JP2000273591A - High corrosion resistance chromium-containing steel with excellent high temperature strength and intergranular corrosion resistance - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide corrosion resistance and high-temperature strength equal to or higher than low Cr stainless steel while having a low chromium content that does not fall into the category of stainless steel, and furthermore, excellent grain resistance unprecedented in the past Providing chromium-containing steel with high corrosion resistance with interfacial corrosion. SOLUTION: In weight%, C, Si, Mn, P, S, N
i, Cr, Al and N in specific amounts.
b, V, and one selected from Zr in a specific ratio, and the contents of Cr, Si, Nb, V, Zr, C, and N [Cr], [Si], [Nb], [V] , [Z
r], [C] and [N] satisfy the following formula: ([Nb] + [V] + [Zr]) / ([C] + [N]) ≧ 75−6 × [Cr] + 12 / [Si] (1) High-corrosion-resistant chromium-containing steel excellent in high-temperature strength and intergranular corrosion resistance consisting of the balance of Fe and other unavoidable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a low chromium (Cr) content which does not fall within the category of stainless steel,
Low Cr stainless steel (Cr content: 11-13% by weight)
The present invention relates to a high-corrosion-resistant chromium-containing steel having corrosion resistance and high-temperature strength equivalent to or higher than that of chromium, and having unprecedented excellent intergranular corrosion resistance.

[0002]

2. Description of the Related Art Stainless steel, which is a typical corrosion-resistant steel, contains chromium in an amount of 11% by weight or more. But C
Since r is an expensive elemental component, there is a need for a steel having a component system in which the characteristics do not deteriorate even if the Cr content is reduced from an economical viewpoint. For example, Japanese Patent Application Laid-Open No. Hei 4-232231 discloses that, with a Cr content of 5.0% by weight to less than 11.0% by weight, Si is 3.0% by weight or less and V is 0.01% by weight.
A high-strength Cr-containing steel sheet containing 0.10% by weight or less and 2.0% by weight or less of Cu is disclosed in JP-A-7-22435.
No. 2 discloses that 2.0 to less than 10.0% by weight of Cr contains S
i is not less than 0.6% by weight and not more than 5.0% by weight;
Alternatively, a Cr-containing steel containing a specific amount of Ta or more has been proposed. These were able to be used as corrosion-resistant steel by adding Si or the like in order to reduce the deterioration of corrosion resistance due to the reduction of the Cr content.

[0003] However, in these conventional chromium-containing steels, firstly, Cr forms a compound with C or N, and sensitization corrosion caused by the formation of a Cr-deficient layer in the periphery thereof is formed. Prevention was not enough. Therefore, for example, for automobile exhaust system members, it is not possible to prevent sensitization caused by welding at the time of molding or heating due to a rise in exhaust gas temperature when mounted on an actual vehicle, and Cr or carbide or nitride cannot be prevented. As a result, intergranular corrosion in which corrosion is accelerated occurs in a portion where the Cr concentration is reduced, and in a severe case, the corroded portion may be damaged. The problem of sensitization corrosion is that Cr
The lower the content, the more significant.

Second, when the conventional chromium-containing steel is used in a high-temperature portion of about 700 ° C., such as an exhaust manifold upstream of an automobile exhaust system, the high-temperature strength is insufficient, and the life of the component is extremely short. Problem.

[0005]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems, and by optimizing the steel composition, to obtain a low Cr stainless steel having a low Cr content which does not fall into the category of stainless steel. Another object of the present invention is to provide a highly corrosion-resistant Cr-containing steel having corrosion resistance and high-temperature strength equivalent to or higher than that of the steel, and having unprecedented excellent intergranular corrosion resistance.

[0006]

Means for Solving the Problems In order to achieve this object, the present inventors have investigated in detail the effects of various additive elements on the corrosion resistance, high-temperature strength and intergranular corrosion resistance of Cr-containing steel. As a result, it contains Si and at least one selected from Nb, V and Zr, and if necessary, Mo or the like is added, and further, depending on the contents of Cr and Si, Nb, V and Zr and (C + N ) Was newly found to have corrosion resistance, high-temperature strength, and intergranular corrosion resistance equal to or higher than that of low Cr stainless steel.

