JP7635793B2 - Heavy steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a thick steel plate and a manufacturing method thereof, and in particular to a thick steel plate having excellent elongation characteristics across the entire thickness, fatigue crack propagation resistance, and toughness, and a manufacturing method thereof. The thick steel plate of the present invention can be suitably used in welded structures where structural safety is highly required, such as ships, marine structures, bridges, buildings, and tanks.

Thick steel plates are widely used in structures such as ships, marine structures, bridges, buildings, and tanks. In addition to excellent mechanical properties such as strength and toughness, and weldability, such thick steel plates are required to have excellent fatigue resistance properties.

When using a welded structure such as the one described above, the structure is subjected to repeated loads, such as those caused by wind or vibrations caused by earthquakes. Therefore, to ensure the safety of the structure even when such repeated loads are applied, thick steel plates are required to have durability against ductile fracture, brittle fracture, and fatigue fracture.

Ductile fracture occurs when tiny voids form under static stress with a small strain rate, and then connect, leading to material damage. Resistance to ductile fracture can be improved by increasing the yield stress, tensile strength, and elongation, which are quantified in tensile tests.

Brittle fracture is an event in which an impact force with a large strain rate causes breakage without any prior signs of damage such as deformation. Resistance to brittle fracture can be improved by increasing the toughness value, which is quantified in a Charpy impact test. It is generally known that toughness value decreases inversely with increasing strength and plate thickness.

Fatigue fracture is a phenomenon in which tiny cracks (fatigue cracks) first occur, and then the cracks spread (progress). In most cases, fatigue cracks occur at welds, propagate through the steel, and eventually destroy the steel. This is believed to be due to the fact that welds tend to become areas of stress concentration due to their shape, and also because tensile residual stress occurs after welding. For this reason, techniques such as peening that introduce compressive residual stress are widely known as a means of suppressing crack generation from welds.

However, applying such treatment to all of the numerous welds present within a structure is not realistic in terms of workability and manufacturing costs. Therefore, even if fatigue cracks do initiate from welds, it is important to extend the fatigue life of the welded structure by slowing the subsequent propagation of the cracks in the steel, and there is a need to improve the fatigue crack propagation resistance of the steel itself.

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

Patent Document 2 also describes a method for manufacturing thick steel plate with small anisotropy in crack propagation properties by combining heating, rolling, accelerated cooling, and heat treatment according to the yield stress.

In Patent Document 3, a dual-phase steel is used that has a microstructure consisting of bainite and 38 to 52% ferrite by area ratio, and the fatigue crack propagation resistance is improved by controlling the Vickers hardness of the ferrite phase and the density of the boundary between the ferrite phase and the bainite phase.

Patent Document 4 proposes a thick steel plate in which, in order to improve excellent fatigue crack propagation resistance and elongation characteristics throughout the entire thickness, the microstructure in the thickness direction from the surface to 100 μm below the surface contains ferrite phase in an area ratio of 80% or more, and the microstructure in the thickness direction from 100 μm below the surface to 1/2 the plate thickness position contains ferrite phase in an area ratio of 80% or less, with the remainder being pearlite phase, bainite phase, or a mixed phase of pearlite phase and bainite phase.

JP 2010-196109 A JP 2007-332402 A Japanese Patent Application Publication No. 08-225882 JP 2019-026927 A

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

In the method described in Patent Document 1, thick steel plates are manufactured by an online process using rolling and accelerated cooling control. Therefore, particularly in thin plates with a thickness of 20 mm or less, temperature deviations are likely to occur at the leading and trailing ends of the steel plate during hot rolling and accelerated cooling, making it impossible to obtain stable mechanical properties over the entire length.

In addition, in the method described in Patent Document 2, if immediate quenching is performed after reheating in the two-phase region, the shape of the thick steel plate is deteriorated due to transformation shrinkage, and the outermost layer of the thick steel plate is refined by quenching and hardened, which deteriorates the elongation characteristics throughout the entire thickness. These tendencies are particularly noticeable when the plate thickness is thin.

In the method described in Patent Document 3, like Patent Document 1, thick steel plates are manufactured by an online process using rolling and accelerated cooling control. Therefore, there is a problem that stable mechanical properties cannot be obtained over the entire length, especially in thin plates with a thickness of 20 mm or less, due to temperature deviations during hot rolling and accelerated cooling.

In Patent Document 4, the reheated hot-rolled sheet is cooled at an average cooling rate of 3 to 20°C/s and quenched. This method does not focus on toughness value, and the toughness value may decrease because the bainite phase may be formed more predominantly than the pearlite phase and because island martensite exists in the bainite phase.

As such, conventional manufacturing methods had the problem of being unable to produce thick steel plates that had all the strength, elongation through the entire thickness (also called total thickness elongation), fatigue crack propagation resistance, and toughness.

The present invention was made in consideration of the above circumstances, and aims to provide a thick steel plate that is high in strength and has excellent elongation characteristics throughout its thickness, fatigue crack propagation resistance, and toughness, as well as a manufacturing method thereof.

As a result of investigations conducted by the present inventors to solve the above problems, the present inventors have obtained the following findings.
(1) After hot rolling and cooling, the thick steel plate has a variation in structure in the rolling direction (longitudinal direction) due to the deviation in the cooling stop temperature. However, by reheating to a temperature range of Ac3 or higher (first reheating step), not only can the variation in structure due to the deviation in the cooling stop temperature after hot rolling be eliminated, but the ferrite phase in the plate thickness after cooling can be refined, thereby improving toughness.
(2) By reheating (second reheating step) to a temperature in the Ac1 temperature range (two-phase range) or higher after the first reheating step, ferrite is generated in the surface layer, and the elongation characteristics are improved.
(3) By providing cooling after the second reheating step, the ferrite phase inside the plate thickness is further refined, and the toughness is further improved.
(4) Even when the plate thickness is thin, by controlling the cooling pattern after the second reheating process, it is possible to achieve both elongation characteristics and fatigue crack propagation resistance throughout the entire thickness while ensuring high strength throughout the entire length.
(5) In the cooling pattern after the second reheating step, the cooling rate is appropriately controlled to generate more pearlite phase than bainite phase, thereby further improving the toughness.

