WO2017047088A1

WO2017047088A1 - High-strength thick steel plate for structural use and manufacturing method therefor

Info

Publication number: WO2017047088A1
Application number: PCT/JP2016/004223
Authority: WO
Inventors: 隆洋 ▲崎▼本; 恒久半田; 聡伊木
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2015-09-18
Filing date: 2016-09-15
Publication date: 2017-03-23
Also published as: CN110878404A; JP6245384B2; KR20180038029A; CN108026618B; JPWO2017047088A1; CN108026618A; KR102092000B1; CN110878404B

Abstract

Provided is a high-strength thick steel plate having a thickness of 70 mm or more and having excellent brittle crack arrestability. This high-strength thick steel plate comprises three layers consisting of an inner region (a center region in the thickness direction) having high brittle crack arrestability and regions having low brittle crack arrestability on both outer sides of the inner region. Specifically, the thickness t of the plate is 70 mm or more and brittle crack arrestability index Y (= vTrs - 12 × I₍₁₀₀₎ - 22 × I₍₂₁₁₎) at positions 1/6t, 1/3t and 1/2t in the thickness direction satisfy Y_1/3t ≤ 0.9Y_1/2t and Y_1/6t ≥ 0.8Y_1/2t. vTrs is the fracture appearance transition temperature (ºC) obtained by the Charpy V-notch impact test. I₍₁₀₀₎ is an X-ray diffraction intensity ratio of the (100) plane parallel to the rolled surface (the surface of the plate). I₍₂₁₁₎ is an X-ray diffraction intensity ratio of the (211) plane parallel to the rolled surface (the surface of the plate).

Description

Structural high-strength thick steel plate and method for producing the same

TECHNICAL FIELD The present invention relates to a structural steel plate suitable for large steel structures such as ships, marine structures, low-temperature storage tanks, and construction / civil engineering structures, and a method for producing the same. It relates to improvement of brittle crack propagation stopping performance.

In large steel structures such as ships, offshore structures, low-temperature storage tanks, and construction / civil engineering structures, large-scale damage or damage associated with brittle fracture will have a significant impact on the economy and the environment. Therefore, especially in large steel structures, improvement of the safety of the structure is always required from the viewpoint of preventing brittle fracture.

In recent years, for example, container ships have become larger, and large ships of 6000 to 20,000 TEU have been built or planned. As the size of the ship increases, the steel plate used as the hull outer plate becomes thicker and stronger, with a thickness of 50 to 100 mm, yield strength of 390 N / mm class ² , and 470 N. / mm ² grade high strength thick steel plate has come to be used. TEU (Twenty feet Equivalent Unit) represents the number of containers converted into a 20-foot container and represents an index of the loading capacity of container ships.

Steel materials such as thick steel plates used in large steel structures such as ships have excellent low temperature toughness and excellent brittle crack propagation stop toughness values at operating temperatures from the viewpoint of ensuring the safety of the structure. It is required to have. In particular, even if a brittle crack occurs, it is necessary to stop the propagation of the brittle crack before it reaches a large-scale fracture. The toughness value (hereinafter also referred to as “arrest performance”) is an important characteristic.

In this situation, various steel materials or large welded structures with improved “arrest performance” have been developed and manufactured.

For example, Patent Document 1 describes a structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics. The technology described in Patent Document 1 includes, in mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.0%, Al: 0.005-0.08%, or Ti: 0.005- 0.03%, Nb: 0.005-0.05%, Cu: 0.01-0.5%, Ni: 0.01-1.0%, Cr: 0.01-0.5%, Mo: 0.01-0.5%, V: 0.001-0.1%, B: 0.003% or less , Ca: 0.005% or less, REM: 0.01% or less steel material containing one or more of them is heated to a temperature of 900-1200 ° C, and the temperature at the center of the plate thickness is Ar ₃ or higher Cumulative rolling reduction: 30% or more, and rolling at a rolling temperature of 30% or more in the temperature range where the temperature at the center of the sheet thickness is Ar ₃ points or less Ar ₃ points -60 ° C or more, then 2 ° C / s or more By cooling to 600 ° C. or less at a cooling rate of (110) plane X on the rolled surface at the rolled surface at the plate thickness center portion, the (100) plane X-ray intensity ratio is 2.0 or more and the thickness is 1/4 portion. Thick steel plate with a texture with a wire strength ratio of 1.5 or more And you can. By applying such a thick steel plate to the flange portion of the T-shaped joint, it is possible to stop the brittle crack that has progressed from the web portion at the flange portion. In the technique described in Patent Document 1, a specific texture is set so that the X-ray intensity ratio of a specific surface parallel to the rolling surface is high at each position of 1/2 part of the plate thickness and 1/4 part of the plate thickness. To improve the arrestability by changing the propagation direction of brittle cracks.

Further, Patent Document 2 describes a thick steel plate having a thickness of 50 mm or more, which is excellent in long brittle crack propagation stopping characteristics, and a manufacturing method thereof. The technique described in Patent Document 2 includes, in mass%, C: 0.15% or less, Si: 0.60% or less, Mn: 0.80 to 1.80%, S: 0.001 to 0.05%, Ti: 0.005 to 0.050%, or Nb : Containing at least one selected from 0.001 to 0.1%, Cu: 2.0% or less, V: 0.2% or less, Ni: 2.0% or less, Cr: 0.6% or less, Mo: 0.6% or less, W : A steel material containing one or more selected from 0.5% or less, B: 0.0050% or less, Zr: 0.5% or less is heated to a temperature of 900-1300 ° C, and a surface temperature of 1000-850 After rolling at a cumulative reduction ratio of 10% or more in the temperature range of ℃, the surface temperature is 900-600 ℃ and the internal temperature is 50-150 ℃ higher than the surface temperature, and then the one-pass reduction ratio is 7% or more In addition, hot rolling is performed in which the rolling reduction temperature is 800 to 500 ° C. with a cumulative reduction ratio of 50% or more. It should be noted that after hot rolling is completed, cooling to 400 ° C. may be performed at a cooling rate of 5 ° C./s or more. As a result, the X-ray intensity ratio of the (211) surface or (100) surface at the rolled surface in the region of at least 20% of the plate thickness at the center of the plate thickness is 1.5 or more, and 1/4 to 1/1 of the plate thickness. When the X-ray intensity ratio of the (211) plane or (100) plane at the rolled surface in the region of 10 or in the region of 3/4 to 9/10 of the plate thickness is 1.3 or more, and a long ESSO test is performed The tip shape of the brittle crack propagation stop in the cross section in the thickness direction of the fracture surface of the steel sheet is from 1/4 to 1/1 / of the plate thickness from the steel plate surface. Thick steel plate that forms a concave recess with a maximum crack length of 10 or 3/4 to 9/10 of the plate thickness, which is at least as long as the plate thickness and shorter than the direction of the brittle crack. It is said that long brittle cracks, which have been difficult in conventional thick steel plates, can be stopped even under conditions without stress reflection. In the technique described in Patent Document 2, an area of at least 20% of the plate thickness at 1/2 part of the plate thickness and 1/4 to 1/10 of the plate thickness or 3/4 to 9/10 of the plate thickness. By adjusting the X-ray intensity ratio of the specific surface parallel to the rolling surface to be high in the region, the long brittle crack propagation stopping property is improved.

