US20050022906A1 - Bainite type non-refined steel for nitriding, method for production thereof and nitrided product - Google Patents

Bainite type non-refined steel for nitriding, method for production thereof and nitrided product Download PDF

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US20050022906A1
US20050022906A1 US10/499,657 US49965704A US2005022906A1 US 20050022906 A1 US20050022906 A1 US 20050022906A1 US 49965704 A US49965704 A US 49965704A US 2005022906 A1 US2005022906 A1 US 2005022906A1
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steel
less
nitrided
rocker arm
sulfide
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Takaaki Harasaki
Takuya Nogami
Noriyuki Yamada
Katsuaki Shiiki
Shinya Takahashi
Takashi Kano
Yutaka Kurebayashi
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Honda Motor Co Ltd
Tanaka Seimitsu Kogyo Co Ltd
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Honda Motor Co Ltd
Tanaka Seimitsu Kogyo Co Ltd
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Priority claimed from JP2002019412A external-priority patent/JP4144224B2/ja
Priority claimed from JP2002019750A external-priority patent/JP4216507B2/ja
Application filed by Honda Motor Co Ltd, Tanaka Seimitsu Kogyo Co Ltd filed Critical Honda Motor Co Ltd
Assigned to HONDA GIKEN KOGYO KABUSHIKI KAISHA, TANAKA SEIMITSU KOGYO CO., LTD. reassignment HONDA GIKEN KOGYO KABUSHIKI KAISHA ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HARASAKI, TAKAAKI, NOGAMI, TAKUYA, SHIIKI, KATSUAKI, TAKAHASHI, SHINYA, YAMADA, NORIYUKI, KANO, TAKASHI, KUREBAYASHI, YUTAKA
Publication of US20050022906A1 publication Critical patent/US20050022906A1/en
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24—Nitriding
    • C23C8/26—Nitriding of ferrous surfaces
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • the present invention relates to a bainite-type untempered steel to be nitrided and a method of manufacturing the same.
  • the present invention also relates to a nitrided product, for example, a rocker arm product manufactured from the steel.
  • nitrided steel including soft nitrided steel, have conventionally an alloy composition containing a comparatively large amount of aluminum. After the steel is shaped into the configuration of a mechanical component part and hardened and tempered, nitriding treatment is carried out to obtain a product.
  • the conventional nitrided steel has many problems as described below.
  • Tempering is generally considered indispensable for securing a mechanical strength of a component part, but is a factor of making a manufacturing cost high. Therefore it is preferable that the steel is not tempered.
  • the steel containing a large amount of Al has problems, for example, in manufacturability peculiar thereto especially castability, and in the quality of a steel ingot especially surface imperfection.
  • free-machining steels have been known: (1) free-machining steel to which elements for improving its machinability such as S, Pb, Bi, Se, and Te are added, (2) free-machining steel to which Ca is added, and (3) free-machining steel to which Ca and S, etc. are compositely added.
  • free-machining steel to which Ca and S, etc. are compositely added Japanese Patent Application Laid-Open No. 49-5815 is publicly known.
  • the composition of non-metal inclusion shown by the ternary system state diagram of CaO—Al 2 O 3 —SiO 2 is present in the mullite region and 5 to 15 ppm of Ca and 0.04 to 0.1% of S are contained therein.
  • the machinability of the said conventional free-machining steel to which Ca and S, etc. are added varies widely, thus it cannot be said that the machinability is sufficiently high.
  • Japanese Patent Application No. 10-287953 “Steel for machine structural use superior in mechanical property and drilling efficiency” is a typical known invention.
  • the free-machining steel is characterized in including calcium-manganese sulfide inclusion containing 1% or less of calcium in a spindle-shape with amajor axis/minor axis ratio of 5, which encloses a calcium aluminate oxide inclusion containing 8 to 62% of CaO.
  • This invention has realized excellent machinability, however, variations in the machinability were sometimes observed during the operation. It can be understood that this is because the calcium-manganese sulfide inclusion has a variety of presence forms.
  • a Ca free-machining steel where machinability of the above free-machining steel to which Ca and S, etc. are compositely added is improved.
  • the Ca free-machining steel described in the publication is a free-machining steel with excellent lathe-turning performance which contains the following elements: C: 0.1 to 0.8%, Si: 0.01 to 2.5%, Mn: 0.1 to 3.5%, P: 0.001 to 0.02%, S: 0.005 to 0.4%, Al: 0.001 to 0.1%, Ca: 0.0005 to 0.02%, O: 0.0005 to 0.01% and N: 0.001 to 0.04%, and the residue composed of Fe and unavoidable impurities, and also satisfies the formulas X/(X+Y+Z) ⁇ 0.3 and Y/(X+Y+Z) ⁇ 0.1, supposing that the ratio of the area of a sulfide containing not less than 40% of Ca to the entire examination and observation visual field is X
  • the steel of the invention is characterized in that it includes not less than five sulfides containing 0.1 to 1% of Ca per 3.3 mm 2 . Each sulfide has a diameter of not less than 5 ⁇ m, supposing that the sulfide is circular.
