JPH09279312A - Ferritic stainless steel excellent in high temperature characteristic, corrosion resistance, and workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel combining high temp. characteristics, corrosion resistance, and excellent workability, for use in automobile exhaust system, electric power plant, etc. SOLUTION: This steel has a composition consisting of, by weight, <=0.020% C, <=0.020% N, 0.1-2.0% Si, 0.1-2.0% Mn, 10.0-30.0% Cr, <=3.0% Mo, <=0.10% Al, <0.10% Cu, <0.10% V, 0.0003-0.0050% B, further either or both of 0.01-0.5% Ti and 0.02-0.8% Nb, and the balance iron with inevitable impurities and satisfying B<=10.8×(N-Ti/48-Al/27)/14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel used for members such as an automobile exhaust system and a power plant which are required to have high temperature characteristics and corrosion resistance as well as workability.

[0002]

2. Description of the Related Art Ferritic stainless steels are less expensive than austenitic stainless steels because they do not contain Ni, but they are inferior in terms of workability, toughness and high temperature strength. On the other hand, ferritic stainless steel has a small amount of thermal expansion, and is therefore used in applications where there is concern about thermal strain in roof materials of buildings, exhaust manifolds of automobile exhaust systems, and the like. However, severe workability is also required for these applications. For example, in steel pipes for exhaust manifolds to which Mo or Nb is added to add high heat resistance, strict workability such as pipe expandability after pipe making is required,
A step such as heat treatment must be added during processing.

Further, Japanese Patent Laid-Open No. 61-261460 discloses that B addition is effective for improving the secondary workability of ferritic stainless steel. However, with respect to heat resistance and corrosion resistance, this is not a component design that fully considers application in the current severe environment. In recent years, environmental problems have been highlighted, and in order to improve maintenance-free and combustion efficiency, improvement of heat resistance and corrosion resistance of materials is desired.

From this viewpoint, a ferritic stainless steel containing a relatively large amount of Mo or Nb has been developed. For example, JP-A-2-175843 and JP-A-3-27
For example, in order to improve high temperature characteristics, Japanese Patent No. 4245 discloses
It is disclosed that a large amount of Mo or Nb is added. However, if Mo or Nb is added excessively, toughness and workability deteriorate.

In order to avoid this, JP-A-57-13
4542 discloses that hot-rolled sheet is annealed during manufacturing in order to improve workability, and JP-A-3-274245 discloses that Cu is added in an amount of 0.1 to 2.5% to improve toughness. ing.

[0006]

As described above, the ferritic stainless steel to which Mo or Nb is added in order to improve the high temperature characteristics and the corrosion resistance is a hot rolled sheet annealed or Cu in order to improve the workability and toughness. Take measures to add expensive elements such as. Therefore, the object of the present invention is to perform hot-rolled sheet annealing and C
The ductility required for automobile exhaust system parts, civil engineering and construction steel materials, etc. by measures that have relatively little cost increase without taking measures that involve a significant cost increase such as adding expensive elements such as u , R value and workability such as pipe expandability in steel pipe are improved, and further considering high temperature members such as power plants, high temperature creep deformation is a big problem, 1000h creep strength at 700 ℃ is the minimum. However, it is an object of the present invention to provide a ferritic stainless steel that satisfies 20 MPa or more and is excellent in high temperature characteristics, corrosion resistance and workability.

[0007]

SUMMARY OF THE INVENTION The present invention is as follows. (1) C: 0.020% or less by weight%, N:
0.020% or less, Si: 0.1 to 2.0%, M
n: 0.1 to 2.0%, Cr: 10.0 to 30.0%,
Mo: 3.0% or less, Al: 0.10% or less, C
u: less than 0.10%, V: less than 0.10%, B
: 0.0003 to 0.0050%, Ti: 0.01 to 0.5%, Nb: 0.02
0.8% of 1 type or more is included and B <= 10.8x (N-T
i / 48-Al / 27) / 14, the balance consisting of iron and unavoidable impurities, and a ferritic stainless steel excellent in heat resistance, corrosion resistance and workability.

(2) C: 0.020% or less, N: 0.020% or less, Si: 0.1 to 0.1% by weight.
Less than 0.8%, Mn: 0.1 to less than 0.8%, Cr:
Over 17.0 to 30.0%, Mo: 0.4 to 3.0%, A
l: 0.20% or less, Cu: less than 0.10%,
V: less than 0.10%, B: 0.0003-
0.0050%, and Ti: 0.01-
0.5%, Nb: contains at least one of 0.02 to 0.8%, and B ≦ 10.8 × (N-Ti / 48-Al /
27) / 14, the balance being iron and inevitable impurities, and ferritic stainless steel excellent in high temperature characteristics, corrosion resistance and workability.

