JPH0151526B2 - - Google Patents
Info
- Publication number
- JPH0151526B2 JPH0151526B2 JP60113731A JP11373185A JPH0151526B2 JP H0151526 B2 JPH0151526 B2 JP H0151526B2 JP 60113731 A JP60113731 A JP 60113731A JP 11373185 A JP11373185 A JP 11373185A JP H0151526 B2 JPH0151526 B2 JP H0151526B2
- Authority
- JP
- Japan
- Prior art keywords
- steel
- toughness
- less
- strength
- temperature
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 229910000831 Steel Inorganic materials 0.000 claims description 55
- 239000010959 steel Substances 0.000 claims description 55
- 238000005496 tempering Methods 0.000 claims description 19
- 230000007797 corrosion Effects 0.000 claims description 16
- 238000005260 corrosion Methods 0.000 claims description 16
- 238000001816 cooling Methods 0.000 claims description 13
- 238000004519 manufacturing process Methods 0.000 claims description 13
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 9
- 238000005098 hot rolling Methods 0.000 claims description 4
- 239000012535 impurity Substances 0.000 claims description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 229910052698 phosphorus Inorganic materials 0.000 claims description 4
- 229910052804 chromium Inorganic materials 0.000 claims description 3
- 229910052720 vanadium Inorganic materials 0.000 claims description 3
- 229910052719 titanium Inorganic materials 0.000 claims description 2
- 229910052748 manganese Inorganic materials 0.000 claims 1
- 229910052750 molybdenum Inorganic materials 0.000 claims 1
- 229910052759 nickel Inorganic materials 0.000 claims 1
- 229910052758 niobium Inorganic materials 0.000 claims 1
- 238000005336 cracking Methods 0.000 description 14
- 239000000126 substance Substances 0.000 description 12
- 230000000694 effects Effects 0.000 description 11
- 238000000034 method Methods 0.000 description 9
- 239000010953 base metal Substances 0.000 description 8
- 238000010791 quenching Methods 0.000 description 8
- 230000000171 quenching effect Effects 0.000 description 8
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 7
- 229910052799 carbon Inorganic materials 0.000 description 7
- 238000010438 heat treatment Methods 0.000 description 7
- 150000004767 nitrides Chemical class 0.000 description 7
- 239000000203 mixture Substances 0.000 description 6
- 238000005096 rolling process Methods 0.000 description 6
- 239000000463 material Substances 0.000 description 5
- 230000008569 process Effects 0.000 description 5
- 239000013535 sea water Substances 0.000 description 5
- 238000003466 welding Methods 0.000 description 5
- 229910001566 austenite Inorganic materials 0.000 description 4
- 229910000734 martensite Inorganic materials 0.000 description 4
- 238000010276 construction Methods 0.000 description 3
