JP6824415B2 - Thick steel sheet with excellent low-temperature impact toughness and CTOD characteristics and its manufacturing method - Google Patents

Thick steel sheet with excellent low-temperature impact toughness and CTOD characteristics and its manufacturing method Download PDF

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JP6824415B2
JP6824415B2 JP2019533569A JP2019533569A JP6824415B2 JP 6824415 B2 JP6824415 B2 JP 6824415B2 JP 2019533569 A JP2019533569 A JP 2019533569A JP 2019533569 A JP2019533569 A JP 2019533569A JP 6824415 B2 JP6824415 B2 JP 6824415B2
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thick steel
steel sheet
impact toughness
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ctod
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ギョム キム，ウ
ギョム キム，ウ
コン ウム，キョン
コン ウム，キョン
ヒョン バン，キ
ヒョン バン，キ
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

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  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Description

本発明は、海洋構造用鋼材に好適に適用することができる低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板及びその製造方法に関する。 The present invention relates to a thick steel sheet having excellent low temperature impact toughness and CTOD characteristics, which can be suitably applied to steel materials for marine structures, and a method for producing the same.

北極は、将来のエネルギー源の宝庫と考えられており、北極圏周辺国を中心に石油及びガス資源開発が徐々に進められている。また、陸上、近海、及び深海地域のエネルギー資源の枯渇に伴い、北極の資源開発は加速化すると考えられている。 The Arctic is considered to be a treasure trove of future energy sources, and oil and gas resource development is gradually progressing mainly in countries around the Arctic Circle. In addition, Arctic resource development is expected to accelerate with the depletion of energy resources on land, in the sea, and in the deep sea.

かかる北極地方の資源開発のための採掘や試錐、保存などの海洋構造設備に適用される鋼材は、−６０℃以下の低温でも靭性が確保される必要があり、疲労破壊特性を示すＣＴＯＤ値も−６０℃が確保される必要がある。また、設備の大型化及び統合化に伴い、鋼材は高強度化及び極厚化しつつある。 Steel materials applied to marine structural equipment such as mining, drilling, and preservation for resource development in the Arctic region need to have toughness even at low temperatures of -60 ° C or less, and have a CTOD value that indicates fatigue fracture characteristics. -60 ° C needs to be secured. In addition, with the increase in size and integration of equipment, steel materials are becoming stronger and extremely thick.

脆性破壊に対する抵抗性は、大きく脆性亀裂発生に対する抵抗性と脆性亀裂伝播に対する抵抗性とに分けられる。脆性亀裂発生とは、構造物内の欠陥部から始まった疲労亀裂が一定の大きさに成長した後、外部から高い応力が加わった際に、成長した疲労亀裂から脆性亀裂が生じることを意味する。このとき、脆性亀裂が生じないようにする材料の抵抗特性を脆性亀裂発生に対する抵抗性と呼び、主にＢＳ ７４４８やＡＳＴＭ １２９０規格に明示されているＣＴＯＤ（亀裂先端開口変位：Ｃｒａｃｋ Ｔｉｐ Ｏｐｅｎｉｎｇ Ｄｉｓｐｌａｃｅｍｅｎｔ）試験法を用いて抵抗性を評価している。すなわち、ＣＴＯＤ特性に優れるとは、脆性亀裂発生に対する抵抗性に優れていることを意味する。 Resistance to brittle fracture can be broadly divided into resistance to brittle crack formation and resistance to brittle crack propagation. Brittle crack generation means that after a fatigue crack starting from a defect in a structure has grown to a certain size, a brittle crack is generated from the grown fatigue crack when a high stress is applied from the outside. .. At this time, the resistance property of the material that prevents brittle cracks from occurring is called resistance to brittle cracks, and is mainly CTOD (crack tip opening displacement) specified in BS 7448 and ASTM 1290 standards. Resistance is evaluated using a test method. That is, excellent CTOD characteristics mean excellent resistance to brittle cracking.

そこで、低温衝撃靭性及びＣＴＯＤ特性を確保するための多くの研究及び開発が行われてきた。例えば、特許文献１には、拡幅圧延のように圧下率の低い圧延工程が含まれる鋼板圧延過程において最終３パス圧下率を一定の水準に維持させることで鋼板のＣＴＯＤ特性に優れるように維持することができる製造方法について開示されている。 Therefore, much research and development have been carried out to ensure low temperature impact toughness and CTOD characteristics. For example, Patent Document 1 maintains the CTOD characteristics of a steel sheet so as to be excellent by maintaining the final 3-pass rolling rate at a constant level in a steel sheet rolling process including a rolling process having a low rolling rate such as widening rolling. The manufacturing methods that can be used are disclosed.

しかし、特許文献１の場合には、十分な低温靭性及びＣＴＯＤ特性を確保することが難しいという問題がある。 However, in the case of Patent Document 1, there is a problem that it is difficult to secure sufficient low temperature toughness and CTOD characteristics.

また、使用環境が徐々に厳しくなるにつれて、−８０℃程度の極低温でも優れた衝撃靭性が確保できる技術の開発が求められており、ＣＴＯＤ特性及び強度にも優れた厚鋼板及びその製造方法に対する開発が求められているのが実情である。 Further, as the usage environment gradually becomes harsher, the development of a technology capable of ensuring excellent impact toughness even at an extremely low temperature of about -80 ° C is required, and for thick steel sheets having excellent CTOD characteristics and strength and their manufacturing methods. The reality is that development is required.

韓国公開特許第１０−２０１０−００６６７５７号公報Korean Publication No. 10-2010-0066757

本発明の目的は、海洋構造用鋼材に好適に適用することができる低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板及びその製造方法を提供することにある。 An object of the present invention is to provide a thick steel sheet having excellent low temperature impact toughness and CTOD characteristics, which can be suitably applied to steel materials for marine structures, and a method for producing the same.

一方、本発明の課題は上述の内容に限定されない。本発明の課題は、本明細書の内容全般から理解できるものであり、本発明が属する技術分野における通常の知識を有する者であれば、本発明の付加的な課題を理解するのに何ら困難はない。 On the other hand, the subject of the present invention is not limited to the above-mentioned contents. The subject of the present invention can be understood from the contents of the present specification in general, and it is difficult for a person having ordinary knowledge in the technical field to which the present invention belongs to understand the additional subject of the present invention. There is no.

本発明の一側面は、重量％で、Ｃ：０．０２〜０．０６％、Ｓｉ：０．００５〜０．０８％、Ｍｎ：１．０〜２．０％、Ｐ：０．０１％以下、Ｓ：０．００３％以下、Ａｌ：０．００１〜０．０１％、Ｎｉ：０．５〜２．０％、Ｔｉ：０．００１〜０．０２％、Ｎｂ：０．００５〜０．０３％、Ｃｕ：０．０５〜０．４％、Ｎ：０．００２〜０．００６％、残部がＦｅと不可避不純物で、かつ下記式１及び式２を満たす組成であり、微細組織が、フェライトを９５面積％以上と、ＭＡとセメンタイトを合計して２面積％以下で含んでいる低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板に関する。 One aspect of the present invention is C: 0.02 to 0.06%, Si: 0.005 to 0.08%, Mn: 1.0 to 2.0%, P: 0.01% in terms of weight%. Hereinafter, S: 0.003% or less, Al: 0.001 to 0.01%, Ni: 0.5 to 2.0%, Ti: 0.001 to 0.02%, Nb: 0.005 to 0 .03%, Cu: 0.05 to 0.4%, N: 0.002 to 0.006%, the balance is Fe and unavoidable impurities, and the composition satisfies the following formulas 1 and 2, and the microstructure is The present invention relates to a thick steel plate having excellent low-temperature impact toughness and CTOD characteristics, which contains ferrite in 95 area% or more and MA and cementite in a total of 2 area% or less.

