JP5051001B2 - High-strength thick steel plate with excellent arrest characteristics in the 45 ° direction with respect to the rolling direction and method for producing the same - Google Patents

Description

  In the present invention, the direction of 45 ° with respect to the rolling direction (hereinafter, simply referred to as “45 ° direction with respect to the rolling direction”) when the distance from the central portion of the plate thickness to the thickness direction is the reference. Regarding a high-strength thick steel plate excellent in arrest properties and a method for producing the same, in detail, even if a brittle crack occurs, 45 ° with respect to the rolling direction can prevent collapse of the entire structure. TECHNICAL FIELD The present invention relates to a high-strength thick steel plate having excellent direction arrest characteristics and a method for producing the same.

  The above-mentioned high-strength thick steel plate is intended for those having a plate thickness exceeding 50 mm, and the strength class is for those having a tensile strength of 490 MPa or more.

  The “arrest characteristic” is a characteristic that can stop the propagation of cracks.

  In recent years, as the scale of various steel structures has increased, the thickness and strength required for various steel sheets used as the material have been increasing. In particular, since international commerce has become active in recent years, commercial ships have become larger in size, and for this reason, high-strength thick steel sheets for hull structures having a plate thickness exceeding 50 mm have been required.

  In such a high-strength thick steel plate, since the mechanical restraint force at the time of use also becomes large, there exists a tendency for the further improvement of the characteristic of a plate | board thickness center part to be requested | required. However, the improvement in the characteristics at the center of the plate thickness is still insufficient.

  In all structures, the collapse due to brittle fracture is the type of fracture that should be avoided because the entire structure collapses instantly and enormous damage is assumed.

  Therefore, although the building is designed to avoid the occurrence of brittle fracture, brittle fracture has occurred due to abnormal situations unexpected by the designer, such as the effects of external force exceeding the design and defects caused by construction. It is necessary to consider the case.

  When a brittle fracture occurs, the brittle fracture spreads over the entire structure due to extremely high-speed crack propagation, and thus the entire structure is destroyed. However, a steel material with extremely high resistance to crack propagation has a characteristic (arrest characteristic) that can stop a crack that has progressed.

  And, a structure with this arresting member in place has double safety at each stage of crack generation and propagation, and as a design philosophy of the structure. It is extremely important.

  For example, in the shipbuilding field, there is a direction in which ships are built under a design philosophy based on this double integrity.

  However, as described above, the thickness of the steel material used is increasing with the increase in the size of structures such as commercial ships. For this reason, the request | requirement with respect to the characteristic improvement of a thick-walled steel plate has become more severe in both material characteristics and mechanical characteristics.

  The simplest method for imparting arrest properties to a steel material is to contain Ni, which is an element that significantly improves toughness.

  The effect of improving the arrest property by adding Ni is large. For example, as a steel material that guarantees double integrity in a cryogenic environment of −165 ° C., so-called “9% Ni steel” containing 9% Ni is used. It is general and is also defined in Japanese Industrial Standards (JIS).

Patent Document 1 discloses that a steel having a specific chemical composition is heated at a low temperature to prevent austenite grains from coarsening during the heating, and then an appropriate amount of rolling (so-called “low temperature” is performed at the lowest possible temperature above the Ar ₃ transformation point. After rolling ”), appropriate rolling is performed in the two-phase region between the Ar ₃ transformation point and Ar ₁ transformation point to promote grain refinement and separation on the fracture surface. “Low temperature steel excellent in brittle fracture initiation resistance characteristics and brittle crack stopping characteristics” is disclosed.

In Patent Document 2, water cooling of a slab made of a specific chemical component is started at a temperature of Ac ₃ or more at a cooling rate of 2 ° C./s or more, and the surface layer from the front and back surfaces of the slab to 1/3 of the thickness. The part is cooled to an Ar ₃ point or less, the water cooling is stopped, finish rolling is started until the reheating of the slab is finished, and after the finish rolling is finished, the thickness of the slab is measured from the front and back surfaces. the until 1/3 that recuperator from front and back surfaces of the template pieces 1/3 or more thickness than the Ac ₃ point while recuperation in Ac less than ₃ points, re by elevated temperature rolling in the non-recrystallization region A “method of manufacturing a steel sheet with excellent brittle crack propagation stopping characteristics” is disclosed, in which a fine structure is secured particularly in the vicinity of the surface layer of the steel sheet by the combination of crystal and reverse transformation, and the shear lip acts effectively.

In Patent Document 3, a steel slab composed of a specific chemical component is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1100 ° C., and after rolling at a cumulative rolling reduction at 950 ° C. or lower is 10 to 50%, the area of the upper and lower surface portion corresponding to the thickness from 2 to 33% of at that stage to begin cooling at 2 ° C. / s or more cooling rate from Ar ₃ transformation point or more of the temperature, cooling below Ar ₃ transformation point In the process of stopping reheating and reheating one or more times, finish rolling is completed until the reheating after the last cooling is completed, and the upper and lower surface layer regions of the steel plate after the completion of rolling are defined as Ac. _By reheating below the ₃ transformation point, above the Ac ₃ transformation point, or to the Ac ₃ transformation point and its upper and lower temperature range, and after completion of rolling, by applying accelerated cooling or accelerated cooling and tempering as necessary, The microstructure of the joints to form shear lip during brittle crack propagation To "method for producing a superior steel plate of brittle crack propagation stopping characteristics and low-temperature toughness" is disclosed.

JP 55-148746 A JP-A-3-2322 JP 7-126798 A

  As described above, the improvement effect of the arrest property by Ni is large and the arrest property can be improved. However, Ni is a very expensive element, and if Ni is contained as much as 9%, the steel material cost is increased. . Therefore, improvement of the arrest characteristics by containing Ni has many problems in terms of cost.

  In addition, according to the technique disclosed in Patent Document 1, which is refined by rolling in a two-phase region and promotes the generation of separation, the arrest characteristics are improved without containing expensive elements such as Ni. It is possible to make it. However, in general, under the two-phase region pressure, the so-called “L direction”, which means a direction parallel to the rolling direction, and the rolling direction in the case where the distance from the center of the plate thickness to the plate thickness direction is the same as the reference, The so-called “C direction” arrest characteristic, which means a perpendicular direction, can be improved, but the arrest characteristic in the 45 ° direction with respect to the rolling direction cannot be improved. Moreover, since the direction in which stress is applied in the structure is not necessarily constant, even if the arrestability is improved by the technique disclosed in Patent Document 1, the safety is not sufficient. Furthermore, Patent Document 1 does not take into consideration the improvement of the arrest characteristics of thick materials.

  In FIG. 1, the above-described directions are schematically shown as “L direction”, “C direction”, and “45 ° direction”.

The combination of recrystallization and reverse transformation by hot rolling in an unrecrystallized region disclosed in Patent Document 2 ensures a fine structure especially in the vicinity of the surface layer of the steel sheet, and shear lip formation during brittle crack propagation According to the urging technique, it is possible to improve the arrest characteristics without adding an expensive element such as Ni. However, in this technique, during the water cooling of the slab, the surface layer portion from the front and back surfaces of the slab to 1/3 of the thickness is cooled to the Ar ₃ point or less to stop the water cooling, and the slab is reheated. Finishing rolling is started until the end, and after finishing rolling, up to 1/3 of the thickness from the front and back surfaces of the slab is reheated to less than Ac ₃ point, and the thickness from the front and back surfaces of the slab is reduced. This requires a costly and complicated process of recovering 1/3 or more to Ac ₃ or more.

  Further, the technique disclosed in Patent Document 3 that makes the surface layer structure of a steel sheet ultrafine-grained to promote the formation of shear lip at the time of brittle crack propagation can be achieved without adding expensive elements such as Ni. It is possible to improve the characteristics. However, even in this technique, since the surface structure of the steel sheet is made ultrafine by cooling and reheating during rolling, a complicated and expensive process is required.

  In addition, in the case of a thick material, an auxiliary heating step is an indispensable requirement in order to realize midway cooling with recuperation. Therefore, the techniques disclosed in Patent Document 2 and Patent Document 3 have poor feasibility because they are unavoidably caused by an increase in equipment cost and energy cost, particularly when applied to thick materials.

