JPS5914099B2 - Duplex stainless steel with excellent hot workability and local corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a duplex stainless steel having excellent hot-hole T properties and local corrosion resistance.

High Cr duplex stainless steel such as SUS329J1 has excellent properties such as excellent pitting corrosion resistance and oxidation resistance and high strength, but has poor hot workability, such as planetary
It has the disadvantage that it is difficult to roll under high strain rates such as rolling by rolling.

In order to improve the drawbacks of the above-mentioned high Cr duplex stainless steel, alloys (a) to (c) having the following compositions have been proposed. (a) Special Public Service Publication No. 51-43807
According to the publication, a high-chromium, low-nickel duplex stainless steel with excellent corrosion resistance and hot and cold workability is proposed.

(D) According to JP-A No. 52-716, high chromium, low nickel duplex stainless steel IJ having properties similar to (D)
3 have been proposed. (c) According to JP-A-51-117916, a ferritic-austenitic duplex stainless steel is proposed, which has excellent general corrosion resistance, local corrosion resistance, hot workability, and mechanical properties. It is stated that

The present invention is based on STJS329J1 type or (a) 2-type)
The object of the present invention is to provide a duplex stainless steel which is even more excellent in hot workability among the various properties of the alloy described in (c), This objective can be achieved by providing stainless steel.

Next, the present invention will be explained in detail.

The present inventors have discovered that high Cr duplex stainless steels such as SUS329J1 type have excellent pitting corrosion resistance and oxidation resistance, but have poor hot corrosion resistance, especially under high strain rates. Since rolling is difficult, various studies were conducted to improve hot workability. (1) The ratio of α2γ two phases was set to a steel with a large amount of α phase.

(2) By balancing the component composition of the steel ingot, when the steel ingot is heated to a high temperature, the strength of the α phase in the steel ingot is brought close to the strength of the α phase, and the difference in strength between the two phases is minimized. Hot rolling can prevent rolling cracks from the boundary between the α phase and the γ phase. In other words, γ in the hot rolling temperature range
The strength of the phase, the mode of change, and the mode of change of the amount of γ phase are determined by Cr, MO, Si, W, Ti, etc. as α phase forming elements,
It is determined by the component balance with C, N, Ni, Mn, Cu, etc. as γ phase forming elements. Therefore, if the amount of γ phase and the strength of γ phase obtained by balancing relatively small amounts of α phase forming elements and γ phase forming elements are achieved with a relatively large amount of elongated phase forming elements, in this case, the obtained γ The strength of the phases and the difference in strength between the phases are reduced, and the rate of decrease in the amount of γ phase with respect to temperature rise can be increased. (3) A small amount of B is added to strengthen the grain boundaries (this also reduces the amount of S which segregates at the grain boundaries and causes cracking during hot rolling).

(4) By making C extremely low, the formation of carbides at low temperatures is suppressed, and cracking from carbides is prevented in this temperature range, improving hot workability, and the γ phase is Increase the rate of decrease in quantity with respect to temperature increase and the rate of decrease in strength of γ phase with respect to temperature increase.

By implementing the above (1) to (4), conventional SUS
329. The present invention was conceived based on the discovery that C exhibits significantly superior hot workability compared to T and type duplex stainless steel Q.

In the present invention, if the ratio of α and γ two phases is set to a force with a large amount of α phase as described in (1) above, corrosion resistance tends to deteriorate. By understanding the relationship quantitatively and setting the amounts of Cr and MO in accordance with the α and γ phase ratios, corrosion resistance is extremely excellent. For example, pitting corrosion does not occur at 3.5% NaCl and 70°C. I succeeded in doing so.

In addition, W has a pitting corrosion resistant Q effect and is a strong material.
It is an iron-forming element that increases the rate of decrease in the amount of γ phase in the upper part of the hot rolling temperature range and improves hot workability in this temperature range.
However, when W is added in excess of Q, the amount of solid solution increases and segregation occurs (thus promoting embrittlement of the steel.
The content range of W was set in balance with the amount of other α-phase forming elements such as CrtMO+S+. Next, the reason for limiting the composition of the alloy of the present invention will be explained.

