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
  • 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
• • 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 high strength and excellent low temperature toughness after quenching and tempering.
• Such a chain is manufactured by cutting a hot-rolled steel bar having a diameter of 50 mm or more into a predetermined length, forming it into an annular shape, and then flash-butt welding the butted end faces. In some cases, a stud is pressed into the center of the chain ring after flash butt welding. Thereafter, the chain is subjected to quenching and tempering treatment to impart high strength and high toughness to the chain.
• Examples of inventions for high strength and high toughness chain steel include, for example, Patent Documents 1 to 6 and the like.
• Patent Documents 1 to 6 are aimed at providing a chain having a tensile strength of 800 to 1000 MPa, and the case where the strength of the steel is set to 1200 MPa or more has not been studied.
• chains have been required to have higher strength, but it is generally known that when steel materials are strengthened, the toughness of the steel materials decreases, thereby reducing the impact value of the steel materials.
• the steel presented in these documents is made to have a strength of 1200 MPa or more, the intended impact value cannot be obtained.
• Japanese Laid-Open Patent Publication No. 58-22361 Japanese Unexamined Patent Publication No. 58-96856 Japanese Unexamined Patent Publication No. 59-159972 Japanese Unexamined Patent Publication No. 59-159969 Japanese Unexamined Patent Publication No. Sho 62-202052 Japanese Unexamined Patent Publication No. Sho 63-203752
• An object of the present invention is to provide a steel having high strength and excellent low temperature toughness (particularly fracture toughness at low temperature) after quenching and tempering. Specifically, it is to provide a steel having a Charpy impact value at ⁇ 20 ° C. of 75 J / cm 2 or more when quenched and tempered so that the tensile strength is 1200 MPa or more.
• the gist of the present invention is as follows.
• the steel according to one embodiment of the present invention is 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: 1.00-2.50%, Cu: 0.01-0.50%, Ni: 0.75-3.20%, 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 0.00.
• the average aspect ratio of the Mn sulfide having a number density of sulfide of 0 to 10 pieces / mm 2 and an equivalent circle diameter of 1.0 to 5.0 ⁇ m is 1.0 or less.
• the upper limit is 10.0 or less.
• the steel described in (1) may contain V: 0.010 to 0.300% in unit mass%.
• the steel described in (1) or (2) is unit mass%, Ca: 0.0005 to 0.0100%, Zr: 0.0005 to 0.0100%, and Mg: 0.0005.
• One or more selected from the group consisting of ⁇ 0.0100% may be contained.
• a steel having a tensile strength of 1200 MPa or more and a Charpy impact value at ⁇ 20 ° C. of 75 J / cm 2 or more after quenching and tempering can be provided.
• the inventor has made various studies in order to realize a steel having high strength and excellent low-temperature toughness, and has obtained the following knowledge.
• the present inventors have found a chemical component, inclusion state, and manufacturing method of steel capable of manufacturing a structural component having high strength and high low temperature toughness, particularly a chain.
• the steel according to the present embodiment is a steel having an effect that the tensile strength after quenching and tempering is 1200 MPa or more and the Charpy impact value at ⁇ 20 ° C. is 75 J / cm 2 or more.
• the impact value is not particularly limited.
• the description explaining the mechanical properties such as strength and toughness relates to the steel according to the present embodiment after quenching and tempering.
• the unit “%” of the content of the alloy element means mass%.
• C 0.08 to 0.12% C is an important element that determines the strength of steel.
• the lower limit of the C content is 0.08%.
• the upper limit of the C content is 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 0.05 to 0.50% Si has an action as a deoxidizer as well as an action for securing the strength of the steel material.
• 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 1.00 to 3.00%
• Mn is an essential component for ensuring the desired hardenability.
• the lower limit value of the Mn content is 1.0%.
• the upper limit value 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 0.040% or less
• P is an impurity mixed in steel in the steel manufacturing process. If the P content exceeds 0.040%, the toughness of the steel is lowered to an allowable limit or more, so the P content is limited to 0.040% or less.
• the upper limit of the P content is preferably 0.030%, 0.025%, or 0.020%. Since the steel according to the present embodiment does not require P, the lower limit value of the P content is 0%, but considering the capacity of the refining equipment and the like, the lower limit value of the P content is 0.001%, 0 It may be 0.002% or 0.003%.
