JP2007126709A - Heat treatment method for making fine of crystal grains in high nitrogen stainless steel, and high nitrogen stainless steel - Google Patents
Heat treatment method for making fine of crystal grains in high nitrogen stainless steel, and high nitrogen stainless steel Download PDFInfo
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- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 title claims abstract description 112
- 229910052757 nitrogen Inorganic materials 0.000 title claims abstract description 60
- 229910001220 stainless steel Inorganic materials 0.000 title claims abstract description 48
- 239000010935 stainless steel Substances 0.000 title claims abstract description 47
- 239000013078 crystal Substances 0.000 title claims abstract description 27
- 238000000034 method Methods 0.000 title claims abstract description 23
- 238000010438 heat treatment Methods 0.000 title claims abstract description 22
- 230000009466 transformation Effects 0.000 claims abstract description 72
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 36
- 150000004767 nitrides Chemical class 0.000 claims abstract description 16
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 16
- 238000003303 reheating Methods 0.000 claims abstract description 8
- 238000007670 refining Methods 0.000 claims description 4
- 229910000831 Steel Inorganic materials 0.000 claims description 2
- 239000010959 steel Substances 0.000 claims description 2
- 229910045601 alloy Inorganic materials 0.000 description 15
- 239000000956 alloy Substances 0.000 description 15
- 239000011651 chromium Substances 0.000 description 12
- 239000000463 material Substances 0.000 description 9
- 229910001199 N alloy Inorganic materials 0.000 description 7
- 238000010521 absorption reaction Methods 0.000 description 6
- 239000012071 phase Substances 0.000 description 6
- 239000002245 particle Substances 0.000 description 5
- 229910000599 Cr alloy Inorganic materials 0.000 description 3
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 3
- 238000000137 annealing Methods 0.000 description 3
- 229910000963 austenitic stainless steel Inorganic materials 0.000 description 3
- 238000001816 cooling Methods 0.000 description 3
- 238000005260 corrosion Methods 0.000 description 3
- 230000007797 corrosion Effects 0.000 description 3
- 238000002844 melting Methods 0.000 description 3
- 230000008018 melting Effects 0.000 description 3
- 239000012299 nitrogen atmosphere Substances 0.000 description 3
- 230000003287 optical effect Effects 0.000 description 3
- 239000007790 solid phase Substances 0.000 description 3
- 210000001519 tissue Anatomy 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 201000004624 Dermatitis Diseases 0.000 description 2
- 238000002441 X-ray diffraction Methods 0.000 description 2
- 239000012298 atmosphere Substances 0.000 description 2
- 229910052804 chromium Inorganic materials 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 238000003672 processing method Methods 0.000 description 2
- 238000000746 purification Methods 0.000 description 2
- 238000001953 recrystallisation Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000005482 strain hardening Methods 0.000 description 2
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- 229910017112 Fe—C Inorganic materials 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 210000000988 bone and bone Anatomy 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 229910001873 dinitrogen Inorganic materials 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- 239000012567 medical material Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 230000006911 nucleation Effects 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
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- 229910052717 sulfur Inorganic materials 0.000 description 1
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Abstract
Description
本発明は、高窒素ステンレス鋼を恒温変態処理した後に逆変態処理することにより結晶粒を微細化することができる高窒素ステンレス鋼の結晶粒微細化熱処理方法及び高窒素ステンレス鋼に関するものである。 TECHNICAL FIELD The present invention relates to a high-nitrogen stainless steel crystal grain refinement heat treatment method and high-nitrogen stainless steel that can refine crystal grains by subjecting high-nitrogen stainless steel to isothermal transformation treatment and then reverse transformation treatment.
