TW201402836A - High-strength thick steel plate for structural use which has excellent brittle crack arrestability, and method for producing same - Google Patents

Abstract

A thick steel plate having a steel composition that comprises, in mass%, 0.03 to 0.20% of C, 0.1% or less of Si, 0.5 to 2.2% of Mn, P, S, 0.005 to 0.05% of Nb, 0.005 to 0.03% of Ti, 0.005 to 0.08% of Al, 0.0075% or less of N, optionally at least one component selected from Cu, Ni, Cr, Mo, V, B, Ca and REM, and a remainder made up by Fe and unavoidable impurities, wherein the microstructure is a structure mainly composed of a worked ferrite, and the Charpy ductile-to-brittle transition temperature at a part positioned at a depth of (1/2 of the thickness (t) of the plate) + 6 mm as observed in the thickness direction is -40 DEG C or lower. A steel material (slab) having the above-mentioned composition is heated to a temperature of 1000 to 1200 DEG C, then rolled at a cumulative rolling reduction ratio of 30% or more in such a temperature range that a thickness center part of the steel material (i.e., a center part of the steel material as observed in the thickness direction) can be in an austenite recrystallization temperature range, then subjected to a first cooling procedure at a cooling rate of 15 DEG C/s or less until the temperature of the thickness center part reaches a temperature point Ar3 or lower, then rolled at a cumulative rolling reduction ratio of 40% or more, then subjected to a second cooling procedure to 600 DEG C or lower at a cooling rate of 4 DEG C/s or more, and then optionally tempered at a temperature equal to or lower than a temperature point Ac1.

Description

High-strength thick steel plate for structural use excellent in brittle crack propagation stop characteristics and manufacturing method thereof

The present invention relates to a high-strength thick steel plate excellent in brittle crack arrestability and a method for producing the same, and particularly to a ship, marine structure, and low-temperature storage tank suitable for use with a steel plate having a thickness of 50 mm or more. ,building. Large structures such as civil structures.

For ships, marine structures, cryogenic storage tanks, construction. In large structures such as civil structures, accidents accompanied by brittle fractures have a large economic or environmental impact, so safety is always required. The steel used is required to have a toughness at the temperature of use or a brittle crack propagation stop characteristic.

For ships such as container ships or bulk carriers, the outer plate of ship's hull uses high-strength thick wall materials. Recently, along with the enlargement of the hull, progress has been made in further high-intensity and thickening. In general, the brittle crack propagation stop characteristic of the steel sheet tends to deteriorate as the strength is increased or the wall material is thicker. Therefore, the requirements for the brittle crack propagation stop characteristics are also increased.

As a method for improving the brittle crack propagation characteristics of steel, a method of increasing the Ni content has been known in the prior art, and a commercial scale of 9% Ni steel is used for a tank for liquefied natural gas (LNG).

However, the increase in the amount of Ni forces the cost to rise sharply, and therefore, it is difficult to apply to applications other than LNG storage tanks.

On the other hand, TMCP (THERMO-MECHANICAL CONTROL PROCESS, thermomechanical control method) can be used for steels with a thickness of less than 50 mm for ships or pipes such as LNG's ultra low temperature. The method seeks to refine the grain and improve the low temperature toughness, and imparts excellent brittle crack propagation stop characteristics.

Further, Patent Document 1 proposes a steel material which is ultra fine crystallization for the structure of the surface layer portion in order to increase the brittle crack propagation stop characteristic without increasing the alloy cost.

The steel material which is excellent in the brittle crack propagation stop characteristic described in the patent document 1 is characterized in that the shearing lip (plastic deformation zone shear-lips) (plastic deformation zone) which is generated in the surface layer part of the steel material when the brittle crack propagates is focused on the brittle turtle When the crack propagation stop characteristic is improved, the crystal grains in the portion of the shear lip (plastic deformation region) are made fine to absorb the propagation energy of the propagated brittle crack.

As a manufacturing method, there is described a case where the surface layer portion is cooled to below the Ar ₃ transformation point by controlled cooling after hot rolling, and then controlled cooling is stopped to reheat the surface layer portion to above the transformation point. The step is repeated one or more times. During this period, the steel material is subjected to rolling deformation to be subjected to transformation or recrystallization, whereby an ultrafine ferrite structure or a toughened iron structure is formed in the surface layer portion.

