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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • 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

Definitions

• the present invention relates to steel having a high strength and an excellent low-temperature toughness after quenching and tempering.
• Such chains are manufactured by cutting a hot rolled steel bar having a diameter of ⁇ 50 mm or more to a predetermined length, forming the steel bar to an annular shape, and welding butted end surfaces through flash butt welding. After flash butt welding, there are cases where a stud is press-fitted into the center of the annular chain. After that, the chain is quenched and tempered, thereby imparting a high strength and a high toughness to the chain.
• Patent Documents 1 to 6 and the like can be exemplified as invention examples of steel for a high strength and high toughness chain.
• all of the documents aim to provide a chain having a tensile strength of 800 MPa to 1,000 MPa and do not study a case where the strength of steel is set to 1,200 MPa or more.
• an increase in the strength of steel generally degrades the toughness of steel and thus decreases the impact value of steel.
• the strength of the steel proposed by the above described documents is set to 1,200 MPa or more, it is not possible to obtain an intended impact value.
• An object of the present invention is to provide a steel having a high strength and an excellent low-temperature toughness (particularly, fracture toughness at a low temperature) after quenching and tempering.
• the object of the invention is to provide a steel in which the Charpy impact value at ⁇ 20° C. reaches 75 J/cm 2 or more, when quenching and tempering are carried out so that the tensile strength reaches 1,200 MPa or more.
• the gist of the present invention is as described below.
• a steel containing, by unit mass %, C: 0.08% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.00% to 3.00%, P: 0.040% or less, S: 0.020% or less, Cr: 1.00% to 2.50%, Cu: 0.01% to 0.50%, Ni: 0.75% to 3.20%, Mo: 0.10% to 0.50%, Nb: 0.005% to 0.050%, Al: 0.010% to 0.100%, N: 0.0050% to 0.0150%, V: 0% to 0.300%, Ca: 0% to 0.0100%, Zr: 0% to 0.0100%, Mg: 0% to 0.0100%, and a remainder including Fe and impurities, in which a number density of Mn sulfides having an equivalent circle diameter of more than 5 ⁇ m is 0 pieces/mm2 to 10 pieces/mm2, and an average aspect ratio of the Mn sulfides having an equivalent circle diameter of
• the steel according to (1) may contain, by unit mass %, V: 0.010% to 0.300%.
• the steel according to (1) or (2) may contain, by unit mass %, one or more selected from the group consisting of Ca: 0.0005% to 0.0100%, Zr: 0.0005% to 0.0100%, and Mg: 0.0005% to 0.0100%.
• the present inventors have continued a variety of researches in order to realize steel having a high strength and an excellent low-temperature toughness, as a result, the present inventors obtained the following findings.
• the present inventors found the chemical composition, inclusion state, and manufacturing method for steel that can be used to manufacture a structural component having a high strength and a high low-temperature toughness, particularly, chains.
• a specific aspect of steel according to the present embodiment will be described.
• the steel according to the present embodiment is steel having an effect in which the tensile strength reaches 1,200 MPa or more and the Charpy impact value at ⁇ 20° C. reaches 75 J/cm 2 or more after quenching and tempering, the strength and the impact value before quenching and tempering are not particularly limited.
• description of mechanical properties such as strength and toughness relates to the steel according to the present embodiment after quenching and tempering.
• the unit “%” of the amounts of the alloying elements indicates mass %.
• the C is an important element that determines the strength of the steel.
• the lower limit of the C content is set to 0.08%.
• the upper limit of the C content is set to 0.12%.
• the upper limit of the C content is preferably 0.11%.
• the lower limit of the C content is preferably 0.09%.
• Si has an action for ensuring the strength of the steel and also an action as a deoxidizing agent.
• the Si content is less than 0.05%, the deoxidizing action cannot be sufficiently obtained, the number of non-metallic inclusions in the steel is increased, and the toughness of the steel is degraded.
• the Si content is more than 0.50%, Si causes the degradation in the toughness of the steel. Therefore, the Si content is set to 0.05% to 0.50%.
• the upper limit of the Si content is preferably 0.40%, 0.30%, or 0.20%.
