JPH11323434A - Production of thick high tensile strength steel excellent in low temperature toughness - Google Patents

Production of thick high tensile strength steel excellent in low temperature toughness

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Publication number
JPH11323434A
JPH11323434A JP10132404A JP13240498A JPH11323434A JP H11323434 A JPH11323434 A JP H11323434A JP 10132404 A JP10132404 A JP 10132404A JP 13240498 A JP13240498 A JP 13240498A JP H11323434 A JPH11323434 A JP H11323434A
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JP
Japan
Prior art keywords
transformation point
temperature
hot rolling
cooling
less
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.)
Withdrawn
Application number
JP10132404A
Other languages
Japanese (ja)
Inventor
Toshinaga Hasegawa
俊永 長谷川
Yukio Tomita
幸男 冨田
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
Nippon Steel Corp
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Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Priority to JP10132404A priority Critical patent/JPH11323434A/en
Publication of JPH11323434A publication Critical patent/JPH11323434A/en
Withdrawn legal-status Critical Current

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Abstract

PROBLEM TO BE SOLVED: To uniformly and stably produce a surface ultrafine granular layer by subjecting a slab having a specified elemental compsn. to hot rolling under specified conditions. SOLUTION: A slab has a compsn. contg., by weight, 0.01 to 0.20% C, 0.01 to 1.0% Si, 0.1 to 2.0% Mn, 0.001 to 0.1% Al, 0.001 to 0.010% N, <=0.025% P, <=0.015% S, and the balance Fe with inevitable impurities. The slab is heated to the Ac3 transformation point to 1050 deg.C, is next cooled to 500 deg.C and is thereafter heated to the Ac3 transformation point to 1150 deg.C, and in the meanwhile in which hot rolling at 20 to 80% cumulative draft is executed, before the start of the hot rolling or on the way thereof, the process in which the surface layer part regions of at least two outer surfaces corresponding to 10 to 33% of the slab thickness in the stage are cooled from the temp. above the Ar3 transformation point at a rate of 2 to 40 deg.C/sec, the cooling is stopped at less than the Ar3 transformation point, and recuperation is executed is repeated for one or more times, and the hot rolling is completed at a temp. of (the Ac1 transformation point -50 deg.C) to (the Ac3 transformation point -50 deg.C).

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】本発明は安全性の観点から良
好な低温靱性を要求される、海洋構造物、圧力容器、船
舶等の構造用鋼材及びその製造方法に関するものであ
る。具体的には鋼材の表層部に平均フェライト粒径が３
μｍ以下の超細粒組織（表層超細粒層）を一定厚みで有
することにより良好な低温靱性、特に良好な母材の脆性
き裂伝播停止特性（アレスト性）を有する鋼材及びその
製造方法に関するものである。特に、本発明は、板厚が
５０mmから１５０mm程度までの厚手材において表層超細
粒層を均一かつ安定に製造する場合に有用である。TECHNICAL FIELD The present invention relates to a structural steel material for marine structures, pressure vessels, ships, etc., which requires good low-temperature toughness from the viewpoint of safety, and a method for producing the same. Specifically, the average ferrite grain size is 3
The present invention relates to a steel material having good low-temperature toughness, particularly good brittle crack propagation arrestability (arrestability) of a base material, by having an ultrafine grain structure (surface layer ultrafine grain layer) of μm or less at a constant thickness, and a method for producing the same. Things. In particular, the present invention is useful in a case where a super-fine grain surface layer is uniformly and stably produced in a thick material having a thickness of about 50 mm to 150 mm.

【０００２】本発明により製造した鋼材は海洋構造物、
圧力容器、造船、橋梁、建築物、ラインパイプなどの溶
接鋼構造物一般に用いることができるが、アレスト性に
極めて優れていることから、特に安全性の配慮が必要
な、海洋構造物、圧力容器、船舶、さらには耐震性を必
要とする建築、橋梁等の構造物用鋼材として有用であ
る。また、鋼材の形態は問わないが、構造部材として用
いられ、低温靭性が要求される鋼板、特に厚板、鋼管素
材、あるいは形鋼で特に有用である。[0002] The steel material produced according to the present invention is an offshore structure,
Can be used in general for welded steel structures such as pressure vessels, shipbuilding, bridges, buildings, line pipes, etc., but because of their excellent arrestability, marine structures and pressure vessels that require special safety considerations It is useful as a steel material for structures such as buildings, bridges, etc. that require earthquake resistance. Although the form of the steel material is not limited, it is particularly useful for a steel plate which is used as a structural member and requires low-temperature toughness, particularly a thick plate, a steel pipe material, or a shaped steel.

【０００３】[0003]

【従来の技術】構造物用鋼材は構造物の安全性確保の観
点から強度とともに低温靭性を要求されることが多い。
低温靭性を向上させる方法は種々提案されているが、Ｎ
ｉのような高価な合金元素を用いずに、他の特性劣化を
生じることなく低温靭性を向上させる方法としては、フ
ェライト粒径の微細化が代表的である。特に、一旦発生
した脆性き裂を停止せしめる能力（アレスト性）を付与
することは構造物の安全性を格段に高めるため、船舶の
重要部材やタンク、圧力容器等、破壊事故による人的、
物的な被害が甚大となる可能性の高い構造物で要求され
る場合が多いが、脆性破壊の発生特性（シャルピー特
性、ＣＴＯＤ特性）が硬さの低下や硬質脆化層の抑制
等、Ｎｉ添加や細粒化以外にも様々な改善策が存在する
のに対して、アレスト性はＮｉの添加と細粒化による以
外には顕著な改善が困難である。2. Description of the Related Art Steel materials for structures are often required to have not only strength but also low-temperature toughness from the viewpoint of ensuring the safety of structures.
Various methods for improving low-temperature toughness have been proposed.
As a method of improving the low-temperature toughness without using an expensive alloy element such as i and deteriorating other characteristics, a finer ferrite grain size is typical. In particular, providing the ability to stop brittle cracks that have occurred once (arrestability) significantly enhances the safety of structures.
In many cases, it is required for a structure having a high possibility that physical damage will be enormous. However, the generation characteristics of brittle fracture (Charpy characteristic, CTOD characteristic) decrease the hardness and suppress the hard embrittlement layer. While there are various improvement measures other than addition and grain refinement, arrestability is difficult to significantly improve except by addition of Ni and grain refinement.

【０００４】Ｎｉ量の増加はミクロ組織にほぼ依存せず
にアレスト性を向上できるが、効果を明確にするために
は０．５〜数％程度の多量添加が必要なためコストの大
幅な増加を必然的に招く。従って、製造方法の工夫によ
り結晶粒径の微細化が可能であれば、その方が当然好ま
しい。[0004] The increase in the amount of Ni can improve the arrestability almost without depending on the microstructure, but in order to clarify the effect, it is necessary to add a large amount of about 0.5 to several%, so that the cost is greatly increased. Inevitably. Therefore, if the crystal grain size can be reduced by devising the manufacturing method, it is naturally preferable.

【０００５】従来から結晶粒径微細化によるアレスト性
を含めた低温靱性改善の方法は種々提案されている。ご
く一般的には、熱間圧延における制御圧延の強化や、制
御圧延を容易にするためのＮｂ添加、さらには制御圧延
後の強制冷却、あるいは熱処理における繰り返し焼入や
焼きならし等が周知されているが、これら既存の技術に
よっては達成されるフェライト粒径はせいぜい５μｍ程
度であり、アレスト性向上効果も小さい。また、制御圧
延では圧延温度待ちのために生産性が劣化し、繰り返し
熱処理では製造工程の増加による生産性の低下や製造コ
ストの増加を必然的に招く点でも好ましくない。Conventionally, various methods for improving low-temperature toughness, including arrestability, by reducing the crystal grain size have been proposed. Very generally, strengthening of controlled rolling in hot rolling, addition of Nb for facilitating controlled rolling, forced cooling after controlled rolling, or repeated quenching or normalizing in heat treatment are well known. However, the ferrite grain size achieved by these existing techniques is at most about 5 μm, and the effect of improving arrestability is small. In addition, in the controlled rolling, the productivity is deteriorated due to the waiting for the rolling temperature, and the repeated heat treatment is not preferable in that the productivity is increased and the manufacturing cost is inevitably increased due to an increase in the number of manufacturing steps.

【０００６】厚鋼板のアレスト性評価試験の脆性き裂の
伝播・停止挙動から、アレスト性に大きく寄与するのは
脆性き裂伝播時に厚鋼板表層部に発生するシアリップと
称する塑性変形量の大きい延性破壊領域であり、脆性き
裂の有する伝播エネルギーがこのシアリップ形成に費や
される場合には、脆性き裂が早期に停止してアレスト性
が飛躍的に向上する。従って、鋼材全体のアレスト性を
均一に向上させなくとも、表層部の一定領域のアレスト
性を高めることで鋼材全体のアレスト性も向上できる。[0006] From the propagation and arrest behavior of brittle cracks in the arrestability evaluation test of thick steel plates, it is the ductility that has a large plastic deformation called shear lip generated on the surface layer of thick steel plates during the propagation of brittle cracks that greatly contributes to arrestability. In the case of the fracture region, when the propagation energy of the brittle crack is used for the formation of the shear lip, the brittle crack stops early and the arrest property is dramatically improved. Therefore, even if the arrestability of the entire steel material is not uniformly improved, the arrestability of the entire steel material can be improved by increasing the arrestability of a certain region of the surface layer portion.

【０００７】このような考え方に立って、鋼材の表層部
に超細粒組織を付与することによって、Ｎｉ添加によら
ずに飛躍的にアレスト性を向上する方法が最近示されて
いる。即ち、特開平４−１４１５１７号公報、特開平６
−８８１６１号公報、等に示されている技術で、基本的
には表層部を急冷後、復熱中の圧延により表層部の結晶
粒径を顕著に微細化してアレスト性を飛躍的に向上させ
るものである。この方法によれば、表層部のフェライト
粒径が約３μｍ以下に超細粒化し、その結果として、脆
性き裂の伝播中に該表層部が早期にシアリップを形成す
るために、その化学組成から通常得られるレベルを遥か
に凌駕するアレスト性が達成される。シアリップの厚み
が不十分であると、たとえシアリップが形成されても脆
性き裂の停止に至らない場合が生じる。脆性き裂の伝播
を確実に停止するには、シアリップはある程度の厚みが
必要となる。Based on such a concept, there has recently been proposed a method of dramatically improving arrestability without imparting Ni by adding an ultrafine grain structure to the surface layer of a steel material. That is, Japanese Patent Application Laid-Open Nos.
No.-88161, basically, the surface layer portion is rapidly cooled, and then the rolling process during recuperation remarkably refines the crystal grain size of the surface layer portion to dramatically improve arrestability. It is. According to this method, the ferrite grain size of the surface layer is ultrafine-grained to about 3 μm or less, and as a result, the surface layer forms a shear lip early during the propagation of a brittle crack. Arrest properties far exceeding the levels normally obtained are achieved. If the thickness of the shear lip is insufficient, even if the shear lip is formed, brittle cracks may not be stopped. In order to reliably stop the propagation of a brittle crack, the shear lip needs to have a certain thickness.

【０００８】従って、超細粒層の厚みも厚ければ厚いほ
どき裂の停止効果が大となるが、必要以上の超細粒層の
厚みを確保しようとすると、製造工程に過大な負荷をか
けたり、製造条件によっては母材の延性や鋼板の形状、
表面性状等の劣化につながる。これらの問題を生じない
範囲は板厚に対する比率で規定され、実験に基づけば、
平均フェライト粒径が３μｍ以下の表層超細粒組織の厚
みは１０以上あれば効果を発揮し、それ以上厚みを増す
ことによって効果も大きくなるが、３３％超では効果が
飽和し、安定製造が困難となることから表層の一つの面
について、表層から板厚の１０％〜３３％の範囲が好ま
しい。なお、該表層超細粒層は鋼材の全ての表面に付与
することが好ましいが、上記条件を満足すれば、最低限
２つの表面に該超細粒層を付与すれば脆性き裂の停止に
は有効である。Accordingly, the effect of stopping the cracks increases as the thickness of the ultrafine-grained layer increases. However, if the thickness of the ultrafine-grained layer is increased more than necessary, an excessive load is imposed on the manufacturing process. Depending on manufacturing conditions, ductility of base material, shape of steel sheet,
This leads to deterioration of surface properties and the like. The range that does not cause these problems is defined by the ratio to the plate thickness, and based on experiments,
The effect is exhibited when the thickness of the surface layer ultrafine grained structure having an average ferrite grain size of 3 μm or less is 10 or more, and the effect is increased by increasing the thickness further. However, when the thickness exceeds 33%, the effect is saturated and stable production is achieved. From the viewpoint of difficulty, it is preferable that one surface of the surface layer has a thickness of 10% to 33% of the plate thickness from the surface layer. It is preferable that the surface ultrafine grain layer is provided on all surfaces of the steel material. However, if the above conditions are satisfied, the brittle crack can be stopped by providing the ultrafine grain layer on at least two surfaces. Is valid.

【０００９】上記方法における表層部の超超細粒化は、
主として復熱中のフェライトの加工・再結晶を利用した
もので、より詳細には、鋼片を熱間圧延するに際し、熱
間圧延中あるいは熱間圧延途中で表層部の適当な厚みの
領域を水冷等の手段によりＡｒ3 変態点よりも低い温度
まで一旦冷却して内部と温度差を付けた後、温度差のつ
いたままの状態からさらに熱間圧延を行うと、Ａｒ3 変
態点よりも低い温度まで一旦冷却された領域は、復熱及
びその過程の加工によりフェライト主体組織となるた
め、該フェライト主体組織を有する表層部は内部の顕熱
により復熱されながら加工を受けることになり、この復
熱中の加工条件を適正化することにより表層部のフェラ
イト結晶粒が顕著に細粒化する。従って、最終的な鋼材
における表層超細粒層の割合は、表層を一旦冷却した際
にＡｒ3 変態点まで低下した領域の割合とほぼ一致する
ことになる。In the above method, ultra-fine graining of the surface layer portion is performed by
It mainly utilizes the processing and recrystallization of ferrite during recuperation.More specifically, when hot rolling a steel slab, a region of an appropriate thickness of the surface layer is water-cooled during hot rolling or during hot rolling. After once cooling to a temperature lower than the Ar 3 transformation point by means such as to give a temperature difference from the inside, and further hot rolling from the state where the temperature difference is kept, it is lower than the Ar 3 transformation point Since the region once cooled to the temperature becomes a ferrite-based structure due to reheating and processing in the process, the surface layer having the ferrite-based structure undergoes processing while being reheated by internal sensible heat. By optimizing the processing conditions during the recuperation, the ferrite crystal grains in the surface layer are remarkably refined. Therefore, the ratio of the surface layer of the ultrafine grain layer in the final steel material substantially coincides with the ratio of the region which has been lowered to the Ar 3 transformation point when the surface layer is once cooled.

