JPH0759740B2 - Ferritic heat resistant steel with excellent toughness and creep strength - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ferritic heat-resistant steel, and more specifically to a ferritic Cr-containing boiler having improved toughness and improved creep characteristics and meltability at high temperatures. The present invention relates to steel for steel pipes.

(Prior Art) In recent years, thermal power generation boilers are becoming larger, higher in temperature, and higher in pressure. However, when the temperature exceeds 550 ° C., when selecting the material, oxidation resistance and high temperature strength are considered
-The current situation is that it has been leapt from 1 / 4Cr-1Mo steel to high-grade austenitic steel such as 18-8 stainless steel.

However, as the materials of low alloy steel, stainless steel, superalloy and high grade materials increase, the cost rises and the construction cost of the boiler becomes expensive.In order to improve the effect of the boiler due to material problems, increase the pressure. A supercritical pressure boiler is used.

Steel materials for filling the middle of 2.1 / 4Cr-1Mo steel and austenitic stainless steel have been sought for the past several decades, but boiler steel pipes of 9Cr, 12Cr, etc., which have an intermediate amount of Cr, can be welded when the strength is increased. However, it is difficult to put it into practical use because the work efficiency is significantly reduced in the construction of the boiler.

From such a viewpoint, the advent of a steel having a creep strength that fills the middle between the 2.1 / 4Cr-1Mo steel and the austenitic stainless steel has been long awaited.

As a process for manufacturing a boiler, a method of performing welding-post-weld heat treatment (hereinafter referred to as PWHT) or post-hot-working welding-PWHT is adopted. Therefore, it is needless to say that the performance required for such a steel for boilers is excellent in weldability and hot workability, and sufficient strength and toughness are maintained even after undergoing the heat history. Is important.

In view of such circumstances, new steel grades have been developed and proposed, which have already been improved in weldability and whose creep rupture strength is significantly higher than that of conventional materials. The steel disclosed in Japanese Examined Patent Publication No. 56-34628 is a steel in which creep rupture strength is ensured and weldability is improved by appropriately adding V and Nb.
The steel of Japanese Patent No. 3865 is a steel in which the correlation between V and Si is determined and the balance between strength and toughness is maintained. In addition, JP-A-60-215746
The steel disclosed in the gazette aims to improve toughness by limiting Si and
This steel aims to improve creep strength by adding B and N and limiting the amount of oxygen. In addition, Japanese Patent Publication No. 58-17820 is W
It is shown that the addition of W is effective in improving the creep strength by defining the optimum range of.

However, depending on the part where the boiler is used, there are parts that require thick material, and among the manufacturing processes, those manufactured by normalization after hot working have a low cooling rate from the austenite region, especially in the center of plate thickness. It becomes difficult to maintain sufficient strength and toughness in the parts. Therefore, development of steel having excellent strength and toughness is desired.

Steel with improved properties has been developed, and (Mo + W) and Nb
Proposals for improving the creep characteristics and toughness by defining the relationship between the amounts are disclosed in JP-A-61-69948, JP-A-61-231139, JP-B-62-297435 and JP-A-62-297436. It is disclosed in the official gazette. Further, it is disclosed in JP-A-63-89644 that the addition of W and Nb in the optimum range is effective for improving creep strength.

(Problems to be solved by the invention) These steels are steels in which W has been added to conventional heat-resistant steels to dramatically increase the creep strength by solid solution strengthening and precipitation strengthening. However, in consideration of post-aging toughness. Was missing.

On the other hand, as a result of reexamination of the toughness of the ferritic heat-resistant steels developed so far, the present inventors have found that the addition of Zr is effective in improving the toughness.

Heat-resistant steels containing 1.0% or less of Zr are disclosed in Japanese Examined Patent Publication No. Sho 60-13060 and Japanese Examined Patent Publication No. 57-30903, but these steels have a low W content of 1% or less, and restrictions on N. Therefore, it is impossible to simultaneously achieve high creep strength like that of the steel of the present invention.

In addition, heat-resistant steel containing 0.5% or less of Zr is disclosed in Japanese Patent Publication Sho 58-17820.
This steel has a W content of 1.5.
%, It is impossible to achieve a high creep strength, and since there is no limitation on Nb, the toughness and strength cannot be the same as those of the steel of the present invention.

