JP2014208891A - Steel sheet and line pipe steel pipe excellent in hydrogen-induced crack resistance and heat affected zone toughness - Google Patents
Steel sheet and line pipe steel pipe excellent in hydrogen-induced crack resistance and heat affected zone toughness Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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Abstract
Description
本発明は、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な、耐水素誘起割れ性と溶接熱影響部の靭性に優れた鋼板、および該鋼板を用いて得られる耐水素誘起割れ性と溶接熱影響部の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 use in natural gas / crude oil transportation line pipes, pressure vessels, storage tanks, etc., and is obtained by using a steel plate excellent in hydrogen-induced crack resistance and toughness of weld heat affected zone, and the steel plate. The present invention relates to a steel pipe for a line pipe having excellent resistance to hydrogen-induced cracking and toughness of a heat affected zone.
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 , N) and other non-metallic inclusions are known to be cracks caused 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、S/Ca<0.5とし、Sに対してCaを多量に含有させるとともに、板厚中心部のMnの偏析度を低減しMnSを抑制することにより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って、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。 Conventionally, several techniques for improving hydrogen-induced crack resistance (hereinafter sometimes referred to as “HIC resistance”) have been proposed. For example, in Patent Document 1, S / Ca <0.5, a large amount of Ca is contained with respect to S, and the degree of segregation of Mn at the center of the plate thickness is reduced and MnS is suppressed to suppress hydrogen-induced cracking. A steel material with improved is disclosed. 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 a Ca-based oxysulfide by a parameter formula including Ca, O, and S contents.
これらの従来技術では、HICの抑制は可能であるものの、表層の溶接熱影響部は、脆性破壊が発生して靭性が著しく劣化する場合があり、耐水素誘起割れ性と溶接熱影響部の靭性(HAZ靭性)を両立させることは困難であった。 Although these conventional technologies can suppress HIC, the weld heat-affected zone on the surface layer may undergo brittle fracture and the toughness may deteriorate significantly. Hydrogen-induced crack resistance and the toughness of the weld heat-affected zone It has been difficult to achieve both (HAZ toughness).
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、耐水素誘起割れ性と溶接熱影響部の靭性(HAZ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 paying attention to the above-mentioned circumstances, and its purpose is to realize a steel plate and a steel pipe excellent in hydrogen-induced crack resistance and weld heat-affected zone toughness (HAZ toughness). is there.
上記課題を解決し得た本発明の耐水素誘起割れ性と溶接熱影響部の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%、
Ti:0.003〜0.030%、
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
前記Ca、前記Sおよび前記Oが(Ca−1.25S)/O ≦ 1.8を満たし、
更に、
板厚方向に表面から深さ5mmまでの領域において、長径または長辺が50μm以上のCa系介在物が2.0個/mm2以下であり、かつ長径または長辺が300nm以下のTiNが5×102個/μm2以上であるところに特徴を有する。
The steel sheet excellent in the hydrogen-induced crack resistance of the present invention and the toughness of the weld heat affected zone, which has solved the above problems,
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%,
Ti: 0.003-0.030%,
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 Ca, S and O satisfy (Ca−1.25S) /O≦1.8,
Furthermore,
In the region from the surface to the depth of 5 mm in the plate thickness direction, the number of Ca inclusions having a long diameter or long side of 50 μm or more is 2.0 pieces / mm 2 or less, and TiN having a long diameter or long side of 300 nm or less is 5 It is characterized in that it is 10 2 / μm 2 or more.
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:1.0%以下(0%を含まない)、
Ni:1.5%以下(0%を含まない)、
Cr:1.0%以下(0%を含まない)、
Mo:1.0%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)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: 1.0% or less (excluding 0%),
Ni: 1.5% or less (excluding 0%),
Cr: 1.0% or less (excluding 0%),
Mo: 1.0% 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) 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、鋼板表層部に存在する介在物を適正に制御しているため、耐水素誘起割れ性と溶接熱影響部の靭性に優れた鋼板や鋼管を提供できる。 According to the present invention, since inclusions existing in the surface layer portion of the steel plate are appropriately controlled, it is possible to provide a steel plate or steel pipe excellent in hydrogen-induced crack resistance and toughness of the weld heat affected zone.
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、種々の鋼板について、NACE(National Association of Corrosion and Engineer)TM0284に規定されたHIC試験を実施し、耐HIC性を評価した。上記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。一方で、溶接熱影響部(HAZ)の靭性を確認するため、後述する実施例に示す通り、溶接入熱が40kJ/cmの溶接を模擬した溶接再現試験を実施し、次いでシャルピー衝撃試験を行った。 The inventors of the present invention have made extensive studies to solve the above problems. First, the present inventors performed HIC tests specified in NACE (National Association of Corrosion and Engineer) TM0284 for various steel plates, and evaluated HIC resistance. In the NACE test, HIC is generated after a test piece, that is, a steel sheet, is immersed in a mixed solution of pH 2.7 of 5% NaCl solution and 0.5% acetic acid saturated with 1 atm hydrogen sulfide gas for 96 hours. It is a test to evaluate. On the other hand, in order to confirm the toughness of the weld heat affected zone (HAZ), a welding reproduction test simulating welding with a heat input of 40 kJ / cm was performed, and then a Charpy impact test was performed as shown in the examples described later. It was.
