JPS5939497B2 - Maraging steel with excellent delayed fracture resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a maraging steel that has excellent delayed fracture resistance in a hydrogen environment.

Maraging steel is an alloy made by adding age-hardening elements to ultra-low carbon, high Ni-based martensitic steel and age-hardening in martensite.
It is well known as an ultra-high strength steel that has a tensile strength of 1 or more.

Maraging steel has superior notch toughness compared to other low-alloy steels and medium-alloy steels, so safety is important.

For example, it is used in various parts for spacecraft, ultra-high pressure parts, etc. However, on the other hand, it is well known that steels with high strength are at risk of time-dependent fracture, that is, delayed fracture, in special environments, and in atmospheric environments they exhibit significantly lower notch toughness than other steels. High maraging steel is no exception. Therefore, the inventors of the present invention have conducted extensive research in order to develop a maraging steel that is safer against delayed fracture, and have discovered that the maraging steel shown below has extremely excellent delayed fracture characteristics in a hydrogen environment. Ta.

That is, in the present invention, C: 0003% or less, Si: 0.10%
% or less, Mn■0.10% or less, P:0.01% or less,
Cu: 0.10% or less, Ni 10.0-20.0%,
Cr: 6.0% or less, Mo■ 2.0-12, 0%, C
o: 16.0% or less, A[: 0.50% or less, Ti: 2
It is a maraging steel with excellent delayed fracture resistance, characterized by comprising 0.0% or less, S2 0.0010% or less, and the balance iron. The present inventors investigated the influence of alloying elements on delayed fracture properties through many experiments using conventional maraging steel, and as a result, they found that S in the impurity element has a large influence.

In other words, in ordinary maraging steel, the amount of impurity elements is strictly limited in consideration of toughness after strengthening, but
．． Usually contains 0.003% or more of S. However, it has been found that when the S content is 0.003% or more, the delayed fracture properties in a hydrogen environment are extremely low and the steel is in an unstable state. Therefore, as a result of investigating the delayed fracture characteristics through accelerated tests using conventional maraging steel as the basic composition and varying the S content, we found that reducing the S content to 0.001% or less improves stability against delayed fracture. was found to increase significantly. The above experimental facts were discovered for the first time by the present inventors, and can greatly contribute to improving the delayed fracture characteristics of maraging steel. Next, the reason for limiting the composition range of the maraging steel according to the present invention will be described below.

The basic composition of the steel of the present invention is 12Ni, which is conventionally used.
, 13Ni, and 18Ni maraging steel, and the composition range that includes these steel types is C: 0.03% or less, Si: 0. I0% or less, Mn: 0. I0% or less, p: 0.0t% or less, Cu
: 0.10% or less, Ni: 10.0-20.0%, Cr
: 6.0% or less, MO: 2.0 to 12.0% or less, CO
: 16.0% or less, Al: 0.50% or less, Ti: 2.
It was set at 0% or less.

It has been confirmed through detailed experiments that within the above range, the influence of S on delayed fracture characteristics shows a similar tendency. As mentioned above, S significantly deteriorates the delayed fracture characteristics, so it needs to be kept in an extremely small amount, with a maximum of 0.
0010% or less. Next, the characteristics of the maraging steel of the present invention will be explained in detail using examples.

Example 1 In order to investigate the influence of S content on delayed fracture characteristics, 18Ni-5M containing various S content as shown in Table 1 was prepared.
A maraging steel whose main component is 0-9C0-0.8Ti was produced.

After hot working the above test steel into a bar, it was solution-treated.
It was tempered to a tensile strength of 210 kg/first grade by aging treatment, and accelerated delayed fracture test pieces were taken. 0.0 with the above test piece.
As a result of determining the stress-rupture time curve in i%H (l aqueous solution), it was confirmed that the smaller the amount of S, the longer the rupture time under the same applied stress. Figure 1 shows an example of many of these data. It is a diagram in which the rupture time in the case of a stress of 300 kg/1 nifL is arranged by the amount of S.

From the figure, it can be seen that steel with a high S content has a short life time until fracture, which greatly affects the delayed fracture characteristics. Normal maraging steel contains about 0.003% S, but compared to this, the maraging steel of the present invention has a S content of 0.0010% or less, which shortens the life before breakage. The time will more than double.

Example 2 Maraging steels with various Ni contents produced on an industrial scale were prepared, and maraging steels of the same composition with the S content adjusted to 0.0010% or less were manufactured.

Table 2 shows the chemical components of each sample material. Large-diameter bolts with M24 threads were formed using the test materials shown in Table 2, and after being tempered to the usual strength of each steel, they were subjected to a delayed fracture test. For the delayed fracture test, an extremely severe corrosive environment was selected as shown in Table 3, and an experimental steel structure tightened with test bolts was exposed to the above corrosive environment. The tightening stress is defined as the tensile stress applied to the bolt when the bolt is 0.
It was tightened to an extent equivalent to 90% of the 2% proof stress value. Table 3 shows the results of measuring the life time until breakage of the test bolts in the above test.

As shown in the table, bolts manufactured using the maraging steel of the present invention with an S content adjusted to 0.0010% or less are:
Compared to conventionally used bolts made from maraging steel produced on an industrial scale, it has a longer lifespan until fracture in any corrosive environment and has stability against delayed fracture. is clearly recognized. As seen in the above examples, the maraging steel of the present invention has improved delayed fracture characteristics by minimizing the amount of S based on the new fact that S, an impurity element, significantly reduces delayed fracture characteristics. This makes it possible to provide highly reliable steel as a material for, for example, aircraft parts or centrifuge rotating bodies, which particularly require high tensile strength and safety against delayed fracture, and the present invention is of great industrial value. It is.

[Brief explanation of the drawing]

Figure 1 shows the effect of S on the delayed fracture characteristics of maraging steel.
FIG.

Claims (1)

[Claims] 1 C: 0.03% or less, Si: 0.10% or less, Mn
: 0.10% or less, P: 0.01% or less, Cu: 0.1
0% or less, Ni: 10.0-20.0%, Cr: 6.0
% or less, Mo: 2.0 to 12.0%, Co: 16.0%
Below, Al: 0.50% or less, Ti: 2.0% or less, S
: A maraging steel with excellent delayed fracture resistance characterized by comprising 0.0010% or less, the balance being iron.