JP2016125138A - Steel sheet and steel pipe for line pipe excellent in hydrogen-induced crack resistance - Google Patents
Steel sheet and steel pipe for line pipe excellent in hydrogen-induced crack resistance Download PDFInfo
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Abstract
Description
本発明は、耐水素誘起割れ性に優れた鋼板に関する。特には、天然ガス・原油輸送用ラインパイプや貯蔵用タンクなどに好適な、耐水素誘起割れ性に優れた鋼板、および該鋼板を用いて得られる耐水素誘起割れ性に優れたラインパイプ用鋼管に関する。 The present invention relates to a steel sheet having excellent resistance to hydrogen-induced cracking. In particular, steel plates excellent in hydrogen-induced crack resistance suitable for natural gas and crude oil transportation line pipes and storage tanks, and steel pipes for line pipes excellent in hydrogen-induced crack resistance obtained using the steel plates About.
主に石油・ガスなどの輸送用ラインパイプや貯蔵用タンクでは、硫化水素を含有する劣質資源の開発に伴い、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。以下では、この耐サワー性を備えた鋼板を「耐サワー鋼板」ということがある。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。 In line with the development of inferior resources containing hydrogen sulfide, so-called sour resistance, such as resistance to hydrogen-induced cracking and stress corrosion cracking, is required mainly for oil and gas transportation line pipes and storage tanks. The Hereinafter, the steel plate having sour resistance may be referred to as “sour-resistant steel plate”. Hydrogen-induced cracking (hereinafter sometimes referred to as “HIC”) is a phenomenon in which hydrogen that has penetrated into a steel material due to the corrosion reaction caused by hydrogen sulfide or the like starts with MnS or Nb (C, N). It is known that it is a crack that accumulates in non-metallic inclusions and is caused by gasification.
HICは、鋳片の中心偏析、内部割れ等を含む偏析部、特にMnS等の介在物を起点に発生しやすいことが知られている。そこで、従来より、耐HIC性を高める技術について幾つか提案されている。例えば特許文献1には、板厚中心部のMn、Nb、Tiの偏析度を抑制することにより耐HIC性を改善した鋼材が開示されている。また特許文献2には、CaとOとSの含有量からなるパラメータ式によりMnSやCa系酸硫化物を起点としたHICを抑制する方法が開示されている。 It is known that HIC is likely to be generated starting from a segregated portion including center segregation of the slab, internal cracks, etc., particularly inclusions such as MnS. Thus, several techniques for improving the HIC resistance have been proposed. For example, Patent Document 1 discloses a steel material having improved HIC resistance by suppressing the segregation degree of Mn, Nb, and Ti at the center of the plate thickness. Patent Document 2 discloses a method of suppressing HIC starting from MnS or Ca-based oxysulfide by a parameter formula including Ca, O, and S contents.
これらの方法により、多くのHICは抑制されるものの、微細なHICが局所的に多数発生する場合がある。 Although many HICs are suppressed by these methods, a large number of fine HICs may be generated locally.
一方、鋼板は、溶製、鋳造、熱間圧延を経て得られた後、製品として出荷前にHIC試験が実施される。しかし、HIC試験は、結果が判明するまでに数週間を要する。また、上記HIC試験でHICが発生すると、上記鋼板を耐水素誘起割れ性に優れた製品として出荷できず、再度製造、即ち再び溶製から行って得られた製品に対し、再度のHIC試験を行う必要がある。そうすると、製造期間が長期化して納期遅れ等の原因となる。 On the other hand, the steel sheet is obtained through melting, casting, and hot rolling, and then subjected to an HIC test before shipping as a product. However, the HIC test takes several weeks before the results are known. In addition, when HIC occurs in the HIC test, the steel sheet cannot be shipped as a product excellent in hydrogen-induced cracking resistance, and the HIC test is performed again on the product obtained by remanufacturing, that is, remelting. There is a need to do. If it does so, a manufacturing period will become long and it will cause a delay in delivery.
そこで、上記熱間圧延後にHIC試験を行うのではなく、前記鋳造後の鋳片の段階で耐HIC性を評価できれば、製造期間を大幅に短縮できると考えられる。HICは、上述したように、偏析部(中心偏析、内部割れ)やMnS等の介在物を起点に発生するため、鋳片の段階でこれらを評価できれば、その評価結果に基づいて耐HIC性を評価できると考えられる。 Therefore, if the HIC resistance can be evaluated at the stage of the cast slab after the casting instead of performing the HIC test after the hot rolling, it is considered that the manufacturing period can be greatly shortened. As described above, since HIC occurs starting from the segregation part (center segregation, internal crack) and inclusions such as MnS, if these can be evaluated at the stage of the slab, the HIC resistance can be improved based on the evaluation result. It can be evaluated.
例えば、圧延後にHIC試験を行う従来の方法では、鋳造から出荷までに下記の長い工程A−1を経る。これに対し、鋳片の段階で耐HIC性を評価できれば、下記工程B−1の通り、HIC試験を行う場合の「(HIC試験のための)サンプル調整→HIC試験」を省略できるため、製品を早期に出荷できる。
工程A−1:鋳造→圧延→(HIC試験のための)サンプル調整→HIC試験→出荷
工程B−1:鋳造→耐HIC性の評価→圧延→出荷
For example, in the conventional method of performing the HIC test after rolling, the following long process A-1 is performed from casting to shipment. On the other hand, if the HIC resistance can be evaluated at the stage of the slab, it is possible to omit “sample adjustment (for HIC test) → HIC test” when performing the HIC test as shown in the following step B-1. Can be shipped early.
Process A-1: Casting → Rolling → Sample preparation (for HIC test) → HIC test → Shipping process B-1: Casting → HIC resistance evaluation → Rolling → Shipping
また、HIC試験の結果がNGであった場合、従来の方法では、鋳造から再溶製までが長い下記の工程A−2を経る。これに対し、下記工程B−2の通り鋳片の段階で耐HIC性を評価できれば、この評価がNGであったとしても、下記工程A−2における「圧延→(HIC試験のための)サンプル調整→HIC試験」を省略でき、早期に再溶製を開始できる。
工程A−2:鋳造→圧延→(HIC試験のための)サンプル調整→HIC試験→再溶製
工程B−2:鋳造→耐HIC性の評価→再溶製
Further, when the result of the HIC test is NG, in the conventional method, the following process A-2 is long from casting to remelting. On the other hand, if HIC resistance can be evaluated at the slab stage as shown in the following process B-2, even if this evaluation is NG, “rolling → (for HIC test) sample in the following process A-2” “Adjustment → HIC test” can be omitted, and remelting can be started at an early stage.
Step A-2: Casting → Rolling → Sample preparation (for HIC test) → HIC test → Remelting Step B-2: Casting → Evaluation of HIC resistance → Remelting
このような方法として、特許文献3には、鋳片の段階で内部割れを評価する方法が開示されている。この方法では、内部割れの評価結果からHCR(Hot Charge Rolling)操業の可否を判断している。 As such a method, Patent Document 3 discloses a method of evaluating internal cracks at the stage of a slab. In this method, whether or not HCR (Hot Charge Rolling) operation is possible is determined from the evaluation result of the internal crack.
ところで、耐サワー性が必要とされる鋼板で問題となる内部割れは、非常に小さい微細な割れであるが、特許文献3では、HCR操業で問題となる内部割れ、即ち、割れ長さが10mm以上の大きな割れを評価している。そのため、上記方法では、耐サワー性が必要とされる鋼板で問題となる微細な内部割れを見逃すことがあるため、鋳片の段階で内部割れが原因の耐HIC性を正確に評価することができない。 By the way, although the internal crack which becomes a problem in the steel plate which requires sour resistance is a very small fine crack, in patent document 3, the internal crack which becomes a problem by HCR operation, ie, the crack length is 10 mm. The above large cracks are evaluated. Therefore, in the above method, since a fine internal crack that causes a problem in a steel sheet that requires sour resistance may be missed, it is possible to accurately evaluate the HIC resistance caused by the internal crack at the slab stage. Can not.
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、耐水素誘起割れ性に優れた鋼板や鋼管を実現すること、更には、HIC試験を行うことなく、鋳片の内部品質から耐HIC性を評価できる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 cracking resistance, and further, without carrying out a HIC test, It is to realize a steel plate and a steel pipe that can evaluate the HIC resistance from the internal quality of the piece.
上記課題を解決し得た本発明の耐水素誘起割れ性に優れた鋼板は、
質量%で、
C:0.02〜0.15%、
Si:0.02〜0.50%、
Mn:0.6〜2.0%、
P:0%超0.030%以下、
S:0%超0.003%以下、
Al:0.010〜0.08%、
Ca:0.0003〜0.0060%、
N:0.001〜0.01%、および
O:0%超0.0045%以下を満たし、更に、
REM:0%超0.02%以下、および
Zr:0%超0.010%以下
よりなる群から選択される1種以上の元素を含み、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 ≦ 1.80を満たし、
更にスラブの段階で、水平割れが存在しない、または、水平割れの最大開孔厚みが閾値tθ以下であって、該閾値tθは、前記スラブを圧延して得た鋼板に水素誘起割れが発生しない水平割れの最大開孔厚みであるところに特徴を有する。
A steel sheet excellent in hydrogen-induced crack resistance of the present invention that has solved the above problems is
% By mass
C: 0.02 to 0.15%,
Si: 0.02 to 0.50%,
Mn: 0.6 to 2.0%,
P: more than 0% and 0.030% or less,
S: more than 0% and 0.003% or less,
Al: 0.010 to 0.08%,
Ca: 0.0003 to 0.0060%,
N: 0.001 to 0.01%, and O: more than 0% and 0.0045% or less,
Including one or more elements selected from the group consisting of REM: more than 0% and 0.02% or less, and Zr: more than 0% and 0.010%, the balance consisting 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≦1.80,
Further, at the stage of the slab, there is no horizontal crack, or the maximum opening thickness of the horizontal crack is equal to or less than a threshold value tθ, and the threshold value tθ does not cause hydrogen-induced cracking in the steel sheet obtained by rolling the slab. It is characterized by the maximum opening thickness of horizontal cracks.
前記閾値tθは、予め、下記(i)〜(iii)の方法で求められた値であってもよい。
(i)前記スラブの最大開孔厚みを測定する。
(ii)前記スラブと同一の鋳造条件で鋳造したスラブを圧延して得られる鋼板に対して
水素誘起割れ試験を行う。
(iii)上記(i)で測定した最大開孔厚みと、上記(ii)の水素誘起割れ試験結果とから、水素誘起割れの発生しない水平割れの最大開孔厚みを求める。
The threshold value tθ may be a value obtained in advance by the following methods (i) to (iii).
(I) The maximum hole thickness of the slab is measured.
(Ii) A hydrogen-induced cracking test is performed on a steel plate obtained by rolling a slab cast under the same casting conditions as the slab.
(Iii) From the maximum hole thickness measured in (i) above and the hydrogen-induced crack test result in (ii) above, the maximum hole thickness of a horizontal crack in which no hydrogen-induced crack occurs is obtained.
前記スラブと同一の鋳造条件で鋳造したスラブは、前記最大開孔厚みを測定したスラブであってもよい。 The slab cast under the same casting conditions as the slab may be a slab in which the maximum hole thickness is measured.
前記鋼板は、API(The American Petroleum Institute)X65グレードであって、前記閾値tθが0.047mmであってもよい。 The steel plate may be API (The American Petroleum Institute) X65 grade, and the threshold value tθ may be 0.047 mm.