これらの試験を行った結果、上記NACE試験で優れた耐HIC性を確保できたものであっても、鋼板表層部のHAZ靭性が他の部位(たとえば板厚中央部)に比べて著しく低下した例があった。この原因について詳細に調査した結果、鋼板表層部には長径または長辺が50μm以上の粗大なCa系介在物が存在し、これが脆性破壊の起点となることをまず見い出した。尚、上記「長径または長辺」とは、後述する実施例で行う介在物のサイズ測定の通り、介在物の形状が、円状や楕円状等の場合は長径をいい、長方形状の場合は長辺をいうことを意味する。 As a result of these tests, the HAZ toughness of the steel sheet surface layer part was significantly reduced compared to other parts (for example, the center part of the plate thickness) even though excellent HIC resistance was ensured in the NACE test. There was an example. As a result of investigating this cause in detail, it was first found that coarse Ca-based inclusions having a long diameter or a long side of 50 μm or more existed in the surface layer of the steel sheet, and this was the starting point for brittle fracture. In addition, the above-mentioned “major axis or long side” means the major axis when the shape of the inclusion is a circle or an ellipse, etc. Means the long side.
これらのことから、従来ではMnSの抑制を目的にCaを多量に添加するため、溶鋼との接触角が大きなCa系介在物、特にはCa系酸化物が生成し、これが製造過程における凝固途中で凝集合体して粗大化、浮上し鋼板表層部に集積しやすく、その結果、表層の溶接熱影響部では、粗大なCa系介在物が脆性破壊の起点となってHAZ靭性が劣化したものと考えられる。 From these facts, conventionally, a large amount of Ca is added for the purpose of suppressing MnS, and therefore, Ca-based inclusions having a large contact angle with the molten steel, particularly Ca-based oxides, are formed, which is in the course of solidification in the manufacturing process. Aggregated and coalesced, coarsened, floated, and easily accumulated on the surface layer of the steel sheet. As a result, in the heat affected zone of the surface layer, it was thought that coarse Ca-based inclusions became the starting point of brittle fracture and the HAZ toughness deteriorated. It is done.
そこで本発明者らは、板厚方向に表面から深さ5mmまでの領域、つまり鋼板表層部に存在する長径または長辺が50μm以上の粗大なCa系介在物とHAZ靭性の関係について調査した。その結果、優れたHAZ靭性、後述する実施例に示す通り、ΔvTrs([1/2tのvTrs]−[表層のvTrs])が0℃以上でかつ表層の破面遷移温度が室温(25℃)以下を達成するには、上記長径または長辺が50μm以上の粗大なCa系介在物の個数密度を2.0個/mm2以下に抑える必要があることを見出した。尚、本発明において、上記「Ca系介在物」とは、後述する実施例に記載の通り、S、O、Nを除く全ての元素を100質量%としたときのCa量(質量%)が60質量%以上の介在物をいう。該Ca系介在物として例えば、Ca酸化物、Ca硫化物、Ca酸硫化物の他、これらと他の介在物との複合介在物などが挙げられる。上記50μm以上のCa系介在物の個数密度は、好ましくは1.8個/mm2以下、より好ましくは1.5個/mm2以下であり、最も好ましくは0個/mm2である。 Therefore, the inventors investigated the relationship between the HAZ toughness and coarse Ca-based inclusions having a major axis or long side of 50 μm or more existing in a region from the surface to a depth of 5 mm in the thickness direction, that is, in the surface layer portion of the steel plate. As a result, excellent HAZ toughness, as shown in the examples described later, ΔvTrs ([1/2 t vTrs] − [surface layer vTrs]) is 0 ° C. or more and the fracture surface transition temperature of the surface layer is room temperature (25 ° C.) In order to achieve the following, it has been found that the number density of coarse Ca-based inclusions having a major axis or long side of 50 μm or more must be suppressed to 2.0 / mm 2 or less. In the present invention, the “Ca inclusions” are, as described in Examples below, the amount of Ca (% by mass) when all elements except S, O, and N are 100% by mass. This refers to inclusions of 60% by mass or more. Examples of the Ca-based inclusion include Ca oxide, Ca sulfide, Ca oxysulfide, and composite inclusions of these and other inclusions. The number density of the Ca inclusions of 50 μm or more is preferably 1.8 pieces / mm 2 or less, more preferably 1.5 pieces / mm 2 or less, and most preferably 0 piece / mm 2 .
