JP2602411B2 - Austenitic stainless steel with excellent hot workability and corrosion resistance in hot water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention is excellent in hot workability and corrosion resistance in hot water, especially in stress corrosion cracking in a chloride-containing hot water environment, and is suitable as a material for oil-fired water heaters and electric water heaters. The present invention relates to a high Si content austenitic stainless steel used for steel.

[0002]

2. Description of the Related Art Due to its excellent corrosion resistance, stainless steel has been widely used as a material for neutral chloride environments such as water heaters, food processing machines, and kitchenware.
In particular, with regard to water heaters, the conversion from conventional enameled products to stainless steel has rapidly progressed, partly due to the increase in the amount of heat generated and the downsizing. As stainless steel used for such applications, ferritic stainless steel such as SUS444, which is unlikely to cause stress corrosion cracking, is generally used. SUS444, which has poor weldability, may not be able to be machined. In addition, when used in high-temperature hot water without corrosion protection, significant corrosion occurs at the gap structure and gas-liquid interface, so it is necessary to take measures such as Al corrosion protection, which leads to higher product costs. I was having.

In order to compensate for such disadvantages of ferritic stainless steel, the use of austenitic stainless steel having good formability and weldability, for example, a general-purpose austenitic stainless steel such as SUS 304 has been studied. Stress corrosion cracking due to residual stress is likely to occur in the formed part and welded structure part, especially when the hot water temperature is high, and there was a defect that water leakage occurred early and it became unusable .

For this reason, efforts have heretofore been made to improve the lacking properties, that is, stress corrosion cracking resistance, crevice corrosion resistance, and acid resistance, while keeping the excellent properties of the above steel types as they are. . For example, JP-A-56-4755
No. 1, JP-A-57-158359, JP-A-60-194016
In order to improve the corrosion resistance described above,
It is a steel type to which about wt% of Cu is added. However, this Cu-added stainless steel is also in neutral chloride hot water, especially
It was insufficient for stress corrosion cracking at around 80 ° C.

In order to solve such a problem, Japanese Patent Laid-Open Publication No. Sho 59-185763 discloses that Cu is contained in an amount of 0.30 to 2.00 wt.
%, And at the same time, a steel type in which Si is added in an amount of 2 wt% or more and 4 wt% or less. The critical temperature of stress corrosion cracking resistance is 80%.
It is higher than ° C. Also, in Japanese Patent Application Laid-Open No. 1-159351, N added positively in the above-mentioned Japanese Patent Application Laid-Open No. 59-185763 promotes the erosion depth when crevice corrosion occurs. In order to find out that corrosion cracking is likely to occur,
A stainless steel with a composition of 0.05 wt% or less is proposed. That is, according to this steel, the critical temperature for stress corrosion cracking resistance exceeded 100 ° C. However, since the steel disclosed in Japanese Patent Application Laid-Open No. 1-159351 is also an element that deteriorates hot workability, Cu is added to a component system containing up to 2% by weight, and further hot ductility is added. And the addition of Si, which easily forms a brittle σ phase during hot working, has a problem that hot rolling is extremely difficult and leads to an increase in cost.

On the other hand, in order to improve the hot workability of a component system containing high Si, a stainless steel to which B, Ca, and REM are added has been proposed (JP-A-52-4418). In the case of this technique, on the other hand, a new problem arises in that corrosion resistance, particularly pitting corrosion resistance, is degraded starting from the precipitates of these components, and improvements have been required. Under the same intention, the present inventor has previously proposed a method of reducing S which is harmful to hot workability (see Japanese Patent Application Laid-Open No. 2-48614). In particular, in the case of steel containing 2 wt% or more of Si, it was extremely difficult to secure sufficient hot ductility by itself. Moreover, recent water heaters are increasingly required to be smaller and have higher performance.
As a result, the temperature of the heat transfer surface greatly exceeds 100 ° C., and the required performance for the critical temperature rise of stress corrosion cracking resistance of the materials used is actually becoming stronger.

An object of the present invention is to propose an austenitic stainless steel which is excellent in hot workability and corrosion resistance in hot water and can solve the problems of the prior art. Another object of the present invention is to enable mass production of an austenitic stainless steel excellent in stress corrosion cracking resistance in a chloride-containing hot water environment and to manufacture it inexpensively.

