JP5910195B2 - Method for evaluating HIC resistance of steel material and method for producing high strength thick steel plate for line pipe using the same - Google Patents
Method for evaluating HIC resistance of steel material and method for producing high strength thick steel plate for line pipe using the same Download PDFInfo
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Description
本発明は、鋼材の耐水素誘起割れ(Hydrogen Induced Cracking:HIC)性向上に係り、とくに、鋼材の耐HIC性の評価方法に関する。なお、ここでいう「鋼材」には、厚鋼板、熱延鋼板が含まれる。 The present invention relates to improvement of hydrogen induced cracking (HIC) resistance of steel materials, and more particularly to a method for evaluating the HIC resistance of steel materials. Here, “steel material” includes thick steel plates and hot-rolled steel plates.
連続鋳造鋳片(以下、単に「鋳片」ともいう)を素材とする鋼材では、連続鋳造時に鋳片中心部に形成される偏析(中心偏析)が、鋼材の品質に大きな悪影響を及ぼすことが知られている。特に、硫化水素が多く含まれる原油、天然ガス等の輸送用ラインパイプでは、表面から鋼中に水素が侵入しやすいため、水素誘起割れ(Hydrogen Induced Cracking:HIC)が多発して問題となってきた。鋼材の中心偏析部には、延伸したMnS、酸化物等の介在物などが多数、存在しており、侵入した水素が集積しやすく、中心偏析部での水素誘起割れが多発しやすい。このため、従来から、中心偏析を軽減することが要望されてきた。 In steel materials that use continuous cast slabs (hereinafter also simply referred to as “slabs”), segregation (center segregation) formed at the center of the slab during continuous casting can have a significant adverse effect on the quality of the steel. Are known. In particular, transportation line pipes for crude oil, natural gas, etc. that contain a large amount of hydrogen sulfide are prone to hydrogen-induced cracking (HIC) because hydrogen easily enters steel from the surface. It was. There are a large number of inclusions such as stretched MnS and oxide in the center segregation part of the steel material, so that the hydrogen that has entered easily accumulates, and hydrogen-induced cracks tend to occur frequently in the center segregation part. For this reason, conventionally, it has been desired to reduce the center segregation.
当初は、中心部におけるMnの偏析を抑制したり、あるいは例えば、特許文献1に記載されるように、Sを0.001%以下に低減し、さらにCaを添加してMnSの形態を制御した鋳片とし、さらに該鋳片に、加熱、圧延、冷却を適正に調整して、HICを抑制しようする技術が提案されている。しかし、これらだけでは、HICの発生を完全には防止できず、更なる検討が行われてきた。 Initially, a slab in which segregation of Mn in the central portion is suppressed or, as described in Patent Document 1, for example, S is reduced to 0.001% or less, and Ca is further added to control the form of MnS In addition, a technique for suppressing HIC by appropriately adjusting heating, rolling, and cooling to the slab has been proposed. However, these alone cannot completely prevent the occurrence of HIC, and further studies have been conducted.
例えば、特許文献2には、中心偏析が低減され、マクロ的な中央偏析が除かれたのちにも、Mnのスポット的な偏析部が存在し、ここを起点に水素誘起割れ(HIC)が発生する場合があることが記載されている。このようなMnのスポット的な偏析部は、群状のMnSが存在し、Pの偏析が認められ、Ca処理が有効に作用していない領域であり、Mnのスポット的な偏析部の大きさを400μm未満、偏析部のP濃度を0.035%未満、かつ有効Ca比を1.7以上とすれば、HICの発生が回避できるとしている。また、特許文献3には、連続鋳造時の2次冷却の比水量を適正量に制限し、中心偏析部のNb、Mn含有量を抑制し、Mn偏析度(偏析部の最大Mn含有量/平均Mn含有量)を1.4以下、かつNb偏析度(偏析部の最大Nb含有量/平均Nb含有量)を4.0以下に調整した熱延鋼板が提案されている。 For example, in Patent Document 2, even after the central segregation is reduced and the macro central segregation is removed, there is a spot segregation portion of Mn, and hydrogen-induced cracking (HIC) occurs from this point. It is stated that there is a case. Such Mn spot-like segregation is a region where group-like MnS exists, P segregation is observed, and Ca treatment does not act effectively, and the size of Mn spot-segregation is large. Is less than 400 μm, the P concentration in the segregation part is less than 0.035%, and the effective Ca ratio is 1.7 or more, the generation of HIC can be avoided. In Patent Document 3, the specific water amount of secondary cooling during continuous casting is limited to an appropriate amount, the Nb and Mn contents in the center segregation part are suppressed, and the Mn segregation degree (the maximum Mn content in the segregation part / A hot rolled steel sheet having an average Mn content) of 1.4 or less and an Nb segregation degree (maximum Nb content of segregation portion / average Nb content) adjusted to 4.0 or less has been proposed.
しかし、特許文献2,3に記載された技術では、各元素の偏析度の測定方法についての記載がなく、また偏析部の形態やその判定方法についての記載もなく、具体的にHICの発生を回避するうえで、不明瞭のままであるという問題がある。
また、特許文献4には、連続鋳造における最終凝固時の軽圧下と、熱間圧延終了後の水冷を調整することにより、最大Mn偏析度:2.0以下、Nb偏析度:4.0以下、Ti偏析度:4.0以下に制限したうえで、S/Ca:0.5以下を満足するようにCa処理を施した耐水素誘起割れ性に優れた高強度ラインパイプ用鋼板が提案されている。しかし、特許文献4に記載された技術では、偏析部における各元素の偏析度は、EPMA(電子プローブマイクロアナライザー、Electron Probe Micro Analyzer)によって測定した値を用いるとしているが、偏析部の形態やその判定方法についてまでの言及はなく、不明瞭のままである。
However, in the techniques described in Patent Documents 2 and 3, there is no description about the measurement method of the segregation degree of each element, and there is no description about the form of the segregation part and its determination method. There is a problem that it remains unclear in avoiding it.
In Patent Document 4, the maximum Mn segregation degree is 2.0 or less, the Nb segregation degree is 4.0 or less, and the Ti segregation degree is adjusted by adjusting the light reduction at the time of final solidification in continuous casting and the water cooling after the hot rolling is completed. : Steel plates for high-strength line pipes, which are excellent in hydrogen-induced cracking resistance and are subjected to Ca treatment so as to satisfy S / Ca: 0.5 or less after limiting to 4.0 or less, have been proposed. However, in the technique described in Patent Document 4, the segregation degree of each element in the segregation part uses values measured by EPMA (Electron Probe Micro Analyzer). There is no mention of the determination method, and it remains unclear.
中心偏析を評価する方法としては、従来から、例えば、スライス法、ドリル法、マクロ腐食法、Hプリント法など、いくつかの方法が開発され、使用されている。
スライス法では、鋳片や厚鋼板を厚さ方向に順次スライスしていき、そのスライスして採取した切粉の成分を分析し、厚さ方向の濃度分布を得る。また、ドリル法では、鋳片の縦断面からマクロプリントを採取して中心偏析領域を特定し、この中心偏析領域上の多数の分析点から、ドリルで切粉サンプルを採取し、この切粉を分析する。
As a method for evaluating center segregation, several methods such as a slice method, a drill method, a macro corrosion method, and an H print method have been developed and used.
In the slicing method, a slab or a thick steel plate is sequentially sliced in the thickness direction, the components of the chips collected by slicing are analyzed, and a concentration distribution in the thickness direction is obtained. In the drill method, a macro print is taken from the longitudinal section of the slab to identify the center segregation region, and a chip sample is collected from a large number of analysis points on the center segregation region with a drill. analyse.
また、マクロ腐食法は、鋳片の切断面を研磨して、偏析部をピクリン酸等の腐食液により腐食させてからインク等を染み込ませた後、一旦、表面のインクを拭き取り、腐食部に残ったインクをセロハン紙等に写し取り、偏析の発生状況を可視化する方法である。また、Hプリント法は、鋳片の切断面を研磨、腐食し、腐食後に写し取ったプリントから中心偏析部の最大偏析粒径等を測定する方法である。 In the macro corrosion method, the cut surface of the slab is polished, the segregated part is corroded with a corrosive liquid such as picric acid, and then ink is soaked, and then the surface ink is wiped off once to the corroded part. This is a method of visualizing the state of occurrence of segregation by copying the remaining ink onto cellophane paper or the like. The H printing method is a method in which the cut surface of the slab is polished and corroded, and the maximum segregation particle size of the center segregation portion is measured from the print copied after the corrosion.
一般に、鋳片の中心偏析は、鋳片のC断面全体、すなわち、鋳造方向に垂直な断面全体についてみると、厚さ方向および幅方向で均一であるとはいえない。そのため、鋳片や厚鋼板の偏析を調べるためには、C断面の広い領域に亘って評価する必要がある。
このような観点から上記した各方法を検討すると、スライス法は、試料調整・分析に時間がかかるため、結果が出るまでに長時間を要するうえ、鋳片や厚鋼板等の中心偏析評価をC断面全体に亘って行うと、分析コストが高騰するという問題がある。また、この方法では、スライスした切粉を分析するため、厚さ方向の平均としての分析値しか得られない。そのため、スライス法は、従来から、一部領域の評価にのみ用いられる程度であった。
Generally, the center segregation of a slab cannot be said to be uniform in the thickness direction and the width direction when viewed over the entire C cross section of the slab, that is, the entire cross section perpendicular to the casting direction. Therefore, in order to investigate the segregation of a slab or a thick steel plate, it is necessary to evaluate over a wide area of the C cross section.
Considering each method described above from this point of view, the slice method takes time to prepare and analyze the sample, so it takes a long time to obtain the result, and the center segregation evaluation of slabs, thick steel plates, etc. If it is performed over the entire cross section, there is a problem that the analysis cost increases. Moreover, in this method, since the sliced chips are analyzed, only an analysis value as an average in the thickness direction can be obtained. For this reason, the slicing method has been conventionally used only for evaluating a partial region.
