JPH06172849A - Heat treatment of cr-mo steel - Google Patents

Abstract

PURPOSE:To attain the same heat treatment effect as isothermal annealing and to reduce a processing time by using a continuous furnace specifically constituted and treating under the specific condition at the heat treatment, which changes a structure to be softened. CONSTITUTION:A vertical feeding type continuous quenching furnace 1 is connected to a horizontal feeding type continuous annealing furnace 5 with a trnsfer path 2 for air cooling and a transfer path 6 for air cooling is arranged on the downstream side of the continuous annealing furnace 5. After a Cr-Mo steel tube 8 containing, by weight, 0.5-10% Cr and 0.4-1.1% Mo is heated at >=austenitizing temp. for >=5 minute in the continuous quenching furnace 1, the steel tube is air-cooled at the transfer path 2. The transfer speed is adjusted in response with a heat capacity of the steel tube 8 and the steel tube is charged into the continuous annealing furnace 5 being kept in the temp. range of >=the martensitic transformation starting temp. and <= the AC transformation point. After the steel tube 8 is held at 700-780 deg.C for 1-4 hours therein, the steel tube is air-cooled at the transfer route 6.

Description

Detailed Description of the Invention

[0001]

The present invention relates to Cr 0.5-1 by weight.
The present invention relates to a heat treatment method for a Cr-Mo steel material containing 0% and 0.4 to 1.1% Mo, and more particularly to a heat treatment method for a Cr-Mo steel material which softens the steel material from an austenitic state to a mixed structure of ferrite and pearlite.

[0002]

2. Description of the Related Art Cr-Mo is used as a heat transfer tube for general boilers.
Steel pipes are often used. However, the Cr-Mo steel is hard as rolled and has low toughness. Therefore, Cr-Mo
When the steel pipe is used, for example, as a heat transfer pipe for a steam generator of a fast breeder reactor, a heat treatment for softening the austenite state into a mixed structure of ferrite and pearlite is performed.

Until now, this heat treatment has been performed by a method called full annealing. The full annealing treatment is a method of austenizing steel and then gradually cooling it at a constant cooling rate. However, since slow cooling is required, the time required for one batch was extremely long, for example, 24 hours.

Therefore, the austenitic steel 725
A so-called isothermal annealing treatment has been developed in which the material is allowed to cool to about 750 ° C., held in that temperature range for 1 hour or more, and then allowed to cool, and many proposals for improvement have been made (Japanese Patent Publication No. 56-21806). JP-A-59-438
No. 21, etc.).

[0005]

However, since the conventional isothermal annealing process has a complicated temperature history, it was customary to carry out the process in a batch furnace. Therefore, the processing time is shortened to only about 10 hours in the case of a material that requires 24 hours for full annealing and processing, which is only about 1/3 or less of the efficiency in continuous furnace operation.

It is an object of the present invention to provide a heat treatment method for a Cr-Mo steel material which has the same heat treatment effect as isothermal annealing in continuous furnace operation.

[0007]

In order to achieve the above object, the present inventor reexamined isothermal annealing in terms of both metallurgy and efficiency. As a result, the following was found.

In the isothermal annealing, as shown in FIG. 2, the steel is 950 ° C. × 2 due to austenitization.
It is heated for about 0 minutes. When this is done in a batch furnace, 95
It takes a long time to raise the temperature to 0 ° C.
It is not particularly important for austenitization, and even if the material is charged into a continuous furnace and heated rapidly, sufficient austenitization will occur if the heating temperature is secured, and rather the heating and holding time will be longer than that of the batch furnace. It can be shortened from 20 minutes to about 5 minutes.

Regarding the cooling after austenitization, it takes time in the batch furnace, but once the material is extracted from the continuous furnace, efficient cooling is performed. Even if the material is once extracted from the continuous furnace, if the material is reloaded into another continuous furnace and tempered before the martensitic transformation starts,
There is no problem in terms of structure even if the ferritization is completed by keeping the temperature constant for more than an hour, preferably about 2 to 3 hours, and then extracted from the continuous furnace and allowed to cool outside the furnace.

