JP7468814B1 - High strength extra thick steel plate and its manufacturing method - Google Patents

Abstract

The present invention provides a high-strength extra thick steel plate and a manufacturing method thereof. The high-strength extra thick steel plate of the present invention has a specific component composition, a texture in which the X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 position of the plate thickness is 1.60 or more, a mixed texture consisting of 80 to 100% bainite phase and 0 to 20% ferrite phase in terms of area fraction, an 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 a circle equivalent diameter of the maximum porosity remaining at the 1/2 position of the steel plate is 200 μm or less, the plate thickness is more than 100 mm, the yield strength at the 1/2 position of the plate thickness is 390 MPa or more, the Charpy fracture appearance transition temperature at the 1/2 position of the plate thickness is vTrs≦−100° C., and the value of Kca(−10° C.) is 8400 N/mm3/2 or more.

Description

The present invention relates to a high-strength, extra-thick steel plate having a thickness of more than 100 mm and used in large structures such as ships, marine structures, low-temperature storage tanks, architectural structures, and civil engineering structures, and which has excellent brittle crack propagation arrest properties, and to 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 conventional decks, and are loaded even onto hatch covers as they travel on the ocean, subjecting them to waves carrying heavy loads. 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, an increase in the Ni content 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 Thermo-Mechanical Control Process (TMCP) method is used to refine the grains and improve low-temperature toughness. This can provide excellent brittle crack propagation arrest properties.

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

In addition, 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 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 so that the propagation energy of the propagating brittle crack is absorbed by the crystal grains. In addition, Patent Document 2 discloses a manufacturing method in which a process is repeated one or more times to cool the surface layer portion to a temperature below the _Ar3 transformation point by controlled cooling after hot rolling, and then stop the controlled cooling and reheat the surface layer portion to a temperature above the _Ar3 transformation point. Patent Document 2 discloses that in this process, a roll reduction is applied to the steel material to repeatedly cause transformation or work recrystallization, thereby generating an ultrafine ferrite structure or bainite structure in the surface layer portion.

Patent Document 3 discloses that in order to improve the brittle crack propagation arrestability of a steel material having a microstructure mainly composed of ferrite-pearlite, both surface portions of the steel material are constituted by layers having a ferrite structure with ferrite grains having a circle 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. As a method for suppressing this variation, Patent Document 3 discloses that the maximum reduction rate per pass during finish rolling is set to 12% or less to suppress local recrystallization.

In addition, a method is also known in which the brittle crack propagation arrestability is 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
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 portion extending from the front and back sides in the thickness direction up to 25% and the remaining central portion of the plate thickness; the front and back layer portion 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 in an area of 5% to 25% of the plate thickness, and the central portion of the plate thickness has a texture in which the (111) and/or (211) X-ray plane intensity ratio parallel to the rolling surface is 2.0 or more. This technology for improving brittle crack propagation arrestability 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 makes it difficult to control on an actual production scale, and places a large load on the rolling and cooling equipment.

In addition, taking into account the manufacturing conditions and the disclosed experimental data, the technologies described in Patent Documents 1 to 5 are primarily intended for steel plates and steel materials with a plate thickness of about 80 mm, and it is unclear whether the specified characteristics can be obtained when each technology is applied to extra-thick steel plates over 100 mm. It is also unclear whether the crack propagation stopping characteristics in the plate thickness direction required for hull structures can be obtained. However, it is predicted that the characteristics required to stop crack propagation will be higher for extra-thick steel plates over 100 mm compared to steel plates with a plate thickness of about 80 mm.

In order to improve brittle crack propagation arrest characteristics while maintaining the rigidity of the hull, even extremely thick steel plates over 100 mm are required to combine high strength and excellent toughness.

The present invention has been made in consideration of the above-mentioned problems, and aims to provide a high-strength extra-thick steel plate and a manufacturing method thereof that have excellent toughness and brittle crack propagation arrest properties even when the plate thickness exceeds 100 mm.

Here, "high strength" in the present invention refers to a yield strength (YS) of 390 MPa or more at 1/2 the plate thickness position of a high-strength extra thick steel plate. "Excellent toughness" in the present invention refers to a Charpy fracture transition temperature of vTrs≦-100°C at 1/2 the plate thickness position of a high-strength extra thick steel plate. "Excellent brittle crack propagation arrestability (arrestability)" in the present invention refers to a Kca value at -10°C (hereinafter also referred to as Kca(-10°C) value (N/mm3 ^/2 )) of 8400 N/mm3 ^/2 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 toughness and brittle crack propagation arrest properties while maintaining strength even when the plate thickness is over 100 mm, and into a manufacturing method for stably obtaining such steel plates. As a result, the following findings were obtained.

Specifically, in order to achieve high strength in extra-thick steel plates with a thickness of more than 100 mm, it is effective to control the carbon equivalent (Ceq) to 0.480 mass% or more, which results in the steel structure at 1/2 the plate thickness being entirely bainite, thereby improving strength and achieving high strength.

