JP2024014856A - Steel plate and method for producing the same - Google Patents

Abstract

To provide a steel plate that can be used in an unpainted state even under outdoor atmospheric corrosion environments such as bridges, especially in harsh corrosion environments such as at sea or near the coast where the air carries a lot of salt, wherein the steel plate offers superior weather resistance, along with enhanced toughness, total elongation, and fatigue crack propagation resistance.SOLUTION: A steel plate has a predetermined composition. The hard structure of the steel plate comprises at least one selected from perlite, bainite and martensite. The hard structure satisfies the following formula (1) and formula (2). L(L)/L(Z)≤5.0 (1) and L(L)/L(C)≤5.0 (2), where L(L): average length of hard structure in rolling direction (L direction), L(Z): average length of hard structure in thickness direction (Z direction), and L(C): average length of hard structure in width direction (C direction).SELECTED DRAWING: None

Description

The present invention relates to a steel plate with excellent weather resistance, total elongation, and fatigue crack propagation resistance, and a method for manufacturing the same.
The steel plate of the present invention can be suitably used for welded structures such as ships, marine structures, bridges, buildings, tanks, etc., which are used in outdoor atmospheric corrosive environments and where structural safety is strongly required. In particular, the steel plate of the present invention can be suitably used for structures such as bridges that are used in severe corrosive environments such as at sea or near the coast where there is a large amount of airborne salt.

Steel structures used outdoors, such as bridges, are usually treated with some type of anti-corrosion treatment.
For example, weathering steel is often used in environments where the amount of airborne salt is low. When weathering steel is used in an environment exposed to the atmosphere, its surface is covered with a highly protective rust layer enriched with alloying elements such as Cu, P, Cr, and Ni, which greatly reduces the corrosion rate. It is made of steel. Bridges made of such weather-resistant steel are known to be able to withstand decades of service without painting in environments with low amounts of airborne salt.

On the other hand, it is known that in a high salt environment, it is difficult for weathering steel to form a highly protective rust layer, making it difficult to obtain practical weather resistance. For this reason, in environments where there is a large amount of airborne salt, such as at sea or near the coast, steel materials that are made of ordinary steel that has been subjected to anti-corrosion treatment such as painting are generally used.

However, painted steel materials require periodic repairs such as repainting due to deterioration of the paint film, generation of rust, and swelling of the paint film over time. Painting work associated with repainting often requires work at high places, which not only makes the work itself difficult but also increases labor costs. Therefore, when painted steel materials are used, the maintenance cost of the structure increases due to the repainting work, which in turn increases the life cycle cost. Therefore, there is a need for steel materials that can be used unpainted even in environments where there is a large amount of airborne salt, such as near the coast.

In response to such demands, steel materials containing various alloying elements, particularly Ni and Cu, have been developed as steel materials that can be used unpainted in environments with a large amount of airborne salt, such as near the coast.

As a steel material with excellent corrosion resistance, for example, Patent Document 1 describes a steel material containing 0.0003 to 0.0050% of B, furthermore 0.1 to 1.5% of Cu, and 0.1 to 6.0% of Ni. A high coastal weathering steel material with excellent earthquake resistance is disclosed, which contains one or more of 0.005 to 0.500% of Mo.

Patent Document 2 discloses that B is contained in an amount of 0.0003 to 0.0050%, Cu is contained in an amount of 0.1 to 2.0%, Ni is contained in an amount of 0.1 to 6.0%, and Mo is contained in an amount of 0.005 to 1.000%. A high coast weather resistant steel material with excellent earthquake resistance is disclosed, which contains one or more of the following.

These steel materials with improved weather resistance are widely used in structures such as ships, marine structures, bridges, buildings, and tanks.
Further, the steel material 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-described structure is used, repeated loads are applied to the structure, such as vibrations caused by wind, waves, and earthquakes. Therefore, 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 required to improve the fatigue crack propagation resistance of steel plates.

That is, various studies are being conducted to improve the fatigue crack propagation resistance of steel plates.
For example, Patent Document 3 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 one or two of ferrite, bainite, and pearlite. Further, in the steel sheet, the average grain size of ferrite is 20 μm or less.

Patent Document 4 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 a difference in hardness between the hard part and the soft part is 150 or more on Vickers hardness.

Patent Document 5 proposes a dual phase steel having a microstructure consisting of bainite and ferrite with an area ratio of 38 to 52%. In the technology proposed in Patent Document 5, fatigue crack propagation resistance is improved by controlling the Vickers hardness of the ferrite phase portion and the number of boundaries between the ferrite phase and the bainite phase that exist per unit length. Improving.

Japanese Patent Application Publication No. 2000-355731 Japanese Patent Application Publication No. 2000-355732 Japanese Patent Application Publication No. 06-322477 Japanese Patent Application Publication No. 07-242992 Japanese Patent Application Publication No. 08-225882

However, although Patent Documents 1 and 2 are both conventional technologies that refer to weather resistance, they do not address the issue of achieving both weather resistance and toughness and fatigue crack propagation resistance, which are important characteristics for steel plates for structures. , was not considered at all. In addition, the microstructure of a steel sheet is important for improving toughness and fatigue crack propagation resistance, and manufacturing methods for controlling the microstructure have not been considered.

Further, 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.

Here, since fatigue crack propagation resistance and total elongation are contradictory properties, conventional techniques such as those described in Patent Documents 1 to 5 do not have excellent fatigue crack propagation resistance and total elongation. I couldn't make it compatible.

In addition, from the perspective of ensuring the safety of structures, steel plates are required to have excellent fatigue crack propagation resistance not only in one direction but also in the thickness direction, rolling direction, and width direction. .

