JP2007039741A - High manganese stainless steel with excellent stress corrosion cracking resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-manganese stainless steel having superior stress corrosion cracking resistance in an aqueous solution containing a high concentration of halide ions at a high temperature. <P>SOLUTION: The high-manganese stainless steel having superior stress corrosion cracking resistance comprises, by mass%, C of 0.001-0.025%, Si of 0.25-2.0%, Mn of 15-30%, P of 0.001-0.045%, S of 0.0001-0.02%, N of 0.3-1.0%, Cr of 14-25%, Ni of 0.01-1.0%, Cu of 0.01-1.0%, and the balance Fe with unavoidable impurities. The high-manganese stainless steel further having improved crevice corrosion resistance further includes at least one or more elements selected from the group consisting of Mo, W and V in an amount of 0.5-4.0%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a high manganese stainless steel excellent in stress corrosion cracking resistance, and particularly suitable for stainless steel used in an aqueous solution containing high-temperature halide ions, and high manganese stainless steel excellent in stress corrosion cracking resistance. Related to steel.

  High manganese stainless steel is an alloy steel that has been developed as a non-magnetic steel using manganese, which is an inexpensive additive element, as an alternative to the expensive nickel-based stainless steel, and is a device member that uses a strong magnetic field such as a superconducting magnet. And is expected to be used for structural members around the equipment.

  In addition, for the purpose of improving the corrosion resistance of high manganese stainless steel, high manganese stainless steel in which chromium is added to the steel has been developed. In addition, a Mn-Ni nonmagnetic steel with improved stress corrosion cracking resistance of such a high manganese stainless steel is disclosed and has excellent stress corrosion cracking resistance in an aqueous solution of sodium chloride at 70 ° C and 8%. (For example, refer to Patent Document 1).

JP-A-57-026146

  However, in a high-temperature aqueous solution containing halide ions such as high-concentration chloride ions, even if it is the Mn-Ni stainless steel of the invention disclosed in Patent Document 1, sufficient stress corrosion cracking resistance There are concerns that do not have.

  Therefore, in view of the above situation, the present invention is suitable for applications such as chemical plants, transportation equipment, energy-related materials, food fields, structural materials, etc., and has high corrosion resistance and resistance to stress corrosion cracking. The object is to provide stainless steel.

The gist of the present invention is as follows.
(1) By mass%, C: 0.001 to 0.025%, Si: 0.25 to 2.0%, Mn: 15 to 30%, P: 0.001 to 0.045%, S: 0 .0001-0.02%, N: 0.3-1.0%, Cr: 14-25%, Ni: 0.01-1.0%, Cu: 0.01-1.0% A high manganese stainless steel excellent in stress corrosion cracking resistance, characterized in that the balance consists of Fe and inevitable impurities.
(2) Further, at least one selected from the group of Mo: 0.5-4.0%, W: 0.5-4.0%, V: 0.5-4.0% by mass The high manganese stainless steel excellent in stress corrosion cracking resistance according to the above (1), wherein the total content is 0.5 to 4.0%.

  The high manganese stainless steel of the present invention has extremely excellent stress corrosion cracking resistance even in a high temperature aqueous solution containing halide ions such as high concentration chloride ions. Corrosion-resistant steel suitable for use as a member and structural member in Japan, and contributes to the improvement of the life of facilities and equipment.

  As a result of performing various studies to improve the stress corrosion cracking resistance of high manganese stainless steel, the present inventors have found that stress corrosion cracking of high manganese stainless steel in an aqueous solution containing a high concentration of halide ions is It was found that there is a high possibility of APC (Active Pass Corrosion). Therefore, as a result of careful examination of the effects of various alloy elements based on prevention of coarse slip during deformation and reduction of dissolution rate at the crack tip, the carbon concentration, nickel concentration, and copper concentration in steel were reduced. At the same time, it has been found that appropriately adjusting the silicon concentration, chromium concentration, nitrogen concentration and manganese concentration works very effectively. The present invention has been completed based on such findings.

  In the following, the present invention will be described in detail. However, the detailed role of each element has many unclear points due to the difficulty of analyzing the film or solution at the crack tip.

  Carbon in steel is an element that promotes the occurrence of stress corrosion cracking, and its content must be 0.025% or less. Even if the carbon concentration is lower than 0.001%, the effect of improving the stress corrosion cracking resistance is almost saturated. Therefore, the lower limit of the carbon concentration in the steel is set to 0.001%.

  Unlike carbon, silicon in steel is an element that effectively acts to improve stress corrosion cracking resistance, and at least its content must be 0.25% or more. However, if added over 2.0%, the elongation and toughness of the material are impaired, so the content is made 2.0% or less.

  Manganese is an indispensable element for improving the stress corrosion cracking resistance and the stability of the austenite structure, and at least 15% is required. However, if added over 30%, the generation of manganese fume at the melting stage becomes remarkable, the workability is reduced, and the effect of improving the stress corrosion cracking resistance is almost saturated. The upper limit.

  Since phosphorus causes a decrease in hot workability, its content is set to 0.045% or less. However, even if the value is lower than 0.001%, the hot workability hardly changes, so 0.001% is set as the lower limit.

