WO2024116737A1 - High-strength ultra-thick steel plate and method for producing same - Google Patents

Abstract

Provided are a high-strength ultra-thick steel plate and a method for producing same. The high-strength ultra-thick steel plate has a specific component composition, includes an aggregate structure in which the X-ray intensity ratio of the (211) plane in a rolled surface at a position of 1/2 the plate thickness is 1.70 or more, and includes a steel structure at the position of 1/2 the plate thickness which is a mixed structure comprising an area fraction of 85-100% of a bainite phase and an area fraction of 0-15% of a ferrite phase. The average grain size of the bainite is 20 μm or less, the circle-equivalent diameter of the maximum porosity remaining in the center of the plate thickness is 200 μm or less, the plate thickness is 50-100 mm, the yield strength at the position of 1/2 the plate thickness is 500 MPa or more, vTrs at the position of 1/2 the plate thickness is -105ºC, and the value of Kca(-10ºC) is 9000 N/mm^3/2 or more.

Description

High strength extra thick steel plate and its manufacturing method

The present invention relates to a high-strength, extra-thick steel plate with excellent brittle crack propagation arrest properties suitable for use in large structures such as ships, marine structures, low-temperature storage tanks, architectural structures, and civil engineering structures, and a method for manufacturing the same.

Large structures such as ships, marine structures, low-temperature storage tanks, architectural structures, and civil engineering structures have a large impact on society, the economy, and the environment if an accident involving brittle fracture occurs. For this reason, there is a constant demand for improving the safety of large structures. Steel materials used in large structures are required to have high levels of toughness and strength at operating temperatures, as well as brittle crack propagation arrestability (arrest performance) that prevents the propagation of brittle cracks.

　Ships such as container ships and bulk carriers do not have standard decks, and are loaded even onto hatch covers, and travel on the ocean while being hit by waves that place heavy loads on them, so for structural and operational reasons they are repeatedly subjected to large bending stresses. For this reason, it is customary to use high-strength, thick steel plates for the hull that can withstand this bending stress. In recent years, as ships have become larger, the steel plates used have become even stronger and thicker.

In general, the higher the strength or thickness of a steel plate, the worse its brittle crack arrestability tends to be. For this reason, it has become difficult to meet the required brittle crack arrestability when steel plates used in container ships and other vessels are made stronger and thicker in recent years.

Increasing the Ni content in steel has been known as a means of improving the brittle crack arrestability of steel materials. For example, 9% Ni steel is used on a commercial scale in liquefied natural gas (LNG) storage tanks, which require brittle crack arrestability at extremely low temperatures. However, increasing the amount of Ni in steel necessitates a significant increase in manufacturing costs, making it difficult to apply to applications other than LNG storage tanks.

On the other hand, for relatively thin steel materials with a plate thickness of less than 50 mm, such as those used in ships and line pipes that do not reach the extremely low temperatures of LNG, the TMCP (Thermo-Mechanical Control Process) method is used to refine the grains and improve toughness. This can give the material excellent brittle crack propagation arrest properties.

For example, in Patent Document 1, the microstructure is a mixed structure of ferrite and bainite, or a mixed structure of ferrite, pearlite and bainite, and the average crystal grain size in the center of the plate thickness is controlled to 5 to 20 μm, thereby improving toughness. Patent Document 1 discloses a technology that improves brittle crack propagation arrestability.

Furthermore, in order to improve the brittle crack propagation arrestability without increasing the alloy cost, a technique for ultra-fine-graining the structure of the surface layer of the steel material is disclosed, for example, in Patent Document 2 and Patent Document 3.

In Patent Document 2, attention is paid to the fact that a shear lip (i.e., a plastic deformation region) that occurs in the surface layer of a steel material when a brittle crack propagates is effective in improving the brittle crack propagation arrest property. Patent Document 2 discloses that the crystal grains in the shear lip portion are refined to absorb the propagation energy of the propagating brittle crack with the crystal grains. In addition, in the manufacturing method of Patent Document 2, the steel material is subjected to a rolling reduction while a process of cooling the surface layer portion to a temperature below the _Ar3 transformation point by controlled cooling after hot rolling and then stopping the controlled cooling to reheat the surface layer portion to a temperature above the _Ar3 transformation point is repeated one or more times. Patent Document 2 discloses that this causes repeated transformation or processing recrystallization to generate an ultrafine ferrite structure or bainite structure in the surface layer portion.

In Patent Document 3, in order to improve the brittle crack propagation arrest properties of a steel material with a microstructure mainly composed of ferrite-pearlite, both surface portions of the steel material are composed of layers having a ferrite structure with ferrite grains having a circular equivalent grain size of 5 μm or less and an aspect ratio of 2 or more at 50% or more by area, while suppressing the variation in ferrite grain size. Patent Document 3 discloses that as a method of suppressing this variation, the maximum reduction rate per pass during finish rolling is set to 12% or less to suppress local recrystallization phenomena.

In addition, a method is also known in which the brittle crack propagation arrest properties are improved by applying a reduction to the transformed ferrite in controlled rolling to develop the texture. For example, the techniques of Patent Document 4 and Patent Document 5 can be mentioned.

In Patent Document 4, the average equivalent circle diameter of effective crystal grains is 25 μm or less in the surface layer portion of a thick steel plate and 35 μm or less in the center portion of the plate thickness, and the texture intensity ratio with respect to the rolling surface and rolling direction is as follows:
I{001}<110>+I{112}<110>+I{332}<113>>5,
I{110}<001>+I{110}<110>+I{001}<010>≦3
Satisfied,
And at the center of the plate thickness,
I{001}<110>+I{112}<110>+I{332}<113>≧3.5
The texture is controlled so as to satisfy the following: Patent Document 4 discloses a technique for improving the brittle crack propagation arrestability by controlling the texture so as to satisfy the following:

In Patent Document 5, the steel plate is divided into three layers: a front and back layer extending from the front and back sides in the thickness direction up to 25%; and the remaining central portion of the plate thickness. In the front and back layer, an area of 5% to 25% of the plate thickness has a texture in which the (100) X-ray plane intensity ratio parallel to the rolling surface is 1.5 or more and less than 2.0, and in the central portion of the plate thickness, a texture in which the (111) and/or (211) X-ray plane intensity ratio parallel to the rolling surface is 2.0 or more is controlled. This technology for improving brittle crack propagation arrest properties is disclosed in Patent Document 5.

International Publication No. 2011/96456 JP 2002-256375 A JP 2011-184754 A JP 2012-172258 A JP 2008-169467 A

However, the technologies described in Patent Documents 2 and 3 involve cooling only the surface layer of the steel material, then reheating it, and processing it during reheating to obtain a specific structure. This means that it is difficult to control on an actual production scale, and there are problems with the large load placed on the rolling and cooling equipment.

Furthermore, the technologies described in Patent Documents 1 to 5 all have a yield strength of 460 MPa or less, taking into consideration the manufacturing conditions and the disclosed experimental data. On the other hand, in order to ensure the strength of the hull, it is necessary to use steel with a yield strength of 500 MPa for the hatch side coamings, for large container ships exceeding the 24,000 TEU class, which are in demand due to the increase in logistics volume in recent years. However, it is unclear whether the manufacturing methods described in the above patent documents can obtain the required characteristics.

Furthermore, since the required brittle crack arrestability increases with increasing strength of steel plate, a steel plate with a yield strength of 500 MPa is required to have an arrestability index, Kca value at -10°C (hereinafter sometimes referred to as "Kca (-10°C)"), of 9000 N/mm3 ^/2 or more. However, such a high-strength arrestable steel and a stable and efficient method for manufacturing it have not yet been established.

In order to improve brittle crack propagation arrestability while maintaining the rigidity of the hull, even thick materials between 50 mm and 100 mm are required to have both high strength and excellent toughness.

The present invention was made in consideration of the above problems, and aims to provide a high-strength, extra-thick steel plate that has excellent toughness and brittle crack propagation arrest properties, even if the steel plate has a yield strength of 500 MPa, and a manufacturing method thereof.

Here, "high strength" in the present invention refers to a high-strength extra thick steel plate having a yield strength (YS) of 500 MPa or more at the 1/2 plate thickness position.
In the present invention, "excellent toughness" refers to a Charpy fracture appearance transition temperature at the 1/2 plate thickness position of a high strength extra thick steel plate being vTrs≦−105° C.
In the present invention, "excellent brittle crack propagation arrestability (arrestability)" refers to a Kca (-10°C) value of 9000 N/mm3 ^/2 or more.
The above-mentioned steel plate having a yield strength of 500 MPa class refers to a steel plate having a yield strength of 500 MPa or more.
The yield strength, vTrs and Kca values can be measured by the methods described in the Examples below.

In order to solve the above problems, the inventors have conducted extensive research into high-strength, extra-thick steel plates that have excellent brittle crack propagation arrest properties and toughness even when the yield strength is 500 MPa or more, and that have good internal quality, as well as a manufacturing method for stably obtaining such steel plates. As a result, the following findings were obtained.

