JP2019112672A - Ferritic stainless steel excellent in rust resistance - Google Patents

Abstract

To provide a ferritic stainless steel excellent in rust resistance and having low price.SOLUTION: There is provided a ferritic stainless steel excellent in rust resistance, having a chemical composition of C:0.020% or less, Si:over 1.0 to 3.5%, Mn:0.02 to 0.80%, S:less than 0.002%, Cr:10.5 to less than 15.0%, Ti:0.03 to 0.35%, Al:0.002% or more and less than 0.5%, Ca:0.0005 to 0.005%, N:0.025% or less, and the balance Fe with inevitable impurities, satisfying Cr+1.8Si of 16.0% or more, and further having a value of CaO/AlOin an oxide-based inclusion containing Ca and Al by mass ratio of 0.75 or more.SELECTED DRAWING: None

Description

  The present invention is a ferritic stainless steel with excellent weather resistance, which is used for members exposed to the air environment and required weather resistance, such as exhaust system members for automobiles, kitchen equipment, water storage tanks, and exterior members. About.

  Automotive exhaust systems include parts such as exhaust manifolds, catalytic converters, flexible pipes, center pipes and mufflers, as well as environmentally compatible parts such as EGR (Exhaust Gas Recirculation), DPF (Diesel Particulate Filter), and urea SCR (Selective Catalytic Reduction) systems. Is attached. Among exhaust systems, parts exposed to high temperature exhaust gas discharged from the engine such as exhaust manifolds and catalytic converters, for example, place importance on heat resistance such as high temperature strength, thermal fatigue characteristics and oxidation resistance, while center pipe And so-called corrosion resistance such as condensed water corrosion resistance and salt corrosion resistance is emphasized in parts located behind the exhaust system, such as mufflers. Ferritic stainless steels are often used for these exhaust system members, but in recent years the importance of appearance has increased. Until now, resistance to damage that penetrates plate thickness has been regarded as important due to corrosion, fatigue, etc. However, when looking at the appearance of exhaust system members at the time of delivery or inspection, changes in color tone due to red rust etc. There are more cases in which users are concerned.

  Many stainless steels are used for kitchen appliances for home and business use, exterior members for buildings, and the like, and ferritic stainless steels that are superior in price are also in use. Although these members are used indoors and outdoors, the appearance is important and the user is often sensitive to changes in color tone due to rust and the like.

  In order to suppress the formation of red rust, it is necessary to improve the corrosion resistance, and in order to improve the corrosion resistance, it is generally by high alloying such as increase of Cr amount and addition of Ni and Mo. Among these, Mo is an expensive alloying element, leading to an increase in price. Therefore, a means for improving the corrosion resistance at low cost without relying on the increase of the amount of Cr or the addition of Ni or Mo has been required.

  In Patent Document 1, C: 0.03% or less, N: 0.03% or less, Cr: 8 to 35%, Mn: 1.5% or less, Si: 0.8 to 2.5% and / or A ferritic stainless steel for petroleum-based fuel reformer characterized in that it contains Al: 0.6 to 6.0% and the total amount of Si and Al is 1.5% or more.

  In Patent Document 2, C: 0.03% or less, N: 0.03% or less, Si: 2.0% or less, Mn: 1.5% or less, S: 0.008% or less, Cr: 11 to A heat collector having an oxide film with a thickness of 0.01 to 10 μm in which the spinel oxide concentration is suppressed to 15% or less on the surface of a steel satisfying 25%, Al: 6.0% or less and Cr + 3Si + 15Al> 22 A heat transfer material is disclosed.

  In Patent Document 3, C: 0.02% or less, N: 0.03% or less, Mn: 1.5% or less, Cr: 5 to 16%, Ni: 1.0% or less, Nb: 0.4 1.51.5%, Co: 0.04 to 0.5%, Si: 2.0% or less, Al: 0.1 to 5.0%, and Si + Al: 1.0 to 5.0 is satisfied A high toughness high toughness ferritic stainless steel is disclosed.

