JP5432548B2 - Thick steel plate with excellent brittle crack propagation stop properties - Google Patents

Description

  The present invention relates to a thick steel plate mainly used as a material for structural materials of ships and bridges, and more particularly to a thick steel plate with improved characteristics (arrest characteristics) for stopping the propagation of a brittle crack that has occurred.

  Thick steel plates used in ships, buildings, tanks, offshore structures, line pipes and other structures have arrest properties that are the ability to suppress breakage due to the propagation of brittle cracks in order to suppress brittle fracture of structures. (Hereinafter, sometimes referred to as “brittle crack propagation stop characteristic”). In recent years, with the increase in size of structures, there are increasing cases of using high-strength thick steel plates having a yield stress of 390 MPa or more and a plate thickness of 50 mm or more. However, it is generally difficult to secure the brittle crack propagation stop characteristics as described above as the steel sheet becomes stronger and thicker.

On the other hand, container ships are also increasing in size for efficiency, and accordingly, thick-walled and high-strength steel sheets are used. Considering the safety of ship hulls, it is first important not to generate brittle fractures, but even if brittle fractures occur, the propagation of cracks is stopped to avoid total collapse of the hull. It is important to provide the hull with brittle crack propagation stopping characteristics. From such a background, it is required to stop a brittle crack generated in the hatch combing portion at the upper deck portion. The brittle crack propagation stop characteristics required for the upper deck to stop brittle cracks have been studied so far, and even if the load stress and brittle crack progress length increase, If the stress intensity factor K value is saturated and the Kca value at -10 ° C. (a numerical value indicating the brittle crack propagation stop characteristic) is about 3500 N / mm ^3/2 , the progress of the brittle crack can be stopped. It is believed that. Therefore, a technique for imparting the above-described brittle crack propagation stopping property to a high-strength thick steel plate is particularly desired in a container ship. Further, it is desired that the Kca value is higher (for example, 5000 N / mm ^3/2 ).

  As a method for improving the brittle crack propagation stop characteristics, (a) a method of adding an alloy element, (b) a method of refining the crystal grain size, and the like are known. Among these, as a method of adding an alloy element, for example, a technique such as Patent Document 1 has been proposed. In this technique, Ni is contained as an alloy element and the cooling rate in the cooling process is controlled to refine the grain size of bainite and improve the brittle crack propagation stop characteristics. However, such a technique also causes an increase in cost due to the addition of alloy elements.

  On the other hand, as a method for improving the brittle crack propagation stop characteristic by reducing the crystal grain size, techniques such as Patent Documents 2 and 3 are known. In these techniques, ferrite is used as a parent phase, and fine brittle crack propagation stop characteristics are ensured by reducing the particle size of the ferrite. However, since these techniques use soft ferrite as a matrix, application to high-strength and thick steel sheets is difficult.

JP 2007-302993 A Japanese Patent No. 3845113 JP 2002-256374 A

The present invention has been made paying attention to the circumstances as described above, and the purpose thereof is high strength (tensile strength of 490 MPa or more) even when the plate thickness exceeds 50 mm without increasing the manufacturing cost. The present invention is to provide a thick steel plate excellent in brittle crack propagation stopping characteristics that satisfies the above-described requirements and satisfies a Kca value of 5,000 N / mm ^3/2 or more at −10 ° C.

The thick steel plate of the present invention that has achieved the above object is C: 0.03 to 0.1% (meaning “mass%”, the same applies to the chemical component composition), Si: 0.5% or less ( Mn: 1.0 to 2.0%, P: 0.015% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0 0.005 to 0.06%, Nb: 0.02 to 0.06%, Ti: 0.008 to 0.03%, N: 0.0020 to 0.010%, and O: 0.01% or less ( 0% is not included) and the solid solution B is suppressed to 0.0005% or less (including 0%), and the depth t / 4 to t / 2 (t represents the plate thickness) from the surface. The same applies hereinafter), the average area ratio of pseudo-polygonal ferrite is 30 to 85% and the depth t / 4 from the surface. When the average grain size of a crystal grain surrounded by a large-angle grain boundary whose orientation difference between two adjacent crystals is 15 ° or more is defined as an effective grain size D (μm), this is the average equivalent circle diameter of island martensite. It has a gist in that it satisfies the following formula (1) in relation to d (μm) and the yield stress YS (MPa) of the steel sheet. The average equivalent circle diameter d refers to the average value of the diameters (diameters) of circles that are assumed to have the same area by paying attention to the size of island martensite. It is that of island martensite observed on the observation surface.
−215 + 1.56 × D + 9.79 × d + 0.24 × YS <−60 (1)