高濃度の窒素を含有する高窒素ステンレス鋼は、窒素による固溶強化作用および加工硬化率を増大させる作用により、通常のステンレス鋼に比べて高い引張強度を有するだけでなく耐熱性や耐食性にも優れているため、高温や腐食環境下で使用される高強度の特殊構造材料として有望視されている。窒素は、オーステナイト系ステンレス鋼に多量に含まれるＮｉと同様にオーステナイト相を熱力学的に安定化する作用があるため、Ｎｉの代替として利用可能である。また、生体用の場合には、Ｎｉが皮膚アレルギーの原因になることが問題となっていることから、Ｎｉの代替としての利用も可能であり、生体用Ｎｉフリーオーステナイト系ステンレス鋼の主要合金元素としても利用が期待されている（非特許文献１〜３参照）。
高窒素ステンレス鋼の優れた強度特性および耐食性を発揮するには、使用前に１２７３Ｋ以上の高温での熱処理が必要であり、それにより著しい結晶粒成長が引き起こされる。とくに、窒素雰囲気中での加熱により固相状態で窒素を吸収させる固相吸収法で作製した高窒素ステンレス鋼の場合、高温保持によりオーステナイト粒が数百μｍにまで粗大化してしまう。結晶粒成長が生じると、加工性および機械的性質は著しく損なわれ、加工時および変形時にしばしば結晶粒界を起点とした破壊を生じることが問題となる。 In order to exhibit the excellent strength characteristics and corrosion resistance of high nitrogen stainless steel, a heat treatment at a high temperature of 1273 K or more is required before use, thereby causing remarkable grain growth. In particular, in the case of a high nitrogen stainless steel produced by a solid-phase absorption method in which nitrogen is absorbed in a solid phase state by heating in a nitrogen atmosphere, austenite grains are coarsened to several hundred μm due to holding at a high temperature. When crystal grain growth occurs, workability and mechanical properties are remarkably impaired, and there is a problem that breakage is often caused at the grain boundary during processing and deformation.
前記問題を解決するため、結晶粒の微細化が有効になると考えられる。一般に、ステンレス鋼の結品粒を微細化する手段として、加工と焼鈍により生じる再結晶を利用した方法が実施される。しかし、前記再結晶を利用した結晶粒微細化法は、高窒素ステンレス鋼が高い耐力と大きな加工硬化能を有することから冷間加工性に乏しく且つ加工後の焼鈍中に窒化物の析出も懸念されるため、適用が困難である。さらに、十分な加工を施すことが可能な大型の板材や棒材への適用に限られ、小型の材料、または既に製品形状とされた材料に適用することはできないという問題もある。 In order to solve the above problem, it is considered that refinement of crystal grains is effective. In general, as a means for refining the resultant grain of stainless steel, a method using recrystallization generated by processing and annealing is performed. However, the crystal grain refining method using recrystallization has poor cold workability because high nitrogen stainless steel has high proof stress and large work hardening ability, and there is concern about precipitation of nitride during post-annealing. Therefore, application is difficult. Furthermore, it is limited to application to a large plate or bar that can be sufficiently processed, and there is a problem that it cannot be applied to a small material or a material that has already been shaped into a product.
そこで、本発明は加工を行うことなく熱処理だけで結品粒を微細化することができる高窒素ステンレス鋼の結晶粒微細化熱処理方法及び高窒素ステンレス鋼を提供するものである。 Therefore, the present invention provides a high-nitrogen stainless steel crystal grain refinement heat treatment method and a high-nitrogen stainless steel that can refine the resultant grains by only heat treatment without processing.
本発明の高窒素ステンレス鋼の結晶粒微細化熱処理方法は、高窒素ステンレス鋼を加熱してオーステナイト化した後、オーステナイトをフェライトと窒化物からなる混合組織へ恒温変態処理し、その後、再加熱して前記混合組織をオーステナイトに逆変態処理することを特徴とする。 The method for heat treatment of grain refinement of the high nitrogen stainless steel of the present invention is to heat the high nitrogen stainless steel to austenite, then subject the austenite to isothermal transformation to a mixed structure composed of ferrite and nitride, and then reheat. The mixed structure is reversely transformed to austenite.
また、逆変態処理後、再びオーステナイトをフェライトと窒化物からなる混合組織へ恒温変態処理し、その後、再加熱して前記混合組織をオーステナイトに逆変態処理して結晶粒をより微細化することもできる。 Further, after the reverse transformation treatment, the austenite is again subjected to isothermal transformation treatment to a mixed structure composed of ferrite and nitride, and then reheated to reverse transform the mixed structure to austenite to further refine the crystal grains. it can.