  In Patent Document 4, C: 0.001 to 0.015%, N: 0.001 to 0.015%, Si: 0.50 to 2.00%, Mn: 0.01 to 1.00%, P: 0.010 to 0.030%, S: 0.003% or less, Cr: 10 to 14%, Nb and Ti any one or two of them: 20 (C + N) ≦ Nb + Ti ≦ 0.6% There is disclosed a ferritic stainless steel for automobile exhaust system equipment which is excellent in high temperature salt corrosion resistance.

  In Patent Document 5, C: 0.001 to 0.1%, Si: more than 1.5 to 4.0%, Mn: 0.05 to 4.0%, Cr: 10.5 to 30%, Ni : 35% or less, Ti: 0.002 to 0.030% and / or Al: 0.002 to 0.10%, N: 0.001 to 0.4% are contained, Si / (Ti + Al) ≧ 40 A stainless steel excellent in brazeability satisfying the above requirements is disclosed.

In Patent Document 6, C: 0.05% or less, N: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0. Containing 02% or less, Cr: 10.0 to 30.0%, Ni: 1.0% or less, Al: 0.2% or less, (O) in steel, Al ₂ O ₃ , CaO is 0.0005 There is disclosed a ferritic stainless steel sheet excellent in weld penetration, characterized by satisfying ≦ O-0.5Al ₂ O ₃ −0.3CaO ≦ 0.012.

In Patent Document 7, the inclusion is 0.1 ≦ (4 × [CaO] + 2 × [MgO] + 3 × [CaS] + 0.4 × [MnS]) / ([TiO ₂ ] + [Al ₂ O ₃ ]. A ferritic stainless steel containing 11% or more of Cr characterized by satisfying + [SiO ₂ ]) ≦ 10 and having a size corresponding to an average equivalent circle diameter of 15 μm or less is disclosed.

In Patent Document 8, C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.015% or less, Cr: 5 to 5 32%, N: 0.02% or less, Al: 0.005% or less, O: 0.01% or less, Ti: 0.08% or more and 6 × (C + N) or more and 0.5% or less, Ca: It is characterized in that it contains 0.0005 to 0.0050%, and the composition of oxide inclusions satisfies 20 to 90% of Ti oxide, 50% or less of Al ₂ O ₃ , and 5 to 50% of CaO. There is disclosed a ferritic stainless steel having good surface properties and excellent corrosion resistance and formability.

Patent Document 9, inclusions are composed of _{CaO-SiO 2 -MgO-Al 2} O 3 -MnO-Cr 2 O 3, its average composition is _{Cr 2 O 3: 10.3~55%,} Al 2 O _3:50% or less, MgO: is 15% or less, stainless steel is disclosed, wherein the maximum equivalent circle diameter of 20μm or less.

In Patent Document 10, C: 0.070% or less, N: 0.02% or less, Si: 0.05 to 0.60% or less, Mn: 0.04 to 0.50%, P: 0.030 %, S: 0.0003 to 0.0020%, Cr: 16 to 21%, Ni: 0.60% or less, Al: 0.002 to 0.14%, Ti: 0.35% or less, The inclusion composition having a maximum diameter of 2 μm or more is [(CaO) + (MgO)] / [(Al ₂ O ₃ ) + (SiO ₂ ) + (TiO ₂ )] ≦ 0.50, (FeO) ≦ 1.5 A ferritic stainless steel excellent in corrosion resistance is disclosed.

JP 2003-160840 JP 2008-101240 A Unexamined-Japanese-Patent No. 5-78791 Unexamined-Japanese-Patent No. 6-248394 International Publication No. 2016/152854 Japanese Patent Application Laid-Open No. 10-102212 Unexamined-Japanese-Patent No. 10-237596 Patent No. 3661418 gazette Patent No. 4285302 Patent No. 5744576

  Although the stainless steels described in Patent Documents 1 to 10 are effective to some extent as means for improving corrosion resistance at low cost, there is room for further improvement.