That is, in general structures, welding is freely performed on steel plates from various directions, so fatigue cracks occur and propagate in various directions. In addition, at welding locations that have included angle corners, fatigue cracks are unavoidable due to their structural characteristics, and the fatigue cracks that do occur tend to first propagate in the thickness direction. Therefore, in order to prevent the collapse of structures due to fatigue cracks, it is necessary to suppress the propagation of fatigue cracks in the width direction and rolling direction even after the fatigue cracks have penetrated the steel plate in the thickness direction. is important.

However, in the conventional techniques described in Patent Documents 1 to 5, the directional dependence of the fatigue crack propagation resistance is not taken into consideration. In addition, the steel materials proposed in Patent Documents 3 to 5 do not have sufficient weather resistance in environments with a large amount of airborne salt, such as near the coast.

The present invention has been developed in view of the above circumstances, and even when used in outdoor atmospheric corrosive environments such as bridges, especially in severe corrosive environments such as at sea or near the coast where there is a large amount of airborne salt, the present invention can be used without painting. The purpose of the present invention is to provide a steel plate that has excellent weather resistance, toughness, total elongation, and fatigue crack propagation resistance, and can be used in.

That is, the gist of the present invention is as follows.
1. The component composition is mass%, C: 0.01% or more and 0.20% or less, Si: 0.05% or more and 1.00% or less, Mn: 0.10% or more and 2.00% or less, P: 0.003% or more, 0.035% or less, S: 0.0001% or more, 0.0350% or less, Al: 0.001% or more, 0.100% or less, and Ni: 0.80% or more, 6.00% or less, and further contains one or two selected from Cu: 1.00% or less and Mo: 1.00% or less, with the remainder being Fe and inevitable impurities. , the microstructure consists of ferrite with an area fraction of 55% or more and a hard structure harder than ferrite with an area fraction of 45% or less, and the hard structure is one selected from pearlite, bainite, and martensite, or A steel plate containing two or more kinds, the hard structure of which satisfies the following formulas (1) and (2).
L(L)/L(Z)≦5.0 (1)
L(L)/L(C)≦5.0 (2)
L (L): Average length of the hard structure in the rolling direction (L direction) L (Z): Average length of the hard structure in the plate thickness direction (Z direction) L (C): Average length of the hard structure in the plate width direction (C average length in direction)

2. The component composition further includes, in mass %, Cr: 1.000% or less, W: 1.00% or less, Co: 1.000% or less, Sn: 0.300% or less, Sb: 0.300% or less. , Nb: 0.100% or less, V: 0.150% or less, Ti: 0.100% or less, B: 0.0050% or less, Zr: 0.1000% or less, Ca: 0.0100% or less, Mg The steel plate according to 1 above, containing one or more selected from: 0.0100% or less and REM: 0.0200% or less.

3. A method for manufacturing a steel plate according to 1 or 2 above, wherein the slab having the component composition according to 1 or 2 above is heated to a temperature range of 1000° C. or higher and 1300° C. or lower, and then Ar ₃ below the slab heating temperature is heated. A method for producing a steel sheet, comprising hot rolling with a cumulative reduction rate of 50% or more in a temperature range above the transformation point to obtain a hot-rolled sheet, and cooling the hot-rolled sheet at an average cooling rate of 0.10° C./s or more. .

4. After the cooling, it is further heated to a reheating temperature of Ac ₁ transformation point or more and less than Ac ₃ transformation point, and after such heating, the heating temperature is 350 to 600°C at an average cooling rate in the range of 2.00 to 7.00°C/s. 3. The method for manufacturing a steel sheet according to 3 above, wherein the steel sheet is cooled to a cooling stop temperature between 1 and 2, and further quenched.

According to the present invention, antifreeze can be used in welded structures used at low temperatures such as in cold regions, especially in outdoor atmospheric corrosive environments such as bridges, and also in areas near the sea or coast where there is a large amount of airborne salt. It is possible to provide a steel plate that can be used without painting even when used in a severe corrosive environment where it is sprayed with.
Further, according to the present invention, it is possible to reduce the maintenance cost of the steel structure of the structure, and by extension, the life cycle cost.
Furthermore, according to the present invention, the steel has excellent fatigue crack propagation resistance and total elongation, and has excellent fatigue crack propagation resistance in all directions, including the plate thickness direction, rolling direction, and width direction. It becomes possible to ensure the safety of the structure.

Hereinafter, the composition and microstructure of the steel sheet, the characteristics of the steel sheet, and the manufacturing method in the present invention will be explained in order. Note that the present invention is not limited to the following embodiments.
[Component composition]
First, the composition of the steel sheet of the present invention will be explained. In the description of the component composition, % indicating the content of each component means mass %.
C: 0.01% or more, 0.20% or less C is an element that increases the strength of steel materials, and must be contained in an amount of 0.01% or more in order to ensure a predetermined strength as a structural steel. On the other hand, when the C content exceeds 0.20%, weldability, total elongation, and toughness deteriorate. Therefore, the C content is set to 0.20% or less. The C content is preferably 0.10% or less. Further, it is more preferably 0.08% or less.

Si: 0.05% or more and 1.00% or less Si needs to be contained in an amount of 0.05% or more in order to ensure deoxidation and the strength of the steel plate. Preferably it is 0.10% or more. On the other hand, if the Si content exceeds 1.00%, toughness and weldability will significantly deteriorate. Therefore, the Si content is set to 1.00% or less. The Si content is preferably 0.80% or less.

Mn: 0.10% or more and 2.00% or less Mn is an element that increases the strength of steel by improving its hardenability. That is, Mn needs to be contained in an amount of 0.10% or more in order to ensure a predetermined strength as a structural steel. Preferably it is 0.20% or more. On the other hand, when the Mn content exceeds 2.00%, total elongation, toughness and weldability deteriorate. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably 1.80% or less.