[0008]

According to the study of the present inventors, it is of course effective to reduce S in order to improve hot workability, but it is necessary to reduce the content of high Si steel, especially 2 wt% or more. of
In the case of steel containing Si, the amount of δ-ferrite at the time of solidification is controlled to an appropriate range, and the Ti content is in the range of 0.05 to 0.20 wt% and 2.0 ≦ Si ≦ 7 (Ti + 0.4). It has been found that by adding the same, hot workability can be remarkably improved, and at the same time, corrosion resistance, particularly stress corrosion cracking resistance, is improved, and the critical temperature of stress corrosion cracking resistance in warm water is increased.

The present invention constructed on the basis of such findings is as follows: C: 0.02 to 0.08 wt%, Si: 2.0 to 4.0 wt%, Mn:
1.0 wt% or less, Cr: 15 to 25 wt%, Ni: 8 to 20 wt%,
Mo: 0.1 to 2.0 wt%, Ti: 0.05 to 0.20 wt%, C
u: 1.5 to 4.0 wt%, N: 0.05 wt% or less, P: 0.030
wt% or less, S: 0.003 wt% or less, and Si and Ti
Is the following formula; 2.0 ≦ Si ≦ 7 (Ti + 0.4) is satisfied, the balance is composed of Fe and inevitable impurities, and the amount of δ-ferrite at the time of solidification shown by the following formula: δ = 1.3 ( 1.5Si (wt%) + Cr (wt%) + Mo (wt%) +
0.5Ti (wt%))-(30C (wt%) + 30N (wt%) + 0.5
This is an austenitic stainless steel excellent in hot workability and corrosion resistance in hot water, in which Mn (wt%) + Ni (wt%) + Cu (wt%)) − 8.5 is in the range of 1.0 to 7.0.

[0010]

The following is a description of how the austenitic stainless steel of the present invention was developed. The inventor first studied on the basis of the previously proposed low S steel (see JP-A-2-48614) with the aim of further improving the corrosion resistance of this steel. Under such circumstances, a method was considered in which a large amount of Si was added, thereby improving pitting corrosion resistance as well as stress corrosion cracking resistance. However, when a large amount of Si is added, hot workability is significantly reduced. That is, 13
Since the hot ductility of more than 00 ° C or less than 1100 ° C sharply decreases with an increase in the amount of Si added, the temperature range in which hot rolling can be performed is substantially narrowed, and thus mass production is hindered. all right. In this respect, in the above-mentioned prior art, S
And O are reduced to 0.0010 wt% or less and 0.0060 wt% or less, respectively, and addition of a predetermined amount of B improves the hot workability. However, it has been found that a high Si-containing austenitic stainless steel-containing steel is not necessarily sufficient by itself, and has little effect on, for example, hot ductility exceeding 1300 ° C or less than 1100 ° C. This is high
This is because the presence of Si makes the steel sensitive to the influence of O in the steel, and the hot strength increases. This is also thought to be due to the fact that even if B is added, only a low-melting-point compound is generated, and ultimately, the fixation of O is insufficient.

Therefore, the present inventor focused on Ti, which has a higher affinity for O. That is, the influence of Ti on the hot workability of the high Si content steel with extremely low S was further studied. As a result, the Ti content was set within the range of 0.05 to 0.20 wt%, and the correlation between the Si content and this Ti content was determined.
It has been found that maintaining the relationship properly improves hot workability. In particular, in hot rolling with a planetary mill, which has a rolling reduction of 90% or more in one pass and the temperature of the slab edge is liable to decrease, the temperature range in which the drawing value in a high-temperature tensile test exceeds 80% is 400%. When the temperature is higher than or equal to ° C., sufficient yield can be ensured without ear cracks, and industrial mass production is possible.

Therefore, based on the above knowledge, the relationship between the contents of Ti and Si was examined in detail.
It was found that if the relational expression of ≦ 7 (Ti% + 0.4) was satisfied, hot ductility would be a region where mass production was possible, and completed the present invention. At the same time, it was also found that the critical temperature of stress corrosion cracking resistance was increased by adding an appropriate amount of Ti. Figure 1 shows 3.5
The effect of the addition of Ti and B on the hot workability of steel containing wt% Si is shown. It can be seen that excellent hot workability can be obtained by adding an appropriate amount of Ti to this type of steel. FIG. 2 shows the effect of Si and Ti on the hot workability. When Ti is in the range of 0.05 to 0.2 wt%, Si% ≦ 7 (Ti% + 0.1%).
In the area 4), the aperture value is 80% or more, which means that the area can be mass-produced and a sufficient yield can be secured, and has a good hot ductility.

