JP5233303B2 - High-strength thick steel plate with excellent Z-direction arrest characteristics and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength thick steel plate having excellent arrest characteristics in the Z direction and a method for producing the same. Specifically, even if a brittle crack occurs, it can prevent the entire structure from collapsing. The present invention relates to a high-strength thick steel plate having excellent Z-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 “Z direction” is a direction connecting the front surface and the back surface of the thick steel plate, and refers to a so-called “plate thickness direction”. 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 sheet 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.

  Furthermore, recently, instead of stopping crack propagation in a steel plate where a brittle crack has occurred, it has been considered to stop crack propagation at the joint with another member in the crack propagation path. ing.

  For example, the site where a brittle crack occurs in a large container ship is hatch side combing, but it is extremely difficult to stop the crack within the hatch side combing. Many attempts have been made to stop cracks in deck materials.

  The upper deck is joined perpendicularly to the hatch side combing. For this reason, the cracks generated by hatch side combing penetrate and propagate into the upper deck material in the direction connecting the front and back surfaces of the steel sheet (Z direction), and the steel sheet rolling direction (hereinafter referred to as the generally accepted direction). , Also referred to as “L direction”) and a direction perpendicular to both the rolling direction of the steel sheet and the Z direction (hereinafter referred to as “C direction”). Therefore, in order to stop the brittle crack with the upper deck material, it is important to improve the brittle crack propagation stopping characteristic in the Z direction.

  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 material has become more severe in both the material characteristic and the mechanical characteristic.

  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 JIS G 3127 (2000).

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.

  Furthermore, in Patent Document 4, as a technique for evaluating the fracture characteristics in the presence of a surface notch, the nominal shelf where the yield shelf after yield in the nominal stress-nominal strain curve is 1% or more and the nominal stress is maximized is disclosed. A “steel material excellent in ductile fracture resistance in a high strain load state” having a work hardening index (n = ln (1 + εu)) determined from strain (εu) of 0.15 or more is disclosed.

JP 55-148746 A JP-A-3-2322 JP 7-126798 A JP 2002-30379 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, rolling in the two-phase region significantly impairs productivity. For this reason, the technique proposed in Patent Document 1 is not necessarily realistic for mass-produced products.

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 an extremely complicated process of recuperating 1/3 or more to Ac ₃ or more. For this reason, the technique proposed in Patent Document 2 is not necessarily realistic for mass-produced products.

  By making the surface layer structure of the steel sheet ultrafine-grained as disclosed in Patent Document 3, the arrest characteristics can be obtained without adding expensive elements such as Ni, even by a technique that promotes shear lip formation during brittle crack propagation. It is possible to improve. 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. For this reason, it is not necessarily realistic for mass-produced products. 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. For this reason, the technique proposed in Patent Document 3 is poor in feasibility because it is inevitable that the equipment cost and the energy cost are increased, particularly when applied to a thick material.

  In addition, none of the techniques disclosed in Patent Documents 1 to 3 described above are intended to improve the crack propagation stopping characteristics in the Z direction of the steel sheet.

  The technique disclosed in Patent Document 4 evaluates the “ductile fracture resistance” by a tensile test using a specimen with a surface notch, and the surface notch exists in the surface direction of the steel sheet when there is a surface notch. This is a technique for evaluating the fracture characteristics when cracks propagate. However, the target is “ductile fracture”, not “brittle fracture”. For this reason, the technique proposed in Patent Document 4 is not necessarily applicable to a high-strength thick steel plate excellent in the arrest characteristics in the surface direction.

  As described above, conventionally, it has been difficult to provide a high-strength thick steel plate having excellent arrest characteristics in the Z direction at a low cost.

Therefore, the present invention is a high-strength thick steel plate excellent in arresting properties in the Z direction, in particular, having an arresting property of 8000 N / mm ^1.5 or more at −10 ° C. and a tensile strength (TS) of 490 MPa or more. The purpose is to provide a high-strength thick steel plate with excellent orientation characteristics 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 (l) was obtained.

  (A) In order to provide a high-strength thick steel plate having excellent arrest characteristics in the Z direction at a low cost, a method of containing a large amount of an expensive alloy component cannot be employed.

  (B) In order to improve the arrest characteristics in the Z direction, it is effective to make the structure of the steel sheet fine as a whole, but this method is from the viewpoint of productivity, equipment cost, energy cost, etc. Not necessarily realistic.

  (C) Even if the structure of the steel plate is not made fine in all directions, the arrest characteristics can be improved if the structure in the direction in which the crack propagates is made fine. Specifically, a boundary having an orientation difference of 15 ° or more using an EBSP method (Electron Back Scattering Pattern Method) is defined as a grain boundary (hereinafter, this grain boundary is referred to as an “effective grain boundary”). Assuming that the crystal grains surrounded by them are called "effective crystal grains"), if the above-mentioned effective crystal grain boundaries are densely distributed in the direction in which the crack propagates, the arrest characteristics are enhanced. Can do. And in order to improve the arrest characteristic of a Z direction, the distance from a plate | board thickness center part to a plate | board thickness direction in a cross section (henceforth "C cross section") perpendicular | vertical to the rolling direction of a thick steel plate is plate | board thickness. Centering on a position that is 1/4 (hereinafter referred to as “plate thickness 1/4 position”), the number of effective grain boundaries Nz that intersect with an arbitrary straight line having a specific length in the Z direction and the C direction It is necessary that the ratio Nz / Nc to the number of grain boundaries Nc of effective crystal grains intersecting with an arbitrary straight line having the same specific length as described above is 1.05 or more.