  Thus, conventionally, it has been difficult to provide a high-strength thick steel plate that is excellent in arrest properties, especially in the 45 ° direction with respect to the rolling direction, at low cost.

In view of such a situation, the present invention is a high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction, in particular, the arrest characteristic at −10 ° C. in the rolling direction is 6000 N / mm ^1.5 or more. And it aims at providing the high intensity | strength thick steel plate excellent in the arrest characteristic of a 45 degree direction with respect to a rolling direction which is 490 Mpa or more in tensile strength (TS) at low cost. It is also an object of the present invention to provide a method for producing the high-strength thick steel plate.

  In order to solve the above problems, the present inventors have conducted various studies and experiments. As a result, the knowledge shown in the following (a) to (j) was obtained.

  (A) In order to provide a high-strength thick steel plate excellent in arrest characteristics in the 45 ° direction with respect to the rolling direction at a low cost, a method of containing a large amount of expensive alloy components cannot be employed.

(B) As another method capable of imparting arrest properties to a thick steel plate, a thermal processing control method (TMCP (Thermo-Mechanical Control Process) method) utilizing the effect of fine graining caused by the reduction of the non-recrystallized region. Can be applied. However, in the thick steel plate to which the normal TMCP method is applied, the fine graining effect due to the reduction in the non-recrystallized region does not penetrate to the center of the plate thickness, so the structure size of the center of the plate thickness tends to be coarsened, The Charpy impact characteristics at the center of the plate thickness are such that the distance from the center of the plate thickness to the plate thickness direction is 1/4 of the plate thickness (hereinafter referred to as “plate thickness 1/4 position”) or The tendency to be worse than the Charpy impact property in the surface layer portion becomes remarkable. Furthermore, the central part of the plate thickness may have a low processing penetration, and the upper bainite structure is mainly used. Generally, the toughness of the upper bainite structure is lower than that of the fine-grained ferrite structure due to the influence of a hard structure between laths called “MA”. As described above, within the range of the TMCP method that has been widely used so far, even when the TMCP conditions are variously adjusted, the arrest characteristic at −10 ° C. is 4000 N due to the lack of toughness in the central structure of the plate thickness. / Mm ^{stays around 1.5} . For this reason, the target value of 6000 N / mm ^1.5 or more is not reached. Therefore, it is necessary to conduct experiments not only in general-purpose TMCP conditions but also in a wider range of TMCP conditions.

(C) Therefore, the present inventors deviated from general-purpose TMCP conditions and conducted various experiments under a wider range of TMCP conditions. As a result, the tensile strength (490 MPa or more), which is the target strength of a high-strength thick steel plate, In order to realize (TS), it is necessary to control the chemical composition with appropriate hardenability, and as an index of hardenability,
Ceq = C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5) + (Mo / 5) + (V / 5) (1)
It has been found that the carbon equivalent Ceq represented by the formula can be used, and the carbon equivalent Ceq needs to be 0.32 to 0.40%.

  However, C, Mn, Cu, Ni, Cr, Mo and V in the above formula (1) mean the content (% by mass) of each element.

  (D) And, the control of the heating condition of the steel ingot which is the material of the thick steel plate is extremely effective. In particular, by lowering the temperature or shortening the time, the ferrite transformation is caused at the transformation after the rolling. It has been found that the initial austenite grain size can be made fine.

Specifically, in the heating process of the steel ingot that is the material of the thick steel plate, the heating temperature Tr (° C.) and the heating time t (h) of the steel ingot are:
^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2)
It is preferable to lower the temperature and shorten the time by heating the steel ingot so as to satisfy the following formula.

  The “steel ingot” is a material that is supplied to each process such as rolling, forging, and extrusion, and is a slab that is manufactured by continuous casting and supplied to the next process by omitting the ingot process. (Continuous cast steel pieces) are also included.

  (E) Next, the ferrite structure fraction in the central portion of the thickness of the thick steel plate obtained after rolling is 80% or more, and the effective crystal grain size in the central portion of the thickness is 25 μm or less. It was found that it is effective for improving the toughness of thick steel plates. Here, the “effective crystal grain” means a crystal grain when a boundary having an orientation difference of 15 ° or more is regarded as a crystal grain boundary using the EBSP method (Electron Back Scattering Pattern Method). This means “effective crystal grain size” means the diameter of a circle having an area equivalent to the area of the effective crystal grain.

  Thus, in order to make the thick steel plate obtained after rolling have a ferrite structure fraction of 80% or more and an effective crystal grain size of 25 μm or less at the center of the plate thickness, the rolling temperature must be lowered. It is effective to control the adjustment plate thickness and the rolling temperature and finish rolling temperature. This is because the dislocation density in austenite before transformation can be increased by increasing the adjustment plate thickness and lowering the temperature.

Specifically, in the rolling process, the finishing at the time of finishing the rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling and the thickness G (mm) of the thick steel plate of the final shape by final rolling. Rolling temperature C (° C.)
A-3.5G ≦ 0 (3)
A-1.5G ≧ 0 (4)
C-670-G ≦ 0 (5)
BC-20- (1400 / G) ≦ 0 (6)
It is preferable to perform rolling so as to satisfy all of the following formulas.

(F) Furthermore, control of the cooling rate and cooling stop temperature in the cooling step after rolling is also effective, and the water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness are ,
E-500 ≦ 0 (7)
F-2 ≧ 0 (8)
It is preferable to perform water cooling so as to satisfy these two equations.

(G) In addition, when tempering at a temperature of Ac ₁ point or less after cooling, the hardened structure in the bainite structure may be partially detoxified.

  (H) As described above, the steel sheet in which the refinement of the ferrite structure is ensured at the center of the sheet thickness exhibits extremely good arrest characteristics despite the thickness, and sufficiently reaches the target characteristics. However, when the heat rolling conditions are inappropriate and the structure size at the center of the plate thickness is coarse, the toughness deteriorates even when the ferrite structure fraction is high.

  (I) By increasing the number of nucleation sites when transforming from austenite to ferrite and making it a fine ferrite-based structure, the austenite orientation is taken over, so the ratio of the bainite structure that tends to form a texture during transformation is low. Thus, the crystal orientation is randomized, so that deterioration of the arrest characteristic can be reduced even in the direction where the arrest characteristic is generally inferior, that is, in the direction of 45 ° with respect to the rolling direction.

  (J) Since the typical cleavage plane in α-iron is the (321) plane, the (211) plane, and the (110) plane, the crystal orientation is randomized to improve the arrest characteristics in the 45 ° direction with respect to the rolling direction. For this purpose, it is necessary to reduce the X-ray intensity ratio of the (321) plane, (211) plane, and (110) plane in the 45 ° direction with respect to the rolling direction to a specific value or less.

  Note that the X-ray intensity ratios of the (321) plane, (211) plane, and (110) plane are the (321) plane, (211) plane, and (110) of the sample taken from a direction of 45 ° with respect to the rolling direction. ) Means the ratio between the X-ray intensity at the plane and the X-ray intensity of a non-oriented sample having a random crystal orientation.

  The high-strength thick steel plate excellent in arrest characteristics in the 45 ° direction with respect to the rolling direction and the method for producing the same according to the present invention have been completed based on such knowledge.

  Here, the gist of the present invention is the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction shown in the following (1) to (6) and the rolling direction shown in (7) and (8). On the other hand, it is in a method for producing a high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction.

