JP3190290B2 - Ferritic stainless steel with excellent corrosion resistance at welds - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Stainless steels are roughly classified into ferritic and austenitic stainless steels. Austenitic steels have excellent corrosion resistance, workability, weldability, high-temperature strength, etc.
Has a wide range of uses. However, they have the disadvantage 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 that there is almost no danger of stress corrosion cracking in a chloride solution. However, taking SUS430, a typical ferritic stainless steel, as an example, it has disadvantages such as insufficient resistance to a slightly harsh environment, and intergranular corrosion caused by heating and cooling during welding. are doing. It is already well known that increasing the amount of Cr, adding Mo, and the like are effective in improving corrosion resistance.

However, an increase in Cr and Mo impairs 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
Is effective in improving intergranular corrosion resistance, but C and N
There is also a limit to the reduction of the intergranular corrosion susceptibility at the C and N content levels that can be reached industrially at present. It is a well-known fact that the adverse effects of C and N on intergranular corrosion can be eliminated by adding a stabilizing element such as Ti or Nb which can fix C and N alone or in combination.

[0004]

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

It is presumed that this is because Cr in the surface layer of the base material is oxidized at a high temperature due to heat input during welding, metal Cr in the surface layer becomes poor, and the corrosion resistance is reduced. For this reason, it has become necessary to develop a material that reduces the Cr oxidation loss during welding and simultaneously satisfies the corrosion resistance of the weld heat affected zone and the weld metal zone. Accordingly, an object of the present invention is to obtain a new ferritic stainless steel that simultaneously satisfies the corrosion resistance of the weld heat affected zone and the weld metal zone.

[0006]

Means for Solving the Problems The object of the present invention is as follows: C: 0.025% or less Si: 0.6% or less Mn: 1.0% or less P: 0.04% or less S: 0.01% by mass %: Ni: 0.6% or less Cr: 16 to 35% Mo: 0.3 to 3.5 % N: 0.025% or less Al: 0.01 to 0.5% Nb: 0.1 to 0.1% 6% Ti: 0.05-0.3% Cu: 0.1-1.0% and between these components in mass%, C + N ≦ 0.04 and Nb + Ti ≧ 7 (C + N) +0.
15 hold, and the following equation: B value = Cr + 3 (Mo + Cu) ≧ 23.5, and P value = 5
The relationship of Ti + 20 (Al-0.01) ≧ 1.5 is satisfied, and the balance is achieved by a ferritic stainless steel having excellent corrosion resistance of a weld substantially consisting of iron and unavoidable impurities.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied in detail the effect of alloying elements on the corrosion resistance of a weld. As a result, the following findings were obtained. That is, the corrosion resistance of the weld metal part is improved by increasing the amounts of Cr and Mo. However, the addition of a certain amount or more of Ti deteriorates the corrosion resistance of the weld metal part. C for improving corrosion resistance of weld heat affected zone
It is effective to increase the amounts of r and Mo, but the corrosion resistance of the heat-affected zone is particularly improved by adding Ti and Al in combination. Further, by adding an appropriate amount of Cu, the corrosion resistance of the welded portion (welded metal portion and weld heat affected zone) is further improved.

Here, the reason why the corrosion resistance of the heat-affected zone is markedly improved by adding Ti and Al in combination as described above is that Ti facilitates the formation of Al oxide during welding, The formation is considered to be effective in preventing the oxidation loss of Cr and to significantly reduce the Cr-poor layer in the steel surface layer immediately below the oxide scale. That is,
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 at the time of welding. It seems that this will happen easily. As described above, Ti is an effective element for improving the corrosion resistance of the heat affected zone. But T
Since the addition of i in a certain amount or more causes surface flaws and deterioration of the corrosion resistance of the weld metal, it is necessary to minimize the addition. When the upper limit of the amount of Ti added is regulated for these reasons,
It is difficult to suppress intergranular corrosion by adding only Ti, and it is necessary to add Nb as a composite.

Hereinafter, the action of each element contained in the steel of the present invention and the reason for limiting the content will be described.

[0010] C and N are elements inevitably contained in steel. When the contents of C and N are reduced, the material becomes soft and the workability is improved, and the generation of carbides and nitrides is reduced,
In order to improve the weldability and the corrosion resistance of the welded portion, the lower the better, the more preferably C ≦ 0.025% and N ≦ 0.025%.

[0011] Si is harmful to the hot cracking of the weld and the toughness of the weld, and also makes the steel harder, so the lower the better, 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 the corrosion resistance is reduced, the lower the one, the more preferable the upper limit is 1.0%.

[0013] P is desirably low because it impairs the toughness of the base metal and the welded portion, but the removal of P from the Cr-containing steel is difficult and raises the production cost, so the upper limit is made 0.04%.

Since S has an adverse effect on corrosion resistance and high-temperature cracking of the welded portion, it is preferably low, and the upper limit is made 0.01%.

Ni is an element effective for improving the toughness of ferritic stainless steel, but if it is too much, the cost increases. The steel of the present invention is also 0.6% or less, 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.
If it exceeds 5%, embrittlement becomes remarkable, and it is difficult to manufacture a thin plate or process a product. Therefore, the Cr content is set to 16 to 35%.

Mo is an effective element that enhances corrosion resistance together with Cr, and the effect increases as the Cr content increases. However, the Cr content level of the steel of the present invention is 0.3.
Small corrosion resistance improving effect is less than%, because rather invited a decrease in ductility when added beyond 3.5%, Mo amount from 0.3 to 3.
5 %.

Al is an important element in constituting the present invention, and by adding it in combination with Ti, an Al film is easily formed on the surface layer of steel at the time of welding and corrosion resistance is prevented by preventing oxidation loss of Cr. improves. However, the Al content is 0.01%
If it is less than 0.5%, an Al film is hardly formed, and if it exceeds 0.5%, the surface quality of the material is deteriorated and the weldability deteriorates. Therefore, the Al content 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 steel of the present invention. However, too much N impairs weld toughness. .6% as the upper limit. The lower limit is determined from the viewpoint of preventing intergranular corrosion and requires 0.1% or more.

Ti is an important element in constituting the present invention. By adding it in combination with Al, Ti easily forms an Al film on the surface layer of steel during welding and prevents oxidation loss of Cr, thereby improving corrosion resistance. improves. Further, Ti also has an effect of fixing C and N. However, if the Ti content is too large,
Since the surface quality of the material and the corrosion resistance of the weld metal are deteriorated, the upper limit is set to 0.3%. Note that the lower limit is determined from the viewpoint of formation of an Al film and prevention of intergranular corrosion, and 0.05% or more is required.

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

Although Cu is an element effective for improving the corrosion resistance of the welded portion, if it is too much, it impairs the toughness of the welded portion.
Is set as the upper limit, and if less than 0.1%, the effect is small, so the lower limit is set to 0.1%.

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

In addition to limiting the content of each component described above, in the present invention, the following limiting formula is established between each component of Cr, Mo, Cu, Al, and Ti, where B value = Cr + 3 (Mo + Cu) ≧
23.5 and P value = 5Ti + 20 (Al−0.01) ≧
1.5 must be satisfied. These formulas have been set through many experimental examples by the present inventors, and the B value regulates the minimum value required to satisfy the corrosion resistance of the weld metal portion and the P value regulates the corrosion resistance of the weld heat affected zone. Is what you do.

In the weld metal, Cr and Mo are basic components for improving the corrosion resistance. Since Mo contributes more to the improvement of the corrosion resistance than Cr, the coefficient is 3% of Cr.
Doubled. Further, since Cu has the same effect as Mo, the coefficient is made the same as Mo. If the B value is less than 23.5%, sufficient corrosion resistance of the weld metal cannot be obtained, so the lower limit is 23.5%.
5%.

In the weld heat affected zone, the addition of Ti and Al in combination facilitates the formation of an Al oxide film on the surface layer of steel during welding, and the formed Al oxide film reduces the oxidation loss of Cr on the surface layer. To prevent As a result, the formation of a Cr-poor layer is suppressed, which is effective for improving the corrosion resistance of the weld heat affected zone. To obtain these effects between Ti and Al, the P value needs to be 1.5 or more.

[0027]

EXAMPLE A stainless steel having the chemical composition shown in Table 1 was melted and hot rolled to produce a hot-rolled sheet having a thickness of 3.5 mm. Then, cold-rolled to a thickness of 1.0 mm,
The sample was subjected to finish annealing at 1050 ° C. and used as a test material.

