JP7468819B2 - Manufacturing method for high-strength hot-dip galvanized steel sheet - Google Patents
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- 229910001335 Galvanized steel Inorganic materials 0.000 title claims description 39
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- UQSXHKLRYXJYBZ-UHFFFAOYSA-N Iron oxide Chemical compound [Fe]=O UQSXHKLRYXJYBZ-UHFFFAOYSA-N 0.000 description 4
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 229910052726 zirconium Inorganic materials 0.000 description 1
- 229910000859 α-Fe Inorganic materials 0.000 description 1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
- C21D9/54—Furnaces for treating strips or wire
- C21D9/56—Continuous furnaces for strip or wire
-
- 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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- 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
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
-
- 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
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
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- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Description
本発明は、耐抵抗溶接割れ特性と耐遅れ破壊特性に優れた溶融亜鉛めっき鋼板の製造方法に関する。The present invention relates to a method for producing a hot-dip galvanized steel sheet having excellent resistance weld cracking resistance and delayed fracture resistance.
近年、地球環境を保護する観点から、自動車の燃費改善が強く求められている。また、衝突時における乗員の安全を確保する観点から、自動車の安全性向上も強く要求されている。これらの要求に応えるためには、自動車車体の軽量化と高強度化とを両立する必要があり、自動車部品の素材となる溶融亜鉛めっき鋼板においては、高強度化による薄肉化が積極的に進められている。しかし、自動車部品の多くは、鋼板を成形加工して製造されることから、これらの鋼板には、高い強度に加えて、優れた成形性が求められる。In recent years, there has been a strong demand for improving the fuel efficiency of automobiles from the viewpoint of protecting the global environment. In addition, there has been a strong demand for improving the safety of automobiles from the viewpoint of ensuring the safety of passengers in the event of a collision. In order to meet these demands, it is necessary to simultaneously reduce the weight and increase the strength of automobile bodies, and for hot-dip galvanized steel sheets, which are used as materials for automobile parts, efforts to reduce the thickness by increasing the strength have been actively promoted. However, since many automobile parts are manufactured by forming steel sheets, these steel sheets are required to have excellent formability in addition to high strength.
溶融亜鉛めっき鋼板の強度を高めるには種々の方法があるが、溶融亜鉛めっき鋼板の成形性を大きく損なわずに高強度化を図ることができる方法としては、Ｃ添加によるマルテンサイトの活用に加え、Ｓｉ添加による固溶強化が挙げられる。一方、自動車部品の製造において、プレス成形された部品は抵抗溶接（スポット溶接）により組み合わせることが多い。鋼板にＣやＳｉが多く添加されていると、抵抗溶接時に、溶接部近傍に残留応力が生成した状態で、めっき層の亜鉛が溶融して結晶粒界に拡散侵入することで、液体金属脆化（ＬｉｑｕｉｄＭｅｔａｌ Ｅｍｂｒｉｔｔｌｅｍｅｎｔ；ＬＭＥ）が起き、鋼板に粒
界割れ（ＬＭＥ割れ）が生じてしまうことが懸念される。特に溶接用の電極が鋼板に対して角度がついた状態で溶接が行われると、残留応力が増加して割れが生成する虞がある。残留応力は鋼板の高強度化に伴い増大すると考えられるため、鋼板の高強度化に伴うＬＭＥ割れの発生が懸念される。 There are various methods for increasing the strength of hot-dip galvanized steel sheets, but as a method for increasing the strength of hot-dip galvanized steel sheets without significantly impairing their formability, in addition to utilizing martensite by adding C, solid solution strengthening by adding Si can be mentioned. On the other hand, in the manufacture of automobile parts, press-formed parts are often combined by resistance welding (spot welding). If a large amount of C or Si is added to a steel sheet, when residual stress is generated near the welded part during resistance welding, the zinc in the coating layer melts and diffuses into the grain boundaries, causing liquid metal embrittlement (LME), which is a concern, and grain boundary cracking (LME cracking) may occur in the steel sheet. In particular, when welding is performed with the welding electrode at an angle to the steel sheet, the residual stress may increase and cracks may occur. Since the residual stress is thought to increase with the increase in the strength of the steel sheet, there is a concern that LME cracking may occur with the increase in the strength of the steel sheet.
さらに、 鋼材の強度の増加に伴い、 水素脆化に起因する遅れ破壊が生じやすくなることも知られており、特に引張り強度が１１８０ＭＰａ以上の高強度鋼ではこの傾向が顕著である。なお、 遅れ破壊とは、 高強度鋼材が静的な負荷応力（引張り強さ未満の負荷応力）を受けた状態で、ある時間が経過したとき、 外見上はほとんど塑性変形を伴うこと
なく、 突然脆性的な破壊が生じる現象である。このような遅れ破壊については、 使用環境によって生じる腐食が原因で、鋼板に侵入した水素によって生じることが多いが、連続溶融亜鉛めっきライン(Ｃｏｎｔｉｎｕｏｕｓ Ｇａｌｖａｎｉｚｉｎｇ Ｌｉｎｅ；ＣＧ
Ｌ)の焼鈍工程で鋼板に侵入した水素も、 特に引張強度が９８０ＰＭａを超える鋼板の機械特性を劣化させ脆性破壊を引き起こす。 It is also known that as the strength of steel increases, delayed fracture due to hydrogen embrittlement becomes more likely to occur, and this tendency is particularly pronounced in high-strength steel with a tensile strength of 1180 MPa or more. Delayed fracture is a phenomenon in which, when high-strength steel is subjected to a static load stress (load stress less than the tensile strength) and a certain amount of time has passed, brittle fracture occurs suddenly, with little or no apparent plastic deformation. Delayed fracture is often caused by hydrogen that has penetrated into the steel plate due to corrosion caused by the usage environment, but it can also occur in continuous hot-dip galvanizing lines (CG lines).
Hydrogen that penetrates into the steel sheet during the annealing process in step L also deteriorates the mechanical properties of steel sheet, particularly those with a tensile strength exceeding 980 PMa, and causes brittle fracture.
以上に述べたように、耐抵抗溶接割れ特性（以下、単に「耐ＬＭＥ割れ性」とも称する）に優れ、鋼中の水素起因によって生じる機械特性の劣化を抑止した高強度鋼板が求められている。As described above, there is a demand for high-strength steel plate that is excellent in resistance weld cracking resistance (hereinafter also simply referred to as "LME cracking resistance") and suppresses deterioration of mechanical properties caused by hydrogen in the steel.
従来、Ｓｉ添加鋼に生じる不めっき欠陥を改善する方法として、特許文献１ではＯ２を含有する雰囲気で７００℃以上まで加熱することでＳｉ添加鋼の表面を酸化し、鋼板表層の酸化物を露点が５℃以上のＨ2を含む雰囲気で還元する方法が開示されている。しかし
、Ｏ２を含有する雰囲気で７００℃以上まで加熱すると、鋼板の酸化量が多く、還元焼鈍時の炉内に酸化物が付着し、鋼板の外観品質を阻害する課題がある。 Conventionally, as a method for improving uncoated defects occurring in Si-added steel,
特許文献２ではＯ２を含有する雰囲気で６００℃以上、８５０℃以下まで加熱することでＳｉ添加鋼の表面を酸化し、鋼板表層の酸化物を露点が５℃以上の５００体積ｐｐｍ以上、５０００体積ｐｐｍ以下のＨ２Ｏ及びＨ２を含む雰囲気で酸化した鋼板を還元する方法が開示されている。特許文献３では同様に直火型加熱炉（ＤＦＦ）の空気比を増加させることでＳｉ添加鋼の表面を酸化し鋼板表層の酸化物をｌｏｇ（ＰＨ２Ｏ/ＰＨ２）が－
３．４以上、－１．１以下となる雰囲気で還元する方法が開示されている。これらの方法では、鋼板の酸化量が調整可能であり、良好な外観品質は確保可能であるものの、焼鈍時に鋼中に侵入した水素が多く残存することで、十分な耐ＬＭＥ割れ性や耐遅れ破壊特性を得ることはできない課題がある。
In the present invention, a method of reducing the steel sheet in an atmosphere having a SiO2 content of 3.4 or more and -1.1 or less has been disclosed. Although these methods make it possible to adjust the amount of oxidation of the steel sheet and ensure good appearance quality, there is a problem in that a large amount of hydrogen that has penetrated into the steel during annealing remains, making it impossible to obtain sufficient LME cracking resistance and delayed fracture resistance.
本発明では、鋼板の酸化量が過剰な場合に生じる、還元焼鈍時の炉内酸化物が鋼板に付着することに因る鋼板の外観品質阻害を防止し、耐ＬＭＥ割れ性や延性に優れ、同時に水素脆化に起因する耐遅れ破壊特性の劣化を抑制可能で高強度溶融亜鉛めっき鋼板の製造方法を提供することを目的とする。An object of the present invention is to provide a method for producing a high-strength hot-dip galvanized steel sheet which prevents deterioration of the appearance quality of the steel sheet due to adhesion of oxides in a furnace during reduction annealing to the steel sheet, which occurs when the amount of oxidation of the steel sheet is excessive, and which has excellent LME cracking resistance and ductility, and at the same time is capable of suppressing deterioration of delayed fracture resistance due to hydrogen embrittlement.
本発明者らは、鋼板の酸化時のＯ2濃度と温度を、 鋼板が含有するＳｉ濃度及びＭｎ濃度に従って適正化し過剰な酸化を抑制することで鋼板の外観品質を確保し、更に還元焼鈍時のＨ２Ｏ濃度、Ｈ２濃度及びｌｏｇ（ＰＨ２Ｏ/ＰＨ２）を最適化することで耐抵抗溶
接割れ特性に優れ、同時に水素脆化に起因する耐遅れ破壊特性の劣化を抑制可能であることを見出し、本発明を完成させた。 The inventors have discovered that by optimizing the O2 concentration and temperature during oxidation of steel plate in accordance with the Si and Mn concentrations contained in the steel plate, excessive oxidation can be suppressed to ensure the appearance quality of the steel plate, and that by optimizing the H2O concentration, H2 concentration and log( PH2O / PH2 ) during reduction annealing, excellent resistance weld cracking properties can be achieved while at the same time deterioration of delayed fracture resistance due to hydrogen embrittlement can be suppressed, and thus completed the present invention.
本発明は、上記知見に基づいてなされたものである。すなわち、本発明の要旨構成は以下の通りである。
［１］質量％で、Ｃ：０．０５％以上０．３０％以下、Ｓｉ：０．４５％以上２．０％以下、Ｍｎ：１．０％以上４．０％以下を含有するスラブを熱間圧延した後、下記式（１）から算出される温度ＴＣ（℃）以下の温度でコイルに巻き取り、酸洗する熱間圧延工程と、前記熱間圧延工程で得られた熱延板に対して冷間圧延を施す冷間圧延工程と、前記冷間圧延工程で得られた冷延鋼板を、直火型の加熱炉と、ラジアントチューブ型の加熱・保持炉を有する焼鈍炉で、連続焼鈍した後、溶融亜鉛めっきを施す高強度溶融亜鉛めっき鋼板の製造方法であって、
前記直火型の加熱炉では、前段で、Ｏ２を１０００体積ｐｐｍ以上、Ｈ２Ｏを１０００体積ｐｐｍ以上含む雰囲気中で鋼板を４００℃以上６７０℃以下まで加熱し、
後段で、Ｏ２を５００体積ｐｐｍ以下含む雰囲気中で鋼板を６００℃以上７００℃以下まで加熱し、
前記加熱・保持炉を有する焼鈍炉では、炉内雰囲気のＨ２Ｏ濃度が５０００体積ｐｐｍ以上４００００体積ｐｐｍ以下、Ｈ２濃度が２体積％以上２０体積％以下、
Ｈ２Ｏの分圧（ＰＨ2Ｏ）とＨ２の分圧（ＰＨ2）の比ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）が－１．
１以上０．５以下を満たす雰囲気に、鋼板温度を６５０℃以上９００℃以下に９０秒以上保持する高強度溶融亜鉛めっき鋼板の製造方法。
ＴＣ＝－３０（［Ｓｉ］＋［Ｍｎ］）＋７７５ ・・・（１）
［Ｓｉ］は鋼板に含まれるＳｉ含有量（質量％）
［Ｍｎ］は鋼板に含まれるＭｎ含有量（質量％）
［２］鋼板に、溶融亜鉛めっきを施した後、合金化処理を行う［１］に記載の高強度溶融亜鉛めっき鋼板の製造方法。
［３］ラジアントチューブ型の加熱・保持炉での加熱および保持の後に、平均冷却速度が１０℃／秒以上の条件で、前記焼鈍での最終保持温度から１５０～３５０℃の温度まで冷却した後、３５０～６００℃の温度まで加熱し１０～６００秒保持する冷却加熱工程をさらに有する［１］～［２］に記載の高強度溶融亜鉛めっき鋼板の製造方法。
［４］前記Ｈ２Ｏの分圧（ＰＨ2Ｏ）とＨ２の分圧（ＰＨ2）の比ｌｏｇ（ＰＨ2Ｏ/ＰＨ2
）が－０．９９以上０．５以下を満たす雰囲気である［１］～［３］のいずれかに記載の高強度溶融亜鉛めっき鋼板の製造方法。
［５］前記Ｈ２Ｏの分圧（ＰＨ2Ｏ）とＨ２の分圧（ＰＨ2）の比ｌｏｇ（ＰＨ2Ｏ/ＰＨ2
）が－０．９以上０．５以下を満たす雰囲気である［１］～［４］のいずれかに記載の高強度溶融亜鉛めっき鋼板の製造方法。
［６］前記Ｈ２Ｏの分圧（ＰＨ2Ｏ）とＨ２の分圧（ＰＨ2）の比ｌｏｇ（ＰＨ2Ｏ/ＰＨ2
）が－０．７以上０．５以下を満たす雰囲気である［１］～［５］のいずれかに記載の高強度溶融亜鉛めっき鋼板の製造方法。 The present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows.