また、本発明の他の一側面は、重量％で、Ｃ：０．０２〜０．０６％、Ｓｉ：０．００５〜０．０８％、Ｍｎ：１．０〜２．０％、Ｐ：０．０１％以下、Ｓ：０．００３％以下、Ａｌ：０．００１〜０．０１％、Ｎｉ：０．５〜２．０％、Ｔｉ：０．００１〜０．０２％、Ｎｂ：０．００５〜０．０３％、Ｃｕ：０．０５〜０．４％、Ｎ：０．００２〜０．００６％、残部がＦｅと不可避不純物で、かつ下記式１及び式２を満たす組成のスラブを１０２０〜１１５０℃に加熱する段階と、上記加熱されたスラブを９００℃以上で再結晶域圧延する段階と、上記再結晶域圧延後に、仕上げ圧延温度がＡｒ３〜８５０℃になるように未再結晶域圧延して厚鋼板を得る段階と、上記厚鋼板を２〜１５℃／ｓｅｃの冷却速度で２５０℃以下に冷却する段階と、上記冷却された厚鋼板を５００〜６５０℃に加熱して焼戻しする段階と、を行う低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板の製造方法に関する。
In addition, another aspect of the present invention is C: 0.02 to 0.06%, Si: 0.005 to 0.08%, Mn: 1.0 to 2.0%, P: in% by weight. 0.01% or less, S: 0.003% or less, Al: 0.001 to 0.01%, Ni: 0.5 to 2.0%, Ti: 0.001 to 0.02%, Nb: 0 .005-0.03%, Cu: 0.05-0.4%, N: 0.002-0.006%, the balance is Fe and unavoidable impurities, and the composition satisfies the following formulas 1 and 2. The step of heating the heated slab to 1020-1150 ° C., the step of recrystallizing the heated slab at 900 ° C. or higher, and after the recrystallization zone rolling, the finish rolling temperature is not re-rolled to Ar3 to 850 ° C. A step of rolling in the crystallographic region to obtain a thick steel sheet, a step of cooling the thick steel sheet to 250 ° C. or lower at a cooling rate of 2 to 15 ° C./sec, and a step of heating the cooled thick steel sheet to 500 to 650 ° C. The present invention relates to a step of tempering and a method for producing a thick steel sheet having excellent low temperature impact toughness and CTOD characteristics.

なお、上記した課題の解決手段は、本発明の特徴をすべて列挙したものではない。本発明の様々且つ有意義な長所及び効果は、本発明の具体的な実施形態を説明する過程でより詳細に理解されることができる。 It should be noted that the means for solving the above-mentioned problems does not list all the features of the present invention. Various and meaningful advantages and effects of the present invention can be understood in more detail in the process of describing specific embodiments of the present invention.

本発明によると、厚さ５０ｍｍ以上の厚鋼板に対して、優れた降伏強度が得られるだけでなく、−８０℃程度の極低温でも優れた衝撃靭性を有し、−６０℃における衝撃靭性及びＣＴＯＤ特性にも優れた厚鋼板及びその製造方法を提供することができるという効果がある。 According to the present invention, not only excellent yield strength can be obtained for a thick steel sheet having a thickness of 50 mm or more, but also excellent impact toughness even at an extremely low temperature of about -80 ° C, and impact toughness at -60 ° C and There is an effect that a thick steel plate having excellent CTOD characteristics and a method for producing the same can be provided.

発明例１の微細組織を撮影した写真である。It is a photograph of the microstructure of Invention Example 1. Ｍｎ＋２Ｎｉ値による降伏強度及び６０℃におけるＣＴＯＤ値を示すグラフである。It is a graph which shows the yield strength by the Mn + 2Ni value and the CTOD value at 60 degreeC.

以下、本発明の好ましい実施形態について説明する。しかし、本発明の実施形態は、いくつかの他の形態に変形することができ、本発明の範囲は以下に説明する実施形態に限定されるものではない。また、本発明の実施形態は、当該技術分野において平均的な知識を有する者にとって本発明をよりよく説明するために提供するものである。 Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention can be transformed into several other embodiments, and the scope of the invention is not limited to the embodiments described below. In addition, embodiments of the present invention are provided to better explain the present invention to those who have average knowledge in the art.

低温靭性及びＣＴＯＤ特性に優れた厚鋼板：
以下、本発明の一側面による低温靭性及びＣＴＯＤ特性に優れた厚鋼板について詳細に説明する。 Thick steel sheet with excellent low temperature toughness and CTOD characteristics:
Hereinafter, a thick steel sheet having excellent low temperature toughness and CTOD characteristics according to one aspect of the present invention will be described in detail.

本発明の一側面による低温靭性及びＣＴＯＤ特性に優れた厚鋼板は、重量％で、Ｃ：０．０２〜０．０６％、Ｓｉ：０．００５〜０．０８％、Ｍｎ：１．０〜２．０％、Ｐ：０．０１％以下、Ｓ：０．００３％以下、Ａｌ：０．００１〜０．０１％、Ｎｉ：０．５〜２．０％、Ｔｉ：０．００１〜０．０２％、Ｎｂ：０．００５〜０．０３％、Ｃｕ：０．０５〜０．４％、Ｎ：０．００２〜０．００６％、残部がＦｅと不可避不純物で、かつ下記式１及び式２を満たす組成であり、微細組織が、フェライトを９５面積％以上と、ＭＡとセメンタイトを合計して２面積％以下で含んでいる。
The thick steel plate having excellent low temperature toughness and CTOD characteristics according to one aspect of the present invention has C: 0.02 to 0.06%, Si: 0.005 to 0.08%, Mn: 1.0 to% by weight. 2.0%, P: 0.01% or less, S: 0.003% or less, Al: 0.001 to 0.01%, Ni: 0.5 to 2.0%, Ti: 0.001 to 0 .02%, Nb: 0.005 to 0.03%, Cu: 0.05 to 0.4%, N: 0.002 to 0.006%, the balance is Fe and unavoidable impurities, and the following formula 1 and The composition satisfies the formula 2, and the microstructure contains ferrite in 95 area% or more, and MA and cementite in a total of 2 area% or less.

まず、本発明の合金組成について詳細に説明する。以下、各元素の含有量の単位は、特に記載しない限り重量％を意味する。 First, the alloy composition of the present invention will be described in detail. Hereinafter, the unit of the content of each element means% by weight unless otherwise specified.

