JP5600986B2 - Structural steel with excellent weather resistance - Google Patents

Description

  The present invention relates to a structural steel material mainly used outdoors such as a bridge, and more particularly to a steel material suitable as a member that requires weather resistance in a high salinity environment such as the vicinity of a coast.

Conventionally, weathering steel is used in steel structures used outdoors such as bridges. Weather-resistant steel is a steel material whose corrosion rate is remarkably reduced by covering the surface with a highly protective rust layer enriched with alloy elements such as Cu, P, Cr, and Ni in an air-exposed environment. Due to its excellent weather resistance, bridges using weathering steel are known to withstand service for decades, often unpainted.
However, it is known that in an environment with a large amount of salinity such as the vicinity of the coast, the highly protective rust layer is difficult to generate and practical weather resistance is difficult to obtain.
According to Non-Patent Document 1, conventional weathering steel (JIS G 3114: weathering hot rolled steel for welded structure) has an incoming salt content of 0.05 mg · NaCl / dm ² / day (hereinafter, unit (mg (NaCl / dm ² / day) may be expressed in mdd) Only in the following areas, it can be used without painting. Therefore, in an environment with a large amount of salinity such as the vicinity of the coast, ordinary steel (JIS G 3106: rolled steel for welded structure) is used after being subjected to anticorrosion measures such as painting.
The coated steel material deteriorates with the passage of time, and regular repair is required. In addition, labor costs increase and repainting becomes difficult. For these reasons, steel materials that can be used without painting are now required, and there is a high demand for steel materials that can be used without painting.
In response to this situation, in recent years, steel materials containing a large amount of various alloy elements, particularly Ni, have been developed as steel materials that can be used without coating in a high-flying salinity environment such as the vicinity of the coast.
For example, Patent Document 1 discloses a high weather resistance steel material to which Cu and 1% or more of Ni are added as a weather resistance improving element.
Patent Document 2 discloses a steel material excellent in weather resistance to which 1% or more of Ni and Mo are added.
Patent Document 3 discloses a weather-resistant steel material in which the amount of C added is kept low and Cu and Ti are added.
Patent Document 4 discloses a steel material for welded structure containing a large amount of Ni and additionally containing Mo, Sn, Sb, P and the like.

Japanese Patent No. 3785271 Japanese Patent No. 3846218 Japanese Patent No. 3367608 Japanese Patent Laid-Open No. 10-251797

Joint Research Report on the Application of Weathering Steel to Bridges (XX), 1993.3, Ministry of Construction Civil Engineering Research Institute, Steel Club, Japan Bridge Construction Association

However, as in Patent Documents 1, 2, and 3, when the Ni content is increased, there is a problem that the price of the steel material increases due to an increase in alloy cost. Furthermore, sufficient weather resistance cannot be ensured in a high flying salinity environment where the flying salt content is about 0.5 mdd to 1.0 mdd.
In addition, as in Patent Document 4, in steel materials that increase the content of Ni and P and contain Cu, Mo, Sn, Sb, etc., the price of the steel materials increases due to an increase in alloy costs, and further, the inclusion of P Due to the high amount, weldability decreases.

  In view of such circumstances, an object of the present invention is to provide a structural steel material that is low in cost and excellent in weather resistance.