In order to achieve excellent toughness in the above-mentioned extra-thick steel plate, it is effective to control the steel structure at 1/2 the plate thickness so that the bainite crystal grains surrounded by grain boundaries with an orientation difference of 15° or more are 20 μm or less, and to control so that the circle equivalent diameter of the maximum porosity remaining in the center of the steel plate is 200 μm or less. This improves toughness due to the grain refinement effect, and the Charpy toughness at 1/2 the plate thickness, where the properties are the lowest, achieves vTrs≦-100°C.

In order to achieve excellent brittle crack propagation arrestability in the extra thick steel plate, it is effective to provide a texture that has excellent toughness and has a (211) plane X-ray intensity of 1.60 or more on the rolled surface at the 1/2 position of the plate thickness. This inhibits the linear propagation of brittle cracks and achieves a Kca (-10°C) value (N/mm3 ^/2 ) of 8400 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 3.25 or more. This allows sufficient rolling stress to be applied to the 1/2 thickness position without being affected by the plate thickness, even for extremely thick steel plates exceeding 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 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 3.25 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 low-temperature toughness while elongating the austenite in the rolling direction and developing a bainite transformation texture in a specific direction. As a result, a texture is obtained in which the X-ray intensity of the (211) plane on the rolled surface at the 1/2 thickness position is 1.60 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 0.150%,
Si: 0.02 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%,
N: 0.0010 to 0.0100% or less, O: 0.0100% or less, and Ceq defined by formula (1) is 0.480 to 0.560%, with 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 1.60 or more,
The steel structure at the 1/2 position of the plate thickness is a mixed structure consisting of 80 to 100% bainite phase and 0 to 20% ferrite phase, in terms of area fraction, 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 the circle equivalent diameter of the maximum porosity remaining at the 1/2 position of the plate thickness is 200 μm or less,
The plate thickness is more than 100 mm,
A high-strength extra-thick steel plate having a yield strength of 390 MPa or more at the 1/2 plate thickness position, a Charpy fracture appearance transition temperature vTrs≦-100°C at the 1/2 plate thickness position, and a Kca(-10°C) value of 8400 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 texture has an X-ray intensity ratio of 0.90 or more for the (110) plane on the rolled surface at the 1/4 position of the sheet thickness,
The high-strength extra thick steel plate according to [1] or [2], which has a steel structure at a 1/4 position in the plate thickness direction, which is a mixed structure consisting of, in terms of area fraction, 80 to 100% bainite phase and 0 to 20% ferrite phase, and in which 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.
[4] A method for producing a high-strength extra thick steel plate according to any one of [1] to [3],
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 5.0% 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 350 mm or more and a reduction ratio, which represents the ratio of the slab thickness to the plate thickness of the final product, of 3.25 or more and less than 4.00.
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 the temperature range of 700 to 500°C at the 1/2 plate thickness position: 2.0°C/s or higher, and a cooling stop temperature at the 1/2 plate thickness position: 500°C or lower.
[5] In the hot rolling, further,
The temperature at the 1/2 position of the plate thickness is the average reduction rate/pass in the austenite recrystallization temperature range: 3.5% or more;
The method for producing a high-strength extra thick steel plate according to [4], wherein the rolling end temperature at the 1/4 position of the plate thickness is _Ar3 point or higher.

According to the present invention, the steel plate has a mixed structure consisting of a bainite phase and a ferrite phase, the average grain size of the bainite crystal grains is 20 μm or less, and the circle equivalent diameter of the maximum porosity remaining in the center of the steel plate is 200 μm or less, so that the porosity at the 1/2 position of the plate thickness is sufficiently compressed. In addition, since the steel plate has the above-mentioned texture at the 1/2 position of the plate thickness, the linear progression of brittle cracks can be inhibited. As a result, the high-strength extra-thick steel plate of the present invention has excellent toughness and brittle crack propagation arrest properties even if the plate thickness is more than 100 mm. In addition, according to the manufacturing method of the present invention, the high-strength extra-thick steel plate having the above-mentioned characteristics can be manufactured by optimizing the rolling conditions.

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 for limiting the composition of the high-strength extra-thick steel plate in this invention. In this specification, "%" regarding the composition means "mass %" unless otherwise specified.

C: 0.040 to 0.150%
C is an element that acts to increase the hardenability of steel and is necessary to achieve a desired strength. 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 0.150%, not only does the weldability deteriorate, but also the toughness is adversely affected. For this reason, the C content is set to a range of 0.040 to 0.150%. The preferred lower limit of the C content is 0.050% or more, more preferably 0.055% or more. The preferred upper limit of the C content is 0.100% or less, more preferably 0.090% or less.

Si: 0.02 to 0.50%
Si is a component necessary for deoxidization. Si is also an element that has the effect of increasing the hardenability of steel by suppressing the generation of coarse carbides, and in order to achieve the desired strength, Si is contained at 0.02% or more. On the other hand, if the Si content exceeds 0.50%, not only the surface properties of the steel are impaired, but also the toughness is extremely deteriorated. For this reason, the Si content is set to a range of 0.02 to 0.50%. The lower limit of the Si content is preferably 0.050% or more, and more preferably 0.10% or more. The upper limit of the Si content is preferably 0.40% or less, and more preferably 0.30% or less.

