KR102483143B1 - Structural Steel and Structures - Google Patents

Abstract

The present invention can reduce the frequency of painting even when used in an atmospheric corrosive environment, especially in a severe corrosive environment such as the sea or near the coast with a large amount of flying salt, and also has good primary rust prevention, excellent paint corrosion resistance, It is an object of the present invention to provide structural steel materials having excellent lamellar tear resistance when used in large and complicated structures such as bridges. The structural steel of the present invention has a predetermined component composition and has a Sn segregation degree of 20 or less.

Description

Structural Steel and Structures

The present invention mainly relates to steel materials applied to structures used under land and outdoor atmospheric corrosive environments such as bridges, particularly under severe corrosive environments such as the sea or coast with high airborne salinity.

Steel structures used outdoors, such as bridges, are normally used after some kind of corrosion prevention treatment is performed. For example, in an environment with a small amount of airborne salt, weathering steel is widely used.

Here, when the weathering steel is used in an environment exposed to the atmosphere, its surface is covered with a highly protective rust layer in which alloying elements such as Cu, P, Cr, and Ni are concentrated, thereby reducing the corrosion rate. It is a greatly deteriorated steel. It is known that bridges using such weathering steel can withstand decades of use without coating in an environment with a small amount of airborne salt.

On the other hand, it is difficult to form a highly protective rust layer in weathering steel in an environment with a large amount of flying salt, such as at sea or near the coast, and it is difficult to use weathering steel as it is unpainted. For this reason, in an environment with a large amount of flying salt, such as on the sea or near the coast, steel materials in which anticorrosive treatment such as painting is applied to steel materials are usually used.

However, in coated steel materials, maintenance such as periodic re-coating is required due to deterioration of the coating film over time, generation of rust, swelling of the coating film, and the like. The painting work accompanying re-coating is often a work at a high place, and while the work itself is difficult, the labor cost required for the work is also required. For this reason, when using painted steel materials, there exists a problem that the maintenance cost of a structure increases, and also the life cycle cost increases.

In addition, for the purpose of preventing corrosion from occurring during the period until the structure is manufactured and while the steel is being stored, rust prevention treatment (primary rust prevention treatment) such as zinc-rich primer is generally applied. all. However, when the storage period of steel materials is long or when the storage location of steel materials is close to the sea and the amount of flying salt is high, corrosion may progress even if the above rust prevention treatment is performed.

From the above points, it is possible to extend the period of re-coating painting, that is, to reduce the frequency of painting, thereby suppressing the maintenance cost of the structure, and further improving the primary rust resistance. Development of this excellent structural steel is desired.

As a technology related to steel materials excellent in corrosion resistance, for example, in Patent Document 1,

"In mass %, C: 0.001 to 0.15%, Si: 2.5% or less, Mn: more than 0.5%, 2.5% or less, P: less than 0.03%, S: 0.005% or less, Cu: 0.05 to 1.0%, Ni: 0.01 to 0.5%, Cr: 0.01 to 3.0%, Al: 0.003 to 2.5%, N: 0.001 to 0.1%, Sn and/or Sb: 0.03 to 0.50%, the remainder being Fe and impurities, , Steel with excellent seaside weather resistance, characterized by having a composition in which the Ni / Cu mass ratio is 0.5 or less.”

is disclosed.

In Patent Document 2,

“In terms of mass%, C: 0.001 to 0.15%, Si: 2.5% or less, Mn: more than 0.5% to 2.5% or less, P: less than 0.03%, S: 0.005% or less, Cu: less than 0.05%, Ni: 0.05 %, Cr: 0.01 to 3.0%, Al: 0.003 to 0.1%, N: 0.001 to 0.1%, and Sn: 0.03 to 0.50%, the remainder being Fe and impurities, and a Cu/Sn ratio of 1 or less. After heating the surface temperature of the slab having a surface temperature of 1050 to 1200 ° C, rolling is performed at 70% or more of the total reduction amount in a temperature range of 900 ° C or higher, and after rolling is finished in a temperature range of 800 ° C or higher, cooling is performed. A method for producing steel materials with excellent corrosion resistance and toughness in the Z direction.”

This is disclosed.

In Patent Document 3,

"In mass %, C: 0.01 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.05 to 3.0%, P: 0.05% or less, S: 0.01% or less, Sn: 0.01 to 0.5%, Cr: 1.0% A steel material with excellent corrosion resistance, characterized in that it contains 13.0% or less of Al and 0.1% or less of Al, the balance consists of Fe and impurities, and the ratio of solid solution Sn in Sn is 95.0% or more.”

is disclosed.

In Patent Document 4,

"In mass %, C: 0.01 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.05 to 3.0%, P: 0.05% or less, S: 0.01% or less, Sn: 0.01 to 0.5%, Al: 0.1% or less A steel material excellent in corrosion resistance, characterized in that it contains Fe and impurities, and the ratio of solid solution Sn in Sn is 95.0% or more.”

is disclosed.