  (D) Increasing the unevenness of the fracture surface by finely bending the propagation path of the brittle crack leads to an increase in the propagation resistance of the brittle crack, which is extremely effective in improving the arrest characteristics.

(E) In order to bend the path of a brittle crack that propagates in the Z direction, it is effective to flatten inclusions in the steel. In particular, inclusions that are thinly stretched by rolling such as MnS are used. It is desirable that the film is flattened equally in the L direction and the C direction. This is because the bending of the fracture surface does not increase if the shape is extended in only one of the above directions. And as concrete manufacturing conditions for flattening the inclusion thinly stretched by rolling such as MnS equally in the L direction and the C direction, the width W0 (mm) of the steel ingot before the rolling start and the after rolling It is desirable that W1 / W0, which is the ratio of the plate width W1 (mm) of the final thick steel plate, is 1.40 or more.

(F) 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 a fine graining effect due to 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 tendency for the Charpy impact characteristics at the center of the plate thickness to become worse than the Charpy impact characteristics at the position of the plate thickness 1/4 of the thick steel plate or the surface layer portion becomes significant. 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 8000 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.

(G) Therefore, the present inventors deviated from the 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 the high-strength thick steel plate) In order to realize (TS), it is necessary to control to a chemical component having an appropriate hardenability, and as an index of the hardenability, the carbon equivalent Ceq represented by the following formula (1) is used. It has been found that it can be used, and its carbon equivalent Ceq needs to be 0.32 to 0.40.
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.

  (H) 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 rolling. It has been found that the initial austenite grain size can be made fine.

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

  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.

  (I) Next, it has been found that it is effective for improving the toughness of the high-strength thick-walled steel sheet that the effective crystal grain size in the central part of the thickness of the thick-walled steel sheet obtained after rolling is 25 μm or less. Here, as described above, 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.

  The “grain size of effective crystal grains” refers to the diameter of a circle having an area equivalent to the area of effective crystal grains.

  Thus, in order to make the thickness of the effective crystal grain 25 μm or less at the center of the thickness of the thick steel plate obtained after rolling, lowering the rolling temperature is effective, and the adjustment plate thickness, Control of rolling temperature and finish rolling temperature is effective. 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. It is preferable to perform rolling so that the rolling temperature C (° C.) satisfies the following formulas (4) and (5).
A-1.5G ≧ 0 (4),
BC-20- (1400 / G) ≦ 0 (5).
Here, A is an arbitrary thickness (mm) during rolling, B is the rolling temperature (° C.) in A, C is the finish rolling temperature (° C.), and G is the thickness of the thick steel plate in the final shape. (Mm) is represented respectively.

(J) Furthermore, control of the cooling rate and cooling stop temperature in the cooling process 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 It is preferable to perform water cooling so as to satisfy the following formulas (6) and (7).
E-500 ≦ 0 (6),
F-2 ≧ 0 (7).
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.

(K) 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.

  (L) Thus, the steel sheet in which the refinement of the effective crystal grain size 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 heating rolling conditions are inadequate and the grain size of the effective crystal grains in the central portion of the plate thickness is coarse, the toughness is poor even when [Nz / Nc] in (c) is 1.05 or more. Become.

  The high-strength thick steel plate excellent in the arresting property in the Z direction and the manufacturing method thereof according to the present invention have been completed based on such knowledge.

  Here, the gist of the present invention is excellent in the Z-direction arrest characteristics shown in (7) and (8) and the high-strength thick steel plates excellent in the Z-direction arrest characteristics shown in (1) to (6) below. It exists in the manufacturing method of a high strength thick steel plate.

(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 a carbon equivalent Ceq represented by the following formula (1) is 0.00. 32 to 0.40 is satisfied, the remainder has a chemical composition composed of Fe and impurities, the effective crystal grain size in the C section at the center of the plate thickness is 25 μm or less, and the plate thickness in the C section is ¼. The number of grain boundaries Nz of effective crystal grains intersecting with an arbitrary straight line having a specific length in the Z direction centering on the position and the grain of effective crystal grains intersecting with an arbitrary straight line having the same specific length as described above in the C direction The ratio Nz / Nc to the number of fields Nc is 1.05 or more, and is excellent in the arrest characteristics in the Z direction. Strength thick steel plate.
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 meanings of the C cross section, effective crystal grains, effective crystal grain diameter, 1/4 thickness position, Z direction and C direction are as follows.
C cross section: a cross section perpendicular to the rolling direction of the thick steel plate,
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,
Z direction: direction connecting the front and back surfaces of a thick steel plate,
C direction: direction perpendicular to the rolling direction of the thick steel plate and the Z direction.

  (2) The high-strength thick steel plate having excellent arrest characteristics in the Z direction according to the above (1), further comprising Ni: 1% or less by mass%.

  (3) Z as described in (1) or (2) above, further comprising one or two elements out of elements of Cu: 2% or less and Cr: 1% or less by mass%. High-strength thick steel plate with excellent orientation characteristics.

  (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 excellent in the arrest characteristics in the Z direction according to any one of (1) to (3) above.

  (5) High strength excellent in the arrest property in the Z direction according to any one of (1) to (4) above, characterized by containing, by mass%, Ti: 0.1% or less 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%. The high-strength thick steel plate excellent in the arrest characteristics in the Z 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 the Z direction: A manufacturing method for high-strength thick steel plates with excellent arrest properties.

[Step 1] A step of heating the ingot so that the heating temperature Tr (° C.) and the heating time t (h) of the ingot satisfy the following expression (2).