The present invention has been made based on the above findings, and has the following gist and configuration.
[1] In mass%,
C: 0.05-0.20%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.00%,
P: 0.05% or less,
S: 0.02% or less, the balance being Fe and unavoidable impurities;
The microstructure is
In the thickness direction, the area ratio of the ferrite phase is 80% or more in the range from the surface to 100 μm below the surface, and at the 1/4 position of the thickness,
Contains a ferrite phase having an area ratio of 90% or less and an average crystal grain size of 25 μm or less,
The remainder of the steel plate is a hard phase, the hard phase is dispersed in a ferrite phase, the hard phase includes a pearlite phase, and the average spacing between the hard phases is less than 25 μm.
[2] The composition further comprises, in mass%,
Cr: 0.01-1.00%,
Cu: 0.01-2.00%,
Ni: 0.01-2.00%,
Mo: 0.01-1.00%,
Co: 0.01 to 1.00%,
Sn: 0.005-0.500%,
Sb: 0.005-0.200%,
Nb: 0.005-0.200%,
V: 0.005-0.200%,
Ti: 0.005-0.050%,
B: 0.0001 to 0.0050%,
Zr: 0.005-0.100%,
Ca: 0.0001-0.020%,
The steel plate according to [1], containing one or more selected from Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%.
[3] The steel plate according to [1] or [2], wherein the hard phase 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 in the mixed phase is larger than the area ratio of the bainite phase.
[4] A heating step of heating a steel material having the component composition according to [1] or [2] to 900 to 1200 ° C.;
a rolling step of subjecting the steel material heated in the heating step to hot rolling at a cumulative rolling reduction rate of 50% or more to obtain a thick steel plate;
A cooling step after the rolling step of cooling the thick steel plate;
A first reheating step in which the steel plate cooled in the cooling step after the rolling step is reheated to a first reheating temperature of not less than the Ac3 transformation point and not more than 950 ° C.;
A first cooling step of cooling the steel plate reheated in the first reheating step;
A second reheating step of reheating the steel plate cooled in the first cooling step to a second reheating temperature of the Ac1 transformation point or higher and 950 ° C. or lower;
A second cooling step of cooling the steel plate reheated in the second reheating step to a second cooling stop temperature of 350 to 600 ° C. at an average cooling rate of 1.5 to 20 ° C./s;
a water cooling step of water cooling the steel plate cooled in the second cooling step;
A method for manufacturing a thick steel plate having the above structure.
[5] The method for producing a steel plate according to [4], wherein the average cooling rate in the second cooling step is 1.5 to 7 ° C./s.

According to the present invention, it is possible to obtain a thick steel plate that is high in strength and has excellent elongation characteristics across the entire thickness, fatigue crack propagation resistance, and toughness. Furthermore, even if fatigue cracks occur over time in the thick steel plate of the present invention from stress concentration areas, welds, etc., the subsequent propagation of the cracks is suppressed, making it possible to increase the safety of the entire steel structure. Furthermore, by suitably using the thick steel plate of the present invention for structures such as bridges, ships, building structures, and construction industry machinery, it is possible to reduce the maintenance costs and therefore the life cycle costs of such structures, making it extremely useful industrially.

FIG. 1 is a schematic diagram of a single-side notched simple tension fatigue test piece used in a fatigue crack propagation test.

Next, a method for implementing the present invention will be specifically described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description in any way.

[Component composition]
The reasons for limiting the chemical composition of the steel plate of the present invention will be described below. In the following description, "%" refers to "mass %" unless otherwise specified.

C: 0.05-0.20%
C is an element that has the effect of increasing the hardness of the matrix and improving the strength. In addition, C has the effect of generating a pearlite phase, which is an aggregate of cementite phases, and therefore the fatigue crack propagation resistance is improved. In order to obtain such an effect, it is necessary to set the C content to 0.05% or more. The C content is preferably 0.08% or more, more preferably 0.10% or more, and even more preferably 0.12% or more. On the other hand, if the C content exceeds 0.20%, the hardness of the matrix phase increases excessively and the elongation in the total thickness deteriorates. For this reason, the C content is set to 0.20% or less. The C content is preferably 0.18% or less, more preferably 0.16% or less, and even more preferably 0.14% or less.

Si: 0.01~0.50%
Si acts as a deoxidizer and is an element that dissolves in steel to increase the hardness of the matrix phase through solid solution strengthening, thereby improving strength. In order to obtain such an effect, the Si content must be 0.01% or more. The Si content is preferably 0.05% or more, more preferably 0.10% or more, even more preferably 0.15% or more, and most preferably 0.20% or more. On the other hand, if the Si content exceeds 0.50%, the elongation and toughness in the total thickness decrease. For this reason, the Si content is set to 0.50% or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less, even more preferably 0.35% or less, and most preferably 0.30% or less.

Mn: 0.50-2.00%
Mn is an element that has the effect of increasing the hardness of the base phase and improving the strength. In order to obtain such an effect, the Mn content needs to be 0.50% or more. The Mn content is preferably 0.60% or more, more preferably 0.70% or more, even more preferably 0.80% or more, and most preferably 1.00% or more. On the other hand, if the Mn content exceeds 2.00%, in addition to the weldability decreasing, the inclusion MnS segregates excessively and the toughness decreases. For this reason, the Mn content is set to 2.00% or less. The Mn content is preferably 1.85% or less, more preferably 1.70% or less, even more preferably 1.55% or less, and most preferably 1.40% or less.

P: 0.05% or less P is an element contained in steel as an inevitable impurity. P segregates at grain boundaries and has adverse effects such as reducing the toughness of the base material and welded parts, so it is preferable to reduce it as much as possible, but a content of 0.05% or less is acceptable. For this reason, the P content is set to 0.05% or less. The P content is preferably 0.04% or less, and more preferably 0.03% or less. On the other hand, although there is no lower limit for the P content, excessive reduction leads to an increase in refining costs, so the P content is preferably 0.001% or more. The P content is preferably 0.002% or more, and more preferably 0.003% or more.

S: 0.02% or less S is an element contained in steel as an inevitable impurity. S exists in steel as sulfide-based inclusions such as MnS, which become the starting point of brittle fracture and deteriorate toughness, so it is preferable to reduce it as much as possible, but a content of 0.02% or less is acceptable. For this reason, the S content is set to 0.02% or less. The S content is preferably set to 0.01% or less. On the other hand, although there is no lower limit for the S content, an excessive reduction leads to an increase in refining costs, so the S content is preferably set to 0.0005% or more.

The balance consists of Fe and unavoidable impurities. If the content of oxygen (O) contained as an unavoidable impurity exceeds 0.0050%, the proportion of inclusions on the steel sheet surface increases, making it easier for cracks to occur starting from the inclusions. Therefore, it is preferable that the O content be 0.0050% or less. Similarly, if the content of N contained as an unavoidable impurity exceeds 0.0050%, the proportion of inclusions on the steel sheet surface increases, making it easier for cracks to occur starting from the inclusions. Therefore, it is preferable that the N content be 0.0050% or less. It is more preferable that the N content be 0.0040% or less. Similarly, if the content of sol. Al contained as an unavoidable impurity exceeds 0.060%, Al is mixed into the weld metal during welding, deteriorating the toughness of the weld. Therefore, it is preferable that the sol. Al content be 0.060% or less. sol. The Al content is more preferably 0.050% or less, and even more preferably 0.040% or less. The oxygen (O) content, sol. Al content, and N content may be 0%.

