WO2013099318A1 - 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 main constituent of the metal structure is bainite; the steel plate has an aggregate structure in which the density (I) in the RD//(110) plane at the plate thickness center is at least 1.5; and the Charpy fracture appearance transition temperature (vTrs) at the surface and plate thickness center is not more than -40°C. More preferably, the Charpy fracture appearance transition temperature (vTrs) at the plate thickness center, and the density (I) in the RD//(110) plane satisfy formula (1). vTrs_(1/2t) - 12 × I_{RD//(110)[1/2t]} ≤ -70 ...(1), wherein, vTrs_(1/2t) is the fracture appearance transition temperature at the plate thickness center (°C), and I_{RD//(110)[1/2t]} is the density in the RD//(110) plane at the plate thickness center.

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 plate 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.

Conventionally, a method of increasing the Ni content is known as a means for improving the brittle crack propagation stopping property of steel. 9% Ni steel is used on a commercial scale in liquefied natural gas storage tanks.

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, TMCP (Thermo-Mechanical Control Process) 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. ) Method, fine graining can be achieved, low temperature toughness can be improved, and excellent brittle crack propagation stopping characteristics can be imparted.

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 excellent in the brittle crack propagation stopping characteristics described in Patent Document 1 has an effect of improving the brittle crack propagation stopping characteristics due to shear lip (plastic deformation region shear-lips) generated in the steel 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. Non-Patent Document 1 evaluates the brittle crack propagation stopping performance of a steel plate having a thickness of 65 mm, and reports a result that the brittle crack does not stop in a large-scale brittle crack propagation stopping test of the base material.

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 above-described steel sheets with excellent brittle crack propagation stopping characteristics described in Patent Documents 1 to 5 are mainly applied to manufacturing conditions and disclosed experimental data up to a plate thickness of about 50 mm, and applied to thick materials exceeding 50 mm. In this case, 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. A standard ESSO test was performed on a thick steel plate having a thickness of more than 50 mm, and it was confirmed that high arrestability was obtained when a short crack branch 3a as shown schematically in FIG. 1A was confirmed. . It is presumed that the stress is relieved by the crack branch 3a. 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 make the structure form to branch the crack. A steel structure mainly composed of bainite having a packet or the like inside is more advantageous than a steel structure mainly composed of ferrite. It is also effective to accumulate the (100) plane, which is a cleavage plane, obliquely with respect to the rolling direction or the plate width direction, which is the crack propagation direction.
3. As a result of detailed observation and analysis of the fracture surface of the standard ESSO test, it is effective to improve the arrest performance by controlling the material of the central part of the plate thickness that becomes the tip of the crack. In particular, it is effective to satisfy the following formula (1) as an index relating to the toughness and texture of the central portion of the plate thickness.
vTrs _{(1 / 2t)} −12 × I _{RD // (110) [1 / 2t]} ≦ −70 (1)
vTrs _{(1 / 2t)} : Fracture surface transition temperature at the thickness center (° C)
I _{RD // (110) [1 / 2t]} : Degree of integration of RD // (110) plane at the center of the plate thickness t: Plate thickness (mm)
4). Furthermore, the structure is refined by carrying out rolling with a cumulative rolling reduction of 20% or more in a state where the temperature is in the austenite recrystallization temperature range. Thereafter, the cumulative rolling reduction is set to 40% or more in a state in the austenite non-recrystallization temperature region. In addition, by rolling the difference between the rolling temperature of the first pass and the rolling temperature of the last pass within 40 ° C., the texture at the center of the plate thickness can be controlled, and the above-described structure 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 bainite, and has a texture with an accumulation degree I of RD // (110) plane (Rolling Direction parallel to (110) plane) at the center of the plate thickness of 1.5 or more, and a surface layer portion and A structural high-strength thick steel plate excellent in brittle crack propagation stop characteristics, characterized in that the Charpy fracture surface transition temperature at the center of the plate thickness is vTrs ≦ −40 ° C.
2. The structural high strength thickness excellent in brittle crack propagation stopping characteristics according to 1, wherein the Charpy toughness value at the center of the plate thickness and the degree of integration I of the RD // (110) plane satisfy the following formula (1): steel sheet.
vTrs _{(1 / 2t)} −12 × I _{RD // (110) [1 / 2t]} ≦ −70 (1)
vTrs _{(1 / 2t)} : Fracture surface transition temperature at the thickness center (° C)
I _{RD // (110) [1 / 2t]} : Degree of integration of RD // (110) plane at the center of the plate thickness t: Plate thickness (mm)
3. Steel composition is mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.5%, 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, containing at least one of them.
The steel material (slab) having the composition described in either 5.3 or 4 is heated to a temperature of 1000 to 1200 ° C., and the sum of the cumulative reduction ratios in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is Roll 65% or more. At this time, the cumulative rolling reduction is 20% or more in a state where the center portion of the plate thickness is in the austenite recrystallization temperature region. Then, in the state where the plate thickness central portion is in the austenite non-recrystallization temperature range, the cumulative rolling reduction is 40% or more, and the first rolling in the state where the plate thickness central portion is in the austenite non-recrystallization temperature range Rolling is performed within a difference of 40 ° C. between the rolling temperature of the pass and the rolling temperature of the last pass. Then, it cools to 450 degrees C or less with the cooling rate of 4 degrees C / s or more, The manufacturing method of the structural high strength thick steel plate excellent in the brittle crack propagation stop characteristic.
6.450 ° C. After accelerated cooling below, further comprising the step of tempering at a temperature below A _c1 point, the method of producing a high strength thick steel plate superior in brittle crack propagation stop characteristics described 5.

