US20150176101A1 - Hollow stabilizer, and steel pipe for hollow stabilizers and method of producing the same - Google Patents

Hollow stabilizer, and steel pipe for hollow stabilizers and method of producing the same Download PDF

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US20150176101A1
US20150176101A1 US14/402,745 US201314402745A US2015176101A1 US 20150176101 A1 US20150176101 A1 US 20150176101A1 US 201314402745 A US201314402745 A US 201314402745A US 2015176101 A1 US2015176101 A1 US 2015176101A1
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steel pipe
electric resistance
hollow stabilizer
content
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US14/402,745
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Inventor
Tetsuo Ishitsuka
Motofumi Koyuba
Masamichi Iwamura
Akira Tange
Ken Takahashi
Kiyoshi Kurimoto
Yutaka Wakabayashi
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NHK Spring Co Ltd
Nippon Steel Corp
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NHK Spring Co Ltd
Nippon Steel and Sumitomo Metal Corp
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Assigned to NHK SPRING CO., LTD., NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NHK SPRING CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KURIMOTO, KIYOSHI, TAKAHASHI, KEN, TANGE, AKIRA, WAKABAYASHI, YUTAKA, ISHITSUKA, TETSUO, IWAMURA, Masamichi, KOYUBA, MOTOFUMI
Publication of US20150176101A1 publication Critical patent/US20150176101A1/en
Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
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    • 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
    • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
    • B21C37/08—Making tubes with welded or soldered seams
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C51/00—Measuring, gauging, indicating, counting, or marking devices specially adapted for use in the production or manipulation of material in accordance with subclasses B21B - B21F
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60G—VEHICLE SUSPENSION ARRANGEMENTS
    • B60G21/00—Interconnection systems for two or more resiliently-suspended wheels, e.g. for stabilising a vehicle body with respect to acceleration, deceleration or centrifugal forces
    • B60G21/02—Interconnection systems for two or more resiliently-suspended wheels, e.g. for stabilising a vehicle body with respect to acceleration, deceleration or centrifugal forces permanently interconnected
    • B60G21/04—Interconnection systems for two or more resiliently-suspended wheels, e.g. for stabilising a vehicle body with respect to acceleration, deceleration or centrifugal forces permanently interconnected mechanically
    • B60G21/05—Interconnection systems for two or more resiliently-suspended wheels, e.g. for stabilising a vehicle body with respect to acceleration, deceleration or centrifugal forces permanently interconnected mechanically between wheels on the same axle but on different sides of the vehicle, i.e. the left and right wheel suspensions being interconnected
    • B60G21/055—Stabiliser bars
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
    • C21D7/06—Modifying the physical properties of iron or steel by deformation by cold working of the surface by shot-peening or the like
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
    • 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
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • 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
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60G—VEHICLE SUSPENSION ARRANGEMENTS
    • B60G2206/00—Indexing codes related to the manufacturing of suspensions: constructional features, the materials used, procedures or tools
    • B60G2206/01—Constructional features of suspension elements, e.g. arms, dampers, springs
    • B60G2206/40—Constructional features of dampers and/or springs
    • B60G2206/42—Springs
    • B60G2206/427—Stabiliser bars or tubes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60G—VEHICLE SUSPENSION ARRANGEMENTS
    • B60G2206/00—Indexing codes related to the manufacturing of suspensions: constructional features, the materials used, procedures or tools
    • B60G2206/01—Constructional features of suspension elements, e.g. arms, dampers, springs
    • B60G2206/70—Materials used in suspensions
    • B60G2206/72—Steel
    • 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
    • C21D2211/004—Dispersions; Precipitations
    • 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
    • C21D2211/008—Martensite
    • 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

Definitions

  • the present invention relates to a hollow stabilizer used for a vehicle such as an automobile, and a steel pipe for hollow stabilizers used as a material for the hollow stabilizer and a method of producing thereof.
  • Stabilizers are applied to vehicles such as automobiles for the purpose of securing the running stability of vehicle bodies during high speed running while suppressing the rolling of vehicle bodies during cornering.
  • the stabilizer is conventionally manufactured by processing a solid material such as a steel bar into an intended shape.
  • hollow stabilizers using a hollow material such as a seamless steel pipe or an electric resistance-welded steel pipe are increasingly used for the purpose of promoting weight reduction.
  • the outer diameter of the hollow stabilizer necessarily becomes larger than that of the solid stabilizer in order to maintain the same roll rigidity.
  • the generated stress with respect to the same load becomes larger in the hollow stabilizer, and therefore, it is necessary to increase a wall thickness/outer diameter (t/D) ratio for suppressing an increase in the generated stress.
  • Thin-walled hollow stabilizers with a t/D of from 0.10 to 0.17 are conventionally applied to compact cars with low design stresses.
  • t/D is required to be increased.