本発明では、更にHAZ靭性確保のために、長径または長辺が300nm以下のTiNを多数分散させる。TiNは溶接加熱時にオーステナイト粒の粗大化を抑制すると共に、溶接加熱後の冷却過程で粒内フェライトの変態核として作用し、溶接熱影響部の組織微細化に寄与する。この効果を得るために、長径または長辺が300nm以下のTiNの個数密度を5×102個/μm2以上とする。上記TiNの個数密度は、好ましくは8×102個/μm2以上、より好ましくは10×102個/μm2以上、更に好ましくは20×102個/μm2以上である。尚、TiNは多ければ多いほど好ましく、特に個数密度の上限は設けないが、本発明の成分組成範囲等から、その上限は150×102個/μm2程度となる。 In the present invention, in order to further secure HAZ toughness, a large number of TiN having a major axis or a long side of 300 nm or less is dispersed. TiN suppresses the coarsening of austenite grains during welding heating, and acts as a transformation nucleus of intragranular ferrite in the cooling process after welding heating, contributing to refinement of the structure of the weld heat affected zone. In order to obtain this effect, the number density of TiN having a long diameter or a long side of 300 nm or less is set to 5 × 10 2 pieces / μm 2 or more. The number density of TiN is preferably 8 × 10 2 pieces / μm 2 or more, more preferably 10 × 10 2 pieces / μm 2 or more, and further preferably 20 × 10 2 pieces / μm 2 or more. The more TiN is, the more preferable, and the upper limit of the number density is not particularly set, but the upper limit is about 150 × 10 2 pieces / μm 2 from the component composition range of the present invention.
尚、上記対象となるTiNのサイズの下限値は、後述する実施例に示す通り、透過型電子顕微鏡(Transmission Electron Microscope、TEM)を用い例えば観察倍率10万倍で認識できる、おおよそ50nm以上である。上記Ca系介在物の個数密度とTiNの個数密度は、後述する実施例に記載の方法で求められる。 The lower limit of the target TiN size is approximately 50 nm or more, which can be recognized, for example, at an observation magnification of 100,000 using a transmission electron microscope (TEM), as shown in the examples described later. . The number density of the Ca-based inclusions and the number density of TiN are obtained by the method described in the examples described later.
優れた耐HIC性とHAZ靭性を確保するには、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、高強度や優れた溶接性等の上記耐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 ensure excellent HIC resistance and HAZ toughness, 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 together with the control of the steel sheet surface layer. Furthermore, in order to ensure properties other than the above-mentioned HIC resistance, such as high strength and excellent weldability, which are required as steel plates for line pipes and steel plates for pressure vessels, for example, 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・進展すると共にHAZ靭性が劣化する。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. On the other hand, if the amount of Si is excessive, island-shaped martensite is generated, and HIC is generated and propagated, and the HAZ toughness deteriorates. 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 made 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.
[Ti:0.003〜0.030%]
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ため、Ti量を0.003%以上とする。Ti量は、好ましくは0.005%以上、より好ましくは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、0.030%以下とする。Ti量は、好ましくは0.025%以下、より好ましくは0.022%以下、更に好ましくは0.020%以下、より更に好ましくは0.018%以下である。
[Ti: 0.003-0.030%]
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, the Ti amount is set to 0.003% or more. The amount of Ti is preferably 0.005% or more, 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 the content is made 0.030% or less. The amount of Ti is preferably 0.025% or less, more preferably 0.022% or less, still more preferably 0.020% or less, and still more preferably 0.018% 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0.0040%以下であり、より好ましくは0.0035%以下、さらに好ましくは0.0030%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 0.0040% or less, more preferably 0.0035% or less, and still more preferably 0.0030% 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。N量は、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. The N amount is preferably 0.008% or less, and more preferably 0.0060% or less.
[O(酸素):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 (oxygen): 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]
前述の通り、Sは硫化物系介在物としてMnSを形成し、圧延により伸展する結果、耐HIC性を最も劣化させる。このため、Caを添加して鋼中の硫化物系介在物をCaSとして形態を制御し、耐HIC性に対するSの無害化を図る。この作用効果を十分に発揮させるには、Ca/Sを2.0以上とする必要がある。Ca/Sは、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]
As described above, S forms MnS as a sulfide inclusion, and as a result of extending by rolling, the HIC resistance is most deteriorated. For this reason, Ca is added to control the form of the sulfide inclusions in the steel as CaS, thereby detoxifying S against HIC resistance. In order to fully exhibit this effect, Ca / S needs to be 2.0 or more. Ca / S is 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.
