TWI463018B - High strength steel plate with excellent crack arrest property - Google Patents

Description

High-strength thick steel plate with excellent crack resistance Field of invention

The present invention relates to a high strength thick steel plate having excellent crack resistance.

The present application claims priority based on Japanese Patent Application No. 2012-087384, filed on Jan.

Background of the invention

Thick steel plates used in structures such as shipbuilding, construction, tanks, marine structures, and pipelines are required to have crack resistance (brittle failure propagation stop performance) capable of suppressing the propagation of brittle failure, and to suppress structures. Brittle damage. In recent years, with the increase in the size of structures, the use of high-strength thick steel sheets having a stress of 390 to 690 MPa and a thickness of 60 to 95 mm has been increased. However, the above-mentioned crack retardation system generally has a tendency to be opposite to the strength and the thickness of the sheet. Therefore, in a high-strength thick steel plate, a technique capable of improving crack retardation is desired.

As a method for improving the crack retardation, for example, a method of controlling the crystal grain size, a method of controlling the embrittlement of the second phase, and a method of controlling the aggregate structure are known.

The method described in Patent Documents 1 to 3 and 21 is a method for controlling the crystal grain size.

In the technique described in Patent Document 1, the crack retardation property is improved by using the ferrite iron as the mother phase and finely granulating the ferrite iron. In order to obtain such fine-grained ferrite, it is cooled in such a manner that 1/8 or more of the thickness of the slab from the front and back layers to the center of the thickness of the slab is Ar3 or less, and rolling is performed in an extremely low temperature region, and then, It is reheated to a temperature greater than Ac3 to recrystallize the ferrite.

In the techniques described in Patent Documents 2 and 3, the ferrite iron is used as the mother phase, and the surface layer portion is temporarily cooled to below Ar1, and then, by rolling in the surface layer portion to obtain fine fertilizer particles. Iron recrystallized grains.

The technique described in Patent Document 21 is such that the average crystal grain size in the longitudinal direction of the ferrite iron is 5 μm or more and the aspect ratio is 2 or more, or the average of the long-axis direction of the old Worthite iron particles. The particle size is 10 μm or more and the aspect ratio is 2 or more to improve the brittle crack propagation stop characteristics.

Further, as a method of controlling the second phase of embrittlement, the technique described in Patent Document 4 is used.

The technique described in Patent Document 4 embrittles the brittle fracture tip portion in the second phase (for example, 麻田散铁) in the ferrite iron of the parent phase. The micro cracks in the phase cause the stress state of the tip end portion of the crack to be moderated.

Further, as a method of controlling the collective organization, the techniques described in Patent Documents 5 to 17 are included. The techniques described in Patent Documents 5 to 17 are used as a collective structure by, for example, a surface layer portion, a quarter portion of a plate thickness, and a plate. The plate thickness position of the 1/2 portion of the thickness controls the X-ray surface intensity ratio to change the propagation direction of the crack and improve the crack retardation.

Further, as a method of controlling both the crystal grain size and the aggregate structure, the techniques described in Patent Documents 18 to 20 are used.

In the technique described in Patent Document 18, the crack retardation is controlled by setting the ratio of the crystal grain size to the X-ray surface intensity ratio by setting the iron fraction of the 1/2 portion of the thickness of the sheet to 80% or more. Upgrade.

The technique described in Patent Document 19 is to prevent crack resistance by controlling the crystal grain size of the surface layer and the plate thickness of 1/2 and the aggregate structure strength ratio measured by X-ray.

The technique described in Patent Document 20 improves the crack retardation by controlling the crystal grain size of the surface layer and the plate thickness of 1/2 and the area ratio of the {100} plane which is perpendicular to the external stress.

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Publication No. 61-235534

Patent Document 2: Japanese Patent Laid-Open Publication No. 2003-221619

Patent Document 3: Japanese Patent Laid-Open No. 5-148842

Patent Document 4: Japanese Patent Laid-Open Publication No. 59-47323

Patent Document 5: Japanese Patent Laid-Open Publication No. 2008-045174

Patent Document 6: Japanese Patent Laid-Open Publication No. 2008-069380

Patent Document 7: Japanese Patent Laid-Open Publication No. 2008-111165

Patent Document 8: Japanese Patent Laid-Open Publication No. 2008-111166

Patent Document 9: Japanese Patent Laid-Open Publication No. 2008-169467

Patent Document 10: Japanese Patent Laid-Open Publication No. 2008-169468

Patent Document 11: Japanese Patent Laid-Open Publication No. 2008-214652

Patent Document 12: Japanese Patent Laid-Open Publication No. 2008-214653

Patent Document 13: Japanese Patent Laid-Open Publication No. 2008-214654

Patent Document 14: Japanese Patent Laid-Open Publication No. 2009-185343

Patent Document 15: Japanese Patent Laid-Open Publication No. 2009-221585

Patent Document 16: Japanese Patent Laid-Open Publication No. 2009-235458

Patent Document 17: Japanese Patent Laid-Open Publication No. 2010-047805

Patent Document 18: Japanese Patent Publication No. 2011-068952

Patent Document 19: Japanese Patent Laid-Open Publication No. 2011-214116

Patent Document 20: Japanese Patent Laid-Open Publication No. 2007-302993

Patent Document 21: Japanese Patent Laid-Open Publication No. 2008-156751

Summary of invention

In the techniques described in Patent Documents 1 to 3, it is difficult to recrystallize the ferrite iron in the back layer portion of the steel sheet and to use the ferrite iron as the main component, and it is difficult to obtain a steel sheet having a high strength and a thick plate thickness. Further, as described in Patent Documents 1 to 3 and 21, when the crystal grain size is controlled, it is difficult to increase the crack resistance in a steel sheet having a high strength and a thick plate thickness. Moreover, since it is necessary to pass the cooling, rolling, and reheating steps, the manufacturing process becomes complicated, and it is very difficult to obtain a stable material. Further, in such a manufacturing process, it is easy to cause a shape defect due to uneven cooling of the steel sheet surface. Shape correction requires considerable cost when producing a poor shape.

Further, in the technique described in Patent Document 4, since the granulated iron is dispersed in the ferrite iron, the brittle crack generation characteristics are remarkably deteriorated. Further, since the ferrite iron is mainly used as the main body, it is difficult to obtain a steel sheet having a high strength and a thick plate thickness as described above. Further, when the second phase is embrittled, it is difficult to increase the crack retardation in a steel sheet having a high strength and a thick plate thickness.

Further, in the techniques described in Patent Documents 5 to 17, 19, and 21, the control of the crystal grain size is not performed, and the control of the crystal grain size is used to improve the crack resistance of the steel sheet having high strength and thick plate thickness. The most effective factor. That is to say, only when the aggregate structure is controlled, the steel sheet having a high strength and a thick plate thickness cannot be lifted by the crack retardation. Further, since the X-ray plane intensity ratio indicates a local aggregate structure, the deviation is large. These techniques are not techniques capable of improving crack retardation and achieving high productivity in hot rolling. The techniques of the prior patent documents 5 to 8, 11 and 21 are techniques for improving the brittle crack propagation stop characteristic in the thickness direction of the sheet, instead of lifting the direction parallel to the surface of the steel sheet as in the present application, for example, perpendicular or parallel to the rolling direction. The technique of brittle fracture propagation characteristics in the direction of the technique. According to this technique, it is not possible to improve the brittle crack propagation stop characteristic in the direction parallel to the surface of the steel sheet.