Mn: 1.00 to 2.50%
Mn is an element that has the effect of increasing 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. The lower limit of the Mn content is preferably 1.50% or more, and more preferably 1.65% or more. The upper limit of the Mn content is preferably 2.35% or less, and more preferably 2.15% 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 more than 100 mm, 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 0.012% 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.

There are no particular lower limits for the P content and S content. However, since excessive reduction in P and S leads to increased manufacturing costs, the P content and S content are preferably set to P: 0.0005% or more and S: 0.0005% or more, respectively. More preferably, P: 0.002% or more and S: 0.001% 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. For this reason, 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 0.030% or more. The upper limit of the Al content is preferably 0.070% or less, and more preferably 0.040% or less.

N: 0.0010 to 0.0100%
N combines with Al in the steel, adjusts the grain size during rolling, and strengthens the steel. To obtain this effect, the N content must be 0.0010% or more. If the N content exceeds 0.0100%, toughness deteriorates. For this reason, 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 0.0025% or more. The upper limit of the N content is preferably 0.0070% or less, and more preferably 0.0055% 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. On the other hand, 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, the O content is preferably 0.0009% or more, and more preferably 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 to 0.560%
In order to improve the hardenability of the extra thick steel plate of the present invention and to realize high strength and improved brittle crack propagation arrestability due to the development of texture, the value of Ceq represented by formula (1) is adjusted to 0.480% or more. This increases 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. On the other hand, if the value of Ceq exceeds 0.560%, the hardenability becomes excessive, the strength of the matrix structure increases excessively, and the amount of MA (martensite) generated, which is the brittle fracture origin, increases. As a result, it is not possible to ensure the weldability and toughness of the base material (steel plate). For this reason, Ceq is set to 0.560% or less. The value of Ceq is preferably set to 0.490% or more, more preferably set to 0.500% or more. The value of Ceq is preferably set to 0.540% or less, more preferably set to 0.530% or less.

The above is the basic composition of the present invention, with the balance 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 properties.

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 the groups A and B described below as necessary. Note that each of the 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: 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 may be reduced. Therefore, when Ti is contained, the Ti content is preferably 0.030% or less, and more preferably 0.025% or less. It is more preferable to set the Ti content to 0.010% 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 (hot rolling) in the austenite region. 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, which may lead to 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.

Cu: 2.00% or less Cu is an element that enhances the hardenability of steel. Cu 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 lead to 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. The Cu content is more preferably 0.05% or more, and even more preferably 0.10% 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 lead to 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. Cr 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, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 2.00%, it will lead to deterioration of weldability and toughness and an increase in alloy cost. Therefore, when Cr is contained, the Cr content is preferably 2.00% or less, and more preferably 1.00% or less. The Cr content is more preferably 0.05% or more, and even more preferably 0.10% 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. Mo 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 to set the Mo content to 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 to set the Mo content to 0.50% or less, and more preferably to 0.30% or less. The Mo content is more preferably 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, and even more preferably 0.010% 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, more preferably 0.40% or less, and even 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 content of B 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, more preferably 0.0005% or more, and even more preferably 0.0010% 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 0.050% 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, and even more preferably 0.0100% 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.

<Texture>
X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 position of the plate thickness: 1.60 or more In order to improve the brittle crack propagation arrestability of an extra-thick steel plate having a plate thickness of more than 100 mm, it is necessary to make the X-ray intensity ratio of the (211) plane on the rolled surface at the 1/2 position of the plate thickness 1.60 or more. If the X-ray intensity ratio of the (211) plane is less than 1.60, the brittle crack is likely 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 1/2 position of the plate thickness is made 1.60 or more. In order to further improve the brittle crack propagation arrestability, the X-ray intensity ratio of the (211) plane is preferably 1.70 or more, more preferably 1.80 or more.

In addition, there is no particular upper limit for the X-ray intensity ratio of the (211) plane. From the viewpoint of rolling efficiency and manufacturing load, the X-ray intensity ratio of the (211) plane is preferably 2.50 or less, and more preferably 2.30 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 (i.e., high-strength extra-thick steel plate). The X-ray intensity ratio of the (211) plane is calculated as the ratio (i.e., I(211)/I0 ₍₂₁₁₎ ) of the X-ray diffraction intensity of the (211) reflection of the target material (I _{( 211 )) to the X-ray diffraction intensity of the (211)} reflection of a random standard sample without texture, as described in the Examples below.

<Steel structure at 1/2 plate thickness>
In the present invention, the steel structure at the 1/2 plate thickness position is a mixed structure consisting of, in terms of area fraction, 80 to 100% bainite phase and 0 to 20% ferrite phase, 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 the circle equivalent diameter of the maximum porosity remaining at the 1/2 plate thickness position (plate thickness center portion) is 200 μm or less.

[Area fraction of bainite phase at 1/2 plate thickness position: 80 to 100%]
In the high-strength extra thick steel plate of the present invention, the area fraction of the bainite phase at the 1/2 plate thickness position must be 80 to 100%. If the area fraction of the bainite phase is less than 80%, the fraction of hard structures 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 the bainite phase at the 1/2 plate thickness position is set to 80% or more. In order to further increase the strength and brittle crack propagation arrestability, the above-mentioned area fraction is preferably 85% or more, and more preferably 90% or more.