- 238000001556 precipitation Methods 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 230000002542 deteriorative effect Effects 0.000 description 2
- 238000005553 drilling Methods 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 238000010248 power generation Methods 0.000 description 2
- 238000004881 precipitation hardening Methods 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- 239000002970 Calcium lactobionate Substances 0.000 description 1
- -1 Mo 2 C increases Chemical class 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- 229910001563 bainite Inorganic materials 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 230000006378 damage Effects 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 238000003860 storage Methods 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
- 239000011593 sulfur Substances 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
- 238000009849 vacuum degassing Methods 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/001—Heat treatment of ferrous alloys containing Ni
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Mechanical Engineering (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Heat Treatment Of Steel (AREA)
Description
(産業上の利用分野)
本発明は、強度と靭性にすぐれ、かつ特に海水
等の応力腐蝕環境中における耐応力腐蝕割れ性の
すぐれた超高張力鋼の製造法に関するものであ
る。
(従来の技術)
近年、海底資源開発や海底地殻地質調査など深
海に対する関心が高まり、この海底開発につなが
る石油掘削用のドリリングプラツトフオーム、海
底石油生産基地などの海洋構造物の建造、あるい
は、原子力および火力発電の夜間余剰電力調整用
の揚水発電用高落差ペンストツクの建造などエネ
ルギー関連の鋼構造物の建設が活発化している。
これらに使用される材料には、何れにせよ安全性
確保から高強度で靭性のすぐれた性能が要求され
ており、さらにこれらの高強度材料が大気中と異
なつた雰囲気、環境中で使用される場合、特に応
力腐蝕割れについて充分検討が行われなければな
らない。
このような、より安全で信頼性の高い材料と云
う要求に応えるため、高張力鋼としてNi含有低
合金鋼の開発、および品質改善が行われている。
例えば、特開昭56−9358号公報のようにC+1/8
Mo+V>0.26で且つCr≦0.8MoであるNi−Cr−
Mo−V系の高強度高靭性鋼、特開昭57−188655
号公報のように焼入処理において広い冷却速度で
高強度高靭性が得られるNi−Cr−Mo−V系の超
高張力鋼、さらに極低燐、極低硫処理した高靭性
の焼入、焼戻型含Ni鋼の製造法など、多くの種
類の製造法が開発されている。
(発明が解決しようとする問題点)
これらの製造法はいずれも鋼の靭性向上には効
果的である。しかしながら作用環境を考えた場
合、例えば海水中で応力腐蝕を考慮に入れた検討
はなされていない。
即ち、高張力鋼の応力腐蝕割れに関しては、線
型破壊力学の理論が取り入れられ、欠陥が存在す
る材料が腐蝕環境に対してどのような破壊挙動を
とるかを亀裂先端のK値を用いて定量化する方法
が用いられている。即ち、作用環境において、予
亀裂付きの試験片を用い、ノツチ先端に非常に苛
酷な条件を作ることにより遅れ破壊を生じ易くし
て、ここに種々のK値のレベルでの定荷重試験を
行うことにより、ある一定のK値以下では破壊を
生じない限界値KISCC値を求めることによつて耐
応力腐蝕割れ性が評価されている。
しかるに、前述の製造法により得られた鋼につ
いてはかかる評価を行う配慮が特になされておら
ず、従つて使用上充分に安全であるとは言い難
い。
そこで本発明者らは、高強度で高靭性を有し、
かつ海水中等における耐応力腐蝕割れ性のすぐれ
た鋼を製造するに際し、主として加熱−圧延−直
接焼入れ工程で製造することを前提に、各種成分
組成の鋼についてそれを製造する手段を検討した
結果、Ni含有鋼にNbを添加し、更に不純物元素
P、N、Oを低減した鋼について圧延後直接焼入
れ、焼戻しの適正条件を採用することによつて目
的の鋼が製造出来ることを知見した。
(問題点を解決するための手段)
本発明はこのような知見に基づいてなされたも
ので、その要旨は、重量%でC0.05〜0.10%、
Si0.20%以下、Mn0.4〜1.5%、Ni5.0〜8.0%、
Mo0.3〜1.5%、Nb0.005〜0.05%を含み、P0.010
%以下、O0.0025%以下、N0.0040%以下に制限
し、更に必要に応じてCu0.2〜1.5%、Cr0.60%以
下、V0.02〜0.12%、Total Al0.08%以下、
Ti0.005〜0.01%、Ca0.003%以下の1種または2
種以上を含み、残部が不可避不純物とFeからな
る鋼片を1050〜1250℃に加熱し、Ar3点以上の温
度で熱間圧延を終了させ、そのまま水冷を行い、
100℃以下の温度になつてから水冷を停止し、そ
の後600℃以下の温度で焼戻すことを特徴とする
