JP2016183387A - Thick steel plate for low temperature and production method therefor - Google Patents

Thick steel plate for low temperature and production method therefor Download PDF

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JP2016183387A
JP2016183387A JP2015064536A JP2015064536A JP2016183387A JP 2016183387 A JP2016183387 A JP 2016183387A JP 2015064536 A JP2015064536 A JP 2015064536A JP 2015064536 A JP2015064536 A JP 2015064536A JP 2016183387 A JP2016183387 A JP 2016183387A
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steel plate
thick steel
cementite
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崇之 加賀谷
Takayuki Kagaya
崇之 加賀谷
学 星野
Manabu Hoshino
学 星野
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Abstract

PROBLEM TO BE SOLVED: To provide a thick steel plate for low temperature which contains less than 9% of Ni and has performance equal to that of 9%Ni steel and excellent properties of suppressing brittle crack initiation, and a production method therefor.SOLUTION: The thick steel plate for low temperature is provided which contains C:0.01 to 0.12%, Si:0.01 to 0.30%, Mn:0.4 to 2.0%, Ni:over 5.0% and less than 8.0% and Al:0.002 to 0.08% and has a metallic structure mainly containing martensite, and in which: residual γ amount at 1/4 position of sheet thickness from a surface of the steel sheet in a sheet thickness direction is 3.0 vol.% or more; average diameter of the equivalent circle of cementite is 1 μm or less; and particle number density of cementite with a diameter of the equivalent circle of 0.5 μm or more is 10000/mmor less. The production method for the thick steel plate for low temperature is also provided in which hot rolling of a billet is terminated at finishing temperature of 650 to 850°C, the billet is hardened from two phase temperature range, heated to a tempering temperature Tt (°C) of Ac1 point+80°C, held for t(min.) or more calculated by a specific formula and then cooled to a temperature T(°C) or less satisfying a specific formula at a cooling rate of 1°C/s or more.SELECTED DRAWING: None

Description

本発明は、低温用厚鋼板及びその製造方法に関する。なお、低温用とは、−60℃以下の低温環境での用途を意味する。また、厚鋼板とは、板厚3mm以上の厚みを有する鋼板を意味する。   The present invention relates to a low-temperature thick steel plate and a method for producing the same. Note that low temperature use means use in a low temperature environment of −60 ° C. or lower. The thick steel plate means a steel plate having a thickness of 3 mm or more.

液化石油ガス(Liquefied Petroleum Gas、LPG)、液化天然ガス(Liquefied Natural Gas、LNG)などの液化ガスを貯蔵するための貯蔵タンクを主な用途とする低温用厚鋼板には、優れた破壊靱性が求められる。これは、液化ガスの貯蔵タンクに地震による大きな外力が負荷された際に、板厚を貫通する破壊の発生が許容されないためである。   Low temperature thick steel sheet, which is mainly used for storage tanks for storing liquefied gas such as liquefied petroleum gas (Liquid Petroleum Gas, LPG) and liquefied natural gas (Liquid Natural Gas, LNG), has excellent fracture toughness. Desired. This is because when a large external force is applied to the liquefied gas storage tank due to an earthquake, it is not allowed to break through the plate thickness.

特に、LNGタンクには、極めて高いレベルの耐破壊性能が求められることから、半世紀に亘り、質量%で9%のNiを含有する鋼(9%Ni鋼ということがある。)が用いられてきた。しかし、Niという高価な合金元素を多量に添加しなければならない9%Ni鋼は、高コストであり、経済的に問題がある。そこで、鋼材価格の抑制のために、Ni含有量を5〜8%程度に低減した鋼材(低Ni鋼ということがある。)が提案されている(例えば、特許文献1〜5、参照)。   In particular, since an extremely high level of fracture resistance is required for the LNG tank, steel containing 9% Ni by mass (sometimes referred to as 9% Ni steel) is used for half a century. I came. However, 9% Ni steel to which a large amount of an expensive alloy element called Ni has to be added has a high cost and is economically problematic. Then, in order to suppress the price of steel materials, steel materials having a Ni content reduced to about 5 to 8% (sometimes referred to as low Ni steel) have been proposed (for example, see Patent Documents 1 to 5).

特許文献1〜3では、残留オーステナイト(残留γということがある。)を安定化させて、9%Ni鋼と同等の性能を有する低Ni鋼が提案されている。一方、特許文献4、5では、液体窒素温度に冷却した際の残留γを制限した低Ni鋼が提案されている。   Patent Documents 1 to 3 propose a low Ni steel that stabilizes retained austenite (sometimes referred to as residual γ) and has the same performance as 9% Ni steel. On the other hand, Patent Documents 4 and 5 propose low Ni steels that limit the residual γ when cooled to liquid nitrogen temperature.

特許文献1には、粒状の微細な残留γが安定であること、また、セメンタイトの析出が強度を低下させ、靱性を劣化させることが記載されている。更に、特許文献2には、個々の残留γを微細にすることにより、不安定破壊抑止特性を高められることが記載されている。また、特許文献3には、残留γにNi及びMnが濃化していると、塑性変形による消失が最小化され、脆性亀裂伝播特性が向上することが記載されている。   Patent Document 1 describes that granular fine residual γ is stable, and that precipitation of cementite reduces strength and deteriorates toughness. Further, Patent Document 2 describes that the unstable fracture suppression characteristics can be enhanced by making individual residual γ fine. Patent Document 3 describes that when Ni and Mn are concentrated in the residual γ, disappearance due to plastic deformation is minimized and brittle crack propagation characteristics are improved.

これに対して、特許文献4、5には、Ni含有量が9%Ni鋼よりも少ない場合、−165℃では残留γが不安定であり、塑性変形によってマルテンサイトに変態し、靱性に悪影響を及ぼすことが記載されている。特許文献4、5で開示された技術は、残留γが安定化しないように、二相域での熱処理を行う際に、オーステナイトへの過剰なCの濃化を抑制し、サブゼロ処理後に含まれる残留γを制限するものである。   On the other hand, in Patent Documents 4 and 5, when the Ni content is less than 9% Ni steel, the residual γ is unstable at −165 ° C., and transformed into martensite by plastic deformation, which adversely affects toughness. Are described. The techniques disclosed in Patent Documents 4 and 5 contain excessive C concentration in austenite after heat treatment in a two-phase region so that residual γ is not stabilized, and are included after sub-zero treatment. It limits the residual γ.

また、従来、9%Ni鋼は、熱間圧延後の鋼板に、焼入工程(Q)、二相域熱処理(L)、焼戻工程(T)を施して製造していた。焼入工程は、金属組織がオーステナイトに変態する温度(Ac3)以上に加熱して急冷する熱処理、二相域熱処理は、オーステナイトとフェライトの2相になる温度(Ac1〜Ac3)に加熱して急冷する熱処理、焼戻工程はフェライト変態が開始する温度(Ac1)以下に加熱して空冷する熱処理である。   Conventionally, 9% Ni steel has been manufactured by subjecting a steel sheet after hot rolling to a quenching step (Q), a two-phase region heat treatment (L), and a tempering step (T). The quenching process is a heat treatment in which the metal structure is transformed to austenite (Ac3) or higher and rapidly cooled, and the two-phase region heat treatment is heated to a temperature at which two phases of austenite and ferrite are formed (Ac1 to Ac3). The heat treatment and tempering step to be performed are heat treatments in which air is cooled by heating below the temperature (Ac1) at which ferrite transformation starts.

二相域熱処理は、金属組織を微細化して靱性を向上させるために施される熱処理であるが、工程が増えると、製造コストが高くなる。そのため、特許文献1〜5では、熱間圧延後にそのまま焼入れを行う直接焼入(DQ)により、焼入工程を省略する製造方法が提案されている。   The two-phase region heat treatment is a heat treatment performed to refine the metal structure and improve the toughness. However, as the number of steps increases, the manufacturing cost increases. Therefore, Patent Documents 1 to 5 propose a manufacturing method in which the quenching process is omitted by direct quenching (DQ) in which quenching is performed as it is after hot rolling.

