JPS582266B2 - High chromium ferritic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high chromium ferritic stainless steel with excellent toughness.

It is known that increasing the amount of Cr is effective in improving the pitting corrosion resistance of stainless steel, and that this effect is further enhanced by the simultaneous addition of Mo.

Therefore, in environments containing chlorides such as seawater, M
It is said that high chromium ferrite stainless steel to which o is added is effective.

However, although increasing the amount of Cr improves corrosion resistance such as pitting corrosion resistance, the toughness deteriorates significantly.

This deterioration of toughness is closely related to the amount of C and N contained in the steel, and even if the amount of Cr is increased, if the amount of C and N is reduced to a predetermined amount or less, the deterioration can be suppressed to some extent. Are known.

For this reason, in recent years, high chromium ferritic stainless steels with considerably reduced amounts of C and N have been developed and produced using special refining methods such as electron beam melting and vacuum induction melting.

However, even with these special refining methods, there is a limit to the reduction of C and N, so the adverse effects of these elements on toughness cannot be completely eliminated.

In other words, the amount of solid solution of C and N in high chromium ferrite steel is extremely small. For example, in high chromium steel with 25% Cr or more, a C amount of only about 30 PPM or a N amount of about 100 PPM is usually used. Approximately 850℃ to 95℃
At a treatment temperature of about 0° C., C and N cannot be completely dissolved, and carbonitrides are formed on the grain boundaries, making it impossible to obtain sufficient toughness.

Furthermore, when the steel is slowly cooled, carbonitrides on the grain boundaries increase and the toughness further deteriorates.

By the way, a widely accepted general method used to measure toughness is the Charpy V-type notch impact test method, and with reference to the information obtained from this test result,
Considering the conditions that steel should have as an equipment material, (1) It should have excellent toughness under optimal heat treatment conditions.

That is, the ductile-brittle transition temperature (hereinafter abbreviated as DBTT) after treatment under the optimal heat treatment conditions is sufficiently low, and the DBT
The impact value at T or higher, that is, within the ductile fracture temperature range, is sufficiently large.

(2) The tendency for the characteristics described in (1) above to deteriorate is small when the cooling rate from high temperatures after hot working and heat treatment is slow, that is, when slow cooling, and when affected by heat such as welding. .

It is required to have two points at the same time.

The inventors of the present invention have kept these points in mind and have conducted extensive tests and detailed studies, and found that the properties of 18αIX2) are closely related to the amounts of C and N, and that the addition of V, AI, Nb, and Ta , each has its own unique effect on improving the properties listed in (IX2) above.Therefore, by reducing C and N as much as possible and adding these elements simultaneously, we can create a high chromium ferritic stainless steel with overall excellent toughness. It was discovered that steel can be obtained, leading to the present invention.

To explain in more detail, the addition of a specific amount of V related to the amount of N has a remarkable effect on improving the property (1), that is, the toughness under optimal heat treatment conditions, and
The addition of I further increases the effect of V, and furthermore, the addition of a specific amount of Nb and/or Ta related to the amount of C particularly
), that is, the toughness under slow cooling, and led to the present invention.

In other words, the high chromium ferritic stainless steel proposed by the present invention has excellent toughness under both rapid cooling and slow cooling conditions, and can be used not only for thin plates that can be rapidly cooled, but also for thick plates, large materials, etc. This steel is ideal as a material for equipment as it has sufficient toughness even for materials that require slow cooling.

The reasons for limiting the various component compositions of the stainless steel according to the present invention will be described in detail below.

(1) Cr Cr forms a stable passive film and is extremely effective in improving corrosion resistance.

On the other hand, as the content increases, embrittlement phenomena peculiar to high chromium steel, such as 475°C brittleness and σ brittleness, become more likely to occur, and C, N,
Since it is difficult to reduce impurities such as .

(2) Mo Mo has a remarkable effect on improving corrosion resistance, especially porosity resistance and crevice corrosion resistance, and for that purpose, it has a minimum of 0.

