JPH0717988B2 - Ferritic stainless steel with excellent toughness and corrosion resistance - Google Patents

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]

2. Description of the Related Art Generally, ferritic stainless steel is known to have improved corrosion resistance as the Cr content increases, and when Mo is added at the same time, the effect becomes even greater. Are known. For this reason,
In environments containing chlorides such as seawater, high Cr ferritic stainless steel containing Mo is effective.

By the way, as described above, this high Cr ferritic stainless steel has an improved corrosion resistance due to the increase of the Cr content and the addition of Mo. On the other hand, for example, SUS 304.
However, there is a problem that the toughness is significantly inferior to that of the austenitic stainless steels.

That is, since ferritic stainless steels such as SUS436L and SUS444 have a remarkably high ductile-brittle transition temperature, there were many difficulties in the treatment after hot working.

The cause is said to be due to Cr and Mo added to improve the corrosion resistance. But,
The deterioration of the toughness due to the addition of Cr and Mo reduces the amount of C and N contained in the steel as much as possible, and
It is known that addition of a stabilizing element such as Ti can suppress it to some extent. However, C and N
Since there is a limit to the amount reduction or addition of Nb and Ti,
It was difficult to completely remove the adverse effects of Cr and Mo on the toughness.

On the other hand, conventionally, a technique for obtaining a high Cr ferritic stainless steel having excellent overall toughness by reducing the amounts of C and N and simultaneously adding V, Al, Nb and Ta has been specially proposed. It is disclosed as Japanese Patent Publication No. 58-2266. That is, in this conventional technique, C: 0.008 wt% or less, Si: 0.30 wt% or less, Cr: 25 to 33 wt%, Ni: 3.0 wt% or less, Mo: 0.5 to 4.0 wt%, V: 0.005 to 0.1 wt% And 4 to 10 times the amount of N, Cu: 2.0 wt% or less, P: 0.025 wt% or less, S: 0.015 wt% or less, Al: 0.03 to 1.0 wt%, N: 0.015 wt% or less and O: 0.010 containing less than wt%, and Nb and Ta alone or in combination, the following formula: 80 × C (wt%) −0.08 ≧ Nb and / or 1 / 2Ta (wt%) ≧ 80 × C (wt%) It is a high-chromium ferritic stainless steel that contains -0.24 so that the balance is essentially Fe. That is, the characteristics of this prior art steel are: (1). The addition of a specific amount of V related to the amount of N has the effect of improving the toughness under optimal heat treatment conditions, and the addition of Al in the presence of V (2) Addition of a specific amount of Nb and / or Ta related to the amount of C improves the toughness under slow cooling.

[0007]

However, it has been confirmed by the present inventors that excellent toughness cannot be obtained even by such a conventional technique. That is, in this prior art, Al having a function of doubling the effect of V is added in an amount of 0.03 to 1.0 so that the impact characteristics can be improved even if a small amount of V is added.
Since a large amount of wt% is added in combination, coarse Al oxides (precipitates) are formed in the steel to provide a starting point for brittle fracture, and sufficient toughness may not be obtained. understood.
In addition, the surface quality of a sliver or the like may be deteriorated.

The object of the present invention is to overcome the problem of the prior art that precipitates are formed in the steel as described above, and simultaneously improve the toughness of ferritic stainless steel in accordance with the improvement of corrosion resistance. To do.

[0009]

[Means for Solving the Problems] As a result of earnest research aiming at the achievement of the above-mentioned object, the inventors of the present invention focused on Co as an element substituting for Al, and this Co causes the formation of precipitates. However, the present invention has been completed by newly discovering that a composite addition with V mainly acts effectively to further improve the toughness.

