Classifications

• • 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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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
  • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires 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
  • C21D9/52—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
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • 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

Definitions

• the present invention relates to a high-strength flat steel wire used for the purpose of realizing reinforcement of tension and the like of a component used under a sour environment containing hydrogen sulfide such as a flexible pipe for transporting a high-pressure fluid such as crude oil.
• a flat steel wire is used as a reinforcing material.
• a digging depth tends to be deep as a demand for petroleum increases, and a demand for increasing strength of the reinforcing material of the flexible pipe is increasing.
• the flat steel wire used for the reinforcing material is required to have a property causing no hydrogen induced cracking (HIC), which is, hydrogen induced cracking resistance.
• Patent Document 2 proposes a hot-rolled wire material containing, by mass %, C: 0.20 to 0.5%, Si: 0.05 to 0.3%, Mn: 0.3 to 1.5%, Al: 0.001 to 0.1%, P: greater than 0% and equal to or less than 0.01%, S: greater than 0% and equal to or less than 0.01%, and the other elements, in which when S amounts are measured by using an electron probe microanalyzer at 300 or more points and at intervals of 200 ⁇ m, and a maximum value Smax (mass %) of the S amounts relative to an average value Save (mass %) of the S amounts is set as a segregation degree (Smax/Save), the segregation degree is 30 or less.
• Patent Document 3 discloses a profiled wire having high mechanical characteristics and excellent resistance with respect to hydrogen embrittlement as a flexible tube component for digging an undersea oil field, characterized in that it contains 0.75 ⁇ C % ⁇ 0.95 and 0.30 ⁇ Mn % ⁇ 0.85, with Cr ⁇ 0.4%, V ⁇ 0.16%, Si ⁇ 1.40% and preferably ⁇ 0.15%, and possibly 0.06% or less of Al, 0.1% or less of Ni, and 0.1% or less of Cu, and in that, starting from a wire material, hot-rolled in its austenite region above 900° C.
• thermomechanical treatment according to two successive and ordered phases, specifically isothermal tempering, which confers on it a homogeneous pearlitic microstructure, following by an operation of cold mechanical transformation, carried out with a global work-hardening ratio of between 50 and 80% at maximum, to give the wire its final profile, and in that the obtained profiled wire is then subjected to short-duration recovery heat treatment at a temperature below the Acl temperature of the steel constituting it, thus conferring on it the desired final mechanical characteristics.
• Patent Document 1 Japanese Laid-open Patent Publication No. 2013-227611
• Patent Document 2 Japanese Laid-open Patent Publication No. 2015-212412
• Patent Document 3 Japanese Translation of PCT International Application Publication No. JP-T-2013-534966
• the Si amount is small and a sulfide is stretched in the longitudinal direction when the steel material is shaped into a flat steel wire, so that when the steel material is turned into a flat steel wire with tensile strength of 1000 MPa or more, the hydrogen induced cracking is caused under a sour environment with pH of less than 5.5, and thus there is a limit to increase strength of the flat steel wire.
• the C amount is high and a hardness distribution in a cross section of the profiled wire is non-uniform, and a sulfide is stretched in the longitudinal direction when the profiled wire is shaped into a flat steel wire, so that when the profiled wire is turned into a flat steel wire with tensile strength of 1000 MPa or more, the hydrogen induced cracking is likely to occur under a severe sour environment with pH of less than 5.5, and thus there is a limit to increase strength of the flat steel wire.
• the present invention has been made in view of the above-described present situation, and an object thereof is to provide a flat steel wire being a high-strength flat steel wire with tensile strength of 1000 MPa or more, which is difficult to cause hydrogen induced cracking even under a severe sour environment with pH of less than 5.5, and which can be used as a reinforcing wire material of a flexible pipe and the like.
• the present inventors conducted various studies for solving the above-described problems. As a result of this, they obtained findings of the following (a) to (d).
• the hydrogen induced cracking of the flat steel wire occurs from a coarse sulfide included in the flat steel wire as a starting point.