国際公開第2007/34576号International Publication No. 2007/34576 国際公開第2012/5330号International Publication No. 2012/5330 特許第5561442号公報Japanese Patent No. 5561442 特開2014−19936号公報JP 2014-19936 A 国際公開第2014/17057号International Publication No. 2014/17057

一般に、脆性破壊は、瞬時に構造物全体を崩壊させるため、避けるべき破壊形態であり、セメンタイトは、マトリックスの界面でのひずみ集中により、脆性破壊を助長させる。したがって、低温用鋼の脆性亀裂発生を抑制するためには、粗大なセメンタイトを低減させることが望ましい。特許文献1には、セメンタイトのアスペクト比や平均円相当径が規定されているが、それらの数密度に関して言及されておらず、理想的なセメンタイトを分散させるような製造方法は不明である。   In general, brittle fracture is a fracture mode to be avoided because it instantaneously collapses the entire structure, and cementite promotes brittle fracture due to strain concentration at the interface of the matrix. Therefore, in order to suppress the occurrence of brittle cracks in the low-temperature steel, it is desirable to reduce coarse cementite. Patent Document 1 defines the aspect ratio and average equivalent circle diameter of cementite, but does not mention their number density, and the manufacturing method for dispersing ideal cementite is unknown.

本発明は、このような実情に鑑み、9%Ni鋼よりも少ないNi含有量で、9%Ni鋼と同等の性能を有し、優れた脆性亀裂発生抑制特性を有する低温用厚鋼板及びその製造方法を提供するものである。   In light of such circumstances, the present invention has a Ni content lower than that of 9% Ni steel, has a performance equivalent to that of 9% Ni steel, and has excellent brittle crack initiation suppressing properties, and its A manufacturing method is provided.

本発明者らは、詳細な検討の結果、低温用厚鋼板の脆性亀裂発生抑制特性を向上させるためには、残留γを確保し、マルテンサイト変態に起因する歪みを除去し、更に、セメンタイトの平均円相当径だけでなく、円相当径が0.5μm以上のセメンタイトの粒子数密度をも制限する必要があることを見出した。そして、残留γを微細化するには、熱間圧延の仕上圧延温度を低温にし、そのまま二相温度域に冷却して焼入れるか、又は、再加熱する場合は加熱温度を二相域温度として焼入れること、歪みの除去には焼戻工程の温度及び保持時間が、粗大なセメンタイトの生成の抑制には、焼戻工程の冷却速度の制御が、それぞれ、重要であることを見出した。本発明は、このような知見に基づいてなされたものであり、その要旨は以下のとおりである。   As a result of detailed studies, the present inventors have confirmed that, in order to improve the brittle crack initiation suppressing property of the low-temperature thick steel plate, the residual γ is secured, the strain caused by the martensitic transformation is removed, and further, the cementite It has been found that it is necessary to limit not only the average equivalent circle diameter but also the number density of cementite having an equivalent circle diameter of 0.5 μm or more. And in order to refine the residual γ, the finish rolling temperature of the hot rolling is lowered and cooled to the two-phase temperature range as it is and quenched, or when reheating, the heating temperature is set to the two-phase region temperature. It has been found that the temperature and holding time of the tempering step are important for quenching and distortion, and the cooling rate of the tempering step is important for suppressing the formation of coarse cementite. This invention is made | formed based on such knowledge, The summary is as follows.

(1)質量%で、
C:0.01〜0.12%、
Si:0.01〜0.30%、
Mn:0.4〜2.0%、
Ni:5.0%を超え8.0%未満、
Al:0.002〜0.08%
を含有し、
P:0.05%以下、
S:0.008%以下、
N:0.0050%以下
に制限し、残部はFe及び不純物からなり、
マルテンサイト主体の金属組織を有し、
板厚方向で鋼板の表面から板厚の1/4の部位での残留γ量が3.0体積%以上であり、
かつセメンタイトの平均円相当径が1μm以下であり、円相当径が0.5μm以上のセメンタイトの粒子数密度が10000個/mm2以下である
ことを特徴とする低温用厚鋼板。
(1) In mass%,
C: 0.01 to 0.12%,
Si: 0.01-0.30%,
Mn: 0.4 to 2.0%,
Ni: more than 5.0% and less than 8.0%,
Al: 0.002 to 0.08%
Containing
P: 0.05% or less,
S: 0.008% or less,
N: limited to 0.0050% or less, the balance is made of Fe and impurities,
It has a martensite-based metal structure,
The amount of residual γ in the region of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is 3.0% by volume or more,
A low temperature thick steel sheet characterized in that the average equivalent circle diameter of cementite is 1 μm or less, and the number density of cementite having an equivalent circle diameter of 0.5 μm or more is 10,000 particles / mm 2 or less.

(2)更に、質量%で、
Cu:2.0%以下、
Cr:1.5%以下、
Mo:0.5%以下、
V:0.10%以下、
B:0.005%以下
の1種又は2種以上を含有することを特徴とする上記(1)に記載の低温用厚鋼板。
(2) Furthermore, in mass%,
Cu: 2.0% or less,
Cr: 1.5% or less,
Mo: 0.5% or less,
V: 0.10% or less,
B: The steel plate for low temperature as described in said (1) characterized by containing 1 type or 2 types or more of 0.005% or less.

(3)更に、質量%で、
Nb:0.10%以下、
Ti:0.10%以下
の一方又は両方を含有することを特徴とする上記(1)又は(2)に記載の低温用厚鋼板。
(3) Furthermore, in mass%,
Nb: 0.10% or less,
Ti: One or both of Ti and 0.10% or less are contained, The low-temperature thick steel plate as described in said (1) or (2) characterized by the above-mentioned.

(4)更に、質量%で、
Ca:0.004%以下、
Mg:0.002%以下、
REM:0.002%以下
の1種又は2種以上を含有することを特徴とする上記(1)〜(3)の何れか1項に記載の低温用厚鋼板。
(4) Furthermore, in mass%,
Ca: 0.004% or less,
Mg: 0.002% or less,
REM: 0.002% or less of 1 type, or 2 or more types, The thick steel plate for low-temperatures of any one of said (1)-(3) characterized by the above-mentioned.

(5)上記(1)〜(4)の何れか1項に記載の化学組成を有する鋼片に、下記[工程1]〜[工程4]の工程を施すことを特徴とする低温用厚鋼板の製造方法。
[工程1]鋼片をAc3〜1000℃に加熱する工程。
[工程2]前記加熱後の鋼片を熱間圧延し、650〜850℃の仕上温度で熱間圧延を終了する工程。
[工程3]前記熱間圧延後、厚鋼板をAr1〜Ar3の焼入開始温度まで冷却し、250℃以下まで3℃/s以上の冷却速度で冷却する工程。
[工程4]前記厚鋼板を、Ac1点+80℃以下の焼戻温度Tt(℃)に加熱し、下記式(1)によって求められるt1(分)以上の時間保持した後、1℃/s以上の冷却速度で、下記(2)式を満足する温度T1(℃)以下まで冷却する工程。
1=350−5×[Ni]+0.4×t−0.5×Tt ・・・ (1)式
1=237−1170×[C]+137×[Si] ・・・ (2)式
ここで、[C]、[Si]、[Ni]は、それぞれ、C、Si、Niの含有量(質量%)、tは板厚(mm)である。
(5) A steel plate for low temperature, characterized by subjecting the steel piece having the chemical composition according to any one of (1) to (4) above to the following [Step 1] to [Step 4]. Manufacturing method.
[Step 1] A step of heating the steel slab to Ac3 to 1000 ° C.
[Step 2] A step of hot-rolling the steel slab after heating and finishing the hot rolling at a finishing temperature of 650 to 850 ° C.
[Step 3] After the hot rolling, the thick steel plate is cooled to a quenching start temperature of Ar1 to Ar3, and is cooled to 250 ° C. or lower at a cooling rate of 3 ° C./s or higher.
[Step 4] The thick steel plate is heated to a tempering temperature Tt (° C.) of Ac1 point + 80 ° C. or lower and held for a time equal to or longer than t 1 (min) determined by the following formula (1). The step of cooling to the temperature T 1 (° C.) or less that satisfies the following equation (2) at the above cooling rate.
t 1 = 350−5 × [Ni] + 0.4 × t−0.5 × Tt (1) Formula T 1 = 237-1170 × [C] + 137 × [Si] (2) Formula Here, [C], [Si], and [Ni] are the contents (% by mass) of C, Si, and Ni, respectively, and t is the plate thickness (mm).