[1] A method for producing a high-strength hot-dip galvanized steel sheet, comprising the steps of: hot-rolling a slab containing, by mass%, C: 0.05% to 0.30%, Si: 0.45% to 2.0%, and Mn: 1.0% to 4.0%, and then winding the slab into a coil at a temperature equal to or lower than T C (° C.) calculated from the following formula (1), and pickling the hot-rolled sheet; cold-rolling the hot-rolled sheet obtained in the hot-rolling step; and continuously annealing the cold-rolled steel sheet obtained in the cold-rolling step in an annealing furnace having a direct-fired heating furnace and a radiant tube-type heating/holding furnace, and then hot-dip galvanizing the cold-rolled steel sheet,
In the direct-fired heating furnace, in the first stage, the steel sheet is heated to 400° C. or higher and 670° C. or lower in an atmosphere containing 1000 ppm by volume or more of O 2 and 1000 ppm by volume or more of H 2 O;
In the latter stage, the steel sheet is heated to 600 ° C. or more and 700 ° C. or less in an atmosphere containing 500 volume ppm or less of O 2 ,
In the annealing furnace having the heating and holding furnace, the H 2 O concentration in the furnace atmosphere is 5,000 volume ppm or more and 40,000 volume ppm or less, and the H 2 concentration is 2 volume % or more and 20 volume % or less;
The ratio of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2 (P H2 ), log (P H2O /P H2 ), is -1.
The method for producing a high-strength hot-dip galvanized steel sheet comprises holding the steel sheet at a temperature of 650° C. or higher and 900° C. or lower for 90 seconds or more in an atmosphere satisfying a ratio of 1 to 0.5.
T C =−30([Si]+[Mn])+775 ... (1)
[Si] is the Si content (mass%) in the steel sheet
[Mn] is the Mn content (mass%) in the steel plate
[2] The method for producing a high-strength hot-dip galvanized steel sheet according to [1], comprising hot-dip galvanizing the steel sheet and then subjecting it to an alloying treatment.
[3] The method for producing a high-strength hot-dip galvanized steel sheet according to [1] to [2], further comprising a cooling and heating step of cooling from the final holding temperature in the annealing to a temperature of 150 to 350°C under conditions of an average cooling rate of 10°C/sec or more, and then heating to a temperature of 350 to 600°C and holding for 10 to 600 seconds, after the heating and holding in a radiant tube type heating and holding furnace.
[4] The ratio of the partial pressure of H 2 O ( PH2O ) to the partial pressure of H 2 ( PH2 ) log ( PH2O / PH2
) is -0.99 or more and 0.5 or less.
[5] The ratio of the partial pressure of H 2 O (P H2 O ) to the partial pressure of H 2 (P H2 ) log (P H2 O /P H2
) is -0.9 or more and 0.5 or less.
[6] The ratio of the partial pressure of H 2 O ( PH2O ) to the partial pressure of H 2 ( PH2 ) log ( PH2O / PH2
) is -0.7 or more and 0.5 or less.
本発明によれば、溶接部における耐抵抗溶接割れ特性に優れ且つ良好な外観品質が得られ、耐遅れ破壊特性の劣化要因となる鋼中の水素を十分に低下させた高強度鋼板を提供することができる。According to the present invention, it is possible to provide a high-strength steel plate which has excellent resistance weld cracking resistance in a welded portion, good appearance quality, and in which the amount of hydrogen in the steel, which is a cause of deterioration of delayed fracture resistance, is sufficiently reduced.
以下、本発明の実施形態について説明する。Hereinafter, an embodiment of the present invention will be described.
なお、以下の説明において、Ｓｉ含有スラブの成分組成の各元素の含有量、めっき層成分組成の各元素の含有量の単位はいずれも「質量％」であり、特に断らない限り単に「％」で示す。また、本明細書中において、「～」を用いて表される数値範囲は、「～」の前後に記載される数値を下限値及び上限値として含む範囲を意味する。また、本明細書において、鋼板が「高強度」であるとは、ＪＩＳ Ｚ ２２４１（２０１１）に準拠して測定した鋼板の引張強さＴＳが５９０ＭＰａ以上であることを意味する。In the following description, the unit of the content of each element in the composition of the Si-containing slab and the content of each element in the composition of the coating layer is "mass%", and is simply indicated as "%" unless otherwise specified. In this specification, a numerical range expressed using "to" means a range including the numerical values written before and after "to" as the lower and upper limits. In this specification, a steel sheet being "high strength" means that the tensile strength TS of the steel sheet measured in accordance with JIS Z 2241 (2011) is 590 MPa or more.
まず、Ｓｉ含有スラブの成分組成について説明する。First, the composition of the Si-containing slab will be described.
＜スラブ成分＞
Ｓｉ：０．４５％以上２．０％以下
Ｓｉは、加工性を大きく損なうことなく、固溶により鋼の強度を高める効果（固溶強化能）が大きいため、鋼板の高強度化を達成するのに有効な元素である。一方で、Ｓｉは溶接部における耐抵抗溶接割れ特性に悪影響を及ぼす元素でもある。Ｓｉを鋼板の高強度化を達成するために添加する場合には、０．４５％以上の添加が必要である。また、Ｓｉが０．４５％未満では、溶接部における耐抵抗溶接割れ特性に特に問題は生じず、本発明を適用する必要性に乏しい。一方、Ｓｉの含有量が３．０％を超えると、熱間圧延性および冷間圧延性が大きく低下し、生産性に悪影響を及ぼしたり、鋼板自体の延性の低下を招いたりする。よって、Ｓｉは０．４５％以上３．０％以下の範囲で添加する。Ｓｉ量は、好ましくは０．７％以上、より好ましくは０．９％以上とする。また、Ｓｉ量は、好ましくは２．５％以下、より好ましくは２．０％以下とする。 <Slab components>
Si: 0.45% or more and 2.0% or less Si is an effective element for achieving high strength of steel plate because it has a large effect of increasing the strength of steel by solid solution (solid solution strengthening ability) without significantly impairing workability. On the other hand, Si is also an element that adversely affects the resistance weld crack resistance property of the welded part. When Si is added to achieve high strength of steel plate, it is necessary to add 0.45% or more. Furthermore, if the Si content is less than 0.45%, there is no particular problem with the resistance weld crack resistance property of the welded part, and there is little need to apply the present invention. On the other hand, if the Si content exceeds 3.0%, the hot rolling property and cold rolling property are greatly reduced, which adversely affects productivity and leads to a decrease in the ductility of the steel plate itself. Therefore, Si is added in the range of 0.45% or more and 3.0% or less. The Si amount is preferably 0.7% or more, more preferably 0.9% or more. Furthermore, the Si amount is preferably 2.5% or less, more preferably 2.0% or less.
Ｃ：０．３０％以下
Ｃは、鋼組織としてマルテンサイトなどを形成させることで鋼板の加工性が向上する。Ｃを含有させる場合、良好な溶接性、耐ＬＭＥ割れ性を得るため、Ｃ量は０．８％以下とすることが好ましく、０．３０％以下とすることがより好ましい。Ｃの下限は特に限定されないが、良好な加工性を得るためにはＣを０．０３％以上とすることが好ましく、０．０５％以上含有させることがより好ましい。 C: 0.30% or less C improves the workability of the steel sheet by forming martensite or the like as a steel structure. When C is contained, in order to obtain good weldability and LME cracking resistance, the C amount is preferably 0.8% or less, and more preferably 0.30% or less. There is no particular limit to the lower limit of C, but in order to obtain good workability, it is preferable to contain C at 0.03% or more, and more preferably 0.05% or more.
Ｍｎ：１．０％以上４．０％以下
Ｍｎは、鋼を固溶強化して高強度化するとともに、焼入性を高め、残留オーステナイト、ベイナイト、およびマルテンサイトの生成を促進する作用を有する元素である。このような効果は、Ｍｎを１．０％以上含有することで発現する。一方、Ｍｎ量が４．０％以下であれば、コストの上昇を招かずに上記効果が得られる。よって、Ｍｎ量は１．０％以上とすることが好ましく、４．０％以下とすることが好ましい。Ｍｎ量は１．８％以上とすることがより好ましい。また、Ｍｎ量は３．３％以下とすることがより好ましい。 Mn: 1.0% or more and 4.0% or less Mn is an element that strengthens steel by solid solution strengthening, improves hardenability, and promotes the formation of retained austenite, bainite, and martensite. Such effects are achieved by including 1.0% or more of Mn. On the other hand, if the Mn content is 4.0% or less, the above effects can be obtained without increasing costs. Therefore, the Mn content is preferably 1.0% or more, and more preferably 4.0% or less. The Mn content is more preferably 1.8% or more. Also, the Mn content is more preferably 3.3% or less.
以下の成分に関してはその含有率は限定されないが、好ましい範囲は下記のとおりである。The contents of the following components are not limited, but the preferred ranges are as follows.
Ｐ：０．１％以下（０％を含まない）
Ｐの含有量を抑制することで、溶接性の低下を防ぐことができる。さらにPが粒界に偏
析することを防いで、延性、曲げ性、および靭性が劣化することを防ぐことができる。また、Ｐを多量に添加すると、フェライト変態を促進することで結晶粒径も大きくなってしまう。そのため、Ｐ量は０．１％以下とすることが好ましい。Ｐの下限は特に限定されず、生産技術上の制約から０％超であり、通常０．００１％以上である。 P: 0.1% or less (excluding 0%)
By suppressing the P content, it is possible to prevent the deterioration of weldability. Furthermore, it is possible to prevent P from segregating at grain boundaries, thereby preventing the deterioration of ductility, bendability, and toughness. Furthermore, if a large amount of P is added, the grain size also becomes large by promoting ferrite transformation. Therefore, it is preferable that the P content is 0.1% or less. There is no particular limit on the lower limit of P, and it is more than 0% due to the constraints of production technology, and is usually 0.001% or more.
Ｓ：０．０３％以下（０％を含まない）
Ｓ量は０．０３％以下とすることが好ましく、０．０２％以下とすることがより好ましい。Ｓ量を抑制することで、溶接性の低下を防ぐとともに、熱間圧延時の延性の低下を防いで、熱間割れを抑制し、表面性状を著しく向上させることができる。さらに、Ｓ量を抑制することで、不純物元素として粗大な硫化物を形成することにより、鋼板の延性、曲げ性、伸びフランジ性の低下を防ぐことができる。これらの問題はＳ量が０．０３％を超えると顕著となり、Ｓの含有量は極力低減することが好ましい。Ｓの下限は特に限定されず、生産技術上の制約から０％超であり、通常０．００１％以上である。 S: 0.03% or less (excluding 0%)
The amount of S is preferably 0.03% or less, more preferably 0.02% or less. By suppressing the amount of S, it is possible to prevent a decrease in weldability and a decrease in ductility during hot rolling, suppress hot cracking, and significantly improve surface properties. Furthermore, by suppressing the amount of S, it is possible to prevent a decrease in the ductility, bendability, and stretch flangeability of the steel sheet by forming coarse sulfides as an impurity element. These problems become significant when the amount of S exceeds 0.03%, so it is preferable to reduce the content of S as much as possible. The lower limit of S is not particularly limited, and is more than 0% due to constraints on production technology, and is usually 0.001% or more.
