JP2009185382A - Ferritic stainless steel sheet having excellent corrosion resistance in welding gap oxide film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel showing excellent corrosion resistance as-welded in warm water environment using clean water. <P>SOLUTION: The ferritic stainless steel having excellent corrosion resistance in a welding gap oxide film is provided which includes a composition comprising, by mass, ≤0.02% C, 0.01 to 0.3% Si, ≤1% Mn, ≤0.04% P, ≤0.03% S, 0.3 to 2% Ni, 22 to 26% Cr, ≤0.8% Mo, 0.10 to 0.6% Nb, 0.15 to 0.4% Ti, ≤0.025% N and 0.04 to 0.3% Al, wherein the content of Cu as impurities is limited to <0.2%, and the balance Fe with the other inevitable impurities, and the ferritic stainless steel is characterized in that a test piece is obtained by forming a TIG welding gap structure in the cold rolled-annealed-pickled sheet of the above ferritic stainless steel at a gap depth of ≥4 mm and a maximum gap spacing of ≤30 μm without using argon back gas seal and in the test piece, an average Cr ratio of oxide scale at the welding gap part within 2 mm from the bond edge part reaches ≥20 mass% at the ratio of all the metallic elements. <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. In this method, since no heating is performed, a flange for inserting a heater is not necessary. However, in order to insert a back gas seal nozzle at the time of TIG welding, the flange cannot be omitted, leading to an increase in cost. Problems 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 part, lowers the corrosion resistance rather in the welded back bead side welded part in TIG welding that does not perform back gas sealing when added over a certain amount. .
(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.
(Vi) The corrosion resistance of the TIG weld gap depends on the corrosion resistance of the scale itself. The corrosion resistance of the scale is strongly related to the scale composition, and the scale containing a certain amount of Cr ₂ O ₃ or more has excellent corrosion resistance.
(Vii) As a problem when the Ar back gas seal is omitted in the TIG welding gap structure, it has been newly found that a scale mainly composed of Fe ₂ O ₃ is generated to promote crevice corrosion. In the gap structure, crevice corrosion occurs and proceeds at a position of 2 mm from the end of the weld bond. The scale mainly composed of Fe ₂ O ₃ is formed at a position of 2 to 4 mm from the bond end. When crevice corrosion occurs, the liquidity in the gap changes to acidic, so Fe ₂ O ₃ is cathodically reduced and promotes the progress of crevice corrosion occurring at a position of 2 mm from the weld bond end.
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%, Cr: 22-26%, Mo: 0.8% or less, Nb: 0.10-0.6%, Ti: 0.15-0.4%, N: 0.00. 025% or less, Al: 0.04 to 0.3%, further limiting the Cu as an impurity to less than 0.2%, a ferritic stainless steel consisting of the remainder Fe and other inevitable impurities, Oxidation of the weld gap within 2 mm from the bond end of the cold-rolled annealed pickled plate with a gap depth of 4 mm or more and a maximum gap distance of 30 μm or less and a TIG weld gap structure formed without an argon back gas seal The average Cr ratio in the region from the outermost layer of the scale to 20 nm is all gold The ratio of the elements is 20% by mass or more, and the average Fe ratio in the region from the outermost layer of the oxide scale at a position of 2 to 4 mm from the bond end to 20 nm is 70% by mass or less in terms of the total metal elements. This ensures the corrosion resistance of the weld gap oxide film.

This steel is a cold-rolled annealed pickled steel sheet, and from a bond part to a test piece in which the steel sheet was formed with a TIG welded gap structure without a back gap seal with a gap depth of 4 mm or more and a maximum gap distance of 30 μm or less. The average Cr ratio of the oxide scale in the weld gap within 2 mm has a ratio of all metal elements to 20% by mass or more. Moreover, the average Fe ratio in the area | region from the outermost layer of the oxide scale of the position of 2-4 mm from a bond edge part to 20 nm is 70 mass% or less in the ratio of all the metal elements.
For the average Cr ratio, for example, arbitrary 10 points in a region within 2 mm from the bond end are selected, and the amount of each metal element at a depth of 5, 10, 15, 20 nm from the outermost layer of the oxide scale at each point is obtained. The Cr ratio is calculated as Cr amount / Σ (total metal element amount), and the average value of the Cr ratio at each measurement point can be calculated.

