JP2010202931A - High-strength thick steel plate for structure excellent in brittle crack propagation arrest property, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength thick steel plate having excellent brittle crack propagation arrest properties and suitably usable for large-sized structures with a plate thickness of &ge;50 mm such as vessels, marine structures, low temperature storage tanks, building or civil engineering structures or the like, and to provide a method for producing the same. <P>SOLUTION: The high-strength thick steel plate has a texture where the intensity in the ä100}&lt;011&gt; orientation in the rolled face in the central part of the plate thickness is &ge;1.7, and also, the intensity in the ä100}&lt;011&gt; orientation in the rolled face in the 1/4 part of the plate thickness is &ge;0.3, in which the aspect ratio in the microstructure in the central part of the plate thickness in the cross-section parallel to the rolling direction is &le;4.0, and has a Charpy fracture transition temperature of &le;-40&deg;C in the 1/4 part of the plate thickness, and preferably has a steel composition containing, by mass, 0.03 to 0.20% C, 0.03 to 0.5% Si, 0.5 to 2.0% Mn, 0.005 to 0.08% Al, and P, S and N by &le;0.0050%, and if required, one or more selected from Ti, Nb, Cu, Ni, Cr, Mo, V, B, Ca and rare earth metals, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  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 in particular, large-sized vessels such as ships, offshore structures, low-temperature storage tanks, construction / civil engineering structures using steel plates having a thickness of 50 mm or more. The present invention relates to a structure suitable for a structure.

  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 because of their structures. Since the brittle crack propagation stop property of steel tends to deteriorate with higher strength or thicker material, the demand for the brittle crack propagation stop 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. ing.

  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 the brittle crack propagation stop characteristic without increasing the alloy cost.

  The steel material having excellent brittle crack propagation stopping characteristics described in Patent Document 1 pays attention to the fact that shear lip (plastic deformation region) generated in the steel surface layer is effective in improving the brittle crack propagation stopping characteristics when the brittle crack propagates. In addition, the crystal grain of the shear lip portion is refined to absorb the propagation energy of the propagating brittle crack.

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.

  Further, in Patent Document 2, in order to improve the brittle crack propagation stop property 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 or more. It is important to suppress the variation in the ferrite grain size with a layer having a ferrite structure with 50% or more of ferrite grains, and the maximum reduction rate per pass during finish rolling is 12% or less as a method to suppress the variation. It is described that local recrystallization phenomenon is suppressed.

  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 surface layer part and then recovering the heat, and by applying processing during the recuperation, a specific structure is obtained. On the actual production scale, control is not easy, 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 technique 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. ing.

  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 stop property is improved by rolling conditions and (d) cooling conditions that suppress 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 stop property by applying a reduction to a transformed ferrite to develop a texture is also known. Separation occurs on the fracture surface of the steel material in a direction parallel to the plate surface, thereby reducing the stress at the tip of the brittle crack, thereby increasing 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 having excellent brittle crack propagation stopping performance in a joint part, the (100) plane X-ray plane strength ratio in the rolled surface inside the plate thickness is 1.5 or more. Steel sheet is disclosed, and it is described that it has excellent brittle crack propagation stoppage characteristics due to the deviation of the angle between the stress load direction and the crack propagation direction due to the texture development.

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

  By the way, in recent large container ships exceeding 6,000 TEU, thick steel plates exceeding 50 mm are used. Inoue et al .: Propagation of long brittle cracks in thick shipbuilding steels No. 3, 2006, pp 359-362 evaluated the brittle crack propagation stopping performance of a steel sheet with a thickness of 65 mm, and reported the result that the brittle crack does not stop in the large brittle crack propagation stopping test of the base material.

In addition, in the ESSO test of the specimen, the Kca value at the use temperature of -10 ° C is less than 3000 N / mm ^3/2, and in the case of a hull structure using a steel plate with a thickness exceeding 50 mm, safety It has been suggested that securing is an issue.

  When the steel plate excellent in the brittle crack propagation stop characteristics described in Patent Documents 1 to 5 described above is applied to a thick material exceeding 50 mm, the main object is a plate thickness of about 50 mm from manufacturing conditions and disclosed experimental data. It is unclear whether a predetermined characteristic can be obtained, and the characteristics against crack propagation in the 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 a method for producing the same The purpose is to provide.