Further, Patent Document 2 describes that in the steel material mainly composed of ferrite-iron-iron, in order to improve the brittle crack propagation stop characteristic, it is important that both surface portions of the steel material have ferrite iron. The structure is composed of 50% or more of the structure, and the unevenness of the particle size of the ferrite is suppressed. The above-mentioned ferrite-iron structure has a circle-equivalent average grain size: 5 μm or less and an aspect ratio of The grains): 2 or more of the ferrite particles, as a method of suppressing unevenness, will be in the finish rolling The maximum rolling reduction per pass is set to 12% or less to suppress local recrystallization.

However, the steel materials excellent in the brittle crack propagation stop characteristics described in Patent Documents 1 and 2 are obtained by reheating only the surface layer portion of the steel material after being temporarily cooled, and performing processing during reheating, thereby obtaining a specific structure. It is difficult to control in terms of the actual production scale, especially in the case of a thick wall material having a thickness of more than 50 mm, which is a treatment for a large load on the rolling and cooling equipment.

On the other hand, Patent Document 3 describes a technique in which attention is paid not only to the refinement of the ferrite grains, but also to the subgrains formed in the ferrite grains to prolong the brittle crack propagation. Stop the TMCP with improved features.

Specifically, in the case of a sheet thickness of 30 to 40 mm, complicated temperature control such as cooling and reheating of the surface layer of the steel sheet is not required, and the brittle crack propagation stop characteristic is improved by the following conditions: (a) ensuring fine ferrite iron crystallizing The rolling conditions of the grain, (b) the rolling condition of the fine ferrite iron structure in the part of the steel plate thickness of 5% or more, and (c) the fine grain iron iron collection texture (developed by heat energy) Disposing a dislocation introduced by processing (rolling) to form a subgrain rolling condition, and (d) suppressing formation of fine ferrite iron crystal grains and fine subgrain grains coarsening Cooling conditions.

Further, a method is also known in which, in the controlled rolling, the metamorphic ferrite iron is rolled and the aggregate structure is developed, whereby the brittle crack propagation stop characteristic is improved. Separation occurs in the direction parallel to the plate surface on the rupture surface of the steel, and the stress at the front end of the brittle fracture is alleviated, thereby improving the resistance to brittle fracture.

For example, Patent Document 4 describes that the (110) plane X-ray intensity ratio is 2 or more by controlled rolling, and the diameter equivalent to a circle in the crystal grains is 20 μm or more. The coarse grain becomes 10% Hereinafter, the brittle fracture resistance is improved.

[Previous Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Special Publication No. 7-100814

Patent Document 2: Japanese Patent Laid-Open Publication No. 2002-256375

Patent Document 3: Japanese Patent No. 3467767

Patent Document 4: Japanese Patent No. 3548349

[Non-patent literature]

Non-Patent Document 1: Inoue et al.: Growth and brittle crack propagation behavior in thick shipbuilding steel, Japanese Society of Marine Engineering, Lectures No. 3, 2006, pp 359-362.

However, in a large container ship of more than 6,000 TEU (Twenty-foot Equivalent Unit), a thick steel plate having a thickness of more than 50 mm is used. In Non-Patent Document 1, the brittle crack propagation of a steel plate having a thickness of 65 mm is used. The cessation characteristics were evaluated, and the following results were reported: the brittle crack did not stop in the base material in the large brittle crack propagation stop test.

Further, in the ESSO test (corresponding to the ESSO test of WES 3003) for the test material, the Kca value at a temperature of -10 ° C (hereinafter, referred to as Kca (-10 ° C)) indicates that it is less than 3000 N/mm ^3/2 . As a result, it is suggested that safety is ensured in the case of applying a hull structure of a steel sheet having a thickness of more than 50 mm.

The brittle crack propagation stop characteristics described in the above Patent Documents 1 to 4 are excellent The steel plate is mainly based on the manufacturing conditions or the experimental data disclosed, with a thickness of about 50 mm. When applied to a thick wall material exceeding 50 mm, it is not clear whether the specified characteristics can be obtained. The necessary characteristics of crack propagation in the direction of the plate thickness have not been verified at all.

Accordingly, an object of the present invention is to provide a high-strength thick steel plate which can be stabilized by an industrially extremely simple process for controlling the aggregate structure in the thickness direction by optimizing the rolling conditions. The product has excellent brittle crack propagation resistance characteristics and a sheet thickness of 50 mm or more.