• the lower limit of the Si content is preferably 0.06%, 0.07%, or 0.08%.
• Mn is an essential element for ensuring a desired hardenability.
• the lower limit of the Mn content is set to 1.0%.
• the upper limit of the Mn content is set to 3.00%.
• the upper limit of the Mn content is preferably 2.90%, 2.80%, or 2.70%.
• the lower limit of the Mn content is preferably 1.10%, 1.20%, or 1.30%.
• P is an impurity that is incorporated into the steel during the manufacturing process of the steel.
• the upper limit of the P content is preferably 0.030%, 0.025%, or 0.020%.
• the steel according to the present embodiment does not need P, and thus the lower limit of the P content is 0%; however, when the capability of a refining facility and the like are taken into account, the lower limit of the P content may be set to 0.001%, 0.002%, or 0.003%.
• S is, similar to P, an impurity that is incorporated into the steel during the manufacturing process of the steel.
• S content exceeds 0.020%, S forms a large amount of Mn sulfides in the steel, and the toughness of the steel is degraded. Therefore, the S content is limited to 0.020% or less.
• the S content is 0.020% or less, the number density of the Mn sulfides is sufficiently decreased, and the toughness of the steel is maintained at a high level.
• the upper limit of the S content is preferably 0.015%, 0.012%, or 0.010%.
• the steel according to the present embodiment does not need S, and thus the lower limit of the S content is 0%; however, when the capability of a refining facility and the like are taken into account, the lower limit of the S content may be set to 0.001%, 0.002%, or 0.003%.
• the Cr has an action for enhancing the hardenability of the steel.
• the lower limit of the Cr content is set to 1.00%.
• the upper limit of the Cr content is set to 2.50%.
• the upper limit of the Cr content is preferably 2.40%, 2.30%, or 2.20%.
• the lower limit of the Cr content is preferably 1.30%, 1.40%, or 1.50%.
• the Cu is an effective element for improving the hardenability and corrosion resistance of the steel.
• the lower limit of the Cu content is set to 0.01%.
• the upper limit of the Cu content is set to 0.50%.
• the upper limit of the Cu content is preferably 0.40%, 0.30%, or 0.20%.
• the lower limit of the Cu content is preferably 0.02%, 0.03%, or 0.05%.
• Ni is an extremely effective element for improving the toughness of the steel and an essential element for increasing the toughness of the steel according to the present embodiment in which a tensile strength after quenching and tempering is 1,200 MPa or more.
• a tensile strength after quenching and tempering is 1,200 MPa or more.
• the Ni content is set to 0.75% to 3.20%.
• the upper limit of the Ni content is preferably 3.15%, 3.10%, or 3.05%.
• the lower limit of the Ni content is preferably 0.80%, 0.85%, or 0.90%.
• Mo has an effect for improving the low-temperature toughness of the steel, when Mo is contained in the steel together with Ni and Nb. This is considered to be because, when the steel contains Mo together with Ni and Nb, cementite in the steel which may generally act as an origin of fracture is refined to a level at which the cementite does not act as a fracture origin.
• the block size of a martensite becomes small, and thus it is assumed that the ductile brittle transition temperature of the steel is decreased and brittle fracture does not easily occur even at a low temperature.
• Mo content is less than 0.10%, it is difficult to sufficiently exhibit the effects.
• the Mo content is set to 0.10% to 0.50%.
• the upper limit of the Mo content is preferably 0.47%, 0.45%, or 0.42%.
• the lower limit of the Mo content is preferably 0.15%, 0.20%, or 0.25%.
• Nb has an effect for improving the low-temperature toughness of the steel, when Nb is contained in the steel together with Ni and Mo. This is considered to be because, when the steel contains Nb together with Ni and Mo, cementite in the steel which may generally act as an origin of fracture is refined to a level at which the cementite does not act as a fracture origin. In addition, when the steel contains Nb together with Ni and Mo, the block size of a martensite becomes small, and thus it is assumed that the ductile brittle transition temperature of the steel is decreased and brittle fracture does not easily occur even at a low temperature. When the Nb content is less than 0.005%, it is difficult to sufficiently exhibit the effects.
