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US20070163687A1 - Component for machine structural use and method for making the same - Google Patents

Component for machine structural use and method for making the same Download PDF

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Publication number
US20070163687A1
US20070163687A1 US11/587,700 US58770006A US2007163687A1 US 20070163687 A1 US20070163687 A1 US 20070163687A1 US 58770006 A US58770006 A US 58770006A US 2007163687 A1 US2007163687 A1 US 2007163687A1
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component
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US11/587,700
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Inventor
Nobutaka Kurosawa
Yasuhiro Omori
Tohru Hayashi
Aklhiro Matsuzaki
Takaaki Toyooka
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: MATSUZAKI, AKIHIRO, HAYASHI, TOHRU, OMORI, YASUHIRO, KUROSAWA, NOBUTAKA, TOYOOKA, TAKAAKI
Publication of US20070163687A1 publication Critical patent/US20070163687A1/en
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    • 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
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/28—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for plain shafts
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/30—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for crankshafts; for camshafts
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
    • F16C19/00—Bearings with rolling contact, for exclusively rotary movement
    • F16C19/02—Bearings with rolling contact, for exclusively rotary movement with bearing balls essentially of the same size in one or more circular rows
    • F16C19/14—Bearings with rolling contact, for exclusively rotary movement with bearing balls essentially of the same size in one or more circular rows for both radial and axial load
    • F16C19/18—Bearings with rolling contact, for exclusively rotary movement with bearing balls essentially of the same size in one or more circular rows for both radial and axial load with two or more rows of balls
    • F16C19/181—Bearings with rolling contact, for exclusively rotary movement with bearing balls essentially of the same size in one or more circular rows for both radial and axial load with two or more rows of balls with angular contact
    • F16C19/183—Bearings with rolling contact, for exclusively rotary movement with bearing balls essentially of the same size in one or more circular rows for both radial and axial load with two or more rows of balls with angular contact with two rows at opposite angles
    • F16C19/184—Bearings with rolling contact, for exclusively rotary movement with bearing balls essentially of the same size in one or more circular rows for both radial and axial load with two or more rows of balls with angular contact with two rows at opposite angles in O-arrangement
    • F16C19/186—Bearings with rolling contact, for exclusively rotary movement with bearing balls essentially of the same size in one or more circular rows for both radial and axial load with two or more rows of balls with angular contact with two rows at opposite angles in O-arrangement with three raceways provided integrally on parts other than race rings, e.g. third generation hubs
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
    • F16C3/00—Shafts; Axles; Cranks; Eccentrics
    • F16C3/04—Crankshafts, eccentric-shafts; Cranks, eccentrics
    • F16C3/06—Crankshafts
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
    • F16C33/00—Parts of bearings; Special methods for making bearings or parts thereof
    • F16C33/30—Parts of ball or roller bearings
    • F16C33/58—Raceways; Race rings
    • F16C33/62—Selection of substances
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16D—COUPLINGS FOR TRANSMITTING ROTATION; CLUTCHES; BRAKES
    • F16D3/00—Yielding couplings, i.e. with means permitting movement between the connected parts during the drive
    • F16D3/16—Universal joints in which flexibility is produced by means of pivots or sliding or rolling connecting parts
    • F16D3/20—Universal joints in which flexibility is produced by means of pivots or sliding or rolling connecting parts one coupling part entering a sleeve of the other coupling part and connected thereto by sliding or rolling members
    • F16D3/22—Universal joints in which flexibility is produced by means of pivots or sliding or rolling connecting parts one coupling part entering a sleeve of the other coupling part and connected thereto by sliding or rolling members the rolling members being balls, rollers, or the like, guided in grooves or sockets in both coupling parts
    • F16D3/223—Universal joints in which flexibility is produced by means of pivots or sliding or rolling connecting parts one coupling part entering a sleeve of the other coupling part and connected thereto by sliding or rolling members the rolling members being balls, rollers, or the like, guided in grooves or sockets in both coupling parts the rolling members being guided in grooves in both coupling parts
    • 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
    • C21D2211/001—Austenite
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
    • F16C2326/00—Articles relating to transporting
    • F16C2326/01—Parts of vehicles in general
    • F16C2326/02—Wheel hubs or castors
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16D—COUPLINGS FOR TRANSMITTING ROTATION; CLUTCHES; BRAKES
    • F16D3/00—Yielding couplings, i.e. with means permitting movement between the connected parts during the drive
    • F16D3/16—Universal joints in which flexibility is produced by means of pivots or sliding or rolling connecting parts
    • F16D3/20—Universal joints in which flexibility is produced by means of pivots or sliding or rolling connecting parts one coupling part entering a sleeve of the other coupling part and connected thereto by sliding or rolling members
    • F16D3/22—Universal joints in which flexibility is produced by means of pivots or sliding or rolling connecting parts one coupling part entering a sleeve of the other coupling part and connected thereto by sliding or rolling members the rolling members being balls, rollers, or the like, guided in grooves or sockets in both coupling parts
    • F16D3/223—Universal joints in which flexibility is produced by means of pivots or sliding or rolling connecting parts one coupling part entering a sleeve of the other coupling part and connected thereto by sliding or rolling members the rolling members being balls, rollers, or the like, guided in grooves or sockets in both coupling parts the rolling members being guided in grooves in both coupling parts
    • F16D2003/22326—Attachments to the outer joint member, i.e. attachments to the exterior of the outer joint member or to the shaft of the outer joint member
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16D—COUPLINGS FOR TRANSMITTING ROTATION; CLUTCHES; BRAKES
    • F16D2250/00—Manufacturing; Assembly
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16D—COUPLINGS FOR TRANSMITTING ROTATION; CLUTCHES; BRAKES
    • F16D2300/00—Special features for couplings or clutches
    • F16D2300/10—Surface characteristics; Details related to material surfaces
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16H—GEARING
    • F16H55/00—Elements with teeth or friction surfaces for conveying motion; Worms, pulleys or sheaves for gearing mechanisms
    • F16H55/02—Toothed members; Worms
    • F16H55/06—Use of materials; Use of treatments of toothed members or worms to affect their intrinsic material properties

Definitions

  • the present invention relates to components for machine structural use, components having hardened layers formed by high-frequency hardening in at least part of the components.
  • Examples of the components for machine structural use described herein include drive shafts, input shafts, output shafts, crankshafts, inner and outer races of constant velocity joints, hubs, and gears for automotive vehicles.
  • one possible approach for increasing the torsional fatigue strength is to increase the depth of hardening by the high-frequency hardening. Although the depth of hardening may be increased, the fatigue strength is saturated at a certain depth.
  • Patent Document 1 reduces the former austenitic grain diameter by dispersing a large amount of fine TiC during heating for high-frequency hardening.
