JPS6289810A - Manufacture of low-alloy steel for nuclear reactor - Google Patents

Abstract

PURPOSE:To obtain a low-alloy steel capable of avoiding He embrittlement due to the presence of B without passing through a complex manufacturing stage by dispersing B in a low-alloy steel for a nuclear reactor having a specified composition as a B-base deposit in the grains of the steel by the recrystallization and growth behavior of B during the manufacture of the steel. CONSTITUTION:A steel contg., by weight, 0.02-0.4% C, 0.005-1.0% Si, 0.1-1.5% Mn, 0.01-5.0% Cr, 0.1-1.5% Mo, <=0.002% B, 0.001-0.1%Al and <=0.006% N or further contg. 0.01-0.05% S is prepd. when the steel is heated, rolled and reheated, it is held at 750-900 deg.C for >=10min in the latter half of the reheating stage. When the steel is heated, rolled and subjected to direct forced cooling, it is held at 750-900 deg.C for >=2min between the rolling and direct forced cooling stages.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing so-called low alloy steel for nuclear reactors, which is a metal material used in fields subject to neutron irradiation such as nuclear reactor structural materials. Carbon steel and low Cr- which do not cause creep embrittlement even under thermal neutron irradiation environment
This is a technology that is advantageously applied to manufacturing Mo steel and the like.

(Prior art) Low carbon steels for nuclear reactor pressure vessels include ASTM standard A 533-CL; 1. Same A308-CL.

3 steel is well known. In addition, 23ACr-1Mo steel with low Cr-Mom is used in fast breeder reactors (FBR).
It is used as a structural material for nuclear reactors such as nuclear fusion reactors (FER).

However, each of the above structural materials for nuclear reactors contains 2 to 3 ppm of boron (B) as an alloying component (even if it is not specifically intended to be added, it is inevitably mixed in as an impurity during the normal steelmaking process). ) is well known.

Boron that is unavoidably mixed in or added as an alloy component causes the following problems under neutron irradiation conditions. In other words, natural boron is 2
It is composed of the radioactive isotopes 103 and nB of the species, of which IQB is ``'B (
n, (Z)' Li nuclear reaction occurs, 103 decays and HC
Produce gas. As a result, 1-1e gas is generated from boron compounds that tend to precipitate at grain boundaries, which weakens the bonding force at grain boundaries and is known to cause so-called creep embrittlement. [Journal of Nuclear Materials
16 ('65) 68-73J Conventionally, as a countermeasure technology for the creep embrittlement caused by Io 3 mentioned above, Japanese Patent Application Laid-open No. 53-88499 discloses that lfl 3 of the total poron contained in steel. It is proposed that the quantity ratio of However, even if this conventional technology is used, in order to achieve this, it is necessary to use a B-containing raw material with a higher 103 ratio than the natural abundance ratio, and it is necessary to avoid increasing costs and complicating the manufacturing process. The situation was such that it was impossible to do so.

In addition, another conventional technology [Journal of Materials (Journal of Nuclear
Materials) No. 103. No
, 104 (1981) J, there is a technique for trapping the generated He to prevent agglomeration, but there are still many problems to be solved in order to realize this technique.

(Problems to be Solved by the Invention) The object of the invention arises when steel containing boron is used as a nuclear reactor structural material subjected to thermal neutron irradiation. We propose an advantageous method for manufacturing alloy steel at low cost without going through complicated manufacturing processes, which can effectively avoid He embrittlement due to the (n, α) reaction caused by the presence of so-called 10[3]. It's there to do.

(Means for Solving the Problems) The present inventors studied the influence of boron using steel that is produced through a normal manufacturing process (in which a predetermined amount of boron is unavoidably mixed). As a result, even if there is a steel containing +03, if it does not exist in the grain boundaries in a solid solution state, it will not become brittle, and the boron present in the steel will be precipitated and dispersed within the grains. I came up with this idea.

However, in the case of boron-free steel, the amount of boron that is unavoidably mixed is extremely small at 1 to 3 ppm, and since boron is an element that tends to segregate at grain boundaries, most of this trace amount of boron is Dispersing it as a precipitate in
This was previously impossible.

However, based on basic research related to recrystallization behavior in recrystallization and/or non-recrystallization regions, boron precipitation, grain boundary segregation, and precipitation behavior, the present inventors have found that boron behavior particularly during recrystallization. According to what I understand,
We have found that it is possible to achieve intragranular precipitation and dispersion of boron, which was previously thought to be difficult.

