KR20130034042A - Ferritic stainless steel - Google Patents

Abstract

(또한, 수학식 1 중의 Al, Ti, Si, Ca는, 강 중의 각 성분의 함유량 [질량％]임)In mass%, C: 0.020% or less, N: 0.025% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.035% or less, S: 0.01% or less, Cr: 16.0 to 25.0%, Al: 0.12 Ferritic stainless steel containing less than or less, Ti: 0.05 to 0.35%, Ca: 0.0015% or less, and the remainder is made of Fe and unavoidable impurities and satisfies the following formula River.
[Equation 1]

(In addition, Al, Ti, Si, and Ca in Formula (1) are content [mass%] of each component in steel.)

Description

Ferritic Stainless Steels {FERRITIC STAINLESS STEEL}

The present invention relates to a ferritic stainless steel with less generation of black spots in TIG welds.

This application claims priority based on Japanese Patent Application No. 2010-177998 for which it applied to Japan on August 6, 2010, and uses the content here.

In general, ferritic stainless steels are not only excellent in corrosion resistance, but also have a low coefficient of thermal expansion compared with austenitic stainless steels, and have excellent corrosion resistance to stress corrosion cracking. For this reason, ferritic stainless steel is widely used for tableware, building exterior materials including kitchen equipment and roofing materials, and storage and storage materials. In recent years, the demand for conversion from austenitic stainless steels has also increased due to an increase in the price of Ni raw materials.

In such a stainless steel structure, welding construction is indispensable. Originally, ferritic stainless steels had a small C and N solid solubility, which caused sensitization in the welded part, and thus had a problem of lowering corrosion resistance. In order to solve this problem, the method of suppressing the sensitization of a weld metal part by reduction of C, N amount, fixation of C, N by addition of stabilizing elements, such as Ti and Nb, etc. (for example, patent document 1 Has been proposed and widely used.

In addition, the corrosion resistance in the weld part of ferritic stainless steel is known to be inferior in corrosion resistance to the scale part generated by the heat input of welding, and it is known that it is important to fully shield by inert gas compared with an austenitic stainless steel. .

In addition, Patent Literature 2 adds Ti and Al so as to satisfy the formula P1 = 5 Ti + 20 (Al-0.01) ≥ 1.5 (Ti and Al in the formula represent the content of each component in the steel), whereby A technique is disclosed in which an Al oxide film for improving corrosion resistance is formed on a surface layer portion of steel during welding.

In addition, Patent Literature 3 discloses a technique for improving the gap corrosion resistance of a welded portion by adding a predetermined amount or more in addition to the complex addition of Al and Ti.

Further, Patent Document 4 satisfies 4Al + Ti ≤ 0.32 (Ti and Al in the formula indicate the content of each component in the steel), thereby reducing heat input during welding, suppressing scale generation of the weld, and improving corrosion resistance of the weld. Techniques for improving are disclosed.

The above-mentioned prior art aims at improving the corrosion resistance of a welded part or a welding heat affected zone.

In addition, there is a technique of actively adding P and adding an appropriate amount of Ca and Al as a means for improving the weather resistance and the gap corrosion resistance of the raw material itself rather than the welded portion (see Patent Document 5, for example). In patent document 5, Ca and Al are added in order to control the shape and distribution of the nonmetallic inclusion in steel. Moreover, the biggest characteristic of patent document 5 is adding P more than 0.04%, and patent document 5 has no description about the effect at the time of welding.

Japanese Patent Publication No. 55-21102 Japanese Patent Application Laid-open No. Hei 5-70899 Japanese Patent Application Publication No. 2006-241564 Japanese Patent Application Publication No. 2007-270290 Japanese Patent Application Laid-open No. Hei 7-34205

In the conventional ferritic stainless steel, even if the shield conditions in a weld part are optimized, black spots generally called a black spot or a slag spot may be dotted on the weld back surface bead after welding. In the black spot, Al, Ti, Si, and Ca, which have strong affinity with oxygen, form an oxide and solidify on the weld metal during solidification of the weld metal in TIG (Tungsten Inert Gas) welding. The generation of black spots greatly affects welding conditions, particularly shielding conditions by inert gas, and the more insufficient shielding, the more black spots are generated.