  The present invention is made in view of the above-mentioned subject, and an object of the present invention is to provide a ferritic stainless steel excellent in corrosion resistance and inexpensive.

The gist of the present invention for solving the above problems is as follows.
[1] In mass%, C: 0.020% or less, Si: more than 1.0 to 3.5%, Mn: 0.02 to 0.80%, S: less than 0.002%, Cr: 10. 5 to less than 15.0%, Ti: 0.03 to 0.35%, Al: 0.002% to 0.5%, Ca: 0.0005 to 0.005%, N: 0.025% CaO / Al in oxide inclusions containing the following, the balance having a chemical composition consisting of Fe and unavoidable impurities, and satisfying Cr. + 1.8Si of 16.0% or more, and further containing Ca and Al: A ferritic stainless steel excellent in weather resistance characterized in that the value of ₂ O ₃ is 0.75 or more in mass ratio.

[2] Furthermore, Nb, 0.03 to 0.60%, Ni: 0.1 to 1.2%, Mo: 0.1 to 2%, W: 0.1 to 1%, V in mass% : 0.05 to 0.5%, Cu: 0.1 to 1.2%, Co: 0.01 to 0.5%, Sn: 0.001 to 0.5%, Sb: 0.001 to 0 .5%, Zr: 0.001 to 0.3%, Ga: 0.0001 to 0.01%, Ta: 0.0001 to 0.01% and any one or more selected from A ferritic stainless steel excellent in weathering resistance according to [1], characterized in that

[3] Further, in mass%, Mg: 0.0002 to 0.005%, REM: 0.005 to 0.1%, B: 0.0002 to 0.005%, any one or two of them A ferritic stainless steel excellent in weathering resistance as described in [1] or [2], characterized in that it contains a seed or more.

  According to the present invention, it is possible to provide a ferritic stainless steel which is excellent in corrosion resistance and inexpensive.

  Hereinafter, embodiments of the present invention will be described in detail.

First, as an explanation of the outline of the present invention, the points particularly studied in the creation of the invention and the examination results will be described.
On the surface of the stainless steel, a Cr-rich (Fe, Cr) oxide film is formed, thereby exhibiting excellent corrosion resistance. This film has a self-healing ability to recover even if the film is broken for some reason, and an oxide film is formed which is more excellent in corrosion resistance as the material is higher in Cr and contains Mo. Therefore, although a material such as SUS436L or SUS444 containing Mo is applied to a member requiring high corrosion resistance, there is a problem that it is expensive.
In view of such background, the present inventors have keenly studied focusing on the composition of inclusions known as the starting point of corrosion and the alloying elements that can improve the weather resistance of stainless steel at low cost compared to Mo.

As a result, while making a stainless steel contain Si more than a fixed amount, it discovered that composition control of an oxide type inclusion was effective in heat resistance. Specifically, it is as follows.
(1) Containing more than 1.0% of Si in the steel.
(2) The value of CaO / Al ₂ O ₃ in oxide inclusions containing Ca and Al is 0.75 or more.

First, with regard to the finding (1), the results of examining the influence of Si on the weatherability by the inventor of the present invention will be described. Although the outer surface side of the member targeted by the present invention is exposed to a salt environment, it has been determined that at least SUS430LX level weather resistance is necessary. Therefore, pitting potential measurement was adopted as a method that can easily evaluate the order of weatherability, and the influence of Si was examined. The measurement was performed in accordance with JIS G0577, and the noblest potential having a current value exceeding 100 μA / cm ² was defined as a pitting potential V'c100. It was found that when the content of Si exceeds 1%, the pitting potential is improved not only by the increase of the Cr content but also by the increase of the Si content, and the effect is about 1.8 times that of Cr. In order to achieve the SUS430LX level at a pitting potential in a 3.5% NaCl solution at 30 ° C., 150 mV was obtained when Ag / AgCl containing saturated KCl as an internal solution was used for the reference electrode. This value can be satisfied if it is 16.0% or more with Cr + 1.8Si. Desirably, it is 160 mV or more, more preferably 170 mV or more.

