JPS5853712B2 - Chromium-molybdenum steel with excellent resistance to temper softening - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a chromium-molybdenum steel with excellent resistance to temper softening, and in particular to a chromium-molybdenum steel for pressure vessels that has a specified normal temperature and high temperature strength and toughness even after long-term tempering at high temperatures. Mo
Regarding steel.

In recent years, tempering and stress relief annealing of pressure vessel steels, such as Cr■0 steel, have become increasingly high temperature and long-term, and the well-known parameter T (20+1og t) shown by Joffe-Hollomon has been increasing. reaches 22×103.

Here, T: Absolute tempering temperature (0K) t: Required time for tempering (hr) In the case of this high-temperature and long-term tempering or stress relief annealing conditions, for example, Cr: 2.25%, Mo: 1% is contained. Conventional steel such as 23ACr IM
It has the disadvantage that it does not even meet the minimum value of 52.5 kg/mm for standard A387, grade 22, class 2, and to satisfy this, A387, grade 21 high alloy steel with Cr: 3% or more is required. This results in an increase in manufacturing costs.

The object of the present invention is to normalize or quench Cr-Mo steel, and then to have the required strength at room temperature and high temperature even after long-term tempering at high temperatures, and without impairing toughness. It is an object of the present invention to provide a Cr-Mo steel having excellent resistance to temper softening.

The reason why the components are limited in the present invention will be explained.

C: If C is less than 0.08%, the required tensile strength cannot be obtained, while if it exceeds 0.21%, weldability and toughness will be impaired, so it is limited to a range of 0.08 to 0.21%.

Si: If Si is less than 0.10%, the deoxidation of the steel will be insufficient, while if it exceeds 0.8%, the carbide precipitation behavior will be saturated and the cost will increase, so the Si content should be 0.1 to 0.8%. did.

Mn: If Mn is less than 0.3%, the required tensile strength and toughness cannot be obtained, while if it exceeds 1.5%, deterioration of weldability and strength will occur, so 0.3 to 1.5%. The range of

P: If P exceeds 0.015%, toughness will be exerted, so it should be set to 0.
．． It should be less than 0.015%.

S: If S exceeds 0.010%, toughness will deteriorate, so 0.
．． It should be less than 0.010%.

Cu: Cu is an effective component for precipitation hardening, but if it is less than 0.1% it is ineffective, while if it exceeds 0.5% it causes deterioration in toughness, so it was set in the range of 0.1 to 0.5%. .

Ni: Ni alone has no effect on tempering softening resistance, but C
It has great softening resistance due to its combination with u.

However, if it is less than 0.1%, it will not be effective, and if it exceeds 0.5%, it will increase the cost, so it is limited to a range of 0.1 to 0.5%.

Cr: If Cr is less than 0.5%, hardenability deteriorates, while if it exceeds 6%, the strength and toughness are saturated, leading to an increase in cost, so it is set to 0.5 to 6.0%.

Mo: If Mo is less than 0.4%, it will not contribute to the required carbide precipitation, while if it exceeds 1.2%, it will impede weldability and result in an increase in cost, which is undesirable.
~1.2%.

5olAl: 5olA1 (acid soluble aluminum) is 0.003%*
If it is less than *, the toughness will deteriorate, while if it exceeds 0.08%, the cleanliness of the steel will deteriorate and at the same time the creep strength will deteriorate, so it was limited to a range of 0.003 to 0.08%.

As described above, the steel of the present invention was able to precipitate fine MozC type carbides in a high density due to the combined effect of trace amounts of Si, Cu, and Ni in order to improve the temper softening resistance. This is a major feature of the present invention.

Embodiments of the present invention will be described in comparison with conventional steel with reference to the accompanying drawings.

Table 1 shows the chemical composition of test steels A, B, C, D, and E.
Table 2 shows the chemical composition of test steels F, G, and H1■.

