JP2022048121A - Plate and production method thereof and structure - Google Patents

Abstract

To provide a plate which is excellent in coating durability, which can extend a cycle required for repaint coating for reducing a coating frequency, even when the plate is used in an outdoor atmosphere corrosion environment such as a bridge, especially in a severe corrosion environment such as sea or the vicinity of a seashore in which an amount of flying salt is large, without excessive increase of alloy cost, the plate having high strength and being excellent in extension property in whole thickness, fatigue crack propagation property and toughness, and to provide a production method of the plate and a structure.SOLUTION: A plate comprises a component composition containing in mass%: 0.05-0.20% of C; 0.01-0.50% of Si; 0.50-2.00% of Mn; 0.05% or less of P; 0.02% or less of S; and 0.005-1.00% of W, containing at least one kind of 0.005-0.500% of Sn and 0.005-0.200% of Sb, and further containing a balance Fe and inevitable impurities. A micro tissue includes a ferrite phase of 80% or more in an area ratio in a range from a surface to 100 μm under the surface, in a plate thickness direction, and includes a ferrite phase of 80% or less in the area ratio in a range from 100 μm under the surface to a 1/4 position of the plate thickness, in the plate thickness direction, and a balance is formed of a pearlitic phase or a mixture phase of the pearlitic phase and a bainite phase, in which an area ratio of the pearlitic phase is greater than that of the bainite phase.SELECTED DRAWING: None

Description

The present invention relates to a thick steel sheet and a method for manufacturing the same, and more particularly to a thick steel sheet having excellent elongation characteristics, fatigue crack propagation characteristics, and toughness at the total thickness and a method for producing the same. The thick steel plate of the present invention is strongly required to have structural safety such as ships, marine structures, bridges, buildings, tanks, etc., and can be suitably used for welded structures. The present invention also relates to a structure that is used in an outdoor corrosive environment such as a bridge, especially in a severe corrosive environment such as on the sea or near a coast where the amount of flying salt is high.

Thick steel plates are widely used in structures such as ships, offshore structures, bridges, buildings and tanks. Such a thick steel sheet is required to have excellent fatigue characteristics in addition to excellent mechanical properties such as strength and toughness and weldability.

When a structure as described above is used, a repetitive load such as vibration due to wind or an earthquake is applied to the structure. Therefore, the thick steel sheet is required to have fatigue characteristics that can ensure the safety of the structure even when such a repetitive load is applied.

Fatigue fracture is a phenomenon in which fine cracks (fatigue cracks) first occur and then the cracks expand (progress). In fatigue fracture, fatigue cracks generally occur from the weld and propagate through the steel material, leading to fracture in many cases. It is said that this is due to the fact that the welded portion tends to be a stress concentration portion due to its shape, and in addition, residual tensile stress is generated after welding. Therefore, as a means for suppressing the generation of cracks from the welded portion, a technique of introducing the residual stress of compression by peening or the like is widely known.

However, it is not realistic from the viewpoint of workability and manufacturing cost to apply such a treatment to all the welded portions existing in a large number in the structure. Therefore, even if fatigue cracks occur from the welded part, it is important to extend the fatigue life of the welded structure by delaying the subsequent crack propagation in the steel material, and the fatigue resistance of the steel material itself is important. It is desired to improve the crack propagation characteristics.

For example, in Patent Document 1, in a method for manufacturing a thick steel sheet having a plate thickness of 20 mm or less, the amount of C added is reduced to control Ceq (carbon equivalent) within a specific range, and the cooling shutdown temperature is lowered to achieve elongation. A thick steel sheet that achieves both fatigue resistance and crack propagation characteristics is described.

Further, Patent Document 2 describes a method for producing a thick steel sheet having a small anisotropy of crack propagation characteristics by combining heating, rolling, accelerated cooling and heat treatment according to the yield stress.

In Patent Document 3, a duplex stainless steel having a microstructure composed of bainite and ferrite having an area ratio of 38 to 52% is used, and the Vickers hardness of the ferrite phase portion and the density of the boundary between the ferrite phase and the bainite phase are controlled. By doing so, the fatigue crack propagation characteristics are improved.

In Patent Document 4, in order to improve excellent fatigue resistance crack propagation characteristics and elongation characteristics at total thickness, the microstructure in the range from the surface to 100 μm below the surface in the plate thickness direction has an area ratio of 80% or more. The microstructure in the range from 100 μm below the surface to the plate thickness 1/2 position contains a ferrite phase with an area ratio of 80% or less, and the balance is a pearlite phase, a bainite phase, or a pearlite phase and a bainite phase. A thick steel plate composed of a mixed phase of is proposed.

By the way, steel structures used outdoors such as bridges are usually used after being subjected to some kind of anticorrosion treatment. For example, weathering steel is often used in an environment where the amount of flying salt is low. Here, the weathering steel is covered with a highly protective rust layer enriched with alloying elements such as Cu, P, Cr, and Ni when used in an air-exposed environment, thereby reducing the corrosion rate. It is a steel material that has been greatly reduced. It is known that a bridge using such weathering steel can withstand several decades of service without painting in an environment where the amount of flying salt is low.

On the other hand, in an environment with a large amount of flying salt such as at sea or near the coast, it is difficult to form a highly protective rust layer in the weathering steel, and it is difficult to use the weathering steel without painting. For this reason, in an environment with a large amount of flying salt such as on the sea or near the coast, a coated steel material obtained by subjecting an ordinary steel material to anticorrosion treatment such as painting is generally used.

However, with coated steel materials, repairs such as periodic repainting are required due to deterioration of the coating film over time, rusting, swelling of the coating film, and the like. The painting work associated with repainting is often done at a high place, which is difficult and requires labor costs. Therefore, when the coated steel material is used, there is a problem that the maintenance cost of the structure increases and the life cycle cost increases.

Therefore, by extending the repainting cycle, it is possible to reduce the frequency of painting and develop a steel material with excellent corrosion resistance that can suppress the maintenance cost of the structure, especially a structural steel material with excellent painting durability. It is desired.

As such a steel material having excellent corrosion resistance, for example, Patent Document 5 discloses a highly weathering steel material containing 0.15 to 0.30% of P and 2.0% or more and less than 3.0% of Cr. Has been done. Patent Document 6 discloses a coated steel material having excellent durability and containing 0.05 to 3.0% of Cu, 0.05 to 6.0% of Ni, and 0.01 to 1.0% of Ti. Has been done. Patent Document 7 describes a coating film containing 0.05 to 3.0% of Cu, 0.05 to 6.0% of Ni, and 0.025 to 0.15% of Ti for coating film having excellent durability. Steel materials are disclosed. Patent Document 8 discloses a highly weldable and highly weathering steel containing 0.30 to 1.00% of Cu and 1.0 to 5.5% of Ni. Patent Document 9 discloses a steel material having excellent corrosion resistance and containing 2.6 to 13.0% of Cr and 0.01 to 0.5% of Sn.

