JP7044089B2 - Structural steel materials with excellent fatigue crack propagation characteristics and coating durability and their manufacturing methods - Google Patents

Description

The present invention relates to a structural steel material having excellent fatigue crack propagation characteristics and coating durability, and a method for producing the same.
The present invention relates to structural steel materials applied to welded structures and the like where structural safety is strongly required, and is mainly used in a land-based and outdoor air-corrosion environment such as a bridge. 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 with some anticorrosion treatment. For example, weathering steel is often used in an environment where the amount of flying salt is low.
Here, the weathering steel is covered with a highly protective rust layer enriched with alloying elements such as Cu, P, Cr, and Ni when used in an air-exposed environment, thereby reducing the corrosion rate. It is a steel material that has been greatly reduced. It is known that a bridge using such weathering steel can withstand several decades of service without painting in an environment where the amount of flying salt is low.

On the other hand, in an environment with a large amount of flying salt such as at sea or near the coast, it is difficult to form a highly protective rust layer in the weathering steel, and it is difficult to use the weathering steel without painting. For this reason, in an environment with a large amount of flying salt such as on the sea or near the coast, a steel material having an anticorrosion treatment such as painting on the surface is generally used.

However, with coated steel materials, repairs such as periodic repainting are required due to deterioration of the coating film over time, rusting, swelling of the coating film, and the like. The painting work associated with repainting is often done at a high place, which is not only difficult but also requires a large amount of labor costs. Therefore, when the coated steel material is used, there is a problem that the maintenance cost of the structure increases and the life cycle cost increases.

Therefore, it is desired to develop a steel material having excellent corrosion resistance, which can reduce the frequency of painting and suppress the maintenance cost of the structure by extending the cycle of repainting, especially a structural steel material having excellent coating durability. ing.

Further, the material used as a member for such a structure is required to have various mechanical properties depending on its use. For example, in recent years, as the number of large structures has increased, high-strength materials are often required. Further, the steel material used for the structural member must ensure the structural safety of the structure against repeated loads caused by repeated loads during constant operation, wind, and vibrations caused by earthquakes and the like. Since repeated loading causes fatigue fracture, it is required that the structural steel used for the above application has excellent fatigue resistance.

In general, fatigue cracks occur from stress-concentrated parts such as toes and roots or scallops in welds, which propagate to steel and lead to eventual fracture of the member. It is important to reduce stress concentration for the occurrence of fatigue cracks, and it is known that improvement of weld toe shape (additional welding, peening treatment, etc.) is effective as such a method. ing.
However, it is impractical to carry out such treatment on hundreds or thousands of welds on an industrial scale in terms of construction time and cost. Therefore, the newly constructed welded structure is inspected regularly, and when fatigue cracks are detected, repairs are repeated to maintain structural safety. Such inspections are carried out. The labor and cost of repairs 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 occur, they do not cause fracture of the members.

As a steel material having excellent corrosion resistance and fatigue crack propagation characteristics, for example, Patent Document 1 contains 0.15 to 0.30% of P and 2.0% or more and less than 3.0% of Cr in mass%. Highly weathering steel materials are disclosed.
Patent Document 2 discloses a super-coating durable steel material containing 0.03 to 0.15% of P and 0.2 to 0.5% of Cu in mass%.
Patent Document 3 has excellent durability in which Cu is contained in an amount of 0.05 to 3.0%, Ni is contained in an amount of 0.05 to 6.0%, and Ti is contained in an amount of 0.01 to 1.0% by mass. Painted steel materials are disclosed.
Patent Document 4 describes the durability of a coating film containing 0.05 to 3.0% of Cu, 0.05 to 6.0% of Ni, and 0.025 to 0.15% of Ti in terms of mass%. Excellent coating steel materials are disclosed.
Patent Document 5 discloses a highly weldable and highly weathering steel containing 0.30 to 1.00% of Cu and 1.0 to 5.5% of Ni in mass%.
Patent Document 6 describes a beach containing 0.05 to 1.0% of Cu, 0.01 to 0.5% of Ni, and 0.03 to 0.50% of Sn and / or Sb in mass%. Steel materials having excellent weather resistance are disclosed.
Patent Document 7 discloses a method for producing a steel material having excellent corrosion resistance and toughness in the Z direction, which contains 0.03 to 0.50% of Sn in mass%.
Patent Document 8 discloses a steel material containing 0.15 to 0.5% of Sn in mass% and having excellent corrosion resistance and used in a dry and wet repeated environment containing chloride.
Patent Document 9 discloses a steel material having excellent corrosion resistance and containing 0.01 to 0.5% of Sn in mass%.

Further, Patent Document 10 discloses a method of reducing the crack propagation velocity in the plate thickness direction by controlling the crystal orientation of ferrite.
Patent Document 11 discloses a technique for improving fatigue characteristics by reducing the ferrite grain size to 1 to 3 μm.
Patent Document 12 describes a fatigue crack growth suppressing effect characterized in that the microstructure is composed of a base material of a hard portion and a soft portion dispersed in the base material, and the hardness difference between the two is 150 or more in Vickers hardness. The steel plate to have is described.

