cold rolling Methods 0.000 claims description 4
- 229910052717 sulfur Inorganic materials 0.000 claims description 4
- 230000007797 corrosion Effects 0.000 description 30
- 238000005260 corrosion Methods 0.000 description 30
- 239000002244 precipitate Substances 0.000 description 15
- 238000005098 hot rolling Methods 0.000 description 9
- 238000003466 welding Methods 0.000 description 9
- 230000000694 effects Effects 0.000 description 7
- 238000010438 heat treatment Methods 0.000 description 7
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 7
- 238000001953 recrystallisation Methods 0.000 description 7
- 239000000463 material Substances 0.000 description 6
- 239000011324 bead Substances 0.000 description 5
- 239000010408 film Substances 0.000 description 5
- 238000005096 rolling process Methods 0.000 description 5
- 239000000243 solution Substances 0.000 description 4
- 230000006866 deterioration Effects 0.000 description 3
- 239000007789 gas Substances 0.000 description 3
- 238000002844 melting Methods 0.000 description 3
- 230000008018 melting Effects 0.000 description 3
- 229910052710 silicon Inorganic materials 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 description 2
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 2
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 2
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 2
- 239000000460 chlorine Substances 0.000 description 2
- 229910052801 chlorine Inorganic materials 0.000 description 2
- 229910052804 chromium Inorganic materials 0.000 description 2
- 238000005336 cracking Methods 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 238000005304 joining Methods 0.000 description 2
- 229910052750 molybdenum Inorganic materials 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 239000010935 stainless steel Substances 0.000 description 2
- 238000009864 tensile test Methods 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 1
- 206010070834 Sensitisation Diseases 0.000 description 1
- QZPSXPBJTPJTSZ-UHFFFAOYSA-N aqua regia Chemical compound Cl.O[N+]([O-])=O QZPSXPBJTPJTSZ-UHFFFAOYSA-N 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 229910000963 austenitic stainless steel Inorganic materials 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 238000005256 carbonitriding Methods 0.000 description 1
- 230000003749 cleanliness Effects 0.000 description 1
- 239000011362 coarse particle Substances 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000005520 cutting process Methods 0.000 description 1
- 230000006378 damage Effects 0.000 description 1
- 238000005261 decarburization Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
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- 230000002542 deteriorative effect Effects 0.000 description 1
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- 238000009413 insulation Methods 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 229910017604 nitric acid Inorganic materials 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 238000005554 pickling Methods 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 230000002787 reinforcement Effects 0.000 description 1
- 230000008313 sensitization Effects 0.000 description 1
- 239000011780 sodium chloride Substances 0.000 description 1
- 238000003756 stirring Methods 0.000 description 1
- 238000005482 strain hardening Methods 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 239000010409 thin film Substances 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
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- Heat Treatment Of Sheet Steel (AREA)
Description
本発明は、温水器用フェライト系ステンレス鋼板、特に、打ち抜き加工性に優れる温水器用フェライト系ステンレス鋼板およびその製造方法に関する。 The present invention relates to a ferritic stainless steel sheet for water heaters, and more particularly to a ferritic stainless steel sheet for water heaters excellent in punching workability and a method for producing the same.
フェライト系ステンレス鋼板は、意匠性や耐食性に優れるため、建築物、輸送機器、家庭電化製品、厨房器具など、様々な用途に用いられている。特に、近年、ヒートポンプ方式の温水器缶体には、応力腐食割れの起きやすいオーステナイト系ステンレス鋼板に代わって、フェライト系ステンレス鋼板の使用される機会が多くなっている。温水器缶体などの構造体は、鋼板を切断、加工、接合する工程を経て製造されるが、さらに補強のために同じ鋼板を打ち抜き加工した部品が接合される場合が多い。しかし、通常のフェライト系ステンレス鋼板を打ち抜き加工すると、打ち抜き加工時に発生するかえり(バリ)の高さが大きく、溶接などで接合するときに隙間が生じ、溶接不良を引き起こすといった問題がある。 Ferritic stainless steel sheets are excellent in design and corrosion resistance, and are used in various applications such as buildings, transportation equipment, home appliances, and kitchen appliances. In particular, in recent years, ferritic stainless steel sheets have been used more frequently in heat pump water heater cans in place of austenitic stainless steel sheets that are susceptible to stress corrosion cracking. A structure such as a water heater can body is manufactured through a process of cutting, processing, and joining steel sheets, and in many cases, parts obtained by punching the same steel sheet for reinforcement are joined. However, when a normal ferritic stainless steel sheet is punched, there is a problem that the height of burr (burr) generated during the punching process is large and a gap is formed when joining by welding or the like, resulting in poor welding.
温水器用フェライト系ステンレス鋼板として、特許文献1には、質量%で、C≦0.08%、Si≦0.3%、Mn≦0.3%、13%≦Cr≦26%、N≦0.03%、P<0.015%、S<0.0010%、0.02%≦Al≦0.20%、残部実質的にFeからなる耐食性を主とする使用性能が優れたフェライト系ステンレス鋼が開示されている。 As a ferritic stainless steel sheet for water heaters, Patent Document 1 includes mass%, C ≦ 0.08%, Si ≦ 0.3%, Mn ≦ 0.3%, 13% ≦ Cr ≦ 26%, N ≦ 0.03%, P <0.015%. , S <0.0010%, 0.02% ≦ Al ≦ 0.20%, and a ferritic stainless steel excellent in use performance mainly composed of corrosion resistance mainly composed of Fe is disclosed.
