Patent Application
20140056752
This disclosure relates to steel structures used mainly outdoors such as bridges and, more particularly, to a steel material preferably used for forming members which are required to have atmospheric corrosion resistance in a high salt environment such as a coastal environment.
Conventionally, as a material for forming steel structures used outdoors such as bridges, weathering steel is used. Weathering steel is a steel material where, in an atmospheric environment, a surface of the steel material is covered with a highly protective rust layer with concentrated alloy elements such as Cu, P, Cr and Ni so that the corrosion rate is remarkably reduced. It has been known that, due to its excellent atmospheric corrosion resistance, a bridge built by using weathering steel can withstand the service for several tens of years in a paintless state in many cases.
However, it has been known that, in an environment where an amount of air-borne salt is large such as a coastal environment, the above-mentioned highly protective rust layer is hardly formed so that it is difficult to acquire the practically effective atmospheric corrosion resistance (hereinafter an amount of air-borne salt being a value measured in accordance with JIS Z2382).
According to Japanese Patent No. 3785271, conventional weathering steel (JIS G3114: Hot-rolled atmospheric corrosion resisting steels for welded structure) can be used in a paintless state only in regions where an amount of air-borne salt is 0.05 mg·NaCl/dm2/day (hereinafter, unit (mg·NaCl/dm2/day) also being expressed as mdd in some cases) or less. Accordingly, in an environment where an amount of air-borne salt is large such as a coastal environment, an ordinary steel material (JIS G3106: Rolled steels for welded structure) is used in the form where a corrosion prevention means such as coating is applied to the steel material. “dm” means decimeter.
With respect to coating, a coating film is deteriorated with time so that periodic maintenance and repair become necessary. Further, it is also necessary to take into account the sharp increase in labor cost and the difficulty in recoating. Due to such reasons, recently, a steel material which can be used in a paintless state is required, and there exists a strong demand for a steel material which can be used in a paintless state.
In view of such current circumstances, recently, as a steel material which can be used in a paintless state in a high air-borne salt environment such as a coastal environment, a steel material which contains various alloy elements, particularly, a large amount of Ni has been developed.
For example, in Japanese Patent No. 3785271, a high atmospheric corrosion resistance steel material to which Cu and 1 mass % or more of Ni are added as an element which enhances atmospheric corrosion resistance is disclosed. In Japanese Patent No. 3846218, a steel material having excellent atmospheric corrosion resistance to which 1 mass % or more of Ni and 1 mass % or more of Mo are added is disclosed.
In Japanese Patent No. 3466076, a weathering steel material to which Ti is added in addition to Cu and Ni is disclosed. Further, in patent document 4, a steel material for welded structures which contains a large amount of Ni and further contains Cu, Mo, Sn, Sb, P and the like is disclosed.
However, when the content of Ni is increased as described in Japanese Patent No. 3785271, Japanese Patent No. 3846218 and Japanese Patent No. 3466076, there arises a drawback that the cost of the steel material is increased due to the increase in cost of the alloy.
With the steel material described in JP-A-10-251797 where the content of Ni and P are increased and the steel material contains Cu, Mo, Sn, Sb and the like, the cost of the steel material is increased due to the increase in cost of the alloy, and weldability is lowered due to the high content of P.
It could therefore be helpful to provide a steel material which can be manufactured at a low cost and has the excellent atmospheric corrosion resistance.
We thus provide:
A steel material having excellent atmospheric corrosion resistance, the steel material having the composition which contains, by mass %, more than 0.06% and less than 0.14% C, 0.05% or more and 2.00% or less Si, 0.20% or more and 2.00% or less Mn, 0.005% or more and 0.030% or less P, 0.0001% or more and 0.0200% or less S, 0,001% or more and 0.100% or less Al, 0.10% or more and 1.00% or less Cu, 0.10% or more and 0.65% or less Ni, 0.001% or more and 1.000% or less Mo, preferably 0.005% or more and 1.000% or less Mo, 0.005% or more and 0.200% or less Nb, and Fe and unavoidable impurities as a balance.
The steel material having excellent atmospheric corrosion resistance described above, wherein the steel material further contains one, two or more kinds selected from a group consisting of, by mass %, 0.2% or more and 1.0% or less Cr, 0.01% or more and 1.00% or less Co, 0.0001% or more and 0.1000% or less REM and 0.005% or more and 0.200% or less Sn.