Japanese Unexamined Patent Publication No. 6-93372 Japanese Unexamined Patent Publication No. 6-143489 Japanese Unexamined Patent Publication No. 10-330881 Japanese Unexamined Patent Publication No. 2000-169939 Japanese Unexamined Patent Publication No. 11-172370 Japanese Unexamined Patent Publication No. 2006-118011 Japanese Unexamined Patent Publication No. 2010-7109 Japanese Unexamined Patent Publication No. 2012-255184 Japanese Unexamined Patent Publication No. 2013-166992 Japanese Unexamined Patent Publication No. 8-199286 Japanese Patent Application Laid-Open No. 2002-363644 Japanese Unexamined Patent Publication No. 7-24292

However, when the content of P is increased as in Patent Documents 1 and 2, the weldability is greatly reduced. Further, in Patent Document 1, no consideration is given to the corrosion resistance of the coated steel material, that is, the coating durability.
Further, as in Patent Documents 3, 4 and 5, if the content of Cu or Ni is excessively increased, the alloy cost is increased, and if the alloy amount is decreased to avoid it, the area where the amount of flying salt is large is large. Then, the problem that the weather resistance becomes insufficient arises. Further, if a large amount of Ti is contained as in Patent Documents 3 and 4, the toughness of the steel material is deteriorated.
In addition, as in Patent Documents 6 to 9, if the content of Sn or the like is excessively increased, the alloy cost is also increased and the toughness of the steel material is deteriorated.

There is a concern that the technique described in Patent Document 10 cannot improve the fatigue crack propagation characteristics that propagate in directions other than the plate thickness direction. In the technique described in Patent Document 11, there is a concern that the load on the rolling mill will be large, the occupancy time of the rolling mill will be long, and the rolling efficiency will be lowered. Patent Document 12 does not describe detailed manufacturing conditions, and it is difficult to manufacture a steel sheet according to the invention described in Patent Document 12. Further, in Patent Document 12, only the retention effect of crack growth is considered. Bending of the crack path must also be considered in order to obtain good fatigue resistance, but no consideration is given to this point.

Furthermore, in each of the above-mentioned patent documents, no study has been made on improving both coating durability and fatigue crack propagation characteristics.

The present invention has been developed in view of the above-mentioned current conditions, and is harsh in an outdoor atmospheric corrosion environment such as a bridge, especially on the sea or near the coast where the amount of flying salt is high, without causing an excessive increase in alloy cost. Even when used in a corrosive environment, it has excellent fatigue crack propagation characteristics and coating durability that can extend the cycle required for repainting, reduce the coating frequency, and delay fatigue crack propagation. It is an object of the present invention to provide structural steel materials together with an advantageous manufacturing method thereof.

“Excellent 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 (where m is the crack length). It means that the fatigue crack propagation velocity at the time of (unit: meter) is 5.0 × ^10-8 m / cycle or less in both the rolling direction and the rolling direction.
・ Fatigue crack propagation test conditions Stress ratio: 0.1, frequency: 20Hz, test environment: room temperature in the atmosphere Compliance 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.)
Further, "excellent in coating durability" means that when a coating film is formed on the surface of a steel material and a corrosion test is performed under the following conditions, the swelling width on one side from the initial defect portion of the coating film is 6.5 mm. It means that it is as follows.
・ Corrosion test conditions Initial defects given to the coating film: Straight cut with a width of 1 mm and a length of 40 mm Adhesion amount of artificial sea salt: 6.0 g / m ²
Test time: 1200 cycles (9600 hours)
Cycle condition: Condition 1 (temperature: 60 ° C, relative humidity: 35%, holding time: 3 hours), condition 2 (temperature: 40 ° C, relative humidity: 95%, holding time: 3 hours), condition 1 to condition 2 And a cycle of 8 hours in total, with each transition time from condition 2 to condition 1 as 1 hour.

By the way, the inventors have obtained excellent coating durability without containing a large amount of Cu, Ni, Sn, etc., which may increase the alloy cost and deteriorate the toughness, and have good fatigue crack propagation characteristics. In order to obtain the above, steel materials having various composition and microstructure morphology were prepared, and their coating durability and fatigue crack propagation characteristics were investigated.
As a result, it was found that coating durability and fatigue crack propagation characteristics are significantly improved by adding an appropriate amount of W and an appropriate amount of Cr, Mo and / or V in a complex manner and forming an appropriate microstructure. ..

