JPH0635615B2 - Manufacturing method of ferritic stainless steel with excellent corrosion resistance of welds - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a ferritic stainless steel having excellent corrosion resistance of a welded portion in a material of a container used in a hot water environment containing a small amount of neutral salt.

[Conventional technology and its problems]

Stainless steel is used as a material for various hot water appliances for water containing a small amount of neutral salt such as tap water.
Austenitic stainless steels such as SUS 304 and SUS 316 are generally used because of their good workability and weldability.However, in view of corrosion resistance, damage due to stress corrosion cracking originating from pitting and crevice corrosion is a concern. It On the other hand, ferritic stainless steels such as SUS 444, in which C and N in the steel are lowered, Ti and Nb are added to suppress intergranular corrosion, and Mo is added, do not cause stress corrosion cracking. Due to its excellent local corrosiveness, it has been widely used as a material for water heaters in recent years. In general, most of the water heater cans are manufactured by welding, and weld oxide scale is inevitably produced and this scale is used without being removed. Corrosion resistance of the substrate directly under the welding scale is unstable, and deterioration of corrosion resistance is observed depending on the scale formation conditions.Especially when this scale occurs in the weld gap, crevice corrosion will occur depending on the quality of water used, but water leakage will occur. May be reached.

As a means to prevent this, 1) corrosion prevention by external power supply method or sacrificial anodic method such as Al alloy or Mg alloy, 2) avoid welding gap structure by devising welding construction method and can structure, 3) Cr
The use of highly corrosion-resistant stainless steel with a high content of Mo and Mo is adopted. In the case of 1) corrosion prevention, there is concern about hydrogen embrittlement due to over-corrosion, and there are problems such as clouding of water due to elution of the sacrificial anode and clogging in the overflow section of equipment, water reactor, etc. . Next, the method of improving the welding method of 2) was also disclosed in Japanese Patent Laid-Open No. 59-61579, but it is difficult to completely control welding conditions and the like in practice, and moreover, productivity is inevitable. There is a problem of lowering. The use of high Cr, high Mo steel of 3) causes an increase in cost and causes problems such as deterioration of workability.

[Purpose of the present invention]

The present invention solves such a problem, and provides a ferritic stainless steel excellent in corrosion resistance of a welded portion and a method for producing the same, in which deterioration of corrosion resistance is extremely small even in a state where oxide scale is attached by welding. Is.

[Technical means for solving problems]

The present inventors believe that the cause of deterioration of corrosion resistance in the welded oxide scale part is that Cr in the stainless steel is oxidized during welding, and the Cr concentration in the material surface layer part immediately below the oxide scale is reduced,
As a result of detailed examination of the method for preventing the oxidation loss of Cr, it was noted that if the surface of stainless steel was previously covered with an oxide film other than Cr, the generation of oxide scale during welding was suppressed. This means that the loss due to the oxidation of Cr in steel is reduced. Therefore, the present inventors have found that ferritic stainless steel has a strong oxide-forming ability of Al.
As a result of repeated studies to develop a ferritic stainless steel that has excellent corrosion resistance in hot water and has little deterioration in corrosion resistance even with oxide scale adhered during welding, a small amount of REM is included with Al. By performing high temperature annealing in an atmosphere of H ₂ or a mixed gas of H ₂ and a non-oxidizing gas to form an Al oxide on the steel surface,
The inventors of the present invention have found that excellent corrosion resistance of welded portions can be obtained in a low-concentration chloride environment, and have reached the present invention.

[Structure of Invention]

The present invention is C: ≤ 0.03%, Si: ≤ 1.0%, Mn: ≤ 0.4%,
P: ≤ 0.04%, S: ≤ 0.005%, 16% ≤ Cr ≤ 30%, N ≤ 0.03
%, 0.15 + 7 (C + N) ≦ Nb ≦ 0.6%, 0.05% ≦ Al ≦ 5.0%, 0.
005% ≤ REM ≤ 0.1% as the basic composition, 0.001% ≤ B ≤ 0.02%, 0.05% depending on the required corrosion resistance level etc.
≦ Ti ≦ 0.5%, 0.4 ≦ Mo ≦ 3.0% and 0.3% ≦ Cu ≦ 0.8%
The steel material of ferritic stainless steel with the balance Fe and unavoidable impurities added with a component selected from the above, in a mixed gas atmosphere of H ₂ or H ₂ and a non-oxidizing gas with a dew point of −35 ° C. or less, Provided is a method for producing a ferritic stainless steel material excellent in corrosion resistance of a welded part, which is formed by annealing at 1100 ° C. to form an oxide film mainly containing Al oxide on the steel surface.

