EP2484790B1

EP2484790B1 - Steel material for structures having excellent weather resistance and steel structure

Info

Publication number: EP2484790B1
Application number: EP10820733.3A
Authority: EP
Inventors: Shinichi Miura; Isamu Kage; Masatsugu Murase; Toshiyuki Hoshino
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2009-09-30
Filing date: 2010-09-28
Publication date: 2018-05-23
Anticipated expiration: 2030-09-28
Also published as: JP2012067377A; JP5691350B2; KR20120040733A; EP2484790A4; WO2011040621A1; MY156959A; EP2484790A1; CN102575327A; US20120183431A1

Description

Technical Field

The present invention generally relates to steel structures such as bridges that are used outdoors and, in particular, to a steel material and a steel structure that are suitable for use in parts required to exhibit atmospheric corrosion resistance in a high air-borne salt environment such as a coastal environment.

Background Art

Conventionally, weathering steel has been used in outdoor steel structures such as bridges. Weathering steel is a steel material that exhibits a significantly low corrosion rate in an atmospheric environment because surfaces thereof are covered with a highly protective rust layer in which alloy elements such as Cu, P, Cr, and Ni are concentrated. Bridges that use paintless weathering steel are known to frequently withstand decades of service owing to the steel's high atmospheric corrosion resistance.
However, it has been known that in an environment with a high amount of air-borne salt, such as a coastal environment, the highly protective rust layer rarely forms and practical atmospheric corrosion resistance is rarely achieved.
According to NPL 1, conventional weathering steel (JIS G 3114: atmospheric corrosion resistant steel for welded structure) can be used paintless only in the regions where the amount of air-borne salt is 0.05 mg·NaCl/dm²/day (hereinafter, the unit (mg·NaCl/dm²/day) may be denoted as mdd) or less. Accordingly, in an environment where the amount of air-borne salt is high, such as a coastal environment, regular steel material (JIS G 3106: rolled steel material for welded structure) subjected to an anticorrosive treatment such as coating has been used. Note that dm denotes decimeter.
With regard to coating, coating films deteriorate with lapse of time and require regular maintenance and repair. In addition, the rise of labor cost and need for recoating add to the difficulty. Due to these reasons, presently, steel materials that can be used paintless are desired and steel materials that can be used paintless are in high demand.
Under such a trend, steel materials that contain various alloy elements, in particular, a large amount of Ni, have been developed as a steel material that can be used paintless in an environment where the amount of air-borne salt is high, such as a coastal environment.
For example, PTL 1 discloses a highly corrosion-resistant steel material containing Cu and 1 wt% or more of Ni as the elements that improve atmospheric corrosion resistance.
PTL 2 discloses a steel material having high atmospheric corrosion resistance and containing 1 mass% or more of Ni and Mo.
PTL 3 discloses a steel material having high atmospheric corrosion resistance and containing Cu and Ti in addition to Ni.
PTL 4 discloses a steel material for welded structure, the steel material containing a large amount of Ni in addition to Mo, Sn, Sb, P, etc.
PTL 5 does not mention atmospheric corrosion resistance in an environment containing a high amount of air-borne salt, such as a coastal environment, but discloses a corrosion-resistant steel material for ships, the corrosion-resistant steel material containing W and Cr in addition to Sb, Sn, Ni, etc., for use as a corrosion-resistant material used in a severe corrosion environment where materials are directly exposed to splash of seawater, such as ballast tanks of JP2006118011 discloses a steel having excellent seaside weather resistance and capable of exhibiting excellent weather resistance even in a severe corrosive environment where the amount of flying salt exceeds 1 mdd.

