JP6169025B2 - Steel plates and line pipe steel pipes with excellent hydrogen-induced crack resistance and toughness - Google Patents

Steel plates and line pipe steel pipes with excellent hydrogen-induced crack resistance and toughness Download PDF

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JP6169025B2
JP6169025B2 JP2014052390A JP2014052390A JP6169025B2 JP 6169025 B2 JP6169025 B2 JP 6169025B2 JP 2014052390 A JP2014052390 A JP 2014052390A JP 2014052390 A JP2014052390 A JP 2014052390A JP 6169025 B2 JP6169025 B2 JP 6169025B2
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喜一郎 田代
喜一郎 田代
加藤 拓
拓 加藤
進佑 佐藤
進佑 佐藤
晴弥 川野
晴弥 川野
孝司 三宅
孝司 三宅
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Kobe Steel Ltd
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Description

本発明は、天然ガス・原油輸送用ラインパイプや圧力容器、貯蔵用タンクなどに好適な、耐水素誘起割れ性と靭性に優れた鋼板と、該鋼板を用いて得られる耐水素誘起割れ性と靭性に優れたラインパイプ用鋼管に関する。   The present invention is suitable for natural gas / crude oil transportation line pipes, pressure vessels, storage tanks, and the like, a steel plate excellent in hydrogen-induced crack resistance and toughness, and hydrogen-induced crack resistance obtained using the steel plate The present invention relates to a steel pipe for line pipe having excellent toughness.

硫化水素を含有する原油、ガスなど劣質資源の開発に伴い、これらの輸送や精製、貯蔵に用いられるラインパイプや圧力容器、貯蔵タンクには、耐水素誘起割れ性や耐応力腐食割れ性などのいわゆる耐サワー性が必要とされる。水素誘起割れ(Hydrogen−Induced Cracking、以下「HIC」ということがある)は、上記硫化水素等による腐食反応に伴って鋼材内部に水素が侵入し、この侵入した水素が、MnSやNb(C,N)をはじめとする非金属介在物などに集積し、ガス化により生じる割れであることが知られている。   With the development of inferior resources such as crude oil and gas containing hydrogen sulfide, the line pipes, pressure vessels, and storage tanks used for transportation, refining, and storage of these materials are resistant to hydrogen-induced cracking resistance and stress corrosion cracking resistance. So-called sour resistance is required. Hydrogen-induced cracking (hereinafter sometimes referred to as “HIC”) causes hydrogen to penetrate into the steel material due to the corrosion reaction caused by hydrogen sulfide or the like, and this penetrated hydrogen is converted into MnS or Nb (C, It is known that the cracks are accumulated in non-metallic inclusions such as N) and are generated by gasification.

特にサワー環境下では、板厚方向に表面から深さ5mmまでの領域(以下、この領域を「鋼板表層部」ということがある)の水素濃度が鋼板中央部に比べて高くなることが知られており、鋼板表層部でCa系酸化物やAl系酸化物などを起点に割れが生じやすいことが知られている。   Particularly in the sour environment, it is known that the hydrogen concentration in the region from the surface to the depth of 5 mm in the thickness direction (hereinafter, this region may be referred to as “steel plate surface layer portion”) is higher than that in the central portion of the steel plate. It is known that cracks are likely to occur in the surface layer portion of the steel sheet starting from Ca-based oxide or Al-based oxide.

従来より、耐水素誘起割れ性(以下「耐HIC性」ということがある)を高める技術について幾つか提案されている。例えば特許文献1には、板厚中心部のMn、Nb、Tiの偏析度を抑制することにより耐水素誘起割れ性を改善した鋼材が開示されている。この方法では、中心偏析部のHIC特性の改善は可能であるが、中心偏析部以外の部位の介在物は十分に制御されていないため、中心偏析部以外の部位の割れを抑制することは困難であると思われる。また特許文献2には、CaとOとSの含有量からなるパラメータ式により、MnSやCa系酸硫化物を起点としたHICを抑制する方法が開示されている。このような方法により耐HIC性は確保できるが、水素濃度が特に高くなる鋼板表層部では、後述する通り、微細なHICが生じ易く表層部の高靭性も併せて確保することは困難であると思われる。   Conventionally, several techniques for improving hydrogen-induced crack resistance (hereinafter sometimes referred to as “HIC resistance”) have been proposed. For example, Patent Document 1 discloses a steel material that has improved resistance to hydrogen-induced cracking by suppressing the segregation degree of Mn, Nb, and Ti at the center of the plate thickness. Although this method can improve the HIC characteristics of the center segregation part, it is difficult to suppress cracks in parts other than the center segregation part because inclusions in parts other than the center segregation part are not sufficiently controlled. It seems to be. Patent Document 2 discloses a method of suppressing HIC starting from MnS or Ca-based oxysulfide by a parameter formula including Ca, O, and S contents. Although HIC resistance can be ensured by such a method, in the steel sheet surface layer portion where the hydrogen concentration is particularly high, as described later, it is difficult to ensure high toughness of the surface layer portion, because fine HIC is likely to occur. Seem.

特開2010−209461号公報JP 2010-209461 A 特開平06−136440号公報Japanese Patent Laid-Open No. 06-136440

本発明は上記の様な事情に着目してなされたものであって、その目的は、サワー環境において特に水素濃度が高く過酷な状況にある鋼板表層部にて、数μm程度の微細なHICも十分に抑制された、耐水素誘起割れ性と靭性に優れた鋼板や鋼管を実現することにある。   The present invention has been made by paying attention to the above-described circumstances, and its purpose is to provide a fine HIC of about several μm in a steel sheet surface layer portion that is particularly severe in a sour environment with a high hydrogen concentration. It is to realize a sufficiently suppressed steel sheet and steel pipe excellent in hydrogen-induced crack resistance and toughness.

上記課題を解決し得た本発明の耐水素誘起割れ性と靭性に優れた鋼板は、
C:0.02〜0.15%(%は質量%の意味。以下同じ)、
Si:0.02〜0.50%、
Mn:0.6〜2.0%、
P:0.030%以下(0%を含まない)、
S:0.003%以下(0%を含まない)、
Al:0.010〜0.08%、
Ca:0.0003〜0.0060%、
N:0.001〜0.01%、および
O(酸素):0.0045%以下(0%を含まない)を満たし、残部が鉄および不可避不純物からなり、
前記Caと前記Sの比(Ca/S)が2.0以上であり、かつ
板厚方向に表面から深さ5mmまでの領域の最大Ca濃度(Cmax)と該領域の平均Ca濃度(Cave)との比(Cmax/Cave)が1.20以下であるところに特徴を有する。
The steel sheet excellent in hydrogen-induced crack resistance and toughness of the present invention that has solved the above problems is
C: 0.02 to 0.15% (% means mass%, the same shall apply hereinafter)
Si: 0.02 to 0.50%,
Mn: 0.6 to 2.0%,
P: 0.030% or less (excluding 0%),
S: 0.003% or less (excluding 0%),
Al: 0.010 to 0.08%,
Ca: 0.0003 to 0.0060%,
N: 0.001 to 0.01%, and O (oxygen): 0.0045% or less (excluding 0%) is satisfied, and the balance consists of iron and inevitable impurities,
The ratio of Ca to S (Ca / S) is 2.0 or more, and the maximum Ca concentration (Cmax) in the region from the surface to the depth of 5 mm in the plate thickness direction and the average Ca concentration (Cave) in the region And the ratio (Cmax / Cave) is 1.20 or less.

前記鋼板は、更に他の元素として、
(a)B:0.005%以下(0%を含まない)、
V:0.1%以下(0%を含まない)、
Cu:1.5%以下(0%を含まない)、
Ni:1.5%以下(0%を含まない)、
Cr:1.5%以下(0%を含まない)、
Mo:1.5%以下(0%を含まない)、および
Nb:0.06%以下(0%を含まない)
よりなる群から選択される1種以上の元素や、
(b)Ti:0.03%以下(0%を含まない)、
Mg:0.01%以下(0%を含まない)、
REM:0.02%以下(0%を含まない)、および
Zr:0.010%以下(0%を含まない)
よりなる群から選択される1種以上の元素を含んでいてもよい。
The steel sheet, as another element,
(A) B: 0.005% or less (excluding 0%),
V: 0.1% or less (excluding 0%),
Cu: 1.5% or less (excluding 0%),
Ni: 1.5% or less (excluding 0%),
Cr: 1.5% or less (excluding 0%),
Mo: 1.5% or less (not including 0%), and Nb: 0.06% or less (not including 0%)
One or more elements selected from the group consisting of:
(B) Ti: 0.03% or less (excluding 0%),
Mg: 0.01% or less (excluding 0%),
REM: 0.02% or less (not including 0%), and Zr: 0.010% or less (not including 0%)
One or more elements selected from the group consisting of may be included.

