EP2799584B1

EP2799584B1 - Production method for a high-strength thick steel plate for construction having excellent characteristics for preventing diffusion of brittle cracks,

Info

Publication number: EP2799584B1
Application number: EP12863408.6A
Authority: EP
Inventors: Yoshiko TAKEUCHI; Kazukuni Hase; Shinji Mitao; Yoshiaki Murakami
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2011-12-27
Filing date: 2012-05-18
Publication date: 2019-01-02
Anticipated expiration: 2032-05-18
Also published as: KR101588258B1; JP2013151732A; BR112014015779B1; KR20140097463A; EP2799584A4; EP2799584A1; BR112014015779A2; CN104024462A; WO2013099318A1; JP5304925B2; BR112014015779A8; CN104024462B

Description

[Technical Field]

The present invention relates to a method for manufacturing a high-strength thick steel plate for structural use excellent in terms of brittle crack arrestability, and in particular to a method for manufacturing a steel plate having a thickness of 50 mm or more which can be preferably used for ships.

[Background Art]

In the case of large-scale structures such as ships, since an accident due to a brittle fracture has a great effect on economy and environment, improvement of safety is always demanded and steel materials used for the structures are required to have good toughness and brittle crack arrestability at a temperature at which the steel materials are used.
In the case of ships such as container carriers and bulk carriers, high-strength steel plates having a heavy thickness are used for the outer plates of the ships' hulls in order to achieve sufficient structural strength, and recently, there is a growing trend toward increasing the strength and thickness of steel materials due to an increase in the size of ships' hulls. Generally, since there is a tendency for the brittle crack arrestability of a steel plate to decrease with increasing strength or thickness of the steel plate, there is a growing demand for improved brittle crack arrestability.
As a method for improving the brittle crack arrestability of a steel material, a method in which Ni content is increased has been known in the past. 9%-Ni steel is commercially used for the storage tanks of liquefied natural gases.
However, since an increase in the amount of Ni added is inevitably accompanied by a large increase in cost, it is difficult to apply the Ni containing steel to any usage other than LNG storage tanks.
On the other hand, in the case of a comparatively thin steel plate having a thickness of less than 50 mm which is applied to ships and line pipes which are not subjected to such an ultra low temperature as that of LNG, it is possible to provide the steel plate with excellent brittle crack arrestability by decreasing a grain size by a TMCP (ThermoMechanical Control Process) method in order to improve low-temperature toughness.
In addition, Patent Literature 1 proposes a steel material having an ultra fine crystallization microstructure in the surface portion in order to improve brittle crack arrestability without an increase in alloy cost.
The steel material having excellent brittle crack arrestability according to Patent Literature 1 is characterized in that, focusing on the fact that shear-lips (plastic deformation areas), which are formed in the surface portion of a steel material when a brittle crack propagates, are effective for improving brittle crack arrestability, the crystal grain size in the shear-lip portions is decreased in order to absorb the propagation energy of a propagating brittle crack.
It is disclosed that, regarding a method for manufacturing the steel material, an ultra fine ferrite structure or bainite structure is formed in the surface portion of the steel material by repeating once or more a process, in which the surface portion of a hot-rolled steel plate is cooled down to a temperature equal to or lower than the A_r3 transformation point by performing controlled cooling and then the controlled cooling is stopped in order to allow the surface portion to recuperate to have a temperature equal to or higher than the transformation point, while the steel material is rolled in order for transformation or recrystallization due to deformation to repeatedly occur.
Moreover, in Patent Literature 2, it is disclosed that, in order to improve the brittle crack arrestability of a steel material having a microstructure mainly including a ferrite-pearlite phase, it is important to form a layer, in either of the surface portions of the steel material, including 50% or more of a ferrite structure having ferrite grains with a circle-equivalent average grain size of 5 µm or less and an aspect ratio of the grains of 2 or more, and to prevent the variation of a ferrite grain size, and that, as a method for preventing the variation, the maximum rolling reduction per pass of finishing rolling is controlled to be 12% or less in order to prevent local recrystallization.
However, in the case of the steel materials having excellent brittle crack arrestability according to Patent Literatures 1 and 2, since the specified microstructure is formed by once cooling only the surface portion of the steel material, by allowing the cooled surface portion to recuperate and by conducting processing of the steel material at the time of the recuperation, it is not easy to perform control on a practical production scale, and, in particular in the case of a thick material having a thickness of more than 50 mm, loads applied by this processing on rolling and cooling equipment are heavy.
On the other hand, Patent Literature 3 discloses a technique which is a modification of TMCP and in which, focusing not only on a decrease in ferrite crystal grain size but also on a subgrain formed in a ferrite grain, brittle crack arrestability is improved.
Specifically, for the case of a thickness of 30 to 40 mm, without the necessity of complicated temperature controlling such as the cooling and recuperation of the surface of a steel plate, brittle crack arrestability is improved by controlling (a) rolling conditions such that fine ferrite crystal grains are achieved, (b) rolling conditions such that a fine ferrite structure is formed in a portion constituting 5% or more of the thickness of the steel material, (c) rolling conditions such that subgrains are formed by growing a texture in the fine ferrite structure and by rearranging dislocations introduced by applying deformation (rolling) using thermal energy and (d) cooling conditions such that an increase in the grain size of the formed fine ferrite crystal grains and an increase in the grain size of the formed fine subgrains are prevented.
In addition, in controlled rolling, a method, in which brittle crack arrestability is improved by applying reduction forth of rolling to a transformed ferrite phase in order to grow a texture, is also known. In this case, resistance to brittle fracture is increased by forming a separation parallel to the plate surface on the fracture surface of a steel material in order to reduce stress at the brittle crack tip.
For example, Patent Literature 4 discloses that brittle fracture resistance is improved by performing controlled rolling in order to form a microstructure having an X-ray plane intensity ratio in the (110) plane showing a texture developing degree of 2 or more and including large-size grains having a diameter equivalent to a circle in the crystal grains of 20 µm or more in an amount of 10% or less.
Patent Literature 5 discloses, as a steel for welded structural use having excellent brittle crack arrestability in the joint part, a steel plate having an X-ray plane intensity ratio in the (100) plane showing a texture developing degree on a plane inside the plate parallel to the rolling surface of the plate of 1.5 or more. It is disclosed that the steel plate has excellent brittle crack arrestability owing to the difference in angle between the direction of applied stress and the direction of crack propagation as a result of the growth of the texture mentioned above.

