JP5365146B2 - Steel plate with excellent toughness of heat affected zone - Google Patents
Steel plate with excellent toughness of heat affected zone Download PDFInfo
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Description
本発明は、大入熱溶接熱影響部の靭性に優れた鋼板に関し、特に５００ｋＪ／ｃｍを超える大入熱溶接を施した場合であっても、溶接熱影響部の靭性に優れる、引張強度が４９０ＭＰａ以上のものに関する。 The present invention relates to a steel sheet excellent in toughness of a high heat input weld heat-affected zone, and in particular, even when subjected to high heat input welding exceeding 500 kJ / cm, excellent tensile strength in the weld heat affected zone. It relates to 490 MPa or more.
建築構造物の一つであるボックス柱の組み立て溶接に適用されるサブマージアーク溶接やエレクトロスラグ溶接等では、施工高能率化のため、５００ｋJ／ｃｍを超える大入熱溶接が施されることがある。 In submerged arc welding and electroslag welding, which are applied to assembly welding of box columns, which is one of the building structures, large heat input welding exceeding 500 kJ / cm may be performed to improve construction efficiency. .
鋼材を溶接すると、溶接入熱の増大に伴い、溶接熱影響部組織（以下、ＨＡＺと記す）が粗大化して靭性が低下するため、従来より種々の靭性改善技術が提案されている。 When steel materials are welded, as the welding heat input increases, the weld heat affected zone structure (hereinafter referred to as HAZ) is coarsened and the toughness is lowered. Therefore, various toughness improving techniques have been proposed.
例えば、特許文献１は大入熱溶接熱影響部の靭性に優れた高張力鋼板に関し、鋼組成を極低Ｃ化して島状マルテンサイト（以下、ＭＡと記す）の生成を抑制し、焼入れ性向上元素であるＭｎ，ＮｉおよびＣｒの含有量を適正化してγ粒界でのフェライトの生成を押えて粒内における変態組織のブロックサイズを微細化し、ＨＡＺ靭性の劣化を抑制することが開示されている。
しかしながら、特許文献１記載の大入熱溶接熱影響部の靭性に優れた高張力鋼板は、ＨＡＺ靭性を改善するため成分組成においてＮｉ添加を必須とし、最近の高合金鋼やステンレス鋼の需要増加によるＮｉ価格の高騰で、建築など大量に使用される用途には適用しがたくなっている。 However, the high-tensile steel plate with excellent toughness of the high heat input welding heat-affected zone described in Patent Document 1 requires the addition of Ni in the component composition in order to improve the HAZ toughness, and the recent increase in demand for high alloy steel and stainless steel Due to soaring Ni prices, it is difficult to apply to large-scale uses such as architecture.
そこで、本発明は、Ｎｉ，Ｍｏ，及びＮｂなど高価な合金元素を用いず、比較的安価なＭｎやＣｒの添加により大入熱溶接のＨＡＺ靭性に優れる、引張強度が４９０ＭＰａ以上の鋼板を提供することを目的とする。 Therefore, the present invention provides a steel sheet having a tensile strength of 490 MPa or more, which is superior in HAZ toughness of high heat input welding by adding relatively inexpensive Mn and Cr without using expensive alloy elements such as Ni, Mo and Nb. The purpose is to do.
