JP5556483B2 - Continuous annealing method for steel sheet using continuous annealing furnace - Google Patents
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- 229910000831 Steel Inorganic materials 0.000 title claims description 134
- 239000010959 steel Substances 0.000 title claims description 134
- 238000000137 annealing Methods 0.000 title claims description 97
- 238000000034 method Methods 0.000 title claims description 28
- 229910052799 carbon Inorganic materials 0.000 claims description 112
- 229910052782 aluminium Inorganic materials 0.000 claims description 57
- 229910000655 Killed steel Inorganic materials 0.000 claims description 54
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 48
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 27
- 238000001816 cooling Methods 0.000 claims description 27
- 238000010438 heat treatment Methods 0.000 claims description 17
- 239000000463 material Substances 0.000 claims description 13
- 239000012535 impurity Substances 0.000 claims description 12
- 229910052698 phosphorus Inorganic materials 0.000 claims description 12
- 229910052717 sulfur Inorganic materials 0.000 claims description 12
- 238000002791 soaking Methods 0.000 claims description 10
- 229910052757 nitrogen Inorganic materials 0.000 claims description 8
- 230000007423 decrease Effects 0.000 claims description 7
- 239000003610 charcoal Substances 0.000 claims description 5
- 238000011144 upstream manufacturing Methods 0.000 claims description 5
- 239000004071 soot Substances 0.000 description 11
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 10
- 230000032683 aging Effects 0.000 description 9
- 239000010960 cold rolled steel Substances 0.000 description 8
- 238000010586 diagram Methods 0.000 description 6
- 230000035882 stress Effects 0.000 description 5
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 4
- 229910020012 Nb—Ti Inorganic materials 0.000 description 4
- 230000003679 aging effect Effects 0.000 description 4
- 238000005097 cold rolling Methods 0.000 description 4
- 238000004519 manufacturing process Methods 0.000 description 4
- 238000005096 rolling process Methods 0.000 description 4
- 239000006104 solid solution Substances 0.000 description 4
- 229910001335 Galvanized steel Inorganic materials 0.000 description 3
- 239000008397 galvanized steel Substances 0.000 description 3
- RQMIWLMVTCKXAQ-UHFFFAOYSA-N [AlH3].[C] Chemical compound [AlH3].[C] RQMIWLMVTCKXAQ-UHFFFAOYSA-N 0.000 description 2
- 238000005275 alloying Methods 0.000 description 2
- 230000000052 comparative effect Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000005246 galvanizing Methods 0.000 description 2
- 239000003973 paint Substances 0.000 description 2
- 238000001953 recrystallisation Methods 0.000 description 2
- 208000024891 symptom Diseases 0.000 description 2
- 238000003466 welding Methods 0.000 description 2
- 229910000975 Carbon steel Inorganic materials 0.000 description 1
- 238000009529 body temperature measurement Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
- 238000007654 immersion Methods 0.000 description 1
- 238000007689 inspection Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 238000007747 plating Methods 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 230000002265 prevention Effects 0.000 description 1
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- 230000009466 transformation Effects 0.000 description 1
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Description
本発明は、連続焼鈍炉を用いた鋼板の連続焼鈍方法に関するものである。 The present invention relates to a method for continuously annealing a steel sheet using a continuous annealing furnace.
冷間圧延を行った鋼板は、そのままでは加工硬化して加工性が極めて悪いので、焼鈍工程を経て良好な加工性を実現している。このような冷延鋼板の焼鈍方法として、バッチ焼鈍方法と連続焼鈍方法が用いられる。本発明は連続焼鈍炉を用いた連続焼鈍方法を対象とする。 The cold-rolled steel sheet is work-hardened as it is and its workability is extremely poor, so that good workability is achieved through an annealing process. As an annealing method for such a cold-rolled steel sheet, a batch annealing method and a continuous annealing method are used. The present invention is directed to a continuous annealing method using a continuous annealing furnace.
連続焼鈍方法においては、コイルから巻き戻された冷延帯鋼が連続焼鈍炉を通過するときに焼鈍され、さらに通常は、コイル冷却後、調質圧延、検査精製まで連続して結合され、焼鈍後の帯鋼がコイルに巻き取られる。図1に示すように、連続焼鈍炉1においては、帯鋼は加熱帯5、均熱帯6、1次冷却帯7、過時効帯8、2次冷却帯9をこの順序で通過し、熱処理を施される。設備の入側において、2基のペイオフリール2、溶接機3、入側ルーパ4が設置され、1本のコイルからの帯鋼の巻き戻しが終了すると、別のコイルからの帯鋼が巻き戻され、溶接機3によって2枚の帯鋼が接合され、当該別のコイルが連続的に連続焼鈍炉1に送り込まれる。
In the continuous annealing method, the cold-rolled steel strip unrolled from the coil is annealed when it passes through the continuous annealing furnace, and more usually, after coil cooling, it is continuously combined from temper rolling to inspection and refinement. The latter steel strip is wound on a coil. As shown in FIG. 1, in the continuous annealing
連続焼鈍装置においては、ペイオフリール2から巻き取りリール12までの帯鋼長さが2000mに近いものとなるので、正常に通板させるため、ロールの水平、直交心の調整、ロール形状とロールクラウン量の選定、自動位置制御装置の導入などが行われている。過時効帯などの炉内には、自動位置制御装置として、中間ガイド方式ステアリング装置などのセンタポジションコントロール(CPC)が設置されている(非特許文献1第654頁)。
In the continuous annealing device, the steel strip length from the
連続焼鈍においては、多品種の鋼板が次々と送り込まれ、順次連続焼鈍が行われる。連続焼鈍に供される代表的な品種として、低炭アルミキルド鋼、極低炭アルミキルド鋼が挙げられ、極低炭アルミキルド鋼はさらに、Ti添加極低炭アルミキルド鋼、Nb−Ti添加極低炭アルミキルド鋼などに分けることができる。連続焼鈍炉の加熱帯における加熱速度、均熱帯における焼鈍温度については、品種毎に最適な焼鈍条件が設定される。 In continuous annealing, various types of steel plates are fed one after another, and continuous annealing is performed sequentially. Typical varieties to be subjected to continuous annealing include low-carbon aluminum killed steel and ultra-low-carbon aluminum killed steel. Can be divided into steel. About the heating rate in the heating zone of the continuous annealing furnace and the annealing temperature in the soaking zone, optimum annealing conditions are set for each type.
