JPWO2011062000A1 - Thick steel plate for hull and manufacturing method thereof - Google Patents

Abstract

This thick steel plate for hulls is mass%, C: more than 0.03 to 0.10%, P: ≦ 0.05%, S: ≦ 0.05%, Al: 0.002 to 0.1%, Containing the chemical component consisting of iron and inevitable impurities, including ferrite, having a microstructure composed of one or more of pearlite and bainite, and having an area ratio of unprocessed ferrite in the microstructure 85% or more, the average crystal grain size of the unprocessed ferrite is 5 to 40 μm, the cementite particles in the ferrite grains are 50,000 particles / mm 2 or less in number density, the yield strength is 235 MPa or more, and the tensile strength is 460 MPa or less, uniform elongation is 15% or more, and Charpy average absorbed energy at 0 ° C. is 100 J or more.

Description

The present invention provides a thick steel plate having a thickness of 8 mm or more for use in a bow structure of a ship having a buffering effect that can prevent damage to a partner ship that has been deformed and collided at the time of a collision, and has improved collision safety. It relates to a manufacturing method.
This application claims priority on November 20, 2009 based on Japanese Patent Application No. 2009-265118 for which it applied to Japan, and uses the content here.

  In today's large ships, thick steel plates with a thickness of 8 mm or more are used in the bow structure of the ship, and a barbus bow (spherical bow) is provided under the bow water line to reduce any energy loss due to wave resistance. It has been.

  Conventionally, a rigid structure in which reinforcing girders are arranged vertically and horizontally has been adopted in the Barbus Bau (see Patent Documents 1 and 2). However, when a ship equipped with a rigid barbasse bow collides with another ship, for example, as shown in FIG. 1A, the barbusbau 20a of the collision ship 20 bites into the hull 21a of the ship 21 to be collided, There is a case where the hull (the hull 21a) of the ship 21 to be collided is damaged to form a fracture (hole) 21b.

  Therefore, in recent years, when the ships collide, especially when the bow of the collision ship collides with the hull of the collision ship, a structure that absorbs collision energy as much as possible so as not to damage the hull of the collision ship. Barbusbau has been proposed (see Patent Documents 3 to 5).

  Patent Document 3 discloses a barbath bow that absorbs collision energy by forming a tip part with a watertight structure and a peripheral part following the tip part with a non-watertight structure (a structure in which the inside of the barbath bow communicates with outside water). . However, since the peripheral part following the front-end | tip part is a non-watertight structure, it is difficult for the Barbusbau disclosed by patent document 3 to fully reduce wave-making resistance.

  In addition, since the Barbasse bow disclosed in Patent Document 3 has a non-watertight structure, the steel sheet must be subjected to anticorrosion, which increases manufacturing costs.

  Patent Document 4 discloses a Barbasse bow in which a thickness reducing portion for reducing lateral bending rigidity is provided on an outer plate at the base of a spherical protrusion. Further, in Patent Document 5, a low strength portion (a portion made of a low yield point steel having a lower yield point or a 0.2% proof stress of 235 MPa or less) having a low bending strength in the lateral direction is formed on the outer plate at the base of the spherical protrusion. The provided Barbus Bau is disclosed.

  In the Barbusbau disclosed in Patent Documents 4 and 5, by providing a thickness-reduced portion or a low-strength portion near the base of the Barbusbau, the base of the Barbusbau is easily bent at the time of a collision.

  In other words, in the Barbus Bau disclosed in Patent Documents 4 and 5, when the ships collide at a desired angle, for example, as shown in FIG. 1B, the Barbus Bau 30a of the collision ship 30 is easily bent at the root portion 30b. Therefore, the surface of the Barbus bow 30a that comes into contact with the hull 31a of the ship to be collided 31 is not the front end face 30c but the body surface 30d, so that damage to the hull 31a of the ship to be collided 31 is reduced.

However, unlike the Barbus bow disclosed in Patent Documents 4 and 5, the Barbus bow itself does not function as a damper that absorbs collision energy. Therefore, when ships collide with each other at an angle close to 90 °, it is assumed that Barbasse bow bites into the flank of the ship to be collided before bending at the root due to the collision reaction force.
The dotted line portion in FIG. 1B indicates the position of the Barbus bow 30a before the collision ship 30 collides with the collision ship 31.

  Eventually, in the Barbusbau disclosed in Patent Documents 4 and 5, there is a limit to the extent to which damage to the hull of the ship to be collided is reduced.

  On the other hand, as a means for improving the collision safety of a ship, the use of a steel plate excellent in energy absorption capability at the time of collision has been studied, and such a steel plate is disclosed in Patent Documents 6 to 10.

  In the steel sheet disclosed in Patent Document 6, the energy absorption capacity is improved by increasing the ferrite occupancy and hardness, reducing the size of the second phase, and increasing the work hardening.

Further, in the steel sheets disclosed in Patent Documents 7 to 9, the retained austenite is dispersed in the steel and the work hardening is increased by utilizing transformation induced plasticity (TRIP), thereby improving the strength and uniform elongation. Increases energy absorption capacity.
Moreover, in the steel sheet disclosed in Patent Document 10, the structure is a fine-grained ferrite main structure, and the strength against fracture is improved by utilizing precipitation strengthening to increase the strength of the ferrite phase.

  However, if the bow of the colliding ship collides with the hull of the collided ship, even if the hull of the collided ship is made of a steel plate with increased energy absorption capacity, the barbasse bow will There is a high risk of passing through. Moreover, the effect of using a steel plate with high energy absorption cannot be expected depending on the collision mode.

  On the other hand, if the collision energy can be absorbed by the Barbus Bau of the collision ship, it is possible to reduce damage to the hull of the ship to be collided as much as possible.

  However, since the steel plates excellent in energy absorbing ability disclosed in Patent Documents 6 to 10 have high strength and large work hardening, the compression force applied to the ship to be collided at the time of collision becomes large. For this reason, there is a high possibility that the barbasse bow will penetrate the hull before the energy of the barbasse bow is sufficiently absorbed without sufficiently progressing.

  Further, in order to produce a curved plate having a large curvature used for the Barbasse bow, bending by linear heating, that is, linear heating, is frequently used in the molding process.

  In this linear heating process, the surface of the steel sheet is locally heated linearly using a gas burner or the like, and the phenomenon that the heated portion thermally expands and is plastically deformed by being constrained from its surroundings. Usually, water cooling is performed immediately after heating in order to increase work efficiency, and the material of the steel sheet after linear heating processing changes according to the change in the microstructure of the base material.

  That is, in the steel plates excellent in energy absorbing ability disclosed in Patent Documents 6 to 10, there is a high possibility that baking is performed in the water-cooled portion after heating, the strength is locally increased, and the elongation is reduced.