If C exceeds 0.02%, intergranular corrosion will occur, pitting corrosion resistance will deteriorate, hot workability will decrease due to precipitation of carbides, and the γ phase will stably exist up to high temperature ranges, making it difficult to work at high temperatures. Since the hot caroe properties decrease, it must be kept at 0.02% or less, and 0.
．． 0.01% or less is more preferable.

Si is effective in improving corrosion resistance, but if it exceeds 2.0%, embrittlement due to α phase grain growth at high temperatures, γ phase embrittlement, and 475°C
Since brittle AO has a negative effect, it must be kept at 2.0% or less, and more preferably 10% or less. If Mn exceeds the 3.0 factor, corrosion resistance will deteriorate and the amount of γ phase will increase beyond the required amount. In addition, if the element is mixed during the steelmaking process and is limited to 3.0% or less, which is the range of components of ordinary ferrite or austenitic stainless steel, it can be easily produced industrially. Ni is an austenite-forming element, and when Ni is less than 3%, the γ phase disappears, while 1
If it is more than 0%, not only will more than the required amount of γ phase be generated, but the manufacturing cost will also increase, so Ni needs to be in the range of 3 to 10%.

Cr is a ferrite-forming element, and if it is less than 20%, corrosion resistance will deteriorate, and if it is more than 35%, toughness will be deteriorated, so Cr must be within the range of 20 to 35%. It is more preferable to be within this range.

MO is a ferrite-forming element, and if MO is less than 0.5%, local corrosion resistance will deteriorate, and if it is more than 6.0%, not only will toughness and local corrosion resistance deteriorate, but also manufacturing costs will increase. MO increases, so it is necessary to keep the MO within the range of 0.5 to 6.0%, and more preferably within the range of 1 to 4%.

Cu is an element that improves stress corrosion cracking resistance, general corrosion resistance, crevice corrosion resistance, and cold stress resistance, but it is an element that deteriorates hot stress resistance.

If Cu is less than 0.2% in the second invention alloy of the present invention, hot workability is excellent but sufficient corrosion resistance cannot be obtained.
On the other hand, if it exceeds 2.0%, excellent corrosion resistance can be obtained, but sufficient hot workability cannot be obtained, so it is necessary to keep it below 2.0%. preferable. N
Like C, N is a strong austenite-forming element, and therefore the N content is determined by the balance with the ferrite-forming elements.

Further, N is an element that is dissolved in large quantities in the γ phase and is effective in improving pitting corrosion resistance. Pitting corrosion resistance improves as the N amount increases, so it is necessary to have at least a coefficient of 0.04 or more.
If the coefficient is more than 0.3, defects such as blowholes will occur in the steel ingot, and the stable presence of the γ phase will deteriorate the hot caloric properties, so it is necessary to keep it within the range of 0.04 to 0.3%. It is more preferably within the range of 0.08 to 0.12%. W and V are ferrite-forming elements, and are useful for improving local corrosion resistance, preventing the stable existence of the γ phase at high temperatures, and improving hot stress PT properties.

Either one or two of W or V is added ρ, and 0.
If it is less than 0.03%, no improvement in local corrosion resistance or hot workability will be observed, and if it is more than 2.0%, it will impair the toughness of the alloy, reduce hot carometry, and increase manufacturing costs. It needs to be within the range of 0.03 to 2.0%, and more preferably within the range of 0.05 to 0.24%. B
When added in a small amount, it exists in the grain boundaries of the alloy and improves hot workability, but if it is less than 0.0005%, the amount of S decreases,
No improvement in hot workability was observed even by adjusting the amount of C.
In addition, if the amount exceeds 0.01%, the amount of segregation at grain boundaries increases, leading to deterioration of hot workability, so 0.0005 to 0.01%
Must be within the range.

The lower the S content, the better in terms of hot stress resistance and corrosion resistance.

Special Q: This is important for hot workability, and S is 0.00.
If it exceeds 5%, S will precipitate at α/γ grain boundaries at high temperatures, making the grain boundaries brittle and deteriorating hot workability, so S is 0.005.
It is necessary to rub less than %C. Next, the present invention will be explained using experimental data. Table 1 shows steels 51 to 10 with typical compositions of the present invention, and Table 2 shows comparative steels [A, b,
c-D,f is shown. As can be seen from the compositions of the alloys of the present invention in Tables 1 and 2 and the comparative alloys, the alloys of the present invention are particularly suitable for the C and S The content of each is 0.022%,
It is about 0.018%, while it is 0.020% respectively.
% or less, 0.01% or less, and contains an appropriate amount of W and/or V.

