CN117580968A

CN117580968A - Thick steel plate and method for manufacturing thick steel plate

Info

Publication number: CN117580968A
Application number: CN202280046480.8A
Authority: CN
Inventors: 村上俊一; 兵藤义浩; 横田智之; 木津谷茂树; 三浦进一
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2021-07-16
Filing date: 2022-06-20
Publication date: 2024-02-20
Also published as: JP7396512B2; JPWO2023286536A1; KR20240021260A; WO2023286536A1

Abstract

The invention provides a thick steel plate which has excellent fatigue crack extension resistance and total elongation and also has excellent elongation in the plate thickness direction. A thick steel plate comprises C:0.01 to 0.16 percent, si: less than 1.00%, mn:0.50 to 2.00 percent, P: less than 0.030%, S: less than 0.020%, al:0.06% or less, the remainder being composed of Fe and unavoidable impurities, and having a microstructure comprising 75 to 97% by area fraction of bainite and 3 to 25% by volume fraction of pearlite, wherein the average equivalent circle diameter of the grains of bainite is 18 μm or less, the average equivalent circle diameter of the grains of pearlite is 10 μm or less, and the reduction of area in the plate thickness direction is 30% or more.

Description

Thick steel plate and method for manufacturing thick steel plate

Technical Field

The present invention relates to a thick steel sheet, and more particularly, to a thick steel sheet having excellent fatigue crack growth resistance and total elongation, and also excellent elongation in the sheet thickness direction. The steel sheet of the present invention can be suitably used for a structure requiring high structural safety such as a ship, a marine structure, a bridge, a building, a tank, etc. The present invention also relates to a method for producing the above-mentioned thick steel plate.

Background

Thick steel plates are widely used for structures such as ships, marine structures, bridges, buildings, tanks, etc. The thick steel sheet is required to have excellent mechanical properties such as strength and toughness and excellent weldability, and also to have fatigue properties.

That is, when the above-described structure is used, the structure receives repeated loads such as vibration due to wind and earthquake. Accordingly, a fatigue property is required for a thick steel plate, which can ensure the safety of a structure even when such repeated loads are applied. In particular, in order to prevent the final damage such as the fracture of the member, the fatigue crack growth resistance of the thick steel plate is improved.

Accordingly, various studies have been made to improve fatigue crack growth resistance of steel sheets.

For example, patent document 1 proposes a steel sheet for oil-tanker which is excellent in fatigue crack growth resistance in a wet hydrogen sulfide environment. The steel sheet has a mixed structure composed of ferrite as a first phase and bainite and/or pearlite as a second phase. In the steel sheet, the average grain size of ferrite is 20 μm or less.

Patent document 2 also proposes a steel sheet excellent in fatigue crack growth resistance. The steel sheet is characterized by having a microstructure composed of hard portions and soft portions, and the hardness difference between the hard portions and the soft portions is 150 or more in terms of Vickers hardness.

Patent document 3 proposes a dual phase steel having a microstructure composed of bainite and ferrite of 38 to 52% in terms of area ratio. 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, a thick-walled steel sheet is generally produced by subjecting a large-sized steel ingot produced by an ingot casting method to cogging rolling and subjecting the obtained cogged sheet to hot rolling. However, in this ingot-cogging step, the thick segregation portion of the tap portion and the negative segregation portion of the bottom of the ingot need to be eliminated, and therefore, the yield is not increased, the manufacturing cost is increased, and the period of time is increased.

On the other hand, in the case of manufacturing a thick steel plate by a process of continuously casting a slab as a blank, the above-described problem does not occur. However, since the continuously cast slab is thinner than the slab produced by the ingot casting method, the reduction in the rolling process up to the product thickness is reduced, and there is a problem that the press center is not loose. If the crimp center is not loose, the elongation due to stretching in the plate thickness direction becomes poor. Further, since the steel plate is used for a structure or the like as described above, high strength is required. Therefore, the amount of the alloying element added to ensure the required strength increases, and as a result, there are new problems such as the occurrence of center porosity due to center segregation, and deterioration of internal quality due to enlargement.

In order to solve such a problem, in the process of manufacturing a thick steel plate from a continuously cast slab, the following technique has been proposed with a view to improving the characteristics of a center segregation portion in the steel plate by loosening the press center.

Patent document 4 proposes a technique of performing forging before hot rolling when a thick steel plate having a cumulative rolling reduction of 70% or less is produced from a continuously cast slab.

Patent document 5 proposes that, in manufacturing an extremely thick steel plate by forging and rolling a continuously cast slab, a plate thickness center portion of the slab is held at a temperature of 1200 ℃ or higher for 20 hours or more before the forging, and then a reduction ratio is performed: forging more than 16%.

Patent document 6 proposes a technique of performing hot rolling after performing cross forging on a continuously cast slab.

Patent document 7 proposes a technique of holding a continuously cast slab at a temperature of 1200 ℃ or higher for 20 hours or longer, and then forging and rolling the slab to produce an extremely thick steel plate. In the above technique, the rolling reduction of the forging and the total rolling reduction of the forging and the thick plate rolling are set to specific ranges, and quenching and tempering are performed under specific conditions after the thick plate rolling.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. H06-322477

Patent document 2: japanese patent laid-open No. 07-242992

Patent document 3: japanese patent laid-open No. 08-225882

Patent document 4: japanese patent laid-open No. 07-232201

Patent document 5: japanese patent laid-open No. 2002-194431

Patent document 6: japanese patent laid-open No. 2000-263103

Patent document 7: japanese patent laid-open No. 2006-111918

Disclosure of Invention

However, the conventional techniques described in patent documents 1 to 7 have the following problems (1) to (4).