  For this reason, the Barbasse bow using such a curved plate is not uniform in strength and is not easily deformed. Therefore, when a ship equipped with this barbus bow collides with another ship, as shown in FIG. 1A, the bow 20a of the collision ship 20 bites into the hull 21a of the collision ship 21, destroys the hull 21a, and further, the destruction part. There is also a great risk that this will expand and form a fracture (hole) 21b in the hull.

  In contrast to Patent Documents 6 to 10, Patent Document 11 discloses a steel sheet having improved bending workability by linear heating by reducing the yield strength at 400 ° C. In this steel plate, the yield strength at room temperature is also reduced in order to reduce the yield strength at 400 ° C. Therefore, when this steel plate is used for a Barbasse bow, deformation easily proceeds at the time of a collision, and the Barbass Bau may be able to sufficiently absorb energy. However, in the unified standard of the International Classification Society (IACS), since the yield strength at room temperature must satisfy 235 MPa or more, dispersion strengthening of fine cementite 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 a predetermined material. Further, when fine cementite is dispersed, the necessary work-hardening characteristics cannot be obtained, and there is a problem that it is difficult to obtain sufficient uniform elongation for absorbing energy.

  Patent Documents 12 and 13 disclose a steel plate for Barbusau that has improved workability and toughness after working by reducing the carbon content and lowering the yield point. However, if such an ultra-low carbon steel sheet is manufactured, the smelting load increases and the cost increases, which is not economically preferable.

  As described above, at the time of navigation, the function of greatly reducing wave resistance to reduce energy loss and further improving the propulsion performance of the ship is demonstrated. There is a need for a thick steel plate for Barbusbau that exhibits a damper function that effectively absorbs energy.

JP 2002-347690 A JP 2005-199736 A Japanese Patent Application Laid-Open No. 08-164877 JP 2004-314824 A JP 2004-314825 A Japanese Patent Laid-Open No. 11-193438 Japanese Patent Laid-Open No. 11-246934 JP 2007-162101 A JP 2008-45196 A JP 2002-105534 A JP 2009-185380 A JP-A-5-70885 JP-A-6-256891

  In view of the above-described requirements for the Barbus Bow, the present invention is an object for Barbus Bow that can effectively absorb the collision energy at the time of collision without significantly changing the design of the hull structure, and can significantly reduce the damage of the hull of the collision ship. It is an object to provide a thick steel plate and a manufacturing method thereof.

  The present invention has been made as a result of intensive studies in order to solve the above-mentioned problems, and the means thereof is as follows.

(1) Thick steel plates for hulls according to one embodiment of the present invention are in mass%, C: more than 0.03 to 0.10%, P: ≦ 0.05%, S: ≦ 0.05%, Al: 0.002 to 0.1%, with the balance having chemical components composed of iron and inevitable impurities, including ferrite, having a microstructure composed of one or more of pearlite and bainite, The area ratio of the unprocessed ferrite is 85% or more, the average crystal grain size of the unprocessed ferrite is 5 to 40 μm, and the cementite particles in the ferrite grains have a number density of 50000 / mm ² or less. The yield strength is 235 MPa or more, the tensile strength is 460 MPa or less, the uniform elongation is 15% or more, and the Charpy average absorbed energy at 0 ° C. is 100 J or more.

(2) In the thick steel plate for a hull as described in (1) above, in mass%, 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 to less than 0.2%, Nb: 0.002 to 0.02%, V: 0.0. 002-0.04%, Ti: 0.002-0.04%, B: 0.0002-0.002%, N: 0.0005-0.008%, Ca: 0.0003-0.005% , Mg: 0.0003 to 0.005%, REM: 0.0003 to 0.005% may be contained as the chemical component, and the carbon equivalent Ceq may be 0.30% or less. .
However, 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], [N] Are the contents in mass% of C, Si, Mn, Cu, Ni, Cr, V, Mo, Nb, Ti, B, and N, respectively.

(3) In the thick steel plate for a hull described in the above (2), the carbon equivalent Ceq may be 0.27% or less in mass%.
(4) In the thick steel plate for a hull described in (1) or (2) above, the yield ratio may be 0.70 or more.

(5) In the manufacturing method of the thick steel plate for ship bodies which concerns on the 1st aspect of this invention, the steel slab which has a chemical component in any one of said (1)-(3) is heated at 1000-1300 degreeC. at a cooling rate of 1 to 50 ° C. / s in thickness average from 760 ° C. or more cooling start temperature;; a _r3 rolling cumulative reduction ratio from 30 to 98% complete the transformation point or more single-phase austenite region to a product thickness After performing accelerated cooling to a temperature of 400 to 650 ° C., air cooling is performed.

(6) In the manufacturing method of the thick steel plate for ship bodies which concerns on the 2nd aspect of this invention, the steel slab which has a chemical component in any one of said (1)-(3) is heated at 1000-1300 degreeC. at a cooling rate of 1 to 50 ° C. / s in thickness average from 760 ° C. or more cooling start temperature;; a _r3 rolling cumulative reduction ratio from 30 to 98% complete the transformation point or more single-phase austenite region to a product thickness After accelerated cooling to a temperature of less than 400 ° C., tempering is performed at a temperature of 400 to 650 ° C.

(7) In the manufacturing method of the thick steel plate for ship bodies which concerns on the 3rd aspect of this invention, the steel slab which has the chemical component in any one of said (1)-(3) is heated at 1000-1300 degreeC. ; performing cooling; performs rolling cumulative rolling reduction 30 to 98% to product thickness at a _r3 transformation point or more austenite single phase region.

  According to the present invention, when an accident occurs in which the bow of a ship having a Barbus bow collides with the hull of another ship, the valve side surface of the Barbus bow in the collision ship (own ship) is more uniformly buckled and deformed. Thus, the collision energy can be greatly absorbed. In addition, by colliding the collision surface while absorbing the collision energy, it is possible to reduce the damage of the ship to be collided (other ships) as much as possible, thereby contributing to the prevention of marine pollution due to the sinking of the ship and the oil spill. be able to.

It is a schematic diagram which shows each deformation | transformation of a collision ship and a ship to be collided when the ship which does not have a buffering effect collides with the bow structure. It is a schematic diagram which shows each deformation | transformation of a collision ship and a ship to be collided when the ship which has a buffer effect collides with the bow structure. It is a schematic diagram which shows the deformation | transformation of each of a collision ship and a ship to be collided when the ship which has the buffer effect using the thick steel plate for ship bodies of this invention collides with the bow structure.

  Hereinafter, an embodiment of the present invention will be described.

When an accident occurs such that the bow of a ship having a barbus bow collides with the hull of another ship, as shown in FIG. Since much collision energy can be absorbed, the impact force caused by the collision is reduced. As a result, local breakage or breakage of the hull 11a of the ship 11 to be collided can be avoided, and the occurrence of breakage (holes) can be prevented.
1C indicates the position of the Barbus bow 10a before the collision ship 10 collides with the collision ship 11.

