US20200123625A1 - Steel wire rod, steel wire, and part - Google Patents

Steel wire rod, steel wire, and part Download PDF

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Publication number
US20200123625A1
US20200123625A1 US16/314,122 US201716314122A US2020123625A1 US 20200123625 A1 US20200123625 A1 US 20200123625A1 US 201716314122 A US201716314122 A US 201716314122A US 2020123625 A1 US2020123625 A1 US 2020123625A1
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steel wire
bainite
mean
less
block size
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Makoto Okonogi
Naoki Matsui
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • 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
    • 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
    • C22C38/02—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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
    • C21D2211/003—Cementite

Definitions

  • the present invention relates to steel wire rod, steel wire manufactured by that steel wire rod, and parts manufactured by that steel wire and having a tensile strength of 700 MPa to 1200 MPa.
  • the “parts” covered by the present invention include machine parts and building parts.
  • Such a steel material contains large amounts of alloy elements, so the steel material becomes high in cost. Further, it requires spheroidizing annealing before formation of a part and quenching and tempering after forming it, so the manufacturing cost swells.
  • Japanese Unexamined Patent Publication No. 2-166229 discloses a method of production of a non-heat treated machine part of a bainite structure comprising rolling steel containing C: 0.03 to 0.20%, Si: 0.10% or less, Mn: 0.7 to 2.5%, a total of one or more of V, Nb, and Ti: 0.05 to 0.30%, and B: 0.0005 to 0.0050% to a steel wire rod, then cooling it by a 5° C./sec or more cooling rate.
  • Japanese Unexamined Patent Publication No. 8-41537 discloses a method of production of a high strength machine part comprising heating steel containing C: 0.05 to 0.20%, Si: 0.01 to 1.0%, Mn: 1.0 to 2.0%, S: 0.015% or less, Al: 0.01 to 0.05%, and V: 0.05 to 0.3% to 900 to 1150° C. in temperature, then hot rolling and finish rolling it, then cooling in a temperature region of 800° C. to 500° C. by a 2° C./sec or more mean cooling rate to thereby obtain a ferrite+bainite structure, then annealing it in a 550 to 700 temperature range.
  • Japanese Unexamined Patent Publication No. 2000-144306 discloses steel for cold forging use containing C: 0.40 to 1.0 mass %, having a chemical composition satisfying a specific condition, and having a structure comprised or pearlite or degenerate pearlite.
  • This steel contains a large amount of C and therefore is inferior in cold forgeability compared with the carbon steel for machine structure use or alloy steel for machine structure use which is used for machine parts in the past.
  • the inventors surveyed the relationship between the chemical composition and structure of steel materials for obtaining high strength parts with a tensile strength of 700 MPa or more for achieving the above object which enables cold forging even if eliminating spheroidizing annealing and even if not performing heat treatment of quenching and tempering.
  • the present invention was made based on the metallurgical findings obtained by such a survey and has as its gist the following:
  • Steel wire rod comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities, in which steel wire rod, 90% or more of the microstructure by area ratio is bainite, a mean bainite block size of the surface layer measured at the cross section is 15 ⁇ m or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size at center part) is less than 1.0, and a mean particle size of cementite dispersed in bainite is 0.1 ⁇ m or less.
  • Steel wire comprised of, by mass %, C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.060%, Ti: 0.005 to 0.030%, B: 0.0003 to 0.0050%, and N: 0.001 to 0.010% and having a balance of Fe and unavoidable impurities and obtained by drawing, in which steel wire, 90% or more of the microstructure by area ratio is bainite, at the surface layer of the steel wire, a mean aspect ratio R of block particles of bainite measured at the longitudinal section is 1.2 to 2.0, a mean bainite block size of the surface layer measured at the cross section is (15/R) ⁇ m or less, a value of a ratio of the mean bainite block size of the surface layer measured at the cross section and the mean bainite block size measured at the center part (mean bainite block size of surface layer)/(mean bainite block size
  • FIG. 1 is an SEM photograph showing the microstructure of a steel wire rod and drawn steel wire according to the present embodiment and parts according to the present embodiment.
