JPS6227555A - Low-alloy tempered forged steel excellent in falling weight characteristics - Google Patents

Abstract

PURPOSE:To obtain a low-alloy tempered forged steel having superior toughness at low temp. when used as thick-walled material by incorporating specific percentage of C, Mn, Cr, Ni, Mo, Al, N, S, Si, and P to Fe. CONSTITUTION:The low-alloy tempered forged steel which consists of, by weight, 0.05-0.15% C, <=0.60% Mn, 1.50-2.50% Cr, 3.00-4.00% Ni, 0.30-0.60% Mo, 0.006-0.060% Al, 0.0015-0.0150% N, <=0.020% S and the balance Fe with inevitable impurities and in which Si and P content is regulated so that 50(%Si)+600(%P) is <=16% is prepared. This steel has such a superior toughness at low temp. as to give an NDT temp. of <=about -100 deg.C by a drop-hammer test even used as a thick-walled material of >=150mm thickness.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a low-alloy heat-treated forged steel with excellent drop weight properties, and particularly relates to a thick-walled forged steel with a wall thickness of 150 mm or more. It is used in fields such as large pressure vessels in the nuclear power or chemical industries, such as casks (called casks).

[Conventional technology]

Nuclear power casks are required to have excellent low-temperature toughness from the perspective of safe transportation. In particular, casks used in cold regions are required to have excellent toughness with an NDT temperature of 1,100° C. or less in the current drop test, in order to prevent brittle fractures due to collisions or falling accidents during transportation.

Furthermore, the wall thickness of the cask is 150m from the viewpoint of radiation velocity.
Thick-walled steel materials with a thickness of m or more are used, but it is difficult to obtain excellent low-temperature toughness with such thick-walled steel materials because it is difficult to ensure sufficient hardenability through conventional hardening.

Conventionally, Ni-Cr for pressure vessels was used as steel material for cask.
-Mo steel (ASME 5A508C/, 4) was used, but thick-walled materials of 150 rITn or more do not have sufficient toughness.

As a means to improve the toughness of Ni-Cr-Mo steel, low Si
A method (Japanese Patent Publication No. 58-53711) of reducing embrittlement and improving toughness by tempering and oxidation is known.

However, none of these methods has been able to ensure low-temperature toughness with an NDT temperature of 110° C. or lower in an electric drop test for thick-walled materials.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the above-mentioned problems of the prior art, and
An object of the present invention is to provide a low-alloy forged steel that can ensure excellent low-temperature toughness with an NDT temperature of 1100° C. or less in an electric drop test even in thick-walled materials of 0 mm or more.

[Means and effects for solving the problem] The present inventors have repeatedly researched the influence of chemical components on the NDT temperature of thick inner materials.
It was found that reducing the content is effective. Especially St.
Regarding the content of P and P, the present invention was completed based on the discovery that a synergistic effect can be obtained by reducing both elements, and an NDT of less than 100 degrees Celsius can be achieved.

The gist of the present invention is as follows.

That is, in weight ratio: C: 0.05 to 0.15 fp Mn: 0.6
0% or less Cr: L50~Z50% I”Ji:
3.00~4.00Mo: 0.30~0.601
Al: 0.006-0.60%N: 0.0015
~0.0150% S: Contains 0.020% or less,
50 (% si) + 600 for Si and P content
Adjust so that (%P) is 16 or less, and the remainder is Fe.
It is a low-alloy tempered forged steel with a wall thickness of 150 plates or more and excellent drop weight properties, consisting of unavoidable impurities.

Next, the reason for limiting the range of components in the present invention will be explained.

C: C increases strength but significantly deteriorates low temperature toughness. To obtain excellent low-temperature toughness, it is necessary to suppress the content to 0.15% or less, and to ensure the specified strength,
Notes that require s 1 or more, Ctto, 05~0.1
It was limited to a range of 5 inches.

Mn: This is an effective element for improving MBI hardenability, but if it exceeds 0.60 mm, susceptibility to temper embrittlement increases and low-temperature toughness decreases, so it was limited to 0.60% or less.

Cr: Cr is an effective element for improving hardenability, and the thickness is 150 m.
In order to obtain sufficient hardenability with the above-mentioned steel materials, a Cr content of 1,501 or more is required.On the other hand, addition of more than 2.50% reduces toughness, so the content of Cr is limited to a range of 1.50-2450.

Ni: Ni is an element that enhances hardenability and improves both strength and toughness, and 3.00% or more is required to obtain the desired strength and toughness. On the other hand, the specified strength and toughness are 4.0 O%
Since the following is sufficient, the upper limit is set to 4,00%, and 3
．． It was limited to a range of OO to 4.00%.

Mo.

MO is an element effective in suppressing temper embrittlement.

Since the steel of the present invention is inherently highly susceptible to tempering, 0.30% or more is required to suppress embrittlement. However, the embrittlement suppressing effect is saturated and small when it exceeds 0.60%, and adding more than necessary is meaningless, so the upper limit is set at 0.60% and 0.3%.
It was limited to a range of 0 to 0.60 inches.