[Citation List]

[Patent Literature]

[PTL 1] Japanese Examined Patent Application Publication No. 7-100814
[PTL 2] Japanese Unexamined Patent Application Publication No. 2002-256375
[PTL 3] Japanese Patent No. 3467767
[PTL 4] Japanese Patent No. 3548349
[PTL 5] Japanese Patent No. 2659661

JP 2007-254767 discloses a welded joint for a high-tensile strength thick steel plate with a thickness of 35 mm or more having both excellent fracture toughness (Kc) and brittle-fracture crack-propagation arrest toughness (Kca), wherein the steel plate has a composition consisting of, by mass%, C: 0.01-0.3%, Si: 0.01-2%, Mn: 0.1-3%, Al: 0.003-0.1%, N: 0.001-0.01%, P: 0.02% or less, S: 0.01% or less, and the balance Fe with unavoidable impurities.
JP 2011-052243 discloses a method for producing a high-tensile strength thick steel plate for structural use having sufficient brittle crack arrestability, comprising heating a steel slab having an appropriate chemical composition to 950-1150°C, rough-rolling the steel slab with a cumulative rolling reduction of 30% or more at 900°C or higher, finish-rolling the rough-rolled material with a cumulative rolling reduction of 40% or more within a surface temperature range of Ar₃ to Ar₃ + 60°C, subsequently acceleration-cooling the surface to 200°C or lower from a temperature of Ar₃ or higher with a cooling rate of 8°C/s or more by average in a sheet thickness and then tempering the steel sheet.

[Non Patent Literature]

[NPL 1] Inoue et al.: Long Brittle Crack Propagation of Heavy-Thick Shipbuilding Steels, Conference proceedings, the Japan Society of Naval Architects and Ocean Engineers (3), 2006, pp. 359 to 362

[Summary of Invention]

[Technical Problem]

Nowadays, a thick steel plate having a thickness of more than 50 mm is used for a mega-container carrier of more than 6,000 TEU (twenty-foot equivalent unit). In Non Patent Literature 1, it is reported that, from the results of the evaluation of the brittle crack arrestability of a steel plate having a thickness of 65 mm, a brittle crack was not arrested in a large brittle crack arrestability test on a base metal.
In addition, it is reported that, from the results of an ESSO test compliant with WES 3003 on the sample, the value of Kca at an operating temperature of -10°C (hereinafter, also expressed by Kca(-10°C)) was less than 3000 N/mm^3/2, which indicates that it is a problem to be solved to ensure the safety of a ship's hull structure built using a steel sheet having a thickness of more than 50 mm.
The steel plates having excellent brittle crack arrestability according to Patent Literatures 1 through 5 described above are mainly intended for a steel plate having a thickness of about 50 mm or less as indicated by the manufacturing conditions and the disclosed experimental data, and therefore it is not clear whether specified properties can be obtained in the case where the disclosed techniques are applied to a steel plate having a thickness of more than 50 mm, and the properties regarding crack propagation in the thickness direction which are required for ship's hull structures have never been tested at all.
Therefore, an object of the present invention is to provide a method for manufacturing a high-strength thick steel plate excellent in terms of brittle crack arrestability which can be stably manufactured using a very simple industrial process in which rolling conditions are optimized in order to control a texture in the thickness direction.

[Solution to Problem]

The present inventors diligently conducted investigations in order to solve the problems described above and found the following knowledge regarding a high-strength thick steel plate having excellent crack arrestability despite the steel plate having a heavy thickness.

1. From the results of an ESSO test compliant with WES 3003 using a thick steel plate having a thickness of more than 50 mm, it was confirmed that, in the case where a short branched crack 3a as schematically illustrated in Fig. 1(a) was recognized, high arrestability was achieved. It is thought to be because stress was relaxed by the branched crack 3a. Figs. 1(a) and 1(b) are schematic diagrams illustrating that a crack 3 which had penetrated from a notch 2 of a test piece 1 for an ESSO test compliant with WES 3003 was arrested with a crack tip shape 4 being left in a base metal 5.
2. In order to obtain the fracture surface shape as described above, it is necessary to form a microstructure in which a crack can branch. It is more advantageous to form a steel microstructure mainly including a bainite phase, in which, for example, packets are present, than a steel microstructure mainly including a ferrite phase. In addition, it is effective to integrate the (100) plane, which is a cleavage plane, diagonally to the rolling direction or the width direction which is a crack propagation direction.
3. From the results of the close observations and analyses regarding the fracture surface in an ESSO test compliant with WES 3003, it was found that it is effective to control the material properties in the central portion in the thickness direction where the crack tip exists in order to improve arrestability. In particular, it is effective for indexes regarding toughness and texture in the central portion in the thickness direction to satisfy relational expression (1) below: ${vTrs}_{(1 / 2 t)} - 12 \times I_{RD / / (110) [1 / 2 t]} \leq - 70$
where vTrs_(1/2t): fracture appearance transition temperature (°C) in the central portion in the thickness direction, I_{RD//(110)[1/2t]} : integration degree of the RD//(110) plane in the central portion in the thickness direction, and t: thickness (mm).
4. Moreover, rolling is performed under the conditions that the cumulative rolling reduction is 20% or more in the austenite recrystallization temperature range in order to decrease a crystal grain size in a microstructure.
Subsequently, rolling is performed under the conditions that the cumulative rolling reduction is 40% or more in the austenite non-recrystallization temperature range. In addition, the difference in rolling temperature between the first and the last passes is 40°C or less, and thereby the texture in the central portion in the thickness direction is controlled so that the microstructure described above can be realized.

