BR112012011685B1 - STEEL SHEET FOR SHIP HULL - Google Patents

Abstract

steel plate for a ship's body and its production method 05% or less, s: 0.05% or less, ai: 0.002% to o, 1%, and the balance being f and e the inevitable impurities such as chemical elements, in which the microstructure includes ferrite and is composed of one or more among pearlite and bainite, the percentage of undeformed ferrite area in the microstructure is 85% or more, the average grain size of undeformed ferrite is 5 j.lm to 40 j..lm, the density of cementite particles in the ferrite grains is 50000 particles/mm2 or less, yield strength is 235 mpa or more, tensile strength is 460 mpa, uniform elongation is 15% or more, tensile strength is 460 mpa or less, uniform elongation is 15 % or more, and the average charpy absorbed energy at 0°C is 100 J or more.

Description

FIELD OF INVENTION

[001] The present invention relates to a steel sheet 8 mm thick or more, which can be deformed in the event of a collision in order to avoid damage to the other ship, thus having a shock-absorbing effect, and is used for a ship's bow structure to improve collision safety and its production method.

[002] Priority is claimed over Japanese Patent Application No. 2009-265118, filed on November 20, 2009, the content of which is incorporated herein by reference.

DESCRIPTION OF RELATED TECHNIQUE

[003] In today's ships, steel sheets 8 mm thick or more are used for the bow structures of ships, and a bulbous bow is provided to a submerged portion of the bow to reduce energy loss caused by wave resistance. as much as possible.

[004] Hitherto a rigid structure having a horizontally and vertically disposed reinforcing beam has been used for the bulbous bow (see Patent Citation 1 and Patent Citation 2). However, for example, when a ship provided with a bulbous bow having a rigid structure collides with another ship, the bulbous bow 20a of the collided ship 20 penetrates into the bottom 21a of the impacted ship 21 as shown in figure 1A, and, in addition, there have been cases where the broken portion is extended so that the hull (bottom 21a) of the collided ship 21 is damaged, and a crack opening (hole) 21b is formed.

[005] Therefore, in recent years, bulbous bows have been suggested that are structured to absorb collision energy as much as possible when two ships collide with each other, particularly when the bow of the colliding ship collides with the bottom of the colliding ship. in order to prevent the hull of the collided ship from breaking (refer to Patent Citations 3 to 5).

[006] Patent Citation 3 describes a bulbous bow having an impermeable structure (a structure in which the interior of the bow bulbs is connected to external water) at the circumferential edge portion continued from the front end portion so as to absorb collision energy. However, since the bulbous bow described in Patent Citation 3 is a non-impermeable structure on the circumferential edge portion continued from the front end portion, it is difficult to sufficiently reduce wave resistance.

[007] In addition, since the bulbous bow described in Patent Citation 3 has a non-waterproof structure, corrosion resistance processing is indispensable for the steel sheet and therefore production costs increase.

[008] Patent Citation 4 describes a bulbous bow provided with a thinning portion that reduces the stiffness of bending in the horizontal direction on the outer base plate of the protruding bulb. In addition, Patent Citation 5 describes a bulbous bow provided with a low strength portion having a low resistance to bending in the horizontal direction (a portion made of a low yield strength steel having a lower yield strength or stress test 235 MPa or less) on the outer base plate of the protruding bulb.

[009] In the bulbous bows as described in Patent Citations 4 and 5, the thinning portion or the low strength portion is provided in the vicinity of the bulbous bow base so that the bulbous bow base is easily folded into collision event.

[0010] That is, in the bulbous bows as described in Patent Citations 4 and 5, when two ships collide with each other at an expected angle, for example, the bulbous bow 30a of the colliding ship 30 is easily folded into the base portion 30b as shown in figure 1B. Therefore, the face of the bulbous bow 30a that contacts the bottom 31a of the collided ship 31 is not the face of the front end 30c, but the face of the trunk 30d, and thus the damage to the bottom 31a of the collided ship is reduced.

[0011] However, unlike the bulbous bow as described in Patent Citation 3, the bulbous bows as described in Patent Citations 4 and 5 do not function as shock absorbers that absorb collision energy. Therefore, when two ships collide with each other at an angle of approximately 90°, the bulbous bow is supposed to receive a collision reaction force and penetrate to the bottom of the impacted ship before the bulbous bow is charged on the base portion.

[0012] Meanwhile, the dotted portion in Figure 1B indicates the position of the bulbous bow 30a before the colliding vessel 30 collides with the colliding vessel 31.

[0013] Consequently, bulbous bows as described in Patent Citations 4 and 5 have a limitation on the degree of damage reduction to the bottom of the collided ship.

[0014] Meanwhile, the use of steel sheets having an excellent energy absorption capacity in the event of a collision is also being studied as a means of increasing the collision safety of ships, and Patent Citations 6 to 10 describe such sheets of steel.

[0015] In steel sheets as described in Patent Citation 6, the amount and hardness of ferrite is increased, the size of the second phase is decreased, and the amount of hardening work is increased so that the ability to energy absorption increases.

[0016] In addition, in steel sheets as described in Patent Citations 7 to 9, the retained austenite is dispersed into the steel, and the amount of work hardening of the steel sheet is increased using induced transformation plasticity ( TRIP) so that the strength and uniform elongation are improved, and the energy-absorbing capacity is increased.

[0017] In addition, in the steel sheet as described in Patent Citation 10, the microstructure is a fine ferrite-based microstructure and the resistance of the ferrite phase is increased by using precipitation resistance so that the fracture resistance is enhanced.

[0018] However, when the bow of the colliding ship collides with the bottom of the collided ship, there is a high risk of the bulbous bow penetrating the bottom as long as the bulbous bow of the colliding ship is not deformed even when the bottom of the ship collided is composed of a steel plate having an increased capacity for energy absorption. In addition, the effects of using a steel sheet having a high absorption capacity cannot be anticipated depending on the aspect of the collision.

[0019] In contrast to the above, the damage to the bottom of the collided ship can be extremely reduced as long as the bulbous bow of the colliding ship can absorb the energy and collision.

[0020] However, since steel sheets having an excellent energy absorbing capacity described in Patent Citations 6 to 10 are made to have a high strength and a high amount of work hardening, the pressing force for- given to the collided ship in the event of a collision becomes large. Therefore. There is a high possibility that the bulbous bow will penetrate the bottom before the bulbous bow is sufficiently deformed, and the energy is sufficiently absorbed.

[0021] In addition, when producing a curved sheet having a large curvature that is used for the bulbous bow a linear heating bending process, ie a linear heating process is often used in a forming process.

[0022] In the linear heating process, a phenomenon is used in which the surface of a steel sheet is locally and linearly heated using a torch or the like, and the heated portion is thermally expanded, but restricted by adjacent portions so to be plastically deformed. Generally, water cooling is performed immediately after heating to increase operating efficiency, and the quality of steel sheet processed by linear heating is varied according to the microstructure of the base metal.

[0023] That is, in steel sheets having an excellent energy absorption capacity as described in Patent Citations 6 to 10 there is a high possibility that the heated and water-cooled portions are quenched, the strength is increased locally, and the elongation is degraded.