また、ドリル法は、スライス法に比較して、迅速性には優れるが、切粉の採取領域がスライス法に比較し、さらに狭くなるため、全体的な評価ができにくいという問題があった。
また、マクロ腐食法は、迅速性の観点から上記した2つの方法と比較すると、優位であるといえる。しかし、マクロ腐食法は、目視による判定となるため、非定量的な評価しか得られないという問題がある。また、Hプリント法は、定量的ではあるが、評価に熟練を要し、しかも長時間を要するため、分析コストが高くなるという問題がある。
In addition, the drill method is superior to the slicing method in terms of rapidity, but there is a problem in that it is difficult to perform an overall evaluation because the chip collection region is further narrowed compared to the slicing method.
Moreover, it can be said that the macro-corrosion method is superior to the above-described two methods from the viewpoint of rapidity. However, the macro-corrosion method has a problem that only non-quantitative evaluation can be obtained because it is judged visually. Further, although the H print method is quantitative, it requires a skill for evaluation and takes a long time, so that there is a problem that the analysis cost becomes high.
また、上記した方法とは別に、中心偏析を評価する方法がいくつか提案されている。例えば、特許文献5には、連鋳鋳片の軸心部の硬度を測定し、その硬度の測定値の平均値、最大値、最大値と最小値との差の中の1種以上から連鋳鋳片の中心偏析度を把握する、簡便な連鋳鋳片中心偏析評価方法が記載されている。
また、特許文献6には、中心偏析部を含む領域の濃度マッピング分析を行って、指標元素の濃度が所定の閾値濃度以上である面積を求め、その面積をもって中心偏析を評価する、連続鋳造鋳片および厚鋼板の中心偏析評価方法が記載されている。なお、特許文献6に記載された方法では、濃度マッピング分析を、EPMA(電子プローブマイクロアナライザー、Electron Probe Micro Analyzer)、発光分光分析およびSEM−EDXのいずれかを用いて行うことが好ましいとしている。
In addition to the methods described above, several methods for evaluating central segregation have been proposed. For example, in Patent Document 5, the hardness of the axial center portion of a continuous cast slab is measured, and the average value, the maximum value, and the difference between the maximum value and the minimum value of the measured values of the hardness are continuously selected. A simple continuous cast slab center segregation evaluation method for grasping the center segregation degree of a cast slab is described.
Patent Document 6 discloses a continuous casting casting in which concentration mapping analysis of a region including a center segregation portion is performed to obtain an area where the concentration of the index element is equal to or higher than a predetermined threshold concentration, and the center segregation is evaluated using the area. The center segregation evaluation method of a piece and a thick steel plate is described. In the method described in Patent Document 6, it is preferable that the concentration mapping analysis is performed using any one of EPMA (Electron Probe Micro Analyzer), emission spectroscopic analysis, and SEM-EDX.
これらの方法によれば、偏析部の存在および偏析度は評価することができるが、特許文献5,6には、偏析度とHICの発生との関連についてまでの言及はない。 According to these methods, the presence of segregation and the degree of segregation can be evaluated, but Patent Documents 5 and 6 do not mention the relationship between the degree of segregation and the occurrence of HIC.
しかし、最近、S量を低減しCaを添加した場合でも、耐HIC性が劣化する場合があることが明らかになっている。とくに、鋼材の中心偏析部に、凝固時に形成され、圧延により伸延したMnSが存在する場合には、中心偏析部の硬さが低くてもHICが発生する場合がある。これは、偏析スポットの形態やCaの反応性の予期せぬバラつきにより、固溶S量が増加して偏析部に局所的にMnSが形成され、圧延により延伸する場合があるためと考えられる。 However, recently, it has been revealed that even when the amount of S is reduced and Ca is added, the HIC resistance may deteriorate. In particular, when MnS formed at the time of solidification and elongated by rolling exists in the center segregation portion of the steel material, HIC may be generated even if the hardness of the center segregation portion is low. This is presumably because the amount of solid solution S increases due to unexpected variations in the form of segregation spots and the reactivity of Ca, MnS is locally formed in the segregation part, and may be stretched by rolling.
このようなことから、とくに高強度鋼板を製造する場合には一層、偏析部でのMnSの形成を抑制する必要があることに想到し、鋼材の耐HIC性を評価するうえでは、中心偏析部等でのMnS形成の有無を判別することが重要になることに思い至った。しかし、MnSの大きさがミクロンオーダーであるため光学顕微鏡による観察では判別できない。また、最近の鋼材では、S量は数ppm程度まで低減されており、 EPMA(電子プローブマイクロアナライザー:Electron Probe Micro Analyzer)等での濃度マッピングによっても、MnS形成を判別することは非常に困難になっている。 For this reason, especially when producing high-strength steel sheets, it is necessary to further suppress the formation of MnS in the segregation part, and in evaluating the HIC resistance of steel materials, the central segregation part It has become important to determine the presence or absence of MnS formation by, for example. However, since the size of MnS is in the micron order, it cannot be determined by observation with an optical microscope. Also, in recent steel materials, the amount of S has been reduced to several ppm, and it is very difficult to determine the formation of MnS by concentration mapping with EPMA (Electron Probe Micro Analyzer). It has become.
本発明は、このような従来技術の問題に鑑みてなされたもので、鋼材の耐HIC性の評価方法およびそれを利用した耐HIC性に優れたラインパイプ用高強度厚鋼板の製造方法を提供することを目的とする。 The present invention has been made in view of such problems of the prior art, and provides a method for evaluating the HIC resistance of steel materials and a method for producing a high-strength thick steel plate for line pipes using the same, which is excellent in HIC resistance. The purpose is to do.
本発明者らは、上記した目的を達成するために、まず、偏析部の微小領域におけるS濃度とMnS形成との関係について、鋭意検討した。その結果、偏析部微小領域のS濃度は、本発明者のうちの一人が開発し、特開2011−169753号公報に記載された「硫黄の分析方法」である高周波燃焼−紫外蛍光法を利用することにより、少量の分析試料でも偏析部微小領域のS濃度を精度高く、定量分析できることを見出し、この分析手法を用いて測定した、偏析部微小領域のS濃度が、MnSの形成とよい相関があることを見出した。 In order to achieve the above-mentioned object, the present inventors first made extensive studies on the relationship between the S concentration and the MnS formation in the minute region of the segregation part. As a result, the S concentration in the segregated portion microregion was developed by one of the present inventors and utilized the high-frequency combustion-ultraviolet fluorescence method, which is the “sulfur analysis method” described in Japanese Patent Application Laid-Open No. 2011-169753. As a result, it was found that the S concentration in the segregated portion microregion can be accurately and quantitatively analyzed even with a small amount of analysis sample, and the S concentration in the segregated portion microregion measured using this analysis method has a good correlation with the formation of MnS. Found that there is.
そして、スラブ(鋳片)であれば、マクロエッチングにより偏析スポット部(偏析部)の位置を特定し、その位置の3×3mmの領域で測定したS濃度がMnSの形成とよい相関があることを見出した。また、鋼板であれば、微小硬さ試験(試験力:490mN;荷重50gf)を行い、HV230を超える箇所では、その圧痕を中心として0.5×3mmの領域で測定したS濃度がMnSの形成とよい相関があることを見出した。具体的には、偏析スポット部でS濃度が質量ppmで18ppmを超える場合に、延伸したMnSが多く形成され、HICが発生する。 And if it is a slab (slab), the position of the segregation spot part (segregation part) is specified by macro-etching, and the S concentration measured in the 3 × 3 mm region at that position has a good correlation with the formation of MnS. I found. In the case of a steel plate, a microhardness test (test force: 490 mN; load 50 gf) is performed, and at a location exceeding HV230, the S concentration measured in the region of 0.5 × 3 mm centering on the indentation is good to form MnS. We found that there is a correlation. Specifically, when the S concentration in the segregation spot portion exceeds 18 ppm in mass ppm, much stretched MnS is formed and HIC is generated.
そして、更なる検討の結果、本発明者らは、鋳片(スラブ)もしくは鋼板の中心偏析部で、特定範囲の微小領域のS濃度を分析し、得られた値が所定値(18質量ppm)を超えるか否かで、MnSの形成の有無を判別し、耐HIC性の優劣を判断できることを見出した。
まず、本発明者らが行った、本発明の基礎となった実験結果について説明する。
質量%で、C:0.02〜0.10%、Si:0.01〜0.80%、P:0.001〜0.02%、S:0.0001〜0.003%、Al:0.01〜0.10%、Nb:0.005〜0.08%、Ti:0.005〜0.04%、Ca:0.0005〜0.005%、N:0.001〜0.01%、Cu:0〜0.7%、Ni:0〜2%、Cr:0〜1%、V:0〜0.5%を含み、Mnを0.8〜1.5%の範囲で変化させた組成の厚鋼板(板厚:32mm)を用意した。
As a result of further studies, the present inventors analyzed the S concentration in a minute region in a specific range at the center segregation part of a slab or a steel plate, and the obtained value was a predetermined value (18 mass ppm). It was found that the presence or absence of MnS can be determined by determining whether the HIC resistance is superior or not.
First, a description will be given of experimental results performed by the present inventors and serving as the basis of the present invention.
In mass%, C: 0.02 to 0.10%, Si: 0.01 to 0.80%, P: 0.001 to 0.02%, S: 0.0001 to 0.003%, Al: 0.01 to 0.10%, Nb: 0.005 to 0.08%, Ti: 0.005 to 0.04%, Ca: 0.0005-0.005%, N: 0.001-0.01%, Cu: 0-0.7%, Ni: 0-2%, Cr: 0-1%, V: 0-0.5%, Mn 0.8 A thick steel plate (plate thickness: 32 mm) having a composition changed in a range of ˜1.5% was prepared.
