JP7327444B2 - Thick steel plate, method for manufacturing thick steel plate, and structure - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel plate, and more particularly to a steel plate having excellent total elongation, fatigue crack propagation resistance, and paint durability. The steel plate of the present invention can be suitably used for welded structures such as ships, offshore structures, bridges, buildings, tanks, etc., which are used outdoors in an atmospheric corrosive environment and strongly required to have structural safety. Among others, the thick steel plate of the present invention can be suitably used for structures such as bridges that are used under severe corrosive environments such as the sea or near the coast where a large amount of airborne salt content is present. The present invention also relates to a method for manufacturing the thick steel plate and a structure using the thick steel plate.

Steel plates are widely used in structures such as ships, offshore structures, bridges, buildings and tanks. The steel plate is required to have excellent mechanical properties such as strength and toughness and excellent weldability, as well as excellent fatigue properties.

That is, when using a structure as described above, a repeated load such as vibration due to wind or an earthquake is applied to the structure. Therefore, steel plates are required to have fatigue properties that can ensure the safety of structures even when such repeated loads are applied. In particular, it is effective to improve fatigue crack propagation resistance of steel plates in order to prevent eventual destruction such as breakage of members.

Therefore, various studies have been made to improve the fatigue crack propagation resistance of steel sheets.

For example, Patent Literature 1 proposes a steel plate for tankers that has excellent fatigue crack propagation resistance in a wet hydrogen sulfide environment. The steel sheet has a mixed structure consisting of ferrite as a first phase and bainite and/or pearlite as a second phase. Further, in the steel sheet, the average grain size of ferrite is 20 μm or less.

Further, Patent Document 2 also proposes a steel sheet having excellent resistance to fatigue crack propagation. The steel sheet has a microstructure consisting of a hard portion and a soft portion, and is characterized in that the hardness difference between the hard portion and the soft portion is 150 or more in terms of Vickers hardness.

Patent Document 3 proposes a duplex steel having a microstructure consisting of bainite and 38 to 52% ferrite in terms of area ratio. In the technique proposed in Patent Document 3, the fatigue crack propagation resistance is improved by controlling the Vickers hardness of the ferrite phase portion and the density of the boundary between the ferrite phase and the bainite phase.

By the way, steel structures used outdoors, such as bridges, are usually provided with measures to ensure some kind of corrosion resistance.

For example, in an environment with a small amount of airborne salt, weather-resistant steel is often used as a material for structures. Weathering steel is steel to which appropriate amounts of alloying elements such as Cu, P, Cr, and Ni are added, as specified in JIS G 3114 as an SMA material. When the weathering steel is used in an atmospheric exposure environment, the surface of the weathering steel is covered with a highly protective rust layer in which the alloying elements are concentrated, thereby greatly reducing the corrosion rate. It is known that bridges using such weather-resistant steel can withstand several decades of service without painting in an environment with a small amount of airborne salt.

On the other hand, it is difficult to use even weather-resistant steel without painting in an environment with a large amount of airborne salt such as the sea or near the coast. This is because a highly protective rust layer is difficult to form in an environment with a large amount of airborne salt. For this reason, in environments where there is a large amount of airborne salt, such as in the sea or near the coast, coated steel materials that have undergone anti-corrosion treatment such as coating are generally used.

However, coated steel requires periodic repairs such as repainting due to deterioration of the coating film, rust generation, blistering of the coating film, etc. over time. The painting work associated with repainting is often done at high places, and the work itself is difficult, and labor costs for the work are also required. Therefore, when using coated steel, there is a problem that the maintenance cost of the structure increases, and thus the life cycle cost increases.

Therefore, by extending the repainting cycle, the frequency of painting can be reduced, and the maintenance cost of structures can be reduced. Steel materials with excellent corrosion resistance, especially structural steel materials with excellent painting durability is desired.

In response to the above demands, in Patent Documents 4 to 7, in addition to alloying elements such as Cu, P, Cr, and Ni, Sn and Sb are added to significantly improve corrosion resistance in a high airborne salt environment. A high weathering steel has been proposed.

JP-A-06-322477 JP-A-07-242992 JP-A-08-225882 Japanese Patent Application Laid-Open No. 2006-118011 JP 2010-007109 A JP 2012-255184 A JP 2013-166992 A

However, it has been found that the conventional techniques described in Patent Documents 1 to 7 have the following problems (1) to (4).

(1) For steel materials used in structures such as ships, offshore structures, bridges, buildings, and tanks, the total elongation value is generally specified in standards. Therefore, even steel sheets with excellent fatigue crack propagation resistance are required to satisfy the standard value for total elongation.

However, since fatigue crack propagation resistance and total elongation are contradictory properties, conventional techniques such as those described in Patent Documents 1 to 7 cannot achieve both excellent fatigue crack propagation resistance and total elongation. I couldn't do it.

That is, the techniques proposed in Patent Documents 1 to 7 do not consider total elongation. In fact, the steel sheets proposed in Patent Documents 1 to 3 all have a microstructure consisting of ferrite as a soft phase and bainite or martensite as a hard phase. In the steel sheet, fatigue crack propagation resistance is improved by increasing the hardness difference between the soft phase and the hard phase. However, if the hardness difference between the soft phase and the hard phase is large, the structure becomes heterogeneous, resulting in a decrease in the total elongation of the steel sheet.

(2) From the viewpoint of ensuring the safety of structures, steel plates have excellent fatigue crack propagation resistance not only in one direction but also in all of the plate thickness direction, rolling direction, and width direction. is required.

That is, in general structures, steel plates are freely welded from various directions. Therefore, fatigue cracks initiate and propagate in various directions. In addition, at a welded portion having narrow-angled corners, the occurrence of fatigue cracks is unavoidable due to its structural characteristics, and the fatigue cracks that have occurred tend to first propagate in the plate thickness direction. However, in order to prevent the structure from collapsing due to fatigue cracks, it is necessary to suppress the propagation of fatigue cracks in the width direction and the rolling direction even after the fatigue cracks have penetrated the steel plate in the thickness direction. is important.

However, in the conventional techniques as described in Patent Documents 1 to 7, the directional dependence of fatigue crack propagation resistance was not taken into consideration.