In order to achieve the above-mentioned problems, the present inventors have made an effort to study the high-strength steel plate having excellent crack propagation stop characteristics even for a thick-walled steel sheet and a method for producing the steel sheet stably, and the following findings have been obtained.

1. In a thick steel plate with a thickness of 50 mm or more, the improvement of the toughness at the central portion of the plate thickness is effective for improving the brittle crack propagation stop characteristic, and the Charpy fracture surface of the plate thickness (t) is 1/2+6 mm. Particularly good results were obtained when the transition temperature was below -40 °C.

2. Reduction of specific chemical components, particularly Si and P as impurity elements, is effective for achieving the above-described toughness value.

3. In the same manner as the chemical composition, the rolling conditions are also important. By rolling under the specific hot rolling conditions in which the temperature at the central portion of the thickness is specified, the microstructure can be made into the processed ferrite. Organization, the result is to further improve the resilience.

The present invention is completed by studying the obtained insights, that is, the present invention is

1. A high-strength steel plate for structural use having excellent brittle crack propagation stop characteristics, characterized in that it has a steel composition containing C: 0.03 to 0.20% by mass%, and Si: 0.1% or less. Mn: 0.5 to 2.2%, P: 0.008% or less, S: 0.01% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.005 to 0.08%, N: 0.0075% or less, and the remainder is Fe. And the composition of the unavoidable impurities, the microstructure is the structure mainly composed of the processed ferrite iron, and the transition temperature of the Xiabi fracture surface of the 1/2+6 mm portion of the plate thickness (t) is -40 ° C or less.

2. A high-strength thick steel plate for structural use having excellent brittle crack propagation resistance characteristics, wherein the steel composition further contains, by mass%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, and Cr: 0.01 to 0.5%. Mo: 0.01 to 0.5%, V: 0.001 to 0.1%, B: 0.003% or less, Ca: 0.005% or less, and REM: 0.01% or less.

3. A high-strength thick steel plate for structural use having excellent brittle crack propagation resistance characteristics, such as 1 or 2, wherein, as the second phase in the microstructure, there are Boron iron, toughened iron, 麻田散铁, and island-shaped 麻田散Iron (MA), and one or more of the unprocessed fat irons after the metamorphosis of the Worthite iron.

4. The high-strength thick steel plate for structural use having excellent brittle crack propagation stop characteristics according to any one of 1 to 3, wherein the plate thickness exceeds 50 mm.

A method for producing a high-strength thick steel sheet for structural use having excellent brittle crack propagation stop characteristics, characterized in that a steel material (slab) having a composition described in 1 or 2 is heated to a temperature of 1000 to 1200 ° C, and a plate When the thick central portion is rolled at a cumulative rolling reduction ratio of 30% or more in the recrystallization temperature region of the Worthite iron, the first cooling is performed at a cooling rate of 15 ° C/s or less until the temperature at the center portion of the thickness becomes Ar ₃ point. Hereinafter, after the rolling at a cumulative rolling reduction ratio of 40% or more in a temperature range of the center portion of the thickness of the sheet thickness of Ar ₃ or less, the second cooling is performed at a cooling rate of 4° C./s or more to 600° C. or less.

6. The method for producing a high-strength thick steel sheet for structural use having excellent brittle crack propagation resistance characteristics, wherein the second cooling is further tempered to a temperature equal to or lower than Ac ₁ point.

According to the present invention, the aggregate structure is appropriately controlled in the thickness direction, and the high-strength thick-walled steel sheet having excellent brittle crack propagation stop characteristics is obtained, and is applied to the sheet thickness because it has a more obvious advantage over the steel of the prior art. A steel sheet of 50 mm or more, preferably a sheet thickness of more than 50 mm, more preferably a sheet thickness of 55 mm or more, and more preferably a sheet thickness of 60 mm or more is effective. Further, for example, in the field of shipbuilding, it is industrially useful for application to a hatch edge or a deck member in a strong deck structure of a large container ship or a bulk cargo ship, thereby contributing to the safety of the ship. .

In the present invention, 1. the base material toughness, 2. chemical composition, and 3. micro-structure are defined.