本発明は、例えば、Ｎを０．５〜１．５質量％含む高窒素ステンレス鋼を１３２０Ｋ〜１５２３Ｋの温度で加熱してオーステナイト化した後、８７３Ｋ〜１２７３Ｋに保持してオーステナイトをフェライトと窒化物からなる混合組織へ恒温変態処理し、その後１３２０Ｋ〜１５２３Ｋの温度に再加熱して前記混合組織をオーステナイトに逆変態処理することで、結晶粒を著しく微細化することができる。さらに、逆変態処理後、再び８７３Ｋ〜１２７３Ｋに保持してオーステナイトをフェライトと窒化物からなる混合組織へ恒温変態処理し、その後１３２０Ｋ〜１５２３Ｋの温度に再加熱して前記混合組織をオーステナイトに逆変態処理することにより結晶粒をより微細化することもできる。 In the present invention, for example, high nitrogen stainless steel containing 0.5 to 1.5% by mass of N is heated to austenite at a temperature of 1320K to 1523K, and then held at 873K to 1273K to hold the austenite with ferrite and nitride. The crystal structure can be remarkably miniaturized by subjecting the mixed structure to a constant temperature transformation treatment, followed by reheating to a temperature of 1320K to 1523K and reverse transformation treatment of the mixed structure to austenite. Further, after the reverse transformation treatment, the austenite is held at 873K to 1273K again, and the austenite is subjected to isothermal transformation treatment to a mixed structure composed of ferrite and nitride, and then reheated to a temperature of 1320K to 1523K to reverse transform the mixed structure to austenite. By processing, the crystal grains can be further refined.
本発明の結晶粒微細化熱処理方法は、Ｎを０．５〜１．５質量％、窒化物生成元素としてＣｒを１０．０〜３０．０質量％含む高窒素ステンレス鋼に適用することができる。高窒素ステンレス鋼は、例えば、ＮｉをＮで置換した高窒素ニッケルフリーオーステナイト系ステンレス鋼などで知られている。高窒素ステンレス鋼は、例えば、高圧力の窒素ガス雰囲気中で溶製を行う加圧溶製法や窒素雰囲気中での焼鈍により固相状態で窒素を吸収させる固相吸収法により製造される。 The grain refinement heat treatment method of the present invention can be applied to high nitrogen stainless steel containing 0.5 to 1.5% by mass of N and 10.0 to 30.0% by mass of Cr as a nitride-forming element. . High nitrogen stainless steel is known as, for example, high nitrogen nickel-free austenitic stainless steel in which Ni is replaced with N. High nitrogen stainless steel is manufactured by, for example, a pressure melting method in which melting is performed in a high-pressure nitrogen gas atmosphere or a solid-phase absorption method in which nitrogen is absorbed in a solid state by annealing in a nitrogen atmosphere.
高窒素ステンレス鋼は１３２０Ｋ〜１５２３Ｋで加熱することにより、オーステナイト化することができる。加熱後は、水冷により室温まで冷却する。 High nitrogen stainless steel can be austenitized by heating at 1320K to 1523K. After heating, it is cooled to room temperature by water cooling.
恒温変態は８７３Ｋ〜１２７３Ｋに保持し、フェライト（α）＋Ｃｒ２Ｎの共析組織にする。なお、オーステナイト化が終了すると、水冷することなく、８７３Ｋ〜１２７３Ｋに降温させて恒温変態させてもよい。恒温変態後に１３２０Ｋ〜１５２３Ｋのγ単相域で逆変態処理を施すことにより、オーステナイト粒が微細化される。 The isothermal transformation is maintained at 873K to 1273K and a eutectoid structure of ferrite (α) + Cr 2 N is formed. When the austenitization is completed, the temperature may be lowered to 873K to 1273K and the temperature may be transformed without cooling with water. Austenite grains are refined by applying a reverse transformation treatment in the γ single phase region of 1320K to 1523K after the isothermal transformation.