そして、それら厚鋼板の板幅中央部から、HIC試験片(大きさ:厚さ全厚または30mm×幅20mm×長さ100mm)を採取した。そして、採取したHIC試験片を、NACE TM0284の規定に準拠して、A規格液中に96時間浸漬した。浸漬後のHIC試験片について、割れの有無を超音波探傷法で調査した。
浸漬したHIC試験片について、とくに、割れが発生した領域で、微小硬度計(試験力:490mN(荷重:50gf))を用いて微小硬さHVを測定し、さらに圧痕を中心として0.5×3mmの領域から分析用試料を採取し、高周波燃焼−紫外蛍光法を用いて、当該領域のS濃度(質量%)を測定した。
Then, HIC test pieces (size: full thickness or 30 mm × width 20 mm × length 100 mm) were sampled from the center of the plate width of these thick steel plates. Then, the collected HIC test piece was immersed in A standard solution for 96 hours in accordance with the regulations of NACE TM0284. The immersed HIC specimens were examined for cracking by ultrasonic flaw detection.
For the immersed HIC test piece, measure the microhardness HV using a microhardness meter (test force: 490 mN (load: 50 gf)), especially in the area where cracking occurred. A sample for analysis was collected from the region, and the S concentration (mass%) in the region was measured using a high-frequency combustion-ultraviolet fluorescence method.
その結果、MnSが形成されている場合(すなわち、S濃度が18質量ppm超えの場合)で、かつ微小硬さがHV230以上の場合には、HICが発生しているが、MnSが形成されていない場合(すなわち、S濃度が18質量ppm以下の場合)では、微小硬さがHV275までは、HICが発生していないことを確認した。
このようなことから、偏析スポット(偏析部)でのS濃度が18質量ppm以下であれば、MnSの形成は抑制されており、優れた耐HIC性を保有していること、一方、偏析スポット(偏析部)でのS濃度が18質量ppmを超えると、伸延したMnSが多く形成され、耐HIC性が著しく低下すること、を知見した。
As a result, when MnS is formed (that is, when the S concentration exceeds 18 mass ppm) and the microhardness is HV230 or more, HIC is generated, but MnS is formed. When it was not (that is, when the S concentration was 18 mass ppm or less), it was confirmed that HIC was not generated until the microhardness reached HV275.
Therefore, if the S concentration at the segregation spot (segregation part) is 18 ppm by mass or less, the formation of MnS is suppressed, and it has excellent HIC resistance, while the segregation spot. It has been found that when the S concentration in (segregation part) exceeds 18 mass ppm, a large amount of elongated MnS is formed and the HIC resistance is remarkably lowered.
本発明は、上記した知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。
(1)鋼材の耐HIC性を評価するにあたり、前記鋼材の中心偏析部を特定し、該中心偏析部を含む微小領域についてS濃度を分析し、得られたS濃度の最大値が質量%で、0.0018%以下である場合を、耐HIC性に優れる鋼材と評価することを特徴とする鋼材の耐HIC性の評価方法。
The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) In evaluating the HIC resistance of a steel material, the central segregation part of the steel material is specified, the S concentration is analyzed for a microregion including the central segregation part, and the maximum value of the obtained S concentration is mass%. A method for evaluating the HIC resistance of a steel material, wherein the steel material is evaluated as a steel material having excellent HIC resistance when 0.0018% or less.
(2)(1)において、前記鋼材が鋳片である場合には、前記微小領域を3×3mm の領域として、前記S濃度の分析を高周波燃焼−紫外蛍光法で行うことを特徴とする鋼材の耐HIC性の評価方法。
(3)(1)において、前記鋼材が鋼板である場合には、前記微小領域を3×0.5 mmの領域とし、該微小領域におけるS濃度の分析を高周波燃焼−紫外蛍光法で行うことを特徴とする鋼材の耐HIC性の評価方法。
(2) In (1), when the steel material is a slab, the fine region is set to a 3 × 3 mm region, and the analysis of the S concentration is performed by a high-frequency combustion-ultraviolet fluorescence method. Evaluation method of HIC resistance.
(3) In (1), when the steel material is a steel plate, the minute region is a region of 3 × 0.5 mm, and analysis of S concentration in the minute region is performed by a high-frequency combustion-ultraviolet fluorescence method. Evaluation method of HIC resistance of steel materials.
(4)鋼材の耐HIC性を評価するにあたり、前記鋼材を鋼板とし、該鋼板の中心偏析部を特定したのち、該中心偏析部を含む微小領域の複数箇所で微小硬さHVを測定し、得られた微小硬さHVが230以上である箇所の圧痕を中心として3×0.5 mmの領域から分析試料を採取して、高周波燃焼−紫外蛍光法を用いてS濃度を分析し、得られたS濃度の最大値が質量%で0.0018%以下である場合を耐HIC性に優れる鋼板と評価することを特徴とする鋼材の耐HIC性の評価方法。 (4) In evaluating the HIC resistance of the steel material, the steel material is a steel plate, and after specifying the center segregation portion of the steel plate, the micro hardness HV is measured at a plurality of locations in a micro area including the center segregation portion, An analysis sample was collected from an area of 3 × 0.5 mm centering on the indentation at a location where the obtained microhardness HV was 230 or more, and the S concentration was analyzed using a high-frequency combustion-ultraviolet fluorescence method. A method for evaluating HIC resistance of a steel material, characterized in that when the maximum value of S concentration is 0.0018% or less by mass, it is evaluated as a steel sheet having excellent HIC resistance.
(5)(1)ないし(4)のいずれかにおいて、前記鋼材が、質量%で、C:0.03%以上0.07%未満、Si:0.01〜0.5%、Mn:0.8〜1.5%、S:0.0008%以下、P:0.008%以下、Al:0.07%以下、Ti:0.005〜0.02%、Nb:0.005〜0.07%、Ca:0.0005〜0.005%、N:0.008%以下、O:0.005%以下を含有し、残部Feおよび不可避的不純物からなる組成を有することを特徴とする鋼材の耐HIC性の評価方法。 (5) In any one of (1) to (4), the steel material is, in mass%, C: 0.03% or more and less than 0.07%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.5%, S: 0.0008% P: 0.008% or less, Al: 0.07% or less, Ti: 0.005-0.02%, Nb: 0.005-0.07%, Ca: 0.0005-0.005%, N: 0.008% or less, O: 0.005% or less, A method for evaluating the HIC resistance of a steel material, characterized by having a composition comprising the balance Fe and inevitable impurities.
(6)(5)において、前記組成に加えてさらに、質量%で、Cu:0.5%以下、Ni:1%以下、Cr:0.5%以下、Mo:0.5%以下、V:0.1%以下のうちから選ばれた1種または2種以上を含有することを特徴とする鋼材の耐HIC性の評価方法。
(7)鋼素材を、熱間圧延し、あるいはさらに急速冷却、あるいはさらに再加熱処理して、厚鋼板とするにあたり、複数の前記鋼素材の中心偏析部を含む微小領域についてS濃度を分析し、その値の最大値が、質量%で0.0018%以下である場合に、該鋼素材をラインパイプ用厚鋼板向け鋼素材とすることを特徴とする耐HIC性に優れたラインパイプ用高強度厚鋼板の製造方法。
(6) In (5), in addition to the above composition, by mass%, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less A method for evaluating the HIC resistance of a steel material, comprising one or more selected from the group consisting of:
(7) When the steel material is hot-rolled, further rapidly cooled, or further reheated to form a thick steel plate, the S concentration is analyzed for a minute region including a central segregation portion of the plurality of steel materials. When the maximum value is 0.0018% by mass or less, the steel material is used as a steel material for thick steel plates for line pipes. High strength thickness for line pipes with excellent HIC resistance A method of manufacturing a steel sheet.
(8)(7)において、前記S濃度を分析する領域を3×3mmの微小領域とし、前記S濃度の分析を高周波燃焼−紫外蛍光法で行うことを特徴とするラインパイプ用高強度厚鋼板の製造方法。
(9)鋼素材を、熱間圧延し、あるいはさらに急速冷却、あるいはさらに再加熱処理して、厚鋼板とするにあたり、得られた厚鋼板について、該厚鋼板の中心偏析部を特定し、該中心偏析部を含む微小領域の複数箇所で微小硬さHVを測定し、得られた微小硬さHVが230以上である箇所の圧痕を中心として3×0.5 mmの領域から分析試料を採取し、高周波燃焼−紫外蛍光法を用いてS濃度を分析し、得られたS濃度の最大値が質量%で0.0018%以下である場合を、ラインパイプ用厚鋼板とすることを特徴とする耐HIC性に優れたラインパイプ用高強度厚鋼板の製造方法。
(8) The high strength thick steel plate for line pipes according to (7), wherein the region for analyzing the S concentration is a 3 × 3 mm minute region, and the analysis of the S concentration is performed by a high-frequency combustion-ultraviolet fluorescence method Manufacturing method.
(9) When the steel material is hot-rolled, further rapidly cooled, or further reheated to obtain a thick steel plate, the center segregation portion of the thick steel plate is specified for the obtained thick steel plate, Measure the microhardness HV at multiple locations in the microregion including the center segregation part, and collect the analysis sample from the 3 × 0.5 mm region centering on the indentation where the microhardness HV is 230 or more. HIC resistance, characterized by analyzing the S concentration using the high frequency combustion-ultraviolet fluorescence method, and using the obtained S concentration maximum value of 0.0018% by mass or less as a thick steel plate for line pipes Of excellent high-strength thick steel plate for line pipes.
(10)(7)ないし(9)のいずれかにおいて、前記鋼素材が、質量%で、C:0.03%以上0.07%未満、Si:0.01〜0.5%、Mn:0.8〜1.5%、S:0.0008%以下、P:0.008%以下、Al:0.07%以下、Ti:0.005〜0.02%、Nb:0.005〜0.07%、Ca:0.0005〜0.005%、N:0.008%以下、O:0.005%以下を含有し、残部Feおよび不可避的不純物からなる組成を有することを特徴とするラインパイプ用高強度厚鋼板の製造方法。 (10) In any one of (7) to (9), the steel material is in mass%, C: 0.03% or more and less than 0.07%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.5%, S: 0.0008 %: P: 0.008% or less, Al: 0.07% or less, Ti: 0.005-0.02%, Nb: 0.005-0.07%, Ca: 0.0005-0.005%, N: 0.008% or less, O: 0.005% or less A method for producing a high-strength thick steel plate for line pipes, which has a composition comprising the balance Fe and inevitable impurities.