Since the black spot itself is an oxide, even if a small amount of black spots are scattered, there is no problem in the corrosion resistance and workability of the welded portion. However, if a large number of black spots are generated or continuously generated, not only the appearance of the welded portion is used without being polished, but also the peeling of the black spot portion may occur when the welded portion is processed. When peeling of a black spot part generate | occur | produces, a problem may arise that a workability falls or a gap corrosion generate | occur | produces in the clearance gap with the peeled black spot part. Moreover, even when a process is not performed after welding, when black spot is thickly formed, when a stress is added to a welding part, a black spot may peel and corrosion resistance may fall.

Therefore, in order to improve the corrosion resistance of a TIG welding part, it is important not only to improve the corrosion resistance of a weld bead part or the welding scale part itself, but also to control the black spot produced | generated in a weld part. However, the scale with discoloration generated during welding can be almost suppressed by the method of strengthening the shielding condition of the welding. However, the black spots generated in the TIG welds can be suppressed even if the shielding conditions are enhanced. The furnace could not be sufficiently suppressed.

This invention is made | formed in view of such a situation, and makes it a subject to provide the ferritic stainless steel which is hard to produce a black spot in a TIG weld part, and excellent in the corrosion resistance and workability of a weld part.

MEANS TO SOLVE THE PROBLEM This inventor repeated earnest research as shown below in order to suppress the production | generation amount of a black spot. As a result, by optimizing the amount of Al, Ti, Si, and Ca, it was found that generation of black spots in the TIG welded section can be suppressed.

The gist of the present invention is as follows.

The 1st aspect of this invention is mass%, C: 0.020% or less, N: 0.025% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.035% or less, S: 0.01% or less, Cr: Ferritic stainless steel containing 16.0 to 25.0%, Al: 0.12% or less, Ti: 0.05 to 0.35%, Ca: 0.0015% or less, consisting of Fe and inevitable impurities as the remainder, and satisfying the following formula to be.

[Equation 1]

(In addition, Al, Ti, Si, and Ca in Formula (1) are content [mass%] of each component in steel.)

In the above ferritic stainless steel, there is little black spot formation in the welded portion.

The 2nd aspect of this invention is the ferritic stainless steel which concerns on the said 1st aspect, It is a ferritic stainless steel which further contains Nb: 0.6% or less by mass%.

3rd aspect of this invention is the ferritic stainless steel which concerns on said 1st or 2nd aspect, It is a ferritic stainless steel which further contains Mo: 3.0% or less in mass%.

The 4th aspect of this invention is the ferritic stainless steel which concerns on any one of said 1st-3rd form, The mass% is 1 type or 2 types chosen from Cu: 2.0% or less and Ni: 2.0% or less. It is a ferritic stainless steel further comprising.

The fifth aspect of the present invention is a ferritic stainless steel according to any one of the first to fourth aspects, and is one or two kinds selected from V: 0.2% or less and Zr: 0.2% or less by mass%. It is a ferritic stainless steel further comprising.

A sixth aspect of the present invention is a ferritic stainless steel according to any one of the first to fifth aspects, and is a ferritic stainless steel further containing B: 0.005% or less by mass.

According to the present invention, it is possible to provide a ferritic stainless steel which is hard to produce black spots on the TIG welds and which is excellent in corrosion resistance and workability of the TIG welds.

1A is a photograph showing the appearance of a black spot generated on the back side during TIG welding.
It is a schematic diagram which shows the external appearance of the black spot which generate | occur | produced in the back surface side at the time of TIG welding, It is a figure corresponding to the photograph shown in FIG.
It is a graph which shows the result of having measured the element depth profile (concentration distribution of the element of a depth direction) in the weld bead part on the back side of a test piece by AES.
It is a graph which shows the result of having measured the element depth profile (concentration distribution of the element of a depth direction) of the black spot in the back side of a test piece by AES.
3 is a graph showing the relationship between the BI value and the black spot generation length ratio.
4 is a graph showing the relationship between BI values and corrosion. The double circle (◎) indicates excellent results, the circle (○) indicates good results, and × indicates poor results.

Hereinafter, the present invention will be described in detail.

The ferritic stainless steel with less generation of black spots in the welded portion of the present invention satisfies the following expression (1).

[Equation 1]

(In addition, Al, Ti, Si, and Ca in Formula (1) are content [mass%] of each component in steel.)

