JPS599620B2 - Low carbon corrosion resistant chromium alloy steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to the improvement of low carbon iron chromium alloy steel with excellent corrosion resistance. steel,
That is, low carbon corrosion resistant Fe-C is treated with calcium in a non-oxidizing atmosphere to lower carbon, oxygen and sulfur and improve deoxidation products, further improving the toughness of -Mo alloy steel. % of improved alloy steel with improved corrosion resistance.

Conventionally, austenitic stainless steels based on 18-8 stainless steel have been mainly used for chemical industrial materials and other applications requiring corrosion resistance. It has drawbacks such as stress corrosion cracking.

On the other hand, ferritic stainless steel has excellent stress corrosion cracking resistance, and is also expected to have improved corrosion resistance against intergranular corrosion, pitting corrosion, and general corrosion by reducing the impurity elements carbon and nitrogen.

The present inventors first added metallic calcium or calcium alloy to chromium-containing molten steel in a vacuum furnace or vacuum degasser in a vacuum or argon gas to perform decarburization, deoxidation, and desulfurization refining. % or less, sulfur 0.0
20% or less, oxygen 0.015% or less, nitrogen 0.025%
Below, we proposed a molybdenum-containing iron-chromium alloy steel with reduced impurity elements and excellent corrosion resistance. Subsequently, as we continued to push forward with practical application, we conducted intensive research to further improve the toughness of welded materials and thick plates of this type of alloy steel. It has been found that when these elements are added alone or in combination, the toughness is further improved and the corrosion resistance is further improved even when the cooling rate of the alloy steel from a high temperature is varied over a wide range. In other words, the steel of the present invention reduces the impurity elements in the steel by adding niobium or tantalum alone or in combination with calcium treatment to the iron-chromium alloy, and improves corrosion resistance against intergranular corrosion, pitting corrosion, and general corrosion. In addition, by changing the form of non-metallic inclusions contained in the steel to CaO-containing non-metallic inclusions, toughness, particularly workability, is improved, and impact properties are prevented from deteriorating during slow cooling. The purpose of the present invention is to provide a high chromium alloy steel that achieves a dramatic improvement in toughness by changing the formation of chromium carbide to niobium or tantalum carbide.

The composition of the steel of the present invention is less than 0.02% carbon and 0.025% nitrogen.
% or less, chromium 25-38 parts, molybdenum 0.5-5 parts, nickel width 5 or less, copper 3 qb or less, calcium 0.0
0.005 to 0.02%, oxygen of 0.015% or less, and sulfur of 0.020% or less, and the sum of sulfur and oxygen is 0.02%.
0.025% or less, for niobium and tantalum, the Nb%+1/2Ta ratio alone or in combination is less than 1 part and the carbon ratio is 20%.
~60 times, and the deoxidation products remaining in the steel are reduced to 0.1
The composition of the deoxidized product is 3 to 20% CaO, Al
2O35-80%, SiO25-80%, and a small amount of C
It is characterized by containing r2O3, MnO+FeO*CaS, etc.

The present invention is achieved by adding metallic calcium or calcium alloy to chromium-containing molten steel in a vacuum furnace or vacuum degassing device and refining it, and then adding niobium or tantalum, which has a high affinity for carbon, either singly or in combination. The aim is to provide a chromium alloy steel with excellent toughness and corrosion resistance by fixing harmful carbon as niobium or tantalum carbide.

Next, the reason for limiting the composition of the steel of the present invention will be described.

(1) Calcium 0.0005 to 0.02: The amount of calcium detected after vacuum treatment of the chromium alloy steel of the present invention is a minimum of 0.0005%, and a maximum of about 0.02% improves corrosion resistance and other properties. It is effective for Calcium 0.0
If 2% or more is added, sufficient decarburization can be achieved, but a large amount of CaO remains, impairing corrosion resistance and other machinability. Furthermore, if the amount of calcium added is less than 0.0005%, decarburization will not proceed sufficiently and the properties will be impaired. For the above reasons, calcium is 0.0005
It is preferable to add about 0.02%. (2) Oxygen 0.01
Reason for limiting the sum of sulfur and oxygen to 0.025% or less with sulfur of 0.020 or less and sulfur of 0.020 or less: Oxygen is 0.015%.
If this is the case, the amount of oxides remaining as impurities will increase, impairing corrosion resistance and machinability.
It is necessary to keep it below 15%.