The reason for this is not always clear, but the inventors think as follows.
(1) W elutes with the anodic reaction and exists as ^WO _4-2 in the rust layer near the surface of the ground iron, so that chloride ions, which are corrosion promoting factors, permeate the rust layer and become the ground iron. Prevent it from reaching.
(2) Further, the anodic reaction is suppressed by the precipitation of the compound containing W on the surface of the steel material.
(3) Further, W forms fine rust to densify the rust layer, thereby preventing chloride ions, which are corrosion factors, from penetrating the rust layer and reaching the ground iron.
(4) In addition, Cr forms fine rust to densify the rust layer, thereby preventing chloride ions, which are corrosion factors, from penetrating the rust layer and reaching the ground iron.
In addition, Mo and V elute with the anodic reaction and are present as ^MoO _4-2 and ^VO ₄₃₃ in the rust layer near the surface of the ground iron, respectively, so that chloride ions, which are corrosion factors, form the rust layer. Prevents permeation and reaching the ground iron. Further, Mo suppresses the anodic reaction by precipitating a compound containing Mo on the surface of the steel material in a corrosive environment.
(5) Therefore, by adding an appropriate amount of W and an appropriate amount of Cr, Mo and / or V in combination, it is possible to improve the coating durability without causing deterioration of toughness.
(6) Fatigue crack propagation characteristics are improved by bending of the crack path when the growth of fatigue cracks is stopped. In addition, the volume fraction of the hard phase changes the likelihood of bending of the crack path when the growth of fatigue cracks is stopped. Therefore, in order to improve the fatigue crack propagation characteristics, it is important to control the volume fraction of the hard phase. In addition, the complex addition of W and Cr, Mo and / or V facilitates the formation of a hard phase and facilitates the volume fraction of the hard phase to a desired value.
The present invention has been completed with further studies based on the above findings.

That is, the gist structure of the present invention is as follows.
1. 1. By mass%,
C: 0.020% or more, 0.200% or less,
Si: 0.05% or more, 1.00% or less,
Mn: 0.20% or more, 2.00% or less,
P: 0.003% or more, 0.030% or less,
S: 0.0001% or more, 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, and
One or more selected from Cr, Mo and V,
Cr: 2.00% or less,
Mo: 0.500% or less and V: 0.200% or less and
Cr (%) / 10 + Mo (%) + V (%) ≧ 0.005%
Has a component composition in which the balance is Fe and unavoidable impurities.
The microstructure is composed of a hard phase and a soft phase, the volume fraction of the hard phase is 0.20 to 0.80, and the average particle size of ferrite in the soft phase is 5 to 50 μm. Structural steel with excellent properties and coating durability.
Here, the soft phase is a structure having a Vickers hardness of less than 225, and the hard phase is a structure having a Vickers hardness of 225 or more.
Further, Cr (%), Mo (%) and V (%) are the contents (mass%) of Cr, Mo and V in the component composition of the steel material, respectively.

2. 2. The component composition is further increased by mass%.
Cu: 0.50% or less,
Ni: 0.50% or less,
The structural steel material having excellent fatigue crack propagation characteristics and coating durability according to 1 above, which contains one or more selected from Sn: 0.200% or less and Sb: 0.200% or less. ..

3. 3. The component composition is further increased by mass%.
Ti: 0.050% or less,
Zr: 0.100% or less,
B: 0.0050% or less,
Ca: 0.0100% or less,
The structure excellent in fatigue crack propagation characteristics and coating durability according to 1 or 2 above, which contains one or more selected from Mg: 0.0100% or less and REM: 0.0100% or less. Steel material.

4. The structural steel material having excellent fatigue crack propagation characteristics and coating durability according to any one of 1 to 3 above, wherein the D value represented by the following equation (1) is 0.5 or more and 10 or less.
D value: 6 x W + 4 x Cr + 2 x Mo + 3 x V ... (1)
However, the element in the formula indicates the content (mass%) of each element in the steel material.

5. The structural steel material having a coating film on the surface and having excellent fatigue crack propagation characteristics and coating durability according to any one of 1 to 4 above.

6. The structural steel material having excellent yield crack propagation characteristics and coating durability according to any one of 1 to 5 above, wherein the yield strength or 0.2% proof stress is 335 MPa or more and the tensile strength is 490 MPa or more.

7. A steel slab having the component 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 rolling reduction in the temperature range from the slab heating temperature to 850 ° C.: 25% or higher, finish rolling. For structures with excellent fatigue crack propagation characteristics and coating durability, which are subjected to hot rolling under conditions of temperature: (Ar ₃ points -40 ° C) or higher and then cooled at a cooling rate of 0.1 ° C / s or higher. How to make steel.

According to the present invention, even when a welded structure for which structural safety is strongly required, such as a bridge, is used outdoors and in a severe corrosive environment such as on the sea or near a coast where a large amount of flying salt is present. It is possible to extend the repainting cycle and reduce the painting frequency, and even if fatigue cracks occur from stress concentration parts or welded parts in structures, delay the growth of fatigue cracks during the use process. It is possible to obtain a structural steel material having excellent fatigue crack propagation characteristics and coating durability at low cost.
Then, the structural steel material having excellent fatigue crack propagation characteristics and coating durability of the present invention is used in an outdoor atmospheric corrosion environment such as a bridge, especially in a severe corrosion environment such as on the sea or near the coast where the amount of flying salt is high. By applying it to the structure to be used, the maintenance cost of the structure and the life cycle cost can be significantly reduced.