Japanese Unexamined Patent Publication No. 2010-196109 Japanese Unexamined Patent Publication No. 2007-332402 Japanese Unexamined Patent Publication No. 08-225882 Japanese Unexamined Patent Publication No. 2019-026927 Japanese Unexamined Patent Publication No. 6-93372 Japanese Unexamined Patent Publication No. 10-330881 Japanese Unexamined Patent Publication No. 2000-169939 Japanese Unexamined Patent Publication No. 11-172370 Japanese Unexamined Patent Publication No. 2013-166992

However, the conventional techniques as described in Patent Documents 1 to 4 have the following problems.

In the method described in Patent Document 1, a thick steel sheet is manufactured by an online process of rolling and accelerated cooling control. Therefore, especially in a thin material having a plate thickness of 20 mm or less, a temperature deviation at the tip and tail of the steel sheet is likely to occur during hot rolling and accelerated cooling, and stable mechanical properties are maintained over the entire length. I can't get it.

Further, in the method described in Patent Document 2, when immediate quenching is performed after reheating in the two-phase region, the shape of the thick steel sheet deteriorates due to transformation shrinkage, and the outermost layer of the thick steel sheet is miniaturized by quenching. Curing deteriorates the elongation characteristics at full thickness. These tendencies are particularly noticeable when the plate thickness is thin.

In the method described in Patent Document 3, as in Patent Document 1, a thick steel sheet is manufactured by an online process by rolling and accelerated cooling control. Therefore, especially in a thin material having a plate thickness of 20 mm or less, a temperature deviation at the tip and tail of the steel sheet is likely to occur during hot rolling and accelerated cooling, and stable mechanical properties are maintained over the entire length. There is a problem that it cannot be obtained.

In Patent Document 4, the reheated hot-rolled plate is cooled and hardened at an average cooling rate of 7.7 to 16.9 ° C./s. In this method, since the cooling rate is high, the bainite phase is predominantly generated over the pearlite phase, and since island-like martensite is present in the bainite phase, the toughness value deteriorates.

Further, the steel materials proposed in Patent Documents 1 to 4 do not have sufficient weather resistance. On the other hand, if an alloy element such as Cr or Sn is excessively contained together with Cu and Ni as in Patent Documents 5 to 9 for the purpose of improving weather resistance, not only the alloy cost increases but also the toughness is increased. It causes deterioration.

As described above, the conventional manufacturing method cannot manufacture a thick steel sheet having all of elongation (also referred to as total thickness elongation) characteristics, fatigue crack propagation characteristics, toughness and coating durability at the total thickness. was there.

The present invention has been made in view of the above circumstances, has high strength, is excellent in elongation characteristics, fatigue crack propagation characteristics and toughness at all thicknesses, and is a bridge without causing an excessive increase in alloy cost. Even when used in an outdoor atmospheric corrosion environment such as, especially in a severe corrosion environment such as on the sea or near the coast where the amount of flying salt is high, the cycle required for repainting is extended to reduce the frequency of painting. It is an object of the present invention to provide a thick steel sheet having excellent coating durability, a method for producing the same, and a structure thereof.

As a result of studies to solve the above problems, the present inventors have obtained the following findings.
(1) The thick steel sheet after hot rolling is completed and cooled has a structure variation due to a cooling deviation. The structure variation should be reheated to a temperature in the two-phase region or higher. Can be resolved by.
(2) Even when the plate thickness is thin, by controlling the cooling pattern after the reheating heat treatment, the strength is high over the entire length, and both the elongation characteristics at the total thickness and the fatigue crack propagation characteristics can be achieved. ..
(3) Further, the toughness value can be improved by producing more pearlite phase than bainite phase.
(4) The content of W remarkably suppresses the formation of rust under the coating film, and is effective in preventing the coating film from swelling and peeling.
(5) In the process of finishing hot rolling and cooling, by appropriately controlling the cooling rate, the variation of the structure is eliminated, the strength is high over the entire length, and the elongation characteristics and resistance at the total thickness are achieved. Both fatigue crack propagation characteristics can be achieved.

The present invention has been made based on the above findings, and its gist structure is as follows.
[1] By mass%,
C: 0.05 to 0.20%,
Si: 0.01-0.50%,
Mn: 0.50 to 2.00%,
P: 0.05% or less,
S: 0.02% or less,
W: 0.005 to 1.00%
It contains at least one of Sn: 0.005 to 0.500% and Sb: 0.005 to 0.200%, and has a component composition consisting of the balance Fe and unavoidable impurities.
The microstructure contains a ferrite phase with an area ratio of 80% or more in the range from the surface to 100 μm below the surface in the plate thickness direction.
In the plate thickness direction, in the range from 100 μm below the surface to the plate thickness 1/4 position.
Contains a ferrite phase with an area ratio of 80% or less,
A thick steel plate in which the balance is a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase.
[2] The composition of the components is further increased by mass%.
Cu: 0.01-1.00%,
Ni: 0.01-1.00%,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
V: 0.005 to 0.100%,
Ti: 0.005 to 0.10%,
B: 0.0001 to 0.0050%,
Ca: 0.0001 to 0.020%,
The thick steel sheet according to [1], which contains one or more selected from Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%.
[3] The thick steel sheet according to [1] or [2], which has a coating film on its surface.
[4] The coating film includes an anticorrosion base layer, an undercoat layer, an intermediate coat layer and a topcoat layer.
The anticorrosion base layer is an inorganic zinc rich paint, the undercoat layer is an epoxy resin paint, the intermediate coat layer is an intermediate coat paint for a fluororesin topcoat paint, and the topcoat layer is a fluororesin topcoat paint [3]. The thick steel plate described.
[5] The steel material having the composition according to the above [1] or [2] is heated to 900 to 1200 ° C.
The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
Cool the hot-rolled sheet and
Then, it is reheated to a reheating temperature equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point.
The steel sheet reheated to a temperature equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
[6] The steel material having the composition according to the above [1] or [2] is heated to 900 to 1200 ° C.
The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
Cool the hot-rolled sheet and
Then, it was reheated to a reheating temperature of Ac3 transformation point or more and 950 ° C. or less.
Keep at the reheating temperature for a holding time of 30 minutes or less.
The steel sheet reheated to a temperature of the Ac3 transformation point or more and 950 ° C. or lower is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
[7] The steel material having the composition according to the above [1] or [2] is heated to 900 to 1200 ° C.
The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
Then
The steel sheet cooled to the temperature above the Ar1 transformation point and below the Ar3 transformation point is cooled to a cooling stop temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened.
[8] A structure using the thick steel plate according to any one of the above [1] to [4].
[9] The structure according to [8], wherein the structure is a bridge.

According to the present invention, it is possible to obtain a thick steel sheet and a structure having high strength and excellent elongation characteristics at full thickness, fatigue crack propagation characteristics, toughness and coating durability. In the thick steel sheet of the present invention, even if fatigue cracks occur over time from stress concentration portions, welded portions, etc., the propagation of subsequent cracks is suppressed, so that the safety of the entire steel structure is enhanced. Is possible. Further, according to the present invention, even when the product is used in an outdoor corrosive environment, especially in a severe corrosive environment such as on the sea or near the coast where the amount of flying salt is high, the repainting cycle is extended to increase the painting frequency. A thick steel plate having excellent coating durability, which can be reduced, can be obtained at low cost.