The crack retardation of the steel sheets disclosed in Patent Documents 9 and 10 is the highest, and the Kca at a plate thickness of 60 mm and at -10 ° C is 6500 to 6600 N. Mm ^-0.5 or so. The Kca is considered to be 6000N at -20 °C. Below mm ^-0.5 , it is necessary to further improve the crack retardation.

Patent Documents 12, 13, 16 and 19 disclose a technique for obtaining high crack retardation. But in order to form an excellent collection group in the thickness direction of the plate Weaving, in the central portion of the thickness of the plate, the temperature is Ar3 point -10 ° C or less, and the temperature range of Ar3 point - 50 ° C or more, the rolling reduction ratio is 30% or more, and the average pass rate is 8% or more. It is necessary. That is, the rolling at a very low temperature is indispensable, resulting in a very low productivity at the time of rolling and being difficult to mass-produce.

The crack resistance in the plate thickness direction of the steel sheets disclosed in Patent Documents 14, 15, and 17 is by a large mixed ESSO test (length of running plate: 1600 mm, length of test plate: 800 mm, width of test body: 2400 mm, load stress) : 235 kg.mm ^-0.5 ) was evaluated. It is considered that its Kca system is 6000N at -20 °C. Mm ^-0.5 or less. Moreover, in order to form the aggregate structure, similarly, the rolling at a low temperature is indispensable, making it difficult to mass-produce.

Further, in the technique described in Patent Document 18, since the crystal grain size and the aggregate structure are controlled only in the 1/2 portion of the thickness of the sheet, it is difficult to increase the crack resistance in the case where the sheet thickness is thick. Further, it is difficult to form a steel sheet having a high strength and a thick plate thickness by using ferrite iron as a main component. Further, since the X-ray plane intensity ratio indicates a local aggregate organizer, the deviation is large and is not suitable as a factor for making the crack retardation. Further, this technique is a technique for improving the crack retardation at an angle of 45° with respect to the rolling direction by forming a collecting structure by rolling at a low temperature. Rolling at low temperatures is indispensable, making it difficult to mass produce.

The technique described in Patent Document 20 controls the crystal grain size and aggregate structure of the surface layer and the thickness of the sheet at 1/2. However, the crack retardation in the case where the thickness of the plate is thick is increased, because the plane stress state and the contribution to the surface layer which is not easily generated by the cleavage damage are very small, and the control system of the surface layer is difficult to make the crack retardation leap. Promote the ground. Further, it has been revealed that the Kca at a plate thickness of 70 mm and at -10 ° C is 210 MPa. The technique of mm ^-0.5 (that is, approximately equivalent to 6600 N.mm ^-0.5 ). The Kca is considered to be 6000N at -20 °C. Below mm ^-0.5 , it is necessary to further improve the crack retardation.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength thick steel plate having excellent crack resistance which is low in manufacturing cost, high in productivity, high in strength, thick in sheet thickness, and not deteriorated in HAZ toughness.

The gist of the present invention is as follows.

(1) A high-strength thick steel plate according to one aspect of the present invention has a composition of C: 0.04 to 0.16% by mass, Si: 0.01 to 0.5%, Mn: 0.75 to 2.5%, and Al: 0.001 to 0.1%, Nb: 0.003 to 0.05%, Ti: 0.003 to 0.05%, and N: 0.001 to 0.008%, and the limit P is 0.03% or less, S is 0.02% or less, Cu is 1% or less, and Ni is 2 % or less, Cr is 1% or less, Mo is 0.5% or less, V is 0.15% or less, B is 0.005% or less, Ca is 0.01% or less, Mg is 0.01% or less, and REM is 0.01% or less, and the remainder contains iron. And an unavoidable impurity, and the carbon equivalent Ceq. of the following formula A is 0.30 to 0.50%; and has the following microstructure: containing ferrite iron having an area ratio of 70% or less, and an area ratio of 30 More than or equal to the toughening iron; further, in the 1/4 portion of the thickness of the plate, the crystal grain boundary density of the total length of the average unit area of the crystal grain boundary having a crystal orientation difference of 15 or more is 400 to 1000 mm/mm ² , and At the same time, the area ratio of the {100} plane constituting the angle within 15° with respect to the plane perpendicular to the main rolling direction is 10 to 40%; and in the 1/2 portion of the thickness of the aforementioned sheet, the crystal grain boundary density is 300 ~ 900mm / mm ² And simultaneously with respect to the rolling direction of the main vertical plane forms an angle of less than 15 ° {110} plane area ratio of 40 to 70%.

Ceq.=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5. . . Type A

(2) The high-strength thick steel plate according to (1) above, wherein the plate thickness may be 60 to 95 mm.

(3) The high-strength thick steel plate according to (1) or (2) above, wherein the relief stress may be 390 to 690 MPa.

(4) The high-strength thick steel plate according to any one of the above items (1) to (3), wherein the microstructure may contain 10% or less of the surface iron.

(5) The high-strength thick steel plate according to any one of the above items (1) to (4), wherein the micro-structure may be a ferrite grain area ratio of less than 50%, a brite iron area ratio of 5% or less, and toughening The iron area ratio is 50% or more.

(6) The high-strength thick steel plate according to any one of (1) to (5) above, wherein the crystal grain boundary density of the 1/4 portion of the thickness of the plate may be 500 to 900 mm/mm ² , and the thickness of the plate The aforementioned crystal grain boundary density of the 1/2 portion may be 400 to 800 mm/mm ² .

(7) The high-strength thick steel plate according to any one of the items (1) to (6), wherein the Cu is 0.5% or less, the Ni is 1% or less, and the Cr is 0.5% or less, and the Mo is further restricted. It is 0.2% or less, and the aforementioned V is 0.07% or less.

(8) The high-strength thick steel plate according to any one of the above (1) to (7), wherein the B is further limited to 0.002% or less.

(9) The high-strength thick steel plate according to any one of the items (1) to (8), wherein the Ca is 0.003% or less, the Mg is 0.003% or less, and the REM is 0.003% or less.

According to the present invention, the crack retardation in the direction parallel to the surface of the steel sheet, for example, perpendicular or parallel to the rolling direction, is excellent, and even if the thickness of the sheet is thick, the steel sheet having high strength and HAZ toughness is not deteriorated. It is possible to reduce the cost and safety of welded steel structures.

Fig. 1A is a photograph showing a state in which a crack is propagated by applying a punch to a V-notch in the left direction of the photograph of the steel sheet according to the embodiment of the present invention.

Figure 1B is a photograph of the fracture surface of the crack shown in Figure 1A.

Fig. 2A is a photograph of a state in which a crack propagated by applying a punch from a V-notch in the left direction of the photograph to the steel sheet of the comparative example.

Figure 2B is a photograph of the fracture surface of the crack shown in Figure 2A.

Fig. 3A is a photograph of a state in which a crack propagated by applying a punch from a V-notch in the left direction of the photograph to the steel sheet of the comparative example.

Figure 3B is a photograph of the fracture surface of the crack shown in Figure 2A.

Form for implementing the invention

In order to solve the problem, the present inventors have conducted intensive studies, and as a result, it has been found that hot rolling can be obtained by controlling the composition of the high-strength steel sheet, the microstructure, the grain boundary density in the thickness direction of the sheet, and the aggregate structure in the thickness direction of the sheet. A high-strength steel sheet having high productivity and improved crack resistance in a direction parallel to the surface of the steel sheet, for example, perpendicular or parallel to the rolling direction.

In the following, a high-strength thick steel plate (hereinafter, simply referred to as a steel plate) according to an embodiment of the present invention based on the above knowledge will be described.

The steel sheet according to the present embodiment controls the component composition, the microstructure, the crystal grain boundary density in the thickness direction of the sheet, and the aggregate structure in the sheet thickness direction to make the direction parallel to the surface of the steel sheet, for example, perpendicular or parallel to the rolling direction. The crack resistance is improved.