The above "good internal quality" refers to a state in which the circle equivalent diameter of the largest porosity remaining in the center of the steel plate is 200 μm or less. In order to achieve even better internal quality, in addition to this condition, it is effective to ensure that the length in the plate thickness direction of the largest porosity remaining in the center of the steel plate is 100 μm or less.

Specifically, in order to achieve high strength with a yield strength of 500 MPa or more, it is effective to control the carbon equivalent (Ceq) to 0.480 mass% or more.

In order to achieve excellent toughness in the above-mentioned extra thick steel plate, it is effective to control the steel structure at the 1/2 position of the plate thickness so that the average grain size of bainite crystal grains surrounded by grain boundaries with an orientation difference of 15° or more is 20 μm or less, and to control so that the circle equivalent diameter of the maximum porosity remaining in the center part of the steel plate (1/2 position of the steel plate) is 200 μm or less. Since the grain size refining effect improves toughness and makes it difficult for porosity to become the starting point of fracture, the Charpy toughness at the 1/2 position of the plate thickness, where the properties are the lowest, achieves vTrs≦−105°C.
As described above, in addition to configuring the steel structure at half the plate thickness as described above, the length in the plate thickness direction of the maximum porosity remaining in the center of the steel plate is controlled to 100 μm or less, thereby further improving toughness.

In order to achieve excellent brittle crack arrestability in the extra thick steel plate, it is effective to provide a texture that has excellent toughness and an X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 position of the plate thickness of 1.70 or more. This inhibits the linear propagation of brittle cracks and achieves a Kca (-10°C) value (N/mm3 ^/2 ) of 9000 N/mm3 ^/2 or more.

In order to obtain the above toughness, it is effective to control the temperature at the 1/2 thickness position in the hot rolling process to a cumulative reduction rate in the austenite non-recrystallization temperature range of 50.0% or more, and to control the reduction ratio, which represents the ratio of the slab thickness to the plate thickness of the final product, to 4.00 or more. This makes it possible to impart sufficient compressive stress to the 1/2 thickness position without being affected by the plate thickness, even in the case of an extra-thick steel plate with a plate thickness of 50 mm to 100 mm. As a result, the average grain size of the bainite crystal grains can be controlled, and the porosity at the 1/2 thickness position of the steel plate, which is the fracture origin, can be compressed, thereby achieving excellent toughness.
In addition, for the above-mentioned "length of maximum porosity in the sheet thickness direction", it is effective to appropriately control the average reduction rate/pass when the temperature at the 1/2 sheet thickness position is in the austenite recrystallization temperature range.

In order to obtain the brittle crack propagation arrestability, the hot rolling process is controlled so that the cumulative reduction in the austenite non-recrystallization temperature range is 50.0% or more, the reduction ratio is 4.00 or more, and the rolling end temperature at the 1/2 thickness position is _Ar3 or more. This control effectively refines the crystal grains of the steel structure at the 1/2 thickness position and controls the texture at the 1/2 thickness position. This makes it possible to achieve excellent toughness while elongating the austenite in the rolling direction and develop a bainite transformation texture in a specific direction. As a result, a texture is obtained in which the X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 thickness position is 1.70 or more.

The present invention has been completed based on the above findings and further investigations, and the gist of the present invention is as follows.
[1] In mass%,
C: 0.040 to 0.130%,
Si: 0.020 to 0.50%,
Mn: 1.00 to 2.50%,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.010 to 0.100%,
The steel sheet has a composition comprising N: 0.0010 to 0.0100%, O: 0.0100% or less, Ceq defined by formula (1) being 0.480% or more, and the balance being Fe and unavoidable impurities;
The X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 position of the sheet thickness is 1.70 or more,
The steel sheet has a steel structure at a 1/2 position in the sheet thickness direction, which is a mixed structure having an area fraction of 85 to 100% of a bainite phase and an area fraction of 0 to 15% of a ferrite phase, and the average grain size of the bainite crystal grains surrounded by grain boundaries having an orientation difference of 15° or more is 20 μm or less, and the circle equivalent diameter of the maximum porosity remaining in the center of the steel sheet is 200 μm or less,
The plate thickness is 50 mm or more and 100 mm or less,
A high-strength extra-thick steel plate having a yield strength of 500 MPa or more at the 1/2 plate thickness position, a Charpy fracture transition temperature vTrs≦-105°C at the 1/2 plate thickness position, and a Kca(-10°C) value of 9000 N/mm3 ^/2 or more.
Ceq = C + Mn/6 + Cu/15 + Ni/15 + Cr/5 + Mo/5 + V/5 ... (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in formula (1) represent the content (mass %) of each element, and the content of an element that is not contained is set to 0.
[2] The high-strength extra thick steel plate according to [1], further comprising, in addition to the above-mentioned chemical composition, one or two of the following groups A and B, in mass %:
Group A: one or more selected from Ti: 0.030% or less, Nb: 0.050% or less, Cu: 2.00% or less, Ni: 2.50% or less, and Cr: 2.00% or less; Group B: one or more selected from Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less. [3] The high-strength extra thick steel plate according to [1] or [2], wherein the steel structure at the 1/2 position of the plate thickness has ^2.0 or less segregation grains having a thickness of 100 μm or more per mm2.
[4] The high-strength extra thick steel plate according to any one of [1] to [3], wherein the steel structure at the plate thickness 1/2 position has a length in the plate thickness direction of the maximum porosity remaining in the center of the steel plate of 100 μm or less.
[5] A method for producing a high-strength extra thick steel plate according to any one of [1] to [4],
A steel material having the above-mentioned composition is heated to a heating temperature of 1000 to 1200°C,
Next, the rolling start temperature at the 1/2 thickness position is ( _Ar3 point + 100) ° C. or higher, the cumulative reduction rate in the austenite recrystallization temperature range at the 1/2 thickness position is 25.0% or higher, the cumulative reduction rate in the austenite non-recrystallization temperature range at the 1/2 thickness position is 50.0% or higher, and the rolling end temperature at the 1/2 thickness position is _Ar3 point or higher.
The slab is hot-rolled under the conditions of a thickness of 200 mm or more and a reduction ratio (representing the ratio of the slab thickness to the plate thickness of the final product) of 4.00 or more.
Next, cooling is performed under the conditions of a cooling start temperature at the 1/2 plate thickness position: Ar ₃ point or higher, an average cooling rate in a temperature range of 700 to 500°C at the 1/2 plate thickness position: 3.5°C/s or higher, and a cooling stop temperature at the 1/2 plate thickness position: 500°C or lower.
[6] In the hot rolling,
The temperature at the 1/2 position of the plate thickness is the average reduction rate/pass in the austenite non-recrystallization temperature range: 4.5% or more;
The method for producing a high-strength extra thick steel plate according to [5] is carried out under the conditions that the rolling end temperature at the 1/4 plate thickness position is (the rolling end temperature at the 1/2 plate thickness position - 20) ° C. or more, and the rolling end temperature at a position 2 mm in the plate thickness direction from the steel plate surface is (the rolling end temperature at the 1/2 plate thickness position - 30) ° C. or more.
[7] In the hot rolling,
The method for producing a high-strength extra thick steel plate according to [5] or [6], wherein the temperature at the 1/2 position of the plate thickness is set under conditions such that the average reduction rate/pass in the austenite recrystallization temperature range is 4.0% or more.

According to the present invention, the texture of the rolled surface at 1/2 the plate thickness position is appropriately controlled by a mixed structure consisting of bainite and ferrite, and the steel structure at 1/2 the plate thickness position is refined, improving toughness and brittle crack propagation properties. In addition, according to the manufacturing method of the present invention, the rolling conditions in the hot rolling process are optimized, so that the extra thick steel plate of the present invention has good internal quality and excellent toughness and brittle crack propagation properties, even though it has a high strength of 500 MPa or more in yield strength.

For example, in the shipbuilding industry, the high-strength extra-thick steel plate of the present invention can be applied to deck members joined to hatch side coamings in the strong deck structures of container ships and bulk carriers, thereby contributing to improved ship safety. In this way, the present invention is extremely useful industrially.

The following describes an embodiment of the present invention. Note that the present invention is not limited to the following embodiment.

First, we will explain the reason why the composition of the high-strength extra-thick steel plate in this invention is limited. In this specification, "%" in relation to the composition means "mass %" unless otherwise specified.

C: 0.040 to 0.130%
C is an element that acts to increase the hardenability of steel and is necessary to achieve the desired strength characteristics. In the present invention, in order to obtain the above effect, the C content is set to 0.040% or more. On the other hand, if the C content exceeds 0.130%, not only does the weldability deteriorate, but also a large amount of coarse carbides that become the starting point of fracture are generated, which has an adverse effect on toughness. Therefore, the C content is set to the range of 0.040 to 0.130%. The preferred lower limit of the C content is 0.050% or more. The preferred upper limit of the C content is 0.100% or less.