The steel material having excellent atmospheric corrosion resistance described above, wherein the steel material further contains one, two or more kinds selected from a group consisting of, by mass %, 0.005% or more and 0.200% or less Ti, 0.005% or more and 0.200% or less V, 0.005% or more and 0.200% or less Zr, 0.0001% or more and 0.0050% or less B and 0.0001% or more and 0.0100% or less Mg.
Our structural steel material having excellent atmospheric corrosion resistance can be acquired at a low cost.
The structural steel material compositely contains elements effective for the enhancement of atmospheric corrosion resistance and hence, the structural steel material can be manufactured at a low cost without containing a large amount of an expensive element such as Ni, has practical weldability, and has excellent atmospheric corrosion resistance in a high salt environment such as a coastal environment. Particularly, our steel materials can acquire outstanding effects in a high air-borne salt environment where an amount of air-borne salt exceeds 0.05 mdd.
We extensively studied the composition of a steel material from a viewpoint of atmospheric corrosion resistance in a high salt environment. As a result, we found that by allowing base steel which contains Cu and Ni to further compositely contain Mo and Nb, the atmospheric corrosion resistance of the steel material in a high salt environment can be enhanced.
Reasons for limiting respective constitutional factors are explained hereinafter.
First, reasons for restricting the composition of steel are explained. Here, % of all components means mass %.
C is an element which enhances the strength of a structural steel material. The structural steel material is required to contain more than 0.06% of C to ensure the predetermined strength. On the other hand, when the structural steel material contains 0.14% or more of C, weldability and toughness of the structural steel material are deteriorated. Accordingly, the content of C is set to a value of more than 0.06% to less than 0.14%. The content of C is preferably 0.08% or more from a viewpoint of ensuring the strength of the structural steel material, and the content of C is more preferably less than 0.10% from a viewpoint of weldability and toughness of the structural steel material.
The structural steel material is, as a deoxidizing agent at the time of steel making and also as an element to enhance the strength of a structural steel material to ensure the predetermined strength, required to contain 0.05% or more of Si. On the other hand, when the structural steel material excessively contains Si exceeding 2.00%, toughness and weldability of the structural steel material are remarkably deteriorated. Accordingly, the content of Si is 0.05% or more to 2.00% or less. The content of Si is preferably 0.10% or more to 0.80% or less.
Mn is an element which enhances the strength of a structural steel material. The structural steel material is required to contain 0.20% or more of Mn for ensuring the predetermined strength. On the other hand, when the structural steel material excessively contains Mn exceeding 2.00%, toughness and weldability of the structural steel material are deteriorated. Accordingly, the content of Mn is 0.20% or more to 2.00% or less.
The content of Mn is preferably 0.20% or more to 1.50% or less.
P is an element which enhances the atmospheric corrosion resistance of a structural steel material. To acquire such an effect, the structural steel material is required to contain 0.005% or more of P. On the other hand, when the structural steel material contains P exceeding 0.030%, weldability of the structural steel material is deteriorated. Accordingly, the content of P is 0.005% or more to 0.030% or less. The content of P is preferably 0.005% or more to 0.025% or less.
When a structural steel material contains S exceeding 0.0200%, weldability and toughness of the structural steel material are deteriorated. On the other hand, when the content of S is lowered to a value less than 0.0001%, a manufacturing cost is increased. Accordingly, the content of S is 0.0001% or more to 0.0200% or less. The content of S is preferably 0.0003% or more to 0.0050% or less.
Al is an element necessary for deoxidization at the time of steel making. To acquire such an effect, a structural steel material is required to contain 0.001% or more of Al. On the other hand, when the content of Al exceeds 0.100%, weldability of the structural steel material is adversely influenced. Accordingly, the content of Al is 0.001% or more to 0.100% or less. The content of Al is preferably 0.010% or more to 0.050% or less, The content of Al is measured in terms of acid-soluble Al,
Cu forms a dense rust layer by making rust particles fine, and has an effect of enhancing atmospheric corrosion resistance of a structural steel material. Such an effect can be acquired when the content of Cu is 0.10% or more. On the other hand, when the content of Cu exceeds 1.00%, the cost is increased along with the increase of a consumption amount of Cu. Accordingly, the content of Cu is 0,10% or more to 1.00% or less. The content of Cu is preferably 0.20% or more to 0.50% or less.
