JP3508520B2 - Cr-containing ferritic steel with excellent high-temperature fatigue properties for welds - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Cr-containing ferritic steel which is suitable for use in high temperature members and has excellent high temperature fatigue properties, particularly excellent high temperature fatigue properties of welds.

[0002]

2. Description of the Related Art Ferritic stainless steel is used for members used at high temperatures such as exhaust system members for automobiles, exhaust duct members for power plants, and members for petroleum combustion equipment. The reason for this is that ferritic stainless steel is cheaper than austenitic stainless steel, and has a small coefficient of thermal expansion and excellent thermal fatigue properties. Here, the high temperature fatigue property is an important property for a high temperature member in view of (1) improvement of durability and (2) weight reduction due to reduction of plate thickness.

By the way, some attempts have been made in the past to improve mechanical properties at high temperature. For example, Japanese Patent Laid-Open No. 8-291374 discloses an improvement in high temperature fatigue property of a base material. Japanese Patent Application Laid-Open No. 9-118961 discloses a technique for suppressing the decrease in high temperature strength during use.

[0004]

However, all of these conventional techniques are effective for improving the durability of only the base metal portion, but the characteristics of the welded portion required for many welded structural members. It didn't respond to the improvement. This is because, among the above-mentioned welded parts, the bond part can be made stronger by optimizing the filler material, but in the weld heat affected zone, it is unavoidable that the high temperature strength is lowered. Met. For example, in conventional ferritic stainless steel, as shown in FIG.
Comparing the high temperature fatigue properties at 0 ℃, the fatigue limit of the weld heat affected zone at 10 ⁶ cycle life is reduced to about 60% of that of the base metal. Here, the material equivalent to the welding heat affected zone is 115O in order to simulate the structure of the welding heat affected zone.
It was annealed at 1 ° C. for 1 minute (hereinafter the same). Fatigue characteristics show the results that take into account the decrease in high-temperature strength over time during use, and are considered to represent the overall characteristics during product use. It is suitable for evaluation.

As described above, the product obtained by subjecting the ferritic stainless steel to the welding process requires the high temperature fatigue characteristics of the welded part, particularly the heat affected zone, but in the conventional material, the welded part The properties were remarkably inferior to those of the base material, and they did not have sufficient properties for practical use. In addition, the conventional material has a problem that the workability represented by ductility is not sufficient. Then, the objective of this invention is solving the problem which the said known technique had, and providing a Cr containing ferritic steel which is excellent in the high temperature fatigue characteristic of a weld zone, especially a weld heat affected zone. Further, another object of the present invention is excellent in workability in addition to the high temperature fatigue characteristics of the weld heat affected zone.
To propose a ferritic steel containing Cr.

[0006]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventors have made a detailed study on the influence of various components on the high temperature fatigue characteristics (high temperature fatigue strength) of the weld heat affected zone. . As a result, for the above characteristics, the total oxygen content (which was conventionally treated as “O” unless otherwise specified)
In addition to the above, it was found that the amounts of oxygen existing as Al ₂ O ₃ had a great influence on each other, and the present invention was completed.

That is, the present invention has the following structures. (1) C: 0.03 wt% or less, Si: 3.0 wt% or less, Mn: 0.58 wt
% Or less, P: 0.06 wt% or less, S: 0.01 wt% or less, Cr: 1.
0 to 20.0 wt%, Al: 0.05 wt% or less, N: 0.030 wt% or less,
B: 0.0002 to 0.005 wt%, Nb: 1.0 wt% or less, O: 0.015
-(48/54) x insol.Al wt% or less, and the balance consisting of Fe and inevitable impurities, a Cr-containing ferritic steel excellent in high temperature fatigue properties of welds.

[0008]

[0009]

(2) In the steel described in (1) above ,
Contains one or more selected from Co: 1.0 wt% or less, Cu: 2.0 wt% or less, Mo: 3.0 wt% or less, Ni: 2.0 wt% or less, with the balance being Fe and unavoidable A Cr-containing ferritic steel excellent in high temperature fatigue properties of welds, which is characterized by containing impurities.

(3) In the steel described in (1) or (2) above , Ca: 0.0003 to 0.003 wt% is further contained, and the balance is Fe.
And a ferritic steel containing Cr, which is excellent in high temperature fatigue properties of welds, characterized by comprising unavoidable impurities.