(1) In steel materials used for structures such as ships, marine structures, bridges, buildings, and tanks, a total elongation value is generally specified in a standard. Therefore, even a steel sheet having excellent fatigue crack growth resistance is required to satisfy the standard value of the total elongation.

However, since the fatigue crack growth resistance and the total elongation are opposite properties, the conventional techniques described in patent documents 1 to 3 cannot achieve both excellent fatigue crack growth resistance and total elongation.

That is, the techniques proposed in patent documents 1 to 3 do not consider the total elongation. In practice, the steel sheets proposed in patent documents 1 to 3 each have a microstructure composed of ferrite as a soft phase and bainite or martensite as a hard phase, and the fatigue crack growth resistance is improved by enlarging the hardness difference between the soft phase and the hard phase. However, when the hardness difference between the soft phase and the hard phase is large, the structure becomes heterogeneous, and as a result, the total elongation of the steel sheet is reduced.

(2) In addition, from the viewpoint of ensuring the safety of the structure, the thick steel plate is required to have excellent fatigue crack growth resistance in the plate thickness direction.

That is, in general structures, since steel plates are welded from various directions, fatigue cracks are generated and propagate in various directions. However, in the welding position of the corner portion having the included angle, the occurrence of fatigue cracks is unavoidable due to the structural characteristics thereof, and the fatigue cracks that occur tend to develop first in the plate thickness direction. Therefore, in order to prevent collapse of the structure due to fatigue cracks, it is important to suppress the development of fatigue cracks in the plate thickness direction.

(3) In addition, it is difficult to control the manufacturing conditions of the conventional steel sheet having the microstructure. That is, in the case of manufacturing the steel sheet by an in-line process, in order to obtain a desired structure, it is necessary to start accelerated cooling from a ferrite-austenite two-phase region and to reduce the cooling stop temperature in the cooling step after hot rolling. 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 the production of the above-described conventional steel sheet, it is necessary to strictly control the cooling conditions in order to obtain a desired microstructure.

(4) The techniques described in patent documents 4 to 7 are effective for reducing center porosity and improving center segregation bands, but require a hot forging step. Further, the processing of the plate thickness center portion becomes insufficient by only ordinary hot rolling, and the center porosity remains, which may deteriorate the tensile characteristics in the plate thickness direction.

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

Specifically, it is an object to provide a steel sheet having excellent characteristics (1) to (4) below.

(1) Has excellent fatigue crack extension resistance and total elongation.

(2) The fatigue crack growth resistance is particularly important in ensuring the safety of the structure, and is excellent in the fatigue crack growth resistance in the plate thickness direction.

(3) Can be manufactured without requiring high cooling control in the two-phase region.

(4) In the production by hot rolling, the elongation by stretching in the plate thickness direction is also excellent.

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

(a) The hardness difference between the soft phase and the hard phase in the microstructure is not as large as in patent documents 1 to 3, and sufficient fatigue crack growth resistance can be obtained.

(b) By using bainite as the first phase, fatigue crack propagation resistance can be improved as compared with the conventional one.

(c) By forming a microstructure including both bainite as a soft phase and pearlite as a hard phase at a specific area fraction, and having the grains of bainite and pearlite in specific ranges, a thick steel plate having both excellent fatigue crack growth resistance and total elongation can be obtained.

(d) The thick steel plate having the above microstructure can be manufactured by controlling manufacturing conditions, particularly conditions in hot rolling and subsequent accelerated cooling. The thick steel plate is preferably manufactured by an in-line process, compared with a conventional steel plate, because the thick steel plate has bainite as the first phase.

(e) Further, in hot rolling, defects such as casting defects are reduced to be harmless by rolling at a reduction ratio of 3 or more, and at least 2 of the final 3 passes are reduced to 10% or more per 1 pass, whereby the whole steel sheet is granulated and the remaining of abnormal coarse grains is suppressed, whereby the tensile characteristics in the sheet thickness direction can be improved.

The present invention has been completed based on the above-described findings, and the gist thereof is as follows.

1. A thick steel plate has the following composition: the alloy comprises the following components in percentage by mass: c:0.01 to 0.16 percent of Si: less than 1.00%, mn:0.50 to 2.00 percent of P: less than 0.030%, S: less than 0.020%, al: the content of the catalyst is less than or equal to 0.06 percent,

The remainder is composed of Fe and unavoidable impurities,

the thick steel plate has the following microstructure:

in terms of the area fraction of the light,

comprises 75-97% of bainite and 3-25% of pearlite,

the average equivalent circle diameter of the grains of the bainite is 18 μm or less,

the average equivalent circle diameter of the pearlite grains is 10 μm or less,

the reduction of area in the thickness direction is 30% or more.

2. The steel sheet according to the above 1, wherein the above composition further comprises, in mass%, a composition selected from the group consisting of Cr:0.01 to 1.00 percent of Cu0.01 to 2.00 percent of Ni:0.01 to 2.00 percent of Mo:0.01 to 1.00 percent of Co:0.01 to 1.00 percent of 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: 0.0001-0.0050%/Zr: 0.005-0.100%, ca: 0.0001-0.020%, mg:0.0001 to 0.020% and REM:0.0001 to 0.020% of 1 or more than 2 kinds.