Citation List

Patent Literature

PTL 1: Japanese Patent No. 3785271 (Japanese Unexamined Patent Application Publication No. 11-172370 )
PTL 2: Japanese Patent No. 3846218 (Japanese Unexamined Patent Application Publication No. 2002-309340 )
PTL 3: Japanese Patent No. 3568760 (Japanese Unexamined Patent Application Publication No. 11-71632 )
PTL 4: Japanese Unexamined Patent Application Publication No. 10-251797
PTL 5: Japanese Unexamined Patent Application Publication No. 2007-254881

Non Patent Literature

NPL 1: Joint study report on use of weathering steel material in bridges [Taikosei kozai no kyouryou heno tekiyou ni kansuru kyodo kenkyu hokokusho] (XX), No. 88, March 1993, Public Works Research Institute in Ministry of Construction, Kozai Club, and Japan Bridge Association

Disclosure of Invention

Problems to be Solved by the Invention

However, when the Ni content is increased as in PTL 1 and PTL 2, the price of the steel material increases due to the alloying cost.
In PTL 3, the Ni content is suppressed to a low level and Cu and Ti are added. In the present invention, additive elements other than these were investigated.
A steel material that contains an increased amount of Ni as well as Cu, Mo, Sn, Sb, P, and the like, such as one disclosed in PTL 4, costs high due to the increase in alloying cost and has low weldability due to a high P content.
The steel material disclosed in PTL 5 has a different usage and a different required atmospheric corrosion resistance. No mention is made as to the atmospheric corrosion resistance in an environment with a high amount of air-borne salt such as a coastal environment.
The present invention has been made under such circumstances and an object thereof is to provide a structural steel material and a steel structure that have high atmospheric corrosion resistance at low cost. Means for Solving the Problems
In the present invention, in order to address the problems described above, the composition of the steel material was investigated from the standpoint of atmospheric corrosion resistance in a high air-borne salt environment. As a result, it has been found that the atmospheric corrosion resistance of a steel material in a high air-borne salt environment improves when W and Sn and/or Nb are contained in a base steel containing Cu and Ni.
Fig. 1 shows the results of a wet and dry cyclic corrosion test conducted on steel materials containing components shown in Table 1. The wet and dry cyclic corrosion test was conducted as follows. A test specimen 35 mm × 35 mm × 5 mm in size was taken from each steel material and a diluted solution of artificial seawater was applied to the test specimen once a week during a dry process so that the amount of salt adhering to the surface was 0.2 mdd. A 24-hour cycle including 11 hours of the dry process at a temperature of 40°C and a relative humidity of 40% RH and 11 hours of a wet process at 25°C and a relative humidity of 95% RH with 1 hour of transition time was performed for 12 weeks (84 cycles). The test specimen was immersed in an aqueous solution prepared by adding hexamethylenetetramine to hydrochloric acid to conduct derusting and then weighed. The decrease in thickness (unit: µm) is an average decrease in thickness at one side of the test specimen and is determined by obtaining the difference between the initial weight and the weight measured as above and then dividing the result by a surface area of the tested portion of the test specimen. The same test was conducted three times for each steel type. The average of the three measurements is marked by a solid circle in Fig. 1 and the minimum and maximum values are indicated by an error bar.
It has been known that 0.2 mdd of adhered salt in this corrosion test is equivalent to about 0.5 mdd in terms of the amount of air-borne salt. The environment with about 0.5 mdd of air-born salt corresponds to a high air-borne salt environment such as a coastal environment.
The amount of corrosion 100 years later is determined by extrapolation from the average decrease in thickness determined by this test. The average decrease in thickness 100 years later is 0.5 mm or less, i.e., rust caused by exfoliation of layers can be prevented, if the average decrease in thickness observed during the period of the corrosion test is 14 µm or less.
In general, whether paintless weathering steel can be used in bridges is determined by whether the decrease in thickness 100 years later is 0.5 mm or less. The steel materials can be used as paintless weathering steel for use in bridges if the average decrease in thickness is 14 µm or less in this atmospheric corrosion resistance test.
Thus, in Fig. 1, steel materials with an average decrease in thickness of 14 µm or less were judged as having high atmospheric corrosion resistance.
The results in Fig. 1 show that the steel (steel type D) composed of a base steel (steel type R), W, and Nb and the steel (steel type C) composed of the same base steel, W, and Sn had an average decrease in thickness less than 14 µm and thus have significantly high atmospheric corrosion resistance compared to a conventional weathering steel (steel type Q), an ordinary steel (steel type S), and steels containing other combinations of elements (steel types A, B, and E to P). Comparison between the steel types C and D and the steel type T with a high Ni content indicates that the atmospheric corrosion resistance of the steel types C and D is superior to that of the steel type T.
The reasons why the steel types C and D exhibited high atmospheric corrosion resistance despite a low Ni content are presumably as follows.
Steel types C and D are each a steel that has a low Ni content and contains Cu, W, Nb and/or Sn. Cu and Ni densify the rust layer and prevent chloride ions, which are corrosion accelerating factors, from permeating through the rust layer and reaching the base iron. W forms a complex oxide with Fe at an anode portion near the interface between the rust layer and the base iron to thereby suppress an anode reaction. Moreover, W exhibits selective permeability for cations by forming tungstic ions distributed in the rust layer and prevents the chloride ions, i.e., corrosion accelerating factors, from permeating through the rust layer and reaching the base iron. Nb is concentrated at the anode portion near the interface between the rust layer and the base iron and suppresses the anode reaction and cathode reaction. Sn, as with Nb, is concentrated at the anode portion near the interface between the rust layer and the base iron and suppresses the anode reaction and cathode reaction. However, these effects are insufficient if these elements are contained alone. The synergetic effect of incorporation of Cu, Ni, W, Nb and/or Sn presumably significantly improves the corrosion suppressing effects of Cu, Ni, W, Nb, and Sn.
In particular, when a steel (steel type V or W) containing Nb or Sn in addition to a steel (steel type U) containing Cu, Ni, and W is compared with a steel (steel type X) containing both Nb and Sn in addition to the steel type U, the atmospheric corrosion resistance of the steel type X is far higher than that of the steel types V and W.
As seen in the steel types C, D, V, and W, the effects of the present invention are achieved as long as at least one of Nb and Sn is contained. However, incorporation of both Nb and Sn more notably improves the atmospheric corrosion resistance as demonstrated by steel type X.
The present invention has been made on the basis of the above-described findings and is summarized as below.

