JPH0321624B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ferritic stainless steel having high toughness, excellent corrosion resistance, and good weldability. In Japanese Patent Application No. 55-59359 filed simultaneously with the present invention,
Ferritic stainless steels are described that are characterized by excellent crevice corrosion and intergranular corrosion resistance. The steel of the above application is disclosed in US Pat. No. 3,932,174 in the following respects:
No. 3929473. That is, the steel contains up to 2% of elements selected from the group consisting of titanium and columbium and more than 275 parts of carbon per million parts, according to the following formula: %Ti/6+%Cb/8%C+%N Plus nitrogen. Due to the high carbon and nitrogen content, it costs less to melt and refine than the steels of both of the above patents. According to the present invention, a steel which is even stronger than the steel according to the above-mentioned co-pending application is provided. In addition to a stabilizer selected from the group consisting of titanium and columbium, and a carbon and nitrogen content of more than 275 parts per million, the steel of the invention also contains from 2.00 to 5.00%, preferably 3.00%. Contains up to 4.50% nickel.
However, the steel according to said application contains less than 2.00%, usually
Contains less than 1.00% nickel. Nickel has been found to increase the toughness of the alloy in the above application. By reason of extension, the alloys of the present invention can be clearly distinguished from those of the aforementioned US Pat. Nos. 3,932,174 and 3,929,473. This is also distinct from that of US Pat. No. 4,119,765. The alloy of U.S. Pat. No. 4,119,765 is below that of the present invention.
The highest content of molybdenum is specified. Another interesting document is “Ferrite
There is a paper titled "Corrosion Resistance and Economics of Stainless Steel." This paper relies on RALula,
Published in the June 1976 issue of Metal Progress, pages 24-29. However, this does not disclose stainless steel according to the present invention. Therefore, the object of the present invention is to obtain one ferrite.
is to provide stainless steel. The ferritic stainless steel of the present invention is characterized by excellent toughness before and after welding, excellent crevice and intergranular corrosion resistance, and good weldability. The ferritic steel of the present invention mainly contains up to 0.08% carbon, up to 0.06% nitrogen, 28.50 to 30.50% chromium, and 3.60 to 5.60% by weight.
% molybdenum, 2.00% manganese, 2.00 to 5.00% nickel, up to 2.00% silicon, up to 0.5% aluminum, and 2.00% of elements selected from the group consisting of titanium and columbium. , consisting mostly of residual iron. The sum of carbon and nitrogen exceeds 0.0275%. Titanium and columbium are represented by the following formulas. %Ti/6+%Cb/8%C+%N In this formula, titanium and columbium are necessary for stabilizing carbon and nitrogen, that is, for corrosion resistance caused by these elements, and especially for preventing deterioration of toughness "as welded". The amount of Note that there is a limit on the amount of titanium and columbium (in this formula, 2.00
% or less (1.00% or less in the formula shown on page 8) because there is a concern that weldability may deteriorate if titanium or columbium is present in excess. Carbon and nitrogen are usually at least 0.005% and 0.010%, respectively, where their sum exceeds 0.0300%.
present in an amount of The amounts of chromium and molybdenum are preferably set at 28.50 to 30.50% and 3.75 to 4.75%, respectively, but such high chromium and significant molybdenum amounts are
This is to improve corrosion resistance, especially crevice corrosion resistance, under conditions where corrosion is likely to accelerate, such as at 50°C. In addition, the amount of manganese and silicon are both
2.00% or less (usually 1.00% or less), aluminum content is limited to 0.5% or less (usually 0.1% or less),
This is due to the following reason. In other words, such elements should be allowed to be contained to some extent in steel, as they are derived from deoxidizing agents during steel manufacturing, but excessive manganese promotes the austenitization of ferrite, which in turn impairs corrosion resistance. This is because excessive aluminum deteriorates weldability. Incidentally, nickel is used in an amount of 2.00 to 5.00% (preferably 3.75 to 4.75%) as an element necessary for improving toughness, but the upper limit is set because an excessive amount also promotes austenitization. Titanium and columbium are added to enhance the crevice and intergranular corrosion resistance of the alloy and represent a high carbon plus nitrogen content variation of the above-cited US Pat. No. 3,929,473.
It has been confirmed that stabilizers can be added to the high carbon and/or nitrogen containing variants of the above US patent without impairing the toughness and/or weldability of the alloy. has been done. Within the scope of the present invention, any desired amount of stable titanium or as columbium may be added, although it is preferred to add at least 0.15% titanium, insofar as the sole presence of columbium can adversely affect the weldability of the alloy. A curing agent can be added.
With regard to the toughness of the alloy, columbium has a superior effect compared to titanium. Certain embodiments of the invention require the addition of at least 0.15% columbium and at least 0.15% titanium. Titanium and columbium are preferably 1.00 according to the following equation:
may be present in amounts up to %. %Ti/6+%Cb/8=1.0 to 4.0 (%C+%N) Nickel is added to the alloy of the present invention to increase its toughness. This is 2.00 to 5.00%,
Preferably, it can be added in an amount of 3.00 to 4.50%. The ferritic stainless steel of the present invention is particularly suitable for use as a welded product. The following examples represent several embodiments of the invention. Ingots made from 24 batches (batches A-X) were heated to 1121°C, hot rolled into 3.2mm thick strips, annealed at temperatures between 1066°C and 1121°C, and rolled into 1.6mm thick strips. It was cold rolled into strips and annealed at temperatures between 1066°C and 1121°C. The hot rolled and cold rolled specimens were subsequently evaluated for their strength. The specimens were TIG (tungsten inert gas) welded and strength tested. The chemical components of each batch sample are shown in Table 1.

