JPH01139717A - Method for working high cr ferritic steel for use at high temperature - Google Patents

Abstract

PURPOSE:To improve the creep strength of a high Cr ferritic steel and the characteristics at high temp. without carrying out normalizing by successively subjecting the steel contg. specified amts. of Nb and V to holding at a specified temp., final hot working, quenching and tempering. CONSTITUTION:A precipitation strengthening high Cr ferritic steel contg., by weight, 0.03-0.3% C, <=0.1% N, 5-13% Cr and 0.01-0.1%, in total, of V and/or Nb is cast. The cast steel is hot rolled and the resulting stock is heated, held at 930-1,300 deg.C for >=1 min and finally hot worked. The stock is then quenched to form a martensite structure and it is tempered at the Ac1 point or below or warm worked after heating to the Ac1 point or below. Stress relief annealing at the Ac1 point or below may further be carried out as required.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention uses one or both of V and Nb! The present invention relates to a method for processing a high-Cr ferritic steel for high temperature use containing L, and more specifically to an improvement in a processing heat treatment method.

[Conventional technology]

High Cr ferritic steel has superior strength and corrosion resistance compared to low alloy steel, and has higher thermal conductivity and lower coefficient of thermal expansion than austenitic stainless steel. Another feature is that it does not cause stress corrosion cracking.

For these reasons, this steel is widely used as a heat-resistant material for boilers, nuclear power, and the chemical industry.

Among these, high-Cr ferritic steel containing precipitation-strengthening elements such as V and Nb has high high-temperature creep strength and is attracting attention as a ferritic steel that can replace austenitic stainless steel.

This precipitation-strengthened Cr-ferritic steel is a 12Cr-IMo steel (DINX20
CrMoWV12)111), modified 9Cr-IMow4 (ASTM A2 j3 T9) developed in the United States
1! II) 9-12Cr steel with addition of V and Nb (Japanese Patent Publication No. 57-36341, Japanese Patent Application Laid-Open No. 58-181849), which was previously developed by the present inventors.

Conventionally, such precipitation-strengthened high Cr ferritic steels are
As shown in Figure 3, after hot working such as hot rolling and hot extrusion, the tempered martensitic structure containing fine precipitates (
Some of them also contain δ-ferrite).

Normalizing treatment is performed by heating Ac, above the transformation point to dissolve precipitates such as coarse carbides into a solid solution, and to homogenize the segregation of various alloy components, followed by rapid cooling to generate a martensitic structure. It is a purpose. On the other hand, the tempering process lowers the dislocation density of the hard martensitic structure, gives the Mi structure and strength that is stable for a long time at high temperatures, and also
The purpose is to disperse and precipitate fine carbonitrides containing carbonitrides such as Fe and Cr, thereby imparting excellent properties such as toughness and high-temperature strength.

When finishing processing such as welding or bending is performed after tempering, post-heat treatment such as stress relief annealing is performed after the processing.

[Problem that the invention seeks to solve]

However, when the plastic working and heat treatment are independent steps as described above, the heat treatment cost is high, especially since the normalizing treatment is a high temperature treatment at or above the Ac= point. Furthermore, the deformation and oxidation caused by heating are significant, and therefore the straightening and descaling performed after the heat treatment further increases the cost of the product.

Note that if this normalizing treatment is omitted in the conventional processing process, properties such as creep strength and toughness will be significantly impaired, resulting in a material with no product value.

The present invention was developed in view of the current situation, and is a novel product that not only ensures predetermined characteristics without the need for normalizing treatment, thereby significantly streamlining the process and reducing costs, but also expects to improve characteristics. The present invention provides a processing heat treatment method.

[Means for solving problems]

As described above, the normalizing treatment performed on precipitation-strengthened Cr ferritic steel is intended to dissolve the precipitates into solid solution, homogenize the components, and generate a martensitic structure. If this normalizing treatment is omitted to streamline the process,
The properties as a high-temperature material, especially the creep strength, decrease.

As a result of repeated experimental research on a method that can omit the normalizing treatment for precipitation-strengthened precipitated Cr ferritic steel without adding any new heat treatment or deteriorating its properties, It was discovered that the heating performed during final hot working such as rolling has an important meaning. In other words, during final hot working such as hot rolling, N
High Cr ferritic steel including b,
It has been discovered that if it is maintained for more than 10 minutes, the same creep characteristics as before can be maintained even if the normalizing treatment is omitted by combining it with tempering treatment, and in some cases, better characteristics than before can be obtained. be.