本発明の高窒素ステンレス鋼は、オーステナイト化された後、フェライトと窒化物からなる混合組織へ恒温変態処理され、その後、再加熱によるオーステナイトへの逆変態処理により微細な結晶粒組織が形成されていることを特徴とする。さらに、逆変態処理された後、再びフェライトと窒化物からなる混合組織へ恒温変態処理され、その後、再加熱によるオーステナイトへの逆変態処理により微細な結晶粒組織が形成されていることを特徴とする。また、結晶粒が２０μｍ以下であることを特徴とする。この高窒素ステンレス鋼は生体用に適している。 The high nitrogen stainless steel of the present invention is austenitized and then subjected to isothermal transformation to a mixed structure composed of ferrite and nitride, and then a fine grain structure is formed by reverse transformation to austenite by reheating. It is characterized by being. Furthermore, after the reverse transformation treatment, it is again subjected to isothermal transformation to a mixed structure composed of ferrite and nitride, and then a fine crystal grain structure is formed by reverse transformation treatment to austenite by reheating. To do. Further, the crystal grains are 20 μm or less. This high nitrogen stainless steel is suitable for biological use.
本発明は、オーステナイトをフェライトと窒化物からなる混合組織へと相変態させ、再加熱して再びオーステナイトに相変態させることにより、結晶粒径が微細化されたオーステナイト組織を得ることができる。また、本発明は加工を必要としないため、小型の材料や既に製品形状とされた材料の結晶粒の微細化にも適用可能である。 In the present invention, an austenite structure with a refined crystal grain size can be obtained by phase-transforming austenite into a mixed structure composed of ferrite and nitride, reheating and phase-transforming into austenite again. In addition, since the present invention does not require processing, the present invention can be applied to miniaturization of crystal grains of a small material or a material already in a product shape.
本発明の結晶粒微細化熱処理方法により得られた高窒素ステンレス鋼は、生体用に適しており、例えば、皮膚アレルギーを起こすＮｉを含まない、医療用・歯科用高窒素ニッケルフリーステンレス鋼を素材とする製品、例えば、歯列矯正器具用ワイヤー・ブラケット等の小型部品、ガイドワイヤ・ステント・骨プレート等の構造材料、耐食材料、医療用材料に用いられる高窒素ステンレス鋼について、その結晶粒の微細化により加工性および機械的性質を改善することが可能となる。また、身体に直接接触する時計や食器類などの用途にも適している。 The high nitrogen stainless steel obtained by the grain refinement heat treatment method of the present invention is suitable for use in living organisms, for example, medical and dental high nitrogen nickel-free stainless steel that does not contain Ni causing skin allergy. For example, high-nitrogen stainless steel used for small parts such as orthodontic appliance wires and brackets, structural materials such as guide wires, stents, and bone plates, corrosion resistant materials, and medical materials. Refinement makes it possible to improve processability and mechanical properties. It is also suitable for applications such as watches and tableware that come into direct contact with the body.
本発明を実施例により説明する。 The present invention is illustrated by examples.
図１は本発明による高窒素ステンレス鋼の結晶粒微細化熱処理方法の一実施例を示す概略図である。本実施例は、窒素吸収処理により高窒素ステンレス鋼を作成し、この過程で高窒素ステンレス鋼をオーステナイト化し、次いで恒温変態処理でオーステナイト（γ）をフェライト（α）と窒化物からなる混合組織へ相変態させ、その後、逆変態処理により再びオーステナイトに相変態させるステップからなる。
（１）オーステナイト化（窒素吸収処理）
高周波真空溶解炉にて純鉄、電解クロムを真空溶解し、鋳造して供試材を得た。供試材の組成（以下「％」は「質量％」である。）は、Ｃ：０．００５％、Ｎ：０．０３３％、Ｏ：０．００７％、Ｐ：０．００６％、Ｓ：０．００２６％、Ｃｒ：２５．４％、Ｍｎ：＜０．０１％、Ｓｉ：０．０３％、Ａｌ：０．０２％、残りＦｅである（以下「Ｆｅ−２５％Ｃｒ合金」という。）。本合金にはγ安定化元素はほとんど含まれておらず、窒素無添加の状態ではα単相組織を有する。
FIG. 1 is a schematic view showing one embodiment of a method for heat treating a crystal grain refinement of high nitrogen stainless steel according to the present invention. In this example, a high nitrogen stainless steel is prepared by nitrogen absorption treatment, and the high nitrogen stainless steel is austenitized in this process, and then austenite (γ) is converted into a mixed structure composed of ferrite (α) and nitride by isothermal transformation treatment. It comprises a step of phase transformation and then a phase transformation to austenite again by reverse transformation treatment.