Ni forms a dense rust layer by making rust particles fine, and has an effect of enhancing atmospheric corrosion resistance of a structural steel material. To sufficiently acquire such an effect, the structural steel material is required to contain 0.10% or more of Ni. On the other hand, when the content of Ni exceeds 0.65%, the cost is increased along with the increase of a consumption amount of Ni. Accordingly, the content of Ni is 0.10% or more to 0.65% or less. The content of Ni is preferably 0,15% or more to 0.50 or less.
Mo is an important factor and, by coexisting with Nb, acquires an effect of remarkably enhancing atmospheric corrosion resistance of a steel material in a high salt environment. Further, Mo forms molybdic acid ions in a rust layer thus preventing chloride ions which are a corrosion accelerating factor from permeating the rust layer and reaching the base iron. Further, MoO42− is dissolved in the steel material along with an anode reaction of the steel material so that the compound containing Mo is precipitated on a surface of the steel material whereby an anode reaction of the steel material is suppressed. To sufficiently acquire such an effect, the structural steel material is required to contain 0.001% or more of Mo. On the other hand, when the content of Mo exceeds 1.000%, the cost is increased along with the increase of a consumption amount of Mo. Accordingly, the content of Mo is 0.001% or more to 1.000% or less. The content of Mo is preferably 0.005% or more to 1.000% or less, and is more preferably 0.10% or more to 0.70% or less.
Nb is an important factor and, by coexisting with Mo, acquires an effect of remarkably enhancing atmospheric corrosion resistance of a steel material in a high salt environment. Nb is concentrated in a rust layer in the vicinity of a surface of the steel material thus acquiring an effect of suppressing an anode reaction of the steel material. To sufficiently acquire such an effect, the structural steel material is required to contain 0.005% or more of Nb. On the other hand, when the content of Nb exceeds 0.200%, the toughness of the steel is deteriorated. Accordingly, the content of Nb is 0.005% or more to 0.200% or less. The content of Nb is preferably 0.010% or more to 0.030% or less.
Although the basic composition of the steel material is as described above, when the enhancement of some desired properties of the structural steel material is further desired, the structural steel material may further contain one, two or more kinds selected from a group consisting of Cr, Co, REM and Sn as selective elements.
Cr forms a dense rust layer by making rust particles fine so that Cr is effective to enhance atmospheric corrosion resistance of a structural steel material. When the structural steel material contains 0.2% or more of Cr, the structural steel material can acquire such an effect, while when the structural steel material contains Cr exceeding 1.0%, the weldability of the structural steel material is deteriorated. Accordingly, when the structural steel material contains Cr, the content of Cr is preferably 0.2% or more to 1.0% or less. The content of Cr is more preferably 0.2% or more to 0.7% or less.
Co is distributed in the whole rust layer, and forms a dense rust layer by making rust particles fine so that Co is effective to enhance atmospheric corrosion resistance of a structural steel material. When the structural steel material contains 0.01% or more of Co, the structural steel material can acquire such an effect, while when the structural steel material contains Co exceeding 1.00%, the cost is increased along with the increase of a consumption amount of Co. Accordingly, when the structural steel material contains Co, the content of Co is preferably 0.01% or more to 1.00% or less. The content of Co is more preferably 0.10% or more to 0.50% or less.
REM is distributed in a whole rust layer, and forms a dense rust layer by making rust particles fine so that REM is effective to enhance atmospheric corrosion resistance of a structural steel material. When the structural steel material contains 0.0001% or more of REM, the structural steel material can acquire such an effect, while when the structural steel material contains REM exceeding 0.1000%, such an effect is saturated. Accordingly, when the structural steel material contains REM, the content of REM is preferably 0.0001% or more to 0.1000% or less. The content of REM is more preferably 0.0010% or more to 0.0100% or less.
Sn is concentrated in a lower layer of a rust layer so that Sn is effective to suppress an anode reaction of the steel material. When the structural steel material contains 0.005% or more of Sn, the structural steel material can acquire such an effect, while when the structural steel material contains Sn exceeding 0.200%, the toughness of the steel material is deteriorated. Accordingly, when the structural steel material contains Sn, the content of Sn is preferably 0.005% or more to 0.200% or less. The content of Sn is more preferably 0.010% or more to 0.100% or less.