  • this technology requires solid solution treatment of TiC prior to the hardening and includes a hot-rolling step of heating the workpiece to a temperature of not less than 1,100° C. Thus, the heating temperature needs to be high during the hot rolling, and the production efficiency is disadvantageously low.
  • Patent Document 1 cannot sufficiently comply with the recent requirements on the fatigue strengths.
  • Patent Document 1 Japanese Unexamined Patent Application Publication No. 2000-154819 (Claims)
  • Patent Document 2 Japanese Unexamined Patent Application Publication No. 8-53714 (Claims)
  • the present invention has been made based on the above-described situations and aims to provide a component for machine structural use with a higher fatigue strength and an advantageous method for manufacturing the component.
  • the inventors have vigorously pursued studies particularly on high-frequency hardened structures to effectively improve the above-described fatigue properties.
  • the inventors have focused attention on the grain diameter distribution of the former austenitic grains of a high-frequency hardened structure and found that the fatigue properties, such as torsional fatigue strength, bending fatigue strength, and rolling fatigue strength, can be improved by reducing the average grain diameter and the maximum grain diameter of the former austenitic grains.
  • a component for machine structural use including a steel material at least part of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 ⁇ m or less and a maximum grain diameter not exceeding four times the average grain diameter.
  • composition for machine structural use according to item 2, the composition further containing, in terms of percent by mass:
  • composition further containing, in terms of percent by mass, at least one selected from among:
  • composition containing, in terms of percent by mass, at least one selected from among:
  • composition further containing, in terms of percent by mass, at least one selected from among:
  • a method for making a component for machine structural use including subjecting at least part of a steel material to high-frequency heating at least once, the steel material containing one or both of a fine bainite structure and a fine martensite structure in a total of 10 percent by volume, wherein the high-frequency heating is conducted at a heating rate of at least 400° C./s and an ultimate temperature of 1,000° C. or less.
  • the steel material further including:
  • the steel material further including, in terms of percent by mass, at least one selected from among:
  • the steel material further including, in terms of percent by mass, at least one selected from among:
  • the steel material further including, in terms of percent by mass, at least one selected from among:
  • FIG. 1 is a front view of a representative example of a shaft.
  • FIG. 3 is an optical microscope image of a hardened structure.
  • FIG. 4 ( a ) and FIG. 4 ( b ) are each a graph showing the relationship between the average former austenitic grain diameter and the torsional fatigue strength.
  • FIG. 6 ( a ) and FIG. 6 ( b ) are each a graph showing the effect of the processing ratio at temperatures lower than 800° C. and the conditions of the high-frequency hardening on the torsional fatigue strength.
  • FIG. 7 ( a ) and FIG. 7 ( b ) are each a graph showing the effect of the former austenitic grain diameter and the maximum former austenitic grain diameter/average former austenitic grain diameter of the hardened layer on the rolling fatigue strength.
  • FIG. 9 is a diagram showing the position of the crankshaft subjected to the high-frequency hardening.
  • FIG. 10 is a diagram showing the scheme of the endurance test.
  • FIG. 15 is a cross-sectional view of a hardened structure in a constant velocity joint inner race.
  • FIG. 16 is another cross-sectional view of a hardened structure in a constant velocity joint inner race.
  • FIG. 17 is a diagram showing a hub and a hub bearing unit.
  • FIG. 18 is another diagram showing a hub and a hub bearing unit.
  • FIG. 19 is a diagram showing the scheme of sliding rolling contact fatigue test.
  • FIG. 20 is a perspective view of a gear.
  • FIG. 21 is a cross-sectional view showing a hardened surface layer of tooth and bottoms of the gear.
  • a component for machine structural use according to the present invention may take various shapes and structures, such as a drive shaft, an input shaft, an output shaft, a crankshaft, an inner or outer race of a constant velocity joint, a hub, and a gear for automobiles.
  • a steel material (150 kg) having a composition indicated as steel a or steel b below was melted in a vacuum melting furnace- and hot-forged into a 150 mm square bar, from which dummy billets were made. Then, rolled steel bar materials were manufactured under various hot-working conditions.
  • Step a C: 0.48 mass %, Si: 0.55 mass %, Mn: 0.78 mass %, P: 0.011 mass %, S: 0.019 mass %, Al: 0.024 mass %, N: 0.0043 mass %, balance: Fe and inevitable impurities
  • Step b C: 0.48 mass %, Si: 0.51 mass %, Mn: 0.79 mass %, P: 0.011 mass %, S: 0.021 mass %, Al: 0.024 mass %, N: 0.0039 mass %, Mo: 0.45 mass %, Ti: 0.021 mass %, B: 0.0024 mass %, balance: Fe and inevitable impurities
  • the steel bar was cut to a predetermined length and subjected to surface cutting and partly to cold-drawing to adjust the diameter. At the same time, the steel bar was rolled to form splines.
  • a shaft 1 with splines 2 having a dimension and the shape shown in FIG. 1 was manufactured as a result.
  • the shaft was heated and hardened under various conditions in a high-frequency hardening device (frequency: 10 to 200 kHz), tempered in a heating furnace at 170° C. for 30 minutes, and then analyzed to determine the torsional fatigue strength.
  • a high-frequency hardening device frequency: 10 to 200 kHz
  • the torsional fatigue strength was evaluated in terms of torque (N ⁇ m) when the number of cycles to fracture was 1 ⁇ 10 5 in the shaft torsional fatigue test.
  • a hydraulic fatigue tester was used, and as shown in FIG. 2 ( a ), splines 2 a and 2 b were respectively fit into disk-shaped chucks 3 a and 3 b and torsional torque was repeatedly applied between the chucks 3 a and 3 b at a frequency of 1 to 2 Hz.
  • the structure of the hardened layer of the same shaft was observed with an optical microscope to determine the average former austenitic grain diameter and the maximum former austenitic grain diameter.
  • the average former austenitic grain diameter was determined as follows. A specimen was observed with an optical microscope at three positions, namely, at 1 ⁇ 5, 1 ⁇ 2, and 4 ⁇ 5 of the thickness of the hardened layer from the surface, and from five field of views for each position under a magnification of ⁇ 400 (the area of one field of view: 0.25 mm ⁇ 0.225 mm) to ⁇ 1,000 (the area of one field of view: 0.10 mm ⁇ 0.09 mm). The average former austenitic grain diameter was measured at each position, and the highest average was defined as the average former austenitic grain diameter. The thickness of the hardened layer was assumed to be the depth of the region from the surface at which the area ratio of the martensitic structure decreased to 98%.
