JPS59159975A - Ferritic chromium stainless steel containing al - Google Patents

Abstract

PURPOSE:To obtain the titled steel having improved toughness and corrosion resistance by specifying a composition consisting of Cr, Mo, Ni, Al and Fe and by reducing the amounts of C, N, S, Si, Mn and O contained in the composition as impurities. CONSTITUTION:Ferritic chromium stainless steel contg. Al is obtd. by providing a composition consisting of, by weight, 15.00-50.00% Cr, <=5.00% Mo, 0.01- 6.00% Ni, <=1.00% Al and the balance Fe with impurities and by reducing the amounts of C, N, S, Si, Mn and O as the impurities to <=0.0060% C, <=0.0150% N, <=0.0020% S, <=0.15% Si, <=0.15% Mn and <=0.015% O by refining. The composition may further contain 5X(C+N)%-1.00% Ti and/or 8X(C+N)% -1.00% Nb. The steel has superior toughness and intergranular corrosion resistance at the weld zone, and it has superior corrosion resistance, especially pitting corrosion resistance and acid resistance at the base metal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ferritic chromium stainless steel having excellent welded part toughness and intergranular corrosion resistance, and base metal corrosion resistance, particularly pitting corrosion resistance and acid resistance.

Ferritic chromium stainless steel does not contain a large amount of Ni and has improved corrosion resistance with Cr and Mo, so the raw material cost during production is low, but it has lower toughness and ductility than austenitic steel ( It has also been said that its corrosion resistance is poor.However, with the progress of steel manufacturing technology in recent years, the impurities in steel, such as C and N, have become the cause of such property deterioration.
As technology has been established to significantly reduce the Studies have been conducted on measures to further improve corrosion resistance.

Typical high-purity ferritic chromium stainless steels include 18%Cr-2%Mo, 26%Cr-1%Mo,
29%Cr-4%Mo, 29%Cr-4%Mo-2
%Ni, 30%Cr-2% Q, etc. The biggest features of these high purity ferritic chrome stainless steels are:
Excellent stress corrosion cracking resistance in solutions containing chlorides (resistance to S,
C,C,Sex-5tress CorrosionCra
The use of this material is expected to be expanded, mainly in neutral environments containing C1- ions, where austenitic stainless steel cannot be used.

By the way, methods of improving the corrosion resistance of ferritic chromium stainless steel that are being implemented today include increasing the Cr content, adding Mo, and adding N.
These include addition of i, extremely low (C+N), and addition of stabilizing elements such as Ti and Nb. However, both the increase in Cr and the addition of Mo act in the direction of deteriorating the toughness of ferritic stainless steel, especially the toughness of welds, and rapid deterioration of toughness is observed below room temperature. This deterioration in toughness has hindered the wide range of applications for high-purity ferritic chromium stainless steel.

In addition, reducing (C+N) is not sufficient to prevent deterioration of the toughness of the weld, and even more so when attempting to reduce the effect of (C+N) by adding stabilizing elements such as Ti and Nb. As a result of the generation of large amounts of carbon and nitrides of these elements, deterioration of the toughness of the weld is inevitable.

Thus, the purpose of the present invention is to eliminate the problems of deterioration in toughness and corrosion resistance, which were considered drawbacks of conventional high-purity ferritic chromium stainless steels, and to create a high-quality stainless steel that dramatically improves both of these properties. Our goal is to provide high purity ferritic chrome stainless steel.

Here, the present inventors investigated various conventional methods of improving toughness and found that in addition to extremely low C and N, low Si and M
Unexpectedly, we discovered that by adding n, ultra-fine sulfur, and old additives, the low-temperature toughness of ferritic chromium stainless steel, including welded parts, was significantly improved as a result of their overall effect. , completed the present invention.

Furthermore, by extremely lowering the S content in the steel to a level that is in line with conventional methods, the present inventors have unexpectedly been able to prevent the decline in hot deformability and deterioration in corrosion resistance that accompany the reduction in Mn. Rather, the present invention was completed by discovering that the synergistic effect of ultra-low S content and low Mn in steel significantly improves the corrosion resistance of the base material, particularly pitting corrosion resistance, corrosion resistance, and acid resistance. It is.

In addition, the present inventors have improved the toughness of the weld zone and the grain resistance of the weld zone by ultra-low C, N content and addition of Al through vacuum refining, and furthermore, if desired, addition of Ti and Nb as stabilizing elements. They discovered that the corrosion resistance was significantly improved and completed the present invention.

