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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing 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 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
  • C21D8/0263—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 following hot rolling
• • 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
  • C22C38/02—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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to low yield ratio extra thick steel plate and a manufacturing method thereof.
• yield strength/tensile strength x 100 (%) yield strength/tensile strength
• the Charpy absorbed energy required for structural steel in Japan is generally 0°C specification, but if the structure is to be constructed in a cold region, excellent low-temperature toughness of -40°C specification may be required.
• Patent Document 1 discloses a method for obtaining low yield ratio steel plates with excellent toughness by controlling the structure to a mixed structure consisting of soft and hard phases, namely fine ferrite, island martensite, and bainite.
• Patent Document 2 discloses a method for obtaining a low yield ratio steel plate with excellent toughness by forming a structure consisting of acicular ferrite and island martensite.
• Patent Document 3 discloses a method for obtaining low-yield-ratio steel plate with excellent toughness by creating a multi-phase structure consisting of fine ferrite and a hard phase.
• Patent Document 4 discloses a method for obtaining a low yield ratio steel plate with excellent toughness by creating a multi-phase structure consisting of fine ferrite and a hard phase.
• Patent Document 1 The technology in Patent Document 1 is an invention intended for thin areas with plate thicknesses of 12 mm or 30 mm, and requires a cooling rate of 10°C/sec or more after hot rolling. This high cooling rate is difficult to achieve with extra-thick steel plates with thicknesses of over 100 mm, and therefore cannot be applied to extra-thick steel plates.
• Patent Document 2 The technology in Patent Document 2 is an invention targeted at the thin region of plate thickness of 15 to 40 mm or less, and requires a cooling rate of 5°C/sec or more after hot rolling, a high cooling rate that is difficult to achieve with extra-thick steel plates with a plate thickness of over 100 mm, and therefore cannot be applied to extra-thick steel plates.
• the addition of Mo, an expensive alloy element is essential, which creates the problem of a significant increase in manufacturing costs when manufacturing extra-thick steel plates.
• Patent Document 3 The technology in Patent Document 3 is an invention that targets the thin region of plate thickness of 10 to 40 mm or less, and requires the addition of expensive alloy elements Cu, Ni, Cr, and Mo, which creates the problem of a significant increase in manufacturing costs when manufacturing extra-thick steel plates.
• Patent Document 4 The technology of Patent Document 4 is an invention that targets plate thicknesses up to 100 mm, and requires a cooling rate of 5 to 40°C/sec after hot rolling.
• the characteristics of the position 1/4 of the way down from the surface of the steel plate in the plate thickness direction hereinafter sometimes referred to as the "plate thickness 1/4 position" that is the target of this invention cannot be applied because it is impossible to obtain a cooling rate of 5°C/sec or more when using a normal water cooling method in plate thicknesses exceeding 100 mm.
• the present invention was developed in consideration of the above-mentioned current situation, and aims to provide a low-yield-ratio extra-thick steel plate with a plate thickness of over 100 mm that has excellent toughness without requiring special equipment.
• the present invention also aims to provide a method for manufacturing the above-mentioned low-yield-ratio extra-thick steel plate.
• the present inventors have conducted extensive research to solve the above problems and have come to the following findings.
• a yield point type where an upper yield point appears
• a roundhouse type where no yield point appears.
• By suppressing the appearance of the upper yield point and creating a roundhouse type it is possible to significantly reduce the yield strength and the yield ratio.
• the present inventors conducted further research and found that in order to achieve a high strength of tensile strength of 520 MPa or more, a low yield ratio of 80% or less, and good toughness of 47 J or more at 0°C, it is necessary to have a component composition satisfying 0.22t0.1 ⁇ Ceq ⁇ 0.50, with the average grain size of parent ferrite being 25 ⁇ m or less, and the area ratio of island martensite being 3 to 20%.
• the present inventors have found that in the process of heating a slab, hot rolling it, and then water cooling it, in order to obtain a structure that combines a high strength of tensile strength of 520 MPa or more, a low yield ratio of 80% or less, and good toughness of 47 J or more at 0 ° C., it is effective to heat it at 1000 to 1150 ° C., reduce it by 30% or more in the range of 800 to 930 ° C., and further cool it after hot rolling at a cooling rate of 2100 t -1.5 or more between 700 and 400 ° C., and set the cooling stop temperature to 350 ° C. or less.