Here, “as-maintained” means that no means (removal such as polishing and chemical removal such as pickling) that removes oxide scale from the welded part has been applied, and welding is performed. It means that it is in an untouched state. 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 plate is opened 10 ° from the horizontal, and the TIG welding arc is moved at a constant speed while the back bead (the surface to which the arc is applied). A method of forming a weld bead under the condition that a weld metal part appearing on the back surface of the weld bead is formed is employed. 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 existence of the gap structure and the presence of the oxide film at the weld-affected zone are the main factors that cause deterioration in corrosion resistance. As a result of extensive preliminary studies, the ratio of Cr in the oxide scale is the ratio of all metal elements within this composition. It has been found that having 20 mass% or more is effective in improving the corrosion resistance of the weld gap structure. Further, the oxide scale at the gap portion 2 to 4 mm away from the bond end portion generates a large amount of Fe2O3 in the surface layer, and this undergoes cathodic reduction in the gap, thereby promoting crevice corrosion of the weld gap structure portion of 2 mm from the bond. . It is newly effective that the average Fe concentration in the oxide scale at the same position is 70% by mass or less in terms of the ratio of all metal elements, and is effective in suppressing the progress of crevice corrosion of the weld gap structure 2 mm from the bond. I found out.

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 the joint is required, including high-pressure type hot water containers that are directly connected to the water supply. Become. As a result of various studies, it is desirable to secure a Si content of 0.01% by mass or more in order to fully enjoy the strength improvement effect 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 with S and forms a chemically stable hanger, the production 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 is a factor that impairs mechanical properties and toughness and increases the cost. In the present invention, in the steel having a Cr content of 22% by mass or more, Ni is used. The effect of improving the corrosion resistance of the weld gap is increased, and even if Cu is mixed at an impurity level, corrosion progresses in the plate thickness direction. Therefore, by limiting the upper limit of Cu, the Cr content can be applied even in severe environments. Without relying on a further increase in the above, the above-mentioned problems can be minimized and sufficient corrosion resistance can be obtained. 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 ferritic 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 TlG welding without back gas sealing, Ti significantly improves the corrosion resistance of the gap and its back bead side weld. 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 the oxidation loss of Cr is suppressed. On the other hand, in the case of TIG welding without a back gas seal, in addition to exerting the effect of promoting repassivation after the occurrence of corrosion in the welded portion, Ti in the gap surface of the TIG welding gap structure together with Al described later It is presumed that the corrosion resistance of the oxide scale is improved by suppressing the formation of Fe ₂ O ₃ at a position 2 to 4 mm away from the bond end and suppressing the promotion of crevice corrosion due to the cathode dissolution of Fe ₂ O ₃ .
In order to fully enjoy such an effect of Ti, it is desirable to secure a Ti content of 0.15% 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 is preferentially oxidized with Ti at the TIG welding heat-affected zone and the inner surface of the weld gap structure by composite addition with Ti, thereby suppressing the formation of Fe ₂ O ₃ and promoting the concentration of Cr ₂ O ₃ , Increase corrosion resistance. In order to obtain the effect sufficiently, it is desirable to secure an Al content of 0.04% 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 weld scale in TIG welding without an Ar back gas seal, increases the amount of chemically stable Cr ₂ O ₃ , and improves the corrosion resistance of the scale. Furthermore, the corrosion resistance of the TIG welded part without the back gas seal is improved by suppressing the progress of corrosion in both the weld metal part (bead part) and the heat-affected part. This effect is greater as the Cr content is higher.
Further, as a means for improving the Cr ratio by the metal element ratio in the oxide film, it is effective not to emit an Fe-based oxide. The effect of Ni is different from that of Ti and Al, and suppresses the oxidation of Fe in the matrix and consequently increases the Cr ratio in the oxide scale. As a result of preliminary studies, Ni is required to be 0.3% by mass or more in order to obtain the effect. However, if a large amount of Ni is contained, the steel is hardened and the workability is hindered.
When two cold-rolled steel sheets with a thickness of 1 mm are stacked to form a TIG welded gap structure under the following conditions, Ni is 0.3 mass% or more, Ti is 0.15 mass% or more, and Al is 0.04 mass% or more. In the added steel, the oxidation of Fe is suppressed, and as a result, Cr ₂ O ₃ in the oxide scale is concentrated and the corrosion resistance of the oxide scale is improved.
Welding conditions: TIG pulse without weld core, lap welding
Welding current Base current 130-145A, Pulse current 120-145A
Welding speed 300mm / min
Ar gas flow rate on the torch seal side 12L / min
Electrode diameter φ2.4mm
Arc length 1mm

