JP3210255B2 - Ferritic stainless steel with excellent corrosion resistance and manufacturability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high alloy ferritic stainless steel excellent in corrosion resistance and manufacturability, and particularly suitable for use in an aqueous environment such as an electric jar pot, a body of an electric water heater, and a water tank member. is there.

[0002]

2. Description of the Related Art As ferritic stainless steels having high corrosion resistance, those containing a large amount of alloy components such as Cr and Mo and containing Nb to stabilize C and N are generally used. I have. In particular, in order to exhibit sufficient corrosion resistance even when used in a water environment, such as the body of an electric jar or an electric water heater, a water tank member, etc.
As in SUS444, high alloying of about Cr% + 3Mo% ≧ 22% is required. Incidentally, in order to stabilize C and N contained in the high alloy ferritic stainless steel, Nb and Ti are generally added.

[0003] However, the addition of Nb increases the recrystallization temperature of steel, which has been a major bottleneck in producing this steel efficiently. This is because recently, as a means of reducing the cost of stainless steel, attempts have been made to produce a cold-rolled stainless steel sheet using a continuous annealing line (hereinafter simply referred to as “CAL”) for cold-rolled ordinary steel sheet. And
This is because the upper limit of the annealing temperature possible in this line is about 900 ° C. That is, the recrystallization temperature of the Nb-added stainless steel depends on the amount of Nb added, and the C, N
Is stabilized by the addition of Nb, the recrystallization temperature of the steel exceeds 900 ° C., which deviates from the upper limit temperature possible with CAL. For this reason, even if the Nb-added steel is annealed with CAL, recrystallization becomes insufficient and workability is significantly deteriorated.

[0004] On the other hand, when Ti is added instead of Nb as a stabilizing element for C and N, the recrystallization temperature is lowered, but on the other hand, a large Ti carbonitride is formed. However, there is a problem that the toughness is deteriorated as compared with the case of
This deterioration of toughness is not a serious problem in the case of low alloy ferritic stainless steel, but in the case of high alloy ferritic stainless steel targeted in the present invention, the toughness is originally inferior and the adverse effect of the above-mentioned addition of Ti Are superimposed and the toughness is significantly deteriorated. As a result, when a relatively thick plate such as a hot rolled plate is handled, cracks are likely to occur, and there is a major problem in manufacturing. In addition, when Ti is added, a surface defect called Ti streak is generated on the surface of the slab, and if it is rolled as it is, the surface quality of the final product is deteriorated. For this reason, in order to manufacture a product having good surface properties, it is necessary to polish and remove these surface defects before rolling, which leads to an increase in the number of steps. As described above, the conventional Ti additive often causes a significant decrease in yield or an increase in process load, resulting in an increase in cost.Stabilizing such a high-alloy ferritic stainless steel with Ti is substantially required. It was a difficult situation.

In order to avoid such toughness deterioration caused by the addition of Ti, a method of adding Ti and Nb in combination is disclosed in Japanese Patent Laid-Open No. 6-2 / 1994.
No. 79951, JP-A-6-279953, JP-A-5-70899, JP-B-55-21102, JP-A-55-158254 and the like. However, even with these methods, if Ti is substituted by Nb to the extent that sufficiently high toughness can be obtained, the recrystallization temperature will still be 900 ° C. or higher. There has been a manufacturing problem that annealing cannot be performed in the process.

[0006]

As described above, in the case of Nb-added high-alloy ferritic stainless steel, the recrystallization temperature is high and CAL cannot be sufficiently annealed. Although the crystallization temperature is low, the toughness is poor, and cracks are likely to occur during manufacturing.
Deterioration of surface properties due to Ti streak, and Nb-
Even for ferritic stainless steel with Ti composite added, CA
L had a problem that sufficient annealing could not be performed.