  • steel having an alloy composition for machine structural use shows stable machinability when the occupation area of sulfide inclusion containing not less than 1.0 wt % of Ca which is present in contact with oxide inclusion containing 8 to 62 wt % of CaO is not less than 2.0 ⁇ 10 ⁇ 4 mm per visual field area of 3.5 mm 2 .
  • the method of manufacturing the steel has been established and proposed (Japanese Patent Application Laid-Open No. 2001-174606).
  • the inclusion having the form in which the oxide inclusion forming the core is surrounded with the inclusion containing the sulfide as its main component which the present applicant calls “sulfide form control type free-machining component”, is applicable to a wide range of steels.
  • the said inclusion is not applicable to steel containing a large amount of Al.
  • the content of Al should be 0.02% or less.
  • nitrided steel in common use contains at least 1% of Al, thus it is impossible to generate the free-machining component of sulfide form control type and utilize it.
  • a steel rocker arm mounted in an internal combustion engine as a component part for machine structural use is formed into a desired final configuration by means of rough processing by plastic work such as forging and following cutting process.
  • a lathe-turning process is a manufacturing process which is applied to almost all component parts. Since the ratio of the cost of lathe-turning process to the manufacturing cost of a component part for machine structural use is considerably high, there have been increasing demands for development of a free-machining steel superior in machinability to reduce the cost.
  • the softening-resistant property means prevention of an inner hardness from being reduced by a treating temperature in nitriding operation.
  • the present invention has been made in view of the above-described situation, and an object of the present invention is, by solving the above-described problems, to provide an untempered free-machining steel to be nitrided having machinability equivalent to that of the known Pb free-machining steel even though Pb is not contained, having superior nitriding properties such as nitrided layer depth, and also having a strength equal to that of the known tempered steel without being tempered. It is also an object of the present invention to provide mechanical component parts using the nitrided steel, for example, steel for a rocker arm and a rocker arm product manufactured from the said steel for a rocker arm.
  • the present inventors have made energetic investigation. As a result, they have succeeded in solving the above problems and developing a bainite-type untempered steel to be nitrided having machinability equivalent to that of the known Pb free-machining steel, superior in its nitriding properties such as nitrided layer depth, and having a strength equal to that of the known tempered steel without being tempered. They have also succeeded in developing mechanical component parts using the nitrided steel such as a rocker arm product.
  • the free-machining steel to be nitrided of the present invention (bainite-type untempered steel) achieving the above-described object contains the following elements including C: 0.05 to 0.8 wt %, Si: 0.01 to 2.5 wt %, Mn: 0.1 to 3.5 wt %, P: 0.001 to 0.2 wt %, S: 0.01 to 0.2 wt %, Cr: 1.0 to 3.5 wt %, V: 0.1 to 0.5 wt %, Al: 0.001 to 0.020 wt %, Ca: 0.0005 to 0.02 wt %, and O: 0.0005 to 0.01 wt % as its basic alloy composition and the residue composed of unavoidable impurities and Fe, wherein an occupation area of a sulfide inclusion present in contact with an oxide inclusion containing 8 to 62 wt % of CaO and containing not less than 1.0 wt % of Ca is not less than 2.0 ⁇ 10 ⁇ 4
  • the steel for the rocker arm according to the present invention is a steel in which a hard oxide in contact with a sulfide composed of MnS and CaS is dispersed in a matrix; the hard oxide contains Al 2 O 3 and CaO, the ratio of the content of the CaO to the entire hard oxide is 8.0 to 62% (wt/wt); the ratio of the content of the Ca to the entire sulfide is 1.0 to 45% (wt/wt); and the occupation area of the sulfide in the steel is 2.0 ⁇ 10 ⁇ 4 to 1.0 ⁇ 10 ⁇ 1 mm 2 per area of 3.5 mm 2 .
  • the present inventors have found that the hard oxide which contains Al 2 O 3 and CaO and is in contact with the sulfide composed of MnS and CaS in the steel of the present invention functions as the free-machining component of sulfide form control type, and improves the grindability of the matrix itself outstandingly without deteriorating the anisotropy.
  • the present inventors have also found that the above-described sulfide generates a tool protective sulfide film on the surface of a machining tool during cutting operation, whereby the life of the tool is greatly improved and its machinability is preferably improved.
  • the use of the steel for the rocker arm allows various rocker arms to be manufactured at a low cost and with high productivity.
  • the hard oxide in contact with the sulfide may be any hard oxide as long as at least one part of the hard oxide is contacting with at least one part of sulfide.
  • the case where the surface of the hard oxide contacts that of the sulfide and the case where a part of or entire surface of the hard oxide is coated with a sulfide or a sulfide film are included.
  • the present inventors have found out that steel for a rocker arm containing C: 0.1 to 0.5% (wt/wt), Si: 0.01 to 2.5% (wt/wt), Mn: 0.1 to 3.5% (wt/wt), P: 0.001 to 0.2% (wt/wt), S: 0.01 to 0.2% (wt/wt), Cr: 1.0 to 3.5% (wt/wt), V: 0.1 to 0.5% (wt/wt), Al: 0.001 to 0.02% (wt/wt), Ca: 0.0005 to 0.02% (wt/wt), and O: 0.0005 to 0.01% (wt/wt) as basic components and the residue composed of Fe and unavoidable impurities, wherein the content of the elements Cr and V satisfies a formula of ([Cr]+1.97 ⁇ [V]) ⁇ 2.15% (wt/wt) facilitates the bond between Ca and O and accelerates the generation of the free-machining component of sulfide form control type
  • the present inventors have also found that, by adjusting the amount of Cr and V to satisfy the above formula, it is possible to improve the softening-resistant property during nitriding treatment and preferably improve the surface hardness and nitrided layer depth after the steel is nitrided, thereby a superior nitriding property is provided.