Further investigations have been conducted on the basis of the obtained knowledge and, as a result, the present invention has been completed. That is to say, the present invention is defined in the claims.

[Advantageous Effects of Invention]

According to the present invention, a method for manufacturing a high-strength thick steel plate having a thickness of 50 mm or more excellent in terms of brittle crack arrestability, in which a texture is appropriately controlled in the thickness direction can be provided, and it is effective to apply the present invention to a steel plate having a thickness of preferably more than 50 mm, more preferably 55 mm or more. In addition, in the field of shipbuilding, the present invention contributes to the improvement of the safety of ships by being applied to hutch side coamings and deck part materials in high-strength deck structures of large container carriers and bulk carriers, which results in a large advantage in industry.

[Brief Description of Drawings]

[Fig. 1] Fig. 1 is a schematic diagram illustrating the fracture surface shape of an ESSO test compliant with WES 3003 of a thick steel plate having a thickness of more than 50 mm, where (a) is a diagram illustrating a plane view of a test piece and (b) is a diagram illustrating the fracture surface of the test piece.

[Description of Embodiments]

In the present disclosure, 1. toughness and a texture in the central portion in the thickness direction and 2. a metallographic structure are specified.

1. Toughness and texture

In the present disclosure, in order to increase crack arrestability against a crack propagating in the horizontal direction (planar direction of a steel plate) such as the rolling direction or a direction at a right angle to the rolling direction, toughness and the integration degree I of the RD//(110) plane in the central portion in the thickness direction are appropriately specified in accordance with desired brittle crack arrestability.
Firstly, since it is prerequisite that the toughness of a base metal be excellent in order to prevent crack propagation, in the case of the steel plate manufactured according to the present invention, it is specified that a Charpy fracture appearance transition temperature in the surface portion and the central portion in the thickness direction be -40°C or lower. It is preferable that the Charpy fracture appearance transition temperature in the central portion in the thickness direction be -50°C or lower.
By increasing the integration degree I of the RD//(110) plane, cleavage planes are integrated diagonally to the main direction of a crack so as to form fine branched cracks, which brings a stress relaxation effect at a brittle crack tip and results in an increase in brittle crack arrestability.
In order to achieve a Kca(-10°C) of 6000 N/mm^3/2 or more which represents a target brittle crack arrestability in order to ensure the structural safety of a thick steel plate having a thickness of more than 50 mm which is increasingly being used nowadays for the outer plates of ships' hulls of container carriers and bulk carriers, it is necessary that the integration degree I of the RD//(110) plane in the central portion in the thickness direction be 1.5 or more, preferably 1.7 or more.
Here, the integration degree I of the RD//(110) plane in the central portion in the thickness direction is defined in the following way. Firstly, by performing mechanical polishing and electrolytic polishing on a surface, being parallel to the steel plate surface, of a sample having a thickness of 1 mm cut out of the central portion in the thickness direction, a test piece for X-ray diffractometry is prepared. By performing X-ray diffraction measurement using a Mo X-ray source on this test piece, the pole figures of (200), (110) and (211) planes are obtained, and then three dimensional orientation distribution function is calculated from the obtained pole figures by using a Bunge method. Subsequently, using the calculated three dimensional orientation distribution function, by integrating the values of the three dimensional orientation distribution function in the orientation in which (110) plane is parallel to the rolling direction in 19 cross sections in total which are selected at intervals of 5° in the range from φ ₂ = 0° to φ ₂ = 90° in terms of Bunge notation, an integrated value is obtained. The value of the integrated value divided by the number of orientations which have been selected for the integration is called the integration degree I of the RD//(110) plane.
In addition to the specification of the toughness and the texture of a base metal described above, it is preferable that the Charpy fracture appearance transition temperature and the integration degree I of the RD//(110) plane in the central portion in the thickness direction satisfy the relational expression (1) below. As a result of the relational expression (1) being satisfied, it is possible to achieve better brittle crack arrestability. ${vTrs}_{(1 / 2 t)} - 12 \times I_{RD / / (110) [1 / 2 t]} \leq - 70$
where vTrs_(1/2t): fracture appearance transition temperature (°C) in the central portion in the thickness direction, I_{RD//(110)[1/2t]}: integration degree of the RD//(110) plane in the central portion in the thickness direction, and t: thickness (mm).

