JP6699633B2 - High-strength cold-rolled steel sheet excellent in corrosion resistance after painting and delayed fracture resistance and method for producing the same - Google Patents
High-strength cold-rolled steel sheet excellent in corrosion resistance after painting and delayed fracture resistance and method for producing the same Download PDFInfo
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- 239000010960 cold rolled steel Substances 0.000 title claims description 38
- 238000004519 manufacturing process Methods 0.000 title claims description 14
- 230000007797 corrosion Effects 0.000 title description 32
- 238000005260 corrosion Methods 0.000 title description 32
- 230000003111 delayed effect Effects 0.000 title description 28
- 238000010422 painting Methods 0.000 title description 6
- 229910000831 Steel Inorganic materials 0.000 claims description 111
- 239000010959 steel Substances 0.000 claims description 111
- 238000005554 pickling Methods 0.000 claims description 80
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 50
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 claims description 40
- 229910017604 nitric acid Inorganic materials 0.000 claims description 40
- 229910052751 metal Inorganic materials 0.000 claims description 34
- 239000002184 metal Substances 0.000 claims description 33
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 claims description 30
- KRHYYFGTRYWZRS-UHFFFAOYSA-N Fluorane Chemical compound F KRHYYFGTRYWZRS-UHFFFAOYSA-N 0.000 claims description 20
- 239000011248 coating agent Substances 0.000 claims description 20
- 238000000576 coating method Methods 0.000 claims description 20
- 150000002500 ions Chemical class 0.000 claims description 19
- 229910052748 manganese Inorganic materials 0.000 claims description 17
- 229910052710 silicon Inorganic materials 0.000 claims description 17
- 239000002253 acid Substances 0.000 claims description 13
- 229910052742 iron Inorganic materials 0.000 claims description 13
- 229910052782 aluminium Inorganic materials 0.000 claims description 12
- 239000000203 mixture Substances 0.000 claims description 8
- 238000000137 annealing Methods 0.000 claims description 6
- 229910052802 copper Inorganic materials 0.000 claims description 6
- 229910052758 niobium Inorganic materials 0.000 claims description 5
- 229910052760 oxygen Inorganic materials 0.000 claims description 5
- 238000004993 emission spectroscopy Methods 0.000 claims description 4
- 229910052717 sulfur Inorganic materials 0.000 claims description 4
- 239000012535 impurity Substances 0.000 claims description 3
- 229910052698 phosphorus Inorganic materials 0.000 claims description 3
- 238000004458 analytical method Methods 0.000 claims description 2
- 238000006243 chemical reaction Methods 0.000 description 72
- 239000000126 substance Substances 0.000 description 65
- 238000012360 testing method Methods 0.000 description 47
- 239000000243 solution Substances 0.000 description 33
- 239000010410 layer Substances 0.000 description 24
- 150000003839 salts Chemical class 0.000 description 14
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 14
- 238000000034 method Methods 0.000 description 12
- 239000007788 liquid Substances 0.000 description 10
- 239000007921 spray Substances 0.000 description 8
- 230000000694 effects Effects 0.000 description 7
- 238000007654 immersion Methods 0.000 description 7
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 6
- 238000004070 electrodeposition Methods 0.000 description 6
- 239000000463 material Substances 0.000 description 5
- 230000001590 oxidative effect Effects 0.000 description 5
- 238000005406 washing Methods 0.000 description 5
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 4
- 238000005516 engineering process Methods 0.000 description 4
- 229910052739 hydrogen Inorganic materials 0.000 description 4
- 239000001257 hydrogen Substances 0.000 description 4
- 238000013507 mapping Methods 0.000 description 4
- 230000008569 process Effects 0.000 description 4
- 230000009257 reactivity Effects 0.000 description 4
- 239000002344 surface layer Substances 0.000 description 4
- 230000001133 acceleration Effects 0.000 description 3
- 238000005097 cold rolling Methods 0.000 description 3
- 239000013078 crystal Substances 0.000 description 3
- 238000005238 degreasing Methods 0.000 description 3
- 238000004090 dissolution Methods 0.000 description 3
- 238000001035 drying Methods 0.000 description 3
- 238000010828 elution Methods 0.000 description 3
- 238000011156 evaluation Methods 0.000 description 3
- 230000001771 impaired effect Effects 0.000 description 3
- 230000006872 improvement Effects 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 238000005096 rolling process Methods 0.000 description 3
- 239000011780 sodium chloride Substances 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- YCKRFDGAMUMZLT-UHFFFAOYSA-N Fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 description 2
- 229910004298 SiO 2 Inorganic materials 0.000 description 2
- 230000002411 adverse Effects 0.000 description 2
- 239000007864 aqueous solution Substances 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 239000003795 chemical substances by application Substances 0.000 description 2
- 230000003750 conditioning effect Effects 0.000 description 2
- 238000001816 cooling Methods 0.000 description 2
- 239000011737 fluorine Substances 0.000 description 2
- 229910052731 fluorine Inorganic materials 0.000 description 2
- 229910044991 metal oxide Inorganic materials 0.000 description 2
- 150000004706 metal oxides Chemical class 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 239000010802 sludge Substances 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 239000002390 adhesive tape Substances 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- 239000010953 base metal Substances 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000005237 degreasing agent Methods 0.000 description 1
- 239000013527 degreasing agent Substances 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 230000002708 enhancing effect Effects 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 230000007935 neutral effect Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 239000003973 paint Substances 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 230000003014 reinforcing effect Effects 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 230000000452 restraining effect Effects 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 238000002791 soaking Methods 0.000 description 1
- 239000002436 steel type Substances 0.000 description 1
- 230000000007 visual effect Effects 0.000 description 1
- 238000009736 wetting Methods 0.000 description 1
- LRXTYHSAJDENHV-UHFFFAOYSA-H zinc phosphate Chemical compound [Zn+2].[Zn+2].[Zn+2].[O-]P([O-])([O-])=O.[O-]P([O-])([O-])=O LRXTYHSAJDENHV-UHFFFAOYSA-H 0.000 description 1
- 229910000165 zinc phosphate Inorganic materials 0.000 description 1
Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
- C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
- C23G1/08—Iron or steel
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
- Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Description
本発明は、自動車構造部材や補強部材などに用いられる高強度冷延鋼板及びその製造方法に関するものである。 The present invention relates to a high-strength cold-rolled steel sheet used for automobile structural members, reinforcing members and the like, and a method for manufacturing the same.
自動車分野で使用される冷延鋼板は、防錆性の観点から塗装用下地処理を施した上で塗装が施されることが多く、優れた塗装後耐食性が得られるようにするために、冷延鋼板は良好な化成処理性を有することが求められる。また、近年、燃費向上の観点から、車体軽量化を目的として鋼板の薄肉化が進んでおり、これに伴って冷延鋼板には高強度化が求められている。しかしながら、高強度化により耐遅れ破壊特性が低下するという問題があり、鉄鋼メーカ各社では、合金元素の添加や様々な熱履歴を付与することで、高強度化に加えて塗装後耐食性と耐遅れ破壊特性の両立を目指している。 Cold-rolled steel sheets used in the automotive field are often coated with a coating pretreatment from the viewpoint of rust resistance, and in order to obtain excellent post-paint corrosion resistance, The rolled steel sheet is required to have good chemical conversion treatability. Further, in recent years, from the viewpoint of improving fuel efficiency, the thickness of steel sheets has been reduced in order to reduce the weight of the vehicle body, and accordingly, the cold rolled steel sheet is required to have high strength. However, there is a problem that delayed fracture resistance deteriorates due to higher strength, and each steel manufacturer has added strength of alloying elements and added various heat histories to increase strength, as well as corrosion resistance and delay after coating. We aim to achieve both breaking characteristics.
また、最近では、化成処理時に発生するスラッジ量やエネルギーコストの削減を目的として、化成処理液の低温化が進んでいる。この化成処理液の低温化によって鋼板と化成処理液の反応性が低下するため、化成処理性を改善することが求められている。このような課題に対して、化成処理液の組成面で鋼板との反応性を高めるために、フリーフッ素濃度を高めに制御し、鋼板との反応性を改善する取り組みなどが行われている。 Further, recently, in order to reduce the amount of sludge generated during chemical conversion treatment and energy cost, the temperature of the chemical conversion treatment liquid has been lowered. Since the reactivity of the steel sheet and the chemical conversion treatment liquid is lowered by lowering the temperature of the chemical conversion treatment liquid, it is required to improve the chemical conversion treatment property. In order to increase the reactivity of the chemical conversion treatment solution with the steel sheet in response to such problems, efforts have been made to control the free fluorine concentration to be high and improve the reactivity with the steel sheet.