[1] A structural steel material with high corrosion resistance including, in terms of mass%, C: 0.020% or more and less than 0.140%, Si: 0.05% or more and 2.00% or less, Mn: 0.20% or more and 2.00% or less, P: 0.005% or more and 0.030% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.001% or more and 0.100% or less, Cu: 0.10% or more and 1.00% or less, Ni: 0.10% or more and less than 0.65%, W: 0.05% or more and 1.00% or less, and one or both of Nb: 0.005% or more and 0.200% or less and Sn: 0.010% or more and 0.050% or less, the balance being iron and unavoidable impurities.
[2] The structural steel material with high corrosion resistance as described in [1], including, in terms of mass%, Nb: 0.005% or more and 0.200% or less and Sn: 0.005% or more and 0.200% or less.
[3] The structural steel material with high corrosion resistance as described in [1] or [2], further including, in terms of mass%, Cr: more than 0.1% and 1.0% or less.
[4] The structural steel material with high corrosion resistance as described in any one of [1] to [3], further including, in terms of mass%, at least one selected from Co: 0.01% or more and 1.00% or less, Mo: 0.005% or more and 1.000% or less, Sb: 0.005% or more and 0.200% or less, and REM: 0.0001% or more and 0.1000% or less.
[5] The structural steel material with high corrosion resistance as described in any one of [1] to [4], further including, in terms of mass%, at least one selected from Ti: 0.005% or more and 0.200% or less, V: 0.005% or more and 0.200% or less, Zr: 0.005% or more and 0.200% or less, B: 0.0001% or more and 0.0050% or less, and Mg: 0.0001% or more and 0.0100% or less.
[6] The structural steel material with high corrosion resistance as described in any one of [1] to [5], in which a weld cracking parameter Pcm defined by formula (1) below is 0.25 mass% or less: $Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 \times [B]$
where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] represent contents (mass%) of respective elements.
[6] A steel structure including the structural steel material with high corrosion resistance as described in any one of [1] to [6].

