JP7396512B2 - Thick steel plate and method for manufacturing thick steel plate - Google Patents

Description

The present invention relates to a thick steel plate, and particularly to a thick steel plate that has both excellent fatigue crack propagation resistance and total elongation, and also has excellent elongation in the thickness direction. The thick steel plate of the present invention can be suitably used in structures where structural safety is strongly required, such as ships, marine structures, bridges, buildings, and tanks. The present invention also relates to a method for manufacturing the thick steel plate.

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

That is, when the above-mentioned structure is used, repeated loads such as vibrations caused by wind and earthquakes are applied to the structure. Therefore, thick steel plates are required to have fatigue properties that can ensure the safety of the structure even when such repeated loads are applied. In particular, in order to prevent ultimate destruction such as breakage of members, it is effective to improve the fatigue crack propagation resistance of thick steel plates.

Therefore, various studies are being conducted to improve the fatigue crack propagation resistance of steel plates.

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

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

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

Here, thick-walled steel plates are usually manufactured by blooming a large steel ingot produced by an ingot-forming method and hot rolling the obtained blooming slab. However, in this ingot making-blooming process, it is necessary to cut off the densely segregated area at the feeder and the negatively segregated area at the bottom of the steel ingot, which does not improve yields, increases manufacturing costs, and shortens the construction period. The problem is that it is long.

On the other hand, if a thick steel plate is manufactured using a continuous casting slab material process, the above problem does not occur. However, since continuous casting slabs are thinner than slabs manufactured by the ingot-forming method, the amount of reduction to the product thickness during the rolling process is small, and there is a problem that the center porosity cannot be crimped. If the center porosity cannot be crimped, the elongation due to tension in the thickness direction will be poor. Furthermore, since thick steel plates are used for structures and the like as mentioned above, high strength is required. Therefore, the amount of alloying elements added to ensure the necessary strength increases, resulting in new problems such as center porosity due to center segregation and deterioration of internal quality due to larger size. .

In order to solve these problems, the following technology has been developed to improve the characteristics of the center segregation area within the steel plate by crimping the center porosity during the process of manufacturing thick steel plates from continuously cast slabs. Proposed.

Patent Document 4 proposes a technique of forging before hot rolling when manufacturing a thick steel plate with a cumulative reduction of 70% or less from a continuously cast slab.

In Patent Document 5, when producing an extra-thick steel plate by subjecting a continuously cast slab to forging and thick plate rolling, prior to the forging, the center of the plate thickness of the slab is held at a temperature of 1200 ° C. or more for 20 hours or more, After that, it is proposed to perform forging at a reduction rate of 16% or more.

Patent Document 6 proposes a technique in which a continuously cast slab is cross-forged and then hot rolled.

Patent Document 7 proposes a technique in which a continuously cast slab is held at a temperature of 1200° C. or higher for 20 hours or more, and then the slab is subjected to forging and plate rolling to obtain an extra-thick steel plate. In the technique, the rolling reduction ratio of the forging and the total rolling ratio of the forging and thick plate rolling are set within a specific range, and quenching and tempering are performed under specific conditions after the thick plate rolling.

Japanese Patent Application Publication No. 06-322477 Japanese Patent Application Publication No. 07-242992 Japanese Patent Application Publication No. 08-225882 Japanese Patent Application Publication No. 07-232201 Japanese Patent Application Publication No. 2002-194431 Japanese Patent Application Publication No. 2000-263103 Japanese Patent Application Publication No. 2006-111918

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

(1) For steel materials used in structures such as ships, offshore structures, bridges, buildings, and tanks, the total elongation value is generally specified in the standard. Therefore, even if the steel plate has excellent fatigue crack propagation resistance, the total elongation is required to meet the standard value.

However, since fatigue crack propagation resistance and total elongation are contradictory properties, conventional techniques such as those described in Patent Documents 1 to 3 do not achieve both excellent fatigue crack propagation resistance and total elongation. I couldn't.

That is, the techniques proposed in Patent Documents 1 to 3 do not take full elongation into consideration. In fact, the steel sheets proposed in Patent Documents 1 to 3 all have a microstructure consisting of ferrite as a soft phase and bainite or martensite as a hard phase, and the hardness of the soft phase and hard phase is By widening the difference, fatigue crack propagation resistance is improved. However, when the difference in hardness between the soft phase and the hard phase is large, the structure becomes inhomogeneous, and as a result, the total elongation of the steel sheet decreases.