1. Base metal toughness

In order to suppress the progress of the crack, it is an important condition that the base material toughness at the center portion of the sheet thickness has good characteristics. In the steel sheet of the present invention, the Charpy fracture surface transition temperature of the 1/2 + 6 mm portion of the sheet thickness (t) is defined.

In the thick wall material with a thickness of 50 mm or more, in order to ensure the structural safety, the brittle crack propagation stop performance of the target Kca (-10 ° C) ≧ 6000 N/mm ^3/2 is obtained, and the thickness is (t The 1/2+6mm portion of the Charpy fracture surface transition temperature is specified to be -40 ° C or less.

Here, the summer-to-fracture surface transition temperature of the 1/2+6 mm portion of the plate thickness (t) refers to the fracture surface transition temperature in the case where the Charpy impact test piece is subjected to the impact test, and the above-mentioned Charpy impact test film system The center position of the Charpy impact test piece was obtained by shifting the center portion of the thickness of 1/2 (that is, the center portion of the plate thickness) by 6 mm.

The reason why the center position of the Charpy impact test piece was shifted by 6 mm from the plate thickness of 1/2 was to avoid the influence of the center segregation portion. The cross-section of the full-width Charpy impact test piece was 10 mm square (excluding the notch portion), and therefore, in the case of the above-described manner, the Charpy impact test piece was deviated by 1 mm from the thickness of 1/2. Thereby, the influence of the center segregation is not disturbed, and the toughness inside the steel sheet can be evaluated.

The above toughness is obtained in the case where the manufacturing conditions are appropriately selected. Hereinafter, the chemical composition, microstructure, and preferable production conditions of the steel in the present invention will be described.

2. Chemical composition

In the description, % is set to mass%.

C: 0.03~0.20%

C is an element which increases the strength of the steel. In the present invention, it is necessary to contain 0.03% or more of C in order to secure the desired strength. However, if it exceeds 0.20%, not only the weldability is deteriorated, but also the toughness is adversely affected. Therefore, the C system is specified in the range of 0.03 to 0.20%. Further, it is preferably 0.05 to 0.15%.

Si: 0.1% or less

The Si system is a deoxidizing element and can be effectively used as a strengthening element for steel. However, if the content is too large, the toughness is extremely deteriorated. Therefore, in order to prevent the toughness of the center part of a steel plate from falling, the content is also made 0.1% or less.

Mn: 0.5~2.2%

Mn is contained as a strengthening element. If it is less than 0.5%, the effect is insufficient. When it exceeds 2.2%, the weldability is deteriorated, and the cost of the steel material also rises, so that it is 0.5 to 2.2%.

P: 0.008% or less

P is an inevitable impurity in steel. Since the increase in the amount of P causes deterioration of toughness, in order to satisfactorily maintain the toughness of the central portion of the steel sheet, it is necessary to set the upper limit to P: 0.008% or less.

S: 0.01% or less

S is the unavoidable impurity in steel like P. When the content exceeds 0.01%, the toughness is deteriorated. Therefore, it is preferably 0.01% or less, and more preferably 0.005% or less.

Nb: 0.005~0.05%

Nb is precipitated in the form of NbC when the ferrite is metamorphosed or reheated, contributing to high strength. Further, in the rolling of the Worthite iron region, the effect of expanding the non-recrystallized region is promoted, which contributes to the fine graining of the ferrite and iron, and is therefore effective for improving the toughness. In order to obtain this effect, it is necessary to contain 0.005% or more of Nb. However, if more than 0.05% of Nb is contained, coarse NbC is precipitated, and conversely, the toughness is lowered. Therefore, it is 0.005 to 0.05%.

Ti: 0.005~0.03%

Ti has an effect of forming a nitride, a carbide, or a carbonitride by containing a trace amount of Ti, thereby refining the crystal grains to improve the toughness of the base material. This effect is obtained by containing 0.005% or more of Ti. However, when Ti is contained in an amount of more than 0.03%, the toughness of the base material and the welded heat-affected zone is lowered, so that it is 0.005 to 0.03%.

Al: 0.005~0.08%

Since Al acts as an oxygen scavenger, it is necessary to contain 0.005% or more of Al. However, if it contains more than 0.08% of Al, the toughness is lowered, and in the case of welding, The toughness of the welded metal portion is reduced. Therefore, the Al system is set to 0.005 to 0.08%. Further, it is preferably 0.02 to 0.04%.

