JP7147663B2 - Structural Steel Material with Excellent Fatigue Crack Propagation Property and Coating Durability, and Manufacturing Method Therefor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a structural steel material having excellent fatigue crack propagation properties and coating durability, and a method for producing the same.
The present invention relates to structural steel materials that are applied to welded structures, etc., for which structural safety is strongly required. It is suitable for use in structural steel materials used in severe corrosive environments such as.

Steel structures used outdoors, such as bridges, are usually used after being subjected to some kind of anticorrosion treatment. For example, weathering steel is often used in environments with a small amount of airborne salt.
Here, when the weathering steel is used in an atmospheric exposure environment, the surface is covered with a highly protective rust layer in which alloy elements such as Cu, P, Cr, and Ni are concentrated. It is a steel material that has been greatly reduced. It is known that bridges using such weather-resistant steel can withstand several decades of service without painting in an environment with a small amount of airborne salt.

On the other hand, it is difficult to form a highly protective rust layer on weathering steel in an environment with a large amount of airborne salt, such as at sea or near the coast, and it is difficult to use weathering steel without coating. For this reason, in environments where there is a large amount of airborne salt, such as in the sea or near the coast, steel materials whose surfaces have been subjected to anti-corrosion treatment such as painting are generally used.

However, coated steel requires periodic repairs such as repainting due to deterioration of the coating film, rust generation, blistering of the coating film, etc. over time. The painting work associated with repainting is often performed at a high place, and not only is the work itself difficult, but the labor cost for the work is also large. Therefore, when using coated steel, there is a problem that the maintenance cost of the structure increases, and thus the life cycle cost increases.

For this reason, it is desired to develop steel materials with excellent corrosion resistance, especially structural steel materials with excellent coating durability, that can reduce the frequency of coating by extending the cycle of recoating and reduce the maintenance cost of structures. ing.

Materials used as such structural members are required to have various mechanical properties depending on their uses. For example, in recent years, the demand for high-strength materials has increased due to the increase in large-sized structures. Furthermore, steel materials used for structural members must ensure the structural safety of structures against repeated loads during constant operation and repeated vibrations caused by wind, earthquakes, and the like. Since cyclic loading causes fatigue fracture, structural steels used for the above applications are required to have excellent fatigue resistance properties.

In general, fatigue cracks are initiated from stress concentration areas such as weld toes, roots, and scallops, and propagate into the steel material, leading to eventual fracture of the member. It is important to reduce stress concentration to prevent fatigue cracks, and it is known that improving the shape of the weld toe (additional welding, peening, etc.) is an effective method for doing so. ing.
However, it is impractical from the standpoint of construction time and cost to carry out such processing on hundreds or thousands of welds on an industrial scale. For this reason, newly constructed welded structures are inspected periodically, and when fatigue cracks are detected, they are repaired repeatedly to maintain structural safety. The labor and cost of maintenance and repair are enormous.

Therefore, it is considered extremely important from the viewpoint of inspection and repair to give the steel material itself the effect of delaying the propagation of fatigue cracks so that even if fatigue cracks do occur, they will not cause destruction of the member.

As a steel material having such excellent corrosion resistance and fatigue crack propagation properties, for example, Patent Document 1 discloses that P is 0.15 to 0.30% and Cr is 2.0% to 3.0% by mass%. A high weathering steel containing less than
Patent Document 2 discloses a super-coated durable steel containing 0.03 to 0.15% P and 0.2 to 0.5% Cu in terms of mass %.
In Patent Document 3, Cu is 0.05 to 3.0%, Ni is 0.05 to 6.0%, and Ti is 0.01 to 1.0% by mass. A coated steel is disclosed.
Patent Document 4 discloses a coating film durability containing 0.05 to 3.0% Cu, 0.05 to 6.0% Ni, and 0.025 to 0.15% Ti by mass%. A superior painting steel is disclosed.
Patent Document 5 discloses a highly weldable and highly weathering steel containing 0.30 to 1.00% Cu and 1.0 to 5.5% Ni by mass.
Patent Document 6 discloses a beach containing 0.05 to 1.0% Cu, 0.01 to 0.5% Ni, and 0.03 to 0.50% Sn and/or Sb in terms of mass %. A steel material having excellent weather resistance is disclosed.
Patent Literature 7 discloses a method of manufacturing a steel material containing 0.03 to 0.50% by mass of Sn and having excellent corrosion resistance and toughness in the Z direction.
Patent Document 8 discloses a steel material containing 0.15 to 0.5% by mass of Sn, which contains chlorides and has excellent corrosion resistance and is used under repeated wet and dry conditions.
Patent Document 9 discloses a steel material with excellent corrosion resistance, containing 0.01 to 0.5% by mass of Sn.

