US20070000582A1 - Steel product for induction hardening, induction-hardened member using the same, and methods for production them - Google Patents

Steel product for induction hardening, induction-hardened member using the same, and methods for production them Download PDF

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US20070000582A1
US20070000582A1 US10/555,511 US55551105A US2007000582A1 US 20070000582 A1 US20070000582 A1 US 20070000582A1 US 55551105 A US55551105 A US 55551105A US 2007000582 A1 US2007000582 A1 US 2007000582A1
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induction
induction quenching
steel
steel material
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Inventor
Akihiro Matsuzaki
Yasuhiro Omori
Nobutaka Kurosawa
Tohru Hayashi
Takaaki Toyooka
Katsumi Yamada
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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, YAMADA, KATSUMI, HAYASHI, TOHRU, KUROSAWA, NOBUTAKA, OMORI, YASUHIRO, TOYOOKA, TAKAAKI
Publication of US20070000582A1 publication Critical patent/US20070000582A1/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
    • C22C38/18—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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C21D1/09—Surface hardening by direct application of electrical or wave energy; by particle radiation
    • C21D1/10—Surface hardening by direct application of electrical or wave energy; by particle radiation by electric induction
    • 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
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/009—Pearlite
    • 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
    • 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
    • Y02P10/25—Process efficiency