前記鋼板は、APIX70グレードであって、前記閾値tθが0.043mmであってもよい。 The steel plate may be APIX 70 grade, and the threshold value tθ may be 0.043 mm.
前記鋼板は、ASME(American Society of Mechanical Engineers)SA516グレード60であって、前記閾値tθが0.047mmであってもよい。 The steel sheet may be ASME (American Society of Mechanical Engineers) SA516 grade 60, and the threshold value tθ may be 0.047 mm.
前記鋼板は、ASMESA516グレード65であって、前記閾値tθが0.047mmであってもよい。 The steel plate may be ASMESA 516 grade 65, and the threshold value tθ may be 0.047 mm.
前記鋼板は、ASMESA516グレード70であって、前記閾値tθが0.043mmであってもよい。 The steel plate may be ASMESA 516 grade 70, and the threshold value tθ may be 0.043 mm.
前記鋼板は、ASTM(American Society for Testing and Materials)A516グレード60であって、前記閾値tθが0.047mmであってもよい。 The steel sheet may be ASTM (American Society for Testing and Materials) A516 grade 60, and the threshold value tθ may be 0.047 mm.
前記鋼板は、ASTMA516グレード65であって、前記閾値tθが0.047mmであってもよい。 The steel plate may be ASTM A516 grade 65, and the threshold value tθ may be 0.047 mm.
前記鋼板は、ASTMA516グレード70であって、前記閾値tθが0.043mmであってもよい。 The steel plate may be ASTM A516 grade 70, and the threshold value tθ may be 0.043 mm.
前記鋼板は、更に他の元素として、下記(A)および(B)のうちのいずれか1以上を含んでいてもよい。
(A)質量%で、B:0%超0.005%以下、V:0%超0.1%以下、Cu:0%超1.5%以下、Ni:0%超1.5%以下、Cr:0%超1.5%以下、Mo:0%超1.5%以下、およびNb:0%超0.06%以下よりなる群から選択される1種以上の元素
(B)質量%で、Ti:0%超0.03%以下、およびMg:0%超0.01%以下よりなる群から選択される1種以上の元素
The said steel plate may contain any one or more of following (A) and (B) as another element.
(A) By mass%, B: more than 0% to 0.005% or less, V: more than 0% to 0.1% or less, Cu: more than 0% to 1.5% or less, Ni: more than 0% to 1.5% or less Cr: more than 0% and 1.5% or less, Mo: more than 0% and 1.5% or less, and Nb: more than 0% and 0.06% or less. One or more elements selected from the group consisting of Ti: more than 0% and 0.03% or less and Mg: more than 0% and 0.01% or less
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、上記鋼板から形成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. The present invention also includes a steel pipe for line pipe formed from the steel plate.
本発明によれば、耐水素誘起割れ性の確実に優れた鋼板や鋼管を提供できる。更には、HIC試験を行うことなく、鋳片の内部品質から耐HIC性を評価できる鋼板や鋼管を提供できる。これらは、天然ガス・原油の輸送用ラインパイプや貯蔵用タンク等の圧力容器などに好適に用いられる。 ADVANTAGE OF THE INVENTION According to this invention, the steel plate and steel pipe which were surely excellent in hydrogen-induced crack resistance can be provided. Furthermore, the steel plate and steel pipe which can evaluate HIC resistance from the internal quality of a slab can be provided, without performing a HIC test. These are suitably used for pressure vessels such as natural gas / crude oil transportation line pipes and storage tanks.
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、HICがMnS介在物を起点に発生しやすいことに着目した。その結果、脱硫作用を有する元素である希土類元素あるいはZrを鋼材に含有させることにより、MnSの生成を抑制し耐水素誘起割れ性を高めることが可能であることに想到した。更に、その脱硫作用を効果的に発揮させるために、後述する適切な含有量を見出すに至った。 The inventors of the present invention have made extensive studies to solve the above problems. First, the inventors focused on the fact that HIC is likely to be generated starting from MnS inclusions. As a result, it was conceived that the rare earth element or Zr, which is an element having a desulfurization action, can be contained in the steel material to suppress the generation of MnS and enhance the resistance to hydrogen-induced cracking. Furthermore, in order to effectively exhibit the desulfurization action, an appropriate content described later has been found.
次に本発明者らは、HICが偏析部を起点に発生しやすいことに着目した。その結果、偏析のうち「水平割れ」、特に水平割れの最大開孔厚みに注目し、スラブの段階においてこれを所定の閾値以下に収めれば、耐水素誘起割れ性の高い鋼板が得られ、更には製品を早期に出荷できることを見出した。この点については後に詳述する。 Next, the present inventors paid attention to the fact that HIC is likely to be generated starting from the segregation part. As a result, paying attention to the `` horizontal crack '' of segregation, especially the maximum hole thickness of the horizontal crack, if this is kept below a predetermined threshold at the slab stage, a steel plate with high hydrogen-induced crack resistance can be obtained, Furthermore, it has been found that products can be shipped early. This point will be described in detail later.
まずは成分組成について説明する。 First, the component composition will be described.
優れた耐HIC性を確保するには、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材として求められるその他の特性として、高強度や優れた溶接性等を確保するにも、鋼板の成分組成を下記の通りとする必要がある。以下、前述した希土類元素およびZrをはじめ、各成分の規定理由について説明する。 In order to ensure excellent HIC resistance, it is necessary to control the composition of the steel material. Furthermore, as another characteristic required as a steel material for line pipes, for example, in order to ensure high strength and excellent weldability, the component composition of the steel sheet needs to be as follows. Hereinafter, the reasons for defining each component including the above-described rare earth element and Zr 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。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. Preferably it is 0.12% or less, More preferably, it is 0.10% or less.
Si:0.02〜0.50%
Siは、脱酸作用を有すると共に、母材および溶接部の強度向上に有効な元素である。これらの効果を得るため、Si量を0.02%以上とする。Si量は、好ましくは0.05%以上であり、より好ましくは0.15%以上である。しかし、Si量が多すぎると溶接性や靭性が劣化する。またSi量が過剰であると、島状マルテンサイトが生じてHICが発生・進展する。よってSi量は、0.50%以下に抑える必要がある。Si量は、好ましくは0.45%以下、より好ましくは0.35%以下である。
Si: 0.02 to 0.50%
Si is an element that has a deoxidizing action and is effective in improving the strength of the base material and the welded portion. In order to obtain these effects, the Si content is set to 0.02% or more. The amount of Si is preferably 0.05% or more, and more preferably 0.15% or more. However, if the amount of Si is too large, weldability and toughness deteriorate. If the amount of Si is excessive, island martensite is generated and HIC is generated and progresses. Therefore, the amount of Si needs to be suppressed to 0.50% or less. The amount of Si is preferably 0.45% or less, more preferably 0.35% or less.
Mn:0.6〜2.0%
Mnは、母材および溶接部の強度向上に有効な元素であり、本発明では0.6%以上含有させる。Mn量は、好ましくは0.8%以上であり、より好ましくは1.0%以上である。しかし、Mn量が多すぎると、MnSが生成されて耐水素誘起割れ性が劣化するだけでなくHAZ靭性や溶接性も劣化する。よってMn量の上限を2.0%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 set to 2.0%. 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%超0.030%以下
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: more than 0% and 0.030% or less P is an element inevitably contained in the steel material. If the P content exceeds 0.030%, the toughness deterioration of the base material and the HAZ part is remarkable, and hydrogen-induced cracking resistance Also deteriorates. 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%超0.003%以下
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: more than 0% and 0.003% or less S is an element that generates a large amount of MnS and significantly deteriorates the resistance to hydrogen-induced cracking if it is too much. Therefore, in the present invention, the upper limit of the amount of S is set to 0.003%. . The amount of S is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. Thus, the smaller one is desirable from the viewpoint of improving hydrogen-induced crack resistance.
Al:0.010〜0.08%
Alは強脱酸元素であり、Al量が少ないと、酸化物中のCa濃度が上昇、即ち、Ca系介在物が鋼板表層部に形成されやすくなり微細なHICが発生する。よって本発明では、Alを0.010%以上とする必要がある。Al量は、好ましくは0.020%以上、より好ましくは0.030%以上である。一方、Al含有量が多すぎると、Alの酸化物がクラスター状に生成し水素誘起割れの起点となる。よってAl量は0.08%以下とする必要がある。Al量は、好ましくは0.06%以下であり、より好ましくは0.05%以下である。
Al: 0.010 to 0.08%
Al is a strong deoxidizing element. When the amount of Al is small, the Ca concentration in the oxide increases, that is, Ca inclusions are easily formed in the surface layer portion of the steel sheet and fine HIC is generated. Therefore, in the present invention, Al needs to be 0.010% or more. The amount of Al is preferably 0.020% or more, more preferably 0.030% or more. On the other hand, when there is too much Al content, the oxide of Al will produce | generate in cluster shape and will become the starting point of a hydrogen induced crack. Therefore, the Al amount needs to be 0.08% or less. The amount of Al is preferably 0.06% or less, and more preferably 0.05% or less.
Ca:0.0003〜0.0060%
Caは、硫化物の形態を制御する作用があり、CaSを形成することによってMnSの形成を抑制する効果がある。この効果を得るには、Ca量を0.0003%以上とする必要がある。Ca量は、好ましくは0.0005%以上であり、より好ましくは0.0010%以上である。一方、Ca量が0.0060%を超えると、Ca系介在物を起点にHICが多く発生する。よって本発明では、Ca量の上限を0.0060%とする。Ca量は、好ましくは0.0045%以下であり、より好ましくは0.0035%以下、さらに好ましくは0.0025%以下である。
Ca: 0.0003 to 0.0060%
Ca has the effect | action which controls the form of sulfide, and there exists an effect which suppresses formation of MnS by forming CaS. In order to obtain this effect, the Ca content needs to be 0.0003% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when the Ca content exceeds 0.0060%, a large amount of HIC is generated starting from Ca-based inclusions. Therefore, in the present invention, the upper limit of the Ca amount is set to 0.0060%. The Ca content is preferably 0.0045% or less, more preferably 0.0035% or less, and still more preferably 0.0025% or less.
N:0.001〜0.01%
Nは、鋼組織中にTiNとして析出し、HAZ部のオーステナイト粒の粗大化を抑制し、さらにフェライト変態を促進させて、HAZ部の靭性を向上させる元素である。この効果を得るにはNを0.001%以上含有させる必要がある。N量は、好ましくは0.003%以上であり、より好ましくは0.0040%以上である。しかしN量が多すぎると、固溶Nの存在によりHAZ靭性がかえって劣化するため、N量は、0.01%以下とする必要がある。好ましくは0.008%以下であり、より好ましくは0.0060%以下である。
N: 0.001 to 0.01%
N is an element that precipitates as TiN in the steel structure, suppresses coarsening of the austenite grains in the HAZ part, further promotes ferrite transformation, and improves the toughness of the HAZ part. In order to acquire this effect, it is necessary to contain N 0.001% or more. The N amount is preferably 0.003% or more, and more preferably 0.0040% or more. However, if the amount of N is too large, the HAZ toughness deteriorates due to the presence of solute N, so the amount of N needs to be 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.0060% or less.
O:0%超0.0045%以下
O、即ち酸素は、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: More than 0% and 0.0045% or less O, that is, oxygen is desirable to be low from the viewpoint of improving cleanliness, and when O is contained in a large amount, in addition to deterioration of toughness, HIC is generated starting from oxide. In addition, the resistance to hydrogen-induced cracking deteriorates. 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.