(3) Furthermore, it is difficult to control manufacturing conditions for conventional steel sheets having the microstructures described above. That is, when the steel sheet is produced by an online process, in order to obtain the desired structure, in the cooling step after hot rolling, accelerated cooling is started from the two-phase region of ferrite and austenite, and the cooling stop temperature is lowered. There is a need to. At that time, the area fraction of the soft phase and the hard phase in the finally obtained microstructure varies greatly depending on the temperature at the start of cooling. Therefore, in the production of conventional steel sheets, it is necessary to strictly control the cooling conditions in order to obtain the desired microstructure.

(4) In addition, the steel materials proposed in Patent Documents 1 to 3 do not have sufficient weather resistance. On the other hand, if alloying elements such as Cr and Sn are excessively contained together with Cu and Ni in order to improve weather resistance as in Patent Documents 4 to 7, not only the alloy cost increases but also the toughness deteriorates.

The present invention has been made in view of the above circumstances, and without causing an excessive increase in alloy cost, it can be used in harsh outdoor atmospheric corrosion environments such as bridges, especially near seas and coasts where there is a large amount of airborne salt. Even when used in a corrosive environment, it has excellent coating durability that can extend the recoating cycle and reduce the frequency of coating, and also has excellent total elongation and fatigue crack propagation resistance. An object is to provide an excellent thick steel plate.

Specifically, the object is to provide a thick steel plate having the following excellent characteristics (1) to (4).
(1) Combines excellent fatigue crack propagation resistance with total elongation.
(2) Excellent resistance to fatigue crack propagation in all of the plate thickness direction, rolling direction and width direction.
(3) It can be manufactured without requiring advanced cooling control in the two-phase region.
(4) Excellent coating durability.

The present inventors have obtained the following knowledge as a result of conducting studies to solve the above problems.

(a) Sufficient fatigue crack propagation resistance can be obtained even if the hardness difference between the soft phase and the hard phase in the microstructure is not as large as in Patent Documents 1 to 3.

(b) By using bainite as the first phase, fatigue crack propagation resistance can be improved more than before.

(c) A microstructure containing both bainite as a soft phase and pearlite as a hard phase in a specific area fraction, and the crystal grain sizes of the bainite and the pearlite are within specific ranges. , a steel plate having both excellent fatigue crack propagation resistance and total elongation can be obtained.

(d) The steel plate having the above microstructure can be produced by controlling production conditions, particularly conditions in hot rolling and subsequent accelerated cooling. Since the thick steel plate has bainite as the first phase, it is more suitable for production by an on-line process than conventional steel plates.

(e) Addition of W remarkably suppresses the formation of rust under the paint film, and is effective in preventing paint film blistering and paint film peeling.

The present invention has been made based on the above findings, and has the following gist.

1. in % by mass,
C: 0.01 to 0.16%,
Si: 1.00% or less,
Mn: 0.50-2.00%,
P: 0.030% or less,
S: 0.020% or less,
Al: 0.06% or less, and W: 0.005 to 1.00%,
Having a component composition in which the balance is Fe and unavoidable impurities,
area fraction,
75-97% bainite and 3-25% perlite,
The bainite crystal grain size is 18 μm or less in terms of the average circle equivalent diameter,
A steel plate having a microstructure in which pearlite crystal grains have an average equivalent circle diameter of 10 µm or less.

2. The component composition further, in mass %,
Cr: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
Co: 0.01 to 1.00%,
Sn: 0.005 to 0.200%,
Sb: 0.005 to 0.200%,
Nb: 0.005 to 0.050%,
V: 0.005 to 0.050%,
Ti: 0.005 to 0.050%,
B: 0.0001 to 0.0050%,
Zr: 0.005-0.100%
Ca: 0.0001 to 0.020%,
2. The steel plate according to 1 above, containing one or more selected from the group consisting of Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%.

3. The microstructure is
3. The steel plate according to 1 or 2 above, containing 81-97% bainite and 6-25% pearlite.

4. 4. The steel plate according to any one of 1 to 3 above, which has a coating film on its surface.

5. the coating comprises an anticorrosion undercoat layer, an undercoat layer, an intermediate coat layer, and a topcoat layer;
The anti-corrosion underlayer comprises an inorganic zinc-rich paint;
The undercoat layer is an epoxy resin paint,
The intermediate coating layer is an intermediate coating for a fluororesin top coating,
5. The steel plate according to 4 above, wherein each of the topcoat layers uses a fluororesin topcoat paint.

6. heating the steel material having the chemical composition described in 1 or 2 above to a heating temperature of 1000° C. or higher and 1250° C. or lower;
Hot-rolling the heated steel material into a hot-rolled steel sheet,
The hot-rolled steel sheet is accelerated and cooled under the following conditions: cooling start temperature: Ar 3 or more, cooling stop temperature: 450 to 700 ° C., average cooling rate on the steel plate surface from cooling start to cooling stop: 20 to 60 ° C./s. A method for manufacturing a thick steel plate,
A steel plate having a cumulative rolling reduction of 80% or more in a temperature range of 950°C or higher and a cumulative rolling reduction of 50% or higher in a temperature range of less than 950°C and an Ar3 point or higher in the hot rolling. manufacturing method.

7. A structure using the thick steel plate according to any one of 1 to 5 above.

8. 8. Structure according to claim 7, wherein said structure is a bridge.

The steel plate of the present invention has both excellent fatigue crack propagation resistance and total elongation, and is excellent in fatigue crack propagation resistance in all of the plate thickness direction, rolling direction and width direction. Furthermore, since the steel plate of the present invention has excellent coating durability, even when it is used outdoors in an atmospheric corrosive environment, especially in a severe corrosive environment such as the sea or near the coast where there is a large amount of airborne salt, The frequency of coating can be reduced by extending the recoating cycle. In addition, the steel plate of the present invention is advantageous in terms of cost because the above-described excellent properties can be achieved without adding a large amount of alloying elements such as Cr and Sn. Moreover, the steel plate of the present invention can be stably produced without requiring advanced cooling control in the two-phase region.

FIG. 3 is a diagram showing the shape and dimensions of a single-side notched simple tensile fatigue test piece used for evaluation of fatigue crack propagation properties in the plate thickness direction.

The present invention will be described in detail below. However, the present invention is not limited to this embodiment.

[Component composition]
First, the chemical composition of the steel plate of the present invention will be described. Unless otherwise specified, "%" representing the content of each component means "% by mass".