• the Nb content is set to 0.005% to 0.050%.
• the upper limit of the Nb content is preferably 0.045%, 0.040%, or 0.035%.
• the lower limit of the Nb content is preferably 0.007%, 0.010%, or 0.015%.
• A1 has an action for adjusting the crystal grain size of a metallographic structure and miniaturizing the metallographic structure when A1 is precipitated as AlN.
• the A1 content is less than 0.010%, it is not possible to obtain a sufficient miniaturizing effect, and thus the toughness of the steel is degraded.
• the A1 content in the steel exceeds 0.100%, the amount of AlN precipitated is saturated, the number of alumina based non-metallic inclusions in the steel is increased, and the toughness of the steel is degraded. Therefore, the A1 content is set to 0.010% to 0.100%.
• the upper limit of the A1 content is preferably 0.090%, 0.070%, or 0.050%.
• the lower limit of the A1 content is preferably 0.012%, 0.015%, or 0.018%.
• N has an action for precipitating AlN, which is effective for adjusting the crystal grain size of the metallographic structure, by bonding to A1.
• the N content is set to 0.0050% to 0.0150%.
• the upper limit of the N content is preferably 0.0140%, 0.0130%, or 0.0120%.
• the lower limit of the N content is preferably 0.0055%, 0.0060%, or 0.0065%.
• V 0% to 0.300%
• the steel according to the present embodiment does not need V. Therefore, the lower limit of the V content is 0%.
• V has an action for adjusting the crystal grain size of the metallographic structure and miniaturizing the metallographic structure when V is precipitated as VN. Therefore, as an optional element, the steel may contain 0.010% or more, 0.020% or more, or 0.030% or more of V.
• the V content in the steel exceeds 0.300%, coarse VN remains in the steel after heating for quenching, and this coarse VN degrades the toughness of the steel after quenching and tempering. Therefore, the V content is set to 0.300% or less.
• the upper limit of the V content is preferably 0.250% or less, 0.200%, or 0.150%.
• the steel according to the present embodiment does not need Ca, Zr, and Mg. Therefore, the lower limit of the V content is 0%.
• all of Ca. Zr, and Mg have an effect for forming an oxide, acting as a crystallization nucleus of MnS, and uniformly and finely dispersing MnS so as to improve the impact value of the steel. Therefore, as an optional element, the steel may contain 0.0005% or more, 0.0010% or more, or 0.0015% or more of Ca, may contain 0.0005% or more, 0.0010% or more, or 0.0015% or more of Zr, and may contain 0.0005% or more, 0.0010% or more, or 0.0015% or more of Mg.
• the upper limits of each of the Ca content, the Zr content, and the Mg content is set to 0.0100% or less.
• the upper limit of the Ca content is preferably 0.0090%, 0.0070%, or 0.0050%
• the upper limit of the Zr content is preferably 0.0090%, 0.0070%, or 0.0050%
• the upper limit of the Mg content is preferably 0.0090%, 0.0070%, or 0.0050%.
• the remainder of the chemical composition of the steel according to the present embodiment consists of Fe and impurities.
• the impurities refer to elements which are incorporated by a raw material such as an ore or a scrap, or a variety of causes in the manufacturing process during the industrial manufacturing of the steel, and the impurities are permitted to an extent in which the steel according to the present embodiment is not adversely influenced.
• Number density of Mn sulfides having an equivalent circle diameter of more than 5 ⁇ m is 0 pieces/mm 2 to 10 pieces/mm 2
• a Mn sulfide having an equivalent circle diameter of more than 5 ⁇ m (hereinafter, referred to as a “coarse Mn sulfide”) significantly degrades the low-temperature toughness of the steel, and thus the number density of the coarse Mn sulfides is preferably set to substantially 0 pieces/mm 2 . Therefore, the lower limit of the number density of the coarse Mn sulfides is 0 pieces/mm 2 . However, when the number density is 10 pieces/mm 2 or less, the low-temperature toughness is not seriously impaired. Therefore, the upper limit of the number density of the coarse Mn sulfides is set to 10 pieces/mm 2 . The upper limit of the number density of the coarse Mn sulfides is preferably 9 pieces/mm 2 , 8 pieces/mm 2 , or 7 pieces/mm 2 .