質量比で示されるCa/S: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、該MnSを起点にHICが発生する。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17程度となる。
Ca / S represented by mass ratio: 2.0 or more As described above, S forms MnS as a sulfide-based inclusion, and HIC is generated starting from the MnS. 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. Incidentally, the upper limit of Ca / S is about 17 from the Ca amount and S amount specified in the present invention.
(Ca−1.25S)/O ≦ 1.80
Ca系酸硫化物によるHICの発生を抑制するには、Ca系介在物の中でも特に凝集合体を形成しやすいCaOを抑制することが有効である。そしてそのためには、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)が鋼板表層部に大量に形成されやすくなる。これらの粗大なCa系介在物はHICの起点となるため、優れた耐HIC性を得るには(Ca−1.25S)/Oを1.80以下とする必要がある。(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≦1.80
In order to suppress the generation of HIC due to Ca-based oxysulfides, it is effective to suppress CaO, which easily forms aggregated coal, among Ca-based inclusions. For this purpose, the Ca amount (Ca-1.25S) obtained by subtracting the Ca component existing as sulfide (CaS) from the total Ca amount in the steel must not be excessive with respect to the O amount. 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 the aggregated coalescence (coarse Ca-based inclusions) of the CaO is the steel sheet surface layer portion. It becomes easy to be formed in large quantities. Since these coarse Ca-based inclusions are the origin of HIC, it is necessary to set (Ca-1.25S) / O to 1.80 or less in order to obtain excellent HIC resistance. (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.
REM:0%超0.02%以下
REM(Rare Earth Metal、希土類元素)は、前述の通り、脱硫作用によりMnSの生成を抑制し耐水素誘起割れ性を高めるのに有効な元素である。このような効果を発揮させるには、REMを0.0002%以上含有させることが好ましい。REM量は、より好ましくは0.0005%以上、更に好ましくは0.0010%以上である。一方、REMを多量に含有させても効果が飽和する。よってREM量の上限は0.02%とすることが必要である。鋳造時の浸漬ノズルの閉塞を抑えて生産性を高める観点からは、REM量を0.015%以下とすることが好ましく、より好ましくは0.010%以下、更に好ましくは0.0047%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、即ちLaからLuまでの15元素と、スカンジウムおよびイットリウムを意味する。
REM: more than 0% and 0.02% or less REM (Rare Earth Metal, a rare earth element) is an element effective for suppressing the generation of MnS by desulfurization and enhancing the resistance to hydrogen-induced cracking as described above. 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 needs to be 0.02%. From the viewpoint of increasing productivity by suppressing the clogging of the immersion nozzle during casting, the REM content is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.0047% or less. is there. In the present invention, the REM means a lanthanoid element, that is, 15 elements from La to Lu, scandium and yttrium.
Zr:0%超0.010%以下
Zrは、脱硫作用により耐HIC性を向上させるとともに、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%以下とすることが必要である。Zr量は、好ましくは0.0070%以下、より好ましくは0.0047%以下、更に好ましくは0.0030%以下である。
Zr: more than 0% and 0.010% or less Zr is an element that improves HIC resistance by desulfurization and contributes to improvement of HAZ toughness by forming an oxide and finely dispersing. 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 amount of Zr needs to be 0.010% or less. The amount of Zr is preferably 0.0070% or less, more preferably 0.0047% or less, and still more preferably 0.0030% or less.
本発明の鋼板、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たり、
(b)下記量のTiおよびMg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靭性の向上や、脱硫が促進されて耐HIC性をより改善することができる。以下、これらの元素について詳述する。
The components of the steel plate and steel pipe of the present invention are as described above, and the balance is composed of iron and inevitable impurities. In addition to the above elements,
(A) By containing one or more elements selected from the group consisting of the following amounts of B, V, Cu, Ni, Cr, Mo, and Nb, the strength and toughness can be further increased,
(B) By containing one or more elements selected from the group consisting of Ti and Mg in the following amounts, the HAZ toughness can be improved and desulfurization can be promoted to further improve the HIC resistance. Hereinafter, these elements will be described in detail.
B:0%超0.005%以下
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量は0.005%以下とすることが好ましい。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: More than 0% and 0.005% or less B enhances hardenability, increases the strength of the base metal and the welded part, and precipitates BN by bonding with N during the process of cooling the heated HAZ part during welding. In order to promote ferrite transformation from within the austenite grains, the 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%超0.1%以下
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: more than 0% and 0.1% or less 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:0%超1.5%以下
Cuは、焼入れ性を向上させて強度を高めるのに有効な元素である。この効果を得るにはCuを0.01%以上含有させることが好ましい。Cu量は、より好ましくは0.05%以上、更に好ましくは0.10%以上である。しかし、Cu含有量が1.5%を超えると靭性が劣化するため、1.5%以下とすることが好ましい。Cu量は、より好ましくは1.0%以下、更に好ましくは0.50%以下である。
Cu: more than 0% and 1.5% or less Cu is an element effective for improving the hardenability and increasing the strength. In order to acquire this effect, it is preferable to contain 0.01% or more of Cu. The amount of Cu is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Cu content exceeds 1.5%, the toughness deteriorates, so it is preferable to set it to 1.5% or less. The amount of Cu is more preferably 1.0% or less, still more preferably 0.50% or less.
Ni:0%超1.5%以下
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: more than 0% and 1.5% or less 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:0%超1.5%以下
Crは、強度の向上に有効な元素であり、この効果を得るには0.01%以上含有させることが好ましい。Cr量は、より好ましくは0.05%以上、更に好ましくは0.10%以上である。一方、Cr量が1.5%を超えるとHAZ靭性が劣化する。よってCr量は1.5%以下とすることが好ましい。Cr量は、より好ましくは1.0%以下、更に好ましくは0.50%以下である。
Cr: more than 0% and 1.5% or less Cr is an element effective for improving the strength. To obtain this effect, it is preferable to contain 0.01% or more. The amount of Cr is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.5%, the HAZ toughness deteriorates. Therefore, the Cr content is preferably 1.5% or less. The amount of Cr is more preferably 1.0% or less, and still more preferably 0.50% or less.
Mo:0%超1.5%以下
Moは、母材の強度と靭性の向上に有効な元素である。この効果を得るには、Mo量を0.01%以上とすることが好ましい。Mo量は、より好ましくは0.05%以上、更に好ましくは0.10%以上である。しかし、Mo量が1.5%を超えるとHAZ靭性および溶接性が劣化する。よってMo量は1.5%以下とすることが好ましく、より好ましくは1.0%以下、更に好ましくは0.50%以下である。
Mo: more than 0% and 1.5% or less Mo is an element effective for improving the strength and toughness of the base material. In order to obtain this effect, the Mo amount is preferably 0.01% or more. The amount of Mo is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Mo amount exceeds 1.5%, the HAZ toughness and weldability deteriorate. Therefore, the Mo amount is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less.
Nb:0%超0.06%以下
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0.047%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: more than 0% and 0.06% or less 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 0.047% or less, still more preferably 0.040% or less, and still more preferably 0.030% or less.
Ti:0%超0.03%以下
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有効な元素である。さらにTiは、脱硫作用を示すため耐HIC性の向上にも有効な元素である。これらの効果を得るには、Tiを0.003%以上含有させることが好ましい。Ti量は、より好ましくは0.005%以上、更に好ましくは0.010%以上である。一方、Ti含有量が過多になると、固溶Tiの増加やTiC析出の増加により母材とHAZ部の靭性が劣化するため、0.03%以下とすることが好ましい。Ti量は、より好ましくは0.02%以下である。
Ti: more than 0% and 0.03% or less Ti precipitates as TiN in the steel to prevent coarsening of austenite grains in the HAZ part during welding and promote ferrite transformation, so that the toughness of the HAZ part It is an effective element for improving Further, Ti is an element effective for improving the HIC resistance since it exhibits a desulfurization action. In order to obtain these effects, it is preferable to contain 0.003% or more of Ti. The amount of Ti is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, if the Ti content is excessive, the toughness of the base material and the HAZ part deteriorates due to an increase in solid solution Ti or an increase in TiC precipitation, so 0.03% or less is preferable. The amount of Ti is more preferably 0.02% or less.
Mg:0%超0.01%以下
Mgは、結晶粒の微細化を通じて靭性の向上に有効な元素であり、また脱硫作用を示すため耐HIC性の向上にも有効な元素である。これらの効果を得るには、Mgを0.0003%以上含有させることが好ましい。Mg量は、より好ましくは0.001%以上である。一方、Mgを過剰に含有させても効果が飽和するため、Mg量の上限は0.01%とすることが好ましい。Mg量は、より好ましくは0.005%以下である。
Mg: more than 0% and 0.01% or less Mg is an element effective for improving toughness through refinement of crystal grains, and is also an element effective for improving HIC resistance because of its desulfurization action. In order to acquire these effects, it is preferable to contain 0.0003% or more of Mg. The amount of Mg is more preferably 0.001% or more. On the other hand, since the effect is saturated even if Mg is contained excessively, the upper limit of the amount of Mg is preferably 0.01%. The amount of Mg is more preferably 0.005% or less.
本発明の鋼板は、スラブの段階において、水平割れが存在しないか、水平割れの最大開孔厚みが閾値以下であって、耐水素誘起割れ性の高い鋼板である。ここで閾値とは、予め求められた、前記スラブを圧延して得た鋼板にHICが発生しない水平割れの最大開孔厚みを意味する。 The steel plate of the present invention is a steel plate having no hydrogen cracking at the stage of slab, or having a maximum horizontal hole thickness of the horizontal crack that is equal to or less than a threshold value. Here, the threshold means the maximum opening thickness of a horizontal crack that is obtained in advance and does not generate HIC in a steel plate obtained by rolling the slab.
この様にスラブの段階で水平割れについて評価、特に水平割れの最大開孔厚みを所定の閾値以下とすることによって、耐水素誘起割れ性の高い鋼板が得られること、また製品を早期に出荷できることについて、以下説明する。 In this way, horizontal cracks are evaluated at the slab stage, and in particular, by setting the maximum open hole thickness of horizontal cracks to a predetermined threshold value or less, it is possible to obtain a steel sheet with high resistance to hydrogen-induced cracking and to be able to ship products early. Will be described below.
まず、上記「水平割れ」から以下に詳述する。 First, the above “horizontal crack” will be described in detail below.
成分の偏析は、スラブの内部割れ部や中心偏析部に存在し、この成分の偏析度が高いほどHICが発生しやすいことが、例えば特開2007−136496号に記載の通り知られている。また偏析により、MA(Martensite−Austenite constituent、島状マルテンサイト)、パーライトバンド等の硬化組織が発生する。偏析度が高いほど硬化組織が発生しやすく、HICは硬化組織に沿って伝播、進展する。本発明では、特に内部割れ部の偏析度を考慮して、耐HIC性を評価する。 It is known that component segregation exists in internal cracks and central segregation portions of the slab, and that the higher the degree of segregation of this component, the more likely HIC is generated, as described in, for example, JP-A-2007-136696. Moreover, segregation produces hardened structures such as MA (Martensite-Austenite constituent, island martensite) and pearlite bands. The higher the degree of segregation, the more easily the hardened structure is generated, and the HIC propagates and propagates along the hardened structure. In the present invention, the HIC resistance is evaluated in consideration of the segregation degree of the internal crack.
