JP2002038248A - Method for manufacturing galvanized steel sheet with high tensile strength and ductility - Google Patents

Method for manufacturing galvanized steel sheet with high tensile strength and ductility

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Publication number
JP2002038248A
JP2002038248A JP2000225413A JP2000225413A JP2002038248A JP 2002038248 A JP2002038248 A JP 2002038248A JP 2000225413 A JP2000225413 A JP 2000225413A JP 2000225413 A JP2000225413 A JP 2000225413A JP 2002038248 A JP2002038248 A JP 2002038248A
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JP
Japan
Prior art keywords
less
steel sheet
seconds
temperature
austenite
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
JP2000225413A
Other languages
Japanese (ja)
Other versions
JP3521851B2 (en
Inventor
Hirotatsu Kojima
啓達 小嶋
Masahiko Hori
雅彦 堀
Hiroyuki Nakagawa
浩行 中川
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Sumitomo Metal Industries Ltd
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Filing date
Publication date
Application filed by Sumitomo Metal Industries Ltd filed Critical Sumitomo Metal Industries Ltd
Priority to JP2000225413A priority Critical patent/JP3521851B2/en
Publication of JP2002038248A publication Critical patent/JP2002038248A/en
Application granted granted Critical
Publication of JP3521851B2 publication Critical patent/JP3521851B2/en
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Expired - Fee Related legal-status Critical Current

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  • Heat Treatment Of Sheet Steel (AREA)

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a galvanized steel sheet with a high tensile strength and ductility, which manufactures a steel sheet suitable for structural members of automobile, having a combination of high tensile strength, high ductility and superior corrosion resistance, by using a conventional facility. SOLUTION: This method comprises, (1) a cold rolled strip which contains 0.05-0.25% C, <=0.7% Si, 0.8-2.5% Mn, 0.4-2.0% sol. Al, and 1.0-2.5% Si+Al, (2) a continuous annealing which includes heating the strip at a two-phase region, cooling at 30 deg.C/second or more between 700-450 deg.C, and holding at 450-350 deg.C for 100-500 seconds, and (3) a hot-dip galvanizing after heating the strip at the two-phase region, cooling at 3 deg.C/second or more to 500 deg.C or lower, and holding at 460-500 deg.C for 15 seconds or longer. A galvannealed strip comprises alloying the galvanized strip by holding at 480-530 deg.C for 10-30 seconds. The steel strip may contain Ti, Nb, Cr, Mo, Cu, Ni, and Co.

Description

【発明の詳細な説明】DETAILED DESCRIPTION OF THE INVENTION

【0001】[0001]

【発明の属する技術分野】本発明は、自動車の構造部材
などに好適な、高強度、高延性、かつ、防錆性能に優れ
た、高張力高延性亜鉛めっき鋼板の製造方法に関する。
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength, high-ductility galvanized steel sheet which is suitable for a structural member of an automobile and has high strength, high ductility, and excellent rust prevention performance.

【0002】[0002]

【従来の技術】自動車の構造部材などには、衝突安全性
確保および軽量化の観点から、従来より強度が高い鋼板
が求められている。一般に、鋼板の強度とプレスなどの
成形性は相反し、鋼板の強度が高くなるほど延性が劣化
し、プレス成形が困難になり、高強度になるほど適用が
可能な部品が制限される。従って高強度鋼板の適用を拡
大するために、高強度でかつ成形性に優れた鋼板が求め
られている。
2. Description of the Related Art Steel plates having higher strength than ever are required for structural members of automobiles and the like from the viewpoint of ensuring collision safety and reducing weight. In general, the strength of a steel sheet and the formability of a press or the like are contradictory, and the higher the strength of the steel sheet, the lower the ductility, the more difficult it is to press form, and the higher the strength, the more applicable parts are limited. Therefore, in order to expand the application of high-strength steel sheets, steel sheets having high strength and excellent formability are required.

【0003】自動車の構造部材などは防錆性が優れてい
ることも重要な要素であるので、安価で優れた防錆性が
得られる溶融亜鉛めっき鋼板が広く用いられている。な
かでも、溶融亜鉛めっきした鋼板を合金化温度に加熱し
てZnめっき層をZn−Fe合金とした合金化溶融亜鉛
めっき鋼板は、摺動性、溶接性や塗装性が優れているの
で特に広く用いられている。
It is also important that structural members of automobiles have excellent rustproofing properties, and therefore galvanized steel sheets that are inexpensive and have excellent rustproofing properties are widely used. Above all, alloyed hot-dip galvanized steel sheets in which a hot-dip galvanized steel sheet is heated to an alloying temperature and a Zn-plated layer is made of a Zn-Fe alloy are particularly widely used because of excellent slidability, weldability and paintability. Used.

【0004】鋼を高強度化するには、Cや、Si、Mn
などの合金元素を含有させることが有効である。しかし
ながらこれらの合金元素を大量に鋼に含有させるとめっ
き前の母材表面の溶融亜鉛に対する濡れ性が低下し、不
めっきが発生しやすいうえ、合金化処理を行う場合に
は、その反応速度が遅くなる。このため、大量の合金元
素を含有させる方法では、表面や性状が良好な溶融めっ
き鋼板あるいは合金化溶融亜鉛めっき鋼板が得られな
い。
[0004] To increase the strength of steel, C, Si, Mn
It is effective to include alloy elements such as. However, if a large amount of these alloying elements are contained in steel, the wettability of the surface of the base material before plating with molten zinc is reduced, non-plating is likely to occur, and the rate of reaction when performing alloying treatment is reduced. Become slow. For this reason, a hot-dip galvanized steel sheet or a galvannealed steel sheet with good surface and properties cannot be obtained by the method of including a large amount of alloying elements.

【0005】オーステナイト系ステンレス鋼板では、オ
ーステナイトが成形中にマルテンサイトに変態すること
によって著しい伸びを呈する「変態誘起塑性(TRI
P)」という現象がよく知られている。これを利用し
て、延性を改善した高強度鋼板を得る技術がすでにいく
つか提案されている。
[0005] In austenitic stainless steel sheet, "transformation-induced plasticity (TRI)" shows a remarkable elongation due to transformation of austenite into martensite during forming.
P) "is well known. Several techniques for obtaining a high-strength steel sheet with improved ductility using this have already been proposed.

【0006】例えば特開昭60−43430号公報で
は、大量のC、Si、Mnを含有する冷間圧延板を、フ
ェライト+オーステナイトの2相域(以下、単に「2相
域」とも記す)に加熱、保持した後に、50℃/秒以上
の冷却速度で450℃〜650℃の温度域に冷却し、そ
の温度域で10〜50秒間保持し、その後30℃/秒以
上の冷却速度で冷却し、最終製品において、フェライト
と残留オーステナイトを各々体積率で10%以上有し、
残部がベイナイトとマルテンサイトからなる結晶組織を
有する高強度高加工性複合組織鋼板の製造方法が開示さ
れている。
[0006] For example, in Japanese Patent Application Laid-Open No. Sho 60-43430, a cold rolled sheet containing a large amount of C, Si, and Mn is placed in a two-phase region of ferrite + austenite (hereinafter, also simply referred to as “two-phase region”). After heating and holding, it is cooled at a cooling rate of 50 ° C./sec or more to a temperature range of 450 ° C. to 650 ° C., kept at that temperature range for 10 to 50 seconds, and then cooled at a cooling rate of 30 ° C./sec or more. In the final product, each of ferrite and retained austenite has a volume ratio of 10% or more,
A method for producing a high-strength and high-workability composite structure steel sheet having a crystal structure composed of bainite and martensite is disclosed.

【0007】しかしながら上記鋼板はC含有量が多いた
めに鋼の強度が高くなり過ぎてプレス成形用途には適さ
ないものであった。また、鋼のSi含有量が多くなるの
で、溶融亜鉛めっきが困難であり、溶融亜鉛めっき鋼板
としての活用ができないという問題を有していた。そし
てこれらの問題を解決するため、次のようにいくつかの
技術が提案されている。
[0007] However, the above steel sheet has a high C content, so that the strength of the steel becomes too high and is not suitable for press forming applications. In addition, since the Si content of the steel is increased, there is a problem that hot-dip galvanizing is difficult and cannot be used as a hot-dip galvanized steel sheet. To solve these problems, several techniques have been proposed as follows.

【0008】特開平5−70886号公報には、C含有
量を低減し、SiをAlで置換することによって、プレ
ス成形用途の強度レベルまで低強度化を図った、残留オ
ーステナイトを有する局部延性の優れた高張力薄鋼板と
その製造法が開示されている。上記公報ではその製造方
法として、所定の化学組成を有する冷間圧延板を2相域
に加熱して焼鈍し(以下、単に「2相域焼鈍」と記
す)、その後の冷却過程の550〜350℃の温度領域
で30秒以上(好ましくは2分以上)保持する低温保持
をおこなえば可能である旨が記載されている。また、パ
ーライト変態を抑制するため、700℃から保持温度ま
での冷却速度を50℃/秒以上とすることが好ましいこ
とも記載されている。
[0008] Japanese Patent Application Laid-Open No. 5-70886 discloses a local ductility having retained austenite in which the C content is reduced and Si is replaced with Al to reduce the strength to the level of strength for press forming applications. An excellent high-strength thin steel sheet and a method for producing the same are disclosed. In the above-mentioned publication, as a manufacturing method, a cold-rolled sheet having a predetermined chemical composition is heated to a two-phase region and annealed (hereinafter simply referred to as “two-phase region annealing”). It is described that this can be achieved by holding at a low temperature for 30 seconds or more (preferably 2 minutes or more) in a temperature range of ° C. It is also described that the cooling rate from 700 ° C. to the holding temperature is preferably set to 50 ° C./sec or more in order to suppress the pearlite transformation.

【0009】特開平5−247586号公報には、鋼の
SiとAlの含有量を特定範囲とすることにより、めっ
き密着性と強度−延性のバランスとを改善した残留オー
ステナイトを含有する高強度高延性溶融亜鉛めっき鋼板
が開示されている。しかしながらその製造方法について
は、連続溶融亜鉛めっきラインにおける2相域での焼鈍
後の冷却速度を、5℃/秒以上とすることが記載されて
いるのみで、他の条件についての記載がなく、実際の生
産における有用性に乏しい。
[0009] Japanese Patent Application Laid-Open No. 5-247586 discloses a high-strength steel containing retained austenite in which the balance between plating adhesion and strength-ductility is improved by setting the content of Si and Al in steel to a specific range. A ductile hot-dip galvanized steel sheet is disclosed. However, the production method only discloses that the cooling rate after annealing in the two-phase region in the continuous hot-dip galvanizing line is set to 5 ° C./sec or more, and there is no description of other conditions. Poor utility in actual production.

【0010】特開平11−222644号公報には、鋼
に適量のTiを含有させることにより、Siによる皮膜
密着性および合金化処理性の劣化を補う技術が開示され
ている。しかしながらその製造方法は通常の方法でよい
とされているだけであり、機械特性とめっき品質を最適
化するための溶融亜鉛めっきラインの条件は開示されて
いない。
[0010] Japanese Patent Application Laid-Open No. 11-222644 discloses a technique in which steel contains an appropriate amount of Ti to compensate for the deterioration of film adhesion and alloying treatment property due to Si. However, it is said that the production method may be any ordinary method, and the conditions of a hot-dip galvanizing line for optimizing mechanical properties and plating quality are not disclosed.