特許文献2には、質量%で、C≦0.025%、Si≦0.6%、Mn≦1.0%、P≦0.04%、S≦0.01%、Ni≦0.6%、16%≦Cr≦35%、0.3%≦Mo≦6%、N≦0.025%、0.01%≦Al≦0.5%、0.1%≦Nb≦0.6%、0.05%≦Ti≦0.3%、0.1%≦Cu≦1.0%を含み、かつこれらの成分の間に質量%において、C+N≦0.04、およびNb+Ti≧7(C+N)+0.15の関係が成立し、なおかつ次の式、B値=Cr+3(Mo+Cu)≧23.5、およびP値=5Ti+20(Al-0.01)≧1.5の関係が成立し、残部は実質的に鉄および不可避的不純物からなる溶接部の耐食性に優れるフェライト系ステンレス鋼が開示されている。 Patent Document 2 includes mass%, C ≦ 0.025%, Si ≦ 0.6%, Mn ≦ 1.0%, P ≦ 0.04%, S ≦ 0.01%, Ni ≦ 0.6%, 16% ≦ Cr ≦ 35%, 0.3%. ≦ Mo ≦ 6%, N ≦ 0.025%, 0.01% ≦ Al ≦ 0.5%, 0.1% ≦ Nb ≦ 0.6%, 0.05% ≦ Ti ≦ 0.3%, 0.1% ≦ Cu ≦ 1.0%, and of these components In the mass%, the relationship of C + N ≦ 0.04 and Nb + Ti ≧ 7 (C + N) +0.15 is established, and the following formula, B value = Cr + 3 (Mo + Cu) ≧ 23.5, And the relationship of P value = 5Ti + 20 (Al-0.01) ≧ 1.5 is established, and the ferritic stainless steel is disclosed which is excellent in the corrosion resistance of the welded portion substantially composed of iron and inevitable impurities.
特許文献3には、質量%で、C≦0.003%、0.1%≦Si≦0.4%、Mn≦0.4%、P≦0.04%、S≦0.01%、16.0%≦Cr≦25.0%、0.8%≦Mo≦2.5%、N≦0.03%、0.1%≦Nb≦0.6%、0.05%≦Ti≦0.3%、0.01%≦Al≦0.5%を含み、かつNb、Ti、CおよびNの間にNb+Ti≧7(C+N)+0.15の関係が成立し、残部が実質的にFeからなるフェライト系ステンレス鋼板から構成され、胴体と上下の鏡板がかしめ接合されている耐食性に優れた温水器缶体が開示されている。 Patent Document 3 includes mass%, C ≦ 0.003%, 0.1% ≦ Si ≦ 0.4%, Mn ≦ 0.4%, P ≦ 0.04%, S ≦ 0.01%, 16.0% ≦ Cr ≦ 25.0%, 0.8% ≦ Mo. ≦ 2.5%, N ≦ 0.03%, 0.1% ≦ Nb ≦ 0.6%, 0.05% ≦ Ti ≦ 0.3%, 0.01% ≦ Al ≦ 0.5%, and Nb + Ti ≧ Nb between Ti, Ti, C and N 7 (C + N) + 0.15 relationship is established, the balance is composed of a ferritic stainless steel plate that is substantially made of Fe, and the body and upper and lower end panels are caulked and joined together. It is disclosed.
特許文献4には、質量%で、0.001%≦C≦0.02%、0.001%≦N≦0.02%、0.01%≦Si≦0.3%、0.05%≦Mn≦1%、P≦0.04%、0.15%≦Ni≦3%、11%≦Cr≦22%、0.01%≦Ti≦0.5%、0.0002%≦Mg≦0.002%を含み、0.5%≦Mo≦3.0%、0.02%≦Nb≦0.6%、0.1%≦Cu≦1.5%の条件で、Mo、Nb、Cuのうち1種または2種以上を、Cr+3Mo+6(Ni+Nb+Cu)≧23を満たす範囲で含み、残部がFeおよび不可避不純物からなる耐すきま腐食性に優れたフェライト系ステンレス鋼が開示されている。
しかしながら、特許文献1〜4に記載されたいずれのフェライト系ステンレス鋼においても、打ち抜き加工時のかえりの高さを必ずしも小さくできない。 However, in any of the ferritic stainless steels described in Patent Documents 1 to 4, the burr height during punching cannot always be reduced.
本発明は、このような課題を解決するためになされたもので、打ち抜き加工時のかえりの高さを確実に小さくできる、すなわち打ち抜き加工性に優れる温水器用フェライト系ステンレス鋼板およびその製造方法を提供することを目的とする。 The present invention has been made to solve such problems, and provides a ferritic stainless steel sheet for water heaters that can reliably reduce the height of burr at the time of punching, that is, excellent in punching workability, and a method for manufacturing the same. The purpose is to do.
本発明者等は、温水器用フェライト系ステンレス鋼板の打ち抜き加工時のかえりの発生挙動について詳細に調査した結果、以下の知見を得た。 As a result of detailed investigations on the occurrence of burr at the time of punching of a ferritic stainless steel sheet for water heaters, the present inventors have obtained the following knowledge.
i)フェライト粒界に、NbとTiを含み、かつNbとTiの組成比[Nb]/[Ti]が1以上10以下の複合炭窒化物(Nb,Ti)(C,N)を析出させると、打ち抜き加工時に亀裂の伝播を促進でき、打ち抜き加工時のかえりの高さの低減に効果的である。 i) Precipitate a composite carbonitride (Nb, Ti) (C, N) containing Nb and Ti and having a composition ratio [Nb] / [Ti] of 1 to 10 at the ferrite grain boundary This can promote the propagation of cracks during the punching process, and is effective in reducing the burr height during the punching process.
ii)フェライト粒径を25μm以下にしてこうした複合炭窒化物を均一に分散させ、さらに降伏比を0.65以上にして著しい加工硬化を抑制することにより、打ち抜き加工時のかえりの高さを確実に小さくできる。 ii) By reducing the ferrite grain size to 25 μm or less and uniformly dispersing these composite carbonitrides, and further suppressing the significant work hardening by setting the yield ratio to 0.65 or more, the burr height at the time of punching is surely reduced. it can.