なお、偏析は2次デンドライト樹間にも存在する。即ちミクロ偏析も生じうる。しかしこの2次デンドライト樹間は非常に小さく、HICが伝播・伸展しないため、品質上、問題とならない。そこで、本発明ではミクロ偏析を考慮しない。 Segregation also exists between secondary dendrite trees. That is, microsegregation can also occur. However, since this secondary dendrite tree is very small and HIC does not propagate or extend, there is no problem in quality. Therefore, in the present invention, microsegregation is not considered.
内部割れには「水平割れ」と「その他の内部割れ」とがあり、これらはロール間バルジングや冷却水のアンバランスや矯正通過時の変形が原因となって生じる。「水平割れ」は、図1(a)に示すように、スラブの幅方向Wで幅端部からスラブ厚さD/2の範囲に存在する割れであり、スラブ幅方向及び鋳造方向に伝播した割れである。一方、「その他の内部割れ」は、図1(a)に示すように、スラブ全幅に存在する割れであり、スラブ厚さ方向及びスラブ幅方向、またはスラブ厚さ方向及びスラブ鋳造方向に伝播した割れである。 Internal cracks include “horizontal cracks” and “other internal cracks”, which are caused by bulging between rolls, unbalanced cooling water, and deformation during correction. As shown in FIG. 1A, the “horizontal crack” is a crack existing in the range of the slab thickness D / 2 from the width end portion in the width direction W of the slab, and propagated in the slab width direction and the casting direction. It is a crack. On the other hand, as shown in FIG. 1A, the “other internal cracks” are cracks existing in the entire width of the slab and propagated in the slab thickness direction and the slab width direction, or in the slab thickness direction and the slab casting direction. It is a crack.
スラブを圧延すると、図1(b)に示すように、「水平割れ」は伸展するが、「その他の内部割れ」は縮小する。上記割れを起点にHICが発生すると、「水平割れ」ではHICが伝播・伸展し易いが、「その他の内部割れ」ではHICが伝播・伸展しないため、品質上、問題とならない。また、HIC試験を実施したところ、「水平割れ」発生部ではHICが発生することがあったが、「その他の内部割れ」発生部ではHICが発生しなかった。そこで、本発明では、内部割れのうち「水平割れ」のみを考慮することとした。 When the slab is rolled, as shown in FIG. 1B, the “horizontal crack” is extended, but the “other internal cracks” are reduced. When HIC occurs starting from the above-mentioned cracks, HIC easily propagates and extends in “horizontal cracks”, but HIC does not propagate and extend in “other internal cracks”, so there is no problem in quality. Further, when the HIC test was performed, HIC sometimes occurred in the “horizontal crack” occurrence portion, but no HIC occurred in the “other internal crack” occurrence portion. Therefore, in the present invention, only “horizontal cracks” among the internal cracks are considered.
そして本発明では、この「水平割れ」の偏析度を、下記に説明する「最大開孔厚み」で評価する。「水平割れ」の発生位置は上記図1(a)の通りであり、凝固時に固液界面で発生する割れである。「水平割れ」はデンドライト樹間に濃化溶鋼が進入して生じた偏析線を伴うものであり、この程度が著しい場合、偏析線に沿って開孔する。水平割れの偏析度と開孔厚み(開孔幅)には相関関係があり、開孔厚みが大きいほど水平割れの偏析度が高い傾向がある。つまり、最大開孔厚みと水平割れの偏析度には相関関係がある。HICは水平割れの偏析度が高いほど発生しやすいため、最大開孔厚みが大きいほどHICが発生しやすいと考えられる。これらのことから耐HIC性は「最大開孔厚み」によって判断でき、この最大開孔厚みを低減すれば、HICを抑制できる、との知見にまず至った。以下、この水平割れの最大開孔厚みを単に「最大開孔厚み」ということがある。 In the present invention, the degree of segregation of this “horizontal crack” is evaluated by “maximum aperture thickness” described below. The occurrence position of the “horizontal crack” is as shown in FIG. 1A, and is a crack generated at the solid-liquid interface during solidification. “Horizontal cracks” are accompanied by segregation lines formed by the intrusion of concentrated molten steel between dendritic trees. When this level is significant, holes are formed along the segregation lines. There is a correlation between the degree of segregation of horizontal cracks and the hole thickness (opening width), and the greater the hole thickness, the higher the degree of segregation of horizontal cracks. That is, there is a correlation between the maximum hole thickness and the degree of segregation of horizontal cracks. Since HIC is more likely to occur as the degree of segregation of horizontal cracks is higher, it is considered that HIC is more likely to occur as the maximum hole thickness increases. From these facts, the HIC resistance can be judged by the “maximum aperture thickness”, and it has first been found that the HIC can be suppressed by reducing the maximum aperture thickness. Hereinafter, the maximum hole thickness of the horizontal crack may be simply referred to as “maximum hole thickness”.
なお、「開孔厚み」が数10μm程度である微細な水平割れは、圧延時に圧着されるため、製品段階でUT(Ultrasonic Testing)欠陥とならないが、HIC発生の原因となることがわかっている。これを考慮すると、HICは、開孔していることが原因で発生するのでなく、水平割れの偏析度が高いことで発生すると考えられる。 In addition, since the fine horizontal crack whose “opening thickness” is about several tens of μm is pressed during rolling, it does not become a UT (Ultrasonic Testing) defect in the product stage, but it is known that it causes HIC generation. . Considering this, it is considered that HIC does not occur due to opening, but occurs due to the high degree of segregation of horizontal cracks.
そして本発明者らは、スラブの段階、つまり鋳造後であって圧延前の鋼片の、上記最大開孔厚みを用いて、圧延後の鋼板の耐HIC性を判断できれば、製品である鋼板に対してHIC試験を行う必要がなくなり、工程を省略できること、その結果、製品を早期に出荷できる、との知見に至った。 And if the present inventors can judge the HIC resistance of the steel plate after rolling by using the maximum hole thickness of the steel slab at the slab stage, that is, after casting and before rolling, the steel plate as a product will be obtained. On the other hand, the present inventors have found that it is not necessary to perform an HIC test, the process can be omitted, and as a result, the product can be shipped early.
以下では、最大開孔厚みの求め方と、最大開孔厚みを用いて圧延後の鋼板の耐HIC性を判断する際に用いる、最大開孔厚みの閾値tθと、について説明する。 Below, the method of calculating | requiring the maximum opening thickness and the threshold value t (theta) of the maximum opening thickness used when judging the HIC resistance of the steel plate after rolling using the maximum opening thickness are demonstrated.
上記水平割れの最大開孔厚みの求め方について説明する。 A method for obtaining the maximum opening thickness of the horizontal crack will be described.
先ず、鋳造して得られたスラブを厚さ方向、即ち、図2に示す通り、鋳造方向に対して垂直な方向に切断し、偏析部の水平割れを調査する。水平割れが発生する位置は、鋳造方向よりもスラブ幅方向及びスラブ厚さ方向にばらつきが生じやすい。そこで上記図2の通り、鋳造方向に対して垂直な切断面を調査対象とすることにより、水平割れが最も悪化している部位を調査できる。 First, the slab obtained by casting is cut in the thickness direction, that is, in a direction perpendicular to the casting direction as shown in FIG. The position where the horizontal crack occurs is more likely to vary in the slab width direction and the slab thickness direction than in the casting direction. Therefore, as shown in FIG. 2 above, by using a cut surface perpendicular to the casting direction as an object to be investigated, it is possible to investigate a portion where the horizontal crack is most deteriorated.
図2のスラブ切断面で、スラブ幅Wの両端からそれぞれスラブ厚さD/2までの領域R1、R2に存在する水平割れの最大開孔厚みt1、t2を測定する。ここで、最大開孔厚みt1は領域R1における最大開孔厚みであり、最大開孔厚みt2は領域R2における最大開孔厚みである。また図2において、前記領域R1とR2をあわせて第1の範囲、図2の領域R3を第2の範囲ということがある。 2, the maximum opening thicknesses t1 and t2 of horizontal cracks existing in the regions R1 and R2 from the both ends of the slab width W to the slab thickness D / 2 are measured. Here, the maximum aperture thickness t1 is the maximum aperture thickness in the region R1, and the maximum aperture thickness t2 is the maximum aperture thickness in the region R2. In FIG. 2, the regions R1 and R2 may be collectively referred to as a first range, and the region R3 in FIG. 2 may be referred to as a second range.
上記領域R1、R2を調査する理由は次の通りである。即ち、水平割れは、凝固がスラブの幅方向両端(狭面)から幅中央に向かって進行する過程で発生する。凝固時、領域R1、R2、即ち、第1の範囲では、狭面側(短辺側)の冷却の影響を受け、凝固が幅方向中央に向かって進行する。一方、幅方向両端からD/2分を除いた幅W−Dの領域R3、即ち第2の範囲では、狭面側(短辺側)の冷却の影響を殆ど受けないため、凝固が幅方向に殆ど進行しない。したがって、水平割れは領域R1、R2で発生すると考えられるため、本発明では上記の通り、領域R1、R2で水平割れを調査する。 The reason for investigating the regions R1 and R2 is as follows. That is, horizontal cracking occurs in the process in which solidification proceeds from both ends (narrow surfaces) in the width direction of the slab toward the center of the width. During solidification, the regions R1, R2, that is, the first range, are affected by cooling on the narrow surface side (short side), and solidification proceeds toward the center in the width direction. On the other hand, in the region R3 having a width WD excluding D / 2 from both ends in the width direction, that is, in the second range, since it is hardly affected by cooling on the narrow surface side (short side), solidification is performed in the width direction. Hardly progress. Accordingly, since horizontal cracks are considered to occur in the regions R1 and R2, in the present invention, as described above, horizontal cracks are investigated in the regions R1 and R2.
ここで、領域R1、R2のそれぞれに、2つ以上の水平割れが存在する場合、各領域R1、R2に存在する複数の開孔の厚みのうちの最大の開孔厚みを、最大開孔厚みt1、t2とする。例えば、領域R1に3つの水平割れが存在する場合、3つの水平割れのうち最も大きな開孔を有する水平割れを選択し、その水平割れの最も開孔している部分、即ち開孔厚みが最も厚い部分の、開孔厚みを「最大開孔厚みt1」とする。 Here, when there are two or more horizontal cracks in each of the regions R1 and R2, the maximum opening thickness among the plurality of opening thicknesses existing in each region R1 and R2 is set to the maximum opening thickness. Let t1 and t2. For example, when there are three horizontal cracks in the region R1, the horizontal crack having the largest opening among the three horizontal cracks is selected, and the most open portion of the horizontal crack, that is, the hole thickness is the largest. The opening thickness of the thick part is defined as “maximum opening thickness t1”.
次に、スラブの耐HIC性評価に用いる閾値tθ、即ち、スラブを圧延して得た鋼板にHICが発生しない最大開孔厚みの求め方について説明する。 Next, the threshold value tθ used for evaluating the slab's HIC resistance, that is, how to obtain the maximum hole thickness at which no HIC occurs in the steel sheet obtained by rolling the slab will be described.
上記閾値tθは、予め求めておくが、その方法は特に制限されない。閾値tθを求める方法として、予め、下記(i)〜(iii)の方法で求めることが挙げられる。以下、詳細について述べる。
(i)前記スラブの最大開孔厚みを測定する。
(ii)前記スラブと同一の鋳造条件で鋳造したスラブを圧延して得られる鋼板に対し、HIC試験を行う。
(iii)上記(i)で測定した最大開孔厚みと、上記(ii)のHIC試験結果とから、水素誘起割れの発生しない水平割れの最大開孔厚みを求める。
The threshold value tθ is obtained in advance, but the method is not particularly limited. As a method for obtaining the threshold value tθ, the following methods (i) to (iii) may be used in advance. Details will be described below.