The present invention solves the above-mentioned problems of the prior art and, despite being a high-alloy ferritic stainless steel having sufficient corrosion resistance, has good toughness and a low recrystallization temperature. An object of the present invention is to provide a ferritic stainless steel that can be annealed at a high temperature and has excellent manufacturability. Further, the present invention is a high alloy ferritic stainless steel having sufficient corrosion resistance, has good toughness, has a low recrystallization temperature, can be annealed with CAL, and has good surface properties. An object of the present invention is to provide a ferritic stainless steel having excellent resistance.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies for realizing the solution of the problems as described above, and as a result, Ti-
It has been found that such a problem can be solved by adding VB in a combined manner, controlling the mutual relationship between the added amounts of Ti and V, and the contents of C and N, and have completed the present invention. That is, the gist configuration of the present invention is as follows. (1) C: 0.02 mass% or less, Si: 0.6 mass% or less,
Mn: 0.6 mass% or less, P: 0.05 mass% or less, S:
0.01 mass% or less, Al: 0.15 mass% or less, N: 0.0
2 mass% or less, Ni: 0.6 mass% or less, Cr: 20.0 ma
More than ss% to less than 25.0 mass% Mo: 0.5 to 4.0 mass%, Ti: 0.02 to 0.3 mass%,
V: 0.02 to 0.3 mass%, B: 0.0002 to 0.005 mass%
And the contents (mass%) of C, N, Ti, and V in the above components are as follows: C + N ≦ 0.02 mass% (Ti + V) / (C + N) = 5 to 50 V / Ti ≧ 0.2 A ferritic stainless steel that is excellent in corrosion resistance and manufacturability, characterized in that it contains Fe and inevitable impurities.

(2) C: 0.02 mass% or less, Si: 0.6
mass% or less, Mn: 0.6 mass% or less, P: 0.05 mass
%, S: 0.01 mass% or less, Al: 0.15 mass% or less, N: 0.02 mass% or less, Ni: 0.6 mass% or less,
Cr: more than 20.0 mass% to less than 25.0 mass% Mo: 0.5 to 4.0 mass%, Ti: 0.02 to 0.3 mass%,
V: 0.02 to 0.3 mass%, B: 0.0002 to 0.005 mass% Ca ≦ 0.005 mass%, and among the above components, C,
The contents (mass%) of N, Ti, and V are as follows: C + N ≦ 0.02 mass% (Ti + V) / (C + N) = 5 to 50 V / Ti ≧ 0.2, the balance being Fe and inevitable impurities A ferritic stainless steel with excellent corrosion resistance and manufacturability, characterized by comprising:

(3) C: 0.02 mass% or less, Si: 0.6
mass% or less, Mn: 0.6 mass% or less, P: 0.05 mass
%, S: 0.01 mass% or less, Al: 0.15 mass% or less, N: 0.02 mass% or less, Ni: 0.6 mass% or less,
Cr: more than 20.0 mass% to less than 25.0 mass% Mo: 0.5 to 4.0 mass%, Ti: 0.02 to 0.3 mass%,
V: 0.02 to 0.3 mass%, B: 0.0002 to 0.005 mass%
And the contents (mass%) of C, N, Ti, and V in the above components are as follows: C + N ≦ 0.02 mass% (Ti + V) / (C + N) = 5 to 50 V / Ti ≧ 0.2 (7Ti + V) A ferritic stainless steel excellent in corrosion resistance and manufacturability, characterized in that the content satisfies × N ≦ 0.0100 and the balance consists of Fe and inevitable impurities.

(4) C: 0.02 mass% or less, Si: 0.6
mass% or less, Mn: 0.6 mass% or less, P: 0.05 mass
%, S: 0.01 mass% or less, Al: 0.15 mass% or less, N: 0.02 mass% or less, Ni: 0.6 mass% or less,
Cr: more than 20.0 mass% to less than 25.0 mass% Mo: 0.5 to 4.0 mass%, Ti: 0.02 to 0.3 mass%,
V: 0.02 to 0.3 mass%, B: 0.0002 to 0.005 mass% Ca ≦ 0.005 mass%, and among the above components, C,
The contents (mass%) of N, Ti and V satisfy the following relationship: C + N ≦ 0.02 mass% (Ti + V) / (C + N) = 5 to 50 V / Ti ≧ 0.2 (7Ti + V) × N ≦ 0.0100 Ferritic stainless steel with excellent corrosion resistance and manufacturability, characterized in that the balance consists of Fe and unavoidable impurities.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for setting the component composition in the present invention within the scope of the gist will be described below. C: 0.02 mass% or less C is an element that deteriorates corrosion resistance and toughness.
It must be limited to 0.02 mass% or less. Note that the content is preferably set to 0.01 mass% or less.