• the gist of the present invention is as follows:
• a low yield ratio type extra thick steel plate having a thickness of more than 100 mm, characterized in that the average grain size of parent phase ferrite is 25 ⁇ m or less, the area ratio of island martensite is 3 to 20%, the tensile strength is 520 MPa or more, the yield ratio is 80% or less, and the Charpy absorbed energy at 0° C. is 47 J or more.
• the composition further comprises, in mass%, Cu: 1.00% or less, Ni: 5.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, V: 0.20% or less, B: 0.0050% or less, Ca: 0.0100% or less,
• the low yield ratio type extra thick steel plate according to claim 1 or 2 comprising one or more selected from the group consisting of Mg: 0.0200% or less, and REM: 0.0500% or less.
• the heating step the slab is heated at 950 to 1150° C.
• a reduction of 30% or more is performed in the range of 800 to 930 ° C.
• the cooling step the rolled steel plate is cooled at a cooling rate of 2100t -1.5 ° C./sec or more between 700 and 400 ° C., and the cooling stop temperature is 350 ° C. or less.
• a method for producing a low yield ratio type extra thick steel plate comprising: a preparation step of preparing a slab having the above component composition; a heating step of heating the slab; a hot rolling step of hot rolling the heated slab to obtain a
• the rolled steel plate is cooled at a cooling rate of 2100t -1.5 or more between 700 and 400 ° C., and the cooling stop temperature is 350 ° C. or less.
• the low yield ratio type extra thick steel plate of the present invention is not particularly limited in its use, but can be applied to a wide range of fields in which thick steel plates are generally used, such as ships, line pipes, buildings, bridges, marine structures, wind power generators, construction and industrial machinery, and pressure vessels.
• the low yield ratio extra thick steel plate of the present invention will be described based on the following embodiment.
• composition of the low yield ratio type extra thick steel plate described in this invention will be explained.
• the unit of the content of elements in the composition is "mass %", and hereinafter, unless otherwise specified, will be simply indicated as “%”.
• a numerical range described as "A to B" using an upper limit value A and a lower limit value B includes the upper limit value A and the lower limit value B.
• C 0.05-0.16% C is the most important element in the formation of island martensite. If the C content is less than 0.05%, the amount of island martensite becomes too small, resulting in a high yield ratio. However, if the C content exceeds 0.16%, the amount of island martensite becomes too large, which reduces the toughness. Therefore, the C content is set to 0.05 to 0.16%. The C content is preferably 0.06% or more. The C content is preferably 0.15% or less. From the viewpoint of ensuring good toughness at -40°C, the C content is preferably 0.05 to 0.12%, and more preferably 0.11% or less.
• Si 0.03 ⁇ 0.70% Silicon is an effective element for deoxidization. If the silicon content is less than 0.03%, the effect is not sufficient. However, if the silicon content exceeds 0.70%, the weldability is deteriorated. Therefore, the Si content is set to 0.03 to 0.70%.
• the Si content is preferably 0.04% or more.
• the Si content is preferably 0.60% or less.
• Mn 0.50-2.50%
• Mn is an element that improves the hardenability and strength of steel at low cost. From the viewpoint of obtaining such effects, the Mn content is set to 0.50% or more. If it exceeds 50%, the weldability will decrease. Therefore, the Mn content is set to 0.50 to 2.50%.
• the Mn content is preferably 0.60% or more. 2.20% or less is preferable.
• P 0.030% or less
• P is an element that has a large effect of embrittling grain boundaries. Therefore, if a large amount of P is contained, the toughness of the steel decreases. Therefore, the P content is set to 0.030% or less.
• the P content is preferably 0.025% or less.
• the lower limit of the P content is not particularly limited and may be 0%.
• P is an element that is inevitably contained in steel as an impurity, and excessively low P content leads to an increase in refining time and an increase in cost. Therefore, the P content is preferably 0.001% or more.
• the S content is set to 0.0200% or less.
• the S content is preferably 0.0100% or less.
• the lower limit of the S content is not particularly limited and may be 0%.
• S is an element that is inevitably contained in steel as an impurity, and excessive reduction in S content leads to an increase in refining time and an increase in cost. Therefore, the S content is preferably 0.0001% or more.
• Nb 0.003-0.100%
• Nb is an element that suppresses recrystallization when strain is applied to the austenite structure through solid solution Nb and fine precipitated NbC.
• Nb is also an element that has the effect of raising the non-recrystallization temperature range. In order to obtain this effect, the Nb content must be 0.003% or more. However, if the Nb content exceeds 0.100%, the weldability decreases.