Further, Patent Document 10 discloses a method for reducing the crack propagation speed in the sheet thickness direction by controlling the crystal orientation of ferrite.
Patent Document 11 discloses a technique for improving fatigue characteristics by refining the ferrite grain size to 1 to 3 μm.
In Patent Document 12, the microstructure is composed of a base of a hard part and a soft part dispersed in this base, and the difference in hardness between the two is 150 or more in Vickers hardness. A steel sheet having a

JP-A-6-93372 JP-A-6-143489 JP-A-10-330881 JP-A-2000-169939 JP-A-11-172370 Japanese Patent Application Laid-Open No. 2006-118011 JP-A-2010-7109 JP 2012-255184 A JP 2013-166992 A JP-A-8-199286 JP-A-2002-363644 JP-A-7-242992

However, when the P content is increased as in Patent Documents 1 and 2, the weldability is greatly reduced. Furthermore, in Patent Document 1, no consideration is given to the corrosion resistance of the coated steel material, that is, the coating durability.
In addition, as in Patent Documents 3, 4 and 5, if the content of Cu or Ni is excessively increased, the alloy cost will increase. Then, the problem of insufficient weather resistance occurs. Furthermore, when a large amount of Ti is contained as in Patent Documents 3 and 4, the toughness of the steel material is deteriorated.
In addition, if the content of Sn or the like is excessively increased as in Patent Documents 6 to 9, the alloy cost will increase and the toughness of the steel material will deteriorate.

There is a concern that the technique described in Patent Literature 10 cannot improve the fatigue crack propagation properties that propagate in directions other than the sheet thickness direction. With the technique described in Patent Document 11, there is a concern that the load on the rolling mill increases, the occupancy time of the rolling mill increases, and the rolling efficiency decreases. Patent Document 12 does not describe detailed manufacturing conditions, and it is difficult to manufacture the steel sheet according to the invention described in Patent Document 12. Furthermore, Patent Document 12 considers only the arresting effect of crack growth. In order to obtain good fatigue resistance, it is necessary to consider bending of the crack path, but no consideration is given to this point.

Furthermore, in each of the patent documents listed above, no consideration is given to improving both coating durability and fatigue crack propagation characteristics.

The present invention has been developed in view of the above-mentioned current situation. Even when used in a corrosive environment, it has excellent fatigue crack propagation characteristics and paint durability that can extend the cycle of repainting, reduce the painting frequency, and delay fatigue crack propagation. It is an object of the present invention to provide a structural steel product with an advantageous manufacturing method.

In addition, "excellent in fatigue crack propagation characteristics" means that when a fatigue crack propagation test is performed under the following conditions, the stress intensity factor range (ΔK) is 20 MPa √ m (here, m is the crack length (unit: meter)) is 5.0×10 ⁻⁸ m/cycle or less in both the direction perpendicular to the rolling direction and the rolling direction.
・Fatigue crack propagation test conditions Stress ratio: 0.1, frequency: 20 Hz, test environment: in air at room temperature Compliant standard: ASTM E647
(However, if the plate thickness exceeds 25 mm, reduce the thickness from one side until the plate thickness reaches 25 mm and collect the test piece.)
In addition, "excellent coating durability" means that when a coating film is formed on the surface of the steel material and a corrosion test is performed under the following conditions, the swelling width on one side from the initial defect part in the coating film is 6.5 mm. means that:
・Corrosion test conditions Initial defects imparted to the coating film: straight line cut with a width of 1 mm and a length of 40 mm Adherence amount of artificial sea salt: 6.0 g / m ²
Test time: 1200 cycles (9600 hours)
Cycle conditions: condition 1 (temperature: 60°C, relative humidity: 35%, retention time: 3 hours), condition 2 (temperature: 40°C, relative humidity: 95%, retention time: 3 hours), condition 1 to condition 2 and a cycle of 8 hours total, with each transition time from condition 2 to condition 1 being 1 hour

By the way, the inventors have obtained excellent coating durability without containing a large amount of Cu, Ni, Sn, etc., which may cause an increase in alloy cost and deterioration of toughness, and a good fatigue crack propagation property. In order to obtain , steel materials with various chemical compositions and microstructural morphologies were produced, and their coating durability and fatigue crack propagation characteristics were investigated.
As a result, the inventors have found that adding an appropriate amount of W and providing an appropriate microstructure greatly improves coating durability and fatigue crack propagation characteristics.

The reason for this is not necessarily clear, but the inventors believe as follows.
(1) W is eluted with the anode reaction and is present as WO ₄ ^2- in the rust layer near the surface of the base iron. prevent it from reaching
(2) In addition, the precipitation of W-containing compounds on the surface of the steel material suppresses the anode reaction.
(3) Fatigue crack propagation properties are improved due to stagnation of fatigue crack propagation and bending of the crack path. In addition, the susceptibility to stoppage of fatigue crack propagation and bending of the crack path changes depending on the volume fraction of the hard phase. Therefore, in order to improve fatigue crack propagation properties, it is important to control the volume fraction of the hard phase. In addition, the addition of W facilitates formation of a hard phase, making it easier to adjust the volume fraction of the hard phase to a desired value.
The present invention has been completed through further studies based on the above findings.