Ｃ：０．０２〜０．０６％
Ｃは、固溶強化に有用な元素であり、Ｎｂなどと炭化物を形成して強度を向上させる役割を果たす元素である。
Ｃの含有量が０．０２％未満では上述した効果が不十分であり、０．０６％を超えるとＭＡの形成を助長するだけでなく、パーライトが生成して、低温における衝撃及び疲労特性を損なう可能性がある。したがって、Ｃの含有量は、０．０２〜０．０６％であることが好ましい。
また、Ｃの含有量のより好ましい下限は０．０２５％、さらに好ましい下限は０．０３％である。また、Ｃの含有量のより好ましい上限は０．０５５％、さらに好ましい上限は０．０５％である。 C: 0.02 to 0.06%
C is an element useful for solid solution strengthening, and is an element that plays a role of forming carbides with Nb and the like to improve the strength.
If the C content is less than 0.02%, the above-mentioned effects are insufficient, and if it exceeds 0.06%, not only the formation of MA is promoted, but also pearlite is generated, which causes impact and fatigue characteristics at low temperature. It can be detrimental. Therefore, the C content is preferably 0.02 to 0.06%.
The more preferable lower limit of the C content is 0.025%, and the more preferable lower limit is 0.03%. The more preferable upper limit of the C content is 0.055%, and the more preferable upper limit is 0.05%.

Ｓｉ：０．００５〜０．０８％
Ｓｉは、Ａｌを補助して溶鋼を脱酸する役割を果たし、降伏強度及び引張強度の向上に役立つ元素であるが、低温における衝撃及び疲労特性に悪影響を及ぼす元素でもある。
Ｓｉの含有量が０．０８％を超えると、Ｃの拡散を妨害してＭＡの形成を助長することにより、低温における衝撃及び疲労特性に悪影響を及ぼす。これに対し、Ｓｉの含有量を０．００５％未満に制御するには、製鋼工程における処理時間が大幅に増え、生産性を低下させる可能性がある。したがって、Ｓｉの含有量は０．００５〜０．０８％であることが好ましい。
また、Ｓｉの含有量のより好ましい下限は０．０１％であり、より好ましい上限は０．０７％、さらに好ましい上限は０．０５５％である。 Si: 0.005 to 0.08%
Si is an element that assists Al to deoxidize molten steel and is useful for improving yield strength and tensile strength, but it is also an element that adversely affects impact and fatigue characteristics at low temperatures.
If the Si content exceeds 0.08%, it interferes with the diffusion of C and promotes the formation of MA, which adversely affects the impact and fatigue characteristics at low temperatures. On the other hand, in order to control the Si content to less than 0.005%, the processing time in the steelmaking process is significantly increased, which may reduce the productivity. Therefore, the Si content is preferably 0.005 to 0.08%.
The more preferable lower limit of the Si content is 0.01%, the more preferable upper limit is 0.07%, and the further preferable upper limit is 0.055%.

Ｍｎ：１．０〜２．０％
Ｍｎは、固溶強化による強度増加の効果が大きいため、１．０％以上添加する。しかし、過度に添加すると、ＭｎＳ介在物の形成、中心部偏析による靭性の低下をもたらす可能性があるため、上限は２．０％であることが好ましい。 Mn: 1.0 to 2.0%
Mn is added in an amount of 1.0% or more because the effect of increasing the strength by strengthening the solid solution is large. However, if it is added excessively, it may cause formation of MnS inclusions and a decrease in toughness due to segregation of the central portion. Therefore, the upper limit is preferably 2.0%.

Ｐ：０．０１％以下
Ｐは、粒界偏析を起こす元素であって、鋼を脆化する原因になることがある。したがって、Ｐは不純物としてできるだけ低く制御する必要があり、Ｐの含有量を０．０１％以下に制御することが好ましい。但し、Ｐを０％に制御することは、実質的に不可能であることがあり、０％でなくともよい。 P: 0.01% or less P is an element that causes grain boundary segregation and may cause embrittlement of steel. Therefore, P needs to be controlled as low as possible as an impurity, and it is preferable to control the P content to 0.01% or less. However, it may be practically impossible to control P to 0%, and it does not have to be 0%.

Ｓ：０．００３％以下
Ｓは、主にＭｎと結合してＭｎＳ介在物を形成する。これらは、低温靭性を阻害する要因となる。したがって、Ｓは不純物としてできるだけ低く制御する必要があり、所望の低温靭性及び低温疲労特性を得るためには、Ｓの含有量を０．００３％以下に制御することが好ましい。但し、Ｓを０％に制御することは、実質的に不可能であることがあり、０％でなくともよい。 S: 0.003% or less S mainly combines with Mn to form MnS inclusions. These are factors that inhibit low temperature toughness. Therefore, S needs to be controlled as low as possible as an impurity, and it is preferable to control the S content to 0.003% or less in order to obtain desired low temperature toughness and low temperature fatigue characteristics. However, it may be practically impossible to control S to 0%, and it does not have to be 0%.

Ａｌ：０．００１〜０．０１％
本発明において、Ａｌは、鋼の主要な脱酸剤として０．００１％以上添加する必要がある。しかし、Ａｌの含有量が０．０１％を超えると、Ａｌ２Ｏ３介在物の分率、大きさが増して低温靭性を低下させる原因となることがある。また、Ｓｉと同様に母材及び溶接熱影響部のＭＡ相の生成を促進して、低温靭性及び低温疲労特性を低下させる可能性がある。したがって、Ａｌの含有量は、０．００１〜０．０１％であることが好ましい。 Al: 0.001 to 0.01%
In the present invention, Al needs to be added in an amount of 0.001% or more as a main deoxidizer for steel. However, if the Al content exceeds 0.01%, the fraction and size of the Al 2 O 3 inclusions may increase, causing a decrease in low temperature toughness. Further, like Si, it may promote the formation of the MA phase of the base metal and the weld heat affected zone, and reduce the low temperature toughness and low temperature fatigue characteristics. Therefore, the Al content is preferably 0.001 to 0.01%.

Ｎｉ：０．５〜２．０％
Ｎｉは、含有量の増加に応じて、強度の向上は大きくないが、強度と靭性をともに向上させることができる元素である。
Ｎｉの含有量が０．５％未満では、上述した効果が不十分であり、２．０％を超えると、硬化能の増加によりＭＡの形成を助長して、衝撃及びＣＴＯＤなどの靭性を阻害するおそれがある。 Ni: 0.5-2.0%
Ni is an element that can improve both strength and toughness, although the strength does not increase significantly as the content increases.
If the Ni content is less than 0.5%, the above-mentioned effects are insufficient, and if it exceeds 2.0%, the hardening ability is increased to promote the formation of MA and inhibit the toughness such as impact and CTOD. There is a risk of

Ｔｉ：０．００１〜０．０２％
Ｔｉは、酸素または窒素と結合して析出物を形成することにより、組織の粗大化を抑制し、微細化に寄与して靭性を向上させる役割を果たす元素である。
Ｔｉの含有量が０．００１％未満では、上述した効果が不十分であり、０．０２％を超えると、析出物の粗大化によって破壊の原因となることがある。 Ti: 0.001 to 0.02%
Ti is an element that plays a role of suppressing the coarsening of the structure, contributing to the miniaturization, and improving the toughness by combining with oxygen or nitrogen to form a precipitate.
If the Ti content is less than 0.001%, the above-mentioned effect is insufficient, and if it exceeds 0.02%, coarsening of the precipitate may cause destruction.

