JPH0570899A - Ferritic stainless steel excellent in corrosion resistance in weld zone - Google Patents

Abstract

PURPOSE:To provide a ferritic stainless steel improved in the corrosion resistance in the weld metal and heat affected zone in a weld zone. CONSTITUTION:The steel has a composition containing, by mass, <=0.025% C, <=0.6% Si, <=1.0% Mn, <=0.04% P, <=0.01% S, <=0.6% Ni, 16-35% Cr, 0.3-6% Mo, <=0.025% N, 0.01-0.5% Al, 0.1-0.6% Nb, and 0.05-0.3% Ti, further containing, if necessary, one or two kind among 0.1-1.0% Cu and/or 0.05-0.3% Zr, satisfying C+N<=0.04%, Nb+Ti>=7(C+N)+0.15, B=Cr+3(Mo+Cu)>=23.5, and P=5(Ti+ Zr)+20(Al-0.01)>=1.5, and having the balance essentially iron with inevitable impurities.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel having excellent corrosion resistance at welded parts.

[0002]

2. Description of the Related Art Although stainless steel is roughly classified into ferrite type and austenitic type, austenitic type is excellent in corrosion resistance, workability, weldability, high temperature strength, etc.
Has a wide range of uses. However, it has the drawback that stress corrosion cracking often occurs in chloride solutions. On the other hand, ferritic stainless steel is relatively inexpensive and does not contain Ni as an alloying element, so there is almost no risk of stress corrosion cracking in a chloride solution. However, taking SUS430, which is a typical type of ferritic stainless steel, as an example, it has the drawbacks that it does not have sufficient resistance to a little harsh environment, and that it causes intergranular corrosion due to heating and cooling during welding. is doing. It is already well known that increasing the amount of Cr and adding Mo are effective for improving the corrosion resistance.

However, the increase of Cr and Mo hinders the toughness of the material itself. It is known that the toughness of Mo-containing high Cr steel can be improved by reducing C and N. This C, N
Reduction is effective for improving the intergranular corrosion resistance, but C, N
There is a limit to the reduction of C., and at present, the susceptibility to intergranular corrosion cannot be completely eliminated at industrially attainable C and N content levels. It is a well-known fact that the adverse effects of C and N on the intergranular corrosiveness can be eliminated by adding a stabilizing element such as Ti or Nb capable of fixing C and N alone or in combination.

[0004]

On the basis of these technical backgrounds, Mo-containing high Cr excellent in corrosion resistance and toughness has been proposed, and in fact, low carbon, low nitrogen 18Cr-2.0Mo-Nb is proposed.
/ Ti steel was developed. However, when used in an environment that contains a lot of ions that cause corrosion such as chlorine ions and residual chlorine, severe corrosion often occurs in the welded part within a short period of time. Not enough yet.

It is presumed that this is because the heat input during welding causes high-temperature oxidation of Cr in the surface layer of the base metal, resulting in poor metal Cr in the surface layer and reduced corrosion resistance. For this reason, it is necessary to reduce the Cr oxidation loss during welding and to develop a material that simultaneously satisfies the corrosion resistance of the weld heat affected zone and the weld metal portion. Therefore, an object of the present invention is to obtain a new ferritic stainless steel that simultaneously satisfies the corrosion resistance of the heat-affected zone of the weld and the weld metal portion.

[0006]

The present inventors have made detailed studies on the influence of alloying elements on the corrosion resistance of welds. As a result, the corrosion resistance of the weld metal is Cr and M
Although it is improved by increasing the amount of o, addition of Ti in a certain amount or more deteriorates the corrosion resistance of the weld metal portion. Increasing the amounts of Cr and Mo is also effective in improving the corrosion resistance of the heat-affected zone, but the addition of Ti and Al in combination also significantly improves the corrosion resistance. Although the details will be shown also in the examples described later, Ti facilitates the formation of Al oxide during welding, and the formation of Al oxide is effective in preventing the oxidation loss of Cr. This is to significantly reduce it. This is because the standard free energy of formation of Al oxide is very low, and the oxygen potential is too high to generate Al oxide in the atmosphere during welding, so that Ti is preferentially oxidized to lower the oxygen potential. Due to Al
It seems that it is to facilitate the oxidation of. As described above, Ti is an effective element for improving the corrosion resistance of the weld heat affected zone.
However, addition of a certain amount or more of Ti causes surface defects and deterioration of the corrosion resistance of the weld metal, so it is necessary to minimize it. For these reasons, when the upper limit of the Ti addition amount is regulated, it is difficult to suppress the intergranular corrosion by adding only Ti, and it is necessary to add Nb in combination. It was also found that the corrosion resistance of the welded portion is further improved by adding proper amounts of Cu and Zr.