^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2).

Here, t represents the heating time (h) of the steel ingot, and Tr represents the heating temperature (° C.).

[Step 2] Rolling temperature B (° C.) at the width W0 (mm) of the steel ingot before the start of rolling, the width W1 (mm) of the thick steel plate of the final shape after rolling, and the arbitrary thickness A (mm) during rolling ) And the final rolling temperature C (° C.) at the time of finishing to the thickness G (mm) of the thick steel plate of the final shape by rolling so that all the following formulas (3) to (5) are satisfied. Process.
W1 / W0 ≧ 1.40 (3),
A-1.5G ≧ 0 (4),
BC-20- (1400 / G) ≦ 0 (5).

Here, W0 is the width (mm) of the steel ingot before the start of rolling, W1 is the plate width (mm) of the thick steel plate of the final shape after rolling, A is the arbitrary thickness (mm) during rolling, B represents the rolling temperature (° C.) in A, C represents the finish rolling temperature (° C.), and G represents the thickness (mm) of the thick steel plate of the final shape.

[Step 3] Water cooling is performed so that 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 satisfy the following formulas (6) and (7). Process.

E-500 ≦ 0 (6),
F-2 ≧ 0 (7).

Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.

(8) A steel ingot having the chemical composition according to any one of (1) to (6) is heated, rolled, cooled, and tempered by the following steps 1 to 4. A method for producing a high-strength thick steel plate with excellent arrest characteristics in the Z direction.

[Step 1] A step of heating the ingot so that the heating temperature Tr (° C.) and the heating time t (h) of the ingot satisfy the following expression (2).

^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2).

Here, t represents the heating time (h) of the steel ingot, and Tr represents the heating temperature (° C.).

[Step 2] Rolling temperature B (° C.) at the width W0 (mm) of the steel ingot before the start of rolling, the width W1 (mm) of the thick steel plate of the final shape after rolling, and the arbitrary thickness A (mm) during rolling ) And the final rolling temperature C (° C.) at the time of finishing to the thickness G (mm) of the thick steel plate of the final shape by rolling so that all the following formulas (3) to (5) are satisfied. Process.
W1 / W0 ≧ 1.40 (3),
A-1.5G ≧ 0 (4),
BC-20- (1400 / G) ≦ 0 (5).

Here, W0 is the width (mm) of the steel ingot before the start of rolling, W1 is the plate width (mm) of the thick steel plate of the final shape after rolling, A is the arbitrary thickness (mm) during rolling, B represents the rolling temperature (° C.) in A, C represents the finish rolling temperature (° C.), and G represents the thickness (mm) of the thick steel plate of the final shape.

[Step 3] Water cooling is performed so that 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 satisfy the following formulas (6) and (7). Process.

E-500 ≦ 0 (6),
F-2 ≧ 0 (7).

Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.

[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 sheet having excellent Z-direction arrest characteristics shown in the above (1) to (6) and the high-strength thick-walling having excellent Z-direction arrest characteristics shown in (7) and (8) The inventions relating to the method for producing steel sheets 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 with excellent Z-direction arrest characteristics according to the present invention is easy to produce on an industrial scale, has a Z-direction arrest characteristic of 8000 N / mm ^1.5 or more at −10 ° C., and is brittle. Since it is possible to prevent the entire structure from collapsing even when a crack occurs, it can be used as a material for various steel structures, particularly commercial vessels such as large container ships. And the high-strength thick steel plate excellent in the arrest property of this Z direction can be manufactured by the method of this invention.

  Below, each requirement of this invention is demonstrated in detail. Here, “%” representing the chemical composition means “mass%” unless otherwise specified.

(A) About chemical composition:
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%.

  In the above range, the C content is the carbon equivalent Ceq represented by the above formula (1), that is, [C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5) + (Mo / 5) + (V / 5)] must satisfy 0.32 to 0.40. This will be described later.

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%. A preferable range of the Mn content is 0.6 to 1.6%.

  As will be described later, the content of Mn is within the above range, the carbon equivalent Ceq represented by the above formula (1), that is, [C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5) + (Mo / 5) + (V / 5)] must satisfy 0.32 to 0.40.

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 refining the structure, improving toughness, improving hardenability, and increasing the strength by precipitation hardening. Especially, Nb 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%.

  In the present invention, the balance is composed of Fe, but other impurities may be included due to various factors in the manufacturing process.

  In addition, the high-strength thick steel plate excellent in the arrest property in the Z direction of the present invention may further include one or more elements selected from the following first group to fifth group as necessary. It can be included.

First group: Ni: 1% or less,
Second group: Cu: 1% or less of elements of 2% or less and Cr: 1% or less,
Third group: Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% or less, one or more elements,
Group 4: Ti: 0.1% or less,
Group 5: Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less, one or more elements.

  That is, one or more elements of the first to fifth groups may be included as optional elements.

  Hereinafter, the above optional elements will be described.

First group: Ni: 1% or less When Ni is contained, the arrest characteristics of the steel sheet can be improved. However, the content of Ni becomes a cost increase factor. For this reason, content of Ni in the case of making it contain shall be 1% or less. When Ni is contained, the preferable content of Ni 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.

  As will be described later, the content of Ni is within the above range, the carbon equivalent Ceq represented by the above formula (1), that is, [C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5) + (Mo / 5) + (V / 5)] must satisfy 0.32 to 0.40.