The ferritic stainless steel according to the present invention is subjected to high temperature annealing in a reducing atmosphere together with the addition of Al and a small amount of REM for improving the corrosion resistance of the welded portion, and a film mainly containing Al oxide is formed on the steel surface. By forming it, the oxidation of Cr during welding is suppressed and deterioration of corrosion resistance is minimized. These effects were discovered as a result of detailed examination of the effects of various alloying elements and atmospheric annealing conditions on pitting corrosion resistance after making TIG welding specimens and dipping them in a saline solution.

The reason why the constituent components of the stainless steel according to the present invention are limited as described above will be described.

C, N: These elements combine with Cr and impair the intergranular corrosion resistance, so it is desirable to reduce them as much as possible. Where C
Since Nb, which has a large bonding force with N and N, is added, a certain amount is acceptable, but if it increases, the amount of Nb added will increase and the characteristics of the weld will deteriorate, so the upper limits of C and N should be 0.03% respectively. did.

Si: Si is not particularly related to the improvement of the corrosion resistance of welded parts, but the upper limit is 1.0 because it is necessary to some extent for deoxidation in the steelmaking process.
%.

Mn: Mn forms sulfide in addition to deoxidizing action. The MnS remaining in the steel is soluble and acts as a starting point for pitting corrosion, etc., impairing the corrosion resistance. Therefore, it is desirable that the amount be as small as possible, but the manufacturing cost increases, so 0.4% was made the upper limit.

P: P is an unavoidable impurity, but it is not necessary to limit it in ferritic stainless steel.
The upper limit was 0.04%, which is about the level of ordinary JIS standard steel.

S: S is also an unavoidable impurity, but M in steel
It remains as nS and impairs corrosion resistance as described above. Further, in the steel of the present invention, REM is added to improve the properties by forming Al oxide, but REM is also likely to combine with S in steel to form a sulfide. Therefore, the effective amount of REM decreases with a large amount of S, so the upper limit was made 0.005%.

Cr: Cr is a basic element of stainless steel. Generally, 11% or more may be added to obtain effective corrosion resistance, but the lower limit is set to 16% in consideration of use in hot water environment. On the other hand, the more Cr is, the better the corrosion resistance is, but if it is too much, the manufacturing becomes difficult and the cost rises, and the workability of the product deteriorates, so the upper limit was made 30%.

Nb: Nb is an important element for fixing C and N and preventing intergranular corrosion. The lower limit of the Nb amount was 0.15 + 7 (C% + N%), which was necessary to prevent sensitization due to TIG welding, and the upper limit was set to 0.6% to avoid hot cracking of the weld.

Al: In the presence of REM, in the steel of the present invention, Al is an element essential for ensuring the corrosion resistance of welds with oxide scale, and an oxide film of Al is formed on the steel surface by reducing atmosphere annealing of cold rolling. Formed to suppress oxidation of Cr during welding and prevent deterioration of corrosion resistance. To obtain this effect, 0.05% or more is required. The higher the Al content, the wider the effective annealing temperature range, but the higher the Al content, the more difficult the production becomes. In addition, the workability and the shape of the weld bead deteriorate, so the upper limit is 5.0%.

REM: REM suppresses the oxidation of Cr and Fe and promotes the oxidation of Al during annealing in a reducing gas atmosphere. Therefore, it is possible to prevent deterioration of the corrosion resistance of the welded scale portion with a relatively small amount of Al. If less than 0.005%, this effect is not fully exerted, while
Addition of more than 0.1% is not preferable from the viewpoints of impairing hot workability and cost and resource saving.

B: B has a function of promoting the formation of an Al film similarly to REM, so it is added for the purpose of obtaining corrosion resistance with a particularly small amount of Al. If it is less than 0.001%, the above effect is not significantly obtained. On the other hand, if it exceeds 0.02%, the amount of compounds such as nitrides increases and steel ingot defects occur. Was set to 0.001 to 0.02%.

