JP6673461B2 - Steel sheet manufacturing method and steel sheet continuous annealing apparatus - Google Patents

Steel sheet manufacturing method and steel sheet continuous annealing apparatus Download PDF

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JP6673461B2
JP6673461B2 JP2018501497A JP2018501497A JP6673461B2 JP 6673461 B2 JP6673461 B2 JP 6673461B2 JP 2018501497 A JP2018501497 A JP 2018501497A JP 2018501497 A JP2018501497 A JP 2018501497A JP 6673461 B2 JP6673461 B2 JP 6673461B2
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steel sheet
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furnace
temperature
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JPWO2017145322A1 (en
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貴幸 北澤
貴幸 北澤
植田　浩平
浩平 植田
裕之 川田
川田　　裕之
薫 平松
薫 平松
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Nippon Steel Corp
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    • 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
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • 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
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
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    • 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
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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Description

本発明は、鋼板の製造方法及び鋼板の連続焼鈍装置に関し、特に、Ｃを０．０５０質量％以上、Ｓｉを０．１０質量％以上、Ｍｎを１．２０質量％以上含有する引張強度７８０ＭＰａ以上の高強度鋼板の製造方法、及び、この鋼板の製造方法に適した鋼板の連続焼鈍装置に関する。   The present invention relates to a method for producing a steel sheet and a continuous annealing apparatus for a steel sheet, and particularly to a tensile strength of 780 MPa or more containing 0.050% by mass or more of C, 0.10% by mass or more of Si, and 1.20% by mass or more of Mn. The present invention relates to a method for manufacturing a high-strength steel sheet, and a continuous annealing apparatus for a steel sheet suitable for the method for manufacturing a steel sheet.

近年、低燃費化やＣＯ２排出量削減を目的とした車体の軽量化および衝突安全性向上を目的として、自動車分野では、車体や部品などに高強度鋼板を使用するニーズが高まっている。最近では、引張強度が７８０ＭＰａ以上、あるいは、９８０ＭＰａ以上といった高強度鋼板も使用されている。
しかしながら、鋼板を高強度化すると、一般的に成形性（加工性）等の材料特性が劣化する。一方で、これら高強度鋼板は、軟鋼板と同様にプレス加工によって大量かつ安価に成形され、各種部材として供されることが求められる。このため、上述の高強度鋼板には、高強度とともに、高い延性、及び、良好な加工性も求められる。In recent years, in the field of automobiles, there is a growing need to use high-strength steel sheets for vehicle bodies and parts for the purpose of reducing the weight of vehicles and improving collision safety in order to reduce fuel consumption and reduce CO 2 emissions. Recently, high-strength steel sheets having a tensile strength of 780 MPa or more or 980 MPa or more have been used.
However, when the strength of a steel sheet is increased, material properties such as formability (workability) generally deteriorate. On the other hand, these high-strength steel sheets are required to be formed in large quantities and at low cost by press working like mild steel sheets, and to be provided as various members. Therefore, the above-mentioned high-strength steel sheet is required to have high strength, high ductility and good workability.

ここで、引張強度が７８０ＭＰａ以上の高強度鋼板において、高い延性、及び、良好な加工性を付与するためには、一般に、ＳｉやＭｎといった合金元素を鋼に添加することが行われている。
ところが、ＳｉやＭｎを含有する鋼板に対して、最高到達温度８００〜９００℃の焼鈍を行った際には、ＳｉやＭｎが鋼板の表層に析出・濃化して酸化し、表面にＳｉ酸化物やＳｉ−Ｍｎ酸化物が露出してしまうおそれがあった。このように鋼板の表面にＳｉ酸化物やＳｉ−Ｍｎ酸化物が露出した場合には、めっき処理時のめっき性劣化や、塗装前の化成処理性の低下といった問題が生じる。Here, in a high-strength steel sheet having a tensile strength of 780 MPa or more, alloy elements such as Si and Mn are generally added to steel in order to impart high ductility and good workability.
However, when a steel sheet containing Si or Mn is annealed at a maximum temperature of 800 to 900 ° C., Si or Mn precipitates and concentrates on the surface layer of the steel sheet, oxidizes, and forms a Si oxide on the surface. Or the Si-Mn oxide may be exposed. When the Si oxide or Si-Mn oxide is exposed on the surface of the steel sheet as described above, problems such as deterioration of plating property at the time of plating treatment and deterioration of chemical conversion treatment property before coating occur.

このような課題に対し、例えば特許文献１には、Ｓｉ含有量が０．４〜２．０質量％である高強度鋼板に対して、熱処理を行うに際して、直火還元炉の直火還元バーナーの空気比を０．６以上０．９未満とした還元雰囲気で鋼板を還元し、Ｓｉ酸化膜を薄く制御した上で、水素還元を行う間接加熱炉で水分圧ＰＨ２Ｏと水素分圧ＰＨ２の対数ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を−１．６以上−０．５以下とすることで、Ｓｉ酸化物が鋼板表面に露出することを抑制して、鋼板のめっき性を向上させる技術が提案されている。In order to solve such a problem, for example, Patent Document 1 discloses that when performing heat treatment on a high-strength steel sheet having a Si content of 0.4 to 2.0% by mass, a direct-fire reduction burner of a direct-fire reduction furnace. The steel sheet is reduced in a reducing atmosphere with an air ratio of 0.6 to less than 0.9, the Si oxide film is controlled to be thin, and the water pressure P H2O and the hydrogen partial pressure P H2 are reduced in an indirect heating furnace that performs hydrogen reduction. By controlling the logarithm log (P H2O / P H2 ) of -1.6 or more to -0.5 or less, a technique of suppressing the Si oxide from being exposed on the steel sheet surface and improving the plating property of the steel sheet is known. Proposed.

また、特許文献２には、直火加熱炉と間接還元炉とを有する連続焼鈍ラインにおいて、還元炉での還元の前に、直火加熱によって鋼板を酸化させた上で、鋼板の最高到達温度をＴ（９２３Ｋ≦Ｔ≦１１７３Ｋ）とした場合に、炉内の雰囲気中の酸素分圧の対数ｌｏｇＰＯ２を、−０．０００．０７４×Ｔ２＋０．１０５×Ｔ−０．２×〔Ｓｉ％〕２＋２．１×〔Ｓｉ％〕−９８．８≦ｌｏｇＰＯ２≦−０．０００．０７８×Ｔ２＋０．１０７×Ｔ−９０．４の範囲で還元することで、成形性に優れた高強度合金化溶融亜鉛めっき鋼板を製造する方法が提案されている。Further, Patent Document 2 discloses that in a continuous annealing line having a direct-fired heating furnace and an indirect reduction furnace, before the reduction in the reduction furnace, the steel sheet is oxidized by direct-fire heating, and then the maximum temperature of the steel sheet is reached. Is T (923K ≦ T ≦ 1173K), the logarithmic log P O2 of the oxygen partial pressure in the furnace atmosphere is −0.000.074 × T 2 + 0.105 × T−0.2 × [Si %] 2 + 2.1 × [Si%] − 98.8 ≦ logPO 2 ≦ −0.000.078 × T 2 + 0.107 × T-90.4 A method for producing a high-strength galvannealed steel sheet has been proposed.

さらに、特許文献３には、冷延鋼板を連続焼鈍する際に、昇温時に鋼板温度が５５０℃以上で空気比０．９５以上の直火バーナーを用いて鋼板を加熱して鋼板の表面を酸化させ、その後、空気比０．８９以下の直火バーナーを用いて鋼板を加熱して鋼板温度が７５０℃以上になるまで昇温した後、露点が−２５℃以下の炉で均熱焼鈍することにより、化成処理性の向上を図った技術が提案されている。   Further, Patent Document 3 discloses that, when a cold-rolled steel sheet is continuously annealed, the steel sheet surface is heated by using a direct-fire burner having a temperature of 550 ° C. or more and an air ratio of 0.95 or more when the temperature is raised. After oxidizing, the steel sheet is heated using a direct fire burner with an air ratio of 0.89 or less and heated to a temperature of 750 ° C. or more, and then annealed in a furnace having a dew point of −25 ° C. or less. Accordingly, a technique for improving the chemical conversion treatment property has been proposed.

また、特許文献４には、鋼板に連続焼鈍を施す際に、加熱過程において、加熱炉内温度が６００℃以上Ａ℃以下（６５０≦Ａ≦７８０）の場合に雰囲気中の露点を−４０℃以下とし、加熱炉内温度がＡ℃超えＢ℃以下（８００≦Ｂ≦９００）の場合に雰囲気中の露点を−１０℃以上とすることにより、化成処理性の向上を図った技術が提案されている。   Further, in Patent Document 4, when performing continuous annealing on a steel sheet, the dew point in the atmosphere is set to −40 ° C. when the temperature in the heating furnace is 600 ° C. or more and A ° C. or less (650 ≦ A ≦ 780) in the heating process. In the case where the temperature in the heating furnace is higher than A ° C. and lower than or equal to B ° C. (800 ≦ B ≦ 900), a technique for improving the chemical conversion treatment property by setting the dew point in the atmosphere to −10 ° C. or higher is proposed. ing.

また、特許文献５、６には、鋼板に予熱工程と昇温工程と再結晶化工程とからなる連続焼鈍を行うに際し、予熱工程において、連続焼鈍雰囲気中の水素分圧に対する水蒸気分圧比（ＰＨ２Ｏ／ＰＨ２）が予熱温度Ｔｐとの関係から下記（１）式の条件を満足するように制御すること、昇温工程において、再結晶化温度Ｔｒを６５０℃〜９００℃とし、焼鈍雰囲気中の水素分圧に対する水蒸気分圧比（ＰＨ２Ｏ／ＰＨ２）が再結晶化温度Ｔｒとの関係から下記（２）式の条件を満足し、かつ昇温速度が１〜２０℃／秒となるように制御すること、及び再結晶化工程において、焼鈍雰囲気の水素分圧に対する水蒸気分圧比（ＰＨ２Ｏ／ＰＨ２）が再結晶化温度Ｔｒとの関係から下記（３）式の条件を満足し、かつ保持時間を４０〜６００秒とするように制御することによって、化成処理性の向上を図った技術が提案されている。
（１）式：ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−２．８×１０−６Ｔｐ２＋６．８×１０−３Ｔｐ−４．８
（２）式：５．３×１０−８×Ｔｒ２＋１．４×１０−５×Ｔｒ−０．０１≦ＰＨ２Ｏ／ＰＨ２≦６．４×１０−７×Ｔｒ２＋１．７×１０−４Ｔｒ−０．１
（３）式：ＰＨ２Ｏ／ＰＨ２＜５．３×１０−８×Ｔｒ２＋１．４×１０−５×Ｔｒ−０．０１Further, Patent Documents 5 and 6 disclose that when a steel sheet is subjected to continuous annealing including a preheating step, a temperature raising step, and a recrystallization step, in the preheating step, a steam partial pressure ratio (P) to a hydrogen partial pressure in a continuous annealing atmosphere is used. H2O / PH2 ) is controlled so as to satisfy the following expression (1) based on the relationship with the preheating temperature Tp. In the heating step, the recrystallization temperature Tr is set to 650 ° C. to 900 ° C. The ratio of the partial pressure of water vapor to the partial pressure of hydrogen (P H2O / P H2 ) satisfies the following expression (2) from the relationship with the recrystallization temperature Tr, and the rate of temperature rise is 1 to 20 ° C./sec. And in the recrystallization step, the ratio of the partial pressure of water vapor to the partial pressure of hydrogen in the annealing atmosphere (P H2O / P H2 ) satisfies the condition of the following formula (3) from the relationship with the recrystallization temperature Tr: And the holding time is 40 to 600 By controlling so as to, technologies with improved chemical conversion treatability has been proposed.
Formula (1): log (P H2O / P H2 ) ≦ −2.8 × 10 −6 Tp 2 + 6.8 × 10 −3 Tp−4.8
Formula (2): 5.3 × 10−8 × Tr 2 + 1.4 × 10 −5 × Tr−0.01 ≦ P H2O / P H2 ≦ 6.4 × 10−7 × Tr 2 + 1.7 × 10 -4 Tr-0.1
Formula (3): P H2O / P H2 <5.3 × 10 −8 × Tr 2 + 1.4 × 10 −5 × Tr-0.01

また、特許文献７には、Ｓｉ、ＭｎおよびＡｌを特定の比率で含有する鋼板を、還元炉中の雰囲気ガスの水素分圧および水蒸気分圧の対数比が、−１．３９≦ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．６９５を満足するように制御する溶融亜鉛めっき鋼板の製造方法が開示されている。Patent Document 7 discloses that a steel sheet containing Si, Mn, and Al at a specific ratio has a logarithmic ratio of a partial pressure of hydrogen and a partial pressure of water vapor of an atmosphere gas in a reduction furnace of -1.39 ≦ log (P method for manufacturing a galvanized steel sheet is controlled so as to satisfy the H2O / P H2) ≦ -0.695 it is disclosed.