加熱帯5、均熱帯6において再結晶および結晶粒成長させた後、1次冷却帯7における急速冷却と過時効帯8における過時効処理が行われ、過飽和固溶Cを析出させる。即ち、加熱・均熱の後で1次冷却帯7において500℃以下まで急冷処理により固溶Cの過飽和固溶状態を作り、析出のための駆動力を与える。ついで、過時効帯8において所定の温度と時間で加熱保持を行い、固溶Cをほぼ完全に析出させる(非特許文献1第638頁)。
After recrystallization and crystal grain growth in the
図2には、連続焼鈍炉の1次冷却帯7から過時効帯8を経て2次冷却帯9に至る設備の概要を示している。1次冷却帯における鋼帯温度が1次冷却帯温度計22によって計測され、過時効帯出側における鋼帯温度が過時効帯出側温度計21によって計測される。鋼帯の自動位置制御のため、CPCが2箇所(20a、20b)に設置されている。
In FIG. 2, the outline | summary of the equipment from the
低炭アルミキルド鋼においては、最も効率的な過時効処理を可能とする板温、即ち、過時効帯入側温度を380〜460℃、過時効帯出側温度を250〜300℃の範囲として過時効処理が行われる。一方、極低炭アルミキルド鋼については、過時効帯における温度は特段に指定されることがない。 In low-carbon aluminum killed steel, the plate temperature that enables the most efficient overaging treatment, that is, overaging with the overaging zone entry side temperature in the range of 380 to 460 ° C and the overaging band exit side temperature in the range of 250 to 300 ° C Processing is performed. On the other hand, for extremely low-carbon aluminum killed steel, the temperature in the overaging zone is not particularly specified.
前述のとおり、連続焼鈍においては、種々の品種が次々に連続焼鈍炉に送り込まれる。過時効帯については、温度範囲が指定されるのは上記のとおり低炭アルミキルド鋼であり、極低炭アルミキルド鋼については特段の指定がない。そこで、過時効帯の温度設定としては、低炭アルミキルド鋼に要求される温度設定としておき、極低炭アルミキルド鋼を焼鈍処理するに際しても同じ温度設定が用いられる。 As described above, in the continuous annealing, various varieties are successively sent to the continuous annealing furnace. Regarding the overaging zone, the temperature range is specified for low-carbon aluminum killed steel as described above, and there is no special specification for extremely low-carbon aluminum killed steel. Therefore, as the temperature setting of the overaging zone, the temperature setting required for the low-carbon aluminum killed steel is set, and the same temperature setting is used when annealing the ultra-low-carbon aluminum killed steel.
特許文献1には、塗装焼付硬化性(BH性)、常温遅時効性、成形性を兼ね備えた合金化溶融亜鉛めっき鋼板の製造方法が開示されている。質量%で、C:0.0014〜0.0025%、Si≦0.5%、Mn:0.03〜1.0%、P:0.01〜0.15%、S≦0.015%、Al:0.005〜0.1%、N≦0.0040%を含有し、残部Feおよび不可避的不純物からなる組成のスラブを、所定条件で熱間圧延・冷間圧延を施し、連続焼鈍ラインにて650℃〜Ac3変態点で焼鈍後に調質圧延を施して、一旦、焼鈍済み冷延鋼板を製造し、引き続き、連続溶融亜鉛めっきラインにて亜鉛めっき浴温度まで加熱してめっきした後、所定条件で合金化熱処理を行い、その後、再度調質圧延を施すものである。焼鈍済み冷延鋼板を用いて、これを急速昇温して、めっき浸漬後に合金化熱処理することで合金化溶融亜鉛めっき鋼板を製造する、新しいめっき鋼板製造プロセスによって、強度と加工性を兼ね備え、更にBH性と常温遅時効性をも兼ね備えた、塗装焼付硬化性能に優れた合金化溶融亜鉛めっき鋼板の製造方法を提供することができる。以下、当該成分を有する鋼を「P添加極低炭BH鋼」と呼ぶ。
連続焼鈍炉を用いた鋼板の連続焼鈍方法については、冷間圧延と連続焼鈍とを直結して行うことも可能である。特許文献2においては、連続冷間圧延設備と連続焼鈍設備を直結した連続冷延鋼板製造設備が開示されている。
About the continuous annealing method of the steel plate using a continuous annealing furnace, it is also possible to carry out by directly connecting cold rolling and continuous annealing.
連続焼鈍炉において、低炭アルミキルド鋼、極低炭アルミキルド鋼を含む品種について連続焼鈍を行ったところ、極低炭アルミキルド鋼のうちの前記P添加極低炭BH鋼に特有の現象として、連続焼鈍終了後の帯鋼表面にスリ疵が発生するという症状が見られた。帯鋼の幅端部付近の表面に、帯鋼の長手方向から数十°程度傾斜し、長さが1mm未満のスリ疵が発生する。このようなスリ疵の発生は、P添加極低炭BH鋼のみに見られる現象であった。 In a continuous annealing furnace, continuous annealing was performed on varieties including low-carbon aluminum killed steel and ultra-low-carbon aluminum killed steel. The symptom that a crack was generated on the surface of the steel strip after completion was observed. On the surface in the vicinity of the width end of the steel strip, there is generated a soot that is inclined by several tens of degrees from the longitudinal direction of the steel strip and has a length of less than 1 mm. Such generation of soot was a phenomenon observed only in the P-added extra low-carbon BH steel.
本発明は、連続焼鈍炉を用いた鋼板の連続焼鈍方法において、P添加極低炭BH鋼に発生するスリ疵を消滅させることのできる連続焼鈍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 continuous annealing method capable of eliminating the soot generated in the P-added ultra low carbon BH steel in a continuous annealing method of a steel sheet using a continuous annealing furnace.