【0011】特開平11−131145号公報には、2
相域で焼鈍した後520℃以下まで冷却し、その後溶融
めっき、付着量調整、合金化処理などの一連の製造工程
において、520℃〜400℃の温度域で90秒以上3
00秒以下の時間留まり、その後200℃以下に冷却す
る、残留オーステナイトを有する高強度高延性溶融亜鉛
めっき鋼板の製造方法が開示されている。
Japanese Patent Application Laid-Open No. H11-131145 discloses 2
After annealing in the phase region, it is cooled to 520 ° C. or lower, and then in a series of manufacturing processes such as hot-dip plating, adhesion control, alloying treatment, etc., at a temperature range of 520 ° C. to 400 ° C. for 90 seconds or more.
A method for producing a high-strength, high-ductility hot-dip galvanized steel sheet having retained austenite, which stays for 00 seconds or less and then cools to 200 ° C. or less, is disclosed.

【0012】特開平11−236621号公報には、特
定の化学組成を有する冷間圧延板に前酸化を施した後に
2相域で焼鈍し、その後、20秒間以上の低温保持をお
こなう高張力高延性亜鉛めっき鋼板の製造方法が開示さ
れている。
JP-A-11-236621 discloses that a cold-rolled sheet having a specific chemical composition is pre-oxidized, then annealed in a two-phase region, and then maintained at a low temperature for at least 20 seconds. A method for producing a ductile galvanized steel sheet is disclosed.

【0013】特開平11−279691号公報には、M
nをC含有量の15倍以上添加することによって、めっ
き直後におこなう合金化処理のための再加熱でパーライ
トおよびベイナイト変態の進行を著しく遅延させる技術
が開示されている。しかし、多量のMn添加は合金コス
ト増になるほか、めっき濡れ性を阻害するという弊害も
ある。
JP-A-11-279691 discloses M
There is disclosed a technique in which the addition of n at least 15 times the C content significantly delays the progress of pearlite and bainite transformation by reheating for alloying treatment performed immediately after plating. However, the addition of a large amount of Mn increases the cost of the alloy and also has the disadvantage of impairing the plating wettability.

【0014】従来から冷間圧延板の連続焼鈍に使用され
ている連続焼鈍設備は、焼鈍温度からの冷却手段として
気水混合したミストや噴流水を冷却媒体とする急速冷
却、および、急速冷却により増加した固溶Cを析出させ
るための、長時間の過時効処理が可能ないわゆる過時効
帯を備えている場合が多い。
Conventionally, continuous annealing equipment used for continuous annealing of cold-rolled sheets is characterized by rapid cooling using gas-water mixed mist or jet water as a cooling medium as a cooling means from the annealing temperature, and rapid cooling. In many cases, a so-called overaging zone capable of performing an overaging treatment for a long time for precipitating the increased solid solution C is provided.

【0015】残留オーステナイトを含む高張力鋼板を製
造するには、パーライト変態を抑制しながら、ベイナイ
ト変態温度域にまで未変態オーステナイトを過冷却する
必要がある。このため、2相域焼鈍後に急速冷却するこ
とが重要である。また、その後には、炭化物生成を伴わ
ないベイナイト変態を促進させ、オーステナイト中にさ
らに炭素を濃縮して安定化させるために、比較的低い温
度で長時間保持する処理を施すことが重要である。
In order to manufacture a high-tensile steel sheet containing retained austenite, it is necessary to supercool untransformed austenite to a bainite transformation temperature range while suppressing pearlite transformation. Therefore, it is important to perform rapid cooling after annealing in the two-phase region. After that, in order to promote the bainite transformation without the formation of carbides and to further concentrate and stabilize carbon in austenite, it is important to perform a treatment of holding at a relatively low temperature for a long time.

【0016】冷間圧延板の再結晶焼鈍に使用される従来
の連続焼鈍設備では、急速冷却装置と過時効帯を備えて
いるのが一般的である。従って残留オーステナイトを含
む高張力鋼板は、冷間圧延板の再結晶焼鈍用の連続焼鈍
設備で製造するのが極めて効率的であり、これにより延
性の優れた高張力鋼板を比較的容易に製造することがで
きる。
Conventional continuous annealing equipment used for recrystallization annealing of a cold-rolled sheet generally has a rapid cooling device and an overaging zone. Therefore, it is extremely efficient to manufacture a high-strength steel sheet containing retained austenite in a continuous annealing facility for recrystallization annealing of a cold-rolled sheet, thereby relatively easily manufacturing a high-tensile steel sheet with excellent ductility. be able to.

【0017】これに対し従来の連続溶融亜鉛めっき設備
においては、焼鈍後に溶融めっきを施す必要があること
からミストや噴流水を冷却媒体とすることが困難であ
り、焼鈍後に急速冷却を施すのは容易ではない。また、
連続溶融亜鉛めっきにおいては、めっき前の鋼板温度を
安定化するために、めっき前の鋼板を比較的低い温度域
で保持する(以下、単に「低温保持」とも記す)。しか
しながらめっき前の鋼板温度が亜鉛の融点以下に低下す
るとめっき浴侵入前に鋼板を再度加熱する必要が生じる
ため、従来の連続溶融亜鉛めっき設備においては長時間
の低温保持を施すのが困難である。
On the other hand, in the conventional continuous hot-dip galvanizing equipment, it is difficult to use hot mist or jet water as a cooling medium because hot-dip coating must be performed after annealing. It's not easy. Also,
In continuous hot-dip galvanizing, in order to stabilize the temperature of a steel sheet before plating, the steel sheet before plating is held in a relatively low temperature range (hereinafter, also simply referred to as “low temperature holding”). However, if the temperature of the steel sheet before plating falls below the melting point of zinc, it is necessary to reheat the steel sheet before entering the plating bath, so it is difficult to maintain a low temperature for a long time in the conventional continuous galvanizing equipment. .

【0018】従って、特開平5−70886号公報、特
開平11−131145号公報あるいは特開平11−2
36621号公報に開示されている急速冷却、あるいは
長時間の低温保持は、一般的な連続溶融亜鉛めっきライ
ンを使用する限り、ライン構成上、容易におこなえるこ
とではない。
Accordingly, JP-A-5-70886, JP-A-11-131145 and JP-A-11-21-2
The rapid cooling or low-temperature holding for a long time disclosed in Japanese Patent No. 36621 cannot be easily performed due to the line configuration as long as a general continuous hot-dip galvanizing line is used.

【0019】溶融めっき温度は460℃前後であり、合
金化処理は460〜500℃の温度領域でおこなわれ
る。残留オーステナイトを有する鋼板をこのような温度
領域に保持するのは、残留オーステナイト中にセメンタ
イトが析出して、炭素を消費するため、残留オーステナ
イトの体積率が減少すると共に、炭素濃度が低下して安
定性が低下するという問題がある。
The hot-dip plating temperature is about 460 ° C., and the alloying treatment is performed in a temperature range of 460 to 500 ° C. Maintaining a steel sheet having retained austenite in such a temperature range is because cementite precipitates in the retained austenite and consumes carbon, so that the volume ratio of the retained austenite is reduced, and the carbon concentration is reduced and the carbon concentration is reduced. There is a problem that the performance is reduced.

【0020】特開平11−131145号公報には、連
続溶融亜鉛めっき設備を用いてオーステナイトを残留さ
せる技術が開示されている。しかしながら一般的に、溶
融亜鉛めっきラインはライン長さが短く、鋼板を520
℃〜400℃での温度範囲に長時間保持するのが困難な
場合がある。ライン速度を遅くして保持時間を長くしよ
うとしても、焼鈍後の冷却速度が6℃/秒に達しないと
いう問題が生じる。
Japanese Patent Application Laid-Open No. 11-131145 discloses a technique in which austenite remains using a continuous galvanizing equipment. However, in general, hot-dip galvanizing lines have a short line length,
In some cases, it is difficult to maintain the temperature in the temperature range of about 400C to about 400C for a long time. Even if the line speed is reduced to increase the holding time, the cooling rate after annealing does not reach 6 ° C./sec.

【0021】特開平11−236621号公報には、時
間および合金化処理温度について詳細に開示されている
が、その好適な条件を既存のラインで実現することは必
ずしも容易ではない。
Japanese Patent Application Laid-Open No. H11-236621 discloses the time and the alloying treatment temperature in detail, but it is not always easy to realize suitable conditions for the existing line.

【0022】これらの問題を避けるために、鋼の化学組
成に対して、例えばTi含有鋼とするなどの改善策を施
すとしても、上記設備差異による性能の差異を化学組成
の改善のみで対応するのは、効果として十分ではない。
In order to avoid these problems, even if improvement measures such as, for example, using Ti-containing steel are applied to the chemical composition of steel, the difference in performance due to the above equipment difference is dealt with only by improving the chemical composition. Is not enough as an effect.

【0023】以上述べたように、TRIP効果を利用し
た成形性に優れた溶融亜鉛めっきおよび合金化溶融亜鉛
めっき高張力鋼板については、種々の提案がされてい
る。しかしながら、既存の一般的な連続溶融亜鉛めっき
ラインにおいて、高張力、高延性、かつ耐食性に優れた
亜鉛めっき鋼板を安定して効率よく製造するのは容易で
はない。
As described above, various proposals have been made for hot-dip galvanized and alloyed hot-dip galvanized high-strength steel sheets having excellent formability utilizing the TRIP effect. However, it is not easy to stably and efficiently produce a galvanized steel sheet having high tensile strength, high ductility, and excellent corrosion resistance in an existing general continuous hot-dip galvanizing line.

【0024】[0024]

【発明が解決しようとする課題】本発明の目的は、上記
のような問題点を解決し、自動車の構造部材などに好適
な、高強度、高延性、および優れた耐食性を兼ね備える
鋼板を、従来の設備を使用して、安定して製造すること
ができる、高張力高延性亜鉛めっき鋼板の製造方法を提
供することにある。
SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet which solves the above problems and has high strength, high ductility and excellent corrosion resistance suitable for structural members of automobiles. It is an object of the present invention to provide a method for producing a high-tension, high-ductility galvanized steel sheet that can be stably produced using the above-mentioned equipment.

【0025】[0025]

【課題を解決するための手段】残留オーステナイトを有
する結晶組織を持つ延性に優れた高強度鋼板を製造する
には、冷間圧延板を焼鈍して、再結晶および炭化物の溶
解を行い、2相域温度での保持によって、オーステナイ
ト中に炭素を濃化させ、その後の冷却時には、パーライ
ト変態を抑制するために冷却速度を数10℃/秒以上と
し、上記冷却終了後には、過冷されたオーステナイトを
ベイナイト変態させ、ベイナイトから排出された炭素を
オーステナイト中にさらに濃化させるために、370℃
以上、450℃以下の温度範囲で長時間保持するのがよ
い。
In order to manufacture a high-strength steel sheet excellent in ductility having a crystal structure having retained austenite, a cold-rolled sheet is annealed, and recrystallization and carbide dissolution are performed. The carbon is concentrated in the austenite by holding at the local temperature, and at the time of subsequent cooling, the cooling rate is set to several tens of degrees Celsius / second or more in order to suppress the pearlite transformation. At 370 ° C. in order to transform bainite into carbon and further enrich carbon contained in bainite in austenite.
As described above, it is preferable that the temperature is maintained for a long time in the temperature range of 450 ° C. or less.