(2) Furthermore, from the perspective of ensuring the safety of structures, thick steel plates are required to have excellent fatigue crack propagation resistance in the thickness direction.

That is, in general structures, steel plates are welded from various directions, so fatigue cracks occur and propagate in various directions. However, due to the structural characteristics of welded areas with narrow corners, fatigue cracks are unavoidable, and the fatigue cracks that do occur tend to propagate in the thickness direction. Therefore, in order to prevent the collapse of a structure due to fatigue cracks, it is important to suppress the growth of fatigue cracks in the thickness direction.

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

(4) Although the techniques described in Patent Documents 4 to 7 are effective in reducing center porosity and improving center segregation zones, they require a hot forging process. In addition, if only normal hot rolling is used, the processing at the center of the sheet thickness will be insufficient, leaving center porosity, and there is a concern that the tensile properties in the thickness direction will deteriorate.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a thick steel plate that has excellent total elongation, elongation in the thickness direction, and fatigue crack propagation resistance.

Specifically, the purpose is to provide a thick steel plate that has the following excellent features (1) to (4).
(1) It has both excellent fatigue crack propagation resistance and total elongation.
(2) The fatigue crack propagation resistance is excellent in the fatigue crack propagation resistance in the plate thickness direction, which is particularly important in ensuring the safety of structures.
(3) It can be manufactured without requiring sophisticated cooling control in the two-phase region.
(4) Excellent elongation by tension in the plate thickness direction also in production by hot rolling.

The present inventors conducted studies to solve the above problems, and as a result, obtained the following findings.

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

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

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

(d) A thick steel plate having the above-mentioned microstructure can be manufactured by controlling the manufacturing conditions, particularly the conditions of hot rolling and subsequent accelerated cooling. Since the thick steel plate has bainite as its first phase, it is more suitable for manufacturing by an online process than conventional steel plates.

(e) Furthermore, in hot rolling, defects such as casting defects are rendered harmless by rolling at a rolling reduction ratio of 3 or more, and at least two of the final three passes are rolled with a rolling reduction ratio of 10% or more per pass. The tensile properties in the thickness direction of the steel sheet can be improved by regulating the grain size of the entire steel sheet and suppressing the remaining abnormally coarse grains.

The present invention has been made based on the above-mentioned knowledge, and has the following gist.

1. In mass%,
C: 0.01-0.16%,
Si: 1.00% or less,
Mn: 0.50-2.00%,
P: 0.030% or less,
S: 0.020% or less,
Contains Al: 0.06% or less,
The remainder has a component composition consisting of Fe and unavoidable impurities,
In area fraction,
Contains 75-97% bainite and 3-25% perlite,
Bainite crystal grain size is 18 μm or less in average circular equivalent diameter,
Having a microstructure in which the pearlite crystal grain size is 10 μm or less in average circular equivalent diameter,
A thick steel plate with an aperture value of 30% or more in the thickness direction.

2. The component composition further includes, 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: 0.005-0.200%,
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 thick steel plate according to 1 above, containing one or more selected from the group consisting of Mg: 0.0001 to 0.020%, and REM: 0.0001 to 0.020%.

3. Heating a steel material having the composition described in 1 or 2 above to a heating temperature of 1000°C or more and 1300°C or less,
The heated steel material is hot-rolled into a hot-rolled steel plate under the conditions of a rolling ratio of 3 or more and a number of passes of 2 or more in which the rolling reduction is 10% or more among the final three passes,
The hot rolled steel sheet is acceleratedly cooled under the following conditions: cooling start temperature: Ar 3 points or higher, cooling stop temperature: 300 to 650 ° C., and average cooling rate on the steel plate surface from the start of cooling to the stop of cooling: 20 to 60 ° C./s. Method for manufacturing thick steel plates.

The thick steel plate of the present invention has both excellent fatigue crack propagation resistance and total elongation, and has excellent fatigue crack propagation resistance in the thickness direction. Furthermore, the reduction of area in the thickness direction is 30% or more, and the elongation by tension in the thickness direction is excellent. In addition, the thick steel plate of the present invention can achieve the above-mentioned excellent properties without adding large amounts of alloying elements such as Cr and Sn, which is advantageous in terms of cost. Further, the thick steel plate of the present invention can be stably manufactured without requiring sophisticated cooling control in the two-phase region.