  • a method of producing a hollow stabilizer in which an electric resistance-welded steel pipe is subjected to diameter-reduction hot rolling and then drawing for example, see Patent Document 1
  • a thick-walled steel pipe for hollow stabilizers produced by subjecting an electric resistance-welded steel pipe to diameter-reduction hot rolling for example, see Patent Document 2
  • Patent Document 1 Japanese Patent Application Laid-Open (JP-A) No. 2000-233625
  • Patent Document 2 International Publication No. WO 2007-023873
  • Patent Document 3 JP-A No. 2007-270349
  • FIG. 1( a ) is a perspective view of an electric resistance-welded steel pipe
  • FIG. 1( b ) is a magnified view of a metal flow 18 in a base metal 17 , observed in the cross section of an electric resistance-welded steel pipe 16 surrounded by circle S 1 in FIG. 1( a );
  • FIG. 1( a ) is a perspective view of an electric resistance-welded steel pipe
  • FIG. 1( b ) is a magnified view of a metal flow 18 in a base metal 17 , observed in the cross section of an electric resistance-welded steel pipe 16 surrounded by circle S 1 in FIG. 1( a );
  • FIG. 1( a ) is a perspective view of an electric resistance-welded steel pipe
  • FIG. 1( b ) is a magnified view of a metal flow 18 in a base metal 17 , observed in the cross section of an electric resistance-welded steel pipe 16 surrounded by circle S 1 in FIG. 1( a );
  • FIG. 1( a )
  • FIG. 1( c ) is a magnified view of a metal flow 18 in a welded portion 19 , observed in the cross section of the electric resistance-welded steel pipe 16 surrounded by circle S 2 in FIG. 1( a ); and FIG. 1( d ) is a magnified view showing a state of presence of MnS in the longitudinal section along the extending direction (L direction) of an electric resistance-welded abutting portion of the electric resistance-welded steel pipe 16 .
  • These figures are schematic diagrams. As is clear from a comparison of FIGS.
  • the metal flow 18 is formed such that the center segregation in the steel plate stands perpendicularly, in the wall thickness direction, due to the intensive upset of abutting surfaces during welding. Therefore, in a case in which MnS 20 elongated in a longitudinal direction by rolling is present at the center segregation of the steel plate, elongated MnS 20 occurs on the surface close to the electric resistance-welded portion 19 after cutting off a welding bead as shown in FIG. 1( d ) and is an origin of fatigue fracture.
  • the present invention is made in consideration of the above situation, and the object of the present invention is to provide a hollow stabilizer having higher strength compared to the conventional hollow stabilizers and having excellent fatigue properties and a steel pipe for hollow stabilizers, which is used as a material for the hollow stabilizer.
  • Hollow stabilizers are produced through quenching and tempering for adjusting quality of material.
  • quenching cracks are generated during quenching in a case in which the C content is too high.
  • Cr is added to ensure hardenability in the invention.
  • Mn, S, Ca, and O it is necessary to restrict Mn, S, Ca, and O. It is preferable to restrict t/D and the thickness of a decarburizing layer in order to suppress the generation of fatigue cracks from the inner surface, and it is more preferable to impart compressive residual stress by shot peening.
  • a hollow stabilizer having a chemical composition comprising, as chemical components, in terms of % by mass: 0.26% to 0.30% of C, 0.05% to 0.35% of Si, 0.5% to 1.0% of Mn, 0.05% to 1.0% of Cr, 0.005% to 0.05% of Ti, 0.0005% to 0.005% of B, and 0.0005% to 0.005% of Ca, wherein: Al, P, S, N, and O are limited to 0.08% or less, 0.05% or less, less than 0.0030%, 0.006% or less, and 0.004% or less, respectively, a remainder of the chemical composition consists of Fe and unavoidable impurities, a value of a product of the Mn content and the S content is 0.0025 or less, and a critical cooling rate Vc90 represented by the following Equation (1) is 40° C./s or less:
  • a steel pipe for a hollow stabilizer used as a material for the hollow stabilizer according to any one of claims 1 to 5 the steel pipe having a chemical composition comprising, as chemical components, in terms of % by mass: 0.26% to 0.30% of C, 0.05% to 0.35% of Si, 0.5% to 1.0% of Mn, 0.05% to 1.0% of Cr, 0.005% to 0.05% of Ti, 0.0005% to 0.005% of B, and 0.0005% to 0.005% of Ca, wherein: Al, P, S, N, and O are limited to 0.08% or less, 0.05% or less, less than 0.0030%, 0.006% or less, and 0.004% or less, respectively, the chemical composition optionally comprises one or more of: 0.05% to 0.5% of Mo, 0.01% to 0.1% of Nb, 0.01% to 0.1% of V, or 0.1% to 1.0% of Ni, a remainder of the chemical composition consists of Fe and unavoidable impurities, a value of a product of the Mn content and
  • a hollow stabilizer for automobiles having an excellent fatigue endurance and higher strength compared to the conventional ones, while maintaining fatigue properties and delayed fracture properties equivalent to those of the conventional hollow stabilizer for automobiles.
  • FIGS. 1( a ) to 1 ( c ) are diagrams showing the relationship between the surface layer of the electric resistance-welded portion and MnS in the center segregation.