  In the thick steel plate of the present invention, (a) Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 2.0% as necessary. 1 or more selected from the group consisting of the following (not including 0%), (b) Mo: 0.5% or less (not including 0%), (c) V: 0.10% or less (0% (D) Mg: 0.005% or less (not including 0%), (e) Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less ( (G) Ca: 0.0035% or less (not including 0%), (h) Co: 2.5% or less (not including 0%) and / or W: 2.5 % (Not including 0%), (i) rare earth elements: 0.01% or less (not including 0), etc. are also useful, depending on the components contained Characteristics of the plate are improved.

  In the steel plate of the present invention, by properly defining and optimizing the chemical composition and structure of the base steel plate, it is possible to realize a thick steel plate having excellent brittle crack propagation stopping characteristics. It is useful as a material for various large structures such as

  In order to achieve the above-described problems, the present inventors have made further studies on factors affecting the brittle crack propagation stop characteristics. As a result, from the viewpoint of fracture mechanics, it was clarified that improving the internal toughness value was superior to the plate thickness surface for improving the brittle crack propagation stopping characteristics, and its technical significance was recognized. The application has already been filed (Japanese Patent Application No. 2007-262872). That is, with regard to the brittle crack propagation stop property, it was thought that the toughness of the steel sheet surface layer portion should be good, but according to the study by the present inventors, the depth t / 4 from the steel plate surface was considered. It has been found that brittle cracks are effectively stopped by increasing the toughness at a position of t / 2 (t: plate thickness).

  Further, the brittle crack propagation stop property of the steel sheet is affected by the form of the bainite structure, the average grain size of the crystal grains surrounded by the large-angle grain boundaries, the size of the island martensite, the yield stress YS of the steel sheet, It was also found that this structure can be controlled by optimizing the chemical composition and rolling / cooling conditions.

Therefore, the inventors select a position at a depth t / 4 to t / 2 (t: thickness) from the surface as the inside of the thickness, and form a predetermined amount of pseudopolygonal ferrite as a microstructure at this position. And an effective crystal grain size D (μm) at a position t / 4 from the surface [that is, an effective crystal grain size D of pseudopolygonal ferrite], an average equivalent circle diameter d (μm) of island martensite (MA), Further, the present inventors have found that if the yield stress YS (MPa) of the steel sheet is controlled in a well-balanced manner so as to satisfy the following formula (1), the brittle crack propagation stopping characteristics of the steel sheet can be improved.
−215 + 1.56 × D + 9.79 × d + 0.24 × YS <−60 (1)

  The process of obtaining the above equation (1) is as follows. First, as the factors affecting the brittle crack propagation stop characteristics of the steel sheet, the present inventors have an effective crystal grain size D (μm) at a position t / 4 from the surface, and an average equivalent circle diameter of island martensite (MA). d (μm) and the yield stress YS (MPa) of the steel sheet were selected.

  Among the above requirements, the yield stress YS of the steel sheet tends to be higher as the strength becomes higher, and it is expected that the brittle crack propagation stop characteristic will be better as the yield stress YS is lower. Since this yield stress YS is generally highly correlated with the hardenability index (Pcm), the hardenability index (Pcm) is used as a control guideline for the yield stress YS. However, in the normal hardenability index (Pcm), the influence of alloy elements such as Nb is not taken into consideration. Therefore, in the present invention, the influence of such an alloy element is also considered, and the brittle crack propagation stop characteristics and the yield stress are considered. The relationship was examined.