[(Ca−1.25S)/O ≦ 1.8]
Ca系介在物の中でも特に凝集合体を形成しやすいCaOを抑制するには、鋼中全Ca量から硫化物(CaS)として存在するCa分を差し引いたCa量(Ca−1.25S)が、O量に対して過剰とならないようにしなければならない。O量に対してCa量(Ca−1.25S)が過剰であると、酸化物系介在物としてCaOが形成され易くなり、該CaOの凝集合体(粗大なCa系介在物)が表層部に大量に形成されやすくなる。これを抑制するため、本発明者らは、(Ca−1.25S)/OとHAZ靭性との関係について検討したところ、優れたHAZ靭性を得るには(Ca−1.25S)/Oを1.8以下とする必要があることを見い出した。(Ca−1.25S)/Oは、好ましくは1.40以下、より好ましくは1.30以下、更に好ましくは1.20以下、特に好ましくは1.00以下である。尚、CaOと同様に凝集合体を形成しやすいAl2O3を抑制する観点から、(Ca−1.25S)/Oの下限値は0.1程度となる。
[(Ca-1.25S) /O≦1.8]
In order to suppress CaO that easily forms agglomerates among Ca-based inclusions, the Ca amount (Ca-1.25S) obtained by subtracting the Ca component existing as sulfide (CaS) from the total Ca amount in the steel, It must be ensured that the amount of O is not excessive. When the amount of Ca (Ca-1.25S) is excessive with respect to the amount of O, CaO is likely to be formed as oxide inclusions, and aggregated coalescence (coarse Ca inclusions) of CaO is formed in the surface layer portion. It becomes easy to be formed in large quantities. In order to suppress this, the present inventors examined the relationship between (Ca-1.25S) / O and HAZ toughness. To obtain excellent HAZ toughness, (Ca-1.25S) / O is used. I found that it was necessary to make it 1.8 or less. (Ca-1.25S) / O is preferably 1.40 or less, more preferably 1.30 or less, still more preferably 1.20 or less, and particularly preferably 1.00 or less. In addition, the lower limit of (Ca-1.25S) / O is about 0.1 from the viewpoint of suppressing Al 2 O 3 that easily forms an aggregated coal as in CaO.
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ることによって強度や靭性をより高めることができ、また、(b)下記量の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ることによって、HAZ靭性をより高めるとともに、脱硫を促進させ耐HIC性をより向上させることができる。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) Strength and toughness can be further increased by containing one or more elements selected from the group consisting of B, V, Cu, Ni, Cr, Mo, and Nb in the following amounts, b) By containing one or more elements selected from the group consisting of Mg, REM, and Zr in the following amounts, HAZ toughness can be further enhanced, desulfurization can be promoted, and HIC resistance can be further improved. . 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:1.0%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1.0%を超えると靭性が劣化するため、1.0%以下とすることが好ましい。Cu量は、より好ましくは0.5%以下、更に好ましくは0.35%以下である。
[Cu: 1.0% 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 1.0%, the toughness deteriorates, so it is preferable to make it 1.0% or less. The amount of Cu is more preferably 0.5% or less, still more preferably 0.35% 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:1.0%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1.0%を超えるとHAZ靭性が劣化する。よってCr量は1.0%以下とすることが好ましい。Cr量は、より好ましくは0.5%以下、更に好ましくは0.35%以下である。
[Cr: 1.0% 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 1.0%, the HAZ toughness deteriorates. Therefore, the Cr content is preferably 1.0% or less. The amount of Cr is more preferably 0.5% or less, and still more preferably 0.35% or less.
[Mo:1.0%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1.0%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1.0%以下とすることが好ましく、より好ましくは0.5%以下、更に好ましくは0.35%以下である。
[Mo: 1.0% 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 1.0%, the HAZ toughness and weldability deteriorate. Therefore, the Mo amount is preferably 1.0% or less, more preferably 0.5% or less, and still more preferably 0.35% 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.
[Mg:0.01%以下(0%を含まない)]
Mgは、結晶粒の微細化を通じてHAZ靭性の向上に有効な元素であり、また、脱硫作用を示し耐HIC特性の向上にも有効な元素である。これらの効果を得るためには、Mg量を0.0003%以上とすることが好ましい。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0.0050%以下である。
[Mg: 0.01% or less (excluding 0%)]
Mg is an element effective for improving the HAZ toughness through refinement of crystal grains, and is an element that exhibits a desulfurization action and is effective for improving the HIC resistance. In order to obtain these effects, the Mg content is preferably 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 0.0050% 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とともに、酸化物を形成してHAZ靭性の向上に有効に作用する元素である。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量の上限を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おさ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 that effectively suppresses the generation of MnS by the desulfurization action and improves the resistance to hydrogen-induced cracking, and forms an oxide to effectively improve the HAZ toughness. 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、脱硫作用を示し耐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 exhibits a desulfurization action and contributes to improvement of HIC resistance, 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(1)〜(4)の全てを満たすように製造する方法が挙げられる。 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. As a method for easily obtaining a steel sheet having a steel sheet surface layer portion as defined above, a method for producing so as to satisfy all of the following (1) to (4) may be mentioned.