(micro organization)

The steel sheet according to the embodiment has a microstructure, which is a mixed structure of ferrite iron and toughened iron, or a mixed structure of ferrite iron, bun iron, and toughened iron, wherein the ferrite iron area ratio is 70. Below %, the toughened iron area ratio is 30% or more.

When the area ratio of the ferrite iron is more than 70%, it is difficult to form a steel sheet having a thick plate and a high strength. As long as the steel sheet having the required thickness and strength can be obtained, the second phase can be made of toughened iron, or ferritic iron and toughened iron. The present invention is directed to thick-walled high-strength steel, and the upper limit of the area ratio of the ferrite iron can be limited to less than 50%, less than 30%, less than 20%, or less than 10%.

When the area ratio of the toughened iron is less than 30%, it is difficult to obtain a steel sheet having a thick plate thickness and a high strength. In order to ensure the area ratio of the ferrite iron and increase the crystal grain boundary which is an obstacle to the propagation of brittle cracks, the upper limit of the area ratio of the toughened iron may be 95%. The present invention is directed to thick-walled high-strength steel, and the lower limit of the area ratio of the toughened iron can be limited to 50% or more, 60% or more, 70% or more, or 80% or more.

As long as the steel sheet having the required thickness and strength of the sheet can be obtained, it may contain Boron. Therefore, it is possible to limit the area ratio of the wave iron to 10% or less, 5% or less, or 3% or less. The lower limit of the area ratio of the Brilliant iron is 0%.

In addition to ferrite iron, bun iron and toughened iron, there may be fine island-like granulated iron (MA: Martensite-Austenite-Consituent), but the area ratio of MA It is set to 5% or less. The MA area ratio can be limited to 3% or less, 2% or less, or 1% or less, and 0% is optimal.

(crystalline grain boundary density in the thickness direction of the plate)

In the dominant factor of crack retardation improvement, the contribution of crystal grain boundaries is the largest. Because the crystal grain boundary system becomes an obstacle to the propagation of brittle cracks. That is, since the crystal orientation between adjacent crystal grains is different at the crystal grain boundary, the crack propagates in the direction in which the portion propagates. Therefore, an unbroken region is generated and the stress is dispersed by the unbroken region, and the cracked closed stress is obtained. Therefore, the driving force for crack propagation is lowered, and the crack retardation is improved. Further, since the unbroken region is finally subjected to ductile fracture, the energy required for brittle fracture is absorbed. Therefore, the crack retardation is improved.

Conventionally, it has been considered that in order to increase the crystal grain boundary, it is necessary to reduce the crystal grain size. This is the case when the ferrite iron is the main body of the structure, but when the steel having a thick plate thickness is high, it is indispensable to use the toughened iron system. Since the toughened iron system is different from the ferrite iron and the shape of the lower part of the structure is complicated, the definition of the crystal grain is very difficult. Therefore, even if it is converted into a circle-equivalent diameter, the relationship between the crystal grain size and the crack retardation is obtained, and the variation is large, and it is difficult to determine the crystal grain size necessary for the crack retardation improvement. Therefore, the basic principle of returning the crystal grain boundary to the crack propagation is defined, and the total length of the crystal grain boundary of the average unit area (hereinafter referred to as crystal grain boundary density) is defined, and is used for finishing When it is related to the retardation of cracks, the knowledge that is most closely related to each other is obtained.

Therefore, in the steel sheet according to the present embodiment, the crystal grain boundary density in the 1/4 portion of the sheet thickness is set to 400 to 1000 mm/mm ² , and (B) is crystallization in the 1/2 portion of the sheet thickness. The grain boundary density is set to 300 to 900 mm/mm ² .

Here, the term "crystalline grain boundary density" means "the total length of the average unit area of the crystal grain boundaries having a crystal orientation difference of 15 or more". When the reason why the crystal orientation difference is 15 or more is less than 15°, the crystal grain boundary system is less likely to be an obstacle to the propagation of brittle cracks, and the effect of improving the crack retardation is reduced.

That is, when the crystal grain boundary density is 1/4 part and 1/2 part of the sheet thickness, and each of them satisfies the requirement of 400 or 300 mm/mm ² or more, the crack retarding value at -20 ° C is exhibited ( Kca) is 6000N. High crack retardation of mm ^-0.5 or more. Further, in order to stably increase the crack retardation, it is preferable to set the crystal grain boundary density to 500 or 400 mm/mm ² or more in each of the 1/4 portion and the 1/2 portion of the sheet thickness, or to set it to 600, respectively. More preferably 400 mm/mm ² or more.

The higher the crystal grain boundary density is, the more the crack retardation system is improved. However, the load of the rolling becomes substantially lower due to the excessive addition, and the upper limit of the crystal grain boundary density is 1/4 of the thickness of the plate. 1/2 parts, each set to 1000, 900mm/mm ² . It is also possible to limit each of the upper limits to 900, 800 mm/mm ² or to each of 800, 700 mm/mm ² .

Crystal grain boundary density is specified in 1/4 part and 1/2 part of the thickness of the plate The reason for this is to increase the crack retardation of the extremely thick material, and it is necessary to increase the crystal grain boundary density of the entire plate thickness by controlling the 1/4 portion and the 1/2 portion of the thickness of the plate because it can be averaged as the thickness of the plate. A representative value of the crystal grain boundary density. In addition, according to the manufacturing method described later, the crystal grain boundary density of the half of the thickness of the main control plate, the other plate thickness position is inevitably low in temperature, the cooling rate is increased, and the crystal grain boundary density tends to increase. It is not necessary to specifically limit the numerical value. However, when the heating method is used, a large temperature gradient is generated in the thickness direction of the sheet, and the crystal grain boundary density of the 1/4 portion and the 1/2 portion of the sheet thickness is reversed, so that the numerical value is intentionally specified.

The measurement of the crystal grain boundary is preferably performed by using an EBSD (Electron Back Scatter Diffraction pattern) method capable of accurately measuring the crystal orientation in a wide field of view. When the EBSD method is used, it is also possible to identify crystal grain boundaries such as complex structures of toughened iron.

In more detail, the crystal grain boundary density can be determined by the EBSD method, and the 1/4 portion of the plate thickness and the 1/2 portion of the 500 μm × 500 μm region are measured at a pitch of 1 μm, and the crystal orientation difference from the adjacent grain is 15° or more. The boundary is defined as a crystal grain boundary, which is obtained by dividing the total length of the crystal grain boundary at this time by the measurement area.

(collection organization in the thickness direction of the board)

In the case of a high-strength steel sheet having a thick plate thickness, it is difficult to stably increase the crack retardation only when the crystal grain boundary density in the thickness direction of the control plate is obtained. Therefore, it is important to control the direction of crack propagation of the living tissue. When the steel sheet is subjected to external stress, the brittle crack generated by the steel sheet propagates along the cracked surface of the {100} plane. Therefore, it is clear that when the {100} plane assembly is developed on the plane perpendicular to the external stress, the crystal grain size is controlled as described above. When the crack retarding effect is reduced, the effect is reduced. External stresses externally impart stress to the steel structure. The brittle fracture system is generated in the direction perpendicular to the highest external stress and is mostly propagated. Therefore, here, the highest stress externally imparted to the steel structure is defined as external stress. Generally, the external stress is applied substantially in parallel with the main rolling direction of the steel sheet. Therefore, it is possible to operate the surface perpendicular to the external stress as a surface perpendicular to the main rolling direction of the steel sheet.