• a sulfide such as MnS is coarse, a gap is generated around the coarse sulfide at a time of performing primary wire drawing and flat rolling required as a step of performing forming from a hot-rolled wire material to a flat steel wire, which becomes a cause of facilitating the hydrogen induced cracking under a severe sour environment with pH of less than 5.5.
• a wire material is worked into a flat steel wire in a manner that, for example, a rolled wire material is subjected to primary wire drawing and then subjected to profiled wire drawing using a die which is worked to have a shape of the flat steel wire or cold rolling using a cold rolling mill.
• hardness at a center portion in a thickness direction of the flat steel wire tends to be high due to a working strain caused by cold working, and a large hardness dispersion is generated in a cross section of the flat steel wire.
• the dispersion of hardness in a cross section thereof causes the hydrogen induced cracking, and thus there is a need to reduce the hardness dispersion in the cross section of the flat steel wire as much as possible.
• the present invention has been completed based on the above-described findings, and the gist thereof lies in a high-strength flat steel wire excellent in hydrogen induced cracking resistance described in the following (1) to (4).
• a high-strength flat steel wire excellent in hydrogen induced cracking resistance is characterized in that it contains, by mass %: C: 0.25 to 0.60%; Si: greater than 0.50% and less than 2.0%; Mn: 0.20 to 1.50%; S: 0.015% or less; P: 0.015% or less; Cr: 0.005 to 1.50%; Al: 0.005 to 0.080%; N: 0.0020 to 0.0080%; and one or two of Ca: 0 to 0.0050% and Mg: 0 to 0.0050% to satisfy the following expression ⁇ 1>, in which components which are optionally contained include: Ti: 0.10% or less; Nb: 0.050% or less; V: 0.50% or less; Cu: 1.0% or less; Ni: 1.50% or less; Mo: 1.0% or less; B: 0.01% or less; REM: 0.10% or less; and Zr: 0.10% or less, the balance is composed of Fe and impurities, tensile strength is 1000 MPa or more, an average value of Hv
• the high-strength flat steel wire excellent in the hydrogen induced cracking resistance described in (1) is characterized in that it contains, by mass %, at least one or two or more selected from Ti: 0.001 to 0.10%, Nb: 0.001 to 0.050%, and V: 0.01 to 0.50%.
• the high-strength flat steel wire excellent in the hydrogen induced cracking resistance described in (1) or (2) is characterized in that it contains, by mass %, at least one or two or more selected from Cu: 0.01 to 1.0%, Ni: 0.01 to 1.50%, Mo: 0.01 to 1.0%, and B: 0.0002 to 0.01%.
• the high-strength flat steel wire excellent in the hydrogen induced cracking resistance described in any one of (1) to (3), is characterized in that it contains, by mass %, at least one or two selected from REM: 0.0002 to 0.10%, and Zr: 0.0002 to 0.10%.
• impurities in “Fe and impurities” being the balance are components which are unintentionally contained in the steel material, and indicate components mixed from an ore as a raw material, a scrap, a manufacturing environment or the like at a time of industrially manufacturing an iron and steel material.
• a flat steel wire of the present invention is difficult to cause hydrogen induced cracking even under a severe sour environment with pH of less than 5.5, while having high tensile strength of 1000 MPa or more, and thus it can be used as a tension reinforcing material of a flexible pipe.
• C is an element which strengthens a steel, and 0.25% or more thereof has to be contained.
• the content of C exceeds 0.60%, when flat steel wires are mutually joined by welding, strength of a joint portion becomes insufficient. Further, dispersion is caused in a hardness distribution in a cross section of the flat steel wire due to segregation, which reduces the hydrogen induced cracking resistance. Therefore, an appropriate content of C is 0.25 to 0.60%.
• the content of C is preferably set to 0.30% or more, and it is more preferably 0.35% or more.
• the content of C is preferably set to 0.55% or less, and it is desirably set to 0.50% or less in order to further improve the hydrogen induced cracking resistance.
• Si is an element solid-dissolved in a matrix and effective for improving the strength of the flat steel wire and improving the hydrogen induced cracking resistance, and it has to be contained in an amount of greater than 0.50%.
• Si is contained in an amount of 2.0% or more, a problem arises such that cracking occurs in a wire material when the wire material is subjected to cold working to have a shape of the flat steel wire. Therefore, the content of Si is greater than 0.50% and less than 2.0%.