2. Metallographic structure

In the present disclosure, a metallographic structure mainly includes a bainite phase. Here, in the present disclosure, "a metallographic structure mainly includes a bainite phase" means that the area fraction of a bainite phase is 80% or more with respect to the whole metallographic structure. The area fraction of the remainder consisting of, for example, a ferrite phase, a martensite phase (including martensite islands) and a pearlite phase is 20% or less.
In order to obtain the toughness and the texture described above, it is effective, after controlled rolling has been performed in the austenite non-recrystallization. temperature range, to perform transformation into a bainite phase. In the case where an austenite phase is transformed into a bainite phase after the rolling, although the desired toughness can be obtained, since there is sufficient transformation time for an austenite phase to transform into a ferrite phase, the obtained texture becomes a random structure, which results in the target value of the integration degree I of the RD//(110) plane of 1.5 or more, preferably 1.7 or more, is not achieved. On the other hand, in the case where the microstructure formed in the rolling performed in the austenite non-recrystallization temperature range is transformed into a bainite phase, since there is not sufficient transformation time, a texture having a specified orientation is preferentially formed, that is, so-called variant selection occurs, which results in the integration degree I of the RD//(110) plane becoming 1.5 or more, preferably 1.7 or more. Therefore, the metallographic structure, which is obtained after rolling and cooling have been performed, mainly includes a bainite phase.

3. Chemical composition

The chemical composition of the slab used in the present invention will be described hereafter. % represents mass% in the description.

C: 0.03% to 0.20%

Although C is a chemical element which increases the strength of steel and it is necessary that the C content be 0.03% or more in order to achieve the desired strength in the present invention, in the case where the C content is more than 0.20%, there is not only a decrease in weldability but also a negative influence on toughness. Therefore, it is preferable that the C content be in the range of 0.03% to 0.20%, more preferably 0.05% to 0.15%.

Si: 0.03% to 0.5%

Although Si is effective as a deoxidizing chemical element and as a chemical element for increasing the strength of steel, the effect cannot be realized in the case where the Si content is less than 0.03%. On the other hand, in the case where the Si content is more than 0.5%, there is not only the deterioration of the surface quality of steel but also a significant decrease in toughness. Therefore, it is preferable that the Si content be 0.03% or more and 0.5% or less.

Mn: 0.5% to 2.5%

Mn is added as a chemical element for increasing strength. Since the effect is insufficient in the case where the Mn content is less than 0.5%, and since there is a decrease in weldability and an increase in steel material cost in the case where the Mn content is more than 2.5%, it is preferable that the Mn content be 0.5% or more and 2.5% or less.

Al: 0.005% to 0.08%

Al is effective as a deoxidizing agent, and it is necessary that the Al content be 0.005% or more in order to realize this effect, but, in the case where the Al content is more than 0.08%, there is not only a decrease in toughness but also a decrease in the toughness of a weld metal when welding is performed. Therefore, it is preferable that the Al content be in the range of 0.005% to 0.08%, more preferably 0.02% to 0.04%.

N: 0.0050% or less

N increases the strength of steel by controlling a crystal grain size as a result of combining with Al in steel to form AlN when rolling is performed, but, since there is a decrease in toughness in the case where the N content is more than 0.0050%, it is preferable that the N content be 0.0050% or less.

P and S

Since P and S are inevitable impurities in steel, and since there is a decrease in toughness in the case where the P content is more than 0.03% or in the case where the S content is more than 0.01%, the contents of P and S are 0.03% or less and 0.01% or less, preferably 0.02% or less and 0.005% or less respectively.

Ti: 0.005% to 0.03%

A small content of Ti is effective for increasing the toughness of a base metal by decreasing a crystal grain size as a result of forming a nitride, carbide or carbonitride. This effect is realized in the case where the Ti content is 0.005% or more, but, since there is a decrease in the toughness of a base metal and a welded heat affected zone in the case where the Ti content is more than 0.03%, it is preferable that the Ti content be in the range of 0.005% to 0.03% .
Although the chemical composition described above is the base chemical composition, one or more of Nb, Cu, Ni, Cr, Mo, V, B, Ca and REM may be added in order to further improve the properties.

Nb: 0.005% to 0.05%

Nb contributes to an increase in strength as a result of precipitating in the form of NbC when ferrite transformation occurs or reheating is performed. In addition, since Nb is effective for expanding a temperature range in which recrystallization does not occur when rolling is performed under conditions for forming an austenite phase, which results in a decrease in bainite packet grain size, Nb contributes to an increase in toughness. This effect is realized in the case where the Nb content is 0.005% or more, but, since there is conversely a decrease in toughness as a result of the precipitation of large-size NbC in the case where the Nb content is more than 0.05%, it is preferable that the upper limit of the Nb content be 0.05%.

Cu, Ni, Cr and Mo

Cu, Ni, Cr and Mo are all chemical elements which increase the hardenability of steel. Since these chemical elements directly contribute to an increase in strength after rolling has been performed and may be added in order to improve functional properties such as toughness, high temperature strength or weather resistance, and since these effects are realized in the case where the contents of these chemical elements are respectively 0.01% or more, it is preferable that the contents of these chemical elements be respectively 0.01% or more in the case where these chemical elements are added. However, since there is a decrease in toughness and weldability in the case where the contents of these chemical elements are excessively large, it is preferable that the upper limits of the contents of Cu, Ni, Cr and Mo be respectively 0.5%, 1.0%, 0.5% and 0.5% in the case where these chemical elements are added.

V: 0.001% to 0.10%

V is a chemical element which increases the strength of steel by precipitation strengthening as a result of precipitating in the form of V(C,N). The V may be contained in the amount of 0.001% or more in order to realize this effect, but there is a decrease in toughness in the case where the V content is more than 0.10%. Therefore, in the case where V is added, it is preferable that the V content be in the range of 0.001% to 0.10%.