• S 0.020% or less S, like P, is an impurity mixed in steel in the steel manufacturing process. If the S content exceeds 0.020%, S forms a large amount of Mn sulfide in the steel and lowers the toughness of the steel. Therefore, the S content is limited to 0.020% or less. When the S content is 0.020% or less, the number density of Mn sulfide is sufficiently reduced, and the toughness of the steel is kept high.
• the upper limit of the S content is preferably 0.015%, 0.012%, or 0.010%.
• the lower limit value of the S content is 0%, but considering the capacity of the refining equipment and the like, the lower limit value of the S content is 0.001%, 0 It may be 0.002% or 0.003%.
• Cr 1.00-2.50% Cr has the effect of increasing the hardenability of the steel.
• the lower limit value of the Cr content is set to 1.00%.
• the upper limit of the Cr content is 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%.
• Cu 0.01 to 0.50%
• Cu is an element effective for improving the hardenability and corrosion resistance of steel.
• the lower limit value of the Cu content is 0.01%.
• the upper limit value of the Cu content is 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 0.75-3.20%
• Ni is an extremely effective element for improving the toughness of the steel, and is an essential element for increasing the toughness of the steel according to this embodiment having a tensile strength after quenching and tempering of 1200 MPa or more. If the Ni content is less than 0.75%, it is difficult to sufficiently exert its effect. On the other hand, if the Ni content exceeds 3.20%, the toughness improving effect is saturated. Therefore, 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 0.10 to 0.50%
• the present inventors have found that when Mo is contained in steel together with Ni and Nb, it has the effect of improving the low temperature toughness of the steel. This is considered to be because when Mo is contained in the steel at the same time as Ni and Nb, cementite in the steel, which can usually be the starting point of fracture, is refined to a level that does not become the starting point of fracture. Moreover, when Mo and Ni and Nb are simultaneously contained in the steel, the block size of the martensite structure becomes fine, so it is estimated that the ductile brittle transition temperature of the steel is lowered and brittle fracture is less likely to occur even at low temperatures. . If the Mo content is less than 0.10%, it is difficult to sufficiently exert its effect.
• 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 0.005 to 0.050%
• Nb has an effect of improving the low temperature toughness of the steel. This is considered to be because when Nb is contained in the steel together with Ni and Mo, cementite in the steel, which can normally be the starting point of fracture, is refined to a level that does not become the starting point of fracture.
• the block size of the martensite structure becomes fine, so the ductile brittle transition temperature of the steel is lowered, and it is estimated that brittle fracture is less likely to occur even at low temperatures. If the Nb content is less than 0.005%, it is difficult to sufficiently exhibit the effect.
• the Nb content is set to 0.005 to 0.050%.
• the upper limit of 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%.
• Al 0.010 to 0.100%
• Al has the action of adjusting the crystal grain size of the metal structure and making the metal structure finer when precipitated as AlN. If the Al content is less than 0.010%, a sufficient grain refining effect cannot be obtained, so that the toughness of the steel decreases. On the other hand, when Al exceeds 0.100%, the precipitation amount of AlN is saturated, alumina nonmetallic inclusions in the steel increase, and the toughness of the steel is lowered. Therefore, the Al content is set to 0.010 to 0.100%.
• the upper limit of the Al content is preferably 0.090%, 0.070%, or 0.050%.
• the lower limit of the Al content is preferably 0.012%, 0.015%, or 0.018%.
• N 0.0050 to 0.0150%
• N binds to Al and has the effect of precipitating AlN effective for adjusting the crystal grain size of the metal structure. If the N content is less than 0.0050%, this effect is not sufficiently exhibited.
• 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 this embodiment does not require V. Therefore, the lower limit of the V content is 0%.
• V when V is precipitated as VN, it has the effect of adjusting the crystal grain size of the metal structure and making the metal structure finer. Therefore, V may be contained as an optional element in an amount of 0.010% or more, 0.020% or more, or 0.030% or more.
• V when V is contained in steel exceeding 0.300%, coarse VN remains in the steel during heating during quenching, and this coarse VN reduces the toughness of the steel after quenching and tempering. Therefore, the V content is set to 0.300% or less.
• the upper limit value of the V content is preferably 0.250% or less, 0.200%, or 0.150%.
• Ca may be contained in the steel in an amount of 0.0005% or more, 0.0010% or more, or 0.0015% or more
• Zr may be contained in the steel in an amount of 0.0005% or more, 0.0010%.