〔製造方法〕
(1)Ca添加速度
上記規定の鋼板表層部を有する鋼板を容易に得るには、たとえばLF、RH処理を行った後のCa添加工程において、Ca(化合物を用いる場合は、Ca単独の量に換算される)の添加速度を0.002kg/min・t〜0.020kg/min・tとすることが推奨される。
〔Production method〕
(1) Ca addition rate In order to easily obtain a steel sheet having the above specified steel sheet surface layer portion, for example, in the Ca addition process after performing LF and RH treatment, Ca (when using a compound, the amount of Ca alone) It is recommended that the rate of addition is 0.002 kg / min · t to 0.020 kg / min · t.
Caの添加速度が0.002kg/min・t未満の場合、Ca添加による化学反応が穏やかなため溶鋼の撹拌が不十分となり、酸化物組成を均質化することができない。その結果、Ca濃度の高い介在物とそれ以外の介在物に分離し、表層部に粗大なCa系介在物が存在しやすくなる。よってCaの添加速度は、0.002kg/min・t以上とすることが好ましく、より好ましくは0.004kg/min・t以上である。一方、Caの添加速度が0.020kg/min・tを超えると、Ca添加による化学反応が過剰に激しくなり、湯面が乱される。よって、溶鋼が直接大気と接触するため酸素の巻き込み量が大きくなり、酸化物の絶対量が増大する。その結果、Ca濃度の高い介在物も相対的に増大し凝集合体化して、表層に粗大なCa系介在物が形成されやすくなる。これらのことからCaの添加速度は、0.020kg/min・t以下とすることが好ましく、より好ましくは0.018kg/min・t以下である。 When the addition rate of Ca is less than 0.002 kg / min · t, the chemical reaction due to the addition of Ca is gentle, so the molten steel is not sufficiently stirred, and the oxide composition cannot be homogenized. As a result, inclusions with high Ca concentration and other inclusions are separated, and coarse Ca inclusions are likely to be present in the surface layer portion. Therefore, the Ca addition rate is preferably 0.002 kg / min · t or more, and more preferably 0.004 kg / min · t or more. On the other hand, when the addition rate of Ca exceeds 0.020 kg / min · t, the chemical reaction due to the addition of Ca becomes excessively intense, and the molten metal surface is disturbed. Therefore, since the molten steel is in direct contact with the atmosphere, the amount of oxygen involved is increased and the absolute amount of oxide is increased. As a result, inclusions with a high Ca concentration also relatively increase and aggregate and coalesce, and coarse Ca-based inclusions are easily formed on the surface layer. For these reasons, the Ca addition rate is preferably 0.020 kg / min · t or less, and more preferably 0.018 kg / min · t or less.
(2)Ca添加後からタンディッシュ(TD)への溶鋼供給開始までの時間
Ca添加後に酸化物組成の均質化を図るため、Ca添加後からタンディッシュへの溶鋼の供給開始までの時間を10分以上確保することが好ましい。該時間は、より好ましくは15分以上である。尚、生産性等の観点から、上記時間の上限は120分程度である。
(2) Time from the addition of Ca to the start of supply of molten steel to the tundish (TD) In order to homogenize the oxide composition after the addition of Ca, the time from the addition of Ca to the start of supply of molten steel to the tundish is set to 10 It is preferable to secure at least minutes. The time is more preferably 15 minutes or longer. From the viewpoint of productivity and the like, the upper limit of the time is about 120 minutes.
(3)タンディッシュ(TD)への溶鋼供給開始から鋳造開始までの時間
取鍋からタンディッシュへの溶鋼供給を開始した後、鋳造開始までに3分以上、より好ましくは5分以上、上限はおおよそ40分以下保持した後に、鋳造を開始することが好ましい。これにより、タンディッシュ内での介在物の凝集・合体を促進でき、一部に残存したCa系介在物を浮上分離させることができる。
(3) Time from supply of molten steel to tundish (TD) to start of casting After starting supply of molten steel from ladle to tundish, 3 minutes or more by start of casting, more preferably 5 minutes or more, upper limit is It is preferable to start casting after holding for approximately 40 minutes or less. Thereby, aggregation and coalescence of inclusions in the tundish can be promoted, and Ca-based inclusions remaining in part can be floated and separated.
(4)鋳造途中の冷却段階における1500℃から1000℃までの平均冷却速度
必要なTiN個数密度を確保する観点から、鋳造途中の冷却段階での1500℃から1000℃までの平均冷却速度は、10℃/min以上とすることが好ましい。これにより、TiNの粗大化を抑制して、本発明で規定量の微細なTiNを確保することができる。上記平均冷却速度は、より好ましくは15℃/min以上である。一方、平均冷却速度が35℃/minを超えると、TiNが析出せず固溶Ti、固溶Nとして鋼材中に残存し、この場合も本発明で規定量の微細なTiNを確保することが難しい。よって上記平均冷却速度は、35℃/min以下とすることが好ましい。上記平均冷却速度は、より好ましくは30℃/min以下である。尚、前記平均冷却速度はスラブ表面の温度が1500℃から1000℃まで冷却される間の平均冷却速度である。
(4) Average cooling rate from 1500 ° C. to 1000 ° C. in the cooling stage during casting From the viewpoint of securing the necessary TiN number density, the average cooling rate from 1500 ° C. to 1000 ° C. in the cooling stage during casting is 10 It is preferable to set it as ° C / min or more. Thereby, coarsening of TiN can be suppressed, and a specified amount of fine TiN can be secured in the present invention. The average cooling rate is more preferably 15 ° C./min or more. On the other hand, when the average cooling rate exceeds 35 ° C./min, TiN does not precipitate and remains in the steel material as solute Ti and solute N, and in this case as well, a specified amount of fine TiN can be secured in the present invention. difficult. Therefore, the average cooling rate is preferably 35 ° C./min or less. The average cooling rate is more preferably 30 ° C./min or less. In addition, the said average cooling rate is an average cooling rate during the temperature of the slab surface being cooled from 1500 degreeC to 1000 degreeC.