Further, in the main rolling direction of the steel sheet, for example, the surface of the steel sheet can be corroded by using picric acid, and the aspect ratio of the old Worthite iron can be measured. That is, the direction in which the aspect ratio of the old Worth iron is large is specified as the main rolling direction of the steel sheet.

It is clear that the aggregate structure of the {100} plane constituting the angle within 15° with respect to the plane perpendicular to the main rolling direction of the steel sheet is 40 to 70% in area ratio of 1/2 of the thickness of the sheet. Since the brittle crack near the 1/2 portion does not spread straight, the crack system propagates obliquely and can reduce the driving force of crack propagation. However, when the same aggregate structure is developed in the plate thickness portion other than the half of the thickness of the plate, the crack propagates in an inclined state, and a sufficient crack retarding effect cannot be exhibited. Therefore, conversely, it is clear that in the 1/4 portion of the thickness of the plate, in order to allow the crack to spread straight, by 1/4 of the thickness of the plate, the surface perpendicular to the main rolling direction of the steel plate is formed. The aggregate structure of the {100} plane at an angle of 15° or less is 10 to 40% in area ratio, and the 1/2-part inclined crack propagation propagates to a plate thickness portion other than the 1/2 portion.

Based on the above knowledge, the steel plate of this embodiment is (C) In the 1/4 portion of the thickness of the plate, the area ratio of the {100} plane forming the angle within 15° with respect to the plane perpendicular to the main rolling direction is set to 10 to 40%, and (D) In the 1/2 portion of the thickness of the sheet, the area ratio of the {100} plane constituting the angle within 15° with respect to the plane perpendicular to the main rolling direction is set to 40 to 70%.

By satisfying the above (C) and (D), as shown in FIG. 1A and FIG. 1B, the 1/2 part of the crack system propagates obliquely, and the 1/4 part of the crack propagates straight, cracked The spread of resistance has gone further. Thereby, the effect of improving the crack retardation due to the increase in the crystal grain boundary density can be exhibited, and the crack retardation system can exhibit a sufficient value.

1A is a photograph showing a state in which a crack propagates from a V-notch in the left direction of the photograph to the crack in the steel sheet according to the embodiment of the present invention, and FIG. 1B shows a photograph of the fracture surface of the crack. .

In the 1/4 portion of the thickness of the plate, the area ratio of the {100} plane which is an angle within 15° with respect to the surface perpendicular to the main rolling direction is 10% or more, because it is less than 10%. The effect of spreading the crack straight can not be obtained.

Further, in the 1/4 portion of the thickness of the plate, the area ratio of the {100} plane which is an angle of 15° or less with respect to the surface perpendicular to the main rolling direction is 40% or less, as shown in FIG. 2A. 2B shows that, because it is greater than 40%, the crack propagation of the 1/4 portion is more dominant than the 1/2 portion and the crack propagates straight, resulting in a low crack retardation.

Further, Fig. 2A shows a steel sheet having an area ratio of a {100} plane which is an angle of 15° or less with respect to a plane perpendicular to the main rolling direction, which is set to be more than 40% in the 1/4 portion of the thickness of the sheet. Applying the steel plate to the V-notch from the left of the photo A photograph of a state in which cracking is generated by punching, and Fig. 2B is a photograph showing a fracture surface of the crack.

The area ratio of the {100} plane constituting the angle within 15° with respect to the plane perpendicular to the main rolling direction in the 1/4 portion of the sheet thickness is preferably 13 to 37%, more preferably 15 to 35. %.

In the half of the thickness of the plate, the area ratio of the {110} plane which is an angle of 15° or less is 40% or more with respect to the surface perpendicular to the main rolling direction, because it is less than 40%. It is impossible to obtain the effect of spreading the crack and spreading it.

In addition, in the 1/2 portion of the thickness of the plate, the area ratio of the {110} plane which is an angle of 15° or less with respect to the surface perpendicular to the main rolling direction is 70% or less, as shown in FIG. 3A. As shown in Fig. 3B, since it is more than 70%, it does not receive the resistance of the 1/4 portion and propagates in a tilted state, resulting in a low crack retardation.

Further, Fig. 3A shows a steel sheet having an area ratio of a {110} plane which is an angle of 15° or less with respect to a surface perpendicular to the main rolling direction with respect to a surface which is perpendicular to the main rolling direction, and is set to be more than 70%. A photograph of the state in which the steel sheet was punched from the V-notch in the left direction of the photograph to cause crack propagation, and FIG. 3B is a photograph showing the fracture surface of the crack.

In the 1/2 portion of the thickness of the plate, the area ratio of the {110} plane constituting the angle within 15° with respect to the surface perpendicular to the main rolling direction is preferably 45 to 65%, more preferably 40 to 40. 60%.

The collection organization is preferably measured by the EBSD method. When measured by the EBSD method, it can be measured accurately compared to the measurement using X-rays. A collection organization with a broader perspective.

More specifically, using the EBSD method, each of the 1/4 portions of the thickness of the sheet is formed with respect to a plane perpendicular to the main rolling direction of the steel sheet to constitute an image of a {100} plane at an angle of 15° or less; The image of the {110} plane at the 1/2 portion of the thickness of the panel can be obtained by dividing the total area by the measured area.

In order to improve the crack retardation as described above, it is possible to apply a steel sheet having a relief stress of 390 to 690 MPa, a tensile strength of 500 to 780 MPa, and a steel sheet having a thickness of 60 to 95 mm. The reason is that in the region where the relief stress is less than 390 MPa or the thickness of the plate is less than 60 mm, even if the means of the invention is not relied on, the crack retardation improvement system is relatively easy, the relief stress is greater than 690 MPa, and the thickness of the plate is greater than 95 mm. The region, even if formed in the crystal grain boundary density and aggregate structure specified in the present invention, because the mechanical conditions are strict, the crack retarding value (Kca) at -20 ° C is 6000 N. High crack retardation of mm ^-0.5 or more is difficult. The lower limit of the lodging stress can be limited to 440 MPa or 470 MPa, and the upper limit can be limited to 640 MPa or 590 MPa. The lower limit of the tensile strength may also be limited to 520 MPa, 540 MPa or 560 MPa, and the upper limit may be limited to 730 MPa, 680 MPa or 630 MPa.

(component composition)

Hereinafter, the chemical composition of the steel sheet according to the embodiment will be described. The "%" of the ingredients means the mass%.

C: 0.04~0.16%

In order to ensure the strength and toughness of the thick base metal, the C system is made to contain 0.04%. on. When the content of C is more than 0.16%, it is difficult to ensure good HAZ toughness, and the content of C is set to be 0.16% or less.

Therefore, the lower limit of C is 0.04%, preferably 0.05%, preferably 0.06%, and the upper limit of C is 0.16%, preferably 0.14%, preferably 0.12%.

Si: 0.01~0.5%

Since Si is effective as a deacidifying element and a strengthening element, it contains 0.01% or more. When the content of Si is more than 0.5%, the HAZ toughness becomes large and deteriorates, and the content of Si is set to 0.5% or less.

Therefore, the lower limit of Si is 0.01%, preferably 0.03%, preferably 0.05%, and the upper limit of Si is 0.5%, preferably 0.4%, preferably 0.35% or 0.3%.

Mn: 0.75~2.5%

In order to economically ensure the strength and toughness of the thick base material, Mn is contained in an amount of 0.75% or more. When the content of Mn is more than 2.5%, the center segregation becomes remarkable, and the toughness of the base material and the HAZ in the portion where the center segregation occurs is deteriorated, and the content of Mn is 2.5% or less.