B: 0.0030% or less

A small amount of B may be added as a chemical element which increases the hardenability of steel. However, in the case where the B content is more than 0.0030%, since there is a decrease in the toughness of a weld zone, it is preferable that the B content be 0.0030% or less in the case where B is added.

Ca: 0.0050% or less and REM: 0.010% or less

Since Ca and REM increase toughness as a result of decreasing a grain size in a microstructure in a welded heat affected zone and there is no decrease in the effect of the present invention even in the case where these chemical elements are added, these chemical elements may be added as needed. However, in the case where the contents of these chemical elements are excessively large, since there is a decrease in the toughness of a base metal as a result of forming large-size inclusions, it is preferable that the upper limit of the contents of Ca and REM be respectively 0.0050% and 0.010% in the case where these are added.

4. Manufacturing conditions

The manufacturing conditions in the present invention will be described hereafter.
It is essential that manufacturing conditions such as the heating temperature of a slab as a steel material, hot rolling conditions and cooling conditions be specified. In particular, regarding hot rolling, it is essential to specify, in addition to total cumulative rolling reduction in the austenite recrystallization temperature range and austenite non-recrystallization temperature range, cumulative rolling reduction for each of the cases where the central portion in the thickness direction has a temperature in the austenite recrystallization temperature range and where the central portion in the thickness direction has a temperature in the austenite non-recrystallization temperature range and to specify a temperature condition in rolling while the central portion in the thickness direction has a temperature in the austenite non-recrystallization temperature range. By specifying these conditions, it is possible to achieve the desired properties of the thick steel plate regarding toughness in the surface portion and in the central portion in the thickness direction, the integration degree I of the RD//(110) plane in the central portion in the thickness direction and strength in a portion located at 1/4 of the thickness.
Firstly, molten steel having the chemical composition described above is produced using, for example, a converter furnace and made into a slab using, for example, a continuous casting method. Subsequently, the slab is heated at a temperature of 1000°C to 1200°C and then hot-rolled.
It is difficult to secure sufficient time for performing rolling in the austenite recrystallization temperature range in the case where the heating temperature is lower than 1000°C. On the other hand, in the case where the heating temperature is higher than 1200°C, since there is not only a decrease in toughness due to an increase in austenite grain size but also a decrease in yield due to a significant loss caused by oxidation, it is preferable that the heating temperature be 1000°C to 1200°C, more preferably in the range of 1000°C to 1150°C from the viewpoint of toughness.
It is essential to specify conditions of hot rolling and subsequent cooling as described below. With this method, since a microstructure which has been formed by rolling performed in the austenite non-recrystallization temperature range is transformed into a bainite phase, a texture having a specified orientation is preferentially formed, that is, so-called variant selection occurs, due to the transformation time being not sufficiently long, which results in the integration degree I of the RD//(110) plane becoming 1.5 or more, preferably 1.7 or more.
It is preferable that hot rolling be performed, firstly, while the central portion in the thickness direction has a temperature in the austenite recrystallization temperature range, under the conditions that the cumulative rolling reduction is 20% or more. By controlling the cumulative rolling reduction to be 20% or more, since an austenite grain size becomes small, a grain size in a metallographic structure which is finally obtained becomes small, which results in an increase in toughness. In the case where the cumulative rolling reduction is less than 20%, since an austenite grain size does not become sufficiently small, there is not an increase in the toughness of the microstructure which is finally obtained.
Subsequently, rolling is performed, while the central portion in the thickness direction has a temperature in the austenite non-recrystallization temperature range, under the conditions that the cumulative rolling reduction is 40% or more. By controlling the cumulative rolling reduction in this temperature range to be 40% or more, since a texture in the central portion in the thickness direction can be sufficiently grown, it is possible to control the integration degree I of the RD//(110) plane in the central portion in the thickness direction to be 1.5 or more, preferably 1.7 or more.
Here, in the case where rolling takes an excessively long time while the central portion in the thickness direction has a temperature in the austenite non-recrystallization temperature range, there is an increase in grain size in a microstructure, which results in a decrease in toughness. Therefore, the difference in rolling temperature between the first and the last passes in rolling performed while the central portion in the thickness direction has a temperature in the austenite non-recrystallization temperature range be 40°C or less. Here, "rolling temperature" means the temperature of the central portion in the thickness direction of a steel material immediately before the steel material is rolled. The temperature of the central portion in the thickness direction can be derived from, for example, the thickness, the surface temperature and the cooling conditions by, for example, simulation calculation. For example, by calculating the temperature distribution in the thickness direction using a difference method, the temperature of the central portion in the thickness direction of the steel plate can be derived.
The rolling is performed so that the total cumulative rolling reduction in the austenite recrystallization temperature range and in the austenite non-recrystallization temperature range be 65% or more. This is because sufficient reduction cannot be applied to a microstructure in the case where the total reduction is small, which results in the target toughness and strength not being achieved, and because, by controlling the total cumulative rolling reduction to be 65% or more, it is possible to apply sufficient reduction to a microstructure, which results in the target toughness and strength being achieved.
The austenite recrystallization temperature range and the austenite non-recrystallization temperature range are determined by performing preliminary experiments using steel having the chemical composition described above in which the steel is subjected to heating and processing history under various conditions.
Here, although there is no limitation on the finishing temperature of hot rolling, it is preferable that the finishing temperature be in the austenite non-recrystallization temperature range from the view point of rolling efficiency.
It is essential that the rolled steel plate be cooled down to a temperature of 450°C or lower at a cooling rate of 4°C/s or more. By controlling the cooling rate to be 4°C/s or more, since there is not an increase in grain size in a
microstructure, and since ferrite transformation is suppressed, a bainite structure having a small grain size can be obtained, which results in the target excellent toughness, texture and strength being achieved. In the case where the cooling rate is less than 4°C/s, there is an increase in grain size in a microstructure and the progression of ferrite transformation throughout the thickness, which results not only in the desired microstructure not being achieved but also in a decrease in strength of the steel plate. By controlling the cooling stop temperature to be 450°C or lower, since bainite transformation can be sufficiently progressed, it is possible to achieve a metallographic structure having the desired toughness and texture. In the case where the cooling stop temperature is higher than 450°C, since bainite transformation is not sufficiently progressed, structures such as a ferrite phase and a pearlite phase are also formed, which results in a microstructure mainly including a bainite phase not being achieved. Here, the cooling rate and cooling stop temperature described above are determined by using the temperature of the central portion in the thickness direction of the steel plate. The temperature of the central portion in the thickness direction can be derived from, for example, the thickness, the surface temperature and the cooling conditions by, for example, simulation calculation. For example, by calculating the temperature distribution in the thickness direction using a difference method, the temperature of the central portion in the thickness direction of the steel plate can be derived.
A temper treatment may be performed on the cooled steel plate. By performing a temper treatment, it is possible to further increase the toughness of a steel plate. By controlling a tempering temperature to be equal to or lower than the A_C1 point in terms of the average temperature of the steel plate, it is possible to prevent the desired microstructure obtained through rolling and cooling from being lost. In the present invention, the A_C1 point (°C) is derived using the equation below. $A_{C 1} point = 751 - 26.6 C + 17.6 Si - 11.6 Mn - 169 Al - 23 Cu - 23 Ni + 24.1 Cr + 22.5 Mo + 233 Nb - 39.7 V - 5.7 Ti - 895 B,$
where an atomic symbol in the equation above represents the content (mass%) of a chemical element in steel represented by the symbol, and where the symbol is assigned a value of 0 in the case where the chemical element is not contained.
The average temperature of the steel plate can also be derived from, for example, the thickness, the surface temperature and the cooling conditions by, for example, simulation calculation, as is the case with the temperature of the central portion in the thickness direction.