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 stop characteristics, and preferably exceeds 50 mm. More preferably, it is effective to apply to a steel plate 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 at the center of the plate thickness Define the metallographic structure.
1. Toughness and texture In the present invention, in order to improve the crack propagation stop characteristic for cracks that progress in the horizontal direction (in-plane direction of the steel sheet) such as the rolling direction or the direction perpendicular to the rolling direction, the toughness and RD at the center of the sheet thickness are improved. // The degree of integration I on the (100) plane is appropriately defined according to the desired brittle crack propagation stop characteristics.

First, since good base material toughness is a precondition for suppressing the progress of cracks, in the steel sheet according to the present invention, the Charpy fracture surface transition temperature in the surface layer part and the center part of the plate thickness is specified to be −40 ° C. or lower. To do. In addition, it is preferable that the Charpy fracture surface transition temperature in a plate | board thickness center part is -50 degrees C or less.

By developing the degree of integration I of the RD // (100) plane, the cleaved surface is accumulated obliquely with respect to the main crack direction, and the effect of stress relaxation at the brittle crack tip by generating fine crack branching causes brittle cracks. Propagation stop performance is improved.

A brittle crack propagation stopping performance, which is a target for ensuring structural safety, is a thick material exceeding 50 mm thick that has been used for hull outer plates such as recent container ships and bulk carriers: Kca (−10 In order to obtain (° C.) ≧ 6000 N / mm ^3/2 , it is necessary that the integration degree I of the RD // (110) plane at the central portion of the plate thickness is 1.5 or more, preferably 1.7 or more.

Here, the degree of integration I of the RD // (110) plane in the central portion of the plate thickness indicates the following. First, a sample having a thickness of 1 mm is collected from the central portion of the plate thickness, and a specimen parallel to the plate surface is mechanically polished / electrolytic polished to prepare a test piece for X-ray diffraction. . Using this test piece, an X-ray diffraction measurement (X-ray diffraction measurement) was performed using a Mo ray source, and (200), (110) and (211) positive figures (pole figures) were 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.