• Average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 ⁇ m to 5.0 ⁇ m is 1.0 or more and 10.0 or less
• a Mn sulfide having an equivalent circle diameter of 1.0 ⁇ m to 5.0 ⁇ m (hereinafter, referred to as a “fine Mn sulfide”) has a smaller adverse influence on the toughness of the steel than the coarse Mn sulfide.
• a fine Mn sulfide in which the aspect ratio of the Mn sulfide that can be calculated by dividing the major axis of the Mn sulfide by the minor axis of the Mn sulfide is excessively large, may act as an origin of fracture and degrade the toughness of the steel, similar to the coarse Mn sulfide.
• the present inventors found that, when the average aspect ratio of the fine Mn sulfides is set to 10.0 or less, it is possible to make the fine Mn sulfides almost harmless.
• a preferred upper limit of the average aspect ratio of the fine Mn sulfides is 9.0, 7.5, or 6.0.
• the major axis and the minor axis of the fine Mn sulfide are equal to each other, the aspect ratio of the fine Mn sulfide reaches 1.0, and thus the lower limit of the average aspect ratio of the fine Mn sulfides is set to 1.0.
• the fine dispersion of Mn sulfides which may act as origins of fracture and a decrease in the aspect ratios thereof are extremely effective for improving the low-temperature toughness of the steel.
• the state of the Mn sulfides does not change before and after quenching and tempering that are carried out under ordinary conditions, the state of the Mn sulfides is maintained even after quenching and tempering as long as the state of the Mn sulfides is controlled as described above before quenching and tempering, and the above described effects can be obtained.
• the number density of the fine Mn sulfides there is no need for limiting the number density of the fine Mn sulfides. Although there is concern that an extremely large amount of the fine Mn sulfides impairs the toughness of the steel, the number density of the fine Mn sulfides does not increase to an extent in which the toughness of the steel is impaired, as long as the S content is in the above described range.
• a Mn sulfide having an equivalent circle diameter of less than 1.0 ⁇ m (hereinafter, referred to as an “ultrafine Mn sulfide”) does not act as an origin of fracture, and thus the aspect ratio and number density of the ultrafine Mn sulfide are not particularly specified.
• the Mn sulfides (the coarse Mn sulfides and the fine Mn sulfides) are almost uniformly dispersed, and thus a place where the state of the Mn sulfide is specified is not particularly limited.
• a method for specifying the state of the Mn sulfide is as described below. First, a cross section of the steel is mirror-polished, and then optical microscopic photographs are captured at 10 or more random places on the cross section at a magnification of 1,000 times. The ten photographs obtained in the above described manner are processed using image analysis software, for example, Luzex (registered trademark) or the like, whereby the state of Mn sulfides in the steel, that is, the number density of the coarse Mn sulfides and the average aspect ratio of the fine Mn sulfides can be obtained.
• image analysis software for example, Luzex (registered trademark) or the like
• Mn sulfides are elongated in a processing direction.
• the Mn sulfides are elongated in a hot rolling direction. Therefore, the cross section where the optical microscopic photographs are captured needs to be formed parallel to the processing direction (for example, the hot rolling direction).
• the Mn sulfides are almost uniformly dispersed in the steel according to the present embodiment, and a place where the optical microscopic photographs are captured is not particularly specified.
• the method for manufacturing the steel according to the present embodiment includes a process of continuously casting molten steel having the chemical composition of the steel according to the present embodiment so as to obtain a slab and a process of soaking the slab twice or more.
• the conditions for continuously casting the molten steel are not particularly limited. In the process of soaking the slab, firstly, the slab is heated up to a temperature range of 1,300° C. to 1,350° C., and then, the temperature of the slab is held in this temperature range for 300 seconds to 18,000 seconds, and furthermore, the slab is cooled to 900° C. or lower. In addition, the soaking is carried out twice or more.
• the soaking is carried out in order to finely disperse Mn sulfides included in the slab.
• coarse Mn sulfides are crystallized in the slab.