It is known that the hot caloric property of duplex stainless steel is largely dependent on the ratio of the two phases, and Alloy No. 53 of the present invention has a BO. This is an alloy to which OO5 is added, but as shown in Fig. 1, this alloy flat plate 3 has a γ over the entire hot rolling temperature range compared to < /Kb.
Since the amount of phases is small, the fracture torsion value, which is one of the characteristic values indicating the ductility of the material when hot rolling property is evaluated by a torsion test, is high, and the hot rolling property is excellent.

Also W, O. Compared to the comparative alloy Me, which contains 20% and no B, the present invention &3 contains less C and S, and contains a trace amount of B, so it cannot be hot-processed in the processing temperature range of 1050°C or less. It can be seen that the fracture twist value, which indicates force and tightness, is improved. By the way, the influence of the increase or decrease of the amount of γ phase on the hot workability in the hot working temperature range becomes more pronounced as the strain rate increases, and especially in two-phase alloys, the hot workability decreases as the strain rate increases. known to deteriorate.

As is clear from Fig. 1, the strain rate Q during actual rolling is
In a hot torsion test under a strain rate of 6.0 sec', comparative alloy Ab was compared to the invention alloy &. It was found that the hot workability was significantly inferior to that of 3C, that is, the alloy A3 of the present invention had excellent hot caroe properties. This is because comparative alloy &b has a significantly larger amount of γ phase in the entire temperature range than alloy carbon 3 of the present invention. The present inventors have found that the increase/decrease behavior TJ3 of the amount of γ phase in the hot working temperature range is important as described above. It has been found through experiments that if the contents of Ni, N, and C are different, the degree of decrease in the amount of γ phase over high temperatures will be different. The results are shown in Figure 2.3.4. Figure 2 Q Accordingly, it is SUS329J, a type of high Cr duplex stainless steel alloy Table 3 & 15 alloy, and W
However, the relationship between the temperature and the amount of γ phase for this alloy is as shown in curve 1.

Now, when C or N is increased in this alloy, as shown in curve 2, the amount of γ phase tends to increase at the same temperature C.
There it is. However, alloy Q with the behavior of curve 1
The alloy containing W in the range of 0.05 to 0.24% has a relationship between temperature and γ phase content shown by curve 1', that is, the γ phase content is smaller than that of the alloy shown by curve 1 at the same temperature. It becomes less. Next, when C or N is increased in an alloy having the behavior shown by curve 1', the temperature and γ shown by curve 2' change.
The alloy has a relationship with the phase amount, that is, a property as shown by curve 2' in which the γ phase amount increases at the same temperature. That is, W
By containing a predetermined amount of C, the amount of γ phase at high temperature is lowered, and when C or N is included, the amount of γ phase at high temperature is increased. A new alloy of the present invention has been discovered which has excellent hot caloerescence properties. Figure 3 shows the same W-free alloy 1 shown in Figure 2 Q.
and Alloy 1′ in which Alloy 1 contains W, and Alloys 3 and 3′ in which Ni is added to each of Alloys 1 and 1′ as a booster. Figure 4 also shows alloy 1 not containing W, alloy 1' containing W in alloy 1, and alloys 1 and 1', which are the same as shown in Figure 2.
, is a diagram showing the relationship between the temperature and the amount of γ phase for alloys 4 and 4' with increased MO content.
It has been found that, unlike the case where 1 or Ni is increased, when Cr or MO is increased, the amount of γ phase tends to decrease at the same temperature.

From the experimental results shown in Figures 2 to 4C above, it is possible to predict the amount of γ phase at a certain temperature, and by extension, the fracture torsion value associated with the amount of γ phase, and it is possible to arrive at the alloy of the present invention. It was done.

However, when the processing temperature is lower than 1050℃ (in this case, impurities such as S, P, or O, or the behavior of carbides have a greater influence on mechanical workability than the two-phase ratio of α and γ). I learned that.