Here, the present invention includes, in weight%, Cr: 15.00-50.00%, Mo: 5
．． 00% or less.

Ni: 0.01-6.00%, Al: 1.
00% or less.

Furthermore, if desired, Ti: 1.00% or less [however, 5x (C+N)% on Ti] and Nb: 1
．． 00% or less [however, 8×(C+N)% on Nb], the remainder consists of Fe and impurities, and by refining, C+ N+ S+ as impurities
Sl + Mn.

Ferritic chromium stainless steel 6Ii, which is characterized by reducing 0 to the following range.
:C: 0.0060% or less, , N: 0.01
50% or less, -3:0.0020% or less, Si
: 0.15% or less.

Mn: 0.15% or less, O: 0.015% or less.

As is clear from the above, in the steel of the present invention, C,
It is characterized by extremely low levels of N, S, Si+Mn, and 0 impurities, and simultaneous reduction of these impurities was made possible for the first time by a newly developed special vacuum refining method described below. It is.

In addition, conventional high-purity ferritic chromium stainless steel has been produced by AOD method, VOD method, VIM method, electron beam melting method, etc., but in these conventional techniques, C+N+S+5IJn and 0 as impurities are removed respectively. At the same time, it was impossible to reduce it to the above range (hereinafter referred to as the steel level of the present invention). That is, for example, in the AOD method, it was possible to reduce S in steel to 0.0020% or less, but on the other hand, C and N in steel
could not be reduced to the level of the steel of the present invention, and furthermore, it was not possible to reduce the content of Si and Mn to the level of the steel of the present invention due to deoxidation restrictions. In addition, VOD method, V
In vacuum melting methods such as IM method and electron beam melting method, carbon in steel is
Although it was possible to reduce 5N+Mn+O to the level of the steel of the present invention, it was not possible to reduce S in the steel to the level of the steel of the present invention.

In this way, even in the conventional technology, each of the above-mentioned impurities C+N
+Si+Mn, although it has been attempted to extremely reduce only a part of 0, it has not been possible to reduce all of them at the same time, and the effect at that time has not been known. That is, like conventional high-purity ferritic chromium stainless steel, C, N, S, and Si
, Mn, and O, the original properties of the reduction of each element that should be exhibited when these elements are simultaneously reduced to the level of the present invention steel are the same as those of the present invention. It can be said that it is hidden by the presence of some impurity elements higher than the steel level. Since the mutual relationship between these impurities was not clarified, the reduction effect as an impurity was, for example, about 50 ppm for S, and for C.
, H were thought to be saturated at about 1100 ppm and 200 ppm, respectively.

Next, the reason why the composition range of each component in the ferritic chromium stainless steel of the steel of the present invention is limited as described above will be explained. Hereinafter, unless otherwise specified, "%" in this specification means "% by weight." Chromium (Cr): Cr is an important element that determines the basic corrosion resistance of the steel of the present invention. Corrosion resistance improves as the content increases, but if the content exceeds 50.00%, forging is impossible, while if it is less than 15.00%, sufficient corrosion resistance is obtained as a ferritic stainless steel. Therefore, its content was set at 15.00-50.00%.

Molybdenum (Mo): 14o is an additive element that has the effect of significantly increasing the corrosion resistance of the steel of the present invention. It has great effects on improving induction resistance, acid resistance, pitting corrosion resistance, and crevice corrosion resistance. If the content exceeds 5.00%, the deterioration of mechanical properties due to the precipitation of intermetallic compounds such as σ (sigma) phase and χ (chi) phase becomes significant, so the upper limit was set at 5.00%.

Nickel (Ni): Ni improves acid resistance and improves the passivation ability of the steel of the present invention. If the content exceeds 6.00%, it becomes difficult to obtain a single ferrite phase, and furthermore, precipitation of the σ phase becomes significant, so the upper limit was set at 6.00%.

The above effect is not observed at less than 0.01%.

Aluminum (At): A1 has the effect of improving the toughness of the weld heat affected zone and reducing toughness deterioration due to N in steel. Furthermore, it has the effect of reducing zero in the steel, reduces nonmetallic inclusions that have a notch effect, and improves the toughness of the base metal by stabilizing solid solution N. On the other hand, if it is added in excess of 1.00%, the base material will become noticeably brittle and cracking will occur in hot conditions, so the upper limit was set at 1.00%.

In the steel of the present invention, Al is an element that is actively added, and the amount of residual Al is distinguished from the amount of unavoidable impurities remaining in normal At deoxidation, and is generally, for example, 0. The lower limit is 0.01%. Preferably it is 0°015%-0.4%.

