JP2011184780A - Stainless steel sheet with austenite-martensite dual-phase structure and method of producing the same - Google Patents
Stainless steel sheet with austenite-martensite dual-phase structure and method of producing the same Download PDFInfo
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- 229910000734 martensite Inorganic materials 0.000 title claims abstract description 56
- 229910001220 stainless steel Inorganic materials 0.000 title claims abstract description 30
- 239000010935 stainless steel Substances 0.000 title claims abstract description 28
- 238000000034 method Methods 0.000 title claims description 16
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 64
- 239000000203 mixture Substances 0.000 claims abstract description 17
- 239000000126 substance Substances 0.000 claims abstract description 14
- 239000011159 matrix material Substances 0.000 claims abstract description 9
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 9
- 229910052796 boron Inorganic materials 0.000 claims abstract description 5
- 229910052802 copper Inorganic materials 0.000 claims abstract description 5
- 229910052750 molybdenum Inorganic materials 0.000 claims abstract description 5
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 4
- 239000012535 impurity Substances 0.000 claims abstract description 4
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 4
- 229910052759 nickel Inorganic materials 0.000 claims abstract description 4
- 229910001566 austenite Inorganic materials 0.000 claims description 67
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 61
- 229910000831 Steel Inorganic materials 0.000 claims description 32
- 238000009826 distribution Methods 0.000 claims description 32
- 238000010438 heat treatment Methods 0.000 claims description 32
- 239000010959 steel Substances 0.000 claims description 32
- 239000000463 material Substances 0.000 claims description 28
- 238000010791 quenching Methods 0.000 claims description 26
- 230000000171 quenching effect Effects 0.000 claims description 26
- 230000008569 process Effects 0.000 claims description 11
- 229910001039 duplex stainless steel Inorganic materials 0.000 claims description 7
- 238000004519 manufacturing process Methods 0.000 claims description 7
- 239000002184 metal Substances 0.000 claims description 6
- 229910052751 metal Inorganic materials 0.000 claims description 6
- 229910018054 Ni-Cu Inorganic materials 0.000 claims description 4
- 229910018481 Ni—Cu Inorganic materials 0.000 claims description 4
- 238000007599 discharging Methods 0.000 claims description 3
- 239000002893 slag Substances 0.000 claims 1
- 238000001816 cooling Methods 0.000 description 11
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 10
- 230000007423 decrease Effects 0.000 description 9
- 238000009792 diffusion process Methods 0.000 description 7
- 239000000243 solution Substances 0.000 description 6
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- 239000002436 steel type Substances 0.000 description 5
- 238000012360 testing method Methods 0.000 description 5
- 238000000137 annealing Methods 0.000 description 4
- 229910001105 martensitic stainless steel Inorganic materials 0.000 description 4
- 230000000717 retained effect Effects 0.000 description 4
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- 238000000638 solvent extraction Methods 0.000 description 4
- 238000005482 strain hardening Methods 0.000 description 4
- 230000009471 action Effects 0.000 description 3
- 239000010960 cold rolled steel Substances 0.000 description 3
- 238000005097 cold rolling Methods 0.000 description 3
- 230000007797 corrosion Effects 0.000 description 3
- 238000005260 corrosion Methods 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
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- 238000009864 tensile test Methods 0.000 description 3
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 230000002411 adverse Effects 0.000 description 2
- 229910000963 austenitic stainless steel Inorganic materials 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 238000002474 experimental method Methods 0.000 description 2
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- 238000002441 X-ray diffraction Methods 0.000 description 1
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- 230000004888 barrier function Effects 0.000 description 1
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Abstract
Description
本発明は、オーステナイト相とマルテンサイト相の複相組織を有するステンレス鋼板であって、特に強度と延性をともに高レベルで両立させた鋼板、およびその製造方法に関する。 The present invention relates to a stainless steel plate having a multiphase structure of an austenite phase and a martensite phase, and more particularly to a steel plate having both strength and ductility at a high level, and a method for producing the same.
自動車部材や機械部品などの用途においては、加工性（延性）の良好な高強度ステンレス鋼材が要求される場合がある。加工性の良好な高強度ステンレス鋼としては、ＳＵＳ３０１ＨＴ、ＳＵＳ３０４ＨＴなどの加工硬化型オーステナイト系ステンレス鋼が挙げられる。この種の鋼は調質圧延によって機械的性質が調整される。その他の高強度ステンレス鋼としては、析出硬化型や変態強化型のマルテンサイト系ステンレス鋼、オーステナイト＋マルテンサイト複相組織高強度ステンレス鋼などがある。これらは主として熱処理によって機械的性質が調整される。 In applications such as automobile parts and machine parts, a high-strength stainless steel material with good workability (ductility) may be required. Examples of the high-strength stainless steel having good workability include work-hardening type austenitic stainless steels such as SUS301HT and SUS304HT. The mechanical properties of this type of steel are adjusted by temper rolling. Other high-strength stainless steels include precipitation hardening type and transformation strengthened martensitic stainless steel, austenite + martensite dual phase high strength stainless steel, and the like. These are mechanically adjusted mainly by heat treatment.
加工硬化型オーステナイト系ステンレス鋼は、Ｎｉを多く含有するため材料コストが高いという問題がある。他方、マルテンサイト系ステンレス鋼やオーステナイト＋マルテンサイト複相組織高強度鋼は、強度には優れるものの同じ強度レベルで比較するとオーステナイト系鋼種に比べ加工性に劣る。 Work-hardening austenitic stainless steel has a problem of high material cost because it contains a large amount of Ni. On the other hand, martensitic stainless steel and austenite + martensitic dual-phase high-strength steel are superior in strength but are inferior in workability compared to austenitic steel types when compared at the same strength level.
一方、特許文献１には、オーステナイト＋マルテンサイト複相組織ステンレス鋼において、焼入れ後に３％以下の歪を付与し、その後、熱処理を施すことによって溶接軟化抵抗を付与する技術が開示されている。しかし、この技術によって得られる材料は比較的良好な延性を示すものの、自動車部材や機械部品などの用途においては 必ずしも満足できる強度レベルを呈するとは限らない。 On the other hand, Patent Document 1 discloses a technique for imparting weld softening resistance by applying a strain of 3% or less after quenching to austenite + martensitic dual-phase stainless steel and then performing heat treatment. However, although the material obtained by this technique exhibits relatively good ductility, it does not always exhibit a satisfactory level of strength in applications such as automobile parts and machine parts.
本発明は、オーステナイト系鋼種より安価で、かつマルテンサイト系鋼種や従来の複相組織高強度ステンレス鋼種よりも優れた強度−延性バランスを有するステンレス鋼板を提供しようというものである。 The present invention is intended to provide a stainless steel plate that is less expensive than an austenitic steel type and has a strength-ductility balance superior to that of a martensitic steel type or a conventional high-strength stainless steel type having a multi-phase structure.
上記目的は特定の化学組成を有するオーステナイト＋マルテンサイト複相組織ステンレス鋼を用いて、熱処理を工夫することにより実現することができる。 The above object can be realized by devising heat treatment using austenite + martensitic dual phase stainless steel having a specific chemical composition.