本発明者等は大入熱溶接ＨＡＺの靭性に及ぼす成分組成の影響について鋭意検討し、その結果、安定して高い靭性を確保するためには、１．Ｃ含有量を０．０３％以下に極低Ｃ化してＭＡの生成を抑制し、Ｍｎ，Cｒを添加して変態点温度を低下させＨＡＺ全体を均一なベイナイト組織とすること、２．更に、ＭｎとＣｒの同時添加において、Ｍｎ量に対してＣｒ量を適正にすること、が有効なことを見出した。本発明は得られた知見を基に更に検討を加えてなされたもので、すなわち、本発明は、
１．鋼組成が、質量％で、Ｃ：０．０１〜０．０２０％、Ｓｉ：０．５０％以下、Ｍｎ：０．９〜２．３％未満、Ｃｒ：１．０〜３．９％、Ａｌ：０．０５％以下、Ｔｉ：０．００５〜０．０５０％を含有し、３５Ｃｒ（％）＋８Ｍｎ（％）≧６３（％）、且つ７Ｃｒ（％）＋１８Ｍｎ（％）≦６１（％）を満足し、残部鉄及び不可避的不純物からなることを特徴とする大入熱溶接熱影響部の靭性に優れた鋼板。但し、各式において各元素は含有量（質量％）とする。
The present inventors diligently studied the influence of the component composition on the toughness of the high heat input welding HAZ, and as a result, in order to ensure high toughness stably, 1. 1. C content is extremely reduced to 0.03% or less to suppress the formation of MA, and Mn and Cr are added to lower the transformation point temperature to make the entire HAZ a uniform bainite structure. Furthermore, it has been found that it is effective to make the Cr amount appropriate with respect to the Mn amount in the simultaneous addition of Mn and Cr. The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. Steel composition, by mass%, C: 0.01~0.0 20%, Si: 0.50% or less, Mn: less than 0.9~2.3%, Cr: 1.0~3.9% Al: 0.05% or less, Ti: 0.005 to 0.050%, 35Cr (%) + 8 Mn (%) ≧ 63 (%), and 7Cr (%) + 18 Mn (%) ≦ 61 (% ), And is composed of the remaining iron and unavoidable impurities, and has excellent toughness in the heat-affected zone of high heat input welding. However, in each formula, each element has a content (mass%).
本発明によれば、近年、高騰しているＮｉ，Ｍｏなど高価な合金元素を添加せず、５００ｋＪ／ｃｍを超える大入熱溶接を施した場合にも、優れたＨＡＺ靭性が確保される鋼板が得られ、安全性の高い建築構造物等を経済的に且つ高能率で製造でき産業上極めて有用である。 According to the present invention, a steel plate that can ensure excellent HAZ toughness even when high heat input welding exceeding 500 kJ / cm is performed without adding expensive alloy elements such as Ni and Mo, which have been rising rapidly in recent years. It is possible to produce highly safe building structures and the like economically and with high efficiency, which is extremely useful industrially.
本発明では成分組成を規定する。説明において％は、質量％とする。
Ｃ：０．０１〜０．０３％
Ｃは、母材強度を確保するとともに、γ粒の粗大化を抑制してＨＡＺ靭性を確保するために必要で、当該効果を発揮させるため、０．０１％以上を添加する。一方、Ｃ量が過剰になるとＭＡが増大してＨＡＺ靭性が劣化するようになるので０．０３％以下とする。
In the present invention, the component composition is defined. In the description,% is mass%.
C: 0.01 to 0.03%
C is necessary for securing the strength of the base material and suppressing the coarsening of the γ grains to ensure the HAZ toughness. In order to exhibit the effect, 0.01% or more is added. On the other hand, if the amount of C becomes excessive, MA increases and the HAZ toughness deteriorates, so the content is made 0.03% or less.
Ｓｉ：０．５０％以下
Ｓｉは、製鋼時の脱酸に必要な元素であるが、脱酸の目的を達すれば、添加量は少なくて良く、一方、０．５０％を超えて過剰に添加するとＭＡが増大してＨＡＺ靭性が劣化するため、０．５０％以下とする。
Si: 0.50% or less Si is an element necessary for deoxidation at the time of steelmaking. However, if the purpose of deoxidation is achieved, the addition amount may be small, while over 0.50% is added excessively. Then, since MA increases and HAZ toughness deteriorates, it is 0.50% or less.
