JPH07109006B2 - Method for producing A-l containing austenitic stainless steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an Al-containing austenitic stainless steel.

Conventional technology It is generally known that when Al is added to stainless steel, it exhibits excellent oxidation resistance in a high-temperature oxidizing atmosphere.
For example, SUH21, FCH2, etc. are widely and generally used for members for combustion appliances, heating wires, and the like. However, these steel types have fundamentally low high-temperature strength as compared with austenitic stainless steels because the matrix phase is ferrite, and are currently unusable for high-temperature structures.

Many attempts have been made so far by adding Al to austenitic stainless steel to produce a steel type having excellent oxidation resistance. For example, Japanese Examined Patent Publication No. 55-43498 proposes that an austenitic steel having excellent oxidation resistance can be obtained by including Al in an amount of more than 4.5%. However, when a large amount of Al is added to an austenitic stainless steel, Ni- An Al-based intermetallic compound precipitates in the austenite matrix, cracks during hot working, or becomes extremely brittle during cooling after hot working, and the manufacturing process during coil rewinding or cold working At present, it is difficult to commercialize austenitic stainless steel containing a large amount of Al because cracks occur in the steel.

Problems to be Solved by the Invention In the present invention, cracks during hot rolling due to precipitation of Ni-Al-based intermetallic compounds as described above, or cracks generated in the manufacturing process after rolling are prevented, and a large amount of Al is contained. The purpose of the present invention is to enable commercial production of austenitic stainless steel having excellent oxidation resistance.

Means for Solving the Problems In the present invention, in an austenitic stainless steel containing 2.0% by weight or more of Al, the billet is heated to 1150 ° C. or more before hot rolling, and hot rolling is performed at 900 ° C. or more. Immediately after the completion, by rapidly cooling to 600 ° C or less at a cooling rate of 5 ° C / sec or more, cracking during hot working is prevented, embrittlement due to slow cooling is suppressed, and in the manufacturing process after hot rolling. It is possible to prevent cracking.

The present invention will be described in detail below.

Action First, the reasons for limiting each component will be described.

C is an element that is unavoidably contained in steel, but when the content is large, a large amount of chromium carbide precipitates at high temperature, and the upper limit for embrittlement the material was set to 0.2%.

Si generally has the effect of improving oxidation resistance, but Al
The effect is small in austenitic steel containing a large amount of. On the other hand, Si tends to concentrate in the Ni-Al intermetallic compound and stabilize the compound. That is, in austenitic stainless steel containing a large amount of Al, if containing a large amount of Si, Ni-
The upper limit was set to 2.0% because it promotes the precipitation of Al-based intermetallic compounds and causes cracking.

Mn is also an element that is unavoidably contained in steel, but it easily concentrates on the outermost surface at high temperature and easily evaporates, which hinders the formation of a protective oxide film on the alumina film, making it the largest alloy of Al-containing stainless steel. Since the characteristic oxidation resistance is deteriorated, the upper limit of the Mn content is set to 2.0%.

Ni is a basic element that makes Al-containing stainless steel an austenitic steel, and is necessary to maintain high temperature strength and creep properties. Within the component range limited by the present invention, it is difficult to maintain the parent phase in austenite unless Ni is added in an amount of at least 15%. Also, when added in excess of 35%, a large amount of Ni-Al-based intermetallic compounds precipitate in the matrix phase, and the embrittlement of the material becomes remarkable, and cracking occurs even if the manufacturing method proposed in the present invention is adopted. Difficult to prevent. Therefore
The Ni content was limited to 15-35%.

Cr is an indispensable additional element in order to obtain a high degree of oxidation resistance together with Al, in the component system limited in the present invention, mainly to form an alumina film on the surface to maintain the oxidation resistance, If Cr is less than 12%, the alumina film is not formed and the oxidation resistance is extremely deteriorated. In other words, if there is no effect of Cr for forming the alumina film, the formation of the alumina film is difficult and the characteristics of the Al-containing stainless steel disappear. However, if the Cr content exceeds 25%, the σ
Since a large amount of phases and carbides precipitate and become brittle, it cannot be used as a high temperature material. In other words, from the performance of the product, the Cr content range was set to 12-25%.