Hereinafter, the present invention will be specifically described.
First, the reason why the composition of steel is limited to the above range in the present invention will be described. The unit of the element content in the composition of steel is "mass%", but hereinafter, it is simply indicated by "%" 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 a structural steel. On the other hand, if the C content exceeds 0.200%, the weldability and toughness deteriorate. Therefore, the C content is 0.020% or more and 0.200% or less.

Si: 0.05% or more, 1.00% or less Si needs to be contained in 0.05% or more in order to secure deoxidation and strength. On the other hand, when the Si content exceeds 1.00%, the toughness and weldability are significantly deteriorated. Therefore, the Si content is 0.05% or more and 1.00% or less.

Mn: 0.20% or more, 2.00% or less Mn is an element that increases the strength of steel materials. Therefore, Mn needs to be contained in an amount of 0.20% or more in order to secure a predetermined strength as a structural steel. On the other hand, if the Mn content exceeds 2.00%, the toughness and weldability deteriorate. Therefore, the Mn content is 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 an effect, P needs to be contained in an amount of 0.003% or more. On the other hand, if the P content exceeds 0.030%, the weldability deteriorates. Therefore, the P content is 0.003% or more and 0.030% or less.

S: 0.0001% or more and 0.0100% or less S is an element that deteriorates weldability and toughness. Therefore, the S content needs to be 0.0100% or less. However, if the S content is reduced to less than 0.0001%, the production cost will increase. Therefore, the S content is 0.0001% or more and 0.0100% or less.

Al: 0.001% or more, 0.100% or less Al is an element necessary for deoxidation during steelmaking. In order to obtain such an effect, Al needs to be contained in an amount of 0.001% or more. On the other hand, if the Al content exceeds 0.100%, the weldability is adversely affected. Therefore, the Al content is 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% or more and 1.000% or less W is an important element for improving fatigue crack propagation characteristics and coating durability. W elutes with the anodic reaction and is distributed as WO ₄₂ ^2- in the rust layer, thereby electrostatically preventing chloride ions of the corrosion promoting factor from penetrating the rust layer and reaching the ground iron. To prevent. Further, the anodic reaction is suppressed by the precipitation of the compound containing W on the surface of the steel material. Further, W forms fine rust and densifies the rust layer to prevent chloride ions, which are corrosion factors, from penetrating the rust layer and reaching the ground iron. In addition, by adding W, it becomes easy to generate a hard phase, and it becomes easy to obtain a volume fraction of a desired hard phase.
In order to obtain these effects sufficiently, it is necessary to contain W in an amount of 0.005% or more. On the other hand, if the W content exceeds 1.000%, the alloy cost increases and the volume fraction of the hard phase becomes too high, resulting in deterioration of fatigue crack propagation characteristics. Therefore, the W content is 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 further preferably 0.050% or more and 0.100% or less.

One or more selected from Cr, Mo and V are Cr: 2.00% or less, Mo: 0.500% or less and V: 0.200% or less, and Cr (%) / 10 + Mo. (%) + V (%) ≥ 0.005% Satisfied Cr, Mo and V all contribute to the improvement of the corrosion resistance of the steel. That is, Cr forms fine rust and densifies the rust layer, thereby preventing chloride ions, which are corrosion factors, from penetrating the rust layer and reaching the ground iron. In addition, Mo and V are eluted with the anodic reaction of the steel material, and are present as MoO ₄₂ ^2- and VO ₄₃ ^3- in the rust layer near the surface of the base iron, so that chloride ions, which are corrosion promoting factors, are present. Prevents it from reaching the ground iron through the rust layer.
Such an effect of improving corrosion resistance can be obtained by satisfying Cr (%) / 10 + Mo (%) + V (%) ≧ 0.005%.
Here, Cr (%), Mo (%) and V (%) are the contents (mass%) of Cr, Mo and V in the component composition of the steel material, respectively.

On the other hand, if the Cr content exceeds 2.00%, the alloy cost will increase. Therefore, the Cr content is set to 2.00% or less.
Further, if the Mo content exceeds 0.500%, the alloy cost will increase. Therefore, the Mo content is 0.500% or less.
Further, when the V content exceeds 0.200%, the above-mentioned effect of improving corrosion resistance is saturated. Therefore, the V content is set to 0.200% or less.

Although the basic components have been described above, in the present invention, the elements described below can be appropriately contained as needed.
Cu: 0.50% or less Cu has the effect of forming a dense rust layer by refining the rust particles in the rust layer and suppressing the permeation of oxygen and chloride ions, which are corrosion promoting factors, into the ground iron. Has. Further, the addition of Cu facilitates the formation of a hard phase. On the other hand, if the Cu content exceeds 0.50%, the alloy cost will increase. Therefore, when Cu is contained, the Cu content is 0.50% or less. It is 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, and particularly preferably 0.05. % Or more and 0.25% or less.

