JP2013136828A - Method for manufacturing high-strength thick steel plate for construction excellent in brittleness-cracking propagation-stopping characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength thick steel plate having ≥50 mm plate thickness, excellent in brittleness-cracking propagation-stopping characteristic and method for manufacturing the same.SOLUTION: The thick steel plate has a texture, in which {100}<011> directional strength on the rolled surface in the center part of the plate thickness is ≥2.0 and {100}<011> directional strength on the rolled surface in the 1/4 part of the plate thickness is ≥1.2, and has -40°C or lower of Charpy fracture transition temperature in the 1/4 part of the plate thickness, and has the steel composition composed by mass of 0.03-0.20% C, 0.03-0.5% Si, 0.5-2.2% Mn, 0.005-0.08% Al, ≤0.03% P, ≤0.01% S, ≤0.0075% N and one or more elements among Ti, Nb, Cu, Ni, Cr, Mo, V, B, Ca, REM. The steel having the composition, is heated to 900-1,200°C temperature and is rolled into ≥30% cumulative rolling-reduction ratio in the temperature-zone becoming recrystal γ-zone at the center part of the plate-thickness and into ≥50% cumulative rolling-reduction ratio in the temperature zone becoming α+γ zone at the center part of the plate thickness by using lubricant and thereafter, is cooled to 600°C or lower at ≥2°C/s cooling rate, or after the accelerated-cooling, tempering is performed at Acpoint temperature or lower.

Description

  TECHNICAL FIELD The present invention relates to a high-strength steel plate excellent in brittle crack propagation stopping characteristics and a method for producing the same, and more particularly to a material suitable for large structures such as ships, marine structures, low-temperature storage tanks, and construction / civil engineering structures. .

  For large structures such as ships, offshore structures, low-temperature storage tanks, and construction / civil engineering structures, accidents associated with brittle fractures have a significant impact on the economy and the environment. Steel materials are required to have toughness at operating temperatures and brittle crack propagation stopping characteristics.

  Ships such as container ships and bulk carriers use high-strength thick materials for the hull outer plates due to their structure. Recently, as the size of the hulls has increased, higher strength and thicker materials have been developed. Since the brittle crack propagation stopping property of steel tends to deteriorate with higher strength or thicker material, the demand for brittle crack propagation stopping property is further advanced.

  As a means of improving the brittle crack propagation stopping characteristics of steel materials, a method of increasing the Ni content has been conventionally known. In a LNG storage tank, 9% Ni steel is used on a commercial scale. Has been.

  However, since the increase in the amount of Ni necessitates a significant increase in cost, it is difficult to apply to applications other than the LNG storage tank.

  On the other hand, for thin steel materials with a plate thickness of less than 50 mm used for ships and line pipes that do not reach extremely low temperatures such as LNG, fine graining is attempted by the TMCP method to achieve low temperature toughness. It can be improved to give excellent brittle crack propagation stopping properties.

  Further, Patent Document 1 proposes a steel material in which the structure of the surface layer portion is made ultrafine in order to improve brittle crack propagation stopping characteristics without increasing the alloy cost.

  In steel materials having excellent brittle crack propagation stopping characteristics described in Patent Document 1, when brittle cracks propagate, shear lip (plastic deformation region) generated in the steel surface layer is effective in improving brittle crack propagation stopping characteristics. In particular, it is characterized in that the propagation energy of the brittle crack propagating is absorbed by refining the crystal grains of the shear lip portion.

As a production method, the process of cooling the surface layer part to the Ar ₃ transformation point or less by controlled cooling after hot rolling, and then repeating the process of stopping the control cooling and returning the surface layer part to the transformation point or more is repeated once or more, During this time, it is described that by rolling down the steel material, it is repeatedly transformed or processed and recrystallized to form an ultrafine ferrite structure or bainite structure in the surface layer portion.

  Furthermore, in Patent Document 2, in order to improve the brittle crack propagation stopping characteristics in a steel material mainly composed of ferrite-pearlite, both surface portions of the steel material have a circle-equivalent particle diameter of 5 μm or less and an aspect ratio of 2 It is important that the ferrite structure with the above ferrite grains is composed of a layer having 50% or more and to suppress the variation in the ferrite particle size. As a method to suppress the variation, the maximum rolling reduction per pass during finish rolling is 12% or less. And suppressing local recrystallization phenomenon.