  • the present inventors have also found that by adjusting the addition amount of elements C, Mn, S, and Cr contained in the steel for a rocker arm having the above basic components in such a way that the carbon equivalent Ceq indicated by theformula:([C]+0.27 ⁇ ([Mn] ⁇ 55 ⁇ [S]/ 32 )+0.31 ⁇ [Cr]+0.3 ⁇ [V]) is set within the range of 0.8 to 1.1% (wt/wt), it is possible to form the structure of ferrite+bainite easily and adjust the inner hardness easily to 20 to 35 HRC when the steel is hot-forged and then allowed to cool. Thereby the steel is allowed to have both excellent machinability and favorable fatigue strength after nitriding treatment is performed, and therefore it is possible to manufacture the rocker arm of fine quality with high productivity.
  • the present inventors have also found that it is possible to easily form the untempered structure of ferrite+bainite formed after the hot-forged and allowed to cool having a strength equivalent to that of tempered steel, by adjusting the content of elements Mn, S and Cr to satisfy the formula:([Mn] ⁇ 55 ⁇ [S]/32+[Cr])>2.0 wt % when Mo is not included among the above elements, and by adjusting the content of the elements Mn, S, Cr and Mo to satisfy a formula:([Mn] ⁇ 55 ⁇ [S]/32+[Cr]+[Mo])>2.0 wt % when Mo is included, whereby a tempering process can be omitted.
  • the method of manufacturing steel for the rocker arm can be embodied by melting an alloy comprising alloy components specified as described above and the residue with a chemical composition composed of unavoidable impurities and Fe, provided that a content (wt %) ofeachofMn, S, Cr, and Mo satisfies the formula: ([Mn] ⁇ 55 ⁇ [S]/32+[Cr]+[Mo])>2.0 wt % and conducting the above operation in such a manner that the said alloy in a molten states satisfies the following conditions 1) through 3): 1) [ H S/[ H O]: 8 to 80, where [ H S] and [ H O] indicate the activity of S and O defined by the formula shown below.
  • [ H S] [S] ⁇ 10 logGs
  • [ H S] and [ H O] indicate the activity of S and O defined by the formula shown below.
  • a bainite-type untempered steel to be nitrided according to the above (1) further contains, in addition to the alloy components specified in the above (1), one or more element(s) selected from Mo: 2.0% or less, Cu: 2.0% or less, Ni: 4.0% or less, and B: 0.0005 to 0.01%, wherein the formula: ([Mn] ⁇ 55 ⁇ [S]/32+[Cr]+[Mo])>2.0 (where [ ] indicates wt % (wt/wt) of each element) is established when Mo is comprised.
  • a method of manufacturing untempered steel according to any one of the above (1) through (5) which comprises an alloy having alloy components specified in any one of the above (1) through (5) and the residue composed of unavoidable impurities and Fe, provided that each content of Mn, S, Cr, and Mo satisfies the formula: ([Mn] ⁇ 55 ⁇ [S]/32+[Cr]+[Mo])>2.0 (where [ ] indicates wt % (wt/wt) of each element), wherein an operation satisfying the following conditions 1) through 3) is performed:
  • a steel for a rocker arm wherein a hard oxide in contact with a sulfide composed of MnS and CaS is dispersed in a matrix; the hard oxide contains Al 2 O 3 and CaO; a ratio of the content of CaO to the hard oxide is 8.0 to 62% (wt/wt); a ratio of the content of Ca to the sulfide i 1.0 to 45% (wt/wt), and an occupation area of the sulfide in the steel is 2.0 ⁇ 10 ⁇ 4 to 1.0 ⁇ 10 ⁇ 1 mm 2 per area of 3.5 mm 2 .
  • FIG. 1 is a graph showing machinability of the free-machining steel of the present invention and a comparison steel and the relationship between [ H S] and [ H O], wherein non-nitrided steels having drilling efficiency (VL5000) of equivalent of or higher than that of free-machining steel containing 0.07% of Pb is denoted by ( ⁇ ) and those having drilling efficiency of lower than that of the free-machining steel containing 0.07% of Pb is denoted by (X) when drilling is conducted on the non-nitrided steel by a high-speed steel drill.
  • VL5000 drilling efficiency of equivalent of or higher than that of free-machining steel containing 0.07% of Pb
  • X drilling efficiency of lower than that of the free-machining steel containing 0.07% of Pb
  • FIG. 2 is a graph showing the relationship between a surface hardness of nitrided free-machining steel of the present invention and nitrided comparison steel and drilling efficiency ratio, in which data of the steel of the present invention is shown by ⁇ and that of the comparison steel is shown with ⁇ .