FIG. 1 is a schematic view of a simple tensile fatigue test piece with a notch on one side used in the fatigue crack propagation test.

Next, a method for carrying out the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description.

[Ingredient composition]
The reasons for limiting the composition of the thick steel sheet of the present invention will be described below. In addition, "%" in the following description shall represent "mass%" unless otherwise specified.

C: 0.05 to 0.20%
C is an element having an effect of increasing the hardness of the matrix phase and improving the strength. In addition, since it has the effect of generating a pearlite phase, which is a set of cementite phases, fatigue resistance is enhanced. In order to obtain such an effect, it is necessary to set the C content to 0.05% or more. On the other hand, when the C content exceeds 0.20%, the hardness of the matrix phase increases excessively, and the elongation at the total thickness deteriorates. Therefore, the C content is set to 0.20% or less.

Si: 0.01-0.50%
Si is an element that acts as a deoxidizing agent and dissolves in steel to increase the hardness of the matrix phase by solid solution strengthening. In order to obtain such an effect, the Si content needs to be 0.01% or more. On the other hand, when the Si content exceeds 0.50%, the elongation and toughness at the total thickness decrease. Therefore, the Si content is set to 0.50% or less.

Mn: 0.50 to 2.00%
Mn is an element having the effect of increasing the hardness of the matrix phase and improving the strength. In order to obtain such an effect, the Mn content needs to be 0.50% or more. On the other hand, when the Mn content exceeds 2.00%, the weldability is lowered, and MnS, which is an inclusion, is excessively segregated and the toughness is deteriorated. Therefore, the Mn content is set to 2.00% or less.

P: 0.05% or less P is an element contained in steel as an unavoidable impurity. P is preferably reduced as much as possible because it segregates at the grain boundaries and has an adverse effect such as lowering the toughness of the base metal and the welded portion. However, a content of 0.05% or less is acceptable. Therefore, the P content is set to 0.05% or less. On the other hand, although the lower limit of the P content is not limited, it is preferable to set the P content to 0.001% or more because an excessive reduction causes an increase in the refining cost.

S: 0.02% or less S is an element contained in steel as an unavoidable impurity. S is present in steel as a sulfide-based inclusion such as MnS and becomes a starting point of brittle fracture and deteriorates toughness. Therefore, it is preferable to reduce S as much as possible, but a content of 0.02% or less is acceptable. Therefore, the S content is 0.02% or less. The S content is preferably 0.01% or less. On the other hand, although the lower limit of the S content is not limited, it is preferable to set the S content to 0.0005% or more because an excessive reduction causes an increase in the refining cost.

W: 0.005 to 1.00%
W elutes with the anode reaction of the thick steel sheet and is distributed as ^WO _4-2 in the rust layer, thereby electrostatically preventing the chloride ions of the corrosion promoting factor from penetrating the rust layer and reaching the ground iron. To prevent. Further, the compound containing W precipitates on the surface of the thick steel sheet, thereby suppressing the anodic reaction of the thick steel sheet. Then, W is added in combination with Sn, Sb, Cu and Ni after adjusting the amount of Si to an appropriate range, thereby increasing the coating durability of the thick steel sheet by the synergistic effect with these elements. Improve. In order to sufficiently obtain such an effect, it is necessary to contain W in an amount of 0.005% or more. It is preferably 0.010% or more, more preferably 0.030% or more, still more preferably 0.050%. On the other hand, if the W content exceeds 1.00%, the alloy cost will increase. Therefore, the W content is set to 1.00% or less. It is preferably 0.70% or less, more preferably 0.50% or less. More preferably, it is 0.30% or less.

At least one of Sn: 0.005 to 0.500% and Sb: 0.005 to 0.200% In the present invention, by containing at least one of Sn and Sb, the coating durability of the thick steel sheet is improved. Let me.

Sn: 0.005 to 0.500%
Sn exists in the rust layer in the vicinity of the surface of the ground iron, and by miniaturizing the rust particles, chloride ions, which are corrosion promoting factors, are prevented from penetrating the rust layer and reaching the ground iron. In addition, Sn suppresses the anodic reaction on the surface of the steel material. Then, when Sn is compoundly added together with W, the coating durability of the thick steel sheet is greatly improved by the synergistic effect with these elements. When Sn is contained, it is necessary to contain 0.005% or more of Sn in order to obtain the above-mentioned effect sufficiently. It is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, if the Sn content exceeds 0.500%, the ductility and toughness of the steel are deteriorated. Therefore, the Sn content is set to 0.50% or less. It is preferably 0.300% or less, more preferably 0.200% or less, still more preferably less than 0.050%.

Sb: 0.005 to 0.200%
Sb exists in the rust layer in the vicinity of the surface of the ground iron, and by refining the rust particles, chloride ions, which are corrosion promoting factors, are prevented from penetrating the rust layer and reaching the ground iron. Further, Sb suppresses the anodic reaction on the surface of the thick steel plate. By containing Sb in combination with W, the coating durability of the thick steel sheet is greatly improved by the synergistic effect with these elements. When Sb is contained, it is necessary to contain 0.005% or more of Sb in order to obtain the above-mentioned effect sufficiently. It is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, if the Sb content exceeds 0.200%, the ductility and toughness of the steel are deteriorated. Therefore, the Sb content is set to 0.200% or less. It is preferably 0.150% or less, more preferably 0.100% or less.

The balance consists of Fe and unavoidable impurities. If the content of oxygen (O) contained as an unavoidable impurity exceeds 0.0050%, the abundance ratio of inclusions on the surface of the steel sheet increases, so that cracks occur starting from the inclusions. It will be easier. Therefore, the O content is preferably 0.0050% or less. Similarly, when the content of N contained as an unavoidable impurity exceeds 0.0050%, the abundance ratio of inclusions on the surface of the steel sheet becomes large, so that cracks are likely to occur starting from the inclusions. .. Therefore, the N content is preferably 0.0050% or less. Similarly, sol., Which is contained as an unavoidable impurity. If the Al content exceeds 0.060%, Al is mixed into the weld metal portion during welding, and the toughness of the weld portion deteriorates. Therefore, sol. Al is preferably 0.060% or less.

Further, in the present invention, at least one of the following elements can be optionally contained. By adding these optional additive elements, the properties such as strength, toughness, weldability, weather resistance, and coating durability of the thick steel sheet can be further improved.

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

Ni: 0.01-1.00%
Ni is an element having an effect of improving low temperature toughness, and improves hot brittleness when Cu is contained. In addition, Ni has the effect of suppressing the permeation of oxygen and chloride ions, which are corrosion promoting factors, into the ground iron by forming a dense rust layer. By containing Ni together with W, the coating durability of the thick steel sheet is further improved. When Ni is contained, the Ni content is set to 0.01% or more in order to obtain the above effect. On the other hand, if the Ni content exceeds 1.00%, the weldability is impaired and the steel material cost increases. Therefore, when Ni is contained, it is set to 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.

