JP3466076B2 - Steel with excellent corrosion resistance and weldability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a steel material suitable for a structural material such as a bridge which is difficult to maintain and manage, and is used for unpainted or painted, and has excellent corrosion resistance and weldability. Related to steel materials.

[0002]

2. Description of the Related Art For example, steel materials used for bridge structures such as road bridges in salt corrosive environments such as salt water and snow melting salt coming in in mountainous areas and coastal areas have been conventionally coated to improve corrosion resistance. It is used. However, since this coating film always deteriorates with time, it is necessary to maintain and maintain the corrosion resistance by recoating the coating film at regular intervals.

On the other hand, in recent years, a small number of main girder bridges with a small number of main girders represented by two main girder bridges have come to be used in many of these bridges in place of conventional multi-girder bridges. There is. This minor girder bridge can reduce the amount of steel used (steel weight) and the number of bridge pieces compared to the majority girder bridge, has good workability, and has advantages in terms of environmental protection and shortening the construction period.
Further, such a minority main girder bridge is strongly required to minimize the load and cost of maintenance and management after the bridge is installed and to extend the life of the bridge itself.

Therefore, steel materials used for structural materials such as steel towers and buildings, including such minority main girder bridges, etc., can be used unpainted (naked) under the salt corrosion environment. Further, there is a strong demand for a steel material that maintains high corrosion resistance so that even if the coating film is used after being painted and the coating film is deteriorated or destroyed during use, maintenance is not required after installation of the bridge in any case.

Conventionally, in order to improve the corrosion resistance of this kind of steel material, various improvement techniques have been proposed from the side of the steel material which is the base material. For example, as a typical example, P: 0.15% or less or C
There is a weathering steel containing u: 0.2 to 0.6%, Cr: 0.3 to 1.25%, Ni: 0.65% or less. Two types of JIS G 3114 (weather resistant hot rolled steel for welded structures) or JIS G 3125 (high weather resistant rolled steel) are standardized for this weather resistant steel. This weather resistant steel, due to the action of the trace elements, rust generated on the steel surface during use of the steel material becomes a dense stable rust layer (weather resistant rust) having high corrosion resistance typified by bare weather resistance. Has a self-corrosion function. Due to such properties, the weather-resistant steel has been basically used unpainted as a maintenance-free structural material for various structures such as the bridge.

However, under the salt corrosive environment, the stable rust layer, which is a characteristic of weather resistant steel, is less likely to be formed due to the influence of salt. If the stable rust layer is not formed, the corrosion resistance of the weather resistant steel will be significantly reduced. This is because in the corrosive environment with a large amount of salt, the pH in the rust film is particularly lowered as the steel corrodes. That is, when corrosion of steel begins even slightly, first, Fe → Fe ²⁺ ＋ 2e ⁻ , followed by Fe ²⁺ ＋ 2H ₂ O → Fe (OH)
_The reaction of ₂ + 2H ⁺ lowers the pH of the steel surface and also lowers the pH in the rust film and at the interface between the rust film and the steel. Then, once the pH is lowered, the transport number of chlorine ions in the rust film increases in order to maintain electrical neutrality, and concentration of chlorine ions occurs at the interface between the rust film and steel. As a result, a hydrochloric acid atmosphere is formed at this interface portion, which promotes corrosion of steel. At the same time, due to the decrease in pH in the rust film, the solubility of iron ions increases, and a phenomenon that hinders the formation of the stable rust layer, which is the key to the anticorrosion mechanism of corrosion-resistant low-alloy steel such as weather-resistant steel, also occurs. , A corrosion acceleration situation is formed.

Therefore, in order to prevent a decrease in pH in the rust film, a technique has been proposed in which the surface of the weather resistant steel is alkalized to prevent the formation of the accelerated corrosion condition. More specifically, Be, Mg, C that alkalizes the surface of weathering steel.
Oxides (chemical species) such as a, Sr, and Ba are dispersed in steel in advance, and at the same time as the corrosion reaction of the steel, these chemical species act to suppress the decrease in pH of the steel surface. , For example,
It is proposed in Japanese Patent Laid-Open No. 58-25458 and Japanese Patent No. 2572447.

[0008]

The technique of adding these oxides to prevent the formation of the accelerated corrosion state is certainly effective in suppressing the influence of salt and the like from the outside. However, the formation of the stable rust layer itself is
As in the case of the weather resistant steel, it is difficult or limited, and in reality, sufficient corrosion resistance is not obtained. Further, there is a concern that the oxide itself added to steel adversely affects properties such as weldability and strength.

According to the findings of the inventor of the present invention, there is a particular demand for a structural material for the minority girder bridge.
The level of corrosion resistance that can be used without painting, that is, the average reduction in steel sheet thickness due to corrosion after exposure to the atmosphere for 1 year (including 5% salt water spraying once a week) is 0.8 mm or less, more preferably 0.5 mm.
Up to the following levels, the corrosion resistance of weathering steel cannot be improved. In addition, there is no preheating (preheating free), which is important for construction of structural materials such as the above-mentioned few main girder bridges, and the heat input is 5 KJ / mm or more,
In some cases, it is not possible to meet the required properties of weldability that enable highly efficient welding such as high heat input welding of 100 to 300 KJ / mm or more. Therefore, in reality, there has been substantially no steel material suitable for a structural material, which is represented by the minority main girder bridge and is used unpainted or painted even in a salt corrosion environment.

Therefore, in view of these problems of the conventional weathering steel, the present invention has corrosion resistance that can be used as a structural material for the minority main girder bridge and the like, and has a heat input amount without preheating.
It is an object of the present invention to provide a steel material having excellent weldability capable of highly efficient welding such as high heat input welding of 5 KJ / mm or more.

[0011]

[Means for Solving the Problem] The gist of the present invention for this purpose is that the compositional components of the steel material, in mass%, C: 0.15% or less,
Si: 0.10 to 1.0%, Mn: 1.5% or less, S: 0.02% or less,
P: 0.05% or less, Cr: 0.05% or less, Ti: 0.01 to 1.0%,
Ca: 0.0001 to 0.01%, Cu: 0.05 to 3.0% and Ni: 0.0
5 to 6.0% 1 or 2 and Mo: 0.05 to 3.0%
And W: 0.05 to 3.0% of 1 type or 2 types, balance Fe
And unavoidable impurities, and the carbon equivalent A (%)
0.20 or less [However, carbon equivalent A = C + Si / 22 + Mn / 6 + P / 10-
Cu / 20-Ni / 15 + Cr / 2-Ti / 2-Ca-Al / 35 + Mo / 4 + W / 6
(%)] And X-ray diffraction of the rust formed on the surface of this steel
If the fraction of the amorphous component obtained by the method is 30 wt% or more, β-
The fraction of the FeOOH component should be 20 wt% or less .

