EP2990499A1 - Fil machine et son procédé de fabrication - Google Patents

Fil machine et son procédé de fabrication Download PDF

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Publication number
EP2990499A1
EP2990499A1 EP14788178.3A EP14788178A EP2990499A1 EP 2990499 A1 EP2990499 A1 EP 2990499A1 EP 14788178 A EP14788178 A EP 14788178A EP 2990499 A1 EP2990499 A1 EP 2990499A1
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EP
European Patent Office
Prior art keywords
wire rod
temperature
rolling
wire
finish
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Granted
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EP14788178.3A
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German (de)
English (en)
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EP2990499A4 (fr
EP2990499B1 (fr
Inventor
Hiroshi Ooba
Arata Iso
Tatsusei TADA
Toshiyuki Manabe
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/613—Gases; Liquefied or solidified normally gaseous material
    • 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
    • C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
    • 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
    • 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
    • C22C38/28—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
    • C21D2211/009—Pearlite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods

Definitions

  • the present invention relates to a wire rod which is raw material of PC steel stranded wire for reinforcing a PC structure used for LNG (Liquefied Natural Gas) Tank of energy relating facility, and which has excellent low-temperature elongation and low-temperature toughness.
  • LNG Liquified Natural Gas
  • Patent document 1 proposes metal double shell type LNG tank including a metal inner tank and a metal outer tank.
  • the PC dike in the technique includes concrete forming circular dike enclosing the LNG tank and PC steel stranded wire embedded in the concrete. Prestress is provided to the PC dike by tensioning the concrete along circumferential direction using the PC steel stranded wire. If LNG outflows from the LNG tank inside the PC dike, although tensile stress is applied to the PC dike along the circumferential direction thereof by the fluid pressure of the outflowing LNG, the prestress provided to the PC dike relieves the tensile strength.
  • Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2006-234137
  • the PC dike is continuously in a low-temperature state since heat is drawn by the LNG in the LNG tank.
  • LNG having temperature of -162°C or lower contacts with the concrete, and thus, the temperature around the PC steel stranded wire reinforcing the PC dike at the inner portion of the concrete may be greatly reduced.
  • low-temperature elongation and low-temperature toughness of the PC steel stranded wire are not sufficiently high, the PC steel stranded wire may break and the PC dike may fracture. Therefore, a wire rod which has better low-temperature elongation and better low-temperature toughness than the conventional wire rod for the PC steel stranded wire is needed.
  • An object of the present invention is to provide a wire rod which has more excellent low-temperature elongation and more excellent low-temperature toughness than the conventional wire rod for the PC steel stranded wire in order to meet the needs of the markets.
  • the inventors performed factual survey of use environment of the PC dike in order to enhance the tension providing effect in the PC dike constructing the PC type LNG tank (LNG tank including PC dike). As a result, in the actual use environment, it was found that there were cases in which the wire rod was exposed to an environmental temperature of about -40°C by heat transfer to the LNG in the LNG tank.
  • the gist of the present invention of which the object is to solve the above-described problems is as follows.
  • a wire rod for PC steel stranded wire which is favorable to usage as tendon of PC dike of PC-type LNG tank and which has more excellent elongation and toughness at about -40°C as compared with the conventional material can be provided by reducing the grain size of pearlite block and limiting the number density of coarse pearlite block.
  • a chemical composition includes, in terms of mass%: C :0.60 to 1.20%; Si:0.30 to 1.30%; Mn:0.30 to 0.90%; P :0.020% or less; S :0.020% or less; N :0.0025 to 0.0060%; Cr:0 to 1.00%; V :0 to 0.800%; one or more selected from the group consisting of A1:0.005 to 0.100%, Ti:0.003 to 0.050%, and B :0.0005 to 0.0040%; and remainder including Fe and impurity, wherein an average value of a grain size of a pearlite block is 23 ⁇ m or less in a cross section perpendicular to a wire rod axial direction, and wherein the number density of the pearlite block having 40 ⁇ m or more of the grain size is 0 to 20
  • C is an element having an effect of increasing a cementite ratio in the wire rod to increase strength of the wire rod.
  • a lamellar spacing of the pearlite can be controlled by adjusting a patenting condition, and the strength of the wire rod can be increased by working.
  • an amount of C is less than 0.60%, a strength which can sufficiently tension a PC dike of a PC-type LNG tank cannot be obtained even if the above-described adjusting the patenting condition is performed.
  • a mesh-shaped cementite forms in a structure in the wire rod.
  • the mesh-shaped cementite may frequently cause breaking during wire drawing and may interfere production activity of the wire rod.