しかしながら、上述の特許文献１〜７に記載された技術には以下の課題がある。
すなわち、特許文献１〜３は、直火加熱部を有し、直火バーナーの空気比を制御することを特徴としている。しかしながら、最近では、鋼板に対して焼鈍を施す連続焼鈍装置としては、直火加熱部を有さずに全て間接加熱炉によって構成されたものが主流となっている。このような直火加熱部を有さない連続焼鈍炉においては、特許文献１〜３に記載されたように、直火バーナーの空気比を制御する方法は適用することができない。また、特許文献１〜３のように、還元前に直火バーナーで鋼板を酸化させる方法では、バーナー設備の高温劣化や燃焼ガスの発熱量変動などに起因して所定の空気比を確保できなくなり、結果として厚く生成した酸化膜によって、炉内のハースロールにビルドアップ（***）が形成されることを防止しきれない。直火加熱炉で生成した酸化膜は、鋼板が炉内ロールに巻きついている間に鋼板から剥離しロール表面に付着することにより、鋼板に押し疵を発生させるので、好ましくない。
また、特許文献４に記載された条件で炉内の雰囲気を制御した場合には、Ｓｉの酸化物が表面に露出することは抑制できるものの、露点を−１０℃以上とする必要があるので、鋼板の脱炭が進行し、鋼板の引張強度や疲労強度が低下してしまうといった問題があった。特に、Ｃを多く含有する高強度鋼板においては、脱炭による強度低下は大きな問題となる。
また、特許文献５及び６に記載された条件について、ガス中の水分量の外的変動や生産設備の劣化により、式（１）、式（３）のＰＨ２Ｏ／ＰＨ２値や露点等の適正条件を担保することが難しいという問題もある。
また、特許文献７では、横型の還元炉を用いて溶融亜鉛めっき鋼板を製造するに際し、炉内の雰囲気ガス中の水素濃度を１０％以上とすることが示唆されている。しかしながら、雰囲気ガス中の水素濃度を１０％以上とするには、特別な設備を必要とするため、適用には大きな設備投資が必要となる。 However, the techniques described in Patent Documents 1 to 7 have the following problems.
That is, Patent Documents 1 to 3 are characterized in that they have a direct fire heating unit and control the air ratio of a direct fire burner. However, recently, as a continuous annealing apparatus for performing annealing on a steel sheet, an apparatus mainly including an indirect heating furnace without a direct-fired heating unit has become mainstream. As described in Patent Documents 1 to 3, the method of controlling the air ratio of the direct fire burner cannot be applied to the continuous annealing furnace having no such direct fire heating section. In addition, in the method of oxidizing a steel plate with an open flame burner before reduction as in Patent Documents 1 to 3, a predetermined air ratio cannot be secured due to high-temperature deterioration of a burner facility or a change in the calorific value of combustion gas. As a result, it is not possible to completely prevent build-up (bulges) from being formed on the hearth roll in the furnace due to the thick oxide film formed. The oxide film generated in the direct-fired heating furnace is not preferable because the steel sheet peels off and adheres to the roll surface while the steel sheet is wound around the roll in the furnace, thereby causing a press flaw on the steel sheet.
Further, when the atmosphere in the furnace is controlled under the conditions described in Patent Document 4, although exposure of the Si oxide to the surface can be suppressed, the dew point needs to be -10 ° C. or higher. There has been a problem that the decarburization of the steel sheet progresses and the tensile strength and fatigue strength of the steel sheet decrease. In particular, in a high-strength steel sheet containing a large amount of C, reduction in strength due to decarburization is a serious problem.
In addition, regarding the conditions described in Patent Documents 5 and 6, due to external fluctuations in the amount of water in the gas and deterioration of the production equipment, the P H2O / P H2 values and the dew point of Expressions (1) and (3) are determined. There is also a problem that it is difficult to secure appropriate conditions.
Further, Patent Document 7 suggests that when producing a hot-dip galvanized steel sheet using a horizontal reduction furnace, the hydrogen concentration in the atmosphere gas in the furnace is set to 10% or more. However, in order to make the hydrogen concentration in the atmosphere gas 10% or more, special equipment is required, so that a large equipment investment is required for application.

日本国特開２００７−１９１７４５号公報Japanese Patent Application Laid-Open No. 2007-191745 日本国特開２００６−２３３３３３号公報Japanese Patent Application Laid-Open No. 2006-233333 日本国特開２０１３−２５３３２２号公報JP 2013-253322 A 日本国特開２０１２−０７２４５２号公報Japanese Patent Application Laid-Open No. 2012-074522 日本国特開２００８−０６９４４５号公報Japanese Patent Application Laid-Open No. 2008-069445 日本国特開２００８−１２１０４５号公報Japanese Patent Application Laid-Open No. 2008-121045 日本国特開２０１２−１２６８３号公報Japanese Patent Application Laid-Open No. 2012-12683

本発明は、前述した状況に鑑みてなされたものであって、直火加熱部を有さない連続焼鈍炉において焼鈍時の雰囲気を制御することにより、Ｓｉを鋼板の内部で酸化させて鋼板の表面にＳｉ酸化物が露出することを抑制し、かつ、鋼板からの脱炭の進行を抑制可能な高強度鋼板の製造方法、及び、この高強度鋼板の製造方法に適した鋼板の連続焼鈍装置を提供することを目的とする。本発明において、高強度とは、引張強度が７８０ＭＰａ以上であることを示す。   The present invention has been made in view of the above-described circumstances, and by controlling the atmosphere during annealing in a continuous annealing furnace having no direct-fired heating section, Si is oxidized inside the steel sheet, thereby A method for producing a high-strength steel sheet capable of suppressing the exposure of Si oxide on the surface and suppressing the progress of decarburization from the steel sheet, and a continuous annealing apparatus for the steel sheet suitable for the method for producing the high-strength steel sheet The purpose is to provide. In the present invention, “high strength” means that the tensile strength is 780 MPa or more.

上記課題を解決するために、本発明者らが鋭意研究した結果、以下のような知見を得た。
（ａ）焼鈍の加熱時において、鋼板温度が７００℃から７５０℃の範囲で、Ｓｉの内部酸化、及び、脱炭が開始される。
（ｂ）７００℃〜８００℃、特に、７００℃から７５０℃の範囲において、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を適正範囲に調整することで、Ｓｉを内部酸化させるとともに脱炭を抑制可能である。
（ｃ）さらに、７００℃以下あるいは、８００℃超で鋼板を加熱する範囲においても、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を適正範囲に調整することで、鋼板の表面の酸酸化を防ぐこと、または内部酸化を促進し、かつ、脱炭を抑制することが可能である。
（ｄ）８００℃超で鋼板を加熱する際の、炉内雰囲気の露点を−１０℃未満とすることで、脱炭を抑制し、強度の低下を防ぐことができる。The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have obtained the following findings.
(A) At the time of heating during annealing, internal oxidation of Si and decarburization are started when the steel sheet temperature is in the range of 700 ° C to 750 ° C.
(B) In the range of 700 ° C. to 800 ° C., particularly 700 ° C. to 750 ° C., a relational expression log (P H2O / P H2 ) consisting of the water pressure P H2O in the atmosphere in the furnace and the hydrogen partial pressure P H2 is obtained. By adjusting to an appropriate range, Si can be oxidized internally and decarburization can be suppressed.
(C) Further, even in a range in which the steel sheet is heated at 700 ° C. or lower or higher than 800 ° C., by adjusting log (P H2O / P H2 ) to an appropriate range, acid oxidation of the surface of the steel sheet is prevented, or It is possible to promote internal oxidation and suppress decarburization.
(D) By setting the dew point of the atmosphere in the furnace to less than −10 ° C. when heating the steel sheet at a temperature exceeding 800 ° C., decarburization can be suppressed and a decrease in strength can be prevented.

本発明は、上述した知見に基づいてなされた。その要旨を以下に示す。
（１）本発明の一態様に係る鋼板の製造方法は、引張強度が７８０ＭＰａ以上、Ｓｉの内部酸化層深さが０．１μｍ以上、かつ脱炭層厚さが７０μｍ以下である高強度鋼板の製造方法であって、化学組成として、質量％で、Ｃ：０．０５０〜０．４０％、Ｓｉ：０．１０〜２．５０％、Ｍｎ：１．２０〜３．５０％、Ｃｒ：０〜０．８０％、Ｎｉ：０〜５．００％、Ｃｕ：０〜３．００％、Ｎｂ：０〜０．１０％、Ｍｇ：０〜０．０１０％、Ｔｉ：０〜０．１０％、Ｂ：０〜０．０１０％、Ｍｏ：０〜０．５％を含有し、残部がＦｅ及び不純物からなり、前記不純物として、Ｐ：０．１００％以下、Ｓ：０．０１０％以下、Ａｌ：１．２００％以下、Ｎ：０．０１００％以下、に制限した鋼板を、８００℃超、９００℃以下の温度範囲まで加熱して、前記温度範囲で０〜３００秒保持することによって連続焼鈍を行う連続焼鈍工程を有し、前記連続焼鈍工程では、前記温度範囲まで前記加熱を行う際、及び前記温度範囲での前記保持を行う際、炉内の雰囲気中の水素濃度を、１０体積％未満とし、前記鋼板の温度が７００℃以下であるときの、前記炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式であるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を、下記式（ｉ）の範囲とし、前記鋼板の温度が７００℃超８００℃以下であるときの、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｉｉ）の範囲とし、前記鋼板の温度が８００℃超であるときの、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｉｉｉ）の範囲で、前記鋼板の温度が７００℃超８００℃以下であるときの前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の前記炉体平均値よりも低く、かつ露点を−１０℃未満とする。
−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７ （ｉ）
−１．３６＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７ （ｉｉ）
−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３ （ｉｉｉ）

The present invention has been made based on the above findings. The summary is shown below.
(1) The method for producing a steel sheet according to one embodiment of the present invention is a method for producing a high-strength steel sheet having a tensile strength of 780 MPa or more, an internal oxide layer depth of Si of 0.1 μm or more, and a decarburized layer thickness of 70 μm or less. In the method, as a chemical composition, in mass%, C: 0.050 to 0.40%, Si: 0.10 to 2.50%, Mn: 1.20 to 3.50%, Cr: 0 to 0% 0.80%, Ni: 0 to 5.00%, Cu: 0 to 3.00%, Nb: 0 to 0.10%, Mg: 0 to 0.010%, Ti: 0 to 0.10%, B: 0 to 0.010%, Mo: 0 to 0.5%, with the balance being Fe and impurities, as the impurities, P: 0.100% or less, S: 0.010% or less, Al : 1.200% or less, N: 0.0100% or less, the limit steel sheet to, 800 ° C. greater, warm to the temperature range of 900 ° C. or less And a continuous annealing step of performing continuous annealing by maintaining the temperature in the temperature range for 0 to 300 seconds. In the continuous annealing step, when the heating is performed up to the temperature range, and the holding in the temperature range is performed. When the hydrogen concentration in the atmosphere in the furnace is set to less than 10% by volume and the temperature of the steel sheet is 700 ° C. or less, the water pressure P H2O and the hydrogen partial pressure P H2 in the atmosphere in the furnace are set. The furnace body average value of log (P H2O / P H2 ) which is a relational expression consisting of the following formula (i) is set in the range of the following formula (i), and when the temperature of the steel plate is more than 700 ° C. and 800 ° C. or less, The furnace body average value of the log (P H2 O / P H2 ) in the atmosphere of the above is set in the range of the following formula (ii), and the log in the atmosphere of the furnace when the temperature of the steel sheet exceeds 800 ° C. under the furnace body average value of (P H2O / P H2) In the scope of formula (iii), lower than the furnace body average value of the log (P H2O / P H2) in the atmosphere of the furnace when the temperature of the steel sheet is below 700 ° C. Ultra 800 ° C., and Dew point should be less than -10 ° C.
−3.01 <log (P H2O / P H2 ) <− 0.07 (i)
−1.36 <log (P H2O / P H2 ) <− 0.07 (ii)
−3.01 <log (P H2O / P H2 ) ≦ −0.53 (iii)

（２）上記（１）に記載の鋼板の製造方法は、前記化学組成が、質量％で、Ｃｒ：０．０１〜０．８０％、Ｎｉ：０．０１〜５．００％、Ｃｕ：０．０１〜３．００％、Ｎｂ：０．００１〜０．１０％、Ｍｇ：０．０００１〜０．０１０％、Ｔｉ：０．００１〜０．１０％、Ｂ：０．０００１〜０．０１０％、Ｍｏ：０．０１〜０．５％、から選択される１種または２種以上を含有してもよい。   (2) In the method for producing a steel sheet according to the above (1), the chemical composition is, in terms of mass%, Cr: 0.01 to 0.80%, Ni: 0.01 to 5.00%, Cu: 0. 0.001 to 3.00%, Nb: 0.001 to 0.10%, Mg: 0.0001 to 0.010%, Ti: 0.001 to 0.10%, B: 0.0001 to 0.010 %, Mo: 0.01 to 0.5%.

（３）上記（１）または（２）に記載の鋼板の製造方法は、前記鋼板の温度が７００℃超８００℃以下であるときの、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｖｉｉ）の範囲としてもよい。
−１．００＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．６７ （ｖｉｉ）(3) The method for producing a steel sheet according to the above (1) or (2), wherein the log (P H2O / P) in the atmosphere in the furnace when the temperature of the steel sheet is more than 700 ° C. and 800 ° C. or less. The average value of the furnace body of H2 ) may be in the range of the following equation (vii).
−1.00 <log (P H2O / P H2 ) <− 0.67 (vii)

（４）本発明の別の態様に係る鋼板の連続焼鈍装置は、化学組成として、質量％で、Ｃ：０．０５０〜０．４０％、Ｓｉ：０．１０〜２．５０％、Ｍｎ：１．２０〜３．５０％、Ｃｒ：０〜０．８０％、Ｎｉ：０〜５．００％、Ｃｕ：０〜３．００％、Ｎｂ：０〜０．１０％、Ｍｇ：０〜０．０１０％、Ｔｉ：０〜０．１０％、Ｂ：０〜０．０１０％、Ｍｏ：０〜０．５％を含有し、残部がＦｅ及び不純物からなり、前記不純物として、Ｐ：０．１００％以下、Ｓ：０．０１０％以下、Ａｌ：１．２００％以下、Ｎ：０．０１００％以下、に制限した鋼板に連続焼鈍を実施する鋼板の連続焼鈍装置であって、炉内の雰囲気中の水素濃度を、１０体積％未満とし、前記鋼板の温度が７００℃以下の場合に、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式であるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｉｖ）の範囲に調整し、前記鋼板の温度が７００℃超８００℃以下の場合に、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｖ）の範囲に調整し、前記鋼板の温度が８００℃超の場合に、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｖｉ）の範囲で、前記鋼板の温度が７００℃超８００℃以下の場合の前記ｌｏｇ（Ｐ Ｈ２Ｏ ／Ｐ Ｈ２ ）の前記炉体平均値よりも低く、かつ露点を−１０℃未満に調整する炉内雰囲気調整手段を備え、（１）に記載の鋼板の製造方法に用いる。
−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７ （ｉｖ）
−１．３６＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７ （ｖ）
−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３ （ｖｉ）