(11)(10)において、前記組成に加えてさらに、質量%で、Cu:0.5%以下、Ni:1%以下、Cr:0.5%以下、Mo:0.5%以下、V:0.1%以下のうちから選ばれた1種または2種以上を含有することを特徴とするラインパイプ用高強度厚鋼板の製造方法。 (11) In (10), in addition to the above composition, by mass%, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less The manufacturing method of the high strength thick steel plate for line pipes characterized by including 1 type, or 2 or more types chosen from these .
本発明によれば、対象とする鋼材の耐HIC性を定量的に精度高く評価でき、産業上格段の効果を奏する。また、本発明によれば、ラインパイプ用として好適な、耐HIC性に優れた高強度厚鋼板を容易に製造できるという効果もある。 ADVANTAGE OF THE INVENTION According to this invention, the HIC resistance of the steel material made into object can be evaluated quantitatively with high precision, and there exists a remarkable effect on industry. In addition, according to the present invention, there is also an effect that a high-strength thick steel plate excellent in HIC resistance suitable for a line pipe can be easily manufactured.
まず、本発明になる鋼材の耐HIC性の評価方法について説明する。
評価の対象とする鋼材を、まずマクロエッチングして、偏析部(中心偏析部)を特定する。なお、マクロエッチングはC断面で行うこととする。
そして、特定された偏析部を含む微小領域でS濃度を測定する。
偏析部(偏析スポット)の大きさは、鋳片であれば直径数mm程度であるため、S濃度を分析する微小領域としては、鋳片であれば、偏析部(偏析スポット)を中心として3mm×3mmの領域とする。なお、鋼板では、偏析部が圧延により板厚方向に潰されているため、S濃度を分析する微小領域は、偏析部(偏析スポット)を中心として幅方向3mm×厚さ方向0.5 mmの領域とする。
First, a method for evaluating the HIC resistance of a steel material according to the present invention will be described.
The steel material to be evaluated is first macro-etched to identify the segregation part (center segregation part). Macro etching is performed on the C cross section.
Then, the S concentration is measured in a minute region including the specified segregation part.
Since the size of the segregation part (segregation spot) is about several millimeters in diameter in the case of a cast slab, the minute region for analyzing the S concentration is 3 mm centering on the segregation part (segregation spot) in the case of a slab. X 3 mm area. In the steel sheet, the segregation part is crushed in the sheet thickness direction by rolling, so the micro area for analyzing the S concentration is an area of 3 mm in the width direction and 0.5 mm in the thickness direction centering on the segregation part (segregation spot). To do.
なお、この微小領域のS濃度は、MnS形成と相関関係を有する。この領域のS濃度が最大値でも18質量ppm以下であれば、HICの発生と関連するMnSの形成は認められない。このようなことから、鋼材の偏析部(偏析スポット)を含む上記した微小領域について測定したS濃度が最大値で質量%で、0.0018%以下である場合を耐HIC性に優れる鋼材と評価することにした。 It should be noted that the S concentration in this minute region has a correlation with MnS formation. If the S concentration in this region is 18 mass ppm or less even at the maximum value, formation of MnS associated with the generation of HIC is not recognized. Therefore, when the S concentration measured for the above-mentioned minute region including the segregation part (segregation spot) of the steel material is the maximum value by mass% and 0.0018% or less, the steel material having excellent HIC resistance is evaluated. I made it.
偏析部を含む微小領域のS濃度が18質量ppmを超えると、MnSの形成頻度が増加し、圧延により伸延したMnS によりHICの感受性が高くなり、耐HIC性が劣化する。S濃度が18質量ppm 以下であれば、偏析部にMnS が形成されず、高強度でも割れ面積率CARが3%以下と、良好な耐HIC性を示す。また、偏析部を含む微小領域のS濃度が10質量ppm以下であれば、厳しいHIC試験においても割れが発生せず、優れた耐HIC性を示す鋼材となる。 If the S concentration in the microscopic region including the segregation part exceeds 18 mass ppm, the frequency of MnS formation increases, and MnS that is extended by rolling increases the sensitivity of HIC and deteriorates the HIC resistance. When the S concentration is 18 ppm by mass or less, MnS is not formed in the segregated portion, and even at high strength, the crack area ratio CAR is 3% or less, indicating good HIC resistance. In addition, if the S concentration in the minute region including the segregation part is 10 ppm by mass or less, cracks do not occur even in a severe HIC test, and the steel material exhibits excellent HIC resistance.
なお、鋼材が鋼板である場合には、偏析部(偏析スポット)が薄く潰されており、HIC発生の危険度を的確に判定するためには、鋼板の偏析部(偏析スポット)を含む微小領域について、荷重50gf(試験力490mN)で微小硬さHVを複数箇所、測定したのち、微小硬さHVが230 HV以上となる箇所についてのみ、該箇所(圧痕)を中心として上記した幅方向3mm×厚さ方向0.5 mmの領域でS濃度を分析することが好ましい。微小硬さHVが230 HV未満であれば、S濃度が0.0018質量%以上と高くても、当該箇所ではHICは発生しないことを確認している。 In addition, when the steel material is a steel plate, the segregation part (segregation spot) is thinly crushed, and in order to accurately determine the risk of occurrence of HIC, a minute region including the segregation part (segregation spot) of the steel plate After measuring multiple locations of microhardness HV at a load of 50 gf (test force 490 mN), only in locations where the microhardness HV is 230 HV or more, the width direction 3 mm described above centering on the location (indentation) It is preferable to analyze the S concentration in an area of 0.5 mm in the thickness direction. If the microhardness HV is less than 230 HV, it has been confirmed that even if the S concentration is as high as 0.0018% by mass or more, HIC does not occur in that location.
また、微小硬さHVが230 HV以上となる箇所であっても、上記のように測定した微小領域のS濃度が最大値でも18質量ppm以下であれば、HICの発生と関連するMnSの形成は認められず、HIC発生の危険性は低い。このようなことから、鋼材が鋼板である場合においては、偏析部(偏析スポット)を含む微小領域の複数箇所で微小硬さを測定し、その値が230 HV以上である箇所について上記した微小領域について測定したS濃度が最大値で質量%で、0.0018%以下である場合を、耐HIC性に優れる鋼材(鋼板)と評価することにした。 Even if the microhardness HV is 230 HV or more, if the S concentration in the microregion measured as described above is 18 mass ppm or less even at the maximum, formation of MnS related to the generation of HIC The risk of HIC is low. For this reason, when the steel material is a steel plate, the microhardness is measured at a plurality of locations in the microregion including the segregation part (segregation spot), and the microregion described above for the portion whose value is 230 HV or more. It was decided to evaluate the case where the S concentration measured for the maximum value was 0.0018% by mass or less as a steel material (steel plate) having excellent HIC resistance.
図1に、本発明で、S濃度を分析する微小領域と偏析部(偏析スポット)との関係を模式的に示す。
本発明では、特定された偏析部(偏析スポット)を含む上記した微小領域から分析試料を採取し、偏析部(偏析スポット)を含む微小領域のS濃度を直接、分析(測定)する。そのため、本発明では、採取した分析試料から精度高くS濃度を分析する必要がある。しかも、対象とする鋼材は、低S含有量の鋼材である。このようなことから、本発明では、本発明者の一人が開発した、特開2011−169753号公報に記載された「硫黄の分析方法」である高周波燃焼−紫外蛍光法を利用することにした。この方法によれば、微小領域のような少ない量の分析試料しか得られない場合でも、鋼材のS濃度を精度よく定量分析することができる。
FIG. 1 schematically shows the relationship between a minute region for analyzing S concentration and a segregation part (segregation spot) in the present invention.
In the present invention, an analysis sample is collected from the above-described minute region including the identified segregation part (segregation spot), and the S concentration in the minute region including the segregation part (segregation spot) is directly analyzed (measured). Therefore, in the present invention, it is necessary to analyze the S concentration with high accuracy from the collected analysis sample. And the steel materials made into object are steel materials with a low S content. Therefore, in the present invention, it was decided to use the high-frequency combustion-ultraviolet fluorescence method that was developed by one of the present inventors and was the “method for analyzing sulfur” described in Japanese Patent Application Laid-Open No. 2011-169753. . According to this method, even when only a small amount of analysis sample such as a minute region can be obtained, the S concentration of the steel material can be quantitatively analyzed with high accuracy.
分析試料は、鋼材から直接切り出しして採取してもよいが、簡便性の観点からは、ドリルや研削により当該箇所から切粉を採取することが好ましい。本発明でS濃度の分析に使用する高周波燃焼−紫外蛍光法では、分析試料は、0.05〜0.2g程度の重量があれば、S:数質量ppm程度の含有であっても十分な精度の分析が可能である。
高周波燃焼−紫外蛍光法で使用する紫外蛍光分析装置の構成の一例を図2に示す。紫外蛍光分析装置1は、高周波誘導加熱炉3と、ダストフィルタ4と、紫外蛍光分析器5から構成される。
The analysis sample may be directly cut out and collected from the steel material, but from the viewpoint of simplicity, it is preferable to collect the chips from the relevant part by drilling or grinding. In the high-frequency combustion-ultraviolet fluorescence method used for the analysis of S concentration in the present invention, if the analysis sample has a weight of about 0.05 to 0.2 g, even if it contains S: about several mass ppm, analysis with sufficient accuracy Is possible.
An example of the configuration of an ultraviolet fluorescence analyzer used in the high frequency combustion-ultraviolet fluorescence method is shown in FIG. The ultraviolet fluorescence analyzer 1 includes a high frequency induction heating furnace 3, a dust filter 4, and an ultraviolet fluorescence analyzer 5.
