JP5034290B2 - Low yield ratio high strength thick steel plate and method for producing the same - Google Patents

Low yield ratio high strength thick steel plate and method for producing the same Download PDF

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JP5034290B2
JP5034290B2 JP2006088159A JP2006088159A JP5034290B2 JP 5034290 B2 JP5034290 B2 JP 5034290B2 JP 2006088159 A JP2006088159 A JP 2006088159A JP 2006088159 A JP2006088159 A JP 2006088159A JP 5034290 B2 JP5034290 B2 JP 5034290B2
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圭治 植田
章夫 大森
茂 遠藤
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JFE Steel Corp
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本発明は、建築・土木等の分野で溶接して用いられる厚鋼板に関し、特に、降伏応力YSが650MPa以上で、降伏比YRが80%以下の特性を有する溶接性に優れる高強度厚鋼板に関するものである。   The present invention relates to a thick steel plate that is used by welding in the field of construction, civil engineering, and the like, and more particularly, to a high-strength thick steel plate excellent in weldability having a yield stress YS of 650 MPa or more and a yield ratio YR of 80% or less. Is.

近年、建築・土木等の分野における鋼構造物の大型化、長スパン化に伴って、使用される鋼材への厚肉化や高強度化に対する要望が強まっている。一方、鋼構造物の安全性を確保する観点から、高い許容応力を有するとともに、降伏応力と引張強さの比である降伏比を低減することが要求されている。それは、降伏比を低くすることにより、降伏応力以上の外部応力が付加されても、塑性変形によりその応力を吸収でき、また、破壊に至るまでの許容応力が大きいので、塑性変形能に優れた鋼材となるからである。   In recent years, with the increase in the size and the span of steel structures in the fields of construction and civil engineering, there is an increasing demand for thickening and increasing the strength of steel materials used. On the other hand, from the viewpoint of ensuring the safety of a steel structure, it is required to have a high allowable stress and to reduce the yield ratio, which is the ratio between the yield stress and the tensile strength. By reducing the yield ratio, even if an external stress higher than the yield stress is applied, the stress can be absorbed by plastic deformation, and since the allowable stress leading to fracture is large, the plastic deformability is excellent. This is because it becomes a steel material.

ところが、降伏応力が650MPaを超える高張力鋼板では、強度を確保するために、合金元素を多量に添加するのが一般的である。そのため、降伏比が上昇するとともに、靭性や溶接性が低下する傾向にある。そこで、高強度であっても、降伏比が低く、溶接性にも優れた厚鋼板の開発が望まれている。   However, in a high-tensile steel sheet having a yield stress exceeding 650 MPa, a large amount of alloy elements is generally added to ensure strength. For this reason, the yield ratio increases and the toughness and weldability tend to decrease. Therefore, it is desired to develop a thick steel plate having a low yield ratio and excellent weldability even with high strength.

このような要望に応える技術として、例えば、特許文献1〜4には、降伏応力が650MPaを超える低降伏比高強度厚鋼板の製造方法が提案されている。特許文献1および2に記載された技術は、熱間圧延後の鋼板を焼入れし、その後、再度フェライト+オーステナイトの2相域まで加熱して焼入れし、高強度化と低降伏比を達成している。また、特許文献3に記載された技術は、圧延後、直ちに焼入れする直接焼入れ法であり、焼入れ後のミクロ組織をベイナイト相あるいはマルテンサイト相としてから、再度フェライト+オーステナイトの2相域まで加熱して焼ならしを行い、高強度化と低降伏比を達成している。また、特許文献4に記載された技術は、圧延後の焼入れ開始を遅らせる直接焼入れ法であり、フェライトを析出させた後、急冷して、フェライト相+マルテンサイト相の2相組織とし、これにより、高強度化と低降伏比を実現している。
特開2001−288512号公報 特開平06−248337号公報 特開平05−230531号公報 特開平07−097626号公報
For example, Patent Documents 1 to 4 propose a method of manufacturing a low yield ratio high strength thick steel plate having a yield stress exceeding 650 MPa. The techniques described in Patent Documents 1 and 2 quench the hot-rolled steel sheet, and then heat and quench again to the two-phase region of ferrite + austenite to achieve high strength and a low yield ratio. Yes. The technique described in Patent Document 3 is a direct quenching method in which quenching is performed immediately after rolling, and the microstructure after quenching is changed to a bainite phase or a martensite phase and then heated again to a two-phase region of ferrite and austenite. Normalizing, achieving high strength and low yield ratio. Further, the technique described in Patent Document 4 is a direct quenching method that delays the start of quenching after rolling, and after ferrite is precipitated, it is rapidly cooled to form a two-phase structure of ferrite phase + martensite phase. High strength and low yield ratio are realized.
JP 2001-288512 A Japanese Patent Laid-Open No. 06-248337 Japanese Patent Laid-Open No. 05-230531 Japanese Patent Application Laid-Open No. 07-097626

しかしながら、特許文献1および2に記載された技術は、煩雑な熱処理プロセスが必要であるため、生産性が低く、製造コストが高くなるという問題がある。また、特許文献3に記載された技術は、直接焼入れ後の焼ならしによって、残留オーステナイトの分解やマルテンサイトの回復、再結晶による強度、靭性、低降伏比の劣化が起こらないようにするために、Mn,Ni,Cu,Coの総量を所定量より多く添加する必要があり、製造コストが高くなるという問題がある。さらに、特許文献4に記載された技術は、製造条件や鋼板内位置によって、フェライトとマルテンサイト相の体積分率がばらつくことから、高強度化と低降伏比を安定して達成するまでには至っていないのが実情である。   However, since the techniques described in Patent Documents 1 and 2 require a complicated heat treatment process, there is a problem that productivity is low and manufacturing cost is high. Moreover, the technique described in Patent Document 3 is to prevent degradation of retained austenite, recovery of martensite, recovery of strength, toughness, and low yield ratio due to recrystallization by normalizing after direct quenching. In addition, it is necessary to add a total amount of Mn, Ni, Cu, Co more than a predetermined amount, and there is a problem that the manufacturing cost becomes high. Furthermore, the technique described in Patent Document 4 varies the volume fraction of the ferrite and martensite phases depending on the manufacturing conditions and the position in the steel plate, so that a high strength and a low yield ratio can be stably achieved. The situation is not reached.

そこで、本発明の目的は、多量の合金元素を添加することなく、また、煩雑な熱処理を施すことなく、高強度でかつ低降伏比で、しかも溶接性にも優れる高強度厚鋼板とその製造方法を提案することにある。具体的な本発明の開発目標は、降伏応力YSが650MPa以上、降伏比YRが80%以下の溶接性に優れた高強度厚鋼板である。   Accordingly, an object of the present invention is to produce a high-strength thick steel plate having high strength, a low yield ratio, and excellent weldability without adding a large amount of alloy elements and without performing complicated heat treatment, and its production. To propose a method. A specific development target of the present invention is a high strength thick steel plate excellent in weldability with a yield stress YS of 650 MPa or more and a yield ratio YR of 80% or less.

発明者らは、上記した課題を達成するために、強度、降伏比および溶接性に及ぼす各種要因について鋭意研究を重ねた。その結果、650MPa以上の降伏応力と、80%以下の低降伏比を安定して実現するとともに、優れた溶接性を確保するためには、厳格な成分調整に加えてさらに、炭素当量Ceqを0.35〜0.50mass%、Pcmを0.28mass%以下に制御することが重要であり、さらに、鋼板組織として、マルテンサイト相を面積分率で60%以上確保することが必要であることを見出した。また、650MPa以上の降伏応力と80%以下の低降伏比を有する厚鋼板を安定して得るためには、上記のように成分調整した鋼素材を熱間圧延し、その後、冷却速度と冷却停止温度を適正化した加速冷却処理、さらには、冷却停止後の昇温速度、再加熱温度、保持時間を適正化した再加熱処理を施すことが必要であることを見出した。
本発明は、上記知見に基づき、さらに検討を加えて完成したものである。
In order to achieve the above-described problems, the inventors have conducted intensive research on various factors affecting strength, yield ratio, and weldability. As a result, in order to stably realize a yield stress of 650 MPa or more and a low yield ratio of 80% or less, and to ensure excellent weldability, in addition to strict component adjustment, a carbon equivalent C eq is further set. 0.35~0.50Mass%, it is important to control the P cm below 0.28Mass%, further, as a steel sheet microstructure, it is necessary to ensure more than 60% of martensite phase at an area fraction I found out. In addition, in order to stably obtain a thick steel plate having a yield stress of 650 MPa or more and a low yield ratio of 80% or less, the steel material adjusted as described above is hot-rolled, and then the cooling rate and cooling stop. It has been found that it is necessary to perform an accelerated cooling process in which the temperature is optimized, and further a reheating process in which the heating rate after the cooling is stopped, the reheating temperature, and the holding time are optimized.
The present invention has been completed based on the above findings and further studies.