耐応力腐蝕割れ性のすぐれた超高張力鋼の製造法
である。
(作用)
最初に本発明における対象鋼の化学成分範囲の
限定理由について説明する。
まず、Cは焼入性および強度を確保するために
必要な元素であるが、0.05%未満の低い含有量で
は所要の強度が得られず、また0.10%超では溶接
熱影響部の硬化が著るしく、靭性および耐応力腐
蝕割れ性が劣化する。従つて0.05〜0.10%とす
る。
Siは高強度化に効果があるが、高Ni鋼の場合
Siが高いと焼戻脆化感受性が大きくなり、特に溶
接熱影響部の切欠靭性やKISCCが損われる。従つ
て、ある程度の強度を確保し、切欠靭性やKISCC
値を低下させないためには上限を0.20%とする。
Mnは焼入性の確保および熱間加工時の割れ、
溶接時の熱間割れ防止のために必要であるが、一
方Ni含有鋼の場合はMnが多いと焼戻脆化感受性
が大きく、1.5%以下にする必要がある。また、
0.40%未満では強度、靭性のバランスを損う危険
性が大きい。従つて0.40〜1.5%とする。
Niは焼入処理によつて下部ベイナイトとマル
テンサイトの混合組織を得、焼戻時に比較的迅速
にオーステナイト中に拡散吸収され、残留オース
テナイトを衝撃応力に対して安定化させる作用を
もつ。そのため、他元素の含有量の兼ね合いによ
り最低5%必要である。一方、8%を超えると焼
戻時に変態した残留オーステナイトを不安定に
し、靭性を劣化させ、また溶接熱影響部の硬さを
上昇させ、靭性あるいは耐応力腐蝕割れ性を劣化
させる。従つて5〜8%とする。
Moは強度を確保するため、またNi含有鋼の焼
戻脆化を防止するために必要である。0.3%未満
では目標とする強度が得られず、また1.5%超で
はMo2C等の炭化物の数が増加し、靭性および耐
応力腐蝕性を低下させる。従つて0.3〜1.5%とす
る。
Nb添加は本発明の加熱−圧延−直接焼入れ−
焼戻しの工程において最も重要なポイントの1つ
である。即ち、Nbを含有せしめるのは鋼片加熱
時に地鉄中に充分Nbの炭・窒化物を溶解させ、
圧延後直接焼入れ−焼戻しの際に一旦溶け込んだ
NbをNbの炭・窒化物として析出させ、それによ
る硬化を期待するためであつて、最低含有量とし
ては0.005%以上が必要である。しかしながら過
多に含有する場合、溶接熱影響部や溶着鋼部の靭
性を劣化させる危険性がある。その限界が0.05%
であるため、これを上限とする。
一方本発明においては、特にP、O、Nを低く
制限することによつて母材および溶接熱影響部の
靭性の向上を期待するものであるが、更に上記
Nbの炭・窒化物の析出による靭性低下を補うこ
とに対しても有効である。
まずPは不可避不純物としてある程度の混入を
回避出来ないものであるが、特に本発明の対象と
なる100Kgf/mm2以上の降伏強度を持つ高張力鋼
では溶接熱影響部の靭性に大きく影響する。特に
耐応力腐蝕割れ性を考慮する場合は、溶接熱影響
部の特に母材側の比較的低温に加熱された焼戻し
脆化域におけるKISCC値に大きく関与し、Pが
0.010%超えるとこの部分のKISCC値が大きく低下
する危険性がある。そこでPを0.010%以下に制
限する。
Oは超高張力鋼では靭性におよぼす影響が大き
く、特にシヤルピー遷移曲線の上棚の吸収エネル
ギー、即ちシエルフエネルギーの値を支配する。
鋼中のOは殆んどが酸化物を形成し、これが破壊
時の吸収エネルギーを低下させるので高張力鋼に
なる程影響力が大きい。従つてOは極力低下させ
ることが望ましく、25ppm以下にすることで本発
明の対象となる鋼の靭性が得られる。そこでOを
0.0025%以下に制限する。
Nは溶接熱影響部の耐応力腐蝕割れ性KISCCに
大きな影響を与えるため、また母材の靭性確保に
も大きく影響するため極力低減する必要がある。
特に溶接熱影響部は充分にAlを含有している場
合でも、母材ではAlと結合してAlNとなつてい
るNが溶接時の高温状態で解離し、そのまま冷却
して固溶Nとなるから高温に加熱された粗粒組織
部の靭性を著しく劣化させる。高張力鋼の場合、
この現象はNが0.0040%を超えると著しい。そこ
でNを0.0040%以下に制限する。
以上が基本的な元素であるが、さらに本発明に
は前記以外の元素としてCu、Cr、V、Al、Ti、
Caのいずれか1種以上を添加することにより、
鋼材の断面厚みに応じて焼入性を確保して強度、
靭性を一層向上せしめることが出来る。
即ち、Cuは靭性を劣化させずに強度を上昇さ
せることに対して有効であるが、0.2%未満の含
有量では有効な効果が得られず、また1.5%を超
えると溶接の際に溶接部に熱間割れを出易くす
る。
Crは焼入性および強度確保の点で効果がある
が、0.6%を超えると溶接熱影響部の硬さを上昇
させ、KISCC値を低下せしめる危険性があるので
これを上限値とするが、好ましい範囲は0.2〜0.5
%である。
Vは焼戻しの時に炭・窒化物を生成して強度確
保のために必要である。Nbと共存させて使用す
ると析出硬化による強度上昇が大きいため、特に
引張強さの大きい超高張力鋼には添加することが
ある。この場合の添加量は強度確保上0.02%は必
要であり、一方0.12%を超えると靭性が劣化す
る。従つて0.02〜0.12%とする。
Alは鋼中のNと結びついてAlNとなり、組織
の微細化に寄与するが、添加量が過多になると却
つてAl2O3等の介在物の量の増大により靭性を阻
害する。従つて上限を0.08%とする。なお、Alは
通常脱酸剤として使用されるが、本発明の場合別
途Oの限界量を規定しているため、真空脱ガス等
Oを低減させる別の手段を用いればAlの添加は
必ずしも必要としない場合もある。
Tiは溶接熱影響部の粗粒化防止を通して同部
分の靭性劣化防止に効果的であるが、添加量が多
いと却つて溶接熱影響部の靭性だけでなく母材の
靭性迄劣化せしめるので、上限を0.01%とする。
また0.005%未満のTi量では上記効果が有効に発
揮出来ない。よつてTiの範囲を0.005〜0.01%と
する。
Caは一般にMnS等の介在物の形状を変え、靭
性の向上や靭性の方向性を少くする効果があり、
その効果の飽和する0.003%を上限として規定す
る。なお、低S鋼を対象とする場合は特にCaを
添加する必要はない。
次に、本発明は以上の化学成分を有する鋼を
1050〜1250℃に加熱し、Ar3点以上の温度で熱間
圧延を終了させ、そのまま水冷を行い、100℃以