Further, the structural steel material can contain one, two or more kinds selected from a group consisting of Ti, V, Zr, B and Mg as selective elements.
Ti is an element effective to enhance the strength of the steel material. When the structural steel material contains 0.005% or more of Ti, the structural steel material can acquire such an effect, while when the structural steel material contains Ti exceeding 0.200%, the toughness of the steel material is deteriorated. Accordingly, when the structural steel material contains Ti, the content of Ti is preferably 0.005% or more to 0.200% or less. The content of Ti is more preferably 0.010% or more to 0.100% or less.
V is an element effective to enhance the strength of the structural steel material. When the structural steel material contains 0.005% or more of V, the structural steel material can acquire such an effect, while when the structural steel material contains V exceeding 0.200%, such an effect is saturated. Accordingly, when the structural steel material contains V, the content of V is preferably 0.005% or more to 0.200% or less. The content of V is more preferably 0.010% or more to 0.100% or less.
Zr is an element effective to enhance the strength of the structural steel material. When the structural steel material contains 0.005% or more of Zr, the structural steel material can acquire such an effect, while when the structural steel material contains Zr exceeding 0.200%, such an effect is saturated. Accordingly, when the structural steel material contains Zr, the content of Zr is preferably 0.005% or more to 0.200% or less. The content of Zr is more preferably 0.010% or more to 0.100% or less.
B is an element necessary to enhance the strength of the structural steel material. However, when the content of B is less than 0.0001%, the structural steel material cannot sufficiently acquire such an effect. On the other hand, when the content of B exceeds 0.0050%, the toughness of the structural steel material is deteriorated. Accordingly, when the structural steel material contains B, the content of B is preferably 0.0001 or more to 0.0050% or less. The content of B is more preferably 0.0005% or more to 0.0040% or less.
Mg is an element effective to enhance toughness of a welded heat affected zone by fixing S in steel. When the structural steel material contains 0,0001 or more of Mg, the structural steel material can acquire such an effect, while when the structural steel material contains Mg exceeding 0.0100%, an amount of inclusions in the steel is increased thus deteriorating the toughness of the structural steel material to the contrary. Accordingly, when the structural steel material contains Mg, the content of Mg is preferably 0,0001% or more to 0.0100% or less. The content of Mg is more preferably 0.0005% or more to 0,0030% or less.
The balance of the structural steel material other than the above-mentioned components is constituted of Fe and unavoidable impurities. As the unavoidable impurities, the structural steel material can contain 0.010% or less of N and 0.010% or less of O. Further, when Ca which the structural steel material contains as an unavoidable impurity is present in a large amount in steel, the toughness of a welded heat affected zone is deteriorated. Accordingly, the content of Ca is preferably 0.0010% or less.
A steel material having excellent atmospheric corrosion resistance is manufactured such that steel having the above-mentioned composition is produced in a molten state, a slab is obtained by usual continuous casting or ingot making, and a steel material such as a steel plate, a shaped steel, a steel plate or a bar steel is manufactured by hot-rolling the slab. Heating and rolling conditions are suitably determined corresponding to required material quality, and the combination of such a manufacturing method with controlled rolling, accelerated cooling or heat treatment such as reheating is also possible.
Steel having the chemical composition shown in Table 1-1 and Table 1-2 was produced in a molten state, a slab was heated up to 1150° C. and, thereafter, hot rolling was performed, and the rolled plate was cooled to a room temperature by air cooling thus manufacturing a steel plate having a thickness of 6 mm for a test. Then, a test specimen having a size of 35 mm×35 mm×4 mm was sampled from the obtained steel plate. The test specimen is subjected to grinding processing such that a surface of the test specimen has surface roughness Ra of 1.6 μm or less. Edge faces and a back side were sealed with a tape, and a front side was also sealed with a tape such that an area of a front-side exposed area becomes 25 mm×25 mm.
An atmospheric corrosion resistance evaluation test was performed with respect to the specimens obtained in the above-mentioned manner, and the atmospheric corrosion resistance of the specimens was evaluated.