  Based on the above findings, the effect of the effective crystal grain size D of pseudopolygonal ferrite and the average equivalent circle diameter d of island martensite MA on yield stress YS was examined based on experiments (regression analysis). It was found that the brittle crack propagation stopping property of the steel sheet is improved when the relationship of the expression (1) is satisfied.

  The “crystal orientation difference” that defines the large-angle grain boundary is also called “shift angle” or “tilt angle”. To measure such a crystal orientation difference, the EBSP method (Electron Backscattering Pattern Method) is used. ) May be employed (see Examples below).

  In order to obtain the structure as described above, it is possible to adjust the conditions of hot rolling and the subsequent cooling conditions after appropriately adjusting the components of the steel sheet, particularly the amount of dissolved B (0.0005% or less). Clarified that it is important.

  Conventionally, it is a general improvement means to lower the transformation temperature to obtain a fine lath (bundle) bainite structure (bainitic ferrite). The new point that the form is effective is an important point. Pseudopolygonal ferrite is a granular (lumped) phase and has a Vickers hardness Hv of about 150 to 200. By forming such a granular phase in a predetermined region, the brittle crack propagation stopping property of the steel sheet becomes good.

  However, in order to improve the brittle crack propagation stopping characteristics by forming pseudopolygonal ferrite, it is necessary to ensure that the average area ratio is at least 30% or more, but the amount becomes excessive. If it exceeds%, the strength decreases. In addition to pseudopolygonal ferrite, lath bainite, martensite, ferrite, cementite, and the like may be included.

  In the present invention, by defining the chemical component composition and the structure in the specific region as described above, a thick steel plate having excellent brittle crack propagation stopping characteristics can be realized. Hereinafter, the toughness of “HAZ” is also basically good. That is, the steel plate of the present invention is applied as a welded structure such as a ship, a building, a tank, a line pipe, and is required to have good toughness of the HAZ when welded. Such HAZ toughness is also good.

  One feature of the steel sheet of the present invention is that the chemical composition is appropriately adjusted. Below, the reason for limiting the range of chemical components will be described.

[C: 0.03-0.1%]
C is an element necessary for ensuring the strength of the steel sheet (welding base metal), and it is necessary to contain 0.03% or more in order to ensure the desired strength. However, if C is contained excessively, the HAZ toughness is reduced instead. For these reasons, the upper limit needs to be 0.1%. The preferable lower limit of the C content is 0.04% (more preferably 0.05%), and the preferable upper limit is 0.09% (more preferably 0.08%).

[Si: 0.5% or less (including 0%)]
Si is an effective element for securing the strength of the steel sheet, and is contained if necessary. However, if it is contained excessively, a large amount of island martensite phase (MA phase) is precipitated in the steel material (base material) to deteriorate the HAZ toughness. For these reasons, the upper limit was made 0.5%. In addition, the minimum with preferable Si content is 0.1%, and a preferable upper limit is 0.4%.

[Mn: 1.0 to 2.0%]
Mn is an element effective in improving the hardenability and ensuring the strength of the steel sheet. In order to exert such effects, it is necessary to contain Mn in an amount of 1.0% or more. However, if Mn is contained excessively, the HAZ toughness of the steel sheet deteriorates, so the upper limit is made 2.0%. The minimum with preferable Mn content is 1.3%, and a preferable upper limit is 1.8%.

[P: 0.015% or less (excluding 0%)]
P is an impurity that is inevitably mixed in, and adversely affects the toughness of the steel sheet and HAZ. From such a viewpoint, P is preferably suppressed to 0.015% or less. The upper limit with preferable P content is 0.01%.

[S: 0.01% or less (excluding 0%)]
S is an impurity that combines with alloy elements in the steel sheet to form various inclusions and adversely affects the ductility and toughness of the steel sheet, so it is preferable that it be as small as possible. Considering the degree, it is preferable to suppress it to 0.01% or less. In addition, S is an impurity inevitably contained in steel, and it is difficult to make the amount 0% in industrial production.