It is preferable that the Charpy toughness value at the center of the plate thickness and the degree of integration I on the RD // (110) plane satisfy the following expression (1) in addition to the above-mentioned base material toughness and texture definition. By satisfying the following formula (1), further excellent brittle crack propagation stopping performance can be obtained.
vTrs _{(1 / 2t)} −12 × I _{RD // (110) [1 / 2t]} ≦ −70 (1)
vTrs _{(1 / 2t)} : Fracture surface transition temperature at the thickness center (° C)
I _{RD // (110) [1 / 2t]} : RD // (110) integration at the center of the plate thickness t: plate thickness (mm)

2. Metal structure In the present invention, the metal structure is mainly composed of bainite. The fact that the metal structure is mainly bainite means that the area fraction of the bainite phase is 80% or more of the whole. The balance is 20% or less of the total area fraction of ferrite, martensite (including island martensite), pearlite, and the like.
In order to obtain the above toughness and texture, it is effective to transform into bainite after performing controlled rolling in the austenite non-recrystallization temperature range. When transforming from austenite to ferrite after rolling, the desired toughness can be obtained, but because there is sufficient transformation time when transforming from austenite to ferrite, the resulting texture becomes random, and the target The degree of integration I on the RD // (110) plane is not more than 1.5, preferably not less than 1.7. On the other hand, when the structure rolled in the austenite non-recrystallization temperature region is transformed into bainite, the transformation time is not sufficient, and a so-called variant is selected in which a texture with a specific orientation is preferentially formed. Is performed, it is possible to obtain an integration degree I of RD // (110) plane of 1.5 or more, preferably 1.7 or more. For this reason, the metal structure obtained after rolling and cooling is mainly bainite.

3. Chemical Components 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%. More 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 preferably 0.03% or more and 0.5% or less.

Mn: 0.5 to 2.5%
Mn is added as a strengthening element. If the content is less than 0.5%, the effect is not sufficient. If the content exceeds 2.5%, the weldability is deteriorated and the steel material cost is increased.

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%, 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, but inclusion 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.

The above is the basic component composition of 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 region in rolling in the austenite region, and contributes to the fine graining of the bainite packet, which is also 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 manufacturing conditions, it is preferable to define the heating temperature, hot rolling conditions, cooling conditions, and the like of the steel material (slab). In particular, for hot rolling, in addition to the cumulative reduction ratio in the sum of the austenite recrystallization temperature range and the austenite non-recrystallization temperature range, the case where the central portion of the plate thickness is in the austenite recrystallization temperature range, It is preferable to define the cumulative rolling reduction for each of the cases in the temperature range and the rolling temperature conditions in a state where the plate thickness center portion is in the austenite non-recrystallized region. By defining these, the toughness at the surface layer portion and the central portion of the plate thickness, the RD // (110) integration degree I at the central portion of the plate thickness, and the strength at ¼ portion of the plate thickness can be obtained as desired. Can be a value.

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, the steel material is heated to a temperature of 1000 to 1200 ° C. and then hot rolled.

If the heating temperature is less than 1000 ° C., sufficient time for rolling in the austenite recrystallization temperature region cannot be secured. If the temperature exceeds 1200 ° C., the austenite grains become coarse, leading to a decrease in toughness, as well as significant oxidation loss and a decrease in yield. Therefore, the heating temperature is preferably 1000 to 1200 ° C. A more preferable heating temperature range is from 1000 to 1150 ° C. from the viewpoint of toughness.

In the present invention, as described below, it is preferable to define hot rolling conditions and subsequent cooling conditions. As a result, the structure rolled in the austenite non-recrystallization temperature region is transformed into bainite, so the transformation time in this case is not sufficient, so the so-called variant selection in which a texture with a specific orientation is preferentially formed. Is performed, the integration degree I of the RD // (110) plane can be 1.5 or more, preferably 1.7 or more.

