JPS5896854A - High toughness steel for pressure vessel - Google Patents

Abstract

PURPOSE:To obtain a high toughness steel for a pressure vessel by hardening a steel contg. specified amounts of C, Si, Mn, Ni, Cr, Mo acid-sol. Al, N, P and S to specify the amounts of N as AlN, P and S contained in the steel. CONSTITUTION:A steel is refined by normalizing, hardening, tempering and stress relieving annealing to obtain a steel having a composition consisting of, by weight, 0.15-0.23% C, 0.15-0.35% Si, 1.25-1.5% Mn, 0.5-0.7% Ni, 0.01- 0.2% Cr, 0.4-0.6% Mo, 0.008-0.025% acid-sol. Al and the balance Fe with inevitable impurities and further contg. 0.004-0.012% N as AlN, P and S satisfying 2P+S/(N as AlN)<=2.6. The amounts of N as AlN, P and S are specified during the heating of the hardening. The resulting steel is used as a steel for a pressure vessel having >=100mm. thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to high toughness steel for pressure vessels, and particularly
Regarding the above steel.

In recent years, with advances in the chemical industry, pressure vessels have become increasingly larger, and the steel plates used therein have also tended to become thicker. Furthermore, nuclear power is attracting attention as an energy source that can replace oil, and the construction of nuclear power reactors is actively underway. The performance requirements for low-temperature toughness of steel materials for pressure vessels such as nuclear reactors are becoming increasingly strict from a safety perspective. Conventionally, this type of steel is used after being quenched and tempered, but by finely dispersing A/N, the crystal grains are made finer, roughly to increase absorbed energy, or anisotropically. Efforts have been made to improve toughness by lowering S to an extremely low level. However, due to reasons such as increased wall thickness, it is becoming increasingly difficult to ensure toughness. Evaluation of the toughness of this type of plate is carried out by the Charpy impact test and the related compliance temperature (hereinafter referred to as RTNDT) determined by the electric drop test. RT N
DT is the non-ductile transition temperature (hereinafter referred to as TNDT) determined by a current drop test and TNDT + 33℃
The individual absorbed energy of the Charpy test conducted at the following test temperatures is 6.91wf-m or more, and the shadow projection ratio is 0.9
m or more, TNDT −RTNDT, and RT
The lower the NDT, the better the toughness. Therefore, from a design perspective, a low RTNDT is required in consideration of safety.However, as nuclear reactors become larger, the thickness of the copper plate increases, making it increasingly difficult to ensure toughness. There is.

The purpose of the present invention is to solve the problems of the prior art described above, and to
An object of the present invention is to provide a steel for pressure vessels with a high toughness of 0- or higher.

From a safety perspective, in nuclear reactors and other pressure vessels,
The internal quality of the steel plate has been thoroughly inspected, so there are no harmful internal defects, but it is possible that defects that occur during construction may exist on the surface. It is thought that such surface defects can cause brittle fractures that can lead to major music accidents, and for this reason, the toughness at the center of the plate thickness is also important, but the toughness of the surface layer is particularly important. Excellent toughness near the surface makes it possible to prevent brittle fracture from occurring due to surface defects. The inventors are RT? [) We investigated and researched the influence of various factors on T, and obtained knowledge that can lower the RTNDT of extremely thick steel sheets, especially the RTNDT of the surface layer of the steel sheet.

The gist of the present invention is as follows.

That is, CIo, 15 to 0.23% by weight, Si+
0.15-0.35%, Mn j 1.25-1.5
0%, Ni+0.50-0.70%, Cr: 0.0
1 to 0.20%, MoC2, 40 to 0.60%, acid-soluble red + o, oos ~ 0.025%, and further N + 0.0040 to 0.0120% as a bunch during quenching heating, ”
The steel is a high-toughness steel for pressure vessels, characterized in that it contains P and S in a range of N)<2.60 as +)/CAth, with the remainder consisting of Fe and inevitable impurities.

The steel of the present invention undergoes heat treatment and skewering (N) - quenching (Q) -
Tempering T) - Used after stress relief annealing (SR).

It is already a well-known fact that making the grain size finer is effective in increasing Charpy impact toughness. However, the method for improving the drop weight characteristics has not been clarified until now.

The present inventors investigated the influence of austenite grain size on RTNDT in a neck drop test and obtained the results shown in FIG. 1.

Figure 1 shows the case where the tempering treatment of skewering, quenching, tempering, and stress relief annealing was performed, and the quenching was performed at a cooling rate of 35°C/mIn, but the austenite grains were refined by RTNDT.
This indicates that the temperature is significantly lower.

In the figure, dr is the diameter of the crystal grain of the γ phase.

Next, we considered the use of ktN as a method of refining the austenite grain size, and we decided to use ktN (hereinafter referred to as NAJN) as A/N.
We investigated the relationship between this and RTNDT. That is, the RTNDT of the same steel plate as in FIG. 1 was investigated by changing the NAo+, and the results are shown in FIG. From Fig. 2, as Nun increases, RTNDT becomes significantly lower.
It can be seen that the drop weight characteristics are improved due to the crystal grain refining effect of AtN. In FIG. 2, the range in which NuN during quenching is 0.0040% or more has a significant effect on lowering the temperature of RTNDT. On the other hand, even if NAJN exceeds 0.0120%, it does not particularly adversely affect RTNDT, but if it is contained in excess of this, problems will arise in melting and processing, so the upper limit was set at 0.0120%.