[Area fraction of ferrite phase at 1/2 sheet thickness position: 0 to 20%]
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 20%. If the area fraction of ferrite phase exceeds 20%, 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 20% or less. In order to further increase the strength and brittle crack propagation arrestability, the above area fraction is preferably set to 15% or less, more preferably 10% or less.

[Average grain size of bainite grains at 1/2 position of plate thickness: 20 μm or less]
In the high-strength extra-thick steel plate of the present invention, the average grain size of bainite grains at the 1/2 position of the plate thickness must be 20 μm or less. If this 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 brittle crack propagation arrestability are reduced. Therefore, the average grain size at the 1/2 position of the plate thickness is set to 20 μm or less. In order to further improve the toughness and brittle crack propagation arrestability, the above average grain size is preferably 19 μm or less, and more preferably 18 μm or less.
The lower limit of the average grain size is not particularly specified. From the viewpoint of rolling efficiency, the average grain size is preferably 5 μm or more, and more preferably 10 μm or more.

[Circle equivalent diameter of porosity remaining at 1/2 plate thickness position: 200 μm or less]
In the high-strength extra-thick steel plate of the present invention, the circle-equivalent diameter of the maximum porosity remaining at the 1/2 position of the plate thickness must be 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 significantly decreases. Therefore, the circle-equivalent diameter of the maximum porosity remaining at the 1/2 position of the plate thickness 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, more preferably set to 50 μm or more, and even more preferably set to 70 μm or more.

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

<Steel structure at 1/4 plate thickness> (optimal conditions)
In the present invention, the steel structure at the 1/4 position of the plate thickness is preferably a mixed structure consisting of 80 to 100% bainite phase and 0 to 20% ferrite phase in terms of area fraction, and the average grain size of bainite crystal grains surrounded by grain boundaries with an orientation difference of 15° or more is preferably 20 μm or less.

[Area fraction of bainite phase at 1/4 position of plate thickness: 80 to 100%]
In the high-strength extra thick steel plate of the present invention, the area fraction of the bainite phase at the 1/4 position of the plate thickness is preferably 80 to 100%. If the area fraction of the bainite phase is less than 80%, the fraction of hard structures is reduced and the texture of the (110) plane is not sufficiently developed, resulting in reduced strength and brittle crack propagation arrestability. Therefore, it is desirable that the area fraction of the bainite phase at the 1/4 position of the plate thickness is 80% or more. In order to further increase the strength and brittle crack propagation arrestability, the above-mentioned area fraction is preferably 85% or more, and more preferably 90% or more.

[Area fraction of ferrite phase at 1/4 position of plate thickness: 0 to 20%]
In the high-strength extra-thick steel plate of the present invention, the area fraction of the ferrite phase at the 1/4 position of the plate thickness is preferably 0 to 20%. If the area fraction of the ferrite phase exceeds 20%, the fraction of hard structures is reduced and the texture of the (110) plane is not sufficiently developed, so that the strength and brittle crack propagation arrestability are reduced. Therefore, it is preferable that the area fraction of the ferrite phase at the 1/4 position of the plate thickness is 20% or less. In order to further increase the strength and brittle crack propagation arrestability, the above-mentioned area fraction is preferably 15% or less, and more preferably 10% or less.

[Average grain size of bainite grains at 1/4 position of plate thickness: 20 μm or less]
In the high-strength extra-thick steel plate of the present invention, the average grain size of bainite grains at the 1/4 position of the plate thickness is preferably 20 μm or less. If this average grain size exceeds 20 μm, the fracture surface unit of the cleavage fracture surface of the grains becomes larger due to the grain size coarsening, and the brittle crack propagation arrestability decreases. Therefore, it is preferable that the average grain size of bainite grains at the 1/4 position of the plate thickness is 20 μm or less. In order to further improve the brittle crack propagation arrestability, the above average grain size is preferably 19 μm or less, and more preferably 16 μm or less.

There is no particular lower limit for the average grain size. From the viewpoint of rolling efficiency, the average grain size is preferably 5 μm or more, and more preferably 10 μm or more.

<Texture> (optimal conditions)
[X-ray intensity ratio of the (110) plane on the rolled surface at the 1/4 position of the plate thickness: 0.90 or more]
In order to improve the brittle crack propagation arrestability of an extra-thick steel plate having a thickness of more than 100 mm, it is desirable to set the X-ray intensity ratio of the (110) plane on the rolled surface at the 1/4 position of the plate thickness to 0.90 or more. If the X-ray intensity ratio of the (110) plane is less than 0.90, the orientation difference with the texture at the 1/2 position of the plate thickness becomes small, and the brittle cracks tend to progress linearly without being dispersed, resulting in a decrease in the brittle crack propagation arrestability of the plate thickness. Therefore, the X-ray intensity ratio of the (110) plane on the rolled surface at the 1/4 position of the plate thickness is set to 0.90 or more. In order to further improve the brittle crack propagation arrestability, the X-ray intensity ratio of the (110) plane is preferably 1.00 or more, more preferably 1.10 or more.

