JPH0125810B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing high chromium steel with excellent toughness, and particularly aims to provide a method for manufacturing high chromium steel with excellent low temperature toughness and heat resistance. Steel materials used in high-temperature parts such as boiler superheaters, steam pipes, chemical plants, and nuclear reactors are required to have excellent properties in terms of not only high-temperature strength but also corrosion resistance, toughness, workability, and weldability. Ru. Particularly in recent years, safety has been emphasized, and existing plants must be used for as long as possible from the perspective of energy conservation, so there is a trend for materials that do not deteriorate even after long-term use, and there is also a need for materials that do not deteriorate even after long-term use. It is also required to be able to withstand the impact loads of
It has also become a necessary condition for heat-resistant steel to have sufficient toughness. Austenitic steel, nickel-based alloy, ferritic stainless steel, 21/4Cr-1Mo steel, 9Cr-1Mo steel, etc. are known as steels used for this type of use. There are problems in that nickel-based alloys are highly susceptible to stress corrosion cracking and are expensive. In addition, ferritic stainless steels generally have low strength and are brittle;
1/4Cr-1Mo steel also has some problems in oxidation resistance due to the Cr content. Among these various steels, 9Cr-1Mo steel is attracting attention because it has excellent high-temperature strength and good oxidation resistance. There are two types of 9Cr-1Mo steels: duplex steels whose structure is composed of ferrite and tempered martensite, and steels whose structure is single-phase tempered martensite. Since steel generally has a high C content of 0.1 to 0.2 wt%, weldability is often insufficient. On the other hand, duplex 9Cr-1Mo steel with a lower carbon content has good weldability but does not necessarily have sufficient high-temperature strength. To compensate for this drawback, 9Cr-1Mo- is a steel of the same series with Nb, V, and B added as needed to improve high-temperature strength.
There is V-Nb(B) steel, but it has the drawback of not having sufficient toughness. As described above, conventional steels do not satisfy all of high-temperature oxidation resistance, high-temperature strength, toughness, and weldability in a well-balanced manner, and have weaknesses in any of the properties. The present invention was newly researched and developed in view of these circumstances, and has improved oxidation resistance, high temperature strength, low temperature toughness,
It is an object of the present invention to provide a steel that satisfies all aspects of weldability in a well-balanced manner, and for this purpose, the present invention adopts a low C 9Cr-1Mo-V-Nb composition and also changes the amount of δ ferrite to By adjusting the relationship with the working ratio (mainly rolling ratio) applied to the steel, we succeeded in improving toughness without impairing high-temperature strength and weldability. That is, the basic characteristics of the first invention of the present application are C:
0.04-0.15%, Si: 1.0% or less, Mn: 1.0% or less,
Cr: 5-12%, one or both of Mo and W in total 0.5-2.0%, V: 0.05-1.5%, Nb (and the sum of Ta that inevitably accompanies this): 0.03-0.6% ,
A steel containing Al: 0.3% or less, N: 0.1% or less, the balance consisting of iron and unavoidable impurities, and a δ ferrite amount of 1 to 50% is prepared according to its δ ferrite amount (δ). The reason is that processing is performed so that P shown in the formula is 0.2 or more. P=logF−3δ/100 where F: processing ratio Furthermore, the basic feature of the second invention of the present application is that C:
0.04-0.15%, Si: 1.0% or less, Mn: 1.0% or less,
Cr: 5-12%, one or both of Mo and W in total 0.5-2.0%, V: 0.05-1.5%, Nb (and the sum of Ta that inevitably accompanies this): 0.03-0.6% ,
Al: 0.3% or less, N: 0.1% or less, and B: 0.01
% or less, Ni: Contains one or two types of 1.0% or less,
A steel whose balance consists of iron and unavoidable impurities and whose δ ferrite content is 1 to 50% has P expressed by the above formula of 0.2 depending on its δ ferrite content (δ).
This is due to the fact that it was processed so that it would be as described above. The reasons for the limitations of the present invention will be explained below. First, the present invention employs the above-described specific component composition. C is required to ensure sufficient high temperature strength.
0.04% or more is necessary, but if it exceeds 0.15% it is not preferable because it impairs weldability. Therefore, the amount of C is
Set at 0.04-0.15%. Si is added as a deoxidizing agent, but the amount is 1.0
%, weldability is impaired, and for this reason the amount of Si is
It is regulated to 1.0% or less. Mn is also added as a deoxidizer along with Si, but
If the amount exceeds 1.0%, workability and weldability deteriorate, and therefore the amount of Mn is also regulated to 1.0% or less. It is necessary to add 5% or more of Cr to ensure sufficient oxidation resistance at high temperatures, but if it exceeds 12%, the amount of ferrite decreases significantly and toughness is impaired. Therefore, the Cr content is set to 5 to 12%. Mo and W are essential elements for increasing high-temperature strength, and it is necessary to add one or both of them in a total amount of 0.5% or more. On the other hand, even if one or both of them are added in a total amount exceeding 2.0%, no improvement in high-temperature strength commensurate with the added amount can be expected, which will actually impair economic efficiency. For this reason, Mo, W