For the above reasons, the NAJN during quenching heating is 0.0040~
It was limited to a range of 0.0120%.

Next, we investigated and researched the relationship between the cooling rate of quenching and RTNDT, and found that RTNDT is xp+d, which is composed of the content% of P and S contained in the steel, in the steel plate that has undergone the above heat treatment treatment.
It was found that there is a relationship with the ux parameter as shown in FIG. That is, in the case of 2000°C/min with a fast cooling rate in the temperature range of 800 to 400°C, i, =
4! ” +/NAtNka2.6 Below 1: Su6
RTNDT becomes extremely low temperature and cooling rate is slow35
A similar tendency is shown in the case of C/min, but the effect is not as pronounced when the speed is high. During quenching, the cooling rate of the surface layer of the plate is fast, so in the present invention, P,
By limiting the relationship between S and NA4N to I P + :/'NA 2.60, the toughness of the steel sheet, particularly the toughness of the surface layer, was significantly improved.

The following components other than the above N, P, and S have a strength of 0.2 after tempering treatment of skewering, quenching, tempering, and stress relief annealing.
% proof stress 35 kff/mJ or more, tensile strength 55 kff/
- As mentioned above, the scope was limited due to the following reasons.

C: If C is less than 0.15%, it is difficult to ensure strength;
If it exceeds 3%, it will have a negative effect on RTNDT, so 0.15~
It was limited to a range of 0.23%.

Si I Si (, below 0.15%, sufficient strength cannot be obtained, and when it exceeds 0.35%, toughness decreases, so 0.15~
It was limited to a range of 0.35%.

The more Mn I Mn, the more advantageous it is to ensure strength, but 1.50%
If the content exceeds 1.25%, the toughness deteriorates, and if it is less than 1.25%, sufficient strength cannot be obtained, so the content was limited to a range of 1.25 to 1.50%.

I I Ni is an element that increases toughness, but if it is less than 0.50%, its effect is small and it is expensive, so is there an upper limit? 0
．． 70% and limited to a range of 0.50 to 0.70%.

r t cr is added to increase the strength, but 0.01
If it is less than 0.2%, it has almost no effect, and if it exceeds 0.20%, it increases the stress relief annealing cracking property, so it is 0.01 to 0.2%.
It was limited to a range of 0%.

MO = MO is an element that increases hardenability and precipitates as MOIC during tempering to increase strength, and is essential as a reinforcing element, but because it is expensive, the upper limit is set at 0.60% and 0.4
If it is less than 0%, the effect will be small.

It was limited to a range of 0.40 to 0.60%.

M: A/ is added as a deoxidizing agent, but it also refines the crystal grains as AIR, so it is N10.00 as the above-mentioned phantom.
For the presence of 40-0.0120%.

The amount of acid-soluble At corresponding to this N amount is 0.008~
Addition was limited to 0.025%.

Example 1 produced by converter - gas - ladle refining - ingot making - rolling
Extra-thick steel plates with thicknesses of 70 and 250 cm and having the chemical composition shown in Table 1 are subjected to the heat treatment shown in Table 2, and JI
A tensile test using a SIO test piece and an electric drop test using a P-3 test piece were conducted, and the results are also shown in Table 1.

In both comparisons @C and D, IP+VNAJN is outside the scope of the present invention.

In Table 1, the RTNDT of the surface of the steels A and B of the present invention is particularly lower than that of the comparative coppers C and D. Furthermore, the temperature of RTNDT in the center is lower in the steel of the present invention. This result shows that the steel sheet according to the present invention has excellent toughness, especially the toughness of the surface layer of the steel sheet.

As is clear from the above examples, the present invention makes it possible to manufacture a high-toughness pressure vessel with particularly excellent surface layer toughness by limiting the chemical composition of steel, particularly AtNi1 during quenching heating. did it.

[Brief explanation of drawings]

Figure 4 shows austenite grain size and electric drop test type) RTN
A correlation diagram showing the relationship with DT, Figure 2 shows N as ktN.
Correlation diagram showing the relationship between the content and RTNDT of the electric drop test,
Figure 3 shows the RTND of “-Shishi’Qhsx” and the current drop test.
It is a correlation diagram showing the relationship with T. Agent Takeo Nakaji Figure 1 Figure 2 Figure 31.11

Claims (1)

[Claims] (1) CIo, 15-0.23% by weight, Si
: 0.15-0.35%, h4n 11.25-1.
50%, Nl + 0.50-0.70%, Cr:
0.01-0.20%, Mo +0.40-0.60
%, acid-soluble A/, 10.008 to 0.025%, and further N as Am during quenching heating: 0.0040 to 0
．． 0120%, ``More 1 (T) N) <, 2.6
1. A high toughness steel for pressure vessels, characterized in that the P and S content is in the range of 0, and the portion thereof is made of Fe and inevitable impurities.