一方、化成処理性の改善を図るための鋼板側での取り組みとして、例えば、以下のようなものがある。
(1)調質圧延を施し、鋼中に転位を導入することにより、化成処理性の改善を図る技術(特許文献1)
(2)鋼中にBを添加し、調質圧延を行うことにより、連続焼鈍材での化成処理性の改善を図る技術(特許文献2)
(3)連続焼鈍材において、鋼板を酸洗後、Feよりも貴な金属(例えばNi等)を表面に薄く付着させて化成結晶の成長サイトを設けることにより、鋼種に関わりなく化成処理性の改善を図る技術(特許文献3、4)
(4)連続焼鈍材において、連続焼鈍時に形成する化成処理性を阻害する酸化物を酸洗により除去することで化成処理性の改善を図る技術(特許文献5)
On the other hand, examples of efforts on the steel sheet side for improving the chemical conversion treatability include the following.
(1) Technology for improving chemical conversion treatability by performing temper rolling and introducing dislocations in steel (Patent Document 1)
(2) Technology for improving chemical conversion treatability in a continuously annealed material by adding B to steel and performing temper rolling (Patent Document 2)
(3) In a continuous annealed material, after pickling a steel sheet, a metal nobler than Fe (such as Ni) is thinly attached to the surface to provide a growth site for chemical conversion crystals, so that the chemical conversion treatability can be improved regardless of the steel type. Technology for improvement (Patent Documents 3 and 4)
(4) A technology for improving chemical conversion treatability in a continuous annealed material by removing oxides which inhibit chemical conversion treatability formed during continuous annealing by pickling (Patent Document 5)
また、高強度鋼板の耐遅れ破壊特性に関しては、Cuの添加が耐遅れ破壊特性の改善に有効であることが知られており、例えば、鋼板表面から0.48μmの深さまでの領域におけるCu濃度をCu0.10%以上とすることで、冷延鋼板の耐遅れ破壊特性の改善を図る技術(特許文献6)などが提案されている。 Regarding delayed fracture resistance of high-strength steel sheets, it is known that addition of Cu is effective in improving delayed fracture resistance. For example, Cu concentration in a region from the steel sheet surface to a depth of 0.48 μm. Has been proposed to improve the delayed fracture resistance of the cold-rolled steel sheet (Patent Document 6) and the like.
冷延鋼板を再結晶させ、所望の組織と強度、加工性を付与するために行われる連続焼鈍工程では、鉄よりも易酸化性の金属元素(Al、Si、Mn)を含む酸化物が鋼板表面に生成する。この易酸化性金属を含む酸化物は、電着塗装の下地処理としてなされる化成処理(一般にリン酸亜鉛処理)における鋼板表面のエッチング性を阻害し、健全な化成処理皮膜の形成に悪影響を及ぼすため、これを除去するために、連続焼鈍された鋼板に対して酸洗が行われる。 In the continuous annealing step performed to recrystallize the cold-rolled steel sheet to impart desired structure, strength, and workability, the steel sheet contains oxides containing metal elements (Al, Si, Mn) that are more easily oxidizable than iron. Generate on the surface. This oxide containing an easily oxidizable metal inhibits the etching property of the steel sheet surface in the chemical conversion treatment (generally zinc phosphate treatment) performed as a base treatment for electrodeposition coating, and adversely affects the formation of a sound chemical conversion treatment film. Therefore, in order to remove this, pickling is performed on the continuously annealed steel sheet.
一方、最近では、化成処理時に発生するスラッジ量やエネルギーコストの削減を目的として、化成処理液の低温化が進み、従来と比較して、化成処理液の鋼板に対する反応性が著しく低い条件で化成処理がなされるようになってきている。従来のように43℃程度で行われる化成処理と比べ、35℃程度に低温化された化成処理では、化成処理性が板の表面性状に非常に大きく左右され、鋼板表面に酸洗で除去されずに残存した金属酸化物(酸洗残渣)や他の生成物質が存在していると化成処理性が著しく低下し、塗装後耐食性を劣化させることが判明した。特に、本発明者らによる検討の結果では、耐遅れ破壊特性の改善のためにCuを添加した高強度冷延鋼板の場合、上述した特許文献1〜5の技術を用いても、低温型化成処理での化成処理性を改善することは難しいことが判った。 On the other hand, recently, in order to reduce the amount of sludge generated during chemical conversion treatment and the energy cost, the chemical conversion treatment liquid has been lowered in temperature, and the chemical conversion treatment liquid has a significantly lower reactivity with the steel sheet compared to the conventional method. Processing is starting to take place. Compared with the conventional chemical conversion treatment performed at about 43°C, in the chemical conversion treatment at a temperature of about 35°C, the chemical conversion treatability is greatly influenced by the surface texture of the plate and is removed by pickling on the steel plate surface. It was revealed that the presence of metal oxides (pickling residue) and other substances that remained without chemical conversion treatment markedly lowered the corrosion resistance after coating. In particular, as a result of the study by the present inventors, in the case of a high-strength cold-rolled steel sheet to which Cu is added for improving delayed fracture resistance, even if the techniques of Patent Documents 1 to 5 described above are used, low temperature type chemical conversion is performed. It has been found difficult to improve the chemical conversion treatability of the treatment.
したがって本発明の目的は、以上のような従来技術の課題を解決し、耐遅れ破壊特性の改善のためにCuを添加した高強度冷延鋼板であって、低温型化成処理での化成処理性が良好であり、優れた塗装後耐食性と耐遅れ破壊特性が得られる高強度冷延鋼板とその製造方法を提供することにある。 Therefore, an object of the present invention is to solve the problems of the prior art as described above, and to provide a high-strength cold-rolled steel sheet containing Cu for improving delayed fracture resistance, which has chemical conversion treatability in low-temperature type chemical conversion treatment. The object is to provide a high-strength cold-rolled steel sheet having good corrosion resistance and excellent post-painting corrosion resistance and delayed fracture resistance, and a method for producing the same.
本発明者らは、Cuを添加した高強度冷延鋼板に関する上記課題を解決すべく、連続焼鈍−酸洗後の鋼板表面について詳細な解析を行い、その結果、酸洗後の鋼板表面に残存する酸化物(酸洗残渣)と、酸洗後から水洗されるまでの間に酸洗液膜と鋼板との反応により鋼板表面に析出した金属Cuの存在が化成処理性を大きく劣化させる要因であること、したがって、化成処理性(特に低温型化成処理での化成処理性)を改善するには、そのような酸洗残渣と析出金属Cuを低減することが極めて重要であることを見出した。 In order to solve the above-mentioned problem regarding the high-strength cold-rolled steel sheet containing Cu, the present inventors performed a detailed analysis of the steel sheet surface after continuous annealing-pickling, and as a result, remained on the steel sheet surface after pickling. The presence of the metal oxide Cu (pickling residue) that has been deposited on the surface of the steel sheet due to the reaction between the pickling liquid film and the steel sheet between pickling and water washing causes a great deterioration in chemical conversion treatability. Therefore, it was found that in order to improve the chemical conversion treatability (particularly, the chemical conversion treatability in the low temperature type chemical conversion treatment), it is extremely important to reduce such pickling residue and precipitated metal Cu.
本発明は、このような知見に基づきなされたもので、以下を要旨とするものである。
[1]Si、Mn、Alのうちの少なくとも1種とCu:0.05〜0.25質量%を含有する引張り強度が1180Mpa以上の鋼板であって、鋼組織が再結晶組織からなり、鋼板表面にAl、Si、Mnのうちの少なくとも1種を含む酸化物層が存在せず(但し、酸化物層が存在しないとは、任意に選ばれた鋼板表面の5箇所をグロー放電発光分光分析で深さ方向に分析したときに、Al、Si、Mn、Oのピークが現れない場合をいう。)、鋼板表面に存在する析出金属Cuによる鋼板表面の被覆率(但し、鋼板断面で測定される鋼板表面の被覆率)が30%以下であることを特徴とする塗装後耐食性と耐遅れ破壊特性に優れた高強度冷延鋼板。
The present invention has been made on the basis of such findings, and has the following gist.