In order to achieve the above-mentioned problems, the present inventors diligently studied the cause of deterioration of brittle crack propagation stopping performance with thick materials, and obtained the following knowledge.
1. The {100} <011> orientation effective for improving the brittle crack propagation stop property changes to the {110} <001> orientation due to 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 material is subjected to controlled rolling in the non-recrystallized austenite region, the {100} <011> azimuth strength decreases by ¼ part of the plate thickness, and the development of the {100} <011> azimuth is The brittle crack propagation stopping performance is limited only in the vicinity of the central portion.
4). By specifying the rolling temperature in the non-recrystallized austenite region, and specifying the cumulative reduction ratio, the ratio of contact arc length (l) for each pass to the thickness (h) of the inlet side of the rolling (l / h) Also in the material, it is possible to increase the ¼ part thickness {100} <011> orientation strength, and further to improve the base metal toughness, an excellent brittle crack propagation stopping performance is achieved.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. A structural high-strength thick steel plate having a texture by controlled rolling in a non-recrystallized austenite region, wherein the {100} <011> orientation strength at the rolling surface in the central portion of the thickness and the thickness 1 {100} <011> orientation strength at the rolling surface at / 4 part, and further, Charpy fracture surface transition temperature at ¼ part of the plate thickness is specified according to the desired brittle crack propagation stop characteristics. Structural high-strength thick steel plate with excellent brittle crack propagation stopping properties.
2. Aggregation in which {100} <011> azimuth strength at the rolling surface at the center of the plate thickness is 1.7 or more and {100} <011> azimuth strength at the rolling surface at ¼ part of the plate thickness is 0.3 or more. The aspect ratio of the microstructure in the central part of the plate thickness of the cross section parallel to the rolling direction is 4.0 or less, and the Charpy fracture surface transition temperature at ¼ part of the plate thickness is −40 ° C. or less. Structural high-strength thick steel plate with excellent brittle crack propagation stopping characteristics.
3. Steel composition is mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.0%, Al: 0.005-0.08 %, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, with the balance being Fe and inevitable impurities, Structural high strength thick steel plate with excellent properties.
4). Steel composition is further mass%, Ti: 0.005-0.03%, Nb: 0.005-0.05%, Cu: 0.01-0.5%, Ni: 0.01-1 0.0%, 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 The structural high-strength thick steel plate having excellent brittle crack propagation stop properties according to 3, characterized by containing at least 1% or less, REM: 0.01% or less.
5). The structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to any one of 1 to 4, wherein the plate thickness exceeds 50 mm.
The steel material having the composition described in 6.3 or 4 is heated to a temperature of 900 to 1200 ° C., and the temperature of the central portion of the plate thickness is (Ar ₃ points + 100) ° C. or more, and the cumulative reduction ratio is 30% or more. The temperature at the center of the plate thickness is (Ar ₃ points + 60) ° C. or less, the cumulative rolling reduction is 50% or more in the temperature range of Ar ₃ points or more, the contact arc length (l) for each pass, and the inlet side plate thickness (h ) Ratio (l / h) after rolling at an average value of 0.6 or higher, cooling to 600 ° C. or lower at a cooling rate of 2 ° C./s or higher is achieved. A method for producing excellent structural high-strength thick steel plates.
7). 6. The method for producing a structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to 6, wherein the plate thickness exceeds 50 mm.

  According to the present invention, it is possible to obtain a high-strength thick steel plate having a thickness exceeding 50 mm, in which the texture is appropriately controlled in the thickness direction and excellent in brittle crack propagation stopping characteristics. It is extremely useful in the industry, for example, it contributes to improving the safety of ships by applying it to hatch side combing and deck members in the strong deck structure of bulk carriers.

(L / h) average effect on {100} <011> azimuth strength of 1/4 part thickness in rolling in a temperature range where the temperature at the center part is (Ar ₃ points +60) ° C. or lower and Ar ₃ points or higher The figure which shows the influence of a value.

In the present invention, 1. Texture inside the steel plate (central part and 1/4 part of the plate thickness); 2. Aspect ratio of microstructure in the center of the plate thickness Define toughness.
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 sheet thickness direction. The {100} <011> orientation strength at the rolling surface and the {100} <011> orientation strength at the rolling surface at a thickness of 1/4 are appropriately defined according to the desired brittle crack propagation stop characteristics.