The method of the present invention was completed based on such knowledge,
Contains 3 to 0.3% by weight of C: o, o, 0.1% or less of N, and 5 to 13% of Cr, and further contains one or two of V and Nb (V+Nb) at 0.3%. The following three processing steps (1) to (3) (FIG. 1 (a) to (c)) are applied to precipitation-strengthened high Cr ferrite steel containing 01 to 1% additive.

■ During the final hot processing, heat and hold at 930-1300℃ for more than 1 minute, and after adding the necessary hot processing during that time,
The Mi weave becomes martensite by rapid cooling, and then AC,
Perform tempering treatment below the dot plate (Figure A).

■ Warm processing is performed during the tempering process performed in step (■), and the heating during the tempering process and the heating during the warm processing are performed (Figure guchi).

■ After the warm processing performed in ■, further Ac.

Perform strain relief annealing under the point plate (Figure C).

Step (2) is the basic process, and the creep characteristics are improved by processing heat treatment during final hot processing such as hot rolling and hot extrusion, and mm adjustment by tempering.

Further, in the steps ① and ②, warm working that also serves as tempering treatment is performed after the final hot working.

[For production]

Below, the reasons for limiting the component composition and heat treatment conditions in the method of the present invention will be described, and the effects will be clarified.

○ Component composition C: An austenite stabilizing element for achieving martensite structure, and an important element for precipitating carbides. When it is less than 0.03%, δ-ferrite a increases significantly, impairs strength and toughness, and does not form stable carbides. Moreover, if it exceeds 0.3%, carbides increase and harden, significantly impairing workability and weldability. Therefore, Ci
was set at 0.03 to 0°3%.

Cr: An essential element from the point of view of oxidation resistance, high Cr for high temperature use.
For steel, if it is less than 5%, sufficient oxidation resistance cannot be obtained;
In addition, when it is added in an amount exceeding 13%, the amount of δ-ferrite increases and the strength and toughness become t-membered.
It was set at 3%.

N is a niostenite stabilizing element and an important element for precipitating nitrides. However, if it exceeds 0.1%, workability is significantly impaired and toughness and strength are reduced, so the upper limit was set at 0.1%. Note that this N does not need to be added.
．． Since it is contained in an amount of about 0.01%, it is desirable to contain it in an amount of 0.02% or more in order to obtain a clear addition effect.

V, Nb: Both combine with C and N to form V (C.

N), Nb (C,N) forms fine precipitates and contributes to increasing creep strength. V (C,N), N
b Since the effect of (C,N) decreases as it becomes coarser, V (C,N), Nb, and (C,N) that are present in undissolved form are harmful to strength and toughness. The present invention has discovered a processing heat treatment method that effectively finely disperses and precipitates these precipitation-strengthening elements. One or two types of V and Nb (V+
Nb) is added in an amount of 0.01% to 1%. If it is less than 0.01%, sufficient precipitation strengthening cannot be obtained, and if it exceeds 1%, coarse V (C,N) generated in the previous process
, Nb (C.

It takes a long time for solid solution of N) and these compounds and M
Undissolved coarse precipitates such as z s Cb remain and impair strength, toughness, and workability.

Components other than the above include Mo and W in a total amount of 0.1 to 3%
and/or Si, Mn, SoρAjl! .

It is desirable to contain Ni and B, however, St is 0°5%
Hereinafter, it is desirable that Mn be 1.5% or less, 5OIA1 be 0.03% or less, Ni be 0.8% or less, and B be 0.01% or less. P is an unavoidable impurity.

It is desirable that S and Cu are each 0.03% or less.

○ Heating during the final hot working in the steps of heat treatment ■~■ (Fig. 1 A~C) eliminates the normalizing treatment, so the V (
C,N), Nb(C,N), M23C.

This is carried out for the purpose of dissolving undissolved carbonitrides such as , CrzN, etc., and making the alloy components uniform.

In particular, as a solid solution condition for carbonitrides of V and Nb, it is necessary to heat the material to 930° C. or higher. When the amount of V or Nb added is large, it is desirable to apply higher temperature heating according to the amount added. If the temperature is lower than 930°C, these precipitates will not be sufficiently dissolved and the strength will be significantly lowered, so the lower limit was set at 930°C.

In addition, for Ac, steel with a transformation point of 930℃ or higher, 930℃~
There is a risk that α-ferrite will remain between the 3 AC points and the uniform solid solution of V and Nb precipitates will be inhibited, so it is desirable to heat to the AC 1 point or higher.The upper limit of the heating temperature is The higher the value, the more uniform solid solution formation is promoted, but 1300
If the temperature exceeds .degree. C., .delta.-ferrite increases, which impairs toughness and strength, so the upper limit is set at 1300.degree.