In this description, % of the component of the steel is mass%. In the present invention, "high atmospheric corrosion resistance" means that the structural steel material satisfies in practice the high atmospheric corrosion resistance required in high air-borne salt environment of 0.5 mdd or less.

Advantageous Effects of Invention

According to the present invention, a structural steel material and a steel structure having high atmospheric corrosion resistance are obtained at low cost. The structural steel material of the present invention is low-cost since plural elements effective for improving the atmospheric corrosion resistance are contained without incorporation of large amounts of expensive elements such as Ni, has practical weldability, and exhibits high atmospheric corrosion resistance in a high air-borne salt environment such as a coastal environment. A particularly notable effect is exhibited in a high air-borne salt environment where the amount of air-borne salt exceeds 0.05 mdd. However, the upper limit of the amount of air-borne salt is preferably 0.5 mdd or less and the upper limit of the amount of salt adhered is preferably 0.2 mdd or less. Brief Description of Drawings

[Fig. 1] Fig. 1 is a graph showing the relationship between the steel types (steel type Nos. A to X) shown in Table 1 and the average decrease in thickness.
[Fig. 2] Fig. 2 is a diagram showing conditions and a cycle of a corrosion test.

Best Modes for Carrying Out the Invention

The present invention will now be described in detail.

C: 0.020% or more and less than 0.140%

Carbon is an element that improves the strength of a structural steel material. The carbon content needs to be 0.020% or more to ensure a required strength. At a C content of 0.140% or more, weldability and toughness are deteriorated. Accordingly, the C content is 0.020% or more and less than 0.140% and preferably in a range of 0.060 to 0.100%.

Si: 0.05% or more and 2.00% or less

Silicon acting as a deoxidizing agent during steel making and an element that improves the strength of the structural steel material to ensure the required strength needs to be contained in an amount of 0.05% or more. Incorporation of excess Si exceeding 2.00% significantly deteriorates toughness and weldability. Accordingly, the Si content is 0.05% or more and 2.00% or less and is preferably in a range of 0.10 to 0.80%.

Mn: 0.20% or more and 2.00% or less

Manganese is an element that improves the strength of the structural steel material and 0.20% or more of Mn needs to be contained in order to ensure a required strength. In contrast, the toughness and weldability are deteriorated if Mn is contained exceeding 2.00%. Accordingly, the Mn content is 0.20% or more and 2.00% or less and preferably in a range of 0.20 to 1.50%.

P: 0.005% or more and 0.030% or less

Phosphorus is an element that improves the atmospheric corrosion resistance of the structural steel material. In order to achieve this effect, 0.005% or more of P needs to be contained. However, if more than 0.030% of P is contained, weldability is deteriorated. Accordingly, the P content is 0.005% or more and 0.030% or less and preferably in a range of 0.005 to 0.025%.

S: 0.0001% or more and 0.0200% or less

At a sulfur content exceeding 0.0200%, the weldability and toughness are deteriorated. If the S content is reduced to less than 0.0001%, the production cost will increase. Accordingly, the S content is 0.0001% or more and 0.0200% or less and preferably in a range of 0.0003 to 0.0050%.

Al: 0.001% or more and 0.100% or less

Aluminum is an element needed in deoxidization during steel making. In order to achieve this effect, the Al content needs to be 0.001% or more. At an Al content exceeding 0.100%, however, the weldability is adversely affected. Thus, the Al content is 0.001% or more and 0.100% or less and preferably in a range of 0.010 to 0.050%. Acid-soluble Al was measured in determining the Al content.