【0026】上記観点からすれば、一般的な溶融亜鉛め
っきラインにおいて製造する際の問題点は、以下の3点
に集約される。 a.2相域焼鈍温度から中間温度領域までの間の冷却速
度が、一般的な溶融亜鉛めっきラインにおいては、数℃
/秒と緩やかである。このため、2相域で存在していた
オーステナイトが冷却過程でパーライトに変態し、最終
製品における残留オーステナイト量が減少する。
From the above point of view, the problems at the time of manufacturing in a general hot-dip galvanizing line are summarized in the following three points. a. In a general hot-dip galvanizing line, the cooling rate between the two-phase annealing temperature and the intermediate temperature range is several degrees Celsius.
/ Slow. For this reason, austenite existing in the two-phase region is transformed into pearlite during the cooling process, and the amount of retained austenite in the final product decreases.

【0027】b.2相域焼鈍温度から中間温度領域に冷
却した後は、その温度で鋼板を保持して、オーステナイ
トをベイナイト変態させる必要がある。しかしながら現
実的には設備制約があるため、2相域焼鈍後の冷却完了
から溶融亜鉛めっき浴に侵入するまでの時間(以下、
「低温保持時間」と記す)を短くせざるを得ず、ベイナ
イト変態が不十分な状態でめっきすることになる。この
ため、未変態オーステナイトが安定化されず、最終的に
はマルテンサイト変態が生じる。
B. After cooling from the two-phase region annealing temperature to the intermediate temperature region, it is necessary to maintain the steel sheet at that temperature to transform austenite to bainite. However, due to equipment limitations in practice, the time from the completion of cooling after annealing in the two-phase region to the intrusion into the hot dip galvanizing bath (hereinafter, referred to as
The plating time must be shortened and the bainite transformation is insufficient. For this reason, untransformed austenite is not stabilized, and finally martensitic transformation occurs.

【0028】また、ベイナイト変態は400℃近傍にノ
ーズがあるが、めっき浴温度より低い温度であるので、
この温度まで鋼板を冷却してしまうと、めっき浴侵入と
同時に亜鉛の凝固が開始し付着量制御が困難になる。し
たがって、鋼板はノーズよりも高い温度で保持せざるを
得ない。このように溶融亜鉛めっきラインで最適なベイ
ナイト変態温度で保持するのが困難であることも溶融め
っき品で残留オーステナイトを得るのが容易でない理由
の一つである。
The bainite transformation has a nose near 400 ° C., but at a temperature lower than the plating bath temperature.
If the steel sheet is cooled to this temperature, solidification of zinc starts at the same time as entering the plating bath, and it becomes difficult to control the amount of adhesion. Therefore, the steel sheet must be maintained at a temperature higher than the nose. As described above, it is difficult to maintain the optimum bainite transformation temperature in the hot-dip galvanizing line, which is one of the reasons why it is not easy to obtain retained austenite in a hot-dip galvanized product.

【0029】c.合金化処理は、ベイナイト変態温度よ
り高温であるため、仮にめっき浴浸漬直後にオーステナ
イトが残留していたとしてもベイナイト変態は進行せ
ず、残留オーステナイトは容易にセメンタイトに分解す
る。特に、Si含有量が多い母材をめっきした場合に
は、母材界面での合金化反応を促進するために合金化温
度を高くする必要があるが、これによりオーステナイト
の分解がさらに加速される。
C. In the alloying treatment, since the temperature is higher than the bainite transformation temperature, even if austenite remains immediately after immersion in the plating bath, bainite transformation does not proceed, and the retained austenite is easily decomposed into cementite. In particular, when a base material having a high Si content is plated, it is necessary to increase the alloying temperature in order to promote the alloying reaction at the base material interface, which further accelerates the decomposition of austenite. .

【0030】これらの要因により、残留オーステナイト
量の確保が困難となり、また、残留オーステナイトの炭
素濃度も低くなる。炭素濃度が低い残留オーステナイト
は不安定で、マルテンサイトに変態し易く、加工初期に
マルテンサイトに変態し、オーステナイトのTRIP効
果への寄与が減少するという悪影響がある。
These factors make it difficult to secure the amount of retained austenite, and also reduce the carbon concentration of the retained austenite. Retained austenite having a low carbon concentration is unstable and easily transforms into martensite, transforms into martensite in the early stage of processing, and has the adverse effect of reducing the contribution of austenite to the TRIP effect.

【0031】本発明者らは、これらの問題を解決するた
めに、種々の研究を重ねた結果、焼鈍を2回施すことに
より、上記の問題点が極めて効率よく解決できることを
見いだした。以下に、本発明者らによる実験結果を説明
する。
The present inventors have conducted various studies to solve these problems, and as a result, have found that the above-mentioned problems can be solved very efficiently by performing annealing twice. Hereinafter, experimental results by the present inventors will be described.

【0032】表1に示す成分の鋼Aを実験室で溶解し、
熱間鍛造を施して厚さが20mmの鋼片とした。
Steel A having the composition shown in Table 1 was melted in a laboratory.
Hot forging was performed to obtain a steel slab having a thickness of 20 mm.

【0033】[0033]

【表1】 これを、1200℃で30分間加熱し、1000℃から
熱間圧延を開始して850℃で完了し、その後、水スプ
レーにより550℃まで冷却し、徐冷炉に装入して55
0℃で30分保持した後に20℃/時の冷却速度で室温
まで冷却し、厚さが3mmの熱間圧延板を得た。これを
酸洗してスケールを除去し、冷間圧延して厚さが1.6
mmの冷間圧延板とした。
[Table 1] This was heated at 1200 ° C. for 30 minutes, hot rolling was started from 1000 ° C. and completed at 850 ° C., then cooled to 550 ° C. by water spray, and charged in a lehr to 55 ° C.
After holding at 0 ° C. for 30 minutes, it was cooled to room temperature at a cooling rate of 20 ° C./hour to obtain a hot-rolled plate having a thickness of 3 mm. This is pickled to remove scale and cold-rolled to a thickness of 1.6.
mm cold-rolled plate.

【0034】この冷間圧延板に、連続焼鈍ライン相当の
熱処理(熱処理A)、連続溶融亜鉛めっきライン相当の
熱処理(熱処理B)、および、連続焼鈍ライン相当の熱
処理に引き続き連続溶融亜鉛めっきライン相当の熱処理
を施した場合(熱処理A+B)の3種類の熱処理を施
し、その後、それぞれの鋼板の機械特性を調査した。
The cold-rolled sheet is subjected to a heat treatment equivalent to a continuous annealing line (heat treatment A), a heat treatment equivalent to a continuous galvanizing line (heat treatment B), and a heat treatment equivalent to a continuous annealing line, followed by a continuous galvanizing line equivalent. (Heat treatment A + B), and then the mechanical properties of each steel plate were investigated.

【0035】図1は、本発明の実施例に係わる、連続焼
鈍ライン相当の熱処理と、連続溶融亜鉛めっきライン相
当の熱処理とを連続して施した場合のヒートパターンを
示すグラフである。
FIG. 1 is a graph showing heat patterns when a heat treatment corresponding to a continuous annealing line and a heat treatment corresponding to a continuous hot-dip galvanizing line according to an embodiment of the present invention are continuously performed.

【0036】図1で線図の左側半分は熱処理Aのヒート
パターン例である。その内容は、10℃/秒の加熱速度
で2相域である830℃に加熱し、この温度で60秒間
保持した後、700℃までを5℃/秒、引き続き400
℃までを50℃/秒の冷却速度で冷却し、400℃で2
40秒間保持した後、20℃/秒の冷却速度で室温まで
冷却するものである。
The left half of the diagram in FIG. 1 is an example of the heat pattern of the heat treatment A. The contents are as follows: heating at a heating rate of 10 ° C./sec to 830 ° C., which is a two-phase region, holding at this temperature for 60 seconds;
At a cooling rate of 50 ° C./sec.
After holding for 40 seconds, it is cooled to room temperature at a cooling rate of 20 ° C./second.

【0037】図1で線図の右側半分は熱処理Bのヒート
パターン例である。その内容は、2相域焼鈍までは熱処
理Aと同一条件で施し、その後、8℃/秒の冷却速度で
480℃まで冷却し、その温度で20秒間保持した後、
めっき浴温度を想定した460℃まで3℃/秒の冷却速
度で冷却するものである。
In FIG. 1, the right half of the diagram is an example of the heat pattern of the heat treatment B. The contents are performed under the same conditions as heat treatment A until the two-phase annealing, then cooled to 480 ° C. at a cooling rate of 8 ° C./sec, and held at that temperature for 20 seconds.
The cooling is performed at a cooling rate of 3 ° C./sec to 460 ° C. assuming a plating bath temperature.

【0038】めっき後に合金化処理をおこなう場合に
は、460℃から5℃/秒の加熱速度で520℃の合金
化温度まで加熱し、その後図示した冷却速度で室温まで
冷却する。合金化処理をおこなわない場合は、上記めっ
き温度の460℃から、破線で図示した冷却速度で室温
まで冷却する。
When an alloying treatment is performed after the plating, the alloy is heated from 460 ° C. to an alloying temperature of 520 ° C. at a heating rate of 5 ° C./sec, and then cooled to room temperature at the illustrated cooling rate. When the alloying process is not performed, cooling is performed from the above plating temperature of 460 ° C. to a room temperature at a cooling rate shown by a broken line.

【0039】次いで上記熱処理後の冷間圧延板に伸び率
1%の調質圧延を施し、圧延方法に平行にJIS−5号
試験片を採取して引張試験をおこなった。図2は、各鋼
板の引張強さ、降伏強さおよび伸び(全伸び)を示すグ
ラフである。図2からわかるように、熱処理Aのみを施
した場合(■印)では、引張強さは610MPa、伸び
は39.0%であり、強度−延性バランス(引張強さ×
伸び)は23000MPa・%以上で、極めて優れた延
性を有している。
Next, the cold-rolled sheet after the heat treatment was subjected to temper rolling at an elongation of 1%, and a JIS-5 test piece was sampled in parallel with the rolling method to conduct a tensile test. FIG. 2 is a graph showing tensile strength, yield strength and elongation (total elongation) of each steel sheet. As can be seen from FIG. 2, when only heat treatment A was applied (marked with ■), the tensile strength was 610 MPa, the elongation was 39.0%, and the strength-ductility balance (tensile strength ×
(Elongation) is 23,000 MPa ·% or more, and has extremely excellent ductility.

【0040】一方、熱処理Bのみを施した場合(○印)
は、熱処理Aのみの場合に比較すると著しく伸びが小さ
くなる。また、延性に対する低温保持温度や合金化温度
の影響はわずかしか認められなかった。
On the other hand, when only heat treatment B was applied (marked with ○)
The elongation is significantly smaller than that of the case of only heat treatment A. In addition, the influence of the low-temperature holding temperature and the alloying temperature on ductility was slightly recognized.

【0041】ところが、熱処理Aと熱処理Bを連続して
施した場合(●印、以下では「熱処理A+B」とも記
す)では、合金化処理をおこなわない場合には熱処理A
よりも強度が上昇し、伸びは低下しているが、強度−延
性バランスは熱処理Bよりも格段に向上している。合金
化処理材の特性は、合金化温度が530℃以下であれ
ば、合金化処理をおこなわない鋼板とほぼ同じ水準の良
好な伸びが得られた。
However, when heat treatment A and heat treatment B are successively performed (indicated by a black circle, hereinafter also referred to as “heat treatment A + B”), heat treatment A is performed when no alloying treatment is performed.
Although the strength increases and the elongation decreases, the strength-ductility balance is remarkably improved as compared with the heat treatment B. As for the characteristics of the alloying material, when the alloying temperature was 530 ° C. or lower, good elongation at substantially the same level as that of the steel sheet not subjected to the alloying treatment was obtained.