[Al: 0.005 to 0.06%]
Al is an element effective as a deoxidizing agent, and also exhibits the effect of improving the toughness of the base material by refining the microstructure of the steel sheet. In order to exert such effects, the Al content needs to be 0.005% or more. However, if it is contained excessively, a large amount of island martensite phase (MA phase) is precipitated on the steel plate (base material) to deteriorate the HAZ toughness. For these reasons, the upper limit was made 0.06%. The preferable lower limit of the Al content is 0.01% (more preferably 0.02%), and the preferable upper limit is 0.04%.

[Nb: 0.02 to 0.06%]
Nb exhibits the effect of improving the hardenability and improving the strength of the base material. However, if it is contained in a large amount, the generation of carbides increases and the brittle crack propagation stop property deteriorates. Therefore, the content is preferably 0.06% or less (more preferably 0.04% or less). In addition, content for exhibiting these effects effectively is 0.020% or more.

[Ti: 0.008 to 0.03%]
Ti has the effect of finely dispersing TiN in the steel to prevent coarsening of the austenite grains during heating, and, like Nb, has the effect of suppressing recrystallization of austenite, so the austenite grains are refined and transformed. Demonstrates the effect of refining the structure. TiN is effective in improving HAZ toughness by refining austenite grains in the HAZ part during welding. In order to exert such effects, it is necessary to contain Ti by 0.008% or more. However, if the Ti content becomes excessive, weldability is impaired, so 0.03% or less.

[N: 0.0020 to 0.010%]
N combines with Al, Ti, Nb, B, etc., and has the effect of forming nitrides and refining the base metal structure, as well as refining austenite grains during welding and refining the intragranular structure. Improve toughness. In order to exert such an effect, N needs to be contained by 0.0020% or more. However, the solid solution N causes the HAZ toughness to deteriorate. As the total nitrogen amount increases, the above-mentioned nitride increases, but the solute N also becomes excessive and harmful, so the content is made 0.010% or less. Preferably, it is 0.008% or less (more preferably 0.006% or less).

[O: 0.01% or less (excluding 0)]
O is contained as an unavoidable impurity, but exists as an oxide in steel. However, if the content exceeds 0.01%, a coarse oxide is generated and the HAZ toughness is deteriorated. For these reasons, the upper limit of the O content is set to 0.01%. The upper limit with preferable O content is 0.005% (more preferably 0.003%).

[Solution B: 0.0005% or less (including 0%)]
The solid solution amount of B needs to be limited because it greatly affects the formation of pseudopolygonal ferrite effective in improving the brittle crack propagation stopping characteristics. When the solid solution B exceeds 0.0005%, pseudo-polygonal ferrite is hardly generated, and the brittle crack propagation stop characteristic is deteriorated. For these reasons, the upper limit of solute B is preferably 0.0005%. The preferable range is good to suppress to 0.0003% or less (more preferably 0.0001% or less). The amount of dissolved B can be controlled by the amount of B added and the heating / rolling conditions. The amount of solid solution B can be reduced (5 ppm or less) by reducing the amount of B added, lowering the heating temperature, or increasing the amount of rolling reduction at low temperature.

  In the steel sheet of the present invention, in addition to the above components, the steel plate is composed of iron and inevitable impurities (for example, Sb, Se, Te, etc.), but may contain trace components (allowable components) to the extent that the properties are not impaired. Such a steel sheet is also included in the scope of the present invention. If necessary, (a) Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 2.0% or less (not including 0%) 1) or more selected from the group consisting of: (b) Mo: 0.5% or less (not including 0%), (c) V: 0.10% or less (not including 0%), (d) Mg: 0.005% or less (not including 0%), (e) Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%), (G) Ca: 0.0035% or less (excluding 0%), (h) Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (including 0%) And (i) rare earth elements: 0.01% or less (not including 0), etc. are also effective. The reasons for limiting the range when these components are contained are as follows.

[Selected from the group consisting of Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 2.0% or less (not including 0%) One or more
Cu, Ni and Cr are all effective elements for improving the hardenability and improving the strength, and are contained as necessary. However, if the content of these elements is excessive, the HAZ toughness is lowered, so that it is preferable that both be 2.0% or less (more preferably 1% or less). A preferable lower limit for exhibiting the above effect is 0.20% (more preferably 0.40%).

