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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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
  • C22C38/14—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

Definitions

• the present invention relates to a steel sheet, particularly a steel sheet applicable to large heat input welding, and a method for manufacturing the same. Specifically, the present invention relates to a steel sheet having excellent toughness in HAZ after high heat input welding. Further, the steel plate of the present invention can be suitably used for large structures such as ships, marine structures, low temperature storage tanks, and construction / civil engineering structures.
• HAZ Heat Affected Zone
• Patent Document 1 describes a technique of finely dispersing TiN in steel to suppress coarsening of austenite grains.
• Patent Document 2 describes a technique for dispersing Ti oxide that is stable at a higher temperature.
• Patent Document 3 discloses a technique for reducing the content of P in addition to reducing the contents of C and Si. ing.
• the weld heat-affected zone is heated to the melting temperature range of TiN when undergoing large heat input welding, so that TiN decomposes and the above dispersion occurs.
• the toughness of the weld heat-affected zone is significantly reduced because the effect disappears or the ground structure of the steel becomes brittle due to the solid-melt Ti and the solid-melt N generated by the decomposition of TiN.
• the present invention provides, in particular, a steel sheet having excellent low-temperature toughness of HAZ (hereinafter referred to as high heat-affected zone) generated when high heat-affected zone welding is performed, and a method for manufacturing such a steel sheet.
• the purpose is to do that.
• the inventors have obtained the following findings as a result of intensive research on a method for improving the low temperature toughness of the large heat-affected zone HAZ.
• the inventors can maintain the effect of suppressing the coarsening of austenite grains by TiN by first dispersing TiN in a fine and large amount and suppressing the decomposition of TiN when undergoing large heat input welding. I thought.
• the amount of Ti and N added is within the range in which Ti / N, which is the mass% ratio of Ti and N, is 2.10 or more and 3.60 or less and the following equation (1) is satisfied.
• the gist of the present invention is as follows. 1. 1. By mass%, C: 0.045% or more and 0.080% or less, Si: 0.02% or more and less than 0.10%, Mn: 1.60% or more and less than 1.95%, P: 0.010% or less. , S: 0.010% or less, Al: 0.010% or more and 0.100% or less, Nb: 0.005% or more and 0.050% or less, O: 0.0100% or less, Cu: 0.50% or less , Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.30% or less, V: 0.30% or less, Ti: 0.010% or more and 0.025% or less and N: 0.
• W 0.30% or less
• Co 0.30% or less
• B 0.0100% or less
• Ca 0.0100% or less
• Mg 0.0100% or less
• REM 0.
• a method for manufacturing steel sheets in terms of mass%, C: 0.045% or more and 0.080% or less, Si: 0.02% or more and less than 0.10%, Mn: 1.60% or more and less than 1.95%.
• P 0.010% or less
• S 0.010% or less
• Al 0.010% or more and 0.100% or less
• Nb 0.005% or more and 0.050% or less
• O 0.0100% or less
• Cu 0.50% or less
• Cr 0.50% or less
• Mo 0.30% or less
• V 0.30% or less
• Ti 0.010% or more 0.
• the molten steel further contains W: 0.30% or less, Co: 0.30% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: The method for producing a steel sheet according to 4 above, which contains one or more selected from 0.0200% or less.
• the present invention it is possible to provide a steel sheet having excellent toughness even in a welded joint having a large heat input, and a method for manufacturing such a steel sheet.
• the steel sheet obtained by the present invention is suitable for large heat input welding, which is excellent in workability at the time of construction of a container ship, for example, and thus exerts a remarkable effect in industry.
• the steel sheet of the present invention has a predetermined composition.
• the contents of each element of C, Si, Mn, P, S, Al, Nb, O, Cu, Ni, Cr, Mo, V, Ti and N are specified, and the content of each element is specified.
• the range of the mass% ratio (Ti / N) of Ti and N and the predetermined formula (1) are defined, and further, the range of Ceq represented by the predetermined formula (2) is defined.