[0024] Therefore, a bulbous bow for which the curved sheet is used has an irregular strength and is not easily deformed. Therefore, when a ship provided with the bulbous bow collides with another ship, there is also a high risk that the bow 20a of the colliding ship 20 will penetrate the bottom 21a of the collided ship 21 as shown in Figure 1A, the bottom 21a is broken and Furthermore, the broken portion is extended so that a broken opening (hole) 21b is formed in the hull of the ship.

[0025] In contrast to Patent Citations 6 to 10, Patent Citation 11 describes a steel sheet, and its workability of bending is improved by linear heating by decreasing the flow tension at 400°C. In such a steel sheet, the yield stress at ambient temperature is also decreased to lower the yield stress at 400°C. Therefore, there is a possibility that the steel sheet is easily deformed in the event of a collision, and the bulbous bow can absorb enough energy when the steel sheet is used for the bulbous bow. However, according to the unified standards of the International Association of Classification Societies (IACS), since the yield strength at room temperature needs to satisfy 235 MPa or more, fine cementite reinforcement is used. However, since cementite is thermally unstable, it is difficult to maintain the dispersion state of cementite in the steel sheet after linear heating and satisfy the predetermined quality. In addition, there are problems in that the dispersion of fine cementite cannot produce the required work hardening characteristics and sufficient uniform elongation to absorb energy.

[0026] In addition, Patent Citations 12 and 13 describe a bulbous bow steel plate for which the working capacity and toughness after forming are increased by reducing the amount of carbon and decreasing the yield point. However, a production such as extremely low carbon steel sheet is not economically preferred as melt loads become large, and costs increase.

[0027] From the above facts, there is currently a demand for steel plates for bulbous bows that present a function with which the resistance to waves is significantly reduced so as to also reduce energy loss and also increase the propulsion performance of a ship during navigation, however a damping function with which the damage to the bottom of a collided ship is reduced so as to effectively absorb the energy of the collision in the event of a collision.

PATENT CITATION

[0028] [Patent Citation 1] Unexamined Japanese Patent Application, First Publication No. 2002-347690

[0029] [Patent Citation 2] Unexamined Japanese Patent Application, First Publication No. 2005-199736

[0030] [Patent Citation 3] Unexamined Japanese Patent Application, First Publication No. H8-164887

[0031] [Patent Citation 4] Unexamined Japanese Patent Application, First Publication No. 2004-314824

[0032] [Patent Citation 5] Unexamined Japanese Patent Application, First Publication No. 2004-314825

[0033] [Patent Citation 6] Unexamined Japanese Patent Application, First Publication No. H11-193438

[0034] [Patent Citation 7] Unexamined Japanese Patent Application, First Publication No. H11-246934

[0035] [Patent Citation 8] Unexamined Japanese Patent Application, First Publication No. 2007-162101

[0036] [Patent Citation 9] Unexamined Japanese Patent Application, First Publication No. 2008-45196

[0037] [Patent Citation 10] Unexamined Japanese Patent Application, First Publication No. 2002-105534

[0038] [Patent Citation 11] Unexamined Japanese Patent Application, First Publication No. 2009-185380

[0039] [Patent Citation 12] Unexamined Japanese Patent Application, First Publication No. H5-70885

[0040] [Patent Citation 13] Unexamined Japanese Patent Application, First Publication No. H6-256891

SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0041] In consideration of the above demand in relation to the bulbous bow, an objective of the present invention is to provide a steel plate for the bulbous bow that can effectively absorb the collision energy and significantly reduce damage to the bottom of a collided ship in the event of collision without changing the design of a hull structure and its production method.

METHODS TO SOLVE THE PROBLEM

[0042] The present invention was made as a result of meticulous studies to solve the above problem, and the methods are as follows.

[0043] (1) A steel plate for a hull in accordance with an aspect of the present invention includes, in % by mass, C: greater than 0.03% to 0.10%, P: 0.05% or less, S: 0.05% or less, Al: 0.002% to 0.1%, and a balance of Fe and the unavoidable impurities as chemical elements, in which the microstructure includes ferrite and is composed of one or more of perlite and bainite, the percentage area of undeformed ferrite in the microstructure is 85% or more, the mean grain size of undeformed ferrite is 5 μm to 40 μm, the density number of cementite particles in the ferrite grains is 50000 particles/mm2 or less, yield strength is 235 MPa or more, tensile strength is 460 MPa or less, uniform elongation is 15% or more, and average Charpy energy absorbed at 0°C is 100 J or more.

[0044] (2) The steel plate for a hull according to item (1) above may also include in % by mass: one or more between Si: 0.03% to 1%, Mn: 0.1% to 1 .5%, Cu: 0.02% to 0.5%, Ni: 0.02% to 0.5%, Cr: 0.02% to 0.5%, Mo: 0.002% less than 0.2 %, Nb: 0.002% to 0.02%, V: 0.002% to 0.04%, Ti: 0.002% to 0.04%, B: 0.0002% to 0.002%, N: 0.0005% to 0.008 %, Ca: 0.0003% to 0.005%, Mg: 0.0003% to 0.005%, and rare earths: 0.0003% to 0.005% as chemical elements, in which the carbon equivalent Ceq below can be 0.30 or less.Ceq = [C] + [Si] / 24 + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [V]) / 10 + ([Mo] + [Nb]) / 5 + [Ti] / 20 + [B] / 3 + [N] / 8.

[0045] Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [V], [Mo], [Nb], [Ti], [B], and [N] are the amounts of C, Si, Mn, Cu, Ni, Cr, V, Mo, Nb, Ti, B, and N, in % by mass, respectively.

[0046] (3) In the hull steel sheet as per item (2) above, the Ceq carbon equivalent may be 0.27% or less in % by mass.

[0047] (4) On the steel plate for hull as per items (1) and (2) above, the yield ratio can be 0.70 or more.

[0048] (5) A method of producing a steel sheet for shell according to the first aspect of the present invention includes: heating a steel including the chemical elements according to any one of items (1) to (3) above to 1000°C to 1300°C; to carry out a rolling with a cumulative reduction of 30% to 98% to the thickness of the sheet product in a temperature range of a single austenite phase from the transformation point Ar3 or more; and perform a quench from a cooling start temperature of 760°C or more to a temperature of 400°C to 650°C at a cooling rate above a plate thickness of 1°C/s to 50°C/s , and then perform an air-cooling.

[0049] (6) A method of producing a steel sheet for shell according to the second aspect of the present invention includes: heating a steel including the chemical elements according to any one of items (1) to (3) to 1000°C to 1300 °C; run a roll with a 30% to 98% cumulative reduction to sheet product thickness over a single-phase austenite temperature range of the transformation point Ar3 or more; and run a quench from a cooling start temperature of 760°C or more to a temperature of less than 400°C at an average cooling rate over plate thickness of 1°C/s to 50°C/s , and then run the temper at a temperature of 400°C to 650°C.

[0050] (7) A method of producing a steel plate for ship's hull in accordance with the third aspect of the present invention includes: heating an steel including the chemical elements in accordance with any one of items (1) to (3) above to 1000° C to 1300°C; run the lamination with a 30% to 98% cumulative reduction to sheet product thickness over a one-phase temperature range [single austenite of transformation point Ar3 or more; and perform air cooling.