(4) The continuous annealing apparatus for a steel sheet according to another embodiment of the present invention has, as a chemical composition, C: 0.050 to 0.40%, Si: 0.10 to 2.50%, and Mn: 1.20 to 3.50%, Cr: 0 to 0.80%, Ni: 0 to 5.00%, Cu: 0 to 3.00%, Nb: 0 to 0.10%, Mg: 0 to 0 0.010%, Ti: 0 to 0.10%, B: 0 to 0.010%, Mo: 0 to 0.5%, with the balance being Fe and impurities. 100% or less, S: 0.010% or less , Al: 1.200% or less, N: 0.0100% or less. the hydrogen concentration in the atmosphere, is less than 10 vol%, when the temperature of the steel sheet of 700 ° C. or less, and water pressure P H2 O in the atmosphere in the furnace The furnace body average value of oxygen partial pressure P H2 Metropolitan is a relational expression consisting of log (P H2O / P H2) is adjusted to the range of the following formula (iv), when the temperature of the steel sheet of 700 ° C. Ultra 800 ° C. or less The furnace body average value of log (P H2O / P H2 ) in the atmosphere in the furnace is adjusted to the range of the following formula (v), and when the temperature of the steel sheet exceeds 800 ° C., said furnace body average value of log (P H2O / P H2) in the atmosphere in the range of the following formula (vi) of the log when the temperature of the steel sheet of 700 ° C. ultra 800 ° C. or less (P H2O / P H2 ), Which is provided with a furnace atmosphere adjusting means for adjusting the dew point to be lower than −10 ° C. and lower than the furnace body average value, and is used in the method for producing a steel sheet according to (1) .
−3.01 <log (P H2O / P H2 ) <− 0.07 (iv)
−1.36 <log (P H2O / P H2 ) <− 0.07 (v)
−3.01 <log (P H2O / P H2 ) ≦ −0.53 (vi)

上述の構成とされた、本発明の上記態様に係る鋼板の製造方法によれば、雰囲気中の水素濃度を１０体積％未満とした炉内において、Ｓｉの内部酸化及び脱炭が開始される７００℃超８００℃以下の温度範囲において、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−０．０７未満としているので、脱炭の発生を抑制することができる。また、上述の鋼板の温度範囲において、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−１．３６超としているので、Ｓｉを鋼板の内部で酸化させることができ、鋼板表面にＳｉ酸化物が露出することを抑制できる。また、７００℃以下の温度範囲については、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７の範囲内に、８００℃超で鋼板を加熱する範囲においては、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３の範囲内とするとともに露点を−１０℃未満とすることで鋼板の表面酸化の防止や内部酸化の促進を可能とする。また、Ｓｉの内部酸化を発生させ、かつ、脱炭の進行を確実に抑制することができる。
本発明の上記態様に係る鋼板の製造方法によれば、伸びや加工性などの特性を従来よりも劣化させることなく、７８０ＭＰａ以上の引張強度が確保され疲労強度、めっき性、及び化成処理性に優れた高強度鋼板を製造することが可能となる。According to the method for manufacturing a steel sheet according to the above-described aspect of the present invention, the internal oxidation and decarburization of Si are started in a furnace in which the hydrogen concentration in the atmosphere is less than 10% by volume. In the temperature range of over 800 ° C. and below 800 ° C., the furnace body average value of the relational expression log (P H2O / P H2 ) consisting of the water pressure P H2O in the furnace atmosphere and the hydrogen partial pressure P H2 is less than −0.07. Therefore, the occurrence of decarburization can be suppressed. Further, since the furnace body average value of log (P H2O / P H2 ) is more than -1.36 in the above-mentioned steel sheet temperature range, Si can be oxidized inside the steel sheet, and the surface of the steel sheet can be oxidized with Si. Exposure of an object can be suppressed. Also, the temperature less than 700 ℃, the furnace body average value of log (P H2O / P H2) , -3.01 <log (P H2O / P H2) <- 0.07 within the range of 800 In a range in which the steel sheet is heated at a temperature higher than 0 ° C., the surface oxidation of the steel sheet is controlled by setting the range of −3.01 <log (P H2O / P H2 ) ≦ −0.53 and the dew point to less than −10 ° C. Prevention and promotion of internal oxidation. Further, internal oxidation of Si can be generated, and the progress of decarburization can be reliably suppressed.
According to the method for manufacturing a steel sheet according to the above aspect of the present invention, a tensile strength of 780 MPa or more is secured without deteriorating properties such as elongation and workability as compared with the related art, and fatigue strength, plating property, and chemical conversion treatment property are improved. It becomes possible to manufacture excellent high-strength steel sheets.

また、上述の構成の鋼板の連続焼鈍装置によれば、加熱時の鋼板の温度が７００℃超８００℃以下の温度範囲において、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を、−１．３６＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７の範囲内とする炉内雰囲気調整手段を備えているので、Ｓｉの内部酸化によりＳｉ酸化物が鋼板の表面に露出することを抑制できるととともに、脱炭を抑制することができる。よって、上述の構成の鋼板の連続焼鈍装置を用いることで、引張強度が７８０ＭＰａ以上で、さらにめっき性、化成処理性に優れた高強度鋼板を製造することが可能となる。また、７００℃以下の温度範囲については、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７の範囲内に、８００℃超で鋼板を加熱する範囲においては、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３の範囲内とすることで表面酸化の防止、内部酸化の促進、脱炭の抑制を可能とする。Further, according to the continuous annealing apparatus for a steel sheet having the above-described configuration, when the temperature of the steel sheet during heating is in a temperature range of more than 700 ° C. and 800 ° C. or less, the water pressure P H2O in the atmosphere in the furnace and the hydrogen partial pressure P H2 are reduced. Furnace atmosphere adjusting means for setting the average value of the furnace body of the relational expression log (P H2O / P H2 ) in the range of −1.36 <log (P H2O / P H2 ) <− 0.07. Therefore, exposure of the Si oxide to the surface of the steel sheet due to internal oxidation of Si can be suppressed, and decarburization can be suppressed. Therefore, by using the continuous annealing apparatus for a steel sheet having the above-described configuration, it becomes possible to manufacture a high-strength steel sheet having a tensile strength of 780 MPa or more and further having excellent plating properties and chemical conversion treatment properties. Also, the temperature less than 700 ℃, the furnace body average value of log (P H2O / P H2) , -3.01 <log (P H2O / P H2) <- 0.07 within the range of 800 In the range where the steel sheet is heated at a temperature exceeding ℃, the surface oxidation is prevented, the internal oxidation is promoted, and the decarburization is suppressed by setting -3.01 <log (P H2O / P H2 ) ≦ −0.53. Is possible.

すなわち、本発明の上記態様によれば、焼鈍時の雰囲気を制御することにより、Ｓｉを鋼板の内部で酸化させて鋼板の表面にＳｉ酸化物が露出することを抑制し、かつ、鋼板からの脱炭の進行を抑制可能な高強度鋼板の製造方法、及び、この高強度鋼板の製造方法に適した鋼板の連続焼鈍装置を提供することができる。また、これらの製造方法及び連続焼鈍装置によって得られた鋼板は、高強度であり、めっき性、及び化成処理性に優れる。   That is, according to the above aspect of the present invention, by controlling the atmosphere during annealing, it is possible to suppress the Si oxide being oxidized inside the steel sheet and to expose the Si oxide on the surface of the steel sheet, and A method for manufacturing a high-strength steel sheet capable of suppressing the progress of decarburization and a continuous annealing apparatus for a steel sheet suitable for the method for manufacturing a high-strength steel sheet can be provided. Moreover, the steel sheet obtained by these manufacturing methods and the continuous annealing apparatus has high strength, and is excellent in plating property and chemical conversion treatment property.

本発明の一実施形態に係る鋼板の製造方法に用いられる、鋼板の連続焼鈍装置を示す概略説明図である。BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic explanatory drawing which shows the continuous annealing apparatus of the steel plate used for the manufacturing method of the steel plate which concerns on one Embodiment of this invention.

以下に、本発明の一実施形態に係る鋼板の製造方法（本実施形態に係る鋼板の製造方法という場合がある）、及び、本発明の一実施形態に係る鋼板の連続焼鈍装置（本実施形態に係る鋼板の連続焼鈍装置と言う場合がある）について、図面を参照して説明する。ただし、本発明は、以下の実施形態に限定されるものではない。
本実施形態においては、例えば自動車用途等に用いられる、引張強度が７８０ＭＰａ以上、好ましくは９８０ＭＰａ以上の高強度鋼板の製造を対象としている。Hereinafter, a method for manufacturing a steel sheet according to an embodiment of the present invention (may be referred to as a method for manufacturing a steel sheet according to the present embodiment), and a continuous annealing apparatus for a steel sheet according to an embodiment of the present invention (the present embodiment) ) May be referred to as a continuous annealing apparatus for a steel sheet. However, the present invention is not limited to the following embodiments.
The present embodiment is directed to the production of a high-strength steel sheet having a tensile strength of 780 MPa or more, preferably 980 MPa or more, which is used for, for example, an automobile.

本実施形態に係る鋼板の製造方法は、質量％で、Ｃ：０．０５０〜０．４０％、Ｓｉ：０．１０〜２．５０％、Ｍｎ：１．２０〜３．５０％、Ｃｒ：０〜０．８０％、Ｎｉ：０〜５．００％、Ｃｕ：０〜３．００％、Ｎｂ：０〜０．１０％、Ｍｇ：０〜０．０１０％、Ｔｉ：０〜０．１０％、Ｂ：０〜０．０１０％、Ｍｏ：０〜０．５％を含有し、残部がＦｅ及び不純物からなる鋼板に連続焼鈍を行う連続焼鈍工程を有する。
ここで、連続焼鈍工程以外の工程については、特に限定されず、所望の鋼板特性に応じて公知の方法で行えばよい。例えば、一般に必要に応じて行われる上記以外の工程として、鋼を鋳造して鋳片を得る鋳造工程、前記鋳片を熱間圧延して鋼板を得る熱間圧延工程、前記鋼板を冷間圧延する冷間圧延工程、酸洗工程、調質圧延工程等を公知の方法で行ってもよい。しかしながら、連続焼鈍工程については、後述する条件で行う必要がある。The manufacturing method of the steel sheet according to the present embodiment is as follows: C: 0.050 to 0.40%, Si: 0.10 to 2.50%, Mn: 1.20 to 3.50%, Cr: 0 to 0.80%, Ni: 0 to 5.00%, Cu: 0 to 3.00%, Nb: 0 to 0.10%, Mg: 0 to 0.010%, Ti: 0 to 0.10 %, B: 0 to 0.010%, and Mo: 0 to 0.5%, and has a continuous annealing step of continuously annealing a steel sheet whose balance is made of Fe and impurities.
Here, steps other than the continuous annealing step are not particularly limited, and may be performed by a known method according to desired steel sheet characteristics. For example, as a step other than the above generally performed as necessary, a casting step of casting steel to obtain a slab, a hot rolling step of hot rolling the slab to obtain a steel sheet, and cold rolling the steel sheet. The cold rolling step, the pickling step, the temper rolling step, and the like may be performed by a known method. However, the continuous annealing step needs to be performed under the conditions described later.

まず、本実施形態において製造の対象とする鋼板（高強度鋼板）の化学組成を限定した理由について説明する。   First, the reason for limiting the chemical composition of the steel sheet (high-strength steel sheet) to be manufactured in the present embodiment will be described.

Ｃ：０．０５０質量％以上、０．４０質量％以下
Ｃは、マルテンサイト、焼戻マルテンサイト、ベイナイト及び残留オーステナイト等の硬質組織を形成し、鋼板の強度を向上させるために必須の元素である。そこで、連続焼鈍工程を経た鋼板の引張強度を７８０ＭＰａ以上とするために、Ｃ含有量を０．０５０質量％以上とする。強度を十分に高めるため、Ｃ含有量は０．０７５質量％以上が好ましい。一方、過度にＣ含有量を高めると鋼板の溶接性が劣化するので、Ｃ含有量は０．４０質量％以下とする。好ましくは０．３０質量％以下である。C: 0.050% by mass or more and 0.40% by mass or less C is an essential element for forming a hard structure such as martensite, tempered martensite, bainite and retained austenite and improving the strength of a steel sheet. is there. Therefore, the C content is set to 0.050% by mass or more in order to make the tensile strength of the steel sheet that has undergone the continuous annealing process 780 MPa or more. In order to sufficiently increase the strength, the C content is preferably 0.075% by mass or more. On the other hand, if the C content is excessively increased, the weldability of the steel sheet deteriorates, so the C content is set to 0.40% by mass or less. Preferably it is 0.30 mass% or less.

Ｓｉ：０．１０質量％以上、２．５０質量％以下
Ｓｉは、鋼板の伸びを確保して、加工性を大きく阻害することなく強度を向上させる作用効果を有する元素である。そこで、加工性と強度とを十分に確保するために、Ｓｉ含有量を０．１０質量％以上とする。加工性と強度とをさらに向上させるため、Ｓｉ含有量を０．４５質量％以上とすることが好ましい。一方、過度にＳｉ含有量を高めると靭性が低下し、却って加工性が劣化する。そのため、Ｓｉ含有量を２．５０質量％以下とする。好ましくは２．３０質量％以下である。Si: 0.10% by mass or more and 2.50% by mass or less Si is an element having an effect of securing the elongation of the steel sheet and improving the strength without significantly impairing the workability. Therefore, in order to ensure sufficient workability and strength, the Si content is set to 0.10% by mass or more. In order to further improve workability and strength, the Si content is preferably set to 0.45% by mass or more. On the other hand, if the Si content is excessively increased, the toughness is reduced, and the workability is rather deteriorated. Therefore, the Si content is set to 2.50% by mass or less. Preferably it is 2.30 mass% or less.

Ｍｎ：１．２０質量％以上、３．５０質量％以下
Ｍｎは、Ｓｉと同等の作用効果を有する元素である。そこで、加工性と強度とを十分に確保するために、Ｍｎ含有量を１．２０質量％以上とする。加工性と強度とをより向上させるため、Ｍｎ含有量を１．５０質量％以上とすることが好ましい。一方、過度にＭｎ含有量を高めると溶接性が劣化する。そのため、Ｍｎ含有量を３．５０質量％以下とする。好ましくは３．３０質量％以下である。Mn: 1.20% by mass or more and 3.50% by mass or less Mn is an element having the same effect as Si. Then, in order to ensure sufficient workability and strength, the Mn content is set to 1.20% by mass or more. In order to further improve workability and strength, the Mn content is preferably set to 1.50% by mass or more. On the other hand, if the Mn content is excessively increased, the weldability deteriorates. Therefore, the Mn content is set to 3.50% by mass or less. Preferably it is 3.30 mass% or less.