  • the cross-section taken in the thickness direction of the hardened layer was reacted with a corrosive liquid containing an aqueous picrinic acid solution (50 g of picrinic acid dissolved in 500 g of water), 11 g of sodium dodecylbenzenesulfonate, 1 g of ferrous chloride, and 1.5 g of oxalic acid so as to expose the former austenitic grain boundaries as shown in FIG. 3 .
  • FIG. 4 shows the relationship between the average former austenitic grain diameter and the torsional fatigue strength.
  • the fatigue strength increased with the decrease in average grain diameter.
  • the fatigue strength differed although the grain diameter was substantially the same. It was found that the cause of this is the grain diameter distribution, in particular, the maximum grain diameter. Further investigations were pursued on this issue and it was found that at a maximum grain diameter of not exceeding four times the average grain diameter, the fatigue strength improves significantly by reducing the average grain diameter.
  • Plotted points in FIG. 4 ( a ) were re-plotted in FIG. 4 ( b ) as open squares and rhombuses for a maximum grain diameter/average grain diameter of 4 or less and solid squares and rhombuses for a ratio grain diameter/average grain diameter exceeding 4.
  • the impurity elements that cause fatigue fracture tend to segregate at the former austenitic boundaries.
  • the area in which segregation occurs increases. This decreases the impurity concentration in the individual segregation sites and thereby increases the fatigue strength.
  • stress concentration to the former austenitic grain boundaries due to cutout or the like is dispersed when the grain diameter is small, thereby decreasing the stress applied onto the individual grain boundaries and increasing the fatigue strength.
  • Such effects are affected not only by the average grain diameter but also by the maximum grain diameter. In other words, in the vicinity of a large grain, the area of the grain boundary is small and the impurity concentration easily increases. Moreover, dispersion of stresses does not readily occur.
  • the fatigue strength is expected to improve significantly and stably for a wide range of component shape. More preferably, the average grain diameter is 5 ⁇ m or less. Most preferably the average grain diameter is 4 ⁇ m or less.
  • FIG. 5 ( a ) and FIG. 5 ( b ) are each a graph showing the effect of the average former austenitic grain diameter and the maximum former austenitic grain diameter/average former austenitic grain diameter in the hardened layer on the torsional fatigue strength.
  • the fatigue strength can be notably increased by adjusting the maximum former austenitic grain diameter/average former austenitic grain diameter to 4 or less.
  • the fatigue strength can be further significantly increased by adjusting the maximum former austenitic grain diameter/average former austenitic grain diameter to 4 or less.
  • FIG. 6 ( a ) and FIG. 6 ( b ) show the effect of processing ratio at temperatures less than 800° C., the maximum ultimate temperature (heating temperature) during high-frequency heating, and the heating rate on the torsional fatigue strength.
  • FIG. 6 ( a ) and FIG. 6 ( b ) show that excellent fatigue properties can be obtained at a processing ratio in the temperature zone less than 800° C. of 25% or more, a maximum ultimate temperature during the high-frequency hardening of 1,000° C. or less, and a heating rate of 400° C./s or more.
  • a steel material (150 kg) having a composition indicated as steel a or steel b above was melted in a vacuum melting furnace and hot-forged into a 150 mm square bar, from which dummy billets were manufactured.
  • the billet was hot-worked and cold-drawn under various conditions and cut to make a steel bar 12 mm in diameter.
  • the surface of the steel bar was subjected to high-frequency hardening under various conditions, and the resulting steel bar was cut to a predetermined length to make a specimen for rolling fatigue test. Radial-type rolling fatigue test shown in FIG. 2 ( b ) was conducted using this specimen.
  • FIG. 7 ( a ) and FIG. 7 ( b ) show the results of the test.
  • the fatigue strength can be notably increased by adjusting the maximum former austenitic grain diameter/average former austenitic grain diameter to 4 or less at an average former austenitic grain diameter of 12 ⁇ m or less.
  • the fatigue strength can be further significantly increased by adjusting the maximum former austenitic grain diameter/average former austenitic grain diameter to 4 or less.
  • Hot-working conditions Total Working HF hardening conditions processing Cooling (cold-working) Pre-hardening Retention ratio (%) at rate (° C./s) ratio (%) at structure Heating time (sec) at Steel 800° C. to after less than Proportion (vol. %) of Heating temperature 800° C. or No. type 1000° C. working 800° C.
  • Hot-working conditions HF hardening conditions Total Working Retention processing (cold- Pre-hardening time ratio (%) at Cooling rate working) ratio structure (sec) at Steel 800° C. to (° C./s) after (%) at less Proportion (vol. %) of Heating rate Heating 800° C. or No. type 1000° C. working than 800° C.
  • bainite structure (° C./s) temperature(° C.) more 35 b 80 0.8 50 82 800 1000 1 36 b 80 0.8 25 82 800 1020 1 37 b 80 0.8 25 82 400 1030 1 38 b 80 0.8 25 82 100 950 1 39 b 80 0.8 0 82 400 1030 1 40 b 80 0.8 25 82 800 1050 1 41 b 80 0.8 10 82 800 1050 1 42 b 80 0.8 0 82 800 1050 1 43 b 80 0.8 0 82 100 1050 1 44 b 80 0.8 50 82 800 950 1 45 b 80 0.8 25 82 800 950 1 46 b 80 0.8 25 82 400 950 1 Hardened layer structure Rolling Average former Maximum former fatigue (ratio austenitic grain austenitic grain based on No.
  • the structure before the high-frequency hardening preferably contains 10 vol % or more and more preferably 25 vol % or more of a bainite structure and/or a martensite structure. Since the bainite structure and the martensite structure are each a structure containing finely dispersed carbides, inclusion of large amounts of bainite structure or martensite structure in the pre-hardening structure will increase the area of the ferrite/carbide boundaries, which are nucleation sites for austenite, during hardening by heating and finer austenite is generated as a result. Thus, this approach effectively contributes to reducing the former austenitic grain diameter of the hardened layer. Since the austenitic grain diameter decreases during the hardening by heating, the grain boundary strength increases and the fatigue strength improves.
  • a steel having the composition described below is preferably hot-worked such that the total processing ratio at temperatures of 800° C. to 1,000° C. is 80% or more, followed by cooling at a cooling rate of 0.2° C./s or more for the temperature range of 700° C. to 500° C.
  • the total proportion of the bainite structure and/or the martensite structure cannot be adjusted to 10 vol % or more unless the cooling rate in the temperature range of 700° C. to 500° C. is 0.2° C./s or more after the hot-working.
  • the working in the temperature range less than 800° C. may be effected in the hot-working step before the cooling at the above-described cooling rate, i.e., may be effected in the temperature range of 700° C. to less than 800° C.