If sulfur remains in an amount of 0.020% or more, the mechanical properties will deteriorate, so it is necessary to keep it at 0.020% or less.

For the same reason as above, if the sum of sulfur and oxygen is not 0.025% or less, the corrosion resistance and other mechanical properties will be impaired, so the sum of sulfur and oxygen is limited to 0.025% or less. (3)
When the deoxidation product is less than 0.1%, the composition of the deoxidation product is CaO
3-20%, AA2O35-80%, SiO25-8
Reason for including Section 0: When metallic calcium or calcium alloy is added to the chromium-containing molten steel in vacuum or argon gas in the chromium alloy steel of the present invention in a vacuum furnace or vacuum degassing device, metallic calcium or This is based on experimental values that measured the upper and lower limits of changes in the composition of nonmetallic inclusions that change depending on the composition of calcium alloys. The product was limited to 0.1 coefficient or less.

In addition to the above three components, the deoxidized product contains a small amount of Cr2O3MnO.
．． FeO. This includes CaS, etc.

(4) Nb, Ta6OXC% ≧Nb% + 1/2Ta Salary ≧20 When used in combination with a calcium addition treatment, it provides even better toughness in both cases of rapid cooling from high temperature and slow cooling, and further improves corrosion resistance.

When the amount of niobium added is 20 to 60 times that of carbon, the effect is most remarkable, but if it exceeds 1 part, solid solution hardening of niobium occurs and the toughness decreases again. Tantalum has the same effect as niobium, but it requires twice the amount of niobium. Therefore, when niopium and tantalum are added alone or in combination, good properties can be obtained by considering Nb%+1/2Ta% as the niobium equivalent and setting the niobium addition range to the above range. (5) Carbon content of 0.02 or less: Carbon is an element that is mixed in during the manufacturing process, and at the same time, carbon is not preferable in ferritic stainless steel because it impairs corrosion resistance.

However, if the carbon content is about 0.02%, the stress corrosion resistance and cracking resistance will not be impaired, and at the same time, if niopium and tantalum are added within the range of the present invention, harmful carbon will not precipitate at the grain boundaries. Therefore, the alloy steel of the present invention also has good seawater resistance.

(6) Nitrogen 0.025% or less: Nitrogen, like carbon, is an element that deteriorates corrosion resistance and toughness, and the lower its content is, the more desirable it is, but its negative effects are not as pronounced as carbon, and it is 0.025% or less. In order to have sufficiently good corrosion resistance and toughness, the nitrogen content should be reduced to 0.02.
It was set to 5% or less.

(7) Chromium 25-38: Chromium is an essential element for improving corrosion resistance, and from this point of view, the more chromium the better.

However, when the chromium content exceeds 38, hot workability is impaired.
At the same time, if the chromium content is less than 25%, not only no improvement in corrosion resistance can be expected, but also the characteristics as a ferritic stainless steel are lost, so the lower limit for chromium was set at 25%. (8) Molybdenum 0.5 to 5: Molybdenum is an element that improves corrosion resistance and has the ability to form ferrite.

Therefore, in order to exhibit this effect, the lower limit of molybdenum was set to 0.5. Furthermore, the reason why we set the upper limit of molybdenum to 5 is that it is no longer necessary from the viewpoint of corrosion resistance due to the relationship with other components, chromium content, carbon content, oxygen content, etc. At the same time, molybdenum is expensive, so the upper limit was set as This is due to the fact that it was assigned to Section 5. (9) Nickel 5% or less; Nickel is added mainly to improve low-temperature impact value.