【００１０】該製造方法において、より均一な超細粒組
織を得て安定したアレスト性を得るためには復熱前の組
織の微細化や累積圧下率の確保が重要になってくる。そ
のため、厚手材になると、表層の冷却・復熱を行う段階
の鋼片厚の増加にともなって、前組織の微細化が困難に
なることや、累積圧下率が不足しがちになることによる
達成粒径の粗大化、混粒度の増加等の問題が顕在化して
くるため、厚手材製造においては、これらの問題が生じ
ないように、注意深い製造条件の選択が必要となる。In the production method, in order to obtain a more uniform ultrafine grain structure and obtain stable arrestability, it is important to refine the structure before reheating and to secure a cumulative rolling reduction. For thicker materials, this is achieved by increasing the thickness of the slab at the stage of cooling and reheating the surface layer, making it difficult to refine the pre-structure and tending to reduce the cumulative draft. Since problems such as coarsening of the particle size and increase of the mixed particle size become apparent, in the production of thick materials, it is necessary to carefully select production conditions so that these problems do not occur.

【００１１】[0011]

【発明が解決しようとする課題】本発明は、特開平４−
１４１５１７号公報、特開平６−８８１６１号公報、特
開平７−１２６７８９号公報等に示されている、表層部
に超細粒層を形成することによりアレスト性を飛躍的に
向上させる鋼材の製造技術において、特に、板厚が５０
〜１５０mm程度の厚手材において、混粒度の少ない平均
粒径が３μｍ以下の超細粒層を表層の一定厚さにわたっ
て確実に付与するための新たな方法を提供するものであ
る。SUMMARY OF THE INVENTION The present invention relates to a method disclosed in
No. 141517, JP-A-6-88161, JP-A-7-126789, and the like, a technique for producing a steel material in which arrestability is drastically improved by forming an ultrafine grain layer in a surface layer portion. In particular, when the plate thickness is 50
It is intended to provide a new method for reliably applying an ultrafine grain layer having a small mixed grain size and an average grain size of 3 μm or less to a thick material of about 150 mm over a certain thickness of a surface layer.

【００１２】[0012]

【課題を解決するための手段】本発明は、表層部に超細
粒層を形成することによりアレスト性を飛躍的に向上さ
せる鋼材の製造技術において、特に、板厚が５０〜１５
０mm程度の厚手材において、混粒度の少ない平均粒径が
３μｍ以下の超細粒層を表層の一定厚さにわたって確実
に付与するための新たな方法を提供するものであり、仕
上げ板厚が厚い場合の復熱段階での加工に入る前の組織
微細化と、累積圧下率が制限される条件における復熱段
階での加工に際しての均一超細粒化条件を詳細に検討し
た結果、発明するに至ったものであり、その要旨は以下
に示す通りである。SUMMARY OF THE INVENTION The present invention relates to a technique for manufacturing a steel material in which arrestability is drastically improved by forming an ultrafine grain layer in a surface layer.
The present invention provides a new method for reliably providing an ultra-fine grain layer having a small mixed grain size and an average grain size of 3 μm or less over a certain thickness of a surface layer in a thick material of about 0 mm. As a result of a detailed study of the conditions for refinement of the structure before processing in the recuperation stage and uniform ultra-fine graining for processing in the recuperation stage under conditions where the cumulative draft is limited, the invention The summary is as follows.

【００１３】（１）重量％で、 Ｃ ：０．０１〜０．２０％ Ｓｉ：０．０１〜１．０％ Ｍｎ：０．１〜２．０％ Ａｌ：０．００１〜０．１％ Ｎ ：０．００１〜０．０１０％を含有し、さらに不純
物として、 Ｐ ：０．０２５％以下 Ｓ ：０．０１５％以下を含有し、残部が鉄及び不可避
不純物からなる鋼片をＡｃ3 変態点以上、１０５０℃以
下の温度に加熱し、０．２〜２０℃/sの冷却速度で５０
０℃以下まで冷却した後、再度鋼片をＡｃ3 変態点以
上、１１５０℃以下の温度に加熱し、累積圧下率が２０
〜８０％の熱間圧延を行う間に、熱間圧延の開始前ある
いは熱間圧延の途中段階で、その段階での鋼片厚みの１
０％〜３３％に対応する少なくとも２つの外表面の表層
部領域をＡｒ3 変態点超の温度から２〜４０℃/sの冷却
速度で冷却し、Ａｒ3 変態点未満で冷却を停止して復熱
させることを１回以上繰り返し、該熱間圧延を（Ａｃ1
変態点−５０℃）〜（Ａｃ3 変態点−５０℃）の温度で
完了させることを特徴とする低温靭性に優れた厚手高張
力鋼材の製造方法。(1) By weight%, C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% N: containing 0.001 to 0.010%, as further impurities, P: 0.025% or less S: containing 0.015% or less, the billet balance of iron and inevitable impurities Ac 3 transformation To a temperature of not less than 1050 ° C. and a cooling rate of 0.2 to 20 ° C./s.
After cooling to 0 ° C. or less, the slab is heated again to a temperature of from the Ac 3 transformation point to 1150 ° C., and the cumulative rolling reduction is 20%.
During the hot rolling of ８０80%, before the start of hot rolling or in the middle of hot rolling, the thickness of the slab at that stage is 1%.
At least the surface layer region of the two outer surfaces corresponding to 0% to 33% is cooled at a cooling rate of 2 to 40 ° C. / s from the temperature of the Ar 3 transformation point greater than stop the cooling at Ar less than 3 transformation point Reheating is repeated at least once, and the hot rolling is performed (Ac 1
A method for producing a thick high-tensile steel excellent in low-temperature toughness, which is completed at a temperature of (transformation point −50 ° C.) to (Ac 3 transformation point −50 ° C.).

【００１４】（２）重量％で、 Ｃ ：０．０１〜０．２０％ Ｓｉ：０．０１〜１．０％ Ｍｎ：０．１〜２．０％ Ａｌ：０．００１〜０．１％ Ｎ ：０．００１〜０．０１０％を含有し、さらに、不
純物として、 Ｐ ：０．０２５％以下 Ｓ ：０．０１５％以下を含有し、残部が鉄及び不可避
不純物からなる鋼片を、Ａｃ3 変態点以上、１１５０℃
以下の温度に加熱した後、累積圧下率が２０〜５０％の
予備熱間圧延を行い、０．２〜２０℃/sの冷却速度で５
００℃以下まで冷却した後、再度鋼片をＡｃ3 変態点以
上、１１５０℃以下の温度に加熱し、累積圧下率が２０
〜８０％の熱間圧延を行う間に、熱間圧延の開始前ある
いは熱間圧延の途中段階で、その段階での鋼片厚みの１
０％〜３３％に対応する少なくとも２つの外表面の表層
部領域をＡｒ3変態点超の温度から２〜４０℃/sの冷却
速度で冷却し、Ａｒ3 変態点未満で冷却を停止して復熱
させることを１回以上繰り返し、該熱間圧延を（Ａｃ1
変態点−５０℃）〜（Ａｃ3 変態点−５０℃）の温度で
完了させることを特徴とする低温靭性に優れた厚手高張
力鋼材の製造方法。(2) By weight%, C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% A steel slab containing N: 0.001 to 0.010%, P: 0.025% or less as an impurity, S: 0.015% or less, and the balance consisting of iron and unavoidable impurities is Ac More than 3 transformation points, 1150 ° C
After heating to the following temperature, preliminary hot rolling is performed with a cumulative draft of 20 to 50%, and a cooling rate of 0.2 to 20 ° C./s is used.
After cooling to below 00 ° C., the slab is again heated to a temperature above the Ac 3 transformation point and below 1150 ° C. and the cumulative draft is 20
During the hot rolling of ８０80%, before the start of hot rolling or in the middle of hot rolling, the thickness of the slab at that stage is 1%.
At least the surface layer region of the two outer surfaces corresponding to 0% to 33% is cooled at a cooling rate of 2 to 40 ° C. / s from the temperature of the Ar 3 transformation point greater than stop the cooling at Ar less than 3 transformation point Reheating is repeated at least once, and the hot rolling is performed (Ac 1
A method for producing a thick high-tensile steel excellent in low-temperature toughness, which is completed at a temperature of (transformation point −50 ° C.) to (Ac 3 transformation point −50 ° C.).

【００１５】（３）重量％で、 Ｃ ：０．０１〜０．２０％ Ｓｉ：０．０１〜１．０％ Ｍｎ：０．１〜２．０％ Ａｌ：０．００１〜０．１％ Ｎ ：０．００１〜０．０１０％を含有し、さらに、不
純物として、 Ｐ ：０．０２５％以下 Ｓ ：０．０１５％以下を含有し、残部が鉄及び不可避
不純物からなる鋼片をＡｃ3 変態点以上、１１５０℃以
下の温度に加熱した後、累積圧下率が２０〜８０％の熱
間圧延を行う間に、熱間圧延の開始前あるいは熱間圧延
の途中段階で、その段階での鋼片厚みの１０％〜３３％
に対応する少なくとも２つの外表面の表層部領域をＡｒ
3 変態点超の温度から２〜４０℃/sの冷却速度で冷却
し、Ａｒ3 変態点未満で冷却を停止して復熱させること
を２回以上繰り返すにおいて、最初の冷却・復熱過程に
おいては、復熱温度をＡｃ3 変態点〜（Ａｃ3 変態点＋
１００℃）の範囲とし、その後の冷却・復熱過程におい
ては復熱温度を（Ａｃ3 変態点−２０℃）以下とし、該
熱間圧延を（Ａｃ1 変態点−５０℃）〜（Ａｃ3 変態点
−５０℃）の温度で完了させることを特徴とする低温靭
性に優れた厚手高張力鋼材の製造方法。(3) C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% by weight% A steel slab containing N: 0.001 to 0.010%, P: 0.025% or less as an impurity, S: 0.015% or less, and the balance of iron and unavoidable impurities is Ac 3 After heating to a temperature not lower than the transformation point and not higher than 1150 ° C., while performing hot rolling with a cumulative rolling reduction of 20 to 80%, before starting hot rolling or in the middle of hot rolling, 10% to 33% of billet thickness
At least two surface layer regions on the outer surface corresponding to
3 is cooled at a cooling rate of temperature from 2 to 40 ° C. / s transformation point than in that to recuperation by stopping cooling at Ar less than 3 transformation point two or more times, in the first cooling-recuperator process Means that the recuperation temperature is from the Ac 3 transformation point to (Ac 3 transformation point +
100 ° C.), and in the subsequent cooling and reheating process, the recuperation temperature is set to (Ac 3 transformation point −20 ° C.) or less, and the hot rolling is performed from (Ac 1 transformation point −50 ° C.) to (Ac 3 transformation point). (Transformation point −50 ° C.).

【００１６】（４）前記熱間圧延の前に、鋼片をＡｃ3
変態点以上、１０５０℃以下の温度に加熱し、０．２〜
２０℃/sの冷却速度で５００℃以下まで冷却することを
特徴とする、前記（３）に記載の低温靭性に優れた厚手
高張力鋼材の製造方法。(4) Before the hot rolling, the billet is made of Ac 3
Heating to a temperature of not less than the transformation point and not more than 1050 ° C.,
The method for producing a thick high-tensile steel excellent in low-temperature toughness according to the above (3), wherein the steel is cooled to 500 ° C. or lower at a cooling rate of 20 ° C./s.

【００１７】（５）前記熱間圧延の前に、鋼片を、Ａｃ
3 変態点以上、１１５０℃以下の温度に加熱した後、累
積圧下率が２０〜５０％の予備熱間圧延を行い、０．２
〜２０℃/sの冷却速度で５００℃以下まで冷却すること
を特徴とする、前記（３）に記載の低温靭性に優れた厚
手高張力鋼材の製造方法。(5) Before the hot rolling, the steel slab is made of Ac
After heating to a temperature not lower than 3 transformation points and not higher than 1150 ° C., preliminary hot rolling with a cumulative rolling reduction of 20 to 50% is performed.
The method for producing a thick high-tensile steel material excellent in low-temperature toughness according to the above (3), wherein the steel material is cooled to 500 ° C. or less at a cooling rate of ２０20 ° C./s.

【００１８】（６）最終の圧延終了後の鋼材を２〜４０
℃/sの冷却速度で２０℃〜６００℃まで加速冷却するこ
とを特徴とする前記（１）〜（５）のいずれかに記載の
低温靭性に優れた厚手高張力鋼材の製造方法。(6) The steel material after the completion of the final rolling is 2 to 40
The method for producing a thick high-tensile steel excellent in low-temperature toughness according to any one of the above (1) to (5), wherein the cooling is performed at an accelerated cooling rate of 20 ° C to 600 ° C at a cooling rate of ° C / s.

【００１９】（７）４５０℃以上、Ａｃ1 変態点以下で
焼戻しを行うことを特徴とする前記（１）〜（６）のい
ずれかに記載の低温靭性に優れた厚手高張力鋼材の製造
方法。(7) The method for producing a thick high-tensile steel excellent in low-temperature toughness according to any one of the above (1) to (6), wherein tempering is performed at a temperature of 450 ° C. or more and an Ac 1 transformation point or less. .

【００２０】（８）重量％で、 Ｃｒ：０．０１〜１．０％ Ｎｉ：０．０１〜３．０％ Ｍｏ：０．０１〜１．０％ Ｃｕ：０．０１〜１．５％ Ｔｉ：０．００３〜０．１０％ Ｖ ：０．００５〜０．５０％ Ｎｂ：０．００３〜０．１０％ Ｚｒ：０．００３〜０．１０％ Ｔａ：０．００５〜０．２０％ Ｗ ：０．０１〜２．０％ Ｂ ：０．０００３〜０．００２０％ の１種または２種以上を含有することを特徴とする前記
（１）〜（７）のいずれかに記載の低温靭性に優れた厚
手高張力鋼材の製造方法。(8) By weight%, Cr: 0.01 to 1.0% Ni: 0.01 to 3.0% Mo: 0.01 to 1.0% Cu: 0.01 to 1.5% Ti: 0.003 to 0.10% V: 0.005 to 0.50% Nb: 0.003 to 0.10% Zr: 0.003 to 0.10% Ta: 0.005 to 0.20% W: 0.01 to 2.0% B: 0.0003 to 0.0020% The low temperature according to any one of the above (1) to (7), wherein one or two or more kinds are contained. A method for producing thick high-tensile steel with excellent toughness.

【００２１】（９）重量％で、 Ｍｇ：０．０００５〜０．０１％ Ｃａ：０．０００５〜０．０１％ ＲＥＭ：０．００５〜０．１０％ のうち１種または２種以上を含有することを特徴とする
前記（１）〜（８）のいずれかに記載の低温靭性に優れ
た厚手高張力鋼材の製造方法。(9) Mg: 0.0005 to 0.01% Ca: 0.0005 to 0.01% REM: One or more of 0.005 to 0.10% by weight% The method for producing a thick high-tensile steel excellent in low-temperature toughness according to any one of the above (1) to (8).

【００２２】[0022]

【発明の実施の形態】本発明者らは詳細な実験・解析の
結果、本発明が目的としている板厚が５０mm〜１５０mm
程度の厚手鋼材において、表層部の冷却・復熱工程中に
圧延（以降復熱中加工）を施すことによるフェライトの
加工・再結晶を利用して表層部に超細粒層を付与するた
めの要件は、復熱中加工の前あるいは初期段階におけ
る、実質的に二相域〜フェライト域加工に入る前の組織
を極力微細化することであることを見出した。BEST MODE FOR CARRYING OUT THE INVENTION As a result of detailed experiments and analyses, the present inventors have found that the plate thickness intended by the present invention is 50 mm to 150 mm.
Requirements for applying an ultrafine grained layer to the surface layer by using ferrite processing and recrystallization by performing rolling (hereinafter processing during recuperation) during the cooling and reheating process of the surface layer for moderately thick steel materials Has found that the structure before the processing during the recuperation or in the initial stage is substantially reduced to a structure substantially before entering the processing from the two-phase region to the ferrite region.

