JP4494903B2 - Continuous annealing equipment for manufacturing high-strength steel sheets - Google Patents
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Description
本発明は、ＤＰ鋼からなる高張力鋼板製造用の連続焼鈍設備に関するものである。ここでＤＰ鋼はDual Phase鋼を意味し、フェライトとマルテンサイトの２相の析出比を制御することにより加工性を確保しつつ強度を高めた鋼である。なお本発明は、冷延鋼板製造のための連続焼鈍処理に限らず、溶融亜鉛めっき製造設備およびこれら両者の兼用製造設備の連続焼鈍処理に対しても適用し得るものである。 The present invention relates to a continuous annealing facility for producing a high-strength steel plate made of DP steel. Here, DP steel means dual phase steel, which is steel whose strength is increased while securing workability by controlling the precipitation ratio of two phases of ferrite and martensite . In addition, this invention is applicable not only to the continuous annealing process for cold-rolled steel plate manufacture, but also to the continuous annealing process of hot dip galvanization manufacturing equipment and these both combined manufacturing equipment.
加熱均熱帯、急冷帯、再均熱帯などを一連に設けて鋼板の連続焼鈍を行う設備は、特許文献１に示すように公知である。この特許文献１に示されるような従来の連続焼鈍設備は軟鋼製造用のものであって、再結晶及びセメンタイト析出による過時効処理のために、例えば７００〜８００℃の加熱・均熱、１０〜１００℃/秒の冷却、３００〜５００℃で６０〜３００秒程度保持の各機能が要求されている。 As shown in Patent Document 1, a facility for continuously annealing a steel sheet by providing a series of heating soaking zones, quenching zones, re-soaking zones and the like in series. The conventional continuous annealing equipment as shown in Patent Document 1 is for mild steel production, and for overaging treatment by recrystallization and cementite precipitation, for example, heating and soaking at 700 to 800 ° C, Each function of cooling at 100 ° C./second and holding at 300 to 500 ° C. for about 60 to 300 seconds is required.
ところがＤＰ鋼などの高張力鋼板の製造にはＡＣ３点付近における高温加熱を必要とするため、従来の連続焼鈍設備では加熱能力が不足して生産性が低下する一方、冷却帯や再加熱帯については過剰設備となり、初期設備投資と生産コストの上昇や、製造条件のばらつきによる品質の不安定などの問題を生じていた。
本発明は上記した従来の問題点を解決し、高張力鋼板の製造に最適な設備構成とすることによって初期設備投資と生産コストを削減し、また製造条件のばらつきを小さくして安定した品質の高張力鋼板の製造を可能とした高張力鋼板製造用の連続焼鈍設備を提供するためになされたものである。 The present invention solves the above-mentioned conventional problems, reduces the initial capital investment and production cost by adopting an optimum equipment configuration for the production of high-tensile steel sheets, and reduces the variation in production conditions to achieve stable quality. It is made in order to provide the continuous annealing equipment for high-tensile steel plate manufacture which enabled manufacture of a high-tensile steel plate.
上記の課題を解決するためになされた本発明の高張力鋼板製造用の連続焼鈍設備は、高温加熱手段と、中速冷却手段と、短時間保持手段とを具備するＤＰ鋼からなる高張力鋼板製造用の連続焼鈍設備であって、高温加熱手段が板温８５０℃以上で４５秒以上保持する均熱帯であり、中速冷却手段が６００〜７５０℃の肩落とし温度まで２０℃/秒以下の冷却速度で徐冷する徐冷帯と、３０〜５０℃/秒の冷却速度の急冷帯であり、短時間保持手段が２５０〜３００℃で１００〜２００秒保持する再均熱帯であることを特徴とするものである。 The continuous annealing equipment for producing the high-strength steel sheet of the present invention made to solve the above-mentioned problems is a high-strength steel sheet made of DP steel having a high-temperature heating means, a medium-speed cooling means, and a short-time holding means. A continuous annealing facility for production, in which the high-temperature heating means is a soaking zone where the plate temperature is kept at 850 ° C. or more and is maintained for 45 seconds or more, and the medium-speed cooling means is 20 ° C./second or less to a shouldering temperature of 600 to 750 ° C. A slow cooling zone that cools slowly at a cooling rate and a rapid cooling zone at a cooling rate of 30 to 50 ° C./second , characterized in that the short-time holding means is a re-equalizing zone that holds 100 to 200 seconds at 250 to 300 ° C. It is what .