本発明は、このような知見に基づきなされたもので、質量%で、0.0030%≦C≦0.012%、0.20%≦Si≦0.50%、Mn≦0.25%、P≦0.04%、S≦0.005%、0.02%≦Al≦0.06%、0.0030%≦N≦0.012%、21.0%≦Cr≦25.0%、0.5%≦Mo≦1.3%、0.3%≦Nb≦0.5%、0.02%≦Ti≦0.15%を含み、残部がFeおよび不可避的不純物からなる成分組成を有し、フェライト粒径が25μm以下で、フェライト粒界にはNbとTiの組成比[Nb]/[Ti]が1以上10以下である複合炭窒化物(Nb,Ti)(C,N)が析出しており、かつ降伏比が0.65以上であることを特徴とする打ち抜き加工性に優れる温水器用フェライト系ステンレス鋼板を提供する。 The present invention was made based on such knowledge, and in mass%, 0.0030% ≦ C ≦ 0.012%, 0.20% ≦ Si ≦ 0.50%, Mn ≦ 0.25%, P ≦ 0.04%, S ≦ 0.005%, 0.02% ≦ Al ≦ 0.06%, 0.0030% ≦ N ≦ 0.012%, 21.0% ≦ Cr ≦ 25.0%, 0.5% ≦ Mo ≦ 1.3%, 0.3% ≦ Nb ≦ 0.5%, 0.02% ≦ Ti ≦ 0.15%, A composite carbon having a composition comprising the balance of Fe and inevitable impurities, a ferrite grain size of 25 μm or less, and a Nb / Ti composition ratio [Nb] / [Ti] of 1 to 10 at the ferrite grain boundary Provided is a ferritic stainless steel sheet for water heaters excellent in punching workability, wherein nitride (Nb, Ti) (C, N) is precipitated and the yield ratio is 0.65 or more.
本発明のフェライト系ステンレス鋼板には、さらに、質量%で、0.15%≦Ni≦0.5%および0.3%≦Cu≦0.6%のうちから選ばれた少なくとも1種の元素を含有させることができる。 The ferritic stainless steel sheet of the present invention may further contain at least one element selected from 0.15% ≦ Ni ≦ 0.5% and 0.3% ≦ Cu ≦ 0.6% by mass%.
本発明のフェライト系ステンレス鋼板は、上記の成分組成を有する鋼スラブを1000℃以上の温度に加熱後、900℃以上の仕上温度で熱間圧延して熱延板とし、前記熱延板を400℃以上550℃以下の巻取温度で巻取り、次いで熱延板焼鈍を行った後、冷間圧延して冷延板とし、前記冷延板を再結晶焼鈍することを特徴とする方法により製造できる。 The ferritic stainless steel sheet of the present invention is a hot-rolled sheet obtained by heating a steel slab having the above component composition to a temperature of 1000 ° C or higher and then hot rolling at a finishing temperature of 900 ° C or higher. Winding at a coiling temperature of 550 ° C to 550 ° C, followed by hot-rolled sheet annealing, cold-rolled into a cold-rolled sheet, and manufactured by a method characterized by recrystallization annealing the cold-rolled sheet it can.
本発明により、打ち抜き加工時のかえりの高さを確実に小さくでき、打ち抜き加工性に優れる温水器用フェライト系ステンレス鋼板を製造できるようになった。 According to the present invention, the height of the burr at the time of punching can be reliably reduced, and a ferritic stainless steel sheet for water heaters having excellent punchability can be manufactured.
以下に、本発明であるフェライト系ステンレス鋼板およびその製造方法について詳述する。 Below, the ferritic stainless steel plate which is this invention, and its manufacturing method are explained in full detail.
1)成分組成(以下の「%」は、「質量%」を表す。)
0.0030%≦C≦0.012%
Cは、フェライト粒界に複合炭窒化物(Nb,Ti)(C,N)として析出し、打ち抜き加工時のかえりの高さを小さくする。そのためには、C量は0.0030%以上とする必要がある。一方、C量が0.012%を超えると、鋼が硬質化し、打ち抜き加工時の端面で破断面比率が多くなり、かえりの高さが大きくなるとともに、Cr炭化物が多量に析出して、鋭敏化により耐食性が劣化する。したがって、C量は0.0030%以上0.012%以下とする。
1) Component composition ("%" below represents "% by mass")
0.0030% ≦ C ≦ 0.012%
C precipitates as composite carbonitride (Nb, Ti) (C, N) at the ferrite grain boundary, and reduces the height of burr during punching. For that purpose, the C content needs to be 0.0030% or more. On the other hand, if the C content exceeds 0.012%, the steel hardens, the fracture surface ratio increases at the end face during punching, the height of burr increases, and a large amount of Cr carbide precipitates, resulting in sensitization. Corrosion resistance deteriorates. Therefore, the C content is 0.0030% or more and 0.012% or less.
0.20%≦Si≦0.50%
Siは、脱酸剤として必要な元素であるとともに、溶接部の耐食性の向上に有効な元素でもある。特に、溶接時の熱影響部で酸化されて緻密な皮膜を形成し、温水器のような残留塩素が存在する溶液中で母材の耐食性の劣化を防止する。そのためには、Si量を0.02%以上、好ましくは0.35%以上とする必要がある。一方、Siは、固溶強化元素であり、その量が0.50%を超えると、鋼が硬質低延性化する。したがって、Si量は0.02%以上、好ましくは0.35%以上0.50%以下とする。
0.20% ≦ Si ≦ 0.50%
Si is an element necessary as a deoxidizer and an element effective for improving the corrosion resistance of the weld. In particular, it is oxidized at the heat-affected zone during welding to form a dense film, and prevents deterioration of the corrosion resistance of the base material in a solution containing residual chlorine such as a water heater. For that purpose, the Si amount needs to be 0.02% or more, preferably 0.35% or more. On the other hand, Si is a solid solution strengthening element, and when its amount exceeds 0.50%, the steel becomes hard and ductile. Therefore, the Si content is 0.02% or more, preferably 0.35% or more and 0.50% or less.
