JPS62228454A - Passivity-strengthened high-purity stainless steel - Google Patents

Abstract

PURPOSE:To strengthen passivation without addition of large amounts of expen sive alloying elements and to improve corrosion resistance, by specifying respec tive amounts of C, Si, Cr, Ni, Mo, Cu, N, Al, Nb, S, P, etc. CONSTITUTION:This stainless steel has a composition consisting of, by weight, 0.005-0.1% C, 0.05-3% Si, 9-27% Cr, 1-22% Ni, 0.02-4% Mo, 0.01-3% Cu, 0.005-0.4% N, 0.01-0.8% of one or more elements among Al, Nb, Ti, and V, and the balance Fe. Moreover, in the above composition, respective amounts of S, P, and Mn as impurities are specified as shown in [P](ppm)+10X[S](ppm)<=400 and [Mn](%)+0.38X[S](ppm)<=11.9. In the above stainless steel, passivity is strengthened, and particularly, resistance of passivity to Cl<->, etc., is increased, so that corrosion resistance can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high purity stainless steel reinforced with tlJB.

The purpose of the present invention is to improve the corrosion resistance of stainless steel by reducing impurities in the steel that impair corrosion resistance to the utmost limit, without relying on the addition of large amounts of expensive alloying elements, and by combining this with the addition of small amounts of elements that are effective for corrosion resistance. Our objective is to supply high-quality, inexpensive stainless steel.

Stainless steel is based on a Fe--Cr alloy, and it is well known that it exhibits excellent corrosion resistance due to the passive film formed on its surface, and its use as an Fe-based corrosion-resistant material has been increasingly expanded. With the expansion of its uses, Fe-Cr
The system has evolved from the Fe-Cr-Ni system to the Fe-Cr-Ni-Mo system, and microstructurally ferritic, martensitic, austenitic, and two-phase alloys are well known. These are the fruit of improved corrosion resistance mainly due to the reinforcement of passivation by main alloying elements.

It is thought that these stainless steels will be increasingly used in the future as their applications expand. However, depending on the application, a large amount of Cr, Nj, Mo, etc., which have insufficient corrosion resistance and are expensive, must be alloyed in large quantities, which inevitably increases costs and becomes an obstacle to expanding the range of applications.

Therefore, there has long been a need for a technology that can greatly improve the passivation properties of stainless steel mesh without using large amounts of these expensive alloys.

Today, progress in refining technology is remarkable, especially in high purification technology. The present inventors have demonstrated the corrosion resistance of stainless steel,
In particular, it has been found that the passivation phenomenon is extremely dependent on the purity of the alloy. As a result, it was found that alloys of normal purity contain many impurities and are unable to fully exhibit the passivation properties that the alloy should originally exhibit. By using high-purity steel, the passivation properties are greatly improved, and as a result, stainless steel with excellent corrosion resistance can be supplied at low cost without alloying large amounts of expensive alloying elements such as Cr, Ni, Mo, etc. There is a possibility that it can be done.

From this point of view, the present inventors have repeatedly investigated the corrosion resistance and passivation properties of high-purity alloys that are currently possible.

The elements considered as impurity elements were s, p, and c.

These include N, O, Mn, Si, etc. As shown in Figure 1, the passivation property is electrochemically 5%11□5O.
The anodic polarization behavior was investigated in 430'C1 degassing, and the peak current of active dissolution (1), passivation potential (V,), and passivation retention current (■p) were measured. In addition, when investigating passivation properties when containing ionic species that destroy passivation such as CX-, 5%
)IZSO4+3%NaC7! , the anodic polarization behavior was investigated in degassing at 30 °C, and the passivation through-potential (V, two conveniently 1 m
The potential was set to a current density of A/cnl).

The sweep speed is 50 mV/min in both cases. Corrosion tests were also conducted under various corrosion conditions.

These investigations have revealed that S, P, and Mn are impurity elements that have a particularly bad effect on the passivation properties of stainless steel. Even when the content of C, N, O, Si, etc. was reduced from the usual level to the technical limit, no significant improvement effect was shown.

Among the three elements S, P, and Mn, S in particular has a large effect on passivation properties, and also has a large effect on passivation destruction due to CX-, and it is desirable to have it as low as technically possible. . Although there are influences from other components, S is 10 pp.
The passivation properties are significantly improved when the concentration is lower than 39 ppm, that is, for example, 50 ppm.
It has been found that a remarkable effect can be obtained, which is different from the extrapolated tendency from the pm level.