Furthermore, in the present invention, Cr: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Co: 0.01 to 1.00%, Sn: 0.005 to 0.500%, Sb: 0.005 to 0.200%, Nb: 0.005 to 0.200%, V: 0.005 to 0. It can optionally contain one or more of the following: 200%, Ti: 0.005-0.050%, B: 0.0001-0.0050%, Zr: 0.005-0.100%, Ca: 0.0001-0.020%, Mg: 0.0001-0.020%, and REM: 0.0001-0.020%.

Cr:0.01~1.00%
Cr is an element that has the effect of further improving strength. In addition, Cr is an element that promotes the formation of cementite, and promotes the formation of pearlite phase, which is advantageous for fatigue crack propagation resistance. When Cr is contained, the Cr content is set to 0.01% or more in order to obtain the above effect. It is preferably set to 0.10% or more. On the other hand, when the Cr content exceeds 1.00%, the weldability and toughness are impaired. Therefore, when Cr is contained, it is set to 1.00% or less. The Cr content is preferably set to 0.80% or less, more preferably 0.50% or less.

Cu: 0.01-2.00%
Cu is an element that further increases strength by solid solution. When Cu is contained, the Cu content is set to 0.01% or more in order to obtain the above effect. The Cu content is preferably set to 0.05% or more, and more preferably set to 0.10% or more. On the other hand, when the Cu content exceeds 1.00%, the weldability is impaired and scratches are easily generated during the production of thick steel plates. Therefore, when Cu is contained, it is set to 2.00% or less. The Cu content is preferably set to 0.70% or less, more preferably 0.60% or less, and even more preferably 0.50% or less.

Ni: 0.01-2.00%
Ni is an element that has the effect of improving low-temperature toughness, and also improves hot brittleness when Cu is contained. When Ni is contained, in order to obtain the above effect, the Ni content is set to 0.01% or more. It is preferable that the Ni content is set to 0.05% or more. On the other hand, when the Ni content exceeds 2.00%, the weldability is impaired and the steel cost increases. Therefore, when Ni is contained, the Ni content is set to 2.00% or less. The Ni content is preferably set to 0.70% or less, more preferably 0.40% or less.

Mo: 0.01~1.00%
Mo is an element that has the effect of increasing the hardness of the matrix phase and improving the strength, and can be optionally contained according to the desired characteristics. When Mo is contained, in order to obtain this effect, the Mo content is set to 0.01% or more. It is preferable that the Mo content is set to 0.05% or more. However, if the Mo content exceeds 1.00%, the weldability and toughness are impaired, so when Mo is contained, the Mo content is set to 1.00% or less. It is preferable that the Mo content is set to 0.80% or less, and more preferably to 0.70% or less.

Co:0.01~1.00%
Co is an element that has the effect of increasing the hardness of the matrix phase and improving the strength, and can be arbitrarily contained according to the desired characteristics. In order to obtain this effect, when Co is contained, the Co content is set to 0.01% or more. The Co content is preferably 0.10% or more, more preferably 0.20% or more, and further preferably 0.35% or more. On the other hand, even if the Co content exceeds 1.00%, the effect is saturated and the alloy cost increases. Therefore, when Co is contained, the Co content is set to 1.00% or less. The Co content is preferably 0.50% or less.

Sn: 0.005-0.500%
Sn is an element that has the effect of increasing the hardness of the matrix phase and improving the strength, and can be arbitrarily contained according to the desired characteristics. In order to fully obtain such effects, when Sn is contained, the Sn content is 0.005% or more. The Sn content is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. On the other hand, if the Sn content exceeds 0.500%, the ductility and toughness of the steel are deteriorated. Therefore, when Sn is contained, the Sn content is 0.500% or less. Preferably, the Sn content is 0.300% or less, more preferably 0.200% or less, and even more preferably 0.100% or less.

Sb: 0.005-0.200%
Sb is an element that has the effect of increasing the hardness of the matrix phase and improving the strength, and can be optionally contained according to the desired characteristics. In order to fully obtain such an effect, when Sb is contained, the Sb content is set to 0.005% or more. The Sb content 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, when Sb is contained, the Sb content is set to 0.200% or less. It is preferably 0.150% or less, more preferably 0.100% or less, even more preferably 0.080% or less, and most preferably 0.050% or less.

Nb: 0.005-0.200%
Nb is an element that has the effect of suppressing the recrystallization of austenite during hot rolling, refining the crystal grains finally obtained, and improving toughness. In addition, Nb precipitates during air cooling after accelerated cooling, further improving strength. When Nb is contained, the Nb content is set to 0.005% or more in order to obtain the above effect. The Nb content is preferably set to 0.007% or more, and more preferably set to 0.010% or more. On the other hand, when the Nb content exceeds 0.200%, the hardenability becomes excessive, and bainite is excessively generated, so that the desired structure cannot be obtained, and the toughness decreases. Therefore, when Nb is contained, the Nb content is set to 0.200% or less. The Nb content is preferably set to 0.070% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.030% or less.

V:0.005~0.200%
Like Nb, V is an element that suppresses the recrystallization of austenite during hot rolling to make it finer, and also has the effect of increasing strength by precipitating during the air cooling process after hot rolling, and can be contained arbitrarily according to the desired characteristics. In order to obtain the above effect, when V is contained, the V content is set to 0.005% or more. The V content is preferably set to 0.010%, more preferably set to 0.020% or more, and even more preferably set to 0.030% or more. However, when the V content exceeds 0.200%, a large amount of VC is precipitated, and toughness is impaired. Therefore, when V is contained, the V content is set to 0.200% or less. The V content is preferably set to 0.150% or less, more preferably set to 0.100% or less, and even more preferably set to 0.070% or less.

Ti: 0.005-0.050%
Ti has a strong tendency to form nitrides, and fixes N to reduce solute N, thus improving the toughness of the base material and the welded portion. In addition, when B is contained, Ti can be added to prevent Ti from fixing N and B from precipitating as BN. As a result, the hardenability improving effect of B is promoted, and the strength can be further improved. Therefore, it can be arbitrarily contained according to the desired characteristics. In order to obtain the above effect, when Ti is contained, it is set to 0.005% or more. The Ti content is preferably set to 0.007% or more, and more preferably set to 0.010% or more. However, when the Ti content exceeds 0.050%, a large amount of TiC is precipitated, and the toughness is impaired. Therefore, when Ti is contained, the Ti content is set to 0.050% or less. The Ti content is preferably set to 0.040% or less, more preferably set to 0.030% or less, and even more preferably set to 0.020% or less.