(6)前記[工程3]に代えて、下記[工程3’]の工程を施すことを特徴とする上記(5)に記載の低温用厚鋼板の製造方法。
[工程3’]前記熱間圧延後、厚鋼板をAr1未満に冷却し、Ac1〜Ac3の焼入開始温度に再加熱し、250℃以下まで3℃/s以上の冷却速度で冷却する工程。
(6) Instead of the [Step 3], the following [Step 3 ′] step is performed, and the method for producing a low-temperature thick steel plate according to the above (5).
[Step 3 ′] After the hot rolling, the thick steel plate is cooled to less than Ar1, reheated to a quench start temperature of Ac1 to Ac3, and cooled to 250 ° C. or less at a cooling rate of 3 ° C./s or more.

低温環境下でも、9%Ni鋼並みの耐破壊安全性に優れたNi低減型の低温用厚鋼板及びその製造方法を提供することができる。Ni含有量を低くすることができるので、低コストでの厚鋼板提供が可能になる。本発明の低温用厚鋼板は、−60℃以下、特に−165℃以下の低温環境での用途に好適であり、例えば、LPGやLNGなどの液化ガスを貯蔵する容器等に使用可能な、9%Ni鋼に比べて安価な鋼材であり、産業上の貢献が極めて顕著である。   Even under a low temperature environment, it is possible to provide a Ni-reduced low-temperature thick steel plate and a method for manufacturing the same, which are excellent in fracture resistance as in 9% Ni steel. Since the Ni content can be reduced, it is possible to provide a thick steel plate at a low cost. The low-temperature steel plate of the present invention is suitable for use in a low-temperature environment of −60 ° C. or less, particularly −165 ° C. or less, and can be used for, for example, a container for storing liquefied gas such as LPG and LNG. Compared to% Ni steel, it is a cheaper steel material, and its industrial contribution is extremely remarkable.

本発明者らは、低温用厚鋼板の脆性亀裂発生抑制特性を上昇させるために検討を行い、以下の(a)〜(c)の知見を得た。   The inventors of the present invention have made studies in order to increase the brittle crack initiation suppressing property of the low-temperature thick steel plate, and have obtained the following findings (a) to (c).

(a)マトリックスの界面でのひずみ集中による脆性破壊の発生を助長する粗大なセメンタイトは、脆性亀裂発生抑制のために低減させることが望ましい。より高い脆性亀裂発生抑制特性を持つために、セメンタイトの平均円相当径を1μm以下、かつ円相当径が0.5μm以上のセメンタイトの粒子数密度を10000個/mm2以下にする必要がある。 (A) Coarse cementite that promotes the occurrence of brittle fracture due to strain concentration at the interface of the matrix is desirably reduced in order to suppress the occurrence of brittle cracks. In order to have higher brittle cracking suppression properties, it is necessary that the average equivalent circle diameter of cementite is 1 μm or less and the number density of cementite having an equivalent circle diameter of 0.5 μm or more is 10,000 particles / mm 2 or less.

(b)残留γは面心立方構造であり、脆性破壊しないので、発生した亀裂の停止に有効である。また、残留γは、組織の分断、微細化に寄与し、鋼板の耐破壊特性を向上させるため、鋼板内に分散して残留γが存在していることが望ましい。低温環境下で高い靭性を示すためには、Ni低減型の厚鋼板の表面から板厚方向に板厚の1/4の部位、すなわち板厚(1/4)t位置で、3.0体積%以上の残留γの存在を必要とする。   (B) Residual γ has a face-centered cubic structure and does not cause brittle fracture, which is effective in stopping the generated crack. Further, the residual γ contributes to the division and refinement of the structure and improves the fracture resistance of the steel sheet, so that it is desirable that the residual γ is dispersed in the steel sheet. In order to exhibit high toughness in a low-temperature environment, 3.0 volume at a position of 1/4 of the plate thickness from the surface of the Ni-reduced thick plate in the plate thickness direction, that is, at the plate thickness (1/4) t position % Of residual γ is required.

(c)残留γを微細化する製造方法としては、熱間圧延後、そのまま二相温度域に冷却して焼入れ、適切な温度で焼戻しを施すことが好ましい。熱間工程では、変形帯を積極的に組織中に導入して最終組織を微細化するため、仕上圧延温度を850℃以下とする。また、焼戻工程では歪みを除去するために、保持時間を下記(1)式のt1(分)以上とし、セメンタイトの粗大化を抑制するには、焼戻工程の冷却を1℃/s以上の冷却速度で、下記(2)式のT1(℃)以下まで行うことが好ましい。また、熱間圧延後、一旦、Ar1未満に冷却した後、二相温度域に再加熱してもよい。
1=350−5×[Ni]+0.4×t−0.5×Tt ・・・ (1)式
1=237−1170×[C]+137×[Si] ・・・ (2)式
ここで、[C]、[Si]、[Ni]は、それぞれ、C、Si、Niの含有量(質量%)、tは板厚(mm)である。
(C) As a manufacturing method for refining the residual γ, it is preferable that the hot rolling is followed by cooling to a two-phase temperature range as it is and quenching and tempering at an appropriate temperature. In the hot process, the final rolling temperature is set to 850 ° C. or lower in order to refine the final structure by positively introducing the deformation zone into the structure. Further, in order to remove distortion in the tempering process, the holding time is set to t1 (min) or more in the following formula (1), and cooling of the tempering process is set to 1 ° C./s or more to suppress cementite coarsening. in the cooling rate is preferably carried out until the following equation (2) T 1 (° C.) or less. Moreover, after hot rolling, after once cooling to less than Ar1, you may reheat to a two-phase temperature range.
t 1 = 350−5 × [Ni] + 0.4 × t−0.5 × Tt (1) Formula T 1 = 237-1170 × [C] + 137 × [Si] (2) Formula Here, [C], [Si] and [Ni] are the contents (mass%) of C, Si and Ni, respectively, and t is the plate thickness (mm).

以下、本発明の低温用厚鋼板及びその製造方法について詳細に説明する。なお、含有量に関する「%」は、特に断らない限り、「質量%」を意味する。   Hereinafter, the low-temperature steel plate and the manufacturing method thereof according to the present invention will be described in detail. In addition, "%" regarding content means "mass%" unless otherwise indicated.

A.化学組成
C:0.01〜0.12%
Cは、母材の強度確保のために必要な元素であり、本発明ではC量を0.01%以上とする。また、C量が0.01%未満であると、溶融線(Fusion Line、FL)でのラス形成が不十分になり、FL近傍の溶接熱影響部(Heat Affected Zone、HAZ)の靭性が低下することがある。好ましくはC量を0.03%以上とする。一方、C量が0.12%を超えると、HAZ、なかでもFL近傍のHAZの靭性劣化が著しくなる。したがって、C量の上限を0.12%以下とする。C量の好ましい上限は0.09%以下である。
A. Chemical composition
C: 0.01 to 0.12%
C is an element necessary for securing the strength of the base material. In the present invention, the C content is 0.01% or more. Also, if the C content is less than 0.01%, lath formation at the fusion line (Fusion Line, FL) becomes insufficient, and the toughness of the weld heat affected zone (Heat Affected Zone, HAZ) near the FL decreases. There are things to do. Preferably, the C content is 0.03% or more. On the other hand, if the amount of C exceeds 0.12%, the toughness deterioration of HAZ, particularly HAZ near FL, becomes significant. Therefore, the upper limit of the C amount is 0.12% or less. The upper limit with the preferable amount of C is 0.09% or less.

Si:0.01〜0.30%
Siは、脱酸剤であり、効果を得るためにSi量を0.01%以上とする。また、Siは、焼戻工程で、過飽和に固溶しているマルテンサイト中からのセメンタイトへの分解析出反応を抑制する元素でもあり、好ましくはSi量を0.02%以上、より好ましくは0.03%以上とする。一方、Si量が0.30%を超えると、母材ではセメンタイトが粗大になり、HAZでは島状マルテンサイトが生成し、靭性が低下するため、上限を0.30%以下とする。好ましくは、Si量の上限を0.15%以下とし、より好ましくは0.10%以下とする。
Si: 0.01-0.30%
Si is a deoxidizer, and the Si content is 0.01% or more in order to obtain the effect. Si is also an element that suppresses the decomposition and precipitation reaction into cementite from martensite dissolved in supersaturation in the tempering step, preferably the amount of Si is 0.02% or more, more preferably 0.03% or more. On the other hand, when the amount of Si exceeds 0.30%, cementite becomes coarse in the base material, and in the HAZ, island-like martensite is generated and toughness is lowered, so the upper limit is made 0.30% or less. Preferably, the upper limit of the Si amount is 0.15% or less, more preferably 0.10% or less.

