JP4294237B2 - High-hardness martensitic stainless steel with excellent corrosion resistance, toughness and cold workability and its products. - Google Patents
High-hardness martensitic stainless steel with excellent corrosion resistance, toughness and cold workability and its products. Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼およびそれを用いたセルフドリリングタッピンネジ等の製品に関するものである。
【０００２】
【従来の技術】
従来から、各種産業機械、自動車、電子機器、化学装置等において高強度と耐食性が要求される部品の素材として利用されるマルテンサイト系ステンレス鋼、例えば、各種シャフト類、耐食軸受等の素材にはＳＵＳ４１０、ＳＵＳ４２０Ｊ２、ＳＵＳ４４０Ｃ等のマルテンサイト系ステンレス鋼が使用されている。しかしながら、これらのマルテンサイト系ステンレス鋼は、一般的なオーステナイト系ステンレス鋼、例えばＳＵＳ３０４と比べて、Ｃｒ量が低くＣ量が高いため、高強度は得られるものの、大気環境中においても容易に発銹する程度の耐食性しかない。一方、高強度と耐食性を両立させたマルテンサイト系ステンレス鋼として、特許第２９６８８４４号公報で示される耐孔食性の優れた高硬度マルテンサイト系ステンレス鋼や特開平６−２６４１９４号公報に示される耐銹性に優れた高強度マルテンサイト系ステンレス鋼およびドリリングタッピンネジ等が提案されている。
【０００３】
【発明が解決しようとする課題】
上述したタッピンネジに求められる特性は、硬度、耐食性、靱性および冷間加工性（冷鍛−転造）である。そこで従来、硬度は高いが耐食性に劣るマルテンサイト系ステンレス鋼には耐食性付与のため、Ｓｎ等のめっきが、硬度の低いオーステナイト系には窒化等の表面硬化法が用いられている。しかしながら、これらの方法は、工数増によるコストアップが不可避である。一方、前述した特許第２９６８８４４号公報や特開平６−２６４１９４号公報に示される耐銹性に優れた高強度マルテンサイト系ステンレス鋼およびドリリングタッピンネジとして開示されているが、いずれも、焼なまし硬さを上げる作用のあるＮｉの積極的添加を必須としたものであり、冷鍛時のプレス負荷が増加して生産性を悪化させるなど、冷間加工性が不十分という問題がある。
【０００４】
【課題を解決するための手段】
本発明は、上述したような問題を解決したもので、耐食性向上に有効なＣｒとＭｏを必要量添加し、硬度と耐食性を上げるＮを添加し、Ｃは必要最小限に抑える。また、マルテンサイト中にフェライトが残留すると耐食性と靱性を著しく低下させるため、成分バランスの調整によりフェライトフリーとすると共に、靱性低下原因のＣ量に制限を加えた耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼およびその製品を提供することにある。
【０００５】
その発明の要旨とするところは、
（１）質量で、Ｃ：０．１７〜０．２２％、Ｓｉ：２．０％以下、Ｍｎ：２．０％以下、Ｃｒ：１４．０〜１５．０％、Ｍｏ：０．７〜１．３％、Ｎｉ：０．１７〜０．６％、
Ｎ：０．１０〜０．１５％、Ｏ：０．００３〜０．０１％を含有し、かつ、Ｃｕ：０．２％以下、Ｐ：０．０５％以下、残部Ｆｅおよび不可避的不純物からなり、かつ下記式によるＰｆ：１４以下、Ｐｈ：５０以上であることを特徴とする耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼。
ただし、
Ｐｆ＝［％Ｃｒ］＋１．７×［％Ｍｏ］＋２．１×［％Ｓｉ］−７×［％Ｃ］−８×［％Ｎ］−３．３×［％Ｎｉ］−１．９×［％Ｍｎ］−０．５×［％Ｃｕ］
Ｐｈ＝５５．７＋４４．５×［％Ｃ］＋８４．７×［％Ｎ］−１．５×［％Ｃｒ］
【０００６】