上記鋼板は、ラインパイプ用や圧力容器用として好適である。また本発明には、上記鋼板を用いて製造されるラインパイプ用鋼管も含まれる。   The steel sheet is suitable for line pipes and pressure vessels. Moreover, the steel pipe for line pipes manufactured using the said steel plate is also contained in this invention.

本発明によれば、鋼板の板厚方向におけるCa濃度の分布を均質化しているため、水素濃度が特に高くなる鋼板表層部において、数μm程度の微細なHICまでも十分に抑制され、その結果、耐水素誘起割れ性と靭性に優れた鋼板や鋼管を提供できる。   According to the present invention, since the Ca concentration distribution in the sheet thickness direction of the steel sheet is homogenized, even a fine HIC of about several μm is sufficiently suppressed in the steel sheet surface layer where the hydrogen concentration is particularly high, and as a result. It is possible to provide a steel plate or a steel pipe excellent in hydrogen-induced crack resistance and toughness.

図1は、HICの起点となった介在物のCa濃度別のHIC発生率を示す図である。FIG. 1 is a diagram showing the HIC generation rate for each Ca concentration of inclusions that are the origin of HIC. 図2は、Cmax/Caveとアッパーシェルフエネルギーの関係を示す図である。FIG. 2 is a graph showing the relationship between Cmax / Cave and upper shelf energy.

本発明者らは、前記課題を解決するために鋭意研究を重ねた。まず本発明者らは、サワー環境において最も過酷な状況にある鋼板表層部のHIC発生について、あらためて原因をつきとめるべく、種々の鋼板を用いて、NACE(National Association of Corrosion and Engineer)TM0284に規定のHIC試験(NACE試験)を実施した。このNACE試験は、1atmの硫化水素ガスを飽和させた5%NaCl溶液と0.5%酢酸のpH2.7の混合水溶液に、試験片、つまり鋼板を96時間浸漬させた後のHICの発生を評価する試験である。   The inventors of the present invention have made extensive studies to solve the above problems. First, the present inventors use various steel sheets to re-specify the cause of the HIC occurrence of the steel sheet surface layer part, which is the most severe situation in the sour environment. An HIC test (NACE test) was performed. In this NACE test, the generation of HIC after a test piece, that is, a steel sheet, was immersed for 96 hours in a mixed aqueous solution of 5% NaCl solution saturated with 1 atm hydrogen sulfide gas and 0.5% acetic acid at pH 2.7. It is a test to evaluate.

次に本発明者らは、HIC試験後の鋼板表面部分について、ASTM A370に従いシャルピー試験を実施した。その結果、前記NACE試験で規定の「倍率100倍での顕微鏡観察」で割れが観察されない場合であっても、HIC試験後のシャルピー試験結果が悪い、つまり靭性に劣る場合があった。   Next, the inventors conducted a Charpy test according to ASTM A370 for the steel plate surface portion after the HIC test. As a result, even when cracks were not observed in the prescribed “microscopic observation at a magnification of 100 times” in the NACE test, the Charpy test result after the HIC test was sometimes poor, that is, the toughness was inferior.

その原因について調査するため、上記顕微鏡観察を倍率を高めて行ったところ、微細な割れが介在物を起点に多数生じていることが判明した。即ち、前記NACE試験で規定の100倍での顕微鏡観察では観察されない観察限界以下の微細なHICが、介在物を起点に多数生成しており、これらがHIC試験後の靭性を劣化させる要因であることをまずつきとめた。   In order to investigate the cause, the above-mentioned microscopic observation was carried out at a higher magnification. As a result, it was found that many fine cracks occurred starting from inclusions. That is, a large number of fine HIC below the observation limit, which is not observed in the NACE test at a magnification of 100 times as specified, is generated starting from inclusions, and these are factors that deteriorate the toughness after the HIC test. I first figured out that.

更に、上記微細なHICを含めたHIC発生起点となっている介在物組成について調査した。詳細には、後述する実施例に記載のHIC試験(NACE試験)を行った鋼板について組織観察を行った。また観察される介在物のCa濃度を求めた。この介在物中Ca濃度は、介在物を構成するOやNを除いた成分組成に占めるCaの割合(質量%、以下、単に%と示す)である。この介在物中Ca濃度が50%以上の介在物のうち、HIC発生起点となっていた介在物の割合(%)と、上記介在物中Ca濃度が20%以下の介在物のうち、HIC発生起点となっていた介在物の割合(%)のそれぞれを求めた。その結果を図1に示す。この図1では、上記HIC発生起点となっていた介在物の割合を、縦軸の「HIC発生率(%)」で示す。この図1に示す通り、特にCa濃度が50%以上と高い介在物(以下、該Ca濃度が50%以上の介在物を「Ca系介在物」という)が、上記微細なHICを含めたHIC発生の起点となりやすいことを見出した。   Furthermore, the inclusion composition which is the origin of HIC generation including the fine HIC was investigated. In detail, the structure observation was performed about the steel plate which performed the HIC test (NACE test) as described in the Example mentioned later. Further, the Ca concentration of the observed inclusions was determined. The Ca concentration in the inclusion is a ratio (% by mass, hereinafter simply referred to as%) of Ca in the component composition excluding O and N constituting the inclusion. Of the inclusions with a Ca concentration in the inclusion of 50% or more, the ratio (%) of the inclusion that was the origin of HIC generation, and among the inclusions with a Ca concentration in the inclusion of 20% or less, HIC generation Each of the ratio (%) of the inclusion which became the starting point was calculated | required. The result is shown in FIG. In FIG. 1, the ratio of inclusions that are the starting point of HIC generation is indicated by “HIC generation rate (%)” on the vertical axis. As shown in FIG. 1, inclusions having a particularly high Ca concentration of 50% or more (hereinafter, inclusions having a Ca concentration of 50% or more are referred to as “Ca inclusions”) include HIC including the fine HIC. We found that it is likely to be the starting point of occurrence.

前記Ca系介在物は、鋳造中に凝集合体し局所的に集積する傾向があり、このCa系介在物が鋼板表層領域に多く存在することによって、このCa系介在物を起点とした、従来の方法では確認し難い微細なHICが局所的に多数発生し、これが靭性の低下を引き起こしているものと考える。   The Ca-based inclusions tend to agglomerate and aggregate locally during casting, and the presence of a large amount of Ca-based inclusions in the surface layer region of the steel sheet makes the Ca-based inclusions a starting point. It is considered that a large number of fine HICs that are difficult to confirm by the method are locally generated, which causes a decrease in toughness.

そして本発明では、板厚方向に表面から深さ5mmまでの領域、つまり鋼板表層部のCa系介在物を制御するにあたり、鋼板表層部にCa系介在物が多く存在する場合には、該鋼板表層部にCa濃度の高い箇所が存在すると考えた。そこで、後述する実施例に示す通り、板厚方向に表面から深さ5mmまでを等間隔にCa濃度を複数箇所測定したときの、最大Ca濃度(Cmax)と、該複数箇所の平均Ca濃度(板厚方向に表面から深さ5mmまでの領域の平均Ca濃度、Cave)との比(Cmax/Cave)を、鋼板表層部のCa系介在物量の制御因子として用いることとした。   And in this invention, when controlling Ca area | region from the surface to a depth of 5 mm in the plate | board thickness direction, ie, the Ca type | system | group inclusion of a steel plate surface layer part, when there are many Ca type | system | group inclusions in a steel plate surface layer part, this steel plate It was considered that a portion with a high Ca concentration was present in the surface layer portion. Therefore, as shown in the examples described later, the maximum Ca concentration (Cmax) when the Ca concentration is measured at a plurality of locations at equal intervals from the surface to the depth of 5 mm in the plate thickness direction, and the average Ca concentration at the plurality of locations ( The ratio (Cmax / Cave) with the average Ca concentration (Cave) in the region from the surface to the depth of 5 mm in the plate thickness direction was used as a control factor for the amount of Ca-based inclusions in the surface layer portion of the steel plate.