  • a separate cold-working may be effected after the cooling or a hot-working may be effected by heating the workpiece at the A 1 transformation temperature or less.
  • the processing ratio at temperatures lower than 800° C. is more preferably 30% or more.
  • Examples of the working method include cold forging, cold ironing, forming rolling, and shotblasting.
  • Carbon is the element having the greatest influence on the hardenability. Carbon effectively contributes to increasing the hardness and the depth of the hardened layer and to improving the fatigue strength. However, at a C content less than 0.3 mass %, the depth of the hardened layer must be significantly increased to securely achieve the required fatigue strength, and this leads to occurrence of significant quenching cracks and difficulties in generating the bainite structure. Thus, 0.3 mass % or more of carbon is contained. At a C content exceeding 1.5 mass %, the grain boundary strength decreases, the fatigue strength decreases, and the machinability, cold-forgeability, and resistance to quenching crack decrease. Therefore, the C content is limited to the range of 0.3 to 1.5 mass %, preferably 0.4 to 0.6 mass %.
  • Silicon not only serves as a deoxidizer but also contributes to effectively improving the strength. At a Si content exceeding 3.0 mass %, the machinability and the forgeability decrease. Thus, the Si content is preferably 3.0 mass % or less.
  • Manganese is added for its effects of improving the hardenability and securing the depth of the hardened layer during the hardening. At a Mn content less than 0.2 mass %, the effect of addition is not sufficient.
  • the Mn content is preferably 0.2 mass % or more, and more preferably 0.3 mass % or more.
  • the Mn content is preferably 2.0 mass % or less.
  • the Mn content is preferably 1.2 mass % or less and more preferably 1.0 mass % or less.
  • Aluminum is effective for deoxidization. It is also effective for reducing the grain diameter of the hardened layer since it restrains the growth of the austenitic grains during the heating for hardening. At an Al content exceeding 0.25 mass %, however, the effect is saturated and the cost for the composition is disadvantageously increased.
  • the Al content is preferably 0.25 mass % or less, and more preferably 0.001 to 0.10 mass %.
  • the above-described four components are the main components.
  • the main components must satisfy formula (1) below: C 1/2 (1+0.7Si)(1+3Mn)>2.0 (1)
  • Chromium is effective for improving the hardenability and for ensuring the hardening depth.
  • An excessive amount of Cr stabilizes the carbides and promotes generation of the residual carbides, thereby decreasing the grain boundary strength and the fatigue strength.
  • the Cr content is preferably as low as possible, but can be up to 2.5 mass %.
  • the Cr content is 1.5 mass % or less.
  • the Cr content is preferably 0.03 mass % or more.
  • Molybdenum is effective for inhibiting the growth of the austenitic grains.
  • the Mo content is preferably 0.05 mass % or more. At a Mo content exceeding 1.0 mass %, the machinability is degraded.
  • the Mo content is preferably 1.0 mass % or less.
  • Copper is effective for improving the hardenability. Copper forms solid solution in the ferrite, and this solid-solution hardening increases the fatigue strength. Copper also inhibits generation carbides, prevents a decrease in grain boundary strength caused by the carbide, and improves the fatigue strength. However, at a Cu content exceeding 1.0 mass %, cracks occur during hot-working. Thus, the Cu content is preferably 1.0 mass % or less, and more preferably 0.5 mass % or less. At a Cu content less than 0.03 mass %, the effect of improving the hardenability and the effect of preventing the decrease in grain boundary strength are small. Thus, the Cu content is preferably 0.03 mass % or more.
  • Nickel improves the hardenability and is used to adjust the hardenability. It also inhibits generation of carbides, prevents a decrease in grain boundary strength caused by carbides, and increases the fatigue strength.
  • Ni is very expensive and increases the cost of steel material if added in an amount exceeding 2.5 mass %.
  • the Ni content is preferably 2.5 mass % or less. At an Ni content less than 0.05 mass %, the effect of improving the hardenability and inhibiting the decrease in brain boundary strength are small.
  • the Ni content is preferably 0.05 mass % or more, and more preferably 0.1 to 1.0 mass %.
  • Cobalt inhibits generation of carbides, inhibits a decrease in grain boundary strength caused by carbides, and increases the fatigue strength.
  • Co is a very expensive element, and incorporation of 1.0 mass % or more of Co will increase the cost of the steel material.
  • the Co content is 1.0 mass % or less. At a Co content less than 0.01 mass %, the effect of inhibiting the decrease in grain boundary strength is small.
  • the Co content is preferably 0.01 mass % or more and more preferably 0.02 to 0.5 mass %.
  • Tungsten is useful for inhibiting the growth of austenitic grains.
  • the W content is preferably 0.005 mass % or more. At a W content exceeding 1.0 mass %, the machinability will be degraded. Thus, the W content is preferably 1.0 mass % or less.
  • At least one selected from Ti: 0.1 mass % or less, Nb: 0.1 mass % or less, Zr: 0.1 mass % or less, B: 0.01 mass % or less, Ta: 0.5 mass % or less, Hf: 0.5 mass % or less, and Sb: 0.015 mass % or less may be further contained.
  • the Ti content is preferably 0.005 mass % or more. At a Ti content exceeding 0.1 mass %, large amounts of TiN are generated, and this starts fatigue fracture and thus causes a significant decrease in fatigue strength.
  • the Ti content is preferably 0.1 mass % or less and more preferably in the range of 0.01 to 0.07 mass %.
  • Niobium not only has hardenability enhancing effect but also serves as a deposition strengthening element by bonding with C and N in the steel. It is also an element that increases the temper softening resistance. These effects improve the fatigue strength. However, at a Nb content exceeding 0.1 mass %, these effects are saturated. Thus, the Nb content is preferably 0.1 mass % or less. At a Nb content less than 0.005 mass %, the effect of deposition strengthening and the effect of enhancing the temper softening resistance are small; thus, the Nb content is preferably 0.005 mass % or more, and more preferably 0.01 to 0.05 mass %.
  • Zirconium not only has a hardenability improvement effect but also serves as a deposition strengthening element by bonding with C and N in the steel. It is also an element that increases the temper softening resistance. These effects improve the fatigue strength. However, at a Zr content exceeding 0.1 mass %, these effects are saturated. Thus, the Zr content is preferably 0.1 mass % or less. At a Zr content less than 0.005 mass %, the effect of deposition strengthening and the effect of enhancing the temper softening resistance are small; thus, the Zr content is preferably 0.005 mass % or more, and more preferably 0.01 to 0.05 mass %.
  • the B content is preferably 0.0003 mass % or more. Since the effect is saturated at a content exceeding 0.01 mass %, the B content is limited to 0.01 mass % or less.
