WO2013099319A1 - High-strength thick steel plate for construction having excellent characteristics for preventing diffusion of brittle cracks, and production method therefor - Google Patents

Abstract

Provided are: a high-strength thick steel plate for construction having excellent characteristics for preventing the diffusion of brittle cracks, said plate being used for ships and having a preferred plate thickness of at least 50 mm; and a method for producing the thick steel plate. The thick steel plate has excellent characteristics for preventing the diffusion of brittle cracks, characterized in that: the thick steel sheet has a specific component composition; the main constituent of the metal structure is a ferrite phase; the thick metal sheet has an aggregate structure in which the density (I) in the RD//(110) plane at the plate thickness surface is at least 1.3, and the density (I) in the RD//(110) plane at the plate thickness center is at least 1.8; the Charpy fracture appearance transition temperature (vTrs) at the surface is not more than -60°C, and the Charpy fracture appearance transition temperature (vTrs) at the plate thickness center is not more than -50°C.

Description

Structural high-strength thick steel plate with excellent brittle crack propagation stopping characteristics and method for producing the same

The present invention relates to a high-strength thick steel plate and a manufacturing method thereof excellent in brittle crack propagation arresting characteristics and particularly for a ship. It relates to a thickness of 50 mm or more.

In large structures such as ships, accidents associated with brittle fractures have a large impact on the economy and the environment, so there is always a need to improve safety. There are demands for toughness and brittle crack propagation stopping properties.

Ships such as container ships and bulk carriers use high-strength thick materials for the outer plate of the ship (outer plate of ship's hull). Fleshing is progressing. In general, since the brittle crack propagation stop property of a steel sheet tends to deteriorate as the strength or thickness of the material increases, 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, and in a liquefied natural gas (Liquid Natural Gas) storage tank, 9% Ni steel is on a commercial scale. in use.

However, an increase in the amount of Ni necessitates a significant increase in cost, making it difficult to apply to applications other than LNG storage tanks.

On the other hand, the so-called TMCP (Thermo-Mechanical Control) is used for relatively thin steel materials with a plate thickness of less than 50 mm used for ships and line pipes that do not reach ultra low temperature such as LNG. It is possible to achieve fine graining by the Process method, improve low-temperature toughness, and impart excellent brittle crack propagation stopping characteristics.

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

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

As a manufacturing method, the surface layer portion is cooled to below _{A r3} transformation point (transformation point) by controlled cooling after hot rolling, then controlled cooling (Controlled Cooling) The Stop recuperator the surface layer portion to the transformation point or higher (Recuperate ) Is repeated one or more times, and during this time, the steel material is subjected to reduction, and repeatedly transformed or processed and recrystallized, so that a superfine ferrite structure or bainite structure is formed on the surface layer portion. Is described.

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 an equivalent circular particle diameter (cycle-equalent average grain). size): 5 μm or less, aspect ratio (aspect ratio of the grains): a layer having a ferrite structure having 50% or more of ferrite grains having two or more ferrite grains, and it is important to suppress variations in the ferrite grain size, and to suppress variations As a method, it is described that a maximum rolling reduction per pass during finish rolling is set to 12% or less to suppress a 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 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, in Patent Document 3, attention is paid not only to the refinement of ferrite crystal grains but also to subgrains formed in ferrite crystal grains, and a technique on the extension of TMCP that improves brittle crack propagation stop characteristics. Is described.

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 The brittle crack propagation stop characteristic is improved by rolling conditions for forming subgrains and (d) cooling conditions for suppressing coarsening of the formed fine ferrite crystal grains and fine subgrain grains.

Also known is a method of improving the brittle crack propagation stop property by controlling the rolling to develop a texture by reducing the transformed ferrite. Separation is produced on the fracture surface of the steel material in a direction parallel to the plate surface, and the stress at the tip of the brittle crack is relaxed, thereby increasing the resistance to brittle fracture.