  When the {100} <011> orientation is developed, the stress intensity factor at the crack tip decreases due to crack bending, that is, the crack deviates from the stress application direction, and the stress relaxation at the brittle crack tip occurs due to the occurrence of fine separation. The effect improves brittle crack propagation stopping performance.

Kca (-50 ° C) ≧ 5000 N / mm, which is a target for securing structural safety, with a thick material exceeding 50 mm thick that has been used for hull outer plates such as recent container ships and bulk carriers. ^In order to obtain a brittle crack propagation stopping performance of ^3/2 , the {100} <011> orientation strength at the rolled surface at the center of the plate thickness is 1.7 or more and {100} at the rolled surface at the 1/4 thickness of the plate. } <011> The orientation strength is 0.3 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. Aspect ratio In order to obtain a suitable texture in the central portion of the plate thickness, the aspect ratio of the microstructure in the central portion of the plate thickness having a cross section parallel to the rolling direction is set to 4.0 or less.

In order to maintain the texture formed by rolling in the non-recrystallized austenite region, it is preferable to end the rolling in a temperature region where the temperature at the central portion of the plate thickness is Ar ₃ points or higher. This is because the aspect ratio of the microstructure in the central portion of the thickness of the cross section parallel to the direction is 4.0 or less.
3. Base material toughness Since the base material toughness is a premise for suppressing the progress of cracks, having good characteristics, the steel sheet according to the present invention also has the desired brittleness of the Charpy fracture surface transition temperature at 1/4 thickness. It is specified as appropriate according to the crack propagation stop characteristics.

In thick material exceeding thickness 50 mm, the case of obtaining the brittle crack propagation stopping performance of Kca (-50 ℃) ≧ 5000N / mm 3/2 the targeted for ensuring the structural safety, the sheet thickness 1/4 The Charpy fracture surface transition temperature in the part is specified to be -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. Hereinafter, preferable chemical components and production conditions of steel in the present invention will be described.
4). In the description of chemical components,% 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: 0.03-0.5%
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 0.5%, 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 0.5% or less.

Mn: 0.5 to 2.0%
Mn is added as a strengthening element. If the content is less than 0.5%, the effect is not sufficient. If the content exceeds 2.0%, the weldability deteriorates and the steel material cost increases, so the content is made 0.5% to 2.0%.

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%.

N: 0.0050% 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 0.0050%, the toughness deteriorates. 0050% or less.

P, S
P and S are inevitable impurities in the steel, but P exceeds 0.03%, and if S exceeds 0.01%, the toughness deteriorates. The following are desirable, and more preferably 0.02% or less and 0.005% or less, respectively.

  The above is the basic component composition of the present invention. In order to further improve the characteristics, it is possible to contain one or more of Nb, Ti, Cu, Ni, Cr, Mo, V, B, Ca, and REM. It is.

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 if added over 0.05%, coarse NbC precipitates and conversely causes a decrease in toughness, so the upper limit is 0.05. % Is preferable.

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, but the content exceeding 0.03% lowers the toughness of the base metal and the weld heat affected zone, so Ti is 0.005 to 0.03%. The range is preferable.

Cu, Ni, Cr, Mo
Cu, Ni, Cr, and Mo are all elements that enhance the hardenability of steel. It contributes directly to strength improvement after rolling, and is added to improve functions such as toughness, high-temperature strength, or weather resistance, but excessive addition degrades toughness and weldability, so the upper limit is 0.5% respectively. 1.0%, 0.5%, 0.5%. On the contrary, if the addition amount is less than 0.01%, the effect does not appear, so 0.01% or more is added.

V: 0.001 to 0.1%
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. Therefore, V is added in the range of 0.001 to 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 contained over 0.003%, the toughness of the welded portion is lowered, so B is added at 0.003% or less.

Ca: 0.005% or less, REM: 0.01% or less Ca, REM is necessary because it refines the structure of the heat affected zone and improves toughness, and even if added, the effect of the present invention is 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, it is preferable to set the upper limit of the addition amount to 0.005% and 0.01%, respectively.