N: 0.0075% or less

The N system is bonded to Al in the steel to adjust the crystal grain size during the rolling process to strengthen the steel. However, if it exceeds 0.0075%, the toughness is deteriorated, so that it is 0.0075% or less.

The above is the basic component composition of the present invention, and the remainder is Fe and unavoidable impurities, and may further contain one or more of Cu, Ni, Cr, Mo, V, B, Ca, and REM in order to further improve the characteristics.

Cu, Ni, Cr, Mo

Cu, Ni, Cr, and Mo are all elements that improve the hardenability of steel. In order to directly contribute to the improvement of strength after rolling, and to improve functions such as toughness, high-temperature strength, and weather resistance, Cu, Ni, Cr, and Mo may be contained. However, if it is contained excessively, toughness or weldability is deteriorated. When Cu, Ni, Cr, and Mo are contained, it is preferable to set each upper limit into Cu: 0.5%, Ni: 1.0%, Cr: 0.5%, and Mo: 0.5%. On the other hand, when the content is less than 0.01%, the above effects are not exhibited. Therefore, when Cu, Ni, Cr, and Mo are contained, it is preferable to set the content to 0.01% or more.

V: 0.001~0.1%

V is an element which increases the strength of steel by precipitation strengthening in the form of V (CN), and may contain V of 0.001% or more. However, if it contains more than 0.1% of V, the toughness is lowered. Therefore, when V is contained, it is preferably set to 0.001 to 0.1%.

B: 0.003% or less

The B system may be contained as an element which slightly increases the hardenability of the steel. However, when B is contained in an amount of more than 0.003%, the toughness of the welded portion is lowered. Therefore, when B is contained, the content is preferably 0.003% or less.

Ca: 0.005% or less, REM: 0.01% or less

Ca and REM refine the structure of the welded heat-affected zone to improve the toughness, and even if it does not impair the effect of the present invention, it can contain Ca and REM as needed. However, when it is contained excessively, a coarse intermediate is formed and the toughness of the base material is deteriorated. Therefore, when Ca or REM is contained, the upper limit of the content is preferably such that Ca is 0.005% and REM is 0.01%.

In addition, in order to secure the weldability as the steel for construction, the carbon equivalent (Ceq) shown by the following formula is preferably 0.45% or less.

Ceq=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5

(The symbol of each element on the right side indicates the content (% by mass) of the element).

3. Micro organization

In addition to the influence of chemical composition, toughness is also affected by the large microstructure. In the steel sheet according to the present invention, the structure which is excellent in toughness, in particular, the processed flattened structure in the ferrite iron structure, that is, the processed ferrite iron (hereinafter, simply referred to as processed ferrite) is used as the main body. Thereby, the structure in the thickness direction is finely granulated to achieve an improvement in toughness.

In the case of insufficient strength only for processed ferrite, one or two kinds of Borne, toughened iron, 麻田散铁, and island-like 麻田散铁 (MA) according to the desired strength level The above dispersion is the second phase, whereby strength and toughness can be simultaneously achieved.

In the present invention, the structure mainly composed of the processed ferrite iron refers to a structure in which the processed ferrite iron has an area fraction of 50% or more. The remainder is selected from the wave One or more types of iron, toughened iron, 麻田散铁, island-shaped 麻田散铁 (MA), and unprocessed fat iron from the Worthite iron metamorphosis.

4. Manufacturing conditions

As the manufacturing conditions of the thick steel plate of the present invention, the slab heating temperature, the cumulative rolling reduction ratio in the re-crystallization temperature region of the Worstian iron during hot rolling, and the self-rolling pressure in the Worstian iron recrystallization temperature region up to Ar ₃ The cooling rate below the point, the cumulative rolling reduction ratio below Ar ₃ point, the cooling rate, the cooling stop temperature, and the temper temperature are specified. In the following description, the temperature (° C.) is the temperature at the center portion (1/2 t portion (t is the plate thickness)) of the thickness of the steel sheet. The temperature at the center of the thickness of the steel sheet is determined by simulation calculation or the like based on the thickness, surface temperature, and cooling conditions. For example, the temperature distribution in the thickness direction is calculated by the difference method, and the temperature at the center portion of the thickness of the steel sheet is obtained.