[0012] Ri by that such a summary, a rust generated on the steel table <br/> surface can be dense stable rust layer, steel even under salt corrosive environment, it can be used without painting It becomes possible to have naked weather resistance. More specifically, the bare weathering resistance of steels is preferably determined by the average amount of reduction in steel thickness due to corrosion after 1 year of atmospheric exposure (including once-weekly salt water spraying).
It can be an excellent one having a thickness of 0.8 mm or less, more preferably 0.5 mm or less.

With the above-mentioned gist, in addition to the above-mentioned naked weather resistance, even if the thickness of the steel material is 50 mm or more, preferably the heat input amount is 5 KJ / mm or more without preheating, and in some cases 100 to 300 KJ / It becomes possible to have weldability capable of performing large heat input welding of mm or more.

The inventors of the present invention have proposed the conventional weather-resistant steel material,
We have diligently studied the reason why weather resistant steel materials in which chemical species to be alkalized are dispersed in steel cannot be improved to the level of corrosion resistance such as weather resistance, which is particularly required for structural materials such as the minority main girder bridge. . As a result, it was found that the reason why the high corrosion resistance of conventional weather-resistant steel materials is not reproducibly exhibited is that S and Cr, which are among the component compositions of steel materials, act as corrosion factors. On the other hand, they have found that the inclusion of Ti significantly improves the high corrosion resistance of the weather resistant steel.

The influence of these elements on the corrosion resistance will be described below. In the case of rust formed on the surface of steel, the amorphousness of rust is important as a measure of whether it is a dense and stable rust layer. That is, the main components of iron rust generated on the steel surface are α-FeOOH, β-FeOOH, γ-FeOOH
And Fe ₃ O ₄ crystalline rust and amorphous rust. Among them, amorphous rust forms a finer and more stable stable rust layer than crystalline rust. Moreover, even if a defective portion as a rust film is formed due to crystalline rust during use of the steel material, the amorphous rust portion also has a defect repair function of filling this hole and reducing the defective portion. As a result, the long-term bare weatherability of the steel material is guaranteed. Therefore, the higher the ratio of amorphous rust in iron rust (amorphous degree), and the finer and finer α-Fe in the crystalline rust component.
The higher the proportion of OOH, the higher the corrosion resistance.

On the other hand, rust other than this, particularly crystalline rust such as β-FeOOH, progresses corrosion starting from this rust even if the ratio of the amorphous or α-FeOOH in the rust is high. Therefore, it is necessary to suppress it as much as possible. Therefore, the proportion of amorphous rust in the iron rust (amorphous degree) and the fine and dense α-FeOOH
It can be said that the higher the rust ratio, the more stable the rust layer. In addition, among crystalline rust components, β-FeOO is particularly apt to promote corrosion.
It can be said that the smaller the proportion of H, the more precise and stable rust layer.

On the other hand, first, if the content of S exceeds 0.02%, the formation of the stable rust layer is hindered and the corrosion resistance deteriorates. Therefore, setting the S content to 0.02% or less is also one of the features of the present invention. .

In addition, Cr is the content of P and P in conventional weather-resistant steel materials.
It is recognized as an additional element indispensable for forming the stable rust layer together with Cu and Ni, and as described above, it is contained in 0.30 to 1.25% even in JIS standards. In addition, the above-mentioned JP-A-58-
Although the addition of Cr is not explicitly disclosed in Japanese Patent No. 25458 and Japanese Patent No. 2572447, the content of Cr is necessarily 0.05% or more as an impurity from the iron raw material or the steelmaking process.

However, when Cr is contained in an amount of 0.05% or more, if corrosion starts even in a microscopic surface defect portion of steel, Cr ions, which are dissolved in a trace amount with iron atoms in a chemical equilibrium, also have the action of Cl ions. It causes a decrease in pH in the microscopic surface defect portion of the steel, has an action of promoting the oxidizability of condensed water in the defect and inducing corrosion. Therefore, even if the dense stable rust layer is generated, Cr has an action of promoting the corrosion of steel in the lower part of the stable rust layer, and inhibits the adhesion between the rust layer and the steel, resulting in the peeling of the rust layer. Or, as a result, inhibits the generation or maintenance of a dense stable rust layer. Therefore, in the present invention, it is necessary to reduce the Cr content as much as possible, and the upper limit is set to less than 0.05% in consideration of the economical efficiency of reducing the Cr content.

In the present invention, Ti is selected as an element for promoting the formation of the stable rust layer instead of Cr. Ti has a unique property that it does not cause the decrease in the pH like Cr and has an effect of promoting the formation of the stable rust layer and remarkably enhances the corrosion resistance. Specifically, while increasing the proportion of amorphous in the iron rust and the proportion of rust of α-FeOOH, suppress the formation of β-FeOOH which is particularly easy to promote corrosion among crystalline rust components, Promotes the formation of a fine and precise stable rust layer. As a result, the penetration of corrosion factors such as chloride ions into the rust layer is prevented, the dense stable rust layer is maintained, and the corrosion resistance is improved.

Incidentally, it is known that Ti is usually added to deoxidize molten steel and maintain the strength of steel materials. For example, the above-mentioned Japanese Patent No. 2572447 has the same purpose.
About 0.03% or less is added. However, in the present invention
The purpose of Ti is to form a dense stable rust layer as described above, and this is one of the features of the present invention together with the reduction of S and Cr.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition and carbon equivalent of the steel material in the present invention will be explained below. C: 0.15% or less. C is 390 for steel structural applications
630N / mm ² grade, or an essential element to secure required strength of more than that, if the content exceeds 0.15%,
Deteriorates weldability and bare weatherability of steel. Therefore, the C content is set to 0.15% or less so that the required strength can be secured.

Si: 0.10 to 1.0%. Si is an essential element for deoxidation and solid solution strengthening of molten steel, and also has an effect of promoting the formation of a dense stable rust layer and improving corrosion resistance such as bare weather resistance. However, if it is less than 0.10%, these effects are insufficient, and conversely if it exceeds 1.0%, the weldability deteriorates. Therefore, the Si content is set to the range of 0.10 to 1.0%.

Mn: 1.5% or less. Mn replaces C, 390-
It is an essential element for securing strength of 630 N / mm ² grade or higher, but if the content exceeds 1.5%, a large amount of MnS will be generated in the steel, and it will be difficult to improve the naked weather resistance. Corrosion resistance may be deteriorated. Therefore, the Mn content should be 1.5% or less.