EFFECTS OF THE INVENTION

[0051] According to the present invention, when an accident occurs in which the bow of the first ship having a bulbous bow collides with the bottom of a second ship, the lateral faces of the bulbous bow portion of the colliding ship (first ship) are bent more evenly so that it is possible to significantly absorb the energy of the collision. In addition, the collapse of the colliding face while absorbing the collision energy makes it possible to reduce the damage to the collided ship (second ship) as much as possible. And therefore it is possible to contribute to the prevention of the sinking of the collided ship or contamination of the sea due to the oil spill.

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1A is a schematic diagram showing the deformation of the colliding vessel and the collided vessel when the colliding vessel having no shock absorbing effect on the bow structure collides with the collided vessel.

[0053] Figure 1B is a schematic diagram showing the deformation of the colliding vessel and the colliding vessel when the colliding vessel having a shock absorbing effect on the bow structure collides with the collided station.

[0054] Figure 1C is a schematic diagram showing the deformation of the colliding ship and the colliding ship when colliding ships in which the hull steel plate according to the present invention is used for the bow structure in order to have a Shock absorption effect collides with the collided ship.

DETAILED DESCRIPTION OF THE INVENTION

[0055] Hereinafter, an embodiment according to the present invention will be described..

[0056] When an accident occurs in which the bow of a ship having a bulbous bow collides with the bottom of another ship, if a bulb portion 10b of a bulbous bow 10a can be bent more uniformly as shown in Figure 1C, it is possible absorb more collision energy, and therefore impact forces due to collision are smoothed out. As a result, it is possible to avoid local breakages or damage to the bottom 11a of a collided ship 11, and the occurrence of broken openings (holes) can be avoided.

[0057] Meanwhile, the portion of the dotted line in figure 1C indicates the position of the bulbous bow 10a before the colliding ship 10 collides with the colliding ship 11.

[0058] As a result of studies of the deformation behavior of the bulbous bow through a FEM analysis, it was found that when the mechanical properties give as follows: the yield strength is 235 MPa or more, the tensile strength is 460 MPa or less, and the uniform elongation is 15% or more, a pressing force supplied to the collided ship in case of collision of two ships is decreased. In addition, it was found that, in this case, the bulbous bow is easily deformed, the contact area between the bulbous bow and the bottom of the collided ship increases so that the bulbous bow effectively absorbs collision energy, and bottom damage of the collided ship can be significantly decreased.

[0059] Hereinafter, the reasons why yield stress, tensile strength, uniform elongation, and average Charpy energy absorbed at 0°C are limited in configuration will be described.

[0060] In current ships, steel sheets having a thickness of 8 mm or more, or a maximum of 100 mm, are used for the bow structures, and basically the yield stress of the steel sheet used for the bow of the colliding vessel needs to be less than the yield stress of the steel plate of the colliding vessel for the bow of the colliding vessel to be bent. However, the yield strength of sheet steel used for ships needs to meet the unified standards of the International Association of Classification Societies (IACS), and it also needs to be a strength that can withstand the impact of waves on bow structures having a frame structure. conventional. Furthermore, when the yield stress is excessively decreased, a large energy-absorbing effect cannot be obtained during deformation. Considering the above facts, the yield strength of the steel sheet needs to be 235 MPa or more. In addition, the upper limit of the yield strength is desirably 400 MPa or less from the standpoint of the yield strength of the steel sheet used for ships. The yield strength of the steel sheet is desirably 320 MPa or less for the bow to bend more reliably in the event of a collision.

[0061] The tensile strength needs to be increased to increase the energy absorption capacity. However, when the tensile strength is increased excessively, a pressing force supplied to the collided ship in the event of a collision has increased. In this case, there is a high possibility that the bulbous bow will penetrate the bottom before the bulbous bow is sufficiently deformed, and the energy is insufficiently absorbed. Therefore, the upper limit of the tensile strength is 460 MPa. To obtain the necessary energy absorption capacity, the lower limit of tensile strength is desirably 300 MPa.

[0062] Similar to tensile strength, uniform elongation needs to be increased to increase energy absorbing capacity, and the lower limit of uniform elongation is 15%. Uniform elongation is desirably 20% or more to also improve energy absorption capacity. In addition, greater uniform elongation is more desirable, and uniform elongation is desirably 50% or less to ensure the required strength;

[0063] In addition, the absorbed energy Charpy at 0°C needs to be 100 J or more to avoid brittle fracture in the event of a collision. That is, when a brittle fracture occurs in the event of a collision, there is an increasing possibility that energy cannot be absorbed by warping deformation, furthermore, fractures propagate to other limbs, and critical fracture accidents occur. Therefore, the lower limit of the average Charpy absorbed energy is 100 J. To avoid brittle fracture and also increase safety, the average Charpy absorbed energy is preferably 150 J or more. In addition, when a level at which the risk of brittle fracture can be almost avoided is taken into account, the upper limit of the average Charpy absorbed energy is desirably 500 J.

[0064] Additionally, the yield ratio is preferably 0.70 or more. When the yield ratio decreases, the amount of work hardening of the steel sheet increases, and therefore the energy absorbing capacity as a steel sheet increases. However, when the amount of work hardening increases, the strength of pressing supplied to the bulbous bow of the colliding vessel for the collided vessel in the event of a collision increases. Therefore, there may be a possibility that the bulbous bow penetrates the bottom before the bulbous bow is sufficiently deformed, and the energy is sufficiently absorbed. Therefore, it is preferable to limit the yield ratio depending on the strength of the steel sheet to sufficiently guarantee the energy absorption capacity like the bulbous bow. To ensure the deformation of the bulbous bow more reliably, the yield ratio is more preferably 0.75 or more, and more preferably 0.80 or more. In addition, to ensure energy absorbing capacity and workability as a bulbous bow steel plate, the yield ratio is preferably 0.95 or less.

[0065] Hereinafter, the reasons why the microstructure is limited in configuration will be described.

[0066] The microstructure of the steel sheet includes ferrite as the matrix (first stage), and is composed of one or more of perlite and bainite in addition to ferrite.

[0067] Initially, the reason why undeformed ferrite is used as the matrix is to increase the energy that is absorbed by the warping deformation in the bulbous bow by using the softest microstructure among the microstructures in the steel sheet in order to suppress strength and improve uniform stretch. In addition, since ferrite deformed by lamination of the dual-phase region or the like becomes the cause of the degradation of the uniform elongation and the average Charpy absorbed energy, the undeformed ferrite is used as the ferrite matrix to prevent its degradation. In addition, since the generation of deformed ferrite increases the anisotropy of the steel sheet, when deformed ferrite is used as a ferrite matrix, it becomes difficult for the bulbous bow to be bent regularly in the event of a collision and for the steel plate absorb energy. Furthermore, when containing 1% or less deformed ferrite, the microstructure is determined not to include deformed ferrite.

