JPS6156234A - Manufacture of austenite stainless fine grain steel - Google Patents

Abstract

PURPOSE:To obtain a fine grain steel without necessitating high temp. softening and cold rolling of especially high draft, by specifying conditions of heat treatment and working in method for obtaining boiler steel by applying heat treatment and working to austenite stainless steel of carbide precipitation type such as of Ti, Nb. CONSTITUTION:Austenite stainless steel contg. by weight >=0.1% Nb and/or >=0.1% Ti, and >=0.04% C is hot worked at >=1,200 deg.C finish working temp. Thereafter, the steel is cooled rapidly at >=500 deg.C/hr rate, successively cold worked by >=20% draft, and solution treated at temp. lower by >=30 deg.C than hot working finishing temp. If hot working temp. of said stainless steel contg. specified quantities of Nb and/or Ti and C is raised sufficiently and carrying out rapid cooling after said hot working in such a way, grain size of the material after hot working is made finer. Further, sufficiently large quantities of Ti and Nb in solid solution state can be ensured and the aim can be achieved.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention is an austenite material that has remarkable resistance to steam oxidation, which is a problem when used as steel pipes for boilers, etc., and has sufficiently high high temperature strength. The present invention relates to a method for manufacturing stainless steel.

Generally, austenitic stainless steels such as 5US321HTB and SUS347HTB are used for boiler superheater tubes, and when manufacturing these products,
Manufacturing conditions are selected to satisfy ASTM standards and thermal power technology standards.

<Prior art> - By the way, austenitic stainless steel pipes for boilers are usually cold-worked to a predetermined final size, but at that time the material is hardened, so it is softened several times and then cooled. It was common practice to complete the preliminary working and then perform solution treatment to ensure the desired high-temperature strength.

In this case, there is no particular specification regarding the intermediate softening temperature or the final softening temperature, but as the solution treatment temperature increases, the high temperature strength also increases.

There is a noticeable tendency to increase the solution treatment temperature higher than the specified temperature in the JIS steel pipe standard.

However, it has been known that increasing the solution treatment temperature causes the disadvantage that the crystal grains of the steel become coarser.

The grain size of steel is affected by water vapor corrosion (°′steam oxidation”).
There is a close relationship between coarse-grained steel and where corrosion is greater. The problems caused by steam oxidation include material thinning and corrosion products (
The latter tends to be a particularly serious problem for boiler steel pipes.

That is, when scale flakes occur, the flaked scales may be carried by water vapor to, for example, the turbine section, causing damage to the turbine blades, or may accumulate in tubes and block the water vapor flow, causing severe damage. This could lead to tube rupture.

As mentioned above, steel pipes for boilers need to have both high-temperature strength and corrosion resistance, so when refining them only by solution treatment, the minimum temperature of solution treatment should be adjusted from the desired creep rupture strength to the desired temperature. Fine crystal grains (
The maximum temperature of the solution treatment is determined from the viewpoint of obtaining sufficient steam oxidation resistance (GSA finer than 7).

However, if the minimum temperature determined by the creep strength is higher than the maximum temperature determined by the grain size, the solution temperature must be selected to ensure satisfactory strength at the expense of corrosion resistance, and then tedious treatment such as 5-surface treatment is necessary. Moreover, the only way to improve corrosion resistance is to use treatment means that lack stability.

Therefore, in order to solve these problems, the present inventors used austenitic stainless steel of the type in which carbides such as rTjc and NbC precipitate, and subjected it to heat treatment and processing under specific conditions. ``A method for achieving a steel material with excellent high-temperature strength and corrosion resistance by maintaining fine grains as they are despite being subjected to high-temperature solution treatment,'' was published in JP-A-58-16'7''. It was proposed earlier as issue i'26.

FIG. 2 schematically shows the method for producing austenitic fine-grained stainless steel according to the above proposal.

<Problems to be Solved by the Invention> The method proposed in JP-A-58-16'7726 certainly achieves good corrosion resistance and high temperature without requiring special post-treatment steps such as surface treatment. Although this was a useful technical means for stably manufacturing steel materials with good strength properties, it still required high-temperature softening treatment, which is undesirable from the standpoint of work efficiency and economy, as is clear from Figure 2. In addition to this, the grain size of the material softened by the high-temperature softening treatment becomes extremely coarse. Subsequent studies by the present inventors have revealed that it is necessary to set the ratio to a high value of 30% or more if possible.

Furthermore, the fact that the working ratio of the cold working must be 30% or more also means that the method cannot be applied depending on the dimensions of the steel pipe, etc. to be manufactured, for example.