  As described above, the surface oxide film was analyzed by X-ray photoelectron spectroscopy (XPS) in order to examine the reason why the pitting potential was improved by the increase of the Si content. It was thought that the pitting potential was improved as a result of the improvement of the protection of the film, because an oxide film enriched with Si was formed on the surface.

  Next, with regard to the finding (2), the results of examining the composition of inclusions which the present inventor exerts on the weather resistance will be described. In the case where corrosion starts from inclusions, it is generally said that the inclusions first dissolve and then the stainless steel base material dissolves. Therefore, in many cases, measures are taken to suppress the occurrence of corrosion by making the inclusions a component that is difficult to dissolve. However, there are also viewpoints of characteristics other than corrosion resistance, manufacturability, etc., and it is not easy to industrially make all inclusions a component that is difficult to dissolve. Therefore, it is the red rust that is caused by the dissolution of stainless steel that hinders the appearance of stainless steel, so we decided to study in the direction of suppressing the corrosion of stainless steel.

  Once the stainless steel base material around inclusions melts, the pH of the portion lowers and corrosion of the stainless steel base material further progresses. Therefore, it was considered that if the reduction in pH could be suppressed, it would be difficult for corrosion of stainless steel to proceed. Therefore, inclusions which are easy to dissolve to some extent and which can raise the pH by dissolution were examined.

As a result of selecting and examining an oxide system which easily forms inclusions, an oxide containing CaO is easily dissolved, and when dissolved, a good pH raising effect is obtained. Oxide inclusions containing CaO because often form mixed oxides with Al ₂ O _3, the pH was measured change of the mixed oxide at the time of dissolution by changing the value of CaO / Al ₂ O ₃ . As a result, when the mass ratio of CaO / Al ₂ O ₃ was 0.7 or more, a good pH increase effect was obtained, and it was found that the effect was saturated at 0.8 or more. Therefore, when a steel material having inclusions having different values of CaO / Al ₂ O ₃ was produced and a short-time dry / wet repeated test was conducted, the value of CaO / Al ₂ O ₃ was 0.75 or more in mass ratio Occasionally, the blasting was alleviated and the weatherability was improved with the increase of the value of CaO / Al ₂ O ₃ . From these results, it was thought that as a result of dissolution of the oxide containing CaO to raise the pH around, dissolution of the stainless steel was suppressed and occurrence was reduced. In order to obtain the effect of improving corrosion resistance, the value of CaO / Al ₂ O ₃ needs to be 0.75 or more in mass ratio, and is preferably 0.8 or more. More preferably, it is 0.85 or more. It is preferable that the value of CaO / Al ₂ O ₃ is high from the viewpoint of the effect of raising the pH, but the solidified structure is coarsened to cause surface defects such as ridging, so the value of CaO / Al ₂ O ₃ should be 2. It is preferable to set it as 0 or less. More preferably, it is 1.8 or less, more preferably 1.6 or less. The content of CaO and Al ₂ O _{3 in} the mixed oxide is preferably 80% by mass or more, more preferably 85% by mass or more, as CaO + Al ₂ O ₃ . In addition, if SiO ₂ which suppresses the dissolution of the oxide is contained in the mixed oxide, the pH increase effect is adversely affected, so SiO ₂ in the mixed oxide is preferably 5% or less. Similarly, since Ti oxide also suppresses dissolution, the Ti oxide (TiO ₂ ) in the mixed oxide is preferably 15% or less.

  The above is a description of the points particularly examined in the creation of the invention and the examination results.