For example, in Patent Document 4, the (110) plane X-ray intensity ratio in the (110) plane showing a texture developing degree) is set to 2 or more by controlled rolling, and the equivalent diameter of the circle (diameter equivalent). To a circle in the crystal grains) It is described that the brittle fracture resistance is improved by making coarse particles of 20 μm or more 10% or less.

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. A steel sheet is disclosed. It is described that it has excellent brittle crack propagation stoppage properties due to the difference in angle between the stress loading 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, a heavy-duty container ship exceeding the recent 6,000 TEU (Twenty-foot Equivalent Unit) uses a thick steel plate exceeding 50 mm in thickness. Inoue et al .: Propagation behavior of long brittle cracks in thick shipbuilding steels, Proceedings of the Japan Society of Marine Science and Technology No. 3, 2006, pp 359 to 362, evaluated the brittle crack propagation stopping performance of steel plates with a thickness of 65 mm. In the large-scale brittle crack propagation stop test of the base metal, the result that the brittle crack does not stop is reported.

Further, in the standard ESSO test (ESSO test comprehensive with WES 3003) of the test material, the value of Kca at the use temperature of −10 ° C. (hereinafter also referred to as Kca (−10 ° C.)) is less than 3000 N / mm ^3/2 . The results are shown, and it is suggested that in the case of a hull structure to which a steel plate having a thickness exceeding 50 mm is applied, ensuring safety 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 production conditions and disclosed experimental data up to a plate thickness of about 50 mm. When it is applied to thick materials exceeding 50 mm, it is unclear whether a predetermined characteristic can be obtained, and the characteristics against 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 a method for producing the same The purpose is to provide.

The inventors of the present invention have made extensive studies to achieve the above-mentioned problems, and have obtained the following knowledge about a high-strength thick steel plate having excellent crack propagation stopping characteristics even with a thick steel plate.
1. As a result of detailed investigation of the fracture surface of the standard ESSO test for thick steel plates exceeding 50 mm in thickness, when the fracture surface form is as shown in FIG. The stress intensity factor at the tip is reduced, and as a result, the arrest performance of the steel sheet is increased. FIGS. 1A and 1B schematically show that the crack 3 that has entered from the notch 2 of the standard ESSO test piece 1 has stopped propagating at the tip shape 4 in the base material 5.
2. In order to obtain the above fractured surface form, it is necessary to improve the arrest performance of the surface layer portion and the plate thickness central portion. As a method for improving the arrest performance of the surface layer portion and the plate thickness center portion, it is effective to improve the toughness of the surface layer portion and the plate thickness center portion. However, a thick steel plate having a thickness exceeding 50 mm has limitations on the cooling rate, the rolling reduction, etc., and there is a limit to improve the toughness of the central portion of the thickness.
3. As a technique for improving arrest performance in addition to improving toughness, it is effective to control the texture at the center of the plate thickness. In particular, it is effective to control the texture so that the (110) planes are accumulated parallel to the rolling direction and cracks extending in the rolling direction or the plate width direction are deflected obliquely from the rolling direction or the plate width direction, respectively. .
4). Further, by making the cumulative reduction ratio in the state where the center part of the plate thickness is in the austenite recrystallization temperature range to 20% or more and the average reduction ratio per pass to 5% or less, the structure of the surface layer part is refined. Plan. Thereafter, by setting the cumulative reduction ratio in the state where the central portion of the plate thickness is in the austenite non-recrystallization temperature range to 40% or more and the average reduction rate per pass to 7% or more, the toughness of the central portion of the plate thickness And the texture can be developed, and the above-described organization can be realized.

