JP2012140690A - Method of manufacturing two-phase stainless steel excellent in toughness and corrosion resistance - Google Patents
Method of manufacturing two-phase stainless steel excellent in toughness and corrosion resistance Download PDFInfo
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本発明は、耐海水用途向けに使用される部材、特にシャフト類、バルブ、フランジ、配管類、計測機器等に使用される靭性および耐食性に優れた二相系ステンレス鋼の製造方法に関するものである。 The present invention relates to a method for producing a duplex stainless steel having excellent toughness and corrosion resistance used for members used for seawater-resistant applications, particularly shafts, valves, flanges, piping, measuring instruments and the like. .
一般に、二相系ステンレス鋼は優れた耐食性と強度を備えていることから、耐海水用途や化学プラント機器材料として広く使用されている。この二相系ステンレス鋼の耐食性は、Ｃｒ，Ｍｏ等の合金元素によってもたらされている。しかし、これらの元素はσ相等の脆い金属間化合物を形成しやすく、製造条件によっては、著しい靭性低下をもたらす。 In general, duplex stainless steels are widely used as seawater resistant applications and chemical plant equipment materials because they have excellent corrosion resistance and strength. The corrosion resistance of this duplex stainless steel is brought about by alloying elements such as Cr and Mo. However, these elements are liable to form brittle intermetallic compounds such as σ phase, and cause a significant decrease in toughness depending on the production conditions.
そこで、従来、二相系ステンレス鋼の製造方法として、多くがσ相析出や４７５℃脆性を防止するため、固溶化熱処理温度の冷却速度を規定しているものが多く、それ以外として固溶化熱処理をインライン化することで製造コストを低減するもので占められている。その代表する例として、特開平１−１６５７２０号公報（特許文献１）に開示されているように、二相系ステンレス鋼素材を熱間圧延した後に急冷する二相系ステンレス鋼の製造方法において、９５０℃以上の仕上げ温度で熱間圧延した直後に圧延ラインにて素材を水冷し、その表面温度を６００℃以下に、一旦低下させた後に放冷する方法が提案されている。 Therefore, conventionally, as a method for producing duplex stainless steel, many of them have prescribed a cooling rate of the solution heat treatment temperature in order to prevent σ phase precipitation and 475 ° C. brittleness. It is occupied by what reduces the manufacturing cost by inlining. As a representative example thereof, as disclosed in JP-A-1-165720 (Patent Document 1), in a method for producing a duplex stainless steel that is rapidly cooled after hot rolling a duplex stainless steel material, There has been proposed a method in which a raw material is water-cooled in a rolling line immediately after hot rolling at a finishing temperature of 950 ° C. or higher, and the surface temperature is once lowered to 600 ° C. or lower and then allowed to cool.
また、特開２００８−２３１４６４号公報（特許文献２）に開示されているように、熱間鍛造または熱間圧延により製造され、または鋳造ままの二相系ステンレス鋼片の固溶化熱処理において、１０５０℃以上の温度で均熱した後、鋼片の外表面温度で９２０〜１０００℃までを１５０℃／ｈｒ以下の冷却速度で冷却し、さらにその温度から少なくとも３００℃までを３０〜４０℃／ｍｉｎの冷却速度で冷却することで、σ相析出や４７５℃脆性を防止しつつ、残留応力の発生を抑制し、内部割れを防止する製造方法が提案されている。 Further, as disclosed in Japanese Patent Application Laid-Open No. 2008-231464 (Patent Document 2), in a solution heat treatment of a duplex stainless steel piece produced by hot forging or hot rolling or as cast, 1050 After soaking at a temperature of ℃ or higher, the outer surface temperature of the steel slab is cooled to 920 to 1000 ℃ at a cooling rate of 150 ℃ / hr or less, and further from that temperature to at least 300 ℃ to 30 to 40 ℃ / min A production method has been proposed in which the generation of residual stress is suppressed and internal cracks are prevented while cooling at a cooling rate of σ, while preventing σ phase precipitation and 475 ° C brittleness.