(1) In mass%, C: 0.01 to 0.12%, Si: 0.50% or less, Mn: 0.4 to 2%, P: 0.05% or less, S: 0.008% or less Al: 0.002 to 0.05%, N: 0.01% or less, Nb: 0.003 to 0.1%, and having a chemical composition consisting of the balance Fe and impurities,
Ceq = C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5) + (Mo / 5) + (V / 5) (1)
The carbon equivalent Ceq represented by the formula is 0.32 to 0.40%, the ferrite structure fraction in the center portion of the plate thickness is 80% or more, and the grain size of the effective crystal grains in the center portion of the plate thickness is 25 μm or less. In addition, the sum of the X-ray intensity ratios of the (321) plane, (211) plane and (110) plane in the 45 ° direction at the plate thickness 1/4 position and the (321) plane in the 45 ° direction at the center of the plate thickness ( 211) plane and (110) plane X-ray intensity ratio sum of arithmetic average value is 3.3 or less, high strength thick steel plate having excellent arrest characteristics in 45 ° direction with respect to rolling direction .
However, C, Mn, Cu, Ni, Cr, Mo, and V in the above formula (1) mean the content (mass%) of each element.
Further, the meaning of the sum of the effective crystal grains, the grain diameter and the 1/4 thickness position of the effective crystal grains, and the X-ray intensity ratio of the (321) plane, (211) plane and (110) plane in the 45 ° direction is Each is as follows.
Effective grain: A grain when a boundary having an orientation difference of 15 ° or more is regarded as a grain boundary using the EBSP method,
Effective grain size: diameter of a circle having an area equivalent to the area of the effective grain,
Plate thickness 1/4 position: the position where the distance from the plate thickness center of the thick steel plate to the plate thickness direction is 1/4 of the plate thickness,
Sum of X-ray intensity ratios of (321) plane, (211) plane and (110) plane in 45 ° direction: with respect to the rolling direction when the distance from the center of the plate thickness to the plate thickness direction is the standard The sum of the ratios of the X-ray intensity of the (321) plane, (211) plane and (110) plane of the sample collected from the direction of 45 ° to the X-ray intensity of the non-oriented sample having a random crystal orientation.

  (2) The high-strength thick steel plate having excellent arrest properties in the 45 ° direction with respect to the rolling direction according to (1) above, further comprising Ni: 1% or less by mass%.

  (3) The rolling according to (1) or (2) above, wherein the rolling further contains one or two elements out of elements of Cu: 2% or less and Cr: 1% or less in mass%. A high-strength thick steel plate with excellent arrest characteristics in the 45 ° direction.

  (4) It is characterized by containing 1 or 2 or more of elements of Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% or less. A high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction according to any one of (1) to (3).

  (5) Arrest characteristics in the direction of 45 ° with respect to the rolling direction according to any one of (1) to (4) above, characterized by further containing Ti: 0.1% or less by mass%. High strength and thick steel plate.

  (6) It is characterized by further containing one or more elements out of elements of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less by mass%. A high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction according to any one of (1) to (5).

(7) A steel ingot having the chemical composition according to any one of (1) to (6) above is heated, rolled, and cooled by the following steps 1 to 3 in a rolling direction. On the other hand, a method for producing a high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction.
[Step 1] The heating temperature Tr (° C.) and heating time t (h) of the steel ingot are
^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2)
The step of heating the steel ingot so as to satisfy the following formula.
[Step 2] Rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling, and finish rolling temperature C (° C.) when finishing to a thickness G (mm) of a thick steel plate having a final shape by final rolling But,
A-3.5G ≦ 0 (3)
A-1.5G ≧ 0 (4)
C-670-G ≦ 0 (5)
BC-20- (1400 / G) ≦ 0 (6)
The process of rolling to satisfy all of the formulas.
[Step 3] The water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness are:
−150 ≦ E−500 ≦ 0 (7)
F-2 ≧ 0 (8)
The process of performing water cooling so that these two formulas may be satisfied.

(8) The steel ingot having the chemical composition according to any one of (1) to (7) is heated, rolled, cooled, and tempered by the following steps 1 to 4. A method for producing a high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction.
[Step 1] The heating temperature Tr (° C.) and heating time t (h) of the steel ingot are
^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2)
The step of heating the steel ingot so as to satisfy the following formula.
[Step 2] Rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling, and finish rolling temperature C (° C.) when finishing to a thickness G (mm) of a thick steel plate having a final shape by final rolling But,
A-3.5G ≦ 0 (3)
A-1.5G ≧ 0 (4)
C-670-G ≦ 0 (5)
BC-20- (1400 / G) ≦ 0 (6)
The process of rolling to satisfy all of the formulas.
[Step 3] The water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness are:
−150 ≦ E−500 ≦ 0 (7)
F-2 ≧ 0 (8)
The process of performing water cooling so that these two formulas may be satisfied.
[Step 4] Tempering at a temperature of Ac ₁ point or less.

  “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM.

  Hereinafter, the invention relating to the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction shown in the above (1) to (6) and the 45 ° direction with respect to the rolling direction shown in (7) and (8) The inventions relating to the method for producing a high-strength thick steel plate having excellent arrest properties are referred to as “present invention (1)” to “present invention (8)”, respectively. Also, it may be collectively referred to as “the present invention”.

The high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction of the present invention is easy to produce on an industrial scale at low cost, and has an arrest characteristic of −10 ° C. in the rolling direction of 6000 N. / Mm ^1.5 or more, and even when a brittle crack is generated, it is possible to prevent the entire structure from collapsing. Therefore, it can be used as a material for various steel structures, particularly large commercial ships. And the high-strength thick steel plate excellent in the arrest characteristic of a 45 degree direction with respect to this rolling direction can be manufactured by the method of this invention.

  Below, the high-strength thick steel plate of this invention and its manufacturing method are demonstrated in detail. As described above, the high-strength thick steel plate of the present invention has a thickness of 50 mm or more, but if the manufacturing method of the present invention is used, an extremely thick thick steel plate having a thickness of 65 mm or more is also usefully manufactured. can do. In addition, although the upper limit of plate | board thickness is not defined, it will be 85 mm or less from restrictions on manufacture.

(A) About chemical composition:
Hereinafter, “%” representing the chemical composition means “% by mass” unless otherwise specified.

C: 0.01 to 0.12%
C is an element necessary for ensuring strength. Unless 0.01% or more is contained, steel having practical strength cannot be produced. On the other hand, if the content exceeds 0.12%, the toughness deterioration of the bainite transformation region becomes remarkable and the toughness of the weld heat affected zone is also impaired. Therefore, the C content is set to 0.01 to 0.12%. A preferable range of the C content from the point of balance between strength and arrest characteristics is 0.03 to 0.10%.

Si: 0.50% or less Si is an element necessary for deoxidation in the refining stage and contributes to an increase in strength. However, if the Si content exceeds 0.50%, the formation of island martensite in the weld heat affected zone is promoted, and the toughness is adversely affected. Therefore, the Si content is 0.50% or less. A preferable Si content is 0.30% or less.

  In addition, in order to express the above-mentioned effect of Si reliably, it is preferable to contain 0.03% or more of Si. For this reason, the Si content is more preferably 0.03 to 0.50%, and even more preferably 0.03 to 0.30%.

Mn: 0.4-2%
Mn is an element necessary for ensuring strength. However, if the content is less than 0.4%, these effects cannot be obtained. On the other hand, when the content of Mn exceeds 2%, the toughness of the weld heat affected zone is significantly deteriorated. Therefore, the Mn content is set to 0.4 to 2%. The minimum with preferable Mn content is 0.6%, and a preferable upper limit is 1.6%.

P: 0.05% or less P is present in steel as an impurity and causes intergranular cracking in the weld heat affected zone. When the content of P increases, especially when it exceeds 0.05%, the occurrence of intergranular cracks in the weld heat affected zone becomes significant. Therefore, the P content is 0.05% or less. In addition, it is preferable to make the mixing amount as low as possible. In order to stably obtain good arrest characteristics, the P content is preferably 0.03% or less.

S: 0.008% or less S is an element which exists in steel as an impurity and forms MnS which becomes a base point of brittle fracture and impairs arrest properties. In particular, when the S content exceeds 0.008%, the deterioration of the arrest characteristics becomes significant. For this reason, content of S shall be 0.008% or less. In addition, the mixing amount is preferably as low as possible. In order to stably obtain good arrest characteristics, the S content is preferably less than 0.003%.

Al: 0.002 to 0.05%
Al is an element necessary for deoxidation of steel, and in the case of the steel according to the present invention, a content of 0.002% or more is necessary. However, when the Al content increases, particularly when it exceeds 0.05%, the deterioration of arrest properties becomes noticeable through the increase of precipitates. Therefore, the Al content is 0.002 to 0.05%. The range of preferable Al content is 0.002 to 0.04%.