(I) The maximum hole thickness of the slab is measured.
(Ii) An HIC test is performed on a steel plate obtained by rolling a slab cast under the same casting conditions as the slab.
(Iii) From the maximum opening thickness measured in (i) above and the HIC test result in (ii) above, the maximum opening thickness of horizontal cracks in which hydrogen-induced cracking does not occur is determined.
上記最大開孔厚みを測定したスラブと同一の鋳造条件で鋳造したスラブを熱間圧延し、閾値測定用の鋼板を製造する。そして鋼板に対してHIC試験を行い、HIC発生の有無を調べる。HIC試験は、後述する実施例に示す通り、NACE(National Association of Corrosion and Engineer) standard TM0284−2003に規定された方法で行うことが挙げられる。 A slab cast under the same casting conditions as the slab whose maximum opening thickness is measured is hot-rolled to produce a steel sheet for threshold measurement. Then, an HIC test is performed on the steel sheet to examine whether or not HIC is generated. The HIC test may be performed by a method defined in NACE (National Association of Corrosion and Engineer) standard TM0284-2003, as shown in the examples described later.
上記「同一の鋳造条件」とは、i)鋳造速度が一定であること、ii)ノズル詰まり等の操業異常が発生していないこと、iii)冷却条件やロール隙間が同じであること等である。閾値tθを決定する際、「スラブを調査して得た偏析度」と「製品に対するHIC試験結果」とを対応させるが、これらの耐HIC性が異なると閾値を決定することができない。i)〜iii)の操業因子は、水平割れ及び中心偏析に大きな影響を与える結果、耐HIC性にも影響する。したがって、操業因子が異なれば耐HIC性も変わる。よって、HIC試験用の鋼板には、最大開孔厚みを調査したスラブと同一の鋳造条件(操業因子)で鋳造したスラブを用い、製造して得られた鋼板を用いることが好ましい。特には、最大開孔厚みを調査したスラブと、HIC試験用のスラブとが同一であることが好ましい。 The above "same casting conditions" are i) that casting speed is constant, ii) that there is no operational abnormality such as nozzle clogging, iii) that cooling conditions and roll gap are the same, etc. . When the threshold value tθ is determined, the “degree of segregation obtained by investigating the slab” and the “HIC test result for the product” are associated with each other. However, if these HIC resistances are different, the threshold value cannot be determined. The operating factors i) to iii) have a great influence on horizontal cracking and center segregation, and as a result, HIC resistance. Accordingly, the resistance to HIC varies with the operation factor. Therefore, it is preferable to use the steel plate obtained by manufacturing using the slab cast on the same casting conditions (operation factor) as the slab which investigated the maximum opening thickness for the steel plate for HIC tests. In particular, it is preferable that the slab whose maximum opening thickness is investigated and the slab for the HIC test are the same.
前記HIC試験では、前記図2に示すスラブの領域R1、R2に対応する、製品(鋼板)の領域でHICが発生しているかを調べる。図2に示すスラブを用いた圧延時の、圧延方向によって、耐HIC性評価対象の領域は、図3に示す通り異なる。 In the HIC test, it is examined whether HIC is generated in the product (steel plate) region corresponding to the slab regions R1 and R2 shown in FIG. Depending on the rolling direction during rolling using the slab shown in FIG. 2, the HIC resistance evaluation target area varies as shown in FIG. 3.
スラブを鋳造方向に圧延した場合、即ち、圧延方向が鋳造方向である場合、図3(a)に示すように、圧延前後で幅は変化しないため、スラブ幅W=製品幅Wである。この場合、図3(a)に示す通り「スラブの領域R1、R2」に対応する製品の領域は「製品の幅方向両端から製品幅D/2の範囲の領域R11、R12」であり、「スラブの領域R3」に対応する製品の領域は「製品の幅方向両端から製品幅D/2分を除く幅W−Dの範囲の領域R13」である。 When the slab is rolled in the casting direction, that is, when the rolling direction is the casting direction, as shown in FIG. 3A, the width does not change before and after rolling, so the slab width W = the product width W. In this case, as shown in FIG. 3A, the product regions corresponding to the “slab regions R1, R2” are “regions R11, R12 in the range of the product width D / 2 from both ends in the product width direction”. The product region corresponding to “slab region R3” is “region R13 in the range of width WD excluding product width D / 2 from both ends in the product width direction”.
一方、スラブを幅方向に圧延した場合、即ち、圧延方向に幅方向が含まれる場合、図3(b)に示すように、幅が圧延前W→圧延後Waに変化するため、スラブ幅W<製品幅Waとなる。この場合、図3(b)に示す通り、スラブの領域R1、R2、R3に対応する領域R21、R22、R23は、圧延比、即ち、製品幅Wa/スラブ幅Wによって決まる。これらのうち領域R21、R22でHICが発生したかを確認する。 On the other hand, when the slab is rolled in the width direction, that is, when the width direction is included in the rolling direction, the width changes from W before rolling to Wa after rolling as shown in FIG. <Product width Wa. In this case, as shown in FIG. 3B, the regions R21, R22, and R23 corresponding to the slab regions R1, R2, and R3 are determined by the rolling ratio, that is, the product width Wa / slab width W. Of these, it is confirmed whether or not HIC has occurred in the regions R21 and R22.
そして『スラブの調査で得た「最大開孔厚みt1、t2」』と『製品に対するHIC試験結果』とから、HICが発生しない「最大開孔厚みの閾値tθ」を決定する。 Then, from the “maximum opening thickness t1, t2” obtained in the slab investigation and the “HIC test result for the product”, the “maximum opening thickness threshold tθ” at which no HIC is generated is determined.
閾値tθを決定するとき、スラブと製品とで互いに対応する領域で得られた結果を対応させる。例えば、
(I)スラブを図3(a)の通り鋳造方向に圧延した場合、HIC試験で、製品領域R11では「HIC発生有」、領域R12では「HIC発生無」であるとき、以下の通り判断する。
(I−1)製品領域R11の結果として、スラブ領域R1の最大開孔厚みt1のときに「HIC発生有」
(I−2)製品領域R12の結果として、スラブ領域R2の最大開孔厚みt2のときに「HIC発生無」
When the threshold value tθ is determined, the results obtained in the areas corresponding to each other are associated with each other between the slab and the product. For example,
(I) When the slab is rolled in the casting direction as shown in FIG. 3A, it is determined in the HIC test that the product region R11 has “HIC generated” and the region R12 has “HIC not generated” as follows. .
(I-1) As a result of the product region R11, “HIC is present” when the maximum opening thickness t1 of the slab region R1
(I-2) As a result of the product region R12, when the maximum opening thickness t2 of the slab region R2 is “HIC not generated”
(II)スラブを図3(b)の通り幅方向に圧延した場合、HIC試験で、製品領域R21では「HIC発生有り」、領域R22では「HIC発生無し」であるとき、以下の通り判断する。
(II−1)製品領域R21の結果として、スラブ領域R1の最大開孔厚みt1のときに「HIC発生有」
(II−2)製品領域R22の結果として、スラブ領域R2の最大開孔厚みt2のときに「HIC発生無」
(II) When the slab is rolled in the width direction as shown in FIG. 3B, it is determined in the HIC test that the product region R21 is “HIC generated” and the region R22 is “HIC not generated” as follows. .
(II-1) As a result of the product region R21, “HIC is present” when the maximum opening thickness t1 of the slab region R1
(II-2) As a result of the product region R22, when the maximum opening thickness t2 of the slab region R2 is “No HIC generation”
上記の複数の結果から、HIC発生有無の境界となる最大開孔厚みの閾値tθを決定する。具体的に例えば、上記(I)の場合、最大開孔厚みt2が閾値tθとなる。また上記(II)の場合も、最大開孔厚みt2が閾値tθとなる。 From the plurality of results, the threshold value tθ of the maximum hole thickness that becomes the boundary of whether or not HIC is generated is determined. Specifically, for example, in the case of (I), the maximum hole thickness t2 is the threshold value tθ. Also in the case of (II) above, the maximum aperture thickness t2 is the threshold value tθ.
また、閾値tθの決定には、複数のスラブの水平割れ・最大開孔厚みの測定結果とHIC試験結果を用いることが好ましい。複数のスラブの水平割れ・最大開孔厚みの測定結果とHIC試験結果を用いることによって、より正確な閾値tθを得ることができ、HIC発生有無の誤判定を減らすことができる。 In addition, for the determination of the threshold value tθ, it is preferable to use the measurement results of the horizontal crack / maximum opening thickness of the plurality of slabs and the HIC test results. By using the measurement results of the horizontal cracking / maximum opening thickness of a plurality of slabs and the HIC test results, it is possible to obtain a more accurate threshold tθ and reduce the erroneous determination of the presence or absence of HIC.
偏析部や耐HIC性の調査は、スラブや製品の1断面から評価してもよく、2断面以上から評価してもよい。以下に、同一チャージのスラブの複数断面を調査した結果を図4に示す。図4において、例1は同一チャージの2断面を調査した例であり、例2は同一チャージの3断面を調査した例であり、いずれもAPIX65グレードに充当可能なスラブで調査を実施した結果である。 The investigation of segregation and HIC resistance may be evaluated from one section of a slab or product, or may be evaluated from two or more sections. FIG. 4 shows the result of examining a plurality of cross sections of the slab having the same charge. In FIG. 4, Example 1 is an example in which two cross sections of the same charge are examined, Example 2 is an example in which three cross sections of the same charge are examined, and both are the results of conducting an investigation with a slab applicable to the APIX65 grade. is there.
上記図4に示すように、例1では、2断面のいずれも最大開孔厚みが0mmであり、かつHIC試験では水平割れ部を起点にHICが発生しなかった。また例2では、3断面それぞれの最大開孔厚みが0.065mm、0.067mm、0.066mmであり、同様な厚みであった。また、全ての断面で、水平割れ部を起点にHICが発生した。 As shown in FIG. 4 described above, in Example 1, the maximum opening thickness was 0 mm in both of the two cross sections, and no HIC occurred from the horizontal crack in the HIC test. In Example 2, the maximum hole thickness of each of the three cross sections was 0.065 mm, 0.067 mm, and 0.066 mm, which were similar thicknesses. In all the cross sections, HIC occurred starting from the horizontal crack.
このように、同一チャージでは、断面が異なっても略同じ結果が得られた。また、50チャージを各チャージにつき1断面ずつ調査した場合も、各チャージ間で略同じ結果が得られ、誤判定がなく、正確な評価ができることを別途確認している。 As described above, in the same charge, substantially the same result was obtained even if the cross sections were different. Further, when 50 charges are examined one section at a time for each charge, it is separately confirmed that substantially the same result is obtained between the charges, there is no erroneous determination, and accurate evaluation can be performed.
上記図4の例ではAPIX65グレードに充当可能なスラブを用いて実施したが、強度グレードが変わり、例えばAPIX70グレード以上でも、内部割れの形成やバラツキは変わらないため、調査断面数は限定されない。 In the example of FIG. 4 described above, the slab applicable to the APIX65 grade was used. However, the strength grade changes. For example, even if the APIX70 grade or higher, the formation and variation of internal cracks do not change, so the number of cross sections to be investigated is not limited.