Cu: 0.10% or more and 1.00% or less

Copper reduces the size of rust grains to help form a dense rust layer and thus has an effect of improving the atmospheric corrosion resistance of the structural steel material. This effect is achieved when the Cu content is 0.10% or more. At a Cu content exceeding 1.00%, the cost will rise due to the increased consumption of Cu. Accordingly, the Cu content is 0.10% or more and 1.00% or less and preferably in a range of 0.20 to 0.50%.
PTL 5 relates to a weathering steel material for ships. Under current technology, the lifetime of corrosion resistant coating of ballast tanks of ships (typically 10 years) is half that of ships (20 years) and the atmospheric corrosion resistance of the remaining 10 years is retained by maintenance and repair of the coating. An object of the weathering steel material described in PTL 5 is to offer high atmospheric corrosion resistance unaffected by the surface condition of the steel material under a severe corrosive environment where the material is directly exposed to seawater and splash thereof such as ballast tanks of ships so that the period up to which the maintenance coating is required can be extended, and to alleviate the load of the maintenance coating. In contrast, the structural steel material according to the present invention is used in outdoor steel structures such as bridges and an object is to achieve a decrease in thickness of 0.5 mm or less 100 years later in a high air-borne salt environment such as a coastal environment. The environment in which the steel material is used and the object significantly differ from those of the steel material described in PTL 5. Accordingly, whereas the steel material described in PTL 5 does not have to contain Cu, the steel material of the present invention needs to contain Cu to help form a dense rust and improve the atmospheric corrosion resistance of the steel material. Thus, in the present invention, Cu content is 0.10% or more.

Ni: 0.10% or more and less than 0.65%

Nickel reduces the size of rust grains to help form a dense rust layer and has an effect of improving the atmospheric corrosion resistance of the structural steel material. In order to fully bring this effect, the Ni content needs to be 0.10% or more. At a Ni content of 0.65% or more, the cost will rise due to the increased consumption of Ni. Accordingly, the Ni content is 0.10% or more and less than 0.65% and preferably in a range of 0.15 to 0.50%.

W: 0.05% or more and 1.00% or less, Nb: 0.005% or more and 0.200% or less and/or Sn: 0.005% or more and 0.200% or less

Tungsten is a important element in the present invention and has an effect of dramatically improving the atmospheric corrosion resistance of the steel material in a high air-borne salt environment when contained in combination with Nb and/or Sn. WO42- elutes as the anode reaction of the steel material proceeds and distributes itself in the rust layer to electrostatically prevent chloride ions, i.e., corrosion accelerating factors, from permeating through the rust layer and reaching the base iron. Moreover, compounds containing W settle on the steel material surface and suppress the anode reaction of the steel material. In order to fully bring this effect, the W content needs to be 0.05% or more. At a W content exceeding 1.00%, the cost will rise due to an increase in consumption of W. Thus, the W content is 0.05% or more and 1.00% or less and preferably in a range of 0.10 to 0.70%.
Niobium is a important element in the present invention and has an effect of dramatically improving the atmospheric corrosion resistance of the steel material in a high air-borne salt environment when contained in combination with W. Niobium is concentrated at the anode portion near the interface between the rust layer and the base iron and suppresses anode reaction and cathode reaction. In order to fully bring this effect, the Nb content needs to be 0.005% or more. At a Nb content exceeding 0.200%, the toughness is decreased. Accordingly, the Nb content is 0.005% or more and 0.200% or less and preferably in a range of 0.010 to 0.030%.
Tin is a important element in the present invention and has an effect of dramatically improving the atmospheric corrosion resistance of the steel material in a high air-borne salt environment when contained in combination with W. Tin helps form an oxide coating film containing Sn on the steel material surface and suppresses anode reaction and cathode reaction of the steel material to improve the atmospheric corrosion resistance of the structural steel material. In order to fully bring these effects, the Sn content needs to be 0.010% or more. At a Sn content exceeding 0.050%, however, the ductility and toughness of the steel are deteriorated. Accordingly the Sn content is in a range of 0.010 to 0.050%.
The effects of the present invention can be achieved as long as one of Nb and Sn is contained. However, incorporation of both Nb and Sn has an effect of notably improving the atmospheric corrosion resistance. The reasons why incorporation of both Nb and Sn brings such an effect are not yet clear. Presumably, conditions (e.g., ambient conditions such as temperature, relative humidity, and salt concentration in the rust) under which Nb exhibits a notable effect are different from conditions under which Sn exhibits a notable effect, and thus Nb and Sn complement one another in an environment in which the dry process and the wet process repetitively occur, thereby notably improving the atmospheric corrosion resistance.
There is also an advantage that the amounts of Nb and Sn added can be decreased without deteriorating the atmospheric corrosion resistance in reliably obtaining the required mechanical properties and weldability of the steel material. Due to these reasons, incorporation of both Nb and Sn is preferred in the present invention.
The balance is Fe and unavoidable impurities.
Allowable unavoidable impurities are N: 0.010% or less, O: 0.010% or less, and Ca: 0.0010% or less. Calcium contained as an unavoidable impurity deteriorates the toughness of the weld heat-affected zone if contained in large amounts and thus the Ca content is preferably 0.0010% or less.
In addition to the elements described above, the following alloy elements may be added as needed.