高周波誘導加熱炉3には、試料2を溶解し、燃焼させるセラミックスるつぼ31と、セラミックスるつぼ31を囲繞するコイル32とが配設される。なお、コイル32には、交流電源(図示せず)が接続されていることは言うまでもない。
ダストフィルタ4は、高周波誘導加熱炉3と紫外蛍光分析器5との間に設置され、高周波誘導加熱炉3で発生したSO2含有ガスから、試料2や助燃剤から発生するダストを除去して、後段に配設された紫外蛍光分析器5を保護する。ダストフィルタ4として使用する素材は、SO2を吸着しない素材、例えば、シリカ繊維やポリテトラフルオロエチレンからなる通気性の優れた素材を用いることが好ましい。
The high frequency induction heating furnace 3 is provided with a
The dust filter 4 is installed between the high frequency induction heating furnace 3 and the ultraviolet fluorescence analyzer 5, and removes dust generated from the sample 2 and the combustion aid from the SO 2 -containing gas generated in the high frequency induction heating furnace 3. The ultraviolet fluorescence analyzer 5 disposed in the subsequent stage is protected. The material used as the dust filter 4 is preferably a material that does not adsorb SO 2 , for example, a material having excellent air permeability made of silica fiber or polytetrafluoroethylene.
紫外蛍光分析器5は、発生したSO2含有ガスに、例えば波長220nmの紫外線を照射し、励起状態となったSO2が基底状態に戻る際に放出する蛍光(波長330nm)を一定時間測定し、測定された蛍光強度の積算値から、予め作成しておいた検量線を用いて試料2中に含まれるS量を算出する。
鋼材の偏析部微小領域から採取された試料2は、セラミックスるつぼ31に収容される。そして、コイル32に交流電流を印加されて、高周波誘導加熱炉3内で純酸素雰囲気下で、迅速に加熱、溶解される。その際、試料2中に含まれているSが酸素と反応して、即ち、Sが酸化(燃焼)してSO2(気体)を生成する。なお、試料2を燃焼する際には、錫やタングステン等の助燃剤を用いることが好ましい。試料2を助燃剤とともに加熱すれば、試料2を迅速に燃焼させることができ、分析を迅速に行うことができる。
The ultraviolet fluorescence analyzer 5 irradiates the generated SO 2 -containing gas with ultraviolet light having a wavelength of 220 nm, for example, and measures the fluorescence (wavelength 330 nm) emitted when the excited SO 2 returns to the ground state for a certain period of time. From the integrated value of the measured fluorescence intensity, the amount of S contained in the sample 2 is calculated using a calibration curve prepared in advance.
The sample 2 collected from the segregated portion micro region of the steel material is accommodated in the
そして、生成したSO2含有ガスは、ダストフィルタ4で塵埃(ダスト)を除去され、ついで、塵埃(ダスト)を除去されたSO2含有ガスは紫外蛍光分析器5に導かれる。紫外蛍光分析器5では、紫外蛍光法によりガス中のSO2量を測定し、予め作成された検量線を用いて、試料2中に含まれているS濃度を定量する。
この紫外蛍光分析装置1では、生成したSO2量を紫外蛍光分析器5で測定するので、従来の赤外線検出器で測定する赤外線吸収法と比較して、測定対象ガス中に含まれる水蒸気や測定対象ガスの温度による影響を受けることがほとんどない。したがって、除湿器を設ける必要がなく、簡便な装置で迅速かつ正確にSの定量分析を行うことができる。さらに、この紫外蛍光分析装置1では、従来技術のように、測定時に参照ガス(比較ガス)を使用する必要もない。
Then, the generated SO 2 -containing gas is dust-removed by the dust filter 4, and then the SO 2 -containing gas from which the dust (dust) has been removed is guided to the ultraviolet fluorescence analyzer 5. The ultraviolet fluorescence analyzer 5 measures the amount of SO 2 in the gas by the ultraviolet fluorescence method, and quantifies the S concentration contained in the sample 2 using a calibration curve prepared in advance.
In this ultraviolet fluorescence analyzer 1, the amount of generated SO 2 is measured by the ultraviolet fluorescence analyzer 5, so that water vapor contained in the measurement target gas and measurement are compared with the infrared absorption method that is measured by a conventional infrared detector. It is hardly affected by the temperature of the target gas. Therefore, it is not necessary to provide a dehumidifier, and the quantitative analysis of S can be performed quickly and accurately with a simple device. Furthermore, in this ultraviolet fluorescence analyzer 1, it is not necessary to use a reference gas (comparison gas) at the time of measurement as in the prior art.
なお、酸素は、励起状態となったSO2が基底状態に戻る際に放出する蛍光を吸収したり、励起状態となったSO2分子と衝突してクエンチング(消光)現象を起こしたりする。そのため、紫外蛍光法では、測定対象ガス中のSO2量が低い場合や、測定対象ガス中に酸素が多量に含まれている場合には、SO2濃度の測定精度が低下することが知られている。しかし、この紫外蛍光分析装置1では、高周波誘導加熱炉3を用いて純酸素雰囲気下で試料2を燃焼させ、試料2中のSを短時間ですべて酸化することができる。このため、測定対象ガス中のSO2濃度が高く、紫外蛍光分析器5で測定される蛍光強度が尖頭状の鋭いピークを示すようになる。このようなことから、この紫外蛍光分析装置1によれば、SO2量を正確に測定することができ、少ない重量の分析試料でも、含まれるS濃度を高い精度で分析することができる。 Note that oxygen absorbs fluorescence emitted when the excited SO 2 returns to the ground state, or collides with the excited SO 2 molecule to cause a quenching (quenching) phenomenon. Therefore, it is known that the measurement accuracy of SO 2 concentration decreases in the ultraviolet fluorescence method when the amount of SO 2 in the measurement target gas is low or when the measurement target gas contains a large amount of oxygen. ing. However, in the ultraviolet fluorescence analyzer 1, the sample 2 can be burned in a pure oxygen atmosphere using the high frequency induction heating furnace 3, and all the S in the sample 2 can be oxidized in a short time. For this reason, the SO 2 concentration in the measurement target gas is high, and the fluorescence intensity measured by the ultraviolet fluorescence analyzer 5 shows a sharp peak. For this reason, according to the ultraviolet fluorescence analyzer 1, the amount of SO 2 can be accurately measured, and the contained S concentration can be analyzed with high accuracy even with an analysis sample having a small weight.
本発明では、上記したように、対象とする鋼材の中心偏析部(偏析スポット)のS濃度を測定し、得られたS濃度が最大値で、0.0018質量%以下であれば、耐HIC性に優れた鋼材と評価する。
なお、耐HIC性に優れた鋼材として、引張強さTS:550MPa以上を有し、ラインパイプ用として好適な高強度厚鋼板がある。このような高強度厚鋼板は、質量%で、C:0.03%以上0.07%未満、Si:0.01〜0.5%、Mn:0.8〜1.5%、S:0.0008%以下、P:0.008%以下、Al:0.07%以下、Ti:0.005〜0.02%、Nb:0.005〜0.07%、N:0.008%以下、Ca:0.0005〜0.005%、O:0.005%以下を含み、残部Feおよび不可避的不純物からなる組成を有することが好ましい。
In the present invention, as described above, the S concentration of the central segregation portion (segregation spot) of the target steel material is measured, and if the obtained S concentration is a maximum value of 0.0018% by mass or less, the HIC resistance is improved. Evaluated as an excellent steel material.
In addition, as a steel material excellent in HIC resistance, there is a high strength thick steel plate having a tensile strength TS: 550 MPa or more and suitable for a line pipe. Such a high-strength thick steel plate is, in mass%, C: 0.03% or more and less than 0.07%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.5%, S: 0.0008% or less, P: 0.008% or less, Al: Contains 0.07% or less, Ti: 0.005-0.02%, Nb: 0.005-0.07%, N: 0.008% or less, Ca: 0.0005-0.005%, O: 0.005% or less, and has a composition composed of the balance Fe and inevitable impurities. It is preferable.
このような高強度厚鋼板の組成限定理由について、つぎに説明する。以下、質量%は単に%で記す。
C:0.03%以上0.07%未満
Cは、強化に寄与する元素である。このような効果を得るためには、0.03%以上の含有を必要とする。Cが0.03%未満では十分な強度を確保できない。一方、0.07%以上含有すると、靭性が低下する。このため、Cは0.03%以上0.07%未満に限定した。
The reason for limiting the composition of such a high-strength thick steel plate will be described next. Hereinafter, mass% is simply expressed as%.
C: 0.03% or more and less than 0.07% C is an element contributing to strengthening. In order to obtain such an effect, the content of 0.03% or more is required. If C is less than 0.03%, sufficient strength cannot be secured. On the other hand, when it contains 0.07% or more, the toughness decreases. For this reason, C was limited to 0.03% or more and less than 0.07%.
Si:0.01〜0.5%
Siは、脱酸剤として作用する元素である。このような効果を得るためには0.01%以上の含有を必要とする。0.01%未満では脱酸効果が十分でない。一方、0.5%を超える含有は、靭性や溶接性を劣化させる。このため、Siは0.01〜0.5%の範囲に限定した。
Mn:0.8〜1.5%
Mnは、強度増加、靭性向上に寄与する元素である。このような効果を得るためには0.8%以上含有する必要がある。一方、1.5%を超える含有は、偏析が顕著となり、中心偏析部にMnS形成の可能性が増して、耐HIC性を低下させる。このため、Mnは0.8〜1.5%の範囲に限定した。
Si: 0.01-0.5%
Si is an element that acts as a deoxidizer. In order to acquire such an effect, 0.01% or more of content is required. If it is less than 0.01%, the deoxidation effect is not sufficient. On the other hand, a content exceeding 0.5% deteriorates toughness and weldability. For this reason, Si was limited to the range of 0.01 to 0.5%.