FIG. 2 is a diagram showing the shape and dimensions of a single-sided notched simple tensile fatigue test piece used for evaluation of fatigue crack propagation characteristics in the plate thickness direction.

The present invention will be explained in detail below. Note that the present invention is not limited to this embodiment.

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

C: 0.01-0.16%
C is an element that has the effect of improving strength. Furthermore, C has the effect of promoting the formation of pearlite phase, which is advantageous for fatigue resistance. If the C content is less than 0.01%, desired strength and fatigue crack propagation resistance cannot be obtained. Therefore, the C content is set to 0.01% or more. On the other hand, when the C content exceeds 0.16%, pearlite is produced excessively or becomes coarse, resulting in deterioration in total elongation and toughness. Therefore, the C content is set to 0.16% or less, preferably 0.15% or less, and more preferably 0.10% or less.

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

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

P: 0.030% or less P deteriorates toughness. Therefore, the P content is set to 0.030% or less. On the other hand, since the lower the P content, the better, the lower limit of the P content is not particularly limited, and the P content may be 0% or more, or more than 0%. However, excessive reduction increases manufacturing costs, so from the viewpoint of manufacturing costs, the P content is preferably 0.001% or more, more preferably 0.002% or more.

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

Al: 0.06% or less Al is an element that acts as a deoxidizing agent and is generally used in the molten steel deoxidizing process. Furthermore, Al fixes N in the steel as AlN, contributing to improving the toughness of the base metal. However, if the Al content exceeds 0.06%, the toughness and total elongation of the base material (thick steel plate) will decrease, and at the same time, Al will be mixed into the weld metal during welding, degrading the toughness of the weld. Therefore, the Al content is 0.06% or less, preferably 0.05% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of enhancing the effect of adding Al, it is preferable that the Al content is 0.01% or more.

The thick steel plate in one embodiment of the present invention can have a composition containing the above elements, with the balance consisting of Fe and inevitable impurities.

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

Cr:0.01~1.00%
Cr is an element that has the effect of further improving strength. Further, Cr is an element that promotes the formation of cementite, and promotes the formation of a pearlite phase that is advantageous for fatigue resistance. When adding Cr, the Cr content is set to 0.01% or more, preferably 0.10% or more in order to obtain the above effects. On the other hand, if the Cr content exceeds 1.00%, weldability and toughness will be impaired. Therefore, when containing Cr, the Cr content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

Cu: 0.01-2.00%
Cu is an element that further increases strength through solid solution. When adding Cu, in order to obtain the above effect, the Cu content is set to 0.01% or more. On the other hand, when the Cu content exceeds 2.00%, weldability is impaired and flaws are likely to occur during the production of thick steel plates. Therefore, when containing Cu, the Cu content is set to 2.00% or less, preferably 0.70% or less, and more preferably 0.60% or less.

Ni: 0.01-2.00%
Ni is an element that has the effect of improving low-temperature toughness, and also improves hot embrittlement when Cu is added. When adding Ni, in order to obtain the above effect, the Ni content is set to 0.01% or more. On the other hand, if the Ni content exceeds 2.00%, weldability will be impaired and steel cost will increase. Therefore, when Ni is contained, the Ni content is set to 2.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.

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

Co:0.01~1.00%
Co is an element that has the effect of increasing the hardness of the matrix phase. When containing Co, in order to obtain the above effects, the Co content is set to 0.01% or more, preferably 0.35% or more. On the other hand, if the Co content exceeds 1.00%, the effect will be saturated and the alloy cost will increase. Therefore, when Co is added, the Co content is 1.00% or less, preferably 0.50% or less.

Sn: 0.005-0.200%
Sn is an element that has the effect of increasing the hardness of the base phase. When adding Sn, in order to obtain the above effect, the Sn content is set to 0.005% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the Sn content exceeds 0.200%, the ductility and toughness of the steel will deteriorate. Therefore, when adding Sn, the Sn content is set to 0.200% or less, preferably 0.100% or less, and more preferably less than 0.050%.

Sb: 0.005-0.200%
Sb is an element that has the effect of increasing the hardness of the base phase. When adding Sb, the Sb content is set to 0.005% or more, preferably 0.010% or more, and more preferably 0.020% or more in order to obtain the above effects. On the other hand, if the Sb content exceeds 0.200%, the ductility and toughness of the steel will deteriorate. Therefore, when adding Sb, the Sb content is set to 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less.

