JPH04280948A - Ferritic stainless steel excellent in tougness and corrosion resistance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

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 having excellent toughness and corrosion resistance.

0002

[Prior Art] Generally, it is known that the corrosion resistance of ferritic stainless steel improves as the Cr content increases, and when Mo is added at the same time,
It is known that the effect is even greater. Therefore, in environments containing chlorides such as seawater, high Cr ferritic stainless steel to which Mo is added is effective.

By the way, as mentioned above, the corrosion resistance of this high Cr ferritic stainless steel is improved by increasing the Cr content and adding Mo, but on the other hand, for example,
There was a problem in that the toughness was significantly inferior compared to austenitic stainless steels such as US 304.

[0004] That is, SUS 436L, SUS 4
Since ferritic stainless steels such as No. 44 have a significantly high ductile-brittle transition temperature, there are many difficulties in processing them after hot working.

[0005] The cause is said to be due to Cr and Mo added to improve corrosion resistance. However, this deterioration in toughness caused by the addition of Cr and Mo can be suppressed to some extent by reducing the amount of C and N contained in the steel as much as possible, or by adding stabilizing elements such as Nb and Ti. Are known. However, since there are limits to reducing the amount of C and N or adding Nb or Ti, it has been difficult to completely eliminate the adverse effects of Cr and Mo on toughness.

[0006] Conventionally, on the other hand, a technique for obtaining high Cr ferritic stainless steel with excellent overall toughness by reducing the amount of C and N and simultaneously adding V, Al, Nb and Ta has been developed. It is disclosed as Publication No. 58-2266. That is, this conventional technology
C: 0.008wt% or less, Si: 0
．． 30wt% or less, Cr: 25 to 33wt%,
Ni: 3.0wt% or less, Mo:
0.5-4.0wt%, V: 0.005
~0.1wt% and 4 to 10 times the amount of N, Cu: 2.0wt% or less, P: 0
．． 025 wt% or less, S: 0.015 wt% or less, Al: 0.03 to 1.0 wt%, N
: 0.015wt% or less and O :
Contains 0.010 wt % or less, and contains Nb and Ta alone or in combination, and has the following formula; 80
×C (wt%) −0.08≧Nb and/or 1
/2Ta (wt%) ≧80×C (wt%) -0.2
It is a high chromium ferritic stainless steel that satisfies the requirements of 4 and the remainder substantially consists of Fe. That is,
The characteristics of this conventional steel are (1). Addition of a specific amount of V in relation to the amount of N has the effect of improving toughness under optimal heat treatment conditions, and addition of Al in the presence of V further enhances the effect of V; (2). A specific amount of N related to the amount of C
The addition of b and/or Ta improves toughness under slow cooling.

[0007]

[Problems to be Solved by the Invention] However, the present inventors have confirmed that even with such conventional techniques, excellent toughness cannot be obtained. In other words, in this prior art, 0.03% of Al, which has the effect of doubling the effect of V, is added so that the impact characteristics can be improved even when a small amount of V is added.
Since the composite addition is relatively large (~1.0 wt%), coarse Al oxides (precipitates) are formed in the steel and provide a starting point for brittle fracture, making it difficult to obtain sufficient toughness. It turned out that I couldn't do it. Moreover, the surface properties of the sliver etc. may deteriorate.

The purpose of the present invention is to overcome the problem of the conventional technology that precipitates are formed in steel as described above, and to simultaneously improve the toughness of ferritic stainless steel and improve its corrosion resistance. It's about doing.

[0009]

[Means for Solving the Problem] As a result of intensive research aimed at achieving the above object, the present inventors focused on Co as an element to replace Al, and found that Co causes the formation of precipitates. The present invention was completed based on the new finding that the addition of V in combination with V effectively works to further improve toughness.