本実施形態で対象とする高強度鋼板は、上記の化学成分を含有し、残部がＦｅ及び不純物からなることを基本とする。しかしながら、要求特性を満たすために必須ではないが、更なる高強度化、または成形性の更なる向上を目的として、Ｃｒ、Ｎｉ、Ｃｕ、Ｎｂ、Ｍｇ、Ｔｉ、Ｂ、Ｍｏを後述する範囲で含有させてもよい。また、Ｃｒ、Ｎｉ、Ｃｕ、Ｎｂ、Ｍｇ、Ｔｉ、Ｂ、Ｍｏの含有量が、下記に示した含有量の下限未満であっても、本発明の効果を損なわない。Ｃｒ、Ｎｉ、Ｃｕ、Ｎｂ、Ｍｇ、Ｔｉ、Ｂ、Ｍｏはいずれも要求特性を満たすために必須ではないので、その含有量の下限は０％である。
不純物とは、鋼材を工業的に製造する際に、鉱石、スクラップ等の原料、その他の要因により混入する成分を意味する。前記不純物は少ない方が好ましいが、不純物のうち、Ｐ、Ｓ、Ａｌ、Ｎについては、特に、Ｐ：０．１００質量％以下、Ｓ：０．０１０質量％以下、Ａｌ：１．２００質量％以下、Ｎ：０．０１００質量％以下に制限することが好ましい。The high-strength steel sheet targeted in the present embodiment basically contains the above-mentioned chemical components, and the balance consists of Fe and impurities. However, although it is not essential to satisfy the required characteristics, Cr, Ni, Cu, Nb, Mg, Ti, B, and Mo are used in the range described below for the purpose of further increasing the strength or further improving the formability. You may make it contain. Further, even if the contents of Cr, Ni, Cu, Nb, Mg, Ti, B, and Mo are less than the lower limits of the contents shown below, the effects of the present invention are not impaired. Since Cr, Ni, Cu, Nb, Mg, Ti, B and Mo are not essential for satisfying the required characteristics, the lower limit of the content is 0%.
The impurity means a component mixed by raw materials such as ore and scrap and other factors when a steel material is industrially manufactured. Although it is preferable that the amount of the impurities is small, among the impurities, P, S, Al, and N are particularly P: 0.100 mass% or less, S: 0.010 mass% or less, and Al: 1.200 mass%. Hereinafter, it is preferable to limit N to 0.0100% by mass or less.

Ｃｒ：０．０１質量％以上、０．８０質量％以下
Ｃｒは、高温での相変態を抑制し、鋼板を高強度化する効果を有する元素である。この効果を得る場合、Ｃｒ含有量を０．０１質量％以上とすることが好ましい。一方、Ｃｒ含有量が０．８０質量％を超えると、熱間での加工性が損なわれ生産性が低下する。そのため、含有させる場合でも、Ｃｒ含有量を０．８０質量％以下とする。好ましくは、０．４０質量％以下である。Cr: 0.01% by mass or more and 0.80% by mass or less Cr is an element having an effect of suppressing phase transformation at high temperatures and increasing the strength of a steel sheet. In order to obtain this effect, the Cr content is preferably set to 0.01% by mass or more. On the other hand, if the Cr content exceeds 0.80% by mass, hot workability is impaired and productivity is reduced. Therefore, even when it is contained, the Cr content is set to 0.80% by mass or less. Preferably, it is 0.40% by mass or less.

Ｎｉ：０．０１質量％以上、５．００質量％以下
Ｎｉは、高温での相変態を抑制し、鋼板を高強度化する効果を有する元素である。この効果を得る場合、Ｎｉ含有量を０．０１質量％以上とすることが好ましい。一方、Ｎｉ含有量が５．００質量％を超えると、溶接性が損なわれる。そのため、含有させる場合でも、Ｎｉ含有量を５．００質量％以下とする。好ましくは、２．００質量％以下である。Ni: 0.01% by mass or more and 5.00% by mass or less Ni is an element that suppresses phase transformation at high temperatures and has the effect of increasing the strength of the steel sheet. To obtain this effect, the Ni content is preferably set to 0.01% by mass or more. On the other hand, if the Ni content exceeds 5.00% by mass, weldability is impaired. Therefore, even when it is contained, the Ni content is set to 5.00% by mass or less. Preferably, it is 2.00% by mass or less.

Ｃｕ：０．０１質量％以上、３．００質量％以下
Ｃｕは、微細な粒子として鋼中に存在することにより鋼板の強度を高める元素である。この効果を得る場合、Ｃｕ含有量を０．０１質量％以上とすることが好ましい。一方、Ｃｕ含有量が５．００質量％を超えると、溶接性が損なわれる。そのため、含有させる場合でも、Ｃｕ含有量を３．００質量％以下とする。好ましくは、２．００質量％以下である。Cu: 0.01% by mass or more and 3.00% by mass or less Cu is an element that enhances the strength of a steel sheet by being present in steel as fine particles. To obtain this effect, the Cu content is preferably set to 0.01% by mass or more. On the other hand, if the Cu content exceeds 5.00% by mass, the weldability is impaired. Therefore, even when it is contained, the Cu content is set to 3.00% by mass or less. Preferably, it is 2.00% by mass or less.

Ｎｂ：０．００１質量％以上、０．１０質量％以下
Ｎｂは、析出強化、フェライト結晶粒の成長抑制による細粒強化及び再結晶の抑制を通じた転位強化にて、鋼板の強度上昇に寄与する元素である。この効果を得る場合、Ｎｂ含有量を０．００１質量％以上とすることが好ましい。一方、Ｎｂ含有量が０．１０質量％を超えると、炭窒化物の析出量が多くなって成形性が劣化する。そのため、含有させる場合でも、Ｎｂ含有量を０．１０質量％以下とする。好ましくは、０．０５質量％以下である。Nb: 0.001% by mass or more and 0.10% by mass or less Nb contributes to an increase in the strength of a steel sheet by precipitation strengthening, fine grain strengthening by suppressing growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization. Element. When this effect is obtained, the Nb content is preferably set to 0.001% by mass or more. On the other hand, when the Nb content exceeds 0.10% by mass, the amount of carbonitride deposited increases and the formability deteriorates. Therefore, even when it is contained, the Nb content is set to 0.10% by mass or less. Preferably, it is 0.05% by mass or less.

Ｍｇ：０．０００１質量％以上、０．０１０質量％以下
Ｍｇは、成形性の改善に有効な元素である。この効果を得る場合、Ｍｇ含有量を０．０００１質量％以上とすることが好ましい。一方、Ｍｇ含有量が０．０１０質量％を超えると、却って延性が損なわれるおそれがある。そのため、含有させる場合でも、Ｍｇ含有量を０．０１０質量％以下とする。好ましくは、０．００５質量％以下である。Mg: 0.0001% by mass or more and 0.010% by mass or less Mg is an element effective for improving formability. To obtain this effect, the Mg content is preferably set to 0.0001% by mass or more. On the other hand, if the Mg content exceeds 0.010% by mass, ductility may be impaired. Therefore, even when it is contained, the Mg content is set to 0.010% by mass or less. Preferably, it is 0.005% by mass or less.

Ｔｉ：０．００１質量％以上、０．１０質量％以下
Ｔｉは、析出強化、フェライト結晶粒の成長抑制による細粒強化及び再結晶の抑制を通じた転位強化によって、鋼板の強度上昇に寄与する元素である。この効果を得る場合、Ｔｉ含有量を０．００１質量％以上とすることが好ましい。一方、Ｔｉ含有量が０．１０質量％を超えると、炭窒化物の析出量が多くなって成形性が劣化する。そのため、含有させる場合でも、Ｔｉ含有量を０．１０質量％以下とする。好ましくは、０．０５質量％以下である。Ti: 0.001% by mass or more and 0.10% by mass or less Ti is an element that contributes to an increase in the strength of a steel sheet by precipitation strengthening, fine grain strengthening by suppressing the growth of ferrite grains, and dislocation strengthening by suppressing recrystallization. It is. When this effect is obtained, the Ti content is preferably set to 0.001% by mass or more. On the other hand, if the Ti content exceeds 0.10% by mass, the amount of carbonitride deposited increases and the formability deteriorates. Therefore, even if it is contained, the Ti content is set to 0.10% by mass or less. Preferably, it is 0.05% by mass or less.

Ｂ：０．０００１質量％以上、０．０１０質量％以下
Ｂは、高温での相変態を抑制し、鋼板の高強度化に有効な元素である。この効果を得る場合、Ｂ含有量を０．０００１質量％以上とすることが好ましい。一方、Ｂ含有量が０．０１０質量％を超えると、熱間での加工性が損なわれ生産性が低下する。そのため、含有させる場合でも、Ｂ含有量を０．０１０質量％以下とする。好ましくは、０．００５質量％以下である。B: 0.0001% by mass or more and 0.010% by mass or less B is an element that suppresses phase transformation at a high temperature and is effective for increasing the strength of a steel sheet. When this effect is obtained, the B content is preferably set to 0.0001% by mass or more. On the other hand, when the B content exceeds 0.010% by mass, hot workability is impaired and productivity is reduced. Therefore, even if it is contained, the B content is set to 0.010% by mass or less. Preferably, it is 0.005% by mass or less.

Ｍｏ：０．０１質量％以上、０．５質量％以下
Ｍｏは、高温での相変態を抑制し、鋼板の高強度化に有効な元素である。この効果を得る場合、Ｍｏ含有量を０．０１質量％以上とすることが好ましい。一方、Ｍｏ含有量が０．５質量％を超えると、熱間での加工性が損なわれ生産性が低下する。そのため、含有させる場合でも、Ｍｏ含有量を０．５質量％以下とする。好ましくは、０．２５質量％以下である。Mo: 0.01% by mass or more and 0.5% by mass or less Mo is an element that suppresses phase transformation at a high temperature and is effective for increasing the strength of a steel sheet. When this effect is obtained, the Mo content is preferably set to 0.01% by mass or more. On the other hand, if the Mo content exceeds 0.5% by mass, hot workability is impaired and productivity is reduced. Therefore, even when Mo is contained, the Mo content is set to 0.5% by mass or less. Preferably, it is 0.25% by mass or less.

次に連続焼鈍工程について説明する。
連続焼鈍工程においては、図１のように鋼板の連続焼鈍装置１０に装入された鋼板１を例えば７５０〜９００℃に加熱し、その温度域で０〜３００秒保持し、その後冷却する。
このとき、炉内の雰囲気を制御しないと鋼板１中のＳｉ、Ｍｎが鋼板１の表層に析出・濃化し、Ｓｉ酸化物やＳｉ−Ｍｎ酸化物として鋼板１の表面に露出することがある。また、焼鈍により、脱炭が発生して鋼板１の強度が低下することがある。０秒保持とは、昇温し、７５０〜９００℃の所定の温度となった時点で、ただちに冷却を行うことを意味する。
本実施形態に係る鋼板の製造方法における連続焼鈍工程では、後述するように炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式であるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を制御することで、鋼板１の表面への酸化物の形成を抑制することができる。上記の制御を行う場合、後述する本実施形態に係る鋼板の連続焼鈍装置を用いることが好ましい。Next, the continuous annealing step will be described.
In the continuous annealing step, as shown in FIG. 1, the steel sheet 1 charged in the continuous steel sheet annealing apparatus 10 is heated to, for example, 750 to 900 ° C., kept at that temperature range for 0 to 300 seconds, and then cooled.
At this time, if the atmosphere in the furnace is not controlled, Si and Mn in the steel sheet 1 may precipitate and concentrate on the surface layer of the steel sheet 1 and may be exposed on the surface of the steel sheet 1 as Si oxides or Si—Mn oxides. In addition, the annealing may cause decarburization and reduce the strength of the steel sheet 1. Holding for 0 seconds means that the temperature is raised to a predetermined temperature of 750 to 900 ° C., and cooling is performed immediately.
In the continuous annealing step in the method for manufacturing a steel sheet according to the present embodiment, as described later, log (P H2O / P H2 ), which is a relational expression composed of the water pressure P H2O in the atmosphere in the furnace and the hydrogen partial pressure P H2 , is used. By controlling the furnace body average value, the formation of oxides on the surface of the steel sheet 1 can be suppressed. When performing the above control, it is preferable to use a steel sheet continuous annealing apparatus according to the present embodiment described later.

＜鋼板の温度が７００℃超８００℃以下であるときの、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値＞
本実施形態に係る鋼板の製造方法における連続焼鈍工程では、鋼板１の温度がＳｉの内部酸化及び脱炭が開始される７００℃超８００℃以下であるときの、連続焼鈍装置１０の炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式であるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−１．３６超、−０．０７未満とする。ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を上記の範囲とすることで、Ｓｉを鋼板１の内部で酸化させることができ、鋼板１の表面にＳｉ酸化物が露出することを抑制できるとともに、脱炭の発生を抑制することができる。その結果、引張強度、疲労強度が確保され、さらにめっき性、化成処理性に優れた高強度鋼板を製造することが可能となる。ここで、ｌｏｇは、常用対数である。<When the temperature of the steel sheet is below 700 ° C. Ultra 800 ° C., the furnace body average value of log (P H2O / P H2) >
In the continuous annealing step in the method for manufacturing a steel sheet according to the present embodiment, when the temperature of the steel sheet 1 is 700 ° C or more and 800 ° C or less at which internal oxidation and decarburization of Si is started, the inside of the furnace of the continuous annealing apparatus 10 is used. The furnace average value of log (P H2O / P H2 ), which is a relational expression consisting of the water pressure P H2O in the atmosphere and the hydrogen partial pressure P H2 , is set to more than -1.36 and less than -0.07. By setting the log (P H2O / P H2 ) to the above range, Si can be oxidized inside the steel sheet 1, and the exposure of the Si oxide on the surface of the steel sheet 1 can be suppressed, and the decarburization can be suppressed. Generation can be suppressed. As a result, tensile strength and fatigue strength are ensured, and a high-strength steel sheet excellent in plating property and chemical conversion treatment can be manufactured. Here, log is a common logarithm.