(4) The steel according to any one of (1) to (3) above , further comprising REM: 0.001 to 0.1 wt%, the balance being Fe and inevitable impurities. Cr-containing ferritic steel with excellent high temperature fatigue properties for welded parts.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION First, the experimental results of investigating the effect of oxygen content on high temperature fatigue characteristics in the present invention will be described. In Figure 2, the composition is almost the same,
The high temperature fatigue properties of the base metal are similar, 0.005 wt% C-0.
16wt% Si-0.09wt% Mn-0.025wt% P-0.004wt% S-
11.8 wt% Cr-0.010 wt% Al-0.43 Nb-0.009 wt% N-0.
Steel consisting of 009 wt% O (Steel C) and 0.005 wt% C
-0.15 wt% Si-0.08 wt% Mn-0.024 wt% P-0.002 wt%
S-11.2wt% Cr-0.004 wt% Al-0.35Nb-0.007 wt% N
For Steel (Steel 1) consisting of -0.008 wt% O, as a high-temperature fatigue properties of welded heat affected zone equivalent material, ^(106-cycle fatigue limit of welded heat affected zone equivalent material) / ^(106-cycle fatigue limit of the base metal ) Is the result of examining the fatigue strength ratio defined at). As shown in FIG. 2, there was a large difference in the fatigue strength ratio between Steel C and Steel 1.

Then, the inventors have investigated the cause of the difference in the fatigue strength ratio (P) between the two steels, and found that the two have a difference in the amount of Al ₂ O ₃ in the steels. FIG. 3 shows the result of a more detailed study focusing on this point. As shown in FIG. 3, in order to increase the fatigue strength ratio, it is important that a certain relationship be satisfied between the oxygen amount as Al ₂ O ₃ and the total oxygen amount (O). all right. In order to increase the fatigue strength ratio, when the oxygen amount as Al ₂ O ₃ is expressed by (48/54) × acid-insoluble Al (insol Al), O ≦
If there is a relationship of 0.015 − (48/54) × (insol Al), the decrease in high temperature fatigue is small even in the heat-affected zone of the weld, and the fatigue limit of the base metal is 70
It was found that the percentage exceeded.

The reason why such a phenomenon is observed is not always clear, but it is considered as follows. In the heat-affected zone of welding, coarse Al ₂ O ₃ exists stably even when subjected to heat history. On the other hand, most of Nb, Ti, Zr, V, Cr carbides, Nb Laves phase (Nb-Fe intermetallic compound), etc. are solid-solved in the heat history of the heat affected zone. After that, solid solution Nb,
When Ti, Zr, V, and Cr are loaded at high temperature and precipitate as carbides or Nb Laves phase, coarse Al ₂ O ₃ becomes the core of these reprecipitations, resulting in coarse grains. It tends to exist on the Al ₂ O ₃ surface. In this way, it is considered that the effect of improving the fatigue limit due to fine precipitates of Nb, Ti, Zr, and Cr does not work in the weld heat affected zone. On the other hand, Al ₂ O
_{When the} amount of ₃ is small, many oxides also dissolve when exposed to welding heat history, and Nb, Ti, Zr, Cr under high temperature fatigue.
When each of these reprecipitates, each finely precipitates. It is considered that the strengthening action of these precipitates reduces the deterioration of the high temperature fatigue properties of the weld heat affected zone.

As described above, the presence of oxygen has a great influence on the high temperature fatigue characteristics of the weld heat affected zone, and in particular, coarse inclusions (Al ₂ O It was found that ₃ ) had a great influence on the decrease in fatigue limit. However, as shown in FIG. 3, the influence of the amount of oxygen existing as Al ₂ O ₃ changes depending on the total amount of oxygen (O), and when the amount of O is small, the amount of oxygen existing as Al ₂ O ₃ is small. Is also eased. The relationship is Al ₂ O ₃
It can be said that the relationship between the existing oxygen amount and the total oxygen amount is represented by the above formula. The analysis method of Al ₂ O ₃ is as described later.