In Patent Document 5,

“Chemical composition, in mass%, C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.05 to 3.00%, Sn: 0.01 to 0.50%, O: 0.0001 to 0.0100%, Cu: 0 to 0.10% less than, Cr: 0 to less than 0.10%, Mo: 0 to less than 0.050%, W: 0 to less than 0.050%, Cu+Cr: 0 to less than 0.10%, Mo+W: 0 to less than 0.050%, Sb: 0 to less than 0.05%, Ni: 0 to 0.05%, Nb: 0 to 0.050%, V: 0 to 0.050%, Ti: 0 to 0.020%, Al: 0 to 0.100%, Ca: 0 to less than 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, P: 0.05% or less, S: 0.01% or less, balance: Fe and impurities; It has a soft structure that is ferrite and a hard structure that is pearlite, bainite, and martensite; Sn concentration ratio, which is the ratio of the Sn concentration in the hard tissue to the Sn concentration in the soft tissue, is 1.2 or more and less than 6.0; Steel material characterized by that.”

is disclosed.

Japanese Unexamined Patent Publication No. 2006-118011 Japanese Laid-Open Patent Publication No. 2010-7109 Japanese Unexamined Patent Publication No. 2013-166992 Japanese Unexamined Patent Publication No. 2012-255184 Japanese Patent No. 5839151

WES3008-1999

However, when a large amount of a component that improves corrosion resistance, such as Cr, is contained, performance other than corrosion resistance may be deteriorated.

For example, in the techniques of Patent Literatures 1 to 3, when the content of Cr is increased, the alloy cost is increased and the toughness of steel materials is deteriorated.

Furthermore, in recent years, in structures such as bridges, the risk of lamellar tear generation has been pointed out. Here, in welded joints that receive tensile stress in the plate thickness direction, such as lamellar tiers, cross joints, T-joints, and corner joints, cracking occurs inside the steel material in a direction parallel to the steel plate surface due to tensile stress This is a phenomenon that progresses and cracks occur.

Regarding the occurrence of such a lamellar tear, for example, in Non-Patent Document 1, the relationship between the drawing value (reduction of area) in the plate thickness direction and the amount of S in steel is shown, and by reducing the amount of S in steel, the plate thickness It is disclosed that the drawing value in the direction is improved and, by extension, the lamellar tear resistance is improved.

However, with the increase in size and complexity of structures in recent years, in such an enlarged and complicated structure, there are many cases in which a large tensile stress is received in the plate thickness direction by welded joints having strict constraint conditions in the constituent members thereof. there is.

In such a case, it cannot be said that sufficient lamellar tear resistance is necessarily obtained only by reducing the amount of S in the steel. In addition, as disclosed in Patent Literatures 1 to 5, the effect of various elements added for the purpose of improving corrosion resistance on lamellar tear resistance is not known. For this reason, when the steel materials with improved corrosion resistance disclosed in Patent Documents 1 to 5 are applied to such a large and complex structure, there is a concern about the occurrence of lamellar tear.

The present invention was developed in view of the above phenomenon, and it is possible to reduce the frequency of painting even when used in an atmospheric corrosive environment, particularly in a severe corrosive environment such as the sea or near the coast with a high airborne salt content, and also 1 It is an object of the present invention to provide a structural steel material having excellent paint corrosion resistance with good secondary rust resistance and excellent lamellar tear resistance when used for large and complex structures such as bridges.

In addition, an object of the present invention is to provide a structure formed by using the structural steel materials described above.

Then, inventors repeated earnest research for the solution of the said subject, and obtained the following recognition.

(1) In order to improve corrosion resistance, in particular coating corrosion resistance, it is effective to add one or a combination of two or more selected from among Cu, Ni, W, Sb, and Si together with Sn.

(2) On the other hand, from the viewpoint of lamellar tear resistance, it is effective to reduce Sn while reducing the amount of S in steel.

(3) In this way, although the addition of Sn is effective from the viewpoint of improving paint corrosion resistance, reducing Sn is effective from the viewpoint of lamellar tear resistance. Then, the inventors repeated examination so that corrosion resistance and lamellar tear resistance could be made compatible further based on said recognition.

As a result,

(4) By suppressing central segregation of Sn and maximally dispersing Sn throughout the steel material, excellent lamellar tear resistance can be obtained even when Sn is contained in a predetermined amount, that is, the amount of Sn is appropriately adjusted from the viewpoint of improving the corrosion resistance of the coating. While adjusting, suppressing the central segregation of Sn and diffusing Sn to the entire steel material, it is possible to achieve both coating corrosion resistance and lamellar tear resistance.

has gained recognition

(5) In addition, by maximally suppressing the thickness of the Sn segregation portion at a concentration equal to or greater than a predetermined level in the plate thickness direction, lamellar tear resistance is further improved,

(6) Further, by strictly controlling the amount of Sn according to the amount of S, the lamellar tear resistance is further improved.

has gained recognition

The present invention was completed through further examination based on the above recognition.

That is, the gist of the present invention is as follows.

1. In mass %,

C: 0.020% or more and 0.200% 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

Sn: 0.005% or more and 0.200% or less

With containing

Cu: 0.010% or more and 0.50% or less;

Ni: 0.010% or more and 0.50% or less;

W: 0.005% or more and 1.000% or less;

Sb: 0.005% or more and 0.200% or less; and

Si: 0.05% or more and 1.00% or less

It contains one or two or more selected from, and has a component composition with the balance consisting of Fe and unavoidable impurities,

Structural steel with a Sn segregation degree of 20 or less.

Here, Sn segregation degree is defined by following formula (1).

[Sn segregation degree] = [Sn concentration of Sn segregation part] / [average Sn concentration] --- (1)

2. The structural steel according to 1 above, wherein the thickness of the Sn segregation portion in the plate thickness direction is 50 μm or less.