Definitions

  • the present invention relates to a steel material for induction quenching, which is suitable for an automobile drive shaft, constant velocity joint, and the like having a hard layer on a surface thereof formed by induction quenching, a induction quenched member using the same, and manufacturing methods thereof.
  • a fatigue strength is imparted to the steel thus treated by induction quenching and tempering, the fatigue strength, such as torsional fatigue strength, bending fatigue strength, rolling contact fatigue strength, and sliding and rolling contact fatigue strength, being important properties for machine structural members.
  • the increase in quenching depth by induction quenching may be considered.
  • the fatigue strength becomes saturated at a certain quenching depth and cannot be further improved.
  • the improvement in grain boundary strength is also effective, and for example, in Japanese Unexamined Patent Application Publication No. 2000-154819, a technique has been proposed in which austenite grains are particularized by precipitating a large amount of fine TiC in heating of induction quenching.
  • this technique although the grain boundary strength can be improved to a certain extent, the recent requirement for the fatigue strength cannot be fully satisfied.
  • a machine structural member having an improved fatigue strength has been proposed in which the fatigue strength is improved by controlling the following properties within respective predetermined ranges in accordance with the amount of C while a ratio CD/R is limited in the range of 0.3 to 0.7, CD being the thickness (depth of quenching) of a hard layer formed by induction quenching of a machine structural member having a circular transverse section, R being the radius thereof.
  • the properties mentioned above are the above CD/R, grain diameter of prior austenite in a region from the surface to 1 mm depth after induction quenching, average Vickers hardness Hf up to a ratio CD/R of 0.1 as obtained by induction quenching, and value A obtained from average Vickers hardness Hc of an axial central portion after induction quenching.
  • the member described above as is the case described above, the recent requirement for the fatigue strength cannot be fully satisfied.
  • the machine structural members such as an automobile drive shaft and constant velocity joint are machined by cutting into a predetermined shape before induction quenching in many cases.
  • a steel material used for the members as described above is required to have superior machinability.
  • the machinability has not been taken into consideration at all. In practice, when the steel disclosed in the publications described above is machined, the life of cutting tool is shortened, and hence the machinability has been a problem.
  • An object of the present invention is to provide a steel material for induction quenching, which has superior machinability and which can obtain a high fatigue strength by induction quenching as compared to that obtained in the past, a induction quenched member using the same, and manufacturing methods thereof.
  • the above object can be achieved by the following steel material for induction quenching and the following induction quenched member.
  • the steel material for induction quenching comprises 0.3% to 0.7% of C, 1.1% or less of Si, 0.2% to 1.1% of Mn, 0.05% to 0.6% of Mo, 0.06% or less of S, 0.025% or less of P, 0.25% or less of Al, 0.3% or less of Cr on a mass basis, and the balance being Fe and unavoidable impurities, in which the steel material has a ferrite structure and a pearlite structure, the total volume fraction of the ferrite structure and the pearlite structure is 90% or more, the thickness of the ferrite structure is 30 ⁇ m or less, and the average prior austenite grain diameter of a hard layer obtained after induction quenching is 12 ⁇ m or less.
  • the induction quenched member is formed using the above steel material for induction quenching, and the average prior austenite grain diameter of the hard layer obtained after induction quenching is 12
  • This steel material for induction quenching can be manufactured by a method for manufacturing a steel material for induction quenching, the method comprising a step of hot working a steel material having the above-described composition at a total reduction rate of 80% or more in a temperature region of more than 850° C. to 950° C., and a step of cooling the steel thus hot-worked to 600° C. or less at a cooling rate of less than of 0.6° C./sec.
  • this induction quenched member can be manufactured by a method for manufacturing an induction quenched member, the method comprising a step of performing induction quenching of a steel material for induction quenching, which is already formed into a predetermined shape, in a heating temperature region of 800° C. to 1,000° C. for 5 seconds or less.
  • FIG. 1 is a schematic view illustrating the thickness of a ferrite structure.
  • FIG. 2 is a graph showing the relationship between the heating temperature in induction quenching and the average prior austenite grain diameter of a hard layer.
  • the total volume fraction of a ferrite structure and a pearlite structure thereof is controlled to be 90% or more, and the thickness of the ferrite structure is controlled to be 30 ⁇ m or less, the machinability can be improved.
  • the average prior austenite grain diameter of a hard layer formed at a surface portion can be decreased to 12 ⁇ m or less, and as a result, a high fatigue strength can be obtained.
  • C has the most significant influence on quenching properties, can make harder the hard layer after quenching, and can make the thickness thereof larger, thereby improving the fatigue strength.
  • the content is less than 0.3 percent by mass, the thickness of the hard layer must be considerably increased in order to ensure a required fatigue strength, and as a result, the generation of quench cracking markedly occurs.
  • the content is more than 0.7 percent by mass, the fatigue strength is decreased due to the decrease in grain boundary strength, and in addition, the machinability, cold forging properties, and quench cracking resistance are also degraded.
  • the content of C is set in the range of 0.3 to 0.7 percent by mass and preferably 0.4 to 0.6 percent by mass.
