EP0674014B1 - Wärmeunbehandelter warmgeschmiedeter stahl mit hervorragender zugfestigkeit, ermüdungsfestigkeit und bearbeitbarkeit - Google Patents

Wärmeunbehandelter warmgeschmiedeter stahl mit hervorragender zugfestigkeit, ermüdungsfestigkeit und bearbeitbarkeit Download PDF

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Publication number
EP0674014B1
EP0674014B1 EP94929026A EP94929026A EP0674014B1 EP 0674014 B1 EP0674014 B1 EP 0674014B1 EP 94929026 A EP94929026 A EP 94929026A EP 94929026 A EP94929026 A EP 94929026A EP 0674014 B1 EP0674014 B1 EP 0674014B1
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EP
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Prior art keywords
machinability
bainite
ratio
tensile strength
comparative example
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EP94929026A
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English (en)
French (fr)
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EP0674014A1 (de
EP0674014A4 (de
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Toshihiko Nippon Steel Corporation Takahashi
Fusao Nippon Steel Corporation Ishikawa
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Nippon Steel Corp
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Nippon Steel Corp
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a non-thermal refined steel, having excellent tensile strength, fatigue strength and machinability simultaneously, and usable as hot forged, without thermal refining process such as quench hardening and tempering after hot forging.
  • Non-thermal refined steels have been widely used for structural machine parts such as an automobile parts from the standpoint of elimination of steps and reduction of production cost.
  • non-thermal refined steels have been developed mainly for their high tensile strength (or hardness), yield strength and toughness.
  • non-thermal refined steels have been proposed that employ V, a typical precipitation strengthening element.
  • the real problem is the fatigue strength and machinability.
  • Fatigue strength is generally understood to depend on the tensile strength and increases as the tensile strength increases.
  • enhancement of tensile strength makes the machinability extremely deteriorated: with a tensile strength exceeding 120 kgf/mm 2 , production with normal efficiency will be impossible.
  • the present inventors noted a structure that includes pearlite, having good machinability and made an invention relating to a non-reinforced refined steel for hot forging excellent in fatigue strength and machinability by combination of two steps to get a metallographic structure that contains fine and precipitation-enriched perlite throughout the structure, the two steps comprising: (1) first step wherein, TiN and VN are compositely precepitated on MnS to refine austenite crystal grains formed when heated for forging and the composite precipitates are utilized as nucleus generating sites for precipitating ferrite very finely; and (2) second step wherein perlite is precipitated and V carbide or V nitride is simultaneously finely precipitated in the ferrite in the precipitated pearlite.
  • the non-thermal refined steel of this type has tensile strength of 100 kgf/mm 2 at best; thus there has been a limit in fatigue strength even if the durability ratio was improved.
  • the present invention is to provide a non-thermal refined steel for hot forging having high fatigue strength, tensile strength and machinability, which has been difficult to realize by the conventional non-thermal refined steel.
  • the easiest way to attain high fatigue strength is to enhance the tensile strength (hardness). This may be realized by introducing structures, such as martensite or bainite, that are transformed at lower temperatures; however, such methods deteriorate the machinability significantly as explained in the description of the prior art.
  • the present inventors have studied the fatigue characteristic and machinability for several kinds of hot forging materials that have metallographic structures containing the ferrite structure mixed with an adequate quantity of the bainite structure.
  • a non-thermal refined steel, of ferrite-bainite type, for hot forging has been invented that improves the tensile strength and fatigue strength and also maintains the machinability allowable in the current machining practices on the basis of the following three points: (1) to use composite precipitates of MnS + TiN + VN as precipitation nuclei for the purpose of making the structure fine; (2) to make a two-phase ferrite-bainite structure that contains an adequate quantity of bainite structure with controled hardness by lowering carbon and nitrogen contents; and (3) to precipitate V carbide in the bainite structure.
  • This element is important for adjusting the structure ratio of bainite structure and accordingly increases tensile strength of the final product.
  • too high contents of this element increase the strength excessively and deteriorate the machinability significantly.
  • contents less than 0.10% make both tensile strength and fatigue strength too low, and the contents exceeding 0.35% make the tensile strength too high causing the machinability significantly to deteriorate.
  • the range of 0.10 - 0.35% is specified.
  • Si This element adjusts deoxidization and the ratio of bainite structure. Si contents less than 0.15% do not give enough effect; the contents exceeding 2.00% lower both durability ratio and machinability. Thus, the range of 0.15 - 2.00% is specified.
  • Mn This element adjusts the ratio of bainite structure and turns to MnS that brings a base of composite precipitates giving the precipitation site for ferrite. Mn contents less than 0.40% do not give enough effect; the contents exceeding 2.00% bring too much formation of bainite causing both durability ratio and machinability to lower. Thus, the range of 0.40 - 2.00% is specified.