Si: 0.020 to 0.50%
Silicon is a component necessary for deoxidization and the like, and is also an element that has the effect of increasing the hardenability of steel by suppressing the formation of coarse carbides. In order to achieve the desired strength characteristics, silicon is contained at 0.020% or more. On the other hand, if the silicon content exceeds 0.50%, not only is the surface quality of the steel impaired, but the toughness is also extremely deteriorated. Therefore, the silicon content is set to a range of 0.020 to 0.50%. The lower limit of the silicon content is preferably 0.050% or more. The upper limit of the silicon content is preferably 0.30% or less.

Mn: 1.00 to 2.50%
Mn is an element that acts to increase the hardenability of steel and is necessary to exhibit the desired strength. In order to obtain the above effect, the Mn content is set to 1.00% or more. On the other hand, if the Mn content is high, the strength increases excessively, and in addition to reducing the toughness, the alloy cost becomes excessively high. From these viewpoints, the Mn content is set to 2.50% or less. Therefore, the Mn content is set to the range of 1.00 to 2.50%. The lower limit of the Mn content is preferably 1.50% or more. The upper limit of the Mn content is preferably 2.35% or less.

P: 0.020% or less, S: 0.010% or less P and S are inevitable impurities in steel. If their contents increase, toughness deteriorates. In order to maintain good toughness in extra-thick steel plates with a plate thickness of 50 mm or more and 100 mm or less, the P content is suppressed to 0.020% or less and the S content is suppressed to 0.010% or less. The P content is preferably 0.015% or less, and more preferably 0.006% or less. The S content is preferably 0.006% or less, and more preferably 0.002% or less.

The lower limits of the P content and S content are not particularly limited. Since excessive reduction of P and S leads to increased manufacturing costs, the P content and S content are each preferably 0.0005% or more, and more preferably 0.0010% or more.

Al: 0.010 to 0.100%
Al is an element that acts as a deoxidizer, and in order to obtain this effect, the Al content must be 0.010% or more. On the other hand, if the Al content exceeds 0.100%, the toughness decreases, and when welding is performed, the toughness of the weld metal part decreases. Therefore, the Al content is set to a range of 0.010 to 0.100%. The lower limit of the Al content is preferably 0.020% or more, and more preferably 0.040% or more. The upper limit of the Al content is preferably 0.080% or less, more preferably 0.070% or less, and even more preferably 0.050% or less.

N: 0.0010 to 0.0100%
N combines with Al in the steel and precipitates as AlN during rolling, resulting in a pinning effect that refines the austenite grain size, thereby strengthening the steel and improving its toughness. To achieve this effect, the N content must be 0.0010% or more. On the other hand, if the N content exceeds 0.0100%, toughness deteriorates. Therefore, the N content is set to 0.0100% or less. Therefore, the N content is set to a range of 0.0010 to 0.0100%. The lower limit of the N content is preferably 0.0020% or more, and more preferably 0.0040% or more. The upper limit of the N content is preferably 0.0070% or less.

O: 0.0100% or less O (oxygen) is an element contained as an inevitable impurity, but since it is an element that should be particularly reduced, its content is specified. O forms oxides and becomes the starting point of brittle fracture, so it has adverse effects such as a decrease in toughness and a decrease in brittle crack propagation arrestability. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0050% or less, and more preferably 0.0030% or less. The lower limit of the O content is not particularly limited and may be 0%. Usually, O is an element that is inevitably contained in steel as an impurity, so industrially it may be more than 0%. In addition, excessive reduction of O leads to an increase in refining costs, so from the viewpoint of cost, it is preferable to set the O content to 0.0020% or more.

In the present invention, each element is contained within the above range, and Ceq (carbon equivalent) (%) defined by formula (1) satisfies the following range.
Ceq=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5... (1)
Here, C, Mn, Cu, Ni, Cr, Mo and V in formula (1) represent the content (mass %) of each element, and the content of an element that is not contained is set to 0.

Ceq: 0.480% or more In order to realize excellent strength due to improved hardenability and improved brittle crack propagation arrestability due to texture development in the extra thick steel plate of the present invention, Ceq represented by formula (1) is adjusted to 0.480% or more. This improves the hardenability of the steel plate, and even at the 1/2 plate thickness position where the cooling rate is the smallest, the steel structure becomes bainite throughout, thereby improving strength. Ceq is preferably 0.495% or more. Note that there is no particular upper limit for Ceq. If Ceq is excessively increased, the alloy cost becomes too high, so Ceq is preferably 0.560% or less, and more preferably 0.545% or less.

The above is the basic composition of the present invention, with the remainder being Fe and unavoidable impurities.

　By having this basic composition and satisfying the above formula (1), the high-strength extra-thick steel plate of the present invention can obtain the desired characteristics.

In the present invention, in order to further improve the properties, in addition to the basic composition, it is possible to contain one or two groups selected from Group A and Group B described below as necessary. Note that each of the following components Ti, Nb, Cu, Ni, Cr, Mo, V, W, Co, B, Ca, Mg, and REM can be contained as necessary, so these components may be 0%.

　Group A: One or more selected from Ti: 0.030% or less, Nb: 0.050% or less, Cu: 2.00% or less, Ni: 2.50% or less, and Cr: 2.00% or less

Ti: 0.030% or less Ti has the effect of forming nitrides, carbides, or carbonitrides by containing a small amount of Ti, refining the crystal grains, and improving the toughness of the base material. In order to obtain this effect, it is preferable to set the Ti content to 0.005% or more. On the other hand, if the Ti content exceeds 0.030%, the toughness of the base material and the welded heat affected zone decreases. Therefore, when Ti is contained, the Ti content is preferably 0.030% or less, and more preferably 0.025% or less. The Ti content is more preferably 0.010% or more, and even more preferably 0.019% or more.

Nb: 0.050% or less Nb has the effect of increasing the hardenability of steel and expanding the non-recrystallization temperature range in rolling in the austenite region (i.e., hot rolling). In order to obtain this effect, it is preferable to set the Nb content to 0.005% or more. On the other hand, if the Nb content exceeds 0.050%, coarse NbC may precipitate, resulting in a decrease in toughness. Therefore, when Nb is contained, the Nb content is preferably 0.050% or less, and more preferably 0.040% or less. The Nb content is more preferably 0.010% or more, and even more preferably 0.020% or more.

Cu: 2.00% or less Cu is an element that enhances the hardenability of steel. This element directly contributes to improving the strength after rolling. In addition, Cu can be contained to improve functions such as toughness, high-temperature strength, and weather resistance. In order to obtain the above effects of this element, it is preferable that the Cu content is 0.01% or more. On the other hand, if the Cu content exceeds 2.00%, it will cause deterioration of weldability and toughness and an increase in alloy cost. Therefore, when Cu is contained, the Cu content is preferably 2.00% or less, and more preferably 1.00% or less. It is more preferable that the Cu content is 0.05% or more.

Ni: 2.50% or less Ni is an element that enhances the hardenability of steel. Ni directly contributes to improving the strength after rolling. In addition, Ni can be contained to improve functions such as toughness, high-temperature strength, and weather resistance. In order to obtain the above effects of this element, it is preferable that the Ni content is 0.01% or more. On the other hand, if the Ni content exceeds 2.50%, it will cause deterioration of weldability and toughness and an increase in alloy cost. Therefore, when Ni is contained, it is preferable that the Ni content is 2.50% or less, and more preferably 2.00% or less. It is more preferable that the Ni content is 0.10% or more.

Cr: 2.00% or less Cr is an element that enhances the hardenability of steel. This element directly contributes to improving the strength after rolling. In addition, Cr can be contained to improve functions such as toughness, high-temperature strength, and weather resistance. In order to obtain the above effects of this element, it is preferable that the Cr content is 0.01% or more. On the other hand, if the Cr content exceeds 2.00%, it will cause deterioration of weldability and toughness and increase in alloy cost. Therefore, when Cr is contained, it is preferable that the Cr content is 2.00% or less, and more preferably 1.00% or less. It is more preferable that the Cr content is 0.05% or more.

　Group B: One or more selected from Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less

Mo: 0.50% or less Mo is an element that enhances the hardenability of steel. This element directly contributes to improving the strength after rolling. In addition, Mo can be contained to improve functions such as toughness, high-temperature strength, and weather resistance. In order to obtain the above effects of this element, it is preferable that the Mo content is 0.01% or more. On the other hand, if the Mo content exceeds 0.50%, it will cause deterioration of weldability and toughness and an increase in alloy cost. Therefore, when Mo is contained, it is preferable that the Mo content is 0.50% or less, and more preferably 0.30% or less. It is more preferable that the Mo content is 0.03% or more.