  • both of DLP (Direct in-Line Patenting) method and stelmor method can be employed as follows. If the wire rod is manufactured with the stelmor method, it is preferable that the amount of C be 0.70 to 0.90%. If the wire rod is manufactured with the stelmor method, a cooling rate of the wire rod is lower than that in the manufacturing with the DLP method, and thus, the elongation and the toughness of the wire rod is relatively low. In order to compensate for the decrease in the strength, it is preferable that the lower limit of the amount of C be 0.70%.
  • the amount of C excessively increases, a pro-eutectoid cementite forms in the mesh shape at a grain boundary of prior austenite during cooling the wire rod, since the wire rod becomes a hyper-eutectoid steel (a steel having a structure containing both of pearlite and cementite).
  • the mesh-shaped pro-eutectoid cementite significantly deteriorates the drawability of the wire rod.
  • the upper limit of the amount of C be 0.90%. It is more preferable that the amount of C be 0.80 to 0.90%.
  • Si is an element which acts as a deoxidizing element during refining. When 0.30% or more of Si is included, the deoxidizing effect is sufficiently expressed. Therefore, the lower limit of the amount of Si of the wire rod according to the present embodiment is 0.30%. Si also includes an effect for enhancing the strength of the wire rod. When 0.80% or more of Si is included, the effect of enhancing the strength is expressed. Therefore, the lower limit of the amount of Si of the wire rod according to the present embodiment may be 0.80%. In addition, although Si solid-solute strengthen ferrite, Si has an effect for rising a nose of isothermal transformation during heat treatment. Therefore, an excessive amount of Si increases cost for the heat treatment. Accordingly, in view of a capacity of the manufacturing equipments, the upper limit of the amount of Si is 1.30%.
  • both of the DLP method and the stelmor method can be employed. If the wire rod is manufactured with the stelmor method, it is preferable that the amount of Si be 0.80 to 1.30%. If the wire rod is manufactured with the stelmor method, a cooling rate of the wire rod is lower than that in the manufacturing with the DLP method, and thus, the elongation and the toughness of the wire rod is relatively low. In order to compensate the decreasing of the strength, it is preferable that the lower limit of the amount of Si be 0.80%. It is more preferable that the amount of Si be 0.90 to 1.25%.
  • Mn is an solid-solute strengthening element and has an effect of enhancing elongation and toughness of the wire rod and an effect of enhancing hardenability.
  • the lower limit of the amount of Mn may be 0.60%.
  • the amount of Mn is higher than 0.90%, a delay of transformation occurs in a center portion of the wire rod to cause a formation of micro martensite at a portion of untransformed austenite during manufacturing the wire rod. The micro martensite at the center portion of the wire rod causes breaking during drawing of the wire rod. Therefore, it is necessary that the upper limit of the amount of Mn is 0.90%.
  • both of the DLP method and the stelmor method can be employed. If the wire rod is manufactured with the stelmor method, it is preferable that the amount of Mn be 0.60 to 0.90%. If the wire rod is manufactured with the stelmor method, the cooling rate of the wire rod is lower than that in the manufacturing with the DLP method, and thus, the elongation and the toughness of the wire rod is relatively low. In order to compensate the decreasing of the strength, it is preferable that the lower limit of the amount of Mn be 0.60%. It is more preferable that the amount of Mn be 0.70 to 0.90%.
  • an upper limit an amount of P is 0.020%.
  • the upper limit of the amount of P may be 0.010%, 0.005%, or 0.001%.
  • S is an element which combines with Mn in the wire rod to form MnS. Since S segregates at a center portion of the steel during refining and solidifying the steel, MnS accumulates at the center portion of the steel. MnS deteriorates the low-temperature elongation of the steel. If an amount of S is more than 0.020%, the low-temperature elongation significantly deteriorates, and thus, the amount of S is 0.020% or lower. When the wire rod is further securely prevented from the low-temperature embrittlement, the upper limit of the amount of S may be 0.010%, 0.005%, or 0.001%.
  • N is an element which combines with Al, Ti, and B to form nitrides.
  • the nitrides act as a nucleuses of precipitation of the austenite, and thus, the grain size of the austenite can be controlled during heating the steel by controlling the number of the nitrides. If the amount of the nitrides increase, the grain is refined. If the amount of N is less than 0.0025%, the nitrides do not sufficiently form and the effect of refining the grain size cannot be sufficiently obtained. On the other hand, If the amount of N is more than 0.0060%, free N, which does not combine with Al, Ti, and B, become excess.
  • the excess amount of free N causes age hardening to deteriorate the elongation and the toughness of the wire rod. Therefore, it is necessary that the amount of N be 0.0025 to 0.0060%. It is preferable that the amount of N be 0.0025 to 0.0040%.
  • the wire rod according to the present embodiment further includes one or more selected from the group consisting of Al: 0.005 to 0.100%, Ti: 0.003 to 0.050%, and B: 0.0005 to 0.0040%.
  • Al acts as a deoxidizing agent during refining the steel.