すなわち本発明では、質量％で、Ｃ：０.２０％以下、Ｓｉ：１.０％以下、Ｍｎ：５.０％以下、Ｎｉ：６.０％以下、Ｃｒ：１０.０〜１８.０％、Ｎ：０.２０％以下、Ｃｕ：０〜３.０％、Ｍｏ：０〜３.０％、Ｂ：０〜０.０５０％、残部がＦｅおよび不可避的不純物であり、下記（１）式で定義されるＣｒ当量が１０.０〜２０.０、下記（２）式で定義されるＮｉ当量が５.０〜１５.０、下記（３）式で定まるＭｓ点が０〜１５０℃である化学組成を有し、マトリクスが、オーステナイト相：１０〜７０体積％、残部：マルテンサイト相である金属組織を有し、引張強さが１２５０Ｎ／ｍｍ2以上、かつ下記（４）式に示す強度・延性バランス指標Ａ値が１７３００以上であるオーステナイト＋マルテンサイト複相組織ステンレス鋼板が提供される。
Ｃｒ当量＝Ｃｒ＋Ｍｏ＋１.５Ｓｉ…（１）
Ｎｉ当量＝Ｎｉ＋３０（Ｃ＋Ｎ）＋０.５Ｍｎ＋０.３Ｃｕ…（２）
Ｍｓ＝｛３０００（０.０６８−Ｃ−Ｎ）＋５０（０.４７−Ｓｉ）＋６０（１.３３−Ｍｎ）＋１１０（８.９−Ｎｉ−Ｃｕ）＋７５（１４.６−Ｃｒ）−３２｝×５／９…（３）
Ａ値＝引張強さ（Ｎ／ｍｍ2）×伸び（％）…（４）
ここで、（１）〜（３）式の元素記号の箇所には質量％で表される当該元素の含有量が代入され、無添加の元素については０（ゼロ）が代入される。また、Ｃｕ、Ｍｏ、Ｂは任意添加元素である。
That is, in the present invention, by mass, C: 0.20% or less, Si: 1.0% or less, Mn: 5.0% or less, Ni: 6.0% or less, Cr: 10.0 to 18.0 %, N: 0.20% or less, Cu: 0 to 3.0%, Mo: 0 to 3.0%, B: 0 to 0.050%, the balance being Fe and inevitable impurities, ) The Cr equivalent defined by the formula is 10.0 to 20.0, the Ni equivalent defined by the following formula (2) is 5.0 to 15.0, and the Ms point determined by the following formula (3) is 0 to 150. It has a chemical composition that is ° C., the matrix has a metal structure that is an austenite phase: 10 to 70% by volume, the balance: a martensite phase, a tensile strength of 1250 N / mm 2 or more, and the following formula (4) An austenite + martensitic duplex stainless steel sheet having a strength / ductility balance index A value of 17300 or more is provided. The
Cr equivalent = Cr + Mo + 1.5Si (1)
Ni equivalent = Ni + 30 (C + N) + 0.5Mn + 0.3Cu (2)
Ms = {3000 (0.068-C-N) +50 (0.47-Si) +60 (1.33-Mn) +110 (8.9-Ni-Cu) +75 (14.6-Cr) -32} × 5/9 (3)
A value = tensile strength (N / mm 2 ) × elongation (%) (4)
Here, the content of the element represented by mass% is substituted for the element symbol in the formulas (1) to (3), and 0 (zero) is substituted for the additive-free element. Cu, Mo, and B are arbitrarily added elements.
上記において「マトリクス」は金属素地を意味し、マトリクス中には析出物や介在物が存在していて構わない。上記（４）式には、引張方向が鋼板の圧延方向である引張試験によって求まる引張強さおよび伸びの値が代入される。上記複相組織ステンレス鋼板は、オーステナイト相中の炭素濃度が、全体の平均炭素濃度（鋼のＣ含有量）よりも高くなっている。また本発明では、特に前記強度・延性バランス指標Ａ値が２３０００以上という、非常に高レベルの強度および延性を具備する材料を提供することができる。 In the above, “matrix” means a metal substrate, and precipitates and inclusions may exist in the matrix. In the above equation (4), values of tensile strength and elongation obtained by a tensile test in which the tensile direction is the rolling direction of the steel sheet are substituted. In the above multiphase stainless steel sheet, the carbon concentration in the austenite phase is higher than the overall average carbon concentration (C content of steel). Further, the present invention can provide a material having a very high level of strength and ductility, in particular, the strength / ductility balance index A value of 23,000 or more.
また本発明では、上記の複相組織ステンレス鋼板の製造方法として、前述の化学組成を有する鋼板を、オーステナイト単相温度域に加熱してオーステナイト単相組織とした後、前記Ｍｓ点より低い温度まで急冷することにより、マトリクスが、オーステナイト相：１０〜７０体積％、残部：マルテンサイト相である焼入れ材を得る工程（複相化処理工程）、
前記焼入れ材を「（３）式によるＭｓ点＋１５０℃」以上、Ａｃ1点未満の温度範囲に加熱してマルテンサイト相中に固溶しているＣの一部をオーステナイト相に排出させることにより、下記（４）式に示す強度・延性バランス指標Ａ値が１７３００以上である特性を付与する工程（炭素分配処理工程）、
を有するオーステナイト＋マルテンサイト複相組織ステンレス鋼板の製造方法が提供される。
In the present invention, as a method for producing the above-mentioned multiphase stainless steel plate, the steel plate having the above-described chemical composition is heated to an austenite single phase temperature range to obtain an austenite single phase structure, and then the temperature is lower than the Ms point. A process of obtaining a quenching material in which the matrix is an austenite phase: 10 to 70% by volume, and the balance: a martensite phase (multiphase treatment process) by rapid cooling,
By heating the quenching material to a temperature range of “Ms point + 150 ° C. according to formula (3)” or more and less than Ac 1 point and discharging a part of C dissolved in the martensite phase to the austenite phase. , A step of imparting the property that the strength / ductility balance index A value shown in the following formula (4) is 17300 or more (carbon distribution treatment step),
A method for producing an austenite + martensitic duplex stainless steel sheet having the following is provided:
本発明によれば、高いレベルで強度と延性を具備し、オーステナイト系鋼よりもＮｉ含有量を節約した安価なステンレス鋼板が提供可能となる。 According to the present invention, it is possible to provide an inexpensive stainless steel plate having strength and ductility at a high level and saving Ni content as compared with austenitic steel.
発明者らは詳細な研究の結果、後述の化学組成に調整したステンレス鋼板を、焼入れによる複相化処理、およびその後の加熱による炭素分配処理に供することにより、強度−延性バランスに優れた複相組織ステンレス鋼板が得られることを見出した。 As a result of detailed studies, the inventors have subjected a stainless steel plate adjusted to the chemical composition described later to a duplex phase treatment by quenching and a carbon distribution treatment by subsequent heating, thereby providing a duplex phase excellent in strength-ductility balance. It has been found that a textured stainless steel plate can be obtained.
図１に、本発明の複相組織ステンレス鋼板を得るためのヒートパターンおよび各段階における金属組織状態を模式的に示す。所定の板厚に調整された圧延材（冷延鋼板または熱延鋼板）を用意し、これを複相化処理に供する。複相化処理では、まず鋼板をオーステナイト（γ）単相温度域に加熱保持して溶体化処理する。オーステナイト単相組織（図１（Ａ））とした後、マルテンサイト（α’）変態開始温度Ｍｓ点より低い温度Ｔｑ（℃）まで急冷することにより、オーステナイト（γ）相：１０〜７０体積％、残部：マルテンサイト（α’）相である組織状態の焼入れ材を得る（図１（Ｂ））。以下において上記Ｔｑを「焼入れ終了温度」ということがある。Ｍｓ点が常温より高い場合、Ｔｑは常温とすることができる。焼入れ材ではオーステナイト相とマルテンサイト相における炭素濃度はそれぞれ、全体の平均炭素濃度（鋼のＣ含有量）と概ね等しい。このとき、Ｃ固溶限の小さいマルテンサイト相には炭素が過飽和に固溶されている。 FIG. 1 schematically shows a heat pattern for obtaining a multiphase stainless steel sheet according to the present invention and a metallographic state at each stage. A rolled material (cold-rolled steel plate or hot-rolled steel plate) adjusted to a predetermined plate thickness is prepared, and this is subjected to a duplexing treatment. In the multiphase treatment, first, the steel sheet is heated and held in the austenite (γ) single-phase temperature range to perform a solution treatment. After making an austenite single phase structure (FIG. 1 (A)), by rapidly cooling to a temperature Tq (° C.) lower than the martensite (α ′) transformation start temperature Ms point, the austenite (γ) phase: 10 to 70 vol% , Remainder: A hardened material in a structural state that is a martensite (α ′) phase is obtained (FIG. 1B). Hereinafter, Tq may be referred to as “quenching end temperature”. When the Ms point is higher than normal temperature, Tq can be normal temperature. In the hardened material, the carbon concentration in the austenite phase and the martensite phase is approximately equal to the overall average carbon concentration (C content of steel). At this time, carbon is supersaturated in the martensite phase having a small C solid solubility limit.