  • This element turns to MnS, bringing a base of composite precipitates which provides the precipition site for ferrite and improves the machinability.
  • the specified range is 0.03 - 0.10%.
  • This element is effective for deoxidizing and making the crystal grains finer. Its contents less than 0.0005% do not give enough effect, and the contents exceeding 0.050% form hard inclusion causing both durability ratio and machinability to lower. Thus, the range of 0.0005 - 0.050% is specified.
  • Ti This element turns to a nitride precipitating on MnS forming the composite precipitates which provides the precipitation site for ferrite.
  • Ti contents less than 0.003% do not give enough effect; the contents exceeding 0.050% promote formation of coarse hard inclusion causing both the durability ratio and machinability to lower. Thus, the range of 0.003 - 0.050% is specified.
  • N This element forms nitrides and carbon nitrides with Ti and V. Its contents less than 0.0020% do not give enough effect, and the contents exceeding 0.070% cause both durability ratio and machinability to lower. Thus, the range of 0.0020 - 0.0070% is specified.
  • V This element forms the composite precipitates with MnS and TiN and enforces the precipitation strengthening of matrix ferrite in bainite. Its contents less than 0.30% do not give enough effect; the contents exceeding 0.70% cause both durability ratio and machinability to lower. Thus, the range of 0.30 - 0.70% is specified.
  • Mo This element has effect similar to Mn and Cr. Its contents less than 0.02% do not give enough effect, and the contents exceeding 1.00% bring too much formation of bainite, causing both durability ratio and machinability to lower. Thus, the range of 0.02 - 1.00% is specified.
  • Nb This element has effect similar to Mn and Cr. Its contents less than 0.001% do not give enough effect, and the contents exceeding 0.20% bring too much formation of bainite causing both durability ratio and machinability to lower. Thus, the range of 0.001 - 0.20% is specified.
  • Pb This element improves the machinability. Its contents less than 0.05% do not give enough effect; the contents exceeding 0.30% saturate such effect and decrease the fatigue strength and durability ratio. Thus, the range of 0.05 - 0.30% is specified.
  • This element has effect similar to Pb. Its contents less than 0.0005% do not give enough effect, and the contents exceeding 0.010% saturate such effect and decreases the fatigue strength and durability ratio. Thus, the range of 0.0005 - 0.010% is specified.
  • the structure ratio of bainite can be controlled by the C content of the steel, the hardening characteristic and the cooling rate from the austenite zone.
  • its structure ratio f is required to be more than 1.4C where C is the carbon content (%).
  • the structure ratio f of bainite is specified as 1.4C or more and 1.4C + 0.4 or less on the basis of the carbon content C (%).
  • the structure ratio f of bainite is determined by observing an etching test piece by optical microscope or the like, and by measuring the structure hardness by a micro-Vickers hardness meter; finally the structure ratio f is determined by measuring the area percentage.
  • Each steel having chemical components shown in Table I was melted in a high frequency furnace to make a steel ingot of 150 kg. From this ingot, a material for forging was cut out, normalized once with heating followed by allowing to cool down, heated up to 1100 - 1250°C and subjected to hot forging at a temperature of 1050 - 1200°C, and thereafter allowed to cool down. From the center part of this material, a JIS No. 4 tensile test piece and a JIS No. I rotary bending test piece were sampled and subjected to the tensile test and rotating bending fatigue test respectively.
  • a specimen for observation by an optical microscope was etched with 5% nital, and observed with a magnification of X200 to determine the structure ratio of bainite.
  • a specimen for machinability test was further sampled from the material, and a blind hole of 30 mm depth was bored therein by a 10 mm ⁇ straight shank drill made of SKH9. Total length of the boring was measured until the drill was broken with life. Machinability was evaluated by the relative total boring length supposing the total boring length of conventional No steel 1.00. The cutting speed was 50 m/min, feed speed was 0.35 mm/rev, and the cutting oil was 7 L/min.
  • Table 2 shows the structure ratio of bainite and results of performance evaluation for each sample.
  • a Comparative Example has a low tensile strength and low fatigue strength since the C content is low.
  • No. 22, a Comparative Example has martensite generated due to the excessive C content and does not satisfy the required range for structure ratio of bainite according to the present invention; although the tensile strength is high, the durability ratio is low compared with Embodying Examples and the machinability is also poor.
  • a Comparative Example has a low degree of deoxidation since the Si content is low, and the durability ratio is low compared with Embodying Examples.
  • No. 24, a Comparative Example has martensite formed due to the excessive Si content and does not satisfy the required range for structure ratio of bainite according to the present invention; the durability ratio is low compared with Embodying Examples and the machinability is also poor.
  • No. 25 a Comparative Example, has a low composite precipitation since the Mn content is low, and has a poor durability ratio compared with Embodying Examples.