  • the inventors as explained above, surveyed in detail the relationship between the chemical composition and structure of steel materials for obtaining high strength parts with a tensile strength of over 700 MPa which enables cold forging even if eliminating spheroidizing annealing and even if not performing heat treatment of quenching and tempering. Further, the inventors proceeded with a comprehensive study of in-line heat treatment utilizing the heat held at the time of hot rolling the steel wire rod and the series of methods of production of the subsequent drawn steel wire and parts based on the metallurgical findings obtained by the survey so as to inexpensively produce high strength parts and reached the following conclusions:
  • Such drawn steel wire forming the material for obtaining a part enabling cold forging even if eliminating the spheroidizing annealing and becoming high strength even without performing the heat treatment of quenching and tempering already has the above characterizing microstructures at the stage of the drawn steel wire. It is effective to work this into a part without performing heat treatment before working.
  • the present invention is more advantageous.
  • the method of production of the steel wire rod forming the material of the drawn steel wire it is possible to utilize the residual heat at the time of hot rolling to immerse the steel wire rod in a molten salt bath immediately after rolling to thereby obtain a steel material of the above-mentioned structure even without adding large amounts of alloy elements.
  • the parts of the present invention are manufactured by a series of methods of production of immersing a steel material adjusted in chemical composition in a molten salt bath utilizing the residual heat at the time of hot rolling so as to form a mainly bainite steel wire rod of a predetermined mean block size and cementite particle size, drawing this at room temperature under specific conditions to prepare high strength bainite, and forming parts.
  • the present invention enables the inexpensive manufacture of parts with a tensile strength of 700 to 1200 MPa.
  • the chemical composition of the steel wire rod and drawn steel wire for part use with a tensile strength of 700 to 1200 MPa according to the present embodiment (below, sometimes respectively referred to as the “steel wire rod” and “drawn steel wire”) and the parts according to the present embodiment (below, sometimes simply referred to as the “parts”) will be explained.
  • the drawn steel wire according to the present embodiment is obtained by drawing the steel wire rod according to the present embodiment.
  • the part according to the present embodiment is obtained by cold forging the drawn steel wire according to the present embodiment or by cold forging and rolling.
  • the drawing, cold forging, and rolling do not affect the chemical composition of the steel. Therefore, the explanation regarding the chemical composition explained below covers all of steel wire rod, drawn steel wire, and parts.
  • “%” means “mass %”. Note that, the balance of the chemical composition is Fe and unavoidable impurities.
  • C 0.15 to 0.30%
  • C is an element required for securing tensile strength. If the C content is less than 0.15%, it is difficult to obtain a 700 MPa or more tensile strength. Preferably, the C content is 0.20% or more. On the other hand, if the C content is over 0.30%, the cold forgeability deteriorates. Preferably, it is 0.25% or less.
  • Si is a deoxidizing element and an element increasing the tensile strength by solid solution strengthening. If the Si content is less than 0.05%, the effect of addition is not sufficiently exhibited. Preferably, the Si content is 0.15% or more. On the other hand, if the Si content is over 0.50%, the effect of addition becomes saturated and the ductility at the time of hot rolling deteriorates resulting in defects easily forming. The preferable Si content is 0.30% or less.
  • Mn is an element raising the tensile strength of steel. If the Mn content is less than 0.50%, the effect of addition is not sufficiently exhibited. Preferably, the Mn content is 0.70% or more. On the other hand, if the Mn content is over 1.50%, the effect of addition becomes saturated, the time for completion of transformation at the time of isothermal transformation of the steel wire rod becomes longer, and the manufacturability deteriorates.
  • the preferable Mn content is 1.30% or less.
  • P is an element segregating at the crystal grain boundaries thereby causing deterioration of the cold formability. If the P content is over 0.030%, the deterioration of the cold formability becomes remarkable.
  • the preferable P content is 0.015% or less.
  • the steel wire rod, drawn steel wire, and parts according to the present embodiment do not have to contain P, so the lower limit of the P content is 0%.
  • S is an element segregating at the crystal grain boundaries thereby causing deterioration of the cold formability. If the S content is over 0.030%, the deterioration of the cold formability becomes remarkable.
  • the preferable S content is 0.015% or less, while the more preferable one is 0.010% or less.