次に、このCmax/Caveと、HIC試験後の鋼板表層部の靭性、具体的にはシャルピー吸収エネルギー、特にはアッパーシェルフエネルギーとの関係について調査した。その結果、両者には、後述する実施例に示す通り明確な相関関係が認められた。つまり本発明者らは、上記(Cmax/Cave)をコントロールすることによって、HIC試験後の鋼板表層部の靭性向上を図ることができることをまず見出した。更に、後述する実施例で評価の通り、優れた靭性としてアッパーシェルフエネルギー:125J以上を達成するには、Cmax/Caveを1.20以下とすればよいことを見出した。該Cmax/Caveは、好ましくは1.19以下、より好ましくは1.18以下、更に好ましくは1.15以下である。靭性向上の観点からは、上記Cmax/Caveは極力小さい方が好ましいが、下限は、鋼板表層部と鋼中のCa量が同じとなる1.00程度である。   Next, the relationship between Cmax / Cave and the toughness of the steel sheet surface layer after the HIC test, specifically Charpy absorbed energy, particularly the upper shelf energy, was investigated. As a result, a clear correlation was recognized between the two as shown in Examples described later. That is, the present inventors first found that the toughness of the steel sheet surface layer portion after the HIC test can be improved by controlling the above (Cmax / Cave). Furthermore, as evaluated in the examples described later, it was found that Cmax / Cave should be 1.20 or less to achieve an upper shelf energy of 125 J or more as excellent toughness. The Cmax / Cave is preferably 1.19 or less, more preferably 1.18 or less, and still more preferably 1.15 or less. From the viewpoint of improving toughness, the Cmax / Cave is preferably as small as possible, but the lower limit is about 1.00 where the amount of Ca in the steel plate surface layer and the steel is the same.

優れた耐HIC性を確保するには、上記鋼板表層部の制御と共に、鋼板や該鋼板を用いて得られる鋼管等の鋼材の成分組成を制御する必要がある。更には例えばラインパイプ用鋼板や圧力容器用鋼板として求められる、優れたHAZ靭性や溶接性等の上記耐HIC性以外の特性を確保するにも、鋼板の成分組成を下記の通りとする必要がある。以下、各成分の規定理由について説明する。   In order to secure excellent HIC resistance, it is necessary to control the component composition of the steel material such as a steel plate and a steel pipe obtained by using the steel plate as well as the control of the surface layer portion of the steel plate. Furthermore, in order to ensure properties other than the above-mentioned HIC resistance such as excellent HAZ toughness and weldability, which are required, for example, as steel plates for line pipes and steel plates for pressure vessels, the component composition of the steel plates needs to be as follows: is there. Hereinafter, the reasons for defining each component will be described.

〔成分組成〕
[C:0.02〜0.15%]
Cは、母材および溶接部の強度を確保するために必要不可欠な元素であり、0.02%以上含有させる必要がある。C量は、好ましくは0.03%以上であり、より好ましくは0.05%以上である。一方、C量が多すぎるとHAZ靭性と溶接性が劣化する。またC量が過剰であると、HICの起点や破壊進展経路となるNbCや島状マルテンサイトが生成しやすくなる。よってC量は0.15%以下とする必要がある。C量は、好ましくは0.12%以下、より好ましくは0.10%以下である。
(Component composition)
[C: 0.02 to 0.15%]
C is an indispensable element for securing the strength of the base material and the welded portion, and needs to be contained by 0.02% or more. The amount of C is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the amount of C is too large, the HAZ toughness and weldability deteriorate. On the other hand, if the amount of C is excessive, NbC and island-shaped martensite that become the starting point of HIC and the fracture propagation path are likely to be generated. Therefore, the C amount needs to be 0.15% or less. The amount of C is preferably 0.12% or less, more preferably 0.10% or less.

[Si:0.02〜0.50%]
Siは、脱酸作用を有する上に、母材および溶接部の強度向上に有効な元素である。これらの効果を得るため、Si量を0.02%以上とする。Si量は、好ましくは0.05%以上であり、より好ましくは0.15%以上である。しかし、Si量が多すぎると溶接性や靭性が劣化する。またSi量が過剰であると、島状マルテンサイトが生じてHICが発生・進展する。よってSi量は、0.50%以下に抑える必要がある。Si量は、好ましくは0.45%以下、より好ましくは0.35%以下である。
[Si: 0.02 to 0.50%]
Si is an element that has a deoxidizing action and is effective in improving the strength of the base material and the welded portion. In order to obtain these effects, the Si content is set to 0.02% or more. The amount of Si is preferably 0.05% or more, and more preferably 0.15% or more. However, if the amount of Si is too large, weldability and toughness deteriorate. If the amount of Si is excessive, island martensite is generated and HIC is generated and progresses. Therefore, the amount of Si needs to be suppressed to 0.50% or less. The amount of Si is preferably 0.45% or less, more preferably 0.35% or less.

[Mn:0.6〜2.0%]
Mnは、母材および溶接部の強度向上に有効な元素であり、本発明では0.6%以上含有させる。Mn量は、好ましくは0.8%以上であり、より好ましくは1.0%以上である。しかし、Mn量が多すぎると、MnSが生成されて耐水素誘起割れ性が劣化するだけでなくHAZ靭性や溶接性も劣化する。よってMn量の上限を2.0%以下とする。Mn量は、好ましくは1.8%以下であり、より好ましくは1.5%以下、さらに好ましくは1.2%以下である。
[Mn: 0.6 to 2.0%]
Mn is an element effective for improving the strength of the base material and the welded portion, and is contained in an amount of 0.6% or more in the present invention. The amount of Mn is preferably 0.8% or more, and more preferably 1.0% or more. However, if the amount of Mn is too large, not only MnS is produced and the hydrogen-induced cracking resistance deteriorates, but also the HAZ toughness and weldability deteriorate. Therefore, the upper limit of the amount of Mn is 2.0% or less. The amount of Mn is preferably 1.8% or less, more preferably 1.5% or less, and still more preferably 1.2% or less.

[P:0.030%以下(0%を含まない)]
Pは、鋼材中に不可避的に含まれる元素であり、P量が0.030%を超えると母材やHAZ部の靭性劣化が著しく、耐水素誘起割れ性も劣化する。よって本発明ではP量を0.030%以下に抑える。P量は、好ましくは0.020%以下、より好ましくは0.010%以下である。
[P: 0.030% or less (excluding 0%)]
P is an element inevitably contained in the steel material. If the amount of P exceeds 0.030%, the toughness of the base material and the HAZ part is significantly deteriorated, and the resistance to hydrogen-induced cracking is also deteriorated. Therefore, in the present invention, the amount of P is suppressed to 0.030% or less. The amount of P is preferably 0.020% or less, more preferably 0.010% or less.

[S:0.003%以下(0%を含まない)]
Sは、多すぎるとMnSを多量に生成し耐水素誘起割れ性を著しく劣化させる元素であるため、本発明ではS量の上限を0.003%とする。S量は、好ましくは0.002%以下であり、より好ましくは0.0015%以下、更に好ましくは0.0010%以下である。この様に耐水素誘起割れ性向上の観点からは少ない方が望ましい。
[S: 0.003% or less (excluding 0%)]
If S is too much, it is an element that produces a large amount of MnS and significantly deteriorates the resistance to hydrogen-induced cracking. Therefore, in the present invention, the upper limit of the amount of S is set to 0.003%. The amount of S is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. Thus, the smaller one is desirable from the viewpoint of improving hydrogen-induced crack resistance.