[Mo: 0.5% or less (excluding 0%)]
Mo is effective for securing the strength by improving the hardenability, and is appropriately used for preventing temper brittleness. Such an effect increases as the content thereof increases. However, if the Mo content becomes excessive, the HAZ toughness deteriorates, so that the content is preferably 0.5% or less. More preferably, it should be 0.30% or less.

[V: 0.10% or less (excluding 0%)]
V exhibits the effect of improving the hardenability and improving the strength of the base material. V also has the effect of increasing the temper softening resistance. However, if it is contained in a large amount, the HAZ toughness deteriorates, so the content is preferably 0.10% or less (more preferably 0.05% or less). Note that the content for effectively exhibiting these effects is 0.01% or more in V.

[Mg: 0.005% or less (excluding 0%)]
Since Mg has the effect of improving HAZ toughness by forming MgO and suppressing the coarsening of austenite grains in the HAZ, it is contained as necessary. However, if the Mg content is excessive, the inclusions become coarse and the HAZ toughness deteriorates. Therefore, the content is preferably 0.005% or less (more preferably 0.0035% or less).

[Zr: 0.1% or less (excluding 0%) and / or Hf: 0.05% or less (0%
Is not included)]
Zr and Hf are elements effective for improving the HAZ toughness by forming a nitride with N as in the case of Ti, refining the austenite grains of the HAZ during welding. However, if excessively contained, the HAZ toughness is reduced. Therefore, when these elements are contained, Zr is set to 0.1% or less and Hf is set to 0.05% or less.

[Ca: 0.0035% or less (excluding 0%)]
Ca is an element that contributes to the improvement of HAZ toughness by controlling the form of sulfide. However, even if it exceeds 0.0035% and contains excessively, HAZ toughness will deteriorate on the contrary. In addition, the preferable upper limit of Ca content is 0.0020% (more preferably 0.0015%).

[Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%)]
Co and W are contained as necessary because they have the effect of improving the hardenability and increasing the strength of the base material. However, since the HAZ toughness deteriorates if contained excessively, the upper limit is set to 2.5%.

[Rare earth element (REM): 0.01% or less (not including 0)]
Rare earth elements (REM) are elements that contribute to improving the toughness of HAZ by refining and spheroidizing the shapes of inclusions (oxides, sulfides, etc.) that are inevitably mixed in steel. Contains if necessary. Such an effect increases as the content thereof increases, but if the content of REM becomes excessive, inclusions become coarse and the HAZ toughness deteriorates. In the present invention, REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium).

In producing the thick steel plate of the present invention, a steel satisfying the above-mentioned chemical composition amount is melted by a normal melting method, the molten steel is cooled to form a slab, and then subjected to a heat treatment (950) before rolling and heating. ˜1100 ° C.), and then re-heated again to the range of 950 to 1300 ° C., followed by hot rolling, and subsequently the cumulative reduction ratio from Ar ₃ transformation point + 150 ° C. to Ar ₃ transformation point + 100 ° C. is set to 10-30. The rolling is finished so that the cumulative rolling reduction of the Ar ₃ transformation point + 50 ° C. to Ar ₃ transformation point is 10 to 20 %, and then the temperature up to 400 ° C. is 0.1 to 20 %. What is necessary is just to cool at the average cooling rate of (degreeC) / second, and to perform a tempering process at the temperature of 500 degreeC or more after that. The reason for setting the range of each condition in this method is as follows. The temperature shown above is controlled by the surface temperature.

[Heating temperature: 950-1100 ° C.]
By performing the heat treatment before rolling and heating, the effective crystal grains can be controlled and the brittle fracture stopping characteristics can be improved. The effective crystal grain size of the steel sheet can be further refined by making the initial γ grains finer, leading to improved characteristics, but this refinement can be achieved by heat treatment once at 950 ° C. or higher before rolling and heating. Is possible. At temperatures below 950 ° C., all austenite is not effective. On the other hand, when the heating temperature exceeds 1100 ° C., the temperature is too high, and the crystal grains become coarse in combination with the subsequent reheating, which deteriorates the characteristics.