  Next, the composition ranges of the elements constituting the ferritic stainless steel excellent in corrosion resistance of the present invention, and the reason for limiting the ranges will be described. In the following description,% of each component represents% by mass unless otherwise specified.

(C: 0.020% or less)
C is an element useful for securing strength, but an excessive addition reduces the intergranular corrosion resistance, so the content of C is made 0.020% or less. Preferably it is 0.002% or more and 0.018% or less.

(Si: more than 1.0 to 3.5%)
Si is the most important element in the present invention, and has the effect of improving the pitting resistance. It is also effective in oxidation resistance, and needs to be contained in excess of 1.0%. Preferably, it is 1.1% or more, more preferably 1.2% or more. However, excessive addition reduces the weldability and processability, so the content of Si is made 3.5% or less. Preferably it is 3.2% or less, more preferably 2.9% or less.

(Mn: 0.02 to 0.80%)
Mn is an element useful as a deoxidizing element, and needs to be contained at least 0.02% or more. Preferably, it is 0.05% or more. However, if it is contained excessively, the corrosion resistance is degraded, so the content of Mn is made 0.80% or less. Preferably it is 0.60% or less, More preferably, it is 0.40% or less.

(S: less than 0.002%)
The content of S needs to be limited to less than 0.002% because S combines with Mn and Ca to form sulfides to deteriorate the corrosion resistance. In particular, it is necessary to suppress the formation of Ca sulfide also from the viewpoint of the amount of CaO important in the present invention. Therefore, the preferable S content is 0.0015% or less, more preferably 0.001% or less.

(Cr: less than 10.5 to 15.0%)
Cr is an element that is fundamental to secure corrosion resistance. Therefore, the content of Cr is required to be at least 10.5% or more. Preferably, it is 11.0% or more, more preferably 11.5% or more, and further preferably 12.5% or more. The corrosion resistance can be improved as the content of Cr is increased, but in order to reduce the workability, it is less than 15.0%. Preferably it is 14.8% or less, more preferably 14.5% or less.

(Ti: 0.03 to 0.35%)
Since Ti is an element which fixes C and N and improves intergranular corrosion resistance and is useful for processability, it is necessary to contain 0.03% or more. However, if it is contained excessively, the productivity is deteriorated, so the content of Ti is made 0.35% or less. Preferably it is 8 * (C + N) or more and 0.32% or less, More preferably, it is 10 * (C + N)% or more and 0.28% or less.

(Al: 0.002% or more and less than 0.5%)
Al is a useful element for refining because it has a deoxidizing effect and the like, and it is necessary to contain 0.002% or more in order to effectively act on corrosion resistance. Preferably it is 0.01% or more. However, if it is contained excessively, the toughness is deteriorated, so the content of Al is less than 0.5%. Preferably it is 0.4% or less.

(Ca: 0.0005 to 0.005%)
Ca is an element useful for refining such as deoxidation effect, and is an important element that affects the composition of the oxide inclusion in the present invention. Therefore, the content of Ca is required to be at least 0.0005% or more. Preferably it is 0.0006% or more, more preferably 0.0008% or more. However, if it is contained excessively, sulfide is formed to adversely affect corrosion resistance, so the content is made 0.005% or less. Preferably it is 0.003% or less, more preferably 0.002% or less.

(N: 0.025% or less)
N is an element useful for strength and pitting corrosion resistance, but an excessive addition reduces the intergranular corrosion resistance, so the content of N is made 0.025% or less. Preferably it is 0.002 to 0.023%, more preferably 0.003 to 0.020%.

  Furthermore, it is preferable to contain any one or two or more of the following components, as necessary.

(Nb: 0.03 to 0.60%)
Nb fixes C and N, improves intergranular corrosion resistance of the weld and improves high-temperature strength, and therefore, Nb can be contained by 0.03% or more as needed. Preferably it is 0.05% or more, more preferably 0.07% or more. Further, Nb / C + N is preferably 8 or more, and more preferably 10 or more Nb / C + N. However, the excessive addition reduces the weldability, so the upper limit of the content of Nb is set to 0.60%. Preferably it is 0.45% or less, More preferably, it is 0.35% or less.