Further, Patent Document 3 describes a method for producing a structural high-strength thick steel plate having a plate thickness of 50 mm or more and excellent brittle crack propagation stopping characteristics. In the technique described in Patent Document 3, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.2%, Al: 0.005-0.08%, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, Ti: 0.005-0.03%, or Nb: 0.005-0.05%, Cu: 0.01-0.5%, Ni: 0.01-1.0%, Cr: 0.01-0.5%, A steel material containing one or more selected from Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: 0.010% or less, 900 to 1150 ° C The total rolling reduction in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more, and the cumulative reduction rate is 20 in the state where the center of the plate thickness is in the austenite recrystallization temperature range. % Rolling and the average reduction rate per pass is 5.0% or less, and then the cumulative reduction is performed in the state where the center of the plate thickness is in the austenite non-recrystallization temperature range. Rolling is performed with a reduction rate of 40% or more and an average reduction rate per pass of 7.0% or more, and then accelerated cooling to 600 ° C or less at a cooling rate of 4.0 ° C / s or more. As a result, the microstructure is mainly composed of ferrite, and the RD // (110) plane integration degree in the surface layer portion is 1.3 or more, and the RD // (110) plane integration degree in the central portion of the plate thickness is 1.8 or more. To obtain a high-strength steel plate for structures with excellent brittle crack propagation stopping characteristics, with a Charpy fracture surface transition temperature of -60 ° C or less at the surface layer and a Charpy fracture surface transition temperature of -50 ° C or less at the center of the plate thickness. I can do it. In the technique described in Patent Document 3, a specific texture is developed in the surface layer portion and the center portion of the plate thickness to improve the brittle crack propagation stop characteristic of the thick steel plate. In Patent Document 3, “RD // (110) plane accumulation degree” is calculated by integrating the values of the three-dimensional crystal orientation density function of the orientation in which the (110) plane is parallel to the rolling direction (RD). The value is obtained and referred to as the value divided by the number of accumulated azimuths.

Patent Document 4 describes a method for producing a structural high-strength thick steel plate having a thickness of 50 mm or more and excellent brittle crack propagation stopping characteristics. In the technique described in Patent Document 4, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.5%, Al: 0.005-0.08%, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, Ti: 0.005-0.03%, or Nb: 0.005-0.05%, Cu: 0.01-0.5%, Ni: 0.01-1.0%, Cr: 0.01-0.5%, A steel material containing at least one selected from Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: 0.010% or less, 1000 to 1200 ° C The total rolling reduction in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more, and the cumulative reduction rate in the state where the center of the plate thickness is in the austenite recrystallization temperature range is 20%. %, And then the cumulative rolling reduction in the state where the center of the plate thickness is in the austenite non-recrystallization temperature range is set to 40% or more and the center of the plate thickness is austenite. Among the rollings in the unrecrystallized temperature range, rolling is performed so that the difference in rolling temperature between the first pass and the final pass is within 40 ° C, and then 450 ° C at a cooling rate of 4 ° C / s or more. It is supposed to cool to below ℃. As a result, the structure is mainly bainite, the RD // (110) surface has a texture of 1.5 or more in the surface layer, and the Charpy fracture surface transition temperature in the surface layer and the center of the plate thickness is -40 ° C or less. It is said that a high-strength thick steel sheet for a structure having excellent brittle crack propagation stopping characteristics can be obtained. In the technique described in Patent Document 4, a specific texture is developed at the surface layer portion and the center portion of the plate thickness to improve the brittle crack propagation stop characteristic of the thick steel plate. In Patent Document 4, “RD // (110) plane accumulation degree” is calculated by integrating the values of the three-dimensional crystal orientation density function of the orientation in which the (110) plane is parallel to the rolling direction (RD). The value is obtained and referred to as the value divided by the number of accumulated azimuths.

Patent Document 5 describes “a high-strength thick steel plate excellent in arrestability”. The thick steel sheet described in Patent Document 5 is mass%, C: 0.04 to 0.16%, Si: 0.01 to 0.5%, Mn: 0.75 to 2.5%, Al: 0.001 to 0.1%, Nb: 0.003 to 0.05%, Ti: 0.005 ~ 0.05%, N: 0.001 ~ 0.008% included, P, S, Cu, Ni, Mo, V, B, Ca, Mg, REM are restricted to below specified values, and the remainder from Fe and inevitable impurities And having a composition with a carbon equivalent Ceq of 0.30 to 0.50%, a ferrite with an area ratio of 70% or less, and a microstructure containing 30% or more of bainite. The grain boundary density, which is the total length per unit area of grain boundaries with an orientation difference of 15 ° or more, is 400 to 1000 mm / mm ² and has an angle within 15 ° with respect to the plane perpendicular to the main rolling direction ( The area ratio of the (100) plane is 10 to 40%, and at 1/2 part of the plate thickness, the grain boundary density is 300 to 900 mm / mm ² and within 15 ° to the plane perpendicular to the main rolling direction. The area ratio of (110) plane with angle is 40-7 It is a 0% high strength thick steel plate. In the technique described in Patent Document 5, a steel slab having the composition described above is charged as a manufacturing method in a heating furnace having an atmospheric temperature of 1000 to 1250 ° C., and then exceeds a center thickness of 850 ° C. to 1150 ° C. Below, rough rolling with a 1-pass reduction ratio of 3 to 30% for 4 to 15 passes, less than 3% within 3 passes, and a cumulative reduction ratio of 15 to 70%, 4 at a center thickness of 750 to 850 ° C. Finishing with ~ 15 passes, shape ratio average of 0.5 ~ 1, cumulative reduction ratio of 40 ~ 80%, and then the sheet thickness center temperature is over 700 ℃, and the thickness is 2 ~ 10 ℃ / s It is preferable to cool to 550 ° C. or lower at the center cooling rate. In the technique described in Patent Document 5, the propagation of brittle cracks is suppressed by changing the texture to be developed between 1/2 part of the plate thickness and 1/4 part of the plate thickness.

JP 2008-045174 JP 2012-180590 A JP 2013-151731 A JP 2013-151732 A Japanese Patent No. 5445720

In the techniques described in Patent Documents 1 to 5, a texture is developed at a specific position in the plate thickness direction to suppress the propagation of a brittle crack and improve the brittle crack propagation stop property in a thick steel plate. . However, even with the techniques described in Patent Documents 1 to 5, in high-strength thick steel plates with a thickness of 70 mm or more, cracks have entered through the full penetration welds, which is the most severe condition in the actual structure. There has been a problem that a sufficiently excellent brittle crack propagation stop property has not been maintained so that a long brittle crack can be stopped even under conditions where there is no stress reflection.

The present invention has been made in view of the problems of the prior art, and an object thereof is to provide a high-strength thick steel plate having a plate thickness of 70 mm or more and excellent brittle crack propagation stopping performance and a method for producing the same. Here, “high strength” refers to the case where the yield strength is 400 N / mm ² or more. The term “excellent in brittle crack propagation stopping performance” here means that the brittle crack propagation stopping toughness value Kca _{-10 °} C at the hull design temperature of -10 ° _C is 9500 N / mm ^3/2 or more. It shall be said.

In order to achieve the above-mentioned object, the present inventors paid attention to the difference in brittle crack propagation stopping performance at each position in the plate thickness direction in the thick steel plate, and distributed the brittle crack propagation stopping performance at each position in the plate thickness direction. The relationship between the state and the brittle crack propagation stopping performance of the entire thick steel plate was studied. As a result, in a thick steel plate with a thickness of 70 mm or more, the brittle crack propagation stopping performance of the entire thick steel plate can be remarkably improved. Found it to exist.

That is, in order to improve the brittle crack propagation stopping performance of the entire thick steel plate, first, an inner region (plate thickness central region) having a high brittle crack propagation stopping performance and the outer sides thereof are relatively compared with each other. It was found that a thick steel plate consisting of three layers having a region with low brittle crack propagation stopping performance is required. In such a thick steel plate, when a brittle crack propagates (propagates), a step occurs in the vicinity of the boundary between the central portion of the plate thickness and the outer portions thereof, and the brittle crack in which the crack tip branches into three layers. It has been found that the brittle crack propagation stopping performance of the entire thick steel plate changes greatly depending on the combination of the thickness of each layer and its characteristics, since it propagates as a crack.