P: 0.001% or more and 0.035% or less P is an element that contributes to improving the weather resistance of steel materials. From the viewpoint of obtaining such an effect, P must be contained in an amount of 0.001% or more. On the other hand, when the P content exceeds 0.035%, weldability and toughness deteriorate. Therefore, the P content is set to 0.035% or less.

S: 0.0001% or more and 0.0350% or less S is an element that deteriorates weldability and toughness. Therefore, the S content needs to be 0.0350% or less. On the other hand, if an attempt is made to reduce the S content to less than 0.0001%, production costs will increase. Therefore, the S content is set to 0.0001% or more.

Al: 0.001% or more and 0.100% or less Al is an element necessary for deoxidation during steel manufacturing. In order to obtain such an effect, Al needs to be contained in an amount of 0.001% or more. The Al content is preferably 0.005% or more, more preferably 0.010% or more. On the other hand, if the Al content exceeds 0.100%, it will adversely affect the total elongation and weldability. Therefore, the Al content is set to 0.100% or less. The Al content is preferably less than 0.080%, more preferably less than 0.060%.

Ni: 0.80% or more, 6.00% or less Ni forms a dense rust layer by refining the rust grains in the rust layer, and prevents oxygen and chloride ions, which are corrosion accelerating factors, from entering the steel base. It has the effect of suppressing permeation. Such effects are obtained when the Ni content is 0.80% or more. Therefore, the Ni content is set to 0.80% or more. The Ni content is preferably 0.90% or more, more preferably 1.00% or more. On the other hand, if the Ni content exceeds 6.00%, weldability will be impaired and the alloy cost will increase excessively. Therefore, the Ni content is set to 6.00% or less. The Ni content is preferably 5.00% or less, more preferably 4.00% or less.

In addition to the above elements, the steel sheet of the present invention must further contain an element selected from Cu: 1.00% or less and Mo: 1.00% or less, either singly or in combination.

Cu: 1.00% or less Cu forms a dense rust layer by making the rust grains in the rust layer finer, and has the effect of suppressing the permeation of oxygen and chloride ions, which are corrosion-promoting factors, into the steel base. have In order to obtain such an effect, when Cu is contained alone, it is desirable to contain it in an amount of 0.05% or more. The Cu content is preferably 0.15% or more, more preferably 0.20% or more. On the other hand, when the Cu content exceeds 1.00%, weldability is impaired and flaws are likely to occur during manufacturing of the steel plate. Therefore, if Cu is contained, the content should be 1.00% or less. The Cu content is preferably 0.80% or less, more preferably 0.60% or less.

Mo: 1.00% or less Mo is eluted with the anode reaction of steel materials, and when MoO ₄ ^2- is distributed in the rust layer, chloride ions, which are corrosion accelerating factors, permeate through the rust layer. Prevent it from reaching the iron. In addition, a compound containing Mo precipitates on the surface of the steel material, thereby suppressing the anodic reaction of the steel material. In order to obtain such an effect, when Mo is contained alone, it is desirable to contain it in an amount of 0.05% or more. On the other hand, if the Mo content exceeds 1.00%, weldability will be impaired and the alloy cost will increase. Therefore, when Mo is contained, the content should be 1.00% or less. The Mo content is preferably 0.80% or less, more preferably 0.50% or less.

In addition, when containing both Cu and Mo, it is desirable that the total content of Cu and Mo is 0.05% or more. On the other hand, when Cu and Mo are contained together, the upper limit of the total is allowed to be 1.00% for both Cu and Mo.

A steel plate in an embodiment of the present invention has a composition containing the above elements, with the balance consisting of Fe and unavoidable impurities.
Moreover, the chemical composition of the steel plate in another embodiment of the present invention can further optionally contain at least one of the elements listed below. By containing these arbitrary elements, properties such as strength, toughness, weldability, and weather resistance of the steel sheet can be further improved.

Cr: 1.000% or less Cr is an element that has the effect of further improving the strength of the steel plate. Further, Cr has the effect of forming a dense rust layer to further improve weather resistance. In order to obtain these effects, when Cr is contained, it is preferable that the Cr content is 0.010% or more. On the other hand, if the Cr content exceeds 1.000%, weldability and toughness will be impaired, and weather resistance will also be adversely affected. Therefore, when containing Cr, the Cr content is set to 1.000% or less. The Cr content is preferably 0.700% or less, more preferably 0.500% or less.

W: 1.00% or less W is an element that improves the weather resistance of steel materials. W elutes with the anode reaction and is distributed as WO ₄ ^2- in the rust layer, thereby electrostatically preventing chloride ions, which are corrosion promoters, from penetrating the rust layer and reaching the steel base. To prevent. Furthermore, a compound containing W is precipitated on the surface of the steel material, thereby suppressing the anodic reaction of the steel material. In addition, by forming fine rust and making the rust layer denser, chloride ions, which are corrosion factors, are prevented from penetrating the rust layer and reaching the steel base. In order to fully obtain these effects, it is preferable to contain W in an amount of 0.01% or more. More preferably, it is 0.03% or more. On the other hand, if the W content exceeds 1.00%, a significant increase in alloy cost will result. Therefore, when containing W, the W content is set to 1.00% or less. The W content is preferably 0.70% or less, more preferably 0.50% or less.

Co: 1.000% or less Co is distributed throughout the rust layer and has the effect of improving weather resistance by forming a dense rust layer. In order to obtain this effect, the Co content is preferably 0.001% or more, more preferably 0.002% or more, and still more preferably 0.010% or more. On the other hand, even if the Co content is made higher than 1.000%, the effect is saturated and the alloy cost increases. Therefore, when containing Co, the Co content should be 1.000% or less. Co content is preferably 0.500% or less.

