JP2010001520A - Thick steel plate excellent in brittle-crack propagation stop property, and producing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide: a thick steel plate excellent in brittle-crack propagation stop property by mainly containing a bainitic structure and also, achieving the fining of crystal granular diameter in a prescribed position in the plate thickness direction; and a useful method for producing such thick steel plate. <P>SOLUTION: The steel plate contains the chemical components, such as C and the balance iron with inevitable impurities and its structure is composed mainly of the bainite, and at the position of the depth t/8-t/4 (t shows the plate thickness) from the surface, when the range surrounded with large angle granular boundary having ≥15° orientation difference of two crystals is made as a crystal grain, the average circle diameter corresponding diameter of these crystal grains is ≤8 μm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a thick steel plate mainly used as a material for structural materials of ships and bridges, and particularly useful for producing such a thick steel plate with improved properties for stopping the propagation of a generated brittle crack. It is about the method.

  Steel sheets used in structures such as ships, buildings, tanks, offshore structures, line pipes, etc., 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, it may be 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 has been demanded to stop the brittle crack generated in the hatch combing portion at the upper deck portion, and a technique for imparting the above-described brittle crack propagation stop property in a high-strength thick steel plate is desired. .

  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 tends to increase the carbon equivalent Ceq, which is not preferable from the viewpoint of weldability. In addition, when the plate thickness is 80 mm, the ability to stop the propagation of brittle cracks is sufficiently exerted even if the surface layer portion (for example, the portion from the surface to a depth of 10% of the plate thickness) is achieved. This is not always possible, and good brittle crack propagation stopping properties cannot always be secured. In addition, the addition of alloy elements also leads to an increase in the cost of the steel sheet.

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 object thereof is to realize a refinement of crystal grain size while mainly using bainite at a predetermined position in the plate thickness direction. It is an object of the present invention to provide a thick steel plate having excellent brittle fracture propagation stopping characteristics and a useful method for producing such a thick steel plate.

The thick steel plate of the present invention that has achieved the above-mentioned object is C: 0.05 to 0.12% (meaning of mass%, the same applies to the chemical composition), Si: 0.05 to 0.30% , Mn: 1.00-1.80%, P: 0.025% or less (excluding 0%), S: 0.01% or less (not including 0%), Al: 0.01-0. 06%, Ti: 0.005 to 0.03%, Nb: 0.005 to 0.05%, B: 0.0005 to 0.003%, and N: 0.0020 to 0.0090%, respectively. The balance is iron and inevitable impurities,
A large angle having a structure mainly composed of bainite and having an orientation difference between two adjacent crystals of 15 ° or more at a position of depth t / 8 to t / 4 (t represents the plate thickness, hereinafter the same) from the surface. When the region surrounded by the grain boundary is a crystal grain, the gist is that the average equivalent circle diameter of the crystal grain is 8 μm or less. In the present invention, “mainly composed of bainite” means a state in which bainite occupies 95% by area or more in the structure. In addition, the average equivalent circle diameter of a crystal grain when a region surrounded by a large-angle grain boundary having an orientation difference of 15 ° or more between two adjacent crystals is defined as a “large-angle grain boundary diameter” is hereinafter abbreviated. Sometimes.

In producing the thick steel plate of the present invention as described above, the slab is heated to a temperature of 1050 to 1150 ° C., and the surface temperature of the steel plate is (Ar ₃ transformation point −90 ° C.) to (Ar ₃ transformation point −) during rolling. Up to 20 ° C.) after water cooling at an average cooling rate of 1 ° C./second or more and completion of reheating from (Ar ₃ transformation point + 10 ° C.) to (Ar ₃ transformation point + 80 ° C.), the cumulative rolling reduction is 60% or more. Then, it is sufficient to perform accelerated cooling to a temperature range of 400 to 500 ° C. at an average cooling rate of 5 ° C./second or more from a temperature higher than (Ar ₃ transformation point −120 ° C.).