In Table 1, steels A and B are the invention steels, steel C is the 23ACr 1% MO steel that has been normalized and tempered, steel D is the low-Si 2% Cr 1%Mo steel, and steel E is the steel that has been quenched and tempered. Both the 23A%Cr-1%MO steel and the C-E steel are the most conventionally known 23ACr-1%Mo steel with a plate thickness of 200mm.
This is a steel plate for pressure vessels.

The mechanical properties obtained from the tensile test and Charpy impact test when all the test steel sheets were annealed (water quenched for E steel) and then tempered at 600 to 760°C are shown in Figures 1 and 2. That's right.

From FIGS. 1 and 4, the steel of the present invention has a higher T than the conventional steel.
The tempering conditions represented by (20+1ogt) are 20
When it is larger than X103, the effect is exhibited and the temperature is 760℃
x20Hr (tempering condition T(20+log t) −
22X 103), its tensile strength is more than 10 kg/mi higher than that of conventional steel, and it is clear that it has excellent resistance to temper softening despite being tempered at high temperatures and for a long time.

Furthermore, as shown in Figures 2 and 5, the steel of the present invention has a Charpy fracture surface transition temperature (vTs) equal to or higher than that of conventional steel even under high temperature and long-term tempering conditions, and is a high-strength and high-toughness material. confirmed.

FIG. 3 is a 10,000x electron microscope micrograph of the steel of the present invention and the conventional steel.

As is clear from Fig. 3, in the present invention, MO2C type fine carbides are densely precipitated, and the present invention exhibits new characteristics not seen in conventional steels regarding the morphology and distribution of carbides that contribute to precipitation hardening. .

As clearly shown in the above examples, the steel of the present invention was able to achieve the following remarkable effects compared to conventional steel.

(a) The steel of the present invention has a tensile strength of 10 kg/mt higher than conventional steel even under high temperature and long-term tempering conditions of, for example, 760 x 20 hours? It is a Cr-MO steel with excellent resistance to temper softening, which has both strength and toughness, and has a toughness equal to or higher than that of conventional steel.

(b) In order to improve the temper softening resistance of the steel of the present invention, fine MO2C type carbides can be precipitated in a high density due to the combined effect of the addition of appropriate amounts of (Si), (Cu), and (Ni), which is different from conventional steel. It is possible to suppress cost increases to a minimum, and conversely, considering that high-alloy steel grades have traditionally been used to improve temper softening resistance, it has made it possible to significantly reduce costs.

[Brief explanation of drawings]

Figure 1 shows examples of the present invention (A steel, H steel) and conventional steels (C, H steel).
D, H steel) and tempering conditions T(20+1ogt)XIO”
Figure 2 is a correlation diagram comparing the relationship between the tensile strength and the tensile strength. Figure 2 shows the same tempering conditions as in Figure 1 for the examples of the present invention (A steel, H steel) and conventional steels (C, D, H steel). and Charpy fracture transition temperature vT
FIGS. 3A and 3B, which are correlation diagrams comparing the relationship with s, are 10,000x electron micrographs showing the structures of the steel of the present invention and the conventional steel, respectively. Figure 4 also shows examples of the present invention (F steel, H steel) and conventional steel (
Figure 5 is a correlation diagram comparing the relationship between the tempering conditions and tensile strength for the steels (G steel, ■ steel), and the relationship between the examples of the present invention (F steel, H steel) and conventional steels (G steel, ■ steel). FIG. 3 is a correlation diagram comparing the relationship between the tempering conditions and the Charpy fracture surface transition temperature.

Claims (1)

[Claims] 1 C: 0.08-0.21%, Si: 0.1% by weight.
1-0.8%, Mn: 0.3-1.5%, P: 0.
0.015% or less, S: 0.010% or less, Cu: 0.015% or less, S: 0.010% or less, Cu: 0.
1-0.5%, Ni: 0.1-0.5%, Cr:
0.5-6.0%, Mo: 0.4-1.2%, 5
olA1: Contains 0.003 to 0.08%, the remainder is substantially composed of Fe, and has a temper softening resistance characterized by having predetermined strength and toughness at room temperature and high temperature even after long-term tempering at high temperatures. Chrome-molybdenum steel with excellent properties.