  • No. 26, a Comparative Example has martensite formed due to the excessive Mn content and does not satisfy the required range for structure ratio of bainite according to the present invention; the durability ratio is low compared with Embodying Examples and the machinability is also poor.
  • a Comparative Example has a low composite inclusion since the S content is low and has a poor durability ratio compared with Embodying Examples; the machinability is also poor since the effect of MnS for improving the machinability is not realized.
  • No. 28 a Comparative Example, has excessive precipitates of Mns since the S content is high, and has a lower durability ratio compared with the Embodying Examples.
  • No. 29, a Comparative Example has a low degree of deoxidation and a smaller effect of making crystals fine since the Al content is low, and has a lower durability ratio compared with the Embodying Examples.
  • No. 30, a Comparative Example has hard inclusion formed due to the high Al content, and has a lower durability ratio compared with the Embodying Examples; the machinability is also poor.
  • a Comparative Example has a small composite precipitates as the Ti content is low, and has a lower durability ratio compared with the Embodying Examples.
  • No. 32 a Comparative Example, has hard inclusions formed due to the high Ti content, and has a lower durability ratio compared with the Embodying Examples; the machinability is also poor.
  • a Comparative Example has small composite precipitates since the N content is low, and has a lower durability ratio compared with the Embodying Examples.
  • No. 34 a Comparative Example, has the matrix hardened due to the high N content, and has a lower durability ratio compared with the Embodying Examples; the machinability is also poor.
  • No. 35 a Comparative Example, has small composite precipitates and has a smaller effect of precipitation strengthening of matrix ferrite due to the low V content; thus, the durability ratio is low compared with the Embodying Examples and the durability ratio is also poor.
  • No. 36 a Comparative Example, has a lower durability ratio compared with the Embodying Examples due to the high V content, and the machinability is also poor.
  • No. 37 a Comparative Example, has martensite formed due to the excessive Cr content and does not satisfy the required range for structure ratio of bainite according to the present invention; the durability ratio is low compared with Embodying Examples and the machinability is also poor.
  • No. 38 a Comparative Example, has martensite formed due to excessive Mo content and does not satisfy the required range for structure ratio of bainite according to the present invention; the durability ratio is low compared with Embodying Examples and the machinability is also poor.
  • No. 40 a Comparative Example, has a poor durability ratio although the machinability is good due to the high Pb content.
  • Each steel having chemical components shown in Table 1 was melted in a high frequency furnace to make a steel ingot of 150 kg. From this ingot, a material for forging was cut out, normalized once with heating at a temperature of 950°C followed by allowing to cool down, heated up to 1100 - 1250°C and subjected to hot forging at a temperature of 1050 - 1200°C, and thereafter allowed to cool down in a way as shown in Table 3. From the center part of this material, the tensile strength, fatigue strength, machinability and ratio of bainite structure were determined in the same procedures as Embodying Example 1. Table 4 shows the ratio of bainite structure and results of performance evaluation for each sample.
  • Nos. 45, 46, 47 and 48 are Embodying Examples satisfying the requirement for structure ratio of bainite according to the present invention, that is, the structure ratio f is, on the basis of the carbon content C (%), represented by: 1.4C + 0 4 ⁇ f ⁇ 1.4C ; all have good machinability nearly 2.5 times as high as No. 42, a conventional thermal refined steel, while the durability ratio is kept 0.55 or more.
  • Nos. 43 and 44 produced with a smaller cooling rate, have a smaller structure ratio of bainite; majority of the structure is ferrite or bainite + spherical cementite.
  • the tensile strength itself is low, furthermore, the effect by the two phase structure of ferrite + bainite disappears and the durability ratio is as low as 0.54 or less; the machinability is poor compared with the Embodying Examples.
  • No. 49 has a structure mainly composed of martensite by increasing the cooling rate; while the tensile strength is enhanced, the durability ratio is extremely low and the machinability is poor with short tool life.
  • No Sample Steel Cooling Method After Forging Average Cooling Speed at 800-500°C 43 No.20 of Table 1 Air cooled after 30min. charge into the furnace at 700 °C ca.0.10 °C/sec. 44 " Cooled in the furnace kept at 200°C ca.0.15 °C/sec. 45 " Gradually cooled in glass wool insulating material ca.0.30 °C/sec. 46 " Nature cooling ca.0.80 °C/sec. 47 " Cooling in air blast ca.1.40 °C/sec.
  • the steel according to the present invention provides high tensile strength while keeping the machinability by making a ferrite-bainite two phase structure, and, further, is able to have improved durability ratio, namely fatigue strength, without sacrificing the machinability by realization of fine metallographic structure by use of composite precipitates formed by MnS, Ti nitride and V nitride and by simultaneous realization of strengthening of the ferrite matrix in bainite by V carbide (or carbon nitride); thus, a non-thermal refined steel for hot forging, which has been eagerly demanded, is now provided, satisfying both high fatigue strength with high tensile strength and machinability and bringing great industrial advantages.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Forging (AREA)