[1] A steel sheet containing at least one of Si, Mn, and Al and Cu: 0.05 to 0.25 mass% and having a tensile strength of 1180 MPa or more, the steel structure of which is a recrystallized structure. There is no oxide layer containing at least one of Al, Si, and Mn on the surface (however, the absence of an oxide layer means that 5 points on the surface of a steel plate arbitrarily selected are analyzed by glow discharge emission spectroscopy. When the peaks of Al, Si, Mn, and O do not appear when analyzed in the depth direction, the coverage of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface (however, measured in the steel sheet cross section) A high-strength cold-rolled steel sheet excellent in corrosion resistance after coating and delayed fracture resistance, which is characterized by having a steel sheet surface coverage ratio of 30% or less.
[2]上記[1]の高強度冷延鋼板において、C:0.08〜0.30質量%、Si:0.1〜2.0質量%、Mn:0.1〜2.5質量%、P:0.020質量%以下、S:0.005質量%以下、Sol.Al:0.01〜0.05質量%、Cu:0.05〜0.25質量%を含有し、残部が鉄及び不可避不純物からなる成分組成を有することを特徴とする塗装後耐食性と耐遅れ破壊特性に優れた高強度冷延鋼板。
[3]上記[2]の高強度冷延鋼板において、さらに、Ni:0.05質量%以下、Ti:0.1質量%以下、Nb:0.1質量%以下、B:5〜30質量ppmの中から選ばれる1種以上を含有することを特徴とする塗装後耐食性と耐遅れ破壊特性に優れた高強度冷延鋼板。
[2] In the high-strength cold-rolled steel sheet according to [1], C: 0.08 to 0.30 mass%, Si: 0.1 to 2.0 mass%, Mn: 0.1 to 2.5 mass%. , P: 0.020 mass% or less, S: 0.005 mass% or less, Sol.Al: 0.01 to 0.05 mass%, Cu: 0.05 to 0.25 mass%, and the balance A high-strength cold-rolled steel sheet excellent in corrosion resistance after coating and delayed fracture resistance, which is characterized by having a component composition consisting of iron and inevitable impurities.
[3] In the high-strength cold-rolled steel sheet according to [2], Ni: 0.05% by mass or less, Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, B: 5 to 30% by mass. A high-strength cold-rolled steel sheet excellent in corrosion resistance after coating and delayed fracture resistance, which is characterized by containing at least one selected from ppm.
[4]上記[1]〜[3]のいずれかに記載の成分組成を有する冷延鋼板を連続焼鈍した後、下記(1)又は(2)の酸洗液で酸洗することを特徴とする塗装後耐食性と耐遅れ破壊特性に優れた高強度冷延鋼板の製造方法。
(1)酸成分が硝酸と塩酸からなり、硝酸濃度が50g/L超110g/L以下、塩酸濃度(g/L)と硝酸濃度(g/L)の比R1[塩酸/硝酸]が0.1〜0.3、Fe3+イオン濃度が3〜30g/Lである酸洗液
(2)酸成分が硝酸と弗酸からなり、硝酸濃度が50g/L超110g/L以下、
弗酸濃度(g/L)と硝酸濃度(g/L)の比R2[弗酸/硝酸]が0.1〜0.3、Fe3+イオン濃度が3〜30g/Lである酸洗液
[5]上記[4]の製造方法において、酸洗液温度20〜70℃、酸洗時間3〜30秒の条件で酸洗を行うことを特徴とする塗装後耐食性と耐遅れ破壊特性に優れた高強度冷延鋼板の製造方法。
[4] A cold-rolled steel sheet having the component composition according to any one of the above [1] to [3] is continuously annealed, and then pickled with a pickling solution according to (1) or (2) below. A method for producing a high-strength cold-rolled steel sheet excellent in corrosion resistance after painting and delayed fracture resistance.
(1) The acid component consists of nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g/L and 110 g/L or less, and the ratio R1 [hydrochloric acid/nitric acid] of hydrochloric acid concentration (g/L) to nitric acid concentration (g/L) is 0. 1 to 0.3, Fe 3+ ion concentration is 3 to 30 g/L pickling solution (2) The acid component consists of nitric acid and hydrofluoric acid, and the nitric acid concentration is more than 50 g/L and 110 g/L or less,
A pickling solution having a ratio R2 of [hydrofluoric acid concentration (g/L) and nitric acid concentration (g/L) [hydrofluoric acid/nitric acid] of 0.1 to 0.3 and Fe 3+ ion concentration of 3 to 30 g/L [ 5] In the production method of [4] above, pickling is performed under the conditions of a pickling solution temperature of 20 to 70° C. and a pickling time of 3 to 30 seconds, which is excellent in post-painting corrosion resistance and delayed fracture resistance. Manufacturing method of high strength cold rolled steel sheet.
本発明の高強度冷延鋼板は、優れた耐遅れ破壊特性を有するとともに、低温型化成処理においても良好な化成処理性が得られるため、塩温水浸漬試験や複合サイクル腐食試験のような過酷な腐食環境下においても優れた塗装後耐食性を有する。また、本発明の製法方法によれば、そのような優れた特性を有する高強度冷延鋼板を安定的に製造することができる。 The high-strength cold-rolled steel sheet of the present invention has excellent delayed fracture resistance and, at the same time, can obtain good chemical conversion treatability even in low-temperature type chemical conversion treatment, so that it is harsh such as salt hot water immersion test or combined cycle corrosion test. It has excellent post-painting corrosion resistance even in corrosive environments. Moreover, according to the manufacturing method of the present invention, a high-strength cold-rolled steel sheet having such excellent characteristics can be stably manufactured.
本発明の鋼板は、Si、Mn、Alのうちの少なくとも1種とCu:0.05〜0.25質量%を含有する引張り強度が1180Mpa以上の高強度冷延鋼板である。この高強度鋼板は、冷間圧延後、連続焼鈍−酸洗を経て製造されるものであり、したがって、鋼組織は再結晶組織からなる。
一般に高強度冷延鋼板は、所定の強度を得るために適量のSi、Mnを含有するとともに、脱酸元素であるAlを含有する。高強度冷延鋼板は遅れ破壊という現象が生じやすく、この現象は強度の増大とともに激しくなり、特に引張り強度1180MPa以上の高強度鋼板で顕著となる。本発明の鋼板は、耐遅れ破壊特性を向上させるためCuを0.05〜0.25質量%含有する。Cuを鋼板に添加することで、鋼板が腐食(アノード反応)しにくくなり、結果として水素発生(カソード反応)が抑制されると考えられる。水素発生が抑制されることから、侵入する水素が減少して遅れ破壊の抑制効果が発現するものと考えられる。ここで、Cu量が0.05質量%未満では、耐遅れ破壊特性の改善が不十分である。一方、Cuを過剰に含有させると、酸洗直後に金属Cu(後述するようにこの金属Cuは化成処理性を阻害する)が鋼板表面に生成しやすくなるので、Cu量は0.25質量%以下とする。
The steel sheet of the present invention is a high-strength cold-rolled steel sheet containing at least one of Si, Mn, and Al and Cu: 0.05 to 0.25 mass% and having a tensile strength of 1180 MPa or more. This high-strength steel sheet is manufactured by continuous annealing-pickling after cold rolling, and therefore the steel structure is a recrystallized structure.
Generally, a high-strength cold-rolled steel sheet contains appropriate amounts of Si and Mn in order to obtain a predetermined strength, and also contains Al which is a deoxidizing element. The phenomenon of delayed fracture easily occurs in the high-strength cold-rolled steel sheet, and this phenomenon becomes more severe as the strength increases, and is particularly remarkable in the high-strength steel sheet having a tensile strength of 1180 MPa or more. The steel sheet of the present invention contains 0.05 to 0.25 mass% of Cu in order to improve delayed fracture resistance. It is considered that the addition of Cu to the steel sheet makes the steel sheet less likely to corrode (anode reaction), and consequently suppresses hydrogen generation (cathode reaction). Since the generation of hydrogen is suppressed, it is considered that the amount of invading hydrogen is reduced and the effect of suppressing delayed fracture is exhibited. Here, if the amount of Cu is less than 0.05% by mass, improvement in delayed fracture resistance is insufficient. On the other hand, if Cu is excessively contained, metal Cu (this metal Cu hinders the chemical conversion treatment property as described later) is likely to be generated on the surface of the steel sheet immediately after pickling, so the Cu content is 0.25 mass %. Below.