That is, according to the present invention, C: 0.025 wt% or less, Si: 0.60 wt% or less, Mn: 0.50 wt% or less, Cr: 1
5〜30wt%, Ni: 1.0wt% or less, Mo: 2.5wt% 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 1.0 wt% and Co: 0.005 to 1.0w
containing t% and combining Nb and Ti individually or in combination, the following formula: 0.1 ≦ Nb (wt%) + Ti (wt%) ≦ 1.0 0 ≦ Nb (wt%) + Ti (wt%) − {8 x (C (wt%) + N (wt%)) +
0.1} ≦ 0.4, and when V and Co are compounded, the following formula; 0.1 ≦ 2 × V (wt%) + Co (wt%) Nb (wt%) + Ti (wt %)-{8 x (C (wt%) + N (wt%)) + 0.1
} ≦ 2 × V (wt%) + Co (wt%) ≦ 3.0, and the above C and N have a combined content of 0.040 wt% or less, and the balance substantially. Is a ferritic stainless steel consisting of Fe. The structure of the present invention will be described in more detail below.

First, the concept of the present invention will be described.
In addition, the amounts of C and N are reduced as much as possible, and the amounts of Nb and / or Ti which are in a complementary relationship with each other are contained.
The toughness is improved to some extent by controlling the addition amount of
In addition, such an alloy design still has a problem in production;
V and Co that have a complementary relationship with the amount of Nb and / or Ti
The combined use of and reduces the amount of Al, which is indispensable in the conventional steel, and, as a result, a ferritic stainless steel that has excellent toughness as well as corrosion resistance (rust resistance) at the same time is obtained. It is the one to get.

[0012]

Next, the composition of the ferritic stainless steel according to the present invention will be described in detail together with its function and the reason for limitation.

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 the steel is extremely high, and therefore, carbide easily precipitates on the grain boundaries, which causes low temperature fracture. In addition, the decrease in Cr concentration due to the formation of carbide causes deterioration of corrosion resistance. As a means to solve such various problems, it is effective to fix C by adding Ti or Nb or to reduce the C content itself. For this reason, the content of C is set to 0.025 wt% or less.

Si is used as a deoxidizing agent, but since it tends to promote the precipitation of the σ phase, it lowers the toughness.
That is, when the Si content exceeds 1 wt%, the low temperature toughness is significantly reduced and the formability is deteriorated. Desirably, it is necessary to reduce to 0.60 wt% or less where the above tendency does not appear. For this reason, the Si content is set to 0.60 wt% or less.

Mn forms MnS in combination with S in steel and can serve as a starting point for rusting. Especially, when it exceeds 0.5 wt%, the tendency becomes remarkable. Therefore, the content of Mn is 0.50 wt%
The following was set.

Cr is an element which forms a stable passivation film and is extremely effective for improving the corrosion resistance, but if it is less than 15 wt%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content increases,
The low temperature toughness becomes conspicuous, especially when it exceeds 30 wt%, it becomes difficult to manufacture, and it is difficult to reduce impurities such as C and N, which cause deterioration of toughness, especially N from the viewpoint of refining. . For this reason, the Cr content is set to 15 to 30 wt%.

Ni is an element effective in corrosion resistance and acid resistance, and also has an effect of improving pitting corrosion resistance and crevice corrosion resistance. However, if it exceeds 1.0 wt%, it becomes difficult to maintain the ferrite structure, and further, the economical efficiency of the steel material is impaired. For this reason, the Ni content is set to 1.0 wt% or less.

Mo is an element effective for corrosion resistance, but as the content thereof increases, the formation of the σ phase is promoted, and the formability and weldability deteriorate significantly. Moreover, the addition of a large amount of expensive Mo impairs the economical efficiency of steel materials. For this reason, the Mo content is set to 2.5 wt% or less.

The addition of Nb and / or Ti fixes C and N in the steel and is effective in improving both toughness and corrosion resistance. Moreover, addition of an appropriate amount of Nb and / or Ti is effective for improving formability. Therefore, in order to obtain desired effects on these actions, as will be described in detail later, in relation to the C and N contents, the following formula: 0.1 ≦ Nb (wt%) + Ti (wt%) ≦ 1.0 …… (1) 0 ≦ Nb (wt%) ＋ Ti (wt%) − {8 × (C (wt%) + N (wt%)) + 0.1} ≦ 0.4 …… (2) It is necessary to contain Nb and / or Ti. This is because even if other components are within the prescribed range, if the Nb and Ti contents do not fall within the prescribed range, they will not have an impact value that enables stripping at the time of manufacturing. is there.