Ni: 0.50% or less Ni has the effect of forming a dense rust layer by refining the rust particles in the rust layer and suppressing the permeation of oxygen and chloride ions, which are corrosion promoting factors, into the ground iron. Has. Further, the addition of Ni facilitates the formation of a hard phase. On the other hand, if the Ni content exceeds 0.50%, the alloy cost will increase. Therefore, when Ni is contained, the Ni content is 0.50% or less. It is 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, and particularly preferably 0. It is 05% or more and 0.15% or less.

Sn: 0.200% or less Sn exists in the rust layer near the surface of the ground iron, and chloride ions, which are corrosion promoting factors, permeate the rust layer and reach the ground iron by refining the rust particles. To prevent. In addition, Sn suppresses the anodic reaction on the surface of the steel material. On the other hand, if the Sn content exceeds 0.200%, the ductility and toughness of the steel are deteriorated. Therefore, when Sn is contained, the Sn content is 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 further 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 ground iron, and chloride ions, which are corrosion promoting factors, permeate the rust layer and reach the ground iron by refining the rust particles. To prevent. Further, Sb suppresses the anodic reaction on the surface of the steel material. On the other hand, if the Sb content exceeds 0.200%, the ductility and toughness of the steel are deteriorated. Therefore, when Sb is contained, the Sb content is 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 further preferably 0.020% or more and 0.100% or less.

Ti: 0.050% or less Ti is an element that enhances strength. On the other hand, if the Ti content exceeds 0.050%, the toughness deteriorates. Therefore, when Ti is contained, the Ti content is set to 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 enhances strength. On the other hand, when the Zr content exceeds 0.100%, the strength improving effect is saturated. Therefore, when Zr is contained, the Zr content is set to 0.100% or less. It is preferably 0.005% or more and 0.100% or less.

B: 0.0050% or less B is an element that enhances strength. On the other hand, if the B content exceeds 0.0050%, the toughness deteriorates. Therefore, when B is contained, the B content is 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 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, which in turn causes deterioration of toughness. Therefore, when Ca is contained, the Ca content is 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 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, which in turn causes deterioration of toughness. Therefore, when Mg is contained, the Mg content is 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, when the REM content exceeds 0.0100%, the amount of inclusions in the steel increases, which in turn causes deterioration of toughness. Therefore, when REM is contained, the REM content is 0.0100% or less. It is preferably 0.0001% or more and 0.0100% or less.

Ingredients other than the above are Fe and unavoidable impurities. Examples of unavoidable impurities include N and O, and N: 0.010% or less and O: 0.010% or less, respectively, are acceptable.

D value: 6 x W + 4 x Cr + 2 x Mo + 3 x V ... (1)
However, each element in the formula indicates the content (mass%) of each element in the steel material.
This equation (1) is an index showing the ease of forming a hard phase. That is, by setting the D value represented by the equation (1) to 0.5 or more, a sufficient amount of hard phase can be generated, and the desired fatigue crack propagation characteristics can be surely obtained. On the other hand, by setting the D value to 10.0 or less, it is possible to prevent the volume fraction of the soft phase from being excessively lowered, and it is possible to surely obtain the desired fatigue crack propagation characteristics. Therefore, it is advantageous to satisfy the D 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 described.
The microstructure of the steel material according to the present invention is a composite structure in which the hard phase is dispersed in the soft phase. When the steel structure is a hard phase single phase or a soft phase single phase, fatigue crack propagation cannot be delayed.
If the fatigue crack tip is present in the soft phase and the hard phase is present in front of it, the fatigue crack will bend or branch and propagate while avoiding the hard phase through restraint of the plastic region and the like. Such bending and branching of cracks bring about a fracture surface roughness-induced crack closure and a stress shielding effect, and reduce the fatigue crack growth driving force.

The soft phase is a phase having a Vickers hardness of less than 225. The main phase of such a soft phase is ferrite. Further, depending on the heat history, tempered martensite and tempered bainite may also have a soft phase. Further, the average particle size of ferrite needs to be 5 to 50 μm. If the average particle size of the ferrite is less than 5 μm, there is a concern that the load on the rolling mill will increase, the occupancy time of the rolling mill will increase, and the rolling efficiency will decrease. Therefore, the average particle size of ferrite is set to 5 μm or more. Further, when the average particle size of the ferrite exceeds 50 μm, the toughness, which is a basic characteristic of the thick steel sheet, deteriorates. Therefore, the average particle size of ferrite is set to 50 μm or less.
The Vickers hardness (HV1) is measured in accordance with JIS Z 2244 (2009) with a test force of 9.807N.

The average particle size of ferrite is measured by the following method.
Using a sample obtained by polishing a sample collected from an arbitrary location, microstructure observation was performed with an optical microscope in any 5 fields at a plate thickness 1/4 position of a cross section parallel to the rolling direction etched with a 3% nital corrosive solution. do. Then, the average particle size of ferrite is obtained by using the line segment method.