Sn: 0.300% or less Sn is an element that improves the weather resistance of steel materials. Further, Sn exists in the rust layer near the surface of the steel base, and by making rust particles finer, it prevents chloride ions, which are corrosion accelerating factors, from penetrating the rust layer and reaching the steel base. Furthermore, Sn suppresses the anodic reaction on the surface of the steel material. In order to sufficiently obtain this effect, it is preferable to contain Sn in an amount of 0.005% or more, more preferably 0.010% or more, still more preferably 0.020% or more. On the other hand, if the Sn content exceeds 0.300%, the ductility and toughness of the steel will deteriorate. Therefore, when Sn is contained, the Sn content is set to 0.300% or less. The Sn content is preferably 0.100% or less, more preferably 0.050% or less.

Sb: 0.300% or less Sb exists in the rust layer near the surface of the steel base, and by making the rust particles finer, chloride ions, which are corrosion accelerators, permeate through the rust layer and reach the steel base. to prevent Moreover, Sb suppresses the anodic reaction on the surface of the steel material. In order to sufficiently obtain this effect, it is preferable to contain Sb in an amount of 0.005% or more, more preferably 0.010% or more, and still more preferably 0.020% or more. On the other hand, when the Sb content exceeds 0.300%, the ductility and toughness of the steel deteriorate. Therefore, when Sb is contained, the Sb content is set to 0.300% or less. The Sb content is preferably 0.150% or less, more preferably 0.100% or less.

Nb: 0.100% or less 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 and further improves the strength. In order to obtain such an effect, when Nb is contained, it is preferable that the Nb content is 0.005% or more. On the other hand, if the Nb content exceeds 0.100%, the hardenability becomes excessive and martensite is generated, making it impossible to obtain the desired structure, resulting in a decrease in toughness. Therefore, when Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.050% or less.

V: 0.150% or less V precipitates during air cooling and further improves strength. In addition, the presence of VO ₄ ³⁻ in the rust layer near the surface of the steel base prevents chloride ions, which are corrosion-promoting factors, from permeating the rust layer and reaching the steel base. In order to sufficiently obtain such effects, it is preferable to contain V in an amount of 0.005% or more. On the other hand, when the V content exceeds 0.150%, the effect is saturated. Therefore, when V is contained, the V content is set to 0.150% or less.

Ti: 0.100% or less Ti is an element that increases strength. In order to sufficiently obtain such effects, it is preferable to contain Ti in an amount of 0.005% or more. On the other hand, if the Ti content exceeds 0.100%, the toughness will deteriorate. Therefore, when Ti is contained, the Ti content is set to 0.100% or less.

B: 0.0050% or less B is an element that has the effect of increasing hardenability and, as a result, further improving strength. In order to obtain such an effect, when B is contained, it is preferable that the B content is 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, the hardenability becomes excessive and martensite is generated, making it impossible to obtain the desired structure, and weldability deteriorates. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0030% or less.

Zr: 0.1000% or less Zr is an element that increases strength. In order to sufficiently obtain such effects, it is preferable to contain Zr in an amount of 0.0050% or more. On the other hand, when the Zr content exceeds 0.1000%, the strength improvement effect is saturated. Therefore, when containing Zr, the Zr content should be 0.1000% or less.

Ca: 0.0100% or less Ca is an element that fixes S in steel and improves the toughness of the weld heat affected zone. In order to sufficiently obtain such effects, it is preferable to contain Ca in an amount of 0.0001% or more. On the other hand, if the Ca content exceeds 0.0100%, the amount of inclusions in the steel will increase, leading to a deterioration in toughness. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less.

Mg: 0.0100% or less Mg is an element that fixes S in steel and improves the toughness of the weld heat affected zone. In order to sufficiently obtain such effects, it is preferable to contain Mg in an amount of 0.0001% or more. On the other hand, if the Mg content exceeds 0.0100%, the amount of inclusions in the steel will increase, leading to a deterioration in toughness. Therefore, when Mg is contained, the Mg content is set to 0.0100% or less.

REM: 0.0200% or less REM (rare earth metal) is an element that fixes S in steel and improves the toughness of the weld heat affected zone. In order to sufficiently obtain such effects, it is preferable to contain REM in an amount of 0.0001% or more. On the other hand, if the REM content exceeds 0.0200%, the amount of inclusions in the steel will increase, leading to a deterioration in toughness. Therefore, when containing REM, the REM content should be 0.0200% or less.

[Microstructure]
Next, the microstructure of the steel plate of the present invention will be explained.
The steel plate in one embodiment of the present invention consists of ferrite with an area fraction of 55% or more and a hard structure of 45% or less.
The hard structure in the present invention means a structure containing one or more selected from pearlite, bainite, and martensite, and is harder than ferrite. Furthermore, the hard tissue needs to satisfy the following formulas (1) and (2). These equations reflect the lumpiness of hard tissue.
L(L)/L(Z)≦5.0 (1)
L(L)/L(C)≦5.0 (2)
L (L): Average length of the hard structure in the rolling direction (L direction) L (Z): Average length of the hard structure in the plate thickness direction (Z direction) L (C): Average length of the hard structure in the plate width direction (C direction)

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 the steel plate. This is because the effects of the present invention can be obtained by defining the microstructure at such locations.
The area fraction of each structure and the length of the hard structure in each direction can be measured by taking a test piece at a depth of 1/4t from the surface of the steel plate, corroding each cross section with nital, and observing the sample. More specifically, the area fraction and the length of the hard tissue in each direction can be determined by the method described in the Examples.