（２）前記（１）に加えて、質量で、Ａｌ：０．０５％以下、Ｃａ：０．００５％以下、Ｍｇ：０．００５％以下、Ｔｉ：０．５％以下、Ｖ：０．５％以下、Ｗ：０．５％以下、Ｎｂ：０．５％以下、Ｔａ：０．５％以下、Ｈｆ：０．５％以下、Ｂ：０．０１％以下、の１種または２種以上を含有することを特徴とする耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼。
（３）前記（１）または（２）に加えて、質量で、Ｓ：０．４％以下、Ｓｅ：０．３％以下、Ｔｅ：０．１％以下、Ｐｂ：０．３％以下、Ｂｉ：０．３％以下、の１種または２種以上を含有することを特徴とする耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼。
【０００７】
（４）前記（１）〜（３）に記載の化学組成において、焼入れ、焼き戻し材の残留フェライトが面積率で５％以下、炭化物の面積率が２％以下であることを特徴とする耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼。
（５）前記（１）〜（４）において、焼なまし硬さが９５ＨＲＢ以下で、引抜き、冷鍛、転造の冷間加工により成形後、熱処理により５０ＨＲＣ以上の硬さとなる耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼製品にある。
【０００８】
【発明の実施の形態】
以下、本発明の耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼の成分範囲の限定理由について具体的に述べる。
Ｃ：０．１７〜０．２２％
Ｃは、Ｎと共に強度確保のため、０．１７％以上必要である。しかし、０．２２％を超えると、本発明のＣｒ量の範囲では、最適焼入条件で固溶しないＣｒ炭化物を多量に生じ、靱性を悪化させる。従って、Ｃ含有量は０．１７〜０．２２％とした。
【０００９】
Ｓｉ：２．０％以下
Ｓｉは、脱酸元素として有効な元素である。しかし、２．０％を超えると焼なまし硬さが高くなる上、焼入れ時の残留フェライト量が増加する。従って、その上限を２．０％とした。
Ｍｎ：２．０％以下
Ｍｎは、脱酸元素であり、Ｎの溶解度を上げる効能がある。しかし、２．０％を超える添加はそれほど効果が得られず、かつ、耐食性を悪化させる。従って、その上限を２．０％とした。
【００１０】
Ｃｒ：１４．０〜１５．０％
Ｃｒは、耐食性を与える基本元素である。しかし、１４．０％未満では耐食性が不十分であり、１５．０％を超えるとフェライトの増加、Ｃｒ炭化物の増加を招き靱性等の劣化を招くことから、その範囲を１４．０〜１５．０％とした。
Ｍｏ：０．７〜１．３％
Ｍｏは、Ｃｒと共存することで大気環境の耐食性を改善する。しかし、０．７％未満では不十分であり、１．３％を超えると偏析を強めフェライトを生じやすくなることから、その範囲を０．７〜１．３％とした。
【００１１】
Ｎ：０．１０〜０．１５％
Ｎは、Ｃと共に硬さを上げ、耐食性にも好影響を与える。しかし、０．１０％未満では十分な硬さが得られず、０．１５％を超えると、本発明範囲ではＮ固溶量の制限のため凝固欠陥の原因となる。従って、その範囲を０．１０〜０．１５％とした。
Ｐ：０．０５％以下
Ｐは、靱性の観点から特に上限を制限した。０．０５％を超えると高靱性が得られない。望ましくは０．０３％未満とする。
【００１２】
Ｏ：０．００３〜０．０１％
Ｏは、冷鍛割れ防止の観点から特に上限を制限した。０．０１％を超えると生成酸化物が加工性を阻害する。従って、その上限を０．０１％とした。望ましくは０．００６％未満とする。また、下限を０．００３％とする。
Ｎｉ：０．１７〜０．６％
Ｎｉは、冷間加工性の観点から特に上限を制限した。０．６％を超えると焼きなましによる軟化が困難になる。従って、その上限を０．６％とした。望ましくは０．４％未満とする。また、下限を０．１７％とする。
【００１３】
Ｃｕ：０．２％以下
Ｃｕは、靱性、熱間加工性、リサイクル性の観点から特に上限を制限した。０．２％を超えると靱性および熱間加工性が悪化する。従って、その上限を０．２％とした。望ましくは０．１％未満とする。
Ａｌ：０．０５％以下
Ａｌは、強力な脱酸元素として有効である。しかし、０．０５％を超えると二次酸化の問題がある。従って、その上限を０．０５％とした。