Mn: 0.8-1.5%
Mn is an element that contributes to increasing strength and improving toughness. In order to acquire such an effect, it is necessary to contain 0.8% or more. On the other hand, if the content exceeds 1.5%, segregation becomes prominent, and the possibility of MnS formation in the central segregation portion increases, thereby reducing HIC resistance. For this reason, Mn was limited to the range of 0.8 to 1.5%.
P:0.008%以下
Pは、偏析傾向が強く、とくに粒界に偏析して粒界の強度を低下させ、HICにおける亀裂進展を助長する。耐HIC性向上のためには、できるだけ低減することが好ましいが、0.008%程度までは許容できる。このため、Pは0.008%以下に限定した。
S:0.0008%以下
Sは、鋼中では延伸したMnSとして中心偏析部(偏析スポット)に存在し、耐HIC性を低下させるため、できるだけ低減することが望ましいが、0.0008%程度までは許容できる。このため、Sは0.0008%以下に限定した。
P: 0.008% or less P has a strong segregation tendency, in particular, segregates at the grain boundary to lower the strength of the grain boundary, and promotes crack propagation in HIC. In order to improve the HIC resistance, it is preferable to reduce as much as possible, but it is acceptable up to about 0.008%. For this reason, P was limited to 0.008% or less.
S: 0.0008% or less S is present in the center segregation part (segregation spot) as stretched MnS in steel, and it is desirable to reduce it as much as possible in order to reduce HIC resistance. However, up to about 0.0008% is acceptable. For this reason, S was limited to 0.0008% or less.
なお、S含有量を低減すれば、中心偏析部(偏析スポット)のS量を低減できる。さらに、中心偏析部(偏析スポット)のS量の低減は、鋳造時のタンディッシュ内で溶鋼を撹拌し、あるいはさらに1.2〜0.8 m/minの範囲の鋳造速度で鋳造したり、あるいはさらに凝固末期に、電磁攪拌を施し偏析部を均一に分散させたり、また、鋳片に軽圧下を施し偏析スポットのサイズを小さくしたりすることで、調整可能である。 In addition, if S content is reduced, S amount of a center segregation part (segregation spot) can be reduced. Furthermore, the amount of S in the center segregation part (segregation spot) can be reduced by stirring the molten steel in the tundish at the time of casting, or casting at a casting speed in the range of 1.2 to 0.8 m / min, or further at the end of solidification. In addition, it can be adjusted by applying electromagnetic stirring to uniformly disperse the segregation part, or by reducing the size of the segregation spot by lightly reducing the slab.
Al:0.07%以下
Alは、脱酸剤として作用する元素であり、このような効果を得るためには0.02%以上含有することが望ましい。一方、0.07%を超えて含有すると、介在物量が増加し、鋼の清浄度が低下し、HICの起点となる介在物が増加する。このため、Alは0.07%以下に限定した。
Al: 0.07% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.02% or more. On the other hand, if the content exceeds 0.07%, the amount of inclusions increases, the cleanliness of the steel decreases, and the inclusions that become the starting point of HIC increase. For this reason, Al was limited to 0.07% or less.
Ti:0.005〜0.02%
Tiは、窒化物TiNを形成し、ピンニング効果により、スラブ加熱時のオーステナイト粗大化を抑制し、母材靭性を向上させる作用を有する。このような効果を得るためには、0.005%以上の含有を必要とする。一方、0.02%を超える含有は、粗大なTi系析出物が析出し、HICの起点になり、耐HIC性が低下する。このため、Tiは0.005〜0.02%の範囲に限定した。
Ti: 0.005-0.02%
Ti forms nitride TiN and has the effect of suppressing the austenite coarsening during slab heating and improving the base material toughness due to the pinning effect. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.02%, coarse Ti-based precipitates are deposited, which becomes the starting point of HIC, and the HIC resistance decreases. For this reason, Ti was limited to 0.005 to 0.02% of range.
Nb:0.005〜0.07%
Nbは、組織の微細粒化に寄与し、靭性を向上させる作用を有する。また、Nb は、析出物を形成し、析出強化による強度上昇に寄与する。このような効果を得るためには、0.005%以上含有する必要があるが、0.07%を超える含有は、粗大なNb系析出物の形成を促進するため、粗大な析出物がHICの起点となり、耐HIC性が低下する。このため、Nbは0.005〜0.07%の範囲に限定した。
Nb: 0.005-0.07%
Nb has the effect of contributing to the refinement of the structure and improving the toughness. Nb also forms precipitates and contributes to strength increase by precipitation strengthening. In order to obtain such an effect, it is necessary to contain 0.005% or more, but the content exceeding 0.07% promotes the formation of coarse Nb-based precipitates, so the coarse precipitates become the starting point of HIC. Reduces HIC resistance. For this reason, Nb was limited to 0.005 to 0.07% of range.
Ca:0.0005〜0.005%
Caは、硫化物系介在物の形態を制御して、耐HIC性を向上させる元素である。このような効果を得るためには0.0005%以上の含有を必要とする。一方、0.005%を超えて含有すると、効果が飽和するとともに、逆に清浄度を低下させてHICの起点となる介在物を形成し、耐HIC性を低下させる。このため、Caは0.0005〜0.005%の範囲に限定した。
Ca: 0.0005 to 0.005%
Ca is an element that improves the HIC resistance by controlling the form of sulfide inclusions. In order to acquire such an effect, 0.0005% or more needs to be contained. On the other hand, if the content exceeds 0.005%, the effect is saturated and, conversely, the cleanliness is lowered to form inclusions as a starting point of HIC, and the HIC resistance is lowered. For this reason, Ca was limited to 0.0005 to 0.005% of range.
N:0.008%以下
Nは、不可避的不純物であるが、0.008%を超える多量の含有は、HICの起点となる粗大なTi−Nb系析出物を形成する。このため、Nは0.008%以下に限定した。
O:0.005%以下
Oは、不可避的不純物であり、粗大で耐HIC性に悪影響を与える酸化物系介在物の生成を抑制するために、Oはできるだけ低減することが望ましいが、0.005%までは許容できる。このため、Oは0.005%以下に限定した。
N: 0.008% or less N is an unavoidable impurity, but a large content exceeding 0.008% forms coarse Ti—Nb-based precipitates that are the starting point of HIC. For this reason, N was limited to 0.008% or less.
O: 0.005% or less O is an inevitable impurity, and it is desirable to reduce O as much as possible in order to suppress the formation of oxide inclusions that are coarse and adversely affect HIC resistance, but up to 0.005% acceptable. For this reason, O was limited to 0.005% or less.
上記した成分に加えてさらに、Cu:0.5%以下、Ni:1%以下、Cr:0.5%以下、Mo:0.5%以下、V:0.1%以下のうちから選ばれた1種または2種以上
Cu、Ni、Cr、Mo、Vはいずれも、強度増加に寄与する元素であり、必要に応じて選択して1種または2種以上含有できる。
Cuは、鋼の焼入性向上を介して、強度増加に寄与する元素である。このような効果を得るためには0.02%以上含有することが好ましい。一方、0.5%を超えて含有すると、靱性劣化が生じる。このため、含有する場合は、Cuは0.5%以下に限定することが好ましい。
In addition to the above components, one or more selected from Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less
Cu, Ni, Cr, Mo, and V are all elements that contribute to an increase in strength, and can be selected as necessary and contained in one or more.
Cu is an element that contributes to an increase in strength through improving the hardenability of steel. In order to acquire such an effect, it is preferable to contain 0.02% or more. On the other hand, if it exceeds 0.5%, toughness deterioration occurs. For this reason, when it contains, it is preferable to limit Cu to 0.5% or less.
Niは、鋼の焼入性向上を介して強度増加に寄与するとともに、多量に含有しても靱性劣化を生じないため、強靱化に有効に寄与する元素である。このような効果を得るためには0.02%以上含有することが望ましい。一方、Niは高価な元素であるため、1%を超える多量の含有は、材料コストが高騰する。このため、含有する場合は、Niは1%以下に限定することが好ましい。 Ni contributes to the increase in strength through improvement of the hardenability of steel and is an element that contributes effectively to toughening because it does not cause toughness deterioration even if contained in a large amount. In order to acquire such an effect, it is desirable to contain 0.02% or more. On the other hand, since Ni is an expensive element, the content cost exceeding 1% increases the material cost. For this reason, when it contains, it is preferable to limit Ni to 1% or less.
Crは、Mnと同様に、低C域で十分な強度を確保すために有効に作用する。このような効果を得るためには、0.1%以上含有することが望ましいが、0.5%を超える過剰な含有は溶接性を低下させる。このため、含有する場合は、Crは0.5%以下に限定することが好ましい。
Moは、焼入性を向上させ、強度を増加させる元素であり、MA生成やベイナイト相を強化することで強度上昇に寄与する元素である。このような効果を得るためには、0.02%以上含有することが好ましい。一方、0.5%を超える含有は、溶接熱影響部靭性の低下を招く。このため、含有する場合は、Moは0.5%以下に限定することが好ましい。さらに好ましくは0.3%以下である。
Cr, like Mn, works effectively to ensure sufficient strength in the low C region. In order to acquire such an effect, it is desirable to contain 0.1% or more, but excessive inclusion exceeding 0.5% reduces weldability. For this reason, when contained, Cr is preferably limited to 0.5% or less.
Mo is an element that improves hardenability and increases strength, and is an element that contributes to increasing strength by strengthening MA formation and the bainite phase. In order to acquire such an effect, it is preferable to contain 0.02% or more. On the other hand, if the content exceeds 0.5%, the weld heat-affected zone toughness is reduced. For this reason, when it contains, it is preferable to limit Mo to 0.5% or less. More preferably, it is 0.3% or less.
Vは、焼入性向上を介し、強度上昇に寄与する元素である。このような効果を得るためには、0.005%以上含有することが好ましい。一方、0.1%を超える含有は、溶接熱影響部靭性を低下させる。このため、含有する場合は、Vは0.1%以下に限定することが好ましい。
なお、上記した選択元素以外に、強度向上、靱性改善の観点から、Mg:0.02%以下、および/または、REM(希土類金属):0.02%以下、および/または、B:0.003%以下を含有してもよい。
V is an element that contributes to an increase in strength through improvement in hardenability. In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, if it exceeds 0.1%, the weld heat-affected zone toughness is lowered. For this reason, when it contains, it is preferable to limit V to 0.1% or less.
