JP5109233B2 - Ferritic / austenitic stainless steel with excellent corrosion resistance at welds - Google Patents

Description

  The present invention relates to a stainless steel, particularly a ferritic / austenitic stainless steel excellent in welded portion corrosion resistance.

  Stainless steel is used as a material with excellent corrosion resistance in a wide range of applications such as automobile parts, building parts, and kitchen appliances. Among these, ferrite and austenitic stainless steels are excellent in strength and corrosion resistance, and are used as corrosion resistant materials for high chloride environments such as seawater and severe corrosive environments such as oil wells. However, SUS 329 ferritic / austenitic stainless steel specified by JIS is expensive because it contains more than 4% (mass ratio, the same below) of expensive Ni, and consumes a large amount of valuable Ni resources. There is a problem.

  In order to solve such problems, ferritic / austenitic stainless steels with reduced Ni content have been demanded. For example, Patent Document 1 discloses that Ni content is limited to more than 0.1 and less than 1%. A ferrite-austenitic stainless steel is disclosed in which the stability is enhanced by taking the IM index defined below as 40 to 115, the Ni content is low, and the tensile elongation is excellent. Here, IM = 551-805 (C + N)%-8.52Si% -8.57% Mn-12.51Cr% -36Ni% -3.45Cu% -14Mo%.

Japanese Patent Laid-Open No. 11-71643

  However, the ferrite-austenitic stainless steel proposed in Patent Document 1 has a problem that the corrosion resistance of the welded portion is inferior. That is, since the ferrite-austenitic stainless steel is used after being welded according to the application, it is necessary that the corrosion resistance of the welded portion is excellent, but the steel proposed in Patent Document 1 is In order to reduce Ni, N, an austenite-forming element, is added in the range of 0.1 to 0.3%. Therefore, N that is dissolved at high temperature in the weld zone and the heat-affected zone in the vicinity thereof precipitates as chromium nitride. There was a problem that the corrosion resistance deteriorated easily due to a chromium-deficient region.

  An object of the present invention is to provide a ferritic / austenitic stainless steel that solves such problems related to the prior art and has excellent weld corrosion resistance while reducing the resources of Ni resources at a relatively low cost. To do.

The ferritic / austenitic stainless steel according to the present invention has a mass ratio of C: 0.007% to 0.069%, Si: 0.31% to 0.41%, Mn: 4.42% to 4.99%, P: 0.1% or less, S : 0.03% or less, Cr: 18.81% or more and 24.01% or less, Ni: 0.12% or more and 0.63% or less, N: 0.16% or more and 0.42% or less, the balance Fe and inevitable impurities, the austenite phase fraction in the metal structure In addition to being 30 vol% or more and 78 vol% or less , the (C + N) content in the austenite phase is 0.28% or more and 0.72% or less in terms of mass ratio and is excellent in welded portion corrosion resistance.

The austenite phase preferably has a processing-induced martensite index Md (γ) determined from the component composition contained in the phase of −30 to 90 or less.
Record
Md (γ) = 551-462C (γ) -462N (γ) -9.2Si (γ) -8.1Mn (γ) -13.7Cr (γ) -29Ni (γ) -29Cu (γ)
Where C (γ), N (γ), Si (γ), Mn (γ), Cr (γ), Ni (γ), Cu (γ) are C, N, Si, Mn, Cr, Ni, Cu content (mass%)

The ferritic / austenitic stainless steel may further contain Al: 0.003% to 0.1%. Moreover, Cu: 0.24% or more and 2.03% or less can be contained. The ferritic / austenitic stainless steel here is a stainless steel containing a ferrite phase and an austenitic phase, and may contain a martensite phase or the like.

  According to the present invention, it is possible to provide a ferritic / austenitic stainless steel excellent in weld corrosion resistance while saving resources of Ni resources. This makes it possible to economically manufacture corrosion resistant materials for high chloride environments such as seawater and severe corrosive environments such as oil wells.

  The composition (%, mass ratio) of the ferritic / austenitic stainless steel according to the present invention is as follows.