  However, the steel materials excellent in brittle crack propagation stopping characteristics described in Patent Documents 1 and 2 are obtained by recooling only the steel material surface layer portion and then reworking and processing during recuperation to obtain a specific structure. However, control is not easy on the actual production scale, and in particular, a thick material with a plate thickness exceeding 50 mm is a process with a heavy load on the rolling and cooling equipment.

  On the other hand, Patent Document 3 describes a technology on the extension of TMCP that focuses on subgrains formed in ferrite crystal grains as well as refinement of ferrite crystal grains and improves brittle crack propagation stopping characteristics. Has been.

  Specifically, in a plate thickness of 30 to 40 mm, without requiring complicated temperature control such as cooling and recuperation of the steel sheet surface layer, (a) rolling conditions for securing fine ferrite crystal grains, (b) steel plate Rolling conditions for generating a fine ferrite structure in a portion of 5% or more of the thickness, (c) A texture is developed in the fine ferrite, and dislocations introduced by processing (rolling) are rearranged by thermal energy to form subgrains. The brittle crack propagation stopping characteristics are improved by rolling conditions and (d) cooling conditions that suppress the coarsening of the formed fine ferrite crystal grains and fine subgrain grains.

  In addition, in controlled rolling, a method of improving the brittle crack propagation stopping property by developing a texture by reducing the transformed ferrite is also known. Separation occurs on the fracture surface of the steel material in a direction parallel to the plate surface, and the stress at the brittle crack tip is relaxed, thereby increasing the resistance to brittle fracture.

  For example, in Patent Document 4, the resistance to brittle fracture is improved by controlling the (110) plane X-ray intensity ratio to 2 or more and controlling coarse grains having an equivalent circle diameter of 20 μm or more to 10% or less by controlled rolling. Is described.

  Patent Document 5 is characterized in that as a welded structural steel excellent in brittle crack propagation stopping performance of a joint part, the X-ray plane strength ratio of the (100) plane in the rolled surface inside the plate thickness is 1.5 or more. A steel sheet is disclosed, and it is described that it has excellent brittle crack propagation stopping characteristics due to a shift in angle in the stress load direction and crack propagation direction due to the texture development.

  By the way, in recent large container ships exceeding 6,000 TEU, thick steel plates having a thickness of more than 50 mm are used. However, Non-Patent Document 1 evaluates the brittle crack propagation stopping characteristics of a steel plate having a thickness of 65 mm, and a base material. In the large brittle crack propagation stop test, the result that the brittle crack does not stop is reported.

In addition, in the ESSO test of the test material, the Kca value at a use temperature of −10 ° C. showed a result of less than 3000 N / mm ^3/2 . It has been suggested that securing is an issue.

  The steel sheets having excellent brittle crack propagation stopping characteristics described in Patent Documents 1 to 5 described above are mainly targeted for a thickness of about 50 mm from manufacturing conditions and disclosed experimental data, and applied to thick materials exceeding 50 mm. In this case, it is unclear whether the predetermined characteristics can be obtained, and the characteristics with respect to crack propagation in the plate thickness direction necessary for the hull structure have not been verified at all.

  Accordingly, the present invention provides a high-strength thick steel plate having excellent brittle crack propagation stopping characteristics that can be stably produced by an industrially simple process that optimizes rolling conditions and controls the texture in the thickness direction, and the production thereof. It aims to provide a method.

The inventors of the present invention have earnestly studied to achieve the above-mentioned problems, and obtained the following knowledge about a high-strength thick steel plate having excellent crack propagation stopping characteristics even with a thick steel plate and a production method for stably obtaining the steel plate. It was.
1. The {100} <011> orientation effective for improving the brittle crack propagation stop property changes to the {110} <001> orientation under the influence of shear strain generated between the roll and the rolled material during rolling.
2. The shear strain occurs near the surface layer when the plate thickness is small, but shows the highest value near the plate thickness ¼ when the plate thickness is large.
3. As a result, when the thick-walled material is subjected to the two-phase rolling, the thickness of the {1/4} portion {100} <011> orientation is lowered, and the {100} <011> orientation develops only in the vicinity of the central portion of the thickness. The brittle crack propagation stopping performance is limited.
4). The shear strain generated between the roll and the steel plate can be reduced by using a lubricant during rolling, and as a result, the thickness of the ¼ part {100} <011> azimuth strength is increased even in a thick material. In addition, by improving the base metal toughness, excellent brittle crack propagation stopping performance is achieved.