上述の関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が−１．３６以下の場合には、Ｓｉの内部酸化が十分に発生せず、Ｓｉ酸化物が鋼板１の表面に露出し、めっき性、化成処理性が劣化する。一方、上述の関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が−０．０７以上となる場合には、脱炭が進行し、鋼板１の強度が低下するおそれがある。
Ｓｉ酸化物の表面への露出をより抑制するとともに鋼板からの脱炭をより少なくするためには、鋼板の温度が７００℃超８００℃以下の温度範囲において、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−１．００≦ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．６７の範囲内とすることが好ましい。When the average value of the furnace body of the above relational expression log (P H2O / P H2 ) is −1.36 or less, the internal oxidation of Si does not sufficiently occur, and the Si oxide is exposed on the surface of the steel sheet 1. In addition, the plating property and the chemical conversion property deteriorate. On the other hand, if the furnace body average value of the above relational expression log (P H2O / P H2 ) is equal to or more than −0.07, decarburization proceeds, and the strength of the steel sheet 1 may be reduced.
In order to further suppress the exposure of the Si oxide to the surface and to reduce the decarburization from the steel sheet, the water pressure P in the atmosphere in the furnace is set in a temperature range of the steel sheet from 700 ° C. to 800 ° C. or less. The furnace body average value of the relational expression log (P H2O / P H2 ) consisting of H 2 O and hydrogen partial pressure P H2 should be in the range of −1.00 ≦ log (P H2O / P H2 ) ≦ −0.67. Is preferred.

ここで、７００℃超８００℃以下の鋼板温度範囲でｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を規定しているのは、内部酸化生成温度域である７００℃から７５０℃の範囲のみでｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−１．３６＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７とした場合には、内部酸化が不均一かつ不十分なケースがあるためである。上記ＰＨ２Ｏ／ＰＨ２で８００℃以下の領域まで加熱することで、内部酸化を十分に生成させ、かつ脱炭も抑制することが可能となる。Here, the reason why the log (P H2O / P H2 ) is defined in the steel plate temperature range from 700 ° C. to 800 ° C. is that the log (P H2O / P H2O ) is limited only in the range of 700 ° C. to 750 ° C. which is the internal oxidation generation temperature range. / P a furnace body average of H2) -1.36 <log (P H2O / P H2) <- 0.07 and when is for the internal oxidation is uneven and insufficient case. By heating to a region of 800 ° C. or less with the above P H2O / P H2 , internal oxidation can be sufficiently generated and decarburization can be suppressed.

＜鋼板の温度が８００℃超であるときの、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値＞
本実施形態に係る鋼板の製造方法において、鋼板の温度が８００℃超の場合（加熱時及び保持時を含む）、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３の範囲内となるように制御する。ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を上記の範囲とすることで内部酸化を十分に満足し、かつ、脱炭の抑制が可能となる。
８００℃超の温度域において、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が−０．５３超となる場合には、脱炭が進行し、鋼板１の強度が低下するおそれがある。脱炭をより少なくするためには、８００℃超の温度範囲におけるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−０．６７未満とすることが好ましい。また、脱炭をより少なくするためには、８００℃超の温度域における、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を、７００℃超８００℃以下のｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値よりも低くすることも好ましい。
一方、鋼板の温度が７００℃以下の場合、及び８００℃超の場合の、各ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の下限値については、７００〜８００℃の範囲外において、実製造の上で達成可能な値として、−３．０１とする。
鋼板の焼鈍温度を８００℃以下とする場合には、８００℃超のｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を考慮する必要はない。<When the temperature of the steel sheet is 800 ° C. greater than the furnace body average value of log (P H2O / P H2) >
In the method for manufacturing a steel sheet according to the present embodiment, when the temperature of the steel sheet is higher than 800 ° C. (including when the steel sheet is heated and held), the furnace body average value of log (P H2O / P H2 ) is set to −3.01 <. Control is performed so that log (P H2O / P H2 ) ≦ −0.53. By setting log (P H2O / P H2 ) within the above range, internal oxidation can be sufficiently satisfied, and decarburization can be suppressed.
If the furnace body average value of log (P H2 O / P H2 ) exceeds −0.53 in a temperature range exceeding 800 ° C., decarburization proceeds, and the strength of the steel sheet 1 may decrease. In order to further reduce the decarburization, it is preferable that the average furnace value of log (P H2O / P H2 ) in a temperature range exceeding 800 ° C. is less than −0.67. Further, in order to further reduce the decarburization, the temperature range of 800 ° C. greater, log the furnace body average value of (P H2O / P H2), 700 ℃ super 800 ° C. The following log (P H2O / P H2) It is also preferable to make the furnace body average value lower than the average value.
On the other hand, the lower limit of each log (P H2O / P H2 ) when the temperature of the steel sheet is 700 ° C. or lower and higher than 800 ° C. is achieved in actual production outside the range of 700 to 800 ° C. A possible value is -3.01.
When the annealing temperature of the steel sheet is set to 800 ° C. or less, it is not necessary to consider a log (P H2O / P H2 ) exceeding 800 ° C.

＜鋼板の温度が７００℃以下であるときの、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値＞
鋼板の温度が７００℃以下の場合、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−３．０１超、−０．０７未満、すなわち、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７となるように制御する。
７００℃以下の温度域においても、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が−０．０７以上となる場合には、鋼板１の表面が酸化し、後の７００℃超８００℃以下において、内部酸化を促進することができなくなる。そのため、ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−０．０７未満とする。好ましくは−０．６７未満である。
一方、鋼板の温度が７００℃以下の場合、及び８００℃超の場合の、各ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の下限値については、７００〜８００℃の範囲外において、実製造の上で達成可能な値として、−３．０１超とする。<When the temperature of the steel sheet is 700 ° C. or less, furnace average value of log (P H2O / P H2) >
When the temperature of the steel sheet is 700 ° C. or less, the furnace body average value of log (P H2O / P H2 ) is more than −3.01 and less than −0.07, that is, −3.01 <log (P H2O / P H2). ) <− 0.07.
Even in the temperature range of 700 ° C. or less, when the furnace body average value of log (P H2 O / P H2 ) is −0.07 or more, the surface of the steel sheet 1 is oxidized, and the temperature exceeds 700 ° C. and 800 ° C. or less. , Internal oxidation cannot be promoted. Therefore, the furnace average value of log (P H2O / P H2 ) is set to less than −0.07. Preferably it is less than -0.67.
On the other hand, the lower limit of each log (P H2O / P H2 ) when the temperature of the steel sheet is 700 ° C. or lower and higher than 800 ° C. is achieved in actual production outside the range of 700 to 800 ° C. A possible value is greater than -3.01.

各温度範囲におけるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を制御する方法は限定されないが、本実施形態に係る鋼板の連続焼鈍装置１０を用いる場合には、所定のＨ２ガスを含む雰囲気に設定した炉内に、雰囲気ガス導入部１５により、水蒸気や加湿ガスを導入することで、制御できる。また、各温度域におけるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）をより細かく制御する場合には、各加熱帯及び均熱帯にそれぞれ雰囲気ガス導入部を設けてもよい。その場合、それぞれの雰囲気ガス導入部からは、異なる組成の雰囲気ガスや水蒸気を導入してもよい。The method of controlling log (P H2O / P H2 ) in each temperature range is not limited, but when using the continuous annealing apparatus 10 for steel sheets according to the present embodiment, a furnace set to an atmosphere containing a predetermined H 2 gas is used. It can be controlled by introducing steam or humidified gas by the atmosphere gas introducing unit 15 therein. When the log (P H2O / P H2 ) in each temperature range is more finely controlled, an atmosphere gas introduction unit may be provided in each of the heating zones and the solitary zone. In that case, atmosphere gas or water vapor having a different composition may be introduced from each atmosphere gas introduction unit.

ここで、Ｓｉの内部酸化とは、鋼板１内に拡散した酸素とＳｉとが反応することでＳｉ酸化物を析出する現象である。この内部酸化は、鋼板１の表面から深さ０．１〜２０μｍ程度の位置で発生する。
本実施形態においては、鋼板の表層部のフェライトにおいて、最大長さが２５ｎｍ以上のＳｉ酸化物が１．０×１０１２個／ｍ２以上存在する領域をＳｉの内部酸化層と定義し、内部酸化層深さが０．１μｍ以上であれば、内部酸化が十分であると判断できる。Ｓｉの内部酸化層深さ位置は、鋼板の圧延方向に平行な板厚断面を観察面として試料を採取し、観察面を研磨、ナイタールエッチングし、電界放射型走査型電子顕微鏡（ＦＥ−ＳＥＭ：Ｆｉｅｌｄ Ｅｍｉｓｓｉｏｎ Ｓｃａｎｎｉｎｇ Ｅｌｅｃｔｒｏｎ Ｍｉｃｒｏｓｃｏｐｅ）により、倍率５０００倍以上で、鋼板の厚さ方向に１．０μｍ、圧延方向に２０μｍの矩形の領域を、３箇所以上設定する。それぞれにおいて、領域内のＳｉ酸化物の個数をカウントし、Ｓｉ酸化物が１０個以上存在した場合に、その領域をＳｉの内部酸化層とし、Ｓｉ酸化物が１０個以上存在した領域の最大深さの平均値をＳｉの内部酸化層深さ位置とすることで得られる。Here, the internal oxidation of Si is a phenomenon in which oxygen diffused into the steel sheet 1 reacts with Si to precipitate Si oxide. This internal oxidation occurs at a depth of about 0.1 to 20 μm from the surface of the steel sheet 1.
In the present embodiment, in the ferrite in the surface layer portion of the steel sheet, a region in which a maximum length of 1.0 × 10 12 Si / m 2 or more having a maximum length of 25 nm or more is defined as an internal oxide layer of Si. If the depth of the oxide layer is 0.1 μm or more, it can be determined that internal oxidation is sufficient. The depth position of the internal oxide layer of Si is obtained by taking a sample with the thickness section parallel to the rolling direction of the steel sheet as an observation surface, polishing the observation surface, performing nital etching, and using a field emission scanning electron microscope (FE-SEM). : Three or more rectangular regions having a magnification of 5,000 or more, 1.0 μm in the thickness direction of the steel sheet, and 20 μm in the rolling direction are set by Field Emission Scanning Electron Microscope. In each case, the number of Si oxides in the region was counted, and when 10 or more Si oxides were present, the region was used as an internal oxide layer of Si, and the maximum depth of the region where 10 or more Si oxides were present. The average value is obtained as the depth position of the internal oxide layer of Si.

このＳｉの内部酸化は、酸素の鋼板１内部への拡散速度がＳｉの鋼板１表面への拡散速度よりも速い場合に発生するものであり、雰囲気の酸素濃度が高く、鋼板１中のＳｉの含有量が少ない場合に起こりやすくなる。このため、鋼板１のＳｉ含有量に応じて、上述の温度範囲における炉内の雰囲気ガスのｌｏｇ（ＰＨ２Ｏ／ＰＨ２）及び露点を調整することが好ましい。
上記では、Ｓｉ酸化物について説明したが、Ｍｎも焼鈍時においてＳｉとともに表層に析出・濃化しやすい元素であり、Ｓｉ−Ｍｎ酸化物として鋼板表面に露出することで、めっき性、化成処理性を劣化させてしまうおそれがある元素である。しかしながら、本実施形態に係る鋼板の製造方法によれば、焼鈍の加熱時の炉内雰囲気を制御することで、Ｓｉ酸化物だけでなく、Ｓｉ−Ｍｎ酸化物が鋼板の表面に露出することを抑制でき、めっき性、化成処理性に優れた高強度鋼板を製造することが可能となる。This internal oxidation of Si occurs when the diffusion rate of oxygen into the steel sheet 1 is higher than the diffusion rate of Si to the surface of the steel sheet 1. The oxygen concentration in the atmosphere is high, and the Si This is likely to occur when the content is small. For this reason, it is preferable to adjust the log (P H2O / P H2 ) and the dew point of the atmospheric gas in the furnace in the above temperature range according to the Si content of the steel sheet 1.
In the above description, the Si oxide has been described. However, Mn is also an element that easily precipitates and concentrates on the surface layer together with Si during annealing, and is exposed to the steel sheet surface as a Si-Mn oxide, thereby improving the plating property and the chemical conversion treatment property. It is an element that may deteriorate. However, according to the method for manufacturing a steel sheet according to the present embodiment, by controlling the furnace atmosphere during heating of annealing, not only the Si oxide but also the Si-Mn oxide is exposed on the surface of the steel sheet. It is possible to manufacture a high-strength steel sheet that can be suppressed and has excellent plating properties and chemical conversion treatment properties.

また、本実施形態において、脱炭層厚さが７０μｍ以下であれば、脱炭の進行を抑制できていると判断できる。本実施形態では、鋼板の板厚の１／４厚における硬質組織の面積分率Ｓ１と、鋼板の表層部における硬質組織の面積分率Ｓ２とを比較し、Ｓ２／Ｓ１が０．４０以下となる最大深さ位置を脱炭層の厚さとした。硬質組織とは、マルテンサイト、焼戻マルテンサイト、ベイナイト及び残留オーステナイトのうちの１つ以上からなる組織である。面積率は、鋼板の圧延方向に平行な板厚断面を観察面として試料を採取し、観察面を研磨、ナイタールエッチングし、電界放射型走査型電子顕微鏡（ＦＥ−ＳＥＭ）により、倍率５００〜３０００倍で３箇所以上の領域を観察して求める。すなわち、各観察領域において、鋼板の板面に５０μｍ以上の平行な線を引き、線が硬質組織と重なる総長さＬを求め、線の長さＬ０との比Ｌ／Ｌ０を求め、これらの平均値を、当該深さ位置における硬質組織の面積分率Ｓ２とすればよい。   In this embodiment, if the decarburized layer thickness is 70 μm or less, it can be determined that the progress of decarburization can be suppressed. In the present embodiment, the area fraction S1 of the hard structure in the 1/4 thickness of the steel sheet is compared with the area fraction S2 of the hard structure in the surface portion of the steel sheet, and S2 / S1 is 0.40 or less. The maximum depth position was defined as the thickness of the decarburized layer. The hard structure is a structure composed of one or more of martensite, tempered martensite, bainite, and retained austenite. The area ratio is determined by taking a sample with the thickness section parallel to the rolling direction of the steel sheet as an observation surface, polishing the observation surface, performing nital etching, and using a field emission scanning electron microscope (FE-SEM) with a magnification of 500 to 500 μm. Observation is made by observing three or more regions at 3000 times magnification. That is, in each observation region, a parallel line of 50 μm or more is drawn on the surface of the steel sheet, the total length L at which the line overlaps the hard structure is determined, the ratio L / L0 to the line length L0 is determined, and the average of these is calculated. The value may be the area fraction S2 of the hard tissue at the depth position.