Ｍｎ：０．９〜２．３％未満
Ｍｎは、強力なオーステナイト安定化元素で、変態温度を低下させて母材の強度を確保するのに有用で、また、ベイナイト変態を促進するため、０．９％以上を添加する。一方、Ｍｎ量が過剰になるとＨＡＺが硬化してＨＡＺ靭性が低下するようになるので、２．３％未満とする。
Mn: 0.9 to less than 2.3% Mn is a strong austenite stabilizing element, useful for lowering the transformation temperature to ensure the strength of the base material, and for promoting bainite transformation. Add 9% or more. On the other hand, if the amount of Mn becomes excessive, the HAZ hardens and the HAZ toughness decreases, so the content is made less than 2.3%.
Ｃｒ：１．０〜３．９％
Ｃｒは、焼入れ性を向上させて母材の強度や靭性を確保するのに有用な元素で、また、フェライト安定化元素のため、Ｍｎによるオーステナイトの過度の安定化を防止し、ＭＡの発生を防止する。このような効果を得るため、１．０％以上添加する。
Cr: 1.0 to 3.9%
Cr is an element useful for improving the hardenability and ensuring the strength and toughness of the base metal, and because it is a ferrite stabilizing element, it prevents excessive stabilization of austenite by Mn and prevents the generation of MA. To prevent. In order to obtain such an effect, 1.0% or more is added.
一方、Ｃｒが過剰に存在すると、ＨＡＺの硬度が増大してＨＡＺ靭性が劣化するので、３．９％以下とする。 On the other hand, if Cr is present excessively, the hardness of the HAZ increases and the HAZ toughness deteriorates.
Ａｌ：０．０５％以下
Ａｌは、製鋼時の脱酸に必要な元素であるが、脱酸の目的が達成されれば添加量は少なくても良く、一方、過剰になると、アルミナ等の粗大介在物が増加して、母材靭性が劣化し、ＭＡも増加してＨＡＺ靭性も劣化させるようになるので、０．０５％以下とする。
Al: 0.05% or less Al is an element necessary for deoxidation at the time of steelmaking. However, if the purpose of deoxidation is achieved, the amount added may be small. Inclusions increase, base metal toughness deteriorates, MA also increases and HAZ toughness deteriorates, so 0.05% or less.
Ｔｉ：０．００５〜０．０５０％
Ｔｉは、Ｎと結合してＴｉＮを形成する。ＴｉＮは、ＨＡＺのγ粒の成長を抑制し、ＨＡＺ靭性を向上させるため、このような効果が得られるように０．００５％以上、好ましくは０．０１０％以上を添加する。一方、Ｔｉが過剰になるとＴｉＮが粗大化し、母材靭性、ＨＡＺ靭性が共に劣化するので、０．０５０％以下、好ましくは０．０２５％以下とする。
Ti: 0.005 to 0.050%
Ti combines with N to form TiN. TiN suppresses the growth of HAZ γ grains and improves the HAZ toughness, so 0.005% or more, preferably 0.010% or more is added so as to obtain such an effect. On the other hand, when Ti becomes excessive, TiN becomes coarse and both the base metal toughness and the HAZ toughness deteriorate, so the content is made 0.050% or less, preferably 0.025% or less.
３５Ｃｒ（％）＋８Ｍｎ（％）≧６３（％）、且つ７Ｃｒ（％）＋１８Ｍｎ（％）≦６１（％）
これらのパラメータ式は、上記成分範囲で構成される鋼の、大入熱溶接ＨＡＺにおけるＭＡの生成量を少なくすると共に、ＨＡＺの硬さを低減して靭性を優れたものとするためのものである。
35Cr (%) + 8Mn (%) ≧ 63 (%) and 7Cr (%) + 18Mn (%) ≦ 61 (%)
These parameter formulas are for reducing the amount of MA generated in the high heat input welding HAZ of the steel composed of the above component ranges, and reducing the hardness of the HAZ to make the toughness excellent. is there.
Ｍｎは強力なオーステナイト安定化元素であり変態温度を低下させてベイナイト変態を促進する。しかし、Ｍｎ単独で添加した場合は、ベイナイトラス間にＭＡが析出してＨＡＺ靭性が低下する。 Mn is a strong austenite stabilizing element and promotes bainite transformation by lowering the transformation temperature. However, when Mn is added alone, MA precipitates between the bainite laths and the HAZ toughness decreases.