The present invention has been made by further study based on the obtained knowledge. That is, the present invention
1. The metal structure is mainly composed of ferrite, the integration degree I of the RD // (110) plane (Rolling Direction parallel to (110) plane) in the plate thickness surface layer portion is 1.3 or more, and the RD // (110 in the plate thickness center portion. ) The surface integration degree I has a texture of 1.8 or more, the Charpy fracture surface transition temperature in the surface layer portion is vTrs ≦ −60 ° C., and the Charpy fracture surface transition temperature in the center portion of the plate thickness is vTrs ≦ −50 ° C. A structural high-strength thick steel plate with excellent brittle crack propagation stopping characteristics.
2. The structure excellent in brittle crack propagation stopping property according to 1, wherein the Charpy toughness value of the surface layer portion and the center portion of the plate thickness and the degree of integration I of the RD // (110) plane satisfy the following formula (1): High strength thick steel plate.
vTrs _{(surface layer)} + 1.9 × vTrs _{(1 / 2t)} −6 × I _{RD // (110) [surface layer]} −84 × I _{RD // (110) [1 / 2t]} ≦ −350 (1 )
vTrs _{(surface layer)} : Fracture surface transition temperature of surface layer (° C)
vTrs _{(1 / 2t)} : Fracture surface transition temperature at the thickness center (° C)
I _{RD // (110) [surface layer]} : Degree of integration of RD // (110) surface of surface layer portion I _{RD // (110) [1 / 2t]} : RD // (110) surface of central portion of plate thickness Integration degree3. Steel composition is mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.2%, Al: 0.005-0.08 %, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, Ti: 0.005-0.03%, with the balance being Fe and inevitable impurities 3. A structural high-strength thick steel plate excellent in brittle crack propagation stopping characteristics according to either 1 or 2.
4). The steel composition is further mass%, Nb: 0.005 to 0.05%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, Cr: 0.01 to 0 0.5%, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: 0.010% or less 3. The structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics as described in 3 above, which contains one or more kinds.
A steel material (slab) having the composition described in either 5.3 or 4 is heated to a temperature of 900 to 1150 ° C. In the state where the sum of the cumulative reduction ratios in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more, and the plate thickness center is in the austenite recrystallization temperature range, the cumulative reduction ratio is 20% or more, and 1 Rolling is performed so that the average rolling reduction per pass is 5.0% or less. Next, in a state where the center portion of the plate thickness is in the austenite non-recrystallization temperature range, rolling is performed so that the cumulative reduction rate is 40% or more and the average reduction rate per pass is 7.0% or more. Then, the manufacturing method of the structural high strength thick steel plate excellent in the brittle crack propagation stop characteristic characterized by accelerated cooling to 600 degrees C or less at the cooling rate of 4.0 degrees C / s or more.
6. The method for producing a structural high-strength thick steel plate having excellent brittle crack propagation stop properties according to 5, further comprising a step of tempering to 600 ° C. or lower and then tempering to a temperature of AC ₁ or lower.

According to the present invention, it is possible to obtain a high-strength thick steel plate having a thickness of 50 mm or more and a method for producing the same, in which the texture is appropriately controlled in the thickness direction and excellent in brittle crack propagation stopping characteristics, and preferably a thickness of 50 mm. It is effective to apply to a steel sheet having a thickness of 55 mm or more. And in the shipbuilding field, it contributes to improving the safety of ships by applying it to hatch side combing and deck members in the structure of large decks of large container ships and bulk carriers.

The figure which shows typically the fracture surface form of the standard ESSO test of the thick steel plate exceeding 50 mm in thickness, (a) is the figure which observed the test piece from the plane side, (b) is the figure which shows the fracture surface of a test piece.

In the present invention, 1. Toughness and texture of the surface layer part and the central part of the plate thickness Define the metallographic structure.
1. Toughness and texture In the present invention, in order to obtain the fracture surface form of FIG. 1 that can improve the crack propagation stop property with respect to a crack that propagates in a horizontal direction (in-plane direction of the steel sheet) such as a rolling direction or a perpendicular direction of rolling. Further, the toughness at the surface thickness layer portion and the central portion and the degree of integration I of the RD // (100) plane are appropriately defined.