Cr: more than 0.1% and 1.0% or less

Chromium is an element that helps form a dense rust layer by decreasing the size of rust grains and improves the atmospheric corrosion resistance. In order to fully bring this effect, the Cr content needs to be more than 0.1%. At a Cr content exceeding 1.0%, the weldability is degraded. Thus, when Cr is to be contained, the Cr content is more than 0.1% and 1.0% or less and preferably in a range of 0.2 to 0.7%.
In the present invention, at least one selected from Co, Mo, Sb, and rare earth metals (REM) may be contained for the following reasons.

Co: 0.01% or more and 1.00% or less

Cobalt distributes itself in the entire rust layer, reduces the size of the rust grains to help form a dense rust layer, and has an effect of improving the atmospheric corrosion resistance of the structural steel material. In order to fully bring this effect, the Co content needs to be 0.01% or more. At a Co content exceeding 1.00%, the cost will rise due to an increase in consumption of Co. Thus, when Co is to be contained, the Co content is 0.01% or more and 1.00% or less and preferably in a range of 0.10 to 0.50%.

Mo: 0.005% or more and 1.000% or less

Molybdenum prevents chloride ions, i.e., corrosion accelerating factors, from permeating through the rust layer and reaching the base iron since MoO42- elutes as the anode reaction of the steel material proceeds and distributes itself in the rust layer. Moreover, compounds containing Mo settle on the steel material surface and suppress the anode reaction of the steel material. In order to fully bring this effect, the Mo content needs to be 0.005% or more. At a Mo content exceeding 1.000%, the cost will rise due to an increase in consumption of Mo. Thus, when Mo is to be contained, the Mo content is 0.005% or more and 1.000% or less and preferably in a range of 0.100 to 0.500%.

Sb: 0.005% or more and 0.200% or less

Antimony is an element that suppresses the anode reaction of the steel material and hydrogen-generating reaction, which is the cathode reaction, to thereby improve the atmospheric corrosion resistance of the structural steel material. In order to fully bring this effect, the Sb content needs to be 0.005% or more. At an Sb content exceeding 0.200%, the toughness is deteriorated. Accordingly, when Sb is to be contained, the Sb content is 0.005% or more and 0.200% or less and preferably in a range of 0.010 to 0.050%.

REM: 0.0001% or more and 0.1000% or less

REM distributes itself to the entire rust layer, reduces the size of the rust grains to help form a dense rust layer, and has an effect of improving the atmospheric corrosion resistance of the structural steel material. In order to fully bring this effect, the REM content needs to be 0.0001% or more. At a REM content exceeding 0.1000%, the effect thereof is saturated. Accordingly, when REM is to be contained, the REM content is 0.0001% or more and 0.1000% or less and preferably in a range of 0.0010 to 0.0100%.
In the present invention, at least one selected from Ti, V, Zr, B, and Mg may be contained for the following reasons.