In order to secure weldability as structural steel, the carbon equivalent (Ceq) represented by the following formula is preferably 0.45% or less.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5
(Each element symbol on the right side indicates the content (mass%) of the element.)
3. Production conditions production conditions slab heating temperature, the cumulative rolling reduction in the hot rolling (Ar ₃ point +100) ° C. or higher, (Ar ₃ point +60) ° C. or less, Ar ₃ point or more cumulative rolling reduction, and (Ar ₃ point The average value of the ratio (l / h) of the contact arc length (l) for each pass at +60) ° C. or lower and ₃ or more points of Ar and the inlet side plate thickness (h) of rolling is calculated on the rolling surface in the central portion of the plate thickness. {100} <011> Orientation strength and {100} <011> orientation strength at the rolling surface at a thickness of ¼ part are appropriately set so as to have values necessary for obtaining desired brittle crack propagation stop characteristics. .

Kca (-50 ° C) ≧ 5000N / mm, which is a target for ensuring structural safety, with thick material exceeding 50mm thickness, which 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, the 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.

  When the heating temperature is less than 900 ° C., the rolling efficiency is lowered. When the heating temperature is more than 1200 ° C., the austenite grains are coarsened and the toughness is lowered. In addition, the oxidation loss becomes remarkable and the yield is lowered. 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 is performed at a central thickness of (Ar ₃ points + 100) ° C. or more and a cumulative reduction ratio of 30% or more. If the cumulative rolling reduction is less than 30%, the austenite is not sufficiently refined and the toughness is not improved.

Although equation for Ar ₃ transformation point in the present invention (℃) is not particularly specified, for example, _{Ar 3 = 910-310C-80Mn-20Cu} -15Cr-55Ni-80Mo, to. In the formula, each element has a steel content (mass%).

Next, rolling is performed at a cumulative reduction of 50% or more in a temperature range where the temperature at the center of the plate thickness is (Ar ₃ points + 60) ° C. or lower and Ar ₃ points or higher. The temperature range of (Ar ₃ points + 60) ° C. or lower and Ar ₃ points or higher is an unrecrystallized austenite region. If the cumulative rolling reduction in this temperature range is not 50% or more, {1/2} parts {100} <011> Azimuth strength does not exceed 1.7.

  Rolling defines the average value of the ratio (l / h) of the contact arc length (l) and the entry side thickness (h) of rolling for each pass. The ratio (l / h) of the contact arc length (l) for each pass and the entry side plate thickness (h) of rolling is an important factor in the present invention. In the finish rolling of thick materials, multipass rolling is usually performed under small reduction, but this hinders the development of {100} <011> orientation with a thickness of 1/4 part.

  Generally, when the ratio (l / h) between the contact arc length (l) and the plate thickness (h) before rolling is large, the shear strain is said to show a small value. In the case of a small pressure, l / h is small, the shear strain is large, and the strength of the {100} <011> orientation obtained as a result is also low. Therefore, the contact arc length (l) of rolling in this temperature range is It is preferable that the average value of the ratio (l / h) of the entry side plate thickness (h) of rolling is higher.

FIG. 1 shows the effect on the {100} <011> azimuth strength at a thickness of 1/4 part in rolling in a temperature range where the temperature at the center of the sheet thickness is (Ar ₃ points + 60) ° C. or lower and Ar ₃ points or higher. h) Indicates the influence of the average value. In order to set the strength of the {100} <011> orientation of the 1/4 thickness part to 0.3 or more, the (l / h) average value is set to 0.6 or more.

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. Here, in order to maintain the texture formed by rolling in the non-recrystallized austenite region, it is preferable to end the rolling in a temperature region where the temperature at the central portion of the plate thickness is Ar ₃ points or higher. In this case, the aspect ratio of the microstructure in the central portion of the thickness of the cross section parallel to the rolling direction is 4.0 or less. Even if the rolling end temperature is lower than the Ar ₃ point, if the aspect ratio is 4.0 or less, the formed texture is substantially a texture by controlled rolling in the non-recrystallized austenite region.

  The rolled steel sheet is cooled to 600 ° C. or lower at a cooling rate of 2 ° C./s or higher. This is to prevent reworking of the work texture introduced in the non-recrystallized austenite region rolling, 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. If the cooling stop temperature is higher than 600 ° C., recrystallization proceeds even after the cooling stop and a desired texture cannot be obtained. Therefore, the cooling stop temperature is preferably 600 ° C. or lower.

  The above explanation was made for a thick material having a plate thickness exceeding 50 mm. However, since the shear strain is generated near the surface layer when the plate thickness is small, { 100} <011> Azimuth strength of 1.7 or more, {100} <011> Azimuth strength of 0.3 or more at a thickness of 1/4 part and Charpy fracture surface transition temperature at a thickness of 1/4 part 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 50 mm or less.