C: 0.01-0.16%
C is an element having the effect of improving the strength. In addition, C has the effect of promoting the formation of pearlite phase, which is advantageous for fatigue resistance. If the C content is less than 0.01%, the desired strength and fatigue crack propagation resistance cannot be obtained. Therefore, the C content is made 0.01% or more. On the other hand, when the C content exceeds 0.16%, pearlite is excessively formed or coarsened, resulting in deterioration of total elongation and toughness. Therefore, the C content should be 0.16% or less, preferably 0.15% or less, and more preferably 0.10% or less.

Si: 1.00% or less Si is an element that has a deoxidizing effect and also has the effect of further improving the strength. Si also has the effect of suppressing excessive cementite formation. Furthermore, Si also has the effect of forming a dense rust layer on the surface of the steel plate and improving the coating durability. However, if the Si content exceeds 1.00%, the weldability and toughness deteriorate, and the formation of the pearlite phase, which is advantageous for fatigue resistance, is suppressed. Therefore, the Si content should be 1.00% or less, preferably 0.50% or less. On the other hand, the lower limit of the Si content is not particularly limited, but from the viewpoint of enhancing the effect of adding Si, the Si content is preferably 0.01% or more, preferably 0.10% or more.

Mn: 0.50-2.00%
Mn is an element that has the effect of increasing the hardenability and, as a result, improving the strength of the steel plate. In order to obtain the above effects, the Mn content should be 0.50% or more, preferably 0.80% or more. On the other hand, if the Mn content exceeds 2.00%, the hardenability becomes too high, and as a result, the formation of the pearlite phase, which is advantageous for fatigue resistance, is suppressed. Moreover, when the Mn content exceeds 2.00%, the total elongation and toughness are lowered. Therefore, the Mn content should be 2.00% or less, preferably 1.65% or less.

P: 0.030% or less P deteriorates toughness. Therefore, the P content is made 0.030% or less. On the other hand, the lower the P content, the better, so the lower limit of the P content is not particularly limited, and the P content may be 0% or more, or may exceed 0%.

S: 0.020% or less S is an element contained in steel plates as an impurity, and deteriorates toughness. Therefore, the S content should be 0.020% or less, preferably 0.010% or less. On the other hand, the lower the S content, the better, so the lower limit of the S content is not particularly limited, and the S content may be 0% or more, or may exceed 0%. However, excessive reduction increases the production cost, so from the viewpoint of production cost, the S content is preferably 0.0005% or more, more preferably 0.001% or more.

Al: 0.06% or less Al is an element that acts as a deoxidizing agent and is generally used in molten steel deoxidizing processes. In addition, Al fixes N in the steel as AlN and contributes to improving the toughness of the base material. However, if the Al content exceeds 0.06%, the toughness and total elongation of the base metal (thick steel plate) are lowered, and Al is mixed into the welded metal portion during welding, deteriorating the toughness of the welded portion. Therefore, the Al content should be 0.06% or less, preferably 0.05% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of increasing the effect of adding Al, the Al content is preferably 0.01% or more.

W: 0.005-1.00%
W is an element that has the effect of greatly improving the coating durability of thick steel plates. That is, W is eluted with the anodic reaction of the steel material in a corrosive environment and distributed as WO ₄ ²⁻ in the rust layer on the surface of the steel material. As a result, chloride ions, which are corrosion-promoting factors, are electrostatically prevented from permeating the rust layer and reaching the steel substrate. Furthermore, the anodic reaction itself of the steel material is suppressed by the precipitation of W dissolved out in the corrosive environment on the surface of the steel material as a compound containing W. Further, when W is added together with at least one alloying element selected from the group consisting of Cu, Ni, Sn, and Sb, the synergistic effect of W and the alloying element further improves the coating durability of the steel plate. In order to obtain the above effects, the W content should be 0.005% or more, preferably 0.010% or more, more preferably 0.030% or more, and still more preferably 0.050% or more. On the other hand, when the W content exceeds 1.00%, the alloy cost rises significantly. Therefore, the W content is 1.00% or less, preferably 0.70% or less, more preferably 0.50% or less, and still more preferably 0.30% or less.

A steel plate according to an embodiment of the present invention may have a chemical composition containing the above elements, with the balance being Fe and unavoidable impurities.

In addition, the chemical composition of the steel plate in another embodiment of the present invention can optionally contain at least one of the elements listed below. By adding these optional additive elements, properties such as strength, toughness, weldability, weather resistance and paint durability of the steel plate can be further improved.

Cr: 0.01-1.00%
Cr is an element that has the effect of further improving the strength. Moreover, Cr is an element that promotes the formation of cementite, and promotes the formation of pearlite phase that is advantageous for fatigue resistance. Moreover, Cr has the effect of forming a dense rust layer and further improving the coating durability. When Cr is added, the Cr content should be 0.01% or more, preferably 0.10% or more, in order to obtain the above effects. On the other hand, when the Cr content exceeds 1.00%, weldability and toughness are impaired. Therefore, the Cr content is 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

Cu: 0.01-1.00%
Cu is an element that has the effect of further increasing the strength through solid solution and improving the weather resistance. In addition, Cu has the effect of suppressing permeation of oxygen and chloride ions, which are corrosion-promoting factors, into the base iron by forming a dense rust layer. By adding Cu together with W, the coating durability of the steel material is greatly improved. When Cu is added, the content of Cu is set to 0.01% or more in order to obtain the above effect. On the other hand, when the Cu content exceeds 1.00%, the weldability is impaired, and defects tend to occur during the production of thick steel plates. Therefore, the Cu content should be 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.

Ni: 0.01-2.00%
Ni is an element that has the effect of improving low-temperature toughness, and improves hot shortness when Cu is added. In addition, Ni has the effect of suppressing permeation of oxygen and chloride ions, which are corrosion-promoting factors, into the base iron by forming a dense rust layer. By adding Ni together with W, the coating durability of the steel material is greatly improved. When Ni is added, the Ni content is made 0.01% or more in order to obtain the above effect. On the other hand, when the Ni content exceeds 2.00%, the weldability is impaired, and the steel material cost increases. Therefore, the Ni content should be 2.00% or less, preferably 1.50% or less, more preferably 1.00% or less, still more preferably 0.70% or less, and most preferably 0.40% or less.