3. The structural steel according to 1 or 2 above, wherein the ST value defined by the following formula (2) is 1.50 or less.

ST=10000×[%S]×[%Sn] ² ---(2)

Here, [%S] and [%Sn] are the contents (mass%) of S and Sn in the above component composition, respectively.

4. The component composition, further, in mass%,

Mo: 0.500% or less and

Co: 1.00% or less

The structural steel according to any one of 1 to 3 above, containing one or two selected from

5. The component composition, further, in mass%,

Ti: 0.050% or less;

V: 0.200% or less;

Nb: 0.200% or less and

Zr: 0.100% or less;

The structural steel according to any one of 1 to 4 above, containing one or two or more selected from

6. The component composition, further, in mass%,

B: The structural steel according to any one of 1 to 5 above, containing 0.0050% or less.

7. The component composition, further, in mass%,

Ca: 0.0100% or less and

Mg: 0.0100% or less;

The structural steel according to any one of 1 to 6 above, containing one or two selected from

8. Structural steel materials according to any one of 1 to 7 above, having a coating film on the surface.

9. The coating film has an anticorrosive base layer, an undercoat layer, an intermediate coating layer and a top coat layer,

The anticorrosion base layer uses inorganic zinc-rich paint, the first coat layer uses an epoxy resin paint, the middle coat layer uses a fluororesin top coat paint, and the top coat layer uses a fluororesin top coat paint. Structural steel materials according to 8 above.

10. The structural steel according to any one of 1 to 7 above, having a zinc-rich primer layer on the surface.

11. A structure formed using the structural steel materials described in any one of 1 to 10 above.

12. The structure described in 11 above, which is a bridge.

According to the present invention, even when used in an atmospheric corrosive environment, particularly in a severe corrosive environment such as at sea or near the coast with high airborne salt content, it is possible to extend the re-coating cycle and reduce the coating frequency, and also has resistance to lamellar tearing. Excellent structural steel can be obtained.

And, by using the structural steel of the present invention for structures such as bridges used in outdoor atmospheric corrosive environments such as bridges, especially in severe corrosive environments such as at sea or near the coast with high airborne salt content, the maintenance cost of these structures, Furthermore, it becomes possible to reduce the life cycle cost, and furthermore, it becomes possible to effectively prevent the occurrence of lamellar tear and ensure high safety even for large and complex structures.

(Mode for implementing the invention)

Hereinafter, the present invention will be described in detail. First, the component composition in one embodiment of the structural steel of the present invention will be described. In addition, the unit of content of an element in a component composition is all "mass %", but below, unless otherwise noted, it is simply expressed as "%".

C: 0.020% or more and 0.200% or less

C is an element that increases the strength of steel materials. For this reason, 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, when the C content exceeds 0.200%, weldability and toughness deteriorate. Therefore, the C content is 0.020% or more and 0.200% or less. Preferably, it is 0.040% or more. Also, preferably, it is 0.180% or less.

Mn: 0.20% or more and 2.00% or less

Mn is an element that increases the strength of steel materials. For this reason, 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, when the Mn content exceeds 2.00%, toughness and weldability deteriorate. Therefore, the Mn content is 0.20% or more and 2.00% or less. Preferably it is 0.75% or more. Moreover, it is preferably 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 corrosion resistance of steel materials. From the viewpoint of obtaining such an effect, it is necessary to contain 0.003% or more of P. On the other hand, when P content exceeds 0.030 %, weldability deteriorates. Therefore, the P content is made 0.003% or more and 0.030% or less.

S: 0.0001% or more and 0.0100% or less

S is an important element involved in lamellar tear resistance. That is, when the amount of S increases, coarse MnS is formed, and this becomes the starting point of the lamellar tear. For this reason, S content needs to be 0.0100% or less. However, if the S content is to be less than 0.0001%, the production cost increases. Therefore, the S content is made 0.0001% or more and 0.0100% or less. Preferably it is 0.0080% or less, More preferably, it is 0.0060% or less.

Al: 0.001% or more and 0.100% or less

Al is an element required for deoxidation during steelmaking. In order to obtain these effects, it is necessary to contain 0.001% or more of Al. On the other hand, when the Al content exceeds 0.100%, weldability is adversely affected. Therefore, the Al content is set to 0.001% or more and 0.100% or less. Preferably it is 0.005% or more, More preferably, it is 0.010% or more. Moreover, it is preferably less than 0.050%, and more preferably less than 0.030%.

Sn: 0.005% or more and 0.200% or less

While having an effect of improving durability of a coating film, Sn is an important element involved in lamellar tear resistance, in other words, an element that reduces lamellar tear resistance while improving coating corrosion resistance.

That is, Sn exists in the rust layer near the surface of the steel substrate, and by miniaturizing the rust particles, chloride ions, which are corrosion promoting factors, are prevented from penetrating the rust layer and reaching the base iron. Also, Sn suppresses the anode reaction on the steel material surface. In order to fully acquire these effects, it is necessary to make the Sn content 0.005% or more. Preferably it is 0.010% or more, More preferably, it is 0.020% or more.

However, Sn tends to segregate at the central portion of the sheet thickness, and in such a Sn segregation portion, a hard, brittle structure is formed, and the structure becomes a starting point of fracture, deteriorating lamellar tear resistance. Therefore, from the viewpoint of ensuring lamellar tear resistance, the content of Sn is set to 0.200% or less. Preferably it is 0.100% or less, More preferably, it is less than 0.050%.