However, this is observed when they act synergistically with the effect of adding V and Co, which will be described later, and these components act in a mutually complementary manner to achieve the desired properties of the present invention. Steel is obtained.

Cu is an element particularly effective for improving the corrosion resistance, but if it is 0.1 wt% or less, the effect cannot be obtained, while on the other hand, 3.0
If it is contained in excess of wt%, hot workability and stress corrosion cracking resistance deteriorate. Therefore, the Cu content is 0.1 to 3.0.
It was set as wt%.

Al is used as a deoxidizing agent during refining of steel. If Al in excess of 0.02 wt% is present, coarse Al oxide is formed in the steel, causing brittle fracture, and lowering toughness. To do. In particular, precipitation of this Al oxide on the steel surface may lead to more serious defects. For this reason, in the present invention, the Al content is 0.02 wt%.
It was decided to regulate the following trace amounts.

Like N, N is an element that causes deterioration of toughness and corrosion resistance of steel, so its content is 0.025 wt%.
Limited to: On top of that, especially when it contains high Cr,
Due to refining problems, it is also necessary to limit the total amount of C and N. That is, the total amount of C and N is 0.04 wt.
%, The toughness is significantly deteriorated, so C + N;
It was determined to be 0.04 wt% or less.

V is an element that works effectively for fixing C and N in combination with the action of Nb and / or Ti. Further, even when the amounts of C and N are small, the grain refinement is promoted and the grain coarsening temperature is raised, which is effective in improving the toughness.

The action of Co is one of the characteristic additional elements in the alloy design of the present invention, and it suppresses the agglomeration of carbides and nitrides precipitated by the addition of Nb and the like, which causes brittle fracture. Prevent the generation of the starting point of. Therefore, the addition of Co is extremely effective in improving the toughness.

These V and Co, which are essential elements in the alloy of the present invention, are extremely effective in improving the toughness of a high Cr ferritic stainless steel. Moreover,
The action of these V and Co is that in addition to the above-mentioned one, solid solution N
b, It also has the effect of suppressing the deterioration of toughness due to solid solution Ti. Then, the effect is further improved by the composite addition (V + Co). V or Co having such an effect
The content of is less than 0.05 wt% or less than 0.005 wt%, respectively, is insufficient. Moreover, the addition of a large amount of expensive V or Co impairs the economical efficiency of the steel material. For these reasons, the V and Co contents in the present invention are V: 0.05 to 1.0 wt.
%, Co: 0.005 to 1.0 wt%, and when compounded, the following formula; 0.1 ≦ 2 × V (wt%) + Co (wt%) ...... (3) Nb (wt%) + Ti (wt%) -{8 x (C (wt%) + N (wt%)) + 0.1} ≤ 2 x V (wt%) + Co (wt%) ... It was found that the amount must satisfy (4) .

Next, the composition of each component at the time of the composite addition of Nb and Ti, and the composite addition of V and Co are as described in the above (1) to
The reason expressed by equation (4) will be explained.

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

That is, for Cr, Mo, Ni and Cu,
Cr: 24-25wt%, Mo: 1.5-1.6wt%, Ni: 0.08-
0.10 wt%, Cu: 0.3-0.4 wt%
i, Mn and Al are approximately constant values within the limits of the present invention, and for Nb, Ti, C and N, Nb: 0.02
~ 1.0 wt%, Ti: 0.02-1.0 wt%, C: 0.001-0.03w
t%, N: 13 charges were melt-produced while varying within the range of 0.001 to 0.03 wt%. After obtaining an ingot with a thickness of 20 mm from each molten steel obtained in this way and forging this, a plate thickness of 5 mm
The material which was hot-rolled to 900 ° C., then annealed at 900 ° C. for 30 minutes, and water-cooled was processed into a JIS No. 2 2 mm V-type notched test piece and subjected to a Charpy impact test. As a general measuring method of toughness, a Charpy V type notch impact test method can be mentioned.