Mo: 0.01-1.00%
Mo is an element that has the effect of further improving the strength. When Mo is added, the Mo content should be 0.01% or more to obtain the above effect. On the other hand, when the Mo content exceeds 1.00%, weldability and toughness are impaired. Therefore, the Mo content is 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.

Co: 0.01-1.00%
Co has the effect of improving the weather resistance by being distributed over the entire rust layer and forming a dense rust layer. In order to obtain this effect, when Co is contained, the Co content is made 0.01% or more, preferably 0.35% or more. On the other hand, even if the Co content is higher than 1.00%, the effect is saturated and the alloy cost increases. Therefore, the Co content should be 1.00% or less, preferably 0.50% or less.

Sn: 0.005-0.200%
Sn is an element that has the effect of suppressing the anode reaction of steel in a corrosive environment. In addition, Sn is present in the rust layer that exists at the interface between the base iron and the corrosive environment. prevent reaching the By adding Sn together with W, the coating durability of the steel plate is greatly improved. In order to sufficiently obtain the above effect, it is necessary to contain 0.005% or more of Sn. Therefore, when Sn is added, the Sn content should be 0.005% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, when the Sn content exceeds 0.200%, the ductility and toughness of the steel deteriorate. Therefore, the Sn content should be 0.200% or less, preferably 0.100% or less, and more preferably less than 0.050%.

Sb: 0.005-0.200%
Sb, like Sn, is an element that has the effect of suppressing the anode reaction of steel materials in a corrosive environment. In addition, Sb is present in the rust layer that exists at the interface between the base iron and the corrosive environment. prevent reaching the By adding Sb together with W, the coating durability of the steel material is greatly improved. In order to sufficiently obtain the above effect, it is necessary to contain 0.005% or more of Sb. Therefore, when Sb is added, the Sb content is set to 0.005% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, when the Sb content exceeds 0.200%, the ductility and toughness of the steel deteriorate. Therefore, the Sb content is 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less.

Nb: 0.005-0.050%
Nb is an element that has the effect of suppressing recrystallization of austenite during hot rolling and refining the finally obtained crystal grains. In addition, Nb precipitates during air cooling after accelerated cooling to further improve the strength. When Nb is added, the Nb content is made 0.005% or more in order to obtain the above effect. On the other hand, if the Nb content exceeds 0.050%, the hardenability becomes excessive and martensite is formed, making it impossible to obtain a desired structure and reducing toughness. Therefore, the Nb content should be 0.050% or less, preferably 0.040% or less.

V: 0.005-0.050%
V is an element that precipitates during air cooling after accelerated cooling and has the effect of further improving strength. When V is added, the V content is made 0.005% or more in order to obtain the above effect. On the other hand, when the V content exceeds 0.050%, the weldability and toughness deteriorate. Therefore, the V content should be 0.050% or less, preferably 0.030% or less.

Ti: 0.005-0.050%
Ti is an element that has the effect of further increasing the strength and improving the weld zone toughness. When Ti is added, the Ti content is made 0.005% or more in order to obtain the above effect. On the other hand, when the Ti content exceeds 0.050%, the cost rises significantly. Therefore, the Ti content should be 0.050% or less, preferably 0.030% or less, and more preferably 0.020% or less.

B: 0.0001 to 0.0050%
B is an element that has the effect of increasing the hardenability and, as a result, further improving the strength. When B is added, the B content is made 0.0001% or more in order to obtain the above effect. On the other hand, if the B content exceeds 0.0050%, the hardenability becomes excessive and martensite is formed, making it impossible to obtain a desired structure and weldability is lowered. Therefore, the B content should be 0.0050% or less, preferably 0.0030% or less.

Zr: 0.005% to 0.100%
Zr is an element that has the effect of further increasing the strength. In order to sufficiently obtain the above effects, it is necessary to contain 0.005% or more of Zr. Therefore, when adding Zr, the Zr content is made 0.005% or more. On the other hand, when the Zr content exceeds 0.100%, the strength improvement effect is saturated. Therefore, when Zr is contained, the Zr content should be 0.100% or less.

Ca: 0.0001-0.020%
Ca is an element that controls the morphology of sulfides and, as a result, has the effect of further improving toughness. When Ca is added, the content of Ca is set to 0.0001% or more in order to obtain the above effect. On the other hand, when the Ca content exceeds 0.020%, the effect is saturated. Therefore, the Ca content is set to 0.020% or less.

Mg: 0.0001-0.020%
Mg is an element that has the effect of improving the toughness through refinement of crystal grains. When adding Mg, the Mg content is made 0.0001% or more in order to obtain the above effect. On the other hand, when the Mg content exceeds 0.020%, the effect is saturated. Therefore, the Mg content is set to 0.020% or less.

REM: 0.0001-0.020%
REM (rare earth metal) is an element that has the effect of improving toughness. When adding REM, the REM content is made 0.0001% or more in order to obtain the above effect. On the other hand, when the REM content exceeds 0.020%, the effect is saturated. Therefore, the REM content is set to 0.020% or less.

[Microstructure]
Next, the microstructure of the steel plate will be explained. The thick steel plate in one embodiment of the present invention contains 75 to 97% bainite and 3 to 25% pearlite in terms of area fraction, and the grain size of the bainite is 18 μm or less in terms of the average circle equivalent diameter, and the pearlite crystal It has a microstructure with an average equivalent circle diameter of 10 µm or less. The microstructure in the present invention refers to the microstructure at the 1/4 position (1/4t position) of the plate thickness t of the steel plate. The area fraction and grain size of each structure can be measured by nital-corroding a section parallel to the rolling direction at a depth of 1/4 from the surface of the steel plate and observing it. More specifically, the area fraction and crystal grain size can be determined by the methods described in the Examples.

Area fraction of bainite: 75-97%
In the present invention, bainite is the first phase in the microstructure and functions as a soft phase. Ferrite is a typical soft phase contained in steel materials, but bainite has a higher effect of suppressing crack growth than ferrite. Therefore, by setting the area fraction of bainite to 75% or more, the propagation of fatigue cracks can be suppressed. If the area fraction of bainite is less than 75%, the desired fatigue crack propagation resistance cannot be obtained. The area fraction of bainite is preferably 80% or more, more preferably 81% or more. On the other hand, if the area fraction of bainite exceeds 97%, pearlite becomes insufficient, and as a result, propagation of fatigue cracks cannot be suppressed. Therefore, the area fraction of bainite is set to 97% or less.