  • Si increases the number of nucleation sites of austenite in heating of quenching and also particularizes the hard layer by suppressing the growth of the austenite grains. In addition, Si suppresses the decrease in grain boundary strength by suppressing the formation of carbides. Hence, Si is an element effective in improving the fatigue strength.
  • the content of Si is set to 1.1 percent by mass or less.
  • the content of Si is preferably set to 0.3 percent by mass or more.
  • the content of Si is preferably set to less than 0.3 percent by mass.
  • Mn is an indispensable element for improving the quenching properties and for ensuring the thickness of the hard layer.
  • the content is less than 0.2 percent by mass, the effect is not significant.
  • the content is more than 2.0 percent by mass, the amount of retained austenite is increased after quenching, the surface hardness is decreased, and as a result, the fatigue strength is decreased.
  • the content of Mn is set to 0.2 percent by mass or more, preferably 0.3 percent by mass or more, and more preferably in the range of 0.5 to 2.0 percent by mass.
  • the content of Mn is high, the hardness of a mother material is increased, and the machinability tends to be disadvantageously degraded; hence, the content is preferably set to 1.2 percent by mass or less and is more preferably set to 1.0 percent by mass or less.
  • Mo decreases the austenite grain diameter in heating of quenching and particularizes the quenched hard layer, so that the fatigue strength is improved.
  • the heating temperature in quenching is set in the range of 800° C. to 1,000° C. and more preferably set in the range of 800° C. to 950° C.
  • Mo is used for the adjustment thereof.
  • Mo prevents the decrease in grain boundary strength by suppressing the formation of carbides.
  • Mo is a very important element of the present invention; however, when the content thereof is less than 0.05 percent by mass, the prior austenite grain diameter of the hard layer cannot be easily decreased to 12 ⁇ m or less all along the depth of the entire hard layer. On the other hand, when the content is more than 0.6 percent by mass, the machinability is degraded. Hence, the content of Mo is set to 0.05 to 0.6 percent by mass and preferably 0.2 to 0.4 percent by mass.
  • S forms MnS in steel and improves the machinability.
  • the content is more than 0.06 percent by mass, S segregates at the grain boundaries, and as a result, the grain boundary strength is decreased.
  • the content of S is set to 0.06 percent by mass or less and preferably set in the range of 0.01 to 0.06 percent by mass.
  • P segregates at the austenite grain boundaries, decreases the grain boundary strength, and decreases the fatigue strength, and in addition, quench cracking is promoted.
  • the content of P is set to 0.02 percent by mass or less, a smaller P content is more preferable.
  • Al is an element effective in deoxidizing steel.
  • Al suppresses the growth of the austenite grains in heating of quenching, and as a result, the hard layer is particularized.
  • the content of Al is set to 0.25 percent by mass or less and is preferably set in the range of 0.01 to 0.05 percent by mass.
  • Cr Cr is an element effective for the quenching properties and increases the thickness of the hard layer, thereby improving the fatigue strength.
  • the content of Cr is set to 0.3 percent by mass or less.
  • the content is preferably set to 0.1 percent by mass or more.
  • the balance that is, elements other than the elements described above, is Fe and unavoidable impurities.
  • the unavoidable impurities are 0, N, B, and the like; however, even when 0.008 percent by mass or less of O, 0.02 percent by mass or less of N, and 0.0003 percent by mass or less of B are contained, the effects of the present invention will not be degraded.
  • B exceeds 0.0003 percent by mass, (Fe, Mo, Mn) 23 (C, B) 6 tends to be stabilized and precipitated in steel before induction quenching, and thereby small and large prior austenite grains are mixed and are present in the hard layer after quenching, so that a high fatigue strength cannot be achieved.
  • Cu is an element effective for the quenching properties.
  • Cu is dissolved in ferrite and improves the fatigue strength by solid-solution strengthening.
  • Cu suppresses the formation of carbides and prevents the decrease in grain boundary strength, so that the fatigue strength is improved.
  • the content of Cu is set to 1.0 percent by mass or less and is preferably set in the range of 0.03 to 0.2 percent by mass.
  • Ni Since being an element improving the quenching properties, Ni is used for adjustment thereof. In addition, Ni suppresses the formation of carbides, prevents the decrease in grain boundary strength, and improves the fatigue strength. However, since Ni is a very expensive element, when the content thereof is more than 3.5 percent by mass, the manufacturing cost is increased. Hence, the content of Ni is set to 3.5 percent by mass or less. In addition, when the content of Ni is less than 0.05 percent by mass, since the effect of improving the quenching properties and the effect of suppressing the decrease in grain boundary strength are not significant, the content is preferably set to 0.05 percent by mass or more. Furthermore, the content is more preferably set in the range of 0.1 to 1.0 percent by mass.
  • Co is an element which suppresses the formation of carbides, prevents the decrease in grain boundary strength, and improves the strength and the fatigue strength.
  • Co is a very expensive element, when the content thereof is more than 1.0 percent by mass, the manufacturing cost is increased. Hence, the content of Co is set to 1.0 percent by mass or less.
  • the content of Co is preferably set to 0.01 percent by mass or more.
  • the content is more preferably set in the range of 0.02 to 0.5 percent by mass.
  • Nb improves the quenching properties, is bonded with C and N to provide precipitation strengthening of steel, and improves the resistance to temper softening, so that the fatigue strength is improved.
  • the content is more than 0.1 percent by mass, the effect thereof is saturated.
  • the content of Nb is set to 0.1 percent by mass or less.
  • the content of Nb is less than 0.005 percent by mass, since the effects of improving the precipitation strengthening and the resistance to temper softening are not significant, the content is preferably set to 0.005 percent by mass or more.