Therefore, the lower limit of Mn is 0.75%, preferably 0.9%, preferably 1.2%, and the upper limit of Mn is 2.5%, preferably 2.0%, preferably 1.8% or 1.6%.

The limit is P: 0.03% or less

P is one of the impurities. In order to stably ensure the HAZ toughness, the content of P may be limited to 0.03% or less. It is preferably 0.02% or less, more preferably 0.015% or less. The lower limit is 0%, but the lower limit may be made 0.0001% in order to reduce the P content.

Limited to S: 0.02% or less

S is one of the elements of the impurity. In order to stably ensure the properties of the base material and the HAZ toughness, the content of S can be limited to 0.02% or less. It is preferably 0.01% or less, more preferably 0.008% or less. The lower limit is 0%, but the lower limit may be made 0.0001% in order to reduce the S content.

Al: 0.001~0.1%

Al is an O which deacidifies and reduces one of the impurities. In addition to Al, Mn and Si also contribute to deacidification. However, even when Mn and Si are added, when the content of Al is less than 0.001%, O cannot be stably reduced. However, when the content of Al is more than 0.1%, the alumina-based coarse oxide and the cluster thereof are formed, and the base material and the HAZ toughness are impaired, and the Al content is set to 0.1% or less.

Therefore, the lower limit of Al is 0.001%, preferably 0.01%, preferably 0.015%, and the upper limit of Al is 0.1%, preferably 0.08%, preferably 0.05%.

Nb: 0.003~0.05%

In the present invention, Nb is an important element. In order to form a predetermined crystal grain boundary density and aggregate structure, a rolling system in which the Worstian iron region is not recrystallized is necessary. Nb is used to expand the effective element in the non-recrystallization temperature region, and contributes to the improvement of the rolling temperature and the improvement of productivity. In order to obtain this effect, it is necessary to contain 0.003% or more. However, when the content of Nb is more than 0.05%, the content of Nb is set to 0.05% or less because the HAZ toughness and weldability are low.

Therefore, the lower limit of Nb is 0.003%, preferably 0.005%, preferably 0.008%, and the upper limit of Nb is 0.05%, preferably 0.025%, preferably 0.018%.

Ti: 0.003~0.05%

In the present invention, Ti is an important element. By forming TiN by containing Ti, it is suppressed that the particle size of the Worthite iron becomes large when the steel sheet is heated. When the particle size of the Worthite iron becomes large, the crystal grain size of the metamorphic structure also becomes large, and it is difficult to obtain a predetermined crystal grain boundary density, resulting in low toughness and crack retardation. In order to obtain a crystal grain boundary density which is not necessary for toughness and crack retardation, it is necessary to make Ti contain 0.003% or more. However, when the content of Ti is more than 0.05%, the Ti content is set to 0.05% or less because TiC is formed and the HAZ toughness is lowered.

Therefore, the lower limit of Ti is 0.003%, preferably 0.006%, preferably 0.008%, and the upper limit of Ti is 0.05%, preferably 0.02%, preferably 0.015%.

N: 0.001~0.008%

In the present invention, N is an important element. As described above, in order to form TiN, it is necessary to suppress the particle size of the Worthite iron from increasing when the steel sheet is heated, and it is necessary to contain 0.001% or more. However, when the content of N is more than 0.008%, the content of N is set to 0.008% or less because the steel material is embrittled.

Therefore, the lower limit of N is 0.001%, preferably 0.0015%, preferably 0.002%, and the upper limit of N is 0.008%, preferably 0.0065%, preferably 0.006%.

In the component composition of the steel sheet according to the present embodiment, the remainder of the above-mentioned elements may be Fe and unavoidable impurities. However, the component composition of the steel sheet according to the present embodiment may contain at least one of Cu, Ni, Cr, Mo, V, B, Ca, Mg, and REM as necessary. The lower limit of the content of these elements is 0%, and the lower limit can be set in order to stably obtain the effect of addition. value. Further, in the present invention, it is possible to allow the element to be contained in a trace amount at an impurity level. Hereinafter, the effect and content of the respective elements will be described. It is considered to be within the scope of the patent application of the present invention to intentionally add such elements, or to incorporate them in the form of unavoidable impurities, which are within the scope of the patent application.

Cu: 0~1%

By adding Cu, the strength and toughness of the base material can be improved. However, when the content of Cu is too large, 1% is set as an upper limit because HAZ toughness and weldability are deteriorated. The lower limit of Cu is 0%, but in order to stably obtain the effect of addition, the lower limit can be made 0.1%.

Therefore, the lower limit of Cu is 0%, but in order to increase the strength and toughness of the base material, the lower limit can be made 0.1% or 0.2%. In order to improve HAZ toughness and weldability, the upper limit of Cu may be limited to 1%, 0.8%, 0.5%, or 0.3% as necessary.

Ni: 0~2%

By adding Ni, the strength and toughness of the base material can be improved. However, when the content of Ni is too large, 2% is set as an upper limit because HAZ toughness and weldability are deteriorated. The lower limit of Ni is 0%, but in order to stably obtain the effect of addition, the lower limit can be made 0.1%.

Therefore, the lower limit of Ni is 0%, but in order to increase the strength and toughness of the base material, the lower limit can be made 0.1% or 0.2%. The upper limit of Ni may also be limited to 2%, 1%, 0.5%, or 0.3% as necessary.

Cr: 0~1%

By adding Cr, the strength and toughness of the base material can be improved. However, when the content of Cr is too large, since HAZ toughness and weldability are deteriorated, 1% is set as limit. The lower limit of Cr is 0%, but in order to stably obtain the effect of addition, the lower limit can be made 0.1% or 0.2%. The upper limit of Cr may also be limited to 1%, 0.8%, 0.5%, or 0.3% as necessary.

Mo: 0~0.5%

By adding Mo, the strength and toughness of the base material can be improved. However, when the content of Mo is too large, 0.5% is set as an upper limit because HAZ toughness and weldability are deteriorated. The lower limit of Mo is 0%, but in order to stably obtain the effect of addition, the lower limit can be made 0.01% or 0.02%. The upper limit of Mo may also be limited to 0.5%, 0.3%, 0.2%, or 0.1% as necessary.

V: 0~0.15%

By adding V, the strength and toughness of the base material can be improved. However, when the content of V is too large, 0.15% is set as an upper limit because HAZ toughness and weldability are deteriorated. The lower limit of V is 0%, but in order to stably obtain the effect of addition, the lower limit can be made 0.01% or 0.02%. The upper limit of V may also be limited to 0.15%, 0.1%, 0.07%, or 0.05% as necessary.

B: 0~0.005%

By adding B, the strength and toughness of the base material can be improved. However, when the content of B is too large, since HAZ toughness and weldability are deteriorated, 0.005% is made the upper limit. The lower limit of B is 0%, but the effect of addition is stably obtained, and the lower limit can be made 0.0002% or 0.0003%. The upper limit of B may also be limited to 0.005%, 0.003%, 0.002%, or 0.001% as necessary.

Ca: 0~0.01%

By adding Ca, the HAZ toughness is improved. However, when the content of Ca is too large, since HAZ toughness and weldability are deteriorated, 0.01% is made the upper limit. Lower limit of Ca The value is 0%, but in order to stably obtain the effect of addition, the lower limit is made 0.0002% or 0.0003%. The upper limit of Ca may also be limited to 0.01%, 0.005%, 0.003%, or 0.001% as necessary.

Mg: 0~0.01%

By adding Mg, HAZ toughness is improved. However, when the content of Mg is too large, 0.01% is set as an upper limit because HAZ toughness and weldability are deteriorated. The lower limit of Mg is 0%, but in order to stably obtain the effect of addition, the lower limit can be made 0.0002% or 0.0003%. The upper limit of Mg may also be limited to 0.01%, 0.005%, 0.003%, or 0.001% as necessary.