3. A method for producing a steel plate, comprising heating a steel blank having the composition of 1 or 2 to a heating temperature of 1000 ℃ to 1300 DEG C

The steel billet to be heated is pressed down: passes with 3 or more and a reduction of 10% or more in the final 3 passes: 2 or more, and hot-rolled into a hot-rolled steel sheet,

The hot rolled steel sheet is cooled at a cooling start temperature: ar3 point or higher, cooling stop temperature: average cooling rate on the surface of the steel sheet from the start of cooling to the stop of cooling at 300 to 650 ℃): and (3) performing accelerated cooling under the condition of 20-60 ℃/s.

Effects of the invention

The thick steel plate has excellent fatigue crack propagation resistance and total elongation, and has excellent fatigue crack propagation resistance in the plate thickness direction. Further, the reduction of area in the plate thickness direction was 30% or more, and the elongation by stretching in the plate thickness direction was excellent. In addition, the thick steel sheet of the present invention is advantageous in terms of cost because the excellent properties can be achieved without adding a large amount of alloy elements such as Cr and Sn. In addition, the thick steel plate of the present invention can be stably manufactured without requiring high cooling control in the two-phase region.

Drawings

Fig. 1 is a diagram showing the shape and size of a single-side-notched simple tensile fatigue test piece used for evaluating fatigue crack growth characteristics in the plate thickness direction.

Detailed Description

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

[ composition of ingredients ]

First, the composition of the steel sheet according to the present invention will be described. Unless otherwise specified, "%" indicating the content of each component means "% by mass".

C：0.01～0.16％

C is an element having an effect of improving strength. In addition, C has an effect of promoting the formation of pearlite phase which is advantageous in fatigue resistance. If the C content is less than 0.01%, the desired strength and fatigue crack growth 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 excessively formed or coarsened, and thus the total elongation and toughness are deteriorated. Therefore, the C content is set to 0.16% or less, preferably 0.15% or less, and more preferably 0.10% or less.

Si: less than 1.00%

Si is an element that also has deoxidizing effect and also has an effect of further improving strength. In addition, si has an effect of suppressing the generation of excessive cementite. However, if the Si content exceeds 1.00%, weldability and toughness are deteriorated, and the formation of pearlite, which is advantageous in 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, and from the viewpoint of improving the effect of adding Si, the Si content is preferably set to 0.01% or more, more preferably set to 0.10% or more.

Mn：0.50～2.00％

Mn is an element that has an effect of improving hardenability, and as a result, has an effect of improving strength of the thick steel plate. 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, when the Mn content exceeds 2.00%, the hardenability becomes excessively high, and as a result, the generation of pearlite phase which is advantageous in fatigue resistance is suppressed. In addition, if the Mn content exceeds 2.00%, the total elongation and toughness are reduced. Therefore, the Mn content is set to 2.00% or less, preferably 1.65% or less.

P: less than 0.030 percent

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

S: less than 0.020%

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

Al: less than 0.06%

Al is an element that functions as a deoxidizer and is used in the molten steel deoxidizing step. In addition, al fixes N in steel in AlN form, contributing to the improvement of toughness of the base material. However, if the Al content exceeds 0.06%, the toughness and the total elongation of the base material (thick steel plate) are reduced, and Al is mixed into the weld metal portion at the time of welding, and the toughness of the welded portion is deteriorated. Therefore, the Al content is set to 0.06% or less, preferably 0.05% or less. On the other hand, the lower limit of the Al content is not particularly limited, and from the viewpoint of improving the effect of adding Al, the Al content is preferably set to 0.01% or more.

The thick steel plate according to an embodiment of the present invention may have a composition including the above elements, and the remainder may be composed of Fe and unavoidable impurities.

The composition of the steel sheet according to the other embodiment of the present invention may further optionally contain at least one of the elements listed below. By adding these optional additives, the properties of the thick steel plate such as strength, toughness, weldability, weather resistance, and paint weather resistance can be further improved.

Cr：0.01～1.00％

Cr is an element having an effect of further improving strength. Cr is an element that promotes cementite formation, and promotes the formation of pearlite phase that is advantageous in fatigue resistance. When Cr is added, the Cr content is set to 0.01% or more, preferably 0.10% or more, in order to obtain the above-described effect. On the other hand, when the Cr content exceeds 1.00%, the weldability and toughness are impaired. Therefore, when Cr is contained, 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 by solid solution. In the case of adding Cu, the Cu content is set to 0.01% or more in order to obtain the above-described effect. On the other hand, when the Cu content exceeds 2.00%, the weldability is impaired, and defects are likely to occur in the production of thick steel plates. Therefore, in the case of 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 having an effect of improving low-temperature toughness, and Ni improves hot shortness when Cu is added. In the case of adding Ni, the Ni content is set to 0.01% or more in order to obtain the above-described effects. 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, preferably 0.70% or less, and more preferably 0.40% or less.

Mo：0.01～1.00％

Mo is an element having an effect of further improving strength. In the case of adding Mo, the Mo content is set to 0.01% or more in order to obtain the above-described effects. On the other hand, when the Mo content exceeds 1.00%, the weldability and toughness are 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 having an effect of increasing the hardness of a base phase (matrix phase). In the case of containing Co, the Co content is set to 0.01% or more, preferably 0.35% or more, in order to obtain the above-mentioned effects. On the other hand, if the Co content exceeds 1.00%, the effect is saturated and the alloy cost increases. Therefore, when Co is added, the Co content is set to 1.00% or less, preferably 0.50% or less.