[Reheating temperature: 950-1300 ° C.]
Although it is necessary to set it as 950 degreeC or more from a viewpoint once the structure of a steel plate is once austenitized, when a reheating temperature exceeds 1300 degreeC, an austenite grain will coarsen and it will become difficult to obtain a desired structure | tissue in a subsequent process.

[Ar ₃ transformation point + 150 ° C. to Ar ₃ transformation point + 100 ° C. Cumulative rolling reduction: 10 to 30%]
By setting the cumulative rolling reduction in this temperature range to 10 to 30%, it can be granulated (granulated) in combination with the subsequent steps. If the temperature falls outside this temperature range, or the cumulative rolling reduction is less than 10% or more than 30%, 30% or more of pseudopolygonal ferrite cannot be secured. In the present invention, the “Ar ₃ transformation point” is a value determined by the following equation (2).
Ar ₃ = 910−230 × [C] + 25 × [Si] −74 × [Mn] −56 × [Cu]
-16x [Ni] -9x [Cr] -5x [Mo] -1620x [Nb] (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo] and [Nb] are respectively C, Si, Mn, Cu, Ni, Cr, Mo and The Nb content (% by mass) is shown.

[Ar ₃ transformation point + 50 ° C. to Ar ₃ cumulative rolling reduction of the transformation point: 10-20%]
By setting the cumulative rolling reduction in this temperature range to 10 % or more, an appropriate amount of granularity (granulation) can be achieved. If the temperature falls outside this temperature range or the cumulative rolling reduction is less than 10 % or more than 20 %, it is not possible to secure 30% or more of pseudopolygonal ferrite. The cumulative rolling reduction is obtained by the following equation (3).
Cumulative rolling reduction = (t ₀ −t ₁ ) / t ₀ × 100 (3)
Wherein (3), t ₀ represents the start rolling thickness of the steel strip in the temperature range (mm), t ₁ represents the end of rolling thickness of the steel strip in the temperature range (mm). ]

Average cooling rate to Ar ₃ transformation point to 400 ° C.: 0.1 to 20 ° C. / sec]
When the average cooling rate during cooling is less than 0.1 ° C./second or more than 20 ° C./second, granulation (granulation) cannot be performed. The reason why the cooling is set to 400 ° C. is that no further structural transformation occurs at a temperature lower than this.

[Tempering treatment]
By performing the tempering process, the MA size can be reduced and the brittle fracture stopping characteristics can be improved. If the temperature is too low, the effect is small and the MA is not miniaturized, so it is necessary to set the temperature to 500 ° C. or higher.

  In addition, although the steel plate which is the object of the present invention basically assumes a thick steel sheet having a thickness of 50 mm or more, it has the same characteristics even in a thickness less than that, and is the object of the present invention. Is included. Further, the heat input when welding the steel sheet of the present invention is assumed to be 2 kJ / mm or more, and good HAZ toughness is exhibited when welding is performed with such a heat input.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and any design modifications may be made in accordance with the gist of the present invention. It is included in the range.

  Various molten steels having chemical composition shown in Tables 1 and 2 below are melted by a normal melting method, and after cooling the molten steel into a slab, hot rolling and cooling are performed under the conditions shown in Tables 3 and 4 below. And various steel plates (thickness: 60 mm) were obtained. In Table 1 below, REM was added in the form of a misch metal containing about 50% La and about 25% Ce. In Tables 1 and 2 below, “-” indicates that no element was added.

  For each steel plate obtained, the matrix structure (pseudopolygonal ferrite hardness, average area ratio, effective crystal grain size (μm), MA average equivalent circle diameter (μm)), solute B content, mechanical properties ( The tensile properties of the base material, the impact properties of the base material, the brittle crack stopping properties) were measured by the following methods, and the HAZ toughness was also evaluated. The measurement results are shown in Tables 5 and 6 below. Tables 5 and 6 also show the value [−215 + 1.56 × D + 9.79 × d + 0.24 × YS] on the left side of the equation (1). In Tables 5 and 6 below, “-” indicates that measurement is impossible.

