TW201333219A - High strength steel plate having excellent brittle crack arrestability and method for manufacturing same - Google Patents

Abstract

Provided are a high strength steel plate having excellent brittle crack arrestability and a method for manufacturing the same. A steel plate, which contains C, Si, Mn, Al, P, S and N each in a definite amount, satisfies the requirement Ceq (=C+Mn/6+(Cu+Ni)/15+(V+Mo+Cr)/5) being 0.34-0.49% inclusive, and may further contain, if required, one or more members selected from among Nb, Ti, Cu, Ni, Cr, Mo, V, Ca, B and REM, wherein a region with an aggregate tissue, said region having an X-ray diffraction intensity ratio according to plane (211) in a plane parallel to the surface of the steel plate of 1.0 or greater, is disposed within at least 1/3 of the sheet thickness including the central part of the sheet thickness, the bainite fraction at the central part of the sheet thickness is 80% or greater, and the Charpy fracture transition temperature at a position of 1/4 of the sheet thickness is -40 DEG C or lower. The method according to the present invention comprises hot rolling a steel material in such a manner that the cumulative rolling reduction ratio is at least 50% within a temperature range at the central part of the sheet thickness from (Ar3+60) DEG C to Ar3 inclusive, and then cooling to 450 DEG C or lower at a cooling speed of 4.0 DEG C/s or higher.

Description

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

The present invention relates to a brittle crack propagation stop characteristic (brittle) which is preferably used as a thick steel plate having a thickness of more than 50 mm in a large structure such as a ship, a marine structure, a low temperature storage tank, or a building/civil structure. Crack arrestability) A high strength steel plate and a method for producing the same.

In large structures such as ships, marine structures, cryogenic storage tanks, and construction/civil structures, safety and environmental impact are always required due to the economic and environmental impacts of accidents involving brittle fractures. For the steel used, it is required to use the toughness or brittle crack propagation stop characteristic at the temperature.

Ships such as container ships or bulk carriers use high-strength thick-walled materials for the outer plate of ship's hull in their structure, but recently they have advanced further strength with the enlargement of the hull. In the case of thickening, in general, the brittle crack propagation stop characteristic of the steel sheet tends to be higher, and the thicker the wall material tends to deteriorate. Therefore, the requirements for the brittle crack propagation stop characteristics are further increased.

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

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

On the other hand, for relatively thin steels with a thickness of less than 50 mm used in ships or line pipes that do not reach the cryogenic temperature of LNG, the heat exchanger control process (TMCP) can be used. The "Thermo-Mechanical Control Process" method achieves fine granulation, improves low temperature toughness, and imparts excellent brittle crack propagation stop characteristics.

Further, Patent Document 1 proposes a steel material in which the microstructure of the surface layer portion is ultrafine-grained so as to improve the brittle crack propagation stop characteristics without increasing the alloy cost.

The steel material excellent in the brittle crack propagation stop characteristic described in Patent Document 1 is characterized in that the shear lip (plastic deformation region shear-lips) which is generated in the surface layer portion of the steel material when the brittle crack propagates is focused on the improvement of the brittle crack propagation. When the stopping characteristic is effective, the crystal grains of the shear lip portion are made fine, thereby absorbing the propagation energy of the propagated brittle crack.

As a manufacturing method, it is described that the surface layer portion is cooled to an Ar ₃ transformation point (Ar ₃ temperature) or less by controlled cooling after one or more times, and then the controlled cooling is stopped and the surface layer is stopped. Partially reheating to a step above the metamorphic point, during which the steel is subjected to rolling and shrinking, thereby causing it to be metamorphosed or processed to recrystallize, thereby generating an ultrafine ferrite structure in the surface layer. Or toughen iron structure (bainite structure).

Further, in Patent Document 2, in order to improve the brittle crack propagation stop characteristic in a steel material which is a microstructure mainly composed of ferrite iron-pearlite, the two surface portions of the steel material are provided. 50% or more of a layer of ferrite-grained iron structure having an average grain diameter equivalent to a circle of 5 μm or less and an aspect ratio of the grains of 2 or more. It is important to suppress the unevenness of the iron particle size of the ferrite grains, and as a method of suppressing unevenness, the average rolling reduction ratio of the average one pass in the finish rolling is set to 12% or less. Inhibit local recrystallization.