REM: 0~0.01%

By adding REM, HAZ toughness is improved. However, when the content of REM is too large, 0.01% is set as an upper limit because HAZ toughness and weldability are deteriorated. The lower limit of the REM is 0%, but in order to stably obtain the effect of addition, the lower limit can be made 0.0003% or 0.0005%. The upper limit of the REM may also be limited to 0.01%, 0.005%, 0.003%, or 0.001% as necessary.

In order to improve the strength, toughness, and the like of the base material, the above-described selection elements can be intentionally added. However, in order to reduce the cost of the alloy, etc., there is no hindrance to not adding these optional elements at all. Even if these elements are not intentionally added, it is possible to contain Cu: 0.1% or less, Ni: 0.1% or less, Cr: 0.1% or less, Mo: 0.01% or less, and V: in an unavoidable impurity. 0.01% or less, B: 0.0002% or less, Ca: 0.0003% or less, Mg: 0.0003% or less, and REM: 0.0003% or less.

(carbon equivalent: 0.30~0.50%)

The steel sheet according to the embodiment is a carbon equivalent obtained by the following formula (1). Ceq. is set to 0.30~0.50%.

Ceq.=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(1)

Here, each component is the mass % of each component contained in a steel plate.

When the carbon equivalent is less than 0.30%, the strength required for a high-strength thick steel plate cannot be obtained. When the carbon equivalent is more than 0.50%, the crack retardation required for the high-strength thick steel plate cannot be satisfied.

Therefore, the lower limit of the carbon equivalent is 0.30%, preferably 0.32%, preferably 0.34%, more preferably 0.36%, and the upper limit of the carbon equivalent is 0.50%, preferably 0.44%, preferably 0.42. %, more preferably 0.40%.

Next, a preferred method of producing the steel sheet according to the embodiment will be described.

First, a molten steel adjusted to have a desired composition is melted by using a well-known melting method such as a converter, and a steel sheet is produced by a well-known casting method such as continuous casting.

The steel sheet is cooled to a thickness of 600 ° C or less, and then placed in a heating furnace having an ambient temperature of 1000 to 1250 ° C for 30 to 600 minutes, and extracted at a center temperature of the sheet thickness of 950 to 1150 ° C.

When the temperature of the cooled steel sheet is greater than 600 ° C, it is charged into the heating furnace, because the transformation from the Worth iron to the ferrite in the cooling is not completed, and the inversion state during heating is to the fine particles caused by the Worthite iron. The effect is not easy to obtain, and in the case of coarse Worthite iron particles, it is difficult to increase the crystal grain boundary density after rolling. It is preferably 500 ° C or less.

When the ambient temperature of heating is less than 1000 ° C, insufficient heating is caused to cause insufficient melt formation. When the ambient temperature is greater than 1250 ° C, the Worthfield iron particles are coarsened, and in the subsequent rolling process, it is difficult to increase the crystal grain boundary density. Better The ambient temperature range is from 1050 to 1200 °C.

Since the time of charging into the heating furnace is less than 30 minutes, the melt is insufficient, and when it is more than 600 minutes, the Woustian iron particles are coarsened. The preferred loading time range is 40 to 500 minutes.

When the center temperature of the sheet thickness at the time of heating extraction is less than 950 ° C, the melt is not sufficiently formed, and the Worstian iron particles are fined, resulting in low quenching property, and it is difficult to form a steel sheet having a thick plate and high strength.

When the center temperature of the sheet thickness at the time of heating extraction is more than 1150 ° C, the Worthfield iron grain is coarsened, and in the subsequent rolling process, it is difficult to increase the crystal grain boundary density, and since the temperature is lowered until the start of the rolling start Time, resulting in low productivity. The preferred heating extraction temperature ranges from 1000 to 1100 °C.

Secondly, in the center temperature of the plate thickness is greater than 850~1150 °C, the rolling reduction rate of 4~15 passes is 3~30%, and less than 3% (including 0) within 3 passes, to carry out the cumulative rolling The shrinkage is 15~70% of rough rolling.

When the center temperature of the sheet thickness is more than 1150 ° C, even if the subsequent finishing rolling does not make the recrystallized Worthfield iron particles fine. When the plate thickness center temperature is less than 850 ° C, the productivity is low. The preferred plate thickness center temperature is 900~1000 °C.

When the rolling reduction rate of one pass is less than 3%, since the Worthite iron grain system grows abnormally, it must be avoided as much as possible. However, when the rolling reduction rate of one pass is less than 3%, the rolling is limited to three passes, and when the rolling reduction of one pass or more is performed, the rolling reduction is 3 to 30%, and sufficient recrystallization can be performed. And fine. However, when it is more than 30%, since the load of the rolling mill is large, when it is more than 15 passes, the productivity is low, The 1 pass reduction ratio is set to 30% as the upper limit, and the number of passes is set to 4 to 15 passes. It is preferable to set the rolling reduction of 5 to 25% in one pass to 6 to 13 passes.

The reason why the cumulative rolling reduction ratio of the rough rolling is 15 to 70% is that, since the cumulative rolling reduction ratio is less than 15%, it is difficult to refine the grain by the recrystallization of the Worth iron, and the pores remain ( Porosity) has the possibility of causing internal cracks, ductility and toughness deterioration. When the porosity is more than 70%, the productivity is low because the number of passes increases. The preferred cumulative rolling reduction ratio is 30 to 60%.

Secondly, the average thickness of the shape ratio (mj) of the following formula (2) is 0.5 to 1 and the cumulative reduction ratio is 40 to 80% at the center temperature of the plate thickness of 750 to 850 °C. Processing rolls.

m _j =2{R(H _j-1 -H _j )} ^1/2 /(H _j-1 +H _j ). . . (2)

Here, j is the number of roll passes, mj is the shape ratio of the jth pass, R is the roll radius (mm), and Hj is the plate thickness (mm) after j passes.

When the center temperature of the sheet thickness is more than 850 ° C, the non-recrystallized region is not sufficiently entered, and the dislocation of the dislocation is suppressed and the crystal grain boundary density cannot be increased. When the plate thickness center temperature is less than 750 ° C, the productivity is low and because a part of the processed ferrite is contained, the 1/2 portion of the plate thickness is perpendicular to the main rolling direction of the steel plate, and is formed within 15°. It is difficult to set the area ratio of the {110} plane of the angle to 40% or more. The preferred plate thickness center temperature is 760~840 °C. When rolling is less than 4 passes, it is difficult to ensure a shape ratio of 1 or less, and when it is more than 15 passes, productivity is low. The number of preferred passes is 5 to 13 passes.

The shape ratio of the formula (2) indicates an index of how the strain component is imparted to the steel sheet by rolling. When the shape ratio is small, the system gives a large amount of shear should become When it is larger, it gives a large amount of compressive strain components. Since the shape has a significant influence on the change of the strain component due to the change, particularly the formation of the aggregate structure at a quarter of the thickness of the sheet, the range is set as described above.

The reason why the average value of the shape ratio is set to 0.5 to 1 is because the shear strain system of the rolling becomes dominant when it is less than 0.5 in the 1/4 portion of the thickness of the sheet, so that the {100} collection structure is developed. It is difficult to set the area ratio of the {100} plane which is an angle of 15° or less to the surface perpendicular to the main rolling direction of the steel sheet to be less than 40%. When it is more than 1, the compressive strain of the rolling becomes It is dominant, so the {110} collection organization is developed so that it is difficult to set the area ratio of the {100} plane to more than 10%. The average of the preferred shape ratios ranges from 0.6 to 0.9. When the cumulative rolling reduction ratio is less than 40%, the crystal grain boundary density due to the accumulation of dislocations increases and the predetermined aggregate structure is difficult to develop, and when it is more than 80%, the crystal grain boundary density due to the accumulation of dislocations increases. The effect is saturated and the productivity is low, so it is set to 40 to 80%. The preferred cumulative reduction ratio is in the range of 45 to 75%.