また、特開平９−２１７１４９号公報（特許文献３）に開示されているように、フェライト相の平均結晶粒径が５０μｍ以上で製品中の残留応力が３００Ｎ／ｍｍ2以下である二相系ステンレス鋼の大型鋳造品、鍛造品として、固溶化熱処理において、１０００〜１１００℃の温度に加熱し、９８０〜７００℃の温度範囲を１０℃／ｍｉｎ以上の速度で冷却し、ついで５００℃までは冷却速度５〜１２．５℃／ｍｉｎの速度で徐冷し、その後少なくとも３００℃までを１０℃／ｍｉｎ以上の速度で急冷する方法が提案されている。 Further, as disclosed in Japanese Patent Application Laid-Open No. 9-217149 (Patent Document 3), a duplex stainless steel in which the average crystal grain size of the ferrite phase is 50 μm or more and the residual stress in the product is 300 N / mm 2 or less. In solid solution heat treatment as steel large castings and forgings, heat to 1000-1100 ° C, cool the temperature range of 980-700 ° C at a rate of 10 ° C / min or more, then cool to 500 ° C A method of slow cooling at a rate of 5 to 12.5 ° C./min and then rapidly cooling to at least 300 ° C. at a rate of 10 ° C./min or more has been proposed.
さらに、特開２００９−２５６７９１号公報（特許文献４）に開示されているように、１１００℃以上の温度で鍛造または圧延後、直ちに冷却速度１００℃／ｍｉｎ以上で８００℃以下まで急冷してフェライト相内に炭窒化物を析出させた後、固溶化熱処理温度９５０〜１１００℃に加熱し、急冷することにより、耐食性に優れた二相系ステンレス鋼を製造する方法が提案されている。
上述した特許文献１は、固溶化熱処理をインライン化することで製造コストを低減するものである。また特許文献２〜３は、σ相析出や４７５℃脆性を防止するため固溶化熱処理時の冷却速度を規定しているものである。しかしながら、発明者は、これらの熱処理によっても脆化が十分に防止されないことを経験しており、靱性は固溶化熱処理の前組織によって大きく変化することを知見した。 Patent document 1 mentioned above reduces manufacturing cost by making solution heat treatment in-line. Patent Documents 2 to 3 specify the cooling rate during solution heat treatment in order to prevent σ phase precipitation and 475 ° C brittleness. However, the inventor has experienced that these heat treatments do not sufficiently prevent embrittlement, and has found that toughness varies greatly depending on the structure before the solution heat treatment.
一方、特許文献４は、鍛造または圧延後は急冷し、その後固溶化熱処理を行うことで、フェライト相内や粒界の炭窒化物を少なくして、耐食性を改善するものである。しかし、炭窒化物を核として生成したオーステナイト相は微細で、亀裂伝播抑制効果が小さいため、十分な靭性が得られない。 On the other hand, Patent Document 4 improves the corrosion resistance by quenching after forging or rolling and then performing solution heat treatment to reduce carbonitride in the ferrite phase and grain boundaries. However, since the austenite phase produced with carbonitrides as nuclei is fine and has a small crack propagation inhibiting effect, sufficient toughness cannot be obtained.
靭性の向上には、固溶化熱処理後の組織において、σ相析出や４７５℃脆性が生じていないことに加え、亀裂伝播抑制に寄与するオーステナイト相の形状が重要である。そこで発明者らは、上述した問題を解消するために鋭意開発を進めた結果、鍛造または圧延後の冷却速度により、固溶化熱処理後の靭性が異なることを新たに知見した。すなわち、本発明は鍛造または圧延時の冷却速度に着目したもので、鍛造または圧延後の冷却速度を５℃／ｍｉｎ以下で徐冷し、ついで固溶化熱処理を行うことにより、靭性と耐食性に優れた二相系ステンレス鋼を提供するものである。 In order to improve toughness, the shape of the austenite phase contributing to crack propagation suppression is important in addition to the occurrence of σ phase precipitation and 475 ° C. brittleness in the structure after solution heat treatment. Thus, the inventors have made extensive developments to solve the above problems, and as a result, have newly found that the toughness after solution heat treatment varies depending on the cooling rate after forging or rolling. That is, the present invention focuses on the cooling rate at the time of forging or rolling, and is excellent in toughness and corrosion resistance by slow cooling the cooling rate after forging or rolling at 5 ° C./min or less and then performing a solution heat treatment. In addition, a two-phase stainless steel is provided.