  In the steel sheet of the present invention, at a predetermined position in the sheet thickness direction, the thickness is excellent in brittle crack propagation stopping characteristics by making the structure mainly composed of bainite and by refining crystal grains having a specific crystal orientation. A steel plate can be realized, and such a steel plate is useful as a material for various large structures such as ships and buildings.

  The present inventors paid attention to a thick steel plate having a bainite structure, and studied means for improving the brittle crack propagation stopping property of the steel plate from various angles. As a result, in the bainite structure, the austenite is generated with some orientation relationship, but each crystal selected by the chemical composition of the steel sheet, the formation temperature of the structure, other conditions, etc. The orientation relation of the lattice will change, and at a predetermined position in the thickness direction of the steel sheet, if the crystal grains having a specific crystal orientation difference are refined, the brittle crack propagation stop property will be improved, The present invention has been completed.

  The steel sheet of the present invention is composed of a structure mainly composed of bainite (a bainite phase is 95 area% or more in the structure) at least at a predetermined position in the sheet thickness direction, but this does not add an expensive alloy element. However, in a thick steel plate having a plate thickness of 50 mm or more, high strength is ensured. For example, when ferrite is a matrix, it is difficult to achieve both thick and high strength.

  In general, it is considered that the energy loss due to the ductile fracture region (shear lip) formed from the surface layer affects the brittle crack propagation stop characteristic. Therefore, the present inventors further examined the requirements for improving the brittle crack propagation stop characteristic from the viewpoint of energy balance during the progress of the brittle crack. As a result, the brittle crack spreads from the steel plate surface to the position where the shear lip has a depth t / 8 to t / 4 (t: plate thickness) (hereinafter sometimes simply referred to as “t / 8 to t / 4 part”). It was found that it stopped.

  Therefore, when the toughness in the surface layer portion from the surface to a depth of 10% of the plate thickness is not increased, but the toughness in the t / 8 to t / 4 portion is increased, the brittle crack propagation stopping characteristic can be improved. This knowledge was obtained. Further, the relationship between the structure size (large-angle grain boundary diameter) at t / 8 to t / 4 part and brittle crack propagation stop characteristics was further studied.

  In a single phase structure mainly composed of bainite phase, it is considered that the grain boundary becomes an obstacle to the propagation of brittle cracks. Can be suppressed. That is, it was found that the frequency of collision with cracks should be increased by reducing the grain size by making the crystal grains finer. However, in a small-angle grain boundary (small tilt boundary) where the orientation difference between both ends forming the grain boundary is small (for example, less than 15 °), the grain boundary energy is small and the effect is small, so the orientation difference is 15 °. It is necessary to target the above large-angle grain boundaries (large tilt boundaries).

  That is, when a region surrounded by a large-angle grain boundary in which two adjacent crystals have an orientation difference of 15 ° or more is defined as a crystal grain, the average equivalent circle diameter (large-angle) of the crystal grain in the t / 8 to t / 4 portion. When the grain boundary diameter was controlled to 8 μm or less, excellent brittle crack propagation stopping characteristics were exhibited.

  The “orientation difference” is also referred to as “shift angle” or “inclination angle”, and may be hereinafter referred to as “crystal orientation difference”. Further, in order to measure such a crystal orientation difference, an EBSP method (Electron Backscattering Pattern method) may be employed as shown in Examples described later.

FIG. 1 shows d ^−1/2 when the large-angle grain boundary diameter at t / 8 to t / 4 is d (μm), and Kca (measurement method) at −10 ° C. representing brittle crack propagation stop characteristics. Is a graph showing the relationship with (described later). From this result, it is clear that the above Kca: 7000 N / mm ^3/2 or more can be secured when d ^−1/2 ≧ 0.35 (ie, d ≦ 8 μm).