• the Nb content is set to 0.003 to 0.100%.
• the Nb content is preferably 0.005% or more.
• the Nb content is preferably 0.075% or less.
• Ti 0.005-0.100%
• Ti is an element that has the effect of pinning the movement of crystal grain boundaries and suppressing grain growth by precipitating as TiN. To obtain this effect, the Ti content is 0.005% or more. However, if the Ti content exceeds 0.100%, the cleanliness of the steel decreases and the ductility decreases. Therefore, the Ti content is set to 0.005 to 0.100%.
• the Ti content is preferably 0.006% or more and 0.080% or less.
• Al 0.001-0.100%
• Al is an effective element for deoxidization. To obtain this effect, the Al content is set to 0.001% or more. On the other hand, if the Al content exceeds 0.100%, the cleanliness of the steel decreases. The Al content is reduced, resulting in reduced ductility and toughness. Therefore, the Al content is set to 0.001 to 0.100%.
• the Al content is preferably 0.005% or more. 080% or less is preferable.
• O 0.0100% or less
• O is an element that reduces ductility. Therefore, the O content is set to 0.0100% or less.
• the lower limit of the O content is not particularly limited and may be 0%.
• O is an element that is inevitably contained in steel as an impurity, and excessive reduction in O content leads to an increase in refining time and an increase in cost. Therefore, the O content is preferably 0.0005% or more.
• N 0.0100% or less
• N is an element that reduces ductility. Therefore, the N content is set to 0.0100% or less.
• the lower limit of the N content is not particularly limited and may be 0%.
• N is an element that is inevitably contained in steel as an impurity, it may be industrially more than 0%. Note that excessive reduction in N content leads to an increase in refining time and an increase in costs. Therefore, the N content is preferably 0.0005% or more.
• the basic component composition of the low yield ratio type extra thick steel plate of the present invention has been described above. However, from the viewpoint of further improving the strength and weldability (such as the toughness and welding workability of the welded portion), one or more of the following optional additive elements may be appropriately contained.
• Cu 1.00% or less
• Cu is an element that improves the strength of steel without significantly deteriorating toughness.
• the Cu content is preferably 1.00% or less.
• the Cu content is preferably 0.01% or more.
• the Cu content is more preferably 0.70% or less.
• Ni 5.00% or less
• Ni is an element that enhances the hardenability of steel.
• Ni is also an element that has the effect of improving toughness.
• the Ni content is preferably 5.00% or less.
• the Ni content is preferably 0.01% or more.
• the Ni content is more preferably 1.50% or less, and even more preferably 1.30% or less.
• Cr 1.00% or less Cr is an element that improves the hardenability of steel, thereby improving the strength of the steel. However, if the Cr content exceeds 1.00%, the weldability decreases. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less. The Cr content is preferably 0.01% or more. The Cr content is more preferably 0.80% or less.
• Mo 1.00% or less Mo is an element that improves the hardenability of steel, thereby improving the strength of the steel. However, if the Mo content exceeds 1.00%, the weldability decreases. Therefore, when Mo is contained, the Mo content is preferably 1.00% or less. The Mo content is preferably 0.01% or more. The Mo content is more preferably 0.80% or less.
• V 0.20% or less
• V is an element that improves the hardenability of steel and generates carbonitrides to improve the strength of steel. However, if the V content exceeds 0.20%, the weldability decreases. Therefore, when V is contained, the V content is preferably 0.20% or less.
• the V content is preferably 0.01% or more.
• the V content is more preferably 0.15% or less.
• B 0.0050% or less
• B is an element that improves the hardenability of steel, thereby improving the strength of the steel. However, if the B content exceeds 0.0050%, the weldability decreases. Therefore, when B is contained, the B content is preferably 0.0050% or less.
• the B content is preferably 0.0001% or more. Furthermore, the B content is more preferably 0.0040% or less.
• Ca 0.0100% or less
• Ca is an element that improves weldability by forming oxysulfides that are highly stable at high temperatures.
• the Ca content exceeds 0.0100%, the effect of adding Ca becomes saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Ca is contained, the Ca content is preferably 0.0100% or less.
• the Ca content is preferably 0.0001% or more.
• the Ca content is more preferably 0.0080% or less.
• Mg 0.0200% or less
• Mg is an element that improves weldability by forming oxysulfides that are highly stable at high temperatures.
• the Mg content exceeds 0.0200%, the effect of adding Mg becomes saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less.