その発明の要旨とするところは、
（１）１０００〜１３００℃の温度域で鍛造または圧延後、冷却速度５℃／ｍｉｎ以下で徐冷した後、固溶化熱処理温度９５０〜１１２５℃で均熱後、急冷することを特徴とする靭性、耐食性に優れた二相系ステンレス鋼の製造方法。
（２）質量％で、Ｃ：０．０８％以下、Ｓｉ：１．００％以下、Ｍｎ：２．００％以下、Ｐ：０．０４０％以下、Ｓ：０．０３０％以下、Ｎｉ：３．０〜８．０％、Ｃｒ：２１〜２６％、Ｍｏ：１．０〜５．０％、Ｃｕ：１．０％以下、Ｗ：２．０％以下、Ｎ：０．１０〜０．３５％、残部を不可避不純物からなる二相系ステンレス鋼において、１０００〜１３００℃の温度域で鍛造または圧延後、冷却速度５℃／ｍｉｎ以下で徐冷した後、固溶化熱処理温度９５０〜１１２５℃に均熱後、急冷することを特徴とする靭性、耐食性に優れた二相系ステンレス鋼の製造方法にある。
The gist of the invention is that
(1) Toughness after forging or rolling in a temperature range of 1000 to 1300 ° C, annealing at a cooling rate of 5 ° C / min or less, soaking at a solution heat treatment temperature of 950 to 1125 ° C, and then rapidly cooling. , A method for producing duplex stainless steel with excellent corrosion resistance.
(2) By mass, C: 0.08% or less, Si: 1.00% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.030% or less, Ni: 3 0.0 to 8.0%, Cr: 21 to 26%, Mo: 1.0 to 5.0%, Cu: 1.0% or less, W: 2.0% or less, N: 0.10 to 0.0. In a duplex stainless steel consisting of 35% and the balance of inevitable impurities, after forging or rolling in a temperature range of 1000 to 1300 ° C., after slow cooling at a cooling rate of 5 ° C./min or less, a solution heat treatment temperature of 950 to 1125 ° C. In the method for producing a duplex stainless steel excellent in toughness and corrosion resistance, characterized by rapid cooling after soaking.
本発明の鍛造または圧延後は徐冷し、ついで固溶化熱処理を行う二相系ステンレス鋼の製造方法によれば、σ相析出や４７５℃脆性を防止することができ、またフェライト相中の炭窒化物を核とした微細オーステナイト相が生成しにくいため、例え同鋼種であっても、靭性、耐食性に優れた二相系ステンレス鋼を提供することができる。 According to the method for producing a duplex stainless steel, which is gradually cooled after forging or rolling according to the present invention and then subjected to solution heat treatment, σ phase precipitation and 475 ° C. brittleness can be prevented, and carbon in the ferrite phase can be prevented. Since a fine austenite phase with nitride as a core is difficult to form, even if it is the same steel type, a duplex stainless steel having excellent toughness and corrosion resistance can be provided.