すなわち、木発明は、C:0.03〜0.2mass%、Si:0.05〜0.5mass%、Mn:0.8〜3mass%、P:0.02mass%以下、S:0.005mass%以下、Al:0.1mass%以下、N:0.007mass%以下を含有し、残部がFeおよび不可避的不純物からなり、下記(l)式;
Ceq=C+Si/24+Mn/6 ・・・(1)
ここで、C,Si,Mn:各元素の含有量(mass%)
で定義されるCeqが0.35〜0.5mass%、かつ、下記(2)式;
cm=C+Si/30+Mn/20 ・・・(2)
ここで、C,Si,Mn:各元素の含有量(mass%)
で定義されるPcmが0.28mass%以下であり、板厚断面の90%以上の領域にいて、マルテンサイト相の面積分率が60%以上で残部がベイナイト相からなり、降伏応力YSが650MPa以上、引張強さTSが826MPa以上、降伏比YRが80%以下の低降伏比高強度厚鋼板である。
That is, the present invention is C: 0.03-0.2 mass%, Si: 0.05-0.5 mass%, Mn: 0.8-3 mass%, P: 0.02 mass% or less, S: 0.005 mass % Or less, Al: 0.1 mass% or less, N: 0.007 mass% or less, with the balance being Fe and inevitable impurities, the following formula (l):
Ceq = C + Si / 24 + Mn / 6 (1)
Here, C, Si, Mn: content of each element (mass%)
The Ceq defined by the formula is 0.35 to 0.5 mass%, and the following formula (2):
P cm = C + Si / 30 + Mn / 20 (2)
Here, C, Si, Mn: content of each element (mass%)
In being defined P cm is less 0.28Mass%, and have your 90% or more areas of ItaAtsudan surface, made balance being bainite phase at an area fraction of martensite phase is 60% or more, yield stress It is a low yield ratio high strength thick steel plate with YS of 650 MPa or more, tensile strength TS of 826 MPa or more, and yield ratio YR of 80% or less.

本発明の厚鋼板は、上記成分組成に加えてさらに、Cu:0.1〜1mass%、Ni:0.1〜1mass%、Cr:1mass%以下、Mo:1mass%以下、Nb:0.1mass%以下、V:0.2mass%以下、Ti:0.03mass%以下、B:0.005mass%以下、Ca:0.005mass%以下、REM:0.02mass%以下およびMg:0.005mass%以下のうちから選ばれる1種または2種以上を含有し、下記(3)式;
eq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 ・・・(3)
ここで、C,Si,Mn,Cr,Ni,Mo,V,:各元素の含有量(mass%)
で定義されるCeqが0.35〜0.50mass%、かつ、下記(4)式;
で定義されるPcmが0.28mass%以下であることを特徴とする。
cm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B ・・・(4)
ここで、C,Si,Mn,Cu,Cr,Ni,Mo,V,B:各元素の含有量(mass%)
In addition to the above component composition, the thick steel plate of the present invention further includes Cu: 0.1 to 1 mass%, Ni: 0.1 to 1 mass%, Cr: 1 mass% or less, Mo: 1 mass% or less, Nb: 0.1 mass. % Or less, V: 0.2 mass% or less, Ti: 0.03 mass% or less, B: 0.005 mass% or less, Ca: 0.005 mass% or less, REM: 0.02 mass% or less, and Mg: 0.005 mass% or less 1 type or 2 types or more selected from among the following (3) formula;
C eq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (3)
Here, C, Si, Mn, Cr, Ni, Mo, V, content of each element (mass%)
C eq defined by the following formula is 0.35 to 0.50 mass%, and the following formula (4):
P cm defined by is 0.28 mass% or less.
P cm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 × B (4)
Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, B: Content of each element (mass%)

また、本発明は、C:0.03〜0.2mass%、Si:0.05〜0.5mass%、Mn:0.8〜3mass%、P:0.02mass%以下、S:0.005mass%以下、Al:0.1mass%以下、N:0.007mass%以下を含有し、残部がFeおよび不可避的不純物からなり、下記(1)式;
eq=C+Si/24+Mn/6 ・・・(1)
ここで、C,Si,Mn:各元素の含有量(mass%)
で定義されるCeqが0.35〜0.5mass%、かつ、下記(2)式;
cm=C+Si/30+Mn/20 ・・・(2)
ここで、C,Si,Mn:各元素の含有量(mass%)
で定義されるPcmが0.28mass%以下である鋼スラブを1000〜1250℃に加熱後、圧延終了温度を800℃以上とする熱間圧延し、Ar変態点以上の温度域から冷却速度5〜60℃/sで350℃以下の温度域まで加速冷却して一旦冷却を中断し、その後、昇温速度2℃/s以上で350〜550℃の温度まで再加熱し、該温度に15min以下保持してから冷却することを特徴とする、降伏応力YSが650MPa以上、引張強さTSが826MPa以上、降伏比YRが80%以下の低降伏比高強度厚鋼板の製造方法を提案する。
In the present invention, C: 0.03 to 0.2 mass%, Si: 0.05 to 0.5 mass%, Mn: 0.8 to 3 mass%, P: 0.02 mass% or less, S: 0.005 mass % Or less, Al: 0.1 mass% or less, N: 0.007 mass% or less, with the balance consisting of Fe and inevitable impurities, the following formula (1):
C eq = C + Si / 24 + Mn / 6 (1)
Here, C, Si, Mn: content of each element (mass%)
C eq defined by the following formula is 0.35 to 0.5 mass%, and the following formula (2):
P cm = C + Si / 30 + Mn / 20 (2)
Here, C, Si, Mn: content of each element (mass%)
A steel slab having a P cm defined by ≦ 0.28 mass% is heated to 1000 to 1250 ° C. and then hot-rolled to a rolling end temperature of 800 ° C. or higher, and a cooling rate from a temperature range of Ar 3 transformation point or higher. Accelerated cooling to a temperature range of 350 ° C. or lower at 5 to 60 ° C./s is temporarily interrupted, and then reheated to a temperature of 350 to 550 ° C. at a rate of temperature rise of 2 ° C./s or higher for 15 minutes. A method for producing a high yield strength steel sheet having a low yield ratio , having a yield stress YS of 650 MPa or more, a tensile strength TS of 826 MPa or more, and a yield ratio YR of 80% or less, characterized by cooling after being held below.

本発明の製造方法は、上記成分組成に加えてさらに、Cu:0.1〜1mass%、Ni:0.1〜1mass%、Cr:1mass%以下、Mo:1mass%以下、Nb:0.1mass%以下、V:0.2mass%以下、Ti:0.03mass%以下、B:0.005mass%以下、Ca:0.005mass%以下、REM:0.02mass%以下およびMg:0.005mass%以下のうちから選ばれる1種または2種以上を含有し、下記(3)式;
eq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 ・・・(3)
ここで、C,Si,Mn,Cr,Ni,Mo,V:各元素の含有量(mass%)
で定義されるCeqが0.35〜0.50mass%、かつ下記(4)式;
cm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B ・・・(4)
ここで、C,Si,Mn,Cu,Cr,Ni,Mo,V,B:各元素の含有量(mass%)
で定義されるPcmが0.28mass%以下であることを特徴とする。
In addition to the above component composition, the production method of the present invention further includes Cu: 0.1 to 1 mass%, Ni: 0.1 to 1 mass%, Cr: 1 mass% or less, Mo: 1 mass% or less, Nb: 0.1 mass. % Or less, V: 0.2 mass% or less, Ti: 0.03 mass% or less, B: 0.005 mass% or less, Ca: 0.005 mass% or less, REM: 0.02 mass% or less, and Mg: 0.005 mass% or less 1 type or 2 types or more selected from among the following (3) formula;
C eq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (3)
Here, C, Si, Mn, Cr, Ni, Mo, V: Content of each element (mass%)
C eq defined by the formula 0.35 to 0.50 mass%, and the following formula (4):
P cm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 × B (4)
Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, B: Content of each element (mass%)
P cm defined by is 0.28 mass% or less.