• the steel plate of the present invention can be suitably used for a large structure such as a container ship because a welded joint having a large heat input can exhibit excellent toughness.
• the steel sheet of the present invention can be obtained, for example, by the manufacturing method described later.
• C (C: 0.045% or more and 0.080% or less) C is an element having an action of increasing the grain boundary strength that contributes to the toughness of HAZ, and is necessary for achieving a desired HAZ toughness value. It is also necessary to achieve the strength of the base metal. In order to obtain these effects, the C content is 0.045% or more. Further, from the viewpoint of increasing the grain boundary strength and increasing the HAZ toughness value, the C content is preferably 0.050% or more, and more preferably 0.055% or more. On the other hand, if the C content is too high, the toughness in HAZ will decrease.
• the C content is 0.080% or less. Further, from the viewpoint of further suppressing the decrease in the toughness of HAZ, the C content is preferably 0.075% or less.
• Si is an element having an action of suppressing the formation of coarse carbides and increasing the toughness of HAZ, and is necessary for achieving a desired HAZ toughness value. It is also a necessary component for ensuring the strength of the base metal and deoxidizing. In order to obtain these effects, the Si content is 0.02% or more. Further, from the viewpoint of increasing the HAZ toughness value, the Si content is preferably 0.03% or more, and more preferably 0.04% or more. On the other hand, if the Si content is too high, MA is generated due to the large heat input welding, and the toughness of HAZ is significantly lowered. Therefore, in order to ensure high HAZ weldability, the Si content is set to less than 0.10%. From the viewpoint of improving the toughness of HAZ, the Si content is preferably 0.09% or less, and more preferably 0.08% or less.
• Mn is an element that increases the hardenability of steel, suppresses the formation of coarse carbides, secures the strength of the base metal, and enhances the toughness of HAZ, and is necessary for achieving the desired HAZ toughness value. be.
• the Mn content is 1.60% or more.
• the Mn content is preferably 1.65% or more, and more preferably 1.70% or more.
• the Mn content is set to less than 1.95%.
• the Mn content is preferably 1.90% or less, and more preferably 1.85% or less.
• P 0.010% or less
• P has an adverse effect such as lowering the toughness of HAZ by segregating at grain boundaries. Therefore, it is desirable to reduce the P content as much as possible, but 0.010% or less is acceptable.
• the lower limit of the P content is not particularly limited and may be 0%. Normally, P is an element that is inevitably contained in steel as an impurity, and therefore may be industrially more than 0%. Further, since excessive reduction of P causes an increase in refining cost, the P content is preferably 0.005% or more from the viewpoint of cost.
• S (S: 0.010% or less) S exists in the steel as a sulfide-based inclusion such as MnS, and has an adverse effect such as lowering the toughness of HAZ and serving as a starting point for brittle fracture. Therefore, it is desirable to reduce the S content as much as possible, but 0.010% or less is acceptable.
• the lower limit of the S content is not particularly limited and may be 0%. Normally, S is an element that is inevitably contained in steel as an impurity, and therefore may be industrially more than 0%. Further, since excessive reduction of S causes an increase in refining cost, it is preferable to set the S content to 0.005% or more from the viewpoint of cost.
• Al 0.010% or more and 0.100% or less
• Al is an element that has an effect of reducing oxide-based inclusions and improving the toughness of HAZ by having an action as a deoxidizing agent. It also has the effect of improving the strength of the base metal.
• the Al content is 0.010% or more.
• the Al content exceeds 0.100%, the oxide-based inclusions increase, the cleanliness decreases, and the toughness of HAZ decreases. Therefore, the Al content is set to 0.100% or less.
• the Al content is preferably 0.050% or less, more preferably 0.040% or less.
• Nb is an element having an action of increasing the toughness of HAZ through the miniaturization of the particle size. It also has the effect of improving the strength and toughness of the base metal.
• the Nb content is set to 0.005% or more.
• the Nb content is preferably 0.007% or more, more preferably 0.009% or more.