The inventors have also found that the addition of B is effective for improving the workability. B
Is generally well known to segregate at grain boundaries and increase the grain boundary strength. On the other hand, in the component composition of the present invention, B enhances workability, and its effect is particularly low Cr
We found that it was remarkable at the level. It is considered that the reason for this is that the oxide, which easily precipitates at the grain boundary, was dispersed and precipitated in the grain due to the addition of B, and the workability was improved.

Next, the reason why the composition of the ferritic steel is limited in the present invention will be explained. C: 0.03 wt% or less C is an element that deteriorates toughness and workability, and is 0.03
If it exceeds 0 wt%, the toughness and workability will be significantly deteriorated, so the content is made 0.03 wt% or less. From the viewpoint of toughness and workability, the lower the C content, the better, and it is desirable to suppress it to 0.010 wt% or less.

Si: 3.0 wt% or less Si is an element that deteriorates the workability, and when it exceeds 3.0 wt%, its effect becomes remarkable, so the upper limit is 3.0 wt%. The Si content is preferably 1.0 wt% or less, more preferably 0.20 wt% or less.

Mn: 0.58 wt% or less Mn is added for deoxidation, but excessive addition causes coarse MnS.
To reduce corrosion resistance and the high temperature fatigue limit of the weld heat affected zone. Therefore, Mn amount is 0.58 wt% or less, good Mashiku is
0.15wt% or less.

P: 0.06 wt% or less Since P adversely affects the corrosion resistance, the smaller the content, the better.
The removal of P treatment involves an increase in cost. Therefore, the upper limit is 0.06wt%, which is acceptable for actual production.

S: 0.01 wt% or less Since S adversely affects the corrosion resistance, the smaller the content, the better.
The cost reduction is accompanied by the low S treatment. Therefore, the upper limit is set to 0.01 wt%, which is acceptable in actual production.

Cr: 1.0 to 20.0 wt% Cr is an element that contributes to the improvement of corrosion resistance and oxidation resistance and the strengthening by Cr carbide, and it is necessary to add 1.0 wt% or more. However, if it is added in excess of 20.0 wt%, the adverse effect of oxygen on workability becomes large, workability deteriorates, and the cost also increases. Therefore, the upper limit of the Cr addition amount is 20.0
wt%, preferably 17.0 wt%, more preferably 12.0 wt%. The most preferable Cr content is 10.0 to 12.0 wt%.

Al: 0.05 wt% or less Since Al adversely affects the workability, it is preferable that the amount is small, but since up to 0.05 wt% is acceptable, it is set to 0.05 wt% or less.

N: 0.030 wt% or less N is an element that deteriorates the toughness and workability of steel.
When it exceeds 0.030 wt%, the degree becomes remarkable, so 0.
It is 030 wt% or less, preferably 0.010 wt% or less.

O: 0.015− (48/54) × insol.Al wt% or less Oxygen coarsens the oxide when it increases, and remains as a stable oxide even after the heat history of the welding heat affected zone is received. It deteriorates the high temperature fatigue characteristics and workability of the heat affected zone. In order to suppress deterioration of the high temperature fatigue characteristics and workability of the weld heat affected zone, as shown in FIG.
− (48/54) x insol.Al wt% or less is required. Here, insol.Al means acid-insoluble Al, and most of them are Al ₂ O _3. Therefore, the oxygen existing as Al ₂ O ₃ can be calculated from (48/54) × insol.Al. In addition,
This insol.Al was measured by the measuring method according to "Study on Extraction Separation and Quantification Method of Oxide Inclusions in Steel" (January 1987) by the Subcommittee for Analysis of Nonmetallic Inclusions in Steel of Japan Iron and Steel Institute. This is a value obtained by analyzing the amount of Al that does not dissolve in. Although the above O is detrimental to workability, it can be excessively reduced.
This leads to lower productivity, higher cost, and deterioration of the weldability of the welded part. Considering these things, the lower limit of the O amount is
It is preferably 0.004 wt%, more preferably 0.006 wt%.

B: 0.0002 to 0.005 wt% B is an element effective for improving workability. The reason for this is not always clear, but it is considered that B segregates at the grain boundaries, which improves the workability due to the action of dispersing any oxides that tend to precipitate at the grain boundaries. The effect appears from 0.0002 wt%, but if it exceeds 0.005 wt%, a large amount of BN is generated and the toughness deteriorates.
˜0.005 wt%, preferably 0.0003 to 0.001 wt%.