Mn:0.4〜2.0%
Mnは、脱酸剤であり、また、焼入れ性を向上させる元素である。本発明では、母材及びHAZの強度及び靭性を確保するために、Mn量を0.4%以上とする。好ましくはMn量を0.5%以上、より好ましくは0.6%以上とする。一方、Mn量が2.0%を超えると、中心偏析に起因して板厚方向での母材特性が不均一になり、靱性が低下することがあるため、Mn量の上限を2.0%以下とする。好ましくはMn量を1.5%以下、より好ましくは1.1%以下とする。
Mn: 0.4 to 2.0%
Mn is a deoxidizer and is an element that improves hardenability. In the present invention, in order to ensure the strength and toughness of the base material and the HAZ, the amount of Mn is set to 0.4% or more. Preferably, the amount of Mn is 0.5% or more, more preferably 0.6% or more. On the other hand, if the amount of Mn exceeds 2.0%, the base material properties in the plate thickness direction become non-uniform due to center segregation and the toughness may be lowered. % Or less. Preferably the amount of Mn is 1.5% or less, more preferably 1.1% or less.

P:0.05%以下
Pは不純物であり、粒界に偏析して靭性を低下させるため、P量を0.05%以下に制限する。好ましくはP量を0.03%以下とする。P量は少ないほど好ましいため、下限は特に規定しないが、製造コストの観点から、0.001%以上を含有してもよい。
P: 0.05% or less P is an impurity, and segregates at grain boundaries to reduce toughness. Therefore, the P content is limited to 0.05% or less. Preferably, the P content is 0.03% or less. Since the lower the amount of P is, the lower the limit, the lower limit is not particularly specified, but 0.001% or more may be contained from the viewpoint of manufacturing cost.

S:0.008%以下
Sは不純物であり、中心偏析を助長したり、脆性破壊の起点となる延伸形状のMnSが生成する原因となることがあるため、S量を0.008%以下に制限する。好ましくはS量を0.003%以下とする。S量は少ないほど好ましいため、下限は特に規定しないが、製造コストの観点から、0.0001%以上を含有してもよい。
S: 0.008% or less S is an impurity, which may promote center segregation or cause the formation of stretched MnS that is the starting point of brittle fracture, so the amount of S is made 0.008% or less. Restrict. Preferably, the S amount is 0.003% or less. Since the lower the amount of S, the better. Therefore, the lower limit is not particularly specified, but 0.0001% or more may be contained from the viewpoint of manufacturing cost.

Ni:5.0%超8.0%未満
Niは低温用鋼として靭性を確保するために必要な最も基本的な元素であり、本発明ではNi量を5.0%超とする。好ましくはNi量を5.5%超、より好ましい範囲は6.0%超とする。Ni量が多いほど高い低温靭性が得られるが、コストが高くなるため、Ni量の上限を8.0%未満とする。
Ni: more than 5.0% and less than 8.0% Ni is the most basic element necessary for securing toughness as a low-temperature steel. In the present invention, the Ni content is more than 5.0%. Preferably, the Ni content is more than 5.5%, and a more preferred range is more than 6.0%. The higher the Ni content, the higher the low temperature toughness, but the higher the cost, so the upper limit of the Ni content is less than 8.0%.

Al:0.002〜0.08%
Alは脱酸剤であり、効果を得るためにAl量を0.002%以上とする。また、Alは、Siと同様、セメンタイトの生成を抑制する元素でもあり、好ましくはAl量を0.005%以上とする。一方、Al量が0.08%を超えると、介在物に起因して靱性が低下するため、上限を0.08%以下とする。また、島状マルテンサイトの生成に起因するHAZの靱性の低下を抑制するため、好ましくはAl量を0.04%以下とする。
Al: 0.002 to 0.08%
Al is a deoxidizer, and in order to obtain an effect, the amount of Al is made 0.002% or more. Al, like Si, is also an element that suppresses the formation of cementite, and the Al content is preferably 0.005% or more. On the other hand, if the Al content exceeds 0.08%, the toughness decreases due to inclusions, so the upper limit is made 0.08% or less. Further, in order to suppress a decrease in the toughness of the HAZ due to the formation of island martensite, the Al content is preferably 0.04% or less.

N:0.005%以下
Nは不純物であり、鋼中に固溶するN量の増加や析出物の生成によって靭性の低下の原因となるため、0.005%以下に制限する。HAZ靭性の確保のためには、N量を0.004%以下にすることが好ましい。N量は少ないほど好ましいため、下限は特に規定しないが、製造コストの観点から、0.001%以上を含有してもよい。
N: 0.005% or less N is an impurity, which causes a decrease in toughness due to an increase in the amount of N dissolved in steel and the formation of precipitates, so it is limited to 0.005% or less. In order to ensure HAZ toughness, the N content is preferably 0.004% or less. The lower the amount of N, the better. Therefore, the lower limit is not particularly specified, but 0.001% or more may be contained from the viewpoint of manufacturing cost.

本発明の低温用厚鋼板は、上記の成分のほか、残部がFeと不純物からなるものである。ここで、不純物とは、低温用厚鋼板を工業的に製造する際に、鉱石やスクラップ等のような原料を始めとして、製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味する。   The low-temperature thick steel plate of the present invention is composed of Fe and impurities in the balance in addition to the above components. Here, an impurity is a component that is mixed due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially producing a low-temperature steel plate, and has an adverse effect on the present invention. It means that it is allowed in the range that does not give.

更に、必要に応じて、Cu、Cr、Mo、V、B、Nb、Ti、Ca、Mg及びREMの1種又は2種以上を含有してもよい。   Furthermore, you may contain 1 type (s) or 2 or more types of Cu, Cr, Mo, V, B, Nb, Ti, Ca, Mg, and REM as needed.

Cu:2.0%以下
Cuは、強度度の上昇に寄与する元素であり、0.1%以上を含有させてもよい。より好ましくは、Cu量を0.2%以上とする。一方、Cuを過剰に含有させると、HAZの靱性が低下することがあるため、Cu量の上限は2.0%以下が好ましい。より好ましくはCu量を1.3%以下、更に好ましくは0.7%以下とする。
Cu: 2.0% or less Cu is an element that contributes to an increase in strength and may contain 0.1% or more. More preferably, the amount of Cu is 0.2% or more. On the other hand, if Cu is contained excessively, the toughness of HAZ may be lowered, so the upper limit of Cu content is preferably 2.0% or less. More preferably, the Cu amount is 1.3% or less, and further preferably 0.7% or less.

Cr:1.5%以下
Crは、耐炭酸ガス腐食性や焼入れ性の向上に寄与する元素であり、0.05%以上を含有させてもよい。より好ましくはCr量を0.1%以上とする。一方、Crを過剰に含有させると、HAZの靱性が低下することがあるため、Cr量の上限は1.5%以下が好ましい。より好ましくはCr量を1.0%以下、更に好ましくは0.5%以下とする。
Cr: 1.5% or less Cr is an element that contributes to improvement of carbon dioxide gas corrosion resistance and hardenability, and may contain 0.05% or more. More preferably, the Cr amount is 0.1% or more. On the other hand, when Cr is excessively contained, the toughness of HAZ may be lowered, so the upper limit of Cr content is preferably 1.5% or less. More preferably, the Cr content is 1.0% or less, and further preferably 0.5% or less.

Mo:0.5%以下
Moは、母材の強度と靱性を向上させる元素であり、0.02%以上を含有させてもよい。より好ましくはMo量を0.05%以上とする。一方、Mo量が過剰であると、HAZの硬度が高くなり、靱性と耐SSC性を損なうことがあるため、Mo量を0.5%以下とすることが好ましい。より好ましくは、Mo量を0.3%以下、更に好ましくは0.2%以下とする。
Mo: 0.5% or less Mo is an element that improves the strength and toughness of the base material, and may contain 0.02% or more. More preferably, the Mo amount is 0.05% or more. On the other hand, if the Mo amount is excessive, the hardness of the HAZ increases, and the toughness and SSC resistance may be impaired. Therefore, the Mo amount is preferably 0.5% or less. More preferably, the Mo amount is 0.3% or less, and further preferably 0.2% or less.