  • Al forms composition with N in the steel, Al has an effect of fixing N. Due to fixing N, the steel can be prevented from age hardening. Furthermore, when B is included together with Al, due to fixing N, the amount of solute B can be increased.
  • the amount of Al is less than 0.005%, the effect of fixing N by Al cannot be sufficiently obtained.
  • the amount of A1 is more than 0.100%, Al 2 O 3 , which is formed by Al combining with oxygen in the steel, forms clusters. The clusters act as origins of cracking during drawing. Therefore, the amount of Al may be 0.005 to 0.100%. It is preferable that the amount of Al be 0.020 to 0.050%.
  • Ti acts as the deoxidizing agent of the steel, similar to Al.
  • Ti forms chemical compound with N in the steel, Ti has an effect of fixing N. Due to fixing N, the steel can be prevented from age hardening. Furthermore, when B is included together with Ti, due to fixing N, the amount of solute B can be increased.
  • the amount of Ti is less than 0.003%, the effect of fixing N by Ti cannot be sufficiently obtained.
  • the amount of Ti is more than 0.050%, TiC, which is formed by Ti combining with carbon in the steel, increase. TiC act as an origins of cracking during drawing. Therefore, the amount of Ti may be 0.003 to 0.050%. It is preferable that the amount of Ti be 0.020 to 0.040%.
  • Al and Ti have similar effects. Therefore, the amount of Al can be decreased by including Ti. In this case, a similar effect can be obtained.
  • B When B exists as the solute B in the austenite, B has an effect of enhancing the hardenability of the wire rod. If the amount of B is less than 0.0005%, the effect of enhancing the hardenability cannot be sufficiently obtained. On the other hand, if the amount of B is more than 0.0040%, B forms chemical compound with Fe and C to form a precipitates such as Fe 23 (C, B) 6 , and the like. The precipitates act as the origins of cracking during drawing. Therefore, the amount of B may be 0.0005 to 0.0040%. It is preferable that the amount of B be 0.0009% to 0.0030%.
  • the wire rod according to the present embodiment may further include, in terms of mass%, Cr: 0 to 1.00% or V: 0 to 0.800%.
  • a lower limit of an amount of Cr is 0%.
  • Cr has an effect of reducing lamellar spacing of the pearlite to enhance the strength of the wire rod. Due to the effect, the degree of increasing of the strength of the wire rod during drawing is enhanced. The effect is obtained in a case in which the amount of Cr is 0.10% or more, and thus, it is preferable that the amount of Cr be 0.10% or more. In addition, when the strength is further enhanced, it is preferable that 0.50% or more of Cr be included.
  • an upper limit of the amount of Cr be 1.00%.
  • both of the DLP method and the stelmor method can be employed. If the wire rod is manufactured with the stelmor method, it is more preferable that the amount of Cr be 0.50 to 1.00%. If the wire rod is manufactured with the stelmor method, the cooling rate of the wire rod is lower than that in the manufacturing with the DLP method, and thus, the elongation and the toughness of the wire rod is relatively low. In order to compensate the decreasing of the strength, it is more preferable that the lower limit of the amount of Cr be 0.50%. It is further preferable that the amount of Cr be 0.50 to 0.90%.
  • V is an element which combines with C to precipitate as carbide in ferrite. Due to the precipitation of the carbide, the ferrite can be hardened and the wire rod can be high-strengthened. The effect can be obtained if 0.005% or more of V is included. However, if the amount of V is more than 0.800%, a coarse carbides precipitate. The coarse carbides act as the origins of cracking in working the wire rod. Therefore, it is preferable that the amount of V be 0.005 to 0.800%.
  • both of the DLP method and the stelmor method can be employed. If the wire rod is manufactured with the stelmor method, it is more preferable that the amount of V be 0.300 to 0.500%. If the wire rod is manufactured with the stelmor method, the cooling rate of the wire rod is lower than that in the manufacturing with the DLP method, and thus, the elongation and the toughness of the wire rod is relatively low. In order to compensate the decreasing of the strength, it is more preferable that the lower limit of the amount of V be 0.300%. In addition, since micro cracks form at boundary portions of VC precipitates and base iron when a processing strain occurs, it is more preferable that an upper limit of the amount of V be 0.500%. It is further preferable that the amount of V be 0.300 to 0.400%.
  • Remainder of the wire rod according to the present embodiment includes Fe and impurity.
  • the impurity is a component which is incorporated by raw materials such as mineral or scrap or various factors of a manufacturing process when the steel is industrially manufactured, and is accepted within a range that does not adversely affect the property of the wire rod according to the present embodiment.
  • a pearlite block boundary is defined as a boundary of adjacent two pearlites between which a difference in crystal orientation is 9° or higher
  • a pearlite block is defined as an area surrounded by the pearlite block boundary
  • PBS Pearlite Block Size
  • an average PBS (Pearlite Block Size) in a cross section perpendicular to a wire rod axial direction is defined to 23 ⁇ m or less.