  As a result of examining the deformation behavior of such a barbasse bow by FEM analysis, when the yield strength is 235 MPa or more, the tensile strength is 460 MPa or less, and the uniform elongation is 15% or more, the ship to be collided at the time of collision between ships. It has been found that the compression force applied to is reduced. Also, in this case, the Barbus bow is easily deformed, the contact area between the Barbus bow and the ship's hull is increased, and the Barbus bow effectively absorbs the collision energy, causing significant damage to the hull of the ship being hit. It has been found that it can be reduced.

  The reasons for limiting the yield strength, tensile strength, uniform elongation, and Charpy average absorbed energy at 0 ° C. in this embodiment will be described below.

  In current large ships, thick steel plates with a thickness of 8 mm or more and a maximum of 100 mm are used for the bow structure, and in order to buckle and deform the bow of the collision ship, it is basically used for the bow of the collision ship. The yield strength of the steel plate to be used needs to be smaller than the yield strength of the steel plate used for the ship to be struck. However, the yield strength of steel plates used in ships must meet the unified standard of the International Classification Society (IACS), and the bow structure with a conventional inner bone structure should be able to withstand wave impact. is necessary. Furthermore, if the yield strength is reduced too much, a large energy absorption effect during deformation cannot be expected. Considering the above points, the yield strength of the steel sheet needs to be 235 MPa or more. In addition, the upper limit of the yield strength is preferably 400 MPa or less in view of the yield strength of steel plates used in ships. In order for the bow to buckle and deform more reliably at the time of a collision, the yield strength of the steel sheet is more preferably 320 MPa or less.

  The tensile strength needs to be increased in order to increase the energy absorption capacity. However, if the tensile strength becomes too high, the compressive force applied to the collision ship at the time of collision increases. In this case, there is a high possibility that the barbasse bow will penetrate the hull before the energy is sufficiently absorbed without the deformation of the barbasse bow sufficiently. Therefore, the upper limit of tensile strength is 460 MPa. In order to obtain a required energy absorption capacity, the lower limit of the tensile strength is desirably 300 MPa.

  Similar to the tensile strength, the uniform elongation needs to be increased in order to increase the energy absorption capacity, and the lower limit of the uniform elongation is 15%. In order to further improve the energy absorption capacity, the uniform elongation is desirably 20% or more. Further, the higher the uniform elongation, the better. However, in order to ensure the required strength, it is desirable that the uniform elongation is 50% or less.

  Further, the Charpy average absorbed energy at 0 ° C. needs to be 100 J or more in order to prevent brittle fracture at the time of collision. That is, if a brittle fracture occurs at the time of a collision, energy cannot be absorbed by buckling deformation, and further, the risk that a crack will propagate to other members and a serious fracture accident will occur. Therefore, the lower limit of the Charpy average absorbed energy is 100J. In order to prevent brittle fracture and enhance safety, the Charpy average absorbed energy is preferably 150 J or more. In consideration of a level at which the risk of brittle fracture can be substantially avoided, the upper limit of Charpy absorbed energy is preferably 500 J.

  In addition, the yield ratio is preferably 0.70 or more. When the yield ratio is lowered, work hardening increases, so that the energy absorbing ability as a steel plate increases. However, as described above, when the work hardening increases, the compression force applied to the ship to be collided by the Barbus Bau of the collision ship at the time of the collision increases. For this reason, there is a possibility that the barbasse bow may penetrate the hull before the barbusbau is sufficiently absorbed without sufficiently deforming. Therefore, in order to sufficiently secure the energy absorption capability as the Barbasse bow, it is preferable to limit the yield ratio depending on the strength of the steel sheet. In order to ensure the deformation of the Barbasu bow more reliably, the yield ratio is more preferably 0.75 or more, and most preferably 0.80 or more. Moreover, in order to ensure the energy absorption ability and the workability as a steel plate for Barbus bow, the yield ratio is preferably 0.95 or less.

  The reason for limiting the microstructure in this embodiment will be described below.

  The microstructure of the steel sheet includes ferrite as a matrix, and is composed of at least one of pearlite and bainite in addition to this ferrite.

  First, the reason why unprocessed ferrite is the parent phase is that it uses the softest structure in the steel sheet structure to suppress the strength while improving the uniform elongation, and to absorb the energy absorbed by buckling deformation of the barbasse bow. This is to increase it. In addition, ferrite processed by two-phase rolling or the like causes a reduction in uniform elongation and Charpy average absorbed energy. Therefore, in order to avoid such a decrease, the parent phase ferrite is an unprocessed ferrite. In addition, since the anisotropy of the steel sheet increases due to the formation of processed ferrite, if the ferrite of the parent phase is processed ferrite, it will be difficult to cause the steel sheet to buckle and deform uniformly and to absorb energy into the steel sheet at the time of collision. . If the processed ferrite is 1% or less, it is determined that there is no processed ferrite in the microstructure.

  The reason for making pearlite and bainite other than unprocessed ferrite is to ensure strength only in the unprocessed ferrite phase, the work hardening characteristics are extremely lowered, and the uniform elongation cannot be made 15% or more. This is because it is difficult to ensure the energy absorption capability at the time of collision. In addition, when martensite is present in the structure, it is difficult to make the tensile strength 460 MPa or less and the Charpy average absorbed energy at 0 ° C. to be 100 J or more. Therefore, it is necessary that martensite is not confirmed by observation of the structure with an optical microscope. It is.

  When the area ratio of the unprocessed ferrite is less than 85%, the hard structure other than unprocessed ferrite, such as processed ferrite, pearlite, bainite, and martensite, exceeds 15%, the tensile strength is 460 MPa or less, and the uniform elongation is 15% or more. It becomes difficult to make. Therefore, the area ratio of unprocessed ferrite is set to 85% or more. The area ratio of unprocessed ferrite is preferably 90 to 95%.

If the unprocessed ferrite grain size is less than 5 μm, it is difficult to ensure a uniform elongation of 15% or more, and if the unprocessed ferrite grain size exceeds 40 μm, the yield strength is 235 MPa or more and 0 ° C. is 100 J or more. It is difficult to ensure the Charpy average absorbed energy. Therefore, the unprocessed ferrite particle diameter is 5 to 40 μm.
Furthermore, if the cementite particles have a number density of more than 50000 / mm ² in the ferrite grains, voids are likely to be generated, so that the uniform elongation is lowered and it is difficult to ensure a uniform elongation of 15% or more. Become. Therefore, the number density of the cementite particles in the ferrite grains is limited to 50000 particles / mm ² or less.