• Si is preferably contained in an amount of 0.70% or more, and it is more preferably contained in an amount of 1.0% or more.
• Si is preferably set to 1.80% or less.
• Mn is a required element for increasing hardenability of the steel to realize high strengthening, and it has to be contained in an amount of 0.20% or more.
• the content of Mn in the flat steel wire of the present invention is 0.20 to 1.50%.
• Mn is only required to be contained in an amount of 0.50% or more, and it is more preferably contained in an amount of 0.70% or more.
• Mn is preferably set to 1.30% or less, and it is more preferably 1.10% or less.
• the content of S in the flat steel wire in the present invention is required to be set to 0.015% or less.
• S has to be contained by considering a balance with elements such as Ca and Mg which are likely to bond with S to generate sulfides.
• the content of S is preferably 0.010% or less, and more preferably 0.008% or less.
• a lower limit value of the S content is not particularly limited, to reduce the S content to 0% is technically difficult, and besides, it causes increase in a steelmaking cost. Therefore, the lower limit value of the S content may be set to 0.0005%.
• P is contained as an impurity. Note that if a content of P exceeds 0.015%, the hydrogen induced cracking is likely to occur, and in the flat steel wire with the tensile strength of 1000 MPa or more, it is not possible to suppress the hydrogen induced cracking under the severe sour environment with pH of less than 5.5. From a viewpoint of improving the hydrogen induced cracking resistance, the content of P is preferably 0.010% or less, and more preferably 0.008% or less. Although a lower limit value of the P content is not particularly limited, to reduce the P content to 0% is technically difficult, and besides, it causes increase in a steelmaking cost. Therefore, the lower limit value of the P content may be set to 0.0005%.
• Cr is a required element for increasing hardenability of the steel to realize high strengthening, similarly to Mn, and it has to be contained in an amount of 0.005% or more.
• an appropriate content of Cr in the flat steel wire of the present invention is 0.005 to 1.50%.
• Cr is preferably contained in an amount of 0.10% or more, and more preferably contained in an amount of 0.30% or more.
• the Cr content is preferably set to 1.30% or less, and it is more preferably 1.10% or less.
• Al not only exhibits deoxidation action but also bonds with N to form AlN, and a pinning effect of AlN brings about an effect of making austenite grains finer during hot rolling, which produces an effect of improving the hydrogen induced cracking resistance of the flat steel wire.
• Al has to be contained in an amount of 0.005% or more.
• the content of Al is desirably set to 0.015% or more, and it is more desirable that Al is contained in an amount of 0.020% or more.
• the content of Al is preferably 0.060% or less, and more preferably 0.050% or less.
• N has an effect of improving the strength of the flat steel wire material by being solid-dissolved in a matrix. Further, N bonds with Al, Ti, or the like to generate a nitride or a carbonitride, which brings about an effect of making austenite grains finer during hot rolling, which produces an effect of improving the hydrogen induced cracking resistance of the flat steel wire. In order to obtain these effects, N has to be contained in an amount of 0.0020% or more, and it is more preferably contained in an amount of 0.0030% or more.
• the N content is preferably set to 0.0060% or less, and more preferably set to 0.0050% or less.
• Ca has an effect of making MnS to be finely dispersed by being solid-dissolved in MnS. By making MnS to be finely dispersed, it is possible to improve the hydrogen induced cracking resistance even in a high-strength flat steel wire. It is possible that Ca is not contained (Ca: 0%), but, in order to obtain the effect of suppressing the hydrogen induced cracking with the use of Ca, Ca is only required to be contained in an amount of 0.0002% or more, and when a higher effect is desired to be achieved, Ca is only required to be contained in an amount of 0.0005% or more.
• the content of Ca is preferably 0.0030% or less, and more preferably 0.0025% or less.
• Mg has an effect of making MnS to be finely dispersed by being solid-dissolved in MnS. By making MnS to be finely dispersed, it is possible to improve the hydrogen induced cracking resistance even in a high-strength flat steel wire. It is possible that Mg is not contained (Mg: 0%), but, in order to obtain the effect of suppressing the hydrogen induced cracking with the use of Mg, Mg is only required to be contained in an amount of 0.0002% or more, and when a higher effect is desired to be achieved, Mg is only required to be contained in an amount of 0.0005% or more.