Ti: 0.005% or more and 0.200% or less

Titanium is an element needed to increase the strength. In order to fully bring this effect, the Ti content needs to be 0.005% or more. At a Ti content exceeding 0.200%, the toughness is deteriorated. Thus, when Ti is to be contained, the Ti content is 0.005% or more and 0.200% or less and preferably in a range of 0.010 to 0.100%.

V: 0.005% or more and 0.200% or less

Vanadium is an element needed to increase the strength. In order to fully bring this effect, the V content needs to be 0.005% or more. At a V content exceeding 0.200%, the effect is saturated. Thus, when V is to be contained, the V content is 0.005% or more and 0.200% or less and preferably in a range of 0.010 to 0.100%.

Zr: 0.005% or more and 0.200% or less

Zirconium is an element needed to increase the strength. In order to fully bring this effect, the Zr content needs to be 0.005% or more. At a Zr content exceeding 0.200%, the effect is saturated. Accordingly, when Zr is to be contained, the Zr content is 0.005% or more and 0.200% or less and preferably in a range of 0.010 to 0.100%.

B: 0.0001% or more and 0.0050% or less

Boron is an element needed to increase the strength. In order to fully bring this effect, the B content needs to be 0.0001% or more. At a B content exceeding 0.0050%, the toughness is deteriorated. Accordingly, when B is to be contained, the B content is 0.0001% or more and 0.0050% or less and preferably in a range of 0.0005 to 0.0020%.

Mg: 0.0001% or more and 0.0100% or less

Magnesium is an element that fixes S in the steel and is effective for improving the toughness of the weld heat-affected zone. In order to fully bring this effect, the Mg content needs to be 0.0001% or more. At a Mg content exceeding 0.0100%, the amounts of inclusions in the steel increase and the toughness is deteriorated. Accordingly, when Mg is to be contained, the Mg content is 0.0001% or more and 0.0100% or less and preferably in a range of 0.0005 to 0.0020%.

Pcm: 0.25 mass% or less

In order to prevent low-temperature cracking by welding and bring the preheating temperature during welding operation to a practical level of 50°C or less, the weld cracking parameter Pcm defined by the formula below is preferably 0.25 mass% or less and more preferably 0.20 mass% or less: $Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 \times [B]$
where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] represent the contents (mass%) of the respective elements.
The structural steel material of the present invention having high atmospheric corrosion resistance is obtained by melting a steel having the above-described composition by using melting means such as a steel converter or an electric furnace by an ordinary method and hot-rolling a slab obtained by ordinary continuous casting or slabbing to prepare a steel material such as a steel plate, a shaped steel, a steel plate, or a bar steel. The heating and rolling conditions may be adequately determined according to the quality of the material used. A combination of controlled rolling, accelerated cooling, and a heat treatment such as reheating can be employed.
When the structural steel material obtained as such is used as a structural member of a steel structure, a steel structure having high atmospheric corrosion resistance in a high air-borne salt environment, such as a coastal environment, can be obtained.