Ｍｎと共にフェライト安定化元素であるＣｒを添加すると、Ｍｎ添加による過度のオーステナイトの安定化が緩和され、ベイナイトラス間のＭＡは減少し、ＨＡＺ靭性に優れる領域が現出する。しかし、Ｃｒを更に添加すると焼入れ性が高まってＨＡＺの硬さが増大してＨＡＺ靭性は低下する。 When Cr, which is a ferrite stabilizing element, is added together with Mn, excessive stabilization of austenite due to the addition of Mn is alleviated, MA between bainite laths is reduced, and a region excellent in HAZ toughness appears. However, when Cr is further added, the hardenability is increased, the hardness of the HAZ is increased, and the HAZ toughness is decreased.
そこで、本発明では、ＭｎとＣｒの添加量を、上述した個々の添加量の範囲内で、３５Ｃｒ（％）＋８Ｍｎ（％）≧６３（％）、且つ７Ｃｒ（％）＋１８Ｍｎ（％）≦６１（％）を満足するように規定する。 Therefore, in the present invention, the addition amounts of Mn and Cr are within the range of the individual addition amounts described above, and 35Cr (%) + 8Mn (%) ≧ 63 (%) and 7Cr (%) + 18Mn (%) ≦ 61 (%) To satisfy.
ＭｎとＣｒの添加量が、３５Ｃｒ（％）＋８Ｍｎ（％）≧６３（％）を満足する場合、Ｍｎ添加によるオーステナイトの過度の安定化が防止されて、ＭＡの発生が抑制されＨＡＺ靭性が向上する。 When the added amount of Mn and Cr satisfies 35Cr (%) + 8 Mn (%) ≧ 63 (%), excessive stabilization of austenite due to the addition of Mn is prevented, the generation of MA is suppressed, and HAZ toughness is improved. To do.
一方、７Ｃｒ（％）＋１８Ｍｎ（％）＞６１（％）となる場合、ＨＡＺ硬さが硬くなりすぎてＨＡＺ靭性が低下するようになるので、７Ｃｒ（％）＋１８Ｍｎ（％）≦６１（％）とする。尚、これらの式においてＣｒ（％），Ｍｎ（％）は添加量（質量％）とする。 On the other hand, if 7Cr (%) + 18Mn (%)> 61 (%), the HAZ hardness becomes too hard and the HAZ toughness decreases, so 7Cr (%) + 18Mn (%) ≦ 61 (%) And In these formulas, Cr (%) and Mn (%) are added amounts (mass%).
本発明に係る鋼板は、常法により溶解、熱間圧延して製造することが可能である。以下、実施例を用いて本発明をより具体的に説明する。 The steel sheet according to the present invention can be manufactured by melting and hot rolling by a conventional method. Hereinafter, the present invention will be described more specifically with reference to examples.
［ＡｓＲｏｌｌ材］
表１に示す成分組成の鋼を溶製しインゴットとした後、１２００℃に加熱し、板厚１３ｍｍまで熱間圧延を行って供試鋼板とし、母材強度とＨＡＺ靭性の評価を行った。
[AsRoll material]
Steels having the component compositions shown in Table 1 were melted into ingots, heated to 1200 ° C., hot-rolled to a plate thickness of 13 mm to obtain test steel plates, and the base metal strength and HAZ toughness were evaluated.
１．母材強度
各鋼板から採取した丸棒試験片（ＡＳＴＭ−Ｆ型）について、ＪＩＳＺ２２４１に準じて引張試験を行い、降伏強度（ＹＳ），引張強度（ＴＳ），伸び（ＥＬ）を測定した。丸棒試験片（ＡＳＴＭ−Ｆ型）は圧延方向から採取した。得られた引張強度が４９０ＭＰａ以上のものを本発明範囲内とした。
1. Base material strength A round bar test piece (ASTM-F type) collected from each steel plate was subjected to a tensile test according to JISZ2241, and yield strength (YS), tensile strength (TS), and elongation (EL) were measured. A round bar specimen (ASTM-F type) was taken from the rolling direction. The obtained tensile strength of 490 MPa or more was made within the scope of the present invention.