• 0.0015% or more may be contained, and Mg may be contained in steel in an amount of 0.0005% or more, 0.0010% or more, or 0.0015% or more.
• the upper limit of each of Ca, Zr, and Mg is 0.0100% or less.
• the upper limit value of Ca content is preferably 0.0090%, 0.0070%, or 0.0050%
• the upper limit value of Zr content is preferably 0.0090%, 0.0070%, or 0.
• the upper limit of the Mg content is preferably 0.0090%, 0.0070%, or 0.0050%.
• Fe and impurities The remainder of the alloy component of the steel according to the present embodiment is composed of Fe and impurities. Impurities are components that are mixed due to various factors of raw materials such as ore or scrap, or manufacturing process when industrially manufacturing steel materials, and a range that does not adversely affect the steel according to the present embodiment. Means what is allowed.
• the number density of Mn sulfide with an equivalent circle diameter of more than 5 ⁇ m is 0 to 10 pieces / mm 2 Mn sulfide having an equivalent circle diameter of more than 5 ⁇ m (hereinafter referred to as “coarse Mn sulfide”) greatly reduces the low-temperature toughness of the steel. Therefore, the number density of coarse Mn sulfide is substantially 0 / mm 2. It is preferable that Therefore, the lower limit of the number density of coarse Mn sulfide is 0 / mm 2 . However, if the number density is 10 pieces / mm 2 or less, the low temperature toughness is not seriously impaired.
• the upper limit of the number density of coarse Mn sulfide is 10 / mm 2 .
• the upper limit of the number density of coarse Mn sulfide is preferably 9 / mm 2 , 8 / mm 2 , or 7 / mm 2 .
• Mn sulfide having an equivalent circle diameter of 1.0 to 5.0 ⁇ m has an average aspect ratio of 1.0 to 10.0 Mn sulfide having an equivalent circle diameter of 1.0 to 5.0 ⁇ m (hereinafter referred to as “fine Mn”). "Sulphides”) have less adverse effects on steel toughness than coarse Mn sulfides. However, a fine Mn sulfide having an excessively large aspect ratio of Mn sulfide, which can be calculated by dividing the major axis of the Mn sulfide by the minor axis of the Mn sulfide, is the starting point of fracture, as in the case of the coarse Mn sulfide. It can reduce the toughness of the steel.
• the present inventors have found that if the average aspect ratio of the fine Mn sulfide is 10.0 or less, the fine Mn sulfide can be made almost harmless.
• the preferable upper limit of the average aspect ratio of the fine Mn sulfide is 9.0, 7.5, or 6.0.
• the aspect ratio of the fine Mn sulfide is 1.0, so the lower limit of the average aspect ratio of the fine Mn sulfide is 1.0.
• Mn sulfide that can be a starting point of fracture and to reduce its aspect ratio in order to improve the low temperature toughness of steel.
• Mn sulfide since the state of Mn sulfide does not change before and after quenching and tempering performed under normal conditions, if the state of Mn sulfide is controlled as described above before quenching and tempering, Mn sulfide can be changed even after quenching and tempering. The state of sulfide is maintained, and the above-described effect is obtained.
• Mn sulfide having an equivalent circle diameter of less than 1.0 ⁇ m (hereinafter referred to as “ultrafine Mn sulfide”) does not serve as a starting point of fracture, and thus the aspect of the ultrafine Mn sulfide.
• the ratio and number density are not particularly specified.
• Mn sulfides (coarse Mn sulfides and fine Mn sulfides) are dispersed almost uniformly, so the location for defining the state of Mn sulfide is not particularly limited.
• the method for specifying the state of Mn sulfide is as follows. First, the cross section of steel is mirror-polished, and then an optical micrograph at a magnification of 1000 is taken at any 10 or more locations in the cross section. By processing the 10 photographs thus obtained using image analysis software such as Luzex (registered trademark), the state of Mn sulfide contained in the steel, that is, the number density of coarse Mn sulfide, The average aspect ratio of the fine Mn sulfide can be obtained.
• the Mn sulfide extends in the processing direction. For example, when steel is hot rolled, the Mn sulfide is stretched in the hot rolling direction.
• the cross section for taking an optical micrograph needs to be formed parallel to the processing direction (for example, the hot rolling direction).
• the processing direction for example, the hot rolling direction.
• the steel manufacturing method includes a step of continuously casting a molten steel having a chemical component of the steel according to the present embodiment to obtain a slab, and a step of performing soaking diffusion treatment on the slab two or more times.