In the atmospheric corrosion resistance evaluation test, a corrosion test which simulates an environment of the inside of a girder having no rain shielding members was performed, wherein such an environment is considered as the severest environment for the structure such as an actual bridge. The corrosion test was performed by repeating a temperature/humidity cycle in a state where salt adheres to surfaces of the above-mentioned specimens.
As shown in
Before starting the temperature/humidity cycle and after every 7 times of temperature/humidity cycle, artificial seawater solution was applied to a surface of the test specimen by dropping before the dry step such that a quantity of salt adhered to the surface of the test specimen becomes 1.4 mg/dm2.
Under such conditions, the test of 182 temperature/humidity cycles was performed over 26 weeks.
After the corrosion test was finished, 1 L(liter) of a rust removing solution was prepared by adding distilled water to 500 mL of 37% hydrochloric acid, 3.5 g of hexamethylene tetramine and 3 mL of HIBIRON (inhibitor produced by AICOH LTD.). The test specimen was immersed in the rust removing solution to remove rust and, thereafter, a weight of the test specimen was measured. The measurement of the weight was carried out in accordance with a method described in a material delivered in the 145th corrosion prevention symposium “The acquisition of high accuracy in a method of evaluating the reduction of corrosion wear”. An average plate thickness reduction amount on one surface of a test specimen was calculated by obtaining the difference between an obtained weight and an initial weight and by dividing the difference in weight by an area of a test subject surface of the test specimen.
The environment where an amount of air-borne salt is approximately 0.5 mdd corresponds to an environment where an amount of air-borne salt is large such as a coastal environment. It is understood from the finding made heretofore that a steel plate thickness reduction amount (182 days) in this corrosion test is substantially equal to a steel plate thickness reduction amount by corrosion when the specimen was exposed to an actual environment where an amount of air-borne salt is approximately 0.5 mdd for 182 days.
Further, in obtaining a corrosion amount after 100 years based on an average plate thickness reduction amount obtained by the test by extrapolation, assuming that the average plate thickness reduction amount obtained during the period of this corrosion test is 22 μm or less, the average plate thickness reduction amount after 100 years is expected to be 0.5 mm or less which means no occurrence of a laminar peeling rust.
In general, it is known that whether or not a paintless weathering steel is applicable to a bridge is determined based on whether or not the plate thickness reduction amount after 100 years is 0.5 mm or less. Accordingly, this corrosion test is performed with respect to various kinds of steel materials, and if an average plate thickness reduction amount obtained by the corrosion test is 22 μm or less, the paintless weathering steel is applicable to a bridge.
Based on such understanding, as shown in Table 1-1 and Table 1-2, it is determined that the steel materials where the average plate thickness reduction amount is 22 μm or less have excellent atmospheric corrosion resistance.
The result of the corrosion test obtained from above is shown in Table 1-1 and Table 1-2 together with the composition.
As shown in Table 1-1 and Table 1-2, in steel kinds No. 1 to 17 and No. 32 to 37 which are our examples, the plate thickness reduction amount is 19.7 to 22.0 μm, that is, all plate thickness reduction amounts of our steel kinds are 22 μm or less so that our examples have excellent atmospheric corrosion resistance.
On the other hand, with respect to the steel kinds No. 18 to No. 31 which are comparison examples, the steel kinds No. 18 to No. 24 do not contain one or more kinds selected from a group consisting of Cu, Ni, Mo and Nb which are indispensable components of our steel material. The content of Cu is less than a lower limit in the steel kind No. 25. The content of Mo is less than a lower limit in the steel kinds No. 26 and No. 29. The content of Nb is less than a lower limit in the steel kind No. 27. The content of Ni is less than a lower limit in the steel kind No. 28. The content of Sn is less than a lower limit in the steel kind No. 30. The content of Nb is less than a lower limit in the steel kind No. 31. Accordingly, in those steel kinds, a plate thickness reduction amount is 24. 3 to 30.7 μm which exceeds 22 μm and hence, it is understood that the atmospheric corrosion resistance of the comparison examples is greatly inferior to the atmospheric corrosion resistance of our examples.
0.05
0.002
0.06
0.003
30.7
28.7
29.5
27.6
27.3
25.4
24.6
24.9
25.1
24.4
24.9
24.8
0.003
24.3
25.2
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-039152 | Feb 2011 | JP | national |
| 2012-035950 | Feb 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/055299 | 2/24/2012 | WO | 00 | 10/16/2013 |