【００１４】
Ｃａ：０．００５％以下
Ｃａは、熱間加工性を改善する元素である。しかし、０．００５％を超えると粗大酸化物が生じて冷間加工性を悪化させる。従って、その上限を０．００５％とした。
Ｍｇ：０．００５％以下
Ｍｇは、Ｃａと同様に、熱間加工性を改善する元素である。しかし、０．００５％を超えると粗大酸化物が生じて冷間加工性を悪化させる。従って、その上限を０．００５％とした。
【００１５】
Ｔｉ：０．５％以下
Ｔｉは、炭窒化物を生成し耐食性を改善する元素である。しかし、０．５％を超えると熱間加工性、冷間鍛造性を悪化させる。従って、その上限を０．５％とした。
Ｖ：０．５％以下、Ｗ：０．５％以下、Ｎｂ：０．５％以下、Ｔａ：０．５％以下、Ｈｆ：０．５％以下
Ｖ，Ｗ，Ｎｂ，Ｔａ，Ｈｆは、Ｔｉと同様、炭窒化物を生成し耐食性を改善する元素である。しかし、０．５％を超えると熱間加工性、冷間鍛造性を悪化させる。従って、それぞれの上限を０．５％とした。
【００１６】
Ｂ：０．０１％以下
Ｂは、熱間加工性を改善する元素である。しかし、０．０１％を超えると逆に熱間加工性を悪化させる。従って、その上限を０．０１％とした。
Ｓ：０．４％以下
Ｓは、被削性を改善する元素である。しかし、０．４％を超えると熱間加工性、冷間鍛造性を悪化させる。従って、その上限を０．４％とした。
【００１７】
Ｓｅ：０．３％以下、Ｔｅ：０．１％以下、Ｐｂ：０．３％以下、Ｂｉ：０．３％以下
Ｓｅ，Ｔｅ，Ｐｂ，Ｂｉは、Ｓと同様、被削性を改善する元素である。しかし、０．３％ないし０．１％を超えると熱間加工性、冷間鍛造性を悪化させる。従って、それぞれの上限を０．３％、ないし０．１％とした。
【００１８】
Ｐｆ（フェライトパラメータ）：１４以下
Ｐｆは、残留フェライト量と相関のあるパラメータで、この値の増加に伴ってフェライト量が増加する。この値が１４を超えると、フェライトフリー化が十分得られない。従って、その上限を１４とした。
Ｐｈ（硬さパラメータ）：５０以下
Ｐｈは、最適焼入温度で焼入れし、２００℃で焼戻ししたときの硬さと相関のあるパラメータである。この値が５０未満では、５０ＨＲＣ以上の高硬度が十分に得られない。従って、その上限を５０とした。
【００１９】
焼入れ、焼き戻し材の残留フェライトが面積率で５％以下、炭化物の面積率が２％以下とした理由は、以下の通りである。残留フェライトが面積率で５％を超えると、フェライト周囲に析出した炭化物との境界で発銹が生じやすく、また靭性にも劣る。また、炭化物の面積率が２％を超えると、冷間加工時に割れを生じやすくなる一方、粒界に析出した炭化物は靭性を悪化させる。従って、その上限をそれぞれ５％、２％とした。
また、焼なまし硬さを９５ＨＲＢ以下で、引抜き、冷鍛、転造等の冷間加工により成形後、熱処理により５０ＨＲＣ以上の硬さとすることにより、セルフドリリングタッピンネジ等の製品に好適な材料となる。
【００２０】
【実施例】
１００ｋｇ真空誘導溶解炉にて鋼塊溶製した。表１に示す各成分の鋼塊を熱間鍛伸した。鍛伸材を８７０℃焼なまし後、硬さ測定、冷間圧縮変形抵抗測定を行った。１０３０℃油冷−２００℃空冷の焼入焼戻し後、硬さ測定、耐食性、シャルピー衝撃試験、フェライト、炭化物量測定を行った。その結果を表２に示す。また、各試験条件および評価方法を以下に示す。
【００２１】
【表１】
【表２】
（１）焼なまし硬さ
φ２０ｍｍに熱間鍛伸後、８７０℃−２時間保持後、５７０℃まで２０℃／ｈで炉冷した後空冷の１回焼なましを施し、Ｔ面中周部をロックウエル硬度計で測定した。
（２）冷間圧縮変形抵抗
ヘッダー加工等冷鍛におけるプレス機への負荷の評価として、焼なまし材をφ１４×Ｌ２１に加工し、長手方向に常温で５０％圧縮したときの変形抵抗を測定した。
【００２４】
（３）焼入焼戻硬さ
１０３０℃−３０ｍｉｎ保持油冷後、２００℃−１時間保持空冷の焼入焼戻しを施し、φ２０Ｔ面中周部をロックウエル硬度計で測定した。以下の調査は、全て上記熱処理を行った。
（４）ＣＡＳＳ試験
焼入焼戻し後、φ１２×Ｌ２１の試験片を用いて、ＣＡＳＳ試験（ＪＩＳ Ｚ２３７１、７２ｈ）を行い、発銹の有無を観察した。
【００２５】
（５）シャルピー衝撃試験