• the content of Mg exceeds 0.0050%, the effect of Mg is saturated, and an oxide of Mg generated through reaction with oxygen in the steel together with Al and Ca becomes coarse, which contrarily causes reduction in the hydrogen induced cracking resistance. Therefore, when Mg is contained, an appropriate content of Mg is 0.0050% or less. From a viewpoint of improving the hydrogen induced cracking resistance, the content of Mg is preferably 0.0030% or less, and more preferably 0.0025% or less.
• the flat steel wire excellent in the hydrogen induced cracking resistance of the present invention has to contain one or two of Ca and Mg, and satisfy a relationship represented by the following expression ⁇ 1>,
• the strength of the flat steel wire material is preferably 1100 MPa or more in a range of causing no hydrogen induced cracking under a certain sour environment.
• the effect of the present invention can be achieved as a result of suppressing the hardness dispersion in a cross section perpendicular to a longitudinal direction by suppressing the component segregation in the cross section perpendicular to the longitudinal direction of the wire material through adjustment of components at a stage of smelting the steel, control of inclusions, and control of rolling and heating conditions, and by manufacturing conditions of the flat steel wire such as removal, through heat treatment and the like, of the working strain that is applied when the working is performed on the flat steel wire material.
• the average value of Hv hardness measured in a cross section perpendicular to a longitudinal direction of the flat steel wire is desirably 430 or less, and more preferably 400 or less.
• the dispersion of the Hv hardness in the cross section perpendicular to the longitudinal direction of the flat steel wire is also required to be controlled at the same time.
• a standard deviation ( ⁇ Hv) of the measured value of each of the flat steel wires was 15 or less.
• the standard deviation ( ⁇ Hv) of the measured value of the Hv hardness in the cross section of each of the flat steel wires in which the hydrogen induced cracking occurred was greater than 15. It can be supposed that the hardness dispersion occurred in the cross section perpendicular to the longitudinal direction due to the composition segregation and the working strain applied at the stage of performing working to obtain the flat steel wire, which exerted an adverse effect on the hydrogen induced cracking. If the hardness dispersion in the cross section of the flat steel wire is as small as possible, it is effective to improve the hydrogen induced cracking resistance, and thus the standard deviation ( ⁇ Hv) of the measured value of the Hv hardness in the cross section is preferably 13 or less. When the hydrogen induced cracking resistance is desired to be improved more, the standard deviation ( ⁇ Hv) is more desirably 11 or less.
• the inclusions are controlled and the component segregation in the cross section perpendicular to the longitudinal direction of the wire material is suppressed by not only the chemical components at the stage of smelting the steel but also the rolling and heating conditions and the manufacturing conditions of the flat steel wire, and the manufacturing conditions of the flat steel wire are controlled such that heat treatment is applied after the working is performed to obtain the flat steel wire, to thereby control the average hardness and the dispersion of hardness in the cross section.
• a wire material is manufactured through the following manufacturing method, and the wire material is used as a raw material to manufacture a flat steel wire.
• a manufacturing process to be described below is one example, and it is needless to say that even in a case where a flat steel wire having chemical components and the other requirements that fall within the range of the present invention is obtained by a process other than the process to be described below, the flat steel wire is included in the present invention.
• a steel ingot or a cast slab smelted and cast by a converter, a normal electric furnace, or the like after chemical components such as C, Si, and Mn are adjusted is subjected to a step of bloom rolling to be a raw material for product rolling being a steel billet.
• the cast steel billet is subjected to heat treatment for 12 hours or more at a temperature of 1250° C. or more. Consequently, a part of MnS is solid-dissolved to be made finer, and it is possible to suppress the component segregation in the rolled wire material.
• the steel billet is reheated and subjected to product rolling in hot working to be finally finished to a bar steel or wire material with a predetermined diameter.
• the rolled wire material is subjected to primary wire drawing and then worked into a flat steel wire.