[EXAMPLES]

By producing molten steels (steel codes A through O) having the chemical compositions given in Table 1 using a converter furnace, by casting the molten steel into slabs as steel material (having a thickness of 250 mm) using a continuous casting method, by hot-rolling the slabs into hot-rolled steel plates having a thickness of 50 to 90 mm and by cooling the hot-rolled steel plate, sample steels No. 1 through No. 30 were obtained. The hot rolling conditions and the cooling conditions are given in Table 2.
By performing a tensile test using a JIS No. 14A test piece having a diameter of φ14 mm cut out of the portion located at 1/4 of the thickness of the obtained thick steel plate such that the longitudinal direction of the test piece was at a right angle to the rolling direction, a yield strength and a tensile strength were determined.
In addition, by performing a Charpy impact test using JIS No. 4 impact test pieces cut out of the portion located at 1/2 of the thickness such that the longitudinal direction of the test pieces was parallel to the rolling direction, a fracture appearance transition temperature was determined. Here, among the surfaces of the impact test piece of the surface portion, one which was the nearest to the surface of the steel plate corresponded to the depth of 1 mm from the surface of the steel plate.
Subsequently, in order to evaluate brittle crack arrestability, Kca value at a temperature of -10°C (Kca(-10°C)) was determined by performing an ESSO test compliant with WES 3003.
Moreover, the integration degree I of the RD//(110) plane in the central portion in the thickness direction was derived in the following way. Firstly, by performing mechanical polishing and electrolytic polishing on the surface parallel to the steel plate surface of a sample having a thickness of 1 mm cut out of the central portion in the thickness direction, a test piece for X-ray diffractometry was prepared. By performing X-ray diffraction measurement using a Mo X-ray source on this test piece, the pole figures of (200), (110) and (211) planes were obtained. Three dimensional orientation distribution function was calculated from the obtained pole figures by using a Bunge method. Subsequently, using the calculated three dimensional orientation distribution function, by integrating the values of the three dimensional orientation distribution function in the orientation in which (110) plane was parallel to the rolling direction in 19 cross sections in total which were selected at intervals of 5° in the range from φ ₂ = 0° to φ ₂ = 90° in terms of Bunge notation, an integrated value was obtained. The value of the integrated value divided by the number 19 of the orientations which had been selected for the integration was defined as the integration degree I of the RD//(110) plane.
The results of these tests are given in Table 3. Sample steel plates (serial Nos. 1 through 13 and 27 through 30) had a Kca(-10°C) of 6000 N/mm^3/2 or more, which means these sample steel plates had excellent brittle crack arrestability. In addition, sample steel plates (serial Nos. 1 through 13), which had the Charpy fracture appearance transition temperature and the integration degrees I of the RD//(110) plane in the surface portion and the central portion in the thickness direction satisfying the relational expression (1), had higher Kca(-10°C) than sample steel plates (serial Nos. 27 through 30), which had the Charpy fracture appearance transition temperature and the integration degree I of the RD//(110) plane in the surface portion and the central portion in the thickness direction not satisfying the relational expression (1).
On the other hand, sample steel plates (serial Nos. 21 through 26), which had chemical compositions of the steel plate within the ranges as described above and were prepared under manufacturing conditions regarding heating and rolling conditions for the steel plate out of the range according to the present invention, had a Kca(-10°C) of less than 6000 N/mm^3/2. The textures of sample steel plates (serial Nos. 22, 23 and 26) did not satisfy the specifications according to the present disclosure. Sample steel plates (serial Nos. 14 through 20), which had chemical compositions of the steel plate out of the preferable ranges as described above, had toughness not satisfying the specifications according to the present disclosure and a Kca(-10°C) of less than 6000 N/mm^3/2.
In addition, sample steel plates (serial Nos. 14 through 26), which had at least one of the toughness and texture in the central portion in the thickness direction out of the ranges according to the present disclosure, had a Kca(-10°C) of less than 6000 N/mm^3/2.