Nb: 0.005-0.200%
Nb is an element that has the effect of suppressing recrystallization of austenite during hot rolling and making the ultimately obtained crystal grains finer. Further, Nb precipitates during air cooling after accelerated cooling, further improving the strength. When adding Nb, in order to obtain the above effect, the Nb content is set to 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, the hardenability becomes excessive and the formation of martensite becomes significant. As a result, the desired structure cannot be obtained and the toughness decreases. Therefore, the Nb content is set to 0.200% or less, preferably 0.050% or less.

V:0.005~0.200%
V is an element that precipitates during air cooling after accelerated cooling and has the effect of further improving strength. When adding V, the V content is set to 0.005% or more in order to obtain the above effect. On the other hand, when the V content exceeds 0.200%, weldability and toughness decrease. Therefore, the V content is set to 0.200% or less, preferably 0.050% or less.

Ti: 0.005-0.050%
Ti is an element that has the effect of further increasing the strength and improving the toughness of the weld zone. When adding Ti, the Ti content is set to 0.005% or more in order to obtain the above effects. On the other hand, if the Ti content exceeds 0.050%, the cost will increase significantly. Therefore, the Ti content is set to 0.050% or less, preferably 0.030% or less, and more preferably 0.020% or less.

B: 0.0001-0.0050%
B is an element that has the effect of increasing hardenability and, as a result, further improving strength. When B is added, the B content is set to 0.0001% or more in order to obtain the above effects. On the other hand, when the B content exceeds 0.0050%, the hardenability becomes excessive and martensite formation becomes significant. As a result, a desired structure cannot be obtained and weldability deteriorates. Therefore, the B content is set to 0.0050% or less, preferably 0.0030% or less.

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

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

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

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

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

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

Bainite crystal grain size: 18 μm or less The bainite crystal grain size is 18 μm or less in average circular equivalent diameter. By refining the bainite, desired toughness and total elongation properties can be obtained. If the crystal grain size of bainite exceeds 18 μm in average circular equivalent diameter, desired toughness cannot be obtained. On the other hand, the lower limit of the crystal grain size of bainite is not particularly limited, but since excessive refinement makes manufacturing difficult, it is preferable that the crystal grain size of bainite is 5 μm or more in actual manufacturing.

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

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

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

Note that pearlite in the present invention includes pearlite and pseudo pearlite.

(other organizations)
The thick steel plate in one embodiment of the present invention can have a microstructure consisting of bainite and pearlite. However, the microstructure may further optionally include other structures. Hereinafter, organizations other than bainite and pearlite will be referred to as "other organizations." The other structure may be, for example, one or both of martensite and ferrite. Here, the martensite includes island-like martensite, lath-like martensite, and lenticular martensite.

When the microstructure of the thick steel plate includes other structures, the area fraction (total area fraction) of the other structures is not particularly limited. However, when martensite is present in excess, a region of high hardness is formed locally, and although the strength increases, the total elongation may deteriorate and the toughness may decrease. In addition, when ferrite is present in excess, the fatigue crack propagation rate is deteriorated, and a locally soft region is formed, which may worsen the total elongation due to the expansion of the hardness difference. Therefore, the lower the area fraction of the other tissues is, the better, but if it is 5% or less, the influence can be ignored. Therefore, it is preferable that the total area fraction of structures other than bainite and pearlite be 5% or less.

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

(plate thickness)
Although the plate thickness of the thick steel plate in the present invention is not particularly limited, it may usually be 6 mm or more. However, since center porosity is likely to occur in thick steel plates having a thickness of 25 mm or more, the present invention is particularly suitably applied to thick steel plates having a thickness of 25 mm or more. Therefore, the thickness of the thick steel plate is preferably 25 mm or more. On the other hand, the upper limit of the plate thickness is not particularly limited, but since fatigue damage is likely to occur in thick steel plates with a plate thickness of 100 mm or less, the present invention is particularly suitably applied to thick steel plates with a plate thickness of 100 mm or less. . Therefore, the thickness of the thick steel plate is preferably 100 mm or less, more preferably 80 mm or less.

