JPS61136661A - Thick martensitic stainless steel having superior toughness - Google Patents

Abstract

PURPOSE:To obtain a thick martensitic stainless steel having superior toughness by controlling N and C contents and vanishing delta-ferrite so as to prevent deterioration in quality caused by thickening. CONSTITUTION:The composition of a thick martensitic stainless steel is composed of, by weight, <0.16% C, <0.3% Si, 0.5-2% Mn, 15-17% Cr, 2-4% Ni, 0.02-0.2% N (2C%+N%=0.22-0.34%) and the balance Fe with inevitable impurities. The ductility and toughness are improved by the reduced C content and reduction in the strength caused by the reduced C content is compensated by the increased N content.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to thick-walled martensitic stainless steel with excellent toughness, and is used for steels, leaps, shafts, pins, etc. that require high strength, high toughness, and corrosion resistance. Used as thick-walled structural steel.

[Conventional technology]

16'4 Cr-r rutinsite stainless steel (SUS4311) is used as a relatively small structural steel material that requires high strength and corrosion resistance.

However, 5US431 stainless steel has a high Cr content.
Although it has excellent strength and corrosion resistance, it is easy to form δ ferrite and has poor toughness. Especially for thick steel materials with mass holes.
When US431 stainless steel is used, a sufficient quenching cooling rate cannot be obtained during quenching, so Cr carbide precipitates at the δ-ferrite grain boundaries during cooling, causing grain boundary embrittlement, resulting in a significant decrease in ductility and toughness. .

Therefore, the current 5US431 stainless steel cannot be applied to materials that require high toughness, especially thick-walled materials that require high toughness.

1'Ji is used as a means to improve the toughness of high Cr martensitic stainless steel such as SUS 431 stainless steel.
# By adding an appropriate amount of M' + W, δ ferrite t-2
Method for suppressing the temperature to below 0 (Japanese Patent Application Laid-Open No. 54-53616),
Alternatively, by adding low amounts of C, Cu, and W, 15
- A method is known (Japanese Unexamined Patent Publication No. 1983-8%57) for suppressing the amount to below. However, in all of these methods, a considerable amount of δ ferrite exists; for example, in actual measurements, the former has 3.8
~17.8 inches, the latter containing 8-11 inches of delta ferrite. Therefore, although good toughness can be obtained with thin-walled materials, good toughness cannot be obtained with thick-walled materials because the quenching cooling rate is slow and carbides precipitate at the δ-ferrite grain boundaries, resulting in significant embrittlement.

[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 to obtain high performance even in thick-walled materials. It is to provide meat martensitic stainless steel.

[Means and operations for solving the problems] The gist of the present invention is as follows.

That is, in terms of weight ratio, C: 0.16% or less, 3i: 0.30% or less Mn
: 0150-λ00S, Cr: 15.00-1
7.00%Ni: λ00~4.00%, N:
Contains 0.02-0.20%, and 2% C+CHN=+
This is a thick-walled martensitic stainless steel with excellent toughness, which satisfies the range of 0.22 to 0.34) with the remainder consisting of Fe and unavoidable impurities.

Material deterioration due to thickening is caused by δ ferrite. If δ-ferrite gold is prevented, the material deterioration caused by thickening can be completely suppressed.

It is effective to reduce δ ferrite by increasing the KC, N, Ni, and Mn contents, and reducing the Si, Cr, and Mo contents.
Among these, the influence of C and N is particularly remarkable.

However, since C promotes grain boundary carbides and completely reduces ductility and toughness, it is desirable to keep the C content low and to increase the N content of gold. In addition, the decrease in strength due to the decrease in C content is due to the decrease in N
It is possible to compensate by increasing the content.

From the above, if the total N content is increased δ ferrei)t-prevented,
It has been found that high Cr martensitic stainless steel with high performance can be produced even in thick-walled materials.

Based on the above findings, the present invention provides a high Cr martensitic stainless steel that has high toughness even in thick-walled materials by increasing the N content and suppressing δ ferrite.

The reasons for limiting the components of the present invention will be explained below.

C: C is an extremely effective element for suppressing δ ferrite, but
Grain boundary carbides are formed and the toughness deteriorates drastically. Especially C
If it exceeds 0.16%k, the toughness deteriorates significantly, so C
was limited to 0.16% or less.

Si: Since Si promotes the formation of δ ferrite, a lower Si content is desirable. If Si is 0.30% or less, the effect on δ ferrite is small, so limit Si to 0.30% or less.

Mn: Mn is an effective element for reducing δ ferrite, but 0.
If it is less than 50%, its effect will not be sufficiently demonstrated. Also λo
Mn was limited to a range of 0.50 to λ00 because if it exceeds os1, the susceptibility to tempering embrittlement will increase and the toughness will deteriorate.