• the Mg content is preferably 0.0001% or more.
• the Mg content is more preferably 0.0180% or less.
• REM 0.0500% or less REM (rare earth metal) is an element that improves weldability by forming oxysulfides that are highly stable at high temperatures. However, if the REM content exceeds 0.0500%, the effect of adding REM becomes saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when REM is contained, the REM content is preferably 0.0500% or less. The REM content is preferably 0.0001% or more. The REM content is more preferably 0.0450% or less.
• the remainder of the composition of the low yield ratio type extra thick steel plate according to one embodiment of the present invention is Fe and unavoidable impurities. Note that, for the elements related to the above-mentioned optional added components, if the content thereof is less than the respective preferred lower limit value, the element is treated as an unavoidable impurity.
• Ceq defined by the following formula (1) and the plate thickness t (mm) of the extra thick steel plate satisfy the relationship 0.22t0.1 ⁇ Ceq ⁇ 0.50.
• Ceq [C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15...(1)
• the [element symbol] in formula (1) represents the content (mass%) of each element in the composition, and when the element is not contained, the element is calculated as 0.
• Ceq is a parameter that correlates with the strength of the steel structure.
• the ratio of this Ceq to the plate thickness t (mm) of the extra thick steel plate is appropriately controlled. Specifically, Ceq is set to 0.22t 0.1 or more. On the other hand, when Ceq exceeds 0.50, the amount of island martensite formed becomes too large, and the desired toughness of the base material at 0° C. cannot be satisfied.
• the range of Ceq is set to 0.22t0.1 ⁇ Ceq ⁇ 0.50. It is preferable that Ceq satisfies 0.23t0.1 ⁇ Ceq ⁇ 0.49.
• the range of Ceq is 0.22t 0.1 ⁇ Ceq ⁇ 0.48, and more preferably 0.23t 0.1 ⁇ Ceq ⁇ 0. It is even more preferable that the saturation temperature satisfies .47.
• Average grain size of parent ferrite 25 ⁇ m or less
• the structure of the extra thick steel plate of the present invention is mainly composed of ferrite with a ferrite phase ratio of more than 51%, and the smaller the ferrite grain size, the better the toughness.
• the average grain size of ferrite In order to obtain the desired toughness at 0° C., the average grain size of ferrite must be 25 ⁇ m or less. Therefore, the average grain size of parent ferrite is set to 25 ⁇ m or less.
• the average grain size is preferably 24 ⁇ m or less.
• brittle fracture is likely to occur starting from low points in the structure, i.e., points where the grain size is coarse.
• the average grain size of the ferrite parent phase is 15 ⁇ m or less and the difference between the maximum grain size and the average grain size is 15 ⁇ m or less. It is more preferable that the average grain size is 14 ⁇ m or less and the difference between the maximum grain size and the average grain size is 14 ⁇ m or less.
• Island martensite is a structure necessary for suppressing the upper yield point and reducing the yield ratio. In order to obtain this effect, the area fraction of island martensite must be 3% or more. On the other hand, if the area fraction of island martensite exceeds 20%, the toughness of the base material deteriorates and the desired toughness cannot be obtained at 0°C. Therefore, the area fraction of island martensite is set to 3 to 20%.
• the area fraction of island martensite is preferably 4 to 19%. From the viewpoint of checking toughness at -40°C, the area ratio of island martensite is preferably 3 to 15%, and more preferably 4 to 14%.
• pearlite, bainite, and cementite may be present as structures other than the parent phase ferrite and island martensite.
• the method for producing a low yield ratio type extra thick steel plate according to the present invention includes a preparation step of preparing a slab (steel material) having the above-described composition, a slab heating step of heating the slab, a hot rolling step of hot rolling the heated slab to obtain a hot rolled steel plate, and a cooling step of cooling the hot rolled steel plate. Each step will be described below.
• temperatures in the heating process, hot rolling process, and cooling process described below are at the 1/4 position in the plate thickness.
• the temperature at the 1/4 position in the plate thickness can be measured using a thermocouple or the like.
• a slab having the above-mentioned composition is prepared.
• the preparation method is not limited.
• molten steel is produced using a converter, an electric furnace, a vacuum melting furnace, or the like.
• secondary refining such as ladle refining may be performed.
• the produced molten steel is turned into a slab by, for example, a continuous casting method or an ingot casting method, to prepare a slab having the above-mentioned composition. Note that each condition may follow a conventional method.