Grain size of bainite: 18 μm or less The grain size of bainite is set to 18 μm or less in terms of average circle equivalent diameter. By refining the bainite, desired toughness and total elongation properties can be obtained. If the grain size of bainite exceeds 18 μm in terms of average equivalent circle diameter, desired toughness cannot be obtained. On the other hand, the lower limit of the grain size of bainite is not particularly limited, but since excessive refinement makes production difficult, the grain size of bainite is preferably 5 μm or more in actual production.

Bainite in the present invention includes upper bainite, acicular ferrite, and granular bainite.

Area fraction of perlite: 3 to 25%
In the present invention, pearlite is the second phase in the microstructure and functions as a hard phase. When a fatigue crack propagating in bainite reaches the hard phase pearlite, the crack stops or bends at the interface between bainite and pearlite. As a result, propagation of cracks is suppressed. In order to obtain the above effect, the area fraction of pearlite is set to 3% or more, preferably 5% or more, and more preferably 6% or more. On the other hand, when the area fraction of pearlite exceeds 25%, the total elongation decreases. Therefore, the area fraction of pearlite is set to 25% or less, preferably 20% or less.

Crystal grain size of pearlite: 10 µm or less The grain size of pearlite is set to 10 µm or less in terms of average circle equivalent diameter. By refining the pearlite, desired toughness and total elongation properties can be obtained. If the pearlite grain size exceeds 10 μm in terms of average circle equivalent diameter, the desired toughness cannot be obtained. On the other hand, the lower limit of the crystal grain size of pearlite is not particularly limited, but may be 1 μm or more, or may be 2 μm or more.

In addition, the pearlite in the present invention includes pearlite and pseudo-perlite.

(other organizations)
The steel plate in one embodiment of the present invention can have a microstructure consisting of bainite and pearlite. However, said microstructure may optionally further comprise other structures. Said other texture may be, for example, one or both of martensite and ferrite. Here, the martensite includes island martensite, lath martensite, and lenticular martensite.

When the tissue exists, the area fraction (total area fraction) of the other tissue is not particularly limited. However, if martensite is excessively present, regions of high hardness are formed locally, and although the strength increases, the total elongation may deteriorate and the toughness may decrease. Further, when ferrite is excessively present, the fatigue crack propagation speed is deteriorated, and in addition, soft regions are locally formed, which may increase the hardness difference and deteriorate the total elongation. Therefore, the lower the area fraction of other structures, the better, but if the area fraction is 5% or less, the effect can be ignored. Therefore, the total area fraction of structures other than bainite and pearlite is preferably 5% or less, more preferably 4% or less.

In other words, the thick steel plate in one embodiment of the present invention is
75-97% bainite,
It can have a microstructure consisting of 3-25% perlite and 0-5% bainite and non-perlite structures.

(Thickness)
In the present invention, a steel plate having a thickness of 6 mm or more is defined as a "thick steel plate" according to the usual definition. On the other hand, the upper limit of the plate thickness is not particularly limited, but the present invention is particularly suitably applied to relatively thin thick steel plates. Therefore, the plate thickness of the thick steel plate in the present invention is preferably 25 mm or less, more preferably less than 20 mm.

(Tensile strength)
The steel plate of the present invention can have excellent tensile strength (TS) as a result of having the above chemical composition and microstructure. Although the value of TS is not particularly limited, it is preferably 500 MPa or more, more preferably 530 MPa or more, and even more preferably 550 MPa or more. On the other hand, the upper limit of TS is also not limited, but may be, for example, 720 MPa or less, 700 MPa or less, 640 MPa or less, or 620 MPa or less.

(yield stress)
Although the yield stress (YS) of the steel plate of the present invention is not particularly limited, it may be 420 MPa or more, 430 MPa or more, or 440 MPa or more. Also, YS may be 560 MPa or less, 530 MPa or less, or 520 MPa or less.

(Toughness)
The steel plate of the present invention has excellent toughness as a result of having the above chemical composition and microstructure. The toughness of the steel plate of the present invention is not particularly limited, but the Charpy absorbed energy _vE0 at 0 ° C., which is one of the indicators of toughness, is preferably 100 J or more, more preferably 130 J or more, and 150 J or more. More preferably, it is most preferably 200 J or more. On the other hand, the upper limit of _vE0 is also not limited, but may be, for example, 400J or less, 300J or less, or 270J or less. In addition, vE ₀ can be measured by the method described in the Examples.

(total elongation)
The steel plate of the present invention has excellent total elongation (EL) as a result of having the above chemical composition and microstructure. Although the value of EL is not particularly limited, it is preferably 15% or more, more preferably 16% or more, even more preferably 17% or more, and most preferably 20% or more. The upper limit of EL is also not particularly limited, but may be 30% or less. Note that EL can be measured by the method described in Examples.

(Fatigue crack propagation resistance)
The steel plate of the present invention has the above chemical composition and microstructure, and as a result, can have excellent resistance to fatigue crack propagation in all of the plate thickness direction, rolling direction, and width direction. A fatigue crack propagation rate (da/dN) can be used as an index of fatigue crack propagation resistance. The value of the fatigue crack propagation rate is not particularly limited.

The fatigue crack propagation speed in the plate thickness direction (Z direction) preferably satisfies the following conditions (a) and (b).
(a) stress intensity factor range ΔK: fatigue crack propagation rate under the condition of 15 MPa/m ^1/2 is 8.75 × 10 ^-9 (m/cycle) or less;
(b) Stress intensity factor range ΔK: The fatigue crack propagation rate under the condition of 25 MPa/m ^1/2 is 4.25 × 10 ^-8 (m/cycle) or less.

In addition, either one of the fatigue crack propagation speed in the rolling direction (L direction) and the fatigue crack propagation speed in the width direction (C direction) preferably satisfies the following conditions (c) and (d), More preferably, both satisfy conditions (c) and (d).
(c) Stress intensity factor range ΔK: fatigue crack propagation rate under the condition of 15 MPa/m ^1/2 is 1.75 × 10 ^-8 (m/cycle) or less,
(d) Stress intensity factor range ΔK: The fatigue crack propagation rate under the condition of 25 MPa/m ^1/2 is 8.50 × 10 ^-8 (m/cycle) or less.