In addition, there is no particular upper limit for the X-ray intensity ratio of the (110) plane. From the viewpoint of rolling efficiency, the X-ray intensity ratio of the (110) plane is preferably 1.50 or less, and more preferably 1.30 or less.

Here, the X-ray intensity ratio of the (110) plane is a value that represents the degree of integration of the (110) crystal plane of the target material (high-strength extra-thick steel plate). The X-ray intensity ratio of the (110) plane is calculated as the ratio (i.e., I(110)/I0 ₍₁₁₀₎ ) of the X-ray diffraction intensity of the (110) reflection of the target material (I ₍ 110 ₎ ) to the X-ray diffraction intensity of the ( ₁₁₀₎ reflection of a random standard sample without texture, as described in the Examples below.

<Strength>
Yield strength at 1/2 plate thickness position: 390 MPa or more The high-strength extra thick steel plate of the present invention has a yield strength of 390 MPa or more, measured using a Φ14 mm JIS 14A tensile test piece taken from a depth of 1/2 the plate thickness (i.e., at the 1/2 plate thickness position). Ships such as container ships and bulk carriers normally use high-strength, thick steel plate base material that can withstand this bending stress, and a yield strength of less than 390 MPa is not suitable for use.

<Charpy fracture transition temperature>
vTrs at 1/2 thickness position: -100°C or less In order to improve the brittle crack propagation arrestability of an extra-thick steel plate having a thickness of more than 100 mm, it is necessary to set the vTrs at 1/2 thickness position to -100°C or less. vTrs: If the transition temperature is higher than -100°C, the brittle crack propagation arrestability of the plate thickness decreases. Therefore, the high-strength extra-thick steel plate of the present invention has a vTrs at 1/2 thickness position of -100°C or less. In order to further improve the brittle crack propagation arrestability, it is preferable to set the vTrs at -110°C or less, and more preferably to set the vTrs at -120°C or less. The lower limit of vTrs is not particularly specified. From the viewpoint of rolling efficiency, the vTrs is preferably -160°C or more, and more preferably -150°C or more.

vTrs can be evaluated by taking a JIS No. 4 impact test specimen from the 1/2 plate thickness position 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: 8400 N/mm3 ^/2 or more Extra thick steel plates with a plate thickness of more than 100 mm need to have a Kca (-10°C) value of 8400 N/mm3 ^{/2 or} more to prevent brittle crack propagation in the hull. In order to further improve the brittle crack propagation arrestability, it is preferable to set the Kca value at -10°C to 9000 N/mm3 ^/2 or more. There is no particular upper limit for the Kca value. From the viewpoint of rolling efficiency, the Kca value at -10°C is preferably 25000 N/mm3 ^/2 or less, and more preferably 20000 N/mm3 ^/2 or less.

The Kca value can be evaluated using a standard temperature gradient ESSO test as described in the examples below.

As explained above, by adjusting the above-mentioned component composition and controlling the texture and toughness at specific positions (i.e., refining the steel structure at 1/2 the plate thickness position), the high-strength extra-thick steel plate of the present invention has high strength, high toughness and high (excellent) brittle crack propagation arrest properties, even in plate thicknesses of more than 100 mm.

There is no particular upper limit to the thickness of the high-strength extra-thick steel plate of the present invention. From the viewpoint of rolling efficiency, it is preferable that the upper limit of the plate thickness be 180 mm or less.

Next, we will explain the manufacturing method of high-strength extra thick steel plate in one embodiment of the present invention.

The high-strength extra-thick steel plate of the present invention is manufactured by subjecting a steel material having the above-mentioned composition to a heating process, a hot rolling process and a cooling process under the specific conditions described below.

Each process is described in detail below. Unless otherwise specified, the temperature in each process refers to the temperature at the center of the thickness of the steel material and hot-rolled plate (i.e., the 1/2 plate thickness position). For example, the temperature distribution in the cross section of the steel plate can be calculated by heat transfer analysis and the result corrected by the surface temperature of the steel plate.

<Heating process>
First, the steel material 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. This makes it difficult to carry out the subsequent hot rolling process. Therefore, the heating temperature is set to 1000°C or higher. 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, oxidation loss increases, and there is a risk of a decrease in yield. For these reasons, the heating temperature is set to 1200°C or lower. The heating temperature is preferably 1050°C or higher, more preferably 1080°C or higher. The heating temperature is preferably 1150°C or lower, more preferably 1130°C or lower.

<Hot rolling process>
Next, the steel material heated in the heating process is subjected to hot rolling 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: 5.0% 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; a slab thickness of 350 mm or more; and a reduction ratio, which represents the ratio of the slab thickness to the plate thickness of the final product, of 3.25 or more and less than 4.00.

[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 plate thickness 1/2 position is less than ( _Ar3 point + 100) ° C., recrystallization does not occur sufficiently in the hot-rolled plate after the hot rolling process. Therefore, the austenite grain size does not become fine, and the toughness decreases. As a result, the desired brittle crack propagation arrest characteristic is not obtained. Therefore, the 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 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 may be in accordance with the heating temperature of the steel material described above. That is, the rolling start temperature is preferably set to 1150 ° C. or lower, and more preferably set to 1130 ° C. or lower.

