JP2009161836A - Ferritic stainless steel sheet excellent in corrosion resistance in welding crevice part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel having excellent corrosion-resistance in a warm water environment using city water under state as welding in even any crevice structure, when a warm water vessel having the crevice structure is constituted with a TIG (tungsten-inserting gas) welding without performing Ar back gas sealing. <P>SOLUTION: The ferritic stainless steel is composed, by mass, of ≤0.02% C, 0.01-0.3% Si, ≤1% Mn, ≤0.04% P, ≤0.03% S, 0.2-2% Ni, 22-26% Cr, ≤0.8% Mo, 0.05-0.6% Nb, 0.05-0.4% Ti, ≤0.025% N, 0.02-0.3% Al and further, limited to <0.2% Cu as impurity and the balance Fe with the other inevitable impurities. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a ferritic stainless steel sheet that is constructed by TIG welding and has a weld gap structure and excellent weld corrosion resistance.

Ferritic stainless steel SUS444 (low C, low N, 18-19Cr-2Mo-Nb, Ti steel) is widely used as a material for hot water containers such as electric water heaters and hot water storage tanks.
SUS444 is a steel type developed mainly for the purpose of improving the corrosion resistance in a hot water environment.

  The mainstream hot water containers have a “weld gap structure” in which constituent members (for example, a mirror and a barrel) are joined by TIG welding. When a hot water container having a weld gap structure is used in a warm water environment, corrosion tends to occur in the weld gap portion. In the case of SUS444, re-passivation tends to occur when the corrosion form is pitting corrosion, and pitting corrosion grows rarely. However, since crevice corrosion is difficult to repassivate, corrosion grows and may penetrate the plate thickness and lead to water leakage. For this reason, it is desirable to make it the structure which avoids formation of the crevice structure which is easy to corrode with a hot water container as much as possible. However, there are some parts where it is difficult to avoid the formation of a gap in construction, such as a welded joint between the mirror and the body.

  When manufacturing a hot water container by TIG welding, in order to reduce the corrosion-resistance fall of a welding part, generally the countermeasure which suppresses the oxidation by the back bead side by performing an Ar back gas seal is taken. However, with electric water heaters, the need for a reheating function has increased, and the number of cans with a structure in which a serpentine tube is inserted has increased. In this case, it becomes difficult to insert a nozzle for performing Ar back gas sealing during welding into the inside of the can body, increasing the number of cases in which TIG welding without back gas sealing has to be adopted, which is a cause of anxiety about a decrease in corrosion resistance. It has become.

In addition, due to global environmental problems, demand for a CO ₂ refrigerant heat pump water heater (EcoCute (registered trademark)) that consumes less power than an electric water heater has increased. Since the heater is not heated by this method, the flange for inserting the heater is originally unnecessary, but the flange cannot be omitted to insert the back gas sealing nozzle at the time of TIG welding. Arise.

  Patent Document 1 describes a stainless steel can for a water heater having a structure in which the depth of insertion of a barrel into a mirror is up to 20 mm and the occurrence of crevice corrosion is avoided. SUS444 equivalent steel is adopted as the steel type. However, according to the investigation by the inventors, the heat affected zone where the corrosion resistance is reduced by welding is in the range of about 10 mm from the weld bead, and the above structure may not provide a sufficient effect of improving the corrosion resistance. In addition, when this SUS444 equivalent steel is subjected to TIG welding without performing Ar back gas sealing, it is expected that a significant reduction in corrosion resistance will occur in the portion where the oxide scale is formed in the back bead portion.

  Patent Document 2 describes a ferritic stainless steel in which Cr and oxidization loss during welding are suppressed by adding Ti and Al in a composite manner, and deterioration in corrosion resistance at the welded portion is improved. By using this steel, the corrosion resistance level of the hot water container can be greatly improved. However, even in the case of this steel, the oxidation loss of Cr cannot be sufficiently suppressed by TIG welding without performing Ar back gas sealing, and a significant reduction in the corrosion resistance of the weld gap is inevitable.

JP 54-72711 A JP-A-5-70899

  In Patent Document 3, a Cr content exceeding 21% by mass is secured as an improvement in the corrosion resistance of the back bead side weld formed by TIG welding without back gas sealing, and the addition of Ni and Cu affects the thermal effect on the back surface of the TIG weld. Steels that greatly improve the corrosion resistance of the parts have been proposed. By using this steel, the corrosion resistance level of the hot water container can be greatly improved. However, depending on the gap structure and the amount of Cu, a sufficient effect of improving the corrosion resistance of the TIG weld gap may not be obtained.