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%) + Ti In the region that satisfies (wt%) − {8 × (C (wt%) + N (wt%)) + 0.1}> 0.4, the impact value is significantly reduced. However, even in the region where 0 ≦ Nb (wt%) + Ti (wt%) − {8 × (C (wt%) + N (wt%)) + 0.1} ≦ 0.4, the range of the impact value obtained is 5 ~ 10kg
It was m / cm ² , and although the toughness was improved, it was not sufficient. As a result, Nb or Ti alone or Nb,
The problem of manufacturing that fracture occurs in the process after hot rolling remains only with the addition of Ti in combination. This is because, in general, the impact value must exceed 10 kg · m / cm ^{2 in order} to pass the plate without causing breakage.

Next, addition of Nb or Ti alone or Nb,
The following formula having a relatively good impact value when Ti was added in combination was 0 ≦ Nb (wt%) + Ti (wt%) − {8 × (C (wt%) + N (wt%)) + 0. Based on the component composition satisfying 1} ≦ 0.4, V and Co were added to examine the effect of improving toughness. That is, Cr: 24-25wt
%, Mo: 1.5 to 1.6 wt%, Ni: 0.08 to 0.10 wt%, Cu:
0.3 to 0.4 wt%, Nb: 0.45 to 0.5 wt%, C: 0.008 to
0.01 wt% and N: 0.006 to 0.01 wt% are set to be almost constant values, Si, Mn and Al are set to be substantially constant values within the range of the present invention, and V is V: 0.02 to 0.7 wt%. By varying the range, 6 charge with Co: 0.1 wt% added and 6 charge without Co: 0.1 wt% were melted.

The ingot of this molten product was processed into a test piece through the above-mentioned process, and a Charpy impact test was conducted. The result is shown in FIG. As is clear from this figure, when the content of V is 0.05 wt% by the composite addition with Co.
In the above cases, the impact value exceeded 10 kg · m / cm ² .

Next, in order to know the optimum composite addition ratio of V and Co, Nb (wt%) + Ti (wt%)-{8 × (C (wt%) + N (wt%)) + 0. 1}, V, and Co added variously to samples 1 to 12, 16, 17, and 17 in Table 1
After ingots having the component compositions shown in 20 to 23 were melted and processed into the above-mentioned test pieces, a Charpy impact test was conducted.

As a result, as shown in FIG. 3, 0 ≦ Nb (wt%) + Ti (wt%) − {8 × (C (wt%) + N (wt%)) + 0.1} ≦ 0.4 is satisfied. Within the range, the addition amount of V and Co is calculated 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%) + Co (wt%) It was recognized that it was an essential condition.

[0035]

EXAMPLE An Al content is 0.02 wt% or less according to the present invention and a composition exceeding 0.02 wt% which deviates from the present invention, and other component compositions are within the suitable range of the present invention 25Cr-1.8Mo. -
0.3Cu-0.5Nb-0.5V-0.2Co ferritic stainless steel 10k
2 g of each g was melted using an atmospheric induction melting furnace (Samples 6, 7, 18, and 19 in Table 1).

From each molten steel thus obtained, a thickness of 20
An ingot of mm was ingot and forged. Then, it was hot rolled into a hot rolled sheet having a thickness of 5 mm, annealed at 900 ° C. for 30 minutes, and further water cooled. The material thus obtained was processed into a JIS No. 2 2 mm V type notch test piece and subjected to a Charpy impact test. Further, the surface of the annealed material obtained by cold rolling the hot-rolled material to a thickness of 1.0 mm was visually observed, and the presence or absence of three bars was also investigated.

As a result, as is clear from Table 2, when the Al content deviates from the present invention (that is, exceeds 0.02 wt%), the impact value is remarkably lowered, and more sliver is generated on the surface. Was confirmed. In particular, ferritic stainless steel is often used as an interior material because of its beautiful finish, and it is necessary to strictly deal with the occurrence of surface flaws. Therefore, this Al content, which adversely affects the toughness and surface properties, is 0.
It was decided to limit the amount to 02wt% or less.

Next, in the same manner as in the above-mentioned embodiment, the impact value in the case where Co was not added (Sample 13), V was not added (Sample 14), and neither Co nor V was added (Sample 15) was examined. As a result, as is clear from Table 2, none exceeding the target of 10 kg · m / cm ² was obtained.