Si: 0.6 mass% or less Si is used as a deoxidizing agent during steelmaking of stainless steel, but if the content is too large, the toughness and workability are deteriorated. Therefore, the upper limit is set to 0.6 mass%.

Mn: 0.6% by mass or less Mn is added as a deoxidizing agent in the same manner as Si, but if contained in an excessively large amount, Mn becomes hard and deteriorates workability and is harmful to corrosion resistance. 6 mass%, preferably 0.3 mass%
And

P: 0.05 mass% or less P is an element that inevitably enters from steelmaking raw materials,
Since the stainless steel is hardened to deteriorate toughness and workability, the content needs to be 0.05 mass% or less.

S: 0.01 mass% or less S is also an element inevitably mixed from steelmaking raw materials, and forms sulfide to deteriorate corrosion resistance.
It is necessary to be 1 mass% or less, preferably 0.006 mass% or less.

Al: 0.15 mass% or less Al is added as a deoxidizing agent at the time of steel making. However, when the added amount is large, large Al-based inclusions are formed and cause surface defects. 15 mass%.

N: 0.02 mass% or less, C + N ≦ 0.0
2 mass% N is an element that deteriorates the corrosion resistance and toughness like C, and therefore it is necessary to limit it to 0.02 mass% or less.
Note that the content is preferably set to 0.01 mass% or less. Further, in the high alloy ferritic stainless steel targeted by the present invention, in order to further exert these effects, it is necessary to further restrict the total amount of C and N to 0.02 mass% or less.

Cr: more than 20.0 mass% to less than 25.0 mass% Cr is basically an essential element for stainless steel in order to impart corrosion resistance to steel. As the Cr content increases, the corrosion resistance improves, but the toughness deteriorates. In particular, in order to obtain sufficient corrosion resistance in a water environment, the Cr content exceeds 20.0 mass%.
Addition is required. However, if the content is 25.0 mass% or more, it is difficult to avoid toughness deterioration due to the addition of Ti, so the upper limit is set to less than 25.0 mass%.

Ni: 0.6% by mass or less Ni is an element that improves the toughness of ferritic stainless steel. However, as the amount of addition increases, the hardness increases, the workability is deteriorated, and the cost is relatively high. Therefore, the cost is increased, so the upper limit is 0.6 mass%, preferably 0.3 mass%.
And

Mo: 0.5 to 4.0 mass% Mo is an element which remarkably improves the corrosion resistance of stainless steel, and in particular, 0.5 mass% or more is essential in a water environment. However, if the addition amount is too large, the stainless steel is hardened and the toughness and workability are deteriorated, so the upper limit is set to 4.0 mass%.

Ti: 0.02 to 0.3 mass% Ti is a particularly important element in the present invention, and improves the corrosion resistance of stainless steel due to its strong C, N stabilizing effect. However, if too much is added, coarse Ti
Precipitation of N deteriorates toughness and surface properties. Therefore, the addition amount of Ti is 0.02 to 0.3 mass
% Range.

V: 0.02 to 0.3 mass% V / Ti ≧ 0.2 (Ti + V) / (C + N) = 5 to 50 V are also particularly important elements in the present invention. originally,
V is a kind of C, N stabilizing element, but in the present invention, a part of Ti is replaced by V and further added in combination with B, thereby significantly improving toughness at a high alloy level. The inventors obtained a result as shown in FIG. 1 for this. That is, from FIG. 1, the brittle transition temperature is B
The Ti-VB system to which Ti is added, the lower the temperature, the higher the V / Ti, and the brittle transition temperature of all the Ti-VB composite-added steels according to the present invention is 0 ° C. or lower, and the productivity is extremely high. It turns out that it becomes favorable. In the case of adding a Ti-V composite, since the recrystallization temperature is not increased unlike the case of adding a Ti-Nb composite, it is also possible to perform annealing with CAL which is advantageous in cost. In order to obtain such an effect, V needs to be added at least 0.02 mass%.
It is necessary to satisfy the relationship of /Ti≧0.2. However, if added in a large amount, it becomes hard and deteriorates workability. Therefore, the upper limit is set to 0.3 mass%. In addition to these conditions,
In order to further fix C and N sufficiently, Ti and V are set in the above range, and (Ti + V) / (C + N) = 5 to 50
Care must be taken to satisfy the relationship.