However, the steel materials excellent in the brittle crack propagation stop characteristics described in Patent Documents 1 and 2 are obtained by temporarily cooling the surface layer portion of the steel material and reheating it, and applying it to the reheating, thereby obtaining a specific organizer. It is difficult to control in the production scale, especially when the thick-walled material with a thickness of more than 50 mm becomes a process with a large load on the cooling equipment.

On the other hand, Patent Document 3 describes the following technique for extending the TMCP: focusing not only on the miniaturization of the ferrite grains, but also on the subgrains formed in the ferrite grains. , thereby improving the brittle crack propagation stop characteristics.

Specifically, when the thickness is 30 to 40 mm, complicated temperature control such as cooling and reheating of the surface layer of the steel sheet is not required, but the brittle crack propagation stop characteristic is improved by the following conditions, that is, (a) ensuring fine fertilizer The rolling conditions of the granular iron grains and (b) the rolling of the fine ferrite iron structure in the part of the steel plate thickness of 5% or more The pressure condition, (c) the development of the texture on the fine ferrite iron, and the dislocation of the dislocation introduced by the processing (rolling) by thermal energy and the formation of the secondary grain rolling conditions, (d) The cooling condition for suppressing the coarsening of the fine ferrite grains and fine crystal grains formed.

Further, in controlling the rolling, a method is also known in which the aggregate structure is developed by applying a shrinkage to the metamorphic ferrite, thereby improving the brittle crack propagation stop characteristic. A crack is formed 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 crack is moderated, thereby improving the impedance against brittle fracture.

For example, Patent Document 4 describes that the (110) plane X-ray intensity ratio (X-ray diffraction intensity according to (110) plane) is set to 2 or more by controlling the rolling pressure, and corresponds to the diameter of the circle ( The coarse grain size of 20 μm or more is set to 10% or less, thereby improving the brittle fracture resistance.

In Patent Document 5, a steel for welded structure excellent in brittle crack propagation resistance of a joint portion is disclosed as a steel sheet characterized by an X-ray plane strength ratio of a (100) plane on a rolling surface inside the sheet thickness. It is 1.5 or more, and it is described that the brittle crack propagation stop characteristic is excellent by the deviation of the angle of the stress load direction and the crack propagation direction by the development of the aggregate structure. Further, in Patent Documents 6 to 9, it is described that the collective structure is developed in each of the thickness direction (1/4 portion of the thickness, the center portion of the thickness, and the like) by controlling the average rolling reduction ratio in the rolling. Brittle crack propagation stop welding joint with excellent performance The method of manufacturing steel is used.

[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

Patent Document 5: Japanese Patent No. 2659661

Patent Document 6: Japanese Patent Laid-Open Publication No. 2008-214652

Patent Document 7: Japanese Patent Laid-Open Publication No. 2010-047805

Patent Document 8: Japanese Patent Laid-Open Publication No. 2009-221585

Patent Document 9: Japanese Patent Laid-Open Publication No. 2010-202931

Non-patent literature

Non-Patent Document 1: Inoue et al.: Long-term brittle crack propagation behavior in thick shipbuilding steel, Nippon Marine Engineering Society Speech Paper No. 3, 2006, pp359-362

Recently, however, thick steel plates with a thickness of more than 50 mm have been used in large container ships of more than 6,000 20 Twenty-foot Equivalent Units, and the growth of brittle cracks in the shipbuilding steel by Inoue et al. Behavior, Proceedings of the Japan Society of Marine and Ocean Engineering Lectures No. 3, 2006, In pp359-362, the brittle crack propagation resistance of the steel plate with a thickness of 65 mm was evaluated, and the results of the brittle cracking in the large brittle crack propagation stop test of the base material were reported.

In addition, in the ESSO test compliant with the guide line for brittle crack arrest design (2009, CLASS NK), the value of Kca at a temperature of -10 ° C is less than 3000 N/mm ^3/2. As a result, it is suggested that safety is ensured as a problem when a hull structure of a steel sheet having a thickness of more than 50 mm is applied.