Therefore, in the alloy of the present invention, it is necessary to suppress carbide precipitation and reduce or fix S, P, or 0, and C and S must be contained in minimal amounts, that is, CO. Below O2 staff, SO. OO
5% or less, extremely small amount, i.e. 0.0005 to 0.0
By adding 10 tons of B to D, we were able to obtain an alloy that has sufficient caloric properties even at temperatures below 1000°C, as shown in Figure 1. FIG. 5 shows that among the alloys of the present invention, for example &. Relationship between temperature and workability (rupture twist value) of 1, 2, 3, 4.6 and comparative steel (SUS329Jl) b and C (however, strain rate Vj = 6.0 s-1
) is shown.

According to the figure, it can be seen that the mechanical strength of the alloy of the present invention is extremely superior to that of comparative steel. It was found that excellent force-PT properties could be obtained by adjusting the ratio of C, Q, and γ phases as well as the content of trace elements, but it was confirmed through experiments that single-strike corrosion resistance varied greatly depending on the amount of α phase. did.

Figure 6 shows the pitting corrosion resistance (1
This figure shows the results of pitting corrosion when a sample was immersed in an aqueous solution of 0% FeCl3.6H20 at 40°C for 4 hours. The samples had the compositions of Alloys 13 to 25 listed in Table 3 above, and Table 3 shows that in addition to the above experimental results, 5%
Corrosion results of 6 hours of immersion in boiling H2SO4 water are also shown.
From this result, from the point of view of pitting corrosion resistance, if the amount of α phase is reduced,
It can be seen that when N is 1.10, it is necessary to increase the amount of Cr and NO. However, as is conventionally known, brittleness becomes significantly increased by increasing the amount of α phase by 1] and increasing the content of Cr, MO, and Si by 7 JD.

Therefore, it is necessary to limit the contents of Cr and MO from the viewpoint of vein formation, so it is necessary to limit the contents of Cr and MO in the alloy of the present invention from this point as well. Figure 7 shows the effect of γ phase ratio and Cr, MO content (Cr+2M0) on pitting corrosion resistance.
) shows the relationship with %.

Note that the ● mark in the figure contains V and W, and NO. O97-011
Invention alloy shown in Table 1 containing 3% &2,3,5,
7,9, and the Δ mark is NO. Comparative alloys Aa, b, c, d shown in Table 2 containing lO%, and from the same figure, V, W
It can be seen that by containing , the pitting corrosion resistant region shown by the dotted line expands as shown by the solid line in the range where the pitting corrosion resistance is excellent, and the pitting corrosion resistance is improved. However, V and W are strong α-phase forming elements, and are not only elements that easily cause embrittlement and segregation, but also V and W are expensive elements, so their use will lead to increased costs, so this is not recommended. In the invented alloy, the content was limited. As described above, the alloy of the present invention is a duplex stainless steel with extremely excellent hot corrosion resistance and local corrosion resistance. It can be advantageously used as a material for plants such as formic acid, acetic acid, and urea plants; petroleum refining plants such as heat exchangers; equipment related to pulp and paper manufacturing equipment; equipment related to the food industry; and equipment related to pollution treatment equipment.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between temperature, γ phase amount, and fracture twist value of the present invention and conventional duplex stainless steel, and FIG.
Figure 3.4 is a diagram showing the relationship between temperature and γ phase amount, Figure 5 is a diagram showing the relationship between temperature and fracture twist value, and Figure 6 is a diagram showing the relationship between temperature and γ phase amount.
FIG. 7 is a diagram showing iso-corrosion lines based on the relationship between the amount of γ phase (g) and Cr+2M0 (a), and FIG.

Claims (1)

[Claims] 1 C 0.02% or less, Si 2.0% or less, Mn 3.0
% or less, Ni3-10%, Cr20-35%, Mo0.
5 to 6.0%, N0.08 to 0.3%, at least one of W or V 0.03 to 2.0%, B0.0005 to
Duplex stainless steel with excellent hot workability and local corrosion resistance, consisting of 0.01% or less S, 0.005% or less, and the remainder substantially Fe. 2 C0.02% or less, Si2.0% or less, Mn3.0
% or less, Ni3-10%, Cr20-35%, Mo0.
5 to 6.0%, N0.08 to 0.3%, at least one of W or V 0.03 to 2.0%, B0.0005 to
0.01%, S 0.005% or less, Cu 2.0% or less,
A duplex stainless steel with excellent hot workability and local corrosion resistance, with the remainder essentially consisting of Fe.