V:0.10%以下
Vは、焼戻工程で炭窒化物を析出し、母材の強度の向上に寄与する元素であり、0.015%以上を含有させてもよい。より好ましくはV量を0.02%以上とする。一方、0.10%超のVを添加しても効果が飽和し、靱性劣化を招くことがあるため、V量を0.10%以下とすることが好ましい。より好ましくは、V量を0.08%以下、更に好ましくは0.05%以下とする。
V: 0.10% or less V is an element that precipitates carbonitride in the tempering step and contributes to improvement of the strength of the base material, and may be contained by 0.015% or more. More preferably, the V amount is 0.02% or more. On the other hand, even if V of more than 0.10% is added, the effect is saturated and the toughness may be deteriorated. Therefore, the V content is preferably 0.10% or less. More preferably, the V amount is 0.08% or less, and more preferably 0.05% or less.

B:0.005%以下
Bは、微量の添加で焼入れ性を向上させる元素であり、母材の強度を向上させるために0.0003%以上を含有させてもよい。より好ましくは、B量を0.0005%以上、更に好ましくは0.0010%以上とする。一方、B量が0.005%を超えると、粗大な硼化合物が析出し、靭性が劣化することがあるため、B量を0.005%以下とすることが好ましい。より好ましくは、B量を0.004%以下、更に好ましくは0.002%以下とする。
B: 0.005% or less B is an element that improves the hardenability by adding a small amount, and may be contained by 0.0003% or more in order to improve the strength of the base material. More preferably, the B amount is 0.0005% or more, and further preferably 0.0010% or more. On the other hand, if the amount of B exceeds 0.005%, a coarse boron compound precipitates and the toughness may be deteriorated. Therefore, the amount of B is preferably 0.005% or less. More preferably, the B amount is 0.004% or less, and further preferably 0.002% or less.

Nb:0.10%以下
Nbは、組織を微細化して低温靭性を向上させる元素であり、0.01%以上を含有させてもよい。より好ましくはNb量を0.02%以上とする。一方、Nbを過剰に添加すると粗大な炭化物や窒化物を形成し、靭性を低下させることがあるため、Nb量を0.10%以下とすることが好ましい。より好ましくはNb量を0.08%以下、更に好ましくは0.05%以下とする。
Nb: 0.10% or less Nb is an element that refines the structure and improves low-temperature toughness, and may contain 0.01% or more. More preferably, the Nb amount is 0.02% or more. On the other hand, if Nb is added excessively, coarse carbides and nitrides may be formed and the toughness may be lowered. Therefore, the Nb content is preferably 0.10% or less. More preferably, the Nb amount is 0.08% or less, and further preferably 0.05% or less.

Ti:0.10%以下
Tiは、脱酸に利用すると、Al、Ti、Mnからなる酸化物相を形成し、組織を微細化する効果が得られることから、0.01%以上のTiを含有させてもよい。より好ましくはTi量を0.02%以上とし、更に好ましくはTi量を0.03%以上とする。一方、Ti量が0.1%を超えると、Ti酸化物やTi−Al酸化物が形成されて分散密度が低下し、小入熱の溶接熱影響部の組織を微細化する効果が低下することがあるため、Ti量は0.1%以下が好ましい。より好ましくはTi量を0.07%以下、更に好ましくは0.03%以下とする。
Ti: 0.10% or less When Ti is used for deoxidation, an oxide phase composed of Al, Ti, and Mn is formed, and the effect of refining the structure is obtained. You may make it contain. More preferably, the Ti amount is 0.02% or more, and further preferably the Ti amount is 0.03% or more. On the other hand, if the amount of Ti exceeds 0.1%, Ti oxide or Ti-Al oxide is formed, the dispersion density is lowered, and the effect of refining the structure of the weld heat affected zone with small heat input is reduced. Therefore, the Ti amount is preferably 0.1% or less. More preferably, the Ti content is 0.07% or less, and further preferably 0.03% or less.

Ca:0.004%以下
Caは、鋼中のSと反応して溶鋼中で酸硫化物(オキシサルファイド)を形成する元素であり、圧延方向に延伸したMnSの生成を抑制して靱性を向上させるために、0.0003%以上を含有させてもよい。より好ましくはCa量を0.0005%以上とする。一方、Ca量が0.004%を超えると、靱性の劣化を招くことがあるので、Ca量は0.004%以下が好ましい。より好ましくはCa量を0.003%以下、更に好ましくは0.002%以下とする。
Ca: 0.004% or less Ca is an element which reacts with S in steel to form oxysulfide (oxysulfide) in molten steel, and suppresses the generation of MnS stretched in the rolling direction to improve toughness. Therefore, 0.0003% or more may be contained. More preferably, the Ca content is 0.0005% or more. On the other hand, if the Ca content exceeds 0.004%, the toughness may be deteriorated, so the Ca content is preferably 0.004% or less. More preferably, the Ca content is 0.003% or less, and further preferably 0.002% or less.

Mg:0.002%以下
Mgは、微細なMg含有酸化物を生成する元素であり、γ粒径を微細化する効果を得るために0.0002%以上を含有させてもよい。より好ましくはMg量を0.0003%以上とする。一方、Mg量が0.002%を超えると、酸化物が多くなりすぎて延性低下をもたらすことがあるので、Mg量を0.002%以下とすることが好ましい。より好ましくはMg量を0.0010%以下とする。
Mg: 0.002% or less Mg is an element that produces a fine Mg-containing oxide, and may be contained in an amount of 0.0002% or more in order to obtain the effect of reducing the γ grain size. More preferably, the Mg amount is 0.0003% or more. On the other hand, if the amount of Mg exceeds 0.002%, the amount of oxide becomes too much and the ductility may be lowered. Therefore, the amount of Mg is preferably 0.002% or less. More preferably, the Mg amount is 0.0010% or less.

REM:0.002%以下
REM(希土類元素)は、溶接熱影響部の組織を微細化し、また、Caと同様、硫化物の形態の制御に有効な元素であり、0.0002%以上含有させることが好ましい。より好ましくはREM量を0.0003%以上とする。一方、REMを過剰に含有させると、介在物を形成し、清浄度を低下させ、靱性を損なうことがあるため、REM量は0.002%以下が好ましい。より好ましくはREM量を0.001%以下とする。
REM: 0.002% or less REM (rare earth element) is an element that refines the structure of the weld heat-affected zone and is effective in controlling the form of sulfides, like Ca, and is contained in an amount of 0.0002% or more. It is preferable. More preferably, the REM amount is 0.0003% or more. On the other hand, when REM is excessively contained, inclusions are formed, the cleanliness is lowered, and the toughness may be impaired. Therefore, the REM content is preferably 0.002% or less. More preferably, the REM amount is 0.001% or less.

ここで、REMとは、ランタノイドの15元素にY及びScを合わせた17元素の総称であり、これらの元素のうちの1種又は2種以上を含有させることができる。なお、REMの含有量はこれらの元素の合計含有量を意味する。   Here, REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. Note that the content of REM means the total content of these elements.

B.金属組織
B−1.板厚(1/4)t位置の残留γ量:3.0体積%以上
本発明の低温用厚鋼板は、焼入れ性を向上させるNiの含有量が多いため、マルテンサイト組織を主体とする金属組織となる。マルテンサイト組織が主体であるとは、分率が最大の相がマルテンサイトであることを意味する。また、残留γとマルテンサイト組織のほかに、25%体積%以下のベイナイト組織などが存在しても、厚鋼板の脆性き裂伝ぱ停止特性に影響を及ぼすことはない。
B. Metallographic structure
B-1. Residual γ amount at position (1/4) t: 3.0% by volume or more The low-temperature thick steel plate of the present invention has a high Ni content to improve hardenability, and therefore a metal mainly composed of a martensite structure Become an organization. The fact that the martensite organization is the main means that the phase with the largest fraction is martensite. In addition to the residual γ and martensite structure, the presence of a bainite structure of 25% by volume or less does not affect the brittle crack propagation stopping characteristics of the thick steel plate.

本発明の低温用厚鋼板の金属組織に含まれる残留γは、脆性き裂伝ぱ停止特性を向上させ、低温環境下での靭性の向上に寄与する。効果を得るには、板厚(1/4)t位置での残留γ量が3.0体積%以上とすることが必要であり、好ましくは5.0体積%以上とする。ここで、板厚(1/4)t位置での残留γ量は、板厚全域の平均的な位置での残留γ量と同等である。したがって、板厚(1/4)t位置での残留γ量は、低温靱性を評価するために、極めて重要である。残留γ量はX線回折法によって測定することができる。   The residual γ contained in the metal structure of the low-temperature thick steel plate of the present invention improves the brittle crack propagation stopping characteristics and contributes to the improvement of toughness in a low-temperature environment. In order to obtain the effect, the amount of residual γ at the position of the plate thickness (1/4) t needs to be 3.0% by volume or more, preferably 5.0% by volume or more. Here, the amount of residual γ at the plate thickness (1/4) t position is equal to the amount of residual γ at the average position throughout the plate thickness. Therefore, the amount of residual γ at the plate thickness (1/4) t position is extremely important for evaluating the low temperature toughness. The amount of residual γ can be measured by an X-ray diffraction method.