S: 0.02% or less. S is 0, as described above.
If the content exceeds 02%, FeS, which becomes the starting point of corrosion,
A large amount of MnS may be generated in the steel, hindering the formation of the stable rust layer, and causing deterioration of corrosion resistance. Further, when Ni or the like is excessively contained, the reaction with S tends to cause the NiS compound having a low melting point to be precipitated in the grain boundaries of the weld metal, and the ductility of the grain boundaries of the solidified metal is likely to be deteriorated. In this respect, the S content is 0.
If it is 02% or less, there is also an advantage that a larger amount of Ni can be contained without precipitating the low-melting NiS compound. For example, if S exceeds 0.02%, Ni
Should be 3.0%, but the S content should be 0.02%.
As described above, it becomes possible to contain Ni up to 6.0% by the following. Therefore, the S content is 0.
02% or less, preferably 0.01% or less, more preferably 0.00
The range is 5% or less.

P: 0.05% or less. P has the effect of preventing chloride ions from entering the rust formed on the steel surface, forming a dense and stable rust layer, and improving the corrosion resistance of the weather resistant steel. Further, in the conventional weathering steel, in order to exert this effect, the content of about 0.05% or more and about 0.15% or less is essential. However, in the present invention, an excessive P content of about 0.05% or more significantly impairs weldability, and is important for construction of the above-mentioned minor girder bridge without preheating (free of preheating) and with high efficiency and high heat input. Weldability is not possible to meet the required properties of weldability. Further, in the present invention, by the inclusion of such Ti, dense stable because the formation of rust layer can be achieved, excessive content is required have the P. Reduction of P content of this contributes to the improvement of weldability.

Cr: 0.05% or less. As described above, Cr causes a decrease in pH in the microscopic surface defects of the steel, promotes the oxidizability of condensed water in the defects, and induces corrosion, which reduces the bare weather resistance of the steel material. Let Therefore, in the present invention, the content of Cr is made as small as possible to less than 0.05%. This reduction in Cr amount also contributes greatly to the improvement of weldability.

Ti: 0.01 to 1.0%. Ti is as described above,
In the present invention, it is an element that is important as an element for promoting the formation of the stable rust layer instead of Cr, and does not have an adverse effect on corrosion resistance such as Cr that causes the decrease in pH. In addition, Ti also has the effect of refining the generated rust by refining the crystal grains of the steel structure, or improving the toughness and weldability. That is, the inclusion of Ti causes TiC, TiN, etc., which are strong ferrite transformation nuclei in the cooling process of the weld, to be dispersed and precipitated in the steel, and contributes greatly to the refinement of ferrite in the structure of the weld heat affected zone. If the Ti content is less than 0.01%, this effect does not occur, and if it exceeds 1.0%, the effect is saturated and it is not economical. In this respect, in order to exert the effect of Ti more, it is preferable to contain 0.05% or more, and if Ti exceeds 0.5%, embrittlement of the steel may become a problem, which is not economical as mentioned above. . Therefore, the Ti content is preferably in the range of 0.05 to 0.5%.

[0029]

Cu: 0.05 to 3.0% and Ni: 0.05 to 6.0% 1
Seed or two species. Cu and Ni are elements that both have the effect of improving corrosion resistance and the effect of improving weldability.
By containing the seed, these effects are exhibited.
Among them, Cu is an element that is electrochemically nobler than iron, and has the effect of densifying the rust formed on the steel surface, promoting the formation of a stable rust layer, and improving the corrosion resistance such as weather resistance. It also contributes to the improvement of weldability. If the Cu content is less than 0.05%, this effect does not occur, and if it exceeds 3.0%, no further effect is obtained, and conversely, the material becomes brittle during processing such as hot rolling for the production of steel. Can cause. From this viewpoint, the upper limit of the Cu content is preferably 0.5% or less. Therefore, the Cu content is in the range of 0.05-3.0%,
It is preferably in the range of 0.05 to 0.5%.

Similar to Cu, Ni has the effect of densifying the rust formed on the steel surface, promoting the formation of a stable rust layer, and improving the corrosion resistance such as weather resistance. It also contributes to the improvement of weldability. Further, Ni also has an effect of suppressing the hot working brittleness of Cu. Therefore, when it is contained together with Cu, a synergistic effect of the corrosion resistance improving effect and the hot working brittleness suppressing effect can be expected. If the Ni content is less than 0.05%, such an excellent effect cannot be obtained. On the other hand, however, excessive Ni content widens the solid-liquid solidification temperature range in the complete austenite structure, promotes segregation of low melting point impurity elements to dendrite grain boundaries, and reacts with S to improve the grain boundary of the weld metal. At the same time, a low melting point NiS compound is deposited to deteriorate the ductility of the grain boundaries of the solidified metal. Therefore, excessive Ni content adversely affects the weld hot cracking resistance, so the upper limit content should be 6.0%, and as a result, the Ni content should be in the range of 0.05 to 6.0%.

Mo: 0.05 to 3.0% and W: 0.05 to 3.0% of 1
Seed or two species. Mo: 0.05 to 3.0% and W are elements that have the effect of improving corrosion resistance when coexisting with Ti or Ni, and are selectively contained. The inclusion of one or two of these has the effect of densifying the rust formed on the steel surface, promoting the formation of a stable rust layer, and improving the corrosion resistance such as weather resistance. More specifically, the generated rust is densified, and the rust property is made to be cation selectivity that is unlikely to be associated with a corrosive anion such as chloride ion, thereby suppressing penetration of the corrosive anion into the rust layer. This effect is a dense stable rust generation effect of Ti and Ni (amorphous rust,
β-FeOO which promotes rust formation and corrosion of α-FeOOH
Suppressing H 2) improves the corrosion resistance of steel. Mo
If the W and W contents are less than 0.05% each, this effect does not occur.
If the content exceeds 3.0%, this effect becomes saturated. Therefore, the Mo and W contents are 0.05 to 3.0%.

Ca: 0.0001 to 0.01%. Ca is an element that further improves the corrosion resistance, and also has the effect of improving the weldability. One of the effects of improving the corrosion resistance of Ca is S which is harmful to the corrosion resistance.
Is to fix and clean the steel matrix.
In addition, as another action, a small amount of Ca dissolved in steel dissolves in a trace amount with the corrosion reaction of iron in the course of corrosion progress on the steel surface or micro defects to become alkaline. Therefore, it is an element that has a solution pH buffering effect on the corrosion (anode) tip and an effect of suppressing corrosion at the corrosion tip. These have an action that is completely opposite to the action of the element such as Cr that lowers the pH during dissolution. Therefore, when Ca is used in combination with Ti, together with the effect of reducing Cr of the present invention and the effect of promoting the formation of a stable rust layer such as Ti, a synergistic effect of improving corrosion resistance such as bare weather resistance occurs. This synergistic effect is not exhibited when the Ca content is less than 0.0001%,
If it is contained excessively, the effect is saturated and it is not economical. In particular, if Ca is contained excessively, the cleanliness of the steel is deteriorated, and there is a possibility that the furnace wall during steelmaking is damaged during the production of the weather-resistant steel. Therefore, the Ca content is 0.0001.
The range is to 0.01%.