Titanium, niobium (Ti, Nb): Ti and Nb
is added, if desired, in order to prevent corrosion resistance deterioration and toughness deterioration in the welded part due to C and N contamination caused by external factors during welding.

When adding Ti and/or Nb, Ti and Nb
b has the effect of suppressing the coarsening of the crystal grains of the base metal and the weld heat affected zone, and it is necessary to sufficiently develop these effects.
In order to drive a car, amounts of Ti≧5x (C+N)% and Nb≧8x (C+N)% are required. However, if both Ti and Nb are present in large amounts, Lav
Since ES phase precipitation becomes noticeable and deterioration of toughness becomes noticeable, the upper limit was set at 1.00%.

Next, the reason for suppressing impurities as an important feature of the steel of the present invention and the effects obtained thereby will be further explained.

Carbon, nitrogen (C, N): C and N are component elements that have a large effect on the toughness of high-purity ferritic chromium stainless steel, as well as the intergranular corrosion resistance, induction resistance, and acid resistance of welded parts. According to their knowledge, the effect of reducing these elements does not reach saturation, and the lower the content of C and N in steel, the more desirable it is. C2N allowed in the steel of the present invention! ! ! The temperature decreases significantly as the Cr concentration increases. If C exceeds 0.0060% and N exceeds 0.015%, the weld toughness deteriorates significantly and intergranular corrosion resistance also deteriorates, so the upper limit for C is set at 0.006%. %, 0.01 for N
It was set at 50%. Preferably CF 0.003% or less, N
: 0.0060% or less.

Sulfur (S): S significantly deteriorates corrosion resistance and hot workability, and also tends to lower the impact value in the ductile fracture region at room temperature. S is 0.0020% or less, preferably 0.0020% or less.
0010% or less. As described later, in the steel of the present invention, the Mn content is kept at 0.15% or less for the purpose of improving toughness, but when the S content is high, the sulfides in the steel are not MnS,
Rather, it takes the form of (Fe, Mn)'', which reduces the corrosion resistance and hot deformability of the base metal.Therefore, it is necessary to limit S to the above range.

Silicon (Si): Si reduces the elongation of the base material through solid solution strengthening and shifts the brittle fracture surface transition temperature to the high temperature side. In the conventional melting method, Si
Usually, about 0.20% of Si is required as a deoxidizing agent, but in the steel of the present invention, the addition of Si as a deoxidizing agent is unnecessary due to the adoption of advanced vacuum refining technology. However, since a very small amount of about 0.15% may be mixed in as unavoidable impurities during melting, the allowable upper limit was set at 0.15%. Preferably Si: 0.10% or less.

Manganese (phylum n): Mn has a remarkable property of shifting the brittle fracture surface transition temperature to the high temperature side, and the lower the Mn content, the more desirable it is. The allowable upper limit was set at 0.15%, which is the level of unavoidable impurities. Preferably it is 0.10% or less.

Oxygen (0): Oxygen exists as oxide-based nonmetallic inclusions in steel and acts as a crack initiation point at the notch, so the presence of oxygen increases the brittle fracture transition temperature. Furthermore, it tends to reduce the impact absorption energy in the ductile fracture region above the brittle fracture transition temperature. Therefore, the upper limit of oxygen is 0.
015%. When oxygen is contained in an amount exceeding 0.015%', not only the toughness but also the corrosion resistance of the base material tends to deteriorate. Preferably the oxygen content is 0.010% or less.

In addition, in the present invention, 0.7% or less of Cu and 0.5% or less of V may be contained as other unavoidable impurities. These unavoidable impurities have no adverse effect on the purpose of the present invention.

Next, the present invention will be explained in more detail with reference to Examples, but first a method for producing the steel of the present invention will be explained.

As already mentioned, the present invention reduces each of the impurities mentioned above to an extremely low level at the same time, and obtains excellent properties through the synergistic effect of the reduction effects of each impurity.

Simultaneous reduction of such impurities can be easily achieved by using the line method.