  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: 0.05 to 0.12%]
C is an element necessary for ensuring the strength of the steel sheet. In order to obtain about 510 MPa with high strength and tensile strength TS, it is necessary to contain 0.05% or more. However, if the content exceeds 0.12%, the weldability deteriorates and the base metal toughness decreases. For these reasons, the C content is set to 0.05 to 0.12%. In addition, the preferable upper limit of C content is 0.10%.

[Si: 0.05-0.30%]
Si is an element necessary for deoxidation and ensuring strength, and for that purpose, it is necessary to contain 0.05% or more. However, if the content exceeds 0.30%, the weldability deteriorates. In addition, the upper limit with preferable Si content is 0.15%.

[Mn: 1.00-1.80%]
Mn is an effective element for increasing the strength of the steel sheet, and in order to exert such an effect, it is necessary to contain 1.00% or more. However, if it is excessively contained, weldability deteriorates, so it is necessary to make it 1.80% or less. In addition, the minimum with preferable Mn content is 1.40%, and a preferable upper limit is 1.60%.

[P: 0.025% or less (excluding 0%)]
P is an impurity that segregates in crystal grains and adversely affects ductility and toughness. Therefore, it is preferable that P be as small as possible. However, considering the degree of cleanliness of practical steel, it is suppressed to 0.025% or less. Is good. In addition, P is an impurity inevitably contained in steel, and it is difficult to make the amount 0% in industrial production.

[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: 0.01 to 0.06%]
Al is an element useful for deoxidation, and is an element effective for forming crystal grains and forming AlN. In order to exert such effects, the Al content needs to be 0.01% or more. However, when the Al content is excessive, the base metal toughness and the toughness of the welded portion are deteriorated, so that it is necessary to be 0.06% or less. In addition, the preferable upper limit of Al content is 0.04%.

[Ti: 0.005 to 0.03%]
Ti has the effect of finely dispersing TiN in the steel to prevent coarsening of the austenite grains during heating and suppressing recrystallization of the austenite. Therefore, the austenite grains are refined and the structure after transformation is refined. Demonstrate the effect. Further, TiN has an effect of preventing the austenite grains from being coarsened in the heat-affected zone (HAZ) during welding and suppressing recrystallization of austenite. Therefore, the austenite grains are refined and effective in improving the HAZ toughness. In order to exert such an effect, it is necessary to contain Ti 0.005% or more (preferably 0.01% or more). However, if the Ti content is excessive, weldability is impaired, so it is necessary to make it 0.03% or less (preferably 0.02% or less).

[Nb: 0.005 to 0.05%]
Nb, like Ti, has the effect of suppressing recrystallization of austenite, and thus exhibits the effect of refining austenite grains and refining the structure after transformation. In order to exert such effects, it is necessary to contain Nb in an amount of 0.005% or more (preferably 0.01% or more). However, if Nb is contained excessively, weldability is impaired, so the Nb content is preferably 0.05% or less (preferably 0.025% or less).

[B: 0.0005 to 0.003%]
B is an element that forms nitrides with N to refine the austenite grain structure of HAZ during welding and is effective in improving HAZ toughness, and free B enhances hardenability and improves the strength of the base metal. . In order to exert such an effect, B needs to be contained in an amount of 0.0005% or more (preferably 0.0010% or more). However, if the B content is excessive, weldability is impaired, so the content was made 0.003% or less (preferably 0.002% or less).

[N: 0.0020 to 0.0090%]
N is an element that combines with elements such as Al, Ti, Nb, and B to form a nitride to refine the matrix structure. In order to exert such an effect, N needs to be contained in an amount of 0.0020% or more (preferably 0.004% or more). However, the solid solution N causes the HAZ toughness to deteriorate. As the total nitrogen amount increases, the above-mentioned nitrides increase, but the solid solution N also becomes excessive and harmful. Therefore, the N content is set to 0.0090% or less (preferably 0.007% or less).

  The basic components in the steel sheet of the present invention are as described above, and the balance consists of iron and inevitable impurities (for example, O).