次に、このような組織状態の焼入れ材を炭素分配処理に供する。炭素分配処理は、焼入れ材をＡｃ1点以下の所定の温度域に加熱保持することにより、過飽和に固溶したマルテンサイト相中の炭素をオーステナイト相中へ拡散させる処理である（図１（Ｃ））。加熱保持後は常温まで冷却する。炭素分配処理によってオーステナイト相中の炭素濃度が上昇するため当該オーステナイト相のＭｓ点は低下しており、常温まで冷却する過程で新たなマルテンサイトの生成はほとんど起こらない。 Next, the hardened material having such a structure is subjected to a carbon distribution treatment. The carbon distribution treatment is a treatment for diffusing carbon in the martensite phase dissolved in supersaturation into the austenite phase by heating and maintaining the quenching material in a predetermined temperature range of Ac 1 point or less (FIG. 1 (C )). Cool to room temperature after heating. Since the carbon concentration in the austenite phase is increased by the carbon distribution treatment, the Ms point of the austenite phase is lowered, and new martensite is hardly generated in the process of cooling to room temperature.
炭素分配処理後においては、全体の平均炭素濃度に対して、オーステナイト相は炭素濃度が高く、マルテンサイト相は炭素濃度が低くなっている。 After the carbon distribution treatment, the austenite phase has a higher carbon concentration and the martensite phase has a lower carbon concentration than the overall average carbon concentration.
このオーステナイト相は、焼入れ後に存在するいわゆる残留オーステナイト相であり、マルテンサイト相との共存により、オーステナイト相自体がクラック進展の障壁として働く。また、応力やひずみが付与されるとマルテンサイト相に変態してエネルギーを吸収し、ＴＲＩＰ効果による高延性を担う。さらに、本来マルテンサイト相に比べ強度の低いオーステナイト相の強度は、炭素濃度の増大によって多少向上している。一方、マルテンサイト相は本来の高強度を担う機能を発揮するとともに、炭素濃度の減少により延性・靱性も向上している。このような作用により本発明のオーステナイト＋マルテンサイト複相組織ステンレス鋼板は、高強度を維持しながら、強度−延性バランスが高い水準に引き上げられている。 This austenite phase is a so-called residual austenite phase that exists after quenching, and the austenite phase itself acts as a barrier against crack growth due to the coexistence with the martensite phase. Further, when stress or strain is applied, it transforms into a martensite phase and absorbs energy, and bears high ductility due to the TRIP effect. Furthermore, the strength of the austenite phase, which is inherently lower in strength than the martensite phase, is somewhat improved by increasing the carbon concentration. On the other hand, the martensite phase exhibits the function of inherently high strength, and also has improved ductility and toughness due to a decrease in carbon concentration. By such an action, the austenite + martensitic duplex stainless steel sheet of the present invention is pulled to a high level of strength-ductility balance while maintaining high strength.
〔化学組成〕
以下、化学組成における「％」は特に断らない限り「質量％」を意味する。
Ｃは、鋼の強度を確保する上で重要な元素である。またＭｓ点を制御する上でもＣ含有量の設定は重要である。本発明では特に低Ｃ化していない一般的な汎用ステンレス鋼種（ＳＵＳ４３０、ＳＵＳ３０４など）と同等以上のＣ含有量とすればよい。例えば０.０３％以上のＣ含有量を確保することがより効果的である。ただし、Ｃ含有量が高くなりすぎるとマルテンサイトが過剰に硬化し、延性・靱性が低下する。この場合、炭素分配処理によってマルテンサイト相中からオーステナイト相中へＣ拡散の進行を図っても、マルテンサイト相の延性・靱性低下を十分に解消することが難しくなることがある。またＣはＣｒ炭化物を形成するので、あまり多量のＣを添加してもその添加効果は飽和する。種々検討の結果、Ｃ含有量は０.２０％以下に抑える必要があり、０.１５％以下とすることがより好ましい。
[Chemical composition]
Hereinafter, “%” in the chemical composition means “% by mass” unless otherwise specified.
C is an important element in securing the strength of steel. Also, setting the C content is important in controlling the Ms point. In the present invention, the C content may be equal to or higher than general general-purpose stainless steel types (SUS430, SUS304, etc.) that are not particularly low C. For example, it is more effective to secure a C content of 0.03% or more. However, if the C content is too high, the martensite is excessively cured, and ductility and toughness are reduced. In this case, even if C diffusion progresses from the martensite phase to the austenite phase by the carbon distribution treatment, it may be difficult to sufficiently eliminate the reduction in ductility and toughness of the martensite phase. Further, since C forms Cr carbide, the effect of addition is saturated even if a large amount of C is added. As a result of various studies, the C content must be suppressed to 0.20% or less, and more preferably 0.15% or less.
Ｓｉは、製鋼での脱酸作用を有する他、Ｃが炭化物形成に消費されることを抑制する作用を有する。これらの作用を十分に発揮させるためには０.２％以上のＳｉ含有量を確保することがより効果的であり、０.４％以上とすることが一層効果的である。ただし、多量のＳｉ含有はＳｉ酸化物を主体とする硬質な介在物の形成を招き、強度、疲労特性に悪影響を及ぼす。したがってＳｉ含有量は１.０％以下に制限され、０.８％以下とすることがより好ましい。 Si has a deoxidizing action in steel making and also has an action of suppressing consumption of C for carbide formation. In order to sufficiently exhibit these functions, it is more effective to secure a Si content of 0.2% or more, and it is more effective to set the content to 0.4% or more. However, the inclusion of a large amount of Si leads to the formation of hard inclusions mainly composed of Si oxide, which adversely affects the strength and fatigue characteristics. Accordingly, the Si content is limited to 1.0% or less, and more preferably 0.8% or less.
Ｍｎは、Ｍｓ点の制御、および適正溶体化処理温度の範囲拡大に有効な元素である。Ｍｎ含有量の下限は特に限定されないが、例えば０.０５％以上の範囲で最適なＭｎ含有量を設定すればよい。ただし、過度にＭｎを含有すると加工割れの起点となるＭｎ系介在物が増大する。種々検討の結果、Ｍｎ含有量の上限は５.０％に制限され、３.０％以下の範囲で調整することがより好ましい。 Mn is an element effective for controlling the Ms point and expanding the range of the appropriate solution treatment temperature. Although the minimum of Mn content is not specifically limited, What is necessary is just to set optimal Mn content in the range of 0.05% or more, for example. However, if Mn is contained excessively, Mn-based inclusions that become the starting point of processing cracks increase. As a result of various studies, the upper limit of the Mn content is limited to 5.0%, and it is more preferable to adjust the content within a range of 3.0% or less.
Ｎｉは、靱性向上に有効な元素である。またＭｓ点の制御にも有効である。Ｎｉ含有量の下限は特に限定されないが、例えば０.５％以上の範囲で最適なＮｉ含有量を設定すればよい。ただし、Ｎｉは高価な元素であるから、本発明では６.０％以下の範囲で含有させる。 Ni is an element effective for improving toughness. It is also effective for controlling the Ms point. Although the minimum of Ni content is not specifically limited, For example, what is necessary is just to set optimal Ni content in the range of 0.5% or more. However, since Ni is an expensive element, it is contained in the range of 6.0% or less in the present invention.
