JPS62267452A - Two-phase stainless steel excellent in corrosion resistance in weld zone - Google Patents

Abstract

PURPOSE:To improve corrosion resistance in a weld zone of a structure manufactured by the use of undermentioned steel, by specifying the relationship between Cr equivalent and Ni equivalent as well as the relationship between N% and Ni% in a two-phase stainless steel having a specific composition. CONSTITUTION:The structural two-phase stainless steel has a composition which consists of, by weight, <=0.03% C, <=1.5% Si, <=2.0% Mn, 18.0-30.0% Cr, 5.0-12.0% Ni, 1.5-5.0% Mo, 0.10-0.30% N, and the balance Fe with inevitable impurities and in which the relationship between N% and Ni% and the F value obtained by subtracting Ni equivalent (represented by equation III) from Cr equivalent (represented by equation II) are regulated to a range represented by inequality I and 12.0-16.0, respectively. By satisfying the above conditions, a weld zone excellent in pitting resistance, crevice corrosion resistance, and intergranular corrosion resistance can be obtained. If necessary, 0.1-1.0% Cu and 0.02-0.10% Sn are added to the above steel composition.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field 111] The present invention relates to a stainless steel consisting of two phases of ferrite and austenite, which has excellent corrosion resistance in welded parts. More specifically, it is a duplex stainless steel for welding that exhibits excellent corrosion resistance in environments such as seawater or chlorides, and has little deterioration in corrosion resistance, especially in the weld metal and weld heat affected zone when welding is involved in welded structures. It's about steel.

[Conventional technology and problems]

Stainless steel is generally used extensively as a corrosion-resistant material, but recently a new application has been found in seawater environmental chemical plants! It is increasingly being used for tubes in heat exchangers in equipment, as well as for oil country tubular goods exposed to environments containing chlorides, hydrogen sulfide, carbon dioxide, and the like. In such a severe corrosive environment, austenitic stainless steel has 2
0Cr-25N i-6Mofi, etc., and high Cr stainless steels.

29Cr-4Mof4, 28Cr-I Mom, etc. are used. However, austenitic stainless steel has problems with stress corrosion cracking resistance. Furthermore, high Cr systems have problems in terms of manufacturability and deterioration in the toughness of welded parts.

On the other hand, duplex stainless steel, which has a dual-phase Mt weave of austenite and ferrite, has excellent stress corrosion cracking resistance, as well as superior manufacturability and mechanical properties compared to high-Cr stainless steel. There is. For this reason, 22Cr-5,5N i-3Mo, 2
3-28Cr-3-6N i=1.5-3. OM.

The series of duplex stainless steels has been published and reprinted.

However, these duplex stainless steels have the disadvantage that when welding is involved in construction, they have inferior corrosion resistance in /8 weld metals and weld heat-affected zones, and the extent of this deterioration is more pronounced than that of austenitic stainless steels. exists. The cause of this is that when the base metal, which has a two-phase structure of ferrite and austenite, undergoes welding, part of the weld metal and the weld heat affected zone becomes a single phase of ferrite, which significantly deteriorates the corrosion resistance. This is caused by For these reasons, the reality is that there is no confidence in the use of 5FI5329J1 type duplex stainless steel as a /8-contact structural material from the viewpoint of corrosion resistance, especially in environments with turbid water or chlorides. It has been avoided for use in welded structural materials.

The present invention was made with the aim of solving the problem of corrosion resistance of welded parts while taking advantage of the above-mentioned advantageous properties of duplex stainless steel.

Means for Solving Problems C] The present invention provides C: 0.03% or less in weight%, Si
: 1.5% or less, Mn: 2.0% or less, Cr:
18.0-30.0%, Ni:5. O~12.0O
A, Ma: 1.5-5.0%, N: o, 1o
~(1,30%.

The remainder: stainless steel consisting of Fe and unavoidable impurities, and N(χ)≧(1/30)x(χN i)−(1/10
) and 12,0≦F value≦16,0 [I, then, Fl direct - Cr equivalent - Ni equivalent.

Cr equivalent=%Cr+%Mo+4X%StN Violet
I = 1.5 x % Ni +3Qx
(XC+XN)+0.5×%Mn] A duplex stainless steel having a composition that satisfies the relationship: (XC+XN)+0.5×%Mn] and has excellent corrosion resistance at a welded part is provided.

Furthermore, the present invention provides a duplex stainless steel with even better corrosion resistance at the welded part, which is made by adding 0.1 to 1.0% Cu to the above-mentioned steel, and Q, L to L, 0% Cu. and 0.02
The object of the present invention is to provide a duplex stainless steel with a composite addition of ~0.10% Sn and which has even better corrosion resistance in welded parts.