(aperture)
The thick steel plate of the present invention is manufactured under the conditions described below, and as a result of crimping the center porosity, it has excellent elongation by tension in the thickness direction. Specifically, the thick steel plate of the present invention has an aperture value (RA) in the thickness direction of 30% or more. The aperture value is preferably 35% or more, more preferably 40% or more. The higher the aperture value, the better the elongation in the thickness direction. In addition, in the present invention, the aperture value refers to the aperture value in the thickness direction measured in accordance with JIS G3199 using a test piece whose parallel portion includes the thickness center part, and more specifically, It can be measured by the method described in Examples.

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

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

(toughness)
The thick steel plate of the present invention has excellent toughness as a result of having the above-mentioned composition and microstructure. The toughness of the thick steel plate of the present invention is not particularly limited, but 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, and 150 J or more. It is more preferable to set it as 200J or more, and it is most preferable to set it as 200J or more. On the other hand, the upper limit of vE ₀ is also not limited, but may be, for example, 400 J or less, 300 J or less, or 270 J or less. Note that vE ₀ can be measured by the method described in Examples.

(total elongation)
The thick steel plate of the present invention has excellent total elongation (EL) as a result of having the above-mentioned composition and microstructure. Although the value of EL is not particularly limited, it is preferably 21% or more, more preferably 22% or more, even more preferably 23% or more, and most preferably 26% or more. The upper limit of EL is also not particularly limited, but may be 36% or less. Note that EL can be measured by the method described in Examples.

(fatigue crack propagation resistance)
As a result of having the above-mentioned composition and microstructure, the thick steel plate of the present invention can have excellent fatigue crack propagation resistance in the thickness direction. Fatigue crack propagation velocity (da/dN) can be used as an index of fatigue crack propagation resistance. The value of the fatigue crack propagation speed is not particularly limited.

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

As mentioned earlier, fatigue cracks tend to propagate in the thickness direction first, so in order to improve fatigue crack propagation resistance, it is especially important to treat fatigue cracks in the thickness direction (Z direction). It will be important to limit progress. However, after the crack propagates in the Z direction, the crack may further propagate in the rolling direction (L direction) or width direction (C direction). Therefore, from the perspective of further improving fatigue crack propagation resistance, either the fatigue crack propagation velocity in the rolling direction (L direction) or the fatigue crack propagation velocity in the width direction (C direction) should be It is preferable that conditions (c) and (d) are satisfied, and it is more preferable that both conditions (c) and (d) are satisfied.
(c) Fatigue crack propagation velocity under the condition of stress intensity factor range ΔK: 15 MPa/m ^1/2 is 1.75 × 10 ^-8 (m/cycle) or less,
(d) Fatigue crack propagation velocity under the condition of stress intensity factor range ΔK: 25 MPa/m ^1/2 is 8.50×10 ^-8 (m/cycle) or less

[Manufacturing conditions]
Next, a method for manufacturing a thick steel plate according to the present invention will be explained. The thick steel plate in one embodiment of the present invention can be manufactured by sequentially performing the following steps (1) to (3) on a steel material having the above-mentioned composition.
(1) Heating (2) Hot rolling (3) Accelerated cooling

The conditions in each step will be explained below. Note that unless otherwise specified, temperature refers to the surface temperature of the object to be treated (steel material or hot rolled steel plate).

(Steel material)
Any steel material can be used as the steel material as long as it has the above-mentioned composition. The composition of the thick steel plate finally obtained is the same as that of the steel material used. As the steel material, for example, one or both of a slab and a bloom can be used. Examples of the slab include continuous casting slabs and ingot slabs.

(1) Heating Heating temperature: 1000-1300℃
First, the steel material is heated to a heating temperature of 1000°C or higher and 1300°C or lower. By the heating, precipitates in the structure are dissolved into solid solution, and the crystal grain size etc. are made uniform. However, when the heating temperature is less than 1000° C., desired characteristics cannot be obtained because the precipitates are not sufficiently dissolved. Therefore, the heating temperature is set to 1000°C or higher, preferably 1050°C or higher, and more preferably 1100°C or higher. On the other hand, if the heating temperature exceeds 1300° C., the material quality deteriorates due to coarsening of the crystal grain size, and in addition, excessive energy is required, resulting in a decrease in productivity. Therefore, the heating temperature is set to 1300°C or lower, preferably 1250°C or lower.

(2) Hot rolling Next, the heated steel material is hot rolled to obtain a hot rolled steel plate. At this time, in order to manufacture a thick steel plate that satisfies the conditions of the present invention, the rolling reduction ratio in the hot rolling needs to satisfy the following conditions.