下の温度迄水冷を行つた後、改めてAc1点以下の
温度に加熱焼戻しを行うことを発明の重要な骨子
の1つとしているが、この工程条件の限定理由に
ついてのべる。
まず鋼片の加熱温度は1050℃以上1250℃迄とし
たが、下限温度を1050℃としたのは、本発明の目
的である耐応力腐蝕割れ性の小さい高張力鋼を得
るための強化機構としてNbの炭・窒化物の析出
による強化を利用するに際し、鋼片の状態で存在
するNbの炭・窒化物を鋼片の加熱時に地鉄中に
充分固溶させる必要があるが、1050℃未満の温度
ではこの固溶作用が充分でなく、従つて水冷後焼
戻の際の充分な析出硬化が期待出来ないからであ
り、一方1250℃超ではNb炭・窒化物の固溶は充
分であるものの、本発明の対象鋼のようにNiを
多く含有する場合は鋼片の表面に特異な酸化物が
出易くなり、これが圧延後鋼板表面に残留して表
面疵を生ずる原因となり易い。従つて加熱温度は
1050〜1250℃とする。
このように加熱したのち熱間圧延を行い、圧延
後熱間のまま鋼板の水冷を行うが、この際の水冷
開始温度は鋼のそれぞれ持つAr3点以上の温度か
らとする必要がある。これは組織を充分にマルテ
ンサイト組織にする必要性のためである。この場
合、焼入後得られるマルテンサイト組織を微細に
すると焼戻後の靭性が更に良好になり、そのため
には水冷前の圧延時Ar3点〜Ar3+100℃の範囲で
充分に圧下を行うなどの圧下スケジユールの採用
も考えられるが、本発明では特に規定は行わな
い。
なお、Ar3点以上の温度から水冷を行う際には
水冷条件を圧延板の厚みによつて加減することが
好ましい。
次に、水冷停止温度を100℃以下と規定したの
は100℃を超える温度で水冷を停止すると本発明
の対象鋼の場合まだマルテンサイト形態が終了し
ていない場合があり、強度、靭性に大きく影響す
る場合があるためである。また、焼戻しは再加熱
焼戻炉により行われるが、本発明で規定する化学
成分を有する鋼のAc1点は540〜570℃にあり、こ
れらをやや超える温度での焼戻しが強度−靭性バ
ランス上最も良好であり、600℃を超えると不安
定オーステナイトの析出により靭性が劣化する。
よつて焼戻温度は600℃以下と限定した。
なお、特に規定しないが、焼戻脆化防止の意味
から焼戻し後に水冷を行つてもよい。
次に、本発明の奏する効果を明確にするため実
施例について説明する。
(実施例)
第1表に示すような化学成分A−1〜A−10、
B−1〜B−5を有する鋼を溶製し、これに熱間
加工を施し32〜180mmの板厚に圧延し、第2表に
示すNo.1〜No.18の条件により直接焼入れ、焼戻
しを行つた。これらについて母材に機械的性質
と、さらに母材および溶接熱影響部のKISCC値を
調べた。
溶接はTIG溶接による入熱25KJ/cmの溶接で
ある。
これら第1表の化学成分を有する鋼と、第2表
に示す熱処理条件とを種々組み合わせて得られた
機械的性質、および3.5%の人工海水中での
ASME E399に示される試験片を使つたKISCC試験
結果を第3表に示す。
なお第1表の化学成分No.B−1〜B−5は本
発明に規定する化学成分範囲を逸脱した成分例で
あり、また第2表の熱処理条件No.16〜18は製造
条件が本発明の規定する範囲を逸脱する例であ
る。
(Field of Industrial Application) The present invention relates to a method for producing ultra-high tensile strength steel that has excellent strength and toughness, and particularly has excellent resistance to stress corrosion cracking in stress corrosion environments such as seawater. (Conventional technology) In recent years, interest in the deep sea has increased, such as undersea resource development and undersea crustal geological surveys, and the construction of offshore structures such as drilling platforms for oil drilling and undersea oil production bases that lead to undersea development, Construction of energy-related steel structures, such as the construction of high-head penstocks for pumped storage power generation to adjust nighttime surplus power from nuclear and thermal power generation, is gaining momentum.
The materials used in these products are required to have high strength and excellent toughness to ensure safety, and furthermore, these high-strength materials are used in atmospheres and environments different from the atmosphere. In this case, sufficient consideration must be given to stress corrosion cracking in particular. In order to meet these demands for safer and more reliable materials, Ni-containing low alloy steels have been developed as high tensile strength steels and quality improvements have been made.