残留γ量の上限は、マルテンサイトの量を超えなければよいので、特に規定しないが、残留γが多く存在しすぎると降伏応力が低下することがある。残留γ量は15.0体積%以下とすることが好ましく、より好ましくは10.0体積%以下とする。   The upper limit of the amount of residual γ should not exceed the amount of martensite and is not particularly defined. However, if too much residual γ is present, the yield stress may be reduced. The residual γ amount is preferably 15.0% by volume or less, and more preferably 10.0% by volume or less.

B−2.セメンタイト数密度
セメンタイトは、鋼の素地(マトリックス)との界面でのひずみ集中によって脆性破壊を助長するため、特に粗大なセメンタイトの生成を抑制することが望ましく、セメンタイトの平均円相当径を1μm以下にすることが必要である。また、平均円相当径の微細化に加えて、セメンタイトの粒子径と数密度も制御する必要がある。これは、円相当径が1μm以下のセメンタイトであっても、粒子が近接していると微小なき裂が連結し、脆性破壊が助長されるためである。より好ましくはセメンタイトの平均円相当径を0.5μm以下とする。セメンタイトが原因となる脆性破壊を抑制するために、円相当径が0.5μm以上のセメンタイトを10000個/mm2以下にする必要があり、好ましくは5000個/mm2以下とする。セメンタイトの平均円相当径、及び、円相当径が0.5μm以上のセメンタイト数密度は、透過型電子顕微鏡によって測定することができる。
B-2. Cementite number density cementite promotes brittle fracture by concentrating strain at the interface with the steel substrate (matrix), so it is desirable to suppress the formation of coarse cementite, and the average equivalent circle diameter of cementite should be 1 μm or less. It is necessary to. In addition to the refinement of the average equivalent circle diameter, it is necessary to control the particle diameter and number density of cementite. This is because even if the cementite has an equivalent circle diameter of 1 μm or less, if the particles are close to each other, minute cracks are connected to promote brittle fracture. More preferably, the average equivalent circle diameter of cementite is 0.5 μm or less. In order to suppress brittle fracture caused by cementite, cementite having an equivalent circle diameter of 0.5 μm or more needs to be 10,000 pieces / mm 2 or less, preferably 5000 pieces / mm 2 or less. The average equivalent circle diameter of cementite and the number density of cementite having an equivalent circle diameter of 0.5 μm or more can be measured with a transmission electron microscope.

以上のような成分組成、金属組織を有する本発明の低温用厚鋼板は、−60℃以下の低温領域、特に、−165℃以下の低温環境での靱性に優れ、LPGやLNGなどの液化ガスを低温域で貯蔵する用途にも好適である。なお、本発明の低温用厚鋼板の板厚は、とりわけ5〜50mmの厚みを有する鋼板である。   The low-temperature steel plate of the present invention having the above component composition and metal structure is excellent in toughness in a low temperature region of −60 ° C. or lower, particularly in a low temperature environment of −165 ° C. or lower, and is a liquefied gas such as LPG or LNG. Is also suitable for use in storing at low temperatures. In addition, the plate | board thickness of the low-temperature thick steel plate of this invention is a steel plate which has a thickness of 5-50 mm especially.

C.製造方法
本発明の低温用厚鋼板は、加熱工程(工程1)、圧延工程(工程2)、焼入工程(工程3、工程3’)、焼戻工程(工程4)を経て製造することができる。ただし、以下の製造方法に限定されるものではない。熱間圧延に供する鋼片については、格別にその鋳造条件を規定するものではなく、造塊−分塊スラブを鋼塊として用いてもよいし、連続鋳造スラブを用いてもよい。製造効率、歩留り及び省エネルギーの観点からは、連続鋳造スラブを用いることが好ましい。
C. Manufacturing Method The steel plate for low temperature of the present invention can be manufactured through a heating step (step 1), a rolling step (step 2), a quenching step (step 3, step 3 ′), and a tempering step (step 4). it can. However, it is not limited to the following manufacturing method. The steel slab to be subjected to hot rolling is not particularly limited in its casting conditions, and an ingot-bundling slab may be used as the steel ingot, or a continuous casting slab may be used. From the viewpoint of production efficiency, yield, and energy saving, it is preferable to use a continuously cast slab.

C−1.加熱工程(工程1)
熱間圧延前に鋼片を加熱する加熱工程では、加熱温度をAc3点〜1000℃に制御するのが好ましい。加熱温度は、組織をオーステナイト変態させるためAc3点以上とすることが好ましい。より好ましくは、鋼片の変形抵抗を小さくして、圧延工程でのロールへの負荷を抑制するために、加熱温度を850℃以上とする。一方、加熱温度が高いほど組織が粗大化するため、1000℃以下にすることが好ましい。
C-1. Heating process (process 1)
In the heating step of heating the steel slab before hot rolling, it is preferable to control the heating temperature to Ac3 point to 1000 ° C. The heating temperature is preferably set to Ac3 point or higher in order to cause the structure to undergo austenite transformation. More preferably, in order to reduce the deformation resistance of the steel slab and suppress the load on the roll in the rolling process, the heating temperature is set to 850 ° C. or higher. On the other hand, since the structure becomes coarser as the heating temperature is higher, the temperature is preferably set to 1000 ° C. or lower.

通常、加熱工程では、加熱炉に挿入後徐々に鋼塊温度が上昇し、均熱帯の温度を超えた後、鋼塊温度が均熱帯の温度に定常化する、いわゆるオーバーシュートが起こりうる。オーバーシュートの発生で鋼塊温度が均熱帯の温度より50℃超となると、鋼塊の組織の粗大化が進み、意図する組織が得られなくなる場合がある。このため、オーバーシュートする温度を50℃以下に制御することが好ましい。   Normally, in the heating process, the steel ingot temperature gradually rises after being inserted into the heating furnace, and after the temperature of the soaking zone is exceeded, so-called overshoot in which the ingot temperature becomes steady at the soaking zone temperature may occur. When the steel ingot temperature exceeds 50 ° C. from the soaking zone temperature due to the occurrence of overshoot, the structure of the steel ingot becomes coarse and the intended structure may not be obtained. For this reason, it is preferable to control the overshooting temperature to 50 ° C. or lower.

C−2.圧延工程(工程2)
圧延工程では、加熱工程で加熱した鋼片の熱間圧延を行う。圧延工程では、熱間圧延によって変形帯を積極的に組織中に導入し、組織を微細化するため、仕上圧延温度が850℃以下となるようにして熱間圧延を行うことが好ましい。一方、仕上圧延温度の下限は、変形抵抗が大きくなり過ぎないように、650℃以上とすることが好ましい。なお、熱間圧延中の温度は、被圧延材(鋼片又は厚鋼板)の表面温度を測定して求めればよい。
C-2. Rolling process (process 2)
In the rolling step, the steel slab heated in the heating step is hot-rolled. In the rolling process, it is preferable to perform hot rolling so that the finish rolling temperature is 850 ° C. or lower in order to positively introduce a deformation zone into the structure by hot rolling and refine the structure. On the other hand, the lower limit of the finish rolling temperature is preferably 650 ° C. or higher so that the deformation resistance does not become too large. The temperature during hot rolling may be determined by measuring the surface temperature of the material to be rolled (steel piece or thick steel plate).