The _Ar3 point ( _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 rate in the austenite recrystallization temperature range: 5.0% or more]
Hot rolling is performed with a cumulative reduction of 5.0% or more in the austenite recrystallization temperature range at the plate thickness center (1/2 plate thickness position). If the cumulative reduction in this temperature range is less than 5.0%, 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 of -100°C or less is not achieved. The cumulative reduction in this temperature range is preferably 10% or more, and more preferably 15% or more.

There is no particular upper limit to the cumulative rolling reduction in this temperature range. Since the effect of improving the grain refinement is saturated, it is preferable that the cumulative rolling reduction in this temperature range be 45% or less, and more preferably 40% or less.

In addition, in the case of the component composition of the present invention, it is preferable that the cumulative reduction rate in the austenite recrystallization temperature range be 5.0% or more in the temperature range of 1100 to 950°C.

[Cumulative reduction rate in the 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 center of the plate thickness (at 1/2 the plate 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, phase transformation occurs along the slip system selected by variant selection, and a texture is obtained in which the X-ray intensity ratio of the (211) plane on the rolled surface at the center of the plate thickness is 1.60 or more.

On the other hand, if the cumulative reduction rate in this temperature range is less than 50%, the austenite does not elongate sufficiently in the rolling direction, reducing the number of nucleation sites during bainite transformation, which results in the formation of coarse bainite and makes it impossible to refine the grain size. As a result, the Charpy fracture transition temperature (vTrs) at the 1/2 position in the plate thickness: -100°C or less is not achieved. In addition, a texture with a (211) plane concentration of 1.60 or more on the rolled surface in the center of the plate thickness cannot be obtained.

Therefore, the cumulative rolling reduction in this temperature range is 50.0% or more. The cumulative rolling reduction is preferably 55% or more, and more preferably 60% or more.

There is no particular upper limit to the cumulative reduction in this temperature range. From the viewpoint of not impairing the rolling efficiency, it is preferable that the cumulative reduction in this temperature range be 75% or less, and more preferably 70% or less.

In addition, in the case of the component composition of the present invention, it is preferable that the cumulative rolling reduction in the austenite non-recrystallization temperature range be 50.0% or more in the temperature range of less than 950°C and 700°C or more.

[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 plate thickness position of _Ar3 point (°C) or higher. If the temperature of the steel plate during hot rolling is lower than _Ar3 point (°C), 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: 3.25 or more and less than 4.00]
In the present invention, it is important that the thickness of the hot-rolled sheet after the hot rolling process is as follows: Specifically, the thickness of the slab is 350 mm or more, and the rolling in the hot rolling process is controlled so that the ratio of the slab thickness to the sheet thickness of the final product (rolling reduction ratio) is 3.25 or more.

If the reduction ratio is less than 3.25, the steel structure is not sufficiently refined, and toughness is not improved. As a result, the Charpy fracture transition temperature at the 1/2 plate thickness position: -100°C or less is not achieved. Also, if the reduction ratio is less than 3.25, the rolling stress applied to the 1/2 plate thickness position is insufficient, and deformation does not proceed in the direction in which the porosity generated inside the steel plate closes, and even when rolling is completed, coarse porosity remains and becomes the starting point of fracture. Therefore, the porosity generated inside the steel plate does not bond, and toughness is significantly reduced. The reduction ratio is preferably 3.40 or more, and more preferably 3.50 or more.

On the other hand, if the reduction ratio is 4.00 or more, the steel structure becomes excessively fine-grained, reducing hardenability, and the area fraction of bainite at the 1/2 plate thickness position in extra-thick steel plates over 100 mm in thickness where the cooling rate is low cannot be increased. As a result, the X-ray intensity ratio of the (211) plane decreases, making it impossible to obtain excellent brittle crack propagation arrest properties. In addition, strength decreases. Therefore, the reduction ratio must be less than 4.00. The reduction ratio is preferably 3.80 or less, and more preferably 3.70 or less.

In the present invention, the above-mentioned effects can be obtained by performing hot rolling under the above-mentioned hot rolling conditions. From the viewpoint of obtaining the effects more effectively, it is also effective to further specify the following hot rolling conditions.
Specifically, the temperature at the 1/2 thickness position is set to an average reduction rate/pass of 3.5% or more in the austenite recrystallization temperature range, and the rolling end temperature at the 1/4 thickness position is set to Ar ₃ point °C or more.

[Average reduction rate/pass in the austenite recrystallization temperature range: 3.5% or more] (optional conditions)
When hot rolling is performed with an average reduction rate/pass of 3.5% or more in the austenite recrystallization temperature range of the plate thickness center (plate thickness 1/2 position), the effect of pressing the porosity at the plate thickness 1/2 position can be obtained. If the average reduction rate/pass in this temperature range is less than 3.5%, sufficient rolling stress cannot be applied during rolling at a high temperature effective for porosity pressing, and porosity tends to remain at the end of rolling. This becomes the fracture origin, making it difficult to achieve a Charpy fracture transition temperature of -100°C or less at the plate thickness 1/2 position. The average reduction rate/pass in this temperature range is preferably 4.0% or more, more preferably 5.0% or more.