Ｃｒは、ステンレス鋼の耐食性を確保するために１０.０％以上の含有量とする。１２.０％以上とすることがより好ましい。ただし、多量にＣｒを含有させるとＣｒ炭化物が生成しやすくなり、炭素分配処理による強度−延性バランスの改善効果が十分に発揮されなくなる場合がある。種々検討の結果、Ｃｒ含有量は１８.０％以下の範囲に制限され、１７.０％以下あるいは１６.０％以下の範囲に管理しても構わない。 In order to ensure the corrosion resistance of stainless steel, the Cr content is 10.0% or more. More preferably, the content is 12.0% or more. However, if a large amount of Cr is contained, Cr carbide tends to be generated, and the effect of improving the strength-ductility balance by the carbon distribution treatment may not be sufficiently exhibited. As a result of various studies, the Cr content is limited to a range of 18.0% or less, and may be controlled to a range of 17.0% or less or 16.0% or less.
Ｎは、Ｃと同様、鋼の強度向上に寄与し、またＭｓ点を制御する上で重要な元素である。Ｎ含有量は例えば０.０２％以上とすることが望ましい。ただし、過度にＮを含有させると熱間圧延時に表面欠陥が生じやすくなる。種々検討の結果、Ｎ含有量は０.２０％以下に制限される。 N, like C, contributes to improving the strength of steel and is an important element in controlling the Ms point. For example, the N content is desirably 0.02% or more. However, if N is excessively contained, surface defects are likely to occur during hot rolling. As a result of various studies, the N content is limited to 0.20% or less.
Ｃｕは、適正溶体化温度の範囲を広げる作用を有し、またＭｓ点の制御にも利用できることから、必要に応じて添加することができる。その場合、０.１％以上の範囲で添加することがより効果的である。ただし、過度のＣｕ添加は耐食性の低下を招くので、Ｃｕを添加する場合は３.０％以下の範囲で行う必要がある。 Cu has the effect of extending the range of the appropriate solution temperature and can also be used to control the Ms point, so it can be added as necessary. In that case, it is more effective to add in the range of 0.1% or more. However, excessive addition of Cu causes a decrease in corrosion resistance. Therefore, when adding Cu, it is necessary to carry out within a range of 3.0% or less.
Ｍｏは、靱性向上に有効であり、また耐食性改善にも有効である。このため必要に応じてＭｏを添加することができる。その場合、０.０３％以上の範囲で添加することがより効果的である。ただし、Ｍｏは高価な元素であるため、Ｍｏを添加する場合は３.０％以下の範囲で行う必要がある。２.０％以下の含有量範囲に管理してもよい。 Mo is effective in improving toughness and effective in improving corrosion resistance. For this reason, Mo can be added as needed. In that case, it is more effective to add in the range of 0.03% or more. However, since Mo is an expensive element, when adding Mo, it is necessary to carry out in the range of 3.0% or less. You may manage to the content range of 2.0% or less.
Ｂは、オーステナイト結晶粒の成長抑制や熱間加工性の改善に有効であり、必要に応じて添加することができる。ただし、多量の添加は延性に悪影響を及ぼすので、Ｂを添加する場合は０.０５０％以下の範囲で行う。 B is effective in suppressing the growth of austenite crystal grains and improving hot workability, and can be added as necessary. However, since addition of a large amount adversely affects ductility, when B is added, it is made in a range of 0.050% or less.
下記（１）式のＣｒ当量および（２）式のＮｉ当量は、オーステナイト安定度に関連する指標である。
Ｃｒ当量＝Ｃｒ＋Ｍｏ＋１.５Ｓｉ…（１）
Ｎｉ当量＝Ｎｉ＋３０（Ｃ＋Ｎ）＋０.５Ｍｎ＋０.３Ｃｕ…（２）
本発明ではＣｒ当量が１０.０〜２０.０、かつＮｉ当量が５.０〜１５.０に調整された鋼を採用することにより、Ｍｓ点以下の所定の温度Ｔｑに焼入れしたときに後述の所定の相比を有するオーステナイト＋マルテンサイト複相組織が得られるようにしている。
The Cr equivalent of the following formula (1) and the Ni equivalent of the formula (2) are indicators related to austenite stability.
Cr equivalent = Cr + Mo + 1.5Si (1)
Ni equivalent = Ni + 30 (C + N) + 0.5Mn + 0.3Cu (2)
In the present invention, when steel having a Cr equivalent of 10.0 to 20.0 and a Ni equivalent of 5.0 to 15.0 is adopted, the steel is later described when quenched to a predetermined temperature Tq below the Ms point. Thus, an austenite + martensite multiphase structure having a predetermined phase ratio is obtained.
下記（３）式のＭｓは、鋼のＭｓ点（℃）を表す指標である。
Ｍｓ＝｛３０００（０.０６８−Ｃ−Ｎ）＋５０（０.４７−Ｓｉ）＋６０（１.３３−Ｍｎ）＋１１０（８.９−Ｎｉ−Ｃｕ）＋７５（１４.６−Ｃｒ）−３２｝×５／９…（３）
本発明で規定する組成範囲の鋼に関しては、（３）式によりＭｓ点が精度良く推定される。したがって、複相化処理における焼入れ終了温度は（３）式により算出されるＭｓ点を基準として設定することができる。本発明ではこのＭｓ点が０〜１５０℃となる組成の鋼を適用する。Ｍｓ点が１５０℃を超えると、後述の焼入れにおいて残留オーステナイト相の割合が１０〜７０体積％である複相組織を得るために焼入れ終了温度Ｔｑを常温よりかなり高い温度に設定する必要が生じやすく、製造コスト低減には不利となる。また、Ｍｓ点が０℃より低い場合は焼入れ終了温度Ｔｑの最適条件が氷点下のかなり低い温度となる場合が多く、やはり製造コスト低減には不利となる。（３）式によるＭｓ点は１５〜１００℃であることがより好ましい。
Ms in the following formula (3) is an index representing the Ms point (° C.) of steel.
Ms = {3000 (0.068-C-N) +50 (0.47-Si) +60 (1.33-Mn) +110 (8.9-Ni-Cu) +75 (14.6-Cr) -32} × 5/9 (3)
For steel in the composition range defined in the present invention, the Ms point is estimated with high accuracy by equation (3). Therefore, the quenching end temperature in the multiphase treatment can be set based on the Ms point calculated by the equation (3). In the present invention, steel having a composition in which the Ms point is 0 to 150 ° C. is applied. When the Ms point exceeds 150 ° C., it is necessary to set the quenching end temperature Tq to be considerably higher than room temperature in order to obtain a multiphase structure in which the ratio of the retained austenite phase is 10 to 70% by volume in quenching described later. This is disadvantageous for manufacturing cost reduction. In addition, when the Ms point is lower than 0 ° C., the optimum condition for the quenching end temperature Tq is often considerably lower than the freezing point, which is also disadvantageous for reducing the manufacturing cost. The Ms point according to the formula (3) is more preferably 15 to 100 ° C.
〔金属組織〕
本発明のオーステナイト＋マルテンサイト複相組織ステンレス鋼板は、マトリクスが、オーステナイト相：１０〜７０体積％、残部：マルテンサイト相である金属組織を有するものである。オーステナイト相が１０体積％より少ないと、延性が不十分となりやすく、またマルテンサイト相中の炭素濃度が十分に低減できないことがあり、靱性に劣る場合がある。一方、オーステナイト相が７０体積％を超えて多くなると、強度が不十分となりやすい。オーステナイト相とマルテンサイト相の相比は、化学組成および複相化処理条件（焼入れ終了温度Ｔｑ）によって制御できる。
[Metal structure]
The austenite + martensite dual phase stainless steel sheet of the present invention has a metal structure in which the matrix is an austenite phase: 10 to 70% by volume and the balance is a martensite phase. If the austenite phase is less than 10% by volume, the ductility tends to be insufficient, the carbon concentration in the martensite phase may not be sufficiently reduced, and the toughness may be inferior. On the other hand, if the austenite phase exceeds 70% by volume, the strength tends to be insufficient. The phase ratio between the austenite phase and the martensite phase can be controlled by the chemical composition and the multiphase treatment conditions (quenching end temperature Tq).