２．ＨＡＺ靭性
ＨＡＺ靭性の評価を再現熱サイクルシャルピー衝撃試験により行った。熱サイクルは、スキンプレート材（５０ｍｍ厚）とダイヤフラム材（５０ｍｍ厚）を組み合わせ、溶接入熱５５０ｋＪ／ｃｍのエレクトロスラグ溶接を行った場合のボンド近傍のＨＡＺにおけるものを模擬したもので、１４００℃で１秒間保持し、８００−５００℃の冷却時間を５１０秒とした。
2. HAZ toughness HAZ toughness was evaluated by a reproducible thermal cycle Charpy impact test. The thermal cycle is a simulation of the HAZ in the vicinity of the bond when electroslag welding with a heat input of 550 kJ / cm is performed by combining a skin plate material (50 mm thickness) and a diaphragm material (50 mm thickness) at 1400 ° C. At 800 to 500 ° C. for 510 seconds.
圧延方向から採取した１２ｍｍ厚さ×１２ｍｍ幅の角棒状試験片に高周波誘導加熱装置で上述の熱サイクルを付与した後、ＪＩＳＺ２２０２のＶノッチシャルピー衝撃試験片を３本採取して、ＪＩＳＺ２２４２に準じてシャルピー衝撃試験を行った。 After applying the above thermal cycle to a 12 mm thick × 12 mm wide square bar specimen taken from the rolling direction with a high-frequency induction heating device, three V-notch Charpy impact specimens of JISZ2202 were collected and in accordance with JISZ2242. A Charpy impact test was performed.
シャルピー衝撃試験は試験温度を０℃として各鋼板について３本の試験片で行った。３本における吸収エネルギー（ｖＥ０）の最低値が５０Ｊ以上、３本の吸収エネルギー（ｖＥ０）の平均値が１００Ｊ以上をＨＡＺ靭性に優れるもの（本発明範囲内）とした。 The Charpy impact test was performed with three test pieces for each steel plate at a test temperature of 0 ° C. The minimum value of the absorbed energy (vE 0 ) of the three pieces was 50 J or more, and the average value of the three absorbed energy (vE 0 ) was 100 J or more, which was excellent in HAZ toughness (within the scope of the present invention).
表２に引張試験結果、表３にシャルピー衝撃試験結果を示す。表１〜３において鋼記号は共通とする。 Table 2 shows the tensile test results, and Table 3 shows the Charpy impact test results. In Tables 1 to 3, the steel symbol is common.
鋼Ａ１，Ｍ１，Ｍ２，Ｍ３，Ｎ１，Ｎ２，Ｎ３は成分組成が本発明範囲内でＭｎ量，Ｃｒ量が関係式を満足する本発明鋼で、引張強度４９０ＭＰａ以上の高強度と、シャルピー衝撃試験における３本の試験片の吸収エネルギー（ｖＥ０）の最低値が５０Ｊ以上、３本の試験片の吸収エネルギー（ｖＥ０）の平均値が１００Ｊ以上の優れたＨＡＺ靭性が得られた。 Steels A1, M1, M2, M3, N1, N2, and N3 are steels of the present invention in which the component composition is within the scope of the present invention and the Mn content and Cr content satisfy the relational expressions. High strength with a tensile strength of 490 MPa or more and Charpy impact Excellent HAZ toughness in which the minimum value of the absorbed energy (vE 0 ) of the three test pieces in the test was 50 J or more and the average value of the absorbed energy (vE 0 ) of the three test pieces was 100 J or more was obtained.