本発明の連続焼鈍設備は、板温を８５０℃以上に保持できる高温加熱手段を備えているので、組織の均一化を図ることができる。また、６００〜７５０℃の肩落とし温度まで２０℃/秒以下の冷却速度で徐冷する徐冷帯と、３０〜５０℃/秒の冷却速度を持つ中速冷却手段を備えているので、強度向上と延性確保に寄与するフェライト及びマルテンサイトを安定に析出させることができる。さらに２５０〜６００℃で１００〜２００秒保持する短時間保持手段を備えているので、急冷により生成したマルテンサイトを適度に焼き戻して硬度と伸びのバランスを取ることができる。このように本発明の連続焼鈍設備は高張力鋼板製造用に適したものであり、初期設備投資と生産コストを抑制しつつ、高張力鋼板を最適条件で安定して生産することができる。以下に本発明の更に具体的な内容を、好ましい実施形態とともに説明する。 Since the continuous annealing equipment of the present invention includes high-temperature heating means that can maintain the plate temperature at 850 ° C. or higher, the structure can be made uniform. In addition, since it has a slow cooling zone that gradually cools at a cooling rate of 20 ° C./second or less to a shoulder temperature of 600 to 750 ° C., and a medium speed cooling means that has a cooling rate of 30 to 50 ° C./second, Ferrite and martensite that contribute to improvement and securing of ductility can be stably precipitated. Furthermore, since a short-time holding means for holding for 100 to 200 seconds at 250 to 600 ° C. is provided, the martensite generated by rapid cooling can be appropriately tempered to balance hardness and elongation. Thus, the continuous annealing equipment of the present invention is suitable for manufacturing high-strength steel sheets, and high-strength steel sheets can be stably produced under optimum conditions while suppressing initial equipment investment and production costs. Hereinafter, more specific contents of the present invention will be described together with preferred embodiments.
図１は本発明の連続焼鈍設備の一例を示す概念図であり、１は加熱帯、２は均熱帯、３は徐冷帯、４は急冷帯、５は再均熱帯、６は最終冷却帯である。高張力鋼板の鋼帯はこれらの各帯域を矢印方向に連続的に走行しつつ順次熱処理を施される。また図２は鋼帯（伸び型のＤＰ鋼）がこれらの各帯域を走行する間に受ける温度変化を示すグラフである。鋼帯はＤＰ鋼などの高張力鋼板であるが、以下の説明では０．１３％Ｃ，１．０％Ｓｉ，２．２％Ｍｎの成分を持つＤＰ鋼を例とする。 FIG. 1 is a conceptual diagram showing an example of continuous annealing equipment of the present invention, where 1 is a heating zone, 2 is a soaking zone, 3 is a slow cooling zone, 4 is a quenching zone, 5 is a re-soaking zone, and 6 is a final cooling zone. It is. The steel strip of the high-tensile steel plate is sequentially heat-treated while continuously running in each of these zones in the direction of the arrow. FIG. 2 is a graph showing the temperature change that the steel strip (elongated DP steel) undergoes while traveling through these zones. The steel strip is a high-strength steel plate such as DP steel , but in the following description, DP steel having components of 0.13% C, 1.0% Si, and 2.2% Mn is taken as an example.