  • the average PBS Pullite Block Size
  • the wire rod according to the present embodiment has 30% or more of the reduction of area.
  • 30% or more of the reduction of area was necessary to prevent breaking from causing during drawing in the drawing working of the wire rod.
  • Figure 1 indicates that 30% or more of the reduction of area can be obtained when the average PBS (Pearlite Block Size) is 23 ⁇ m or less.
  • the average PBS Peak Block Size
  • frequency of branching of the crack end decreases. Since a branch of the crack end has an effect of suppressing crack propagation, decreasing the frequency of branching of the crack end enlarges a fracture facet to deteriorate the low-temperature elongation and the low-temperature toughness. Therefore, the average PBS (Pearlite Block Size) is 23 ⁇ m or less. It is preferable that the average pearlite block size be 18 ⁇ m or less.
  • the average value of the equivalent circle diameter of the pearlite block in any size of field of view at any position of the cross section perpendicular to the wire rod axial direction can be obtained by using EBSD device.
  • the average PBS in the cross section perpendicular to the wire rod axial direction according to the present embodiment can be obtained by the following procedures. At first, average values (primary average values) of equivalent circle diameters of pearlite blocks in 300 ⁇ m ⁇ 180 ⁇ m of fields of view in each of five area consisting of
  • the number density of the pearlite block having 40 ⁇ m or more of grain size in the cross section perpendicular to the wire rod axial direction is 0 to 20 pieces/mm 2 .
  • the number density of the pearlite block having a grain size of 40 ⁇ m or more in the cross section perpendicular to the wire rod axial direction is defined to 0 to 20 pieces/mm 2 .
  • pearlite blocks having 40 ⁇ m or more of grain size act as fracture origins, the pearlite blocks having 40 ⁇ m or more of grain size deteriorate the elongation and the toughness of the wire rod even if the number of the pearlite blocks having 40 ⁇ m or more of grain size is small.
  • pearlite block having a grain size of 40 ⁇ m or more may be referred to as “coarse pearlite block" or "coarse PB".
  • the number density of the coarse pearlite blocks in the cross section perpendicular to the wire rod axial direction is more than 20 pieces/mm 2 , the elongation and the toughness of the wire rod do not satisfy required levels. Therefore, it is necessary that the number density of the coarse pearlite blocks in the cross section perpendicular to the wire rod axial direction is limited to 0 to 20 pieces/mm 2 . It is preferable that an upper limit of the number density of the coarse pearlite blocks in the cross section perpendicular to the wire rod axial direction be 18 pieces/mm 2 . The less the amount of the coarse pearlite blocks are, the more preferable it is, and therefore, the lower limit of the number density of the coarse pearlite blocks in the cross section perpendicular to the wire rod axial direction is 0 pieces/mm 2 .
  • the number density of the pearlite blocks having a grain size of 40 ⁇ m or more in any size of field of view at any position of the cross section perpendicular to the wire rod axial direction can be obtained by using EBSD device.
  • the number density of the pearlite blocks having 40 ⁇ m or more of grain size in the cross section perpendicular to the wire rod axial direction according to the present embodiment can be obtained by the following procedures. At first, the number densities of the pearlite blocks having a grain size of 40 ⁇ m or more in 300 ⁇ m ⁇ 180 ⁇ m of fields of view in each of five areas of
  • the wire rod according to the present embodiment is manufactured by the stelmor method, as shown in Figure 10 , (1) heating a steel piece having a chemical composition including, in terms of mass%, C: 0.70 to 0.90%; Si: 0.80 to 1.30%; Mn: 0.60 to 0.90%; P: 0.020% or less; S: 0.020% or less; N: 0.0025 to 0.0060%; Cr: 0 to 1.00%; V: 0 to 0.500%; one or more selected from the group consisting of Al: 0.005 to 0.100%, Ti: 0.003 to 0.050%, and B: 0.0005 to 0.0040%; and remainder including Fe and impurity to a rough-rolling temperature of 950 to 1040°C and then rough-rolling, (2) finish-wire-rolling at a finish-rolling temperature of 750 to 950°C, (3) coiling at a coiling temperature of 730 to 840°C, and (4) air blast cooling to a normal temperature with a cooling rate of 15°C/sec
  • d ⁇ / dt expresses the strain rate during the finish-wire-rolling in terms of s -1 and T expresses the finish-rolling temperature in terms of °C.
  • the steel piece may further include, in terms of mass%, one or more selected from the group consisting of Cr: 0.50 to 1.00%, and V: 0.300 to 0.500%.
  • the stelmor method is a manufacturing method in which air blast cooling is performed after coiling, and the cooling rate by the air blast cooling is lower than a cooling rate of a direct heat treatment by a molten salt in a DLP method described below.