First, since good base material toughness is a premise for suppressing the progress of cracks, in the steel sheet according to the present invention, the Charpy fracture surface transition temperature vTrs in the surface layer portion is −60 ° C. or lower and the thickness in the central portion. The Charpy fracture surface transition temperature vTrs is defined as −50 ° C. or lower. The Charpy fracture surface transition temperature vTrs at the center of the plate thickness is preferably −60 ° C. or lower.

In addition, by developing a texture of the RD // (100) plane, the cleavage plane is accumulated obliquely with respect to the main crack direction, and fine crack branching is generated to reduce stress at the brittle crack tip. The effect of stopping brittle crack propagation is improved. Kca (-10 ° C) ≧ 6000 N / mm, which is a target for ensuring structural safety, with thick materials exceeding 50 mm thick that have been used for hull outer plates such as recent container ships and bulk carriers. ^When obtaining a brittle crack propagation stopping performance of ^3/2, the degree of integration I of the RD // (110) plane in the plate thickness surface layer portion is 1.3 or more, preferably 1.6 or more, and RD // in the plate thickness center portion. The degree of integration I on the (110) plane needs to be 1.8 or more, preferably 2.0 or more.

Here, the integration degree I of the RD // (110) plane at the plate thickness surface layer portion or the plate thickness central portion indicates the following. First, a sample having a plate thickness of 1 mm is taken from the plate thickness surface layer portion or the plate thickness central portion, and a surface parallel to the plate surface is mechanically polished / electrolytic polished, thereby obtaining an X-ray diffraction sample. Prepare a specimen. In the case of the plate thickness surface layer portion, the surface closest to the outermost surface is polished. Using this test piece, X-ray diffraction measurement is performed using a Mo ray source, and (200), (110) and (211) positive figures (pole figures) are obtained. A three-dimensional crystal orientation density function is calculated from the obtained positive pole figure by the Bunge method. Next, from the obtained three-dimensional crystal orientation density function, the (110) plane is parallel to the rolling direction in a total of 19 cross-sectional views at 5 ° intervals from ψ ₂ = 0 ° to 90 ° in Bunge notation. The integrated value is obtained by integrating the values of the three-dimensional crystal orientation density function of the orientation. A value obtained by dividing the integrated value by the number of integrated directions is referred to as an integration degree I of the RD // (110) plane.

In addition to the above-mentioned base material toughness and texture definition, it is preferable that the Charpy toughness value of the surface layer portion and the central portion of the plate thickness and the degree of integration I of the RD // (110) surface satisfy the following formula (1). .
vTrs _{(surface layer)} + 1.9 × vTrs _{(1 / 2t)} −6 × I _{RD // (110) [surface layer]} −84 × I _{RD // (110) [1 / 2t]} ≦ −350 (1 )
vTrs _{(surface layer)} : Fracture surface transition temperature of surface layer (° C)
vTrs _{(1 / 2t)} : Fracture surface transition temperature at the thickness center (° C)
I _{RD // (110) [surface layer]} : Degree of integration of RD // (110) surface of surface layer portion I _{RD // (110) [1 / 2t]} : RD // (110) surface of central portion of plate thickness Degree of integration When the above formula (1) is satisfied, further excellent brittle crack propagation stopping performance can be obtained.

2. Metal structure In the present invention, it is assumed that the metal structure is mainly composed of ferrite. Here, in the present invention, the fact that the metal structure is mainly composed of ferrite is that the area fraction of the ferrite phase is 60% or more of the whole. The balance of bainite, martensite (including island-like martensite), pearlite, etc. is 40% or less in total area fraction.

In a structure mainly composed of ferrite, if a metal structure mainly composed of ferrite is obtained by rolling conditions in normal austenite rolling, the target toughness can be obtained, but transformation occurs when transforming from austenite to ferrite after rolling. Since sufficient time exists, the resulting texture becomes random, and the integration degree I of the RD // (110) plane is 1.3 or more, preferably 1.6 or more in the target plate thickness surface layer portion. The degree of integration I of the RD // (110) plane at the center of the plate thickness cannot be achieved at 1.8 or more, preferably 2.0 or more. Therefore, by devising the rolling conditions as described later, the degree of integration I of the RD // (110) plane is 1.3 or more, preferably 1.6 or more in the plate thickness surface layer portion even in the structure mainly composed of ferrite. The degree of integration I of the RD // (110) plane at the center of the plate thickness can be 1.8 or more, preferably 2.0 or more.