V: 0.50% or less V is an element that improves the strength of steel by precipitation strengthening, which is precipitated as V(CN). This effect is exhibited by making the V content 0.001% or more. However, if the V content exceeds 0.50%, the toughness may decrease. Therefore, when V is contained, the V content is preferably 0.50% or less, and more preferably 0.30% or less. The V content is more preferably 0.005% or more.

W: 0.50% or less W is an element that has the effect of improving the strength of the steel plate. In order to obtain this effect, the W content is preferably 0.001% or more. On the other hand, if the W content exceeds 0.50%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when W is contained, the W content is preferably 0.50% or less, and more preferably 0.30% or less. The W content is more preferably 0.005% or more.

Co: 0.50% or less Co is an element that has the effect of improving the strength of the steel plate. In order to obtain this effect, the Co content is preferably 0.001% or more. On the other hand, if the Co content exceeds 0.50%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when Co is contained, the Co content is preferably 0.50% or less, and more preferably 0.30% or less. The Co content is more preferably 0.005% or more.

B: 0.0100% or less B is an element that enhances the hardenability of steel in small amounts. However, if the B content exceeds 0.0100%, the toughness of the welded part is reduced. Therefore, when B is contained, the B content is preferably 0.0100% or less, and more preferably 0.0030% or less. In order to improve the strength of the steel plate, the B content is preferably 0.0001% or more, and more preferably 0.0005% or more.

Ca: 0.0100% or less Ca refines the structure of the weld heat affected zone and improves toughness. In order to obtain this effect, when Ca is contained, the Ca content is preferably 0.0005% or more, more preferably 0.0020% or more. On the other hand, when the Ca content exceeds 0.0100%, coarse inclusions are formed, which deteriorates toughness. Therefore, when Ca is contained, the Ca content is preferably 0.0100% or less, more preferably 0.0050% or less.

Mg: 0.0100% or less Like Ca, Mg refines the structure of the welded heat affected zone and improves toughness. In order to obtain this effect, when Mg is contained, the Mg content is preferably 0.0005% or more, and more preferably 0.0020% or more. On the other hand, when the Mg content exceeds 0.0100%, coarse inclusions are formed, which deteriorates toughness. Therefore, when Mg is contained, the Mg content is preferably 0.0100% or less, and more preferably 0.0050% or less.

REM: 0.0200% or less Like Ca, REM (rare earth metal) refines the structure of the weld heat affected zone and improves toughness. In order to obtain this effect, when REM is contained, the REM content is preferably 0.0005% or more, more preferably 0.0015% or more. On the other hand, when the REM content exceeds 0.0200%, coarse inclusions are formed, which deteriorates toughness. Therefore, when REM is contained, the REM content is preferably 0.0200% or less, more preferably 0.0100% or less.

<Texture>
[X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 position of the plate thickness: 1.70 or more]
In order to improve the brittle crack propagation arrestability of an extra-thick steel plate having a plate thickness of 50 mm or more and 100 mm or less, it is necessary to make the X-ray intensity ratio of the (211) plane on the rolled surface at the plate thickness 1/2 position 1.70 or more. If the X-ray intensity ratio of the (211) plane is less than 1.70, the brittle crack tends to progress linearly, and as a result, the brittle crack propagation arrestability of the plate thickness is reduced. Therefore, the X-ray intensity ratio of the (211) plane on the rolled surface at the plate thickness 1/2 position is made 1.70 or more. In order to further improve the brittle crack propagation arrestability, the X-ray intensity ratio of the (211) plane is preferably made 1.80 or more, more preferably 1.90 or more.
The upper limit of the X-ray intensity ratio of the (211) plane is not particularly specified. From the viewpoints of rolling efficiency and production load, the X-ray intensity ratio of the (211) plane is preferably 2.50 or less, and more preferably 2.25 or less.

Here, the X-ray intensity ratio of the (211) plane is a value that represents the degree of integration of the (211) crystal plane of the target material (high-strength extra-thick steel plate). The X-ray intensity ratio of the (211) plane is calculated as the ratio (I(211)/I0 _{(211)) of the X-ray diffraction intensity of the reflection of the (211) plane of the target material (I(} 211)) to the X-ray diffraction intensity of the reflection of the (211) plane of a random standard sample without texture (I0 _{( 211) )} , as described in the Examples below.

<Steel structure at 1/2 plate thickness>
In the present invention, the steel structure at the 1/2 sheet thickness position is a mixed structure having an area fraction of 85 to 100% of bainite phase and an area fraction of 0 to 15% of ferrite phase, the average grain size of bainite crystal grains surrounded by grain boundaries having an orientation difference of 15° or more is 20 μm or less, and the circle equivalent diameter of the maximum porosity remaining in the center of the steel sheet is 200 μm or less.

[Area fraction of bainite phase: 85 to 100%]
In the present invention, the area fraction of the bainite phase at the 1/2 sheet thickness position must be 85% or more. By controlling the steel structure in this way, it is possible to increase the (211) plane orientation, which is advantageous for brittle crack propagation properties. It is more preferable that the area fraction of the bainite phase is 90% or more. The upper limit of the area fraction of the bainite phase is 100%.

[Area fraction of ferrite phase at 1/2 sheet thickness position: 0 to 15%]
The high-strength extra-thick steel plate of the present invention needs to have an area fraction of ferrite phase at 1/2 thickness position of 0 to 15%. If the area fraction of ferrite phase exceeds 15%, the fraction of hard structure decreases and the texture of the (211) plane does not develop sufficiently, resulting in a decrease in strength and brittle crack propagation arrestability. Therefore, the area fraction of ferrite phase at 1/2 thickness position is set to 15% or less. In order to further increase the strength and brittle crack propagation arrestability, the area fraction of the ferrite phase is preferably set to 10% or less, more preferably 5% or less.

The area fraction in the present invention can be measured by the method described in the Examples below.

[Average grain size of bainite grains at 1/2 position of plate thickness: 20 μm or less]
In order to improve the toughness and the brittle crack arrestability of the high-strength extra-thick steel plate of the present invention, the average grain size of the bainite grains in the steel structure at the 1/2 position of the plate thickness (average grain size of bainite) must be 20 μm or less. If the average grain size exceeds 20 μm, the grain size becomes coarse and the fracture surface unit of the cleavage fracture surface of the grain becomes large, so that the toughness and the brittle crack arrestability are reduced, and as a result, the brittle crack arrestability of the plate thickness is reduced. The average grain size is preferably 18 μm or less, and more preferably 15 μm or less.

On the other hand, the smaller the average grain size, the more advantageous the toughness and brittle crack propagation arrest properties, so there is no particular limit to the lower limit. From the viewpoint of rolling efficiency, the lower limit of the average grain size is 1 μm. Therefore, the average grain size is preferably 1 μm or more, and more preferably 3 μm or more.

Here, "grain size" is defined as the circle-equivalent diameter of a grain surrounded by a grain boundary (high-angle grain boundary) with an orientation difference of 15° or more with adjacent grains. In addition, the above "average grain size" refers to the average grain size at the 1/2 position of the plate thickness.

The average grain size at the 1/2 plate thickness position can be measured using the method described in the examples below.

[Circle equivalent diameter of maximum porosity remaining in the center of steel sheet: 200 μm or less]
The high-strength extra-thick steel plate of the present invention needs to have a circle-equivalent diameter of the maximum porosity remaining in the center of the steel plate (at 1/2 the plate thickness position) of 200 μm or less. If the circle-equivalent diameter of the maximum porosity exceeds 200 μm, the porosity becomes the fracture origin and the toughness is significantly reduced. Therefore, the circle-equivalent diameter of the maximum porosity remaining at the 1/2 plate thickness position is set to 200 μm or less. In order to further increase the toughness, the circle-equivalent diameter is preferably set to 150 μm or less, and more preferably set to 100 μm or less. The lower limit of the circle-equivalent diameter is not particularly specified. From the viewpoint of rolling efficiency, the circle-equivalent diameter is preferably set to 25 μm or more, and more preferably set to 50 μm or more.

As mentioned above, in order to further improve the internal quality, it is effective to make the length in the plate thickness direction of the maximum porosity remaining in the center of the steel plate 100 μm or less, in addition to the circle equivalent diameter of the maximum porosity mentioned above. The reasons for this are as follows.

In the high-strength extra-thick steel plate of the present invention, it is desirable that the length in the thickness direction of the maximum porosity remaining in the center of the steel plate is 100 μm or less. If the length in the thickness direction of the maximum porosity exceeds 100 μm, the porosity may become the starting point of fracture, resulting in a significant decrease in toughness. In order to further increase toughness, the length in the thickness direction is more preferably 75 μm or less, and even more preferably 50 μm or less. There is no particular lower limit for the length in the thickness direction. From the viewpoint of rolling efficiency, the length in the thickness direction is preferably 10 μm or more, and more preferably 25 μm or more. The length in the thickness direction of the maximum porosity at the 1/2 position of the plate thickness can be measured by the method described in the examples below. Note that the "length in the thickness direction of the maximum porosity" and the "circle equivalent diameter of the maximum porosity" are different. The former is an indicator of the defect size of the porosity, and the latter is an indicator of the shape of the porosity.