The present invention has been made by further studying the obtained knowledge, that is, the present invention,
1. Steel composition is mass%, C: 0.03-0.20%, Si: 0.03-0.50%, Mn: 0.5-2.2%, A1: 0.005-0.08 %, P: 0.03% or less, S: 0.010% or less, N: 0.0075% or less, Ti: 0.005 to 0.03%, Nb: 0.005 to 0.05% In addition, after heating the steel material consisting of Fe and unavoidable impurities to a temperature of 900 to 1200 ° C., the cumulative reduction ratio is 30% or more in the temperature range in which the center portion of the plate thickness becomes the recrystallization γ region, Brittle crack propagation stop characterized by rolling at a cumulative reduction ratio of 50% or more in a temperature range of α + γ using a lubricant and then cooling to 600 ° C. or less at a cooling rate of 2 ° C./s or more. A manufacturing method for structural high strength thick steel plates with excellent properties.
2. The steel composition is further mass%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0 0.5%, V: 0.001 to 0.10%, B: 0.003% or less, Ca: 0.005% or less, REM: 0.01% or less, or two or more 2. A method for producing a structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to 1.
3. The method for producing a high strength thick steel sheet for structural use having excellent brittle crack propagation stopping characteristics according to 1 or 2, wherein tempering is performed at an Ac1 point or less after the accelerated cooling.

  According to the present invention, a high-strength thick steel plate having a texture controlled appropriately in the plate thickness direction and excellent in brittle crack propagation stopping characteristics is obtained, and a steel plate having a plate thickness of 50 mm or more, more preferably 55 mm or more. It is effective to apply to. For example, in the shipbuilding field, it is extremely useful in the industry, for example, by contributing to improved safety of ships by applying it to hatch side combing and deck members in the structure of large decks of bulk container ships and bulk carriers.

In the present invention, 1. Texture inside the plate thickness (plate thickness central part and plate thickness ¼ part); 2. toughness, and Define manufacturing conditions. The central portion of the plate thickness (sometimes referred to as 1/2 plate thickness) is the 1/2 position of the plate thickness, and the 1/4 thickness portion is the 1/4 position of the plate thickness or 3/4 position of the plate thickness. .
1. BACKGROUND OF THE INVENTION The present invention mainly aims at obtaining a texture with a developed {100} <011> orientation effective in improving crack propagation stopping characteristics in a wide region in the thickness direction. The {100} <011> orientation strength at the rolling surface in the part and the {100} <011> orientation strength at the rolling surface at the ¼ part thickness are appropriately defined according to the desired brittle crack propagation stop characteristics. To do. When the {100} <011> orientation is developed, the effect of reducing the stress intensity factor at the crack tip due to crack bending, that is, the crack escaping from the direction of stress application, and the occurrence of a fine separation, The effect of stress relaxation improves brittle crack propagation stopping performance.

Kca (-10 ° C) ≧ 6000N / mm, which is a target for securing structural safety, with thick material of 50mm or more thickness that has been used for hull outer plates such as recent container ships and bulk carriers. ^When a brittle crack propagation stopping performance of ^3/2 is obtained, the {100} <011> orientation strength at the rolled surface in the center portion of the plate thickness is 2.0 or more, and the rolled surface at the 1/4 portion of the plate thickness is { 100} <011> The azimuth strength is 1.2 or more.

Here, the azimuth intensity is obtained by using an X-ray diffractometer (manufactured by Rigaku Corporation) and obtaining (200), (110) and (211) positive electrode dot diagrams using a Mo ray source. The three-dimensional crystal orientation density function was calculated from the figure.
2. Base material toughness Since the base material toughness is a premise for suppressing the growth of cracks since it has good characteristics, the steel sheet according to the present invention also has a desired brittleness at the Charpy fracture surface transition temperature at 1/4 thickness. It is specified appropriately according to the crack propagation stop characteristics.

When a brittle crack propagation stopping performance of Kca (−10 ° C.) ≧ 6000 N / mm ^3/2 which is a target for ensuring structural safety is obtained with a thick material having a thickness of 50 mm or more, the thickness 1 / The Charpy fracture surface transition temperature at 4 parts is defined as -40 ° C or lower.

The texture having the above-described features can be obtained when the chemical components and production conditions of steel are appropriately selected.
3. In the description of the chemical composition of steel,% is mass%.

C: 0.03-0.20%
C is an element that improves the strength of steel. In the present invention, it is necessary to contain 0.03% or more in order to ensure a desired strength, but if it exceeds 0.20%, the weldability deteriorates. As well as adversely affecting toughness. For this reason, C was specified in the range of 0.03 to 0.20%. In addition, Preferably it is 0.05 to 0.15%.