  • the steel wire rod, drawn steel wire, and parts according to the present embodiment do not have to contain S, so the lower limit of the S content is 0%.
  • Al is a deoxidizing element. Further, it is an element which forms AlN functioning as pinning particles. AlN refines the crystal grains and due to this raises the cold formability. Further, Al is an element having the action of reducing the solute N to suppress dynamic strain aging. If the Al content is less than 0.005%, the above-mentioned effect cannot be obtained.
  • the preferable Al content is 0.020% or more. If the Al content is over 0.060%, the above-mentioned effect becomes saturated and defects easily form at the time of hot rolling.
  • the preferable Al content is 0.050% or less.
  • Ti is a deoxidizing element. Further, it is an element which forms TiN and has the action of reducing the solute N to suppress dynamic strain aging. If the Ti content is less than 0.005%, the above-mentioned effect cannot be obtained.
  • the preferable Ti content is 0.010% or more. If the Ti content is over 0.030%, the above-mentioned effect becomes saturated and defects easily form at the time of hot rolling. The preferable Ti content is 0.025% or less.
  • B suppresses intergranular ferrite and has the effect of improving cold formability and the effect of promoting bainite transformation and improving strength. If less than 0.0003%, the effect is not sufficient, while if over 0.0050%, the effect becomes saturated.
  • N is an element sometimes causing cold formability to deteriorate due to dynamic strain aging. To avoid such a detrimental effect, the N content is made 0.0100% or less. Further, N forms AlN or TiN to refine the crystal grain size and has the effect of improving the cold formability. For this reason, the lower limit was made 0.0010%. The preferable content of N is 0.0020 to 0.0040%.
  • one or two of Cr: 0.01 to 0.40%, Nb: 0 to 0.03%, and V: 0 to 0.10% may be included.
  • the inclusion of Cr, Nb, and V is optional. 0% is also possible.
  • Cr has the effect of increasing the tensile strength of the steel.
  • Nb and V have the effect of reducing the solute N to suppress dynamic strain aging and the effect of promoting bainite transformation to increase the strength.
  • Cr is an element increasing the tensile strength of the steel. If the Cr content is less than 0.01%, the above-mentioned effect is not sufficiently obtained. On the other hand, if the Cr content is over 0.40%, martensite easily forms. Due to this, the drawability and the cold forgeability deteriorate.
  • the preferable content of Cr is 0.03 to 0.30%.
  • Nb is an element which forms NbN and has the action of reducing the solute N to suppress dynamic strain aging. If the Nb content is over 0.03%, the above-mentioned effect becomes saturated and defects easily form at the time of hot rolling.
  • the Nb content is preferably 0.025% or less.
  • V is an element which forms VN and has the action of reducing the solute N to suppress dynamic strain aging. If the V content is over 0.10%, the above-mentioned effect becomes saturated and defects easily form at the time of hot rolling.
  • the preferable V content is 0.05% or less.
  • O is present as an oxide of Al and Ti etc. in the steel wire rod, drawn steel wire, and parts (for example, machine parts). If the O content is over 0.0030%, coarse oxides are formed in the steel resulting in fatigue fracture easily occurring.
  • the preferable O content is 0.0020% or less.
  • the lower limit value of the O content is 0%.
  • unavoidable impurities mean constituents contained in the raw materials or entering in the manufacturing process and not intentionally included in the steel. Further, “unavoidable impurities” specifically include for example Sb, Sn, W, Co, As, Mg, Pb, Bi, and H.
  • Sb, Sn, W, Co, As, Mg, Pb, Bi, and H respectively are allowed to be contained in amounts of up to 0.010%, 0.10%, 0.50%, 0.50%, 0.010%, 0.010%, 0.10%, 0.10%, and 0.0010% for realizing the effects of the present application.
  • the drawn steel wire according to the present embodiment is obtained by drawing the steel wire rod according to the present embodiment.
  • the parts according to the present embodiment are obtained by cold forging or by cold forging and rolling the drawn steel wire according to the present embodiment.
  • the effect of cold forging and rolling on the microstructure of the parts is small. This is because the amount of work on the parts by the cold forging and rolling is small.
  • the effect of drawing, cold forging, and rolling on the bainite area ratio of the microstructure is small, so the following explanation applies to all of the steel wire rod, drawn steel wire, and parts.