Molten steel (steel symbols A to N) 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 35 to 75 mm, cooling is performed. No. 1-28 test steels were obtained. Table 2 shows hot rolling conditions and cooling conditions. The Ar ₃ transformation point (° C.) was calculated by the following formula.
Ar ₃ = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
However, each element has a steel content (mass%).

  About the obtained thick steel plate, a JIS14A test piece of Φ14 was collected from 1/4 part of the plate thickness 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 Denki Co., Ltd.), 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.

  The aspect ratio of the microstructure in the central part of the plate thickness of the cross section parallel to the rolling direction is revealed by corroding the microstructure of the cross section in the rolling direction with a 3% nital aqueous solution, and then observed with a 500 × optical microscope for image processing. The average value obtained by

  Next, in order to evaluate the brittle crack propagation stop characteristic, a temperature gradient type ESSO test was performed to obtain Kca (−50 ° C.).

Table 3 shows the results of these tests. In the case of a test steel plate (manufacturing number 1 to 10) in which the texture at the center of the plate thickness and 1/4 of the plate thickness is within the scope of the present invention, Kca (−50 ° C.) is 5000 to 8100 N / mm ^3/2. And showed excellent brittle crack propagation stopping performance.

On the other hand, the steel plate whose component composition is outside the scope of the present invention (manufacturing numbers 11 to 16) and the manufacturing conditions are outside the scope of the present invention, and the texture of the steel sheet does not meet the provisions of the present invention (manufacturing numbers 17 to In 28), the value of Kca was 4100 N / mm ^3/2 or less, which was not equivalent to the example of the present invention.

Claims (7)

  A structural high-strength thick steel plate having a texture by controlled rolling in a non-recrystallized austenite region, wherein the {100} <011> orientation strength at the rolling surface in the central portion of the thickness and the thickness 1 {100} <011> orientation strength at the rolling surface at / 4 part, and further, Charpy fracture surface transition temperature at ¼ part of the plate thickness is specified according to the desired brittle crack propagation stop characteristics. Structural high-strength thick steel plate with excellent brittle crack propagation stopping properties.   Aggregation in which {100} <011> orientation strength at the rolling surface at the center of the plate thickness is 1.7 or more and {100} <011> orientation strength at the rolling surface at the 1/4 thickness portion is 0.3 or more. The aspect ratio of the microstructure in the central part of the thickness of the cross section parallel to the rolling direction is 4.0 or less, and the Charpy fracture surface transition temperature at ¼ part of the thickness is −40 ° C. or less. Structural high-strength thick steel plate with excellent brittle crack propagation stopping characteristics.   Steel composition is mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.0%, Al: 0.005-0.08 %, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, and the balance is made of Fe and inevitable impurities. Structural high-strength thick steel plate with excellent propagation stop characteristics.   Steel composition is further mass%, Ti: 0.005-0.03%, Nb: 0.005-0.05%, Cu: 0.01-0.5%, Ni: 0.01-1 0.0%, Cr: 0.01-0.5%, Mo: 0.01-0.5%, V: 0.001-0.1%, B: 0.003% or less, Ca: 0.005 % Or less, REM: 0.01% or less, or 2 or more types, The structural high-strength thick steel plate excellent in the brittle crack propagation stop characteristic of Claim 3 characterized by the above-mentioned.   The structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to any one of claims 1 to 4, wherein the plate thickness exceeds 50 mm. The steel material having the composition according to claim 3 or 4 is heated to a temperature of 900 to 1200 ° C, and the cumulative reduction rate is 30% or more at a temperature of the central part of the plate thickness of (Ar ₃ points +100) ° C or more. The temperature at the center of the plate thickness is (Ar ₃ points + 60) ° C. or lower, the cumulative rolling reduction is 50% or higher in the temperature range of Ar ₃ points or higher, the contact arc length (l) for each pass, and the inlet side plate thickness (h ) Ratio (l / h) after rolling at an average value of 0.6 or higher, cooling to 600 ° C. or lower at a cooling rate of 2 ° C./s or higher is achieved. A method for producing excellent structural high-strength thick steel plates.   The method for producing a structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to claim 6, wherein the plate thickness exceeds 50 mm.

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