Furthermore, heat-resistant steel containing 0.02% to 0.1% Zr is disclosed in Japanese Patent Laid-Open No.
As disclosed in Japanese Unexamined Patent Publication No. 8-217661, since this steel has a W content of 1% or less and there is no limitation of B and N, it is impossible to achieve high creep strength and toughness like the steel of the present invention. It is impossible.

The present invention improves on the above-mentioned conventional drawbacks at 500 to 600 ° C.
The purpose is to increase the creep rupture strength at 1, and to improve the toughness as well as the impact absorption energy at 0 ° C. in the Charpy test.

(Means for Solving the Problems) The present invention provides the following steel in order to solve the above problems.

That is, C: 0.01% to 0.30%, Si: 0.01% to 0.08%, Mn: 0.10
% ~ 1.50%, Cr: 8.00% ~ 13.00%, Mo: 0.005% ~ 1.00
%, W: 0.20% to 3.00%, V: 0.05% to 0.50%, Nb: 0.02%
~ 0.10%, B: 0.0003% ~ 0.008%, Zr: 0.0005% ~ 0.10
%, N: 0.01% to 0.10%, Al: 0.0005% to 0.050%, Co: 0.0
1% to 1.00%, P: 0.050% or less, S: 0.010% or less, O: 0.020% or less, or Ni: 0.05% or less
It is a ferritic heat-resistant steel with excellent toughness, characterized by containing one or two of 1.00% and Ti: 0.01% to 0.10%.

The present invention will be described in detail below.

(Operation) First, the reason why each component range is limited as described above in the present invention will be described below.

C is necessary for maintaining the strength, and if it is less than 0.01%, it is insufficient to secure the strength. The upper limit was set to 0.30% from the viewpoint of weldability.

That is, due to the relationship with the amount of Cr described later, this steel has very good hardenability and the heat-affected zone of welding is significantly hardened, which causes 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, if C is kept at 0.30% or less, the maximum hardness of the weld heat affected zone will be lowered and welding cracks can be easily prevented, so the upper limit was made 0.30%.

Si is added for deoxidation hardening, ensuring strength and oxidation resistance, but is an element that adversely affects toughness. The lower limit was 0.01% from the viewpoint of deoxidation, strength and oxidation resistance, and the upper limit was 0.80% from the viewpoint of toughness.

Not only for Mn deoxidation, but also for maintaining strength. The upper limit of 1.50% is unfavorable from the viewpoint of toughness above this range, and the lower limit was set to 0.10% as the minimum amount required for deoxidation.

Cr is an indispensable element for oxidation resistance and is always added to heat-resistant steel. Due to fine precipitation of M ₂₃ C ₆ and M ₆ C (M is a metal element) in the matrix, high temperature strength is obtained. Is increasing. The lower limit was set to 8.00%, at which the precipitation effect is remarkably recognized and contributing to oxidation resistance, and the upper limit was set to 13.00% from the viewpoint of weldability and toughness.

W is an element that remarkably enhances high temperature strength by solid solution strengthening and precipitation strengthening by precipitating as a carbide.
It is effective for strengthening for a long time beyond 600 ° C. If added in excess of 3.00%, the weldability and oxidation resistance will be impaired, so the upper limit is set.
It was set to 3.00%. Also, the lower limit was made 0.20% in order to exert the effect in the coexistence with Mo.

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, the weldability and oxidation resistance are impaired, so the upper limit was made 1.00%. In addition, the lower limit was made 0.005% in order to exert an effect of improving the creep strength in the coexistence with W.

V, like W, is an element that remarkably enhances the high temperature strength of steel even if it forms a solid solution in the matrix or precipitates as a precipitate.
In particular, in the case of precipitation, V ₄ C ₃ serves as precipitation nuclei for other M ₂₃ C ₆ , M ₆ C and M ₂ and shows a remarkable effect on fine dispersion of the precipitate. The lower limit is 0.05% to exert the effect of improving creep strength.
And Further, if it exceeds 0.50%, the strength lowers, so the upper limit was made 0.50%.

Nb enhances high temperature strength by precipitation of Nb (CN), and long-term creep is used to finely control the precipitation state of M ₂₃ C ₆ , M ₆ C, M ₂ C, etc., which is followed by initial fine dispersion precipitation. It also contributes to strength. The lower limit is 0.02% to exert the effect of Nb
The upper limit was made 0.12% because if it exceeds 0.12%, cohesive coarsening of precipitates occurs and the strength is reduced.