• a total reduction of area by working when the rolled wire material is worked into the flat steel wire is desirably 80% or less.
• the flat steel wire is adjusted to have a predetermined size by performing cold rolling on the primary-drawn wire material by using a cold rolling mill. In a state where the cold rolling is performed and nothing is performed thereafter, the hardness dispersion in a cross section perpendicular to a longitudinal direction is large, and thus heat treatment is performed on the flat steel wire.
• a heating temperature may be set to a temperature of not less than 400° C. nor more than Al point.
• quenching and tempering treatment in which after the flat steel wire is reheated to an austenite region, it is subjected to oil quenching and then tempered at a temperature of 400° C. or more.
• both end faces in a thickness direction are parallel and cross sections perpendicular to the longitudinal direction of both end faces in a width direction respectively have a semi-elliptic shape or an arc shape. It is also possible that the flat steel wire is finished to have the same shape by wire drawing using a profiled die.
• a width/thickness ratio being a ratio between a maximum width and a thickness in the width direction of the flat steel wire is less than 1.5, an amount of working with respect to the flat steel wire is small, and it is sometimes impossible to obtain sufficient tensile strength.
• the high-strength flat steel wire of the present invention may contain, according to need, at least one or two or more of elements selected from Ti: 0.10% or less, Nb: 0.050% or less, V: 0.50% or less, Cu: 1.0% or less, Ni: 1.50% or less, Mo: 1.0% or less, B: 0.01% or less, REM: 0.10% or less, and Zr: 0.10% or less.
• elements selected from Ti: 0.10% or less, Nb: 0.050% or less, V: 0.50% or less, Cu: 1.0% or less, Ni: 1.50% or less, Mo: 1.0% or less, B: 0.01% or less, REM: 0.10% or less, and Zr: 0.10% or less.
• Ti, Nb, V, Cu, Ni, Mo, B, REM, and Zr being the optional elements and reasons for limiting the contents thereof will be described.
• % regarding the optional components indicates mass %.
• Ti is only required to be contained in an amount of 0.001% or more.
• the content of Ti is desirably set to 0.005% or more, and it is more desirable that Ti is contained in an amount of 0.010% or more.
• the content of Ti exceeds 0.10%, the effect of Ti is saturated, and not only that, a large number of coarse TiN are generated, which contrarily reduces the hydrogen induced cracking resistance of the flat steel wire. Accordingly, the content of Ti is preferably 0.050% or less, and more preferably 0.035% or less.
• Nb is only required to be contained in an amount of 0.001% or more.
• the content of Nb is desirably set to 0.005% or more, and it is more desirable that Nb is contained in an amount of 0.010% or more.
• the content of Nb is preferably 0.035% or less, and more preferably 0.030% or less.
• 0.01% or more of V may be contained, but, if the content of V exceeds 0.50%, the strength of the flat steel wire is increased by the carbide or the carbonitride to be precipitated, which contrarily reduces the hydrogen induced cracking resistance.
• the amount of V is preferably 0.20% or less, and more preferably 0.10% or less. Note that in order to stably achieve the aforementioned effect of V, V is preferably contained in an amount of 0.02% or more.
• Cu is an element which increases the hardenability of the steel, and thus it may be contained. Note that in order to achieve the effect of increasing the hardenability, 0.01% or more of Cu is only required to be contained. However, if the content of Cu exceeds 1.0%, the strength of the wire material becomes too high, and there arises a problem such that cracking occurs in the wire material when the wire material is worked into the flat steel wire. Therefore, when Cu is contained, the content of Cu is 0.01 to 1.0%. From a viewpoint of improving the hardenability, when Cu is contained, the content of Cu is preferably 0.10% or more, and it is more preferable that 0.30% or more of Cu is contained. Note that by considering the workability of the wire material into the flat steel wire, when Cu is contained, the content of Cu is preferably set to 0.80% or less, and it is more preferably 0.50% or less.