Ti: Ti has the ability to form oxides next to Al, and has the effect of forming oxides and making the Al film more stable in the region with a relatively high oxidation potential corresponding to the dew point of the atmosphere and the annealing temperature. . Further, Ti combines with S in steel to form a chemically stable sulfide, which easily becomes the starting point of corrosion.
By preventing the formation of MnS, it also has the effect of improving the corrosion resistance of steel, especially pitting corrosion resistance and crevice corrosion resistance. In order to exert these effects, 0.05% or more of Ti is required. On the other hand, if the amount of Ti increases, the problem of surface flaws of the product becomes apparent, so the upper limit was made 0.5%.

Both Mo and Cu are effective components for improving the pitting corrosion resistance and crevice corrosion resistance of steel in chloride solution environment, but they may be used individually or in combination depending on the environment used and the crevice structure of the can body. Add in. If the Mo content is less than 0.4%, the effect of improving the corrosion resistance is insufficient, and the upper limit was set to 3.0% in consideration of cost and corrosion resistance because it is intended for hot water environments. Cu
For less than 0.3%, the effect of improving corrosion resistance cannot be obtained,
On the other hand, even if added over 0.8%, the effect is saturated, so the content was limited to 0.3-0.8%.

Next, the reasons for limiting the heat treatment conditions will be described.

When annealing is performed in H ₂ or a mixed gas atmosphere of non-oxidizing gas such as H ₂ and N ₂ , oxides with different composition are generated on the surface of stainless steel sheet depending on the oxidation potential of the atmosphere, dew point and temperature. . If the oxidation potential and the dew point of the atmosphere are high, the steel sheet surface after annealing is strongly colored. In particular, the steel of the present invention contains Al, which has a strong affinity for oxygen, so the dew point of the atmosphere must be controlled at -35 ° C or lower. The lower limit of the annealing temperature is set to 900 ° C, but a sufficient recrystallized structure cannot be obtained at 900 ° C or lower, and mechanical properties and formability are deteriorated. Moreover, the Al oxide in the film on the surface of the steel sheet is not sufficiently formed, and the initial corrosion resistance is not exhibited. On the other hand, 1100
If the temperature exceeds ℃, the coarsening of crystal grains becomes remarkable and the skin becomes rough after processing the product, which is not preferable.

It should be noted that the heat treatment time is not preferable because a long heat treatment causes the oxide film to become thicker and the degree of coloring to be improved, and the cost to be increased. BA with bright annealing finish
A line is used, but if the plate thickness is about 1 mm, a soaking time of about 5 to 60 seconds is appropriate.

[Example] Steels having the components shown in Table 1 were melted by a vacuum melting method, forged and hot rolled, and then cold-rolled steel sheets having a thickness of 1 mm were manufactured.

Steels A and E to I are 19Cr series steels, and steels B to D and J to
O steel is 18Cr-2Mo steel.

Atmosphere annealing is performed in H ₂ alone and in a mixed gas of H ₂ / N ₂ = 3/1,
The temperature was changed in the range of 800 to 1100 ° C. Retention time
30 seconds.

Figure 1 shows the test steel I under the above conditions at 850 ℃, 900 ℃ and 10 ℃.
The chemical composition of the surface layer when annealed at 00 ° C is ESCA (Elec
It is a figure which shows the result analyzed by the tron spectroscopy of chemical analysis. The horizontal axis represents the etching time, that is, the depth from the surface. It can be seen that a large amount of Al ₂ O ₃ is generated in the outermost layer when heated to 900 ° C or higher. at the same time,
The metal components of Fe and Cr in the surface layer are significantly reduced, and they are not observed in the surface layer when heated at 1000 ° C. That is, the method of the present invention is intended to form an Al oxide on the surface of a steel material having the above composition by annealing it under the above conditions.

Table 2 shows the results of the pitting corrosion test of TIG-welded specimens.
In TIG welding, the Ar gas shield was not applied to the backside of the weld of the specimen, which promoted the formation of oxide scale. The pitting test is 50 mm x 25 mm so that the TIG weld bead is centered from the specimen after welding.
The sample was cut into a shape of No. 1 and was immersed in a 5% NaCl solution containing 2% H ₂ O ₂ at 50 ° C. without removing the scale. Immersion is 19
The Cr-based steel was used for 4 hours, and the 18Cr-2Mo-based steel was used for 24 hours. The corrosion resistance was evaluated by calculating the degree of corrosion from the weight loss before and after the test, and by checking the presence or absence of pits in the heat affected zone (HAZ) with oxide scale.