3. Hereinafter, preferable chemical components in the present invention will be described. In the description,% 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 is preferably specified in the range of 0.03 to 0.20%. Furthermore, it is preferably 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 preferably 0.03% or more and 0.5% 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. Therefore, it is preferably 0.5% or more and 2.2% or less. .

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. Therefore, Al is preferably specified in the range of 0.005 to 0.08%, and more preferably 0.02 to 0.04%.

N: 0.0050% or less N combines with Al in the steel to form AlN, thereby adjusting the crystal grain size during rolling and strengthening the steel, but if it exceeds 0.0050%, the toughness Since it deteriorates, it is preferable to make it 0.0050% or less.

P, S
P and S are inevitable impurities in the steel. However, if P exceeds 0.03%, the toughness deteriorates when S exceeds 0.01%. % Or less is desirable, and 0.02% or less and 0.005% or less are more desirable, respectively.

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, so 0.005 to 0.03% is set.

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

Nb: 0.005 to 0.05%
Nb precipitates as NbC at the time of ferrite transformation or reheating, and contributes to the increase in strength. In addition, it has the effect of expanding the non-recrystallization temperature range in rolling in the austenite region, and contributes to the refinement of ferrite and is effective in improving toughness. The effect is exhibited by addition of 0.005% or more, but if added over 0.05%, coarse NbC precipitates and conversely causes a decrease in toughness, so the upper limit is made 0.05%. Is preferred.

Cu, Ni, Cr, Mo
Cu, Ni, Cr, and Mo are all elements that enhance the hardenability of steel. While contributing directly to strength enhancement after rolling, it can be added to improve functions such as toughness, high-temperature strength, or weather resistance, since these effects are exhibited by containing 0.01% or more, When contained, the content is preferably 0.01% or more. However, when it contains excessively, toughness and weldability will deteriorate, when containing, upper limit is 0.5% for Cu, 1.0% for Ni, 0.5% for Cr, and 0.5% for Mo. % Is preferable.

V: 0.001 to 0.10%
V is an element that improves the strength of the steel by precipitation strengthening as V (C, N). In order to exhibit this effect, 0.001% or more may be contained, but if it exceeds 0.10%, toughness is reduced. For this reason, when it contains V, it is preferable to set it as 0.001 to 0.10% of range.

B: 0.0030% or less B may be added as an element that enhances the hardenability of steel in a small amount. However, if it exceeds 0.0030%, the toughness of the welded portion is lowered. Therefore, when B is contained, the content is preferably 0.0030% or less.

Ca: 0.0050% or less, REM: 0.010% 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 it is excessively contained, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, when it is included, the upper limit of Ca is preferably 0.0050% and REM is preferably 0.010%. .

4). Manufacturing conditions Hereinafter, preferable manufacturing conditions in the present invention will be described.
As production conditions, it is preferable to define the heating temperature, hot rolling conditions, cooling conditions, and the like of the steel material. In particular, for hot rolling, in addition to the overall cumulative reduction ratio, the cumulative reduction ratio and the case where the sheet thickness central portion is in the austenite recrystallization temperature range and in the austenite non-recrystallization temperature range, respectively. It is preferable to define the average rolling reduction per pass. By defining these, desired characteristics can be obtained with respect to toughness in the surface layer portion and the central portion of the thick steel plate, and the degree of integration I of the RD // (110) plane and the strength at the 1/4 thickness portion. .

First, the molten steel having the above composition is melted in a converter or the like, and is made into a steel material by continuous casting or the like. Next, hot rolling is performed after the steel material is heated to a temperature of 900 to 1150 ° C.