[Reduction ratio in hot rolling: 3 or more]
If the hot rolling reduction ratio is less than 3, it is not possible to obtain the effect of improving the tensile properties in the sheet thickness direction by crimping the center porosity. Furthermore, if the hot rolling reduction ratio is less than 3, the promotion of recrystallization by rolling and the effect of grain size regulation thereby will be insufficient, and coarse austenite grains will remain. As a result, properties such as strength and toughness deteriorate. Therefore, the reduction ratio is set to 3 or more, preferably 4 or more, and more preferably 5 or more. On the other hand, there is no need to particularly limit the upper limit of the rolling reduction ratio, but it is preferably 50 or less. This is because when the reduction ratio exceeds 50, the anisotropy of mechanical properties becomes significantly large. Here, the rolling reduction ratio in hot rolling is defined as "thickness of steel material/thickness of steel plate after rolling."

[Number of passes where the reduction rate is 10% or more among the final 3 passes: 2 or more]
In the present invention, it is important that the number of passes in which the rolling reduction is 10% or more is 2 or more among the final three passes of the hot rolling. In other words, rolling is performed at a rolling reduction ratio of 10% or more in at least two of the final three passes of the hot rolling. As a result, in addition to being able to compress and reliably render defects existing in the steel material (such as casting defects) harmless, it is also possible to regularize the grains of the entire steel plate and prevent abnormally coarse grains from remaining. Specifically, the number of bainite crystal grains having an equivalent circle diameter of 100 μm or more can be reduced to 3 or less per unit area (1 mm ² ). As a result, the aperture value in the plate thickness direction can be made 30% or more. If the above conditions are not met, an aperture value of 30% or more cannot be secured. Here, the rolling rate refers to the rolling reduction rate in each pass.

From the above point of view, it is preferable that the number of passes in which the rolling reduction is 10% or more among the final three passes of the hot rolling is three. In other words, it is preferable to perform rolling at a rolling reduction rate of 10% or more in all three final passes of hot rolling. On the other hand, from the viewpoint of crimping defects, the higher the rolling reduction ratio is, the better, so the upper limit of the rolling reduction ratio in each of the final three passes is not particularly limited. However, if the rolling reduction ratio is increased, the rolling load increases, so in view of equipment constraints, it is preferable that the rolling reduction ratio in each of the final three passes is 30% or less.

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

Cooling start temperature: Ar3 point or higher If the cooling start temperature in the accelerated cooling is less than Ar3 point, ferrite and coarse pearlite will precipitate excessively, resulting in a decrease in strength and fatigue crack propagation resistance. Therefore, the cooling start temperature is set to Ar3 or higher. On the other hand, the upper limit of the cooling start temperature is not particularly limited, but is preferably 870° C. or lower from the viewpoint of ensuring a cumulative reduction rate in a temperature range of Ar3 or higher.

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

Furthermore, the fact that the cooling start temperature is at least Ar3 points necessarily means that the rolling end temperature in the hot rolling is at least Ar3 points. If the rolling end temperature is less than the Ar3 point, rolling will occur in a two-phase region and the total elongation will deteriorate; however, if the rolling end temperature is higher than the Ar3 point, rolling will be performed in the austenite single phase region, resulting in deterioration in the total elongation. can be prevented.

Cooling stop temperature: 300-650℃
In order to transform untransformed austenite into a hard phase (pearlite), the cooling stop temperature in the accelerated cooling is set to 650°C or lower, preferably 600°C or lower. When the cooling stop temperature is higher than 650° C., ferrite and coarse pearlite are excessively produced, making it impossible to obtain desired fatigue crack propagation resistance and strength. On the other hand, when the cooling stop temperature is less than 300° C., the amount of martensite produced increases, resulting in a failure to obtain a desired microstructure and a decrease in toughness and total elongation. Furthermore, since pearlite is insufficiently produced, the desired fatigue crack propagation resistance cannot be obtained. Therefore, the cooling stop temperature is set to 300°C or higher, preferably 350°C or higher, and more preferably over 400°C.