(Ni: 0.1 to 1.2%)
Ni is contained in an amount of 0.1% or more as needed to improve the corrosion resistance. Preferably, it is 0.2% or more, more preferably 0.25% or more. It is preferable to add 1.2% or less, since the addition of excess causes the cost increase. More preferably, it is at most 1.1%, more preferably at most 1.0%.

(Mo: 0.1 to 2%)
Mo is contained in an amount of 0.1% or more as needed to improve strength and corrosion resistance. Preferably it is 0.2% or more, more preferably 0.3% or more. It is preferable to add 2% or less, since the addition of excess causes the cost increase. More preferably, it is 1.5% or less, still more preferably 0.9% or less.

(W: 0.1 to 1%)
W is contained in an amount of 0.1% or more as needed to improve the strength and corrosion resistance. Preferably it is 0.2% or more. It is preferable to contain 1% or less, since the addition of excess results in cost increase. More preferably, it is 0.9% or less.

(V: 0.05 to 0.5%)
V can be contained 0.05% or more as needed in order to improve corrosion resistance. It is preferable to add 0.5% or less, since excessive addition degrades processability and leads to cost increase because it is expensive.

(Cu: 0.1 to 1.2%)
Cu can be contained in an amount of 0.1% or more and 1.2% or less as needed to improve the corrosion resistance. Excess addition degrades processability. Preferably they are 0.2% or more and 0.9% or less.

(Co: 0.01 to 0.5%)
Co can be contained 0.01% or more as needed in order to improve corrosion resistance. Since addition of excess leads to cost increase, it is preferable to contain 0.5% or less. Preferably it is 0.03% or more and 0.4% or less.

(Sn: 0.001 to 0.5%)
In order to improve corrosion resistance, Sn can be contained 0.001% or more as needed. However, it is preferable to add 0.5% or less because excessive addition reduces the productivity and toughness. More preferably, it is 0.01% or more and 0.3% or less, and further preferably 0.05 to 0.25%.

(Sb: 0.001 to 0.5%)
Sb can be contained 0.001% or more as needed in order to improve corrosion resistance. However, it is preferable to add 0.5% or less because excessive addition reduces the productivity and toughness. More preferably, it is 0.005% or more and 0.4% or less.

(Zr: 0.001 to 0.3%)
In order to improve corrosion resistance, 0.001% or more can be contained as necessary. However, since addition in excess leads to cost increase, it is preferable to contain 0.3% or less. More preferably, they are 0.01% or more and 0.2% or less.

(Ga: 0.0001 to 0.01%)
Since Ga is an element that improves the corrosion resistance and the resistance to hydrogen embrittlement, it may contain 0.001% or more as needed. However, if the Ga content exceeds 0.01%, the cost increases. Therefore, the Ga content is 0.01% or less. More preferable Ga content is 0.0005% or more and 0.005% or less.

(Ta: 0.0001 to 0.01%)
Since Ta is an element that improves the corrosion resistance, 0.001% or more may be contained as necessary. However, if the Ta content exceeds 0.01%, the cost increases. Therefore, the Ta content is 0.01% or less. A more preferable Ta content is 0.0005% or more and 0.005% or less.

(Mg: 0.0002 to 0.005% or less)
Mg is a useful element for refining since it has a deoxidizing effect and the like, and therefore, it can be contained in an amount of 0.0002% or more and 0.005% or less as necessary. Preferably it is 0.0004 to 0.002%.

(REM: 0.005 to 0.1%)
Since REM has a deoxidizing effect and the like, it is an element useful for refining, and is also useful for oxidation resistance, and therefore can be contained in an amount of 0.005% or more and 0.1% or less as necessary. Preferably it is 0.008% or more and 0.08% or less.