  • the cooling rate is low, the elongation and the toughness of the wire rod finally obtained are relatively low.
  • the wire rod according to the present embodiment is manufactured by the stelmor method, it is necessary that the amount of C, Mn, and Si which are alloy elements for enhancing the elongation and the toughness are more than that in a case of performing the direct heat treatment by the molten salt in the DLP method.
  • the amount of Cr and V be also more than that in the case of performing the direct heat treatment by the molten salt in the DLP method.
  • a heating temperature of the steel piece before the rough-rolling (rough-rolling temperature) is 950 to 1040°C. If the heating temperature of the steel piece before rough-rolling is lower than 950°C, roll reaction forth in wire rod rolling may rapidly increase to cause trouble of the equipment such as cracking of roll. Therefore, the heating temperature of the steel piece before the rough-rolling is 950°C or more. On the other hand, if the heating temperature of the steel piece before the rough-rolling is more than 1040°C, solutionizing of aluminum nitride (AlN) precipitated in the steel piece progresses excessively.
  • AlN aluminum nitride
  • AlN act as the nucleuses of precipitation of the austenite to contribute to refine the grain size of the austenite.
  • the grain size of the pearlite of the wire rod finally obtained can be refined by refining the grain size of the austenite of the wire rod during manufacturing stage.
  • the solutionizing of AlN progresses excessively, coarsening the grain size of the austenite progresses.
  • PBS of the wire rod is coarsened after manufacturing the wire rod.
  • the average value of the grain size of the pearlite block in the cross section perpendicular to the wire rod axial direction becomes more than 23 ⁇ m, and/or the number density of the pearlite block having 40 ⁇ m or more of the grain size becomes more than 20 pieces/mm 2 .
  • the heating temperature of the steel piece before the rough-rolling is 1040°C or lower.
  • finish-wire-rolling is performed at a temperature range of 750 to 950°C. If the temperature in the finish-wire-rolling (finish-rolling temperature) is less than 750°C, roll reaction forth during rolling may increase to cause trouble of the equipment such as cracking of roll, and thus, the temperature in the finish-wire-rolling is 750°C or more. On the other hand, if the temperature during the finish-wire-rolling is more than 900°C, the grain size of the austenite coarsens.
  • the average value of the grain size of the pearlite block in the cross section perpendicular to the wire rod axial direction becomes more than 23 ⁇ m, and/or the number density of the pearlite blocks having 40 ⁇ m or more of the grain size becomes more than 20 pieces/mm 2 .
  • the elongation of the wire rod is deteriorated by coarsening the pearlite block.
  • the temperature during the finish-wire-rolling is 900°C or lower.
  • "d ⁇ / dt” expresses the strain rate in terms of s -1 .
  • "R” is gas constant and the value thereof is 1.98 cal/mol ⁇ deg.
  • T expresses the finish-rolling temperature in terms of K. If the term of the finish-rolling temperature is °C, it is necessary that "T” in the expression 2 is replaced by "(T + 273.15)”.
  • Q expresses the activation energy of the plastic deformation of the wire rod. It is assumed that the activation energy of the plastic deformation of the wire rod according to the present embodiment having the above-described chemical composition is 63800 cal/mol.
  • the strain rate of the wire rod at the finish-wire-rolling d ⁇ / dt can be obtained by the following expression.
  • d ⁇ / dt ⁇ ⁇ 2 ⁇ ⁇ ⁇ N / 60 ⁇
  • L d ⁇ ln h 2 / h 1
  • ⁇ is an amount of strain during the finish-wire-rolling and is a dimensionless parameter.
  • “h1” is a diameter of the wire rod before the finish-wire-rolling in terms of mm
  • “h2” is a diameter of the wire rod after the finish-wire-rolling in terms of mm.
  • N is a number of revolution of a roll performing the finish-wire-rolling in terms of rpm.
  • “L d " is a projected roll contact length during the finish-wire-rolling in terms of mm. The projected roll contact length is the length of an area, at which the roll contacts with a rolled material (wire rod) during the rolling, along the rolling direction.
  • “r” is a radius of the roll at the finish-wire-rolling in terms of mm.
  • “ ⁇ h” is a rolling reduction during the finish-wire-rolling in terms of mm. As shown in expressions 3 to 6, the strain rate is a rolling condition depending on both of rolling speed and rolling reduction.
  • the upper limit of log 10 Z is 16.5. It is preferable that log 10 Z be 14.0 to 16.0. If log 10 Z is lower than above-described range, the average value of the grain size of the pearlite block in the cross section perpendicular to the wire rod axial direction becomes more than 23 ⁇ m, and/or the number density of the pearlite block having 40 ⁇ m or more of the grain size becomes more than 20 pieces/mm 2 .