  • FIG. 3 shows transition of a cutting resistance and a cutting torque in relation to the number of processes when a ⁇ 16.5 opening is formed with a ⁇ 16.5 drill.
  • FIG. 4 shows transition of a cutting resistance and a cutting torque in relation to the number of processes when a finish machining of a ⁇ 18 opening is performed with a ⁇ 18 reamer.
  • FIG. 5 shows a worn state of a cutting face of a tool when a ⁇ 16.5 drill was used to process 700 workpieces.
  • FIG. 6 shows a nitrided layer depth (transition of sectional hardness) after nitriding treatment.
  • FIG. 7 shows the form of the free-machining component of sulfide form control type (background color is black to clarify the form).
  • FIG. 8 shows the result of a face analysis (mapping).
  • FIG. 9 shows the structure of an untempered steel cooled after hot forging is performed.
  • FIG. 10 shows transition of carbon of a ⁇ 20 round rod cooled after hot forging is performed and an inner hardness thereof.
  • FIG. 11 shows the relationship between the structure of a JIS-14A specimen cooled after hot forging is performed and a yield strength ⁇ 0.2 .
  • Carbon is a necessary component for securing the strength of the steel. If the content of carbon is less than 0.05%, the steel has an insufficient strength. On the other hand, if the steel contains a large amount of carbon, the steel has low toughness and machinability. Therefore the upper limit of carbon is set to 0.8%.
  • Silicon makes a component of the steel acting as a deoxidizing agent in an operation of melting an alloy and has a function of improving the hardenability. Thus, Si is necessary for untempered steel. This effect cannot be obtained if the content of Si is less than 0.01%. If a large amount of more than 2.5% of Si is added, ductility is lost and cracking tends to take place in plastic working.
  • Manganese is an important element for generating a sulfide. If the content of manganese is 0.1%, the amount of the inclusion is insufficient. On the other hand, if the content of manganese exceeds 3.5%, the steel becomes hard and its machinability deteriorates.
  • Phosphorous is positively contained at not less than 0.001% in the steel as a component for improving the machinability, particularly the property of a machined surface.
  • phosphorous is disadvantageous for toughness. Thus phosphorous cannot be contained at 0.2% or more in the steel.
  • sulfur is a useful, or indispensable to be more accurate, component for improving the machinability of the steel, not less than 0.01% thereof is contained in the steel. If the content of the sulfur exceeds 0.2%, the steel loses its toughness and ductility, and the sulfur reacts with calcium to generate CaS. Since CaS has a high melting point, it presents obstacles to a casting process.
  • Chromium 1.0 to 3.5%
  • chromium is added to the steel. If a large amount of chromium is added, hot workability deteriorates and the steel cracks in a forging operation. Thus the upper limit of the addition amount of chromium is set at 3.5%.
  • Vanadium 0.1 to 0.5%
  • V is added to the steel.
  • the upper limit of the addition amount of V is set at 0.5%.
  • Aluminum is necessary for appropriately adjusting the composition of the oxide inclusion, thus not less than 0.001% of aluminum is added to the steel.
  • Calcium is a very important component for the steel of the present invention. To contain Ca in a sulfide, it is essential to add not less than 0.0005% of Ca. On the other hand, if not less than 0.02% of Ca is added, said calcium sulfide having a high melting point is formed. Calcium sulfide presents obstacles to a casting process.
  • Oxygen 0.0005 to 0.01%
  • Oxygen is an element necessary for forming an oxide.
  • a large amount of CaS having a high melting point is formed in steel excessively deoxidized and presents obstacles to acastingprocess. Therefore the addition of at least 0.0005% and preferably more than 0.0015% of oxygen to the steel is necessary.
  • O is added, a large amount of a hard oxide is formed, which deteriorates the machinability of the steel and makes it difficult to form a desired calcium sulfide.
  • the condition shown by the above formula must be satisfied to form the structure of the to-be-nitrided untempered steel not of pearlite but of the structure containing bainite as its main component and a small amount of ferrite.
  • the structure containing the bainite as its main component which is only hot-forged has a strength almost equal to that of hardened and tempered steel even, and allows nitrogen to have a higher diffusion speed than a structure containing the pearlite as its main component. Therefore the structure containing the bainite as its main component has an advantage of faster nitriding speed.
  • the to-be-nitrided untempered free-machining steel of the present invention is capable of additionally containing one or more element (s) belonging to the following group in a specified composition range in dependence on a demand for a product. Description is made on the action of each alloy component which can be arbitrarily added in its altered form and the reason the composition range thereof is limited.
  • Molybdenum 2.0% or less
  • Copper makes the structure of the steel dense and increases its strength.
  • the addition of a large amount of Cu is not preferable for both the hot workability and the machinability, thus upper limit of the addition of Cu is set at 2.0%.
  • Ni also enhances the hardenability, but is disadvantageous for the machinability of the steel. Taking this disadvantage and the manufacturing cost into account, the upper limit of the addition amount of Ni is set at 4.0%.
  • the hardenability can be enhanced by the addition of a slight amount of B. To obtain this effect, it is necessary to add not less than 0.0005% of B to the steel. It is disadvantageous to add more than 0.01% of B to the steel because the hot workability deteriorates.