In order to obtain good toughness, it is effective to lower the heating temperature and reduce the crystal grain size before rolling, but if the heating temperature is less than 900 ° C, sufficient time for rolling in the austenite recrystallization temperature range is secured. Can not. Further, if the temperature exceeds 1150 ° C., the austenite grains are coarsened and the toughness is lowered, and the oxidation loss becomes remarkable and the yield is lowered. Therefore, the heating temperature is preferably 900 to 1150 ° C. A more preferable heating temperature range is 1000 to 1100 ° C. from the viewpoint of toughness.

Generally, when a metal structure mainly composed of ferrite is obtained by carrying out normal austenite rolling, the target toughness is obtained, but there is sufficient transformation time when transforming from austenite to ferrite after rolling. Therefore, the resulting texture becomes random. Therefore, the integration degree I of the RD // (110) plane is 1.3 or more, preferably 1.6 or more in the plate thickness surface layer portion targeted in the present invention, and the RD // (110) plane in the center portion of the plate thickness. The degree of integration I is 1.8 or higher, preferably 2.0 or higher. Therefore, in the present invention, it is preferable to define hot rolling conditions as described below. As a result, even if the structure is mainly composed of ferrite, the degree of integration I of the RD // (110) plane is 1.3 or more, preferably 1.6 or more in the plate thickness surface layer portion, and RD // (110 in the plate thickness center portion. ) Surface integration degree I is 1.8 or more, preferably 2.0 or more.

In the hot rolling, first, in a state where the central portion of the plate thickness is in the austenite recrystallization temperature range, the rolling is performed so that the cumulative reduction rate is 20% or more and the average reduction rate per pass is 5.0% or less. Is preferred. By setting the cumulative rolling reduction to 20% or more, austenite is refined and the finally obtained metal structure is also refined to improve toughness. On the other hand, by setting the average rolling reduction per pass in this temperature range to 5.0% or less, strain can be introduced particularly in the vicinity of the surface layer of the steel material, and thereby RD // (110 in the plate thickness surface layer portion. ) Surface integration degree I can be 1.3 or more, preferably 1.6 or more, and the surface layer portion is further refined to obtain the effect of improving the toughness of the surface layer portion.

Next, it is preferable to perform rolling so that the cumulative reduction ratio is 40% or more and the average reduction ratio per pass is 7.0% or more in a state where the temperature at the center of the plate thickness is in the austenite non-recrystallization temperature range. By setting the cumulative rolling reduction in this temperature range to 40% or more, the texture at the center of the plate thickness is sufficiently developed. Further, by setting the average rolling reduction per pass to 7.0% or more, the integration degree I of the RD // (110) plane at the center of the plate thickness is set to 1.8 or more, preferably 2.0 or more. it can.

In addition, it is preferable that the cumulative rolling reduction is 65% or more as a whole by combining the austenite recrystallization temperature range and the austenite non-recrystallization temperature range. This is because by setting the total cumulative rolling reduction to 65% or more, a sufficient rolling reduction can be ensured for the structure, and the toughness and strength can achieve the target values.

The austenite recrystallization temperature range and the austenite non-recrystallization temperature range can be grasped by conducting a preliminary experiment in which the steel having the component composition is given a heat / working history with varying conditions.

In addition, although the completion | finish temperature of hot rolling is not specifically limited, From a viewpoint of rolling efficiency, it is preferable to complete | finish in the austenite non-recrystallization temperature range.

It is preferable that the rolled steel sheet is cooled to 600 ° C. or lower at a cooling rate of 4.0 ° C./s or higher. By setting the cooling rate to 4.0 ° C./s or more, a fine-grained structure can be obtained without coarsening the structure, and excellent target toughness can be obtained. When the cooling rate is less than 4.0 ° C./s, the structure becomes coarse, and the target toughness cannot be obtained. By setting the cooling stop temperature to 600 ° C. or less, the progress of recrystallization can be avoided, and the desired texture obtained by hot rolling and subsequent cooling can be maintained. When the cooling stop temperature is higher than 600 ° C., recrystallization proceeds even after the cooling stop and a desired texture cannot be obtained. 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.