For example, as in Japanese Patent Application Laid-Open No. 56-9358, C+1/8
Ni−Cr− where Mo+V>0.26 and Cr≦0.8Mo
Mo-V series high strength and high toughness steel, JP-A-57-188655
As mentioned in the publication, Ni-Cr-Mo-V ultra-high tensile strength steel can achieve high strength and toughness at a wide range of cooling rates during quenching treatment, and also has high toughness quenched with ultra-low phosphorus and ultra-low sulfur treatment. Many types of manufacturing methods have been developed, including methods for manufacturing tempered Ni-containing steel. (Problems to be Solved by the Invention) All of these manufacturing methods are effective in improving the toughness of steel. However, when considering the working environment, for example, no study has been conducted that takes into account stress corrosion in seawater. In other words, regarding stress corrosion cracking in high-strength steel, the theory of linear fracture mechanics is adopted, and the K value of the crack tip is used to quantify the fracture behavior of the material in which the defect exists in a corrosive environment. A method is used to That is, in a working environment, a pre-cracked test piece is used to create extremely harsh conditions at the notch tip to facilitate delayed fracture, and constant load tests are conducted at various K value levels. Therefore, the stress corrosion cracking resistance is evaluated by determining the limit K ISCC value at which destruction does not occur below a certain K value. However, no particular consideration has been taken to perform such evaluation on the steel obtained by the above-mentioned manufacturing method, and therefore it is difficult to say that it is sufficiently safe for use. Therefore, the present inventors have developed a method that has high strength and high toughness.
In order to manufacture steel with excellent resistance to stress corrosion cracking in seawater etc., we have studied methods for manufacturing steel of various compositions, assuming that it is mainly manufactured through a heating-rolling-direct quenching process. It has been found that the desired steel can be produced by adding Nb to Ni-containing steel and employing appropriate conditions for direct quenching and tempering after rolling for steel with reduced impurity elements P, N, and O. (Means for solving the problem) The present invention was made based on such knowledge, and the gist thereof is that C0.05 to 0.10% by weight,
Si0.20% or less, Mn0.4~1.5%, Ni5.0~8.0%,
Contains Mo0.3~1.5%, Nb0.005~0.05%, P0.010
% or less, O 0.0025% or less, N 0.0040% or less, and if necessary, Cu 0.2 to 1.5%, Cr 0.60% or less, V 0.02 to 0.12%, Total Al 0.08% or less,
Ti0.005~0.01%, Ca0.003% or less type 1 or 2
A steel billet containing at least 100% Fe, with the remainder consisting of unavoidable impurities and Fe, is heated to 1050 to 1250°C, hot rolling is completed at a temperature of Ar 3 or higher, and water cooling is performed as it is.
This is a method for producing ultra-high tensile strength steel with excellent stress corrosion cracking resistance, which is characterized by stopping water cooling after the temperature reaches 100°C or lower, and then tempering at a temperature of 600°C or lower. (Function) First, the reason for limiting the chemical composition range of the target steel in the present invention will be explained. First, C is an element necessary to ensure hardenability and strength, but if the content is low, less than 0.05%, the required strength cannot be obtained, and if it exceeds 0.10%, the weld heat affected zone will be hardened significantly. toughness and stress corrosion cracking resistance. Therefore, it is set at 0.05 to 0.10%. Si is effective in increasing strength, but in the case of high Ni steel
A high Si content increases the susceptibility to temper embrittlement, which particularly impairs the notch toughness and K ISCC of the weld heat affected zone. Therefore, a certain degree of strength is secured, and notch toughness and K ISCC
In order not to reduce the value, the upper limit is set at 0.20%. Mn ensures hardenability and prevents cracking during hot working.
It is necessary to prevent hot cracking during welding, but in the case of Ni-containing steel, a high Mn content increases the susceptibility to temper embrittlement, so it must be kept at 1.5% or less. Also,
If it is less than 0.40%, there is a great risk of losing the balance between strength and toughness. Therefore, it is set at 0.40 to 1.5%. Ni obtains a mixed structure of lower bainite and martensite through quenching, is diffused and absorbed into austenite relatively quickly during tempering, and has the effect of stabilizing residual austenite against impact stress. Therefore, a minimum content of 5% is required depending on the content of other elements. On the other hand, if it exceeds 8%, the retained austenite transformed during tempering becomes unstable, the toughness deteriorates, and the hardness of the weld heat affected zone increases, causing the toughness or stress corrosion cracking resistance to deteriorate. Therefore, it is set at 5 to 8%. Mo is necessary to ensure strength and to prevent Ni-containing steel from tempering embrittlement. If it is less than 0.3%, the target strength cannot be obtained, and if it exceeds 1.5%, the number of carbides such as Mo 2 C increases, reducing toughness and stress corrosion resistance. Therefore, it is set at 0.3 to 1.5%. Nb addition is the heating-rolling-direct quenching process of the present invention.