In other words, the method of the present invention is applicable to wIA manufacturing of nuclear reactor metal materials (low-alloy steel) that are subjected to neutron irradiation during use.
By utilizing the recrystallization and growth behavior of boron that is inevitably mixed in or added as an alloy component during the manufacturing process, it is precipitated and dispersed within the crystal grains as boron-based precipitates, significantly reducing the amount of boron at grain boundaries. The decision was made to do so.

The process unique to the present invention in which boron in the grooves is precipitated and dispersed within the crystal grains is a process at which boron-based precipitates are precipitated at the subsequent stage of reheating or direct forced cooling of the heated and rolled steel. In the former case, at 750-900℃ for 10 minutes or more,
The latter case is achieved by holding it for 2 minutes or more.

That is, in the manufacturing method of the present invention, C: 0.02 to 0.4 wt%, Si: 0.
005~1.0wt%, Mn: 0.1~? ,5
wt%, (:, r: 0.01-5.0wt%,
Mo: 0.1~1.5wt%, ! 1 < 0
,002wt%, Aβ: o,ooi~0.1wt%
, and N2O, 006wt% and 3:0.01~
During the process of heating, rolling, and subsequent reheating or direct forced cooling of steel containing 0.05 V1t%, at least after the reheating or between rolling and direct forced cooling, the The above problem is solved by a method for producing low alloy steel for nuclear reactors, which is characterized by carrying out a boron precipitation dispersion treatment achieved by holding the steel at a temperature of 10 minutes or more or 2 minutes or more. be.

(Function) Next, the idea that led to the present invention and the specific solution will be described. Figure 1 shown in the drawings shows the behavior of boron and grain boundaries observed during holding at a predetermined temperature range (reheating treatment), which is the above-mentioned precipitation and dispersion treatment of boron within the grains, and the resulting grain boundaries. This shows the change in boron concentration.

In general, boron has the property of being easily segregated at grain boundaries, and N also has the same property. Therefore, boron and N at grain boundaries
The concentration of boron is higher than the concentration within the grains, and boron-based precipitates are likely to precipitate at the grain boundaries even if there is only a small amount of boron. (Figure 1 (b)) Boron-based precipitates that tend to precipitate at grain boundaries will end up being left behind within the grains if the grain boundaries themselves move over time ( Figure 1 (c, d)). If this process is repeated, the grain boundary boron concentration will gradually decrease. Therefore, at the reheating stage (at least in the latter stage)
It is effective to maintain the temperature at 50-900°C.

The above-mentioned effects will be even greater if the same treatment as described above is applied before quenching by direct forced cooling (hereinafter referred to as DQ) after rolling. In other words, the γ grain boundaries in the DQ process change greatly from the deformed grain boundaries immediately after rolling to the grain boundary movement accompanying recrystallization behavior during subsequent holding, resulting in the intragranular dispersion effect of boron-based precipitates. It's for a reason.

In order to obtain the above-mentioned desired effects aimed at by the present invention, the temperature and holding time at which the boron-based precipitate is dispersed are important. In the case of reheating treatment in the present invention, 750
The reason for holding the temperature at ~900°C for 10 minutes or more was determined by considering the temperature range in which boron-based precipitates are likely to precipitate, the precipitation rate, and the grain boundary movement time.

In addition, when the present invention is applied to the above-mentioned DQ process, since the boron-based precipitates can be dispersed within a short time, the holding time in this case is different from that in the above-mentioned reheating process. It is sufficient if the rolling time is 2 minutes or more after rolling.

As mentioned above, the holding time in the boron precipitation and dispersion treatment proposed in the method of the present invention differs between the reheating treatment and the direct quenching treatment. This is based on the fact that movement of the grain boundaries themselves is an important factor in the treatment process of the present invention, which reduces grain boundary boron by dispersing boron precipitates within the grains. In other words, direct quenching can reduce the processing time because the amount of grain boundary movement is greater.

In addition, since the movement speed of grain boundaries and the precipitation rate of boron change depending on the holding temperature, the holding time also changes depending on the temperature relationship.

FIG. 3 shows the relationship between holding temperature and time at which the effects of the present invention are sufficiently achieved. If the holding temperature is high, the precipitation of boron is slow, and if the temperature is low, the grain boundary movement speed is low, so the required holding time becomes longer in both cases. The relationship between holding temperature and holding time is determined based on curve A when the manufacturing process is reheating treatment and curve B when direct quenching is performed.