• the Nb content exceeds 0.050%, MA is generated in HAZ and the toughness is lowered. Therefore, the upper limit of the Nb content is 0.050%.
• the Nb content is preferably 0.045% or less, more preferably 0.040% or less, and even more preferably 0.035% or less.
• O is an element that can be contained as an unavoidable impurity, but in the present invention, since it is an element that should be particularly reduced, its content is specified. O forms an oxide, becomes a starting point for brittle fracture, and has an adverse effect such as lowering the toughness of HAZ. Therefore, the O content is limited to 0.0100% or less.
• the O content is preferably 0.0050% or less, more preferably 0.0030% or less.
• the lower limit of the O content is not particularly limited and may be 0%. Normally, O is an element that is inevitably contained in steel as an impurity, and therefore may be industrially more than 0%. Further, since excessive reduction of O causes an increase in refining cost, it is preferable to set the O content to 0.0020% or more from the viewpoint of cost.
• Cu is an element having an action of increasing the hardenability of steel and improving the strength of the steel sheet (base material), and can be arbitrarily added.
• the Cu content is preferably 0.01% or more in order to obtain the above effect.
• the Cu content exceeds 0.50%, the toughness of HAZ deteriorates and the alloy cost increases. Therefore, when Cu is added, the Cu content is 0.50% or less.
• the Cu content is more preferably 0.20% or more.
• the Cu content is more preferably 0.40% or less, further preferably 0.30% or less.
• Ni is an element having an effect of improving the strength of the steel sheet (base material) and can be arbitrarily added.
• Ni it is preferable to add 0.01% or more of Ni content in order to obtain the above effect.
• the Ni content exceeds 0.50%, the weldability is deteriorated and the alloy cost is increased. Therefore, when Ni is added, the Ni content is 0.50% or less.
• the Ni content is more preferably 0.20% or more.
• the Ni content is more preferably 0.40% or less, and further preferably 0.30% or less.
• Cr 0.50% or less
• Cr is an element having an action of improving the strength of the steel sheet (base material) like Cu, and can be arbitrarily added.
• the Cr content is preferably 0.01% or more.
• the Cr content is more preferably 0.05% or more.
• the Cr content is more preferably 0.40% or less, and further preferably 0.30% or less.
• Mo is an element having an action of improving the strength of the steel sheet (base material) like Cu, and can be arbitrarily added.
• the Mo content is preferably 0.01% or more.
• the Mo content is set to 0.30% or less.
• the Mo content is more preferably 0.05% or more.
• the Mo content is more preferably 0.20% or less.
• V is an element having an action of improving the strength of the steel sheet (base material) like Cu, and can be arbitrarily added.
• the V content is preferably 0.01% or more.
• the V content is set to 0.30% or less.
• the V content is more preferably 0.05% or more.
• the V content is more preferably 0.20% or less.
• Ti 0.010% or more and 0.025% or less
• Ti is an element that precipitates as TiN during solidification of steel and contributes to the suppression of coarse-grained austenite in the weld heat-affected zone and the ferrite transformation nucleus to increase toughness, and is important in the present invention. It is one of the elements.
• it is preferable to contain 0.010%.
• the Ti content is more preferably 0.012% or more, and further preferably 0.014% or more. On the other hand, if it is added in excess of 0.025%, a large amount of TiN is generated or the problem of coarsening of TiN particles occurs, and the expected effect cannot be obtained.
• the toughness of the welded portion is rather lowered. Therefore, the upper limit of the Ti content is preferably 0.025%. Further, from the viewpoint of improving toughness, it is more preferably 0.023% or less, further preferably 0.021% or less, and further preferably 0.019% or less.
• N is an element necessary for the formation of TiN described above, and it is preferable to contain TiN in an amount of 0.0038% or more in order to secure a required amount from the viewpoint of toughness of HAZ.
• the N content is more preferably 0.0040% or more, and further preferably 0.0042% or more.