  Cu suppresses the occurrence of pitting corrosion at the heat-affected zone on the back of the weld in the corrosion resistance of the TIG butt weld without an Ar back gas seal, and reduces the crevice corrosion area in the TIG weld gap. Depending on conditions, 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 action 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%, preferably less than 0.1%.

  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.

  About the steel plate of each test material, the TIG welding clearance 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, and after bending one steel plate at an angle of 10 °, welding is performed with the surface to be the gap exposed to the atmosphere. It was. The 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, “gap depth” was defined as the distance (mm) from the center of the weld bead to the bending position, and each having a weld gap with gap depths of 5 mm, 7 mm, and 10 mm was produced. A 15 × 40 mm test piece was cut out from a sample from which the oxidized scale generated by welding was not removed (uncleaned sample), and was subjected to an immersing 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 immerse test piece includes a weld bead portion, a heat affected zone, and a base metal 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.

Immersion test of 80 ℃ 2000ppmCl ^- was carried out for 24 hours in an aqueous solution. FIG. 3 schematically shows the immersion test method. A Pt auxiliary cathode 1 was connected to the immersion test piece 2.
The Pt auxiliary cathode 1 is obtained by performing Pt plating on the surface of a 40 × 60 mm Ti plate. This auxiliary cathode has a cathode capacity corresponding to a 300 L (liter) hot water can body with respect to the test piece here. The immersion test piece 2 and the Pt auxiliary cathode 1 were immersed in the test liquid 3, and air was fed into the test liquid 3 from the aeration nozzle 4 during the test. The test was performed at n = 3. During the test, the corrosion current was monitored. The progress of corrosion can be determined by the change in corrosion current over time.

  The weld bead of the weld gap test piece after the immersion test was cut to open the gap surface, the gap surface was observed with a microscope, and the maximum erosion depth was measured. The results are shown in Table 2. The erosion depth values shown in Table 2 are the maximum erosion depths for all n = 3 specimens. In any of the test pieces, the maximum erosion depth due to crevice corrosion was observed within 2 mm from the end of the weld bond at the place where the oxide scale of the weld gap shown in FIG. 1 occurred.

  Further, an oxide film analysis at a position 2 mm from the bond end portion, which was the maximum erosion position in the Pt auxiliary cathode test 24-hour immersion test, was performed by a micro part X-ray emission electron analysis method (μ-XPS). The beam diameter is 50 μm. In this way, the amount of each metal element at a depth of 5, 10, 15, and 20 nm was determined from the outermost layer of the oxide scale, and the Cr ratio was calculated as Cr amount / Σ (total metal element amount). And the average value of Cr ratio in each measurement position was made into the average Cr rate. The results are shown in Table 2.