〔強度−延性バランス〕
本発明のオーステナイト＋マルテンサイト複相組織ステンレス鋼板は、引張強さが１２５０Ｎ／ｍｍ2以上、かつ下記（４）式に示す強度・延性バランス指標Ａ値が１７３００以上という、優れた強度−延性バランスを呈するものである。
Ａ値＝引張強さ（Ｎ／ｍｍ2）×伸び（％）…（４）
自動車部材をはじめとする多くの高強度部材用途においては、引張強さ１２５０Ｎ／ｍｍ2以上の強度レベルが要求される場合が多々ある。このような強度レベルにおいてＡ値が１７３００以上となるステンレス鋼板は、従来、加工硬化型オーステナイト系鋼種など、高価な元素を多用する一部の鋼種を除き実現が難しかったものである。
本発明のオーステナイト＋マルテンサイト複相組織ステンレス鋼板における強度−延性バランスは、化学組成、複相化処理条件（焼入れ終了温度Ｔｑ）、および炭素分配処理条件によって制御できる。
[Strength-ductility balance]
The austenite + martensitic duplex stainless steel sheet of the present invention has an excellent strength-ductility balance in which the tensile strength is 1250 N / mm 2 or more and the strength / ductility balance index A value shown in the following formula (4) is 17300 or more. It presents.
A value = tensile strength (N / mm 2 ) × elongation (%) (4)
In many high-strength member applications including automobile members, a strength level of 1250 N / mm 2 or more is often required. A stainless steel plate having an A value of 17300 or higher at such a strength level has been difficult to realize except for some steel types that frequently use expensive elements such as work-hardening austenitic steel types.
The strength-ductility balance in the austenite + martensitic duplex stainless steel sheet of the present invention can be controlled by the chemical composition, the duplexing treatment conditions (quenching finish temperature Tq), and the carbon partitioning treatment conditions.
〔複相化処理〕
複相化処理に供する鋼板は、通常のステンレス鋼板製造工程によって得られた圧延材を適用することができる。例えば、「連続鋳造→熱間圧延→焼鈍・酸洗→冷間圧延」の工程で所定の製品板厚に調整した鋼板を使用することができる。冷間圧延工程では、必要に応じて中間焼鈍を挟んだ複数回の冷間圧延を行ってもよい。
[Multi-phase treatment]
A rolled material obtained by a normal stainless steel sheet manufacturing process can be applied to the steel sheet to be subjected to the duplexing treatment. For example, it is possible to use a steel plate that has been adjusted to a predetermined product thickness in a process of “continuous casting → hot rolling → annealing / pickling → cold rolling”. In the cold rolling step, a plurality of cold rollings with intermediate annealing may be performed as necessary.
複相化処理においては、まず圧延材をオーステナイト単相温度域に加熱保持して炭素および窒素が完全固溶するように溶体化処理する。例えば９００〜１２００℃×均熱０〜１０ｍｉｎの条件が適用できる。過度に高温または長時間の保持を行うとオーステナイト粒が粗大化し、焼入れ後の複相組織も粗大化するので特性低下を招く要因となる。ここで均熱０ｍｉｎとは、材料の板厚中心部の温度が所定温度に到達した後、直ちに冷却することをいう。次いで溶体化処理温度からＭｓ点より低い温度Ｔｑまで急冷することにより、焼入れ処理を行う。急冷の手法は水あるいはその他の冷媒中に鋼板を浸漬する方法や、通板中の鋼帯に水等の液状冷媒を吹き付ける方法が採用できる。冷却速度は従来一般的なマルテンサイト系ステンレス鋼の焼入れ処理と同程度とすればよい。 In the multi-phase treatment, first, the rolled material is heated and held in the austenite single-phase temperature region, and solution treatment is performed so that carbon and nitrogen are completely dissolved. For example, the conditions of 900-1200 ° C. × soaking 0-10 min can be applied. If held at an excessively high temperature or for a long time, the austenite grains become coarse, and the double phase structure after quenching also becomes coarse, which causes a decrease in characteristics. Here, the soaking of 0 min refers to cooling immediately after the temperature at the center of the plate thickness of the material reaches a predetermined temperature. Next, quenching is performed by quenching from the solution treatment temperature to a temperature Tq lower than the Ms point. As a rapid cooling method, a method of immersing a steel plate in water or other refrigerant, or a method of spraying a liquid refrigerant such as water on a steel strip in the plate is adopted. The cooling rate may be about the same as the conventional quenching treatment of martensitic stainless steel.
焼入れ後のオーステナイト相とマルテンサイト相の相比は、焼入れ終了温度Ｔｑによって制御可能である。成分組成に応じて、所定の相比が得られるＴｑの条件を予備実験により求めておき、そのデータに基づいてＴｑを設定することにより精度よく相比を制御することができる。 The phase ratio between the austenite phase and the martensite phase after quenching can be controlled by the quenching end temperature Tq. Depending on the component composition, the Tq condition for obtaining a predetermined phase ratio is obtained by preliminary experiments, and the phase ratio can be accurately controlled by setting Tq based on the data.
〔炭素分配処理〕
焼入れ後の鋼板は、常温で保管したのち炭素分配処理に供しても構わないし、連続ラインにおいて上記複相化処理と炭素分配処理を連続的に実施しても構わない。ただし、複相化処理後には調質圧延などの加工工程を挿入することなく、そのまま炭素分配処理に供することが好ましい。
[Carbon distribution treatment]
The steel plate after quenching may be stored at room temperature and then subjected to carbon distribution treatment, or the above-described multiphase treatment and carbon distribution treatment may be continuously performed in a continuous line. However, it is preferable to use the carbon distribution treatment as it is without inserting a processing step such as temper rolling after the multiphase treatment.
炭素分配処理は前述のように焼入れ後のマルテンサイト相中に過飽和に存在している炭素をオーステナイト相中に吐き出させるための加熱処理である。その加熱は「（３）式によるＭｓ点＋１５０℃」以上、Ａｃ1点未満の温度域で行う。下限は「（３）式によるＭｓ点＋２００℃」以上に管理してもよい。また上限は６００℃以下、あるいは５５０℃以下に管理してもよい。炭素分配処理後の強度−延性バランスは、オーステナイト相とマルテンサイト相の相比、およびマルテンサイト相からオーステナイト相への炭素の拡散の程度によって変化する。一般的にはマルテンサイト相の体積率が大きいほど強度が高くなる。また、炭素の拡散が（ある程度までは）進行することにより伸びが増大する。本発明で対象とする化学組成の鋼の場合、オーステナイト相とマルテンサイト相の相比は前工程の複相化処理によって決定づけられ、炭素分配処理ではほとんど変化しない。したがって、炭素分配処理によってマルテンサイト相からオーステナイト相への炭素の拡散を適度に進行させると、主として伸びが増大することにより、上記（４）式で表される強度・延性バランス指標Ａ値は向上する。 As described above, the carbon distribution treatment is a heat treatment for discharging carbon that is supersaturated in the martensite phase after quenching into the austenite phase. The heating is performed in a temperature range of “Ms point + 150 ° C. according to equation (3)” or more and less than Ac 1 point. The lower limit may be managed to be “Ms point + 200 ° C. according to equation (3)” or higher. Moreover, you may manage an upper limit at 600 degrees C or less, or 550 degrees C or less. The strength-ductility balance after the carbon distribution treatment varies depending on the phase ratio between the austenite phase and the martensite phase and the degree of carbon diffusion from the martensite phase to the austenite phase. Generally, the greater the volume fraction of the martensite phase, the higher the strength. Elongation increases as carbon diffusion proceeds (to some extent). In the case of the steel having the chemical composition targeted by the present invention, the phase ratio between the austenite phase and the martensite phase is determined by the dual phase treatment in the previous step, and hardly changes in the carbon distribution treatment. Therefore, when carbon diffusion from the martensite phase to the austenite phase is appropriately advanced by the carbon partitioning process, the elongation increases mainly, thereby improving the strength / ductility balance index A value expressed by the above formula (4). To do.