B: 0.0001-0.0050%
B is an element that has the effect of significantly improving hardenability and increasing strength even when contained in a small amount, and can be contained according to the desired characteristics. In order to obtain the above effect, when B is contained, the B content is set to 0.0001% or more. The B content is preferably set to 0.0005% or more, and more preferably set to 0.0010% or more. However, when the B content exceeds 0.0050%, not only the effect is saturated but also the weldability is reduced, so when B is contained, the B content is set to 0.0050% or less. The B content is preferably set to 0.0040% or less, more preferably set to 0.0030% or less, and even more preferably set to 0.0020% or less.

Zr: 0.005-0.100%
Zr is an element that has the effect of further increasing strength. In order to fully obtain the above effect, when Zr is contained, the Zr content is set to 0.005% or more. The Zr content is preferably set to 0.010% or more, more preferably set to 0.030% or more, and even more preferably set to 0.050% 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 is set to 0.100% or less.

Ca: 0.0001-0.020%
Ca combines with S to suppress the formation of MnS and the like that elongates in the rolling direction, controls the shape of sulfide-based inclusions so that they are spherical, and contributes to improving the toughness of welds and the like, so it can be contained according to the desired characteristics. When Ca is contained, in order to obtain this effect, the Ca content is set to 0.0001% or more. The Ca content is preferably set to 0.0005% or more, and more preferably set to 0.0010% or more. However, when the Ca content exceeds 0.020%, not only is the effect saturated, but the cleanliness of the steel decreases, surface defects occur frequently, and the surface properties deteriorate. For this reason, when Ca is contained, the Ca content is set to 0.020% or less. The Ca content is preferably set to 0.010% or less, more preferably set to 0.006% or less, and even more preferably set to 0.002% or less.

Mg: 0.0001-0.020%
Mg is an element that has the effect of improving toughness through the refinement of crystal grains. When Mg is contained, the Mg content is set to 0.0001% or more in order to obtain the above effect. The Mg content is preferably set to 0.0003% or more, and more preferably set to 0.0005% or more. On the other hand, when the Mg content exceeds 0.020%, the effect is saturated. Therefore, when Mg is contained, the Mg content is set to 0.020% or less. The Mg content is preferably set to 0.015% or less, more preferably set to 0.010% or less, and even more preferably set to 0.005% or less.

REM: 0.0001-0.020%
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. The REM content is preferably set to 0.0003% or more. On the other hand, when the REM content exceeds 0.020%, the effect is saturated. Therefore, when REM is contained, the REM content is set to 0.020% or less. The REM content is preferably set to 0.010% or less, more preferably set to 0.005% or less, and even more preferably set to 0.001% or less.

[Microstructure]
Next, the reasons for limiting the microstructure of the steel plate will be described. Note that "%" in the description of the microstructure refers to the area ratio unless otherwise specified.

The structure in the range from the surface to 100 μm below the surface (surface layer structure)
In the steel plate of the present invention, the microstructure in the thickness direction from the surface to 100 μm below the surface (hereinafter, sometimes simply referred to as the "surface layer") contains 80% or more of ferrite phase in terms of area ratio. In the two-phase region from the Ac1 transformation point to the Ac3 transformation point, a surface layer decarburization reaction occurs, and 80% or more of ferrite phase is generated in the surface layer to soften the surface layer of the steel plate, thereby significantly improving the elongation characteristics in the entire thickness. This surface layer decarburization reaction occurs by passing through or holding in the two-phase region in the second reheating step. On the other hand, if the area ratio of the ferrite phase in the surface layer is less than 80%, a large amount of hard residual consisting of a bainite phase, a pearlite phase, a martensite phase, or a mixed phase thereof will be present. As a result, the hardness of the surface layer increases, and the desired elongation characteristics in the entire thickness cannot be obtained. In addition, the tensile strength may become excessive.

Here, the area ratio of the ferrite phase in the surface layer 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 plate. The microstructure in the surface layer refers to the microstructure of the surface layer at two locations taken from any position in the rolling direction of the thick steel plate. Therefore, the area ratio of the ferrite phase in the range from the surface to 100 μm below the surface is measured at two locations taken from any position in the rolling direction of the thick steel plate, and the average value is 80% or more. Normally, if the microstructure of the surface layer at two locations taken from any position in the rolling direction of the thick steel plate satisfies the above condition, the thick steel plate satisfies the above condition over the entire length in the rolling direction. Therefore, it can be said that the area ratio of the ferrite phase in the surface layer of the thick steel plate of the present invention is 80% or more over the entire length in the rolling direction. Regarding the upper limit, the ferrite phase in the surface layer may be 100% in order to ensure the desired strength.

The remainder of the microstructure of the surface layer other than the ferrite phase is preferably a hard phase, and the hard phase is preferably a pearlite phase or a mixed phase of bainite and pearlite phases. However, since the bainite phase contains island martensite and reduces toughness, the less bainite phase there is, the better, and it is even more preferable to have only pearlite phase.

Structure at 1/4 of plate thickness (internal structure of plate thickness)
In the thick steel plate of the present invention, the microstructure at the 1/4 position of the plate thickness (hereinafter, sometimes simply referred to as "plate thickness interior") contains 90% or less of ferrite phase in terms of area ratio. In order to suppress excessive strength increase and toughness decrease, it is preferable to contain 50% or more of ferrite phase in terms of area ratio. When the microstructure in the plate thickness interior satisfies the above conditions, the desired strength and fatigue crack propagation resistance can be obtained. In addition, the average crystal grain size of the ferrite phase is 25 μm or less. When the microstructure in the plate thickness interior satisfies the above conditions, the desired toughness can be obtained. It is preferably 23 μm or less, more preferably 20 μm or less. On the other hand, the average crystal grain size of the ferrite phase is preferably 3 μm or more in order to suppress excessive strength increase and toughness decrease.

Here, the area ratio of the ferrite phase in the thickness direction refers to the average value of the area ratio of the ferrite phase at the 1/4 position of the thickness of the thick steel plate. Here, the microstructure in the thickness direction refers to the microstructure in the thickness direction at two locations taken from any position in the rolling direction of the thick steel plate. Therefore, in the thick steel plate of the present invention, the microstructure in the thickness direction at the 1/4 position at two locations taken from any position in the rolling direction of the thick steel plate satisfies the above condition. Note that, as with the structure of the surface layer, if the microstructure in the thickness direction at two locations taken from any position in the rolling direction of the thick steel plate satisfies the above condition, the above condition is usually satisfied over the entire length of the thick steel plate in the rolling direction. Therefore, it can be said that the microstructure in the thickness direction of the thick steel plate of the present invention is a ferrite phase with an area ratio of 90% or less and an average crystal grain size of 25 μm or less over the entire length of the rolling direction.