In order to solve the above problems, the present invention has intensively studied the composition of steel materials from the viewpoint of weather resistance in a high salinity environment. As a result, it was found that the weather resistance of the steel material in a high salinity environment is improved by compositely containing Mo and Sb and / or Sn in the base steel containing Cu and Ni.
FIG. 1 shows the results of repeated wet and dry corrosion tests on steel materials having the components shown in Table 1. In the wet and dry repeated corrosion test, a 35 mm × 35 mm × 5 mm test piece was taken from the steel material, and then the artificial seawater solution adjusted so that the salt content attached to the surface was 0.2 mdd was added to the test piece. Once, during the drying process, it was applied to the surface of the test piece, the drying process at a temperature of 40 ° C. and a relative humidity of 40% RH for 11 hours, the temperature at 25 ° C. and the relative humidity of 95% RH for 11 hours, The transition time was 1 hour, 1 cycle was 24 hours, and 12 weeks (84 cycles) were performed. Then, the test piece was immersed in an aqueous solution obtained by adding hexamethylenetetramine to hydrochloric acid to derust, and the weight was measured. The thickness reduction amount (unit: μm) in Fig. 1 is the average thickness reduction amount on one side of the test piece. The difference between the weight obtained above and the initial weight is obtained, and that value is the test target of the test piece. It is the value divided by the surface area of the part. Further, the same test was performed three times for each steel type, the average value of the three measurements was indicated by a symbol ● in FIG. 1, and the maximum value and the minimum value were indicated by an error bar.

In addition, from the knowledge so far, it is known that the attached salt content of 0.2 mdd in this corrosion test is about 0.5 mdd when converted to the amount of incoming salt, and this amount of incoming salt of about 0.5 mdd is around the coast, etc. Corresponds to an environment with a large amount of incoming salt.
Further, when the amount of corrosion after 100 years is obtained by extrapolation from the average thickness reduction amount obtained by the test, if the average thickness reduction amount obtained in the period of this corrosion test is less than 14 μm, 100 years The subsequent reduction in the average thickness is 0.5 mm or less, at which no delamination rust occurs.
In general, the standard of applicability of unpainted weathering steel to bridges is known to have a thickness reduction of 0.5 mm or less after 100 years. If the average thickness reduction amount obtained is less than 14 μm, it is possible to apply unpainted weathering steel to bridges.
From the above, in FIG. 1, it was determined that the steel plate having an average thickness reduction of less than 14 μm has excellent weather resistance.

  From the results shown in FIG. 1, steel (steel D) containing Mo and Sb in combination with the base steel (steel M), steel containing Mo and Sn (steel E), and steel containing Mo, Sb and Sn ( Steel type I) has an average thickness reduction of less than 14 μm, and is a conventional weather resistant steel (steel type J), ordinary steel (steel type K), and combinations of other elements (steel types A to C, F to H). It can be seen that the weather resistance is remarkably excellent even when compared with. Further, when compared with steel types D, E, and I and steel type L having a high Ni content, the weather resistance of steel types D, E, and I is superior to that of steel type L.

  Thus, the reason why steel types D, E, and I showed excellent weather resistance despite the low Ni content is estimated as follows.

Steel types D, E, and I are steels in which the Ni content is reduced, Cu is contained, and Mo, Sb, and / or Sn are compositely contained. Cu and Ni densify the rust layer and prevent chloride ions, which are corrosion promoting factors, from passing through the rust layer and reaching the iron core. Mo forms molybdate ions in the rust layer, thereby preventing chloride ions, which are corrosion accelerating factors, from passing through the rust layer and reaching the base iron. Further, MoO ₄ ²⁻ is eluted with the anode reaction of the steel material, and the compound containing Mo is precipitated on the steel material surface, thereby suppressing the anode reaction of the steel material. Sb is concentrated near the interface between the rust layer and the ground iron in the anode part to suppress the anode reaction. Similar to Sb, Sn is concentrated near the interface between the rust layer and the ground iron in the anode portion, and suppresses the anode reaction. However, these effects are insufficient when contained alone, and the corrosion inhibition effect of Cu, Ni, Mo, Sb, Sn is greatly improved by the synergistic effect of the combined inclusion of Cu, Ni, Mo and Sb and / or Sn. Presumed.