• the slab having the above-mentioned component composition is heated.
• the slab heating temperature is 950 to 1150°C. If the slab heating temperature is less than 950°C, a portion that is not transformed into austenite remains, and the desired structure cannot be obtained. On the other hand, if the slab heating temperature exceeds 1150°C, the austenite grain size during heating becomes coarse, and the grain size of the finally obtained ferrite becomes coarse, and the desired toughness cannot be obtained. Therefore, the slab heating temperature is set to 950 to 1150°C. It is preferable that the slab heating temperature is set to 970 to 1140°C. From the viewpoint of ensuring toughness at -40°C, the heating temperature of the slab is preferably 950 to 1100°C, and more preferably 970 to 1100°C.
• l d /h m is a parameter called the rolling shape ratio, which is expressed by the following formula, and the larger this value is, the more easily shear strain during rolling is introduced from the steel plate to the plate thickness center side.
• rolling reduction under conditions where l d /h m is 0.3 or more is necessary.
• l d and h m are defined by the following formulas (2) and (3).
• t iN is the thickness of the plate before rolling in the Nth rolling pass among the rolling passes under the predetermined conditions
• t fN is the plate thickness after rolling in the Nth rolling pass among the rolling passes under the specified conditions.
• the total reduction rate of reduction with l d /h m of 0.3/pass or more in the range of 800 to 930° C. is preferably 40% or more, more preferably 41% or more.
• the rolling efficiency decreases, so that it is preferably 80% or less.
• the rolled steel sheet is cooled.
• the upper limit of the cooling rate is not particularly specified, but since special cooling equipment is required to significantly increase the cooling rate, it is preferable to set it to 6000t -1.5 or less.
• Cooling stop temperature 350°C or less In the cooling process, it is essential that the cooling stop temperature is 350°C or less. If the cooling stop temperature is higher than 350°C, the amount of island martensite formed decreases and the yield ratio increases. Therefore, the cooling stop temperature is set to 350°C or less.
• cooling methods include water cooling and gas cooling.
• cooling speed in temperature ranges other than those mentioned above there are no particular limitations on the cooling speed in temperature ranges other than those mentioned above, and any cooling method may be used to cool to room temperature.
• Example 1 Molten steel having the composition shown in Table 1 was melted, and slabs with a thickness of 260 to 600 mm were prepared by continuous casting, ingot casting, etc. Note that blanks in the element columns in Table 1 indicate that the element was not intentionally added, and include not only cases where the element was not contained (0%), but also cases where the element was unavoidably contained.
• the prepared slab was hot-rolled and cooled under the conditions shown in Table 2 to obtain a low yield ratio type extra thick steel plate having a plate thickness t (mm) shown in Table 2.
• the temperature at the 1/4 plate thickness position was measured using a thermocouple.
• t0 indicates the plate thickness when the temperature at the 1/4 position of the plate thickness reaches 930°C
• t1 indicates the plate thickness when the temperature at the 1/4 position of the plate thickness reaches 800°C.
• the area ratio of island martensite was determined by mirror-polishing a sample for microstructural observation, etching it with 3% nital, and then electrolytically polishing it with an aqueous solution of 200 g of sodium hydroxide and 40 g of picric acid dissolved in 800 ml of water to remove cementite, and then taking five scanning electron microscope photographs at a magnification of 2000 times and determining the average value by image analysis.
• Tensile test pieces were taken from each of the obtained extra-thick steel plates at the center position of the longitudinal direction (rolling direction) of the extra-thick steel plate so that the longitudinal direction of the tensile test piece was parallel to the plate width direction (direction perpendicular to rolling) of the extra-thick steel plate.
• the tensile test pieces were taken so that the center position in the thickness direction of the tensile test piece was at 1/4 of the plate thickness position of the extra-thick steel plate.
• the shape of the tensile test piece was a JIS No. 4 shape.
• a tensile test was performed in accordance with JIS Z2241 (2011) using each of the taken tensile test pieces, and the yield strength and tensile strength were measured and the yield ratio was obtained.
• any one of the tensile strength, toughness, and yield ratio was insufficient. That is, in No. 16, the C content was too low, resulting in a small amount of island martensite and a high yield ratio. In No. 17, the C content was too high, resulting in a large amount of island martensite and low toughness. In Nos. 18 and 19, the amount of island martensite was large due to too high Ceq, and the toughness was low. In No. 20, the plate thickness was too large for the Ceq, and therefore the tensile strength was low. In No. 21, the heating temperature was too high, so the ferrite grain size was large and the toughness was low. In Comparative Example No.