Japanese Patent Application No. 2007-088124

As described above, in recent hot water containers, there are an increasing number of structures that are difficult to carry out Ar back gas sealing when manufactured by TIG welding. On the other hand, it is also difficult to design a hot water container having a structure that does not form a gap in the weld due to a demand for manufacturing cost reduction or the like. In view of such a current situation, the present invention, when constructing a hot water container having a gap structure by TIG welding without performing Ar back gas seal, does not cause any gap structure to be in the state of welding. The purpose is to develop and provide ferritic stainless steel exhibiting excellent corrosion resistance in a hot water environment.

As a result of detailed studies to achieve the above object, the inventors have found the following.
(I) The corrosion resistance of the weld gap depends on the gap structure such as the gap clearance and gap depth as well as the weld scale. In particular, the depth of the gap from the gap opening to the welded portion (weld bond) is important. Crevice corrosion grows in structures with a range of gap depths. That is, if the gap depth is shallow, corrosion does not grow, and the same is true if the gap depth is too deep.
(Ii) It is extremely effective to improve the corrosion resistance of the back bead side weld gap formed by TIG welding without back gas sealing to ensure the Cr content exceeding 22% by mass and improve the basic corrosion resistance level. is there.
(Iii) The effect of improving the corrosion resistance of the weld gaps of Ni and Cu is different. Ni has a great effect of suppressing the growth in the thickness direction of crevice corrosion occurring in the weld gap. On the other hand, Cu suppressed lateral growth of crevice corrosion, but the effect of suppressing growth in the plate thickness direction was small, and it was found that erosion deepened in some cases. Therefore, it is necessary to regulate the amount of Cu in applications that require corrosion resistance in the weld gap structure.
(Iv) Si, which has been said to be effective for improving the corrosion resistance of the welded portion, in TIG welding that does not perform back gas sealing when added over a certain amount, in the back bead side welded portion as-welded,
Rather, it reduces the corrosion resistance.
(V) Mo, which is known as an element for improving corrosion resistance, does not effectively act to suppress oxidation on the surface of stainless steel, that is, to improve the corrosion resistance of welds.
The present invention provides a new ferritic stainless steel whose components are designed based on such knowledge.

  That is, in the present invention, in mass%, C: 0.02% or less, Si: 0.01 to 0.3%, Mn: 1% or less, P: 0.04% or less, S: 0.03% or less, Ni: 0.3-2% or less, Cr: 22-26%, Mo: 1.0% or less, Nb: 0.05-0.6%, Ti: 0.05-0.4%, N: 0 0.025% or less, Al: 0.02 to 0.3%, Cu as an impurity is limited to less than 0.3%, and the corrosion resistance of the weld gap is excellent, with the balance being Fe and other inevitable impurities. A molten ferritic stainless steel sheet is provided.

  This steel sheet is cold-rolled annealed and pickled, and the steel sheet is within 1 mm from the bond end with respect to a test piece in which a TIG weld gap structure is formed with a gap depth of 7 mm and a maximum gap distance of 20 μm or less and without an argon back gas seal. The density of the oxide scale in the weld gap is 80% or more.

Here, “uncleaned” means that no means for removing the oxide scale generated in the welded part (mechanical removal means such as sharpening and chemical removal means such as pickling) has been applied. It means that it has been done.
The “welded part” is an area composed of a weld bead part and a heat affected part. In order to form a welding gap for use in the immersion test, two steel plates are stacked, one steel rice is opened from the horizontal by 10 °, and the back bead (arc is applied while moving the TIG welding arc at a constant speed. A technique is adopted in which a weld bead is formed under the condition that a weld metal portion appearing on the back surface of the surface is formed. At that time, no back gas sealing is performed on the part that becomes the welding gap and the back bead side. Also, no filler material is used. The test piece should include the weld gap and the base metal parts on both sides.

  For stainless steel, the gap structure and the presence of the oxide film in the weld heat affected zone are the main factors that cause deterioration in corrosion resistance. As a result of preliminary studies, the density in the oxide scale is 80% or more within the range of this composition, in other words, It has been found that by setting the ratio of voids present in the oxide scale to 20% or less, the corrosion resistance of the weld gap structure is effective.