From the above, if two continuous furnaces are connected by a carrier path for cooling, and a furnace equipment having a carrier path for cooling is provided on the downstream side of the subsequent continuous furnace, heat treatment equivalent to isothermal annealing is performed. The effect is obtained, and the treatment time is shortened to about 3 to 4 hours in the case of a material which requires about 10 hours for isothermal annealing.

The material temperature at the time of charging into the tempering furnace must be higher than the martensitic transformation start temperature and lower than the AC ₁ transformation point. That is, in the conveying path connecting the continuous furnace for quenching and the continuous furnace for tempering, the material temperature should not be lowered below the austenite transformation start temperature, but on the other hand, it should be lower than the AC ₁ transformation point.

However, the cooling rate in the conveying path depends on the heat capacity of the material, and if the material moving speed in the conveying path is constant, the heat capacity is small, for example, in the case of a thin-walled tube, the material is up to the AC ₁ transformation point or lower. Even if it is allowed to cool, if it has a large heat capacity, such as a thick-walled tube, the cooling is insufficient and the charging temperature is A
It may be higher than the C ₁ transformation point. Then, as shown in FIG. 3, the ferritization time in the tempering furnace is shortened, and the ferrite-pearlite transformation does not proceed. As a countermeasure against this, it is preferable to adjust the material moving speed in the conveying path according to the heat capacity of the material.

The present invention has been made based on the above findings, and a Cr-Mo steel material containing Cr 0.5 to 10% and Mo 0.4 to 1.1% by weight is mixed with ferrite and pearlite from the austenitic state. Cr-M to soften into tissue
o A heat treatment method for steel material, wherein a continuous tempering furnace is provided on the downstream side of the continuous quenching furnace via a conveying path, and a conveying path is provided on the downstream side of the continuous tempering furnace, so that the steel material is After heating to the austenitizing temperature or higher for 5 minutes or more, it is left to cool in the transfer path between the continuous quenching furnace and the continuous tempering furnace to 500
It is charged into a continuous tempering furnace at ℃ or more,
A heat treatment method for a Cr-Mo steel material is characterized in that it is held at 0 ° C for 1 to 4 hours and then allowed to cool in a conveying path on the downstream side of a continuous tempering furnace.

In the conveying path between the continuous quenching furnace and the continuous tempering furnace, the heat capacity of the steel material is adjusted so that the temperature of the steel material at the time of charging the continuous tempering furnace is not less than the martensite transformation start temperature and not more than the AC ₁ transformation point. It is better to adjust the transport speed accordingly.

[0015]

A typical Cr-Mo steel material targeted by the method of the present invention is C0.02-0.2% by weight, Si0.05-1.0%,
Mn 0.3-0.8%, P 0.03% or less, S 0.03% or less,
Cr 0.5 to 10%, Mo 0.4 to 1.1% are included, and if necessary, Cu 0.1% or less, Ni 0.5% or less, Al 0.08
% Or less, and the balance consists of Fe and unavoidable impurities. Here, the reason why each component is limited as described above is as follows.

C is indispensable as a strength improving element, and for this reason, 0.02% or more is required. However, if it exceeds 0.2%, the toughness and workability deteriorate.

Si is contained as a deoxidizing agent in an amount of 0.05% or more. However, if it exceeds 1.0%, the toughness, surface properties and weldability are impaired.

Mn acts as a deoxidizing agent and a desulfurizing agent, contributes to securing the strength, and requires 0.3% or more. However, if it exceeds 0.8%, the strength becomes excessive, workability is deteriorated, and the cleanliness of steel is impaired.

The upper limit of P is 0.03% because it impairs the cleanliness of steel and deteriorates toughness and ductility.

Similar to P, S also impairs the cleanliness of steel and deteriorates toughness and ductility, so it is limited to 0.03% or less.