即ち、本発明の要旨とするところは以下のとおりである。
(1)加熱帯、均熱帯、1次冷却帯、過時効帯、2次冷却帯をこの順序で有する連続焼鈍炉を用いた鋼板の連続焼鈍方法であって、
質量%で、C:0.03〜0.05%、Si≦0.02%、Mn:0.1〜0.2%、P≦0.02%、S≦0.01%、Al:0.005〜0.1%、残部Feおよび不可避的不純物からなる鋼(以下「低炭アルミキルド鋼」という。)、
質量%で、C :0.0014〜0.0025%、Si≦0.5%、Mn:0.03〜1.0%、P:0.01〜0.15%、S≦0.015%、Al:0.005〜0.1%、N≦0.0040%を含有し、残部Feおよび不可避的不純物からなる鋼(以下「P添加極低炭BH鋼」という。)の2種を含む複数品種の鋼板を連続して焼鈍し、
前記過時効帯は鋼帯の温度が上流から下流に向けて順次下がっていく温度パターンを有し、該過時効帯の出側温度は、前記低炭アルミキルド鋼焼鈍時においては250〜300℃の範囲とし、前記P添加極低炭BH鋼焼鈍時においては300℃を超える温度とすることを特徴とする鋼板の連続焼鈍方法。
(2)加熱帯、均熱帯、1次冷却帯、過時効帯、2次冷却帯をこの順序で有する連続焼鈍炉を用いた鋼板の連続焼鈍方法であって、
質量%で、C:0.03〜0.05%、Si≦0.02%、Mn:0.1〜0.2%、P≦0.02%、S≦0.01%、Al:0.005〜0.1%、残部Feおよび不可避的不純物からなる鋼(以下「低炭アルミキルド鋼」という。)、
質量%で、C :0.0014〜0.0030%、Si≦0.5%、Mn:0.03〜1.0%、P:0.01〜0.15%、S≦0.015%、Al:0.005〜0.1%、N≦0.0040%を含有し、さらにTi:0.002〜0.015%およびNb:0.002〜0.015%のうち1種または2種をTi+Nb=0.002〜0.015%となるように含有し、残部Feおよび不可避的不純物からなる鋼(以下「P添加極低炭BH鋼」という。)の2種を含む複数品種の鋼板を連続して焼鈍し、
前記過時効帯は鋼帯の温度が上流から下流に向けて順次下がっていく温度パターンを有し、該過時効帯の出側温度は、前記低炭アルミキルド鋼焼鈍時においては250〜300℃の範囲とし、前記P添加極低炭BH鋼焼鈍時においては300℃を超える温度とすることを特徴とする鋼板の連続焼鈍方法。
(3)前記P添加極低炭BH鋼は、さらに、質量%で、B:0.0001〜0.0040%を含有することを特徴とする上記(1)又は(2)に記載の鋼板の連続焼鈍方法。
(4)前記低炭アルミキルド鋼が、更に、質量%で、B:0.0020〜0.0035%を含有することを特徴とする上記(1)乃至(3)のいずれかに記載の鋼板の連続焼鈍方法。
(5)前記過時効帯の出側温度は、前記P添加極低炭BH鋼焼鈍時においては300℃超305℃以下の温度とすることを特徴とする上記(1)乃至(4)のいずれかに記載の鋼板の連続焼鈍方法。
(6)前記低炭アルミキルド鋼とP添加極低炭BH鋼の2種を含む複数品種の鋼板を連続して焼鈍するに際し、低炭アルミキルド鋼とP添加極低炭BH鋼との間には、Ti添加極低炭アルミキルド鋼又はNb−Ti添加極低炭アルミキルド鋼を緩衝材として挿入し、低炭アルミキルド鋼とP添加極低炭BH鋼とが入れ替わる際には、低炭アルミキルド鋼とP添加極低炭BH鋼との間に挿入された緩衝材の処理時間中に過時効帯の温度を変更することを特徴とする上記(1)乃至(5)のいずれかに記載の鋼板の連続焼鈍方法。
That is, the gist of the present invention is as follows.
(1) A method for continuous annealing of a steel sheet using a continuous annealing furnace having a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone in this order,
In mass%, C: 0.03-0.05%, Si ≦ 0.02%, Mn: 0.1-0.2%, P ≦ 0.02%, S ≦ 0.01%, Al: 0 0.005 to 0.1% steel, balance Fe and inevitable impurities (hereinafter referred to as “low-carbon aluminum killed steel”),
In mass%, C: 0.0014 to 0.0025%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015% , Al: 0.005 to 0.1%, N ≦ 0.0040%, including two types of steel (hereinafter referred to as “P-added ultra-low carbon BH steel”) composed of Fe and inevitable impurities. Continuously anneal multiple types of steel plates,
The overaging zone has a temperature pattern in which the temperature of the steel strip gradually decreases from upstream to downstream, and the outlet temperature of the overaging zone is 250 to 300 ° C. during annealing of the low-carbon aluminum killed steel. A continuous annealing method for a steel sheet, characterized in that the temperature is over 300 ° C. during annealing of the P-added ultra-low carbon BH steel.
(2) A continuous annealing method for a steel sheet using a continuous annealing furnace having a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone in this order,
In mass%, C: 0.03-0.05%, Si ≦ 0.02%, Mn: 0.1-0.2%, P ≦ 0.02%, S ≦ 0.01%, Al: 0 0.005 to 0.1% steel, balance Fe and inevitable impurities (hereinafter referred to as “low-carbon aluminum killed steel”),
By mass%, C: 0.0014 to 0.0030%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015% Al: 0.005 to 0.1%, N ≦ 0.0040%, Ti: 0.002 to 0.015% and Nb: 0.002 to 0.015% A plurality of varieties including two kinds of steel (hereinafter referred to as “P-added ultra-low-carbon BH steel”) containing seeds such that Ti + Nb = 0.002 to 0.015% and the balance Fe and inevitable impurities . Annealing steel plates continuously,
The overaging zone has a temperature pattern in which the temperature of the steel strip gradually decreases from upstream to downstream, and the outlet temperature of the overaging zone is 250 to 300 ° C. during annealing of the low-carbon aluminum killed steel. A continuous annealing method for a steel sheet, characterized in that the temperature is over 300 ° C. during annealing of the P-added ultra-low carbon BH steel .
(3) The P-added ultra-low charcoal BH steel further contains B: 0.0001 to 0.0040% in mass%, and the steel sheet according to (1) or (2) above Continuous annealing method.
(4) The steel sheet according to any one of (1) to (3), wherein the low-carbon aluminum killed steel further contains B: 0.0020 to 0.0035% by mass. Continuous annealing method.