This invention is made | formed based on the above knowledge, The summary is as follows.
[1] By mass%, C: more than 0.06% and less than 0.14%, Si: 0.05% or more and 2.00% or less, Mn: 0.2% or more and 2.0% or less, P: 0.0. 005% to 0.030%, S: 0.0001% to 0.0200%, Al: 0.001% to 0.100%, Cu: 0.1% to 1.0%, Ni: 0.1% to 5.0%, Mo: 0.005% to 1.000%, Sb: 0.005% to 0.200%, Sn: 0.005% to 0 Structural steel with excellent weather resistance, characterized in that it contains one or two types of 100% or less, and the balance consists of iron and inevitable impurities.
[2] In the above [1], the composition further contains one or more selected from Co: 0.01% to 1.00% and REM: 0.0001% to 0.1000% by mass%. Structural structural steel with excellent weather resistance.
[3] In the above [1] or [2], further, by mass, Ti: 0.005% to 0.200%, V: 0.005% to 0.200%, Mg: 0.0001 A structural steel material having excellent weather resistance, comprising at least one selected from% to 0.0100%.
[4] In any one of the above [1] to [3], the structural steel material having excellent weather resistance, further comprising Cr: 0.2% to 1.0% by mass% .
In addition, in this specification,% which shows the component of steel is mass% altogether. Further, in the present invention, “excellent weather resistance” is a structural steel material that practically satisfies the high weather resistance applicable in a high flying salinity environment of 1.0 mdd or less.

According to the present invention, a structural steel material having a low cost and excellent weather resistance can be obtained.
The structural steel material of the present invention has a practical weldability at a low cost without containing a large amount of an expensive element such as Ni by containing a combination of elements effective for improving weather resistance, It can have excellent weather resistance in a high salinity environment such as the vicinity. In particular, it has a remarkable effect in a high flying salt environment where the flying salt amount exceeds 0.05 mdd. However, the range of flying salt content is 0.05 mdd or more and 0.5 mdd or less flying salt content and Ni content is 1.0% when Ni content is 0.1% or more and 1.0% or less. In the case of exceeding 5.0% or less, it is desirable that the amount of incoming salt is 0.5 mdd or more and 1.0 mdd or less.

It is a figure which shows the relationship between the combination (steel types AM) of an containing element, and an average sheet thickness reduction amount.

The present invention is described in detail below.
C: More than 0.06% and less than 0.14% C is an element that improves the strength of the structural steel material, and needs to be contained in an amount exceeding 0.06% in order to ensure a predetermined strength. On the other hand, if it is 0.14% or more, weldability and toughness deteriorate. Therefore, the C content is more than 0.06% and less than 0.14%. Preferably, it is 0.08% or more from the viewpoint of securing strength, and more preferably less than 0.10% from the viewpoint of improving weldability and toughness.

Si: 0.05% or more and 2.00% or less Si must be contained in an amount of 0.05% or more as a deoxidizer during steelmaking and as an element for improving the strength of structural steel materials and ensuring a predetermined strength. is there. On the other hand, if it exceeds 2.00% and it contains excessively, toughness and weldability will deteriorate remarkably. Therefore, the Si content is 0.05% or more and 2.00% or less.

Mn: 0.2% or more and 2.0% or less Mn is an element that improves the strength of the structural steel material, and it is necessary to contain 0.2% or more in order to ensure a predetermined strength. On the other hand, if the content exceeds 2.0%, the toughness and weldability deteriorate. Therefore, the Mn content is 0.2% or more and 2.0% or less.

P: 0.005% to 0.030% P is an element that improves the weather resistance of the structural steel. In order to acquire such an effect, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.030%, weldability deteriorates. Therefore, the P content is 0.005% or more and 0.030% or less.

S: 0.0001% or more and 0.0200% or less If S exceeds 0.0200%, weldability and toughness deteriorate. On the other hand, when the content is reduced to less than 0.0001%, the production cost increases. Therefore, the S content is set to be 0.0001% or more and 0.0200% or less.

Al: 0.001% or more and 0.100% or less Al is an element necessary for deoxidation during steelmaking. In order to acquire such an effect, it is necessary to contain 0.001% or more as Al content. On the other hand, if it exceeds 0.100%, the weldability is adversely affected. Therefore, the Al content is set to be 0.001% or more and 0.100% or less.