Al, Ti, Si, and Ca are elements with particularly strong affinity with oxygen, and are elements that generate black spots during TIG welding. In addition, as the content of Al, Ti, Si, and Ca contained in the steel increases, black spots tend to be formed. The coefficient of content of Al, Ti, Si, and Ca in the said Formula (1) is determined based on the magnitude | size (strength) of the effect | action which promotes generation | occurrence | production of a black spot, and content in steel. In more detail, as Al shows in the experiment example mentioned later, Al is contained in black spot at the highest density, and an element which has the effect | action which promotes production | generation of a black spot is especially large. For this reason, the coefficient of Al content is set to 3 in the said Formula (1). In addition, although Ca is small in content in steel, it is contained in black spots in high concentration | density, and it is a big element which has the effect | action which promotes generation | occurrence | production of a black spot. For this reason, the coefficient of Ca content is set to 200.

When the BI value exceeds 0.8, the generation of black spots becomes remarkable. On the other hand, when BI value is 0.8 or less, generation | occurrence | production of the black spot of a TIG weld part will become small enough, and it will become excellent in corrosion resistance. In addition, when the BI value is 0.6 or less, the generation of black spots can be more effectively suppressed, and when the BI value is 0.4 or less, generation of the black spots can be almost suppressed, and the corrosion resistance of the TIG weld portion can be further improved. Can be.

Under conditions where a large amount of black spots occur, the thickness of the black spots also becomes thick, and therefore, it is presumed that the black spots are easily peeled off during processing. On the contrary, since the thickness becomes thinner on the condition that the occurrence rate of black spots is small, it is estimated that even if a black spot is produced, it is hard to peel. Therefore, it is considered that corrosion resistance of a weld part can be improved by suppressing generation | occurrence | production of a black spot.

Next, the component composition of the ferritic stainless steel of this invention is demonstrated in detail.

First, each element stipulating Equation 1 will be described.

Aluminum (Al): 0.012% or less by mass%

Al is important as a deoxidation element, and there is also an effect of making the structure finer by controlling the composition of nonmetallic inclusions. However, Al is an element most contributing to the generation of black spots. In addition, excessive addition of Al may cause coarsening of a nonmetallic interference | inclusion, and may become a starting point of a flaw generation of a product. Therefore, the upper limit of Al content was made into 0.12%. It is preferable to contain Al 0.01% or more for deoxidation. Al content becomes like this. More preferably, they are 0.03%-0.10%.

Titanium (Ti): 0.05% to 0.35% by mass

Ti is a very important element in terms of fixing C and N, suppressing intergranular corrosion of the welded portion, and improving workability. However, excessive addition of Ti not only produces black spots but also causes surface scratches at the time of manufacture. For this reason, the range of Ti content was made into 0.05%-0.35%. More preferably, they are 0.07%-0.20%.

Silicon (Si): 1.0% or less by mass%

Si is an important element as a deoxidation element and is effective also in improving corrosion resistance and oxidation resistance. However, excessive addition of Si not only promotes the formation of black spots, but also lowers workability and manufacturability. Therefore, the upper limit of content of Si was made into 1.0%. It is preferable to contain Si 0.01% or more for deoxidation. Si content becomes like this. More preferably, they are 0.05%-0.55%.

Calcium (Ca): 0.0015% or less by mass%

Ca is very important as a deoxidation element and is contained in trace amount in steel as a nonmetallic inclusion. However, since Ca is very easily oxidized, it becomes a big factor in generating a black spot during welding. In addition, Ca may generate | occur | produce a water-soluble inclusion and may reduce corrosion resistance. For this reason, it is preferable that content of Ca is as low as possible, and the upper limit of content of Ca was made into 0.0015%. More preferably, the content of Ca is 0.0012% or less.

Next, the other element which comprises the ferritic stainless steel of this invention is demonstrated.

Carbon (C): 0.020% or less by mass%

Since C lowers grain boundary corrosion resistance and workability, it is necessary to reduce the content. For this reason, the upper limit of content of C was made into 0.020%. However, when the content of C is excessively reduced, the refining cost is deteriorated. Therefore, the content of C is more preferably 0.002% to 0.015%.