That is, the gist and configuration of the present invention are as follows.
1. in % by mass,
C: 0.020% or more and 0.200% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 0.20% or more and 2.00% or less,
P: 0.003% or more and 0.030% or less,
S: 0.0001% or more and 0.0100% or less,
Al: 0.001% or more and 0.100% or less and W: 0.005% or more and 1.000% or less, with the balance being Fe and unavoidable impurities,
Fatigue crack propagation in which the microstructure is composed of a hard phase and a soft phase, the volume fraction of the hard phase is 0.20 to 0.80, and the average grain size of ferrite in the soft phase is 5 to 50 μm Structural steel with excellent properties and paint durability.
Here, the soft phase is a structure with a Vickers hardness of less than 225, and the hard phase is a structure with a Vickers hardness of 225 or more.

2. The component composition further, in mass %,
Cu: 0.50% or less,
Ni: 0.50% or less,
Sn: 0.200% or less,
1. Structural steel material excellent in fatigue crack propagation properties and coating durability according to 1 above, containing one or more selected from Sb: 0.200% or less and Mo: 0.500% or less .

3. The component composition further, in mass %,
V: 0.200% or less,
Ti: 0.050% or less,
Zr: 0.100% or less,
B: 0.0050% or less,
Ca: 0.0100% or less,
3. The structure excellent in fatigue crack propagation properties and coating durability according to 1 or 2 above, containing one or more selected from Mg: 0.0100% or less and REM: 0.0100% or less steel material.

4. 4. A structural steel material having excellent fatigue crack propagation properties and coating durability according to any one of 1 to 3 above, wherein the P value represented by the following formula (1) satisfies 0.5 or more and 10 or less.
Note P value: 10 × W + 2 × Cu + 1.5 × Ni + 3 × Mo (1)
However, each element in the formula indicates the content (% by mass) of each element in the steel material.

5. 5. A structural steel material having excellent fatigue crack propagation properties and coating durability according to any one of 1 to 4 above, which has a coating film on its surface.

6. The coating film has an anticorrosion undercoat layer, an undercoat layer, an intermediate coat layer and a topcoat layer, wherein the anticorrosion undercoat layer is an inorganic zinc-rich paint, the undercoat layer is an epoxy resin paint, and the intermediate coat layer is fluorine for an intermediate paint. 5. Structural steel material excellent in fatigue crack propagation property and coating durability as described in 5 above, wherein resin and fluorocarbon resin for topcoat paint are used as the topcoat layer.

7. 7. The structural steel material having excellent fatigue crack propagation properties and coating durability according to any one of 1 to 6 above, which has a yield strength or 0.2% proof stress of 335 MPa or more and a tensile strength of 490 MPa or more.

8. A steel slab having the chemical composition according to any one of 1 to 4 above is heated to 1000° C. or higher and 1300° C. or lower, and then the reduction ratio in the temperature range from the slab heating temperature to 850° C.: 25% or more, finish rolling Temperature: After hot rolling under conditions of (Ar ₃ point - 40 ° C.) or higher, cooling rate 5 ° C./s or higher from the temperature range of Ar ₃ point to (Ar ₃ point - 80 ° C.) to 650 ° C. or lower 400 A method for manufacturing structural steel materials with excellent fatigue crack propagation properties and coating durability, in which accelerated cooling is performed to a temperature range of ℃ or higher.

According to the present invention, even when a welded structure such as a bridge, for which structural safety is strongly required, is used outdoors in a severe corrosive environment such as the sea or near the coast where a large amount of airborne salt is present, It is possible to extend the repainting cycle to reduce the frequency of painting, and even if fatigue cracks occur from stress concentration parts or welded parts when used in structures, the propagation of fatigue cracks can be delayed during use. It is possible to obtain a structural steel material with excellent fatigue crack propagation properties and coating durability at low cost.
Then, the structural steel material of the present invention, which has excellent fatigue crack propagation properties and coating durability, is subjected to outdoor atmospheric corrosive environments such as bridges, especially under severe corrosive environments such as seas and near coasts where there is a large amount of airborne salt. By applying it to the structures that are used, it is possible to greatly reduce the maintenance costs of the structures and, in turn, the life cycle costs.

The present invention will be specifically described below.
First, the reason why the chemical composition of steel is limited to the above range in the present invention will be explained. Although the unit of content of elements in the chemical composition of steel is "% by mass", hereinafter, it is indicated simply as "%" unless otherwise specified.
C: 0.020% or more and 0.200% or less C is an element that increases the strength of the steel material and increases the volume fraction of the hard second phase. Therefore, C needs to be contained in an amount of 0.020% or more in order to secure a predetermined strength as structural steel. On the other hand, when the C content exceeds 0.200%, weldability and toughness deteriorate. Therefore, the C content should be 0.020% or more and 0.200% or less.

Si: 0.05% or more and 1.00% or less Si must be contained in an amount of 0.05% or more to ensure deoxidation and strength. On the other hand, when the Si content exceeds 1.00%, the toughness and weldability deteriorate significantly. Therefore, the Si content should be 0.05% or more and 1.00% or less.