各温度範囲におけるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値は、以下のようにして測定することができる。
まず、公知の測定装置を用いて、炉内の７００℃以下になっている位置、７００℃超８００℃以下になっている位置、８００℃超になっている位置のそれぞれにおいて、炉体の上段、中断、下段の最低１ヶ所ずつを含む計５か所において、露点、及び水素濃度を測定する。そして、平均した値を、当該温度域での露点、及び水素濃度とする。そして、測定した露点に基づいて、Ｔｅｔｅｎｓ式を用いて、当該温度域の水蒸気圧（ＰＨ２Ｏ）を求める。
なお、炉内における鋼板の温度は、炉内の温度と同等となる。すなわち、例えば炉内の７００℃超８００℃以下になっている位置では鋼板の温度も７００℃超８００℃以下となる。The furnace average value of log (P H2O / P H2 ) in each temperature range can be measured as follows.
First, using a known measuring device, the upper stage of the furnace body at each of a position of 700 ° C. or lower, a position of higher than 700 ° C. and 800 ° C. or lower, and a position of higher than 800 ° C. in the furnace. The dew point and the hydrogen concentration are measured at a total of five locations including at least one location at the bottom, interruption, and at the bottom. Then, the averaged values are used as the dew point and the hydrogen concentration in the temperature range. Then, based on the measured dew point, the water vapor pressure (P H2O ) in the temperature range is determined using the Tetens equation.
The temperature of the steel sheet in the furnace is equal to the temperature in the furnace. That is, for example, at a position in the furnace where the temperature is higher than 700 ° C. and equal to or lower than 800 ° C., the temperature of the steel sheet is also higher than 700 ° C. and equal to or lower than 800 ° C.

本実施形態に係る鋼板の製造方法によればめっき性及び化成処理性に優れた、引張強度７８０ＭＰａ以上の高強度鋼板が製造できる。   According to the method for manufacturing a steel sheet according to the present embodiment, a high-strength steel sheet excellent in plating property and chemical conversion treatment and having a tensile strength of 780 MPa or more can be manufactured.

次に、本実施形態に係る鋼板の連続焼鈍装置１０について、図を参照して説明する。本実施形態に係る鋼板の連続焼鈍装置１０は、上述した本実施形態に係る連続焼鈍工程を実施するのに適している。   Next, a continuous annealing apparatus 10 for a steel sheet according to the present embodiment will be described with reference to the drawings. The continuous annealing apparatus 10 for a steel sheet according to the present embodiment is suitable for performing the above-described continuous annealing step according to the present embodiment.

図１に示す鋼板の連続焼鈍装置１０は、鋼板１を走行させながら焼鈍する装置であり、鋼板１は、図１の左下から鋼板の連続焼鈍装置１０に装入される。鋼板の連続焼鈍装置１０は、鋼板１の走行方向上流側に位置する７００℃以下まで鋼板を加熱する第１加熱帯１１、この第１加熱帯１１の下流側に位置する７００℃超８００℃以下まで鋼板を加熱する第２加熱帯１２、第２加熱帯１２の下流側に位置する８００℃超の温度域に鋼板を加熱する第３加熱帯１３、及び第３加熱帯１３の後段側に位置する均熱帯１４を備えている。この連続焼鈍装置１０においては、第１加熱帯１１、第２加熱帯１２、第３加熱帯１３、及び均熱帯１４は、いずれも間接加熱方式の雰囲気炉である。炉内は、所定の水素濃度を有する雰囲気に制御されている。   A continuous annealing apparatus 10 for a steel sheet illustrated in FIG. 1 is an apparatus that performs annealing while running the steel sheet 1. The steel sheet 1 is charged into the continuous annealing apparatus 10 for a steel sheet from the lower left of FIG. The continuous annealing apparatus 10 for a steel sheet includes a first heating zone 11 for heating the steel sheet up to 700 ° C. or less located upstream of the steel sheet 1 in the traveling direction, and a 700 ° C. or more and 800 ° C. or less located downstream of the first heating zone 11. A second heating zone 12 for heating the steel sheet up to the second heating zone 12, a third heating zone 13 for heating the steel sheet to a temperature range higher than 800 ° C. located downstream of the second heating zone 12, and a third heating zone 13 located downstream of the third heating zone 13. Tropics 14 are provided. In the continuous annealing apparatus 10, the first heating zone 11, the second heating zone 12, the third heating zone 13, and the soaking zone 14 are all indirect heating type atmosphere furnaces. The inside of the furnace is controlled to an atmosphere having a predetermined hydrogen concentration.

そして、本実施形態に係る鋼板の連続焼鈍装置１０の第２加熱帯１２には、鋼板１の走行方向上流側に向けて、水蒸気または加湿ガスを炉内に供給する雰囲気ガス導入部１５（炉内雰囲気調整手段）が設けられている。
この雰囲気ガス導入部１５からの水蒸気等の供給によって、鋼板１の温度が７００℃超８００℃以下で鋼板を加熱する際、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−１．３６＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７の範囲内に制御できる。In the second heating zone 12 of the continuous steel sheet annealing apparatus 10 according to the present embodiment, an atmosphere gas introduction unit 15 (furnace) for supplying steam or humidified gas into the furnace toward the upstream side in the traveling direction of the steel sheet 1. Internal atmosphere adjusting means).
When the steel sheet 1 is heated at a temperature higher than 700 ° C. and equal to or lower than 800 ° C. by the supply of water vapor or the like from the atmospheric gas introduction unit 15, the water pressure P H2O in the atmosphere in the furnace and the hydrogen partial pressure P H2 are used. The furnace body average value of the relational expression log (P H2O / P H2 ) can be controlled within the range of −1.36 <log (P H2O / P H2 ) <− 0.07.

また、雰囲気ガス導入部１５からの水蒸気等の供給によって、第１加熱帯１１ではｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７、第３加熱帯１３の８００℃超の範囲においては、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３、均熱帯１４においては、その温度が８００℃以下の場合には、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．０７、８００℃超の場合には、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３の範囲内に制御できる。第１加熱帯１１、第３加熱帯１３または均熱帯１４のｌｏｇ（ＰＨ２Ｏ／ＰＨ２）をより精度良く制御するため、第１加熱帯１１、第３加熱帯１３または均熱帯１４に、雰囲気ガス導入部をさらに設けても構わない
また、雰囲気ガス導入部からの水蒸気等の供給によって、鋼板の温度が８００℃超であるときの、炉内の雰囲気の露点や炉内の炉内の雰囲気中の水素濃度も制御することができる。Further, by the supply of water vapor or the like from the atmospheric gas introduction unit 15, a furnace body average value of the first heating zone 11 log (P H2O / P H2 ) is -3.01 <log (P H2O / P H2) <- 0.07, -3.01 <log (P H2O / P H2 ) ≦ −0.53 in the third heating zone 13 exceeding 800 ° C., In this case, -3.01 <log (P H2O / P H2 ) ≦ −0.07, and in the case of exceeding 800 ° C., −3.01 <log (P H2O / P H2 ) ≦ −0.53. Can be controlled within the range. In order to more accurately control the log (P H2O / P H2 ) of the first heating zone 11, the third heating zone 13, or the solitary tropical zone 14, the atmosphere is added to the first heating zone 11, the third heating zone 13, or the soothing tropical zone 14. A gas introduction unit may be further provided. Also, when the temperature of the steel sheet exceeds 800 ° C. by supplying steam or the like from the atmospheric gas introduction unit, the dew point of the atmosphere in the furnace or the atmosphere in the furnace in the furnace. The concentration of hydrogen in it can also be controlled.

本実施形態に係る鋼板の連続焼鈍装置１０を用いて連続焼鈍を行えば、所定の強度を備え、めっき性及び化成処理性に優れた高強度鋼板が製造できる。   If continuous annealing is performed using the steel sheet continuous annealing apparatus 10 according to the present embodiment, a high-strength steel sheet having a predetermined strength and excellent in plating properties and chemical conversion treatment properties can be manufactured.

以上、本実施形態に係る鋼板の製造方法、及び、鋼板の連続焼鈍装置について説明した。しかしながら、本発明はこれらに限定されることはなく、その発明の技術的思想を逸脱しない範囲で適宜変更可能である。
例えば、鋼板の組成は、本実施形態に例示されたものに限定されることはなく、その他の元素を要求特性に応じて含有させたものであってもよい。
また、本実施形態に係る鋼板の製造方法では、図１に示す連続焼鈍装置によって連続焼鈍工程を実施することが好ましいとして説明したが、これに限定されることはない。すなわち、加熱時の鋼板１の温度が７００℃超８００℃以下となるよう鋼板を加熱する範囲において、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を、−１．３６≦ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．０７、鋼板１の温度が７００℃以下となるよう鋼板を加熱する範囲において、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７、８００℃超で鋼板を加熱する範囲においては、−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３、最高加熱温度による保熱帯においては、上記加熱時の温度における制御に準じた範囲内にｌｏｇ（ＰＨ２Ｏ／ＰＨ２）を制御することのできる炉内雰囲気調整手段を備えた他の連続焼鈍炉を用いてもよい。The method for manufacturing a steel sheet and the continuous annealing apparatus for a steel sheet according to the present embodiment have been described above. However, the present invention is not limited to these, and can be appropriately modified without departing from the technical idea of the invention.
For example, the composition of the steel sheet is not limited to those exemplified in the present embodiment, and may include other elements in accordance with required characteristics.
Further, in the method for manufacturing a steel sheet according to the present embodiment, it has been described that the continuous annealing step is preferably performed by the continuous annealing apparatus illustrated in FIG. 1, but the present invention is not limited thereto. That is, in a range in which the steel sheet 1 is heated so that the temperature of the steel sheet 1 at the time of heating becomes higher than 700 ° C. and equal to or lower than 800 ° C., a relational expression log (P) composed of the water pressure P H2O in the atmosphere in the furnace and the hydrogen partial pressure P H2. The average value of the furnace body of H 2 O / P H2 ) is -1.36 ≦ log (P H2O / P H2 ) ≦ −0.07, and the steel sheet 1 is heated so that the temperature of the steel sheet 1 becomes 700 ° C. or less. 3.01 <log (P H2O / P H2 ) <− 0.07, and in the range where the steel sheet is heated above 800 ° C., −3.01 <log (P H2O / P H2 ) ≦ −0.53, the highest In the preservation by heating temperature, another continuous annealing furnace equipped with a furnace atmosphere adjusting means capable of controlling log (P H2 O / P H2 ) within a range similar to the control of the temperature at the time of heating is used. You may.

本発明の効果を確認すべく実施した実験結果について説明する。
上述の実施形態において説明した鋼板の連続焼鈍装置を用いて、公知の方法で製造した、板厚１．２ｍｍの冷延鋼板に対して、連続焼鈍を実施した。焼鈍に供した鋼板の組成は、表１に示す通りであった。The results of experiments performed to confirm the effects of the present invention will be described.
Using the continuous annealing apparatus for steel sheets described in the above embodiment, continuous annealing was performed on a cold-rolled steel sheet having a thickness of 1.2 mm manufactured by a known method. The composition of the steel sheet subjected to the annealing was as shown in Table 1.

Figure 0006673461
Figure 0006673461

連続焼鈍工程では、鋼板の連続焼鈍装置により、表２〜８に示す条件（保持板温、保持時間）で連続焼鈍を実施した。最高加熱温度の上限は実製造の上で達成可能な値として９００℃とした。また、加熱、保持時における炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を表２〜８に示す条件とした。炉内の雰囲気中の水素濃度は、１．０〜５．０％であり、鋼板温度が８００℃超であるときの露点は、いずれも−１０℃未満であった。In the continuous annealing process, continuous annealing was performed by the steel sheet continuous annealing apparatus under the conditions (holding plate temperature, holding time) shown in Tables 2 to 8. The upper limit of the maximum heating temperature was 900 ° C. as a value achievable in actual production. Further, the furnace body average value of the relational expression log (P H2O / P H2 ) consisting of the water pressure P H2O in the atmosphere in the furnace during heating and holding and the hydrogen partial pressure P H2 was determined according to the conditions shown in Tables 2 to 8. did. The hydrogen concentration in the furnace atmosphere was 1.0 to 5.0%, and the dew point was less than -10 ° C when the steel sheet temperature was higher than 800 ° C.

Figure 0006673461
Figure 0006673461

Figure 0006673461
Figure 0006673461

Figure 0006673461
Figure 0006673461

Figure 0006673461
Figure 0006673461

Figure 0006673461
Figure 0006673461

Figure 0006673461
Figure 0006673461

Figure 0006673461
上述のようにして焼鈍した鋼板に関して、Ｓｉの内部酸化層深さ及び脱炭層厚さを評価した。
Figure 0006673461
 With respect to the steel sheet annealed as described above, the depth of the internal oxide layer of Si and the thickness of the decarburized layer were evaluated.