Si: 0.03-0.50%
Si is effective as a deoxidizing element and as a strengthening element for steel, but if its content is less than 0.03%, it has no effect. On the other hand, if it exceeds 0.50%, not only the surface properties of the steel are impaired, but also the toughness is extremely deteriorated. Therefore, the addition amount is set to 0.03% or more and 0.50% or less.

Mn: 0.5 to 2.2%
Mn is added as a strengthening element. If it is less than 0.5%, the effect is not sufficient, and if it exceeds 2.2%, the weldability deteriorates and the steel material cost also rises.

Al: 0.005 to 0.08%
Al acts as a deoxidizer, and for this purpose, it needs to contain 0.005% or more. However, if it contains more than 0.08%, it reduces the toughness and, when welded, weld metal Reduce the toughness of the part. For this reason, Al was specified in the range of 0.005 to 0.08%. In addition, Preferably, it is 0.02 to 0.04%.

P: 0.03% or less, S: 0.010% or less P and S are inevitable impurities in the steel, but P exceeds 0.03%, and if S exceeds 0.010%, the toughness deteriorates. Therefore, the content is set to 0.03% or less and 0.010% or less, respectively. Desirably, they are 0.02% or less and 0.005% or less, respectively.

N: 0.0075% or less N combines with Al in the steel, adjusts the crystal grain size at the time of rolling, and strengthens the steel. However, if it exceeds 0.0075%, the toughness deteriorates. 0075% or less.

Ti: 0.005 to 0.03%,
Ti has the effect of forming nitrides, carbides, or carbonitrides by adding a small amount, and refining crystal grains to improve the base material toughness. The effect is obtained by addition of 0.005% or more. However, if the content exceeds 0.03%, the toughness of the base metal and the weld heat-affected zone is lowered. % Is preferable.

Nb: 0.005 to 0.05%
Nb precipitates as NbC during ferrite transformation or reheating, and contributes to increasing the strength. In addition, it has the effect of expanding the non-recrystallized region in rolling in the austenite region, and contributes to the refinement of ferrite, so it is also effective in improving toughness. In order to obtain the effect, addition of 0.005% or more is necessary, but when adding over 0.05%, coarse NbC precipitates, and conversely, the toughness is reduced. The upper limit is preferably 0.05%.

  The above is the basic component composition of the present invention, and the balance is Fe and inevitable impurities. However, in order to further improve the characteristics, one or more of Cu, Ni, Cr, Mo, V, B, Ca, and REM are contained. Is possible.

Cu, Ni, Cr, Mo
Cu, Ni, Cr, and Mo are all elements that enhance the hardenability of steel. While contributing directly to strength improvement after rolling, it can be added to improve functions such as toughness, high-temperature strength, or weather resistance, but excessive addition degrades toughness and weldability, so when adding The upper limit is preferably 0.5%, 1.0%, 0.5%, and 0.5%, respectively. On the other hand, if the addition amount is less than 0.01%, the effect does not appear. Therefore, when it is added, the addition amount is preferably 0.01% or more.

V: 0.001 to 0.10%
V is an element that improves the strength of the steel by precipitation strengthening as V (CN), and may be contained in an amount of 0.001% or more, but if it exceeds 0.10%, the toughness is lowered. For this reason, when adding, it is preferable to set it as 0.001-0.10%.

B: 0.003% or less B may be added as an element that enhances the hardenability of steel in a small amount. However, if the content exceeds 0.003%, the toughness of the welded portion is lowered, so when added, the content is preferably 0.003% or less.

Ca: 0.005% or less, REM: 0.01% or less Ca and REM are necessary because the structure of the weld heat-affected zone is refined to improve toughness, and even if added, the effects of the present invention are not impaired. It may be added accordingly. However, if added excessively, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, when added, the upper limit is preferably made 0.005% and 0.01%, respectively.
4). Manufacturing conditions Manufacturing conditions include slab heating temperature, cumulative rolling reduction in recrystallization γ region (austenite recrystallization temperature region) in hot rolling, two-phase region that becomes α + γ (two-phase region of ferrite + austenite, simply α + γ region) Set the cumulative reduction ratio and lubrication conditions.

Kca (-10 ° C) ≧ 6000N / mm, which is a target for securing structural safety, with thick material of 50mm or more thickness that has been used for hull outer plates such as recent container ships and bulk carriers. ^In order to obtain ^3/2 brittle crack propagation stopping performance, first, molten steel having the above composition is melted in a converter or the like, and is made into a steel material (slab) by continuous casting or the like. Next, hot rolling is performed after the steel material is heated to a temperature of 900 to 1200 ° C.