It is also possible to perform a tempering process on the steel plate that has been cooled. By performing tempering, the toughness of the steel sheet can be further improved. A tempering temperature can be made not to impair the desired structure obtained by rolling and cooling by carrying out by making steel sheet average temperature below _AC1 point. In the present invention, the _AC1 point (° C.) is obtained by the following equation.
A _C1 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 can also be obtained by simulation calculation or the like from the sheet thickness, surface temperature, cooling conditions, etc., similarly to the temperature at the center of the sheet thickness.

Molten steel (steel symbols A to O) of each composition shown in Table 1 is melted in a converter and made into a steel material (slab thickness 250 mm) by a continuous casting method. After hot rolling to a plate thickness of 50 to 80 mm, cooling is performed. No. 1 to 29 test steels were obtained. Some were tempered after cooling. Table 2 shows hot rolling conditions, cooling conditions, and tempering conditions.

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 a yield point (Yield Strength) was obtained. Tensile Strength was measured.

Further, a JIS No. 4 impact test piece was taken from the surface layer part and the center part of the plate thickness (hereinafter also referred to as 1 / 2t part) so that the direction of the longitudinal axis of the test piece was parallel to the rolling direction, and the Charpy impact test was performed. The fracture surface transition temperature (vTrs) was obtained. Here, the impact test piece of the surface layer part is assumed to have a surface closest to the surface at a depth of 1 mm from the steel sheet surface.

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

Furthermore, the degree of integration I of the RD // (110) plane at the central portion of the plate thickness was determined as follows. First, a sample having a plate thickness of 1 mm was collected from the central portion of the plate thickness, and a test piece for X-ray diffraction was prepared by mechanically polishing and electrolytic polishing a surface parallel to the plate surface. Using this test piece, X-ray diffraction measurement was performed using a Mo ray source, and (200), (110) and (211) positive electrode dot diagrams were obtained. A three-dimensional crystal orientation density function was calculated from the obtained positive electrode dot diagram by the Bunge method. Next, from the obtained three-dimensional crystal orientation density function, (110) plane is parallel to the rolling direction in a total of 19 cross-sectional views at 5 ° intervals in the Bunge notation from ψ ₂ = 0 ° to 90 °. The integrated value was obtained by integrating the values of the three-dimensional crystal orientation density function of the orientation to be. A value obtained by dividing the integrated value by the integrated number of azimuths 19 was defined as an integration degree I of the RD // (110) plane.

Table 3 shows the results of these tests. In the case of a test steel plate (production Nos. 1 to 13, 27 to 29) in which the toughness value and texture in the surface layer portion and the center portion of the plate thickness are within the scope of the present invention, Kca (−10 ° C.) is 6000 N / mm ³ / ² or more, the brittle crack propagation stopping performance was excellent. Further, in the test steel plates (manufacturing numbers 1 to 13) in which the Charpy toughness values of the surface layer portion and the central portion of the plate thickness and the degree of integration I of the RD // (110) surface satisfy the formula (1) (Production numbers 1 to 13), (1) A higher Kca (−10 ° C.) value was obtained as compared with the test steel sheets (Product Nos. 27 to 29) not satisfying the formula.

On the other hand, in the case of the test steel sheets (manufacturing numbers 21 to 26) in which the manufacturing conditions are outside the scope of the present invention and the toughness or texture of the steel sheets does not satisfy the provisions of the present invention, the value of Kca (−10 ° C.) is 6000 N / mm ^{3 /} It was less than ² . The test steel sheets (Product Nos. 14 to 20) in which the component composition of the steel sheet was outside the preferred range of the present invention, the toughness of the steel sheet did not satisfy the provisions of the present invention, and the value of Kca (−10 ° C.) was 6000 N / mm. ^It did not reach ^3/2 .