Although B is originally well known as an element that remarkably enhances hardenability, addition of a small amount of B improves creep strength. In order to exert the effect of B, the lower limit is made 0.0003%,
Also, the upper limit is 0.00 so as not to impair hot workability and weldability.
8%.

Zr is the main component of this invention and has a strong affinity with N.
ZrN is formed, BN precipitation due to nitriding of B is suppressed, and the effect of B addition is prevented from being impaired when a large amount of nitrogen is added. Further, Zr controls the deoxidization equilibrium in the steel and suppresses the formation of oxides by significantly reducing the oxygen activity. The lower limit was set to 0.0005% in order to exert the effect of controlling the deoxidation equilibrium, and the upper limit was set to 0.10% to prevent a large amount of coarse ZrN and ZrC from precipitating and significantly reducing the toughness of the base material.

N is an element that dissolves in the matrix as a solid solution or precipitates as a nitride or carbonitride and enhances the creep strength. However, from the viewpoint of securing the creep strength, the lower limit was made 0.01%, and the occurrence of blowholes during casting was avoided to ensure soundness. Cap to get steel ingot
It was set to 0.10%.

Although it has the effect of enhancing the hardenability of B by refining the Al crystal grains and fixing the solid solution nitrogen, on the other hand, excessive addition produces coarse nitrides and impairs toughness as in Ti described below.
It was set to 0005% to 0.050%.

Co is an element that precipitates as a carbide, improves the high temperature strength of the base material, and suppresses the formation of δ ferrite.
Therefore, it is possible to suppress the deterioration of high temperature strength and toughness due to the formation of δ ferrite. If it is less than 0.01%, there is no effect, and if it is added excessively, coarse carbides may precipitate and the toughness may decrease, so the Co addition range is 0.01% to 1.
It was set to 00%.

Since P adversely affects temper embrittlement and reheat cracking susceptibility, the condition was set to 0.050%.

Since S causes deterioration of toughness, anisotropy and increase in reheat cracking susceptibility, the upper limit was made 0.010%.

Since O causes the formation of oxides which adversely affects toughness, the upper limit was made 0.020%.

The above are the basic components of the steel of the present invention, but in the present invention, Ni: 0.05% to 1.00%, T
i: 0.01% to 0.10% of 1 type or 2 types can be contained.

Ni is effective in increasing hardenability and improving toughness, but even if it is added in an amount exceeding 1.00%, the effect cannot be expected to be improved, so the content was made 0.05% to 1.00%.

Ti is an element that precipitates as a carbide and improves the high temperature strength of the base material. If it is less than 0.01%, there is no effect, and if it is added in excess, coarse nitrides may precipitate and the toughness may decrease, so the content was made 0.01% to 0.10%.

The above alloy components may be added individually or in combination.

Since the present invention provides a heat-resistant steel having high toughness and high creep rupture strength, the steel of the present invention can be subjected to various production methods and heat treatments depending on the purpose of use, and It does not hinder the effect of the present invention.

First, VIM (vacuum induction heating furnace), EF
(Electric furnace) and LD (converter) can be used and are useful. Subsequently, ESR (Electro Slag Remelting), AOD (Arg
on Oxygen Decarbrization), VAD (Vacum Argon Decarb
rization), VOD (Vacum Oxygen Decarbrization), and LF (Ladle Furnace) and other processes using vacuum degassing or powder blowing smelting equipment (eg, RH, DH, CAS, etc.) alone or in combination It is possible and suitable.

Molten steel can be cast into a mold and then cast into a slab by a continuous casting device or a billet to form a steel ingot, which can be processed into a shape suitable for various manufacturing processes.

As a manufacturing process, after processing into a round billet or a square billet, hot extrusion, or a method of processing into seamless pipes and tubes by various seamless rolling methods, hot rolling into thin plates, cold rolling followed by electrical resistance welding ERW steel pipe method, TIG, MIG, SAW,
The method of making welded steel pipe by LASER, EB welding (single or in combination) can be applied, and after each of the above methods, SR (drawing rolling) or standard rolling is additionally performed hot or warm. It is also possible to expand the range of applicable dimensions of the steel of the present invention.

The steel of the present invention can be provided not only as a steel pipe but also in the form of a thick plate and a thin plate, and can be used in the form of various heat-resistant materials by using the plate as hot-rolled or subjected to the necessary heat treatment. It can be used without affecting the effects of the present invention.

The above-mentioned steel pipes, plates, and heat-resistant members of various shapes can be subjected to various heat treatments depending on the purpose and application, and are important for sufficiently exerting the effects of the present invention.