Nitrogen (N): 0.025% or less by mass%

Since N reduces grain boundary corrosion resistance and workability similarly to C, it is necessary to reduce the content. For this reason, the upper limit of content of N was made into 0.025%. However, when the content of N is excessively reduced, the refining cost is deteriorated. Therefore, the content is more preferably 0.002% to 0.015%.

Manganese (Mn): 1.0% or less by mass%

Although Mn is an important element as a deoxidation element, when added excessively, MnS becomes easy to produce | generate MnS which becomes a starting point of corrosion, and destabilizes a ferrite structure. For this reason, content of Mn was made into 1.0% or less. It is preferable to contain Mn 0.01% or more for deoxidation. More preferably, they are 0.05%-0.5%. More preferably, they are 0.05%-0.3%.

Phosphorus (P): 0.035% or less by mass%

P not only lowers the weldability and workability but also tends to generate grain boundary corrosion, and therefore P needs to be kept low. Therefore, content of P was made into 0.035% or less. More preferably, they are 0.001%-0.02%.

Sulfur (S): 0.01% or less by mass%

S produces | generates the water-soluble inclusions which become a starting point of corrosion, such as CaS and MnS, and needs to reduce it. Therefore, content of S is made into 0.01% or less. Excessive reductions, however, lead to deterioration of costs. For this reason, it is more preferable that content of S is 0.0001%-0.005%.

Chromium (Cr): 16.0 to 25.0% by mass

Cr is the most important element in securing corrosion resistance of stainless steel, and it is necessary to contain Cr 16.0% or more in order to stabilize a ferrite structure. However, since Cr deteriorated workability and manufacturability, the upper limit was made into 25.0%. Content of Cr becomes like this. Preferably it is 16.5%-23.0%, More preferably, it is 18.0%-22.5%.

Niobium (Nb): 0.6% or less by mass%

Nb can be added alone or in combination with Ti in view of its properties. It is preferable to satisfy (Ti + Nb) / (C + N) ≧ 6 (Ti, Nb, C, N in the formula is the content [mass%] of each component in the steel) when Nb is contained together with Ti.

Nb is an element which fixes C and N similarly to Ti, suppresses grain boundary corrosion of a weld part, and improves workability. However, since excessive addition of Nb reduces workability, it is preferable to make the upper limit of content of Nb into 0.6%. Moreover, in order to improve said characteristic by containing Nb, it is preferable to contain Nb 0.05% or more. The content of Nb is preferably 0.15% to 0.55%.

Molybdenum (Mo): 3.0% or less by mass%

Mo is effective in repairing a passivation film and is a very effective element for improving corrosion resistance. In addition, since Mo is contained together with Cr, Mo has an effect of effectively improving pitting resistance. In addition, Mo contains the Ni together with the effect of improving the flow resistance. However, when Mo increases, workability will fall and cost will increase. For this reason, it is preferable to make the upper limit of content of Mo into 3.0%. Moreover, in order to improve said characteristic by containing Mo, it is preferable to contain Mo 0.30% or more. Content of Mo becomes like this. Preferably it is 0.60%-2.5%, More preferably, it is 0.9%-2.0%.

Nickel (Ni): 2.0% or less by mass%

Ni has an effect of suppressing the active dissolution rate and is excellent in repassivation characteristics because of a small hydrogen overvoltage. However, excessive addition of Ni reduces workability and makes the ferrite structure unstable. For this reason, it is preferable to make the upper limit of content of Ni into 2.0%. Moreover, in order to improve said characteristic by containing Ni, it is preferable to contain Ni 0.05% or more. The content of Ni is preferably 0.1% to 1.2%, more preferably 0.2% to 1.1%.

Copper (Cu): 2.0% or less by mass%

Like Ni, Cu not only lowers the active dissolution rate but also has the effect of promoting repassivation. However, excessive addition of Cu reduces workability. For this reason, when adding Cu, it is preferable to make an upper limit into 2.0%. In order to improve the said characteristic by containing Cu, it is preferable to contain Cu 0.05% or more. The content of Cu is preferably 0.2% to 1.5%, more preferably 0.25% to 1.1%.