  • FIG. 1( a ) shows the electric resistance-welded steel pipe
  • FIG. 1( b ) is magnified view of a metal flow in a base metal, observed in the cross section of an electric resistance-welded steel pipe surrounded by circle S 1 in FIG. 1( a )
  • FIG. 1( c ) is magnified view of a metal flow in the welded portion, observed in the cross section of the electric resistance-welded steel pipe surrounded by circle S 2 in FIG. 1( a );
  • FIG. 1( d ) is magnified view of the longitudinal sectional along the extending direction (L direction) of the electric resistance-welded abutting portion of the electric resistance-welded steel pipe.
  • FIG. 2 shows an example of the stabilizer.
  • FIGS. 3( a ) and 3 ( b ) are diagrams illustrating the method of producing a plate for a plane bending fatigue test specimen from the electric resistance-welded steel pipe.
  • FIG. 3( a ) is a perspective view of the electric resistance-welded steel pipe after making a slit in a longitudinal direction; and
  • FIG. 3( b ) is a perspective view of the electric resistance-welded steel pipe of FIG. 3( a ) developed in a plane.
  • FIGS. 4( a ) and 4 ( b ) are diagram illustrating the plane bending fatigue test specimen produced using the plate of FIG. 3( b ).
  • FIG. 4( a ) is a plane view
  • FIG. 4( b ) is a side view.
  • FIGS. 5( a ) and 5 ( b ) show a fracture surface of the specimen after the fatigue test.
  • FIG. 5( a ) is the SEM photograph showing the fracture surface of the specimen
  • FIG. 5( b ) shows the result of the EDX analysis at the position enclosed with the dashed oval in FIG. 5( a ).
  • FIG. 6 is the photograph showing the metal flow of the cross section perpendicular to the fracture surface of the specimen after the fatigue test shown by sticking the photographs at the fracture position.
  • FIG. 7 show an example of the relationship between the cooling rate and the hardness during quenching.
  • FIG. 8 shows an example of the process of manufacturing the stabilizer by cold forming.
  • FIG. 9 shows an example of the process of manufacturing the stabilizer by hot forming.
  • FIG. 10 shows an example of the temper-softening curve of the steel pipe for hollow stabilizers.
  • a stabilizer 10 includes a torsion portion 11 which is extended in a width direction of a vehicle body (not shown) and a left-and-right pair of arm portions 12 which are connected to either end of the torsion portion 11 .
  • the torsion portion 11 is fixed to the body side via a bush 14 or the like.
  • the terminals 12 a of the arm portions 12 are connected to suspension mechanisms 15 on the left and right via stabilizer links (not shown) or the like.
  • several portions or ten or more portions are usually subjected to bending to avoid interference with other components.
  • the hollow stabilizer according to the invention has a maximum hardness of HRC 50 as an achievable hardness and a minimum hardness of HRC 40 which is a substantial upper limit of the conventional material.
  • a wall thickness/outer diameter ratio (t/D) is set to 0.14 or more, such that fatigue fracture initiates from the outer surface.
  • t/D wall thickness/outer diameter ratio
  • the upper limit of the t/D is not particularly limited. Since the stabilizer with a t/D of 0.5 is theoretically solid, the upper limit of the t/D in the invention is substantially less than 0.5.
  • each of the HRC and the t/D is a value at a portion not subjected to bending during the production of the hollow stabilizer.
  • elongated MnS sometimes is an origin of fatigue fracture.
  • the inventors produced a plate 22 for a plane bending fatigue test specimen from an electric resistance-welded pipe 21 as shown in FIGS. 3( a ) and 3 ( b ). Further, a plane bending fatigue test was conducted using a specimen 24 in which an electric resistance-welded portion 23 of the electric resistance-welded pipe 21 is located at a central position in a longitudinal direction of the specimen 24 for the plane bending fatigue test, in which the electric resistance-welded portion 23 extends in a direction perpendicular to the longitudinal direction of the specimen 24 , as shown in FIGS. 4( a ) and 4 ( b ). After the test, the fracture surface of the specimen 24 was observed under a scanning electron microscope (SEM), and the composition of an inclusion present at a fracture origin was analyzed using an energy dispersive X-ray spectrometry (EDS) attached to the SEM.
  • SEM scanning electron microscope
  • FIGS. 5( a ) and 5 ( b ) it was confirmed that MnS was present at the fracture origin in the fractured specimen.
  • the observation result of metal flow of the cross section perpendicular to the fracture surface of the specimen after the fatigue test is shown in FIG. 6 .
  • the fracture surface in the specimen was located at a position somewhat removed from the position of the welded portion, rather than the welded portion. It was also confirmed that the surfaces of portions in the vicinity of both sides sandwiching the electric resistance-welded portion therebetween corresponded to the segregation zone located at the center part in a thickness direction of the base metal.
  • the length of elongated MnS present at the center part in a thickness direction of the base metal is required to be limited for preventing the generation of fatigue fracture from the electric resistance-welded portion.
  • the length of elongated MnS present at the center part in a thickness direction is set to 150 ⁇ m or less.
  • elongated MnS serves as an origin of fatigue fracture of the electric resistance-welded portion.
  • a 10 mm-length segment, as a specimen for observing the cross sectional structure is cutout from the hollow stabilizer in a longitudinal direction, and at a center part in a thickness direction of the hollow stabilizer present in the cross section of the specimen, the length of MnS is confirmed with an optical microscope.