以下、本発明について詳細に説明する。
二相系ステンレス鋼とは、主にＪＩＳＧ４３０３のステンレス鋼棒やＪＩＳＧ３４５９の配管用ステンレス鋼管およびＪＩＳＧ３４６３のボイラ・熱交換器用ステンレス鋼鋼管、或いはＡＳＴＭではＡ３２１やＡ４７９、Ａ７８９、Ａ７９０で規定されているオーステナイト・フェライト系に分類されるもので、代表的な鋼種としてＳＵＳ３２９Ｊ１、ＳＵＳ３２９Ｊ３Ｌ、ＳＵＳ３２９Ｊ４Ｌ、Ｓ３１８０３、Ｓ３２７５０等が挙げられる。
Hereinafter, the present invention will be described in detail.
The duplex stainless steel is mainly a stainless steel rod of JIS G4303, a stainless steel pipe for piping of JIS G3459 and a stainless steel pipe for boiler / heat exchanger of JIS G 3463, or austenite specified by ASTM A321, A479, A789 and A790. -It is classified into a ferrite type, and typical steel types include SUS329J1, SUS329J3L, SUS329J4L, S31803, S32750, and the like.
二相系ステンレス鋼は、オーステナイト相とフェライト相がほぼ等分に含有し、熱履歴によりこの相の分配率が大きく変化する。１１００℃を超えるとフェライト相が増加し、逆にオーステナイト相が減少するため、ＣやＮの固溶度が高いオーステナイト相の減少により固溶できなかったＣやＮがフェライト相内で炭窒化物として析出する。この現象は二相系ステンレス鋼特有の現象であり、オーステナイト系ステンレス鋼やマルテンサイト系ステンレス鋼ではみられない。 Duplex stainless steel contains an austenite phase and a ferrite phase almost equally, and the distribution ratio of this phase varies greatly depending on the thermal history. When the temperature exceeds 1100 ° C., the ferrite phase increases, and conversely, the austenite phase decreases. Therefore, C and N, which cannot be dissolved due to the decrease in the austenite phase with high C and N solid solubility, are carbonitrides in the ferrite phase. To be deposited. This phenomenon is unique to duplex stainless steels, and is not seen in austenitic stainless steels or martensitic stainless steels.
以下、本発明の成分組成の限定理由を説明する。
Ｃ：０．０８％以下
Ｃは、オーステナイト相の体積分率を高めると共にオーステナイト相中に濃化して、オーステナイト相の安定度を高める元素である。しかし、０．０８％を超えて含有させるとＣｒ炭化物が生成して耐食性、靱性が劣化することから、その上限を０．０８％とした。
Hereinafter, the reason for limitation of the component composition of the present invention will be described.
C: 0.08% or less C is an element that increases the volume fraction of the austenite phase and concentrates in the austenite phase to increase the stability of the austenite phase. However, if the content exceeds 0.08%, Cr carbide is generated and corrosion resistance and toughness deteriorate, so the upper limit was made 0.08%.
Ｓｉ：１．００％以下
Ｓｉは、脱酸のための元素である。しかし、１．００％を超えて添加すると耐食性、靱性が劣化することから、その上限を１．００％とした。
Ｍｎ：２．００％以下
Ｍｎは、脱酸のための元素である。しかし、２．００％を超えて添加すると耐食性、靱性が劣化することから、その上限を２．００％とした。
Si: 1.00% or less Si is an element for deoxidation. However, if added over 1.00%, corrosion resistance and toughness deteriorate, so the upper limit was made 1.00%.
Mn: 2.00% or less Mn is an element for deoxidation. However, if added over 2.00%, corrosion resistance and toughness deteriorate, so the upper limit was made 2.00%.
Ｐ：０．０４０％以下
Ｐは、耐食性および靱性が劣化することから、その上限を０．０４０％とした。
Ｓ：０．０３０％以下
Ｓは、耐食性および靱性が劣化することから、その上限を０．０３０％とした。
P: 0.040% or less P has an upper limit of 0.040% because corrosion resistance and toughness deteriorate.
S: 0.030% or less S has an upper limit of 0.030% because corrosion resistance and toughness deteriorate.