As is clear from the above description,
The austenitic stainless steel of the present invention exhibits excellent corrosion resistance without adding B by adding a predetermined amount of Ti on the premise of lowering S and lowering O, and has a hot working property that enables mass production. It has.

Next, the reason for limiting the composition of each component element in the steel of the present invention will be described. C: 0.02 to 0.08 wt% C is an element stabilizing austenite, and addition of an appropriate amount contributes to improvement of stress corrosion cracking resistance, but excessive addition increases the intergranular corrosion susceptibility during welding and increases the welding heat. Since the corrosion resistance of the affected area is deteriorated, the content is set to 0.02 to 0.08 wt%. Incidentally, the content is preferably 0.03 to 0.07 wt%.

Si: 2.0 to 4.0 wt% Si is an important element in the present invention, and has an extremely strong effect of improving stress corrosion cracking resistance. Less than 2.0 wt% is not enough to achieve the effect, and it is necessary to add 2.0 wt% or more. However, if added in excess of 4.0 wt%, the hot ductility is significantly degraded.
wt%. As described above, the addition of an appropriate amount of Ti
Since good hot ductility is obtained even if Si is contained, a preferable range from the viewpoint of stress corrosion cracking resistance is 2.5 to 4.0
wt%.

Mn: 1.0 wt% or less Mn is used as a decarburizing agent in steel making. If it is contained in a large amount as an impurity, it forms a compound with S in the steel and becomes a starting point of pitting corrosion, resulting in corrosion resistance. Therefore, the content is set to 1.0 wt% or less.

Cr: 15 to 25 wt% Cr is an essential element from the viewpoint of ensuring corrosion resistance, and it is necessary to add 15 wt% or more. The higher the amount of Cr added, the better, but up to 25 wt% provides sufficient corrosion resistance, so the range of Cr is 15 to 25 wt%.

Ni: 8 to 20% by weight Ni is an austenite-forming element, and at least 8% by weight is required to obtain an austenite single-phase structure.
% Or more is required. However, Ni is a very expensive element, and a sufficient austenitic single-phase structure can be obtained by adding as much as 20 wt%.
20 wt%.

Mo: 0.1 to 2.0 wt% Mo is an element effective for pitting corrosion resistance and crevice corrosion resistance, but is harmful to stress corrosion cracking resistance. In the steel of the present invention, the harmfulness of Mo to stress corrosion cracking resistance is suppressed by adding a large amount of Si or an appropriate amount of Ti, but Mo is an expensive element, and the addition of a large amount makes it easy to generate a σ phase Therefore, the range of Mo is set to 0.1 to 2.0 wt%.

Ti: 0.05 to 0.20 wt% Ti is an element that plays a very important role in the present invention, and has a great effect of improving hot workability and stress corrosion cracking resistance. The effect is not sufficient if the addition is less than 0.05% by weight, and if the addition amount exceeds 0.20% by weight, the ductility during hot work is reduced. Therefore, the range of Ti is set to 0.05 to 0.20% by weight. Note that Ti is further restricted in relation to Si, as described later.

Cu: 1.5 to 4.0 wt% Cu is an element that is extremely effective for stress corrosion cracking resistance, acid resistance, and crevice corrosion resistance, and plays an extremely important role in the present invention. 1.5 to get the required corrosion resistance
It is necessary to add more than wt%, but if it exceeds 4%, hot workability deteriorates rapidly, so the range of Cu is 1.5-4.0 wt%.
Restrict to Preferably, it is 1.5 to 3.0 wt%.

N: 0.05 wt% or less N is an element that improves pitting corrosion resistance and crevice corrosion resistance, but is harmful to stress corrosion cracking resistance and hot workability. Further, if a large amount of N is added, it is combined with Ti which is essential in the steel of the present invention to form coarse precipitates, and a sliver-like flaw is easily generated during steel sheet production. desirable. Therefore, the upper limit of N is set to 0.05 wt% or less. Preferably 0.03wt
% Or less.

P: 0.030 wt% or less P deteriorates stress corrosion cracking resistance and hot ductility, so the upper limit is 0.030 wt%.