Further, from the viewpoint of improving the coating corrosion resistance of steel materials, it is necessary to add one or two or more compounds selected from Cu, Ni, W, Sb, and Si together with Sn. That is, as described above, Sn improves paint corrosion resistance, but cannot be contained in a large amount from the viewpoint of lamellar tear resistance. For this reason, from the viewpoint of improving the paint corrosion resistance of steel materials while ensuring good lamellar tear resistance, it is necessary to add one or more selected from among Cu, Ni, W, Sb, and Si in combination with Sn. Do.

Cu: 0.010% or more and 0.50% or less

Cu forms a dense rust layer by refining rust particles in the rust layer, and has an effect of suppressing permeation of oxygen and chloride ions, which are corrosion promoting factors, into the base iron. These effects are obtained when the Cu content is 0.010% or more. On the other hand, when the Cu content exceeds 0.50%, an increase in alloy cost is caused. Therefore, the content for obtaining the effect of adding Cu is 0.010% or more and 0.50% or less. It is preferably 0.030% or more, more preferably 0.040% or more, and still more preferably 0.050% or more. Further, it is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.25% or less.

Ni: 0.010% or more and 0.50% or less

Ni forms a dense rust layer by minimizing the rusting of the rust layer, and has an effect of suppressing the permeation of oxygen and chloride ions, which are corrosion promoting factors, into the base iron. These effects are obtained when the Ni content is 0.010% or more. On the other hand, when the Ni content exceeds 0.50%, an increase in alloy cost is caused. Therefore, the content for obtaining the effect of adding Ni is 0.010% or more and 0.50% or less. It is preferably 0.030% or more, more preferably 0.040% or more, and still more preferably 0.050% or more. Further, it is preferably less than 0.40%, more preferably 0.30% or less, and still more preferably 0.15% or less.

W: 0.005% or more and 1.000% or less

W elutes with the anode reaction of the steel material and is distributed as WO ₄ ^2- in the rust layer, thereby electrostatically preventing chloride ions of corrosion-accelerating factors from penetrating the rust layer and reaching the base iron. Moreover, the anode reaction of steel materials is suppressed by the compound containing W precipitating on the surface of steel materials. These effects are obtained when the W content is 0.005% or more. On the other hand, when the W content exceeds 1.000%, an increase in alloy cost is caused. Therefore, the content for obtaining the effect of adding W is 0.005% or more and 1.000% or less. It is preferably 0.010% or more, more preferably 0.030% or more, and still more preferably 0.050% or more. Further, it is preferably 0.700% or less, more preferably 0.500% or less, and still more preferably 0.300% or less.

Sb: 0.005% or more and 0.200% or less

Sb exists in the rust layer near the surface of the base iron, and suppresses chloride ions, which are corrosion promoting factors, from penetrating the rust layer and reaching the base iron by miniaturizing the rust particles. Also, Sb suppresses the anode reaction on the surface of steel materials. These effects are obtained when the Sb content is 0.005% or more. On the other hand, when the Sb content exceeds 0.200%, deterioration of ductility and toughness of steel is caused. Therefore, the content for obtaining the effect of adding Sb is 0.005% or more and 0.200% or less. Preferably it is 0.010% or more, More preferably, it is 0.020% or more. Moreover, it is preferably 0.150% or less, more preferably 0.100% or less.

Si: 0.05% or more and 1.00% or less

Si has an effect of minimizing the rust of the entire rust layer to form a dense rust layer and improving the coating corrosion resistance of steel materials. In addition, Si is an element required for deoxidation during steelmaking. These effects are obtained when the Si content is 0.05% or more. On the other hand, when Si content exceeds 1.00%, toughness and weldability deteriorate remarkably. Therefore, the content for obtaining the effect of adding Si is 0.05% or more and 1.00% or less. Preferably it is 0.15% or more. Also, it is preferably 0.80% or less.

From the viewpoint of further improving the paint corrosion resistance of steel materials, the Si content is more preferably 0.40% or more and 0.60% or less.

As mentioned above, although the basic component was demonstrated, the element described below can be contained suitably as needed.

Mo: 0.500% or less

Mo elutes with the anode reaction of steel materials, and MoO ₄ ^2- distributes in the rust layer, thereby suppressing chloride ions, which are corrosion-accelerating factors, from permeating the rust layer and reaching the base iron. Moreover, the anode reaction of steel materials is suppressed by the compound containing Mo precipitating on the surface of steel materials. However, when the Mo content exceeds 0.500%, an increase in alloy cost is caused. Therefore, when containing Mo, Mo content is made into 0.500% or less. In order to obtain the above effect, the Mo content is preferably 0.005% or more.

Co: 1.00% or less

Co is distributed throughout the entire rust layer and forms a dense rust layer by minimizing the rust, thereby having an effect of improving the weather resistance of the steel material. However, when the Co content exceeds 1.00%, an increase in alloy cost is caused. Therefore, when Co is contained, Co content is made into 1.00% or less. Preferably it is 0.50% or less, More preferably, it is 0.35% or less. In order to obtain the above effect, the Co content is preferably 0.01% or more, more preferably 0.03% or more, and still more preferably 0.10% or more.

Ti: 0.050% or less

Ti is an element that increases strength. However, when the Ti content exceeds 0.050%, deterioration in toughness may be caused. Therefore, when it contains Ti, Ti content is made into 0.050 % or less. More preferably, it is 0.030% or less. In order to obtain the above effect, the Ti content is preferably 0.001% or more, more preferably 0.005% or more.