  • the content is more preferably set in the range of 0.01 to 0.05 percent by mass.
  • Ti is bonded with C and N to provide precipitation strengthening of steel and improves the resistance to temper softening, so that the fatigue strength is improved.
  • the content is set to 0.1 percent by mass or less.
  • the content is preferably set to 0.01 percent by mass or more.
  • V is bonded with C and N to provide precipitation strengthening of steel and improves the resistance to temper softening, so that the fatigue strength is improved.
  • the content is set to 0.5 percent by mass or less.
  • the content of V is preferably set to 0.01 percent by mass or more.
  • the content is more preferably set in the range of 0.03 to 0.3 percent by mass.
  • the machinability is more effectively improved by the following reasons.
  • Ca Since forming a sulfide, which functions as a chip breaker, in combination with MnS and thereby improving the machinability, Ca may be added whenever necessary. However, when the content is more than 0.005 percent by mass, the effect is not only saturated but the manufacturing cost is also increased. Hence, the content of Ca is set to 0.005 percent by mass or less. When the content of Ca is less than 0.0001 percent by mass, since the effect of improving the machinability is not significant, the content is preferably set to 0.0001 percent by mass or more.
  • Mg Since being not only a deoxidizing element but also concentrating stress so as to improve the machinability, Mg may be added whenever necessary. However, when the content is excessive, the effect is not only saturated but the manufacturing cost is also increased. Hence, the content of Mg is set to 0.005 percent by mass or less. When the content of Mg is less than 0.0001 percent by mass, since the effect of improving the machinability is not significant, the content is preferably set to 0.0001 percent by mass or more.
  • Te Since Te is bonded with Mn to form MnTe which functions as a chip breaker, the machinability is improved. However, when the content is more than 0.005 percent by mass, the effect is not only saturated but the manufacturing cost is also increased. Hence, the content of Te is set to 0.005 percent by mass or less. When the content of Te is less than 0.003 percent by mass, since the effect of improving the machinability is not significant, the content is preferably set to 0.003 percent by mass or more.
  • Bi has melting, lubricating, and embrittlement functions in cutting, and thereby the machinability is improved.
  • the content of Bi is set to 0.5 percent by mass or less.
  • the content of Bi is preferably set to 0.01 percent by mass or more.
  • Pb has melting, lubricating, and embrittlement functions in cutting, and thereby the machinability is improved.
  • the content of Pb is set to 0.5 percent by mass or less.
  • the content of Pb is preferably set to 0.01 percent by mass or more.
  • Zr Since Zr forms in combination with MnS a sulfide which functions as a chip breaker, the machinability is improved. However, when the content is more than 0.01 percent by mass, the effect is not only saturated but the manufacturing cost is also increased. Hence, the content of Zr is set to 0.01 percent by mass or less. When the content of Zr is less than 0.003 percent by mass, since the effect of improving the machinability is not significant, the content is preferably set to 0.003 percent by mass or more.
  • the microstructure of steel is composed of a ferrite structure and a pearlite structure, the total volume fraction of the ferrite structure and the pearlite structure is set to 90% or more, and the thickness of the ferrite structure is set to 30 ⁇ m or less.
  • the thickness of the ferrite structure is defined as follows.
  • the steel material of the present invention has a microstructure composed of a pearlite structure and a ferrite structure in the form of a rosary disposed therearound.
  • the width of the ferrite structure in the direction perpendicular to the circumferential direction thereof is called the thickness of the ferrite structure. For this measurement, the image of an optical microscope was traced.
  • the machinability is considerably degraded.
  • the width of the ferrite structure exceeds 30 ⁇ m, since hard phases and soft phases are coarsely dispersed, the number of hard phases which function as a chip breaker in machining is decreased, and as a result, the machinability cannot be sufficiently ensured.
  • the width of the ferrite structure is decreased to 30 ⁇ m or less, superior machinability can be obtained, and in addition to that, the average prior austenite grain diameter of the hard layer after induction quenching is advantageously decreased to 12 ⁇ m or less.
  • the reason for this is that since the nucleation of austenite grains in heating of quenching occurs from the ferrite/pearlite interfaces and cementite interfaces, the number of nucleation sites is increased as the thickness of the ferrite structure is decreased, and that as a result, the size of the generated austenite grains is decreased.
  • the average prior austenite grain diameter of the hard layer formed by induction quenching on the surface of the induction quenched member must be controlled to be 12 ⁇ m or less, preferably 10 ⁇ m or less, and more preferably 5 ⁇ m or less.
  • the average prior austenite grain diameter of the hard layer is more than 12 ⁇ m, a sufficient grain boundary strength cannot be obtained, and as a result, the improvement in fatigue strength cannot be expected.
  • the prior austenite grain diameter of the hard layer is measured as described below.
  • the topmost layer of the member thus processed has 100% of a martensite structure on an area fraction basis. From the surface toward the inside, a region composed of 100% of a martensite structure is present to a certain depth, and from this certain depth, the area fraction of the martensite structure is rapidly decreased. In the present invention, a region from the surface of the member processed by induction quenching to a depth at which the area fraction of the martensite structure is decreased to 98% is defined as the hard layer.