In the hot rolling, first, it is preferable to perform rolling with a cumulative reduction ratio of 20% or more in a state where the central portion of the plate thickness is in the austenite recrystallization temperature region. 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. If the cumulative rolling reduction is less than 20%, austenite is not sufficiently refined and the toughness is not improved in the finally obtained structure.

Next, it is preferable to perform rolling at a cumulative reduction of 40% 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 reduction ratio in this temperature range to 40% or more, the texture at the central portion of the plate thickness is sufficiently developed, and the integration degree I of the RD // (110) plane at the central portion of the plate thickness is 1.5. As mentioned above, Preferably it can be 1.7 or more.

In addition, if it takes too much time for rolling in a state where the temperature at the center of the plate thickness is in the austenite non-recrystallization temperature range, the structure becomes coarse and the toughness is lowered. Therefore, it is preferable that the difference between the rolling temperature of the first pass and the rolling temperature of the last pass in the rolling in a state where the central portion of the plate thickness is in the austenite non-recrystallized region is within 40 ° C. Here, the rolling temperature refers to the temperature at the center of the plate thickness of the steel just before rolling. The temperature at the center of the plate thickness is obtained by simulation calculation or the like from the plate thickness, surface temperature, thermal history, 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 cumulative rolling reduction is preferably 65% or more as a whole by combining the austenite recrystallization temperature range and the austenite non-recrystallization temperature range. When the overall rolling reduction is small, the rolling of the structure is not sufficient, and the toughness and strength cannot achieve the target values. 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 450 ° C. or lower at a cooling rate of 4 ° C./s or higher. By making the cooling rate 4 ° C./s or more, the structure is not coarsened, and by suppressing the ferrite transformation, a fine-grained bainite structure is obtained, and the excellent excellent toughness and texture are obtained. Strength can be obtained. When the cooling rate is less than 4 ° C./s, coarsening of the structure and ferrite transformation proceed at each plate thickness position, so that not only a desired structure cannot be obtained but also the strength of the steel sheet is lowered. By setting the cooling stop temperature to 450 ° C. or less, the bainite transformation can be sufficiently advanced, and a metal structure having desired toughness and texture can be obtained. If the cooling stop temperature is higher than 450 ° C., the bainite transformation does not proceed sufficiently, and a structure such as ferrite or pearlite is also produced, and the bainite-based structure intended by the present invention 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 implementing as 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, made into a steel material (slab thickness 250 mm) by a continuous casting method, hot-rolled to a sheet thickness of 50 to 90 mm, and then cooled. No. 1-30 test steels were obtained. Table 2 shows hot rolling conditions and cooling conditions.

With respect to the obtained thick steel plate, a JIS 14A test piece having a diameter of 14 mm was collected from a 1/4 part of the plate thickness so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and a tensile test was performed, yield point (Yield Strength). Tensile Strength was measured.

Also, a JIS No. 4 impact test piece was taken from 1/2 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. 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 sheet (manufacturing numbers 1 to 13, 27 to 30) in which the toughness value and texture at the center of the plate thickness are within the scope of the present invention, Kca (−10 ° C.) is 6000 N / mm ^3/2 or more. Excellent brittle crack propagation stopping performance was demonstrated. Further, in the test steel sheets (manufacturing numbers 1 to 13) in which the Charpy toughness value and RD // (110) integration degree I of the surface layer portion and the center portion of the plate thickness satisfy the equation (1), the equation (1) is A higher Kca (−10 ° C.) value was obtained as compared with the test steel plates (Product Nos. 27 to 30) which were not satisfied.

On the other hand, although the component composition of the steel sheet is within the preferable range of the present invention, the steel sheet (manufacturing numbers 21 to 26) whose heating and rolling conditions in the manufacturing conditions of the steel sheet deviate from the preferable range of the present invention is Kca (−10 ° C.). The value did not reach 6000 N / mm ^3/2 . In the steel plate (manufacturing numbers 22, 23, and 26), the texture of the steel plate does not satisfy the provisions of the present invention. For the test steel sheets (Production Nos. 14 to 20) in which the component composition of the steel sheets was outside the preferred range of the present invention, the toughness of the steel sheets did not satisfy the provisions of the present invention, and the value of Kca (−10 ° C.) was 6000 N / It did not reach mm ^3/2 .