Steel containing 0.05% or more of Al and REM has a dew point of -35 ° C or less.
When annealed at 900 ℃ ~ 1100 ℃ in H ₂ or H ₂ / N ₂ mixed gas atmosphere, the corrosion degree due to pitting corrosion is low in both 19Cr and 18Cr-2Mo steels. Moreover, pitting corrosion does not occur even in the heat-affected zone of the weld backside where the weld oxide scale is thick,
No deterioration of corrosion resistance is observed in the oxide scale. In 19Cr steel, F steel is 0.08% Al, but by adding B 1.
It shows the same corrosion resistance as E steel containing 27% Al. Further, the effect of adding Ti is exhibited in the G steel, the corrosion degree is smaller than the E and F steels, and the corrosion rates of the H and I steels to which Cu is added are low. Next, regarding 18Cr-2Mo system steel, even if the C steel is annealed under the conditions shown in the present invention, the Al content is 0.019%.
Since it is low, pitting corrosion often occurs at the weld. Also, D steel is 1.
Pitting corrosion occurs in the scale part because it contains 45% Al but does not contain REM. Steel J is the steel specified in the present invention, but when the annealing temperature was 800 ° C., the degree of corrosion tended to increase and pitting corrosion was observed in the scale. The K steel contained B and Ti and showed excellent corrosion resistance. L, M steel is 0.
Although it contained B and B and Cu, although it contained 38% and 0.33% Al, it showed corrosion resistance equal to or higher than that of N steel containing 1.21% Al. O steel had a high Al content of 4.19% and, in addition to containing Cu, exhibited excellent corrosion resistance.

[Effects of the Invention] As described above, the steel material produced by the method of the present invention does not impair the corrosion resistance due to the oxide scale generated during welding, so that it is possible to obtain excellent corrosion resistance in a low-concentration chloride environment, and tap water or medium water. It has extremely high industrial value because there are no restrictions on corrosion protection and welding work for hot water equipment for tap water, and the durability of these equipment can be improved.

[Brief description of drawings]

Figure 1 shows the dew point of -38 for steel containing 0.20% Al and REM.
850 ℃ for 30 seconds in a mixed gas atmosphere of ℃, H ₂ / N ₂ = 3/1, 90
It is the figure which analyzed the composition of the steel plate surface produced by annealing at the temperature of 0 degreeC x 30 second and 1000 degreeC x 30 second by ESCA.

Claims (7)