B: 0.0002 to 0.005 mass% B is also a particularly important element in the present invention. This B
Has an effect of further enhancing the effect of improving the toughness by adding a Ti-V composite. Although the mechanism is not always fully understood, TiN cannot be sufficiently suppressed by V alone.
It is considered that the precipitation of is suppressed by the formation of BN. B also exerts an effect even when added in a small amount, so that the influence on the grain boundary strength may be considered. In order to obtain such a combined effect of B, at least 0.0002
It is necessary to add more than mass%. However, if added in an excessively large amount, the hot workability deteriorates, so the upper limit is made 0.005 mass%.

Ca: 0.005 mass% or less Ca is an element for improving nozzle clogging which is likely to occur during continuous casting of Ti-added steel, and is added as necessary. However, if the added amount is too large, the corrosion resistance is deteriorated, so the upper limit is made 0.005 mass%. The content of Ca as an impurity is usually about 0.0002 mass%.

(7Ti + V) × N ≦ 0.0100 There is a good correlation between the parameter (7Ti + V) × N and the surface flaw of the slab, and when this value exceeds 0.0100 (mass%), the surface of the slab Flaws are likely to become apparent. Therefore, when economically producing a product having good surface properties, it is preferable to adjust the components so as to satisfy (7Ti + V) × N ≦ 0.0100.

[0027]

EXAMPLE A 50 kg steel ingot having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, and heated to 1200 ° C. and hot-rolled repeatedly to obtain a 4 mm-thick steel sheet. I got From this hot-rolled sheet, a Charpy impact test specimen was sampled, subjected to an impact test according to JIS Z2242, and the brittle fracture rate was measured. From the results obtained, the temperature at which the brittle fracture rate was 50% was defined as the brittle transition temperature. In addition,
The brittle transition temperature has a good correlation with cracking during manufacturing, and it has been confirmed that cracking occurs when this temperature exceeds 0 ° C.

Next, the hot-rolled sheet was annealed at 1050 ° C. for 1 minute to remove scale, and then cold-rolled to a thickness of 1.2 mm. When the obtained cold-rolled sheet was annealed for 30 seconds while changing the temperature, the temperature at which the material rapidly softened and the hardness became almost constant was determined as the recrystallization temperature. Furthermore, each cold-rolled sheet was annealed at the determined recrystallization temperature to obtain a cold-rolled annealed sheet.
From this cold-rolled annealed plate, a test piece for measuring electric potential of 20 mm × 20 mm was cut out, and the surface was polished by wet # 800, followed by 3.5% NaCl, 70 according to JIS G0577.
The pitting potential was measured under the condition of ° C. Moreover, about some test materials, the surface property was investigated by grind | polishing the surface of the slab about 1 mm with a grinder, and visually counting the flaw of 1 mm or more by the penetrant flaw detection method. The obtained results are also shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

From Table 2, it can be seen that all of the inventive materials obtained by adding the Ti-VB composite have good toughness (brittle transition temperature of 0 ° C. or less) and a recrystallization temperature of 900 ° C. or less. In addition, the inventive material has sufficient pitting corrosion resistance as a high alloy ferritic stainless steel, and exhibits corrosion resistance comparable to conventional materials. In particular, the parameter (7Ti + V)
It can be seen that those with adjusted × N show excellent surface properties. On the other hand, the conventional Nb-added steel and Nb-Ti composite-added steel have a recrystallization temperature of 950 ° C. or higher.