【００２３】そして、そのための具体的方法の基本的要
件としては、冷却・復熱工程を含む熱間圧延工程前の
鋼片を予め熱処理あるいは熱間圧延を施すことにより鋼
片の粗大な凝固組織を微細化することで二相域〜フェラ
イト域加工前の組織を微細化する、繰り返しの冷却・
復熱工程において、冷却・復熱中のオーステナイト／フ
ェライト変態を利用して、二相域〜フェライト域加工前
の組織を微細化する、前記とを組み合わせる、の
３種類が実用的に最適であるとの結論に至った。即ち、
請求項１及び２に記載の方法が上記に基づく製造方法
であり、請求項３に記載の方法が上記に基づく製造方
法であり、請求項４及び５に記載の方法がこれらの組み
合わせである。従って、請求項１〜３に示す製造方法が
本発明の製造方法の具体的要件になる。以下に、請求項
１〜３についてさらに詳細に説明する。The basic requirements of a specific method for this purpose are that the steel slab before the hot rolling step including the cooling and recuperation steps is subjected to heat treatment or hot rolling in advance, thereby forming a coarse solidified structure of the steel slab. Refinement of the two-phase region to the ferrite region
In the recuperation step, the three types of two-phase region to refine the structure before the ferrite region are processed by utilizing the austenite / ferrite transformation during cooling and recuperation, and the combination with the above, are considered to be practically optimal. Came to the conclusion. That is,
The methods described in claims 1 and 2 are manufacturing methods based on the above, the method described in claim 3 is a manufacturing method based on the above, and the methods described in claims 4 and 5 are combinations thereof. Therefore, the manufacturing method described in claims 1 to 3 is a specific requirement of the manufacturing method of the present invention. Hereinafter, claims 1 to 3 will be described in more detail.

【００２４】先ず、請求項１に示す製造方法は、「本発
明を満足する適正な化学組成を有する鋼片をＡｃ3 変態
点以上、１０５０℃以下の温度に加熱し、０．２〜２０
℃/sの冷却速度で５００℃以下まで冷却した後、さらに
鋼片をＡｃ3 変態点以上、１１５０℃以下の温度に加熱
し、熱間圧延の開始前あるいは熱間圧延の途中段階で、
その段階での鋼片厚みの１０％〜３３％に対応する少な
くとも２つの外表面の表層部領域をＡｒ3 変態点超の温
度から２〜４０℃/sの冷却速度で冷却を開始し、Ａｒ3
変態点未満で冷却を停止して復熱させることを１回以上
経由させる過程で、最後の復熱が終了するまでの間に累
積圧下率が２０〜８０％の仕上げ圧延を行い、該圧延を
（Ａｃ1 変態点−５０℃）〜（Ａｃ3 変態点−５０℃）
の温度で完了させること」を特徴とする低温靭性に優れ
た厚手高張力鋼材の製造方法である。First, a manufacturing method according to claim 1 is that a steel slab having an appropriate chemical composition satisfying the present invention is heated to a temperature not lower than the Ac 3 transformation point but not higher than 1050 ° C.
After cooling to 500 ° C. or lower at a cooling rate of 500 ° C./s, the slab is further heated to a temperature of not less than the Ac 3 transformation point and 1150 ° C., and before the start of hot rolling or in the middle of hot rolling,
At this stage, at least two outer surface regions corresponding to 10% to 33% of the thickness of the slab are cooled from a temperature higher than the Ar 3 transformation point at a cooling rate of 2 to 40 ° C./s. Three
In the process of stopping cooling at less than the transformation point and performing reheating at least once, finish rolling with a cumulative reduction ratio of 20 to 80% is performed until the final reheating is completed, and the rolling is performed. (Ac 1 transformation point -50 ° C) to (Ac 3 transformation point -50 ° C)
At high temperature "is a method for producing a thick high-tensile steel excellent in low-temperature toughness.

【００２５】上記方法においては、冷却・復熱工程を含
む熱間圧延を行う前に、鋼片の粗大な凝固組織を解消し
て、最終の冷却・復熱工程を含む熱間圧延工程における
冷却・復熱工程中の超細粒化のための二相域〜フェライ
ト域での加工に入る前の組織を微細化する。その際、鋼
片をＡｃ3 変態点以上、１０５０℃以下の温度に加熱す
るが、これは、加熱温度がＡｃ3 変態点未満であると、
その後の冷却条件如何によらず粗大な組織が残存し、１
０５０℃超であると加熱時のオーステナイト粒径が粗大
化して冷却後の組織の微細化が不十分となるためであ
る。In the above method, before performing the hot rolling including the cooling / reheating step, the coarse solidification structure of the billet is eliminated, and the cooling in the hot rolling step including the final cooling / reheating step is performed. -Refine the structure before processing in the two-phase region to the ferrite region for ultra-fine graining during the recuperation process. At this time, the slab is heated to a temperature of not less than the Ac 3 transformation point and not more than 1050 ° C. This is because when the heating temperature is less than the Ac 3 transformation point,
Regardless of the subsequent cooling conditions, a coarse structure remains and 1
If the temperature exceeds 050 ° C., the austenite particle size during heating becomes coarse, and the microstructure after cooling becomes insufficient.

【００２６】また、加熱保持後の冷却も０．２〜２０℃
/sの冷却速度で５００℃以下まで制御する必要がある。
該条件は加熱温度を前記のように規定して加熱オーステ
ナイト粒径を微細化しても、その後の冷却による変態組
織を微細化するために必要である。即ち、鋼片は一般的
に厚手であるために、放冷程度でも鋼片内部の冷却は徐
冷となる場合が多く、その場合には変態組織は粗大とな
る。そのため、冷却後の組織を微細化するためには０．
２℃/s以上で冷却する必要がある。冷却速度は大きけれ
ば大きいほど組織微細化には好ましいが、板厚の大きい
鋼片では冷却速度を極端に大きくすることは現実的でな
く、２０℃/s以上であれば最終的なフェライトの加工・
再結晶組織に有害な極端な粗大組織の出現は抑制できる
ことから、本発明では上限を２０℃/sとした。また、こ
のような粗大組織の出現は、該制御冷却を５００℃まで
実施すれば、その後の冷却条件によらずに抑制できる。
なお、鋼片厚さが比較的小さく、空冷によっても０．２
℃/s以上で冷却される場合には当然空冷でも構わない。The cooling after heating and holding is also performed at 0.2 to 20 ° C.
It is necessary to control the temperature to 500 ° C. or less at a cooling rate of / s.
Even if the heating temperature is specified as described above and the heated austenite grain size is reduced, it is necessary to refine the transformed structure by subsequent cooling. That is, since the steel slab is generally thick, the inside of the steel slab is often gradually cooled even if it is allowed to cool, in which case the transformed structure becomes coarse. Therefore, to reduce the size of the microstructure after cooling, it is necessary to use 0.1.
It is necessary to cool at 2 ° C / s or more. The higher the cooling rate is, the better the microstructure is. However, it is not realistic to make the cooling rate extremely large for a steel slab with a large thickness.・
Since the appearance of an extremely coarse structure harmful to the recrystallized structure can be suppressed, the upper limit is set to 20 ° C./s in the present invention. Further, the appearance of such a coarse structure can be suppressed regardless of the subsequent cooling conditions if the controlled cooling is performed up to 500 ° C.
Note that the billet thickness is relatively small, and 0.2
In the case of cooling at a temperature of ° C./s or more, air cooling may of course be used.

【００２７】このようにして鋼片の組織を微細化するこ
とで、以後の工程によって厚手材の表層部の超細粒化が
確実に達成される。表層部に必要厚み（板厚の１０〜３
３％）の超細粒層付与のためには、鋼片をＡｃ3 変態点
以上、１０５０℃以下の温度に加熱し、０．２〜２０℃
/sの冷却速度で５００℃以下まで冷却して鋼片組織の微
細化を図った後、さらに鋼片をＡｃ3 変態点以上、１１
５０℃以下の温度に加熱し、熱間圧延の開始前あるいは
熱間圧延の途中段階で、その段階での鋼片厚みの１０％
〜３３％に対応する、少なくとも２つの外表面の表層部
領域をＡｒ3 変態点超の温度から２〜４０℃/sの冷却速
度で冷却を開始し、Ａｒ3 変態点未満で冷却を停止して
復熱させることを１回以上経由させる過程で、最後の復
熱が終了するまでの間に、累積圧下率が２０〜８０％の
仕上げ圧延を行い、該圧延を（Ａｃ1 変態点−５０℃）
〜（Ａｃ3 変態点−５０℃）の温度で完了させる必要が
ある。By refining the structure of the steel slab in this manner, the ultrafine graining of the surface layer of the thick material is reliably achieved by the subsequent steps. The thickness required for the surface layer (10-3
3%), the steel slab is heated to a temperature of from the Ac 3 transformation point to 1050 ° C.
After cooling to 500 ° C. or less at a cooling rate of / s to reduce the structure of the steel slab, the steel slab is further cooled to an Ac 3 transformation point or higher and 11
Heat to a temperature of 50 ° C. or less, and before the start of hot rolling or in the middle of hot rolling, 10% of the billet thickness at that stage
Starting cooling at a cooling rate of 2 to 40 ° C./s from a temperature above the Ar 3 transformation point to at least two outer surface regions corresponding to ３３33%, and stopping the cooling below the Ar 3 transformation point. In the process of passing the heat recuperation one or more times, until the final reheat is completed, finish rolling is performed with a cumulative rolling reduction of 20 to 80%, and the rolling is performed (Ac 1 transformation point −50). ℃)
It is necessary to complete at a temperature of (Ac 3 transformation point −50 ° C.).

【００２８】即ち、鋼片を熱間圧延するに際し、熱間圧
延中あるいは熱間圧延途中で表層部の適当な厚みの領域
を水冷等の手段により、Ａｒ3 変態点よりも低い温度ま
で一旦冷却して内部と温度差を付けた後、温度差のつい
たままの状態からさらに熱間圧延を行うと、Ａｒ3 変態
点よりも低い温度まで一旦冷却された領域は、復熱及び
その過程の加工によりフェライト主体組織となるため、
該フェライト主体組織を有する表層部は、内部の顕熱に
より復熱されながら加工を受けることになり、この復熱
中の加工条件を適正化することにより、表層部のフェラ
イト結晶粒が顕著に細粒化する。従って、最終的な鋼材
における表層超細粒層の割合は、表層を一旦冷却した際
にＡｒ3 変態点未満まで低下した領域の割合とほぼ一致
することになる。That is, when hot rolling a steel slab, a region having an appropriate thickness in the surface layer portion is once cooled to a temperature lower than the Ar 3 transformation point by means such as water cooling during hot rolling or hot rolling. After a temperature difference between the inside and the temperature difference, if hot rolling is further performed from the state where the temperature difference is maintained, the region once cooled to a temperature lower than the Ar 3 transformation point is used for reheating and the process of the process. Since it becomes a ferrite-based structure by processing,
The surface layer having the ferrite-based structure undergoes processing while being recuperated by the internal sensible heat, and by optimizing the processing conditions during the recuperation, the ferrite crystal grains in the surface layer become extremely fine. Become Therefore, the ratio of the surface layer of the ultrafine grain layer in the final steel material substantially coincides with the ratio of the region which has been reduced to less than the Ar 3 transformation point when the surface layer is once cooled.

【００２９】本発明においては、該超細粒化のための上
記熱間圧延工程を、以下に示すような条件に限定するこ
とによって超細粒化が達成される。先ず、鋼片をオース
テナイト域に再加熱するが、この場合の温度としてはＡ
ｃ3 変態点以上、１１５０℃以下が好ましい。即ち、Ａ
ｃ3 変態点未満ではオーステナイト単相にならず、フェ
ライト相が残存し、該フェライト相が残存すると後の工
程の如何によらず、表層に均一な超細粒組織を形成する
ことができない。また、内部も二相域加工されるため、
鋼材の異方性が増大する問題も生じる。一方、再加熱温
度が高すぎると加熱オーステナイト粒径が粗大化して、
折角鋼片の粗大な凝固組織を前述した熱処理により微細
化しても、最終圧延前の変態組織が粗大化するため、好
ましくない。特に、厚手材の場合は鋼片が厚いために冷
却速度が遅い、累積圧下率が限定される等のために、超
細粒化及び中心部の靱性確保に不利な条件が多く、最終
圧延前の組織粗大化は避ける必要がある。そのため、再
加熱温度の上限は薄手材よりも低めの１１５０℃にする
ことが好ましい。即ち、本発明では鋼片の加熱温度をＡ
ｃ3 変態点〜１１５０℃に限定する。In the present invention, ultra-fine graining is achieved by limiting the hot rolling step for ultra-fine graining to the following conditions. First, the slab is reheated to the austenite region.
The temperature is preferably from the c 3 transformation point to 1150 ° C. That is, A
c not to single-phase austenite is less than 3 transformation point, ferrite phase remains, regardless of whether the process after the said ferrite phase remains, it is impossible to form a uniform ultrafine grain structure in the surface layer. Also, since the inside is also processed in the two-phase region,
There is also a problem that the anisotropy of the steel material increases. On the other hand, if the reheating temperature is too high, the heated austenite particle size becomes coarse,
Even if the coarse solidified structure of the bent steel slab is refined by the above-described heat treatment, the transformed structure before final rolling becomes coarse, which is not preferable. In particular, in the case of thick materials, the cooling rate is slow due to the thick steel slab, the cumulative draft is limited, etc., so there are many disadvantageous conditions for ultra-fine graining and securing toughness in the center, and before the final rolling It is necessary to avoid coarsening of the organization. Therefore, the upper limit of the reheating temperature is preferably set to 1150 ° C., which is lower than that of the thin material. That is, in the present invention, the heating temperature of the steel slab is set to A
c 3 be limited to the transformation point ~1150 ℃.