C−3.焼入工程(工程3、工程3’)
焼入工程は、組織の粗大化を防止するために、焼入開始温度を二相域温度とし、冷却速度を3℃/s以上にすることが好ましい。また、焼入工程は、熱間圧延後、そのまま、厚鋼板をAr1点〜Ar3点に冷却した後、焼入れる工程(工程3)、又は、一旦、Ar1点未満に冷却した後、Ac1点〜Ac3点に再加熱して焼入れる工程(工程3’)の何れかを選択することができる。焼入れの方法はスプレー法など手段を問わない。また、冷却停止温度は250℃以下とすることが好ましい。なお、焼入工程の冷却速度は、厚鋼板の板厚tの中心部、すなわち、板厚(1/2)t位置での冷却速度である。
C-3. Quenching process (process 3, process 3 ')
In the quenching process, in order to prevent the coarsening of the structure, it is preferable that the quenching start temperature is a two-phase region temperature and the cooling rate is 3 ° C./s or more. In addition, after the hot rolling, the quenching process is performed by cooling the thick steel plate to the Ar1 point to the Ar3 point, and then quenching the process (step 3), or once cooled to less than the Ar1 point, and then the Ac1 point to the Ar1 point. Any of the steps (step 3 ′) of reheating to the Ac3 point and quenching can be selected. The quenching method may be any means such as a spray method. The cooling stop temperature is preferably 250 ° C. or lower. The cooling rate in the quenching step is the cooling rate at the center of the plate thickness t of the thick steel plate, that is, at the plate thickness (1/2) t position.

工程3では、熱間圧延後、焼入開始温度まで空冷してもよく、加速冷却してもよい。熱間圧延後、そのまま冷却速度を3℃/s以上として焼入れしてもよい。工程3’では、熱間圧延後の厚鋼板を、空冷してもよく、加速冷却してもよい。また、工程3’では、熱間圧延後の厚鋼板を、一旦、室温まで冷却した後、再加熱してもよい。   In step 3, after hot rolling, air cooling to the quenching start temperature or accelerated cooling may be performed. After hot rolling, it may be quenched as it is at a cooling rate of 3 ° C./s or more. In step 3 ', the hot-rolled thick steel plate may be air-cooled or accelerated. In step 3 ', the hot-rolled thick steel plate may be once cooled to room temperature and then reheated.

C−4.焼戻工程(工程4)
焼戻工程では、焼入れによって生じたマルテンサイト中の歪みを除去するため、厚鋼板を[Ac1点+80℃]以下の温度に加熱する。焼戻しを[Ac1点+80℃]以下の温度で行うことにより、焼入れままのマルテンサイト組織を高靭性化し、残留γ量を増加させることができる。なお、効果的に歪みを除去するためには、焼戻工程の加熱温度を500℃以上とすることが好ましく、保持時間を下記(1)式のt1(分)以上とすることが好ましい。
C-4. Tempering process (process 4)
In the tempering step, the thick steel plate is heated to a temperature of [Ac1 point + 80 ° C.] or lower in order to remove distortion in martensite caused by quenching. By performing tempering at a temperature of [Ac1 point + 80 ° C.] or lower, the as-quenched martensite structure can be made tough and the residual γ amount can be increased. In addition, in order to remove distortion effectively, it is preferable that the heating temperature of a tempering process shall be 500 degreeC or more, and it is preferable that holding time shall be t1 (min) or more of following (1) Formula.

更に、セメンタイトの粗大化の抑制や数密度の増加の防止のために、焼戻工程の冷却速度を1℃/sとすることが好ましい。焼戻工程の冷却停止温度が高いと、セメンタイトの粗大化を防止する効果が不十分になることがあるため、下記(2)式のT1(℃)以下まで行うことが好ましい。
1=350−5×[Ni]+0.4×t−0.5×Tt ・・・ (1)式
1=237−1170×[C]+137×[Si] ・・・ (2)式
ここで、[C]、[Si]、[Ni]は、それぞれ、C、Si、Niの含有量(質量%)、tは板厚(mm)である。
Furthermore, it is preferable to set the cooling rate in the tempering step to 1 ° C./s in order to suppress the cementite coarsening and to prevent the number density from increasing. If the cooling stop temperature in the tempering step is high, the effect of preventing the cementite from becoming coarse may be insufficient. Therefore, it is preferable to carry out to T 1 (° C.) or less of the following formula (2).
t 1 = 350−5 × [Ni] + 0.4 × t−0.5 × Tt (1) Formula T 1 = 237-1170 × [C] + 137 × [Si] (2) Formula Here, [C], [Si], and [Ni] are the contents (% by mass) of C, Si, and Ni, respectively, and t is the plate thickness (mm).

表1に示す化学組成を有する厚さ300mmの鋼片を、表2に示す条件で熱間圧延及び熱処理を施し、厚鋼板を製造した。なお、各鋼のAc1、Ac3、Ar3、Ar1を熱膨張挙動御から求め、熱間圧延の加熱温度がAc3以上であり、焼戻し温度がAc1+80℃以下であることを確認した。鋼1〜9、13、16〜18、20、21、24、26〜29、31〜35、x1、x2、x5、x6は、熱間圧延後に250℃以下まで加速冷却した[工程3]の例であり、熱間圧延後の冷却開始温度がAr1〜Ar3であることを確認した。鋼10〜12、14、15、19、22、23、25、30、x3、x4は、熱間圧延後に、一旦、冷却し、二相域温度(Ac1〜Ac3)に再加熱し、250℃以下まで加速冷却した[工程3’]の例であり、熱間圧延後の再加熱を開始した温度がAr1未満であることを確認した。製造後の厚鋼板の板厚は6〜50mmである。得られた各厚鋼板の板厚(1/4)t位置から試験片を採取し、金属組織の観察、残留オーステナイト量の測定、セメンタイトの平均円相当径及び円相当径が0.5μm以上の粒子密度の測定、機械試験を行った。   Steel strips having a chemical composition shown in Table 1 and having a thickness of 300 mm were subjected to hot rolling and heat treatment under the conditions shown in Table 2 to produce thick steel plates. In addition, Ac1, Ac3, Ar3, Ar1 of each steel was calculated | required from thermal expansion behavior control, and it was confirmed that the heating temperature of hot rolling is Ac3 or more and the tempering temperature is Ac1 + 80 degreeC or less. Steels 1 to 9, 13, 16 to 18, 20, 21, 24, 26 to 29, 31 to 35, x1, x2, x5, and x6 were accelerated and cooled to 250 ° C. or lower after hot rolling [Step 3]. It was an example, and it was confirmed that the cooling start temperature after hot rolling was Ar1 to Ar3. Steels 10-12, 14, 15, 19, 22, 23, 25, 30, x3, x4 are once cooled after hot rolling and reheated to a two-phase temperature (Ac1-Ac3) at 250 ° C. It is an example of [Step 3 ′] accelerated and cooled to the following, and it was confirmed that the temperature at which reheating after hot rolling was started was less than Ar1. The plate thickness of the steel plate after manufacture is 6-50 mm. A specimen is collected from the position (1/4) t of each thick steel plate obtained, observation of the metal structure, measurement of the amount of retained austenite, the average equivalent circle diameter and the equivalent circle diameter of cementite are 0.5 μm or more. Particle density measurement and mechanical testing were performed.

Figure 2016183387
Figure 2016183387

Figure 2016183387
Figure 2016183387

金属組織は、光学顕微鏡で観察し、面積率が最大の相がマルテンサイト(マルテンサイト主体)の組織であることを確認した。残留オーステナイト量(体積%)はX線回折法により測定した。より詳細には、製造した全ての試験片は主としてマルテンサイト組織で構成されていたため、面心立方構造を有する残留γと体心立方構造を有するマルテンサイトの格子構造の違いを利用して、X線ピークの積分強度比から残留γ量を測定した。セメンタイトは透過型電子顕微鏡により10000倍の倍率にて20視野観察し、平均円相当径及び円相当径が0.5μm以上のセメンタイトの粒子密度を算出した。   The metal structure was observed with an optical microscope, and it was confirmed that the phase with the largest area ratio was a martensite structure (mainly martensite). The amount of retained austenite (% by volume) was measured by the X-ray diffraction method. More specifically, since all the manufactured specimens were mainly composed of a martensite structure, the difference between the lattice structure of the residual γ having a face-centered cubic structure and the martensite having a body-centered cubic structure was used to calculate X The amount of residual γ was measured from the integrated intensity ratio of the line peak. The cementite was observed in 20 visual fields with a transmission electron microscope at a magnification of 10,000 times, and the average equivalent circle diameter and the particle density of cementite having an equivalent circle diameter of 0.5 μm or more were calculated.