Group 2: Cu: 2% or less and Cr: 1% or less of elements of 1% or less Cu and Cr have the effect of increasing the strength. Therefore, in order to obtain this effect, the above elements are included. Also good. Hereinafter, the above Cu and Cr will be described in detail.

Cu: 2% or less When Cu is contained, strength can be improved 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, if Cu is included, the content of Cu is 2% or less. The upper limit with preferable Cu content 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.

  In the above range, the Cu content is the carbon equivalent Ceq represented by the above formula (1), that is, [C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5). ) + (Mo / 5) + (V / 5)] must satisfy 0.32 to 0.40. This will be described later.

Cr: 1% or less When Cr is contained, the strength can be increased. 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 Cr content when contained is 1% or less. The upper limit with preferable Cr content 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.

  Regarding the Cr content, as described later, within the above range, the carbon equivalent Ceq represented by the above formula (1), that is, [C + (Mn / 6) + (Cu / 15) + (Ni / 15) ) + (Cr / 5) + (Mo / 5) + (V / 5)] needs to satisfy 0.32 to 0.40.

  In addition, said Cu and Cr can be contained only in any 1 type in them, or 2 types of composite.

Group 3: Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% or less, one or more elements Mo, V, and B improve hardenability and strength In order to obtain these effects, the above elements may be included. Hereinafter, the above Mo, V and B will be described in detail.

Mo: 0.5% or less When Mo is contained, hardenability and strength can be improved. 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 Mo content in the case of inclusion is 0.5% or less. The upper limit with preferable Mo content 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 Mo 0.02% or more.

  As will be described later, the content of Mo is within the above range, the carbon equivalent Ceq represented by the above formula (1), that is, [C + (Mn / 6) + (Cu / 15) + (Ni / 15) + (Cr / 5) + (Mo / 5) + (V / 5)] must satisfy 0.32 to 0.40.

V: 0.1% or less V is effective for improving the hardenability and improving the strength by precipitation hardening. However, if the V content exceeds 0.1%, the toughness is significantly deteriorated. For this reason, when V is contained, the content of V is set to 0.1% or less. The upper limit with preferable V content 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.

  In addition, also about content of V, in said range, carbon equivalent Ceq shown by said Formula (1), ie, [C + (Mn / 6) + (Cu / 15) + (Ni / 15) + ( The value of (Cr / 5) + (Mo / 5) + (V / 5)] needs to satisfy 0.32 to 0.40.

B: 0.005% or less When B is contained, the ferrite transformation from the austenite grain boundary is suppressed, and the hardenability is improved. Further, the strength can be increased. However, if the B content exceeds 0.005%, the toughness deteriorates. Therefore, when B is included, the B content is 0.005% or less. The upper limit with preferable B content in the case of making it contain is 0.0015%. In order to ensure the effect of improving the hardenability and strength by B, it is preferable to contain B by 0.0003% or more.

  In addition, said Mo, V, and B can be contained only in any 1 type in them, or 2 or more types of composites.

Group 4: Ti: 0.1% or less When Ti is contained, it is effective as a constituent element of oxide particles, and is effective in preventing cracking of a steel ingot produced by continuous casting by increasing high temperature ductility. Become. However, if the Ti content exceeds 0.1%, TiC is generated and the toughness is deteriorated. For this reason, when Ti is contained, the content of Ti is set to 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.

Group 5: Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less, one or more elements Ca, Mg, and REM are inclusion shape control effects In order to obtain this effect, the above elements may be included. Hereinafter, the above Ca, Mg, and REM will be described in detail.

Ca: 0.004% or less When Ca is contained, it has an effect of controlling the shape 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, content of Ca in the case of making it contain shall be 0.004% or less. The upper limit with preferable Ca content 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 When Mg is contained, it has a form control effect of inclusions. Specifically, the dispersion density of the fine oxide can be increased, thereby improving the toughness of the weld heat affected zone. However, if the Mg content exceeds 0.002%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced. Therefore, when Mg is contained, the Mg content is set to 0.002% or less. The upper limit with preferable Mg content 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 When REM is contained, the inclusion has a shape control effect. Specifically, the dispersion density of the fine oxide can be increased, thereby improving the toughness of the weld heat affected zone. Moreover, the effect of fixing excess S as a sulfide can also be obtained by including REM. However, if the content of REM exceeds 0.002%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced. For this reason, the content of REM in the case of making it contain shall be 0.002% or less. The upper limit with preferable REM content 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 making REM contain in molten steel, before making Al contain in molten steel, you may isolate | separate into each REM element and make it contain in steel, and the state mixed as misch metal And may be contained in the steel.

Ceq value: 0.32 to 0.40%
Since the high-strength thick steel plate defined in the present invention is used as a strength member, it needs to have sufficient strength as a standard material. Therefore, it is necessary that the chemical composition of the high-strength thick steel plate not only satisfies each specified range but also has an 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 having a tensile strength of 490 MPa or more, a carbon equivalent formula defined by IIW (International Welding Association) can be used. That is, it can be arranged using the carbon equivalent Ceq represented by the following formula (1).
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.

  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. Significantly damaged. Therefore, the carbon equivalent Ceq is defined as 0.32 to 0.40%. In addition, the preferable range of carbon equivalent Ceq is 0.32-0.38%.

  For the above reasons, the high-strength thick steel plate having excellent arrest characteristics in the Z direction according to the present invention (1) contains the elements from C to Nb in the above-mentioned range, and is represented by the above formula (1). The carbon equivalent Ceq was 0.32 to 0.40, and the balance was defined as having a chemical composition composed of Fe and impurities.