Sn：0.005～0.200％

Sn is an element having an effect of increasing the hardness of the base phase. In the case of adding Sn, the Sn 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-described effects. On the other hand, when the Sn content exceeds 0.200%, the ductility and toughness of the steel are deteriorated. Therefore, when Sn is added, 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 having an effect of increasing the hardness of the base phase. In the case of 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-described effect. On the other hand, if the Sb content exceeds 0.200%, deterioration of ductility and toughness of the steel occurs. Therefore, when Sb is added, 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 having an effect of suppressing recrystallization of austenite during hot rolling and refining the finally obtained crystal grains. In addition, nb precipitates during air cooling after accelerated cooling, and the strength is further improved. In the case of adding Nb, the Nb content is set to 0.005% or more in order to obtain the above-described effects. On the other hand, if the Nb content exceeds 0.200%, hardenability becomes excessive, and the formation of martensite becomes remarkable. As a result, a desired structure cannot be obtained, and toughness is lowered. 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 having an effect of being precipitated during air cooling after accelerated cooling to further improve the strength. When V is added, the V content is set to 0.005% or more in order to obtain the above-described effect. On the other hand, when the V content exceeds 0.200%, the weldability and toughness are reduced. 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 improving the strength and toughness of the welded portion. In the case of adding Ti, the Ti content is set to 0.005% or more in order to obtain the above-described effects. On the other hand, when the Ti content exceeds 0.050%, the cost increases 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 having an effect of improving hardenability, and as a result, strength is further improved. In the case of adding B, the B content is set to 0.0001% or more in order to obtain the above-described effects. On the other hand, when the B content exceeds 0.0050%, hardenability becomes excessive, and martensite generation becomes remarkable. As a result, a desired structure is not obtained, and weldability is lowered. Accordingly, the B content is set to 0.0050% or less, preferably 0.0030% or less.

Zr：0.005～0.100％

Zr is an element having an effect of further improving strength. In order to sufficiently obtain the above effects, it is necessary to contain Zr at 0.005% or more. Therefore, when Zr is added, the Zr content is set to 0.005% or more. On the other hand, when the Zr content exceeds 0.100%, the strength-improving effect is saturated. Therefore, when Zr is contained, the Zr content is set to 0.100% or less.

Ca：0.0001～0.020％

Ca is an element having an effect of controlling the form of sulfide, and as a result, the toughness is further improved. In the case of adding Ca, the Ca content is set to 0.0001% or more in order to obtain the above-described effect. On the other hand, if 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 having an effect of further improving toughness through grain refinement. In the case of adding Mg, the Mg content is set to 0.0001% or more in order to obtain the above-described effects. On the other hand, if the Mg content exceeds 0.020%, the effect thereof 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 having an effect of further improving toughness. In the case of adding REM, the REM content is set to 0.0001% or more in order to obtain the above-described 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 described. The thick steel plate according to an embodiment of the present invention has the following microstructure: the surface area ratio of the alloy comprises 75-97% of bainite and 3-25% of pearlite, wherein the average equivalent circle diameter of the grains of the bainite is less than 18 mu m, and the average equivalent circle diameter of the grains of the pearlite is less than 10 mu m. The microstructure of the present invention means a microstructure at the 1/4 position (1/4 t position) of the plate thickness t of the thick steel plate. The area fraction and the crystal grain of each structure can be measured by subjecting a section parallel to the rolling direction at a depth of 1/4 from the surface of the thick steel plate to the corrosion with nitric alcohol, and observing the same. More specifically, the area integration rate and the crystal grain can be obtained by the method described in the examples.

Area fraction of bainite: 75 to 97 percent

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

And (3) the grains of the bainite: 18 μm or less

The grain size of bainite is set to 18 μm or less in terms of average equivalent circle diameter. By refining bainite, desired toughness and total elongation characteristics can be obtained. When the average equivalent circle diameter of the bainite exceeds 18 μm, the desired toughness cannot be obtained. On the other hand, the lower limit of the bainite crystal grains is not particularly limited, but excessive miniaturization makes production difficult, so that in actual production, it is preferable to set the bainite crystal grains to 5 μm or more.

The bainite of the present invention includes upper bainite, acicular ferrite, and granular bainite.

Area fraction of pearlite: 3 to 25 percent

In the present invention, pearlite is a second phase in the microstructure and functions as a hard phase. When fatigue cracks that propagate in bainite reach pearlite as a hard phase, the cracks stay or bend at the interface between bainite and pearlite. As a result, crack growth 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.

Grains of pearlite: less than 10 mu m

The grains of pearlite are set to 10 μm or less in terms of average equivalent circle diameter. By making pearlite fine, desired toughness and total elongation characteristics can be obtained. If the average equivalent circle diameter of the pearlite grains exceeds 10 μm, the desired toughness is not obtained. On the other hand, the lower limit of the pearlite grains is not particularly limited, and may be 1 μm or more, or may be 2 μm or more.

The pearlite of the present invention includes pearlite and pseudo pearlite.

(other organizations)

The thick steel plate according to an embodiment of the present invention may have a microstructure composed of bainite and pearlite. However, the microstructure may further optionally include other tissues. Hereinafter, the structure other than bainite and pearlite will be referred to as "other structure". The other structure may be, for example, one or both of martensite and ferrite. Here, the martensite includes island martensite, lath martensite, and lenticular martensite.