引張試験は、JIS Z 2241に準拠して、10号引張試験片、5号引張試験片を採取し、室温で行い、引張強さTS(MPa)、降伏強さYS(MPa)を測定した。シャルピー衝撃試験は、JIS Z 2242に準拠してVノッチシャルピー試験片を板厚(1/4)t位置より採取して−196℃で行い、シャルピー吸収エネルギーvE−196(J)を測定した。限界CTOD値(3本の平均値)は、BS7448(British Standard)に準拠して、B×2BタイプのCTOD試験片を用いて測定した。表3に試験結果を示す。   In the tensile test, No. 10 tensile test piece and No. 5 tensile test piece were sampled in accordance with JIS Z 2241 and performed at room temperature, and tensile strength TS (MPa) and yield strength YS (MPa) were measured. In the Charpy impact test, a V-notch Charpy test piece was sampled from the plate thickness (1/4) t position in accordance with JIS Z 2242 and conducted at -196 ° C, and Charpy absorbed energy vE-196 (J) was measured. The limit CTOD value (average value of three) was measured using a B × 2B type CTOD test piece in accordance with BS7448 (British Standard). Table 3 shows the test results.

Figure 2016183387
Figure 2016183387

なお、強度の良否の判断基準は以下の通りである。常温における降伏強度YS:590MPa以上、常温における引張強度TS:690MPa以上、Vノッチシャルピー吸収エネルギーvE−196(J):150J以上、限界CTOD値δC-165(mm):0.30mm以上を合格とした。表3に示すように、鋼1〜35は本発明例であり、引張特性及び靭性が良好である。一方、C量が過剰なx1はセメンタイトの析出により、靱性が低下している。Si量が過剰なx2はセメンタイトの粗大化によって靭性が低下している。また、Mn量が過剰なx3及びS量が過剰なx4は、中心偏析に起因して靭性が低下している。x5は焼戻し工程の冷却速度が遅いためにセメンタイトの粗大化や数密度が増加し、x6は冷却停止温度が高いためにセメンタイトが粗大化し、靱性が低下している。 The criteria for judging whether the strength is good or bad are as follows. Yield strength at normal temperature YS: 590 MPa or higher, Tensile strength TS at normal temperature: 690 MPa or higher, V-notch Charpy absorbed energy vE-196 (J): 150 J or higher, critical CTOD value δ C-165 (mm): Passed 0.30 mm or higher It was. As shown in Table 3, Steels 1 to 35 are examples of the present invention and have good tensile properties and toughness. On the other hand, x1 having an excessive amount of C has reduced toughness due to precipitation of cementite. X2 having an excessive amount of Si has reduced toughness due to coarsening of cementite. Further, x3 having an excessive amount of Mn and x4 having an excessive amount of S have reduced toughness due to center segregation. Since x5 has a slow cooling rate in the tempering process, cementite coarsening and number density increase, and x6 has a high cooling stop temperature, resulting in coarse cementite coarsening and reduced toughness.

Claims (6)

質量%で、
C:0.01〜0.12%、
Si:0.01〜0.30%、
Mn:0.4〜2.0%、
Ni:5.0%を超え8.0%未満、
Al:0.002〜0.08%
を含有し、
P:0.05%以下、
S:0.008%以下、
N:0.0050%以下
に制限し、残部はFe及び不純物からなり、
マルテンサイト主体の金属組織を有し、
板厚方向で鋼板の表面から板厚の1/4の部位での残留γ量が3.0体積%以上であり、
かつセメンタイトの平均円相当径が1μm以下であり、円相当径が0.5μm以上のセメンタイトの粒子数密度が10000個/mm2以下である
ことを特徴とする低温用厚鋼板。
% By mass
C: 0.01 to 0.12%,
Si: 0.01-0.30%,
Mn: 0.4 to 2.0%,
Ni: more than 5.0% and less than 8.0%,
Al: 0.002 to 0.08%
Containing
P: 0.05% or less,
S: 0.008% or less,
N: limited to 0.0050% or less, the balance is made of Fe and impurities,
It has a martensite-based metal structure,
The amount of residual γ in the region of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is 3.0% by volume or more,
A low temperature thick steel sheet characterized in that the average equivalent circle diameter of cementite is 1 μm or less, and the number density of cementite having an equivalent circle diameter of 0.5 μm or more is 10,000 particles / mm 2 or less.
更に、質量%で、
Cu:2.0%以下、
Cr:1.5%以下、
Mo:0.5%以下、
V:0.10%以下、
B:0.005%以下
の1種又は2種以上を含有することを特徴とする請求項1に記載の低温用厚鋼板。
Furthermore, in mass%,
Cu: 2.0% or less,
Cr: 1.5% or less,
Mo: 0.5% or less,
V: 0.10% or less,
B: One type or two or more types of 0.005% or less are contained, The low-temperature thick steel plate of Claim 1 characterized by the above-mentioned.
更に、質量%で、
Nb:0.10%以下、
Ti:0.10%以下
の一方又は両方を含有することを特徴とする請求項1又は2に記載の低温用厚鋼板。
Furthermore, in mass%,
Nb: 0.10% or less,
The steel plate for low temperature according to claim 1 or 2, characterized by containing one or both of Ti: 0.10% or less.
更に、質量%で、
Ca:0.004%以下、
Mg:0.002%以下、
REM:0.002%以下
の1種又は2種以上を含有することを特徴とする請求項1〜3の何れか1項に記載の低温用厚鋼板。
Furthermore, in mass%,
Ca: 0.004% or less,
Mg: 0.002% or less,
REM: 0.002% or less of 1 type or 2 types or more, The thick steel plate for low temperature of any one of Claims 1-3 characterized by the above-mentioned.
請求項1〜4の何れか1項に記載の化学組成を有する鋼片に、下記[工程1]〜[工程4]の工程を施すことを特徴とする低温用厚鋼板の製造方法。
[工程1]鋼片をAc3〜1000℃に加熱する工程。
[工程2]前記加熱後の鋼片を熱間圧延し、650〜850℃の仕上温度で熱間圧延を終了する工程。
[工程3]前記熱間圧延後、厚鋼板をAr1〜Ar3の焼入開始温度まで冷却し、250℃以下まで3℃/s以上の冷却速度で冷却する工程。
[工程4]前記厚鋼板を、Ac1点+80℃以下の焼戻温度Tt(℃)に加熱し、下記式(1)によって求められるt1(分)以上の時間保持した後、1℃/s以上の冷却速度で、下記(2)式を満足する温度T1(℃)以下まで冷却する工程。
1=350−5×[Ni]+0.4×t−0.5×Tt ・・・ (1)式
1=237−1170×[C]+137×[Si] ・・・ (2)式
ここで、[C]、[Si]、[Ni]は、それぞれ、C、Si、Niの含有量(質量%)、tは板厚(mm)である。
The manufacturing method of the steel plate for low temperature characterized by performing the process of the following [process 1]-[process 4] to the steel piece which has a chemical composition of any one of Claims 1-4.
[Step 1] A step of heating the steel slab to Ac3 to 1000 ° C.
[Step 2] A step of hot-rolling the steel slab after heating and finishing the hot rolling at a finishing temperature of 650 to 850 ° C.
[Step 3] After the hot rolling, the thick steel plate is cooled to a quenching start temperature of Ar1 to Ar3, and is cooled to 250 ° C. or lower at a cooling rate of 3 ° C./s or higher.
[Step 4] The thick steel plate is heated to a tempering temperature Tt (° C.) of Ac1 point + 80 ° C. or lower and held for a time equal to or longer than t 1 (min) determined by the following formula (1). The step of cooling to the temperature T 1 (° C.) or less that satisfies the following equation (2) at the above cooling rate.
t 1 = 350−5 × [Ni] + 0.4 × t−0.5 × Tt (1) Formula T 1 = 237-1170 × [C] + 137 × [Si] (2) Formula Here, [C], [Si], and [Ni] are the contents (% by mass) of C, Si, and Ni, respectively, and t is the plate thickness (mm).
前記[工程3]に代えて、下記[工程3’]の工程を施すことを特徴とする請求項5に記載の低温用厚鋼板の製造方法。
[工程3’]前記熱間圧延後、厚鋼板をAr1未満に冷却し、Ac1〜Ac3の焼入開始温度に再加熱し、250℃以下まで3℃/s以上の冷却速度で冷却する工程。
6. The method for producing a low-temperature thick steel plate according to claim 5, wherein the following step [Step 3 ′] is performed instead of the above [Step 3].
[Step 3 ′] After the hot rolling, the thick steel plate is cooled to less than Ar1, reheated to a quench start temperature of Ac1 to Ac3, and cooled to 250 ° C. or less at a cooling rate of 3 ° C./s or more.
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