Vanadium (V) and / or zirconium (Zr): 0.2% or less by mass%

V and Zr improve weather resistance and gap corrosion resistance. In addition, by suppressing the use of Cr and Mo and adding V, excellent workability can be ensured. However, since excessive addition of V and / or Zr reduces workability, and also the effect of improving corrosion resistance is saturated, it is preferable that the upper limit of the content in the case of containing V and / or Zr is 0.2%. Moreover, in order to improve said characteristic by containing V and / or Zr, it is preferable to contain V and / or Zr 0.03% or more. The content of V and / or Zr is more preferably 0.05% to 0.1%.

Boron (B): 0.005% or less by mass%

Although B is an effective grain boundary strengthening element for improving secondary work brittleness, excessive addition causes solid solution strengthening of ferrite to cause ductility reduction. For this reason, when adding B, it is preferable to make a minimum into 0.0001% and an upper limit into 0.005%, and it is more preferable to set it as 0.0002%-0.0020%.

Example

The test piece which consists of ferritic stainless steel which has a chemical component (composition) shown in Table 1 was manufactured by the method shown below. First, the cast steel of the chemical component (composition) shown in Table 1 was melted by vacuum melting, the ingot of 40 mm thickness was produced, and this was rolled to 5 mm thickness by hot rolling. Then, based on each recrystallization behavior, heat processing was performed for 1 minute at the temperature of 800-1000 degreeC, the scale was ground and removed, and the steel plate of thickness 0.8mm was produced again by cold rolling. Thereafter, heat treatment is performed at a temperature of 800 to 1000 ° C. for 1 minute based on the respective recrystallization behavior as final annealing, and the oxidation scale on the surface is pickled and removed to prepare test specimens. Prepared. In addition, in the chemical component (composition) shown in Table 1, content of each element is represented by the mass%, and remainder is iron and an unavoidable impurity. Underlines represent numerical values outside the scope of the present invention.

Thus, the test piece of No.1-28 obtained was TIG-welded on the welding conditions shown below, and the black spot generation length ratio was computed as shown below. Moreover, the corrosion test shown below was done about the test piece of No.1-28.

`` Welding condition ''

TIG welding was performed with the same steel grade at the feed rate of 50 cm / min and the heat input of 550-650 J / cm <2>. As the shield, argon was used for both the torch side and the back side.

`` Black spot generation length ratio ''

The black spot generation length ratio was determined as a reference indicating the amount of black spots generated after TIG welding. The black spot generation length ratio was calculated by integrating the length in the welding direction of each black spot generated in the welded portion, and dividing the accumulated value by the total welding length. About 10 cm of welding length was image | photographed with the digital camera, the length of each black spot was measured, and it calculated | required by calculating the ratio with respect to the welding length of the total of the length of the black spot in a welding length using image processing.

Corrosion Test

As a corrosion test piece, what bulged the TIG weld part was used. The bulging conditions were Ericsen test conditions based on JIS 2247, and used the punch of 20 mm (phi) as the surface of the back surface of a welding test piece as a surface. However, the bulging height stopped processing in order to match the processing conditions. The stop height (bulging height) was unified to 6 mm and 7 mm. Corrosion evaluation was carried out in accordance with JIS Z 2371, a continuous spray test of 5% NaCl, and evaluated for the presence or absence of flow rust after 48 hours. In the continuous spray test of 5% NaCl in the workpiece having a bulging height of 6 mm, the flowable rust was not confirmed in the welded portion. The case where flow rust generate | occur | produced in the continuous spray test was made into "bad."

Table 2 shows the BI values, the black spot growth length ratios and the results of the corrosion test obtained from the chemical components of Table 1.

As shown in Table 2, in test pieces Nos. 1 to 21 in which the chemical composition (composition) was within the scope of the present invention and the BI value was 0.8 or less, the black spot generation length ratio was small, and the generation of black spots after TIG welding was small.

Among these, the production of black spots is more suppressed in Nos. 1 to 15, 18, and 19 with a BI value of 0.6 or less, and in Nos. 1 to 13 with a BI value of 0.4 or less, the generation length is 10% or less. The occurrence was almost suppressed.

Further, in test pieces Nos. 1 to 21 having a bulging height of 6 mm, no rust from the weld was observed in the continuous spray test of 5% NaCl in the corrosion resistance test piece after processing with an Eriksen tester. Further, in test pieces Nos. 1 to 21 having a strict bulging height of 7 mm, rust of the weld portion was not confirmed in the test piece having a BI value of 0.4 or less, and rust was observed in the test piece exceeding 0.4.