That is, 2 having a high-frequency induction heating coil having oxygen top-blowing ability, gas bottom-blowing ability, and powder top-blowing ability.
, a 5-ton vacuum smelting furnace is used, and a slag-forming agent for desulfurization is used for desulfurization, and oxide powder as an oxygen supply source is used for decarburization and denitrification using a special porous slag with an Ar gas carrier at the speed of sound. It is supplied onto the molten metal surface by top blowing using a lance. This smelting furnace is already in use in actual production, and is configured to allow casting under reduced pressure (vacuum). In this way, by top-blowing the powder as described above at the sonic speed level under vacuum, the ability to stir molten steel is dramatically improved. Therefore, by employing such a refining method, in a preferred embodiment of the present invention, 26% C
For example, C in r&li is less than 0.0010% (1
N is less than 0.0040% (4
less than 0 ppm), and S is 0.0002% (2 ppm
m), which was previously unimaginable, becomes possible. Therefore, in the present invention, as a result of sufficient stirring of molten steel under vacuum, the effect of C as a deoxidizing element can be effectively utilized, so that Si, Mn, etc., which have conventionally been required as deoxidizing elements, etc. It becomes possible to actively reduce the addition of elements, and the excellent effects of reducing such elements can also be utilized. At the same time, it became possible to actively and effectively utilize AI as an alloying element rather than as a deoxidizer, and the excellent effects of adding AI as an alloying element became possible. .

Ultra-high purity ferritic chromium stainless steel having the steel composition shown in Table 1 melted by the method described above was cast into 50kg round ingots.After casting, each ingot was heated to 1200℃ after external cutting. After heating for half an hour, 30mm
Forged to 130mm x 100mm. Then,
After heating to 1200°C, hot rolling was performed at a temperature of 950°C or higher to obtain a hot rolled sheet with a thickness of 7 mm and a width of 130 mm, and the thus obtained hot rolled sheet was annealed at 850°C and air cooled. Alternatively, after annealing at 1000°C or 1100°C and then water cooling, the specimens were processed into test pieces.

The analysis of C and S in steel uses a high frequency combustion infrared absorption method with a sensitivity of 0. L, an ultra-high performance analyzer with lppm
This was carried out using a C5-144 device manufactured by ECO.

The pitting corrosion resistance, brittleness, high temperature deformability, acid resistance, and intergranular corrosion resistance of each test piece thus obtained were examined. The results of these tests are the same (collectively shown in Table 1).

(1) Pitting corrosion resistance: 0.01 mol NaCl aqueous solution (6
The pitting potential at 0°C was measured. The measurement is 200
After degassing with Ar gas at m1/min for 1 hour, 20mV/min
Evaluation was made using v'ci=IQDAA when swept forward at min. The measurement data are summarized in Table 1.

Incidentally, the changes in pitting corrosion potential with respect to S in steel are summarized in a graph based on the data for test steel 2.3.4.12.13.14 and are shown in FIG. This means that the Mn content is 0.10% or less, and the S amount in the steel is 0.0002%-0,0048% (2ppm-48ppm).
This figure shows the influence of S in steel on the pitting corrosion resistance when it is changed to m).

As is clear from the results shown in the graph of the same figure, the higher the pitting corrosion potential, the better the corrosion resistance, but the pitting corrosion potential begins to deteriorate significantly at 34 degrees, which is around 0.002%. On the other hand, it can be seen that it is stable at 0.0010% or less.

In the figure, the numbers indicate the test steel numbers in Table 1.

(2) Brittleness: In the case of this example, the hot-rolled sheets obtained for each of the above-mentioned test steels were annealed at 1000°C in an Ar gas atmosphere for 20 minutes, and then water-cooled. A Charpy impact test was conducted on the test piece thus obtained, and the brittle fracture surface transition temperature was measured. The test piece size was a JIS No. 4 full size Charpy impact test piece. The results are summarized in Table 1.

In addition, the data obtained for sample steel 5.16.17 are summarized in a graph and shown in FIG. These data show 2 when the S content is less than 0.0010% and the Mn content is changed.
Brittle fracture transition temperature ('vTrs
'C) shows the change.

As is clear from the graph shown, vTrs increases as the amount of Mn increases, and it can be seen that the lower the amount of Mn, the more desirable it is. Mn content is 0.20% compared to the present invention steel level
For test steel 16, which is as high as the conventional steel level, vTrs is 3.
It can be seen that the temperature is 5°C, which is higher than room temperature, which is a problem even when used at room temperature.

The test steels listed in Table 1 are as, e, and 7. Use +5 and T
IG tanning was used to examine the toughness of the weld heat affected zone. After hot rolling, the material was annealed at 1000°C for 20 minutes in an Ar gas atmosphere, then water cooled, further cold rolled by 60%, and finally annealed at 920°C for 2 minutes in an Ar gas atmosphere. The sample was cooled with water after being held. Ar gas flow rate 10
j2/min, current 65-75A, welding speed 120 m
After TIG licking at m/min, the Charpy impact test piece was machined in the rolling longitudinal direction so that the weld heat affected zone became the notch part of the Charpy impact test piece. A Charpy impact test was conducted on the test piece thus obtained, and the brittle fracture surface transition temperature was measured. The test piece size was a JIS d half-Is Charby impact test piece. The obtained results are summarized in a graph in FIG. As is clear from the graph shown, the addition of AI not only improves the low-temperature toughness of the base metal, but also improves the toughness in the weld heat affected zone.