[Al:0.010〜0.08%]
Alは強脱酸元素であり、Al量が少ないと、酸化物中のCa濃度が上昇、即ち、Ca系介在物が鋼板表層部に形成されやすくなり微細なHICが発生する。よって本発明では、Alを0.010%以上とする必要がある。Al量は、好ましくは0.020%以上、より好ましくは0.030%以上である。一方、Al含有量が多すぎると、Alの酸化物がクラスター状に生成し水素誘起割れの起点となる。よってAl量は0.08%以下とする必要がある。Al量は、好ましくは0.06%以下であり、より好ましくは0.05%以下である。
[Al: 0.010 to 0.08%]
Al is a strong deoxidizing element. When the amount of Al is small, the Ca concentration in the oxide increases, that is, Ca inclusions are easily formed in the surface layer portion of the steel sheet and fine HIC is generated. Therefore, in the present invention, Al needs to be 0.010% or more. The amount of Al is preferably 0.020% or more, more preferably 0.030% or more. On the other hand, when there is too much Al content, the oxide of Al will produce | generate in cluster shape and will become the starting point of a hydrogen induced crack. Therefore, the Al amount needs to be 0.08% or less. The amount of Al is preferably 0.06% or less, and more preferably 0.05% or less.

[Ca:0.0003〜0.0060%]
Caは、硫化物の形態を制御する作用があり、CaSを形成することによってMnSの形成を抑制する効果がある。この効果を得るには、Ca量を0.0003%以上とする必要がある。Ca量は、好ましくは0.0005%以上であり、より好ましくは0.0010%以上である。一方、Ca量が0.0060%を超えると、Ca系介在物を起点にHICが多く発生する。よって本発明では、Ca量の上限を0.0060%とする。Ca量は、好ましくは0.0045%以下であり、より好ましくは0.0035%以下、さらに好ましくは0.0025%以下である。
[Ca: 0.0003 to 0.0060%]
Ca has the effect | action which controls the form of sulfide, and there exists an effect which suppresses formation of MnS by forming CaS. In order to obtain this effect, the Ca content needs to be 0.0003% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when the Ca content exceeds 0.0060%, a large amount of HIC is generated starting from Ca-based inclusions. Therefore, in the present invention, the upper limit of the Ca amount is set to 0.0060%. The Ca content is preferably 0.0045% or less, more preferably 0.0035% or less, and still more preferably 0.0025% or less.

[N:0.001〜0.01%]
Nは、鋼組織中にTiNとして析出し、HAZ部のオーステナイト粒の粗大化を抑制し、さらにフェライト変態を促進させて、HAZ部の靭性を向上させる元素である。この効果を得るにはNを0.001%以上含有させる必要がある。N量は、好ましくは0.003%以上であり、より好ましくは0.0040%以上である。しかし、N量が多すぎると、固溶Nの存在によりHAZ靭性がかえって劣化するため、N量は、0.01%以下とする必要がある。好ましくは0.008%以下であり、より好ましくは0.0060%以下である。
[N: 0.001 to 0.01%]
N is an element that precipitates as TiN in the steel structure, suppresses coarsening of the austenite grains in the HAZ part, further promotes ferrite transformation, and improves the toughness of the HAZ part. In order to acquire this effect, it is necessary to contain N 0.001% or more. The N amount is preferably 0.003% or more, and more preferably 0.0040% or more. However, if the amount of N is too large, the HAZ toughness deteriorates due to the presence of solute N, so the amount of N needs to be 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.0060% or less.

[O:0.0045%以下(0%を含まない)]
O(酸素)は、清浄度向上の観点から低いほうが望ましく、Oが多量に含まれる場合、靭性が劣化することに加え、酸化物を起点にHICが発生し、耐水素誘起割れ性が劣化する。この観点から、O量は0.0045%以下とする必要があり、好ましくは0.0030%以下、より好ましくは0.0020%以下である。
[O: 0.0045% or less (excluding 0%)]
O (oxygen) is preferably low from the viewpoint of improving cleanliness. When a large amount of O is contained, in addition to deterioration of toughness, HIC is generated starting from oxide, and resistance to hydrogen-induced cracking is deteriorated. . From this viewpoint, the amount of O needs to be 0.0045% or less, preferably 0.0030% or less, more preferably 0.0020% or less.

[Ca/S(質量比):2.0以上]
Caに対してSが過剰となる場合、板厚中央部を中心にMnSが生成し、MnSを起点にHICが発生する。これを抑制するためにはCa/Sを2.0以上とする必要があり、好ましくは2.5以上、より好ましくは3.0以上である。尚、本発明で規定するCa量とS量からCa/Sの上限は15程度となる。
[Ca / S (mass ratio): 2.0 or more]
When S is excessive with respect to Ca, MnS is generated around the center of the plate thickness, and HIC is generated starting from MnS. In order to suppress this, Ca / S needs to be 2.0 or more, preferably 2.5 or more, more preferably 3.0 or more. The upper limit of Ca / S is about 15 from the Ca amount and S amount specified in the present invention.

本発明の鋼材(鋼板、鋼管)の成分は、上記の通りであり、残部は鉄および不可避不純物からなる。また、上記元素に加えて更に、
(a)下記量のB、V、Cu、Ni、Cr、Mo、およびNbよりなる群から選択される1種類以上の元素を含有させて、強度や靭性をより高めることや、
(b)下記量のTi、Mg、REM、およびZrよりなる群から選択される1種類以上の元素を含有させて、HAZ靭性をより高めるとともに、脱硫を促進させ耐HIC性をより改善することができる。以下、これらの元素について詳述する。
The components of the steel material (steel plate, steel pipe) of the present invention are as described above, and the balance consists of iron and inevitable impurities. In addition to the above elements,
(A) Inclusion of one or more elements selected from the group consisting of the following amounts of B, V, Cu, Ni, Cr, Mo, and Nb to further increase strength and toughness,
(B) To contain one or more elements selected from the group consisting of Ti, Mg, REM, and Zr in the following amounts to increase HAZ toughness and promote desulfurization to further improve HIC resistance. Can do. Hereinafter, these elements will be described in detail.

[B:0.005%以下(0%を含まない)]
Bは、焼入れ性を高め、母材および溶接部の強度を高めるとともに、溶接時に、加熱されたHAZ部が冷却する過程でNと結合してBNを析出し、オーステナイト粒内からのフェライト変態を促進するため、HAZ靭性を向上させる。この効果を得るためには、B量を0.0002%以上含有させることが好ましい。B量は、より好ましくは0.0005%以上であり、更に好ましくは0.0010%以上である。しかし、B含有量が過多になると、母材とHAZ部の靭性が劣化したり、溶接性の劣化を招くため、B含有量は0.005%以下とするのが好ましい。B量は、より好ましくは0.004%以下、更に好ましくは0.0030%以下である。
[B: 0.005% or less (excluding 0%)]
B enhances hardenability, enhances the strength of the base metal and the welded part, and bonds with N during the process of cooling the heated HAZ part during welding, thereby precipitating BN and causing ferrite transformation from within the austenite grains. In order to promote, HAZ toughness is improved. In order to acquire this effect, it is preferable to contain B amount 0.0002% or more. The amount of B is more preferably 0.0005% or more, and further preferably 0.0010% or more. However, if the B content is excessive, the toughness between the base material and the HAZ part deteriorates or weldability deteriorates, so the B content is preferably 0.005% or less. The amount of B is more preferably 0.004% or less, and still more preferably 0.0030% or less.

[V:0.1%以下(0%を含まない)]
Vは、強度の向上に有効な元素であり、この効果を得るには0.003%以上含有させることが好ましい。より好ましくは0.010%以上である。一方、V含有量が0.1%を超えると溶接性と母材靭性が劣化する。よってV量は0.1%以下とすることが好ましく、より好ましくは0.08%以下である。
[V: 0.1% or less (excluding 0%)]
V is an element effective for improving the strength. To obtain this effect, V is preferably contained in an amount of 0.003% or more. More preferably, it is 0.010% or more. On the other hand, if the V content exceeds 0.1%, weldability and base metal toughness deteriorate. Therefore, the V amount is preferably 0.1% or less, and more preferably 0.08% or less.

[Cu:1.5%以下(0%を含まない)]
Cuは、焼入れ性を向上させて強度を高めるのに有効な元素である。この効果を得るにはCuを0.01%以上含有させることが好ましい。Cu量は、より好ましくは0.05%以上、更に好ましくは0.10%以上である。しかし、Cu含有量が1.5%を超えると靭性が劣化するため、1.5%以下とすることが好ましい。Cu量は、より好ましくは1.0%以下、更に好ましくは0.50%以下である。
[Cu: 1.5% or less (excluding 0%)]
Cu is an element effective for improving the hardenability and increasing the strength. In order to acquire this effect, it is preferable to contain 0.01% or more of Cu. The amount of Cu is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Cu content exceeds 1.5%, the toughness deteriorates, so it is preferable to set it to 1.5% or less. The amount of Cu is more preferably 1.0% or less, still more preferably 0.50% or less.