  The steel sheet of the present invention is composed of a structure mainly composed of bainite at a predetermined position in the sheet thickness direction. However, by performing accelerated cooling in the austenite state, the steel sheet becomes a supercooled state and can have a bainite structure. . The thick steel plate of the present invention is characterized in that the structure is a bainite structure at t / 8 to t / 4, and the large-angle grain boundary diameter is refined. The method for manufacturing the will be described.

In producing the thick steel plate of the present invention, a slab satisfying the above-mentioned chemical component composition is heated to a temperature of 1050 to 1150 ° C., and the steel plate surface temperature during the rolling is (Ar ₃ transformation point −90 ° C.) to ( Up to (Ar ₃ transformation point−20 ° C.) with an average cooling rate of 1 ° C./second or more, and after completion of reheating from (Ar ₃ transformation point + 10 ° C.) to (Ar ₃ transformation point + 80 ° C.), cumulative reduction The rolling may be performed at a rate of 60% or more, and then accelerated cooling from a temperature higher than (Ar ₃ transformation point −120 ° C.) to a temperature range of 400 to 500 ° C. at an average cooling rate of 5 ° C./second or higher.

  The steel sheet of the present invention has excellent brittle crack propagation stopping characteristics by refining the structure at t / 8 to t / 4 part of the steel sheet. In order to obtain such a steel sheet, in the production method described above, the sheet thickness position (t / 8 to t / 4 part) is controlled to an appropriate temperature range for rolling, and the austenite low-temperature-side recrystallization temperature range (hereinafter simply referred to as “reducing temperature range”) By refining austenite grains by rolling in “recrystallization temperature range” and rolling in austenite non-recrystallization temperature range (hereinafter sometimes simply referred to as “non-recrystallization temperature range”) The structure at the plate thickness position is refined by increasing the number of nucleation sites at the time of transformation due to the introduction of deformation strain. Hereinafter, each requirement will be described in order.

  First, the heating temperature of a slab shall be 1050-1150 degreeC. The reason why the heating temperature is set to 1000 ° C. or more is that it is necessary to ensure the strength by homogenizing the material and dissolving Nb. However, if the heating temperature exceeds 1150 ° C., a microstructure cannot be obtained due to the coarsening of the austenite grains during heating.

After heating to the above temperature range, the steel sheet surface temperature is (Ar ₃ transformation point −90 ° C.) to (Ar ₃ transformation point −20 ° C.) during rolling at a mean cooling rate of 1 ° C./second or more. When the recrystallization temperature was examined in the component system of the steel sheet of the present invention, the temperature at t / 8 to t / 4 part of the steel sheet was recrystallized from (Ar ₃ transformation point + 110 ° C.) to (Ar ₃ transformation point + 180 ° C.). After the temperature range, less than (Ar ₃ transformation point + 110 ° C.) is defined as the non-recrystallization temperature range, and the temperature at the position of t / 8 to t / 4 (t: plate thickness) is cooled to the recrystallization temperature range It was found that finish rolling should be started.

Therefore, after completion of rough rolling, water cooling at an average cooling rate of 1 ° C./second or more is performed until the steel sheet surface temperature is (Ar ₃ transformation point −90 ° C.) to (Ar ₃ transformation point −20 ° C.). Cooling in this process is called “water cooling” because it takes a long time to cool the temperature at the plate thickness position to the recrystallization temperature range by air cooling, and the austenite grains grow during cooling. This is because it becomes difficult to achieve finer structure. In addition, since air cooling causes a decrease in productivity, water cooling is also used from the viewpoint of productivity. The cooling stop temperature at this time is (Ar ₃ transformation point −90 ° C.) to (Ar ₃ transformation point −20 ° C.) in terms of the steel sheet surface temperature. The temperature of t / 8 to t / 4 part is the recrystallization temperature. This is to make it an area.