Based on the above knowledge, the TIG welding gap test piece was produced under the condition of a gap depth of 7 mm, and the immersion test was performed on the steel sheets of the respective test materials shown in Table 1. The test piece, test conditions and test method are the same as above. The test results are shown in Table 3.

As can be seen from Table 2, all of the examples of the present invention having the chemical composition defined in the present invention were judged to have passed the corrosion resistance evaluation in the immersion test. That is, it was confirmed that the weld gap in which the oxide scale is formed by performing TIG welding without a back gas seal has excellent corrosion resistance in a hot water environment. From the comparison of No. 1 steel (23Cr-0.3Ni-0.8Mo), No. 3 steel (24Cr-0.5Ni-0.5Mo) and No. 4 steel (25Cr-0.3Ni-0.7Mo) In steel added with 0.1 mass% or more of Ni, the corrosion current tends to disappear more stably and disappear earlier and the erosion depth becomes shallower as the Cr content increases. In particular, No. 4 steel lost its corrosion current within 7 days, the maximum erosion depth was extremely shallow at 0.08 mm, and the corrosion resistance of the weld gap was excellent. The maximum erosion depth of No. 2 steel (23Cr-1Ni-0.3Mo) and No. 1 steel (23Cr-0.3Ni-0.8Mo) is almost the same, and in the TIG weld gap without back gas seal Regarding the corrosion resistance, the effect of improving the corrosion resistance by increasing Mo is not recognized.
Compared with No. 1 steel (23Cr-0.3Ni-0.8Mo) and No. 5 steel (23Cr-0.07Ni-1Mo), the effect of improving the corrosion resistance of the TIG weld gap by adding Ni is remarkable.
From the comparison of No. 3 steel (24Cr-0.5Ni-0.5Mo-0.06Cu) and No. 6 steel (24Cr-0.5Ni-0.5Mo-0.28Cu), the corrosion current is increased by the addition of Cu. It is clear that it will continue to flow and the maximum erosion depth may exceed 0.2 mm.

  This material can be applied not only to the water heater can body but also to an oil supply pipe having a weld gap structure, an oil supply system member of a fuel tank, a fuel injection rail, and a heat exchanger member.

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 It is a schematic diagram of the structure of an immersion test.

Explanation of symbols

1 Auxiliary cathode 2 Sample 3 Test solution 4 Aeration nozzle 5 Saturation reference electrode

Claims (2)

% 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: 0.3-2%,
Cr: 22-26%
Mo: 0.8% or less,
Nb: 0.10 to 0.6%,
Ti: 0.15-0.4%,
N: 0.025% or less,
Al: 0.04 to 0.3%,
Further, it is a ferritic stainless steel plate that limits Cu as an impurity to less than 0.2%, and the balance Fe and other inevitable impurities, the cold-rolled annealed pickled plate having a gap depth of 4 mm or more and a maximum gap interval For a test piece having a TIG weld gap structure formed at 30 μm or less and without an argon back gas seal, the average Cr ratio in the region from the outermost layer of the oxide scale of the weld gap within 2 mm from the bond end to 20 nm is all metal elements A ferritic stainless steel sheet excellent in the corrosion resistance of the weld gap oxide film, wherein the ratio is 20% by mass or more.   A ferritic stainless steel sheet according to claim 1, wherein the cold-rolled annealed pickled steel sheet has a gap depth of 4 mm or more and a maximum gap interval of 30 µm or less and a TIG welded gap structure formed without an argon back gas seal. On the other hand, the average Cr ratio in the region from the outermost layer of the oxide scale of the weld gap within 2 mm from the bond end to 20 nm is 20% by mass or more in terms of the total metal elements, and further 2 to 4 mm from the bond end. A ferritic stainless steel sheet excellent in the corrosion resistance of the weld gap oxide film, characterized in that the average Fe ratio in the region from the outermost layer of the oxide scale at the position of 20 to 20 nm is 70% by mass or less in terms of the ratio of all metal elements.

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