Ａｌ：０．１％以下（０％を含まない）
Ａｌは熱力学的に最も酸化しやすいため、ＳｉおよびＭｎに先だって酸化し、ＳｉおよびＭｎの鋼板最表層での酸化を抑制し、ＳｉおよびＭｎの鋼板内部での酸化を促進する効果がある。この効果はＡｌ量が０．０１％以上で得られる。一方、Ａｌ量が０．１％を超えるとコストアップになる。したがって、添加する場合、Ａｌ量は０．１％以下とすることが好ましい。Ａｌの下限は特に限定されず、０％超であり、通常０．００１％以上である。 Al: 0.1% or less (excluding 0%)
Since Al is thermodynamically the most easily oxidized, it oxidizes before Si and Mn, and has the effect of suppressing the oxidation of Si and Mn at the outermost layer of the steel sheet and promoting the oxidation of Si and Mn inside the steel sheet. This effect is obtained when the Al content is 0.01% or more. On the other hand, if the Al content exceeds 0.1%, the cost increases. Therefore, when Al is added, the Al content is preferably 0.1% or less. There is no particular lower limit for Al, and it is more than 0%, and is usually 0.001% or more.
Ｎ：０．０１０％以下（０％を含まない）
Ｎの含有量は０．０１０％以下とすることが好ましい。Ｎの含有量を０．０１０％以下とすることで、ＮがＴｉ、Ｎｂ、Ｖと高温で粗大な窒化物を形成し、これにより、Ｔｉ、Ｎｂ、Ｖ添加による鋼板の高強度化の効果が損なわれることを防ぐことができる。また、Ｎの含有量を０．０１０％以下とすることで靭性の低下も防ぐことができる。さらに、Ｎの含有量を０．０１０％以下とすることで、熱間圧延中にスラブ割れ、表面疵が発生することを防ぐことができる。Ｎの含有量は、好ましくは０．００５％以下であり、より好ましくは０．００３％以下であり、さらに好ましくは０．００２％以下である。Ｎの含有量の下限は特に限定されず、生産技術上の制約から０％超であり、通常０．０００５％以上である。 N: 0.010% or less (excluding 0%)
The content of N is preferably 0.010% or less. By setting the content of N to 0.010% or less, N forms coarse nitrides with Ti, Nb, and V at high temperatures, which can prevent the effect of increasing the strength of the steel plate by adding Ti, Nb, and V from being impaired. In addition, by setting the content of N to 0.010% or less, it is possible to prevent a decrease in toughness. Furthermore, by setting the content of N to 0.010% or less, it is possible to prevent slab cracks and surface defects from occurring during hot rolling. The content of N is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. The lower limit of the content of N is not particularly limited, and is more than 0% due to constraints on production technology, and is usually 0.0005% or more.
成分組成はさらに、任意で、Ｂ：０．００５％以下、Ｔｉ：０．２％以下、Ｃｒ：１．０％以下、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下、Ｍｏ：１．０％以下、Ｎｂ：０．２０％以下、Ｖ：０．５％以下、Ｓｂ：０．２００％以下、Ｔａ：０．１％以下、Ｗ：０．５％以下、Ｚｒ：０．１％以下、Ｓｎ：０．２０％以下、Ｃａ：０．００５％以下、Ｍｇ：０．００５％以下およびＲＥＭ（Ｒａｒｅ Ｅａｒｔｈ Ｍｅｔａｌ）：０．００５％以下からなる群から選ばれる１種または２種以上を含有してもよい。The component composition may further optionally contain one or more selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.200% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM (Rare Earth Metal): 0.005% or less.
Ｂ：０．００５％以下
Ｂは鋼の焼入れ性を向上させるのに有効な元素である。焼入れ性を向上するためには、Ｂ量は０．０００３％以上とすることが好ましく、０．０００５％以上とすることがより好ましい。しかし、Ｂを過度に添加すると成形性が低下するため、Ｂ量は０．００５％以下とすることが好ましい。 B: 0.005% or less B is an element effective in improving the hardenability of steel. In order to improve the hardenability, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. However, since excessive addition of B reduces formability, the B content is preferably 0.005% or less.
Ｔｉ：０．２％以下
Ｔｉは鋼の析出強化に有効である。Ｔｉの下限は特に限定されないが、強度調整の効果を得るためには、０．００５％以上とすることが好ましい。しかし、Ｔｉを過度に添加すると、硬質相が過大となり、成形性が低下するため、Ｔｉを添加する場合、Ｔｉ量は０．２％以下とすることが好ましく、０．０５％以下とすることがより好ましい。 Ti: 0.2% or less Ti is effective for precipitation strengthening of steel. The lower limit of Ti is not particularly limited, but in order to obtain the effect of adjusting the strength, it is preferable to set it to 0.005% or more. However, if Ti is added excessively, the hard phase becomes excessively large and the formability decreases, so when Ti is added, the Ti amount is preferably 0.2% or less, and more preferably 0.05% or less.
Ｃｒ：１．０％以下
Ｃｒ量は０．００５％以上とすることが好ましい。Ｃｒ量を０．００５％以上とすることで、焼き入れ性が向上し、強度と延性とのバランスを向上させることができる。Ｃｒを添加する場合、コストアップを防ぐ観点から、Ｃｒ量は１．０％以下とすることが好ましい。 Cr: 1.0% or less The Cr content is preferably 0.005% or more. By making the Cr content 0.005% or more, the hardenability is improved, and the balance between strength and ductility can be improved. When Cr is added, the Cr content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.
Ｃｕ：１．０％以下
Ｃｕ量は０．００５％以上とすることが好ましい。Ｃｕ量を０．００５％以上とすることで、残留γ相の形成を促進することができる。また、Ｃｕを添加する場合、コストアップを防ぐ観点から、Ｃｕ量は１．０％以下とすることが好ましい。 Cu: 1.0% or less The Cu content is preferably 0.005% or more. By making the Cu content 0.005% or more, the formation of the residual γ phase can be promoted. In addition, when Cu is added, the Cu content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.
Ｎｉ：１．０％以下
Ｎｉ量は０．００５％以上とすることが好ましい。Ｎｉ量を０．００５％以上とすることで、残留γ相の形成を促進することができる。また、Ｎｉを添加する場合、コストアップを防ぐ観点から、Ｎｉ量は１．０％以下とすることが好ましい。 Ni: 1.0% or less The Ni content is preferably 0.005% or more. By making the Ni content 0.005% or more, the formation of the residual γ phase can be promoted. In addition, when Ni is added, the Ni content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.
Ｍｏ：１．０％以下
Ｍｏ量は０．００５％以上とすることが好ましい。Ｍｏ量を０．００５％以上とすることで、強度調整の効果を得ることができる。Ｍｏ量はより好ましくは０．０５％以上とする。また、Ｍｏを添加する場合、コストアップを防ぐ観点から、Ｍｏ量は１．０％以下が好ましい。 Mo: 1.0% or less The Mo content is preferably 0.005% or more. By making the Mo content 0.005% or more, the effect of adjusting the strength can be obtained. The Mo content is more preferably 0.05% or more. In addition, when Mo is added, the Mo content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.
Ｎｂ：０．２０％以下
Ｎｂは、０．００５％以上含有することで強度向上の効果が得られる。また、Ｎｂを含有する場合、コストアップを防ぐ観点から、Ｎｂ量は０．２０％以下とすることが好ましい。 Nb: 0.20% or less The effect of improving strength can be obtained by including 0.005% or more of Nb. Furthermore, when Nb is included, the Nb content is preferably 0.20% or less from the viewpoint of preventing an increase in costs.
Ｖ：０．５％以下
Ｖは、０．００５％以上含有することで強度向上の効果が得られる。また、Ｖを含有する場合、コストアップを防ぐ観点から、Ｖ量は０．５％以下とすることが好ましい。 V: 0.5% or less The effect of improving strength can be obtained by including 0.005% or more of V. Furthermore, when V is included, from the viewpoint of preventing an increase in costs, the V content is preferably 0.5% or less.
Ｓｂ：０．２００％以下
Ｓｂは鋼板表面の窒化、酸化、あるいは酸化により生じる鋼板表面から数十ミクロンの深さまでの領域の脱炭を抑制する観点から含有することができる。Ｓｂは、鋼板表面の窒化および酸化を抑制することで、鋼板表面においてマルテンサイトの生成量が減少するのを防止し、鋼板の疲労特性および表面品質を改善する。このような効果を得るために、Ｓｂ量は０．００１％以上とすることが好ましい。一方、良好な靭性を得るためには、Ｓｂ量は０．２００％以下とすることが好ましい。 Sb: 0.200% or less Sb can be contained from the viewpoint of suppressing nitridation and oxidation of the steel sheet surface, or decarburization of the region from the steel sheet surface to a depth of several tens of microns caused by oxidation. Sb suppresses the nitridation and oxidation of the steel sheet surface, thereby preventing a decrease in the amount of martensite formed on the steel sheet surface, and improving the fatigue properties and surface quality of the steel sheet. In order to obtain such an effect, the Sb content is preferably 0.001% or more. On the other hand, in order to obtain good toughness, the Sb content is preferably 0.200% or less.
Ｔａ：０．１％以下
Ｔａは、０．００１％以上含有することで強度向上の効果が得られる。また、Ｔａを含有する場合、コストアップを防ぐ観点から、Ｔａ量は０．１％以下とすることが好ましい。 Ta: 0.1% or less When Ta is contained in an amount of 0.001% or more, the effect of improving strength can be obtained. Furthermore, when Ta is contained, the Ta content is preferably 0.1% or less from the viewpoint of preventing an increase in costs.
Ｗ：０．５％以下
Ｗは、０．００５％以上含有することで強度向上の効果が得られる。また、Ｗを含有する場合、コストアップを防ぐ観点から、Ｗ量は０．５％以下とすることが好ましい。 W: 0.5% or less The effect of improving strength can be obtained by including 0.005% or more of W. Furthermore, when W is included, from the viewpoint of preventing an increase in costs, the W content is preferably 0.5% or less.
Ｚｒ：０．１％以下
Ｚｒは、０．０００５％以上含有することで強度向上の効果が得られる。また、Ｚｒを含有する場合、コストアップを防ぐ観点から、Ｚｒ量は０．１％以下とすることが好ましい。 Zr: 0.1% or less The effect of improving strength can be obtained by including 0.0005% or more of Zr. Furthermore, when Zr is included, the Zr content is preferably 0.1% or less from the viewpoint of preventing an increase in costs.
Ｓｎ：０．２０％以下
Ｓｎは脱窒、脱硼等を抑制して、鋼の強度低下抑制に有効な元素である。こうした効果を得るには０．００２％以上とすることが好ましい。一方、良好な耐衝撃性を得るために、Ｓｎ量は０．２０％以下とすることが好ましい。 Sn: 0.20% or less Sn is an element that is effective in suppressing denitrification, deboronization, etc., and suppressing the decrease in strength of steel. To obtain such effects, it is preferable to set the Sn content to 0.002% or more. On the other hand, in order to obtain good impact resistance, it is preferable to set the Sn content to 0.20% or less.
Ｃａ：０．００５％以下
Ｃａは、０．０００５％以上含有することで硫化物の形態を制御し、延性、靭性を向上させることができる。また、良好な延性を得る観点から、Ｃａ量は０．００５％以下とすることが好ましい。 Ca: 0.005% or less Ca can control the morphology of sulfides and improve ductility and toughness by containing 0.0005% or more of Ca. In addition, from the viewpoint of obtaining good ductility, the Ca content is preferably 0.005% or less.
Ｍｇ：０．００５％以下
Ｍｇは、０．０００５％以上含有することで硫化物の形態を制御し、延性、靭性を向上させることができる。また、Ｍｇを含有する場合、コストアップを防ぐ観点から、Ｍｇ量は０．００５％以下とすることが好ましい。 Mg: 0.005% or less By including 0.0005% or more of Mg, the form of sulfides can be controlled and ductility and toughness can be improved. In addition, when Mg is included, the Mg content is preferably 0.005% or less from the viewpoint of preventing an increase in costs.
ＲＥＭ：０．００５％以下
ＲＥＭは、０．０００５％以上含有することで硫化物の形態を制御し、延性、靭性を向上させることができる。また、ＲＥＭを含有する場合、良好な靭性を得る観点から、ＲＥＭ量は０．００５％以下とすることが好ましい。 REM: 0.005% or less When REM is contained in an amount of 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. In addition, when REM is contained, the REM content is preferably 0.005% or less from the viewpoint of obtaining good toughness.
本実施形態のＳｉ含有スラブは、上記成分以外の残部はＦｅおよび不可避的不純物である。ここで、Ｓｉ含有鋼板は、冷延鋼板、熱延鋼板のいずれでもよい。The Si-containing slab of this embodiment contains Fe and unavoidable impurities as the balance other than the above-mentioned components. Here, the Si-containing steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet.