Ｎｂ：０．００５〜０．０３％
Ｎｂは、鋼中に固溶されるか、または炭窒化物を析出することにより、圧延または冷却中に再結晶を抑制し、組織を微細化するとともに、強度を増加させる元素である。
Ｎｂの含有量が０．００５％未満では、上述した効果が不十分であり、０．０３％を超えると、Ｃの親和力によってＣ集中が発生し、ＭＡ相の生成を促進し、低温における靭性及び破壊特性を低下させるおそれがある。 Nb: 0.005 to 0.03%
Nb is an element that suppresses recrystallization during rolling or cooling by solid-solving or precipitating carbonitride in steel, making the structure finer and increasing the strength.
If the Nb content is less than 0.005%, the above-mentioned effect is insufficient, and if it exceeds 0.03%, C concentration occurs due to the affinity of C, the formation of MA phase is promoted, and toughness at low temperature is promoted. And there is a risk of degrading the breaking characteristics.

Ｃｕ：０．０５〜０．４％
Ｃｕは、衝撃特性を大幅に低下させない成分であって、固溶及び析出によって強度を向上させる元素である。
Ｃｕの含有量が０．０５％未満では、上述した効果が不十分であり、０．４％を超えると、Ｃｕの熱衝撃によって鋼板表面にクラックを発生させるおそれがある。 Cu: 0.05-0.4%
Cu is a component that does not significantly reduce the impact characteristics, and is an element that improves the strength by solid solution and precipitation.
If the Cu content is less than 0.05%, the above-mentioned effect is insufficient, and if it exceeds 0.4%, cracks may occur on the surface of the steel sheet due to the thermal shock of Cu.

Ｎ：０．００２〜０．００６％
Ｎは、Ｔｉ、Ｎｂ、Ａｌなどとともに析出物を形成して再加熱時にオーステナイト組織を微細にすることで強度及び靭性の向上に役立つ元素であって、０．００２％以上添加することが好ましい。
しかし、Ｎの含有量が０．００６％を超えると、高温で表面クラックを誘発し、析出物を形成して残留するＮは原子状態で存在して靭性を低下させるおそれがある。したがって、Ｎの含有量は０．００２〜０．００６％であることが好ましい。 N: 0.002 to 0.006%
N is an element that helps to improve strength and toughness by forming a precipitate together with Ti, Nb, Al and the like and making the austenite structure finer at the time of reheating, and it is preferable to add 0.002% or more.
However, if the N content exceeds 0.006%, surface cracks may be induced at high temperatures, and N remaining after forming precipitates may exist in an atomic state to reduce toughness. Therefore, the N content is preferably 0.002 to 0.006%.

本発明の他の成分は鉄（Ｆｅ）である。但し、通常の製造過程では、原料や周囲の環境から意図しない不純物が必然的に混入する可能性があり、これを排除することはできない。これらの不純物は、通常の製造過程における技術者であれば誰でも分かることであり、そのすべての内容を具体的に言及することはしない。 The other component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be mixed from the raw materials and the surrounding environment, and this cannot be eliminated. These impurities are known to any engineer in the normal manufacturing process, and all the contents are not specifically mentioned.

本発明の合金組成は、上述した各元素の含有量を満たすばかりでなく、下記式１及び式２を満たすようにする必要がある。
The alloy composition of the present invention needs to satisfy not only the content of each element described above but also the following formulas 1 and 2.

上記式１及び式２は、強度を低下させることなく、優れた低温衝撃靭性及びＣＴＯＤ特性を確保するためのものであって、ＭＡの抑制効果と強度に及ぼす影響に関する相関関係を考慮して設計した式である。
式２により、ＭＡの抑制のためにＣ、Ｓｉ、及びＡｌの含有量を制御し、これによる強度低下を補うために式１に基づいてＭｎ及びＮｉを添加する必要がある。
式１の値が３．０未満であると、強度向上の効果が不十分であり、４．３を超えると、低温衝撃靭性及びＣＴＯＤ特性を低下させるおそれがある。
式２の値は、脱酸などの製鋼工程のために、０．０５以上であることが好ましい。さらに、式２の値が０．０５未満であると、強度を満足させることが難しくなり、０．２５を超えると、ＭＡ相が大量に形成して、低温衝撃靭性及びＣＴＯＤ特性を低下させるおそれがある。 The above formulas 1 and 2 are for ensuring excellent low temperature impact toughness and CTOD characteristics without lowering the strength, and are designed in consideration of the correlation between the MA suppressing effect and the influence on the strength. Is the formula.
According to Formula 2, it is necessary to control the contents of C, Si, and Al to suppress MA, and to add Mn and Ni based on Formula 1 to compensate for the resulting decrease in strength.
If the value of Equation 1 is less than 3.0, the effect of improving the strength is insufficient, and if it exceeds 4.3, the low temperature impact toughness and CTOD characteristics may be deteriorated.
The value of Equation 2 is preferably 0.05 or more for the steelmaking process such as deoxidation. Further, if the value of Equation 2 is less than 0.05, it becomes difficult to satisfy the strength, and if it exceeds 0.25, a large amount of MA phase is formed, which may reduce low temperature impact toughness and CTOD characteristics. There is.

一方、本発明の合金組成は、上述した元素の他に、さらに、重量％で、Ｍｏ：０．００１〜０．０５％及びＣａ：０．０００２〜０．００５％のうち１種以上を含むことができる。
Ｍｏ：０．００１〜０．０５％
Ｍｏは、硬化能を増加させて強度を高めるのに効果的な役割を果たす元素である。このためには、Ｍｏを０．００１％以上添加することが好ましいが、０．０５％を超えて添加すると、硬化能の増大による靭性低下、及びモリブデンカーバイドの析出物を生成して靭性を低下させるという問題がある。
Ｃａ：０．０００２〜０．００５％
製鋼中の溶鋼にＡｌを脱酸した後にＣａを添加すると、主にＭｎＳとして存在するＳと結合してＭｎＳの生成を抑制するとともに、球状のＣａＳを形成して鋼材の中心部の亀裂クラックを抑制するという効果を奏する。このためには、Ｃａを０．０００２％以上添加することが好ましいが、０．００５％を超えて添加すると、余剰のＣａがＯと結合して粗大な酸化性介在物を生成し、後の圧延過程で延伸、破折して低温における亀裂開始点として作用することになる。 On the other hand, the alloy composition of the present invention further contains one or more of Mo: 0.001 to 0.05% and Ca: 0.0002 to 0.005% by weight in addition to the above-mentioned elements. be able to.
Mo: 0.001-0.05%
Mo is an element that plays an effective role in increasing curability and increasing strength. For this purpose, it is preferable to add 0.001% or more of Mo, but if it is added in excess of 0.05%, the toughness is lowered due to the increase in curability and the toughness is lowered by forming a precipitate of molybdenum carbide. There is a problem of letting it.
Ca: 0.0002 to 0.005%
When Ca is added to the molten steel during steelmaking after deoxidizing Al, it mainly combines with S existing as MnS to suppress the formation of MnS, and forms spherical CaS to form cracks in the center of the steel material. It has the effect of suppressing. For this purpose, it is preferable to add 0.0002% or more of Ca, but if it is added in excess of 0.005%, excess Ca binds to O to form coarse oxidizing inclusions, which is followed later. It stretches and breaks during the rolling process and acts as a crack starting point at low temperatures.