Cu: 0.1% or more and 1.0% or less Cu has an effect of forming a dense rust layer by refining rust grains and improving the weather resistance of the structural steel material. Such an effect is obtained when the content is 0.1% or more. On the other hand, if it exceeds 1.0%, a cost increase accompanying an increase in Cu consumption is caused. Therefore, the Cu content is set to 0.1% or more and 1.0% or less.

Ni: 0.1% or more and 5.0% or less Ni has the effect of forming a dense rust layer by refining rust grains and improving the weather resistance of the structural steel material. In order to obtain this effect sufficiently, it is necessary to contain 0.1% or more. On the other hand, if it exceeds 5.0%, the effect of improving weather resistance is saturated. Therefore, the Ni content is 0.1% or more and 5.0% or less.

However, when used in an environment where the amount of incoming salt is 0.05 mdd or more and 0.5 mdd (corresponding to the amount of attached salt 0.2 mdd) or less, as is clear from the above-described examination results shown in FIG. Even if the content is low, sufficient corrosion resistance can be obtained by compounding at least one of Sb and Sn and Mo. Therefore, in this case, the upper limit of the Ni content that can prevent the cost increase due to Ni while ensuring the corrosion resistance is 1.0%. For these reasons, when used in an environment where the amount of incoming salt is 0.05 mdd or more and 0.5 mdd or less, the Ni content is preferably 0.1% or more and 1.0% or less.
On the other hand, when used in a very severe environment where the amount of incoming salt exceeds 0.5 mdd, in addition to the combined effect of at least one of Sb and Sn and Mo, the formation of a dense rust layer with Ni It is desirable to actively utilize the effects. As will be described later, the improvement in corrosion resistance due to the inclusion of Mo and Sb and / or Sn seems to be due to a mechanism different from that of Ni. For this reason, it has been found by the inventors' studies relating to the present invention that the improvement in corrosion resistance due to Ni and the improvement in corrosion resistance due to the combined inclusion of Mo, Sb and / or Sn can be obtained cumulatively. Usually, in a very severe environment where the amount of incoming salt exceeds 0.5 mdd, since the steel is used after being subjected to anticorrosion measures such as painting, the cost increases due to personnel costs. On the other hand, the steel material of the present invention can be used barely, and an increase in cost due to painting or the like can be avoided. Therefore, an increase in cost due to an increase in Ni content does not become a significant problem. For these reasons, when used in an environment where the amount of incoming salt is more than 0.5 mdd to 1.0 mdd or less, the Ni content is preferably more than 1.0% and 5.0% or less.

Mo: 0.005% to 1.000%, Sb: 0.005% to 0.200% and / or Sn: 0.005% to 0.100% Mo is an important requirement in the present invention. In addition, coexistence with Sb and / or Sn has the effect of significantly improving the weather resistance of the steel material in a high salinity environment. In addition, by forming molybdate ions in the rust layer, the chloride ions of the corrosion promoting factor are prevented from passing through the rust layer and reaching the base iron. Further, MoO ₄ ²⁻ is eluted with the anode reaction of the steel material, and the compound containing Mo is precipitated on the steel material surface, thereby suppressing the anode reaction of the steel material. In order to obtain these effects sufficiently, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 1.000%, the cost will increase with the increase in Mo consumption. Therefore, the Mo content is 0.005% or more and 1.000% or less.

  Sb is an important requirement in the present invention, and by coexisting with Mo, there is an effect of remarkably improving the weather resistance of the steel material in a high salinity environment. Moreover, it concentrates near the interface between the rust layer and the ground iron in the anode part to suppress the anode reaction. In order to obtain these effects sufficiently, it is necessary to contain 0.005% or more. On the other hand, when it exceeds 0.200%, embrittlement of steel occurs. Therefore, the Sb content is 0.005% or more and 0.200% or less.