【0042】すなわち、以上の実験結果から、連続溶融
亜鉛めっきする前に、冷間圧延板の再結晶焼鈍に使用さ
れているのと類似の焼鈍後の急速冷却と長時間の低温保
持を含む連続焼鈍を施すことにより、安定して強度−延
性バランスに優れた高張力高延性亜鉛めっき鋼板が得ら
れることが明らかになった。
That is, from the above experimental results, before continuous hot dip galvanizing, continuous cooling including rapid cooling after annealing and long-time low-temperature holding similar to that used for recrystallization annealing of a cold-rolled sheet. It has been clarified that by performing the annealing, a high-tensile-high-ductility galvanized steel sheet excellent in strength-ductility balance can be obtained stably.

【0043】このメカニズムは必ずしも明らかでない
が、以下のように推察される。熱処理Bの場合、熱処理
Aの場合と同様、2相域への加熱途中に再結晶とセメン
タイト(パーライト中のセメンタイトも含む)の溶解が
同時に進行する。このため、2相域温度域におけるオー
ステナイト粒の分布はランダムとなる。しかしながら2
相域温度で保持した後の冷却では、熱処理Aの場合より
も冷却速度が遅いためにオーステナイトを過冷すること
ができず、冷却途中でパーライトが析出してオーステナ
イト量が減少する。
This mechanism is not necessarily clear, but is presumed as follows. In the case of the heat treatment B, as in the case of the heat treatment A, recrystallization and dissolution of cementite (including cementite in pearlite) proceed simultaneously during heating to the two-phase region. Therefore, the distribution of austenite grains in the two-phase temperature range is random. However 2
In the cooling after maintaining at the phase region temperature, the austenite cannot be supercooled because the cooling rate is slower than in the case of heat treatment A, and pearlite precipitates during the cooling to reduce the amount of austenite.

【0044】また、熱処理Bにおいては、低温保持温度
がベイナイト変態の最適温度よりも高いため、オーステ
ナイト中への炭素の濃縮が不十分となり、オーステナイ
トを安定化できない。したがって、最終的な残留オース
テナイト量も減少し、強度−延性バランスが低下したも
のと考えられる。
In the heat treatment B, since the low-temperature holding temperature is higher than the optimum temperature of the bainite transformation, the concentration of carbon in the austenite becomes insufficient and the austenite cannot be stabilized. Therefore, it is considered that the final amount of retained austenite also decreased, and the strength-ductility balance decreased.

【0045】一方、熱処理Aで焼鈍された鋼板は、一般
に知られているように、フェライト主体の組織に、残留
オーステナイト粒が孤立して分布するか、ベイナイトの
層間にフィルム状になって分布している。
On the other hand, in the steel sheet annealed by heat treatment A, as is generally known, the retained austenite grains are distributed in a ferrite-based structure in isolation or in a film form between bainite layers. ing.

【0046】熱処理A+Bのように、熱処理Aを施して
得られた結晶組織を有する鋼板を再度2相域の温度に加
熱すると、残留オーステナイトから炭素が周囲へ拡散し
ながらオーステナイトの体積率が増加する。この時、オ
ーステナイト粒内は一様な炭素分布ではない。すなわ
ち、当初から存在していた残留オーステナイトの部分は
炭素濃度が極めて高く、上記残留オーステナイトの周辺
部分は、同じオーステナイト粒内でありながら炭素濃度
が希薄であると推定される。
When the steel sheet having the crystal structure obtained by performing the heat treatment A as in the heat treatment A + B is heated again to the temperature in the two-phase region, the volume fraction of the austenite increases while carbon diffuses from the residual austenite to the surroundings. . At this time, the inside of the austenite grains does not have a uniform carbon distribution. That is, it is presumed that the portion of the retained austenite existing from the beginning has an extremely high carbon concentration, and the peripheral portion of the retained austenite has a low carbon concentration even within the same austenite grains.

【0047】このようなC濃度分布を備えたオーステナ
イト粒が冷却されると、その冷却速度が緩冷却であって
も、周辺部は炭素が希薄ためパーライト変態できずにフ
ェライトとなる。他方、その中心部は炭素濃度が高いた
めにオーステナイトのままベイナイト変態温度域まで過
冷され、パーライトの析出が抑制されるものと考えられ
る。
When the austenite grains having such a C concentration distribution are cooled, even if the cooling rate is slow cooling, the peripheral portion becomes carbon-diluted and cannot be transformed into pearlite to become ferrite. On the other hand, it is considered that the center portion is supercooled to the bainite transformation temperature range while maintaining austenite because the carbon concentration is high, thereby suppressing the precipitation of pearlite.

【0048】さらに、この過冷オーステナイトは既に炭
素濃度が高い。これらのことから、熱処理Bのように、
ベイナイト変態に最適とされる温度より高い温度で保持
しても、熱処理Aと同様な残留オーステナイトが得ら
れ、その結果、強度−延性バランスが優れているものと
考えられる。
Further, the supercooled austenite already has a high carbon concentration. From these, as in heat treatment B,
Even when the temperature is maintained at a temperature higher than the optimum temperature for bainite transformation, the same retained austenite as in heat treatment A is obtained, and as a result, it is considered that the strength-ductility balance is excellent.

【0049】本発明はこれらの知見を基にして完成され
たものであり、その要旨は下記(1)〜(5)に記載の
高張力高延性亜鉛めっき鋼板の製造方法にある。 (1)質量%で、C:0.05〜0.25%、Si:
0.7%以下、Mn:0.8〜2.5%、P:0.05
%以下、S:0.01%以下、sol.Al:0.4〜
2.0%、N:0.01%以下、かつ、Si+Al:
1.0〜2.5%を満足する化学組成を有する冷間圧延
板に、下記条件を満足する連続焼鈍を施し、次いで下記
条件を満足する溶融亜鉛めっきを施すことを特徴とする
高張力高延性亜鉛めっき鋼板の製造方法; 連続焼鈍条件:鋼を780℃以上、860℃以下の2相
域に加熱し、該2相域で30秒以上、90秒以下保持し
た後、700℃以下、450℃以上の温度範囲を30℃
/秒以上の冷却速度で冷却し、次いで450℃以下、3
50℃以上の温度範囲で100秒以上、500秒以下保
持した後、室温まで冷却する、 溶融亜鉛めっき条件:鋼を780℃以上、860℃以下
の2相域に加熱し、該温度域で10秒以上、90秒以下
保持した後、500℃以下まで3℃/秒以上の冷却速度
で冷却し、500℃以下、460℃以上の温度範囲で1
5秒以上保持した後、溶融亜鉛浴に浸漬してめっきし、
室温まで冷却する。
The present invention has been completed on the basis of these findings, and the gist of the invention resides in the following method (1) to (5) for producing a high-tensile, high-ductility galvanized steel sheet. (1) In mass%, C: 0.05 to 0.25%, Si:
0.7% or less, Mn: 0.8 to 2.5%, P: 0.05
%, S: 0.01% or less, sol. Al: 0.4-
2.0%, N: 0.01% or less, and Si + Al:
A cold-rolled sheet having a chemical composition satisfying 1.0 to 2.5% is subjected to continuous annealing satisfying the following conditions, and then hot-dip galvanized satisfying the following conditions. Manufacturing method of ductile galvanized steel sheet; Continuous annealing condition: Heat steel in two-phase region of 780 ° C or more and 860 ° C or less, hold in the two-phase region for 30 seconds or more and 90 seconds or less, and then 700 ° C or less and 450 ° C or less. 30 ℃ over the temperature range above ℃
/ Second cooling rate, then 450 ° C or less,
After holding at a temperature range of 50 ° C. or more for 100 seconds or more and 500 seconds or less, it is cooled to room temperature. Hot-dip galvanizing condition: Heat the steel to a two-phase region of 780 ° C. or more and 860 ° C. After holding for not less than 90 seconds and not more than 500 seconds, it is cooled to not more than 500 ° C. at a cooling rate of not less than 3 ° C./sec.
After holding for more than 5 seconds, immerse in a molten zinc bath and plate
Cool to room temperature.

【0050】(2)上記(1)に記載の化学組成を有す
る冷間圧延板に、上記(1)に記載の条件を満足する連
続焼鈍および溶融亜鉛めっきを施した後、480℃以
上、530℃以下の温度範囲に加熱し、該温度範囲で1
0秒以上、30秒以下保持してめっき層を合金化させて
合金化溶融亜鉛めっき鋼板を製造することを特徴とする
高張力高延性亜鉛めっき鋼板の製造方法。
(2) The cold-rolled sheet having the chemical composition described in (1) above is subjected to continuous annealing and hot-dip galvanization satisfying the conditions described in (1) above, and then subjected to 480 ° C. or more and 530 Heat to a temperature range of less than or equal to
A method for producing a galvanized steel sheet having a high tensile strength and a high ductility, wherein a galvannealed steel sheet is produced by alloying a plating layer while holding the plating layer for at least 0 second and at most 30 seconds.

【0051】(3)鋼の前記化学組成がさらにTiおよ
び/またはNbを質量%の合計で、0.003〜0.0
5%含有するものであることを特徴とする上記(1)ま
たは(2)に記載の高張力高延性亜鉛めっき鋼板の製造
方法。
(3) The chemical composition of the steel further includes Ti and / or Nb in a total of 0.003 to 0.03% by mass.
The method for producing a high-tension, high-ductility galvanized steel sheet according to the above (1) or (2), wherein the galvanized steel sheet contains 5%.

【0052】(4)鋼の前記化学組成がさらに質量%
で、Cr:0.05〜1.0%および/またはMo:
0.01〜0.2%を含有するものであることを特徴と
する上記(1)〜(3)のいずれかに記載の高張力高延
性亜鉛めっき鋼板の製造方法。
(4) The chemical composition of the steel is further
And Cr: 0.05-1.0% and / or Mo:
The method for producing a high-tension, high-ductility galvanized steel sheet according to any one of the above (1) to (3), characterized by containing 0.01 to 0.2%.

【0053】(5)鋼の前記化学組成がさらに質量%
で、Cu:0.1〜1.0%、Ni:0.05〜0.5
%、Co:0.0005〜1.0%からなる群の内の1
種または2種以上を、質量%の合計で1.5%以下含有
するものであることを特徴とする上記(1)〜(4)の
いずれかに記載の高張力高延性亜鉛めっき鋼板の製造方
法。
(5) The chemical composition of the steel is further
And Cu: 0.1-1.0%, Ni: 0.05-0.5
%, Co: 1 in the group consisting of 0.0005 to 1.0%
The production of a high-tensile high-ductility galvanized steel sheet according to any one of the above (1) to (4), characterized in that the steel sheet contains at least 1.5% by mass of two or more kinds. Method.

【0054】[0054]

【発明の実施の形態】以下に本発明の実施の形態を詳細
に述べる。なお、以下の説明において化学組成の%表示
は質量%を意味する。
DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail. In the following description, the percentage of chemical composition means% by mass.