The heating holding time is a condition for converting the precipitate into a solid solution.

Holding time also includes processing. Even if processing is performed immediately after heating, it is sufficient if processing is performed at the above temperature for at least 1 minute.
If the holding time is less than 1 minute, the solid solution of the precipitates and the homogenization of the alloy components will be insufficient, and many undissolved coarse precipitates will remain, impairing the strength. Although it is not particularly specified as it is convenient for securing the wall characteristics, in practice, it is desirable to have a thickness of about (lh/25 m thick) from the viewpoint of maintaining uniform heat of the member.

Processing also includes a case where the processing is performed while being maintained at the above heating temperature. There is no limit to the finishing temperature, but preferably AC
It is sufficient if the processing is completed at a temperature higher than the transformation point, more preferably at the AC2 transformation point or higher, and the subsequent rapid cooling can achieve martensitic structure.

The purpose of rapid cooling after processing is to create a martensitic structure. If the processing end temperature is above the A Cz transformation point, it is performed at a rate of 500°C/h or more. If cooling is less than 500°C/h, carbide + ferrite is formed during cooling. The structure changes and does not become a healthy martensitic structure.

In this precipitate, it is desirable that the AC be rapidly cooled from the transformation point to the Mf point where martensitic transformation is completed.
In this case, the cooling rate measurements are made for an average cooling rate between 800° C. and 500° C., this region being the ferrite formation nose region. The upper limit of the cooling rate is not particularly specified because the faster the cooling, the better the &l weave can be obtained, but in actual operation, care should be taken to ensure that thick materials do not crack or deform due to rapid cooling.

In the method of the present invention, the martensitic structure after quenching may partially contain δ-ferrite, and processing may be performed during the quenching.According to the present invention, normalizing treatment is unnecessary. , and then undergoes tempering treatment.

Tempering treatment is performed to lower the dislocation density and stabilize the structure, as the dislocation density of the martensitic structure is extremely high and the structure lacks long-term stability at high temperatures. , V, Nb and other carbides are finely precipitated. The reason why the temperature of the tempering treatment is set to be below the ACI point is to prevent re-austenization of the structure.

Steps ① and ② are cases in which shaping and finishing processing is performed during the tempering process. This process assumes that dimensional adjustment and deformation correction are performed warmly after the above-mentioned tempering treatment, and the heating for this one-time processing is performed at the same time as the heating during tempering to streamline the process. Contribute. Heating temperature is Ac
If it is 1 or more, the martensitic Mi weave undergoes austenite transformation again, which is inappropriate. Therefore, the upper limit of the heating temperature was set to be below the Ac+ point. If machining strain remains after warm working, stress relief annealing is performed as a post-heat treatment to remove this residual stress.The stress relief annealing temperature has an upper limit temperature of AC, which is the transformation point, to prevent re-austenitization.
An appropriate temperature is selected as the lower limit depending on necessity. Depending on the part, softening treatment may not be necessary, so two processes (1) and (2) were defined.

When heating to Ac1 or less in the steps of ■ to ■, boiler heat exchanger materials and boiler piping materials have a
It is preferable to heat it at ℃ for about 1 hour.

Through the steps (1) to (2), it is possible to omit the normalizing treatment. Furthermore, in the step (2), since the heating during tempering also serves as the heating during warm processing, thermal deformation of the finished product and scale formation are suppressed, making it possible to significantly reduce costs and rationalize.

In addition, the finished product has fine V (C
.

〔Example〕

Table 1 shows the chemical composition of the test steel.AfI4 is an American improved 9Cr-IMo steel (ASTM T91-A2) 3) steel with V and Nb composite additions, and steel B is a 9Cr 2Mo' steel.
A steel with a small amount of Nb added to iA, C steel is European 12Cr-I
These are Mo-V steels (DINX20CrMoWV12) steels, and both are representative steel types with a high Cr ferritic tone.

First, 150 kg of each steel is melted in a vacuum heating furnace, and the ingot is hot-forged at 1150°C to 900°C.
0Xj! Next, this material was made into 200 blocks and subjected to the processing heat treatments 1 to 3 shown in FIGS. 1(a) to 3(c) and the conventional processing heat treatment shown in FIG. 3.

Table 2 shows the thermal history of each heat treatment method.

As hot processing in steps Φ~■, 1100~900
After giving a working degree of 60 to 80% by roll rolling in the temperature range of .degree. C., the material was rapidly cooled to below the Ms transformation point. As warm working in (2) and (2), after soaking at 780°C for 1 hour, a working degree of 30% was given by roll rolling. The stress relief annealing in (2) was performed at 760 to 780°C.