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HAZ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 HAZ 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、下記のCa添加方法および鋳造方法により、厚みが280mmである鋼片(スラブ)を得た。製造工程におけるCa添加から連続鋳造までの条件は、表2に示す通りである。即ち、LF、RH処理を行った後のCa添加工程で、Caの添加速度を0.002kg/min・t〜0.020kg/min・tとした場合を、表2の「(1)Ca添加速度」の欄において「○」、そうでない場合を「×」とした。また、Ca添加後からタンディッシュ(TD)への溶鋼供給開始までの時間を10分以上とした場合を、表2の「(2)Ca添加後からTD供給開始までの時間」の欄において「○」、そうでない場合を「×」とした。タンディッシュ(TD)への溶鋼供給開始から鋳造開始までの時間を3分以上とした場合を、表2の「(3)TD供給開始から鋳造開始までの時間」の欄において「○」、そうでない場合を「×」とした。更に鋳造途中の冷却段階における1500℃から1000℃までの平均冷却速度を10〜35℃/minとした場合を、表2の「(4)鋳造時の冷却段階の1500〜1000℃の平均冷却速度」の欄において「○」、そうでない場合を「×」とした。 Steel with the component composition shown in Table 1 was melted, and a steel piece (slab) having a thickness of 280 mm was obtained by the following Ca addition method and casting method. The conditions from Ca addition to continuous casting in the production process are as shown in Table 2. That is, in the Ca addition step after the LF and RH treatments, the case where the Ca addition rate is 0.002 kg / min · t to 0.020 kg / min · t is shown in Table 2, “(1) Ca addition In the column of “Speed”, “◯” is indicated, and otherwise “X” is indicated. In addition, when the time from the addition of Ca to the start of molten steel supply to the tundish (TD) is set to 10 minutes or more, in the column of “(2) Time from the addition of Ca to the start of TD supply” in Table 2, “ ◯ ”, otherwise“ × ”. When the time from the start of the molten steel supply to the tundish (TD) to the start of casting is 3 minutes or more, “○” in the column of “(3) Time from the start of TD supply to the start of casting” in Table 2 When it was not, it was set as "x". Furthermore, when the average cooling rate from 1500 ° C. to 1000 ° C. in the cooling stage during casting is 10 to 35 ° C./min, “(4) Average cooling rate of 1500 to 1000 ° C. in the cooling stage during casting” is shown in Table 2. "" In the column of "", and "x" otherwise.
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:20〜51mm)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 indicated by (Quenching and Tempering), steel plates (plate thickness: 20 to 51 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、Ca系介在物とTiNの個数密度の測定を行った。また、HIC試験を行って耐HIC性の評価、HAZ靭性の評価を行った。 And using each steel plate, as shown below, the number density of Ca inclusions and TiN was measured. Moreover, the HIC test was conducted to evaluate the HIC resistance and the HAZ toughness.
[Ca系介在物の個数密度の測定]
Ca系介在物の測定は、走査型電子顕微鏡(Scanning Electron Microscope、SEM)を用い、次の様にして行った。まず、観察倍率を400倍とし、鋼板表面から板厚方向に深さ5mmまでを等間隔に、圧延方向に垂直な断面(板幅方向×板厚方向の面)を10断面以上観察した。該観察の1視野サイズは50mm2程度である。
[Measurement of number density of Ca inclusions]
The Ca inclusions were measured using a scanning electron microscope (SEM) as follows. First, the observation magnification was 400 times, and the cross section perpendicular to the rolling direction (surface in the plate width direction × plate thickness direction) was observed at 10 or more at equal intervals from the steel plate surface to a depth of 5 mm in the plate thickness direction. One visual field size of the observation is about 50 mm 2 .