(5) Any of the above (1) to (4), wherein the exit temperature of the overaging zone is a temperature of more than 300 ° C. and not more than 305 ° C. during annealing of the P-added ultra low carbon BH steel. A method for continuous annealing of a steel sheet according to
(6) When continuously annealing a plurality of types of steel plates including two types of low-carbon aluminum killed steel and P-added ultra-low-carbon BH steel, between low-carbon aluminum-killed steel and P-added extremely low-carbon BH steel When Ti-added ultra-low-carbon aluminum killed steel or Nb-Ti-added ultra-low-carbon aluminum killed steel is inserted as a buffer material, The continuous steel sheet according to any one of the above (1) to (5), wherein the temperature of the overaging zone is changed during the treatment time of the buffer material inserted between the added extra low carbon BH steel. Annealing method.
本発明は、連続焼鈍炉を用いた鋼板の連続焼鈍方法において、過時効帯の温度パターンを品種によって変化させ、過時効帯の出側温度を、低炭アルミキルド鋼焼鈍時においては250〜300℃の範囲とし、P添加極低炭BH鋼焼鈍時においては300℃を超える温度とすることにより、P添加極低炭BH鋼において発生していたスリ疵を大幅に減少することができる。 In the continuous annealing method of the steel sheet using the continuous annealing furnace, the present invention changes the temperature pattern of the overaging zone depending on the type, and the exit temperature of the overaging zone is 250 to 300 ° C. when annealing the low-carbon aluminum killed steel. When the P-added ultra-low carbon BH steel is annealed, the temperature exceeding 300 ° C. can be used to significantly reduce the soot generated in the P-added ultra-low carbon BH steel.
前述のとおり、連続焼鈍炉内の温度パターンのうち、過時効帯における温度パターンが指定されるのは低炭アルミキルド鋼のみであり、極低炭アルミキルド鋼については特段に指定されることがない。極低炭アルミキルド鋼の一種であるP添加極低炭BH鋼も同様である。 As described above, among the temperature patterns in the continuous annealing furnace, the temperature pattern in the overaging zone is specified only for the low-carbon aluminum killed steel, and the ultra-low-carbon aluminum killed steel is not particularly specified. The same applies to P-added ultra-low-carbon BH steel, which is a kind of ultra-low-carbon aluminum killed steel.
本発明においてP添加極低炭BH鋼とは、第1に、質量%で、C :0.0014〜0.0025%、Si≦0.5%、Mn:0.03〜1.0%、P:0.01〜0.15%、S≦0.015%、Al:0.005〜0.1%、N≦0.0040%を含有し、残部Feおよび不可避的不純物からなる鋼をいう。第2に、質量%で、C :0.0014〜0.0030%、Si≦0.5%、Mn:0.03〜1.0%、P:0.01〜0.15%、S≦0.015%、Al:0.005〜0.1%、N≦0.0040%を含有し、さらにTi:0.002〜0.015%およびNb:0.002〜0.015%のうち1種または2種をTi+Nb=0.002〜0.015%となるように含有し、残部Feおよび不可避的不純物からなる鋼をいう。第3に、さらに、質量%で、B:0.0001〜0.0040%を含有する。C含有量を第1の範囲とすることにより、十分なBH性と常温非時効性を確保する。Ti、Nbを含有する場合はC含有量を第2の範囲まで拡張できる。Si含有量を表記範囲とすることにより、プレス成形性が良好となる。Mn含有量を表記範囲とすることにより、Sによる耳割れを防止するとともに好適な強度を確保する。Pを表記範囲で含有することにより、強度上昇と加工性向上を実現する。P含有量下限を0.045質量%とするとより好ましい。Alは脱酸及びNの固定に使用する。Ti、NbはN、C、Sの一部を固定することにより、常温遅時効性を確保する役割を有する。Bは2次加工脆化の防止に有効であるほか、再結晶温度を低くする効果を有する。 In the present invention, P-added ultra-low carbon BH steel is, first, in mass%, C: 0.0014 to 0.0025%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015%, Al: 0.005 to 0.1%, N ≦ 0.0040%, steel that consists of the balance Fe and inevitable impurities . Second, in mass%, C: 0.0014 to 0.0030%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015%, Al: 0.005 to 0.1%, N ≦ 0.0040%, and Ti: 0.002 to 0.015% and Nb: 0.002 to 0.015% One or two types are contained so that Ti + Nb = 0.002 to 0.015%, and the steel consists of the balance Fe and inevitable impurities. Thirdly, it further contains B: 0.0001 to 0.0040% by mass. By setting the C content within the first range, sufficient BH properties and room temperature non-aging properties are ensured. When Ti and Nb are contained, the C content can be expanded to the second range. By making Si content into the description range, press moldability becomes favorable. By making Mn content into the description range, an ear crack caused by S is prevented and a suitable strength is secured. By containing P in the notation range, strength is increased and workability is improved. The lower limit of the P content is more preferably 0.045% by mass. Al is used for deoxidation and N fixation. Ti and Nb have a role of securing a slow aging property at room temperature by fixing a part of N, C, and S. B is effective in preventing secondary work embrittlement and has the effect of lowering the recrystallization temperature.
本発明において低炭アルミキルド鋼とは、質量%で、C;0.03〜0.05%、Si≦0.02%、Mn:0.1〜0.2%、P≦0.02%、S≦0.01%、Al:0.005〜0.1%、残部Feおよび不可避的不純物からなる鋼を意味する。このような成分を有する低炭アルミキルド鋼においては、過時効帯入側温度を380〜460℃、過時効帯出側温度を250〜300℃の範囲として過時効処理が行われる。即ち、最も効率的な過時効処理を可能とする板温としている。 In the present invention, the low-carbon aluminum killed steel is, in mass%, C; 0.03-0.05%, Si ≦ 0.02%, Mn: 0.1-0.2%, P ≦ 0.02%, It means steel composed of S ≦ 0.01%, Al: 0.005 to 0.1%, balance Fe and inevitable impurities. In the low-carbon aluminum killed steel having such a component, the overaging treatment is performed with the overaging zone entry side temperature in the range of 380 to 460 ° C. and the overaging band exit side temperature in the range of 250 to 300 ° C. That is, the plate temperature enables the most efficient overaging treatment.