＜熱間圧延＞
熱間圧延工程とは、前述のスラブを熱間圧延した後、下記式（１）から算出される温度ＴＣ（℃）以下の温度でコイルに巻き取り、酸洗する工程である。 <Hot rolling>
The hot rolling step is a step in which the above-mentioned slab is hot-rolled, then wound into a coil at a temperature equal to or lower than a temperature T C (° C.) calculated from the following formula (1), and pickled.
熱間圧延工程の技術的意義について説明する。通常の熱間圧延では、圧延が完了してコイルとして巻き取り後、冷却される過程において酸化スケールから酸素が鋼板の内方へ拡散するために、鋼板表面より内部にＳｉやＭｎの内部酸化物が形成される。 しかし、圧延後に形成されるＳｉやＭｎの内部酸化物は不均一に形成するため、その後のＣＧＬで溶融めっき処理を施した場合に、めっき密着性のムラや、合金化処理を行った後の合金化ムラなどの外観不良の原因となる。そのために、熱間圧延では、内部酸化の形成を抑制させることが重要である。ＳｉやＭｎの内部酸化物を抑制するためには、圧延後の巻き取り温度を低温化することが有効である。また、酸化物として形成するＳｉやＭｎの含有量が多い鋼を用いる場合には、巻取り温度をより低温化する必要がある
さらに調査を進めた結果、コイル長手中央部、かつ幅方向中央部での内部酸化量（熱延板の、スケール直下の鋼板表面から１０μｍ以内の鋼板表層部に生成したＳｉ内部酸化物及びＭｎ内部酸化物の合計。圧延後の巻き取りコイルの長手方向および幅方向の中央位置において酸素量として表したものを内部酸化量とする。）を０．１０ｇ／ｍ２以下に制御することで、ＳｉやＭｎの内部酸化がより均一化され、その後に溶融めっき処理を施してもめっき密着性のムラや、合金化処理後の外観ムラの発生をより抑制できることが分かった。ここで、ＳｉおよびＭｎの含有量を変化させた鋼を用いて、熱間圧延を施し、冷却した後に形成されたコイル長手方向中央部、かつ幅方向中央部での内部酸化量を調査した結果、下記式（１）から算出される温度ＴＣ（℃）以下の温度でコイルに巻き取ることで、熱間圧延工程で形成するＳｉ内部酸化物及びＭｎ内部酸化物の合計を、０．１０ｇ／ｍ２以下に制御することができる。 The technical significance of the hot rolling process will be explained. In normal hot rolling, after the rolling is completed and the steel sheet is wound into a coil, oxygen diffuses from the oxide scale toward the inside of the steel sheet during the cooling process, so that internal oxides of Si and Mn are formed inside the steel sheet surface. However, the internal oxides of Si and Mn formed after rolling are formed non-uniformly, and therefore, when hot-dip galvanizing is performed in the subsequent CGL, this causes poor appearance such as uneven plating adhesion and uneven alloying after alloying. For this reason, it is important to suppress the formation of internal oxides in hot rolling. In order to suppress the formation of internal oxides of Si and Mn, it is effective to lower the coiling temperature after rolling. Furthermore, when using steel with a high content of Si or Mn, which form as oxides, it is necessary to lower the coiling temperature. As a result of further investigation, it was found that by controlling the amount of internal oxidation at the longitudinal center and widthwise center of the coil (the total of Si internal oxides and Mn internal oxides generated in the steel sheet surface layer portion within 10 μm from the steel sheet surface directly below the scale of the hot-rolled sheet. The amount of internal oxidation is defined as the amount of oxygen at the longitudinal and widthwise central positions of the wound coil after rolling) to 0.10 g/ m2 or less, the internal oxidation of Si and Mn can be made more uniform, and the occurrence of uneven plating adhesion and uneven appearance after alloying treatment can be further suppressed even when a hot-dip plating treatment is subsequently performed. Here, using steels with different Si and Mn contents, the amount of internal oxidation formed in the longitudinal center and width center of the coil after hot rolling and cooling was investigated. As a result, it was found that the total amount of Si internal oxide and Mn internal oxide formed in the hot rolling process can be controlled to 0.10 g/ m2 or less by winding the steel into a coil at a temperature equal to or lower than T C (° C.), which is calculated from the following formula (1):
Ｔｃ＝－３０（［Ｓｉ］＋［Ｍｎ］）＋７７５ ・・・（１）
ここで、Ｔｃは圧延後の巻き取り温度、［Ｓｉ］、［Ｍｎ］はそれぞれ鋼中のＳｉ含有量、Ｍｎ含有量である。なお、Ｔｃは４００℃以上が好ましい。 Tc=-30([Si]+[Mn])+775... (1)
Here, Tc is the coiling temperature after rolling, [Si] and [Mn] are the Si content and Mn content in the steel, respectively. Note that Tc is preferably 400° C. or higher.
なお、熱間圧延前の加熱温度と熱間圧延の仕上げ温度は特に制限されるものではないが、組織制御の観点において、スラブを１１００～１３００℃に加熱、均熱し、８００～１０００℃で仕上げ圧延を完了することが好ましい。The heating temperature before hot rolling and the finishing temperature of hot rolling are not particularly limited, but from the viewpoint of controlling the structure, it is preferable to heat and soak the slab to 1100 to 1300°C, and then complete the finish rolling at 800 to 1000°C.
本発明では、以上の圧延後に、スケールを除去するために酸洗を行う。酸洗方法は特に限定されず、常法を採用すればよい。In the present invention, after the above rolling, pickling is carried out to remove scale. The pickling method is not particularly limited, and a conventional method may be adopted.
＜冷間圧延工程＞
冷間圧延工程とは、上記熱間圧延工程で得られた熱延板に対して、冷間圧延を施す工程である。冷間圧延の条件は特に限定されず、例えば、冷却された熱延板を、３０～８０％の所定の圧下率で冷間圧延すればよい。 <Cold rolling process>
The cold rolling process is a process of cold rolling the hot-rolled sheet obtained in the hot rolling process. The conditions of the cold rolling are not particularly limited, and for example, the cooled hot-rolled sheet may be cold-rolled at a predetermined rolling reduction of 30 to 80%.
＜焼鈍工程＞
本発明の焼鈍工程は、上記冷間圧延工程で得られた冷延板に対して、２つ以上に分離された区域を有する直火加熱炉を用いて鋼板を酸化する工程とラジアントチューブ型加熱炉や保持炉を用いて酸化した鋼板を還元する工程から成る。 <Annealing process>
The annealing process of the present invention comprises a step of oxidizing the cold-rolled steel sheet obtained in the above-mentioned cold rolling process using a direct-fire heating furnace having two or more separated zones, and a step of reducing the oxidized steel sheet using a radiant tube-type heating furnace or a holding furnace.
先ず、直火加熱炉(鋼板の酸化焼鈍工程)について説明する。First, the direct flame heating furnace (the oxidation annealing process for steel sheets) will be described.
鋼の高強度・高加工性を実現するためにＣ、ＳｉやＭｎを添加することが有効である。しかし、これらの元素を添加した鋼板を用いると、溶融亜鉛めっき処理を施す前に実施する焼鈍過程（酸化処理＋還元焼鈍）において、鋼板表面にＳｉ、Ｍｎの酸化物が生成し、めっき性を確保することが困難になる。そのために、ＳｉやＭｎを鋼板内部で酸化させ、鋼板表面でのこれらの元素の酸化を防ぐことが有効であるが、前述したように、本発明においてはめっき密着性や合金化のムラの観点から熱間圧延後に形成する内部酸化を抑制することが必須である。このように熱間圧延後に内部酸化の形成が少ない場合においても、溶融亜鉛めっき処理を施す前の焼鈍条件（酸化処理条件＋還元焼鈍条件）を厳密に制御することで、ＳｉおよびＭｎを鋼板内部で酸化させ、めっき性を向上させ、更にはめっきと鋼板の反応性を高めることができ、めっき密着性を改善することができる。そして、焼鈍工程において、ＳｉおよびＭｎを鋼板内部で酸化させ、鋼板表面での酸化を防ぐために、酸化処理を行う。特に、酸化処理で一定量以上の鉄酸化物量を得ることが必要である。その後、還元焼鈍、溶融めっきおよび必要に応じて合金化処理を行うことが有効である。In order to realize high strength and high workability of steel, it is effective to add C, Si, and Mn. However, when using a steel sheet to which these elements have been added, oxides of Si and Mn are generated on the surface of the steel sheet in the annealing process (oxidation treatment + reduction annealing) performed before hot-dip galvanizing, making it difficult to ensure galvanizing properties. For this reason, it is effective to oxidize Si and Mn inside the steel sheet to prevent oxidation of these elements on the surface of the steel sheet, but as described above, in the present invention, it is essential to suppress internal oxidation formed after hot rolling from the viewpoint of plating adhesion and uneven alloying. Even in this case where the formation of internal oxidation is small after hot-dip rolling, by strictly controlling the annealing conditions (oxidation treatment conditions + reduction annealing conditions) before hot-dip galvanizing, Si and Mn can be oxidized inside the steel sheet, improving galvanizing properties, and further increasing the reactivity of the plating with the steel sheet, thereby improving plating adhesion. Then, in the annealing process, an oxidation treatment is performed to oxidize Si and Mn inside the steel sheet and prevent oxidation on the surface of the steel sheet. In particular, it is necessary to obtain a certain amount of iron oxide or more in the oxidation treatment. Thereafter, it is effective to carry out reduction annealing, hot dip plating, and, if necessary, alloying treatment.
十分な量の鉄酸化物を得るためには、加熱する雰囲気と温度を管理することが必要となる。雰囲気の制御については直火型加熱炉の空気比を制御することで行う。直火型加熱炉は、製鉄所の副生ガスであるコークス炉ガス（ＣＯＧ）等の燃料と空気を混ぜて燃焼させたバーナー火炎を直接鋼板表面に当てて鋼板を加熱するものである。空気比を高くし、燃料に対する空気の割合を多くすると、未反応の酸素が火炎中に残存し、その酸素で鋼板の酸化を促進することが可能となる。ここで、直火型加熱炉の燃料としてはコークス炉ガスの他に、天然ガス、水素ガス、アンモニアガスなどを用いても良い。これらの燃料が燃焼した際に発生する酸化生成物としてはＣＯ、ＣＯ２、Ｈ２Ｏ、ＮＯＸ等がある。また、雰囲気中には燃焼用空気中のＮ２も存在する。 In order to obtain a sufficient amount of iron oxide, it is necessary to control the atmosphere and temperature for heating. The atmosphere is controlled by controlling the air ratio of the direct-fired heating furnace. The direct-fired heating furnace heats the steel sheet by directly applying a burner flame, which is a mixture of fuel such as coke oven gas (COG), a by-product gas from a steel mill, and air, to the surface of the steel sheet. If the air ratio is increased and the ratio of air to fuel is increased, unreacted oxygen remains in the flame, and the oxygen can promote the oxidation of the steel sheet. Here, in addition to coke oven gas, natural gas, hydrogen gas, ammonia gas, etc. may be used as fuel for the direct-fired heating furnace. Oxidation products generated when these fuels are burned include CO, CO 2 , H 2 O, NOX , etc. In addition, N 2 in the combustion air is also present in the atmosphere.
一方、鋼板を酸化しすぎると、続く還元焼鈍工程で、酸化物が剥離し、ロールに付着するピックアップという現象を引き起こす。ロールにピックアップが生じると、亜鉛めっき鋼板の外観を大きく阻害してしまう。そこで、直火加熱炉を用いて鋼板を酸化する工程は、２つ以上に分離された区域を有し、２つ以上の異なる雰囲気で加熱することが必要となる。次に、加熱帯前段、加熱帯後段について説明する。On the other hand, if the steel sheet is oxidized too much, the oxide peels off and adheres to the rolls in the subsequent reduction annealing process, causing a phenomenon called pickup. If pickup occurs on the rolls, it significantly impairs the appearance of the galvanized steel sheet. Therefore, the process of oxidizing the steel sheet using a direct-fire heating furnace needs to have two or more separated zones and to heat the steel sheet in two or more different atmospheres. Next, the front heating zone and the rear heating zone will be described.