As a conventional process carried out for comparison, 1000℃
After heating, 60% hot rolling was performed at 1000 to 800°C, and then gradual cooling was performed at 250°C/h to 300°C, and then left to cool.

Then, J
IS4 Charpy impact test piece and φ5XGL30n
A tensile test piece was taken and subjected to a room temperature tensile test, a 0°C Charpy impact M test, and a 600°C creep rupture test. The results are shown in Table 3.

Conventional method A4. B4. C4 is a standard normalizing and tempering process. In comparison, A to which the method of the present invention was applied
I, A2. As, A6. Bl, B2, B5. B6. C.I.
, C2, C5, and C'6 all showed strength characteristics that were equivalent to or slightly higher than those of the respective conventional methods. Also A
3. B3. C3 is a case where process (1) is applied to each steel, but the material heating temperature is set at 900°C, which is lower than the AC2 transformation point, so the creep rupture strength is extremely low.

On the other hand, C7 is a case in which the process (■) is applied to CIl, the cooling rate is slow cooling outside of the present invention, and the creep strength is extremely low.
FIG. 2 shows the creep rupture test results for C steel as an example.

As a result of structural examination, CI has a structure that is almost the same as that of conventional normalized and tempered material (C4), and has fine V (C
, N) were dispersed and precipitated, whereas C, which had low strength,
No. 3 is a fine tempered martensitic structure in which the processed structure remains, and many undissolved precipitates that are thought to have become coarse during processing were confirmed.

(Effects of the Invention) As is clear from the above description, the method of the present invention can be applied to boilers that have been subjected to normalizing and tempering to ensure creep properties when forming plate materials, pipe materials, and forged products.
It is clear that it is possible to impart properties equivalent to or better than conventional ones to precipitation-strengthened pit Cr ferritic steel for the nuclear power and chemical industries without normalizing treatment, and this greatly simplifies the process. This is highly effective in reducing manufacturing costs.

[Brief explanation of the drawing]

112I(a) to 4 are heat pattern diagrams of the processing steps in the method of the present invention, FIG. 2 is a graph showing the test results of high temperature creep strength for the method of the present invention and the conventional method, and FIG. 3 is the processing process in the conventional method. It is a process step pattern diagram. Procedural amendment (released) January 927, 1988 Director General of the Japan Patent Office Yoshi 1) Tsuyoshi Moon ■, Indication of the case 1988 Patent Application No. 190790 2 Title of the invention Processing method of high Cr ferritic steel for high temperature use 3 , Relationship with the case of the person making the amendment Patent applicant address 5-15 Kitahama, Higashi-ku, Osaka Name (2)
1) Sumitomo Metal Industries Co., Ltd. Representative Yasuo Shingu 4, Agent 6, drawings subject to amendment. 7. Details of the amendment Figure 1 of the drawing will be corrected as shown in the attached sheet. (Figure 3 is without correction.)

Claims (3)

[Claims] (1) C: 0.03-0.3%, N: 0.1% in weight%
Hereinafter, high carbon containing 5 to 13% Cr and further adding one or two of V and Nb (V+Nb) at 0.01 to 1%
r In final hot working of ferritic steel, 930-13
A method for processing high-Cr ferritic steel for high temperature use, which comprises performing hot working by heating and holding at 00° C. for 1 minute or more, converting the structure to martensite by rapid cooling, and then performing tempering treatment to Ac_1 point or less. (2) C: 0.03-0.3%, N: 0.1% in weight%
Hereinafter, high carbon containing 5 to 13% Cr and further adding one or two of V and Nb (V+Nb) at 0.01 to 1%
r In final hot working of ferritic steel, 930-13
A method for processing high-temperature high Cr ferritic steel, which is characterized by performing hot working by heating and holding at 00°C for 1 minute or more, converting the structure to martensite by rapid cooling, and then performing warm working by heating to Ac_1 or less. . (3) C: 0.03-0.3%, N: 0.1% in weight%
Hereinafter, high carbon containing 5 to 13% Cr and further adding one or two of V and Nb (V+Nb) at 0.01 to 1%
r In final hot working of ferritic steel, 930-13
Hot working is performed by heating and holding at 00℃ for 1 minute or more, and the structure is martensited by rapid cooling, then warm working is performed by heating to Ac_1 point or less, and then stress relief annealing is performed to Ac_1 point or less. A method of processing high Cr ferritic steel for high temperature use, which is characterized by: