Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
• • 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
  • C22C38/06—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
  • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

• the present invention relates to a steel plate, and in particular, to a high-strength steel plate suitable for cryogenic applications that can stably ensure excellent cryogenic toughness and brittle crack initiation suppression performance over a wide range of plate thickness, and a method for producing the same.
• INDUSTRIAL APPLICABILITY The steel sheet and manufacturing method of the present invention can be suitably used for structures used in cryogenic environments, such as liquefied gas storage tanks for ships and land.
• cryogenic toughness When hot-rolled steel sheets are used in structures such as liquefied gas storage tanks, the operating environment is extremely low, so not only the strength of the steel sheets but also the toughness under extremely low temperatures (cryogenic toughness) It is required to be excellent in For example, when a hot-rolled steel plate is used in a tank for storing liquefied natural gas, it is necessary to ensure excellent toughness at cryogenic temperatures below ⁇ 164° C., which is the boiling point of liquefied natural gas. If the cryogenic toughness of the steel material is poor, it may become impossible to maintain the safety of the structure for cryogenic storage. Conventionally, 7% Ni or 9% Ni steel sheets have been used to meet this requirement.
• Patent Document 1 A 7% Ni steel plate is proposed in Patent Document 1, for example.
• Patent Document 1 discloses a heavy steel plate for cryogenic use containing more than 5.0% to less than 10.0% Ni and predetermined amounts of C, Si, Mn, and Al.
• the average value of absorbed energy vE-196 per unit area is 1.25 J/mm 2 or more over a plate thickness of 6 to 50 mm.
• 7% Ni steel plate (hereinafter also referred to as a 7% Ni steel plate) with Ni: about 6.0 to 7.5%. It was found that the risk of unstable fracture (brittle fracture) of the steel plate due to the decrease in absorbed energy (toughness) in the Charpy test and the occurrence of brittle cracks increases when the steel plate is directly quenched and tempered later.
• Patent Literature 1 does not discuss these problems, especially the occurrence of brittle fracture.
• the present invention has been devised in view of the above circumstances, and an object of the present invention is to provide a high-strength steel sheet that is excellent in cryogenic toughness and brittle crack initiation suppression performance, on the premise that it is manufactured by a reduced process that uses direct quenching and tempering.
• the decrease in absorbed energy described above is caused mainly by separation occurring in the Mn-enriched region due to the formation of the Mn-enriched region and the Mn-depleted region.
• the separation is perpendicular to the fracture surface that occurs during a toughness measurement test typified by the Charpy impact test and CTOD test, and occurs in a direction parallel to the rolled surface as a result of brittle fracture.
• the present invention has been completed based on the above findings, and the gist thereof is as follows. [1] % by mass, C: 0.01 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.05-0.40%, Ni: 6.0% or more and less than 7.0%, Cr: 0.01 to 1.00%, Mo: 0.01-0.50%, P: 0.030% or less, S: 0.0050% or less, N: 0.0010-0.0080% and Al: 0.008-0.100% and the balance has a component composition consisting of Fe and unavoidable impurities, The amount of retained austenite at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate is less than 1.7% by volume, A steel sheet having a Charpy absorbed energy at -196°C in a Charpy impact test according to JIS Z 2242 of 200 J or more when using a full size test piece and 100 J or more when using a half size test piece.
• the component composition further includes, in % by mass, Cu: 0.40% or less, Nb: 0.05% or less, V: 0.05% or less,
• the component composition further contains, in % by mass, Ca: 0.007% or less,
• the steel material having the chemical composition according to any one of [1] to [3] above has a cumulative reduction rate of 15 to 75% at 870 ° C. or less and a final rolling end temperature of 830 to 700 in steel plate surface temperature.
• the hot-rolled sheet is obtained by hot rolling at a temperature of 300° C.
• the hot-rolled sheet is subjected to a temperature of 600° C. or less at a depth position of 1/4 of the thickness in the thickness direction from the surface of the hot-rolled sheet.