以下、本発明による厚鋼板の微細組織について詳細に説明する。
本発明による厚鋼板の微細組織は、フェライトが９５面積％以上で、ＭＡとセメンタイトが合計して２面積％以下でなっている。
フェライトが９５面積％未満であると、−８０℃における衝撃靭性及び−６０℃におけるＣＴＯＤ特性が低下するおそれがある。
低温衝撃靭性及びＣＴＯＤ特性を確保するためには、母材の組織及びＭＡの分率が重要である。ＭＡは圧延及び冷却中にＣが集積されて濃化して高まった硬化能により、高硬度のマルテンサイトへ変態するか、またはオーステナイトとして残るが、これをＭＡ（マルテンサイト−オーステナイト）と呼ぶ。かかるＭＡは、高硬度である特性により、破壊に対して脆弱であり、周辺の軟質フェライトの変形時に応力を集中させて破壊の開始点として作用することになる。
また、セメンタイトは、ＭＡと同様の性質により、母材アシキュラーフェライトよりも高硬度を有する硬質相であって、低温衝撃靭性及びＣＴＯＤ特性を低下させる。
したがって、優れた低温衝撃靭性及びＣＴＯＤ特性を確保するためには、ＭＡとセメンタイトの合計を２面積％以下に制御することが重要である。
このとき、上記フェライトは、円相当直径で測定した平均結晶粒サイズが２０μｍ以下であるのがよい。結晶粒サイズが２０μｍを超えると、フェライト内部の転位が増加して破壊伝播を容易にすることで、低温衝撃靭性及びＣＴＯＤ特性が損なわれる可能性がある。結晶粒サイズは、小さいほど低温衝撃靭性及びＣＴＯＤ特性を満たすに有利であり、その下限は特に限定しない。 Hereinafter, the fine structure of the thick steel sheet according to the present invention will be described in detail.
In the microstructure of the thick steel sheet according to the present invention, ferrite is 95 area% or more, and MA and cementite are 2 area% or less in total.
If the amount of ferrite is less than 95 area%, the impact toughness at -80 ° C and the CTOD characteristics at -60 ° C may deteriorate.
In order to ensure low temperature impact toughness and CTOD characteristics, the structure of the base metal and the fraction of MA are important. MA is transformed into high-hardness martensite or remains as austenite due to the hardening ability that C is accumulated and concentrated during rolling and cooling, and this is called MA (martensite-austenite). Due to its high hardness, such MA is vulnerable to fracture, and when the surrounding soft ferrite is deformed, stress is concentrated and acts as a starting point of fracture.
In addition, cementite is a hard phase having a higher hardness than the base material cyclic ferrite due to the same properties as MA, and lowers low temperature impact toughness and CTOD characteristics.
Therefore, in order to ensure excellent low temperature impact toughness and CTOD characteristics, it is important to control the total of MA and cementite to 2 area% or less.
At this time, it is preferable that the average crystal grain size of the ferrite measured with a diameter equivalent to a circle is 20 μm or less. If the grain size exceeds 20 μm, dislocations inside the ferrite increase and fracture propagation is facilitated, which may impair low temperature impact toughness and CTOD characteristics. The smaller the crystal grain size, the more advantageous it is to satisfy the low temperature impact toughness and the CTOD characteristics, and the lower limit thereof is not particularly limited.

また、上記フェライトは、ポリゴナルフェライトと針状フェライトで構成されることがあるが、本発明はこの分率を具体的に限定しない。 Further, the above-mentioned ferrite may be composed of polygonal ferrite and acicular ferrite, but the present invention does not specifically limit this fraction.

このとき、本発明の厚鋼板は、降伏強度が４２０ＭＰａ以上、衝撃靭性が−８０℃において２００Ｊ以上、ＣＴＯＤが−６０℃において０．５ｍｍ以上であることができる。かかる物性を満たすことにより、極低温環境で用いられる海洋構造用鋼材などに好適に適用することができる。より好ましくは、ＣＴＯＤは、−６０℃において１．０ｍｍ以上である。 At this time, the thick steel sheet of the present invention can have a yield strength of 420 MPa or more, an impact toughness of 200 J or more at −80 ° C., and a CTOD of 0.5 mm or more at −60 ° C. By satisfying such physical properties, it can be suitably applied to steel materials for marine structures used in an extremely low temperature environment. More preferably, the CTOD is 1.0 mm or more at −60 ° C.

また、本発明の厚鋼板は、引張強度が５００ＭＰａ以上、伸びが２５％以上、衝撃靭性が−６０℃において４００Ｊ以上であることができる。 Further, the thick steel sheet of the present invention can have a tensile strength of 500 MPa or more, an elongation of 25% or more, and an impact toughness of 400 J or more at −60 ° C.

また、本発明の厚鋼板は、厚さが５０〜１００ｍｍであることができる。 Further, the thick steel plate of the present invention can have a thickness of 50 to 100 mm.

低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板の製造方法：
以下、本発明の他の一側面である低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板の製造方法について詳細に説明する。 Manufacturing method of thick steel sheet with excellent low temperature impact toughness and CTOD characteristics:
Hereinafter, a method for producing a thick steel sheet having excellent low temperature impact toughness and CTOD characteristics, which is another aspect of the present invention, will be described in detail.

本発明の他の一側面である低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板の製造方法は、上述した合金組成を満たすスラブを１０２０〜１１５０℃に加熱する段階と、上記加熱されたスラブを９００℃以上で再結晶域圧延する段階と、上記再結晶域圧延後に、仕上げ圧延温度がＡｒ３〜８５０℃になるように、未再結晶域圧延して厚鋼板を得る段階と、上記厚鋼板を２〜１５℃／ｓｅｃの冷却速度で２５０℃以下に冷却する段階と、上記冷却された厚鋼板を５００〜６５０℃に加熱して焼戻しする段階と、を行うことからなっている。 A method for producing a thick steel sheet having excellent low-temperature impact toughness and CTOD characteristics, which is another aspect of the present invention, includes a step of heating a slab satisfying the above-mentioned alloy composition to 1020-1150 ° C. A step of recrystallizing area rolling at ° C. or higher, a step of obtaining a thick steel sheet by unrecrystallizing area rolling so that the finish rolling temperature becomes Ar3 to 850 ° C. after the recrystallization area rolling, and 2 It comprises a step of cooling to 250 ° C. or lower at a cooling rate of ~ 15 ° C./sec and a step of heating the cooled thick steel sheet to 500 to 650 ° C. and tempering.