一方、前記規定された成分を有するP添加極低炭BH鋼については、過時効帯における温度パターンが指定されない。特許文献1の段落[0033]に「熱処理条件」と記載されているのが過時効帯における温度条件であり、同段落に記載のとおり、特別に限定するものではない。
On the other hand, the temperature pattern in the overaging zone is not specified for the P-added extra low-carbon BH steel having the specified components. Paragraph [0033] of
連続焼鈍においては、上記低炭アルミキルド鋼、P添加極低炭BH鋼、左記品種以外の極低炭アルミキルド鋼などの多くの品種の鋼板を順次供給して焼鈍が行われる。このうち、過時効帯における温度パターンが規定されるのは低炭アルミキルド鋼のみであり、その他の品種は特段の指定がされていないので、過時効帯の温度パターンとしては、低炭アルミキルド鋼で規定される温度パターンに固定し、いずれの品種においても同じ温度パターンを用いることとしていた。 In the continuous annealing, annealing is performed by sequentially supplying many types of steel plates such as the above-mentioned low-carbon aluminum killed steel, P-added extra-low-carbon BH steel, and extra-low-carbon aluminum-killed steel other than those listed on the left. Of these, the temperature pattern in the overaging zone is specified only for low-carbon aluminum killed steel, and other grades are not specifically specified. The temperature pattern is fixed to the specified temperature pattern, and the same temperature pattern is used for all varieties.
ここにおいて、前述のとおり、P添加極低炭BH鋼に特有の現象として、連続焼鈍終了後の帯鋼表面にスリ疵が発生するという症状が見られた。帯鋼の幅端部付近の表面に、帯鋼の長手方向から数十°程度傾斜し、長さが1mm未満のスリ疵が発生する。このようなスリ疵の発生は、P添加極低炭BH鋼のみに見られる現象であった。また、連続焼鈍炉の過時効帯内部に設けられたセンタポジションコントロール(CPC)の動作量が、P添加極低炭BH鋼において特に大きいことがわかった。さらに、P添加極低炭BH鋼の処理において、CPCの動作量が大きいほど、スリ疵の発生程度が大きくなるという傾向も見られた。 Here, as described above, as a phenomenon peculiar to the P-added ultra-low carbon BH steel, a symptom in which a soot was generated on the surface of the steel strip after the continuous annealing was observed. On the surface in the vicinity of the width end of the steel strip, there is generated a soot that is inclined by several tens of degrees from the longitudinal direction of the steel strip and has a length of less than 1 mm. Such generation of soot was a phenomenon observed only in the P-added extra low-carbon BH steel. Further, it was found that the operation amount of the center position control (CPC) provided in the overaging zone of the continuous annealing furnace is particularly large in the P-added ultra low carbon BH steel. Furthermore, in the treatment of P-added ultra-low charcoal BH steel, there was a tendency that the greater the amount of CPC operation, the greater the degree of generation of soot.
そこで、P添加極低炭BH鋼について、過時効帯の温度領域における応力−歪挙動の評価を行った。低炭アルミキルド鋼及びNb−Ti添加極低炭素鋼の挙動と対比を行った。評価に用いた鋼の成分は表1に示すとおりである。 Therefore, the stress-strain behavior in the temperature region of the overaged zone was evaluated for the P-added extremely low carbon BH steel. The behavior and contrast of low-carbon aluminum killed steel and Nb-Ti added ultra-low carbon steel were compared. The components of the steel used for the evaluation are as shown in Table 1.
応力−歪挙動の評価は、150℃〜400℃の温度範囲において、50℃きざみで温度を設定し、各温度において0〜15%の歪を付与して応力の挙動を評価した。歪を一定速度で増大させ、応力と歪の関係について評価した。通常、歪が増大するにしたがって、応力も単調に増大する。ところが、特定の温度における応力−歪測定において、歪を一定速度で増大していく際、応力が時間の経過と共に増大と減少を繰り返す現象として観察されることがある。動的歪時効(セレーション)によるものである。そこで図3においては、各品種において、室温から400℃までの各温度において、応力−歪線図としてグラフ化した。 The stress-strain behavior was evaluated by setting the temperature in steps of 50 ° C. in a temperature range of 150 ° C. to 400 ° C., and applying 0 to 15% strain at each temperature to evaluate the stress behavior. Strain was increased at a constant rate, and the relationship between stress and strain was evaluated. Usually, as the strain increases, the stress increases monotonously. However, in the stress-strain measurement at a specific temperature, when the strain is increased at a constant speed, the stress may be observed as a phenomenon in which the stress repeatedly increases and decreases over time. This is due to dynamic strain aging (serration). Therefore, in FIG. 3, each product was graphed as a stress-strain diagram at each temperature from room temperature to 400 ° C.
その結果、図3に示すように、P添加極低炭BH鋼に特有の現象として、300℃付近の温度域で動的歪時効(セレーション)が観察されることがわかった。動的歪時効が観察される温度域において連続焼鈍炉の過時効帯を通過させると、炉内において帯鋼に不均一伸びが発生し、帯鋼が偏りやすく、その偏りを修正するためにCPCを動作させる際に帯鋼がロールとの間でミクロスリップを発生させ、最終的に特有のスリ疵発生に至ると推定される。 As a result, as shown in FIG. 3, it was found that dynamic strain aging (serration) was observed in the temperature range near 300 ° C. as a phenomenon peculiar to the P-added ultra low carbon BH steel. When passing the over-aging zone of a continuous annealing furnace in a temperature range where dynamic strain aging is observed, non-uniform elongation occurs in the steel strip in the furnace, the steel strip tends to be biased, and CPC is used to correct the bias. It is presumed that the steel strip generates micro-slip between the rolls when it is operated, and eventually leads to the generation of a unique thread.
以上の観察結果から、P添加極低炭BH鋼を連続焼鈍するに際しては、過時効帯における温度パターンとして、動的歪時効が発生する温度域を回避することが有効であることが判明した。ここにおいて、過時効帯における温度パターンとして、動的歪時効が発生する温度域を高温側に回避するのか、低温側に回避するのかという選択が必要となる。 From the above observation results, it has been found that it is effective to avoid a temperature range in which dynamic strain aging occurs as a temperature pattern in the overaging zone when P-added ultra low carbon BH steel is continuously annealed. Here, as the temperature pattern in the overaging zone, it is necessary to select whether to avoid the temperature range where dynamic strain aging occurs on the high temperature side or on the low temperature side.