N: 0.01% or less N is an element that exists in steel as an impurity and causes toughness deterioration by forming precipitates. For this reason, in order to ensure low temperature toughness, it is better that the N content is low. In particular, when the N content exceeds 0.01%, the arrest characteristics are significantly deteriorated. Therefore, the N content is 0.01% or less. The preferable N content is 0.006% or less.

Nb: 0.003 to 0.1%
Nb is an element effective for refinement of structure, improvement of toughness, improvement of hardenability and strength increase by precipitation hardening, and in particular, a steel material to which the TMCP method is applied because it has a large effect of expanding the non-recrystallized region. Is a necessary element. The above effect is exhibited when the Nb content is 0.003% or more. However, if the Nb content exceeds 0.1%, the toughness is deteriorated due to an increase in precipitates. Therefore, the Nb content is set to 0.003 to 0.1%. The preferable range of Nb content is 0.003 to 0.04%.

  The main component of the balance of the present invention (1) is composed of Fe, but contains other impurities due to various factors in the manufacturing process.

  Since the high-strength thick steel plate according to the present invention (1) is used as a strength member, it is necessary to have sufficient strength as a standard material, but C, Si, Mn, P, S, Al, Even if the contents of N and Nb are within the above-described range and the balance has a chemical composition composed of Fe and impurities, the strength may not be ensured. For this reason, the chemical composition of the high-strength thick steel plate of the present invention (1) not only satisfies each specified range but also needs to have appropriate hardenability.

Carbon equivalent can be used as a parameter representing the hardenability of the high-strength thick steel plate. In particular, in the case of a high-strength thick steel plate of a strength class having a tensile strength of 490 MPa or more, IIW (international welding) Association),
Ceq = C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5) + (Mo / 5) + (V / 5) (1)
The carbon equivalent formula can be used.
Here, C, Mn, Cu, Ni, Cr, Mo, and V in the above formula (1) mean the content (% by mass) of each element.

  And in order to ensure sufficient intensity | strength to a high intensity | strength thick steel plate, the carbon equivalent Ceq represented by the said (1) formula must be 0.32% or more. On the other hand, if the carbon equivalent Ceq increases and exceeds 0.40%, the hardenability becomes excessive and particularly the toughness of the weld heat affected zone is significantly impaired. Therefore, the carbon equivalent Ceq represented by the formula (1) needs to satisfy 0.32 to 0.40%. A preferred range for the carbon equivalent Ceq is 0.32 to 0.38%.

  For the above reasons, the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction according to the present invention (1) contains the elements from C to Nb in the above-described range, with the balance being Fe and impurities. And the carbon equivalent Ceq represented by the above formula (1) is defined to be 0.32 to 0.40%.

  Note that the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction of the present invention may further include Ni, Cu, Cr, Mo, V, B, Ti, Ca, Mg, and REM as necessary. One or more elements selected from among them can be contained.

  Hereinafter, the above optional elements will be described.

Ni: 1% or less Ni has an effect of improving the arrest properties of the steel sheet, and may be contained in order to obtain this effect. However, the content of Ni becomes a cost increase factor. For this reason, the amount of Ni when contained is 1% or less. The preferable amount of Ni when contained is 0.6% or less. In order to surely exhibit the effect of improving the arrest characteristics by Ni, it is preferable to contain Ni by 0.03% or more.

  Even in the case where Ni is contained, the carbon equivalent Ceq represented by the formula (1) needs to satisfy 0.32 to 0.40% within the above range.

  This is because when the carbon equivalent Ceq is lower than 0.32%, sufficient strength is not ensured. On the other hand, when the carbon equivalent Ceq is higher than 0.40%, the hardenability becomes excessive, and the ferrite in the central portion of the plate thickness. This is because the tissue fraction decreases.

  Since Cu and Cr have a function of increasing the strength, the above elements may be contained in order to obtain this effect. Hereinafter, the above Cu and Cr will be described in detail.

Cu: 2% or less Cu has an effect of improving strength without deteriorating toughness. However, if the content exceeds 2%, the arrest properties are deteriorated due to an increase in precipitates, and further, minute cracks are generated on the surface during hot working. Therefore, the amount of Cu in the case of containing is 2% or less. The upper limit with the preferable quantity of Cu in the case of making it contain is 1%. In addition, in order to express the strength improvement effect by Cu reliably, it is preferable to contain 0.03% or more of Cu.

Cr: 1% or less Cr has an action of increasing strength. However, if the content exceeds 1%, the toughness is deteriorated, and further, a hardened structure is formed in the weld heat affected zone to deteriorate the toughness. Therefore, the amount of Cr when contained is 1% or less. The upper limit with the preferable amount of Cr in the case of making it contain is 0.6%. In addition, in order to express the strength improvement effect by Cr reliably, it is preferable to contain Cr 0.05% or more.

  In addition, although said Cu and Cr can be contained only in any 1 type of them, or 2 types of composites, the carbon equivalent shown by said Formula (1) in the above-mentioned range also in that case Ceq needs to satisfy 0.32 to 0.40%.

  As already described, when the carbon equivalent Ceq is lower than 0.32%, sufficient strength is not secured, whereas when it is higher than 0.40%, the hardenability becomes excessive. This is because the ferrite structure fraction at the center of the plate thickness is lowered.

  Since Mo, V, and B have the effect of improving hardenability and increasing strength, the above elements may be included in order to obtain these effects. Hereinafter, the above Mo, V and B will be described in detail.

Mo: 0.5% or less Mo has an effect of improving hardenability and strength. However, the content of Mo becomes a cost increase factor, and if the content exceeds 0.5%, the toughness of the weld heat affected zone is deteriorated. Therefore, the amount of Mo when contained is 0.5% or less. The upper limit with the preferable amount of Mo in the case of making it contain is 0.3%. In addition, in order to express the hardenability and strength improvement effect by Mo reliably, it is preferable to contain 0.02% or more of Mo.

V: 0.1% or less V has an effect of improving strength by improving hardenability and precipitation hardening. However, if the V content exceeds 0.1%, the toughness is significantly deteriorated. For this reason, the amount of V in the case of making it contain shall be 0.1% or less. The upper limit with preferable V amount in the case of making it contain is 0.06%. In order to ensure the effect of improving the hardenability and strength by V, it is preferable to contain V by 0.003% or more.

B: 0.005% or less B has the effect of suppressing the ferrite transformation from the austenite grain boundary and improving the hardenability. B also has the effect of increasing strength. However, if the B content exceeds 0.005%, the toughness deteriorates. Therefore, the amount of B when contained is 0.005% or less. The upper limit with the preferable amount of B in the case of making it contain is 0.0015%. In order to ensure the effect of improving hardenability and strength by B, it is preferable to contain B by 0.0003% or more.

  In addition, although said Mo, V, and B can be contained only in one of those, or 2 or more types of composites, in that case, in the above-mentioned range, it is shown by said Formula (1). The carbon equivalent Ceq must be 0.32 to 0.40%.

  As already described, when the carbon equivalent Ceq is lower than 0.32%, sufficient strength is not secured, whereas when it is higher than 0.40%, the hardenability becomes excessive. This is because the ferrite structure fraction at the center of the plate thickness is lowered.

Ti: 0.1% or less Ti has the effect of forming oxide particles to refine the structure, and increasing the hot ductility to prevent cracking of a steel ingot produced by continuous casting. You may make it contain in order to obtain. However, if the Ti content exceeds 0.1%, TiC is generated and the toughness is deteriorated. For this reason, the amount of Ti when contained is 0.1% or less. The upper limit with preferable Ti content in the case of making it contain is 0.04%. In addition, in order to express these effects by Ti reliably, it is preferable to contain Ti 0.003% or more.

  Since Ca, Mg, and REM have an inclusion shape control effect, the above elements may be included to obtain this effect. Hereinafter, the above Ca, Mg, and REM will be described in detail.

Ca: 0.004% or less Ca has an effect of controlling the form of inclusions, thereby improving the arrest characteristics. However, if the Ca content exceeds 0.004%, the cleanliness of the steel itself is greatly reduced. For this reason, the quantity of Ca in the case of making it contain shall be 0.004% or less. The upper limit with the preferable amount of Ca in the case of making it contain is 0.002%. In addition, in order to express the said effect by Ca reliably, it is preferable to contain 0.0003% or more of Ca.

