WO2011027847A1

WO2011027847A1 - Low ni stainless steel having excellent corrosion resistance

Info

Publication number: WO2011027847A1
Application number: PCT/JP2010/065091
Authority: WO
Inventors: 道郎金子; 正治秦野; 美茶崎谷
Original assignee: 新日鐵住金ステンレス株式会社
Priority date: 2009-09-02
Filing date: 2010-08-27
Publication date: 2011-03-10
Also published as: TW201113382A; CN102471855A; CN102471855B; JP5528459B2; JPWO2011027847A1; TWI412610B

Abstract

Disclosed is stainless steel having excellent corrosion resistance, which is characterized by containing, in mass%, 0.1% or less of C, 1.0% or less of Si, 0.045% or less of P, 0.005% or less of S, 17-22% of Cr, 4-12% of Mn, 2-6% of Ni, 0.5-3% of Cu and 0.05-0.3% of N, and additionally containing either La and/or Ce respectively in an amount of 0.006% or more but 0.2% or less in total, with the balance made up of Fe and unavoidable impurities.

Description

Ni-saving stainless steel with excellent corrosion resistance

The present invention relates to stainless steel used for automobiles, home appliances, kitchens, buildings, etc., and particularly relates to stainless steel that is inexpensive and excellent in cold workability and corrosion resistance.

Austenitic stainless steel is represented by SUS304 steel, and is the steel type used for a wide range of applications among stainless steels. However, since SUS304 steel contains Ni, it has a disadvantage of being expensive.
On the other hand, there is ferritic stainless steel as a stainless steel that does not contain Ni or contains a small amount, but it has a drawback that it is generally inferior in cold workability as compared with austenitic stainless steel such as SUS304 steel.
Therefore, attempts have been made to replace Ni with an inexpensive alloy element in austenitic stainless steel. Steel types in which Ni is partially replaced with Mn and N are standardized by JIS as SUS201 and 202.
For example, Patent Document 1 discloses a high-strength non-containing material having a low Ni content and containing Si: 1% or less, Mn: 14-16%, Cr: 15-19%, and N: 0.3-0.4%. Magnetic stainless steel is disclosed.
Patent Document 2 includes Si: 1 to 5%, Mn: 16 to 25%, Cr: 5 to 12%, N: 0.1 to 0.3%, and high strength and high ductility not containing Ni. High Mn steel is shown.
However, although these steel types have the advantage that high strength is easily obtained and become non-magnetic, there is a disadvantage that the corrosion resistance is inferior due to the addition of Mn.

JP 60-197853 A Japanese Patent Laid-Open No. 2-8351

As described above, the high-strength nonmagnetic austenitic stainless steel in which Ni, which is an expensive alloy additive element, is substituted with Mn has a problem that the corrosion resistance is reduced due to Mn substitution. An object of the present invention is to provide a stainless steel having excellent corrosion resistance even when Ni is replaced with Mn.

The inventors diligently studied stainless steel that can ensure corrosion resistance even when Ni is replaced with Mn.
As a result, it was found that the corrosion resistance can be ensured even when Ni is replaced with Mn by adding a predetermined amount of one or both of La and Ce while reducing the S concentration in the stainless steel.
Further, it has been found that the corrosion resistance is remarkably improved when the total concentration of La and Ce and the S concentration have a predetermined relationship.
Furthermore, in the stainless steel which substituted Ni with Mn, it also discovered that addition of excess Si reduced corrosion resistance.
The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.1% or less, Si: 1.0% or less, P: 0.045% or less, S: 0.005% or less, Cr: 17 to 22%, Mn: 4 to 12%, Ni: 2 to 6%, Cu: 0.5 to 3%, N: 0.05 to 0.3%, and at least one of La and Ce is 0.006% or more and in total Stainless steel excellent in corrosion resistance, characterized by comprising 0.2% or less and the balance being Fe and inevitable impurities.
(2) The stainless steel having excellent corrosion resistance according to the above (1), further comprising B: 0.0002 to 0.015% by mass%.
(3) When the total concentration of La and Ce is (A) mass% and the S concentration is (B) mass%, the relationship of the following formula (1) is satisfied: Stainless steel excellent in corrosion resistance as described in (2).
0.005% + 25 (B)% ≦ (A)% ≦ 0.02% + 36 (B)% (1)

According to the present invention, stainless steel having excellent corrosion resistance can be obtained even when Ni in stainless steel is replaced with Mn and the amount of expensive Ni added is reduced.
The stainless steel of the present invention is particularly effective for applications such as automobiles, home appliances, kitchens, and construction because the amount of Ni added is small.