For thick wall materials with a thickness of 50 mm or more used in the outer plate of ship's hull of recent container ships or bulk carriers, in order to ensure structural safety, it is necessary to obtain a Kca value at -10 °C. That is, Kca (-10 ° C) is a brittle crack propagation stop performance of 6000 N/mm ^3/2 or more. First, a molten steel of the above composition is melted by a converter or the like, and a steel material (steel billet) is produced by continuous casting or the like. Then, the obtained steel material (steel billet) is heated to a temperature of 1000 to 1200 ° C and then hot rolled.

If the heating temperature is less than 1000 ° C, the rolling time in the recrystallization temperature region of the Worthite iron is insufficient, and if it exceeds 1200 ° C, the Worthite iron particles are coarsened, which not only causes the toughness to decrease, but also the oxidation loss becomes obvious. The yield is lowered, so the heating temperature is set to 1000 to 1200 °C. From the viewpoint of toughness, the preferred heating temperature ranges from 1,000 to 1,150 ° C, more preferably from 1,000 to 1,050 ° C.

In the hot rolling, first, when the temperature in the center portion of the sheet thickness is in the Vostian iron recrystallization temperature region, the cumulative rolling reduction ratio is set to 30% or more. If the cumulative rolling reduction rate is not When the temperature is over 30%, the fine graining of the Worthite iron is insufficient, and the toughness is not improved.

After rolling in the recrystallization temperature region of the Worthite, the first cooling is performed until the temperature at the center portion of the thickness becomes Ar ₃ or less. Here, if it is excessively quenched, the time for sufficient recrystallization cannot be obtained. Therefore, the cooling rate up to or below Ar ₃ is set to 15 ° C/s or less. In the present invention, Ar ₃ point (°C) is obtained by the following formula.

Ar ₃ (°C)=910-273C-74Mn-57Ni-16Cr-9Mo-5Cu

In the formula, each element symbol is set to the content (% by mass) in steel, and is referred to as 0 when it is not.

By performing the first cooling, it is possible to reduce the coarse-grained Worthite iron obtained by the rolling at the center of the thickness of the sheet thickness in the recrystallization temperature region of the Worstian iron. temperature of the central portion of the thickness of said rolling pressure at a temperature range of _3:00 or less Ar, and therefore also contributes to fine granulation tissue of the finally obtained.

Then, at a temperature in the central portion of the thickness of the sheet at a temperature of Ar ₃ or less, a rolling reduction of a cumulative rolling reduction ratio of 40% or more is performed. When the cumulative rolling reduction ratio in this temperature region is not 40% or more, the structure cannot be sufficiently granulated and the toughness is deteriorated.

Furthermore, compared with the rolling pressure in the non-recrystallization zone, the rolling pressure in the temperature region below the Ar ₃ point has a greater effect on improving the crack propagation characteristics, so it is necessary to distribute the effective rolling to the temperature region as much as possible. . Therefore, in the present invention, the rolling in the non-recrystallization zone is not performed.

The steel sheet after completion of the rolling is subjected to the second cooling to a temperature of 600 ° C or lower at a cooling rate of 4 ° C /s or more. When the cooling rate is less than 4 ° C / s, the structure is coarsened and the toughness is lowered. Further, when the cooling stop temperature is higher than 600 ° C, recrystallization is performed even after the cooling is stopped, and the desired aggregate structure cannot be obtained. Therefore, the cooling stop temperature is set to 600 ° C or lower.

The tempered steel sheet can also be tempered. The toughness of the steel sheet can be further improved by performing tempering. The tempering temperature is performed at _a point below the Ac ₁ point without loss of use of rolling and cooling. In the present invention, Ac ₁ point (°C) is obtained by the following formula. Ac ₁ point=751-26.6C+17.6Si-11.6Mn-169Al-23Cu-23Ni+24.1Cr+22.5Mo+233Nb-39.7V-5.7Ti-895B

In the formula, each element symbol is set to the content (% by mass) in steel, and is referred to as 0 when it is not.

[Examples]

The molten steel (steel marks A to P) of each composition shown in Table 1 was melted by a converter, and a steel material (steel billet) (slab 280 mm thick) was formed by a continuous casting method, and the first cooling was inserted and hot rolled into a plate on the way. After the thickness is 50 to 80 mm, the second cooling is performed, and the test steel No. 1 to 22 is obtained. Table 2 shows hot rolling conditions and cooling conditions.