• the upper limit of the N content is preferably 0.0084%. From the viewpoint of improving toughness, the N content is more preferably 0.0082% or less, further preferably 0.0080% or less, and even more preferably 0.0078% or less.
• Ti and N precipitate as TiN during solidification of steel which contributes to the suppression of coarse graining of austenite in the weld heat-affected zone and the ferrite transformation nucleus to increase the toughness. It is an element and is contained in the following range.
• Ti / N (Ti / N: 2.10 or more and 3.60 or less) If the mass% ratio (Ti / N) of Ti and N is less than 2.10, the solid solution N that does not become TiN increases, and the toughness of HAZ decreases. Therefore, Ti / N is set to 2.10 or more.
• the Ti / N is preferably 2.20 or more, and more preferably 2.30 or more.
• the upper limit of Ti / N is 3.60. Further, from the viewpoint of improving the toughness of HAZ, it is preferably 3.50 or less, and more preferably 3.40 or less. Further, in Ti / N, each element is the content in steel (mass%).
• the value of the formula (1) is preferably more than 169, more preferably 175 or more, and further preferably 180 or more.
• the value of the above formula (1) exceeds 309, a large amount of TiN is generated, which in turn lowers the toughness of HAZ. Therefore, the value of the above equation (1) is set to 309 or less.
• the value of the formula (1) is preferably less than 309, more preferably 280 or less, and further preferably 260 or less.
• Ceq C + Mn / 6 + (Cu + Ni) / 15+ (V + Cr + Mo) / 5 ...
• Ceq is preferably 0.410 or more, more preferably 0.420 or more, and even more preferably 0.430 or more.
• Ceq is set to 0.500 or less. From the viewpoint of component cost, Ceq is preferably 0.490 or less, and more preferably 0.480 or less.
• the basic composition of the steel sheet of the present invention contains each element having the contents described above, and the balance is Fe and other unavoidable impurities.
• This basic composition may optionally have W: 0.30% or less, Co: 0.30% or less, B: It can further contain one or more selected from 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less.
• W is an element having an action of improving the strength of the steel sheet (base material) like Cu, and can be arbitrarily added.
• the W content is preferably 0.01% or more.
• the W content is set to 0.30% or less.
• the W content is more preferably 0.05% or more.
• the W content is more preferably 0.20% or less.
• Co is an element having an action of improving the strength of the steel sheet (base material) like Cu, and can be arbitrarily added.
• the Co content is preferably 0.01% or more.
• the Co content is set to 0.30% or less.
• the Co content is more preferably 0.05% or more.
• the Co content is more preferably 0.20% or less.
• B is an element having an action of remarkably improving hardenability even when added in a small amount. Therefore, the strength of the steel plate (base material) can be improved. In addition, by contributing to the improvement of hardenability in HAZ, it suppresses the formation and growth of a coarse ferrite structure, and by forming a precipitate with N, it acts as a transformation nucleus and contributes to the miniaturization of the structure. , HAZ toughness can also be improved.
• the B content is preferably 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, coarse Fe-B-based carbides may be produced.
• Such coarse Fe-B-based carbides serve as a starting point of fracture, and the toughness of the base metal and HAZ is significantly reduced. Therefore, when B is added, the B content is 0.0100% or less.
• the B content is more preferably 0.0050% or less, more preferably 0.0030% or less, more preferably 0.0012% or less, and more preferably 0.0010% or less. Is even more preferable. Further, from the viewpoint of avoiding high alloying and suppressing the cost, it is desirable that the upper limit of the B content is as described above when B is added.
• Ca (Ca: 0.0100% or less) Ca is an element that binds to S and has an effect of suppressing the formation of MnS or the like that extends long in the rolling direction. Therefore, by adding Ca, the morphology of the sulfide-based inclusions can be controlled so as to have a spherical shape, and the toughness of the welded joint or the like can be improved.
• the Ca content is preferably 0.0005% or more, more preferably 0.0020% or more.
• the Ca content exceeds 0.0100%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in the toughness of HAZ. Therefore, when Ca is added, the Ca content is set to 0.0100% or less.