Next, the selective addition element of the steel material of the present invention will be explained. Al: 0.05 to 0.50%, La: 0.0001 to 0.05%, C
e: 0.0001~ 0.05%, Mg: 0.0001~0.05% of one or
More than seeds. Al, La, Ce, and Mg are elements that have the effect of improving corrosion resistance, and are selectively contained. The inclusion of one or more of these has the effect of densifying the rust formed on the steel surface, promoting the formation of a stable rust layer, and improving the corrosion resistance such as weather resistance.

Of these, Al also has the effect of further promoting the formation of a stable rust layer when added in combination with Ti. Al also has the effect of improving weldability. Therefore, in the case of further improving the corrosion resistance of the steel material of the present invention, Al is 0.05 to 0.50.
Include in the range of%. Further, Al is an element effective as a deoxidizing element for molten steel, capturing solid solution oxygen, preventing blowholes from occurring, and improving the toughness of steel. If the Al content is less than 0.05%, these sufficient effects cannot be obtained.On the other hand, if the Al content exceeds 0.50%, the corrosion resistance improving effect is saturated, and conversely, the weldability is deteriorated or the alumina is deteriorated. The toughness of steel deteriorates due to the increase of system inclusions.

Further, La, Ce and Mg are dissolved in a trace amount with the corrosion reaction of iron and become alkaline in the course of corrosion progress on the steel surface or micro defects. Therefore, it is an element that has a solution pH buffering effect on the corrosion (anode) tip and an effect of suppressing corrosion at the corrosion tip. They are,
It has a completely opposite action to the action of the element such as Cr that lowers the pH when dissolved. Therefore, when used together with the effect of reducing Cr and the effect of promoting formation of a stable rust layer such as Ti of the present invention, a synergistic effect of further improving corrosion resistance can be expected. This effect is not exhibited when the content of each is less than 0.0001%, but even if it is excessively contained, the effect is saturated and it is not economical, and the mechanical properties of steel deteriorate. Therefore, the content of each is La: 0.0001 to 0.05%, Ce: 0.0001 to 0.0
5%, Mg 0.0001-0.05%.

One or two of Zr, Ta, Nb, V and Hf
0.50% or less in total for seeds and above. Zr, Ta, Nb, and V have the same effect as Ti, and amorphize the generated rust and α-FeOOH.
To form a fine and dense rust and form a stable rust layer in which β-FeOOH is suppressed. However, its effect is inferior to that of Ti. Therefore,
These elements are selectively contained as a complement to the effect of Ti.

The effect of these Zr, Ta, Nb, and V is that the total (total amount) of one or more of these elements should be at least the required content of Ti, preferably at least 0.1%. Exerted by. However, even if the content exceeds 0.50%, the effect is the same, top quantifiers is 0.50% of total (total amount).

Further, in the present invention, the carbon equivalent A (%) of the steel material is
0.20 or less [However, carbon equivalent A = C + Si / 22 + Mn / 6 + P / 10
−Cu / 20 −Ni / 15 ＋ Cr / 2−Ti / 2−Ca−Al / 35 ＋ Mo / 4 + W /
6 (%)] as low as possible. This is because in addition to the excellent weather resistance of steel materials for structures such as minority main girder bridges, weldability is ensured even when the plate thickness is large. More specifically, 50
Even for thick plates with a thickness of mm or more, or even 80 mm or more, without preheating, and without causing welding defects such as welding cracks,
This is to secure weldability that enables high-efficiency welding such as heat input of 5 KJ / mm or more, and in some cases large heat input welding of 100 to 300 KJ / mm or more. The reduction of the carbon equivalent of this steel lowers the hardenability of the steel matrix and is also effective for refining the ferrite in the structure of the heat affected zone during welding. Therefore, when the carbon equivalent A (%) of steel exceeds 0.20,
Weldability deteriorates, and it is not possible to perform high-efficiency welding work such as large heat input welding with a heat input of 5 KJ / mm or more with a thick plate with a thickness of 50 mm or more, and it is suitable for a small number of girder bridge applications of the present invention Cannot be used.

In order to reliably secure the weldability of a thick plate having a thickness of 50 mm or more, or 80 mm or more, or to improve the weldability, the carbon equivalent A is set when the thickness of the steel material is 50 mm or more. (%) Is 0.19 or less, and when the steel material has a thickness of 80 mm or more, the carbon equivalent A (%) is preferably 0.18 or less. Further, when the thickness of the steel material is 100 mm or more, the carbon equivalent A (%) is more preferably 0.16 or less.

Usually, the weldability of steel material largely changes depending on the plate thickness, and the greater the plate thickness, the worse the weldability. Also,
The weather resistance and weldability of steel materials are also issues that often conflict with each other. If an alloy element is added to improve the weather resistance, the weldability is sacrificed. However, in the present invention, Cr and P are mainly reduced from the viewpoint of weather resistance, but this also leads to the improvement of weldability, and the weather resistance and weldability of steel materials, which are the conflicting problems, are Both are improving and improving.

In the present invention, the carbon equivalent A is the carbon equivalent Ceq of the Japanese welding industry standard [carbon equivalent Ceq = C + Si / 24
+ Mn / 6 + Cr / 5 + Mo / 4 + Ni / 40 + V / 14 (%)], the carbon equivalent A of the present invention is set. this is,
Tested on the steel material within and outside the composition range of the present invention, the result of evaluating from both corrosion resistance and weldability, the action of each contained element, there is a part different from the concept of conventional carbon equivalent Ceq,
This is because it was found that the steel within the composition range of the present invention needs to be improved in order to have both corrosion resistance and weldability. For example, Cu and Ni are elements that hinder the weldability in the conventional carbon equivalent Ceq, but in the present invention, they contribute to the improvement of the corrosion resistance and, on the contrary, have the effect of improving the weldability. In addition, Ti, Ca, and Al are not considered in the conventional carbon equivalent Ceq, but have the effect of improving weldability. Furthermore, P also impairs weldability, although it is not considered in the conventional carbon equivalent Ceq. Therefore, in the present invention, the carbon equivalent A = C + Si / 22 + Mn / 6 + P / 10-Cu / 20 is added in consideration of both the corrosion resistance and weldability of these elements.
-Ni / 15 + Cr / 2-Ti / 2-Ca-Al / 35 + Mo / 4 + W / 6 (%) was specified.