スラブの調査位置(調査面)は、下記実施例に示す通り定常部が好ましいが、非定常部でもよい。「非定常部」とは、鋳造条件の変化時に鋳造された部分であり、鋳造速度の上昇時といった鋳造初期や、鋳造速度の下降時といった鋳造末期に鋳造された部分等が挙げられる。非定常部で調査する場合、図5に示すように、HIC試験を実施する部位に隣接する部分を調査することが好ましい。このような部分はHIC試験結果と同様な耐HIC性を示すため、より正確な評価を行うことができる。 The investigation position (survey surface) of the slab is preferably a stationary part as shown in the following examples, but may be an unsteady part. The “unsteady portion” is a portion cast when a casting condition is changed, and includes a portion cast at an early stage of casting such as when the casting speed is increased, or a portion cast at the end of casting such as when the casting speed is decreased. When investigating in the unsteady part, it is preferable to investigate the part adjacent to the site where the HIC test is performed, as shown in FIG. Since such a part shows the same HIC resistance as the HIC test result, more accurate evaluation can be performed.
本発明の鋼板は、上記の通り、その圧延前の状態であるスラブの段階において、水平割れが存在しない、または、水平割れの最大開孔厚みが閾値tθ以下の鋼板である。この様に、スラブ切断面の前記領域R1、R2に水平割れが存在しない場合、水平割れ部の偏析度が低いため、水平割れ起因のHICが発生しない。また、スラブ切断面の前記領域R1、R2の水平割れの最大開孔厚みが閾値tθ以下の場合も、水平割れ部の偏析度が低いため、水平割れが原因のHICが発生しない。 As described above, the steel plate of the present invention is a steel plate in which no horizontal cracks are present or the maximum opening thickness of the horizontal cracks is equal to or less than the threshold value tθ at the stage of the slab before rolling. As described above, when there is no horizontal crack in the regions R1 and R2 of the slab cut surface, the segregation degree of the horizontal crack portion is low, so that HIC due to the horizontal crack does not occur. In addition, even when the maximum opening thickness of the horizontal cracks in the regions R1 and R2 on the slab cut surface is equal to or less than the threshold value tθ, the segregation degree of the horizontal cracks is low, so that HIC due to horizontal cracks does not occur.
また本発明によれば、耐HIC性の評価に「水平割れの最大開孔厚み」を用いている。これから鋳片の内部品質を正確に評価できるため、この評価結果を基に鋳片の段階で耐HIC性を評価できる。これにより、数週間を要するHIC試験を省略できるため、製造から出荷までの期間を大幅に短縮することができる。 Further, according to the present invention, the “maximum opening thickness of horizontal cracks” is used for evaluating the HIC resistance. Since the internal quality of the slab can be accurately evaluated from this, the HIC resistance can be evaluated at the stage of the slab based on this evaluation result. Thereby, since the HIC test which requires several weeks can be omitted, the period from manufacture to shipment can be greatly shortened.
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1−1、表1−2、図6および図7には、閾値tθを決定するための実験条件および実験結果を示す。APIX65グレード相当およびAPIX70グレード相当のスラブをそれぞれ21チャージずつ、ASMESA516グレード60相当、ASME SA516グレード65相当、およびASME SA516グレード70相当のスラブを、それぞれ1チャージずつ鋳造し、下記の通り水平割れを調査した。尚、前記の表1−1、表1−2、および後記の表3において、「X70」はAPI X70グレード、「X65」はAPI X65グレード、「SA516 60」はASME SA516グレード60、「SA516 65」はASME SA516グレード65、「SA516 70」はASME SA516グレード70を示す。 Table 1-1, Table 1-2, FIG. 6, and FIG. 7 show experimental conditions and experimental results for determining the threshold value tθ. Cast slabs equivalent to APIX65 grade and APIX70 grade each with 21 charges, slabs equivalent to ASMESA516 grade 60, ASME SA516 grade 65, and ASME SA516 grade 70, respectively, and investigate horizontal cracks as shown below. did. In Table 1-1, Table 1-2, and Table 3 below, “X70” is API X70 grade, “X65” is API X65 grade, “SA516 60” is ASME SA516 grade 60, “SA516 65”. "Indicates ASME SA516 grade 65 and" SA516 70 "indicates ASME SA516 grade 70.
ここで、表1−1および表1−2に示す条件を説明する。
<タンディッシュ内溶鋼の成分>
C、Mn、Nb、P、Caの濃度を発光分光分析法によって測定した。S濃度は低いため、発光分光分析法による測定が困難であった。そこで、S濃度の測定に燃焼−赤外線吸収法を用いた。
<鋳造条件>
・比水量
比水量=(鋳型直下から連鋳機最終ロールまでの単位時間当たりの全二次冷却水量[L/min.])/(単位時間当たりの鋳造鋳片質量[kg/min.])
・鋳造速度
鋳片の引き抜き速度[m/min.]であり、鋳片に接触するロール(メジャーロール)の直径(周長)と回転速度(単位時間当たりの回転数)から算出した。
Here, the conditions shown in Table 1-1 and Table 1-2 will be described.
<Components of molten steel in tundish>
The concentrations of C, Mn, Nb, P, and Ca were measured by emission spectroscopy. Since the S concentration was low, it was difficult to measure by emission spectroscopic analysis. Therefore, the combustion-infrared absorption method was used for measuring the S concentration.
<Casting conditions>
Specific water amount Specific water amount = (total secondary cooling water amount per unit time from directly under the mold to the final roll of the continuous casting machine [L / min.]) / (Cast slab mass per unit time [kg / min.])
・ Casting speed Pulling speed of slab [m / min. It was calculated from the diameter (peripheral length) of the roll (major roll) in contact with the slab and the rotation speed (number of rotations per unit time).
(鋳造)
本発明で規定の成分組成の範囲内であって、タンディッシュ内溶鋼の成分組成が表1−1および表1−2に示す通りである鋼を溶製し、連続鋳造により、厚みが280mmである鋳片、即ちスラブを得た。
(casting)
In the present invention, a steel having a component composition within the range defined by the present invention and having the composition of the molten steel in the tundish as shown in Table 1-1 and Table 1-2 is melted, and the thickness is 280 mm by continuous casting. A slab or slab was obtained.
(水平割れの調査)
スラブを全長が10〜15mの位置であって定常部で切断し、下記の通り水平割れを調査した。ここで、「定常部」とは下記の条件を満たす部位である。水平割れ調査断面数は表1−1、表1−2に示す通りである。
1)鋳造速度が一定である。
2)浸漬ノズル詰まり等の操業異常が発生していない。
3)冷却条件が変化していない。
4)ロール隙間が変化していない。
(Investigation of horizontal cracking)
The slab was cut at the steady portion at a length of 10 to 15 m, and horizontal cracks were examined as follows. Here, the “stationary part” is a part that satisfies the following conditions. The number of cross sections for the horizontal crack investigation is as shown in Table 1-1 and Table 1-2.
1) The casting speed is constant.
2) No abnormal operation such as clogged immersion nozzle has occurred.
3) Cooling conditions have not changed.
4) The roll gap has not changed.
水平割れの調査手順
(1)スラブ切断面の幅方向両端からD/2の範囲を#800まで研磨した。
(2)研磨面を、ピクリン酸20g/L、塩化第二銅5g/L及び表面活性剤60ml/Lで腐食した。
(3)腐食面を目視で確認し、水平割れが存在する部分を40mm×70mmの大きさに切り出した。
(4)切り出した試料をバフ研磨し、1μm以下の粗さに仕上げた。
(5)EPMA(Electron Probe MicroAnalyser)を用いてビーム径20μmで試料中の水平割れ部のMn偏析度をライン分析した。この水平割れ部のMn偏析度をCmax(Mn)で示す。
(6)鋳造時に測定したタンディッシュ内溶鋼のMn濃度、即ちC0(Mn)と、前記Cmax(Mn)から、Cmax(Mn)/C0(Mn)を算出した。
(7)EPMA分析を実施した部分の水平割れを顕微鏡(20倍〜50倍)で観察し、開孔厚みを測定した。
Investigation procedure for horizontal crack (1) The range of D / 2 from both ends in the width direction of the slab cut surface was polished to # 800.
(2) The polished surface was corroded with 20 g / L picric acid, 5 g / L cupric chloride and 60 ml / L surfactant.
(3) The corroded surface was visually confirmed, and a portion where a horizontal crack was present was cut out to a size of 40 mm × 70 mm.
(4) The cut sample was buffed and finished to a roughness of 1 μm or less.
(5) Line analysis was performed for the degree of Mn segregation of horizontal cracks in the sample with a beam diameter of 20 μm using EPMA (Electron Probe MicroAnalyzer). The Mn segregation degree of this horizontal crack portion is indicated by Cmax (Mn).
(6) Cmax (Mn) / C 0 (Mn) was calculated from the Mn concentration of the molten steel in the tundish measured during casting, that is, C 0 (Mn) and the Cmax (Mn).
(7) The horizontal crack of the part which performed the EPMA analysis was observed with the microscope (20 times-50 times), and the hole thickness was measured.
(圧延)
その後、APIX65グレード相当およびAPIX70グレード相当のスラブを、1050〜1250℃となるよう加熱した後、鋼板の表面温度で900℃以上、下記の通り計算により求められる鋼板平均温度が1000℃以上の累積圧下率が40%以上であり、かつ1パス当りの圧下率が10%以上であるパスが2パス以上になるよう熱間圧延を行う。その後さらに、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℃の温度で停止し、更にその後、室温まで空冷して、板厚45mmの鋼板を得た。また、ASME SA516グレード60相当、ASME SA516グレード65相当およびASME SA516グレード70相当のスラブを、圧延終了温度が850℃以上になるように熱間圧延した後、室温まで空冷し、そして更に、850℃以上950℃以下の温度に再加熱して焼入れした後、600〜700℃で焼き戻し処理を行って、板厚40mmの鋼板を得た。なお、いずれもスラブ幅方向に圧延を実施しなかった。
(rolling)
Then, after heating the slabs corresponding to APIX65 grade and APIX70 grade to 1050 to 1250 ° C, the surface temperature of the steel plate is 900 ° C or higher, and the average steel plate temperature calculated by the following calculation is 1000 ° C or higher. Hot rolling is performed so that a pass having a rate of 40% or more and a reduction rate per pass of 10% or more becomes 2 passes or more. Thereafter, hot rolling was performed so that the cumulative rolling reduction at 700 ° C. or more and less than 900 ° C. was 20% or more, so that the rolling end temperature was 700 ° C. or more and less than 900 ° C. Then, water cooling was started from the temperature of 650 degreeC or more, it stopped at the temperature of 350-600 degreeC, and it further air-cooled to room temperature, and obtained the steel plate with a plate thickness of 45 mm. Also, slabs equivalent to ASME SA516 grade 60, ASME SA516 grade 65 and ASME SA516 grade 70 were hot-rolled so that the rolling end temperature was 850 ° C. or higher, then air-cooled to room temperature, and further 850 ° C. After reheating to 950 ° C. or lower and quenching, a tempering treatment was performed at 600 to 700 ° C. to obtain a steel plate having a plate thickness of 40 mm. In all cases, rolling was not performed in the slab width direction.
