JPS5818425B2 - Austenitic stainless steel with excellent cracking resistance and press formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an austenitic stainless steel sheet with excellent resistance to cracking and press formability, which is seen in utensils made by deep drawing an austenitic stainless steel plate.

This relates to tenitic stainless steel.

Generally J I S-8TJS 301 and 5TJS
Austenitic stainless steel such as No. 304 has good press formability because it is rich in ductility and toughness and has appropriate work hardenability.

Therefore, after going through a single-stage or multi-stage press processing process,
It is often deep drawn; for example, deep containers such as deep sinks, pots or pasta tubs are made from this type of stainless steel.

However, even if the drawing process is performed successfully, vertical cracks will occur in the molded product after the fact, and the process up to that point will be wasted, resulting in economical problems and problems.
It happens all the time, incurring a lot of time loss.

This vertical cracking phenomenon that occurs in deep-drawn products is called placement cracking or period cracking, and the ease with which it occurs mainly depends on the chemical composition of the steel, especially the Ni and H2 content. is said to be dominant.

. An example of the influence of Ni content is as follows.

Steel with the composition shown in Table 1 is made into a 30 kg steel ingot, which is then hot rolled and cold rolled to 0.8 mr.
n thickness, the plate is air annealed and pickled using the conditions normally applied to the production of austenitic stainless steel plates, and this plate is drawn in multiple stages to produce flat bottoms with various drawing ratios. Table 1 shows the results of investigating the maximum drawing ratio that does not cause cracking when processed into a cylindrical cup and left at room temperature for two days and nights.

As is clear from the table, increasing the Ni content in steel
Increasing the drawing ratio that does not cause cracking due to placement, that is, the effect of improving cracking resistance due to placement is remarkable.

Therefore, in high Ni-containing steel such as JIs-8 US305 steel, there is no need to worry about occurrence of cracking in practice.

However, the use of such a high Ni-containing steel is not only expensive, but also has the disadvantage that it is prone to breakage during forming because its press formability, particularly its stretch formability, is not satisfactory.

On the other hand, a plate brightly annealed in an H2 or decomposed ammonia gas atmosphere absorbs H2 during annealing, so if the steel has the same composition, it is much more prone to cracking than a plate that has been air annealed and pickled. .

This situation is shown in Table 2. Regarding steels with different compositions, the ranking of cracking resistance of each steel quantity obtained by air annealing and pickling finished plates (plates with a small amount of H2 in the steel) is as follows. Even if a bright annealed plate is compared, the results are not reversed.

However, when deep drawing is performed at a large drawing ratio where there is a concern that cracks may occur, bright annealed plates are not used.
It is clearly better to use an air annealed-pickled finish.

Now, in order to suppress and prevent the number of cracks in deep-drawn austenitic stainless steel products using conventional methods, or to increase the drawing ratio that does not cause cracks, it is necessary to increase the N1 content of the steel or press In this case, it is necessary to preform with a deep drawing amount that does not cause cracking, then anneal it and then draw it again to form the desired shape, or draw it to the desired depth and then anneal it, or It was necessary to take steps such as dehydrogenation heat treatment on the steel plate in order to significantly reduce the amount of H2 in the steel sheet.

However, such measures waste valuable Ni resources, increase the price of steel sheets, and require extra labor and energy in the process of manufacturing steel sheets or deep-drawn products.

The inventors conducted experiments and studies aimed at overcoming these problems and providing an inexpensive austenitic stainless steel with excellent resistance to cracking due to aging.In addition to the effects of the elements mentioned above, the inventors found that We obtained the knowledge that C increases susceptibility to cracking in place, but N has little effect on this property, and we developed an austenitic system with excellent resistance to setting cracking without impairing hot workability or press formability. Succeeded in developing stainless steel.

That is, the weight percentage is 0; 0.04% or less, 8111.
0% or less, Mn: 1.0-4.0%, Nt;
7.5-9.5%, Or; 17.0-19.0%
, N: 0.03 to 0.08%, and C+N: 0.04
The remaining ~0.10% is an austenitic stainless steel consisting of Fe and unavoidable impurities.

In this invention, the reason why each alloy component of the steel is limited as described above is as follows.

(1) O: A small amount improves the cracking resistance, but it provides a suitable working hardness and is effective in improving formability, so the limit of 0.
The upper limit was set at 0.04%.

(2) N: In this invention, C is 0.0 as described above.
Although the cracking resistance was improved by limiting the amount to 4% or less, the amount of δ-ferrite phase in the cast structure increased accordingly.
The lower limit was set at 0.03% in order to prevent damage to the surface of the steel sheet by inhibiting heat workability during the manufacturing process of the steel sheet, and to help stabilize the austenite phase.