Mn: 0.20% to 2.00% Mn is an element that increases the strength of steel materials. Therefore, Mn must be contained in an amount of 0.20% or more in order to secure a predetermined strength as structural steel. On the other hand, when the Mn content exceeds 2.00%, toughness and weldability deteriorate. Therefore, the Mn content should be 0.20% or more and 2.00% or less. It is preferably 0.75% or more and 1.80% or less.

P: 0.003% or more and 0.030% or less P is an element that contributes to the improvement of coating durability of steel materials. From the viewpoint of obtaining such effects, it is necessary to contain 0.003% or more of P. On the other hand, when the P content exceeds 0.030%, the weldability deteriorates. Therefore, the P content should be 0.003% or more and 0.030% or less.

S: 0.0001% to 0.0100% S is an element that deteriorates weldability and toughness. Therefore, the S content should be 0.0100% or less. However, an attempt to reduce the S content to less than 0.0001% results in an increase in production costs. Therefore, the S content should be 0.0001% or more and 0.0100% or less.

Al: 0.001% or more and 0.100% or less Al is an element necessary for deoxidation during steelmaking. In order to obtain such effects, it is necessary to contain 0.001% or more of Al. On the other hand, if the Al content exceeds 0.100%, the weldability is adversely affected. Therefore, the Al content should be 0.001% or more and 0.100% or less. It is preferably 0.005% or more and less than 0.050%, more preferably 0.010% or more and less than 0.030%.

W: 0.005% to 1.000% W is an important element for improving fatigue crack propagation properties and coating durability. W is eluted with the anode reaction and distributed as WO ₄ ²⁻ in the rust layer, thereby electrostatically preventing chloride ions, which are corrosion-promoting factors, from penetrating the rust layer and reaching the base iron. To prevent. Furthermore, precipitation of compounds containing W on the surface of the steel suppresses the anodic reaction of the steel. In addition, the addition of W facilitates the formation of a hard phase, making it easier to obtain a desired volume fraction ratio between the hard phase and the soft phase.
In order to sufficiently obtain these effects, it is necessary to contain 0.005% or more of W. On the other hand, if the W content exceeds 1.000%, the alloy cost increases and the hard phase fraction becomes too high, deteriorating the fatigue crack propagation characteristics. Therefore, the W content should be 0.005% or more and 1.000% or less. It is preferably 0.010% or more and 0.700% or less, more preferably 0.030% or more and 0.500% or less, and still more preferably 0.050% or more and 0.100% or less.

Although the basic components have been described above, in the present invention, the elements described below can be appropriately contained as necessary.
Cu: 0.50% or less Cu forms a dense rust layer by refining the rust grains in the rust layer, and has the effect of suppressing the permeation of oxygen and chloride ions, which are factors that promote corrosion, into the base iron. have Also, the addition of Cu facilitates the formation of a hard phase. On the other hand, when the Cu content exceeds 0.50%, the alloy cost increases. Therefore, when Cu is contained, the Cu content is set to 0.50% or less. Preferably 0.01% or more and 0.50% or less, more preferably 0.03% or more and 0.40% or less, still more preferably 0.04% or more and 0.30% or less, particularly preferably 0.05% % or more and 0.25% or less.

Ni: 0.50% or less Ni forms a dense rust layer by refining the rust grains in the rust layer, and has the effect of suppressing the penetration of oxygen and chloride ions, which are factors that promote corrosion, into the base iron. have Also, the addition of Ni facilitates the formation of a hard phase. On the other hand, when the Ni content exceeds 0.50%, the alloy cost increases. Therefore, when Ni is contained, the Ni content should be 0.50% or less. Preferably 0.01% or more and 0.50% or less, more preferably 0.03% or more and 0.40% or less, still more preferably 0.04% or more and 0.30% or less, particularly preferably 0.04% or more and 0.30% or less. 05% or more and 0.15% or less.

Sn: 0.200% or less Sn is present in the rust layer near the surface of the base iron, and refines the rust particles so that chloride ions, which are corrosion-promoting factors, permeate the rust layer and reach the base iron. to prevent In addition, Sn suppresses the anode reaction on the surface of the steel material. On the other hand, when the Sn content exceeds 0.200%, the ductility and toughness of the steel deteriorate. Therefore, when Sn is contained, the Sn content shall be 0.200% or less. It is preferably 0.005% or more and 0.200% or less, more preferably 0.010% or more and 0.100% or less, and still more preferably 0.020% or more and 0.050% or less.

Sb: 0.200% or less Sb exists in the rust layer near the surface of the base iron, and refines the rust particles so that chloride ions, which are corrosion-promoting factors, permeate the rust layer and reach the base iron. to prevent Moreover, Sb suppresses the anode reaction on the surface of the steel material. On the other hand, when the Sb content exceeds 0.200%, the ductility and toughness of the steel deteriorate. Therefore, when Sb is contained, the Sb content shall be 0.200% or less. It is preferably 0.005% or more and 0.200% or less, more preferably 0.010% or more and 0.150% or less, and still more preferably 0.020% or more and 0.100% or less.

