BRPI0808347A2 - HIGH RESISTANCE THICK STEEL SHEET AND SAME PRODUCTION METHOD - Google Patents

Description

Report of the Invention Patent for "HIGH RESISTANCE THICK STEEL PLATE AND SAME PRODUCTION METHOD".

TECHNICAL FIELD

The present invention relates to a high strength thick steel plate (hereinafter also referred to as high strength high detention thick steel plate or high detention steel plate) which is capable of stopping the sudden spread of brittle cracks. (hereinafter referred to as holding capacity) and method of production thereof.

Specifically, the present invention relates to a high strength thick steel sheet which has an excellent ability to stop the sudden spread of brittleness cracks and a production method thereof where the thickness of the steel sheet is 50 µm. mm or more (hereinafter referred to as heavy plate), and the temperature at which Kca = 6000N / mm15 is satisfied (hereinafter referred to as holding capacity indicator) is at most -10 ° C.

A steel plate according to the present invention may be applied to welded constructions such as shipbuilding, buildings, bridges, tanks, and offshore structures. The steel plate according to the present invention may also be worked for steel pipes, steel beams or the like, and may be distributed as a by-product.

Priority is sought over Japanese Patent Application ης 2007-54279 filed March 5, 2007, the wording of which is incorporated herein by reference.

BACKGROUND ART

In line with the recent trend of increasing the size of steel structures, the use of thick and high strength steel plates is required. In addition, in terms of ensuring safety, it is strictly necessary for the steel plate to be capable of stopping the sudden spread of fragility cracks (holding capacity). On the other hand, increasing the strength or thickness of a steel generally results in abruptly increasing the difficulty in securing holding capacity, thereby disrupting the application of high strength thick steel plate in steel constructions. In addition, there is a growing demand from consumers for reduced lead times. Therefore, increased productivity in the steel production process is strongly required.

Known metallurgical factors for increasing steel holding capacity include (i) refining crystal grain size, (ii) Ni addition, (iii) brittle secondary phase control, (iv) texture control, or the like.

For example, Reference Patent 1 (Patent Application

Unexamined Plaque, First Publication, No. 2 H02-1293189) describes a method

(i) refining the size of the crystal grain. In this method, the steel is rolled with a reduction of 50% or more at a

non-recrystallization region at a temperature not less than Ar 3 and is subsequently subjected to a 30 to 50% reduction lamination in the two-phase region of 700 to 750 ° C. As a specific method for refining the crystal grain size of a steel plate, Reference Patent 2 (Japanese Patent Application examined, Second Pu

H06-004903) and Reference Patent 3 (Unexamined Japanese Patent Application, First Publication No. 2003-221619) each describe a method including cooling a surface of a plate prior to lamination or after lamination. preliminary lamination, and make the plate recover by the beginning of lamination while maintaining the temperature difference

between the inner portion and the surface of the plate, thereby generating fine ferrites on the surface of the plate.

(ii) It is reported that the addition of Ni suppresses the propagation of fragility cracks by promoting transverse landslides in a low temperature region (Non-Patent Reference 1: Imao Tamura,

"Study on steel strength" published by Nikkan Kogyo Shinbunsha, July 5, 1969, p. 125), and thereby increasing the holding capacity of a matrix (Non-Patent Reference 2: Hasebe and Kawaguchi, "Brittle fracture propagation arrestability of Ni-added steel plate examined by taper-shaped DCB test", Iron and Steel, Vol. 61, 1975, p. 875).

(iii) Reference Patent 4 (Japanese Patent Application, First Publication, No. S59-047323) describes a method of controlling the

fragile secondary phase (embrittlement). In this method, the occurrence of martensite as a fragile phase is controlled to occur as a thin dispersed phase dispersed in a ferrite matrix.

(iv) With regard to texture control, Reference Patent 5 (Japanese Patent Application, First Publication No. 2002-241891)

provides a method including the rolling of low carbon bainite steel under low temperature and heavy reduction conditions, thereby accumulating (211) planes parallel to the cylinder plane.

DESCRIPTION OF THE INVENTION

Problems to be solved by the invention However, the method described in Patent Reference 1 is directed to

It is made of steels used in low temperature conditions, where steel has relatively low strength due to its microstructure composed mainly of ferrite, and the sheet thickness is up to 20 mm. Therefore, if the method is applied to a thick plate 50 mm or more thick as designated by the present invention, it is difficult to guarantee a sufficient reduction ratio in terms of plate thickness. In addition, there is another problem that prolonged waiting time before reaching the processing temperature reduces steel productivity.

In addition, it is difficult to guarantee a yield strength of 390MPa or more by the method described in that reference.

Application of the method described in Patent References 2 and 3 to the production of a coarse product having a plate thickness of

50 mm or more as designated in the present invention also causes problems. Although a similar microstructure is obtained, it is difficult to guarantee the holding capacity. The effect of refined ferrite on the surface portion is relatively small. Furthermore, the production process itself includes a problem that thermal control along the plate thickness is also made difficult, and it is impossible to avoid further reduction during the recovery process, thereby greatly disrupting productivity.

On the other hand, a very high cost is required to form an alloy that provides the desired holding capacity for a steel sheet only by adding Ni as described in item (ii) above. It is considered that microstructure refining could be combined with the addition of Ni to ensure holding capacity while reducing the addition of Ni. However, there is no examination to separate and quantify the influence of different factors of Ni addition on holding capacity. In the current circumstance, there is still no clear guideline for the production of high-detention Ni-type steel sheets.

In addition, in a thick steel plate, it is difficult to disperse fine martensite in steel as described in Patent Reference 4. In a high strength steel plate, such brittle phase possibly deteriorates the initiation property of brittle cracks.

If the invention described in Patent Reference 5 is applied to a thick plate, the lamination efficiency is extremely low. Thus, such a method is not applicable in industrial production.

As described above, there is no technology established

for the stable and effective production of a high detention thick steel sheet as directed by the present invention having a sheet thickness of 50 mm or more, the holding capacity indicator TKCa = 60 ° -10 ° C or less even when the yield limit is 25 in a range of 390 to 460 MPa, and applicable to large constructions.

Based on the circumstances described above, it is an object of the present invention to provide a high strength thick steel plate which has excellent arrestability of the sudden spread of 30 fragility cracks that is sufficient as a steel for large constructions, and to provide a method which allows for an industrially stable and efficient production of the same steel sheet. Means to solve problems

The present invention relates to a high strength thick steel plate having excellent ability to arrest sudden crack propagation and a method of producing it which solves the problems described above, and includes the aspects described below.