[Reference Signs List]

1: test piece for ESSO test compliant with WES 3003
2: notch
3: crack
3a: branched crack
4: tip shape
5: base metal

[Table 1]

Table 1

(mass%)
Steel Code	C	Si	Mn	P	S	Al	Nb	Ti	V	Cu	Ni	Cr	Mo	N	B	Ca	REM	Ceq
A	0.06	0.16	1.66	0.006	0.001	0.040	-	0.008	-	0.35	-	0.21	-	0.0031	-	-	-	0.40
B	0.05	0.15	1.75	0.005	0.002	0.070	0.014	0.015	-	-	0.51	-	-	0.0026	-	0.0015	-	0.38
C	0.07	0.17	1.55	0.006	0.002	0.060	-	0.012	0.03	0.30	0.40	-	-	0.0047	0.0013	-	0.005	0.38
D	0.07	0.17	1.45	0.007	0.003	0.030	0.020	0.023	-	-	-	0.42	-	0.0034	-	-	-	0.40
E	0.05	0.18	1.56	0.005	0.001	0.050	0.011	0.009	0.05	-	0.38	-	0.13	0.0045	0.0009	-	-	0.37
F	0.04	0.15	1.88	0.008	0.002	0.056	0.008	0.017	-	-	0.71	-	-	0.0022	-	-	-	0.40
G	0.06	0.18	1.50	0.006	0.002	0.060	-	0.013	-	0.46	-	0.24	-	0.0037	-	-	0.002	0.39
H	0.05	0.14	1.83	0.012	0.003	0.050	-	0.007	-	-	-	-	0.28	0.0040	-	0.0011	-	0.41
I	0.07	0.17	1.71	0.006	0.002	0.110	-	0.014	0.04	-	0.36	-	-	0.0027	-	-	-	0.39
J	0.05	0.06	1. 61	0.062	0.001	0.050	-	0.011	-	-	0.85	0.11	-	0.0044	0.0008	0.0024	-	0.40
K	0.27	0.14	0.87	0.007	0.002	0.040	0.013	0.015	-	-	-	-	-	0.0031	-	-	-	0.42
L	0.06	0.15	1.33	0.006	0.001	0.040	-	0.024	-	-	-	-	0.74	0.0027	-	-	-	0.43
M	0.05	0.05	1.71	0.005	0.002	0.070	-	0.016	-	0.94	0.03	-	-	0.0041	-	-	-	0.40
N	0.06	0.16	1.47	0.006	0.001	0.060	0.013	0.017	-	-	0.35	0.24	-	0.0074	-	0.0013	-	0.38
O	0.07	0.67	1.66	0.008	0.001	0.050	0.011	0.006	-	-	-	-	0.22	0.0033	-	-	-	0.39

Note 1: Ceq = C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5
(An atomic symbol represents the content (mass%) of a chemical element represented by the symbol.)

[Table 2]

Table 2

Serial No.	Steel Code	Thickness (mm)	Heating and Rolling Conditions							Tempering Temperature (°C)
Serial No.	Steel Code	Thickness (mm)	Heating Temperature (°C)	Cumulative Rolling Reduction in γ Recrystallization Range (%)	Cumulative Rolling Reduction in γ Non-Recrystallization Range (%)	Total Cumulative Rolling Reduction (%)	Rolling Temperature Difference in γ Non-Recrystallization Range (°C)	Cooling Rate (°C/s)	Cooling Stop Temperature (°C)	Tempering Temperature (°C)
1	A	65	1140	33	61	74	27	6.7	270	-
2	B	55	1070	55	51	78	35	7.9	140	550
3	C	60	1050	40	60	76	14	7.4	400	-
4	D	70	1200	53	41	72	22	6.3	330	-
5	E	65	1000	52	46	74	32	7.1	390	-
6	F	50	1060	45	64	80	29	9.8	420	-
7	G	60	1100	33	64	76	11	7.6	370	-
8	H	80	1100	38	48	68	17	4.8	310	-
9	C	60	1120	42	58	76	26	7.6	280	-
10	E	70	1150	38	55	72	23	6.5	170	580
11	F	75	1080	45	46	70	9	5.8	340	-
12	B	60	1100	47	51	76	31	7.3	360	-
13	H	55	1120	35	66	78	16	8.4	440	-
14	I	70	1040	28	61	72	34	5.9	320	-
15	J	65	1120	40	57	74	24	6.6	370	-
16	K	70	1060	45	49	72	28	6.4	340	-
17	L	55	1170	42	62	78	27	8.2	440	-
18	M	70	1150	52	41	72	19	6	320	-
19	N	80	1060	37	49	68	25	5.2	340	-
20	O	60	1010	29	66	76	26	7.2	410	-
21	A	70	1100	12	68	72	17	6.2	290	-
22	C	60	1050	57	44	76	27	Air Cooling (≤0.5)	-	-
23	D	90	1150	62	25	64	32	4.3	410	-
24	F	65	1340	29	63	74	18	6.9	380	-
25	H	75	1170	47	43	70	59	5.7	340	-
26	F	80	1080	42	45	68	33	5.4	610	-
27	C	75	1030	45	46	70	22	5.7	410	-
28	B	70	1080	37	56	72	18	6.4	350	-
29	F	80	1120	32	53	68	26	5.2	370	-
30	G	60	1100	52	50	76	11	7.4	330	-