In addition to the above-mentioned selected elements, Mg: 0.02% or less and / or REM (rare earth metal): 0.02% or less and / or B: 0.003% or less are contained from the viewpoint of improving strength and toughness. May be.
上記した成分以外の残部は、Feおよび不可避的不純物からなる。
従来から、厚鋼板は、鋼素材(スラブ)を、熱間圧延し、あるいはさらに急速冷却、あるいはさらに再加熱処理して、製造されている。上記した高強度厚鋼板の好ましい製造方法はつぎのとおりである。
上記した組成の溶鋼を、転炉等の常用の溶製方法で溶製し、連続鋳造法等の鋳造方法でスラブ等の鋳片(鋼素材)としたのち、好ましくは1020〜1200℃に加熱し、930℃以下の温度域で累積圧下率:65%以上の熱間圧延を施し、800℃以上の温度で圧延を終了する制御圧延と、圧延終了後、平均冷却速度で20℃/s以上40℃/s以下で、380〜550℃の冷却停止温度まで冷却し、あるいはさらに誘導加熱装置で600〜680℃の温度範囲に再加熱する、製造工程で厚鋼板とすることが好ましい。これにより、引張強さ:550MPa以上で、例えば、試験温度:−5℃でのシャルピー衝撃試験で210J以上の吸収エネルギーを確保でき、ラインパイプ用として好適な、高靭性高強度の厚鋼板とすることができる。上記した条件を外れる工程では、所望の高強度と高靭性を確保することができない。
The balance other than the components described above consists of Fe and inevitable impurities.
Conventionally, a thick steel plate has been manufactured by hot rolling, further rapid cooling, or further reheating treatment of a steel material (slab). A preferred method for producing the above-described high-strength thick steel plate is as follows.
The molten steel having the above composition is melted by a conventional melting method such as a converter, and is made into a slab or other slab (steel material) by a casting method such as a continuous casting method, and is preferably heated to 1020 to 1200 ° C. And a rolling reduction at a temperature of 930 ° C or lower, with a rolling reduction of 65% or higher, and controlled rolling at a temperature of 800 ° C or higher, and an average cooling rate of 20 ° C / s or higher after the rolling. It is preferable to use a thick steel plate in the production process where it is cooled to a cooling stop temperature of 380 to 550 ° C. at 40 ° C./s or lower, or further reheated to a temperature range of 600 to 680 ° C. with an induction heating device. As a result, the tensile strength: 550 MPa or more, for example, the absorbed energy of 210 J or more can be secured in the Charpy impact test at a test temperature: −5 ° C., and a high toughness and strength steel plate suitable for line pipes is obtained. be able to. In a step outside the above conditions, desired high strength and high toughness cannot be ensured.
さらに、上記した高強度厚鋼板の好ましい製造方法において、本発明の鋼材の耐HIC性の評価方法を適用すれば、耐サワー性能に優れた高強度ラインパイプの製造が可能な、耐HIC性に優れたラインパイプ用高強度厚鋼板が容易に製造できる。
たとえば、上記した高強度厚鋼板の好ましい製造方法において、鋼素材を、上記した本発明の鋼材の耐HIC性の評価方法を用いて評価し、耐HIC性に優れた鋼素材であると評価された鋼素材を用いて、厚鋼板を製造することが好ましい。得られた厚鋼板は、耐HIC性に優れ、ラインパイプ用として好適である。
Furthermore, in the preferable manufacturing method of the high-strength thick steel plate described above, if the method for evaluating the HIC resistance of the steel material of the present invention is applied, it is possible to manufacture a high-strength line pipe excellent in sour resistance performance. Excellent high-strength thick steel plate for line pipes can be easily manufactured.
For example, in a preferable manufacturing method of the above-described high-strength thick steel plate, the steel material is evaluated using the above-described evaluation method for HIC resistance of the steel material of the present invention, and is evaluated as a steel material having excellent HIC resistance. It is preferable to manufacture a thick steel plate using a steel material. The obtained thick steel plate has excellent HIC resistance and is suitable for line pipe use.
また、例えば、上記した高強度厚鋼板の好ましい製造方法で製造された厚鋼板について、上記した本発明の鋼材の耐HIC性の評価方法を用いて、耐HIC性を評価し、耐HIC性に優れた厚鋼板であると評価された厚鋼板のみを、ラインパイプ用厚鋼板として適用すれば、耐サワー性能に優れた高強度ラインパイプの製造が可能な、耐HIC性に優れたラインパイプ用高強度厚鋼板が確実に製造できる。 Further, for example, with respect to the thick steel plate manufactured by the preferable manufacturing method of the above-described high-strength thick steel plate, the HIC resistance is evaluated by using the above-described method for evaluating the HIC resistance of the steel material of the present invention. For line pipes with excellent HIC resistance, high-strength line pipes with excellent sour resistance can be manufactured by applying only thick steel plates that have been evaluated as excellent steel sheets as thick steel sheets for line pipes. High strength thick steel plates can be manufactured reliably.
表1に示す組成の溶鋼を転炉で溶製し、連続鋳造法でスラブ(鋼素材)(肉厚:250 mm)とした。なお、鋼No.9は、鋳造速度を、通常の約1.3倍(1.6 m/min)として、中心偏析部の偏析度を変化させた鋳片(スラブ)を鋼素材とした。また、鋼板No.12以外は、凝固末期に0.9mm/minの圧下を加えた鋳片(スラブ)を用いた。
得られた鋼素材を、ついで、1150℃に加熱し、圧延開始温度:1050℃、圧延終了温度:870℃とする熱間圧延を施した。熱間圧延終了後、830℃から急速水冷却を施し、表2に示すほぼ23〜28℃/sの冷却速度で、表2に示す冷却停止温度:400〜450℃の範囲の冷却停止温度まで冷却し、表2に示す板厚:29 mmまたは32 mmの厚鋼板とした。なお、一部の厚鋼板には、誘導加熱装置により、表2に示す520〜650℃の範囲の温度に再加熱する再加熱処理を施した。
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) (wall thickness: 250 mm) by a continuous casting method. In Steel No. 9, the casting speed was about 1.3 times the normal (1.6 m / min), and the slab (slab) in which the segregation degree of the central segregation part was changed was used as the steel material. For steel plates other than steel plate No. 12, cast slabs (slabs) with a reduction of 0.9 mm / min at the end of solidification were used.
The obtained steel material was then heated to 1150 ° C. and subjected to hot rolling at a rolling start temperature of 1050 ° C. and a rolling end temperature of 870 ° C. After the hot rolling is completed, rapid water cooling is performed from 830 ° C., and at a cooling rate of about 23 to 28 ° C./s shown in Table 2, the cooling stop temperature shown in Table 2 to a cooling stop temperature in the range of 400 to 450 ° C. The plate was cooled to a thick steel plate having a thickness of 29 mm or 32 mm shown in Table 2. A part of the thick steel plates was subjected to a reheating treatment for reheating to a temperature in the range of 520 to 650 ° C. shown in Table 2 using an induction heating device.
なお、鋼板No.1〜No.4、No.9、No.10は、鋼素材(スラブ)段階で試験材を採取し、C断面をマクロエッチングして、偏析部(偏析スポット)を特定し、該偏析部(偏析スポット)を含む微小領域のS濃度を測定した。S濃度の測定は、特定した中心偏析部(偏析スポット)を含み3×3mmの領域から、ドリルで分析試料(切粉)を採取して行った。採取した切粉の重量を秤量し、助燃剤としてタングステン1.5gとスズ0.3gとともに、分析に供した。なお、採取した切粉の重量は0.05〜0.2gの範囲であった。採取した切粉を、助燃剤とともに、セラミックルツボに入れ、高周波燃焼−紫外蛍光法でS量を分析し、偏析部(偏析スポット)のS濃度とした。 For steel plates No. 1 to No. 4, No. 9, and No. 10, test materials were collected at the steel material (slab) stage, and the C section was macro-etched to identify the segregation part (segregation spot). Then, the S concentration in a minute region including the segregation part (segregation spot) was measured. The S concentration was measured by collecting an analysis sample (chip) with a drill from an area of 3 × 3 mm including the specified center segregation part (segregation spot). The collected chips were weighed and analyzed with 1.5 g of tungsten and 0.3 g of tin as a combustion aid. The weight of the collected chips was in the range of 0.05 to 0.2 g. The collected chips were put in a ceramic crucible together with a combustion aid, and the amount of S was analyzed by a high-frequency combustion-ultraviolet fluorescence method to obtain the S concentration in the segregation part (segregation spot).
得られた厚鋼板のうち、鋼板No.5、No.6、No.7、No.8、No.11、No.12については、厚鋼板のC断面をマクロエッチングして、偏析部(偏析スポット)を特定し、該偏析部を含む微小領域の複数箇所で、荷重50gf(試験力490mN)で微小硬さHVを測定したのち、微小硬さHVが230 HV以上となる箇所についてのみ、該箇所(圧痕)を中心として幅方向3mm×厚さ方向0.5 mmの領域から研削で分析試料(切粉)を採取した。このとき、分析試料の重量が0.05〜0.2gとなるように採取した。得られた分析試料は、助燃剤とともに、セラミックルツボに入れ、高周波燃焼−紫外蛍光法でS量を分析し、偏析部(偏析スポット)含む微小領域のS濃度とした。 Among the obtained thick steel plates, for steel plates No. 5, No. 6, No. 7, No. 8, No. 11, and No. 12, the C cross section of the thick steel plate is macro-etched and segregated (segregated) Spot), and after measuring the microhardness HV at a load of 50 gf (test force 490 mN) at a plurality of locations in the microscopic area including the segregated portion, only the locations where the microhardness HV is 230 HV or higher An analysis sample (chip) was collected by grinding from an area of 3 mm in the width direction and 0.5 mm in the thickness direction centered on the spot (indentation). At this time, it collected so that the weight of an analysis sample might be 0.05-0.2g. The obtained analysis sample was put in a ceramic crucible together with a combustion aid, and the amount of S was analyzed by a high-frequency combustion-ultraviolet fluorescence method to obtain an S concentration in a minute region including a segregation part (segregation spot).