  Moreover, the high-strength thick steel plate excellent in the arrest property in the Z direction according to the present invention (2) is further added to the high-strength thick steel plate excellent in the arrest property in the Z direction according to the present invention (1). % Or less was specified.

  Similarly, the high-strength thick steel plate excellent in the Z-direction arrest properties according to the present invention (3) is the high-strength thick steel plate excellent in the Z-direction arrest properties of the present invention (1) or (2). Furthermore, it was specified that one or two elements out of Cu: 2% or less and Cr: 1% or less were contained.

  The high-strength thick steel plate excellent in the arrest property in the Z direction according to the present invention (4) is a high-strength thick steel plate excellent in the arrest property in any Z direction from the present invention (1) to (3). Furthermore, it was specified that one or more elements of Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% or less were contained.

  The high-strength thick steel plate excellent in the arresting property in the Z direction according to the present invention (5) is one of the high-strength thick steel plates excellent in the arresting property in the Z direction from the present invention (1) to (4). Furthermore, it was specified that Ti: 0.1% or less was contained.

  The high-strength thick steel plate excellent in the arrest characteristics in the Z direction according to the present invention (6) is any one of the high-strength thick steel plates excellent in the arrest characteristics in the Z direction from the present invention (1) to (5). Furthermore, it was specified that one or more elements of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less were contained.

(B) About the grain size of the effective crystal grains in the C cross section at the center of the plate thickness:
It was found that the toughness of the high-strength thick steel plate is improved when the grain size of effective crystal grains in the C cross section at the center of the thickness of the thick steel plate obtained after rolling is 25 μm or less.

  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 Z direction according to the present invention, it is defined that the effective crystal grain size at the central portion of the plate thickness is 25 μm or less.

(C) About [Nz / Nc] at a portion centered at a position of 1/4 of the plate thickness in the C cross section:
When the effective grain size in the Z direction is small, the arrest characteristics in the Z direction can be improved. This is because a crack progresses by cleavage fracture within the effective crystal grains, whereas a ductile fracture region called tear ridge exists at the grain boundary, leading to an increase in crack propagation resistance. Improvement of brittle crack propagation resistance due to microstructure refinement is considered to be basically this mechanism.

  According to the study by the present inventors, the number of grain boundaries Nz of effective crystal grains intersecting with an arbitrary straight line having a specific length in the Z direction with the plate thickness 1/4 position in the C cross section as the center, and the C direction. When the ratio Nz / Nc to the number of grain boundaries Nc of the effective crystal grains intersecting an arbitrary straight line having the same specific length as Nz / Nc is 1.05 or more, the arrest characteristics in the Z direction can be reliably improved. It was revealed.

  Therefore, in the high-strength thick steel plate excellent in the arresting property in the Z direction of the present invention, the above [Nz / Nc] is defined as 1.05 or more.

  If the number of grain boundaries of effective crystal grains intersecting the same number of four or more straight lines each having a length of 200 μm in each of the Z direction and the C direction is obtained, it is possible to eliminate variations in measurement positions. .

  In addition, the high-strength thick steel plate excellent in the arrest property in the Z direction according to the present inventions (1) to (6) described so far can be obtained, for example, by the production method of the present invention (7) or the present invention (8). Can be manufactured.

(D) Manufacturing conditions The manufacturing conditions described in detail below are one of the methods for economically and efficiently realizing the above-described high-strength thick steel plate having excellent surface-direction arrest characteristics. The technical scope of itself is not defined by this manufacturing condition.

  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, leading to coarsening of crystal grains in the final structure. 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 is experimentally clarified, in the heating step of the steel ingot which is the material of the thick steel plate, the heating temperature Tr (° C.) and the heating time t (h) of the steel ingot are the following (2 It has been found that satisfying the expression (1) is preferable as a condition for producing an economically excellent thick high arrested steel sheet.

^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2).
Here, t represents the heating time (h) of the steel ingot, and Tr represents the heating temperature (° C.).

  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} ) ] above is preferably 50 or more to avoid inhibition of rolling efficiency due to an increase in deformation resistance.

  Next, the so-called “bending ratio” at the time of hot rolling of a steel plate has a great influence on the arrest characteristics in the surface direction through shape control of inclusions.

  That is, finely bending the propagation path of the brittle crack to increase the unevenness of the fracture surface leads to an increase in the propagation resistance of the brittle crack, which is extremely effective in improving the arrest characteristics.

  And in order to bend the path of the brittle crack that propagates in the Z direction finely, it is effective to flatten the inclusions in the steel. Desirably flattened in the direction and C direction. This is because the bending of the fracture surface does not increase if the shape is extended in only one of the above directions.

Specific manufacturing conditions for flattening the inclusions thinly rolled by rolling such as MnS equally in the L direction and the C direction include the width W0 (mm) of the steel ingot before rolling and the final shape after rolling. It is preferable that W1 / W0 (breadth ratio) which is the ratio of the plate width W1 (mm) of the thick steel plate is 1.40 or more. That is, rolling is preferably performed so that the aforementioned W1 / W0 satisfies the following expression (3).

W1 / W0 ≧ 1.40 (3)

Here, W0 represents the width (mm) of the steel ingot before the start of rolling, and W1 represents the sheet width (mm) of the thick steel plate having the final shape after rolling.

The above W1 / W0 is preferably set to 4 or less in order to avoid the yield deterioration due to the disordered rolling shape.

  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 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 finer structure 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, It is rolling so that the finish rolling temperature C (° C.) at the time of finishing to the thickness G (mm) of the thick steel plate of the final shape by the final rolling satisfies the following formulas (4) and (5). .