Note 1: "Cumulative Rolling Reduction in γ Recrystallization Range" means cumulative rolling reduction when the central portion of the thickness has a temperature in the austenite recrystallization temperature range.
Note 2: "Cumulative Rolling Reduction in γ Non-Recrystallization Range" means cumulative rolling reduction when the central portion of the thickness has a temperature in the austenite non-recrystallization temperature range.
Note 3: "Rolling Temperature Difference in γ Non-Recrystallization Range" means difference in rolling temperature between the first and the last passes in rolling performed while the central portion of the thickness has a temperature in the austenite non-recrystallization temperature range.

[Table 3]

Table 3

Serial No.	Steel Code	Thickness (mm)	YS (MPa)	TS (MPa)	vTrs of 1/2t Portion (°C)	I_RD//(110) of 1/2t Portion	Left Side of Relational Expression (1)	Kca(-10°C) (N/mm^3/2)
1	A	65	557	672	-74	2.2	-100.4	7900
2	B	55	562	677	-117	4.6	-172.2	15400
3	C	60	561	668	-72	2.3	-99.6	8400
4	D	70	578	685	-64	2.3	-91.6	7400
5	E	65	524	641	-82	2.4	-110.8	9100
6	F	50	581	702	-84	2.6	-115.2	9300
7	G	60	572	683	-68	2.6	-99.2	8200
8	H	80	564	687	-59	2.1	-84.2	6200
9	C	60	574	678	-76	2.4	-104.8	8700
10	E	70	513	637	-66	2.2	-92.4	7100
11	F	75	575	688	-63	1.7	-83.4	6500
12	B	60	547	653	-97	3.7	-141.4	12500
13	H	55	607	711	-88	2.7	-120.4	10200
14	I	70	547	665	-32	2.4	-60.8	3200
15	J	65	588	694	-24	2.3	-51.6	2200
16	K	70	594	708	-21	2.2	-47.4	1800
17	L	55	584	702	-25	3.1	-62.2	3600
18	M	70	577	698	-18	2.3	-45.6	1400
19	N	80	522	627	-23	1.9	-45.8	1600
20	O	60	587	692	-34	2.3	-61.6	3300
21	A	70	565	679	-22	2.0	-46	1500
22	C	60	342	487	-13	1.3	-28.6	1300
23	D	90	426	543	-32	1.1	-45.2	1400
24	F	65	572	713	-24	2.5	-54	2500
25	H	75	574	687	-17	1.9	-39.8	1500
26	F	80	411	517	-56	1.0	-68	4200
27	C	75	534	645	-43	1.8	-64.6	6000
28	B	70	534	656	-62	1.5	-80	6100
29	F	80	575	677	-48	1.6	-67.2	6000
30	G	60	554	657	-45	1.5	-63	6100

Note 1: Underlined value is out of the range according to the present disclosure.
Note 2: Relational Expression (1) vTrs_(1/2t)-12xI_{RD//(110)[1/2t]}≤-70
Note 3: 1/2t portion represents the central portion of the thickness.

Claims

A method for manufacturing a high-strength thick steel plate having a thickness of 50 mm or more for structural use with excellent brittle crack arrestability, the method comprising:
heating a slab at a temperature of 1000°C to 1200°C, wherein the slab has a chemical composition consisting of, by mass%, C: 0.03% or more and 0.20% or less, Si: 0.03% or more and 0.5% or less, Mn: 0.5% or more and 2.5% or less, Al: 0.005% or more and 0.08% or less, P: 0.03% or less, S: 0.01% or less, N: 0.0050% or less, Ti: 0.005% or more and 0.03% or less, and optionally one or more of Nb: 0.005% or more and 0.05% or less, Cu: 0.01% or more and 0.5% or less, Ni: 0.01% or more and 1.0% or less, Cr: 0.01% or more and 0.5% or less, Mo: 0.01% or more and 0.5% or less, V: 0.001% or more and 0.10% or less, B: 0.0030% or less, Ca: 0.0050% or less, and REM: 0.010% or less, and the balance being Fe and inevitable impurities;

performing rolling so that the total cumulative rolling reduction in an austenite recrystallization temperature range and in an austenite non-recrystallization temperature range is 65% or more, in which,
firstly, while the central portion in the thickness direction has a temperature in the austenite recrystallization temperature range, the cumulative rolling reduction is controlled to be 20% or more, and

subsequently, while the central portion in the thickness direction has a temperature in the austenite non-recrystallization temperature range, the cumulative rolling reduction is controlled to be 40% or more, wherein the difference in rolling temperature between the first pass and the last pass is controlled to be 40 °C or less, wherein the rolling temperature means the temperature of the central portion in the thickness direction of a steel material immediately before the steel material is rolled; and

performing cooling on the rolled steel plate down to a temperature of 450°C or lower at a cooling rate of 4°C/s or more, wherein said temperature and cooling rate are determined by using the temperature of the central portion in the thickness direction of the steel plate.
The method according to Claim 1, the method further comprising a process in which the steel plate which has been subjected to accelerated cooling down to the temperature of 450°C or lower is tempered at a temperature equal to or lower than the A_C1 point.