V: 0.200% or less

V is an element that increases strength. However, when the V content exceeds 0.200%, the effect is saturated. Therefore, when V is contained, the V content is made 0.200% or less. Further, in order to obtain the above effect, the V content is preferably 0.005% or more.

Nb: 0.200% or less

Nb is an element that increases strength. However, when the Nb content exceeds 0.200%, deterioration in toughness may be caused. Therefore, when Nb is contained, Nb content is made into 0.200% or less. In order to obtain the above effect, the Nb content is preferably 0.005% or more.

Zr: 0.100% or less

Zr is an element that increases strength. However, when the Zr content exceeds 0.100%, the effect is saturated. Therefore, when Zr is contained, the Zr content is made 0.100% or less. In order to obtain the above effects, the Zr content is preferably 0.005% or more.

B: 0.0050% or less

B is an element that increases strength. However, when the B content exceeds 0.0050%, deterioration in toughness may be caused. Therefore, when B is contained, the B content is made 0.0050% or less. In order to obtain the above effect, the B content is preferably 0.0001% or more.

Ca: 0.0100% or less

Ca is an element that fixes S in steel and improves the toughness of the weld heat-affected zone. However, when the Ca content exceeds 0.0100%, the amount of inclusions in the steel increases, resulting in deterioration of toughness on the contrary. Therefore, when Ca is contained, Ca content is made into 0.0100% or less. In order to obtain the above effect, the Ca content is preferably 0.0001% or more.

Mg: 0.0100% or less

Mg is an element that fixes S in steel and improves the toughness of the weld heat-affected zone. However, when the Mg content exceeds 0.0100%, the amount of inclusions in the steel increases, resulting in deterioration of toughness on the contrary. Therefore, when Mg is contained, Mg content is made into 0.0100% or less. In order to obtain the above effects, the Mg content is preferably 0.0001% or more.

Components other than the above are Fe and unavoidable impurities. Incidentally, N and O (oxygen) are exemplified as unavoidable impurities, and N: 0.010% or less and O: 0.010% or less are permissible.

In addition, in the structural steel of the present invention, it is very important to control the Sn segregation degree as follows.

Sn segregation degree: 20 or less

As described above, Sn tends to segregate at the center of the plate thickness. In this way, in the portion where Sn is segregated (hereinafter, also referred to as Sn segregation portion), an embrittled structure with high hardness is generated, and the structure becomes a starting point of fracture. As a result, the internal lamellar tear of the steel sheet lower the gender Therefore, in order to ensure excellent lamellar tear resistance, it is important to suppress the central segregation of Sn, in other words, to reduce the Sn segregation degree defined by the following formula (1). For this reason, Sn segregation degree is made into 20 or less. Preferably it is 18 or less. More preferably, it is 15 or less. More preferably, it is 12 or less. Since Sn segregation is preferably less, the lower limit is not particularly limited, but is preferably 1, more preferably 5.

[Sn segregation degree] = [Sn concentration of Sn segregation part] / [average Sn concentration] --- (1)

Here, the Sn segregation degree, more specifically, in a cross section cut parallel to the rolling direction of the steel material (cross section perpendicular to the steel material surface), of an electron beam microanalyzer (hereinafter referred to as EPMA) It is the ratio of the maximum Sn concentration of the Sn segregation part to the average Sn concentration obtained by line analysis.

That is, when the thickness of the steel is t (mm) and the width (direction perpendicular to the rolling direction and thickness direction of the steel) is W (mm), first, a cross section cut parallel to the rolling direction of the steel (on the surface of the steel) Vertical cross-section) in the thickness direction of the steel: (0.5±0.1) × t, in the rolling direction: 15 mm surface area (ie, surface area including the central position in the thickness direction of the steel material), beam diameter: 20 μm , EPMA surface analysis of Sn is performed under the condition of pitch: 20 μm. In addition, EPMA surface analysis of Sn is performed in three cross-sectional fields of view at positions of 1/4×W, 1/2×W, and 3/4×W.

Then, from the above EPMA surface analysis, the position where the Sn concentration is highest in each cross-sectional field of view is selected, and at that position, in the thickness direction of the steel material, under the conditions of a beam diameter: 5 μm and a pitch: 5 μm, EPMA of Sn Conduct line analysis. In addition, in the implementation of the EPMA line analysis, regions up to 25 μm from the front and back surfaces of the steel materials are excluded.

Next, the maximum value of the Sn concentration (mass concentration) is obtained for each measurement line, and the average value thereof is taken as the Sn concentration (mass concentration) of the Sn segregation portion. And the value which divided the Sn concentration of this Sn segregation part by the average Sn concentration (mass concentration) which is the arithmetic mean value of all the measured values of the measurement line is set as Sn segregation degree.

Thickness in the plate thickness direction of the Sn segregation part: 50 μm or less

In addition, by suppressing the thickness of the Sn segregation portion in the plate thickness direction as much as possible, the lamellar tear resistance is further improved. For this reason, the thickness of the Sn segregation portion is preferably set to 50 µm or less. More preferably, it is 40 μm or less, and still more preferably 30 μm or less. The lower limit is not particularly limited, and may be 0 µm.