Average cooling rate: 20-60℃/s
The average cooling rate in the accelerated cooling is 20° C./s or more. When the average cooling rate is lower than 20° C./s, ferrite is generated and the desired microstructure is not obtained, resulting in a decrease in fatigue crack propagation resistance. In addition, the desired total elongation cannot be obtained because the toughness is reduced. On the other hand, when the average cooling rate exceeds 60° C./s, residual stress and excessive martensite are generated due to cooling strain, resulting in deterioration of total elongation and toughness. Furthermore, since pearlite is insufficiently produced, the desired fatigue crack propagation resistance cannot be obtained. Therefore, the average cooling rate is 60°C/s or less, preferably 50°C/s or less. In addition, the said average cooling rate shall refer to the average cooling rate on the steel plate surface from the start of accelerated cooling to the stop of accelerated cooling.

The method for performing the accelerated cooling is not particularly limited, and any method can be used, but water cooling is preferably used.

The treatment after the above-mentioned accelerated cooling is not particularly limited. For example, a thick steel plate after accelerated cooling can be allowed to cool in an atmosphere. In the cooling, for example, it can be cooled to room temperature. Further, after the accelerated cooling is completed, the warpage of the thick steel plate can be corrected by a hot leveler if desired.

Note that after hot rolling, the steel sheet temperature immediately decreases. Therefore, the thick steel plate of the present invention is preferably manufactured by an online process using equipment provided with a rolling device and an accelerated cooling device on a conveyance line.

Hereinafter, the functions and effects of the present invention will be explained using Examples. Note that the present invention is not limited to the following examples.

A thick steel plate was manufactured using the following procedure.

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

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

The microstructure, mechanical properties, and fatigue crack propagation characteristics of each of the obtained thick steel plates were evaluated. The evaluation method will be explained below. Table 3 shows the results of each evaluation.

(microstructure)
First, a sample for microstructure observation was taken from a 1/4 t position in the thickness direction of a thick steel plate so that the longitudinal section was the observation surface. Here, the longitudinal cross section refers to a cross section perpendicular to the width direction of the thick steel plate. Next, the surface of the sample was etched with nital, and the structure was photographed using an optical microscope (100x and 400x) and a scanning electron microscope (SEM) at 2000x. Using the captured images, the existing tissues were identified, and the images were analyzed to determine the area fraction of bainite, the area fraction of pearlite, and the total area fraction of other tissues. Note that SEM images were used to identify pearlite structures, and optical microscope images were used to measure the area fraction of each structure.

(Bainite grain size)
Furthermore, the crystal grain size of bainite was measured using the sample for microstructure observation. In the measurement, first, the surface of the sample was mirror-polished, and the crystal orientation was measured from an electron backscattered diffraction image using an electron back-scattering pattern (EBSP) device attached to a SEM. A region surrounded by 200 μm squares is measured at 0.3 μm intervals, and a region surrounded by grain boundaries where the crystal orientation difference between adjacent grains is 15° or more is defined as a crystal grain. The equivalent circle diameter was determined. The average value of the obtained equivalent circle diameters was taken as the crystal grain size of bainite.

(Number density of coarse B crystal grains)
Further, the observation surface of the sample for microstructural observation after nital corrosion was imaged using an optical microscope at a magnification of 100 times to obtain an optical microscope image. Bainite crystal grains are observed as white particles in the optical microscope image. Therefore, the optical microscope image was analyzed to calculate the number density of bainite crystal grains having an equivalent circular diameter of 100 μm or more, that is, the number per 1 mm ² .

(Crystal grain size of pearlite)
When the observed surface of the sample for microstructural observation after nital corrosion was observed with an optical microscope image at 400x magnification, a region that appeared black was observed with a SEM, and was identified as pearlite having a lamellar structure. Thereafter, using image analysis software (Image-J), the area was determined from the number of pixels in the black area in the optical microscope image, and the area was converted to the average circular equivalent diameter of pearlite. The obtained average equivalent circle diameter is regarded as the crystal grain size of pearlite.

(mechanical properties)
A full-thickness tensile test piece was taken from the width direction (C direction) of a thick steel plate. Using the full thickness tensile test piece, a tensile test was conducted in accordance with JIS Z 2241 to measure yield strength (YS), tensile strength (TS), and total elongation (EL). In addition, in the measurement, the type of test piece used was selected according to the regulations of JIS Z 2241. Specifically, first, a tensile test was conducted using a JIS No. 4 test piece, and the results showed that Example No. 1 had a tensile strength of less than 570 MPa and a final plate thickness of 50 mm or less. For No. 4 and No. 6, the tensile test was conducted again using the JIS No. 1A test piece, and the results of the tensile test using the JIS No. 1A test piece were adopted.