Cr is required to be 0.5% or more in order to secure high temperature strength and corrosion resistance. However, a large amount is economically disadvantageous, so the content is made 10% or less.

Similarly, Mo and Cr are elements indispensable for securing high temperature strength and corrosion resistance and require 0.4% or more, but in order to ensure economic efficiency as well, they are set to 1.1% or less.

Cu is effective for improving corrosion resistance, but 0.1%
If it exceeds, red heat embrittlement occurs.

Ni is effective in improving strength, toughness and corrosion resistance. However, it is not effective for high temperature strength and is expensive, so the upper limit is made 0.5%.

Although Al is added as a deoxidizer and is an element effective for ensuring toughness, if it is contained in a large amount, the cleanliness of steel is impaired, so the content is set to 0.08% or less.

In the method of the present invention, the Cr-Mn steel material is charged into the continuous quenching furnace and rapidly heated to a temperature above the austenitizing temperature, and the heating time to the heating temperature is shortened. In the continuous quenching furnace, the steel material rises above the austenitizing temperature 5
It is heated for more than a minute. The steel material that has been heated is efficiently left to cool and is charged into the continuous tempering furnace while proceeding along the transfer path between the continuous quenching furnace and the continuous tempering furnace. However, the charging temperature is controlled to be higher than the martensite transformation start temperature and lower than the AC ₁ transformation point. In the continuous tempering furnace, the steel material
Hold at 780 ° C for 1-4 hours. The steel material that has been heated and held is efficiently cooled while traveling along the transport path on the downstream side.

In the method of the present invention, the holding time at the austenitizing temperature or higher is set to 5 minutes or longer because the austenitization does not proceed sufficiently if the holding time is shorter than 5 minutes. A desirable holding time is 5 to 20 minutes.

The charging temperature into the continuous tempering furnace is set to be not less than the martensitic transformation start temperature and not more than the AC ₁ transformation point, because a perfect ferrite / pearlite structure cannot be obtained in tempering at a temperature below the martensite transformation start temperature, and thus AC ₁ This is because even if the transformation point is exceeded, the ferrite-pearlite transformation does not proceed in the tempering furnace and the austenite structure remains (see FIG. 3).

The tempering temperature in the continuous tempering furnace is 700 ° C.
If it is less than the above, it takes a long time to transform austenite into ferrite, and the productivity is lowered. Further, even if the temperature exceeds 780 ° C., it takes time for the ferrite transformation, and the productivity is lowered. Therefore, the tempering temperature was 700 to 780 ° C.

The holding time at the tempering temperature is set to 1 to 4 hours because the ferrite transformation is not completed in less than 1 hour.
This is because if the time is exceeded, heating will be performed more than necessary and productivity will be reduced.

[0031]

EXAMPLES Examples of the present invention will be described below. FIG. 1 is a schematic diagram showing a furnace equipment suitable for carrying out the method of the present invention and a heat pattern by the equipment.

A walking beam type continuous tempering furnace 5 is provided on the downstream side of the barrel type four continuous quenching furnace 1 via a conveying path 2. The steel pipe 8 as a Cr—Mn steel material extracted from the continuous quenching furnace 1 after completion of austenitization is laterally fed by the skid 3 of the transport path 2,
It is charged into the continuous tempering furnace 5.

At this time, in the case where the steel pipe 8 is a thin material having a small heat capacity, the conveying speed in the conveying passage 2 is increased and the continuous tempering furnace is set so that the tip temperature of the steel pipe 8 does not drop too much and martensite transformation does not start. The surface temperature of the steel pipe 8 is controlled by the thermometer 4 at the inlet of 5. When the steel pipe 8 is a thick material having a large heat capacity, the transport speed in the transport path 2 is slowed and the temperature is controlled by the thermometer 4 so that the rear end temperature does not exceed the AC ₁ transformation point. .