Cr: 0.01-1.00%
Cr is an element having an effect of improving the strength. Further, Cr is an element that promotes the formation of cementite, and promotes the formation of a pearlite phase that is advantageous in fatigue resistance characteristics. When Cr is contained, the Cr content is set to 0.01% or more in order to obtain the above effect. It is preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.00%, weldability and toughness are impaired. Therefore, when Cr is contained, it is set to 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less.

Mo: 0.01-1.00%
Mo is an element having an effect of increasing the hardness of the matrix phase, and can be arbitrarily added depending on the desired properties. When Mo is contained, the Mo content is set to 0.01% or more in order to obtain this effect. However, if the Mo content exceeds 1.00%, weldability and toughness are impaired. Therefore, when the Mo content is contained, the Mo content is set to 1.00% or less.

V: 0.005 to 0.100%
Similar to Nb, V is an element having the effect of suppressing recrystallization of austenite during hot rolling to make it finer and precipitating in the air cooling process after hot rolling to increase the strength, which is desired. It can be arbitrarily contained depending on the characteristics to be rolled. In order to obtain the above effect, when V is contained, the V content is set to 0.005% or more. However, if the V content exceeds 0.200%, a large amount of VC is deposited and the toughness is impaired. Therefore, when V is contained, the V content is set to 0.100% or less.

Ti: 0.005 to 0.10%
Ti has a strong tendency to form a nitride and fixes N to reduce the solid solution N, so that it has an effect of improving the toughness of the base metal and the welded portion. Further, when B is contained, by including Ti together, it is possible to prevent Ti from fixing N and B from precipitating as BN. As a result, the hardenability improving effect of B can be promoted, and the strength can be further improved. Therefore, it can be arbitrarily contained depending on the desired characteristics. In order to obtain the above effect, when Ti is contained, the content is 0.005% or more. However, if the Ti content exceeds 0.10%, a large amount of TiC is deposited and the toughness is impaired. Therefore, when Ti is contained, the Ti content is set to 0.10% or less.

B: 0.0001 to 0.0050%
B is an element having an effect of significantly improving hardenability and increasing strength even when contained in a small amount, and can be contained according to desired properties. In order to obtain the above effect, when B is contained, the content is 0.0001% or more. However, if the B content exceeds 0.0050%, the effect is not only saturated but also the weldability is deteriorated. Therefore, when B is contained, the B content is set to 0.0050% or less.

Ca: 0.0001 to 0.020%
Ca binds to S, suppresses the formation of MnS and the like that extend long in the rolling direction, controls the morphology of the sulfide-based inclusions so as to have a spherical shape, and contributes to the improvement of the toughness of the welded portion, which is desired. It can be contained according to the characteristics. When Ca is contained, the Ca content is set to 0.0001% or more in order to obtain this effect. However, when the Ca content exceeds 0.020%, not only the effect is saturated, but also the cleanliness of the steel is lowered, surface defects occur frequently, and the surface texture is deteriorated. Therefore, when Ca is contained, the Ca content is set to 0.020% or less.

Mg: 0.0001 to 0.020%
Mg is an element having an effect of improving toughness through the miniaturization of crystal grains. When Mg is contained, it is preferable that the Mg content is 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, when Mg is contained, the Mg content is 0.020% or less.

REM: 0.0001 to 0.020%
REM (rare earth metal) is an element that has the effect of improving toughness. When REM is contained, the REM content is set to 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, when REM is contained, the REM content is 0.020% or less.

[Micro organization]
Next, the reason for limiting the microstructure of the thick steel sheet will be described. In addition, "%" in the description of the microstructure shall indicate the area ratio unless otherwise specified. Further, the "tip" of the thick steel sheet in the following description is defined as a position 100 mm from the tip of the steel sheet in the rolling direction to the tail end side. Similarly, the "tail end" of a thick steel sheet is defined as a position 100 mm from the tail end in the rolling direction of the steel sheet to the tip end side. Further, the "center" of the thick steel sheet is defined as the position at the center of the steel sheet in the rolling direction (longitudinal direction).

Tissue in the range from the surface to 100 μm below the surface (surface structure)
In the thick steel sheet of the present invention, the microstructure in the range from the surface to 100 μm below the surface (hereinafter, may be simply referred to as “surface layer portion”) in the plate thickness direction is assumed to contain a ferrite phase having an area ratio of 80% or more. .. In the two-phase region where the Ac1 transformation point or more and less than the Ac3 transformation point, the surface layer decarburization reaction produces 80% or more ferrite on the surface layer to soften the surface layer of the thick steel sheet, thereby improving the elongation characteristics at the total thickness. It can be significantly improved. This surface decarburization reaction occurs by passing through or retaining the two-phase region in the reheating process. When the area ratio of the ferrite phase in the surface layer portion is less than 80%, a large amount of a hard residual structure composed of a bainite phase, a pearlite phase, a martensite phase, or a mixed phase thereof is present. As a result, the hardness of the surface layer portion increases, and it is not possible to obtain the desired elongation characteristics at the total thickness. In addition, the tensile strength may become excessive.

Here, the area ratio of the ferrite phase in the surface layer portion refers to the average value of the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface of the thick steel sheet. Further, the microstructure in the surface layer portion refers to the microstructure of the surface layer portion at the tip, center and tail end of the thick steel sheet in the rolling direction. Therefore, in the thick steel sheet of the present invention, the average value of the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface at the tip, center and tail end in the rolling direction of the thick steel sheet is 80% or more. Normally, if the microstructure of the surface layer portion at the tip, center and tail end satisfies the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. Therefore, it can be said that the thick steel sheet of the present invention has an area ratio of the ferrite phase in the surface layer portion of 80% or more over the entire length in the rolling direction.

The rest of the microstructure of the surface layer other than the ferrite phase is preferably composed of a pearlite phase or a mixed phase of a pearlite phase and a pearlite phase, but the pearlite phase contains island-like martensite and deteriorates toughness. Is preferable, and it is more preferable to use only the pearlite phase.

Structure in the range from 100 μm below the surface to 1/4 of the plate thickness (plate thickness internal structure)
In the thick steel plate of the present invention, the microstructure in the range from 100 μm below the surface to the 1/4 position of the plate thickness (hereinafter, may be simply referred to as “inside the plate thickness”) in the plate thickness direction is 80% or less in area ratio. It shall contain the ferrite phase of. When the microstructure inside the plate thickness satisfies the above conditions, desired strength and fatigue-resistant crack propagation characteristics can be obtained.