Mn≦0.25%
Mn量が0.25%を超えると、耐食性を劣化させる元素であるとともに、フェライト粒界にMnSとして析出し、打ち抜き加工時のかえりの高さを大きくする。このため、Mn量は0.25%以下とする。
Mn ≦ 0.25%
If the Mn content exceeds 0.25%, it is an element that degrades the corrosion resistance, and precipitates as MnS at the ferrite grain boundary, increasing the burr height during punching. For this reason, the amount of Mn is made 0.25% or less.
P≦0.04%
P量が0.04%を超えると、鋼を固溶強化するとともに、脆化させる。このため、P量は0.04%以下とする。
P ≦ 0.04%
If the P content exceeds 0.04%, the steel is strengthened by solid solution and embrittled. Therefore, the P content is 0.04% or less.
S≦0.005%
S量が0.005%を超えると、フェライト粒界にMnSとして析出し、耐食性を劣化させるとともに、打ち抜き加工時のかえりの高さを大きくする。このため、S量は0.005%以下とする。
S ≦ 0.005%
When the amount of S exceeds 0.005%, it precipitates as MnS at the ferrite grain boundary, which deteriorates the corrosion resistance and increases the burr height during punching. Therefore, the S content is 0.005% or less.
0.02%≦Al≦0.06%
Alは、脱酸剤であり、鋼の清浄度を向上させるためには積極添加が望まれる。また、Siと同様に、溶接時の熱影響部で酸化されて緻密な皮膜を形成し、温水器のような残留塩素が存在する溶液中で母材の耐食性の劣化を防止する。こうした効果を得るためには、Al量は0.02%以上とする必要がある。一方、Al量が0.06%を超えると、AlNや微細酸化物が生成し、打ち抜き加工時のかえりの高さを大きくする。したがって、Al量は0.02%以上0.06%以下とする。
0.02% ≦ Al ≦ 0.06%
Al is a deoxidizer, and positive addition is desired to improve the cleanliness of steel. Also, like Si, it is oxidized at the heat-affected zone during welding to form a dense film and prevents deterioration of the corrosion resistance of the base material in a solution containing residual chlorine such as a water heater. In order to obtain such an effect, the Al content needs to be 0.02% or more. On the other hand, when the Al content exceeds 0.06%, AlN and fine oxides are generated, and the burr height during punching is increased. Therefore, the Al content is 0.02% or more and 0.06% or less.
0.0030%≦N≦0.012%
Nは、Cと同様に、フェライト粒界に複合炭窒化物(Nb,Ti)(C,N)として析出し、打ち抜き加工時のかえりの高さを小さくする。そのためには、N量は0.0030%以上とする必要がある。一方、N量が0.012%を超えると、複合炭窒化物の量が増大し、鋼が硬質化して、打ち抜き加工時の端面で破断面比率が多くなり、かえりの高さが大きくなる。したがって、N量は0.0030%以上0.012%以下とする。
0.0030% ≦ N ≦ 0.012%
N, like C, precipitates as composite carbonitrides (Nb, Ti) (C, N) at the ferrite grain boundaries, and reduces the burr height during punching. For that purpose, the N amount needs to be 0.0030% or more. On the other hand, if the N content exceeds 0.012%, the amount of composite carbonitride increases, the steel becomes hard, the fracture surface ratio increases at the end face during stamping, and the burr height increases. Therefore, the N content is 0.0030% or more and 0.012% or less.
21.0%≦Cr≦25.0%
Crは、ステンレス鋼表面に不動態被膜を形成し、耐食性を向上させる元素である。通常のフェライト系ステンレス鋼板は、SUS430に代表されるように、18%のCrを含有している。温水器缶体製造におけるTIG溶接などではガスシールドが十分でなく、ビード部などにCrを含むテンパーカラーと呼ばれる酸化皮膜が生成して、母材のCr濃度を低下させ、溶接部の耐食性を劣化させる。特に、1000℃以上で生成する酸化皮膜にはCrが選択的に多量に含まれ、温水中での溶接部の耐食性を極端に劣化させてしまう。そこで、本発明者等がこうした耐食性の極端な劣化を起こすCr量について検討したところ、Cr量が21.0%未満ではMoやその他の元素を添加しても、温水中での耐食性は不安定となり、すきま部などでは孔食の原因となることが明らかになった。一方、Cr量が25.0%を超えると、加工性が顕著に低下する。このため、Cr量は21.0%以上25.0%以下、好ましくは22.0%以上24.0%以下とする。
21.0% ≦ Cr ≦ 25.0%
Cr is an element that forms a passive film on the surface of stainless steel and improves corrosion resistance. A typical ferritic stainless steel sheet contains 18% Cr, as represented by SUS430. Gas shield is not enough in TIG welding etc. in water heater can manufacturing, and an oxide film called temper color containing Cr is generated in the bead part etc., lowering the Cr concentration of the base material and deteriorating the corrosion resistance of the welded part Let In particular, the oxide film generated at 1000 ° C. or higher contains a large amount of Cr selectively, which extremely deteriorates the corrosion resistance of the weld in hot water. Therefore, the present inventors examined the amount of Cr causing such extreme deterioration of corrosion resistance.When the amount of Cr is less than 21.0%, even when Mo or other elements are added, the corrosion resistance in warm water becomes unstable. It became clear that pitting corrosion was caused in the gaps. On the other hand, when the Cr content exceeds 25.0%, the workability is significantly reduced. Therefore, the Cr content is 21.0% or more and 25.0% or less, preferably 22.0% or more and 24.0% or less.