鋼帯は高温加熱手段である加熱炉１において加熱昇温され、均熱帯２で図２中にＡとして示したＡＣ３点付近の高温に保持される。その温度は８５０℃以上９００℃未満とし、保持時間は４５秒以上とすることが好ましい。高張力鋼板のＡＣ３点は成分によって異なるがこの例では８５０℃程度であり、高温保持温度がＡＣ３点を下回ると出来上り組織が不均一となるので、８５０℃以上とする。しかし高温保持温度が９００℃を越えるとオーステナイト組織の結晶粒粗大化のため出来上がり組織でのフェライト組織生成が抑制されるので好ましくない。更に保持時間が４５秒未満では出来上がり組織が不均一となるため、上記の数値範囲とすることが好ましい。 The steel strip is heated and heated in a heating furnace 1 as a high-temperature heating means, and is maintained at a high temperature in the vicinity of the AC3 point indicated as A in FIG. The temperature is preferably 850 ° C. or higher and lower than 900 ° C., and the holding time is preferably 45 seconds or longer. C3 points A high-tensile steel sheet depends component but is about 850 ° C. In this example, a high temperature holding temperature is so ready tissue below the C3 point A becomes nonuniform, and 850 ° C. or higher. However, if the high temperature holding temperature exceeds 900 ° C., it is not preferable because the formation of ferrite structure in the finished structure is suppressed due to the coarsening of the austenite structure. Further, when the holding time is less than 45 seconds, the finished structure becomes non-uniform, and thus the above numerical range is preferable.
均熱帯２を出た鋼板は、中速冷却手段の一部である徐冷帯３において図２中にＢとして示した温度まで徐冷される。この温度（肩落とし温度＝急冷開始温度）によりフェライトとマルテンサイトとの比率が決定される。本発明においては肩落とし温度は６００〜７５０℃とすることが好ましい。肩落とし温度をこの範囲内とすることにより、強度や伸び等の特性が向上する。なお、徐冷帯３における冷却速度は２０℃/秒以下好ましくは５℃／秒以下と低速にすることが好ましい。これはフェライトとマルテンサイトとの比率を決定する肩落とし温度を、正確に制御するためである。 The steel sheet that has left the soaking zone 2 is gradually cooled to the temperature indicated as B in FIG. 2 in the slow cooling zone 3 that is part of the medium speed cooling means. This temperature (shoulder drop temperature = rapid cooling start temperature) determines the ratio of ferrite to martensite. In the present invention, the shoulder drop temperature is preferably 600 to 750 ° C. By setting the shoulder drop temperature within this range, properties such as strength and elongation are improved. The cooling rate in the slow cooling zone 3 is preferably 20 ° C./second or less, preferably 5 ° C./second or less. This is to accurately control the shoulder drop temperature that determines the ratio of ferrite to martensite.
徐冷帯３を出た鋼帯は、中速冷却手段の一部である急冷帯４において図２中にCとして示した３０〜５０℃/秒の冷却速度で２５０〜３００℃まで急冷される。冷却速度はパーライトの生成を回避し、マルテンサイトを効率よく生成させるために重要であり、３０℃/秒未満では強度向上に寄与するマルテンサイトの生成が不十分となり、５０℃/秒を越えると冷却終了温度を厳密に管理することがむずかしくなる。図２中にＤとして示した冷却終了温度は、マルテンサイト生成による強度確保のために重要であり、ＤＰ鋼の場合には２５０〜３００℃が好ましい。 The steel strip exiting the slow cooling zone 3 is rapidly cooled to 250 to 300 ° C. at a cooling rate of 30 to 50 ° C./second shown as C in FIG. 2 in the quench zone 4 which is a part of the medium speed cooling means. . The cooling rate is important in order to avoid the formation of pearlite and to efficiently generate martensite. If it is less than 30 ° C / second, the generation of martensite that contributes to strength improvement becomes insufficient, and if it exceeds 50 ° C / second, It becomes difficult to strictly control the cooling end temperature. The cooling end temperature indicated as D in FIG. 2 is important for securing the strength by martensite generation, and is preferably 250 to 300 ° C. in the case of DP steel.