本発明によれば、優れた溶接性を有するともに、降伏応力が650MPa以上で、80%以下の低降伏比を有する厚鋼板を、煩雑な熱処理を施すことなく、安定して製造することができるので、鋼構造物の大型化や耐震性の向上、施工能率向上に大きく寄与することができる。   According to the present invention, it is possible to stably manufacture a thick steel plate having excellent weldability and having a yield stress of 650 MPa or more and a low yield ratio of 80% or less without performing complicated heat treatment. Therefore, it can greatly contribute to the enlargement of steel structures, the improvement of earthquake resistance, and the improvement of construction efficiency.

まず、本発明の厚鋼板における鋼組織について説明する。
従来、低降伏比高強度厚鋼板の製造プロセスとしては、フェライト+オーステナイト2相域への再加熱と焼入れを含む多段熱処理が一般的である。このようなプロセスで得られる厚鋼板の組織は、フェライト相を主体とし、硬質の第2相としてベイナイトあるいはマルテンサイトを分散させたものであり、フェライト相の体積分率によっては、降伏応力650MPa以上の高強度を安定して達成できないという問題がある。
First, the steel structure in the thick steel plate of the present invention will be described.
Conventionally, a multistage heat treatment including reheating and quenching to a ferrite + austenite two-phase region is generally used as a manufacturing process of a low yield ratio high strength thick steel plate. The structure of the thick steel plate obtained by such a process is mainly composed of a ferrite phase and dispersed bainite or martensite as a hard second phase. Depending on the volume fraction of the ferrite phase, the yield stress is 650 MPa or more. There is a problem that the high strength of can not be achieved stably.

一方、降伏比を考慮していない、従来の降伏応力650MPa以上の高強度厚鋼板の製造プロセスは、圧延後、直ちにあるいは再加熱後、Ac変態点以上の温度から比較的低温度域まで焼入れし、次いで、Ac変態点以下の温度域で、熱処理炉を用いた長時間の焼戻し処理を施すのが一般的であり、この場合の鋼組織は、焼戻しマルテンサイト相が主体となり、80%以下の降伏比を達成できないという問題がある。 On the other hand, the conventional manufacturing process of high-strength thick steel sheets with yield stress of 650 MPa or higher, which does not consider the yield ratio, is quenched from the temperature above the Ac 3 transformation point to a relatively low temperature range immediately after rolling or after reheating. Then, it is common to perform a long-time tempering treatment using a heat treatment furnace in a temperature range below the Ac 1 transformation point. In this case, the steel structure is mainly composed of a tempered martensite phase, and 80% There is a problem that the following yield ratio cannot be achieved.

そこで、本発明は、降伏応力650MPa以上の高強度と80%以下の低降伏比を安定して達成するための鋼板組織として、可動転位を多量に含むマルテンサイト相を面積分率で60%以上とすることを必須の要件とする。というのは、圧延後、直ちに焼入れたマルテンサイト組織は、転位密度が非常に高く、また非常に硬い相であるために、高い引張強度を有するが、多量に導入された可動転位がYSの極端な上昇を抑制するため、高強度と低降伏比を両立することができるからである。   Therefore, the present invention provides a steel sheet structure for stably achieving a high strength with a yield stress of 650 MPa or more and a low yield ratio of 80% or less, and a martensite phase containing a large amount of movable dislocations in an area fraction of 60% or more. Is an essential requirement. This is because the martensite structure quenched immediately after rolling has a very high dislocation density and a very hard phase, and thus has a high tensile strength. This is because it is possible to achieve both high strength and a low yield ratio in order to suppress excessive increase.

しかし、焼入れたままのマルテンサイト組織は、母材の延性や靭性を低下させることから、低降伏比高強度厚鋼板の組織としては、これまで積極的に利用されていない。そこで、従来、焼入れままのマルテンサイト鋼の延性、靭性を向上するために、熱処理炉を用いた長時間の焼戻し処理を行い、焼戻しマルテンサイト相としているのが一般的である。しかし、この場合、可動転位が消失し、降伏応力の大幅な上昇を招くため、80%以下の低降伏比を達成できないという問題がある。   However, as-quenched martensite structure reduces the ductility and toughness of the base metal, and thus has not been actively used as a structure of a low yield ratio high-strength thick steel sheet. Therefore, conventionally, in order to improve the ductility and toughness of the as-quenched martensitic steel, it is common to perform a tempering treatment for a long time using a heat treatment furnace to obtain a tempered martensite phase. However, in this case, the movable dislocation disappears and the yield stress is significantly increased, so that there is a problem that a low yield ratio of 80% or less cannot be achieved.

そこで、本発明では、厳格な成分調整を行うとともに、熱間圧延後の加速冷却と、冷却停止後の再加熱処理条件を厳格に規定することによって、延性と靭性の向上を図りつつ、高強度と低降伏比を両立させるための好適なミクロ組織を、以下のように規定する。   Therefore, in the present invention, while strict component adjustment is performed and accelerated cooling after hot rolling and reheating treatment conditions after stopping cooling are strictly defined, while improving ductility and toughness, high strength is achieved. A suitable microstructure for achieving both a low yield ratio and a low yield ratio is defined as follows.

まず、本発明の厚鋼板が有するミクロ組織は、板厚断面の90%以上の領域において、マルテンサイトが面積分率で60%以上であることが必要である。マルテンサイト相を除く相としては、実質的にベイナイト相である。マルテンサイトの面積分率が60%未満では、降伏応力650MPa以上の高強度と80%以下の低降伏比を両立させることができない。このようなミクロ組織は、板厚断面の90%以上の領域において満足していればよい。 First, the microstructure included in the steel plate of the present invention, at 90% or more of the regions of the plate thickness cross section, it is necessary that martensite is in an area fraction of 60% or more. The phase excluding the martensite phase is substantially a bainite phase. If the area fraction of martensite is less than 60%, it is impossible to achieve both high strength with a yield stress of 650 MPa or more and low yield ratio of 80% or less . Such a microstructure may be satisfied in an area of 90% or more of the plate thickness cross section.

マルテンサイト相以外の相として、フェライト、パーライトおよびセメンタイト等の組織が混在する場合には、強度が低下するため、面積分率は少ない方が好ましい。また、島状マルテンサイトが混在する場合には、靭性が低下するので、この面積分率も少ない方が好ましい。ただし、マルテンサイト相およびベイナイト相以外に混在する組織が、面積分率で10%以下の場合には、それらの影響が無視することができる。   When a structure such as ferrite, pearlite, and cementite is mixed as a phase other than the martensite phase, the strength is lowered, so that the area fraction is preferably small. Further, when island martensite coexists, the toughness is lowered, so it is preferable that this area fraction is also small. However, when the structure mixed in addition to the martensite phase and the bainite phase is 10% or less in area fraction, the influence can be ignored.

次に、本発明の厚鋼板の成分組成を限定する理由について説明する。
C:0.03〜0.2mass%、
Cは、鋼の強度を高める効果が大きく、構造用鋼材に求められる強度を確保するのに必要な成分である。上記効果を得るためには、Cを0.03mass%以上含有させる必要がある。一方、0.2mass%を超える添加は、溶接熱影響部(HAZ)靭性や耐溶接割れ性を低下させるとともに、母材の靭性をも低下させる。そのため、本発明では、Cは0.03〜0.2mass%の範囲とする。好ましくは0.05〜0.15mass%の範囲である。
Next, the reason which limits the component composition of the thick steel plate of this invention is demonstrated.
C: 0.03-0.2 mass%,
C has a large effect of increasing the strength of steel and is a component necessary for ensuring the strength required for structural steel. In order to acquire the said effect, it is necessary to contain C 0.03 mass% or more. On the other hand, the addition exceeding 0.2 mass% reduces the weld heat affected zone (HAZ) toughness and weld crack resistance, as well as the toughness of the base material. Therefore, in the present invention, C is in the range of 0.03 to 0.2 mass%. Preferably it is the range of 0.05-0.15 mass%.