【0055】(1)鋼板の化学組成 C:Cは最も強力なオーステナイト安定化元素であり、
本発明の必須構成要素の一つである。室温においてオー
ステナイトを安定化するためには、焼鈍工程においてオ
ーステナイト中に1%程度以上に濃縮する必要がある。
そのために鋼のC含有量を0.05%以上とする。鋼の
C含有量を増すにつれて、鋼の強度を高めることができ
る。しかしながら、Cを0.25%を超えて含有させる
と鋼の強度が高くなりすぎて塑性加工に適さなくなるう
え、溶接性も劣化する。従ってC含有量は0.25%以
下とする。好ましくは0.20%以下である。
(1) Chemical composition of steel sheet C: C is the most powerful austenite stabilizing element,
This is one of the essential components of the present invention. In order to stabilize austenite at room temperature, it is necessary to concentrate to about 1% or more in austenite in the annealing step.
Therefore, the C content of steel is set to 0.05% or more. As the C content of the steel increases, the strength of the steel can be increased. However, if C is contained in excess of 0.25%, the strength of the steel becomes too high, which makes the steel unsuitable for plastic working and also deteriorates the weldability. Therefore, the C content is set to 0.25% or less. Preferably it is 0.20% or less.

【0056】SiおよびAl:SiとAlは、いずれも
フェライト安定化元素であり、これらの元素を含有させ
ると、2相域での焼鈍において、フェライトの体積率が
増加し、フェライトと平衡するオーステナイト相のC濃
度が高くなり、オーステナイトが安定化する効果が得ら
れる。さらに、Siは炭化物の析出を抑制する作用があ
るので、2相域での焼鈍後の冷却におけるベイナイト変
態時にもオーステナイト中にCを濃縮させる効果があ
る。これらの効果を得るためには、SiとAlの合計量
で1.0%以上含有させる。
Si and Al: Si and Al are both ferrite stabilizing elements. When these elements are contained, the volume fraction of ferrite increases during annealing in the two-phase region, and austenite which equilibrates with ferrite is obtained. The C concentration of the phase is increased, and the effect of stabilizing austenite is obtained. Furthermore, since Si has an effect of suppressing the precipitation of carbides, it has an effect of enriching C in austenite even during bainite transformation during cooling after annealing in the two-phase region. In order to obtain these effects, the total content of Si and Al is set to 1.0% or more.

【0057】これらのフェライト安定化元素を多量に添
加すると、オーステナイトにCが濃縮されるが、体積率
が減少してしまうので、SiとAlの含有量は合計で
2.5%以下とする。
When a large amount of these ferrite stabilizing elements is added, C is concentrated in austenite, but the volume ratio is reduced. Therefore, the total content of Si and Al is set to 2.5% or less.

【0058】他方、Siには溶融亜鉛めっき時の母材と
溶融めっき浴との間の濡れ性を阻害する作用があり、S
iを過度に含有させると不めっきが生じるおそれがあ
る。また、Siは亜鉛めっき層の合金化を遅延させる作
用もある。従ってSiは含有させなくても構わないが、
含有させる場合でも、上記の悪影響を避けるために、S
i含有量は0.7%以下とする。好ましくは0.6%以
下である。
On the other hand, Si has an effect of inhibiting wettability between the base material and the hot-dip galvanizing bath during hot-dip galvanizing.
If i is contained excessively, non-plating may occur. Si also has the effect of delaying alloying of the galvanized layer. Therefore, Si does not have to be contained,
Even if it is contained, in order to avoid the above-mentioned adverse effects, S
The i content is 0.7% or less. Preferably it is 0.6% or less.

【0059】Alは、上述のフェライト安定化作用があ
るうえ、めっき濡れ性を阻害しない元素であるのでSi
よりも好適な元素である。フェライト安定化のためにA
lは0.4%以上含有させる。また、Alは製鋼時に脱
酸材として使われるが、2.0%を超えて含有させると
鋼板中に介在物が多くなり延性を損なうので、Alの含
有量は2.0%以下とする。
Al has an effect of stabilizing ferrite as described above and does not inhibit plating wettability.
It is a more preferable element. A for stabilizing ferrite
1 is contained at least 0.4%. Further, Al is used as a deoxidizer during steelmaking, but if it is contained in excess of 2.0%, inclusions increase in the steel sheet and impair ductility, so the Al content is 2.0% or less.

【0060】Mn:Mnはオーステナイト安定化元素で
あり、本発明の必須元素の一つである。2相域での焼鈍
後の冷却過程において、オーステナイトをマルテンサイ
トに変態させることなく室温まで残留させるために、M
nを0.8%以上含有させる。他方Mnは鋼が凝固する
際に偏析する傾向があり、過剰にMnを含有させると偏
析に起因する縞状組織が生じ、延性が損なわれる。これ
を避けるためにMn含有量は2.5%以下とする。好ま
しくは、2.0%以下である。
Mn: Mn is an austenite stabilizing element and is one of the essential elements of the present invention. In order to allow austenite to remain at room temperature without transforming to martensite in the cooling process after annealing in the two-phase region, M
n is contained 0.8% or more. On the other hand, Mn tends to segregate when the steel is solidified. If Mn is excessively contained, a striped structure due to segregation occurs, and ductility is impaired. To avoid this, the Mn content is set to 2.5% or less. Preferably, it is 2.0% or less.

【0061】P:Pは鋼の凝固時に偏析する傾向があ
り、過剰に含有させると偏析に起因する縞状組織が生
じ、延性を損なう。また、溶接性も損なう。これらの害
を避けるためにP含有量は0.05%以下とする。好ま
しくは0.02%以下である。ただし、PはCuと共存
させると鋼板表層に安定な皮膜を形成し、耐食性を向上
させる場合がある。従ってCuを含有する鋼において
は、Pを0.01%以上、0.05%以下含有させるこ
とが好ましい。
P: P tends to segregate during solidification of steel. If P is contained excessively, a striped structure is generated due to segregation, and ductility is impaired. Also, weldability is impaired. In order to avoid these harms, the P content is set to 0.05% or less. Preferably it is 0.02% or less. However, when P coexists with Cu, a stable film may be formed on the surface layer of the steel sheet to improve the corrosion resistance in some cases. Therefore, in steel containing Cu, it is preferable to contain P in an amount of 0.01% or more and 0.05% or less.

【0062】S:不純物元素として鋼中に不可避的に含
有される。S含有量が多いとMnSとしての析出量が増
し、延性が阻害されるうえ、オーステナイト安定化元素
としてのMnを消費する。従ってS含有量は0.01%
以下とする。
S: Inevitably contained in steel as an impurity element. If the S content is large, the amount of precipitation as MnS increases, ductility is inhibited, and Mn as an austenite stabilizing element is consumed. Therefore, the S content is 0.01%
The following is assumed.

【0063】N:不純物元素として鋼中に不可避的に含
有され、低い方が好ましい。N含有量が多いとAlNに
よるスラブ割れの原因になるほか、製品でもAlNは延
性を低下させるので、上限は0.01%とする。より加
工性を重視する場合は、0.005%以下とすることが
望ましい。
N: Inevitably contained in steel as an impurity element, the lower the better. If the N content is high, it causes slab cracking due to AlN, and AlN also reduces ductility in products, so the upper limit is made 0.01%. When more emphasis is placed on workability, the content is desirably 0.005% or less.

【0064】TiおよびNb:これらの元素はいずれも
炭化物生成元素であり、鋼中において微細な析出物を形
成して鋼板を強化すると共に、結晶組織を微細にしてめ
っき層の合金化を促進する効果がある。これらの効果を
得るために、両者の合計で0.003%以上含有させて
も構わない。これらの元素の含有量が合計で0.05%
を超えると強度の上昇よりも延性の低下が顕著になるの
で、含有させる場合の合計量は0.05%と以下する。
Ti and Nb: These elements are carbide forming elements, and form fine precipitates in the steel to strengthen the steel sheet and promote the alloying of the plating layer by making the crystal structure fine. effective. In order to obtain these effects, the total content of both may be 0.003% or more. The content of these elements is 0.05% in total
If it exceeds 300, the decrease in ductility is more remarkable than the increase in strength, so that the total amount when contained is 0.05% or less.

【0065】また、Tiは鋼中のNと結合し易く、Al
Nの析出に優先してTiNが析出することにより、Al
Nの析出に起因するスラブ割れを防止する作用がある。
この効果を確実に得るには、(Ti/48)/(N/1
4)≧2となるように、Tiを含有させるのが好まし
い。
Further, Ti is easily bonded to N in steel, and Al
The precipitation of TiN in preference to the precipitation of N leads to Al
This has the effect of preventing slab cracking due to the precipitation of N.
To ensure this effect, (Ti / 48) / (N / 1
4) It is preferable to include Ti so that ≧ 2.

【0066】CrおよびMo:これらの元素はパーライ
ト変態を抑制する作用があり、2相域での焼鈍後の冷却
において、パーライト変態を抑制してオーステナイトの
減少を抑える効果がある。逆にこれらの元素を過剰に含
有させると、ベイナイト変態を遅延させてしまい、オー
ステナイトへの炭素濃縮が不十分となる。従って適正な
残留オーステナイトを得るために、Crは0.05%以
上、1.0%以下、Moは0.01%以上、0.2%以
下の範囲でこれらのいずれか、または双方を含有させて
も構わない。
Cr and Mo: These elements have the effect of suppressing the pearlite transformation, and have the effect of suppressing the pearlite transformation and the reduction of austenite during cooling after annealing in the two-phase region. Conversely, when these elements are excessively contained, the bainite transformation is delayed, and the carbon concentration in austenite becomes insufficient. Therefore, in order to obtain an appropriate retained austenite, Cr is contained in a range of 0.05% or more and 1.0% or less, and Mo is contained in a range of 0.01% or more and 0.2% or less. It does not matter.

【0067】Cu、NiおよびCo:これらの元素はい
ずれも鉄炭化物中に溶け難い元素であり、ベイナイト変
態中に炭化物の析出を抑制し、オーステナイトを残留さ
せる作用がある。この効果を得るために、Cuは0.1
%以上、Niは0.05%以上、Coは0.0005%
以上の範囲で、1種または2種以上を含有させても構わ
ない。これらの元素は過剰に含有させると、ベイナイト
変態が不十分になり、連続焼鈍設備における過時効帯、
または連続溶融めっきにおける低温保持においてオース
テナイト中に炭素が十分濃縮されないので、含有させる
場合でも、Cuは1.0%以下、Niは0.5%以下、
Coは1.0%以下、合計で1.5%以下とする。
Cu, Ni and Co: All of these elements are hardly soluble in iron carbide, and have the effect of suppressing the precipitation of carbides during bainite transformation and leaving austenite. To obtain this effect, Cu should be 0.1
%, Ni is 0.05% or more, Co is 0.0005%
One or more kinds may be contained within the above range. If these elements are contained excessively, bainite transformation becomes insufficient, and the overaging zone in the continuous annealing equipment,
Or, since carbon is not sufficiently concentrated in austenite during low-temperature holding in continuous hot-dip plating, even if it is contained, Cu is 1.0% or less, Ni is 0.5% or less,
Co is 1.0% or less, and 1.5% or less in total.

【0068】また、CuはPと共存すると耐食性を向上
するのでこの目的のために含有させても構わない。な
お、Cuはスラブ割れの要因となるので、Cu添加時に
はNiをNi≧Cu/2を満足して複合添加することが
好ましい。
Further, Cu improves the corrosion resistance when coexisting with P, and may be contained for this purpose. Since Cu causes slab cracking, it is preferable to add Ni at the time of adding Cu so as to satisfy Ni ≧ Cu / 2.