C: 0.007% to 0.069%
C is an element effective for increasing the strength, and is preferably contained in an amount of 0.003% or more. Specifically, the C amount is set to 0.007% or more based on Reference Examples and Examples (see Tables 8 to 10) shown later. However, if it exceeds 0.2%, the heat treatment temperature for solid solution becomes remarkably high, which impairs economy. Therefore, the C content is limited to 0.2% or less. In particular, as can be seen from Tables 8 and 9 shown as examples, when the C content is 0.110% or less, the corrosion resistance of the welded portion is excellent at any of the weld bead, the heat-affected zone and the base metal. However, when the C content is 0.10% or more, the stress corrosion cracking resistance is remarkably deteriorated as can be confirmed from Reference Example 2 described later. Therefore, the C content in the present invention is set to 0.069% or less based on the examples.

Si: 0.31% to 0.41%
Si is an effective element as a deoxidizing material, and is preferably contained in an amount of 0.01% or more. However, if its content exceeds 1.2%, the hot workability deteriorates, so it is limited to 1.2% or less, preferably 1.0% or less. In the present invention, in addition to this, the content is limited to 0.31% or more and 0.41% or less based on Examples . In order to further suppress the deterioration of corrosion resistance due to sensitization (deterioration of corrosion resistance due to the formation of chromium carbide and chromium nitride at grain boundaries), the Si content is preferably 0.4% or less.

Mn: 4.42% to 4.99%
Mn is a particularly important element for obtaining excellent weld corrosion resistance. FIG. 1 shows that a welding material including a welded portion, a heat-affected zone, and a base metal portion is 100 to 300 mV vs. SCE. In a 0.035% (mass ratio) sodium chloride solution. 5 is a graph showing the relationship between the presence or absence of corrosion and the Mn content when held at a potential of 30 min. The presence or absence of corrosion was determined as “corrosion” when the current value was 1 mA or more, and as “no corrosion” when the current value was less than 1 mA.

As is apparent from FIG. 1, it is clear that the corrosion resistance of the welding material is remarkably improved when the Mn content is 4% or more. According to the inventor's view, this cause is that when the Mn content is increased to 4% or more, the precipitation temperature of chromium nitride is lowered, and the chromium nitride in the heat affected zone near the weld and the weld is reduced. This is because the generation of the chromium-deficient region is suppressed. However, as apparent from FIG. 1, when the Mn content exceeds 12%, excellent corrosion resistance cannot be obtained. This is presumably because a large number of corrosion starting points such as MnS are formed in the base material part when the Mn content exceeds 12%. In addition to these findings, based on the examples shown later, in the present invention, the Mn content is limited to 4.42% or more and 4.99% or less.

P: 0.1% or less, S: 0.03% or less
P is an element harmful to the corrosion resistance of the gap-resistant portion. Particularly, if it exceeds 0.1%, the effect becomes remarkable, so 0.1%, preferably 0.05% or less. S is an element harmful to hot workability. If it exceeds 0.03%, the effect becomes significant, so 0.03% or less, preferably 0.02% or less.

Cr: 18.81-24.01%
Cr is an important component that imparts corrosion resistance to stainless steel, and in the present invention, if it is less than 15%, sufficient weld corrosion resistance cannot be obtained. However, if the Cr content exceeds 35%, it becomes difficult to form an austenite phase in the steel, and the desired ferrite-austenite stainless steel cannot be obtained. Therefore, Cr is limited to 15% or more and 35% or less. In addition, Preferably it is 17% or more and less than 30%, More preferably, it is restricted to 18% or more and 28% or less. In the present invention, based on these findings, the content is further limited to 18.81% or more and 24.01% or less based on examples shown later.

Ni: 0.12% to 0.63%
Ni is an austenite formation promoting element and is useful for forming a ferrite-austenite structure. However, it is an expensive alloy element and needs to be reduced as much as possible in terms of resource protection. From these viewpoints, the Ni content is limited to 1% or less, preferably 0.9% or less, and more preferably less than 0.5%. However, when the Ni content is 0.10% or less, the toughness of the base material and the welded portion is lowered. Therefore, in order to improve the toughness including the welded part, it is preferable to contain Ni at least over 0.10% (see Reference Example 3). In addition to these findings, based on examples shown later, in the present invention, the Ni content is limited to 0.12% or more and 0.63% or less .

