JPS61149437A - Heat treatment of heat-resistant high-chromium ferritic steel pipe - Google Patents

Abstract

PURPOSE:To prevent reduction in the creep rupture strength of a steel pipe of 9%Cr-1%Mo steel contg. V and Nb in case of a large thickness when the steel pipe having a restricted outside diameter and a restricted thickness is heat treated, by specifying conditions during the heat treatment. CONSTITUTION:A steel pipe consisting of 0.04-0.15% C, <=1.0% Si, <=1.0% Mn, 7-12% Cr, 0.5-1.5% Mo, 0.005-0.10% N, 0.1-0.45% V and/or 0.05-0.60% Nb and the balance Fe or further contg. one or more among <=0.05% Al, <=0.5% Ni and 5-100ppm B and having <=50mm outside diameter and >=15mm thickness or >=50mm outside diameter and >=20mm thickness is normalized by heating to 1,000-1,200 deg.C and cooling to 750 deg.C at >=1 deg.C/sec average cooling rate, and then it is cooled to room temp. and tempered at 700-800 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a heat treatment method for high chromium ferrite heat-resistant copper tubes.

<Prior art> In recent years, with the improvement of boiler steam conditions, there has been an active review of the optimal material for heat-resistant copper tubes, and among these, 9%Cr-1% MO steel with V and chicken added is 9%CrCr. -1s
It is attracting attention as a copper with improved high-temperature strength. A wide variety of products are made from this steel, from small-diameter pipes to thick, large-diameter pipes.

In these copper tubes, the high-temperature strength is increased by dissolving carbides of V and Nb by high-temperature heat treatment (normalizing treatment) and re-precipitating them. ' However, there was a problem in that the creep rupture strength decreased when the wall thickness of the copper pipe was thick.

<Summary of the invention> The present invention provides a 9% Cr-1% Mo
This was done for the purpose of improving the creep rupture strength of M, and after performing normalizing treatment at a heating temperature of 1000 to 1200.750°C and an average cooling rate of 1°C/sec or more, and then cooling to room temperature. It is characterized by tempering at 700 to 800°C.

As a result of various studies on the causes of decrease in creep rupture strength, the present inventors found that Nb, which contributes to increase in high-temperature strength,
The behavior of carbides in V is greatly affected by the cooling rate during normalization, and when using normal air cooling (air cooling) for thick materials, carbides are not very effective in improving strength during cooling in high temperature ranges. We have come to the conclusion that this is because it precipitates in a different form.

In other words, if the outer diameter of the heat-treated copper tube is relatively large or the wall thickness is relatively thin, the cooling rate of the copper tube during normalizing will be within a range where the reduction in high-temperature strength will not be a problem; As the wall thickness becomes smaller and the cooling rate becomes slower, the temperature range where the diffusion rate is sufficiently large (750℃
(above)), carbides are precipitated which are coarser and contribute less to increases in strength, particularly creep and creep rupture strength, than those that precipitate at subsequent low temperatures. Along with this, the amount of solid solution decreases, so the amount of subsequent precipitation on the low temperature side decreases.

As mentioned above, this phenomenon is observed in thick-walled materials, and in copper pipes with an outer diameter of 50 or less, the wall thickness is 15 mm or more, and in copper pipes with an outer diameter of 50 m or more, the wall thickness is 2 (in the case of 1 m+ or more). This led to the finding that it is recognized in

The present invention is based on this knowledge, and when heat-treating and normalizing copper pipes of such dimensions, instead of normal air cooling, appropriate methods such as forced air cooling, mist cooling, water cooling, etc. are used.
This is intended to improve creep rupture strength by increasing the cooling rate in a high temperature range of 50°C or higher.

First, the steel to be subjected to the heat treatment of the present invention will be described. The steel targeted by the present invention is limited to high-strength steel containing V and C, more specifically, C: 0.04 to 0.15.
%, Si: 1.0% or less, Mrl: 1.0% or less,
Cr ~12%, Mo2 0.5~1.5%, N:
0.005-0.10%, plus V: 0.1-0.4
5%, Nb: 0.05-0.60% (7) Lai or more, and if necessary At: 0.05% or less,
It is limited to steel containing one or more of N1: 0.5% or less, B: 5 to looppm, and the balance consisting of iron and inevitable impurities. The reason for this is as follows.

For use at high temperatures such as boilers, Cr is used from the viewpoint of corrosion resistance.
The amount needs to be 7 inches or more. Further, in order to ensure high temperature strength, Mo needs to be at least 0.5%. The upper limit of cr, M is set by increasing the amount of δ-ferrite to 40 to ensure toughness.
It is necessary to set the above value to limit it to % or less.

V.