  • the speed of the finish-wire-rolling is not limited as long as log 10 Z is within the range of 13.7 to 16.5. It is preferable that, if a wire rod having a diameter of 12mm or more is manufactured, the speed of the finish-wire-rolling be 15.5 to 25.2 m/sec. If the speed of the finish-wire-rolling is less than 15.5 m/sec, the strain rate decreases. In this case, PBS may not be sufficiently reduced. Therefore, it is preferable that the speed of the finish-wire-rolling be 15.5 m/sec or more. On the other hand, if the speed of the finish-wire-rolling is more than 25.2 m/sec, exotherm during working may increase to coarsen the grain size of austenite.
  • the wire rod is coiled at a temperature of 730 to 840°C, and then, air blast cooling is performed with a cooling rate of 15°C/sec or more. If the coiling temperature is lower than 730°C, the amount of scale decreases to deteriorate mechanical descalability, and thus, the coiling temperature is 730°C or more. If the coiling temperature is higher than 840°C, the grain size of the austenite increases.
  • the coiling temperature is 840°C or lower. It is preferable that the coiling temperature be 750 to 820°C.
  • the wire rod after the coiling is cooled by air blast cooling to a normal temperature.
  • the normal temperature basically indicates a temperature range of 5 to 35°C as defined in JIS Z 8703.
  • the cooling rate at the air blast cooling is 15°C/sec or more. It is preferable that the cooling rate at the air blast cooling be 25°C/sec or more.
  • the wire rod according to the present embodiment is manufactured by the DLP method, (1) heating a steel piece having a chemical composition including, in terms of mass%, C: 0.60 to 1.20%; Si: 0.30 to 1.30%; Mn: 0.30 to 0.90%; P: 0.020% or less; S: 0.020% or less; N: 0.0025 to 0.0060%; Cr: 0 to 1.00%; V: 0 to 0.500%; one or more selected from the group consisting of Al: 0.005 to 0.100%, Ti: 0.003 to 0.050%, and B: 0.0005 to 0.0040%; and remainder including Fe and impurity to a temperature of 950 to 1040°C and wire rod rolling, (2) coiling at a temperature range of 750 to 800°C, and (3) direct heat treating with 500 to 600°C of a molten salt just after termination of the coiling are performed.
  • the steel piece which is raw material, may further include, in terms of mass%, one or more selected from the group consisting of Cr: 0.10 to 1.00%, and V: 0.005 to 0.800%.
  • a method for manufacturing with DLP method has an advantage that a wire rod having excellent elongation and toughness can be obtained from a steel piece of which alloy elements for enhancing the elongation and the toughness is relatively low.
  • direct heat treatment with molten salt is essential the for the method for manufacturing with the DLP method, and thus, in order to bring the method for manufacturing with the DLP method into operation, much more equipment is required than that with the stelmor method.
  • the heating temperature of the wire rod before wire rod rolling the steel piece is 950 to 1040°C. If the heating temperature is lower than 950°C, roll reaction forth during the wire rod rolling may considerably increase to cause troubles in equipments such as cracking of roll, and thus, the heating temperature before the wire rod rolling is 950°C or more. On the other hand, if the heating temperature before the wire rod rolling is higher than 1040°C, solutionizing of aluminum nitride (AlN) precipitated in the steel piece progresses, and thus, coarsening the grain size of the austenite progresses to coarsen the pearlite block size (PBS) of the wire rod finally obtained.
  • AlN aluminum nitride
  • the average value of the grain size of the pearlite block in the cross section perpendicular to the wire rod axial direction becomes more than 23 ⁇ m, and/or the number density of the pearlite blocks having 40 ⁇ m or more of the grain size becomes more than 20 pieces/mm 2 .
  • the heating temperature before the wire rod rolling is 1040°C or lower. It is preferable that the heating temperature before the wire rod rolling be 980 to 1030°C.
  • the finish temperature at the wire rod rolling is not limited, and thus, a reasonable temperature can be arbitrarily selected.
  • coiling temperature after the wire rod rolling is 750 to 850°C. If the coiling temperature is lower than 750°C, unevenness along longitudinal direction of the wire rod regarding tensile strength increases after isothermal transformation in the following isothermal transformation treating. Therefore, the coiling temperature is 750°C or higher. If the coiling temperature is higher than 800°C, the grain size of austenite increases. In this case, the grain size of the pearlite block of the wire rod finally obtained cannot be controlled to 23 ⁇ m or less and the number density of the pearlite block having 40 ⁇ m or more of the grain size cannot be controlled to be 20 pieces/mm 2 or less, and thus, the coiling temperature is 800°C or lower.
  • the wire rod according to the present embodiment is manufactured by the DLP method, immediately after coiling the wire rod, the wire rod is immersed into 500 to 600°C of molten salt to perform isothermal transformation treating. If the isothermal transformation treating temperature is lower than 500°C, many amount of non-pearlite structure forms at the surface part of the wire rod. In this case, unevenness of processing strain occurs at boundary between pearlite structure forming at internal part of the wire rod and the non-pearlite structure at the surface part, and the unevenness may cause breaking during wire drawing stage. Therefore, the isothermal transformation treating temperature is 500°C or higher.
