JPH0627302B2 - High-strength ferrite heat-resistant steel with excellent weldability and toughness - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength ferritic heat-resistant steel having excellent weldability and toughness, and is particularly suitable for use in the fields of boilers, oil refining, nuclear power, etc. The present invention relates to a suitable high strength ferritic heat resistant steel.

(Prior Art) In recent years, thermal power generation boilers are becoming larger and higher in temperature and pressure. However, when the temperature exceeds 550 ° C., when selecting the material, from the viewpoint of oxidation resistance and high temperature strength, ferrite type At present, it is being leapt from steel to high-grade austenitic steel such as 18-8 stainless steel.

However, as low-alloy steel, stainless steel, superalloys, and higher-grade materials become more expensive, the cost of building boilers becomes higher, and the steam temperature of the boiler is conversely restricted due to material problems. The upper limit is 566 ° C.
Therefore, in order to increase the efficiency of the boiler, an ultra-supercritical pressure boiler with increased pressure is used.

by the way Steel materials for filling the middle of steel and austenitic stainless steel have been sought for several decades, but the Cr content is 5Cr, which is in the middle.
Since the weldability of 9Cr, 12Cr and other boiler steel pipes deteriorates when the strength is increased, much research has been conducted, but it is difficult to put it into practical use because it significantly reduces the work efficiency in the construction of the boiler. From this perspective The advent of new steels with creep strength that fills the gap between steels and austenitic stainless steels has long been awaited.

(Problems to be Solved by the Invention) In view of such circumstances, the present inventors have developed a new steel type that has already improved weldability and has a creep rupture strength significantly higher than that of conventional materials. Kosho 56-34628
Gazette, (b) Japanese Patent Laid-Open No. 59-153865, or (c) Japanese Patent Application No. 59
Proposed by No. -68377. These (a)
All of the steels (c) to (c) are excellent steels that can be used for a long time at 600 ° C, but all of them are disadvantageous in terms of heat embrittlement because they contain a large amount of Mo.

(Means for Solving Problems) Therefore, the present inventors considered reducing the amount of Mo while maintaining the creep rupture strength at 600 ° C.
It has been clarified that it can be compensated by adding a considerable amount of Si and reducing the Si content or further adding B. Also, the range of (C + N) content and Si content necessary to secure high toughness was clarified. Based on these findings, we have succeeded in developing a ferritic heat-resistant steel with excellent weldability, toughness, and creep rupture strength.

(Structure / Operation of the Invention) The composition range of the present invention is shown in Table 1.

The present invention will be described in detail below.

First, the reason for limiting each component contained in the steel of the present invention will be described. C is necessary for maintaining strength, but the upper limit was made 0.14% from the viewpoint of weldability. That is, in relation to the amount of Cr described later,
This type of steel has very good hardenability and the heat-affected zone of the weld hardens significantly, causing cold cracking during welding. Therefore, in order to complete welding, preheating at a considerably high temperature is required, and welding workability is significantly impaired. However, C is 0.14
%, The maximum hardness of the heat-affected zone of the weld is reduced and welding cracks can be easily prevented. Therefore, the upper limit was made 0.14%.
Regarding the lower limit, if the C content is less than 0.04%, it becomes difficult to secure the creep rupture strength, so the lower limit is 0.04%.
I decided.

Mn is a component necessary not only for deoxidation but also for maintaining strength. The upper limit of 0.8% is not preferable from the viewpoint of toughness if it exceeds this, and the lower limit was set to 0.2% as the minimum amount necessary for deoxidation.

Cr is an essential element for oxidation resistance, and is inevitably added to heat-resistant steel. It enhances high temperature strength by fine precipitation of M ₂₃ C ₆ and M ₆ C (where M is a metal element). However, the lower limit was set to 8% at which precipitation hardening was remarkably recognized, and the upper limit was set to 9.5% from the viewpoint of weldability and toughness.