Here, the area ratio of the ferrite phase inside the plate thickness refers to the average value of the area ratio of the ferrite phase in the range from 100 μm below the surface to the 1/4 position of the plate thickness of the thick steel plate. Further, here, the microstructure inside the plate thickness refers to the microstructure inside the plate thickness at the tip, center and tail end of the thick steel sheet in the rolling direction. Therefore, in the thick steel sheet of the present invention, the microstructure in the range from 100 μm below the surface to the 1/4 position of the plate thickness satisfies the above conditions at the tip, center and tail end of the thick steel sheet in the rolling direction. As with the structure of the surface layer portion, normally, if the microstructure inside the plate thickness at the tip, center and tail end satisfies the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. .. Therefore, in the thick steel sheet of the present invention, it can be said that the microstructure inside the plate thickness is a ferrite phase having an area ratio of 80% or less over the entire length in the rolling direction. That is, in the present invention, the area ratio of the ferrite phase in the surface layer portion is 80% or more, which means that the area ratio of the ferrite phase in the surface layer portion is 80% or more at any of the tip, the center, and the tail end over the entire length in the rolling direction. Means that is obtained.

In the present invention, the remainder in the microstructure inside the plate thickness is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase. .. The bainite phase contains island-like martensite and deteriorates toughness. Therefore, the desired toughness can be obtained by increasing the surface integral of the pearlite phase to be larger than the surface integral of the bainite phase. The surface integral of the bainite phase is preferably 15% or less. It is more preferably 13% or less, still more preferably 11% or less.
The remaining portion of the thick steel plate of the present invention refers to the remaining portion inside the surface layer portion and the plate thickness at the tip, center and tail end. That is, over the entire length of the thick steel plate in the rolling direction, the balance of the microstructure is composed of a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase.

The microstructure inside the surface layer portion and the plate thickness can be evaluated by the method described in the examples.
[Total thickness growth]
The total thickness elongation of the thick steel sheet is not particularly limited, but is preferably 19% or more when the plate thickness exceeds 16 mm and 15% or more when the plate thickness is 16 mm or less. In the present invention, it is preferable that the above-mentioned total thickness elongation condition is satisfied at the tip, center and tail end of the thick steel sheet in the rolling direction. Normally, if the tip, center and tail end satisfy the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. Further, the total thickness elongation can be measured by the method described in Examples.

[Tensile strength]
The tensile strength (TS) of the thick steel sheet is not particularly limited, but is preferably 490 MPa or more. Further, the upper limit of TS is not particularly limited, but for example, in the case of 490 MPa (50 kgf / mm ² ) class in JIS, TS may be 610 MPa or less. Further, in the case of 570 MPa (60 kgf / mm ² ) class in JIS, the upper and lower limits of TS may be set to 570 MPa and 720 MPa, respectively. In the present invention, it is preferable that the above TS conditions are satisfied at the tip, center and tail end of the thick steel sheet in the rolling direction. Normally, if the tip, center and tail end satisfy the above conditions, the above conditions are satisfied over the entire length of the thick steel sheet in the rolling direction. Further, TS can be measured by the method described in Examples.

[Toughness]
The thick steel sheet of the present invention has excellent toughness as a result of having the above-mentioned composition and microstructure. The toughness of the thick steel sheet of the present invention is not particularly limited, but when the test piece thickness is 10 mm, the Charpy absorption energy vE ₀ at 0 ° C., which is one of the indicators of toughness, is preferably 100 J or more, preferably 130 J or more. More preferably, it is more preferably 150 J or more, and most preferably 200 J or more. On the other hand, the upper limit of vE ₀ is not limited, but may be, for example, 400 J or less, 300 J or less, or 270 J or less. When the test piece thickness is 5 mm, it is preferable that the Charpy absorption energy vE ₀ at 0 ° C. is 50 J or more. On the other hand, the upper limit of vE ₀ is not limited, but may be 200 J or less, 150 J or less, and 135 J or less. In addition, vE ₀ can be measured by the method described in the Example.

[Fatigue crack propagation characteristics]
As a result of having the above-mentioned composition and microstructure, the thick steel sheet of the present invention can have excellent fatigue crack propagation characteristics. As an index of the fatigue crack propagation characteristic, the fatigue crack propagation velocity (da / dN) can be used. The value of the fatigue crack propagation velocity is not particularly limited, but in the present invention, the fatigue crack propagation velocity at ΔK = 25 MPa√m is preferably 4.25 × ^10-8 m / cycle or less.

[Plate thickness]
The "thick steel sheet" in the present invention refers to a steel sheet having a thickness of 6 mm or more according to the usual definition in the present technical field. On the other hand, the upper limit of the plate thickness of the thick steel plate in the present invention is not particularly limited and can be any value. However, as described above, the effect of the present invention is particularly remarkable in a thin material in which the temperature deviation at the tail end of the steel sheet tends to be large and the elongation characteristics at the total thickness are required to be excellent. Therefore, the thickness of the thick steel plate is preferably 25 mm or less, more preferably 20 mm or less.

[Coating film]
The thick steel plate of the present invention may have a coating film on its surface. As the coating film, any coating film can be used without particular limitation.

Further, examples of the coating film include a coating film having an anticorrosion base layer, an undercoat layer, an intermediate coat layer, and a topcoat layer in this order. The anticorrosion base layer is preferably an inorganic zinc rich paint formed by using an inorganic zinc rich paint, and more preferably a layer containing an alkyl silicate resin and zinc powder. The inorganic zinc rich paint is not particularly limited, and examples thereof include SD zinc 1500 manufactured by Kansai Paint Co., Ltd. The undercoat layer is preferably formed by using an epoxy resin paint (epoxy resin coating film). The epoxy resin paint is not particularly limited, and examples thereof include Epomarin HB (K) manufactured by Kansai Paint Co., Ltd. The intermediate coating layer is preferably formed by using an intermediate coating paint for a fluororesin topcoat coating, and more preferably an epoxy resin coating film. The intermediate coating for the fluororesin topcoat is not particularly limited, and examples thereof include Ceratecto F intermediate coating manufactured by Kansai Paint Co., Ltd., which is a polyamide-curable epoxy resin intermediate coating. The topcoat layer is preferably formed by using a fluororesin topcoat paint (fluororesin coating film). The fluororesin topcoat is not particularly limited, and examples thereof include Ceratecto F (K) topcoat manufactured by Kansai Paint Co., Ltd., which is a low-contamination isocyanate-curable fluororesin paint.

[Structure]
By using the thick steel plate of the present invention for structures such as bridges that are used in an outdoor atmospheric corrosion environment such as a bridge, especially in a severe corrosion environment such as on the sea or near the coast where the amount of flying salt is high. It is possible to reduce the maintenance cost of various structures and, by extension, the life cycle cost, and further, it is possible to prevent deterioration of strength characteristics in the event of a fire and ensure high safety.

[Production method]
The thick steel sheet of the present invention is a method in which a steel material having the above-mentioned composition is sequentially subjected to heating, hot rolling, cooling, reheating, cooling, and quenching, or heating, hot rolling, cooling, and quenching. It can be obtained by a method of sequentially performing treatment. First, a method of sequentially performing heating, hot rolling, cooling, reheating, cooling, and quenching will be described.

Steel Material As the steel material of the present invention, any steel material having the above-mentioned composition and capable of hot rolling can be used, but usually a steel slab may be used. For example, molten steel having the above-mentioned composition can be melted by means such as a converter and used as a steel material such as a slab by a casting method such as a continuous casting method. Further, a steel material such as a slab can be used by the ingot-decomposition rolling method.