Area fraction of ferrite: 55% or more, area fraction of hard structure: 45% or less The microstructure of the steel sheet in the present invention is a composite structure in which hard structure is dispersed in a ferrite phase. If a hard structure exists at the tip of a fatigue crack, bending or branching of the fatigue crack will occur, resulting in fracture surface roughness-induced crack closure and stress shielding effects, reducing the driving force for fatigue crack propagation. Fatigue crack propagation characteristics are improved.
Furthermore, since ferrite is effective in improving total elongation, if the area fraction of ferrite is less than 55% (that is, the area fraction of hard structure exceeds 45%), it is difficult to obtain the desired total elongation. Can not. Therefore, the area fraction of ferrite is 55% or more (that is, the area fraction of hard tissue is 45% or less). The area fraction of ferrite is preferably 60% or more (that is, the area fraction of hard tissue is 40% or less). On the other hand, the upper limit of the area fraction of ferrite is not particularly limited, but it is preferably 97% or less (that is, the area fraction of the hard structure is 3% or more).

Note that the organization in the present invention is as follows.
Ferrite includes polygonal ferrite, pearlite includes pearlite and pseudo pearlite, bainite includes upper bainite, acicular ferrite, and granular bainite, and martensite includes island martensite, lath marten, site, and lenticular martensite.

L(L)/L(Z)≦5.0 (1)
L(L)/L(C)≦5.0 (2)
In the present invention, the average length (L (L)) of the hard structure in the rolling direction (L direction), the average length (L (Z)) of the hard structure in the plate thickness direction (Z direction), and the average length (L (Z)) of the hard structure in the rolling direction (L direction), The relationship between the average length (L(C)) and the plate width direction (C direction) is defined by the above equations (1) and (2).
Note that the above equations (1) and (2) reflect the lumpiness of the hard tissue, and if the above equations (1) and (2) are not satisfied, the hard tissue becomes band-like and fatigue There is a direction in which the frequency of contact between the tip of the fissure and the hard tissue decreases.
Therefore, desired fatigue crack propagation resistance cannot be obtained in all directions. That is, in the present invention, desired fatigue crack propagation resistance can be obtained in all directions of the steel plate by satisfying both the above equations (1) and (2).

In the present invention, the total elongation, tensile strength, toughness, and fatigue crack propagation properties are as described below.
[Total elongation]
The steel sheet of the present invention has excellent total elongation (EL) as a result of having the above-described composition and microstructure. The value of EL is not particularly limited and can follow standards such as JIS G 3106, but it is preferably 15% or more, more preferably 16% or more, even more preferably 17% or more, and 20% or more. % or more is most preferable. On the other hand, the upper limit of EL is also not particularly limited, but is preferably 40% or less, for example. Note that EL can be measured by the method described in Examples described later.

[Tensile strength]
The steel sheet of the present invention has the above-described composition and microstructure, and as a result, it can have excellent tensile strength (TS). Although the value of TS is not particularly limited, it is preferably 400 MPa or more. On the other hand, the upper limit of TS is not limited either, but for example, when the steel plate is 400 MPa (50 kgf/mm ² ) class according to JIS, it is sufficient that TS is 510 MPa or less. Further, when the steel plate is 490 MPa (60 kgf/mm ² ) class according to JIS, the upper and lower limits of TS may be 490 MPa and 610 MPa, respectively.

[Toughness]
The steel plate of the present invention has excellent toughness as a result of having the above-described composition and microstructure. The toughness of the 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 150 J or more, and 200 J or more. It is even more preferable that On the other hand, the upper limit of vE ₀ is not particularly limited because the higher the value, the better. Note that vE ₀ can be measured by the method described in Examples described later.

[Fatigue crack propagation resistance]
As a result of having the above-described composition and microstructure, the steel plate of the present invention can have excellent fatigue crack propagation resistance in all directions, including the plate thickness direction, rolling direction, and width direction.
Fatigue crack propagation velocity (da/dN) can be used as an index of fatigue crack propagation resistance in the present invention. Further, the value of the fatigue crack propagation speed is not particularly limited.
In addition, the fatigue crack propagation speed in the plate thickness direction (Z direction) under the condition of stress intensity factor range ΔK: 25 MPa・m ^1/2 has sufficient fatigue durability compared to conventional steel. It is preferable that the value is 4.25×10 ⁻⁸ (m/cycle) or less, which can be confirmed to be good for good performance. In addition, for both the fatigue crack propagation speed in the rolling direction (L direction) and the fatigue crack propagation speed in the width direction (C direction), fatigue cracks under the condition of stress intensity factor range ΔK: 25 MPa・m ^1/2 It is preferable that the propagation velocity is 8.50×10 ⁻⁸ (m/cycle) or less, which confirms that the fatigue durability is sufficiently good compared to conventional steel.

[Plate thickness]
In the present invention, the thickness of the steel plate is not particularly limited and can be set to any value.
In the present invention, the thickness is preferably 6 mm or more, which is commonly used as a structural member. Furthermore, as described above, the effects of the present invention are particularly noticeable in steel plates where the temperature deviation at the leading and trailing ends of the steel plate tends to be large and where excellent elongation characteristics are required throughout the thickness. Therefore, the thickness of the steel plate is preferably 50 mm or less, more preferably 25 mm or less.