  • Nb and Ti are useful for preventing crystalline grains from becoming coarse at a high temperature.
  • the extent of the effect is maximized and does not change with an increase of the amount thereof. Thus it is advisable to add them at 0.2% or less.
  • Tantalum 0.5% or less
  • zirconium 0.5% or less
  • manganese 0.02% or less
  • These elements have an action of making crystalline grains fine and improving the toughness of the steel.
  • Each of these elements has an action of improving the machinability.
  • Lead is present independently or attaches to the periphery of a sulfide, and lead itself improves the machinability.
  • the upper limit of 0.4% is determined for the reason that, if not less than 0.4% of Pb is added it is not dissolved in the steel but is aggregated and deposited, making the steel defective. The same reason is applied to Bismuth.
  • the upper limit of the addition amount of each of selenium and tellurium is set in consideration of bad influence on hot workability.
  • the inclusion present inside the to-be-nitrided untempered free-machining steel according to the present invention has a double structure, as described above.
  • An analysis made by EPMA JXA8800 manufactured by Nippon Denshi Kabushiki Kaisha
  • the core of the inclusion is composed of an oxide of each Ca, Mg. Si, and Al, and Manganese sulfide containing CaS surrounds the core.
  • the presence form of the inclusion is that the occupation area of the sulfide inclusion present in contact with the oxide inclusion containing 8 to 62 wt % of CaO and containing not less than 1.0 wt % of Ca is not less than 2.0 ⁇ 10 ⁇ 4 mm 2 per visual field area of 3.5 mm 2 .
  • Such a form of the inclusion is necessary to achieve a high machinability targeted in the present invention through a mechanism which will be described later.
  • the condition for realizing such a form of the inclusion becomes the operation condition for the said manufacture. The significance of the condition is described below.
  • FIG. 1 Data obtained from experiments relating to the condition 1) is as shown in FIG. 1 .
  • the graph of FIG. 1 shows the plotted relationship between [ H S]and [ H O] in nitrided steel having not less than 1 drilling efficiency ratio to that of the material before nitrided ( ⁇ ), and nitrided steel having less than 1 drilling efficiency ratio to that of the material before nitrided (X).
  • the word “drilling efficiency ratio” is defined as a value obtained by comparing processability of steel for which a high-speed steel drill is used with processability of a conventional Al—Cr nitrided steel whose machinability has been improved by adding 0.07% of Pb thereto.
  • FIG. 1 indicates that when the activity of S and that of O are combined at an appropriate ratio, preferable machinability can be obtained.
  • the core of the free-machining component of sulfide form control type is composed of a CaO.Al 2 O 3 -based composite oxide surrounded with a (Ca, Mn)S-based composite sulfide.
  • the said oxide has a low melting point
  • the said composite sulfide has a higher melting point than a simple sulfide MnS.
  • the sulfide securely deposits with the sulfide surrounding the oxide when the oxide is composed of the CaO—Al 2 O 3 -based oxide having a low melting point.
  • thermal diffusion wear The significance of the coating the (Ca, Mn)S-based composite sulfide forms on the surface of the tool is admitted most clearly in cutting made by a carbide tool. That is, significance of the coating is wear-suppressing effect of a carbide tool called “thermal diffusion wear”. Describing the thermal diffusion wear, when the tool contacts chip generated from a workpiece at a high temperature, a carbide represented by tungsten-carbide constructing the material of the tool is thermally decomposed. As a result, C is lost by diffusing into the metal of the chip and thus the tool becomes frail, which makes wear progress. Temperature rise of the tool is prevented by the coating having high lubricating property formed on the surface of the tool, thereby diffusion of C is suppressed.
  • the free-machining component of sulfide form control type CaO—Al 2 O 3 /(Ca, Mn)S of the free-machining steel according to the present invention has both advantages of MnS which is the inclusion of the conventional sulfur free-machining steel and anorthite CaO.Al 2 O 3 .2SiO 2 which is the inclusion of the conventional calcium free-machining steel.
  • MnS on the surface of the tool shows lubricating property but the stability of the coating is not sufficient. Thus it is vulnerable to the thermal diffusion wear.
  • the CaO.Al 2 O 3 .2SiO 2 prevents the thermal diffusion wearby forming a stable coating, but has low lubricating property.
  • the free-machining component of sulfide form control type according to the present invention effectively prevents the thermal diffusion wear by forming a stable coating and shows favorable lubricating property.
  • the generation of the free-machining component of sulfide form control type starts from preparation of the composite oxide having a low melting point as described above, so the amount of aluminum is important, and at least 0.001 wt % of the Al is necessary.
  • the amount of Al should be 0.020 wt % or less, because, if the amount of Al is too much, the melting point of the composite oxide becomes high.
  • [Ca]X [S] and [Ca]/[S] are adjusted to the above-described values.
  • the steel for a rocker arm of the present invention has machinability equivalent to that of the known Pb free-machining steel, nitriding property superior in a nitrided layer depth or the like, and a strength equal to that of the known tempered steel, although the steel is not tempered.
  • the ratio of the content of CaO to the entire hard oxide is 8.0 to 62%.
  • the ratio of the content of Ca to the entire sulfide is 1.0 to 45%.