The lower limit of the amount of Nb is determined as above in order to ensure high temperature strength, and the upper limit is determined as above because excessive addition exceeding this amount will deteriorate high temperature strength and toughness. The lower limit of Ci- is set to ensure normal and high temperature strength, and the upper limit is set to the above value in consideration of weldability and cold workability. The range of the amount of N needs to be within the above value for the same reason as for C. Sl is an effective element that has a deoxidizing effect, but if it is contained in an amount exceeding 1.0%, toughness deteriorates, so the upper limit is set at 1.0%. Mn has a deoxidizing effect and also has the effect of reducing hot embrittlement by combining with S, which is an impurity present in steel. The above action is 1
%, and even if the content exceeds this, there is no effect associated with the increase, so the upper limit is set at 1%.

The steel targeted by the present invention has the above-mentioned elements as essential constituents, but may further contain one or more of Kl, B, and Ni, if necessary. At is effective in improving toughness, but if added in excess, it is harmful to high temperature strength, so it is set to less than o, oss. B is effective for high-temperature strength, but it is less effective below 5 ppm;
Exceeding this will have an adverse effect on workability, etc. Ni is an element effective in improving toughness, but if the content exceeds 0.5%, it has an adverse effect on stress corrosion cracking resistance, so the upper limit is set at 0.5%.

The wall thickness of the copper tube having the above composition is 15 cm or more if the outer diameter is 50 cm or less, and 20 cm or more if the outer diameter is 50 cm or more.
m or more.

This is because, as mentioned above, the creep rupture strength of a copper tube with a wall thickness within this range decreases.

In the method of the present invention, a round copper tube limited as described above is normalized at a heating temperature of 1000 to 1200°C and an average cooling rate of 1°C/sec or more in a high temperature range up to 750°C.

The reason why the heating temperature is set to 1000° C. or higher is that it is necessary to set the heating temperature to 1000° C. or higher in order to dissolve carbonitrides of V and rabbit into the matrix. Moreover, if the temperature exceeds 1200°C, the crystal grains become too coarse and the toughness decreases, so this temperature is set as the upper limit.

Next, as for the cooling rate, in a high temperature range up to 750° C., the distribution of carbides is greatly affected, so as mentioned above, it is necessary to cool at a certain rate or higher.

FIG. 1 shows the relationship between the cooling rate and creep rupture time from 1050°C to 750°C. The numbers in each plot in the figure are the steel type numbers in Table 1, which will be described later, and are divided into component systems.

As can be seen from this graph, the higher the cooling rate, the higher the high temperature strength.
It can be seen that it is better to set it to ec or higher. Therefore, in the present invention, the average cooling rate up to 750°C is limited to 1°C/sec or more.

Note that Steel 16 is not on the line of Steel 11-Steel 15, but this is because the heat treatment is performed with more emphasis on toughness.

The cooling rate at temperatures lower than 750°C does not have a major effect on the properties of the material, but for thick-walled products, the cooling rate at temperatures below 400°C should be set at 21:/sec to prevent cracking.
It is necessary to do the following.

Thereafter, it is cooled to room temperature and then tempered at 700 to 800°C. This is because if the tempering treatment is performed at a temperature lower than 700°C, the room temperature strength will increase and the workability will deteriorate. Furthermore, if the temperature exceeds SOO°C, there is a risk of ausbunite being formed and there is no meaning, so the upper limit is set at 800°C.

In addition, the tempering time should be 3 to obtain the desired tempering effect.
0 minutes or more is required.

<Example> Copper tubes and steel plates whose compositions and dimensions are shown in Table 1 were subjected to standard-tempering treatment under the conditions shown in Table 2. The creep rupture time of the obtained steel materials under a load of 650° C. is shown on the right side of Table 2. Steel 1 subjected to normal cooling during standardization
．． For 4.9.11 and 14.20, the average cooling rate in the high temperature range up to 750°C is less than 1°C/sec, and 650
The creep rupture time at ℃ is also not good.

On the other hand, when forced air cooling, water cooling, etc. are performed to increase the average cooling rate in the high temperature range to 1° C./sec or more, it can be seen that the creep rupture time increases as the cooling rate increases.

This confirmed the effectiveness of the method of the present invention and resulted in 7 tons.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between average cooling rate and creep rupture time.

Claims (1)

[Claims] C: 0.04 to 0.15%, Si: 1.0% or less, Mn
: 1.0% or less, Cr: 7-12%, Mo: 0.5-1
．． 5%, N: 0.005-0.10%, and V: 0.
1 to 0.45%, Nb: 1 out of 0.05 to 0.60%
species or more, and if necessary Al: 0.05% or less, Ni
: 0.5% or less, B: Contains one or more of 5 to 100 ppm, the balance consists of iron and unavoidable impurities, and has an outer diameter of 50 mm or less, a wall thickness of 15 mm or more, or an outer diameter of 50 mm.
As described above, in the method of heat treating a copper tube with a wall thickness of 20 mm or more, normalizing treatment is performed under the conditions of a heating temperature of 1000 to 1200° and an average cooling rate of 1°C/sec or more to 750°C, and then cooling to room temperature. A heat treatment method for a high chromium ferrite heat-resistant copper tube, which is characterized by tempering at 700 to 800°C.