[0007]

The present invention has been completed on the basis of this finding, and the gist thereof is, in mass%, C: 0.025% or less, Si: 0.6% or less, Mn: 1.0% or less P : 0.04% or less S: 0.01% or less Ni: 0.6% or less Cr: 16 to 35% Mo: 0.3 to 6% N: 0.025% or less Al: 0.01 to 0.5% % Nb: 0.1-0.6% Ti: 0.05-0.3%, depending on the case, Cu: 0.1-1.0%, Zr: 0.06-0.3% One or two are contained, and C + N ≦ 0.04% and Nb + Ti ≧ 7 (C + N) +0.02 between these components.
15 is satisfied, and the following formula B = Cr + 3 (Mo + Cu) ≧ 2
3.5 and P = 5 (Ti + Zr) +20 (Al-0.0)
1) The relationship is maintained in the range of ≧ 1.5, and the balance is ferritic stainless steel which is excellent in corrosion resistance of the weld metal portion and the weld heat affected zone which are substantially composed of iron and unavoidable impurities.

[0008]

The function of each component contained in the steel of the present invention and the reason for limiting the content will be described below. C and N are elements inevitably contained in steel. C, N
When the content is reduced, it becomes soft and the workability is improved, and the generation of carbides and nitrides is reduced, and the weldability and the corrosion resistance of the weld are improved.
≦ 0.025% and N ≦ 0.025%.

Si is harmful to the hot cracking of the weld and the toughness of the weld, and since it hardens the steel, it is preferably low so that the upper limit is 0.6%.

Mn combines with S present in a trace amount in steel,
Since MnS, which is a soluble sulfide, is formed and corrosion resistance is lowered, the lower limit is preferable and the upper limit is set to 1.0%.

[0011] P is preferable to be low because it impairs the toughness of the base metal and the welded portion, but it is difficult to remove P of Cr-containing steel and causes an increase in manufacturing cost, so the upper limit is made 0.04%.

Since S adversely affects the corrosion resistance and the hot cracking of the welded portion, it is preferably low so that the upper limit is 0.01%.

Ni is an element effective for improving the toughness of ferritic stainless steel, but if it is too much, the cost will increase. The steel of the present invention is also set to not more than 0.6% which is regulated by ordinary ferritic stainless steel.

Cr is a main element that enhances the corrosion resistance of steel and significantly improves pitting corrosion resistance, crevice corrosion resistance and general corrosion resistance, but if it is less than 16%, its effect is small.
When it exceeds 5%, embrittlement becomes remarkable and it is difficult to manufacture a thin plate or to process a product. Therefore, the Cr content is set to 16 to 35%.

Mo is an effective element that enhances the corrosion resistance together with Cr, and the effect increases as the amount of Cr increases. However, at the Cr content level of the steel of the present invention, it is 0.3
If it is less than 6%, the effect of improving the corrosion resistance is small, and if it is added in excess of 6%, the ductility is lowered and it is difficult to work.

Al is an important element for constituting the present invention. By adding Ti together with Al, an Al film is easily formed on the surface layer of steel at the time of welding, and corrosion loss is prevented by preventing Cr oxidation loss. improves. However, the amount of Al is 0.01%
If it is less than 0.5%, an Al film is hard to be formed, and if it exceeds 0.5%, the surface quality of the raw material is deteriorated and the weldability is deteriorated. Therefore, the Al amount is set to 0.01 to 0.5%.