Al, N: Al and N effectively act to improve toughness and suppress temper embrittlement through refinement of crystal grains. Get fine grain effect +z Alhto o, o 061 or higher, Nt! 0.00
15 inches or more is required. On the other hand, A/ exceeds 0.060% or N is 0. Note that when O exceeds 150 degrees, toughness decreases, At: 0.006 to 0.060%, N
: Limited to the range of 0.0015 to 0.015096. However, Al: 0.010-0.030S, N
: A range of 0.0050 to 0.0120 is desirable.

S: Since S reduces toughness, it is limited to 0.020% or less, but is preferably 0.010% or less.

A feature of the present invention is that both Si, P; St and P are limited to low levels. Toughness can be improved by reducing only either Si or P, but by regulating both elements, it is possible to improve toughness. Better toughness can be ensured.

We will explain the systematic investigation of Si and P contents with respect to NDTH degree in electric drop tests. That is, the composition of sample m was ttc: 0.09 to 0.14%, Mn: 0.
22~0.50%, Cr: 1.65~Z28
%, Ni: 3.30-3.95%, Mo: 0.
35-0.55%, A/: 0.015-0.
028chi, N: 0.0068~0.01 1 8
CH, S: 0.005 to O,OOOB
After heat treatment by temperature tempering, an electric drop test was performed to
, P and the NDT temperature in the current drop test were investigated and shown in Figure 1.

In Figure 1, NDT @ -100°C or less is obtained in the diagonally shaded range, which is 50 (Si) + 600 (
%P) Since the area is 16 cm, in the present invention, S
The range of i and P was limited to the above region.

The method for manufacturing the low-alloy heat-treated forged steel of the present invention having the above-mentioned limited components is the same as conventional methods, and a steel ingot is produced using a molten metal having a predetermined composition, and after forging, annealing, quenching, and tempering are performed.

Next, the relationship between the wall thickness and toughness of the low-alloy tempered forged steel of the present invention containing the above-mentioned limited ingredients will be explained. Two specimens of the invention steel (1) and comparative steel (2) whose compositions are shown in Table 1 were annealed at 900°C, quenched at 880°C, and tempered at 640°C for various wall thicknesses, and subjected to an electric drop test. ND in
The TQ degree was investigated and the results are shown in Figure 2.

In Fig. 2 of the broom, it can be seen that comparative steel (2) exhibits a significant decrease in toughness as the wall thickness increases, whereas inventive steel (2) exhibits a small decrease in toughness as the wall thickness increases.

That is, in thin-walled materials, there is not much difference between the steel of the present invention and the comparative steel, but in thick-walled buttons, the superiority of the steel of the present invention becomes remarkable. Therefore, in the present invention, the target wall thickness is 150 m.
Limited to thick-walled materials of m or more.

〔Example〕

The 68i inventive steel and seven comparative steels whose compositions are shown in Table 1 were used as test materials. In addition, the 7 types of comparison steel are conventional ty)
Ni-Cr-Mo steel (A S M E S A
508C1, 4), and the numerical values that do not satisfy the limiting conditions of the present invention are underlined. Comparative steel ■ has C omitted, and comparative steels ■ and ■ have 50 (ChiSi) and 600 (ChiP).
) exceeds 1696, comparative steel [phase], ■, 0 Table 2, 0 is Ni, Cr, Mo, A/, respectively.
does not satisfy the conditions of the present invention.

These test materials were melted in a vacuum melting furnace, forged into steel ingots, and then subjected to annealing, quenching, and tempering, and then subjected to tensile tests, Charpy impact tests, and electric drop tests, and the results were evaluated. It is shown in Table 2.

As is clear from Table 2, comparison steels that are currently in use: ■,■
, ■ Inventive steels ■ to ■ have lower impact toughness and drop TI.
L411! The quality is extremely good.

t, comparison m@〜@ノ! 5K Ni, Cr,
Mo.

It is clear that when the content of AI deviates from the compatible range of the present invention, the impact toughness and drop weight properties deteriorate.

〔Effect of the invention〕

As is clear from the above examples, the present invention makes it possible to obtain a thick-walled material with excellent low-temperature toughness by limiting the components of low-alloy tempered forged steel, and in particular by reducing the contents of Si and P. Ta.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the St and P contents and the NDT temperature in the current drop test, and Figure 2 shows the relationship between the steel material thickness and the ND temperature in the current drop test.
It is a diagram showing the relationship with T temperature. Agent: Patent Attorney Takeo Nakaji cL, > NDTAIL (oC)

Claims (1)

[Claims] (1) Weight ratio: C: 0.05-0.15% Mn: 0.60% or less Cr: 1.50-2.50% Ni: 3.00-4.0
0 Mo: 0.30-0.60% Al: 0.006-0.
060% N: 0.0015-0.0150% S: 0.020%
Contains the following and for Si and P content: 50 (%Si
)+600(%P) is adjusted to be 16% or less, and the remainder is Fe and unavoidable impurities.The wall thickness is 150 mm.
Low-alloy tempered forged steel with excellent drop weight properties.