高強度冷延鋼板の製造では、冷間圧延後に連続焼鈍が行われ、引き続き、鋼板表面に生成した易酸化性元素(Al、Si、Mnのうちの少なくとも1種)を含む酸化物層の除去を目的とした酸洗が行われるが、本発明者らによる研究の結果、上記のように耐遅れ破壊特性の改善のために適量のCuを含有する高強度冷延鋼板について、良好な化成処理性(特に低温型化成処理での化成処理性)が得られるようにするには、酸洗後の鋼板の表面性状を以下のようにする必要があることが判った。 In the production of a high-strength cold-rolled steel sheet, continuous annealing is performed after cold rolling, and subsequently, an oxide layer containing easily oxidizable elements (at least one of Al, Si and Mn) formed on the surface of the steel sheet is removed. However, as a result of research conducted by the present inventors, a high-strength cold-rolled steel sheet containing an appropriate amount of Cu for improving delayed fracture resistance as described above is subjected to a good chemical conversion treatment. It has been found that the surface properties of the steel sheet after pickling have to be set as follows in order to obtain good properties (particularly chemical conversion treatability in low temperature type chemical conversion treatment).
まず、酸洗により鋼板表面の酸化物層(Al、Si、Mnのうちの少なくとも1種を含む酸化物層。以下同様)がほぼ完全に除去され、鋼板表面に酸化物層が実質的に存在しなくなることが必要である。ここで、鋼板表面に酸化物層が実質的に存在しないとは、任意に選ばれた鋼板表面の5箇所をグロー放電発光分光分析(GDS)で深さ方向に分析したときに、Al、Si、Mn、Oのピークが現れない場合をいう。すなわち、このようなレベルまで、酸洗により鋼板表面の酸化物層が除去される必要がある。 First, the oxide layer on the surface of the steel sheet (oxide layer containing at least one of Al, Si, and Mn. The same applies hereinafter) is almost completely removed by pickling, and the oxide layer is substantially present on the surface of the steel sheet. It is necessary to stop doing it. Here, the fact that the oxide layer does not substantially exist on the surface of the steel sheet means that Al and Si are determined when five locations on the surface of the steel sheet arbitrarily selected are analyzed in the depth direction by glow discharge emission spectroscopy (GDS). , Mn, O peaks do not appear. That is, it is necessary to remove the oxide layer on the surface of the steel sheet by pickling to such a level.
さらに、酸洗後から水洗までの間に、鋼板表面上の酸洗液膜が鋼板と反応し続け、鋼板から溶出したCuが鋼板表面に析出し、この析出した金属Cuが、その後の化成処理工程(特に低温型化成処理工程)において化成結晶形成反応を阻害すること、すなわち化成処理性を低下させることが判った。これは、鋼板表面に析出した金属Cuにより、化成処理における鉄の溶解反応が阻害され、化成結晶の成長が抑制されるためであると考えられ、良好な化成処理性を得るためには、鋼板表面での析出金属Cuの被覆率を抑える必要があることが判った。具体的には、鋼板表面に存在する析出金属Cuによる鋼板表面の被覆率(但し、鋼板断面で測定される鋼板表面の被覆率。以下、説明の便宜上「断面被覆率」という。)を30%以下にする必要があり、これにより良好な化成処理性が得られることが判った。このため本発明では、鋼板表面に存在する析出金属Cuによる鋼板表面の断面被覆率を30%以下とする。 Furthermore, the pickling liquid film on the steel sheet surface continues to react with the steel sheet between the pickling and the water washing, Cu eluted from the steel sheet is deposited on the steel sheet surface, and the deposited metal Cu is the chemical conversion treatment after that. It has been found that in the step (particularly the low temperature type chemical conversion treatment step), the chemical conversion crystal forming reaction is inhibited, that is, the chemical conversion treatability is lowered. It is considered that this is because the metal Cu deposited on the surface of the steel plate inhibits the dissolution reaction of iron in the chemical conversion treatment and suppresses the growth of the chemical conversion crystals. It was found that it is necessary to suppress the coverage of the deposited metal Cu on the surface. Specifically, the coverage rate of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface (however, the coverage rate of the steel sheet surface measured in the steel sheet cross section, hereinafter referred to as “cross section coverage rate” for convenience of description) is 30%. It was necessary to make the amount below, and it was found that good chemical conversion treatability was obtained. Therefore, in the present invention, the cross-section coverage of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface is set to 30% or less.
ここで、鋼板表面の析出金属Cuの断面被覆率とは、以下のようにして求められる値である。すなわち、鋼板の表層断面について、極表層情報を検出できる極低加速電圧の走査型電子顕微鏡(ULV−SEM)を用い、加速電圧1kV、作動距離3.0mm、倍率20000倍程度で5視野を観察し、同一視野における加速電圧15kVでの元素マッッピング像を得る。図1(A)にULV−SEMによる二次電子像の一例を示し、図1(B)に同一視野における元素マッッピング像を示す。そして、得られた元素マッピング像において、鋼板表層に金属Cuが存在する領域(図1(B)において白矢印で示される領域)の幅A1、A2・・・をそれぞれ算出し、その和を求める。5視野において求めた和が5視野全体に占める割合を金属Cuの断面被覆率として定義する。
析出金属Cuによる鋼板表面の断面被覆率を30%以下にする方法としては、後述するように特定の条件で酸洗する方法がある。
Here, the cross-section coverage of the deposited metal Cu on the surface of the steel sheet is a value obtained as follows. That is, with respect to the surface layer cross section of the steel sheet, 5 fields of view are observed at an accelerating voltage of 1 kV, a working distance of 3.0 mm, and a magnification of about 20000 times by using a scanning electron microscope (ULV-SEM) with an extremely low accelerating voltage capable of detecting information on the surface layer. Then, an elemental mapping image at an acceleration voltage of 15 kV in the same field of view is obtained. FIG. 1A shows an example of a secondary electron image by ULV-SEM, and FIG. 1B shows an element mapping image in the same visual field. Then, in the obtained element mapping image, the widths A1, A2,... Of the regions where the metal Cu is present in the surface layer of the steel sheet (regions indicated by white arrows in FIG. 1B) are respectively calculated, and the sum thereof is obtained. .. The ratio of the sum obtained in the 5 fields of view to the entire 5 fields of view is defined as the cross-sectional coverage of the metallic Cu.
As a method of reducing the sectional coverage of the surface of the steel sheet by the precipitated metal Cu to 30% or less, there is a method of pickling under specific conditions as described later.
次に、本発明の高強度冷延鋼板の好ましい成分組成について説明する。
本発明の鋼板は、C:0.08〜0.30質量%、Si:0.1〜2.0質量%、Mn:0.1〜2.5質量%、P:0.020質量%以下、S:0.005質量%以下、Al:0.01〜0.05質量%、Cu:0.05〜0.25質量%を含有し、残部が鉄及び不可避不純物からなる成分組成を有することが好ましく、さらに必要に応じて、Ni:0.05質量%以下、Ti:0.1質量%以下、Nb:0.1質量%以下、B:5〜30質量ppmの中から選ばれる1種以上を含有することができる。これら各元素の含有量の限定理由は、以下の通りである。
Next, a preferable component composition of the high strength cold rolled steel sheet of the present invention will be described.
The steel sheet of the present invention is C: 0.08 to 0.30 mass%, Si: 0.1 to 2.0 mass%, Mn: 0.1 to 2.5 mass%, P: 0.020 mass% or less. , S: 0.005% by mass or less, Al: 0.01 to 0.05% by mass, Cu: 0.05 to 0.25% by mass, with the balance being a component composition consisting of iron and unavoidable impurities. Is preferable, and if necessary, one kind selected from Ni: 0.05 mass% or less, Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, and B: 5 to 30 mass ppm. The above can be contained. The reasons for limiting the contents of these elements are as follows.
・C:0.08〜0.30質量%
Cは、鋼板の強度を確保するために必要な元素であり、C量が0.08質量%未満では強度を確保することが難しくなるおそれがあるので、C量は0.08質量%以上とすることが好ましい。一方、C量が過剰になると溶接性が劣化するため、C量は0.30質量%以下とすることが好ましい。
・Si:0.1〜2.0質量%
Siは、固溶強化元素であり、Si量が0.1質量%未満では鋼板の硬質化が不十分となるおそれがあるので、Si量は0.1質量%以上とすることが好ましい。一方、Siを過剰に含有させると靱性が劣化し、また、焼鈍時に形成するSi系酸化物量が増加し、酸洗設備が長大になって設備コストの増大を招く。そのため、Si量は2.0質量%以下とすることが好ましい。
-C: 0.08 to 0.30 mass%
C is an element necessary to secure the strength of the steel sheet, and if the C content is less than 0.08 mass %, it may be difficult to secure the strength, so the C content is 0.08 mass% or more. Preferably. On the other hand, if the C content becomes excessive, the weldability deteriorates, so the C content is preferably 0.30 mass% or less.