Next, after completion of recuperation from (Ar ₃ transformation point + 10 ° C.) to (Ar ₃ transformation point + 80 ° C.), finish rolling is started. If rolling is performed before the recuperation is completed, rolling is performed with a large temperature difference between t / 8 to t / 4 part and t / 2 part (plate thickness center part), and the strength is relatively low. The rolling strain is preferentially introduced into the t / 2 part, but the rolling strain is difficult to be introduced into the t / 8 to t / 4 part, and the structure of the t / 8 to t / 4 part is refined. Becomes difficult. After cooling to (Ar ₃ transformation point −90 ° C.) to (Ar ₃ transformation point −20 ° C.) at the steel sheet surface temperature, reheating is completed at (Ar ₃ transformation point + 10 ° C.) to (Ar ₃ transformation point +80). This was regarded as the recuperation completion temperature. In addition, in the state where the recuperation is not sufficiently performed, as described above, the temperature difference in the thickness direction is large, so that “warping” due to the difference in deformation resistance in the thickness direction is likely to occur during finish rolling. From the viewpoint of reducing the occurrence of warpage, it is preferable to start the finish rolling after completion of recuperation.

The temperature at t / 8 to t / 4 part of the steel sheet is from (Ar ₃ transformation point + 110 ° C.) to (Ar ₃ transformation point + 180 ° C.) austenite low temperature side recrystallization temperature range (recrystallization temperature range), (Ar ₃ transformation temperature) The point below + 110 ° C. is defined as the austenite non-recrystallization temperature range (non-recrystallization temperature range), and when the cumulative rolling reduction in each temperature range is Rr and Rd, the relationship of the following formula (1) is satisfied. When the relationship between R (reduced contribution reduction coefficient) and d ^−1/2 (d: equivalent diameter of crystal grains) was examined, the results shown in FIG. 2 were obtained. The following equation (1) is obtained by experiments based on the ratio of the cumulative reduction ratio in each temperature region to contribute to the refinement, and the above R is an index for refinement of the grain. It is.
R = (0.44 × Rr) + (0.56 × Rd) (1)

The cumulative rolling reduction in each temperature region is obtained by the following equation (2).
Cumulative rolling reduction = (t ₀ −t ₁ ) / t ₀ × 100 (2)
Wherein (2), 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). ]

As is apparent from the results of FIG. 2, when R ≧ 35, d ^−1/2 ≧ 0.35 can be basically satisfied. From the relationship with FIG. 1, Kca ≧ 7000 N / mm ^3/2 is satisfied. It turns out that it is satisfactory. Even if R ≧ 35, if Rr ≦ 10 (%), the amount of reduction in the austenite low-temperature recrystallization temperature range is insufficient, and the austenite grains before transformation cannot be made fine, and d ^{−1 It} can be seen that brittle crack propagation characteristics deteriorate without satisfying /2≧0.35 (Experiment No. 11 in Examples described later). Thus, Kca ≧ 7000 N / mm ^3/2 can be satisfied by setting the R defined by the above formula (1) to 35 or more. It is necessary to make the cumulative rolling reduction in rolling 60% or more. In the production method of the present invention, after slab heating, in order to make the cast structure uniform, rough rolling is performed in the high temperature range of austenite. In order to ensure at least%, it is preferable to ensure that the finished thickness of the rough rolling is at least 2.5 times the final thickness.

After finish rolling is completed, it is necessary to perform accelerated cooling from a temperature of (Ar ₃ transformation point −120 ° C.) or higher to an average cooling rate of 5 ° C./second to a temperature range of 400 to 500 ° C. This step is for ensuring a high strength in a thick steel plate having a thickness of 50 mm or more with a structure of t / 8 to t / 4 part of the steel plate being a bainite single phase. From this point of view, the accelerated cooling stop temperature is set to 500 ° C. or lower because it is necessary to cool to a temperature at which the structure is mainly composed of bainite. However, when the accelerated cooling stop temperature is less than 400 ° C., an island-like martensite phase is generated and the base material toughness is deteriorated, so 400 ° C. is the lower limit.