  • the isothermal transformation treating temperature is higher than 600°C, operational problems such as increasing heat deformation of the equipment occurs, and thus, the isothermal transformation treating temperature is 600°C or lower.
  • the isothermal transformation treatment is performed with direct heat treatment (online heat treatment). If the direct heat treatment is not performed (i.e. the isothermal transformation treatment is performed with offline heat treatment), reheating included in the offline heat treatment causes growth of y grain. The phenomenon prevents the grain size of PBS of the wire rod from being controlled to 23 ⁇ m or less.
  • the conditions in manufacturing the examples are an example condition employed to confirm the operability and the effects of the present invention, so the present invention is not limited to the example condition.
  • the present invention can employ various types of conditions as long as the conditions do not depart from the scope of the present invention and can achieve the object of the present invention.
  • molten steel having chemical composition shown in Table 1-1 was continuously casted so as to be a cast bloom of 300mm ⁇ 500mm, and the cast bloom is rolled so as to be a steel billet of 122mm square by blooming. Then, the steel billet was heated to a heating temperature shown in Table 1-2 and rolled under a condition shown in the Table 1-2 to obtain a wire rod of 12mm ⁇ . A radius of a roll in finish-wire-rolling was 75.5mm.
  • No. S1 to S16 are examples which satisfy the condition according to the present invention and No. S 17 to S41 are comparative examples which do not satisfy the condition according to the present invention.
  • a tensile test pieces shown in Figure 3 were manufactured from the rolled wire rods. Tensile test was performed to the tensile test pieces at a low temperature atmosphere of -40°C while controlling the temperature using dry ice and alcohol to measure tensile strength and elongation at -40°C of the wire rod.
  • a charpy impact test pieces defined in JIS Z 2202 were extracted from the above-described wire rods in a manner of extracting shown in Figure 4 to manufacture 5mm subsize 2mm U-notch charpy impact test pieces.
  • Charpy impact test at -40°C was performed to the charpy impact test pieces to obtain impact value of the wire rods at a temperature of -40°C, which was similar to actual using environment temperature of PC dike of PC-type LNG tank.
  • the average PBS (Pearlite Block Size) of the wire rod was obtained by the following procedures. At first, average values (primary average values) of an equivalent circle diameter of a pearlite block in 300 ⁇ m ⁇ 180 ⁇ m of fields of view in each of five area of
  • the number density of the coarse pearlite blocks of the wire rod was obtained by the following procedures. At first, number densities of the pearlite block having 40 ⁇ m or more of grain size in 300 ⁇ m ⁇ 180 ⁇ m of fields of view in each of five area of
  • the heating temperature, the finishing temperature, and the coiling temperature of the examples were within the adequate temperature range. Therefore, the pearlite block of the example was refined and the average PBS and the number density of the coarse PB of the example were controlled in the adequate level. On the other hand, the average PBS and the number density of the coarse PB of the comparative examples of which the heating temperature, the finishing temperature, and/or the coiling temperature were higher than the adequate temperature range were out of range defined by the present invention. The examples demonstrated better property than the comparative examples in low-temperature strength, low-temperature toughness, and room-temperature toughness. In example S36, the finish-rolling temperature was lower than the adequate temperature range, and thus, mill load increased to prevent rolling from performing.
  • Figure 5A shows SEM picture of the pearlite block of the example and Figure 5B shows SEM picture of the pearlite block of the comparative example. It could be found from the SEM pictures that the grain size of the pearlite block of the example was smaller than the grain size of the pearlite block of the comparative example.
  • Figure 6 shows a result of tensile test at -40°C of the example (No. S6) and the comparative example (No. S17). It could be found that the elongation of the example was higher and better than the elongation of the comparative example in low-temperature atmosphere of -40°C. In addition, it could be found from Table 2 that the elongation of the example tended to be higher than the elongation of the comparative example. It was assumed that the difference in elongation was caused by the difference in the grain size of the pearlite block shown in Figure 5A and Figure 5B .
  • molten steel having chemical composition shown in Table 3 was continuously casted so as to be a cast bloom of 300mm ⁇ 500mm, and the cast bloom was rolled so as to be a steel billet of 122mm square. Then, the steel billet was heated to a heating temperature shown in Table 3 and rolled, coiled and heat treated using molten salt under a condition shown in the Table 3 to obtain a wire rod of 12mm ⁇ .
  • No. D1 to D16 and No. D30 to D36 were manufactured with a heat treatment (direct heat treatment) in which they were immersed into the molten salt without reheating after coiling.