In the case where 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, if martensite is excessively present, a region of high hardness is locally formed, and the strength is increased, but the total elongation is deteriorated, and the toughness may be lowered. In addition, if ferrite is excessively present, the fatigue crack growth rate may deteriorate, and in addition, soft regions may be locally formed, and the total elongation may deteriorate due to the expansion of the hardness difference. Therefore, the lower the area fraction of the above-mentioned other tissue is, the more preferable, but if it is 5% or less, the influence can be ignored. Therefore, the total area fraction of the structure other than bainite and pearlite is preferably set to 5% or less.

In other words, the thick steel plate according to one embodiment of the present invention has a microstructure composed of 75 to 97% of bainite, 3 to 25% of pearlite, and 0 to 5% of bainite and a microstructure other than pearlite.

(plate thickness)

The thickness of the thick steel plate of the present invention is not particularly limited, and may be usually 6mm or more. However, since center porosity is likely to occur in a thick steel plate having a plate thickness of 25mm or more, the present invention is particularly suitable for application to a thick steel plate having a plate thickness of 25mm or more. Therefore, the thickness of the thick steel plate is preferably set to 25mm or more. On the other hand, the upper limit of the plate thickness is not particularly limited, but a thick steel plate having a plate thickness of 100mm or less is likely to cause fatigue damage, and therefore the present invention is particularly suitable for application to a thick steel plate having a plate thickness of 100mm or less. Therefore, the thickness of the thick steel plate is preferably set to 100mm or less, more preferably 80mm or less.

(reduced area)

The thick steel plate of the present invention is produced under the conditions described below, whereby the center thereof is loosened and pressure-bonded, and as a result, the elongation due to stretching in the plate thickness direction is excellent. Specifically, the Reduction of Area (RA) in the thickness direction of the thick steel plate of the present invention is 30% or more. The reduction of area is preferably 35% or more, more preferably 40% or more. The higher the reduction of area means that the elongation in the plate thickness direction is excellent. In the present invention, the above-mentioned reduction of area means the reduction of area in the plate thickness direction measured in accordance with JIS G3199 using a test piece having a plate thickness center portion in a parallel portion, and more specifically, the reduction of area can be measured by the method described in examples.

(tensile Strength)

The steel sheet of the present invention has the above-described composition and microstructure, and as a result, can have excellent Tensile Strength (TS). The value of TS is not particularly limited, but is preferably 500MPa or more, more preferably 530MPa or more, and still more preferably 550MPa or more. On the other hand, the upper limit of TS is not limited, but may be, for example, 720MPa or less, 700MPa or less, 640MPa or less, or 620MPa or less.

(yield stress)

The Yield Stress (YS) of the steel sheet of the invention is not particularly limited, and may be 420MPa or more, 430MPa or more, or 440MPa or more. In addition, YS may be 560MPa or less, 530MPa or less, or 520MPa or less.

(toughness)

The steel sheet of the present invention has the above-described composition and microstructure, and as a result, has excellent toughness. The toughness of the steel sheet of the present invention is not particularly limited, but it is preferable that the Charpy impact absorption energy vE at 0℃is one of the indexes of toughness ₀ It is set to 100J or more, more preferably 130J or more, still more preferably 150J or more, and most preferably 200J or more. On the other hand, for vE ₀ The upper limit of (2) is not limited, and may be, for example, 400J or less, 300J or less, or 270J or less. It should be noted that vE ₀ The measurement can be performed by the method described in examples.

(Total elongation)

The steel sheet of the present invention has the above-described composition and microstructure, and as a result, has excellent total Elongation (EL). The value of EL is not particularly limited, but is preferably 21% or more, more preferably 22% or more, further preferably 23% or more, and most preferably 26% or more. The upper limit of EL is not particularly limited, but may be 36% or less. EL can be measured by the method described in examples.

(fatigue crack growth resistance)

The steel sheet according to the present invention has the above-described composition and microstructure, and as a result, is excellent in fatigue crack growth resistance in the sheet thickness direction. As an index of fatigue crack growth resistance, a fatigue crack growth rate (da/dN) may be used. The value of the fatigue crack growth rate is not particularly limited.

It is preferable that the fatigue crack growth rate in the plate thickness direction (Z direction) satisfies the conditions (a) and (b) below.

(a) Stress magnification coefficient range Δk:15MPa/m ^1/2 The fatigue crack growth rate under the condition of (2) is 8.75X10 ^－9 The ratio of the number of the groups to the number of the groups (m/cycle) is less than or equal to,

(b) Stress magnification coefficient range Δk:25MPa/m ^1/2 The fatigue crack growth rate under the condition of (2) is 4.25X10 ^－8 (m/cycle) or below

Since the fatigue cracks first tend to develop in the plate thickness direction as described above, it is particularly important to suppress the development of the fatigue cracks particularly in the plate thickness direction (Z direction) in terms of improving the fatigue crack propagation resistance. However, after the crack progresses in the Z direction, the crack may progress further in the rolling direction (L direction) or the width direction (C direction). Therefore, from the viewpoint of further improving the fatigue crack growth resistance, it is preferable that either one of the fatigue crack growth rate in the rolling direction (L direction) and the fatigue crack growth rate in the width direction (C direction) satisfies the following conditions (C) and (d), and more preferably that both satisfy the conditions (C) and (d).