The total amount of one or both of them is in the range of 0.5 to 2.0%. V is an effective element for high-temperature strength, and is added in an amount of 0.05% or more to obtain the necessary high-temperature strength. but
If it exceeds 1.5%, the toughness will be significantly impaired, so the V content should be 0.05 to 1.5%. Nb is an element that contributes to creep strength along with C, but its effect is weak when it is less than 0.03% in the above-mentioned range of C, so it is added in an amount of 0.03% or more. However, if it exceeds 0.6%, weldability will be significantly impaired. When Nb is added, it inevitably accompanies Ta, which has similar properties.
Therefore, the total amount of Nb including Ta is set to 0.03 to 0.6%. Al is added as a deoxidizing agent, and when added in an appropriate amount, forms AlN together with nitrogen, which improves toughness through the fine graining effect. However, if the amount added exceeds 0.3%, it is undesirable because it reduces the creep strength, and therefore the amount of Al is
0.3% or less. N is an element that increases strength, but if it exceeds 0.1%, it impairs toughness and tends to cause bubbles when high energy welding such as electron beam welding is performed on thick plates, so the amount of N is set to 0.1% or less. In the second invention of the present application, one or two types of B and Ni are added to the above basic components. Among these, B is an element that increases creep strength, but if its amount exceeds 0.01%, it impairs weldability, so the B amount is set to 0.01% or less. Further, although Ni has the effect of increasing toughness when added in an appropriate amount, addition of more than 1.0% is not preferable because it may cause chloride stress corrosion cracking, and therefore the amount of Ni is set to 1.0% or less. In the present invention, in addition to the composition conditions as described above,
Furthermore, regulations are added on the amount of δ ferrite in the tissue. That is, in order to improve weldability, it is desirable to have a mixed structure of δ ferrite + tempered martensite rather than a single phase structure of tempered martensite, and therefore the amount of δ ferrite is required to be at least 1%. However, if the content of δ ferrite exceeds 50%, the rolling/forging ratio required to ensure the desired toughness must be significantly increased, and as a result,
In order to create a material with a predetermined size, it is necessary to create and process an extremely large steel ingot, which is extremely uneconomical. For this reason, in the present invention, the amount of δ ferrite in the steel is limited to a range of 1 to 50%. In addition, as the amount of δ ferrite, it is possible to use a value obtained by examining steel that has been subjected to a prescribed heat treatment with an optical microscope and determining the area ratio of ferrite in the form of bands or islands. Although it is common, if sufficient accuracy is ensured, values obtained based on empirical formulas from the content of each element contained in the steel can also be used. In the present invention, in addition to the above compositional and structural restrictions, certain restrictions are also imposed on processing such as rolling. The present inventors have discovered that, in addition to the above-mentioned compositional and structural regulations, excellent toughness can be obtained by performing processing such as rolling at a processing ratio above a certain level in relation to the amount of δ-ferrite. , this is a major feature of the present invention. In other words, when F is the processing ratio (material cross-sectional area A before processing/product cross-sectional area B), the processing ratio (F ) to perform processing. P=logF-3δ/100 Here, processing may be any processing that destroys the cast structure, such as rolling, forging, extrusion, etc., and it does not matter whether it is hot working or cold working. However, the processing is preferably carried out at a temperature range of 1250 to 1000°C from the viewpoint of ease of processing. 1250℃ for processing
Heating above 1000°C significantly shortens the life of the furnace, and heating below 1000°C is undesirable because the deformation resistance becomes too large. In addition, the processing ratio F is defined as the cross-sectional area of the material before processing is A, and the cross-sectional area of the product is B.
In this case, it is expressed as F=A/B, where the cross-sectional area A of the material before processing is
When manufacturing by general ingot making, it refers to the cross-sectional area of the steel ingot itself, not the cross-sectional area of the semi-finished product after blooming.
Furthermore, in the case of manufacturing using a slab or billet obtained by continuous casting, it means the cross-sectional area of the slab or billet. The heat treatment generally adopted in the present invention is a normal normalizing-tempering treatment, in which the molten steel is forged and tempered.
After rolling, normalize at 950℃ or higher, then 700℃ or higher Ac ₁
It is tempered at the following temperature. [Example] Steel with the composition shown in Table 1 was melted, rolled, normalized and tempered, and subjected to a full-size Sharpi impact test and a JIS maximum hardness test (JISZ3101). . The results are also shown in the same table. As a comparative material, commercially available tempered martensite single-phase 2
1/4Cr-1Mo steel, 9Cr-1Mo steel, X20CrMoV steel, etc. were used.