上記鋼板平均温度は、次の様にして求められる。即ち、圧延中の圧延パススケジュールやパス間の冷却方法(水冷あるいは空冷)などのデータに基づいて、板厚方向の任意の位置における温度を差分法など計算に適した方法を用いて計算し、求められた鋼片の表面から裏面までの温度の平均値を鋼板平均温度とする。鋼板平均温度について以下同じである。 The steel sheet average temperature is determined as follows. That is, based on data such as a rolling pass schedule during rolling and a cooling method (water cooling or air cooling) between passes, the temperature at an arbitrary position in the plate thickness direction is calculated using a method suitable for calculation such as a difference method, Let the average value of the temperature from the surface of the calculated | required steel piece to the back surface be steel plate average temperature. The same applies to the average temperature of the steel sheet.
(HIC試験)
閾値tθ決定のために、本実施例では圧延後にHIC試験を行った。
(a)圧延後の製品からサンプルを切り出し、HIC試験を実施した。HIC試験はNACE standard TM0284−2003に規定された方法に従って実施した。
(b)HIC試験後、サンプルを3箇所で切断し、各断面(3断面)を顕微鏡で観察し、HICの有無を確認した。ここで図3(a)に示した「製品の幅方向両端からD/2の範囲の領域R11、R12」で割れの有無を確認した。
(HIC test)
In order to determine the threshold value tθ, in this example, an HIC test was performed after rolling.
(A) A sample was cut out from the rolled product and an HIC test was performed. The HIC test was performed according to the method specified in NACE standard TM0284-2003.
(B) After the HIC test, the sample was cut at three locations, and each cross section (three cross sections) was observed with a microscope to confirm the presence or absence of HIC. Here, the presence or absence of cracks was confirmed in “regions R11 and R12 in the range of D / 2 from both ends in the width direction of the product” shown in FIG.
(最大開孔厚みの閾値tθの決定)
図6、7には『「水平割れ開孔厚み」及び「Cmax(Mn)/C0(Mn)」』と、前記HIC試験により確認した「HIC発生の有無」との関係を示している。図6は、表1−2に示す強度クラスがAPIX65グレード相当、ASME SA516グレード60相当およびASME SA516グレード65相当の成分でHICが発生する閾値tθを調査した結果であり、図7は、表1−1および表1−2に示す強度クラスがAPIX70グレード相当およびASME SA516グレード70相当の成分でHICが発生する閾値tθを調査した結果である。
(Determination of threshold tθ for maximum hole thickness)
FIGS. 6 and 7 show the relationship between ““ horizontal crack opening thickness ”and“ Cmax (Mn) / C 0 (Mn) ”” and “presence or absence of HIC generation” confirmed by the HIC test. FIG. 6 is a result of investigating a threshold value tθ at which HIC occurs in components having strength classes shown in Table 1-2 equivalent to APIX65 grade, ASME SA516 grade 60 and ASME SA516 grade 65. FIG. -1 and Table 1-2 are the results of investigating the threshold value tθ at which HIC occurs in components corresponding to APIX70 grade and ASME SA516 grade70.
図6から、APIX65グレードに充当可能なスラブでは、最大開孔厚み≦0.047mmのときHICが発生しなかったが、最大開孔厚み>0.047mmのときHICが発生することがあった。そこで、APIX65グレードに充当可能なスラブでは、最大開孔厚みの閾値tθを0.047mmとし、下記の通り判断した。
最大開孔厚み≦0.047mmのとき、HICが発生しないと判断する。
最大開孔厚み>0.047mmのとき、HICが発生すると判断する。
From FIG. 6, in the slab applicable to the APIX65 grade, HIC did not occur when the maximum hole thickness ≦ 0.047 mm, but HIC sometimes occurred when the maximum hole thickness> 0.047 mm. Therefore, in the slab applicable to the APIX65 grade, the threshold tθ of the maximum opening thickness was set to 0.047 mm, and the determination was made as follows.
When the maximum aperture thickness ≦ 0.047 mm, it is determined that no HIC occurs.
It is determined that HIC occurs when the maximum hole thickness> 0.047 mm.
また、ASMESA516グレード60、グレード65、および、ASTM A516グレード60、グレード65は、APIX65グレード相当の成分であるため、最大開孔厚みの閾値tθを0.047mmとし、下記の通り判断した。
最大開孔厚み≦0.047mmのとき、HICが発生しないと判断する。
最大開孔厚み>0.047mmのとき、HICが発生すると判断する。
In addition, ASMESA516 grade 60, grade 65, and ASTM A516 grade 60, grade 65 are components equivalent to APIX65 grade, and therefore the threshold tθ of the maximum opening thickness was set to 0.047 mm, and the following judgment was made.
When the maximum aperture thickness ≦ 0.047 mm, it is determined that no HIC occurs.
It is determined that HIC occurs when the maximum hole thickness> 0.047 mm.
一方、図7から、APIX70グレードに充当可能なスラブでは、最大開孔厚み≦0.043mmのときHICが発生しなかったが、最大開孔厚み>0.043mmのときHICが発生することがあった。そこで、APIX70グレードに充当可能なスラブでは、最大開孔厚みの閾値tθを0.043mmとし、下記の通り判断した。
最大開孔厚み≦0.043mmのとき、HICが発生しないと判断する。
最大開孔厚み>0.043mmのとき、HICが発生すると判断する。
On the other hand, from FIG. 7, in the slab applicable to the APIX 70 grade, HIC did not occur when the maximum hole thickness ≦ 0.043 mm, but HIC sometimes occurred when the maximum hole thickness> 0.043 mm. It was. Therefore, in the slab applicable to the APIX70 grade, the threshold tθ of the maximum opening thickness was set to 0.043 mm, and the determination was made as follows.
When the maximum hole thickness ≦ 0.043 mm, it is determined that no HIC occurs.
It is determined that HIC is generated when the maximum hole thickness> 0.043 mm.
また、ASMESA516グレード70、および、ASTM A516グレード70は、APIX70グレード相当の成分であるため、最大開孔厚みの閾値tθを0.043mmとし、下記の通り判断した。
最大開孔厚み≦0.043mmのとき、HICが発生しないと判断する。
最大開孔厚み>0.043mmのとき、HICが発生すると判断する。
Moreover, since ASMESA516 grade 70 and ASTM A516 grade 70 are components equivalent to APIX70 grade, the threshold tθ of the maximum opening thickness was set to 0.043 mm, and the determination was made as follows.
When the maximum hole thickness ≦ 0.043 mm, it is determined that no HIC occurs.
It is determined that HIC is generated when the maximum hole thickness> 0.043 mm.
なお、図6、7ではいずれも、開孔していない、即ち最大開孔厚み=0mmの水平割れではHICが発生しなかった。 In FIGS. 6 and 7, no HIC occurred in horizontal cracks where no holes were formed, that is, the maximum hole thickness = 0 mm.
(判定対象のスラブの耐HIC性評価)
判定対象のスラブの耐HIC性を、上記閾値tθを用い、下記の手順で評価した。まず表2に示す成分組成の鋼を溶製し、連続鋳造により、スラブ厚Dが280mmであってスラブ幅Wが2100mmである判定対象のスラブを得た。そしてこのスラブを用いて、下記の手順で評価した。
(1)判定対象のスラブ切断面の幅方向両端から幅D/2の範囲をフライス加工し、染色浸透探傷試験(JIS Z2343)を実施した。
(2)水平割れが検出されなかった場合、最大開孔厚みが検出下限以下(10μm程度以下)と判断した。この場合、最大開孔厚みは、閾値tθ以下、即ちAPIX65グレードでは0.047mm以下、APIX70グレードでは0.043mm以下であるため、水平割れ起因のHICが発生しないと判断した。
(3)水平割れが検出された場合、開孔していた部位をバフ研磨し、研磨面を20倍〜50倍の顕微鏡で観察して最大開孔厚みを、上述した通り測定した。
(3−1)そして、上記「最大開孔厚みの閾値tθの決定」で示した通り、APIX65グレードに充当可能なスラブでは、上記最大開孔厚みが閾値tθ:0.047mm以下のとき、水平割れ起因のHICが発生しない、即ちスラブの耐HIC性評価がOKであり、得られた鋼板は耐HIC性に優れていると判断した。一方、上記最大開孔厚みが閾値tθ:0.047mmを超えたとき、水平割れ起因のHICが発生する、即ちスラブの耐HIC性評価がNGであり、得られた鋼板は耐HIC性に劣ると判断した。
(3−2)APIX70グレードに充当可能なスラブでは、上記最大開孔厚みが閾値tθ:0.043mm以下のとき、水平割れ起因のHICが発生しない、即ちスラブの耐HIC性評価がOKであり、得られた鋼板は耐HIC性に優れていると判断した。一方、上記最大開孔厚みが閾値tθ:0.043mmを超えたとき、水平割れ起因のHICが発生する、即ちスラブの耐HIC性評価がNGであり、得られた鋼板は耐HIC性に劣ると判断した。
(HIC resistance evaluation of the slab to be judged)
The HIC resistance of the judgment target slab was evaluated by the following procedure using the threshold value tθ. First, steel having the component composition shown in Table 2 was melted, and a slab to be determined having a slab thickness D of 280 mm and a slab width W of 2100 mm was obtained by continuous casting. And it evaluated in the following procedure using this slab.
(1) A range of width D / 2 from both ends in the width direction of the slab cut surface to be judged was milled, and a dye penetration test (JIS Z2343) was performed.
(2) When a horizontal crack was not detected, it was judged that the maximum hole thickness was not more than a detection lower limit (about 10 μm or less). In this case, the maximum aperture thickness was not more than the threshold value tθ, that is, 0.047 mm or less for the APIX65 grade and 0.043 mm or less for the APIX70 grade, so it was determined that no HIC due to horizontal cracking occurred.
(3) When a horizontal crack was detected, the portion that had been opened was buffed, and the polished surface was observed with a 20 to 50 times microscope, and the maximum opening thickness was measured as described above.
(3-1) And, as shown in the above “determination of the maximum opening thickness threshold tθ”, in the slab applicable to the APIX65 grade, when the maximum opening thickness is the threshold tθ: 0.047 mm or less, the horizontal It was judged that no HIC due to cracking occurred, that is, the slab HIC resistance evaluation was OK, and the obtained steel sheet was excellent in HIC resistance. On the other hand, when the maximum hole thickness exceeds the threshold value tθ: 0.047 mm, HIC due to horizontal cracking occurs, that is, the slab has a HI resistance evaluation of NG, and the obtained steel sheet is inferior in HIC resistance. It was judged.
(3-2) In the slab applicable to the APIX70 grade, when the maximum hole thickness is the threshold value tθ: 0.043 mm or less, HIC due to horizontal cracking does not occur, that is, the slab has an HIC resistance evaluation of OK. The obtained steel sheet was judged to be excellent in HIC resistance. On the other hand, when the maximum hole thickness exceeds the threshold value tθ: 0.043 mm, HIC due to horizontal cracking occurs, that is, the slab has a HI resistance evaluation of NG, and the obtained steel sheet is inferior in HIC resistance. It was judged.