In other words, the present inventors have conducted extensive research on the ferrite-austenite phase ratio and corrosion resistance of duplex stainless steel welds, as well as the influence of alloying elements on the phase ratio.
We discovered the ferrite-austenite phase ratio necessary to improve the corrosion resistance of welded parts and the alloy composition necessary to obtain this phase ratio, and developed a two-phase stainless steel with significantly superior corrosion resistance in welded parts compared to the conventional 5US329J1 type duplex stainless steel. Phase stainless steel is obtained.

The reason for limiting the action and content of each component in the steel of the present invention is as follows.

The lower the CTC, the better from the viewpoint of corrosion resistance and pitting resistance.
Since it is necessary to consider manufacturability issues when reducing C, the upper limit was set at 0.03%.

Si: Si needs to be added for deoxidation during steel manufacturing, but if the amount is large, weldability and workability will deteriorate, and it will also stabilize the ferrite phase at high temperatures and reduce the austenite phase in the rapidly quenched composition. The upper limit is set to 1 to suppress precipitation.
．． It was set at 5%.

Mn: Mn needs to be added for deoxidation during steel manufacturing and as an austenite stabilizing element, but too much Mn deteriorates corrosion resistance, so the upper limit was set at 2.0%.

Cr: Cr is an important element for obtaining a ferrite Mi texture in duplex stainless steel, and is extremely important for obtaining corrosion resistance and pitting corrosion resistance of stainless steel. In order to obtain good pitting corrosion resistance, 18.0% or more of Cr is required, but if it exceeds 30.0%, the manufacturability and workability of the steel deteriorates, so the content is set to 30.0% or less.

Ni: Ni is an important element for obtaining an austenitic Mi texture in duplex stainless steel. It is necessary to add 5.0% or more to generate the austenite phase, but adding too much increases manufacturing cost and tends to cause σ embrittlement during manufacturing, so it is set to 12.0% or less.

Mo: Mo, together with Cr, Ni, and N, effectively acts to improve the corrosion resistance, particularly the pitting corrosion resistance, of the steel of the present invention. M
If o is less than 1.5%, sufficient pitting corrosion resistance cannot be obtained, while if it exceeds 5.0%, the toughness will deteriorate and the manufacturing cost will increase, so Mo should be in the range of 1.5 to 5.0%. It was inside.

SUN effectively acts to increase the corrosion resistance, especially the pitting corrosion resistance, of the steel of the present invention.In the present invention, instead of increasing the amount of Ni, which can increase the corrosion resistance but is likely to cause σ embrittlement, the austenite phase is increased. It is necessary to add 0.10% or more of N, which is an element that can be produced. However, if N exceeds 0.30%, blowholes are likely to occur during manufacturing, so there is a limit. Therefore, the N content is 0. t
It is within the range of 0 to 0°30%, and the related amount with Ni is expressed by the following formula (
The N content is set to satisfy condition 11.

N(χ)≧(1/30) x (χN i) −(1/
10) Reason for regulating formula (1)+11: Formula (1) determines the minimum value of N that ensures the corrosion resistance of the steel of the present invention. Ni equivalent is C, N, M n + and Ni
However, the distribution of N and Ni is a problem in improving the corrosion resistance of the austenite phase precipitated in the 17-conductor joint. In other words, the corrosion resistance of the austenite phase precipitated in the weld zone varies significantly depending on the N content. For example, when the N content is less than 0.10%, the corrosion resistance of the precipitated austenite phase is poor, but as the N content increases, the corrosion resistance of the austenite phase decreases. Corrosion resistance improved = Cr + Mo? It was found that the corrosion resistance of the entire weld was improved due to the synergistic effect with the compressed ferrite phase. Therefore, in order to improve the corrosion resistance of welds, the distribution of N in the precipitated austenite phase is extremely important, and the minimum value of N should be regulated by (1) 2 in consideration of the balance with the amount of Ni. This is an important requirement for achieving the purpose of the present invention, and this point is one of the special strengths of the steel of the present invention.

Cu: Cu improves stress corrosion cracking resistance, general corrosion resistance, and crevice corrosion resistance in the steel of the present invention, but this effect is obtained when it is added in an amount of 0.10% or more. However, l,
Adding more than 0% impairs hot workability, so the upper limit is set at 1.0%.

Sn: Sn improves crevice corrosion resistance when coexisting with Cu. This effect can be obtained by adding 0.02% or more, but if it exceeds 0.10%, hot workability is impaired, so the upper limit is set at 0.10%.