This is one of the most important points in the tempering process. In other words, Nb is added by sufficiently dissolving Nb carbon and nitride in the steel base when heating the steel billet.
Direct quenching after rolling - once melted during tempering
This is to precipitate Nb as Nb carbon/nitride and to expect hardening due to this, and the minimum content is required to be 0.005% or more. However, if it is contained in an excessive amount, there is a risk of deteriorating the toughness of the weld heat affected zone and welded steel part. The limit is 0.05%
Therefore, this is the upper limit. On the other hand, in the present invention, it is expected that the toughness of the base metal and the weld heat-affected zone will be improved by particularly limiting P, O, and N to a low level.
It is also effective in compensating for the decrease in toughness due to the precipitation of Nb carbon/nitride. First, P is an unavoidable impurity that cannot be avoided to some extent, but it greatly affects the toughness of the weld heat-affected zone, especially in high-strength steel with a yield strength of 100 Kgf/mm 2 or more, which is the subject of the present invention. Particularly when stress corrosion cracking resistance is considered, P has a large influence on the K ISCC value in the temper embrittlement region heated to a relatively low temperature in the weld heat affected zone, especially on the base metal side.
If it exceeds 0.010%, there is a risk that the K ISCC value in this area will decrease significantly. Therefore, P is limited to 0.010% or less. O has a large influence on the toughness of ultra-high-strength steels, and in particular controls the absorbed energy on the upper shelf of the Shallpy transition curve, that is, the value of the Shelf energy.
Most of the O in steel forms oxides, which lower the absorbed energy at fracture, so the higher the tensile strength of the steel, the greater its influence. Therefore, it is desirable to reduce O as much as possible, and by reducing it to 25 ppm or less, the toughness of the steel, which is the object of the present invention, can be obtained. So O
Limit to 0.0025% or less. N has a large effect on the stress corrosion cracking resistance K ISCC of the weld heat affected zone, and also has a large effect on ensuring the toughness of the base metal, so it must be reduced as much as possible.
In particular, even if the weld heat-affected zone contains sufficient Al, the N that combines with Al to form AlN in the base metal dissociates in the high temperature state during welding and cools as it is, becoming solid solution N. The toughness of coarse grain structures heated to high temperatures is significantly deteriorated. For high-strength steel,
This phenomenon is remarkable when N exceeds 0.0040%. Therefore, N is limited to 0.0040% or less. The above are the basic elements, but the present invention further includes Cu, Cr, V, Al, Ti,
By adding one or more types of Ca,
Ensures hardenability and strength according to the cross-sectional thickness of the steel material.
Toughness can be further improved. In other words, Cu is effective in increasing strength without deteriorating toughness, but if the content is less than 0.2%, no effective effect will be obtained, and if it exceeds 1.5%, the welded part will be damaged during welding. make hot cracks more likely to occur. Cr is effective in ensuring hardenability and strength, but if it exceeds 0.6%, it increases the hardness of the weld heat affected zone and risks lowering the K ISCC value, so this is the upper limit. , the preferred range is 0.2-0.5
%. V is necessary to generate carbon/nitride during tempering and ensure strength. When used in combination with Nb, the strength increases due to precipitation hardening, so it is sometimes added to ultra-high tensile strength steels with especially high tensile strength. In this case, the addition amount is required to be 0.02% to ensure strength, while if it exceeds 0.12%, toughness will deteriorate. Therefore, it is set at 0.02 to 0.12%. Al combines with N in the steel to form AlN, which contributes to the refinement of the structure, but when added in an excessive amount, it actually inhibits toughness due to an increase in the amount of inclusions such as Al 2 O 3 . Therefore, the upper limit is set at 0.08%. Note that Al is normally used as a deoxidizing agent, but in the case of the present invention, the limit amount of O is separately specified, so if another means to reduce O, such as vacuum degassing, is used, it is not necessary to add Al. In some cases, this is not the case. Ti is effective in preventing deterioration of the toughness of the weld heat-affected zone by preventing coarse grains in the weld heat-affected zone, but if it is added in a large amount, it will deteriorate not only the toughness of the weld heat-affected zone but also the toughness of the base metal. The upper limit shall be 0.01%.
Furthermore, if the Ti content is less than 0.005%, the above effects cannot be effectively exhibited. Therefore, the range of Ti is set to 0.005 to 0.01%. Ca generally has the effect of changing the shape of inclusions such as MnS, improving toughness and reducing the directionality of toughness.
The upper limit is set at 0.003%, where the effect is saturated. Note that when low S steel is targeted, it is not necessary to particularly add Ca. Next, the present invention uses steel having the above chemical composition.