When the ferritic stainless steel of the present invention is used, the corrosion resistance of the weld in a warm water environment is significantly improved. In particular, even when a weld gap formed by TIG welding without a back gas seal is exposed to high temperature water without maintenance, excellent corrosion resistance is maintained for a long time. That is, when manufacturing the hot water container by TIG welding, high reliability can be obtained even if the Ar back gas seal is omitted. Therefore, according to the present invention, it is possible to expand the degree of design freedom in a hot water container in a water supply environment where high corrosion resistance is required. Further, in the hot water can body of the CO ₂ refrigerant heat pump water heater that is expected to increase in demand in the future, the flange for the back gas seal becomes unnecessary, and the cost can be reduced.

The component elements constituting the ferritic stainless steel of the present invention will be described.
C and N are elements inevitably contained in the steel. When the content of C and N is reduced, the steel becomes soft and the workability is improved, and the formation of carbides and nitrides is reduced, and the weldability and the corrosion resistance of the welded portion are improved. Therefore, in the present invention, it is better that the contents of both C and N are small, and C is allowed to be contained up to 0.02% by mass and N is allowed to be contained up to 0.025% by mass.

  Si effectively acts to improve the corrosion resistance of the weld when performing Ar gas sealing and TIG welding. However, according to detailed examinations by the inventors, it has been found that when TIG welding is performed without a gas seal, Si becomes a factor that inhibits corrosion resistance of the welded portion. For this reason, in terms of corrosion resistance, the Si content is preferably low. In the present invention, the Si content is specified to be 0.3% by mass or less. However, since Si contributes to the hardening of ferritic steel, the addition of Si is advantageous in applications where the strength of joints is required, such as high-pressure type hot water containers directly connected to waterworks. Become. As a result of various studies, it is desired to secure a content of 0.01% by mass or more in order to fully enjoy the effect of improving the strength of Si. Therefore, in the present invention, the Si content is controlled in the range of 0.01 to 0.3% by mass.

  Mn is used as a deoxidizer for stainless steel. However, Mn lowers the Cr concentration in the passive film and causes a decrease in corrosion resistance. Therefore, in the present invention, the Mn content is preferably low, and is defined as a content of 1% by mass or less. Since some amount of Mn is unavoidable in the stainless steel made from scrap, it is necessary to manage it so that it is not excessively contained.

  P is desirable to be low because it impairs the toughness of the base metal and the weld. However, since dephosphorization by refining is difficult in the production of Cr-containing steel, excessively increasing the cost, such as careful selection of raw materials, is required to minimize the P content. Therefore, in the present invention, the P content up to 0.04% by mass is allowed as in the case of general ferritic stainless steel.

  It is known that S forms MnS that tends to be a starting point of pitting corrosion and inhibits corrosion resistance. However, since an appropriate amount of Ti is essentially added in the present invention, it is not necessary to regulate S particularly severely. That is, since Ti has a strong affinity for S and forms a chemically stable sulfide, the generation of MnS that causes a decrease in corrosion resistance is sufficiently suppressed. On the other hand, if too much S is contained, hot cracking of the welded portion is likely to occur, so the S content is specified to be 0.03% by mass or less.

Cr is a main constituent element of the passive film, and improves local corrosion properties such as pitting corrosion resistance and crevice corrosion resistance. Cr is a particularly important element in the present invention because the corrosion resistance of a welded portion TIG welded without a back gas seal depends greatly on the Cr content. As a result of investigations by the inventors, it has been found that a Cr content exceeding 21% by mass should be ensured in order to impart corrosion resistance required in a hot water environment to a welded portion welded without a back gas seal. The corrosion resistance improving effect is improved as the Cr content is increased. However, when the Cr content increases, it becomes difficult to reduce C and N, which causes a deterioration in mechanical properties and toughness and an increase in cost.
In the present invention, the steel having a Cr content of 22% by mass or more has a large effect of improving the corrosion resistance of the Ni weld gap, and Cu progresses in the plate thickness direction even if it is mixed with an impurity level. By restricting the upper limit, the above-mentioned problems can be minimized and sufficient corrosion resistance can be obtained without depending on further increase in the Cr content even in severe environment applications. Therefore, in this invention, Cr content shall be 22-26 mass%.

  Mo is an effective element for improving the corrosion resistance level together with Cr, and it is known that the effect of improving the corrosion resistance increases as the Cr content increases. However, according to detailed investigations by the inventors, it has been found that the corrosion resistance improving effect brought about by Mo is not so great for the weld gap portion and the back bead side weld portion which are TIG welded without a back gas seal. Although it is effective to contain 0.2% by mass or more of Mo for the warm water environment of clean water, which is the main use of the present invention, even if the amount exceeds 0.8% by mass, it is resistant to gaps. The effect of improving corrosivity is small. Therefore, the Mo content is 0.8% by mass or less.