Therefore, the present inventors have determined the brittle crack propagation stopping performance at each position in the plate thickness direction by the following equation: Y = vTrs-12 × I ₍₁₀₀₎ −22 × I ₍₂₁₁₎ (A)
Where, vTrs: V notch Charpy impact test fracture surface transition temperature (° C),
I ₍₁₀₀₎ : X-ray diffraction intensity ratio of (100) plane parallel to the rolling surface (plate surface),
I ₍₂₁₁₎ : X-ray diffraction intensity ratio of (211) plane parallel to the rolling surface (plate surface),
I came up with the idea that it can be evaluated easily using the index Y (° C) defined in. This index Y is based on the fracture surface transition temperature (vTrs) of the Charpy impact test and improves the brittle crack propagation stop toughness by taking into account the degree of texture development that affects the improvement of brittle crack propagation stop performance. This is a parameter introduced by some of the present inventors in order to define the fracture surface transition temperature (vTrs) necessary to achieve this. If a texture favorable for improving the brittle crack propagation stopping performance develops and I ₍₁₀₀₎ and I ₍₂₁₁₎ become large, the index Y becomes low temperature.

The present inventors use this index Y as an index of brittle crack propagation stopping performance at each position in the plate thickness direction, and in particular, brittle crack propagation stopping performance index Y _{1 / 6t} at 1/6 position of the plate thickness, The brittle crack propagation stopping performance index Y _{1 / 3t at} the 1/3 position of the plate thickness and the brittle crack propagation stopping performance index Y _{1 / 2t at} the 1/2 position of the sheet thickness are _expressed by the following equations (1) and (2 ) Formula Y _{1 / 3t} ≦ 0.9Y _{1 / 2t} (1)
Y _{1 / 6t} ≧ 0.8 Y _{1 /} _2t (2)
It was newly found out that by adjusting so as to satisfy the above, a steel plate having a significantly improved brittle crack propagation stop toughness value Kca in the entire thickness was obtained. Formula (1) means that an internal region (plate thickness central region) having high brittle crack propagation stopping performance exists over a region between at least two plate thickness 1/3 positions. Moreover, Formula (2) means that the area | region which has a relatively low brittle crack propagation stop performance exists in the both outer sides of the said internal area | region. Here, Y _{1 / 2t} , Y _{1 / 3t} and Y _{1 / 6t} are less than zero.

First, the experimental results that are the basis of the present invention will be described.

Various types of thick steel plates with a thickness of 70 mm or more were prepared by changing the composition, manufacturing conditions, and the like so that the characteristics at each position in the thickness direction changed. From each of the obtained thick steel plates, a full thickness ESSO test piece (size: t × 500 × 500 mm) and a reduced thickness ESSO test piece (size: 20 × centered on each position in the plate thickness direction) 500 × 500 mm) was collected.

Using these specimens, a temperature gradient type ESSO test was conducted in accordance with Annex A of Brittle Crack Arrest Design Guidelines (Nippon Kaiji Kyokai, September 2009). The relationship between the crack stop temperature and the brittle crack propagation stop toughness value Kca at each position in the plate thickness direction was obtained.

From the obtained results, an example of a thick steel plate having a thickness of 85 mm is shown in FIG.

Figure 1 shows a steel plate with brittle crack propagation stopping performance, which shows a high brittle crack propagation stopping toughness value Kca in the plate thickness internal region at 1/2 the plate thickness and 1/3 the plate thickness. Thickness indicating the distribution of brittle crack propagation stopping performance at each position in the plate thickness direction, consisting of three layers, showing a lower brittle crack propagation stopping toughness value Kca at 1/6 of the plate thickness on both outer sides. It is a steel plate.

The brittle crack propagation stop toughness value Kca at each position in the plate thickness direction is a result obtained using a test piece with a plate thickness of 20 mm. In order to eliminate the influence of the plate thickness, the Japan Welding Association standard WES 3003 In accordance with the following formula: f (t) = 1−0.05 (t−30); t ≦ 35 mm
= 54 / 65-3t / 1300; 35mm ≦ t ≦ 100mm
Using the plate thickness effect coefficient f (t) defined by the following equation, from the Kca obtained by using a test piece with a plate thickness of 20 mm (hereinafter referred to as Kca _{t = 20 mm} ), the following formula Kca _{t = 85 mm} = Kcat _{= 20 mm} × f (85) / f (20)
Thus, Kcat _{= 85} mm with a plate thickness of 85 _mm is converted into that shown in FIG.

From FIG. 1, in the thick steel plate showing the distribution of brittle crack propagation stopping performance at each position in the plate thickness direction, the brittle crack propagation stopping toughness value Kca of the full thickness is the brittle crack at each position in the plate thickness direction. It is much higher than the crack propagation stop toughness value.

This thick steel plate was tested from each position in the plate thickness direction (1/6 position of plate thickness, 1/3 position of plate thickness, 1/2 position of plate thickness) in accordance with the provisions of JIS Z 2242. V-notch Charpy impact test pieces were collected so that the longitudinal direction of the piece was the rolling direction, and the fracture surface transition temperature vTrs at each position in the plate thickness direction was determined. In addition, a specimen was taken so that each position in the plate thickness direction (1/6 position of the plate thickness, 1/3 position of the plate thickness, 1/2 position of the plate thickness) of this thick steel plate becomes the measurement surface. The X-ray diffraction intensity ratio between the (100) plane and the (211) plane parallel to the rolling surface (plate surface) was determined by the line diffraction method. From these values, the index Y (Y _{1 / 6t} , Y _{1 / 3t} , Y _{1 / 2t} ) at each position in the plate thickness direction is calculated using the above-described equation (A). Y _{1 / 6t} = −129 ° C., Y _{1 / 3t} = −168 ° C., Y _{1 / 2t} = −170 ° C., which satisfies both the above formulas (1) and (2).

That is, a thick steel plate having a thickness of 70 mm or more has a high brittleness having a thickness of 1/3 of the total thickness centering on the central position of the plate thickness so as to satisfy the above formulas (1) and (2). An internal region having a crack propagation stopping performance and a region having a thickness of 1/3 of the total plate thickness having a brittle crack propagation stopping performance relatively lower than the internal region are arranged on both outer sides thereof, It is possible to adjust the distribution of brittle crack propagation stop performance at each position in the plate thickness direction so that there are three layers with different brittle crack propagation stop performance in the plate thickness direction. It was found that Kca is important for making a thick steel plate.

The present invention has been completed based on such knowledge and further investigation. That is, the gist of the present invention is as follows.