  • No. D17 to D29 were coiled under conditions shown in the Table 3, and then, subjected to a heat treatment (offline heat treatment) in which they were reheated to 950°C and subjected to lead patenting treatment.
  • Tensile strength at -40°C, elongation at -40°C, and impact value at -40°C of the above-described wire rods were obtained.
  • the test methods for obtaining the values were same to each test methods performed to the above-described No. S1 to No. S41.
  • charpy impact test at room temperature was performed to the above-described wire rods to obtain impact value of the wire rods at the room temperature.
  • the method for performing the charpy impact test at the room temperature was same to the above-described method for performing the charpy impact test at -40°C except test temperature.
  • the average PBS and the number density of the coarse pearlite blocks of the above-described wire rod were measured.
  • No. D1 to D16 of the Table 3 were examples satisfying the condition according to the present invention.
  • No. D17 to D38 of the Table 4 were comparative examples which did not satisfy the conditions according to the present invention.
  • the grain size of the pearlite block and the number density of the coarse PB of the example were controlled to the adequate level, the grain size of the pearlite block and the number density of the coarse PB of the comparative example were out of the range defined by the present invention.
  • the examples demonstrated more excellent low-temperature strength, low-temperature toughness, and room-temperature toughness as compared with the comparative examples.
  • Figure 7A shows SEM picture of the pearlite block of the example and Figure 7B shows SEM picture of the pearlite block of the comparative example. It could be found from the SEM pictures that the grain size of the pearlite block of the example was clearly different from that of the comparative example.
  • Figure 8 shows a relationship between the grain size of the pearlite block ( ⁇ m) and the impact value based on the impact value shown in Table 4. It could be found from the Figure 8 that the impact value of the example (PBS: 15 to 23 ⁇ m) was higher than the impact value of the comparative example (PBS: 30 to 45 ⁇ m) in both of room temperature and -40°C.
  • Figure 9A and Figure 9B show SEM observation result of the fracture surface of the charpy impact test piece of the example and comparative example.
  • Figure 9A shows fracture facet of the example
  • Figure 9B shows fracture facet of the comparative example.
  • the fracture facet of the example was finer than the fracture facet of the comparative example. This indicates that the example had more excellent toughness than the comparative example. In view of the point, the effect of refining PBS could be found.
  • the toughness of the example was higher than the toughness of the comparative example at both of room temperature and the environment of -40°C, to which the wire rod was exposed when the wire rod was used as additional PC of LNG tank.
  • a wire rod for PC steel stranded wire which is used as tendon of PC dike of PC-type LNG tank and which has more excellent elongation at about -40°C as compared with the conventional material can be provided by reducing grain size of pearlite block. Therefore, the present invention contributes to enhancing reliability of the PC steel stranded wire, which is a member constructing equipments concerning LNG tank which is much in demand these days, under low-temperature usage environment, and thus, the present invention has significant industrial applicability.

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Publication number Priority date Publication date Assignee Title
EP3165623A4 (fr) * 2014-07-01 2018-04-04 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Matériau de fil pour fil d'acier, et fil d'acier
CN112176258A (zh) * 2020-09-30 2021-01-05 江苏省沙钢钢铁研究院有限公司 2500MPa级钢绞线用盘条及其制造方法

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MX2017007665A (es) * 2014-12-15 2017-10-27 Nippon Steel & Sumitomo Metal Corp Alambron.

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JP2000192148A (ja) * 1998-12-25 2000-07-11 Kobe Steel Ltd 冷間加工性に優れた鋼線材およびその製造方法
DE602005019268D1 (de) * 2004-12-22 2010-03-25 Kobe Steel Ltd Hochkohlenstoff Stahldraht mit hervorragenden Zieheigenschaften und Verfahren zu seiner Herstellung
JP2006234137A (ja) 2005-02-28 2006-09-07 Mitsubishi Heavy Ind Ltd 地上式ｌｎｇタンク
JP5098444B2 (ja) * 2006-06-01 2012-12-12 新日鐵住金株式会社 高延性の直接パテンティング線材の製造方法
BRPI0903902B1 (pt) * 2008-03-25 2017-06-06 Nippon Steel & Sumitomo Metal Corp arame de aço de alta resistência e seu método de produção
JP2010229469A (ja) * 2009-03-26 2010-10-14 Nippon Steel Corp 冷間加工特性に優れる高強度線材及びその製造方法
EP2687619A4 (fr) * 2011-03-14 2014-11-26 Nippon Steel & Sumitomo Metal Corp Matériau de fil-machine et procédé pour sa production

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3165623A4 (fr) * 2014-07-01 2018-04-04 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Matériau de fil pour fil d'acier, et fil d'acier
CN112176258A (zh) * 2020-09-30 2021-01-05 江苏省沙钢钢铁研究院有限公司 2500MPa级钢绞线用盘条及其制造方法

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