  By the manufacturing method as described above, it is possible to manufacture a thick steel plate that satisfies the chemical component composition requirements and the structural requirements of the present invention and has a tensile strength TS of 510 MPa or more. It is preferable that the plate | board thickness in the steel plate of this invention is 50-80 mm.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Steels having the chemical composition shown in Table 1 below were melted in a converter, and steel sheets were produced under various cooling and rolling conditions. The manufacturing conditions at this time are shown in Table 2 below. The temperature of t / 8 to t / 4 part of the steel slab was calculated by a process computer using a difference method. The specific temperature management procedure is as follows. The Ar ₃ transformation point in the present invention employs a value calculated by the following equation (3).

Ar ₃ transformation point = 910-310 [C] -80 [Mn ] -20 [Cu] -15 [Cr] -
55 [Ni] -80 [Mo] +0.35 (t-8) (3)
Where t is the plate thickness, and [C], [Mn], [Cu], [Cr], [Ni] and [Mo] are the contents of C, Mn, Cu, Cr, Ni and Mo, respectively ( [In the thick steel sheet of the present invention, Cu, Cr, Ni and Mo are calculated as not containing in the above formula (3)].

[Temperature measurement method during rolling]
1. Using a process computer, the heating temperature at a predetermined position (t / 8 to t / 4 part) of the steel slab is calculated based on the atmospheric temperature from the start of heating to the end of heating and the in-furnace time.
2. Using the calculated heating temperature, based on the rolling pass schedule during rolling and the data of the cooling method (water cooling or air cooling) between passes, a method suitable for calculation such as the difference method is used to calculate the rolling temperature at any position in the plate thickness direction. The rolling is carried out while using the calculation.
3. The surface temperature of the steel sheet is measured using a radiation type thermometer installed on the rolling line. However, the theoretical value is also calculated in the process computer.
4). The surface temperature of the steel sheet measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is collated with a calculated temperature calculated from a process computer.
5). If the difference between the calculated temperature and the measured temperature is ± 30 ° C or more, recalculate the calculated surface temperature so that it matches the measured temperature to obtain the calculated temperature on the process computer. The calculated temperature is used as it is.
6). Using the calculated temperature calculated above, the rolling temperature in the region to be controlled is managed.

For each steel plate obtained, the fraction of ferrite and bainite (area ratio), the large-angle grain boundary diameter d (and d ^−1/2 ) at t / 8 to t / 4 part, mechanical properties (yield point YP, tensile) Strength TS, impact characteristics (impact characteristics of base material), brittle crack propagation stop characteristics (arrest characteristics of base material), and HAZ toughness were measured by the following methods, and the results are collectively shown in Table 3 below. Show.

[Ferrite and bainite fraction]
A sample was cut from t / 8 to t / 4 part of the steel sheet so that a plane parallel to the rolling direction of the steel sheet and perpendicular to the surface of the steel sheet was exposed, and this was processed into wet emery from # 150 to # 1000. It grind | polished using abrasive paper, and mirror-finished using diamond slurry as an abrasive | polishing agent after that. After this mirror-polished piece was etched with a 2% nitric acid-ethanol solution (Nital solution), a 150 μm × 200 μm field of view was observed at an observation magnification of 400 times, and the ferrite fraction was measured by image analysis. Here, all lath structures other than ferrite were regarded as bainite. And the ferrite and bainite fraction of 5 visual fields in total were calculated | required, and the average value was employ | adopted.