Mo: 0.500% or less Mo is eluted with the anode reaction of the steel material, and MoO ₄ ^2- is distributed in the rust layer. Prevents reaching iron. In addition, a compound containing Mo precipitates on the surface of the steel material, thereby suppressing the anode reaction of the steel material. Furthermore, the addition of Mo makes it easier to generate a hard phase. On the other hand, if the Mo content exceeds 0.500%, the alloy cost will increase. Therefore, when Mo is contained, the Mo content shall be 0.500% or less. It is preferably 0.005% or more and 0.500% or less.

V: 0.200% or less V is an element that increases strength. On the other hand, when the V content exceeds 0.200%, the effect of improving strength is saturated. Therefore, when V is contained, the V content should be 0.200% or less. It is preferably 0.005% or more and 0.200% or less.

Ti: 0.050% or less Ti is an element that increases strength. On the other hand, when the Ti content exceeds 0.050%, deterioration of toughness is caused. Therefore, when Ti is contained, the Ti content should be 0.050% or less. It is preferably 0.005% or more and 0.050% or less.

Zr: 0.100% or less Zr is an element that increases strength. On the other hand, when the Zr content exceeds 0.100%, the strength improvement effect is saturated. Therefore, when Zr is contained, the Zr content shall be 0.100% or less. It is preferably 0.005% or more and 0.100% or less.

B: 0.0050% or less B is an element that increases strength. On the other hand, when the B content exceeds 0.0050%, deterioration of toughness is caused. Therefore, when B is contained, the B content shall be 0.0050% or less. It is preferably 0.0001% or more and 0.0050% or less.

Ca: 0.0100% or less Ca is an element that fixes S in the steel and improves the toughness of the weld heat affected zone. On the other hand, when the Ca content exceeds 0.0100%, the amount of inclusions in the steel increases, rather causing deterioration in toughness. Therefore, when Ca is contained, the Ca content shall be 0.0100% or less. It is preferably 0.0001% or more and 0.0100% or less.

Mg: 0.0100% or less Mg is an element that fixes S in the steel and improves the toughness of the weld heat affected zone. On the other hand, when the Mg content exceeds 0.0100%, the amount of inclusions in the steel increases, rather causing deterioration of toughness. Therefore, when Mg is contained, the Mg content shall be 0.0100% or less. It is preferably 0.0001% or more and 0.0100% or less.

REM: 0.0100% or less REM is an element that fixes S in steel and improves the toughness of the weld heat affected zone. On the other hand, if the REM content exceeds 0.0100%, the amount of inclusions in the steel will increase, resulting in deterioration of toughness. Therefore, when REM is contained, the REM content shall be 0.0100% or less. It is preferably 0.0001% or more and 0.0100% or less.

Components other than the above are Fe and unavoidable impurities. Incidentally, the inevitable impurities include N and O, and N: 0.010% or less and O: 0.010% or less, respectively, are permissible.

P value: 10 x W + 2 x Cu + 1.5 x Ni + 3 x Mo (1)
However, each element in the formula indicates the content (% by mass) in the steel material.
This formula (1) is an index showing the easiness of forming a hard phase. That is, if the P value shown in formula (1) is less than 0.5, a sufficient amount of hard phase is not generated, and desired fatigue crack propagation characteristics cannot be obtained. On the other hand, if the P-value exceeds 10.0, the volume fraction of the soft phase decreases, and in this case also the desired fatigue crack propagation properties cannot be obtained. Therefore, it is advantageous to satisfy a P value of 0.5 or more and 10.0 or less. More preferably, it is in the range of 0.6 or more and 9.0 or less.

Next, the microstructure morphology in the present invention will be explained.
The microstructure of the steel material according to the present invention has a composite structure in which a hard phase is dispersed in a soft phase. Fatigue crack propagation cannot be delayed when the steel material structure is a hard phase single phase or a soft phase single phase.
When the fatigue crack tip exists in the soft phase and the hard phase exists in front of it, the fatigue crack avoids the hard phase and propagates by bending or branching through the constraint of the plastic region. Such bending and branching of cracks bring about fracture surface roughness-induced crack closure and stress shielding effect, which reduces the driving force for fatigue crack propagation.

A soft phase is a phase with a Vickers hardness of less than 225. A main phase of such a soft phase is ferrite. Tempered martensite and tempered bainite may also be soft phases depending on the thermal history. Also, the average grain size of ferrite must be 5 to 50 μm. If the average grain size of ferrite is less than 5 µm, there is concern that the load on the rolling mill will increase and the rolling mill will be occupied for a longer period of time, resulting in a decrease in rolling efficiency. Therefore, the average grain size of ferrite is set to 5 μm or more. Further, when the average grain size of ferrite exceeds 50 μm, toughness, which is a basic characteristic of steel plates, deteriorates. Therefore, the average grain size of ferrite is set to 50 μm or less.
The Vickers hardness (HV1) is measured according to JIS Z 2244 (2009) with a test force of 9.807N.