P is related to S, and it was found that the lower it is, the more the resistance of passive state to CX- increases, and the lower the Mn is, the more the resistance of passive state to CX- increases. In order to study this in more detail, various types of stainless steel were melted in the laboratory and the allowable levels of these impurities were investigated.The composition range of the stainless steel studied was as follows.

C0,005~0.10%, Si0.05~3%, Cr
9 to 27%, N41% to 22% or less, NO,005
~0.4%, Mo0.01~4.0%, Cub, 01
~2.8%, Ti0.02~0.9%, Nb0.02~
0.6%, /'40.01~0.6%, 80.001~
The alloy steel contains 0.05%, Vo, 01-0.7%, etc., and is made into ferritic, martensitic, austenitic, or ferrite + austenite two-phase alloys depending on the alloying amount of Cr and Ni. We investigated the amounts of the three elements S, P, and Mn in these alloy systems, and the lower limits were all as low as possible (for S, from 51) m to soppm, and for P, from 50 ppm to 4001) fl. , Mn was studied at 0.05% to 2% (mainly ferritic and martensitic) and 0.3 to 12% (mainly austenitic and two-phase). Of these, Slop
For the melting of alloys of less than pm, the highest purity electrolytic alloys were used for Cr, Ni, etc., and the base Fe was also melted by sufficiently pre-desulfurizing electrolytic iron with desulfurization flux.

In addition, the analysis of S has large variations when S is less than 10 ppm using the conventional JIS method, and a new method using 2I is used when S is less than 20 ppm.
) An infrared absorption method based on the reduced distilled methylene blue method, which can be analyzed with high accuracy down to the pm level, was developed and analyzed.

The tests were already mentioned with 5% H2SO, anodic polarization curve in 30 °C, as well as 5% tlzsOi + 3% NaCj
! Positive polarization curves at 30°C were measured. In the case of high alloys, some have a high night temperature. 18Cr
The effect of S reduction on the active dissolution peak current value Ip of the anodic polarization curve in 5% 11□SO in the -8Ni system is the second
As shown in the figure, the S reduction effect is extremely significant below 320 to 30 ppm.

Figure 3 shows the positive polarization curve and the effect of reducing p and s in 17Cr-based 5% 1lZsO4+ 3% NaCR. Figure 4 shows that in a low S system, reducing Mn suppresses the passivity destruction of CX- to a level of 14].

Based on these, the following five systems can be used to determine the permissible range of these impurity elements in many alloy systems:
2Cr series, 17Cr series, 18Cr-8Ni series, 18Cr
-8N+-2Mo system, 25Cr 5Ni 2M.

The normal content of impurities in the system (S=50 ppm, P=
230 ppm, Mn 0.33-0°99%) was set as the standard alloy. From the test results of a large number of alloys, the current density (■,) at the active peak during anodic polarization in 5% H2SOJ is 1/10 or less of that of the reference alloy, and Cff- As a guideline to express the resistance to
The permissible limits of S, P, and Mn are determined based on the precondition that the potential corresponding to the current density of The results obtained for steel basically depend on the amount of Ni in the alloy, that is, in alloys where Ni is 1% or less, as shown in Figures 5 and 6, both S and P, and S and Mn are below the diagonal line. It is necessary to regulate the On the other hand, in alloys containing more than 1% Ni, it is necessary to control S and P within the diagonal lines in FIGS. 7 and 8, and it is particularly desirable that they be low even within the diagonal lines. Alloys containing more than 1% Ni have the characteristic that restrictions on Mn can be largely relaxed only when the content is less than 330 ppm. Thus, for alloys containing more than 1% Ni, the SP regulation shown in FIG. 7 and the S-Mn regulation shown in FIG. 8 are required.

In this way, for many stainless steels, by reducing the impurity elements S, P, and Mn in steel to the industrially possible limit for the passivation property and the passivity destruction property due to Cf-, etc. It was clarified that it was possible to strengthen the passive state over a wide range, and therefore, a large improvement in corrosion resistance was obtained, and the amounts of S, P, and Mn that produced this effect in a large number of stainless steels were determined.

In stainless steel containing more than 1% Ni and 9% to 27% Cr, it is necessary to control S, P, and Mn as described below.

In this way, particularly harmful impurities are reduced to the utmost in stainless steel, and Mo is an element that improves corrosion resistance.

It has a synergistic effect with various elements normally contained in stainless steel, such as Cu and Co, and exhibits even better corrosion resistance.

Next, preferred ranges of the content of each element that can be contained in the alloy steel of the present invention will be explained.