A-1.5G ≧ 0 (4),
BC-20- (1400 / G) ≦ 0 (5).
Here, A is an arbitrary thickness (mm) during rolling, B is the rolling temperature (° C.) in A, C is the finish rolling temperature (° C.), and G is the thickness of the thick steel plate in the final shape. (Mm) is represented respectively.

  When both of the above formulas (4) and (5) are not satisfied, the dislocation density before ferrite transformation is insufficient, and the structure size in the final structure at the center of the plate thickness becomes coarse. Thick high arrested steel plates cannot be obtained well.

  In addition, in order to avoid the fall of rolling efficiency, it is preferable to set the value of [A-1.5G] in said (4) Formula to 5 or less.

  Further, the value of [BC-20- (1400 / G) 1.5G] in the formula (5) 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 is performed so that the water cooling stop temperature E (° C.) and the average cooling rate F (° C./s) during the water cooling at the central portion of the plate thickness satisfy the following equations (6) and (7). preferable.

E-500 ≦ 0 (6),
F-2 ≧ 0 (7).
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.

  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 the arresting property in the Z direction according to the present invention (7) is any of the present inventions (1) to (6) described in the item (A). It was defined that the steel ingot having the chemical composition described in the above was heated, rolled and cooled by the steps 1 to 3 described above.

  Moreover, the manufacturing method of the high intensity | strength thick steel plate excellent in the arrest characteristic of the Z direction which concerns on this invention (8) is described in any one of this invention (1) to (6) as described in said (A) term. The steel ingot having the chemical composition of the above was prescribed to be heated, rolled, cooled, and tempered by 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 steel ingots tested this time. In Tables 1 and 2, the steel No. 1 to 37 are steel ingots whose chemical compositions are within the range defined by the present invention. On the other hand, steel No. 38 to 43 are steel ingots of comparative examples whose chemical compositions deviate from the conditions defined in the present invention.

  Using these various steel ingots, various high-strength thick steel plates were manufactured based on the manufacturing 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 plate thus obtained, tensile properties, grain size of effective crystal grains at the center of the plate thickness, [Nz / Nc] at the center of the plate thickness 1/4 position in the C cross section and the Z direction The arrest characteristics were investigated.

  The tensile test was performed by collecting a tensile test piece according to JIS Z 2201 (1998) in the C direction centering on the 1/4 thickness position, and performing a tensile test speed of 10 N / (mm · s) at room temperature. Strength (YS) and tensile strength (TS) were measured.

  In addition, said yield strength was calculated | required from the yield point, and when a clear yield point did not appear, 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 effective crystal grain size in the center portion of the plate thickness is as follows. First, a test piece is collected from each steel plate so as to include the center portion of the plate thickness so that the test surface has a C cross section. After embedding in resin and mirror polishing, EBSP apparatus is used to observe 5 fields of view at a magnification of 300 times, and a boundary having an orientation difference of 15 ° or more is regarded as a crystal grain boundary, and a portion surrounded by the boundary is considered. After determining the effective crystal grain, and further by image analysis, the area inside each effective crystal grain is obtained, converted into the diameter of a circle having an area equivalent to the area, and the grain size of each effective crystal grain As evaluated.

  [Nz / Nc] at a portion centered on the position of the plate thickness ¼ in the C cross section was obtained by the method illustrated in FIG. 1 using an EBSP apparatus.

  That is, a test piece is taken from each steel plate so that the test surface has a C cross section and includes a ¼ position of the plate thickness, and the test piece is embedded in resin and mirror-polished, and then an EBSP apparatus is used. The magnification was set to 300 times, and five fields of view were observed. A boundary having an orientation difference of 15 ° or more was regarded as a crystal grain boundary, and the grain boundary was displayed. Next, on the display screen, a straight line was drawn in the Z direction at an arbitrary position centered on the ¼ thickness position, and the number of grain boundaries of effective crystal grains intersecting the straight line was counted. Similarly, on the display screen, a straight line was drawn in the C direction at an arbitrary position centered on the ¼ thickness position, and the number of effective grain boundaries intersecting the straight line was counted. In order to eliminate variations in the measurement position, four straight lines each having a length of 200 μm were drawn in the Z direction and the C direction on each display screen of the five fields of view.

  For the Z direction and the C direction, arithmetic average values of the number of grain boundaries of effective crystal grains intersecting a straight line having a length of 200 μm drawn four by four were obtained and evaluated as [Nz] and [Nc].

  The arrest characteristics in the surface direction were evaluated by assembling a test body having the shape shown in FIG.

  That is, as shown in FIG. 2, the embrittled plates are arranged at the upper and lower portions so that cracks propagate in the test steel plate in the Z direction connecting the front and back surfaces of the plate thickness, Each of the test steel plate, the lower embrittlement plate, and the test steel plate was assembled by electron beam welding, and a mixed ESSO test was performed at −10 ° C.

Since the above ESSO test is a “Go or No-Go test”, the maximum K value that was “No-Go” was evaluated as the Kca of the material. The goal of the arrest characteristic is to have a Kca of 8000 N / mm ^1.5 or higher.

In FIG. 2, “EBW” means an electron beam welding, and “EH36” is an E grade yield point of 353 MPa (36 kgf / mm ² ) class with a Charpy required temperature of −40 ° C. It means a steel plate. A steel plate having a length of 500 mm is processed so that the test loading direction is the L direction with the original thickness of the steel plate to be evaluated, and is joined to the upper and lower steel plates by electron beam welding and used for a mixed-type ESSO test.