・Si: 0.1 to 2.0 mass%
Si is a solid solution strengthening element, and if the Si amount is less than 0.1% by mass, the steel sheet may be insufficiently hardened, so the Si amount is preferably 0.1% by mass or more. On the other hand, if Si is excessively contained, the toughness deteriorates, the amount of Si-based oxide formed during annealing increases, and the pickling equipment becomes long, resulting in an increase in equipment cost. Therefore, the Si amount is preferably 2.0% by mass or less.
・Mn:0.1〜2.5質量%
Mnは、鋼板の強度を確保する元素であり、Mn量が0.1質量%未満では鋼板の強度が不十分となるおそれがあるので、Mn量は0.1質量%以上とすることが好ましい。一方、Mnを過剰に含有させると偏析の発生が多くなって加工性が低下し、かつ溶接性も劣化するので、Mn量は2.5質量%以下とすることが好ましい。
・P:0.020質量%以下、S:0.005質量%以下
P、Sは、加工性を考慮した場合、含有量はなるべく低い方が好ましく、このためP量は0.020質量%以下とすることが好ましい。また、Sを過剰に含有させると介在物(MnS)が増加し、加工性に悪影響を及ぼすので、S量は0.005質量%以下とすることが好ましい。
・Mn: 0.1 to 2.5 mass%
Mn is an element that secures the strength of the steel sheet. If the Mn content is less than 0.1 mass %, the strength of the steel sheet may be insufficient, so the Mn content is preferably 0.1 mass% or more. .. On the other hand, if Mn is excessively contained, segregation is increased, workability is deteriorated, and weldability is deteriorated. Therefore, the Mn content is preferably 2.5% by mass or less.
-P: 0.020 mass% or less, S: 0.005 mass% or less P, S is preferably as low as possible in view of workability, so that the P content is 0.020 mass% or less. It is preferable that Further, when S is contained excessively, inclusions (MnS) increase, which adversely affects the workability, so the S content is preferably 0.005 mass% or less.
・Sol.Al:0.01〜0.05質量%
Alは、脱酸のために添加される元素であり、Sol.Al量が0.01質量%未満ではシリケート介在物が残り、鋼の加工性が劣化するおそれがあるため、Sol.Al量は0.01質量%以上とすることが好ましい。一方、Sol.Al量が多すぎると表面疵の増加を招くため、Sol.Al量は0.05質量%以下とすることが好ましい。
・Cu:0.05〜0.25質量%
Cu量は上述した通りである。すなわち、Cu量が0.05質量%未満では耐遅れ破壊特性の改善が不十分であり、一方、Cu量が0.25質量%を超えると、酸洗直後に金属Cuが鋼板表面に析出しやすくなるので、Cu量は0.05〜0.25質量%とする。
・Sol.Al: 0.01 to 0.05 mass%
Al is an element added for deoxidation, and if the amount of Sol.Al is less than 0.01% by mass, silicate inclusions may remain and the workability of steel may deteriorate. It is preferably 0.01% by mass or more. On the other hand, when the amount of Sol.Al is too large, surface defects are increased, so the amount of Sol.Al is preferably 0.05% by mass or less.
-Cu: 0.05 to 0.25 mass%
The amount of Cu is as described above. That is, if the amount of Cu is less than 0.05% by mass, the improvement of delayed fracture resistance is insufficient, while if the amount of Cu exceeds 0.25% by mass, metallic Cu precipitates on the surface of the steel sheet immediately after pickling. Since it becomes easy, the amount of Cu is set to 0.05 to 0.25 mass %.
・Ni:0.05質量%以下
Niは、固溶強化元素であるが、Niを過剰に含有させても鋼板の機械的性質の向上効果が飽和し、却ってコスト増を招くため、Ni量は0.05質量%以下とすることが好ましい。
・Ti:0.1質量%以下、Nb:0.1質量%以下
Ti及びNbは、鋼組織を細粒化する元素であり、靱性を損なうことなく強度を向上させるのに非常に有用な元素である。しかし、Ti、Nbを過剰に含有させても鋼板の機械的性質の向上効果が飽和し、却ってコスト増を招くため、Ti量及びNb量はそれぞれ0.1質量%以下とすることが好ましい。
・B:5〜30質量ppm
Bは、焼き入れ性を高めるために非常に有用な元素であり、また、粒界を強化して耐遅れ破壊特性を向上させる効果があり、これらの効果を十分に発現させるためには、B量は5質量ppm以上とすることが好ましい。一方、Bを過剰に含有させると熱間加工性が劣化するため、B量は30質量ppm以下とすることが好ましい。
-Ni: 0.05% by mass or less Ni is a solid solution strengthening element, but the effect of improving the mechanical properties of the steel sheet is saturated even if Ni is excessively contained, and the cost is rather increased. It is preferably 0.05% by mass or less.
-Ti: 0.1 mass% or less, Nb: 0.1 mass% or less Ti and Nb are elements that refine the steel structure, and are very useful elements for improving strength without impairing toughness. Is. However, even if Ti and Nb are excessively contained, the effect of improving the mechanical properties of the steel sheet is saturated and the cost is rather increased. Therefore, the Ti amount and the Nb amount are preferably 0.1% by mass or less.
・B: 5 to 30 mass ppm
B is a very useful element for enhancing the hardenability, and has the effect of strengthening the grain boundaries and improving the delayed fracture resistance. To fully exhibit these effects, B is The amount is preferably 5 mass ppm or more. On the other hand, if B is contained excessively, the hot workability is deteriorated, so the amount of B is preferably 30 mass ppm or less.
次に、本発明の高強度冷延鋼板の製造方法について説明する。
さきに述べたように、良好な化成処理性(特に低温型化成処理での化成処理性)が得られるようにするには、(i)酸洗により鋼板表面の酸化物層がほぼ完全に除去され、鋼板表面に酸化物層が実質的に存在しなくなること、(ii)酸洗後から水洗までの間に酸洗液膜が鋼板と反応することによって金属Cuが鋼板表面に析出するのを抑制する(金属Cuによる断面被覆率を30%以下とする)こと、が必要である。
このため本発明の高強度冷延鋼板の製造工程では、連続焼鈍することにより易酸化性の金属元素を含む酸化物が表面に生成した冷延鋼板を、以下のような条件で酸洗することが好ましい。
Next, a method for manufacturing the high strength cold rolled steel sheet of the present invention will be described.
As mentioned above, in order to obtain good chemical conversion treatability (particularly low temperature type chemical conversion treatment), (i) pickling removes almost completely the oxide layer on the steel plate surface. That the oxide layer does not substantially exist on the surface of the steel sheet, and (ii) the pickling liquid film reacts with the steel sheet between after pickling and washing with water to prevent metal Cu from precipitating on the surface of the steel sheet. It is necessary to suppress it (to make the cross-section coverage by metallic Cu 30% or less).
Therefore, in the manufacturing process of the high-strength cold-rolled steel sheet of the present invention, the cold-rolled steel sheet on the surface of which an oxide containing an easily oxidizable metal element is formed on the surface by continuous annealing is pickled under the following conditions. Is preferred.
易酸化性の金属のうち特にSiを含有する酸化物の中には、SiO2のように酸に対して難溶性を示す酸化物が存在する。このような難溶性のSiO2を含めた易酸化性の金属元素を含む酸化物層を完全に除去するためには、酸洗により鋼板表面の酸化物層を地鉄ごと取り除く必要がある。また、酸洗では、易酸化性の金属元素を含む酸化物層を短時間で効率的に除去する必要がある。
このため酸洗液は、強酸化性の酸である硝酸をベースとした酸成分からなることが好ましい。すなわち、易酸化性の金属元素を含む酸化物層を短時間で効率的に除去するには、硝酸の酸化力(=鉄を酸化(溶解)させる作用)を利用することが有効である。一方、そのような酸洗液を用いると、鉄の再表層が酸化(=Fe系酸化物が形成)されやすく、また、FeとともにCuも酸洗液に溶出し、CuはFeよりも貴な元素であるため、溶出後に鋼板表面にすぐ金属Cuとして析出してしまう。すなわち、酸洗によって鋼中のCuがFeとともに溶出するが、鋼板表面に析出する金属Cuの断面被覆率を低減させるには、そのCuの溶出を抑制することが重要である。したがって、「易酸化性の金属元素を含む酸化物層の除去」と「再表層でのFe系酸化物の生成・金属Cuの析出」はトレードオフの関係であり、このため鋼板のCu添加量に加えて、硝酸をベースとした酸洗液の組成・濃度を最適化する必要がある。
Among the easily oxidizable metals, particularly oxides containing Si include oxides that are hardly soluble in acid, such as SiO 2 . In order to completely remove such an oxide layer containing an easily oxidizable metal element including hardly soluble SiO 2 , it is necessary to remove the oxide layer on the surface of the steel sheet together with the base metal by pickling. Further, in the pickling, it is necessary to efficiently remove the oxide layer containing the easily oxidizable metal element in a short time.