Nb: 1.0 wt% or less Nb contributes to improvement of high temperature fatigue characteristics as NbC and Nb Laves phase. However, if added in excess of 1.0 wt%, a large amount of Laves phase is precipitated, which significantly increases the room temperature strength of the steel and deteriorates formability. Therefore, the amount of Nb is 1.0 wt%
Below, preferably 0.1 to 0.5 wt%, more preferably 0.
2 to 0.4 wt%

[0029]

[0030]

[0031]

Co: 1.0 wt% or less Co is an element effective for improving the toughness and high temperature strength of the weld heat affected zone, but is 1.0% by weight or less because it is an expensive element. In addition, it is desirable to add 0.03 wt or more in order to exert the above effects.

Cu: 2.0 wt% or less Cu is added as necessary in order to improve the corrosion resistance and the toughness of the weld heat affected zone. The effect is exhibited by the addition of 0.05 wt% or more, but since addition of a large amount deteriorates workability, it is limited to 2.0 wt% or less. The desirable addition range is 0.15 to 0.30 wt%.

Mo: 3.0 wt% or less Mo has an effect of solid solution strengthening and is an element effective for improving high temperature strength, but it is an expensive element and causes high cost.
Limited to 3.0 wt% or less.

Ni: 2.0 wt% or less Ni is an element effective for improving the toughness of the heat-affected zone of welding, and its effect becomes remarkable when it is added in an amount of 0.6 wt% or more, but the addition of a large amount increases the cost. , 2.0 wt% or less.

Ca: 0.0003 to 0.003 wt% Ca has the effect of suppressing nozzle clogging due to Ti-based inclusions during slab casting, and is added as necessary.
The effect appears when 0.0003 wt% or more is added, but 0.003 wt%
Not only does the effect saturate even if added in excess of wt%,
Inclusions containing Ca act as the starting point for pitting corrosion and deteriorate corrosion resistance, so 0.003 wt% is the upper limit.

REM: 0.001 to 0.1 wt% REM means a group of rare earth elements, that is, Y and lanthanoid elements such as La and Ce. If it is 0.001 wt% or more, it has the effect of improving the oxidation resistance, but if it is added in an amount of 0.1 wt% or more, the workability is significantly deteriorated and it is also expensive, so it is limited to 0.001 to 0.1 wt%. The so-called misch metal containing La and Ce is also a kind of REM. The lanthanoid element group such as Y, La and Ce contributes to the improvement of the oxidation resistance whether added alone or in combination.

The method for producing the steel of the present invention may be a general process used for producing this type of steel. For example, a steel having a predetermined component composition is smelted by a conventional steelmaking method such as a converter or an electric furnace, and a steel piece is formed by a continuous casting method or an ingot making method, and then hot rolling- (hot rolled sheet annealing). -Pickling-Cold rolling-Finishing annealing-Pickling, and depending on the application, cold rolling-Finishing annealing-
A method of repeatedly performing pickling may be used. In particular, a process of omitting hot-rolled sheet annealing is advantageous in terms of cost and is desirable. However, in order to control the oxygen amount so as to satisfy O ≦ 0.015 − (48/54) × insol.Al, for example, Al
It is necessary to suppress the addition as much as possible. Thereby inso
Since l.Al is kept low, the upper limit regulation of oxygen is relaxed. However, since Al is a strong deoxidizer, it is necessary to supplement the decrease in oxygen with another deoxidizer such as Si or Mn.

[0039]

EXAMPLE Steels having the composition shown in Table 1 were used for the total oxygen content and Al ₂ O ₃ content.
The amount of oxygen as, ie, (48/54) × insol.Al, was taken into consideration for melting, heating to 1150 ° C., and hot rolling to a sheet thickness of 5 mm to obtain a hot rolled sheet. This hot rolled plate is pickled,
It was cold-rolled to a plate thickness of 2 mm and finish annealed at 900 to 950 ° C. The cold rolled annealed sheet thus obtained was evaluated for workability at room temperature and high temperature fatigue property at 600 ° C. by the following methods.