It needs to be 5% or more. However, as the content increases, intermetallic compounds such as σ phase

(3) C, N In ferritic stainless steel, especially chromium ferrite stainless steel, the solid capacity of C and N is extremely small, and the diffusion rate of C and N, which generally have small atomic radii, is fast. Carbides, nitrides, or carbonitrides are formed on grain boundaries even during rapid cooling, such as water cooling treatment after high-temperature annealing, which is employed as a method of high-temperature annealing.

These precipitates on the grain boundaries deteriorate the mechanical properties of the steel, especially the toughness at low temperatures, and when these precipitates are formed, Cr, Mo, etc. are effective in improving corrosion resistance in the vicinity of these precipitates. Corrosion resistance also deteriorates because the concentration of these components decreases.

Considering this point and further considering that C is more harmful than N, c: 0.0080% or less N: 0.015
% or less.

(4) Most of 00 exists in steel as nonmetallic inclusions,
It acts as a notch, moves DBTT to the high temperature side, and has a tendency to lower the impact value in the ductile fracture region above DBTT, so it needs to be kept low, and 0.010%
(5) Si limited as follows Si causes DBTT to move to the high temperature side by solid solution hardening.

Further, Si is limited to 0.3% or less because it tends to promote precipitation of the σ phase that impairs toughness.

(6) PP P shifts the DBTT of the steel to the high temperature side and impairs toughness, so it was limited to 0.025% or less.

(7) SS, like O, forms non-metallic inclusions in steel, shifts DBTT to the high temperature side, and lowers the impact value in the ductile fracture region above DBTT, so it should be 0.025% or less. limited to.

(8) Ni Ni increases corrosion resistance against H2S04, HC1, etc.
For this purpose, a maximum of 3% is sufficient, and addition of more than that increases susceptibility to stress corrosion cracking and promotes precipitation of the σ phase, so it was limited to 3% or less.

(9) Cu Cu, like Ni, increases the corrosion resistance against H2S04, but it also increases stress corrosion cracking susceptibility and impairs hot workability, so it was limited to 2% or less.

(9) Al Al acts as a deoxidizing agent during melting and refining, and must be contained at least 0.03% in order to reduce the zero content to 0.010% or less as described above.

In addition, AI doubles the effect of ■ in the presence of a trace amount of V, and D
It has the effect of increasing the impact value at BTT or higher.

In order to fully exhibit its effect, AI needs to be 0.03% or more.

However, since 1% or more of AI promotes 475° C. brittleness and also lowers the impact value, it is set to 0.03 or more and 1% or less.

(8)■ In high chromium ferritic stainless steel, precipitation of carbides, nitrides, or carbonitrides occurs during cooling, and
In order to prevent embrittlement due to brittleness, etc., the faster the cooling rate after heat annealing is, the better; water cooling treatment is generally employed, and such heat treatment conditions are considered optimal.

The addition of a small amount of (2) has a remarkable effect on improving the impact properties under such optimal heat treatment conditions, especially on improving the impact value in the ductile fracture region of DBTT or higher.

If the content of (①) is less than 0.005% or less than I times the nitrogen content, it is not sufficient to improve the impact properties, or 0.1%.
Alternatively, if the amount exceeds 10 times the nitrogen content, the effect of addition is saturated.

Therefore, a range of 4×N to 10N is appropriate in the range of 0.005 to 0.1%.

(IQ) Nb, Ta By adding Nb and Ta alone or in combination, C, which is harmful to corrosion resistance and toughness, is fixed and these properties are improved.

The appropriate amount of addition depends on the C content, and in the case of composite N
If b%+-a-Ta% is less than 80×C%-0.24%, sufficient toughness cannot be obtained because C in the steel cannot be sufficiently fixed. 80×C%-0.
At 0.8% or higher, C and N are sufficiently fixed, but excessive N
(b) or Ta solid solution in the steel causes deterioration of toughness.

This also applies to the case of Nb and Ta alone.

Furthermore, since the C content of the steel of the present invention is suppressed to a maximum of 80 PPM, the Nb and Ta contents are 0.40% and 0.40%, respectively. Do not exceed SO%.