In addition, the Sn segregation part referred to here is a region where the ratio of the Sn concentration (mass concentration) to the average Sn concentration (mass concentration) obtained from the above EPMA line analysis is 5 or more.

In addition, the thickness of the Sn segregation portion is obtained by averaging the thicknesses in the plate thickness direction in the above region obtained for each of the above measurement lines.

ST value: 1.50 or less

Moreover, lamellar tear resistance can further be improved by making ST value defined by following formula (2) into 1.50 or less. More preferably, it is 1.20 or less. The lower limit is not particularly limited, but is about 0.0000005.

ST=10000×[%S]×[%Sn] ² ---(2)

Here, [%S] and [%Sn] are the contents (mass%) of S and Sn in the above component composition, respectively.

In addition, the structural steel of one embodiment of the present invention is used by painting the surface of the steel. Here, the coating film on the surface of the steel material is not particularly limited, but examples include a coating film having an anticorrosive base layer, a priming layer, an intermediate coating layer, and a top coating layer in this order.

In addition, the anticorrosive base layer is inorganic zinc rich paint (eg SD zinc 1500), the first coat layer is an epoxy resin paint (eg Epomarine HB (K)), and the middle coat layer is an intermediate for fluorine resin finish paint It is preferable to form the coating material (for example, Cellatect F intermediate coating) and the topcoat layer using a fluororesin topcoat paint (for example, Cellatect F (K) topcoat).

In addition, at the time of product shipment, it is preferable to form a zinc-rich primer layer on the surface of steel materials for the purpose of primary rust prevention.

Note that the zinc-rich primer layer is a primer layer formed using a zinc-rich primer defined in JIS K 5552 (2002).

Next, a manufacturing method according to an embodiment of the structural steel described above will be described.

That is, the steel prepared with the above component composition is melted using a known refining process such as a converter, an electric furnace, or vacuum degassing, and a continuous casting method or an ingot-bulge rolling method It is manufactured by making a steel material (slab) by (ingot casting-blooming), then reheating this steel material as needed and then hot rolling it to make it into a steel plate or a shaped steel.

In addition, the thickness of the steel is not particularly limited, but is preferably 2 to 100 mm. More preferably, it is 3 mm or more, and still more preferably 4 mm or more. Moreover, it is more preferably 80 mm or less, and still more preferably 60 mm or less.

However, as described above, in order to obtain excellent lamellar tear resistance, it is very important to suppress central segregation of Sn, specifically, to control the Sn segregation degree to 20 or less. Here, even if the component composition is the same, Sn segregation degree changes greatly depending on manufacturing conditions. For this reason, in order to suppress center segregation of Sn, it is important to appropriately control manufacturing conditions, in particular, casting conditions and hot rolling.

That is, in the case of continuous casting, a light reduction method in which a cast steel having an unsolidified layer at the end of solidification is cast while gradually being reduced by a reduction roll group at a total reduction amount and a reduction speed corresponding to the sum of the amount of solidification shrinkage and the amount of heat shrinkage. It is desirable to do

In this case, the casting speed (drawing speed) is preferably 0.50 to 2.80 m/min.

Here, when the casting speed is less than 0.50 m/min, the working efficiency deteriorates. In addition, solidification is completed before the cast steel reaches the light reduction rolling zone, and the rolling in the unsolidified layer cannot be sufficiently performed, and the effect of suppressing Sn segregation by the light reduction method is not sufficiently obtained. , central segregation of Sn is promoted. More preferably, it is 0.70 m/min or more, More preferably, it is 0.80 m/min or more.

On the other hand, when the casting speed exceeds 2.80 m/min, uneven surface temperature occurs, and supply of molten steel to the inside of the cast steel becomes insufficient, which promotes central segregation of Sn. In addition, the position where solidification is completely completed becomes the position past the lower pressure zone, and the effect of suppressing the segregation of Sn by the light pressure lowering method is not sufficiently obtained, and the central segregation of Sn is promoted. More preferably, it is 2.50 m/min or less, More preferably, it is 1.20 m/min or less.

In addition, when hot-rolling the steel raw material into desired dimensions and shapes, it is preferable to heat at a temperature of 1000°C to 1350°C. That is, since the diffusion of Sn in the central segregation portion is promoted as the heating temperature is higher, it is advantageous from the viewpoint of ensuring lamellar tear resistance. From this point of view, the heating temperature is preferably 1000°C or higher. However, when the heating temperature exceeds 1350°C, surface flaws occur, or scale loss and fuel consumption rate increase. Therefore, the upper limit of the heating temperature is preferably 1350°C.

In addition, at the heating temperature, it is preferable to soak so that the temperature difference between the surface layer and the center of the steel material (slab) is 50 ° C or less. As a result, the diffusion of Sn in the central segregation portion is sufficiently promoted. For this reason, it is preferable to set the soaking time at the heating temperature to 30 min or more. More preferably, it is 60 min or more. More preferably, it is 90 min or more. The upper limit is not particularly limited, but is preferably 1000 min.

In addition, when the temperature of the raw material is originally in the range of 1000 to 1350 ° C. and has been maintained in that temperature range for 30 minutes or more, it may be subjected to hot rolling as it is without heating. Alternatively, the hot-rolled sheet obtained after hot rolling may be subjected to reheating treatment, pickling, and cold rolling to obtain a cold-rolled sheet having a predetermined sheet thickness.