（Ｓｉの内部酸化層深さ位置）
鋼板の表層部のフェライトにおいて、最大長さが２５ｎｍ以上のＳｉ酸化物が１．０×１０１２個／ｍ２以上存在する領域をＳｉの内部酸化層と定義した。
具体的には、鋼板の圧延方向に平行な板厚断面を観察面として試料を採取し、観察面を研磨、ナイタールエッチングし、電界放射型走査型電子顕微鏡（ＦＥ−ＳＥＭ：Ｆｉｅｌｄ Ｅｍｉｓｓｉｏｎ Ｓｃａｎｎｉｎｇ Ｅｌｅｃｔｒｏｎ Ｍｉｃｒｏｓｃｏｐｅ）により、倍率５０００倍以上で観察した。鋼板の厚さ方向に１．０μｍ、圧延方向に２０μｍの矩形の領域を、３箇所任意に設定した。その３箇所それぞれにおいて、領域内のＳｉ酸化物の個数をカウントし、Ｓｉ酸化物が１０個以上存在した場合に、その領域をＳｉの内部酸化層とし、Ｓｉ酸化物が１０個以上存在した領域の最大深さの３箇所の平均値をＳｉの内部酸化層深さ位置とした。評価結果を表１に示す。内部酸化層深さが０．１μｍ以上であれば、内部酸化が十分であると判断した。(Depth position of internal silicon oxide layer)
In the ferrite in the surface layer portion of the steel sheet, a region where the maximum length of Si oxide having a maximum length of 25 nm or more was 1.0 × 10 12 / m 2 or more was defined as an internal oxide layer of Si.
Specifically, a sample is sampled by using a plate thickness section parallel to the rolling direction of the steel sheet as an observation surface, the observation surface is polished and nital-etched, and a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron) is used. Microscope) at a magnification of 5000 or more. Three rectangular regions of 1.0 μm in the thickness direction of the steel sheet and 20 μm in the rolling direction were arbitrarily set. At each of the three places, the number of Si oxides in the region was counted, and when 10 or more Si oxides were present, the region was used as an internal oxide layer of Si, and the region where 10 or more Si oxides were present The average value of the three maximum depths was determined as the internal oxide layer depth position of Si. Table 1 shows the evaluation results. When the depth of the internal oxide layer was 0.1 μm or more, it was determined that internal oxidation was sufficient.

（脱炭層厚さ）
鋼板の板厚の１／４厚における硬質組織の面積分率Ｓ１と、鋼板の表層部における硬質組織の面積分率Ｓ２とを比較し、Ｓ２／Ｓ１が０．４０以下となる最大深さ位置を脱炭層の厚さとした。硬質組織とは、マルテンサイト、焼戻マルテンサイト、ベイナイト及び残留オーステナイトのうちの１つ以上からなる組織である。面積率は、鋼板の圧延方向に平行な板厚断面を観察面として試料を採取し、観察面を研磨、ナイタールエッチングし、電界放射型走査型電子顕微鏡（ＦＥ−ＳＥＭ）により、倍率５００〜３０００倍で３箇所の領域を観察して求めた。すなわち、各観察領域において、鋼板の板面に５０μｍ以上の平行な線を引き、線が硬質組織と重なる総長さＬを求め、線の長さＬ０との比Ｌ／Ｌ０を求め、これらの平均値を、当該深さ位置における硬質組織の面積分率Ｓ２とした。評価結果を表１に示す。脱炭層厚さが７０μｍ以下であれば、脱炭の進行を抑制できていると判断した。(Decarburized layer thickness)
The area fraction S1 of the hard structure at a thickness of 1/4 of the thickness of the steel sheet is compared with the area fraction S2 of the hard structure in the surface portion of the steel sheet, and the maximum depth position where S2 / S1 is 0.40 or less. Was the thickness of the decarburized layer. The hard structure is a structure composed of one or more of martensite, tempered martensite, bainite, and retained austenite. The area ratio is determined by taking a sample with the thickness section parallel to the rolling direction of the steel sheet as an observation surface, polishing the observation surface, performing nital etching, and using a field emission scanning electron microscope (FE-SEM) with a magnification of 500 to 500 μm. It was determined by observing three regions at 3000 times magnification. That is, in each observation region, a parallel line of 50 μm or more is drawn on the surface of the steel sheet, the total length L at which the line overlaps the hard structure is determined, the ratio L / L0 to the line length L0 is determined, and the average of these is calculated. The value was defined as the area fraction S2 of the hard tissue at the depth position. Table 1 shows the evaluation results. When the thickness of the decarburized layer was 70 μm or less, it was determined that the progress of decarburization could be suppressed.

さらに、これらの鋼板について、化成処理性、めっき性、及び引張強度の評価を行った。   Furthermore, these steel sheets were evaluated for chemical conversion property, plating property, and tensile strength.

（化成処理性）
まず、連続焼鈍後の鋼板を７０ｍｍ×１５０ｍｍの試験片に切断し、これに日本パーカライジング社製の脱脂剤（商品名：ファインクリーナーＥ２０８３）の１８ｇ／ｌ水溶液を、４０℃で１２０秒間スプレーし、水洗することで脱脂を行った。次に、脱脂した冷延鋼板を日本パーカライジング社製の表面調整剤（商品名：プレパレンＸＧ）の０．５ｇ／ｌ水溶液に常温で６０秒間浸漬した。その後、日本パーカライジング社製のりん酸亜鉛処理剤（商品名：パルボンドＬ３０６５）に１２０秒間浸漬し、水洗、乾燥することで、化成処理を施した。その後、化成処理を施した試験片の長さ方向に沿って３か所（中央部および両端部）を、走査型電子顕微鏡（ＳＥＭ）を使って１０００倍の倍率で観察し、りん酸亜鉛被膜の結晶粒の付着度合いを観察した。
化成処理被膜のりん酸亜鉛結晶が緻密に付着していた場合は「ＧＯＯＤ」、りん酸亜鉛結晶が疎で、隣り合う結晶間に僅かな隙間（りん酸亜鉛被膜が付着していない、一般に「スケ」と呼ばれる部分）が見られる場合を「ＦＡＩＲ」、明らかに化成処理被膜が被覆されていない箇所が見られる場合を「ＰＯＯＲ」と評価した。(Chemical conversion treatment)
First, the steel sheet after continuous annealing was cut into a test piece of 70 mm × 150 mm, and an 18 g / l aqueous solution of a degreasing agent (trade name: Fine Cleaner E2083) manufactured by Nippon Parkerizing Co. was sprayed at 40 ° C. for 120 seconds, Degreasing was performed by washing with water. Next, the degreased cold-rolled steel sheet was immersed in a 0.5 g / l aqueous solution of a surface conditioner (trade name: Preparen XG) manufactured by Nippon Parkerizing Co., Ltd. at room temperature for 60 seconds. Thereafter, a chemical conversion treatment was performed by immersing in a zinc phosphate treating agent (trade name: Palbond L3065) manufactured by Nippon Parkerizing Co., Ltd. for 120 seconds, washing with water and drying. Thereafter, three points (center and both ends) along the length direction of the test piece subjected to the chemical conversion treatment were observed at a magnification of 1000 times using a scanning electron microscope (SEM), and the zinc phosphate coating was observed. The degree of adhesion of the crystal grains was observed.
If the zinc phosphate crystals in the chemical conversion treatment film were densely attached, "GOOD" indicates that the zinc phosphate crystals were sparse, and a slight gap between adjacent crystals (the zinc phosphate film was not attached, generally " (A part called "scale") was evaluated as "FAIR", and a case where a part not clearly coated with the chemical conversion treatment film was evaluated as "POOR".

（引張強度）
連続焼鈍後の鋼板から、圧延方向に直角方向にＪＩＳ Ｚ２２０１に記載の５号試験片を切り出し、ＪＩＳ Ｚ２２４１に準拠して、常温で引張試験を行うことにより引張強度及び伸びを求めた。
そして、引張強度が７８０ＭＰａ以上であって、かつ板厚中心から厚み方向で８割の範囲外を削りとったときの試験片の引張強度と比較して引張強度の低下が１．０％未満であった場合を「ＧＯＯＤ」とした。一方、引張強度が７８０ＭＰａ未満であった場合、及び、引張強度が７８０ＭＰａ以上であっても、板厚中心から厚み方向で８割の範囲外を削りとったときの試験片の引張強度と比較して１．０％以上低下していた場合を「ＰＯＯＲ」とした。(Tensile strength)
From the steel sheet after continuous annealing, a No. 5 test piece described in JIS Z2201 was cut out in a direction perpendicular to the rolling direction, and a tensile test was performed at room temperature in accordance with JIS Z2241 to determine tensile strength and elongation.
When the tensile strength is 780 MPa or more, and the tensile strength of the test piece is less than 1.0% as compared with the tensile strength of the test piece when the outside of the range of 80% is cut off in the thickness direction from the center of the plate thickness. When there was, it was set as "GOOD". On the other hand, when the tensile strength was less than 780 MPa, and even when the tensile strength was 780 MPa or more, the tensile strength was compared with the tensile strength of the test piece when the outside of the range of 80% was cut off in the thickness direction from the plate thickness center. Was decreased by 1.0% or more as "POOR".

（めっき性）
連続焼鈍後の鋼板に、公知の方法で溶融亜鉛めっき処理を施し、溶融亜鉛めっき処理された鋼板について、目視にて外観を評価するとともに、めっき剥離試験を行ってめっき密着性を評価した。具体的には以下のように評価した。
「外観検査」
溶融亜鉛めっき鋼板の表面の外観について、該鋼板から全幅に亘る長さ１ｍのサンプルを５枚連続で採取し、目視で不めっきの発生状況を以下の基準で判断した。
ＧＯＯＤ：直径０．５ｍｍ以上の不めっきは観察されなかった（実用上許容しうる外観）
ＰＯＯＲ：直径０．５ｍｍ以上の不めっきが観察された（外観上の許容範囲を逸脱）
「めっき剥離試験」
鋼板に圧縮応力が加わる加工時におけるめっき密着性を評価する、ＪＩＳＺ ２２４８に記載の「金属材料曲げ試験方法」に従い、めっき剥離試験を行った。具体的には、文献「溶融亜鉛めっき鋼鈑マニュアル，ｐ５３−５５」に開示されているように、各鋼板を用いて６０°Ｖ字曲げ試験を行った後、曲げ部の内側にテープを貼り、そのテープを引き剥がした。そして、テープとともに剥離しためっき層の剥離状況からめっき密着性を以下の基準で評価した。テープには、ニチバン製「セロテープ」（登録商標）を用いた。
ＧＯＯＤ：剥離幅が７．０ｍｍ未満（実用上許容し得る）
ＰＯＯＲ：剥離幅が７．０ｍｍ以上（実用上許容不可）(Plating property)
The steel sheet after the continuous annealing was subjected to a hot-dip galvanizing treatment by a known method, and the steel sheet subjected to the hot-dip galvanizing treatment was visually evaluated for its appearance, and subjected to a plating peeling test to evaluate the plating adhesion. Specifically, evaluation was made as follows.
"Visual inspection"
Regarding the appearance of the surface of the hot-dip galvanized steel sheet, five samples each having a length of 1 m over the entire width were continuously sampled from the steel sheet, and the occurrence of non-plating was visually determined based on the following criteria.
GOOD: no plating with a diameter of 0.5 mm or more was not observed (appearance acceptable for practical use)
POOR: non-plating with a diameter of 0.5 mm or more was observed (outside the allowable range in appearance)
"Plating peel test"
A plating peeling test was performed according to “Metallic material bending test method” described in JISZ2248, which evaluates plating adhesion during processing in which compressive stress is applied to a steel sheet. Specifically, as disclosed in the document “Hot Dip Galvanized Steel Sheet Manual, p53-55”, a 60 ° V-shaped bending test was performed using each steel sheet, and then tape was applied inside the bent portion. The tape was peeled off. Then, the plating adhesion was evaluated based on the peeling state of the plating layer peeled with the tape according to the following criteria. The tape used was "Cellotape" (registered trademark) manufactured by Nichiban.
GOOD: Peeling width is less than 7.0 mm (practically acceptable)
POOR: Peeling width is 7.0 mm or more (practically unacceptable)

試験Ｎｏ．１、Ｎｏ．３０、Ｎｏ．５９、Ｎｏ．８８、Ｎｏ．１１７は、焼鈍時の鋼板温度が７００℃以下であり、Ｓｉの内部酸化及び脱炭がともに発生していない。試験Ｎｏ．１９、Ｎｏ．４８、Ｎｏ．７７、Ｎｏ．１０６、Ｎｏ．１３５では、加熱時の鋼板温度が７００℃超８００℃以下で鋼板を加熱する範囲において、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が−１．３６以下であり、Ｓｉの内部酸化が不十分である。
試験Ｎｏ．２、Ｎｏ．３１、Ｎｏ．６０、Ｎｏ．８９、Ｎｏ．１１８では、加熱保持時の前記鋼板の温度が９００℃であるが、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が−０．０７以上であり、脱炭厚さが過剰に厚くなっている。
試験Ｎｏ．３〜５、Ｎｏ．３２〜３４、Ｎｏ．６１〜６３、Ｎｏ．９０〜９２、Ｎｏ．１１９〜１２１では、８００℃超のｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が−０．５４以上であり、脱炭厚さが過剰に厚くなっている。
また、試験Ｎｏ．１４６〜１４８、Ｎｏ．１５０、Ｎｏ．１５１、Ｎｏ．１５３〜１５５、Ｎｏ．１５７、Ｎｏ．１５８、Ｎｏ．１６０〜１６２、Ｎｏ．１６４、Ｎｏ．１６５、Ｎｏ．１６７〜Ｎｏ．１６９、Ｎｏ．１７１、Ｎｏ．１７２、Ｎｏ．１７２〜１７６、Ｎｏ．１７８、Ｎｏ．１７９、Ｎｏ．１８１〜１８６では、鋼板温度が７００℃以下、７００℃超８００℃以下、８００℃超のいずれかのｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値が本発明を外れており、内部酸化が不十分であったり、脱炭層厚さが過剰であったり、引張強度やめっき性に劣っていた。Test No. 1, No. 30, no. 59, no. 88, no. In No. 117, the steel sheet temperature during annealing was 700 ° C. or less, and neither internal oxidation nor decarburization of Si occurred. Test No. 19, no. 48, no. 77, No. 106, no. In 135, the relational expression log (P H2O / P) comprising the water pressure P H2O in the atmosphere in the furnace and the hydrogen partial pressure P H2 in a range in which the steel sheet is heated at a temperature of over 700 ° C. and not more than 800 ° C. The average value of the furnace body of H2 ) is -1.36 or less, and the internal oxidation of Si is insufficient.
Test No. 2, No. 31, No. 60, no. 89, no. At 118, the temperature of the steel sheet at the time of heating and holding is 900 ° C., but the furnace body of the relational expression log (P H2O / P H2 ) consisting of the water pressure P H2O and the hydrogen partial pressure P H2 in the atmosphere in the furnace. The average value is -0.07 or more, and the decarburized thickness is excessively large.
Test No. Nos. 3 to 5; 32 to 34; Nos. 61 to 63; Nos. 90 to 92; In the case of 119 to 121, the furnace average value of log (P H2O / P H2 ) exceeding 800 ° C. is −0.54 or more, and the decarburized thickness is excessively large.
Test No. 146-148, no. 150, no. 151, no. Nos. 153-155; 157, no. 158, no. No. 160 to 162; 164, no. 165, no. 167-No. 169, no. 171, No. 1; 172, no. Nos. 172 to 176; 178, no. 179, no. 181 to 186, the furnace body average value of any log (P H2O / P H2 ) of the steel sheet temperature of 700 ° C. or lower, 700 ° C. or higher and 800 ° C. or lower and higher than 800 ° C. is out of the range of the present invention. It was insufficient, the thickness of the decarburized layer was excessive, or the tensile strength and plating properties were poor.