  • the presence of MnS may be confirmed by electron scanning microscopy together with energy dispersive X-ray spectrometry.
  • the length of MnS is determined by observing the center part in a thickness direction present in the cross section with an optical microscope or a scanning electron microscope with respect to 10 samples for each, and measuring the length of MnS having the largest size among the MnS present in the observed region.
  • the depth of a decarburized layer at the inner surface part is set to 20 ⁇ m or less from the inner surface. It is preferable to include no decarburized layer since the decarburized layer has lower strength than the base metal and tends to serve as the origin of fatigue fracture.
  • the t/D is set to 0.14 or more, the generation of fatigue fracture from the inner surface can be prevented by setting the depth of the decarburized layer at the inner surface part to 20 ⁇ m or less.
  • the depth of the decarburized layer means a maximum depth from the inner surface of ferrite present at the inner surface.
  • the grain size of ferrite on the inner surface of the steel pipe for stabilizers before quenching is approximately from 10 ⁇ m to 20 ⁇ m.
  • the width corresponding to the grain size of the ferrite grains connectively present on the inner surface is regarded as the width of a layer
  • the depth of the ferrite decarburized layer of the steel pipe can be set to 20 ⁇ m or less by limiting the width of the layer up to the one-layer size.
  • the depth of the decarburized layer can be set to 20 ⁇ m or less.
  • the decarburized layer is formed in the dual phase range during cooling from a high temperature to the room temperature.
  • the dual phase range is a temperature range below the Ar 3 transformation temperature at which austenite-to-ferrite transformation starts, and is a temperature range at which austenite and ferrite coexist.
  • the hollow stabilizer according to the invention fatigue strength is improved when compressive residual stress is imparted to the outer surface, and the effect can be obtained significantly when the maximum compressive residual stress on the outer surface is 400 MPa or more. While the stress generated at the inner surface of the hollow stabilizer is lower compared to the outer surface, it is sometimes preferable to impart compressive residual stress to the inner surface in order to improve fatigue endurance.
  • the method of imparting compressive residual stress is not particularly limited, and shot peening is the simplest method.
  • the compressive residual stress can be determined by an X-ray diffraction method.
  • % indicating the content of respective components means “% by mass”.
  • C is an element that determines the strength of the hollow stabilizer. In order to realize higher strength compared to the conventional hollow stabilizer, it is necessary to set the C content to 0.26% or more. However, when the C content exceeds 0.30%, quenching cracks are generated. Therefore, the upper limit of the C content is set to 0.30%.
  • Si is a deoxidizing element and contributes to solid-solution strengthening. Si also has the effect of increasing temper-softening resistance. In order to obtain these effects, the content of Si is required to be 0.05% or more. However, when the Si content exceeds 0.35%, toughness is decreased. Therefore, the Si content is set to a range of from 0.05% to 0.35%. It is preferable that the lower limit of the Si content is set to 0.20% and the upper limit of the Si content is set to 0.30%.
  • Mn is an element that improves hardenability.
  • the Mn content is less than 0.5%, the sufficient effect of improving hardenability cannot be ensured.
  • the Mn content exceeds 1.0%, delayed fracture properties tends to be deteriorated and MnS easily precipitates, thereby decreasing fatigue strength in the vicinity of the electric resistance-welded portion. Therefore, the Mn content is set to a range of form 0.5% to 1.0%, and preferably to 0.5% or more and less than 0.8%.
  • the P content is an element that has an adverse effect on weld crack resistance and toughness. Therefore, the P content is limited to 0.05% or less. The P content is preferably 0.03% or less.
  • the S content is limited to less than 0.0030%.
  • the S content is preferably 0.0026% or less.
  • the value of the product of the Mn content and the S content is limited to 0.0025 or less. This is because, in a case in which the value of the product of the Mn content and the S content exceeds 0.0025, sufficient fatigue strength cannot be obtained in the vicinity of the electric resistance-welded portion even when each of the Mn content and the S content satisfies the above appropriate range.
  • Cr is an element that improves hardenability. In a case in which the Cr content is less than 0.05%, these function and effect cannot be expected sufficiently. In a case in which the Cr content exceeds 1.0%, defects are easily generated during electric resistance-welding. Therefore, the Cr content is set to a range of from 0.05% to 1.0%.
  • Al is an element that is useful as an agent for deoxidizing a molten steel, and it is preferable to add 0.01% or more of Al.
  • Al is also an element for fixing N and, hence, the Al content has a significant influence on crystal grain sizes and mechanical properties of the steel. In a case in which the Al content exceeds 0.08%, large amounts of non-metallic inclusions are formed and surface defects are easily generated in the final product. Therefore, the Al content is set to 0.08% or less.
  • the Al content is preferably 0.05% or less, and more preferably 0.03% or less.
  • Ti acts to stably and effectively improve hardenability obtained by the addition of B, by suppressing the precipitation of BN by fixing nitrogen in the steel in the form of TiN. Therefore, in order to satisfy the stoichiometry of TiN, Ti needs to be added in an amount that is at least 3.42 or more times the N content, and the range of the Ti content is also automatically determined based on the range of the N content. However, since some Ti may precipitate to form a carbide, the Ti content is set to be larger than a theoretical value, i.e., a range of from 0.005% to 0.05%, in order to more surely fix N. The Ti content is preferably from 0.01% to 0.02%.