• the Ca content is more preferably 0.0050% or less, and further preferably 0.0025% or less.
• Mg is an element that binds to S and has an effect of suppressing the formation of MnS or the like that extends long in the rolling direction. Therefore, by adding Mg, the morphology of the sulfide-based inclusions can be controlled so as to have a spherical shape, and the toughness of the welded joint or the like can be improved.
• the Mg content is preferably 0.0005% or more, more preferably 0.0020% or more.
• the Mg content exceeds 0.0100%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in the toughness of HAZ. Therefore, when Mg is added, the Mg content is set to 0.0100% or less.
• the Mg content is more preferably 0.0050% or less, and further preferably 0.0025% or less.
• REM 0.0200% or less
• REM rare earth metal
• the morphology of the sulfide-based inclusions can be controlled so as to have a spherical shape, and the toughness of the welded joint or the like can be improved.
• the REM content is preferably 0.0005% or more, more preferably 0.0015% or more.
• the REM content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in the toughness of HAZ. Therefore, when REM is added, the REM content is set to 0.0200% or less.
• the REM content is more preferably 0.0100% or less, further preferably 0.0080% or less, and even more preferably 0.0050% or less.
• TiN particle size and density TiN in the steel sheet of the present invention, particularly TiN particles existing at a depth of 1 mm from the surface of the steel sheet will be described.
• Average grain size of TiN particles at a depth of 1 mm from the surface of the steel sheet 20 nm or more and 50 nm or less
• TiN precipitates during solidification of steel, suppresses coarse graining of austenite in the heat-affected zone of welding, and becomes ferrite transformation nuclei. It is a precipitate that contributes to high toughness and plays an important role in the present invention. If the average particle size of the TiN particles is less than 20 nm, TiN decomposes during welding and its dispersion effect disappears, or the solid-melted Ti and solid-melted N produced by such decomposition weaken the ground structure of the steel. The toughness of HAZ is significantly reduced.
• the average particle size of TiN particles at a depth of 1 mm from the surface of the steel sheet is set to 20 nm or more.
• the average particle size is preferably 25 nm or more, and more preferably 30 nm or more.
• the average particle size of TiN at a depth of 1 mm from the surface is set to 50 nm or less.
• the average particle size is preferably 45 nm or less, and more preferably 40 nm from the viewpoint of improving HAZ toughness.
• Density of TiN particles at a depth of 1 mm from the surface of the steel sheet 5.0 ⁇ 10 8 particles / cm 2 or more.
• the density is preferably 8.0 ⁇ 10 8 pieces / cm 2 or more, and more preferably 1.0 ⁇ 10 9 pieces / cm 2 or more.
• the upper limit of the density is not particularly limited, but from the viewpoint of the content of Ti and N defined in the present invention, and if the density is too high, the average particle size of TiN becomes too fine, so that it is practically. 1.0 ⁇ 10 10 pieces / cm 2 or less.
• the TiN particles refer to precipitates containing 10% or more of Ti and N, respectively. Further, the average particle size and density of the above-mentioned TiN particles are set in the range of 10 ⁇ m ⁇ 10 ⁇ m which is arbitrarily selected and observed by a microscope when a sample is taken so that the observation surface is at a depth of 1 mm from the surface of the steel sheet. , The area equivalent circle diameter and the number of TiN particles can be specified and calculated from these.
• the characteristics of the joint manufactured by high heat input welding are mainly achieved by the component design of the steel material and the manufacturing method of the steel material, which are within the range of the component composition of the steel sheet, and the hot rolling during the steel sheet manufacturing. It is not affected by the influence or the characteristics after hot rolling.
• the plate thickness, strength, toughness, and microstructure of the steel plate (base material) suitable for application to the hatch side combing portion in a container ship are as follows.
• the thickness of the steel plate of the present invention is not particularly limited, but for example, when applied to a hatchside combing portion in a container ship, a steel plate having a thickness of 50 mm or more is substantially applied, and a steel plate having a thickness of 75 mm or more is more preferable.