The present invention deals with the passivation properties of stainless steel and the passivation breakdown properties due to CX-, etc. Among the impurities in steel, S, P,
Based on the knowledge that Mn has a particularly bad effect, the purpose of this method is to reduce the content of these elements as much as possible industrially to obtain stainless steel with excellent corrosion resistance.

S has a negative effect on both the passivation properties of stainless steel and the passivity destruction properties due to CX-, etc., and the lower the S content, the more
In particular, the improvement effect is particularly significant below 10 ppm. However, the allowable limit is determined in relation to the amounts of P and Mn described below, and is influenced by the amount of Ni in the alloy.

P has an adverse effect on the passive state destruction properties caused by CX-, etc., and the lower the P content, the more desirable it is. Tolerance limits are determined in relation to Sl and are also influenced by Ni in the alloy.

Like P, Mn also has an adverse effect on the passive state destruction properties caused by CX-, etc., and the lower the Mn is, the more desirable it is.

However, the degree of influence differs depending on the alloy, and the adverse effect is particularly large in stainless steel containing 1% Ni or less, and the allowable limit is determined in relation to S. However, even in stainless steel containing more than 1% Ni, the lower the Mn content, the better, but the degree of negative effect becomes smaller.
The tolerance limit is determined in relation to S.

Stainless steel containing more than 1% Ni must satisfy both of the following conditions in order to obtain the desired effect.

(pH (ppm) +10 X (S) (ppm)≦
400 [Mn:l (%) +0.38x [S)
(ppm) ≦11.9 (11) It has been confirmed that the above-mentioned effects are valid for an extremely wide range of stainless steels described below.

Cr is a basic component for passivation of stainless steel, and if it is less than 9%, the desired effect cannot be obtained, and the higher the Cr content, the greater the desired effect, but if it exceeds the upper limit of 27%, it becomes expensive. %. The effect is particularly large under the regulated conditions of S-P and S-Mn.

Regarding Ni, the more Ni there is in controlled alloys of S-P and S-Mn, the more the desired effect is exhibited. In other words, it was found that when Ni is high, the tolerance limit for Mn expands especially in relation to S-Mn, and the branching point of its action and effect is at 1% Ni, and when it exceeds 1%, the tolerance limit for S-Mn expands. do. Therefore, Ni should be more than 1%, and the effect becomes greater as the amount added increases, but if it exceeds 22%, the alloy becomes extremely expensive.

C has no significant effect on the intended effect, and the lower limit is 0.00.
5% is the industrial technical lower limit, and if it exceeds the upper limit of 0.10%, corrosion resistance deteriorates.

N has no significant effect on the intended effect, and the lower limit is 0.00.
5% is the industrial technical lower limit, and is added to improve phase stability and mechanical properties, but if it exceeds the upper limit of 0.4%, addition becomes difficult.

Even a small amount of water is effective for the desired effect, and it is at the impurity level (0
, 01%), and the effect increases with the amount added, but if it exceeds the upper limit of 4.0%, it becomes extremely expensive. When SP and S-Mn are regulated, the effect of MO is particularly large.

Similar to Mo, Cu is also effective in achieving the desired effect and is effective when added at 0.01% or more, which is about the same as an impurity, and when added in a large amount (it is true that the effect is large, but the effect is saturated when the upper limit of 3% is exceeded.S) −
The effect is particularly great when P and S-Mn are regulated.

Si is effective in increasing strength. This effect becomes apparent from an impurity level of about 0.05% and saturates at 3.0%. Therefore, the Si content should be 0.05 to 3.0%.
shall be.

Ti, Nb, A! , V shows the desired effect by containing one or more types within the range of 0.01 to 0.8%, and normal stainless steel is added to the alloy with regulated S-P and S-Mn. As in , one or more types can be selectively added. The upper limit of each component is preferably 0.8%, and if it exceeds this, the effects will be saturated.

In addition, Sn, B, etc. do not have a very large effect and can be selectively added in one or two types as in ordinary stainless steel. The upper limit of each component is preferably 0.05%. If this value is exceeded, the effects of each component will be saturated.

Examples of the present invention are shown below.

As shown in Table 1, 18Cr-8Ni system, 17Cr-12
Ni 3Mo -ICu system, 25Cr 13Ni-
0.8M.