In addition to the above-mentioned steel structure and texture, the present invention may also have the configurations described below.

[Number density of segregated grains at 1/2 sheet thickness position] (preferred conditions)
Number of segregation grains of 100 μm or more per ^mm2 : 2.0 or less Large segregation grains of 100 μm or more deteriorate toughness, so in the high-strength extra thick steel plate of the present invention, the number density of segregation grains of 100 μm or more at the 1/2 position of the plate thickness is desirably 2.0 or less per ^mm2 . Note that, since the segregation grains have an elliptical shape stretched in the rolling direction when viewed from the plate thickness direction, the length of the ellipse in the plate thickness direction is referred to as the "thickness" in the present invention.

Furthermore, if the number density of the segregated grains exceeds 2.0 grains/ ^mm2 , the number of segregated grains that can become fracture initiation points increases, making the material more susceptible to brittle fracture and reducing toughness. In order to further increase the strength and brittle crack propagation arrestability, the number density is preferably 1.5 grains/ ^mm2 or less, and more preferably 1.0 grains/ ^mm2 or less.

The thickness and number density of the segregated grains described above can be measured using the method described in the examples below.

<Strength>
[Yield strength at 1/2 plate thickness: 500 MPa or more]
The high-strength extra-thick steel plate of the present invention has a yield strength of 500 MPa or more, measured using a JIS 14A tensile test piece with a diameter of 14 mm taken from the 1/2 position of the plate thickness. From the viewpoint of ensuring the hull strength of large container ships exceeding the 24,000 TEU class, it is necessary to use a high-strength and thick steel plate. From the viewpoint of use in the above-mentioned large container ships, the plate thickness of the extra-thick steel plate is set to 50 mm or more and less than 100 mm.

<Charpy fracture transition temperature>
[vTrs at 1/2 plate thickness: -105°C or less]
In order to improve the brittle crack propagation arrestability of a high-strength extra-thick steel plate having a yield strength of 500 MPa or more, it is necessary to set vTrs at the 1/2 plate thickness position to -105°C or less. vTrs: When the transition temperature is higher than -105°C (i.e., when vTrs exceeds -105°C), cracks tend to propagate in one direction, and as a result, the brittle crack propagation arrestability of the plate thickness decreases. Therefore, in the high-strength extra-thick steel plate of the present invention, vTrs at the 1/2 plate thickness position is set to -105°C or less. In order to further improve the brittle crack propagation arrestability, it is preferable to set vTrs to -110°C or less, and more preferably to set vTrs to -120°C or less.

Note that there is no particular lower limit for vTrs. From the viewpoint of rolling efficiency, the vTrs is preferably -160°C or higher, and more preferably -150°C or higher.

vTrs can be evaluated by taking a JIS No. 4 impact test specimen from the 1/2 position of the plate thickness of a high-strength extra-thick steel plate and conducting a Charpy test, as described in the examples below.

<Brittle crack propagation arrest properties>
[Kca (-10°C) value: 9000 N/mm3 ^/2 or more]
High-strength extra thick steel plate with a yield strength of 500 MPa or more must have a Kca (-10°C) value of 9000 N/mm3 ^/2 or more to prevent brittle crack propagation in the hull. To further improve brittle crack propagation arrestability, the Kca value at -10°C is preferably 10000 N/mm3 ^/2 or more, and more preferably 11000 N/mm3 ^/2 or more.

There is no particular lower limit for the Kca value. From the viewpoint of rolling efficiency, the Kca value at -10°C is preferably 25000 N/mm ^3/2 or less, and more preferably 20000 N/mm ^3/2 or less.
The Kca value can be evaluated by a standard temperature gradient ESSO test as described in the Examples below.

As explained above, by adjusting the composition of the components and refining the texture and the steel structure at the 1/2 plate thickness position, the high-strength extra-thick steel plate of the present invention has high strength and high (excellent) brittle crack propagation arrest properties and toughness.

Next, a method for producing a high-strength extra thick steel plate according to one embodiment of the present invention will be described.
The high-strength extra thick steel plate of the present invention is produced by subjecting a steel material having the above-mentioned composition to a heating step, a hot rolling step, and a cooling step under specific conditions described below.

Each process is explained in detail below. Unless otherwise specified, the temperature in each process refers to the temperature at the 1/2 plate thickness position of the steel material and hot-rolled plate. For example, it can be determined by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis and correcting the result by the surface temperature of the steel plate.

<Heating process>
First, a steel material (slab) having the above-mentioned composition is heated to a heating temperature of 1000 to 1200°C.

[Heating temperature of steel material: 1000-1200°C]
If the heating temperature of the steel material is less than 1000°C, the heating temperature is too low, resulting in high deformation resistance and an increased load on the hot rolling machine, making the subsequent hot rolling difficult. On the other hand, if the heating temperature of the steel material is high and exceeds 1200°C, the austenite grains become coarse, resulting in a decrease in toughness and a failure to obtain the desired brittle crack propagation arrest characteristic. In addition, the strength of the steel plate decreases. Furthermore, oxidation becomes severe, increasing oxidation loss and possibly decreasing yield. For these reasons, the heating temperature is set to 1200°C or less. The above heating temperature is preferably 1050°C or more, more preferably 1080°C or more. The above heating temperature is preferably 1150°C or less, more preferably 1130°C or less.

<Hot rolling process>
Next, the heated steel material is hot rolled under the following conditions: a rolling start temperature at the 1/2 thickness position: ( _Ar3 point + 100)°C or higher; a cumulative reduction rate in the austenite recrystallization temperature range at the 1/2 thickness position: 25.0% or higher; a cumulative reduction rate in the austenite non-recrystallization temperature range at the 1/2 thickness position: 50.0% or higher; and a rolling end temperature at the 1/2 thickness position: _Ar3 point or higher, the slab thickness is 200 mm or more, and the reduction ratio, which represents the ratio of the slab thickness to the plate thickness of the final product, is 4.00 or more.

[Rolling start temperature: ( _Ar3 point + 100) ° C. or higher]
When hot rolling the steel material heated in the above-mentioned heating process, if the temperature at which hot rolling is started at the 1/2 plate thickness position is less than ( _Ar3 point + 100) ° C., recrystallization does not occur sufficiently in the hot-rolled plate after the hot rolling process is completed. Therefore, the austenite grain size does not become fine, and the toughness decreases. As a result, the desired brittle crack propagation arrest property is not obtained. Therefore, the above-mentioned rolling start temperature is set to ( _Ar3 point + 100) ° C. or higher. From the viewpoint of securing the time to perform hot rolling in the non-recrystallized region described later, the above-mentioned rolling start temperature is preferably set to ( _Ar3 point + 150) ° C. or higher, and more preferably set to ( _Ar3 point + 200) ° C. or higher.

The upper limit of the rolling start temperature should be in accordance with the heating temperature of the steel material described above. That is, the rolling start temperature is preferably 1150°C or less, more preferably 1130°C or less, and even more preferably 1000°C or less.

The _Ar3 point (also referred to as the " _Ar3 transformation point") (°C) can be calculated according to the following formula (3).
Ar ₃ point (°C) = 910 - 273 x C - 74 x Mn - 57 x Ni - 16 x Cr - 9 x Mo - 5 x Cu ... (3)
In formula (3), each element symbol represents the content (mass%) of the corresponding element in the steel, and elements that are not contained are represented as 0.

[Cumulative reduction in austenite recrystallization temperature range: 25.0% or more]
Hot rolling is performed such that the cumulative reduction in the austenite recrystallization temperature range at the temperature at 1/2 the plate thickness is 25.0% or more. If the cumulative reduction in this temperature range is less than 25.0%, the austenite grains are not sufficiently refined, and the toughness is not improved, and as a result, the Charpy fracture transition temperature at the 1/2 plate thickness position: -105°C or less is not achieved. In addition, reduction in the high temperature range effective for pressing the voids inside the steel plate cannot be achieved, and as a result, defects remain inside the steel plate. The cumulative reduction in this temperature range is preferably 40.0% or more, more preferably 50.0% or more, and even more preferably 55.0% or more.

The upper limit of the cumulative rolling reduction in this temperature range is not particularly limited. Since the effect of improving grain refinement described above becomes saturated, the cumulative rolling reduction in this temperature range is preferably 75.0% or less, more preferably 70.0% or less, even more preferably 65.0% or less, and even more preferably 60.0% or less.

In the case of the component composition of the present invention, the cumulative rolling reduction is preferably 25.0% or more in the temperature range of 1100 to 950°C.