[Ni:1.5%以下(0%を含まない)]
Niは、母材および溶接部の強度と靭性の向上に有効な元素である。この効果を得るためには、Ni量を0.01%以上とすることが好ましい。Ni量は、より好ましくは0.05%以上、更に好ましくは0.10%以上である。しかしNiが多量に含まれると、構造用鋼材として極めて高価となるため、経済的な観点からNi量は1.5%以下とすることが好ましい。Ni量は、より好ましくは1.0%以下、更に好ましくは0.50%以下である。
[Ni: 1.5% or less (excluding 0%)]
Ni is an element effective for improving the strength and toughness of the base material and the welded portion. In order to obtain this effect, the Ni content is preferably 0.01% or more. The amount of Ni is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if Ni is contained in a large amount, it becomes extremely expensive as a structural steel material. Therefore, the Ni content is preferably 1.5% or less from an economical viewpoint. The amount of Ni is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Cr:1.5%以下(0%を含まない)]
Crは、強度の向上に有効な元素であり、この効果を得るには0.01%以上含有させることが好ましい。Cr量は、より好ましくは0.05%以上、更に好ましくは0.10%以上である。一方、Cr量が1.5%を超えるとHAZ靭性が劣化する。よってCr量は1.5%以下とすることが好ましい。Cr量は、より好ましくは1.0%以下、更に好ましくは0.50%以下である。
[Cr: 1.5% or less (excluding 0%)]
Cr is an element effective for improving the strength, and in order to obtain this effect, it is preferable to contain 0.01% or more. The amount of Cr is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.5%, the HAZ toughness deteriorates. Therefore, the Cr content is preferably 1.5% or less. The amount of Cr is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Mo:1.5%以下(0%を含まない)]
Moは、母材の強度と靭性の向上に有効な元素である。この効果を得るには、Mo量を0.01%以上とすることが好ましい。Mo量は、より好ましくは0.05%以上、更に好ましくは0.10%以上である。しかし、Mo量が1.5%を超えるとHAZ靭性および溶接性が劣化する。よってMo量は1.5%以下とすることが好ましく、より好ましくは1.0%以下、更に好ましくは0.50%以下である。
[Mo: 1.5% or less (excluding 0%)]
Mo is an element effective for improving the strength and toughness of the base material. In order to obtain this effect, the Mo amount is preferably 0.01% or more. The amount of Mo is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Mo amount exceeds 1.5%, the HAZ toughness and weldability deteriorate. Therefore, the Mo amount is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less.

[Nb:0.06%以下(0%を含まない)]
Nbは、溶接性を劣化させることなく強度と母材靭性を高めるのに有効な元素である。この効果を得るには、Nb量を0.002%以上とすることが好ましい。Nb量は、より好ましくは0.010%以上、更に好ましくは0.020%以上である。しかし、Nb量が0.06%を超えると母材とHAZの靭性が劣化する。よって、本発明ではNb量の上限を0.06%とすることが好ましい。Nb量は、より好ましくは0.050%以下、更に好ましくは0.040%以下、より更に好ましくは0.030%以下である。
[Nb: 0.06% or less (excluding 0%)]
Nb is an element effective for increasing strength and base metal toughness without degrading weldability. In order to obtain this effect, the Nb content is preferably 0.002% or more. The Nb amount is more preferably 0.010% or more, and still more preferably 0.020% or more. However, if the Nb content exceeds 0.06%, the toughness of the base material and the HAZ deteriorates. Therefore, in the present invention, the upper limit of the Nb amount is preferably 0.06%. The Nb amount is more preferably 0.050% or less, still more preferably 0.040% or less, and still more preferably 0.030% or less.

[Ti:0.03%以下(0%を含まない)]
Tiは、鋼中にTiNとして析出することで、溶接時のHAZ部でのオーステナイト粒の粗大化を防止しかつフェライト変態を促進するため、HAZ部の靭性を向上させるのに必要な元素である。さらにTiは、脱硫作用を示すため耐HIC性の向上にも有効な元素である。これらの効果を得るには、Tiを0.003%以上含有させることが好ましい。Ti量は、より好ましくは0.005%以上、更に好ましくは0.010%以上である。一方、Ti含有量が過多になると、Tiの固溶やTiCの析出により母材とHAZ部の靭性が劣化するため、0.03%以下とすることが好ましい。Ti量は、より好ましくは0.02%以下である。
[Ti: 0.03% or less (excluding 0%)]
Ti is an element necessary for improving the toughness of the HAZ part in order to prevent coarsening of austenite grains in the HAZ part during welding and promote ferrite transformation by precipitating as TiN in the steel. . Further, Ti is an element effective for improving the HIC resistance since it exhibits a desulfurization action. In order to obtain these effects, it is preferable to contain 0.003% or more of Ti. The amount of Ti is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, if the Ti content is excessive, the toughness of the base material and the HAZ part deteriorates due to solid solution of Ti and precipitation of TiC, so 0.03% or less is preferable. The amount of Ti is more preferably 0.02% or less.

[Mg:0.01%以下(0%を含まない)]
Mgは、結晶粒の微細化を通じて靭性の向上に有効な元素であり、また脱硫作用を示すため耐HIC性の向上にも有効な元素である。この効果を得るためには、Mgを0.0003%以上含有させることが好ましい。Mg量は、より好ましくは0.001%以上である。一方、Mgを、過剰に含有させても効果が飽和するため、Mg量の上限は0.01%とすることが好ましい。Mg量は、より好ましくは0.005%以下である。
[Mg: 0.01% or less (excluding 0%)]
Mg is an element effective for improving toughness through refinement of crystal grains, and is an element effective for improving HIC resistance since it exhibits a desulfurization action. In order to acquire this effect, it is preferable to contain Mg 0.0003% or more. The amount of Mg is more preferably 0.001% or more. On the other hand, since the effect is saturated even if Mg is excessively contained, the upper limit of the Mg content is preferably 0.01%. The amount of Mg is more preferably 0.005% or less.

[REM:0.02%以下(0%を含まない)]
REM(希土類元素)は、脱硫作用によりMnSの生成を抑制し耐水素誘起割れ性を高めるのに有効な元素である。このような効果を発揮させるには、REMを0.0002%以上含有させることが好ましい。REM量は、より好ましくは0.0005%以上、更に好ましくは0.0010%以上である。一方、REMを多量に含有させても効果が飽和する。よってREM量の上限は0.02%とすることが好ましい。鋳造時の浸漬ノズルの閉塞を抑えて生産性を高める観点からは、REM量を0.015%以下とすることがより好ましく、更に好ましくは0.010%以下、より更に好ましくは0.0050%以下である。尚、本発明において、上記REMとは、ランタノイド元素(LaからLuまでの15元素)とSc(スカンジウム)およびYを意味する。
[REM: 0.02% or less (excluding 0%)]
REM (rare earth element) is an element effective for suppressing the formation of MnS by the desulfurization action and enhancing the resistance to hydrogen-induced cracking. In order to exhibit such an effect, it is preferable to contain REM 0.0002% or more. The amount of REM is more preferably 0.0005% or more, and further preferably 0.0010% or more. On the other hand, the effect is saturated even if a large amount of REM is contained. Therefore, the upper limit of the REM amount is preferably 0.02%. From the viewpoint of increasing productivity by suppressing the clogging of the immersion nozzle during casting, the REM content is more preferably 0.015% or less, still more preferably 0.010% or less, and still more preferably 0.0050%. It is as follows. In the present invention, the REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y.