加熱帯前段
Ｏ２を１０００体積ｐｐｍ以上、Ｈ２Ｏを１０００体積ｐｐｍ以上含む雰囲気中で鋼板を４００℃以上６７０℃以下まで加熱
加熱帯前段では、Ｏ２を１０００体積ｐｐｍ以上、Ｈ２Ｏを１０００体積ｐｐｍ以上の雰囲気になるよう空気比を調整し、上記冷延鋼板を加熱する。ここで、Ｏ２が１０００体積ｐｐｍ以下、Ｈ２Ｏが１０００体積ｐｐｍ以下であると、鋼板の酸化が不十分となる。一方、Ｏ２が１０００体積ｐｐｍ未満、Ｈ２Ｏが１０００体積ｐｐｍ未満では、鋼板の酸化へのＯ２、Ｈ２Ｏ濃度の影響が小さく、鋼板の温度の影響が大きくなるため、上限は特に設けない。好ましくは、設備の劣化の観点から、Ｏ２は１００００体積ｐｐｍ以下、Ｈ２Ｏが１００００体積ｐｐｍ以下であることが好ましい。鋼板の温度が４００℃以上、６７０℃以下の範囲になるように加熱する。鋼板の温度が４００℃未満であると、鋼板の酸化が不十分であり、６７０℃を超えると鋼板の酸化が過剰になり、前述のロールへのピックアップが生じてしまう。そのため、本発明では鋼板の温度が４００℃以上、６７０℃以下の範囲になるように加熱することが必須条件となる。 Heating zone front stage The steel sheet is heated to 400 ° C. or more and 670 ° C. or less in an atmosphere containing 1000 volume ppm or more of O2 and 1000 volume ppm or more of H2O . In the heating zone front stage, the air ratio is adjusted so that the atmosphere contains 1000 volume ppm or more of O2 and 1000 volume ppm or more of H2O , and the cold-rolled steel sheet is heated. Here, if O2 is 1000 volume ppm or less and H2O is 1000 volume ppm or less, the oxidation of the steel sheet is insufficient. On the other hand, if O2 is less than 1000 volume ppm and H2O is less than 1000 volume ppm, the effect of O2 and H2O concentrations on the oxidation of the steel sheet is small, and the effect of the temperature of the steel sheet is large, so no upper limit is particularly set. From the viewpoint of equipment deterioration, it is preferable that O2 is 10,000 volume ppm or less and H2O is 10,000 volume ppm or less. The steel sheet is heated so that its temperature is in the range of 400°C or more and 670°C or less. If the temperature of the steel sheet is less than 400°C, the oxidation of the steel sheet is insufficient, and if it exceeds 670°C, the oxidation of the steel sheet becomes excessive, causing the above-mentioned pickup to the roll. Therefore, in the present invention, it is an essential condition to heat the steel sheet so that its temperature is in the range of 400°C or more and 670°C or less.
加熱帯後段
Ｏ２を５００体積ｐｐｍ以下含む雰囲気中で鋼板を６００℃以上７００℃以下まで加熱
加熱後段は、前述のロールピックアップを抑制し、押し疵などのない美麗な表面外観を得るために本発明において重要な要件である。ピックアップ現象の発生を防止するためには、一旦酸化された鋼板表面の一部（表層）を還元処理することが重要である。このような還元処理を行うには、加熱帯後段では、Ｏ２を５００体積ｐｐｍ以下の雰囲気になるよう空気比を調整し、加熱帯前段を通過した鋼板を加熱する。ここで、Ｏ２が５００体積ｐｐｍを超えると、鋼板の酸化が過剰になり、前述のロールへのピックアップが生じてしまう。鋼板の温度が６００℃以上、７００℃以下の範囲になるように加熱する。鋼板の温度が６００℃未満であると、鋼板表面の一部（表層）の還元が不十分であり、７００℃を超えると鋼板表面の一部（表層）が還元せず、酸化が促進され、前述のロールへのピックアップが生じてしまうことがある。そのため、本発明では鋼板の温度が６００℃以上、７００℃以下の範囲になるように加熱することが必須条件となる。 Heating zone rear stage: Heat the steel sheet to 600°C or more and 700°C or less in an atmosphere containing 500 ppm or less by volume of O2 . The heating zone rear stage is an important requirement in the present invention in order to suppress the above-mentioned roll pickup and obtain a beautiful surface appearance without indentations. In order to prevent the occurrence of the pickup phenomenon, it is important to reduce a part of the steel sheet surface (surface layer) that has been oxidized once. To perform such reduction treatment, the air ratio is adjusted in the heating zone rear stage so that the atmosphere contains 500 ppm or less by volume of O2 , and the steel sheet that has passed through the heating zone front stage is heated. Here, if O2 exceeds 500 ppm by volume, the oxidation of the steel sheet becomes excessive, and the above-mentioned pickup to the roll occurs. The steel sheet is heated so that the temperature is in the range of 600°C or more and 700°C or less. If the temperature of the steel sheet is less than 600°C, the reduction of a part of the steel sheet surface (surface layer) is insufficient, and if it exceeds 700°C, a part of the steel sheet surface (surface layer) is not reduced, oxidation is promoted, and the above-mentioned pickup to the roll may occur. Therefore, in the present invention, it is an essential condition that the steel sheet is heated to a temperature in the range of 600°C or more and 700°C or less.
次に、ラジアントチューブ型加熱炉や保持炉(鋼板の還元焼鈍工程)について説明する。Next, the radiant tube type heating furnace and holding furnace (reduction annealing process for steel sheets) will be described.
これまで述べたように、鋼の高強度・高加工性を実現するためにＣ、ＳｉやＭｎを添加することが有効である。しかし、特にＣやＳｉを多く添加した鋼板を用いると、めっき層の亜鉛が溶融して結晶粒界に拡散侵入することで、ＬＭＥが起き、鋼板に粒界割れ（ＬＭＥ割れ）が生じてしまうことが懸念される。さらに、鋼材の強度の増加に伴い、 水素脆
化に起因する遅れ破壊が生じやすくなることも知られている。このような遅れ破壊については、使用環境によって生じる腐食が原因で、鋼板に侵入した水素によって生じることが多いが、ＣＧＬの焼鈍工程で鋼板に侵入した水素も、 特に引張強度が９８０ＰＭａを超
える鋼板の耐遅れ破壊特性の劣化を引き起こす。 As described above, it is effective to add C, Si, or Mn to achieve high strength and high workability of steel. However, when using steel sheets with a large amount of C or Si added, zinc in the coating layer melts and diffuses into the grain boundaries, which may cause LME and grain boundary cracking (LME cracking) in the steel sheet. Furthermore, it is known that delayed fracture due to hydrogen embrittlement is more likely to occur as the strength of steel increases. Such delayed fracture is often caused by hydrogen that penetrates into the steel sheet due to corrosion caused by the usage environment, but hydrogen that penetrates into the steel sheet during the annealing process of CGL also causes deterioration of the delayed fracture resistance, especially in steel sheets with a tensile strength of more than 980 PMa.
これらの課題を解決するためには、溶融亜鉛めっき処理を施す前に実施する焼鈍過程（酸化処理＋還元焼鈍）のうち、還元焼鈍の雰囲気を制御することが重要である。このメカニズムは明らかではないが、還元焼鈍の雰囲気を制御することにより、形成するＳｉやＭｎの内部酸化層の周囲の固溶ＳｉやＭｎが減少する。更にＣは雰囲気中のＨ２Ｏによって、酸化され、ＣＯガスとして炉内に放出されるため、鋼板表層のＣ濃度が低下する。結果としてＬＭＥ割れの原因となる固溶ＣとＳｉが欠乏した領域が表層に形成されるため、結果としてＬＭＥ割れが生じにくいものと考えられる。また、鋼板に侵入した水素については、同様にＳｉやＭｎの内部酸化層が鋼板表層に存在することで、めっき層と下地鋼を合金化する際、鋼板表層に形成したＳｉやＭｎの内部酸化物がめっき層中に分散し、これによって、製造後の鋼板からの脱水素が促進されることで、良好な耐遅れ破壊特性が得られるものと考えられる。 In order to solve these problems, it is important to control the atmosphere of the reduction annealing in the annealing process (oxidation treatment + reduction annealing) performed before the hot-dip galvanizing treatment. Although the mechanism is not clear, by controlling the atmosphere of the reduction annealing, the amount of solute Si and Mn around the internal oxide layer of Si and Mn formed is reduced. Furthermore, C is oxidized by H 2 O in the atmosphere and released into the furnace as CO gas, so the C concentration in the surface layer of the steel sheet is reduced. As a result, a region lacking in solute C and Si, which causes LME cracking, is formed in the surface layer, and it is considered that LME cracking is unlikely to occur as a result. In addition, as for hydrogen that has entered the steel sheet, it is similarly considered that the internal oxide layer of Si and Mn exists in the surface layer of the steel sheet, and when the coating layer and the base steel are alloyed, the internal oxides of Si and Mn formed in the surface layer of the steel sheet are dispersed in the coating layer, which promotes dehydrogenation from the steel sheet after production, thereby obtaining good delayed fracture resistance.
還元焼鈍には、ラジアントチューブ型の加熱や保持を用いることができる。この時、雰囲気のＨ２Ｏ濃度を５０００体積ｐｐｍ以上、４００００体積ｐｐｍ以下に制御することで、ＬＭＥ割れを抑止し、脱水素を促進することができる。Ｈ２Ｏ濃度が５０００体積ｐｐｍ未満であると、耐ＬＭＥ割れ性や、脱水素促進効果が十分とは言えない。一方、Ｈ２Ｏ濃度が４００００体積ｐｐｍを超えると、設備ダメージが懸念されるため、４００００体積ｐｐｍ以下であることが好ましい。ここで、炉内の上部と下部のＨ２Ｏ濃度の差は２０００体積ｐｐｍ以下である必要がある。炉内の上部と下部のＨ２Ｏ濃度の差が２０００体積ｐｐｍを超えると、鋼中のＳｉやＭｎが内部酸化されずに、外部に酸化し、めっき性を阻害し、不めっき欠陥を生じることがある。また、十分な内部酸化層が形成されず、耐ＬＭＥ割れ性や、脱水素促進効果が十分ではない場合がある。 For reduction annealing, radiant tube type heating and holding can be used. At this time, by controlling the H 2 O concentration of the atmosphere to 5000 volume ppm or more and 40000 volume ppm or less, LME cracking can be suppressed and dehydrogenation can be promoted. If the H 2 O concentration is less than 5000 volume ppm, it cannot be said that the LME cracking resistance and dehydrogenation promotion effect are sufficient. On the other hand, if the H 2 O concentration exceeds 40000 volume ppm, equipment damage is a concern, so it is preferable that the H 2 O concentration is 40000 volume ppm or less. Here, the difference in H 2 O concentration between the upper and lower parts of the furnace needs to be 2000 volume ppm or less. If the difference in H 2 O concentration between the upper and lower parts of the furnace exceeds 2000 volume ppm, Si and Mn in the steel are not internally oxidized but are oxidized to the outside, which may hinder plating properties and cause non-plating defects. In addition, a sufficient internal oxide layer may not be formed, and the LME cracking resistance and dehydrogenation promotion effect may be insufficient.
内部酸化層の形成には、還元焼鈍時のＨ２濃度も大きく影響する。Ｈ２濃度は２体積％以上～２０体積％以下である必要がある。また、Ｈ２Ｏの分圧（ＰＨ2Ｏ）とＨ２の分圧
（ＰＨ2）の比が下記式（２）を満たす必要がある。Ｈ２濃度２体積％未満であると、酸
化した鋼板の還元が不十分で、溶融亜鉛めっきをした際に不めっき欠陥が生じたり、めっき密着性を阻害したりすることがある。一方、水素濃度が２０体積％を超えると、鋼板に水素が多く残存し、脱水素が促進されていても、鋼中に残存する水素が多くなり、良好な耐遅れ破壊特性が得られないことがある。内部酸化層の形成に対しては、Ｈ２Ｏの分圧（ＰＨ2Ｏ）とＨ２の分圧（ＰＨ2）の比が影響する。良好な耐ＬＭＥ割れ性や、脱水素促進効果を得るためには、ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）が－１．１以上、０．５以下である必要
がある。ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）が－１．１未満であると、十分な内部酸化層を形成せ
ず、良好な耐ＬＭＥ割れ性や、脱水素促進効果が得られないことがある。一方、ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）が０．５を超えると、設備ダメージが懸念されるため、ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）は０．５以下であることが好ましい。 The formation of the internal oxide layer is also greatly affected by the H2 concentration during reduction annealing. The H2 concentration must be 2 vol.% or more to 20 vol.% or less. In addition, the ratio of the partial pressure of H2O (P H2O ) to the partial pressure of H2 (P H2 ) must satisfy the following formula (2). If the H2 concentration is less than 2 vol.%, the reduction of the oxidized steel sheet is insufficient, which may cause non-coating defects or inhibit coating adhesion when hot-dip galvanizing is performed. On the other hand, if the hydrogen concentration exceeds 20 vol.%, a large amount of hydrogen remains in the steel sheet, and even if dehydrogenation is promoted, the amount of hydrogen remaining in the steel increases, and good delayed fracture resistance properties may not be obtained. The ratio of the partial pressure of H2O (P H2O ) to the partial pressure of H2 (P H2 ) affects the formation of the internal oxide layer. In order to obtain good LME cracking resistance and dehydrogenation promotion effect, log(P H2O /P H2 ) needs to be -1.1 or more and 0.5 or less. If log(P H2O /P H2 ) is less than -1.1, a sufficient internal oxide layer is not formed, and good LME cracking resistance and dehydrogenation promotion effect may not be obtained. On the other hand, if log(P H2O /P H2 ) exceeds 0.5, there is a concern of equipment damage, so log(P H2O /P H2 ) is preferably 0.5 or less.