[Equivalent circle diameter of large-angle grain boundary diameter]
(A) A sample including the front and back surfaces of the plate thickness cut in parallel with the rolling direction of the steel plate was prepared.
(B) A cross-section is polished using a wet emery polishing paper of # 150 to # 1000 or a polishing method having a function equivalent to that, and mirror-finished using an abrasive such as diamond slurry.
(C) At t / 8 to t / 4 part of the steel plate, large-angle grains are obtained by FE-SEM-EBSP (electron backscattering diffraction imaging using an electron emission scanning electron microscope) in a cross section parallel to the rolling direction of the steel plate. The field diameter was measured. Specifically, an EBSP apparatus (trade name: “OIM”) manufactured by Tex SEM Laboratories is used in combination with an FE-SEM, and a boundary having an inclination (crystal orientation difference) of 15 ° or more is used as a grain boundary. The field diameter was measured. The measurement conditions at this time are as follows: measurement area: 200 × 200 (μm), measurement step: 0.5 μm interval, and a measurement index having a confidence index (Confidence Index) indicating the reliability of the measurement orientation is smaller than 0.1. Excluded from analysis. The average value of the large-angle grain boundary diameters thus obtained was calculated and used as the “large-angle grain boundary diameter (average circle equivalent diameter d)” in the present invention.
(D) As a method for analyzing text data, those having a large-angle grain boundary diameter (average equivalent circle diameter d) of 2.5 μm or less were determined to be measurement noise and excluded from the target of average value calculation.

[Tensile properties of base material]
NK U14A test specimens are taken from a portion of each steel sheet having a depth t / 4 (direction perpendicular to the rolling direction: C direction), and a tensile test is performed according to JIS Z2241, thereby measuring the yield point YP and the tensile strength TS. did. Yield point YP: 390 MPa or more and tensile strength TS: 510 MPa or more were regarded as acceptable.

[Base material toughness]
A V-notch Charpy test was conducted (test method based on JIS Z 2242), and a brittle fracture surface transition temperature vTrs was determined from a transition curve. As a test piece, a U4 test piece determined by NK (Japan Maritime Association) classification was collected from t / 4 part (direction parallel to rolling direction: L direction). At this time, for each temperature measurement (minimum 4 temperatures or more), the test was carried out 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. The temperature at which the area ratio was 50% was calculated as the brittle fracture surface transition temperature vTrs (a line was drawn so that vTrs was on the highest temperature side). When vTrs was −80 ° C. or lower, it was determined to be acceptable (good base material toughness).

[Brittle crack stopping characteristics]
The brittle crack arrest 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 represents the length from the propagation portion entrance to the brittle crack tip, σ represents the length from the propagation portion entrance to the brittle crack tip, and W represents the propagation portion width.

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 value at −10 ° C. is shown in Table 3 below. In the present invention, a case where Kca at −10 ° C. is 7000 N / mm ^3/2 or more is regarded as acceptable (excellent in brittle crack propagation stopping characteristics).

[Evaluation of HAZ toughness]
Repetitive HAZ thermal cycle test (heating rate up to 1400 ° C: 50 ° C / second, keeping time at maximum heating temperature of 1400 ° C: 30 seconds, cooling time from 800 to 500 ° C during cooling Tc: 300 seconds) The amount of heat: 40 to 45 kJ / mm The heat history of the bond part during high heat input welding was simulated, and the HAZ part was subjected to a Charpy impact test at −20 ° C. to measure the absorbed energy (vE ₋₂₀ ). At this time, the absorbed energy (vE _-20 ) was measured for the three test pieces, and the average value was obtained. And the thing whose average value of vE- ₂₀ is 100J or more was evaluated as excellent in HAZ toughness.

  From the results in Table 3, it can be considered as follows. First, experiment no. 1-7 satisfy | fills all the requirements prescribed | regulated by this invention, and the brittle crack propagation stop characteristic is favorable. On the other hand, those lacking any of the requirements of the present invention (Experiment Nos. 8-32) are inferior in any of the characteristics. Details are as follows.

  Experiment No. In No. 8, since the heating temperature was lower than the range specified in the present invention, the large-angle grain boundary diameter was refined and showed good arrest characteristics, but the strength was insufficient due to the lack of solid solution of Nb. ing.