[Zr:0.010%以下(0%を含まない)]
Zrは、脱硫作用により耐HIC性の向上に寄与するとともに、酸化物を形成し微細に分散することでHAZ靭性の向上にも寄与する元素である。これらの効果を発揮させるには、Zr量を0.0003%以上とすることが好ましい。Zr量は、より好ましくは0.0005%以上、更に好ましくは0.0010%以上、より更に好ましくは0.0015%以上である。一方、Zrを過剰に添加すると粗大な介在物を形成して耐水素誘起割れ性および母材靭性を劣化させる。よってZr量は0.010%以下とすることが好ましい。Zr量は、より好ましくは0.0070%以下、更に好ましくは0.0050%以下、より更に好ましくは0.0030%以下である。
[Zr: 0.010% or less (excluding 0%)]
Zr is an element that contributes to improvement of HIC resistance by desulfurization and also contributes to improvement of HAZ toughness by forming an oxide and finely dispersing it. In order to exert these effects, the Zr content is preferably 0.0003% or more. The amount of Zr is more preferably 0.0005% or more, still more preferably 0.0010% or more, and still more preferably 0.0015% or more. On the other hand, when Zr is added excessively, coarse inclusions are formed and the hydrogen-induced crack resistance and the base metal toughness are deteriorated. Therefore, the Zr content is preferably 0.010% or less. The amount of Zr is more preferably 0.0070% or less, still more preferably 0.0050% or less, and still more preferably 0.0030% or less.

以上、本発明で規定する鋼板について説明した。本発明の鋼板を製造する方法は上記規定の鋼板表層部が得られる方法であれば特に限定されない。上記規定の鋼板表層部を有する鋼板を容易に得る方法として下記の方法が挙げられる。   In the above, the steel plate prescribed | regulated by this invention was demonstrated. The method for producing the steel plate of the present invention is not particularly limited as long as it is a method capable of obtaining the steel plate surface layer defined above. The following method is mentioned as a method for easily obtaining a steel plate having a steel plate surface layer part as defined above.

〔製造方法〕
上記成分組成となるように溶製した後、溶鋼は、取鍋、タンディッシュを経て鋳型に注入されるが、本発明で規定の鋼板表層部を有する鋼板を得るには、上記タンディッシュに溶鋼を注入し連続鋳造を行う工程において、下記(1)〜(3)の全てを満たすことが推奨される。
(1)タンディッシュにおいて、取鍋からの溶鋼注入位置での流路断面積よりも、鋳型への溶鋼注入位置における流路断面積が大きくなるようにする。具体的には、各流路断面積がこの様に設計されたタンディッシュを用いる。
(2)注入ノズルの吐出孔上部から50mm以上の位置からArを0.04〜9.7L(リットル)/t(ton)の流量で吹き込みつつ、鋳造する。
(3)鋳型内溶鋼のメニスカス位置から引き抜き方向に向かって1〜3mの位置の凝固速度を0.26mm/s以下とする。
〔Production method〕
After melting to have the above component composition, the molten steel is poured into a mold through a ladle and tundish, but in order to obtain a steel sheet having a specified steel sheet surface layer portion in the present invention, the molten steel is poured into the tundish. It is recommended that the following (1) to (3) be satisfied in the process of injecting and continuously casting.
(1) In the tundish, the flow path cross-sectional area at the molten steel injection position into the mold is made larger than the flow path cross-sectional area at the molten steel injection position from the ladle. Specifically, a tundish in which each channel cross-sectional area is designed in this way is used.
(2) Casting while blowing Ar at a flow rate of 0.04 to 9.7 L (liter) / t (ton) from a position of 50 mm or more from the upper part of the discharge hole of the injection nozzle.
(3) The solidification rate at a position of 1 to 3 m in the drawing direction from the meniscus position of the molten steel in the mold is set to 0.26 mm / s or less.

上記(1)〜(3)の各条件について以下、順に説明する。
(1)流路断面積
Ca系介在物は高融点であり、溶鋼との接触角が大きいため凝集合体を形成しやすく粗大な介在物となりやすい。よってこのCa系介在物をタンディッシュ内部で十分に浮上分離させる必要がある。この浮上分離が不十分な場合には、例えば連続鋳造時の湾曲部で上記粗大なCa系介在物が浮上し、表層に集積しやすくなる。タンディッシュ内で上記介在物を十分に浮上分離させるには、タンディッシュ内での溶鋼平均流速を小さくするのがよい。溶鋼平均流速を小さくすることによって、浮上時間を長時間化でき、また取鍋注入時の乱流により浮上分離を促進させることができる。タンディッシュ内での溶鋼平均流速を小さくするには、タンディッシュにおける鋳型への溶鋼注入位置における流路断面積が、取鍋からの溶鋼注入位置での流路断面積よりも大きいタンディッシュを用いる。(鋳型への溶鋼注入位置における流路断面積)/(取鍋からの溶鋼注入位置での流路断面積)で表される比が1.00超であればよいが、上記比は好ましくは1.50以上である。尚、上記比の上限は5.0程度である。
Each of the conditions (1) to (3) will be described in order below.
(1) Channel cross-sectional area Ca-based inclusions have a high melting point and have a large contact angle with molten steel, so that agglomerates are easily formed and coarse inclusions are easily formed. Therefore, it is necessary to sufficiently float and separate this Ca-based inclusion inside the tundish. If this floating separation is insufficient, for example, the coarse Ca-based inclusion floats at the curved portion during continuous casting, and tends to accumulate on the surface layer. In order to sufficiently float and separate the inclusions in the tundish, it is preferable to reduce the average molten steel flow velocity in the tundish. By reducing the average molten steel flow velocity, the ascent time can be extended, and levitation separation can be promoted by turbulent flow during ladle injection. To reduce the average molten steel flow velocity in the tundish, use a tundish whose cross-sectional area at the pouring position of molten steel into the mold in the tundish is larger than the cross-sectional area at the pouring position of molten steel from the ladle. . The ratio represented by (flow path cross-sectional area at the molten steel injection position into the mold) / (flow path cross-sectional area at the molten steel injection position from the ladle) may be more than 1.00, but the ratio is preferably 1.50 or more. The upper limit of the ratio is about 5.0.

(2)Ar吹込み
ノズル内の溶鋼が非充満となる吐出孔上部から50mm以上の位置でArを吹込みつつ鋳造を行うことで、ノズルおよび鋳型内でCa系介在物とAr気泡を合体させ浮上分離を促進させることができる。この効果を得るには、Ar流量を0.04L/t以上とするのが好ましい。前記Ar流量は、より好ましくは0.10L/t以上、更に好ましくは0.20L/t以上である。一方、Ar流量が9.7L/tを上回る場合、鋼片表層にAr気泡が残存し、鋼板に欠陥として残存しやすくなる。よってAr流量は、9.7L/t以下とすることが好ましく、より好ましくは9.0L/t以下、更に好ましくは8.0L/t以下である。
(2) Ar blowing By casting while blowing Ar at a position of 50 mm or more from the upper part of the discharge hole where the molten steel in the nozzle is not filled, the Ca inclusions and Ar bubbles are combined in the nozzle and the mold. Floating separation can be promoted. In order to obtain this effect, the Ar flow rate is preferably 0.04 L / t or more. The Ar flow rate is more preferably 0.10 L / t or more, and still more preferably 0.20 L / t or more. On the other hand, when the Ar flow rate exceeds 9.7 L / t, Ar bubbles remain in the steel slab surface layer and easily remain as defects in the steel sheet. Therefore, the Ar flow rate is preferably 9.7 L / t or less, more preferably 9.0 L / t or less, and still more preferably 8.0 L / t or less.

(3)凝固速度
一般に、凝固速度が大きい場合は、凝固界面近傍に存在する介在物が界面に取り込まれやすく、凝固速度が小さい場合は、介在物の一部が凝固界面から未凝固の中央部に押し出される。本発明では凝固速度を小さくすることによって、鋼板表層部に介在物が集積しないようにする。具体的には、本発明で対象とする「表面から深さ5mmまでの領域」が凝固する、鋳型内溶鋼のメニスカス位置から引き抜き方向に向かって1〜3mの位置の、凝固速度を0.26mm/s以下とする。凝固速度は好ましくは0.22mm/s以下、より好ましくは0.18mm/s以下である。尚、凝固速度の下限値は、生産性等の観点からおおよそ0.05mm/sとなる。上記凝固速度は、冷却水の水量密度や鋳造速度の制御によって調整することができる。
(3) Solidification rate In general, when the solidification rate is high, inclusions existing in the vicinity of the solidification interface are easily taken into the interface, and when the solidification rate is low, some of the inclusions are unsolidified from the solidification interface to the central part. Extruded. In the present invention, by reducing the solidification rate, inclusions are prevented from accumulating in the steel sheet surface layer. Specifically, the solidification rate at a position of 1 to 3 m from the meniscus position of the molten steel in the mold where the “region from the surface to a depth of 5 mm” targeted in the present invention is solidified is 0.26 mm. / S or less. The solidification rate is preferably 0.22 mm / s or less, more preferably 0.18 mm / s or less. The lower limit of the solidification rate is approximately 0.05 mm / s from the viewpoint of productivity and the like. The solidification speed can be adjusted by controlling the water density of the cooling water and the casting speed.