That is, Nb or -}Ta or Nb + Ta (these are expressed as Nb and/or +Ta) is 80 x C% -
0.24% or more 80×C%10. Must be less than 0.08%.

Here, Ta is twice as large as Nb because its atomic weight is approximately twice that of Nb.

When Nb and Ta are added alone or in combination within the above range, steel tends to precipitate carbonitrides tzN600-9
When the cooling rate is slow in the range of 00°C, a remarkable effect of improving toughness appears.

The improvement in toughness of the steel of the present invention will be explained below with reference to Examples.

Figure 1 shows Cr 29-30%, Mo 1.8-2.0%
, C0.0025-0.0032%, NbO. 11-0
．． 12%, NO. 0060-0.0072%, NiO.
13 to 0.15%, CuO. 1 to 0.2%, and a 20'kg vacuum induction melting furnace to keep Si, S, P, and 0 at almost constant values within the range of the present invention.
7 and 9 when AI is added within the range of 0.006 to 0.07% (addition amount 0.20%) and when it is not, respectively.
The charge is melted and these ingots (AI content 0
．． 13%) was forged into a billet with a thickness of 50 mm, hot-rolled to a plate thickness of 5 mm, annealed at 900°C for 20 minutes, and then water-cooled. Charpy impact test (using JIS No. 4 2 mm V notch test piece) was performed on the material. It shows the impact value at °C.

As a result, when the V content is less than 3 times the N content, sufficient impact properties as a material are not obtained, but when it is 4 times or more, the impact value becomes high and reaches a certain value.

Furthermore, the impact value when AI is added is much improved compared to when V alone is added, and it is clearly observed that the combination of V and AI improves the impact properties.

Figure 2 shows Cr29-30%, Mo 1.8-2.0%,
No. 0060~0.

0080%, AI0.11-0.13%, V0.035
~0. 040%, NiO. 13~0.15%,
Cu is almost constant at 0.1 to 0.2%, Si, S,
The Nb and C contents were adjusted to 0 to 0.26% and 0.0%, respectively, so that P and 0 were constant values within the range of the present invention.
The material was melted in a vacuum induction melting furnace for 22 charges with varying values within the range of 0.011 to 0.0060%, then forged and hot rolled to a thickness of 5 mm as in the case of Fig. 1, annealed at 900°C for 20 minutes, and then air cooled. This figure shows the ductile-brittle transition temperature when a Charpy impact test was conducted on Nb and C contents.

The shaded area in the figure corresponds to the scope of the present invention, that is, the C with Nb described above.
This is a diagram illustrating the relational expression.

As is clear from FIG. 1, when the C content increases and the amount of Nb added is increased at a constant ratio within the range of the present invention, a sufficiently low transition temperature is obtained even in the material air-cooled after annealing.

Figure 3 shows the impact value at θ°C obtained in the impact test in Figure 2, and as in the case of the ductile-brittle transition temperature, it is practical if Nb is added according to the C content within the range of the present invention. A sufficient impact value was obtained.

Note that when Ta was added in twice the amount (weight) of Nb, exactly the same effect as Nb was obtained.

Therefore, it goes without saying that these may be used in combination.

[Brief explanation of the drawings] Figure 1 shows the V/
Relationship diagram between N and impact value at 0°C. FIG. 2 is a diagram showing the relationship between Nb and C on the ductile-brittle transition temperature, and FIG. 3 is a diagram showing the relationship between Nb and C on the impact value.

Claims (1)

[Claims] 1 Cr: 25-33%, Mo: 0.5-4.0%
, C: 0.0080% or less, N: 0.0150% or less,
0: 0.010% or less, Si: 0.30% or less, P: 0
．． 025% or less, S: 0.025% or less, Ni: 3.0
% or less, Cu: 2.0% or less, V: 0.005 to 0.1
0% and 4 to 10 times the amount of N, AI: 〇, 03 to 1
．． 0%, Nb and Ta are singly or in combination, and their content is within the range of the following formula: 80×C%-0.08%≧Nb and/or -J! -Ta
≧80×C%-0.24%, a high chloroferrite stainless steel with the remainder substantially consisting of Fe.