  Experiment No. In No. 9, the heating temperature is higher than the range specified in the present invention, so that austenite during heating is coarsened, and sufficient fine structure cannot be obtained, and good arrest characteristics are not obtained. Experiment No. Nos. 10 and 16 to 25 have insufficient cumulative rolling reduction at the time of finish rolling, and cannot ensure the refinement of the large-angle grain boundary diameter, so that good arrest characteristics are not obtained.

  Experiment No. Nos. 11 and 12 are those in which the temperature after water cooling after rough rolling is out of the range specified in the present invention, and the cumulative reduction ratio in the austenite low-temperature side recrystallization temperature or the austenite non-recrystallization temperature range becomes low. Further, the refinement of the large-angle grain boundary diameter cannot be ensured, and good arrest characteristics are not obtained.

  Experiment No. In No. 13, the cooling start temperature is outside the range defined in the present invention, the bainite fraction is lowered, and high strength is not obtained. Experiment No. In No. 14, the cooling stop temperature is outside the range defined in the present invention (see FIG. 1), island martensite is generated, toughness is deteriorated, and good arrest characteristics are obtained. Absent.

  Experiment No. No. 15 was air cooled without performing accelerated cooling after completion of rolling, so it became a structure mainly composed of ferrite, not a bainite structure, so that not only the strength was insufficient, but the structure was coarse and good. Arrest characteristics are not obtained.

  Experiment No. Nos. 26 to 32 have chemical composition compositions that do not satisfy any of the requirements defined in the present invention, and have not been able to obtain good arrest characteristics, or have been insufficient in strength or deteriorated in HAZ toughness.

FIG. 1 shows the relationship between d ^−1/2 and Kca based on the results in Table 3. Further, FIG. 2 shows the relationship between R (fine grain contribution reduction coefficient) satisfying the relationship of the expression (1) and d ^−1/2 (d: equivalent diameter of crystal circle). .

It is a graph which shows the relationship between d- ^{1 / 2} when the large angle grain boundary diameter in t / 8-t / 4 part is made into d (micrometer), and Kca at -10 degreeC showing a brittle crack propagation stop characteristic. . It is the graph shown about the relationship of R (fine grain contribution reduction coefficient) which satisfies the relationship of (1), and d- ^{1 / 2} (d: circle equivalent diameter of a crystal grain).

Claims (2)

C: 0.05 to 0.12% (meaning by mass, the same applies to the chemical composition), Si: 0.05 to 0.30%, Mn: 1.00 to 1.80%, P: 0.00. 025% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.06%, Ti: 0.005 to 0.03%, Nb: 0.005 to 0.05%, B: 0.0005 to 0.003% and N: 0.0020 to 0.0090%, respectively, the balance being iron and inevitable impurities,
A large angle having a structure mainly composed of bainite and having an orientation difference between two adjacent crystals of 15 ° or more at a position of depth t / 8 to t / 4 (t represents the plate thickness, hereinafter the same) from the surface. A thick steel plate excellent in brittle crack propagation stopping characteristics, characterized in that, when a region surrounded by a grain boundary is a crystal grain, an average equivalent circle diameter of the crystal grain is 8 μm or less. In producing the steel sheet according to claim 1, the slab is heated to a temperature of 1050 to 1150 ° C., and the steel sheet surface temperature is (Ar ₃ transformation point −90 ° C.) to (Ar ₃ transformation point −20 ° C.) during rolling. ) Is cooled with water at an average cooling rate of 1 ° C./second or more, and after completion of reheating from (Ar ₃ transformation point + 10 ° C.) to (Ar ₃ transformation point + 80 ° C.), the cumulative rolling reduction becomes 60% or more. The brittle crack propagation stop characteristic is characterized by performing rolling and then performing accelerated cooling from a temperature higher than (Ar ₃ transformation point −120 ° C.) to an average cooling rate of 5 ° C./second to a temperature range of 400 to 500 ° C. A method for producing thick steel plates with excellent resistance.

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