【００３０】最終板厚と鋼片厚みに応じて、工程負荷軽
減、表層超細粒層を得るための復熱後の必要圧下率の確
保の観点から判断して、鋼片を加熱後、鋼片ままか、粗
圧延により鋼片厚みを適当な厚みに減厚した後、該鋼材
の超細粒層とすべき表層部を水冷等の手段により冷却
し、該鋼材の水冷前の熱間圧延時点での板厚の１０〜３
３％に対応する各表層部の領域をＡｒ3 変態点未満まで
冷却するとともに、表層部と内部に温度差をつけるが、
その際、該鋼材の水冷前の熱間圧延時点での板厚の１０
〜３３％に対応する各表層部の領域の冷却速度は２℃/s
以上にする必要がある。これは冷却速度が２℃/s未満で
は冷却前の熱間圧延により、オーステナイトを微細化し
ておいても冷却後の変態組織が粗大となり、その後の復
熱中の圧延で均一な超微細フェライト組織を得ることが
困難となるためである。冷却速度は大きい方が組織微細
化の観点からは好ましいが、４０℃/sを超えて急冷して
も効果が飽和する上に、不必要に急冷することは鋼板の
形状維持のためには好ましくないため、上限を４０℃/s
とする。Heating the steel slab according to the final sheet thickness and the thickness of the slab is determined from the viewpoint of reducing the process load and securing the required rolling reduction after reheating to obtain a superfine grain layer of the surface. After the thickness of the steel slab is reduced to an appropriate thickness by flakes or rough rolling, the surface layer portion of the steel material to be an ultrafine grain layer is cooled by means of water cooling or the like, and hot rolling of the steel material before water cooling is performed. 10-3 of the plate thickness at the time
While cooling the area of each surface layer corresponding to 3% to less than the Ar 3 transformation point, a temperature difference is created between the surface layer and the inside.
At this time, the steel sheet had a thickness of 10 at the time of hot rolling before water cooling.
Cooling rate of each surface layer area corresponding to ~ 33% is 2 ° C / s
It is necessary to do above. This is because when the cooling rate is less than 2 ° C / s, the transformation structure after cooling becomes coarse even if the austenite is refined by hot rolling before cooling, and a uniform ultrafine ferrite structure is obtained by rolling during subsequent recuperation. It is because it becomes difficult to obtain. A higher cooling rate is preferable from the viewpoint of microstructural refinement, but the effect is saturated even if quenching exceeds 40 ° C./s, and unnecessary quenching is preferable for maintaining the shape of the steel sheet. No upper limit of 40 ° C / s
And

【００３１】なお、Ａｒ3 変態点未満まで冷却する表層
部の厚みを、該鋼材の水冷前の熱間圧延時点での板厚の
１０〜３３％とするのは、超細粒層となるのがＡｒ3 変
態点未満まで冷却された領域であるため、前述の通り、
鋼材のアレスト性を発揮させるために必要かつ十分な超
細粒層の厚みである、最終板厚の１０〜３３％となる表
層超細粒層を付与するためである。The reason why the thickness of the surface layer cooled to less than the Ar 3 transformation point is 10 to 33% of the sheet thickness at the time of hot rolling before the water cooling of the steel material becomes an ultrafine grain layer. Is a region cooled to less than the Ar 3 transformation point,
This is for providing a surface superfine grain layer which is 10 to 33% of the final sheet thickness, which is a necessary and sufficient thickness of the ultrafine grain layer for exhibiting the arrestability of the steel material.

【００３２】また、上記の冷却はＡｒ3 変態点超から開
始する。これは、単相オーステナイトから冷却すること
で表層超細粒層を均一に形成させるためである。即ち、
該表層部が強制冷却前にＡｒ3 変態点以下になると、フ
ェライトが一部粗大に生成し、その部分での超細粒化が
阻害されるためである。The above cooling is started from the Ar 3 transformation point. This is because the surface ultrafine grain layer is formed uniformly by cooling from single-phase austenite. That is,
If the surface layer portion becomes lower than the Ar 3 transformation point before forced cooling, ferrite is partially coarsely formed, and ultrafine graining at that portion is hindered.

【００３３】表層超細粒層付与のための冷却前に必要に
応じて行う鋼片厚み調整のための粗圧延の条件は特に規
定するものではないが、内部組織の微細化のためには、
オーステナイトの未再結晶域での圧延を行う方が有利で
ある。ただし、オーステナイトの未再結晶域での圧延は
必然的に低温圧延となり、生産性の低下、表層部の復熱
のための内部の顕熱の減少等の悪影響も生ずる。生産性
の極端な低下を招かず、表層超細粒層の形成に不利にな
らない条件として、オーステナイトの未再結晶域での圧
延を行う場合の累積圧下率は５０％以下が好ましい。The conditions of the rough rolling for adjusting the thickness of the slab, which is performed as necessary before cooling for providing the superfine grain layer, are not particularly specified.
It is advantageous to perform rolling in the austenite unrecrystallized region. However, rolling in the non-recrystallized region of austenite necessarily involves low-temperature rolling, and adverse effects such as a decrease in productivity and a decrease in internal sensible heat due to reheating of the surface layer also occur. As a condition that does not cause an extreme decrease in productivity and is not disadvantageous for the formation of the superfine grain layer, the rolling reduction in the austenite unrecrystallized region is preferably 50% or less.

【００３４】以上の理由により、該鋼材の冷却前の熱間
圧延時点での板厚の１０〜３３％に対応する各表層部の
領域を２℃/s〜４０℃/sの冷却速度でＡｒ3 変態点未満
まで冷却し、その後仕上げ圧延を行う際、内部の顕熱に
よるか、及び／または外部からの加熱を利用して板厚の
１０〜３３％に対応する各表層部の領域を昇温中に圧延
を施すことにより該領域の組織が超微細化し、脆性亀裂
伝播停止特性向上に寄与できるようになる。For the above reasons, the area of each surface layer, which corresponds to 10 to 33% of the sheet thickness at the time of hot rolling before cooling the steel, is cooled at a cooling rate of 2 ° C./s to 40 ° C./s by Ar. When cooled to less than 3 transformation point and then finish rolling, the surface area corresponding to 10 to 33% of the sheet thickness is raised by internal sensible heat and / or by using external heating. By performing rolling in a warm state, the structure in the region becomes ultrafine, and it is possible to contribute to improvement in brittle crack propagation stopping characteristics.

【００３５】後述するように、上記復熱過程の加工は１
回もしくは２回以上繰り返してもよいが、最後の冷却後
の復熱過程での圧延後の復熱温度は（Ａｃ1 変態点−５
０℃）〜（Ａｃ3 変態点−５０℃）の範囲にする必要が
ある。即ち、該最終復熱温度が（Ａｃ1 変態点−５０
℃）よりも低いと、加工後の加工フェライトの回復・再
結晶が十分でないため、超細粒化が不十分で、脆性き裂
伝播停止特性が向上しない。一方、該最終復熱温度が
（Ａｃ3 変態点−５０℃）よりも高いと、加工により超
細粒化したフェライトの一部が再度オーステナイトに逆
変態することによって消失してしまい、その割合が無視
できないほど多くなるため、また、超細粒化したフェラ
イトが粒成長により粗大化するため、靭性及び脆性き裂
伝播停止特性を損なう。従って、本発明においては、最
後の冷却後の復熱過程での圧延後の復熱温度は、（Ａｃ
1 変態点−５０℃）〜（Ａｃ3 変態点−５０℃）の範囲
に限定する。As will be described later, the processing in the recuperation process is one
Or two or more times, but the recuperation temperature after rolling in the recuperation process after the last cooling is (Ac 1 transformation point −5).
0 ° C.) to (Ac 3 transformation point−50 ° C.). That is, the final reheat temperature is (Ac 1 transformation point −50).
If the temperature is lower than (° C.), recovery and recrystallization of the processed ferrite after processing is not sufficient, so that ultrafine graining is insufficient and brittle crack propagation stopping characteristics are not improved. On the other hand, if the final recuperation temperature is higher than (Ac 3 transformation point −50 ° C.), part of the ferrite ultrafine-grained by the working is transformed back into austenite again and disappears, and the proportion is reduced. Since it is not negligible, and the ultrafine-grained ferrite is coarsened by grain growth, the toughness and brittle crack propagation arresting properties are impaired. Therefore, in the present invention, the recuperation temperature after rolling in the recuperation process after the last cooling is (Ac
It is limited to the range of ( 1 transformation point -50 ° C) to (Ac 3 transformation point -50 ° C).

【００３６】以上の、Ａｒ3 変態点未満への冷却と復熱
中の加工工程は１回でも良いが、複数回繰り返すことに
より効果が重畳するため、２回以上繰り返しても所望の
微細組織を得ることが可能である。その場合、各復熱段
階の最高温度あるいは最低温度は任意であっても、本発
明の温度条件に従えば超細粒化する。即ち、最後の冷却
後の復熱過程での圧延後の復熱温度を（Ａｃ1 変態点−
５０℃）〜（Ａｃ3 変態点−５０℃）とすればよい。た
だし、好ましくは途中の復熱温度の上限はＡｃ3 変態点
以下とする方が細粒化の効果が確実に重畳する点で好ま
しい。The above-described processing steps during cooling to less than the Ar 3 transformation point and reheating may be performed once, but the effect is superimposed by repeating a plurality of times, so that a desired microstructure can be obtained even if the processing is repeated twice or more. It is possible. In this case, even if the maximum temperature or the minimum temperature in each recuperation stage is arbitrary, ultrafine granulation is performed according to the temperature conditions of the present invention. That is, the recuperation temperature after rolling in the recuperation process after the last cooling is represented by (Ac 1 transformation point−
(50 ° C.) to (Ac 3 transformation point−50 ° C.). However, it is preferable to set the upper limit of the reheating temperature in the middle to the Ac 3 transformation point or lower, since the effect of the grain refining is surely superimposed.

【００３７】最初の冷却後から最後の復熱に至るまでの
圧延としての熱間圧延の累積圧下率は、大きい方が均一
かつ安定に超細粒組織を得られる。厚手材の場合には、
粗大な凝固組織を有する鋼片を用いると、該圧延に入る
前の組織が粗大となって超細粒化のために必要な累積圧
下率を大きくする必要がある一方で、厚手材故に累積圧
下率を大きくすることには限度があるため、鋼片厚や鋼
種によっては超細粒化が困難となる場合が多くなるが、
本発明のように、鋼片組織を適正条件により微細化して
おけば、薄手材と同程度の２０％以上の累積圧下率であ
れば超細粒化が可能となる。累積圧下率は大きいほど超
細粒化には有利であるが、圧下率が８０％を超えるよう
な圧延は効果が飽和し、生産性を極端に阻害するため好
ましくなく、また、厚手材では実質的にこのような過大
な圧下率の圧延を加えることができない場合が多い。従
って、本発明では該熱間圧延の累積圧下率は２０〜８０
％に限定する。The larger the cumulative rolling reduction of the hot rolling from the first cooling to the last reheating, the more uniform and stable the ultrafine grain structure can be obtained. For thick materials,
When a slab having a coarse solidified structure is used, the structure before entering the rolling becomes coarse, and it is necessary to increase the cumulative rolling reduction required for ultra-fine graining. Since there is a limit to increasing the rate, it is often difficult to make ultra-fine grains depending on the billet thickness and steel type,
As in the present invention, if the microstructure of the steel slab is refined under appropriate conditions, ultra-fine graining can be achieved with a cumulative reduction ratio of 20% or more, which is about the same as that of a thin material. The higher the cumulative rolling reduction is, the more advantageous for ultra-fine graining. However, rolling with a rolling reduction exceeding 80% saturates the effect and extremely impairs productivity, which is not preferable. In many cases, it is not possible to apply such an excessive rolling reduction. Therefore, in the present invention, the cumulative rolling reduction of the hot rolling is 20 to 80.
%.

【００３８】次いで、請求項２に示す方法は、請求項１
に示す方法と同様、鋼片の粗大な凝固組織を微細化する
ことにより、厚手材の表層部の超細粒化を達成する別の
方法を示したものである。従って、鋼片の組織微細化工
程以外は請求項１の方法と同一である。Next, the method according to claim 2 is based on claim 1.
This shows another method of achieving ultra-fine graining of the surface layer portion of a thick material by refining a coarse solidified structure of a slab similarly to the method shown in FIG. Therefore, the steps other than the step of refining the structure of the steel slab are the same as the method of the first aspect.

【００３９】請求項２における鋼片組織微細化のための
要件は、「本発明を満足する適正な化学組成を有する鋼
片をＡｃ3 変態点以上、１１５０℃以下の温度に加熱し
た後、累積圧下率が２０〜５０％の予備熱間圧延を行
い、０．２〜２０℃/sの冷却速度で５００℃以下まで冷
却すること」にある。即ち、請求項１の方法では熱処理
により組織微細化を図るが、請求項２の方法では予備熱
間圧延により鋼片組織を微細化する。予備熱間圧延を行
う方が組織微細化には有利であるが、鋼片厚みを減ずる
ことにもなるため、鋼片厚みが仕上げ板厚に対して厚
く、累積圧下率の裕度が高い場合に好ましい方法とな
る。The requirement for refining the structure of the steel slab according to claim 2 is that a steel slab having an appropriate chemical composition that satisfies the present invention is heated to a temperature from the Ac 3 transformation point to 1150 ° C. Preliminary hot rolling at a rolling reduction of 20 to 50%, and cooling to 500 ° C or lower at a cooling rate of 0.2 to 20 ° C / s ”. That is, in the method of claim 1, the structure is refined by heat treatment, but in the method of claim 2, the steel slab structure is refined by preliminary hot rolling. Pre-hot rolling is more advantageous for refining the structure, but it also reduces the thickness of the slab, so the slab thickness is thicker than the finished plate thickness and the tolerance of the cumulative draft is high. This is a preferred method.

【００４０】該方法において、先ず鋼片をＡｃ3 変態点
以上、１１５０℃以下の温度に再加熱する。Ａｃ3 変態
点以上に再加熱するのは請求項１と同じ理由であり、一
方、後の圧延がオーステナイトの細粒化効果を有するた
め、再加熱温度の上限は圧延のない場合に比べて緩和さ
れるが、再加熱オーステナイト粒径が過大であると、後
の圧延によっても細粒化が不十分となるため、本発明で
は上限を１１５０℃とする。In the method, the steel slab is first reheated to a temperature not lower than the Ac 3 transformation point and not higher than 1150 ° C. The reason for reheating above the Ac 3 transformation point is the same as in claim 1. On the other hand, since the subsequent rolling has an austenite grain refinement effect, the upper limit of the reheating temperature is relaxed as compared with the case without rolling. However, if the reheated austenite particle size is excessively large, the grain refinement becomes insufficient even in the subsequent rolling, so the upper limit is set to 1150 ° C. in the present invention.

【００４１】Ａｃ3 変態点以上、１１５０℃以下の温度
に再加熱した後、オーステナイト組織微細化のために熱
間圧延を行うが、その際、該熱間圧延の累積圧下率を２
０〜５０％とする。これはＡｃ3 変態点〜１１５０℃の
再加熱に続く圧延の場合に、累積圧下率が２０％未満で
あると、折角圧延を行ってもオーステナイト粒の微細化
とその結果としての変態組織の微細化が不十分であり、
一方、５０％超では、最終的なフェライトに加工を加え
る復熱段階での圧延段階での累積圧下率が十分とれなく
なるためである。熱間圧延後の冷却は請求項１と同じ
で、変態組織を十分微細化するための条件として、０．
２〜２０℃/sの冷却速度で５００℃以下まで冷却する。After reheating to a temperature not lower than the Ac 3 transformation point and not higher than 1150 ° C., hot rolling is carried out to refine the austenite structure.
0% to 50%. This is because, in the case of rolling subsequent to reheating from the Ac 3 transformation point to 1150 ° C., if the cumulative rolling reduction is less than 20%, the austenite grains are refined and the resulting transformed microstructure is refined even when angle rolling is performed. Is not enough,
On the other hand, if it exceeds 50%, the cumulative rolling reduction in the rolling stage in the reheating stage in which the final ferrite is worked cannot be sufficiently obtained. The cooling after the hot rolling is the same as that in claim 1, and the condition for sufficiently refining the transformed structure is 0.1 mm.
Cool to 500 ° C. or less at a cooling rate of 2 to 20 ° C./s.