S: 0.003 wt% or less S is an element extremely harmful to hot workability. In the present invention, since the main purpose is to improve hot workability, S content is reduced as much as possible. Need to be done. However, there is an industrial limit, so the upper limit is 0.00
It should be 3 wt% or less.

The above-mentioned Si must be added so as to satisfy the following formula in relation to Ti. 2.0 ≦ Si ≦ 7 (Ti + 0.4) In order to improve the stress corrosion cracking resistance, it is necessary to add Si in an amount of 2.0 wt% or more as in the above formula. to degrade. To compensate for this,
Although it is necessary to add an appropriate amount of Ti, it is necessary to adjust the components so as to satisfy the following expression indicating the amount of δ-ferrite, in order to make hot workability possible for mass production. δ = 1.3 (1.5Si (wt%) + Cr (wt%) + Mo (wt%) +
0.5Ti (wt%)-(30C (wt%) + 30N (wt%) + 0.5M
n (wt%) + Ni (wt%) + Cu (wt%)-8.5

Δ at the time of solidification which can be expressed by the above equation
-Ferrite content is an index that affects hot workability,
If it is less than 1.0 or more than 7.0, the hot workability is remarkably reduced, so the range of the amount of δ-ferrite represented by the above formula is from 1.0 to 7.0.

[0027]

EXAMPLES Steel having the composition shown in Table 1 was melted on a laboratory scale to form a 10 kg steel ingot, and test samples were prepared from the steel ingot. That is, a part of the sample was subjected to an ultra-high temperature tensile test in a cast state to evaluate the hot workability from the drawn value, and the other was subjected to hot forging and cold rolling for accuracy test.
It was made to have a size of mmt × 100 mmw × l and subjected to a heat treatment of water cooling at 1100 ° C. × 10 minutes to obtain a test material. All of these test pieces had a # 400 wet polishing finish. In Table 2,
The results of hot workability and accuracy test are shown. As can be seen from the results shown in Table 2, the steels 1 to 6 of the present invention have a reduction of more than 80% at 900 ° C., which is close to the final hot rolling temperature, even though the steel has a high Si content. The temperature at which the value sharply drops (Ni1 temperature) increases with respect to the comparative steel, and the aperture value decreases to 80.
% Is over 400 ° C., and it is clear that mass production is possible, especially with planetary mills.
Further, it is understood that the steels of the present invention all have a critical temperature of stress corrosion cracking resistance in hot water exceeding 100 ° C, and have improved stress corrosion cracking resistance as compared with the comparative steel with the addition of high Si and Ti composites. .

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

As described above, the present invention relates to an austenitic stainless steel to which a large amount of Si has been added in order to improve the stress corrosion cracking resistance in hot water, and to further increase the Ti content.
By adding an appropriate amount in connection with Si, it also has hot workability for enabling mass production without deteriorating its corrosion resistance. Moreover, a high Si-containing austenitic stainless steel excellent in corrosion resistance can be provided at relatively low cost. In particular, stainless steel according to the present invention,
It can be seen that it can be used stably for a long time in a high temperature neutral chloride water environment such as a water heater or a boiler.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the addition of B and Ti on the hot workability of steel containing 3.5 wt% of Si.

FIG. 2 is a graph showing the effect of a composite addition of Si and Ti on the hot workability of a high Si content steel.

Claims (1)

(57) [Claims] 1. C: 0.02 to 0.08 wt%, Si: 2.0 to 4.0 wt%
%, Mn: 1.0 wt% or less, Cr: 15 to 25 wt%, Ni: 8 to 20 wt%, Mo: 0.1 to 2.0 wt%, Ti: 0.05 to 0.20 wt%, Cu: 1.5 to 4.0 wt%, N: 0.05 wt% or less, P: 0.030 wt% or less, S: 0.003 wt% or less, and Si and Ti satisfies the following formula: 2.0 ≦ Si ≦ 7 (Ti + 0.4), with the balance being Fe And unavoidable impurities, and the amount of δ-ferrite during solidification as shown in the following formula: δ = 1.3 (1.5Si (wt%) + Cr (wt%) + Mo (wt%) +
0.5Ti (wt%))-(30C (wt%) + 30N (wt%) + 0.5
Austenitic stainless steel with excellent hot workability and corrosion resistance in hot water, with Mn (wt%) + Ni (wt%) + Cu (wt%) − 8.5 in the range of 1.0 to 7.0.