  Nb has a strong affinity for C and N like Ti, and is an element effective in preventing intergranular corrosion, which is a problem in ferrite stainless steel. In order to sufficiently exhibit the effect, it is desirable to secure an Nb content of 0.05% by mass or more. However, if added in excess, weld hot cracking occurs and the weld zone toughness also decreases, so the upper limit of the Nb content is 0.6% by mass.

  Ti is an element that contributes to improving the corrosion resistance of welds in conventional TIG welding that performs Ar back gas sealing. However, even in TIG welding without back gas sealing, the corrosion resistance of the gap and its back bead side welds is remarkable. It was found to have an improving effect. Although the mechanism is not necessarily clear, in the case of TIG welding with Ar back gas sealing, an oxide film mainly composed of Al is preferentially formed on the steel surface during the welding due to the combined addition with Al. It is thought that oxidation loss is suppressed. On the other hand, in the case of TIG welding without a back gas seal, it is presumed that Ti exhibits an action of promoting repassivation after the occurrence of corrosion, thereby improving corrosion resistance. In order to fully enjoy such an action of Ti, it is desirable to secure a Ti content of 0.05% by mass or more. However, if the Ti content is increased, the surface quality of the material is deteriorated, or oxide is generated in the weld bead and the weldability is likely to be lowered. Therefore, the upper limit of the Ti content is 0.4% by mass.

  Al suppresses a decrease in corrosion resistance due to welding by the combined addition with Ti. In order to obtain the effect sufficiently, it is desirable to secure an Al content of 0.02% by mass or more. On the other hand, excessive Al content causes deterioration of the surface quality of the material and weldability, so the Al content is 0.3% by mass or less.

Ni increases the Cr concentration in the welding scale in TIG welding without Ar back gas sealing, it increased to improve the corrosion resistance of scale production of chemically stable Cr ₂ 0 _3. Furthermore, it has been found that Ni is effective in increasing the density of both Cr ₂ 0 ₃ and Fe ₂ 0 ₃ scales. In order to obtain the effect, Ni needs to be 0.2% or more. However, if a large amount of Ni is contained, the steel is hardened and the workability is hindered.

  Cu suppressed the occurrence of pitting corrosion at the heat-affected zone on the back side of the weld in the corrosion resistance of the TIG butt weld without an Ar back gas seal, but reduces the crevice corrosion area in the TIG weld gap, but the erosion depth is Depending on the gap condition, erosion may be deepened. Therefore, Cu may interfere with corrosion resistance in applications where gaps are formed by TIG welding without a back gas seal. For this reason, Cu is not added in the present invention. Furthermore, since the effect of inhibiting crevice corrosion resistance of Cu appears even at the impurity level, the upper limit of Cu is restricted to less than 0.2%.

In addition, when a TIG welded gap structure is formed under the following welding conditions without using an argon back gas seal, a cold rolled annealed pickled steel sheet according to the present invention has a gap depth of 7 mm and a maximum gap distance of 20 μm or less. The density density of the oxide scale in the weld gap within 1 mm is 80% or more.
(Welding conditions)
Welding method: Butt welding without welding core wire Welding current: 60A Welding speed: 300mm / min
Ar gas flow rate on the torch seal side: 12 L / min
Electrode diameter: φ1.6mm

The oxide scale generated at this time is corundum type Cr ₂ O ₃ or Fe ₂ O ₃ .
During the oxide scale generation process, peeling may occur at the oxide scale / base metal interface or within the oxide scale due to the thermal history during welding. This peeling lowers the density density of the oxide scale, that is, increases the porosity. It will be. As a result, penetration of the corrosive liquid through the oxide scale is promoted and corrosion is accelerated.
Since Ni has an effect of suppressing this peeling, it leads to prevention of corrosion.

  Stainless steel having the chemical composition shown in Table 1 was melted, and a hot-rolled sheet having a thickness of 3 mm was produced by hot rolling. Thereafter, the plate thickness was 1.0 mm by cold rolling, finish annealing was performed at 1000 to 1070 ° C., and pickling was performed to obtain a test material.