Next, the components and composition of rust in the present invention will be described below. In the present invention, the main component of rust is α-Fe.
It shall consist of OOH and / or amorphous rust . Among them, especially amorphous rust forms a stable rust layer that is extremely finer and more dense than crystalline rust, and as described above, even if a "defect part" as a rust film is formed, it is amorphous. The quality rust part has a "defect repair function" that fills this hole. Therefore, the higher the ratio of amorphous rust in iron rust (amorphous degree), the higher the corrosion resistance. Therefore, in the present invention, the rust generated on a steel material surface, defining the fraction of amorphous component as determined by X-ray diffraction method and 30 wt% or more.

On the other hand, rust other than this, particularly crystalline rust such as β-FeOOH, progresses as the rust becomes a regulation even if the ratio of the amorphous or α-FeOOH in the rust is high. Therefore, it is necessary to suppress it as much as possible. Therefore, in the present invention ,
Rust generated on a steel material surface, it was determined by X-ray diffraction method β-
The fraction of FeOOH component is restricted to 20wt% or less. When the fraction of the amorphous component of rust is less than 20 wt% and the fraction of the β-FeOOH component exceeds 20 wt%, the above α-FeOOH, β-FeOO
Since H, γ-FeOOH, and Fe ₃ O ₄ have a large amount of crystalline rust components and the rust on the surface of the steel material does not form a dense stable rust layer, it may not be possible to guarantee high corrosion resistance of the steel material.

In the present invention, the high corrosion resistance of the rust formed on the surface of the steel material is the corrosion resistance of the steel material in a salt corrosion environment. Therefore, in order to guarantee this high corrosion resistance, the corrosion resistance of steel materials after atmospheric exposure including atmospheric exposure of steel materials for 1 year, and salt water spray simulating a salt corrosive environment (5% salt water spray once a week) Need to be evaluated. Whether or not the rust is such that the average thickness reduction of the steel after exposure to the atmosphere is preferably 0.80 mm or less, more preferably 0.50 mm or less is an important measure of the corrosion resistance of the steel. If the corrosion weight loss (thickness reduction) exceeds this level, it is not possible to guarantee high corrosion resistance exceeding the above-mentioned conventional techniques.

As a means for measuring the amorphousness of rust, quantification of iron rust component by the powder X-ray diffraction method and its measurement in "Corrosion protection 95 C-306 (pp. 341-344)" The powder X-ray diffraction method disclosed in “Application” is effective. In this document, the powder X-ray diffraction method was applied to weather-resistant steel materials, and an attempt was made to quantify the iron rust component on the steel surface.The higher the proportion of amorphous rust in iron rust (amorphous degree), It supports the corrosion resistance improvement model that becomes a precise and stable rust layer. As a more specific powder X-ray diffraction method, in the same literature, a standard X-ray diffraction method was used in which a fixed weight ratio of CaF ₂ or ZnO as an internal standard was mixed with a rust sample taken from steel and powdered. The rust component of each crystallinity was quantified from the integrated intensity ratio of the unique diffraction peaks of each of the above 5 types of rust and the calibration curve of each rust component obtained in advance, and the total rust The ratio of the amorphous component is calculated by subtracting the amount of each crystalline rust component from the amount. This is because it is difficult to obtain the integrated intensity ratio of the diffraction peak of the amorphous component itself and it is difficult to quantify it.

Incidentally, as disclosed in the same document,
In addition to X-ray diffractometry, other analytical methods such as infrared spectroscopic analysis can perform qualitative analysis of rust components, but quantitative analysis is difficult. There is no. Therefore, the amorphousness of rust on the surface of steel referred to in the present invention means the X-ray diffraction method, especially the powder disclosed in the above-mentioned document.
It is measured quantitatively by the X-ray diffraction method.

Regarding the method of forming the dense stable rust layer of the present invention, the steel material of the present invention is not particularly positively treated, or even under a salt corrosive environment such as salt water or snow melting salt coming in. It is a great advantage that a dense stable rust layer is generated during use as a structural material for bridges and the like. However, from the viewpoint of quality assurance that guarantees reliable corrosion resistance such as bare weather resistance, after the steel material is manufactured, if necessary, pretreatment such as pickling is performed, and then heat treatment is performed in an atmosphere such as a gas with a controlled oxidation potential. Or, it is chemically surface-treated with chemicals such as phosphates, chromates, oxidizers, etc. to amorphize the rust generated during the steel manufacturing process,
A dense stable rust layer may be positively formed.

The target steel material for determining the degree of rust amorphous on the surface of the steel material may be, for example, a steel material that is not actually used as a structural material for a bridge, or may be exposed to the atmosphere for 1 year (1 week per week). It may be a steel material that has been sprayed with salt water once) or a steel material that has been actually used as a structural material for a bridge.

Further, although the steel material structure of the present invention is basically a mixed structure of ferrite and pearlite, for example, the required strength as a structural material for a structure such as a bridge.
In order to secure strength and toughness of 390 to 630 N / mm ² grade or higher and to have excellent corrosion resistance, the amount of ferrite is preferably 90% or more. As the amount of ferrite increases and the steel structure approaches a ferrite phase single layer, the steel structure itself is less likely to form a micro battery, and corrosion resistance such as bare weather resistance is improved. Therefore, it is more preferable that the steel material structure has a ferrite content of 95% or more.

Next, a method for manufacturing the steel material of the present invention will be described.
The steel material of the present invention can be manufactured by a normal method for manufacturing a thick steel plate. That is, after steel is continuously cast or melted by an ingot making method, slab rolling, hot forging, or hot working such as thick plate rolling is performed to manufacture a predetermined product sheet thickness. The hot working conditions and the cooling and heat treatment conditions after hot working are
For example, depending on the requirements and specifications of mechanical properties such as strength of 390 to 630 N / mm ² grade or higher as a structural material for bridges,
It is decided as appropriate. Therefore, in addition to normal hot working,
After ensuring low alloying or low carbon equivalent to ensure weldability, ensure mechanical properties such as the strength, the steel structure of the present invention, preferably in order to have a ferrite content of 90% or more, Forced cooling such as accelerated cooling after hot working or controlled rolling may be performed. Further, the heat treatment after the hot working may be appropriately performed such as rolling online direct quenching (DQ) or offline quenching and tempering (QT).