Therefore, the pickling solution preferably comprises an acid component based on nitric acid, which is a strongly oxidizing acid. That is, in order to efficiently remove the oxide layer containing the easily oxidizable metal element in a short time, it is effective to utilize the oxidizing power of nitric acid (=the action of oxidizing (dissolving) iron). On the other hand, when such a pickling solution is used, the resurface layer of iron is easily oxidized (=Fe-based oxide is formed), and Cu is eluted in the pickling solution together with Fe, and Cu is noble than Fe. Since it is an element, it immediately precipitates as metallic Cu on the surface of the steel sheet after elution. That is, Cu in the steel is eluted together with Fe by pickling, but in order to reduce the cross-sectional coverage of metallic Cu deposited on the surface of the steel sheet, it is important to suppress the elution of Cu. Therefore, there is a trade-off relationship between "removal of the oxide layer containing an easily oxidizable metal element" and "formation of Fe-based oxide/precipitation of metal Cu in the resurface layer". In addition, it is necessary to optimize the composition and concentration of the nitric acid-based pickling solution.
具体的には、酸成分が硝酸をベースとし、これに適量の塩酸又は弗酸を加えた下記(1)又は(2)の酸洗液を用いることが好ましい。
(1)酸成分が硝酸と塩酸からなり、硝酸濃度が50g/L超110g/L以下、塩酸濃度(g/L)と硝酸濃度(g/L)の比R1[塩酸/硝酸]が0.1〜0.3、Fe3+イオン濃度が3〜30g/Lである酸洗液
(2)酸成分が硝酸と弗酸からなり、硝酸濃度が50g/L超110g/L以下、
弗酸濃度(g/L)と硝酸濃度(g/L)の比R2[弗酸/硝酸]が0.1〜0.3、Fe3+イオン濃度が3〜30g/Lである酸洗液
Specifically, it is preferable to use a pickling solution of the following (1) or (2) in which the acid component is nitric acid as a base, and an appropriate amount of hydrochloric acid or hydrofluoric acid is added thereto.
(1) The acid component consists of nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g/L and 110 g/L or less, and the ratio R1 [hydrochloric acid/nitric acid] of hydrochloric acid concentration (g/L) to nitric acid concentration (g/L) is 0. 1 to 0.3, Fe 3+ ion concentration is 3 to 30 g/L pickling solution (2) The acid component consists of nitric acid and hydrofluoric acid, and the nitric acid concentration is more than 50 g/L and 110 g/L or less,
Pickling solution having a ratio R2 of [hydrofluoric acid concentration (g/L) and nitric acid concentration (g/L) of 0.1 to 0.3 and Fe 3+ ion concentration of 3 to 30 g/L]
上記(1)の酸洗液は、酸成分が硝酸をベースとし、これに酸化膜破壊効果がある塩酸を適量加えた酸洗液である。ここで、硝酸濃度が50g/L以下では、易酸化性の金属元素を含む酸化物層を適切に除去できず、残存した酸化物が化成処理性を阻害して耐食性の低下を招く。また、酸化物に起因して鋼板表面が黒色を呈し、鋼板表面の美観を損ねる。一方、硝酸濃度が110g/Lを超えると、易酸化性の金属元素を含む酸化物層は容易に除去できるが、FeやCuの溶解が激しく起こるため、金属Cuが析出しやすくなり、鋼板表面に析出する金属Cuの断面被覆率を低減させることができない。また、鋼板表面の凹凸が激しくなり塗装表面の美観を損ねる。また、塩酸濃度(g/L)と硝酸濃度(g/L)の比R1[塩酸/硝酸]が0.1未満では、十分な酸洗速度は得られるが、硝酸比率が高いと、硝酸の強い酸化力により地鉄の溶解が促進されてCuの溶出量が多くなるため、鋼板表面に金属Cuが析出しやすくなり、鋼板表面に析出する金属Cuの断面被覆率を低減させることができない。また、酸洗に伴う反応熱が大きくなるため温度制御が非常に難しくなり、冷却設備が必要となるので製造設備が複雑になる。一方、比R1[塩酸/硝酸]が0.3を超えると、硝酸の酸化力が塩酸により抑制されるため、所望の酸洗速度が得られず、鋼板表面の酸化物を効率よく除去することができない。 The pickling solution of (1) above is a pickling solution in which the acid component is nitric acid as a base, and an appropriate amount of hydrochloric acid having an oxide film destroying effect is added thereto. Here, when the nitric acid concentration is 50 g/L or less, the oxide layer containing the easily oxidizable metal element cannot be removed properly, and the remaining oxide impairs the chemical conversion treatment property and causes a decrease in corrosion resistance. In addition, the surface of the steel sheet is black due to the oxide, and the appearance of the surface of the steel sheet is impaired. On the other hand, when the nitric acid concentration exceeds 110 g/L, the oxide layer containing an easily oxidizable metal element can be easily removed, but since Fe and Cu are strongly dissolved, metal Cu easily precipitates, and the steel sheet surface It is not possible to reduce the cross-sectional coverage of the metal Cu deposited on the surface. In addition, the unevenness of the steel plate surface becomes severe and the appearance of the painted surface is impaired. When the ratio R1 [hydrochloric acid/nitric acid] of the hydrochloric acid concentration (g/L) to the nitric acid concentration (g/L) is less than 0.1, a sufficient pickling rate can be obtained. Since the dissolution of base iron is promoted by the strong oxidizing power and the elution amount of Cu is increased, metal Cu is easily deposited on the surface of the steel sheet, and the cross-sectional coverage of the metal Cu deposited on the surface of the steel sheet cannot be reduced. Further, since the reaction heat accompanying pickling becomes large, it becomes very difficult to control the temperature, and a cooling facility is required, which complicates the manufacturing facility. On the other hand, if the ratio R1 [hydrochloric acid/nitric acid] exceeds 0.3, the oxidizing power of nitric acid is suppressed by hydrochloric acid, so the desired pickling rate cannot be obtained, and oxides on the surface of the steel sheet must be removed efficiently. I can't.
酸洗液中のFe3+イオン濃度が3g/L未満では、Fe3+イオンによる鉄の酸化反応が不十分であるため、所望の酸洗速度が得られず、鋼板表面の酸化物を効率よく除去することができない。一方、Fe3+イオン濃度が30g/Lを超えると、Fe3+イオンによる鉄の酸化反応が十分に行われ、所望の酸洗速度は得られるものの、酸洗液中のFe3+イオンが多いために、鋼板表面にFe系酸化物が多く形成してしまい、酸洗により新たに形成されるFe系酸化物を除去しきれず、化成処理性及び塗装後耐食性を改善することができない。 If the concentration of Fe 3+ ions in the pickling solution is less than 3 g/L, the oxidation reaction of iron by Fe 3+ ions is insufficient, so the desired pickling rate cannot be obtained, and the oxides on the steel plate surface are efficiently removed. Can not do it. On the other hand, when the Fe 3+ ion concentration exceeds 30 g/L, the iron is sufficiently oxidized by the Fe 3+ ion to obtain a desired pickling rate, but the Fe 3+ ion in the pickling solution is large. However, a large amount of Fe-based oxide is formed on the surface of the steel sheet, the Fe-based oxide newly formed by pickling cannot be completely removed, and chemical conversion treatability and corrosion resistance after coating cannot be improved.
上記(2)の酸洗液は、酸成分が硝酸をベースとし、これに酸化膜破壊効果がある弗酸を適量加えた酸洗液である。ここで、硝酸濃度とFe3+イオン濃度の限定理由は上記(1)の酸洗液と同様であり、また、弗酸濃度(g/L)と硝酸濃度(g/L)の比R2[弗酸/硝酸]の限定理由も、上記(1)の酸洗液と同様である。
ここで、(1)又は(2)の酸洗液のFe3+イオン濃度は、次のようにして調整することができる。すなわち、鋼板を酸洗液に浸漬すると、鋼板が溶解してFe2+イオンとなり、これが硝酸で酸化されてFe3+イオンとなり、その濃度が経時的に増加していく。このため、酸洗液中のFe3+イオン濃度を測定・監視し、管理濃度(例えば30g/L)を超える前に酸洗液の一部を抜き出し、Fe3+イオンを含有しない新液を補給することにより、酸洗液のFe3+イオン濃度を所定のレベルに調整することができる。
The pickling solution of (2) above is a pickling solution in which the acid component is nitric acid as a base, and an appropriate amount of hydrofluoric acid which has an oxide film destroying effect is added thereto. Here, the reason for limiting the nitric acid concentration and the Fe 3+ ion concentration is the same as in the pickling solution of (1) above, and the ratio R2 [fluorine concentration (g/L) to nitric acid concentration (g/L)] The reason for limiting [acid/nitric acid] is the same as that of the pickling solution in (1) above.