[0010] That is, in the present invention, C: 0.025 wt% or less, Si: 0
．． 60wt% or less, Mn: 0.50wt
% or less, Cr: 15-30wt%, Ni:
4.0wt% or less, Mo: 4.0w
t% or less, Cu: 0.1 to 3.0 wt%, Al:
0.02 wt% or less, N: 0.025
wt% or less, V: 0.05 to 1.0wt% and C
o: Contains 0.005 to 1.0 wt%, and Nb and Ti each alone or in combination, as shown in the following formula;
0.1≦ Nb (wt%) + Ti (wt%)≦
1.0 0≦Nb (wt%) + Ti (wt%
) − { 8×(C(wt%)+N(wt%))+0.1
}≦0.4, and the above V
and Co, when combined, have the following formula: 0.1≦2×V(wt%) +Co(wt%)
Nb (wt%) + Ti (wt%) - { 8×
(C(wt%)+N(wt%))+0.1 }
≦ 2×V (wt%) +Co (wt%)
and the above C and N have a content of 0.040 wt when combined.
% or less, and the remainder is substantially Fe. Below, the configuration of the present invention will be explained in more detail.

First, to explain the concept of the present invention, the first
Second, the amount of C and N is reduced as much as possible, and the amount of Nb and/or Ti added, which is in a mutually complementary relationship with the amount of C and N, is controlled to improve the toughness to a certain extent. Such an alloy design still has manufacturing problems; in other words, it is not possible to obtain an impact value that allows for sheet passing, so V and Co, which have a mutually complementary relationship with the Nb and/or Ti amounts, are added in combination. By using this method, we aim to reduce the amount of Al, which is considered essential in the conventional steel, and thereby obtain a ferritic stainless steel that has excellent properties in both toughness and corrosion resistance (rust resistance). It is.

0012

[Function] Next, the composition of the ferritic stainless steel according to the present invention will be explained in detail, as well as the reasons for its limitations.

C is an element that plays an extremely important role in terms of toughness in ferritic stainless steel. That is, the diffusion rate of C in steel is extremely fast, and therefore carbides easily precipitate on grain boundaries, causing low-temperature fracture. Moreover, a decrease in Cr concentration due to the formation of carbides leads to deterioration of corrosion resistance. As a means to solve these various problems, Ti and Nb
It is effective to fix C by adding or to reduce the C content itself. For this reason,
The content of C was set at 0.025 wt% or less.

[0014]Si is used as a deoxidizing agent, but it tends to promote precipitation of the σ phase, thereby reducing toughness. That is, when the Si content exceeds 1 wt%, low-temperature toughness is significantly reduced and formability is also deteriorated. Desirably, it is necessary to reduce the content to 0.60 wt% or less, at which the above-mentioned tendency does not appear. For this reason, the Si content was set to 0.
It was set at 60 wt% or less.

[0015] Mn combines with S in steel to form MnS, which can become the starting point for rusting. In particular, this tendency becomes remarkable when the content exceeds 0.5 wt%. Therefore, the Mn content was determined to be 0.50 wt% or less.

[0016] Cr forms a stable passive film and is an extremely effective element for improving corrosion resistance, but if it is less than 15 wt%, sufficient corrosion resistance cannot be obtained. On the other hand, as the Cr content increases, the decrease in low-temperature toughness becomes noticeable, especially at 30
If it exceeds wt%, manufacturing becomes difficult, and impurities such as C and N that cause deterioration of toughness, especially N, become difficult to reduce from a refining point of view. For this reason, the Cr content was set at 15 to 30 wt%.

[0017] Ni is an element effective in corrosion resistance and acid resistance,
It also has the effect of improving pitting corrosion resistance and crevice corrosion resistance. However, if it exceeds 4 wt%, it becomes difficult to maintain the ferrite structure, and moreover, the economic efficiency of the steel material is impaired. For this reason, the Ni content was determined to be 4 wt% or less.

[0018] Mo is an effective element for corrosion resistance, but as its content increases, the formation of the σ phase is promoted and the deterioration of formability and weldability becomes noticeable. Moreover, addition of a large amount of expensive Mo impairs the economic efficiency of the steel material. For this reason, Mo
The upper limit of the content was 4 wt%.

Addition of Nb and/or Ti fixes C and N in steel and is effective in improving both toughness and corrosion resistance. Furthermore, addition of an appropriate amount of Nb and/or Ti is also effective in improving moldability. Therefore, in order to obtain the desired effects on these actions, as will be explained in detail later,
In relation to C and N content, the following formula; 0.1≦Nb (wt%) + Ti (wt%)≦
1.0...(1) 0≦Nb(wt
%) + Ti (wt%) - { 8 x (C (wt%) + N
(wt%))+0.1}≦0.4...(2) It is necessary to contain Nb and/or Ti in an amount that satisfies the following. This is because even if the other components are within the specified ranges, unless the Nb and Ti contents fall within the specified ranges, the material will not have the impact value to allow threading during manufacturing. be.