Nb, together with Ti, is an essential element for preventing intergranular corrosion, which is a problem in ferritic stainless steels having a C content level of the present invention steel, but if too much, it hinders weld toughness. The upper limit is 0.6%. The lower limit is determined from the viewpoint of preventing intergranular corrosion, and requires 0.1% or more.

Ti is an important element for constituting the present invention, and by adding Al together with Al, a Al film is easily formed on the surface layer of steel during welding, and corrosion resistance is prevented by preventing Cr oxidation loss. improves. Further, Ti also has a function of fixing C and N. However, if the Ti content is too high,
The upper limit is 0.3% because it deteriorates the surface quality of the material and the corrosion resistance of the weld metal. The lower limit is determined from the viewpoint of forming an Al film and preventing intergranular corrosion, and requires 0.05% or more.

In the present invention, from the viewpoint of preventing intergranular corrosion, the combined addition amount of Nb and Ti is limited to the limited formula [N
b + Ti ≧ 7 (C + N) +0.15] must be satisfied.

Cu and Zr are both effective elements for improving the corrosion resistance of the welded portion, but if they are too large, they impair the toughness of the welded portion, so Cu is 1.0% and Zr is 0.3%, and C is the upper limit.
If u is less than 0.1% and Zr is less than 0.05%, the effect is small, so the lower limits are made 0.1% and 0.05%, respectively.

When a fixed element is added as in the steel of the present invention, the fixed element is combined with N as well as C and consumed.
It is necessary to control both elements with the sum of (C + N). When a large amount of (C + N) is contained, the amounts of Ti and Nb must be correspondingly increased, but this deteriorates the cleanliness of steel, so the upper limit of the amount of (C + N) is 0.04.
%. On the other hand, it is desirable that the amount of (C + N) is low, and there is no particular lower limit.

In addition to the above content limits of the respective components, in the present invention, the following limited formula between each component of Cr, Mo, Cu, Al, Ti and Zr, B = Cr + 3 (Mo + C)
u) ≧ 23.5 and P = 5 (Ti + Zr) +20 (Al
-0.01) ≧ 1.5 must be satisfied. These equations are set through the experiment according to the present invention, and the B value regulates the minimum value required to satisfy the corrosion resistance of the weld metal part and the P value satisfies the weld heat affected zone. ..
In the weld metal part, Cr and Mo are the basic components for improving the corrosion resistance, and Mo contributes to the improvement of the corrosion resistance more than Cr. Therefore, the coefficient is set to 3 times that of Cr. Further, since Cu has the same effect as Mo, the coefficient is set to be the same as that of Mo. If the B value is less than 23.5%, sufficient corrosion resistance of the weld metal cannot be obtained, so the lower limit was made 23.5%.

In the heat-affected zone of welding, by adding Ti and Al in combination, it is possible to facilitate the formation of an Al oxide film on the surface layer of the steel during welding, and the formed Al oxide film causes an oxidation loss of Cr on the surface layer. Prevent. As a result, the formation of the Cr-poor layer is suppressed, which is effective in improving the corrosion resistance of the weld heat affected zone. Further, since Zr has almost the same standard free energy of formation as Ti, it seems that Zr has the same effect as Ti in forming the Al oxide film. Ti, Z
To obtain these effects between r and Al, P
It is necessary that the value is 1.5 or more.

[0024]

DETAILED DESCRIPTION OF THE INVENTION The working effects of the steel of the present invention will be specifically described below with reference to examples. A stainless steel having the chemical composition shown in Table 1 is melted and hot-rolled to a plate thickness of 3.5 mm.
The hot-rolled sheet was manufactured. Then, it was cold-rolled to a plate thickness of 1.0 mm, finish-annealed at 1000 to 1050 ° C., and used as a sample material.

[0025]

[Table 1]

In Table 1, No. 1 to No. 10 are steels having a composition range regulated by the present invention, and all of them are Nb as a fixed element.
It is a steel to which Al and Ti are added together and which contains Al as a trace element. No. 7 is Cu-containing steel, No. 8 is Zr
No. 9 is a steel containing Cu and Zr. No. 11
No. 18 is a comparative steel, and the production history is the same as the steel of the present invention. Of these, No. 11-No. 13, No. 15-N
o.18 is steel that does not satisfy the limiting formula, and No. 11,
No. 12 is Ti, No. 13 is Ti and Al, No. 18 is A
It is a steel to which 1 is not added. No. 14 is a steel to which Nb is not added.