That is, according to the present inventors, in order to improve the corrosion resistance and high-temperature strength of chromium-containing steel, first, Si and at least one selected from Nb, V and Zr must have a specific value or more. It has been found that it is effective to add an element selected from Mo, Cu, Co, Ca and B if necessary. On the other hand, in order to improve the intergranular corrosion resistance, first, the above-mentioned Si, Nb,
At least one selected from V and Zr;
o to improve the corrosion resistance of the material.
And the content of Si [Cr] and [Si],
The sum of the contents [Nb], [V] and [Zr] of Nb, V and Zr, the content of C [C] and the content of N [N]
Of the sum of: [[Nb] + [V] + [Zr]) / ([C]
+ [N]) is found to be more than a specific value.

Here, as an experimental example, Fe-9% by weight C
FIG. 1 shows the results of an intergranular corrosion test performed on a chromium-containing steel based on the composition of r and changing the contents of Si, Nb, V, C, and N.

In the intergranular corrosion test, a test piece prepared by butt-welding two test materials by TIG welding is immersed in a boiling 2% sulfuric acid + 6% copper sulfate solution for 16 hours.
= 2 mm and a bending test at an angle of 180 ° were performed, and then the presence or absence of intergranular corrosion was confirmed by observing the cross-sectional structure. FIG.
In the above, ○ indicates no intergranular corrosion, and ● indicates intergranular corrosion.

As shown in FIG. 1, in a steel having a Cr content of 9% by weight, ([Nb] + [V]) / ([C] +
[N]) is changed to (21 + 12 / [S
It has been found that by setting the value of i)) or more, excellent intergranular corrosion resistance can be obtained.

As a result of a more detailed investigation, it was found that, in order to obtain a steel having excellent intergranular corrosion resistance, a higher ([Nb] + [V]) accompanied by a decrease in [Cr] and [Si]. +
[Zr]) / ([C] + [N]) is required, and as a result of deriving the relational expression, this value becomes (75−6 × [Cr]).
+ 12 / [Si]) was confirmed to be necessary, and the present invention was reached.

[0012] Therefore, based on the above findings, the present invention firstly provides, by weight%, C: 0.015% or less and Si:
0.6% or more, 3% or less, Mn: 0.5% or less, P:
0.05% or less, S: 0.01% or less, Ni: 1% or less, Cr: 5% or more, 10.4% or less, Al: 0.1%
Below, N: 0.015% or less, provided that the content of C and N
(C + N): 0.02% or less;
b: 0.6% or less, V: 0.6% or less, and Zr:
0.6% or less, one or more selected from the group consisting of 0.35% or more and 0.6% or less in total, and Cr, S
i, Nb, V, Zr, C and N content [Cr],
[Si], [Nb], [V], [Zr], [C] and [N] are in a relationship satisfying the following formula: ([Nb] + [V] + [Zr]) / ([C] + [N]) ≧ 75−6 × [Cr] + 12 / [Si] (1) To provide a high-corrosion-resistant chromium-containing steel excellent in high-temperature strength and intergranular corrosion resistance consisting of the balance of Fe and other unavoidable impurities. It is.

Second, Nb: more than 0.3% and 0.5% or less, and V: more than 0.1% and 0.3% or less instead of Nb, V and Zr of the above-mentioned components. And C
r, Si, Nb, V, C and N content [Cr],
A high corrosion resistance chromium-containing steel excellent in high-temperature strength and intergranular corrosion resistance in which [Si], [Nb], [V], [C] and [N] satisfy the following formula (2). It is. ([Nb] + [V]) / ([C] + [N]) ≧ 75−6 × [Cr] + 12 / [Si] (2)

Third, in addition to the above-mentioned components, Mo: 0.02% or more and 2% or less by weight, and Cu: 0.02% or less.
02% or more, 2% or less, and Co: 0.02% or more,
An object of the present invention is to provide a high-corrosion-resistant chromium-containing steel excellent in high-temperature strength and intergranular corrosion resistance containing at least one selected from 2% or less.

Fourth, in addition to the above components, Ca: 0.0005% or more and 0.003% or less in weight%
And B: a highly corrosion-resistant chromium-containing steel containing at least one selected from 0.0002% or more and 0.005% or less and having excellent high-temperature strength and intergranular corrosion resistance.

Hereinafter, the highly corrosion-resistant chromium-containing steel of the present invention (hereinafter referred to as “steel of the present invention”) will be described in detail.
In the steel of the present invention, C is an element having an adverse effect on corrosion resistance and intergranular corrosion resistance. When the content exceeds 0.015% by weight, the effect becomes remarkable. Therefore, C is limited to 0.015% by weight or less. You. In particular, from the viewpoint of improving corrosion resistance and intergranular corrosion resistance, the lower the C content, the better.
% By weight or less is desirable.