  Hereinafter, the reason for limiting the amount of each element will be described. The “%” below is “mass%” unless otherwise specified.

C: Over 0.03 to 0.10%
C is an element that increases the strength of steel, and in order to secure a yield strength at room temperature of 235 MPa or more and reduce a smelting load, C of more than 0.03% is required. However, if the amount of C exceeds 0.10%, the area ratio of the second phase such as pearlite increases, and it is difficult to make the tensile strength 460 MPa or less and the uniform elongation 15% or more. Therefore, the upper limit of the C amount is 0.10%. In order to more surely control the yield strength, tensile strength, and uniform elongation, the C content is preferably 0.04 to 0.08%.

P: ≦ 0.05%, S: ≦ 0.05%
P is an impurity element, and it is necessary to reduce P as much as possible in order to increase the yield strength at high temperature by solid solution strengthening and deteriorate toughness. However, when the P amount is 0.05% or less, those adverse effects can be tolerated, so the upper limit of the P amount is 0.05%. Since S is also an impurity element and deteriorates the toughness and ductility of steel, it is desirable to reduce it as much as possible. However, if the S amount is 0.05% or less, these adverse effects are acceptable, so the upper limit of the S amount is 0.05%.

Al: 0.002 to 0.1%
Al is an important element in the present invention, and is added mainly for the purpose of deoxidation. In order to perform sufficient deoxidation, Al needs to be 0.002% or more. However, if the Al content exceeds 0.1%, alumina-based coarse oxides and clusters thereof are generated and the toughness is impaired, so the upper limit of the Al content is 0.1%. In order to perform deoxidation more reliably and to secure toughness more, the Al content is preferably 0.01 to 0.07%.

  The above components are basic components of the steel sheet of the present invention. A steel plate containing at least these basic components is a thick steel plate for Barbusbau that is excellent in the collision energy absorption capability of the present invention (target thickness is 8 mm or more. Note that the upper limit of the plate thickness is not particularly limited. , About 100 mm is realistic). Furthermore, Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, B, and N can be added as selective elements to the steel for the purpose of adjusting strength and toughness. These selective elements increase the hardenability of the steel even when added in a small amount, and therefore contribute to the improvement of strength such as solid solution strengthening and precipitation strengthening in addition to the improvement of strength and toughness by crystal grain refinement. In order to obtain the effect, the lower limit of the Si amount is 0.03%, the lower limit of the Mn amount is 0.1%, the lower limits of the Cu amount, Ni amount, and Cr amount are 0.02%, Mo amount, and Nb amount, respectively. The lower limit of the V amount and the Ti amount is 0.002%, the lower limit of the B amount is 0.0002%, and the lower limit of the N amount is 0.0005%. However, if any of the selective elements is added excessively, a low-temperature transformation structure such as bainite is likely to be generated, and it is difficult to increase the area ratio of ferrite to 90% or more. In this case, since the strength increases and the uniform elongation decreases, it is difficult to make the tensile strength 460 MPa or less and the uniform elongation 15% or more. Therefore, it is necessary to set an upper limit for the amount of each selected element. About this upper limit, Si amount is 1%, Mn amount is 1.5%, Cu amount, Ni amount, Cr amount is 0.5%, Mo amount is 0.2% (not including 0.2%), The Nb amount is 0.02%, the V amount and the Ti amount are 0.04%, the B amount is 0.002%, and the N amount is 0.008%. Regarding this upper limit, preferably, the Si amount is 0.8% or less, the Mn amount is 1.2% or less, the Cu amount, the Ni amount, and the Cr amount are each 0.3% or less, and the Mo amount is 0.05%. Hereinafter, the Nb content is 0.01% or less, the V content and the Ti content are 0.02% or less, the B content is 0.001% or less, and the N content is 0.006% or less.

In addition, when adding Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, and B, the carbon equivalent Ceq shown in the following formula (1) needs to be 0.30% by mass or less. When the above elements are added excessively and the carbon equivalent Ceq exceeds 0.30% by mass, a low-temperature transformation structure such as bainite is likely to be formed as described above, and the area ratio of ferrite should be 85% or more. Have difficulty. In this case, since the strength increases and the uniform elongation decreases, it is difficult to make the tensile strength 460 MPa or less and the uniform elongation 15% or more. In addition, since the yield strength increases, the yield strength of the collision ship may exceed the yield strength of the collision ship. In that case, the impact force on the ship to be collided is not relieved at the time of collision, and as a result, there is an increased risk that a broken hole (hole) is generated due to local breakage or damage of the ship to be collided. Further, when a linear heating process is performed to produce a curved plate having a large curvature used for the Barbasse bow, the portion that is water-cooled after heating is baked, the strength is locally increased, and the uniform elongation is decreased. In order to prevent these, the carbon equivalent Ceq needs to be 0.30 mass% or less. In order to control the tensile strength, yield strength, and uniform elongation more reliably, the preferred carbon equivalent Ceq is 0.27% by mass or less. The carbon equivalent Ceq is an equation in which a coefficient is determined by investigating the correlation between the amount and strength of the element and performing multiple regression analysis.
Ceq = [C] + [Si] / 24 + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [V]) / 10 + ([Mo] + [Nb]) / 5+ [ Ti] / 20 + [B] / 3 + [N] / 8 (1)
Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [V], [Ti], [B], [N] Is the amount of each element added (% by mass).
Therefore, the carbon equivalent Ceq of the formula (1) is different from standardized carbon equivalents such as the carbon equivalent Ceq (JIS) defined by JIS and the carbon equivalent Ceq (IIW) defined by the International Welding Society. (See formulas (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)
Further, even when the selective element is included as an inevitable impurity, the carbon equivalent Ceq of the formula (1) needs to be 0.30% by mass or less.

Further, in addition to the above-mentioned selective elements, in the present invention, 0.0003 to 0.005% Ca, 0.0003 to 0.005% Ca is contained in the steel for the purpose of improving the ductility and HAZ toughness of the steel sheet. Mg, 0.0003 to 0.005% REM may be contained as a selective element. By adding these, ductility and HAZ toughness are ensured. If the amount of each of Ca, Mg, and REM is less than 0.003%, it is difficult to obtain the effect of improving the ductility and improving the HAZ toughness of the steel sheet. On the other hand, when each of Ca, Mg, and REM is added exceeding 0.005%, these effects are saturated. Therefore, the amounts of Ca, Mg, and REM are 0.0003 to 0.005%, respectively.
Accordingly, in the steel, one or more of Si, Mn, Cu, Ni, Cr, Mo, Nb, V, Ti, B, N, Ca, Mg, and REM are included as selective elements within the above-described content range. May be.
As described above, a steel plate having a chemical composition containing the basic component and, if necessary, the above-described selective element and having the balance composed of iron and unavoidable impurities is used as a steel plate for a hull.
As described above, in the steel plate according to the present embodiment, when an accident occurs such that the bow of the own ship having a barbus bow collides with the hull of another ship without changing the design of the hull structure, Collision energy can be effectively absorbed by the valve part side surface of the barbus bow in the ship) being more uniformly buckled and deformed. Further, the collision surface is crushed while absorbing the collision energy, so that damage to the hull of the ship to be collided (other ship) can be remarkably reduced.