• Ni is an element which increases the hardenability of the steel, and thus it may be contained. Note that in order to achieve the effect of increasing the hardenability, 0.01% or more of Ni is only required to be contained. However, if the content of Ni exceeds 1.50%, the strength of the wire material becomes too high, and there arises a problem such that cracking occurs in the wire material when the wire material is worked into the flat steel wire. Therefore, when Ni is contained, the content of Ni is 0.01 to 1.50%. From a viewpoint of improving the hardenability, when Ni is contained, the content of Ni is preferably 0.10% or more, and it is more preferable that 0.30% or more of Ni is contained. Note that by considering the workability of the wire material into the flat steel wire, when Ni is contained, the content of Ni is preferably set to 1.0% or less, and it is more preferably 0.60% or less.
• Mo is an element which increases the hardenability of the steel, and thus it may be contained. Note that in order to achieve the effect of increasing the hardenability, 0.01% or more of Mo is only required to be contained. However, if the content of Mo exceeds 1.0%, the strength of the wire material becomes too high, and there arises a problem such that cracking occurs in the wire material when the wire material is worked into the flat steel wire. Therefore, when Mo is contained, the content of Mo is 0.01 to 1.0%. From a viewpoint of improving the hardenability, when Mo is contained, the content of Mo is preferably 0.02% or more, and it is more preferable that 0.05% or more of Mo is contained. Note that by considering the workability of the wire material into the flat steel wire, when Mo is contained, the content of Mo is preferably set to 0.50% or less, and it is more preferably 0.30% or less.
• B is effective for increasing the hardenability of the steel when a slight amount thereof is added, and in order to achieve the effect, 0.0002% or more of B may be contained. Even if B is contained in an amount exceeding 0.01%, the effect is saturated, and not only that, a coarse nitride is generated, resulting in that the hydrogen induced cracking is likely to occur. Therefore, when B is contained, the content of B is 0.0002 to 0.01%. When the hardenability is desired to be increased more, the content of B is only required to be set to 0.001% or more, and it is more preferably 0.002% or more. Note that by considering the hydrogen induced cracking, when B is contained, the content of B is preferably set to 0.005% or less, and more preferably 0.003% or less.
• REM is a general term of rare earth elements, and a content of REM is a total content of rare earth elements.
• REM has an effect of making MnS to be finely dispersed by being solid-dissolved in MnS, similarly to Ca and Mg.
• MnS finely dispersed, it is possible to improve the hydrogen induced cracking resistance, so that REM may be added.
• REM is only required to be contained in an amount of 0.0002% or more, and when a higher effect is desired to be achieved, 0.0005% or more of REM is only required to be contained.
• the content of REM is 0.10% or less. From a viewpoint of improving the hydrogen induced cracking resistance, the content of REM is preferably 0.05% or less, and more preferably 0.03% or less.
• Zr reacts with O to generate an oxide, and if a slight amount thereof is added, it exhibits an effect of making the oxide to be finely dispersed to suppress the hydrogen induced cracking, so that it may be added when the effect is desired to be achieved.
• 0.0002% or more of Zr is only required to be contained, and when a higher effect is desired to be achieved, 0.001% or more of Zr is only required to be contained.
• the content of Zr exceeds 0.10%, the effect of Zr is saturated, and Zr reacts with N or S in the steel to generate a coarse nitride or sulfide, which contrarily causes reduction in the hydrogen induced cracking resistance.
• the content of Zr is 0.10% or less.
• the content of Zr is preferably 0.08% or less, and more preferably 0.05% or less.
• the balance is composed of “Fe and impurities”.
• the “impurities” are components which are unintentionally contained in the steel material, and indicate components mixed from an ore as a raw material, a scrap, a manufacturing environment or the like at a time of industrially manufacturing an iron and steel material.
• Each of steels A, B having chemical components represented in Table 1 was smelted in an electric furnace, the obtained steel ingot was heated at 1250° C. for 12 hours and then subjected to bloom rolling into a steel billet of 122 mm square, which was set as a raw material for rolling. Next, the raw material for rolling was heated at 1050° C. to be rolled to a wire material with a diameter of 12 mm. After performing the rolling, a surface of the wire material was subjected to lubricating treatment, and then primary wire drawing was performed to obtain a wire material with a diameter of 11 mm. Thereafter, the drawn wire material was rolled by a cold rolling mill to be formed into a flat steel wire.