【００４２】以上が、厚手材の表層部の超細粒化を達成
するための、冷却・復熱工程を含む熱間圧延工程前の
鋼片を予め熱処理あるいは熱間圧延を施すことで、鋼片
の粗大な凝固組織を微細化することで二相域〜フェライ
ト域加工前の組織を微細化する、繰り返しの冷却・復
熱工程において、冷却・復熱中のオーステナイト／フェ
ライト変態を利用して、二相域〜フェライト域加工前の
組織を微細化する、の２種類の方法のうちのの基本要
件を示したものである。次にの方法に関する詳細な説
明を以下に示す。As described above, the steel slab before the hot rolling step including the cooling and recuperating steps is subjected to heat treatment or hot rolling in advance to achieve ultra-fine graining of the surface layer of the thick material, In the repetitive cooling and reheating process, the austenite / ferrite transformation during cooling and reheating is used in the repetitive cooling and reheating process, in which the coarse solidification structure of the piece is refined to refine the structure before processing in the two-phase region to ferrite region. This shows the basic requirements of the two types of methods, namely, the two-phase region to the finer structure before the ferrite region processing. A detailed description of the following method is provided below.

【００４３】表層部の冷却・復熱過程において、冷却停
止温度と復熱温度とを規定し、フェライトを加工・再結
晶させることで表層部の一定厚領域を超細粒化させる。
具体的には、熱間圧延の開始前あるいは熱間圧延の途中
段階で、その段階での鋼片厚みの１０％〜３３％に対応
する少なくとも２つの外表面の表層部領域を、Ａｒ3変
態点超の温度から２〜４０℃/sの冷却速度で冷却を開始
し、Ａｒ3 変態点未満で冷却を停止して復熱させること
を２回以上経由させる過程で所定の累積圧下率の圧延を
加える。請求項１、２の方法では、前述したように、鋼
片の前組織微細化を前提とすれば、最終的な仕上げ温度
が該圧延を（Ａｃ1 変態点−５０℃）〜（Ａｃ3 変態点
−５０℃）の温度であれば超細粒化が達成される。ただ
し、前組織の工程が増える分、生産性は若干低下する。In the process of cooling and reheating the surface layer portion, the cooling stop temperature and the reheating temperature are specified, and the ferrite is worked and recrystallized to make the constant thickness region of the surface layer portion ultrafine.
Specifically, before the start of hot rolling or in the middle of hot rolling, at least two surface layer regions corresponding to 10% to 33% of the thickness of the slab at that stage are subjected to Ar 3 transformation. Rolling at a predetermined cumulative draft in the process of starting cooling at a cooling rate of 2 to 40 ° C./s from a temperature above the point, stopping cooling at a temperature lower than the Ar 3 transformation point, and performing recuperation at least twice. Add. According to the method of claims 1 and 2, as described above, assuming that the microstructure of the steel slab is refined, the final finishing temperature is determined by changing the rolling from (Ac 1 transformation point -50 ° C.) to (Ac 3 transformation). At a temperature of (point -50 ° C), ultrafine graining is achieved. However, as the number of processes in the previous organization increases, productivity slightly decreases.

【００４４】請求項３の方法は鋼片組織の微細化工程を
行わずに厚手材の表層部の超細粒化を図るための方法で
あって、その要件は、「本発明を満足する適正な化学組
成を有する鋼片をＡｃ3 変態点以上、１１５０℃以下の
温度に加熱した後、熱間圧延の開始前あるいは熱間圧延
の途中段階で、その段階での鋼片厚みの１０％〜３３％
に対応する少なくとも２つの外表面の表層部領域をＡｒ
3 変態点超の温度から２〜４０℃/sの冷却速度で冷却を
開始し、Ａｒ3 変態点未満で冷却を停止して復熱させる
ことを２回以上経由させる過程で、最初の冷却・復熱過
程においては、復熱温度をＡｃ3 変態点〜（Ａｃ3 変態
点＋１００℃）の範囲とし、その後の冷却・復熱過程に
おいては復熱温度を（Ａｃ3 変態点−２０℃）以下と
し、最後の復熱が終了するまでの間に累積圧下率が２０
〜８０％の仕上げ圧延を行い、該圧延を（Ａｃ1 変態点
−５０℃）〜（Ａｃ3 変態点−５０℃）の温度で完了さ
せること」にある。The method of claim 3 is a method for achieving ultra-fine graining of the surface layer portion of a thick material without performing a step of refining a steel slab structure. After heating a slab having a high chemical composition to a temperature of not less than the Ac 3 transformation point and 1150 ° C. or less, before the start of hot rolling or in the middle of hot rolling, 10% to 10% of the thickness of the slab at that stage. 33%
At least two surface layer regions on the outer surface corresponding to
In the process of starting cooling at a cooling rate of 2 to 40 ° C./s from a temperature exceeding the 3 transformation point, stopping cooling at a temperature lower than the Ar 3 transformation point, and performing recuperation twice or more, the first cooling In the recuperation process, the recuperation temperature is set in the range from the Ac 3 transformation point to (Ac 3 transformation point + 100 ° C.), and in the subsequent cooling and recuperation process, the recuperation temperature is not more than (Ac 3 transformation point −20 ° C.). And the cumulative rolling reduction is 20 until the last reheating is completed.
仕 上 げ 80% finish rolling, and complete the rolling at a temperature of (Ac 1 transformation point -50 ° C.) to (Ac 3 transformation point -50 ° C.) ”.

【００４５】即ち、請求項３の方法は、「最初の冷却・
復熱過程においては、復熱温度をＡｃ3 変態点〜（Ａｃ
3 変態点＋１００℃）の範囲とし、その後の冷却・復熱
過程においては復熱温度を（Ａｃ3 変態点−２０℃）以
下とし、最後の復熱が終了するまでの間に累積圧下率が
２０〜８０％の仕上げ圧延を行う」ことが請求項１、２
と異なり、該工程によって粗大な凝固組織を有する鋼片
を用いても、厚手材において表層部に超細粒層を形成さ
せることが可能となる。That is, the method according to claim 3 is characterized in that "the first cooling
In the recuperation process, the recuperation temperature is changed from the Ac 3 transformation point to (Ac
3 Transformation point + 100 ° C), and in the subsequent cooling and reheating process, the recuperation temperature is set to (Ac 3 transformation point-20 ° C) or less, and the cumulative rolling reduction until the last recuperation is completed. 20-80% finish rolling ".
Unlike this, even if a steel slab having a coarse solidified structure is used by this step, it becomes possible to form an ultrafine grain layer on the surface layer of a thick material.

【００４６】本発明の請求項３の方法における最初の冷
却・復熱工程は、フェライト→オーステナイト→フェラ
イト変態を繰り返し行うことにより、加工前あるいは加
工中の組織を微細化させる目的の工程である。従って、
本発明の請求項３の方法における最初の冷却・復熱工程
においては、変態による組織微細化を確実する必要性か
ら、復熱温度をＡｃ3 変態点以上とする。また、該復熱
温度が（Ａｃ3 変態点＋１００℃）を超えるとオーステ
ナイト粒径の粗大化を招き、組織微細化が不十分となる
ことから、上限を（Ａｃ3 変態点＋１００℃）とする。
なお、上述のように最初の冷却・復熱工程は組織微細化
を目的としているので、加工をともなっても、ともなわ
なくとも構わない。該冷却・復熱中に加工を行えば、そ
の後の加工前の組織微細化が進むが、一方でフェライト
に対する累積加工率が減少するため、効果は相殺され
る。また、二相域加工前組織微細化を目的とする、該Ａ
ｃ3変態点〜（Ａｃ3 変態点＋１００℃）の範囲への復
熱工程は、繰り返しても構わないものではあるが、その
効果は加算的でなく、２回目以降は効果が飽和してしま
う一方で、超細粒化に最も重要なフェライトへの加工段
階での加工温度確保が容易でなかったり、内部の温度低
下が著しくなって、材質の劣化や形状の圧下を招く可能
性が高くなるため、本発明においては該冷却−復熱工程
は最初のみとした。従って、冶金的効果上、該冷却−復
熱工程を繰り返すこと自体は本発明の範疇を逸脱するも
のではない。The first cooling / reheating step in the method according to the third aspect of the present invention is a step for refining the structure before or during processing by repeatedly performing ferrite → austenite → ferrite transformation. Therefore,
In the first cooling / reheating step in the method according to the third aspect of the present invention, the reheating temperature is set to the Ac 3 transformation point or higher because of the necessity of ensuring the refinement of the structure by transformation. Further, if the reheat temperature exceeds (Ac 3 transformation point + 100 ° C.), the austenite grain size becomes coarse and the microstructure becomes insufficient, so the upper limit is set to (Ac 3 transformation point + 100 ° C.). .
Note that, as described above, since the first cooling / reheating step is aimed at miniaturization of the structure, it does not matter whether or not processing is performed. If processing is performed during the cooling and reheating, the structure becomes finer before the subsequent processing, but the effect is offset because the cumulative processing rate for ferrite decreases. In addition, the A
The reheating step to the range from the c 3 transformation point to the (Ac 3 transformation point + 100 ° C.) may be repeated, but the effect is not additive and the effect is saturated after the second time. On the other hand, it is not easy to secure the processing temperature at the stage of processing to ferrite, which is the most important for ultra-fine graining, or the internal temperature drops significantly, increasing the possibility of causing material degradation and shape reduction Therefore, in the present invention, the cooling and reheating step is performed only at the beginning. Therefore, in terms of metallurgical effects, repeating the cooling / reheating step itself does not depart from the scope of the present invention.

【００４７】また、請求項３に示す方法においては、鋼
片組織の微細化を考慮しない分、上記の最初の冷却・復
熱の温度を限定するとともに、その後の加工においても
さらに復熱温度が過大にならないよう注意すべきで、請
求項１、２と同程度のフェライトの超細粒化を得るため
には、復熱温度が（Ａｃ3 変態点−２０℃）を超えない
ようにする必要がある。その他の加工上の要件は請求項
１、２と同じであり、最後の復熱が終了するまでの間
に、累積圧下率が２０〜８０％の熱間圧延を行うことに
より、厚手材においても表層部に均一な超細粒層を安定
して形成することが可能となる。Further, in the method according to the third aspect, the temperature of the first cooling and reheating is limited because the miniaturization of the billet structure is not considered, and the reheating temperature is further increased in the subsequent processing. Care must be taken not to make the temperature excessive, and in order to obtain ultra-fine ferrite grains equivalent to those of the first and second aspects, it is necessary that the reheat temperature does not exceed (Ac 3 transformation point −20 ° C.). There is. Other processing requirements are the same as those in claims 1 and 2. By performing hot rolling with a cumulative reduction ratio of 20 to 80% until the last reheating is completed, even in thick materials. It is possible to stably form a uniform ultrafine grain layer on the surface layer.

【００４８】請求項４及び５の方法は、請求項１、２に
示す鋼片組織微細化のための方法と請求項３に示す冷却
・復熱中の組織微細化の方法とを組み合わせたものであ
る。各々加算的な効果が期待されるため、方法について
の限定条件は今までに述べた請求項１〜３の方法におけ
るものと同様であり、また、その超細粒化の達成度、均
一度とも請求項１〜３の方法と比べて一段と優れてお
り、さらなる靱性、アレスト性向上が期待できる。The method according to claims 4 and 5 is a combination of the method for refining the structure of the steel slab according to claims 1 and 2 and the method for refining the structure during cooling and reheating as set forth in claim 3. is there. Since each additive effect is expected, the limiting conditions for the method are the same as those in the methods of claims 1 to 3 described above. It is much better than the methods of claims 1 to 3, and further improvement in toughness and arrestability can be expected.

【００４９】以上が、本発明における基本要件について
の説明であるが、その他、製造方法に関する要件の限定
理由を以下に付け加える。即ち、鋼材の強度調整、靱性
向上を目的として、最終の圧延終了後の鋼材を２〜４０
℃/sの冷却速度で２０℃〜６００℃まで加速冷却するこ
とができる。加速冷却により内部の組織が微細化すると
ともに、第二相がより硬質なものへと変化することによ
る。この効果を発揮するためには冷却速度は、２℃/s以
上必要である。冷却速度は大きい方が組織微細化、硬質
相形成には有利であるが、厚手材では無制限に冷却速度
を大きくすることは実用上困難であることから、組織制
御に対する加速冷却効果が飽和しない範囲として、本発
明においてはその上限を４０℃/sとする。また、該加速
冷却は２０℃〜６００℃まで行うことが好ましい。即
ち、２０℃未満まで冷却しても組織制御に有効でなく、
加速冷却の停止温度が６００℃超では、組織微細化が十
分でなく、また硬質相が形成され難いために強度調整に
有効でなく、靱性が劣化する場合があるためである。ま
た、本発明で規定した加速冷却条件によれば、一旦形成
された表層部の超細粒組織の粒成長抑制にも有効であ
る。The above is a description of the basic requirements in the present invention. Other reasons for limiting the requirements relating to the manufacturing method will be added below. That is, for the purpose of adjusting the strength of the steel material and improving the toughness, the steel material after the final rolling is finished in 2 to 40.
Accelerated cooling from 20 ° C to 600 ° C at a cooling rate of ° C / s is possible. This is because the internal structure is refined by the accelerated cooling and the second phase changes to a harder one. In order to exhibit this effect, the cooling rate needs to be 2 ° C./s or more. A higher cooling rate is advantageous for microstructural refinement and hard phase formation, but it is practically difficult to increase the cooling rate unlimitedly for thick materials, so that the accelerated cooling effect on structure control is not saturated. In the present invention, the upper limit is set to 40 ° C./s. Preferably, the accelerated cooling is performed at a temperature of from 20C to 600C. That is, even if cooled to less than 20 ° C., it is not effective for tissue control,
If the stop temperature of the accelerated cooling is higher than 600 ° C., the structure is not sufficiently refined, and the hard phase is hardly formed, so that it is not effective for adjusting the strength and the toughness may be deteriorated. Further, according to the accelerated cooling conditions defined in the present invention, it is also effective in suppressing the grain growth of the ultrafine grain structure of the surface layer once formed.

【００５０】またさらに、強度調整、靭性向上、形状改
善の目的で、焼き戻し処理を施すことも可能である。そ
の場合には、表層部に形成された超細粒組織を損なわな
いことが必須要件となる。本発明では焼き戻し温度を、
４５０℃〜Ａｃ1 変態点に限定するが、これは、４５０
℃未満では焼き戻しの効果が明確ではなく、Ａｃ1 変態
点超では表層部の超細粒組織の形態を損なうためであ
る。ただし、超細粒層の粒成長抑制をより確実にするた
めには、焼戻し温度は７００℃を超えないことがより好
ましい。なお、本発明の焼き戻し温度範囲であれば、焼
き戻しの加熱保持時間は任意であるが、同様に表層部の
超細粒組織保存の観点からは、保持時間は５ｈ以内であ
ることが好ましい。Further, for the purpose of adjusting the strength, improving the toughness, and improving the shape, a tempering treatment can be performed. In this case, it is essential that the ultrafine grain structure formed on the surface layer is not damaged. In the present invention, the tempering temperature is
Limited to 450 ° C. to the Ac 1 transformation point,
If the temperature is lower than ° C, the effect of tempering is not clear, and if the temperature exceeds the Ac 1 transformation point, the form of the ultrafine grain structure in the surface layer is impaired. However, in order to more reliably suppress the grain growth of the ultrafine grain layer, it is more preferable that the tempering temperature does not exceed 700 ° C. The heating and holding time of the tempering is arbitrary within the tempering temperature range of the present invention. Similarly, from the viewpoint of preserving the ultrafine grain structure of the surface layer, the holding time is preferably within 5 hours. .