The scope of the present invention is defined by each curve in FIG. 3, and the conditions under which the effects of the present invention can be stably obtained industrially are limited. In other words, conditions were selected that were not affected by the temperature distribution existing within the steel plate. As a result, the holding temperature was limited to 900 to 750°C and the holding time was limited to 2 minutes or more in the case of DQ and 10 minutes or more in the case of reheating treatment.

The manufacturing process of the present invention is characterized by the addition of boron precipitation and dispersion treatment, the temperature and time of which are limited for the reasons mentioned above.

However, in order to obtain the effects of the present invention, the timing of performing the dispersion processing is also important. For processes that include reheating, see Figure 4 (
As shown in a') to (C), it is necessary to carry out the process of the present invention at least after the reheating process, and when forced cooling is performed directly after rolling, the process shown in (d) and (e) in the figure It is necessary to perform this between the rolling and the direct forced cooling as shown in FIG.

In FIG. 4, TO is the slab heating temperature for rolling, which is usually 1000 to 1200°C. (2) is often air-cooled for cooling after rolling, and (2) is for cooling after reheating, which includes quenching, normalizing, etc. (2) Direct quenching or accelerated cooling is applied. Furthermore, the effects of the present invention can be exhibited even if tempering is performed after the above steps.

Next, the chemical composition of the base steel to which the method of the present invention is applied is limited and listed as follows.

C is an essential component for ensuring hardenability and strength, and forms carbides with other alloying elements such as Cr and MO to increase high-temperature strength.
%), the effect is small and no strength is obtained. Moreover, if it exceeds 0.4%, problems such as weld cracking will occur, so it is set at 0.02 to 0.4%.

Si is added to deoxidize and ensure strength, but 0.00
If it is less than 5%, the effect will not be sufficient, and if it exceeds 1.0%, the toughness will deteriorate.゛Mn is an element useful for ensuring the hardenability of steel and improving hot workability, and it is necessary to add 0.1% or more. However, adding a large amount will improve the toughness and weldability The content was set at 1.5% or less because it had an adverse effect on performance.

Even when B is not added as an alloy component, 2-3 ppm of B is inevitably mixed into the steel. In addition, when actively using BN as a precipitate (dispersion strengthening, etc.), 2
Add up to 0 ppm. However, addition of more than this will impair the effects of the present invention.

S is an impurity element, and when present in large amounts, toughness,
Since it has an adverse effect on hot workability, it is usually desirable to keep it at 0.01% or less. However, when trying to further improve the effect of the present invention by promoting the precipitation of BN, the presence of a predetermined amount of S promotes the precipitation of BN using MnS as a nucleus, so 5ffi is set to It is effective to add it in a range of 0.05%. Even in this case, addition of S exceeding 0.05% significantly deteriorates the toughness of the steel, so the upper limit needs to be set to 0.05%.

Cr is an element that improves hardenability, oxidation resistance, and high-temperature strength, and is added in an amount of 0.01% or more; however, addition of more than 5% deteriorates weldability and base metal toughness, so Cr is an element that improves hardenability, oxidation resistance, and high-temperature strength.
Limited to 5.0%.

MO is an element that ensures hardenability and improves high-temperature strength through solid solution strengthening and precipitation strengthening, so it is added in an amount of 0.1% or more, but if it exceeds 1.5%, it causes a decrease in weldability and an increase in cost. Therefore, the upper limit was set at 1.5%.

Although 0.001% or more of A° C. is added as a deoxidizer, adding too much deteriorates the toughness, so it was set to 0.10% or less.

N is necessary to disperse unavoidably mixed B into the grains as BN and to lower the grain boundary B111 degree. However, since N in an amount of 0.0060% or more has an adverse effect on toughness, it is set to 0.0060% or less.

Furthermore, in this invention, v, Nb, Ni and C
One or more types of LI may be added.

The effects of these additions and preferred groups of additions will be explained below.

V: 0.001% or more may be added to ensure strength, but if the amount added exceeds 0.06%, the base material toughness and weldability will be significantly deteriorated, so 0.01 to 0.06% shall be.

Nb can be added in an amount of 0.01 to 2.0% to ensure double strength.

Ni: To ensure hardenability and improve low thirst,
Although it may be added in an amount of 0.01% or more, addition in excess of 1.0% is not appropriate from an economic standpoint.

Cu: Add 0.01% or more as necessary to improve strength and corrosion resistance, but addition of more than 0.5% is not appropriate as it deteriorates hot workability, toughness, and weldability.

Note that the addition of these alloy components does not have any adverse effect on the intragranular dispersion treatment of boron, which is the subject of the present invention, and its effects.