• test numbers A1 to A5 flat steel wires each of which was cold-rolled to have a width of 15 mm and a thickness of 3 mm were heated at 900° C. for 15 minutes, then subjected to quenching treatment by being immersed into a cold oil, and subjected to heat treatment at a temperature of 400 to 600° C. for 1 minute or 60 minutes, to thereby produce flat steel wires with different tensile strengths.
• a test number A6 the heat treatment was not performed after the cold rolling.
• test numbers B1 to B4 the cold rolling was performed to obtain flat steel wires each having a width of 13.5 mm and a thickness of 5 mm, and then, without performing the quenching treatment, the heat treatment was performed at 600° C. for 10 minutes regarding the test number B1, the heat treatment was performed at 450° C. for 30 seconds regarding the test number B2, the heat treatment was performed at 600° C. for 240 minutes regarding B3, and then cooling was performed to a room temperature. Regarding the test number B4, no heat treatment was performed. Further, regarding a test number B5, the cold rolling was performed to obtain a flat steel wire having a width of 10 mm and a thickness of 8 mm, and the heat treatment was not performed.
• the flat steel wires in which the tensile strengths, the hardness dispersions in cross sections perpendicular to the longitudinal direction, and the shapes were different, were produced.
• a test number B6 a flat steel wire having a width of 17 mm and a thickness of 1.5 mm was produced, heated at 900° C. for 15 minutes, and then immersed into a cold oil to be subjected to the quenching treatment. At that time, large warpage occurred in the longitudinal direction of the flat steel wire, so that a test thereafter was canceled.
• Each of steels in test Nos. 1 to 31 having chemical components represented in Table 2 was smelted in an electric furnace, the obtained steel ingot was heated at 1250° C. for 12 hours and then subjected to bloom rolling into a steel billet of 122 mm square, which was set as a raw material for rolling. Next, the raw material for rolling was heated at 1050° C. to be rolled to a wire material with a diameter of 12 mm. After performing the rolling, a surface of the wire material was subjected to lubricating treatment, and then primary wire drawing was performed to obtain a wire material with a diameter of 11 mm.
• the drawn wire material was rolled by a cold rolling mill to be formed into a flat steel wire having a width of 15 mm and a thickness of 3 mm or a width of 13.5 mm and a thickness of 5 mm.
• the formed flat steel wires after being subjected to the cold rolling were heated at 900° C. for 15 minutes, then immersed into a cold oil to be subjected to quenching treatment, and subjected to heat treatment at a temperature of 450 to 500° C. for 60 minutes.
• the quenching treatment was not performed, and heating at 600° C. for 2 minutes or heating at 580° C.
• the tensile strength, the average hardness and the standard deviation representing the hardness dispersion in the cross section perpendicular to the longitudinal direction, and the hydrogen induced cracking resistance of the flat steel wires were respectively examined through methods to be described below.
• the tensile strength of the flat steel wire was measured by a break test described in JIS G 3546. The break test was carried out at a room temperature by setting a gage length to 30 mm, thereby determining the tensile strength. Note that a cross-sectional area (S (mm 2 )) of the flat steel wire was calculated by using the following expression ⁇ 2>, and the tensile strength was determined by dividing a maximum test force until when the test piece breaks by the cross-sectional area.
• w indicates a width (mm) of the flat steel wire
• t indicates a thickness (mm) of the flat steel wire.
• the flat steel wire was cut into a length of 10 mm, resin embedding and mirror polishing were performed so that a transverse section (a cross section perpendicular to the longitudinal direction) thereof becomes a specimen plane, and the Hv hardness was measured by using a Vickers hardness tester.
• a test load was 100 gf, and measurement at ten points at equal intervals in a thickness direction from a position separated by 50 ⁇ m or more from a surface was repeated nine times or more while being displaced by 1 mm in a width direction to measure a hardness distribution in the cross section, thereby determining the average hardness and the standard deviation ( ⁇ Hv) as an index of the hardness dispersion.
• the standard deviation ⁇ Hv to be the index of the hardness dispersion may be determined through the following expression ⁇ 3>.