  Table 5 summarizes the above test results. In addition, “the grain size of the effective crystal grains in the center portion of the plate thickness” in each test steel sheet in Table 5 is the maximum grain size among the grain sizes of the effective crystal grains obtained as described above.

From Table 5, the steel numbers of Test No. 1-2, Test No. 1-3, Test No. 2-6, Test No. 3-1, and Test Nos. Despite being thick, Kca at −10 ° C. is 8000 N / mm ^1.5 or more, excellent arrest properties in the Z direction, and the target of tensile strength (TS) is 490 MPa or more. Obviously.

On the other hand, in the case of the steel plates having the test numbers of the comparative examples that deviate from the conditions specified in the present invention, the Kca at −10 ° C. does not reach 8000 N / mm ^1.5 and is inferior in the Z-direction arrest characteristics. Is. Moreover, the tensile strength (TS) of some of them does not reach the target of 490 MPa or more and is inferior in tensile properties.

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. In spite of using 1, the [Nz / Nc] is 1.01, which is lower than the lower limit specified in the present invention, so that Kca at −10 ° C. is as low as 4822 N / mm ^1.5, and Z Poor direction arrest characteristics.

The steel plates of Test Nos. 1-4 are steel Nos. That satisfy the conditions specified in the present invention by the chemical composition of the steel. In spite of the use of 1, the effective crystal grain size at the central portion of the plate thickness is 27 μm, which exceeds the upper limit specified in the present invention, so the Kca at −10 ° C. is 7250 N / mm ^1.5 . The target of 8000 N / mm ^1.5 is not reached, and the arrest characteristics in the Z direction are inferior.

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. Although [Nz / Nc] is 1.02 and is lower than the lower limit specified in the present invention, Kca at −10 ° C. is as low as 6610 N / mm ^1.5. Poor direction arrest characteristics.

All of the steel plates with test numbers 2-2 to 2-5 are steel Nos. That satisfy the conditions specified by the present invention in terms of the chemical composition of the steel. In spite of the use of 2, the grain sizes of the effective crystal grains in the center portion of the plate thickness are 30 μm, 30 μm, 32 μm and 41 μm, respectively, which are higher than the upper limit specified in the present invention. Each Thus, the Kca at ^{-10 ℃, 7150N / mm 1.5,} 6550N / mm 1.5, a 6220N / mm ^1.5 and 6920N / mm ^1.5, not reached the goal of 8000 N / mm ^1.5, the Z-direction Inferior to arrest properties. Among the steel plates with the above test numbers, the steel plates with test numbers 2-3 and 2-4 have tensile strengths (TS) of 456 MPa and 465 MPa, respectively, and the target of 490 MPa or higher is not reached. The characteristics are also inferior.

The steel plate of Test No. 3-2 is a steel No. 3 whose chemical composition satisfies the conditions specified by the present invention. 3 is used, the effective crystal grain size at the center of the plate thickness is 32 μm, which exceeds the upper limit specified in the present invention, so the Kca at −10 ° C. is 6790 N / mm ^1.5 . The target of 8000 N / mm ^1.5 is not reached, the arrest property in the Z direction is inferior, and the tensile strength (TS) is 485 MPa, the target of 490 MPa or more is not reached, and the tensile property is also inferior. ing.

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 grain diameter of the effective crystal grains in the central portion of the plate thickness is 45 μm, which exceeds the upper limit specified in the present invention. Therefore, the Kca at −10 ° C. is 6630 N / mm ^1.5, which is the target. It does not reach 8000 N / mm ^1.5 and is inferior in the arrest characteristic in the Z direction.

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. Due to the use of 39, Kca at -10 ° C. does not reach the target of 8000 N / mm ^1.5 at 6780N / mm ^1.5, it is inferior to the arrest characteristics of the Z-direction.

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 grain size of the effective crystal grains in the central portion of the plate thickness is 59 μm, which exceeds the upper limit specified in the present invention. Therefore, the Kca at −10 ° C. is 7050 N / mm ^1.5, which is the target. It does not reach 8000 N / mm ^1.5 and is inferior in the arrest characteristic in the Z direction.

The steel plate of test No. 41 has a steel No. whose chemical composition deviates from the conditions specified in the present invention. Due to the use of 41, Kca at -10 ° C. does not reach the target of 8000 N / mm ^1.5 at 7230N / mm ^1.5, it is inferior to the arrest characteristics of the Z-direction.

The steel plate of test number 42 is a steel No. whose chemical composition deviates from the conditions specified in the present invention. 42, and the effective crystal grain size at the center of the plate thickness is 35 μm, which exceeds the upper limit defined in the present invention. Therefore, the Kca at −10 ° C. is 6490 N / mm ^1.5, which is the target. It does not reach 8000 N / mm ^1.5 and is inferior in the arrest characteristic in the Z direction.

The steel plate of test number 43 is a steel No. whose chemical composition deviates from the conditions specified in the present invention. Due to the use of 43, Kca at -10 ° C. does not reach the target of 8000 N / mm ^1.5 at 7600N / mm ^1.5, it is inferior to the arrest characteristics of the Z-direction.

As described above, the high-strength thick steel plate having excellent Z-direction arrest characteristics according to the present invention is easy to produce on an industrial scale and has a Z-direction arrest characteristic of 8000 N / mm ^1.5 or more at −10 ° C. However, even when a brittle crack occurs, it is possible to prevent the entire structure from collapsing, so that it can be used as a material for various steel structures, in particular, commercial vessels such as large container ships. And the high-strength thick steel plate excellent in the arrest property of this Z direction can be manufactured by the method of this invention.