Moreover, in hot rolling, it is preferable to make a reduction ratio into 3.0 or more. By setting the reduction ratio to 3.0 or more, the thickness of the Sn segregation portion in the plate thickness direction is reduced. More preferably, it is 3.2 or more, More preferably, it is 4.0 or more. The upper limit of the reduction ratio is preferably about 60. In addition, it is preferable to set the finish rolling end temperature to 650°C or higher. If the finish rolling end temperature is less than 650°C, the rolling load increases due to the increase in deformation resistance, making it difficult to perform rolling. The upper limit of the finishing temperature is preferably 950°C.

In addition, although any method of air cooling or accelerated cooling may be used for cooling after hot rolling, when higher strength is desired, it is preferable to perform accelerated cooling.

Here, when accelerated cooling is performed, it is preferable to set the cooling rate to 2 to 100°C/s and the cooling stop temperature to 700 to 400°C. That is, if the cooling rate is less than 2°C/s and/or the cooling stop temperature is more than 700°C, the effect of accelerated cooling is small, and sufficient high strength may not be achieved. Moreover, from a viewpoint of facility capacity, it is preferable that a cooling rate be 100 degrees C/s or less. In addition, if the cooling stop temperature is less than 400°C, the toughness of steel materials may decrease or deformation may occur in the shape of steel materials. Further, when the cooling stop temperature is less than 400°C, it is preferable to perform tempering heat treatment in a temperature range of 400°C to 700°C in the post-process.

Example

Steel having the component composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted and continuously cast under the conditions shown in Table 2 to obtain a steel slab. In addition, continuous casting was performed by the light pressure method. Then, under the conditions shown in Table 2, these steel slabs were reheated, cracked, and then subjected to hot rolling to obtain various steel plates. In addition, cooling after hot rolling was made into air cooling to room temperature.

Then, the Sn segregation degree and the thickness of the Sn segregation portion were determined in the obtained steel sheet by the method described above. The results are listed together in Table 2.

In addition, when the Sn segregation degree is less than 5, all descriptions in the columns of the Sn segregation degree and the thickness of the Sn segregation portion in Table 2 are set to "-".

(1) Evaluation of coating corrosion resistance

Moreover, about the steel plate obtained by the above, the coating corrosion resistance was evaluated in the following way.

That is, a test piece of 70 mm × 50 mm × 5 mm was sampled from the steel sheet obtained as described above. The surface of this test piece was shot blasted so that the rust removal degree Sa specified in JIS Z 0313 (2004) was 2.5, ultrasonically degreased in acetone for 5 minutes, and air dried. Next, one side of the test piece was used as a painted surface, and inorganic zinc rich paint (SD zinc 1500A manufactured by Kansai Paint Co., Ltd., thickness: 75 μm) was applied as an anticorrosive base, and then epoxy resin paint (Kansai Paint Co., Ltd.) was applied as a mist coating. Epomalin priming by Kaisha (for mist coating) is applied, then epoxy resin paint (Kansai Paint Co., Ltd. Epomalin HB (K), thickness: 120 μm) is applied as priming, followed by fluororesin top coating as intermediate coating An intermediate coat paint for paint (Kansai Paint Co., Ltd. Cellatekt F intermediate coat paint, thickness: 30 μm) is applied, followed by fluororesin paint topcoat paint (Kansai Paint Co., Ltd. Cellatekt F topcoat) as a topcoat paint, thickness: 25 μm) was applied to form a coating film composed of an anticorrosive layer, a priming layer (including a coating film formed by mist coating), an intermediate coating layer, and a top coating layer. Further, 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 coating, a straight cut with 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 iron, thereby forming an initial defect. Then, based on ISO 16539 2013, a corrosion test was conducted under the conditions shown below.

That is, a solution in which artificial sea salt was diluted with pure water to a predetermined concentration was sprayed so that the amount of artificial sea salt attached to the surface of the test piece was 6.0 g/m 2 , and the artificial sea salt was adhered to the test piece. Next, 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 cycle for a total of 8 hours, in which each transition time from condition 1 to condition 2 and condition 2 to condition 1 is 1 hour, was repeated 1200 cycles. A corrosion test was conducted. In addition, the adhesion of artificial sea salt was made once a week.

After completion of the corrosion test, the swelling width from the initial defective portion in the coating (hereinafter referred to as the coating swelling width) was measured to evaluate the coating corrosion resistance. Here, the coating blistering width is the average value of the coating blistering widths in the width direction of the initial defects described above (the sum of blistering widths on both sides), and specifically, the above initial defects are measured at equal intervals in the longitudinal direction by 10 The paint swelling width in the width direction at the position of the location (the total swelling width of both sides) was obtained, and these were averaged. Moreover, it was judged that it was excellent in paint corrosion resistance as the paint blistering width|variety was 12.0 mm or less.

Here, the reason why it was judged that the coating corrosion resistance was excellent when the coating swelling width was 12.0 mm or less is as follows.

That is, when C5-based coating, which is usually a coating for new bridges, is applied to normal steel, the coating has a lifespan of about 50 years in a mild corrosive environment (general atmospheric corrosion environment). On the other hand, in a severe corrosive environment such as the sea or the coast, the life of the coating is about 30 years. Here, for example, in order to extend the life of the coating from 30 to 50 years in a harsh corrosive environment such as the sea or the coast, it is necessary to suppress the development of swelling of the coating accompanying corrosion of the steel through a coating film defect there is Here, (a) the paint swelling area is represented by a linear expression of the exposure period, (b) the swelling shape is a circle or rectangle starting from a pinhole or the like, and the paint swelling area is 2 Assuming that it is proportional to the power, the coating swelling area after exposure for a certain period of time in a given environment is 60% or less of the coating swelling area when common steel is exposed under the same conditions (in terms of coating swelling width, 77.5% or less), and from a more safe point of view, when it becomes 56.25% or less (about 75% or less in terms of paint swelling width), it can be considered that the life of the paint is extended from 30 to 50 years.