(1) Austenitization (nitrogen absorption treatment)
Pure iron and electrolytic chromium were vacuum-melted in a high-frequency vacuum melting furnace and cast to obtain a test material. The composition of the test material (hereinafter “%” is “mass%”) is C: 0.005%, N: 0.033%, O: 0.007%, P: 0.006%, S : 0.0026%, Cr: 25.4%, Mn: <0.01%, Si: 0.03%, Al: 0.02%, remaining Fe (hereinafter referred to as “Fe-25% Cr alloy”) .) This alloy contains almost no γ-stabilizing element, and has an α single-phase structure when nitrogen is not added.
Ｆｅ−２５％Ｃｒ合金を金型鋳造した後、鋳造組織を壊すため熱間圧延し、その後、Ａｒ雰囲気中にて均質化焼鈍を施した。得られたＦｅ−２５％Ｃｒ合金を４０ｍｍ×１５ｍｍ×１ｍｍの薄板に切り出した後、研磨を施し、１気圧の窒素雰囲気中にて１４７３Ｋで１０８ｋｓの窒素吸収処理をした後水冷した。 After casting the Fe-25% Cr alloy, it was hot-rolled to break the cast structure, and then homogenized and annealed in an Ar atmosphere. The obtained Fe-25% Cr alloy was cut out into a thin plate of 40 mm × 15 mm × 1 mm, polished, and subjected to a nitrogen absorption treatment at 1473 K at 1473 K in a nitrogen atmosphere at 1 atm, followed by water cooling.
高温での窒素吸収処理後の試料は、試料の表面から窒素が内部へと拡散し、それに伴いαがγに相変態し、その粒径は４００μｍ程度にまで粗大になっている。この試料について化学分析を行った結果、固溶窒素濃度は約１．０１％であった（以下「Ｆｅ−２５％Ｃｒ−１％Ｎ合金」という。）。 In the sample after the nitrogen absorption treatment at a high temperature, nitrogen diffuses from the surface of the sample into the inside, and α is phase-transformed into γ, and the particle size is coarsened to about 400 μm. As a result of chemical analysis of this sample, the solid solution nitrogen concentration was about 1.01% (hereinafter referred to as “Fe-25% Cr-1% N alloy”).
（２）恒温変態処理
ついで、オーステナイト化した高窒素ステンレス鋼を各種温度で０．３〜１８ｋｓ恒温保持した。
(2) Isothermal transformation treatment Next, the austenitized high nitrogen stainless steel was kept at a constant temperature of 0.3 to 18 ks at various temperatures.
１２７３Ｋを超える温度で種々の時間恒温保持したＦｅ−２５％Ｃｒ−１％Ｎ合金の光顕組織およびＸ線回折の結果、この１２７３Ｋを超える温度での恒温保持ではα＋Ｃｒ２Ｎから構成された共析組織は試料全面で得られなかった。 As a result of optical microstructure and X-ray diffraction of an Fe-25% Cr-1% N alloy held at a temperature exceeding 1273 K for various times, eutectoid composed of α + Cr 2 N was observed at the constant temperature holding above 1273 K. Tissue was not obtained on the entire sample surface.