Ｎｉ：３．０〜８．０％
Ｎｉは、オーステナイト安定化元素として用いられ、二相系ステンレス鋼を得るための重要な元素である。オーステナイト・フェライト相比を望ましい値とするため、Ｎｉ含有量は、３．０〜８．０％とする。
Ni: 3.0-8.0%
Ni is used as an austenite stabilizing element and is an important element for obtaining a duplex stainless steel. In order to set the austenite-ferrite phase ratio to a desirable value, the Ni content is set to 3.0 to 8.0%.
Ｃｒ：２１〜２６％
Ｃｒは、耐食性を確保する必須元素であり、その効果を十分に得るためには２１％を必要とする。しかし、２６％を超えると、金属間化合物が析出しやすくなり靱性を阻害し、鋼中にオーステナイト相を生成させることが困難になる。したがって、その範囲を２１〜２６％とした。
Cr: 21-26%
Cr is an essential element that ensures corrosion resistance, and 21% is required to obtain the effect sufficiently. However, if it exceeds 26%, intermetallic compounds are likely to precipitate, impairing toughness, and it becomes difficult to generate an austenite phase in the steel. Therefore, the range was 21 to 26%.
Ｍｏ：１．０〜５．０％
Ｍｏは、ステンレス鋼の耐食性を付加的に高める有効な元素であり、その効果を得るためには、１．０％必要である。しかし、５．０％を超えるとＣｒと共に金属間化合物の析出を促進する元素であるため、その上限を５．０％とした。
Mo: 1.0-5.0%
Mo is an effective element that additionally enhances the corrosion resistance of stainless steel, and 1.0% is necessary to obtain the effect. However, if it exceeds 5.0%, it is an element that promotes precipitation of intermetallic compounds together with Cr, so the upper limit was made 5.0%.
Ｃｕ：１．０％以下
Ｃｕは、ステンレス鋼の耐食性を付加的に高める有効な元素である。しかし、１．０％を超えると熱間加工性を劣化させることから、その上限を１．０％とした。
Ｗ：２．０％以下
Ｗは、Ｍｏと同様に、ステンレス鋼の耐食性を付加的に高める有効な元素である。しかし、２．０％を超えて含有させると金属間化合物が析出し靱性が低下することから、その上限を２．０％とした。
Cu: 1.0% or less Cu is an effective element that additionally enhances the corrosion resistance of stainless steel. However, if it exceeds 1.0%, the hot workability deteriorates, so the upper limit was made 1.0%.
W: 2.0% or less W, like Mo, is an effective element that additionally enhances the corrosion resistance of stainless steel. However, if the content exceeds 2.0%, an intermetallic compound precipitates and the toughness decreases, so the upper limit was made 2.0%.
Ｎ：０．１０〜０．３５％
Ｎは、Ｎｉと同様に、オーステナイト相を安定化させる作用を持つ元素である。また、Ｎは、ＣｒとＭｏと同様に、耐食性を高める効果を有する元素である。しかし、その含有量が０．１０％未満では、これらの効果が十分でない。また、０．３５％を超えて含有させるとＣｒ窒化物析出を促進することから、その範囲を０．１０〜０．３５％とした。
N: 0.10 to 0.35%
N, like Ni, is an element that has the effect of stabilizing the austenite phase. N, like Cr and Mo, is an element having an effect of improving corrosion resistance. However, if the content is less than 0.10%, these effects are not sufficient. Moreover, since Cr nitride precipitation will be accelerated | stimulated if it contains exceeding 0.35%, the range was made into 0.10-0.35%.
１０００℃〜１３００℃の温度域で鍛造または圧延する理由は、熱間での延性を確保するためであり、１０００℃未満になると高Ｃｒ，Ｍｏの鋼種ではσ相析出温度域となるため、熱間加工性が顕著に悪化するためである。鍛造または圧延後、冷却速度５℃／ｍｉｎ以下で徐冷し、フェライト相内における析出炭窒化物の微細化を抑制し、後の固溶化熱処理時にこれらを核として析出するオーステナイト相の微細化を防止する。 The reason for forging or rolling in the temperature range of 1000 ° C. to 1300 ° C. is to ensure hot ductility. When the temperature is less than 1000 ° C., the high Cr and Mo steel grades are in the σ phase precipitation temperature range. This is because the inter-workability is significantly deteriorated. After forging or rolling, the steel is gradually cooled at a cooling rate of 5 ° C./min or less to suppress the refinement of the precipitated carbonitride in the ferrite phase, and to refine the austenite phase that precipitates as a nucleus during the subsequent solution heat treatment. To prevent.