• the strength of the steel plate (base material) of the present invention is not particularly limited, but for example, when applied to a hatchside combing portion in a container ship, the yield strength at a plate thickness of 1/2 position (also referred to as 1 / 2t) is 390 MPa or more. Steel plate is recommended. It is preferably 430 MPa or more, more preferably 460 MPa or more.
• the toughness of the steel sheet of the present invention is not particularly limited, but for example, when applied to a hatchside combing portion in a container ship, it is recommended that the toughness at the plate thickness 1/4 position be 53 J or more in absorption energy at ⁇ 40 ° C. To. It is preferably 64 J or more, more preferably 75 J or more.
• the microstructure of the steel sheet of the present invention which is ideal for obtaining the plate thickness and the strength of the base material, will be described.
• the pearlite fraction decreases in the ferrite-pearlite structure. Therefore, it becomes difficult to secure a predetermined base material strength. Therefore, the volume fraction of bainite at 1 / 2t, which is the central portion of the plate thickness, is preferably 80% or more, more preferably 90% or more, and may be 100%.
• the volume fraction of bainite within the above range, it is possible to secure a predetermined base material strength and base material toughness. Further, as long as the volume fraction of bainite satisfies the above range, a structure other than bainite such as ferrite and pearlite, which is usually recognized as a structure of a steel sheet, may be present in the remaining fine structure.
• ⁇ Manufacturing method of steel sheet> The particle size and density of TiN particles that affect the properties of the joint are influenced by the composition of the steel material and the casting process. Therefore, in the method for producing a steel sheet of the present invention, in addition to the above-mentioned composition of steel, only the conditions of the casting process for obtaining a steel material to be subjected to hot rolling are specified. Other manufacturing methods and conditions thereof are not particularly limited, but a heating step may be performed after the casting step and before the hot rolling step, or a cooling step may be performed after the hot rolling step.
• the heating temperature of the heating process when applied to a hatchside combing portion in a container ship, the heating temperature of the heating process; the rolling start temperature of the hot rolling process, the cumulative rolling reduction rate in the unrecrystallized region, and the rolling end temperature; the cooling start of the cooling process. It is preferable to adjust the temperature, the average cooling rate, and the cooling stop temperature; to the following conditions, respectively.
• a steel sheet obtained by a manufacturing method that satisfies these conditions has excellent base material strength and excellent toughness for large heat-affected zone HAZ, and thus can be suitably used for manufacturing large structures such as container ships. Is.
• the manufacturing conditions for steel materials are not particularly limited except for limiting the cooling rate when obtaining steel materials such as slabs.
• a molten steel having the above-mentioned composition is produced by a known melting method such as a converter, and a steel material such as a slab having a desired dimension by a known casting method such as a continuous casting method. It is preferable to obtain as.
• molten steel having the above-mentioned composition can be used. Further, the temperature of the molten steel used for casting can be 1400 ° C. or higher.
• the cooling conditions at the time of casting are important. That is, when the steel material is cast, if the average cooling rate is less than 100 ° C./min in the temperature range of 1400 to 1250 ° C. where TiN is deposited at a position 1 mm from the surface of the steel material, the base material (steel plate) in the product steel sheet. ) TiN size becomes coarse. When the TiN size becomes coarse, the TiN density of the base material (steel plate) decreases, the austenite structure in the large heat-affected zone becomes coarse, and the toughness of the HAZ may decrease.
• the average cooling rate at the time of casting (the average cooling rate of the steel material) is set to 100 ° C./min or more.
• the average cooling rate is preferably 150 ° C./min or higher, more preferably 200 ° C./min or higher.
• the cooling rate of the steel material exceeds 500 ° C./min, the density of TiN increases, but the size of TiN becomes finer, and TiN melts and the austenite grains become coarser during high heat input welding. Therefore, the toughness of HAZ deteriorates.
• the cost for removing the cracks and the yield of the material may decrease.