【0069】残部はFeおよび不可避的不純物である。 (2)圧延、焼鈍およびめっき条件 上記化学組成を有する冷間圧延板は、所定の化学組成を
有する鋼を常法に従い転炉などで溶製し、連続鋳造して
スラブとし、熱間圧延したのち、酸洗などの処理を経て
冷間圧延するなどの方法で得られる。
The balance is Fe and inevitable impurities. (2) Rolling, Annealing and Plating Conditions The cold-rolled sheet having the above chemical composition is prepared by melting steel having a predetermined chemical composition in a converter or the like according to a conventional method, continuously casting to form a slab, and hot rolling. After that, it is obtained by a method such as cold rolling through a treatment such as pickling.

【0070】熱間圧延は公知の方法でおこなえばよい。
鋳込まれたスラブを直接熱間圧延してもよいし、再加熱
してから熱間圧延してもよい。圧延温度が低すぎると、
圧延中にフェライト変態して加工フェライト組織が現れ
るため、仕上圧延出側温度(仕上温度)を800℃以上
とするのが好ましい。鋼板の全幅、全長にわたって仕上
温度を確保するため、粗圧延と仕上圧延の間で補助加熱
をおこなってもよい。仕上圧延後の冷却は特に限定しな
いが、熱間圧延板の組織を微細化するために、仕上圧延
直後から水冷することが好ましい。
The hot rolling may be performed by a known method.
The cast slab may be directly hot-rolled, or may be re-heated and then hot-rolled. If the rolling temperature is too low,
Since the ferrite is transformed during rolling to form a processed ferrite structure, the finish-rolling exit temperature (finish temperature) is preferably set to 800 ° C. or higher. In order to secure the finishing temperature over the entire width and the entire length of the steel sheet, auxiliary heating may be performed between rough rolling and finish rolling. Cooling after finish rolling is not particularly limited, but it is preferable to perform water cooling immediately after finish rolling in order to make the structure of the hot-rolled sheet finer.

【0071】巻取温度を低くしすぎると、第2相として
ベイナイトおよびマルテンサイトが生成し、冷間圧延が
困難になるので、500℃以上で巻取ることが好まし
い。巻取温度が高すぎると、スケールロスが増加すると
ともに、鋼板が軟質になりすぎて巻姿が崩れやすくなる
ので、680℃以下とするのが好ましい。
If the winding temperature is too low, bainite and martensite are formed as the second phase, making cold rolling difficult. Therefore, it is preferable to wind at 500 ° C. or higher. If the winding temperature is too high, the scale loss increases, and the steel sheet becomes too soft, so that the rolled form is likely to collapse.

【0072】冷間圧延:冷間圧延は常法によっておこな
えばよいが、圧下率を低くしすぎると圧延効率が低下
し、高くしすぎると圧延荷重が増大し圧延が困難になる
ので、40%以上、70%以下の範囲とするのが好まし
い。
Cold rolling: Cold rolling may be carried out by a conventional method. However, if the rolling reduction is too low, the rolling efficiency decreases, and if the rolling reduction is too high, the rolling load increases and rolling becomes difficult. As described above, the content is preferably in the range of 70% or less.

【0073】上記冷間圧延板には連続焼鈍(1回目の焼
鈍)を施した後に連続溶融亜鉛めっきを施す。連続溶融
亜鉛めっきでは、めっき前に焼鈍(2回目の焼鈍)がお
こなわれる。
The cold-rolled sheet is subjected to continuous annealing (first annealing) and then to continuous galvanizing. In continuous galvanizing, annealing (second annealing) is performed before plating.

【0074】連続焼鈍:連続焼鈍(1回目の焼鈍)は、
冷間圧延板の再結晶焼鈍に使用される公知の連続焼鈍設
備、すなわち、入り側から順に加熱帯、均熱帯、冷却
帯、過時効帯を備えた設備が望ましい。冷間圧延板は、
再結晶をおこなわせると共に、Cをオーステナイトに濃
縮させるため、加熱帯において、780℃以上、860
℃以下の2相域温度に加熱して焼鈍する。
Continuous annealing: Continuous annealing (first annealing)
Known continuous annealing equipment used for recrystallization annealing of a cold-rolled sheet, that is, equipment provided with a heating zone, a soaking zone, a cooling zone, and an overaging zone in order from the entry side is desirable. Cold rolled sheet is
In the heating zone, 780 ° C. or higher, 860
The steel is annealed by heating to a temperature in a two-phase region of not more than ℃.

【0075】焼鈍温度が780℃に満たない場合には再
結晶が不十分になるうえ、セメンタイトが再固溶するの
に時間がかかりすぎる。逆に、焼鈍温度が860℃を超
えると、焼鈍温度での鋼のオーステナイト体積率が過大
になり、オーステナイト中のC濃度が低下する。好まし
くは800℃以上、850℃以下である。
When the annealing temperature is lower than 780 ° C., recrystallization becomes insufficient and it takes too much time for cementite to form a solid solution again. Conversely, if the annealing temperature exceeds 860 ° C., the austenite volume fraction of the steel at the annealing temperature becomes excessive, and the C concentration in austenite decreases. Preferably it is 800 to 850 ° C.

【0076】上記2相域での保持時間(均熱時間)が3
0秒に満たない場合にはセメンタイトの再溶解が不十分
となり、90秒を超える場合にはオーステナイト粒が粗
大化するので好ましくない。従って、保持時間は30秒
以上、90秒以下とする。
The holding time (soaking time) in the two-phase region is 3
When the time is less than 0 second, the re-dissolution of cementite becomes insufficient, and when the time is more than 90 seconds, austenite grains become undesirably coarse. Therefore, the holding time is 30 seconds or more and 90 seconds or less.

【0077】2相域焼鈍を施した後は、パーライト変態
を抑制するために、700℃以下、450℃以上の温度
範囲を30℃/秒以上の冷却速度で冷却する。冷却速度
の上限は特に限定するものではないが、過度に早くしす
ぎると、パーライト変態抑制効果が飽和するうえ、冷却
終了温度の制御が困難となる場合があるので、100℃
/秒以下とするのが好ましい。
After the two-phase annealing, the temperature range of 700 ° C. or lower and 450 ° C. or higher is cooled at a cooling rate of 30 ° C./sec or higher in order to suppress the pearlite transformation. Although the upper limit of the cooling rate is not particularly limited, if it is too fast, the effect of suppressing the pearlite transformation is saturated, and it may be difficult to control the cooling end temperature.
/ Sec or less.

【0078】上記冷却の開始点は限定しないが、フェラ
イトの体積率を増やして、オーステナイト中にCを濃縮
するために、焼鈍温度から700℃までは10℃/秒以
下の冷却速度で冷却することが好ましい。
The starting point of the above-mentioned cooling is not limited, but in order to increase the volume fraction of ferrite and to enrich C in austenite, the cooling is performed at a cooling rate of 10 ° C./sec or less from the annealing temperature to 700 ° C. Is preferred.

【0079】上記冷却に引き続き、450℃以下、35
0℃以上の温度範囲で100秒以上、500秒以下保持
する。この保持とは、一定温度に保持しても良いし、上
記温度範囲内で徐々に温度を低下させてもよい。この温
度範囲あるいは保持時間範囲の上限、下限いずれを外れ
ても、ベイナイト変態が不十分にとなり、オーステナイ
トへのCの濃縮があまり起こらなくなる。好ましくは4
50℃以下、370℃以上の温度範囲で200秒以上、
400秒以下保持する。その後の冷却については限定し
ないが、強制冷却をおこなっても良い。
Following the above cooling, the temperature was lowered to 450 ° C.
Hold at a temperature range of 0 ° C. or more for 100 seconds or more and 500 seconds or less. The holding may be performed by maintaining the temperature at a constant value or by gradually lowering the temperature within the above temperature range. Beyond either the upper limit or the lower limit of the temperature range or the holding time range, bainite transformation becomes insufficient and the concentration of C in austenite hardly occurs. Preferably 4
200 seconds or more in a temperature range of 50 ° C. or less and 370 ° C. or more,
Hold for 400 seconds or less. The subsequent cooling is not limited, but forced cooling may be performed.

【0080】溶融亜鉛めっき:溶融亜鉛めっき(2回目
の焼鈍を含む)は、公知の連続溶融亜鉛めっき設備、す
なわち、入り側から順に加熱帯、均熱帯、冷却帯、低温
保持帯、溶融亜鉛浴、合金化処理帯を備えた設備を用い
るのが望ましい。合金化処理をおこなわない場合には、
合金化処理帯は必要ない。
Hot-dip galvanizing: Hot-dip galvanizing (including the second annealing) is performed by known continuous hot-dip galvanizing equipment, that is, a heating zone, a soaking zone, a cooling zone, a low-temperature holding zone, and a hot-dip zinc bath in order from the entrance side. It is desirable to use equipment provided with an alloying zone. If alloying is not performed,
No alloying zone is required.

【0081】連続焼鈍された鋼板は、加熱帯で780℃
以上、860℃以下の2相域に加熱して焼鈍する(2回
目の焼鈍)。1回目の焼鈍で形成された残留オーステナ
イトは、上記焼鈍の加熱途中にフェライトとセメンタイ
トに分解するが、Ac1点以上で再びオーステナイトに
変態する。焼鈍温度が780℃に満たない場合には、残
留オーステナイトが分解したままであり、オーステナイ
トの形成量が少ない。逆に焼鈍温度が860℃を超える
場合には、鋼板の結晶組織がオーステナイト単相に近く
なるため、オーステナイトへのC濃縮が進行しない。
The steel sheet annealed continuously was heated at 780 ° C.
As described above, annealing is performed by heating to a two-phase region of 860 ° C. or lower (second annealing). The residual austenite formed in the first annealing is decomposed into ferrite and cementite during the heating in the above-mentioned annealing, but is transformed into austenite again at an Ac point or higher. If the annealing temperature is lower than 780 ° C., the retained austenite remains decomposed and the amount of austenite formed is small. Conversely, when the annealing temperature exceeds 860 ° C., the crystal structure of the steel sheet becomes close to the austenite single phase, and thus C enrichment in austenite does not proceed.

【0082】2相域での滞留時間(均熱時間)が10秒
に満たない場合には、セメンタイトの溶解が不十分とな
るので、上記温度域での保持時間は10秒以上とする。
保持時間が過度に長くなるとオーステナイト粒内の炭素
濃度分布(前述した、中心が高濃度で周辺低濃度)が拡
散によって消失し、炭素濃度が高い領域が無くなってし
まうため、1回目の焼鈍を行わない時と同じになってし
まうのでよくない。これを避けるために、2相域での保
持時間は90秒以下とする。
If the residence time (soaking time) in the two-phase region is less than 10 seconds, the dissolution of cementite becomes insufficient, so that the retention time in the above-mentioned temperature region is 10 seconds or more.
If the holding time is excessively long, the carbon concentration distribution in the austenite grains (the above-described high concentration in the center and low concentration in the periphery) disappears by diffusion, and a region having a high carbon concentration disappears, so the first annealing is performed. It is not good because it will be the same as when there is no. In order to avoid this, the holding time in the two-phase region is set to 90 seconds or less.

【0083】オーステナイトを過冷状態にするためには
焼鈍後の冷却速度を速くするのがよいが、一般的な溶融
亜鉛めっきラインは強力な冷却設備を備えていないため
に、冷間圧延板の連続焼鈍設備のような急速冷却は困難
である。
It is preferable to increase the cooling rate after annealing in order to make the austenite in a supercooled state. However, since a general hot-dip galvanizing line is not equipped with a powerful cooling facility, the hot-dip galvanizing line is not used. Rapid cooling such as continuous annealing equipment is difficult.