(c) Stress magnification coefficient range Δk:15MPa/m ^1/2 The fatigue crack growth rate under the condition of (2) is 1.75X10 ^－8 The ratio of the number of the groups to the number of the groups (m/cycle) is less than or equal to,

(d) Stress magnification coefficient range Δk:25MPa/m ^1/2 The fatigue crack growth rate under the condition of (2) was 8.50X10 ^－8 (m/cycle) or below

[ production conditions ]

Next, a method for manufacturing a thick steel plate according to the present invention will be described. The thick steel plate according to an embodiment of the present invention can be produced by sequentially performing the following steps (1) to (3) on a steel blank having the above-described composition.

(1) Heating

(2) Hot rolling

(3) Accelerated cooling

The conditions in each step will be described below. Unless otherwise specified, the temperature refers to the surface temperature of the object to be treated (steel slab or hot-rolled steel sheet).

(Steel blank)

Any material may be used as long as it has the above-described composition. The composition of the finally obtained steel slab was the same as that of the steel slab used. The steel blank may be, for example, one or both of a slab and a ingot. Examples of the slab include a continuous casting slab and an ingot slab.

(1) Heating

Heating temperature: 1000-1300 DEG C

First, the steel blank is heated to a heating temperature of 1000 ℃ to 1300 ℃. By the heating, the precipitates in the structure are solid-dissolved, and the crystal grains and the like are homogenized. However, when the heating temperature is lower than 1000 ℃, the precipitate is not sufficiently solid-dissolved, and thus desired characteristics cannot be obtained. Therefore, the heating temperature is set to 1000 ℃ or higher, preferably 1050 ℃ or higher, and more preferably 1100 ℃ or higher. On the other hand, if the heating temperature exceeds 1300 ℃, the material is deteriorated due to coarsening of crystal grains, and excessive energy is required, and productivity is lowered. Therefore, the heating temperature is 1300 ℃ or lower, preferably 1250 ℃ or lower.

(2) Hot rolling

Then, the heated steel blank is hot-rolled to produce a hot-rolled steel sheet. In this case, in order to produce a thick steel plate satisfying the conditions of the present invention, the reduction ratio in the hot rolling needs to satisfy the following conditions.

Reduction ratio in hot rolling: 3 or more ]

When the reduction ratio of the hot rolling is less than 3, the effect of improving the tensile characteristics in the plate thickness direction by the press-bonding with loose center cannot be obtained. When the reduction ratio of hot rolling is less than 3, the effect of promoting recrystallization by rolling and granulating by the same becomes insufficient, and coarse austenite grains remain. As a result, the properties such as strength and toughness are deteriorated. Therefore, the reduction ratio is set to 3 or more, preferably 4 or more, and more preferably 5 or more. On the other hand, the upper limit of the reduction ratio is not particularly limited, but is preferably 50 or less. This is because, in the case where the reduction ratio exceeds 50, the anisotropy of mechanical properties becomes significantly large. The reduction ratio in hot rolling is defined by "plate thickness of steel slab/plate thickness of steel sheet after rolling".

[ number of passes with a reduction of 10% or more in the final 3 passes: 2 or more ]

In the present invention, it is important to set the number of passes in the final 3-pass hot rolling to a reduction of 10% or more to 2 or more. In other words, in at least 2 of the final 3 passes of the hot rolling, the reduction is performed at a reduction ratio of 10% or more. This makes it possible to reliably render the steel blank harmless by pressure-bonding defects (casting defects, etc.), and to prevent the occurrence of abnormal coarse grain residues by granulating the entire steel sheet. Specifically, the number of bainite grains having an equivalent circle diameter of 100 μm or more can be set to be 1mm (per unit area ² ) Three or less. As a result, the reduction in area in the plate thickness direction can be made 30% or more. If the above condition is not satisfied, a reduction of area of 30% or more cannot be ensured. Here, the rolling reduction refers to the reduction in each pass.

From the above point of view, the number of passes in which the reduction ratio in the final 3 passes of the hot rolling is 10% or more is preferably set to 3. In other words, it is preferable that the reduction is performed at a reduction ratio of 10% or more in the final 3 passes of hot rolling. On the other hand, from the viewpoint of the press-bonding defect, the higher the rolling reduction, the better, and therefore the upper limit of the rolling reduction of each of the final 3 passes is not particularly limited. However, if the rolling reduction is increased, the rolling load increases, and therefore, from the viewpoint of restrictions on equipment, it is preferable to set the rolling reduction of each of the final 3 passes to 30% or less.

(3) Accelerated cooling

Subsequently, the hot-rolled steel sheet obtained in the hot-rolling step is subjected to accelerated cooling. The conditions in the above-described accelerated cooling are required as follows.

Cooling start temperature: ar3 point or more

When the cooling start temperature in the accelerated cooling is less than the Ar3 point, excessive precipitation of ferrite and coarse pearlite is caused, and strength and fatigue crack growth resistance are reduced. Therefore, the cooling start temperature is set to be equal to or higher than the Ar3 point. On the other hand, the upper limit of the cooling start temperature is not particularly limited, and is preferably set to 870 ℃ or lower from the viewpoint of securing the cumulative reduction in the temperature range of Ar3 point or higher.

Here, the Ar3 point can be obtained by the following formula.

Ar3(℃)＝910－310C－80Mn－20Cu－15Cr－55Ni－80Mo

The element symbol of the above formula represents the content (mass%) of the element in the steel blank, and is set to zero when the element is not contained in the steel blank.