焼入焼戻しを行い、１０×１０×５５のＪＩＳ Ｚ ２２０２（２ｍｍＵノッチ）シャルピー衝撃試験片を用いてシャルピー衝撃値を求めた。
（６）フェライト面積率
焼入焼戻し状態におけるマルテンサイト中の残留フェライト量に及ぼす成分元素の影響を調べるため、φ２０中心部のフェライト量を、１００倍写真×５視野について画像解析装置で測定した。
（７）炭化物面積率
φ２０焼入焼戻し材中心部の炭化物面積率を、５００倍×５視野について画像解析装置で測定した。
【００２６】
表１に示すように、Ｎｏ．１〜Ｎｏ．１７を本発明の実施例としての化学成分を掲げ、Ｎｏ．１８〜Ｎｏ．３３を比較例の化学成分組成とした。その試験結果、比較例Ｎｏ．１８〜Ｎｏ．３３における成分組成は本発明の条件と外れ、その結果としての表２は、いずれかの特性において劣り、特に、比較例Ｎｏ．２７は鋼塊に欠点があり、比較例Ｎｏ．３２は圧縮時に割れが発生した。
【００２７】
【発明の効果】
以上述べたように、本発明は耐食性、靱性および冷間加工性に優れた硬さ５０ＨＲＣ以上の高硬度マルテンサイト系ステンレス鋼およびタッピンネジ用材として好適な材料を提供することが出来る、優れた効果を奏するものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-hardness martensitic stainless steel excellent in corrosion resistance, toughness, and cold workability, and a product such as a self-drilling tapping screw using the same.
[0002]
[Prior art]
Conventionally, martensitic stainless steels used as materials for parts that require high strength and corrosion resistance in various industrial machines, automobiles, electronic equipment, chemical equipment, etc., for example, various shafts, corrosion resistant bearings, etc. Martensitic stainless steel such as SUS410, SUS420J2, and SUS440C is used. However, these martensitic stainless steels have a low Cr content and a high C content compared to general austenitic stainless steels such as SUS304, so that although high strength is obtained, they are easily generated even in the atmospheric environment. It has only a certain degree of corrosion resistance. On the other hand, as a martensitic stainless steel having both high strength and corrosion resistance, the high hardness martensitic stainless steel with excellent pitting corrosion resistance disclosed in Japanese Patent No. 2968844 and the resistance to resistance shown in JP-A-6-264194 are disclosed. High-strength martensitic stainless steel with excellent inertia and drilling tapping screws have been proposed.