N: 0.16% to 0.42%
N is also an austenite formation promoting element and is alloyed as an alternative component of Ni in the present invention. When the Ni content is limited to 1% or less, a sufficient amount of austenite phase cannot be formed unless the N content is 0.05% or more. However, if it exceeds 0.6%, blowholes are generated in the welded part, which causes a decrease in weldability. In addition to these findings, the Ni content is limited to 0.16% or more and 0.42% or less in the present invention based on examples shown later. Note that N is preferably 0.18% or more from the viewpoint of austenite phase generation, and 0.34% or less from the viewpoint of hot workability.

  In the present invention, in addition to the above elements, the following elements can be contained as required.

Al: 0.003% to 0.1%
Al can be used as a deoxidizer and can be contained as much as necessary as a deoxidizer. The effect as a deoxidizing agent is recognized at 0.003% or more, but when it exceeds 0.1%, nitrides are formed and cause flaws in the steel sheet. Therefore, the content (residual amount) is preferably 0.1% or less, preferably Is 0.02% or less.

Cu: 0.24% to 2.03%
Cu is useful for improving the corrosion resistance, and in order to exhibit the effect, it is preferable to contain all at 0.1% or more. For Cu, if it exceeds 4%, the hot workability deteriorates significantly, so its content should be 4% or less. In addition to these findings, based on examples shown later, the present invention limits the Cu content to 0.24% or more and 2.03% or less.

Inevitable impurities except the remaining Fe Components other than the above components are Fe except for inevitable impurities. Inevitable impurities include deoxidation products such as O (oxygen). It is desirable to reduce these as much as possible, including cases where they inevitably remain.

  The steel according to the present invention has the above composition, and the metal structure thereof needs to be a ferrite-austenitic stainless steel having an austenite phase fraction in the structure of 10 vol% or more and 85 vol% or less. FIG. 2 is a graph showing the influence of the austenite phase fraction on the corrosion resistance of the weld material including the base metal part. The method for measuring corrosion resistance is the same as in FIG. As apparent from FIG. 2, when the austenite phase fraction is 10 vol% or more, the corrosion resistance of the welded portion is remarkably improved.

  Although this reason does not affect the interpretation of the technical scope of the present invention, the inventors presume as follows. That is, in general, ferrite and austenitic stainless steels with low Ni content and high N content have a high diffusion rate of Cr and N during cooling after welding, so chromium nitride precipitates at the grain boundaries containing the ferrite phase. Therefore, it is considered that a chromium-deficient region is likely to occur. However, in the ferrite-austenitic stainless steel having an austenite phase of 10 vol% or more, particularly 15 vol% or more as in the present invention, since the austenite phase forming ability is high, even if Cr decreases at the crystal grain boundary including the ferrite phase. This part is transformed into an austenite phase, so that the solubility of chromium nitride is increased, and as a result, the chromium-deficient region is decreased.

However, when the austenite phase fraction exceeds 85 vol%, the stress corrosion cracking sensitivity is remarkably increased. For the above reasons, in the present invention, the austenite phase fraction is 10 to 85 vol%, preferably 15 to 85 vol%, more preferably 25 to 75 vol%. In addition to these findings, it is preferable in the present invention to limit the austenite phase fraction to 30 vol% or more and 78 vol% or less based on the examples shown later.