[Coating film]
A thick steel plate in one embodiment of the present invention has a coating film on its surface. Any coating film can be used as the coating film without any particular limitation.

Examples of the coating film include a coating film having an anticorrosive undercoat layer, an undercoat layer, an intermediate coat layer, and a topcoat layer in this order.

The anticorrosive base layer is preferably an inorganic zinc-rich paint layer formed using an inorganic zinc-rich paint, and more preferably a layer containing an alkylsilicate resin and zinc powder. Examples of the inorganic zinc-rich paint include, but are not limited to, SD Zinc 1500 manufactured by Kansai Paint Co., Ltd.

The undercoat layer is preferably an epoxy resin coating film formed using an epoxy resin coating. The epoxy resin paint is not particularly limited, but an example thereof includes EPOMARINE HB(K) manufactured by Kansai Paint Co., Ltd.

The intermediate coating layer is preferably a coating film formed using an intermediate coating material for a fluororesin top coating material, and more preferably an epoxy resin coating material. Although the intermediate coating for the fluororesin top coating is not particularly limited, for example, Ceratect F intermediate coating manufactured by Kansai Paint Co., Ltd., which is a polyamide-cured epoxy resin intermediate coating, can be used.

The topcoat layer is preferably a fluororesin coating film formed using a fluororesin topcoat. The fluororesin topcoat is not particularly limited, but includes, for example, Ceratect F(K) topcoat manufactured by Kansai Paint Co., Ltd., which is a low-staining isocyanate-curable fluororesin paint.

[Manufacturing conditions]
Next, a method for manufacturing a thick steel plate according to the present invention will be described. A thick steel plate according to an embodiment of the present invention can be produced by sequentially performing the following steps (1) to (3) on a steel material having the chemical composition described above.
(1) Heating (2) Hot rolling (3) Accelerated cooling Hereinafter, conditions in each step will be described. Unless otherwise specified, the temperature refers to the surface temperature of the object to be treated (steel material or hot-rolled steel sheet).

(steel material)
As the steel material, any material can be used as long as it has the chemical composition described above. The chemical composition of the finally obtained thick steel plate is the same as the chemical composition of the steel material used. For example, a steel slab can be used as the steel material.

(1) Heating Heating temperature: 1000 to 1250°C
First, the steel material is heated to a heating temperature of 1000° C. or higher and 1250° C. or lower. If the heating temperature is less than 1000°C, the temperature necessary for the subsequent hot rolling cannot be ensured. On the other hand, if the heating temperature exceeds 1250° C., the crystal grains of the steel become coarse and the toughness deteriorates.

(2) Hot Rolling Next, the heated steel material is hot rolled to form a hot rolled steel sheet. At that time, in order to manufacture a thick steel plate that satisfies the conditions of the present invention, the cumulative rolling reduction in the hot rolling must satisfy the following conditions.

Cumulative rolling reduction in a temperature range of 950° C. or higher: 80% or higher By setting the cumulative rolling reduction in a temperature range of 950° C. or higher to 80% or higher, the austenite grains are refined. As a result, bainite generated by transformation during accelerated cooling and pearlite generated from untransformed austenite are refined. If the cumulative rolling reduction is less than 80%, bainite and pearlite are insufficiently refined, resulting in deterioration of toughness. On the other hand, the upper limit of the cumulative rolling reduction in the temperature range of 950° C. or higher is not particularly limited, but may be, for example, 90% or lower.

Cumulative reduction rate in the temperature range of less than 950 ° C. and Ar 3 points or more: 50% or more By setting the cumulative reduction rate in the temperature range of less than 950 ° C. and Ar 3 points or more to 50% or more, the austenite grains are refined and accelerated. Bainite generated by transformation during cooling and pearlite generated from untransformed austenite are refined. If the rolling reduction at the Ar point of 3 or more is less than 50%, bainite and pearlite are not sufficiently refined, and the toughness deteriorates. On the other hand, the upper limit of the cumulative rolling reduction in the temperature range of less than 950° C. and 3 points or more of Ar is not particularly limited, but may be, for example, 80% or less, or 75% or less.

Here, the Ar3 point can be obtained by the following formula.
Ar3 (°C) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
However, the element symbol in the above formula represents the content (% by mass) of the element in the steel material, and is zero when the element is not contained in the steel material.

(3) Accelerated cooling Next, the hot-rolled steel sheet obtained in the hot rolling process is acceleratedly cooled. The conditions for the accelerated cooling must be as follows.

Cooling start temperature: Ar 3 point or more If the cooling start temperature in the accelerated cooling is less than Ar 3 point, ferrite and coarse pearlite are excessively precipitated, and the strength and fatigue crack propagation resistance are lowered. Therefore, the cooling start temperature is set to Ar3 or higher. On the other hand, although the upper limit of the cooling start temperature is not particularly limited, it is preferably 870° C. or less from the viewpoint of ensuring the cumulative rolling reduction in the temperature range of Ar 3 or higher.

Further, the fact that the cooling start temperature is Ar 3 or higher necessarily means that the rolling end temperature is Ar 3 or higher. If the rolling end temperature is less than the Ar3 point, the rolling is performed in a two-phase region, and the total elongation is deteriorated. can be prevented.

Cooling stop temperature: 450-700°C
In order to transform untransformed austenite into a hard phase (pearlite), the cooling stop temperature in the accelerated cooling is set to 700° C. or less, preferably 650° C. or less. If the cooling stop temperature exceeds 700° C., ferrite is formed and the desired resistance to fatigue crack propagation cannot be obtained. On the other hand, if the cooling stop temperature is lower than 450° C., the amount of martensite produced increases, resulting in failure to obtain a desired microstructure and reduced toughness and total elongation. Moreover, the desired fatigue crack propagation resistance cannot be obtained because the formation of pearlite is insufficient. Therefore, the cooling stop temperature should be 450°C or higher, preferably 500°C or higher, and more preferably higher than 550°C.

Average cooling rate: 20-60°C/s
The average cooling rate in the accelerated cooling is set to 20° C./s or higher. If the average cooling rate is lower than 20° C./s, ferrite is generated and the desired microstructure cannot be obtained, resulting in a decrease in fatigue crack propagation resistance. Also, the desired total elongation cannot be obtained due to the reduced toughness. On the other hand, if the average cooling rate exceeds 60° C./s, residual stress and excessive martensite are generated due to cooling strain, resulting in deterioration of total elongation and toughness. Moreover, the desired fatigue crack propagation resistance cannot be obtained because the formation of pearlite is insufficient. Therefore, the average cooling rate is set to 60° C./s or less, preferably 50° C./s or less. The average cooling rate refers to the average cooling rate on the surface of the steel sheet from the start of accelerated cooling to the stop of accelerated cooling.

A method for performing the accelerated cooling is not particularly limited, and any method can be used, but it is preferable to use water cooling.

The treatment after the end of the accelerated cooling is not particularly limited. For example, the steel plate after accelerated cooling can be allowed to cool in the atmosphere. In the cooling, for example, cooling to room temperature is possible. Further, after the accelerated cooling is finished, the warp of the steel plate can be arbitrarily corrected by a hot leveler.

It should be noted that the temperature of the steel sheet immediately drops after hot rolling. Therefore, the thick steel plate of the present invention is preferably manufactured by an online process using equipment having a rolling device and an accelerated cooling device on a transfer line.

Hereinafter, the action and effects of the present invention will be described using examples. In addition, the present invention is not limited to the following examples.

A thick steel plate was manufactured by the following procedure.

First, a steel slab (steel material) having the chemical composition shown in Table 1 was produced by a converter-continuous casting method. The thickness of the steel slab was as shown in Table 2.

Next, the steel slab was heated to the heating temperature shown in Table 2, and then hot-rolled at the cumulative rolling reduction shown in Table 2 to obtain a hot-rolled steel sheet. Table 2 also shows the rolling end temperature in the hot rolling and the thickness (final thickness) of the obtained hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was acceleratedly cooled under the conditions shown in Table 2 to obtain a thick steel sheet. The plate thickness of the obtained thick steel plate is the same as the final plate thickness.

Microstructures, mechanical properties, fatigue crack propagation properties, and coating durability were evaluated for each of the obtained steel plates. Evaluation methods are described below. Table 3 shows the results of each evaluation.

(Microstructure)
First, a sample for microstructure observation was taken from a 1/4t position in the plate thickness direction of a thick steel plate so that the cross section in the longitudinal direction was the observation surface. Here, the longitudinal cross section refers to a cross section perpendicular to the width direction of the steel plate. Next, after the surface of the sample was nital-eroded, the structure was photographed with an optical microscope (400x) and a scanning electron microscope (SEM) at 2000x. The captured images were used to identify the structures present, and the images were analyzed to determine the area fraction of bainite, the area fraction of pearlite, and the total area fraction of other structures. SEM images were used to identify the pearlite structure, and optical microscope images were used to measure the area fraction of each structure.

(Crystal grain size of bainite)
Furthermore, the crystal grain size of bainite was measured using the microstructure observation sample. In the measurement, first, the surface of the sample was mirror-polished, and the crystal orientation was measured from the electron beam backscatter diffraction pattern using an electron back-scattering pattern (EBSP) device attached to the SEM. A region surrounded by 200 μm squares is measured at intervals of 0.3 μm, and a region surrounded by grain boundaries having a crystal orientation difference of 15° or more with adjacent crystal grains is defined as a crystal grain. An average circle equivalent diameter was obtained. The obtained average equivalent circle diameter is regarded as the grain size of bainite.

(Crystal grain size of pearlite)
When the observation surface of the microstructure observation sample after nital corrosion was observed with an optical microscope image, the region appearing black was observed with an SEM, and it was identified as perlite having a lamellar structure. After that, using image analysis software (Image-J), the area was determined from the number of pixels in the black region in the optical microscope image, and converted to the average circle equivalent diameter of perlite. The obtained average equivalent circle diameter is regarded as the grain size of pearlite.

(mechanical properties)
A full-thickness tensile test piece was taken from the plate width direction (C direction) of the thick steel plate. Using the full-thickness tensile test piece, a tensile test was performed according to JIS Z 2241 to measure yield strength (YS), tensile strength (TS), and total elongation (EL). In the above measurements, the type of test piece used was selected according to JIS Z 2241. Specifically, first, a tensile test was performed using a JIS No. 1A test piece. For 4, 6, 15, 17, and 24, the tensile test was performed again using the JIS No. 5 test piece, and the results of the tensile test using the JIS No. 5 test piece were adopted.

In addition, a Charpy impact test piece was sampled parallel to the rolling direction (L direction) from the center of the plate thickness of the thick steel plate, and the Charpy impact test was performed at 0 ° C. in accordance with JIS Z 2202, and the absorbed energy vE ₀ was measured. It was measured.

(Fatigue crack propagation resistance)
As an index of fatigue crack propagation resistance, the fatigue crack propagation speed (da/dN) in the plate thickness direction (Z direction), rolling direction (L direction), and width direction (direction perpendicular to the rolling direction, C direction) was measured under two conditions of stress intensity factor range ΔK: 15 MPa/m ^1/2 and 25 MPa/m ^1/2 respectively.

· Rolling direction and width direction The fatigue crack propagation speed in the rolling direction (L direction) was measured using a test piece taken from a thick steel plate so that the load application direction was the rolling direction. Similarly, the fatigue crack propagation speed in the width direction (C direction) was measured using a test piece taken from a thick steel plate so that the load application direction was the width direction. The test piece was a compact tension test piece conforming to ASTM E647. In the measurement, a fatigue crack propagation test was performed based on the crack gauge method to obtain the fatigue crack propagation speed.

- Plate thickness direction On the other hand, in the measurement of the fatigue crack propagation speed in the plate thickness direction (Z direction), a one-sided notched simple tensile fatigue test piece shown in Fig. 1 was used. The test piece was taken from the thick steel plate, and the fatigue crack propagation speed was measured when the crack propagated in the plate thickness direction.

(Paint durability)
A test piece of 70 mm×50 mm×5 mmt was taken from each of the obtained thick steel plates. Coating durability was evaluated using the test piece. Specifically, first, the surface of the test piece was subjected to shot blasting so that the degree of rust removal Sa defined in ISO 8501-1 was 2.5. The specimens were then ultrasonically degreased in acetone for 5 minutes and then air dried.