• the amount of retained austenite at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the plate is less than 1.7% in volume ratio, and the Charpy impact test conforming to JIS Z 2242 Charpy absorption at -196 ° C.
• a high-strength steel sheet with excellent cryogenic toughness and brittle crack initiation suppression performance can be provided with high productivity.
• the steel sheet of the present invention has a predetermined chemical composition. Further, the steel material used in the manufacturing method of the present invention, which is suitably used for manufacturing the steel sheet of the present invention, also has the above-described predetermined chemical composition. Each element contained in this component composition will be described below. In this specification, unless otherwise specified, "%" as a unit of content of each element means “% by mass”.
• C 0.01% or more and 0.15% or less C is an element that has the effect of improving the strength of the steel sheet.
• the C content should be 0.01% or more. Preferably, it is 0.03% or more.
• the C content is made 0.15% or less. Preferably, it is 0.12% or less.
• Si 0.01% or more and 0.50% or less Si is an element that contributes to improving the strength of the steel sheet, and is also an element that acts as a deoxidizing agent.
• the Si content is set to 0.01% or more. Preferably, it is 0.03% or more.
• the Si content is set to 0.50% or less. Preferably, it is 0.30% or less.
• Mn 0.05% or more and 0.40% or less
• Mn is an element effective in increasing the hardenability of steel and increasing the strength of the steel sheet.
• Mn is added at 0.05% or more.
• it is 0.10% or more.
• Mn content exceeds 0.40%, separation tends to occur and the toughness decreases, so the Mn content is limited to 0.40% or less.
• Mn content exceeds 0.40%, the Mn concentration in the Mn segregation band tends to increase, and the brittle crack initiation suppressing performance is lowered, so the content is limited to 0.40% or less.
• the Mn content is preferably 0.35% or less, more preferably less than 0.35%, still more preferably less than 0.20%.
• Ni 6.0% or more and less than 7.0%
• Ni is an extremely effective element for improving the cryogenic toughness of the steel sheet, and also contributes to the improvement of brittle crack initiation suppression performance.
• the Ni content is set to 6.0% or more. Preferably, it is 6.5% or more.
• the Ni content is set to less than 7.0%.
• Cr 0.01% or more and 1.00% or less Cr is an element that can improve the strength of the steel sheet without significantly impairing the cryogenic toughness.
• the Cr content should be 0.01% or more. Preferably, it is 0.30% or more.
• the Cr content is set to 1.00% or less. Preferably, it is 0.80% or less.
• Mo 0.01% or more and 0.50% or less
• Mo is an element that can improve the strength of the steel sheet without significantly impairing the cryogenic toughness.
• the Mo content is set to 0.01% or more. Preferably, it is greater than 0.10%.
• Mo content shall be 0.50% or less. Preferably, it is 0.30% or less. More preferably, it is 0.25% or less.
• P 0.030% or less
• P is an unavoidable impurity and is a harmful element that adversely affects the cryogenic toughness and brittle crack initiation suppression performance of the steel sheet.
• the P content is suppressed to 0.030% or less.
• the lower the P content the better. be.
• the lower limit of the P content is preferably 0.001%.
• S 0.0050% or less S forms MnS in steel and significantly deteriorates cryogenic toughness and brittle crack initiation suppression performance.
• the S content is preferably 0.0020% or less.
• the lower the S content the better, so the lower limit is not particularly limited, and it may be 0%, but even in that case, it is allowed to be contained as an unavoidable impurity.
• N 0.0010% or more and 0.0080% or less N forms precipitates in steel, and if the content exceeds 0.0080%, it causes a decrease in the toughness of the base material. In addition, brittle crack generation suppression performance is also lowered.
• N is also an element that contributes to grain refinement of the base material by forming AlN, and such an effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content should be 0.0010% or more and 0.0080% or less.
• the N content is preferably 0.0020% or more. Preferably, it is 0.0060% or less.