【００５１】以上が、本発明の低温靱性に優れた高張力
鋼材の製造方法に関する要件であるが、該製造方法によ
り効果を発揮するためには、個々の化学成分についても
下記に述べる理由により、各々限定する必要がある。The above is the requirement relating to the method for producing a high-tensile steel material having excellent low-temperature toughness according to the present invention. In order to exhibit the effect by the production method, the individual chemical components are also required for the following reasons. Each must be limited.

【００５２】即ち、Ｃは鋼の強度を向上させる有効な成
分として含有するもので、０．０１％未満では構造用鋼
に必要な強度の確保が困難であるが、０．２０％を超え
る過剰の含有は母材及び溶接部の靭性や耐溶接割れ性を
低下させるので、０．０１〜０．２０％の範囲とした。That is, C is contained as an effective component for improving the strength of the steel. If the content is less than 0.01%, it is difficult to secure the strength required for structural steel, but the excess exceeds 0.20%. Content decreases the toughness and weld crack resistance of the base metal and the welded portion, so that the content was made 0.01 to 0.20%.

【００５３】次に、Ｓｉは脱酸元素として、また、母材
の強度確保に有効な元素であるが、０．０１％未満の含
有では脱酸が不十分となり、また強度確保に不利であ
る。逆に１．０％を超える過剰の含有は粗大な酸化物を
形成して延性や靭性の劣化を招く。そこで、Ｓｉの範囲
は０．０１〜１．０％とした。Next, Si is an element effective as a deoxidizing element and for ensuring the strength of the base material. However, if the content is less than 0.01%, deoxidation becomes insufficient and disadvantageous for securing the strength. . Conversely, an excessive content exceeding 1.0% forms a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is set to 0.01 to 1.0%.

【００５４】また、Ｍｎは母材の強度、靭性の確保に必
要な元素であり、最低限０．１％以上含有する必要があ
るが、過剰に含有すると、硬質相の生成や粒界脆化等に
より母材靱性や溶接部の靭性、さらに溶接割れ性など劣
化させるため、材質上許容できる範囲で上限を２．０％
とした。Further, Mn is an element necessary for securing the strength and toughness of the base material, and it is necessary to contain at least 0.1% or more. Degrades base metal toughness, weld toughness, weld cracking, etc. due to the above factors.
And

【００５５】Ａｌは脱酸、オーステナイト粒径の細粒化
等に有効な元素であるが、効果を発揮するためには０．
００１％以上含有する必要がある。一方、０．１％を超
えて過剰に含有すると、粗大な酸化物を形成して延性を
極端に劣化させるため、０．００１％〜０．１％の範囲
に限定する必要がある。Al is an element effective for deoxidation, reduction of austenite grain size, and the like.
001% or more must be contained. On the other hand, if it is contained in excess of 0.1%, a coarse oxide is formed and ductility is extremely deteriorated. Therefore, it is necessary to limit the content to the range of 0.001% to 0.1%.

【００５６】ＮはＡｌやＴｉと結びついてオーステナイ
ト粒微細化に有効に働くため、微量であれば機械的特性
に有効に働く。また、工業的に鋼中のＮを完全に除去す
ることは不可能であり、必要以上に低減することは製造
工程に過大な負荷をかけるため好ましくない。そのた
め、工業的に制御が可能で、製造工程への負荷が許容で
きる範囲として下限を０．００１％とする。過剰に含有
すると、固溶Ｎが増加し、延性や靭性に悪影響を及ぼす
可能性があるため、許容できる範囲として上限を０．０
１０％とする。N is effective in refining austenite grains in combination with Al and Ti, so that a small amount of N effectively works on mechanical properties. Further, it is impossible to industrially completely remove N in steel, and it is not preferable to reduce N more than necessary because an excessive load is applied to a manufacturing process. Therefore, the lower limit is set to 0.001% as a range in which industrial control is possible and load on the manufacturing process can be tolerated. If it is contained excessively, the amount of dissolved N increases, which may adversely affect ductility and toughness.
10%.

【００５７】Ｐ、Ｓは不純物元素で、延性、靭性を劣化
させる元素であり、極力低減することが好ましいが、材
質劣化が大きくなく、許容できる量として、Ｐの上限を
０．０２５％，Ｓの上限を０．０１５％に限定する。P and S are impurity elements which deteriorate ductility and toughness, and are preferably reduced as much as possible. However, the deterioration of the material is not so large, and the upper limit of P is set to 0.025% and S is set as an allowable amount. Is limited to 0.015%.

【００５８】以上が本発明の鋼材の基本成分の限定理由
であるが、本発明においては、強度・靭性の調整のため
に、必要に応じて、Ｃｒ、Ｎｉ、Ｍｏ、Ｃｕ、Ｔｉ、
Ｖ、Ｎｂ、Ｚｒ、Ｔａ、Ｗ、Ｂの１種または２種以上を
含有することができる。The reasons for limiting the basic components of the steel material of the present invention have been described above. In the present invention, Cr, Ni, Mo, Cu, Ti,
One, two or more of V, Nb, Zr, Ta, W, and B can be contained.

【００５９】Ｃｒ及びＭｏはいずれも母材の強度向上に
有効な元素であるが、明瞭な効果を生じるためには０．
０１％以上必要であり、一方、１．０％を超えて添加す
ると、靭性及び溶接性が劣化する傾向を有するため、各
々０．０１〜１．０％の範囲とする。Although Cr and Mo are both effective elements for improving the strength of the base material, in order to produce a clear effect, the content of Cr is not limited to 0.1.
The content is required to be not less than 01%. On the other hand, if added in excess of 1.0%, the toughness and weldability tend to be deteriorated.

【００６０】また、Ｎｉは母材の強度と靭性を同時に向
上でき、非常に有効な元素であるが、効果を発揮させる
ためには０．０１％以上含有させる必要がある。含有量
が多くなると強度、靭性は向上するが３．０％を超えて
添加しても効果が飽和する一方で、溶接性が劣化するた
め、上限を３．０％とする。また、３．０％を超える添
加では焼入性が過大となるため、フェライトの生成が抑
制されるためにフェライトの超細粒化に好ましくない効
果も顕在化する。Ni is a very effective element that can simultaneously improve the strength and toughness of the base material, but must be contained at 0.01% or more in order to exert its effect. When the content is increased, the strength and toughness are improved, but if the content exceeds 3.0%, the effect is saturated, but the weldability is deteriorated. Therefore, the upper limit is set to 3.0%. Further, if the addition exceeds 3.0%, the hardenability becomes excessive, and the formation of ferrite is suppressed, so that an unfavorable effect on ultrafine graining of ferrite also becomes apparent.

【００６１】次に、ＣｕもほぼＮｉと同様の効果を有す
るが、１．５％超では熱間加工性に問題を生じるため、
０．０１〜１．５％の範囲に限定する。Next, Cu has almost the same effect as Ni, but if it exceeds 1.5%, there is a problem in hot workability.
It is limited to the range of 0.01 to 1.5%.

【００６２】Ｔｉは析出強化により母材強度向上に寄与
するとともに、ＴｉＮの形成により加熱オーステナイト
粒径微細化にも有効な元素であり、靭性向上にも有効な
元素であるが、効果を発揮するためには０．００３％以
上の含有が必要である。一方、０．１０％を超えると、
粗大な析出物、介在物を形成して靭性や延性を劣化させ
るため、上限を０．１０％とする。Ti is an element that contributes to the improvement of the base metal strength by precipitation strengthening and is also effective in reducing the austenite grain size by forming TiN, and is also effective in improving the toughness. For this purpose, a content of 0.003% or more is required. On the other hand, if it exceeds 0.10%,
The upper limit is 0.10% because coarse precipitates and inclusions are formed to deteriorate toughness and ductility.

【００６３】ＶもＶＮを形成して強度向上に有効な元素
であるが、過剰の含有では析出脆化により靭性が劣化す
る。従って、靭性の大きな劣化を招かずに、効果を発揮
できる範囲として、０．００５〜０．５０％の範囲に限
定する。V is also an element effective for improving the strength by forming VN, but if it is contained excessively, the toughness is deteriorated due to precipitation embrittlement. Therefore, the range in which the effect can be exerted without causing significant deterioration in toughness is limited to the range of 0.005 to 0.50%.

【００６４】ＮｂはＮｂ（Ｃ、Ｎ）を形成することで強
度・靭性の向上に有効な元素であるが、過剰の含有では
析出脆化により靭性が劣化する。従って、靭性の劣化を
招かずに、効果を発揮できる範囲として、０．００３〜
０．１０％の範囲に限定する。Nb is an element effective for improving strength and toughness by forming Nb (C, N). However, if Nb is excessively contained, toughness deteriorates due to precipitation embrittlement. Therefore, the range in which the effect can be exhibited without causing the deterioration of toughness is 0.003 to 0.003.
It is limited to the range of 0.10%.

【００６５】Ｚｒも窒化物を形成する元素であり、Ｔｉ
と同様の効果を有するが、その効果を発揮するためには
０．００３％以上の含有が必要である。一方、０．１０
％を超えると、Ｔｉと同様、粗大な析出物、介在物を形
成して靭性や延性を劣化させるため、０．００３〜０．
１０％の範囲に限定する。Zr is also an element forming nitride, and Ti
Has the same effect as described above, but in order to exhibit the effect, the content of 0.003% or more is required. On the other hand, 0.10
%, Coarse precipitates and inclusions are formed to deteriorate toughness and ductility, similarly to Ti.
Limit to 10% range.

【００６６】Ｔａも強度・靭性の向上に有効な元素であ
るが、効果を発揮するためには０．００５％以上の含有
が必要である。一方、０．２０％を超えると、析出脆化
や粗大な析出物、介在物による靭性劣化を生じるため、
上限を０．２０％とする。Ta is also an element effective for improving the strength and toughness, but it is necessary to contain 0.005% or more in order to exhibit the effect. On the other hand, if it exceeds 0.20%, precipitation embrittlement, coarse precipitates, and toughness degradation due to inclusions occur,
The upper limit is set to 0.20%.

【００６７】Ｗは固溶強化及び析出強化により母材強度
の上昇に有効であるが、効果を発揮するためには０．０
１％以上必要である。一方、２．０％を超えて過剰に含
有すると、靭性劣化が顕著となるため、上限を２．０％
とする。W is effective in increasing the strength of the base material by solid solution strengthening and precipitation strengthening.
1% or more is required. On the other hand, if the content exceeds 2.0%, the toughness is remarkably deteriorated.
And

【００６８】Ｂは微量で確実にＮと結びつくため、固溶
Ｎ固定により靭性向上や、焼入性向上による強度・靭性
向上に有効な元素であるが、効果を発揮するためには
０．０００３％以上必要である。一方、０．００２０％
を超えて過剰に含有するとＢＮが粗大となり、延性や靭
性に悪影響を及ぼす。また溶接性も劣化させるため、上
限を０．００２０％とする。B is an element effective for improving toughness by solid solution N fixation and for improving strength and toughness by improving hardenability, since it is surely linked with N in a trace amount. % Is required. On the other hand, 0.0020%
If the content exceeds BN, the BN becomes coarse and adversely affects ductility and toughness. Further, the weldability is also deteriorated, so the upper limit is made 0.0020%.

【００６９】さらに、延性の向上、継手靭性の向上のた
めに、必要に応じて、Ｍｇ、Ｃａ、ＲＥＭの１種または
２種以上を含有することができる。Further, in order to improve ductility and joint toughness, one or more of Mg, Ca and REM can be contained as necessary.

【００７０】Ｍｇ、Ｃａ、ＲＥＭはいずれも硫化物の熱
間圧延中の展伸を抑制して延性特性向上に有効である。
酸化物を微細化させて継手靭性の向上にも有効に働く。
その効果を発揮するための下限の含有量は、Ｍｇ及びＣ
ａは０．０００５％、ＲＥＭは０．００５％である。一
方、過剰に含有すると、硫化物や酸化物の粗大化を生
じ、延性、靭性の劣化を招くため、上限を各々、Ｍｇ、
Ｃａは０．０１％、ＲＥＭは０．１０％とする。Mg, Ca, and REM are all effective in suppressing ductility of sulfide during hot rolling and improving ductility.
It also works effectively to improve the joint toughness by making the oxide finer.
The lower limit contents for exhibiting the effect are Mg and C.
a is 0.0005% and REM is 0.005%. On the other hand, if contained excessively, sulfides and oxides are coarsened, and ductility and toughness are deteriorated.
Ca is 0.01% and REM is 0.10%.

【００７１】[0071]

【実施例】以上が、本発明の要件についての説明である
が、さらに、実施例に基づいて本発明の効果を示す。表
１に示す化学組成の供試鋼を用いて、表２、表３に示す
製造条件で板厚が５０mm以上の厚手鋼板を製造した。製
造した鋼板の、本発明の特徴となる表層超細粒層の厚さ
や該超細粒層のフェライト組織の特徴（該超細粒層の中
央部の平均粒径、混粒度、粒形態等）や機械的性質（強
度、２mmＶノッチシャルピー衝撃特性、ＥＳＳＯ特性）
の測定結果も合わせて表２、表３に示す。The above has been a description of the requirements of the present invention. The effects of the present invention will be further shown based on examples. Using test steels having the chemical compositions shown in Table 1, thick steel plates having a thickness of 50 mm or more were manufactured under the manufacturing conditions shown in Tables 2 and 3. The thickness of the surface superfine grain layer and the characteristics of the ferrite structure of the superfine grain layer, which are features of the present invention, of the manufactured steel sheet (average grain size, mixed grain size, grain morphology, etc. at the center of the ultrafine grain layer) And mechanical properties (strength, 2 mm V notch Charpy impact characteristics, ESSO characteristics)
Tables 2 and 3 also show the measurement results.

【００７２】表層部の組織観察、粒径の測定は、本発明
鋼及び比較例のうちのフェライト粒径の比較的粒径の微
細な鋼については、倍率１０００倍から５０００倍の走
査型電子顕微鏡（ＳＥＭ）組織写真に基づいて実施し、
粒径が粗い比較例については倍率５００倍の光学顕微鏡
組織写真に基づいて実施した。引張特性は、圧延方向に
直角な方向（Ｃ方向）の板厚中心部から丸棒引張試験片
を採取して実施した。２mmＶノッチシャルピー衝撃試験
は、引張試験板厚１０mmの標準試験片で行い、板厚中心
部に加えて、表層部の靱性評価のために、表面下１mm
（試験片中心部が鋼板表面下６mm）についても評価し
た。両位置ともＣ方向から試験片を採取し、破面遷移温
度（ vＴrS）で靱性を評価した。ＥＳＳＯ試験もＣ方向
で試験片を採取し、Ｋcaが４００kgf/mm1.5 となる温度
（ＴKca400）でアレスト性を評価した。The structure observation of the surface layer portion and the measurement of the grain size were carried out by using a scanning electron microscope with a magnification of 1000 to 5000 times for the steels of the present invention and comparative examples having a relatively small ferrite grain size. (SEM) Based on the micrograph,
The comparative example having a coarse particle diameter was carried out based on an optical microscope structure photograph at a magnification of 500 times. The tensile properties were measured by collecting a round bar tensile test specimen from the center of the sheet thickness in the direction (C direction) perpendicular to the rolling direction. The 2 mm V notch Charpy impact test was performed on a standard test piece having a thickness of 10 mm in a tensile test, and 1 mm below the surface to evaluate the toughness of the surface layer in addition to the center of the thickness.
(The center of the test piece was 6 mm below the surface of the steel sheet) was also evaluated. Specimens were sampled from both directions from the C direction, and the toughness was evaluated at the fracture surface transition temperature (vTrS). ESSO also tested specimens were taken at the C direction, Kca and rated the arrestability temperature (TKca400) to be 400 kgf / mm 1.5.