【0084】しかしながら本発明の製造方法において
は、前述したように、1回目の焼鈍によって炭素が濃化
した残留オーステナイト相を形成させ、2回目の焼鈍に
おいてはその炭素濃化領域を壊すことなく均熱するの
で、1回目の焼鈍の均熱の時よりも、Cが高濃度に濃化
したオーステナイトが形成される。従って、2回目の焼
鈍に際しては、焼鈍温度からパーライト変態のノーズを
下回る500℃までの間の冷却速度を3℃/秒以上とす
ることができれば、パーライトの析出を抑制し、オース
テナイト量を維持することができる。
However, in the manufacturing method of the present invention, as described above, the residual austenite phase in which carbon is concentrated is formed by the first annealing, and the residual austenite phase is uniformly formed in the second annealing without breaking the carbon-enriched region. Because of heating, austenite in which C is concentrated at a higher concentration is formed than at the time of soaking in the first annealing. Therefore, in the second annealing, if the cooling rate from the annealing temperature to 500 ° C., which is lower than the nose of the pearlite transformation, can be set to 3 ° C./sec or more, the precipitation of pearlite is suppressed and the austenite amount is maintained. be able to.

【0085】上記冷却に引き続き、460℃〜500℃
の温度領域で15秒以上保持する低温保持を施す。この
低温保持を施すことによりベイナイト変態を進行させ、
オーステナイト中にさらにCを濃縮して安定化させる。
この低温保持の温度は、通常の連続焼鈍で用いられてい
る過時効温度よりも高温である。しかしながら、上述の
ように焼鈍を2回施すことによりオーステナイト中の炭
素濃度が高くなっているので、上記のような低温保持に
おいてもベイナイト変態が十分に進行するものと考えら
れる。
Following the above cooling, 460 ° C. to 500 ° C.
Low-temperature holding is performed for 15 seconds or more in the above temperature range. By performing this low-temperature holding, the bainite transformation proceeds,
C is further concentrated and stabilized in austenite.
The temperature for maintaining the low temperature is higher than the overaging temperature used in ordinary continuous annealing. However, since the carbon concentration in the austenite is increased by performing the annealing twice as described above, it is considered that the bainite transformation sufficiently proceeds even in the low-temperature holding as described above.

【0086】低温保持温度が500℃を超える場合に
は、保持中にパーライト変態が進行し、所望の残留オー
ステナイトが得られない。低温保持温度を460℃に満
たない低温度域とすると、めっき浴より温度が低いた
め、鋼板がめっき浴に侵入すると同時にめっき浴の凝固
が開始し、めっき付着量制御が困難になる。めっき浴直
前で再加熱などをおこなう場合には、低温保持を460
℃に満たない温度領域でおこなうことも可能であるが、
その場合でも、350℃以上とするのがよい。
If the low-temperature holding temperature exceeds 500 ° C., the pearlite transformation proceeds during the holding, and the desired retained austenite cannot be obtained. If the low-temperature holding temperature is lower than 460 ° C., since the temperature is lower than that of the plating bath, solidification of the plating bath starts at the same time as the steel sheet enters the plating bath, and it becomes difficult to control the amount of plating applied. If reheating is performed immediately before the plating bath, keep the low temperature at 460.
Although it is possible to perform in the temperature range below ℃,
Even in that case, the temperature is preferably 350 ° C. or more.

【0087】低温保持時間が15秒に満たない場合に
は、オーステナイトへのC濃縮が不十分で残留オーステ
ナイトの安定化が不十分となり、合金化処理時にオース
テナイトが容易に分解してしまう。低温保持時間を長く
すると、より高濃度に炭素が濃縮されて合金化処理中の
分解を抑える利点がある。従って低温保持時間は20秒
以上とするのが好ましい。
If the low-temperature holding time is less than 15 seconds, the concentration of C in austenite is insufficient and the stabilization of the retained austenite becomes insufficient, and the austenite is easily decomposed during the alloying treatment. Increasing the low-temperature holding time has the advantage that carbon is concentrated at a higher concentration and decomposition during the alloying treatment is suppressed. Therefore, the low-temperature holding time is preferably set to 20 seconds or more.

【0088】しかしながら低温保持時間を過度に長くす
ると残留オーステナイトの安定化作用が飽和し、それ以
上の保持は生産性を損なうのみであるので、120秒以
下とするのがよい。
However, if the low-temperature holding time is excessively long, the stabilizing action of the retained austenite is saturated, and if the holding time is longer than this, the productivity is only impaired.

【0089】溶融亜鉛めっきは常法によりおこなえばよ
い。めっき浴から鋼板を引き上げた後、常法によりめっ
き付着量を調整した後、合金化処理を施さない場合はそ
のまま室温まで冷却する。
The hot-dip galvanizing may be performed by a conventional method. After pulling up the steel sheet from the plating bath, the amount of coating is adjusted by a conventional method, and if no alloying treatment is performed, the steel sheet is cooled to room temperature.

【0090】合金化処理を施す場合には、めっき付着量
を調整した後、合金化炉にて480℃以上、530℃以
下の合金化温度に加熱し、その温度領域で10秒以上、
30秒以下保持する。合金化温度が480℃に満たない
場合、あるいは保持時間が10秒に満たない場合には合
金化が十分におこなえない。合金化温度が530℃を超
える場合には、残留オーステナイトがセメンタイトに分
解してしまい、機械特性が劣化する。保持時間が30秒
を超える場合には、合金化が過度に進行し、加工時にめ
っき皮膜が剥離するパウダリング不良が生じるのでよく
ない。
In the case of performing the alloying treatment, after adjusting the coating weight, the alloy is heated to an alloying temperature of 480 ° C. or more and 530 ° C. or less in an alloying furnace, and is heated for 10 seconds or more in the temperature range.
Hold for 30 seconds or less. When the alloying temperature is less than 480 ° C. or the holding time is less than 10 seconds, alloying cannot be performed sufficiently. When the alloying temperature exceeds 530 ° C., the retained austenite is decomposed into cementite, and the mechanical properties are deteriorated. If the holding time exceeds 30 seconds, alloying proceeds excessively, and a powdering defect in which a plating film peels off during processing is not good.

【0091】上記条件以外は常法により製造すればよ
い。例えば、連続焼鈍後や溶融亜鉛めっき後に、鋼板の
平坦矯正、表面粗度調整、降伏点伸びの低減のための調
質圧延などをおこなっても構わない。
Except for the above conditions, it may be manufactured by a conventional method. For example, after continuous annealing or hot-dip galvanizing, flatness correction of the steel sheet, surface roughness adjustment, and temper rolling for reducing the yield point elongation may be performed.

【0092】[0092]

【実施例】表2に示す化学組成を有する鋼を実験室で溶
解し、熱間鍛造をおこない、厚さが20mmの鋼片とし
た。
EXAMPLE Steel having the chemical composition shown in Table 2 was melted in a laboratory and hot forged to obtain a steel slab having a thickness of 20 mm.

【0093】[0093]

【表2】 上記鋼片を1200℃に加熱して30分間保持し、10
00℃から熱間圧延を開始して850℃で完了し、次い
で水スプレーにより550℃まで冷却し、徐冷炉に装入
して550℃で30分保持し、その後20℃/時の冷却
速度で室温まで徐冷して厚さが3mmの熱間圧延板を得
た。これらを酸洗してスケールを除去し、冷間圧延を施
して厚さが1.6mmの冷間圧延板を得た。
[Table 2] The slab was heated to 1200 ° C. and held for 30 minutes.
Hot rolling is started at 00 ° C. and completed at 850 ° C., then cooled to 550 ° C. by water spray, placed in a lehr, kept at 550 ° C. for 30 minutes, and then cooled at a cooling rate of 20 ° C./hour to room temperature. This was gradually cooled to obtain a hot-rolled plate having a thickness of 3 mm. These were pickled to remove scale and cold-rolled to obtain a cold-rolled plate having a thickness of 1.6 mm.

【0094】この冷間圧延板から得たサンプルを用い
て、図1に示すヒートパターンでの連続焼鈍ライン相当
の熱処理Aを1回目の焼鈍として施した後、引き続き連
続溶融亜鉛めっきライン相当の熱処理Bを2回目の焼鈍
を兼ねて施した。一部の鋼については、従来例として熱
処理Bのみを施した場合も試験、評価した。その際、焼
鈍条件(熱処理Aの焼鈍均熱条件:RA1、熱処理Bの焼
鈍均熱条件:RA2、冷却速度:CR2、低温保持条件TLH
は、表3に示すように種々変更した。
Using the sample obtained from the cold-rolled sheet, heat treatment A corresponding to the continuous annealing line in the heat pattern shown in FIG. 1 was performed as the first annealing, and then heat treatment corresponding to the continuous galvanizing line was performed. B was also used for the second annealing. Some steels were also tested and evaluated when only heat treatment B was applied as a conventional example. At that time, annealing conditions (annealing and heating conditions of heat treatment A: R A1 , annealing and soaking conditions of heat treatment B: R A2 , cooling rate: C R2 , low temperature holding conditions T LH
Was variously changed as shown in Table 3.

【0095】[0095]

【表3】 上記熱処理後の鋼板には伸び率1%の調質圧延を施し、
圧延方法に平行にJIS−5号試験片を採取し、引張試
験をおこなった。得られた試験結果を表4に示す。
[Table 3] The steel sheet after the above heat treatment is subjected to temper rolling at an elongation of 1%,
A JIS-5 test piece was sampled in parallel with the rolling method, and a tensile test was performed. Table 4 shows the obtained test results.

【0096】[0096]

【表4】 本発明が規定する化学組成範囲の成分を有する鋼Bを用
い、本発明が規定する範囲の条件で製造した試験番号1
〜3は、優れた強度−延性バランスを示していた。合金
化処理を施さなかった試験番号3は、試験番号1と同様
の優れた強度−延性バランスを示していた。試験番号4
は、熱処理Aを行わなかったため、試験番号1と同じ熱
処理Bを施したにもかかわらず、伸びが小さく強度−延
性バランスがよくなかった。
[Table 4] Test No. 1 manufactured using steel B having a component in the chemical composition range specified by the present invention under the conditions specified in the present invention
-3 showed an excellent strength-ductility balance. Test No. 3 which was not subjected to the alloying treatment showed the same excellent strength-ductility balance as Test No. 1. Test number 4
Since heat treatment A was not performed, heat treatment B same as Test No. 1 was performed, but the elongation was small and the strength-ductility balance was not good.