In the steel of the present invention, Si is a component effective for improving corrosion resistance, high-temperature strength and intergranular corrosion resistance.
In order to exert these effects, it is necessary to add 0.6% by weight or more. From the viewpoints of improving corrosion resistance, high-temperature strength, and intergranular corrosion resistance, the higher the Si content, the better, and the content of more than 1% by weight is desirable. If the content exceeds 3% by weight, the effect is not only saturated, but also the steel is hardened and the formability is deteriorated.

Mn has a deoxidizing and desulfurizing action and is an element necessary for steelmaking. However, if added in excess, the corrosion resistance and formability of steel are deteriorated.
Limited to:

The lower the P content is, the better the corrosion resistance and the formability of the obtained steel can be improved. However, it is desirable that the content of P is not more than 0.05% by weight in view of economical restrictions on steel making. In the steel of the present invention, the lower the content of S, the higher the corrosion resistance. However, the content of S is limited to 0.01% by weight or less due to an economic limitation in the de-S treatment during steel making. Further, from the viewpoint of corrosion resistance and intergranular corrosion resistance, the lower the S content, the better, and it is preferable that the content of S is 0.005% by weight or less.

In the steel of the present invention, Ni is an elemental component for improving corrosion resistance, but is expensive, and aggressive addition causes an increase in cost, so Ni is limited to 1.0% by weight or less. In the steel of the present invention, Cr is an element component effective for improving corrosion resistance, high-temperature strength, and intergranular corrosion resistance. To obtain corrosion resistance equivalent to or higher than that of stainless steel, 5.0% by weight is required. The above is necessary, and this is the lower limit. Corrosion resistance, high-temperature strength and intergranular corrosion resistance improve with an increase in the Cr content, and therefore are preferably 8.0% by weight or more. However, Cr is expensive, and excessive addition increases the cost, so that it is limited to 10.4% by weight or less. However, even if the Cr content is less than 10% by weight, the corrosion resistance, high-temperature strength and intergranular corrosion resistance, which are the gist of the present invention, can be sufficiently obtained.

Al is necessary as a deoxidizing agent on steelmaking.
Excessive addition deteriorates the corrosion resistance and surface properties due to the formation of inclusions, so is limited to 0.1% by weight or less. In the steel of the present invention, N is an element that has an adverse effect on corrosion resistance and intergranular corrosion resistance. If the content exceeds 0.015% by weight, the effect becomes remarkable, so the content is limited to 0.015% by weight or less. In particular, from the viewpoint of improving corrosion resistance and intergranular corrosion resistance, the N content is preferably as low as possible, and is preferably 0.008% by weight or less.

Further, from the viewpoint of improving corrosion resistance and intergranular corrosion resistance, the sum of the C content and the N content (C + N) is 0.02% by weight.
Limited to the following. In particular, from the viewpoint of improving the corrosion resistance and the intergranular corrosion resistance, the lower the (C + N), the better, and 0.015.
% By weight or less is desirable.

The steel of the present invention contains at least one selected from Nb, V and Zr. Nb, V
And Zr may contain only one type or a combination of two or more types. Nb, V and Zr are elements that fix C and N in the steel, improve corrosion resistance and intergranular corrosion resistance, and improve high-temperature strength. This effect is found in the steel of the present invention. Is exerted when the sum of is 0.35% by weight or more. In particular, the sum of the contents of Nb, V and Zr is 0.40
When it is not less than% by weight, the high temperature strength is improved. On the other hand, N
If the sum of the contents of b, V and Zr exceeds 0.60% by weight, the effect is not only saturated, but also the workability is deteriorated due to the hardening of the steel. The sum of the amounts is 0.6% by weight or less.

Further, in the steel of the present invention, the addition of Nb and V together (composite addition) is effective for significantly improving the strength at high temperatures. In the case of this complex addition, an Nb content exceeding 0.3% by weight and a V content exceeding 0.1% by weight are required. Further, from the viewpoint of preventing deterioration of workability due to hardening of steel, the upper limit of the Nb content is 0.5% by weight or less, and the upper limit of the V content is 0.3% by weight or less.