Also, the average grain size of ferrite is measured by the following method.
Using a polished sample taken from an arbitrary location, the structure was observed with an optical microscope in arbitrary 5 fields at 1/4 position of the thickness of the cross section parallel to the rolling direction etched with 3% nital corrosive solution. do. Obtain the average grain size of ferrite using the line segment method.

A hard phase is a phase with a Vickers hardness of 225 or more. Such hard phases include pearlite, bainite, and martensite, and depending on the thermal history, tempered martensite and tempered bainite may also become hard phases.
In addition, discrimination between the soft phase and the hard phase is carried out by measuring the Vickers hardness of each microstructure by the method described above.

In the present invention, the volume fraction of the hard phase is 0.20 to 0.80. When the volume fraction of the hard phase is less than 0.20, the ratio of the soft phase is large, so in many cases, the fatigue crack tip exists in the soft phase, and the hard phase exists in front of it. few situations. Therefore, the effect of lowering the driving force for fatigue crack growth due to bending and branching of the fatigue crack due to the presence of the hard phase in front of the tip of the fatigue crack present in the soft phase cannot be obtained. On the other hand, when the volume fraction of the hard phase is greater than 0.80, the fatigue crack tip often exists in the hard phase because the ratio of the soft phase is small. Therefore, the effect of lowering the driving force for fatigue crack growth due to bending and branching of the fatigue crack due to the presence of the hard phase in front of the tip of the fatigue crack present in the soft phase cannot be obtained.
Therefore, the volume fraction of the hard phase is set to 0.20 to 0.80.
The volume fraction of the hard phase is determined as follows.
That is, the structure is observed by an optical microscope at the position of 1/4 of the thickness of the cross section of the steel material parallel to the rolling direction, and the area of each microstructure is calculated by image processing. Then, 5 points are randomly selected from each microstructure and the Vickers hardness (HV1) is measured. discriminate. Then, the value obtained by dividing the total area of the microstructure determined as the hard phase by the area of the entire observation field is defined as the volume fraction of the hard phase.

Moreover, the steel material of the present invention is usually used after coating the surface of the steel material. Here, the coating film on the surface of the steel material includes, for example, a coating film having an anticorrosion base layer, an undercoat layer, an intermediate coating layer and a top coating layer in this order.
An inorganic zinc-rich paint (e.g., SD Zinc 1500 manufactured by Kansai Paint Co., Ltd.) is used as the anticorrosive base layer, and an epoxy resin paint (e.g., Epomarine HB (K), manufactured by Kansai Paint Co., Ltd.) is used as the undercoat layer. As a layer, a fluororesin for intermediate coating (for example, manufactured by Kansai Paint Co., Ltd.: Ceratect F intermediate coating), and as a topcoat layer, a fluororesin for top coating (eg, manufactured by Kansai Paint Co., Ltd.: Ceratect F (K) topcoat) is advantageously suitable. The coating film on the surface of the steel material may be a coating film having a primer layer, an undercoat layer, an intermediate coating layer and a topcoat layer in this order, or a coating film having an anticorrosive base layer, an undercoat layer and a topcoat layer in this order.

In addition, the structural steel material of the present invention is produced by forming a slab from molten steel having the above chemical composition by ordinary continuous casting or blooming, and hot rolling the slab into thick plates, shaped steels, thin steel plates, steel bars, etc. It is obtained by

In particular, the steel material manufactured by the following manufacturing method has a yield strength or 0.2% yield strength of 335 MPa or more and a tensile strength of 490 MPa or more.
The slab heating temperature may be determined as appropriate. However, heating the slab above 1300° C. leads to reduced yield and increased energy consumption due to excessive scaling. On the other hand, deterioration of toughness due to coarsening of crystal grains becomes a problem. Moreover, if the slab is heated below 1000° C., the deformation resistance of the slab increases, and rolling may become difficult due to an increase in the rolling load in the subsequent rolling step. Therefore, the slab heating temperature should be in the range of 1000 to 1300.degree. It is preferably in the range of 1050-1250°C. It is more preferably in the range of 1080 to 1200°C.

The heated slab is hot-rolled at a temperature of (Ar ₃ point-40° C.) or higher, and then made into a steel material having desired dimensions and shape. If the finish rolling temperature is less than (Ar ₃ point -40°C), a large amount of ferrite phase is formed, and the desired volume fraction of the hard phase cannot be obtained. Therefore, the finish rolling temperature should be (Ar ₃ point - 40°C) or higher. Ar is preferably ₃ points or more.
In addition, the reduction ratio in the temperature range from the slab heating temperature to 850 ° C. in hot rolling (= ([thickness of slab before the start of hot rolling] - [thickness of material to be rolled at 850 ° C.]) ÷ [hot rolling The thickness of the slab before the start]×100) is 25% or more. By this rolling, the austenite grains are recrystallized or partially recrystallized, so ferrite having an average grain size of 5 μm to 50 μm can be obtained.
The Ar ₃ point may be measured by a known method, but in this application, the value obtained by the formula (1) described in "Tetsu to Hagane Vol. 67 (1981) p147" shown below Using.
Ar ₃ points (°C)
= 910-310 x C-80 x Mn-20 x Cu-55 x Ni-80 x Mo
However, the elements in the formula indicate the content (% by mass) in the steel material.