マルテンサイト相からオーステナイト相への炭素の拡散は、加熱温度が高いほど進行しやすい。また加熱温度が同じであれば、加熱時間が（ある程度までは）長いほど進行しやすい。ところが、加熱温度が過度に高い場合や、加熱時間が過度に長い場合には、逆に強度・延性バランス指標Ａ値が低下してくることがわかった。その原因は現時点で必ずしも明確ではないが、一つには固溶炭素や固溶窒素が析出してくることが考えられる。オーステナイト相中における炭素、窒素の固溶量が減少するとオーステナイト安定度が低下してしまう。また、加熱温度が高い場合にはマルテンサイト相の分解（マルテンサイト相→フェライト相＋炭化物）が進行しやすくなり、これによって強度が低下することも要因になると考えられる。 The diffusion of carbon from the martensite phase to the austenite phase tends to proceed as the heating temperature increases. Further, if the heating temperature is the same, the longer the heating time (up to a certain extent), the easier it proceeds. However, when the heating temperature is excessively high, or when the heating time is excessively long, the strength / ductility balance index A value decreases. The cause of this is not necessarily clear at the present time, but one reason is that solute carbon and solute nitrogen precipitate. If the solid solution amount of carbon and nitrogen in the austenite phase decreases, the austenite stability decreases. Further, when the heating temperature is high, the decomposition of the martensite phase (martensite phase → ferrite phase + carbide) is likely to proceed, and this is considered to cause a decrease in strength.
強度−延性バランスは、化学組成、オーステナイト相とマルテンサイト相の相比に応じて、炭素分配処理における加熱温度および加熱時間を適正化することにより制御できる。炭素の拡散は加熱温度に到達するまでの昇温過程においても生じうるので、使用する加熱炉における「材料温度−時間曲線」を考慮する必要もある。具体的には、化学組成、相比、板厚毎の材料温度−時間曲線に応じて、引張強さが１２５０Ｎ／ｍｍ2以上、かつ（４）式に示す強度・延性バランス指標Ａ値が１７３００以上となる適正な加熱条件（炉温、在炉時間）を予備実験において求めておき、そのデータに基づいて熱処理を施せばよい。種々検討の結果、炭素分配処理における加熱温度（材料の最高到達温度）は上述のように「（３）式によるＭｓ点＋１５０℃」以上、Ａｃ1点未満の範囲とする。本発明対象鋼種においては、この加熱温度範囲内で適正条件を見出すことができる。 The strength-ductility balance can be controlled by optimizing the heating temperature and the heating time in the carbon distribution treatment according to the chemical composition and the phase ratio of the austenite phase to the martensite phase. Since carbon diffusion can occur in the temperature raising process until the heating temperature is reached, it is necessary to consider the “material temperature-time curve” in the heating furnace to be used. Specifically, the tensile strength is 1250 N / mm 2 or more according to the chemical composition, the phase ratio, and the material temperature-time curve for each plate thickness, and the strength / ductility balance index A value shown in the formula (4) is 17300. Appropriate heating conditions (furnace temperature, in-furnace time) as described above may be obtained in a preliminary experiment, and heat treatment may be performed based on the data. As a result of various studies, the heating temperature in the carbon distribution process (the maximum temperature reached by the material) is set in the range of “Ms point + 150 ° C. according to the equation (3)” to less than Ac 1 point as described above. Appropriate conditions can be found in this heating temperature range in the steel grade subject to the present invention.
また、例えば「熱処理」第４２巻第３号の１６３頁に説明されているＬａｒｓｏｎ−ＭｉｌｌｅｒパラメータＰLM（下記（５）式参照）を用いて加熱条件を管理しても構わない。この場合、ＰLMが１５０００以下となるように加熱条件を設定すると良好な結果が得られる。
ＰLM＝Ｔn（ｌｏｇｔn＋２０）…（５）
ただし、
Ｔn＝Ｔn-1＋αΔＴ
ｔn＝１０P＋Δｔ
Ｐ＝｛（Ｔn-1／Ｔn）・（ｌｏｇｔn-1＋２０）−２０｝
ｔ1＝Δｔ
である。ここで、Ｔn、Ｔn-1、ΔＴの単位は絶対温度（Ｋ）、ｔn、ｔn-1、Δｔの単位は時間（ｈ）であり、αは温度Ｔn-1での昇温または降温速度（Ｋ／ｈ）である。
Further, for example, the heating conditions may be managed using the Larson-Miller parameter P LM (see the following formula (5)) described in “Heat Treatment” Vol. 42, No. 3, page 163. In this case, good results can be obtained by setting the heating conditions so that PLM is 15000 or less.
P LM = T n (log n +20) (5)
However,
T n = T n-1 + αΔT
t n = 10 P + Δt
P = {(T n−1 / T n ) · (logt n−1 +20) −20}
t 1 = Δt
It is. Here, the unit of T n , T n-1 and ΔT is absolute temperature (K), the unit of t n , t n-1 and Δt is time (h), and α is an increase at temperature T n-1. It is a temperature or a cooling rate (K / h).
加熱後、常温までの冷却速度については特にこだわる必要はないが、炭化物が析出するような極端に遅い冷却速度は避けるべきである。析出により固溶炭素が減少すると分配処理による残留オーステナイトの安定度向上効果が十分に得られなくなる場合がある。 There is no need to pay particular attention to the cooling rate to room temperature after heating, but an extremely slow cooling rate at which carbides precipitate should be avoided. When solid solution carbon decreases due to precipitation, the stability improvement effect of retained austenite by distribution treatment may not be sufficiently obtained.
表１に示す鋼を溶製し、そのスラブを抽出温度１２００℃にて熱間圧延して板厚４.０ｍｍの熱延鋼帯とし、６００℃×１２ｈ、炉冷の熱延板焼鈍および酸洗を施し、冷間圧延して板厚２.０ｍｍとし、６００〜７００℃×均熱１ｍｉｎの中間焼鈍および酸洗を施し、仕上げ冷間圧延を行って板厚１.０ｍｍの冷延鋼板を得た。この冷延鋼板に対して、「１１００℃×均熱１ｍｉｎの溶体化処理→約２０℃に調整された水中へ浸漬して急冷する焼入れ処理」からなる複相化処理を施し、次いで「大気中４００〜５００℃×在炉３〜６０ｍｉｎの加熱→常温まで炉外で放冷（空冷）」による炭素分配処理を施すことにより、供試材を得た。炭素分配処理の加熱条件は表２中に示してある。一部、炭素分配処理を省略した供試材（焼入れ材）も用意した。なお、上記炭素分配処理の加熱温度はＡｃ1点未満である。 The steel shown in Table 1 was melted and the slab was hot-rolled at an extraction temperature of 1200 ° C. to form a hot-rolled steel strip having a thickness of 4.0 mm, 600 ° C. × 12 h, furnace-cooled hot-rolled sheet annealing and acid Washed, cold rolled to a thickness of 2.0 mm, subjected to intermediate annealing and pickling at 600-700 ° C. × soaking for 1 min, and finished cold rolled to produce a cold rolled steel sheet having a thickness of 1.0 mm. Obtained. The cold-rolled steel sheet was subjected to a dual phase treatment consisting of “solution treatment of 1100 ° C. × soaking for 1 min → quenching treatment that was quenched in water adjusted to about 20 ° C.” The sample material was obtained by performing the carbon distribution process by "400-500 degreeC x in-furnace 3-60min heating-> cooling outside the furnace to room temperature (air cooling)". The heating conditions for the carbon distribution treatment are shown in Table 2. In some cases, a test material (quenched material) in which carbon distribution treatment was omitted was also prepared. The heating temperature of the carbon distribution processing is Ac less than 1 point.
