JPS63149360A - Two-phase stainless steel having high corrosion resistance and superior weldability - Google Patents

Abstract

PURPOSE:To suppress reduction of the amt. of austenite in the weld heat-affected zone of a steel as a base metal and the weld zone formed with no filler metal by specifying the ratio of the Cr equiv. to the Ni equiv. of the steel having a specified compsn. and by considerably increasing the amt. of N and the total amt. of all the alloying elements except C and Si. CONSTITUTION:The compsn. of a steel is composed of, by weight, <=0.025% C, 0.01-0.2% Si, 1.0-5.0% Mn, 28-35% Cr, 6-16% Ni, 3.1-6.0% Mo, 0.2-0.4% N and the balance Fe. The ratio of the Cr equiv. represented by formula I to the Ni equiv. represented by formula II is regulated to 1.9-2.4 and the total amt. of all the alloying elements except C and Si is regulated to >=45%. 0.1-3.0% Cu may be added to the compsn. so as to regulate the ratio of the Cr equiv. represented by the formula I to the Ni equiv. represented by formula III to 1.9-2.4 and the total amt. of all the alloying elements except C and Si to >=45%. The steel has high corrosion resistance and ensures extremely superior corrosion resistance for the weld heat-affected zone or the weld zone formed with no filler metal.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to ferrite-austenite dual-phase stainless steel, and particularly to changes in the ferrite-austenite phase ratio in the weld heat-affected zone or in welds where no filler metal is used. The present invention relates to highly corrosion-resistant duplex stainless steel with low corrosion resistance and good corrosion resistance.

[Conventional technology]

Ferritic-austenitic stainless steel (hereinafter referred to as
Duplex stainless steel (duplex stainless steel) has a two-phase structure in which 40 to 65% of austenite is finely mixed in a ferrite phase matrix, and as it combines the advantages of austenitic stainless steel and ferritic stainless steel, it has recently been used as a corrosion-resistant structural material. Its application is active as a For such duplex stainless steel, JIS 5US329 and DIN
There are standards such as 1.4462, and JP-A-55-4452 has excellent base material corrosion resistance and mechanical properties.
No. 8, JP-A-56-127753, JP-A-57-47
Publication G such as No. 852 has been disclosed.

However, it is known that when conventional duplex stainless steel is welded, the phase balance between ferrite and austenite is disrupted in the heat affected zone, and the austenite ffi is considerably lower than the level of the base metal. Further, the weld metal obtained by dip welding, for example, by TIG welding, plasma welding, or electron beam (EB) welding without using a filler metal, is similar to the heat affected zone and has a considerably low austenite content. Therefore, the filler metal for welding duplex stainless steel is usually designed to increase the content of austenite-forming elements and suppress the decrease in the amount of austenite. However, a reduction in the amount of austenite in the weld heat-affected zone of the base metal is essentially unavoidable in conventional duplex stainless steels, and when applying these welded structures to more severe corrosive environments, corrosion resistance in this area is It is thought that this will become a big problem.

[Problem that the invention seeks to solve]

Conventional duplex stainless steel has a melting point of about 100 to 20
Ferrite is a single phase in the range of about 0°C, and austenite gradually becomes stable at temperatures below that, and ferrite and austenite coexist in two phases. In other words, in the case of the base metal, the phase balance between ferrite and austenite can be ensured by rolling and heat treating it at a temperature in the two-phase range after casting, but when welding, the base metal is heated to a high temperature. In the heat-affected zone, the austenite phase becomes unstable as the heating peak temperature increases, so austenite M gradually decreases and the phase balance collapses. Furthermore, when heated to a higher temperature, it becomes a single phase of ferrite and austenite is produced during the subsequent cooling process, but in a non-equilibrium process of rapid cooling such as a welding thermal cycle, austenite changes during the cooling process. Precipitation is considerably suppressed, and the structure consists of coarse ferrite grains and a small amount of austenite formed at the grain boundaries, so the amount of austenite is significantly reduced in these regions.