In addition, in the present invention, it is important that the hard phase is dispersed in the ferrite phase. By dispersing the hard phase in the ferrite phase, fatigue cracks propagating through the ferrite phase bend or branch when they encounter the hard phase. As a result, fatigue crack propagation resistance is improved. If the average spacing between hard phases is 25 μm or more, fatigue crack propagation resistance is deteriorated. Therefore, when dispersing the hard phase in the ferrite phase, it is necessary to make the average spacing between hard phases less than 25 μm. The average spacing between hard phases refers to the distance obtained by measuring the spacing between hard phases dispersed in the ferrite phase in two observation fields taken from any position in the rolling direction of the thick steel plate and averaging them. Specifically, for example, when the hard phase is composed of bainite and pearlite, it refers to the distance obtained by averaging the distances between all bainite phases and bainite phases, the distance between pearlite phases and pearlite phases, and the distance between bainite phases and pearlite phases. It is preferably less than 20 μm. More preferably, it is less than 10 μm. In addition, since the hard phase needs to be dispersed in the ferrite phase, the average spacing between the hard phases needs to be greater than 0 μm. It is preferably 2 μm or greater.

In addition, if the hard phase is present in a band shape over the entire surface, it is not preferable because the desired fatigue crack propagation resistance cannot be obtained. Here, a band-shaped structure refers to a structure in which the hard phase is continuously formed over 100 μm or more in the rolling direction when observed at a position 1/4 of the sheet thickness on a plane (L cross section) formed by the rolling direction and sheet thickness direction of the steel sheet. Therefore, in order to obtain the desired fatigue crack propagation resistance, it is preferable that the hard phase has a major axis of less than 100 μm in the rolling direction. More preferably, it is 25 μm or less. It is preferable that the hard phase has a major axis of 1 μm or more in the rolling direction.

The remainder of the microstructure within the plate thickness must be a hard phase containing pearlite phase in order to improve fatigue crack propagation characteristics without deteriorating toughness, and the hard phase preferably consists of pearlite phase or a mixed phase of pearlite phase and bainite phase.
The bainite phase contains island martensite and reduces toughness. For this reason, it is preferable to make the area ratio of the pearlite phase greater than the area ratio of the bainite phase. The area ratio of the bainite phase is preferably 15% or less. The area ratio of the bainite phase is more preferably 13% or less, further preferably 11% or less, and further preferably 9% or less. The lower limit of the bainite phase may be 0%. The area ratio of the pearlite phase is preferably 5% or more, and preferably 30% or less.
In addition, since the martensite phase reduces elongation and toughness, the dispersion of the martensite phase as a hard phase is not preferable, and in particular, the dispersion of the martensite phase alone is even more unpreferable.
Here, the remainder of the microstructure within the plate thickness refers to the remainder at 1/4 of the plate thickness of the thick steel plate at two locations taken from arbitrary positions in the rolling direction of the thick steel plate. That is, over the entire length of the rolling direction of the thick steel plate, the remainder of the microstructure is a hard phase, including a pearlite phase, and preferably, the hard phase is a pearlite phase or a mixed phase of a pearlite phase and a bainite phase, and the hard phase is present dispersed in the ferrite phase.

The microstructure at the surface and inside the plate thickness can be evaluated using the method described in the examples.

[Total thickness elongation]
The total thickness elongation of the thick steel plate 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 two points taken from any position in the rolling direction of the thick steel plate. Note that, usually, if two points taken from any position in the rolling direction of the thick steel plate satisfy the above-mentioned condition, the thick steel plate satisfies the above-mentioned condition over the entire length in the rolling direction. Moreover, the total thickness elongation can be measured by the method described in the examples.

[Tensile strength]
The tensile strength (TS) of the thick steel plate is not particularly limited, but is preferably 490 MPa or more. The upper limit of TS is also not particularly limited, but for example, when the 490 MPa (50 kgf/mm ² ) class in JIS is used, the TS may be set to 610 MPa or less. When the 570 MPa (60 kgf/mm ² ) class in JIS is used, 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 two points taken from any position in the rolling direction of the thick steel plate. Note that, usually, if two points taken from any position in the rolling direction of the thick steel plate satisfy the above conditions, the above conditions are satisfied over the entire length of the thick steel plate in the rolling direction. The TS can be measured by the method described in the examples.

[Toughness]
The steel plate of the present invention has excellent toughness as a result of having the above-mentioned composition and microstructure. The toughness of the steel plate of the present invention is not particularly limited, but in the case of a test piece having a thickness of 10 mm, the Charpy absorbed energy vE ₀ at 0° C., which is one of the indicators of toughness, is preferably 100 J or more, more preferably 130 J or more, even more preferably 150 J or more, and most preferably 270 J or more. In the case of a test piece having a thickness of 5 mm, the Charpy absorbed energy vE ₀ is preferably 50 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. Note that vE ₀ can be measured by the method described in the examples.

[Fatigue crack propagation characteristics]
The steel plate of the present invention has the above-mentioned composition and microstructure, and as a result, has excellent fatigue crack propagation resistance. The 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, but in the present invention, a fatigue crack propagation rate of 8.50×10 ⁻⁸ m/cycle or less at ΔK=25 MPa√m is preferable. The fatigue crack propagation rate (da/dN) can be measured by the method described in the examples.

[Thickness]
In the present invention, the term "thick steel plate" refers to a steel plate having a thickness of 6 mm or more, in accordance with the usual definition in this 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.

[Production method]
The thick steel plate of the present invention is manufactured by sequentially subjecting a steel material having the above-mentioned component composition to a heating process, a rolling process, a cooling process after the rolling process, a first reheating process, a first cooling process, a second reheating process, a second cooling process, and a water cooling process.

Steel material As the steel material of the present invention, any material having the above-mentioned composition and capable of being hot-rolled can be used, but typically, a steel slab is sufficient. For example, molten steel having the above-mentioned composition can be produced by means of a converter or the like, and can be made into a steel material such as a slab by a casting method such as a continuous casting method. Also, a steel material such as a slab can be made by an ingot casting-splitting rolling method.

Heating step The steel material having the above-mentioned 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 rolling step increases, the load on the hot rolling machine increases, and hot rolling becomes difficult. Therefore, the heating temperature is set to 900°C or higher. The heating temperature is preferably set to 950°C or higher. On the other hand, if the heating temperature exceeds 1200°C, the reverse transformed austenite becomes coarse, and the crystal grains of the ferrite phase inside the final plate thickness become coarse, resulting in a decrease in toughness. Therefore, the heating temperature is set to 1200°C or lower. The heating temperature is preferably set to 1150°C or lower.

When the steel material (slab) is produced by a method such as continuous casting, the slab may be directly subjected to the above heating process without being cooled, or may be subjected to the above heating process after being cooled. The heating method is not particularly limited, but it may be heated in a heating furnace according to the usual method, for example.