0.5%≦Mo≦1.3%
Mo量が0.5%以上であると、耐食性が顕著に向上するが、1.3%を超えると、本発明のCr含有量の範囲では靭性が顕著に低下する。このため、Mo量は0.5%以上1.3%以下、好ましくは0.8%以上1.2%以下とする。
0.5% ≦ Mo ≦ 1.3%
When the Mo content is 0.5% or more, the corrosion resistance is remarkably improved. However, when it exceeds 1.3%, the toughness is remarkably lowered within the range of the Cr content of the present invention. Therefore, the Mo content is 0.5% or more and 1.3% or less, preferably 0.8% or more and 1.2% or less.
0.3%≦Nb≦0.5%
Nbは、本発明において重量な元素であり、複合炭窒化物(Nb,Ti)(C,N)としてフェライト粒界に析出し、打ち抜き加工時のかえりの高さを効果的に小さくする。Nb量が0.3%を下回ると、(Nb,Ti)(C,N)の析出量が少なくなり、こうした効果が得られなくなるばかりか、Cr炭化物が析出しやすくなって、溶接部の耐食性が劣化する。一方、Nb量が0.5%を超えると、フェライト粒内の粗大なTiNにNbCが複合析出し、打ち抜き加工時のかえりの高さを低減する効果がなくなる。したがって、Nb量は0.3%以上0.5%以下とする。
0.3% ≦ Nb ≦ 0.5%
Nb is a heavy element in the present invention, and is precipitated as a composite carbonitride (Nb, Ti) (C, N) at the ferrite grain boundary, effectively reducing the height of burr during punching. If the Nb content is less than 0.3%, the precipitation amount of (Nb, Ti) (C, N) decreases, and not only these effects can be obtained, but Cr carbide is liable to precipitate, and the corrosion resistance of the weld zone deteriorates. To do. On the other hand, if the amount of Nb exceeds 0.5%, NbC is complex-precipitated on coarse TiN in the ferrite grains, and the effect of reducing the burr height during punching is lost. Therefore, the Nb amount is set to 0.3% to 0.5%.
0.02%≦Ti≦0.15%
Tiは、Nbと同様、本発明において重量な元素であり、複合炭窒化物(Nb,Ti)(C,N)としてフェライト粒界に析出し、打ち抜き加工時のかえりの高さを効果的に小さくする。Ti量が0.02%を下回ると、(Nb,Ti)(C,N)の析出量が少なくなり、こうした効果が得られなくなる。一方、Ti量が0.15%を超えると、多量にTiCが析出し、鋼が硬質化して、打ち抜き加工時の破断面比率が多くなり、かえりの高さが大きくなる。したがって、Ti量は0.02%以上0.15%以下とする。
0.02% ≦ Ti ≦ 0.15%
Ti, like Nb, is a heavy element in the present invention, and precipitates at the ferrite grain boundary as composite carbonitride (Nb, Ti) (C, N), effectively increasing the burr height during punching. Make it smaller. If the Ti content is less than 0.02%, the amount of (Nb, Ti) (C, N) deposited decreases and such an effect cannot be obtained. On the other hand, if the Ti content exceeds 0.15%, a large amount of TiC precipitates, the steel hardens, the fracture surface ratio during punching increases, and the burr height increases. Therefore, the Ti content is 0.02% or more and 0.15% or less.
残部は、Feおよび不可避的不純物であるが、次の理由により、さらに、質量%で、0.15%≦Ni≦0.5%および0.3%≦Cu≦0.6%のうちから選ばれた少なくとも1種の元素を含有させることができる。 The balance is Fe and inevitable impurities, but for the following reason, at least one element selected from 0.15% ≦ Ni ≦ 0.5% and 0.3% ≦ Cu ≦ 0.6% is further added by mass%. It can be included.
0.15%≦Ni≦0.5%
Ni量を0.15%以上にすると、耐食性、特に、すきま腐食に対する耐食性を向上させる効果がある。一方、その量が0.5%を超えると、鋼を硬質低延性化させる。したがって、Ni量は0.15%以上0.5%以下とする。
0.15% ≦ Ni ≦ 0.5%
When the Ni content is 0.15% or more, there is an effect of improving the corrosion resistance, in particular, the corrosion resistance against crevice corrosion. On the other hand, if the amount exceeds 0.5%, the steel is made hard and ductile. Therefore, the Ni content is 0.15% or more and 0.5% or less.
0.3%≦Cu≦0.6%
Cuは、本発明鋼のようなCr量が21.0%以上のフェライト系ステンレス鋼に対し、耐食性、特に、すきま腐食に対する耐食性を向上させる働きがある。そのためには、Cu量を0.3%以上とする必要があるが、0.6%を超えると、CuSが析出して、打ち抜き加工時のかえりの高さを大きくする。したがって、Cu量は0.3%以上0.6%以下、好ましくは0.5%以下とする。
0.3% ≦ Cu ≦ 0.6%
Cu has a function of improving the corrosion resistance, particularly the resistance to crevice corrosion, for ferritic stainless steel having a Cr content of 21.0% or more like the steel of the present invention. For that purpose, it is necessary to make the amount of Cu 0.3% or more, but when it exceeds 0.6%, CuS is precipitated, and the burr height at the time of punching is increased. Therefore, the Cu content is 0.3% or more and 0.6% or less, preferably 0.5% or less.
なお、不可避的不純物として、B≦0.001%、V≦0.05%、Mg≦0.01%、Ca≦0.01%などが混入しても、本発明の効果は得られるが、これらの元素は少ないほど好ましい。 The effects of the present invention can be obtained even if B ≦ 0.001%, V ≦ 0.05%, Mg ≦ 0.01%, Ca ≦ 0.01%, etc. are mixed as inevitable impurities, but the smaller the number of these elements, the better.