  • the B is an element for significantly enhancing the hardenability of the steel material with addition in a small quantity.
  • the B content is set to a range of from 0.0005% to 0.005%.
  • the B content is preferably from 0.001% to 0.002%.
  • N is an element that has the function of enhancing steel strength via the precipitation in the form of nitrides or carbonitrides.
  • a deterioration of hardenability caused by the precipitation of BN, or deterioration of hot workability, fatigue strength, or toughness caused by the precipitation of TiN as a result of Ti added to prevent the precipitation of BN as described above becomes problematic.
  • TiN has the effect of suppressing the coarsening of ⁇ -grains at a high temperature to improve toughness. Therefore, in order to achieve an optimum balance between hot workability, fatigue strength, and toughness, the N content is set to 0.006% or less.
  • the N content is preferably from 0.001% to 0.005%, and more preferably from 0.002% to 0.004%.
  • Ca is an element that improves toughness and that suppresses the reduction of fatigue strength caused by MnS in the vicinity of the electric resistance-welded portion, by the fixation of S in the form of CaS.
  • the Ca content is less than 0.0005%, these effects cannot be expected sufficiently.
  • the Ca content exceeds 0.005%, toughness is deteriorated due to the increase in oxides in the steel. Therefore, the Ca content is set to a range of from 0.0005% to 0.005%.
  • O is an element that neutralizes the effect obtained by the addition of Ca via the formation of CaO. Therefore, the 0 content is limited to 0.004% or less.
  • the hollow stabilizer according to the invention has the chemical composition including the above essential components, and may further include Mo, Nb, V, and Ni if necessary.
  • Mo is an element that has the effect of improving hardenability. In a case in which the Mo content is less than 0.05%, the effect cannot be expected sufficiently. On the other hand, in a case in which the Mo content exceeds 0.5%, the alloy cost is increased. Therefore, the Mo content is set to a range of from 0.05% to 0.5%.
  • Nb has the effect of precipitation strengthening by forming Nb carbonitrides, and also has the effect of improving toughness by reducing the crystal grain size of the steel material. In a case in which the Nb content is less than 0.01%, sufficient effect of improving strength and toughness cannot be obtained. On the other hand, in a case in which the Nb content exceeds 0.1%, more effects cannot be expected and merely the alloy cost is increased. Therefore, the Nb content is set to a range of from 0.01% to 0.1%.
  • V has the effect of precipitation strengthening by V carbonitrides.
  • the V content is set to a range of from 0.01% to 0.1%.
  • Ni is an element that has the effect of improving hardenability and toughness. In a case in which the Ni content is less than 0.1%, the effect cannot be expected sufficiently. On the other hand, in a case in which the Ni content exceeds 1%, the alloy cost is increased. Therefore, the Ni content is set to a range of from 0.1% to 1.0%.
  • the critical cooling rate Vc90 (° C./s) conventionally known by “TETSU-TO-HAGANE” 74 (1998), P. 1073, may be used for example.
  • This is an index represented by the following Equation (1), and means the cooling rate at which the volume ratio of martensite is 90% or more. Therefore, hardenability is higher as Vc90 is lower, and the martensite structure can be obtained even when the cooling rate is reduced.
  • the inventors produced electric resistance-welded steel pipes with various compositions and examined the relationship between Vc90 and the hardness after quenching. As a result, it was found that, in a case in which Vc90 is 40° C./s or less, the martensite structure can be surely formed up to the inside by water quenching. For this reason, the upper limit of Vc90 is limited to 40° C./s in the invention.
  • the inventors further examined the relationship between the cooling rate and the Rockwell C hardness at a center part in a thickness direction using the electric resistance-welded steel pipe of steel No. 1 containing 0.30% of C, 0.30% of Si, and 0.35% of Cr and having a Vc90 of 27.1° C./s as shown in Table 1.
  • the Rockwell C hardness (HRC) was measured in accordance with JIS Z 2245. As shown in FIG. 7 , in a case in which the cooling rate is 20° C./s or more, the hardness corresponding to that of the structure composed of 90% of martensite can be obtained. Since the cooling rate during water quenching is 20° C./s or more, the structure composed of 90% or more of martensite can be obtained by water quenching.
  • the metallic structure of the hollow stabilizer according to the invention is limited to a tempered martensite. This is because variation in the structure and hardness is suppressed, and the hardness is easily adjusted. In order to ensure the formation of the martensite structure up to the inside by quenching, Vc90 is set to 40° C./s or less to obtain the sufficient hardenability of the material. Whether the metallic structure of the hollow stabilizer is a tempered martensite or not can be determined by the observation under an optical microscope.