また、得られた厚鋼板から、試験片を採取し、引張試験、衝撃試験、HIC試験を実施し、各厚鋼板の強度靭性、耐HIC性を評価した。試験方法はつぎのとおりとした。
(1)引張試験
得られた厚鋼板から、API−5Lの規定に準拠して、全厚引張試験片を採取し、引張試験を実施し、引張特性(引張強さTS)を評価した。
(2)衝撃試験
得られた厚鋼板の板厚方向1/4T位置から、長さ方向が圧延方向に直角方向となるようにVノッチ試験片を採取し、JIS Z 2242の規定に準拠してシャルピー衝撃試験を実施し、試験温度:−5℃で、シャルピー衝撃試験を実施し、吸収エネルギーvE-5 (J)を求め、靭性を評価した。
(3)HIC試験
得られた厚鋼板の幅方向板中央位置(W/2部)で、板厚中央部を含むように3個ずつ、HIC試験片(大きさ:板厚全厚〜30mm×幅20mm×長さ100mm)を採取した。得られたHIC試験片を、NACE TM0284に準拠して、A規格液中に96h浸漬するHIC試験を実施した。浸漬後、試験片の割れ面積率(CAR%)を超音波探傷法で測定した。得られた各試験片のCARの平均値をもとめ、3.0%以下である場合を耐HIC性が良好であるとした。
Further, test pieces were collected from the obtained thick steel plates, and subjected to tensile tests, impact tests, and HIC tests, and the strength toughness and HIC resistance of each thick steel plate were evaluated. The test method was as follows.
(1) Tensile test From the obtained thick steel sheet, a full-thickness tensile test piece was sampled according to API-5L, a tensile test was performed, and tensile properties (tensile strength TS) were evaluated.
(2) Impact test V-notch specimens were taken from the 1 / 4T position in the plate thickness direction of the obtained thick steel plate so that the length direction was perpendicular to the rolling direction, and in accordance with the provisions of JIS Z 2242 A Charpy impact test was conducted, and a Charpy impact test was conducted at a test temperature of −5 ° C. to obtain an absorbed energy vE −5 (J), and toughness was evaluated.
(3) HIC test At the center position (W / 2 part) in the width direction of the obtained thick steel sheet, three HIC test pieces (size: total thickness ~ 30mm x 3) so as to include the central part of the thickness. (Width 20 mm x length 100 mm). Based on NACE TM0284, the obtained HIC test piece was subjected to an HIC test in which it was immersed for 96 hours in a standard A solution. After immersion, the crack area ratio (CAR%) of the test piece was measured by an ultrasonic flaw detection method. The average value of CAR of each obtained test piece was obtained, and when it was 3.0% or less, the HIC resistance was considered good.
得られた結果を表3に併記する。
なお、得られた厚鋼板の強度は、560MPa〜660MPaの範囲であった。
The results obtained are also shown in Table 3.
In addition, the strength of the obtained thick steel plate was in the range of 560 MPa to 660 MPa.
鋳片で偏析部を含む微小領域のS濃度を測定し18質量ppm以下である鋳片を鋼素材とする厚鋼板はいずれも、HICが発生しても、軽微であり、また、鋼板で偏析部を含む微小領域のS濃度を測定し18質量ppm以下である場合はいずれも、HICが発生しても、軽微であり、偏析部を含む微小領域のS濃度を測定し18質量ppm以下である鋼材を耐HIC性に優れた鋼材とする、本発明の鋼材の耐HIC性の評価方法は有効であるといえる。 Thick steel plates made from steel slabs with a slab measuring 18% by mass or less of S concentration in a small area including the segregation part are insignificant even if HIC occurs. In any case where the S concentration in the micro area including the part is 18 ppm by mass or less, even if HIC occurs, the S concentration in the micro area including the segregation part is 18 ppm or less. It can be said that the method for evaluating the HIC resistance of a steel material of the present invention, in which a certain steel material is a steel material having excellent HIC resistance, is effective.
素材である鋳片または厚鋼板の偏析部を含む微小領域のS濃度が10質量ppm以下の厚鋼板(厚鋼板No.1、No.5)では、HIC割れが発生せず、あるいは発生しても軽微であり、耐HIC性に優れた厚鋼板であるといえる。また、素材である鋳片または厚鋼板の偏析部を含む微小領域のS濃度が18質量ppm以下である厚鋼板(厚鋼板No.2、No.3、No.4、No.6〜No.8)では、HICが発生する場合もあるが、HICが発生しても平均のCARは2%以下であり、良好な耐HIC性を有する厚鋼板といえる。 Thick steel plates (thick steel plates No. 1 and 5) with an S concentration of 10 mass ppm or less including the segregation part of the cast slab or thick steel plate, which are raw materials, have no or no HIC cracks. The steel plate is light and has excellent HIC resistance. In addition, thick steel plates (thick steel plates No. 2, No. 3, No. 4, No. 6 to No. 6) having an S concentration of 18 ppm by mass or less in a minute region including segregated portions of cast slabs or thick steel plates. In 8), HIC may occur, but even if HIC occurs, the average CAR is 2% or less, and it can be said that the steel sheet has good HIC resistance.
一方、鋳片の鋳造速度を速くした鋳片を鋼素材とした厚鋼板No.9、鋳造末期の圧下を施さなかった鋳片を鋼素材とした厚鋼板No.12では、中心偏析が著しくなり、偏析部を含む微小領域のS濃度が18質量ppmを超え、MnSが形成されて、耐HIC特性が低下している。また、成分含有量(Mn)が好適範囲を外れた厚鋼板No.10、Sが好適範囲を外れた厚鋼板No.11では、偏析部を含む微小領域のS濃度が18質量ppmを超え、MnSが形成されて、耐HIC特性が低下している。 On the other hand, in the case of thick steel plate No. 9 using steel slabs with increased casting speed as the steel material and thick steel plate No. 12 using steel slabs that were not subjected to the reduction at the end of casting, the center segregation was significant. The S concentration in the minute region including the segregation part exceeds 18 mass ppm, MnS is formed, and the HIC resistance is deteriorated. In addition, in the thick steel plate No. 10 in which the component content (Mn) is out of the preferred range, in the thick steel plate No. 11 in which the S is out of the preferred range, the S concentration in the micro area including the segregation portion exceeds 18 mass ppm, MnS is formed and the anti-HIC properties are degraded.
このように、本発明によれば、鋼材の中心偏析部を含む微小領域のS濃度を測定し、あるいは鋼材の中心偏析部を含む微小領域で微小硬さを測定して、MnSの形成の有無の判定を介して、鋼材の耐HIC性の良否を精度高く評価することが可能であることがわかる。 Thus, according to the present invention, the presence or absence of the formation of MnS is measured by measuring the S concentration in the minute region including the central segregation portion of the steel material or by measuring the microhardness in the minute region including the central segregation portion of the steel material. It can be seen that the quality of the HIC resistance of the steel material can be evaluated with high accuracy through this determination.
1 紫外蛍光分析装置
2 分析試料
3 高周波誘導加熱炉
31 セラミックスるつぼ
32 コイル
4 ダストフィルタ
5 紫外蛍光分析器
1 Ultraviolet fluorescence analyzer 2 Analytical sample 3 High frequency induction heating furnace
31 Ceramic crucible
32 Coil 4 Dust filter 5 Ultraviolet fluorescence analyzer
Claims (11)
C:0.03%以上0.07%未満、 Si:0.01〜0.5%、
Mn:0.8〜1.5%、 S:0.0008%以下、
P:0.008%以下、 Al:0.07%以下,
Ti:0.005〜0.02%、 Nb:0.005〜0.07%、
Ca:0.0005〜0.005%、 N:0.008%以下、
O:0.005%以下
を含有し、残部Feおよび不可避的不純物からなる組成を有することを特徴とする請求項1ないし4のいずれかに記載の鋼材の耐HIC性の評価方法。 The steel material is mass%,
C: 0.03% or more and less than 0.07%, Si: 0.01 to 0.5%,
Mn: 0.8 to 1.5%, S: 0.0008% or less,
P: 0.008% or less, Al: 0.07% or less,
Ti: 0.005-0.02%, Nb: 0.005-0.07%,
Ca: 0.0005 to 0.005%, N: 0.008% or less,
The method for evaluating the HIC resistance of a steel material according to any one of claims 1 to 4, wherein the composition contains O: 0.005% or less and has a balance of Fe and inevitable impurities.
C:0.03%以上0.07%未満、 Si:0.01〜0.5%、
Mn:0.8〜1.5%、 S:0.0008%以下、
P:0.008%以下、 Al:0.07%以下,
Ti:0.005〜0.02%、 Nb:0.005〜0.07%、
Ca:0.0005〜0.005%、 N:0.008%以下、
O:0.005%以下
を含有し、残部Feおよび不可避的不純物からなる組成を有することを特徴とする請求項7ないし9のいずれかに記載のラインパイプ用高強度厚鋼板の製造方法。 The steel material is mass%,
C: 0.03% or more and less than 0.07%, Si: 0.01 to 0.5%,
Mn: 0.8 to 1.5%, S: 0.0008% or less,
P: 0.008% or less, Al: 0.07% or less,
Ti: 0.005-0.02%, Nb: 0.005-0.07%,
Ca: 0.0005 to 0.005%, N: 0.008% or less,
The method for producing a high-strength thick steel plate for a line pipe according to any one of claims 7 to 9, wherein the composition contains O: 0.005% or less and the balance is Fe and inevitable impurities.
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