更に、高強度鋼板の成形性に必要な曲げ性に対しても、ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）を高
めることが有効であることが分かった。このメカニズムは明らかでは無いが、鋼板中の水素が低下することによる成形性向上効果と、内部酸化層が存在することで表層に比較的成形性が良好な層が存在することにより歪分散能が変化することによるものと考えられる。ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）を－１．１以上とすることで曲げ性も向上するが、ｌｏｇ（Ｐ
Ｈ2Ｏ/ＰＨ2）を－０．９９以上とすることで更に曲げ性が向上し、－０．９０以上とし
ても良く、－０．７以上とすることで更に向上することができる。なお、いずれの場合も、ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）の上限は、０．５以下が好ましい。 Furthermore, it was found that increasing log(P H2O /P H2 ) is also effective for the bendability required for the formability of high-strength steel plate. Although the mechanism behind this is not clear, it is thought to be due to the formability-improving effect of reducing hydrogen in the steel plate, and the presence of an internal oxide layer, which creates a surface layer with relatively good formability, changing the strain dispersion ability. Bendability is also improved by increasing log(P H2O /P H2 ) to -1.1 or more, but
The bendability can be further improved by making log (P H2O /P H2 ) -0.99 or more, and can be further improved by making it -0.90 or more, and by making it -0.7 or more. In any case, the upper limit of log(P H2O /P H2 ) is preferably 0.5 or less.
なお、還元焼鈍雰囲気は、Ｈ２ＯとＨ２以外については、コストの観点からＮ２を使用することが好ましい。その他、ＮＯＸやＳＯＸ、ＣＯ、ＣＯ２などの混入があり得る。 From the viewpoint of cost, it is preferable to use N2 in the reducing annealing atmosphere other than H2O and H2 . Other contaminants such as NOx , SOx , CO, and CO2 may be mixed in.
還元焼鈍の温度は６５０℃以上、９００℃以下であることが必要である。６５０℃未満であると、耐ＬＭＥ割れ性の向上や、脱水素促進に必要な、内部酸化層の形成が不十分となる場合がある。また、９００℃を超えると、焼鈍炉の炉体へのダメージが懸念されるため、９００℃以下であることが好ましい。The reduction annealing temperature must be 650° C. or higher and 900° C. or lower. If the temperature is lower than 650° C., the formation of an internal oxide layer necessary for improving LME cracking resistance and promoting dehydrogenation may be insufficient. If the temperature exceeds 900° C., damage to the furnace body of the annealing furnace may occur, so the temperature is preferably 900° C. or lower.
上記で説明した還元雰囲気は、炉内の一部又は全部が満たしていれば良い。一部が、上記で説明した雰囲気を満たす場合は、指定した雰囲気で焼鈍される時間が９０秒以上必要である。９０秒以上、指定された雰囲気で焼鈍されていれば、還元焼鈍の雰囲気は炉内の全部が制御されていなくても構わない。The reducing atmosphere described above may be filled in part or all of the furnace. If only a part of the furnace is filled with the atmosphere described above, the time for annealing in the specified atmosphere must be 90 seconds or more. If annealing is performed in the specified atmosphere for 90 seconds or more, the atmosphere for the reducing annealing does not need to be controlled in the entire furnace.
＜冷却加熱工程＞
冷却加熱工程とは、還元焼鈍の後に、平均冷却速度が１０℃／秒以上の条件で、還元焼鈍での最終保持温度から１５０～３５０℃の冷却到達温度まで冷却した後、３５０～６００℃の再加熱温度まで加熱し、該温度で１０～６００秒保持する工程である。この冷却加熱工程を行うこと、機械特性をさらに高めることができる。なお、本発明において、冷却加熱工程は必須の工程ではないため、必要に応じて行えばよい。 <Cooling and heating process>
The cooling and heating step is a step in which, after reduction annealing, the steel is cooled from the final holding temperature in reduction annealing to a cooling end temperature of 150 to 350°C under conditions of an average cooling rate of 10°C/sec or more, and then heated to a reheating temperature of 350 to 600°C and held at that temperature for 10 to 600 seconds. By carrying out this cooling and heating step, the mechanical properties can be further improved. Note that in the present invention, the cooling and heating step is not an essential step, and may be carried out as necessary.
還元焼鈍での最終保持温度からの冷却速度が１０℃／秒未満ではパーライトが生成し、ＴＳ×ＥＬおよび穴拡げ性が低下する。従って、還元焼鈍での最終保持温度からの冷却速度は１０℃／秒以上が好ましい。ここで、還元焼鈍おける最終保持温度は、前記還元焼鈍の焼鈍温度、水素濃度、露点、保持時間の要件を満たす範囲で焼鈍を行った鋼板が前記要件の少なくとも一つを外れる時の温度を指す。If the cooling rate from the final holding temperature in reduction annealing is less than 10° C./sec, pearlite is generated, and TS×EL and hole expandability are reduced. Therefore, the cooling rate from the final holding temperature in reduction annealing is preferably 10° C./sec or more. Here, the final holding temperature in reduction annealing refers to the temperature at which the steel sheet annealed within the range satisfying the requirements of the annealing temperature, hydrogen concentration, dew point, and holding time of the reduction annealing falls outside at least one of the requirements.
冷却到達温度が６００℃より高い温度では、続く、溶融めっき工程において、めっき浴の温度が上昇し、表面外観品質を阻害するドロスの発生を促進してしまうことがある。従って、冷却到達温度は６００℃以下が好ましい。
冷却到達温度を３５０℃以下とすることで機械特性を高めることが出来る。また、冷却到達温度が１５０℃より低くなると、冷却中にオーステナイトがほとんどマルテンサイトに変態し未変態オーステナイト量が減少する。従って冷却到達温度は１５０～３５０℃の範囲であることが好ましい。冷却の方法については、目標の冷却速度と冷却停止温度（冷却到達温度）が達成できれば、ガスジェット冷却、ミスト冷却、水冷、メタルクエンチ等のいかなる冷却方法を用いてもよい。 If the cooling temperature is higher than 600° C., the temperature of the plating bath will rise in the subsequent hot dip plating step, which may promote the generation of dross that impairs the surface appearance quality. Therefore, the cooling temperature is preferably 600° C. or lower.
By setting the ultimate cooling temperature at 350°C or less, it is possible to improve mechanical properties. Furthermore, if the ultimate cooling temperature is lower than 150°C, most of the austenite is transformed into martensite during cooling, and the amount of untransformed austenite decreases. Therefore, it is preferable that the ultimate cooling temperature is in the range of 150 to 350°C. Regarding the cooling method, any cooling method such as gas jet cooling, mist cooling, water cooling, metal quenching, etc. may be used as long as the target cooling rate and cooling stop temperature (ultimate cooling temperature) can be achieved.
ここで、冷却到達温度までの冷却後、場合によっては、再加熱温度まで加熱し、１０秒以上保持しても良い。１０秒以上保持することで、冷却時に生成したマルテンサイトが焼戻され焼戻しマルテンサイトとなる。その結果、穴拡げ性が向上し、さらに冷却時にマルテンサイトに変態しなかった未変態オーステナイトが安定化され、最終的に十分な量の残留オーステナイトが得られ、延性が向上する場合がある。Here, after cooling to the cooling temperature, in some cases, it may be heated to the reheating temperature and held for 10 seconds or more. By holding for 10 seconds or more, the martensite generated during cooling is tempered to become tempered martensite. As a result, the hole expandability is improved, and the untransformed austenite that was not transformed into martensite during cooling is stabilized, and finally a sufficient amount of retained austenite is obtained, and ductility may be improved.
また、再加熱する場合は、再加熱温度が６００℃を超えると、冷却停止時の未変態オーステナイトがパーライトに変態し、最終的に面積率で３％以上残留オーステナイトが得られなくなる。再加熱時の保持時間が１０秒未満ではオーステナイトの安定化が不十分となり、また６００秒を超えると冷却停止時の未変態オーステナイトがベイナイトに変態し、最終的に十分な量の残留オーステナイトが得られなくなる。従って、再加熱する場合の温度は３５０～６００℃の範囲とし、その温度域での保持時間は１０～６００秒とする。Furthermore, in the case of reheating, if the reheating temperature exceeds 600°C, the untransformed austenite at the time of cooling being stopped will be transformed into pearlite, and ultimately, retained austenite of 3% or more in area ratio will not be obtained. If the holding time during reheating is less than 10 seconds, the stabilization of austenite will be insufficient, and if it exceeds 600 seconds, the untransformed austenite at the time of cooling being stopped will be transformed into bainite, and ultimately, a sufficient amount of retained austenite will not be obtained. Therefore, the temperature during reheating is set to a range of 350 to 600°C, and the holding time in that temperature range is set to 10 to 600 seconds.
＜溶融亜鉛めっき処理工程＞
鋼板に、溶融亜鉛めっきを施した後、合金化処理を行っても良い。溶融亜鉛めっき処理工程とは、焼鈍工程後の焼鈍板に対して、０．１２～０．２２質量％のＡｌを含有した溶融亜鉛めっき浴で溶融亜鉛めっき処理を施す工程である。 <Hot-dip galvanizing process>
The steel sheet may be subjected to hot-dip galvanization and then alloyed. The hot-dip galvanization process is a process in which the annealed sheet after the annealing process is subjected to hot-dip galvanization in a hot-dip galvanization bath containing 0.12 to 0.22 mass % Al.
本発明では、亜鉛めっき浴中のＡｌ濃度を０．１２～０．２２質量％とする。０．１２質量％未満ではめっき時にＦｅ－Ｚｎ合金相が形成し、めっき密着性が劣化したり、外観のムラが発生したりすることがある。０．２２質量％超えでは、めっき時にめっき／地鉄界面に生成するＦｅ－Ａｌ合金相が厚く生成するため、溶接性が劣化する。また、浴中Ａｌが多いために、めっき鋼板表面にＡｌ酸化皮膜が多量に生成し、溶接性だけでなく外観性も損なわれる場合がある。In the present invention, the Al concentration in the zinc plating bath is set to 0.12 to 0.22 mass%. If it is less than 0.12 mass%, an Fe-Zn alloy phase is formed during plating, which may deteriorate the plating adhesion and cause uneven appearance. If it exceeds 0.22 mass%, a thick Fe-Al alloy phase is formed at the plating/base steel interface during plating, which deteriorates weldability. In addition, since the bath contains a large amount of Al, a large amount of Al oxide film is formed on the surface of the plated steel sheet, which may impair not only weldability but also appearance.
合金化処理を行う場合のめっき浴中Ａｌ濃度は０．１２～０．１７質量％が好ましい。０．１２質量％未満ではめっき時にＦｅ－Ｚｎ合金相が形成し、めっき密着性が劣化したり、外観のムラが発生したりすることがある。０．１７質量％超えでは、めっき時にめっき／地鉄界面に生成するＦｅ－Ａｌ合金相が厚く生成し、Ｆｅ－Ｚｎ合金化反応の障壁となるために合金化温度が高温化し、機械特性が劣化する場合がある。The Al concentration in the plating bath when alloying is preferably 0.12 to 0.17 mass%. If it is less than 0.12 mass%, an Fe-Zn alloy phase is formed during plating, which may deteriorate the plating adhesion or cause uneven appearance. If it exceeds 0.17 mass%, a thick Fe-Al alloy phase is formed at the plating/base steel interface during plating, which acts as a barrier to the Fe-Zn alloying reaction, and the alloying temperature increases, which may cause deterioration of mechanical properties.
溶融亜鉛めっき時のその他の条件は制限されるものではないが、例えば、溶融亜鉛めっき浴温度は通常の４４０～５００℃の範囲で、板温４４０～５５０℃で鋼板をめっき浴中に浸入させて行い、ガスワイピングなどで付着量を調整することが出来る。Other conditions during hot-dip galvanization are not limited, but for example, the hot-dip galvanization bath temperature is usually in the range of 440 to 500° C., and the steel sheet is immersed in the plating bath at a sheet temperature of 440 to 550° C., and the coating amount can be adjusted by gas wiping or the like.