The steel sheets having excellent brittle crack propagation stop characteristics described in the above Patent Documents 1 to 5 are mainly used for a thick wall material of more than 50 mm, depending on the production conditions or the experimental data disclosed, with a thickness of about 50 mm. In the case, it is not clear whether the specific characteristics can be obtained, and the characteristics of the hull structure for the crack propagation in the required thickness direction are not verified at all.

Further, in Patent Documents 6 to 9, in order to develop the aggregate structure at the center portion of the thickness of the sheet, it is necessary to set the rolling reduction ratio of one pass at a high level during the rolling, so that it is produced in terms of production conditions, steel sheet size, and the like. Various constraints to seek for improvement.

Accordingly, it is an object of the present invention to provide an industrially extremely simple process which is capable of optimizing the rolling conditions and controlling the assembly in the thickness direction even in a thick-walled steel sheet having a thickness of more than 50 mm. A high-strength thick steel plate having excellent brittle crack propagation resistance characteristics and a method for producing the same.

The inventors of the present invention have conducted intensive studies on the high-strength thick steel plate having excellent brittle crack propagation stop characteristics even for a thick-walled steel sheet having a thickness of more than 50 mm and the method for stably obtaining the steel sheet. The influence of the aggregate structure in the steel plate on the propagation characteristics of the brittle crack propagation was investigated in detail. As a result, the following findings were obtained: a region having a (211) plane X-ray intensity ratio of 1.0 or more on a plane parallel to the surface of the steel sheet It is present in a region of 1/3 or more of the total thickness of the plate thickness including the center portion of the plate thickness, whereby excellent brittle crack propagation stop characteristics can be obtained. Further, it is also known that in order to obtain such a thick steel plate, it is preferable to combine a chemical component of a specific range with a production condition of a specific range, in particular, a rolling/cooling condition at a central portion of the thickness.

The present invention is based on the insights obtained, that is, the present invention is:

A high-strength thick steel plate excellent in brittle crack propagation stop characteristics, characterized in that it has a surface parallel to the surface of the steel sheet in a region of 1/3 or more of the total thickness of the plate thickness including the center portion of the plate thickness The (211) plane X-ray intensity ratio is 1.0 or more, and the toughness iron fraction in the central portion of the sheet thickness is 80% or more, and the Charpy break transition temperature at the 1/4 position of the sheet thickness (fracture) The transition temperature) is below -40 °C.

2. A high-strength thick steel plate having excellent brittle crack propagation resistance characteristics, wherein the chemical composition of the steel is in mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.50%, and Mn: 0.50 to 2.20%. , P: 0.030% or less, S: 0.010% In the following, Al: 0.005 to 0.08%, N: 0.0045% or less, and the carbon equivalent (Ceq) expressed by the following formula (1) is 0.34% or more and 0.49% or less, and the remainder contains Fe and unavoidable impurities.

Ceq=C+Mn/6+(Cu+Ni)/15+(V+Mo+Cr)/5 (1)

Here, each element symbol indicates the content (% by mass) of each component.

3. A high-strength thick steel plate having excellent brittle crack propagation resistance characteristics, wherein the chemical composition of the steel is further selected from the group consisting of Ti: 0.005 to 0.030%, Nb: 0.005 to 0.050%, and Cu: 0.01. ~0.50%, Ni: 0.01 to 1.00%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, and REM: 0.010% or less One or two or more.

A method for producing a high-strength thick steel plate excellent in brittle crack propagation stop characteristics, characterized in that a steel material having a chemical composition such as 2 or 3 is heated to a temperature of 900 to 1200 ° C, and is subjected to the following rolling After that, it is cooled to 450 ° C or lower at a cooling rate of 4.0 ° C / s or more, and the rolling is accumulated in a temperature region at a central portion of the thickness of the hot rolling at a temperature range of (Ar ₃ point + 100) ° C or more. The shrinkage is 30% or more, and when the temperature in the central portion of the thickness is at (Ar ₃ + 60) ° C or lower and the temperature is within Ar ₃ or more, the cumulative reduction ratio is 50% or more, and the average rolling is 1 pass. The average shrinkage ratio is 6.0% or more, and the rolling reduction ratio of each pass is 5.0 to 20.0%.