本発明では、上記の様にして鋳造した後の工程については特に問わず、常法に従って熱間圧延を行うか、または前記熱間圧延後、更に再加熱して熱処理を行うことにより、鋼板を製造することができる。また、該鋼板を用い、一般的に行われている方法でラインパイプ用鋼管を製造することができる。本発明の鋼板を用いて得られるラインパイプ用鋼管もまた耐HIC性および靭性に優れている。   In the present invention, the process after casting as described above is not particularly limited. Hot rolling is performed according to a conventional method, or after the hot rolling, the steel sheet is further reheated and heat treated. Can be manufactured. Moreover, the steel pipe for line pipes can be manufactured by the method generally performed using this steel plate. The steel pipe for line pipes obtained using the steel sheet of the present invention is also excellent in HIC resistance and toughness.

以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。   EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

表1に示す成分組成の鋼を溶製し、連続鋳造により、厚みが280mmである鋼片(スラブ)を得た。製造工程における連続鋳造の条件は、表2に示す通りである。表2の「(1)流路断面積」の欄において、鋳型への溶鋼注入位置における流路断面積が、取鍋からの溶鋼注入位置での流路断面積よりも大きいタンディッシュを用いた場合には「○」とし、そうでない場合を「×」とした。尚、本実施例において前記「○」の場合は、(取鍋からの溶鋼注入位置での流路断面積)/(鋳型への溶鋼注入位置における流路断面積)の比が1.05以上であるタンディッシュを用いた。また、表2の「(2)Ar吹込み」の欄において、注入ノズルの吐出孔上部から50mm以上の位置からArを0.04〜9.7L/tの流量で吹き込みつつ、鋳造した場合を「○」とし、そうでない場合を「×」とした。更に表2の「(3)凝固速度」の欄において、鋳型内溶鋼のメニスカス位置から引き抜き方向に向かって1〜3mの位置の凝固速度を0.26mm/s以下とした場合を「○」とし、該凝固速度で行わなかった場合を「×」とした。   Steel having the component composition shown in Table 1 was melted and a steel piece (slab) having a thickness of 280 mm was obtained by continuous casting. The conditions for continuous casting in the production process are as shown in Table 2. In the column of “(1) Channel cross-sectional area” in Table 2, a tundish in which the channel cross-sectional area at the molten steel injection position into the mold is larger than the channel cross-sectional area at the molten steel injection position from the ladle was used. In some cases, “◯” was indicated, and in other cases, “X” was indicated. In this example, in the case of “◯”, the ratio of (flow channel cross-sectional area at the molten steel injection position from the ladle) / (flow path cross-sectional area at the molten steel injection position into the mold) is 1.05 or more. A tundish was used. In addition, in the column of “(2) Ar blowing” in Table 2, the casting was performed while blowing Ar at a flow rate of 0.04 to 9.7 L / t from a position of 50 mm or more from the upper part of the discharge hole of the injection nozzle. “○” was given, and “x” was given otherwise. Furthermore, in the column of “(3) Solidification rate” in Table 2, “○” indicates that the solidification rate at a position of 1 to 3 m from the meniscus position of the molten steel in the mold toward the drawing direction is 0.26 mm / s or less. The case where the solidification rate was not performed was defined as “x”.

その後、連続鋳造により製造した鋼片を、1050〜1250℃となるよう加熱してから、表2の「熱間圧延・冷却方法」の欄に「TMCP」(Thermo Mechanical Control Process)または「QT」(Quenching and Tempering)と示す通り、2パターンの熱間圧延・冷却方法により、成分組成が種々の鋼板(板厚:12〜90mm)を得た。前記「TMCP」では、鋼板の表面温度で900℃以上の累積圧下率が30%以上になるよう熱間圧延し、更に、700℃以上900℃未満の累積圧下率が20%以上となるよう熱間圧延を行い、圧延終了温度が700℃以上900℃未満となるようにした。その後、650℃以上の温度から水冷を開始し、350〜600℃の温度で水冷を停止し、更にその後、室温まで空冷した。また前記「QT」では、熱間圧延した後室温まで空冷し、850℃以上950℃以下の温度に再加熱して焼入れた後、600〜700℃で焼き戻し処理を行った。   Thereafter, the steel slab produced by continuous casting is heated to 1050 to 1250 ° C., and then “TMCP” (Thermo Mechanical Control Process) or “QT” in the “Hot rolling / cooling method” column of Table 2. As shown in (Quenching and Tempering), steel plates (plate thickness: 12 to 90 mm) with various component compositions were obtained by two patterns of hot rolling and cooling methods. In the “TMCP”, hot rolling is performed so that the cumulative reduction rate of 900 ° C. or more at the surface temperature of the steel sheet is 30% or more, and further, the cumulative reduction rate of 700 ° C. or more and less than 900 ° C. is 20% or more. Rolling was performed so that the rolling end temperature was 700 ° C. or higher and lower than 900 ° C. Then, water cooling was started from the temperature of 650 degreeC or more, water cooling was stopped at the temperature of 350-600 degreeC, and also it cooled to room temperature after that. In the “QT”, after hot rolling, air-cooled to room temperature, reheated to a temperature of 850 ° C. or higher and 950 ° C. or lower and quenched, and then tempered at 600 to 700 ° C.

そして各鋼板を用いて、下記に示す通りCmax/Caveの測定を行った。また、HIC試験を行って耐HIC性の評価を行い、シャルピー衝撃試験を行って靭性を評価した。   And using each steel plate, Cmax / Cave was measured as shown below. Moreover, the HIC test was conducted to evaluate the HIC resistance, and the Charpy impact test was conducted to evaluate the toughness.

[Cmax/Caveの測定]
鋼板の板厚方向に表面から深さ5mmまでの領域のCa濃度の分布を蛍光分光分析により測定した。具体的には、最初に鋼板のスケール層を剥離するため鋼板表面から0.5mmまでを研削し、鋼板の表面に相当する該研削面のCa濃度を測定した。次いで、板厚方向に0.5mm研削してから該研削面のCa濃度測定を行った。これを板厚方向に0.5mmピッチで繰り返し行い、表面から板厚方向に深さ5mmまでの計10断面のCa濃度を測定した。そして10断面におけるCa濃度の最大値をCmax、10断面のCa濃度の平均値をCaveとし、Cmax/Caveを求めた。
[Measurement of Cmax / Cave]
The distribution of Ca concentration in the region from the surface to a depth of 5 mm in the plate thickness direction of the steel plate was measured by fluorescence spectroscopic analysis. Specifically, in order to first peel off the scale layer of the steel sheet, 0.5 mm from the steel sheet surface was ground, and the Ca concentration of the ground surface corresponding to the steel sheet surface was measured. Next, after grinding 0.5 mm in the plate thickness direction, the Ca concentration of the ground surface was measured. This was repeated at a pitch of 0.5 mm in the plate thickness direction, and the Ca concentration of 10 cross sections from the surface to the depth of 5 mm was measured. Cmax / Cave was determined by setting Cmax as the maximum value of Ca concentration in 10 cross sections and Cave as the average value of Ca concentration in 10 cross sections.