  • the occupation area of the sulfide in the entire steel is 2.0 ⁇ 10 ⁇ 4 mm 2 to 1 ⁇ 10 ⁇ 1 mm 2 per 3.5 mm 2 , which can be confirmed by face analysis using an electron microscope EPMA (JXA8800 manufactured by Nippon Denshi Kabushiki Kaisha).
  • the sulfide contains MnS as its main component, and CaS is formed by substituting a part of Mn with Ca. It is necessary, however, to prevent the sulfide from having various properties in dependence on the extent of the substitution of Mn with Ca and prevent the formation of the tool protection coating from being difficult. To obtain desired machinability, it is preferable to adjust the content of the Ca to not more than 45%.
  • the occupation area of the sulfide in the steel is preferably not less than 2.0 ⁇ 10 ⁇ 4 mm 2 per3.5 mm 2 and more preferably not more than 1.0 ⁇ 10 ⁇ 1 mm 2 per 3.5 mm 2 to also obtain superior castability.
  • Carbon is a necessary component for securing the strength of the steel. To allow the steel to securely obtain a higher strength, not less than 0.1% of carbon is necessary. To avoid decrease of the toughness and machinability of the steel, it is preferable to set the content of carbon to at 0.5% or less.
  • Silicon is contained as a deoxidizing agent in a melting operation.
  • Si has an action of improving the hardenability.
  • it is necessary to add not less than 0.01% of Si to the steel.
  • it is preferable to add not more than 2.5% of Si to the steel.
  • Manganese is an element for forming the free-machining component of sulfide form control type. To form better free-machining component, it is preferable to add not less than 0.1% of Mn to the steel. To obtain superior machinability, it is preferable to add not more than 3.5% of Mn to the steel.
  • Phosphorous is added to the steel to improve the machinability, and particularly the property of the machined surface of the steel. To improve the machinability and the property of the machined surface of the steel to a higher extent, it is preferable to add not less than 0.001% of P to the steel. To allow the steel to have a higher ductility, it is preferable to add not more than 0.2% of P to the steel.
  • Sulfur is an element for forming the free-machining component of sulfide form control type. To form better free-machining components, it is preferable to add not less than 0.01% of S to the steel. To obtain superior machinability, it is preferable to add not more than 0.2% of S to the steel.
  • Chromium 1.0 to 3.5%
  • Chromium is an element effective for improving hardenability and securing surface hardness, depth, and softening-resistant property in nitriding treatment by an appropriate combination of the addition amount of Cr with that of V. To allow the steel to have the said better properties, it is preferable to add not less than 1.0% of Cr. To manufacture the steel at a low cost and prevent cracking of the steel in a hot working operation, it is more preferable that the addition amount of Cr is not more than 3.5%.
  • Vanadium 0.1 to 0.5%
  • Vanadium is an element effective for securing surface hardness, depth, and softening-resistant property in nitriding treatment by an appropriate combination of the addition amount of Cr with that of V.
  • the V combines with C and N to form a carbonitride and has the effect of making crystalline grains fine.
  • Aluminum is an element necessary for deoxidization. To obtain sufficient deoxidizing effect, addition of not less than 0.001% of Al is preferable. The addition of not more than 0.02% of Al is preferable to prevent generation of a hard alumina cluster which deteriorates the machinability of the steel, makes it difficult to form the free-machining component of sulfide form control type, and deteriorates the anisotropy of the steel.
  • Ca is an element very important for the steel of the present invention.
  • Ca is an element for forming the free-machining component of sulfide form control type.
  • Oxygen 0.0005 to 0.01%
  • Oxygen is an element necessary for forming an oxide in the free-machining component of sulfide form control type.
  • the addition of not less than 0.0005% of O is preferable.
  • the addition of not less than 0.0015% is preferable.
  • the addition of less than 0.01% of O is preferable.
  • one or not less than two elements selected from Mo, Cu, Ni, and B may be added to the steel for a rocker arm of the present invention.
  • one or two elements selected from Nb and Ti and one or more element(s) selected from Ta, Zr and Mg may be added to the steel.
  • These elements of alloy may be contained in the steel of the present invention in various combinations in dependence on purpose. The effect of these elements of alloy and the reason for controlling the content thereof will be described below.
  • molybdenum is an effective element for improving hardenability. It is preferable to add not more than 2.0% of Mo to allow the steel to be manufactured at a low cost, to be highly machinable, and improve hot workability.
  • Copper is an element effective for making the structure of the steel dense and increasing its strength. It is preferable to add not more than 2.0% of Co to allow the steel to be highly machinable and improve hot workability.
  • Ni is an effective element for improving the hardenability of the steel. It is preferable to add not more than 4.0% of Ni to allow the steel to be manufactured at a low cost and to be highly machinable.
  • the hardenability of the steel can be enhanced by the addition of a slight amount of B. To obtain better hardenability, it is preferable to add not less than 0.0005% of B to the steel. It is preferable to add not more than 0.01% of B to improve the hot workability and prevent crystalline grains from becoming coarse.
  • Niobium ⁇ 0.2%
  • Nb is an effective element for preventing crystalline grains from becoming coarse at high temperatures. It is preferable to add not more than 0.2% of Nb to manufacture the steel at a low cost and also obtain a higher effect.