  • Tantalum may be contained since it is effective against delay of microstructural changes and prevents degradation of fatigue strength, in particular, rolling fatigue strength.
  • Ta does not contribute to further strength improvements even when the Ta content is increased over 0.5 mass %.
  • the Ta content is set to 0.5 mass % or less.
  • the Ta content is preferably 0.02 mass % or more.
  • Hafnium may be contained since it is effective against delay of microstructural changes and prevents degradation of fatigue strength, in particular, rolling fatigue strength.
  • Hf does not contribute to further strength improvements even when the Hf content is increased over 0.5 mass %.
  • the Hf content is 0.5 mass % or less.
  • the Hf content is preferably 0.02 mass % or more.
  • Antimony may be contained since it is effective against delay of microstructural changes and prevents degradation of fatigue strength, in particular, rolling fatigue strength.
  • the toughness is decreased.
  • the Sb content is thus 0.015 mass % or less and preferably 0.010 mass % or less.
  • the Sb content is preferably 0.005 mass % or more to allow Sb to exhibit an effect of improving the fatigue strength.
  • Si 0.1 mass % or less
  • Pb 0.1 mass % or less
  • Bi 0.1 mass % or less
  • Se 0.1 mass % or less
  • Te 0.1 mass % or less
  • Ca 0.01 mass % or less
  • Mg 0.01 mass % or less
  • REM 0.1 mass % or less
  • Sulfur is a useful element that forms MnS in the steel and improves the machinability. At a S content exceeding 0.1 mass %, however, S segregates in the grain boundaries and thereby decreases the grain boundary strength. Thus, the S content is limited to 0.1 mass %or less and preferably 0.04 mass % or less.
  • both Pb and Bi improve machinability since they melt during machining to provide lubricating and embrittlement effects. Thus they may be added for these effects. However, at a Pb content exceeding 0.1 mass % and a Bi content exceeding 0.1 mass %, the effects are saturated and the cost of the components increases. Thus, the Pb and Bi contents are set to be within the above-described ranges.
  • the Pb content is preferably 0.01 mass % or more and the Bi content is preferably 0.01 mass % or more.
  • Se and Te each bond with Mn to form MnSe and MnTe, which function as chip-breakers and thereby improve the machinability.
  • the content is set to be 0.1 mass % or less for both the elements.
  • the Se content is preferably 0.003 mass % or more and the Te content is preferably 0.003 mass % or more.
  • Ca and REM each bond with MnS to form sulfides, which function as chip-breakers and thereby improve the machinability.
  • the Ca content is preferably 0.0001 mass % or more and the REM content is preferably 0.0001 mass % or more.
  • Magnesium is not only an oxidizer but also functions as the stress concentration source thereby improving the machinability.
  • Mg may added as necessary.
  • the Mg content is set to be 0.01 mass % or less.
  • the Mg content is preferably 0.0001 mass % or more.
  • a steel material having a particular composition described above is subjected to bar-mill rolling and then to hot-working such as hot-forging to form a component-shaped product.
  • the component is at least partially subjected to high-frequency hardening at a heating temperature of 800° C. to 1,000° C. This portion of the component subjected to the hardening is set to be the site where fatigue strength is required.
  • a hardened structure having an average former austenitic grain diameter of 12 ⁇ m or less and a maximum grain diameter not more than four times the average diameter can be obtained by conducting the hot-working such that the total processing ratio in the temperature range of 800° C. to 1,000° C. is at least 80%, cooling at a rate of at least 0.2° C./s in the temperature range of 700° C. to 500° C., and then effecting at least 20% or processing in the temperature range of below 800° C. or by conducting the hot-working such that the total processing ratio in the temperature range of 800° C. to 1,000° C.
  • the total processing ratio in the range of 800° C. to 1,000° C. during the hot-working is set to be at least 80%, and the cooling rate in the temperature range of 700° C. to 500° C. is set to be at least 0.2° C./s.
  • the pre-hardening structure can be obtained as homogeneous, fine bainite and/or martensitic structure, and the austenitic grains become finer during the subsequent heating for high-frequency hardening.
  • the cooling rate is at least 0.5° C./s.
  • the working in the temperature range of 800° C. or lower may be conducted in the hot-working step and before the cooling at the above-described cooling ratio (the temperature range from 700° C. to less than 800° C.); alternatively, a separate cold working may be effected after the cooling or a hot-working may be effected by re-heating the workpiece at the A 1 transformation temperature or less.
  • the processing ratio at a temperature below 800° C. is preferably at least 30%. Examples of the working methods include cold forging, cold ironing, forming rolling, and shotblasting. By working at a temperature of 800° C.
  • the bainite or martensitic structure before the high-frequency hardening becomes finer; therefore, the average former austenitic grain diameter in the hardened layer obtained by the high-frequency hardening will be 12 ⁇ m or less and the maximum grain diameter does not exceed four times the average grain diameter. As a result, the fatigue strength is improved.
  • processing ratio used herein is defined as the reduction ratio of the cross-sectional area before and after working for the cases concerning rolling, forging, and drawing.
  • the processing ratio is determined by the change in hardness corresponding to the reduction ratio.
  • the heating temperature is set to be 800° C. to 1,000° C., and the range 600° C. to 800° C. is heated at a rate of 400° C./s or more. If the heating temperature is less than 800° C., generation of the austenitic structure is insufficient, and a hardened layer cannot be obtained. On the other hand, when the heating temperature exceeds 1,000° C., the rate of austenitic grain growth increases substantially. This increases the average grain diameter and, at the same time, the individual grains frequently grow at rates significantly different from one another in the temperature region where rapid growth occurs. As a result, the maximum grain diameter exceeds four times the average grain diameter, thereby inflicting a decrease in fatigue strength.
  • the heating ratio in the range of 600° C. to 800° C. is less than 400° C./s, the austenitic grain growth is promoted and the size of the individual grains becomes nonuniform.
  • the maximum grain diameter exceeds four times the average grain diameter, which results in a decreased fatigue strength. This is presumably due to the fact that when the heating rate is low, reverse transformation from ferrite to austenite starts at a lower temperature and nonuniform grain growth tends to occur depending on the position of the grains.
  • shafts simulating a drive shaft, an output shaft, and an input shaft of automobiles were prepared.
  • each of the steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting.
  • the size of the cast slab was 300 ⁇ 400 [mm].
  • the cast slab was subjected to a breakdown step and rolled into a 150 mm square billet.
  • the billet was rolled into a steel bar under the hot-working conditions shown in Tables 4-1 and 4-2 while setting the finishing temperature to 800° C. or more.
  • the total processing ratio in the range of 800° C. to 1,000° C. is the ratio of reduction of the cross section in this temperature range.