[HIC試験(NACE試験)]
HIC試験は、NACE standard TM0284−2003に従って実施・評価した。詳細には、各鋼板の幅方向における1/4W位置と1/2W位置から、それぞれ3本、計6本の試験片(サイズ:板厚×(幅)100mm×(圧延方向)20mm)を採取した。そして該試験片を、1atmの硫化水素を飽和させた25℃の0.5%NaClと0.5%酢酸を含む混合水溶液中に96時間浸漬し、断面評価をNACE standard TM0284−2003 FIGURE3に従って行い、CLR(Crack Length Ratio、試験片幅に対する割れ長さ合計の割合(%)、割れ長さ率)を測定した。そして、前記CLRが3%以下の場合を耐HIC性に優れる(○)と評価し、CLRが3%超の場合を耐HIC性に劣る(×)と評価した。
[HIC test (NACE test)]
The HIC test was performed and evaluated according to NACE standard TM0284-2003. Specifically, three test pieces (size: plate thickness x (width) 100 mm x (rolling direction) 20 mm) were collected from each 1/4 W position and 1/2 W position in the width direction of each steel sheet. did. The test piece was immersed in a mixed aqueous solution containing 0.5% NaCl and 0.5% acetic acid at 25 ° C. saturated with 1 atm hydrogen sulfide for 96 hours, and the cross-sectional evaluation was performed according to NACE standard TM0284-2003 FIGURE3. , CLR (Crack Length Ratio, ratio (%) of crack length to crack width, crack length ratio) was measured. And when the said CLR was 3% or less, it evaluated that it was excellent in HIC resistance ((circle)), and the case where CLR was over 3% was evaluated as inferior to HIC resistance (x).

[シャルピー衝撃試験]
NACE試験後、試験片の表面直下よりASTM A370に従い、板厚方向5mm×圧延方向10mmのシャルピー試験片を圧延方向に垂直な方向で3本採取し、鋼板の板厚方向にノッチを施した。シャルピー衝撃試験はASTM A370に従い実施し、試験温度は0℃〜80℃まで種々変化させ、脆性破面率が0%でのシャルピー吸収エネルギー、つまりアッパーシェルフエネルギーを求めた。そして、このアッパーシェルフエネルギーが125J以上の場合を靭性に優れると評価した。
[Charpy impact test]
After the NACE test, three Charpy test pieces having a thickness of 5 mm and a rolling direction of 10 mm were sampled in a direction perpendicular to the rolling direction from directly under the surface of the test piece, and notched in the thickness direction of the steel sheet. The Charpy impact test was performed according to ASTM A370, and the test temperature was variously changed from 0 ° C. to 80 ° C., and the Charpy absorbed energy, that is, the upper shelf energy when the brittle fracture surface ratio was 0% was obtained. And it evaluated that the case where this upper shelf energy was 125J or more was excellent in toughness.

これらの結果を表2に示す。   These results are shown in Table 2.

表1および表2より次のことがわかる。No.1〜13、およびNo.22〜26は、本発明で規定の成分組成を満たすと共に、鋼板表層部のCmax/Caveが本発明で規定の範囲を満たしているため、耐HIC性に優れ、かつ靭性にも優れていることがわかる。   Table 1 and Table 2 show the following. No. 1-13, and No.1. Nos. 22 to 26 satisfy the specified component composition in the present invention, and Cmax / Cave of the steel sheet surface layer portion satisfies the specified range in the present invention, so that the HIC resistance is excellent and the toughness is also excellent. I understand.

これに対し、No.14および27は、鋼板表層部のCmax/Caveは本発明で規定の範囲を満たしているが、成分組成(Ca/S)が本発明の規定を外れているため、耐HIC性に劣る結果となった。またNo.15〜21およびNo.28〜31は、鋼板表層部のCmax/Caveが本発明で規定の範囲を満たしていないため、靭性が悪くなった。特にNo.15〜19、21、28、29および31は、耐HIC性は確保できているものの靭性が悪くなった。   In contrast, no. 14 and 27, Cmax / Cave of the steel sheet surface layer portion satisfies the range specified in the present invention, but the component composition (Ca / S) deviates from the range specified in the present invention, resulting in poor HIC resistance. became. No. 15-21 and no. In Nos. 28 to 31, because Cmax / Cave of the steel sheet surface layer portion did not satisfy the specified range in the present invention, the toughness deteriorated. In particular, no. Although 15-19, 21, 28, 29, and 31 were able to ensure HIC resistance, the toughness deteriorated.

図2は、上記表2の結果を用いて得られた、Cmax/Caveとアッパーシェルフエネルギーの関係を示す図である。この図2から、アッパーシェルフエネルギーが125J以上の優れた靭性を得るには、Cmax/Caveを1.20以下とすればよいことがわかる。   FIG. 2 is a graph showing the relationship between Cmax / Cave and upper shelf energy obtained using the results in Table 2 above. From FIG. 2, it can be seen that Cmax / Cave should be 1.20 or less in order to obtain excellent toughness with an upper shelf energy of 125 J or more.

本発明に係る鋼板は、耐水素誘起割れ性と靭性に優れているので、これらは、天然ガス・原油の輸送用ラインパイプや圧力容器、貯蔵用タンクなどに好適に用いられる。   Since the steel plates according to the present invention are excellent in hydrogen-induced crack resistance and toughness, they are suitably used for natural gas / crude oil transportation line pipes, pressure vessels, storage tanks, and the like.

Claims (6)

C:0.02〜0.15%(%は質量%の意味。以下同じ)、
Si:0.02〜0.50%、
Mn:0.6〜2.0%、
P:0.030%以下(0%を含まない)、
S:0.003%以下(0%を含まない)、
Al:0.010〜0.08%、
Ca:0.0003〜0.0060%、
N:0.001〜0.01%、および
O(酸素):0.0045%以下(0%を含まない)を満たし、残部が鉄および不可避不純物からなり、
前記Caと前記Sの比(Ca/S)が2.0以上であり、かつ
板厚方向に表面から深さ5mmまでの領域の最大Ca濃度(Cmax)と該領域の平均Ca濃度(Cave)との比(Cmax/Cave)が1.20以下であることを特徴とする耐水素誘起割れ性と靭性に優れた鋼板。
C: 0.02 to 0.15% (% means mass%, the same shall apply hereinafter)
Si: 0.02 to 0.50%,
Mn: 0.6 to 2.0%,
P: 0.030% or less (excluding 0%),
S: 0.003% or less (excluding 0%),
Al: 0.010 to 0.08%,
Ca: 0.0003 to 0.0060%,
N: 0.001 to 0.01%, and O (oxygen): 0.0045% or less (excluding 0%) is satisfied, and the balance consists of iron and inevitable impurities,
The ratio of Ca to S (Ca / S) is 2.0 or more, and the maximum Ca concentration (Cmax) in the region from the surface to the depth of 5 mm in the plate thickness direction and the average Ca concentration (Cave) in the region A steel sheet excellent in hydrogen-induced crack resistance and toughness, wherein the ratio (Cmax / Cave) is 1.20 or less.
更に他の元素として、
B:0.005%以下(0%を含まない)、
V:0.1%以下(0%を含まない)、
Cu:1.5%以下(0%を含まない)、
Ni:1.5%以下(0%を含まない)、
Cr:1.5%以下(0%を含まない)、
Mo:1.5%以下(0%を含まない)、および
Nb:0.06%以下(0%を含まない)
よりなる群から選択される1種以上の元素を含む請求項1に記載の鋼板。
As other elements,
B: 0.005% or less (excluding 0%),
V: 0.1% or less (excluding 0%),
Cu: 1.5% or less (excluding 0%),
Ni: 1.5% or less (excluding 0%),
Cr: 1.5% or less (excluding 0%),
Mo: 1.5% or less (not including 0%), and Nb: 0.06% or less (not including 0%)
The steel plate according to claim 1, comprising at least one element selected from the group consisting of:
更に他の元素として、
Ti:0.03%以下(0%を含まない)、
Mg:0.01%以下(0%を含まない)、
REM:0.02%以下(0%を含まない)、および
Zr:0.010%以下(0%を含まない)
よりなる群から選択される1種以上の元素を含む請求項1または2に記載の鋼板。
As other elements,
Ti: 0.03% or less (excluding 0%),
Mg: 0.01% or less (excluding 0%),
REM: 0.02% or less (not including 0%), and Zr: 0.010% or less (not including 0%)
The steel plate according to claim 1 or 2, comprising one or more elements selected from the group consisting of:
ラインパイプ用である請求項1〜3のいずれかに記載の鋼板。   It is an object for line pipes, The steel plate in any one of Claims 1-3. 請求項1〜4のいずれかに記載の鋼板を用いて製造されるラインパイプ用鋼管。   The steel pipe for line pipes manufactured using the steel plate in any one of Claims 1-4. 圧力容器用である請求項1〜3のいずれかに記載の鋼板。   The steel plate according to any one of claims 1 to 3, which is used for a pressure vessel.
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