各供試材について、以下の調査を行った。
〔残留オーステナイト量〕
供試材から採取した試料について透過型電子顕微鏡（ＴＥＭ）を用いて組織観察を行うことにより残留オーステナイト量（体積％）を求めた。なお、いずれの供試材もマトリクスを構成するオーステナイト相以外の相はマルテンサイト相であった。
The following investigation was conducted for each specimen.
[Residual austenite amount]
The amount of retained austenite (% by volume) was determined by observing the structure of the sample collected from the test material using a transmission electron microscope (TEM). In all the test materials, the phases other than the austenite phase constituting the matrix were martensite phases.
〔硬さ〕
ＪＩＳ Ｚ２２４０に従って荷重１０ｋｇで鋼板表面のビッカース硬さを測定した。
〔引張強さ、伸び〕
供試材から圧延方向を長手方向とするＪＩＳ１３Ｂ号引張試験片を採取し、引張速度１ｍｍ／ｍｉｎにて破断するまで引張試験を行って、引張強さおよび破断伸びを求めた。また、この引張強さおよび伸びの値を上記（４）式に代入して強度・延性バランス指標Ａ値を求めた。
〔Hardness〕
The Vickers hardness of the steel sheet surface was measured with a load of 10 kg according to JIS Z2240.
[Tensile strength, elongation]
A JIS No. 13B tensile test piece with the rolling direction as the longitudinal direction was taken from the test material, and a tensile test was performed until it broke at a tensile speed of 1 mm / min to obtain the tensile strength and elongation at break. Further, the tensile strength and elongation values were substituted into the above equation (4) to obtain the strength / ductility balance index A value.
上記供試材の他、市販のＪＩＳ規格ステンレス鋼板の中からＳＵＳ３０１Ｈ、ＳＵＳ３０４Ｈ、ＳＵＳ４２０Ｊ２焼戻し材を入手し、硬さ、引張強さ、伸びを測定した。
これらの結果を表２に示す。表２中には参考のため、炭素分配処理の加熱条件についての前述（５）式によるＬａｒｓｏｎ−ＭｉｌｌｅｒパラメータＰLMを併記した。
In addition to the above test materials, SUS301H, SUS304H, and SUS420J2 tempered materials were obtained from commercially available JIS standard stainless steel plates, and the hardness, tensile strength, and elongation were measured.
These results are shown in Table 2. For reference, Table 2 also shows the Larson-Miller parameter P LM according to the above equation (5) for the heating conditions of the carbon distribution treatment.
表２からわかるように、適正な条件で複相化処理および炭素分配処理を施した本発明例のものは、引張強さが１２５０Ｎ／ｍｍ2以上の高強度を有し、かつ上記（４）式の強度・延性バランス指標Ａ値が１７３００以上という、優れた強度−延性バランスを呈するものである。特に、炭素分配処理の条件によって、引張強さが１２５０Ｎ／ｍｍ2以上の高強度を有しながら、（４）式のＡ値が２３０００以上、あるいは２５０００以上、あるいは３００００以上という、非常に高いレベルの強度−延性バランスを実現することができる。 As can be seen from Table 2, the example of the present invention which was subjected to the duplex phase treatment and the carbon distribution treatment under appropriate conditions had a high strength of a tensile strength of 1250 N / mm 2 or more and (4) The strength-ductility balance index A value of the formula exhibits an excellent strength-ductility balance of 17300 or more. In particular, depending on the conditions of the carbon distribution treatment, the A value of the formula (4) is a very high level of 23,000 or more, 25000 or more, or 30000 or more while having a high strength of 1250 N / mm 2 or more. Strength-ductility balance can be achieved.
なお、本発明例の複相化処理後（焼入れ材）と炭素分配処理後のサンプルについて、それぞれＸ線回折によりオーステナイト相（面心立方格子）の格子定数を測定したところ、いずれも炭素分配処理によって格子定数が増大していることが確認された。例えば表２のＮｏ.１のデータを例示すると、オーステナイト相の格子定数は、焼入れまま；ａ＝０.３５８３４ｎｍ、炭素分配処理後；ａ＝０.３５９１８ｎｍであった。この格子定数の増大は、炭素分配処理によってオーステナイト結晶の侵入型位置に固溶する炭素量が増大したことに起因するものであると考えられる。なお、窒素についてもマルテンサイト相中からオーステナイト相中への拡散が生じると考えられ、上記の格子定数増大には窒素の拡散も一部寄与しているものと考えられる。 In addition, when the lattice constant of the austenite phase (face-centered cubic lattice) was measured by X-ray diffraction for the sample after the multiphase treatment (quenched material) and the carbon distribution treatment of the example of the present invention, respectively, the carbon distribution treatment As a result, it was confirmed that the lattice constant increased. For example, when the data of No. 1 in Table 2 is illustrated, the lattice constant of the austenite phase was as-quenched; a = 0.35834 nm, after carbon partitioning treatment; a = 0.35918 nm. This increase in the lattice constant is considered to be caused by the increase in the amount of carbon dissolved in the interstitial position of the austenite crystal by the carbon partitioning process. Nitrogen is also considered to diffuse from the martensite phase to the austenite phase, and it is considered that the diffusion of nitrogen partially contributes to the increase in the lattice constant.
一方、比較例であるＮｏ.１−１、５−１は炭素分配処理を省略したものであり、焼入れにより高い強度を有する反面、伸びが低く、強度−延性バランスに劣る。Ｎｏ.５−５は炭素分配処理の加熱が過剰な条件（ＰLM＞１５０００）であったことにより伸びが低下し、強度−延性バランスに劣る。Ｎｏ.２１−１、２１−２はＭｓ点が高すぎる鋼を使用したことにより、常温付近への焼入れではオーステナイト相を十分に残留させることができず、炭素分配処理の有無にかかわらず強度−延性バランスに劣る。Ｎｏ.２２−１、２２−２はＭｓ点が低すぎる鋼を使用したことにより、常温付近までの急冷ではマルテンサイト変態が生じておらず、炭素分配処理の有無にかかわらず強度−延性バランスに劣る。ＳＵＳ３０１Ｈ、ＳＵＳ３０４Ｈは加工硬化型オーステナイト系ステンレス鋼の鋼板であり、高強度を維持しながら優れた強度−延性バランスを実現することは容易でないことがわかる。ＳＵＳ４２０Ｊ２焼戻し材はマルテンサイト系ステンレス鋼の鋼板であり、強度レベルは高いが強度−延性バランスに劣る。 On the other hand, Nos. 1-1 and 5-1 which are comparative examples omit carbon distribution treatment and have high strength by quenching, but have low elongation and inferior strength-ductility balance. No. 5-5 is inferior in the strength-ductility balance due to a decrease in elongation due to excessive heating (P LM > 15000) in the carbon distribution treatment. Nos. 21-1 and 21-2 used steel having an Ms point that was too high, so that the austenite phase could not be sufficiently retained by quenching to near room temperature, and the strength was obtained regardless of the presence or absence of carbon distribution treatment. Poor balance of ductility. No. 22-1 and 22-2 used steel with an Ms point that was too low, so martensitic transformation did not occur during rapid cooling to near room temperature, and the strength-ductility balance was maintained regardless of the presence or absence of carbon distribution treatment. Inferior. SUS301H and SUS304H are work hardening type austenitic stainless steel sheets, and it is not easy to achieve an excellent strength-ductility balance while maintaining high strength. The SUS420J2 tempered material is a steel plate of martensitic stainless steel and has a high strength level but is inferior in strength-ductility balance.