The thick steel plate obtained by the present invention has a plate thickness of 50 mm or more, Since the aggregate structure is appropriately controlled according to each position in the thickness direction, the brittle crack propagation stop characteristic is excellent. When the present invention is applied to a steel sheet having a thickness of 50 mm or more, preferably a thickness of more than 50 mm, more preferably a thickness of 55 mm or more, and more preferably a thickness of 60 mm or more, it is more effective than the steel of the prior art. Significant superiority, so effective. Among them, as a structural member for a ship, for example, by applying the present invention to a deck member joined to a hatch side coaming in a strong deck structure of a container ship or a bulk cargo ship, it is helpful to improve The safety of the ship is extremely useful in industry.

In the present invention, the assembly structure inside the steel sheet, the microstructure of the center portion of the sheet thickness, and the toughness of the base material are defined.

1. The organization of the inside of the steel plate

In the present invention, in order to improve the crack propagation stop characteristic for the crack propagated in the direction parallel to the plate surface in the direction of the rolling direction or the direction perpendicular to the rolling direction, the surface parallel to the surface of the steel sheet, that is, the rolling surface is formed. The (211) surface is developed in parallel. When the (211) surface is developed on the surface parallel to the surface of the steel sheet at the center portion of the sheet thickness, a micro crack is generated before the crack progresses, and the crack progresses.

Since the microscopic crack is generated before the crack progresses, the (211) plane X-ray intensity on the surface parallel to the surface of the steel sheet is in the region of more than 1/3 of the total thickness of the plate thickness including the center portion of the plate thickness. A collection organization with a ratio of 1.0 or more. The above-mentioned micro cracks occur before the crack progresses, and the crack progresses. The effect of the impedance is obtained as long as the region having the aggregated structure is one-third or more of the total thickness of the plate thickness including the center portion of the plate thickness. Therefore, the upper limit is not particularly specified. If the area having the aggregate structure is increased, the above-described effects are further exerted. However, even if the area is increased to more than 3/4 of the total thickness of the plate thickness, the increase in the effect is saturated, so that it is not necessary to have the assembly. The area is up to 3/4 of the total thickness of the board. However, of course, even if the total thickness of the plate is the aggregate structure, the above effects are exerted.

Here, the (211) plane X-ray intensity ratio refers to a value indicating the (X-ray diffraction intensity ratio of texture) of the target material, and refers to the (211) reflection of the target material. the X-ray diffraction intensity _{(I (211)), (} 211) X -ray diffraction intensity of the reflection _{(I 0 (211))} and the ratio of the non-random texture of the standard sample _{(I (211)} / I _{0 (211)} ).

2. Microstructure in the central part of the plate thickness

In order to obtain a preferred aggregate structure in the central portion of the thickness of the sheet, the tough iron fraction in the central portion of the thickness of the cross section parallel to the rolling direction is set to be at least 80%. The toughened iron fraction is set to be expressed by area fraction.

The (211) plane on the surface parallel to the surface of the steel sheet is developed by the austenite structure which is processed at the time of rolling and is transformed into a ferrite iron or a toughened iron structure. In the case of the fermented iron-cementite carbonite (cementite) structure, the aggregated structure is not developed in a wide range in the thickness direction due to the influence of recovery or the like. The highest (211) plane X-ray intensity ratio can be maintained over a wide range by metamorphizing the transformed tissue into a toughened iron structure. In the present invention The microstructure of the central portion of the plate thickness means the microstructure of the region including at least 1/3 of the thickness of the center portion of the plate thickness. The present invention includes a steel sheet having a whole cross section in the thickness direction of the microstructure.

3. Base metal toughness

In the steel sheet of the present invention, it is prescribed that the ratio of the thickness of the base material is 1/4 of the thickness of the steel sheet. The Charpy break transition temperature in the Charpy impact test of the test piece.