First, the reasons for limiting chemical components in the present invention will be described. In addition, "%" display of the content of each element means "mass%".
C: 0.1% or less,
C precipitates as Cr carbide at the grain boundary during the cooling process after the solution heat treatment, thereby forming a chromium-deficient layer and lowering the corrosion resistance. Moreover, C addition becomes a solid solution strengthening and reduces cold workability. For this reason, the upper limit is made 0.1%. A preferable upper limit is 0.06%. In order to stabilize the austenite structure, 0.04% or more is preferably added.
Si: 1.0% or less,
Si is an element that serves as a deoxidizer during dissolution, but when added in excess, it promotes the formation of a δ ferrite phase at high temperatures and reduces hot workability.
Further, although it has been reported that Si is effective for improving corrosion resistance, the present inventors have newly found that, in high Mn Ni-saving stainless steel, excessive addition of Si causes a decrease in corrosion resistance.
Therefore, the upper limit of Si is 1.0%. A preferable upper limit of Si is 0.6%. Further, the lower limit of Si is preferably 0.1%.
P: 0.045% or less Since P deteriorates corrosion resistance and hot workability, the upper limit is made 0.045%.
S: 0.005% or less S forms inclusions and lowers corrosion resistance, so the upper limit is made 0.005%. The relationship between the total concentration of La and Ce and the S concentration will be described later.
Cr: 17-22%
Cr is the most important element for the corrosion resistance of stainless steel, and at least 17% or more is necessary. However, if over 22% is added, hot workability is reduced due to the formation of δ ferrite at high temperatures, so 22% is made the upper limit. A preferable upper limit of Cr is 20%. Further, the lower limit of Cr is preferably 18%.
Mn: 4-12%
Mn is an austenite-forming element that replaces Ni, and at least 4% or more must be added. However, excessive addition of Mn degrades the corrosion resistance, so the upper limit is 12%. In order to ensure the corrosion resistance in the acid rain air environment, it is more preferable to set it to 10% or less. More preferably, it is 8% or less.
Ni: 2-6%
Since it is difficult to obtain an austenite structure with Mn alone, Ni that is an austenite-generating element is required to be at least 2%. It is also effective for improving corrosion resistance.
However, excessive addition of Ni causes an increase in manufacturing cost, so the upper limit of Ni is 6%. Preferably it is 5.5% or less, More preferably, it is 5% or less.
Ni is most preferably in the range of 2.5 to 4%.
Cu: 0.5 to 3%
Cu is an austenite-generating element and an element that improves acid resistance, and at least 0.5% or more must be added. However, excessive addition of Cu reduces the hot workability by forming a low melting point Mn—Cu phase or Cu phase, so the upper limit of Cu addition is 3%. In particular, when emphasizing hot workability, it is more preferable to set it to 2.5% or less.
N: 0.05 to 0.3%
N is an austenite-forming element and an element effective for improving corrosion resistance, and at least 0.05% is necessary. However, excessive addition of N causes a decrease in cold workability due to a significant increase in strength or causes blowholes during solidification, so the upper limit is made 0.3%. In consideration of the corrosion resistance, the stability of the austenite structure, and the decrease in cold workability, a more preferable range of N is 0.07 to 0.15%.
In addition to the elements described so far, La and Ce alone or both are added. These elements contribute to the control of the form and properties of oxides and sulfides, and are indispensable for improving the corrosion resistance of stainless steel in which Ni is replaced with Mn. At least 0.006% or more of each element must be added. There is.
However, if these elements are added in excess of 0.2% in total, the cleanliness of the stainless steel is lowered, and coarse oxides of several μm or more of La and Ce are formed, which becomes the starting point of pitting corrosion.
When the corrosion resistance is particularly important, such as sea salt particle scattering or acid rain environment, La and Ce are each preferably set to 0.10% or less.
Although there are many unclear points regarding the mechanism of improving corrosion resistance by adding La and Ce alone or both, it is related to pitting corrosion of Ni-rich high-Mn stainless steel starting from water-soluble MnS. It is considered a thing. That is, when La or Ce alone or both is added to Ni-saving high-Mn stainless steel, a sulfide containing La or Ce that is poorly water-soluble alone or both is formed, and the formation of MnS is inhibited, thereby improving the corrosion resistance. Is presumed to bring about.
It is more preferable to add La and Ce in combination. This is presumed to contain more sparingly water-soluble sulfides by containing both La and Ce.
Furthermore, hot workability and corrosion resistance can be improved by adding B. In order to obtain such an effect, it is necessary to add at least 0.0002% or more of B.
However, B is an element that easily segregates at the grain boundary, and excessive addition lowers the corrosion resistance at the grain boundary, so 0.015% is made the upper limit.
Further, when adding La and Ce alone or in combination, it is preferable that the total concentration of La and Ce and the S concentration have a predetermined relationship after the S concentration is reduced as described above. The reason for this will be explained based on the results of the following studies.
C: 0.05%, Si: 0.25%, P: 0.02%, Cr: 18 to 19%, Mn: 6.0 to 8.0%, Ni: 3.0 to 4.0%, Cu: 2.0-3%, N: 0.09-0.11% as the basic component system, S concentration in the range of 0.0005-0.0050%, La and Ce concentrations, respectively, the analysis limit A steel ingot of 40 kinds of components varied in a range from 0.10% to 0.10% was prepared using 10 kg vacuum melting.
This steel ingot was hot-rolled to a thickness of 4 mm, annealed at 1200 ° C. for 5 minutes, then dipped in nitric hydrofluoric acid to remove the scale, and then cold-rolled to a thickness of 1 mm to 1080 ° C. After annealing for 3 minutes, the surface was ground 0.1 mm or more by wet emery paper polishing to prepare a test piece.
The test piece thus obtained was tested according to “Method for measuring pitting corrosion potential of stainless steel” of JIS G 0577.
For each specimen, the pitting potential at which the current density is 100 μA / cm ² is 300 mV or more on the basis of the SCE (standard calomel electrode), and the extension of the total concentration range of La and Ce is S When investigated in relation to the concentration, it was found that the total concentration of La and Ce and the S concentration have a very strong correlation and can be expressed by a linear function of the S concentration.
Therefore, as a result of regression analysis, it was found that the pitting corrosion potential was 300 mV or higher when the relationship of the following formula (1) was satisfied, and excellent corrosion resistance was exhibited.
That is, when the total concentration of La and Ce is (A) mass% and the S concentration is (B) mass%, the steel components preferably satisfy the following formula (1).
0.005% + 25 (B)% ≦ (A)% ≦ 0.02% + 36 (B)% (1)
This is presumed to be a result of the degree of poor water solubility of sulfides containing La and Ce alone or both being affected by the S concentration in the steel.
Therefore, it is estimated that when the steel component is in the range represented by the above formula (1), a sulfide containing extremely poorly water-soluble La and Ce alone or both is formed, and particularly contributes to improvement of corrosion resistance. Is done.
On the other hand, when the steel component is out of the range of the above formula (1), the corrosion resistance is slightly lowered.
That is, when the total concentration of La and Ce is smaller than the S concentration, the formation of sulfide containing La or Ce alone or both is insufficient, and the generation of MnS cannot be sufficiently suppressed. Is the starting point of
On the other hand, when the total concentration of La and Ce is excessive with respect to the S concentration, a large amount of oxide containing La or Ce alone or both is generated, and a coarse inclusion in which sulfide is combined is formed. . Since the inclusions composed of oxides and sulfides are coarse of several μm or more, they become a starting point for pitting corrosion.
That is, the lower limit of the above formula (1) is that the addition of La and Ce alone or in combination suppresses the precipitation of water-soluble MnS, and sufficiently produces sulfide containing single or both of poorly soluble La and Ce. And the total concentration of La and Ce necessary for suppressing pitting corrosion in the pitting corrosion potential test.
On the other hand, the upper limit of the above formula (1) is that when La and Ce are added singly or in combination, a complex oxide inclusion containing a coarse oxide of several μm or more including sulfide and oxide is generated. This is considered to correspond to the starting point of pitting corrosion.
Therefore, better corrosion resistance can be obtained by adjusting the contents of the three components La, Ce, and S so as to satisfy the above formula (1).