Next, the following paints (a) to (d) were sequentially applied to one surface of the test piece to form a coating film consisting of an anticorrosion base layer, undercoat layer, intermediate coat layer, and top coat layer. The thickness of each layer is also listed below.
(a) Anti-corrosion base layer: inorganic zinc rich paint (SD zinc 1500A manufactured by Kansai Paint Co., Ltd.), thickness: 75 μm
(b) Undercoat layer: epoxy resin paint (EPOMARINE HB (K) manufactured by Kansai Paint Co., Ltd.), thickness: 120 μm
(c) Intermediate coating layer: Intermediate coating for fluorine resin top coating (Ceratect F intermediate coating manufactured by Kansai Paint Co., Ltd.), thickness: 30 μm
(d) Topcoat layer: fluororesin topcoat (Ceratect F topcoat manufactured by Kansai Paint Co., Ltd.), thickness: 25 μm

After the coating film was formed, the other surface and the end surface of the test piece were sealed with a solvent type epoxy resin paint, and further coated with a silicone sealant.

A linear cut with a width of 1 mm and a length of 40 mm reaching the steel substrate was cut as an initial defect in the center of the coating film formed on the test piece. Then, a corrosion test was carried out under the conditions shown below.

・Corrosion test A solution obtained by diluting artificial sea salt with pure water to a predetermined concentration is sprayed so that the amount of artificial sea salt adhering to the surface of the test piece is 6.0 g / m ² , and the artificial sea salt is applied to the test piece. attached. Then, using this test piece, a corrosion test was conducted in which the following cycle (8 hours in total) was repeated 1200 times. The artificial sea salt was applied once a week.
(cycle)
・Condition 1: Temperature 60°C, Relative Humidity 35%, Holding Time 3 hours ・Transition from Condition 1 to Condition 2: 1 hour ・Condition 2: Temperature 40°C, Relative Humidity 95%, Holding Time 3 hours ・From Condition 2 Transition to condition 1: 1 hour

After completion of the corrosion test, the area of the coating film swelling from the initial defect portion was measured and used as an index of coating durability.

As can be seen from the results shown in Table 3, the steel plate satisfying the conditions of the present invention had extremely excellent properties satisfying all of the following conditions. In particular, it had excellent fatigue crack propagation resistance and total elongation, and was excellent in fatigue crack propagation resistance in all of the plate thickness direction, rolling direction, and width direction. On the other hand, the thick steel plate of the comparative example, which did not satisfy the conditions of the present invention, did not satisfy at least one of the following conditions.
・TS: 500 MPa or more ・EL: 15% or more (when using JIS No. 1A test piece),
EL: 19% or more (when using JIS No. 5 test piece)
・vE ₀ : 100 J or more ・Fatigue crack propagation speed in the L and C directions:
ΔK: 1.75 × 10 ^-8 (m/cycle) or less under the condition of 15 MPa/m ^1/2 ,
ΔK: 8.50 × 10 ^-8 (m/cycle) or less under conditions of 25 MPa/m ^1/2 Fatigue crack propagation rate in the Z direction:
ΔK: 8.75 × 10 ^-9 (m/cycle) or less under the conditions of 15 MPa/m ^1/2 ,
ΔK: 4.25 × 10 ^-8 (m/cycle) or less under conditions of 25 MPa/m ^1/2・Paint swelling area is 480 mm ² or less

Claims (8)

in % by mass,
C: 0.01 to 0.16%,
Si: 1.00% or less,
Mn: 0.50-2.00%,
P: 0.015 % or less,
S: 0.010 % or less,
Al: 0.06% or less, and W: 0.005 to 1.00%,
Having a component composition in which the balance is Fe and unavoidable impurities,
area fraction,
75-97% bainite and 3-25% perlite,
The total area fraction of structures other than bainite and pearlite is 5% or less, and
The bainite crystal grain size is 18 μm or less in terms of the average circle equivalent diameter,
A thick steel plate having a microstructure in which the grain size of pearlite is 10 μm or less in terms of average equivalent circle diameter. The component composition further, in mass %,
Cr: 0.01 to 1.00%,
Cu: 0.01 to 1.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
Co: 0.01 to 1.00%,
Sn: 0.005 to 0.200%,
Sb: 0.005 to 0.200%,
Nb: 0.005 to 0.050%,
V: 0.005 to 0.050%,
Ti: 0.005 to 0.050%,
B: 0.0001 to 0.0050%,
Zr: 0.005-0.100%
Ca: 0.0001 to 0.020%,
The steel plate according to claim 1, containing one or more selected from the group consisting of Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%. The microstructure is
Steel plate according to claim 1 or 2, comprising 81-97% bainite and 6-25% perlite. The steel plate according to any one of claims 1 to 3, which has a coating film on its surface. the coating comprises an anticorrosion undercoat layer, an undercoat layer, an intermediate coat layer, and a topcoat layer;
The anti-corrosion underlayer comprises an inorganic zinc-rich paint;
The undercoat layer is an epoxy resin paint,
The intermediate coating layer is an intermediate coating for a fluororesin top coating,
5. The steel plate according to claim 4, wherein each of said topcoat layers is made of a fluororesin topcoat paint. Heating the steel material having the chemical composition according to claim 1 or 2 to a heating temperature of 1000 ° C. or higher and 1250 ° C. or lower,
Hot-rolling the heated steel material into a hot-rolled steel sheet,
The hot-rolled steel sheet is accelerated and cooled under the following conditions: cooling start temperature: Ar 3 or more, cooling stop temperature: 450 to 700 ° C., average cooling rate on the steel plate surface from cooling start to cooling stop: 20 to 60 ° C./s. A method for manufacturing a thick steel plate,
In the hot rolling, the cumulative rolling reduction in the temperature range of 950 ° C. or higher is 80% or more, and the cumulative rolling reduction in the temperature range of less than 950 ° C. and Ar3 point or higher is 50% or more,
The thick steel plate is
area fraction,
75-97% bainite, and
Contains 3-25% perlite,
The total area fraction of structures other than bainite and pearlite is 5% or less, and
The bainite crystal grain size is 18 μm or less in terms of the average circle equivalent diameter,
A method for producing a thick steel plate having a microstructure in which the pearlite crystal grain size is 10 µm or less in terms of average equivalent circle diameter . A structure using the thick steel plate according to any one of claims 1 to 5. 8. The structure of claim 7, wherein said structure is a bridge.

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