  • the microstructure of the steel wire rod, drawn steel wire, and parts according to the present embodiment includes an area ratio of 90% or more of bainite.
  • 1 refers to a structure in which it is deemed needle-shaped or particle-shaped cementite is dispersed if etching a cross section of an object (steel wire rod, drawn steel wire, or parts) (cross-section perpendicular to axis of steel material (drawn steel wire)) by Nital, then photographing a position of a predetermined depth (for example, depth from surface layer of 0.25 time diameter) from the surface layer of the object by a scan type electron microscope (SEM).
  • SEM scan type electron microscope
  • the bainite area ratio of steel wire rod, drawn steel wire, and parts is determined by the following procedure. That is, first, the cross section of the object is etched by Nital to bring out the structure. Next, if the diameter of the object is D, a total of nine locations are specified: four locations determined by rotation every 90° about the longitudinal direction axis of the object at a depth position of a depth from the surface layer of the object of 50 ⁇ m, four locations determined by rotation every 90° about the above axis at a depth position of a depth from the surface layer of the object of 0.25D, and one location determined at the center part of the above axis (depth position of a depth from the surface layer of 0.5D).
  • these nine locations are photographed for structure by a power of 1000 ⁇ using an SEM.
  • the nonbainite structures in the structural photographs (ferrite, pearlite, and martensite structures) are visually marked and the regions of the structures are found by image analysis.
  • the region containing the bainite is found by subtracting the nonbainite regions from the overall field of observation.
  • the area ratio of this region is made the area ratio of the bainite. Note that, this operation measures and calculates the values for at least two samples, finds the mean value of the same, and uses the mean value as the bainite area ratio in the present embodiment.
  • the KAM method is the method of performing a calculation averaging the difference in orientation between pixels in first approximation of six adjoining pixels of a regular hexagonal shape in the measurement data, second approximation of the 12 at the outside of that, or third approximation of the 18 at the further outside of that and using this value as the value of the center pixel for the different pixels. This calculation is performed so that the grain boundaries are not exceeded and thereby a map expressing a change in orientation in the particles can be prepared.
  • Bainite is larger is dislocation density and larger in strain in the particles compared with the polygonal pro-eutectoid ferrite transformed at a high temperature, so the difference in particles in crystal orientation is large. Therefore, in the analysis in the present embodiment, the conditions for calculation of the difference in orientation between adjoining pixels are made third approximation, particles with that difference in orientation of 5° or less are displayed, and particles with a difference in orientation among these of over 1° are defined as bainite.
  • the area ratio of bainite of the steel wire rod is less than 90%, the area ratio of bainite of the drawn steel wire obtained by drawing this steel wire rod and parts obtained by cold forging the drawn steel wire becomes less than 90%.
  • the permanent elongation at the time of use as a machine part deteriorates.
  • the drawn steel wire preferably does not contain any microstructures other than bainite, so the upper limit value of the area ratio of bainite of the drawn steel wire is 100%.
  • the mean block size of the bainite measured at the cross section is 15 ⁇ m or less.
  • the “cross section” means the plane perpendicular to the axial direction of the steel wire rod. If the mean bainite block size measured at the cross section of the steel wire rod is over 15 ⁇ m, the ductility of the drawn drawn steel wire falls and due to this the cold formability of the drawn steel wire falls. Furthermore, the mean block size of the bainite of the part obtained by cold working this drawn steel wire becomes coarse. If the mean block size of the bainite becomes coarser, the yield ratio falls. Note that, the mean block size of the bainite of the steel wire rod is preferably smaller, so there is no need to prescribe the lower limit value.
  • the mean aspect ratio R of the block particles of bainite measured at the longitudinal section of the drawn steel wire is 1.2 to 2.0.
  • the “longitudinal section” means the plane parallel to the axial direction of the steel wire rod and including the center axis. If the mean aspect ratio of the bainite block is less than 1.2, the hydrogen embrittlement resistance of parts manufactured by cold forging the drawn steel wire deteriorates. Further, if the mean aspect ratio exceeds 2.0, the yield ratio falls and the permanent elongation when used as parts deteriorates.
  • the mean aspect ratio R of the block particles of bainite of the drawn steel wire and parts is determined as follows.