The austenite phase fraction is the volume fraction of austenite in the metal structure. Typically, the steel structure of the steel plate cross section parallel to the rolling direction is observed under an optical microscope, and the austenite phase fraction in the organization The ratio can be determined by measuring the line segment method or the surface segment method. Specifically, after polishing the sample, the red blood salt solution (potassium ferricyanide (K ₃ [Fe (CN) ₆ ]): 30 g + potassium hydroxide (KOH): 30 g + water (H ₂ O): 60 ml) Under the optical microscope, the ferrite phase is identified as gray, and the austenite phase and martensite phase are identified as white.Therefore, the proportion of the gray portion and the white portion is obtained by image analysis, and the ratio of the white portion is determined as the austenite phase fraction. It is a rate. Strictly speaking, in this method, the austenite phase and the martensite phase cannot be distinguished, and not only the austenite phase but also the martensite phase may be included in the white portion. Even when the austenite phase fraction and other conditions measured by this method are satisfied, the desired effect can be obtained. Such austenite phase fraction can be controlled by the steel composition and the steel sheet production heat history.

Ferrite and austenitic stainless steels with the above basic composition and an austenite phase fraction in the metal structure of 30 vol% or more and 78 vol% or less are relatively low-cost, aiming to save Ni resources. However, it has excellent corrosion resistance at welds. However, in order to further secure formability (ductility, deep drawability), and in the ferrite and austenitic stainless steels of the present invention, the amount of C + N contained in the austenitic phase of the steel structure is 0.16% or more and 2% or less. Is preferable. This is because if the amount of C + N contained in the austenite phase of the steel structure is less than 0.16%, sufficient formability cannot be obtained, while it is difficult to contain more than 2%. The content is preferably 0.2 to 2%, more preferably 0.3 to 1.5%. In the present invention, in addition to these findings, the amount of C + N is limited to 0.28% or more and 0.72% or less based on examples shown later.

  The amount of C and N in the austenite phase can be adjusted by adjusting the steel composition and annealing conditions (temperature, time). The relationship between the steel structure and annealing conditions and the amount of C and N in the austenite phase is not clear, but when the amount of Cr, C and N in the steel structure is large, the amount of C and N in the austenite phase is often increased. In addition, when the steel component composition is the same, the knowledge gained from experience such as that the lower the austenite phase fraction determined by annealing conditions, the higher the amount of C and N in the austenite phase often. Based on the above, an appropriate amount of C and N can be contained. In addition, the measurement of the content of C and N in the austenite phase can be performed by, for example, EPMA.

  The amount of C + N contained in the austenite phase first affects the ductility. The reason is not clear, but the present inventor thinks as follows. That is, when the steel slab is pulled and deformed, necking (necking) usually occurs and eventually breaks, but in the stainless steel of the present invention, since austenite phase exists, The austenite phase undergoes work-induced transformation to the martensite phase and becomes harder than other parts. For this reason, further necking does not proceed, and as a result, deformation progresses uniformly over the entire steel piece, resulting in high ductility. In particular, when the amount of C + N in the austenite phase is high, the hardness of the martensite phase generated in the initial necking portion is high, and the austenite fraction is the same amount compared to stainless steel with a small amount of C + N in the austenite phase. However, it is estimated that the effect of improving the ductility due to the work-induced martensite phase works very effectively.

It is estimated that the improvement of ductility is caused by the processing-induced transformation of the austenite phase to the martensite phase as described above, and the processing-induced martensite index Md (γ) is used as the index, and this is −30 It is effective to adjust to a range of ˜90 to obtain higher ductility. Here, the processing-induced martensite index Md (γ) is determined by the following formula from the composition components contained in the austenite phase.
Md (γ) = 551-462C (γ) -462N (γ) -9.2Si (γ) -8.1Mn (γ) -13.7Cr (γ) -29Ni (γ) -29Cu (γ)
Where C (γ), N (γ), Si (γ), Mn (γ), Cr (γ), Ni (γ), Cu (γ) are C, N, Si, Mn, Cr, Mo, Ni, Cu content