[1] Plate thickness t: 70 mm or more, and a brittle crack propagation stopping performance index Y _{1 / 2t} (° C.) at the plate thickness 1 / 2t position defined by the following equation (a), and the following equation (b) The brittle crack propagation stoppage performance index Y _{1 / 3t} (° C) at the plate thickness 1 / 3t position defined by, and the brittle crack propagation stoppage performance index at the plate thickness 1 / 6t position defined by the following formula (c) Y _{1 / 6t} (° C) satisfies the following formulas (1) and (2): a high strength thick steel sheet for structural use.
Y _{1 / 3t} ≦ 0.9Y _{1 / 2t} (1)
Y _{1 / 6t} ≧ 0.8 Y _{1 /} _2t (2)
Y _{1 / 2t} = (vTrs) _{1 / 2t} −12 × {I ₍₁₀₀₎ } _{1 / 2t} −22 × {I ₍₂₁₁₎ } _{1 / 2t} (a)
Y _{1 / 3t} = (vTrs) _{1 / 3t} −12 × {I ₍₁₀₀₎ } _{1 / 3t} −22 × {I ₍₂₁₁₎ } _{1 / 3t} (b)
Y _{1 / 6t} = (vTrs) _{1 / 6t} −12 × {I ₍₁₀₀₎ } _{1 / 6t} −22 × {I ₍₂₁₁₎ } _{1 / 6t} (c)
Here, (vTrs) _{1 / 2t} , (vTrs) _{1 / 3t} , (vTrs) _{1 / 6t} : Fracture surface transition temperature (° C) of V notch Charpy impact test at each thickness position,
{I ₍₁₀₀₎ } _{1 / 2t} , {I ₍₁₀₀₎ } _{1 / 3t} , {I ₍₁₀₀₎ } _{1 / 6t} : X-ray diffraction intensity ratio of (100) plane parallel to the plate surface at each plate thickness position ,
{I ₍₂₁₁₎ } _{1 / 2t} , {I ₍₂₁₁₎ } _{1 / 3t} , {I ₍₂₁₁₎ } _{1 / 6t} : X-ray diffraction intensity ratio of (211) plane parallel to the plate surface at each thickness position

[2] Sheet thickness: The structural high-strength thick steel sheet according to [1], which is 100 mm or less.

[3] The structural high-strength thick steel plate according to the above [1] or [2], wherein the brittle crack propagation stopping performance index Y _{1 / 3t} (° C.) is −150 ° C. or less.

[4] By mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.08%, Nb: 0.005 -0.05%, Ti: 0.005-0.03%, N: 0.0050% or less, the composition comprising the balance Fe and inevitable impurities,
One or more selected from ferrite phase, pearlite, and martensite with a volume ratio of 80% or more as the main component and a total volume ratio of 20% or less (including 0%) as the second phase. An organization consisting of
The structural high-strength thick steel plate according to any one of the above [1] to [3].

[5] The composition is further in terms of mass%, Ni: 0.05 to 3%, Cu: 0.05 to 1.5%, Cr: 0.02 to 1.0%, Mo: 0.005 to 1.0%, V: 0.002 to 0.10%, B: 0.0002 The structural high-strength thick steel plate according to the above-mentioned [4], which contains one or more selected from ˜0.003%.

[6] The above composition according to [4] or [5], wherein the composition further contains, by mass%, one or two selected from Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.010% Structural high-strength thick steel plate.

[7] A manufacturing method of a structural high-strength thick steel plate, in which a steel material is subjected to a heating step and a hot rolling step to obtain a steel plate having a thickness t: 70 mm or more,
The steel material is, in mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.08%, Nb : 0.005 to 0.05%, Ti: 0.005 to 0.03%, N: 0.0050% or less, steel material having a composition consisting of the balance Fe and inevitable impurities,
The heating step is a step of heating the steel material to a heating temperature: 900 to 1200 ° C.,
In the hot rolling process, primary rolling is performed at a surface temperature of 1000 to 850 ° C. in a temperature range of 9% or less, and then the surface temperature is 900 to 600 ° C. in a temperature range of 1 pass: Secondary rolling at 7% or more, cumulative rolling reduction: 55% or more, rolling finish temperature: 800 to 550 ° C at surface temperature, then 5 ° C / average on the surface temperature range of 790 to 540 ° C Cooling stop temperature: at a cooling rate of over s
A method for producing a structural high-strength thick steel plate, characterized in that a brittle crack propagation stopping performance index Y at each position in the thickness direction satisfies the following formula (1) and the following formula (2).
Y _{1 / 3t} ≦ 0.9Y _{1 / 2t} (1)
Y _{1 / 6t} ≧ 0.8 Y _{1 /} _2t (2)
Here, Y _{1 / 2t} = (vTrs) _{1 / 2t} −12 × {I ₍₁₀₀₎ } _{1 / 2t} −22 × {I ₍₂₁₁₎ } _{1 / 2t} (a)
Y _{1 / 3t} = (vTrs) _{1 / 3t} −12 × {I ₍₁₀₀₎ } _{1 / 3t} −22 × {I ₍₂₁₁₎ } _{1 / 3t} (b)
Y _{1 / 6t} = (vTrs) _{1 / 6t} −12 × {I ₍₁₀₀₎ } _{1 / 6t} −22 × {I ₍₂₁₁₎ } _{1 / 6t} (c)
(VTrs) _{1 / 2t} , (vTrs) _{1 / 3t} , (vTrs) _{1 / 6t} : Fracture transition temperature (° C) of V-notch Charpy test at each thickness position,
{I ₍₁₀₀₎ } _{1 / 2t} , {I ₍₁₀₀₎ } _{1 / 3t} , {I ₍₁₀₀₎ } _{1 / 6t} : X-ray diffraction intensity ratio of (100) plane parallel to the plate surface at each plate thickness position ,
{I ₍₂₁₁₎ } _{1 / 2t} , {I ₍₂₁₁₎ } _{1 / 3t} , {I ₍₂₁₁₎ } _{1 / 6t} : X-ray diffraction intensity ratio of (211) plane parallel to the plate surface at each thickness position

[8] The composition is further mass%, Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02-1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002 The method for producing a structural high-strength thick steel plate according to the above [7], which contains one or more selected from ˜0.003%.

[9] The composition according to [7] or [8], wherein the composition further contains, by mass%, one or two selected from Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.010% A manufacturing method for structural high-strength thick steel plates.

According to the present invention, the plate thickness: 70 mm or more, yield strength: 400 N / mm ² or more, and hull design temperature: −10 ° C., the brittle crack propagation stop toughness value Kca _{−10 ° C.} is 9500 N / mm ^{3 /} A high-strength thick steel plate excellent in brittle crack propagation stopping performance of ² or more can be easily produced, and has a remarkable industrial effect. In addition, by applying the high-strength thick steel plate according to the present invention to hatch side combing and deck members in the strong deck structure of large container ships and bulk carriers, there is also a great effect of contributing to the improvement of ship safety. is there.

It is a graph which shows an ESSO test result by the relationship between the brittle crack propagation stop toughness value Kca and the brittle crack propagation stop temperature Tk.

The high-strength thick steel plate of the present invention has a high brittle crack propagation stopping performance in the plate thickness central section including the plate thickness center position and 1/3 of the plate thickness in the plate thickness direction cross section. This is a thick steel plate having a distribution of brittle crack propagation stopping performance in the three-layer thickness direction, having relatively low brittle crack propagation stopping performance in each outer region having a thickness of 1/3 of the plate thickness. The presence of a region exhibiting the development of a brittle crack different from the region of the center of the plate thickness in the regions on both outer sides of the plate thickness center, the development of the brittle crack is different in each layer, and as a result, for example, Compared to a uniform thick steel plate that has the same brittle crack propagation stopping performance as the central portion of the plate thickness, the brittle crack propagation stopping toughness value of the entire thick steel plate is high.

In the high-strength thick steel sheet of the present invention, as described above, the distribution of brittle crack propagation stopping performance in the three-layer thickness direction, that is, the brittle crack propagation stopping performance index Y at each position in each sheet thickness direction is ,
Next (1) Formula Y _{1 / 3t} ≦ 0.9Y _{1 / 2t} (1)
And the following formula (2) Y _{1 / 6t} ≧ 0.8Y _{1 / 2t} (2)
Satisfied.