<Means to Solve the Problem> From the above-mentioned viewpoint, the present inventors have developed a method that eliminates the need for the high-temperature softening tube that adversely affects product quality, work efficiency, economic efficiency, etc. As a result of further research aimed at stably and reliably producing austenitic fine-grained stainless steel with high high-temperature strength and suitable for boiler steel pipes, etc., we found that ``austenite containing a specific amount of Nb, one or more of T1, and C.'' If the hot working temperature of stainless steel is sufficiently raised and the cooling after hot working is rapid cooling, the grain size of the material after hot working will be 3 to 4 in the GS version (with the method proposed earlier). After the high-temperature softening treatment, the material becomes fine (about SAO ~ 2), and a sufficiently large amount of Ti and Nb in solid solution can be secured.

They came to the conclusion that fine-grained steel with high creep strength can be obtained through solution treatment without high-temperature softening treatment or cold working at extremely high working rates.''

This invention was made based on the above findings.

Austenite containing Nb: 0.1% or more (hereinafter, component proportions are indicated on a weight basis), Ti: 0.1% or more, singly or in combination, and also containing C: 0.04% or more After hot working on stainless steel at a processing end temperature of 1200°C or higher, cooling rate: 5
By rapidly cooling at 00℃/hr or more, followed by cold working at a working rate of 20 or more, and then solution treatment at a temperature 30℃ or more lower than the hot working end temperature, high-temperature strength and Easy and stable production of fine-grain austenitic stainless steel with excellent corrosion resistance (steam oxidation resistance).

It has the following characteristics.

5. Figure 1 schematically shows the manufacturing method of the austenitic fine-grained stainless steel of the present invention, and by comparing it with Figure 2, the features of the present invention should become clearer. .

Next, in the method of this invention, Nb, Ti and C
The reason why the content, hot working end temperature, cooling rate, cold working rate, and solution treatment temperature are each numerically limited as described above will be explained.

(a) Nb, Tj, and C content Nb, T:i and C components precipitate as NbC, TiC, etc. during solution treatment and have the effect of refining the structure of the steel material, but Nb If the content of T1 and T1 is less than 0.1%, or if the C content is less than 0.04%, the amount of precipitation of NbC, TiC, etc. will be insufficient, and the desired grain refinement of the steel structure will not be achieved. Nb of 01 or 0.1% is added to the steel because it is not possible to achieve this and causes deterioration of corrosion resistance.
It was decided to contain the above Tj alone or in combination, and the amount of C was also set to be 0.04% or more.

In addition, if the content of Nb or Tj exceeds 3%, addition
□ There is a risk of deterioration of workability and embrittlement due to significant precipitation of the σ phase during use of the steel, and if the C content exceeds 03, carbides (M2., C6) will precipitate at the grain boundaries. There is a risk that the corrosion resistance will become severe and the corrosion resistance will be impaired.
It is preferable to suppress the Nb and T1 contents to 3 parts or less, and the C content to 03 parts or less.

(b) Hot working end temperature When the hot working end temperature is less than 120C)
Since it becomes impossible to secure a solid solution amount of b or T1 and it becomes difficult to realize fine crystal grains through solution treatment, the hot working end temperature was set at 1200° C. or higher.

(C) Cooling rate after completion of hot working If the cooling rate after completion of hot working is less than 500 °C/hr, precipitation of NbC and TjC will occur, and Nb or TjC will precipitate.
Since a sufficient amount of solid solution of j could not be secured, the cooling rate was set at 500° C./hr or more.

Figure 3 is a diagram showing the relationship between NbC or uTiC precipitation and the cooling rate after hot working in SUS 321H steel and 5US34'7H steel. When the cooling rate between 12oO and 950℃ is 500℃/hr, 80% of the carbon content is Nb,
C or Tj, which intersects the nose that precipitates as C.

As described above, it is clear from FIG. 3 that the cooling rate after hot working must be faster than 500° C./1]r.

(d) Reduction rate of cold working Cold working after hot working is essential for making the recrystallized grains produced during solution treatment into the desired fine grains. Since the desired microstructure (grain size with a GS diameter of 7 or more) cannot be achieved when the working rate is less than 20 times, the cold working rate was set at 20 times or higher.

Note that, as is clear from the above description, in order to obtain fine-grained steel, the higher the working rate is, the better, and from this point of view, there is no upper limit to the working rate.