また、固溶化熱処理温度９５０〜１１２５℃で均熱後、急冷することに限定した理由は、この温度域はオーステナイト相が元々あるオーステナイト相を核として成長しやすく、かつσ相や炭窒化物の析出温度以上であるためであり、急冷することで、σ相析出や４７５℃脆性を防止すると共に、炭窒化物の析出を軽減するためである。 Further, the reason for limiting to rapid cooling after soaking at a solution heat treatment temperature of 950 to 1125 ° C. is that this temperature range is easy to grow with the austenite phase originally having an austenite phase as the core, and the σ phase and carbonitride This is because the temperature is equal to or higher than the precipitation temperature, and by rapid cooling, σ phase precipitation and 475 ° C. brittleness are prevented and carbonitride precipitation is reduced.
以下、本発明について実施例によって具体的に説明する。
表１に示す各種化学成分１００ｋｇ鋼塊を真空溶解炉にて溶解し、１２００℃に加熱し、径６０ｍｍに鍛延したものを試験に供した。これらの鋼材を鍛造或いは圧延終止温度である加熱温度１０００〜１２５０℃で３０ｍｉｎ加熱し、冷却速度５℃／ｍｉｎ以下、または６〜２０℃／ｍｉｎで冷却した後、９５０〜１１２５℃で３０ｍｉｎの固溶化熱処理をした後、冷却速度２０℃／ｍｉｎ以上で冷却した結果を表２に示す。
Hereinafter, the present invention will be specifically described with reference to examples.
100 kg steel ingots having various chemical components shown in Table 1 were melted in a vacuum melting furnace, heated to 1200 ° C., and forged to a diameter of 60 mm, and then used for the test. These steel materials are heated for 30 minutes at a heating temperature of 1000 to 1250 ° C., which is a forging or rolling end temperature, cooled at a cooling rate of 5 ° C./min or less, or 6 to 20 ° C./min, and then solidified at 950 to 1125 ° C. for 30 minutes. Table 2 shows the results of cooling at a cooling rate of 20 ° C./min or higher after the solution heat treatment.
靭性はシャルピー衝撃試験で評価した。試験片はＴ方向採取−Ｌ方向ノッチとし、鋼種Ａは２ｍｍ−Ｕノッチ、鋼種Ｂ〜Ｃは２ｍｍ−Ｖノッチで作製した。また試験条件は、試験温度２３℃、各条件につきＮ＝２である。
ミクロ組織観察は、供試材のＬ面中周部近傍を鏡面仕上げし、１０％シュウ酸電解腐食（１Ａ／ｃｍ2、３０ｓｅｃ）の腐食試験を行い、光学顕微鏡にて組織観察を行った。
Toughness was evaluated by Charpy impact test. The test piece was made with a T-direction sampling-L-direction notch, steel type A with a 2 mm-U notch, and steel types B to C with a 2 mm-V notch. The test conditions are a test temperature of 23 ° C. and N = 2 for each condition.
In the microstructure observation, the vicinity of the L surface middle periphery of the test material was mirror finished, a 10% oxalic acid electrolytic corrosion (1 A / cm 2 , 30 sec) corrosion test was performed, and the structure was observed with an optical microscope.