[0052]

EXAMPLES Next, the significance of each requirement of the steel material of the present invention described above will be explained with reference to examples. Steel ingots having the chemical compositions and carbon equivalents shown in Tables 1 and 2 were melted, and these steel ingots were hot-rolled and then forcedly cooled by accelerated cooling to produce thick steel plates (Tables 1 and 2 The thickness of Comparative Example No. 3 is 16 mm
In other Comparative Examples Nos. 1, 2, and 4 to 10, the plate thickness was 30 mm, and the plate thicknesses of the present invention examples were all 50 mm). The balance of the chemical components shown in Tables 1 and 2 is Fe and inevitable impurities.

(Corrosion resistance test 1; Tables 3 and 4). For each of these thick plates, test specimens were collected and simulated for painting use.
Prepare test pieces coated with μm-thick butyral resin, make cuts in the coating film in advance to create artificial coating film defects, and then test each test piece with a corrosion acceleration test with 0.1% salt water spray. The corrosion acceleration test by 0.5% salt water spray and the atmospheric exposure test for 1 year including 5% salt water spray once a week were performed to evaluate the long-term durability. Evaluation is the appearance evaluation of the steel material of the test piece after each test, the bulge width (mm) of the coating film defect part
Was measured, and a comprehensive evaluation (◎ ○ △ ×) was performed. In addition,
Since the results of the corrosion acceleration test with 0.1% salt spray were poor in each comparative example, the corrosion acceleration test with 0.5% salt spray was not performed. The results are shown in Tables 3 and 4.

In addition to the relatively short-term corrosion acceleration test by salt spray, the purpose of carrying out the atmospheric exposure test for one year was that the steel of the present invention was used for structural materials such as bridges in a salt corrosive environment. Therefore, accurate evaluation cannot be performed unless the test is suitable for corrosion under the actual use conditions.

Regarding specific conditions of the corrosion test, in the corrosion acceleration test by salt spray, 0.1% and 0.5% salt spray was irradiated with ultraviolet rays (8 hr) → soaking in salt water (2 minutes) → constant temperature and constant humidity (60
The conditions were such that 60 cycles of (° C, 95RH, 16hr) were repeated. In addition, the conditions for the one-year atmospheric exposure test including salt spray are 1 week per week in accordance with the actual salt corrosion environment.
After spraying 5% salt water once, the test material is facing south and 30
It was installed at an inclination of °. Also, the appearance evaluation of the test piece is JIS
Rating number method (RN method) based on the method specified in
It was evaluated by 10 grades, with the best being 10 and the worst being 1.

(Corrosion resistance test 2; Table 5). In addition, in order to see the effect of the resin type on the coating of the test piece, of the test materials shown in Tables 1 and 2, for Comparative Example Nos. 1 and 2 and Invention Examples No. 17 and 28, the type of resin to be applied Prepare test pieces with different coatings, make cuts in the coating film in advance to make artificial coating film defects, and then, as in the above corrosion resistance test 1, expose to atmospheric air for one year including once-weekly salt water spray. Perform a test, after this, the appearance of the steel material of the test piece, the coating film bulge width of the coating film defective portion (mm) is measured,
A comprehensive evaluation (◎ ○ △ ×) was performed. The results are shown in Table 5.

In Table 5, the appearance of the test pieces was evaluated by the RN method, and the best one was evaluated as ◯, the worst one was evaluated as x, and the middle was evaluated as Δ. Also, the bulge width is 0.80 mm or more ×, 0.5 to 0.8 mm △, 0.5 mm
The following was evaluated as ○.

(Corrosion resistance test 3; Table 6). Furthermore, simulating the use of bare steel (unpainted), the test pieces of the test materials shown in Tables 1 and 2 were used as they were in the same manner as in the above corrosion resistance test 1, in air for one year including salt water spraying once a week. After performing an exposure test, measure the average thickness reduction (mm) of the steel material of the test material and the rust structure (amorphous degree) on the surface of the test piece, and then perform a comprehensive evaluation (◎ ○ △
X) was performed. The results are shown in Table 6.

The average plate thickness reduction amount of the steel material of the sample material is measured by measuring the average plate thickness of the sample material before and after the corrosion acceleration test and the atmospheric exposure test with a micrometer, and taking the density into consideration, the average plate thickness reduction The amount (mm) was calculated.

As a means for measuring the degree of amorphousness,
Performed by the powder X-ray diffraction method disclosed in "Corrosion protection 95 C-306 (pp. 341 to 344)", and ZnO of a certain weight ratio as an internal standard was mixed with a rust sample taken from steel and powdered. Identified by X-ray diffraction, the α-FeOOH, β
−FeOOH, γ-FeOOH, and Fe ₃ O ₄ The crystalline intensity of each crystalline rust component was determined from the integrated intensity ratio of the unique diffraction peaks of each of the 5 types of crystalline rust and the calibration curve of each rust component determined in advance. Was quantified, and the amount (%) of the amorphous component was calculated by subtracting the amount of each crystalline rust component from the total amount of rust. In addition, in Tables 6 and 4, the fraction of the amorphous rust component is A: 0 to
30wt%, B: 31-40wt%, C: 40wt% or more.

(Corrosion resistance test 4; Table 7). Further, while forming rust in advance on the surface of the test steel materials shown in Tables 1 and 2 (Comparative Examples No. 1 and 2, Invention Example No. 17), among the rust components, particularly β-FeOOH
Of crystalline steel and amorphous rust by changing the composition ratio of the amorphous rust by heat treatment that controls the oxidation potential, and a plurality of steel materials each positively provided with amorphous rust, the corrosion resistance test
In the same manner as 1), an atmospheric exposure test including salt water spraying once a week was conducted for 1 year, and thereafter, along with the average amount of reduction in steel thickness,
The rust amorphousness of the surface of the test piece and the proportion of β-FeOOH were measured by the powder X-ray diffraction method, respectively, and comprehensive evaluation (◎ ○ △
X) was performed. The results are shown in Table 7. In Table 7, the fractions of amorphous rust component are A: 0 to 30 wt%, B: 31 to 40w.
t%, C: 40 wt% or more, the proportion of crystalline rust component of β-FeOOH is A: 20 wt% or more, B: 10 to 20 wt%, C: 10 wt% or less, the average plate thickness of steel Reduction: ○: 0.5 mm or less, △: 0.5
~ 0.8 mm, x: 0.8 mm or more.