1 Standard ESSO test piece 2 Notch 3 Crack 4 Tip shape 5 Base material

Claims (8)

The metal structure is mainly composed of ferrite, and the integration degree I of the RD // (110) plane in the plate thickness surface layer portion is 1.3 or more, and the integration degree I of the RD // (110) plane in the plate thickness center portion is 1.8. Brittle crack propagation stop characterized by having the above texture, Charpy fracture surface transition temperature vTrs in the surface layer portion being −60 ° C. or lower, and Charpy fracture surface transition temperature vTrs in the central portion of the plate thickness being −50 ° C. or lower. Structural high-strength thick steel plate with excellent characteristics. The brittle crack propagation stop characteristic according to claim 1, wherein the Charpy fracture surface transition temperature of the surface layer portion and the central portion of the plate thickness and the degree of integration I of the RD // (110) surface satisfy the following formula (1): Excellent structural high strength thick steel plate.
vTrs _{(surface layer)} + 1.9 × vTrs _{(1 / 2t)} −6 × I _{RD // (110) [surface layer]} −84 × I _{RD // (110) [1 / 2t]} ≦ −350 (1 )
vTrs _{(surface layer)} : Fracture surface transition temperature of surface layer (° C)
vTrs _{(1 / 2t)} : Fracture surface transition temperature at the thickness center (° C)
I _{RD // (110) [surface layer]} : Degree of integration of RD // (110) surface of surface layer portion I _{RD // (110) [1 / 2t]} : RD // (110) surface of central portion of plate thickness Integration degree Steel composition is mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.2%, Al: 0.005-0.08 %, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, Ti: 0.005-0.03%, with the balance being Fe and inevitable impurities The structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to any one of claims 1 and 2. The steel composition is further mass%, Nb: 0.005 to 0.05%, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, Cr: 0.01 to 0 0.5%, Mo: 0.01 to 0.5%, V: 0.001 to 0.10%, B: 0.0030% or less, Ca: 0.0050% or less, REM: 0.010% or less The structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to claim 3, comprising at least one kind. The steel material having the composition according to claim 3 is heated to a temperature of 900 to 1150 ° C., and the sum of the cumulative rolling reductions in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more, In a state where the part is in the austenite recrystallization temperature range, rolling is performed so that the cumulative reduction rate is 20% or more and the average reduction rate per pass is 5.0% or less, and then the central part of the thickness is austenite. Rolling with a cumulative reduction ratio of 40% or more and an average reduction ratio per pass of 7.0% or more in a state where the temperature is in the non-recrystallization temperature range, and then cooling at 4.0 ° C./s or more A method for producing a structural high-strength thick steel plate excellent in brittle crack propagation stopping characteristics, characterized by accelerated cooling to 600 ° C. or lower at a speed. After accelerated cooling to 600 ° C. or less, further, the method of producing a high strength steel plate for better structure to brittle crack propagation stop characteristics of claim 5 including the step of tempering at a temperature below point _C1 A. The steel material having the composition according to claim 4 is heated to a temperature of 900 to 1150 ° C., and the sum of the cumulative reduction ratios in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more, In a state where the part is in the austenite recrystallization temperature range, rolling is performed so that the cumulative reduction rate is 20% or more and the average reduction rate per pass is 5.0% or less, and then the central part of the thickness is austenite. Rolling with a cumulative reduction ratio of 40% or more and an average reduction ratio per pass of 7.0% or more in a state where the temperature is in the non-recrystallization temperature range, and then cooling at 4.0 ° C./s or more A method for producing a structural high-strength thick steel plate excellent in brittle crack propagation stopping characteristics, characterized by accelerated cooling to 600 ° C. or lower at a speed. The method for producing a structural high-strength thick steel sheet having excellent brittle crack propagation stopping characteristics according to claim 7, further comprising a step of tempering to 600 ° C. or lower and further tempering to a temperature of AC ₁ point or lower.

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