Sn is an important requirement in the present invention, and by coexisting with Mo, there is an effect of remarkably improving the weather resistance of the steel material in a high salinity environment. Moreover, the weather resistance of the structural steel material is improved by forming an oxide film containing Sn on the steel material surface and suppressing the anode reaction of the steel material. In order to obtain these effects sufficiently, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.100%, the ductility and toughness of the steel are deteriorated. Therefore, the Sn content is 0.005% or more and 0.100% or less.
Moreover, the effect of this invention can be show | played if Sb and Sn contain at least any 1 type. However, if both Sb and Sn are contained, the weather resistance is more remarkably improved. Moreover, when securing the mechanical properties, weldability, and the like of the steel material, there is also an advantage that the addition amounts of Sb and Sn can be reduced without deteriorating the weather resistance. For these reasons, containing both Sb and Sn is a preferred invention form.
The balance is Fe and inevitable impurities. Here, N: 0.010% or less, O: 0.010% or less, and Ca: 0.0010% or less are acceptable as inevitable impurities. Moreover, since Ca contained as an unavoidable impurity is present in a large amount in steel, the toughness of the weld heat affected zone is deteriorated, so 0.0010% or less is preferable.

In addition to the above component elements, the following alloy elements can be added as necessary.
Co: 0.01% or more and 1.00% or less Co is distributed over the entire rust layer, and has the effect of improving the weather resistance of the structural steel by forming a fine rust layer by refining the rust grains. In order to sufficiently obtain such an effect, it is necessary to contain 0.01% or more. On the other hand, if it exceeds 1.00%, the cost increases with the increase in Co consumption. Therefore, when it contains, Co content shall be 0.01% or more and 1.00% or less.

REM: 0.0001% or more and 0.1000% or less REM is distributed over the entire rust layer, and by forming fine rust layers by refining rust grains, it has the effect of improving the weather resistance of structural steel materials. In order to obtain this effect sufficiently, it is necessary to contain 0.0001% or more. On the other hand, if it exceeds 0.1000%, the effect is saturated. Therefore, when it contains, REM content shall be 0.0001% or more and 0.1000% or less.

  Furthermore, in this invention, 1 or more types chosen from Ti, V, and Mg can be included for the following reasons.

Ti: 0.005% or more and 0.200% or less Ti is an element necessary for increasing the strength of the steel material. In order to obtain this effect sufficiently, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.200%, the toughness is deteriorated. Therefore, when it contains, Ti content shall be 0.005% or more and 0.200% or less.

V: 0.005% to 0.200% V is an element necessary for increasing the strength. In order to obtain this effect sufficiently, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.200%, the effect is saturated. Therefore, when it contains, V content shall be 0.005% or more and 0.200% or less.

Mg: 0.0001% or more and 0.0100% or less Mg is an element effective for fixing S in steel and improving the toughness of the heat affected zone. In order to obtain this effect sufficiently, it is necessary to contain 0.0001% or more. On the other hand, if it exceeds 0.0100%, the amount of inclusions in the steel increases, leading to deterioration of toughness. Therefore, when it contains, Mg content shall be 0.0001% or more and 0.0100% or less.

  Furthermore, in this invention, Cr can be contained for the following reasons.

Cr: 0.2% or more, 1.0% or less Cr is an element that improves the weather resistance by forming a fine rust layer by refining rust grains. To obtain this effect sufficiently , 0.2% or more must be contained. On the other hand, if it exceeds 1.0%, the weldability is lowered. Therefore, when it contains, Cr content shall be 0.2% or more and 1.0% or less.

  The structural steel material having excellent weather resistance according to the present invention is a steel plate having the above composition, by hot rolling a slab obtained by ordinary continuous casting or a block method, a thick plate, a shape steel, a thin steel plate, a steel bar. It is manufactured and obtained in steel materials such as. The heating and rolling conditions may be appropriately determined according to the required material, and combinations such as controlled rolling, accelerated cooling, or reheating heat treatment are also possible.

  Steels having chemical compositions shown in Tables 2 and 3 were melted, heated to 1150 ° C., hot-rolled, air-cooled to room temperature, and a steel plate having a thickness of 6 mm was prototyped. Next, a test piece of 35 mm × 35 mm × 5 mm was collected from the obtained steel plate. The test piece was ground so that the surface roughness Ra was 1.6 or less, the end surface and the back surface were tape-sealed, and the surface was also tape-sealed so that the area of the exposed surface portion was 25 mm × 25 mm.