[1] A high strength thick steel plate having excellent ability to stop the sudden spread of fragility cracks, having:

a composition containing by weight% C: 0.01 to 0.14%, Si: 0.03 to 0.5%, Mn: 0.3 to 2.0%, P: 0.020% or less, S: 0.010% or less, Ni: 0.5 to 4.0%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.050%, Al: 0.002 to 0.10%, N: 0.0010 to 0.0080%, and the balance consisting of Fe and the inevitable impurities, where the Ceq defined by formula (1) described below is 0.30 to 0.50%;

a bainite-dominated microstructure of 60% or more by volume fraction, where the perlite fraction is less than or equal to 5%,

the equivalent diameter of the cementite semicircle is less than or equal to 0.5 pm,

the crude ferrite fraction having a circle equivalent diameter greater than 25 pm is less than or equal to 10% in surface portions each having a depth of 5% of plate thickness from the front and rear surfaces of the plate, and

the mean equivalent circle diameter of iso-crack propagation resistant domains is greater than or equal to 8 pm and less than or equal to d (pm) defined by the formula (2) described below,

where each of the domains resistant to crack propagation

The default is defined so that the section (flat section) perpendicular to the plate rolling direction is defined as a T section, and a direction parallel to the plate surface is defined as T direction in the T section, the inner portion in the T section. is defined as the portion that excludes surface portions,

the T-section microstructure is divided into domains each having the same crystal orientation (iso-orientation domain) by analyzing the internal portion orientation using Electron Back Scattering Pattern (EBSP),

an arbitrary measuring line is drawn on the split T-section microstructure for iso-orientation domains by applying an examination method according to JIS G0551,

a plurality of iso-orientation domains having a circle equivalent diameter of 8μιη continuously aligned and adjacent to one another on the measuring line are identified excluding the isoorientation domains having an equivalent circle diameter of less than 8μιτι,

the <001> axis closest to the T direction is selected from the

three axes <001> of each of the identified iso-orientation domains,

an angle (slit propagation deviation angle) formed by the axis <001> closest to the T-direction of the identified iso-oriented domains is measured on each adjacent pair of the measuring line isoorienting domains,

continuously aligned iso-orientation domains satisfying an angle no greater than 20 ° and adjacent iso-orientation domains having a circle equivalent diameter of less than 8μηη are estimated to be a domain that resists propagation of the standard crack.

Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ... (1)

d = (7,11 x [Ni] +11) x (1,2-t / 300) ... (2) where [X] denotes the content (in mass%) of an Xet element denotes the thickness of the plate (mm).

[2] A high strength thick steel plate having excellent ability to stop the sudden spread of fragility cracks.

as described in item [1] above, also containing by weight one or two or more elements selected from Cu: 0,05 to 1,5%, Cr: 0,05 to 1,0%, Mo: 0 1.0 to 1.0%, V: 0.005 to 0.10%, B: 0.0002 to 0.0030%.

[3] A high strength thick steel plate having excellent ability to stop the sudden spread of fragility cracks.

as described in items [1] and [2] above, also containing by weight one or two or more elements selected from Mg: 0,0003 to 0,0050%, Ca: 0,0005 to 0,0030% , REM: 0.0005 to 0.010%.

[4] A method of producing a high strength thick steel plate having excellent ability to stop the sudden spreading of brittleness cracks, including: reheating a steel plate having

a composition as described in any of [1] to [3] above to 950 to 1150 ° C;

perform cold rolling (preliminary rolling) on steel at a temperature of at least 900 ° C with a cumulative reduction of 30% or more;

perform finishing lamination on steel at a temperature

a minimum of Ar3 and a maximum of T (0C) as defined by formula (3) described below, with a cumulative reduction of 40%;

perform accelerated cooling of the steel with a cooling rate of 8 ° C / s or more on average across the plate thickness from

from a temperature of at least Ar3 to a temperature of a maximum of 500 ° C.

T = (37 * [Ni] +810) x (1,1-1 / 500) ... (·) where [Ni] denotes Ni content (in mass%) and denotes sheet thickness ( mm).

[6] A method of producing thick steel plate with the exception of

lens ability to arrest sudden crack propagation as described in item [5] above, also including execution of steel hardening at a temperature of 300 to 600 ° C after accelerated cooling is completed.

Effect of the Invention

Applying the present invention, high holding steel plates applicable to large buildings can be provided by a stable and effective production method, where the holding capacity indicators T 'ca = 60 ° of the plates are -10 ° C. or less even

when steel sheets are thick sheets having a thickness of 50 mm or more, and the yield strength of the steel is in the range of 390 to 460 MPa. Therefore, the present invention has great industrial efficiency. BRIEF EXPLANATION OF DRAWINGS

Figure 1 shows an example of crystal orientation mapping according to EBSP and boundary analysis of the standard crack propagation resistance domains.

Figure 2 is a graph showing a change in capacity.

depending on the amount of Ni added.

Figure 3 is a graph showing the effects of Ni addition and effective grain size on holding capacity.

Figure 4 is a graph showing the relationship between the perlite fraction and holding capacity.

Figure 5 is a graph showing the relationship between the cementite circle equivalent diameter and holding capacity.

Figure 6 is a graph showing the relationship between holding capacity and crude ferrite fraction having a circle equivalent diameter of 25 pm or more in surface portions having a depth of 5% of the plate thickness from the front surfaces. and back of the plate.

Figure 7 is a graph showing the relationship between Ni content and effective grain size required to provide a predetermined holding capacity for a steel.

Figure 8 is a graph showing the dependence of plate thickness on the effective grain size required to provide a predetermined holding capacity for a steel.

Figure 9 is a graph showing the relationship between the Ni content and the lamination end temperature required to provide a predetermined holding capacity for a steel.

Figure 10 is a graph showing the dependence of the plate thickness on the lamination end temperature required to provide a predetermined holding capacity to steel.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, a configuration of the present invention is explained in detail. The inventors performed experimental research on factors that dominate the holding capacity of a steel having a yield limit of 390 to 460 MPa and a bainite-dominated microstructure (60% or more by volume fraction). As a result, the inventors have discovered a method that ensures detentability even on a thick steel plate having a thickness of 50 mm or more. Important points in the present invention include new discoveries described in items (1) to (5) below.

(1) The fracture unit during propagation of a fragility crack does not correspond to the apparent limits of the crystal grain, but

approximately corresponds to the crystal grain boundaries obtained by crystal orientation analysis using EBSP. Specifically, an equivalent mean circle diameter (effective crystal grain size) of a standard crack propagation resistant domain (including grains having a circle equivalent diameter of less than 8 pm) shows a good correlation with the holding capacity. where the domain boundaries are defined by identifying the axes <001> closest to a direction T perpendicular to the rolling direction. That is, where angles of 20 ° or more are required for the identified grain axes (excluding grains 20 having an equivalent circle diameter of less than 8 pm) to coincide with each other, the boundaries of the domains are defined. between the grains.

(2) Where Ni is added in an amount of 0.5% or more, the apparent effect of improved holding capacity appears. The Ni effect and the grain size refining effect are independent of each other, and,

therefore, a substantially additive effect can be obtained. That is, by the addition of Ni, a similar holding capacity can be guaranteed even in a gross microstructure. As a result, it is possible to reduce the load on the production process such as increasing the lamination termination temperature.