Furthermore, since too much content increases intergranular corrosion and stress corrosion cracking susceptibility of the molded product, the upper limit was set at 0.08%.

The lower limit of the sum of C and N was set at 0.04% because if the amount is less than this, a large amount of δ-ferrite phase will be generated in the cast structure, and the steel will become too soft and press formability will deteriorate. The reason for setting the upper limit to 0.100% is that the austenite phase becomes too stable and the stretchability of the steel plate decreases. This is because chromium compounds of N are precipitated, resulting in a decrease in corrosion resistance.

(3) Si: The upper limit was set at 1.0% as the amount of Si remaining in the steel used for recovering Or and deoxidizing to improve cleanliness during steel melting.

(4) The amount required to stabilize the Mn niostenite phase and provide appropriate strength to the steel is 1.0 to 4.
゜0%.

(5) Ni: An essential component to maintain and improve cracking resistance, and if it is less than 7.5%, the austenite phase will be stabilized by reducing the C content, which is a component of the present invention, and adding abundant N. However, not only does the aging cracking resistance not improve to the desired degree, but also the deep drawability deteriorates.

In addition, if the concentration exceeds 9.5%, even if the cracking resistance is relatively good even without taking the above measures, press formability, especially stretch formability, will decrease, furthermore, Ni resources will be wasted and steel sheet prices will increase. Therefore, the above range was set.

(6) Or: Lower limit value is set to 17 to maintain corrosion resistance.
．． It was set to 0%.

On the other hand, the higher the content, the better the corrosion resistance (ζ), but since a large amount of δ-ferrite phase is generated in the cast structure, impairing hot workability during the steel plate manufacturing process, the upper limit is set at 19.0.
%.

The steel of this invention may be melted and refined in an electric furnace, a converter furnace, other ordinary melting furnaces, or a combination of these and a vacuum degassing furnace, but the method of adding N to the steel is N2 It is preferable to blow gas into the molten steel.

The reason for this is not only the economic efficiency of not using a nitride alloy, but also the fact that dehydrogenation of the steel by N2 gas injection at the time of molten steel reduces the hydrogen content of the plate after atmospheric annealing and pickling. This is because it helps to keep the temperature low and is effective in reducing the susceptibility of steel plates to cracking.

Next, embodiments of the present invention will be described.

Example Steel having the chemical composition shown in Table 3 was vacuum melted to form a 30 kg steel ingot, which was hot rolled and cold rolled to a thickness of 0.8 mm.
After making it thicker, it was made into a plate with an air annealing and pickling finish.

Three types of raw plates of 76.78 mm and 82 mm diameter taken from these plates were squeezed out with a 40 mm diameter flat-bottom cylindrical punch, left at room temperature for 48 hours, and checked for cracks. Those with no cracks. For that, further increase it to 37mmφ
The material was drawn out with a punch, left at room temperature for 48 hours again, and checked for cracks, and then deep drawn with the punch shown in Table 4 to a maximum drawing ratio of 3.28 to avoid cracking. The maximum aperture ratio was determined.

Note that a large drawing ratio means that the cracking resistance is excellent.

The above test results are also listed in Table 3.

Table 5 shows the results of a press formability test conducted on some of the steels shown in Table 3.

Steel B and Steel H both have a C+N content of approximately 0.07% and a Ni content of approximately the same level, but Steel B has a critical drawing ratio of 2.71 and has a longer shelf life than Steel H. The excellent crackability is thought to be due to the low C content of 0.04% or less.

Comparing Steels A, B, and F, it can be seen that the addition of an amount of N within the range according to the present invention improves the hot workability of the steel without impeding the resistance to aging cracking.

Also, even when comparing steel C and steel, C, which is the steel of the present invention,
It exhibits cracking resistance that exceeds that of conventional steel containing about 1% more Ni.

As shown in Table 5, the steel of the present invention is far superior to the conventional steel in terms of press formability, and it is clear that the steel is highly practical.

As described above, according to the present invention, the cracking resistance can be improved without wasting Ni or requiring any special process.
This can be achieved particularly advantageously and with good press formability.

Claims (1)

[Claims] 1 O: 0.04% or less, N: 0.03, in weight percentage
~0.08% and C1N; 0.04-0.10%. St + 1.0% or less, Mn: 1.0 to 4.0%
, Ni near, 5-95%, Or: 17.0-19.
An austainer having excellent resistance to aging cracking and press formability, characterized by containing 0% Fe and the remainder being substantially Fe. Ito-based stainless steel. □