• the conditions for the process of continuously casting molten steel are not particularly limited.
• the slab is first heated to a temperature range of 1300 ° C. to 1350 ° C., and then the temperature of the slab is maintained within this temperature range for 300 to 18000 seconds. Is cooled to 900 ° C. or lower.
• the soaking diffusion process is performed twice or more.
• the soaking process is performed to finely disperse Mn sulfide contained in the slab.
• coarse Mn sulfide crystallizes in the slab.
• the coarse Mn sulfide is solutionized, and the slab is cooled to 900 ° C. or less.
• Mn sulfide is deposited.
• the Mn sulfide is refined by solution and precipitation.
• the holding temperature of the slab is less than 1300 ° C. and when the temperature holding time of the slab is less than 300 seconds, the Mn sulfide is not sufficiently solutionized.
• the soaking diffusion treatment is performed only once, the Mn sulfide is not sufficiently refined.
• the cooling stop temperature of the slab is over 900 ° C. and the next soaking diffusion treatment is started, Mn sulfide does not precipitate during cooling, so the Mn sulfide is not sufficiently refined.
• the heating temperature of a slab is more than 1350 degreeC, the ductility of a slab falls and the problem of a crack arises. Further, when the heating time of the slab is longer than 18000 seconds, it is not preferable in view of economy.
• the slab in which the Mn sulfide is sufficiently refined by the above-described treatment can be subjected to arbitrary processing and heat treatment thereafter.
• this slab can be subjected to partial rolling and hot rolling to form a steel bar, and this steel bar can be chain processed to obtain a chain. Further, the chain can be quenched and tempered during or after the chain processing. Since the Mn sulfide contained in the slab obtained by the above-described method is sufficiently refined, the slab is obtained by the partial rolling, hot rolling, chain processing, and quenching and tempering performed under normal conditions. It is presumed that the contained fine Mn sulfide does not fall outside the specified range described above.
• the steel according to this embodiment is particularly preferably used as a steel for quenching and tempering.
• the steel according to the present embodiment is subjected to a quenching process in which the steel is heated to 900 ° C. and held for 30 minutes and then cooled with water, and further tempered to be heated to 135 ° C. and held for 30 minutes, the tensile strength is 1200 MPa.
• a steel having a Charpy impact value at ⁇ 20 ° C. of 75 J / cm 2 or more is obtained.
• the steel according to this embodiment after being heat-treated under the quenching and tempering conditions has a number density of Mn sulfides with an equivalent circle diameter of more than 5 ⁇ m of 0 to 10 pieces / mm 2 and an equivalent circle diameter of 1.0 to 1.0.
• the average aspect ratio of the Mn sulfide of 5.0 ⁇ m is 1.0 or more and 10.0 or less, the average particle size of cementite is 0.05 ⁇ m or less, and the average size of the martensite block is 5.5 ⁇ m or less. is there.
• the steel according to the present embodiment contains 0.08% or more of C, it has a tensile strength of 1200 MPa or more when heat-treated under this quenching and tempering condition. Normally, when the tensile strength of steel is 1200 MPa or more, low temperature toughness (particularly low temperature toughness) is impaired. However, the steel according to the present embodiment contains 0.75 to 3.20% Ni, 0.10 to 0.50% Mo, and 0.005 to 0.050% Nb.
• the martensite block and cementite are sufficiently refined and have high low temperature toughness.
• the steel according to the present embodiment after being heat-treated under the quenching and tempering conditions has a number density of Mn sulfide having an equivalent circle diameter of more than 5 ⁇ m of 0 to 10 like the steel according to the present embodiment before quenching and tempering. a number / mm 2, a circular average aspect ratio of the Mn sulfide equivalent diameter of 1.0 ⁇ 5.0 .mu.m 1.0 or more, since at 10.0, with a high low-temperature toughness.
• the quenching and tempering under the above-described conditions is merely an example of the use of the steel according to the present embodiment.
• the steel according to the present embodiment can be heat-treated under any conditions depending on the purpose.
• the characteristics of the steel according to the present embodiment after the heat treatment based on the example of the quenching and tempering condition described above is not limited to the technical scope of the steel according to the present embodiment.
• the problem with the steel according to this embodiment is that the Charpy impact value at ⁇ 20 ° C. is 75 J / cm 2 or more after heat treatment is performed so that the tensile strength is 1200 MPa.