The present invention will be further described in the examples. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. It is not something. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
A steel ingot of 85 mm x 90 mm x 250 mm is prepared in a high-wave vacuum melting furnace, the surface is mechanically ground, then heated in an electric furnace at 1200 ° C for 60 minutes, and the plate thickness is 5 mm with a four-stage rolling mill Until hot rolled.
The obtained hot-rolled sheet was annealed at 1200 ° C. for 5 minutes, the scale was removed by immersion in nitric hydrofluoric acid, and then cold-rolled to 1 mm with a four-high rolling mill. The obtained cold-rolled sheet was annealed at 1080 ° C. for 3 minutes, and the scale was removed by immersion in hydrofluoric acid.
Corrosion resistance was evaluated by a rating number (RN) based on JISG0595 “Stainless steel surface rust generation degree evaluation method” after conducting an exposure test for 6 months in Gushigami Village, Okinawa Prefecture.
The reason for adopting RN as the evaluation method is that it is closer to the actual corrosive environment than the pitting corrosion test. In addition, since the result of a pitting corrosion test and RN are qualitatively positive, those with excellent pitting corrosion test results tend to have a high RN in the actual exposure test.
Tables 1 and 2 show the components of the inventive examples and comparative examples, the degree of ear cracking during hot rolling, and the RN based on JISG0595. In Tables 1 and 2, component values that are outside the scope of the present invention are underlined.