In particular, if the cooling rate is fast, it may become a single-phase ferrite. Even in the case of a part that is melted and solidified with the same components during welding without filler metal, it solidifies as a single phase of ferrite, so it has a structure similar to the heat affected zone of the base metal that is heated to a high temperature and becomes a single phase of ferrite. After all, the structure has a significantly reduced amount of austenite. By the way, in duplex stainless steel and especially in its welded parts, corrosion resistance such as pitting corrosion resistance and intergranular corrosion resistance strongly depends on the amount of austenite, and if the phase balance is disrupted, the corrosion resistance will decrease, and if the amount of austenite is approximately It is often reported that the corrosion resistance is the best when the content is 40 to 65%. Therefore, in the weld heat-affected zone of the base metal and the weld zone without filler metal, corrosion resistance deteriorates due to a decrease in the amount of austenite. Such local deterioration of corrosion resistance becomes a big problem especially when the corrosive environment is severe. Furthermore, the coarsening of crystal grains and the decrease in the amount of austenite in the structure caused by passing through the ferrite single phase region also impairs the toughness and ductility of these regions. These problems are essentially problems associated with conventional duplex stainless steels, and remain as drawbacks when duplex stainless steels are applied to welded structures in severe corrosive environments in the future.

The present invention has been made in view of the above-mentioned points, and particularly improves austenite in base metal weld heat-affected zones and welded areas without filler metal. The objective is to provide a highly corrosion-resistant duplex stainless steel that suppresses deterioration of properties such as corrosion resistance in those areas by suppressing the deterioration.

[Means and actions for solving the problem] That is, the present inventors have developed the structures of welded parts welded without base metal, weld heat-affected zone, and filler metal in duplex stainless steels of various composition systems. As a result of a systematic study of the effects of component elements on corrosion resistance, the following findings were obtained.

First, as shown in Fig. 1, the amount of austenite in the base metal structure subjected to solution treatment at 1050°C after hot rolling is Cr equivalent = Cr (wt%) + 1.5 × Si (wt
%) + Mo (wt%), N1 equivalent = Ni (wt%) +
30×C(wt%)+0.5×Mn(wt%)+3
When 0×N (wt%) + Cu (wt%), Cr
It is approximately proportional to the equivalent/Ni equivalent ratio, the larger the value, the lower the austenite content, and the smaller the value, the higher the austenite content. Furthermore, when this ratio is 1.9 or more and 2.4 or less, the austenite content in the base metal structure is A relationship in the range of approximately 40-65% was obtained.

On the other hand, in the base metal weld heat affected zone heated by the welding thermal cycle, 5OS329 type or DIN 1.4462
In the case of the type, the amount of austenite decreases when the peak temperature is about 1,200°C or higher, and in particular, at the peak temperature of about 1,300°C or higher, the structure changes to a structure consisting of coarse ferrite grains and grain boundary austenite, which seems to have passed through the ferrite single phase region, and the austenite The decrease in quantity is also significant.

In addition, the structure of the weld metal without filler metal also has a peak temperature of 130
This is similar to the base metal weld heat affected zone that is heated to 0°C or higher.

In response to such a significant decrease in the amount of austenite in the base metal heat affected zone of conventional duplex stainless steel and the weld metal welded without filler metal, the amount of nitrogen was increased significantly compared to the conventional method as shown in Figure 2. Furthermore, it has been found that in duplex stainless steel in which the total amount of alloying elements is greatly increased, the degree of decrease in the amount of austenite is significantly improved. In particular, regarding the latter, the total amount of alloying elements excluding C and Si is 4
At 5-t% or more, as shown in Figure 3, the increase in the amount of ferrite is suppressed, and at the same time, as shown in Figure 4, the thermal effect of coarse-grained base metal welding, which is formed via the ferrite single phase region, is reduced. It was also found that the width of the area was also significantly reduced.

On the other hand, regarding its corrosion resistance, it is generally said that an increase in Cr and Mo improves the corrosion resistance.
In the heat-affected zone of base metal welds heated above ℃ or in weld metal without filler metal, increasing them alone does not improve corrosion resistance and increases the amount of austenite in the structure by 40°C.
It was also found that the same effect as the base material can be achieved only by adjusting the chemical composition to ~65%.

That is, the gist of the present invention is that C: 0.025 wt% or less, Si: 0.01 to 0.2 wt%, Mn: 1.0
~5.0wt%, Cr:28~35wt%, Ni:
6-16wt%, Mo: 3.1-6.0wt%, N
: Contains 0.2 to 0.4 wt%, the balance consists of Fe and unavoidable impurities, and Cu as necessary:
Duplex stainless steel containing 0.1 to 3.0 wt%, and Cr equivalent = Cr Cwt%) + 1.5xSi
(wt%) +Na (wt%), i1 per N1 = Nt(
wt%) + 30×C(wt%) + 0.5 X Mn
(wt%) + 30xN (wt%) + Cu (wt%), the i1/Ni equivalent ratio per Cr is 1.9 or more and 2.4 or less, and the alloying elements other than C1Si in the above equivalent formula are It is a ferrite-austenite duplex stainless steel that satisfies the requirement that the total amount is 45-t% or more.

Next, the use of limited ingredients according to the present invention will be described.

★C: 0.025 sot% or less C combines with Cr, Mo, etc. during the welding heat cycle,
In particular, carbides precipitate in the base metal heat affected zone and weld metal.
This significantly deteriorates the corrosion resistance of these areas.

Therefore, from the viewpoint of improving corrosion resistance, it is necessary to reduce it as much as possible, and it is limited to 0.025 wt% or less.

★Si: 0.01 to 0.2wt% Si is essential as a deoxidizing element in steelmaking reactions, but if it is contained in large amounts, corrosion resistance may deteriorate when the base metal heat affected zone or weld metal undergoes multiple welding heat cycles. , significantly accelerates the precipitation of σ phase, which significantly deteriorates mechanical properties. Therefore, it was limited to 0.01 to 0.2 wt%, which is effective as a deoxidizer and does not affect the precipitation of the σ phase.

★Mn: 1. 0-5. Like 0wt%Si, it is added as a deoxidizing agent, but at the same time, it is also an effective element for increasing the solid solution amount of nitrogen, which is particularly effective for austenite formation in the base metal heat affected zone and weld metal, and is 1.0wt% or less. Preferably. However, if it contains 5.01wt% or more,
Since it promotes the formation of MnS, which is harmful to pitting corrosion resistance, and also impairs toughness, the upper limit was set at 5.0 wt%.

★Cr: 28-35wt% Main element that provides corrosion resistance and oxidation resistance, and M
Similarly to n, the solid solution amount of nitrogen is increased. From these points of view,
A high content is desirable, and the total amount of alloying elements described below is 45-t.
% or more, the content must be at least 2-t% or more. However, if the content exceeds 35 wt%, the ductility and toughness will be significantly lowered, and σ phase precipitation will also be promoted during the welding heat cycle, so 35 wt% is set as the upper limit.

★Ni: 6-16-t% It is the main element that forms austenite. It is the most effective element for improving toughness and ductility, and from this point of view it is necessary to contain at least 6 wt%. On the other hand, below-mentioned Cr
In order to satisfy the condition of the it/Ni equivalent ratio, it is necessary that the content be small (both 16-t% or less).

★Mo: 3.1~6. 0wt% A main element that improves corrosion resistance, and is particularly distributed in the ferrite phase to improve the corrosion resistance of the ferrite phase, such as pitting corrosion resistance in environments containing chlorides. at least 3.1
The content is preferably at least wt%.

On the other hand, if the content exceeds 6.0 wt%, harmful intermetallic compounds such as elongated phase and χ phase will precipitate during the welding heat cycle, and ductility will also decrease, so 6.0 wt% is set as the upper limit.

★N: 0.2 to 0.4 wt% It is an austenite-forming element, and is an extremely effective element, especially in suppressing the decrease in the amount of austenite in the base metal weld heat affected zone and in the weld metal welded without filler metal. . In addition, it is effective in improving strength and pitting corrosion resistance of the austenite phase, so it is desirable to contain as much as possible. From these viewpoints, the content was set to at least 0.2 wt%.

However, since a large amount of content causes an increase in nitride precipitation and a decrease in hot workability and ductility, the upper limit was set here as 0.4 wt%, which is within the solid solubility limit.