The fact that the cooling start temperature is equal to or higher than the Ar3 point necessarily means that the rolling end temperature in the hot rolling is equal to or higher than the Ar3 point. If the rolling end temperature is less than Ar3 point, the dual-phase region rolling is performed, and the total elongation is deteriorated, but if the rolling end temperature is equal to or more than Ar3 point, the rolling is performed in the austenite single-phase region, so that the deterioration of the total elongation can be prevented.

Cooling stop temperature: 300-650 DEG C

In order to change the non-deformed austenite to the hard phase (pearlite), the cooling stop temperature during the accelerated cooling is set to 650 ℃ or less, preferably 600 ℃ or less. When the cooling stop temperature is higher than 650 ℃, ferrite and coarse pearlite are excessively generated, and thus the desired fatigue crack growth resistance and strength are not obtained. On the other hand, when the cooling stop temperature is less than 300 ℃, the amount of martensite generated increases, and as a result, a desired microstructure cannot be obtained, and toughness and total elongation decrease. In addition, the formation of pearlite is insufficient, and thus the desired fatigue crack growth resistance is not obtained. Therefore, the cooling stop temperature is 300 ℃ or higher, preferably 350 ℃ or higher, and more preferably more than 400 ℃.

Average cooling rate: 20-60 ℃/s

The average cooling rate in the accelerated cooling is 20 ℃/s or more. When the average cooling rate is less than 20 ℃/s, ferrite is generated and a desired microstructure cannot be formed, so that fatigue crack propagation resistance is reduced. Further, the toughness is lowered, and thus the desired total elongation is not obtained. On the other hand, when the average cooling rate exceeds 60 ℃/s, residual stress and excessive martensite due to cooling strain are generated, and deterioration of the total elongation and toughness is generated. In addition, the formation of pearlite is insufficient, and thus the desired fatigue crack growth resistance is not obtained. Therefore, the average cooling rate is 60 ℃ per second or less, preferably 50 ℃ per second or less. The average cooling rate refers to an average cooling rate of the steel sheet 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 may be used, and water cooling is preferably used.

The treatment after the completion of the accelerated cooling is not particularly limited. For example, the thick steel plate after the completion of the accelerated cooling may be naturally cooled in an atmosphere. In the above natural cooling, for example, the cooling may be performed to room temperature. After the completion of the accelerated cooling, the warp of the thick steel plate may be optionally corrected by a hot straightener.

After hot rolling, the steel sheet temperature was immediately lowered. Accordingly, the steel sheet of the present invention is preferably produced by an in-line process using equipment provided with a rolling device and an accelerated cooling device on a conveying line.

Examples

The operation and effects of the present invention will be described below with reference to examples. The present invention is not limited to the following examples.

The following procedure was followed to produce a thick steel plate.

First, billets (billets) having the composition shown in table 1 were produced by a converter-continuous casting method.

Next, the billets were heated to the heating temperatures shown in table 2, and then hot rolled at the reduction ratios shown in table 2 to produce hot rolled steel sheets. Table 2 shows the reduction ratios of the final 3 passes in the hot rolling, the rolling end temperatures, and the plate thicknesses (final plate thicknesses) of the obtained hot-rolled steel sheets. Then, the hot-rolled steel sheet was accelerated cooled under the conditions shown in table 2 to obtain a thick steel sheet. The thickness of the obtained thick steel plate is the same as the final thickness.

The microstructure, mechanical properties, and fatigue crack growth properties of the obtained thick steel plate were evaluated. The evaluation method is described below. The results of each evaluation are shown in table 3.

(microstructure)

First, a microstructure observation sample was collected from a 1/4t position in the plate thickness direction of a thick steel plate so that a longitudinal cross section became an observation surface. Here, the longitudinal cross section refers to a cross section perpendicular to the width direction of the thick steel plate. Next, after the surface of the above sample was subjected to the nitric acid ethanol etching, the tissue was photographed with an optical microscope of 100 times and 400 times and a Scanning Electron Microscope (SEM) of 2000 times. Using the captured image, the existing structure is identified, and the image is analyzed to determine the area fraction of bainite, the area fraction of pearlite, and the total area fraction of other structures. The pearlite structure was identified by using SEM images, and the area fraction of each structure was measured by using an optical microscope image.

(grains of bainite)

Further, the above microstructure observation sample was used to measure the crystal grains of bainite. In the measurement, the surface of the sample was mirror polished first, and the crystal orientation was measured from an Electron Back-scattered diffraction image using an Electron Back-Scattering Pattern (EBSP) apparatus attached to an SEM. The equivalent circle diameter of each crystal grain was determined by measuring the area surrounded by 200 μm square at 0.3 μm intervals, and defining the area surrounded by the boundary with the crystal orientation difference of 15 ° or more from the adjacent crystal grain as the crystal grain. The average value of the equivalent circle diameters obtained was used as the grains of bainite.

(number density of coarse B grains)

The observation surface of the sample for observing a microstructure was photographed at 100 times by using an optical microscope, and an optical microscope image was obtained. Bainite grains are white grains in the optical microscope imageThe child is observed. Therefore, the number density of bainite grains having an equivalent circle diameter of 100 μm or more, i.e., 1mm per unit area, was calculated by image analysis of the optical microscope image ² Is a number of (3).

(pearlitic grains)

When the observation surface of the sample for observing a microstructure after nitric acid etching was observed with an optical microscope image at 400 times, the region mapped to black was observed by SEM, and it was identified as pearlite having a lamellar structure. Then, the area was obtained from the Pixel number of the black region in the above optical microscope Image using Image analysis software (Image-J), and converted into the average equivalent circle diameter of pearlite. The average equivalent circle diameter obtained was regarded as the grain size of pearlite.