[0068] The reason that the steel microstructure includes pearlite or bainite in addition to undeformed ferrite is that the inclusion of undeformed ferrite alone cannot guarantee strength and obtain a uniform elongation of 15% or more by extremely degrading the characteristics of hardening work and makes it difficult to guarantee the energy absorption capacity in the event of a collision. In addition, since it becomes difficult to obtain a tensile strength of 460 MPa or less and an average Charpy energy absorbed at 0°C of 100 J or more when martensite is present in the microstructure, it is necessary that no martensite be confirmed by observation cross-section using an optical microscope.

[0069] When the percentage area of undeformed ferrite is less than 85%, hard microstructures other than undeformed ferrite, such as deformed ferrite, pearlite, bainite, and martensite, exceed 15%, and it becomes difficult to obtain a strength at a tension of 460 MPa or less and a uniform elongation of 15% or more. Therefore, the percentage of undeformed ferrite area is 85% or more. The percentage of undeformed ferrite surface is preferably 90% to 95%.

[0070] When the grain size (diameter) of the undeformed ferrite is less than 5 μm, it is difficult to guarantee a uniform elongation of 15% or more, and when the grain size of the undeformed ferrite exceeds 40 μm, it is difficult to guarantee a yield strength of 235 MPa or more and an average Charpy energy absorbed at 0°C of 100 J or more. Therefore, the grain size of undeformed ferrite is 5 µm to 40 µm.

[0071] Furthermore, when cementite particles are present in the ferrite grains at a density of more than 50,000 particles/mm2, gaps become likely to occur, and therefore the uniform elongation is degraded so that it becomes difficult to guarantee a uniform elongation of 15% or more. Therefore, the density of cementite particles in the ferrite grains is limited to 50000 particles/mm2 or less.

[0072] Hereinafter, the reasons why the amount of each of the chemical elements is limited will be described. Here, "%" indicates "% by mass" unless particularly described otherwise.

[0073] C: more than 0.03% to 0.10%

[0074] C is a chemical element that increases the strength of steel, and more than 0.03% C is required to ensure a yield strength at room temperature of 235 MPa or more and to reduce melt loads. However, when the C content exceeds 0.10%, for example, the area percentage of a second phase such as pearlite increases, and it is difficult to obtain a tensile strength of 460 MPa or less and uniform elongation. 15% or more. Therefore, the upper limit of the C content is 0.10%. The C content is preferably 0.04% to 0.08% to more reliably control yield strength, tensile strength and uniform elongation.

[0075] P: < 0.05%, S: < 0.05%

[0076] P is an impurity element, and it is necessary to reduce the P content as much as possible to increase the yield strength at a high temperature by strengthening the solid solution and degrading the toughness. However. when the P content is 0.05% or less, since its adverse effects may be allowed, the upper limit of the P content is 0.05. S is also an impurity element, and it is desirable to reduce the S content as much as possible so as not to degrade the toughness and ductility of the steel. However, when the S content is 0.05% or less, since its adverse effects may be allowed, the upper limit of the S content is 0.05%.

[0077] Al: 0.002% to 0.1%

[0078] Al is an important element in the present invention, and added primarily for the purpose of deoxidation. To perform sufficient deoxidation, the Al content needs to be 0.002% or more. However, when the Al content exceeds 0.1%, once raw alumina-based oxides and their groups are generated, and toughness is impaired, the upper limit of the Al content is 0.1%. In order to carry out the deoxidation more reliably and also to guarantee the toughness, the amount of Al is preferably 0.01% to 0.07%.

[0079] The above chemical elements are fundamental chemical elements of the steel sheet of the present invention. Sheet steel including at least the above fundamental chemical elements can be used as a bulbous bow steel sheet having excellent energy absorption capacity on collision (sheet thickness of 8 mm or more is applicable, however the upper limit of the thickness of the plate is not particularly limited, but about 100 mm is a realistic upper limit), which is an object of the present invention. Furthermore, it is possible to add Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, B, and N to steel as optional chemical elements for the purpose of adjusting strength and toughness. Since these optional chemical elements increase the hardenability of steel to even a small amount added, the optional chemical elements not only improve strength and toughness by refining the crystal grains, but also contribute to improved strength by reinforcing the steel. solid solution, enhanced precipitation, etc. To obtain the effects, the lower limit of the Si content is 0.03%, the lower limit of the Mn content is 0.1%, the lower limits of the Cu content, the Ni content and the Cr content are 0 .02%, the lower limits of Mo content, Nb content, V content, and Ti content are 0.002%, and the lower limit of N content is 0.0005%. However, when any of the optional chemical elements are added excessively, low temperature transformation microstructures such as bainite are likely to be generated, and it is difficult to obtain a ferrite area percentage of 90% or more. In that case, since strength is increased, and uniform elongation is decreased, it is difficult to obtain a tensile strength of 460 MPa or less and a uniform elongation of 15% or more. Therefore, it is necessary to provide upper limits for each of the optional chemical elements. The upper limits are 1% for Si content, 1.5% for Mn content, 0.5% for Cu content, Ni content and Cr content 0.2% ( 0.2% not included) for Mo content, 0.02% for Nb content, 0.04% for V content and Ti content, 0.002% for B content, and 0.008% for the N content. In addition, the preferred upper limits are 0.8% or less for Si content, 1.2% or less for Mn content, 0.3% or less for Cu content, for the Ni content, and for the Cr content, 0.05% or less for the Mo content, 0.01% or less for the Nb content, 0.02% or less for the V content and for the Ti content, 0.001% or less for B content, and 0.006% or less for N content.

[0080] In addition, when Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, and B are added, it is necessary that the carbon equivalent as shown in equation (1) is 0.30% by mass or less . When chemical elements are added excessively, and the equivalent carbon exceeds 0.30% by mass, as described above, low-temperature transformation microstructures, such as bainite, are likely to be generated, and it is difficult to obtain the percentage of ferrite area of 85% or more. In that case, since strength is increased, and uniform elongation is decreased, it is difficult to obtain a tensile strength of 460 MPa or less and a uniform elongation of 15% or more. In addition, since the yield stress also increases, there is a possibility that the yield stress of the colliding vessel is above the yield stress of the colliding vessel. In this case, the impact forces on the collided ship in the event of a collision are not smoothed out and, consequently, there is an increasing risk of fractured openings (holes) occurring due to local ruptures or damage to the collided ship. Furthermore, when a linear heating process is performed to produce a curved sheet having a large curvature, which is used for the bulbous bow, portions that are water-cooled after heating are tempered, strength is increased locally, and elongation uniform is diminished. The carbon equivalent needs to be 0.30% by mass or less to avoid the above facts. To more reliably control tensile strength, yield stress, and uniform elongation, the carbon equivalent is preferably 0.27% by mass or less. However, the carbon equivalent Ceq is an equation for which the coefficient is specified by investigating the mutual relationship between the amounts of the above chemical elements and strength and running multiple regression analyses.Ceq=[C]+[Si]/ 24+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[V])/10+([Mo]+[Nb])/5+[Ti] /20+ [B]/3+[N]/8 ... (1)

[0081] Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [V], [Ti], [B], and [N] are the added amounts of the corresponding elements (% by mass).