Claims (4)
Ｃｒ当量＝Ｃｒ＋Ｍｏ＋１.５Ｓｉ…（１）
Ｎｉ当量＝Ｎｉ＋３０（Ｃ＋Ｎ）＋０.５Ｍｎ＋０.３Ｃｕ…（２）
Ｍｓ＝｛３０００（０.０６８−Ｃ−Ｎ）＋５０（０.４７−Ｓｉ）＋６０（１.３３−Ｍｎ）＋１１０（８.９−Ｎｉ−Ｃｕ）＋７５（１４.６−Ｃｒ）−３２｝×５／９…（３）
Ａ値＝引張強さ（Ｎ／ｍｍ2）×伸び（％）…（４）
ここで、（１）〜（３）式の元素記号の箇所には質量％で表される当該元素の含有量が代入され、無添加の元素については０（ゼロ）が代入される。 In mass%, C: 0.20% or less, Si: 1.0% or less, Mn: 5.0% or less, Ni: 6.0% or less, Cr: 10.0 to 18.0%, N: 0 .20% or less, Cu: 0 to 3.0%, Mo: 0 to 3.0%, B: 0 to 0.050%, the balance being Fe and inevitable impurities, defined by the following formula (1) The chemical composition in which the Cr equivalent is 10.0 to 20.0, the Ni equivalent defined by the following formula (2) is 5.0 to 15.0, and the Ms point determined by the following formula (3) is 0 to 150 ° C. The matrix has a metal structure that is an austenite phase: 10 to 70% by volume, the balance: a martensite phase, a tensile strength of 1250 N / mm 2 or more, and the strength and ductility shown in the following formula (4) An austenite + martensite multiphase stainless steel plate having a balance index A value of 17300 or more.
Cr equivalent = Cr + Mo + 1.5Si (1)
Ni equivalent = Ni + 30 (C + N) + 0.5Mn + 0.3Cu (2)
Ms = {3000 (0.068-C-N) +50 (0.47-Si) +60 (1.33-Mn) +110 (8.9-Ni-Cu) +75 (14.6-Cr) -32} × 5/9 (3)
A value = tensile strength (N / mm 2 ) × elongation (%) (4)
Here, the content of the element represented by mass% is substituted for the element symbol in the formulas (1) to (3), and 0 (zero) is substituted for the additive-free element.
前記焼入れ材を「（３）式によるＭｓ点＋１５０℃」以上、Ａｃ1点未満の温度範囲に加熱してマルテンサイト相中に固溶しているＣの一部をオーステナイト相に排出させることにより、下記（４）式に示す強度・延性バランス指標Ａ値が１７３００以上である特性を付与する工程（炭素分配処理工程）、
を有するオーステナイト＋マルテンサイト複相組織ステンレス鋼板の製造方法。
Ｃｒ当量＝Ｃｒ＋Ｍｏ＋１.５Ｓｉ…（１）
Ｎｉ当量＝Ｎｉ＋３０（Ｃ＋Ｎ）＋０.５Ｍｎ＋０.３Ｃｕ…（２）
Ｍｓ＝｛３０００（０.０６８−Ｃ−Ｎ）＋５０（０.４７−Ｓｉ）＋６０（１.３３−Ｍｎ）＋１１０（８.９−Ｎｉ−Ｃｕ）＋７５（１４.６−Ｃｒ）−３２｝×５／９…（３）
Ａ値＝引張強さ（Ｎ／ｍｍ2）×伸び（％）…（４）
ここで、（１）〜（３）式の元素記号の箇所には質量％で表される当該元素の含有量が代入され、無添加の元素については０（ゼロ）が代入される。 In mass%, C: 0.20% or less, Si: 1.0% or less, Mn: 5.0% or less, Ni: 6.0% or less, Cr: 10.0 to 18.0%, N: 0 .20% or less, Cu: 0 to 3.0%, Mo: 0 to 3.0%, B: 0 to 0.050%, the balance being Fe and inevitable impurities, defined by the following formula (1) The chemical composition in which the Cr equivalent is 10.0 to 20.0, the Ni equivalent defined by the following formula (2) is 5.0 to 15.0, and the Ms point determined by the following formula (3) is 0 to 150 ° C. The steel sheet is heated to an austenite single phase temperature range to obtain an austenite single phase structure, and then rapidly cooled to a temperature lower than the Ms point, whereby the matrix becomes an austenite phase: 10 to 70% by volume, and the balance: martensite phase. A process for obtaining a quenching material (double phase treatment process),
By heating the quenching material to a temperature range of “Ms point + 150 ° C. according to formula (3)” or more and less than Ac 1 point and discharging a part of C dissolved in the martensite phase to the austenite phase. , A step of imparting the property that the strength / ductility balance index A value shown in the following formula (4) is 17300 or more (carbon distribution treatment step),
A method for producing austenite + martensitic duplex stainless steel sheet having a slag.
Cr equivalent = Cr + Mo + 1.5Si (1)
Ni equivalent = Ni + 30 (C + N) + 0.5Mn + 0.3Cu (2)
Ms = {3000 (0.068-C-N) +50 (0.47-Si) +60 (1.33-Mn) +110 (8.9-Ni-Cu) +75 (14.6-Cr) -32} × 5/9 (3)
A value = tensile strength (N / mm 2 ) × elongation (%) (4)
Here, the content of the element represented by mass% is substituted for the element symbol in the formulas (1) to (3), and 0 (zero) is substituted for the additive-free element.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2016065289A (en) * | 2014-09-25 | 2016-04-28 | 日新製鋼株式会社 | High strength tabular steel material, manufacturing method therefor and discharge valve part |
| EP3031942A1 (en) | 2014-12-09 | 2016-06-15 | voestalpine Precision Strip AB | Stainless steel strip for flapper valves |
| JP2016163390A (en) * | 2015-02-27 | 2016-09-05 | 株式会社デンソー | Piezoelectric actuator |
| JP2017526818A (en) * | 2014-07-30 | 2017-09-14 | アルセロールミタル | Manufacturing method of high strength billet |
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| JP2000129401A (en) * | 1998-10-21 | 2000-05-09 | Nisshin Steel Co Ltd | High toughness skin pass-rolled martensitic stainless steel plate having high spring characteristic and its production |
| JP2000129400A (en) * | 1998-10-21 | 2000-05-09 | Nisshin Steel Co Ltd | Annealed martensitic stainless steel excellent in strength, toughness, and spring characteristic |
| JP2001234236A (en) * | 2000-02-28 | 2001-08-28 | Nisshin Steel Co Ltd | Producing method for martensitic stainless steel excellent in strength, ductility and toughness |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2000129401A (en) * | 1998-10-21 | 2000-05-09 | Nisshin Steel Co Ltd | High toughness skin pass-rolled martensitic stainless steel plate having high spring characteristic and its production |
| JP2000129400A (en) * | 1998-10-21 | 2000-05-09 | Nisshin Steel Co Ltd | Annealed martensitic stainless steel excellent in strength, toughness, and spring characteristic |
| JP2001234236A (en) * | 2000-02-28 | 2001-08-28 | Nisshin Steel Co Ltd | Producing method for martensitic stainless steel excellent in strength, ductility and toughness |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2017526818A (en) * | 2014-07-30 | 2017-09-14 | アルセロールミタル | Manufacturing method of high strength billet |
| JP2016065289A (en) * | 2014-09-25 | 2016-04-28 | 日新製鋼株式会社 | High strength tabular steel material, manufacturing method therefor and discharge valve part |
| EP3031942A1 (en) | 2014-12-09 | 2016-06-15 | voestalpine Precision Strip AB | Stainless steel strip for flapper valves |
| JP2016163390A (en) * | 2015-02-27 | 2016-09-05 | 株式会社デンソー | Piezoelectric actuator |
| DE102016103007B4 (en) | 2015-02-27 | 2021-07-15 | Denso Corporation | Piezoelectric actuator |
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