  This high work-induced martensite index Md (γ) is an index indicating the ease of work-induced martensitic transformation by austenite phase processing, and the lower this index is, the harder the martensitic transformation associated with processing occurs. The higher the induced martensite index, the more likely the martensitic transformation associated with processing occurs. When the processing-induced martensite index Md (γ) is less than −30, it is considered that the amount of processing-induced martensite generated in the micro-necking portion is small when micro-necking begins to occur during processing. On the other hand, when Md (γ) exceeds 90, the austenite phase undergoes martensitic transformation throughout the steel before micronecking begins to occur. It is thought that the austenite phase that is the basis of the presence or absence of the austenite phase is extremely small. Therefore, by adjusting the processing-induced martensite index Md (γ) in the range of -30 to 90, the amount of martensite at the necking site when micronecking starts during processing is optimized, resulting in extremely high ductility. It is estimated that

  When the above-described conditions are satisfied, not only ductility but also high deep drawability are provided. The reason for this is the same as described above regarding the influence of the austenite phase fraction and the amount of C + N in the austenite phase on the ductility described above. This is thought to be because hardening due to the martensite phase occurs and local deformation is suppressed.

  Thus, in addition to the basic composition, the austenite phase fraction in the metal structure, the C + N amount in the austenite phase, and further by adjusting the above-mentioned work-induced martensite index to an appropriate value, the steel of the present invention In addition to excellent weld corrosion resistance, which is a typical property, it also has high formability (ductility, deep drawability).

  The effect of the present invention can be obtained by any of a hot rolled sheet, a hot rolled annealed sheet, and a cold rolled annealed sheet. In addition, the effects of the present invention can be obtained in any surface finishing state such as No. 2A, No. 2B, BA, and polishing finishing regardless of the finishing state. The effect of the present invention can be obtained regardless of the shape of the product. For example, the effect can be obtained even in the shape of a wire, a steel bar, a shape steel, a steel pipe, or the like.

(Reference Example 1)
Various steels having the compositions shown in Tables 1 and 2 were melted in an atmosphere in which vacuum melting or nitrogen partial pressure was controlled within a range of up to 0.9 atm (882 hPa) to obtain a steel slab (or steel ingot, ingot). Then, in accordance with a conventional method, it hot-rolled, annealed, and cold-rolled, and then finish-annealed at the temperature of 900-1300 degreeC, and the cold-rolled annealing board with a board thickness of 2.25mm was obtained. About the obtained cold-rolled annealed plate, the austenite phase fraction was measured, and a weld bead with a width of about 5 mm was placed under the conditions of an input power of 900 W and a speed of 30 cm / min using a TIG welder.

The obtained test piece including a weld bead, a heat-affected zone, and a base metal with a side of 25 mm was ground at 100, 200 and 300 mV vs. SCE. In a 0.035% (mass ratio) sodium chloride aqueous solution after grinding the surface scale. A sample that was held at a potential for 30 minutes and generated a current of 1 mA or more was evaluated as “corrosion”, and a sample that did not generate a current of 1 mA or more was evaluated as “no corrosion”. The test results are shown in Table 3. In Table 3, the mark “◯” indicates “no corrosion” and the mark “×” indicates “corrosion”. It is clear that the welding material of this reference example does not generate corrosion up to a potential of 200 mV vs SCE. And is excellent in corrosion resistance of the welded portion.

(Reference Example 2)
In the same manner as in Reference Example 1 , a steel having the composition shown in Table 4 was melted to form a steel slab (or steel ingot, ingot), and then hot rolled, annealed, and cold rolled according to a conventional method. Thereafter, finish annealing was performed at a temperature of 1050 ° C. to obtain a cold-rolled annealed plate having a thickness of 2.25 mm. The austenite phase fraction was measured about the obtained cold-rolled annealing board.

  On the cold-rolled sheet obtained as described above, using a TIG welder, a welding bead with a width of about 5 mm was placed in a direction perpendicular to the rolling direction under the conditions of an input power of 900 W and a speed of 30 cm / min, A specimen having a width of 10 mm and a length of 75 mm parallel to the rolling direction from the welded portion was used, and this was a U-bend specimen having a bending radius of 10 mm. In the specimen cut out from the welded portion, the bottom of the U-bend specimen was made the welded portion. The U-bend test piece thus adjusted was immersed in a magnesium chloride aqueous solution (temperature 80 ° C.) having a concentration of 42 mass%, and the presence or absence of the crack was visually observed every 24 hours. The survey results are shown in Table 5. As apparent from Table 5, the stress corrosion cracking resistance of the base metal and the welded portion is remarkably improved by setting the C content to 0.1% or less.