図3から明らかなように、P添加極低炭BH鋼の高温特性として、動的歪時効が発生することに加え、高温強度が非常に高いという特徴が見られた。Nb−Ti添加極低炭鋼と対比すると、350℃付近における差が一番大きく、P添加極低炭BH鋼の方が約2倍の強度を有している。連続焼鈍炉の過時効帯内部におけるロールと帯鋼との接触について検討すると、当該過時効帯の温度域における帯鋼の強度が高いほど、帯鋼とロールとの間の摩擦係数が小さくなり、結果として帯鋼がロールの一方の側に偏りやすくなり、その偏りを修正するためにCPCを動作させる際に帯鋼がロールとの間でミクロスリップを発生させ、最終的に特有のスリ疵発生に至ると推定される。P添加極低炭BH鋼の高温強度が高いこととあいまって、過時効帯の温度パターンとして動的歪時効が発生する温度域を低温側に回避しようとすると、高温強度が増してスリ疵発生を助長させる可能性がある。 As is clear from FIG. 3, the high temperature characteristics of the P-added ultra low carbon BH steel were characterized by the fact that the high temperature strength was very high in addition to the occurrence of dynamic strain aging. Compared with the Nb—Ti-added ultra-low carbon steel, the difference in the vicinity of 350 ° C. is the largest, and the P-added ultra-low carbon BH steel has about twice the strength. When considering the contact between the roll and the steel strip in the overaging zone of the continuous annealing furnace, the higher the strength of the steel strip in the temperature range of the overaging zone, the smaller the friction coefficient between the steel strip and the roll, As a result, the steel strip tends to be biased to one side of the roll, and when the CPC is operated to correct the bias, the steel strip generates a microslip with the roll, and finally a unique slip occurs. It is estimated that Combined with the high-temperature strength of P-added ultra-low-carbon BH steel, when trying to avoid the temperature range where dynamic strain aging occurs as a temperature pattern of the overaging zone on the low temperature side, the high-temperature strength increases and the generation of cracks. May be encouraged.
連続焼鈍炉の過時効帯における帯鋼の蛇行防止のための方策として、ロールのサーマルクラウンが積極的に利用されている。ロールを軸方向に見たとき、ロールの中央部は高温の帯鋼が常時通過するので温度が上昇して熱膨張し、ロールの両端部は帯鋼が通過しないので温度上昇が少なく熱膨張も少ない。その結果、連続焼鈍に使用中のロールは常温時に比較して中央部の径が両端部に比較して増大しており、クラウンを形成した形となる。このような現象をサーマルクラウンと称している。ロールのサーマルクラウンが発生していることにより、帯鋼の蛇行発生が軽減されている。これに対して、過時効帯の温度パターンとして動的歪時効が発生する温度域を低温側に回避しようとすると、ロール温度が低下してサーマルクラウンが減少し、帯鋼の蛇行防止効果が低下する可能性がある。 As a measure for preventing the meandering of the steel strip in the overaging zone of the continuous annealing furnace, the thermal crown of the roll is actively used. When the roll is viewed in the axial direction, since the hot steel strip always passes through the center of the roll, the temperature rises and thermally expands, and since the steel strip does not pass through both ends of the roll, the temperature rise is small and thermal expansion also occurs. Few. As a result, the roll in use for continuous annealing has a shape in which the diameter of the central portion is larger than that at both ends as compared to that at the normal temperature, and a crown is formed. Such a phenomenon is called a thermal crown. Due to the occurrence of the thermal crown of the roll, the occurrence of meandering of the steel strip is reduced. On the other hand, if you try to avoid the temperature range where dynamic strain aging occurs as a temperature pattern of the overaging zone on the low temperature side, the roll temperature will drop and the thermal crown will decrease, reducing the meandering prevention effect of the strip steel there's a possibility that.
以上のような状況から、P添加極低炭BH鋼を連続焼鈍するに際して、過時効帯における温度パターンとして、動的歪時効が発生する温度域を高温側に回避することが有効であると判断することができる。そして、過時効帯の出側温度を、P添加極低炭BH鋼焼鈍時においては300℃を超える温度とすることにより、スリ疵の発生を防止することが可能となる。過時効帯において鋼帯の温度は上流から下流に向けて順次下がっていくので、過時効帯出側温度を300℃超とすれば、過時効帯の全域において鋼帯温度を300℃超とすることができる。ここで過時効帯出側温度とは、2次冷却帯入側温度を意味する。具体的には、図2に示す過時効帯出側温度計21の測温結果をもって過時効帯出側温度とする。
From the above situation, when continuously annealing the P-added ultra low carbon BH steel, it is judged to be effective to avoid the temperature range where dynamic strain aging occurs as a temperature pattern in the overaging zone on the high temperature side. can do. And it becomes possible to prevent generation | occurrence | production of a soot by making the outgoing side temperature of an overaging zone into the temperature which exceeds 300 degreeC at the time of P addition ultra-low carbon BH steel annealing. In the overaging zone, the temperature of the steel strip gradually decreases from the upstream to the downstream, so if the overaging zone outlet side temperature exceeds 300 ° C, the steel strip temperature should exceed 300 ° C throughout the overaging zone. Can do. Here, the overaging zone outlet side temperature means the secondary cooling zone inlet side temperature. Specifically, the temperature measurement result of the overaging band-
一方、低炭アルミキルド鋼においては、過時効帯出側温度を250〜300℃とすることが必要である。また、低炭アルミキルド鋼とP添加極低炭BH鋼以外の品種については、過時効帯の温度パターンが特段に指定されない。 On the other hand, in low-carbon aluminum killed steel, it is necessary to set the overaging band temperature to 250 to 300 ° C. In addition, for varieties other than the low-carbon aluminum killed steel and the P-added extra-low-carbon BH steel, the temperature pattern of the overaging zone is not particularly specified.
