Ferritic stainless steel excellent in corrosion resistance and strength of weld zone and tig welded structure

Abstract

Ferritic stainless steel excellent in corrosion resistance of the weld zone, in particular the corrosion resistance to crevice corrosion, even if eliminating the Ar gas shield at the time of TIG welding and, furthermore, excellent in weld zone strength and a structure of the same are provided. By making the ingredient system one satisfying the following formula (1) in ferritic stainless steel which has predetermined contents of ingredients by mass % under a usual corrosive environment, it is possible to raise the corrosion resistance. Furthermore, under a severely corrosive environment, by making the relationship between the back surface exposed width of the weld bead and the steel sheet thickness satisfy the formula (2) in addition to the relationship of the ingredients prescribed in formula (1), it is possible to obtain a welded structure strong in crevice corrosion. Furthermore, by improving the weld strength by making the contents of Nb and V satisfy formula 3, it is possible to obtain a welded structure with a high weld strength:

Si+2.5Al+3Ti≦1.50  (formula (1))

0<Wb/2t≦1.50  (formula (2))

(t: sheet thickness of welded metal, Wb: length of bead width exposed at back surface)

Nb×V≧0.01  (formula 3)

Description

TECHNICAL FIELD[0001]The present invention relates to ferritic stainless steel excellent in corrosion resistance and strength of a weld zone and a TIG welded structure of the same.BACKGROUND ART[0002]Ferritic stainless steel is generally excellent in corrosion resistance. Not only this, it is small in coefficient of thermal expansion and excellent in stress corrosion cracking resistance compared with austenitic stainless steel.[0003]When using such stainless steel as a structure, welding work is essential. General use ferritic stainless steel such as SUS430 has small C and N solid solubility limits, so there was the problem that it becomes sensitized at the weld zones (sensitization: meaning Cr is consumed by formation of carbides and, locally, the matrix Cr drops sharply, so the corrosion susceptibility rises) and falls in corrosion resistance. To solve this problem, high purity ferrite steel has been developed which reduces the amounts of C and N or adds Ti, Nb, or other stabilizi...

AI Technical Summary

Benefits of technology

[0030]According to the present invention, it is possible to provide ferritic stainless steel which is excellent in the corrosion resistance of a weld zone even if omitting an Ar gas shield. Further, it is possible to provide a welded structure which is excellent in the crevice corrosion and the strength resistance of the weld zone.

Problems solved by technology

General use ferritic stainless steel such as SUS430 has small C and N solid solubility limits, so there was the problem that it becomes sensitized at the weld zones (sensitization: meaning Cr is consumed by formation of carbides and, locally, the matrix Cr drops sharply, so the corrosion susceptibility rises) and falls in corrosion resistance.

On the other hand, it is known that high purity ferrite steel deteriorates in corrosion resistance at the scale parts formed due to the input heat in welding (oxide film on the surface).

Furthermore, if issues with the water quality causes the concentration of residual chlorine or the concentration of chloride ions to rise, the environment becomes a highly corrosive one.

Further, when it is necessary to add chlorine or another oxidizing agent for sterilization etc., the environment becomes a severely corrosive one in increasing cases.

In such an environment, the problem of the corrosion of the weld zone sometimes occurs.

However, PLT 1 secures the corrosion resistance by the formation of a Cr oxide film even if omitting the shield, but in a severe corrosive environment like the inside of a hot water tank where the crevice is extremely narrow, even if the Cr film is thick, there is an extremely high possibility of the formation of crevice corrosion.

For this reason, it is difficult to control the structure by just the ingredients of one of the materials.

Furthermore, PLT 2 shows to add a large amount of expensive Ni to a high purity ferrite system.

Ni and, further, Si, Nb, and Cu are elements which promote solution strengthening, so the material rises too much in strength or hardness and the workability deteriorates.

Furthermore, the formation of CaS, which is considered a problem in corrosion resistance in the literature, is greatly affected by the production conditions of the steel sheet and other added ingredients, so is not determined by just the Ca concentration.

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