Here, Y _{1 / 2t} is a brittle crack propagation stoppage performance index at the thickness 1 / 2t position, and the following equation (a) Y _{1 / 2t} = (vTrs) _{1 / 2t} −12 × {I ₍₁₀₀₎ } _{1 / 2t} −22 × {I ₍₂₁₁₎ } _{1 / 2t} (a)
(Where (vTrs) _{1 / 2t} : Fracture surface transition temperature (° C) of V-notch Charpy test at plate thickness 1 / 2t position, {I ₍₁₀₀₎ } _{1 / 2t} : Plate surface at plate thickness 1 / 2t position X-ray diffraction intensity ratio of the (100) plane parallel to the surface, {I ₍₂₁₁₎ } _{1 / 2t} : X-ray diffraction intensity ratio of the (211) plane parallel to the plate surface at the plate thickness 1 / 2t position) The In the high-strength thick steel sheet of the present invention, Y _{1 / 2t} preferably satisfies −150 ° C. or less in order to ensure a high brittle crack propagation toughness value in the entire thickness.

Y _{1 / 3t} is a brittle crack propagation stop performance index at the position of the plate thickness 1 / 3t, and the following equation (b) Y _{1 / 3t} = (vTrs) _{1 / 3t} −12 × {I ₍₁₀₀₎ } _{1 / 3t} -22 x {I ₍₂₁₁₎ } _{1 / 3t} (b)
(Where, (vTrs) _{1 / 3t} : Fracture transition temperature (° C) of V-notch Charpy test at _{1 / 3t} plate thickness, {I ₍₁₀₀₎ } _{1 / 3t} : Plate surface at 1 / 3t plate thickness X-ray diffraction intensity ratio of (100) plane parallel to, {I ₍₂₁₁₎ } _{1 / 3t} : X-ray diffraction intensity ratio of (211) plane parallel to plate at 1 / 3t thickness The

Y _{1 / 6t} is a brittle crack propagation stoppage performance index at a thickness of 1 / 6t, and the following formula (c) Y _{1 / 6t} = (vTrs) _{1 / 6t} −12 × {I ₍₁₀₀₎ } _{1 / 6t} -22 × {I ₍₂₁₁₎ } _{1 / 6t} ...... (c)
(Here, (vTrs) _{1 / 6t} : Fracture transition temperature of V-notch Charpy test at 1 / 6t thickness (° C), {I ₍₁₀₀₎ } _{1 / 6t} : Plate thickness at 1 / 6t thickness X-ray diffraction intensity ratio of (100) plane parallel to, {I ₍₂₁₁₎ } _{1 / 6t} : X-ray diffraction intensity ratio of (211) plane parallel to plate at 1 / 6t thickness The

If the above formula (1) is not satisfied, the brittle crack propagation stopping performance at the 1/3 position of the plate thickness is lowered, and the thickness of the central portion of the plate thickness having high brittle crack propagation stopping capability is reduced. Therefore, the desired brittle crack propagation stop toughness value Kca cannot be secured. Incidentally, the brittle crack arrest performance Y _{1 / 3t} in the sheet thickness 1 / 3t position is preferably -150 ° C. or less.

On the other hand, if the above equation (2) is not satisfied, the brittle crack propagation stopping performance at the 1 / 6th plate thickness will be too high, and the progress of the brittle crack will be the same as in the central region of the plate thickness. Since the brittle crack fracture surface branched into three layers is not formed, the desired brittle crack propagation stop toughness value Kca cannot be secured.

For this reason, in the present invention, the thickness 1 / 3t brittle crack propagation in position stopping performance Y _{1 / 3t,} stop brittle crack propagation in the sheet thickness 1 / 2t position Performance Y _{1 / 2t,} and the plate thickness 1 The brittle crack propagation stopping performance Y _{1 / 6t at the} / 6t position is limited to a thick steel plate that satisfies the above-described formulas (1) and (2). Such a thick steel plate is a thick steel plate having the desired high brittle crack propagation stop toughness value Kca. Furthermore, in order to obtain a thick steel plate having a brittle crack fracture surface with the crack tip branched into three layers stably and having a high brittle crack propagation toughness value in the entire thickness, Y _{1 / 6t} ≦ 0.7Y _{1 /} It is desirable to satisfy _2t .

In addition, if it is a thick steel plate that satisfies the above-mentioned conditions, the composition and structure are not particularly limited, but in mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5- Containing 2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005 to 0.08%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.03%, N: 0.0050% or less, or optionally Ni: One or two selected from 0.05 to 3%, Cu: 0.05 to 1.5%, Cr: 0.02 to 1.0%, Mo: 0.005 to 1.0%, V: 0.002 to 0.10%, B: 0.0002 to 0.003% The composition containing one or two selected from the above and / or Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.010%, with the balance consisting of Fe and unavoidable impurities, and a volume ratio of 80 % Selected from ferrite phase, pearlite, and martensite with a total volume ratio of 20% or less (including 0%) as the second phase. It is preferable to use a high-strength thick steel plate having a seed or two or more kinds of structures.

Hereinafter, the reason for limiting the preferred composition will be described first. Hereinafter, the mass% related to the composition is simply expressed as%.

C: 0.03-0.20%
C is an element that contributes to an increase in strength. In order to ensure the desired strength of the thick steel plate of the present invention, it is necessary to contain 0.03% or more. On the other hand, if the content exceeds 0.20%, the toughness of the weld heat-affected zone decreases. Therefore, the C content is limited to the range of 0.03 to 0.20%. Preferably, it is 0.05 to 0.09% from the viewpoint of texture control. More preferably, it is 0.05 to 0.07%.

Si: 0.03-0.5%
Si is an element that acts as a deoxidizer and contributes to an increase in strength by solid solution. In order to obtain such an effect, the content of 0.03% or more is required. On the other hand, if the content exceeds 0.5%, the toughness of the weld heat-affected zone decreases. Therefore, the Si content is limited to the range of 0.03 to 0.5%. Preferably, the content is 0.14 to 0.28%. More preferably, it is 0.14 to 0.17%.

Mn: 0.5-2.5%
Mn is an element that contributes to an increase in strength through solid solution strengthening and hardenability, as well as an improvement in toughness and further a transformation texture. In order to acquire such an effect, 0.5% or more of content is required. On the other hand, if the content exceeds 2.5%, there is a concern that the toughness of the base material will decrease. Therefore, the Mn content is limited to the range of 0.5 to 2.5%. It is preferably 1.5 to 2.4%. More preferably, it is 1.8 to 2.0%.

P: 0.03% or less P is an element that exists in steel as an impurity and segregates at grain boundaries and adversely affects the toughness of the base metal, and is desirably reduced as much as possible. However, up to 0.03% is acceptable. For this reason, the P content is limited to 0.03% or less. In addition, Preferably it is 0.006% or less. On the other hand, from the viewpoint of dephosphorization cost, it is 0.0001% or more industrially.

S: 0.01% or less S is an element that exists as sulfide inclusions in steel and reduces hot workability, base material toughness, base material ductility, and the like, and is preferably reduced as much as possible. However, up to 0.01% is acceptable. For this reason, S content was limited to 0.01% or less. In addition, Preferably it is 0.003% or less. On the other hand, from the viewpoint of desulfurization cost, it is 0.0001% or more industrially.

Al: 0.005-0.08%
Al acts as a deoxidizer and also has a function of binding to nitrogen and precipitating as AlN to suppress coarsening of crystal grains. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.08%, the amount of oxide inclusions increases and the cleanliness of the steel decreases. For this reason, the Al content is limited to a range of 0.005 to 0.08%. Note that the content is preferably 0.02 to 0.04%.