(3) Where the perlite fraction exceeds 5%, the crude perlite tends to

act as a starting point for brittleness cracks thereby decreasing holding capacity even when steel has an effective fine grain size. To avoid such a phenomenon, it is necessary to control the rate of accelerated cooling and the desired (terminating) temperature at which cooling is stopped.

(4) Fine Cementites having an average equivalent circle diameter of 0.5 mm or less contributes to improved holding capacity. To maintain the fine grain size of cementite, accelerated cooling after rolling and subsequent heat treatment conditions must be controlled.

(5) Although steel has an effective fine grain size for plate thickness, the steel holding capacity deteriorates when the crude ferrite fraction generated on the surface portion exceeds 10%. To avoid such deterioration, it is necessary to control the lamination end temperature and the cooling start temperature not to be too low.

In the following, the constitutions of the present invention are explained. In general, the basic structural unit that dominates toughness

Bainite steel is not the grain size of the previous austenite, but the size of a region of a so-called package or block (for a package or block see Non-Patent Reference: Matsuda, Inoue, Mimura, and Okamura , "Toughness and effective grain size of low-alloy heat-treated 20 high-tensile steel," Proc. Of Int. Symp. On Towered Improved Ductility and Toughness. Climax Molybdenum Co., Kyoto (1971), p. 47). The toughness is improved since the region is small in size.

However, according to microstructural observation using an optical microscope, it is difficult to measure the size of packets and blocks. Furthermore, where there is mixed ferrite in the microstructure, it is very difficult to define the basic frame unit objectively.

Based on the circumstances described above, the inventors initially produced 50 mm thick steel plates under various conditions using a non-Ni plate, worked the 30 mm square specimen steel plates each notched to a depth. 29 mm, and subjected the specimens to an ESSO test-type temperature gradient to assess detention capacity according to the method described in WES 3003. Thereafter, by observing the fracture surface of the specimen with the using a scanning electron microscope, and measuring the unit (fracture face unit) of the cleavage surface surrounded by the fracture portion called the rupture crest, and confirmed its good correlation with the holding capacity.

Next, the EBSP measurement was performed in a section plane perpendicular to the fracture surface described above. The result of analyzing the crystal orientation of the underlying grains at the fracture surface was compared with a photograph. Thus, the boundary conditions of the 10 fracture unit were carefully examined. Figure 1 shows an example of the exam. The analysis was performed at the representative points and the iso-orientation domains were analyzed based on the orientation maps obtained by EBSP. Cubes made up of {100} planes (which are considered to be division planes) and the direction of crack propagation assuming that the crack propagates along the {100} planes were shown in Figure 1.

The numbers in figure 1 denote the angles (crack propagation deviation angles) required to make the nearest <001> axes coincide with each other, that is, the angles required to make the pla20 at {100} perpendicular to the T direction coincide. each other by allowing it to rotate. By this analysis it was confirmed that the orientation of crack propagation appears to change where the angle of deviation of crack propagation was 20 ° or more as shown in (a), (b), (c), and (f). As a result of observing the fracture surface, it was confirmed that the limits of the critical conditions described above currently constitute the limits of the fracture face unit size. On the other hand, in some cases as shown in (d) to (e), crack propagation does not change its direction in small size domains even when the angle of deviation is greater than 20 ° C. Such a phenomenon is considered to correspond to a fracture envelope and 30 of the ductile fracture surface. Such a phenomenon is observed in domains that have an equivalent circle diameter of less than 8 pm. As a result of observing the fracture surface, it was also confirmed that apparent boundaries were not constituted by such small domains. In determining the effective grain size, existing domains smaller than 8 pm may be joined to either adjacent grain on either side, and the boundaries of the standard crack propagation resistant domain may be determined by examining the angles. of deviation between the two adjacent domains. As described above, the effective grain size can be estimated by excluding grains smaller than 8pm from the EBSP analysis result, determining the boundaries of the domains that have a fracture propagation deviation angle of at least 20 °, and calculating 10 if the equivalent mean circle diameter of the domains involved by the boundaries.

As a result of careful examination of the relationship between the effective grain size thus measured and the steel holding capacity, it was found that the finishing lamination had to be performed at a low temperature of at most 800 ° C in order to provide a holding capacity of sufficient level to be applied to steel for large constructions, and cooling required to be started from the high temperature in order to control the flow limit to be 390MPa or more, making it very difficult to produce steel effectively and stably.

Therefore, the effect of Ni addition was carefully examined.

see it in detail as a solution to the problem described above. Plates were laminated by varying the balance of Ni and Mn contents so that substantially the same microstructures and strengths were obtained. Under the same conditions, 50 mm thick steel sheets, and 80 mm thick steel sheets were produced from the plates, and their holding capacity was examined based on an ESSO test. As a result, it was confirmed that holding capacity tended to increase with increasing Ni content although the effective grain size was almost unchanging. This trend is shown in figure 2.

Here the holding capacity was assessed at a temperature (T <ca = 60 ° C) at which the holding capacity for the sudden propagation of fragility cracks Kca satisfied Kca = 6000N / min1 · 5. From figure 2, it was confirmed that the holding capacity was apparently improved where the Ni content was 0.5% or more. By observing the fracture surfaces of the ESSO1 specimens, the appearance of three-dimensional irregularities 5 apparently increased as the Ni content increased. This phenomenon can be explained by soluble Ni increasing cross slips thus making the propagation direction of fractures random.

Next, to individually distinguish and quantify the effects of Ni addition and refining effective grain size, the steel sheets obtained by rolling the Ni-containing steel plate described above under various conditions were subjected to examination for their holding capacity. . As a result, it was found that the grain refining enhancing effect did not depend on the Ni content, and can be substantially summarized by the Ni content effect. This behavior is shown in figure 3. That is, by adding an adequate amount of Ni, it is possible to ensure the holding capacity without refining the effective grain size. Therefore, where high steelmaking efficiency is required, regardless of the cost of Ni bonding, it is possible to increase the finishing temperature of the rolling mill by adding Ni, thereby shortening the lead time for the process temperature. As a result, it is possible to significantly increase the productivity of a thick steel.

The inventors also examined the influence of microstructure factors other than effective grain size on holding capacity, as it has been confirmed that thin-grain effective size steel plates occasionally showed insufficient holding capacity.

One of the factors is the occurrence of mixed perlite in the bainite dominated structure. Where the perlite structure fraction increases, the holding capacity tends to deteriorate due to the increasing amount of crude perlite that acts as the starting point for the fragile fracture. Therefore, as shown in figure 4, it is necessary to control the perlite fraction to be 5% or less. In addition, it was confirmed that the holding capacity was also affected by the size of the cementite included in the bainite. As shown in figure 5, the holding capacity is reduced where the average equivalent diameter of the cementite circle exceeds 0.5 pm. It can be estimated5 that thin cementites generate microfractures in their grain boundaries in the matrix prior to propagation of the main tent, thereby decreasing the stress state at the fracture tip. On the other hand, where cementite is hardened, such as perlite, cementite acts as a source for causing fragile fractures to occur, thereby decreasing the holding capacity.