The results of visual observation of the presence or absence of surface flaws on these steels are also shown in Table 1. As shown in Table 1,
It is clear that excessive addition of Ti causes surface flaws. In the present invention, since the upper limit of the amount of (C + N) is 0.04%, in order to prevent the occurrence of intergranular corrosion, T of 0.43% or more is required.
i must be included. In this case, since surface flaws are generated, it can be seen that it is effective to reduce the Ti content and add a fixed element such as Nb as in the present invention.

The steels produced as described above were TIG-welded and subjected to surface analysis and immersion test. TIG welding was performed on a plate having a plate thickness of 1 mm by bead on plate. The welding conditions are a current of 80 A, a speed of 50 cm / min, and an Ar gas seal of the torch seal side and the back seal side of 10 l / min.

TIG of the invention steel No. 1 and comparative steel No. 11
ESC of the oxide film formed on the heat-affected zone on the back of the welding material
The analysis in the depth direction was performed by A. FIG. 1 shows the results organized by plotting the atomic concentration on the vertical axis and the etching time on the horizontal axis. It can be seen from FIG. 1 that in the steel No. 1 of the present invention containing both Ti and Al, the Cr atom concentration in the oxide film was extremely small, and the oxidation loss of Cr from the base material was hardly generated. Further, in the steel No. 1 of the present invention, Al and T
The atomic concentration of i is not detected almost at the same time, and T
It is speculated that the presence of i contributes to the formation of the Al oxide film. On the other hand, Comparative Steel No. 1 containing Al but not Ti
It can be seen that in No. 1, the Cr atom concentration in the oxide film is high and the oxidation loss of Cr from the base material occurs.

Next, in order to examine the corrosion resistance of the welded portion, Table 1
Steel No. 1 to No. 18 are TIG welded and the weld is 1
It was cut out so as to be 5 mm and 40 mm in the direction perpendicular to the welded portion, and an immersion test was performed. The immersion test was carried out for 30 days by placing a test piece in 500 ml of water having a liquid temperature of 80 ° C., +1000 ppm Cl ⁻ +10 ppm Cu ²⁺ solution. Cu ²⁺ was replenished every week.

FIG. 2 shows the results obtained by arranging the maximum erosion depth of the weld metal portion by the B value shown in Table 1, and FIG. 3 shows the results by arranging the maximum erosion depth of the heat affected zone by the P value shown in Table 1. 22C in FIG.
FIG. 5 shows the results of rearranging the maximum erosion depth of r-1Mo-0.3Nb-0.15Ti steel by Al content. 1,
The results of sorting by weight loss for 3, 4, 6, 11, and 12 are shown.

2 and 3, it can be seen that the steels (1 to 10) of the present invention have shallow erosion depths of 0.1 mm or less in both the weld metal portion and the heat-affected zone. Further, it can be seen that the corrosion depth of the heat-affected zone of No. 14 of Ti-added steel was about 0.07 mm, but that of the weld metal was about 0.13 mm.

Further, it is apparent from FIG. 4 that as the Al content increases, the erosion depth becomes shallower, and the addition of Al is effective for improving the corrosion resistance. It can be seen from FIG. 5 that the steel of the present invention has a much smaller corrosion weight loss than the comparative steel and is excellent in the corrosion resistance of the welded portion.

[0034]

As described above, according to the present invention, a ferritic stainless steel satisfying the corrosion resistance of the weld metal portion of the TIG welded portion and the heat affected zone at the same time was obtained. Since this steel has excellent corrosion resistance in the welded portion, it can be used in a corrosive environment without being subjected to post-treatment such as pickling and polishing, as it is as a welding process. Further, compared to steel added with a large amount of Ti, surface scratches are less likely to occur, so that the yield in the cold rolling process is high and it is possible to manufacture at a relatively low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a result of ESCA analysis of an oxide film in a heat-affected zone of welding.