これに対して、加熱保持時の前記鋼板の温度を７５０〜９００℃で鋼板を加熱する連続焼鈍工程において、鋼板の７００℃超８００℃以下での炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−１．３６＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７の範囲内とし、鋼板の温度が７００℃以下のｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７、８００℃超でのｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３とした試験Ｎｏ．６〜１８、Ｎｏ．２０〜２９、Ｎｏ．３５〜４７、Ｎｏ．４９〜５８、Ｎｏ．６４〜７６、Ｎｏ．７８〜８７、Ｎｏ．９３〜１０５、Ｎｏ．１０７〜１１６、Ｎｏ．１２２〜１３４、Ｎｏ．１３６〜１４５、Ｎｏ．１４９、Ｎｏ．１５２、Ｎｏ．１５６、Ｎｏ．１５９、Ｎｏ．１６３、Ｎｏ．１６６、Ｎｏ．１７０、Ｎｏ．１７３、Ｎｏ．１７７、Ｎｏ．１８０は、Ｓｉの内部酸化層の深さ位置が十分に深く、かつ、脱炭層厚さが薄くなっている。
以上のことから、本発明によれば、Ｓｉを鋼板の内部で酸化させて鋼板の表面にＳｉ酸化物が露出することを抑制し、かつ、鋼板からの脱炭の進行を抑制できることが確認された。On the other hand, in the continuous annealing step of heating the steel sheet at a temperature of 750 to 900 ° C. during the heating and holding, the water pressure P H2O and hydrogen in the atmosphere in the furnace between 700 ° C. and 800 ° C. The furnace body average value of the relational expression log (P H2O / P H2 ) composed of the partial pressure P H2 is set within the range of −1.36 <log (P H2O / P H2 ) <− 0.07, and the temperature of the steel sheet is The furnace average value of log (P H2O / P H2 ) of 700 ° C. or less is −3.01 <log (P H2O / P H2 ) <− 0.07, and log (P H2O / P H2 ) above 800 ° C. Test No. with the furnace body average value of −3.01 <log (P H2O / P H2 ) ≦ −0.53. Nos. 6 to 18; 20 to 29; Nos. 35 to 47; Nos. 49 to 58; 64 to 76, No. 7; 78-87, No. 7; Nos. 93 to 105; Nos. 107 to 116; 122 to 134; 136-145, No. 149, no. 152, no. 156, no. 159, no. 163, no. 166, no. 170, no. 173, no. 177, no. In reference numeral 180, the depth position of the internal oxide layer of Si is sufficiently deep and the thickness of the decarburized layer is small.
From the above, according to the present invention, it has been confirmed that Si can be oxidized inside the steel sheet to suppress the exposure of the Si oxide to the surface of the steel sheet, and that the progress of decarburization from the steel sheet can be suppressed. Was.

焼鈍時の雰囲気を制御することにより、Ｓｉを鋼板の内部で酸化させて鋼板の表面にＳｉ酸化物が露出することを抑制し、かつ、鋼板からの脱炭の進行を抑制可能な高強度鋼板の製造方法、及び、この高強度鋼板の製造方法に適した鋼板の連続焼鈍装置を提供することができる。   By controlling the atmosphere during annealing, high-strength steel sheet that can oxidize Si inside the steel sheet and suppress the exposure of Si oxide on the surface of the steel sheet, and can also suppress the progress of decarburization from the steel sheet And a continuous annealing apparatus for a steel sheet suitable for the method for manufacturing a high-strength steel sheet.

１ 鋼板
１０ 鋼板の連続焼鈍装置
１１ 第１加熱帯
１２ 第２加熱帯
１３ 第３加熱帯
１４ 均熱帯
１５ 雰囲気ガス導入部（炉内雰囲気調整手段）DESCRIPTION OF SYMBOLS 1 Steel plate 10 Continuous annealing apparatus of steel plate 11 1st heating zone 12 2nd heating zone 13 3rd heating zone 14 Uniform tropical zone 15 Atmospheric gas introduction part (furnace atmosphere adjusting means)

Claims (4)

引張強度が７８０ＭＰａ以上、Ｓｉの内部酸化層深さが０．１μｍ以上、かつ脱炭層厚さが７０μｍ以下である高強度鋼板の製造方法であって、
化学組成として、質量％で、Ｃ：０．０５０〜０．４０％、Ｓｉ：０．１０〜２．５０％、Ｍｎ：１．２０〜３．５０％、Ｃｒ：０〜０．８０％、Ｎｉ：０〜５．００％、Ｃｕ：０〜３．００％、Ｎｂ：０〜０．１０％、Ｍｇ：０〜０．０１０％、Ｔｉ：０〜０．１０％、Ｂ：０〜０．０１０％、Ｍｏ：０〜０．５％を含有し、残部がＦｅ及び不純物からなり、前記不純物として、Ｐ：０．１００％以下、Ｓ：０．０１０％以下、Ａｌ：１．２００％以下、Ｎ：０．０１００％以下、に制限した鋼板を、８００℃超、９００℃以下の温度範囲まで加熱して、前記温度範囲で０〜３００秒保持することによって連続焼鈍を行う連続焼鈍工程を有し、
前記連続焼鈍工程では、前記温度範囲まで前記加熱を行う際、及び前記温度範囲での前記保持を行う際、
炉内の雰囲気中の水素濃度を、１０体積％未満とし、
前記鋼板の温度が７００℃以下であるときの、前記炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式であるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を、下記式（ｉ）の範囲とし、
前記鋼板の温度が７００℃超８００℃以下であるときの、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｉｉ）の範囲とし、
前記鋼板の温度が８００℃超であるときの、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｉｉｉ）の範囲で、前記鋼板の温度が７００℃超８００℃以下であるときの前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の前記炉体平均値よりも低く、かつ露点を−１０℃未満とする、
ことを特徴とする鋼板の製造方法。
−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７ （ｉ）
−１．３６＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７ （ｉｉ）
−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３ （ｉｉｉ） A method for producing a high-strength steel sheet having a tensile strength of 780 MPa or more, an internal oxide layer depth of Si of 0.1 μm or more, and a decarburized layer thickness of 70 μm or less,
As the chemical composition, in mass%, C: 0.050 to 0.40%, Si: 0.10 to 2.50%, Mn: 1.20 to 3.50%, Cr: 0 to 0.80%, Ni: 0 to 5.00%, Cu: 0 to 3.00%, Nb: 0 to 0.10%, Mg: 0 to 0.010%, Ti: 0 to 0.10%, B: 0 to 0 0.010%, Mo: 0 to 0.5%, with the balance being Fe and impurities, P: 0.100% or less, S: 0.010% or less, Al: 1.200% Hereinafter, a continuous annealing step in which a steel sheet limited to N: 0.0100% or less is heated to a temperature range of more than 800 ° C and 900 ° C or less , and is maintained at the temperature range for 0 to 300 seconds to perform continuous annealing. Has,
In the continuous annealing step, when performing the heating to the temperature range, and when performing the holding in the temperature range,
The hydrogen concentration in the furnace atmosphere is less than 10% by volume,
Furnace average value of log (P H2O / P H2 ), which is a relational expression consisting of the water pressure P H2O in the atmosphere in the furnace and the hydrogen partial pressure P H2 when the temperature of the steel sheet is 700 ° C. or less. Is the range of the following formula (i),
When the temperature of the steel sheet is higher than 700 ° C. and equal to or lower than 800 ° C., the furnace body average value of the log (P H2 O / P H2 ) in the furnace atmosphere is defined as a range of the following formula (ii):
When the temperature of the steel sheet exceeds 800 ° C., the furnace body average value of the log (P H2O / P H2 ) in the atmosphere in the furnace is set within the range of the following formula (iii), and the temperature of the steel sheet is 700 When the log (P H2 O / P H2 ) in the atmosphere in the furnace is lower than the furnace body average value and the dew point is less than −10 ° C.
A method for producing a steel sheet, comprising:
−3.01 <log (P H2O / P H2 ) <− 0.07 (i)
−1.36 <log (P H2O / P H2 ) <− 0.07 (ii)
−3.01 <log (P H2O / P H2 ) ≦ −0.53 (iii) 前記化学組成が、質量％で
Ｃｒ：０．０１〜０．８０％、
Ｎｉ：０．０１〜５．００％、
Ｃｕ：０．０１〜３．００％、
Ｎｂ：０．００１〜０．１０％、
Ｍｇ：０．０００１〜０．０１０％、
Ｔｉ：０．００１〜０．１０％、
Ｂ：０．０００１〜０．０１０％、
Ｍｏ：０．０１〜０．５％、
から選択される１種または２種以上を含有する
ことを特徴とする請求項１に記載の鋼板の製造方法。 The chemical composition is represented by mass% Cr: 0.01 to 0.80%,
Ni: 0.01-5.00%,
Cu: 0.01 to 3.00%,
Nb: 0.001 to 0.10%,
Mg: 0.0001 to 0.010%,
Ti: 0.001 to 0.10%,
B: 0.0001 to 0.010%,
Mo: 0.01-0.5%,
The method for producing a steel sheet according to claim 1, wherein the steel sheet contains one or more kinds selected from the group consisting of: 前記鋼板の温度が７００℃超８００℃以下であるときの、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｖｉｉ）の範囲とすることを特徴とする請求項１または２に記載の鋼板の製造方法。
−１．００＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．６７ （ｖｉｉ） When the temperature of the steel sheet is more than 700 ° C. and 800 ° C. or less, the furnace body average value of the log (P H2O / P H2 ) in the atmosphere in the furnace is set to be in a range of the following formula (vii). The method for producing a steel sheet according to claim 1.
−1.00 <log (P H2O / P H2 ) <− 0.67 (vii) 化学組成として、質量％で、Ｃ：０．０５０〜０．４０％、Ｓｉ：０．１０〜２．５０％、Ｍｎ：１．２０〜３．５０％、Ｃｒ：０〜０．８０％、Ｎｉ：０〜５．００％、Ｃｕ：０〜３．００％、Ｎｂ：０〜０．１０％、Ｍｇ：０〜０．０１０％、Ｔｉ：０〜０．１０％、Ｂ：０〜０．０１０％、Ｍｏ：０〜０．５％を含有し、残部がＦｅ及び不純物からなり、前記不純物として、Ｐ：０．１００％以下、Ｓ：０．０１０％以下、Ａｌ：１．２００％以下、Ｎ：０．０１００％以下、に制限した鋼板に連続焼鈍を実施する鋼板の連続焼鈍装置であって、
炉内の雰囲気中の水素濃度を、１０体積％未満とし、前記鋼板の温度が７００℃以下の場合に、炉内の雰囲気中の水分圧ＰＨ２Ｏと水素分圧ＰＨ２とからなる関係式であるｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｉｖ）の範囲に調整し、前記鋼板の温度が７００℃超８００℃以下の場合に、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｖ）の範囲に調整し、前記鋼板の温度が８００℃超の場合に、前記炉内の雰囲気中の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の炉体平均値を下記式（ｖｉ）の範囲で、前記鋼板の温度が７００℃超８００℃以下の場合の前記ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）の前記炉体平均値よりも低く、かつ露点を−１０℃未満に調整する炉内雰囲気調整手段を備え、
請求項１に記載の鋼板の製造方法に用いる
ことを特徴とする鋼板の連続焼鈍装置。
−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７ （ｉｖ）
−１．３６＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）＜−０．０７ （ｖ）
−３．０１＜ｌｏｇ（ＰＨ２Ｏ／ＰＨ２）≦−０．５３ （ｖｉ） As the chemical composition, in mass%, C: 0.050 to 0.40%, Si: 0.10 to 2.50%, Mn: 1.20 to 3.50%, Cr: 0 to 0.80%, Ni: 0 to 5.00%, Cu: 0 to 3.00%, Nb: 0 to 0.10%, Mg: 0 to 0.010%, Ti: 0 to 0.10%, B: 0 to 0 0.010%, Mo: 0 to 0.5%, with the balance being Fe and impurities, P: 0.100% or less, S: 0.010% or less, Al: 1.200% Hereinafter, a continuous annealing apparatus for a steel sheet that performs continuous annealing on a steel sheet limited to N: 0.0100% or less,
When the hydrogen concentration in the atmosphere in the furnace is less than 10% by volume and the temperature of the steel sheet is 700 ° C. or less, a relational expression consisting of the water pressure P H2O in the furnace atmosphere and the hydrogen partial pressure P H2 is given. The furnace body average value of a certain log (P H2O / P H2 ) is adjusted to the range of the following formula (iv), and when the temperature of the steel sheet is more than 700 ° C. and 800 ° C. or less, the log in the atmosphere in the furnace is The average value of the furnace body of (P H2O / P H2 ) was adjusted to the range of the following formula (v), and when the temperature of the steel plate exceeded 800 ° C., the log (P H2O / P H2 ) within the range of the following formula (vi), lower than the log (P H2O / P H2 ) furnace average value when the temperature of the steel sheet is more than 700 ° C and 800 ° C or less, And a furnace atmosphere adjusting means for adjusting the dew point to less than −10 ° C.,
A continuous annealing apparatus for a steel sheet, which is used in the method for manufacturing a steel sheet according to claim 1.
−3.01 <log (P H2O / P H2 ) <− 0.07 (iv)
−1.36 <log (P H2O / P H2 ) <− 0.07 (v)
−3.01 <log (P H2O / P H2 ) ≦ −0.53 (vi)
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