(Reference Example 3)
In the same manner as in Reference Example 1 , a steel having the composition shown in Table 6 was melted to form a steel slab (or steel ingot, ingot), and then hot-rolled, annealed, and cold-rolled according to a conventional method. Thereafter, finish annealing was performed at a temperature of 1050 ° C. to obtain a cold-rolled annealed plate having a thickness of 2.25 mm. The austenite phase fraction was measured about the obtained cold-rolled annealing board.

  On the cold-rolled annealed plate obtained as described above, a weld bead having a width of about 5 mm was placed in a direction perpendicular to the rolling direction under the conditions of an input power of 900 W and a speed of 30 cm / min using a TIG welder. A Charpy impact test piece was cut out from the cold-rolled annealed plate on which the weld bead was placed so that the 2 mm V notch was perpendicular to the rolling direction, and an impact test was performed at 0 ° C. The impact test results are shown in Table 7. As is apparent from Table 7, the impact absorption energy of the base material and the weld is significantly improved by setting the Ni content to 0.1% or more.

(Example)
In the same manner as in Reference Example 1, after melting the steel having the composition shown in Table 8 into a steel slab (or steel ingot, ingot), hot rolling, annealing, cold rolling according to a conventional method, Finish annealing was performed at a temperature of 1050 ° C. to obtain a cold-rolled annealing plate having a thickness of 2.25 mm. About the obtained cold-rolled annealed plate, the austenite phase fraction was measured, and a weld bead with a width of about 5 mm was placed under the conditions of an input power of 900 W and a speed of 30 cm / min using a TIG welder.

  As in Reference Example 1, for a test piece having a side of 25 mm including a weld bead, a heat-affected zone, and a base material, after grinding the surface scale, it is 100, 200, and 0.02% in a sodium chloride aqueous solution of 0.035% (mass ratio). The sample was held at a potential of 300 mV vs SCE for 30 minutes, and a sample in which a current of 1 mA or more was generated was evaluated as “corrosion”, and a sample in which a current of 1 mA or more was not generated was evaluated as “no corrosion”. Table 9 shows the austenite phase fraction measurement results and the corrosion test results. In Table 9, the mark “◯” indicates “no corrosion” and the mark “×” indicates “corrosion”. It is clear that the welding material of the steel of the present invention does not corrode up to a potential of 300 mV vs SCE and is excellent in the corrosion resistance of the welded portion.

  Steel having the composition shown in Table 8 is melted to form a steel slab (or steel ingot or ingot), followed by hot rolling, annealing and cold rolling according to conventional methods, and then finishing at a temperature of 1050 ° C. Annealing was performed to obtain a cold-rolled annealed sheet having a thickness of 0.8 mm. The austenite phase fraction was measured for the cold-rolled annealed plate obtained as described above, and further component analysis in the austenite phase, tensile test, and measurement of the limit drawing ratio were performed by the following methods.

(Analysis of components in austenite phase)
A sample having a length of 15 mm was cut out along the rolling direction (L direction) from the substantially central portion in the width direction of the obtained cold-rolled annealed plate, and a sample was prepared by polishing the cross section in the L direction. Qualitative mapping of C and / or N was performed on the entire cross section of the sample by EPMA, and the austenite phase was identified by utilizing the feature that C and N are concentrated in the austenite phase. C, N, Si, Mn, Cr, Ni, Cu and Mo were quantitatively analyzed by EPMA for the almost central part of the austenite phase thus identified. At that time, an electron beam was not applied to the ferrite phase. Further, the electron irradiation region was set to a range of about 1 μm in diameter. The measurement was performed on at least three austenite phases for each sample, and the average value thereof was used as a representative value, and a work-induced martensite index Md (γ) defined by the following formula was obtained based on the measured value.
Md (γ) = 551-462C (γ) -462N (γ) -9.2Si (γ) -8.1Mn (γ) -13.7Cr (γ) -18.5Mo (γ) -29Ni (γ) -29Cu (γ)
here,
C (γ), N (γ), Si (γ), Mn (γ), Cr (γ), Mo (γ), Ni (γ), Cu (γ) are respectively C, N, Si, austenite, Mn, Cr, Mo, Ni, Cu content (mass%)