[0003]
[Problems to be solved by the invention]
The characteristics required for the tapping screw described above are hardness, corrosion resistance, toughness, and cold workability (cold forging-rolling). Therefore, conventionally, plating of Sn or the like is used for martensitic stainless steel having high hardness but poor corrosion resistance, and surface hardening methods such as nitriding are used for austenite having low hardness. However, these methods inevitably increase costs due to an increase in man-hours. On the other hand, it is disclosed as high strength martensitic stainless steel and drilling tapping screw having excellent weather resistance shown in the above-mentioned Japanese Patent No. 2968844 and JP-A-6-264194, both of which are annealed hard There is a problem that cold workability is inadequate, such as positive addition of Ni, which has an effect of increasing the thickness, which is essential, and the press load at the time of cold forging increases to deteriorate productivity.
[0004]
[Means for Solving the Problems]
The present invention solves the above-mentioned problems, adds necessary amounts of Cr and Mo effective for improving corrosion resistance, adds N which increases hardness and corrosion resistance, and keeps C to the minimum necessary . In addition, if ferrite remains in martensite, the corrosion resistance and toughness are remarkably reduced. Therefore, it is made ferrite-free by adjusting the balance of ingredients, and the corrosion resistance, toughness and cold workability are limited by limiting the amount of C that causes toughness reduction. The object is to provide an excellent high-hardness martensitic stainless steel and its products.
[0005]
The gist of the invention is that
(1) By mass, C: 0.17 to 0.22%, Si: 2.0% or less, Mn: 2.0% or less, Cr: 14.0 to 15.0%, Mo: 0.7 to 1.3%, Ni: 0.17 to 0.6%,
N: 0.10 to 0.15%, O: 0.003 to 0.01%, Cu: 0.2% or less, P: 0.05% or less, balance Fe and inevitable impurities And a high-hardness martensitic stainless steel excellent in corrosion resistance, toughness and cold workability, characterized in that Pf: 14 or less and Ph: 50 or more according to the following formula.
However,
Pf = [% Cr] + 1.7 × [% Mo] + 2.1 × [% Si] −7 × [% C] −8 × [% N] −3.3 × [% Ni] −1.9 × [% Mn] -0.5 × [% Cu]
P h = 55.7 + 44.5 × [ % C] + 84.7 × [% N] -1.5 × [% Cr]
[0006]
(2) In addition to the above (1), in terms of mass, Al: 0.05% or less, Ca: 0.005% or less, Mg: 0.005% or less, Ti: 0.5% or less, V: 0.00. One or two of 5% or less, W: 0.5% or less, Nb: 0.5% or less, Ta: 0.5% or less, Hf: 0.5% or less, B: 0.01% or less A high-hardness martensitic stainless steel excellent in corrosion resistance, toughness and cold workability characterized by containing the above.
(3) In addition to the above (1) or (2), by mass, S: 0.4% or less, Se: 0.3% or less, Te: 0.1% or less, Pb: 0.3% or less, Bi: High hardness martensitic stainless steel excellent in corrosion resistance, toughness and cold workability, characterized by containing one or more of 0.3% or less.
[0007]
(4) Corrosion resistance characterized in that in the chemical composition according to the above (1) to (3), the residual ferrite of the quenched and tempered material is 5% or less in area ratio and the area ratio of carbide is 2% or less. , High-hardness martensitic stainless steel with excellent toughness and cold workability.
(5) the (1) to (4), with annealing hardness 95HRB less, drawing, cold forging, rolling after molding by forming the cold working, the corrosion resistance to be 50HRC or more hardness by heat treatment, the toughness and High hardness martensitic stainless steel products with excellent cold workability.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the reason for limiting the component range of the high hardness martensitic stainless steel excellent in corrosion resistance, toughness and cold workability of the present invention will be specifically described.
C: 0.17 to 0.22%
C, together with N, needs to be 0.17% or more for securing the strength. However, if it exceeds 0.22%, within the range of the Cr content of the present invention, a large amount of Cr carbide that does not dissolve in optimum quenching conditions is produced, and the toughness is deteriorated. Therefore, the C content is set to 0.17 to 0.22%.