• Al 0.008% to 0.100%
• Al is an element contained in the deoxidizing agent. If the Al content is less than 0.008%, the effect as a deoxidizing agent is poor. Al is also an element that contributes to grain refinement of the base material by forming AlN. Therefore, the Al content is set to 0.008% or more. Preferably, it is 0.020% or more. On the other hand, if the Al content exceeds 0.100%, the cleanliness of the steel is impaired, and the toughness, especially the Charpy absorbed energy at extremely low temperatures, is lowered. Therefore, the Al content is set to 0.100% or less. Preferably, it is 0.050% or less.
• the component composition in one embodiment of the present invention can be composed of the above-described predetermined amount of elements, and the balance of Fe and unavoidable impurities.
• the above component composition optionally further contains one or more selected from Cu, Nb, V, Ti and B, preferably in the amounts described below. can be done.
• Cu 0.40% or less
• Cu is an element that has the effect of increasing the strength of the steel sheet by improving the hardenability.
• the Cu content exceeds 0.40%, the cryogenic toughness of the steel sheet is lowered, and the properties of the surface of the steel material (slab) after casting are deteriorated. Therefore, when Cu is added, the Cu content is preferably 0.40% or less. More preferably, it is 0.30% or less.
• the lower limit of the Cu content is not particularly limited, the Cu content is preferably 0.10% or more in order to obtain the above effect.
• Nb 0.05% or less
• Nb is an effective element for increasing the strength of the steel sheet by precipitation strengthening.
• the Nb content is preferably 0.05% or less. More preferably, it is 0.03% or less.
• the lower limit of the Nb content is not particularly limited, but the Nb content is preferably 0.01% or more in order to obtain the above effect.
• V 0.05% or less
• Nb is an effective element for increasing the strength of the steel sheet by precipitation strengthening.
• the V content is preferably 0.05% or less. More preferably, it is 0.04% or less.
• the lower limit of the V content is not particularly limited, but the V content is preferably 0.01% or more in order to obtain the above effect.
• Ti 0.03% or less
• Ti is an element that has the effect of increasing the toughness of the weld zone without degrading the mechanical properties of the base metal when steel sheets are welded to form a welded structure. For that purpose, it is preferable to add 0.003% or more. On the other hand, when the Ti content exceeds 0.03%, the toughness is rather lowered, so the Ti content is preferably in the range of 0.03% or less.
• B 0.0030% or less
• B is an element that enhances hardenability when added in a small amount. In order to effectively exhibit this effect, it is preferable to contain B at 0.0003% or more. On the other hand, if the B content exceeds 0.0030%, the toughness deteriorates. Therefore, when B is contained, the content is preferably 0.0030% or less.
• the component composition may optionally further contain one or more selected from Ca, REM and Mg, preferably in the amounts described below.
• Ca 0.007% or less Ca is an element that has the effect of improving the cryogenic toughness of the steel sheet by controlling the form of inclusions in the steel.
• Charpy toughness the Charpy absorbed energy at cryogenic temperatures
• the Ca content is preferably 0.007% or less. More preferably, it is 0.004% or less.
• the lower limit of the Ca content is not particularly limited, it is preferably 0.001% or more in order to obtain the above effect.
• REM 0.010% or less REM (rare earth metal), like Ca, is an element that has the effect of improving the cryogenic toughness of a steel sheet by controlling the form of inclusions in the steel. However, excessive REM impairs the cleanliness of the steel and reduces the Charpy toughness. Therefore, when REM is added, the REM content is preferably 0.010% or less. More preferably, it is 0.008% or less. On the other hand, the lower limit of the REM content is not particularly limited, but the REM content is preferably 0.001% or more in order to obtain the above effects.
• REM is a general term for 17 elements including Y and Sc in addition to 15 lanthanoid elements, and these elements can be contained singly or in combination. The content of REM means the total content of these elements.
• Mg 0.070% or less
• Mg is an element that has the effect of improving the cryogenic toughness of the steel sheet by controlling the form of inclusions in the steel.
• excessive Mg impairs the cleanliness of the steel and reduces the Charpy toughness. Therefore, when Mg is added, the Mg content is preferably 0.070% or less. More preferably, it is 0.004% or less.