[Claims] 1. C: <0.03% Si: ≤1.0% Mn: ≤0.4% P: ≤0.04% S: ≤0.005% Cr: 16-30% N: ≤0.03% Nb: 0.15 + 7 (C% + N% ) To 0.6% Al: 0.05 to 5.0% REM: 0.005 to 0.1%, with a balance of Fe and unavoidable impurities and a ferritic stainless steel material in the furnace Atmosphere: H ₂ or H ₂ mixed with non-oxidizing gas Gas dew point: -35 ° C or less Temperature: 900-1100 ° C Annealing is carried out in the atmosphere to form an oxide film mainly of Al compound on the steel surface. Ferrite with excellent corrosion resistance of welded parts. Of producing stainless steel. 2. C: ≤0.03% Si: ≤1.0% Mn: ≤0.4% P: ≤0.04% S: ≤0.005% Cr: 16-30% N: ≤0.03% Nb: 0.15 + 7 (C% + N% ) To 0.6% Al: 0.05 to 5.0% REM: 0.005 to 0.1% B: 0.001 to 0.02%, with a balance of Fe and unavoidable impurities, a ferritic stainless steel material with a furnace atmosphere of H ₂ or H ₂ . Mixed gas of non-oxidizing gas Dew point: -35 ° C or lower Temperature: Welding characterized by forming an oxide film mainly of Al oxides on the surface of steel by annealing in the atmosphere at 900 to 1100 ° C Method for producing ferritic stainless steel material with excellent corrosion resistance in parts. 3. C: ≤0.03% Si: ≤1.0% Mn: ≤0.4% P: ≤0.04% S: ≤0.005% Cr: 16-30% N: ≤0.03% Nb: 0.15 + 7 (C% + N% ) To 0.6% Al: 0.05 to 5.0% REM: 0.005 to 0.1% Ti: 0.05 to 0.5%, with a balance of Fe and unavoidable impurities, a ferritic stainless steel material in the furnace atmosphere: H ₂ or H ₂ Mixed gas of non-oxidizing gas Dew point: -35 ° C or lower Temperature: Welding characterized by forming an oxide film mainly of Al oxides on the surface of steel by annealing in the atmosphere at 900 to 1100 ° C Method for producing ferritic stainless steel material with excellent corrosion resistance in parts. 4. C: ≤0.03% Si: ≤1.0% Mn: ≤0.4% P: ≤0.04% S: ≤0.005% Cr: 16-30% N: ≤0.03% Nb: 0.15 + 7 (C% + N% ) To 0.6% Al: 0.05 to 5.0% REM: 0.005 to 0.1%, Mo: 0.4 to 3.0%, Cu: 0.3 to 0.8%, one or two, and the balance Fe and unavoidable impurities. Steel atmosphere in the furnace: H ₂ or a mixed gas of H ₂ and non-oxidizing gas Dew point: −35 ° C or less Temperature: 900 to 1100 ° C Annealing is performed in the atmosphere, and Al oxide is mainly applied to the steel surface. A method for producing a ferritic stainless steel material having excellent corrosion resistance of a welded portion, which comprises forming an oxide film. 5. C: ≤0.03% Si: ≤1.0% Mn: ≤0.4% P: ≤0.04% S: ≤0.005% Cr: 16-30% N: ≤0.03% Nb: 0.15 + 7 (C% + N% ) To 0.6% Al: 0.05 to 5.0% REM: 0.005 to 0.1% B: 0.001 to 0.02%, Mo: 0.4 to 3.0%, Cu: 0.3 to 0.8%, 1 or 2 types, balance Fe and unavoidable A ferritic stainless steel material consisting of impurities in a furnace atmosphere: H ₂ or a mixed gas of H ₂ and a non-oxidizing gas Dew point: −35 ° C. or less Temperature: 900 to 1100 ° C. Atmosphere annealing is performed and the steel surface A method for producing a ferritic stainless steel material having excellent corrosion resistance of a weld, which comprises forming an oxide film mainly of Al oxide on. 6. C: ≤0.03% Si: ≤1.0% Mn: ≤0.4% P: ≤0.04% S: ≤0.005% Cr: 16-30% N: ≤0.03% Nb: 0.15 + 7 (C% + N% ) ~ 0.6% Al: 0.05 ~ 5.0% REM: 0.005 ~ 0.1% Ti: 0.05 ~ 0.5% and Mo: 0.4 ~ 3.0%, Cu: 0.3 ~ 0.8% One or two kinds, balance Fe and unavoidable A ferritic stainless steel material consisting of impurities in a furnace atmosphere: H ₂ or a mixed gas of H ₂ and a non-oxidizing gas Dew point: −35 ° C. or less Temperature: 900 to 1100 ° C. Atmosphere annealing is performed and the steel surface A method for producing a ferritic stainless steel material having excellent corrosion resistance of a weld, which comprises forming an oxide film mainly of Al oxide on. 7. C: ≤0.03% Si: ≤1.0% Mn: ≤0.4% P: ≤0.04% S: ≤0.005% Cr: 16-30% N: ≤0.03% Nb: 0.15 + 7 (C% + N% ) To 0.6% Al: 0.05 to 5.0% REM: 0.005 to 0.1% B: 0.001 to 0.02% Ti: 0.05 to 0.5%, Mo: 0.4 to 3.0%, Cu: 0.3 to 0.8%, including one or two , A ferritic stainless steel consisting of balance Fe and unavoidable impurities Furnace atmosphere: H ₂ or mixed gas of H ₂ and non-oxidizing gas Dew point: −35 ° C or less Temperature: 900 to 1100 ° C atmosphere A method for producing a ferritic stainless steel material having excellent corrosion resistance of a weld, which is characterized by forming an oxide film mainly of Al oxide on the surface of the steel material by annealing.