  • the metallic structure of the steel pipe for hollow stabilizers which is used as a material for the hollow stabilizer according to the invention, comprises a mixed structure of ferrite and perlite. Whether the metallic structure of the steel pipe for hollow stabilizers comprises a mixed structure of ferrite and perlite or not can be determined by the observation under an optical microscope. The hollow stabilizer is often produced by cold bending of the steel pipe. Therefore, in order to ensure sufficient workability, the Rockwell B hardness (HRB) is preferably 95 or less. In a case in which the metallic structure comprises a mixed structure of ferrite and perlite, workability can be ensured.
  • the Rockwell B hardness (HRB) of the steel pipe for hollow stabilizers can be measured in accordance with JIS Z 2245.
  • the depth of the ferrite decarburized layer on the inner surface of the steel pipe for hollow stabilizers according to the invention is set to 20 ⁇ m or less.
  • the depth of the decarburized layer on the inner surface of the hollow stabilizer after quenching can be reduced to less than 20 ⁇ m.
  • the depth of the ferrite decarburized layer is a maximum depth measured from the inner surface of a region at which only ferrite grains are arranged and no cementite is present in an L direction, when the metallic structure of the longitudinal section (L section) of the steel pipe is observed with an optical microscope.
  • the decarburized layer on the inner surface of the steel pipe for hollow stabilizers is, for example, formed in the dual phase range during cooling to the room temperature after subjecting the electric resistance-welded steel pipe to diameter-reduction hot rolling.
  • the decarburized layer is formed easily on the inner surface of the steel pipe for hollow stabilizers when the steel pipe passes through the dual phase range during cooling from a high temperature at which the metallic structure is composed of an austenite single phase.
  • the decarburized layer has a metallic structure composed of ferrite, since the content of C, which is an element for stabilizing austenite, is reduced therein. In order to suppress the formation of the decarburized layer on the inner surface of the steel pipe for hollow stabilizers, it is preferable to shorten the time required for passing through the dual phase range.
  • the depth of the decarburized layer generated at the inner surface of the steel pipe for a hollow stabilizer can be reduced to 20 ⁇ m or less by, for example, supplying water to the outer surface of the steel pipe obtained by subjecting the electric resistance-welded steel pipe to diameter-reduction hot rolling and adjusting the cooling rate during passage through the dual phase range to 5° C./s or more. While the water for cooling may be supplied only to the outer surface of the steel pipe for hollow stabilizers, it is possible to supply water to the inner surface in addition to the outer surface. By increasing the cooling rate of the inner surface of the steel pipe for hollow stabilizers, the depth of the decarburized layer can be further reduced.
  • a molten steel, smelted to provide a required chemical composition is casted as a slab, or the molten steel is formed as an ingot and subsequently processed into a billet by hot rolling, and the slab or the billet are then subjected to hot rolling to obtain a hot rolled steel sheet.
  • the hot rolled steel sheet is processed into an electric resistance-welded steel pipe by a method of producing the conventional electric resistance-welded steel pipe, for example, hot or cold electric resistance-welding or high-frequency induction heating.
  • the obtained electric resistance-welded steel pipe may be further subjected to diameter-reduction hot rolling to produce a thick walled steel pipe.
  • the diameter-reduction rolling may be conducted using a stretch reducer.
  • the stretch reducer is a rolling machine provided with a plurality of rolling stands arranged in series with the rolling axis, the plurality of rolling stands each having 3 or 4 rolls disposed around the rolling axis.
  • the tension in the pipe axis direction (rolling direction) of the steel pipe and the compression force in the circumferential direction of the steel pipe are controlled by adjusting the number of revolutions of the rolls and the rolling force in each of the rolling stands of the rolling machine, whereby the diameter-reduction rolling for increasing the wall thickness/outer diameter ratio can be achieved.
  • the outer diameter is reduced by the rolling force with respect to the outer diameter of the steel pipe and the wall thickness is increased at the same time.
  • the wall thickness is reduced by tension acting in the pipe axis direction of the steel pipe. Therefore, the final wall thickness is determined by the balance therebetween. Since the wall thickness of the steel pipe obtained by diameter-reduction rolling is mainly determined in accordance with tension between the rolling stands, it is necessary to calculate tension between the rolling stands for obtaining the desired wall thickness based on the rolling theory or the like, and to set the number of revolutions of a roll in each of the rolling stands for exerting the tension.
  • the diameter-reduction rolling is preferably conducted at a reduction in cross sectional area of from 40% to 80% using the electric resistance-welded steel pipe heated to a temperature of from 800° C. to 1200° C.
  • the steel pipe for hollow stabilizers is preferably an electric resistance-welded steel pipe obtained by diameter-reduction hot rolling, but not limited thereto.
  • the steel pipe for hollow stabilizers may be an electric resistance-welded steel pipe as electric resistance-welded state or a drawn pipe obtained by cold drawing after electric resistance-welding.
  • a steel pipe e.g., an electric resistance-welded steel pipe, a seamless pipe, a pipe obtained by diameter-reduction hot rolling, or a drawn pipe thereof
  • bend forming (bend forming process) to provide the intended shape shown in FIG. 2 .
  • the bend formed steel pipe is heated to an austenitizing temperature region (heating process) by furnace heating or electric heating, or using a high-frequency heater, and then subjected to quenching (quenching process) in water (or another quenching medium).