In the steel of the present invention, Cr, Si,
V, Zr, C and N contents [Cr], [Si],
[Nb], [V], [Zr], [C] and [N] are
When [Cr] is in the range of 5 to 10.4% by weight, the following formula (1) is satisfied. ([Nb] + [Zr] + [V]) / ([C] + [N]) ≧ 75−6 × [Cr] + 12 / [Si] (1)

In the steel of the present invention, the left side: ([Nb] +
If the value of [Zr] + [V]) / ([C] + [N]) is smaller than the value of the right side: 75−6 × [Cr] + 12 / [Si], C and N become Nb. , V or Zr is not sufficiently fixed, and a large amount of the compound with Cr is formed, so that a Cr-deficient layer is formed at the grain boundary, and grain boundary corrosion is likely to occur. Usually, in stainless steel having a Cr content of 11% by weight or more, ([Nb] + [Zr] + [V])
A value of / ([C] + [N]) of 15 or more is sufficient. However, according to our studies, less than 11% by weight of C
In steels with an r content, the Cr in the base metal is small,
[Nb] + [Zr] +
[V]) / ([C] + [N]) needed to be larger. Further, as the content of Cr and Si decreases, ([Nb] + [Zr] + [V]) / ([C] +
[N]) was found to be required to be larger, and the relationship of the above formula (1) was obtained.

In the steel of the present invention, Nb and V
And the contents of Cr, Si, Nb, V, C, and N [Cr], [Si], [Nb], [V], [C], and [N] are calculated by the above formula (2). When the relationship is satisfied, the high-temperature strength is particularly improved. This is considered to be because the effect of solid solution strengthening of Nb and V is remarkably exhibited by the composite addition.

In the steel of the present invention, it is effective to improve corrosion resistance by adding at least one element component selected from Mo, Cu and Co, in addition to the above essential components. One of these Mo, Cu and Co may be added alone, or a combination of two or more may be added. In particular, the addition of Mo has the greatest effect, and has the advantage that the high-temperature strength can be improved. This M
Each of o, Cu and Co has an effect of improving the corrosion resistance by adding 0.02% by weight or more. In order to obtain a more excellent effect of improving corrosion resistance, it is preferable to add 0.1% by weight or more. However, when each of Mo, Cu and Co exceeds 2% by weight, not only the effect is saturated, but also the surface quality and economy are impaired.
This value is the upper limit.

Further, when the steel of the present invention contains at least one of Ca and B, it is effective for improving the high-temperature strength. In order to improve the high temperature strength, Ca is 0.000.
It is necessary to add 5% by weight or more and B to 0.0002% by weight or more. However, if added excessively, the corrosion resistance of the steel is reduced.
30% by weight, B is 0.0050% by weight.

The method for producing the steel of the present invention is not particularly limited, and a method generally used for producing Cr-containing steel such as stainless steel can be applied almost as it is. For example, in steelmaking, a method in which the essential components and components added as necessary are melted in a converter or an electric furnace or the like and secondary refining is performed by VOD is preferable. The melted steel can be made into a steel material according to a known casting method. In particular, from the viewpoint of productivity and quality,
Preferably, a continuous casting method is applied.

The steel material obtained by continuous casting is heated to a predetermined temperature if necessary, and then hot-rolled into a hot-rolled sheet having a desired thickness. This hot rolled sheet, if necessary,
Preferably, after performing box annealing or continuous annealing at 700 to 1050 ° C, pickling and cold rolling are performed to obtain a cold-rolled sheet having a predetermined thickness.

The cold rolled sheet is preferably 700 to 1030 ° C.
After continuous annealing, it is preferable to perform pickling to obtain a cold-rolled annealed plate. Depending on the application, it is also possible to use the hot rolled sheet or the hot rolled annealed sheet as it is.

Furthermore, the shape and form of the steel of the present invention are not particularly limited, and the present invention can be applied to any shape and form of not only plate materials but also any processed products such as pipes, pressed products and wire rods. Can be.

[0034]

The present invention will be described in more detail with reference to the following examples and comparative examples.

(Examples 1 to 12, Comparative Examples A to K) A chromium-containing steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace to form a 50 kg steel ingot and hot-rolled to a 3 mm-thick plate. And
After annealing at 800 ° C. for 8 hours, the sheet was cold-rolled into a 1 mm-thick plate. Thereafter, finish annealing at 900 ° C. for 1 minute and then pickling were performed to obtain a cold-rolled annealed sheet having a thickness of 1 mm. This cold-rolled annealed plate was used as a test material, and the corrosion resistance, high-temperature strength and intergranular corrosion resistance were evaluated according to the following methods. Table 1 shows the results.