[Production method]
Next, a method for manufacturing a steel plate according to the present invention will be explained. The steel plate in one embodiment of the present invention can be obtained by subjecting a steel material having the above-mentioned composition to the following steps (1), (2), and (3), and if necessary, (4) ) (5) and (6), it is possible to produce a steel plate with even better fatigue crack propagation resistance and total elongation.
(1) Heating (2) Hot rolling (3) Cooling (4) Reheating (5) Cooling (6) Quenching

The conditions in each step will be explained below. In the present invention, unless otherwise specified, temperature refers to the surface temperature of the object to be treated (steel material or hot rolled steel plate). Further, the cooling rate is the cooling rate of the surface temperature. Note that the above-mentioned surface temperature can be measured using, for example, a radiation thermometer.

Any steel material can be used as the steel material as long as it has the above-mentioned composition. The composition of the steel plate finally obtained is the same as that of the steel material used. As the steel material, for example, a steel slab can be used.
(1) Heating process Heating temperature: 1000°C or higher and 1300°C or lower First, the above steel material is heated to a temperature range of 1000°C or higher and 1300°C or lower. If the heating temperature is less than 1000°C, the deformation resistance of the steel material in the next hot rolling step will be high, the load on the hot rolling mill will increase, and hot rolling will become difficult. On the other hand, if the heating temperature exceeds 1300° C., the grain size of the steel sheet structure becomes too large and the toughness deteriorates. When holding in the heating step, the holding time is preferably 1 hour or more.

(2) Hot Rolling Next, the heated steel material is hot rolled to form a hot rolled sheet. At this time, in order to ensure toughness, which is the basic performance of the product steel sheet, the cumulative reduction rate in the temperature range below the slab heating temperature and above the Ar ₃ transformation point is set to 50% or more. If the cumulative rolling reduction is less than 50%, the ferrite grains within the thickness of the sheet become coarse and locally low ductility regions occur, making brittle cracks more likely to occur and the toughness to deteriorate. Other conditions regarding the hot rolling process are not particularly limited, and may be based on known conditions.
In addition, the rolling reduction ratio in the recrystallization temperature range is preferably 30% or more, more preferably 45% or more, so that grain refinement by recrystallization can be effectively performed. Further, by setting the reduction ratio in the non-recrystallization temperature range to preferably 60% or less, more preferably less than 30%, elongation of crystal grains can be effectively prevented.
The recrystallization temperature is determined by determining the softening curve of the steel material in a two-stage compression test, and the temperature at which the softening degree is 50% is determined as the recrystallization temperature.
Further, the Ar ₃ transformation point can be determined, for example, by the following equation (3).
Ar ₃ (°C) = 910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo+0.35...(3)
Here, the element symbol in the above formula (3) means the content (mass %) of each element in the steel, and is zero if the element is not contained.

(3) Cooling Next, cooling is performed, that is, cooling the hot rolled sheet after hot rolling (first cooling step). Such cooling can be performed by any method, such as air cooling or accelerated cooling. The average cooling rate is 0.10° C./s or more. If the average cooling rate is less than 0.10° C./s, the shape of the hard structure becomes band-like, and desired fatigue crack propagation resistance cannot be obtained.
Note that other cooling conditions are not particularly limited, and may be any known cooling conditions.
In addition, there is no problem with the dispersibility of the hard phase in the microstructure of the steel manufactured by the manufacturing steps (1), (2), and (3) above.

Next, by performing the following steps (4), (5), and (6), fatigue crack propagation resistance and total elongation can be further improved.
(4) Reheating The steel plate that has been cooled as described in (3) above is heated to a temperature (reheating temperature) that is higher than or equal to the Ac ₁ transformation point and lower than the Ac ₃ transformation point (hereinafter referred to as reheating treatment). That is, by heating to a reheating temperature that results in a two-phase region of ferrite and austenite, variations in the microstructure caused by cooling deviation can be eliminated without damaging the structure before heating. As a result, fatigue crack propagation resistance can be further improved in all directions: rolling direction, sheet width direction, and sheet thickness direction.
If the reheating temperature is above the Ac ₃ transformation point, the decarburization reaction specific to the temperature range above the Ac ₁ transformation point and below the Ac ₃ transformation point will not proceed, making it impossible to further improve the fatigue crack propagation resistance. . On the other hand, if the reheating temperature is less than the Ac ₁ transformation point, it is not possible to eliminate variations in the microstructure caused by cooling deviations, and the fatigue crack propagation resistance cannot be further improved.

Note that the Ac ₁ transformation point can be determined by, for example, the following equation (4).
Ac ₁ (℃) = 723 + 29.1 x Si - 10.7 x Mn - 16.9 x Ni + 16.9 x Cr... (4)
Further, the Ac ₃ transformation point can be determined by, for example, the following equation (5).
Ac ₃ (℃)=961.6-311.9×C+49.5×Si-36.4×Mn+438.1×P-2818×S+12.7×Al-51×Cu-29×Ni-8.7× Cr+13.5×Mo+308.1×Nb-140×V+318.9×Ti+611.2×B-969×N…(5)
Here, the element symbol in the above formulas (4) and (5) means the content (mass %) of each element in the steel, and is zero if the element is not contained.

In the reheating treatment, it is preferable to heat to the reheating temperature and then maintain the temperature. At this time, if the holding time is less than 10 minutes, the reverse transformation to the austenite phase will not start over the entire length of the steel sheet, and the hardenability may be significantly reduced in some regions. Therefore, the holding time is preferably 10 minutes or more.

(5) Cooling The steel plate heated in the reheating process is cooled to a cooling stop temperature arbitrarily set in the range of 350 to 600°C (second cooling process). At this time, the average cooling rate is set to 2 to 7°C/s. A lower average cooling rate is preferable in terms of improving toughness because it promotes pearlite transformation. However, if the average cooling rate is less than 2°C/s, pearlite tends to form in a band shape and forms along the band structure. Cracks propagate more easily. Therefore, the fatigue crack propagation resistance cannot be further improved. On the other hand, when the average cooling rate exceeds 7° C./s, pearlite transformation does not progress sufficiently in the microstructure inside the steel sheet, and bainite transformation and martensitic transformation tend to progress. In this case, the total elongation deteriorates because the hard tissue increases. For this reason, the average cooling rate is set to 2 to 7°C/s. The average cooling rate is more preferably 5° C./s or less.