表２に示すように、Ｎｏ．１〜６は鋼種Ａ、Ｎｏ．７〜１０は鋼種Ｂ、Ｎｏ．１１〜１４は鋼種Ｃである。また、冷却速度は加熱温度からのは冷却速度が５℃／ｍｉｎ以下である本発明例と、加熱温度からの冷却速度が６〜２０℃／ｍｉｎの比較例を示す。 As shown in Table 2, no. 1 to 6 are steel types A and No. 7 to 10 are steel types B and No. 11 to 14 are steel types C. Moreover, the cooling rate shows the example of the present invention in which the cooling rate is 5 ° C./min or less from the heating temperature and the comparative example in which the cooling rate from the heating temperature is 6 to 20 ° C./min.
加熱温度からの冷却速度を５℃／ｍｉｎ以下と６〜２０℃／ｍｉｎとした場合のミクロ組織の代表例として、鋼種Ａのミクロ組織を図１に示す。比較例である、図１（ａ）の１１００℃での急冷材はフェライト相内に炭窒化物が多数析出しているが、本発明例である、図１（ｂ）の１１００℃での徐冷材には炭窒化物の析出は認められない。 As a representative example of the microstructure when the cooling rate from the heating temperature is 5 ° C./min or less and 6 to 20 ° C./min, the microstructure of steel type A is shown in FIG. The quenching material at 1100 ° C. in FIG. 1 (a), which is a comparative example, has many carbonitrides precipitated in the ferrite phase, but the slow cooling material at 1100 ° C. in FIG. 1 (b), which is an example of the present invention. No carbonitride precipitation is observed.
また、図１に示すミクロ組織となった鋼材の固溶化熱処理後のミクロ組織を図２に示す。図２（ａ）と図２（ｂ）は、同条件で固溶化熱処理を実施しているため、フェライト相量は同等となっている。しかしながら、固溶化熱処理を同条件で行った場合、いずれの鋼種においても、本発明例である冷却速度が５℃／ｍｉｎ以下での鋼材は、比較例である６〜２０℃／ｍｉｎの比較例鋼材よりも靭性が優れていた。 Moreover, the microstructure after the solution heat treatment of the steel material having the microstructure shown in FIG. 1 is shown in FIG. Since the solution heat treatment is performed under the same conditions in FIGS. 2A and 2B, the ferrite phase amounts are equal. However, when the solution heat treatment is performed under the same conditions, the steel material with a cooling rate of 5 ° C./min or less, which is an example of the present invention, is a comparative example of 6 to 20 ° C./min which is a comparative example. The toughness was superior to that of steel.
これは、比較例である加熱温度から急冷した鋼材は、フェライト相内に炭窒化物が多数析出していたため、固溶化熱処理時にこれらの炭窒化物を核とした、亀裂伝播抑制効果の小さい微細なオーステナイト相が生成しやすかったためと考えられる。
すなわち、圧延後の冷却速度を５℃／ｍｉｎ以下の徐冷とすることで、ミクロ組織のオーステナイト・フェライト相比は同等であっても、オーステナイト相の亀裂伝播抑制効果が向上した。
This is because the steel material rapidly cooled from the heating temperature as a comparative example had a large amount of carbonitrides precipitated in the ferrite phase. This is thought to be because a large austenite phase was easily generated.
That is, the effect of suppressing the propagation of cracks in the austenite phase was improved even if the austenite / ferrite phase ratio of the microstructure was the same by gradually cooling the rolling after the rolling at 5 ° C./min.
本発明は、二相系ステンレス鋼が熱履歴によりフェライト相内に炭窒化物を析出することに着目したものであり、二相系ステンレス鋼全般に共通する現象である。
したがって、本発明は、靭性、耐食性に優れた二相系ステンレス鋼全般に適用可能な製造方法であり、工業的に極めて優れた効果を奏するものである。
The present invention focuses on the fact that duplex stainless steel precipitates carbonitride in the ferrite phase due to thermal history, and is a phenomenon common to all duplex stainless steels.
Therefore, the present invention is a production method applicable to all of the duplex stainless steels having excellent toughness and corrosion resistance, and exhibits extremely excellent effects industrially.
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