(Welding test 5; Table 8). Furthermore, a part of the sample steel materials shown in Tables 1 and 2 (Comparative Examples No. 1 to 3, Invention Example No. 23),
After hot rolling, accelerated cooling or direct quenching was performed to obtain a thick plate with the width and strength varied from 16 to 100 mm. Large heat input welding is basically performed without preheating these steel sheets, and the weld zone has a minimum preheating temperature that can prevent hot cracking and cold cracking,
Tensile strength (N / mm ² ) and toughness (vE-40) were evaluated and comprehensive evaluation (◎
○ △ ×) was performed. The results are shown in Table 8.

Of these, in particular, the invention example steel material No. 23 was tested by preparing 7 cases in which the plate thickness and the tensile strength were variously changed. Large heat input welding was performed by the submerged arc welding method with a heat input of 35 KJ / cm. Regarding the method and conditions of the specific evaluation test, the high-temperature cracking ratio of the test steel material was determined by the C-type jig constrained butt welding cracking test (hot-cracking test) established by JIS standard. The low temperature cracking of the test steel material is the diagonal Y-type restraint butt welding cracking test (cold cracking test) established by JIS standard.
The evaluation was made based on the preheating temperature of the test material that can prevent the occurrence of cracks. The toughness is evaluated by the absorbed energy vE-40 (N / mm ² ) of the welded joint bond portion at −40 ° C. Table 8 shows the No. of the test steel materials. (Table abbreviations) are Tables 1, 2 and 3, respectively.
Corresponds to No. 4 (abbreviation in table).

(Corrosion resistance test 6; Table 9). In addition, No. 1 in Tables 1 and 2
Regarding the bare weather resistance of the specimen steels of 1, 2, 12, and 17 with varying amounts of ferrite in the microstructure, the test pieces were
Table 9 shows the results of evaluation by the average amount of reduction in steel sheet thickness after 1 year of atmospheric exposure (including salt water spraying once a week) under the same conditions as above.
Shown in. In Table 9, the average reduction in plate thickness of the steel material is indicated by ○: 0.5 mm or less, Δ: 0.5 to 0.8 mm, ×: 0.8 mm or more.

The test results of the test steel materials shown in Tables 1 and 2 will be described below, and the evaluation results will be described using the tables.

Comparative Example: In the comparative example of Table 1, No. 1 is a conventional ordinary steel, and Nos. 2 to 8 are conventional weathering steels. Of these, N
In o.1 and 6, the content of Cu and Ni is too small and the content of Ti is also absent, and the carbon equivalent A of the present invention exceeds 0.20. N
In o.2, 3, and 7, the content of Cr is too large and the content of Ti is also absent, and the carbon equivalent A of the present invention exceeds 0.20. No.
Each of 4, 5, 7, and 8 has too much content of C, P, Cr, and S, and has no Ti content except for No. 8, and the carbon equivalent A of the present invention exceeds 0.20. Furthermore, Nos. 9 and 10 do not contain Ti.

Therefore, as shown in Tables 3 and 4, in each of the comparative examples, the results of each corrosion resistance test showed that the appearance of the steel surface was poor, and the swelling width of the coating film was 1.42 mm or more in the corrosion acceleration test. It was as large as 0.39 mm or more in the atmospheric exposure test, and its corrosion resistance was extremely poor. Further, as shown in Table 5, regardless of the type of resin used for coating, the appearance was poorly evaluated, the blister width of the coating film was large, and the corrosion resistance was extremely poor.

Further, as shown in Table 6, these comparative examples are remarkably inferior in corrosion resistance even when they are used without coating. And
The reason why the corrosion resistance in these coated or unpainted comparative examples is inferior is that, as can be seen from the same Table 6, the amorphousness of rust on the steel surface is low (A level), and as shown in Table 7, the proportion of β rust is Because there are many. The reason why stable rust intended by the present invention is not formed is that the composition of the steel material of each comparative example is out of the range of the present invention.

Further, in all of these comparative examples, the carbon equivalent A of the present invention exceeds 0.20, and the weldability is poor as shown in Table 8. Specifically, even at a level of a relatively small plate thickness of 16 to 30 mm, low temperature cracking occurs without preheating, but low temperature cracking occurs, and preheating is necessary to prevent the occurrence of this cracking. Furthermore, the toughness of the welded joint bond is extremely low.

As shown in Table 1, Comparative Example No. 8 has S
The amount was out of the range of the present invention, and hot embrittlement cracking occurred during hot rolling, so that the thick plate itself could not be manufactured. Therefore, Comparative Example No. 8 was not evaluated, such as a corrosion resistance test. Further, although not clearly shown in Table 3, the rust on the steel surface of Comparative Examples No. 1 to 10 is β-
While the ratio (fraction) of the crystalline rust component of FeOOH exceeds 20 wt%, all of Invention Examples No. 11 to 30 have β-FeOO
The ratio of crystalline rust component of H (β rust) is 20 wt% or less.

(Evaluation of Corrosion Resistance of Invention Examples): In contrast, Table 1
Inventive Examples Nos. 11 to 30 of No. 2 and No. 2 all satisfy the scope of the present invention in the composition of the steel material, and the accelerated corrosion test and the atmospheric exposure in Tables 3, 4, 5 and 6 with or without coating. Appearance evaluation was good in all of the tests, and the swelling width of the coating film was 0.50 mm or less in the corrosion acceleration test (0.1% salt water) and 0.33 in the atmospheric exposure test.
mm or less, the average thickness reduction of unpainted material in the atmospheric exposure test is 0.8 mm or less, and those with more excellent corrosion resistance are 0.8 mm or less. From these results, it can be seen that the steel material of the present invention has excellent corrosion resistance and is particularly suitable for use in a salt corrosion environment. The reason why the corrosion resistance of these invention examples is excellent in coated or unpainted is that, as can be seen from the same Table 6, the amorphousness of rust on the steel surface is high (B and C levels), and as shown in Table 7, β This is because the proportion of rust is low. And, the reason why the stable rust intended by these present invention is formed is that the composition of the steel material of each invention example includes Ti, and has a small amount of Cr,
This is because it is within the scope of the present invention such as containing one or more of Mo and W.

Among these invention examples, the invention examples of Nos. 15 and 16 are inferior in corrosion resistance to the other invention examples because the carbon equivalent A is relatively high. Invention examples of No. 18 and 22
The corrosion resistance is inferior to the other invention examples, but the Cr content and the carbon equivalent A are relatively high although they are within the specifications, and as shown in Tables 6 and 7, the corrosion resistance is lower than that of the other invention examples. This is because the crystallinity is inferior (B) and the proportion of β-rust is relatively large.