• the slab is heated up to a temperature range of 1,300° C. to 1,350° C. and then held in this temperature range for 300 seconds to 18,000 seconds, the coarse Mn sulfides are solutionized, and the Mn sulfides are precipitated when the slab is cooled to 900° C. or lower.
• the Mn sulfides are refined by solutionizing and precipitation.
• the holding temperature of the slab is lower than 1,300° C. and when the holding time of the slab at the temperature is shorter than 300 seconds, the Mn sulfides are not sufficiently solutionized. In addition, when the soaking is carried out only once, the Mn sulfides are not sufficiently refined. In order to set the dispersion state of the Mn sulfides in the steel in the above described range, the soaking under the above described conditions needs to be carried out twice or more. When the cooling stop temperature of the slab is set to higher than 900° C. and the subsequent soaking is initiated, the Mn sulfides are not precipitated during cooling, and thus the refinement of the Mn sulfides becomes insufficient.
• the heating temperature of the slab is higher than 1,350° C.
• the ductility of the slab is degraded, and a problem of cracking is caused.
• the heating time of the slab is longer than 18,000 seconds, it is not preferable in consideration of the economic efficiency.
• the slab in which the Mn sulfides are sufficiently refined by the above described treatment it is possible to carry out an optional processing and an optional heat treatment afterwards. For example, blooming and hot rolling are performed on this slab so as to produce a steel bar, and a chain processing is performed on this steel bar, whereby a chain can be obtained. In addition, during or after the chain processing, it is possible to quench and temper the chain. Since the Mn sulfides included in the slab obtained using the above described method are sufficiently refined, and it is assumed that the fine Mn sulfides included in the slab are not outside the specification range described above due to blooming, hot rolling, the chain processing, and quenching and tempering that are carried out under ordinary conditions.
• the steel according to the present embodiment is particularly preferably used as steel for quenching and tempering.
• the number density of Mn sulfides having an equivalent circle diameter of more than 5 ⁇ m is 0 pieces/mm 2 to 10 pieces/mm 2
• the average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 ⁇ m to 5.0 ⁇ m is 1.0 or more and 10.0 or less
• the average grain size of cementite is 0.05 ⁇ m or less
• the average size of martensite blocks is 5.5 ⁇ m or less.
• the steel according to the present embodiment contains 0.08% or more of C and thus has a tensile strength of 1,200 MPa or more, when the heat treatment under the above described quenching and tempering conditions is performed.
• the low-temperature toughness (particularly, low-temperature toughness) is impaired when the tensile strength of the steel is 1,200 MPa or more.
• the steel according to the present embodiment contains 0.75% to 3.20% of Ni, 0.10% to 0.50% of Mo, and 0.005% to 0.050% of Nb, and thus, when the heat treatment under the above described quenching and tempering conditions is performed, martensite blocks and cementite are sufficiently refined, and the steel has a high low-temperature toughness.
• the number density of Mn sulfides having an equivalent circle diameter of more than 5 ⁇ m is 0 pieces/mm 2 to 10 pieces/mm 2
• the average aspect ratio of the Mn sulfides having an equivalent circle diameter of 1.0 ⁇ m to 5.0 ⁇ m is 1.0 or more and 10.0 or less, and thus the steel has a high low-temperature toughness.
• quenching and tempering under the above described conditions are simply an example of the use of the steel according to the present embodiment.
• a heat treatment under optional conditions can be performed on the steel according to the present embodiment.
• the properties of the steel according to the present embodiment on which the heat treatment is performed on the basis of an example of the above described quenching and tempering conditions do not limit the technical scope of the steel according to the present embodiment.
• the object of the steel according to the present embodiment is to obtain a Charpy impact value at ⁇ 20° C. of 75 J/cm 2 or more after a heat treatment is carried out so that the tensile strength reaches 1,200 MPa.
• the steel according to the present embodiment is capable of exhibiting particularly excellent effects, when the steel according to the present embodiment is used as a material for chains for mooring offshore oil-drilling rigs which needs to have a high tensile strength and a high low-temperature toughness.