In addition, it was confirmed that the crude ferrite generated in the

surface conditions also reduced holding capacity. This rough surface ferrite is generated where a relatively low hardening steel (temperobility) is rolled at a temperature below Ar3 or where the rolling is finished at a temperature not below Ar3, but where the temperature at start of cooling is below temperature Ar3. As shown in Figure 6, a noticeable reduction in holding capacity can be prevented where the ferrite fraction having a circle equivalent diameter greater than 25 pm is not greater than 10% in the surface portions each having a depth of 5%. the thickness of the plate from the front and rear surfaces of the plate.

To clarify the principle of producing a high strength, high detention thick steel plate while taking into account the metallurgical factors described above, the influence of effective grain size, Ni content, and plate thickness on the Holding capacity were examined in greater detail using steel sheets that satisfied the conditions described above for perlite, cementite and surface ferrite. As a result, it was found that the effective grain size needed to be equal to or less than the d value described below.

d = (7.11 x [Ni%] + 11) x (1.2 —1/300),

where [Ni] denotes the Ni content (in% by mass) and t denotes the plate thickness (mm). In determining the d-value described above, the linear equation derived from Figure 7 based on the influence of the effective grain size and Ni on the holding capacity of sheet having a thickness of 50 mm was combined for the derived thickness effect equation. Fig. 58 based on the result of tests in which the plate thickness was varied by polishing the front and rear surfaces of the 80 mm thick plate containing 2% Ni. Where the effective grain size is greater than the value d described above, the breaking chain is not formed often enough when a fragility crack propagates from one grain 10 to another. Therefore, the effect of suppressing the spread of a fracture is reduced, and the holding capacity is reduced.

The following explains the reason for the limitation of the conditions of

production.

In the present invention, the reheat temperature of a plate was controlled to give 950 to 1150 ° C. Where the reheat temperature is below 950 ° C, the homogenization of the coupling elements is insufficient, thus causing inhomogeneous properties. Where the reheat temperature exceeds 1150 ° C, the austenite grain sizes are stiffened. Therefore, there is a possibility that it is difficult to obtain a thin microstructure in the final state.

Subsequent gross rolling should be performed at a temperature of 900 ° C or more with a cumulative reduction of 30% or more. If the conditions described above are not met, the recrystallization of austenite grains does not proceed sufficiently, resulting in a mixed grain microstructure that can cause inhomogeneous properties.

The following finishing lamination is the most important process for refining the effective grain size that dominates holding capacity. Finishing lamination is performed at a temperature of at least Ar3 temperature (temperature at which 30 ferrite formation begins during steel cooling) and at most the temperature T (° C) described below with a cumulative reduction of 40% or more.

T = (37x [Ni] +810) x (1.1 -t / 500), where [Ni] denotes the Ni content (in mass%) and t denotes the plate thickness (mm).

At the temperature T described above, a linear equation is combined with a thickness effect equation, where the linear equation is derived from Figure 9 showing the relationship between Ni content and finishing lamination temperature required to satisfy TKca = 6000. Based on the results of the experiments described above, and the thickness effect equation is derived from Figure 10 based on the experimental results obtained while varying the thickness and finish lamination temperature using plates containing 2 % Ni. Where the temperature is less than Ar3, crude ferrite having an equivalent circle diameter greater than 25μηι is generated, thereby decreasing the steel sheet's holding capacity, strength, toughness, and ductility. On the other hand, where the temperature exceeds the temperature T described above, or where the cumulative reduction is less than 40%, the holding capacity is reduced since the effective grain size is not sufficiently refined. By selecting a temperature slightly lower than the temperature T described above according to the amount of Ni added, it is possible to shorten the lead time for the process temperature prior to finishing lamination, thus making it possible to effectively produce high-grade thick steel sheets. resistance.

Upon completion of the finishing lamination, the steel plate is subjected to accelerated cooling from a temperature of at least Ar3 to a temperature of 500 ° C or less with a cooling rate of 8 ° C / s or more. Where the cooling onset temperature is lower than Ar3, the fraction of crude ferrite in the surface portions exceeds 10%, thus deteriorating holding capacity. Where the cooling rate is less than 8 ° C / s, or where the cooling termination temperature is greater than 500 ° C, sufficient resistance is not obtained. In addition, the holding capacity is reduced by insufficient refining of the effective grain size, cementite stiffening which may contribute to improved holding capacity, or by the generation of perlite exceeding 5%.

After accelerated cooling, a quenching treatment may be performed at a temperature of 300 to 600 ° C to control the strength and toughness of the steel sheet. Where the tempering temperature is below 300 ° C, the ductility and toughness are not sufficiently improved. Where the tempering temperature exceeds 600 ° C, the holding capacity is reduced by cementite stiffening.

The following are reasons for limiting the composition of the present invention.

C (carbon) is an element that contributes to the generation of c

and prevention of microstructural stiffening. In addition, carbon is an unavoidable element for increasing steel strength at low cost. Therefore, carbon is added in an amount of 0.01% or more. On the other hand, a very large carbon addition makes it difficult to ensure a heat-affected zone (ZAC) toughness at the time of the large heat input of the weld, and easily foules the cementite. Therefore, the upper limit of carbon content is controlled to be 0.14%.

Si (silicon) is an economical deoxidizing element and is added in an amount of 0.03% or more for solid solution hardening. On the other hand, a silicon content exceeding 0.5% deteriorates ZAC's weldability and toughness. Therefore, the upper limit of silicon content is controlled to be 0.5%.

Mn (manganese) is added in an amount of 0.3% or more since manganese is an effective element in improving steel strength and toughness. On the other hand, Mn deteriorates the toughness of ZAC and the fracture property of the weld. Therefore, the upper limit of manganese content is controlled to be 2.0%.

Although P (phosphorus) and S (sulfur) levels are preferably controlled to be as low as possible, a large cost is required to industrially reduce P and S levels. Therefore, the upper limits are controlled to be 0, 02% for P and 0.01% for S. Ni (nickel) is added in an amount of 0.5% or more since nickel is effective to ensure strength and to improve toughness holding ability. from ZAC. The Ni content is controlled to be at most 4.0% as increasing amounts of Ni result in rising costs of a plate.

Where added in small quantities, Nb (niobium) is an element that contributes to microstructure refining, transformation enhancement and precipitation enhancement, and is effective in ensuring matrix strength. Therefore, Nb is added in an amount of 0.005% or more.

On the other hand, excessive Nb addition hardens ZAC and noticeably deteriorates toughness. Therefore, the upper limit of Nb is controlled to be 0.050%.