(3) C: 0.020% or less, N: 0.020% or less, Si: more than 0.7% by weight.
1.0%, Mn: over 0.8 to 1.0%, Cr: 1
More than 3.5-30.0%, Mo: 0.4-3.0%, A
l: 0.20% or less, Cu: less than 0.10%,
V: less than 0.10%, B: 0.0003-
0.0050%, and either one of Ti or both Ti and Nb, Ti: 0.01
.About.0.5%, Nb: 0.2 to 0.8%, and B.ltoreq.10.8.times. (N-Ti / 48-Al / 27) /
14, Ti / 48 ≧ C / 12 + N / 14, Cu + V ≦ 5
00 × B, the balance consisting of iron and unavoidable impurities, and the creep rupture strength at 700 ° C. for 1000 hours is 20M.
Ferritic stainless steel having Pa or higher and excellent in high temperature characteristics, corrosion resistance and workability.

(4) C: 0.010% or less, N: 0.010% or less, Si: 0.1 to 0.
Less than 8%, Mn: 0.1 to less than 0.8%, Cr: 10.
0-20.0%, Mo: 0.05-2.0%, Al:
0.20% or less, Cu: less than 0.10%, V:
Less than 0.10%, B: 0.0003 to 0.00
50%, and is either one of Ti or both Ti and Nb, and Ti: 0.01 to 0.5.
%, Nb: 0.2 to 0.8%, and B ≦
10.8 x (N-Ti / 48-Al / 27) / 14, T
i / 48 + Nb / 93 ≧ C / 12 + N / 14, Cu + V
Ferritic stainless steel with ≦ 500 × B and the balance consisting of iron and unavoidable impurities and having excellent high temperature characteristics, corrosion resistance and workability.

(5) An automobile exhaust system component made of the ferritic stainless steel plate or steel pipe according to any one of (1) to (4).

(6) A power-generating member made of a ferritic stainless steel plate or steel pipe according to any one of (1) to (4) above.

(7) A member for civil engineering and construction made of the ferritic stainless steel plate or steel pipe according to any one of (1) to (4).

The ferritic stainless steel described in (2) is particularly corrosion resistant, the ferritic stainless steel described in (3) is particularly heat resistant, and the ferritic stainless steel described in (4) is particularly workable. Are better.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In order to improve the workability of ferritic stainless steel to which Mo or Nb is added in order to improve heat resistance and corrosion resistance, in the present invention, in particular, B,
Focused on Cu and V. The present inventors added B to Cu
The inventors have found a method of dramatically improving the workability of ferritic stainless steel containing Mo and Nb by limiting the addition amount of V and V. That is, in the ferritic stainless steel containing Mo and Nb, the increase in the solid solution amount of B in the state where the contents of Cu and V are suppressed means that the workability can be improved more than the addition of B alone. I found it.

The reasons for limiting the components of the steel of the present invention will be described below. B: It greatly contributes to the improvement of r value and ductility. However,
Since the effect is exhibited in the solid solution state, the relationship between N and Ti and Al, which have a higher nitride forming ability than B, is B ≦ 10.8 (N-Ti / 48-Al / 27) / 1.
4 must be met. Further, when the addition amount of Mo or Nb is particularly large, the workability is not sufficiently improved only by adding B. Therefore, it is desirable to satisfy Cu + V ≦ 500 × B in relation to Cu and V.

Cu: Improves corrosion resistance but is also a strengthening element. 0.1% in order to fully bring out the effect of B addition
It is necessary to: Further, in order to further improve the workability, it is desirable to satisfy Cu + V ≦ 500 × B.

V: Has a strong affinity with C and N to form carbonitrides, and thus generally contributes to improvement of workability. However, when Ti and Nb are added, V is Nb,
Since it has a weaker affinity for C and N than Ti, Ti or N
In the case of complex addition with b, most of them are solid solutions. In this case, the workability does not have a good effect. Also, the oxidation resistance is deteriorated. Further, in order to exert the workability improving effect of B, the content has to be regulated. Therefore,
It should be 0.1% or less. In order to further improve the workability, it is desirable to satisfy Cu + V ≦ 500 × B.

C: 0.020% or less because it is an element that deteriorates workability and promotes intergranular corrosion. Further, it is preferably low in order to improve workability and intergranular corrosion resistance, and when 0.010% or less, workability and intergranular corrosion resistance are improved.

N: Since it is an austenite former in addition to degrading workability, it was set to 0.020% or less. In addition, it is desirable to be low for improving workability, and 0.010%
The workability becomes good in the following.