次ぎに、Ｆｅ−２５％Ｃｒ−１％Ｎ合金に１２２３Ｋ以下の種々の温度で恒温変態処理を施した。その結果、いずれの温度においても、γ→α＋Ｃｒ２Ｎの共析変態が起こり、試料全面で共析組織が確認された。α＋Ｃｒ２Ｎ共析組織の生成挙動は恒温保持の温度により大きく異なっており、１２２３Ｋで恒温保持した試料では、共析組織がγ粒界から生成して片側のγ粒内に向かって成長していくのに対し、低温の８７３Ｋで恒温保持した試料では、共析組織はγ粒内からも核生成していることがわかった。このことは、恒温保持の温度が低温になるほど、共析変態の核生成の頻度が高くなっていることを示唆している。 Next, a constant temperature transformation treatment was performed on the Fe-25% Cr-1% N alloy at various temperatures of 1223K or less. As a result, the eutectoid transformation of γ → α + Cr 2 N occurred at any temperature, and the eutectoid structure was confirmed on the entire surface of the sample. The formation behavior of the α + Cr 2 N eutectoid structure varies greatly depending on the temperature at which the temperature is maintained. In the sample maintained at a constant temperature of 1223K, the eutectoid structure is generated from the γ grain boundary and grows into the γ grain on one side. On the other hand, it was found that the eutectoid structure nucleated from the γ grains in the sample kept at a low temperature of 873 K. This suggests that the frequency of nucleation of eutectoid transformation increases as the temperature of the isothermal holding decreases.
一方、各温度での変態挙動を比較すると、１１７３Ｋではわずか０．３ｋｓ程度で変態は完了するが、それよりも高温の１２２３Ｋや低温の８７３Ｋでは、いずれも共析変態完了時間が長時間側へ移行している。
On the other hand, when the transformation behavior at each temperature is compared, the transformation is completed at about 0.3 ks at 1173K, but the eutectoid transformation completion time is longer for both the
図２は恒温保持温度が異なる他の試料でも同様な組織観察を行って作製したＴ．Ｔ．Ｔ．図である。高窒素γ鋼における、γ→α＋Ｃｒ２Ｎの共析変態の開始（Ｔｓ）・完了（Ｔｆ）はＦｅ−Ｃ系合金のパーライト変態と同様に典型的なＣ曲線となっており、１１７３Ｋ付近にノーズ（ｎｏｓｅ）が存在している。 FIG. 2 is a graph showing the structure of T.P. T.A. T.A. FIG. The onset (T s ) and completion (T f ) of the eutectoid transformation of γ → α + Cr 2 N in the high nitrogen γ steel is a typical C curve, similar to the pearlite transformation of the Fe—C alloy, 1173K There is a nose in the vicinity.
図３は各温度での恒温保持後に得られる共析組織の模式図である。高温域では、共析組織が３０μｍ程度のブロックから構成され、その内部には板状や塊状のＣｒ２Ｎが不均一に分布しているのに対して、低温域では、変態温度の低下に伴い、ブロックサイズは細かくなり、Ｃｒ２Ｎの分散状態も均一となる。したがって、共析組織を緻密化させるという観点からは、ノーズ以下の温度域で、しかもなるべく低い温度で恒温変態させることが有効であると判断される。 FIG. 3 is a schematic view of a eutectoid structure obtained after holding at a constant temperature at each temperature. In the high temperature region, the eutectoid structure is composed of blocks of about 30 μm, and plate-like or block-like Cr 2 N is unevenly distributed inside, whereas in the low temperature region, the transformation temperature decreases. As a result, the block size becomes finer and the dispersion state of Cr 2 N becomes uniform. Therefore, from the viewpoint of densifying the eutectoid structure, it is judged that it is effective to perform isothermal transformation at a temperature as low as possible in the temperature range below the nose.
（３）逆変態処理
１２２３Ｋ、１１７３Ｋおよび８７３Ｋの各温度でそれぞれ恒温変態後、γ単相域の１４７３Ｋで１．８ｋｓの逆変態処理を施した試料についてＸ線回折した結果、いずれの試料においても、αとＣｒ２Ｎの回折ピークは確認されず、α＋Ｃｒ２Ｎからγへの逆変態が完了していた。
(3) Reverse transformation treatment As a result of X-ray diffraction on a sample subjected to a constant transformation at 1223K, 1173K, and 873K, respectively, and subjected to a reverse transformation treatment of 1.8 ks at 1473 K in the γ single phase region, The diffraction peaks of α and Cr 2 N were not confirmed, and the reverse transformation from α + Cr 2 N to γ was completed.