【0097】熱処理Aの焼鈍温度が低すぎた試験番号5
は、熱処理Aの均熱中にオーステナイトが形成されず、
熱処理Aの後に残留オーステナイトが存在しないため、
本発明が規定する条件に従って熱処理Bを施しても、オ
ーステナイトに炭素を濃縮することができず、残留オー
ステナイトが形成されなかったため、伸びが低かった。
試験番号6は、熱処理Aでオーステナイトは形成されて
いるが、均熱時間が短かすぎたためセメンタイトの溶解
が不完全で、Cのオーステナイトへの濃化が不十分であ
り、伸びが低かった。試験番号7は、熱処理Aで残留オ
ーステナイトが形成されているが、熱処理Bでの焼鈍温
度が低すぎたために、残留オーステナイトがセメンタイ
トに分解したままで、残留オーステナイトが得られず、
伸びが低かった。試験番号8は、熱処理Bでの焼鈍温度
が高すぎたため、熱処理Aで形成された炭素の濃化領域
が拡散によって消失し、その後の冷却で残留オーステナ
イトを残すことができなかったために、伸びが低かっ
た。試験番号9は、熱処理Bにおいて均熱後の冷却速度
が低すぎたため、パーライトが析出してオーステナイト
量が減少し、伸びが低かった。試験番号10は、熱処理
Bでの冷却保持温度が高過ぎたためにパーライトが析出
し、伸びが低かった。試験番号11は、合金化処理条件
の内の合金化温度が高すぎたので、残留オーステナイト
が分解してしまったため、伸びが低かった。試験番号1
2は、合金化処理条件の保持時間が長すぎたためにオー
ステナイトが分解し、その結果として良好な伸びが得ら
れなかった。
Test No. 5 where the annealing temperature of heat treatment A was too low
Austenite is not formed during soaking in heat treatment A,
Since there is no residual austenite after heat treatment A,
Even when heat treatment B was performed under the conditions specified by the present invention, carbon could not be concentrated in austenite, and no residual austenite was formed, resulting in low elongation.
In Test No. 6, austenite was formed by heat treatment A, but the soaking time was too short to completely dissolve cementite, the concentration of C to austenite was insufficient, and the elongation was low. In Test No. 7, the retained austenite was formed in the heat treatment A, but the annealing temperature in the heat treatment B was too low, so that the retained austenite was decomposed into cementite, and the retained austenite was not obtained.
Growth was low. In Test No. 8, the annealing temperature in heat treatment B was too high, so that the carbon-enriched region formed in heat treatment A disappeared by diffusion, and the remaining austenite could not be left by the subsequent cooling. It was low. In Test No. 9, in the heat treatment B, the cooling rate after soaking was too low, so that pearlite was precipitated, the amount of austenite was reduced, and the elongation was low. In Test No. 10, pearlite was precipitated and elongation was low because the cooling holding temperature in heat treatment B was too high. In Test No. 11, the elongation was low because the retained austenite was decomposed because the alloying temperature among the alloying treatment conditions was too high. Test number 1
In No. 2, austenite was decomposed because the holding time of the alloying treatment conditions was too long, and as a result, good elongation was not obtained.

【0098】本発明の規定する化学組成を備えた鋼C、
D、G、H、Iを用い、本発明の規定する条件で製造し
た試験番号13、14、17、18、19、20は、優
れた強度−延性バランスを示した。試験番号15は、用
いた鋼EのMnが少なすぎたためにオーステナイト安定
化効果が不足し、残留オーステナイトが得られず、伸び
が低かった。試験番号16は、用いた鋼Fの(Si+A
l)含有量が少なすぎたためにベイナイト変態中にセメ
ンタイトが析出し、オーステナイトにCを濃縮する効果
が不足して残留オーステナイトが得られず、伸びが低く
なった。
A steel C having the chemical composition defined by the present invention,
Test Nos. 13, 14, 17, 18, 19, and 20 produced using D, G, H, and I under the conditions specified by the present invention exhibited excellent strength-ductility balance. In Test No. 15, the Mn of the steel E used was too small, the austenite stabilizing effect was insufficient, no retained austenite was obtained, and the elongation was low. Test No. 16 is (Si + A) of the steel F used.
l) Since the content was too small, cementite was precipitated during the transformation of bainite, the effect of concentrating C in austenite was insufficient, and no residual austenite was obtained, resulting in low elongation.

【0099】[0099]

【発明の効果】本発明の製造方法によれば一般的な既存
の連続焼鈍設備と連続溶融亜鉛めっき設備を使用し、冷
間圧延板に特定条件で2回の熱処理を施すことにより、
高強度と高延性を兼ね備えた溶融亜鉛めっき鋼板、ある
いは、合金化溶融亜鉛めっき鋼板を、容易に製造するこ
とができる。従って本発明の製造方法は、自動車、その
他種々の構造部材や製品などへの高強度鋼板の適用を容
易にし、自動車の衝突安全性向上や、鋼製品の軽量化推
進に大きく寄与するので、本発明により得られる工業的
メリットは大きい。
According to the production method of the present invention, a general existing continuous annealing equipment and continuous hot-dip galvanizing equipment are used, and a cold-rolled sheet is subjected to two heat treatments under specific conditions.
A hot-dip galvanized steel sheet having both high strength and high ductility or an alloyed hot-dip galvanized steel sheet can be easily manufactured. Therefore, the manufacturing method of the present invention facilitates the application of high-strength steel sheets to automobiles and various other structural members and products, and contributes greatly to improving the collision safety of automobiles and promoting the reduction of weight of steel products. The industrial merit obtained by the invention is great.

【図面の簡単な説明】[Brief description of the drawings]

【図1】本発明の実施例に係わる、冷間圧延板の熱処理
サイクルを示すグラフである。
FIG. 1 is a graph showing a heat treatment cycle of a cold-rolled sheet according to an example of the present invention.

【図2】引張り特性に及ぼす熱処理条件の影響を示すグ
ラフである。
FIG. 2 is a graph showing the effect of heat treatment conditions on tensile properties.

───────────────────────────────────────────────────── フロントページの続き (51)Int.Cl.7 識別記号 FI テーマコート゛(参考) C22C 38/06 C22C 38/06 38/58 38/58 (72)発明者 中川 浩行 茨城県鹿嶋市大字光3番地 住友金属工業 株式会社鹿島製鉄所内 Fターム(参考) 4K027 AA02 AA05 AA23 AB02 AB42 AC12 AC32 AC73 AE12 AE22 4K037 EA01 EA05 EA06 EA10 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA31 FH01 FJ05 FJ06 FK02 FK03 FK08 GA05 GA07 ──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. 7 Identification symbol FI Theme coat ゛ (Reference) C22C 38/06 C22C 38/06 38/58 38/58 (72) Inventor Hiroyuki Nakagawa Omitsu, Kajimu, Ibaraki Pref. No. 3 Sumitomo Metal Industries, Ltd. Kashima Works, Ltd. F-term (reference) 4K027 AA02 AA05 AA23 AB02 AB42 AC12 AC32 AC73 AE12 AE22 4K037 EA01 EA05 EA06 EA10 EA11 EA13 EA15 EA16 EA17 EA18 F05 F03 F03 F03 F03 F05 GA07

Claims (5)

【特許請求の範囲】[Claims] 【請求項1】 質量%で、C:0.05〜0.25%、
Si:0.7%以下、Mn:0.8〜2.5%、P:
0.05%以下、S:0.01%以下、sol.Al:
0.4〜2.0%、N:0.01%以下、かつ、Si+
Al:1.0〜2.5%を満足する化学組成を有する冷
間圧延板に、下記条件を満足する連続焼鈍を施し、次い
で下記条件を満足する溶融亜鉛めっきを施すことを特徴
とする高張力高延性亜鉛めっき鋼板の製造方法; 連続焼鈍条件:鋼を780℃以上、860℃以下の2相
域に加熱し、該2相域で30秒以上、90秒以下保持し
た後、700℃以下、450℃以上の温度範囲を30℃
/秒以上の冷却速度で冷却し、次いで450℃以下、3
50℃以上の温度範囲で100秒以上、500秒以下保
持した後、室温まで冷却する、 溶融亜鉛めっき条件:鋼を780℃以上、860℃以下
の2相域に加熱し、該温度域で10秒以上、90秒以下
保持した後、500℃以下まで3℃/秒以上の冷却速度
で冷却し、500℃以下、460℃以上の温度範囲で1
5秒以上保持した後、溶融亜鉛浴に浸漬してめっきし、
室温まで冷却する。
1. A mass% of C: 0.05 to 0.25%,
Si: 0.7% or less, Mn: 0.8 to 2.5%, P:
0.05% or less, S: 0.01% or less, sol. Al:
0.4 to 2.0%, N: 0.01% or less, and Si +
Al: A cold rolled sheet having a chemical composition satisfying 1.0 to 2.5% is subjected to continuous annealing satisfying the following conditions, and then hot-dip galvanized satisfying the following conditions. Method for producing high-tensile ductile galvanized steel sheet; Continuous annealing conditions: Heat steel in a two-phase region of 780 ° C or more and 860 ° C or less, hold it in the two-phase region for 30 seconds or more and 90 seconds or less, and then 700 ° C or less. Temperature range of 450 ° C or higher at 30 ° C
/ Second cooling rate, then 450 ° C or less,
After holding at a temperature range of 50 ° C. or more for 100 seconds or more and 500 seconds or less, it is cooled to room temperature. Hot-dip galvanizing condition: Heat the steel to a two-phase region of 780 ° C. or more and 860 ° C. After holding for not less than 90 seconds and not more than 500 seconds, it is cooled to not more than 500 ° C. at a cooling rate of not less than 3 ° C./sec.
After holding for more than 5 seconds, immerse in a molten zinc bath and plate
Cool to room temperature.
【請求項2】 請求項1に記載の化学組成を有する冷間
圧延板に、請求項1に記載の条件を満足する連続焼鈍お
よび溶融亜鉛めっきを施した後、480℃以上、530
℃以下の温度範囲に加熱し、該温度範囲で10秒以上、
30秒以下保持してめっき層を合金化させて合金化溶融
亜鉛めっき鋼板を製造することを特徴とする高張力高延
性亜鉛めっき鋼板の製造方法。
2. A cold-rolled sheet having the chemical composition described in claim 1 is subjected to continuous annealing and hot-dip galvanization satisfying the conditions described in claim 1, and then subjected to 480 ° C. or higher and 530 ° C.
Heating to a temperature range of not more than 10 ° C.
A method for producing a high-tension, high-ductility galvanized steel sheet, wherein the galvannealed steel sheet is manufactured by holding the alloy layer for 30 seconds or less to alloy the plating layer.
【請求項3】 鋼の前記化学組成がさらにTiおよび/
またはNbを質量%の合計で、0.003〜0.05%
含有するものであることを特徴とする請求項1または2
に記載の高張力高延性亜鉛めっき鋼板の製造方法。
3. The steel of claim 1 wherein said chemical composition further comprises Ti and / or
Or Nb is 0.003 to 0.05% by mass in total.
3. The composition according to claim 1, wherein
The method for producing a high-tensile-high-ductility galvanized steel sheet according to item 1.
【請求項4】 鋼の前記化学組成がさらに質量%で、C
r:0.05〜1.0%および/またはMo:0.01
〜0.2%を含有するものであることを特徴とする請求
項1〜3のいずれかに記載の高張力高延性亜鉛めっき鋼
板の製造方法。
4. The steel according to claim 1, wherein the chemical composition is further
r: 0.05 to 1.0% and / or Mo: 0.01
The method for producing a high-tension, high-ductility galvanized steel sheet according to any one of claims 1 to 3, characterized in that the steel sheet contains 0.1 to 0.2%.
【請求項5】 鋼の前記化学組成がさらに質量%で、C
u:0.1〜1.0%、Ni:0.05〜0.5%、C
o:0.0005〜1.0%からなる群の内の1種また
は2種以上を、質量%の合計で1.5%以下含有するも
のであることを特徴とする請求項1〜4のいずれかに記
載の高張力高延性亜鉛めっき鋼板の製造方法。
5. The steel according to claim 1, wherein said chemical composition further comprises:
u: 0.1 to 1.0%, Ni: 0.05 to 0.5%, C
o: One or two or more of the group consisting of 0.0005 to 1.0% are contained in a total of 1.5% or less by mass%. The method for producing a high-tension, high-ductility galvanized steel sheet according to any one of the above.
JP2000225413A 2000-07-26 2000-07-26 Manufacturing method of high tensile high ductility galvanized steel sheet Expired - Fee Related JP3521851B2 (en)

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