[0020] However, this is observed when it acts synergistically with the effect of adding V and Co, which will be described later.
These components act in a mutually complementary manner to obtain steel having the desired properties of the present invention.

[0021]Cu is an element that is particularly effective in improving corrosion resistance; however, if the content is less than 0.1 wt%, this effect cannot be obtained; on the other hand, if it is contained in an amount exceeding 3.0 wt%, hot workability and stress resistance are Corrosion cracking resistance deteriorates. Therefore, the content of Cu was determined to be 0.1 to 3.0 wt%.

Al is used as a deoxidizing agent during steel refining, but if Al exceeds 0.02 wt%, coarse Al oxides are formed in the steel, causing brittle fracture and reducing toughness. descend. In particular, if this Al oxide precipitates on the steel surface, it may lead to more serious defects. For these reasons, in the present invention, the content of Al is limited to a trace amount of 0.02 wt% or less.

[0023] Like C, N is an element that causes a decrease in the toughness and corrosion resistance of steel, so its content is reduced to 0.02.
Limit to 5wt% or less. Furthermore, especially when containing a high Cr content, it is necessary to limit the total amount of C and N due to refining problems. That is, if the total amount of C and N exceeds 0.04 wt%, the deterioration of toughness becomes significant, so C+N was set at 0.04 wt% or less.

V is an element that effectively works to fix C and N in conjunction with the action of Nb and/or Ti. In addition, even when the amounts of C and N are small, it promotes grain refinement and increases the crystal grain coarsening temperature, which is effective in improving toughness.

[0025] The action of Co is one of the characteristic additive elements in the alloy design of the present invention, and it suppresses the agglomeration of carbides and nitrides precipitated by the addition of Nb etc., thereby preventing brittle fracture. Prevent the generation of origins. Therefore, the addition of Co is extremely effective in improving toughness.

V and Co, which are essential elements in the alloy of the present invention, are particularly effective in improving the toughness of high Cr ferritic stainless steel. Moreover, in addition to the effects described above, the effects of V and Co also have the effect of suppressing the decrease in toughness due to solid solution Nb and solid solution Ti. The effect is further improved by adding a combination (V+Co). V with such effects
Or, if the Co content is less than 0.05 wt% or 0.005 wt%, respectively, it is insufficient. Moreover, adding a large amount of expensive V or Co impairs the economic efficiency of the steel material. For these reasons, in the present invention, the respective contents of V and Co are V: 0.05 to 1.0 wt%, Co: 0.05 to 1.0 wt%.
005 to 1.0 wt% and when combined, the following formula: 0.1≦2×V (wt%) +Co (wt%)
...(3) Nb (wt%) + Ti (w
t%) - { 8×(C(wt%)+N(wt%))+0
．． 1 } ≦ 2×V (wt%)
+Co (wt%) It was found that the amount must satisfy the following (4).

Next, combined addition of Nb and Ti and V
The reason why each component composition at the time of addition of Co and Co is shown as in the above formulas (1) to (4) will be explained.

First, in order to investigate the effect of Nb or Ti on improving toughness, the present inventors obtained a sample by melting 10 kg of stainless steel having the following composition using an atmospheric induction melting furnace. Ta.

That is, regarding Cr, Mo, Ni and Cu, Cr: 24 to 25 wt%, Mo:
1.5-1.6wt%, Ni: 0.08-0.1
0 wt%, Cu: 0.3 to 0.4 wt%, approximately constant values, Si, Mn, and Al approximately constant values within the limited range of the present invention, and Nb, Ti, C, and N. , Nb: 0.02 to 1.0 wt%
, Ti: 0.02-1.0 wt%, C: 0.
001-0.03wt%, N: 0.001-0.0
Thirteen charges were produced by changing the amount within a range of 3 wt%. An ingot with a thickness of 20 mm was obtained from each of the molten steels obtained in this way, which was then forged, hot-rolled to a plate thickness of 5 mm, annealed at 900 °C for 30 minutes, and water-cooled. Processed into a No. 2 mm V-shaped notch test piece,
A Charpy impact test was conducted. Note that a general method for measuring toughness includes the Charpy V-type notch impact test method.