一方、鋼Ａ２は、Ｃｒ量が過剰で、且つ７Ｃｒ＋１８Ｍｎ≦６１を満足せず、引張強度４９０ＭＰａ以上の高強度は得られるもののシャルピー衝撃試験において３本の平均値は１００J以上となるが、１本のシャルピー衝撃値が５０Ｊ未満となり安定性に欠ける。 On the other hand, the steel A2 has an excessive amount of Cr and does not satisfy 7Cr + 18Mn ≦ 61, and although a high strength of 490 MPa or higher is obtained, the average value of three in the Charpy impact test is 100 J or more. The Charpy impact value is less than 50 J and lacks stability.
鋼Ａ３，Ａ４は、Ｃｒ量が過剰で、且つ７Ｃｒ＋１８Ｍｎ≦６１を満足せず、鋼Ａ３は引張強度４９０ＭＰａ以上を満足するもののシャルピー衝撃試験において３本の平均値は１００J未満で、１本のシャルピー衝撃値が５０Ｊ未満となり安定性に欠ける。 Steels A3 and A4 have an excessive amount of Cr and do not satisfy 7Cr + 18Mn ≦ 61, and steel A3 satisfies a tensile strength of 490 MPa or more, but the average value of three in the Charpy impact test is less than 100 J, and one Charpy. The impact value is less than 50 J and lacks stability.
鋼Ａ４は、引張強度４９０ＭＰａ以上を満足するもののシャルピー衝撃試験において３本の平均値は１００J未満で、３本全てのシャルピー衝撃値が５０Ｊ未満でＨＡＺ靭性に劣る。 Steel A4 satisfies the tensile strength of 490 MPa or more, but in the Charpy impact test, the average value of three is less than 100 J, and all three Charpy impact values are less than 50 J, and the HAZ toughness is inferior.
鋼Ｂ２は、７Ｃｒ＋１８Ｍｎ≦６１を満足するもののＣｒ量が過剰で、引張強度（ＴＳ）が４９０ＭＰａ未満と低く、シャルピー衝撃試験において３本の平均値は１００J未満で、２本のシャルピー衝撃値が５０Ｊ未満となり安定性に欠ける。 Steel B2 satisfies 7Cr + 18Mn ≦ 61, but has an excessive amount of Cr and has a low tensile strength (TS) of less than 490 MPa. It becomes less than that and lacks stability.
鋼Ｂ３は、Ｃｒ量が過剰で、且つ７Ｃｒ＋１８Ｍｎ≦６１を満足せず、引張強度４９０ＭＰａ以上を満足するもののシャルピー衝撃試験において３本の平均値は１００J未満で、３本全てのシャルピー衝撃値が５０Ｊ未満でＨＡＺ靭性に劣る。 Steel B3 has an excessive amount of Cr, does not satisfy 7Cr + 18Mn ≦ 61, and satisfies a tensile strength of 490 MPa or more, but in the Charpy impact test, the average value of three is less than 100 J, and all three Charpy impact values are 50 J Less than the HAZ toughness.
鋼Ｂ４は、Ｃｒ量が過剰で、且つ７Ｃｒ＋１８Ｍｎ≦６１を満足せず、引張強度４９０ＭＰａ以上を満足するもののシャルピー衝撃試験において３本の平均値は１００J未満で、３本全てのシャルピー衝撃値が５０Ｊ未満でＨＡＺ靭性に劣る。 Steel B4 has an excessive amount of Cr, does not satisfy 7Cr + 18Mn ≦ 61, and satisfies a tensile strength of 490 MPa or more, but in the Charpy impact test, the average value of three is less than 100 J, and all three Charpy impact values are 50 J Less than the HAZ toughness.
鋼Ｃ１は、７Ｃｒ＋１８Ｍｎ≦６１を満足するもののＣｒ量が過剰で、引張強度（ＴＳ）が３４８ＭＰａと低く、シャルピー衝撃試験において３本の平均値は１００J未満で、２本のシャルピー衝撃値が５０Ｊ未満となり安定性に欠ける。 Steel C1 satisfies 7Cr + 18Mn ≦ 61, but has an excessive amount of Cr and has a low tensile strength (TS) of 348 MPa. In the Charpy impact test, the average value of three samples is less than 100 J, and the two Charpy impact values are less than 50 J. And lacks stability.