【table】

TABLE It is noted in Table 1 that the compositions of batches A to F do not belong to the present invention. That is, they do not contain 2.00 to 5.00% nickel. The invention relies on a nickel content of 2.00% or more. The table represents data regarding the above chemical components of the batches.

[Table] Next, we used a small transversely pierced V-shaped notched specimen to examine the hot-rolled and annealed material (3.2
× 10 mm specimen), cold rolled and annealed material (1.6 × 10 mm specimen), as-welded specimen (1.6
Toughness was evaluated by determining the transition temperature for a welded and annealed specimen (1.6 × 10 mm specimen) and a welded annealed specimen (1.6 × 10 mm specimen). The transition temperature is 50
Appearance was based on % malleability and 50% brittle fracture. The table shows the transition temperatures for hot and cold rolled specimens. Batch A to L
were annealed at 1066℃, and the others were annealed at 1121℃.

[Table] Furthermore, the transition temperatures for as-welded specimens and post-weld annealed specimens are shown in the table. Batches A through F were annealed at 1066°C prior to welding, and the other specimens were annealed at 1121°C. Each batch was quenched with water. Batch A
F to F were annealed at 1066°C after welding, and the others were annealed at 1121°C. Each specimen welded,
After annealing, it was quenched with water.

【table】

【table】

[Table] From the table and table above, the effect of Nickel is clear. The transition temperatures of batches G to X in the above table are significantly lower than those of batches A to F.
It is extremely strong. It goes without saying that batches G to X belong to the present invention, but batches A to F do not belong to the present invention. Batch G to X is 2.00%
Contains more than nickel. The lower transition temperature for batches G to X is
Although illustrated in a table, the table is a combination of tables and.

Note that in each of the above examples, the highest transition temperature for batches G through X is lower than the lowest transition temperature for batches A through F. This data clearly shows that batches G-X are more intense than batches A-F. Additional batches G-X specimens were evaluated for crevice and intergranular corrosion resistance.
These specimens were prepared similarly to the specimens listed above. Crevice corrosion resistance was evaluated by immersing 25.4 mm x 50.8 mm polished specimens in a 10% ferric chloride solution for 72 hours. The exam is 50
It was carried out at a temperature of °C. The gap is at 32℃.
It was made by using front and back Teflon blocks that were stretched lengthwise and crosswise and held in place by a pair of rubber bands. Testing was conducted according to American Testing and Materials Institute G-48-76. The results of the above evaluation are shown in Table 1.
Three specimens were prepared: cold rolled and annealed, as welded, and welded and annealed.

[Table] From the table, it is noted that Batch G to X have extremely excellent crevice corrosion resistance. The alloy of the present invention was confirmed to have excellent crevice corrosion resistance. Intergranular corrosion resistance was evaluated by immersing 25.4 mm x 50.8 mm polished specimens in a boiling copper sulfate/50% sulfuric acid solution for 120 hours. The usual pass/fail criteria for this test are an annual corrosion rate of 0.61 mm (50.8 microns per month) and satisfactory microscopic examination. This test is recommended for stabilized high chromium ferrite stainless steels. The table shows the results of the above evaluation. Specimens were presented in as-welded and as-welded and annealed conditions.

[Table] From the table, batches G to L and S to X
However, it is recognized that all specimens passed the inspection, exhibiting excellent intergranular corrosion resistance.

Claims (1)

[Claims] 1. up to 0.08% by weight of carbon;
up to 0.06% nitrogen, 28.50 to 30.50% chromium, 3.60 to 5.60% molybdenum, up to 2.00% manganese, and 2.00 to 5.00% nickel.
up to 2.00% silicon, up to 0.5% aluminum, up to 2.00% elements from the group consisting of titanium and columbium, and the balance substantial iron, the sum of said carbon and nitrogen. but
A ferritic stainless steel in which the content of titanium and columbium exceeds 0.0275%, the titanium and columbium satisfy the following relationship, and the steel has high toughness, excellent corrosion resistance, and good weldability. %Ti/6+%Cb/8%C+%N2 Ferritic stainless steel according to claim 1, containing 3.00 to 4.50% nickel. 3 Contains at least 0.005% carbon and at least 0.010% nitrogen, and the sum of carbon and nitrogen is 0.0300%
The ferritic stainless steel according to claim 1, which exceeds the scope of claim 1. 4. Ferritic stainless steel according to claim 1, containing 3.75 to 4.75% molybdenum. 5. Ferritic stainless steel according to claim 1, containing up to 1.00% of elements selected from the group consisting of titanium and columbium, according to the following equation: %Ti/6+%Cb/8 = (1.0-4.0) (%C+%N) 6 Ferritic stainless steel according to claim 1, comprising at least 0.15% titanium. 7. A ferritic stainless steel according to claim 6 containing at least 0.15% columbium. 8 At least 0.005% carbon and at least 0.010%
of nitrogen, 28.50 to 30.50% chromium, 3.75 to 4.75% molybdenum, and 3.00 to 4.50% nickel, and furthermore, the sum of the carbon and nitrogen is
Ferritic stainless steel according to claim 1, containing more than 0.0300% and also up to 1.00% of elements selected from the group consisting of titanium and columbium, according to the following equation: %Ti/6+%Cb/8 = (1.0~4.0) (%C+%N)