[0009]
Si: 2.0% or less Si is an effective element as a deoxidizing element. However, if it exceeds 2.0%, the annealing hardness increases and the amount of residual ferrite during quenching increases. Therefore, the upper limit was made 2.0%.
Mn: 2.0% or less Mn is a deoxidizing element and has the effect of increasing the solubility of N. However, the addition exceeding 2.0% is not so effective and deteriorates the corrosion resistance. Therefore, the upper limit was made 2.0%.
[0010]
Cr: 14.0 to 15.0%
Cr is a basic element that provides corrosion resistance. However, if it is less than 14.0%, the corrosion resistance is insufficient, and if it exceeds 15.0%, ferrite increases and Cr carbide increases, leading to deterioration of toughness and the like. 0%.
Mo: 0.7 to 1.3%
Mo improves the corrosion resistance of the atmospheric environment by coexisting with Cr. However, if it is less than 0.7%, it is insufficient, and if it exceeds 1.3%, segregation is strengthened and ferrite tends to be formed, so the range was made 0.7 to 1.3%.
[0011]
N: 0.10 to 0.15%
N increases the hardness together with C and has a positive effect on the corrosion resistance. However, if it is less than 0.10%, sufficient hardness cannot be obtained, and if it exceeds 0.15%, it causes solidification defects in the scope of the present invention due to the limitation of the amount of N solid solution. Therefore, the range was made 0.10 to 0.15%.
P: 0.05% or less P particularly limited the upper limit from the viewpoint of toughness. If it exceeds 0.05%, high toughness cannot be obtained. Desirably, it is less than 0.03%.
[0012]
O: 0.003 to 0.01%
O particularly limited the upper limit from the viewpoint of preventing cold forging cracks. When it exceeds 0.01%, the generated oxide inhibits workability. Therefore, the upper limit was made 0.01%. Desirably, the content is less than 0.006%. Further, the lower limit is made 0.003%.
Ni: 0.17 to 0.6%
Ni particularly limited the upper limit from the viewpoint of cold workability. If it exceeds 0.6%, it becomes difficult to soften by annealing. Therefore, the upper limit was made 0.6%. Desirably, the content is less than 0.4%. Further, the lower limit is made 0.17%.
[0013]
Cu: 0.2% or less Cu particularly limited the upper limit from the viewpoints of toughness, hot workability, and recyclability. If it exceeds 0.2%, toughness and hot workability deteriorate. Therefore, the upper limit was made 0.2%. Desirably, the content is less than 0.1%.
Al: 0.05% or less Al is effective as a powerful deoxidizing element. However, if it exceeds 0.05%, there is a problem of secondary oxidation. Therefore, the upper limit was made 0.05%.
[0014]
Ca: 0.005% or less Ca is an element that improves hot workability. However, if it exceeds 0.005%, a coarse oxide is generated, and cold workability is deteriorated. Therefore, the upper limit was made 0.005%.
Mg: 0.005% or less Mg, like Ca, is an element that improves hot workability. However, if it exceeds 0.005%, a coarse oxide is generated, and cold workability is deteriorated. Therefore, the upper limit was made 0.005%.
[0015]
Ti: 0.5% or less Ti is an element that generates carbonitrides and improves corrosion resistance. However, when it exceeds 0.5%, hot workability and cold forgeability are deteriorated. Therefore, the upper limit was made 0.5%.
V: 0.5% or less, W: 0.5% or less, Nb: 0.5% or less, Ta: 0.5% or less, Hf: 0.5% or less V, W, Nb, Ta, Hf are Like Ti, it is an element that produces carbonitrides and improves corrosion resistance. However, when it exceeds 0.5%, hot workability and cold forgeability are deteriorated. Therefore, the upper limit of each is 0.5%.
[0016]
B: 0.01% or less B is an element that improves hot workability. However, if it exceeds 0.01%, the hot workability is adversely affected. Therefore, the upper limit was made 0.01%.
S: 0.4% or less S is an element that improves machinability. However, when it exceeds 0.4%, hot workability and cold forgeability are deteriorated. Therefore, the upper limit was made 0.4%.