There is no particular upper limit to the average rolling reduction/pass in this temperature range. From the viewpoint of production load, it is preferable that the average rolling reduction/pass in this temperature range be 10% or less, and more preferably 8.0% or less.

[Rolling end temperature at 1/4 plate thickness: Ar ₃ point or higher] (arbitrary condition)
In the hot rolling process, if the rolling end temperature at 1/4 of the plate thickness is at or above the _Ar3 point (°C) at the 1/4 plate thickness position, the effect of improving the brittle crack propagation arrestability can be more effectively obtained. If the temperature at the 1/4 plate thickness position of the hot rolled plate during hot rolling is less than the _Ar3 point (°C), processed ferrite is generated at the 1/4 plate thickness position, and the fraction of bainite that increases the X-ray intensity of the (110) plane decreases, so the X-ray intensity ratio of the (110) plane at the 1/4 plate thickness position decreases. As a result, excellent brittle crack propagation arrestability 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. It is preferable that the rolling end temperature is _Ar3 point + 20) °C or higher.

The upper limit of the rolling end temperature is not particularly limited. From the viewpoint of rolling efficiency, the rolling end temperature is preferably ( _Ar3 point + 50) ° C. or less, and more preferably ( _Ar3 point + 40) ° C. or less.

<Cooling process>
Next, the hot-rolled sheet hot-rolled in the hot rolling process 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: 2.0°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.: 2.0 ° C./s or more]
If the average cooling rate in the temperature range from 700°C at the 1/2 position of the plate thickness to the cooling stop temperature (500°C or less) is less than 2.0°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 arrestability cannot be obtained. In addition, the strength decreases.

On the other hand, the temperature range for measuring this average cooling rate is set to 700 to 500°C, from the viewpoint of obtaining an effect in which the transformation of most of the austenite structure occurs and which contributes greatly to the properties of the steel plate.

That is, 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 set to 2.0°C/s or more. This has the effect of increasing the proportion of bainite, improving strength, and developing the texture of the (211) plane, improving brittle crack propagation arrest properties. The average cooling rate in this temperature range is preferably 3.0°C/s or more.

The upper limit of the average cooling rate is not particularly limited. In order to avoid an increase in cooling costs due to excessive quenching, the average cooling rate is preferably 20°C/s or less, and more preferably 10°C/s or less.

The average cooling rate outside the above temperature range of 700 to 500°C is not specified because it does not have a significant effect on the formation of bainite. From the viewpoint of production efficiency, it is preferable to set it to 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 that the area 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 described in detail based on examples. Note that the following examples are merely 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 steels, and Table 2 shows the manufacturing conditions. Molten steels (steel symbols: A to AG) with the chemical compositions shown in Table 1 were produced in a converter and made into steel material by continuous casting. The heating process, hot rolling process, and cooling process were then carried out in that order under the manufacturing conditions shown in Table 2, to produce high-strength extra thick steel plates (steel symbols: 1 to 42) with the plate thicknesses shown in Table 3. Note that blank spaces in Table 1 indicate that no element was intentionally added, and include not only cases where no element is contained (i.e. 0%), but also cases where an element is unavoidably contained.

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

(1) Strength A Φ14 JIS 14A test piece was taken from the obtained high-strength extra thick steel plate at a position halfway through the plate thickness so that the longitudinal axis of the test piece was perpendicular to the rolling direction. 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 390 MPa or more was evaluated as being high strength, and a tensile strength of 570 MPa or more was evaluated as being high strength.

(2) 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 carried out in accordance with the provisions of JIS Z 2242 (2005) to determine the Charpy fracture transition temperature (vTrs) in the range of -40°C to -160°C. Three pieces were tested per test temperature, and the brittle fracture rate was calculated from the average value of the three pieces.
Here, vTrs≦−100° C. was evaluated as being excellent in low temperature toughness.

(3) Texture 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 with the plate thickness 1/2 position as the plate thickness center, 22 mm in the direction parallel to the rolling direction, and 25 mm in the direction perpendicular to the rolling direction was taken, and a surface parallel to the surface of the sample was mechanically polished and electrolytically polished to prepare a test piece for X-ray diffraction. Using this test piece, X-ray diffraction measurement was performed using an X-ray diffractometer using a Mo radiation source, and the X-ray intensity ratio of the (211) plane was obtained.

In addition, the X-ray intensity ratio of the (110) plane at a position corresponding to 1/4 of the sheet thickness of the steel sheet was measured.
From the obtained high-strength extra-thick steel plate, a plate-shaped sample having dimensions of 1 mm in thickness with the 1/4 position at the plate thickness center, 22 mm in the direction parallel to the rolling direction, and 25 mm in the direction perpendicular to the rolling direction was taken, and a surface parallel to the surface of the sample was mechanically polished and electrolytically polished to prepare a test piece for X-ray diffraction. Using this test piece, X-ray diffraction measurement was performed using an X-ray diffractometer using a Mo radiation source, and the X-ray intensity ratio of the (110) plane was obtained.