  • cooling was conducted under the conditions described in Tables 4-1 and 4-2.
  • the steel bar was cut to a predetermined length and subjected to surface cutting and partly cold-drawing to adjust the diameter.
  • splines of the steel bar are formed by rolling to prepare a shaft 1 having splines 2 with dimensions and a shape shown in FIG. 1 .
  • the cold-working ratio is the ratio of reduction of the cross-section.
  • This shaft was hardened under conditions shown in Tables 4-1 and 4-2 using a high-frequency hardening apparatus with a frequency of 15 kHz and then tempered at 170° C. for 30 minutes in a heating furnace. The torsional fatigue strength was tested. For some of the shafts, tempering was omitted and their torsional fatigue strength was tested.
  • the hardened layer of the same shaft was etched using an etchant whose main component is picric acid, e.g., a mixture of an aqueous picric acid solution prepared by dissolving 50 g of picric acid in 500 g of water, 11 g of sodium dodecylbenzenesulfonate, 1 g of ferrous chloride, and 1.5 g of oxalic acid. Subsequently, its structure was observed with an optical microscope to determine the average diameter and maximum diameter of the former austenitic grains. The average diameter and the maximum diameter were determined by the same method as one described above.
  • picric acid e.g., a mixture of an aqueous picric acid solution prepared by dissolving 50 g of picric acid in 500 g of water, 11 g of sodium dodecylbenzenesulfonate, 1 g of ferrous chloride, and 1.5 g of oxalic acid.
  • picric acid e.g., a mixture of an a
  • the resistance to quenching crack was evaluated in terms of the number of quenching cracks observed, with an optical microscope (magnification: ⁇ 100 to ⁇ 200), in five polished C-cross-sections of the splines after the high-frequency hardening.
  • Tables 4-1 and 4-2 clearly show that every shaft with a hardened structure in which the average former austenitic grain diameter was 12 ⁇ m or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited high torsional fatigue strength and excellent resistance to quench cracking, i.e., the number of quench cracking being zero.
  • a crankshaft shown in FIG. 8 was prepared as a component for machine structural use of the present invention.
  • a crankshaft 4 has journals 5 to a cylinder, crank pins 6 , which are bearings for a piston connecting rod, crank webs 7 , and counter weights 8 .
  • the journals 5 and the crank pins 6 are subjected to high-frequency hardening to increase the fatigue strength.
  • the bending fatigue life of the crankshaft was evaluated as follows.
  • the average diameter and the maximum grain diameter of the former austenitic grains in the hardened layer of the same crankshaft were determined by the same method as one described above.
  • Tables 5-1 and 5-2 clearly show that every shaft with a hardened structure where an average grain diameter of former austenitic grains in the hardened layer was 12 ⁇ m or less and a maximum grain diameter did not exceed four times the average diameter exhibited excellent bending fatigue life, i.e., the number of times of application of the load to breaking being 9 ⁇ 10 6 or more.
  • a constant velocity joint 12 for transmitting power from a drive shaft 10 to a hub 11 of a wheel was prepared as a component for machine structural use of the present invention.
  • the constant velocity joint 12 is a combination of an outer race 13 and an inner race 14 .
  • the inner race 14 is movably fixed to the inner side of a mouth 13 a of the outer race 13 via balls 15 fit in a ball track groove in the inner surface of the mouth 13 a and is connected to the drive shaft 10 ; meanwhile, a stem 13 b of the outer race 13 is for example splined to the hub 11 to transmit power from the drive shaft 10 to the hub 11 of the wheel.
  • Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting.
  • the size of the cast slab was 300 ⁇ 400 [mm].
  • the cast slab was subjected to a breakdown step and rolled into a 150 mm square billet. The billet was then rolled to prepare a steel bar having a diameter of 50 mm.
  • the steel bar was cut to a predetermined length, hot-forged under the conditions set forth in Table 6-1, 6-2, 7-1, or 7-2 at a temperature 800° C. or more, and formed such that the mouth (outer diameter: 600 mm) and the stem (diameter: 20 mm) of the constant velocity joint outer race were integrally combined.
  • the tracking groove for balls is formed in the inner surface of the mouth of the constant velocity joint outer race by cutting or cold forging, and, at the same time, the stem of the constant velocity joint outer race was formed into a spline shaft by cutting or rolling.
  • the cooling after the hot-forging were conducted under the conditions set forth in Tables 6-1, 6-2, 7-1, and 7-2.
  • the total processing ratio in the hot-forging and the forming by rolling was adjusted by controlling the rate of change in cross-sectional area of a cross section taken in the direction orthogonal to the axis direction of the component to which the high-frequency hardening was effected.
  • the inner surface of the mouth 13 a or the outer surface of the stem 13 b of the constant velocity joint outer race 13 was hardened using a 15 kHz high-frequency hardening apparatus to form a hardened structure layer 16 and then tempered at 180° C. for 2 hours in a heating furnace to prepare a product.
  • the conditions for the tempering were set forth in Tables 6-1, 6-2, 7-1, and 7-2.
  • the tempering was omitted for some of the constant velocity joint outer races.
  • the resulting constant velocity joint outer race was attached, via balls (steel balls) in the mouth, to the inner race connected to the drive shaft, and the stem was fit into the hub to prepare a constant velocity joint unit (see FIG. 11 ).
  • the specifications for the balls, the inner race, and the hub were as follows:
  • the endurance test for determining the torsional fatigue strength was conducted.
  • the working angle of the constant velocity joint unit (the angle defined by the axis line of the outer race and the axis line of the drive shaft) was set to 0° and a torsional fatigue tester with a maximum torque of 4900 N ⁇ m was used to apply torsional force between the hub and the drive shaft, and the maximum torque of the stem was changed so that the stress conditions are fully reversed.
  • the stress at which the number of cycles to fracture was 1 ⁇ 10 5 was evaluated as the torsional fatigue strength.
  • the constant velocity joint outer races prepared under the same conditions were analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened layer by the above-described process.
  • Tables 6-1, 6-2, 7-1, and 7-2 clearly show that every constant velocity joint outer race with a hardened structure in which the average former austenitic grain diameter was 12 ⁇ m or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited excellent rolling fatigue properties and torsional fatigue strength.
  • a constant velocity joint 12 shown in FIG. 14 for transmitting power from a drive shaft 10 to a hub 11 of a wheel was prepared as the component for a machine structural use according to the present invention.
  • the constant velocity joint 12 was a combination of an outer race 13 and an inner race 14 .
  • the inner race 14 was movably fixed to the inner side of a mouth 13 a of the outer race 13 via balls 15 fit in a ball tracking groove in the inner surface of the mouth 13 a and was connected to the drive shaft 10 ; meanwhile, a stem 13 b of the outer race 13 was for example splined to the hub 11 to transmit power from the drive shaft 10 to the hub 11 of the wheel.
  • Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting.
  • the size of the cast slab was 300 ⁇ 400 [mm].
  • the cast slab was subjected to a breakdown step and rolled into a 150 mm square billet. The billet was then rolled to prepare a steel bar having a diameter of 55 mm.
  • the steel bar was cut to a predetermined length and formed into a constant velocity joint inner race (outer diameter: 45 mm, inner diameter: 20 mm) by hot-forging.
  • the fitting surface thereof was cut or rolled to form riffles for splining.
  • a surface for contact rolling with balls was formed by cutting or cold forging.
  • the cooling conditions after the hot-forging are shown in Tables 8-1, 8-2, 9-1, and 9-2.
  • the total processing ratio in the hot-forging and cold forging was adjusted by controlling the reduction ratio of area of a cross-section taken in the direction orthogonal to the axis direction of the contact rolling surface.
  • a fitting surface 14 b of the constant velocity joint inner race into which the drive shaft fits and a contact rolling surface 14 a onto which the balls interposed between the inner race and the constant velocity joint outer race make rolling contact were hardened with a 15 Hz high-frequency hardening apparatus under the conditions set forth in Tables 8-1, 8-2, 9-1, and 9-2 to thereby form a hardened structure layer 16 .
  • tempering at 180° C. was conducted for 2 hours in a heating furnace to harden the layer. The tempering was omitted for some of the constant velocity joints.
  • the drive shaft was fitted to the fitting surface of the resulting constant velocity joint inner race, and the constant velocity joint inner race was attached to the mouth of the constant velocity joint outer race via balls (steel balls).
  • the stem of the constant velocity joint outer race was fit to the hub to prepare a constant velocity joint unit (see FIG. 11 ).
  • the specifications for the balls, the outer race, the drive shaft, and the hub were as follows:
  • outer race high-frequency hardened, tempered carburized steel for machine structural use
  • drive shaft high-frequency hardened, tempered carburized steel for machine structural use
  • the endurance test for determining rolling fatigue strength was conducted for those in which the fitting surface with the drive shaft was subjected to high-frequency hardening, and the endurance test for determining torsional fatigue strength was conducted for those in which the contact rolling surface with the balls was subjected to high-frequency hardening, both the tests being conducted in a power transmission system for transmitting the rotary action of the drive shaft to the hub through the inner race of the constant velocity joint.
  • the constant velocity joint inner races prepared under the same conditions were analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened layers by the above-described process.
  • Tables 8-1, 8-2, 9-1, and 9-2 clearly show that every constant velocity joint inner race with a hardened structure in which the average former austenitic grain diameter was 12 ⁇ m or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited excellent fatigue properties.
  • a hub shown in FIG. 17 for an automobile wheel was prepared as a component for machine structural use according to the present invention.
  • Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting.
  • the size of the cast slab was 300 ⁇ 400 [mm].
  • the cast slab was subjected to a breakdown step and rolled into a 150 mm square billet.
  • the billet was then rolled to prepare a steel bar having a diameter of 24 mm.
  • the steel bar was then cut to a predetermined length, formed into the shape of a hub by hot-forging, and cooled at a cooling rate set forth in Tables 10-1 and 10-2.
  • the outer surface onto which the bearing balls for the hub shaft make rolling contact was formed by cutting or cold forging and subjected to high-frequency hardening under the conditions set forth in Tables 10-1 and 10-2 to form a hardened structure layer.
  • the hub was tempered at 170° C. for 30 minutes in a heating furnace and subjected to finishing to prepare a product. Tempering was omitted for some of the hubs.
  • the total processing ratio in the hot-forging and cold forging was controlled by adjusting the ratio of change in area of the cross-section taken in the direction orthogonal to the axis direction of the contact rolling surface.
  • the rolling fatigue life of the hub was evaluated as follows.
  • Bearing balls were arranged on the outer surface of the shaft of the hub, and an outer race was attached. While fixing the hub, a predetermined load (900 N) was applied onto the outer race 20 of the hub as shown in FIG. 17 and the outer race 20 of the hub was rotated at a predetermined rate (300 rpm) to conduct the endurance test. The time taken until occurrence of the rolling fatigue fracture of a high-frequency hardened structure layer 22 was evaluated as the rolling fatigue life.
  • the rolling fatigue life was indicated in terms of the ratio with respect to the rolling fatigue life of Sample No. 22, which is a related-art sample prepared under the hot working and high-frequency hardening conditions outside the range of the present invention, in Tables 10-1 and 10-2, the rolling fatigue life of the related-art sample being defined as 1.
  • the dimensions and shape of the outer race, steel balls, and the like were adjusted so that the shaft contact rolling surface of the hub was the weakest part in the endurance test.
  • the same hub was analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened structure layer by the above-described process.
  • Tables 10-1 and 10-2 clearly show that every hub with a hardened structure in which the average former austenitic grain diameter was 12 ⁇ m or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited a rolling fatigue life at least 10 times that of the related art sample.
  • a hub shown in FIG. 18 was prepared as in EXAMPLE 5 as an component for machine structural use according to the present invention.
  • each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting.
  • the size of the cast slab was 300 ⁇ 400 [mm].
  • the cast slab was subjected to a breakdown step, rolled into a 150 mm square billet, and rolled to prepare a steel bar having a diameter of 24 mm.
  • the steel bar was cut to a predetermined length, formed into a shape of a hub by hot-forging, and cooled at a rate shown in Tables 11-1 and 11-2. Subsequently, the hub shaft was cut or rolled for splining for fitting with the shaft of the constant velocity joint.
  • the inner surface (fitting portion 23 in FIG. 18 ) of the hub shaft that fits with the shaft of the constant velocity joint was subjected to high-frequency hardening under the conditions set forth in Tables 10-1 and 10-2 to form a hardened structure layer.
  • the hub was then tempered at 170° C. for 30 minutes in a heating furnace and subjected to finishing to prepare a product. Tempering was omitted for some of the hubs.
  • the total processing ratio in the hot-forging and forming rolling was adjusted by controlling the ratio of change in cross-section in the axis direction of the fitting portion of the hub shaft with the constant velocity joint.
  • the resulting hub was analyzed to determine the sliding rolling contact fatigue life of the inner surface that fits with the constant velocity joint. The results are shown in Tables 11-1 and 11-2.
  • the sliding rolling contact fatigue life of the hub was evaluated as follows:
  • a shaft 24 of the constant velocity joint was fit into the inner surface of the shaft of the hub, and while fixing the hub, fully reversed cycles of torsional force were applied to the shaft of the constant velocity joint (maximum torque: 700 N, 2 cycles per second). The number of cycles until occurrence of fracture of the hub spline due to the sliding rolling contact fatigue was evaluated as the fatigue life.