Claims (2)

  1. Nicht-wärmebehandelter warmgeschmiedeter Ferrit-Bainit-Stahl, der eine Zusammensetzung in Gewichts-% aufweist von
    C: 0,10 -0,35%,
    Si: 0,15 - 2,00%,
    Mn: 0,40 - 2,00%,
    S: 0,03 - 0,10%,
    Al: 0,0005 - 0,050%,
    Ti: 0,003 - 0,050%,
    N: 0,0020 - 0,0070%,
    V: 0,30 - 0,70%,
    wobei er optional eines oder mehr Elemente enthält, die ausgewählt werden aus
    Cr: 0,02 - 1,50%,
    Mo: 0,02 - 1,00%,
    Nb: 0,001 - 0,20%,
    Pb: 0,05 - 0,30% und
    Ca: 0,0005 - 0,010%,
    und wobei der Rest aus Fe und Verunreinigungen besteht; und er ein Strukturverhältnis f einer Bainit-Struktur in der metallographischen Struktur nach dem Abkühlen vom Warmschmieden auf Raumtemperatur aufweist, wobei das Strukturverhältnis f auf der Basis des Kohlenstoff-Gehaltes C (%) dargestellt wird durch: 1,4C + 0,4 ≥ f ≥ 1,4C.
  2. Stahl nach Anspruch 1, der ein oder zwei oder mehr Elemente enthält, die ausgewählt werden aus
    Cr: 0,02 - 1,50%,
    Mo: 0,02 - 1,00%,
    Nb: 0,001 - 0,20%,
    Pb: 0,05 - 0,30%, und
    Ca: 0,0005 - 0,010%.
EP94929026A 1993-10-12 1994-10-11 Wärmeunbehandelter warmgeschmiedeter stahl mit hervorragender zugfestigkeit, ermüdungsfestigkeit und bearbeitbarkeit Expired - Lifetime EP0674014B1 (de)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
JP25433593A JP3241897B2 (ja) 1993-10-12 1993-10-12 引張強度、疲労強度および被削性に優れる熱間鍛造用非調質鋼
JP25433593 1993-10-12
JP254335/93 1993-10-12
PCT/JP1994/001693 WO1995010637A1 (fr) 1993-10-12 1994-10-11 Acier de forgeage a chaud sans traitement thermique, presentant d'excellentes caracteristiques de resistance a la traction et a la fatigue et une tres bonne aptitude a l'usinage