  • Titanium ⁇ 0.2%
  • Ti combines with N in nitriding treatment to form TiN, thus allowing B to display hardenability-improving effect. It is preferable to add not more than 0.2% of Nb to form TiN favorably and improve the hot workability.
  • Tantalum ⁇ 0.5%
  • Ta is an element effective for making crystalline grains fine and improving the toughness of the steel. It is preferable to add not less than 0.5% of Ta to manufacture the steel at a low cost and also obtain higher effect.
  • Zr has properties similar to that of Ta, and is an effective element for making crystalline grains fine and improving the toughness of the steel. It is preferable to add not more than 0.5% of Zr to manufacture the steel at a low cost and also obtain higher effect.
  • the content of Cr and V are adjusted to satisfy the formula of ([Cr]+1.97 ⁇ [V]) ⁇ 2.15% (wt/wt).
  • the content of Cr and V are adjusted to satisfy the formula of ([Cr]+1.97 ⁇ [V]) ⁇ 2.15 wt %.
  • the surface hardness of not less than 750 HV is obtained in a gas nitrocarburizing condition (for example, 580° C. ⁇ 3 hr) and the nitrided layer depth and the softening-resistant property in the nitriding treatment can be preferably improved. That is, the steel is allowed to have a preferable nitriding property.
  • the content of Mn, S, and Cr are adjusted to satisfy the formula of ([Mn] ⁇ 55 ⁇ [S]/32+[Cr])>2.0 wt % or the content of Mn, S, Cr, and Mo are adjusted to satisfy the formula of ([Mn] ⁇ 55 ⁇ [S]/32+[Cr]+[Mo])>2.0 wt %.
  • the structure of the untempered ferrite+bainite which is formed after the hot-forged steel is cooled and which has a strength almost equal to that of the tempered steel.
  • the Ceq is adjusted within the range of 0.8 to 1.1 wt %.
  • the present inventors have found that to allow the rocker arm product to have superior machinability and preferable fatigue strength after nitriding treatment is performed in manufacturing the rocker arm product by hot forging from the steel for a rocker arm of the present invention, it is preferable to set the inner hardness of the steel within the range of 20 to 35 HRC, supposing that a test is conducted in accordance with JIS;Z2245.
  • the said inner hardness can be easily obtained by adjusting the content of C, Mn, S, Cr, and V in the steel for a rocker arm having above-mentioned basic components within the above range of Ceq.
  • examples A1 to A17 and comparison examples a1 to a8 steels each having an alloy composition shown in tables 1 and 2 were melted in a 5-ton arc furnace to cast them into ingots.
  • the content of elements S, Al, Ca, and O in each steel was adjusted to values shown in tables 1 and 2 using an FS shot, a CaSi shot, and an Al shot as a sulfur source, a calcium source, and an aluminum source respectively.
  • Each ingot was hot-forged to a round rod having a diameter of 15 mm or 22 mm.
  • Hot-forged steels were allowed to air-cooled and then gas-nitrided at 580° C. for three hours.
  • the surface hardness (HV) of each nitrided steel, the depth of hardened layer (depth of layer having hardness not less than 450HV) thereof, the hardness (HRC) of the core thereof, and the ductility thereof were measured.
  • FIGS. 3 and 4 show the results and machinability (indicated by drilling efficiency ratio mentioned above).
  • FIG. 2 shows the results.
  • a nitride product of the steel of the present invention achieved the target surface hardness of not less than 750 HV and secured 1 in the drilling efficiency ratio.
  • Some nitrided steels excellent in the machinability thereof showed more than 3 in the drilling efficiency ratio, achieving high machinability.
  • an SCr435HL steel (comparison steel; b1) shown in table 5 was adopted to examine the machinability of each steel
  • an SAC430AL steel (comparison steel; b2) shown in table 6 was adopted and to examine the nitriding property of each steel.
  • the machinability of the steel of the present invention was compared with the cutting resistance and cutting torque of the known SCr435HL steel containing 0.2% of Pb serving as a free-machining component in the same cutting condition.
  • the wear degree of a tool in a cutting operation was evaluated by comparing the observation of the wear state of the cutting face of the tool after the cutting operation finished.
  • the smaller the value of the cutting resistance and the cutting torque are in relation to the SCr435HL steel the better the machinability of the steel is.
  • the lower the wear degree of the tool is, the better the machinability of the steel is.
  • synthetic evaluation of the machinability of the steel of the present invention shown in Table 5 is judged based on the above-described determining method.
  • a commercially-available rotary cutting dynamometer 9123 C manufactured by Kisler Inc.
  • VF-Center manufactured by Enshu Seisakusho NC milling machine
  • cutting processing was carried out with a ⁇ 16.5 drill provided on a chucking portion of the said rotary cutting dynamometer when a ⁇ 16.5 opening was formed and with a ⁇ 18 reamer provided on the chucking portion thereof to perform finish machining of a ⁇ 18 opening.
  • a voltage output value obtained from the rotary cutting dynamometer was measured in each cutting operation.