Heating A steel material having the above composition is heated to 900 to 1200 ° C. If the heating temperature is less than 900 ° C., the deformation resistance of the steel material in the next hot rolling step increases, the load on the hot rolling mill increases, and hot rolling becomes difficult. Therefore, the heating temperature is set to 900 ° C. or higher. The heating temperature is preferably 950 ° C. or higher. On the other hand, when the heating temperature exceeds 1200 ° C., the toughness decreases. Therefore, the heating temperature is set to 1200 ° C. or lower. The heating temperature is preferably 1150 ° C. or lower.

When the steel material (slab) is manufactured by a method such as continuous casting, the slab may be directly subjected to the heating step without being cooled, or may be subjected to the heating step after being cooled. The heating method is not particularly limited, but for example, heating can be performed in a heating furnace according to a conventional method.

Hot rolling Next, the heated steel material is hot-rolled to obtain a hot-rolled plate. At that time, in order to secure the toughness which is the basic performance of the product steel sheet, the cumulative reduction rate is set to 50% or more. When the cumulative reduction rate is less than 50%, the ferrite grains inside the plate thickness become coarse and a region with low brittleness is locally generated, brittle cracks are likely to occur, and the toughness deteriorates. Other conditions relating to the hot rolling process are not particularly limited.

Cooling Next, the steel sheet after hot rolling is cooled (first cooling step). In the cooling step, it is preferable to cool to room temperature. The cooling can be performed by any method, for example, air cooling or accelerated cooling. Further, the cooling conditions are not particularly limited.

Reheating Next, the cooled steel sheet is reheated to 950 ° C. or higher at the Ac1 transformation point or higher. The reheating temperature is preferably less than the Ac3 transformation point. By heating to a temperature range including the austenite phase of the Ac1 transformation point or more and 950 ° C. or less in this way, the variation in the microstructure due to the cooling deviation can be eliminated, and as a result, the variation in the mechanical properties is eliminated. be able to. When the reheating temperature is lower than the Ac3 transformation point, the structure before reheating is not damaged, which is preferable.

When the reheating temperature is equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point, the decarburization reaction peculiar to the two-phase region proceeds, and the area ratio of the ferrite phase in the surface layer portion can be 80% or more. On the other hand, when the reheating temperature is not less than the Ac3 transformation point and 950 ° C., the austenite phase is the surface ferrite phase generated by the surface decarburization reaction when passing through the two-phase region by shortening the holding time at the reheating temperature. The reaction of reverse transformation to is suppressed, and the area ratio of the surface ferrite phase can be set to 80% or more.
When the reheating temperature exceeds 950 ° C., the reaction in which the surface ferrite phase generated by the surface decarburization reaction when passing through the two-phase region is reverse-transformed into the austenite phase is promoted, and the area ratio of the ferrite phase in the surface layer is less than 80%. Will be. As a result, the hardness of the surface layer portion increases, and it is not possible to obtain the desired elongation characteristics at the total thickness.
When the reheating temperature is above the Ac3 transformation point and below 950 ° C., the crystal grain size of the austenite phase inside the plate thickness becomes coarser than when the reheating temperature is below the Ac3 transformation point. It was found that the toughness was not excessively deteriorated. Furthermore, in this temperature range, the rate at which the reverse transformation to the austenite phase inside the plate thickness proceeds increases. Therefore, since the desired matrix structure is obtained in a short heating time, the number of thick steel sheets that can be manufactured in a predetermined time increases, and the productivity is improved.
On the other hand, when the temperature exceeds 950 ° C., the austenite phase reverse-transformed inside the plate thickness grows and becomes coarse, and as a result, a region having low toughness locally is generated and the toughness decreases.
On the other hand, when the reheating temperature is lower than the Ac1 transformation point, the reaction of reverse transformation to the austenite phase does not occur, and the ferrite phase, the pearlite phase and the bainite phase inside the plate thickness after cooling do not have the desired area ratio. As a result, fatigue characteristics (crack propagation characteristics) deteriorate. Further, it is not possible to eliminate the variation in mechanical properties due to the cooling deviation in the cooling process after hot rolling.

The Ac1 transformation point can be obtained, for example, by the following equation (1).
Ac1 (° C.) = 723 + 29.1 x Si-10.7 x Mn-16.9 x Ni + 16.9 x Cr ... (1)
Further, the Ac3 transformation point can be obtained by, for example, the following equation (2).
Ac3 (° C.) = 961.6-311.9 x C + 49.5 x Si-36.4 x Mn + 438.1 x P-2818 x S + 12.7 x Al-51 x Cu-29 x Ni-8.7 x Cr + 13 .5 x Mo + 308.1 x Nb-140 x V + 318.9 x Ti + 611.2 x B-969 x N ... (2)
Here, the element symbol in the above equations (1) and (2) means the content (mass%) of each element, and is set to zero when the element is not contained.
In the above reheating treatment, it is preferable to heat the temperature to the reheating temperature and then keep the temperature at that temperature. When the reheating temperature is above the Ac1 transformation point and below the Ac3 transformation point, if the holding time is less than 10 minutes, the reverse transformation to the austenite phase is not started over the entire length of the steel sheet, and it is hardenable in some regions. May decrease significantly. Therefore, the holding time is preferably 10 minutes or more. Although no upper limit of the holding time is set, the holding time is preferably 10 hours or less from the viewpoint of efficiency in production. On the other hand, when the reheating temperature is equal to or higher than the Ac3 transformation point and lower than 950 ° C., the austenite phase grows and becomes coarse when the holding time exceeds 30 minutes. Therefore, the holding time is set to 30 minutes or less. Preferably, when the Ac3 transformation point or more and 950 ° C. or lower, the holding time at the reheating temperature is 20 minutes or less.

Cooling The steel sheet reheated in the above reheating step is cooled to a cooling shutdown temperature of 350 to 600 ° C. (second cooling step). At that time, the average cooling rate is 2 to 7 ° C./s. A lower average cooling rate is preferable in terms of improving toughness because pearlite transformation is promoted more. However, if the average cooling rate is less than 2 ° C./s, the grain growth of ferrite becomes excessive and coarse-grained, resulting in deterioration of toughness. Therefore, the average cooling rate is set to 2 ° C./s or higher.
On the other hand, when the average cooling rate exceeds 7 ° C./s, the pearlite transformation does not sufficiently proceed in the microstructure inside the steel sheet, and the bainite transformation and the martensitic transformation are likely to proceed. In this case, since the fractions of the bainite phase and the martensite phase increase, the elongation characteristics and toughness at the total thickness deteriorate. Therefore, the average cooling rate is set to 7 ° C./s or less. The average cooling rate is preferably 5 ° C./s or less, more preferably 4 ° C./s or less, and even more preferably less than 3 ° C./s.
Further, when the cooling shutdown temperature is less than 350 ° C., ferrite is excessively generated inside the plate thickness, so that the entire steel sheet is softened and the desired tensile strength cannot be obtained. Therefore, the cooling shutdown temperature is set to 350 ° C. or higher. On the other hand, when the cooling shutdown temperature exceeds 600 ° C., quenching is performed with a large amount of untransformed austenite remaining, so that hard bainite and martensite are excessively generated. As a result, the elongation characteristics at the total thickness are deteriorated, and the toughness is also deteriorated. Therefore, the cooling shutdown temperature is set to 600 ° C. or lower.