Further, if the cooling stop temperature is less than 350° C., pearlite tends to form in a band shape, and crack propagation along the band structure tends to occur, making it impossible to further improve fatigue crack propagation characteristics. On the other hand, if the cooling stop temperature exceeds 600° C., hard bainite and martensite will be produced excessively since the steel will be quenched with a large amount of untransformed austenite remaining. As a result, the total elongation cannot be further improved.

(6) Hardening In the present invention, the steel plate cooled to the cooling stop temperature can be hardened. The quenching temperature at that time is preferably in the range of 350 to 600°C. The other conditions for quenching are not particularly limited and can be carried out under any known conditions, but it is preferable to water-cool to a temperature below the Ms point, preferably below 200°C. Note that the Ms point can be determined by, for example, the following equation (6).
Ms(℃)=517-300×C-11×Si-33×Mn-17×Ni-22×Cr-11×Mo…(6)
Here, the element symbol in the above formula (6) means the content (mass%) of each element in the steel, and is zero if the element is not contained.

In addition, in the manufacturing method according to the present invention, conventional methods can be used for any items not described in this specification.

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

A steel plate was manufactured using the following procedure.
First, a steel slab (steel material) having the chemical composition shown in Table 1 with the balance being Fe and unavoidable impurities was produced by a converter-continuous casting method.
Next, such a steel slab is heated to the heating temperature shown in column (1) of Table 2, and then the cumulative reduction rate, the reduction rate in the recrystallization zone, and the Each was hot-rolled at the reduction rate in the recrystallization region to produce a hot-rolled steel sheet. The plate thickness (final plate thickness) of the hot rolled steel plate thus obtained is also listed in Table 2.
Thereafter, the hot rolled steel plate was cooled under the conditions shown in column (3) of Table 2 to obtain a steel plate. The thickness of such a steel plate is the same as the final thickness described above. In addition, some of the samples were subjected to the steps described in (4), (5), and (6) in Table 2.

The microstructure, mechanical properties, weather resistance, and fatigue crack propagation resistance of each of the steel plates were evaluated. The evaluation method will be explained below.
In addition, the average length of the hard structure in the rolling direction (L direction) is (L (L)), the average length of the hard structure in the plate thickness direction (Z direction) is (L (Z)), and the plate width of the hard structure Let the average length in the direction (C direction) be (L(C)).

(microstructure)
First, from the 1/4t position in the plate thickness direction of the steel plate, a sample for microstructure observation is made so that the observation plane is the rolling direction (L direction) cross section, the plate width direction (C direction) cross section, and the plate thickness direction (Z direction) cross section. was collected. Note that the rolling direction (L direction) cross section is a cross section perpendicular to the sheet width direction, the sheet width direction (C direction) cross section is a cross section perpendicular to the sheet thickness direction, and the sheet thickness direction (Z direction) cross section is a cross section perpendicular to the rolling direction. shall refer to.
Next, after the surface of the sample was subjected to nital corrosion, the structure was photographed using a 400x optical microscope and a 2000x scanning electron microscope (SEM). The images were used to identify the tissue present. Furthermore, by analyzing the optical microscope image using image analysis software (Photoshop) and binarizing the ferrite region and the hard tissue region, the area fraction of ferrite, the area fraction of hard tissue, and The length of the hard tissue in each direction was measured. These values were obtained by observing 5 visual fields for each sample and using the average value of the 5 visual fields.

(mechanical properties)
A full-thickness tensile test piece was taken from the width direction (C direction) of the steel plate. Using a full thickness tensile test piece, a tensile test was conducted in accordance with JIS Z 2241 to measure tensile strength (TS) and total elongation (EL). In addition, a Charpy impact test piece was taken from the center of the thickness of the 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 measure the absorbed energy vE ₀ . did.

(Weatherability)
A test piece with a size of 50 mm x 50 mm x 4 mm was taken from each of the steel plates, and the end and back sides of the test piece were tape-sealed, and the surface was also tape-sealed so that the area of the surface exposed part was 40 mm x 40 mm. . The weather resistance of the thus obtained test piece was evaluated.
As a method to evaluate weather resistance, we conducted a corrosion test that simulated the environment inside a girder without rain exposure, which is considered to be the most severe environment in actual structures such as bridges. This corrosion test was conducted by repeating temperature and humidity cycles with salt attached to the sample surface.
The temperature/humidity cycle described above includes a drying process at a temperature of 40°C and a relative humidity of 40% RH for 11 hours, followed by a transition time of 1 hour, and a further moistening process at a temperature of 25°C and a relative humidity of 95% RH for 11 hours. , and then a transition time of 1 hour was taken, making one cycle for a total of 24 hours, simulating the temperature and humidity cycle of an actual environment.
Before the start of the temperature/humidity cycle and every 7 cycles, artificial seawater was dropped onto the surface of the test piece before the drying process so that the salt content adhering to the test piece surface was 1.4 mg/dm ² .
Under these conditions, a test was conducted with 182 cycles of temperature and humidity cycles over 26 weeks.