Rolling process Next, the heated steel material is hot-rolled to obtain a thick steel plate. In this process, the cumulative rolling reduction is set to 50% or more in order to ensure toughness. If the cumulative rolling reduction is less than 50%, the crystal grains of the ferrite phase inside the thickness of the finally obtained plate become coarse, resulting in localized regions of low brittleness, which makes brittle cracks more likely to occur and reduces toughness. There is no particular upper limit, but it is preferably set to 99% or less from the viewpoint of suppressing efficiency reduction due to load restrictions of the rolling machine. Other conditions related to the hot rolling process are not particularly limited.

Cooling step after rolling step Next, the thick steel plate after the rolling step is cooled. By cooling the thick steel plate after the rolling step, the crystal grains of the ferrite phase inside the plate thickness finally obtained can be refined and the toughness can be improved. 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 water cooling. However, water cooling is more preferable from the viewpoint of suppressing the coarsening of crystal grains. In addition, there is no particular limitation on the cooling conditions.

First reheating step In the first reheating step, the thick steel plate after the cooling step is reheated to a temperature between the Ac3 transformation point and 950 ° C. (first reheating temperature). By reheating to a temperature between the Ac3 transformation point and 950 ° C. in this way, the microstructure of the entire steel plate is reverse transformed to the austenite phase, and the variation in the microstructure in the rolling direction (longitudinal direction) caused by the deviation in the cooling stop temperature can be eliminated. As a result, the variation in mechanical properties (strength) can be eliminated, and further, the crystal grains of the ferrite phase inside the plate thickness after the first cooling step are also refined, so that the toughness is also improved. On the other hand, when the first reheating temperature exceeds 950 ° C., the crystal grains of the reverse transformed austenite phase grow and become coarse, so that the crystal grains of the ferrite phase inside the plate thickness finally obtained also become coarse, and the toughness is reduced.

In the first reheating step, it is preferable to heat the steel plate to the first reheating temperature and then hold the temperature at that temperature. If the holding time exceeds 60 minutes, the crystal grains of the reverse-transformed austenite phase grow excessively, which tends to reduce toughness. On the other hand, if the holding time is less than 10 minutes, the reverse transformation to the austenite phase does not proceed sufficiently, resulting in a non-uniform microstructure within the plate thickness and a deterioration in toughness in some parts of the steel plate. Therefore, it is preferable that the holding time in the first reheating step be 10 minutes or more and 60 minutes or less.

First cooling step In the first cooling step, the thick steel plate after the first reheating step is cooled. By cooling the thick steel plate after the first reheating step, the crystal grains of the ferrite phase inside the plate thickness finally obtained can be further refined, and the toughness can be further improved. If the plate thickness is 50 mm or less, the cooling in the first cooling step can be performed by any method, for example, air cooling or water cooling, and water cooling is preferable from the viewpoint of suppressing the coarsening of crystal grains. For this reason, 1 ° C./s or more is preferable, more preferably 20 ° C./s or more, and even more preferably 40 ° C./s or more. The upper limit does not need to be particularly limited, but it is preferable to set it to 100 ° C./s or less from the viewpoint of effective use of water resources used for cooling. The cooling stop temperature (first cooling stop temperature) may be lower than the second reheating temperature, and is more preferably 300 ° C. or less, and room temperature (20 ° C.) or more. Furthermore, for thick steel plates having a thickness of more than 50 mm, it may be difficult to achieve a cooling rate sufficient to obtain a desired structure inside the plate thickness if air cooling is used in the first cooling step, whereas water cooling can achieve an average cooling rate of 20°C/s or more, so it is preferable to apply water cooling.

In the second reheating step, the steel plate after the first cooling step is reheated to a temperature (second reheating temperature) of not less than the Ac1 transformation point and not more than 950° C. The second reheating temperature is preferably lower than the Ac3 transformation point.

When the second reheating temperature is between the Ac1 transformation point and the Ac3 transformation point, the decarburization reaction specific to the two-phase region proceeds, and the area ratio of the ferrite phase in the surface layer can be 80% or more. On the other hand, when the second reheating temperature is between the Ac3 transformation point and 950°C, the holding time at the second reheating temperature is set to a short time, so that the reaction in which the ferrite phase in the surface layer generated by the surface decarburization reaction during passage through the two-phase region is reverse transformed to the austenite phase is suppressed, and the area ratio of the ferrite phase in the surface layer can be 80% or more. When the second reheating temperature is below the Ac1 transformation point, the surface decarburization reaction specific to the two-phase region does not proceed, and the area ratio of the ferrite phase in the surface layer is less than 80%. As a result, the hardness of the surface layer increases and the desired elongation characteristics at the entire thickness cannot be obtained. Furthermore, the reverse transformation reaction to the austenite phase does not occur inside the plate thickness, and the grain refinement of the ferrite phase inside the plate thickness does not occur after the second cooling process. As a result, the toughness decreases. On the other hand, if the second reheating temperature is above 950°C, the reaction in which the ferrite phase in the surface layer formed by the surface decarburization reaction during passage through the two-phase region reverse-transforms into the austenite phase is promoted, and the area ratio of the ferrite phase in the surface layer becomes less than 80%. As a result, the hardness of the surface layer increases and the desired elongation characteristics over the entire thickness cannot be obtained. Furthermore, the austenite phase reverse-transformed inside the plate thickness grows and coarsens, resulting in localized areas of low brittleness and reduced toughness.

In addition, when the second reheating temperature is equal to or higher than the Ac3 transformation point and equal to or lower than 950°C, the crystal grains of the austenite phase inside the plate thickness become coarser than when the second reheating temperature is lower than the Ac3 transformation point, but the toughness does not deteriorate excessively. Furthermore, in this temperature range, the rate of reverse transformation to the austenite phase inside the plate thickness increases. Therefore, the reverse transformation reaction to the austenite phase is completed in a short heating time, and the number of thick steel plates that can be manufactured in a given time increases, improving productivity.

The Ac1 transformation point can be calculated, for example, by the following formula (1).
Ac1 (℃) = 723 + 29.1 x Si - 10.7 x Mn - 16.9 x Ni + 16.9 x Cr... (1)
The Ac3 transformation point can be calculated, for example, by the following formula (2).
Ac3(℃)=961.6-311.9×C+49.5×Si-36.4×Mn+438.1×P-2818×S+12.7×Al-51×C u-29×Ni-8.7×Cr+13.5×Mo+308.1×Nb-140×V+318.9×Ti+611.2×B-969×N…(2)
Here, the element symbols in the above formulas (1) and (2) indicate the content (mass%) of each element, and when the element is not contained, it is set to zero.