Mg: 0.002% or less Mg has a shape control effect of inclusions. Specifically, by containing Mg, the dispersion density of the fine oxide can be increased, whereby the toughness of the weld heat affected zone can be increased. However, if the Mg content exceeds 0.002%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced. Therefore, the amount of Mg in the case of inclusion is 0.002% or less. The upper limit with the preferable amount of Mg in the case of making it contain is 0.0015%. In addition, in order to make the above-mentioned effect by Mg surely manifest, it is preferable to contain 0.0002% or more of Mg. Here, it is preferable to perform the process of containing Mg in molten steel before containing Al in molten steel.

REM: 0.002% or less REM also has a shape control effect of inclusions. Specifically, the dispersion density of the fine oxide can be increased by including REM, and thereby the toughness of the weld heat affected zone can be increased. Moreover, the effect of fixing excess S as a sulfide can also be obtained by including REM. However, if the REM content exceeds 0.002%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced. For this reason, the amount of REM in the case of making it contain shall be 0.002% or less. The upper limit with the preferable amount of REM in the case of making it contain is 0.0015%. In addition, in order to express these effects by REM reliably, it is preferable to contain REM 0.0002% or more.

  As described above, the above “REM” is a generic name of a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of one or more elements of REM. .

  In addition, it is preferable to perform the process of containing REM in molten steel, before making Al contain in molten steel, and it may isolate | separate into each REM element and may make it contain in steel, and the state called the mixture of misch metal And may be contained in the steel.

  For the above reasons, the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction according to the present invention (2) is excellent in the arrest characteristics in the 45 ° direction with respect to the rolling direction of the present invention (1). Further, it was specified that Ni: 1% or less was further contained in the high-strength thick steel plate.

  Similarly, the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction according to the present invention (3) is the arrest characteristic in the 45 ° direction with respect to the rolling direction of the present invention (1) or (2). It was specified that the high-strength thick steel plate excellent in the above contained one or two elements out of Cu: 2% or less and Cr: 1% or less.

  The high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction according to the present invention (4) is an arrest in the 45 ° direction with respect to any of the rolling directions from the present invention (1) to (3). The high strength thick steel plate having excellent characteristics further contains one or more elements of Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% or less. Stipulated.

  The high-strength thick steel plate excellent in the arrest characteristics in the 45 ° direction with respect to the rolling direction according to the present invention (5) is excellent in the arrest characteristics in the 45 ° direction with respect to the rolling directions from the present invention (1) to (4). It was specified that any of the high-strength thick steel plates contained Ti: 0.1% or less.

  The high-strength thick steel plate excellent in the arrest characteristics in the 45 ° direction with respect to the rolling direction according to the present invention (6) is excellent in the arrest characteristics in the 45 ° direction with respect to the rolling directions from the present invention (1) to (5). Further, any one of the elements of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less was contained in any of the high-strength thick steel plates. Stipulated.

(B) Ferrite structure fraction at the center of the plate thickness:
The central part of the plate thickness may have a low processing penetration, and the upper bainite structure is the main component. In general, the upper bainite structure has lower toughness than the fine-grained ferrite structure due to the influence of the hard structure between the laths called “MA”.

  Thus, since the toughness tends to deteriorate as the upper bainite structure increases, it is necessary to increase the ferrite structure. Examples of methods for increasing the ferrite structure include an increase in austenite grain boundaries due to low-temperature heating and an expansion of the ferrite generation range due to processing induction.

  As a result of testing on steels having various ferrite ratios, the present inventors have found that steels having a ferrite structure fraction of 80% or more at the center of the plate thickness possess excellent arrest properties. Therefore, the ferrite structure fraction at the center of the plate thickness is defined as 80% or more. The ferrite structure fraction is preferably 85% or more.

  The ferrite structure fraction can be evaluated based on observation using a scanning electron microscope or a transmission electron microscope having an acceleration voltage of 100 to 200 kV in addition to the optical microscope. The ferrite structure fraction here is evaluated by the area ratio of ferrite. Specifically, with respect to 100 visual fields observed by the above-described observation method, after calculating the area ratio of ferrite to the total visual field area in each visual field, the average value of the area ratios of ferrite of 100 visual fields is obtained.

  In addition, it is thought that an arrest characteristic improves, so that the ferrite structure fraction of the said plate | board thickness center part is high. However, in order to increase the ferrite structure fraction while ensuring the strength, it is necessary to reduce the C content and add other alloy elements, which leads to an increase in cost. For this reason, it is preferable that the ferrite structure fraction of the center portion of the plate thickness is 95% or less.

(C) About the grain size of the effective crystal grains at the center of the plate thickness:
The toughness of the high-strength thick steel plate is defined by the fact that the ferrite structure fraction at the center of the thickness of the thick steel plate obtained after rolling is defined as 80% or more, and the effective grain size at the center of the plate thickness is 25 μm or less. It turns out that it improves by doing.

  As described above, the effective crystal grain indicates a crystal grain when a boundary having an orientation difference of 15 ° or more is regarded as a crystal grain boundary using the EBSP method.

  Specifically, using the EBSP method, observations of 5 fields or more were performed at a magnification of 300 times, and a boundary having an orientation difference of 15 ° or more was regarded as a crystal grain boundary. And the area inside each effective crystal grain was calculated | required, and the diameter of the circle which has an area equivalent to the area was evaluated as a particle size of each effective crystal grain.

  This is because when the effective crystal grain size is quantified based on the grain boundary recognized by an optical microscope or scanning electron microscope, it corresponds to the fracture surface unit when the orientation difference between adjacent crystal grains is small. This is because the value is not good and cannot be a numerical value representative of the tissue size.

  Therefore, in the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction of the present invention, it is defined that the effective crystal grain size at the center of the plate thickness is 25 μm or less.

  The effective crystal grain size at the center of the plate thickness is better as it is finer. However, the realization of ultrafine grains of less than 5 μm generally requires special production conditions, which leads to a reduction in production efficiency. The thickness is preferably 5 μm or more. More preferably, it is 15 μm or more.

(D) X-ray intensity ratio:
The arrest characteristic in the direction of 45 ° with respect to the rolling direction is the sum of the X-ray intensity ratios of the (321) plane, (211) plane and (110) plane in the 45 ° direction at the quarter thickness position and the central portion of the thickness. When the arithmetic average value with the sum of the X-ray intensity ratios of the (321) plane, (211) plane and (110) plane in the 45 ° direction is 3.3 or less, it is remarkably improved.

  The reason for this is not necessarily clear, but if the structure is mainly composed of fine ferrite by increasing the number of nucleation sites when transforming from austenite to ferrite, the orientation of austenite is taken over, so that a texture is easily formed during transformation. The ratio of the bainite structure is lowered, the crystal orientation is randomized, and the accumulation of cleaved surfaces in the direction where the arrest properties are generally inferior, that is, the direction of 45 ° with respect to the rolling direction is alleviated, and in the thickness direction As a result, it is estimated that the arrest characteristics in the 45 ° direction with respect to the rolling direction are improved. And, in the case of directly under the surface of the steel sheet, the temperature history during rolling is greatly different from the inside of the steel sheet, so the X-ray intensity ratio varies greatly. When the sample is taken from the center, the variation in the X-ray intensity ratio is small, so it is considered that the arrest characteristic in the 45 ° direction with respect to the rolling direction and the arithmetic average value of the X-ray intensity ratio show a good correlation. .

  The X-ray intensity ratio of the (321) plane, (211) plane and (110) plane in the 45 ° direction at the plate thickness ¼ position and the plate thickness center is 45 ° with respect to the rolling direction. Means the ratio of the X-ray intensity at the (321) plane, (211) plane and (110) plane of the sample taken from the direction to the X-ray intensity of the non-oriented sample with random crystal orientation. It can be measured using a line diffractometer.

  For the above reasons, in the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction of the present invention, the (321) plane, (211) plane in the 45 ° direction at the ¼ thickness position, and The arithmetic average value of the sum of the X-ray intensity ratios of the (110) plane and the sum of the X-ray intensity ratios of the (321) plane, (211) plane, and (110) plane in the 45 ° direction at the center of the plate thickness is 3. It was specified that it was 3 or less.