以下、各段階で詳細に説明する。
＜スラブ加熱段階＞
上述した合金組成を満たすスラブを１０２０〜１１５０℃に加熱する。
スラブ加熱温度が１１５０℃を超えると、オーステナイトの結晶粒が粗大化して靭性を低下させるおそれがあり、１０２０℃未満では、Ｔｉ、Ｎｂなどが十分に固溶しない場合が発生し、強度の低下をもたらすことがある。 Hereinafter, each step will be described in detail.
<Slab heating stage>
A slab satisfying the alloy composition described above is heated to 1020 to 1150 ° C.
If the slab heating temperature exceeds 1150 ° C, the crystal grains of austenite may become coarse and the toughness may decrease. If the temperature is less than 1020 ° C, Ti, Nb, etc. may not be sufficiently solid-solved, resulting in a decrease in strength. May bring.

＜再結晶域圧延段階＞
上記加熱されたスラブを９００℃以上で再結晶域圧延する。９００℃未満では、オーステナイトの十分な再結晶が困難になることがある。
このとき、上記再結晶域圧延は、最後の２パスの圧下率がそれぞれ１５〜２０％になるように行うことができる。これは、均一でありながらも微細な最終微細組織を確保するためである。 <Recrystallization area rolling stage>
The heated slab is rolled in a recrystallization region at 900 ° C. or higher. Below 900 ° C, sufficient recrystallization of austenite may be difficult.
At this time, the recrystallization region rolling can be performed so that the rolling reduction of the last two passes is 15 to 20%, respectively. This is to secure a uniform but fine final microstructure.

＜未再結晶域圧延段階＞
上記再結晶域圧延後に、仕上げ圧延温度がＡｒ３〜８５０℃になるように未再結晶域圧延して厚鋼板を得る。
上記仕上げ圧延温度がＡｒ３未満では、冷却開始前に厚鋼板の表面温度が二相域領域に入り、表面〜１／４ｔの厚さで二相組織が形成されて衝撃靭性が低下するおそれがあり、８５０℃を超えると、結晶粒微細化の不足によって強度及び靭性が低下するおそれがある。
このとき、上記未再結晶域圧延は、厚鋼板の厚さが５０〜１００ｍｍとなるように行うことができる。 <Rolling stage in unrecrystallized area>
After the recrystallization region rolling, the unrecrystallized region is rolled so that the finish rolling temperature becomes Ar3 to 850 ° C. to obtain a thick steel sheet.
If the finish rolling temperature is less than Ar3, the surface temperature of the thick steel sheet may enter the two-phase region region before the start of cooling, and a two-phase structure may be formed at a thickness of about 1/4 t on the surface to reduce impact toughness. If the temperature exceeds 850 ° C., the strength and toughness may decrease due to insufficient grain refinement.
At this time, the unrecrystallized area rolling can be performed so that the thickness of the thick steel sheet is 50 to 100 mm.

＜冷却段階＞
上記厚鋼板を２〜１５℃／ｓｅｃの冷却速度で２５０℃以下に冷却する。
冷却速度が１５℃／ｓｅｃを超えると、厚鋼板の表面と中心部の冷却速度の差によって物性差が生じることがある。これに対し、２℃／ｓｅｃ未満では、アシキュラーフェライトの分布が減少し、ポリゴナルフェライトの分布が増加するおそれがある。
冷却終了温度が２５０℃を超えると目標強度に達しないおそれがある。 <Cooling stage>
The thick steel sheet is cooled to 250 ° C. or lower at a cooling rate of 2 to 15 ° C./sec.
If the cooling rate exceeds 15 ° C./sec, a difference in physical properties may occur due to a difference in the cooling rate between the surface and the center of the thick steel sheet. On the other hand, if the temperature is lower than 2 ° C./sec, the distribution of acicular ferrite may decrease and the distribution of polygonal ferrite may increase.
If the cooling end temperature exceeds 250 ° C., the target strength may not be reached.

＜焼戻し段階＞
上記冷却された厚鋼板を５００〜６５０℃に加熱して焼戻しする。これは、ＭＡ相とフェライト内部の転位が低温衝撃靭性とＣＴＯＤ特性に大きな影響を与える因子であって、焼き戻しを介してＭＡ相の分解及びフェライト内部の転位を下げるためである。
焼戻し温度が５００℃未満であると、上述した効果が不十分であり、６５０℃を超えると、カーバイドが形成されて靭性が低下するおそれがある。 <Tempering stage>
The cooled thick steel sheet is heated to 500 to 650 ° C. and tempered. This is because the dislocations inside the MA phase and the ferrite are factors that have a great influence on the low temperature impact toughness and the CTOD characteristics, and the decomposition of the MA phase and the dislocations inside the ferrite are reduced through tempering.
If the tempering temperature is less than 500 ° C., the above-mentioned effect is insufficient, and if it exceeds 650 ° C., carbide may be formed and the toughness may be lowered.

以下、実施例を通じて本発明をより詳細に説明する。しかし、かかる実施例の記載は、本発明の実施を例示するためのものであって、かかる実施例の記載によって本発明を制限するものではない。本発明の権利範囲は、特許請求の範囲に記載された事項とそれから合理的に類推される事項によって決定される。 Hereinafter, the present invention will be described in more detail through examples. However, the description of such examples is for exemplifying the practice of the present invention, and the description of such examples does not limit the present invention. The scope of rights of the present invention is determined by the matters stated in the claims and the matters reasonably inferred from them.

（実施例）
下記表１に示す成分組成を有する溶鋼を連続鋳造してスラブを製造した。上記スラブを下記表２の製造条件で加熱、再結晶域圧延、未再結晶域圧延、冷却、及び焼戻し工程を介して厚さ８０ｍｍの厚鋼板を製造した。このとき、上記再結晶域圧延は、最後の２パスの圧下率がそれぞれ１８％となるようにした。 (Example)
A slab was produced by continuously casting molten steel having the composition shown in Table 1 below. The slab was heated under the production conditions shown in Table 2 below, rolled in the recrystallized region, rolled in the unrecrystallized region, cooled, and tempered to produce a thick steel plate having a thickness of 80 mm. At this time, in the recrystallization region rolling, the rolling reduction of the last two passes was set to 18%, respectively.