[0017]
Se: 0.3% or less, Te: 0.1% or less, Pb: 0.3% or less, Bi: 0.3% or less Se, Te, Pb, and Bi improve machinability in the same manner as S. It is an element. However, when it exceeds 0.3% to 0.1%, hot workability and cold forgeability are deteriorated. Therefore, the upper limit of each is set to 0.3% or 0.1%.
[0018]
Pf (ferrite parameter): 14 or less Pf is a parameter correlated with the amount of residual ferrite, and the amount of ferrite increases as this value increases. If this value exceeds 14, a ferrite-free structure cannot be obtained sufficiently. Therefore, the upper limit is set to 14.
Ph (Hardness parameter): 50 or less Ph is a parameter correlated with hardness when quenched at an optimum quenching temperature and tempered at 200 ° C. If this value is less than 50, a high hardness of 50 HRC or higher cannot be obtained sufficiently. Therefore, the upper limit is set to 50.
[0019]
The reason why the residual ferrite of the quenched and tempered material is 5% or less in area ratio and the area ratio of carbide is 2% or less is as follows. If the residual ferrite exceeds 5% in terms of area ratio, it tends to cause erosion at the boundary with the carbide precipitated around the ferrite and is inferior in toughness. On the other hand, when the area ratio of carbide exceeds 2%, cracking is likely to occur during cold working, while carbide precipitated at the grain boundary deteriorates toughness. Therefore, the upper limit was made 5% and 2%, respectively.
In addition, the material having a suitable hardness for a product such as a self-drilling tapping screw can be obtained by forming it by cold working such as drawing, cold forging, rolling, etc. with an annealing hardness of 95 HRB or less and then heat-treating it to a hardness of 50 HRC or more. Become.
[0020]
【Example】
The ingot was melted in a 100 kg vacuum induction melting furnace. The steel ingot of each component shown in Table 1 was hot forged. After annealing the forged material at 870 ° C., hardness measurement and cold compression deformation resistance measurement were performed. After quenching and tempering at 1030 ° C. oil cooling to 200 ° C. air cooling, hardness measurement, corrosion resistance, Charpy impact test, ferrite, and carbide content measurement were performed. The results are shown in Table 2. Each test condition and evaluation method are shown below.
[0021]
[Table 1]
[Table 2]
(1) Annealing hardness φ20mm, hot forging, holding at 870 ° C for 2 hours, furnace cooling to 570 ° C at 20 ° C / h, then air-cooled once annealing The parts were measured with a Rockwell hardness meter.
(2) Cold compression deformation resistance As an evaluation of the load on the press in cold forging such as header processing, the deformation resistance is measured when the annealed material is processed to φ14 × L21 and compressed in the longitudinal direction by 50% at room temperature. did.
[0024]
(3) Quenching and tempering hardness 1030 ° C. for 30 minutes Holding oil cooling, 200 ° C. for 1 hour holding air cooling quenching and tempering was performed, and the φ20T surface middle circumference was measured with a Rockwell hardness meter. All the following investigations performed the above heat treatment.
(4) CASS test After quenching and tempering, a CASS test (JIS Z2371, 72h) was performed using a φ12 × L21 test piece, and the presence or absence of rusting was observed.
[0025]
(5) Charpy impact test Quenching and tempering were performed, and a Charpy impact value was determined using a 10 × 10 × 55 JIS Z 2202 (2 mm U notch) Charpy impact test piece.
(6) In order to investigate the influence of the component elements on the amount of residual ferrite in the martensite in the ferrite area ratio quenching and tempering state, the ferrite amount at the center of φ20 was measured with an image analysis device with respect to a 100 × photograph × 5 field of view.
(7) Carbide area ratio φ20 The carbide area ratio at the center of the quenching and tempering material was measured with an image analyzer for 500 × 5 visual fields.
[0026]
As shown in Table 1, no. 1-No. No. 17 is a chemical component as an example of the present invention. 18-No. 33 was set as the chemical component composition of the comparative example. The test results, Comparative Example No. 18-No. The component composition in No. 33 deviates from the conditions of the present invention, and the resulting Table 2 is inferior in any of the properties. No. 27 has a defect in the steel ingot. No. 32 cracked during compression.