Al is the greatest feature of stainless steel that can be produced by the method of the present invention, and austenitic stainless steel containing 2% or more of Al cracks during hot rolling, especially heat treatment, unless the method described in the present invention is used. It is difficult to prevent embrittlement during the cooling process after completion of rolling. In addition, in order for the produced Al-containing stainless steel to stably form a uniform alumina protective film on the surface of the steel material during high temperature use and to exhibit outstanding oxidation resistance,
It is desirable that Al is contained at 4.0% or more.

However, if the Al content exceeds 6.0%, it will be 1150 ° C before hot rolling.
Even if heated above, it is difficult to completely eliminate the Ni-Al-based intermetallic compound, during or after hot rolling, the remaining compound becomes the starting point of cracking, so the Al content is 6.0.
% Or less is desirable.

Addition of an appropriate amount of REM or Ca causes S, which deteriorates workability,
Although it has the effect of fixing O and reducing cracks due to these impurities during hot rolling, addition of these elements alone is not effective in suppressing embrittlement during cooling after hot rolling. Furthermore, excessive addition promotes crack generation with the opposite effect.

Next, the hot rolling method proposed by the present invention and its effect will be described in more detail.

Figure 1 shows the TTP (Time-temperature-Precipitation) curve of austenitic stainless steel containing 5.0% Al. From this figure, it can be seen that the NiAl intermetallic compound precipitates in the austenite matrix at 850 ° C in the shortest time.
At 750 ° C or lower, Ni ₃ Al type intermetallic compounds are finely precipitated in the austenite matrix. When this Ni ₃ Al type intermetallic compound precipitates, this steel type becomes extremely brittle. However
This intermetallic compound does not precipitate below 600 ° C.

From these findings and various experimental data by the present inventor, as shown in claim 1, heating before hot rolling is performed at 1150 ° C. or higher, and hot rolling is completed at 900 ° C. or higher, and immediately after that. The inventor of the present invention has found that quenching to 600 ° C. or lower can prevent cracking during hot rolling and embrittlement that occurs during subsequent cooling.

The reason for heating to 1150 ° C or higher before hot rolling is that in order to eliminate the NiAl-based intermetallic compound precipitated in the austenite matrix during solidification, it must be heated to 1150 ° C or higher. Remains, not only becomes brittle after hot rolling, but also becomes a starting point of cracking during hot rolling.
Further, the steel type containing a large amount of Al has a higher deformation resistance at high temperature than that of conventional austenitic stainless steel, and also has a high recrystallization temperature. However, when the heating temperature exceeds 1300 ° C, the grain boundaries are melted in the part and the hot workability is deteriorated. Therefore, the heating temperature is preferably 1300 ° C or lower.

When the hot rolling end temperature is lower than 900 ° C, a large amount of NiAl-type intermetallic compound is precipitated in the austenite matrix and embrittlement after hot rolling becomes significant. Therefore, the hot rolling end temperature is 900 ° C or higher. Even if hot rolling is performed at 900 ° C or higher, a small amount of N
The iAl-type intermetallic compound precipitates but does not cause cracking during hot rolling or embrittlement after hot rolling.

The greatest feature of the present invention is that it is rapidly cooled to 600 ° C. or lower immediately after the hot rolling is finished. This is to prevent embrittlement due to a Ni—Al-based intermetallic compound, particularly a Ni ₃ Al type intermetallic compound that precipitates at over 600 ° C. to 750 ° C. For this purpose, a cooling rate of 5 ° C / sec or more is required. However, in a normal hot rolling process, the coil is wound on a coil and air-cooled, so that at a cooling rate of 0.01 to 0.1 ° C./sec, a large amount of Ni ₃ Al type or NiAl type intermetallic compound is precipitated and becomes brittle. Therefore, in actual hot rolling, it is effective to water cool to 600 ° C or lower after finishing rolling and then wind the coil.

Another rapid cooling method is to wind the coil at a high temperature of 900 ° C or higher, and then cool the coil with water, but considering the ease of operation and safety, the former method after water cooling is preferable. desirable. If the cooling rate is low and the precipitation of Ni ₃ Al type or NiAl type intermetallic compound cannot be suppressed, the hot-rolled sheet becomes brittle and cracks occur when the coil is unwound or when it is passed through the factory thereafter.

Further, claim 2 proposes to use a steel piece in which δ ferrite phase is crystallized in an amount of 20% or less at the time of solidification. It has the effect of reducing the Al concentration in the austenite matrix. As a result, during solidification, Ni that precipitates in the austenite phase
It is possible to reduce the amount of the -Al-based intermetallic compound and easily eliminate it in a short time during heating before hot rolling.