★1i per Cr/Ni equivalent ratio = 1.9 or more, 2.4
Below (however, Cr equivalent - Cr (wt%) + 1
．． 5 ×Si (evt%) +Mo (wt%), N
1 equivalent - Ni (wt%) + 30 x C (wt%) + 0.
5 × Mn (ivt%) + 30 × N (wt%) 10C
u (wt%)) From the viewpoint of corrosion resistance, the optimal amount of austenite in the matrix is in the range of 40 to 65%, but if the Cr equivalent/Ni equivalent ratio is less than 1.9, austenite formation will occur, and on the other hand, If it exceeds 2.4, it becomes ferrite rich. Therefore, we have added conditions for the notation.

★The total amount of alloying elements other than C and Si is 45wt% or more In order to suppress the decrease in the amount of austenite in the weld metal welded without base metal heat affected zone or filler metal, increase the total amount of alloying elements other than C and Si. This is extremely important, and the effect becomes particularly pronounced when the total amount is 45 wt% or more. Also,
When the total amount of alloying elements is 45-t% or more, the heat-affected zone width is significantly reduced with a large structural change.

Furthermore, in the present invention, 0.1 to 3.0 wt% of Cu can also be added. In other words, Cu is an effective element for improving corrosion resistance, especially corrosion resistance in a reducing atmosphere, but on the other hand, 3.0 wt%
This is because if it exceeds this, the workability will deteriorate.

Within the composition range that satisfies the above conditions, the corrosion resistance is extremely excellent, and the loss of austenite I in the base metal heat-affected zone due to welding and the weld metal welded without filler metal is small, and the phase difference between the base metals, etc. It is possible to obtain a duplex stainless steel that maintains balance and exhibits extremely little deterioration in corrosion resistance, etc.

〔Example〕

EXAMPLES Hereinafter, the effects of the present invention will be explained in detail with reference to Examples.

Duplex stainless steels having the 16 component compositions shown in Table 1 were produced by vacuum melting, and the steel ingots were hot-rolled and solution heat-treated in the usual manner to form a true plate with a thickness of 51 mm. . Steels N111 to 8 in Table 1 are the steels of the present invention, and steels 9 to 16 are comparative steels. Among the comparison steels, the compositions of commercial 5US329 type and DIN 1.4462 type were also included for reference. Table 1 also shows that Cr equivalent = Cr (w
t%) + 1.5 ×Si (wt%) + Mo(&
4t%), Ni equivalent=Ni(int%)+30×C(w
t%) + 〇, 5 × Mn (wt%) + 30 × N (wt
%) + Cu (wt%), Cr equivalent fJ / Ni equivalent ratio of each component composition, and C in the equivalent formula
, the total amount of alloying elements excluding Si is also shown.

After measuring the austenite content of each of these steel plates, TI was applied to each steel plate without using filler metal.
Perform G bead on plate (heat input 15kJ/cm)
In addition to measuring the amount of austenite in the heat-affected zone of the base metal and the weld metal, and the width of the heat-affected zone of the base metal, corrosion test pieces including the welded zone were taken to evaluate the corrosion resistance. Austenite l-ff1 was evaluated by optical microscopic observation and was found to have a structure consisting only of ferrite and austenite61! m
! After that, the amount of ferrite was measured at multiple points using a magnetic method, and the average was calculated. Furthermore, this result was confirmed by the point counting method. In addition, as a corrosion resistance test, we conducted a 65% nitric acid test (J
IS G0573-1980) and ferric chloride corrosion test (JIS G0578-1981). Figure 5 shows a welded part test piece for corrosion testing. In the same figure, 1 is the base metal, 2 is the TIG welding bead, and a is 5mmSb.
, c are each 30 pieces.

Table 2 shows the measurement results of the base metal, the weld heat affected zone, the amount of austenite in the weld metal, and the weld heat affected zone width, as well as the results of various corrosion resistance tests of the weld.

As is clear from the comparison between Tables 1 and 2, the amount of austenite in the base metal is
4 when the value of Cr equivalent/Ni equivalent ratio is in the range of 1.9 to 2.4.
It is in the range of 0 to 65%, and when the value of Cr equivalent / Ni equivalent ratio is less than 1.9, it becomes austenite rich than 65%, and when the value of Cr equivalent / Ni equivalent ratio exceeds 264, it becomes ferrite lynch and the amount of austenite is will be less than 40%. Therefore, the base material component has at least Cr equivalent/
The Ni equivalent ratio needs to be in the range of 1.9 to 2.4.