  When the arithmetic average value of the X-ray intensity ratio is less than 0.5, there is a possibility that a texture develops in directions other than the 45 ° direction with respect to the rolling direction. For this reason, it is preferable to set it as 0.5 or more.

(F) Manufacturing conditions The manufacturing conditions described in detail below are one of the methods for economically realizing a high-strength thick steel plate excellent in arrest characteristics in the 45 ° direction with respect to the rolling direction described above. Yes, the technical scope of the thick steel plate itself is not stipulated by the manufacturing conditions.

  The control of the heating conditions of the steel ingot, which is the material of the thick-walled steel plate, that is, the control of the heating temperature and the heating time, is the main manufacturing condition that controls the refinement of the initial austenite grain size during reheating of the steel ingot. It is extremely important in the invention.

  Heating at a high temperature or heating for a long time promotes the growth of austenite grains, so that the number of ferrite nuclei during ferrite transformation decreases, the ferrite structure fraction in the final structure decreases, and the waiting time during rolling As a result, the economic efficiency is impaired. Therefore, it is necessary to control the heating temperature low and the heating time short. However, since temperature and time are equivalent, it is sufficient to satisfy one of the conditions.

  That is, by lowering the heating temperature or shortening the heating time, ferrite transformation can occur during transformation after rolling, and the initial austenite grain size can be made fine.

When the above equivalence was experimentally clarified, the heating temperature Tr (° C.) and the heating time t (h) of the steel ingot in the heating process of the steel ingot, which is the material of the thick steel plate,
^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2)
It has been found that satisfying the following formula is preferable as a condition for producing an economically excellent thick high arrested steel sheet.

  In addition, when the heating temperature is extremely low, it becomes difficult to realize rolling due to an increase in deformation resistance or the like. Therefore, the heating temperature is preferably 800 ° C. or higher. However, the heating temperature is preferably 1050 ° C. or lower.

The value of ^{[t × exp (Tr 3/270000000} ) ] described above, to avoid inhibition of rolling efficiency due to an increase in deformation resistance, preferably 50 or more.

  Next, as a method for economically obtaining a thick high arrested steel plate, it is also effective to control the adjustment plate thickness, rolling temperature, and finish rolling temperature in the subsequent rolling process. By applying TMCP technology that promotes ferrite transformation by increasing the amount of rolling in the non-recrystallized region and increasing the dislocation density in the austenite before ferrite transformation, it is sufficient even at the thickness center of thick materials This is because a good ferrite transformation can be expected. As manufacturing parameters for controlling the amount of reduction in the non-recrystallized region, it has been found that the adjustment plate thickness, the rolling start temperature during adjustment, and the finishing temperature are important.

The conditions obtained by many experiments by the present inventors are, in the rolling process, rolling temperature (adjusting rolling temperature) B (° C.) at an arbitrary thickness (adjusted plate thickness) A (mm) during rolling, The finishing rolling temperature C (° C.) when finishing to the thickness G (mm) of the thick steel plate of the final shape by the final rolling,
A-3.5G ≦ 0 (3)
A-1.5G ≧ 0 (4)
C-670-G ≦ 0 (5)
BC-20- (1400 / G) ≦ 0 (6)
It is rolling so that all the formulas may be satisfied.

  If any one of the above formulas (3) to (6) is not satisfied, the dislocation density before ferrite transformation is insufficient, and the ferrite structure fraction in the structure at the center of the plate thickness is reduced. It is not possible to obtain a thick high arrested steel plate efficiently.

  In addition, it is preferable that the value of [A-3.5G] in the above formula (3) is −200 or more in order to secure the ferrite structure fraction.

  In addition, the value of [A-1.5G] in the above formula (4) is preferably 5 or less in order to avoid a reduction in rolling efficiency.

  The value of [C-670-G] in the formula (5) is preferably set to −100 or more because if it becomes too low, the rolling efficiency is lowered.

  The value of [B-C-20- (1400 / G) 1.5G] in the formula (6) is preferably set to −60 or more in order to avoid a reduction in rolling efficiency.

In order to ensure sufficient strength, in the case of such a thick material, it is also effective to control the cooling rate and cooling stop temperature in the cooling process after rolling, and the cooling rate during water cooling is 2 ° C / The water cooling stop temperature is preferably 500 ° C. or lower. That is, the water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness are
E-500 ≦ 0 (7)
F-2 ≧ 0 (8)
It is preferable to perform water cooling so as to satisfy these two equations.

  In addition, the value of the water cooling stop temperature E (° C.) is naturally equal to or higher than the normal temperature because the normal temperature industrial water is used.

  In addition, the value of the average cooling rate F (° C./s) during the water cooling in the center portion of the plate thickness is about 50 or less from the theoretical limit cooling rate by the water cooling.

In addition, when tempering at a temperature of Ac ₁ point or less after cooling, the hardened structure in the bainite structure may be partially detoxified, and therefore may be carried out as necessary.

  In addition, when tempering, it is preferable to set it as 200 degreeC or more which is the minimum temperature which expresses the effect of tempering.

  For the above reasons, the method for producing a high-strength thick steel plate excellent in arrest properties in the 45 ° direction with respect to the rolling direction according to the present invention (7) is the present invention (1) described in the item (A) ( It was defined that the steel ingot having the chemical composition according to any one of 6) was heated, rolled, and cooled by the above steps 1 to 3.

  Moreover, the manufacturing method of the high intensity | strength thick steel plate excellent in the arrest characteristic of a 45 degree direction with respect to the rolling direction which concerns on this invention (8) is from this invention (1) to (6) as described in said (A) term. It was defined that the steel ingot having the chemical composition described in any of the above was heated, rolled, cooled, and tempered according to the steps 1 to 4 described above.

  As described above, the high-strength thick steel plate of the present invention is intended to have a thickness exceeding 50 mm, but if the manufacturing method of the present invention (7) or the present invention (8) is used, the thickness is 65 mm or more. An extremely thick thick steel plate can also be produced usefully.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Tables 1 and 2 show the chemical components of the steels tested this time. In Tables 1 and 2, the steel No. 1 to 37 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steel No. Nos. 38 to 43 are comparative steels whose chemical compositions deviate from the conditions defined in the present invention.

  Using these various steel ingots, various high-strength thick steel plates were produced based on the production conditions shown in Tables 3 and 4.

  Test number 1-2 is not clearly shown in Table 3, but is tempered at 520 ° C. after cooling.

  For each steel sheet thus obtained, the ferrite structure fraction and the grain size of the effective crystal grains in the center of the plate thickness, the tensile properties, the (321) plane in the 45 ° direction at the 1/4 position of the plate thickness, and the (211) plane. The X-ray intensity of the (110) plane, the X-ray intensity of the (321) plane, the (211) plane and the (110) plane in the 45 ° direction at the center of the plate thickness, the arrest characteristics and the Charpy impact characteristics were investigated.

  The ferrite structure fraction in the center part of the plate thickness is obtained by taking a test piece from the center part of the thickness of each steel plate so that a so-called “L cross section” which is a plane parallel to the rolling surface becomes the test surface, The specimen was embedded in resin and mirror-polished, then corroded with nital, and observed with 100 optical fields using an optical microscope at a magnification of 500, and after calculating the area ratio of ferrite to the total visual field area in each field, the 100 fields The area ratio of ferrite was obtained by arithmetic averaging.

  In addition, with regard to the grain size of the effective crystal grains in the central portion of the plate thickness, a test piece is collected from the central portion of the plate thickness of each steel plate so that the L cross section becomes the test surface, and after mirror polishing, using an EBSP apparatus , Observing 10 fields of view at a magnification of 300 times, considering a boundary having an orientation difference of 15 ° or more as a crystal grain boundary, determining a portion surrounded by the boundary as an effective crystal grain, and then further by image analysis, The area inside each effective crystal grain was determined, converted into the diameter of a circle having an area equivalent to the area, and evaluated as the grain diameter of each effective crystal grain.

  For tensile properties, a tensile test piece according to JIS Z 2201 (1998) was sampled in the “C direction”, which is a direction perpendicular to the rolling direction, centered on the ¼ position of the plate thickness, and JIS Z 2241 (1998). A tensile test at room temperature was conducted by the method described, and the yield strength (YS) and tensile strength (TS) were measured.