FIG. 1 shows the effect of C+ on the impact value at 0°C.
The influence of N content and Nb+Ti content is shown. As is clear from this figure, Nb (wt%) + Ti (wt%) - {
8×(C(wt%)+N(wt%))+0.1}<
0 and Nb (wt%) + T
i (wt%) - { 8 × (C (wt%) + N (wt%)
)+0.1 }>0.4, the impact value decreases significantly. However, 0≦Nb (wt%) + Ti (wt%
) − { 8×(C(wt%)+N(wt%))+0.1
Even in the region satisfying }≦0.4, the range of impact values obtained was 5 to 10 kg·m/cm 2 , and although an improvement in toughness was observed, it was not sufficient. As a result, there remained a manufacturing problem in that the addition of Nb or Ti alone or the combined addition of Nb and Ti caused breakage in the steps after hot rolling. This is because, generally, in order to pass through the plate without causing breakage, it is necessary that the impact value exceeds 10 kg·m/cm2.

Next, when Nb or Ti is added alone or when Nb and Ti are added in combination, the following formula, which has a relatively good impact value: 0≦Nb (wt%) + Ti (wt%)
) − { 8×(C(wt%)+N(wt%))+0.1
}≦0.4, the effect of improving toughness by adding V and Co was investigated. That is,
Cr: 24-25wt%, Mo: 1.5-1
．． 6 wt%, Ni: 0.08 to 0.10 wt%
, Cu: 0.3 to 0.4 wt%, Nb: 0.
45 ~ 0.5 wt%, C: 0.008 ~ 0.0
1wt%, and N: approximately constant values of 0.006 to 0.01wt%, Si, Mn, and Al are approximately constant values within the range of the present invention, and V:
C by varying it in the range of 0.02 to 0.7 wt%.
o: 6 charges with 0.1 wt% added and Co:
Six charges were prepared without adding 0.1 wt%.

[0032] An ingot of this melted product was processed into a test piece through the process described above, and a Charpy impact test was conducted. The results are shown in FIG. As is clear from this figure, the V content is 0.0 when combined with Co.
When the content was 5 wt% or more, the impact value exceeded 10 kg·m/cm2.

Next, in order to find the optimal composite addition ratio of V and Co, Nb (wt%) + Ti (wt%) - { 8
×(C(wt%)+N(wt%))+0.1}, Samples 1 to 12 in Table 1 with various added amounts of V and Co
, 16, 17, 20 to 23 were melted and processed into the above-mentioned test pieces, which were then subjected to a Charpy impact test.

As a result, as shown in FIG.
0≦ Nb (wt%) + Ti (wt%) - { 8 × (
C(wt%)+N(wt%))+0.1}≦0.4
Within the range that satisfies the above, the amount of V and Co added is determined by the following formula;
2×V (wt%) +Co (wt%) ≧
0.1 Nb (wt%) + Ti (wt%) -
{8×(C(wt%)+N(wt%))+0.1}
≦ 2×V (wt%)
It was recognized that being within the range of +Co (wt%) is an essential condition.

[0035]

[Example] Al content is 0.02wt according to the present invention
% or less, and 25Cr-1.8Mo-0.3Cu-0.5Nb-0 with a composition exceeding 0.02wt% which deviates from the present invention, and other component compositions are within the appropriate range of the present invention.
．． 5V-0.2Co ferrite stainless steel 10kg
Two charges of each were melted using an atmospheric induction melting furnace (Samples 6, 7, 18, and 19 in Table 1).

[0036] From each molten steel thus obtained, a thickness of 2
A 0 mm ingot was made into an ingot, and this was forged. After that, after hot-rolling to make a hot-rolled plate with a thickness of 5 mm,
C. for 30 minutes and further water-cooled. The material thus obtained was processed into a JIS No. 4 2 mm V-shaped notch test piece and subjected to a Charpy impact test. Furthermore, the hot rolled material was cold rolled to a thickness of 1.0 mm, and the surface of the annealed material was visually observed and the presence or absence of slivers was investigated.