In order to obtain a brittle crack propagation stop characteristic of Kca (-10 ° C) ≧ 7000 N/mm ^3/2 which is regarded as a target in terms of structural safety, a thick-walled material having a thickness of 50 mm or more is used. The Charpy break transition temperature in the Charpy impact test of the test piece extracted at a quarter of the thickness was specified to be -40 ° C or less.

The chemical composition and manufacturing conditions of steel which is preferable for the steel sheet having the above-mentioned aggregate structure and base material toughness are as follows. Hereinafter, % of the description of the chemical components is made by mass%.

C: 0.03~0.20%

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

Si: 0.03~0.50%

Si is effective as a deoxidizing element, and is effective as a strengthening element of steel, but This effect is not obtained when the content is less than 0.03%. On the other hand, when it exceeds 0.50%, not only the surface property of steel but also the toughness will be extremely deteriorated. Therefore, the content thereof is preferably set to 0.03% or more and 0.50% or less. More preferably, it is 0.05 to 0.45%.

Mn: 0.50~2.20%

Mn can be contained as a strengthening element. When the amount is less than 0.50%, the effect is insufficient. When the amount exceeds 2.20%, the toughness or weldability of the base material is deteriorated, and the cost of the steel material also increases. Therefore, it is preferably 0.50% or more and 2.20% or less. More preferably, it is 0.60 to 2.15%.

P, S

P, S, and the inevitable impurities in the S-based steel. However, if P exceeds 0.030% and S exceeds 0.010%, the toughness deteriorates. Therefore, it is preferably 0.030% or less and 0.010% or less, and more preferably 0.020% or less and 0.005, respectively. %the following.

Al: 0.005~0.08%

Al acts as an oxygen scavenger, and is preferably contained in an amount of 0.005% or more. However, if it is more than 0.08%, the toughness is lowered, and the toughness of the welded metal portion is lowered in the case of welding. Therefore, Al is preferably in the range of 0.005 to 0.08%. Furthermore, it is more preferably 0.02 to 0.04%.

N: 0.0045% or less

The N system is combined with Al in the steel to adjust the crystal grain size during the rolling process and to strengthen the steel. However, if it exceeds 0.0045%, the toughness deteriorates, so it is preferably 0.0045% or less. More preferably, it is 0.0040% or less.

Carbon equivalent (Ceq): 0.34% or more and 0.49% or less

Carbon equivalent is an important indicator for predicting the strength and abnormal behavior of tissues. If the carbon equivalent is less than 0.34%, it is difficult to obtain the above-mentioned tough iron fraction in the center portion of the sheet thickness. Moreover, when it exceeds 0.49%, since toughness deteriorates, it is preferable to set it as 0.34 % or more and 0.49% or less. More preferably, it is 0.35 to 0.48%.

Further, the carbon equivalent (Ceq) is obtained by the formula shown below.

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

Each element symbol is a content (% by mass), and is set to 0 when it is not contained.

The above is a preferred basic component composition of the present invention and the remaining part of Fe and unavoidable impurities. As an unavoidable impurity, for example, O is allowed to be 0.0050% or less.

In order to further improve the characteristics, one or two or more selected from the group consisting of Ti, Nb, Cu, Ni, Cr, Mo, V, B, Ca, and REM may be contained.

Ti: 0.005~0.030%

Ti forms a nitride, a carbide, or a carbonitride by containing a trace amount, and has an effect of refining crystal grains and improving the toughness of the base material. This effect is obtained by containing 0.005% or more. However, if the content exceeds 0.030%, the toughness of the base material and the heat-affected zone of the weld is lowered. Therefore, when Ti is contained, it is preferably 0.005 to 0.030%. range. More preferably, it is 0.008 to 0.028%.

Nb: 0.005~0.050%

Nb is precipitated in the form of NbC when the ferrite is metamorphosed or reheated. Helps to increase strength. Further, in the rolling of the Worthite iron region, the effect of expanding the non-recrystallized region is promoted, and the fine graining of the ferrite iron is facilitated, so that the improvement of the toughness is also effective. This effect is obtained by containing 0.005% or more. However, when the content exceeds 0.050%, coarse NbC is precipitated, which in turn causes a decrease in toughness. Therefore, when Nb is contained, the upper limit is preferably set to 0.050%. More preferably, it is 0.008 to 0.040%.

Cu, Ni, Cr, Mo

Any of Cu, Ni, Cr, and Mo is an element that improves the hardenability of steel. It directly contributes to the improvement of the strength after rolling, and can be contained in order to improve functions such as toughness, high-temperature strength, and weather resistance. However, excessive inclusion may deteriorate toughness or weldability, so it contains Cu, Ni, Cr, and Mo. In the case, it is preferable to set the upper limit of Cu to 0.50%, the upper limit of Ni to 1.00%, the upper limit of Cr to 0.50%, and the upper limit of Mo to 0.50%. More preferably, the upper limit of Cu is set to 0.45%, the upper limit of Ni is set to 0.95%, the upper limit of Cr is set to 0.45%, and the upper limit of Mo is set to 0.45%. On the other hand, if the content of each element is less than 0.01%, the effect does not appear. Therefore, when it is contained, it is preferable to contain 0.01% or more with respect to each element.

V: 0.001~0.10%

V is an element which increases the strength of steel by precipitation strengthening in the form of V (CN), and may contain 0.001% or more in order to exhibit this effect. However, if it contains more than 0.10%, the toughness will fall. Therefore, in the case where V is contained, it is preferably contained in the range of 0.001 to 0.10%. More preferably 0.008~ 0.095%.

B: 0.0030% or less

B is an element which slightly increases the hardenability of steel, and its effect is exhibited by containing 0.0006% or more. However, when the content exceeds 0.0030%, the toughness of the welded portion is lowered. Therefore, when B is contained, it is preferably 0.0030% or less. More preferably, it is 0.0028% or less.

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

Ca and REM make the structure of the welded heat-affected zone finer and improve the toughness, and even if it is contained, the effect of the present invention is not impaired, and may be contained as needed. However, when it is contained in a large amount, 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 amount is preferably made 0.005% or 0.01%.

Hereinafter, preferred manufacturing conditions in the present invention will be described.

The production conditions are preferably slab heating conditions, hot rolling conditions, and cooling conditions after hot rolling.

[Steel billet heating]

It is preferable to melt the molten steel of the above composition in a converter or the like, and to form a steel material (steel billet) by continuous casting or the like, and to perform hot rolling after heating to 900 to 1200 °C.

When the heating temperature is less than 900 ° C, the rolling time in the recrystallization temperature region of the Worthite iron cannot be sufficiently ensured, and if it exceeds 1200 ° C, the Worthite iron particles are coarsened, which not only causes a decrease in toughness. Further, the oxidation loss is remarkable, so that the yield is lowered, so the heating temperature is set to 900 to 1200 °C. on The preferred heating temperature range from 1000 to 1150 ° C, more preferably from 1000 to 1050 ° C.

[hot rolling]

It is preferable that the temperature at the central portion of the thickness of the sheet during hot rolling (the temperature at a position equal to 1/2 of the sheet thickness, the same applies hereinafter) is the cumulative rolling reduction ratio (Ar ₃ point + 100) ° C or more, (Ar ₃ points + 60) °C or less and the cumulative rolling reduction ratio of Ar ₃ or more, (Ar ₃ + 60) ° C or less, and the average of 1 rolling pass of Ar ₃ points or more, and (Ar ₃ The range of the rolling reduction ratio of the average one pass of the point +60) ° C or less and the Ar ₃ point or more.

In the hot rolling system, when the temperature in the central portion of the thickness is (Ar _{3 +} 100) ° C or more, the rolling reduction is 30% or more, and the Worthite iron is finely granulated, thereby achieving the final result. Fine granulation of the microstructure to improve the toughness of the base metal. The cumulative rolling reduction ratio in this temperature region is further preferably 35% or more. In the present invention, the Ar ₃ point (°C) is obtained by the following formula.

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

In the formula, the content of each element symbol steel (% by mass) is set to 0 when it is not contained.