[0027]
【The invention's effect】
As described above, the present invention can provide a material suitable as a material for high-hardness martensitic stainless steel having a hardness of 50 HRC or more excellent in corrosion resistance, toughness, and cold workability, and a tapping screw material. It is what you play.
Claims (5)
Ｃ：０．１７〜０．２２％、
Ｓｉ：２．０％以下、
Ｍｎ：２．０％以下、
Ｃｒ：１４．０〜１５．０％、
Ｍｏ：０．７〜１．３％、
Ｎｉ：０．１７〜０．６％、
Ｎ：０．１０〜０．１５％、
Ｏ：０．００３〜０．０１％
を含有し、かつ、
Ｃｕ：０．２％以下、
Ｐ：０．０５％以下、
残部Ｆｅおよび不可避的不純物からなり、かつ下記式によるＰｆ：１４以下、Ｐｈ：５０以上であることを特徴とする耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼。
ただし、
Ｐｆ＝［％Ｃｒ］＋１．７×［％Ｍｏ］＋２．１×［％Ｓｉ］−７×［％Ｃ］−８×［％Ｎ］−３．３×［％Ｎｉ］−１．９×［％Ｍｎ］−０．５×［％Ｃｕ］
Ｐｈ＝５５．７＋４４．５×［％Ｃ］＋８４．７×［％Ｎ］−１．５×［％Ｃｒ］By mass
C: 0.17 to 0.22%,
Si: 2.0% or less,
Mn: 2.0% or less,
Cr: 14.0 to 15.0%,
Mo: 0.7 to 1.3%,
Ni: 0.17 to 0.6%,
N: 0.10 to 0.15%,
O: 0.003-0.01%
Containing, and
Cu: 0.2% or less,
P: 0.05% or less,
A high-hardness martensitic stainless steel excellent in corrosion resistance, toughness, and cold workability, characterized by comprising the balance Fe and unavoidable impurities and having Pf: 14 or less and Ph: 50 or more according to the following formula.
However,
Pf = [% Cr] + 1.7 × [% Mo] + 2.1 × [% Si] −7 × [% C] −8 × [% N] −3.3 × [% Ni] −1.9 × [% Mn] -0.5 × [% Cu]
P h = 55.7 + 44.5 × [ % C] + 84.7 × [% N] -1.5 × [% Cr]
Ａｌ：０．０５％以下、
Ｃａ：０．００５％以下、
Ｍｇ：０．００５％以下、
Ｔｉ：０．５％以下、
Ｖ：０．５％以下、
Ｗ：０．５％以下、
Ｎｂ：０．５％以下、
Ｔａ：０．５％以下、
Ｈｆ：０．５％以下、
Ｂ：０．０１％以下、
の１種または２種以上を含有することを特徴とする耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼。In addition to claim 1, by mass,
Al: 0.05% or less,
Ca: 0.005% or less,
Mg: 0.005% or less,
Ti: 0.5% or less,
V: 0.5% or less,
W: 0.5% or less,
Nb: 0.5% or less,
Ta: 0.5% or less,
Hf: 0.5% or less,
B: 0.01% or less,
A high-hardness martensitic stainless steel excellent in corrosion resistance, toughness and cold workability, characterized by containing one or more of the following.
Ｓ：０．４％以下、
Ｓｅ：０．３％以下、
Ｔｅ：０．１％以下、
Ｐｂ：０．３％以下、
Ｂｉ：０．３％以下、
の１種または２種以上を含有することを特徴とする耐食性、靱性および冷間加工性に優れた高硬度マルテンサイト系ステンレス鋼。In addition to claim 1 or 2, by mass,
S: 0.4% or less,
Se: 0.3% or less,
Te: 0.1% or less,
Pb: 0.3% or less,
Bi: 0.3% or less,
A high-hardness martensitic stainless steel excellent in corrosion resistance, toughness and cold workability, characterized by containing one or more of the following.
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