(B: 0.0002 to 0.005%)
B can be contained 0.0002% or more as needed in order to improve the brazeability. The addition of B is also effective in improving the secondary processability. However, it is preferable to add 0.005% or less, since excessive addition reduces the intergranular corrosion resistance. More preferably, it is 0.0004 to 0.004%.

In the stainless steel of the present invention, the balance other than the above-described elements consists of Fe and unavoidable impurities.
Among the unavoidable impurities, P is preferably 0.05% or less, more preferably 0.04% or less, from the viewpoint of weldability.

  The stainless steel of the present invention is basically manufactured by the general process of manufacturing stainless steel. For example, after making it into molten steel which has the above-mentioned chemical composition with an electric furnace, refining with an AOD furnace, a VOD furnace, etc. and making it into a steel piece by a continuous casting method or a block formation method, hot rolling-annealing of a hot-rolled sheet-acid It manufactures through the process of washing-cold rolling-finish annealing-pickling. If necessary, annealing of the hot-rolled sheet may be omitted, and cold rolling-finishing annealing-pickling may be repeated. The composition of the oxide inclusions important in the present invention can be controlled by adjusting the basic material of the flux and the addition conditions, etc., for the deoxidizer at the time of refining and its addition method.

  Hereinafter, the effects of the present invention will be made more apparent by examples. The present invention is not limited to the following examples, and can be appropriately modified and implemented without changing the gist of the invention.

A stainless steel having a steel composition and an inclusion composition within the scope of the present invention, and a stainless steel having a steel composition and / or an inclusion composition outside the scope of the present invention were manufactured and evaluated for corrosion resistance. The specific procedure is as follows.
First, a steel having the chemical composition shown in Table 1 was vacuumed while changing the amount of flux containing CaO and CaF ₂ in the range of 1 to ₂ kg and the processing time after adding the flux in the range of 5 to 30 minutes. It melted in a melting furnace and produced a 17 kg flat steel ingot. Thereafter, hot rolling, hot rolled sheet annealing, and shot were performed at a heating temperature of 1200 ° C. to a thickness of 4.5 mm, and cold rolling was performed to a thickness of 1 mm. Thereafter, finish annealing was performed according to the recrystallization temperature of the material to obtain a cold rolled annealed sheet.

[Inclusion composition]
The sample for L cross-section observation was created from the heat-rolled steel plate, and composition analysis using SEM-EDX was used. Composition analysis of the 20 oxide inclusions was carried out, inclusions containing both CaO and Al ₂ O ₃ were selected, and the average was used as the composition of the inclusions. The Ti oxide in the inclusions was determined as TiO ₂ .

  Next, the weathering resistance of the obtained stainless steel was evaluated. Corrosion resistance was evaluated by pitting potential and CCT (Cyclic Corrosion Test) as described below.

[Pitting potential]
A test piece having a width of 15 mm and a length of 20 mm was cut out from the cold-rolled steel plate and wet-polished to # 600 with emery paper. The periphery was coated with a resin so as to expose a central portion 10 mm × 10 mm of the plate, and used as an electrode for measurement. The pitting potential (V'c 100: the most noble potential with a current value exceeding 100 μA / cm ² ) was measured in accordance with JIS G0577 in a 3.5% aqueous NaCl solution at 30 ° C. Note that Ag / AgCl containing saturated KCl as an internal solution was used for the reference electrode, and the potential sweep rate was set to 20 mV / min. The number of test pieces was 5 and was evaluated by the average value of the pitting potential obtained. An average value of 150 mV or more was regarded as “good”, and less than 150 mV as “defective”.