2)フェライト粒径:25μm以下
フェライト粒径が25μmを超えると、複合炭窒化物(Nb,Ti)(C,N)が不均一に分散したり、打ち抜き加工時に一つ一つのフェライト粒の変形量が大きくなって、かえりの高さが大きくなる。このため、フェライト粒径は25μm以下とする。
2) Ferrite grain size: 25 μm or less If the ferrite grain size exceeds 25 μm, composite carbonitride (Nb, Ti) (C, N) is dispersed unevenly or deformation of each ferrite grain during punching The amount increases and the height of the burr increases. For this reason, the ferrite grain size is set to 25 μm or less.
3)フェライト粒界上の複合炭窒化物(Nb,Ti)(C,N)
打ち抜き加工時の亀裂は、フェライト粒界に存在する析出物とフェライト粒の界面で発生し、伝播とともに合体し、材料の破壊を引き起こす。そして、材料が破壊したところまでの塑性変形量で、かえりの高さが決定される。析出物として、NbとTiの組成比[Nb]/[Ti]が1以上10以下である複合炭窒化物(Nb,Ti)(C,N)を用いると、亀裂発生が促進され、かえりの高さが小さくなる。この原因は、必ずしも明らかではないが、組成比[Nb]/[Ti]により格子定数と析出物の格子欠陥が変化し、フェライト界面での亀裂が発生しやすくなるためと推察される。
3) Composite carbonitrides (Nb, Ti) (C, N) on ferrite grain boundaries
Cracks at the time of punching occur at the interface between precipitates and ferrite grains present in the ferrite grain boundary, coalesce with propagation, and cause destruction of the material. The height of burr is determined by the amount of plastic deformation up to the point where the material breaks. When composite carbonitride (Nb, Ti) (C, N) with a composition ratio [Nb] / [Ti] of Nb and Ti of 1 to 10 is used as the precipitate, cracking is promoted and The height becomes smaller. The reason for this is not necessarily clear, but it is presumed that the lattice constant and the lattice defects of the precipitates change depending on the composition ratio [Nb] / [Ti], and cracks are likely to occur at the ferrite interface.
4)降伏比:0.65以上
降伏比が0.65未満だと、加工硬化しやすいため、フェライト粒が変形しやすくなり、打ち抜き加工時に一つ一つのフェライト粒の変形量が大きくなって、かえりの高さが大きくなる。このため、降伏比は0.65以上とする。
4) Yield ratio: 0.65 or more If the yield ratio is less than 0.65, it is easy to work harden, so the ferrite grains are easily deformed, and the amount of deformation of each ferrite grain increases during punching, resulting in high burr height. Becomes larger. For this reason, the yield ratio is 0.65 or more.
5)製造条件
上述したように、本発明のフェライト系ステンレス鋼板は、例えば、上記の成分組成を有する鋼スラブを1000℃以上の温度に加熱後、900℃以上の仕上温度で熱間圧延して熱延板とし、前記熱延板を400℃以上550℃以下の巻取温度で巻取り、次いで熱延板焼鈍を行った後、冷間圧延して冷延板とし、前記冷延板を再結晶焼鈍することを特徴とする方法により製造できる。
5) Manufacturing conditions As described above, the ferritic stainless steel sheet of the present invention is, for example, hot rolled at a finishing temperature of 900 ° C. or higher after heating a steel slab having the above component composition to a temperature of 1000 ° C. or higher. A hot-rolled sheet is obtained, the hot-rolled sheet is wound at a coiling temperature of 400 ° C. or higher and 550 ° C. or lower, and then subjected to hot-rolled sheet annealing, and then cold-rolled to obtain a cold-rolled sheet. It can be manufactured by a method characterized by crystal annealing.
鋼スラブの加熱温度:1000℃以上
鋼スラブの加熱温度が1000℃未満だと、加熱前にスラブ中に存在しているNbやTiの炭窒化物を溶解できず、その後の工程における複合炭窒化物(Nb,Ti)(C,N)の析出状態制御が困難になる。このため、鋼スラブの加熱温度は1000℃以上とする。
Heating temperature of steel slab: 1000 ° C or more If the heating temperature of steel slab is less than 1000 ° C, Nb and Ti carbonitride existing in the slab cannot be dissolved before heating, and composite carbonitriding in the subsequent process It becomes difficult to control the precipitation state of the product (Nb, Ti) (C, N). For this reason, the heating temperature of a steel slab shall be 1000 degreeC or more.
熱間圧延の仕上温度:900℃以上
熱間圧延の仕上温度が900℃未満だと、熱間圧延後に未再結晶組織が残り、結果的に冷間圧延、再結晶焼鈍後のフェライト粒が25μmを超えた粒径の展伸した粗大粒になる。このため、熱間圧延の仕上温度は900℃以上とする。
Hot rolling finishing temperature: 900 ° C or higher If the hot rolling finishing temperature is less than 900 ° C, an unrecrystallized structure remains after hot rolling, resulting in 25 μm of ferrite grains after cold rolling and recrystallization annealing. Expanded coarse particles with a particle size exceeding. For this reason, the finishing temperature of hot rolling shall be 900 degreeC or more.