The amount of δ-ferrite crystallized during solidification is determined by the balance of the constituent elements, and the amount of δ-ferrite phase described here is
It is the amount measured by a commercially available ferrite meter.

δ-ferrite has the effect of reducing the amount of Ni-Al-based intermetallic compounds as described above, but if crystallization exceeds 20%, a large amount of δ-ferrite phase remains even after hot rolling annealing, and The upper limit was set to 20% because productivity deteriorates, such as the occurrence of ear cracks during rolling. In addition, in order to maintain the high temperature strength and creep strength of the product, the amount of δ ferrite during solidification should be 10
% Or less is desirable.

Example Steel billets having the components listed in Table 1 were melted and hot-rolled under the heating temperature, hot-rolling end temperature, and hot-rolling cooling conditions shown in Table 2. After that, a Charpy test piece was cut out from the hot rolled sheet, and the impact value at room temperature (20 ° C) was measured. Except for the hot rolling conditions shown in Table 2 (heating time, rolling pass schedule, etc.), all conditions were fixed, the final thickness was 6 mm, and the Charpy test piece (JIS No. 4 V notch, 5 mm subsize) was rolled in the rolling direction. Was collected in parallel. The measured impact values are also shown in Table 2.

The hot rolling conditions proposed in the present invention from this table, the impact value of the hot rolled plate falls below ² kgm / cm ² when deviating from the cooling conditions, and in the actual hot rolling process, the risk of cracking during coil rewinding or during cold rolling Is fully conceivable.

On the other hand, it can be seen that the sheet hot rolled under the hot rolling conditions and cooling conditions proposed in the present invention shows a high impact value and is not embrittled.

Next, using No. 12 steel slab in Table 1, heat it to 1250 ° C.,
A hot-rolled sheet that had been hot-rolled at 950 ° C was water-cooled to 500 ° C and wound into a coil, and immediately after the hot-rolled sheet was wound into a coil, the coil was unwound. The average cooling rate from 950 ℃ to 500 ℃ during water cooling is about 20 ℃ / scc.
When wound on a coil, the average was 0.1 ° C / scc.
A large number of cracks occurred when the coil wound up without water cooling was unwound, but the coil cooled to 500 ° C did not crack when unwound.

From the above examples, it can be understood that embrittlement of the hot rolled coil can be prevented by performing hot rolling and cooling after hot rolling under the conditions of the present invention.

Effects of the Invention By satisfying the hot rolling conditions described in the claims as described above, it is possible to produce a hot rolled sheet of Al-containing austenitic stainless steel without embrittlement, and during the threading due to embrittlement Can be prevented from cracking. Also, cracking during hot rolling can be suppressed. That is, the present invention has an extremely excellent effect in industrially producing 2% or more Al-containing austenitic stainless steel.

[Brief description of drawings]

Figure 1 shows TTP showing precipitation behavior of Ni-Al intermetallic compound in austenitic stainless steel containing 5.0% Al.
It is a (Time-temperature-Precipitation) curve figure.
The vertical axis represents temperature and the horizontal axis represents holding time.

Claims (4)

[Claims] 1. In weight percent, C is 0.2% or less and Si is
2.0% or less, Mn 2.0% or less, Ni 15-35%, Cr 12-25
%, Al of 2.0% or more and the balance of Fe and inevitable impurities are heated to 1150 ° C or higher, hot rolling is completed at 900 ° C or higher, and immediately after that, rapidly cooled to 600 ° C or lower. A method for producing an Al-containing austenitic stainless steel. 2. In weight percent, C is 0.2% or less and Si is
2.0% or less, Mn 2.0% or less, Ni 15-35%, Cr 12-25
%, Al of 2.0% or more, the balance of Fe and unavoidable impurities, and a slab of ferrite of 20% or less in the solidification stage are heated to 1150 ° C or more and hot rolled at 900 ° C or more. A method for producing an Al-containing austenitic stainless steel, which comprises quenching to 600 ° C or less immediately after the completion. 3. The method for producing an Al-containing austenitic stainless steel according to claim 1, wherein the quenching is performed at a cooling rate of 5 ° C./sec or more. 4. The method for producing an Al-containing austenitic stainless steel according to claim 2, wherein the quenching is performed at a cooling rate of 5 ° C./sec or more.