  Hereinafter, the reason for limiting the production method of the present invention will be described using an embodiment.

  First, the molten steel adjusted to the appropriate chemical composition is melted by a generally known melting method such as a converter, and a steel material (steel piece) is produced by a generally known casting method such as continuous casting.

  Next, this steel material is heated to a temperature of 1000 ° C. to 1300 ° C. in a heating furnace, and the structure of the steel material is changed to an austenite single phase (γ single phase). When the heating temperature is less than 1000 ° C, the structure of the steel material does not sufficiently transform into an austenite single phase, and when the heating temperature exceeds 1300 ° C, the heated γ particle size (γ particle size resulting from this heating) becomes extremely coarse. To do. In these cases, since it is difficult to obtain a fine structure after rolling, the toughness is reduced, and the hardenability is increased and the hard second phase is increased, so that the elongation is reduced. In order to ensure toughness and elongation more reliably, a preferable heating temperature is more than 1050 ° C. to 1250 ° C.

Subsequent rolling is the most important step of the present invention. That is, it is necessary to carry out the product thickness (final thickness) cumulative rolling reduction 30 to 98% of rolling up at A _r3 transformation point or more austenite single phase region.

First, the reason for rolling in the austenite single phase region above the _{Ar 3} transformation point is that when the dislocation is introduced into the ferrite by two-phase rolling below the _{Ar 3} transformation point, the uniform elongation is remarkably lowered and the uniform elongation is 15%. It is because it is difficult to make it more than%. In addition, it is necessary to avoid two-phase rolling because the interface between the ferrite in which dislocations are introduced and the ferrite in which dislocations are not introduced tends to be the starting point of brittle fracture and lower the toughness. Further, when the two-phase rolling is performed, separation is likely to occur due to the development of the texture, and it becomes difficult to secure Charpy average absorbed energy at 0 ° C. of 100 J or more. Further, in this case, since the anisotropy of the steel sheet is increased, it is difficult to cause the barbasse bow to be uniformly buckled and deformed at the time of collision so that the barbasse bow can absorb energy. From the above, rolling is performed in the austenite single phase region above the _Ar3 transformation point. In addition, although the upper limit of rolling temperature is not specifically limited, Since the heating temperature in the said heating furnace is 1300 degreeC, it can set to 1300 degreeC. However, in order to sufficiently secure a rolling temperature range at a temperature higher than the two-phase region and efficiently refine the austenite recrystallized grains, it is preferable that the temperature is as high as possible. However, considering the temperature drop from the heating furnace to the start of rolling, about (heating temperature−50) ° C. can be set as the upper limit of the rolling temperature.

  Next, the reason why rolling is performed at a cumulative rolling reduction of 30 to 98% will be described. If the cumulative rolling reduction is less than 30%, austenite is not sufficiently refined by recrystallization, and control of the crystal grain size by subsequent cooling becomes difficult. In order to more easily control the crystal grain size, the cumulative rolling reduction of rolling is preferably 50% or more, and more preferably 70% or more. Further, when rolling is performed at a cumulative reduction ratio exceeding 98%, the effect of crystal grain refinement is almost saturated, resulting in a reduction in rolling productivity. Therefore, the upper limit of the cumulative rolling reduction is set to 98%. In order to further secure the rolling productivity, the upper limit of the cumulative rolling reduction is preferably 95%.

  After the rolling, it is necessary to perform cooling under any of the following cooling conditions (first cooling condition, second cooling condition, and third cooling condition). In the first cooling condition, air cooling is performed after accelerated cooling from a cooling start temperature of 760 ° C. or higher to a temperature of 400 to 650 ° C. (cooling stop temperature) at a cooling rate of 1 to 50 ° C./s on the average thickness. I do. In the second cooling condition, after performing accelerated cooling from a cooling start temperature of 760 ° C. or higher to a temperature of less than 400 ° C. (cooling stop temperature) at a cooling rate of 1 to 50 ° C./s on the average thickness. Tempering is performed at a temperature of 400 to 650 ° C. (tempering temperature). In the third cooling condition, cooling by air cooling is performed. In general, there are two types of air cooling: natural cooling in still air and forced cooling by artificially generated wind (air). Here, natural cooling in a static atmosphere including that performed while passing through the plate was used.

  With respect to the first cooling condition, if the cooling start temperature is less than 760 ° C., ferrite is generated by the transformation before the cooling and grows, making it difficult to control the crystal grain size by cooling. In this case, since C is concentrated in the austenite and the hardenability is increased, a hard second phase is generated after cooling and elongation is reduced. Therefore, the cooling start temperature needs to be 760 ° C. In order to ensure sufficient elongation while controlling the crystal grain size more easily, the cooling start temperature is preferably 770 ° C. or higher.

  Moreover, the reason for having made the cooling rate at the time of accelerated cooling into 1-50 degrees C / s on the board thickness average about 1st cooling conditions is demonstrated. If the cooling rate during accelerated cooling is less than 1 ° C./s, it is difficult to control the cooling, so uniform cooling cannot be performed, and the plate shape deteriorates or the material varies. Therefore, the lower limit of the cooling rate during accelerated cooling is 1 ° C./s. On the other hand, if the cooling rate during accelerated cooling exceeds 50 ° C./s, the crystal grain size becomes less than 5 μm and becomes too fine, so that it is difficult to ensure a uniform elongation of 15% or more. Therefore, the upper limit of the cooling rate during accelerated cooling is 50 ° C./s. In order to more easily control the cooling and ensure a higher uniform elongation, the cooling rate during accelerated cooling is preferably 5 to 40 ° C./s on average in the plate thickness.

Furthermore, the reason for accelerated cooling to a temperature of 400 to 650 ° C. (accelerated cooling stop temperature) will be described for the first cooling condition. When the accelerated cooling stop temperature exceeds 650 ° C., the crystal grain refinement effect due to cooling disappears due to the crystal grain growth after the cooling stop. Therefore, the upper limit of the accelerated cooling stop temperature is 650 ° C. In addition, if accelerated cooling is stopped in a transition boiling region below 400 ° C., uniform cooling cannot be performed and the materials vary, so that it is difficult to uniformly deform the barbasse bow, and at the same time, the plate shape is remarkably reduced. Further, in this case, pearlite or bainite cannot be generated, and it is difficult to ensure a uniform elongation of 15% or more. In addition, cementite particles may be generated in the ferrite grains in a number density exceeding 50000 particles / mm ² , and the uniform elongation may be reduced. Therefore, the lower limit of the accelerated cooling stop temperature is 400 ° C. In order to enhance the effect of crystal grain refinement and ensure higher uniform elongation, the range of the accelerated cooling stop temperature is preferably 450 to 600 ° C.