Publications (3)

Publication Number Publication Date
EP0674014A1 EP0674014A1 (de) 1995-09-27
EP0674014A4 EP0674014A4 (de) 1996-02-07
EP0674014B1 true EP0674014B1 (de) 1999-07-28

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EP94929026A Expired - Lifetime EP0674014B1 (de) 1993-10-12 1994-10-11 Wärmeunbehandelter warmgeschmiedeter stahl mit hervorragender zugfestigkeit, ermüdungsfestigkeit und bearbeitbarkeit

Country Status (6)

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EP (1) EP0674014B1 (de)
JP (1) JP3241897B2 (de)
KR (1) KR0180938B1 (de)
CN (1) CN1039035C (de)
DE (1) DE69419720T2 (de)
WO (1) WO1995010637A1 (de)

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FR2774098B1 (fr) * 1998-01-28 2001-08-03 Ascometal Sa Acier et procede pour la fabrication de pieces de mecanique secables
KR20010048426A (ko) * 1999-11-26 2001-06-15 이계안 앞차축 빔용 저 탄소계 고인성 비조질강 단조품
KR100435461B1 (ko) * 1999-12-20 2004-06-10 주식회사 포스코 재질편차가 적은 냉간압조용 강재의 제조방법
JP3888865B2 (ja) * 2000-10-25 2007-03-07 株式会社ゴーシュー 鍛造方法
KR100401571B1 (ko) * 2001-02-20 2003-10-17 한국기계연구원 고강도 베이나이트계 열간단조용 비조질강재 및 그제조방법
EP1605071B1 (de) * 2003-03-18 2008-10-15 Sumitomo Metal Industries, Ltd. Nicht abgeschreckte/getemperte pleuelstange und zugehöriges herstellungsverfahren
CA2531615A1 (en) * 2004-12-28 2006-06-28 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) High strength thin steel sheet having high hydrogen embrittlement resisting property
CN100500918C (zh) * 2007-03-29 2009-06-17 攀钢集团成都钢铁有限责任公司 大口径非调质石油套管用钢
CN102851471A (zh) * 2012-09-27 2013-01-02 攀枝花学院 低碳合金钢快速获得细小铁素体晶粒的热处理方法
DK3168312T3 (da) * 2015-11-16 2019-07-01 Deutsche Edelstahlwerke Specialty Steel Gmbh & Co Kg Konstruktionsædelstål med bainitisk struktur, smedeemne fremstillet deraf og fremgangsmåde til fremstilling af et smedeemne
CN105734414A (zh) * 2016-02-19 2016-07-06 唐山钢铁集团有限责任公司 一种中碳非调质钢及其生产方法
CN111295457A (zh) * 2017-10-31 2020-06-16 日本制铁株式会社 热锻钢材
CN111154945B (zh) * 2020-01-17 2021-05-28 中天钢铁集团有限公司 一种Ti、V微合金化铝脱氧含硫非调质钢中液析氮化物的控制方法
CN115896614A (zh) * 2022-10-31 2023-04-04 中国第一汽车股份有限公司 一种含铌贝氏体非调质钢材料、转向节以及制备方法

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Publication number Publication date
KR950704530A (ko) 1995-11-20
CN1039035C (zh) 1998-07-08
EP0674014A1 (de) 1995-09-27
EP0674014A4 (de) 1996-02-07
DE69419720T2 (de) 1999-12-16
JP3241897B2 (ja) 2001-12-25
KR0180938B1 (ko) 1999-02-18
JPH07109545A (ja) 1995-04-25
WO1995010637A1 (fr) 1995-04-20
CN1115582A (zh) 1996-01-24
DE69419720D1 (de) 1999-09-02

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