  • the shape of the heat-deformed stabilizer bar is corrected to an intended stabilizer shape (shape correction process), and is subjected to tempering (tempering process).
  • the tempered pipe is subjected to shot peening (shot peening process) with respect to only the outer surface, or the outer and inner surfaces, and then coated using an intended coating material (coating process).
  • the shape correction shape correction process may be omitted if restrained quenching is performed.
  • a steel pipe e.g., an electric resistance-welded steel pipe, a seamless pipe, a pipe obtained by diameter-reduction hot rolling, or a drawn pipe thereof
  • a steel pipe cut to a predetermined length is heated to an austenitizing temperature region (heating process) by furnace heating or electric heating, or using a high-frequency heater, and then subjected to bend forming (bend forming process) to provide the intended shape shown in FIG. 2 .
  • the bend formed steel pipe is then subjected to quenching (quenching process) in water (or another quenching medium).
  • the shape of the heat-deformed stabilizer bar is corrected to an intended stabilizer shape (shape correction process), and is subjected to tempering (tempering process).
  • the tempered pipe is subjected to shot peening (shot peening process) with respect to only the outer surface, or the outer and inner surfaces, and then coated using an intended coating material (coating process).
  • the shape correction shape correction process may be omitted if restrained quenching is performed.
  • the heating temperature is preferably 900° C. or higher, and more preferably 950° C. or higher.
  • the tempering temperature is determined based on the temper-softening curve of the steel pipe for hollow stabilizers.
  • FIG. 10 shows the temper-softening curve of the electric resistance-welded steel pipe of steel No. 1 containing 0.30% of C, 0.30% of Si, and 0.35% of Cr and having a Vc90 of 27.1° C. as shown in Table 1.
  • the tempering temperature at which the Rockwell C hardness (HRC) of from 40 to 50 is obtained can be determined based on the temper-softening curve shown in FIG. 9 .
  • Each of steels having the compositions shown in Table 1 was melted and cast into a slab.
  • the slab was then heated to 1200° C. and hot-rolled into a steel sheet of 5 mm in thickness at a hot finishing temperature of 890° C. and a coiling temperature of 630° C.
  • the obtained steel sheet was slit to a predetermined width, roll-formed into a tubular shape, and then subjected to high frequency electric resistance welding to produce a steel pipe of 90 mm in outer diameter.
  • the obtained electric resistance welded steel pipe was subsequently heated to 980° C.
  • the metallic structure of the obtained steel pipes for hollow stabilizers were observed under an optical microscope, and it was confirmed that all of the obtained steel pipes have a metallic structure composed of a mixed structure of ferrite and perlite with depth of a decarburized layer at the inner surface of 15 ⁇ m or less.
  • the Rockwell hardness was determined in accordance with JIS Z 2245, and as a result of which the Rockwell hardness of all of the steel pipes for hollow stabilizers found to have a Rockwell B hardness (HRB) of 95 or less.
  • MnS having a length exceeding 150 ⁇ m was confirmed using an optical microscope, an SEM, and an EDS in combination, and as a result of which MnS having a length exceeding 150 ⁇ m was present in Comparative Examples of Nos. H to K as shown in Table 2.
  • the obtained steel pipes for hollow stabilizers were cut at the electric resistance-welded portion or at a position located at 180° opposite side of the electric resistance-welded portion as shown in FIG. 3( a ) and cold developed, thereby obtaining a plate-like piece as shown in FIG. 3( b ). Furthermore, the steel pipes for hollow stabilizers were heated at 950° C. for 10 minutes and water-quenched, and then tempered at different temperature. The Rockwell C hardness (HRC) of the resultant was determined in accordance with JIS Z 2245 to obtain a temper-softening curve.
  • HRC Rockwell C hardness
  • the plate-like piece obtained by cutting the steel pipe for a hollow stabilizer at a position located at a 180° opposite side of the electric resistance-welded portion and spread out as shown in FIGS. 3( a ) and 3 ( b ) was processed into a plane bending fatigue test specimen in which the electric resistance-welded portion is located at a central position in a longitudinal direction as shown in FIGS. 4( a ) and 4 ( b ).
  • the plate-like piece obtained by cutting the steel pipe for hollow stabilizers at the electric resistance-welded portion and developing was processed into a plane bending fatigue test specimen in which the base metal is located at a central position in a longitudinal direction.
  • the thickness (ta) was set to 3 mm and the width (Wa) was set to 15 mm.
  • Each of the specimens was tempered to impart a Rockwell C hardness (HRC) of 40 based on the temper-softening curve, and subjected to a plane bending fatigue test with a fatigue limit of 5 million cycles.
  • HRC Rockwell C hardness
  • Table 2 the holding time for tempering was set to 30 minutes.
  • each of the steel pipes of Nos. A to K of the examples of the invention has good fatigue properties at the electric resistance-welded portion since the difference in fatigue limit between the base metal and the electric resistance-welded portion is as small as 15 MPa or less.
  • each of the steel pipes of Nos. L to O of the comparative examples has significantly deteriorated fatigue properties at the electric resistance-welded portion compared to the base metal since the difference in fatigue limit between the base metal and the electric resistance-welded portion is as large as 140 MPa or more.