そして観察中に検出された介在物について、介在物の形状が、円状や楕円状等の場合は長径を測定し、長方形状の場合は長辺を測定して、介在物のサイズとした。上記測定では、介在物と介在物の間隔が10μm以下のものは一つの介在物として扱った。次いで、前記長径または長辺が50μm以上の介在物について、EDX(Energy Dispersive X−ray spectrometry)で定量分析を実施した。そして、検出された元素からS、O、Nを除く全ての元素を100質量%としたときのCa量(質量%)を求め、このCa量が60質量%以上の介在物をCa系介在物とした。上記10断面以上のそれぞれの断面において、上記Ca系介在物の個数を測定し、単位面積(mm2)あたりの個数に換算した。そして、複数断面において求めた個数密度のうち最大値を、前記長径または長辺が50μm以上のCa系介在物の個数密度とした。 And about the inclusion detected during observation, when the shape of the inclusion was circular or elliptical, the major axis was measured, and when the inclusion was rectangular, the long side was measured to determine the size of the inclusion. In the above measurement, one having an interval between inclusions of 10 μm or less was treated as one inclusion. Next, quantitative analysis was performed by EDX (Energy Dispersive X-ray spectroscopy) for inclusions having a major axis or a long side of 50 μm or more. And the Ca amount (mass%) when all elements except S, O, and N are detected as 100% by mass from the detected elements is obtained, and the inclusion whose Ca amount is 60% by mass or more is determined to be a Ca-based inclusion. It was. In each of the 10 or more cross sections, the number of Ca inclusions was measured and converted to the number per unit area (mm 2 ). Then, the maximum value among the number densities obtained in a plurality of cross sections was the number density of Ca-based inclusions having a major axis or a long side of 50 μm or more.
[TiNの個数密度の測定]
TiNの測定は、透過型電子顕微鏡(TEM)を用い、次の様にして行った。まず、鋼板表面から板厚方向に深さ5mmの位置において、任意の5箇所を観察した。観察倍率は6万倍以上、1視野サイズは1.5μm×1.5μm以上とした。この様に観察倍率を大きくすることで、介在物の個数をより正確に計測することができる。また観察視野を広く、かつ観察数を多くしてその平均値を採用することにより、観察箇所によるTiNの個数のばらつきを小さくすることができる。
[Measurement of TiN Number Density]
TiN was measured using a transmission electron microscope (TEM) as follows. First, arbitrary five places were observed in the position of depth 5mm from the steel plate surface to the plate thickness direction. The observation magnification was 60,000 times or more, and the visual field size was 1.5 μm × 1.5 μm or more. Thus, by increasing the observation magnification, the number of inclusions can be measured more accurately. In addition, by widening the observation field and increasing the number of observations and adopting the average value, variation in the number of TiNs depending on the observation location can be reduced.
そして観察中に検出された介在物について長径または長辺を測定し、介在物のサイズとした。さらに、前記長径または長辺が300nm以下の介在物について、EDXで定量分析を実施し、TiおよびNをそれぞれ10質量%以上含有する介在物をTiNと確認した。そして該TiNの個数を測定して、単位面積(μm2)あたりの個数を計算して求め、上記5箇所の平均値を、長径または長辺が300nm以下のTiNの個数密度とした。 And the major axis or long side was measured about the inclusion detected during observation, and it was set as the size of the inclusion. Further, the inclusion having the major axis or the long side of 300 nm or less was quantitatively analyzed by EDX, and the inclusion containing 10% by mass or more of Ti and N was confirmed as TiN. Then, the number of TiN was measured, and the number per unit area (μm 2 ) was calculated and obtained. The average value of the five locations was defined as the number density of TiN having a major axis or a long side of 300 nm or less.
[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 an 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).
[HAZ靭性の評価]
溶接熱影響部(HAZ)の靭性を評価するため、各鋼板について溶接入熱量が40kJ/cmの溶接を模擬して、次の溶接再現試験を行った。即ち、表層(鋼板表面下6mm)および板厚中央部(1/2t)のそれぞれから切り出したサンプル(いずれもサイズは12mm×33mm×55mm)を、熱サイクル試験後のシャルピー試験片でノッチ位置になる部分が1350℃となるよう加熱した後、5秒保持し、冷却した。このときの平均冷却速度は800〜500℃への冷却時間が27秒となるよう調整した。
[Evaluation of HAZ toughness]
In order to evaluate the toughness of the weld heat affected zone (HAZ), the following welding reproduction test was performed by simulating welding with a heat input of 40 kJ / cm for each steel plate. That is, samples (each size is 12 mm × 33 mm × 55 mm) cut out from the surface layer (6 mm below the surface of the steel plate) and the central part (1/2 t) of the plate thickness are placed at the notch position with the Charpy test piece after the thermal cycle test. After heating to a temperature of 1350 ° C., it was held for 5 seconds and cooled. The average cooling rate at this time was adjusted so that the cooling time to 800 to 500 ° C. was 27 seconds.