Although the said accelerated cooling may be performed to the temperature below 400 degreeC, in this case, it is necessary to perform tempering at the temperature of 400-650 degreeC after accelerated cooling (2nd cooling conditions). The reason for this is to recover the material variation, plate shape deterioration, and uniform elongation deterioration caused by stopping the cooling at less than 400 ° C. by tempering. Therefore, in order to obtain the effect, the tempering temperature needs to be 400 ° C. or higher. Further, at a tempering temperature exceeding 650 ° C., softening associated with the coarsening of crystal grains proceeds, and it may be difficult to ensure yield strength and toughness. Further, even when cementite particles are produced in the ferrite grains with a number density exceeding 50000 / mm ² , the tempering can reduce the cementite particles and improve the uniform elongation. Therefore, the upper limit of the tempering temperature is 650 ° C. In order to ensure uniform elongation, yield strength, and toughness more reliably, the tempering temperature is preferably set to 450 to 600 ° C. The cooling after tempering is preferably air cooling.

Further, as described above, in order to control the crystal grain size to 5 to 40 μm and ensure a uniform elongation of 15% or more, air cooling may be performed without water cooling (third cooling condition). In the case of this air cooling, uniform cooling can be easily performed, the material variation is small, and the plate shape is also good. Furthermore, air cooling is preferable in that a sufficient ferrite structure can be secured, but the cooling time becomes longer and productivity is lowered. For this reason, when there is a sufficient margin in productivity, it is preferable to select air cooling.
However, in order to ensure the impact characteristics of the base material and the strength of the base material while achieving both productivity and a certain degree of hardenability, it is preferable to perform cooling by the first cooling method or the second cooling method.

  As described above, according to the present embodiment, a thick steel plate for barbusbau that can effectively absorb collision energy and significantly reduce damage to the hull of the ship to be collided without changing the design of the hull structure is manufactured. be able to. That is, the steel plate manufactured according to the present embodiment has a valve portion side surface of the collision bus (own ship) in the case of an accident in which the bow of the own ship having the Barbus bow collides with the hull of another ship. By more uniformly buckling and deforming, collision energy can be effectively absorbed. Moreover, the thick steel plate manufactured by this embodiment can reduce the damage of the hull of a ship to be collided (another ship) remarkably by colliding a collision surface, absorbing a collision energy.

After adjusting the chemical composition of the molten steel in the steel making process, a slab was produced by continuous casting. The chemical components of these slabs are shown in Tables 1 and 2. Here, the _Ar3 transformation point is obtained by giving a thermal history of cooling at 0.5 ° C./s after performing an austenitizing treatment at 1200 ° C. using a formaster test piece taken from these slabs. Obtained from the obtained thermal expansion curve. A thick steel plate (steel plate) having a thickness of 8 to 30 mm was manufactured using the cast pieces of Table 1 and Table 2. Table 3 shows the manufacturing conditions for each thick steel plate. In accelerated cooling in Table 3, the cooling rate was controlled by water cooling.

  Table 4 shows the area ratio of the microstructure (non-processed ferrite, processed ferrite, second phase) of each steel sheet, the average crystal grain size of the ferrite phase, and the number density of cementite particles in the ferrite grains. The area ratio of the microstructure of each steel plate and the average crystal grain size of the ferrite phase are measured values obtained from the plate thickness center position not including central segregation, and these measured values were used as representative values of each steel plate. The area ratio of the microstructure was measured by image analysis using an optical micrograph of 100 times or 500 times. At this time, in order to distinguish between the processed ferrite stretched in the rolling direction and the unprocessed ferrite, the dimensions of the ferrite grains in the rolling direction and the dimensions in the plate thickness direction were measured. Ferrite grains whose length (aspect ratio) divided by the length of the ferrite grains in the rolling direction is 1.5 or more are processed ferrite, and ferrite whose aspect ratio is less than 1.5 are unprocessed ferrite Defined. The average crystal grain size of the ferrite phase was measured in accordance with JIS G 0551 (2005) “Microscopic test method for steel grain size” using an optical microscope photograph in which the area ratio of the microstructure was measured. The number density of the cementite particles in the ferrite grains was measured using a scanning electron microscope to photograph 5 fields of view at a magnification of 20000 with respect to the region contained in the ferrite grains, counting the number of cementite particles, It was determined by dividing by the photo area.

Table 5 shows the mechanical properties and energy absorption of each thick steel plate. Tensile properties (yield strength, tensile strength, uniform elongation) are measured using full-thickness test pieces, and Charpy impact properties are measured using test pieces taken from the center of the plate thickness. It was set as the representative value of the steel plate. Yield strength, tensile strength, and uniform elongation were measured by a tensile test in accordance with JIS Z 2241 (1998) “Metal material tensile test method” using a JIS No. 1B tensile test piece (see JIS Z 2201 (1998)). It was done. In this tensile test, each of the two tensile test pieces was tested, and the average of the measured values is shown in Table 5. The Charpy average absorbed energy at 0 ° C. was measured by a Charpy impact test in accordance with “Charpy impact test method of metal material” of JIS Z 2242 (2005) using a 2 mmV notch Charpy impact test piece. In this Charpy impact test, each of the three Charpy impact test pieces was tested at 0 ° C., and the average of the measured values is shown in Table 5. The amount of energy absorption is generally determined by the area of the stress-strain curve. However, here, using the tensile properties obtained above, the amount of energy absorption was approximately obtained by the following equation (4), and the energy absorption capacity was evaluated.
EA = (YS + TS) / 2 × uEL (4)
Here, EA is energy absorption (MPa), YS is yield strength (MPa), TS is tensile strength (MPa), and uEL is uniform elongation (-).

  Steel numbers 1 to 20 are examples of the thick steel plate of the present invention. Since the chemical components shown in Table 1 and the production method shown in Table 3 satisfied the conditions of the present invention, the microstructure shown in Table 4 satisfied the conditions of the present invention. Therefore, in the steel numbers 1-20, the mechanical properties shown in Table 5 satisfied the conditions of the present invention. As a result, the energy absorption capacity of steel Nos. 1-20 is superior to the energy absorption capacity of the comparative examples described later. It was sufficient as a thick steel plate for Barbusbau with a buffering effect that could be effectively prevented. Further, among these examples, in the method for producing thick steel plates having steel numbers 1 to 8, 11 to 15, 17, 18, and 20, since accelerated cooling was performed after rolling, steel numbers 9, 10, 16, and 19 were performed. Compared with the manufacturing method of the thick steel plate, the manufacturing time could be greatly shortened. In addition, when using a steel plate as a thick steel plate for Barbus Bau, in Table 5, the energy absorption amount needs to be 60 Mpa or more.