It is heated to 1050-1250℃, hot rolling is completed at a temperature of Ar 3 points or higher, water-cooled as it is, water-cooled to a temperature of 100℃ or lower, and then heated and tempered again to a temperature of Ac 1 or lower. One of the important points of the invention is to carry out this process, and the reasons for limiting the process conditions will be described below. First, the heating temperature of the steel slab was set at 1050°C to 1250°C, but the lower limit temperature was set at 1050°C as a strengthening mechanism to obtain high-strength steel with low stress corrosion cracking resistance, which is the objective of the present invention. When utilizing the strengthening caused by the precipitation of Nb carbon and nitrides, it is necessary to fully dissolve the Nb carbon and nitrides present in the steel slab into the steel base when heating the steel slab, but the temperature is lower than 1050℃. This is because at temperatures above 1250°C, this solid solution effect is not sufficient, and therefore sufficient precipitation hardening cannot be expected during tempering after water cooling.On the other hand, at temperatures above 1250°C, solid solution of Nb carbon and nitrides is sufficient. However, when the steel contains a large amount of Ni, such as the steel subject to the present invention, specific oxides tend to appear on the surface of the steel slab, and these tend to remain on the surface of the steel sheet after rolling and cause surface flaws. Therefore, the heating temperature is
The temperature shall be 1050-1250℃. After heating in this way, hot rolling is performed, and after rolling, the steel plate is water-cooled while still hot, but the water-cooling start temperature at this time needs to be at least 3 points of Ar, which each steel has. This is due to the necessity of making the structure sufficiently martensitic. In this case, if the martensitic structure obtained after quenching is made finer, the toughness after tempering will be even better.To achieve this, sufficient rolling should be carried out within the range of Ar 3 points to Ar 3 +100℃ during rolling before water cooling. It is also possible to adopt a reduction schedule such as the following, but this invention does not particularly specify it. In addition, when performing water cooling from a temperature of Ar 3 or higher, it is preferable to adjust the water cooling conditions depending on the thickness of the rolled plate. Next, the reason why the water-cooling stop temperature is specified as 100℃ or less is because if water-cooling is stopped at a temperature exceeding 100℃, the martensitic morphology may not yet be completed in the case of the steel subject to the present invention, which significantly affects the strength and toughness. This is because it may have an impact. In addition, tempering is performed in a reheating tempering furnace, but since the Ac 1 point of steel with the chemical composition specified in the present invention is 540 to 570°C, tempering at a temperature slightly higher than this is necessary in order to improve the strength-toughness balance. This is the best, and when the temperature exceeds 600℃, the toughness deteriorates due to the precipitation of unstable austenite.
Therefore, the tempering temperature was limited to 600°C or less. Note that, although not particularly stipulated, water cooling may be performed after tempering in order to prevent tempering embrittlement. Next, examples will be described in order to clarify the effects of the present invention. (Example) Chemical components A-1 to A-10 as shown in Table 1,
Steel having B-1 to B-5 is produced, hot worked and rolled to a thickness of 32 to 180 mm, and directly quenched under the conditions of No. 1 to No. 18 shown in Table 2. Tempering was performed. For these, the mechanical properties of the base metal and the K ISCC values of the base metal and weld heat affected zone were investigated. Welding is TIG welding with a heat input of 25KJ/cm. Mechanical properties obtained by various combinations of steels having chemical compositions shown in Table 1 and heat treatment conditions shown in Table 2, and mechanical properties in 3.5% artificial seawater.
Table 3 shows the K ISCC test results using the test pieces specified in ASME E399. Chemical components No. B-1 to B-5 in Table 1 are examples of components that deviate from the chemical component range specified in the present invention, and heat treatment conditions No. 16 to 18 in Table 2 are examples of chemical components that are outside the range of chemical components specified in the present invention. This is an example of deviating from the scope defined by the invention.
【表】
鋼塊より直接試験板厚みに圧延。
○ Ar3点(℃)は下記の式により計算した。
Ar3(℃)=863−396C+24.6Si−68.1Mn−36.
1Ni−20.7Cu−24.8Cr+29.6Mo
[Table] Rolled directly from steel ingot to test plate thickness.
○ Ar 3 points (°C) were calculated using the following formula.
Ar 3 (℃)=863−396C+24.6Si−68.1Mn−36.