Here, the Fe 3+ ion concentration of the pickling solution of (1) or (2) can be adjusted as follows. That is, when a steel sheet is dipped in a pickling solution, the steel sheet dissolves and becomes Fe 2+ ions, which are oxidized by nitric acid to become Fe 3+ ions, and the concentration thereof increases with time. Therefore, the Fe 3+ ion concentration in the pickling solution is measured and monitored, a part of the pickling solution is extracted before the control concentration (for example, 30 g/L) is exceeded, and a new solution containing no Fe 3+ ion is replenished. As a result, the Fe 3+ ion concentration of the pickling solution can be adjusted to a predetermined level.
また、上記の(1)又は(2)の酸洗液を用いる場合には、酸洗液温度20〜70℃、酸洗時間3〜30秒の条件で酸洗を行うのが好ましい。
酸洗液温度が20℃未満では、易酸化性の金属元素を含む酸化物層の除去が不十分となりやすく、残存した酸化物が化成処理性を阻害して耐食性の低下を招きやすい。また、酸化物に起因して鋼板表面が黒色を呈し、鋼板表面の美観を損ねる。一方、酸洗液温度が70℃を超えると、十分な酸洗速度は得られるが、酸洗に伴う反応熱が大きくなるため温度制御が非常に難しくなり、冷却設備が必要となるので製造設備が複雑になる。また、酸洗時間が3秒未満の場合、易酸化性の金属元素を含む酸化物層を十分に除去するには硝酸濃度を高める必要があり、これにより上述したような問題を生じやすい。一方、酸洗時間が30秒を超えても性能上は問題ないが、設備が長くなり、設備コストが増加する。
また、酸洗を行うに当たっては、酸洗液に酸洗促進剤を添加したり、電解処理を併用したりして、地鉄の溶解を促進することも有効である。
When the above-mentioned pickling solution (1) or (2) is used, it is preferable to carry out pickling under the conditions of pickling solution temperature of 20 to 70° C. and pickling time of 3 to 30 seconds.
When the pickling solution temperature is lower than 20° C., the oxide layer containing the easily oxidizable metal element is liable to be insufficiently removed, and the remaining oxide tends to impair the chemical conversion treatment property and reduce corrosion resistance. In addition, the surface of the steel sheet is black due to the oxide, and the appearance of the surface of the steel sheet is impaired. On the other hand, when the pickling solution temperature exceeds 70° C., a sufficient pickling rate can be obtained, but the reaction heat accompanying pickling becomes large, which makes temperature control extremely difficult and requires cooling equipment. Becomes complicated. When the pickling time is less than 3 seconds, the nitric acid concentration needs to be increased in order to sufficiently remove the oxide layer containing the easily oxidizable metal element, which easily causes the above-mentioned problems. On the other hand, even if the pickling time exceeds 30 seconds, there is no problem in performance, but the equipment becomes long and the equipment cost increases.
Further, in the case of pickling, it is also effective to add a pickling accelerator to the pickling solution or use an electrolytic treatment together to accelerate the dissolution of the base iron.
一般に高強度冷延鋼板の製造工程では、連続焼鈍した冷延鋼板を水焼入れした後、酸洗し、その後、調質圧延等の通常の処理工程を経て製品鋼板とする。 Generally, in the manufacturing process of a high-strength cold-rolled steel sheet, a cold-rolled steel sheet that has been continuously annealed is water-quenched, then pickled, and then subjected to a normal treatment step such as temper rolling to obtain a product steel sheet.
供試材として表1に示す成分組成と引張り強度TSを有する連続焼鈍材(冷延鋼板)を使用した。これらの鋼板に対して表2及び表3に示す条件で酸洗を行い、水洗し、乾燥した後、伸び率0.7%の調質圧延を施してNo.1〜24の高強度冷延鋼板を製造した。なお、酸洗液のFe3+イオン濃度は、上述した方法により調整した。
製造された各鋼板から試験片を採取し、任意に選ばれた試験片表面の5箇所をグロー放電発光分光分析(GDS)で深さ方向に分析し、Al、Si、Mn、Oのピークが現れるか否かを調べ、Al、Si、Mn、Oのうちの1つ以上のピークが現れた場合を「酸化物層が存在(残存)する」、いずれのピークも現れない場合を「酸化物層が存在(残存)しない」と評価した。また、金属Cuの断面被覆率は、試験片の表層断面について、さきに説明した手法で求めた。
As a test material, a continuous annealed material (cold rolled steel sheet) having the composition and tensile strength TS shown in Table 1 was used. These steel sheets were pickled under the conditions shown in Tables 2 and 3, washed with water, dried, and then temper-rolled with an elongation of 0.7% and subjected to high-strength cold rolling of Nos. 1 to 24. A steel plate was manufactured. The Fe 3+ ion concentration of the pickling solution was adjusted by the method described above.
A test piece was taken from each of the manufactured steel plates, and five spots on the surface of the arbitrarily selected test piece were analyzed in the depth direction by glow discharge emission spectroscopy (GDS), and the peaks of Al, Si, Mn, and O were found. It is checked whether or not it appears, and when one or more peaks of Al, Si, Mn, and O appear, "the oxide layer exists (remains)", and when none of the peaks appears, "oxide. There is no layer (remaining)”. The cross-sectional coverage of the metallic Cu was obtained by the method described above for the surface layer cross section of the test piece.
製造された各鋼板から試験片を採取し、これらの試験片に下記条件で化成処理(化成処理液温度が35℃の低温型化成処理)と塗装を施した後、塩温水浸漬試験、塩水噴霧試験及び複合サイクル腐食試験の3種類の腐食試験に供して塗装後耐食性を評価した。また、製造された各鋼板(化成処理・塗装されていない鋼板)から試験片を採取し、この試験片で耐遅れ破壊特性を評価した。なお、化成処理では皮膜付着量が1.7〜3.0g/m2となるように化成処理時間を調整した。
(1)化成処理条件
・脱脂工程
脱脂剤:日本パーカライジング社製「FC−E2011」
脱脂方法:スプレー脱脂
処理温度:40℃
処理時間:120秒
・表面調整工程
表面調整剤:日本パーカライジング社製「PL−X」
表面調整剤pH:9.5
処理温度:室温
処理時間:20秒
・化成処理工程
化成処理剤:日本パーカライジング社製「パルボンドPB−SX」
化成処理液温度:35℃
処理時間:90秒
(2)塗装条件
化成処理を施した試験片の表面に、日本ペイント社製の電着塗料「GT−100」を用いて、膜厚が15μmとなるように電着塗装を施した。
After collecting test pieces from each manufactured steel sheet, applying chemical conversion treatment (low temperature type chemical conversion treatment with chemical conversion treatment liquid temperature of 35° C.) and coating to these test pieces under the following conditions, salt hot water immersion test, salt water spray The test and the combined cycle corrosion test were subjected to three types of corrosion tests to evaluate the corrosion resistance after coating. In addition, test pieces were taken from each of the manufactured steel sheets (steel sheets that had not been subjected to chemical conversion treatment/painting), and the delayed fracture resistance was evaluated using the test pieces. In the chemical conversion treatment, the chemical conversion treatment time was adjusted so that the coating adhesion amount was 1.7 to 3.0 g/m 2 .
(1) Chemical conversion treatment conditions-Degreasing process Degreasing agent: "FC-E2011" manufactured by Nippon Parkerizing Co., Ltd.
Degreasing method: Spray degreasing Treatment temperature: 40°C
Treatment time: 120 seconds ・Surface conditioning process Surface conditioning agent: "PL-X" manufactured by Nihon Parkerizing Co., Ltd.
Surface conditioner pH: 9.5
Treatment temperature: Room temperature Treatment time: 20 seconds ・Chemical conversion treatment step Chemical conversion treatment agent: "Palbond PB-SX" manufactured by Nippon Parkerizing Co., Ltd.
Chemical conversion treatment liquid temperature: 35°C
Treatment time: 90 seconds (2) Coating conditions Electrodeposition coating was applied to the surface of the test piece that had been subjected to chemical conversion treatment, using the electrodeposition coating "GT-100" manufactured by Nippon Paint Co., Ltd., so that the film thickness was 15 μm. gave.