[0082] Therefore, the Ceq carbon equivalent in equation (1) is different from the Ceq carbon equivalent (JIS) as defined in the JIS or standardized equivalent carbons such as the Ceq carbon equivalent (IIW) as defined by the International Institute of Welding (see refer to equations (2) and (3) below). Ceq (JIS)=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo ]/4+[V]/14 . (2) Ceq (IIW)=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5 . (3)

[0083] In addition, even when optional chemical elements are included as unavoidable impurities, the carbon equivalent Ceq in equation (1) must be 0.30% by mass or less.

[0084] Furthermore, in addition to the optional chemical elements above, in the present invention, steel may be 0.0003% to 0.005% Ca, 0.0003% to 0.005% Mg, and 0.0003% to 0.005% rare earths as optional chemical elements for the purpose of improving ductility and toughness in HAZ. The addition of the above elements guarantees the ductility and tenacity of HAZ. When the amounts of Ca, Mg or rare earths are less than 0.003%, the effects of improving the ductility of the steel sheet and improving the toughness of HAZ are not easily obtained. Meanwhile, when more than 0.005% Ca, Mg, or rare earths are added, the effects are saturated. Therefore, the amounts of Ca, Mg and rare earths are 0.0003% to 0.005%.

[0085] Therefore, steel can include one or more elements among Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, B, N, Ca, Mg, and rare earths in ranges of the above grades as elements optional chemicals.

[0086] As described above, steel sheets having a chemical composition including the fundamental chemical elements, the optional chemical elements as needed, and the composite balance of Fe and the unavoidable impurities are used as steel sheets for ship hulls.

[0087] As described above, on the steel plate according to the configuration, when an accident occurs in which the bow of a colliding ship having a bulbous bow collides with the bottom of the collided ship, the sides of the bulb portion of the bulbous bow in the colliding ship (first ship) they are warped more evenly without changing the design of the hull structure so that it is possible to absorb the collision energy. In addition, the colliding face is contracted while the collision energy is absorbed, which makes it possible to greatly reduce the damage to the bottom of the collided ship (second ship).

[0088] Hereinafter, using a configuration, the reasons why the production method according to the present invention is limited will be described

[0089] Initially, cast steel having the chemical composition suitably adjusted above is prepared by a well-known common smelting method, such as a converter, and a steel (plate) is produced by a well-known casting method, such as ingot- continuous treatment.

[0090] Next, the steel is heated to a temperature of 1000°C to 1300°C in a heating oven. And the steel microstructure is transformed into a single austenite phase (single Y phase). When the heating temperature is less than 1000°C, the microstructure of the steel is not sufficiently transformed into an austenite single phase, and when the heating temperature exceeds 1300°C, the grain size of the heated y (grain size of the y resulting from heating) is extremely coarse. In such cases, since it is difficult to obtain a fine microstructure after lamination, the toughness is degraded, or once the hardening capacity increases, and a second hard phase increases, the elongation is degraded. To more reliably ensure tenacity and elongation, the heating temperature is preferably greater than 1050°C to 1250°C.

[0091] Subsequent lamination is the most important process in the present invention. That is, it is necessary to carry out the lamination having a cumulative reduction of 30% to 98% up to a thickness of the plate product (final thickness of the plate) in a temperature range of a single austenite phase (austenite phase) of the transformation point Ar3 or most.

[0092] Initially, the reason why the lamination is performed in a single austenite phase temperature range of the Ar3 or more transformation point is that when displacement is introduced into the ferrite by the lamination in a dual phase temperature range (austenite plus ferrite) of less than the Ar3 transformation point, the uniform elongation is significantly degraded, and it is difficult to obtain a uniform elongation of 15% or more. In addition, the boundaries between the ferrite introduced by displacements and the ferrite introduced by non-displacements are likely to act as a brittle fracture starting point, and toughness is degraded, and therefore it is necessary to avoid rolling in a temperature range of a double phase. Furthermore, when lamination over a dual-phase temperature range is performed, the development of a texture makes separation likely to occur, and therefore it becomes difficult to guarantee an average Charpy absorbed energy at 0°C of 100 J or more . In addition, in this case, as the anisotropy of the steel sheet increases, it becomes difficult for the bulbous bow to be warped regularly and absorb energy in the event of a collision. Due to the above, the lamination is performed in a temperature range of a single austenite phase from the transformation point Ar3 or more. Meanwhile, the upper limit of the rolling temperature is not particularly limited; however, since the heating temperature in the heating oven is 1300°C, it is possible to set the upper limit of the rolling temperature to 1300°C. However, to sufficiently ensure a temperature range for rolling at a temperature greater than the dual phase temperature range in order to sufficiently refine the recrystallized austenite grains, it is preferable to increase the temperature as much as possible. However, when the decrease in temperature from the heating oven to the start of rolling is taken into account, it is possible to set about (heating temperature - 50)°C as the upper limit of the rolling temperature.

[0093] The reason why the lamination is performed at a cumulative reduction of 30% to 98% will be described below. When the cumulative reduction is less than 30%, the refining of the austenite grain by recrystallization is insufficient, and it becomes difficult to control the grain size through subsequent cooling. To more easily control grain size, the cumulative reduction in rolling is preferably 50% or more, and more preferably 70% or more. In addition, when the rolling is performed at a cumulative reduction exceeding 98%, the grain refining effect is nearly saturated, and the rolling productivity is degraded. Therefore, the upper limit of the cumulative reduction is set to 98%. To ensure more rolling productivity, the upper limit of cumulative reduction is preferably 95%.

[0094] After rolling, it is necessary to perform cooling under any condition of the following cooling conditions (a first cooling condition, a second cooling condition, and a third cooling condition). Under the first cooling condition, quenching is performed from a cooling start temperature of 760°C or more to a temperature of 400°C to 650°C (cooling stop temperature) at an average rate of cooling over the plate thickness of 1°C/s to 50°C/s, then air-cooling is performed. In addition, under the second cooling condition, quenching is performed from a cooling start temperature of 760°C or more to a temperature less than 400°C (cooling stop temperature) at an average cooling rate on the sheet thickness of 1°C/s to 50°C/s, and then tempering is carried out at a temperature of 400°C to 650°C (tempering temperature). In addition, under the third cooling condition, cooling is performed by air cooling. Commonly, air cooling is classified into two types, which are natural cooling in calm air and forced cooling by artificial wind (air current). Here, natural cooling in calm air is employed, which also includes cooling while feeding the steel.

[0095] Regarding the first cooling condition, when the cooling start temperature is less than 760°C, since the ferrite is generated and developed by the transformation before cooling, it becomes difficult to control the grain size through of cooling. In addition, in this case, once the C is condensed into the austenite, and the hardening capacity, a second hard phase is generated, and the elongation is degraded after cooling. Therefore, the start temperature of cooling needs to be 760°C. To more easily control the grain size and ensure sufficient elongation, the start temperature of cooling is preferably 770°C or more.

[0096] In addition, in relation to the first cooling condition, the reason why the average cooling rate over the sheet thickness is set to 1°C/sec to 50°C/s during quenching will be described. When the cooling rate during quenching is less than °C/s, since cooling control is difficult, uniform cooling is not possible, and the sheet shape deteriorates, or the material quality becomes unstable . Therefore, the lower limit of the cooling rate during quenching is 1°C/s. In addition, when the cooling rate exceeds 50°C/s, since the grain size becomes less than 5 µm and the crystal grains become excessively fine, it becomes difficult to guarantee a uniform 15% elongation. or more. Therefore the upper limit of the cooling rate during quenching is 50°C/s. To more easily perform quench control and ensure greater uniform elongation, the average rate of quench during quenching over the plate thickness is preferably 5°C/s to 40°C/s.