The holding time in the second reheating step is explained below.

In the second reheating step, it is preferable to heat the steel plate to the second reheating temperature and then hold the temperature. When the second reheating temperature is equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point, if the holding time is less than 10 minutes, the reverse transformation to the austenite phase does not start over the entire length of the steel plate, and it is difficult to obtain the desired microstructure inside the plate thickness after the second cooling step. In addition, in the surface layer, the decarburization reaction does not proceed sufficiently, and it is difficult to obtain the desired microstructure even in the surface layer. Therefore, it is preferable to hold the temperature for 10 minutes or more. There is no need to particularly limit the upper limit, but it is preferable to hold the temperature for 120 minutes or less from the viewpoint that long-term heating leads to an increase in the operating costs of maintaining the heating furnace. On the other hand, when the second reheating temperature is equal to or higher than the Ac3 transformation point and lower than 950 ° C., if the holding time exceeds 30 minutes, the reversely transformed austenite phase grows inside the plate thickness, the crystal grains become coarse, and the toughness decreases. In addition, in the surface layer, the reverse transformation to the austenite phase proceeds more, and it is difficult to obtain the desired microstructure even in the surface layer. Therefore, the holding time is preferably 30 minutes or less. There is no particular lower limit, but it is preferably 5 minutes or more to allow the phase transformation to proceed sufficiently.

Second Cooling Step In the second cooling step, the thick steel plate reheated in the second reheating step is cooled to a second cooling stop temperature of 350 to 600 ° C. At that time, the second average cooling rate is set to 1.5 to 20 ° C. / s. If the second average cooling rate is less than 1.5 ° C. / s, pearlite is generated in a band shape as described above, and the frequency of encounter between the fatigue crack and pearlite increases, so that the fatigue crack propagation resistance property decreases. Therefore, the second average cooling rate is set to 1.5 ° C. / s or more, and the pearlite phase is dispersed in the ferrite phase. On the other hand, if the second average cooling rate exceeds 20 ° C. / s, the pearlite transformation does not proceed sufficiently in the microstructure inside the plate thickness, and the bainite transformation tends to proceed. Since the bainite contains island martensite, the toughness decreases with an increase in the bainite area ratio. If the cooling rate is 20°C/s or less, the pearlite transformation in the microstructure inside the steel sheet proceeds moderately, and the grain size of the ferrite phase is refined in the process prior to the second cooling process, so that the desired toughness can be obtained. Furthermore, if the cooling rate is 7°C/s or less, the area ratio of the bainite phase becomes smaller than the area ratio of the pearlite phase, and the toughness is further improved. The second 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 average cooling rate can be obtained by measuring the temperature distribution of the steel sheet after cooling using a non-contact type radiation thermometer and calculating the temperature history before and after cooling. In addition, the temperature history from before cooling to after cooling can be constantly measured and calculated using a contact type thermocouple or the like.

Furthermore, if the second cooling stop temperature is less than 350°C, excessive ferrite will be produced within the plate thickness, softening the entire steel plate and making it impossible to obtain the desired tensile strength. For this reason, the second cooling stop temperature is set to 350°C or higher. On the other hand, if the second cooling stop temperature is more than 600°C, the plate will be quenched with a large amount of untransformed austenite remaining, resulting in the excessive production of hard bainite and martensite within the plate thickness. As a result, the elongation characteristics throughout the entire thickness will decrease, and the toughness will also decrease. For this reason, the second cooling stop temperature is set to 600°C or lower.

According to the manufacturing method of the present application, the steel sheet is subjected to a total of three processes of repeated normal and reverse transformations, including the first reheating, the first cooling process from the first reheating, and the second cooling process from the second reheating. The repeated normal and reverse transformations make the crystal grains finer. Therefore, this process is important for refining the crystal grains, and the desired average crystal grain size of the ferrite phase can be obtained.

Water-cooling step In the water-cooling step, the steel plate after the second cooling step is water-cooled to prevent the formation of band-shaped structures. Therefore, the water-cooling temperature is in the range of 350 to 600°C. Water-cooling is not particularly limited and can be performed under any conditions, but it is preferable to water-cool the steel plate to a temperature below the Ms point, preferably below 200°C. The Ms point can be calculated, for example, by the following formula (3). Water-cooling refers to cooling at an average cooling rate of 20°C/s or more. The upper limit of the average cooling rate is preferably 100°C/s or less.
Ms(℃)=517-300×C-11×Si-33×Mn-17×Ni-22×Cr-11×Mo…(3)
Here, the element symbols in the above formula (3) indicate the content (mass%) of each element, and are set to zero when the element is not contained.

There are no particular limitations on the manufacturing conditions other than those mentioned above, but it is preferable to carry out the process under the following conditions.

Cooling After the Water-Cooling Step In the present invention, the cooling method after the water-cooling step is not particularly limited, and any method such as air cooling or water cooling can be used.

The effects of the present invention will be specifically explained below based on examples, but the present invention is not limited to these examples.

Molten steel having the composition shown in Table 1 was melted and used as steel material (slab). The values of Ac1 point, Ac3 point, and Ms point shown in Table 1 were calculated using the above-mentioned formulas (1), (2), and (3), respectively.

Next, the obtained slab was heated (heating process) and hot rolled (rolling process) under the conditions shown in Table 2 to obtain a thick steel plate with a total length of 20 m and a plate thickness shown in Table 2. The thick steel plate was then cooled to room temperature by the cooling method shown in Table 2 (cooling process), reheated to the first reheating temperature shown in Table 2, and held for the holding time shown in Table 2 (first reheating process). Then, it was cooled to the first cooling stop temperature at the first average cooling rate by the cooling method shown in Table 2 (first cooling process). Next, it was reheated to the second reheating temperature shown in Table 2, and held for the holding time shown in Table 2 (second reheating process). Then, it was cooled to the second cooling stop temperature at the second average cooling rate by the cooling method shown in Table 2 (second cooling process). Then, it was subjected to water cooling treatment (water cooling process). In the water cooling process, it was water cooled to 150°C or less.

For comparison, in some comparative examples (No. 31 in Table 2), water cooling was performed immediately after the second reheating without performing the second cooling process. The water cooling conditions in this comparative example were an average cooling rate of 44.0°C/s and a cooling stop temperature of 110°C.

The resulting steel plates were evaluated for (1) microstructure, (2) total thickness elongation and tensile strength (TS), (3) fatigue crack propagation properties, and (4) toughness. To evaluate the variability in properties in the rolling direction and width direction of the steel plate, test specimens were taken from any position in the rolling direction of the steel plate, and the two positions with the greatest variability (position 1 and position 2) were evaluated. The test method is as follows. Each of the above test specimens was taken from a position 100 mm in from the end of the steel plate in the rolling direction.