Next, when the temperature in the central portion of the plate thickness is within a temperature range of (Ar ₃ + 60) ° C or less and Ar ₃ or more, the cumulative rolling reduction ratio is 50% or more and the average value of the rolling reduction is 1 pass. It is a rolling pressure of 6.0% or more. If the cumulative rolling reduction ratio in this temperature region is less than 50%, the toughness of the steel sheet is deteriorated. In addition, since the (211) plane X-ray intensity ratio is 1.0 or more, cumulative rolling is performed in a temperature range of (Ar ₃ point + 60) ° C or less and Ar ₃ point or more in the non-recrystallized Worstian iron region. The shrinkage ratio is set to 50% or more. The cumulative rolling reduction ratio in this temperature region is further preferably 55% or more.

In the finish rolling of the thick-walled material, the rolling is usually performed by a small rolling and multi-pass rolling. Therefore, there is a tendency that the area of the (211) plane X-ray intensity ratio which is parallel to the surface of the steel sheet is 1.0 or more. Therefore, in the present invention, the average value of the rolling rate of the average one pass in the temperature range of (Ar ₃ point + 60) ° C or less and Ar ₃ point or more is set to 6.0% or more. And the range of rolling reduction of each pass is specified as 5.0~20.0%. Thereby, the region in which the (211) plane X-ray intensity ratio is 1.0 or more can be set to a region of 1/3 or more of the total thickness of the plate thickness including the center of the plate thickness. When the average value of the average rolling reduction is less than 6.0%, or when the minimum value of each pass is less than 5.0%, the toughness will decrease and the (211) surface will not be able to be used. A region in which the X-ray intensity ratio is 1.0 or more is set to a region of 1/3 or more of the total thickness of the plate including the center of the plate thickness. On the other hand, if the maximum value of the rolling reduction ratio of each pass exceeds 20.0%, the toughness is deteriorated due to the influence of the processing strain. The average value of the average rolling reduction ratio in the temperature region is further preferably 6.5% or more, and the rolling reduction ratio of each pass is further preferably 5.5 to 18.0%. Further, in the hot rolling, the rolling pressure outside the predetermined temperature range can also be performed. The rolling pressure including the above-described predetermined rolling reduction ratio may be performed in the temperature range specified above.

[Cooling after hot rolling]

After the rolling is completed, the steel plate is cooled to a cooling rate of 4.0 ° C / s or more to Below 450 °C. If the cooling rate is less than 4.0 ° C / s, the transformation to the toughened iron will not be sufficiently performed. Therefore, the region where the (211) plane X-ray intensity ratio is 1.0 or more cannot be set to the thickness including the center of the plate thickness. More than 1/3 of the total thickness, and thus the desired microstructure, that is, the structure in which the toughness iron fraction in the central portion of the plate thickness is 80% or more is not obtained. Further, when the cooling stop temperature exceeds 450 ° C, the transformation to the toughened iron is not sufficiently performed, and thus the desired microstructure is still not obtained. As the cooling method, water cooling, gas cooling, or the like can be used.

According to the above manufacturing conditions, not only the desired aggregate structure but also the fracture facet size in the Charpy impact test can be made fine, so that the Charpy break transition temperature at the 1/4 position of the sheet thickness reaches - Below 40 °C.

In the above description, the temperature at the center portion of the thickness is determined by heat transfer calculation based on the surface temperature of the plate measured by a radiation thermometer. The temperature condition during cooling after hot rolling is also set to the temperature at the center of the sheet thickness.

[Examples]

The molten steel (steel symbol A~T) of each composition shown in Table 1 was melted in a converter, and steel material (slab 280 mm thick) was formed by continuous casting, after hot rolling to a thickness of 50 to 75 mm Cooling was carried out to obtain test steel No. 1 to 28. Table 2 shows hot rolling conditions and cooling conditions. Ar ₃ point (°C) is calculated by the following formula.

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

Here, the content (% by mass) of each element symbol steel is set to 0 when it is not contained.

For the obtained thick steel plate, the direction orthogonal to the rolling direction is taken as the length direction from the 1/4 portion of the plate thickness. A test piece of JIS 14A No. 14 was subjected to a tensile test, and the tensile strength (YS) and tensile strength (TS) were measured.