Further, when Co and V are added together, as shown in FIG. 3, Nb (wt%) + Ti (wt%)-{8 × (C (wt%) + N (wt%)) + 0. 1} ≤ 0.4 Nb (wt%) + Ti (wt%)-{8 x (C (wt%) + N (wt%)) + 0.1} ≤ 2 x V (wt%) + Co (wt%) is not satisfied The thing was found to be less than 10 kg · m / cm ² .

According to the present invention, Nb is reduced with the reduction of C + N.
The toughness can be improved by the combined addition of V and Co and the reduction of Al in addition to the addition of Ti and / or Ti, but such an alloy design deteriorates the excellent corrosion resistance originally possessed by ferritic stainless steel. Must not be

In order to secure this point, the composition of the components according to the present invention (Samples 1 to 12) shown in Table 1 and the composition of the components deviating from the scope of the present invention (Samples 13 to 21) are described below. A corrosion resistance comparison test was conducted. This test method uses 3.5% NaCl
A pitting potential measurement method evaluated with V _C ' ₁₀₀ under conditions of aqueous solution, 30 ° C, Ar deaeration, and No. 600 emery polished surface, and artificial seawater (NaC
l: 28g / l, MgCl ₂ : 5g / l, MgSO ₄ : 7g / l, CaCl ₂ : 2.4g / l, NaH
CO ₃ : 0.2g / l) is sprayed and kept at constant temperature and humidity, and evaluated by rusting rating number (3 (2-logX) where X is the rusting area ratio to the area before the sample) A laboratory rusting acceleration test method was used.

As a result, as is clear from Table 2, the corrosion resistance of the steel having the composition satisfying the conditions of the present invention is not deteriorated, and in some cases, the corrosion resistance is rather improved. Was recognized.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

As described above, according to the present invention,
Along with the addition of Nb and / or Ti as well as the reduction of C + N, the combined addition of V and Co and the reduction of Al make ferritic stainless steel excellent in both toughness and corrosion resistance. It can be manufactured without causing deterioration.

[Brief description of drawings]

FIG. 1 is a relationship diagram of Nb + Ti content and C + N content that affect impact value.

FIG. 2 is a diagram showing the relationship between the V content and the impact value when V is added alone and V + Co is added.

[Fig. 3] Impact on impact value Nb (wt%) + Ti (wt%)-{8 x
(C (wt%) + N (wt%)) + 0.1} value and 2 x V (wt%) + Co (wt%)
It is a relationship diagram of a value.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Saihito Fujiwara 4-2 Kojima-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa Nihon Metallurgical Industry Co., Ltd. Research & Development Headquarters, Technical Research Laboratory (72) Yutaka Kobayashi Kawasaki, Kanagawa , Kojima-cho, Kawasaki-ku, Ichi, Japan Technical Research Institute, Research & Development Division, Nihon Metallurgical Industry Co., Ltd. (56) References Japanese Patent Laid-Open No. 53-86617 (JP, A)

Claims (1)

[Claims] 1. C: 0.025 wt% or less, Si: 0.60 wt%
Below, Mn: 0.50wt% or below, Cr: 15 to 30wt%, Ni:
1.0 wt% or less, Mo: 2.5 wt% or less, V: 0.05 ~
1.0 wt%, Cu: 0.1 to 3.0 wt%, Al: 0.02 wt% or less,
N: 0.025 wt% or less, and Co: 0.005 to 1.0 wt
%, And Nb and Ti are used alone or in combination, respectively, and have the following formula; 0.1 ≦ Nb (wt%) + Ti (wt%) ≦ 1.0 0 ≦ Nb (wt%) + Ti (wt%) − { 8 x (C (wt%) + N (wt%)) +
0.1} ≦ 0.4, and when V and Co are compounded, the following formula; 0.1 ≦ 2 × V (wt%) + Co (wt%) Nb (wt%) + Ti (wt %)-{8 x (C (wt%) + N (wt%)) + 0.1
} ≦ 2 × V (wt%) + Co (wt%) ≦ 3.0, and the above C and N have a combined content of 0.040 wt% or less, and the balance substantially. A ferritic stainless steel consisting of Fe with excellent toughness and corrosion resistance.