Where added in a small amount, Ti (titanium) is effective in refining the structure, enhancing precipitation that improves the strength

base metal toughness and TiN generation that improves weld joint ZAC toughness. Therefore, Ti is added in an amount of 0.005% or more. On the other hand, an excessive addition of Ti noticeably deteriorates the toughness of ZAC. Therefore, the upper limit of Ti is controlled to be 0.050%.

Al (aluminum) is an important deoxidizing element and is added

in an amount of 0.002% or more. On the other hand, excessive aluminum addition deteriorates the surface quality of the plate and forms inclusions that disturb the toughness of the steel. Therefore, the upper limit of aluminum is controlled to be 0.10%.

N (nitrogen) is combined with Ti and forms better nitrides

ZAC's toughness. Therefore, N is added in an amount of 0.010% or more. On the other hand, an excessive addition of N generates soluble N brittleness. Therefore, the amount of Ni is controlled to be 0.0080% or less.

Additional optional elements are limited for the reason described.

below, down, beneath, underneath, downwards, downhill.

Each of the elements between Cu (copper), Cr (chrome), and Mo (molybdenum) increases the hardening capacity and is effective in reinforcing steel. Therefore, they are added in an amount of 0.05% or more. On the other hand, Cu is limited to 1.5% or less, and Cr and Mo are limited to 1.0% or less since their excessive addition deteriorates ZAC toughness.

V (vanadium) contributes to the increase of resistance by the effect of precipitation reinforcement. Therefore, V is added and an amount of 0.005% or more. On the other hand, its upper limit is controlled to be 0.10% since an addition of V exceeding 0.10% reduces ZAC toughness.

B (boron) is an element for improving hardening ability and is effective in increasing the strength of steel by adding it in an appropriate amount. On the other hand, an excessive addition of B deteriorates the weldability. Therefore, the boron content is controlled to be 0.0002 to 0.0030%.

Mg (magnesium), Ca (calcium), and REM form fine oxides or sulfides and contribute to ZAC toughness improvement. On the other hand, too much addition dulls inclusions and reduces toughness. Therefore, Mg is controlled to be in the range of 0.0003 to 0.0050%, Ca is controlled to be in the range of 0.0005 to 0.0030%, REM is controlled to be in the range of 0 , 0005 to 0.010%. REM denotes a rare earth element (rare earth metal) such as La, Ce, or the like.

In addition, to ensure base metal strength and joint strength consistently, the Ceq shown by the formula described below 25 must be controlled to be within a range of 0.30 to 0.50%. Where Ceq is less than 0.30%, it is difficult to guarantee the yield strength to be 390 MPa or more on a base metal made of thick steel having a sheet thickness of 50 mm or more. Where Ceq exceeds 0.50%, it is difficult to guarantee weldability and joint toughness. In addition, there is a possibility that the holding capacity is reduced by very high resistance.

Ceq = [C] + [Mn] / 6 + (rCu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5, where [element symbol] denotes the content (in % by mass) of the element. That is, where X denotes the symbol of an element, [X] denotes the content (in% by mass) of the element X.

Examples

The effects of the present invention are shown below.

according to examples. The present invention is not limited to the examples described below, and may be modified in any way within the scope of the invention.

Steel sheets each having a thickness of 50 to 80 mm were produced according to the production method shown in Tables 2 and 3 using plates each having the composition shown in Table 1. The microstructure, the strength of the base metal, and the retention capacity of the steel sheets are shown in Tables 4 and 5.

The crude surface ferrite fraction (surface gross fraction) was measured based on the image analysis of a T-section optical micrograph of an outer surface portion of a steel plate.

The perlite fraction was measured based on an optical microscopic photograph of a T-section obtained from 5 mm deep from the plate surface, a 1/4 portion of the plate thickness, and the central portion of the plate thickness. plate.

Replica samples were obtained from the three portions described above along the plate thickness. The cementite grain size (diameter Θ) was determined as the equivalent mean circle diameter calculated from photographs taken by an electron transmission microscope from the replica samples.

EBSP samples had been obtained from the three portions described above along the plate thickness so that T sections were subjected to measurement. In each sample, after measuring the 500 χ 500 pm region with all 1 pm peaks, orientation analysis was performed with a peak of 3 to 5 pm for a length of 2 mm based on a crystal orientation map. . Thus, the grain boundaries were determined. Then, the effective grain size was calculated by the dissection method according to JIS G0551.

Tensile strength (YP) and tensile strength (TS) were evaluated using a tensile strength specimen having a JIS Z 2201 dimension 4 taken along the T direction from the central portion of plate thickness.

The holding capacity was assessed by the temperature gradient ESSO test based on a temperature at which Kca = 6000N / mm1 · 5 was satisfied.

Table 1

Steel C Si Mn PS Ni Nb Ti Al NA 0.033 0.22 1.42 0.013 0.003 0.56 0.036 0.018 0.026 0.0037 B 0.090 0.15 0.72 0.005 0.003 1.80 0.007 0.015 0.025 0.0041 C 0.072 0 , 36 1.03 0.006 0.004 1.22 0.011 0.007 0.011 0.0021 D 0.061 0.24 0.56 0.008 0.002 3.54 0.006 0.009 0.022 0.0031 E 0.080 0.45 1.00 0.007 0.003 1.41 0.018 0.022 0.075 0.0054 F 0.110 0.33 0.65 0.009 0.002 1.98 0.009 0.013 0.030 0.038 G 0.077 0.20 1.50 0.004 0.002 1.16 0.013 0.009 0.039 0.0018 H 0.080 0.18 1.03 0.006 0.004 1.56 0.011 0.010 0.006 0.0041 I 0.014 0.14 1.71 0.010 0.003 0.75 0.014 0.042 0.019 0.0035 J 0.132 0.23 0.79 0.008 0.002 0.97 0.009 0.008 0.028 0.0036 K 0.083 0.27 1.44 0.006 0.002 0.63 0.005 0.009 0.020 0.0027 L 0.093 0.08 1.37 0.007 0.005 1.11 0.012 0.012 0.015 0.0030 M 0.058 0.04 1.05 0.005 0.002 1.46 0.017 0.018 0.024 0.0031 N 0.083 0.16 1.16 0.004 0.003 1.37 0.025 0.020 0.038 0.0071 O 0.052 0.25 1.20 0.006 0.004 2.88 0.011 0.011 0.029 0.0046 P 0.030 0.30 0.97 0.012 0.008 2.35 0.020 0.007 0.012 0.0022 Q 0.060 0.12 1.02 0.008 0.001 0.52 0.008 0.014 0.032 0.0025 R 0.144 0.94 0.92 0.003 0.003 1.20 0.010 0.008 0.018 0.0026 S 0.099 0.45 0.98 0.010 0.009 0.30 0.015 0.020 0.072 0.0050 T 0.071 0.30 1.76 0.017 0.008 2.38 0.020 0.008 0.014 0.0025 Table 1 -continuation