Mo is an element that remarkably enhances high temperature strength by solid solution strengthening, so it is usually added to heat-resistant steel, but if added in a large amount, weldability and oxidation resistance are impaired, so the upper limit is 1.2%. On the other hand, even if the amount of N is increased, the effect of improving the creep rupture strength is from 0.8% or more, so the lower limit is 0.8.
% Fixed.

V, like Mo, is an element that remarkably enhances the high temperature strength of steel, whether it is solid-solved in the matrix or precipitated as a precipitate. Particularly in the case of precipitation enters the part of the M ₂₃ C _6, M ₆ C as V ₄ C _3, shows a remarkable effect in suppressing the coarsening of precipitates. However, 600 ° C
In order to obtain creep rupture strength exceeding SUS 304 stainless steel before and after, less than 0.15% is not sufficient, and above 0.25%, weldability is impaired, so an upper limit of 0.25%,
The lower limit was 0.15%.

Nb is increasing the high temperature strength by precipitation of Nb (CN), also contribute to long-term creep strength to finely control the precipitation conditions such as M ₂₃ C _6, M ₆ C where fine dispersion precipitation followed . If the amount is less than 0.04%, there is no effect, and
If it exceeds 0%, aggregation coarsening occurs rather to lower the strength, so the upper limit was made 0.10% and the lower limit was made 0.04%.

N is an element that dissolves in the matrix or precipitates as a nitride or carbonitride and increases the creep rupture strength. However, if it is less than 0.02%, the strength decreases sharply, and if it exceeds 0.05%, it is cast. Since blowholes sometimes occur and it is difficult to form a sound steel ingot, the upper limit is set to 0.
05% and the lower limit was 0.02%.

Si is an element that is originally added for deoxidation, but is an element that adversely affects toughness in terms of material. Therefore, when the influence on the toughness was investigated, it was found that the toughness was improved when the content was suppressed to less than 0.14%, so the content of Si was limited to less than 0.14%. The preferable range is 0.095%.
It is the following.

Next, the relationship between (C + N) and the amount of Si will be described with reference to FIG.

Both C and N significantly improve the creep rupture strength, but considering the strength, toughness, weldability, etc., there is an optimum range for the addition amount, and it must be within the range surrounded by ABCDEF in FIG. I understand. That is, the straight line AB is a line in which (C + N) is 0.16%, and if it exceeds this, the toughness decreases and the tendency of the precipitates to agglomerate becomes strong, resulting in a decrease in long-term creep rupture strength. Straight line BC
Is a boundary line for maintaining good toughness from the balance of (C + N) and Si content. The straight line CD has a Si content of 0.14%, and good toughness cannot be expected if the Si content is more than this. Straight line D
E is a line in which (C + N) is 0.06%, and if it is cut, sufficient creep rupture strength cannot be expected. The straight line EF is a limit line for keeping the creep rupture strength at a certain level or more from the balance of (C + N) and the amount of Si. That is, basically, the upper or right side of ABC is a portion with poor toughness, and the lower side of the straight line DEF is a portion with insufficient strength.

The above are the basic components of the present invention, but in the present invention, 0.001 to 0.008 is used for the purpose of further improving the creep strength.
% B can be included. That is, B is originally well known as an element that remarkably enhances hardenability, but it is also extremely effective from the viewpoint of creep strength, and the addition of a trace amount significantly improves creep strength. The amount is 0.00
If it is less than 1%, there is almost no effect, and if it exceeds 0.008%, the hot workability and weldability are impaired, so the upper limit is 0.008%,
The lower limit was made 0.001%.

Next, the effects of the present invention will be described more specifically with reference to Examples.

Examples Table 2 shows the chemical composition of the test steel, the creep rupture time at a stress of 650 ° C. 11 kg / mm ² , the elongation at break, and the preheating temperature for preventing cracking in the y-type constrained cracking test, which indicates weldability, 60
The impact value after aging at 0 ° C for 1000 hours and the tensile properties at room temperature are shown.

No. 5, 6, 9, 11, 13, among those shown in Table 2
Steels 15 to 17 are the invention steels, and the others are comparative steels.