【００７３】なお、表２は、本発明の第１の方法、即
ち、“冷却・復熱工程を含む熱間圧延工程前の鋼片を予
め熱処理あるいは熱間圧延を施すことにより鋼片の粗大
な凝固組織を微細化することで二相域〜フェライト域加
工前の組織を微細化する”方法に関する本発明の方法に
従って製造した鋼板と、その比較例の製造方法、組織、
特性を示したものであり、表３は本発明の第２の方法、
即ち、“繰り返しの冷却・復熱工程において、冷却・復
熱中のオーステナイト／フェライト変態を利用して、二
相域〜フェライト域加工前の組織を微細化する”方法
と、本方法と第１の方法を組み合わせた方法に従って製
造した鋼板と、その比較例の製造方法、組織、特性を示
したものである。Table 2 shows that the first method of the present invention, that is, “coarse slabs are obtained by subjecting a slab to a heat treatment or hot rolling before a hot rolling step including a cooling / reheating step in advance. A steel sheet manufactured according to the method of the present invention relating to a method of “refining the microstructure before working in the two-phase region to the ferrite region by refining a solidified structure, and the manufacturing method, structure,
Table 3 shows the characteristics of the second method of the present invention.
In other words, a method of "using an austenite / ferrite transformation during cooling / reheating in a repetitive cooling / reheating process to refine the structure before working in the two-phase region to the ferrite region", the present method and the first method It shows a steel sheet manufactured according to a method obtained by combining the methods, and a manufacturing method, a structure, and characteristics of a comparative example.

【００７４】表２のうちの試験Ｎｏ．Ａ１〜Ａ９と表３
のうちの試験Ｎｏ．Ａ１０〜Ａ１８は本発明の化学組成
を有する鋼番１〜１２を用いて、本発明の製造方法によ
り製造した鋼板であり、厚手であるにもかかわらず、良
好な靱性、アレスト性が得られている。即ち本発明によ
れば、全厚でのＥＳＳＯ特性が、板厚が５０mmから１５
０mmの鋼板において、ＴKca400で−９５℃〜−１４０℃
と、極めて良好となり、鋼材の安全性が飛躍的に増加し
ている。Test No. 1 in Table 2 A1 to A9 and Table 3
Test No. A10 to A18 are steel sheets manufactured by the manufacturing method of the present invention using steel numbers 1 to 12 having the chemical composition of the present invention, and have good toughness and arrestability despite being thick. I have. That is, according to the present invention, the ESSO characteristic at the entire thickness is changed from 50 mm to 15 mm.
For 0mm steel plate, -95 ° C to -140 ° C with TKca400
, And the safety of the steel material has been dramatically increased.

【００７５】一方、表２、表３の結果から、本発明の範
囲を逸脱している試験Ｎｏ．Ｂ１〜Ｂ１４の鋼板は本発
明により製造された試験Ｎｏ．Ａ１〜Ａ１８の鋼板に比
べて、靱性、アレスト性が大幅に劣っていることが明ら
かである。On the other hand, based on the results of Tables 2 and 3, Test Nos. The steel sheets of Nos. B1 to B14 were the test Nos. Manufactured according to the present invention. It is clear that the toughness and arrestability are significantly inferior to those of the steel sheets A1 to A18.

【００７６】試験Ｎｏ．Ｂ１〜Ｂ４、及びＮｏ．Ｂ８〜
Ｂ１０は、化学組成が本発明を満足していないために、
製造方法は本発明を満足しているものの、十分な靱性、
アレスト性が達成できなかった例である。即ち、試験Ｎ
ｏ．Ｂ１は、表層部の超細粒層の粒径、厚さともに十分
であるが、Ｃ量が過剰なために、第二相による脆化が大
となり、特に板厚中心部の vＴrSとアレスト性が劣る。
試験Ｎｏ．Ｂ２は、Ｍｎ量が過剰なために全般的にシャ
ルピー特性が劣り、全厚でのアレスト性も不十分であ
る。試験Ｎｏ．Ｂ３は、合金元素量の化学組成におい
て、不純物であるＰ量が過剰なため、表層部、板厚中心
部ともシャルピー特性が本発明に比べて劣り、全厚での
アレスト性も不十分である。試験Ｎｏ．Ｂ４は、同様に
不純物としてのＳ量が過剰なために、特に板厚中心部の
靱性が顕著に劣化しており、その結果、全厚でのアレス
ト性も向上していない。Test No. B1 to B4, and No. B8 ~
B10 is because the chemical composition does not satisfy the present invention,
Although the production method satisfies the present invention, sufficient toughness,
This is an example in which arrestability could not be achieved. That is, test N
o. In B1, both the particle size and thickness of the ultrafine grain layer in the surface layer are sufficient, but embrittlement due to the second phase becomes large due to the excessive amount of C, and particularly vTrSS and arrestability in the central portion of the sheet thickness. Is inferior.
Test No. B2 has inferior Charpy properties as a whole due to an excessive amount of Mn, and has insufficient arrestability at all thicknesses. Test No. B3 has an excessive amount of P, which is an impurity, in the chemical composition of the alloying element, so that both the surface layer portion and the center portion of the plate thickness have inferior Charpy characteristics as compared with the present invention, and the arrestability at the entire thickness is insufficient. . Test No. Similarly, in B4, since the amount of S as an impurity is excessive, the toughness particularly in the central portion of the sheet thickness is significantly deteriorated, and as a result, the arrestability in the entire thickness is not improved.

【００７７】試験Ｎｏ．Ｂ８は、試験Ｎｏ．Ｂ１と同じ
Ｃ量の過剰な鋼片を用いた例で、製造方法をＮｏ．Ｂ１
と変えているが、Ｃ量が過剰によることの靱性劣化要因
は同様に働くために、やはり靱性、アレスト性は大幅に
劣る。試験Ｎｏ．Ｂ９は、Ｃｒ、Ｍｏの添加が過剰であ
るために、強度は高くなるが、靱性に好ましくない硬質
相の割合が増加するため、靱性、アレスト性が向上しな
い。試験Ｎｏ．Ｂ１０は、Ｃｒに加えてＮｉが過剰な化
学組成を有しているために、過剰なＣｒによる靱性劣化
が生じている。Ｎｉはマトリクスの靱性改善には有利で
あるが、焼入性が過剰となって、表層部の組織の一部が
超細粒化せず、伸長粒が混在して組織の均一さが損なわ
れているため、Ｎｉを多量に添加している割には靱性、
アレスト性の向上が認められない。Test No. B8 is the test No. In this example, an excess steel slab having the same C content as that of B1 was used. B1
However, the factor of toughness degradation due to excessive C content works in the same way, so that toughness and arrestability are also significantly inferior. Test No. In B9, the strength is increased due to excessive addition of Cr and Mo, but the ratio of a hard phase unfavorable in toughness increases, so that toughness and arrestability are not improved. Test No. In B10, since Ni has an excessive chemical composition in addition to Cr, the toughness is deteriorated due to excessive Cr. Although Ni is advantageous for improving the toughness of the matrix, the hardenability becomes excessive, and a part of the structure of the surface layer does not become ultrafine, and the elongated grains are mixed to deteriorate the uniformity of the structure. Therefore, although a large amount of Ni is added, the toughness,
No improvement in arrestability is observed.

【００７８】一方、試験Ｎｏ．Ｂ５〜Ｂ７、Ｂ１１〜Ｂ
１４は、化学組成は本発明を満足しているが、製造法が
本発明の範囲を逸脱しているために、表層超細粒層を有
する厚手鋼材としては、本発明により製造したものに比
べて劣っている例である。On the other hand, Test No. B5 to B7, B11 to B
14 shows that the chemical composition satisfies the present invention, but the production method deviates from the scope of the present invention. This is a poor example.

【００７９】試験Ｎｏ．Ｂ５は、“冷却・復熱工程を含
む熱間圧延工程前の鋼片を予め熱処理あるいは熱間圧延
を施すことにより鋼片の粗大な凝固組織を微細化するこ
とで二相域〜フェライト域加工前の組織を微細化する”
表層部超細粒層を有する厚保で鋼材を製造する方法にお
いて、最も重要な、鋼片組織微細化のための工程が施さ
れていないために、薄手材では十分超細粒化する製造方
法にも関わらず、板厚が７５mmの厚手材である故に、表
層部の超細粒層が若干混粒となり、十分な靱性、アレス
ト性が達成されていない。Test No. B5 is a process in which the steel slab before the hot rolling process including the cooling and recuperation process is subjected to heat treatment or hot rolling in advance to refine the coarse solidified structure of the steel slab, thereby forming a two-phase region to a ferrite region. Refine the previous organization ”
In the method of manufacturing steel material with a thickness layer having a superfine grain layer in the surface layer, the most important step is to perform a process for refining the billet structure. Nevertheless, since the material is a thick material having a thickness of 75 mm, the superfine grain layer in the surface layer is slightly mixed, and sufficient toughness and arrestability have not been achieved.

【００８０】試験Ｎｏ．Ｂ６は、前組織微細化処理は施
しているが、表層部の冷却−復熱工程での圧延の終了温
度が、Ａｃ3 変態点を超えており、高すぎるために、フ
ェライトの加工再結晶により形成された超細粒組織の大
部分がフェライトからオーステナイトへの逆変態で解消
されてしまい、超細粒化が達成されず、そのため、靱
性、アレスト性の改善が見られない。Test No. B6 is pre tissue and refining process performs cooling of the surface layer portion - end temperature of rolling in the recuperation process, exceeds the Ac 3 transformation point, in order too high, the ferrite processing recrystallization Most of the formed ultrafine-grained structure is eliminated by the reverse transformation from ferrite to austenite, and ultrafine-graining is not achieved, and therefore, improvement in toughness and arrestability is not observed.

【００８１】試験Ｎｏ．Ｂ７は、鋼片組織微細化工程は
実施しているものの、表層の急冷工程を全く含まないた
めに、本発明鋼に特徴的な内部と全く組織形態の異なる
表層組織層が形成されない。即ち、通常の熱間圧延と異
なることがないため、表層から内部にわたって、連続的
に通常程度のフェライト粒径の組織で構成されているた
めに、靱性、アレスト性も従来レベルに止まっている。Test No. In B7, although the step of refining the slab structure is performed, since the step of quenching the surface layer is not included at all, a surface layer having a structure different from the internal structure characteristic of the steel of the present invention is not formed. That is, since there is no difference from ordinary hot rolling, the structure is continuously formed from the surface layer to the inside with a ferrite grain size of a normal level, so that toughness and arrestability remain at conventional levels.

【００８２】試験Ｎｏ．Ｂ１１〜Ｂ１４は本発明のうち
の、“繰り返しの冷却・復熱工程において、冷却・復熱
中のオーステナイト／フェライト変態を利用して、二相
域〜フェライト域加工前の組織を微細化する”製造方法
に関連した比較例である。Test No. B11 to B14 of the present invention, "In the repetitive cooling / reheating process, utilizing the austenite / ferrite transformation during cooling / reheating, refining the structure before working in the two-phase region to the ferrite region" It is a comparative example related to the method.

【００８３】試験Ｎｏ．Ｂ１１は、鋼片の組織微細化を
目的とした最初の冷却−復熱工程における復熱温度が本
発明の上限である、Ａｃ3 変態点＋１００℃よりも高い
ために、表層組織の微細化が十分でなく、本発明鋼に比
べて、表層部のシャルピー衝撃特性、全厚のＥＳＳＯ特
性がともに若干劣る。Test No. In B11, since the recuperation temperature in the first cooling and recuperation step for the purpose of miniaturizing the structure of the steel slab is higher than the upper limit of the present invention, the Ac 3 transformation point + 100 ° C., the microstructure of the surface layer is reduced. It is not sufficient, and both the Charpy impact characteristics of the surface layer portion and the ESSO characteristics of the entire thickness are slightly inferior to those of the steel of the present invention.

【００８４】試験Ｎｏ．Ｂ１２は、逆に最初の復熱温度
が本発明の下限未満となっているために、やはり表層部
におけるフェライト加工前の組織微細化が不十分で、靱
性、アレスト性が本発明鋼に比べて若干劣化する。Test No. On the other hand, B12, on the contrary, has an initial reheat temperature lower than the lower limit of the present invention, so that the microstructure of the surface layer before the ferrite processing is still insufficient, and the toughness and arrestability are lower than those of the steel of the present invention. Degrades slightly.

【００８５】試験Ｎｏ．Ｂ１３は、冷却復熱工程での累
積圧下率が過少なため、その他の条件については本発明
の要件を満足しているものの、表層部の超細粒化が十分
でなく加工組織が回復していない伸長粒主体組織となて
いるため、靱性、アレスト性が顕著に劣っている。Test No. In B13, since the cumulative draft in the cooling and reheating step is too small, the other conditions satisfy the requirements of the present invention, but the superfine graining of the surface layer portion is not sufficient and the processed structure is recovered. Due to the non-elongated grain structure, toughness and arrestability are remarkably inferior.

【００８６】試験Ｎｏ．Ｂ１４は、冷却−復熱工程が１
回のみで、最初の加工前組織微細化を目的としたＡｃ3
変態点〜（Ａｃ3 変態点＋１００℃）の温度への復熱工
程のみとなっているため、本発明で最も重要な、フェラ
イトへの加工・再結晶による超細粒化工程が行われてお
らず、その結果、表層部の超細粒化が達成されていない
ため、飛躍的なアレスト性の向上は実現されていない。Test No. In B14, the cooling-recuperation step is 1
Ac 3 for the first time to refine the microstructure before processing
Since only the reheating step to the temperature of the transformation point to (Ac 3 transformation point + 100 ° C.) is performed, the ultra-fine graining step by processing and recrystallization into ferrite, which is the most important in the present invention, is performed. As a result, the superfine graining of the surface layer portion has not been achieved, so that a dramatic improvement in arrestability has not been realized.

【００８７】以上のように、本発明によれば、５０mm以
上の厚手鋼材においても均一な超細粒層を表層部に安定
して一定厚み付与することが可能となり、アレスト性が
飛躍的な厚手高張力鋼材の製造が可能になることは明白
である。As described above, according to the present invention, a uniform ultrafine grain layer can be stably provided to a surface layer at a constant thickness even in a thick steel material having a thickness of 50 mm or more. Obviously, the production of high strength steel is possible.