Then, in the hot rolling, the plate thickness center temperature is 700 ° C or higher, and the temperature is cooled to 550 ° C or lower at a plate thickness central cooling rate of 2 to 10 ° C / s.

When the center temperature of the sheet thickness at the start of cooling is less than 700 ° C, it is difficult to increase the crystal grain boundary density because the ferrite grains are metamorphosed and coarsened. When the plate thickness center cooling rate is less than 2 ° C / s, it becomes difficult to increase the crystal grain boundary density. When the plate thickness center cooling rate is more than 10 ° C / s, it is difficult to achieve a steel plate having a plate thickness of 60 mm or more, so this is made the upper limit. When the cooling stop temperature is greater than 550 ° C, the crystal grain boundary density is increased and changed. For the sake of difficulty. Although the necessity of the lower limit of the cooling stop temperature is not particularly specified, since the temperature below the water temperature is impossible, the water temperature or the room temperature is set as the lower limit. The conditions for preferably accelerated cooling are a plate thickness center temperature of 720 ° C or more at the start of cooling, a cooling rate of 3 to 8 ° C / s, and a cooling stop temperature of 500 ° C or less.

Moreover, the steel sheet of the present embodiment can be manufactured by controlling the production using the plate thickness center temperature of the steel sheet. By using the plate thickness center temperature, the steel sheet can be appropriately controlled, and the steel sheet having a small material variation and good quality can be efficiently produced, even when the thickness of the sheet is changed, in comparison with the surface temperature of the steel sheet.

In the rolling step, the temperature distribution inside the steel sheet is calculated while measuring the surface temperature of the steel sheet from the heating to the rolling, and the rolling reaction force is predicted from the calculation result of the temperature distribution. Rolling control. In this way, the center temperature of the steel sheet in the rolling can be easily obtained. In the case of accelerated cooling, it is also controlled to accelerate the cooling while predicting the temperature distribution inside the thickness of the sheet.

After the accelerated cooling is performed, tempering can be performed at 300 to 650 ° C as necessary.

When tempering is performed at less than 300 ° C, it is not easy to obtain the effect of tempering. When the tempering temperature is more than 650 ° C, the amount of softening becomes large and it becomes difficult to ensure strength. The preferred tempering temperature is 400~600 °C.

The steel sheet according to the present embodiment can be applied to a steel sheet having a sheet thickness of 60 to 95 mm and a relief stress of 390 to 690 MPa. In particular, it can be applied to the 390 MPa class of the undulating stress for the manufacture of hull and marine structures. Steel plate of 460 MPa class or higher.

As described above, according to the present embodiment, the Kca exhibiting crack retardation at -20 ° C can be 6000 N. Mm ^-0.5 or more, which improves crack retardation. Further, it is possible to produce a high-strength thick steel plate having excellent crack resistance, which is low in production cost, high in productivity, and not deteriorated in HAZ toughness.

[Examples]

The effects of the present invention will be described based on the following examples.

The composition of the molten steel was adjusted in the steel making step, and then the steel sheets A to Z were produced by continuous casting. Steel sheets A to O are invented steel, and steel sheets P to Z are comparative steel.

In Examples 1 to 20 and Comparative Examples 21 to 55, steel sheets A to Z were reheated, and thick steel sheets were rolled to obtain thick steel sheets having a thickness of 60 to 95 mm, and then the thick steel sheets were water-cooled. However, Comparative Example 53 was subjected to air cooling instead of water cooling. Subsequently, heat treatment is performed as necessary.

Table 1 and Table 2 show the composition of the steel sheets A to Z. The bottom lines of Tables 1 and 2 show that the values are outside the scope of the present invention, and the oblique system shows the analytical value of the amount contained in the form of unavoidable impurities.

The manufacturing method is shown in Tables 3 to 6. For the rolling, a roll mill having a roll radius of 600 mm was used. The productivity is evaluated by the time required from the time of extraction from the heating furnace to the completion of rolling and cooling, and it is preferable that the production time is less than 1000 s. The bottom line of Tables 3 to 6 indicates a poor condition, or indicates that the productivity is a good value from the above-mentioned regulations. Further, the temperature at the temperature of the manufacturing method and the cooling rate are the values at the center position of the thickness of the plate, and are obtained from the measured surface temperature by heat conduction analysis using a well-known differential method.

The micro-structure phase fraction, aggregate structure, crystal grain boundary density, and mechanical properties were measured for each of the manufactured thick steel sheets.

The microstructural phase fraction was obtained by photographing the micro-tissue at a magnification of 500 times using an optical microscope with a half of the thickness of the plate, and calculating the total area of each phase by image analysis and dividing by the measurement area.

The crystal grain boundary density is determined by the EBSD method, and a region of 1/4 of the thickness of the plate and a region of 500 μm × 500 μm of the 1/2 portion are measured at a pitch of 1 μm, and a boundary of a crystal orientation difference of the adjacent particles of 15° or more is defined as a crystal crystal. The boundary is obtained by dividing the total length of the crystal grain boundaries at this time by the measurement area.

When the 1/4 portion of the thickness of the tissue plate is collected, an image of a {100} plane constituting an angle of 15° or less and a thickness of 1/1 of the plate thickness are prepared for each of the faces perpendicular to the main rolling direction of the steel sheet. The image of the {110} plane at the time of 2 parts, and the total area is divided by the measured area to obtain the area ratio of the equal surface.

Among the mechanical properties, the fall stress and the absorbing energy of the base material were tested using a test piece taken from the center portion of the thickness of the plate, and the results were taken as representative values of the respective steel sheets.

The tensile test was carried out in accordance with JIS Z 2241 (1998) "Metal Material Tensile Test Method", and each of the two test pieces was measured and the average value thereof was determined. The tensile test piece was made into a test piece No. 4 of JIS Z 2201 (1998).

The Chabai-type absorption energy system uses a 2 mmV notch-like smashing test piece and tests each of the three pieces at -40 ° C according to JIS Z 2242 (2005) "The slashing test method for metallic materials" and seeks absorption. The average of the energy.

The crack retardation of the base metal is determined by the standard ESSO test of the temperature gradient type (original thickness and plate width is 500 mm) to obtain -20 °C. The crack retards the toughness value Kca.

Joint toughness is made by welding a submerged arc welding method with a heat of 10 kJ/mm to make a butt welded joint, and cutting a 2 mmV notch along a melting line (FL) of a quarter of the thickness of the sheet. The notch of the Chabai type test piece was taken, and the average value of the absorbed energy of each of the three samples was obtained at -20 °C. The Chabai type punching test is based on JIS Z 2242 (2005) "Testing method for metal materials."

The measurement results of the thick steel sheets of Examples 1 to 20 and Comparative Examples 21 to 55 are shown in Table 7. Here, it is stipulated that the energy of the worship type is 100J or more and the Kca is 6000N. Mm ^-0.5 or more is good.

The bottom line conditions in Table 7 are outside the scope of the present invention, or indicate that the characteristics of the steel sheet are out of the above-mentioned specifications to be good values.

In all of Examples 1 to 20, strength, toughness, crack retardation, joint toughness, and productivity were good because all of the conditions of the present invention were satisfied.