Therefore, these results support the importance of the regulation of Cr content and P content and the inclusion of one or more of Ti and Mo and W in the present invention. Inventive Examples Nos. 25, 26, and 30 having a relatively high Ni content can also be manufactured without causing high temperature embrittlement cracking during hot rolling because the S content is low.
Moreover, the characteristics were good as in the other invention examples.

(Evaluation of Weldability of Inventive Example): Further, as is clear from Table 8, inventive example steel material No. 23 in Table 2 is 50
Even at the level of relatively thick plates up to ~ 100 mm,
Excellent weldability even for steel plates of 610 to 630 N / mm ² grade with relatively high strength. More specifically, it can be seen that there is no hot cracking, the preheating temperature for preventing cold cracking is 25 ° C or less, and high heat input welding is possible without preheating. Furthermore, the strength is high and the toughness of the welded joint is significantly higher than that of the comparative example.

(Carbon Equivalent and Weldability of Inventive Example): In FIG.
The results obtained by arranging the relationship between the carbon equivalent A defined by the present invention and the weldability of the test materials of Nos. 1 to 30 in Tables 1 and 2 are shown. In Fig. 1, ● indicates that the comprehensive evaluation including bare weather resistance and weldability in Tables 3 and 4 is poor, and ○ indicates that the overall evaluation is good. As is clear from FIG. 1, the carbon equivalent A specified in the present invention is 0.20, good and bad weldability is separated, and the carbon equivalent A specified in the present invention has a critical meaning that it is 0.20 or less. I understand.

(Rust Composition and Corrosion Resistance of Invention Examples):
As is clear from the above, in the comparative examples of Test Nos. 43 to 48, β
-Regardless of the proportion (fraction) of crystalline rust in FeOOH, the average thickness reduction of steel exceeds 0.8 mm because the proportion of amorphous rust component is less than 30 wt%. In contrast, the test
The invention examples of Nos. 49 to 54 all have an amorphous rust component ratio of 30.
Since it is more than wt%, the average thickness reduction of steel is 0.8 mm.
It is below. However, in the invention example of Test No. 49 in which the proportion of β-FeOOH crystalline rust exceeds 20 wt%, the average thickness reduction of the steel material is 0.5 to 0.8 mm, and the proportion of β-FeOOH crystalline rust is However, the bare corrosion resistance is slightly inferior to the average steel sheet thickness reduction amount of the other steels of the invention examples of 20 wt% or less being 0.5 mm or less. Therefore, from this result, it is understood that the amorphization of rust on the steel surface of the present invention and the suppression of crystalline rust of β-FeOOH are important from the viewpoint of bare weather resistance.

(Ferrite Amount and Corrosion Resistance of Steels of Invention Examples): Further, in Table 9, the average plate thickness reduction amount of the steel material is the same as in Table 7, ○: 0.5 mm or less, △: 0.5 to 0.8 mm, ×: Shown at 0.8 mm or more. As is clear from Table 9, in all the comparative examples of Test Nos. 55 to 60, the average sheet thickness reduction amount of the steel material exceeds 0.8 mm, and the bare weather resistance is poor. In particular, in the comparative examples of Test Nos. 57 and 60, the average plate thickness reduction amount of the steel material exceeded 0.8 mm, even though the ferrite content was 90% or more. This is because the P and Cr contents of the steel materials 1 and 2 (Table 1) are high, and the range of the present invention is deviated from the upper limit. On the other hand, in the invention examples of Test Nos. 61 to 66, the ferrite content was 90%.
In the invention examples of Test Nos. 62, 63, and 65, which have a ferrite content of less than 90%, the invention example of Test Nos. 62, 63, and 65 has an average thickness reduction of 0.5 mm or less. The average reduction in plate thickness is 0.5 to 0.8 mm, and it is clear that a ferrite content of 90% or more is advantageous for improving the bare weather resistance.

[0078]

[Table 1]

[0079]

[Table 2]

[0080]

[Table 3]

[0081]

[Table 4]

[0082]

[Table 5]

[0083]

[Table 6]

[0084]

[Table 7]

[0085]

[Table 8]

[0086]

[Table 9]

[0087]

EFFECTS OF THE INVENTION According to the present invention, particularly as a structure such as a minority main girder bridge under a salt corrosion environment, it has an excellent bare weather resistance that can be used without painting, and the heat input amount without preheating.
It is possible to provide a steel material capable of highly efficient welding such as high heat input welding of 5 KJ / mm or more. Therefore, in particular, the use of the steel having excellent weather resistance of this kind is newly and greatly expanded, and its industrial value is great.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a relationship between a carbon equivalent A specified in the present invention and a comprehensive evaluation of weather resistance and weldability of steel materials.

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-6-240406 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (8)

(57) [Claims] 1. In mass%, C: 0.15% or less, Si: 0.10
~ 1.0%, Mn: 1.5% or less, S: 0.02% or less, P: 0.05%
Below, Cr: 0.05% or less, Ti: 0.01 to 1.0%, Ca: 0.0001
~ 0.01%, and Cu: 0.05-3.0% and Ni: 0.05-6.0%
1 or 2, and Mo: 0.05 to 3.0% and W: 0.05
〜3.0% of 1 type or 2 types, balance Fe and unavoidable impurities, and carbon equivalent A (%) of 0.20 or less
[However, carbon equivalent A = C + Si / 22 + Mn / 6 + P / 10-Cu / 20 -N
i / 15 + Cr / 2-Ti / 2-Ca-Al / 35 + Mo / 4 + W / 6 (%)]
Then, the rust generated on the surface of this steel material was analyzed by the X-ray diffraction method.
When the obtained amorphous component fraction is 30 wt% or more, β-FeOOH composition
Steel with excellent corrosion resistance and weldability, characterized by a fraction of 20 wt% or less . 2. The steel material further comprises Al: 0.05 to 0.50%, L
a: 0.0001 to 0.05%, Ce: 0.0001 to 0.05%, M g: 0.00
The steel material having excellent corrosion resistance and weldability according to claim 1, which contains 01 to 0.05% of one kind or two or more kinds. 3. Excellent corrosion resistance and weldability according to claim 1 or 2, wherein the steel material further contains one or more of Zr, Ta, Nb, V and Hf in a total content of 0.50% or less. Steel material. 4. The steel material having excellent corrosion resistance and weldability according to claim 1, wherein the amount of ferrite in the steel structure is 90% or more. 5. The steel material is for structural purposes.
Steel excellent in corrosion resistance and weldability according to any one of 4
Material . 6. The structure according to claim 5, wherein the structure is a bridge.
Steel material with excellent corrosion resistance and weldability . 7. The method according to any one of claims 1 to 4.
Bridge made of abandoned steel . 8. The method according to any one of claims 1 to 4.
Welded joint made from broken steel .