After the corrosion test was completed, the test piece was immersed in a rust removal solution made by adding distilled water to 500 mL of 37% hydrochloric acid, 3.5 g of hexamethylenetetramine, and 3 mL of Hibilon (Inhibitor manufactured by Iko Chemical Co., Ltd.) to make 1 L (liter). I removed the rust. In addition, the measurement of mass was based on the method described in the 145th Corrosion Prevention Symposium material "Improving the accuracy of corrosion and wear evaluation method".
Furthermore, the difference between the obtained mass and the initial mass was calculated, and the difference was divided by the area of the surface to be tested of the test piece, thereby calculating the average plate thickness reduction amount on one side of the test piece. In this example, the amount of decrease in average plate thickness was used as an index of weather resistance.
Note that the amount of airborne salt of about 0.5mdd corresponds to an environment with a large amount of airborne salt, such as near the coast, but based on previous knowledge, the amount of steel plate thickness reduction (182 days) in this corrosion test is due to the amount of airborne salt. It has been found that this is equivalent to the amount of steel sheet thickness reduction due to corrosion when exposed to an actual environment of approximately 0.5 mdd for 182 days.

When calculating the amount of corrosion after 100 years by extrapolation from the average amount of plate thickness reduction, if the average amount of plate thickness reduction obtained during the period of this corrosion test is 22 μm or less, the average amount of plate thickness reduction after 100 years. is evaluated to be 0.5 mm or less without the occurrence of delaminated rust.

In general, it is known that the standard for applicability of unpainted weathering steel to bridges is that the plate thickness decreases after 100 years is 0.5 mm or less, so a full corrosion test was conducted on various steel materials. If the resulting average plate thickness reduction is 22 μm or less, unpainted weathering steel can be applied to bridges. Therefore, in Table 3, it was determined that the weather resistance was excellent when the average plate thickness reduction amount was 22 μm or less.

(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 the following conditions with a stress intensity factor range ΔK: 25 MPa·m ^1/2 .
- Rolling direction and width direction The fatigue crack propagation rate in the rolling direction (L direction) was measured using a test piece taken from a 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 steel plate so that the load direction was in the width direction. These test pieces were compact tension test pieces based on ASTM E647. In addition, in the above-mentioned measurements, a fatigue crack propagation test was conducted based on the crack gauge method to determine the fatigue crack propagation speed.
- Plate thickness direction On the other hand, in measuring the fatigue crack propagation speed in the plate thickness direction (Z direction), a single-sided notched simple tension type fatigue test piece was used. Such a test piece was taken from a steel plate, and the fatigue crack propagation speed when the crack propagated in the thickness direction of the plate was measured.

Table 3 shows the results of each evaluation.

As can be seen from the results shown in Table 3, the steel plate that met the conditions of the present invention had extremely excellent characteristics that satisfied all of the following five conditions. In particular, it had both excellent fatigue crack propagation resistance and total elongation, and was also excellent in fatigue crack propagation resistance in all directions, including the plate thickness direction, rolling direction, and width direction. On the other hand, the steel plate of the comparative example that does not satisfy the conditions of the present invention did not satisfy at least one of the following five conditions.
・EL: 15% or more ・vE ₀ : 100J or more ・Fatigue crack propagation velocity in L direction and C direction: ΔK: 8.50×10 ^-8 (m/cycle) or less under the condition of 25 MPa・m ^1/2・Fatigue crack propagation speed in Z direction: ΔK: 4.25×10 ^-8 (m/cycle) or less under the condition of 25 MPa・m ^1/2・Average plate thickness reduction: 22 μm or less

Claims (4)

The component composition is in mass%,
C: 0.01% or more, 0.20% or less,
Si: 0.05% or more, 1.00% or less,
Mn: 0.10% or more, 2.00% or less,
P: 0.003% or more, 0.035% or less,
S: 0.0001% or more, 0.0350% or less,
Contains Al: 0.001% or more and 0.100% or less and Ni: 0.80% or more and 6.00% or less,
moreover,
Contains one or two selected from Cu: 1.00% or less and Mo: 1.00% or less, the remainder being Fe and inevitable impurities,
The microstructure consists of ferrite with an area fraction of 55% or more and a hard structure harder than ferrite with an area fraction of 45% or less,
The hard structure contains one or more selected from pearlite, bainite, and martensite,
A steel plate in which the hard structure satisfies the following formulas (1) and (2).
L(L)/L(Z)≦5.0 (1)
L(L)/L(C)≦5.0 (2)
L (L): Average length of the hard structure in the rolling direction (L direction) L (Z): Average length of the hard structure in the plate thickness direction (Z direction) L (C): Average length of the hard structure in the plate width direction (C average length in direction) The component composition further includes, in mass%,
Cr: 1.000% or less,
W: 1.00% or less,
Co: 1.000% or less,
Sn: 0.300% or less,
Sb: 0.300% or less,
Nb: 0.100% or less,
V: 0.150% or less,
Ti: 0.100% or less,
B: 0.0050% or less,
Zr: 0.1000% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
The steel plate according to claim 1, containing one or more selected from REM: 0.0200% or less. A method for manufacturing the steel plate according to claim 1 or 2, comprising:
After heating the slab having the component composition according to claim 1 or 2 to a temperature range of 1000 ° C. or higher and 1300 ° C. or lower,
Hot rolling is performed at a cumulative reduction rate of 50% or more in the temperature range of Ar ₃ or higher than the slab heating temperature to obtain a hot rolled sheet, and the hot rolled sheet is heated at an average cooling rate of 0.10°C/s or more. A method of manufacturing a steel plate for cooling. After the cooling, further heating to a reheating temperature of not less than Ac ₁ transformation point and less than Ac ₃ transformation point,
After such heating, cooling to a cooling stop temperature between 350 and 600 °C at an average cooling rate in the range of 2.00 to 7.00 °C/s,
The method for manufacturing a steel plate according to claim 3, further comprising quenching.