  In addition, said yield strength is calculated | required from the yield point when a tensile test speed | rate is 10 N / (mm * s), and when a yield point is not clear, 0.2% yield strength was made into yield strength. . The target for tensile properties was to have a TS of 490 MPa or higher.

  The X-ray intensity of the (321) plane, (211) plane and (110) plane in the 45 ° direction at each of the plate thickness 1/4 position and the plate thickness central portion is 45 ° to the rolling direction at each of the above positions. A test piece of 2 mm × 20 mm × 20 mm was taken from a direction having an angle of and measured by an X-ray diffraction test. Then, the ratio of the X-ray intensity obtained as described above and the X-ray intensity of the non-oriented sample having a random crystal orientation is obtained, and from these values, (321) in the 45 ° direction at the two positions. The arithmetic average value of the sum of the X-ray intensity ratios of the plane, (211) plane and (110) plane was derived.

For the arrest characteristics, a plurality of temperature gradient type ESSO tests were conducted, and the obtained results were plotted on an Arrhenius graph to perform linear approximation, and the Kca value at −10 ° C. was evaluated as the arrest characteristics of the steel sheet. . The target of the arrest characteristic is to have a Kca value of 6000 N / mm ^1.5 or more. The specimen for the ESSO test was taken from the so-called “L direction” parallel to the rolling direction.

  It is known that the arrest characteristic and the fracture surface transition temperature (vTrs) of the Charpy impact test have a certain degree of correlation. Therefore, the quality of the arrest characteristic can be roughly estimated by vTrs.

  Therefore, in order to confirm that the arrest characteristic in the 45 ° direction with respect to the rolling direction is not significantly different from the arrest characteristic in the L direction, a V-notch test piece having a width of 10 mm described in JIS Z 2242 (2005) is used. Samples are taken from two directions, the so-called “L direction” and “rolling direction and 45 ° direction” parallel to the rolling direction centering on the thickness 1/4 position, and a Charpy impact test is performed to determine the fracture surface transition temperature (vTrs). It was.

Next, from the value of vTrs in the L direction and the value of vTrs in the rolling direction and 45 ° obtained as described above,
ΔvTrs = vTrs (L) × (−1) −vTrs (45 °) × (−1) (9)
ΔvTrs was derived by the following formula, and the difference in toughness between the L direction and the 45 ° direction with respect to the rolling direction was confirmed. The target of ΔvTrs was set to 10 ° C. or lower. Note that vTrs (L) and vTrs (45 °) in the above equation (9) mean vTrs in the L direction and vTrs in the 45 ° direction with respect to the rolling direction, respectively.

  Table 5 summarizes the above test results. “Effective crystal grain size at the center of thickness” in each test steel sheet in Table 5 is the maximum grain size among the effective crystal grain sizes determined as described above.

  In Table 5, the sum of the X-ray intensity ratios of the (321) plane, (211) plane, and (110) plane in the 45 ° direction at the 1/4 thickness position and the (321) in the 45 ° direction at the central portion of the thickness. ) Plane, (211) plane, and (110) plane, the arithmetic average value with the sum of X-ray intensity ratios was expressed as “sum of X-ray intensity ratio sums”. Therefore, in the following description, this “arithmetic average value of the sum of X-ray intensity ratios” will be used.

  From Table 5, it is clear that the strength target and the arrest property target cannot be achieved at the same time in the case of the steel plate with the test number of the comparative example that deviates from the conditions specified in the present invention.

  That is, the steel plate of test number 1-1 is a steel No. 1 that satisfies the conditions specified by the present invention in terms of the chemical composition of the steel. 1 is used, the ferrite structure fraction at the center of the plate thickness is 59%, which is lower than the lower limit specified in the present invention. Conversely, the effective grain size of the center of the plate thickness is 32 μm. In addition, the “arithmetic average value of the sum of X-ray intensity ratios” is 3.47, which exceeds the upper limit value defined in the present invention, and therefore is inferior in arrest characteristics.

  Similarly, the steel numbers of Test Nos. 1-4 are steel Nos. That satisfy the conditions specified by the present invention in the chemical composition of the steel. 1 is used, the ferrite structure fraction at the center of the plate thickness is 55%, which is lower than the lower limit defined in the present invention. Conversely, the effective grain size of the center of the plate thickness is 27 μm. The “arithmetic mean value of the sum of X-ray intensity ratios” is 4.02, which exceeds the upper limit specified in the present invention, and is inferior in arrest characteristics.

  The steel plate of test number 2-1 is a steel No. 1 that satisfies the conditions defined by the present invention in terms of the chemical composition of the steel. 2 is used, the ferrite structure fraction of the center portion of the plate thickness is 59%, which is lower than the lower limit specified in the present invention. Conversely, the effective grain size of the center portion of the plate thickness is 34 μm. In addition, the “arithmetic average value of the sum of X-ray intensity ratios” is 3.45, which exceeds the upper limit value defined in the present invention, and is inferior in tensile strength and arrest properties.

  Similarly, the steel plate of test number 2-2 is a steel No. 2 whose chemical composition satisfies the conditions specified by the present invention. 2 is used, the ferrite structure fraction at the center of the plate thickness is 68%, which is lower than the lower limit specified in the present invention. Conversely, the effective grain size of the center of the plate thickness is 30 μm. In addition, the “arithmetic average value of the sum of X-ray intensity ratios” is 4.52, which exceeds the upper limit value defined in the present invention, and is inferior in arrest characteristics.

  The steel plate of Test No. 2-3 has a steel No. 2 that satisfies the conditions specified in the present invention by the chemical composition of the steel. 2 is used, the ferrite structure fraction at the center of the plate thickness is 62%, which is lower than the lower limit specified in the present invention. Conversely, the effective grain size of the center of the plate thickness is 30 μm. In addition, the “arithmetic mean value of the sum of X-ray intensity ratios” is 3.43, which exceeds the upper limit value defined in the present invention, and is inferior in tensile strength.

  Similarly, the steel plates of test numbers 2 to 4 are steel Nos. That satisfy the conditions specified by the present invention in terms of the chemical composition of the steel. 2 is used, the ferrite structure fraction at the center of the plate thickness is 57%, which is lower than the lower limit specified in the present invention. Conversely, the effective grain size of the center of the plate thickness is 32 μm. In addition, the “arithmetic average value of the sum of X-ray intensity ratios” is 3.39, which exceeds the upper limit value defined in the present invention, and is inferior in tensile strength.

  The steel plates of Test Nos. 2 to 5 are steel Nos. That satisfy the conditions specified in the present invention by the chemical composition of the steel. 2 is used, the ferrite structure fraction of the center portion of the plate thickness is 63%, which is lower than the lower limit specified in the present invention. Conversely, the effective grain size of the center portion of the plate thickness is 41 μm. In addition, the “arithmetic average value of the sum of X-ray intensity ratios” is 3.43, which exceeds the upper limit value defined in the present invention, and is inferior in arrest characteristics.

  The steel plate of Test No. 3-2 is a steel No. 3 whose chemical composition satisfies the conditions specified by the present invention. Even though 3 is used, the ferrite structure fraction at the central portion of the plate thickness is 75%, which is lower than the lower limit specified in the present invention, so that the tensile strength and arrest properties are inferior.

  The steel plate of test number 38 is a steel No. whose chemical composition deviates from the conditions specified in the present invention. 38, and the ferrite structure fraction at the center of the plate thickness is 30%, which is lower than the lower limit specified in the present invention. Conversely, the effective grain size of the center of the plate thickness is 45 μm, The “arithmetic mean value of the sum of the X-ray intensity ratios” is 4.03, which exceeds the upper limit value defined in the present invention, and is inferior in arrest characteristics.

  The steel plate of test number 39 has a steel No. of which the chemical composition of the steel is out of the conditions specified in the present invention. 39, and the ferrite structure fraction at the center of the plate thickness is 20%, which is below the lower limit specified in the present invention. Conversely, the “arithmetic average value of the sum of X-ray intensity ratios” is 3.62. Therefore, the arrest characteristic is inferior because it exceeds the upper limit defined in the present invention.