[Hardness measurement of pseudopolygonal ferrite]
The hardness (Hv) for discrimination of pseudo-polygonal ferrite was measured as follows. A 2 cm square specimen taken from t / 4 and t / 2 (t: thickness) of each steel plate is mirror-polished, etched with a nital etchant (2% nitric acid-ethanol solution), and then nanoindentered. (Elionix, “ENT-1100”) was used to measure the TD surface at a load of 250 mg. In the observation range of 150 μm × 200 μm, first, a granular structure (aspect ratio of 3 or less) was selected, the hardness of the granular structure was measured, and the granular structure in the range of Hv 150 to 200 was identified as pseudopolygonal ferrite.

[Measurement of pseudopolygonal ferrite fraction]
In order to measure the pseudo-polygonal ferrite fraction obtained from the hardness measurement results, first, the sample observation surface on which the hardness measurement was performed was photographed with an optical microscope, and then image analysis software (Media Cybernetics) was used. (Made by Image-Pro Plus) was used to quantify the area fraction (average value) of the structure determined to be pseudopolygonal ferrite from the hardness measurement results.

[Measurement of the amount of dissolved B]
The amount of solid solution B was quantified by a chemical analysis test of the extraction residue. Test specimens were taken at 10 mm × 10 mm squares from the positions of t / 4 parts and t / 2 parts (t: plate thickness), and 10 mA acetylacetone-1 mass% tetramethylammonium chloride-methanol solution was used as the electrolyte, and 200 mA. Extraction was performed under a current of less than / m ² and a 0.1 μm filter was used.

[Evaluation of tensile properties of base metal]
By collecting a JIS No. 4 test piece from the position of t / 4 (t: plate thickness) of each steel plate and performing a tensile test according to JIS Z2241, the yield stress YS (yield point YP) and the tensile strength TS are measured, Yield ratio YR was calculated.

[Effective crystal grain size D]
The effective crystal grain size D was measured using an EBSP analyzer (manufactured by TexSEM Laboratories) and an FE-SEM (electrolytic emission scanning electron microscope) “XL30S-FEG” manufactured by Philips. The crystal grain size of the pseudopolygonal ferrite surrounded by a large-angle grain boundary with a crystal orientation difference of 15 ° or more was determined as the effective crystal grain size D. At this time, the measurement area was 250 μm, the measurement step was 0.4 μm, and measurement points having a confidence index (Confidence Index) of 0.1 or less indicating the reliability of the measurement direction were deleted from the analysis target. In addition, those having an effective crystal grain size D of 2.0 μm or less were judged as measurement noise and excluded from the target of the effective crystal grain size D ( target of calculation of the average value of crystal grain size ) .

[Average circle equivalent diameter d of island martensite (MA)]
The specimens mirror-polished at the t / 4 position of each steel plate were repeller-corroded, the structure was observed with an optical microscope, a magnification: 1000 times, a 50 μm square area was photographed at n = 10, and an image analyzer (manufactured by Media Cybernetics) : Image-Pro Plus), the average equivalent circle diameter d was measured.

[Evaluation of impact properties (toughness) of base metal]
For the impact properties (toughness) of the base material, a V-notch Charpy test was performed, and vTrs (brittle fracture surface transition temperature) was obtained from a transition curve. A JIS No. 4 test piece was taken from the position of t / 4 (t: plate thickness), and the test was carried out according to JIS Z2242. At this time, for each temperature measurement (at least 4 temperatures), the test was conducted at n = 3, and a brittle fracture surface transition curve was drawn so as to pass through the point with the highest brittle fracture surface ratio among the three points. A temperature of 50% was calculated as the brittle fracture surface transition temperature vTrs (a line was drawn so that vTrs was on the highest temperature side).