【table】

[Table] ○ Invention steel *1 0°C Sialpi test *2 JIS Z 3101
As can be seen by comparing Steel B and Steel D in the same table, the impact test results show that the smaller the amount of δ ferrite, the better the toughness, but regarding the absorbed energy (Sharpi test at 0°C), steels A to F As can be seen, the P value increases due to the high rolling ratio and the relationship with the amount of δ ferrite.
If it is not 0.2 or more, preferred toughness cannot be obtained.
Figure 1 shows the relationship between the P values of several test steels, including the test steels in Table 1 above, and the Syarpy absorbed energy at 0°C.
It can be seen that excellent toughness with an absorbed energy of 5 kg·m or more at 0°C can be obtained at a temperature of 0.2 or more. On the other hand, single-phase steels such as G steel and H steel are promising in terms of their high toughness, but as shown in Table 1, compared to two-phase materials, the maximum hardness of welding Problems with sexuality remain. Further, although the steel 21/4Cr-1Mo steel has excellent toughness and weldability, it has a small amount of chromium and has insufficient corrosion resistance. Figure 2 shows Steel A, which is the steel of the present invention, and Steel G, which is the comparative steel (tempered martensitic 9Cr-1Mo steel,
This figure shows a comparison of the creep strength (at 650°C) of STBA26), which shows that the steel of the present invention has excellent high-temperature strength. According to the present invention described above, it has excellent high temperature strength,
Furthermore, it is possible to produce high chromium steel with excellent low-temperature toughness and weldability, and it can be said to be extremely useful as a method for producing steel materials for boilers, chemical plants, nuclear reactors, etc.

[Brief explanation of drawings]

FIG. 1 shows the relationship between the P value and the Syarpy absorbed energy for the test steels shown in Table 1. FIG. 2 shows a comparison of the creep strength of steel A and steel G in Table 1.

Claims (1)

[Claims] 1 C: 0.04 to 0.15%, Si: 1.0% or less, Mn: 1.0
% or less, Cr: 5 to 12%, one or two of Mo and W
Total seeds: 0.5-2.0%, V: 0.05-1.5%, Nb (and the sum of Ta that inevitably accompanies this): 0.03-0.6
%, Al: 0.3% or less, N: 0.1% or less, the balance consists of iron and unavoidable impurities, and the amount of δ ferrite is 1 to 50%, depending on the amount of δ ferrite (δ). A method for producing high chromium steel with excellent toughness, characterized by processing the steel so that P expressed by the following formula is 0.2 or more. P=logF-3δ/100 However, F: Processing ratio 2 C: 0.04 to 0.15%, Si: 1.0% or less, Mn: 1.0
% or less, Cr: 5 to 12%, one or two of Mo and W
Total seeds: 0.5-2.0%, V: 0.05-1.5%, Nb (and the sum of Ta that inevitably accompanies this): 0.03-0.6
%, Al: 0.3% or less, N: 0.1% or less, and B:
0.01% or less, Ni: 1.0% or less, the balance consists of iron and unavoidable impurities, and the amount of δ ferrite is 1 to 50%.
Depending on the amount of ferrite (δ), P expressed by the following formula is
A method for manufacturing high-chromium steel with excellent toughness, which is characterized by processing it to a toughness of 0.2 or higher. P=logF−3δ/100, where F: processing ratio