Si: Effective as a deoxidizer. Further, although it improves the oxidation resistance and the high temperature salt resistance, it deteriorates the workability, so the content was made 0.1% to 2.0%. Further, it is preferably less than 0.8% from the viewpoint of workability, and more than 0.7% is desirable in consideration of heat resistance.

Mn: Effective as a deoxidizer. Further, in order to deteriorate the oxidation resistance and workability, 0.1% to 2.
0%. Further, it is preferably less than 0.8% from the viewpoint of workability, and is 1.0% or less in consideration of heat resistance.

Cr: Corrosion resistance and oxidation resistance are improved, but the addition amount increases 10.0 to 30.
0%. From the viewpoint of ensuring corrosion resistance, addition of more than 17% is desirable, and considering the oxidation resistance at 900 to 1000 ° C., it is good in the region of more than 13.5%. Further, from the viewpoint of workability, it can be said that 20% or less is a desirable range.

Mo: An additive element effective for corrosion resistance, high temperature salt damage resistance, high temperature strength and oxidation resistance, but excessive addition thereof causes a marked deterioration in workability. Therefore, 0.05 to 3.0%
And From the balance of these characteristics, 0.4-
2.0% is desirable.

Al: Particularly effective as a strong deoxidizer,
Since the addition of more than 0.20% is meaningless from the viewpoint of oxygen reduction, this was set as the condition. Further, from the viewpoint of oxidation resistance and high temperature salt damage resistance, it is desirable to add 0.010% or more.

Nb: Improves high-temperature strength and workability, but from the viewpoint of ensuring toughness, it was set to 0.02% to 0.80%. Especially when high temperature strength is required, the content of 0.2% or more is desirable.

Ti: The workability is improved, but from the viewpoint of preventing the occurrence of flaws, it is set to 0.01 to 0.5%. Further, from the viewpoint of workability and intergranular corrosion prevention, at least Ti / 48 ≧
It is desirable to add C / 12 + N / 14. In addition, when combined with Nb for heat resistance, Ti /
Addition satisfying 48 + Nb / 93 ≧ C / 12 + N / 14 is desirable.

[0028]

【Example】

[Example 1] Each of the steels shown in Table 1 was melted in a vacuum high-frequency furnace in an amount of 20 kg, and these steel ingots were heated at 1250 ° C.
Hot rolled to a thickness of 4 mm, pickled and cold rolled to a thickness of 1 mm
Then, a cold rolled sheet annealed at 900 to 1000 ° C. was produced. Then, each material evaluation test of high temperature creep property, oxidation resistance, corrosion resistance and normal temperature tensile strength was performed. The conditions for each test are as follows.

High temperature creep properties: 1000 at 700 ° C
The breaking strength of h was measured. Very good with 30 MPa or more (⊚), good with 20 to 30 MPa (∘), 20M
Less than Pa was judged to be defective (x mark).

Oxidation resistance: 1000 in dry air
After continuously heating at 100 ° C. for 100 hours, it was examined whether abnormal oxidation occurred. Further, the amount of increase in oxidation was also investigated, and 2 mg / cm ² or less was very good (⊚), ² to 10 mg / cm ² was good (○), 10
More than mg / cm ² was regarded as a defect (marked with X). In most cases, abnormal oxidation occurs when the increased oxidation amount exceeds 10 mg / cm ² .

Corrosion resistance: A one-month exposure test was conducted in the coastal area to examine the presence or absence of rust. The rusted area ratio was measured by image analysis, and was very good when it was less than 0.1% (◎
Mark), 0.1. 5% was good (◯), and more than 0.5% was bad (x).

Normal temperature tensile test and workability evaluation: elongation at break and r value were measured, and elongation at break was 29% or more, and r value was 1.0 or more, which was considered good. Further, a TIG welded pipe having an inner diameter of 40 mm was produced, and when a 1.4 D pipe was expanded at room temperature, it was examined whether or not cracking occurred.

Table 2 shows the test results. As is clear from Table 2, the steel of the present invention has good characteristics such as high temperature creep strength, oxidation resistance, corrosion resistance and workability. Further, P2, 3, 4 and 7 in the component range described in claim 2 have extremely good corrosion resistance. Despite having a relatively low Cr content, P1 satisfies the component range of claim 3 and is excellent in high temperature creep and oxidation resistance. Furthermore, P1 and P2 satisfy the component conditions described in claim 4, and exhibit excellent characteristics in both fracture ductility and r value in normal temperature tensile.