(e) Solution treatment temperature If the difference between the hot working end temperature and the solution treatment temperature is less than 30°C, the difference in solubility during single treatment is small, so Nb-jTi dissolved in the hot working process is dissolved in the solution treatment. Sometimes NbC or TiC
Since the amount of precipitation is small and the desired fine grain structure cannot be achieved, the solution treatment temperature was determined to be ``a temperature 30° C. or more lower than the hot working end temperature''.

Note that if the solution treatment temperature is less than 1100°C, it may be difficult to secure the leap rupture strength required as a high-temperature material, and if it exceeds 1300°C, NbC or TiC
It is preferable to adjust the temperature of the 5-solution treatment to 1,100 to 1,300°C because the amount of solid solution increases, leading to the formation of grains.

Needless to say, the steel targeted by the method of this invention is SUS 321H steel and 5US347H steel.
It is needless to say that the present invention is not limited to steel, and may be any austenitic stainless steel containing Nb and T] singly or in combination, and containing a predetermined amount of C. will be explained using examples and comparing with comparative examples.

In Examples, steels A to Q having chemical compositions as shown in Table 1 were prepared.

Next, these steels were subjected to forging, hot rolling (hot working), cold rolling (cold working), and solution treatment in the steps shown in FIG. 1 to produce cold rolled sheets.

These processing conditions as well as the grain size of the obtained cold-rolled sheet (
A, STMGSA) and creep rupture strength are shown in Table 2.

1 in Table 2 is for reference, the method of JP-A-58-16'i"i'26 (method shown in Fig. 2) previously proposed for steel with the same composition was applied. The grain size and creep rupture strength of the obtained cold-rolled sheets are also shown.

The following five points are clear from the results shown in Table 2. That is, 5. (7) According to the method according to the conditions of the present invention, fine grain steel having high creep strength and a GSA of 7 or more can be stably produced.

(a) Even if high-temperature hot working is performed, if the subsequent cooling rate is slow, the crystal grain size of the resulting steel becomes coarse (Test Nos. 3 and 7).

(1) The difference between the hot working end temperature and the solution treatment temperature is 30
It is difficult to refine the steel material below ℃ (Test No. 9
and 12).

b) If the cold working rate is less than 20%, the steel material obtained becomes a slightly coarse-grained material (Test No. 11).

(2) Even if steel with Nb content or T1 content of less than 0.1% is used, the desired fine-grained steel cannot be obtained (Test Nos. 24 and 25).

(If a steel material with an AC content of less than 0.04% is used, the desired fine-grained steel cannot be achieved even if the Ti and Nb contents are sufficiently high (Test No. 26).

In addition, Figure 4 shows the dimensions of steel pipes that can be manufactured by the method of the present invention from the perspective of the processing rate of cold working (manufacturing method of steel pipes: hot extrusion followed by cold drawing finishing method). Showa 58-
This drawing is a comparison with the dimensional range possible by the method of No. 167726, and it is clear from FIG. 4 that the dimensional range that can be manufactured by the method of the present invention is expanded.

<Overall Effects> As explained above, according to the present invention, ■ it becomes possible to manufacture austenitic fine-grained stainless steel easily and economically by omitting the high-temperature softening process; ■ cold working; Since the grain size of the previous material is smaller than that of conventionally processed materials, the cold working rate can be lowered, making it possible to manufacture steel pipes, etc. in a wide range of dimensions. ■ The resulting austenitic stainless steel material Since it is a fine-grained steel, it has good steam oxidation resistance, which is a problem in steel pipes for boilers.

etc., extremely useful industrial effects are brought about.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the method for producing fine grain steel of the present invention; Fig. 2 is a schematic diagram showing the conventional method for producing fine grain steel; Fig. 3 is a schematic diagram showing the method for producing fine grain steel of the present invention;
FIG. 4 is a diagram showing the relationship between temperature and cooling time on C or TiC precipitation. FIG. 4 is a drawing comparing the dimensions of a steel pipe that can be produced by the method of the present invention and the dimensions that can be produced by the conventional method. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and one other person. )
Nozomi? Keto

Claims (1)

[Claims] An austenitic stainless steel containing Nb: 0.1% or more, Ti: 0.1% or more singly or in combination, and also containing C: 0.04% or more, in terms of weight percentage. Processing end temperature: After hot processing at 1200℃ or higher, cooling rate: 5
Rapid cooling at 00°C/hr or more, followed by cold working at a processing rate of 20% or more, and then
characterized by solution treatment at a temperature lower than 0°C,
Manufacturing method for high-temperature austenitic fine-grain stainless steel.