  • the fracture surface of the specimen after the plane bending fatigue test was observed under an SEM, and the composition of the inclusion present at the fracture origin of fatigue fracture was analyzed using an EDS attached to the SEM. As a result, the presence of MnS at the fracture origin was confirmed in comparative examples L to O in which the electric resistance-welded portion is located at a central position in a longitudinal direction.
  • Hot rolling 40 480 465 15 Example of B Hot rolling 40 480 465 15 Invention C Hot rolling 40 480 465 15 D Hot rolling 40 480 465 15 E Hot rolling 40 480 465 15 F Hot rolling 40 480 465 15 G Hot rolling 40 480 465 15 H Cold drawing 40 480 465 15 I Hot rolling 40 480 470 10 J Hot rolling 40 480 465 15 K Hot rolling 40 480 465 15 L Hot rolling 40 485 310 175 Comparative M Hot rolling 40 480 340 140 Example N Hot rolling 40 485 300 185 O Hot rolling 40 480 300 180 P Hot rolling 40 480 465 15 Q Hot rolling 40 480 465 15 R Hot rolling 40 480 465 15 Hot rolling: diameter reduction hot rolling
  • the underline in the table means that the value is outside the range of the invention.
  • a 1000 mm-length segment was cut out from each of the steel pipes A to R produced in Example 1, and cold bended to a 90° angle in a position at 200 mm from the each pipe end, thereby forming to have a U-shape.
  • the U-shape pipe was formed such that the electric resistance-welded portion was located at the part that can be seen when viewed from the direction giving a U-shaped appearance, that is, along the side surface of the U-shape pipe, thereby obtaining respective Test Materials a to v.
  • the Test Materials a to n (but not Test Material c) as examples of the invention were heated to 950° C. for 10 minutes and subjected to water quenching, and then tempered at 200° C.
  • Test Material c was heated to 950° C. for 10 minutes and subjected to water quenching, and then tempered at 350° C. for 30 minutes, thereby adjusting the HRC thereof to 43. Furthermore, with respect to Test Materials a to v, after heating, the outer surface of each Test Material was subjected to shot peening to impart a compressive residual stress on 450 MPa. In addition, with respect to Test Materials b and f, the inner surface thereof was also subjected to shot peening to have compressive residual stress on 450 MPa.
  • Test Materials r and s were produced using, as a material, the comparative steel (Steel 12 in Table 1) having a typical chemical composition for conventionally used hollow stabilizers, and thus contained low levels of C in the steel and could not achieve an HRC of 49. Therefore, Test Material r was adjusted to have an HRC of 47, which is the upper limit of achievable hardness, and Test Material s was adjusted to have an HRC of 40, which is the upper limit of hardness in practical use.
  • test materials using as a material Steel 1, which is an adapted steel shown in Table 1 were prepared: Test Material o in which the depth of the decarburized layer at the inner surface part is 25 ⁇ m; Test Material p in which the wall thickness/outer diameter ratio is 0.13; and Test Material q having a bainite structure with a HRC of 35 produced with a cooling rate during quenching of 15° C./s, which is slower compared to the case of water quenching.
  • Test Materials a to v were fixed at a central position in a longitudinal direction, and fatigue endurance was tested up to 1 million cycles by vibrating respective ends thereof in the opposite direction under a condition of a maximum main stress on the outer surface of the bending portion of 500 MPa. Each test was conducted with respect to 20 samples for each Test Material. The observation of the metallic structure of Test Materials and the measurement of the depth of the decarburized layer at the inner surface part were carried out using an optical microscope. The Rockwell hardness was measured in accordance with JIS Z 2245. The presence of MnS having a length exceeding 150 ⁇ m was confirmed using an SEM and an EDS in combination.
  • Test Material o in which the depth of the decarburized layer at the inner surface part exceeds 20 ⁇ m and Test Material p in which t/D is as small as 0.13, the number of samples exhibiting inner surface breakage is large, and the number of cycles to fatigue was not always reached to half-million, which did not meet a standard for fatigue endurance.
  • Test Material q having a bainite structure with an HRC as low as 35, the number of cycles to fatigue was significantly low.
  • a hollow stabilizer for automobiles having an excellent fatigue endurance and higher strength compared to the conventional ones, while maintaining fatigue properties and delayed fracture properties equivalent to those of the conventional hollow stabilizers for automobiles.
  • Such a hollow stabilizer can contribute considerably to weight reduction in automobiles.

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CN104395487A (zh) 2015-03-04
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CN104395487B (zh) 2017-02-22
EP2857537A4 (en) 2016-04-13
MX358844B (es) 2018-09-05
MX2014014205A (es) 2015-02-12
ES2691085T3 (es) 2018-11-23
KR101706839B1 (ko) 2017-02-14
EP2857537B1 (en) 2018-07-25
HUE040155T2 (hu) 2019-02-28
JP6225026B2 (ja) 2017-11-01
BR112014028915B1 (pt) 2019-05-14
WO2013175821A1 (ja) 2013-11-28
EP2857537A1 (en) 2015-04-08
BR112014028915A2 (pt) 2017-06-27

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