Nb: 0.005-0.05%
Nb contributes to strength increase through precipitation strengthening and has the effect of suppressing recrystallization of austenite, facilitates processing (rolling) in the austenite non-recrystallization temperature range, and contributes to refinement of crystal grains Element. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.05%, the amount of precipitates becomes excessive and the toughness tends to decrease. Therefore, the Nb content is limited to a range of 0.005 to 0.05%. Note that the content is preferably 0.02 to 0.04%.

Ti: 0.005-0.03%
Ti forms a nitride, suppresses coarsening of austenite grains, contributes to refinement of crystal grains of the base material, improves base material toughness, and contributes to refinement of the structure of the heat affected zone of the weld, Improve toughness of weld heat affected zone. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.03%, toughness is reduced. For this reason, the Ti content is limited to the range of 0.005 to 0.03%. Preferably, the content is 0.008 to 0.015%.

N: 0.0050% or less N combines with Ti, Nb, and the like, contributes to refinement of crystal grains as a nitride, and contributes to improvement of base material toughness and toughness of weld heat affected zone. In order to obtain such an effect, the content of 0.002% or more is required. However, if the content exceeds 0.0050%, the toughness of the welded portion is lowered. For this reason, N content was limited to 0.0050% or less.

The above-described components are basic components. In the present invention, in addition to the basic composition, Ni: 0.05 to 3%, Cu: 0.05 to 1.5%, Cr: 0.02 to 1.0%, Mo: One or more selected from 0.005 to 1.0%, V: 0.002 to 0.10%, B: 0.0002 to 0.003%, and / or Ca: 0.0005 to 0.003%, REM: 0.0005 to 0.010% 1 type or 2 types chosen from these can be contained.

One selected from Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02-1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003% Two or more types Ni, Cu, Cr, Mo, V, and B are all elements that increase the strength, and can be selected as necessary and contained in one or more types.

Ni is an element that has a function of solid solution to improve toughness in addition to increasing strength, and also to prevent hot cracking when Cu is contained. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 3%, the material cost will rise. Therefore, when Ni is contained, the Ni content is preferably limited to a range of 0.05 to 3%. More preferably, the content is 0.2 to 1%.

Cu is an element that contributes to an increase in strength by solid solution. To obtain such an effect, it needs to be contained in an amount of 0.05% or more. On the other hand, if the content exceeds 1.5%, the strength increases excessively and the toughness decreases. For this reason, when Cu is contained, the Cu content is preferably limited to a range of 0.05 to 1.5%. More preferably, it is 0.2 to 0.5%.

¡Cr is an element that contributes to the increase in strength by solid solution, and in order to obtain such an effect, the content of 0.02% or more is required. On the other hand, if the content exceeds 1.0%, the toughness of the heat affected zone decreases. Therefore, when Cr is contained, the Cr content is preferably limited to a range of 0.02 to 1.0%. More preferably, the content is 0.1 to 0.6%.

Mo has the effect of suppressing the decrease in strength by forming a solid solution or further forming carbides. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, a large content exceeding 1.0% reduces the toughness of the weld heat affected zone. For this reason, when Mo is contained, the Mo content is preferably limited to a range of 0.005 to 1.0%. More preferably, it is 0.005 to 0.01%.

V is an element that contributes to an increase in strength by forming a solid solution or further forming a precipitate (carbide). In order to acquire such an effect, 0.002% or more of content is required. On the other hand, if the content exceeds 0.10%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, when V is contained, the V content is preferably limited to a range of 0.002 to 0.10%. More preferably, the content is 0.002 to 0.02%.

B is an element that segregates at the grain boundaries, improves the hardenability by containing a trace amount, and contributes to an increase in strength. In order to acquire such an effect, it is necessary to contain 0.0002% or more. On the other hand, if the content exceeds 0.003%, the toughness is decreased. For this reason, when B is contained, the B content is preferably limited to a range of 0.0002 to 0.003%. More preferably, the content is 0.0002 to 0.001%.

One or two types selected from Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.010%. Both Ca and REM can improve ductility and toughness through the form control action of sulfide inclusions. It is a contributing element. In order to obtain such an effect, it is necessary to contain Ca: 0.0005% or more and REM: 0.0005% or more. On the other hand, even if the content exceeds Ca: 0.003% and REM: 0.010%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, when one or both of Ca and REM are contained, it is preferable to limit the range to Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.010%.

The balance other than the above components is composed of Fe and inevitable impurities. Inevitable impurities include As: 0.03% or less, Sb: 0.01% or less, Sn: 0.02% or less, Pb: 0.01% or less, Bi: 0.01% or less.

Next, the reason for limiting the structure of the high-strength thick steel plate of the present invention will be described.

The high-strength thick steel sheet of the present invention has the above composition and the whole thickness direction, excluding the steel sheet surface layer, with a bainite phase of 80% or more in volume ratio as the main phase and a total volume ratio of 20% or less as the second phase. (Including 0%) ferrite phase, pearlite, and martensite.

The high-strength thick steel plate of the present invention has a bainite phase as the main phase in order to maintain a high strength and a high toughness with a plate thickness of 70 mm or more and a yield strength of 400 N / mm ² or more. When the main phase is a phase other than the bainite phase, it becomes difficult to combine the above-described high strength and high toughness in a thick steel plate having a thickness of 70 mm or more. Here, the “main phase” refers to a phase occupying 80% or more by volume.

In the high-strength thick steel plate of the present invention, the second phase other than the main phase is one or two selected from ferrite phase, pearlite, and martensite having a total volume ratio of 20% or less (including 0%). That's it. If the second phase exceeds 20% by volume, the desired high strength cannot be ensured. For this reason, the second phase is limited to one or more selected from ferrite phase, pearlite, and martensite having a total volume ratio of 20% or less (including 0%). The second phase contains 0% by volume. That is, the bainite phase may be 100%.

Next, a preferred method for producing the high strength thick steel plate of the present invention will be described.

In the present invention, the steel material having the above composition is subjected to a heating step and a hot rolling step to obtain a thick steel plate having a thickness of 70 mm or more. In addition, the manufacturing method of the steel material is not particularly limited, the molten steel having the above composition is melted in a conventional melting furnace such as a converter, and a slab is formed by a conventional casting method such as a continuous casting method. A method using a steel material is preferable from the viewpoint of productivity. Needless to say, a steel material may be used as a steel slab by the ingot-bundling rolling method.

The obtained steel material is then subjected to a heating process for hot rolling. In the heating process, the steel material is heated to a temperature of 900 to 1200 ° C.

Heating temperature: 900 ~ 1200 ℃
When the heating temperature is less than 900 ° C., the hot deformation resistance becomes too high, the load on the rolling mill increases, and it becomes difficult to obtain a thick steel plate having a predetermined shape. On the other hand, when the heating temperature is higher than 1200 ° C., the oxidation becomes remarkable and the yield is lowered, and the crystal grains become coarse, and the desired high toughness cannot be ensured. For this reason, the heating temperature was limited to a temperature in the range of 900 to 1200 ° C. The temperature is preferably 1050 to 1150 ° C. from the viewpoint of forming a transformation texture having a desired degree of accumulation. In addition, when the temperature of the steel material is high enough to enable hot rolling, the steel material is left as it is without being heated separately, or after a short time inside the furnace, Hot rolling may be performed.

The heated steel material is subjected to a hot rolling process. The hot rolling step is a step consisting of primary rolling, secondary rolling and cooling after rolling.

Primary rolling is rolling with a cumulative rolling reduction of 9% or less in the surface temperature range of 1000 to 850 ° C.