Si:0.05〜0.5mass%
Siは、脱酸材として添加される場合には、少なくとも0.05mass%の添加が必要である。しかし、0.5%を超えて添加すると、母材の靭性が劣化するとともに、溶接性、HAZ靭性が顕著に低下する。そのため、Siは0.05〜0.5mass%の範囲とする。好ましくは0.05〜0.35mass%の範囲である。
Si: 0.05-0.5 mass%
When Si is added as a deoxidizing material, it is necessary to add at least 0.05 mass%. However, if added over 0.5%, the toughness of the base material deteriorates, and the weldability and the HAZ toughness significantly decrease. Therefore, Si is set to a range of 0.05 to 0.5 mass%. Preferably it is the range of 0.05-0.35 mass%.

Mn:0.8〜3mass%
Mnは、鋼の強度を高める効果があり、本発明では、降伏応力650MPa以上を確保するため、0.8mass%以上の含有を必要とする。一方、3mass%を超えて含有すると、母材の靭性およびHAZ靭性が著しく低下する。よって、Mnは0.8〜3mass%の範囲とする。好ましくは1.0〜2.5mass%である。
Mn: 0.8-3 mass%
Mn has an effect of increasing the strength of steel, and in the present invention, it is necessary to contain 0.8 mass% or more in order to ensure a yield stress of 650 MPa or more. On the other hand, when it contains exceeding 3 mass%, the toughness of a base material and HAZ toughness will fall remarkably. Therefore, Mn is set to a range of 0.8 to 3 mass%. Preferably it is 1.0-2.5 mass%.

P:0.02mass%以下
Pは、鋼中に不可避的不純物として混入し、鋼の強度を高めると共に、靭性を劣化させる元素であり、特に、溶接部の靭性を大きく劣化させるので、できるだけ低減することが望ましい。Pが0.02mass%を超えると、この傾向が顕著となるため、この値を上限とする。なお、過度のPの低減は、精錬コストの上昇を招くため、Pの下限は0.005mass%程度とするのが望ましい。
P: 0.02 mass% or less P is an element that is mixed as an inevitable impurity in the steel, increases the strength of the steel, and degrades the toughness. In particular, it greatly reduces the toughness of the welded portion, so it is reduced as much as possible. It is desirable. When P exceeds 0.02 mass%, this tendency becomes remarkable, so this value is set as the upper limit. In addition, since excessive reduction of P leads to an increase in refining cost, it is desirable that the lower limit of P is about 0.005 mass%.

S:0.005mass%以下
Sは、鋼中に不可避的不純物として混入し、母材および溶接部の靭性を劣化させる元素であり、できるだけ低減することが望ましい。特に、Sが0.005mass%を超えると、この傾向が顕著となる。よって、Sは0.005mass%以下とする。
S: 0.005 mass% or less S is an element that is mixed as an inevitable impurity in steel and deteriorates the toughness of the base metal and the welded portion, and is desirably reduced as much as possible. In particular, when S exceeds 0.005 mass%, this tendency becomes remarkable. Therefore, S is set to 0.005 mass% or less.

Al:0.1mass%以下
Alは、脱酸剤として添加される成分であり、高張力鋼の溶鋼脱酸に於いては、もっとも汎用的に使われている。また、鋼中のNをAlNとして固定し、母材の靭性向上に寄与する。このような効果は、Al:0.005mass%以上の添加で認められる。一方、0.1mass%を超える添加は、母材の靭性の低下を招くとともに、溶接時に溶接金属部に混入して、靭性を低下させる。このため、Alの含有量は0.1mass%以下とする。好ましくは、0.01〜0.07mass%である。
Al: 0.1 mass% or less Al is a component added as a deoxidizer, and is most commonly used in molten steel deoxidation of high-tensile steel. In addition, N in the steel is fixed as AlN, which contributes to improving the toughness of the base material. Such an effect is recognized by addition of Al: 0.005 mass% or more. On the other hand, addition exceeding 0.1 mass% leads to a decrease in the toughness of the base material, and is mixed into the weld metal part during welding to decrease the toughness. For this reason, content of Al shall be 0.1 mass% or less. Preferably, it is 0.01-0.07 mass%.

N:0.007mass%以下
Nは、不可避的に鋼中に含まれる不純物成分である。Nが0.007mass%を超えて含有すると、母材および溶接部靭性が著しく低下する。このため、Nの含有量は0.007mass%以下とする。
N: 0.007 mass% or less N is an impurity component inevitably contained in the steel. When N exceeds 0.007 mass%, the base material and weld zone toughness are significantly reduced. For this reason, content of N shall be 0.007 mass% or less.

eq:0.35〜0.50mass%
本発明では、上記した成分組成範囲内で、下記(1)式で定義される炭素当量Ceqが0.35〜0.50mass%となるように、各成分の含有量を調整する必要がある。
eq=C+Si/24+Mn/6 ・・・(1)
ここで、C,Si,Mn:各元素の含有量(mass%)
eqが0.35mass%未満では、圧延後の加速冷却における焼入れ性が不足し、所望の降伏応力650MPa以上を確保できなくなる。一方、Ceqが0.50mass%を超えると、母材靭性および一様伸びが低下する。そのため、Ceqは0.35〜0.50mass%の範囲とする必要がある。
C eq : 0.35 to 0.50 mass%
In the present invention, it is necessary to adjust the content of each component so that the carbon equivalent C eq defined by the following formula (1) is 0.35 to 0.50 mass% within the above-described component composition range. .
C eq = C + Si / 24 + Mn / 6 (1)
Here, C, Si, Mn: content of each element (mass%)
When C eq is less than 0.35 mass%, the hardenability in accelerated cooling after rolling becomes insufficient, and a desired yield stress of 650 MPa or more cannot be secured. On the other hand, if C eq exceeds 0.50 mass%, the base material toughness and the uniform elongation decrease. Therefore, C eq needs to be in the range of 0.35 to 0.50 mass%.

本発明では、さらに、上記した成分組成範囲内で、下記(2)式で定義されるPcmが0.28mass%以下となるように、各成分の含有量を調整する必要がある。
cm=C+Si/30+Mn/20 ・・・(2)
ここで、C,Si,Mn:各元素の含有量(mass%)
cmは、溶接部の低温割れ性の指標であり、できるだけ低いことが望ましい。Pcmが0.28mass%を超えると、溶接性が著しく低下するため、Pcmは0.28mass%以下に調整する必要がある。
In the present invention, it is further necessary to adjust the content of each component so that P cm defined by the following formula (2) is 0.28 mass% or less within the above component composition range.
P cm = C + Si / 30 + Mn / 20 (2)
Here, C, Si, Mn: content of each element (mass%)
P cm is an index of the cold cracking property of the weld, and is desirably as low as possible. When P cm exceeds 0.28 mass%, the weldability is remarkably lowered. Therefore, P cm needs to be adjusted to 0.28 mass% or less.

本発明の厚鋼板は、上記必須成分に加えて、必要に応じて、Cu,Ni,Cr,Mo,Nb,V,Ti,B,Ca,REMおよびMgのうちから選ばれる1種または2種以上を以下の範囲で含有することができる。
Cu:0.1〜1mass%およびNi:0.1〜1mass%から選ばれる1種または2種
CuおよびNiは、高靭性を保ちつつ強度を高める効果があり、HAZ靭性への悪影響も小さい。そのため、高強度化には有用な元素であり、必要に応じて含有させることができる。
Cuは0.1%以上含有するのが好ましいが、含有量が1mass%を超えると、熱間脆性を起こして鋼板の表面性状を劣化させる。よって、Cuは0.1〜1mass%の範囲で添加するのが好ましい。より好ましくは0.2〜0.7mass%の範囲である。
Niは、0.1mass%以上含有するのが好ましいが、1mass%を超えて含有しても、効果が飽和し、含有量に見合う効果が得られず、コスト上昇を招くだけである。よって、Niは0.1〜1mass%の範囲とするのが好ましい。より好ましくは0.2〜0.8mass%である。
In addition to the above essential components, the thick steel plate of the present invention is optionally selected from one or two selected from Cu, Ni, Cr, Mo, Nb, V, Ti, B, Ca, REM and Mg. The above can be contained in the following ranges.
One or two kinds selected from Cu: 0.1 to 1 mass% and Ni: 0.1 to 1 mass% Cu and Ni have an effect of increasing strength while maintaining high toughness, and have a small adverse effect on HAZ toughness. Therefore, it is an element useful for increasing the strength, and can be contained as necessary.
Cu is preferably contained in an amount of 0.1% or more. However, if the content exceeds 1 mass%, hot brittleness is caused to deteriorate the surface properties of the steel sheet. Therefore, Cu is preferably added in the range of 0.1 to 1 mass%. More preferably, it is the range of 0.2-0.7 mass%.
Ni is preferably contained in an amount of 0.1 mass% or more, but even if contained in excess of 1 mass%, the effect is saturated, an effect commensurate with the content cannot be obtained, and only an increase in cost is caused. Therefore, Ni is preferably in the range of 0.1 to 1 mass%. More preferably, it is 0.2-0.8 mass%.