• the lower limit of the Mg content is not particularly limited, but the Mg content is preferably 0.001% or more in order to obtain the above effect.
• the steel sheet of the present invention has a retained austenite amount (hereinafter also referred to as a retained ⁇ amount) at a depth position of 1/4 of the sheet thickness t (hereinafter also referred to as 1/4 t) in the thickness direction from the surface of the steel sheet. It has a texture that is less than 1.7% by volume.
• a representative position when the amount of residual ⁇ at the above position is 1.7% or more, it can be read that a relatively large amount of unstable ⁇ was generated in the concentrated region of the Mn segregation band, and as a result, brittle cracks occurred. becomes more likely to occur. From the viewpoint of enhancing brittle crack initiation suppression performance, the smaller the amount of residual ⁇ at the above position, the better. 1% or less is more preferable, and 0% is even more preferable.
• the amount of residual ⁇ can be measured by the method described in the examples below, and can be measured on either the front side or the back side of the steel sheet.
• the structure of the steel sheet is preferably a structure mainly composed of martensite and bainite, specifically, the total area ratio of martensite and bainite is preferably 98.3% or more, and 99.0% or more. 100% is more preferable. This is because, as described above, if the structure is mainly composed of martensite and/or bainite, it is easy to obtain sufficient strength while ensuring excellent cryogenic toughness. Note that the ratio of martensite and bainite is arbitrary and does not matter. Also, the type of structure other than martensite and bainite is not particularly limited.
• the thickness of the steel plate is not particularly limited, and can be any thickness. For example, it is preferably 6 mm or more and 50 mm or less.
• the lower limit of the tensile strength of the steel sheet is not particularly limited, but the lower limit is preferably 690 MPa. More preferably, it is 720 MPa or more.
• the upper limit of the tensile strength is not particularly limited, it is preferable to set the upper limit to 930 MPa. More preferably, it is 900 MPa or less.
• the tensile strength can be measured by the method described in Examples below.
• the Charpy absorbed energy (vE -196°C ) at -196°C must be 200 J or more in a Charpy impact test using a full size test piece. On the other hand, it is preferably 350J or less, more preferably 280J or less. In the Charpy impact test using a half size test piece, vE -196°C must be 100J or more. On the other hand, it is preferably less than 200J, more preferably 150J or less.
• the cryogenic toughness can be measured by the method described in Examples below.
• the production method of the present invention uses a steel material having the above-described predetermined chemical composition; the cumulative reduction rate and final rolling end temperature in hot rolling; the average cooling rate and cooling stop temperature in quenching; and the temperature range in tempering. is controlled to a predetermined condition to obtain a steel sheet that satisfies the above-described predetermined amount of retained ⁇ .
• the temperature refers to the temperature at the center of the plate thickness unless otherwise specified.
• the temperature at the center of the plate thickness can be obtained, for example, by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer.
• the steel sheet of the present invention can be suitably produced by sequentially performing the following steps (1) to (4). (1) Heating of steel material (2) Hot rolling (3) Quenching (accelerated cooling) (4) Tempering
• the steel material must have the above-described chemical composition.
• the steel material is preferably heated to a temperature of 900°C or higher and 1200°C or lower.
• the method of manufacturing the steel material is not particularly limited, but it can be manufactured, for example, by melting molten steel having the chemical composition described above and casting it by a conventional method. Melting can be performed by any method such as a converter, an electric furnace, or an induction furnace. Casting is preferably carried out by a continuous casting method from the viewpoint of productivity, but it can also be carried out by an ingot casting-decomposition rolling method. For example, a steel slab can be used as the steel material.
• the heating of the steel material may be performed after the steel material obtained by a method such as casting is once cooled, or the obtained steel material may be directly heated without being cooled.