＜合金化処理工程＞
合金化処理工程とは、溶融亜鉛めっき処理工程後の鋼板に対して、温度４５０～５５０℃の範囲で１０～６０秒間の合金化処理を施す工程である。 <Alloying Treatment Step>
The alloying treatment step is a step in which the steel sheet after the hot-dip galvanizing treatment step is subjected to an alloying treatment at a temperature in the range of 450 to 550° C. for 10 to 60 seconds.
合金化処理後の合金化度（めっき層内のＦｅ濃度）は特に制限されるものではないが、７～１５質量％の合金化度が好ましい。７質量％未満ではη相が残存してプレス成形性に劣り、１５質量％を超えるとめっき密着性に劣る。Although there is no particular limitation on the degree of alloying after the alloying treatment (Fe concentration in the plating layer), a degree of alloying of 7 to 15 mass% is preferable. If it is less than 7 mass%, the η phase remains and press formability is poor, and if it exceeds 15 mass%, plating adhesion is poor.
表１に示す化学成分の鋼を溶製した後に、連続鋳造によってスラブとした。
これらのスラブを１２００℃で加熱した後に、仕上げ温度８９０℃で板厚２．６ｍｍとなるように熱間圧延を施し、表２に示す巻き取り温度でコイルとして巻き取り、冷却した後に酸洗によって黒皮スケールを除去して、熱延板とした。この時のコイル長手方向かつ幅方向の中央部のＳｉおよび／またはＭｎの内部酸化量を下記に示す方法で測定した。 Steel having the chemical composition shown in Table 1 was melted and then formed into a slab by continuous casting.
These slabs were heated at 1200°C, and then hot-rolled to a thickness of 2.6 mm at a finishing temperature of 890°C, coiled at the coiling temperature shown in Table 2, cooled, and then pickled to remove black scale to obtain hot-rolled sheets. The amount of internal oxidation of Si and/or Mn in the center of the coil in the longitudinal and transverse directions was measured by the method described below.
内部酸化量は、「インパルス炉溶融―赤外線吸収法」により測定する。熱延板両面の表層部（コイルの中央（幅方向中央かつ長手方向中央））の１０ｍｍ×７０ｍｍの領域を１０μｍ研磨する前と後でのそれぞれの鋼中酸素濃度を測定した。更に、それらの測定値の差から、鋼板表面から１０μｍの領域に存在する片面単位面積当たりの酸素量を求め、Ｓｉおよび／またはＭｎの内部酸化量（ｇ／ｍ２）とした。熱延板の表層部に形成した内部酸化物が、Ｓｉおよび／またはＭｎの酸化物であることは、熱延板を樹脂に埋め込み断面を研磨した後に、ＳＥＭによる観察およびＥＤＳ（エネルギー分散型Ｘ線分光器）による元素分析によって確認した。内部酸化量を表３に示した。
The amount of internal oxidation is measured by the "Impulse Furnace Melting-Infrared Absorption Method". The oxygen concentration in the steel was measured before and after polishing 10 μm of a 10 mm×70 mm area of the surface layer (the center of the coil (the center in the width direction and the center in the length direction)) on both sides of the hot-rolled sheet. Furthermore, the amount of oxygen per unit area on one side present in a region 10 μm from the steel sheet surface was calculated from the difference between these measured values, and this was taken as the amount of internal oxidation of Si and/or Mn (g/m 2 ). The fact that the internal oxide formed in the surface layer of the hot-rolled sheet is an oxide of Si and/or Mn was confirmed by embedding the hot-rolled sheet in resin, polishing the cross section, observing with a SEM, and performing elemental analysis with an EDS (energy dispersive X-ray spectrometer). The amount of internal oxidation is shown in Table 3.
次いで、冷間圧延によって板厚を１．２ｍｍの冷延板とした後に、ＣＧＬでの焼鈍および溶融めっき処理を行った。加熱炉前段はノズルミックス型バーナーを有する直火型加熱炉により表２に示す条件にて加熱を行った。次いでプレミックス型バーナーを有する直火型加熱炉にて表２に示す条件で加熱炉後段の加熱を行った。なお、酸化開始温度は３００℃とした。酸化開始温度は特にめっき外観に影響がないため、４００℃未満を酸化雰囲気としても良い。還元焼鈍はラジアントチューブ型の加熱・保持炉で表２に示す条件にて行い冷却した。引き続き、表２に示す０．１３５％のＡｌを含有した４６０℃の亜鉛浴を用いて溶融亜鉛めっき処理を施した後にガスワイピングで目付け量を約５０ｇ／ｍ２に調整した。一部の条件では、合金化処理を行った。 Next, the plate was cold-rolled to a thickness of 1.2 mm, and then annealed and hot-dip galvanized in a CGL. The front stage of the heating furnace was heated under the conditions shown in Table 2 using a direct-fired heating furnace having a nozzle mix type burner. Next, the rear stage of the heating furnace was heated under the conditions shown in Table 2 using a direct-fired heating furnace having a premix type burner. The oxidation start temperature was set to 300 ° C. Since the oxidation start temperature does not particularly affect the plating appearance, a temperature of less than 400 ° C may be used as the oxidizing atmosphere. Reduction annealing was performed in a radiant tube type heating and holding furnace under the conditions shown in Table 2, and the plate was cooled. Subsequently, a hot-dip galvanizing process was performed using a 460 ° C zinc bath containing 0.135% Al shown in Table 2, and the basis weight was adjusted to about 50 g / m 2 by gas wiping. Under some conditions, an alloying process was performed.
続いて、以上により得られた高強度溶融亜鉛めっき鋼板について対して、外観性を評価し、引張特性について調査した。更に、耐ＬＭＥ割れ性、脱水素挙動及び炉体へのダメージを評価した。以下に、測定方法および評価方法を示す。Next, the appearance of the high-strength hot-dip galvanized steel sheets obtained as described above was evaluated, and the tensile properties were investigated. Furthermore, the LME cracking resistance, dehydrogenation behavior, and damage to the furnace body were evaluated. The measurement and evaluation methods are shown below.
＜外観性＞
鋼板の外観を目視観察し、不めっき、ピックアップ現象による押し疵、または合金化ムラなどの外観不良がないものを「◎」、外観不良がわずかにあるが製品として許容範囲であるものを「〇」、明瞭な合金化ムラ、不めっき、または押し疵があるものは「×」とした。上記評価が「〇」、「◎」であれば、外観良好と判定した。 <Appearance>
The appearance of the steel sheet was visually observed, and those without defects in appearance such as bare spots, scratches due to pick-up phenomenon, or uneven alloying were rated as "◎", those with slight defects in appearance but within the acceptable range for the product were rated as "◯", and those with obvious uneven alloying, bare spots, or scratches were rated as "×". If the above evaluation was "◯" or "◎", the appearance was judged to be good.
＜引張特性＞
圧延方向を引張方向としてＪＩＳ５号試験片を用いてＪＩＳ Ｚ２２４１に準拠した方法で行った。ＴＳ（ＭＰａ）×ＥＬ（％）が８０００（ＭＰａ・％）以上を良好と判定した。 <Tensile properties>
The test was performed using a JIS No. 5 test piece with the rolling direction as the tensile direction in accordance with a method conforming to JIS Z 2241. A TS (MPa) x EL (%) of 8000 (MPa·%) or more was judged to be good.
＜耐ＬＭＥ割れ性＞
溶融亜鉛めっき鋼板から圧延直角方向（ＴＤ）を長手、圧延方向を短手として、長手方向１５０ｍｍ×短手方向５０ｍｍに切り出した試験片を、同サイズに切り出した、溶融亜鉛めっき層の片面あたりのめっき付着量が５０ｇ／ｍ２である試験用溶融亜鉛めっき鋼板（板厚１．２ｍｍ、ＴＳ：９８０ＭＰa級）と重ねて板組とした。この板組は、試験片の
溶融亜鉛めっき層と、市販の溶融亜鉛めっき鋼板の溶融亜鉛めっき層面とを合わせるように組み立てた。図１（Ａ）に示すように、この板組を、厚さ２．０ｍｍのスペーサーを介して、一部の部品形状で想定される最大の傾きである５°傾けた状態で固定台に固定した。スペーサーは、長手方向５０ｍｍ×短手方向４５ｍｍ×厚さ２．０ｍｍの一対の鋼板であり、この一対の鋼板各々の長手方向端面が、板組の短手方向両端面とそろうように配置した。したがって、スペーサーを構成する一対の鋼板間の距離は６０ｍｍとなる。固定台は、中央部に穴が開いた一枚の板である。 <LME crack resistance>
A test piece cut from a hot-dip galvanized steel sheet with a length of 150 mm in the longitudinal direction and a length of 50 mm in the transverse direction, with the direction perpendicular to the rolling (TD) as the longitudinal direction and the rolling direction as the transverse direction, was stacked with a test hot-dip galvanized steel sheet (sheet thickness 1.2 mm, TS: 980 MPa class) cut to the same size and having a coating weight of 50 g/ m2 per side of the hot-dip galvanized layer to form a plate assembly. This plate assembly was assembled so that the hot-dip galvanized layer of the test piece and the hot-dip galvanized layer surface of the commercially available hot-dip galvanized steel sheet were aligned. As shown in FIG. 1(A), this plate assembly was fixed to a fixing table via a spacer with a thickness of 2.0 mm in a state of inclination of 5°, which is the maximum inclination expected for some part shapes. The spacer was a pair of steel sheets with a length of 50 mm in the longitudinal direction, a transverse direction of 45 mm in the transverse direction, and a thickness of 2.0 mm, and was arranged so that the longitudinal end faces of each of the pair of steel sheets were aligned with both transverse end faces of the plate assembly. Therefore, the distance between the pair of steel plates constituting the spacer is 60 mm. The fixing base is a plate with a hole in the center.
次いで、サーボモータ加圧式で単相交流（５０Ｈｚ）の抵抗溶接機を用い、板組を一対の電極（先端径：６ｍｍ）で加圧しつつ板組をたわませた状態で、加圧力：３．５ｋＮ、ホールドタイム：０．１０秒または０．１６秒、溶接部のナゲット径が５．９ｍｍになる溶接電流および溶接時間の条件（すなわち、溶接電流および溶接時間は、板組毎にナゲット径が５．９ｍｍとなるよう適宜調整する）にて抵抗溶接を施して溶接部付き板組とした。このとき、一対の電極は、鉛直方向の上下から板組を加圧し、下側の電極は、固定台の穴を通じて試験片を加圧した。加圧に際しては、一対の電極のうち下側の電極がスペーサーと固定台とが接する面を延長した平面に接するように、下側の電極と固定台とを固定し、上側の電極を可動とした。また、上側の電極が試験用溶融亜鉛めっき鋼板の中央部に接するようにした。なお、ホールドタイムとは、溶接電流を流し終わってから電極を開放し始めるまでの時間を指す。また、ナゲット径とは、図２に示すように板組の長手方向におけるナゲットの端部１０の距離を指す。Next, a resistance welding machine with a servo motor pressure and single-phase AC (50 Hz) was used to apply pressure to the plate assembly with a pair of electrodes (tip diameter: 6 mm) while bending the plate assembly, and resistance welding was performed under the conditions of pressure: 3.5 kN, hold time: 0.10 seconds or 0.16 seconds, welding current and welding time conditions that result in a nugget diameter of 5.9 mm for the welded portion (i.e., the welding current and welding time were appropriately adjusted so that the nugget diameter was 5.9 mm for each plate assembly) to form a plate assembly with a welded portion. At this time, the pair of electrodes applied pressure to the plate assembly from above and below in the vertical direction, and the lower electrode applied pressure to the test piece through the hole in the fixing base. When applying pressure, the lower electrode and the fixing base were fixed so that the lower electrode of the pair of electrodes was in contact with a plane extending from the surface where the spacer and the fixing base were in contact, and the upper electrode was made movable. In addition, the upper electrode was made to contact the center of the test hot-dip galvanized steel sheet. The hold time refers to the time from the end of the welding current flow to the start of opening the electrodes. The nugget diameter refers to the distance of an end 10 of the nugget in the longitudinal direction of the plate assembly as shown in FIG.
