JPH04354856A - Ferritic heat resistant steel excellent in touchness and creep strength and its production - Google Patents

Abstract

PURPOSE:To manufacture a heat resistant steel excellent in toughness and creep strength by preparing a ferritic heat resistant steel having a specified compsn. in which the content of Al and N is controlled. CONSTITUTION:A ferritic heat resistant steel contg., by weight, 0.01 to 0.30% C, 0.01 to 0.80% Si, 0.10 to 1.50% Mn,8.00 to 13.00% Cr, 0.20 to 3.00% W, 0.005 to 1.00% Mo, 0.05 to 0.50% V, 0.02 to 0.15% Nb, 0.0003 to 0.008% B, 0.03 to 0.11% N and 0.001 to 0.10% Al, in which the content of P is limited to <=0.030%, S to <=0.010% and O to <=0.015%, moreover, satisfying the relationship of N (mass%)>14/27XAl (mass%)+0.025 and the balance Fe with inevitable impurities is prepd. Furthermore, the cooling of this steel after hot working or the cooling in normalizing or hardening is executed from the austenitic area to 500 deg.C of the martensitic area at the cooling rate of >=5 deg.C/min.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic heat-resistant steel, and more particularly to a ferritic Cr-containing boiler steel pipe steel with improved toughness and excellent creep properties at high temperatures.

0002

[Prior Art] In recent years, thermal power generation boilers have become larger in size, at higher temperatures, and at higher pressures. However, when the temperature exceeds 550°C, it is important to choose ferrite-based materials from the viewpoint of oxidation resistance and high-temperature strength. /4Cr-1Mo steel to 18-8
Currently, the use of high-grade austenitic steels such as stainless steel is rapidly increasing. However, as materials become more high-grade, such as low-alloy steel, stainless steel, and superalloys,
The cost increases and the boiler construction cost becomes expensive.

Therefore, research on steel materials to fill the gap between 2 1/4Cr-1Mo steel and austenitic stainless steel has been conducted for the past several decades, but boiler steel pipes with intermediate Cr content, such as 9Cr and 12Cr, In reality, increasing the strength deteriorates weldability and significantly reduces work efficiency in boiler construction, making it difficult to put it into practical use. In view of these circumstances, new steel types have already been developed and proposed that have improved weldability and creep rupture strength that is significantly higher than conventional materials.

However, depending on the part of the boiler used, there are parts that require thick wall materials, and in the manufacturing process, especially those manufactured by normalizing after hot working, the cooling rate from the austenite region is slow, especially for the plate. It becomes difficult to maintain sufficient strength and toughness in the thick central portion. Therefore, the development of steel with excellent strength and toughness has been desired.

[0005] Steels with improved these required properties have been developed, and the relationship between (Mo+W) and the amount of Nb has been determined to improve creep properties and toughness, as disclosed in Japanese Patent Laid-Open No. 61-69948 and Japanese Patent Laid-Open No. Publication No. 61-231139, JP-A-62-
It is disclosed in Japanese Patent Application Laid-open No. 297435 and Japanese Patent Application Laid-Open No. 62-297436. In addition, it has been shown in JP-A-63-8 that adding W and Nb in the optimum range is effective for improving creep strength.
It is disclosed in Japanese Patent No. 9644.

[0006] These steels are made by adding W to conventional heat-resistant steel,
Although this steel has dramatically increased creep strength through solid solution strengthening and precipitation strengthening, consideration was not given to post-aging toughness. On the other hand, the present inventors reexamined the toughness of the ferritic heat-resistant steels that had been developed and found that the addition of Al was effective in improving the toughness. Al was 0.
High Cr steel with less than 0.05% addition is designated as
A ferritic steel containing 0.001 to 0.5% of solid solution Al is disclosed in JP-A-60-155649, but since there is no restriction on N or cooling rate, the steel of the present invention It is not possible to achieve such high creep strength.

[0007]

[Problems to be Solved by the Invention] The present invention improves the above-mentioned conventional drawbacks, improves toughness by improving impact absorption energy at 0°C in the Charpy impact test, and improving creep rupture at 500 to 650°C. The purpose is to increase strength.

[0008]

[Means for Solving the Problems] The present invention aims to solve the above problems, and the gist thereof is as follows. (1) C: 0.01-0.30%, Si: 0.01-0.80 in mass%
%, Mn: 0.10-1.50%, Cr: 8.00-1
3.00%, W: 0.20-3.00%, Mo: 0.0
05-1.00%, V: 0.05-0.50%, Nb:
0.02-0.15%, B: 0.0003-0.008
%, N: 0.03-0.11%, Al: 0.001-0
．． Contains 10%, P: 0.030% or less, S: 0.0
10% or less, O: 0.015% or less, and N(
mass%)>14/27×Al(mass%)+0.
A ferritic heat-resistant steel having excellent toughness and creep strength, the remainder being Fe and inevitable impurities.