After hot rolling is completed, accelerated cooling is performed.
The cooling start temperature is in the temperature range from Ar ₃ point to (Ar ₃ point - 80°C). As a result, a soft phase represented by ferrite is generated from the austenite phase.
The cooling rate should be 5° C./s or more. By this accelerated cooling, the remaining austenite portion can be made into a hard phase such as bainite or martensite, and a steel material consisting of a hard phase and a soft phase can be produced.
The cooling stop temperature is in the temperature range of 650°C or lower and 400°C or higher. If the cooling stop temperature exceeds 650° C., the amount of soft phases such as ferrite produced increases, and the desired hard phase volume fraction cannot be obtained. On the other hand, if the cooling stop temperature is lower than 400° C., the steel material is likely to warp, and there is concern about an increase in production costs due to straightening and the like.

The content of each element in steel can be determined by spark discharge emission spectrometry, fluorescent X-ray analysis, ICP emission spectrometry, ICP mass spectrometry, combustion method, or the like.

(Example 1)
A steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted, continuously cast into a slab (thickness: 250 mm), and then hot rolled under various conditions shown in Table 2. A steel sheet having a thickness of 12 to 80 mm after rolling was obtained. In addition, No. In 15, it was cooled to room temperature by air cooling.
Corrosion tests, structure observations, tensile tests, and fatigue crack propagation tests were performed on the obtained steel sheets under the following conditions. Table 3 shows the results.

- Corrosion test A test piece of 70 mm x 50 mm x 5 mm was taken from the steel plate obtained as described above. The surface of this test piece was shot-blasted so that the surface roughness was Sa 2.5 of ISO 25178, ultrasonically degreased in acetone for 5 minutes, and air-dried. Next, one side of the test piece was used as a painted surface, and an inorganic zinc-rich paint, SD Zinc 1500 (thickness: 75 μm) was applied as an anticorrosion base, and an epoxy resin paint, Epomarine HB (K) (thickness: 120 μm), was applied as an undercoat. and then apply Ceratect F intermediate paint (thickness: 30 μm) as an intermediate coat, then apply Ceratect F (K) top paint (thickness: 25 μm) as a top coat, anticorrosion base layer, undercoat layer, A coating film consisting of an intermediate coating layer and a top coating layer was formed. The other side and the end face of the test piece were sealed with a solvent type epoxy resin paint and further coated with a silicon-based sealing agent.

After the above coating, a linear cut with a width of 1 mm and a length of 40 mm was made in the central part of the coating film formed on the test piece so as to reach the base steel, thereby forming an initial defect. Then, a corrosion test was carried out under the conditions shown below.
That is, a solution obtained by diluting artificial sea salt with pure water to a predetermined concentration was sprayed so that the amount of artificial sea salt adhered to the surface of the test piece was 6.0 g/m ² , and the artificial sea salt adhered to the test piece. let me Then, using this test piece, condition 1 (temperature: 60 ° C., relative humidity: 35%, holding time: 3 hours), condition 2 (temperature: 40 ° C., relative humidity: 95%, holding time: 3 hours) A corrosion test was carried out by repeating 1200 cycles of a total of 8 hours, with each transition time from condition 1 to condition 2 and from condition 2 to condition 1 being 1 hour. The artificial sea salt was applied once a week.
After the corrosion test was completed, the width of the swelling on one side of the coating film from the initial defective portion was measured to evaluate the coating durability.
Table 3 shows the results obtained. It was judged that the durability of the coating was excellent when the swelling width on one side was 6.5 mm or less.

- Observation of the structure The observation of the structure (observation with an optical microscope) is performed using a polished sample taken from an arbitrary location, and etched with a 3% nital corrosive solution. implemented. The structure was observed in 5 fields of view by the method described above, and the volume fraction of the hard phase was determined as an average value in all the fields of view. In addition, in order to distinguish between the soft phase and the hard phase, five points were randomly selected for each microstructure such as ferrite, martensite, bainite, tempered martensite, and tempered bainite, and the Vickers hardness (HV1) was measured. , a phase having a Vickers hardness of less than 225 was classified as a soft phase, and a phase having a Vickers hardness of 225 or more as a hard phase.
In addition, No. Except for No. 13, the area ratio of ferrite in the soft phase was 95% or more.

・ Tensile test Among the obtained steel sheets, steel sheets with a thickness of less than 50 mm use JIS Z 2241-compliant No. 1A or No. 5 tensile test pieces, and the full-thickness yield strength or 0.2% yield strength and tensile strength evaluated. For steel sheets with a thickness of 50 mm or more, a No. 4 tensile test piece conforming to JIS Z 2241 was used to evaluate the yield strength or 0.2% yield strength at the position of 1/4 thickness and the tensile strength. The number of test pieces was two, and the arithmetic mean was evaluated as the yield strength or 0.2% yield strength and tensile strength of the steel plate.