It is a figure explaining an example of the method of calculating | requiring [Nz / Nc] in the site | part centering on the plate | board thickness 1/4 position in C cross section using an EBSP apparatus. It is a figure explaining the test body used in the mixed molding ESSO test performed in order to evaluate the arrest characteristic of a Z direction.

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: 0.002 to 0.05%, N: 0.01% or less, Nb: 0.003 to 0.1%, and a carbon equivalent Ceq represented by the following formula (1) is 0.32 to 0 .40, and the balance has a chemical composition composed of Fe and impurities, the grain size of the effective crystal grains in the C section at the center of the plate thickness is 25 μm or less, and the center of the plate thickness is 1/4 in the C section The number of grain boundaries Nz of effective crystal grains intersecting with an arbitrary straight line having a specific length in the Z direction and the number of grain boundaries Nc of effective crystal grains intersecting with an arbitrary straight line having the same specific length as described above in the C direction Nz / Nc ratio is 1.05 or more, and high strength thickness with excellent Z-direction arrest characteristics Meat steel plate.
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 meanings of the C cross section, effective crystal grains, effective crystal grain diameter, 1/4 thickness position, Z direction and C direction are as follows.
C cross section: a cross section perpendicular to the rolling direction of the thick steel plate,
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,
Z direction: direction connecting the front and back surfaces of a thick steel plate,
C direction: direction perpendicular to the rolling direction of the thick steel plate and the Z direction   The high-strength thick steel plate having excellent arrest characteristics in the Z direction according to claim 1, further comprising Ni: 1% or less in mass%.   It is excellent in the arresting property in the Z direction according to claim 1 or 2, characterized in that it further contains one or two elements out of elements of Cu: 2% or less and Cr: 1% or less. High strength thick steel plate.   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 excellent in arrest characteristics in the Z direction according to any one of 1 to 3.   The high-strength thick steel plate having excellent arrest properties in the Z direction according to any one of claims 1 to 4, further comprising Ti: 0.1% or less by mass.   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 excellent in arrest characteristics in the Z direction according to any one of 1 to 5. A 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 is excellent in the arrest property in the Z direction. A manufacturing method for high strength thick steel sheets.
[Step 1] A step of heating the ingot so that the heating temperature Tr (° C.) and the heating time t (h) of the ingot satisfy the following expression (2).
^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2)
Here, t represents the heating time (h) of the steel ingot, and Tr represents the heating temperature (° C.).
[Step 2] Rolling temperature B (° C.) at the width W0 (mm) of the steel ingot before the start of rolling, the width W1 (mm) of the thick steel plate of the final shape after rolling, and the arbitrary thickness A (mm) during rolling ) And the final rolling temperature C (° C.) at the time of finishing to the thickness G (mm) of the thick steel plate of the final shape by rolling so that all the following formulas (3) to (5) are satisfied. Process.
W1 / W0 ≧ 1.40 (3)
A-1.5G ≧ 0 (4)
B−C−20− (1400 / G) ≦ 0 (5)
Here, W0 is the width (mm) of the steel ingot before the start of rolling, W1 is the plate width (mm) of the thick steel plate of the final shape after rolling, A is the arbitrary thickness (mm) during rolling, B represents the rolling temperature (° C.) in A, C represents the finish rolling temperature (° C.), and G represents the thickness (mm) of the thick steel plate of the final shape.
[Step 3] Water cooling is performed so that 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 satisfy the following formulas (6) and (7). Process.
E-500 ≦ 0 (6)
F-2 ≧ 0 (7)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness. The steel ingot having the chemical composition according to any one of claims 1 to 6 is heated, rolled, cooled, and tempered according to the following steps 1 to 4, to an arrest characteristic in a Z direction. A method for producing excellent high-strength thick steel plates
[Step 1] A step of heating the ingot so that the heating temperature Tr (° C.) and the heating time t (h) of the ingot satisfy the following expression (2).
^{t × exp (Tr 3/270000000} ) ≦ 580 ··· (2)
Here, t represents the heating time (h) of the steel ingot, and Tr represents the heating temperature (° C.).
[Step 2] Rolling temperature B (° C.) at the width W0 (mm) of the steel ingot before the start of rolling, the width W1 (mm) of the thick steel plate of the final shape after rolling, and the arbitrary thickness A (mm) during rolling ) And the final rolling temperature C (° C.) at the time of finishing to the thickness G (mm) of the thick steel plate of the final shape by rolling so that all the following formulas (3) to (5) are satisfied. Process.
W1 / W0 ≧ 1.40 (3)
A-1.5G ≧ 0 (4)
B−C−20− (1400 / G) ≦ 0 (5)
Here, W0 is the width (mm) of the steel ingot before the start of rolling, W1 is the plate width (mm) of the thick steel plate of the final shape after rolling, A is the arbitrary thickness (mm) during rolling, B represents the rolling temperature (° C.) in A, C represents the finish rolling temperature (° C.), and G represents the thickness (mm) of the thick steel plate of the final shape.
[Step 3] Water cooling is performed so that 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 satisfy the following formulas (6) and (7). Process.
E-500 ≦ 0 (6)
F-2 ≧ 0 (7)
Here, E represents the water cooling stop temperature (° C.), and F represents the average cooling rate F (° C./s) during water cooling at the center of the plate thickness.
[Step 4] Tempering at a temperature of Ac ₁ point or less.

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