• the heating temperature of the steel material is less than 900°C, the deformation resistance of the steel material is high, so the load on the rolling mill in subsequent hot rolling increases, making hot rolling difficult. Therefore, the heating temperature of the steel material is preferably 900° C. or higher. On the other hand, if the heating temperature of the steel material is higher than 1200° C., the oxidation of the steel becomes remarkable, and the loss due to removal of the oxide film due to oxidation increases, resulting in a decrease in yield. Therefore, it is preferable to set the heating temperature of the steel material to 1200° C. or less.
• Hot rolling Cumulative reduction rate at 870°C or less is 15% or more and 75% or less.
• the cumulative rolling reduction is preferably 30% or more, preferably 70% or less, and more preferably 30 to 70%.
• the average cooling rate is preferably 5° C./s or higher, more preferably 10° C./s or higher.
• the upper limit of the average cooling rate is not particularly limited. easier. As a result, material properties such as tensile properties and toughness tend to vary. Therefore, the average cooling rate is preferably 200° C./s or less.
• the cooling stop temperature should be 300° C. or less at 1/4t.
• the cooling stop temperature is preferably 250° C. or lower, more preferably 200° C. or lower. Under such conditions, the hot-rolled sheet is satisfactorily quenched by directly subjecting the hot-rolled sheet to accelerated cooling after hot rolling.
• the cooling treatment in quenching is not particularly limited as long as the above conditions are met, and can be performed by any method.
• one or both of air cooling and water cooling can be used.
• water cooling any cooling method using water (eg, spray cooling, mist cooling, laminar cooling, etc.) can be used.
• tempering temperature 550°C or more and less than the Ac 1 transformation point
• the tempering temperature should be 550° C. or higher and lower than the Ac 1 transformation point. If the tempering temperature is less than 550°C, the tempering is insufficient and the Charpy toughness is lowered. Moreover, when the tempering temperature is higher than the Ac 1 transformation point, the strength is lowered and the ability to suppress brittle crack initiation is lowered due to the generation of unstable ⁇ .
• the Ac 1 transformation point can be obtained by the following formula (1).
• a C1 transformation point (°C) 750.8 - 26.6 x C + 17.6 x Si - 11.6 x Mn - 22.9 x Cu - 23 x Ni + 24.1 x Cr + 22.5 x Mo - 39.7 x V-5.7 ⁇ Ti+232.4 ⁇ Nb-169.4 ⁇ Al (1)
• the element symbol in the above formula (1) represents the content (% by mass) of each element, and is set to 0 when the element is not contained.
• Any heating method can be used for heating in the tempering process as long as the heating temperature can be controlled as described above.
• An example of a heating method includes furnace heating.
• the furnace heating is not particularly limited, and a general heat treatment furnace can be used.
• the holding time is not particularly limited, but is preferably 5 minutes or longer.
• a steel plate was produced by the procedure described below, and its properties were evaluated. First, molten steel having the chemical composition shown in Table 1 was melted in a converter, and a steel slab (thickness: 200 mm) was produced as a steel material by a continuous casting method. Table 1 also shows the AC1 transformation points (° C.) obtained by the above-described formula (1).
• the obtained steel slabs were heated and hot-rolled into hot-rolled sheets having respective thicknesses (final thicknesses). Further, the obtained hot-rolled sheets were quenched and tempered according to the conditions shown in Table 2 to obtain steel sheets.
• the microstructure total area ratio of martensite + bainite
• the amount of retained ⁇ the tensile strength (TS)
• the Charpy absorbed energy at -196 ° C vE -196 ° C
• the evaluation results are also shown in Table 2.
• a test piece for microstructure observation was taken from each steel plate so that the position of 1/4t was the observation position. This test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror-polished. Next, after performing nital corrosion, the tissue was observed with a scanning electron microscope at magnifications of 2,000 and 10,000, and an image of the tissue was taken. The images obtained were analyzed to identify the microstructure.
• Tensile strength A JIS No. 4 tensile test piece was taken from the position of 1/4t of the steel plate. Using this tensile test piece, a tensile test was performed according to JIS Z 2241 to evaluate the tensile strength (TS) of the steel plate. If the tensile strength was 690 MPa or more, the strength was high and it was considered acceptable.

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