巻取温度:400℃以上550℃以下
熱間圧延後の巻取温度が400℃未満だと、巻取り時に複合炭窒化物(Nb,Ti)(C,N)が析出せず、続く熱延板焼鈍時に圧延方向にそった粒界のみに析出し、その結果、熱延板焼鈍後のフェライト粒は圧延方向に展伸した粗大粒となる。このフェライト粒は、その後の再結晶焼鈍後も残り、展伸した粗大粒となるので、打ち抜き加工時のかえりの高さが大きくなる。一方、巻取温度が550℃を超えると、複合炭窒化物(Nb,Ti)(C,N)がフェライト粒内の粗大なTiNを核として粗大析出物としてに析出し、冷間圧延、再結晶焼鈍後もフェライト粒内に存在するようになり、本発明の効果が得られなくなる。したがって、巻取温度は400℃以上550℃以下とする。
Winding temperature: 400 ° C or more and 550 ° C or less If the winding temperature after hot rolling is less than 400 ° C, composite carbonitride (Nb, Ti) (C, N) does not precipitate during winding, and the hot rolling continues. At the time of sheet annealing, it precipitates only at the grain boundaries along the rolling direction, and as a result, the ferrite grains after the hot-rolled sheet annealing become coarse grains expanded in the rolling direction. The ferrite grains remain after the subsequent recrystallization annealing and become expanded coarse grains, so that the burr height at the time of punching increases. On the other hand, when the coiling temperature exceeds 550 ° C, the composite carbonitride (Nb, Ti) (C, N) precipitates as coarse precipitates with coarse TiN in the ferrite grains as the core, and cold rolling, re- Even after the crystal annealing, it is present in the ferrite grains, and the effect of the present invention cannot be obtained. Therefore, the coiling temperature is set to 400 ° C. or more and 550 ° C. or less.
なお、上記Nb、Ti、C、Nの含有量とこの巻取温度により、NbとTiの組成比[Nb]/[Ti]が1以上10以下の複合炭窒化物(Nb,Ti)(C,N)が形成される。そして、この複合炭窒化物は、その後の熱延板焼鈍や再結晶焼鈍では変わらず、再結晶焼鈍時に粒成長を抑制してフェライト粒径を25μm以下にするとともに、フェライト粒界に留まって打ち抜き加工性を向上させることになる。 The Nb, Ti, C, N content and the coiling temperature depend on the composite carbonitride (Nb, Ti) (C) with a composition ratio [Nb] / [Ti] of Nb to Ti of 1 to 10. , N) is formed. This composite carbonitride does not change during subsequent hot-rolled sheet annealing or recrystallization annealing, and suppresses grain growth during recrystallization annealing to reduce the ferrite grain size to 25 μm or less, and remains at the ferrite grain boundary for punching. Workability will be improved.
巻取り後の熱延板は、熱延板焼鈍が施された後、酸洗によりスケールが除去され、冷間圧延により冷延板とされ、再結晶焼鈍が施される。これらの条件は、通常のフェライト系ステンレス鋼板の場合と同様である。さらに、再結晶焼鈍後には、伸長率0.5%以上1.5%以下のスキンパス圧延を行うことが好ましい。 The hot-rolled sheet after winding is subjected to hot-rolled sheet annealing, the scale is removed by pickling, the sheet is cold-rolled by cold rolling, and subjected to recrystallization annealing. These conditions are the same as those of a normal ferritic stainless steel sheet. Further, after recrystallization annealing, it is preferable to perform skin pass rolling with an elongation of 0.5% or more and 1.5% or less.
熱間圧延前の鋼の製造方法は、特に限定しないが、転炉、電気炉などで本願発明の範囲内の成分組成を有する鋼を溶製し、強攪拌真空酸素脱炭処理などにより二次精錬を行い、連続鋳造法などによってスラブにすればよい。 The method for producing the steel before hot rolling is not particularly limited, but the steel having the component composition within the scope of the present invention is melted in a converter, an electric furnace, etc., and then secondary by strong stirring vacuum oxygen decarburization treatment or the like. Refining and slab by continuous casting method.
表1に示す化学成分の鋼No.1〜26を50kgの小型真空溶解炉で溶製し、鋼塊とした後、表2、3に示す熱延条件で、実験室的に、熱間圧延しその後保熱ボックスに装入して巻取り相当の熱処理を行い、板厚3mmの熱延板とした。この熱延板を、表2、3に示す熱延板焼鈍で焼鈍し、酸洗後、冷間圧延し、板厚0.8mmの冷延板とし、表2、3に示す焼鈍温度で再結晶焼鈍し、鋼板No.1〜30を作製した。そして、作製した鋼板No.1〜30に対し、以下の方法で、フェライト粒径、複合炭窒化物(Nb,Ti)(C,N)中におけるNbとTiの組成比[Nb]/[Ti]、機械的性質、かえりの高さ、孔食電位を測定した。 Steel Nos. 1 to 26 of the chemical composition shown in Table 1 are melted in a 50 kg small vacuum melting furnace to form a steel ingot, and then hot rolled in the laboratory under the hot rolling conditions shown in Tables 2 and 3. After that, it was placed in a heat insulation box and subjected to a heat treatment equivalent to winding to obtain a hot rolled sheet having a thickness of 3 mm. This hot-rolled sheet was annealed by hot-rolled sheet annealing shown in Tables 2 and 3, pickled, cold-rolled to form a cold-rolled sheet having a thickness of 0.8 mm, and recrystallized at the annealing temperatures shown in Tables 2 and 3 It annealed and produced steel plate No. 1-30. And with respect to the produced steel plates No. 1 to 30, the ferrite grain size, the composition ratio of Nb and Ti in the composite carbonitride (Nb, Ti) (C, N) [Nb] / [Ti ], Mechanical properties, burr height, and pitting potential were measured.
フェライト粒径:鋼板から圧延方向にそって切断した板厚断面を、研磨後、薄い王水(硝酸:塩酸=1:3)でエッチングして、光学顕微鏡で観察し、その平均粒径を測定した。 Ferrite grain size: Thickness section cut from steel sheet along rolling direction is polished, etched with thin aqua regia (nitric acid: hydrochloric acid = 1: 3), observed with optical microscope, and average grain size is measured did.