[0097] Furthermore, in relation to the first cooling condition, the reason why the hardening is performed at a temperature of 400°C to 650°C (temper stop temperature) will be described. When the quench stop temperature exceeds 650°C, the refining effect of the grain by cooling is lost due to grain growth after cooling. Therefore, the upper limit of the quench stop temperature is 650°C. In addition, when quenching is stopped at a transition boil temperature range of less than 400°C, since uniform cooling is not possible and the quality becomes unstable, it is difficult to regularly deform the bulb. good, and the shape of the sheet is significantly deteriorated. Furthermore, in this case, it is not possible to generate perlite or bainite, and it is difficult to guarantee a uniform elongation of 15% or more. Additionally, there are cases where cementite particles are generated in the ferrite grains at a density of more than 50000 particles/mm2 and the uniform elongation is degraded. Therefore, the lower limit of the quench stop temperature is 400°C. To increase the grain refining effect and ensure greater uniform elongation, the quench stop temperature range is preferably from 450°C to 600°C;

[0098] The tempering can be performed at a temperature of less than 400°C, however, in this case, it is necessary to perform the tempering at a temperature of 400°C to 650°C after the tempering (the second condition cooling system). The reason is because tempering improves non-uniform quality, plate shape deterioration, and uniform elongation deterioration, which are caused by stopping cooling to less than 400°C. Therefore, to obtain the effect, it is necessary to adjust the tempering temperature to 400°C or more. In addition, when the tempering temperature exceeds 650°C, there are cases where the steel becomes soft from the roughening of the grain, and it becomes difficult to guarantee the yield strength and toughness. In addition, even when cementite particles are generated in the ferrite grains at a density of more than 50,000 particles/mm2, tempering can reduce the cementite particles and improve uniform elongation. Therefore, the upper limit of the tempering temperature is 650°C. To more reliably ensure uniform elongation, yield stress and toughness, the annealing temperature is preferably adjusted to 450°C to 600°C. Meanwhile, cooling after tempering is desirably air cooling.

[0099] In addition, to ensure a uniform elongation of 15% or more by controlling the grain size to 5 μm to 40 μm as described above, air cooling can be employed without water cooling (the third condition of cooling). In the case of air-cooling, uniform cooling can be carried out easily, the quality variation is small, and the shape of the sheet is also favorable. Furthermore, air-cooling is preferred as it becomes possible to sufficiently secure the ferrite, but the cooling time becomes long, and productivity is degraded. Therefore, when productivity is not a high priority, air cooling is preferably selected.

[00100] However, to satisfy both the productivity and a certain degree of hardening and to ensure the impact characteristics of the base metal and the strength of the base metal, it is preferable to perform the cooling by the first cooling method or by the second cooling method.

[00101] As described above, according to the configuration, it is possible to produce a bulbous bow steel plate that can effectively absorb the collision energy and significantly reduce the damage to the bottom of the collided ship without changing the design of the hull structure. That is, when an accident occurs in which the bow of the colliding ship having a bulbous bow collides with the bottom of a collided ship, the steel sheet as produced by the configuration makes the sides of the bulbous portion of the bulbous bow in the colliding ship (first ship) be warped more regularly so that it is possible to effectively absorb the collision energy. In addition, the steel sheet as produced according to the configuration breaks the colliding face while absorbing the collision energy so that it is possible to greatly reduce the damage to the bottom of the collided ship (second ship).

[EXAMPLE]

[00102] After the chemical composition of the molten steel was adjusted in the refining process, slabs were produced by continuous casting. The chemical elements of the plates are shown in tables 1 and 2. Here, the transformation point Ar3 was obtained from the thermal expansion curves obtained by performing an austenitization treatment at 1200°C using sampled Formastor specimens of the plates and then applying a thermal history in which the specimens were cooled to 0.5°C/sec. Steel sheets having a thickness of 8mm to 30mm were produced using the plates in tables 1 and 2. Table 3 shows the production conditions for each of the steel sheets. Meanwhile, in tempering table 3, the cooling rate was controlled using water cooling.

[00103] Table 4 shows the area percentages of the microstructures (undeformed ferrite, deformed ferrite, and the second phase), the average grain size of the ferrite phase, and the density of cementite particles in the ferrite grains in each of the plates of steel. The area percentages of the microstructures in each of the steel sheets and the average grain size of the ferrite phase were measured values obtained from a central portion of the sheet thickness not including the central segregation, and these measured values were used as representative values of the steel sheets. steel. The area percentage of the microstructure was measured by an image analysis using an optical microscope photograph at a magnification of 100 times to 500 times. At that moment, to differentiate the elongated deformed ferrite in the rolling direction and the undeformed ferrite, the dimensions of the ferrite grains in the rolling direction and their dimensions in the direction of the thickness of the plate were measured. A ferrite having the value of the length of the ferrite grains in the rolling direction divided by the length of the ferrite grains in the sheet thickness direction (aspect ratio) of 1.5 or more was defined as a deformed ferrite. A ferrite having an aspect ratio value of less than 1.5 was defined as undeformed ferrite. The mean grain size of the ferrite phase was measured using an optical microscope photograph with which the area percentages of the microstructures were measured based on the "Steels - Micrographic Determination of the Apparent Grain Size" of JIS G 0551 (2005). Density of the cementite particles in the ferrite grains was obtained by performing the photograph of 5 fields at a magnification of 20000 times in relation to the region included in the ferrite grains using a scanning electron microscope, counting the number of cementite particles, and dividing across the entire area of the photograph.

[00104] Table 5 shows the mechanical properties and the amount of energy absorption of each of the steel sheets. Tensile characteristics (yield stress, tensile strength, and uniform elongation) were measured using full thickness specimens, Charpy impact characteristics were measured using specimens sampled from the central portion of the plate thickness, and the values Measured values were used as representative values for the steel sheets. Yield stress, tensile strength, and uniform elongation were measured by tensile tests using JIS1B specimens (refer to JIS Z 2201 (1998)) based on the "Method of Tensile Test for Metallic Materials" from JIS Z 2241 (1998). In tensile tests, two tensile specimens were tested for each of the steel sheets, and the average of the measured values is shown in Table 5. The average Charpy energy absorbed at 0°C was measured by Charpy impact tests using 2mm V-notch specimens for Charpy impact testing based on JIS Z 2242 (2005) "Method for Charpy Pendulum Impact Test of Metallic Materials". In the Charpy impact tests, three specimens for the Charpy impact test were tested for each of the steel sheets at 0°C, and the average of the measured values is shown in Table 5. if the area of the stress-strain curve. However, here, the amount of energy absorption was obtained approximately using the voltage characteristics as obtained and equation (4) below, and the energy absorption capacity was evaluated.EA = (YS + TS) / 2 x uEL ... (4)

[00105] Here, EA is the amount of energy absorption (MPa), YS is the yield stress (MPa), TS is the tensile strength (MPa), and uEL is the uniform elongation (-).