(1) Workability at room temperature From the directions of 0 °, 45 ° and 90 °, JIS 13
A tensile test piece (plate thickness mm) of No. B was sampled, and the elongation El was measured by the following formula. El = (El ₀ +2 x El ₄₅ + El ₉₀ ) / 4, where El ₀ , El ₄₅ , and El ₉₀ are 0 with respect to the rolling direction, respectively.
Elongation values in °, 45 ° and 90 ° directions. The value of El was evaluated as 38% or more as AA, 35% or more and less than 38% as A, and less than 35% as B.

(2) Fatigue characteristics at high temperature With respect to the material corresponding to the weld heat affected zone which was subjected to heat treatment at 115O ° C for 1 minute as it was, the Schenk fatigue test was conducted at 600 ° C to measure the 10 ⁶ fatigue limit. . The fatigue strength ratio P was calculated by the following equation as a parameter showing how much the fatigue limit of the weld heat affected zone deteriorates from the base metal. P = (10 ⁶ cycle fatigue limit of materials equivalent to welding heat affected zone) /
(10 ⁶ cycle fatigue limit of base metal)

The test results obtained are also shown in Table 1. In the invention steels 1 to 16, the high temperature fatigue limit of the weld heat affected zone is 70% of the base metal.
And shows excellent high temperature fatigue properties. In addition, it has excellent workability. On the other hand, in the comparative steels out of the range of the present invention, the high temperature fatigue limit of the weld heat affected zone is 70% or less of the base metal. Also, the workability is generally poor.

[0043]

[Table 1]

As described above, the steel obtained by the present invention has a good workability even if it contains a relatively large amount of oxygen by simplifying the deoxidizing step which is essential for cost reduction. It also has good high temperature fatigue characteristics of the heat affected zone.

[0045]

As described above, according to the present invention, it is possible to provide an inexpensive material which is excellent in the high temperature fatigue characteristics of the weld heat affected zone, and also excellent in workability. Therefore, the present invention is applied to the fields where workability and weldability are required, for example, exhaust system materials for automobiles and motorcycles, exhaust path members for thermal power generation systems, kitchen products, etc., and its durability is improved. It greatly contributes to.

[Brief description of drawings]

1] Conventional steel base metal and welding heat affected zone equivalent member 60
It is a graph which shows the high temperature fatigue characteristic in 0 degreeC.

FIG. 2 is a graph comparing the fatigue strength ratios of two types of steel having similar components.

FIG. 3 is a graph showing the influence of the oxygen amount as Al ₂ O ₃ and the total oxygen amount on the fatigue strength ratio.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-8-170154 (JP, A) JP-A-6-228715 (JP, A) JP-A-8-260107 (JP, A) JP-A-4- 228547 (JP, A) JP 9-296249 (JP, A) JP 6-88168 (JP, A) JP 5-43986 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 301 C22C 38/00 302 C22C 38/18 C22C 38/54

Claims (4)

(57) [Claims] 1. C: 0.03 wt% or less, Si: 3.0 wt% or less, M
n: 0.58 wt% or less, P: 0.06 wt% or less, S: 0.01 wt% or less, Cr: 1.0 to 20.0 wt%, Al: 0.05 wt% or less, N: 0.030
wt% or less, B: 0.0002 to 0.005 wt%, Nb: 1.0 wt% or less,
O: 0.015− (48/54) × insol.Al wt% or less, the balance being Fe and unavoidable impurities, and a Cr-containing ferritic steel excellent in high-temperature fatigue properties of welds. 2. The steel according to claim 1 , further comprising Co:
1.0wt% or less, Cu: 2.0wt% or less, Mo: 3.0wt% or less, N
i: Cr containing at least one selected from the group consisting of 2.0 wt% or less, and the balance being Fe and unavoidable impurities, and being excellent in high temperature fatigue properties of the weld
Containing ferritic steel. 3. The steel according to claim 1 , further comprising Ca: 0.0003 to 0.003 wt% and the balance being Fe and unavoidable impurities, which is excellent in high temperature fatigue properties of the welded portion. Cr-containing ferritic steel. 4. The steel according to claim 1 , further comprising REM: 0.001 to 0.1 wt%, the balance being Fe and inevitable impurities. Cr-containing ferritic steel with excellent high temperature fatigue properties.