FIG. 2 shows the immersion test results of the example with Cr + 3 as the horizontal axis.
(Mo + Cu), the figure which arranged the vertical axis as the maximum erosion depth of the welded part.

FIG. 3 shows the results of the immersion test of the example in which the horizontal axis represents 5 (Ti
+ Zr) +20 (Al-0.01), the vertical axis is a diagram in which the maximum erosion depth of the heat affected zone is arranged.

FIG. 4 shows the immersion test results of No. 3, No. 5 and No. 18 in which only the Al content was changed among the steels shown in the examples, the horizontal axis represents the Al content, and the vertical axis represents the maximum of the heat-affected zone. A diagram arranged as erosion depth.

FIG. 5: No. 1, No. 3, N among the steels shown in the examples
Fig. 4 shows the results of immersion tests of No. 4, No. 6, No. 11 and No. 12 as the steel type on the horizontal axis and the corrosion weight loss on the vertical axis.

Claims (4)

[Claims] 1. Mass% C: 0.025% or less Si: 0.6% or less Mn: 1.0% or less P: 0.04% or less S: 0.01% or less Ni: 0.6% or less Cr: 16-35% Mo: 0.3-6% N: 0.025% or less Al: 0.01-0.5% Nb: 0.1-0.6% Ti: 0.05-0.3 %, And between these components, C + N ≦ 0.04% and Nb + Ti ≧ 7 (C + N) +
0.15 is satisfied, and the following formula B ₁ = Cr + 3Mo ≧ 23.5
And a relationship in which P ₁ = 5Ti + 20 (Al-0.01) ≧ 1.5 is maintained, and the balance substantially consists of iron and unavoidable impurities. 2. Mass% C: 0.025% or less Si: 0.6% or less Mn: 1.0% or less P: 0.04% or less S: 0.01% or less Ni: 0.6% or less Cr: 16-35% Mo: 0.3-6% N: 0.025% or less Al: 0.01-0.5% Nb: 0.1-0.6% Ti: 0.05-0.3 % Cu: 0.1-1.0%, and between these components, C + N≤0.04% and Nb + Ti≥7 (C + N) + 0.04%.
15 is satisfied, and the following equation B ₂ = Cr + 3 (Mo + Cu) ≧ 23.
5 and P ₁ = 5Ti + 20 (Al-0.01) ≧ 1.5
A ferritic stainless steel in which the relationship is maintained within the range, and the balance substantially consists of iron and inevitable impurities. 3. Mass% C: 0.025% or less Si: 0.6% or less Mn: 1.0% or less P: 0.04% or less S: 0.01% or less Ni: 0.6% or less Cr: 16-35% Mo: 0.3-6% N: 0.025% or less Al: 0.01-0.5% Nb: 0.1-0.6% Ti: 0.05-0.3 % Zr: 0.05-0.3%, and between these components, C + N ≦ 0.04% and Nb + Ti ≧ 7 (C + N) +0.0.
15 is satisfied and the following formula B ₁ = Cr + 3Mo ≧ 23.5,
And P ₂ = 5 (Ti + Zr) +20 (Al-0.01) ≧ 1.5
A ferritic stainless steel in which the relationship is maintained within the range, and the balance substantially consists of iron and inevitable impurities. 4. C: 0.025% or less Si: 0.6% or less Mn: 1.0% or less P: 0.04% or less S: 0.01% or less Ni: 0.6% or less Cr: 16-35% Mo: 0.3-6% N: 0.025% or less Al: 0.01-0.5% Nb: 0.1-0.6% Ti: 0.05-0.3 % Cu: 0.1 to 1.0% Zr: 0.05 to 0.3%, and between these components, C + N ≦ 0.04% and Nb + Ti ≧ 7 (C + N) +0.0.
15 is satisfied, and the following formula B ₂ = Cr + 3 (Mo + Cu) ≧ 2
3.5, and _{P 2 = 5 (Ti + Zr} ) +20 (Al-0.0
1) A ferritic stainless steel in which the relationship is maintained within the range of ≧ 1.5, and the balance substantially consists of iron and inevitable impurities. [0001]