The aperture value (RA) by a tensile test in the plate thickness direction was evaluated in accordance with JIS G3199. For the measurement of the aperture value, a Type A test piece taken from the thick steel plate was used. At this time, the test piece was taken so that the parallel part of the test piece included the center of the thickness of the thick steel plate.

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

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

In measuring the fatigue crack propagation speed in the plate thickness direction (Z direction), a single-sided notched simple tension type fatigue test piece shown in FIG. 1 was used. The test piece was taken from a thick steel plate, and the fatigue crack propagation speed when the crack propagated in the thickness direction of the plate was measured.

The fatigue crack propagation rate in the rolling direction (L direction) was measured using a test piece taken from a thick steel plate so that the load direction was in the rolling direction. Similarly, the fatigue crack propagation speed in the width direction (C direction) was measured using a test piece taken from a thick steel plate so that the load direction was in the width direction. The test piece was a compact tension test piece based on ASTM E647.

As can be seen from the results shown in Table 3, the thick steel plate that satisfied the conditions of the present invention had extremely excellent characteristics that satisfied all of the following conditions. In particular, it had both excellent fatigue crack propagation resistance and total elongation, and was also excellent in fatigue crack propagation resistance in the thickness direction. Therefore, the thick steel plate of the present invention can be extremely suitably used as a material for structures in which structural safety is strongly required, such as ships, marine structures, bridges, buildings, and tanks. On the other hand, the thick steel plate of the comparative example that did not satisfy the conditions of the present invention did not satisfy at least one of the following conditions.
・Number of bainite crystal grains with a circular equivalent diameter of 100 μm or more per 1 mm ² : 3 or less ・TS: 500 MPa or more ・EL: 21% or more (when using a JIS No. 1A test piece),
EL: 23% or more (when using JIS No. 4 test piece)
・RA: 30% or more (JIS G3199 Type A test piece)
・vE ₀ : 100J or more ・Fatigue crack propagation speed in Z direction:
ΔK: 8.75×10 ⁻⁹ (m/cycle) or less under the condition of 15 MPa/m ^1/2 ,
ΔK: 4.25×10 ^-8 (m/cycle) or less under the condition of 25 MPa/m ^1/2

Furthermore, the thick steel plate satisfying the conditions of the present invention also satisfied the following conditions and had excellent fatigue crack propagation resistance in the rolling direction (L direction) and width direction (C direction).
・Fatigue crack propagation speed in L direction and C direction:
ΔK: 1.75×10 ⁻⁸ (m/cycle) or less under the condition of 15 MPa/m ^1/2 ,
ΔK: 8.50×10 ^-8 (m/cycle) or less under the condition of 25 MPa/m ^1/2

Claims (3)

In mass%,
C: 0.01-0.16%,
Si: 1.00% or less,
Mn: 0.50-2.00%,
P: 0.030% or less,
S: 0.020% or less,
Contains Al: 0.06% or less,
The remainder has a component composition consisting of Fe and unavoidable impurities,
In area fraction,
Contains 75-97% bainite and 3-25% perlite,
Bainite crystal grain size is 18 μm or less in average circular equivalent diameter,
Having a microstructure in which the pearlite crystal grain size is 10 μm or less in average circular equivalent diameter,
The plate thickness is 25 mm or more, and
A thick steel plate with a reduction of area in the thickness direction of 30% or more. The component composition further includes, 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: 0.005-0.200%,
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 thick steel plate according to claim 1, containing one or more selected from the group consisting of Mg: 0.0001 to 0.020%, and REM: 0.0001 to 0.020%. The method for manufacturing a thick steel plate according to claim 1 or 2,
Heating a steel material having the above-mentioned composition to a heating temperature of 1000°C or more and 1300°C or less,
The heated steel material is hot-rolled into a hot-rolled steel plate under the conditions of a rolling ratio of 3 or more and a number of passes of 2 or more in which the rolling reduction is 10% or more among the final three passes,
The hot rolled steel sheet is acceleratedly cooled under the following conditions: cooling start temperature: Ar 3 points or higher, cooling stop temperature: 300 to 650 ° C., and average cooling rate on the steel plate surface from the start of cooling to the stop of cooling: 20 to 60 ° C./s. Method for manufacturing thick steel plates.

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