Cr:1mass%以下、Mo:1mass%以下、Nb:0.1mass%以下、V:0.2mass%以下、Ti:0.03mass%以下およびB:0.005mass%以下のうちから選ばれる1種または2種以上
Cr,Mo,Nb,V,Ti,Bは、いずれも鋼の強度向上に寄与する元素であり、必要に応じて添加することができる。
Crは、0.05mass%以上含有するのが好ましいが、1mass%を超えると、HAZ靭性を劣化させるため、1mass%以下とするのが望ましい。
Moは、0.05mass%以上含有するのが好ましいが、1mass%を超えると母材靭性およびHAZ靭性に悪影響を及ぼす。よって、Moは1mass%以下添加するのが望ましい。
Nbは、0.005mass%以上含有するのが好ましいが、0.1mass%を超えると、母材靭性およびHAZ靭性を劣化させるので、0.1mass%以下とするのが好ましい。
Vは、0.01mass%以上含有するのが好ましいが、0.2mass%を超えると、HAZ靭性が低下するので、0.2mass%以下が望ましい。
Tiは、0.005mass%以上含有することにより、強度の向上に寄与する。また、Nとの親和力が強く、凝固時にTiNとして析出して、HAZでのオーステナイト粒の粗大化を抑制し、HAZの高靭性化に寄与する。一方、0.03mass%を超えて含有すると、母材靭性を劣化させる。よって、Tiは0.03mass%以下添加することが望ましい。
Bは、焼入れ性の向上を介して、鋼の強度を増加させる作用を有する。しかし、Bの0.005mass%を超える含有は、焼入れ性を著しく高めて、母材の靭性、延性の劣化をもたらす。そのため、Bは0.005mass%以下添加するのが好ましい。より好ましくは、0.0003〜0.002mass%の範囲である。
Cr: 1 mass% or less, Mo: 1 mass% or less, Nb: 0.1 mass% or less, V: 0.2 mass% or less, Ti: 0.03 mass% or less, and B: 0.005 mass% or less Alternatively, two or more of Cr, Mo, Nb, V, Ti, and B are elements that contribute to improving the strength of steel, and can be added as necessary.
Cr is preferably contained in an amount of 0.05 mass% or more. However, if it exceeds 1 mass%, the HAZ toughness is deteriorated.
Mo is preferably contained in an amount of 0.05 mass% or more, but if it exceeds 1 mass%, it adversely affects the base material toughness and the HAZ toughness. Therefore, it is desirable to add 1 mass% or less of Mo.
Nb is preferably contained in an amount of 0.005 mass% or more. However, if it exceeds 0.1 mass%, the base material toughness and the HAZ toughness are deteriorated. Therefore, the Nb content is preferably 0.1 mass% or less.
V is preferably contained in an amount of 0.01 mass% or more, but if it exceeds 0.2 mass%, the HAZ toughness decreases, so 0.2 mass% or less is desirable.
Ti contributes to the improvement of strength by containing 0.005 mass% or more. Further, it has a strong affinity with N, and precipitates as TiN during solidification, thereby suppressing the austenite grain coarsening in the HAZ and contributing to high toughness of the HAZ. On the other hand, when it contains exceeding 0.03 mass%, base material toughness will deteriorate. Therefore, Ti is preferably added in an amount of 0.03 mass% or less.
B has the effect of increasing the strength of the steel through improving hardenability. However, if the content of B exceeds 0.005 mass%, the hardenability is remarkably increased, and the toughness and ductility of the base material are deteriorated. Therefore, B is preferably added in an amount of 0.005 mass% or less. More preferably, it is the range of 0.0003-0.002 mass%.

Ca:0.005mass%以下、REM:0.02mass%以下およびMg:0.005mass%以下のうちから選ばれる1種または2種以上
Ca,REMおよびMgは、いずれも靭性向上に寄与する成分であり、必要に応じて添加することができる。
Caは、結晶粒の微細化を介して靭性を向上させる有用な成分であり、0.001mass%以上含有することが好ましい。しかし、0.005mass%を超えて含有しても、上記効果が飽和するため、0.005mass%を上限とするのが好ましい。
REMは、0.002mass%以上含有するのが好ましいが、0.02mass%を超えて含有しても効果が飽和するため、0.02mass%を上限とするのが好ましい。
Mgは、結晶粒の微細化を介して靭性を向上させる効果のある成分であり、0.001mass%以上添加するのが好ましい。しかし、0.005mass%を超えて添加しても、その効果が飽和するため、0.005mass%を上限とするのが好ましい。
One or more selected from Ca: 0.005 mass% or less, REM: 0.02 mass% or less, and Mg: 0.005 mass% or less Ca, REM, and Mg are all components that contribute to improved toughness. Yes, it can be added as needed.
Ca is a useful component that improves toughness through refinement of crystal grains, and is preferably contained in an amount of 0.001 mass% or more. However, even if it contains exceeding 0.005 mass%, since the said effect is saturated, it is preferable to make 0.005 mass% into an upper limit.
Although it is preferable to contain REM 0.002 mass% or more, since an effect will be saturated even if it contains exceeding 0.02 mass%, it is preferable to make 0.02 mass% into an upper limit.
Mg is a component having an effect of improving toughness through refinement of crystal grains, and is preferably added in an amount of 0.001 mass% or more. However, even if added over 0.005 mass%, the effect is saturated, so it is preferable to set the upper limit to 0.005 mass%.

なお、本発明の厚鋼板に上記の選択元素を添加する場合には、上記の成分組成範囲内において、下記(3)式で定義される炭素当量Ceqが0.35〜0.5mass%となるように、各成分の含有量を調整する必要がある。
eq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 ・・・(3)
ここで、C,Si,Mn,Ni,Cr,Mo,V:各元素の含有量(mass%)
eqが0.35mass%未満では、圧延後の加速冷却における焼入れ性が不足し、所望の降伏応力650MPa以上を確保できなくなる。一方、Ceqが0.50mass%を超えると、母材靭性および一様伸びが低下する。そのため、Ceqは0.35〜0.50mass%の範囲とする必要がある。
In the case of adding the above selection element steel plate of the present invention, within the chemical composition range of the above, and 0.35~0.5Mass% carbon equivalent C eq, defined by the following equation (3) Therefore, it is necessary to adjust the content of each component.
C eq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (3)
Here, C, Si, Mn, Ni, Cr, Mo, V: Content of each element (mass%)
When C eq is less than 0.35 mass%, the hardenability in accelerated cooling after rolling becomes insufficient, and a desired yield stress of 650 MPa or more cannot be secured. On the other hand, if C eq exceeds 0.50 mass%, the base material toughness and the uniform elongation decrease. Therefore, C eq needs to be in the range of 0.35 to 0.50 mass%.

さらに、本発明の厚鋼板に上記の選択元素を添加する場合には、上記した成分組成範囲内において、下記(4)式で定義されるPcmが0.28mass%以下となるように、各成分の含有量を調整する必要がある。
cm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B ・・・(4)
ここで、C,Si,Mn,Cu,Cr,Ni,Mo,V,B:各元素の含有量(mass%)
cmは、溶接部の低温割れ性の指標であり、できるだけ低いことが望ましい。Pcmが0.28mass%を超えると、溶接性が著しく低下するため、Pcmは0.28mass%以下に調整する必要がある。
Further, when the above-mentioned selective element is added to the thick steel plate of the present invention, each P cm defined by the following formula (4) is 0.28 mass% or less within the above component composition range. It is necessary to adjust the content of the components.
P cm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 × B (4)
Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, B: Content of each element (mass%)
P cm is an index of the cold cracking property of the weld, and is desirably as low as possible. When P cm exceeds 0.28 mass%, the weldability is remarkably lowered. Therefore, P cm needs to be adjusted to 0.28 mass% or less.