The JIS No. 4 impact test piece was taken from the quarter of the plate thickness in the direction of the long axis of the test piece in parallel with the rolling direction, and the Charpy impact test was performed to determine the fracture transition temperature (vTrs). ). It is within the scope of the invention to set the Charpy to break transition temperature in the 1/4 portion of the sheet thickness to -40 °C or less.

Regarding the tough iron fraction in the central portion of the plate thickness, the plate thickness of the plate thickness section parallel to the rolling length direction of the central portion of the plate thickness is mirror-polished, and then the metal structure emerging by etching is taken. An optical microscope photograph was taken and determined by imaging analysis.

Further, in order to evaluate the aggregate structure of the steel sheet, the (211) plane X-ray intensity ratio on the surface parallel to the surface of the steel sheet was measured from the surface of the steel sheet to the back surface in units of 1 mm, thereby obtaining a (211) plane X-ray intensity ratio. It is an area of 1.0 or more.

Then, in order to evaluate the brittle crack propagation stop characteristic, a temperature gradient type ESSO test was performed to obtain Kca (-10 ° C) (N/mm ^3/2 ).

The results of these tests are shown in Table 3.

Further, in any of Nos. 1 to 28, the (211) plane X-ray intensity on the surface parallel to the surface of the steel sheet is 1.0 or more in the central portion of the sheet thickness.

The transition temperature of the Charpy impact test in the 1/4 portion of the sheet thickness, the toughening iron fraction at the center of the sheet thickness, and the (211) plane X-ray intensity ratio on the surface parallel to the surface of the steel sheet are 1.0 or more. In the case of the test steel sheets (manufactured No. 1 to 13) in the range of the present invention, Kca (-10 ° C) is an excellent brittle crack propagation stop performance of 7000 N/mm ^3/2 or more.

Claims (4)

A high-strength thick steel plate characterized in that the (211) plane X-ray intensity ratio on a surface parallel to the surface of the steel sheet is in a region of more than 1/3 of the total thickness of the plate thickness including the center portion of the plate thickness In the aggregate structure of 1.0 or more, the toughening iron fraction in the central portion of the sheet thickness is 80% or more, and the Charpy break transition temperature at the quarter of the sheet thickness is -40 ° C or lower. For example, the high-strength thick steel plate of the first application of the patent scope, wherein the chemical composition of the steel is in mass%, C: 0.03 to 0.20%, Si: 0.03 to 0.50%, Mn: 0.50 to 2.20%, P: 0.030% Hereinafter, S: 0.010% or less, Al: 0.005 to 0.08%, and N: 0.0045% or less, and the carbon equivalent (Ceq) expressed by the following formula (1) is 0.34% or more and 0.49% or less, and the remainder includes Fe and not. Impurities to be avoided; Ceq=C+Mn/6+(Cu+Ni)/15+(V+Mo+Cr)/5 (1) wherein each element symbol indicates the content (% by mass) of each component. For example, the high-strength thick steel plate of the second application of the patent scope, wherein the chemical composition of the steel is further selected from the group consisting of Ti: 0.005 to 0.030%, Nb: 0.005 to 0.050%, Cu: 0.01 to 0.50%, Ni. : 0.01 to 1.00%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, and REM: 0.010% or less. More than one species. A method for producing a high-strength thick steel plate, characterized in that a steel material having a chemical composition as in the second or third aspect of the patent application is heated to a temperature of 900 to 1200 ° C, and after being rolled as follows, at 4.0 ° C The cooling rate of /s or more is cooled to 450 ° C or less; and the rolling reduction is 30% when the temperature in the central portion of the thickness of the hot rolling is in a temperature range of (Ar _{3 +} 100) ° C or more. In the above, when the temperature in the central portion of the thickness is at (Ar ₃ + 60) ° C and below, and the temperature is within the temperature range of Ar ₃ or more, the cumulative reduction ratio is 50% or more, and the average of the average rolling reduction is 1 pass. It is 6.0% or more, and the rolling reduction ratio of each pass is 5.0 to 20.0%.