By subjecting steel with the above-mentioned composition to the necessary treatments through the prescribed steps, the unavoidably mixed boron is precipitated and dispersed within the grains as boron-based precipitates, thereby significantly reducing the grain boundary 811 degrees. As a result, steel with excellent neutron irradiation embrittlement resistance can be manufactured industrially and at low cost.

(Example) Examples of the present invention will be described below along with comparative examples.

Steels with chemical compositions shown in Table 1 were melted in a 100 kg vacuum melting furnace, A steel (88 g) was subjected to the DQ process, and C to E steels were subjected to boron precipitation dispersion treatment according to the present invention in the reheating furnace conditioning process. Table 3 shows the results of examining the presence of grain boundary B by Auger analysis and fission track etching method, with and without application under the conditions shown in Table 2.

Among steels A to E, steels B and D have an increased Sl content in order to utilize the effect of promoting BN precipitation due to MnS. In both cases, grain boundary boron was reduced to such an extent that it was impossible to detect it within the range of the present invention, confirming that the present invention is highly effective. This effect could be similarly obtained for steels with compositions other than those of the examples, without impairing other properties, as long as the compositions were within the range of the present invention.

In addition, although the above-mentioned illustrations all showed the Example about a steel plate, the manufacturing method of this invention is applicable regardless of the material and product shape.

(Effects of the Invention) As explained above, according to the present invention, low alloy steel that does not undergo creep embrittlement even when subjected to neutron irradiation can be produced relatively easily and at low cost. .

[Brief explanation of drawings]

Fig. 1 is a model diagram explaining the mechanism of BN intragranular precipitation dispersion treatment by heating and holding according to the present invention, and Fig. 2 is a model diagram explaining the mechanism of BN intragranular precipitation dispersion treatment by the direct forced cooling process of the present invention. FIG. 3 is a graph showing the holding temperature and time for obtaining the effects of the present invention, and FIGS. 4 a to 4 are diagrams showing the procedure for heating and heat treatment of the present invention.

Claims (1)

[Claims] 1. C: 0.02~0.4wt%, Si: 0.005~
1.0wt%, Mn: 0.1-1.5wt%, Cr: 0
．． 01 to 5.0 wt%, Mo: 0.1 to 1.5 wt%,
B≦0.002wt%, Al: 0.001-0.1wt
% and N≦0.006wt% by heating,
During the process of rolling and subsequent reheating,
A method for producing low alloy steel for a nuclear reactor, characterized in that, at least after the reheating treatment, boron precipitation and dispersion treatment is carried out by holding the temperature at a temperature of 750 to 900°C for 10 minutes or more. 2, C: 0.02~0.4wt%, Si: 0.005~
1.0wt%, Mn: 0.1-1.5wt%, Cr: 0
．． 01 to 5.0 wt%, Mo: 0.1 to 1.5 wt%,
B≦0.002wt%, Al: 0.001-0.1wt
%, N≦0.006wt% and S: 0.01-0.0
During the process of heating, rolling, and subsequent reheating, steel containing 5 wt% is maintained at a temperature of 750 to 900°C for 10 minutes or more at least after the reheating treatment. 1. A method for producing low alloy steel for nuclear reactors, characterized by performing a precipitation dispersion treatment. 3, C: 0.02~0.4wt%, Si: 0.005~
1.0wt%, Mn: 0.1-1.5wt%, Cr: 0
．． 01 to 5.0 wt%, Mo: 0.1 to 1.5 wt%,
B≦0.002wt%, Al: 0.001-0.1wt
% and N≦0.006wt% by heating,
During the process of rolling and subsequent direct forced cooling, between the rolling and the direct forced cooling, 750 to 90
A method for producing low alloy steel for a nuclear reactor, characterized by performing boron precipitation and dispersion treatment achieved by holding the temperature at 0° C. for 2 minutes or more. 4, C: 0.02~0.4wt%, Si: 0.005~
1.0wt%, Mn: 0.1-1.5wt%, Cr: 0
．． 01 to 5.0 wt%, Mo: 0.1 to 1.5 wt%,
B≦0.002wt%, Al: 0.001-0.1wt
%, N≦0.006wt% and S:0.0. ~0.0
Achieved by holding steel containing 5 wt% at a temperature of 750 to 900 ° C for 2 minutes or more between the rolling and direct forced cooling during the process of heating, rolling, and subsequent direct forced cooling. A method for producing low alloy steel for nuclear reactors, which comprises performing a boron precipitation and dispersion treatment.