• JIS No. 14A tensile test pieces and four JIS No. 4 V-notch Charpy impact test pieces were produced from a 1 ⁇ 4D portion (a region at a depth of approximately 1 ⁇ 4 of a diameter D of the round steel bar from the surface of the round steel bar) of a C cross section of each of the quenched and tempered round steel bars Nos. A1 to A5.
• a tensile test was carried out at normal temperature and a rate of 20 mm/min according to JIS Z 2241.
• a Charpy impact test was carried out at ⁇ 20° C. according to JIS Z 2242.
• Table 1 shows the chemical compositions of the steel A (that is, the chemical compositions of the steels No. A1 to No. A5).
• Table 2 shows soaking conditions and the number of times of soaking during the manufacturing of the steels No. A1 to No. A5, and the average aspect ratios of Mn sulfides having an equivalent circle diameter of 1.0 ⁇ m to 5.0 ⁇ m, the number densities of Mn sulfides having an equivalent circle diameter of more than 5.0 ⁇ m, the tensile strengths, the impact values, the average grain sizes of the cementite, and the average sizes of the martensite blocks in the steels No. A1 to No.
• the steels No. A1 and No. A2 which were the present invention example, the chemical compositions and the manufacturing conditions were appropriate, and thus the form of the Mn sulfides was in the specification range of the present invention. Therefore, the steels No. A1 and A2 had a tensile strength of 1,200 MPa or more and a Charpy impact value at ⁇ 20° C. of 75 J/cm 2 or more after quenching and tempering. In contrast, in the steels No.
• Each of steel B to AH having a chemical composition shown in Table 3 was continuously cast so as to obtain a slab, then, a soaking in which the holding temperature was 1,300° C. and the holding time was 7,200 seconds was performed on the slab twice, and furthermore, blooming was performed on the slab, thereby obtaining a 162 mm ⁇ 162 mm rolled material. After that, hot rolling was performed on the rolled material, thereby producing a round steel bar having a diameter of 86 mm. Next, a quenching treatment in which the round steel bar was cut, heated to 900° C., held for 30 minutes, and furthermore, cooled with water was carried out, and then a tempering treatment in which the round steel bar was heated to 135° C. and held for 30 minutes was carried out, thereby obtaining round steel bars Nos. B to AH. These quenching conditions and tempering conditions are the same as heat treatment conditions that are recommended for the production of chains using the present invention steel.
• JIS No. 14A tensile test pieces and four JIS No. 4 V-notch Charpy impact test pieces were produced from a 1 ⁇ 4D portion of a C cross section of each of the quenched and tempered round steel bars Nos. B to AH.
• a tensile test was carried out at normal temperature and a rate of 20 mm/min according to JIS Z 2241.
• a Charpy impact test was carried out at ⁇ 20° C. according to JIS Z 2242.
• a 10 mm ⁇ 10 mm sample was cut out from the 1 ⁇ 4D portion of the C cross section of each of the quenched and tempered round steel bars Nos. B to AH, and the metallographic structure of the steel and the state of inclusions were observed on a cross section parallel to a rolling direction.
• the cross section was mirror-polished, then, 10 metallographic photographs were captured using an optical microscope at a magnification of 1,000 times, and the equivalent circle diameters and aspect ratios of Mn sulfides included in the photographs were obtained by means of an image analysis (Luzex (registered trademark)).
• Table 3 shows the chemical compositions of the steels Nos. B to AH.
• Table 4 shows the aspect ratios of Mn sulfides having an equivalent circle diameter of 1.0 ⁇ m to 5.0 ⁇ m, the number densities of Mn sulfides having an equivalent circle diameter of more than 5.0 ⁇ m, the tensile strengths, the impact values, the average grain sizes of the cementite, and the average sizes of the martensite blocks in the steels Nos. B to AH which were quenched and tempered under the above described conditions.
• values outside the specification ranges of the present invention are underlined.
• the steels Nos. B to U As shown in Table 3 and Table 4, in all of the steels Nos. B to U which were the present invention example, the chemical compositions and the states of the Mn sulfides were in the specification range of the present invention. Therefore, the steels Nos. B to U had a tensile strength of 1,200 MPa or more and a Charpy impact value at ⁇ 20° C. of 75 J/cm 2 or more after quenching and tempering.

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