In this temperature range, by performing primary rolling, the austenite grains are uniformized without being coarsened, so that the variation of the transformation texture is reduced. When the rolling temperature exceeds 1000 ° C. as the surface temperature, the austenite grains become too coarse and the desired structure cannot be obtained even by subsequent hot rolling. On the other hand, if the rolling temperature is less than 850 ° C. at the surface temperature, it becomes an austenite non-recrystallization temperature range, which adversely affects the uniformity of crystal grains. For this reason, primary rolling was performed in the temperature range of 1000 to 850 ° C. as the surface temperature.

Also, if the cumulative rolling reduction in this temperature range exceeds 9%, the desired rolling reduction in the secondary rolling cannot be secured, and the desired distribution of brittle crack propagation stopping performance in the thickness direction can be achieved. Disappear. For this reason, the primary rolling is limited to rolling with a cumulative rolling reduction of 9% or less in the temperature range of 1000 to 850 ° C. at the surface temperature. In primary rolling, the rolling reduction per pass is preferably about 3 to 5% from the viewpoint of regulating the austenite grain size.

The secondary rolling is a rolling with a one-pass reduction ratio of 7% or more, a cumulative reduction ratio of 55% or more, and a rolling end temperature of 800 to 550 ° C in a surface temperature range of 900 to 600 ° C. .

By performing secondary rolling in this temperature range, it is possible to promote the development of a texture that improves brittle crack propagation stopping performance in the inner region of the plate thickness.

In the surface temperature range of 900 to 600 ° C, the vicinity of the steel sheet surface is a two-phase temperature range, and inside the steel sheet is an austenite region. When rolling at a 1-pass reduction ratio of 7% or more, rolling strain is generated inside the steel sheet. Is intensively introduced, and the development of the texture is promoted. As a result, the X-ray diffraction intensity ratio of the (100) plane and (211) plane parallel to the plate surface (rolled surface), which is effective for improving the brittle crack propagation stopping performance, is increased in the central region of the plate thickness. If the one-pass rolling reduction is less than 7%, the introduction of rolling strain into the steel sheet is weak and a desired texture cannot be formed. The one-pass reduction ratio is preferably 9% or more from the viewpoint of securing the width of the texture development region in the desired center portion of the plate thickness.

In addition, if the rolling reduction in this temperature range is less than 55%, the (100) plane parallel to the plate surface (rolled surface) after transformation is effective in improving the brittle crack propagation stopping performance in the central region of the plate thickness. The X-ray diffraction intensity ratio of the (211) plane is lower than the desired value, making it difficult to ensure the desired brittle crack propagation stopping performance. For this reason, the secondary rolling was limited to rolling with a one-pass reduction ratio of 7% or more and a cumulative reduction ratio of 55% or more in a temperature range of 900 to 600 ° C. at the surface temperature. Preferably, the one-pass rolling reduction is 9% or more, and the cumulative rolling reduction is 60% or more. On the other hand, from the viewpoint of overloading the rolling mill, the one-pass rolling reduction is preferably 15% or less, and the cumulative rolling reduction is preferably 75% or less.

The rolling end temperature of secondary rolling is 800 to 550 ° C. When the rolling end temperature is higher than 800 ° C., the texture development is insufficient. On the other hand, if the rolling end temperature is less than 550 ° C., the plastic strain accumulated in the grains becomes too much and the toughness is lowered, so that it becomes difficult to ensure the desired brittle crack propagation stopping performance.

After hot rolling, cool down to a cooling stop temperature of 450-400 ° C at a cooling rate of 5 ° C / s or more on average in the temperature range of 790-540 ° C at the surface temperature.

When the cooling rate after hot rolling is less than 5 ° C./s, the cooling is too slow, and even in the composition within the scope of the present invention, the structure up to the plate thickness 1 / 2t position is the structure in which the bainite phase is the main phase. I can't. The upper limit of the cooling rate is not particularly limited, but is preferably 30 ° C./s or less from the viewpoint of suppressing the formation of the martensite phase. The average cooling rate is preferably 5 to 15 ° C./s.

Also, if the cooling stop temperature exceeds 450 ° C., the amount of the second phase other than the bainite phase exceeds 20% by volume, and a desired thick steel plate structure cannot be secured. On the other hand, if it is less than 400 ° C., a martensite phase appears and a desired thick steel plate structure cannot be secured. For this reason, cooling after hot rolling was performed at a cooling rate of 5 ° C / s or more at an average temperature range of 790 to 540 ° C at the surface temperature to a cooling stop temperature of 450 to 400 ° C. .

Molten steel having the composition shown in Table 1 was melted in a converter, and was cast into a slab (thickness: 300 mm) by a continuous casting method to obtain a steel material. These steel materials were subjected to a heating step under the conditions shown in Table 2 and a hot rolling step consisting of primary rolling, secondary rolling and cooling to obtain thick steel plates having the thicknesses shown in Table 2.

Specimens were collected from the obtained thick steel plates and subjected to a structure observation, a tensile test, an impact test, and a texture measurement test, and the strength and brittle crack propagation stoppage performance index Y of each thick steel plate were evaluated. The test method is as follows.

(1) Structure observation From each position (1 / 6t, 1 / 3t, 1 / 2t) of the thickness direction of the obtained thick steel plate, specimens for structure observation were collected and rolled in the thickness direction cross section. Grind and corrode (corrosion solution: nital solution) so that the cross section parallel to the observation surface becomes an observation surface, and use an optical microscope (magnification: 500 times) or a scanning electron microscope (magnification: 1000 times) Were observed and imaged. The tissue was observed at least 2 fields of view.

Using the obtained tissue photograph, by tissue identification and image analysis, the tissue fraction (volume%) of each phase is obtained for each visual field, the tissue fraction in each visual field is arithmetically averaged, and the thickness It was set as the structure fraction of the position of the steel plate.

(2) Tensile test JIS 4 test so that the longitudinal direction of the specimen is the rolling direction from each position (1 / 6t, 1 / 3t, 1 / 2t of the thickness) of the obtained thick steel plate Pieces were collected and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to determine tensile properties (yield strength YS, tensile strength TS).

(3) Impact test From each position in the plate thickness direction (1 / 6t, 1 / 3t, 1 / 2t of plate thickness) of the obtained thick steel plate, the test piece longitudinal direction is rolled in accordance with the provisions of JIS Z 2242. A V-notch Charpy impact test piece was collected so as to be parallel to the direction, and a Charpy impact test was performed in accordance with the provisions of JIS Z 2242 to determine the fracture surface transition temperature vTrs (° C.).

(4) Texture measurement test X-ray diffraction specimen (size) parallel to the plate surface from each position (1 / 6t, 1 / 3t, 1 / 2t of the plate thickness) of the obtained thick steel plate : 1.5mm thickness x 24mm width x 25mm length), and after removing the processed layer by applying mechanical polishing and chemical polishing so that the measurement surface (24x25mm) is at each position in the plate thickness direction The X-ray diffraction intensity of the (100) plane and (211) plane was measured by X-ray diffraction. In addition, the random test piece was prepared and the X-ray diffraction intensity was measured similarly. The ratio between the obtained X-ray diffraction intensity and the X-ray diffraction intensity of the random specimen is obtained, and the (100) plane X-ray diffraction intensity ratio parallel to the plate plane at each position in the plate thickness direction, (211) plane X-ray diffraction intensity ratio.

Regarding the obtained results, Table 3 shows the results at the plate thickness 1 / 6t position, Table 4 shows the results at the plate thickness 1 / 3t position, and Table 5 shows the results at the plate thickness 1 / 2t position.