Usually, the product is often subjected to the normalization + tempering process, but in addition to this, the quenching, tempering, and normalizing processes are performed separately.
Alternatively, they can be used in combination and are also useful. It is also possible to apply each of the above steps a plurality of times repeatedly within a range necessary for sufficient expression of material properties, and does not affect the effect of the present invention.

The above steps may be appropriately selected and applied to the steel production process of the present invention.

(Examples) The steels shown in Tables 1 and 2 were melted in a 50 kg vacuum furnace and hot-rolled to produce a plate having a thickness of 15 mm. After normalizing each test material at 1050 ° C. for 1 hour, it was tempered at 780 ° C. for 1 hour.

As for the creep characteristics, as shown in Fig. 1, a 6 mmφ × GL30 mm creep test piece 2 was cut out parallel to the rolling direction of the rolled steel sheet 1, tested at 600 ° C, and the breaking stress of 600 ° C × 10 ⁵ hours was extrapolated. , Evaluated.

Further, the Charpy characteristics are as shown in FIG.
After 10 ³ hours aging at 600 ° C., the rolling direction and parallel to No. JIS 4 2 mm V
Notch Charpy impact test piece 3 was cut out and tested at 0 ° C. for evaluation.

The creep rupture stress at 600 ° C × 10 ⁵ hours and the 0 ° C toughness after aging at 600 ° C × 10 ³ hours are shown in Tables 1 and 2 at the same time. still,
The toughness test result in the table is an average of 3 points of the Charpy test at 0 ° C.

For comparison, as shown in Table 3, steels having components not falling within the scope of the present invention were melted, manufactured, and evaluated by the same method.

Figure 3 shows the effect of Zr addition on toughness.

When Zr is 0.005% or more, the toughness is remarkably improved, and when it exceeds 0.10%, the toughness is decreased.

Figure 4 shows the effect of Zr addition on creep rupture strength.

Creep rupture strength does not decrease even with the addition of Zr, and exceeds the target of 16 kg f / mm ² .

Since Zr is not added to the steels A and B of the comparative examples shown in Table 3, the creep strength of 16 kg f / mm ² can be secured, and the post-aging toughness is low. In steel, the amount of Zr added was too large, resulting in coarsening of ZrN and deterioration of toughness after aging. In E steel, G steel, I steel, K steel, M steel, and O steel, Zr is not added. Examples where the toughness could not be secured after aging, F steel, H
Steel, J-steel, L-steel, N-steel, and P-steel had too much Zr added.
Is an example in which the toughness deteriorates after aging.

(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 cope with high temperature and high pressure of the equipment, as compared with the conventional ferritic heat-resistant steel, and has practical toughness and weldability It has excellent characteristics and contributes greatly to the industrial world.

[Brief description of drawings]

FIG. 1 is a perspective view showing a procedure for collecting creep rupture test pieces from a rolled steel sheet, FIG. 2 is a perspective view showing a procedure for collecting Charpy impact test pieces from a rolled steel sheet, and FIG. 3 is an effect on the toughness of Zr addition. The chart shown in FIG. 4 is a chart showing the effect of Zr addition on the creep rupture strength. 1 ... Rolled steel plate, 2 ... Creep test piece 3 ... Charpy impact test piece

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hisashi Naoi 5-10-1 Fuchinobe, Sagamihara City, Kanagawa Pref., Second Research Laboratory, Nippon Steel Corporation (56) Reference Japanese Patent Laid-Open No. 62-103345 (JP, A) JP-A-55-110758 (JP, A)

Claims (2)

[Claims] 1. By weight%, C: 0.01% to 0.30% Si: 0.01% to 0.08% Mn: 0.10% to 1.50% Cr: 8.00% to 13.00% W: 0.20% to 3.00% Mo: 0.005% to 1.00% V: 0.05% to 0.50% Nb: 0.02% to 0.12% B: 0.0003% to 0.008% Zr: 0.0005% to 0.10% N: 0.01% to 0.10% Al: 0.0005% to 0.050% Co: 0.01% to 1.00% A ferritic heat-resistant steel with excellent toughness and creep strength, characterized in that P: 0.050% or less, S: 0.010% or less, O: 0.020% or less, and the balance being Fe and inevitable impurities. 2. A ferritic heat-resistant steel excellent in toughness and creep strength according to claim 1, which contains one or two of Ni: 0.05% to 1.00% Ti: 0.01% to 0.10% in weight%.