• the average cooling rate during casting is set to 500 ° C./min or less.
• the average cooling rate is preferably 400 ° C./min or less, more preferably 300 ° C./min or less.
• the temperature range for measuring the average cooling rate is in the range of 1400 to 1250 ° C.
• the temperature in each process described below shall be the temperature at the center of the plate thickness (1 / 2t) of each steel material.
• the heating temperature of the steel material in the heating step is preferably 950 ° C. or higher and 1250 ° C. or lower. If the heating temperature is less than 950 ° C., the heating temperature is too low and the deformation resistance becomes high, which increases the load on the hot rolling mill, which may make it difficult to perform the subsequent hot rolling. On the other hand, when the heating temperature is higher than 1250 ° C., the austenite grains are coarsened, which not only lowers the toughness of the steel sheet base material and the large heat-affected zone HAZ, but also significantly oxidizes and increases the oxidation loss, resulting in a decrease in yield. There is a risk of The heating temperature is more preferably 1000 ° C. or higher. On the other hand, the heating temperature is more preferably 1150 ° C. or lower.
• the rolling start temperature is preferably Ar 3 points + 100 ° C. or higher. From the viewpoint of securing the time for rolling in the unrecrystallized region described later, the rolling start temperature is more preferably Ar 3 points + 150 ° C.
• Ar 3 points + 200 ° C. or higher The upper limit of the rolling start temperature is not particularly limited, but is about the heating temperature of the steel material described above.
• the Ar 3 points (° C) can be obtained according to the following equation (3).
• Ar 3 points (°C) 910-273C-74Mn-57Ni-16Cr-9Mo-5Cu ... (3)
• each element symbol represents the content (mass%) of the element in steel, and 0 is set for the element not contained.
• Rolling end temperature Ar 3 points or more It is desirable that the hot rolling process is completed at a temperature equal to or higher than the Ar 3 transformation point (° C.). If the temperature is lower than the Ar 3 transformation point (° C.) during hot rolling, a large amount of ferrite is generated in the steel, so that the volume fraction of bainite cannot be increased, and there is a possibility that a predetermined strength cannot be obtained. Further, the lower the temperature, the higher the deformation resistance, so that the load on the hot rolling mill increases. From the viewpoint of ensuring the cooling start temperature in the subsequent process, the hot rolling temperature is preferably Ar 3 points + 20 ° C. or higher.
• the average cooling rate until the temperature becomes 500 ° C. or lower in the cooling step is preferably 2.0 ° C./s or higher.
• the upper limit of the average cooling rate is not particularly limited, but the industrial cooling rate at the plate thickness 1/2 position of the plate thickness 50 mm is 20 ° C./s at the maximum, and an increase in cooling cost due to excessive quenching is avoided. Therefore, it is preferably 20 ° C./s or less.
• the temperature range for measuring the average cooling rate is in the range of 600 to 500 ° C.
• the cooling step for cooling is performed until the temperature at 1 / 2t becomes 500 ° C. or lower, that is, the cooling shutdown temperature: 500 ° C. or lower. If the cooling shutdown temperature exceeds 500 ° C., a large amount of ferrite is generated in the steel, and the volume fraction of bainite cannot be increased, so that the base metal may not have a predetermined strength.
• the lower limit of the cooling stop temperature is not limited, but if the cooling stop temperature is too low, the shape of the steel sheet deteriorates, so that the temperature is preferably about 200 ° C, more preferably about 300 ° C.
• a base material (steel plate) having a fine structure according to the present invention can be obtained.
• the steel sheet thus obtained has excellent toughness due to the large heat input HAZ, and becomes a suitable steel sheet when applied to a hatchside combing portion in a container ship.
• the toughness of HAZ the case where the absorption energy at ⁇ 20 ° C. (vE-20 ° C.): 46J or more is regarded as excellent toughness.
• the toughness of HAZ the case where vE-20 ° C. is 53J or more is regarded as more excellent toughness, the case where vE-20 ° C. is 64J or more is regarded as further excellent toughness, and the case where vE-20 ° C. is 92J or more is regarded as particularly excellent toughness.