EXAMPLES

Steels having chemical compositions shown in Table 2 were melted, heated to 1150°C, hot rolled, and air-cooled to room temperature to prepare steel plates 6 mm in thickness. Then a test specimen 35 mm × 35 mm × 5 mm in size was taken from each of the steel plates obtained. The test specimen was subjected to grinding processing so that the surface had a surface roughness Ra of 1.6 µm or less. An edge face and a back side were sealed with a tape and the surface was also sealed with a tape so that the area of the exposed area was 25 mm × 25 mm.
The test specimens obtained as such were subjected to a wet and dry cyclic corrosion test to evaluate the atmospheric corrosion resistance.
A corrosion test employed as the wet and dry cyclic corrosion test simulated an environment of inside girders not under eaves, which is presumably the severest environment for actual structures such as bridges. The conditions for the corrosion test were as follows: One 24-hour cycle included 11 hours of a dry process at a temperature of 40°C and a relative humidity of 40% RH, 1 hour of transition time, 11 hours of a wet process at a temperature of 25°C and a relative humidity of 95% RH, and 1 hour of transition time to simulate the temperature-humidity cycle of actual environments. A diluted solution of artificial seawater was applied to the test specimen once a week during the dry process so that the amount of salt adhering to the test specimen surface was 0.2 mdd. Under these conditions, 84 cycles of testing were conducted in 12 weeks. The conditions and the cycle of the corrosion test are schematically illustrated in Fig. 2. After completion of the corrosion test, the test specimen was immersed in an aqueous solution of hexamethylenetetramine in hydrochloric acid to remove rust and weighed, and an average decrease in thickness at one side of the test specimen was obtained from the difference between the observed weight and the initial weight. Test specimens having an average decrease in thickness of 14 µm or less were evaluated as having high atmospheric corrosion resistance.
The weldability of the test specimen was also evaluated. A y-slit weld cracking test that studies the cold cracking susceptibility of a welded zone was conducted as the evaluation method, and the preheating temperature for prevention of weld cracking was determined. Test specimens having high preheating temperature for prevention of weld cracking were evaluated as having low weldability.
The results of the corrosion test and the results of evaluation of weldability obtained as above are shown in Table 2 along with the compositions.
In Invention Examples (steel type Nos. 1 to 25), the decrease in thickness was 11.8 to 13.8 µm and high atmospheric corrosion resistance was exhibited. Although No. 25 has high atmospheric corrosion resistance, Pcm was more than 0.25 mass%. Thus, the preheating temperature for prevention of weld cracking was as high as 100°C and the weldability was low.
In particular, steel type No. 7 containing both Nb and Sn has significantly improved atmospheric corrosion resistance compared to steel type Nos. 2 and 5 that contain substantially the same amounts of Cu, Ni, and W and Nb or Sn, where only one of Nb and Sn is contained. Similarly, steel type No. 8 containing both Nb and Sn has significantly improved atmospheric corrosion resistance compared to steel types 1 and 4. Similarly, steel type Nos. 11 and 12 containing both Nb and Sn have improved atmospheric corrosion resistance compared to steel type 10.
In contrast, Comparative Examples (steel type Nos. 26 to 42) outside the range of the present invention have a decrease in thickness of 14.3 to 17.7 µm and are thus inferior to the invention examples in terms of atmospheric corrosion resistance. Although Comparative Examples (steel type Nos. 41 and 42) have a decrease in thickness of 14.0 µm and 12.5 µm, respectively, and thus have high atmospheric corrosion resistance, the alloy cost is high due to a large amount of Ni and thus the price of the steel material is high. Comparative Example steel type No. 42 has Pcm exceeding 0.25 mass% and thus the preheating temperature for prevention of weld cracking was as high as 100°C, resulting in low weldability.

Claims

A structural steel material consisting of, in terms of mass%, C: 0.020% or more and less than 0.140%, Si: 0.05% or more and 2.00% or less, Mn: 0.20% or more and 2.00% or less, P: 0.005% or more and 0.030% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.001% or more and 0.100% or less, Cu: 0.10% or more and 1.00% or less, Ni: 0.10% or more and less than 0.65%, W: 0.05% or more and 1.00% or less, Nb: 0.005% or more and 0.200% or less and Sn: 0.010 or more and 0.050% or less, optionally at least one selected from Cr: more than 0.1% and 1.0% or less, Co: 0.01% or more and 0.50% or less, Mo: 0.005% or more and 0.500% or less, Sb: 0.005% or more and 0.200% or less, REM: 0.0001% or more and 0.1000% or less, Ti: 0.005% or more and 0.100% or less, V: 0.005% or more and 0.100% or less, Zr: 0.005% or more and 0.100% or less, B: 0.0001% or more and 0.0020% or less, Mg: 0.0001% or more and 0.0100% or less and Ca: 0.0010% or less, the balance being iron and unavoidable impurities.
The structural steel material according to Claim 1, wherein a weld cracking parameter Pcm defined by formula (1) below is 0.25 mass% or less: $Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 \times [B]$
where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] represent contents (mass%) of respective elements.
A steel structure comprising the structural steel material according to any one of Claims 1 to 2.