  For each specimen, a TIG welding gap was formed by the method shown in FIG. Welding was performed without an Ar back gas seal. That is, when two steel plates are stacked and TIG welded, a gap opening is made. After bending one steel plate at an angle of 10 °, welding is performed with the surface that becomes the gap exposed to the atmosphere. The 0 welding conditions were such that the penetration (welded metal part) reached the back surface and a “back bead” having a width of about 4 mm was formed on the back surface. In the case of this condition, the welding heat affected zone (HAZ) is in the center of the plate thickness and the distance from the bead center is in the range of about 10 mm.

Prior to the evaluation of the test steel, the comparative steel No. 1 shown in Table 1 was used. 7 was used to investigate the gap structure, particularly the relationship between the gap depth and the erosion depth due to crevice corrosion. As shown in FIG. 1, the “gap depth” was defined as the distance (mm) from the center of the weld bead to the bending position, and the gap depth was fixed to 7 mm.
A 15 × 40 mm test piece was cut out from a sample from which the oxide scale generated by welding had not been removed (an uncleaned sample), and was subjected to an immersion test in warm water.
FIG. 2 schematically shows the appearance of the weld gap test piece. The specimen was collected so that the weld bead crossed the center position in the longitudinal direction of the specimen. This immersion test piece includes a weld bead portion, a maturation-affected portion, and a base material portion. A lead wire was connected to the end of the base material portion by spot welding, and only the lead wire and its connecting portion were coated with resin.

The immersion test was immersed in 0.5 mol / L H ₂ SO ₄ at 80 ° C. for 48 hours, and Table 2 shows the results of the presence or absence of erosion of the base material by observing 10 corrosion cross-sectional structures.

Also, high resolution RBS (High Resolution Rutherford Backscattering Spectrometry) is used to analyze the density of the oxide film at a position of 1 mm from the bond edge, which was the maximum erosion position when immersed in 0.5 mol / L H ₂ SO ₄ for 48 hours. went. This is a method of measuring the density with the intensity of scattered He ⁺ ions, measuring the film thickness with TEM at a sample current of 25 nA / irradiation amount of 20 μC, and measuring the density by dividing the He ⁺ ion intensity by the scale thickness. did.
The measurement of the density is according to the following procedure.
The composition of the oxide scale is _Cr 2 _{O 3} or _Fe 2 _{O 3} of corundum, each density of 5.21 g ^/ cm 3, approximately equal to 5.24 g / cm ^3. Therefore, the density of both compositions was regarded as the same, and Fe ₂ O ₃ having a thickness of 100 nm and having no voids was used as a standard test piece. Density measurement was performed on the standard test piece with high resolution RBS, and the He ⁺ ion intensity per unit thickness was set to a value of 100% dense density. The He ⁺ strength by RBS of the oxide of each experimental material was determined, and the dense density was calculated from the following equation.

Density (%) = (He ⁺ ionic strength by RBS of each test material) / (He ⁺ ionic strength by RBS of standard test piece) × 100

  It is considered that none of the present examples had erosion of the base material after 48 hours in the above-mentioned pickled test, and there was no opening through which the corrosive solution penetrated due to the high density of the oxide film. In Comparative Example 7, there is no erosion of the base material, but the maximum erosion depth is much larger due to the addition of Cu on the 30th day of the Pt auxiliary immersed cathode. As described above, the steel according to the present invention can achieve improved corrosion resistance of the welded portion of the gap.

  This material is not only used for water heater cans used in Eco Cute, electric water heaters, stationary fuel cells, Eco Will, etc., but also in oil pipes and fuel tanks with welding gap structures, fuel injection rails and fuel exchange rails It can also be applied to.

It is a schematic diagram of the section of a welding sample. It is an overview of the weld sample used in the corrosion test

Claims (1)

% By mass
C: 0.02% or less,
Si: 0.01 to 0.3%
Mn: 1% or less,
P: 0.04% or less,
S: 0.03% or less,
Ni: 0.2-2%,
Cr: 22-26%
Mo: 0.8% or less,
Nb: 0.05 to 0.6%,
Ti: 0.05 to 0.4%,
N: 0.025% or less,
Al: 0.02 to 0.3%,
Further, Cu as an impurity is limited to less than 0.2%, and is a cold-rolled annealed pickled steel plate made of ferritic stainless steel made of the remaining Fe and other inevitable impurities, and has a gap depth of 7 mm and a maximum gap distance of 20 μm. In the following, when a TIG weld gap structure is formed without an argon back gas seal, the density of the oxide scale in the weld gap within 1 mm from the bond end is 80% or more. Ferritic stainless steel sheet with excellent corrosion resistance.

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