• the steel sheet of the present invention can effectively avoid coarsening of austenite grains in a large heat-affected zone, and can obtain a high vE-20 ° C. in a welded joint containing such HAZ.
• the steel sheet of the present invention is suitable for use in large heat input welding.
• a molten steel having the component composition shown in Table 1 is prepared, cast under the conditions shown in Table 2 to obtain a steel material (slab), and then the steel material is heated in a heating process and hot under the conditions also shown in Table 2. A rolling process and a cooling process were sequentially performed to obtain each steel sheet.
• the average particle size and density of TiN particles at a depth of 1 mm from the surface and the depth of 1/2 of the plate thickness from the surface (also referred to as 1 / 2t at the center of the plate thickness in this embodiment).
• the volume ratio of bainite was measured.
• the yield strength (YS) and the base material toughness (vE-40 ° C.) were evaluated as the base material characteristics.
• the joint test plates collected from each of the above steel plates are subjected to V-groove processing, and a large heat input welding of 200 kJ / cm is performed using a commercially available welding wire for low temperature steel. A joint was manufactured by heat input welding. Then, the toughness of HAZ was evaluated using the obtained joint.
• Each test method is as follows. The characteristics evaluated using the obtained joints as described above were defined as joint characteristics.
• a sample was taken from the steel sheet so that the observation surface was at a depth of 1 mm from the surface of the steel sheet.
• a thin film sample was prepared from the collected sample by an extraction replica method, and a range of 10 ⁇ m ⁇ 10 ⁇ m was photographed using a transmission electron microscope (TEM).
• TEM transmission electron microscope
• EDX analysis for precipitates containing 10% or more of Ti and N, the diameter and number of precipitates corresponding to the area circle are analyzed from the captured images using an image analysis device, and the average particle size and density are calculated. bottom.
• the bainite structure was determined as follows when determining the fraction of the microstructure. That is, the above imaging was performed at a magnification of 500 to 3000 times to obtain an SEM image. In such an SEM image, a structure having an elongated lath-shaped ferrite structure and containing carbides having a diameter equivalent to a circle of 0.05 ⁇ m or more was determined to be a bainite structure.
• Base material characteristics (Base material strength) A JIS Z 2201 No. 14A test piece was taken from the center of the steel plate thickness in a direction perpendicular to the rolling direction so that the center of the plate thickness (1/2 position of the plate thickness) was the center of the test piece. The collected test pieces were subjected to a tensile test in a manner in accordance with JIS Z 2241, and the yield strength YS (unit: MPa) was measured as the base metal strength.
• HAZ characteristics HAZ toughness
• the NK U4 impact test piece was sampled so that the surface layer of the test piece was from the surface of the joint obtained by high heat input welding to a depth of 1 mm and the HAZ was the notch position.
• the collected test pieces were subjected to a Charpy impact test at a test temperature of -20 ° C, and the average value vE-20 ° C (unit: J) of the absorbed energy of the three test pieces conducted under the same conditions was defined as the toughness of HAZ. ..
• the evaluation results thus obtained are also shown in Table 2.
• the vE-20 ° C. of the large heat input HAZ is 46 J or more.
• all the steel sheets manufactured in a suitable range show a high strength of the base material YS of 390 MPa or more and a high base material toughness of vE-40 ° C. of 53 J or more, and both the base material strength and the base material toughness are compatible. ..
• the steel sheet of the invention example is excellent in high heat input weldability.
• the toughness of HAZ is low because any of the component compositions of the steel material does not satisfy the conditions of the present invention, or the YS of the base metal is low in addition to the toughness of HAZ. I understand.
• the steel plate No. corresponding to the comparative example.
• the composition of the steel material satisfies the conditions of the present invention
• the average cooling rate at the time of casting does not satisfy the conditions of the present invention. It can be seen that at least one of the densities is outside the scope of the present invention, and thus the toughness of HAZ is low.

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