  • rotational frequency of the spindle 2500 rpm, feeding amount: 150 m/min, and cutting amount: 0.080 mm per rotation were employed; and in the finish machining of the ⁇ 18 reamer opening, rotational frequency of the spindle: 2000 rpm, feeding amount: 400 m/min, and cutting amount: 0.200 mm per rotation were employed. Both operations were performed bywet process (non-aqueous cutting oil; Cutting Lubricant X-5 produced by General Sekiyu).
  • FIGS. 3 and 4 show the machinability of the steel (B3) of the present invention and the comparison steel (b1) shown in table 5.
  • FIG. 3 shows the transition of the cutting resistance and the cutting torque in relation to the number of processes when the an opening was formed with the ⁇ 16.5 drill.
  • FIG. 4 shows the transition of the cutting resistance and the cutting torque in relation to the number of processes when the finish machining of an opening was performed with the ⁇ 18 reamer.
  • Photographs, shown in FIG. 5 taken with a stereoscopic microscope show the worn state of the cutting face of the tool when the ⁇ 16.5 drill was used to process 700 workpieces.
  • Table 5 and FIGS. 3 through 5 indicate that the steel of the present invention has equivalent or higher machinability than the known Pb free-machining steel. That is, the steel of the present invention has favorable machinability.
  • a graph of FIG. 6 shows the nitrided layer depth (transition of sectional hardness) after nitriding treatment of the steels of the present invention (B6 to B9) and the comparison steel (b2) shown in table 6 was made in the above-described gas nitrocarburizing condition.
  • the steel of the present invention is almost equal to the comparison steel concerning the sectional hardness in the neighborhood of the surface thereof. Further the nitrided layer depth of the former is larger than that of the latter in every case, and the decrease of the sectional hardness is suppressed inward in the steel of the present invention, meaning that the softening-resistant property is secured. Therefore the steel of the present invention has a favorable nitriding property.
  • FIG. 7 shows the form of the free-machining component of sulfide form control type contained in the steel of the present invention.
  • FIG. 8 shows the result of componential analysis and examination of the free-machining component shown in FIG. 7 .
  • the photograph of FIG. 7 shows the result of observation made by an electron microscope EPMA (JXA8800 manufactured by Nippon Denshi Kabushiki Kaisha).
  • the photograph of FIG. 8 shows the result of a face analysis (mapping) made by the EPMA.
  • the generation of the hard oxide which improves the grindability of the matrix itself and the generation of the sulfide which forms a tool protection coating of the sulfide on the surface of the tool and thereby improves the life of the tool are controlled to have the spherical shape. Further the free-machining component of sulfide form control type is dispersed uniformly in the matrix. Accordingly, the steel of the present invention can have superior machinability.
  • FIG. 9 shows the structure of the untempered steel of the present invention allowed to cool after it is hot-forged at 1200° C.
  • the untempered steel of the present invention can be manufactured with high productivity and can have strength equivalent to that of the tempered steel.
  • Table 7 and FIG. 10 indicate that the internal hardness of the steel of the present invention manufactured by hot forging and cooling is controlled in the range of 20 to 35 HRC.
  • the softening-resistant property after nitriding treatment is secured in the steel of the present invention, it is possible to restrain nitriding-caused reduction of the internal hardness and impart good fatigue strength to the steel after nitriding treatment is performed. Thereby it is possible to produce rocker arm products superior in the strength property.
  • the inclusion providing high machinability, specifically the free-machining component of sulfide form control type is present in the most suitable form in the to-be-nitrided bainite-type untempered steel of the present invention. Therefore it becomes possible to realize the superior machinability.
  • the present invention has overcome the problem of the conventional art in this regard, and is capable of manufacturing the bainite-type untempered free-machining steel to-be-nitrided which constantly has excellent machinability by producing in such a way satisfying the above-described operating conditions.
  • the distinctive advantage of the to-be-nitrided bainite-type untempered steel of the present invention is that, without adding Pb which has been indispensable for improving the machinability of this type of nitrided steel in conventional methods, the steel is provided with machinability equal to or higher than that of the conventional steel containing Pb. It is well known that the use of Pb tend to cause problems in environment where steel is manufactured or processed, and using and discarding of Pb are not preferable, therefore nowadays efforts are made to the utmost to avoid the use of Pb.
  • the steel for the rocker arm of the present invention has machinability equivalent to that of the known Pb free-machining steel and is superior in nitriding property such as a nitrided layer depth. Further, a strength equivalent to that of the known tempered steel can be obtained even it is not tempered. Therefore it is possible to manufacture rocker arm products from various kinds of steel at a low cost and with high productivity.

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DE102011088234A1 (de) * 2011-12-12 2013-06-13 Aktiebolaget Skf Bauteil
RU2535148C2 (ru) * 2013-01-09 2014-12-10 Открытое акционерное общество "Машиностроительный концерн ОРМЕТО-ЮУМЗ" Инструментальная сталь для горячего деформирования
DE102017117483A1 (de) * 2017-08-02 2019-02-07 Schaeffler Technologies AG & Co. KG Verfahren zur Herstellung einer Wälzlagerkomponente aus Stahl
CN113075211B (zh) * 2021-03-29 2023-10-10 安徽工业大学 一种用于钢中氧化夹杂物在高温下演化过程的检测方法

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