Cr: Cr needs to be 15.0 OS or higher to ensure good corrosion resistance, but if it exceeds 17.0 O1', δ ferrite tends to be generated, so it is limited to a range of 15.00 to 17.00 OS. My friend.

Ni: Ni is necessary to suppress the formation of δ ferrite and ensure good toughness. But 400%t-
Ni is limited to a range of 200 to 4.00 because if it exceeds this, soft austenite increases and strength decreases.

N: N is an extremely effective element for suppressing δ ferrite.
For complete prevention of ferrite 1, 0.02 or more is required. However, if it exceeds 0.20% T, the toughness deteriorates, so N was limited to a range of 0.02 to 0.20 T. - Next, the reason for limiting the range to 2C1%N=+0.22 to 0.34) will be explained. As mentioned above, C,
Although any of N is an element effective in suppressing δ ferrite, C in particular significantly reduces toughness, and when N is increased more than necessary, toughness is also reduced.

The appropriate amounts of C and N for toughness were investigated, and the relationship between the C1N content and the Charpy impact value UE20 of the impact test is shown in FIG. In FIG. 1, the numbers inside the circle indicate the Charpy impact value UE20 (kff - m/cd). The shaded area in Figure 1 is 2C+4N=(0,2
2 to 0.3411, and it was found that high toughness could be obtained only in this region. Therefore, regarding C2N, I/c2 % C+1N= l O,22
~0°34) Satisfying H is added to the requirements.

Furthermore, the reason for limiting the use to thick-walled steel materials is as follows.

All impact tests were conducted for various wall thicknesses on the inventive steel whose components are shown in Table 1 and the conventional comparative steel, and the relationship between Charpy impact value UE20 and wall thickness was shown in Figure 2 In contrast, the material quality deteriorates significantly as a result of thickening, that is, a decrease in the quenching cooling rate, whereas the steel of the present invention exhibits less material quality deterioration due to thickening, and has a thick wall thickness of 504 or more. This is because it is especially superior when it comes to materials.

Note that the manufacturing method of the present invention is the same as the conventional method, and there is no need for particular limitations, as it is sufficient to manufacture a steel ingot from molten steel of a predetermined composition, perform hot forging, and then perform quenching and tempering treatments.

〔Example〕

Based on 5US431 stainless steel and having the chemical composition shown in Table 2, ■, 0,0. [F], [F] steel invention steel, ■, 0. Comparison of steel pipes (1), (2), (2), (2), (2) Steel pipes were melted and forged plates were made by simulating the manufacturing conditions of large thick-walled forged steel, and then tempered.

The refining treatment consisted of holding at 1025°C for 5 hours, quenching, and tempering at 680°C for 10 hours. Quenching cooling is approximately 15%, which is equivalent to oil quenching a 200m thick product.
It was set as °C/IwI. Tensile tests and impact tests were carried out on each of the steels manufactured in this manner, and the results are shown in Table 3. In Table 1, numerical values that do not satisfy the conditions of the present invention are underlined.

Comparative steel O, which corresponds to the current 5US431 stainless steel, has a large number of δ ferrites, whereas the present invention's ■～
[F] Steel has very little δ ferrite, less than 1 inch.

Regarding the mechanical properties, compared to Comparative Steel 0, the steel No. 2 to [F] of the present invention shows no clear difference in tensile strength, but is significantly superior in tensile ductility and impact toughness.

Even if you simply increase the C and N contents, if the C content is 0.19%, which is higher than the limited range, as in ■ steel, or the sum of 210 + SN as in comparative steels, ■, ■, and [Stock]. is 0.
When the content is outside the limited range of 22 to 0.34%, the tensile ductility and impact toughness are poorer than the steel of the present invention. Si, M
Comparative steels O and Q, in which the n, Ni, and Cr contents were outside the limited range, also had poor toughness.

As described above, by controlling the N and C contents,
1-It has been found that in the steel of the present invention, which has disappeared, material deterioration due to thickening can be prevented and excellent toughness can be obtained even in thick-walled materials.

〔Effect of the invention〕

As is clear from the above examples, the present invention provides C9Si,
By limiting the content of Mn, Cr, Ni, N, etc. and suppressing the formation of δ ferrite, we were able to produce high-Cr martensitic stainless steel with excellent strength, toughness, and corrosion resistance even in thick-walled steel materials. .

[Brief explanation of the drawing]

Figure 1 shows the Charpy impact value (UE) of 200+aa thick-walled material.
Fig. 2 is a diagram showing the influence of steel material thickness on Charpy impact value.

Claims (1)

[Claims] (1) Weight ratio: C: 0.16% or less, Si: 0.30% or less, Mn: 0
．． 50-2.00%, Cr: 15.00-17.00%
Ni: 2.00-4.00%, N: 0.02-0.20
%, and 2%C+%N=(0.22-0.34
) %, with the remainder consisting of Fe and unavoidable impurities.