(% in large scale)

Steel Cu Cr Mo VB Mg Ca REM Ceq A --- --- --- --- --- 0.0016 --- 0.31 B --- --- --- 0.0011 --- --- --- 0.33 C 0.28 0.18 --- --- --- --- --- 0.38 D --- --- --- --- --- --- 0.39 E 0.45 --- --- 0.044 - --- --- 0.38 F - --- 0.56 --- --- 0.46 G - --- 0 .0015 --- 0.0041 0.40 H 0.72 --- --- --- 0.0023 --- --- 0.50 I 1.39 --- --- 0.066 0.0025 --- --- 0.45 J --- --- --- --- --- 0.0018 0.0012 --- 0.33 K --- --- --- --- --- --- --- 0.37 L 0.84 - --- --- --- --- --- 0.45 M _ 0 68 --- --- --- - - --- --- 0.47 N. --- 0.089 --- --- --- 0.39 O _ _ --- --- 0.0030 ---- 0.44 P _ _ --- --- --- 0.0028 0.35 Q _ 0.50 0.34 --- --- 0.0009 --- 0.43 R --- --- --- --- --- 0.38 S 0.44 0 , 0.15 0.050 --- --- 0.38 T --- --- --- 0.0070 0.52 0 underscore denotes values outside this invention Table 2 Heating Thickness Gross rolling CLASS N5 Steel Thickness Temp. (0C) Temperature Temperature Reduction Initial plate (mm) (0C) Fin. (0C) Cumulative (%) O 1 A 180 60 1135 1009 992 31 ■ <O 2 B 240 65 1058 980 964 33 3 B 220 55 1072 1005 978 48 I- 4 C 280 70 1050 983 966 43 Z LU LU 5 D 300 80 997 921 902 39 I CL <6 E 220 50 1088 1011 985 49 UJ Ct 7 E 220 60 1065 998 975 41 OZ 8 F 200 65 1026 960 932 40 O W 9 F 250 75 1020 954 927 40 O _I CL 10 G 250 70 1120 1014 987 38 LU X 11 H 300 80 10 75 1009 984 40 LU ro

ω Table 2 - continuedCLASS N2 Steel Heating Thickness Thickness Gross lamination Temp. (° C) Temperature Temperature Initial Reduction (0C) Fin. (0C) Cumulative (%) O 12 H 250 60 1070 1010 981 58 13 I 250 60 1056 984 962 40 14 I 220 50 1055 986 960 50 J 200 55 1090 1003 981 45 16 K 200 60 1010 970 948 40 17 L 220 70 1076 996 970 36 18 M 280 80 1095 1013 988 38 19 N 220 65 1141 1057 1020 34 O 250 75 970 926 907 44 21 P 200 50 1122 1024 1001 53 22 Q 200 60 1020 965 934 45 EXAMPLES ACCORDING TO THIS INVENTION Table 2 - continuation

CLASS

N2

10

11

Steel

G

H

Finishing Lamination

Temperature

initial

_ (° C)

805

825

850

810

860

833

825

819

815

798

791

Temperature

over

_ (° c)

778

803

829

792

841

806

801

798

792

775

773

Ar3

(0C)

774

745

742

752

677

733

737

690

694

724

731

T

(0C)

814

850

868

821

885

862

845

857

839

819

816

Cumulative Reduction _ (%)

52

59

52

56

57 55

54 46 50

55

56

Accelerated cooling

Cooling Rate

(° C / s)

16

15

10

12

9th

12

11

15

9th

13

9th

Initial Temperature (0C)

748

786

799

770

822

788

785

784

778

760

761

End temperature (0C)

458

306

475

213

255

280

116

141

184

239

275

Treatment

thermal

Temperature

(0C)

550

395

485

N>

Ol Table 2 - continuedCLASS Ns Steel Finishing Lamination Accelerated cooling Thermal treatment O Temperature Temperature Ar3 T Reduction Temperature Rate Temperature Temperature> <initial fin. (0C) (0C) Cumulative Initial cooling (0C) fin. (0C) (0C) O (0C) (0C) (%) (0C Is) 12 H 810 792 724 850 43 12 775 306 _ 13 I 805 784 718 821 60 8 765 425 586 14 I 820 797 715 838 55 14 773 384 J 815 795 769 837 50 12 776 269 16 K 800 782 753 817 50 11 765 346 17 L 805 786 715 817 50 10 770 280 18 M 800 781 743 812 54 8 769 135 523 19 N 822 803 736 835 55 10 788 304 O 845 824 663 871 46 9 807 286 21 P 880 860 709 897 47 16 842 401 22 Q 810 789 765 813 45 12 773 346 500 T = (37x [Ni] + 810x (1.1 -t / 500). EXAMPLES ACCORDING TO THIS INVENTION Table 3

N5 Steel Plate Thickness Plate Thickness Heating Raw Lamination Temperature Temperature Fin. Initial (0C) Rate (0C) Cooling (0C) (° C / s) 23 A 180 60 1135 1006 990 31 24 B 240 65 1060 983 965 33 B 220 55 1071 1005 980 48 26 C 280 70 1055 987 962 43 27 D 300 80 996 918 901 58 28 E 220 50 1090 1012 982 49 29 E 220 60 1164 998 977 41 F 200 65 1025 959 930 40 31 F 200 75 1020 955 927 25 32 G 210 70 1186 107 1 1036 26 33 H 300 80 1075 1005 981 40 34 I 250 60 1055 985 960 40 Table 3 -continuationHeating Gross rolling CLASS N2 Steel Plate Thickness Plate Thickness Temperature Temperature Fin. Initial (mm) (mm) (0C) rate TC) Cooling (0C) (° C Is) O 35 J 200 55 1093 1002 979 58 36 K 200 60 1010 970 948 40 37 L 220 70 1076 997 972 36 1- - 38 M 280 80 1097 1013 990 38 39 N 200 65 1141 1056 1018 28 <40 O 250 75 973 928 908 44 LU 2 41 P 200 50 1122 1024 1001 53 Cd OZ 42 Q 200 60 1020 965 934 45 43 R 280 70 1050 985 966 43 CL 44 S 220 50 1085 1012 983 49 45 T 200 50 1120 1029 1005 53 Table 3 -continuedCLASS N2 Steel Finishing Lamination Accelerated Cooling Heat Treatment Temperature Air Rate3 T Reduction Temperature Rate Temp. (0C) Cooling (0C) (0C) Cumulative Initial cooling fin. (0C) (° C Is) (%) (0C / s) (0C) TC) EXAMPLES ACCORDING TO THIS INVENTION 23 A 782 758 774 814 52 15 739 467 24 B 827 804 745 850 59 5 788 303 B 891 870 742 868 52 10 826 480 26 C 812 793 752 821 56 12 772 218 635 27 D 860 840 677 885 36 9 821 263 28 E 835 807 733 862 55 12 705 251 29 E 825 802 737 845 54 11 785 110 395 F 875 847 690 857 46 15 825 192 487 31 F 815 790 694 839 50 9 778 182. 32 G 795 772 724 819 55 13 758 235 _ 33 H 790 772 731 816 56 9 763 528 _ N>