その後、上記スラブを、1050〜1250℃となるよう加熱した後、表3の「熱間圧延・冷却方法」の欄に「TMCP」または「QT」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(9〜90mm板厚×2000〜3500mm幅×12000〜35000mm長さ)を得た。前記「TMCP」は、鋼板の表面温度で900℃以上、計算により求められる鋼板平均温度が1000℃以上の累積圧下率が40%以上であって、かつ1パス当りの圧下率が10%以上であるパスが2パス以上になるよう熱間圧延を行った。その後さらに、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、圧延終了表面温度が850℃となるようにした後、冷却開始表面温度:950℃から平均冷却速度:10℃/sで冷却を開始し、350〜600℃の温度で停止し、更にその後、室温まで空冷する方法である。前記「QT」は、圧延終了温度が850℃以上になるように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した後、600〜700℃で焼き戻し処理を行う方法である。 Thereafter, the slab was heated to 1050 to 1250 ° C., and then “TMCP” or “QT” in the column of “Hot rolling / cooling method” in Table 3, two patterns of hot rolling / cooling. By the method, various steel compositions (9 to 90 mm plate thickness x 2000 to 3500 mm width x 12000 to 35000 mm length) were obtained. The “TMCP” is a steel sheet surface temperature of 900 ° C. or higher, an average steel plate temperature calculated by calculation of 1000 ° C. or higher, a cumulative rolling reduction of 40% or higher, and a rolling reduction per pass of 10% or higher. Hot rolling was performed so that a given pass was 2 passes or more. Thereafter, hot rolling is performed so that the cumulative rolling reduction of 700 ° C. or more and less than 900 ° C. becomes 20% or more, and the rolling end surface temperature is set to 850 ° C. Then, the cooling start surface temperature: average cooling from 950 ° C. Speed: Cooling is started at 10 ° C./s, stopped at a temperature of 350 to 600 ° C., and then cooled to room temperature. The “QT” is hot-rolled so that the rolling end temperature is 850 ° C. or higher, air-cooled to room temperature, reheated to a temperature of 850 ° C. or higher and 950 ° C. or lower, quenched, and then baked at 600 to 700 ° C. This is a method of performing a return process.
(HIC試験)
上記鋼板を用い、HIC試験を実施した。該HIC試験はNACE standard TM0284−2003に規定された方法に従って実施した。HIC試験後、サンプルを3箇所で切断し、各断面(3断面)を顕微鏡で観察し、HICの有無を確認した。その結果を表3に示す。
(HIC test)
The HIC test was implemented using the said steel plate. The HIC test was performed according to the method specified in NACE standard TM0284-2003. After the HIC test, the sample was cut at three locations, and each cross section (three cross sections) was observed with a microscope to confirm the presence or absence of HIC. The results are shown in Table 3.
表2および表3より次のことがわかる。No.1〜7、10、12および14〜17は、規定の成分組成を満たし、かつスラブの水平割れの最大開孔厚みが閾値tθ以下に抑えられており、耐HIC性に優れた本発明の鋼板である。 Table 2 and Table 3 show the following. No. 1 to 7, 10, 12, and 14 to 17 satisfy the prescribed composition, and the maximum open-hole thickness of horizontal cracks in the slab is suppressed to a threshold value tθ or less, and the steel plate of the present invention having excellent HIC resistance It is.
これに対し、No.11および13はスラブの水平割れの最大開孔厚みが閾値tθを超えているため、スラブの耐HIC性評価はNGであった。また圧延後に行うHIC試験では、鋼板に割れが生じ、耐HIC性に劣ることを確認した。No.8、9、18および19は、スラブの水平割れの最大開孔厚みは閾値tθ以下に抑えられているものの、鋼板の化学成分組成が本発明の規定を外れた例である。即ち、No.8の鋼板は、REMおよびZrが0%であり、かつ(Ca/S)の値が規定を外れており、No.9の鋼板は、REMおよびZrが0%であり、かつ(Ca−1.25S)/Oの値が規定を外れているため、いずれも耐HIC性に劣った。またNo.18は(Ca/S)の値が規定を外れており、No.19は(Ca−1.25S)/Oの値が規定を外れているため、いずれも耐HIC性に劣った。 In contrast, no. In Nos. 11 and 13, the maximum opening thickness of the horizontal crack of the slab exceeded the threshold value tθ, and thus the HIC resistance evaluation of the slab was NG. Further, in the HIC test performed after rolling, it was confirmed that the steel plate was cracked and inferior in HIC resistance. No. Nos. 8, 9, 18 and 19 are examples in which the chemical composition of the steel sheet deviates from the definition of the present invention, although the maximum hole thickness of the horizontal crack of the slab is suppressed to a threshold value tθ or less. That is, the steel plate of No. 8 has REM and Zr of 0%, and the value of (Ca / S) is out of regulation, the steel plate of No. 9 has REM and Zr of 0%, and Since the value of (Ca-1.25S) / O was not specified, all were inferior in HIC resistance. No. No. 18 has a value of (Ca / S) that is out of regulation. No. 19 was inferior in HIC resistance because the value of (Ca-1.25S) / O was out of specification.
スラブでの耐HIC性評価がOKであった例では、鋳造開始から製品である鋼板、即ち、耐サワー鋼板の出荷までの期間(鋳造→圧延→出荷)が19日であった。これに対し、圧延後に得られた鋼板を用いてHIC試験を行い、耐HIC性を評価した場合には、鋳造開始から出荷までの期間(鋳造→圧延→HIC試験→出荷)が28日と長期間を要した。本実施例では、前記圧延後のHIC試験を省略できたため、鋳造開始から出荷までの期間を28日→19日へ大幅に短縮できた。 In the example in which the HIC resistance evaluation on the slab was OK, the period (casting → rolling → shipment) from the start of casting to the shipment of the product steel plate, that is, the sour-resistant steel plate, was 19 days. On the other hand, when the HIC test is performed using the steel sheet obtained after rolling and the HIC resistance is evaluated, the period from casting start to shipping (casting → rolling → HIC test → shipping) is as long as 28 days. It took a period. In this example, since the HIC test after rolling could be omitted, the period from the start of casting to shipment could be greatly shortened from 28 days to 19 days.
また、スラブでの耐HIC性評価がNGであった例では、スラブの段階で再溶製を開始したところ、鋳造開始から製品である鋼板、即ち、耐サワー鋼板の出荷までの期間(鋳造→再溶製→圧延→出荷)は54日であった。これに対し、圧延後に得られた鋼板を用いてHIC試験を行い、製品の耐HIC性を評価した結果、評価がNGであった場合は、上記HIC試験を行った後に再溶製を開始したため、鋳造開始から製品である鋼板の出荷までの期間(鋳造→圧延→HIC試験→再溶製→圧延→HIC試験→出荷)が72日と長期間を要した。本実施例では、前記圧延後のHIC試験を省略できたため、再溶製が必要な場合であっても、鋳造開始から出荷までの期間を72日→54日へ大幅に短縮できた。 Further, in the case where the HIC resistance evaluation on the slab was NG, when remelting was started at the slab stage, the period from the start of casting to the shipment of the product steel plate, that is, the sour steel plate (casting → Remelting → rolling → shipping) was 54 days. On the other hand, as a result of performing the HIC test using the steel sheet obtained after rolling and evaluating the HIC resistance of the product, if the evaluation is NG, remelting was started after the HIC test was performed. The period from the start of casting to the shipment of the steel sheet as a product (casting → rolling → HIC test → remelting → rolling → HIC test → shipping) required 72 days. In this example, since the HIC test after rolling could be omitted, even when remelting was necessary, the period from the start of casting to shipment could be greatly shortened from 72 days to 54 days.
以上のように、本発明によると、圧延後のHIC試験を行うことなく、鋳片であるスラブの段階で耐HIC性を評価できたため、製造リードタイムを大幅に短縮できた。尚、本実施例では、スラブの耐HIC性評価用閾値tθ決定のためのHIC試験と、確認用のHIC試験とが同じであったため、本発明の判定方法は精度が高いといえる。
As described above, according to the present invention, since the HIC resistance could be evaluated at the stage of the slab, which is a slab, without performing the HIC test after rolling, the manufacturing lead time could be greatly shortened. In the present example, the determination method of the present invention is highly accurate because the HIC test for determining the threshold tθ for evaluating the HIC resistance of the slab and the HIC test for confirmation are the same.
Claims (16)
C:0.02〜0.15%、
Si:0.02〜0.50%、
Mn:0.6〜2.0%、
P:0%超0.030%以下、
S:0%超0.003%以下、
Al:0.010〜0.08%、
Ca:0.0003〜0.0060%、
N:0.001〜0.01%、および
O:0%超0.0045%以下を満たし、更に、
REM:0%超0.02%以下、および
Zr:0%超0.010%以下
よりなる群から選択される1種以上の元素を含み、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 ≦ 1.80を満たし、
更にスラブの段階で、水平割れが存在しない、または、水平割れの最大開孔厚みが閾値tθ以下であって、該閾値tθは、前記スラブを圧延して得た鋼板に水素誘起割れが発生しない水平割れの最大開孔厚みであることを特徴とする耐水素誘起割れ性に優れた鋼板。 % By mass
C: 0.02 to 0.15%,
Si: 0.02 to 0.50%,
Mn: 0.6 to 2.0%,
P: more than 0% and 0.030% or less,
S: more than 0% and 0.003% or less,
Al: 0.010 to 0.08%,
Ca: 0.0003 to 0.0060%,
N: 0.001 to 0.01%, and O: more than 0% and 0.0045% or less,
Including one or more elements selected from the group consisting of REM: more than 0% and 0.02% or less, and Zr: more than 0% and 0.010%, the balance consisting 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≦1.80,
Further, at the stage of the slab, there is no horizontal crack, or the maximum opening thickness of the horizontal crack is equal to or less than a threshold value tθ, and the threshold value tθ does not cause hydrogen-induced cracking in the steel sheet obtained by rolling the slab. A steel plate with excellent resistance to hydrogen-induced cracking, characterized by having the maximum hole thickness for horizontal cracking.
(i)前記スラブの最大開孔厚みを測定する。
(ii)前記スラブと同一の鋳造条件で鋳造したスラブを圧延して得られる鋼板に対して水素誘起割れ試験を行う。
(iii)上記(i)で測定した最大開孔厚みと、上記(ii)の水素誘起割れ試験結果とから、水素誘起割れの発生しない水平割れの最大開孔厚みを求める。 The steel sheet according to claim 1, wherein the threshold value tθ is a value obtained in advance by the following methods (i) to (iii).
(I) The maximum hole thickness of the slab is measured.
(Ii) A hydrogen-induced cracking test is performed on a steel plate obtained by rolling a slab cast under the same casting conditions as the slab.
(Iii) From the maximum hole thickness measured in (i) above and the hydrogen-induced crack test result in (ii) above, the maximum hole thickness of a horizontal crack in which no hydrogen-induced crack occurs is obtained.
B:0%超0.005%以下、
V:0%超0.1%以下、
Cu:0%超1.5%以下、
Ni:0%超1.5%以下、
Cr:0%超1.5%以下、
Mo:0%超1.5%以下、および
Nb:0%超0.06%以下
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1〜11のいずれかに記載の鋼板。 Furthermore, as other elements,
B: more than 0% and 0.005% or less,
V: more than 0% and 0.1% or less,
Cu: more than 0% and 1.5% or less,
Ni: more than 0% and 1.5% or less,
Cr: more than 0% and 1.5% or less,
The steel plate according to any one of claims 1 to 11, comprising one or more elements selected from the group consisting of Mo: more than 0% and 1.5% or less, and Nb: more than 0% and 0.06% or less.
Ti:0%超0.03%以下、および
Mg:0%超0.01%以下
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1〜12のいずれかに記載の鋼板。 Furthermore, as other elements,
The steel plate according to any one of claims 1 to 12, comprising one or more elements selected from the group consisting of Ti: more than 0% and 0.03% or less, and Mg: more than 0% and 0.01% or less.
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JP2016125139A (en) * | 2014-12-26 | 2016-07-11 | 株式会社神戸製鋼所 | Steel sheet and steel pipe for line pipe excellent in hydrogen-induced crack resistance |
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