この溶接を模擬したサンプルから、表層(鋼板表面下6mm)および板厚中央部(1/2t)のそれぞれにおいて、ASTM A370に規定の通り、鋼板の板厚方向にVノッチを施した一辺が10mmのシャルピー試験片を各3本採取した。そして、ASTM A370に規定の方法で、シャルピー衝撃試験を実施して破面遷移温度を測定した。本実施例では、表層の破面遷移温度(表層のvTrs)と板厚中央部の破面遷移温度(1/2tのvTrs)の差:ΔvTrs([1/2tのvTrs]−[表層のvTrs])が0℃以上でかつ表層の破面遷移温度が室温(25℃)以下の場合を、HAZ靭性に優れると評価した。 From the sample simulating this welding, each surface layer (6 mm below the surface of the steel plate) and the center of the plate thickness (1/2 t) has a side of 10 mm with a V notch in the plate thickness direction of the steel plate as defined in ASTM A370. Three Charpy test pieces were collected. And the Charpy impact test was implemented by the method prescribed | regulated to ASTM A370, and the fracture surface transition temperature was measured. In this example, the difference between the fracture surface transition temperature of the surface layer (vTrs of the surface layer) and the fracture surface transition temperature of the central part of the plate thickness (vTrs of 1/2 t): ΔvTrs ([1/2 t vTrs] − [vTrs of the surface layer] ]) Was 0 ° C. or higher and the fracture surface transition temperature of the surface layer was room temperature (25 ° C.) or lower, it was evaluated as having excellent HAZ toughness.
表1および表2より次のことがわかる。No.1〜14およびNo.23〜26は、鋼板表層部において、粗大なCa系介在物が抑制され、かつTiNの個数密度が一定以上であるため、耐HIC性に優れ、かつHAZ靭性にも優れていることがわかる。 Table 1 and Table 2 show the following. No. 1-14 and no. It can be seen that Nos. 23 to 26 are excellent in HIC resistance and HAZ toughness because coarse Ca-based inclusions are suppressed in the steel sheet surface layer portion and the number density of TiN is not less than a certain value.
これに対し、No.15および27はTiNの個数密度が不足したため、HAZ靭性に劣る結果となった。No.16〜20およびNo.28〜30は、鋼板表層部にCa系介在物が多く存在したため、HAZ靭性に劣る結果となった。尚、No.20では鋼板表層部にCa系介在物が著しく多く存在したため、HAZ靭性もかなり劣る結果となった。No.21および31は、Ca/Sの値が小さく、MnSが多く形成されたため、HAZ靭性に劣る結果となった。またNo.22および32は、(Ca−1.25S)/Oの値が大きく、Ca系介在物、特にCaOが多く形成されたため、HAZ靭性に劣る結果となった。 In contrast, no. Nos. 15 and 27 were inferior in HAZ toughness due to insufficient TiN number density. No. 16-20 and no. Nos. 28 to 30 were inferior in HAZ toughness because many Ca-based inclusions were present in the surface layer portion of the steel sheet. No. In No. 20, since there were significantly more Ca-based inclusions in the surface layer of the steel sheet, the HAZ toughness was considerably inferior. No. Nos. 21 and 31 had a low Ca / S value and a large amount of MnS, resulting in poor HAZ toughness. No. Nos. 22 and 32 had a large value of (Ca-1.25S) / O, and a large amount of Ca-based inclusions, particularly CaO, resulted in inferior HAZ toughness.
本発明に係る鋼板は、耐水素誘起割れ性とHAZ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 plate according to the present invention is excellent in hydrogen-induced cracking resistance and HAZ toughness, they are suitably used for natural gas / crude oil transportation line pipes, pressure vessels, storage tanks, and the like.
Claims (6)
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%、
Ti:0.003〜0.030%、
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
前記Ca、前記Sおよび前記Oが(Ca−1.25S)/O ≦ 1.8を満たし、
更に、
板厚方向に表面から深さ5mmまでの領域において、長径または長辺が50μm以上のCa系介在物が2.0個/mm2以下であり、かつ長径または長辺が300nm以下のTiNが5×102個/μm2以上であることを特徴とする耐水素誘起割れ性と溶接熱影響部の靭性に優れた鋼板。 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%,
Ti: 0.003-0.030%,
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 Ca, S and O satisfy (Ca−1.25S) /O≦1.8,
Furthermore,
In the region from the surface to the depth of 5 mm in the plate thickness direction, the number of Ca inclusions having a long diameter or long side of 50 μm or more is 2.0 pieces / mm 2 or less, and TiN having a long diameter or long side of 300 nm or less is 5 A steel sheet having excellent resistance to hydrogen-induced cracking and toughness of the heat affected zone, characterized by being × 10 2 pieces / μm 2 or more.
B:0.005%以下(0%を含まない)、
V:0.1%以下(0%を含まない)、
Cu:1.0%以下(0%を含まない)、
Ni:1.5%以下(0%を含まない)、
Cr:1.0%以下(0%を含まない)、
Mo:1.0%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: 1.0% or less (excluding 0%),
Ni: 1.5% or less (excluding 0%),
Cr: 1.0% or less (excluding 0%),
Mo: 1.0% 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:
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,
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:
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