  • the average prior austenite grain diameters are measured at depths of one fifth, one half, and four fifths of the thickness of the hard layer from the surface thereof, and when being 12 ⁇ m or less at all the depths described above, the average prior austenite grain diameter is regarded as 12 ⁇ m or less all along the depth of the hard layer.
  • the average prior austenite grain diameter After the cross-section of the hard layer was processed by using an etching solution which was prepared by dissolving 50 g of picric acid into 500 g of water, followed by addition of 11 g of sodium dodecylbenzene sulfonate, 1 g of ferrous chloride, and 1.5 g of oxalic acid, five visual fields of each measurement point were observed at a magnification from 400 (area of one visual field being 0.25 mm ⁇ 0.225 mm) to 1,000 (area of one visual field being 0.10 mm ⁇ 0.09 mm) using an optical microscope, and the average grain diameter was measured by an image analysis device.
  • the thickness of the hard layer is preferably set to 2 mm or more.
  • the thickness described above is more preferably 2.5 mm or more and even more preferably 3 mm or more.
  • a steel material for induction quenching according to the present invention is obtained in which a ferrite structure and a pearlite structure are present, and the total volume fraction of the ferrite structure and the pearlite structure is 90% or more, and the thickness of the ferrite structure is 30 ⁇ m or less.
  • the thickness of the ferrite structure surrounding the pearlite structure cannot be decreased to 30 ⁇ m or less.
  • the cooling rate after the hot working is set to 0.6° C./s or more, a martensite structure and/or a bainite structure may be generated, and as a result, it becomes difficult to control the total volume fraction of the ferrite structure and the pearlite structure to be 90% or more.
  • the prior austenite grain diameter can be decreased to 12 ⁇ m or less all along the depth of the entire hard layer, so that a induction quenched member having a high fatigue strength can be obtained.
  • the heating temperature is less than 800° C., since the generation of the austenite grains is not sufficient, the formation of the hard layer becomes insufficient, and as a result, a high fatigue strength cannot be obtained.
  • the heating temperature is more than 1,000° C., since the growth of the austenite grains is promoted so as to form coarser and larger grains, the hard layer are formed of coarser and larger grains, resulting in decrease in fatigue strength.
  • the heating temperature is more preferably in the range of 800° C. to 950° C.
  • FIG. 2 is a graph showing the relationship between the average prior austenite grain diameter of the hard layer and the heating temperature in induction quenching for steel containing Mo (Mo: 0.05 to 0.6 percent by mass) and steel containing no Mo.
  • the prior austenite grain diameter of the hard layer is decreased.
  • a hard layer formed of significantly fine grains can be obtained at a heating temperature of 1,000° C. or less and preferably 950° C. or less.
  • the heating time in induction quenching must be set to 5 seconds or less and is preferably set to 3 seconds or less.
  • the heating rate in induction quenching is increased, the grain growth of austenite is likely to be suppressed, and hence the heating rate is preferably set to 200° C./sec or more. Furthermore, the heating rate is more preferably set to 500° C./sec or more.
  • Steel Nos. 1 to 31 having the compositions shown in Table 1 were each molten in a converter and then formed into a steel sheet having a cross-sectional size of 300 ⁇ 400 mm by continuous casting. After the steel sheets were rolled into billets having a size of 150 ⁇ 150 mm through a breakdown step, rolling was performed under the hot working conditions shown in Table 2, so that steel bars having a diameter of 24 to 60 mm were formed.
  • Test pieces having a parallel portion diameter of 8 mm for rotary bending fatigue test were sampled from the above steel bars and were then processed by induction quenching under the conditions shown in Tables 2-1 and 2-2 using a induction quenching device at a frequency of 15 kHz, followed by tempering at 170° C. for 30 minutes, so that test pieces of steel materials No. 1 to 43 were obtained. Subsequently, the rotary bending fatigue test was performed for the test pieces thus obtained at an rpm of 3,000 using an Ono's rotary bending fatigue testing machine by changing stress conditions so as to measure a stress at which a life of 1 ⁇ 10 8 times was obtained, and this stress was defined as the fatigue strength.
  • Test pieces for machinability evaluation were also sampled from the above steel bars and were then repeatedly drilled at an rpm of 1,500 using an SKH-made drill having a diameter of 4.4 mm to form a hole having a length of 12 mm, and the total hole length (mm) was measured at which the drilling could not be further continued, so that the machinability was evaluated. When the total hole length is larger, the machinability is regarded as superior.
  • microstructure of the steel material before induction quenching was observed using an electron microscope and an optical microscope.
  • the thickness of the ferrite structure, the thickness of the hard layer, and the average prior austenite grain diameter were measured by the methods described above.
  • austenite grains having a grain diameter three times or more the average austenite grain diameter of the hard layer was present at an area fraction of 30% or more, the hard layer was defined as a mixed grain layer.
  • All the steel materials according to the present invention have an average prior austenite grain diameter of the hard layer of 12 ⁇ m or less after induction quenching and exhibit a superior bending fatigue strength, the above steel materials having the compositions within the range of the present invention and having microstructures each composed of a ferrite structure and a pearlite structure before induction quenching, the total volume fraction of the ferrite structure and the pearlite structure being 90% or more, the thickness of the ferrite structure being 30 ⁇ m or less.
  • the steel materials described above also have superior machinability.
  • steel materials Nos. 4 and 19 have a small total reduction rate in a temperature region of more than 850° C. to 950° C., the thickness of the ferrite structure is increased, and hence the machinability is degraded.