The hard phase is a phase having a Vickers hardness of 225 or more. Examples of such a hard phase include pearlite, bainite, and martensite, and depending on the heat history, tempered martensite and tempered bainite may also be a hard phase.
The soft phase and the hard phase are discriminated by measuring the Vickers hardness of each microstructure by the above method.

In the present invention, the volume fraction of the hard phase is set to 0.20 to 0.80. When the volume fraction of the hard phase is smaller than 0.20, the ratio of the soft phase is large, so that the fatigue crack tip is often present in the soft phase, and the hard phase is present in front of the fatigue crack tip. There are few situations. Therefore, the effect of reducing the fatigue crack growth driving force due to bending or branching of the fatigue crack cannot be obtained due to the presence of the hard phase in front of the fatigue crack tip existing in the soft phase. On the other hand, when the volume fraction of the hard phase is larger than 0.80, the ratio of the soft phase is small, so that the fatigue crack tip is often present in the hard phase. Therefore, the effect of reducing the fatigue crack growth driving force due to bending or branching of the fatigue crack cannot be obtained due to the presence of the hard phase in front of the fatigue crack tip existing in the soft phase. Therefore, the volume fraction of the hard phase is set to 0.20 to 0.80.
The volume fraction of the hard phase is obtained as follows.
That is, the structure is observed by optical microscope observation at a position of 1/4 of the plate thickness of the cross section parallel to the rolling direction, and the area ratio of each microstructure is calculated by image processing. Next, 5 points were randomly extracted from each microstructure and the Vickers hardness (HV1) was measured. The phase with a Vickers hardness of less than 225 was designated as the soft phase, and the phase with Vickers hardness of 225 or higher was designated as the hard phase. Classify. Then, the value obtained by dividing the total area of the microstructures classified into the hard phase by the area of the entire observation field is defined as the volume fraction of the hard phase.

Further, the steel material of the present invention is usually used by coating the surface of the steel material with anticorrosion coating.
The anticorrosive coating film may be applied to the surface of a steel material at the time of product shipment for primary rust prevention, or may be applied after being installed as a structure. Examples of the coating for primary rust prevention include zinc rich primers specified in JIS K 5552.
Here, examples of the coating film on the surface of the steel material after being installed as a structure include a coating film having an anticorrosion base layer, an undercoat layer, an intermediate coat layer, and a topcoat layer in this order.
Examples of the anticorrosion base layer include inorganic zinc rich paint (for example, manufactured by Kansai Paint Co., Ltd .: SD zinc 1500) defined by JIS K 5535. The undercoat layer is, for example, manufactured by Kansai Paint Co., Ltd .: Epomarin HB (K), the intermediate coat layer is, for example, manufactured by Kansai Paint Co., Ltd .: CERATEC F middle coat, and the top coat layer is manufactured by, for example, Kansai Paint Co., Ltd. : Seratecto F (K) Topcoat can be given. The coating film on the surface of the steel material may be a coating film having a primer layer, an undercoat layer, an intermediate coat layer, and a topcoat layer in this order, or a coating film having an anticorrosion base layer, an undercoat layer, and a topcoat layer in this order.

When manufacturing outdoor structures such as bridges, after shipping the product (steel material), the zinc rich primer coating film for primary rust prevention is removed at the site by blasting, etc., and then the above anticorrosive coating is applied. It is common.

Further, in the structural steel material of the present invention, molten steel having the above composition is made into a slab by ordinary continuous casting or a slabbing method, and this slab is manufactured into thick plates, shaped steels, thin steel plates, steel bars, etc. by hot rolling. It is obtained by doing.

In particular, the steel material produced by the following production method has characteristics of yield strength or 0.2% proof stress of 335 MPa or more and tensile strength of 490 MPa or more.
The slab heating temperature may be appropriately determined. However, heating the slab above 1300 ° C. leads to a decrease in yield and an increase in energy consumption due to excessive scale generation. On the other hand, deterioration of toughness due to coarsening of crystal grains becomes a problem. Further, when the slab is heated at a temperature lower than 1000 ° C., the deformation resistance of the slab increases, and the rolling load in the subsequent rolling step may increase, which may make rolling difficult. Therefore, the slab heating temperature is in the range of 1000 to 1300 ° C. It is preferably in the range of 1050 to 1250 ° C. More preferably, it is in the range of 1080 to 1200 ° C.

The heated slab is made into a steel material having a desired size and shape after hot rolling is completed at a temperature of (Ar ₃ points −40 ° C.) or higher. If the finish rolling temperature is less than (Ar ₃ points −40 ° C.), a large amount of ferrite phase is generated, and the desired volume fraction of the hard phase cannot be obtained. Therefore, the finish rolling temperature is set to (Ar ₃ points −40 ° C.) or higher. Ar ₃ points or more are preferable.
Further, the rolling reduction in the temperature range from the slab heating temperature in hot rolling to 850 ° C. (= ([thickness of slab before start of hot rolling]-[thickness of material to be rolled at 850 ° C.]) ÷ [hot rolling Rolling is performed so that 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 that ferrite having an average particle size of 5 μm to 50 μm can be obtained.
The Ar ₃ points may be measured by a known method, but in the present application, the value obtained by the formula (1) described in "Iron and Steel Vol. 67 (1981) p147" shown below is used. Using.
Ar ₃ points (℃)
= 910-310 x C-80 x Mn-20 x Cu-55 x Ni-80 x Mo
However, the element in the formula indicates the content (mass%) in the steel material.