続いて鋼板は短時間保持手段である再均熱帯５において、図２中にＥとして示すように２５０〜３００℃で１００〜２００秒保持される。この短時間保持は、マルテンサイトの適度な焼きもどしによる加工性向上を目的として行われる。１００秒未満では局部延性が不足してもろくなるが、２００秒を越えると延性が大きくなる反面、強度が低下してしまうため１００〜２００秒が適当であり、１２０〜１７０秒の範囲が特に好ましい。 Subsequently, the steel sheet is held at 250 to 300 ° C. for 100 to 200 seconds as indicated by E in FIG. This short-time holding is performed for the purpose of improving workability by appropriate tempering of martensite. If it is less than 100 seconds, it becomes brittle even if the local ductility is insufficient. However, if it exceeds 200 seconds, the ductility increases, but the strength decreases, so that 100 to 200 seconds is appropriate, and the range of 120 to 170 seconds is particularly preferable. .
なお、従来の軟鋼の連続焼鈍設備においては、図２中に破線で示したように３００秒程度の長時間にわたる再加熱が行われていたが、これは過時効処理によりセメンタイトを析出させる目的であり、結晶組織中のカーボンを拡散移動させるために長時間を要していた。これに対して本発明ではマルテンサイトの焼きなましを目的としているため、カーボンは格子間移動を行うのみでよく、２００秒以下の短時間で十分である。 In addition, in the conventional continuous annealing equipment for mild steel, as shown by the broken line in FIG. 2, reheating was performed for a long time of about 300 seconds. This is for the purpose of precipitating cementite by overaging treatment. In addition, it took a long time to diffuse and move the carbon in the crystal structure. On the other hand, since the present invention aims at annealing of martensite, carbon only needs to perform interstitial movement, and a short time of 200 seconds or less is sufficient.
このようにして得られたＤＰ鋼は、例えばフェライトが約６５％、マルテンサイトと焼き戻しマルテンサイトとが合計で約３０％、残部がベイナイト等である相構成を持ち、９８０ＭＰａ以上の強度と１７％以上の伸びとを備えた高張力鋼板となる。なおフェライトは伸びに寄与し、マルテンサイトと焼き戻しマルテンサイトとは強度に寄与する。中でも焼き戻しマルテンサイトは焼き戻し時にフェライトとマルテンサイト間の粒界強度差を低減し、局部成形性の向上に寄与する。本発明の連続焼鈍設備は高張力鋼板の製造に最適な設備構成であり、初期設備投資と生産コストを削減し、また製造条件のばらつきを小さくして安定した品質の高張力鋼板の製造を可能としたものである。 The DP steel thus obtained has a phase structure in which, for example, ferrite is about 65%, martensite and tempered martensite are about 30% in total, and the balance is bainite and the like. % High-strength steel sheet having an elongation of at least%. Ferrite contributes to elongation, and martensite and tempered martensite contribute to strength. Among them, tempered martensite reduces the grain boundary strength difference between ferrite and martensite during tempering, and contributes to the improvement of local formability. The continuous annealing equipment of the present invention is the optimum equipment configuration for the production of high-strength steel sheets, reducing initial equipment investment and production costs, and enabling the production of high-strength steel sheets with stable quality by reducing variations in production conditions. It is what.
次に参考のため、ＴＲＩＰ鋼について記載する。
図３は、鋼帯（ＴＲＩＰ鋼）がこれらの各帯域を走行する間に受ける温度変化を示すグラフである。ここでは０．１１％Ｃ、１．３％Ｓｉ、１．６％Ｍｎの成分を持つＴＲＩＰ鋼を例とする。加熱帯１で加熱された鋼帯は均熱帯２で高温に保持されるが、残留オーステナイトの生成を促進するためにその温度はＡＣ３点以下の８５０℃程度とするのが好ましい。
Next, TRIP steel is described for reference.