  • the sliding rolling contact fatigue life was indicated in terms of the ratio with respect to the sliding rolling contact fatigue life of Sample No. 22, which is a related art sample prepared under the hot-working and high-frequency hardening conditions outside the range of the present invention), in Tables 11-1 and 11-2, the sliding rolling contact fatigue life of the related-art sample being defined as 1.
  • the same hub was analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened structure layer by the above-described process.
  • Tables 11-1 and 11-2 clearly show that every hub with a hardened structure in which the average former austenitic grain diameter was 12 ⁇ m or less and the maximum grain diameter did not exceed four times the average grain diameter exhibited a sliding rolling contact fatigue life at least 10 times that of the related-art sample.
  • a gear 25 shown in FIG. 20 was prepared as a component for machine structural use according to the present invention.
  • a gear 25 shown in FIG. 20 was a representative example of a gear and has many teeth 26 in the edge face.
  • the gear of the present invention as shown in FIG. 21 , had many teeth 26 and bottoms 27 between the teeth 26 , and a hardened structure layer 28 was formed in the surface layer of the teeth and the bottoms.
  • the hardened structure layer 28 was formed in the surface portion of the teeth 26 and the bottoms 27 ; alternatively, it was possible to form a hardened structure layer in the inner surface of a shaft hole 29 into which a driving shaft of a variety of type was inserted.
  • Each of steel materials having compositions shown in Table 3 was melted in a converter and cast into a cast slab by continuous casting.
  • the size of the cast slab was 300 ⁇ 400 [mm].
  • the cast slab was subjected to a breakdown step and rolled into a 150 mm square billet.
  • the billet was then rolled under hot working conditions indicated in Tables 12-1 and 12-2 to prepare a steel bar having a diameter of 90 mm.
  • the processing ratio here is defined as the ratio of reduction in the cross section in the individual temperature ranges.
  • a gear was prepared from this steel bar by cutting as follows:
  • small gear outer diameter: 75 mm, module: 2.5, number of teeth: 28, reference diameter: 70 mm
  • large gear outer diameter: 85 mm, module: 2.5, number of teeth: 32, reference diameter: 80 mm
  • This gear was hardened under the conditions set forth in Tables 12-1 and 12-2 using a 200 kHz high-frequency hardening apparatus and tempered at 180° C. for 2 hours in a heating furnace, followed by fatigue test of actual gears. The tempering was omitted for some of the gears.
  • the fatigue test of actual gears was conducted by meshing the small and large gears and rotating them at a speed of 3000 rpm and a load torque of 245 N ⁇ m. The number of torque application cycles until fracture of one of the gears was evaluated.
  • Gears prepared under same conditions were analyzed to determine the average diameter and the maximum diameter of the former austenitic grains in the hardened structure layer by the above-described process.
  • Tables 12-1 and 12-2 clearly show that when the hardened layer had an average former austenitic grain diameter of 12 ⁇ m or less and a maximum grain diameter not exceeding four times the average grain diameter, excellent fatigue properties, i.e., the number of torque load application cycles being about 1000 ⁇ 10 4 or more, was achieved.
  • gears having hardened structures with an average former austenitic grain diameter exceeding 12 ⁇ m and a maximum grain diameter exceeding four times the average grain diameter exhibited poor fatigue properties.
  • a component for machine structural use that excels in all the fatigue properties, such as torsional fatigue property, the bending fatigue property, the rolling fatigue property, and the sliding rolling contact fatigue property, can be stably produced. Therefore, the invention has a significant effect on the demand of weight-reduction of automobile components and the like.

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Cited By (17)

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US20110052442A1 (en) * 2008-03-25 2011-03-03 Aktiebolaget Skf Bearing component
US20110126946A1 (en) * 2008-07-31 2011-06-02 Harshad Kumar Dharamshi Hansraj Bhadeshia Bainite steel and methods of manufacture thereof
US8956470B2 (en) 2008-07-31 2015-02-17 The Secretary Of State For Defence In Her Britannic Majesty's Government Of The United Kingdom Of Great Britain And Northern Ireland Bainite steel and methods of manufacture thereof
US20130006542A1 (en) * 2010-03-16 2013-01-03 Ntn Corporation Assessment of shear fatigue property of rolling contact metal material and estimation of fatigue limit maximum contact pressure using same assessment
US9234826B2 (en) * 2010-03-16 2016-01-12 Ntn Corporation Assessment of shear fatigue property of rolling contact metal material and estimation of fatigue limit maximum contact pressure using same assessment
US20170073785A1 (en) * 2014-03-05 2017-03-16 Daido Steel Co., Ltd. Age hardening non-heat treated bainitic steel
US10745772B2 (en) * 2014-03-05 2020-08-18 Daido Steel Co., Ltd. Age hardening non-heat treated bainitic steel
CN104087832A (zh) * 2014-06-06 2014-10-08 马鞍山市恒毅机械制造有限公司 铌微合金钢制备汽车轮毂轴承单元的方法
US10266908B2 (en) * 2014-07-03 2019-04-23 Nippon Steel & Sumitomo Metal Corporation Rolled steel bar for machine structural use and method of producing the same
US10260123B2 (en) * 2014-07-03 2019-04-16 Nippon Steel & Sumitomo Metal Corporation Rolled steel bar for machine structural use and method of producing the same
CN109112420A (zh) * 2017-06-26 2019-01-01 通用汽车环球科技运作有限责任公司 细晶粒钢合金及其形成的汽车部件
US20180372146A1 (en) * 2017-06-26 2018-12-27 GM Global Technology Operations LLC Fine grain steel alloy and automotive components formed thereof
US20210230724A1 (en) * 2018-05-31 2021-07-29 Nippon Steel Corporation Steel material for steel piston
US12134812B2 (en) * 2018-05-31 2024-11-05 Nippon Steel Corporation Steel material for steel piston
DE102018212009A1 (de) * 2018-07-18 2020-01-23 Volkswagen Aktiengesellschaft Hochfestes Lenkritzel
US12522886B2 (en) 2019-09-04 2026-01-13 Posco Steel plate having excellent strength and low-temperature impact toughness and method for manufacturing same
CN110715005A (zh) * 2019-09-25 2020-01-21 南阳理工学院 一种具有取向结构的高导热铜基刹车片的制备方法

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CN1950531A (zh) 2007-04-18
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EP1741798A1 (en) 2007-01-10
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KR20070004055A (ko) 2007-01-05
CN1950531B (zh) 2010-05-05

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