Corrosion resistance A salt spray test was performed for 1 hour in accordance with JIS Z2371 and evaluated according to the following criteria based on the area ratio of rust. ◎ The best corrosion resistance is obtained when the rusting area ratio is 5% or less. ○ Good corrosion resistance is obtained when the rusting area ratio is more than 5% and 20% or less. × The rusting area ratio exceeds 20% and the corrosion resistance is significantly deteriorated.

High Temperature Strength Tensile strength at 700 ° C. was measured according to JIS G0567. Grain boundary corrosion resistance The test material was subjected to TIG welding (voltage: 12 V, current: 150
A, shield gas: Ar, flow rate: front (electrode side) 10 l / min, back 5 l / min, electrode moving speed: 60 cm
/ Min), butted and boiled 2%
It was immersed in a sulfuric acid + 6% copper sulfate solution for 16 hours. afterwards,
A bending test was performed at R = 2 mm and an angle of 180 °, the cross-sectional structure was observed, and evaluation was made based on the presence or absence of intergranular corrosion.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

As is clear from Table 1, the chromium-containing steel of the present invention has excellent corrosion resistance, high temperature strength and intergranular corrosion resistance.

[0043]

The steel of the present invention is a chromium-containing steel excellent in corrosion resistance, high temperature strength and intergranular corrosion resistance. This steel is more economical than conventional stainless steel containing 11% by weight or more of expensive Cr, but has corrosion resistance, high temperature strength and intergranular corrosion resistance equal to or higher than that of low Cr stainless steel. Therefore, it can be widely applied to applications where low Cr stainless steel is currently used. In particular, it is suitable as a material such as an exhaust manifold for an automobile exhaust system or a pipe (front pipe) immediately below the exhaust manifold, which requires corrosion resistance and high-temperature strength of the material and the welded portion.

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of a grain boundary corrosion test performed on a chromium-containing steel having a basic composition of Fe-9% by weight chromium and further changing the contents of Nb, V, Si, C and N. .

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuhide Ishii 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Susumu Sato Susumu 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corp.

Claims (4)

[Claims] (1) C: 0.015% or less by weight, Si:
0.6% or more, 3% or less, Mn: 0.5% or less, P:
0.05% or less, S: 0.01% or less, Ni: 1% or less, Cr: 5% or more, 10.4% or less, Al: 0.1%
Below, N: 0.015% or less, provided that the content of C and N
(C + N): 0.02% or less;
b: 0.6% or less, V: 0.6% or less, and Zr:
0.6% or less, one or more selected from the group consisting of 0.35% or more and 0.6% or less in total, and Cr, S
i, Nb, V, Zr, C and N content [Cr],
[Si], [Nb], [V], [Zr], [C] and [N] are in a relationship satisfying the following formula: ([Nb] + [V] + [Zr]) / ([C] + [N]) ≧ 75−6 × [Cr] + 12 / [Si] (1) High corrosion-resistant chromium-containing steel excellent in high-temperature strength and intergranular corrosion resistance consisting of the balance of Fe and other unavoidable impurities. 2. C: 0.015% or less by weight, Si:
0.6% or more, 3% or less, Mn: 0.5% or less, P:
0.05% or less, S: 0.01% or less, Ni: 1% or less, Cr: 5% or more, 10.4% or less, Al: 0.1%
Below, N: 0.015% or less, provided that the content of C and N
(C + N): 0.02% or less, Nb: 0.
More than 3% and 0.5% or less, and V: more than 0.1% and 0.3% or less, and Cr, Si, Nb,
Content of V, C and N [Cr], [Si], [N
b], [V], [C] and [N] satisfy the following formula: ([Nb] + [V]) / ([C] + [N]) ≧ 75−6 × [Cr] + 12 / [Si] (2) High-corrosion-resistant chromium-containing steel excellent in high-temperature strength and intergranular corrosion resistance consisting of the balance and other unavoidable impurities. 3. In addition to the above components, further in weight%
2. Mo: 0.02% or more and 2% or less, Cu: 0.02% or more and 2% or less, and Co: 0.02% or more and 2% or less.
Or a highly corrosion-resistant chromium-containing steel excellent in high-temperature strength and intergranular corrosion resistance according to 2. 4. In addition to the above-mentioned components, further by weight C
The high-temperature strength according to any one of claims 1 to 3, which contains at least one selected from the group consisting of a: 0.0005% or more and 0.003% or less, and B: 0.0002% or more and 0.005% or less. High corrosion resistance chromium-containing steel with excellent intergranular corrosion resistance.