[Brittle crack propagation stopping characteristics]
The brittle crack propagation stop property (arrest property) was performed in accordance with the “brittle fracture propagation stop test” defined by the steel type qualification test method (established on March 31, 2003) issued by the Japan Welding Association (WES). . In the test, a test piece having the shape shown in FIG. 7.2 of the brittle fracture propagation stop test method is used, and a temperature gradient is applied to the test piece in an arbitrary temperature range selected from the range of −190 ° C. to + 60 ° C. A total of 4 specimens were used. The Kca value was calculated by the following formula (4). In the following formula (4), c is the length (mm) from the propagation part inlet to the brittle crack tip, σ is the stress (MPa) for propagating the brittle crack, and W is the propagation part width (mm). .

Create a graph showing the correlation between 1 / T and Kca value, where T is the temperature of the brittle crack tip (unit is K), X axis is 1 / T, and Y axis is the calculated Kca value. And the Kca value at −10 ° C. The Kca values at −10 ° C. are shown in Tables 5 and 6 below. In the present invention, a case where Kca at −10 ° C. is 5000 N / mm ^3/2 or more is regarded as acceptable (excellent in brittle crack propagation stopping characteristics).

[HAZ toughness test]
As a HAZ toughness evaluation method simulating a thermal cycle when submerged arc welding (2 kJ / mm) is performed, a heating temperature is maintained at 1400 ° C. for 5 seconds, and then a cooling time is 800 to 500 ° C. (Tc): 25 seconds. After heat-treating each test steel plate in the thermal cycle, the Charpy absorbed energy (V notch) at a temperature of −15 ° C. was measured. In addition, as a test piece, it is a rod shape of size 10 mm × 10 mm × 55 mm taken from the position of the plate thickness t / 4 part (t: plate thickness) and has a V notch having a depth of 2 mm on one side of the central part. used. At this time, the V Charpy impact value (vE- ₁₅ ) was 50 J or more.

  As is clear from these results, Experiment No. Nos. 1 to 24 are examples satisfying the requirements defined in the present invention, and a thick steel plate excellent in brittle crack propagation stopping characteristics is obtained while satisfying high strength. In contrast, Experiment No. Nos. 25 to 49 are examples that do not satisfy any of the requirements defined in the present invention, and it is understood that any characteristic is not obtained.

Claims (9)

C: 0.03 to 0.1% (meaning “mass%”, the same applies to the chemical component composition), Si: 0.5% or less (including 0%), Mn: 1.0 to 2.0% , P: 0.015% or less (excluding 0%), S: 0.01% or less (not including 0%), Al: 0.005-0.06%, Nb: 0.02-0. It contains 06%, Ti: 0.008 to 0.030%, N: 0.0020 to 0.010%, and O: 0.01% or less (not including 0%), respectively, and solid solution B: 0.0005% or less (including 0%), the balance is iron and inevitable impurities, and the depth t / 4 to t / 2 (t represents the plate thickness, the same shall apply hereinafter) from the surface In the microstructure, the average area ratio of pseudo-polygonal ferrite is 62 to 85%, and it is adjacent to the surface at a depth t / 4. When the grain size of a crystal grain surrounded by a large-angle grain boundary where the orientation difference between two crystals is 15 ° or more is defined as an effective crystal grain size D (μm), this is the average equivalent circular diameter d (μm) of island martensite and A thick steel plate having excellent brittle crack propagation stopping characteristics, which satisfies the following formula (1) in relation to the yield stress YS (MPa) of the steel plate.
−215 + 1.56 × D + 9.79 × d + 0.24 × YS <−60 (1)   Further, Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 2.0% or less (not including 0%) The thick steel plate according to claim 1, which contains one or more selected.   The thick steel plate according to claim 1 or 2, further comprising Mo: 0.5% or less (not including 0%).   Furthermore, it contains V: 0.10% or less (it does not contain 0%), The thick steel plate in any one of Claims 1-3.   Furthermore, Mg: 0.005% or less (0% is not included) The thick steel plate in any one of Claims 1-4.   Furthermore, it contains Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%). Thick steel plate.   Furthermore, Ca: 0.0035% or less (0% is not included) The thick steel plate in any one of Claims 1-6.   Furthermore, Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%) are contained. Thick steel plate.   Furthermore, it contains a rare earth element: 0.01% or less (not including 0%).