In Comparative Examples 21 to 55, since the conditions of the bottom line portion were deviated from the scope of the present invention, good results were not obtained at the following points.

In Comparative Examples 21 to 31, since the component range was deviated from the range of the present invention, at least one of strength, toughness, crack retardation, and joint toughness was problematic.

In Comparative Example 32, since the temperature before heating of the steel sheet was too high, the crystal grain boundary density was small, and the toughness and crack retardation were low.

In Comparative Example 33, since the ambient temperature of the heating furnace was too high, the crystal grain boundary density of the 1/4 portion of the sheet thickness was small, and the crack retardation was low.

In Comparative Example 34, since the heating time was too short, the crystal grain boundary density was small, and the toughness and crack retardation were low.

In Comparative Example 35, since the heating time was too long, the crystal grain boundary density was small, and the toughness and crack retardation were low.

In Comparative Example 36, since the heating extraction temperature was too high, productivity was low, crystal grain boundary density was small, and toughness and crack retardation were low.

In Comparative Example 37, since the heating extraction temperature was too low, the ferrite particles had a high iron fraction and a low strength.

In Comparative Example 38, since the number of passes of less than 3% of the rough rolling was too large, the crystal grain boundary density was small, and the toughness and the crack retardation were low.

In Comparative Example 39, since the number of passes of 3 to 30% of the rough rolling was too small, the crystal grain boundary density of the 1/2 portion of the plate thickness was small, and the crack retardation was low.

Comparative Example 40 is because the number of passes of 3 to 30% of the rough roll is too More, the productive line is significantly lower.

In Comparative Example 41, since the heating extraction temperature was high, the coarse rolling temperature was too high, the crystal grain boundary density was small, the toughness, the crack retardation property, and the productivity were low.

In Comparative Example 42, since the cumulative rolling reduction ratio of the rough rolling was too small, the crystal grain boundary density was small, and the toughness and the crack retardation were low.

In Comparative Example 43, since the number of passes of 3 to 30% of the rough rolling was large, the cumulative rolling reduction ratio of the rough rolling was too large, and the productivity was remarkably low.

In Comparative Example 44, since the finishing rolling temperature was too high, the crystal grain boundary density was small, and the toughness and crack retardation were low.

In Comparative Example 45, since the finishing rolling temperature was too low, the {110} area ratio of the 1/2 portion of the sheet thickness was small, the crack resistance and the productivity were low.

In Comparative Example 46, since the number of passes of the finish rolling was small, the shape ratio was too large, and the {100} area ratio of the 1/4 portion of the plate thickness was small, and the crack resistance was low.

In Comparative Example 47, since the number of passes of the finishing rolling was too large, the productivity was low.

In Comparative Example 48, since the average shape ratio of the finish rolling was too large, the {100} area ratio of the 1/4 portion of the sheet thickness was small, and the crack retardation was low.

In Comparative Example 49, since the average shape ratio of the finishing rolling was too small, the {100} area ratio of the 1/4 portion of the sheet thickness was large, and the crack retardation was low.

In Comparative Example 50, since the cumulative rolling reduction ratio of the finishing rolling was too small, the {100} area ratio of the 1/4 portion of the sheet thickness, the {110} area ratio of the 1/2 portion of the sheet thickness, and the crystal grain boundary density were small. , toughness and low crack resistance.

In Comparative Example 51, since the cumulative rolling reduction ratio of the finishing rolling was too large, the productivity was low.

In Comparative Example 52, since the starting cooling temperature was too low, the {100} area ratio of the 1/4 portion of the sheet thickness, the {110} area ratio of the 1/2 portion of the sheet thickness, and the crystal grain boundary density were small, strength, toughness, and crack. Blocking and low productivity.

Comparative Example 53 was cooled by air cooling, and the {100} area ratio of the 1/4 portion of the sheet thickness, the {110} area ratio of the 1/2 portion of the sheet thickness, and the crystal grain boundary density were small, strength, toughness, and crack. Low retardation.

In Comparative Example 54, since the cooling stop temperature was too high, the crystal grain boundary density was small, and the toughness and the crack retardation were low.

Comparative Example 55 is because the tempering temperature is too high and the strength is low.

From the above examples, it has been confirmed that by applying the present invention, it is possible to provide a high-strength thick steel sheet having excellent crack resistance which is low in production cost, high in productivity, high in strength, thick in sheet thickness, and not deteriorated in HAZ toughness.

Further, the present invention is not limited to the above embodiments. Various modifications can be made without departing from the spirit and scope of the invention.

Industrial availability

According to the present invention, it is possible to provide a high-strength thick steel plate having excellent crack resistance which is low in manufacturing cost, high in productivity, high in strength, thick in plate thickness, and not deteriorated in HAZ toughness.

Claims (8)

A high-strength thick steel plate characterized by having the following composition: C: 0.04 to 0.16%, Si: 0.01 to 0.5%, Mn: 0.75 to 2.5%, Al: 0.001 to 0.1%, Nb : 0.003 to 0.05%, Ti: 0.003 to 0.05%, and N: 0.001 to 0.008%, and the limit P is 0.03% or less, S is 0.02% or less, Cu is 1% or less, Ni is 2% or less, and Cr is 1 % or less, Mo is 0.5% or less, V is 0.15% or less, B is 0.005% or less, Ca is 0.01% or less, Mg is 0.01% or less, and REM is 0.01% or less, and the balance is iron and unavoidable impurities. The carbon equivalent Ceq. of the following formula (1) is 0.30 to 0.50%; and has the following microstructure: containing ferrite iron having an area ratio of 70% or less, and an area ratio of 30% or more The toughness iron; in the 1/4 portion of the plate thickness, the crystal grain boundary density of the total length of the average unit area of the crystal grain boundary having a crystal orientation difference of 15 or more is 400 to 1000 mm/mm ² , and at the same time relative to the pair The main rolling direction is a vertical surface, and the area ratio of the {100} plane constituting the angle within 15° is 10 to 40%; and in the 1/2 part of the thickness of the sheet, the crystal grain boundary density is 300 to 900 mm/ Mm ² , and At the same time, with respect to the surface perpendicular to the main rolling direction, the area ratio of the {110} plane constituting the angle within 15° is 40 to 70%; the lodging stress is 390 MPa or more and 690 MPa or less; Ceq.=C+Mn/ 6+(Cu+Ni)/15+(Cr+Mo+V)/5. . . (1) Formula. For example, the high-strength thick steel plate of the first application of the patent scope, wherein the thickness of the aforementioned plate is 60 to 95 mm. The high-strength thick steel plate according to claim 1 or 2, wherein the micro-structure contains spin iron having an area ratio of 10% or less. For example, the high-strength thick steel plate according to the first or second aspect of the patent application, wherein the micro-structured ferrite grain area ratio is less than 50%, the brite iron area ratio is 5% or less, and the toughened iron area ratio is 50% or more. The high-strength thick steel plate according to claim 1 or 2, wherein the crystal grain boundary density of the 1/4 portion of the plate thickness is 500 to 900 mm/mm ² , and the crystal grain boundary of the plate thickness 1/2 portion The density is 400~800mm/mm ² . The high-strength thick steel plate according to claim 1 or 2, wherein the Cu is further limited to 0.5% or less, the Ni is 1% or less, the Cr is 0.5% or less, the Mo is 0.2% or less, and the V is It is 0.07% or less. The high-strength thick steel plate according to claim 1 or 2 further restricts the above B to 0.002% or less. The high-strength thick steel plate according to claim 1 or 2 further restricts the Ca to 0.003% or less, the Mg to 0.003% or less, and the REM to 0.003% or less.