(3) High-temperature deformability: High-Cr ferritic stainless steel of 26% Cr or higher was evaluated for high-temperature deformability by conducting a falling weight test in the manner described below.

Here, the above-mentioned falling weight test means that a cylindrical test piece with a diameter of 15 mm and a height of 20 mm heated to 1200'C is instantaneously made into a flat piece with a height of 7 mm using a hammer press. This is to evaluate the deformability of.

If the deformability is good, cracks will not occur on the end face, but if the deformability is insufficient, cracks will occur. Table 1 summarizes the results. It can be seen that cracks become large when the Mn content is low and the sM in the steel is higher than the level of the steel of the present invention.

(4) Acid resistance: 26% Cr test steel 5, 6, 7, 15, 16, 17.1
A boiling test was conducted on test steels 8, 9, and 19.20 of 8 and 29% Cr in a (7) pH 1, H2SO4 solution containing 15% NaCl as follows. The immersion time was 6 hours, and the evaluation was based on the average corrosion rate per 6 hours.

The test piece dimensions are 3mm x 10mm x 40m
m and was polished No. 600 with wet emery. Then, acid resistance was evaluated using the average value of two test pieces.

The results are also summarized in Table 1, and it can be seen that the acid resistance of Ni-containing test steel 9 was significantly improved.

(5) Intergranular corrosion resistance: In order to evaluate the intergranular corrosion resistance, the sulfuric acid-ferric sulfate test (JIS
G'0572) was carried out. For TIG tanning, static gas flow rate is 10j2/min, current is 65 to 75A, and welding speed is 120.
It was carried out under the condition of mm/min. The results of these tests are summarized in Table 1.

As is clear from these results, the intergranular corrosion resistance of the weld heat-affected zone was improved by the addition of AI;
．． In the AI-free materials of No. 12, 13, 14, and 15, several dales of crystal grains were observed to have fallen from the surface. Symbol G in the table,
L indicates crystal grain dropout in the heat affected zone.

The present invention has been described in detail above, but the high-purity ferritic chromium stainless steel according to the present invention simultaneously reduces C+N+S+Si, Mn, and o as impurities in the steel to a level that is in line with conventional methods. , which significantly improves the toughness and intergranular corrosion resistance of chromium stainless steel including welded parts, as well as the corrosion resistance of the base metal, especially pitting corrosion resistance and acid resistance, and has extremely high industrial value.

@1 table continued)

[Brief explanation of the drawing]

Figure 1 is a graph showing the effect of S in steel on corrosion resistance; Figure 2 is a graph showing the effect of Mn in steel on toughness; and Figure 3 is the effect of adding 1 to steel on steel toughness. This is a graph showing. Patent Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Akira Hirose - 8 Content (ppm) Figure 2 Mn('/,,P

Claims (2)

[Claims] (1) Weight%: Cr: 15.00-50.00%, Mo: 5
．． 00% or less, -Ni: 0.01-6.00%,
Al: 1.00% or less. The remainder consists of Fe and impurities, and as a result of refining, impurities such as C, N, S,
Al-containing ferritic chromium stainless steel characterized by reducing Si+Mn+0 to the following ranges: C: o, ooeo% or less, N: 0.0
150% or less. S: 0.0020% or less, Si: 0.15
%below. Mn: 0.15% or less, O: 0.015% or less. (2) Weight%: Cr: 15.00-50.00%, Mo: 5
．． 00% or less. Ni: 0.01-6.00%, Al: 1.
00% or less. Furthermore, Ti: 1.00% or less [however, 5% on Ti
x (C1N)%] and Nb: 1.00% or less [
However, 1 or 2 types of 8x (C+N)% on Nb
Contains seeds, the balance consists of Fe and impurities, and by refining, C, N, s, Si as impurities
, Al-containing ferritic chromium stainless steel characterized by reducing Mn・0 to the following ranges: c: 0.0060% or less, N: 0.015
Less than 0%. S: 0.0020% or less, Si: 0.15
%below. Mn: 0.15% or less, 0:0.015
%below.