・Fatigue crack propagation test In the fatigue crack propagation test, a CT test piece of full thickness (thickness on one side is reduced to 25 mmt for plates with a thickness exceeding 25 mm) is collected, stress ratio 0.1, frequency 20 Hz, room temperature atmosphere Conducted according to ASTM E647. The test was conducted in the case where the crack propagated in the direction perpendicular to the rolling direction and in the case where the crack propagated in the rolling direction.
Then, the fatigue crack propagation speed when the stress intensity factor range (ΔK) is 20 MPa√m (here, m indicates the crack length (unit: meter)) is 5 in both the direction perpendicular to the rolling direction and the rolling direction. A case of 0×10 ⁻⁸ m/cycle or less was regarded as acceptable.

From Table 3, all invention examples had a coating film swelling width of 6.5 mm or less, and a fatigue crack propagation speed of 5.0 × 10 ^-8 m/cycle or less in both the rolling direction and the rolling direction. It can be seen that both coating durability and fatigue crack propagation properties are excellent.
On the other hand, in the comparative example, the swelling width of the coating film exceeds 6.5 mm, the fatigue crack propagation speed exceeds 5.0 × 10 ^-8 m / cycle, and sufficient coating durability and fatigue crack Propagation characteristics were not obtained.

Claims (7)

in % by mass,
C: 0.051% or more and 0.200% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 0.20% or more and 2.00% or less,
P: 0.003% or more and 0.030% or less,
S: 0.0001% or more and 0.0100% or less,
Al: 0.001% or more and 0.100% or less and W: 0.034% or more and 1.000% or less, with the balance being Fe and unavoidable impurities,
The microstructure is composed of a hard phase and a soft phase, the volume fraction of the hard phase is 0.29 to 0.80, the average grain size of ferrite in the soft phase is 5 to 36 μm, and the soft layer has Structural steel material having excellent fatigue crack propagation properties and coating durability, wherein the area ratio of ferrite is 95% or more, and the P value represented by the following formula (1) satisfies 0.5 or more and 10 or less.
Here, the soft phase is a structure with a Vickers hardness of less than 225, and the hard phase is a structure with a Vickers hardness of 225 or more.
In addition, observation of the microstructure was carried out at a position of 1/4 of the plate thickness of a cross section parallel to the rolling direction etched with a 3% nital corrosive solution using a polished sample taken from an arbitrary location. , The average grain size of the ferrite is obtained using the line segment method, and the volume fraction is obtained by observing the structure by optical microscope observation, calculating the area of each microstructure by image processing, and determining the hard phase. The volume fraction of the hard phase is obtained by dividing the total area of the tissue by the area of the entire observation field.
Note P value: 10 × W + 2 × Cu + 1.5 × Ni + 3 × Mo (1)
However, each element in the formula indicates the content (% by mass) of each element in the steel material. The component composition further, in mass %,
Cu: 0.50% or less,
Ni: 0.50% or less,
Sn: 0.200% or less,
Sb: 0.200% or less and Mo: 0.500% or less The structural use having excellent fatigue crack propagation properties and coating durability according to claim 1, containing one or more selected from 0.500% or less steel. The component composition further, in mass %,
V: 0.200% or less,
Ti: 0.050% or less,
Zr: 0.100% or less,
B: 0.0050% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less and REM: Excellent in fatigue crack propagation properties and coating durability according to claim 1 or 2, containing one or more selected from 0.0100% or less Structural steel. A structural steel material having excellent fatigue crack propagation properties and coating durability according to any one of claims 1 to 3, which has a coating film on its surface. The coating film has an anticorrosion undercoat layer, an undercoat layer, an intermediate coat layer and a topcoat layer, wherein the anticorrosion undercoat layer is an inorganic zinc-rich paint, the undercoat layer is an epoxy resin paint, and the intermediate coat layer is fluorine for an intermediate paint. 5. A structural steel material having excellent fatigue crack propagation properties and coating durability according to claim 4, wherein said topcoat layer comprises a fluorocarbon resin for topcoat paint. The structural steel material having excellent fatigue crack propagation properties and coating durability according to any one of claims 1 to 5, wherein the yield strength or 0.2% proof stress is 335 MPa or more and the tensile strength is 490 MPa or more. A method of manufacturing the structural steel material according to any one of claims 1 to 6, wherein the steel slab having the chemical composition according to any one of claims 1 to 3 is heated to 1000° C. or higher and 1300° C. or lower. After heating, hot rolling is performed under the conditions of a reduction ratio of 25% or more in a temperature range from the slab heating temperature to 850 ° C. and a finish rolling temperature of (Ar ₃ points - 40 ° C.) or higher, and then Ar ₃ points. For structures with excellent fatigue crack propagation characteristics and paint durability, which performs accelerated cooling from a temperature range of ~ (Ar ₃ points -80 ° C) to a temperature range of 650 ° C or lower and 400 ° C or higher at a cooling rate of 5 ° C/s or more. A method of manufacturing steel.