• the rolling reduction ratio between 800 and 930° C. was too low, so the ferrite grain size was large and the toughness was low.
• the cooling rate was too low, so sufficient tensile strength was not obtained, and the amount of island martensite was too small, so a low yield ratio was not obtained.
• the cooling stop temperature was too high, so the amount of island martensite was small and the yield ratio was high.
• the P content was too high, which resulted in embrittlement of the structure and low toughness.
• the S content was too high, which caused the structure to become brittle and resulted in low toughness.
• the Nb content was too low, so the structure was coarse (i.e., the ferrite grain size was large) and the toughness was low.
• the Ti content was too low, so the structure became coarse (i.e., the ferrite grain size became large) and the toughness was low.
• Example 2 Molten steel having the composition shown in Table 3 was produced, and slabs with a thickness of 260 to 600 mm were prepared by continuous casting, ingot casting, etc. Note that blanks in the columns for elements in Table 3 indicate that the element was not added intentionally, and include not only cases where the element was not contained (0%), but also cases where the element was unavoidably contained.
• the prepared slab was subjected to hot rolling and cooling under the conditions shown in Table 4 to obtain extra thick steel plates having the plate thickness t (mm) shown in Table 4.
• the temperature at the 1/4 plate thickness position was measured using a thermocouple.
• t0 indicates the plate thickness when the temperature at the 1/4 position of the plate thickness reaches 930°C
• t1 indicates the plate thickness when the temperature at the 1/4 position of the plate thickness reaches 800°C.
• the area ratio of island martensite was determined by mirror-polishing a sample for microstructural observation, etching it with 3% nital, and then electrolytically polishing it with an aqueous solution of 200 g of sodium hydroxide and 40 g of picric acid dissolved in 800 ml of water to remove cementite, and then taking five scanning electron microscope photographs at a magnification of 2000 times and determining the average value by image analysis.
• Tensile test pieces were taken from each of the obtained extra-thick steel plates at the center position of the longitudinal direction (rolling direction) of the extra-thick steel plate so that the longitudinal direction of the tensile test piece was parallel to the plate width direction (direction perpendicular to rolling) of the extra-thick steel plate.
• the tensile test pieces were taken so that the center position in the thickness direction of the tensile test piece was at 1/4 of the plate thickness position of the extra-thick steel plate.
• the shape of the tensile test piece was a JIS No. 4 shape.
• a tensile test was performed in accordance with JIS Z2241 (2011) using each of the taken tensile test pieces, and the yield strength and tensile strength were measured and the yield ratio was obtained.
• the extra thick steel plates did not have sufficient tensile strength, toughness, or yield ratio.
• Nos. 15', 17' and 19' had a high C content and a large amount of island martensite, and therefore had low Charpy absorbed energy at -40°C.
• Nos. 16', 20' and 21' had high Ceq and a large amount of island martensite, and therefore had low Charpy absorbed energy at -40°C.
• the C content was low, so the island martensite ratio was low and the yield ratio was high.
• No. 22' the Charpy absorbed energy at -40°C was low because the heating temperature of the slab was high and the average grain size of ferrite was large.
• No. 15', 17' and 19' had a high C content and a large amount of island martensite, and therefore had low Charpy absorbed energy at -40°C.
• No. 16', 20' and 21' had high Ceq and a large amount of island martensite, and therefore had low Charpy absorbed energy at -40°C.
• the C content was low, so the island marten
• the Charpy absorbed energy at -40°C was low because the heating temperature of the slab was high, and the average grain size of ferrite and the difference between the average grain size and the maximum grain size were large.
• the total reduction ratio of ld / hm between 800 and 930°C was low at 0.3/pass or more, and the difference between the average grain size and the maximum grain size of ferrite was large, so that the Charpy absorbed energy at -40°C was low.
• No. 23' the total reduction ratio of ld / hm between 800 and 930°C was low at 0.3/pass or more, and the difference between the average grain size and the maximum grain size of ferrite was large, so that the Charpy absorbed energy at -40°C was low.
• the P content was too high, which resulted in embrittlement of the structure and low toughness.
• the S content was too high, which caused the structure to become embrittled and resulted in low toughness.
• the Nb content was too low, so the average grain size of ferrite became coarse and the toughness was low.
• the Ti content was too low, so the average grain size of ferrite became coarse and the toughness was low.

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