そこで本発明においては、連続焼鈍炉における過時効帯の出側温度パターンとして、低炭アルミキルド鋼焼鈍時においては250〜300℃の範囲とし、P添加極低炭BH鋼焼鈍時においては300℃を超える温度とするように温度を制御する。 Therefore, in the present invention, as the outgoing temperature pattern of the overaging zone in the continuous annealing furnace, the range is 250 to 300 ° C. at the time of annealing the low carbon aluminum killed steel, and 300 ° C. at the time of annealing the P-added ultra low carbon BH steel. The temperature is controlled so as to exceed the temperature.
通常の連続焼鈍炉の温度制御において、過時効帯出側温度を250℃から300℃に昇温する時間は90分程度、300℃から250℃に降温するのに要する時間は90分程度である。そこで、連続焼鈍炉に供給するコイルの順番として、低炭アルミキルド鋼とP添加極低炭BH鋼とを隣り合わせとすることをせず、両者の間にそれ以外の品種(以下「緩衝材」ともいう。)を挿入することとすると好ましい。緩衝材の帯鋼が過時効帯を通過している間に昇温あるいは降温を行うこととする。緩衝材については、過時効帯の温度パターンが特に限定されないので、緩衝材が過時効帯を通過している任意の時間帯に昇温又は降温を行うことができる。 In the temperature control of a normal continuous annealing furnace, the time for raising the overaging zone temperature from 250 ° C. to 300 ° C. is about 90 minutes, and the time required for lowering the temperature from 300 ° C. to 250 ° C. is about 90 minutes. Therefore, as the order of the coils supplied to the continuous annealing furnace, the low-carbon aluminum killed steel and the P-added extra-low-carbon BH steel are not placed next to each other, and other varieties (hereinafter referred to as “buffer material”) between them. It is preferable to insert. The temperature of the buffer material is increased or decreased while it passes through the overaging band. With respect to the buffer material, the temperature pattern of the overaging zone is not particularly limited, and therefore the temperature can be raised or lowered in any time zone during which the buffer material passes through the overaging zone.
本発明は、冷間圧延が終了した冷延鋼帯からなるコイルを用い、当該コイルから冷延鋼帯を巻き戻して連続焼鈍炉で焼鈍を行う方法のほか、冷間圧延と連続焼鈍とを直結して行う場合にも適用することができる。 The present invention uses a coil made of a cold-rolled steel strip that has been cold-rolled, rewinds the cold-rolled steel strip from the coil, and performs annealing in a continuous annealing furnace, as well as cold rolling and continuous annealing. The present invention can also be applied when directly connected.
加熱帯、均熱帯、1次冷却帯、過時効帯、2次冷却帯をこの順序で有する連続焼鈍炉を用いた鋼板の連続焼鈍において、本発明を適用した。処理する品種は、低炭アルミキルド鋼、P添加極低炭BH鋼、その他の極低炭素鋼(Ti添加極低炭アルミキルド鋼、Nb−Ti添加極低炭アルミキルド鋼など)である。 The present invention was applied to continuous annealing of a steel sheet using a continuous annealing furnace having a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone in this order. The varieties to be treated are low-carbon aluminum killed steel, P-added ultra-low-carbon BH steel, and other ultra-low-carbon steels (Ti-added ultra-low-carbon aluminum killed steel, Nb-Ti-added ultra-low-carbon aluminum killed steel, etc.).
過時効帯における温度パターンとして、従来例では、すべての品種に共通の温度パターンを採用していた。低炭アルミキルド鋼において必要とされる温度パターンである。1次冷却帯出側温度を430℃とし、過時効帯のヒーターをオフとし、過時効帯出側温度を270℃とした。 In the conventional example, a temperature pattern common to all varieties has been adopted as the temperature pattern in the overaging zone. It is a temperature pattern required in low-carbon aluminum killed steel. The primary cooling zone outlet temperature was 430 ° C., the overaging zone heater was turned off, and the overaging zone outlet side temperature was 270 ° C.
これに対し本発明例では、低炭アルミキルド鋼の処理時には従来例と同じ温度パターンとし、P添加極低炭BH鋼の処理時には1次冷却帯出側温度を460℃に上昇し、過時効帯のヒーターをオンとし、過時効帯出側温度を305℃とした。低炭アルミキルド鋼とP添加極低炭BH鋼との間には必ずそれ以外の材料(Ti添加極低炭アルミキルド鋼、Nb−Ti添加極低炭アルミキルド鋼など)を緩衝材として挿入した。低炭アルミキルド鋼とP添加極低炭BH鋼とが入れ替わる際には、両者の間に挿入された緩衝材の処理時間中に過時効帯の温度を変更し、昇温速度40℃/時間での昇温、または降温速度40℃/時間での降温を行った。 On the other hand, in the example of the present invention, the same temperature pattern as in the conventional example is used when processing the low-carbon aluminum killed steel, and the primary cooling zone outlet temperature is increased to 460 ° C. when processing the P-added extremely low-carbon BH steel, The heater was turned on and the overaging zone temperature was 305 ° C. Other materials (Ti-added ultra-low-carbon aluminum killed steel, Nb-Ti-added ultra-low-carbon aluminum killed steel, etc.) were always inserted between the low-carbon aluminum killed steel and the P-added ultra-low-carbon BH steel as buffer materials. When the low-carbon aluminum killed steel and the P-added extra-low-carbon BH steel are replaced, the temperature of the overaging zone is changed during the treatment time of the buffer material inserted between the two, and the heating rate is 40 ° C / hour. The temperature was lowered at a rate of 40 ° C./hour.
P添加極低炭BH鋼及び比較材としてNb−Ti極低炭鋼を処理する際における過時効帯のCPC偏差量について調査した。CPC偏差量とは、過時効帯の最終段直前のトップハースロールに配置されたCPCロールの出側のセンターからの板偏差量を意味する。結果を表2に示す。 The amount of CPC deviation of the overaged zone when processing P-added ultra-low carbon BH steel and Nb-Ti ultra-low carbon steel as a comparative material was investigated. The CPC deviation amount means a plate deviation amount from the center on the exit side of the CPC roll arranged in the top hearth roll immediately before the last stage of the overaging zone. The results are shown in Table 2.