Further, in the case of a test steel plate (manufacturing number 14 to 26) in which at least one of the toughness value and texture in the central portion of the plate thickness is outside the scope of the present invention, Kca (−10 ° C.) is 6000 N / mm ^{3 / 2} was not reached.

1 Standard ESSO Test Piece 2 Notch 3 Crack 3a Branch 4 Tip Shape 5 Base Material

Claims (8)

1. The metal structure is mainly bainite, the RD // (110) plane has a texture I of 1.5 or more in the central part of the sheet thickness, and the Charpy fracture surface transition temperature in the surface layer part and the central part of the sheet thickness. A structural high-strength thick steel plate excellent in brittle crack propagation stopping characteristics, characterized in that vTrs is −40 ° C. or lower.
2. The structure excellent in brittle crack propagation stopping characteristics according to claim 1, wherein the Charpy fracture surface transition temperature in the central 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 _{(1 / 2t)} −12 × I _{RD // (110) [1 / 2t]} ≦ −70 (1)
   vTrs _{(1 / 2t)} : Fracture surface transition temperature at the thickness center (° C)
   I _{RD // (110) [1 / 2t]} : Degree of integration of RD // (110) plane at the center of the plate thickness t: Plate thickness (mm)
3. Steel composition is mass%, C: 0.03-0.20%, Si: 0.03-0.5%, Mn: 0.5-2.5%, 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.
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 The structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to claim 3, comprising at least one of them.
5. The steel material having the composition according to claim 3 is heated to a temperature of 1000 to 1200 ° C., and rolling is performed such that 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 this state, the cumulative reduction ratio is 20% or more in the state where the central portion of the plate thickness is in the austenite recrystallization temperature range, and then the cumulative reduction in the state where the central portion of the plate thickness is in the austenite non-recrystallization temperature region. The difference between the rolling temperature of the first pass and the rolling temperature of the last pass in rolling in a state where the rate is 40% or more and the sheet thickness central portion is in the austenite non-recrystallization temperature range is within 40 ° C, Then, it cools to 450 degrees C or less with the cooling rate of 4 degrees C / s or more, The manufacturing method of the structural high strength thick steel plate excellent in the brittle crack propagation stop characteristic.
6. The method for producing a high-strength structural steel plate excellent in brittle crack propagation stopping property according to claim 5, further comprising a step of tempering to a temperature of not more than _Ac1 after accelerated cooling to 450 ° C or lower.
7. The steel material having the composition according to claim 4 is heated to a temperature of 1000 to 1200 ° C., and rolling is performed in which the total rolling reduction in the austenite recrystallization temperature range and the austenite non-recrystallization temperature range is 65% or more. In this state, the cumulative reduction ratio is 20% or more in the state where the central portion of the plate thickness is in the austenite recrystallization temperature range, and then the cumulative reduction in the state where the central portion of the plate thickness is in the austenite non-recrystallization temperature region. The difference between the rolling temperature of the first pass and the rolling temperature of the last pass in rolling in a state where the rate is 40% or more and the sheet thickness central portion is in the austenite non-recrystallization temperature range is within 40 ° C, Then, it cools to 450 degrees C or less with the cooling rate of 4 degrees C / s or more, The manufacturing method of the structural high strength thick steel plate excellent in the brittle crack propagation stop characteristic.
8. The method for producing a structural high-strength thick steel plate having excellent brittle crack propagation stopping characteristics according to claim 7, further comprising a step of tempering to a temperature of not more than _{Ac 1} point after accelerated cooling to 450 ° C. or less.

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