Next, from the results obtained, the following equation Y = vTrs−12 × I ₍₁₀₀₎ −22 × I ₍₂₁₁₎ (A)
Where, vTrs: Fracture surface transition temperature (° C) of Charpy impact test,
I ₍₁₀₀₎ : X-ray diffraction intensity ratio of (100) plane parallel to the rolling surface (plate surface),
I ₍₂₁₁₎ : X-ray diffraction intensity ratio of (211) plane parallel to the rolling surface (plate surface),
Was used to evaluate the brittle crack propagation stopping performance index Y at each position in the plate thickness direction (1 / 6t, 1 / 3t, 1 / 2t of the plate thickness) of each steel plate. The obtained index Y at each position in the plate thickness direction is also shown in Tables 3-5. Next, using the obtained index Y at each position in the plate thickness direction, the following formula (1), formula (2) Y _{1 / 3t} ≦ 0.9Y _{1 / 2t} (1)
Y _{1 / 6t} ≧ 0.8 Y _{1 /} _2t (2)
In the case of conformity, “○” was evaluated, and “X” was evaluated otherwise. Table 6 shows the obtained evaluation results.

In addition, ESSO test specimens (full thickness) are collected from each steel plate, and a temperature gradient type ESSO test is conducted in accordance with Annex A of the Brittle Crack Arrest Design Guidelines (Japan Maritime Association (2009)). Ship hull design temperature: The brittle crack propagation toughness value Kca _{-10 °} C at the full thickness at _{-10 ° C} was determined. The results of the obtained ESSO test are shown in Table 6 together with the results of the brittle crack propagation stopping performance index Y.

Any of the examples of the present invention in which the brittle crack propagation stopping performance index Y at each position in the plate thickness direction (1 / 6t, 1 / 3t, 1 / 2t) satisfies the specific relationship of the formulas (1) and (2) , yield strength YS: 400 N / mm ² or more and temperature and the brittle crack arrest toughness Kca _{-10 ° C.} in total thickness is 9500N / mm ^3/2 or more at -10 ° C.,-out high brittle crack propagation stop It is a high-strength thick steel plate with performance. On the other hand, the brittle crack propagation stop performance index Y of the 1 / 3t thickness is high, and the brittle crack propagation stop performance index Y at each position in the thickness direction (1 / 6t, 1 / 3t, 1 / 2t) is ( In the comparative example that does not satisfy the specific relationship of the formulas (1) and (2), the brittle crack propagation stopping performance is low at Kca- _{10 ° C.} of less than 9500 N / mm ^3/2 .

Claims

Plate thickness t: 70 mm or more, and is defined by the brittle crack propagation stop performance index Y 1 / 2t (° C) at the plate thickness 1 / 2t position defined by the following equation (a) and the following equation (b): The brittle crack propagation stoppage performance index Y 1 / 3t (° C) at the 1 / 3t thickness position and the brittle crack propagation stoppage performance index Y 1 / 6t (° C.) satisfies the following formulas (1) and (2): a structural high strength thick steel plate.
Y 1 / 3t ≦ 0.9Y 1 / 2t (1)
Y 1 / 6t ≧ 0.8 Y 1 / 2t (2)
Y 1 / 2t = (vTrs) 1 / 2t −12 × {I (100) } 1 / 2t −22 × {I (211) } 1 / 2t (a)
Y 1 / 3t = (vTrs) 1 / 3t −12 × {I (100) } 1 / 3t −22 × {I (211) } 1 / 3t (b)
Y 1 / 6t = (vTrs) 1 / 6t −12 × {I (100) } 1 / 6t −22 × {I (211) } 1 / 6t (c)
Here, (vTrs) 1 / 2t , (vTrs) 1 / 3t , (vTrs) 1 / 6t : Fracture surface transition temperature (° C) of V notch Charpy impact test at each thickness position,
{I (100) } 1 / 2t , {I (100) } 1 / 3t , {I (100) } 1 / 6t : X-ray diffraction intensity ratio of (100) plane parallel to the plate surface at each plate thickness position ,
{I (211) } 1 / 2t , {I (211) } 1 / 3t , {I (211) } 1 / 6t : X-ray diffraction intensity ratio of (211) plane parallel to the plate surface at each thickness position
Plate thickness: 100 mm or less The structural high-strength thick steel plate according to claim 1.
The structural high-strength thick steel plate according to claim 1 or 2, wherein the brittle crack propagation stopping performance index Y 1 / 3t (° C) is -150 ° C or less.
In mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.08%, Nb: 0.005-0.05% , Ti: 0.005 to 0.03%, N: 0.0050% or less, the composition comprising the balance Fe and inevitable impurities,
One or more selected from ferrite phase, pearlite, and martensite with a volume ratio of 80% or more as the main component and a total volume ratio of 20% or less (including 0%) as the second phase. An organization consisting of
The structural high-strength thick steel plate according to any one of claims 1 to 3, wherein
The composition is further in terms of mass%, Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02-1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003% The structural high-strength thick steel plate according to claim 4, comprising one or more selected from among the above.
The structural high-strength thick steel plate according to claim 4 or 5, wherein the composition further contains, by mass%, one or two selected from Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.010%. .
A steel material is subjected to a heating process and a hot rolling process to obtain a steel sheet having a thickness t of 70 mm or more, and a manufacturing method of a structural high-strength thick steel sheet,
The steel material is, in mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.005-0.08%, Nb : 0.005 to 0.05%, Ti: 0.005 to 0.03%, N: 0.0050% or less, steel material having a composition consisting of the balance Fe and inevitable impurities,
The heating step is a step of heating the steel material to a heating temperature: 900 to 1200 ° C.,
In the hot rolling process, primary rolling is performed at a surface temperature of 1000 to 850 ° C. in a temperature range of 9% or less, and then the surface temperature is 900 to 600 ° C. in a temperature range of 1 pass: Secondary rolling at 7% or more, cumulative rolling reduction: 55% or more, rolling finish temperature: 800 to 550 ° C at surface temperature, then 5 ° C / average on the surface temperature range of 790 to 540 ° C Cooling stop temperature: at a cooling rate of over s
A method for producing a structural high-strength thick steel plate, characterized in that a brittle crack propagation stopping performance index Y at each position in the thickness direction satisfies the following formula (1) and the following formula (2).
Y 1 / 3t ≦ 0.9Y 1 / 2t (1)
Y 1 / 6t ≧ 0.8 Y 1 / 2t (2)
Here, Y 1 / 2t = (vTrs) 1 / 2t −12 × {I (100) } 1 / 2t −22 × {I (211) } 1 / 2t (a)
Y 1 / 3t = (vTrs) 1 / 3t −12 × {I (100) } 1 / 3t −22 × {I (211) } 1 / 3t (b)
Y 1 / 6t = (vTrs) 1 / 6t −12 × {I (100) } 1 / 6t −22 × {I (211) } 1 / 6t (c)
(VTrs) 1 / 2t , (vTrs) 1 / 3t , (vTrs) 1 / 6t : Fracture transition temperature (° C) of V-notch Charpy test at each thickness position,
{I (100) } 1 / 2t , {I (100) } 1 / 3t , {I (100) } 1 / 6t : X-ray diffraction intensity ratio of (100) plane parallel to the plate surface at each plate thickness position ,
{I (211) } 1 / 2t , {I (211) } 1 / 3t , {I (211) } 1 / 6t : X-ray diffraction intensity ratio of (211) plane parallel to the plate surface at each thickness position
The composition is further in terms of mass%, Ni: 0.05-3%, Cu: 0.05-1.5%, Cr: 0.02-1.0%, Mo: 0.005-1.0%, V: 0.002-0.10%, B: 0.0002-0.003% The manufacturing method of the structural high strength thick steel plate of Claim 7 containing 1 type, or 2 or more types chosen from these.
The structural high-strength thick steel plate according to claim 7 or 8, wherein the composition further contains, by mass%, one or two selected from Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.010%. Manufacturing method.