About the test piece obtained by the above, the weather resistance test was done and the weather resistance was evaluated.
As a weather resistance evaluation test, a corrosion test was performed that simulates the environment inside a girder without rain, which is considered to be the most severe environment in structures such as actual bridges. The conditions of the corrosion test are as follows. After drying for 10 hours at a temperature of 40 ° C. and a relative humidity of 40% RH, the transition time was set to 1 hour, and the wetting process at 25 ° C. and a relative humidity of 95% RH was set to 11 hours. Time was taken to make one cycle for a total of 24 hours, simulating the actual temperature and humidity cycle. In addition, an artificial seawater solution adjusted to have a salt content of 0.2 mdd for the steel shown in Table 2 and 0.4 mdd for the steel shown in Table 3 once a week during the drying process. Applied to the surface. Under these conditions, 84 cycles of tests were conducted in 12 weeks.
After completion of the corrosion test, the test piece was immersed in an aqueous solution of hexamethylenetetramine in hydrochloric acid and derusted, and then the weight was measured. The difference between the obtained weight and the initial weight was determined to determine the average plate on one side. The amount of thickness reduction was determined. When this average thickness reduction amount was 14 μm or less, it was evaluated that the weather resistance was excellent.

  The corrosion test results obtained as described above are shown in Table 2 and Table 3 together with the component composition.

From Table 2, in the present invention examples (steel types Nos. 1 to 18), the thickness reduction amount is 12.5 to 13.9 μm, and the environment has an attached salt content of 0.2 mmd (corresponding to an incoming salt content of 0.5 mmd). In, it has excellent weather resistance.
On the other hand, in the comparative examples (steel type Nos. 19 to 31) that deviate from the range specified in the present invention, it is understood that the plate thickness reduction amount is 15.3 to 17.3 μm, which is greatly inferior in weather resistance compared to the present invention examples. .

Moreover, from Table 3, in the present invention examples (steel types Nos. 32-45), the thickness reduction amount is 12.1-13.8 μm, and the amount of adhered salt is 0.4 mmd (corresponding to the amount of incoming salt is 1.0 mmd). It has excellent weather resistance under the environment. Also, from this result, it is clear that the improvement in corrosion resistance by Ni and the improvement in corrosion resistance by the combined inclusion of Mo, Sb and / or Sn are obtained cumulatively.
On the other hand, in the comparative example (steel type Nos. 46 to 58) that deviates from the range defined in the present invention, it is understood that the plate thickness reduction amount is 17.3 to 35.0 μm, which is greatly inferior in weather resistance compared to the present invention example. .

Claims (4)

In mass%, C: more than 0.06% and less than 0.14%, Si: 0.17 % or more and 2.00% or less, Mn: 0.2% or more and 2.0% or less, P: 0.005% or more 0.030% or less, S: 0.0001% to 0.0200%, Al: 0.001% to 0.100%, Cu: 0.1% to 1.0%, Ni: 0.1 %: 5.05% or more, Mo: 0.005% or more and 1.000% or less, Sb: 0.005% or more and 0.200% or less, Sn: 0.005% or more and 0.100% A structural steel material having excellent weather resistance, comprising one or two of the following, the balance being iron and inevitable impurities.   2. The composition according to claim 1, further comprising at least one selected from Co: 0.01% to 1.00% and REM: 0.0001% to 0.1000% in mass%. Structural steel with excellent weather resistance.   Further, at least one selected from the group consisting of Ti: 0.005% to 0.200%, V: 0.005% to 0.200%, Mg: 0.0001% to 0.0100% in mass%. The structural steel material having excellent weather resistance according to claim 1, wherein the structural steel material is contained.   Furthermore, the structural steel material excellent in the weather resistance in any one of Claims 1-3 which contains Cr: 0.2% or more and 1.0% or less by mass%.

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