• other configurations, such as control of the martensite and cementite states before heat treatment are not required for solving the problems of the steel according to the present embodiment.
• the steel according to the present embodiment can exhibit particularly excellent effects when used as a material such as a submarine oil drilling rig mooring chain that requires high tensile strength and high low temperature toughness.
• Steel A having the chemical composition shown in Table 1 is continuously cast to obtain a slab, and then the slab is subjected to one or more soaking and diffusion processes, and the slab is subjected to a block rolling process.
• a 162 mm square rolled material was obtained.
• Table 2 shows the soaking diffusion treatment conditions and the number of soaking diffusion treatments. Thereafter, the rolled material was hot-rolled to obtain a round steel bar having a diameter of 86 mm. Next, the round bar steel was cut, heated to 900 ° C. and held for 30 minutes, further subjected to a quenching treatment with water cooling, and then subjected to a tempering treatment heated to 135 ° C. and held for 30 minutes. A1 to A5 were obtained. These quenching conditions and tempering conditions are the same as the heat treatment conditions recommended when creating a chain using the steel of the present invention.
• Table 1 shows the chemical components of Steel A (that is, the chemical components of No. A1 to No. A5 steels).
• Table 2 shows no. A1-No.
• A1-No. The average aspect ratio of Mn sulfide with an equivalent circle diameter of 1.0 to 5.0 ⁇ m in A5 steel, the number density, tensile strength, impact value, and average particle diameter of cementite of Mn sulfide with an equivalent circle diameter of more than 5.0 ⁇ m. , And the average size of the martensite block.
• Steels B to AH having chemical compositions shown in Table 3 are continuously cast to obtain slabs, and then the slabs are subjected to a soaking and diffusion treatment twice at a holding temperature of 1300 ° C. and a holding time of 7200 seconds, and further slabs are cast.
• the piece was subjected to ingot rolling to obtain a 162 mm square rolled material. Then, it hot-rolled to the rolling raw material and obtained the round bar steel of diameter 86mm.
• these round bar steels are cut, heated to 900 ° C. and held for 30 minutes, further subjected to quenching treatment with water cooling, and then subjected to tempering treatment heated to 135 ° C. and held for 30 minutes to obtain round bar steel No. . B to AH were obtained.
• These quenching conditions and tempering conditions are the same as the heat treatment conditions recommended when creating a chain using the steel of the present invention.
• the cross section was corroded with a nital etchant, and five structure photographs at a magnification of 5000 times were taken using a scanning electron microscope. The average of cementite contained in these photographs was taken.
• the particle size was determined by image analysis (Luzex®). Furthermore, a crystal orientation analysis using a backscattered electron diffraction pattern is performed on the sample, and an area weighted average circle of crystal grains surrounded by a large angle grain boundary having an orientation difference angle of 15 degrees or more is obtained by this analysis.
• the equivalent diameter was defined as the average particle diameter of the martensite block.
• Tables 3 and 4 show no. The chemical composition of steels B to AH is shown.
• Table 4 shows No. after quenching and tempering under the above conditions. Average aspect ratio of Mn sulfide with equivalent circle diameter of 1.0 to 5.0 ⁇ m, number density, tensile strength, impact value, average of cementite of equivalent circle diameter of more than 5.0 ⁇ m for steels of B to AH The particle size and the average size of the martensite block are shown. In Tables 3 and 4, values outside the specified range of the present invention are underlined. In addition, since quenching and tempering under the above conditions does not affect the state of Mn sulfide, no. The states of Mn sulfides in the steels B to AH are No. before quenching and tempering. Equivalent to that of steels B to AH.
• No. 1 is an example of the present invention.
• All of the steels B to U have a chemical composition and a state of Mn sulfide within the specified range of the present invention. As a result, no.
• the steels B to U had a tensile strength of 1200 MPa or more after quenching and tempering, and a Charpy impact value at ⁇ 20 ° C. of 75 J / cm 2 or more.
• No. which is a comparative example.
• the steels of V, W, X, Y, Z and AA have a content of one or more of Mo, Nb, and Ni, or are not contained. Therefore, after quenching and tempering, The cementite became coarse, the average size of the martensite block became coarse, and the low temperature toughness was insufficient.
• AD steel has an excessive Si content.
• the AE steel had an excessive Mn content. Since these excessive Si or Mn lowered the toughness of the steel, No. after quenching and tempering. AD and No. The low temperature toughness of AE steel was insufficient.

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• 2016
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