CO Table 3 -continuedFinishing Rolling Accelerated Cooling Heat Treatment CLASS Ne Steel Temperature Air Rate3 T Reduction Temperature Rate Temp. (0C) Cooling (0C) (0C) Cumulative Initial cooling fin. (0C) CC Is) (%) (0CIs) (0C) TC) O 34 I 805 786 718 821 60 0Ji - - 'O 35 J 815 794 769 837 35 12 777 258 Z Lll> 36 K 840 822 753 817 50 11 805 375 Z ULI 37 L 805 786 715 817 50 4 771 276 IZ CO 38 M 800 780 743 812 54 8 769 138 620 39 N 820 801 736 835 55 10 788 303 LU 40 O 845 825 663 871 46 9 808 543 41 P 705 693 709 897 47 16 680 395 O 42 Q 810 790 765 813 45 12 741 334 503 O CO 43 R 810 794 747 820 56 12 768 220 575 O -I 0_ 44 S 804 780 776 821 55 13 759 286 2 LU LU 45 T 878 860 631 898 47 15 844 405 T = (37x [Ni] + 810 * (1.1 -t / 500). 0 underscore denotes values outside the range of the invention Table 4 Bake Microstructure N2 Steel Plate Thickness Bainite Fraction Perlite Fraction Super Fraction ¬ Average Size Average Size ((pm) grain Θ (mm) (%) (%) α Gross Surface ( %) (pm) effective grain (pm) O 1 A 60 63 1,5 5,3 0,24 14 15> <O 2 B 65 73 3,0 1,8 0,37 21 23 Z UJ Z 3 B 55 75 3.4 1.4 0.39 23 24 LU I-4 C 70 74 2.3 1.2 0.40 17 19 Z LU UJ 5 D 80 72 1.0 0.8 0.17 31 34 tr CL <6 E 50 80 2.2 2.5 0.28 20 22 LU QL 7 E 60 79 2.5 3.0 0.30 20 21 OZ 8 F 65 76 3.4 1.3 0.32 23 25 9 F 75 79 4, 1 2.2 0.36 23 24 CL 10 G 70 87 0.9 2.3 0.28 18 19 11 H 80 89 1.6 1.6 0.39 20 21 Table 4-continuationMicrostructure Class N2 Steel Sheet thickness Bainite fraction Perlite fraction Super¬ fraction Average size Average size ((Mm) (mm) (%) (%) gross surface area (%) effective grain θ (μηι) effective grain (μιη) O 1 H 60 92 1.4 1.1 0.31 20 22 '<13 I 60 90 0.2 8.4 0.10 15 16 z UJ Z 14 I 50 95 0.1 7.2 0.08 16 17 UJ 1- - 15 J 55 71 2.5 1.3 0.37 16 18 16 K 60 75 2.0 4.4 0.30 14 15 <17 L 70 86 2.3 2.8 0.35 16 18 UJ cr 18 M 80 84 1.8 2.5 0.40 17 20 19 N 65 78 2.1 3.0 0.31 18 20 O 75 85 1.5 2 .0 0.24 26 30 CL 21 P 50 66 0.8 4.6 0.16 25 29 LLI X 22 Q 60 91 1.2 2.5 0.22 14 15 LiJ Table 4 -continuationBase metal resistance holding Class N2 Steel YP TS TKca = 6000 (MPa) (MPa) (0C) O 1 A 443 566 -24 2 B 450 568 -21 2 3 B 467 577 -28 UJ l · --- 4 C 462 550 -20 2 UJ LU 5 D 426 539 -23 cc Cl <6 E 485 605 -36 UJ Ct 7 E 512 602 -32 O 2 8 F 488 584 -30 9 F 483 601 -28 CL 10 G 474 580 -23 2 LU X 11 H 469 570 -21 UJ Table 4 -continuedClass N5 Steel Base Metal Resistance Holding Capacity YP TS TKca = 6000 (MPa) (MPa) (0C) EXAMPLES ACCORDING TO THIS INVENTION 12 H 478 593 -24 1 I 502 604 -22 14 I 519 626 -27 J 469 561 -25 16 K 458 573 -24 17 L 490 620 -20 18 M 486 572 -19 19 N 464 587 -23 O 471 599 -20 21 P 453 574 -27 22 Q 467 597 -24 CO

d = (7.11 x [Ni] +11) x (1.2-t / 300) EXAMPLES AS PRESENT INVENTION Table 5

Class

Nq Steel Thickness Microstructure Sheet Metal Holding Capacity Resistance Fraction Fraction Fraction Fraction ¬ Mean Size ¬ Size 9 (Mm) YP TS T Kca = 6000 Perlite Bainite Surface α Gross Grain Average (MPa) (MPa) (0C) (%) (%) (%) (μηη) grain (Mm) 23 H 60 58 1.8 12 0.46 15 15 410 521 11 24 B 65 68 LZ 2.8 0.61 23 23 412 523 14 B 55 78 3.8 1.0 0.40 26 24 470 582 12 26 C 70 73 2.3 1.1 0.82 17 19 452 541 16 27 D 80 74 1.1 0.8 0.15 37 34 431 544 13 28 E 50 72 2.3 14 0.30 21 22 443 557 3 29 E 60 79 2.6 3.8 0.34 24 21 510 602 8 F 65 77 3.5 0.5 0.23 29 25 486 585 5 31 F 75 79 4.5 2.6 0.33 26 24 480 597 9 32 G 70 87 0.9 3.6 0.31 22 19 472 583 12 33 H 80 82 1.2 1, 3 0.55 20 21 427 531 21 Table 5 -continuationPerformance Resistance Microstructure metal holding base Class N2 Steel Thickness Fraction Fraction Fr F fraction m Size m m¬ Θ (Mm) YP TS T Kca = 6000 Perlite Bainite Plate gross α gio effective rate of (MPa) (MPa) (0C) (mm) (%) (%) (%) (μη) grain (μιτι) O 34 I 60 80 6j2 9.7 0.60 22 16 454 567 10 J 55 70 2.7 2.0 0.36 21 18 465 556 4 36 K 60 77 2.1 4.0 0.36 19 15 463 580 8 I- 37 L 70 81 5.3 3.5 0.53 18 18 480 607 11 Z LL 1 CO 38 M 80 83 1.9 2.5 0.76 19 20 475 559 15 39 N 65 79 2.3 3.1 0.35 22 20 465 586 12 LU 40 O 75 80 1.7 2.7 0.54 28 30 462 585 14 41 P 50 61 0.9 16 0.18 26 29 464 583 7 Z 42 Q 60 86 1.4 13 0.26 15 15 455 576 10 OO ω 43 R 70 82 4.2 1.0 0.57 18 19 483 576 20 O CL 44 S 50 85 2.0 3, 2 0.29 16 14 482 603 7 45 T 50 93 0.4 0.5 0.48 28 29 521 667 17 d = {7.11 * [Ni] +11) x (1.2-t / 300) Underline denotes values outside The sheet steel plates 1 to 22 as examples according to the present invention had chemical compositions within a predetermined range and were produced under predetermined conditions. Therefore, each of the plates had sufficient strength as a YP steel: grade 390 to 460MPa, and had a satisfactory holding capacity.