(4) Steel structure [Steel structure]
The steel structure at the 1/2 sheet thickness position was measured as follows.
A sample was taken from the width center 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 portion was at 1/2 the plate thickness position, and the magnification was 200 times. The area fractions of the bainite phase and ferrite phase were evaluated by calculating the area fractions by image analysis, assuming that the structure revealed as white chunks in the obtained photograph was ferrite and the remaining part was bainite.

The steel structure at the 1/4 position of the plate thickness was measured as follows.
A sample was taken from the center of the width of the steel plate, at 1/4 of 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 mirror polishing the surface perpendicular to the rolling direction of this sample, an optical microscope photograph of the metal structure revealed by nital etching was taken. The photographed area was at 1/4 of the plate thickness position, and the magnification was 200 times. The area fractions of the bainite phase and ferrite phase were evaluated by image analysis of the obtained photograph.

[Average grain size of bainite grains]
The average grain size of bainite grains at the 1/2 position of the sheet thickness 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. After mirror polishing the surface perpendicular to the rolling direction of this sample, 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.
(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
The average grain size of bainite grains at the 1/4 position of the plate thickness was measured as follows.
A sample was taken from the center of the width of the steel plate, at 1/4 of 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 mirror polishing the surface perpendicular to the rolling direction of this sample, EBSP analysis was performed at the 1/4 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.
(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 number density of porosity. First, one or two cross-sections of samples for observation were taken parallel to the sheet width direction at the half-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 largest porosity was calculated.

(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 propagation arrestability, a temperature gradient type standard ESSO test was carried out to determine the Kca value at -10°C (Kca (-10°C) value (N/mm ^3/2 )).
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 calculated from the results of the three-body test.
In this case, a value of Kca (-10°C) of 8400 N/mm ^3/2 or more was evaluated as having excellent brittle crack propagation arrest properties.

The results of these tests are shown in Table 3.

From the results shown in Table 3, the following was found. In the case of the inventive examples (production Nos. 1 to 17), the chemical composition and production conditions were within the range of the present invention, the texture at the 1/2 thickness position satisfied the X-ray intensity ratio of the (211) plane: 1.60 or more, and the above-mentioned steel structure at the 1/2 thickness position was also obtained. As a result, the properties were obtained in which the yield strength at the 1/2 thickness position was 390 MPa or more, the Charpy fracture appearance transition temperature at the 1/2 thickness position was -100°C or less, and the Kca (-10°C) value was 8400 N/mm3 ^/2 or more.

On the other hand, in the case of the comparative examples (production Nos. 18 to 42), one or more of the component composition and manufacturing conditions were outside the range of the present invention, so the texture and steel structure at the 1/2 plate thickness position could not be obtained, and the target characteristics at the 1/2 plate thickness position were not satisfied.

Claims (6)

In mass percent,
C: 0.040 to 0.150%,
Si: 0.02 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%,
N: 0.0010 to 0.0100% or less, O: 0.0100% or less, and Ceq defined by formula (1) is 0.480 to 0.560%, with 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 1.60 or more,
The steel structure at the 1/2 position of the plate thickness is a mixed structure consisting of 80 to 100% bainite phase and 0 to 20% ferrite phase, in terms of area fraction, 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 the circle equivalent diameter of the maximum porosity remaining at the 1/2 position of the plate thickness is 200 μm or less,
The plate thickness is more than 100 mm,
A high-strength extra-thick steel plate having a yield strength of 390 MPa or more at the 1/2 plate thickness position, a Charpy fracture appearance transition temperature vTrs≦-100°C at the 1/2 plate thickness position, and a Kca(-10°C) value of 8400 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. 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. The texture has an X-ray intensity ratio of 0.90 or more for the (110) plane on the rolled surface at a quarter-thickness position of the sheet thickness,
2. The high-strength extra thick steel plate according to claim 1, having a steel structure at a 1/4 position in the plate thickness direction, which is a mixed structure consisting of, in terms of area fraction, 80 to 100% of a bainite phase and 0 to 20% of a ferrite phase, and in which 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. The texture has an X-ray intensity ratio of 0.90 or more for the (110) plane on the rolled surface at a quarter-thickness position of the sheet thickness,
3. The high-strength extra thick steel plate according to claim 2, having a steel structure at a 1/4 position in the plate thickness direction, which is a mixed structure consisting of, in terms of area fraction, 80 to 100% of a bainite phase and 0 to 20% of a ferrite phase, and in which 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. A method for producing a high-strength extra thick steel plate according to any one of claims 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 5.0% 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 350 mm or more and a reduction ratio, which represents the ratio of the slab thickness to the plate thickness of the final product, of 3.25 or more and less than 4.00.
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 the temperature range of 700 to 500°C at the 1/2 plate thickness position: 2.0°C/s or higher, and a cooling stop temperature at the 1/2 plate thickness position: 500°C or lower. In the hot rolling,
The temperature at the 1/2 position of the plate thickness is the average reduction rate/pass in the austenite recrystallization temperature range: 3.5% or more;
The method for producing a high strength extra thick steel plate according to claim 5 , wherein the rolling is performed under conditions such that the rolling end temperature at the 1/4 position of the plate thickness is _Ar3 point or higher.