1Ni−20.7Cu−24.8Cr+29.6Mo
【表】【table】
【表】【table】
【表】【table】
【表】
第3表の結果からわかる如く、母材の強度、靭
性および溶接熱影響部の良好なKISCCを得るには
本発明の規定する化学成分と本発明の規定する製
造条件を遵守することを前提にした直接焼入れ、
焼戻しによる製造方法をとることによつてのみ達
成できており、化学成分が本発明の規定する範囲
を逸脱するもの、或いは化学成分が規定内に入つ
ていても本発明の規定する製造条件を外れるもの
は本発明の目的とする耐応力腐蝕割れ性のすぐれ
た高張力鋼は得られていない。
(発明の効果)
以上の実施例からも明らかなように、本発明に
よれば靭性にすぐれ且つ海水等の腐蝕環境下にお
いても充分な健全性を示す高張力鋼を製造するこ
とが可能となり、産業上の効果は極めて顕著なも
のがある。[Table] As can be seen from the results in Table 3, in order to obtain good K ISCC for the strength and toughness of the base metal and the weld heat affected zone, it is necessary to comply with the chemical composition specified by the present invention and the manufacturing conditions specified by the present invention. Direct quenching based on the premise that
This can only be achieved by using a manufacturing method using tempering, and even if the chemical components are outside the range specified by the present invention, or even if the chemical components are within the specified range, the manufacturing conditions specified by the present invention cannot be met. If this is not the case, the high tensile strength steel with excellent stress corrosion cracking resistance, which is the object of the present invention, has not been obtained. (Effects of the Invention) As is clear from the above examples, according to the present invention, it is possible to produce high-strength steel that has excellent toughness and exhibits sufficient soundness even in corrosive environments such as seawater. The industrial effects are extremely significant.
Claims (1)
物とFeからなる鋼片を1050〜1250℃に加熱し、
Ar3点以上の温度で熱間圧延を終了させ、そのま
ま水冷を行い、100℃以下の温度になつてから水
冷を停止し、その後600℃以下の温度で焼戻すこ
とを特徴とする耐応力腐蝕割れ性のすぐれた超高
張力鋼の製造法。[Claims] 1. Contains C 0.05-0.10%, Si 0.20% or less, Mn 0.4-1.5%, Ni 5.0-8.0%, Mo 0.3-1.5%, Nb 0.005-0.05%, P 0.010% Below, O is limited to 0.0025% or less, N is limited to 0.0040% or less, and if necessary, Cu 0.2 to 1.5%, Cr 0.60% or less, V 0.02 to 0.12%, Total Al 0.08% or less, Ti 0.005 to 0.01%, Ca. A steel billet containing 0.003% or less of one or more kinds, with the remainder consisting of unavoidable impurities and Fe is heated to 1050-1250℃,
Stress corrosion resistance characterized by finishing hot rolling at a temperature of Ar 3 or higher, directly water cooling, stopping water cooling when the temperature reaches 100°C or lower, and then tempering at a temperature of 600°C or lower. A manufacturing method for ultra-high tensile strength steel with excellent crackability.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP11373185A JPS61272316A (en) | 1985-05-27 | 1985-05-27 | Manufacture of high tension steel having more than 100kgf/mm2 yield strength and superior in stress corrosion cracking resistance |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP11373185A JPS61272316A (en) | 1985-05-27 | 1985-05-27 | Manufacture of high tension steel having more than 100kgf/mm2 yield strength and superior in stress corrosion cracking resistance |
Publications (2)
Publication Number | Publication Date |
---|---|
JPS61272316A JPS61272316A (en) | 1986-12-02 |
JPH0151526B2 true JPH0151526B2 (en) | 1989-11-06 |
Family
ID=14619708
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
JP11373185A Granted JPS61272316A (en) | 1985-05-27 | 1985-05-27 | Manufacture of high tension steel having more than 100kgf/mm2 yield strength and superior in stress corrosion cracking resistance |
Country Status (1)
Country | Link |
---|---|
JP (1) | JPS61272316A (en) |
Families Citing this family (5)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPS63241114A (en) * | 1986-11-14 | 1988-10-06 | Nippon Steel Corp | Manufacture of high toughness and high tension steel having superior resistance to stress corrosion cracking |
JPH01230713A (en) * | 1988-03-08 | 1989-09-14 | Nippon Steel Corp | Production of high-strength and high-toughness steel having excellent stress corrosion cracking resistance |
US5827379A (en) * | 1993-10-27 | 1998-10-27 | Nippon Steel Corporation | Process for producing extra high tensile steel having excellent stress corrosion cracking resistance |
BR102015018600A2 (en) * | 2015-08-03 | 2017-02-07 | Inst Alberto Luiz Coimbra De Pós Graduação E Pesquisa De Engenharia - Coppe/Ufrj | 9% modified nickel steel alloy, 9% modified nickel steel alloy composition and its uses |
CN112342458B (en) * | 2020-09-01 | 2022-01-11 | 南京钢铁股份有限公司 | Low-yield-ratio stress corrosion cracking resistant high-strength steel and preparation method thereof |
Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPS569358A (en) * | 1979-07-03 | 1981-01-30 | Nippon Steel Corp | High strength high toughness steel |
-
1985
- 1985-05-27 JP JP11373185A patent/JPS61272316A/en active Granted
Patent Citations (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPS569358A (en) * | 1979-07-03 | 1981-01-30 | Nippon Steel Corp | High strength high toughness steel |
Also Published As
Publication number | Publication date |
---|---|
JPS61272316A (en) | 1986-12-02 |
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