(mechanical Properties)

Total thickness tensile test pieces were collected from the plate width direction (C direction) of the thick steel plate. Using the total thickness tensile test piece, a tensile test was performed based on JIS Z2241 to measure Yield Strength (YS), tensile Strength (TS), and total Elongation (EL). In the above measurement, the type of the test piece to be used was selected in accordance with the regulation of JIS Z2241. Specifically, first, a tensile test was performed using a JIS No.4 test piece, and as a result, for example Nos. 4 and 6 having a tensile strength of less than 570MPa and a final plate thickness of 50mm or less, a tensile test was performed again using a JIS No. 1A test piece, and a tensile test using a JIS No. 1A test piece was employed.

The Reduction of Area (RA) in the plate thickness direction based on the tensile test was evaluated in accordance with JIS G3199. In the above measurement of the reduction of area, a TypeA test piece collected from the above thick steel plate was used. In this case, the test piece is collected so that the parallel portion of the test piece includes the center portion of the thick steel plate.

In addition, a Charpy impact test piece was collected from the center portion of the thickness of the thick steel plate in parallel with the rolling direction (L direction), and a Charpy impact test was performed at 0℃in accordance with JIS Z2202 to measure the absorption energy vE ₀ 。

(fatigue crack growth resistance)

As an index of fatigue crack growth resistance, respectivelyStress magnification coefficient range Δk:15MPa/m ^1/2 And 25MPa/m ^1/2 The fatigue crack growth rate (da/dN) in the plate thickness direction (Z direction), the rolling direction (L direction) and the width direction (direction perpendicular to the rolling direction, C direction) were measured under the 2 conditions of (a). In the above measurement, a fatigue crack growth test was performed based on a crack measurement method, and a fatigue crack growth rate was obtained.

For the measurement of the fatigue crack growth rate in the plate thickness direction (Z direction), a sheet side cut simple tensile type fatigue test piece shown in fig. 1 was used. The above test piece was taken from a thick steel plate, and the fatigue crack growth rate was measured as it progressed in the plate thickness direction.

The fatigue crack growth rate in the rolling direction (L direction) was measured using a test piece collected from a thick steel plate so that the load direction became the rolling direction. Similarly, the fatigue crack growth rate in the width direction (C direction) was measured using a test piece collected from a thick steel plate so that the load direction became the width direction. The test piece is a compact tensile test piece according to ASTM E647.

As is clear from the results shown in table 3, the steel sheet satisfying the conditions of the present invention has extremely excellent characteristics satisfying all of the following conditions. In particular, the steel sheet has excellent fatigue crack growth resistance and total elongation, and also excellent fatigue crack growth resistance in the sheet thickness direction. Therefore, the steel sheet of the present invention can be used as a material for ships, marine structures, bridges, buildings, tanks, and other structures where structural safety is strongly required. In contrast, the thick steel plate of the comparative example, which does not satisfy the conditions of the present invention, does not satisfy at least one of the following conditions.

Bainitic grains having an equivalent circle diameter of 100 μm or more per 1mm ² Is the number of: less than three

TS:500MPa or more

EL:21% or more (in the case of using JIS No. 1A test piece)

EL:23% or more (in the case of using JIS No. 4 test piece)

RA:30% or more (JIS G3199 TypeA test piece)

·vE ₀ :100J or more

Fatigue crack propagation speed in Z direction:

Δk: at 15MPa/m ^1/2 Under the condition of 8.75X10) ^－9 (m/cycle) or below

Δk: at 25MPa/m ^1/2 Is 4.25X10 g ^－8 (m/cycle) or below

The thick steel sheet satisfying the conditions of the present invention also satisfies the following conditions, and is excellent in fatigue crack growth resistance in the rolling direction (L direction) and the width direction (C direction).

Fatigue crack propagation speed in L and C directions:

Δk: at 15MPa/m ^1/2 Is 1.75X10 g ^－8 (m/cycle) or below

Δk: at 25MPa/m ^1/2 Is 8.50X10 g ^－8 (m/cycle) or below

Claims

1. A thick steel plate has the following composition: the alloy comprises the following components in percentage by mass: c:0.01 to 0.16 percent of Si: less than 1.00%, mn:0.50 to 2.00 percent of P: less than 0.030%, S: less than 0.020%, al: less than 0.06%, the rest is composed of Fe and unavoidable impurities,

the thick steel plate has the following microstructure:

based on area fraction, contains 75-97% of bainite and 3-25% of pearlite,

the reduction of area in the thickness direction is 30% or more.

2. The thick steel plate according to claim 1, wherein the composition of the components further comprises, in mass%, a composition selected from the group consisting of Cr:0.01 to 1.00 percent of Cu:0.01 to 2.00 percent of Ni:0.01 to 2.00 percent of Mo:0.01 to 1.00 percent of Co:0.01 to 1.00 percent of 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: 0.0001-0.0050%, zr: 0.005-0.100%, ca: 0.0001-0.020%, mg: 0.0001-0.020% and REM:0.0001 to 0.020% of 1 or more than 2 kinds.

3. A method for producing a steel plate, comprising heating a steel blank having the composition of claim 1 or 2 to a heating temperature of 1000 to 1300 ℃,

the heated billet is hot-rolled under the conditions that the reduction ratio is more than 3 and the pass of the final 3 passes with the reduction ratio of more than 10% is more than 2 to prepare a hot-rolled steel plate,