Quenching The steel sheet cooled to the above cooling shutdown temperature is quenched. Therefore, the quenching temperature is in the range of 350 to 600 ° C. Quenching can be performed under any conditions without particular limitation, but it is preferably water-cooled to a temperature of Ms point or lower, preferably 200 ° C. or lower. The Ms point can be obtained by, for example, the following equation (3).
Ms (° C.) = 517-300 x C-11 x Si-33 x Mn-17 x Ni-22 x Cr-11 x Mo ... (3)
Here, the element symbol in the above formula (3) means the content (mass%) of each element, and is set to zero when the element is not contained.

Next, a method of sequentially performing heating, hot rolling, cooling, and quenching will be described.

As the steel material, the same steel material as that described above is used. The heating and hot rolling can be carried out in the same manner as the heating and hot rolling described above.
For cooling after hot rolling, first, the temperature is cooled to the temperature above the Ar1 transformation point and below the Ar3 transformation point, and then the average cooling is 2 to 7 ° C./s from the temperature above the Ar1 transformation point and below the Ar3 transformation point (cooling start temperature). Cool to a cooling stop temperature of 350-600 ° C. at a rate. The reason why the cooling start temperature is set to the temperature above the Ar1 transformation point and below the Ar3 transformation point (two-phase region) is that the decarburization reaction peculiar to the two-phase region proceeds and the area ratio of the ferrite phase in the surface layer portion is 80% or more. Because it can be done.
Further, the reasons for setting the average cooling rate after that to 2 to 7 ° C./s include the following reasons. If the average cooling rate is less than 2 ° C./s, the grain growth of ferrite becomes excessive and coarse-grained, resulting in deterioration of toughness. Therefore, the average cooling rate is set to 2 ° C./s or more. On the other hand, when the average cooling rate exceeds 7 ° C./s, the pearlite transformation does not sufficiently proceed in the microstructure inside the steel sheet, and the bainite transformation and the martensitic transformation are likely to proceed. In this case, since the fractions of the bainite phase and the martensite phase increase, the elongation characteristics and toughness at the total thickness deteriorate. Therefore, the average cooling rate is set to 7 ° C./s or less. The average cooling rate is preferably 5 ° C./s or less, more preferably 4 ° C./s or less, and even more preferably less than 3 ° C./s.
The reasons for setting the cooling shutdown temperature to 350 to 600 ° C. are as follows. When the cooling shutdown temperature is less than 350 ° C., ferrite is excessively generated inside the plate thickness, so that the entire steel sheet is softened and the desired tensile strength cannot be obtained. Therefore, the cooling shutdown temperature is set to 350 ° C. or higher. On the other hand, when the cooling shutdown temperature exceeds 600 ° C., quenching is performed with a large amount of untransformed austenite remaining, so that hard bainite and martensite are excessively generated. As a result, the elongation characteristics at the total thickness are deteriorated, and the toughness is also deteriorated. Therefore, the cooling shutdown temperature is set to 600 ° C. or lower.
Subsequent quenching can be carried out in the same manner as the quenching described above.
The Ar1 transformation point can be obtained, for example, by the following equation (4).
Ar1 = 712-17.8 x C-19.1 x Ni + 20.1 x Si + 11.9 x Cr + 9.8 x Mo ... (4)
Further, the Ar3 transformation point can be obtained by, for example, the following equation (5).
Ar3 = 910-310 x C-80 x Mn-20 x Cu-15 x Cr-55 x Ni-80 x Mo ... (5)
Here, the element symbol in the above equations (4) and (5) means the content (mass%) of each element, and is set to zero when the element is not contained.

Hereinafter, the effects of the present invention will be specifically described based on examples, but the present invention is not limited to these examples.

The molten steel having the composition shown in Table 1 was melted and used as a steel material (slab). The values of Ac1, Ac3, Ms, Ar1, and Ar3 shown in Table 1 are the above-mentioned equations (1), (2), (3), (4), and (5), respectively. This is the calculated value.

Next, the obtained slab was heated and hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled plate having a total length of 20 m and a plate thickness shown in Table 2. Then, the hot-rolled sheet was cooled to room temperature by the cooling method shown in Table 2, reheated to the reheating temperature shown in Table 2, and maintained. Next, cooling water was sprayed on both sides of the steel sheet, cooled to the cooling stop temperature at the average cooling rate shown in Table 2, and then quenched. In the quenching treatment, it was water-cooled to 150 ° C. or lower.

For comparison, in some comparative examples (No. 26 in Table 2), quenching was performed immediately after reheating without cooling to satisfy the conditions of the present invention. The quenching conditions in this comparative example were an average cooling rate of 44.0 ° C./s and a cooling shutdown temperature of 110 ° C.

Regarding the obtained thick steel sheet, (1) microstructure, (2) total thickness elongation, (3) tensile strength (TS), (4) fatigue crack propagation characteristics, (5) toughness, (6) coating durability. , Each was evaluated. For the tests (1) to (5), in order to evaluate the characteristics of the thick steel sheet over the entire length, the test pieces were taken from each of the tip, the center, and the tail end of the thick steel sheet in the rolling direction. For the test (6), test pieces were collected from the central part in the rolling direction. The test method is as follows. The test pieces at the tip and the tail end were taken from a position 100 mm from the end of the steel sheet in the rolling direction.

(1) Observation of microstructure The microstructure was observed by the following procedure.
・ Area ratio of ferrite phase in the surface layer, area ratio of ferrite phase inside the plate thickness, area ratio of pearlite phase and bainite phase inside the plate thickness First, from the obtained thick steel plate, the observation surface is a cross section perpendicular to the rolling direction. A test piece for microstructure observation is collected so as to be (cross-sectional in the plate thickness direction), polished to a mirror surface, corroded with a corrosive solution (methanol nitrate solution), and used with an optical microscope (magnification: 400 times). Observation was performed from the surface of the steel plate to the position of 1/4 of the plate thickness in the plate thickness direction, and images were taken so that the screen was continuous. Using the obtained microstructure photograph, the phase was identified by image analysis, (a) the average value of the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface of the thick steel plate, and (b) the plate thickness from 100 μm below the surface. The average value of the area ratio of the ferrite phase in the range up to the 1/4 position and (c) the area ratio of the pearlite phase and the bainite phase in the range from 100 μm below the surface to the 1/4 position of the plate thickness were obtained.

Table 3 shows the measurement results of the microstructure.