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US10/555,511 2003-09-29 2004-07-16 Steel product for induction hardening, induction-hardened member using the same, and methods for production them Abandoned US20070000582A1 (en)

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
JP2003338634 2003-09-29
JP2003339634 2003-09-30
JP2004181467 2004-06-18
JP2004181467 2004-06-18
PCT/JP2004/010570 WO2005031020A1 (fr) 2003-09-29 2004-07-16 Produit en acier pour trempe par induction, element trempe par induction l'utilisant et procedes de production correspondant

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US (1) US20070000582A1 (fr)
EP (1) EP1669468B1 (fr)
KR (1) KR100726251B1 (fr)
DE (1) DE602004032363D1 (fr)
TW (1) TWI276690B (fr)
WO (1) WO2005031020A1 (fr)

Cited By (4)

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US20110002807A1 (en) * 2009-01-16 2011-01-06 Nippon Steel Corporation Steel for induction hardening
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
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
US10808304B2 (en) 2016-07-19 2020-10-20 Nippon Steel Corporation Steel for induction hardening

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20110002807A1 (en) * 2009-01-16 2011-01-06 Nippon Steel Corporation Steel for induction hardening
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
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
US10808304B2 (en) 2016-07-19 2020-10-20 Nippon Steel Corporation Steel for induction hardening

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TW200512302A (en) 2005-04-01
EP1669468A1 (fr) 2006-06-14
KR20060016788A (ko) 2006-02-22
KR100726251B1 (ko) 2007-06-08
TWI276690B (en) 2007-03-21
EP1669468A4 (fr) 2007-03-07
EP1669468B1 (fr) 2011-04-20
WO2005031020A1 (fr) 2005-04-07
DE602004032363D1 (de) 2011-06-01

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