• n indicates the number of hardness measurement points in the cross section
• Hv AV indicates the average hardness
• Hv i indicates the hardness at a position of a measurement point i.
• the flat steel wire cut into a length of 150 mm was used to evaluate the hydrogen induced cracking resistance.
• HCl was applied to adjust a pH to be 5.0.
• the deaeration was performed using a nitrogen gas, and then a mixed gas of hydrogen sulfide (H 2 S) and carbon dioxide (CO 2 ) was introduced, and the flat steel wire was immersed into the solution to examine occurrence of cracking.
• a partial pressure of the hydrogen sulfide is 0.01 MPa
• a test temperature is 25° C.
• a test time is 96 hours.
• a f indicates the total area (mm 2 ) of the cracking-occurred portion measured by the UST
• w indicates the width (mm) of the flat steel wire
• L indicates the length (mm) of the flat steel wire.
• each of the test numbers A2 to A5, and B1 being the examples of the present invention satisfied the chemical components and the requirements of the present invention, and the manufacturing conditions of the steel materials were appropriate, so that although the tensile strength thereof was 1000 MPa or more, the hydrogen induced cracking did not occur and thus there is no problem.
• the working was performed to obtain the flat steel wire and then the heat treatment was conducted, in which the standard deviation of hardness ( ⁇ Hv) in the cross section was 15 or more and thus the dispersion of hardness in the cross section was large, resulting in that the hydrogen induced cracking occurrence rate was 10% or more.
• ⁇ Hv standard deviation of hardness
• the working was performed to obtain the flat steel wire and then the heat treatment was conducted, in which the average hardness was less than Hv320, and the tensile strength was less than 1000 MPa.
• the shape of the flat steel wire was out of the range of the present invention, and since the amount of working with respect to the flat steel wire was small, the tensile strength was less than 1000 MPa. Besides, since no heat treatment was conducted, the standard deviation of hardness ( ⁇ Hv) in the cross section was 15 or more, and the hydrogen induced cracking occurrence rate was 10% or more.
• the shape of the flat steel wire was out of the range of the present invention, so that large warpage occurred in the longitudinal direction of the flat steel wire during the quenching treatment, and thus the test such as the tensile test was not carried out.
• each of the test numbers 1 to 19 being the examples of the present invention satisfied the chemical components and the requirements of the present invention, and the manufacturing conditions of the steel materials were appropriate, so that although the tensile strength thereof was 1000 MPa or more, the hydrogen induced cracking did not occur or the hydrogen induced cracking occurrence rate was less than 10%, and thus there is no problem.
• any of the chemical components or the expression ⁇ 1> was not satisfied, so that the hydrogen induced cracking occurred at the hydrogen induced cracking occurrence rate of 10% or more.
• any of the chemical components of the steel was out of the range of the present invention, and the cracking occurred in the flat steel wire when the cold rolling was performed to obtain the flat steel wire, so that the test was canceled without performing the step of heat treatment and thereafter.
• the content of Si was out of the range of the present invention, so that the hydrogen induced cracking occurred at the hydrogen induced cracking occurrence rate of 10% or more.
• the content of S was out of the range of the present invention, so that the hydrogen induced cracking occurred at the hydrogen induced cracking occurrence rate of 10% or more.
• the content of C was out of the range of the present invention, and the standard deviation representing the hardness dispersion in the cross section exceeded 15, so that the hydrogen induced cracking occurred at the hydrogen induced cracking occurrence rate of 10% or more.
• the content of Si was out of the range of the present invention, and the cracking occurred in the flat steel wire when the cold rolling was performed to obtain the flat steel wire.
• the content of Mn was out of the range of the present invention, and the cracking occurred in the flat steel wire material when the cold rolling was performed to obtain the flat steel wire.
• the content of Cr was out of the range of the present invention, and the cracking occurred in the flat steel wire material when the cold rolling was performed to obtain the flat steel wire.
• the content of P was out of the range of the present invention, so that the hydrogen induced cracking occurred at the hydrogen induced cracking occurrence rate of 10% or more.
• the content of N was out of the range of the present invention, and the cracking occurred in the flat steel wire material when the cold rolling was performed to obtain the flat steel wire.

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