Here, when the above corrosion test was performed by applying C5-based coating to common steel, the coating blistering width was 16.0 mm. judged to be excellent.

In addition, the primary rust prevention property of each steel plate was evaluated in the following way.

That is, in accordance with the salt water spray resistance test described in JIS K 5552 (2002): "Zinc Rich Primer", a zinc rich primer (Kansai Paint Co., Ltd. Production SD zinc 1000) was applied and dried, and then a salt spray test was conducted using these test pieces.

Then, the number of days until red rust was confirmed in the zinc rich primer layer by visual inspection was measured, and the primary rust prevention property was evaluated according to the following evaluation criteria.

In addition, in this evaluation, the test period was longer than the test period prescribed by JIS K 5552 (2002) in order to evaluate the primary rust prevention property of the steel sheet serving as the base of the zinc-rich primer layer.

A (pass, excellent): 120 days or more until red rust is confirmed

B (Pass, particularly excellent): The number of days until red rust is confirmed is 90 days or more and less than 120 days

C (Pass, Excellent): The number of days until red rust is confirmed is 60 days or more and less than 90 days

D (Pass): The number of days until red rust is confirmed is 30 days or more and less than 60 days

E (disqualified): The number of days until red rust is confirmed is less than 30 days

In addition, the lamellar tear resistance was evaluated in the following manner.

(2) Evaluation of lamellar tear resistance

In accordance with JIS G 3199, a tensile test was conducted on the steel sheet obtained in the above manner in the thickness direction (Z direction) of the steel sheet, and a drawing value was calculated. And based on the calculated drawing value, the lamellar tear resistance was evaluated according to the following criteria.

A (pass, very good): 85% or more

B (passed, particularly excellent): 75% or more and less than 85%

C (pass, excellent): 65% or more and less than 75%

D (Pass): 35% or more and less than 65%

E (disqualified): less than 35%

The evaluation results of (1) and (2) are listed together in Table 2.

(Table 1-1)

(Table 1-2)

(Table 2-1)

(Table 2-2)

As shown in Table 2, all examples of the invention have both excellent coating corrosion resistance and lamellar tear resistance.

On the other hand, in the comparative examples, sufficient characteristics were not obtained for at least one of paint corrosion resistance and lamellar tear resistance.

Claims (25)

in mass percent,
C: 0.020% or more and 0.200% 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
Sn: 0.005% or more and 0.200% or less
With containing
Cu: 0.010% or more and 0.50% or less;
Ni: 0.010% or more and 0.50% or less;
W: 0.005% or more and 1.000% or less;
Sb: 0.005% or more and 0.200% or less and
Si: 0.05% or more and 1.00% or less
It contains one or two or more selected from, and has a component composition with the balance consisting of Fe and unavoidable impurities,
Sn segregation degree is 20 or less,
The thickness of the Sn segregation portion in the plate thickness direction is 50 μm or less,
Structural steel materials having an ST value defined by the following formula (2) of 1.50 or less.
Here, Sn segregation degree is defined by following formula (1).
[Sn segregation degree] = [Sn concentration of Sn segregation part]/[Average Sn concentration] --- (1)
Here, the Sn segregation portion is a region where the ratio of the Sn concentration (mass concentration) to the average Sn concentration (mass concentration) obtained from line analysis with an electron beam microanalyzer is 5 or more.
ST=10000×[%S]×[%Sn] ² ---(2)
Here, [%S] and [%Sn] are the contents (mass%) of S and Sn in the above component composition, respectively. According to claim 1,
Structural steel materials, wherein the component composition includes at least one group selected from the following (A) to (D).
(A) in mass %,
Mo: 0.500% or less and
Co: 1.00% or less
One or two selected from
(B) in mass %;
Ti: 0.050% or less;
V: 0.200% or less;
Nb: 0.200% or less and
Zr: 0.100% or less
One or two or more selected from
(C) in mass %;
B: 0.0050% or less
(D) in mass %;
Ca: 0.0100% or less and
Mg: 0.0100% or less
One or two selected from According to claim 1 or 2,
Structural steel materials with a coating on the surface. According to claim 3,
The coating film has an anticorrosive base layer, a priming layer, an intermediate coating layer and a finishing coating layer,
Structural steel material in which the anticorrosive base layer is inorganic zinc-rich paint, the priming layer is epoxy resin paint, the intermediate coating layer is fluororesin topcoat paint, and the topcoat layer is fluororesin topcoat paint. . According to claim 1 or 2,
Structural steel having a zinc rich primer layer on its surface. A structure made of the structural steel according to claim 1 or 2. A structure formed using the structural steel materials according to claim 3. A structure formed using the structural steel materials according to claim 4. A structure formed using the structural steel materials according to claim 5. According to claim 6,
bridges, structures. According to claim 7,
bridges, structures. According to claim 8,
bridges, structures. According to claim 9,
bridges, structures. delete delete delete delete delete delete delete delete delete delete delete delete