  If the heating temperature is less than 900 ° C., sufficient time for rolling in the austenite recrystallization temperature region cannot be secured, and if it exceeds 1200 ° C., the austenite grains become coarse, leading to a decrease in toughness, and oxidation loss becomes significant. Since the yield is lowered, the heating temperature is set to 900 to 1200 ° C. The range of preferable heating temperature from a viewpoint of toughness is 1000-1150 degreeC, More preferably, it is 1000-1050 degreeC.

  In the hot rolling, first, rolling with a cumulative reduction ratio of 30% or more is performed in a temperature range where the central portion of the plate thickness becomes the recrystallization γ region. If the cumulative rolling reduction is less than 30%, the austenite is not sufficiently refined and the toughness is not improved. Next, rolling is performed at a cumulative reduction of 50% or more in a temperature range where the central portion of the plate thickness is in the α + γ range. If the cumulative rolling reduction in this temperature range is not 50% or more, the development of the texture is small, and the {100} <011> azimuth strength at the central portion of the plate thickness does not become 2.0 or more.

  The recrystallized γ region can be grasped by conducting a preliminary experiment in which a steel having the component composition is given a heat / working history in which conditions are changed.

  The type of lubricant used at the time of rolling is not particularly defined, but a lubricant that satisfies the following requirements is used. In order to make the {100} <011> azimuth strength at a thickness of ¼ part 1.2 or more, it is preferable to use a lubricant having a friction coefficient of 0.35 or less during rolling. Further, unlike the rolling of a thin plate, the thick plate cannot be tensioned from the outside unlike the rolling of a thin plate, so that the steel plate cannot be caught in the roll with a lubricant having an extremely low friction coefficient. Therefore, it is preferable to use a lubricant having a friction coefficient during rolling of 0.10 or more. For example, a solid lubricant such as mineral oil, synthetic ester or graphite can be used.

  The coefficient of friction between the roll and the steel sheet is determined by performing hot rolling using a roll having various surface roughnesses in advance using a test material, and hot rolling conditions including deformation resistance, sheet thickness, and reduction ratio. It calculates | requires by calculation from the measured rolling load. The method for obtaining the friction coefficient from the rolling load is based on the rolling theory. For example, numerical calculation based on the Orowan theory is preferably used.

  The friction coefficient specified in the present invention only needs to be achieved at the time of rolling in the α + γ region, and the friction coefficient is not an important control factor in rolling in the recrystallized γ region. However, since the friction coefficient does not change greatly in a series of rolling processes, the same friction coefficient is usually obtained in rolling in the recrystallization γ region.

  In hot rolling, rolling outside the specified temperature range is not limited. If the specified cumulative reduction is performed in the specified temperature range, the specified structure can be obtained.

  The end temperature of the hot rolling is a temperature region in which the center portion of the plate thickness is in the α + γ region, and immediately after the end, it is cooled to 600 ° C. or less at a cooling rate of 2 ° C./s or more. This is in order to prevent transformation and processing texture introduced by rolling from being reduced by recrystallization, and it is necessary to cool the steel sheet to a low temperature after rolling.

  When the cooling rate is less than 2 ° C./s, not only the desired texture is not obtained, but also the strength of the steel sheet is lowered. Therefore, the cooling rate is 2 ° C./s or more. When the cooling stop temperature exceeds 600 ° C., recrystallization proceeds even after the cooling stop and a desired texture cannot be obtained, so the cooling stop temperature is set to 600 ° C. or lower.

  In addition, let these cooling rate and cooling stop temperature be the temperature of the plate | board thickness center part of a steel plate. The temperature at the central portion of the plate thickness is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the temperature at the center of the plate thickness of the steel sheet is obtained by calculating the temperature distribution in the plate thickness direction using the difference method.

  The above explanation was made for a thick material having a plate thickness of 50 mm or more. However, when the plate thickness is small, the shear strain is generated near the surface layer, so that { 100} <011> Azimuth strength of 2.0 or more, Charpy fracture surface transition temperature at 1/4 thickness of {100} <011> orientation strength at a rolling surface at 1/4 thickness However, it is clear that a steel sheet having a characteristic of −40 ° C. or less has excellent brittle crack propagation characteristics even when the thickness is less than 50 mm.