[00106] Steels Nos. 1 to 20 are examples of the present invention. Since the chemical elements as shown in table 1 and the production method shown in table 3 satisfied the conditions of the present invention, the microstructures as shown in table 4 satisfied the conditions of the present invention. Therefore, in steels Nos. 1 to 20, the mechanical properties as shown in table 5 satisfied the conditions of the present invention. As a result, the energy absorption capacities of steels Nos. 1 to 20 were superior to the energy absorbing capacities of the comparative examples as described below, and the characteristics of the steel sheets of steels Nos. 1 to 20 were sufficient as steel plate for bulbous bow having a shock absorbing effect with which damage to the collided ship can be effectively avoided in the event of a collision. In addition, among the examples, in the steel sheet production methods of steels Nos. 1 to 8, 11 to 15. 17, 18 and 20, since the hardening was performed after rolling, the production time may be significantly shortened compared to the steel plate production methods of No. 9, 10, 16 and 19 steels. Meanwhile, when a steel plate is used as a steel plate for bulbous bow, the amount of absorption of energy needs to be 60 MPa or more in table 5.

[00107] In contrast to the above, steels Nos. 21 to 37 are the comparative examples of the steel sheet, Among them, for steels numbers 21 to 27, the chemical elements satisfied the conditions of the present invention as shown in table 2 , but the production method in table 3 did not satisfy the conditions of the present invention, and therefore the microstructure does not satisfy the conditions of the present invention. In addition, for steels Nos. 28 to 32, the production methods satisfied the conditions of the present invention, but the chemical elements did not satisfy the conditions of the present invention. In addition, steels Nos. 33 to 37 did not satisfy the conditions of the present invention both in chemical elements and in production methods.

[00108] Hereinafter, the reasons why the comparative examples of the steel sheets were inferior to the steel sheet of the present invention will be described.

[00109] For steel number 21, the rolling was performed at a temperature lower than the Ar3 transformation point in the production method. In summary, the rolling in a dual-phase temperature range was performed on steel number 21. Consequently, the start temperature of the cooling was less than 760°C. Therefore, the percentage of undeformed ferrite area was less than 85%, and the uniform elongation was less than 15%. Therefore, the energy absorption amount of steel No. 21 was deteriorated more than the energy absorption amounts of steel No. 1 to 20. In addition, the average Charpy absorbed energy was less than 100 J once separation occurred. . Therefore, when steel plate of No. 21 steel is applied to the bulbous bow, there is a possibility that a fragile fracture will occur in the bulbous bow in case of collision, and energy absorption by the warping is not possible. Therefore, it is difficult to use steel sheet having excellent collision energy absorption capacity.

[00110] For steel n 22, the cooling rate was greater than 50 °C/s in the production method. Therefore, the ferrite grain size was less than 5 μm, and the uniform elongation was less than 15%. In addition, the cooling stop temperature was less than 400°C, and tempering was not performed. Therefore, the density of cementite particles was greater than 50000 particles/mm2. This results from the fact that the uniform elongation was greater than 15%. From the above results, the energy absorption amounts of steel number 22 were more deteriorated than the energy absorption amounts of steels number 1 to 20.

[00111] For number 23, the start temperature of cooling was less than 760°C in the production method. Therefore, the carbon was concentrated into the austenite, and the hardening ability was greatly improved. Therefore, ferrite was not easily generated during cooling and more than 10% hard martensite rather than perlite or bainite was generated as a second phase. Therefore, the tensile strength exceeded 460 MPa, and the uniform elongation was less than 15%. In addition, the average Charpy absorbed energy was also less than 100 J. From the above, the energy absorption amount of steel number 23 was more deteriorated than the energy absorption amounts of steels number 1 to 20, and sheet steel No. 23 steel is not suitable as sheet steel which has excellent energy absorption capacity in collision.

[00112] For steel No. 24, the heating temperature exceeded 1300°C in the production method. Therefore, the heated austenite grains were coarsened, and the ferrite grain size became greater than 40 µm after cooling. As a result, the yield stress was less than 235 MPa. And the absorbed energy Charpy was less than 100 J. Although the amount of energy absorption of steel No. 24 is equivalent to the amount of energy absorption of Steels Nos. 1 to 20, the characteristics of Steel No. 24 as a structural steel were more deteriorated than the characteristics of steels Nos. 1 to 20. Therefore, it is difficult to use steel plate of steel No. 24 as a bulbous bow steel plate having excellent collision energy absorption capacity.

[00113] For steel No. 25, tempering was performed at 700°C in the production method. Therefore, the ferrite grain size became greater than 40 μm, the yield stress was less than 235 MPa, and the absorbed energy Charpy was less than 100 J. Similar to No. 24 steel, although the amount of absorption of The energy of steel No. 25 was equivalent to the energy absorbed amounts of Steel No. 1 to 20, the characteristics of Steel No. 25 as a structural steel were more deteriorated than the characteristics of Steel No. 1 to 20. Therefore, it is difficult to use No. 25 steel sheet steel as bulbous bow steel sheet having excellent collision energy absorbing capacity.

[00114] For steels no. 26 and 27, the cooling stop temperature was less than 400°C in the production method. In this case, although it is necessary to properly perform the temper, it was not performed in steel no. 26, and the tempering temperature was less than 400°C in steel No. 27. Therefore, the density of cementite particles in the ferrite grains was greater than 50000 particles/mm2. As a result, since the uniform elongation was lowered to less than 15%, the energy absorption amounts of steels Nos. 26 and 27 were more deteriorated than the energy absorption amounts of steels Nos. 1 to 20.

[00115] For steel No. 28, the amount of C exceeded 0.10% in the chemical elements. Therefore, the area percentage of undeformed ferrite became less than 85%, and, conversely, the area percentage of the second stage increased. Therefore, the tensile strength became greater than 460 MPa, and the uniform elongation was less than 15%. Therefore, the energy absorption amount of steel No. 28 was more deteriorated than the energy absorption amounts of Steel No. 1 to 20.

[00116] For steel No. 29, the amount of C was 0.03% or less in the chemical elements. In No. 29 steel, since the hardenability was extremely degraded, the size of the ferrite grains was coarsened to more than 40 μm. As a result, the yield strength was less than 235 MPa, and the absorbed energy Charpy was less than 100 J. Although the energy absorption amount of steel #29 is equivalent to the energy absorption amounts of steels #1 to 20 , steel No. 29 did not meet the required characteristics as structural steel.

[00117] For steel No. 30, the amount of Mn was more than 1.5%, the amount of Nb was more than 0.02%, the amount of V was more than 0.04%m and the carbon equivalent was more than 0.30% in the chemical elements. In addition, for steel No. 31, the amount of Ni was more than 0.5%, the amount of Mo was 0.2% or more, and the carbon equivalent was more than 0.30% in the chemical elements . Therefore, in steels 30 and 31, the area percentage of the undeformed ferrite became less than 85%, and the area percentage of the second phase increased. As a result, the tensile strength became greater than 460 MPa, and the uniform elongation was less than 15%. Therefore, the energy absorption amounts of steels Nos. 30 and 31 were more deteriorated than the energy absorption amounts of steels Nos. 1 to 20.