本発明の厚鋼板は、上記以外の成分は、Feおよび不可避的不純物からなることが好ましい。ただし、本発明の効果を害しない範囲であれば、上記以外の成分の含有を拒むものではない。   In the thick steel plate of the present invention, the components other than the above are preferably composed of Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

次に、本発明に係る厚鋼板の製造方法について説明する。なお、以降説明する温度は、特に断らない限り、板厚中央部(1/2t)の温度を意味する。
本発明で厚鋼板の製造に使用する素材は、上記した成分組成を有する溶鋼を転炉、電気炉、真空溶解炉等で溶製し、その後、造塊−分塊圧延法あるいは連続鋳造法等、通常公知の方法で鋼素材(スラブ)としたものである。
これらの鋼素材は、熱間圧延を行うに先立って、1000〜1250℃に加熱する。加熱温度が1000℃未満では、熱間圧延での変形抵抗が高くなり、1パス当たりの圧下量が大きく取れなくなるため、圧延パス数が増加し、圧延能率の低下を招くとともに、鋼素材中の鋳造欠陥を圧着することができない場合がある。一方、加熱温度が1250℃を超えると、加熱時に生成するスケールによって表面疵が生じやすく、圧延後の手入れ負荷が増大する。
Next, the manufacturing method of the thick steel plate concerning this invention is demonstrated. In addition, the temperature demonstrated hereafter means the temperature of plate | board thickness center part (1 / 2t) unless there is particular notice.
The material used for the production of the thick steel plate in the present invention is the melting of molten steel having the above-described composition in a converter, electric furnace, vacuum melting furnace, etc., and then ingot-bundling rolling method or continuous casting method, etc. The steel material (slab) is obtained by a generally known method.
These steel materials are heated to 1000 to 1250 ° C. prior to hot rolling. When the heating temperature is less than 1000 ° C., the deformation resistance in hot rolling is high, and the amount of reduction per pass cannot be increased. Therefore, the number of rolling passes increases, and the rolling efficiency is lowered. Sometimes casting defects cannot be crimped. On the other hand, when the heating temperature exceeds 1250 ° C., surface flaws are likely to occur due to the scale generated during heating, and the maintenance load after rolling increases.

加熱した鋼素材は、圧延終了温度を800℃以上とする熱間圧延により所定の板厚まで圧延する。圧延終了温度を800℃以上とする以外の熱間圧延条件は、所定の板厚および形状を満足できればよく、特に制限されない。なお、板厚が80mmを超える極厚鋼板の場合には、ザク疵を圧着するために、1パスあたりの圧下率が15%以上の圧延パスを、少なくとも1パス以上確保することが望ましい。また、圧延終了温度が800℃未満では、変形抵抗が高くなりすぎて、圧延荷重が増大し、圧延機への負担が大きくなる。さらに、厚肉材の圧延温度を800℃未満まで低下させるには、圧延途中で待機する必要があり、生産性を大きく阻害する。以上のことから、本発明では、圧延終了温度を800℃以上とする。   The heated steel material is rolled to a predetermined plate thickness by hot rolling with a rolling end temperature of 800 ° C. or higher. The hot rolling conditions other than setting the rolling end temperature to 800 ° C. or higher are not particularly limited as long as the predetermined plate thickness and shape can be satisfied. In the case of an extremely thick steel plate having a thickness exceeding 80 mm, it is desirable to secure at least one or more rolling passes with a reduction rate of 15% or more per pass in order to press-fit the zaku soot. Moreover, if rolling end temperature is less than 800 degreeC, a deformation resistance will become high too much, a rolling load will increase and the burden to a rolling mill will become large. Furthermore, in order to lower the rolling temperature of the thick material to less than 800 ° C., it is necessary to wait in the middle of rolling, which greatly hinders productivity. From the above, in the present invention, the rolling end temperature is set to 800 ° C. or higher.

圧延終了後の厚鋼板は、Ar変態点以上の温度域から平均冷却速度5〜60℃/sので、350℃以下の温度域まで加速冷却する必要がある。圧延終了後の冷却速度が5℃/s未満では、加速冷却後のミクロ組織中のフェライト相やパーライト相の面積分率が増加し、所望のミクロ粗織を得ることができず、結果として降伏応力650MPa以上を確保することができなくなる。一方、冷却速度が60℃/sを超えると、温度制御や均一冷却が困難となり、鋼板位置による材質のばらつきが生じる。また、加速冷却の冷却停止温度が350℃よりも高くなると、ベイナイト相やフェライト相の面積分率が増加し、目的とするマルテンサイト相の面積分率60%以上とすることができず、結果として、降伏応力650MPa以上を確保できなくなる。なお、加速冷却の停止温度は、室温以上であればよいが、その後の再加熱処理の能率を考慮すると、150℃以上とするのが好ましい。 The thick steel plate after rolling is required to be accelerated and cooled to a temperature range of 350 ° C. or lower because the average cooling rate is 5 to 60 ° C./s from the temperature range of the Ar 3 transformation point or higher. If the cooling rate after rolling is less than 5 ° C / s, the area fraction of the ferrite phase and pearlite phase in the microstructure after accelerated cooling increases, and the desired microrough weave cannot be obtained, resulting in yielding. It becomes impossible to ensure a stress of 650 MPa or more. On the other hand, when the cooling rate exceeds 60 ° C./s, temperature control and uniform cooling become difficult, and the material varies depending on the steel plate position. In addition, when the cooling stop temperature of accelerated cooling is higher than 350 ° C., the area fraction of the bainite phase and the ferrite phase increases, and the area fraction of the target martensite phase cannot be set to 60% or more. As a result, a yield stress of 650 MPa or more cannot be secured. The accelerated cooling stop temperature may be room temperature or higher, but it is preferably 150 ° C. or higher in consideration of the efficiency of the subsequent reheating treatment.

加速冷却終了後の厚鋼板は、一旦冷却を中断し、350〜550℃の温度域まで2℃/s以上の昇温速度で再加熱し、その温度で15min以下保持し、その後、冷却する。昇温速度が2℃/s未満では、目的とする再加熱温度までの昇温時間が長くなるため、生産性が低下する。また、再加熱温度が350℃未満であると、母材靭性および延性の改善効果が発現しない。一方、再加熱温度が550℃を超えると、マルテンサイト相中の可動転位が消失し、大幅な降伏強度の上昇を招き、降伏比80%以下を得ることができない。再加熱後の保持時間は、鋼板の全板厚において目標の再加熱温度まで加熱されれば短時間でもよいが、生産性を阻害しないため、保持時間を15min以下とする。ここで、再加熱の手段としては、雰囲気加熱炉、ガス炎、誘導加熱等の方法があるが、経済性、制御性等を考慮すると誘導加熱が好ましい。再加熱後の冷却方法、条件は特に制限されない。   After the accelerated cooling is finished, the thick steel plate is temporarily cooled, reheated to a temperature range of 350 to 550 ° C. at a heating rate of 2 ° C./s or more, held at that temperature for 15 minutes or less, and then cooled. When the rate of temperature increase is less than 2 ° C./s, the temperature increase time to the target reheating temperature becomes long, so that productivity is lowered. Further, if the reheating temperature is less than 350 ° C., the effect of improving the base material toughness and ductility is not exhibited. On the other hand, if the reheating temperature exceeds 550 ° C., movable dislocations in the martensite phase disappear, resulting in a significant increase in yield strength, and a yield ratio of 80% or less cannot be obtained. The holding time after reheating may be a short time as long as it is heated to the target reheating temperature for the entire thickness of the steel sheet, but the holding time is set to 15 min or less in order not to impede productivity. Here, as means for reheating, there are methods such as an atmosphere heating furnace, a gas flame, induction heating, etc., but in consideration of economy, controllability, etc., induction heating is preferable. The cooling method and conditions after reheating are not particularly limited.

上記成分組成の鋼素材を用いて、上記条件の熱間圧延、加速冷却および再加熱処理を施すことにより、板厚断面の90%以上の領域において、鋼板組織が、マルテンサイトの面積分率が60%以上で残部がベイナイト相からなり、優れた母材靭性、延性を有するとともに、降伏応力650MPa以上と降伏比80%以下とを兼備する、溶接性に優れた低降伏比高強度鋼板を得ることができる。 Using a steel material of the chemical composition, hot rolling under the above conditions, by performing accelerated cooling and reheating, at 90% or more of the regions of the plate thickness cross section, the steel plate structure is, the area fraction of martensite Is a low-yield-strength steel sheet with excellent weldability, having a base metal toughness and ductility, and having a yield stress of 650 MPa or more and a yield ratio of 80% or less. Obtainable.