However, nickel is expensive and is a strong austenite-forming element, and too much nickel impairs the properties of ferritic steel, so the upper limit for nickel is 5.
I was in charge. (10 Copper 3% or less: Copper penetrates to some extent during the manufacturing process and is effective for corrosion resistance, stress corrosion cracking resistance, and intergranular corrosion. From this point of view, the upper limit of copper was set at 3%.

Next, the present invention will be specifically explained using examples.

EXAMPLE Samples of the alloy steel of the present invention, the previously developed alloy steel (A) (Japanese Patent Application No. 47-33437), and the comparative alloy steel (B) were prepared and tested for toughness and corrosion resistance.

First, after melting low nitrogen, low carbon Fecachrome (0.009% nitrogen, 0.03% carbon), electrolytic iron, and ferromolybdenum in a vacuum induction melting furnace, sample plate 2, /F64, A
For samples 6.5 and 6, 0.5% of calcium alloy was added, and for samples 43, 46.4, scrap 5, and flat 6, 0.15% of ferronniobium was added and refined, and these molten steels were refined. The cast steel ingot is hot forged and hot rolled to a thickness of 107 mm.
After forming a plate of rLm, it was cold rolled into a plate with a thickness of 3 mm, and this plate was subjected to various tests. The compositions of these steels are shown in Table 1. A Charpy impact test was conducted to evaluate the toughness of the material.

First, a cold-rolled plate was annealed at 900°C for 10 minutes, then water-cooled and air-cooled. Charpy impact test pieces with a 2 mm V notch were prepared from each plate and impact tests were conducted in the range of -120°C to -30°C. The results are summarized in Table 2, and the melt number A
Comparative alloy steel (A) invented before 6”2 (patent application 1972-
33437), comparative alloy steel FB with only niobium addition as a comparative material of melt number /163> and melt number. The impact test results of No. 44 alloy steel of the present invention (C) were divided into cases of annealing water cooling treatment and air cooling treatment, and are shown in FIGS. 1 and 2, respectively.

As is clear from these results, the toughness of the alloy steel of the present invention is far superior to that obtained only by calcium treatment or niopium addition treatment. It is very effective in improving toughness when In the corrosion resistance test, each cold rolled steel plate was annealed at 900°C for 10 minutes and then air cooled, which is a condition that is bad for corrosion resistance.
A pitting corrosion resistance test was conducted in a 3+1/21M HclAl) solution at 60°C, and the results are shown in Table 3.

As is clear from this, the pitting corrosion resistance of the alloy steel of the present invention is superior to that of an earlier invention! It can be seen that this is superior to TA) (I2).

The acid resistance of the alloy steel of the present invention was investigated in boiling solution of 51H2Sα.
It was slightly better than I62), and showed stable corrosion resistance with little variation in the degree of corrosion.

[Brief explanation of the drawing]

The drawings are characteristic diagrams showing the results of impact property tests of the steel of the present invention and comparative steel, in which Figure 1 shows the impact properties of the material treated with water cooling after annealing, and Figure 2 shows the impact properties of the material treated with air cooling after annealing. .

Claims (1)

[Claims] 1 Chromium 25-38%, molybdenum 0.5-5%, nickel 5% or less, pot 3% or less, carbon 0.02% or less, calcium 0.0005-0.02%, oxygen 0.015%
Below, nitrogen is 0.025% or less, sulfur is 0.020% or less, and the sum of sulfur and oxygen is 0.025% or less, and niobium or tantalum alone or in combination is 60×C%.
≧Nb%+1/2Ta%≧20×C% and Nb
%+1/2Ta%≦1, and the deoxidation product remaining in the steel is 0.1% or less, and the composition of the deoxidation product is CaO3~20%, Al_2O_35~
A low carbon corrosion resistant chromium alloy steel with high toughness characterized by 80% SiO_25 to 80%.