【００８８】[0088]

【表１】 [Table 1]

【００８９】[0089]

【表２】 [Table 2]

【００９０】[0090]

【表３】 [Table 3]

【００９１】[0091]

【表４】 [Table 4]

【００９２】[0092]

【表５】 [Table 5]

【００９３】[0093]

【表６】 [Table 6]

【００９４】[0094]

【表７】 [Table 7]

【００９５】[0095]

【表８】 [Table 8]

【００９６】[0096]

【表９】 [Table 9]

【００９７】[0097]

【発明の効果】本発明は、従来はＮｉの多量添加でしか
得られなかったようなレベルのアレスト性を板厚が５０
mm以上の厚手高張力鋼材にも付与することを可能にした
画期的な製造方法を提供するものであり、構造物の安全
性を飛躍的に高めることが可能な厚手鋼材を経済性、生
産性を損なうことなく製造できる手段として、産業上の
効果は極めて大きい。According to the present invention, the arrestability of a level which can be obtained only by adding a large amount of Ni in the past can be obtained at a plate thickness of 50%.
It provides a revolutionary manufacturing method that can be applied to thick high-tensile steel materials of mm or more, and enables economical production of thick steel materials that can dramatically improve the safety of structures. The industrial effect is extremely large as a means for producing without impairing the properties.

Claims (9)

【特許請求の範囲】[Claims] 【請求項１】 重量％で、 Ｃ ：０．０１〜０．２０％ Ｓｉ：０．０１〜１．０％ Ｍｎ：０．１〜２．０％ Ａｌ：０．００１〜０．１％ Ｎ ：０．００１〜０．０１０％を含有し、さらに不純
物として、 Ｐ ：０．０２５％以下 Ｓ ：０．０１５％以下を含有し、残部が鉄及び不可避
不純物からなる鋼片をＡｃ3 変態点以上、１０５０℃以
下の温度に加熱し、０．２〜２０℃/sの冷却速度で５０
０℃以下まで冷却した後、再度鋼片をＡｃ3 変態点以
上、１１５０℃以下の温度に加熱し、累積圧下率が２０
〜８０％の熱間圧延を行う間に、熱間圧延の開始前ある
いは熱間圧延の途中段階で、その段階での鋼片厚みの１
０％〜３３％に対応する少なくとも２つの外表面の表層
部領域をＡｒ3 変態点超の温度から２〜４０℃/sの冷却
速度で冷却し、Ａｒ3 変態点未満で冷却を停止して復熱
させることを１回以上繰り返し、該熱間圧延を（Ａｃ1
変態点−５０℃）〜（Ａｃ3 変態点−５０℃）の温度で
完了させることを特徴とする低温靭性に優れた厚手高張
力鋼材の製造方法。C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% N by weight% : containing 0.001 to 0.010%, as further impurities, P: 0.025% or less S: containing 0.015% or less, the billet balance of iron and inevitable impurities Ac 3 transformation point Above, heated to a temperature of 1050 ° C or less, and cooled at a cooling rate of 0.2 to 20 ° C / s.
After cooling to 0 ° C. or less, the slab is heated again to a temperature of from the Ac 3 transformation point to 1150 ° C., and the cumulative rolling reduction is 20%.
During the hot rolling of ８０80%, before the start of hot rolling or in the middle of hot rolling, the thickness of the slab at that stage is 1%.
At least the surface layer region of the two outer surfaces corresponding to 0% to 33% is cooled at a cooling rate of 2 to 40 ° C. / s from the temperature of the Ar 3 transformation point greater than stop the cooling at Ar less than 3 transformation point Reheating is repeated at least once, and the hot rolling is performed (Ac 1
A method for producing a thick high-tensile steel excellent in low-temperature toughness, which is completed at a temperature of (transformation point −50 ° C.) to (Ac 3 transformation point −50 ° C.). 【請求項２】 重量％で、 Ｃ ：０．０１〜０．２０％ Ｓｉ：０．０１〜１．０％ Ｍｎ：０．１〜２．０％ Ａｌ：０．００１〜０．１％ Ｎ ：０．００１〜０．０１０％を含有し、さらに、不
純物として、 Ｐ ：０．０２５％以下 Ｓ ：０．０１５％以下を含有し、残部が鉄及び不可避
不純物からなる鋼片を、Ａｃ3 変態点以上、１１５０℃
以下の温度に加熱した後、累積圧下率が２０〜５０％の
予備熱間圧延を行い、０．２〜２０℃/sの冷却速度で５
００℃以下まで冷却した後、再度鋼片をＡｃ3 変態点以
上、１１５０℃以下の温度に加熱し、累積圧下率が２０
〜８０％の熱間圧延を行う間に、熱間圧延の開始前ある
いは熱間圧延の途中段階で、その段階での鋼片厚みの１
０％〜３３％に対応する少なくとも２つの外表面の表層
部領域をＡｒ3変態点超の温度から２〜４０℃/sの冷却
速度で冷却し、Ａｒ3 変態点未満で冷却を停止して復熱
させることを１回以上繰り返し、該熱間圧延を（Ａｃ1
変態点−５０℃）〜（Ａｃ3 変態点−５０℃）の温度で
完了させることを特徴とする低温靭性に優れた厚手高張
力鋼材の製造方法。2. C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% N by weight% : containing from 0.001 to 0.010%, further, as an impurity, P: 0.025% or less S: containing 0.015% or less, the slab balance of iron and inevitable impurities, Ac 3 Above the transformation point, 1150 ° C
After heating to the following temperature, preliminary hot rolling is performed with a cumulative draft of 20 to 50%, and a cooling rate of 0.2 to 20 ° C./s is used.
After cooling to below 00 ° C., the slab is again heated to a temperature above the Ac 3 transformation point and below 1150 ° C. and the cumulative draft is 20
During the hot rolling of ８０80%, before the start of hot rolling or in the middle of hot rolling, the thickness of the slab at that stage is 1%.
At least the surface layer region of the two outer surfaces corresponding to 0% to 33% is cooled at a cooling rate of 2 to 40 ° C. / s from the temperature of the Ar 3 transformation point greater than stop the cooling at Ar less than 3 transformation point Reheating is repeated at least once, and the hot rolling is performed (Ac 1
A method for producing a thick high-tensile steel excellent in low-temperature toughness, which is completed at a temperature of (transformation point −50 ° C.) to (Ac 3 transformation point −50 ° C.). 【請求項３】 重量％で、 Ｃ ：０．０１〜０．２０％ Ｓｉ：０．０１〜１．０％ Ｍｎ：０．１〜２．０％ Ａｌ：０．００１〜０．１％ Ｎ ：０．００１〜０．０１０％を含有し、さらに、不
純物として、 Ｐ ：０．０２５％以下 Ｓ ：０．０１５％以下を含有し、残部が鉄及び不可避
不純物からなる鋼片をＡｃ3 変態点以上、１１５０℃以
下の温度に加熱した後、累積圧下率が２０〜８０％の熱
間圧延を行う間に、熱間圧延の開始前あるいは熱間圧延
の途中段階で、その段階での鋼片厚みの１０％〜３３％
に対応する少なくとも２つの外表面の表層部領域をＡｒ
3 変態点超の温度から２〜４０℃/sの冷却速度で冷却
し、Ａｒ3 変態点未満で冷却を停止して復熱させること
を２回以上繰り返すにおいて、最初の冷却・復熱過程に
おいては、復熱温度をＡｃ3 変態点〜（Ａｃ3 変態点＋
１００℃）の範囲とし、その後の冷却・復熱過程におい
ては復熱温度を（Ａｃ3 変態点−２０℃）以下とし、該
熱間圧延を（Ａｃ1 変態点−５０℃）〜（Ａｃ3 変態点
−５０℃）の温度で完了させることを特徴とする低温靭
性に優れた厚手高張力鋼材の製造方法。3. In% by weight, C: 0.01 to 0.20% Si: 0.01 to 1.0% Mn: 0.1 to 2.0% Al: 0.001 to 0.1% N : containing 0.001 to 0.010%, further, as an impurity, P: 0.025% or less S: containing 0.015% or less, the billet balance of iron and inevitable impurities Ac 3 transformation After heating to a temperature of not less than the point and 1150 ° C. or less, while performing the hot rolling with the cumulative rolling reduction of 20 to 80%, before starting the hot rolling or in the middle of the hot rolling, the steel at that stage 10% to 33% of piece thickness
At least two surface layer regions on the outer surface corresponding to
3 is cooled at a cooling rate of temperature from 2 to 40 ° C. / s transformation point than in that to recuperation by stopping cooling at Ar less than 3 transformation point two or more times, in the first cooling-recuperator process Means that the recuperation temperature is from the Ac 3 transformation point to (Ac 3 transformation point +
100 ° C.), and in the subsequent cooling and reheating process, the recuperation temperature is set to (Ac 3 transformation point −20 ° C.) or less, and the hot rolling is performed from (Ac 1 transformation point −50 ° C.) to (Ac 3 transformation point). (Transformation point −50 ° C.). A method for producing a thick high-tensile steel excellent in low-temperature toughness, characterized by being completed at a temperature of (transformation point −50 ° C.). 【請求項４】 前記熱間圧延の前に、鋼片をＡｃ3 変態
点以上、１０５０℃以下の温度に加熱し、０．２〜２０
℃/sの冷却速度で５００℃以下まで冷却することを特徴
とする、請求項３に記載の低温靭性に優れた厚手高張力
鋼材の製造方法。4. Prior to the hot rolling, the slab is heated to a temperature not lower than the Ac 3 transformation point and not higher than 1050 ° C.
4. The method for producing a thick high-tensile steel material having excellent low-temperature toughness according to claim 3, wherein cooling is performed at a cooling rate of 500C / s to 500C or lower. 【請求項５】 前記熱間圧延の前に、鋼片を、Ａｃ3 変
態点以上、１１５０℃以下の温度に加熱した後、累積圧
下率が２０〜５０％の予備熱間圧延を行い、０．２〜２
０℃/sの冷却速度で５００℃以下まで冷却することを特
徴とする、請求項３に記載の低温靭性に優れた厚手高張
力鋼材の製造方法。5. Before the hot rolling, the slab is heated to a temperature not lower than the Ac 3 transformation point and not higher than 1150 ° C., and then subjected to preliminary hot rolling at a cumulative rolling reduction of 20 to 50%. .2 to 2
The method for producing a thick high-tensile steel excellent in low-temperature toughness according to claim 3, wherein the steel is cooled to 500 ° C or less at a cooling rate of 0 ° C / s. 【請求項６】 最終の圧延終了後の鋼材を２〜４０℃/s
の冷却速度で２０℃〜６００℃まで加速冷却することを
特徴とする請求項１〜５のいずれか１項に記載の低温靭
性に優れた厚手高張力鋼材の製造方法。6. The steel material after the final rolling is finished at 2 to 40 ° C./s.
The method for producing a thick high-tensile steel material having excellent low-temperature toughness according to any one of claims 1 to 5, wherein accelerated cooling is performed at a cooling rate of 20 ° C to 600 ° C. 【請求項７】 ４５０℃以上、Ａｃ1 変態点以下で焼戻
しを行うことを特徴とする請求項１〜６のいずれか１項
に記載の低温靭性に優れた厚手高張力鋼材の製造方法。7. The method according to claim 1, wherein tempering is performed at a temperature of 450 ° C. or more and an Ac 1 transformation point or less. 【請求項８】 重量％で、 Ｃｒ：０．０１〜１．０％ Ｎｉ：０．０１〜３．０％ Ｍｏ：０．０１〜１．０％ Ｃｕ：０．０１〜１．５％ Ｔｉ：０．００３〜０．１０％ Ｖ ：０．００５〜０．５０％ Ｎｂ：０．００３〜０．１０％ Ｚｒ：０．００３〜０．１０％ Ｔａ：０．００５〜０．２０％ Ｗ ：０．０１〜２．０％ Ｂ ：０．０００３〜０．００２０％ の１種または２種以上を含有することを特徴とする請求
項１〜７のいずれか１項に記載の低温靭性に優れた厚手
高張力鋼材の製造方法。8. In weight%, Cr: 0.01 to 1.0% Ni: 0.01 to 3.0% Mo: 0.01 to 1.0% Cu: 0.01 to 1.5% Ti : 0.003 to 0.10% V: 0.005 to 0.50% Nb: 0.003 to 0.10% Zr: 0.003 to 0.10% Ta: 0.005 to 0.20% W : 0.01 to 2.0% B: 0.0003 to 0.0020% of one or more of the following: the low-temperature toughness according to any one of claims 1 to 7, An excellent method for producing thick, high-tensile steel. 【請求項９】 重量％で、 Ｍｇ：０．０００５〜０．０１％ Ｃａ：０．０００５〜０．０１％ ＲＥＭ：０．００５〜０．１０％ のうち１種または２種以上を含有することを特徴とする
請求項１〜８のいずれか１項に記載の低温靭性に優れた
厚手高張力鋼材の製造方法。9. The composition contains one or more of Mg: 0.0005 to 0.01% Ca: 0.0005 to 0.01% REM: 0.005 to 0.10% by weight% The method for producing a thick high-tensile steel excellent in low-temperature toughness according to any one of claims 1 to 8, characterized in that:
JP10132404A 1998-05-14 1998-05-14 Production of thick high tensile strength steel excellent in low temperature toughness Withdrawn JPH11323434A (en)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102828116A (en) * 2011-06-14 2012-12-19 鞍钢股份有限公司 TMCP (thermal mechanical control processing) process-based surface-layer ultrafine-grain high-strength steel plate and manufacturing method thereof
CN104018064A (en) * 2014-06-18 2014-09-03 内蒙古包钢钢联股份有限公司 Low-cost Q345R steel plate and production method thereof
JP2020066047A (en) * 2018-10-26 2020-04-30 日本製鉄株式会社 Manufacturing method of steel piece

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JPH06104860B2 (en) * 1989-03-25 1994-12-21 新日本製鐵株式会社 Manufacturing method of high heat input welding steel with excellent low temperature toughness
JPH1017982A (en) * 1996-06-28 1998-01-20 Nippon Steel Corp High tensile strength steel material with low yield ratio for construction use, excellent in fracture resistance, and its production
JPH1017981A (en) * 1996-06-28 1998-01-20 Nippon Steel Corp High tensile strength steel material with low yield ratio for construction use, excellent in brittle crack arrest property, and its production

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JPS62180012A (en) * 1986-02-01 1987-08-07 Kobe Steel Ltd Manufacture of high toughness thick steel plate
JPH06104860B2 (en) * 1989-03-25 1994-12-21 新日本製鐵株式会社 Manufacturing method of high heat input welding steel with excellent low temperature toughness
JPH1017982A (en) * 1996-06-28 1998-01-20 Nippon Steel Corp High tensile strength steel material with low yield ratio for construction use, excellent in fracture resistance, and its production
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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102828116A (en) * 2011-06-14 2012-12-19 鞍钢股份有限公司 TMCP (thermal mechanical control processing) process-based surface-layer ultrafine-grain high-strength steel plate and manufacturing method thereof
CN104018064A (en) * 2014-06-18 2014-09-03 内蒙古包钢钢联股份有限公司 Low-cost Q345R steel plate and production method thereof
CN104018064B (en) * 2014-06-18 2016-08-17 内蒙古包钢钢联股份有限公司 The production method of low cost Q345R steel plate
JP2020066047A (en) * 2018-10-26 2020-04-30 日本製鉄株式会社 Manufacturing method of steel piece

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