表1に示した各種成分組成を有するA〜Oの鋼を転炉で溶製し、取鍋精錬で調整し、連続鋳造法で鋼スラブとした。その後、これらの鋼スラブを、表2に示す条件で、熱間圧延し、加速冷却し、再加熱し、その後、冷却し、表2に示す板厚の厚鋼板を得た。この厚鋼板から試験材を採取し、下記のマルテンサイト面積分率の測定、引張試験、衝撃試験および溶接部の衝撃試験に供した。
<マルテンサイト面積分率の測定>
鋼板(試験材)の圧延方向に垂直な断面を鏡面研摩し、ナイタール液に浸漬し、腐食してマルテンサイト組織を現出させてから、この断面組織を走査型電子顕微鏡を用いて1000倍の倍率で写真撮影し、その結果を画像解析することによってマルテンサイトの面積分率を求めた。
<引張試験>
上記のようにして得た各厚鋼板の板厚1/2位置から、JIS4号引張試験片を採取し、JIS Z2241の規定に準拠して引張試験を実施し、引張特性(降伏応力YS、引張り強さTS、伸びEl)を測定した。なお、本実施例では、降伏応力YS:650MPa以上、降伏比YR:80%以下、全伸び:15%以上を目標特性とした。
<衝撃試験>
各厚鋼板の板厚1/2位置から、JIS Z2202の規定に準拠してVノッチ試験片を採取し、JIS Z22242の規定に準拠してシャルピー衝撃試験を実施し、0℃における吸収エネルギー(vE)を求め、母材靭性を評価した。なお、本実施例では、試験数は各3ずつ行い、すべてvE>100Jを目標とした。
<斜めy形溶接割れ試験>
各厚鋼板から、JIS Z3158に準拠して、斜めy形溶接割れ試験を実施し、予熱50℃におけるルート部割れ発生率を測定した。なお、供給ワイヤは、JIS Z3212相当を使用した。
Steels A to O having various component compositions shown in Table 1 were melted in a converter, adjusted by ladle refining, and made into a steel slab by a continuous casting method. Thereafter, these steel slabs were hot-rolled under the conditions shown in Table 2, acceleratedly cooled, reheated and then cooled to obtain thick steel plates having the thicknesses shown in Table 2. Test materials were sampled from the thick steel plates and subjected to the following martensite area fraction measurement, tensile test, impact test, and weld impact test.
<Measurement of martensite area fraction>
A cross section perpendicular to the rolling direction of the steel sheet (test material) is mirror-polished, dipped in a nital solution, corroded to reveal a martensite structure, and this cross-sectional structure is 1000 times larger using a scanning electron microscope. The area fraction of martensite was obtained by taking a picture at a magnification and analyzing the result.
<Tensile test>
JIS No. 4 tensile test specimens were collected from the thickness 1/2 position of each thick steel plate obtained as described above, and subjected to a tensile test in accordance with the provisions of JIS Z2241, tensile properties (yield stress YS, tensile Strength TS, elongation El) were measured. In this example, the target characteristics were yield stress YS: 650 MPa or more, yield ratio YR: 80% or less, and total elongation: 15% or more.
<Impact test>
A V-notch specimen was taken from the position of half the thickness of each steel plate in accordance with JIS Z2202, the Charpy impact test was conducted in accordance with JIS Z22242, and the absorbed energy at 0 ° C. (vE 0 ) and the toughness of the base metal was evaluated. In this example, the number of tests was 3 each, and all targets were vE 0 > 100 J.
<Slant y-shaped weld crack test>
From each thick steel plate, an oblique y-type weld crack test was carried out in accordance with JIS Z3158, and the root crack generation rate at 50 ° C. preheating was measured. In addition, JIS Z3212 equivalent was used for the supply wire.

上記測定の結果を表2に併記して示した。表2から、本発明の条件に適合する発明例の厚鋼板は、いずれも、降伏応力が650MPa以上で、降伏比が80%以下、全伸びが15%以上、0℃での吸収エネルギーvE>100Jの高強度、低降伏比、高延性かつ高靭性の母材特性を有するとともに、斜めy形溶接割れ試験においても、ルート部での割れ発生のない溶接性に優れたものであることがわかる。
これに対して、本発明の条件を外れる比較例の厚鋼板は、母材強度、降伏比、延性、母材靭性および溶接割れ性のうち、いずれか1つ以上の特性が劣ったものしか得られない。
例えば、圧延終了後の冷却速度が本発明範囲より低い比較例(No.3)は、マルテンサイト相が生成されず、降伏応力が目標値に対して大きく劣っている。また、再加熱処理を行わない比較例(No.4)は、マルテンサイト相の脆化が著しく、目標とする靭性が得られない。また、再加熱保持時間が本発明範囲に対して長い比較例(No.10)は、マルテンサイト相中の可動転位が消失して降伏応力の上昇を招き、結果として低降伏比が得られない。
再加熱温度が本発明範囲より高い比較例(No.5)は、マルテンサイト相中の可動転位が消失して降伏応力が高く、結果として低降伏比が得られない。一方、再加熱温度が本発明範囲より低い比較例(No.15)は、マルテンサイト相の靭性向上が不十分で、母材靭性が劣っている。
圧延終了後の冷却停止温度が本発明範囲より高い比較例(No.7)は、マルテンサイトの面積分率が低くなり、降伏応力が目標値より低い。一方、圧延終了温度が本発明範囲より低い比較例(No.16)は、一部にフェライト相が生成してマルテンサイト面積分率が低くなり、やはり降伏応力が目標値より低い。
C量が本発明範囲より高い比較例(No.19)は、マルテンサイト相の靭性、延性の劣化が著しく、また溶接性も劣っている。一方、C量が本発明範囲より低い比較例(No.22)は、加速冷却時にマルテンサイト相が生成しないため、降伏応力が目標値に達していない。CeqおよびPcmが本発明範囲より高い比較例(No.20)は、母材靭性、延性および溶接性が目標特性より劣っている。Ceqが本発明範囲より低い比較例(No.23)は、加速冷却時にマルテンサイト相が生成せず、降伏応力が低い。さらに、Mn量が本発明範囲より低い比較例(No.21)は、加速冷却時の焼入性が低いため、マルテンサイト相の面積分率が低くなり、降伏応力が目標値より低い。
The results of the above measurements are shown together in Table 2. From Table 2, all the thick steel plates of the inventive examples that meet the conditions of the present invention have a yield stress of 650 MPa or more, a yield ratio of 80% or less, a total elongation of 15% or more, and the absorbed energy vE 0 at 0 ° C. It has a high strength of> 100 J, a low yield ratio, high ductility and high toughness, and excellent weldability without occurrence of cracks in the root portion even in the oblique y-type weld cracking test. Recognize.
On the other hand, the thick steel plate of the comparative example which deviates from the conditions of the present invention can obtain only one inferior in any one or more of the base material strength, yield ratio, ductility, base material toughness and weld cracking property. I can't.
For example, in the comparative example (No. 3) in which the cooling rate after the end of rolling is lower than the range of the present invention, the martensite phase is not generated, and the yield stress is greatly inferior to the target value. Moreover, the comparative example (No. 4) which does not perform a reheating process remarkably embrittles the martensite phase, and the target toughness cannot be obtained. Further, in the comparative example (No. 10) in which the reheating holding time is long with respect to the range of the present invention, the movable dislocation in the martensite phase disappears, resulting in an increase in yield stress, and as a result, a low yield ratio cannot be obtained. .
In the comparative example (No. 5) in which the reheating temperature is higher than the range of the present invention, the movable dislocation in the martensite phase disappears and the yield stress is high, and as a result, a low yield ratio cannot be obtained. On the other hand, the comparative example (No. 15) whose reheating temperature is lower than the range of the present invention is insufficient in improving the toughness of the martensite phase and inferior in the base material toughness.
In the comparative example (No. 7) in which the cooling stop temperature after the end of rolling is higher than the range of the present invention, the area fraction of martensite is low and the yield stress is lower than the target value. On the other hand, in the comparative example (No. 16) in which the rolling end temperature is lower than the range of the present invention, a ferrite phase is partially generated and the martensite area fraction is low, and the yield stress is also lower than the target value.
In the comparative example (No. 19) in which the C content is higher than the range of the present invention, the martensite phase is severely deteriorated in toughness and ductility, and weldability is also inferior. On the other hand, in the comparative example (No. 22) in which the amount of C is lower than the range of the present invention, since the martensite phase is not generated during the accelerated cooling, the yield stress does not reach the target value. In the comparative example (No. 20) in which C eq and P cm are higher than the range of the present invention, the base material toughness, ductility and weldability are inferior to the target characteristics. In the comparative example (No. 23) in which C eq is lower than the range of the present invention, a martensite phase is not generated during accelerated cooling, and the yield stress is low. Furthermore, since the hardenability at the time of accelerated cooling is low in the comparative example (No. 21) in which the amount of Mn is lower than the range of the present invention, the area fraction of the martensite phase is low and the yield stress is lower than the target value.