After the hot rolling is completed, the mixture is cooled to room temperature at a cooling rate of 0.1 ° C./s or higher by, for example, air cooling. By setting the cooling rate to 0.1 ° C./s or higher, the remaining austenite portion can be a hard phase such as bainite or martensite, and a steel material composed of a hard phase and a soft phase can be produced.
Further, in the temperature range from Ar ₃ points to (Ar ₃ points −80 ° C.) to 650 ° C. or lower and 400 ° C. or higher, accelerated cooling may be performed by water cooling or the like at a cooling rate of 5 ° C./s or higher. However, if accelerated cooling is performed in a temperature range of less than 400 ° C., the steel material tends to warp, and there is a concern that the manufacturing cost will increase due to straightening or the like. Therefore, when accelerating cooling is performed, the cooling shutdown temperature of the accelerated cooling is preferably 400 ° C. or higher.

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

(Example 1)
Steel having the component composition shown in Table 1 (the balance is Fe and unavoidable impurities) is melted to form a slab (thickness 250 mm) by continuous casting, and then hot-rolled under various conditions shown in Table 2 at room temperature. After cooling to, a steel sheet having a thickness of 15 to 80 mm after hot rolling was obtained. In addition, No. which performed accelerated cooling. In 2, 9, 17 to 20, after the acceleration cooling was completed, air cooling was performed so that the cooling rate became 0.1 ° C./s or more, and the mixture was cooled to room temperature.
Corrosion test, microstructure observation, tensile test and fatigue crack propagation test were carried out on the obtained steel sheet under the following conditions. The results are shown in Table 3.

-Corrosion test A 70 mm × 50 mm × 5 mm test piece was collected from the steel sheet 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, then ultrasonic degreasing in acetone was performed for 5 minutes, and the test piece was air-dried. Next, one side of the test piece is used as a painted surface, SD zinc 1500 (thickness: 75 μm) is applied as an inorganic zinc rich paint as an anticorrosion base, and then Epomarin HB (K) (thickness: 120 μm) is applied as an epoxy resin paint as an undercoat. Then, apply Ceratecto F middle coat paint (thickness: 30 μm) as an intermediate coat, and then apply Ceratecto F (K) top coat paint (thickness: 25 μm) as a top coat, and then apply the anticorrosion base layer, undercoat layer, etc. A coating film consisting of an intermediate coating layer and a top coating layer was formed. The other side and end face of the test piece were sealed with a solvent-type epoxy resin paint, and further coated with a silicone-based sealant.

After the above coating, a straight line cut having a width of 1 mm and a length of 40 mm was made in the central portion of the coating film formed on the test piece so as to reach the base metal, and an initial defect was provided. Then, a corrosion test was carried out under the conditions shown below.
That is, a solution of artificial sea salt diluted to a predetermined concentration with pure water is sprayed so that the amount of artificial sea salt attached to the surface of the test piece is 6.0 g / m ² , and the artificial sea salt is attached to the test piece. I let you. 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 in which each transition time from condition 1 to condition 2 and condition 2 to condition 1 was set to 1 hour, and a total cycle of 8 hours was set as one cycle, and this was repeated 1200 cycles. The artificial sea salt was attached once a week.
Then, after the corrosion test was completed, the swelling width on one side from the initial defective portion of the coating film was measured to evaluate the coating durability.
The results obtained are shown in Table 3. If the swelling width on one side is 6.5 mm or less, it is judged that the durability of the coating is excellent.

・ Structure observation For structure observation (observation with an optical microscope), a sample collected from an arbitrary location is used as a polished sample, and the sample is etched with a 3% nital corrosive solution at a position 1/4 of the plate thickness of the cross section parallel to the rolling direction. It was carried out. The microstructure observation was carried out in 5 visual fields by the method described above, and the volume fraction of the hard phase was obtained as the average value in those total visual fields. In addition, in order to distinguish between the soft phase and the hard phase, 5 points were randomly extracted for each microstructure such as ferrite, martensite, bainite, tempered martensite, and tempered bainite, and the Vickers hardness (HV1) was measured. A phase having a bainite hardness of less than 225 was discriminated as a soft phase, and a phase having a bainite hardness of 225 or more was discriminated as a hard phase.
In addition, No. Except for 15, the area ratio of ferrite in the soft phase was 95% or more.