上記厚鋼板の微細組織、機械的物性、低温衝撃靭性、及びＣＴＯＤ特性を測定して下記表３に記載した。
微細組織は、走査電子顕微鏡（ＳＥＭ）と透過電子顕微鏡（ＴＥＭ）で観察し、ＭＡとセメンタイトの合計（第２相）を分析して下記表３に記載した。第２相を除いた部分は、ポリゴナルフェライト及び針状フェライトで構成されたフェライトであった。
フェライトの結晶粒サイズは、円相当直径で測定した平均値を下記表３に記載した。
降伏強度、引張強度、及び伸びは引張試験を通じて測定した。
低温衝撃靭性は、−６０℃及び−８０℃においてシャルピー衝撃試験を通じて測定した。
ＣＴＯＤ特性は、ＢＳ ７４４８規格に基づいて圧延方向に垂直に６０ｍｍ×１２０ｍｍ×３００ｍｍサイズで試験片を加工し、疲労亀裂の長さが試験片の幅の５０％になるように疲労亀裂を入れた後、−６０℃においてＣＴＯＤ試験を行った。各鋼板に対して、ＣＴＯＤ試験をそれぞれ３回行い、３回の試験値のうちの最小値を下記表３に記載した。 The microstructure, mechanical properties, low temperature impact toughness, and CTOD characteristics of the thick steel sheet were measured and shown in Table 3 below.
The microstructure was observed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM), and the total of MA and cementite (Phase 2) was analyzed and shown in Table 3 below. The portion excluding the second phase was a ferrite composed of polygonal ferrite and acicular ferrite.
As for the crystal grain size of ferrite, the average value measured with the diameter equivalent to a circle is shown in Table 3 below.
Yield strength, tensile strength, and elongation were measured through tensile tests.
Cold impact toughness was measured through a Charpy impact test at −60 ° C. and −80 ° C.
As for the CTOD characteristics, the test piece was processed in a size of 60 mm × 120 mm × 300 mm perpendicular to the rolling direction based on the BS 7448 standard, and fatigue cracks were formed so that the length of the fatigue crack was 50% of the width of the test piece. After that, a CTOD test was performed at −60 ° C. The CTOD test was performed three times for each steel sheet, and the minimum value among the three test values is shown in Table 3 below.

上記表１において、各元素の含有量の単位は重量％である。但し、Ｐ＊、Ｓ＊、及びＮ＊の単位は重量ｐｐｍである。
式１はＭｎ＋２Ｎｉを計算した値であり、式２はＣ＋Ｓｉ＋１０Ａｌを計算した値であり、式１及び式２において、各元素記号は、各元素の含有量を重量％で表した値である。 In Table 1 above, the unit of the content of each element is% by weight. However, the unit of P * , S * , and N * is ppm by weight.
Equation 1 is a value calculated for Mn + 2Ni, Equation 2 is a value calculated for C + Si + 10Al, and in Equations 1 and 2, each element symbol is a value representing the content of each element in% by weight.

本発明で提示した合金組成及び製造条件をすべて満たす発明例は、降伏強度４２０ＭＰａ以上を確保することができ、−８０℃における衝撃靭性が２００Ｊ以上、−６０℃におけるＣＴＯＤ値が０．５ｍｍ以上と、低温衝撃靭性及びＣＴＯＤ特性に優れることが確認できる。 In the invention example satisfying all the alloy composition and the production conditions presented in the present invention, a yield strength of 420 MPa or more can be secured, the impact toughness at -80 ° C is 200 J or more, and the CTOD value at -60 ° C is 0.5 mm or more. It can be confirmed that the low temperature impact toughness and CTOD characteristics are excellent.

図１は発明例１の微細組織を撮影した写真であって、ＭＡとセメンタイトの形成が少なく、結晶粒サイズも微細であることが分かる。 FIG. 1 is a photograph of the microstructure of Invention Example 1, and it can be seen that the formation of MA and cementite is small and the crystal grain size is also fine.

比較例１〜３は、本発明で提示した合金組成を満たしているが、製造条件を満たしていない場合である。
比較例１及び２では、−８０℃における衝撃靭性と−６０℃におけるＣＴＯＤ特性が劣ることが確認でき、比較例３では、−８０℃における衝撃靭性が低下し、強度を確保することが困難であることが分かる。
比較例４〜７は、本発明で提示した製造条件は満たしたが、合金組成を満たしていない場合である。
比較例４では、Ｃの含有量が範囲を超え、比較例５では、Ｍｎ＋２Ｎｉ値が範囲を超えていて、強度は優れるが、−８０℃における衝撃靭性と−６０℃におけるＣＴＯＤ特性に急激な低下が確認できる。
比較例６では、Ｍ＋２Ｎｉ値が範囲より小さく、強度と−８０℃における衝撃靭性が劣っていることが分かる。
比較例７では、Ｃ＋Ｓｉ＋１０Ａｌ値が範囲を超えていて、−８０℃における衝撃靭性と−６０℃におけるＣＴＯＤ特性が極めて劣っていることが分かる。 Comparative Examples 1 to 3 are cases where the alloy composition presented in the present invention is satisfied, but the production conditions are not satisfied.
In Comparative Examples 1 and 2, it was confirmed that the impact toughness at -80 ° C and the CTOD characteristic at -60 ° C were inferior, and in Comparative Example 3, the impact toughness at -80 ° C was lowered and it was difficult to secure the strength. It turns out that there is.
Comparative Examples 4 to 7 are cases where the production conditions presented in the present invention are satisfied, but the alloy composition is not satisfied.
In Comparative Example 4, the C content exceeded the range, and in Comparative Example 5, the Mn + 2Ni value exceeded the range, and the strength was excellent, but the impact toughness at -80 ° C and the CTOD characteristics at -60 ° C dropped sharply. Can be confirmed.
In Comparative Example 6, it can be seen that the M + 2Ni value is smaller than the range, and the strength and impact toughness at −80 ° C. are inferior.
In Comparative Example 7, it can be seen that the C + Si + 10Al value exceeds the range, and the impact toughness at −80 ° C. and the CTOD characteristics at −60 ° C. are extremely inferior.

図２は、Ｍｎ＋２Ｎｉ値による降伏強度及び６０℃におけるＣＴＯＤ値を示すグラフである。降伏強度４２０ＭＰａ以上を満たすとともに、０．５ｍｍ以上のＣＴＯＤ値を確保するためには、３．０≦Ｍｎ＋２Ｎｉ≦４．３を満たす必要があることが確認できる。Ｍｎ＋２Ｎｉ値が３．０未満では、強度の低下を示し、４．３を超えた場合には、−６０℃におけるＣＴＯＤ値が著しく低くなることが分かる。 FIG. 2 is a graph showing the yield strength based on the Mn + 2Ni value and the CTOD value at 60 ° C. It can be confirmed that it is necessary to satisfy 3.0 ≦ Mn + 2Ni ≦ 4.3 in order to satisfy the yield strength of 420 MPa or more and to secure the CTOD value of 0.5 mm or more. It can be seen that when the Mn + 2Ni value is less than 3.0, the strength is lowered, and when it exceeds 4.3, the CTOD value at −60 ° C. is significantly lowered.

以上、実施例を参照して説明したが、当該技術分野における熟練した当業者は、添付の特許請求の範囲に記載された本発明の思想及び領域を外れない範囲内で本発明を多様に修正及び変更させることができることを理解することができる。

Although the above description has been made with reference to the examples, a skilled person skilled in the art will modify the present invention in various ways within the scope of the idea and domain of the present invention described in the appended claims. And can be understood that it can be changed.