[Cumulative reduction in austenite non-recrystallization temperature range: 50.0% or more]
Furthermore, hot rolling is performed with a cumulative reduction of 50.0% or more when the temperature at the 1/2 thickness position is in the austenite non-recrystallization temperature range. By setting the cumulative reduction in this temperature range to 50.0% or more, the rolling texture developed when rolling the austenite phase is transformed from austenite to bainite. At this time, by performing phase transformation along the slip system selected by variant selection, a texture is obtained in which the X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 thickness position is 1.70 or more. On the other hand, if the cumulative reduction in this temperature range is less than 50.0%, the refinement of the bainite crystal grains becomes insufficient, and the Charpy fracture transition temperature (vTrs): -105°C or less at the 1/2 thickness position is not achieved. In addition, a texture in which the X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 thickness position is 1.70 or more is not obtained. The cumulative rolling reduction in this temperature range is preferably 55% or more, and more preferably 60% or more.

The upper limit of the cumulative reduction in this temperature range is not particularly limited. From the viewpoint of not impeding the rolling efficiency, the cumulative reduction in this temperature range is preferably 75% or less, and more preferably 70% or less.

In the case of the component composition of the present invention, the cumulative rolling reduction is preferably 50.0% or more in the temperature range of less than 950°C and equal to or greater than 700°C.

[Rolling end temperature at 1/2 plate thickness position: Ar ₃ point or higher]
The hot rolling process must be completed at a rolling end temperature at the 1/2 thickness position of _Ar3 point (°C) or higher. If the temperature of the hot rolled sheet becomes lower than _Ar3 point (°C) during hot rolling, phase transformation to ferrite begins, and the fraction of bainite that increases the X-ray intensity of the (211) plane decreases, so the X-ray intensity ratio of the (211) plane decreases. As a result, excellent brittle crack propagation arrest properties cannot be obtained. Furthermore, the lower the temperature, the higher the deformation resistance, which causes a problem of increased load on the hot rolling machine. In addition, the rolling end temperature is preferably ( _Ar3 point + 10) °C or higher from the viewpoint of setting the cooling start temperature of the subsequent process to _Ar3 point (°C) or higher.
There is no particular upper limit to the rolling end temperature. From the viewpoint of rolling efficiency, the rolling end temperature is preferably ( _Ar3 point + 60)°C or less, and more preferably ( _Ar3 point + 50)°C or less.

[Reduction ratio: 4.00 or more]
In the present invention, in addition to the above-mentioned hot rolling conditions, it is also important to control the thickness of the hot-rolled sheet after the hot rolling process is completed.

Specifically, the thickness of the slab is set to 200 mm or more, and the rolling in the hot rolling process is controlled so that the ratio of the slab thickness to the plate thickness of the final product (i.e., the reduction ratio) is 4.00 or more. If the reduction ratio is less than 4.00, the bainite grains are not sufficiently refined, and the toughness is not improved. As a result, the Charpy fracture transition temperature at the 1/2 plate thickness position: -105°C or less is not achieved. The thickness of the slab is preferably 300 mm or more, and more preferably 350 mm or more. The reduction ratio is preferably 4.50 or more, and more preferably 5.00 or more.
The upper limit of the reduction ratio is not particularly limited. From the viewpoint of rolling efficiency, the reduction ratio is preferably 7.50 or less, and more preferably 7.00 or less.

In the present invention, the hot rolling step is performed under the above-mentioned hot rolling conditions, thereby obtaining the effects. From the viewpoint of obtaining the effects more effectively, it is also effective to further have the following hot rolling conditions in the hot rolling step.
Specifically, the average reduction rate/pass in the austenite recrystallization temperature range at the 1/2 position of the plate thickness is set to 4.0% or more.
More specifically, the temperature at the 1/2 thickness position is set to an average reduction rate/pass in the austenite non-recrystallization temperature region of 4.5% or more, the rolling end temperature at the 1/4 thickness position is set to be (the rolling end temperature at the 1/2 thickness position-20)°C or more, and the rolling end temperature in the surface layer is set to be (the rolling end temperature at the 1/2 thickness position-30)°C or more.

[Average reduction rate/pass in the austenite recrystallization temperature range: 4.0% or more] (optional conditions)
In the hot rolling process, it is preferable to perform hot rolling with an average reduction rate/pass of 4.0% or more in the austenite recrystallization temperature range at the temperature at 1/2 the plate thickness. If the average reduction rate/pass in this temperature range is less than 4.0%, the grain refinement effect due to the recrystallization of austenite grains becomes insufficient, and the grain refinement of bainite after transformation may become insufficient. In addition, the strain caused by rolling in a high temperature range, which is effective in reducing the thickness of the voids inside the steel plate, cannot be applied to the 1/2 plate thickness position of the steel plate, and as a result, defects inside the steel plate that are the fracture origin may remain large, and the toughness may decrease. The above-mentioned adjustment of the length in the plate thickness direction of the maximum porosity remaining in the center of the steel plate can be achieved by controlling the hot rolling conditions. The average reduction rate/pass in this temperature range is more preferably 4.5% or more, even more preferably 5.0% or more, and even more preferably 6.0% or more.

The upper limit of the average rolling reduction/pass in this temperature range is not particularly limited. From the viewpoint of production load, the average rolling reduction/pass in this temperature range is preferably 10.0% or less, more preferably 8.0% or less, and even more preferably 7.0% or less.

[Average reduction rate/pass in austenite non-recrystallization temperature range: 4.5% or more] (optional condition)
In the hot rolling process, it is preferable to perform hot rolling with an average reduction rate/pass of 4.5% or more in the austenite non-recrystallization temperature range at the 1/2 thickness position. If the average reduction rate/pass in this temperature range is less than 4.5%, the austenite grains may not be elongated sufficiently, and the bainite grains may not be refined sufficiently. In addition, the reduction rate may be insufficient in the low temperature range when the deformation resistance difference between the thicknesses is large, and the amount of strain applied to the 1/2 thickness position may decrease, and as a result, the segregation where the alloy elements are concentrated, which is the fracture origin, may not be dispersed, and the toughness may decrease. The average reduction rate/pass in this temperature range is more preferably 5.0% or more, and even more preferably 6.0% or more.

The upper limit of the average rolling reduction/pass in this temperature range is not particularly limited. From the viewpoint of production load, the average rolling reduction/pass in this temperature range is preferably 10% or less, more preferably 7.0% or less, and even more preferably 6.0% or less.

The above "average reduction rate/pass" refers to the average reduction rate per pass during rolling within a certain temperature range.

[Rolling end temperature at 1/4 plate thickness position: (Rolling end temperature at 1/2 plate thickness - 20) ° C or less] (arbitrary condition)
In the hot rolling process, it is preferable that the rolling end temperature at the 1/4 thickness position is at a temperature of (rolling end temperature at 1/2 thickness - 20) ° C or less at the 1/4 thickness position. If the temperature at the 1/4 thickness position of the hot rolled sheet during hot rolling exceeds (rolling end temperature at 1/2 thickness - 20) ° C, the rolling reduction in the state where the deformation resistance difference between the sheet thicknesses is large in the low temperature range becomes insufficient, and the amount of strain applied to the 1/2 thickness position decreases. In addition, the segregation where the alloy elements that are the fracture origin are concentrated cannot be dispersed. As a result, the toughness decreases. This rolling end temperature is preferably (Ar ₃ point - 30) ° C or less.

The upper limit of the rolling end temperature at the 1/4 sheet thickness position is not particularly limited. From the viewpoint of rolling efficiency, the rolling end temperature is preferably ( _Ar3 point - 50) ° C. or higher, and more preferably ( _Ar3 point - 40) ° C. or higher.

[Rolling end temperature at a position 2 mm from the steel plate surface in the plate thickness direction: (Rolling end temperature at 1/2 plate thickness - 30) ° C or less] (arbitrary condition)
The hot rolling process is preferably completed at a rolling end temperature at a position 2 mm from the steel sheet surface in the sheet thickness direction at a temperature of (rolling end temperature at 1/2 sheet thickness - 30) ° C or less. If the temperature at a position 2 mm from the steel sheet surface in the sheet thickness direction of the hot rolled sheet during hot rolling exceeds (rolling end temperature at 1/2 sheet thickness - 30) ° C, the rolling reduction in the state where the deformation resistance difference between the sheet thicknesses is large in the low temperature range becomes insufficient, and the amount of strain applied to the sheet thickness 1/2 position decreases. In addition, the segregation where the alloy elements that are the fracture origin are concentrated cannot be dispersed. As a result, the toughness decreases. This rolling end temperature is preferably (Ar ₃ point - 40) ° C or less.

In addition, there is no particular upper limit to the rolling end temperature at a position 2 mm from the steel sheet surface in the sheet thickness direction. From the viewpoint of production load, the rolling end temperature of the surface layer is preferably ( _Ar3 point - 60) ° C. or more, and more preferably ( _Ar3 point - 50) ° C. or more.
The above-mentioned "surface layer" refers to a position 2 mm from the surface of the steel sheet in the sheet thickness direction.