C1 containing less B than P1 has good high-temperature characteristics and corrosion resistance, but is inferior in elongation at break, r-value and hole expandability of steel pipe. Further, C2 and C3 to which an excessive amount of B is added have many cracks during the hot rolling, which does not lead to the production of a cold rolled steel sheet. This also applies to C7 in which Nb and V are excessively added. C3 and C4 have good high-temperature characteristics and corrosion resistance, but have poor workability because they contain Cu. Further, C6 has a large amount of V added and is inferior in oxidation resistance and workability. Regarding C8, the workability is low due to the addition of excessive Mo. C9 has excellent workability by adding B, but does not contain Mo and has a small amount of Nb.
High temperature creep and poor oxidation resistance. Also, C10 is
A large amount of C and N added, and a small amount of B added. Furthermore, M
o Since it is not added, it is inferior in workability and corrosion resistance.

Based on the above evaluation results, in FIG. 1, it is possible to expand the steel pipe on the matrix of the amount of B added and (Cu + V) / 500 (◯: workable, Δ: partial work cracking, × : Frequent occurrence of processing cracks). The amount of B added is (C
It can be seen that the workability is good in a region larger than the value of (u + V) / 500.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

The ferritic stainless steel of the present invention is
It is possible to obtain high temperature characteristics and corrosion resistance as well as excellent workability.

[Brief description of drawings]

FIG. 1 is a chart showing whether or not expansion of steel pipes is possible (○: machinable, Δ: partial work cracks occur, x: work cracks occur frequently) on a matrix of B addition amount and (Cu + V) / 500. is there.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuo Sumitomo 1-1 Hibahata-cho, Tobata-ku, Kitakyushu City Nippon Steel Yawata Works Co., Ltd.

Claims (7)

[Claims] 1. By weight%, C: 0.020% or less, N: 0.020% or less, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, Cr: 10 0.0 to 30.0%, Mo: 0.05 to 3.0%, Al: 0.20% or less, Cu: less than 0.10%, V: less than 0.10%, B: 0.0003 to 0 0.0050%, Ti: 0.01 to 0.5%, Nb: 0.02 to 0.8%, and at least one of B ≦ 10.8 × (N-Ti / 48-Al / 27) / 14
And the balance is iron and unavoidable impurities, and ferritic stainless steel excellent in high temperature characteristics, corrosion resistance and workability. 2. By weight%, C: 0.020% or less, N: 0.020% or less, Si: 0.1 to less than 0.8%, Mn: 0.1 to less than 0.8%, Cr : More than 17.0 to 30.0%, Mo: 0.4 to 3.0% Al: 0.20% or less, Cu: less than 0.10%, V: less than 0.10%, B: 0.0003 To 0.0050%, and further contains at least one of Ti: 0.01 to 0.5%, Nb: 0.02 to 0.8%, and B ≦ 10.8 × (N -Ti / 48-Al / 27) / 14
And the balance is iron and unavoidable impurities, and ferritic stainless steel excellent in high temperature characteristics, corrosion resistance and workability. 3. In% by weight, C: 0.020% or less, N: 0.020% or less, Si: more than 0.7 to 1.0%, Mn: more than 0.8 to 1.0%, Cr. : More than 13.5 to 30.0%, Mo: 0.4 to 3.0%, Al: 0.20% or less, Cu: less than 0.10%, V: less than 0.10%, B: 0. 0003 to 0.0050%, and either one of Ti or both Ti and Nb, Ti: 0.01 to 0.5%, Nb: 0.2 to 0.8 %, And B ≦ 10.8 × (N-Ti / 48-Al / 27) / 1
4, Ti / 48 ≧ C / 12 + N / 14, Cu + V ≦ 500 × B, the balance consisting of iron and unavoidable impurities, creep rupture strength of 1000 MPa at 700 ° C. of 20 MPa or more, high temperature characteristics, corrosion resistance and workability. Excellent ferritic stainless steel. 4. By weight%, C: 0.010% or less, N: 0.010% or less, Si: 0.1 to less than 0.8%, Mn: 0.1 to less than 0.8%, Cr. : 10.0 to 20.0%, Mo: 0.05 to 2.0%, Al: 0.20% or less, Cu: less than 0.10%, V: less than 0.10%, B: 0.0003 .About.0.0050%, and either one of Ti or both Ti and Nb, Ti: 0.01 to 0.5%, Nb: 0.2 to 0.8% And B ≦ 10.8 × (N-Ti / 48-Al / 27) / 1
4, Ti / 48 + Nb / 93 ≧ C / 12 + N / 14, Cu + V ≦ 500 × B, and the balance consisting of iron and unavoidable impurities, a ferritic stainless steel excellent in high temperature characteristics, corrosion resistance and workability. 5. An automobile exhaust system component made of the ferritic stainless steel steel plate or steel pipe according to any one of claims 1 to 4. 6. A power generating member made of the ferritic stainless steel steel plate or steel pipe according to claim 1. 7. A member for civil engineering and construction made of the ferritic stainless steel steel plate or steel pipe according to any one of claims 1 to 4.