鋼Ｃ２は、７Ｃｒ＋１８Ｍｎ≦６１を満足するもののＣｒ量が過剰で、引張強度（ＴＳ）が４０８ＭＰａと低く、シャルピー衝撃試験において３本の平均値は１００J未満で、２本のシャルピー衝撃値が５０Ｊ未満となり安定性に欠ける。 Steel C2 satisfies 7Cr + 18Mn ≦ 61, but has an excessive amount of Cr and has a low tensile strength (TS) of 408 MPa. In the Charpy impact test, the average value of three samples is less than 100 J, and the two Charpy impact values are less than 50 J. And lacks stability.
鋼Ｃ３は、７Ｃｒ＋１８Ｍｎ≦６１を満足するもののＣｒ量が過剰で、引張強度４９０ＭＰａ以上を満足するものの、シャルピー衝撃試験において３本の平均値は１００J未満で、３本全てのシャルピー衝撃値が５０Ｊ未満となり安定性に欠ける。 Steel C3 satisfies 7Cr + 18Mn ≦ 61, but has an excessive Cr content and satisfies a tensile strength of 490 MPa or more. However, in the Charpy impact test, the average value of three samples is less than 100 J, and all three Charpy impact values are less than 50 J. And lacks stability.
鋼Ｍ４は、Ｃｒ量は本発明範囲内であるが７Ｃｒ＋１８Ｍｎ≦６１を満足せず、
引張強度（ＴＳ）は４９０ＭＰａ以上であり、シャルピー衝撃試験において３本の平均値は１００J以上を満足するものの、１本のシャルピー衝撃値が５０Ｊ未満となり安定性に欠ける。
Steel M4 has a Cr content within the range of the present invention, but does not satisfy 7Cr + 18Mn ≦ 61.
The tensile strength (TS) is 490 MPa or more, and in the Charpy impact test, the average value of three satisfies 100 J or more, but one Charpy impact value is less than 50 J and lacks stability.
鋼Ｎ４は、７Ｃｒ＋１８Ｍｎ≦６１を満足するが、Ｃｒ量が本発明範囲外で引張強度（ＴＳ）は４９０ＭＰａ以上であり、シャルピー衝撃試験において３本の平均値は１００J以上を満足するものの、１本のシャルピー衝撃値が５０Ｊ未満となり安定性に欠ける。 Steel N4 satisfies 7Cr + 18Mn ≦ 61, but the Cr amount is outside the range of the present invention, and the tensile strength (TS) is 490 MPa or more. In the Charpy impact test, the average value of the three samples satisfies 100 J or more. The Charpy impact value is less than 50 J and lacks stability.
［ＱＴ材］
表４に示す成分組成の鋼を溶製しインゴットとした後、１２００℃に加熱後、板厚１３ｍｍまで熱間圧延し、室温まで冷却した。その後、９００℃に再加熱し１０分間保持後水焼入れして試験材とした。
[QT material]
Steels having the composition shown in Table 4 were melted into ingots, heated to 1200 ° C., hot-rolled to a plate thickness of 13 mm, and cooled to room temperature. Thereafter, it was reheated to 900 ° C., held for 10 minutes, and then water-quenched to obtain a test material.
表５，６に母材強度とＨＡＺ靭性を示す。引張強度５９０ＭＰａ以上、シャルピー衝撃値として３本の平均値３００Ｊ以上、３本の個々の値も３００Ｊ以上が得られた。母材強度とＨＡＺ靭性は前述のＡｓＲｏｌｌ材の試験方法に準じて求めた。 Tables 5 and 6 show the base metal strength and HAZ toughness. A tensile strength of 590 MPa or more and a Charpy impact value of three average values of 300 J or more and three individual values of 300 J or more were obtained. The base material strength and the HAZ toughness were determined in accordance with the aforementioned AsRoll material test method.
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