次いで、図２に示すように、前記溶接部付き板組を、溶接部（ナゲット）を含むように切断して、該溶接部の断面を光学顕微鏡（２００倍）で観察し、以下の基準で溶接部における耐抵抗溶接割れ特性を評価した。ここで、図２の上の図は溶接部付き板組の平面図であり、切断位置を示す。図２の下の図は切断後の板組の板厚方向断面を示す図面であり、試験片に発生したき裂を模式的に示してある。なお、試験用溶融亜鉛めっき鋼板に割れが発生した場合、試験片の応力が分散し、適切な評価とならない。このため、試験用溶融亜鉛めっき鋼板に割れが発生していないデータを実施例として採用した。Next, as shown in Fig. 2, the plate assembly with the weld was cut to include the weld (nugget), and the cross section of the weld was observed with an optical microscope (200x), and the resistance weld cracking properties of the weld were evaluated according to the following criteria. Here, the upper diagram in Fig. 2 is a plan view of the plate assembly with the weld, showing the cutting position. The lower diagram in Fig. 2 is a drawing showing the cross section of the plate assembly in the plate thickness direction after cutting, and shows a schematic of cracks that occurred in the test piece. Note that if cracks occur in the test hot-dip galvanized steel sheet, the stress of the test piece is dispersed, and an appropriate evaluation cannot be performed. For this reason, data in which no cracks occurred in the test hot-dip galvanized steel sheet was adopted as an example.
下記の評価が「〇」、「◎」であれば、溶接部における耐抵抗溶接割れ特性はそれぞれ良好、優良と判断し、「×」であれば、溶接部における耐抵抗溶接割れ特性に劣ると判断した。If the evaluation was "◯" or "◎", the resistance weld crack resistance characteristics of the welded part were judged to be good and excellent, respectively, and if the evaluation was "×", the resistance weld crack resistance characteristics of the welded part were judged to be poor.
◎：ホールドタイム０．１０秒で０．１ｍｍ以上の長さのき裂が認められない。⊚: No cracks longer than 0.1 mm were observed with a hold time of 0.10 seconds.
○：ホールドタイム０．１０秒で０．１ｍｍ以上の長さのき裂が認められるが、ホールドタイム０．１６秒で０．１ｍｍ以上の長さのき裂が認められない。◯: A crack having a length of 0.1 mm or more is observed with a hold time of 0.10 seconds, but no crack having a length of 0.1 mm or more is observed with a hold time of 0.16 seconds.
×：ホールドタイム０．１６秒で０．１ｍｍ以上の長さのき裂が認められる。×: A crack having a length of 0.1 mm or more was observed at a hold time of 0.16 seconds.
＜脱水素挙動＞
溶融亜鉛めっき鋼板の幅中央部から、長軸長さ３０ｍｍ、短軸長さ５ｍｍの短冊状の試験片を採取し、その試験片のめっき層をリューターで除去し、直ちに、昇温脱離分析装置を用いて分析開始温度２５℃、分析終了温度３００℃、昇温速度２００℃／時間の条件で水素分析し、各温度において試験片表面から放出される水素量である放出水素量（質量ｐｐｍ／ｍｉｎ）を測定した。分析開始温度から３００℃までの放出水素量の合計を鋼中拡散性水素量として算出した。ここで、鋼中拡散性水素量が０．１０質量ｐｐｍ以下のものを良好「◎」とし、０．３０質量ｐｐｍ以下を合格「〇」とした。また、経験上、鋼中拡散性水素量が０．３０質量ｐｐｍを超えると、鋼板の耐遅れ破壊特性が低下することが多いことから、０．３０質量ｐｐｍ以上は「×」とした。脱水素挙動は「◎」と「〇」の場合が優れると判定した。 <Dehydrogenation behavior>
A rectangular test piece with a major axis length of 30 mm and a minor axis length of 5 mm was taken from the center of the width of the hot-dip galvanized steel sheet, the plating layer of the test piece was removed with a router, and immediately hydrogen analysis was performed using a thermal desorption analyzer under the conditions of an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C/hour, and the amount of released hydrogen (mass ppm/min), which is the amount of hydrogen released from the surface of the test piece at each temperature, was measured. The total amount of released hydrogen from the analysis start temperature to 300°C was calculated as the amount of diffusible hydrogen in steel. Here, a steel with a diffusible hydrogen amount of 0.10 mass ppm or less was evaluated as good "◎", and a steel with a diffusible hydrogen amount of 0.30 mass ppm or less was evaluated as pass "◯". In addition, from experience, when the diffusible hydrogen amount in steel exceeds 0.30 mass ppm, the delayed fracture resistance of the steel sheet often deteriorates, so a steel with a diffusible hydrogen amount of 0.30 mass ppm or more was evaluated as "×". The dehydrogenation behavior was judged to be excellent in the cases of "◎" and "◯".
＜炉体ダメージ＞
炉体へのダメージは、焼鈍炉内の鉄皮(ＳＵＳ３１０Ｓ)に変色が認められたかどうか、目視によって評価した。ここで、鉄皮に変色が認められなかったものを「〇」とし、炉体ダメージを与えないと判定した。明らかに変色が認められたものを「×」とし炉体ダメージを与えると判定した
＜曲げ性の評価方法＞
製造しためっき鋼板から、圧延方向に並行方向が短辺となるように、２５×１００ｍｍの短冊試験片を切り出した。次いで圧延方向が曲げたときの稜線になるように９０°Ｖ曲げ試験を行った。ストリーク速度は５０ｍｍ/ｍｉｎとし、荷重１０トンで５秒間ダイス
に押し付ける決め押しをした。Ｖ型パンチの先端Ｒを０．５ステップで種々変化させて試験を行い、試験片稜線近傍を２０倍のレンズで観察して亀裂（割れ）の有無を確認した。亀裂が発生しなかった最小のＲと、試験片の板厚（ｔｍｍ、千分の一の位で四捨五入した百分の一の位までの値を使用）から、Ｒ／ｔを算出し、これを曲げ性の指標とした。Ｒ／ｔが小さいほど曲げ性は良好である。ここで、Ｒ／ｔが１．０未満のものを極めて良好「◎+」、１．５未満のものを良好「◎」、２．０未満のものを合格「〇」、４．０未満の
ものを普通「△」、４．０以上を「×」とした。 <Furnace body damage>
Damage to the furnace body was evaluated by visual inspection to see whether discoloration was observed on the steel shell (SUS310S) inside the annealing furnace. Here, if no discoloration was observed on the steel shell, it was rated as "O" and it was determined that the furnace body would not be damaged. If obvious discoloration was observed, it was rated as "X" and it was determined that the furnace body would be damaged. <Evaluation method for bendability>
From the manufactured plated steel sheet, a strip test piece of 25×100 mm was cut out so that the short side was parallel to the rolling direction. Then, a 90° V-bend test was performed so that the rolling direction was the ridgeline when bent. The streak speed was 50 mm/min, and the test piece was pressed against the die for 5 seconds with a load of 10 tons. The V-shaped punch tip R was changed in 0.5 steps and the test was performed, and the vicinity of the ridgeline of the test piece was observed with a 20x lens to confirm the presence or absence of cracks (cracks). R/t was calculated from the minimum R at which no cracks occurred and the plate thickness of the test piece (t mm, the value rounded off to the nearest hundredth was used), and this was used as an index of bendability. The smaller R/t, the better the bendability. Here, R/t less than 1.0 was rated as extremely good "◎+", less than 1.5 was rated as good "◎", less than 2.0 was rated as pass "◯", less than 4.0 was rated as normal "△", and 4.0 or more was rated as x.
以上により得られた結果を製造条件と併せて表３に示す。The results thus obtained are shown in Table 3 together with the production conditions.
本発明の製造方法で得られた高強度溶融亜鉛めっき鋼板は、外観品質、耐抵抗溶接割れ特性に優れ、同時に水素脆化に起因する耐遅れ破壊特性の劣化を抑制可能であり、自動車の車体そのものを軽量化かつ高強度化するための表面処理鋼板として利用することができる。The high-strength hot-dip galvanized steel sheet obtained by the manufacturing method of the present invention has excellent appearance quality and resistance weld cracking resistance, and at the same time, is capable of suppressing deterioration of delayed fracture resistance caused by hydrogen embrittlement, and can be used as a surface-treated steel sheet for reducing the weight and increasing the strength of the vehicle body itself.
１ 試験用溶融亜鉛めっき鋼板
２ 試験片
３ スペーサー
４ 電極
５ 固定台
６ ナゲット
７ ナゲット径
８ 切断線Reference Signs List 1: Test hot-dip galvanized steel sheet 2: Test piece 3: Spacer 4: Electrode 5: Fixing stand 6: Nugget 7: Nugget diameter 8: Cutting line
Claims (16)
前記直火型の加熱炉では、前段で、Ｏ２を１０００体積ｐｐｍ以上、Ｈ２Ｏを１０００体積ｐｐｍ以上含む雰囲気中で鋼板を４００℃以上６７０℃以下まで加熱し、後段で、Ｏ２を５００体積ｐｐｍ以下含む雰囲気中で鋼板を６００℃以上７００℃以下まで加熱し、
前記加熱・保持炉を有する焼鈍炉では、炉内雰囲気のＨ２Ｏ濃度が５０００体積ｐｐｍ以上４００００体積ｐｐｍ以下、Ｈ２濃度が２体積％以上２０体積％以下、
Ｈ２Ｏの分圧（ＰＨ2Ｏ）とＨ２の分圧（ＰＨ2）の比ｌｏｇ（ＰＨ2Ｏ/ＰＨ2）が－１．１以上０．５以下を満たす雰囲気に、鋼板温度を６５０℃以上９００℃以下で９０秒以上保持する高強度溶融亜鉛めっき鋼板の製造方法。
ＴＣ＝－３０（［Ｓｉ］＋［Ｍｎ］）＋７７５ ・・・（１）
［Ｓｉ］は鋼板に含まれるＳｉ含有量（質量％）
［Ｍｎ］は鋼板に含まれるＭｎ含有量（質量％） A method for producing a high-strength hot-dip galvanized steel sheet, comprising the steps of: hot-rolling a slab containing, by mass%, C: 0.05% to 0.30%, Si: 0.45% to 2.0%, and Mn: 1.0% to 4.0%, and then winding the slab into a coil at a temperature equal to or lower than T C (° C.) calculated from the following formula (1), and pickling the hot-rolled sheet; cold-rolling the hot-rolled sheet obtained in the hot-rolling step; and continuously annealing the cold-rolled steel sheet obtained in the cold-rolling step in an annealing furnace having a direct-fired heating furnace and a radiant tube-type heating/holding furnace, and then hot-dip galvanizing the cold-rolled steel sheet,
In the direct-fired heating furnace, in the first stage, the steel sheet is heated to 400°C or more and 670°C or less in an atmosphere containing 1000 volume ppm or more of O2 and 1000 volume ppm or more of H2O , and in the second stage, the steel sheet is heated to 600°C or more and 700°C or less in an atmosphere containing 500 volume ppm or less of O2 ;
In the annealing furnace having the heating and holding furnace, the H 2 O concentration in the furnace atmosphere is 5,000 volume ppm or more and 40,000 volume ppm or less, and the H 2 concentration is 2 volume % or more and 20 volume % or less;
A method for manufacturing a high - strength hot - dip galvanized steel sheet, comprising: maintaining the steel sheet temperature at 650°C or higher and 900°C or lower for 90 seconds or more in an atmosphere in which the ratio log(P H2O /P H2 ) of the partial pressure of H 2 O (P H2O ) to the partial pressure of H 2 (P H2 ) is -1.1 or higher and 0.5 or lower.
T C =−30([Si]+[Mn])+775 ... (1)
[Si] is the Si content (mass%) in the steel sheet
[Mn] is the Mn content (mass%) in the steel plate
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| WO2015087549A1 (en) | 2013-12-13 | 2015-06-18 | Ｊｆｅスチール株式会社 | Method for manufacturing high-strength hot-dip galvanized steel sheet |
| JP2015113504A (en) | 2013-12-12 | 2015-06-22 | Ｊｆｅスチール株式会社 | High strength hot-dip galvanized steel sheet excellent in processability and method for manufacturing the same |
| WO2016038801A1 (en) | 2014-09-08 | 2016-03-17 | Ｊｆｅスチール株式会社 | Method and apparatus for producing high-strength hot-dipped galvanized steel sheet |
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| JP2015113504A (en) | 2013-12-12 | 2015-06-22 | Ｊｆｅスチール株式会社 | High strength hot-dip galvanized steel sheet excellent in processability and method for manufacturing the same |
| WO2015087549A1 (en) | 2013-12-13 | 2015-06-18 | Ｊｆｅスチール株式会社 | Method for manufacturing high-strength hot-dip galvanized steel sheet |
| WO2016038801A1 (en) | 2014-09-08 | 2016-03-17 | Ｊｆｅスチール株式会社 | Method and apparatus for producing high-strength hot-dipped galvanized steel sheet |
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