-0,4N system, 25Cr -5Ni -2Mo -2,
Four types of 5Cu-based stainless steel were melted and refined in an electric furnace and AOD, and a de-S and de-P flux was blown into the ladle from the bottom to reach the specified S and P levels. Thereafter, the slab obtained by a conventional method was hot-rolled, further cold-rolled, annealed, and pickled to obtain a cold-rolled product with a thickness of 11 mm. Many properties of this product board, including mechanical properties, were investigated. Various corrosion resistance tests were carried out, including the passivation property and Cp-induced passivity destruction property. Table 1 also shows the component systems of the three types of stainless steels mentioned above as comparative materials. All of the steels of the present invention satisfy the regulation values of S-P and S-Mn, and the comparative steels do not satisfy the regulations, including S.

The passivation properties were determined by measuring the anodic polarization curve in 5% 1lzsOa, and the peak current density of active dissolution was 1. and 5%1hso4
It was evaluated based on the immersion corrosion test value. The passive state destruction properties due to chlorine ions, etc.
A pitting corrosion test in C was also used. The test results are shown in Table 2.

Passivation properties in Cp-free acids, i.e. 5%
Anodic polarization behavior in 11□SO4 solution, for example (■,), as well as 5% 11□SO4? Also in the corrosion test in liquid, the steel of the present invention showed superior corrosion resistance compared to the comparative steel. In addition, it is widely used as a pitting corrosion test as well as passivation breakdown property in a solution containing Cβ-, that is, anodic polarization behavior in a 5% 11□SO4 + 3% NaCl solution, such as passive penetration potential (VS). There are 50 g / (!
FeC1,J+1/2ONIICJ! ? Also in the pitting corrosion test in liquid, the steel of the present invention showed significantly superior properties compared to the comparative steel. From this result, according to the present invention, C
Even if the amount of alloys such as r, Ni, Mo etc. is small, S, P, M
It is clear that high purification technology that regulates n will promote alloy steels that can replace high alloy steels among comparative steels.

[Brief explanation of drawings]

Figure 1 ta+ is 5% ll2So, positive polarization curve of stainless steel in solution, same (b) is 5% H2S0J + 3%
NaCI! Positive polarization curve of stainless steel in solution, Figure 2 shows 18Cr-8Ni in 5%++2sO430°C degassing.
Active dissolution peak current value of anodic polarization curve of stainless steel [3
Figure 3 shows the influence of S on 5%) 12SOa
A diagram showing the influence of S and P on the positive polarization curve of 17Cr stainless steel during +3% NaC130'C degassing,
Figure 4 shows the influence of Mn on the positive polarization curve of 17Cr stainless steel during degassing with 5% IIZSO4 + 3% NaC130"C, and Figure 5 shows the influence of Mn on the positive polarization curve of 17Cr stainless steel during degassing with 5% IIZSO4 + 3% NaC130"C. Figure 5 shows the effect of Mn on the passive state of stainless steel with Ni 1% or less The relationship between S and Pi allowed for, ([P) (ppm) +10X (S) (ppm)
≦350), Figure 6 shows the allowable s, Mnft to strengthen the passivity of stainless steel with Ni 1% or less.
The relationship ((Mn) (%) + 0.018 x (S)
(ppm) ≦0.15), Figure 7 shows Ni1
Permissible P, S barrier (CP) (ppm) to strengthen the passivity of stainless steel exceeding % → -10X C
Figure 8 shows Ni
Allowable relationship between Mn and S to strengthen the passivity of stainless steel exceeding 1% (CMn] (χ) + 0.3
8 X (S) (ppm)≦11.9). Figure 3: Curve ■ (P: 50ppm, S: 10p
4) ■(P: 50ppm, S: 30ppm)■
(P: 50ppm, S: 60ppm) ■(P:
50ppm, S: 140ppm) Curve I (S
:20ppm, P :50ppm)I[(S :2
0ppm, P: 150ppm>Iil (S:
20ppm, P:250ppm)r/(S:
20ppm, P: 340ppm) Figure 4: Curve ■
(S: 10ppm, Mn: 0.07χ)■(S:
10ppm, Mn:0.18χ)■(S:10ppm
, Mn:0.38χ)■(S:10ppm, Mn:
0.80 X) Rod/Mouth (α) (b) 3rd

Claims (1)

[Claims] C: 0.005 to 0.10%, Si: 0.05% by weight
~3%, Cr: 9-27%, Ni: more than 1% and 22% or less, Mo: 0.01-4.0%, Cu: 0.01-3%
, N: 0.005-0.4% and Al, Nb, Ti
and one or two V within the range of 0.01 to 0.8%.
Species or more, S, P and Mn are in amounts defined by the following formula,
High-purity stainless steel with reinforced passivation, the remainder of which is essentially Fe. [P](ppm)+10×[S](ppm)≦400[
Mn](%)+0.38×[S](ppm)≦11.9