  • the bainite block grain boundary is determined using EBSD for the longitudinal section of the drawn steel wire.
  • the crystal orientations of bcc-Fe at the measurement points in the regions are measured in measurement steps of 0.3 ⁇ m and a boundary with a difference in orientation of 15 degrees or more is defined as a “bainite block boundary”.
  • the region surrounded by this boundary is defined as a “bainite block grain”.
  • a map of bainite block grains at a total of two regions at the two sides of one longitudinal section is obtained. This is performed for four samples to obtain a map of bainite block grains at a total of eight regions. 10 bainite block grains are selected in order from the one with the largest circle equivalent diameter from the map of bainite block grains obtained. The selected 10 bainite block grains are measured for aspect ratio of the block grains. Finally, the mean value of these is made the mean aspect ratio R of the block grains of the bainite.
  • the mean block size of the bainite of the surface layer measured at the cross section is (15/R) ⁇ m or less.
  • the “cross section” means the plane perpendicular to the axial direction of the drawn steel wire. If the mean bainite block size of the surface layer of the drawn steel wire measured at the cross section is over (15/R) ⁇ m, the ductility of the drawn steel wire falls and due to this the cold formability of the drawn steel wire falls. Furthermore, the mean block size of the bainite of the parts obtained by cold working this drawn steel wire becomes coarser and the yield strength falls. Note that, the mean block size of the bainite at the surface layer part of the drawn steel wire is preferably small, so the lower limit value does not have to be prescribed.
  • the mean bainite block size at the surface layer of the steel wire rod (similar for drawn steel wire and parts) is determined as follows: First, at the cross section of the steel wire rod, the region extending 500 ⁇ m in the peripheral direction from the surface layer in the center axial direction by 500 ⁇ m width is determined. Four regions rotated every 90° about the center axis in this region are specified. Further, the block particle sizes of these four regions measured by an EBSD apparatus are averaged to obtain the mean bainite block size at the surface layer of the steel wire rod (similar for drawn steel wire and parts).
  • the mean bainite block size of the surface layer measured at the cross section is (15/R) ⁇ m or less.
  • the “cross section” means the plane perpendicular to the axial direction of the parts. If the mean bainite block size of the surface layer of the parts measured at the cross section is over (15/R) ⁇ m, the yield ratio falls.
  • the mean bainite block size of the surface layer of the drawn steel wire is preferably small, so there is no need to prescribe the lower limit value.
  • the method of determination of the mean bainite block size of the parts is the same as the above-mentioned method of determination of the mean bainite block size of the steel wire rod.
  • the ratio of the mean bainite block size of the surface layer measured at the cross section and the mean bainite block size of the center part measured at the cross section is less than 1.0. If the ratio is over 1.0, the cold forgeability of the drawn steel wire deteriorates and the yield ratio of the parts deteriorates.
  • the mean block size of the bainite at the center part of the steel wire rod is determined as follows: First, in the cross section of the steel wire rod, a region of 500 ⁇ m ⁇ 500 ⁇ m centered about the center axis is determined. The block particle size of this region is measured by an EBSD apparatus. Next, three different cross sections are similarly measured, then the block particle sizes of the four samples are averaged to obtain the mean block size of the bainite at the center part of the steel wire rod (similar for drawn steel wire and parts).
  • the ratio of the block particle size of the surface layer and the block particle size of the center part is found by (mean bainite block size of surface layer)/(mean bainite block size at center part).
  • the mean particle size of cementite dispersed in the bainite is 0.1 ⁇ m or less. If the mean particle size of the cementite exceeds 0.1 ⁇ m, the cold forgeability of the drawn steel wire deteriorates. Furthermore, the yield ratio of the parts fall and the permanent elongation when used as for example a machine part deteriorates.
  • the mean particle size of cementite in the bainite according to the present embodiment is determined by the following procedure. First, picral is used to etch the cross section of the object (steel wire rod, drawn steel wire, or parts) to bring out the structure. Next, if the diameter of the object is D, a total of nine locations are specified: four locations determined by rotation every 90° about the longitudinal direction axis of the object at a depth position of a depth from the surface layer of the object of 50 ⁇ m, four locations determined by rotation every 90° about the above axis at a depth position of a depth from the surface layer of the object of 0.25D, and one location determined at the center part of the above axis (depth position of a depth from the surface layer of 0.5D).