複合炭窒化物(Nb,Ti)(C,N)中におけるNbとTiの組成比[Nb]/[Ti]:鋼板の板厚中央部より薄膜をツインジェット法で作製し、透過型電子顕微鏡で観察して、EDXによる分析強度により、観察された複合炭窒化物(Nb,Ti)(C,N)中のNbとTiの組成比[Nb]/[Ti]の平均を求めた。 Composition ratio [Nb] / [Ti] of Nb and Ti in composite carbonitride (Nb, Ti) (C, N): A thin film is prepared by the twin jet method from the center of the plate thickness of the steel plate, and a transmission electron microscope The average of the composition ratio [Nb] / [Ti] of Nb and Ti in the observed composite carbonitride (Nb, Ti) (C, N) was determined from the analytical strength by EDX.
機械的性質:鋼板から引張方向が圧延方向に平行になるようにJIS 13号B引張試験片を採取し、歪速度10mm/minで引張試験を行って、引張強度TS、伸びEl、降伏比(降伏強度/引張強度)を求めた。 Mechanical properties: JIS 13B tensile test specimens were taken from the steel sheet so that the tensile direction was parallel to the rolling direction, and a tensile test was performed at a strain rate of 10 mm / min. Tensile strength TS, elongation El, yield ratio ( Yield strength / tensile strength).
かえりの高さ:鋼板から100mm角の試料を切り出し、その中央に10mmφの穴を打ち抜き、かえりの高さを測定した。こうして求めたかえりの高さが50μm以下であれば、打ち抜き加工性に優れるといえる。 Height of burr: A 100 mm square sample was cut out from a steel sheet, a 10 mmφ hole was punched in the center, and the burr height was measured. When the height of the burr thus obtained is 50 μm or less, it can be said that the punching workability is excellent.
孔食電位:鋼板から試験片を採取し、電極:1.6mmタングステン電極、溶接電圧:10V、溶接電流:60〜100A、溶接速度:600mm/min、表ビード側シールドガス:Ar(20L/min)、裏ビード側シールドガス:Ar+5%O2(20L/min)の条件で、裏ビード幅が2mm以上になるように、試験片にビード・オン・プレートのTIG溶接を行った。そして、JIS G 0577「ステンレ鋼の孔食電位測定方法」に基づき、温水器環境をシミュレートした85℃の200ppmNaCl溶液中で、溶接部の裏ビード面および溶接部から離れた部分(素材)の孔食電位を測定した。なお、試験前の試験片の研磨は行わず、また、溶液浸漬後10分間放置はせずに直ちに電位測定を行った。孔食電位は、非溶接部は溶解電流値が10μA/cm2になる電位Vc10(mV vs SCE)で、溶接部は溶解電流値が100μA/cm2になる電位Vc100(mV vs SCE)で評価した。こうして求めた溶接部の孔食電位(Vc100)が100mV以上であれば、溶接部の耐食性に優れ、温水器用として問題ないといえる。 Pitting potential: Specimen taken from steel plate, electrode: 1.6mm tungsten electrode, welding voltage: 10V, welding current: 60-100A, welding speed: 600mm / min, front bead side shielding gas: Ar (20L / min) The bead-on-plate TIG welding was performed on the test piece so that the back bead width was 2 mm or more under the condition of back bead side shielding gas: Ar + 5% O 2 (20 L / min). Then, based on JIS G 0577 “Method for measuring pitting corrosion potential of stainless steel”, the back bead surface of the welded part and the part (material) away from the welded part in a 85 ppm 200 ppm NaCl solution that simulates the water heater environment. Pitting potential was measured. In addition, the potential was measured immediately without polishing the test piece before the test and without leaving it for 10 minutes after the solution was immersed. The pitting corrosion potential is evaluated by the potential Vc10 (mV vs SCE) at which the melting current value is 10 μA / cm 2 for the non-welded part and the potential Vc100 (mV vs SCE) at which the melting current value is 100 μA / cm 2 for the welded part. did. If the pitting potential (Vc100) of the welded portion thus obtained is 100 mV or more, it can be said that the welded portion has excellent corrosion resistance and is not problematic for a water heater.
結果を表2、3に示す。本発明の成分組成を有し、フェライト粒径が25μm以下で、フェライト粒界にはNbとTiの組成比[Nb]/[Ti]が1以上10以下である複合炭窒化物(Nb,Ti)(C,N)が析出しており、かつ降伏比が0.65以上である鋼板No.2〜4、7〜9、12〜14、17〜19、21、22、28、29では、かえりの高さが50μm以下で、孔食電位が100mV以上であり、打ち抜き加工性と溶接部の耐食性に優れていることがわかる。なお、フェライト粒径、NbとTiの組成比[Nb]/[Ti]、降伏比が本発明範囲内であっても、Si、Cr、Moの量が本発明範囲より低い鋼板No.23〜25では、50μm以下のかえりの高さが得られるが、溶接部の孔食電位が著しく低く、溶接部の耐食性に劣っている。 The results are shown in Tables 2 and 3. A composite carbonitride (Nb, Ti) having the composition of the present invention, having a ferrite grain size of 25 μm or less, and a ferrite grain boundary having a composition ratio [Nb] / [Ti] of Nb to Ti of 1 to 10. Steel plates No. 2-4, 7-9, 12-14, 17-19, 21, 22, 28, 29 where (C, N) is precipitated and the yield ratio is 0.65 or more It can be seen that the height is 50 μm or less and the pitting potential is 100 mV or more, which is excellent in punching workability and corrosion resistance of the weld. Incidentally, even when the ferrite grain size, the composition ratio of Nb and Ti [Nb] / [Ti], and the yield ratio are within the range of the present invention, the amount of Si, Cr, Mo is less than the range of the present invention. In 25, a burr height of 50 μm or less is obtained, but the pitting corrosion potential of the welded portion is extremely low, and the corrosion resistance of the welded portion is inferior.
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