Comparative Examples 1 and 2 correspond to martensitic SUS430 steel and austenitic SUS304 steel, respectively, but the inventive examples all have RNs higher than Comparative Example 1 which is equivalent to martensitic SUS403 steel. It was confirmed that it was high and excellent in corrosion resistance.
In addition, it was confirmed that Examples 1 to 3 and 5 to 28 of the present invention had corrosion resistance equivalent to or higher than that of Comparative Example 2 corresponding to austenitic SUS304 steel.
That is, it was confirmed that Examples 1-3, 5-28 of the present invention had corrosion resistance equivalent to or higher than that of SUS304 steel without adding a large amount of Ni as in SUS304 steel.
In addition, in Example 4 of the present invention, Ni is close to the lower limit of the range of the present invention, and Si that causes a decrease in the corrosion resistance of the high Mn-saving Ni-type stainless steel is high within the range of the present invention.
Therefore, RN of Invention Example 4 is 6, which is slightly inferior in corrosion resistance as compared with SUS304 steel, but considering that the addition amount of Ni is near the lower limit of the range of the present invention and is economical, it is practical. It has sufficient corrosion resistance.
And among the inventive examples, in inventive examples 5, 6, 11, 12, 17, 23, and 26 that satisfy the above formula (1), RN was 8, and it was confirmed that the corrosion resistance was particularly excellent.
In Tables 1 and 2, Examples 1 to 7 and 10 to 19 of the present invention in which the addition amount (content) of B is 0.0001% do not actively add B, and are inevitable impurity levels. Indicates that there is.
Among Invention Examples, Invention Examples 8 and 9 where B is not added, and Invention Examples 1 to 7 and 10 to 19 where the addition amount (content) of B is an inevitable impurity level, ear cracks are generated during hot rolling. In contrast, in Examples 20 to 28 in which B was added and 0.0002% or more of B was added to the steel, no ear cracks occurred during hot rolling.
On the other hand, in Comparative Examples 3 to 10, since the concentrations (contents) of La and Ce were outside the lower limit of the present invention, the corrosion resistance was inferior and the RN was low.
Further, Comparative Example 11 had poor corrosion resistance because La and Ce were out of the lower limit, Si was out of the upper limit, and Cu was out of the lower limit.
In Comparative Example 12, B is outside the upper limit, in Comparative Examples 13 to 15, the total concentration of La and Ce (total content) is outside the upper limit, and in Comparative Example 16, S is outside the upper limit. The corrosion resistance was poor.
In addition, the place mentioned above is only what illustrated embodiment of this invention, and this invention can add a various change within the description range of a claim.

As described above, according to the present invention, even if the amount of expensive Ni added is reduced, corrosion resistance comparable to that of SUS304 steel can be obtained, so that a member requiring corrosion resistance can be provided at low cost. The present invention has high utility value industrially.

Claims

In mass%, C: 0.1% or less, Si: 1.0% or less, P: 0.045% or less, S: 0.005% or less, Cr: 17-22%, Mn: 4-12%, Ni: 2 to 6%, Cu: 0.5 to 3%, N: 0.05 to 0.3%, and at least one of La and Ce is 0.006% or more and 0.2 in total. % Stainless steel excellent in corrosion resistance, characterized by comprising Fe or less and the balance Fe and unavoidable impurities.
The stainless steel excellent in corrosion resistance according to claim 1, further comprising B: 0.0002 to 0.015% by mass%.
3. The relationship of the following formula (1) is satisfied when the total concentration of La and Ce is (A) mass% and the S concentration is (B) mass%. Stainless steel with excellent corrosion resistance.
0.005% + 25 (B)% ≦ (A)% ≦ 0.02% + 36 (B)% (1)