図４は恒温変態処理した後逆変態処理を施した合金の光顕組織を示す。恒温変態前のＦｅ−２５％Ｃｒ−１％Ｎ合金に比べて、共析変態処理した後逆変態処理を施したいずれの合金も、γ粒は著しく微細化されていた。例えば、１２２３Ｋ恒温保持の合金では、γ粒の粒径が５０μｍ程度であり、恒温変態前の合金の粒径（５００μｍ）の１／１０程度にまで微細化されていた。 FIG. 4 shows a light microstructure of an alloy that has been subjected to a constant temperature transformation treatment and then a reverse transformation treatment. Compared with the Fe-25% Cr-1% N alloy before isothermal transformation, in any alloy subjected to the eutectoid transformation treatment and then subjected to the reverse transformation treatment, the γ grains were remarkably refined. For example, in an alloy maintained at a constant temperature of 1223K, the particle size of γ grains is about 50 μm, and the particle size is reduced to about 1/10 of the particle size (500 μm) of the alloy before the isothermal transformation.
図５はＦｅ−２５％Ｃｒ−１％Ｎ合金の応力−歪み曲線を示す図である。オーステナイト化し恒温変態処理後逆変態処理して微細化した合金１は、オーステナイト化しただけの合金２に比べて、機械的性質が優れていることが確認できた。
FIG. 5 is a diagram showing a stress-strain curve of an Fe-25% Cr-1% N alloy. It was confirmed that the
本実施例は恒温変態→逆変態処理を２回繰り返すことによってさらに結晶粒を微細化するものである。 In the present embodiment, the crystal grains are further refined by repeating the isothermal transformation → reverse transformation treatment twice.
図６は高窒素ステンレス鋼の結晶粒微細化熱処理方法の別実施例を示す概略図、図７は恒温変態処理→逆変態処理を２回繰り返した合金の光顕組織を示す図である。 FIG. 6 is a schematic view showing another embodiment of the method for heat treatment of grain refinement of high nitrogen stainless steel, and FIG. 7 is a view showing an optical microscopic structure of an alloy in which the isothermal transformation treatment → reverse transformation treatment is repeated twice.
図６に示すように、実施例１と同様に、１４７３Ｋ、１０８ｋｓでオーステナイト化し、１１７３Ｋ、０．３ｋｓで恒温変態処理した後、１４７３Ｋ、０．３ｋｓで逆変態処理したＦｅ−２５％Ｃｒ−１％Ｎ合金を再び１１７３Ｋ、０．３ｋｓで恒温変態処理し、次いで１４７３Ｋ、０．３ｋｓで逆変態処理した。 As shown in FIG. 6, similarly to Example 1, Fe-25% Cr-1 was austenitized at 1473 K and 108 ks, subjected to isothermal transformation treatment at 1173 K and 0.3 ks, and then reverse transformed at 1473 K and 0.3 ks. The% N alloy was again subjected to isothermal transformation treatment at 1173 K and 0.3 ks, and then reverse transformation treatment was conducted at 1473 K and 0.3 ks.
その結果、図７に示すように初回の恒温変態処理前の合金の粒径５００μｍが微細化されてγ粒の粒径が２０μｍ程度であり、さらに実施例１の初回の恒温変態処理後逆変態処理させた粒径５０μｍのγ粒よりさらに微細化されるとともに、より均一な組織になっていた。 As a result, as shown in FIG. 7, the grain size of the alloy before the first isothermal transformation treatment is refined to 500 μm, and the γ grain size is about 20 μm. Further, the reverse transformation after the first isothermal transformation treatment of Example 1 is performed. It was further refined from the treated γ grains with a particle diameter of 50 μm and a more uniform structure.
１：オーステナイト化し恒温変態処理後逆変態処理して微細化した合金
２：オーステナイト化しただけの合金
1: Alloy austenitized and refined by isothermal transformation after reverse transformation 2: Alloy just austenitized
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