  • the critical upsetting rate is used as an indicator showing the cold formability.
  • the “critical upsetting rate” means the maximum compression rate where no cracking occurs when preparing a sample with a height of 1.5 times the diameter from drawn steel wire by machining and compressing the end face of the sample in the axial direction using a die formed with grooves concentrically.
  • the “compression rate” is the value shown by ((H ⁇ H1)/H) ⁇ 100 where the height before compression in the drawing (axial direction dimension) is “H” and the height after compression in the drawing (axial direction dimension) is “H1”.
  • the critical upsetting rate can be made 80% or more and an excellent cold formability can be realized.
  • This steel slab is heated to 1000 to 1150° C., then hot rolled by a finish rolling temperature of 800 to 950° C. to thereby obtain a steel wire rod.
  • this 800 to 950° C. steel wire rod is cooled by a mean cooling rate of 40° C./s or more down to 600° C., then is cooled by a mean cooling rate of 25° C./s or more down to 480° C.
  • this steel wire rod is held thermostatically in a 400 to 480° C. temperature zone for 15 seconds or more (first isothermal holding) and further is held thermostatically in a 530 to 600° C. temperature zone for 25 seconds or more (second isothermal holding). Further, after that, the material is water cooled to obtain a steel wire rod.
  • the two-stage cooling after finish rolling and the first isothermal holding are performed by immersing the steel wire rod in 400 to 480° C. molten salt inside a first molten salt tank. Further, the second isothermal holding is performed by immersing the steel wire rod in 530 to 600° C. molten salt inside a second molten salt tank.
  • the cooling of the steel wire rod from 800 to 950° C. is divided into two stages of cooling down to 600° C. and cooling from 600° C. down to 480° C.
  • the cooling rate can be made 25° C./s or more to thereby control the mean block size of the bainite to 15 ⁇ m or less.
  • the molten salt bath temperature in the first molten salt tank is made 400 to 480° C. and the immersion time is made 15 to 50 seconds.
  • the molten salt bath temperature 400° C. or more the entry of martensite is suppressed and excellent cold forgeability is obtained.
  • the 480° C. or less it is possible to reduce the mean particle size of the cementite to obtain excellent cold forgeability and eliminate bluing.
  • the immersion time 15 seconds or more the entry of nonbainite structures is suppressed and excellent cold forgeability is obtained.
  • it 50 seconds or less it is possible to reduce the mean particle size of the cementite to obtain excellent cold forgeability and eliminate bluing.
  • the molten salt bath temperature in the second molten salt tank is made 530 to 600° C. and the immersion time is made 25 to 80 seconds.
  • the molten salt bath temperature 530° C. or more the entry of martensite is suppressed and excellent cold forgeability is obtained.
  • the immersion time 25 seconds or more it is possible to reduce the mean particle size of the cementite to obtain excellent cold forgeability and eliminate bluing.
  • the immersion time 25 seconds or more the entry of martensite is suppressed and excellent cold forgeability is obtained.
  • it 80 seconds or less it is possible to reduce the mean particle size of the cementite to obtain excellent cold forgeability and eliminate bluing.
  • the drawn steel wire according to the present embodiment can as one example be manufactured by the following method. That is, the steel wire rod manufactured by the above-mentioned method is drawn by a total area reduction rates of 10 to 55%. The total area reduction rate 10 to 55% in the drawing operation may be reached by a single drawing operation or may be realized by a plurality of drawing operations. The drawn steel wire according to the present embodiment is obtained in this way.
  • the parts of the present embodiment can as one example be manufactured by the following method. That is, the above-mentioned drawn steel wire is worked into the shape of the individual parts by cold forging or cold forging and rolling to obtain parts with a tensile strength of 700 to 1200 MPa.
  • tensile test pieces were sampled from the shaft parts of the parts, subjected to tensile tests, and measured for tensile strength and 0.2% proof stress. Parts with a yield ratio (0.2% proof stress/tensile strength) of 0.90 or more were judged excellent in yield ratio. Note that, for each of steel materials, drawn steel wires, and parts, the Levels 1 to 7 and Levels 14 to 20 are invention examples while the Levels 8 to 13 and Levels 21 to 28 are comparative examples.