[CCT]
Three test pieces each having a width of 70 mm and a length of 150 mm were cut out from the cold-rolled steel plate and wet-polished to # 600 using emery paper. After degreasing using acetone, the back and end faces were sealed and placed in the combined cycle tester. Thereafter, a cycle consisting of artificial sea water spray (35 ° C., 4 h) -dry (60 ° C., 2 h) -wet (50 ° C., 2 h) was performed three times. After the test piece was taken out of the testing machine, the surface appearance was evaluated for rating number (hereinafter, RN) in accordance with JIS G0595. The determined RN was 4 or more as good weather resistance and 3 or less as poor.

  Table 2 shows the composition of the oxide inclusions, the pitting potential and RN.

No. 1 to No. No. 12 had a steel composition and an inclusion composition within the scope of the present invention, and exhibited good characteristics in pitting potential and RN.
On the other hand, no. 13-No. No. 19 was out of the scope of the present invention in at least one of the steel composition and the inclusion composition, and at least one of the pitting potential or RN was defective.
In more detail, no. In No. 13, CaO / Al ₂ O ₃ was less than 0.75 and the pitting potential and RN were poor because the processing time after the flux addition was short. Si was below the lower limit of the range of the present invention, and Cr + 1.8Si was less than 16.0%.
No. In No. 14, CaO / Al ₂ O ₃ was less than 0.75 and the pitting potential and RN were poor because the processing time after the flux addition was short. Cr + 1.8Si was less than 16.0%.
No. Although No. 15 satisfied Cr + 1.8Si ≧ 16.0%, CaO / Al ₂ O ₃ was less than 0.75 and the RN was defective because the processing time after the addition of flux is short.
No. No. 16 had less than 16.0% of Cr + 1.8Si, and the pitting potential and RN were poor.
No. In No. 17, Ti was not more than the lower limit of the range of the present invention, and pitting potential and RN were poor.
No. In the case of No. 18, Al was not more than the lower limit of the range of the present invention, and SiO ₂ and TiO ₂ in the oxide inclusions were increased to suppress dissolution, so that RN was defective.
In No. 19, Ca was not more than the lower limit of the range of the present invention, and although the treatment time after the addition of the flux was not short, CaO / Al ₂ O ₃ was less than 0.75, and the RN was defective.
From the above results, it was found that by controlling the steel composition and the inclusion composition within the range of the present invention, it is possible to obtain a ferritic stainless steel that is inexpensive and has excellent corrosion resistance.

  The ferritic stainless steel excellent in weathering resistance of the present invention is suitable as a material for automobile exhaust system members, kitchen appliances for home and business use, and exterior members for buildings.

Claims (3)

C: 0.020% or less, Si: more than 1.0 to 3.5%, Mn: 0.02 to 0.80%, S: less than 0.002%, Cr: 15 to 15% by mass Less than 0%, Ti: 0.03 to 0.35%, Al: 0.002% or more and 0.5%, Ca: 0.0005 to 0.005%, N: 0.025% or less , CaO / Al ₂ O _{3 in} oxide inclusions containing Cr and 1.8 Si and having a chemical composition in which the balance is Fe and unavoidable impurities, and Cr + 1.8 Si and further containing Ca and Al. A ferritic stainless steel excellent in weathering resistance characterized in that the mass ratio is 0.75 or more.   Furthermore, Nb, 0.03 to 0.60%, Ni: 0.1 to 1.2%, Mo: 0.1 to 2%, W: 0.1 to 1%, V: 0 by mass%. 05 to 0.5%, Cu: 0.1 to 1.2%, Co: 0.01 to 0.5%, Sn: 0.001 to 0.5%, Sb: 0.001 to 0.5% And Zr: 0.001 to 0.3%, Ga: 0.0001 to 0.01%, Ta: 0.0001 to 0.01% and any one or more of them are contained. The ferritic stainless steel excellent in corrosion resistance according to claim 1.   Furthermore, in mass%, any one or two or more of Mg: 0.0002 to 0.005%, REM: 0.005 to 0.1%, B: 0.0002 to 0.005% The ferritic stainless steel excellent in corrosion resistance according to claim 1 or 2 characterized by containing.