No.2 steel is usually used as low alloy heat resistant steel No. 1 steel is an alloy steel pipe for boiler heat exchangers with further improved high-temperature corrosion resistance, but since the creep rupture strength is low, the steel pipe developed to improve this is No. 3
It is steel. However, this steel has a drawback that its toughness decreases when it is used at a high temperature for a long time.

The steel of the present invention reduces the embrittlement during use and has an improved creep rupture strength.
The relationship between N) amount and Si amount must be within the range of ABCDEF in FIG. The No. 4 and 8 steels have a C content below the lower limit of the range of the present invention, and are located below the EFD line in FIG. 1, and have only a low creep rupture strength. No.7,
12 and 14 steels have a high (C + N) amount, and are shown in FIG. 1A-B-C.
Since it is located on the line, embrittlement due to aging is large. Moreover, the No. 7 steel has a C content exceeding the upper limit of the range of the present invention, and the preheating temperature for stopping cracking in the y crack test is 1
It reaches 50 ° C., and there is a problem in terms of weldability. No. 1
No. 0 steel has an Si content exceeding the upper limit of the range of the present invention and has a large age embrittlement.

On the other hand, the steel of the present invention has a better balance of creep rupture strength, toughness after aging, and weldability than the comparative steels described above. Comparative steel No. 2 steel which is steel, comparative steel which is commercially available 9Cr-1Mo steel
It has much higher strength than No. 1 steel and can be used at considerably higher temperatures at the same stress level.

The No. 15, 16 and 17 steels are obtained by further adding B to the basic composition of the present invention. The No. 15 steel is compared to the No. 11 steel, and the No. 16 and 17 steels are the No. 13 steels. Compared with each, the creep rupture strength is further improved.

In addition, the steel of the present invention is also weldable. It is almost the same as steel and is extremely easy to use.

(Effects of the Invention) As described above, the steel of the present invention is a steel that achieves an increase in high-temperature strength that can withstand high temperatures and high pressures of equipment, as compared with the conventional ferritic heat-resistant steel, and has practically high weldability and toughness. It has excellent characteristics and contributes greatly to the industrial world.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between (C + N) and the amount of Si in the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsukuni Hashimoto 5-10-1, Fuchinobe, Sagamihara-shi, Kanagawa Inside the 2nd Technical Research Laboratory, Nippon Steel Corporation (56) Reference JP-A-59-232254 (JP) , A) JP 59-211553 (JP, A) JP 60-24353 (JP, A) JP 60-215746 (JP, A)

Claims (2)

[Claims] 1. C0.04-0.14% by weight, Mn 0.
2 to 0.8%, Cr 8.0 to 9.5%, Mo 0.8 to 1.
2%, V 0.15-0.25%, Nb 0.04-0.1
0%, N 0.02 to 0.05% contained, Si 0.14
%, And the relationship between (C + N) and the amount of Si is surrounded by ABCDEF in Fig. 1 (but not on the CD line)
The high-strength ferritic heat-resistant steel with excellent weldability and toughness, characterized in that the balance is Fe and inevitable impurities. Si%, (C + N)% Si%, (C + N)% A (0, 0.16) D (0.14, 0.06) B (0.05, 0.16) E (0. 08,0.06) C (0.14,0.115) F (0,0.1) 2. C0.04-0.14% by weight, Mn 0.
2 to 0.8%, Cr 8.0 to 9.5%, Mo 0.8 to 1.
2%, V 0.15-0.25%, Nb 0.04-0.1
0%, N 0.02-0.05%, B 0.001-0.0
08%, Si is limited to less than 0.14%, and the relationship between (C + N) and Si content is in the range surrounded by ABCDEF in FIG. 1 (but not on the CD line), the balance Fe and unavoidable. High-strength ferritic heat-resistant steel with excellent weldability and toughness, which is characterized by containing impurities. Si%, (C + N)% Si%, (C + N)% A (0, 0.16) D (0.14, 0.06) B (0.05, 0.16) E (0. 08,0.06) C (0.14,0.115) F (0,0.1)