In the continuous tempering furnace 5, the steel pipe 8 is transversely fed in the direction opposite to the traveling direction in the transport path 2 to complete the ferrite transformation. Steel pipe 8 extracted from continuous tempering furnace 5
Is allowed to cool while being laterally fed on the skid 7 of the transfer path 6 provided on the downstream side of the continuous tempering furnace 5.

Using the furnace equipment of FIG. 1, Cr-Mn steel pipes of the same material 2 size whose materials are shown in Table 1 and whose dimensions are shown in Table 2 were continuously heat-treated. Table 3 shows the heat treatment conditions and the mechanical properties after the heat treatment. For comparison, an isothermal annealing process using a batch furnace was also performed. The processing conditions and the mechanical properties after the processing are shown in Table 3.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

Compared with isothermal annealing, the continuous treatment according to the method of the present invention is particularly short in temperature rising time and holding time in austenitization, and further in cooling time, and the treatment time can be shortened to 1/3. The efficiency (Ton / hr) can be tripled. Further, since heating and cooling of a furnace such as a batch furnace are not required, heat treatment cost can be reduced to 1/10 or less of that of isothermal annealing. Then, the quality of the steel pipe after the heat treatment is equivalent to that of the isothermal annealed product (invention example A).

However, even in the case of continuous treatment, when the thick material is conveyed at the same speed as the thin material in the conveying path between the quenching furnace and the tempering furnace, the material temperature at the time of charging in the tempering furnace is the AC ₁ transformation point. And the mechanical properties after heat treatment deteriorated. However, by extending the transportation time from 3 minutes to 10 minutes, this deterioration was prevented, and good mechanical properties equivalent to those of thin-walled materials were obtained. The total processing time will be longer due to the extension of the transportation time, but the degree is about 1.5 in terms of the total processing time.
%, Which is a level that can be ignored (Example B of the present invention).

[0041]

As is apparent from the above description, the heat treatment method for Cr—Mn steel according to the present invention can obtain the heat treatment quality equivalent to that of the isothermal annealing in the batch furnace in the continuous furnace operation and the treatment efficiency. It has a great effect on the improvement of temperature and the reduction of processing cost.

[Brief description of drawings]

FIG. 1 is a schematic view showing a layout of furnace equipment suitable for carrying out the method of the present invention and a heat treatment pattern by the equipment.

FIG. 2 is a heat pattern diagram of isothermal annealing.

FIG. 3 is a TTT diagram of a thin material and a Cr—Mo steel material showing the thin material.

[Explanation of symbols]

 1 Continuous quenching furnace 2,6 Conveying path 5 Continuous tempering furnace 8 Steel pipe (Cr-Mo steel material)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C22C 38/00 301 Z 302 302 Z 38/22

Claims (2)

[Claims] 1. A weight ratio of Cr 0.5 to 10% and Mo 0.4 to
A Cr-Mo steel material heat treatment method for softening a Cr-Mo steel material containing 1.1% from an austenitic state to a mixed structure of ferrite and pearlite, which is downstream of a continuous quenching furnace via a conveying path. , A continuous tempering furnace is provided, and a conveying path is provided on the downstream side of the continuous tempering furnace, and the steel material is heated in the continuous quenching furnace at a temperature above the austenitizing temperature for 5 minutes or more, and then the continuous quenching furnace and the continuous tempering furnace are combined. It is allowed to cool in the conveyance path between them and charged into a continuous tempering furnace at a temperature not lower than the martensitic transformation start temperature and not higher than the AC ₁ transformation point, where 700 to 78
A method for heat treating a Cr—Mo steel material, which comprises holding the material at 0 ° C. for 1 to 4 hours and then allowing it to cool in a conveying path on the downstream side of a continuous tempering furnace. 2. The heat capacity of the steel material so that the temperature of the steel material at the time of charging the continuous tempering furnace is not less than the martensitic transformation start temperature and not more than the AC ₁ transformation point in the conveying path between the continuous quenching furnace and the continuous tempering furnace. The heat treatment method for a Cr-Mo steel material according to claim 1, wherein the conveying speed is adjusted according to the above.