In the thick steel plate obtained, a JIS 14A test piece of Φ 14 mm was taken from the quarter of the thickness (t) so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and the tensile test was performed to measure the drop point ( Yield Strength) (YS), Tensile Strength (TS). Regarding the microstructure, the 1/4 portion of the thickness of the sheet was observed by an optical microscope at a magnification of 400 times and a field of view parallel to the rolling direction, thereby confirming the type of the structure.

In addition, the JIS No. 4 impact test piece was taken in such a manner that the 1/2+6 mm portion of the plate thickness (t) was the center of the test piece, and the longitudinal direction of the test piece was parallel to the rolling direction, and the Charpy impact test was performed. Find the fracture surface transition temperature (vTrs). It is within the scope of the present invention to set the Charpy fracture surface transition temperature of the 1/2 + 6 mm portion of the sheet thickness (t) to -40 ° C or lower.

Then, in order to evaluate the brittle crack propagation stop characteristic, a temperature gradient type ESSO test (corresponding to the ESSO test of WES 3003) was performed, and Kca (-10 ° C) was determined.

The results of these tests are shown in Table 3. When the toughness of the central portion of the thickness is the test steel sheet (manufacturing No. 1 to 11) within the scope of the present invention, if the Kca (-10 ° C) is 6000 N/mm ^3/2 or more, the excellent brittle turtle is displayed. Crack propagation stops performance. Furthermore, the microstructure of the test steel sheets of No. 1 to 11 was 50% or more of the processed ferrite.

On the other hand, the steel sheet having the composition of the steel sheet outside the scope of the present invention (manufacturing No. 12 to 18) and the steel sheet having the manufacturing conditions outside the scope of the present invention and having the assembly of the steel sheet not satisfying the requirements of the present invention (manufactured) In No. 19 to 22), the value of Kca (-10 ° C) was 3,800 N/mm ³ / ² or less, which was inferior to the example of the present invention.

Claims (6)

A high-strength thick steel plate for structural use having excellent brittle crack propagation stop characteristics, characterized in that it has a steel composition containing C: 0.03 to 0.20%, Si: 0.1% or less, and Mn: 0.5 to 2.2%, P in mass%. : 0.008% or less, S: 0.01% or less, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.03%, Al: 0.005 to 0.08%, N: 0.0075% or less, and the balance being Fe and inevitable impurities. The microstructure is a structure mainly composed of processed ferrite iron, and the transition temperature of the Charpy fracture surface of the 1/2+6 mm portion of the sheet thickness (t) is -40 ° C or less. A high-strength steel plate for structural use having excellent brittle crack propagation resistance characteristics according to the first aspect of the patent application, wherein the steel composition contains, in mass%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, and Cr: 0.01 to 0.5%, Mo: 0.01 to 0.5%, V: 0.001 to 0.1%, B: 0.003% or less, Ca: 0.005% or less, and REM: 0.01% or less. A high-strength thick steel plate for structural use having excellent brittle crack propagation stop characteristics as in the first or second aspect of the patent application, wherein the second phase in the microstructure has a Borne iron, a toughened iron, a granulated iron, and an island. One or two or more types of granulated iron (MA) and unprocessed fertilized iron from the Worthite iron. A high-strength thick steel plate for structural use having excellent brittle crack propagation stop characteristics according to any one of claims 1 to 3, wherein the plate thickness exceeds 50 mm. A method for producing a high-strength thick steel plate for structural use having excellent brittle crack propagation stop characteristics, characterized in that a steel material (slab) having a composition of claim 1 or 2 is heated to a temperature of 1000 to 1200 ° C, The central portion of the thickness of the sheet is rolled at a rolling reduction rate of 30% or more in the recrystallization temperature region of the Worthite iron, and the first cooling is performed at a cooling rate of 15 ° C/s or less until the temperature at the center portion of the thickness becomes Ar _{3 .} Below the point, after the rolling in which the cumulative rolling reduction ratio is 40% or more in the temperature region where the temperature in the central portion of the thickness is not less than ₃ points, the second cooling is performed at a cooling rate of 4° C./s or more to 600° C. or lower. A method for producing a high-strength thick steel sheet for structural use having excellent brittle crack propagation stop characteristics according to the fifth aspect of the patent application, wherein the second cooling is further tempered to a temperature equal to or lower than Ac ₁ point.