Claims (6)

重量％で、Ｃ：０．０２〜０．０６％、Ｓｉ：０．００５〜０．０８％、Ｍｎ：１．０〜２．０％、Ｐ：０．０１％以下、Ｓ：０．００３％以下、Ａｌ：０．００１〜０．０１％、Ｎｉ：０．５〜２．０％、Ｔｉ：０．００１〜０．０２％、Ｎｂ：０．００５〜０．０３％、Ｃｕ：０．０５〜０．４％、Ｎ：０．００２〜０．００６％、残部がＦｅと不可避不純物で、かつ下記式１及び式２を満たす組成であり、
［数１］
式１：３．０≦Ｍｎ＋２Ｎｉ≦４．３
式２：０．０５≦Ｃ＋Ｓｉ＋１０Ａｌ≦０．２５
（式１、式２において、各元素記号は、各元素の含有量を重量％で表した数値である。）
微細組織が、フェライトを９８面積％以上と、ＭＡとセメンタイトを合計して２面積％以下で含み、
前記フェライトは、円相当直径で測定した平均結晶粒サイズが２０μｍ以下であり、
降伏強度が４２０ＭＰａ以上、衝撃靭性が−８０℃において２００Ｊ以上、ＣＴＯＤが−６０℃において０．５ｍｍ以上であり、
厚さが５０〜１００ｍｍであることを特徴とする低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板。 By weight%, C: 0.02 to 0.06%, Si: 0.005 to 0.08%, Mn: 1.0 to 2.0%, P: 0.01% or less, S: 0.003 % Or less, Al: 0.001 to 0.01%, Ni: 0.5 to 2.0%, Ti: 0.001 to 0.02%, Nb: 0.005 to 0.03%, Cu: 0 .05-0.4%, N: 0.002-0.006%, the balance is Fe and unavoidable impurities, and the composition satisfies the following formulas 1 and 2.
[Number 1]
Equation 1: 3.0 ≤ Mn + 2Ni ≤ 4.3
Equation 2: 0.05 ≦ C + Si + 10Al ≦ 0.25
(In Equations 1 and 2, each element symbol is a numerical value representing the content of each element in% by weight.)
Microstructure, only including a ferrite 98% or more by area, MA and cementite total of the 2 area% or less,
The ferrite has an average crystal grain size of 20 μm or less measured with a diameter equivalent to a circle.
The yield strength is 420 MPa or more, the impact toughness is 200 J or more at -80 ° C, and the CTOD is 0.5 mm or more at -60 ° C.
Excellent steel plate in low temperature impact toughness and CTOD properties thickness, characterized in that a 50 to 100 mm. 前記厚鋼板は、さらに、重量％で、Ｍｏ：０．００１〜０．０５％とＣａ：０．０００２〜０．００５％の１種以上を含むことを特徴とする請求項１に記載の低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板。 The low temperature according to claim 1, wherein the thick steel sheet further contains one or more of Mo: 0.001 to 0.05% and Ca: 0.0002 to 0.005% in weight%. Thick steel plate with excellent impact toughness and CTOD characteristics. 前記フェライトは、ポリゴナルフェライト及び針状フェライトで構成されることを特徴とする請求項１に記載の低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板。 The thick steel sheet having excellent low-temperature impact toughness and CTOD characteristics according to claim 1, wherein the ferrite is composed of polygonal ferrite and acicular ferrite. 前記厚鋼板は、引張強度が５００ＭＰａ以上、伸びが２５％以上、衝撃靭性が−６０℃において４００Ｊ以上であることを特徴とする請求項１に記載の低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板。 The thick steel sheet having excellent low-temperature impact toughness and CTOD characteristics according to claim 1, wherein the thick steel sheet has a tensile strength of 500 MPa or more, an elongation of 25% or more, and an impact toughness of 400 J or more at −60 ° C. .. 請求項１に記載の厚鋼鈑を製造するための方法であって、
重量％で、Ｃ：０．０２〜０．０６％、Ｓｉ：０．００５〜０．０８％、Ｍｎ：１．０〜２．０％、Ｐ：０．０１％以下、Ｓ：０．００３％以下、Ａｌ：０．００１〜０．０１％、Ｎｉ：０．５〜２．０％、Ｔｉ：０．００１〜０．０２％、Ｎｂ：０．００５〜０．０３％、Ｃｕ：０．０５〜０．４％、Ｎ：０．００２〜０．００６％、残部がＦｅと不可避不純物で、かつ下記式１及び式２を満たす組成のスラブを１０２０〜１１５０℃に加熱する段階と、
［数１］
式１：３．０≦Ｍｎ＋２Ｎｉ≦４．３
式２：０．０５≦Ｃ＋Ｓｉ＋１０Ａｌ≦０．２５
（式１、式２において、各元素記号は、各元素の含有量を重量％で表した数値である。）
前記加熱されたスラブを９００℃以上で再結晶域圧延する段階と、
前記再結晶域圧延後に、仕上げ圧延温度がＡｒ３〜８５０℃になるように、未再結晶域圧延して厚鋼板を得る段階と、
前記厚鋼板を２〜１５℃／ｓｅｃの冷却速度で２５０℃以下に冷却する段階と、
前記冷却された厚鋼板を５００〜６５０℃に加熱して焼戻しする段階と、
を含み、
前記再結晶域圧延は、最後の２パスの圧下率がそれぞれ１５〜２０％になるように行い、
前記未再結晶域圧延は、厚鋼板の厚さが５０〜１００ｍｍとなるように行うことを特徴とする低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板の製造方法。 The method for manufacturing the thick steel plate according to claim 1.
By weight%, C: 0.02 to 0.06%, Si: 0.005 to 0.08%, Mn: 1.0 to 2.0%, P: 0.01% or less, S: 0.003 % Or less, Al: 0.001 to 0.01%, Ni: 0.5 to 2.0%, Ti: 0.001 to 0.02%, Nb: 0.005 to 0.03%, Cu: 0 A step of heating a slab having a composition of .05 to 0.4%, N: 0.002 to 0.006%, the balance of Fe and unavoidable impurities, and satisfying the following formulas 1 and 2 to 1020-1150 ° C.
[Number 1]
Equation 1: 3.0 ≤ Mn + 2Ni ≤ 4.3
Equation 2: 0.05 ≦ C + Si + 10Al ≦ 0.25
(In Equations 1 and 2, each element symbol is a numerical value representing the content of each element in% by weight.)
The step of rolling the heated slab in the recrystallization region at 900 ° C. or higher, and
After the recrystallization region rolling, a step of obtaining a thick steel sheet by rolling in the unrecrystallized region so that the finish rolling temperature becomes Ar3 to 850 ° C.
The step of cooling the thick steel sheet to 250 ° C. or lower at a cooling rate of 2 to 15 ° C./sec, and
The step of heating the cooled thick steel sheet to 500 to 650 ° C. and tempering it.
Including
The recrystallization region rolling is performed so that the rolling reduction of the last two passes is 15 to 20%, respectively.
A method for producing a thick steel sheet having excellent low-temperature impact toughness and CTOD characteristics, wherein the unrecrystallized area rolling is performed so that the thickness of the thick steel sheet is 50 to 100 mm. 前記スラブは、さらに、重量％で、Ｍｏ：０．００１〜０．０５％及びＣａ：０．０００２〜０．００５％のうち１種以上を含むことを特徴とする請求項５に記載の低温衝撃靭性及びＣＴＯＤ特性に優れた厚鋼板の製造方法。
The low temperature according to claim 5 , wherein the slab further contains at least one of Mo: 0.001 to 0.05% and Ca: 0.0002 to 0.005% by weight. A method for manufacturing a thick steel sheet having excellent impact toughness and CTOD characteristics.
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