  Sulfur tends to be a starting point of pitting corrosion due to a decrease in hot workability or formation of sulfide inclusions, so its content is set to 0.02% or less. However, even if the value is lower than 0.0001%, the effect on hot workability and corrosion resistance is small, so 0.0001% is set as the lower limit.

  N is an element effective for increasing the strength, and its content is required to be at least 0.3%. However, if it exceeds 1.0%, the strength is too high and the workability deteriorates, so 1.0% is made the upper limit.

  Chromium is an element that forms a passive film mainly composed of chromium oxide on the surface of the material and is indispensable for improving the corrosion resistance, and its content is required to be at least 14% or more. However, if added over 25%, the proportion of the ferrite phase increases, causing elongation and a decrease in toughness, so 25% is made the upper limit.

  Both nickel and copper are elements that are often used to improve the corrosion resistance of stainless steel. However, the steel of the present invention has been found to promote the occurrence of stress corrosion cracking, and the content of any element is 1 0.0% or less is necessary. However, even if it is reduced from 0.01%, the improvement effect is saturated, so 0.01% is made the lower limit.

  Furthermore, it is effective to add 0.5% or more in total of at least one selected from the group of molybdenum, tungsten and vanadium for the purpose of improving pitting corrosion resistance and crevice corrosion resistance, as necessary. To do. However, if added over 4%, these elements are all additive elements that stabilize the ferrite phase, and the stability of the austenite phase of the parent phase is lowered, so 4% is made the upper limit.

  The production of the high manganese stainless steel, which is the steel of the present invention, is produced by the same production process as that of an ordinary austenitic stainless steel containing nickel. That is, it is usually melted in an electric furnace, subjected to secondary refining and alloy adjustment by AOD (Argon Oxygen Decarburization) or VOD (Vacuum Oxygen Decarburization), and then continuously cast. Furthermore, it is processed into the form of a plate, a rod, a tube, etc. according to the form used. After the processing, it is usually subjected to a solution heat treatment which is heated and cooled in the range of 1000 ° C. to 1100 ° C.

  Table 1 shows the analysis results of the components in the steels of the present invention steel and the comparative steel and the results of the stress corrosion cracking test. Components that fall outside the scope of the present invention are indicated by underlines. For the stress corrosion cracking test piece, a test piece having a thickness of 2 mm, a width of 10 m and a length of 75 mm was taken from the center of a hot rolled plate having a thickness of 5 mm, and a U-bend test piece was prepared in accordance with JIS G 0576. The stress corrosion cracking resistance was evaluated by two types of stress corrosion cracking test methods.

  In Test Method 1, 0.01 cc of artificial seawater is placed on the top of the U-bend test piece, placed in a constant temperature and humidity chamber, maintained at a temperature of 85 ° C. and a relative humidity of 35% for 2 weeks, and tested. Then, the crack was observed using the optical microscope. Those that were broken are indicated by a cross, and those that were not broken are indicated by a circle. Note that those that were not cracked and that did not generate rust in the appearance observation were also marked with ◎ because they were excellent in pitting corrosion resistance.

  In test method 2, stress corrosion cracking resistance was evaluated using a U-bend specimen in a boiling 42% magnesium chloride solution in accordance with JIS G 0576 (A) method b. The test time is 240 hours, and those that have cracked within this time are indicated by x, and those that have not cracked are indicated by ○.

  These two types of stress corrosion cracking tests are cases in which Test Method 1 leads to cracking starting from pitting corrosion. Since Test Method 2 is an extremely severe corrosive environment, pitting corrosion etc. However, it is considered that this is a test method in which cracks are generated directly from the deformed portion of the surface of the test piece. As is clear from Table 1, the steel of the present invention has such a severe high temperature and high concentration chloride solution. It can be seen that cracks are not generated in the inside, and that it has extremely excellent stress corrosion cracking resistance. Moreover, it turns out that the invention steel which added Mo, W, and V also improved the pitting corrosion resistance, and the generation | occurrence | production of rust is suppressed.

  Since the comparative steel 1 has a low manganese content and a high nickel content, both stress corrosion cracking tests 1 and 2 have stress corrosion cracking. Since the comparative steel 2 had a nickel content lowered to 2.5%, no stress corrosion cracking occurred in Test 1, but stress corrosion cracking occurred in Test 2, which was severer than Test 1. Since Comparative Steel 3 contains 2.5% of copper, it was not cracked in Test 1 as in Comparative Steel 2, but cracks were generated in Test 2. Since the comparative steel 4 has a high carbon content of 0.2%, both tests 1 and 2 were cracked.

Claims (2)

% By mass
C: 0.001 to 0.025%,
Si: 0.25 to 2.0%,
Mn: 15-30%,
P: 0.001 to 0.045%,
S: 0.0001 to 0.02%,
N: 0.3 to 1.0%
Cr: 14 to 25%,
Ni: 0.01 to 1.0%,
Cu: 0.01 to 1.0%
A high manganese stainless steel excellent in stress corrosion cracking resistance, characterized in that the balance is made of Fe and inevitable impurities. Furthermore, in mass%,
Mo: 0.5-4.0%
W: 0.5-4.0%
V: 0.5-4.0%
The high resistance to stress corrosion cracking according to claim 1, comprising at least one selected from the group consisting of 0.5 to 4.0% in total. Manganese stainless steel.