(1) Microstructure Observation The microstructure was observed by the following procedure.

Area ratio of ferrite phase in surface layer, area ratio of ferrite phase in plate thickness, area ratio of pearlite phase and bainite phase in plate thickness First, a test piece for microstructure observation was taken from the obtained thick steel plate so that the observation surface was a cross section perpendicular to the rolling direction (plate thickness direction cross section), polished to a mirror surface, and then corroded with an etching solution (nitric acid methanol solution). Using an optical microscope (magnification: 400 times) or a scanning electron microscope (magnification: 400 times), the steel plate was observed from the surface to the 1/4 position 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, and (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, (b) the average value of the area ratio of the ferrite phase at the 1/4 position of the plate thickness, the average value of the ferrite grain size obtained by the cutting method described in JIS G 0551, and (c) the area ratio of the pearlite phase and the bainite phase at the 1/4 position of the plate thickness were obtained. In the above image, the white parts are judged as ferrite phase, the black parts as pearlite phase, and the remaining parts as bainite phase. The average spacing between hard phases is calculated as the distance between edges by performing image analysis on the above image using Image-J. Specifically, the phase boundaries in each region of the hard phase, specifically the pearlite phase and the bainite phase, were detected and drawn as edges (contour lines), and the distance between the edges of the closest hard phases (pearlite phase or bainite phase) was measured. The above evaluation was performed on 10 fields of view, and the average value was calculated as the average spacing between the hard phases.

The size of the hard phase is measured by detecting and drawing the edges (contour lines) of the phase boundaries in each region of the pearlite phase and bainite phase, and measuring the width in the rolling direction.

The martensite phase was identified based on the pattern of the hard phase in the scanning electron microscope (magnification: 400x) image taken above.

The microstructure measurement results are shown in Table 3.

(2) Tensile test Tensile tests were conducted using full-thickness test pieces of JIS Z 2201 No. 1A taken from the width center of a thick steel plate so that the plate width direction coincided with the tensile direction, and the tensile strength (TS) and full-thickness elongation were obtained. A tensile strength of 490 MPa or more was considered to pass. Elongation properties were considered to pass when the plate thickness was 16 mm or less, and when the plate thickness was more than 16 mm, when it was 15% or more.

(3) Fatigue crack propagation test A fatigue crack propagation test was performed using a single-side notched simple tension fatigue test piece shown in Figure 1 to evaluate the fatigue crack propagation behavior when the crack propagates in the width direction of the steel plate. The test piece was a compact tension test piece taken from the 1/4 position of the plate thickness, and the test conditions were in accordance with ASTM E647, with a stress ratio of 0.1 and a frequency of 10 Hz, and the test was performed in air at room temperature. Since the purpose of the present invention is to reduce the propagation speed of a crack generated from a weld or the like in a welded structure when it propagates through the steel material, assuming such a situation, the test was performed with a stress intensity factor range (ΔK) of 10 to 30 MPa√m. A fatigue crack propagation speed of 8.50 × 10 ^-8 m/cycle or less at ΔK = 25 MPa√m was considered to be acceptable.

(4) Toughness Charpy impact test specimens were taken from the center of the thickness of the thick steel plate parallel to the rolling direction (L direction). The thickness of the test specimen was 10 mm when the plate thickness was 10 mm or more, and 5 mm when the plate thickness was less than 10 mm. The test was performed at 0°C in accordance with JIS Z 2202 to measure the absorbed energy vE _0. The test specimens with a thickness of 10 mm were considered to have passed if the absorbed energy was 100 J or more. The test specimens with a thickness of 5 mm were considered to have passed if the absorbed energy was 50 J or more.

The measurement results are shown in Table 4. As can be seen from these results, in the examples that satisfy the conditions of the present invention, thick steel plates that have elongation characteristics through the entire thickness, fatigue crack propagation resistance, and toughness are obtained. On the other hand, in the comparative examples that do not satisfy the conditions of the present invention, at least one of the strength characteristics, elongation characteristics through the entire thickness, fatigue crack propagation rate, or toughness is inferior at one of the two locations evaluated in the rolling direction of the thick steel plate.

Claims (5)

In mass percent,
C: 0.05-0.20%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.00%,
P: 0.05% or less,
S: 0.02% or less, the balance being Fe and unavoidable impurities;
The microstructure is
In the thickness direction, the area ratio of the ferrite phase is 80% or more in the range from the surface to 100 μm below the surface, and at the 1/4 position of the thickness,
Contains a ferrite phase having an area ratio of 90% or less and an average crystal grain size of 25 μm or less,
The balance of the steel plate is a hard phase, the hard phase is dispersed in a ferrite phase, the hard phase includes a pearlite phase, and the average spacing between the hard phases is less than 25 μm. The composition further comprises, in mass%,
Cr: 0.01-1.00%,
Cu: 0.01-2.00%,
Ni: 0.01-2.00%,
Mo: 0.01-1.00%,
Co: 0.01 to 1.00%,
Sn: 0.005-0.500%,
Sb: 0.005-0.200%,
Nb: 0.005-0.200%,
V: 0.005-0.200%,
Ti: 0.005 to 0.050%,
B: 0.0001 to 0.0050%,
Zr: 0.005-0.100%,
Ca: 0.0001-0.020%,
The steel plate according to claim 1, containing one or more selected from Mg: 0.0001 to 0.020% and REM: 0.0001 to 0.020%. The steel plate according to claim 1 or 2, wherein the hard phase is a pearlite phase or a mixed phase of pearlite and bainite, and the area ratio of the pearlite phase in the mixed phase is larger than the area ratio of the bainite phase. The method for producing a steel plate according to claim 1 or 2,
A heating step of heating a steel material having the above-mentioned composition to 900 to 1200°C;
a rolling step of subjecting the steel material heated in the heating step to hot rolling at a cumulative rolling reduction rate of 50% or more to obtain a thick steel plate;
A cooling step after the rolling step of cooling the thick steel plate;
A first reheating step in which the steel plate cooled in the cooling step after the rolling step is reheated to a first reheating temperature of not less than the Ac3 transformation point and not more than 950 ° C.;
A first cooling step of cooling the steel plate reheated in the first reheating step;
A second reheating step of reheating the steel plate cooled in the first cooling step to a second reheating temperature of the Ac1 transformation point or higher and 950 ° C. or lower;
A second cooling step of cooling the steel plate reheated in the second reheating step to a second cooling stop temperature of 350 to 600 ° C. at an average cooling rate of 1.5 to 20 ° C./s;
a water cooling step of water cooling the steel plate cooled in the second cooling step;
A method for manufacturing a thick steel plate having the above structure. The method for manufacturing thick steel plates according to claim 4, wherein the average cooling rate in the second cooling step is 1.5 to 7°C/s.