F value: The reason for setting a limit on F (ii) is based on the fi maintenance and manufacturability of the austenite phase necessary for improving the corrosion resistance of welded metal boxes, which is the objective of the present invention (i.e., the F value is set at 12.
If it is less than 0, the content of the ferrite phase at room temperature will be 40% or less, and in this case, edge cracking is likely to occur during hot rolling during manufacturing, resulting in poor hot workability and poor quality of the hot rolled sheet. There are problems such as the generation of σ phase during cooling, which significantly reduces toughness. On the other hand, when F(fi) exceeds 16.0, the ferrite phase increases and stabilizes in the high temperature range, and the amount of austenite precipitated in the structure that is cooled after melting such as welding becomes 5% or less, and the Therefore, in order to improve the corrosion resistance of the weld metal, which is the object of the present invention, and maintain good manufacturability, the F value must be set to 1.
Must be between 2.0 and 16.0.

〔Example〕

Table 1 shows the chemical composition values and ferrite t of the steel subjected to the corrosion test for the base metal and welded parts.

Samples A to 1 (comparison m<However, A, B, and C are commercially available materials), and Sample 1-P is the steel of the present invention.

Among the comparative steels, samples D-H are similar to the steels of the present invention, but their F values are outside the range of 12.0 to 16.0. Regarding the valley-shaped fAD-P other than A-C, which is a commercially available material, it is melted in a 30 kg vacuum melting furnace, then forged, hot rolled, and cold rolled to a plate thickness of 2 ml, and then melted at 1050°C.
After holding for 30 minutes, final annealing with air cooling was performed. The amount of ferrite in Table 1 was determined by observing the structure using an optical microscope.

It is shown as an area percentage based on point count.

Figure 1 shows the F value, Cr equivalent, and N of the base material for each sample.
The relationship between the i equivalent and the F value and the amount of austenite precipitation in the weld zone are shown in FIG. The amount of austenite phase precipitated in the welded part was determined by applying a current of 12 OA and a voltage of 13 V to a plate with a thickness of 211111 mm.

TTG at welding speed of 30cm/min? 8-contact (bead-on-plate) was performed, the structure was observed with an optical microscope, and the amount of austenite was determined by the point counting method.

From FIG. 1 and FIG. 2, it can be seen that in order to make the precipitation of austenite phase in the weld zone 105 or more, F(11! must be made 16.O or less. That is, F(11!) must be made 16.O or less.

By balancing the Cr equivalent and Ni equivalent so that the F value is 16.0 or less, 10% or more of austenite can be precipitated in the weld.

FIG. 3 shows the results of high temperature tensile tests on samples with different F values. As seen in FIG. 3, when the F number is lower than 12.0, the aperture ratio becomes significantly small.

Defects such as edge cracks occur during hot rolling.

Figure 4 summarizes the results of the corrosion tests on the base metal and welded parts of each sample. The corrosion test was conducted assuming use in a seawater environment, and the base metal and weld metal parts of the inventive steel and comparative steel were soaked in a (1/20)N llCl aqueous solution with 50 g/l FeC.
The pitting corrosion resistance was tested by immersing it in a 50°C test solution to which 1* was added for 48 hours. The evaluation of corrosion rate was measured as weight per square meter per hour (g/m"·hr).

As can be seen from the results in FIG. 4, it can be seen that all of the steels of the present invention have extremely superior pitting corrosion resistance in both the base metal and the weld zone, compared to the comparative steels.

Figure 5 shows the pitting corrosion potential of the base metal and weld zone for samples C, F, I, and P using 3.5% NaCl, 50°C to 70'C.
This shows the results measured by potentiodynamic method in a solution.

From the results shown in Fig. 5, the pitting potential of both the base metal and the weld zone decreases as the temperature increases for each sample, but the weld zone of sample 2 and sample P, which are the invention steels, is lower than that of sample C and the weld zone, which are the comparative steels. It is observed that the pitting corrosion potential decreases less than that of the welded part of Sample F. In particular, when comparing Sample 1, which is the steel of the present invention, and Sample F, which is the comparative steel, the Mo content of the former is 2.1%,
Although the M0 content of the latter is 3.2%, the pitting corrosion potential of the welded part of the former has a small decrease. This also shows that the steel of the present invention is significantly improved.

Next, in order to investigate crevice corrosion resistance, samples B, C, K, 0
, the base metal and weld metal part for P are (1/20) 11C of N! water? An infiltration test was carried out in a test solution in which 50 g/1 of FeC1z was added to solution 8. The test conditions are shown in Table 2. The crevice corrosion property was determined by using the temperature at which corrosion occurs in the crevices as the critical temperature of the steel.

Table 2 The results of this test are shown in FIG.