表2から明らかなように、P添加極低炭BH鋼のCPC偏差量は、平均値と標準偏差がともに、従来例に対して本発明例は大幅に改善され、比較材であるNb−Ti極低炭鋼に匹敵する値となった。 As is apparent from Table 2, the CPC deviation amount of the P-added extra low-carbon BH steel, both the average value and the standard deviation, is significantly improved in the present invention example compared to the conventional example, and Nb-Ti which is a comparative material The value was comparable to ultra-low charcoal steel.
次に、P添加極低炭BH鋼について、焼鈍処理後の鋼板スリ疵発生レベル比較を行った。スリ疵の程度を、スリ疵の深さで評価し、深さ5μm以下を「小」、5〜10μmを「中」、10〜15μmを「大」とした。スリ疵「小」または「なし」を合格とする。従来例においては「中」「大」が合計で80%発生していたが、本発明例はスリ疵「中」「大」の発生がなくなり、「小」が70%、「なし」が30%となり、合格率が大幅に向上した。 Next, for the P-added ultra-low carbon BH steel, a comparison was made on the level of steel plate slag generation after annealing. The degree of the soot was evaluated by the depth of the soot, and the depth of 5 μm or less was “small”, 5 to 10 μm was “medium”, and 10 to 15 μm was “large”. Three passes “Small” or “None”. In the conventional example, “medium” and “large” occur 80% in total, but in the example of the present invention, the occurrence of “medium” and “large” is eliminated, “small” is 70%, and “none” is 30. %, And the pass rate improved significantly.
1 連続焼鈍炉
2 ペイオフリール
3 溶接機
4 入側ルーパ
5 加熱帯
6 均熱帯
7 1次冷却帯
8 過時効帯
9 2次冷却帯
10 出側ルーパ
11 調質圧延機
12 巻き取りリール
20 CPC
21 過時効帯出側温度計
22 1次冷却帯温度計
DESCRIPTION OF
21
Claims (6)
質量%で、C:0.03〜0.05%、Si≦0.02%、Mn:0.1〜0.2%、P≦0.02%、S≦0.01%、Al:0.005〜0.1%、残部Feおよび不可避的不純物からなる鋼(以下「低炭アルミキルド鋼」という。)、
質量%で、C :0.0014〜0.0025%、Si≦0.5%、Mn:0.03〜1.0%、P:0.01〜0.15%、S≦0.015%、Al:0.005〜0.1%、N≦0.0040%を含有し、残部Feおよび不可避的不純物からなる鋼(以下「P添加極低炭BH鋼」という。)の2種を含む複数品種の鋼板を連続して焼鈍し、
前記過時効帯は鋼帯の温度が上流から下流に向けて順次下がっていく温度パターンを有し、該過時効帯の出側温度は、前記低炭アルミキルド鋼焼鈍時においては250〜300℃の範囲とし、前記P添加極低炭BH鋼焼鈍時においては300℃を超える温度とすることを特徴とする鋼板の連続焼鈍方法。 A continuous annealing method for a steel sheet using a continuous annealing furnace having a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone in this order,
In mass%, C: 0.03-0.05%, Si ≦ 0.02%, Mn: 0.1-0.2%, P ≦ 0.02%, S ≦ 0.01%, Al: 0 0.005 to 0.1% steel, balance Fe and inevitable impurities (hereinafter referred to as “low-carbon aluminum killed steel”),
In mass%, C: 0.0014 to 0.0025%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015% , Al: 0.005 to 0.1%, N ≦ 0.0040%, including two types of steel (hereinafter referred to as “P-added ultra-low carbon BH steel”) composed of Fe and inevitable impurities. Continuously anneal multiple types of steel plates,
The overaging zone has a temperature pattern in which the temperature of the steel strip gradually decreases from upstream to downstream, and the outlet temperature of the overaging zone is 250 to 300 ° C. during annealing of the low-carbon aluminum killed steel. A continuous annealing method for a steel sheet, characterized in that the temperature is over 300 ° C. during annealing of the P-added ultra-low carbon BH steel.
質量%で、C:0.03〜0.05%、Si≦0.02%、Mn:0.1〜0.2%、P≦0.02%、S≦0.01%、Al:0.005〜0.1%、残部Feおよび不可避的不純物からなる鋼(以下「低炭アルミキルド鋼」という。)、
質量%で、C :0.0014〜0.0030%、Si≦0.5%、Mn:0.03〜1.0%、P:0.01〜0.15%、S≦0.015%、Al:0.005〜0.1%、N≦0.0040%を含有し、さらにTi:0.002〜0.015%およびNb:0.002〜0.015%のうち1種または2種をTi+Nb=0.002〜0.015%となるように含有し、残部Feおよび不可避的不純物からなる鋼(以下「P添加極低炭BH鋼」という。)の2種を含む複数品種の鋼板を連続して焼鈍し、
前記過時効帯は鋼帯の温度が上流から下流に向けて順次下がっていく温度パターンを有し、該過時効帯の出側温度は、前記低炭アルミキルド鋼焼鈍時においては250〜300℃の範囲とし、前記P添加極低炭BH鋼焼鈍時においては300℃を超える温度とすることを特徴とする鋼板の連続焼鈍方法。 A continuous annealing method for a steel sheet using a continuous annealing furnace having a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone in this order,
In mass%, C: 0.03-0.05%, Si ≦ 0.02%, Mn: 0.1-0.2%, P ≦ 0.02%, S ≦ 0.01%, Al: 0 0.005 to 0.1% steel, balance Fe and inevitable impurities (hereinafter referred to as “low-carbon aluminum killed steel”),
By mass%, C: 0.0014 to 0.0030%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015% Al: 0.005 to 0.1%, N ≦ 0.0040%, Ti: 0.002 to 0.015% and Nb: 0.002 to 0.015% A plurality of varieties including two kinds of steel (hereinafter referred to as “P-added ultra-low-carbon BH steel”) containing seeds such that Ti + Nb = 0.002 to 0.015% and the balance Fe and inevitable impurities . Annealing steel plates continuously,
The overaging zone has a temperature pattern in which the temperature of the steel strip gradually decreases from upstream to downstream, and the outlet temperature of the overaging zone is 250 to 300 ° C. during annealing of the low-carbon aluminum killed steel. A continuous annealing method for a steel sheet, characterized in that the temperature is over 300 ° C. during annealing of the P-added ultra-low carbon BH steel .
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