-Tensile test For steel sheets with a thickness of less than 50 mm, use No. 1A or No. 5 tensile test pieces conforming to JIS Z 2241, and yield strength or 0.2% proof stress of the total thickness and tensile strength. Was evaluated. For steel sheets with a plate 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% proof stress at the position of 1/4 of the plate thickness and the tensile strength. The number of test pieces was two each, and the arithmetic mean was evaluated as the yield strength or 0.2% proof stress and tensile strength of the steel sheet.

・ Fatigue crack propagation test In the fatigue crack propagation test, CT test pieces of total thickness (thickness on one side up to 25 mmt for those with a plate thickness of more than 25 mm) are sampled, and the stress ratio is 0.1, the frequency is 20 Hz, and the temperature is in the atmosphere. It was performed in accordance with ASTM E647. The test was carried out in each of the case where the crack was propagated in the direction perpendicular to the rolling direction and the case where the crack was propagated in the rolling direction.
Then, the fatigue crack propagation velocity at 20 MPa√m (where m indicates the crack length (unit: meter)) in the stress intensity factor range (ΔK) is 5 in both the rolling direction and the rolling direction. A case of 0 × ^10-8 m / cycle or less was regarded as a pass.

From Table 3, in each of the invention examples, the swelling width of the coating film was 6.5 mm or less, and the fatigue crack propagation speed was 5.0 × 10 ^-8 m / cycle or less in both the rolling direction and the rolling direction. It can be seen that both the coating durability and the fatigue crack propagation characteristics are excellent.
On the other hand, in the comparative example, the swelling width of the coating film exceeds 6.5 mm, and the fatigue crack propagation speed exceeds 5.0 × ^10-8 m / cycle, so that sufficient coating durability and fatigue cracks are obtained. No propagation characteristics were obtained.

Claims (6)

By mass%,
C: 0.051 % or more, 0.200% or less,
Si: 0.05% or more, 1.00% or less,
Mn: 0.20% or more, 2.00% or less,
P: 0.003% or more, 0.030% or less,
S: 0.0001% or more, 0.0100% or less,
Al: 0.001% or more, 0.100% or less and W: 0.021 % or more, 1.000% or less, and
One or more selected from Cr, Mo and V,
Cr: 2.00% or less,
Mo: 0.500% or less and V: 0.200% or less and
Cr (%) / 10 + Mo (%) + V (%) ≧ 0.005%
Has a component composition in which the balance is Fe and unavoidable impurities.
The D value represented by the following equation (1) satisfies 0.5 or more and 10 or less.
The microstructure is composed of a hard phase and a soft phase, the volume fraction of the hard phase is 0.30 to 0.80, and the area fraction of ferrite in the soft phase is 95% or more . A structural steel material with an average ferrite grain size of 5 to 43 μm and excellent fatigue crack propagation characteristics and coating durability.
Here, the soft phase is a structure having a Vickers hardness of less than 225, and the hard phase is a structure having a Vickers hardness of 225 or more. The microstructure was observed with an optical microscope at a position of 1/4 of the plate thickness of the cross section parallel to the rolling direction, and the value obtained by dividing the total area of the microstructure classified into the hard phase from the Vickers hardness by the area of the entire observation field. , The volume fraction of the hard phase. The average particle size of ferrite is determined by the line segment method after observing the structure.
Further, Cr (%), Mo (%) and V (%) are the contents (mass%) of Cr, Mo and V in the component composition of the steel material, respectively.
Record
D value: 6 x W + 4 x Cr + 2 x Mo + 3 x V ... (1)
However, the element in the formula indicates the content (mass%) of each element in the steel material. The component composition is further increased by mass%.
Cu: 0.50% or less,
Ni: 0.50% or less,
The structure having excellent fatigue crack propagation characteristics and coating durability according to claim 1, which contains one or more selected from Sn: 0.200% or less and Sb: 0.200% or less. Steel material. The component composition is further increased by mass%.
Ti: 0.050% or less,
Zr: 0.100% or less,
B: 0.0050% or less,
Ca: 0.0100% or less,
The fatigue crack propagation property and coating durability according to claim 1 or 2, which contain one or more selected from Mg: 0.0100% or less and REM: 0.0100% or less, are excellent. Structural steel material. The structural steel material having a coating film on the surface and having excellent fatigue crack propagation characteristics and coating durability according to any one of claims 1 to 3 . The structural steel material having excellent yield crack propagation characteristics and coating durability according to any one of claims 1 to 4 , wherein the yield strength or 0.2% proof stress is 335 MPa or more and the tensile strength is 490 MPa or more. The method for producing a structural steel material having excellent fatigue crack propagation characteristics and coating durability according to any one of claims 1 to 5, wherein the component composition according to any one of claims 1 to 3 is used. The steel slab to have is heated to 1000 ° C. or higher and 1300 ° C. or lower, and then the rolling reduction in the temperature range from the slab heating temperature to 850 ° C.: 25% or higher and the finish rolling temperature: (Ar ₃ points -40 ° C) or higher. A method for producing a structural steel material having excellent fatigue crack propagation characteristics and coating durability, which is cooled at a cooling rate of 0.1 ° C./s or higher after being hot-rolled in.