FIG. 3 is a graph showing temperature changes that a steel strip (TRIP steel) undergoes while traveling in each of these zones. Here, TRIP steel having components of 0.11% C, 1.3% Si, and 1.6% Mn is taken as an example. The steel strip heated in the heating zone 1 is kept at a high temperature in the soaking zone 2, but the temperature is preferably about 850 ° C. below the AC 3 point in order to promote the formation of retained austenite.
次いで鋼帯は徐冷帯３を経て急冷帯で冷速５０℃／秒以上の冷却速度で４００〜４５０℃まで急冷される。これは冷却中のパーライト生成を回避し、残留オーステナイト量を安定確保することによって、目標とする強度と延性を得るためである。なお、徐冷帯３における冷却速度は、急冷開始温度を正確に制御するために１０℃／秒以下と低速にすることが好ましいのは、前出のＤＰ鋼の例と同じである。次にＴＲＩＰ鋼は再均熱帯５において、４００〜４５０℃で１００〜２００秒保持される。これによってオーステナイト中のカーボン濃度を高め、残留オーステナイトの生成を促す。
Next, the steel strip passes through the slow cooling zone 3 and is rapidly cooled to 400 to 450 ° C. at a cooling rate of 50 ° C./second or more in the quench zone. This is to avoid the formation of pearlite during cooling and to obtain a target strength and ductility by ensuring a stable amount of retained austenite. It is to be noted that the cooling rate in the slow cooling zone 3 is preferably set to a low speed of 10 ° C./second or less in order to accurately control the rapid cooling start temperature, as in the case of the DP steel described above. Next, the TRIP steel is held at 400 to 450 ° C. for 100 to 200 seconds in the
この図３に示す温度条件により製造されたＴＲＩＰ鋼は、例えばフェライトを主とする組織中に約１０％の残留オーステナイトを持ち、この残留オーステナイトが加工の際の歪みによって硬いマルテンサイトに変わる現象によって、高い延性と強度を有する高強度鋼板となる。 The TRIP steel manufactured under the temperature condition shown in FIG. 3 has a retained austenite of about 10% in a structure mainly composed of ferrite, for example, and this retained austenite is transformed into hard martensite due to strain during processing. It becomes a high-strength steel sheet having high ductility and strength.
以上に説明したとおり、本発明の連続焼鈍設備を用いれば初期設備投資と生産コストを削減し、また製造条件のばらつきを小さくして安定した品質の高張力鋼板の製造が可能となる。しかし本発明の連続焼鈍設備は低炭素鋼の製造には不向きであるものの、極低炭素鋼の製造には支障なく用いることができる。 As described above, if the continuous annealing equipment of the present invention is used, initial equipment investment and production costs can be reduced, and variations in manufacturing conditions can be reduced, and stable high-tensile steel sheets can be manufactured. However, although the continuous annealing equipment of the present invention is not suitable for the production of low carbon steel, it can be used without difficulty for the production of ultra low carbon steel.
１ 加熱帯
２ 均熱帯
３ 徐冷帯
４ 急冷帯
５ 再均熱帯
６ 最終冷却帯
1 Heating zone 2 Soaking zone 3 Slow cooling zone 4
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| JPH05117761A (en) * | 1991-10-28 | 1993-05-14 | Nippon Steel Corp | Manufacture of high strength thin steel sheet extremely excellent in workability |
| JPH05179345A (en) * | 1991-12-27 | 1993-07-20 | Nkk Corp | Production of compound structure steel plate having high workability and high strength |
| JPH09235626A (en) * | 1995-12-26 | 1997-09-09 | Nippon Steel Corp | Primary cooling method at continuous annealing for steel strip |
| JP2002363650A (en) * | 2001-06-07 | 2002-12-18 | Kobe Steel Ltd | Method for producing ultrahigh strength cold rolled steel sheet having excellent seam weldability |
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