本発明は、建築、土木の分野のほかに、造船や橋梁、ラインパイプ、圧力容器などの溶接鋼構造物に用いられる厚鋼板にも適用することができる。   The present invention can be applied to thick steel plates used for welded steel structures such as shipbuilding, bridges, line pipes, pressure vessels, in addition to the fields of architecture and civil engineering.

Claims (4)

C:0.03〜0.2mass%、
Si:0.05〜0.5mass%、
Mn:0.8〜3mass%、
P:0.02mass%以下、
S:0.005mass%以下、
Al:0.1mass%以下、
N:0.007mass%以下を含有し、
残部がFeおよび不可避的不純物からなり、
下記(l)式で定義されるCeqが0.35〜0.5mass%、かつ、下記(2)式で定義されるPcmが0.28mass%以下であり、板厚断面の90%以上の領域において、マルテンサイト相の面積分率が60%以上で残部がベイナイト相からなり、降伏応力YSが650MPa以上、引張強さTSが826MPa以上、降伏比YRが80%以下の低降伏比高強度厚鋼板。

eq=C+Si/24+Mn/6 ・・・(1)
cm=C+Si/30+Mn/20 ・・・(2)
ここで、C,Si,Mn:各元素の含有量(mass%)
C: 0.03-0.2 mass%,
Si: 0.05-0.5 mass%,
Mn: 0.8-3 mass%,
P: 0.02 mass% or less,
S: 0.005 mass% or less,
Al: 0.1 mass% or less,
N: 0.007 mass% or less,
The balance consists of Fe and inevitable impurities,
Below (l) C eq is 0.35~0.5Mass% defined by the equation, and the following (2) P cm defined by the equation is not more than 0.28Mass%, ItaAtsudan surface 90% In the above regions, the martensite phase area fraction is 60% or more, the balance is a bainite phase, the yield stress YS is 650 MPa or more, the tensile strength TS is 826 MPa or more, and the yield ratio YR is 80% or less. High strength thick steel plate.
C eq = C + Si / 24 + Mn / 6 (1)
P cm = C + Si / 30 + Mn / 20 (2)
Here, C, Si, Mn: content of each element (mass%)
上記成分組成に加えてさらに、Cu:0.1〜1mass%、Ni:0.1〜1mass%、Cr:1mass%以下、Mo:1mass%以下、Nb:0.1mass%以下、V:0.2mass%以下、Ti:0.03mass%以下、B:0.005mass%以下、Ca:0.005mass%以下、REM:0.02mass%以下およびMg:0.005mass%以下のうちから選ばれる1種または2種以上を含有し、下記(3)式で定義されるCeqが0.35〜0.50mass%かつ下記(4)式で定義されるPcmが0.28mass%以下であることを特徴とする請求項1に記載の低降伏比高強度厚鋼板。

eq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 ・・・(3)
cm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B ・・・(4)
ここで、C,Si,Mn,Cu,Cr,Ni,Mo,V,B:各元素の含有量(mass%)
In addition to the above component composition, Cu: 0.1 to 1 mass%, Ni: 0.1 to 1 mass%, Cr: 1 mass% or less, Mo: 1 mass% or less, Nb: 0.1 mass% or less, V: 0.00. 1 mass% or less, Ti: 0.03 mass% or less, B: 0.005 mass% or less, Ca: 0.005 mass% or less, REM: 0.02 mass% or less, and Mg: 0.005 mass% or less or comprise two or more, the P cm to C eq, defined by the following equation (3) is defined by 0.35~0.50Mass% and the following equation (4) is not more than 0.28Mass% The low yield ratio high-strength thick steel plate according to claim 1,
C eq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (3)
P cm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 × B (4)
Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, B: Content of each element (mass%)
C:0.03〜0.2mass%、
Si:0.05〜0.5mass%、
Mn:0.8〜3mass%、
P:0.02mass%以下、
S:0.005mass%以下、
Al:0.1mass%以下、
N:0.007mass%以下を含有し、残部がFeおよび不可避的不純物からなり、下記(1)式で定義されるCeqが0.35〜0.5mass%、かつ、下記(2)式で定義されるPcmが0.28mass%以下である鋼スラブを1000〜1250℃に加熱後、圧延終了温度を800℃以上とする熱間圧延し、Ar変態点以上の温度域から冷却速度5〜60℃/sで350℃以下の温度域まで加速冷却して一旦冷却を中断し、その後、昇温速度2℃/s以上で350〜550℃の温度まで再加熱し、該温度に15min以下保持してから冷却することを特徴とする、降伏応力YSが650MPa以上、引張強さTSが826MPa以上、降伏比YRが80%以下の低降伏比高強度厚鋼板の製造方法。

eq=C+Si/24+Mn/6 ・・・(1)
cm=C+Si/30+Mn/20 ・・・(2)
ここで、C,Si,Mn:各元素の含有量(mass%)
C: 0.03-0.2 mass%,
Si: 0.05-0.5 mass%,
Mn: 0.8-3 mass%,
P: 0.02 mass% or less,
S: 0.005 mass% or less,
Al: 0.1 mass% or less,
N: 0.007 mass% or less, the balance being Fe and inevitable impurities, C eq defined by the following formula (1) is 0.35 to 0.5 mass%, and the following formula (2) A steel slab having a defined P cm of 0.28 mass% or less is heated to 1000 to 1250 ° C., then hot-rolled to a rolling end temperature of 800 ° C. or higher, and a cooling rate of 5 from a temperature range of Ar 3 transformation point or higher. Accelerated cooling to a temperature range of 350 ° C. or lower at ˜60 ° C./s, once cooling is interrupted, and then reheated to a temperature of 350 to 550 ° C. at a temperature rising rate of 2 ° C./s or higher, and the temperature is reduced to 15 minutes or less. A method of manufacturing a low yield ratio high strength thick steel sheet having a yield stress YS of 650 MPa or more, a tensile strength TS of 826 MPa or more, and a yield ratio YR of 80% or less.
C eq = C + Si / 24 + Mn / 6 (1)
P cm = C + Si / 30 + Mn / 20 (2)
Here, C, Si, Mn: content of each element (mass%)
上記成分組成に加えてさらに、Cu:0.1〜1mass%、Ni:0.1〜1mass%、Cr:1mass%以下、Mo:1mass%以下、Nb:0.1mass%以下、V:0.2mass%以下、Ti:0.03mass%以下、B:0.005mass%以下、Ca:0.005mass%以下、REM:0.02mass%以下およびMg:0.005mass%以下のうちから選ばれる1種または2種以上を含有し、下記(3)式で定義されるCeqが0.35〜0.50mass%かつ下記(4)式で定義されるPcmが0.28mass%以下であることを特徴とする請求項3に記載の低降伏比高強度厚鋼板の製造方法。

eq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14
・・・(3)
cm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B ・・・(4)
ここで、C,Si,Mn,Cu,Cr,Ni,Mo,V,B:各元素の含有量(mass%)
In addition to the above component composition, Cu: 0.1 to 1 mass%, Ni: 0.1 to 1 mass%, Cr: 1 mass% or less, Mo: 1 mass% or less, Nb: 0.1 mass% or less, V: 0.00. 1 mass% or less, Ti: 0.03 mass% or less, B: 0.005 mass% or less, Ca: 0.005 mass% or less, REM: 0.02 mass% or less, and Mg: 0.005 mass% or less or comprise two or more, the P cm to C eq, defined by the following equation (3) is defined by 0.35~0.50Mass% and the following equation (4) is not more than 0.28Mass% The method for producing a low yield ratio high strength thick steel plate according to claim 3.
C eq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
... (3)
P cm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 × B (4)
Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, B: Content of each element (mass%)
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