<Cooling process>
Next, the hot-rolled hot-rolled sheet is cooled under the following conditions: a cooling start temperature at 1/2 thickness position: Ar ₃ point or higher, an average cooling rate in the temperature range of 700 to 500°C at the 1/2 thickness position: 3.5°C/s or higher, and a cooling stop temperature at the 1/2 thickness position: 500°C or lower.

[Cooling start temperature: Ar ₃ point or higher]
It is necessary to start cooling the hot-rolled sheet obtained through the above-mentioned hot rolling process at a temperature at 1/2 the sheet thickness position of the _Ar3 point (°C) or higher. If the cooling start temperature is lower than the _Ar3 point (°C), a large amount of ferrite is generated in the steel, making it impossible to increase the strength. As a result, the X-ray intensity ratio of the (211) plane decreases, and excellent brittle crack propagation arrest properties cannot be obtained. Therefore, the cooling start temperature is set to be _Ar3 point (°C) or higher. The cooling start temperature is preferably set to be ( _Ar3 point + 10) °C or higher, and is preferably set to be ( _Ar3 point + 40) °C or lower.

[Average cooling rate in the temperature range of 700 to 500 ° C: 3.5 ° C/s or more]
If the average cooling rate in the temperature range from 700°C to the cooling stop temperature (500°C or less) at the 1/2 plate thickness position is less than 3.5°C/s, a large amount of ferrite is generated in the steel by slow cooling, and the volume fraction of bainite cannot be increased. As a result, the X-ray intensity ratio of the (211) plane decreases, and excellent brittle crack propagation arrest properties cannot be obtained. In addition, strength decreases. In the present invention, the average cooling rate in the temperature range of 700 to 500°C at the 1/2 plate thickness position is 3.5°C/s or more. The average cooling rate in this temperature range is preferably 4.0°C/s or more.

On the other hand, there is no particular upper limit to the average cooling rate in the above temperature range. In order to avoid an increase in cooling costs due to excessive rapid cooling, the average cooling rate in the above temperature range is preferably 20°C/s or less, and more preferably 10°C/s or less.

The temperature range for measuring this average cooling rate is 700 to 500°C, where most of the austenite structure transforms and contributes greatly to the properties.

　Average cooling rates outside the above temperature range of 700 to 500°C are not specified because they do not have a significant effect on bainite formation. From the standpoint of manufacturing efficiency, it is preferable to set the rate at 0.5 to 0.1°C/s.

[Cooling stop temperature: 500°C or less]
The cooling process needs to be performed until the temperature at the 1/2 position of the plate thickness is 500°C or less. In other words, it needs to be performed until the cooling stop temperature is 500°C or less. If the cooling stop temperature exceeds 500°C, a large amount of ferrite is generated in the steel, so the volume fraction of bainite cannot be increased. As a result, the X-ray intensity ratio of the (211) plane decreases, and excellent brittle crack propagation stopping properties cannot be obtained. In addition, the strength decreases. The cooling stop temperature is preferably 450°C or less, more preferably 400°C or less. On the other hand, although the lower limit of the cooling stop temperature is not limited, if the cooling stop temperature is too low, the shape of the steel plate will be deteriorated. Therefore, the cooling stop temperature is preferably 200°C or more, more preferably 300°C or more.

Next, the present invention will be specifically described based on examples. Note that the following examples are only preferred examples of the present invention, and the present invention is not limited to these examples.

Table 1 shows the chemical composition of the test steel, and Table 2 shows the manufacturing conditions. Molten steel (steel symbols: A to AD) with each chemical composition shown in Table 1 was melted in a converter and made into steel material using a continuous casting method. After that, the heating process, hot rolling process, and cooling process were carried out in that order under the manufacturing conditions shown in Table 2 to manufacture high-strength extra thick steel plates (steel symbols: 1 to 43) with thicknesses of 50 to 100 mm. Note that blank spaces in Table 1 indicate that no element was intentionally added, and include not only cases where no element is contained (0%), but also cases where an element is unavoidably contained.

Each of the obtained high-strength extra-thick steel plates was evaluated for (1) texture, (2) steel structure, (3) strength, (4) toughness, and (5) brittle crack propagation arrest properties using the methods described below.

(1) Texture [X-ray intensity ratio of (211) plane]
The texture was evaluated by measuring the X-ray intensity ratio of the (211) plane at the 1/2 position of the sheet thickness of the steel sheet.
From the obtained high-strength extra-thick steel plate, a plate-shaped sample having dimensions of 1 mm in thickness, 22 mm in a direction parallel to the rolling direction, and 25 mm in a direction perpendicular to the rolling direction was taken from the plate thickness center at the plate thickness 1/2 position, and a test piece for X-ray diffraction was prepared by mechanically polishing and electrolytically polishing a surface parallel to the surface of the sample. Using this test piece, X-ray diffraction measurement was performed using an X-ray diffractometer using a Mo radiation source to determine the X-ray intensity ratio of the (211) plane. The obtained values are listed in the "X-ray intensity ratio of X(211) plane" column in Table 3.

(2) Steel structure [Steel structure at 1/2 plate thickness position]
The steel structure at the 1/2 sheet thickness position was measured as follows.
A sample was taken from the center of the width of the steel plate at 1/2 the plate thickness so that the surface perpendicular to the rolling direction was the observation surface. The dimensions of the sample were 20 mm in the plate thickness direction, 10 mm in the direction parallel to the rolling direction, and 1 mm in the direction perpendicular to the rolling direction. The surface perpendicular to the rolling direction of this sample was mirror-polished, and then an optical microscope photograph of the metal structure revealed by nital etching was taken. The photographed part was at 1/2 the plate thickness position and the magnification was 200 times. The area fraction of the bainite phase and the ferrite phase was evaluated by calculating the area fraction by image analysis, assuming that the structure revealed as a white mass in the obtained photograph was ferrite and the remaining part was bainite. The obtained values are shown in the columns of "Bainite Phase Area Fraction" and "Ferrite Phase Area Fraction" in Table 3.

[Average grain size of bainite grains at 1/2 position of plate thickness]
A sample was taken from the center of the width of the steel plate, at 1/2 the plate thickness, so that the surface perpendicular to the rolling direction was the observation surface. The dimensions of the sample were 20 mm in the plate thickness direction, 10 mm in the direction parallel to the rolling direction, and 1 mm in the direction perpendicular to the rolling direction. After the surface perpendicular to the rolling direction of this sample was mirror-polished, EBSP analysis was performed at the 1/2 plate thickness position under the following conditions. From the obtained crystal orientation map, the circle equivalent diameter of the structure surrounded by high-angle grain boundaries with an orientation difference of 15° or more with adjacent crystal grains was obtained, and the average value of the circle equivalent diameter in the following analysis region was taken as the average effective crystal grain size. The obtained values are shown in the "Average crystal grain size of bainite" column in Table 3.
(EBSP conditions)
Acceleration voltage: 20 KV, irradiation current: 50 nA
Beam diameter: 50 nm
Analysis area: 1 mm x 1 mm area at the center of plate thickness Step size: 0.4 μm

[Circle equivalent diameter of maximum porosity]
Ultrasonic testing is often used to detect defects inside steel sheets because it allows non-destructive testing, but in this embodiment, direct observation was performed to accurately confirm the state of the defective parts and measure the size of the porosity. First, one or two cross-sections of samples for observation were taken parallel to the plate width direction at the 1/2 plate thickness position of the rolled material and mirror polished. Next, the prepared samples were observed with an optical microscope and photographed. The magnification was 400 times. The obtained photographs were subjected to image analysis to determine the circle equivalent diameter of each of the existing porosities, and the circle equivalent diameter of the maximum porosity was calculated. The obtained values were recorded in the "Circle equivalent diameter of maximum porosity" column in Table 3.

[Maximum porosity length in plate thickness direction]
Ultrasonic testing is often used to detect defects inside a steel plate because it can be inspected non-destructively, but in this embodiment, direct observation was performed to accurately confirm the state of the defective portion, and the size of the porosity was measured. First, one or two cross-sections of samples for observation were taken parallel to the plate width direction at the plate thickness 1/2 position of the rolled material, and mirror polished. Next, the prepared samples were observed with an optical microscope and photographed. The magnification was 400 times. The obtained photographs were subjected to image analysis to measure the length of each existing porosity in the plate thickness direction, and the length of the maximum porosity in the plate thickness direction was calculated. Specifically, the length of the plate thickness direction was obtained at a pitch of 20 μm along the plate width direction for each porosity, and the maximum value was taken as the above-mentioned length in the plate thickness direction. Similarly, the length of the remaining porosity in the plate thickness direction was measured, and the maximum of the obtained values was taken as the length of the maximum porosity in the plate thickness direction. The obtained values were recorded in the "Length of the maximum porosity in the plate thickness direction" column in Table 3.