[0032]

As described above, according to the ferritic stainless steel according to the present invention, the brittle transition temperature is zero.
° C or less, and the recrystallization temperature is also reduced to 900 ° C or less, so that good manufacturability can be provided without impairing corrosion resistance. Therefore, according to the ferritic stainless steel according to the present invention, annealing can be performed using the cold-rolled sheet annealing line (CAL) of ordinary steel, so that productivity,
It is very economical. Further, in the present invention, by adjusting (7Ti + V) × N, it is possible to provide a ferritic stainless steel having more excellent surface properties in addition to the above effects.

[Brief description of the drawings]

Fig. 1 Influence of component system and V / Ti on brittle transition temperature of hot rolled sheet
5 is a graph showing the effect of the above.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Makoto Kitazawa 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (72) Inventor Susumu Suto 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba (56) References JP-A-6-158233 (JP, A) JP-A-4-228542 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38 / 00 C22C 38/54

Claims (4)

(57) [Claims] [Claim 1] C: 0.02 mass% or less, Si: 0.6 mass% or less, Mn: 0.6 mass% or less, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.15 mass% or less, N: 0.02 mass% or less, Ni: 0.6 mass% or less, Cr: more than 20.0 mass% to less than 25.0 mass% Mo: 0.5 to 4.0 mass%, Ti: 0.02 to 0.3 mass%, V: 0.02 to 0.3 mass%, B: 0.0002 to The content of C, N, Ti, and V (mass%) among the above components is as follows: C + N ≦ 0.02 mass% (Ti + V) / (C + N) = 5 to 50 V / Ti ≧ 0.2 A ferritic stainless steel excellent in corrosion resistance and manufacturability, characterized by containing and containing Fe and inevitable impurities. 2. C: 0.02 mass% or less, Si: 0.6 mass% or less, Mn: 0.6 mass% or less, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.15 mass% or less, N: 0.02 mass% or less, Ni: 0.6 mass% or less, Cr: more than 20.0 mass% to less than 25.0 mass% Mo: 0.5 to 4.0 mass%, Ti: 0.02 to 0.3 mass%, V: 0.02 to 0.3 mass%, B: 0.0002 to 0.005 mass% Ca ≦ 0.005 mass%, and among the above components, the contents (mass%) of C, N, Ti, and V are as follows: C + N ≦ 0.02 mass% (Ti + V) / (C + N) = 5 to 50 A ferritic stainless steel excellent in corrosion resistance and manufacturability, characterized by satisfying V / Ti ≧ 0.2, with the balance being Fe and unavoidable impurities. C: 0.02 mass% or less, Si: 0.6 mass% or less, Mn: 0.6 mass% or less, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.15 mass% or less, N: 0.02 mass% or less, Ni: 0.6 mass% or less, Cr: more than 20.0 mass% to less than 25.0 mass% Mo: 0.5 to 4.0 mass%, Ti: 0.02 to 0.3 mass%, V: 0.02 to 0.3 mass%, B: 0.0002 to The content of C, N, Ti, and V (mass%) among the above components is as follows: C + N ≦ 0.02 mass% (Ti + V) / (C + N) = 5 to 50 V / Ti ≧ 0.2 (7Ti + V) × N ≦ 0.0100. Ferritic stainless steel excellent in corrosion resistance and manufacturability, characterized in that the balance is Fe and unavoidable impurities. C: 0.02% by mass or less, Si: 0.6% by mass or less, Mn: 0.6% by mass or less, P: 0.05% by mass or less, S: 0.01% by mass or less, Al: 0.15% by mass or less, N: 0.02% mass% or less, Ni: 0.6 mass% or less, Cr: more than 20.0 mass% to less than 25.0 mass% Mo: 0.5 to 4.0 mass%, Ti: 0.02 to 0.3 mass%, V: 0.02 to 0.3 mass%, B: 0.0002 to 0.005 mass% Ca ≦ 0.005 mass%, and among the above components, the contents (mass%) of C, N, Ti, and V are as follows: C + N ≦ 0.02 mass% (Ti + V) / (C + N) = 5 to 50 A ferritic stainless steel excellent in corrosion resistance and manufacturability, characterized by satisfying V / Ti ≧ 0.2 (7Ti + V) × N ≦ 0.0100, with the balance being Fe and inevitable impurities.