On the other hand, in test piece No. 22, 24, 26-28 whose BI value exceeds 0.8, the black spot formation length ratio after TIG welding was large, and all the rust from the weld part was confirmed in the corrosion test. When the rust generation part of test piece No. 22, 24, 26-28 was magnified and observed with the magnifier, peeling was confirmed at the boundary of a black spot and a welding bead part. Nos. 22, 26, 27, and 28, in which Al, Ti, Si, and Ca had a concentration above a specified level, were rusted in the corrosion test.

Moreover, in test piece No. 25 whose composition ratio of Cr was less than 16%, and test piece No. 23 whose composition ratio of Ti is less than 0.05%, generation | occurrence | production of rust was confirmed by the corrosion test.

Experimental Example 1

The test material was manufactured like the manufacturing method of the test piece of No. 1 except having produced the steel plate of thickness 1mm by cold-rolling the ferritic stainless steel which has the chemical component (composition) shown below. This was used to obtain test piece A and test piece B.

`` Chemical ingredient (composition) ''

Specimen A

C: 0.007%, N: 0.011%, Si: 0.12%, Mn: 0.18%, P: 0.22%, S: 0.001%, Cr: 19.4%, Al: 0.06%, Ti: 0.15%, Ca: 0.0005%, Remaining part: Iron and unavoidable impurity

Test piece B

C: 0.009%, N: 0.010%, Si: 0.25%, Mn: 0.15%, P: 0.21%, S: 0.001%, Cr: 20.2%, Al: 0.15%, Ti: 0.19%, Ca: 0.0015%, Remaining part: Iron and unavoidable impurity

Thus, about the test piece A and the test piece B obtained, TIG welding was carried out on the welding conditions similar to the test piece of No. 1, and the external appearance of the black spot which generate | occur | produced on the back surface side at the time of TIG welding was observed.

The results are shown in Figs. 1A and 1B.

FIG. 1A is a photograph showing the appearance of a black spot generated on the back side during TIG welding. 1B is a schematic diagram which shows the external appearance of the black spot which generate | occur | produced on the back surface side at the time of TIG welding, Comprising: It is a figure corresponding to the photograph shown to FIG. 1A.

In FIG. 1A and FIG. 1B, the left side is the photograph and the figure of the test piece A whose BI value is 0.49, and the right side is the photograph and the figure of the test piece B whose BI value is 1.07.

As shown by the arrow in FIG. 1A and FIG. 1B, the spot-shaped black spot is seen here and there on both sides of test piece A with a BI value of 0.49 and test piece B with a BI value of 1.07. However, it turns out that the black spot generate | occur | produced more in test piece B (right photograph) with a big BI value.

Moreover, Auger Electron Spectroscopy (AES) measurement was performed about the test piece B whose BI value is 1.07 about two places of a weld bead part and a black spot part. The results are shown in Figs. 2A and 2B.

In addition, in AES measurement, using a scanning type FE Auger electron spectrometer, it measures until oxygen intensity is hardly observed on the conditions of an acceleration voltage of 10 keV, a spot diameter of about 40 nm, and a sputter rate of 15 nm / min. Was carried out. In addition, since the measurement spot of AES is small, an error may generate | occur | produce according to a measurement position, but it employ | adopted this time as showing the approximate thickness.

2A and 2B are graphs showing the results obtained by measuring the element depth profile (concentration distribution of elements in the depth direction) of the black spot and the weld bead portion on the back side of the test piece by AES. 2A is the result of the weld bead portion, and FIG. 2B is the result of the black spot.

As shown in FIG. 2A, the weld bead part was an oxide having a thickness of several hundred Pas, mainly composed of Ti and containing Al and Si. On the other hand, as shown in FIG. 2B, the black spot was a thick oxide having a thickness of several thousand Pas, mainly including Al, and containing Ti, Si, and Ca. In addition, from the graph of the black spot shown in FIG. 2B, Al was contained in the black spot at the highest concentration, and it was confirmed that Ca was contained in the black spot at high concentration, although the content in steel was small.