  • Level 10 is an example of not performing isothermal transformation after hot rolling, but immersing the sample in a boiling water tank for production.
  • Level 11 is an example of not performing isothermal transformation after hot rolling, but cooling by cooling air for production.
  • Level 13 is an example of cooling the hot rolled steel wire rod once down to room temperature, then reheating it to 1000° C. and immersing it in one molten salt tank for production.
  • Table 3 shows the results relating to the structure of the steel wire rod
  • Table 4 shows the results relating to the structure of the drawn steel wire
  • Table 5 shows the results for the cold forgeability of the drawn steel wire and the characteristics of the parts.
  • Levels 1 to 7 and Levels 14 to 20 (invention examples) where all of the manufacturing conditions prescribed in the present application are inside the prescribed ranges, in each case, a good result is obtained for the cold forgeability of the drawn steel wire and characteristics of the parts. That is, regarding Levels 1 to 7 and Levels 14 to 20, it will be understood that in each case, the tensile strength of the parts is 700 to 1200 MPa and that a yield ratio of 0.90 or more is obtained even without so-called bluing after forming the parts.
  • Levels 8 to 13 and Levels 21 to 28 (comparative examples) where at least one of the manufacturing conditions prescribed in the present application is outside the prescribed range, it will be understood that the result shows that at least one of the cold forgeability of the drawn steel wire and characteristics of the part is not excellent.
  • parts with a tensile strength of 700 to 1200 MPa which can be inexpensively manufactured are obtained.
  • drawn steel wire used for manufacture of the parts and enabling elimination of spheroidizing annealing, quenching and tempering, and bluing after cold forging and steel wire rod for manufacturing that drawn steel wire can be obtained. Therefore, the present invention has high applicability in the field of manufacturing of steel members, so is promising.

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CN111690801A (zh) * 2020-05-25 2020-09-22 中天钢铁集团有限公司 一种获得全贝氏体组织的合金工具钢盘条生产工艺
CN113416884A (zh) * 2021-06-07 2021-09-21 宁夏建龙龙祥钢铁有限公司 一种高延耐蚀钢筋生产方法

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JP2731797B2 (ja) * 1988-12-20 1998-03-25 トーア・スチール株式会社 非調質ボルト用鋼線材の製造方法
JP3737323B2 (ja) * 1999-09-17 2006-01-18 株式会社神戸製鋼所 球状化後の冷間鍛造性に優れた鋼線材・棒鋼およびその製造方法
JP4349732B2 (ja) * 2000-09-20 2009-10-21 Ｊｆｅ条鋼株式会社 溶接性および加工性に優れたばね用線材および鋼線
JP5092554B2 (ja) * 2007-01-17 2012-12-05 Ｊｆｅスチール株式会社 高強度鉄筋用鋼材の製造方法
WO2012161322A1 (ja) * 2011-05-26 2012-11-29 新日鐵住金株式会社 機械構造用鋼部品およびその製造方法
CN103906853B (zh) * 2011-08-26 2016-01-20 新日铁住金株式会社 非调质机械部件用线材、非调质机械部件用钢线和非调质机械部件及它们的制造方法
CN105612269B (zh) * 2013-10-08 2017-11-14 新日铁住金株式会社 线材、过共析贝氏体钢丝及它们的制造方法
WO2015133614A1 (ja) * 2014-03-06 2015-09-11 新日鐵住金株式会社 伸線加工性に優れた高炭素鋼線材とその製造方法
CN104018076B (zh) * 2014-06-25 2016-06-15 武汉钢铁(集团)公司 一种耐高温钢筋及生产方法

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CN111690801A (zh) * 2020-05-25 2020-09-22 中天钢铁集团有限公司 一种获得全贝氏体组织的合金工具钢盘条生产工艺
CN113416884A (zh) * 2021-06-07 2021-09-21 宁夏建龙龙祥钢铁有限公司 一种高延耐蚀钢筋生产方法

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JP6673478B2 (ja) 2020-03-25
KR102154575B1 (ko) 2020-09-10
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