[0028]

[Table 1]

In Table 1, No. 7 is the steel of the present invention, in which Nb and Ti are added as fixing elements in a complex manner, and Al is used as a trace element.
Is a steel to which Cu is further added. No. 11-
No. 18 is a comparative steel, and the production history is the same as that of the steel of the present invention. Among them, No. 11 to No. 13, No. 15 to No.
Reference numeral 18 denotes steel that does not satisfy the limiting formula.
No. 2 is Ti, No. 13 is Ti and Al, No. 18 is Al
Is a steel to which is not added. No. 14 is steel to which Nb has not been added.

The results of visual observation of the presence or absence of surface flaws of these steels are also shown in Table 1. From the results in Table 1, it can be seen that Ti
It can be seen that excessive addition of manganese causes surface flaws. In the present invention, since the upper limit of the (C + N) amount is 0.04%, it is necessary to contain 0.43% or more of Ti or Nb in order to prevent the occurrence of intergranular corrosion. In this case, if Ti is contained in an amount of 0.43% or more, a surface flaw is generated (see Comparative Steel No. 14). Therefore, as in the present invention, the Ti content is reduced and a fixing element such as Nb is added accordingly. It is effective.

Each of the steels produced as described above was subjected to TIG welding and subjected to an immersion test in order to examine the corrosion resistance of the weld. TIG welding was performed on a 1 mm-thick plate with a bead-on-plate. The welding conditions were a current of 80 A, a speed of 50 cm / min, and an Ar gas seal on the torch seal side and a back seal side of 10 L / min. The immersion test piece used had a weld bead portion substantially at the center thereof, and was cut out so that the direction parallel to the weld bead was 15 mm and the perpendicular direction was 40 mm. The immersion test was carried out at a water temperature of 80 ° C.
The test piece was placed in 500 ml of 0 ppm Cl ⁻ +10 ppm Cu ²⁺ solution and the test was performed for 30 days. Note that Cu ²⁺ was replenished every week.

FIG. 1 shows the maximum pit depth of the weld metal.
2 shows the results arranged by the B value, and FIG. 2 shows the results arranged by the P value shown in Table 1 of the maximum erosion depth of the heat-affected zone.

1 and 2, it can be seen that the steel of the present invention (No. 7) has a shallow erosion depth of 0.1 mm or less in both the weld metal portion and the heat-affected zone. The erosion depth of the heat-affected zone was as shallow as about 0.07 mm in the comparative steel No. 14 containing only Ti, but was as deep as about 0.13 mm in the weld metal part. A scratch has occurred. In other comparative steels, none of the weld metal portions and the heat-affected zone simultaneously had an erosion depth of 0.1 mm or less.

[0034]

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

[Brief description of the drawings]

FIG. 1 shows a horizontal axis of Cr + 3 (Mo +
Cu), a graph in which the vertical axis is arranged as the maximum erosion depth of the welded portion.

FIG. 2 shows the results of the immersion test, where the horizontal axis is 5Ti + 20 (A).
1-0.01), a graph in which the vertical axis is arranged as the maximum erosion depth of the heat-affected zone.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Mitsuaki Nishikawa 4976 Nomura Minami-cho, Shinnanyo-shi, Yamaguchi Nisshin Steel Works, Ltd., Steel Research Laboratory, Stainless Steel and High Alloy Research Laboratory (56) References JP, A) JP-A-57-126954 (JP, A) JP-A-53-149111 (JP, A) JP-A-51-110415 (JP, A) JP-A-3-264652 (JP, A) Hei 4-280948 (JP, A) Special table Hei 4-504140 (JP, A) (58) Fields studied (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (1)

(57) [Claims] 1. 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 3.5 % N: 0.025% or less Al: 0.01 to 0.5% Nb: 0.1 to 0.6% Ti: 0.05 to 0% 0.3% Cu: 0.1 to 1.0% and in mass% between these components, C + N ≦ 0.04 and Nb + Ti ≧ 7 (C + N) +0.0.
15 hold, and the following equation: B value = Cr + 3 (Mo + Cu) ≧ 23.5, and P value = 5
Ti + 20 (Al-0.01) ≧ 1.5 is satisfied, and the balance is substantially ferritic stainless steel with excellent corrosion resistance at the welded portion consisting of iron and unavoidable impurities.