Experimental Example 2

C: 0.002 to 0.015%, N: 0.02 to 0.015%, Cr: 16.5 to 23%, Ni: 0 to 1.5%, and Mo: 0 to 2.5% as base compositions, and Al, Ti, and Si, which are main components of the black spot, The test material of the ferritic stainless steel which has various chemical components (compositions) in which content, and Ca etc. differs was manufactured by the manufacturing method similar to the test piece A. Using this, the some test piece was obtained.

TIG welding was carried out on the welding conditions similar to the test piece of No. 1 about the some test piece obtained in this way, and black spot generation length ratio was computed similarly to the test piece of No. 1.

As a result, the black spot generation length ratio showed a tendency to increase as Al, Ti, Si, and Ca increased. These elements are particularly strong in affinity with oxygen. Among them, Al has a large effect, and Ca has a large effect on the black spot, although the content of steel is low. It was also found that Ti and Si also contributed to the generation of black spots.

From this, when the addition amount of Al, Ti, Si, and Ca was large, there was a possibility that a black spot might generate | occur | produce even if shielding was performed, In particular, it turned out that Al and Ti have a big influence on formation of a black spot.

Moreover, BI value represented by following formula (1) was computed about each of some test piece, and the relationship with the black spot generation length ratio was investigated.

[Equation 1]

(In addition, Al, Ti, Si, and Ca in Formula (1) are content [mass%] of each component in steel.)

The results are shown in Fig. 3 is a graph showing the relationship between the BI value and the black spot generation length ratio. As shown in FIG. 3, it turns out that black spot generation length ratio becomes large, so that BI value is large.

In addition, the corrosion test was done about each test piece similarly to the test piece of No.1. The result is shown in FIG. 4 is a graph showing the relationship between the BI value and the corrosion resistance evaluation result after the spray test after processing. In the figure, double circles (?) Indicate excellent results, circles (○) indicate good results, and × indicates poor results. As shown in FIG. 4, when BI value is 0.8 or less, corrosion does not generate | occur | produce in the test piece with a bulging height of 6 mm, and especially 0.4 or less, since corrosion is not confirmed also in the test piece with a bulging height of 7 mm, it is very favorable. did.

The ferritic stainless steel of the present invention is an exterior material, a building material, an outdoor device, a water storage tank, a home appliance, a bathtub, a kitchen appliance, a drain water recovery device of a latent heat recovery type gas water heater (hot water supply), its heat exchanger, and various welding pipes. For example, it can be used suitably for the member which needs corrosion resistance in the structure formed by TIG welding for other general use of outdoor and indoor. In particular, the ferritic stainless steel of the present invention is suitable for a member to be processed after TIG welding. Moreover, since the ferritic stainless steel of this invention is excellent not only in corrosion resistance but also in the workability of a TIG welding part, it is applicable widely also in the use which has severe processing.

Claims (11)

In mass%, C: 0.020% or less, N: 0.025% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.035% or less, S: 0.01% or less, Cr: 16.0 to 25.0%, Al: 0.12 A ferritic stainless steel containing% or less, Ti: 0.05 to 0.35%, Ca: 0.0015% or less, and the remainder being made of Fe and unavoidable impurities and satisfying the following formula (1).
[Equation 1]

(In addition, Al, Ti, Si, and Ca in Formula (1) are content [mass%] of each component in steel.) The ferritic stainless steel according to claim 1, further comprising Nb: 0.6% or less by mass%. The ferritic stainless steel according to claim 1, further comprising Mo: 3.0% or less by mass%. The ferritic stainless steel according to claim 2, further comprising Mo: 3.0% or less by mass%. The ferritic stainless steel according to any one of claims 1 to 4, further comprising one or two types selected from Cu: 2.0% or less and Ni: 2.0% or less. The ferritic stainless steel according to any one of claims 1 to 4, further comprising one or two kinds selected from V: 0.2% or less and Zr: 0.2% or less by mass%. The ferritic stainless steel according to claim 5, further comprising one or two kinds selected from V: 0.2% or less and Zr: 0.2% or less by mass%. The ferritic stainless steel according to any one of claims 1 to 4, which further contains B: 0.005% or less by mass%. The ferritic stainless steel of Claim 5 which further contains B: 0.005% or less by mass%. The ferritic stainless steel of Claim 6 which further contains B: 0.005% or less by mass%. The ferritic stainless steel according to claim 7, which further contains, in mass%, B: 0.005% or less.

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