  On the other hand, steel numbers 21 to 37 are comparative examples of thick steel plates. Among these, as for steel numbers 21-27, as shown in Table 2, the chemical composition satisfies the conditions of the present invention, but the manufacturing method of Table 3 does not satisfy the conditions of the present invention. The conditions of the present invention were not satisfied. Steel Nos. 28 to 32, although the production method satisfied the conditions of the present invention, the chemical components did not satisfy the conditions of the present invention. Steel numbers 33 to 37 did not satisfy the conditions of the present invention in terms of chemical composition and production method.

  The reason why the comparative example of the thick steel plate is inferior to the thick steel plate of the present invention will be described below.

In steel No. 21, rolling was performed to a temperature _lower than the _Ar3 transformation point in the production method. That is, in this steel No. 21, two-phase rolling was performed. Accordingly, the cooling start temperature was less than 760 ° C. Therefore, the area ratio of the unprocessed ferrite was less than 85%, and the uniform elongation was less than 15%. Therefore, the energy absorption amount of steel number 21 was inferior to the energy absorption amounts of steel numbers 1-20. The Charpy average absorbed energy was less than 100 J due to separation. Therefore, when the steel plate of steel No. 21 is applied to the Barbasse bow, brittle fracture occurs in the Barbasse bow at the time of collision, and there is a possibility that energy cannot be absorbed by buckling deformation. Therefore, it is difficult to use the steel plate having the steel number 21 as a steel plate having excellent collision energy absorption capability.

In steel No. 22, the cooling rate was higher than 50 ° C./s in the production method. Therefore, the ferrite crystal grain size was less than 5 μm, and the uniform elongation was less than 15%. Moreover, the cooling stop temperature was less than 400 ° C., and tempering was not performed. Therefore, the number density of cementite particles was larger than 50000 / mm ² . This is also the reason why the uniform elongation was less than 15%. From the above results, the energy absorption amount of steel number 22 was inferior to the energy absorption amounts of steel numbers 1-20.

  In Steel No. 23, the cooling start temperature was less than 760 ° C. in the production method. Therefore, carbon was concentrated in austenite and the hardenability was extremely improved. For this reason, ferrite is not easily generated during cooling, and hard martensite is generated in the second phase in excess of 10% instead of pearlite or bainite. Therefore, the tensile strength exceeded 460 MPa and the uniform elongation was less than 15%. Moreover, Charpy average absorbed energy was also less than 100J. As mentioned above, the energy absorption amount of the steel number 23 is inferior to the energy absorption amount of the steel numbers 1-20, and the steel plate of the steel number 23 is unsuitable as a steel plate excellent in the collision energy absorption ability.

  In the steel No. 24, the heating temperature exceeded 1300 degreeC in the manufacturing method. Therefore, the heated austenite grains are coarsened, and the crystal grain size of the ferrite after cooling is larger than 40 μm. As a result, the yield strength was less than 235 MPa, and the Charpy absorbed energy was less than 100 J. The energy absorption amount of steel number 24 is equivalent to the energy absorption amount of steel numbers 1 to 20, but the properties of steel number 24 as structural steel were inferior to those of steel numbers 1 to 20. Therefore, it is difficult to use the steel plate having the steel number 24 as a steel plate having excellent collision energy absorbing ability for Barbus Bau.

Steel No. 25 was tempered at 700 ° C. in the production method. Therefore, the crystal grain size of ferrite was larger than 40 μm, the yield strength was less than 235 MPa, and the Charpy absorbed energy was less than 100 J. Similar to Steel No. 24, the energy absorption amount of Steel No. 25 is equivalent to the energy absorption amount of Steel Nos. 1 to 20, but the properties of Steel No. 25 as a structural steel are those of Steel Nos. 1 to 20. Was inferior. Therefore, it is difficult to use the steel plate of steel No. 25 as a steel plate excellent in the collision energy absorbing ability for Barbus Bau.
In steel numbers 26 and 27, the cooling stop temperature was less than 400 ° C. in the manufacturing method. In this case, it is necessary to appropriately perform tempering. However, in steel number 26, tempering was not performed, and in steel number 27, the tempering temperature was less than 400 ° C. For this reason, the number density of cementite particles in the ferrite grains was larger than 50000 / mm ² . As a result, since the uniform elongation was reduced to less than 15%, the energy absorption amounts of steel numbers 26 and 27 were inferior to the energy absorption amounts of steel numbers 1 to 20.

  In Steel No. 28, the C content exceeded 0.10% in the chemical composition. Therefore, the area ratio of unprocessed ferrite was less than 85%, and conversely the area ratio of the second phase increased. Therefore, the tensile strength was greater than 460 MPa and the uniform elongation was less than 15%. For this reason, the energy absorption amount of the steel number 28 was inferior to the energy absorption amount of the steel numbers 1-20.

  In Steel No. 29, the C content was 0.03% or less in chemical composition. In this steel No. 29, since the hardenability was extremely lowered, the crystal grain size of ferrite was coarsened to over 40 μm. As a result, the yield strength was less than 235 MPa, and the Charpy absorbed energy was less than 100 J. Although the energy absorption amount of the steel number 29 is equivalent to the energy absorption amount of the steel numbers 1 to 20, the steel number 29 does not satisfy the characteristics required for structural steel.

  In Steel No. 30, the chemical composition was such that the Mn amount was more than 1.5%, the Nb amount was more than 0.02%, the V amount was more than 0.04%, and the carbon equivalent was more than 0.30%. Steel No. 31 had a Ni content of over 0.5%, a Mo content of 0.2% or more, and a carbon equivalent of over 0.30% in chemical composition. Therefore, in these steel numbers 30 and 31, the area ratio of unprocessed ferrite became less than 85%, and the area ratio of the second phase increased. As a result, the tensile strength was greater than 460 MPa and the uniform elongation was less than 15%. Therefore, the energy absorption amount of the steel numbers 30 and 31 was inferior to the energy absorption amount of the steel numbers 1-20.

  In Steel No. 32, the chemical content was such that the Si content exceeded 1% and the Cr content exceeded 0.5%. As a result, the area ratio of unprocessed ferrite was less than 85%, and the area ratio of the second phase increased. Therefore, in this steel number 32, uniform elongation was less than 15%, and the energy absorption amount of the steel number 32 was inferior to the energy absorption amounts of the steel numbers 1-20. Moreover, since the alloy was added excessively, the Charpy absorbed energy was reduced to less than 100 J, and the characteristics as structural steel were not satisfied.