(2) Tensile test A tensile test was carried out using a full-thickness test piece of JIS Z 2201 1A collected so that the plate width direction coincided with the tensile direction from the center of the width of the thick steel plate, and the tensile strength (TS) and We asked for full-thickness growth. The tensile strength of 490 MPa or more was accepted. The elongation characteristics were accepted as 15% or more when the plate thickness was 16 mm or less, and 19% or more when the plate thickness exceeded 16 mm.

(3) Fatigue crack propagation test A fatigue crack propagation test was conducted using the one-sided notch simple tensile type fatigue test piece shown in Fig. 1, and the fatigue crack propagation behavior when the crack propagated in the plate thickness direction was evaluated. .. The test conditions were based on ASTM E647, a stress ratio of 0.1, a frequency of 10 Hz, and the test was carried out in room temperature atmosphere. Since the object of the present invention is to reduce the propagation speed when cracks generated from a welded portion or the like propagate in a steel material in a welded structure, the stress intensity factor range (ΔK) is assumed in such a situation. Was tested in the range of 10 to 30 MPa√m. Fatigue crack propagation speeds of 4.25 × ^10-8 m / cycle or less at ΔK = 25 MPa√m were accepted.

(4) Toughness A Charpy impact test piece was taken from the center of the thick steel plate in parallel with the rolling direction (L direction). When the plate thickness was 10 mm or more, the test piece thickness was 10 mm, and when the plate thickness was less than 10 mm, the test piece thickness was 5 mm. The test was carried out in accordance with JIS Z 2202 by performing a Charpy impact test at 0 ° C., and the absorbed energy vE ₀ was measured. A test piece having a thickness of 10 mm was accepted as having an absorption energy of 100 J or more. A test piece with a thickness of 5 mm was accepted as having an absorption energy of 50 J or more.

(5) Coating durability With respect to the obtained thick steel sheet, test pieces of 70 mm × 50 mm × 5 mmt were collected and evaluated for coating durability. Specifically, first, the surface of the test piece was shot blasted so that the rust removal degree Sa specified in ISO 8501-1 was 2.5. The test piece was 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 composed of an anticorrosion base layer, an undercoat layer, an intermediate coat layer, and a topcoat layer. The thickness of each layer is also described below.
(A) Anticorrosion base layer: Inorganic zinc rich paint (SD zinc 1500A manufactured by Kansai Paint Co., Ltd.), Thickness: 75 μm
(B) Undercoat layer: Epoxy resin paint (Epomarin HB (K) manufactured by Kansai Paint Co., Ltd.), Thickness: 120 μm
(C) Intermediate coating layer: Intermediate coating paint for fluororesin top coating (Ceratect F intermediate coating manufactured by Kansai Paint Co., Ltd.), thickness: 30 μm
(D) Topcoat layer: Fluororesin topcoat paint (Ceratect F topcoat manufactured by Kansai Paint Co., Ltd.), Thickness: 25 μm
After forming the coating film, 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-based sealant. As an initial defect, a linear cut having a width of 1 mm and a length of 40 mm to reach the base metal was made in the central portion of the coating film formed on the test piece. Then, a corrosion test was carried out under the conditions shown below.
・ Corrosion test Spray a solution of artificial sea salt diluted with pure water to a predetermined concentration so that the amount of artificial sea salt adhered to the surface of the test piece is 6.0 g / m ² , and apply the artificial sea salt to the test piece. Attached. Then, using this test piece, a corrosion test was carried out in which the following cycles (total of 8 hours) were repeated for 1200 cycles. The artificial sea salt was attached 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: After the 1-hour corrosion test was completed, the swelling area of the coating film from the initial defect portion was measured and used as an index of coating durability. A paint swelling area of 480 mm ² or less was considered acceptable.

The measurement results are shown in Table 4. As can be seen from this result, in the examples satisfying the conditions of the present invention, a thick steel sheet having elongation characteristics at full thickness, fatigue crack propagation characteristics, toughness and coating durability is obtained. On the other hand, in the comparative example that does not satisfy the conditions of the present invention, at least one of the elongation characteristics at the total thickness, the fatigue crack propagation rate, the toughness, and the coating durability at at least one position of the tip and the tail of the steel sheet. One is inferior.

Claims (9)

By mass%,
C: 0.05 to 0.20%,
Si: 0.01-0.50%,
Mn: 0.50 to 2.00%,
P: 0.05% or less,
S: 0.02% or less,
W: 0.005 to 1.00%
It contains at least one of Sn: 0.005 to 0.500% and Sb: 0.005 to 0.200%, and has a component composition consisting of the balance Fe and unavoidable impurities.
The microstructure contains a ferrite phase with an area ratio of 80% or more in the range from the surface to 100 μm below the surface in the plate thickness direction.
In the plate thickness direction, in the range from 100 μm below the surface to the plate thickness 1/4 position.
Contains a ferrite phase with an area ratio of 80% or less,
A thick steel plate in which the balance is a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the area ratio of the pearlite phase is larger than the area ratio of the bainite phase. The component composition is further increased by mass%.
Cu: 0.01-1.00%,
Ni: 0.01-1.00%,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
V: 0.005 to 0.100%,
Ti: 0.005 to 0.10%,
B: 0.0001 to 0.0050%,
Ca: 0.0001 to 0.020%,
The thick steel sheet according to claim 1, which contains one or more selected from Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%. The thick steel sheet according to claim 1 or 2, which comprises a coating film on the surface. The coating film includes an anticorrosion base layer, an undercoat layer, an intermediate coat layer and a topcoat layer.
The third aspect of claim 3, wherein the anticorrosion base layer is an inorganic zinc rich paint, the undercoat layer is an epoxy resin paint, the intermediate coat layer is an intermediate coat paint for a fluororesin topcoat paint, and the topcoat layer is a fluororesin topcoat paint. The thick steel plate described. The steel material having the composition according to claim 1 or 2 is heated to 900 to 1200 ° C.
The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
Cool the hot-rolled sheet and
Then, it is reheated to a reheating temperature equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point.
The steel sheet reheated to a temperature equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened. The steel material having the composition according to claim 1 or 2 is heated to 900 to 1200 ° C.
The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
Cool the hot-rolled sheet and
Then, it was reheated to a reheating temperature of Ac3 transformation point or more and 950 ° C. or less.
Keep at the reheating temperature for a holding time of 30 minutes or less.
The steel sheet reheated to a temperature of the Ac3 transformation point or more and 950 ° C. or lower is cooled to a cooling shutdown temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened. The steel material having the composition according to claim 1 or 2 is heated to 900 to 1200 ° C.
The heated steel material is hot-rolled with a cumulative reduction rate of 50% or more to form a hot-rolled plate.
Then
The steel sheet cooled to the temperature above the Ar1 transformation point and below the Ar3 transformation point is cooled to a cooling stop temperature of 350 to 600 ° C. at an average cooling rate of 2 to 7 ° C./s.
A method for manufacturing a thick steel sheet, in which a steel sheet cooled to a cooling shutdown temperature of 350 to 600 ° C. is hardened. A structure using the thick steel plate according to any one of claims 1 to 4. The structure according to claim 8, wherein the structure is a bridge.

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