On the other hand, even if the base metal contains an appropriate amount of austenite, as can be seen from Table 2, the amount of austenite in the weld heat-affected zone and the weld zone without filler metal decreases significantly in the comparative steel. The decrease in the amount of austenite is extremely small. Correspondingly, the results of the 65% nitric acid test and the ferric chloride corrosion test of the weld zone show that the corrosion resistance of the steel according to the invention is considerably improved. In the comparison steel, localized corrosion such as intergranular corrosion or pitting observed after the 65% nitric acid test and ferric chloride corrosion test was concentrated from the weld heat affected zone to the weld metal, and the corrosion loss was extremely large. On the other hand, in the steel of the present invention, almost no local corrosion was observed even in those areas, and the corrosion loss was almost the same as when the base metal was tested alone, and was extremely small. This improvement in the corrosion resistance of the weld zone in the steel of the present invention is due to the austenite 1 of the weld zone.
-ffi is in an appropriate range compared to that of the comparative steel, and together with this, the alloying elements Cr, Mo, and Ni are higher than the comparative steel.

〔Effect of the invention〕

As described above, the steel of the present invention not only has high corrosion resistance itself (but also has high corrosion resistance in the weld metal without weld heat affected zone or filler metal, which has been a problem with conventional duplex stainless steel). It also has extremely excellent corrosion resistance, and can fully meet the demands for highly corrosion-resistant structural materials, which are expected to become increasingly strict in the future.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the amount of base metal austenite after solution treatment and the Cr equivalent fJ/Ni equivalent ratio, where C
r equivalent = Cr (wt%) + l, 5 × Si
(wt%) +Mo (wt%), Nu equivalent = Ni
(it%) + 30 × C (wt%) + 〇, 5 × Mn (
wt%) + 30xN (wt%) + Cu (wj%), Figure 2 shows the amount of austenite, base metal, and filler metal when the maximum heating temperature is varied in a simulated welding thermal cycle equivalent to a heat input of 15 kJ/cm. A diagram showing the change in austenite 1-ffi in the case of TIG lick welding, which is not used.
Figure 4 is a diagram showing the influence of the total amount of alloying elements other than Si. Figure 4 is a diagram showing the influence of alloying elements other than C and Si on the coarse grained HAZO width. Figure 5 is a perspective view of a corrosion-resistant test piece. It is. 1... Base material, 2... TIG lick bead. Cr Yuyaku/wi Equivalent Keikan 0 Jujutsu ('c) ”“1 Figure 3

Claims (2)

[Claims] (1) C: 0.025 wt% or less Si: 0.01-0.2 wt% Mn: 1.0-5.0 wt% Cr: 28-35 wt% Ni: 6-16 wt% Mo: 3.1-6. Contains 0 wt% N: 0.2 to 0.4 wt%, the remainder consists of Fe and unavoidable impurities, and Cr equivalent = Cr (wt%) + 1.5 x Si (wt%) +
Mo (wt%), Ni equivalent = Ni (wt%) + 30 × C
(wt%)+0.5×Mn(wt%)+30×N(wt
%), the Cr equivalent/Ni equivalent ratio is 1.9 or more,
2.4 or less, and the total amount of alloying elements (Cr, Ni, Mo, Mn, N) excluding C and Si in the above equivalent formula is 45wt
% or more. Highly corrosion resistant duplex stainless steel with excellent weldability. (2) C: 0.025 wt% or less Si: 0.01-0.2 wt% Mn: 1.0-5.0 wt% Cr: 28-35 wt% Ni: 6-16 wt% Mo: 3.1-6. Contains 0 wt% Cu: 0.1 to 3.0 wt% N: 0.2 to 0.4 wt%, the remainder consists of Fe and inevitable impurities, and Cr equivalent = Cr (wt%) + 1.5 × Si ( wt%) +
Mo (wt%), Ni equivalent = Ni (wt%) + 30 × C
(wt%)+0.5×Mn(wt%)+30×N(wt
%) + Cu (wt%), the Cr equivalent/Ni equivalent ratio is 1.9 or more and 2.4 or less, and C in the above equivalent formula
, alloying elements other than Si (Cr, Ni, Mo, Mn, Cu
, N) in a total amount of 45 wt% or more, a highly corrosion-resistant duplex stainless steel with excellent weldability.