As a result, as is clear from Table 2, Al
deviates from the present invention (i.e. 0.02wt
%), the impact value decreased significantly and moreover, it was confirmed that many slivers were generated on the surface. especially,
In the case of ferritic stainless steel, it is often used for interior materials because of its beautiful finish, and it is necessary to take strict measures against the occurrence of surface flaws. Therefore, the Al content, which adversely affects toughness and surface properties, is limited to a trace amount of 0.02 wt% or less as in the present invention.

Next, in the same manner as in the above embodiment, Co
The impact values were investigated with no additive (Sample 13), without V (Sample 14), and with neither Co nor V added (Sample 15). As a result, as is clear from Table 2, no product exceeding the target of 10 kg·m/cm2 was obtained.

Furthermore, when Co and V are added in combination, Nb (wt%) + Ti (wt%) - {8×(C
(wt%)+N(wt%)) +0.1 }≦0.4
Nb (wt%) + Ti (wt%) −{8×(C
(wt%)+N(wt%)) +0.1 }
≦ 2×V(wt%) +Co(w
t%), 10kg・m/cm2
It was found that less than

According to the present invention, as well as reducing C+N, N
Along with the addition of b and/or Ti, V and Co
Although the toughness can be improved by the combined addition of and the reduction of Al, such alloy design must not deteriorate the excellent corrosion resistance inherent in ferritic stainless steel.

[0041] In order to ensure this point, regarding the steels having the component compositions according to the present invention (samples 1 to 12) shown in Table 1 and the component compositions (samples 13 to 21) that deviate from the scope of the present invention,
A comparative test of their corrosion resistance was conducted. This test method consists of 3
．． Pitting corrosion potential measurement method evaluated with VC '100 under the conditions of 5% NaCl aqueous solution, 30°C, Ar degassing, No. 600 emery polished surface, and artificial seawater (NaCl: 28 g/l, M
gCl2: 5g/l, MgSO4: 7g/l, Ca
Cl2: 2.4g/l, NaHCO3: 0.2g/l
) is maintained at constant temperature and humidity after spraying, and is evaluated using the post-rusting rating number (3 (2-log method was used.

As a result, as is clear from Table 2, the corrosion resistance of steel having a composition that satisfies the conditions of the present invention does not deteriorate, and in some cases, the corrosion resistance actually improves. was recognized.

[0043]

[0044]

[0045]

[Effects of the Invention] As explained above, according to the present invention,
By reducing C+N, adding Nb and/or Ti, combined addition of V and Co, and reducing Al, ferritic stainless steel has excellent toughness and corrosion resistance. It can be manufactured without causing any deterioration.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between Nb+Ti content and C+N content on impact value.

[Figure 2] Relationship diagram between V content and impact value in the case of adding V alone and adding V+Co in combination

[Figure 3] Effect of Nb (wt%) + Ti (w
t%) - { 8×(C(wt%)+N(wt%))+0
．． 1 } value and 2×V (wt%) +Co (wt%) value.

Claims (1)

[Claims] [Claim 1] C: 0.025wt% or less,
Si: 0.60wt% or less, Mn: 0.50wt% or less, Cr:
15 to 30wt%, Ni: 4.0wt% or less,
Mo: 4.0wt% or less, V: 0.
05-1.0wt%, Cu: 0.1-3
．． 0 wt%, Al: 0.02 wt% or less,
N: 0.025 wt% or less, and Co
: Contains 0.005 to 1.0 wt%, and contains Nb and Ti, each alone or in combination, according to the following formula: 0.1≦Nb (wt%) + Ti (wt%)≦
1.0 0≦Nb (wt%) + Ti (wt%
) − { 8×(C(wt%)+N(wt%))+0.1
}≦0.4, and the above V
and Co, when combined, have the following formula: 0.1≦2×V(wt%) +Co(wt%)
Nb (wt%) + Ti (wt%) - { 8×
(C(wt%)+N(wt%))+0.1 }
≦ 2×V (wt%) +Co (wt%)
and the above C and N have a content of 0.040 wt when combined.
% or less, the remainder being substantially Fe, and having excellent toughness and corrosion resistance.