On the other hand, in steel plates 23-45 as comparative examples, the chemical composition or production condition was outside the predetermined range of the present invention. As a result, detention capacity was reduced in each case.

At Nos 23 and 41, finishing lamination temperatures

were smaller than Ar3, and crude ferrite was generated in large quantities in the surface portions, resulting in reduced holding capacity.

At ns 28 and 42, the finishing lamination temperature was not below Ar3, but accelerated cooling was initiated at a temperature below Ar3. Therefore, surface rough ferrite fractions were increased resulting in reduced holding capacity.

At paragraphs 24 and 37, accelerated cooling was performed at a low cooling rate.

At ns 33 and 40, the cooling interruption temperature was

greater than 500 ° C.

In Nos. 26 and 38, heat treatment temperatures were greater than 600 ° C. Therefore, the cementite was large in each case and sufficient holding capacity cannot be obtained.

# 34 underwent air cooling without accelerated cooling. Therefore, the effective grain size has not been refined, resulting in a reduction in holding capacity.

Nos. 27 and 35 were laminated with a small cumulative reduction of the finishing lamination.

At ns 25, 30, and 36, the effective grain size was stiffened due to the high finishing lamination temperatures, resulting in reduced holding capacity.

Ne 29 was heated to high temperature.

Nos 31 and 39 were roughly rolled with a small cumulative reduction.

The ns 32 was heated to a high temperature and was cold rolled.

with a small cumulative reduction. Therefore, the effective grain size was hardened, resulting in reduced holding capacity.

At 43, the cementite was oversized due to the excessive C content. As a result, the holding capacity was reduced and the toughness of the ZAC was reduced.

In paragraph 44, the holding capacity was insufficient due to the low Ni content.

At No. 45, resistance was greatly increased due to the high Ceq, resulting in reduced holding capacity.

INDUSTRIAL APPLICABILITY

Applying the present invention, high holding steel plates applicable to large buildings may be provided by a stable and effective production method, where the holding capacity indicators TKCa = 6o good plates are -10 ° C or below, even 20 when the steel sheets are thick sheets having a thickness of 50 mm or more, and the yield strength of the steel is in the range of 390 to 460 MPa. Therefore, the present invention has great industrial applicability.

Claims (6)

1. High-strength thick steel sheet, comprising: a composition containing by weight% C: 0,01 to 0,14% Si: 0,03 to 0,5% Mn: 0,3 to 2 , 0%, P: 0.020% or less, S: 0.010% or less, Ni: 0.5 to 4.0%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.050%, Al: 0.002 to 0.10 %, N: 0.0010 to 0.0080%, and the balance consisting of Fe and the inevitable impurities, where the Ceq defined by formula (1) described below is 0.30 to 0.50%; a bainite-dominated microstructure of 60% or more by volume fraction, where the perlite fraction is less than or equal to 5%, the crude ferrite fraction having a circle equivalent diameter greater than 25 pm is less than or equal to 10% in surface portions each having a depth of 5% of the plate thickness from the front and rear surfaces of the plate, the equivalent average cementite circle diameter is less than or equal to 0.5 pm, and the average diameter The equivalent of the circle of standard crack propagation resistant domains is greater than or equal to 8 pm and less than or equal to d (pm) defined by the formula (2) described below, where each of the standard crack propagation resistant domains is defined that the section perpendicular to the plate rolling direction is defined as section T, the direction parallel to the plate surface is defined as direction T in section T, the inner portion of section T is defined as the excluding surface portions, the T-section microstructure is divided into domains that have the same crystal orientation (iso-orientation domain) by internal portion orientation analysis using the Electron Back Scattering Pattern, an arbitrary measuring line. is drawn on the T-section microstructure divided into iso-orientation domains by applying the dissection method according to JIS G0551, a plurality of iso-orientation domains having an equivalent circle diameter of 8 pm continuously aligned and adjacent to each other in the Measurement lines are identified by excluding iso-orientation domains that have a circle equivalent diameter of less than 8μm, the axis <001> closest to the T direction is selected from three axes <001> of each of the iso domains. -identified orientation, an angle (crack propagation deviation angle) formed by the nearest <001> axes T-direction of the isoorienting domains is measured on each adjacent pair of the measurement line iso-orientation domains, continuously aligned iso-orientation domains satisfying an angle of less than or equal to 20 °, and adjacent iso-orientation domains having an equivalent circle diameter of less than 8pm are considered to constitute the standard crack propagation resistance domain. Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 ... (1) d = (7.11 x [Ni] +11) * (1,2-t / 300) ... (2) where [X] denotes the content (in% by mass) of an element X and t denotes the plate thickness (mm). Thick high strength steel plate according to claim 1, also containing by weight% one or two or more elements selected from Cu: 0,05 to 1,5%, Cr: 0,05 to 1 , 0%, Mo: 0.05 to 1.0%, V: 0.005 to 0.10%, and B: 0.002 to 0.0030%. The high strength thick steel plate of claim 1, also containing by weight% one or two or more elements selected from Mg: 0.0003 to 0.0050%, Ca: 0.0005 to 0 , 0030%, and REM: 0.0005 to 0.010%. Thick high strength steel plate according to claim 2, also containing by weight% one or two or more elements selected from Mg: 0,0003 to 0,0050%, Ca: 0,0005 to 0 , 0030%, and REM: 0.0005 to 0.010%. A method of producing a high strength thick steel plate, comprising: reheating a steel plate having a composition as defined in any one of claims 1 to 4, above to 950 to 1150 ° C; rough rolling steel at a temperature of at least 900 ° C with a cumulative reduction of 30% or more; perform finishing lamination on steel at a temperature of at least Ar3 and T (0C) defined by formula (3) described below, with a cumulative reduction of 40%; Perform accelerated cooling of the steel with a cooling rate of 8 ° C / s or more on average through the plate thickness from a temperature of at least Ar3 to a temperature of at most 500 ° C. T = (37x [Ni] +810) x (1,1-t / 500) ... (3) where [Ni] denotes Ni content (in mass%) and t denotes sheet thickness (mm) . The sheet steel production method of claim 5, further comprising quenching the steel at a temperature of 300 to 600 ° C upon completion of accelerated cooling.