  The steel plate of test No. 40 is a steel No. whose chemical composition deviates from the conditions specified in the present invention. 40, and the ferrite structure fraction in the center portion of the plate thickness is 10%, which is lower than the lower limit specified in the present invention. On the contrary, the effective crystal grain size in the center portion of the plate thickness is 59 μm. The “arithmetic mean value of the sum of the X-ray intensity ratios” is 3.98, which exceeds the upper limit value defined in the present invention, and is inferior in arrest characteristics.

  In Test Nos. 38 to 40, since the ferrite structure fraction is low, it can be estimated that excessive hardenability is the cause of insufficient arrest characteristics.

  The steel plate of test No. 41 has a steel No. whose chemical composition deviates from the conditions specified in the present invention. 41, and the “arithmetic mean value of the sum of the X-ray intensity ratios” is 4.88, which exceeds the upper limit value defined in the present invention, and is inferior in the arrest characteristics.

  The steel plate of test number 42 is a steel No. whose chemical composition deviates from the conditions specified in the present invention. 42 is used, and the “arithmetic mean value of the sum of X-ray intensity ratios” is 4.25, which exceeds the upper limit value defined in the present invention, and is therefore inferior in arrest characteristics.

  The steel plate of test number 43 is a steel No. whose chemical composition deviates from the conditions specified in the present invention. 43, and the “arithmetic mean value of the sum of the X-ray intensity ratios” is 5.10, which exceeds the upper limit value defined in the present invention, and is inferior in arrest characteristics.

  In addition, among the steel sheets having the test numbers of the comparative examples, all the steel sheets in which the ferrite structure fraction in the center portion of the plate thickness is lower than the lower limit specified in the present invention have a texture in the 45 ° direction with respect to the rolling direction. Since it becomes prominent and inferior in toughness in the 45 ° direction, ΔvTrs is large and does not reach the target of 10 ° C. or less.

  On the other hand, the test number 1-2, the test number 1-3, the test number 2-6, the test number 3-1, and the test numbers 4 to 37 of the examples of the present invention that satisfy the conditions specified in the present invention are: In spite of being thick, both have high arrest characteristics while ensuring necessary strength characteristics. In addition, no deterioration was observed with respect to the toughness in the 45 ° direction, suggesting that it exhibits high arrest characteristics regardless of the stress load direction.

As described above, the high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction of the present invention is easy to produce on an industrial scale at low cost, and is −10 ° C. in the rolling direction. Since the arrest property is 6000 N / mm ^1.5 or more and it is possible to prevent the entire structure from collapsing even when a brittle crack occurs, it is used as a material for various steel structures, especially large commercial ships. be able to. And the high-strength thick steel plate excellent in the arrest characteristic of a 45 degree direction with respect to this rolling direction can be manufactured by the method of this invention.

About a direction parallel to the rolling direction (denoted as “L direction”), a direction perpendicular to the rolling direction (denoted as “C direction”), and a direction of 45 ° relative to the rolling direction (denoted as “45 ° direction”) It is a figure explaining typically.

Claims (8)

In mass%, C: 0.01 to 0.12%, Si: 0.50% or less, Mn: 0.4 to 2%, P: 0.05% or less, S: 0.008% or less, Al: It contains 0.002 to 0.05%, N: 0.01% or less, Nb: 0.003 to 0.1%, has a chemical composition consisting of the balance Fe and impurities, and has the following formula (1): The carbon equivalent Ceq shown is 0.32 to 0.40%, the ferrite structure fraction at the center of the plate thickness is 80% or more, the grain size of the effective crystal grains at the center of the plate thickness is 25 μm or less, Sum of X-ray intensity ratios of (321) plane, (211) plane and (110) plane in 45 ° direction at 1/4 thickness position, and (321) plane and (211) plane in 45 ° direction at the center of plate thickness And an arithmetic average value with the sum of the X-ray intensity ratios of the (110) plane is 3.3 or less. A high-strength thick steel plate with excellent arrest characteristics in the 45 ° direction.
Ceq = C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5) + (Mo / 5) + (V / 5) (1)
However, C, Mn, Cu, Ni, Cr, Mo and V in the formula (1) mean the content (mass%) of each element.
Further, the meaning of the sum of the effective crystal grains, the grain diameter and the 1/4 thickness position of the effective crystal grains, and the X-ray intensity ratio of the (321) plane, (211) plane and (110) plane in the 45 ° direction is Each is as follows.
Effective grain: A grain when a boundary having an orientation difference of 15 ° or more is regarded as a grain boundary using the EBSP method,
Effective grain size: diameter of a circle having an area equivalent to the area of the effective grain,
Plate thickness 1/4 position: the position where the distance from the plate thickness center of the thick steel plate to the plate thickness direction is 1/4 of the plate thickness,
Sum of X-ray intensity ratios of (321) plane, (211) plane and (110) plane in 45 ° direction: with respect to the rolling direction when the distance from the center of the plate thickness to the plate thickness direction is the standard The sum of the ratios of the X-ray intensity of the (321) plane, (211) plane and (110) plane of the sample collected from the direction of 45 ° to the X-ray intensity of the non-oriented sample having a random crystal orientation.   The high-strength thick steel plate having excellent arrest characteristics in the 45 ° direction with respect to the rolling direction according to claim 1, further comprising Ni: 1% or less by mass%.   The mass direction further includes one or two elements of Cu: 2% or less and Cr: 1% or less, and 45 ° direction with respect to the rolling direction according to claim 1 or 2. High strength thick steel plate with excellent arrest properties.   The composition further comprises one or more elements selected from the group consisting of Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% or less. A high-strength thick steel plate having excellent arrest characteristics in a 45 ° direction with respect to the rolling direction according to any one of 1 to 3.   The high-strength thickness excellent in arrest properties in the direction of 45 ° with respect to the rolling direction according to any one of claims 1 to 4, characterized by containing Ti: 0.1% or less in mass%. Meat steel plate.   The composition further comprises one or more elements selected from the group consisting of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. A high-strength thick steel plate having excellent arrest characteristics in a 45 ° direction with respect to the rolling direction according to any one of 1 to 5. The steel ingot having the chemical composition according to any one of claims 1 to 6 is heated, rolled, and cooled by the following steps 1 to 3, and the arrest is in a 45 ° direction with respect to the rolling direction. A method for producing high-strength thick steel plates with excellent characteristics.
[Step 1] The heating temperature Tr (° C.) and heating time t (h) of the steel ingot are
^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2)
The step of heating the steel ingot so as to satisfy the following formula.
[Step 2] Rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling, and finish rolling temperature C (° C.) when finishing to a thickness G (mm) of a thick steel plate having a final shape by final rolling But,
A-3.5G ≦ 0 (3)
A-1.5G ≧ 0 (4)
C-670-G ≦ 0 (5)
BC-20- (1400 / G) ≦ 0 (6)
The process of rolling to satisfy all of the formulas.
[Step 3] The water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness are:
−150 ≦ E−500 ≦ 0 (7)
F-2 ≧ 0 (8)
The process of performing water cooling so that these two formulas may be satisfied. The steel ingot having the chemical composition according to any one of claims 1 to 7 is heated, rolled, cooled, and tempered by the following steps 1 to 4 at 45 ° with respect to a rolling direction. Of high-strength thick-walled steel plate with excellent orientation characteristics.
[Step 1] The heating temperature Tr (° C.) and heating time t (h) of the steel ingot are
^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2)
The step of heating the steel ingot so as to satisfy the following formula.
[Step 2] Rolling temperature B (° C.) at an arbitrary thickness A (mm) during rolling, and finish rolling temperature C (° C.) when finishing to a thickness G (mm) of a thick steel plate having a final shape by final rolling But,
A-3.5G ≦ 0 (3)
A-1.5G ≧ 0 (4)
C-670-G ≦ 0 (5)
BC-20- (1400 / G) ≦ 0 (6)
The process of rolling to satisfy all of the formulas.
[Step 3] The water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during water cooling at the center of the plate thickness are:
−150 ≦ E−500 ≦ 0 (7)
F-2 ≧ 0 (8)
The process of performing water cooling so that these two formulas may be satisfied.
[Step 4] Tempering at a temperature of Ac ₁ point or less.

Images