  In steel No. 33, in the chemical composition, the C content is more than 0.1%, the Cu content and the Ni content are more than 0.5%, and the carbon equivalent is more than 0.30%. Was done. Therefore, the area ratio of unprocessed ferrite was less than 85%, the tensile strength was greater than 460 MPa, and the uniform elongation was less than 15%. As a result, the energy absorption amount of steel number 33 was significantly inferior to the energy absorption amounts of steel numbers 1-20. Moreover, since the processed ferrite increased by the two-phase rolling, the Charpy absorbed energy was also less than 100J. Therefore, since the risk of brittle fracture increases, the steel plate No. 33 is not suitable as a thick steel plate for Barbasse bow.

  In steel No. 34, the Mo content was 0.2% or more, and the carbon equivalent was more than 0.30%. In the manufacturing method, the heating temperature was higher than 1300 ° C. In addition to chemical components having high hardenability, excessive quenching was performed in a state where the heated austenite grains were coarsened, so that the area ratio of unprocessed ferrite was less than 85% and the area ratio of the second phase increased. As a result, in Steel No. 34, the uniform elongation was less than 15%, and the energy absorption amount of Steel No. 34 was inferior to that of Steel Nos. 1-20.

  In Steel No. 35, the C content was 0.03% or less in the chemical composition, and the cooling start temperature was less than 760 ° C. in the production method. In addition to chemical components that are extremely hard to harden, proper cooling has not been performed, so the crystal grain size of ferrite has increased to over 40 μm. Accordingly, the yield strength was less than 235 MPa, and the Charpy absorbed energy was less than 100 J. The energy absorption amount of the steel number 35 is equivalent to the energy absorption amount of the steel numbers 1 to 20, but the steel plate of the steel number 35 does not have the characteristics as a thick steel plate for Barbusau.

  In Steel No. 36, the amount of Ti was more than 0.04% in the chemical components, and the tempering temperature was more than 650 ° C. in the production method. As a result, in addition to the coarsening of the ferrite crystal grain size to more than 40 μm, TiC precipitation embrittlement caused the Charpy absorbed energy to be significantly reduced to less than 100 J. In this case, even if the amount of energy absorption calculated from the strength and elongation is high, there is a high risk of brittle fracture occurring when the Barbasse bow is deformed at the time of collision, so that sufficient energy absorption due to buckling deformation cannot be expected.

  In steel No. 37, in the chemical composition, the C amount is more than 0.10%, the Si amount is more than 1%, the Mn amount is more than 1.5%, and the Mo amount is 0.2% or more. , B amount was more than 0.002%, and carbon equivalent was 0.30%. Moreover, in this steel No. 37, the cooling rate was larger than 50 degrees C / s in the manufacturing method. For this reason, excessive quenching was performed, the area ratio of unprocessed ferrite was less than 85%, and the ferrite crystal grain size was less than 5 μm. As a result, the tensile strength was significantly higher than 460 MPa, and the uniform elongation was significantly lower than 15%. As a result, the energy absorption amount of steel number 37 was inferior to that of steel numbers 1-20. Moreover, Charpy absorbed energy is also less than 100 J, and it is difficult to apply the steel plate of steel No. 37 as a thick steel plate for Barbus Bau.

  From the above examples, it was confirmed that by applying the present invention, it is possible to provide a thick steel plate for Barbusbau having excellent collision energy absorption capability and a method for manufacturing the same. If this steel plate is used in a Barbus bow, an accident may occur in which the bow of a ship (barreled ship) with the barbus bow collides with the hull of another ship (a collision ship) without changing the design of the hull structure. In this case, the side of the valve part of the Barbus Bau in the collision ship is more uniformly buckled and deformed, so that the collision energy can be absorbed effectively, and the collision surface collapses while absorbing the collision energy. Damage to the hull of the ship can be significantly reduced.

  Note that the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

  It is possible to provide a thick steel plate for a bow structure having a buffering effect that can effectively prevent damage to a collision ship at the time of collision between ships without causing a change in hull structure design, and a manufacturing method thereof.

DESCRIPTION OF SYMBOLS 10 Collision ship 10a Barbusbau 10b Valve part 11 Collision ship 11a The hull of a collision ship

Claims (7)

% By mass
C: more than 0.03 to 0.10%,
P: ≦ 0.05%,
S: ≦ 0.05%,
Al: 0.002 to 0.1%,
Containing the chemical component consisting of iron and inevitable impurities,
It contains a ferrite and has a microstructure composed of one or more of pearlite and bainite, the area ratio of unprocessed ferrite in the microstructure is 85% or more, and the average crystal grain size of the unprocessed ferrite is 5 to 40 μm The number of cementite particles in the ferrite grains is 50000 particles / mm ² or less, the yield strength is 235 MPa or more, the tensile strength is 460 MPa or less, the uniform elongation is 15% or more, and the Charpy average absorption at 0 ° C. A thick steel plate for a hull characterized by having an energy of 100 J or more. % By mass
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 to 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%,
REM: 0.0003 to 0.005%
The thick steel plate for a hull according to claim 1, wherein one or more of the following are contained as the chemical component and the carbon equivalent Ceq is 0.30% or less.
However, 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], [N] Are the contents in mass% of C, Si, Mn, Cu, Ni, Cr, V, Mo, Nb, Ti, B, and N, respectively.   The thick steel plate for a hull according to claim 2, wherein the carbon equivalent Ceq is 0.27% or less in terms of mass%.   The thick steel plate for a hull according to claim 1 or 2, wherein a yield ratio is 0.70 or more. A steel slab having the chemical component according to any one of claims 1 to 3 is heated to 1000 to 1300 ° C;
Perform rolling cumulative rolling reduction 30 to 98% to product thickness at A _r3 transformation point or more austenite single-phase region;
Accelerated cooling is performed from a cooling start temperature of 760 ° C. or higher to a temperature of 400 to 650 ° C. at a cooling rate of 1 to 50 ° C./s on the average sheet thickness, followed by air cooling;
A method for producing a thick steel plate for a hull. A steel slab having the chemical component according to any one of claims 1 to 3 is heated to 1000 to 1300 ° C;
Perform rolling cumulative rolling reduction 30 to 98% to product thickness at A _r3 transformation point or more austenite single-phase region;
Accelerating cooling from a cooling start temperature of 760 ° C. or higher to a temperature of less than 400 ° C. at a cooling rate of 1 to 50 ° C./s on an average sheet thickness, followed by tempering at a temperature of 400 to 650 ° C .;
A method for producing a thick steel plate for a hull. A steel slab having the chemical component according to any one of claims 1 to 3 is heated to 1000 to 1300 ° C;
Perform rolling cumulative rolling reduction 30 to 98% to product thickness at A _r3 transformation point or more austenite single-phase region;
Air cooling;
A method for producing a thick steel plate for a hull.

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