It is also possible to perform tempering after accelerated cooling. By performing tempering, the toughness of the steel sheet can be further improved. In this case, in order not to reduce the texture obtained by rolling, the tempering temperature is set to an average steel plate temperature of Ac1 point or less. In this invention, Ac1 point (degreeC) is calculated | required by the following Formula.
Ac1 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 the content (% by mass) in steel, and 0 if not contained.

  The average temperature of the steel sheet is also obtained by simulation calculation or the like from the sheet thickness, surface temperature, cooling conditions, and the like, similarly to the temperature at the center of the sheet thickness.

  Molten steel (steel symbols A to J) of each composition shown in Table 1 is melted in a converter, made into a steel material (slab 280 mm thick) by a continuous casting method, and after hot rolling to a plate thickness of 50 to 80 mm, cooling is performed. No. 1-25 test steels were obtained. Table 2 shows the manufacturing conditions. In this example, mineral oil was used as the lubricant.

  About the obtained thick steel plate, a JIS14A test piece having a diameter of 14 mm from 1/4 part of the plate thickness was collected so that the longitudinal direction of the test piece was perpendicular to the rolling direction, a tensile test was performed, and the yield point (YS), Tensile strength (TS) was measured. In addition, a JIS No. 4 impact test piece was taken from 1/4 part of the plate thickness so that the direction of the longitudinal axis of the test piece was parallel to the rolling direction, and a Charpy impact test was conducted to determine the fracture surface transition temperature (vTrs). Asked. The Charpy fracture surface transition temperature at ¼ part of the plate thickness was within the range of the present invention within −40 ° C.

  Further, in order to evaluate the texture of the steel sheet, the {100} <011> azimuth strength at the rolling surface at the central portion of the plate thickness and the {100} <011> azimuth strength at the rolling surface at the ¼ portion of the plate thickness. It was measured. The azimuth strength is 3 from the obtained positive electrode point diagram using an X-ray diffractometer (manufactured by Rigaku Corporation), obtaining (200), (110) and (211) positive electrode point diagrams using a Mo ray source. It was determined by calculating the dimensional crystal orientation density function.

  Next, in order to evaluate the brittle crack propagation stop characteristic, a temperature gradient type ESSO test was performed to obtain Kca (−10 ° C.).

Table 3 shows the results of these tests. In the case of a test steel plate (manufacturing numbers 1 to 13) in which the texture at the center of the plate thickness and the 1/4 thickness is within the scope of the present invention, Kca (−10 ° C.) = 6200 to 7600 N / mm ^3/2 And showed excellent brittle crack propagation stopping performance.

On the other hand, in the case of a test steel plate whose component composition is outside the scope of the present invention and the manufacturing conditions are outside the scope of the present invention and the steel sheet whose texture does not meet the provisions of the present invention (manufacturing numbers 14 to 25), the value of Kca is It was 5300 N / mm ^3/2 or less and did not reach the example of the present invention.

Japanese Patent Publication No. 7-100814 JP 2002-256375 A Japanese Patent No. 3467767 Japanese Patent No. 3548349 Japanese Patent No. 2659661

Inoue et al .: Propagation behavior of long brittle cracks in thick shipbuilding steels, Proceedings of Japan Society of Marine Science and Technology No. 3, 2006, pp 359-362.

Claims (3)

  Steel composition is mass%, C: 0.03-0.20%, Si: 0.03-0.50%, Mn: 0.5-2.2%, A1: 0.005-0.08 %, P: 0.03% or less, S: 0.010% or less, N: 0.0075% or less, Ti: 0.005 to 0.03%, Nb: 0.005 to 0.05% In addition, after heating the steel material consisting of Fe and unavoidable impurities to a temperature of 900 to 1200 ° C., the cumulative reduction ratio is 30% or more in the temperature range in which the center portion of the plate thickness becomes the recrystallization γ region, Brittle crack propagation stop characterized by rolling at a cumulative reduction of 50% or more in a temperature range of α + γ using a lubricant and then cooling to 600 ° C. or less at a cooling rate of 2 ° C./s or more. A manufacturing method for structural high strength thick steel plates with excellent properties.   The steel composition is further mass%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0 0.5%, V: 0.001 to 0.10%, B: 0.003% or less, Ca: 0.005% or less, REM: 0.01% or less, or two or more The manufacturing method of the structural high strength thick steel plate excellent in the brittle crack propagation stop characteristic of Claim 1 characterized by performing.   The method for producing a high strength thick steel sheet for structural use having excellent brittle crack propagation stopping characteristics according to claim 1 or 2, wherein after the accelerated cooling, tempering is performed at an Ac1 point or less.