[Number of segregation grains of 100 μm or more at the 1/2 position per ¹ mm2]
Plate-shaped samples were taken parallel to the plate width direction of the obtained high-strength extra thick steel plate, and EPMA analysis of Mn was performed so as to include the central segregation part. From the obtained data, the part where the Mn concentration was 1.25 times or more the average value of the EPMA measurement range was determined as the segregation part, and the thickness and number were measured.
The thickness of the segregation grains was determined by measuring the length in the sheet thickness direction as viewed from the rolling direction.
The number per ^mm2 was calculated by dividing the number of segregated grains having a thickness of 100 μm or more in the measurement area by the area of the measurement area. The obtained values are shown in the “Number density of segregated grains” column in Table 3.
(EPMA measurement conditions)
Irradiation current: 100 nA
Beam diameter: 20 μm
Measurement area: 3 mm height x 20 mm width including central segregation area Step: 20 μm

(3) Strength A Φ14 JIS 14A test piece was taken from the 1/2 position of the plate thickness of the obtained extra thick steel plate so that the longitudinal axis of the test piece was perpendicular to the rolling direction, and a tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011) to measure the yield strength (YS) and tensile strength (TS). Here, a yield strength of 500 MPa or more was evaluated as high strength.

(4) Toughness The toughness was evaluated in terms of the Charpy fracture transition temperature at the 1/2 plate thickness position.
A JIS No. 4 impact test piece was taken from the 1/2 position of the plate thickness of the obtained high-strength extra thick steel plate so that the longitudinal axis of the test piece was parallel to the rolling direction, and a Charpy impact test was performed in accordance with the provisions of JIS Z 2242 (2005) to determine the Charpy fracture transition temperature (vTrs). Three pieces were tested per test temperature, and the Charpy fracture transition temperature was calculated from the average brittle fracture rate of the three pieces. Here, vTrs≦-105°C was evaluated as excellent toughness.

(5) Brittle Crack Propagation Arrest Property The brittle crack propagation arrest property was evaluated by determining the Kca value.
In order to evaluate the brittle crack arrest properties, a temperature gradient type standard ESSO test was conducted to determine the Kca value at -10°C (Kca (-10°C) value (N/mm3 ^/2 )). Three specimens were taken from each of the obtained high strength extra thick steel plates. The size of the specimen was total thickness x L: 500 x W: 500 (mm). The Kca value was determined from the results of the three-body test. Here, specimens with a Kca (-10°C) value of 9000 N/mm3 ^/2 or more were evaluated as having excellent brittle crack arrest properties.

The test results are shown in Table 3.

From the results shown in Table 3, the following was found. In the case of the invention examples (production Nos. 1 to 16, 35, 38, 42, and 43), the component composition and production conditions were within the range of the invention, the texture at the 1/2 sheet thickness position satisfied the X-ray intensity ratio of the (211) plane: 1.70 or more, the area fraction of the bainite phase satisfied 85% or more, and the average effective grain size of the bainite phase satisfied 20 μm or less. As a result, the properties were obtained in which the yield strength at the 1/2 sheet thickness position was 500 MPa or more, the Charpy fracture appearance transition temperature at the 1/2 sheet thickness position was -105°C or less, and the Kca (-10°C) value was 9000 N/mm3 ^/2 or more.

On the other hand, in the case of the comparative examples (production Nos. 17-34, 36-37, 39-41), one or more of the component composition and manufacturing conditions were outside the range of the present invention, so the texture at the 1/2 plate thickness position and the steel structure at the 1/2 plate thickness position were not obtained, and the target properties at the 1/2 plate thickness position were not met.

Claims (12)

1. In mass percent,
   C: 0.040 to 0.130%,
   Si: 0.020 to 0.50%,
   Mn: 1.00 to 2.50%,
   P: 0.020% or less,
   S: 0.010% or less,
   Al: 0.010 to 0.100%,
   The steel sheet has a composition comprising N: 0.0010 to 0.0100%, O: 0.0100% or less, Ceq defined by formula (1) being 0.480% or more, and the balance being Fe and unavoidable impurities;
   The X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 position of the sheet thickness is 1.70 or more,
   The steel sheet has a steel structure at a 1/2 position in the sheet thickness direction, which is a mixed structure having an area fraction of 85 to 100% of a bainite phase and an area fraction of 0 to 15% of a ferrite phase, and the average grain size of the bainite crystal grains surrounded by grain boundaries having an orientation difference of 15° or more is 20 μm or less, and the circle equivalent diameter of the maximum porosity remaining in the center of the steel sheet is 200 μm or less,
   The plate thickness is 50 mm or more and 100 mm or less,
   A high-strength extra-thick steel plate having a yield strength of 500 MPa or more at the 1/2 plate thickness position, a Charpy fracture appearance transition temperature vTrs≦-105°C at the 1/2 plate thickness position, and a Kca(-10°C) value of 9000 N/mm3 ^/2 or more.
   Ceq = C + Mn/6 + Cu/15 + Ni/15 + Cr/5 + Mo/5 + V/5 ... (1)
   Here, C, Mn, Cu, Ni, Cr, Mo and V in formula (1) represent the content (mass %) of each element, and the content of an element that is not contained is set to 0.
2. The high-strength extra thick steel plate according to claim 1, further comprising, in addition to the above-mentioned composition, one or two of the following groups A and B, in mass %:
   Group A: one or more selected from Ti: 0.030% or less, Nb: 0.050% or less, Cu: 2.00% or less, Ni: 2.50% or less, and Cr: 2.00% or less. Group B: one or more selected from Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less.
3. 2. The high-strength extra thick steel plate according to claim 1, wherein the steel structure at the 1/2 plate thickness position has a number of segregation grains of 100 μm or more in thickness of 2.0 or less per mm ² .
4. 3. The high-strength extra thick steel plate according to claim 2, wherein the steel structure at the 1/2 plate thickness position has a number of segregation grains of 100 μm or more in thickness of 2.0 or less per mm ² .
5. The high-strength extra-thick steel plate according to claim 1, wherein the steel structure at the 1/2 plate thickness position has a maximum porosity remaining in the center of the steel plate with a length in the plate thickness direction of 100 μm or less.
6. The high-strength extra-thick steel plate according to claim 2, wherein the steel structure at the 1/2 plate thickness position has a length in the plate thickness direction of the maximum porosity remaining in the center of the steel plate of 100 μm or less.
7. The high-strength extra-thick steel plate according to claim 3, wherein the steel structure at the 1/2 plate thickness position has a length in the plate thickness direction of the maximum porosity remaining in the center of the steel plate of 100 μm or less.
8. The high-strength extra-thick steel plate according to claim 4, wherein the steel structure at the 1/2 plate thickness position has a maximum porosity remaining in the center of the steel plate with a length in the plate thickness direction of 100 μm or less.
9. A method for producing a high-strength extra thick steel plate according to any one of claims 1 to 8,
   A steel material having the above-mentioned composition is heated to a heating temperature of 1000 to 1200°C,
   Next, the rolling start temperature at the 1/2 thickness position is ( _Ar3 point + 100) ° C. or higher, the cumulative reduction rate in the austenite recrystallization temperature range at the 1/2 thickness position is 25.0% or higher, the cumulative reduction rate in the austenite non-recrystallization temperature range at the 1/2 thickness position is 50.0% or higher, and the rolling end temperature at the 1/2 thickness position is _Ar3 point or higher.
   The slab is hot-rolled under the conditions of a thickness of 200 mm or more and a reduction ratio (representing the ratio of the slab thickness to the plate thickness of the final product) of 4.00 or more.
   Next, cooling is performed under the conditions of a cooling start temperature at the 1/2 plate thickness position: Ar ₃ point or higher, an average cooling rate in a temperature range of 700 to 500°C at the 1/2 plate thickness position: 3.5°C/s or higher, and a cooling stop temperature at the 1/2 plate thickness position: 500°C or lower.
10. In the hot rolling,
    The temperature at the 1/2 position of the plate thickness is the average reduction rate/pass in the austenite non-recrystallization temperature range: 4.5% or more;
    The rolling end temperature at the 1/4 plate thickness position is (the rolling end temperature at the 1/2 plate thickness position-20) ° C. or higher, and the rolling end temperature at a position 2 mm in the plate thickness direction from the steel plate surface is (the rolling end temperature at the 1/2 plate thickness position-30) ° C. or higher. The method for producing a high-strength extra thick steel plate according to claim 9,
11. In the hot rolling,
    The method for producing a high strength extra thick steel plate according to claim 9, wherein the rolling is performed under conditions where the temperature at the 1/2 position of the plate thickness is an average reduction rate/pass of 4.0% or more in the austenite recrystallization temperature range.
12. In the hot rolling,
    The method for producing a high strength extra thick steel plate according to claim 10, wherein the rolling is performed under conditions where the temperature at the 1/2 position of the plate thickness is an average reduction rate/pass of 4.0% or more in the austenite recrystallization temperature range.