(Tensile test)
JIS13B tensile test specimens were taken from cold-rolled annealed plates from 0 ° (parallel to the rolling direction), 45 ° and 90 ° (perpendicular to the rolling direction), at room temperature, A tensile test was performed in the atmosphere at a tensile speed of 10 mm / min, and the total elongation until fracture in each direction was measured. The average elongation (El) was calculated by the following formula, and this was evaluated as the total elongation.
El = {El (0 °) + 2El (45 °) + El (90 °)} / 4

(Limit aperture ratio)
Round test pieces of various diameters (blank diameters) are punched from cold-rolled annealed plates, and the punched circular test pieces are cylindrically drawn under the conditions of a punch diameter of 35 mm and a plate pressing force of 9.8 kN, and can be drawn without breaking. The maximum blank diameter was divided by the punch diameter to determine the limit drawing ratio (LDR), and the deep drawability was evaluated. The punching diameter of the test piece used for the cylindrical drawing was changed so that the drawing ratio was 0.1 interval.

The results of the above test are shown in Table 10. The example of the present invention in which the amount of C + N in the austenite phase is 0.16 to 2% is much higher in total elongation than the comparative example in which the amount of C + N in the austenite phase is 0.10% (steel 69) or 0.07% (steel 70). It is clear that it has excellent drawing ratio and deep drawability. Furthermore, the present invention examples (steel Nos. 51, 52, 53, 54, 56, 57, 58, 60) in which the work-induced martensite index Md (γ) determined from the components in the austenite phase is in the range of -30 to 90. , 61,63,64,65,66,67,68) are invention examples departing from the scope of this (steel No.55, compared to 59, 62), have a higher total elongation and limitations aperture It is clear that the ductility and deep drawability are excellent.

A welding material including a welded portion, a heat-affected zone, and a base metal portion is placed in a 0.035% (mass ratio) sodium chloride solution at 100 to 300 mV vs. SCE. 6 is a graph showing the relationship between the presence or absence of corrosion and the Mn content when held at a potential of 30 minutes. It is a graph which shows the influence of the austenite phase fraction which acts on the corrosion resistance of the welding material containing a base material part.

Claims (4)

By mass ratio, C: 0.007% to 0.069%, Si: 0.31% to 0.41%, Mn: 4.42% to 4.99%, P: 0.1% or less, S: 0.03% or less, Cr: 18.81% to 24.01% Ni: 0.12% or more and 0.63% or less, N: 0.16% or more and 0.42% or less, balance Fe and inevitable impurities, the austenite phase fraction in the metal structure is 30 vol% or more and 78 vol% or less , and the austenite Ferritic / austenitic stainless steel with excellent weld corrosion resistance, characterized in that the (C + N) content in the phase is 0.28% to 0.72% by mass. 2. The ferrite with excellent weld corrosion resistance according to claim 1, wherein the following processing-induced martensite index Md (γ) determined from the component composition contained in the austenite phase is −30 to 90 or less.・ Austenitic stainless steel.
Record
Md (γ) = 551-462C (γ) -462N (γ) -9.2Si (γ) -8.1Mn (γ) -13.7Cr (γ) -29Ni (γ) -29Cu (γ)
Where C (γ), N (γ), Si (γ), Mn (γ), Cr (γ), Ni (γ), Cu (γ) are C, N, Si, Mn, Cr, Ni, Cu content (mass%) The ferritic / austenitic stainless steel excellent in welded portion corrosion resistance according to claim 1 or 2, further comprising Al: 0.003% to 0.1% . The ferritic / austenitic stainless steel excellent in welded portion corrosion resistance according to any one of claims 1 to 3, further comprising Cu: 0.24% or more and 2.03% or less .

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