(3)腐食試験
<塩温水浸漬試験>
化成処理と電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片を5質量%NaCl溶液(60℃)に240時間浸漬し、その後、水洗し、乾燥し、クロスカット疵部に粘着テープを貼り付けた後、引き剥がすテープ剥離試験を行い、クロスカット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が5.0mm以下であれば、塩温水浸漬試験における耐食性は良好と評価することができる。
(3) Corrosion test <Salt hot water immersion test>
After a cross-cut flaw having a length of 45 mm was formed by a cutter on the surface of the above-mentioned test piece (n=1) that had been subjected to chemical conversion treatment and electrodeposition coating, this test piece was added to a 5 mass% NaCl solution (60° C.) 240 After soaking for a period of time, followed by washing with water, drying, adhering an adhesive tape to the cross-cut flaw portion, a tape peeling test for peeling off was performed, and the maximum peeling total width of the left and right cross-cut flaw portions was measured. When this maximum peeling overall width is 5.0 mm or less, the corrosion resistance in the salt hot water immersion test can be evaluated as good.
<塩水噴霧試験(SST)>
化成処理と電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片に対して、5質量%NaCl水溶液を用いてJIS Z2371:2000に規定される中性塩水噴霧試験に準拠して960時間の塩水噴霧試験を行った後、クロスカット疵部についてテープ剥離試験を実施し、クロスカット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が4.0mm以下であれば、塩水噴霧試験における耐食性は良好と評価することができる。
<Salt spray test (SST)>
After a cross-cut flaw having a length of 45 mm was formed by a cutter on the surface of the test piece (n=1) that had been subjected to chemical conversion treatment and electrodeposition coating, a 5 mass% NaCl aqueous solution was used for this test piece. After performing a salt spray test for 960 hours in accordance with the neutral salt spray test specified in JIS Z2371:2000, a tape peeling test was performed on the crosscut flaws, and maximum peeling was performed by combining the left and right crosscut flaws. Full width was measured. If this maximum peeling overall width is 4.0 mm or less, it can be evaluated that the corrosion resistance in the salt spray test is good.
<複合サイクル腐食試験(CCT)>
化成処理と電着塗装を施した上記試験片(n=1)の表面に、カッターで長さ45mmのクロスカット疵を付与した後、この試験片に対して、「塩水噴霧(5質量%NaCl水溶液:35℃、相対湿度:98%)×2時間→乾燥(60℃、相対湿度:30%)×2時間→湿潤(50℃、相対湿度:95%)×2時間」を1サイクルとして、これを50サイクル繰り返す腐食試験を実施し、水洗し、乾燥した後、クロスカット疵部についてテープ剥離試験を実施し、クロスカット疵部左右を合わせた最大剥離全幅を測定した。この最大剥離全幅が6.0mm以下であれば、複合サイクル腐食試験での耐食性は良好と評価できる。
<Compound Cycle Corrosion Test (CCT)>
After a cross-cut flaw having a length of 45 mm was formed by a cutter on the surface of the above-mentioned test piece (n=1) that had been subjected to chemical conversion treatment and electrodeposition coating, the test piece was subjected to "salt spray (5 mass% NaCl). Aqueous solution: 35° C., relative humidity: 98%)×2 hours→drying (60° C., relative humidity: 30%)×2 hours→wetting (50° C., relative humidity: 95%)×2 hours” as one cycle, After repeating this cycle for 50 cycles, a corrosion test was carried out, followed by washing with water and drying, and then a tape peeling test was carried out on the crosscut flaws, and the maximum peeling total width of the left and right crosscut flaws was measured. If the maximum peeling total width is 6.0 mm or less, the corrosion resistance in the combined cycle corrosion test can be evaluated as good.
(4)耐遅れ破壊特性
製造された各鋼板をそれぞれ幅35mm×長さ100mmにせん断し、幅が30mmになるまで研削加工を施し、試験片を作製した。図2に示すように、この試験片1をU字形状に曲げて、ボルト2とナット3で拘束して試験片形状を固定し、遅れ破壊評価用試験片を得た。このようにして作製した遅れ破壊評価用試験片に対し、米国自動車技術会で定めたSAE J2334に規定された、乾燥・湿潤・塩水浸漬の工程からなる複合サイクル腐食試験(図3参照)を、最大20サイクルまで実施した。各サイクルの塩水浸漬の工程前に目視により割れの発生の有無を調査し、割れ発生サイクル数を測定した。また、本試験は、各鋼板3検体ずつ実施し、その平均値をもって割れ発生サイクル数とした。
(4) Delayed fracture resistance properties Each of the manufactured steel plates was sheared to a width of 35 mm and a length of 100 mm, and ground to give a width of 30 mm to prepare test pieces. As shown in FIG. 2, this test piece 1 was bent into a U-shape, and the shape of the test piece was fixed by restraining it with a bolt 2 and a nut 3 to obtain a test piece for delayed fracture evaluation. The delayed fracture evaluation test piece thus produced was subjected to a combined cycle corrosion test (see FIG. 3) consisting of the steps of dry, wet, and salt water immersion specified in SAE J2334 defined by the American Society of Automotive Engineers. Up to 20 cycles were carried out. Before the process of immersion in salt water in each cycle, the presence or absence of cracks was visually inspected, and the number of crack generation cycles was measured. In addition, this test was carried out for each of three steel plates, and the average value was used as the number of crack generation cycles.
上記試験の結果を、酸洗条件及び鋼板表面性状とともに表2及び表3に示す。これによれば、本発明例の鋼板は、優れた耐遅れ破壊特性を有するとともに、塩温水浸漬試験、塩水噴霧試験及び複合サイクル腐食試験のいずれにおいても最大剥離全幅が小さく、極めて優れた塗装後耐食性を有していることが判る。
なお、表3のNo.28〜32の比較例は、本発明例に較べて耐遅れ破壊特性が劣っているが、これは、鋼板表面に酸化物層があるか若しくは金属Cuの断面被覆率が高いために化成処理性が悪く、塗装後耐食性が不十分であるため、暴露に伴う腐食反応で水素が多く発生し、割れが発生し易いためであると考えられる。
The results of the above tests are shown in Tables 2 and 3 together with pickling conditions and steel plate surface properties. According to this, the steel sheet of the example of the present invention has excellent delayed fracture resistance, and also has a maximum maximum peeling width in any of the salt hot water immersion test, the salt spray test and the combined cycle corrosion test, which is extremely excellent after coating. It can be seen that it has corrosion resistance.
The comparative examples of Nos. 28 to 32 in Table 3 are inferior in delayed fracture resistance to the inventive examples, but this is due to the presence of an oxide layer on the steel plate surface or the cross-section coverage of metal Cu. It is considered that this is because the chemical conversion treatment property is poor due to the high value and the corrosion resistance after coating is insufficient, so that a large amount of hydrogen is generated due to the corrosion reaction associated with exposure and cracking is likely to occur.
Claims (5)
(1)酸成分が硝酸と塩酸からなり、硝酸濃度が50g/L超110g/L以下、塩酸濃度(g/L)と硝酸濃度(g/L)の比R1[塩酸/硝酸]が0.1〜0.3、Fe3+イオン濃度が3〜30g/Lである酸洗液
(2)酸成分が硝酸と弗酸からなり、硝酸濃度が50g/L超110g/L以下、
弗酸濃度(g/L)と硝酸濃度(g/L)の比R2[弗酸/硝酸]が0.1〜0.3、Fe3+イオン濃度が3〜30g/Lである酸洗液 A high-strength cold-rolled steel sheet characterized by being subjected to pickling with a pickling solution according to the following (1) or (2) after continuously annealing a cold-rolled steel sheet having the component composition according to claim 1. Manufacturing method.
(1) The acid component consists of nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g/L and 110 g/L or less, and the ratio R1 [hydrochloric acid/nitric acid] of hydrochloric acid concentration (g/L) to nitric acid concentration (g/L) is 0. 1 to 0.3, Fe 3+ ion concentration is 3 to 30 g/L pickling solution (2) The acid component consists of nitric acid and hydrofluoric acid, and the nitric acid concentration is more than 50 g/L and 110 g/L or less,
Pickling solution having a ratio R2 of [hydrofluoric acid concentration (g/L) and nitric acid concentration (g/L) [hydrofluoric acid/nitric acid] of 0.1 to 0.3 and Fe 3+ ion concentration of 3 to 30 g/L.
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