[00118] For steel No. 32, the amount of Si was more than 1%, and the amount of Cr was more than 0.5% in the chemical elements. As a result, the undeformed ferrite area percentage became less than 85%, and the second stage area percentage increased. Therefore, in No. 32 steel, the uniform elongation was less than 15%, and the energy absorption amount of No. 32 steel was more deteriorated than the energy absorption amounts of No. 1 to 20 steels. once an alloy was added excessively, the energy absorbed by Charpy was decreased to less than 100 J, and characteristics such as structural steel were not satisfied either.

[00119] For steel No. 33, the amount of C was more than 0.1%, the amount of Cu and the amount of Ni was more than 0.5%, and the equivalent carbon was more than 0 .30% in the chemical elements, and lamination was carried out in a dual phase temperature range in the production method. Therefore, the undeformed ferrite area percentage became less than 85%, the tensile strength became greater than 460 MPa, and the uniform elongation was less than 15%. As a result, the energy absorption amount of steel No. 33 was significantly more deteriorated than the energy absorption amounts of steel No. 1 to 20. In addition, since the deformed ferrite was increased by rolling in one range of double-phase temperature, the absorbed energy Charpy was also less than 100 J. Therefore, since there is an increasing risk of brittle fracture, the steel plate of steel No. 33 is not suitable as a steel plate for bulbous bow.

[00120] For steel No. 34, the amount of Mo was 0.2% or more, and the equivalent carbon was more than 0.30%. In addition, the heating temperature was greater than 1300°C in the production method. Since the chemical composition had a high hardening capacity, and the over-cooling was performed in a state of heated austenite grains being coarsened, the percentage of undeformed ferrite area became less than 85%, and the percentage of area of the second phase has increased. As a result, in steel No. 34, the uniform elongation became less than 15%, and the energy absorption amount of steel No. 34 was more deteriorated than the energy absorption amounts of steels No. 1 to 20.

[00121] For steel No. 35, the amount of C was 0.03% or less in the chemical elements, and the start temperature of cooling was less than 760°C in the production method. Since the chemical elements resulted in an extreme lack of hardenability, and adequate cooling was not performed, the ferrite grain size was coarsened to more than 40 µm. Consequently, the yield strength was less than 235 MPa and the absorbed energy Charpy was less than 100 J. Although the energy absorption amount of steel No. 35 is equivalent to the energy absorption amounts of steels No. 1 to 20, the No.35 steel plate does not have bulbous bow steel plate characteristics.

[00122] For steel No. 36, the amount of Ti was more than 0.04% in the chemical elements, and the tempering temperature was greater than 650°C in the production method. As a result, the ferrite grain size was blunted to more than 40 μm, and the absorbed energy Charpy was significantly decreased due to blunting by precipitation of TiC to be less than 100 J. In this case, the amount of absorption of energy calculated from strength and elongation was high, but there is a high risk of brittle fracture occurring during bulbous bow deformation in the event of a collision, and therefore sufficient energy absorption by the warp can never be anticipated.

[00123] For steel No. 37, the amount of C was more than 0.10%, the amount of Si was more than 1%, the amount of Mn was more than 1.5%, the amount of Mo was 0.2% or more, the amount of B was greater than 0.002%, and the carbon equivalent was 0.30% in the chemical elements. In addition, for steel No. 37, the cooling rate was greater than 50°C/s in the production method. Therefore, over-cooling was performed, the percentage of undeformed ferrite area became less than 85%, and the ferrite grain size became less than 5 µm. As a result, the tensile strength was well above 460 MPa, and the uniform elongation was also well below 15%. As a result, the energy absorption amount of steel #37 was more deteriorated than the energy absorption amounts of steels #1 to 20. In addition, the absorbed energy Charpy was also less than 100 J, and it is difficult to apply the steel plate of steel n 37 as steel plate for bulbous bow.

[00124] From the above examples, it was confirmed that the application of the present invention can provide a bulbous bow steel plate having an excellent collision energy absorption capacity and its production method. When steel plate is used for a bulbous bow, and an accident occurs in which the bow of a ship (colliding ship) having a bulbous bow collides with the bottom of another ship (collided ship), the side faces of the portion of Bulbous bow bulbs on the colliding ship warp more regularly without changing the design of the hull structure so that it is possible to effectively absorb the collision energy, and breaking the colliding face while absorbing makes it possible to greatly reduce damage to the bottom of the crashed ship.

[00125] Meanwhile, the present invention is not limited to the above configuration, and a variety of modifications can be made without departing from the scope of the present invention.

INDUSTRIAL APPLICABILITY

[00126] It is possible to provide a steel plate for a bow structure provided with a shock absorbing effect whereby damage to the bottom of a collided ship can be effectively avoided in the event of a collision of two ships without changing the design of the hull structure and its production method.REFERENCE SYMBOLS LIST10 COLLIDING SHIP 10a BULB HEAD 10b BULB PORTION 11 COLLIDED SHIP 11a COLLIDED SHIP BOTTOM

Claims (4)

1. Sheet steel for a ship's hull, characterized by the fact that it comprises: in % by mass, as chemical elements, C: more than 0.03% to 0.10%, P: 0.05% or less, S: 0.05% or less, Al: 0.002% to 0.1%, and a balance of Fe and the unavoidable impurities where its microstructure includes ferrite and is composed of one or more of perlite and bainite, an area percentage of ferrite undeformed in the microstructure of 85% or more, an average undeformed ferrite grain size is 5 μm to 40 μm, the density of cementite particles in the ferrite grains is 50000 particles/mm2 or less, its yield strength is 235 MPa or more, its tensile strength is 460 MPa or less, its uniform elongation is 15% or more, and the average Charpy energy absorbed at 0°C is 100 J or more. 2. Steel sheet for the hull according to claim 1, characterized in that it further comprises: In % by mass, as chemical elements, one or more between: 0.03% to 1%, Mn: 0.1 % to 1.5%, Cu: 0.02% to 0.5%, Ni: 0.02% to 0.5%, Cr: 0.02% to 0.5%, Mo: 0.002% less than 0.2%,Nb: 0.002% to 0.02%,V: 0.002% to 0.04%, Ti: 0.002% to 0.04%,B: 0.0002% to 0.002%,N: 0.0005 % to 0.008%, Ca: 0.0003% to 0.005%, Mg: 0.0003% to 0.005%, Rare Earths: 0.0003% to 0.005%, Where the carbon equivalent Ceq is 0.30% or less, where Ceq = [C] + [Si] / 24 + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [V]) / 10 + ([Mo] + [Nb]) / 5 + [Ti] / 20 + [B] / 3 + [N] / 8. Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [V] , [Mo], [Nb], [Ti], [B], and [N] are amounts of C, Si, Mn, Cu, Ni, Cr, V, Mo, Nb, Ti, B, and N in % in mass respectively. 3. Hull steel sheet according to claim 2, characterized in that the equivalent carbon Ceq is 0.27% or less in % by mass. 4. Hull steel sheet according to claim 1 or 2, characterized in that its yield ratio is 0.70 or more.

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