As seen in the results in Figure 6, O
, P have a higher limit temperature at which crevice corrosion occurs in both the base metal and the welded part than comparative steel samples B and C, indicating that they have excellent crevice corrosion resistance. In addition, O
, P, the sample P with 0.55% Cu addition and the composite addition of 0.0.48% Cu and 0.05% Sn has a lower resistance to the base metal and welded area than the sample K with no additives. Both have high critical temperatures for pitting corrosion to occur. This shows the effect of Cu and Sn on improving crevice corrosion resistance. Therefore, when crevice corrosion resistance is particularly required, addition of Cu or Cu-3n
The combined addition of is advantageous.

FIG. 7 shows the results of the intergranular corrosion resistance test.

The test was conducted according to the JIS G 0575 method on the gap welded area using a sulfuric acid-copper sulfate test solution, and the results were evaluated according to the same method.

As seen in the results in Figure 7, the F value is 12.0 to 16.
．． The welded parts of the inventive steel (in which 10% or more of austenite is precipitated) in the range of 0 are the comparative steel samples C, D,
It can be seen that the intergranular corrosion resistance is superior to that of E.

As mentioned above, the duplex stainless steel of the present invention has pitting corrosion resistance,
It can have welded parts with excellent crevice corrosion resistance and intergranular corrosion resistance, and is extremely useful as a material with welded structures for applications in seawater environments, such as heat exchanger tubes and condensers for chemical plant power generation. It is.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the Cr equivalent and Ni equivalent of the inventive steel and the comparative steel. Figure 2 shows the F value of the inventive steel and the comparative steel. Figure 3 is a diagram showing the relationship between the amount of austenite precipitation at the 8-joint joint, Figure 3 is a diagram showing the relationship between temperature and reduction ratio in a high-temperature tensile test of the inventive steel and comparative steel, and Figure 4 is the base metal and welding ratio of the invention steel and comparative steel. Figure 5 is a diagram showing the relationship between temperature and pitting corrosion potential in a 3.5χN a Cl 78 solution of the base metal and welded part of the inventive steel and comparative steel. FIG. 6 is a diagram showing the results of crevice corrosion resistance in a solution in which 50 g/f FeCIt was added to a 1/20 N MCI aqueous solution for the base metal and welded parts of the inventive steel and comparative steel, and FIG. 7 is a diagram showing the results of intergranular corrosion tests of welded parts of the steel of the present invention and comparative steel.

Claims (1)

[Claims] (1) In weight%, C: 0.03% or less, Si: 1.5%
Below, Mn: 2.0% or less, Cr: 18.0 to 30.0
%, Ni: 5.0-12.0%, Mo: 1.5-5.0
%, N: 0.10 to 0.30%, balance: stainless steel consisting of Fe and inevitable impurities, and N (%) ≧ (1/30) × (%Ni) - (1/10) and 12.0≦F value≦16.0 [However, F value = Cr equivalent - Ni equivalent, Cr equivalent =
%Cr+%Mo+4×%Si Ni equivalent=1.5×%Ni+30×(%C+%N)+0
．． A duplex stainless steel having a composition that satisfies the following relationship: 5×%Mn and having excellent corrosion resistance in welded parts. (2) In weight%, C: 0.03% or less, Si: 1.5%
Below, Mn: 2.0% or less, Cr: 18.0 to 30.0
%, Ni: 5.0-12.0%, Mo: 1.5-5.0
%, N: 0.10-0.30%, Cu: 0.1-1.0
%, balance: stainless steel consisting of Fe and unavoidable impurities, and N (%) ≧ (1/30) × (% Ni) - (1/10) and 12.0≦F value≦16.0 [However, F value = Cr equivalent - Ni equivalent, Cr equivalent =
%Cr+%Mo+4×%Si Ni equivalent=1.5×%Ni+30×(%C+%N)+0
．． A duplex stainless steel having a composition that satisfies the following relationship: 5×%Mn and having excellent corrosion resistance in welded parts. (3) In weight%, C: 0.03% or less, Si: 1.5%
Below, Mn: 2.0% or less, Cr: 18.0 to 30.0
%, Ni: 5.0-12.0%, Mo: 1.5-5.0
%, N: 0.10-0.30%, Cu: 0.1-1.0
%, Sn: 0.02 to 0.10%, balance: stainless steel consisting of Fe and inevitable impurities, and N (%) ≧ (1/30) × (%Ni) - (1/10) and 12.0≦F value≦16.0 [However, F value = Cr equivalent - Ni equivalent, Cr equivalent =
%Cr+%Mo+4×%Si Ni equivalent=1.5×%Ni+30×(%C+%N)+0
．． A duplex stainless steel having a composition that satisfies the following relationship: 5×%Mn and having excellent corrosion resistance in welded parts.