(2) Cooling after hot working, or cooling during normalizing or quenching of the steel described in item 1 above is carried out at a cooling rate of 5° C./min or more from the austenite region to the martensite region of 500° C. A method for producing ferritic heat-resistant steel with excellent toughness and creep strength. The present invention will be explained in detail below.

0010

[Operation] First, the reason why the range of each component and the cooling rate are limited as described above in the present invention will be described below. C is necessary for maintaining strength, and less than 0.01% is insufficient for maintaining strength. Also, from the viewpoint of weldability, the upper limit was set at 0.30%.
And so. In other words, in relation to the Cr content described below, this steel has very good hardenability, and the weld heat affected zone hardens significantly, which causes cold cracking during welding. Therefore, in order to complete welding, It requires preheating at a considerably high temperature, and as a result, welding workability is significantly impaired. However, C is 0.30
% or less, the maximum hardness of the weld heat affected zone will decrease and weld cracking can be easily prevented, so the upper limit is set at 0.30%.
And so.

[0011]Si is added to have a deoxidizing effect, ensure strength, and provide oxidation resistance, but it is an element that adversely affects toughness. The lower limit was set to 0.01% from the viewpoint of deoxidizing effect, securing strength, and imparting oxidation resistance, and the upper limit was set to 0.80% from the viewpoint of toughness. Mn is a necessary component not only for deoxidizing but also for maintaining strength. The reason why the upper limit was set at 1.50% is that exceeding this is not preferable from the viewpoint of toughness, and the lower limit was set at 0.10% as the minimum amount necessary for deoxidation.

Cr is an essential element for oxidation resistance,
It is always added to heat-resistant steel, M23C6, M6
High-temperature strength is increased by fine precipitation of C (M refers to a metal element) in the matrix. The lower limit is 8.00, where the precipitation effect is noticeable and also contributes to oxidation resistance.
%, and the upper limit is 13.00% from the viewpoint of weldability and toughness.
And so.

[0013] W is an element that significantly increases high-temperature strength through solid solution strengthening and precipitation strengthening by precipitating as carbides, and is particularly effective for long-term strengthening at temperatures exceeding 600°C. Adding more than 3.00% impairs weldability and oxidation resistance, so the upper limit was set at 3.00%. Further, in order to exhibit the effect in coexistence with Mo, the lower limit was set to 0.20%.

Mo is an element that significantly increases high-temperature strength through solid solution strengthening, so it is usually added to heat-resistant steel, but if added in large amounts, it impairs weldability and oxidation resistance, so the upper limit is set at 1.00. %. In addition, in order to exhibit the effect of improving creep strength in coexistence with W, the lower limit was set to 0.00.
It was set at 5%. Like W, V is an element that significantly increases the high-temperature strength of steel, whether dissolved in the matrix or precipitated as a precipitate. In particular, in the case of precipitation, V4 C3 or VN becomes a precipitation nucleus for other M23C6, M6 C, and M2 C, and has a remarkable effect on fine dispersion of precipitates. In order to be effective in improving creep strength, the lower limit is set to 0.
05%. Moreover, since strength decreases when the content exceeds 0.50%, the upper limit was set at 0.50%.

[0015] Nb increases the high temperature strength by precipitation of Nb (C, N), and the initial fine dispersed precipitation increases the subsequent M
It also contributes to long-term creep strength by finely controlling the precipitation state of 23C6, M6C, M2C, etc. The lower limit was set to 0.02% in order to exhibit the effect of Nb, and the upper limit was set to 0.15% because if it exceeded 0.15%, agglomeration and coarsening of precipitates would occur and the strength would decrease.

B is well known as an element that significantly improves hardenability, but creep strength is improved by adding a small amount of B. In order to achieve the effect of B, the lower limit is set to 0.0.
The upper limit was set at 0.003%, and the upper limit was set at 0.008% so as not to impair hot workability and weldability. N is an element that is dissolved in the matrix or precipitated as a nitride or carbonitride, and increases the creep strength. However, in order to form a nitride with Al, which will be described later, the N used as AlN is 14
/27×Al, and the amount of N necessary for forming nitrides such as Nb and V is 0.025%, so the necessary N to be added is N>14/27×Al+0.025. did. Furthermore, in order to ensure creep strength, the lower limit was set to 0.0.
The upper limit was set at 3%, and the upper limit was set at 0.11% in order to avoid blowholes during casting and obtain a sound steel ingot.

[0017] Al is a main component of the present invention, and combines with N to form AlN. During heating in the austenite region during normalizing treatment, this AlN acts as a pinning effect and prevents coarsening of crystal grains. It plays the role of forming fine crystal grains. In addition, fixation of solid solution nitrogen has the effect of improving the hardenability of B, but on the other hand, excessive addition produces coarse nitrides and impairs toughness, so 0.001-0.
It was set at 10%.

[0018] P has an adverse effect on temper embrittlement and reheat cracking susceptibility, so the upper limit was set at 0.030%. Since S causes deterioration of toughness, anisotropy, and an increase in reheat cracking susceptibility, the upper limit was set at 0.010%. O causes the formation of oxides that have an adverse effect on toughness, so the upper limit is set at 0.015.
%.

[0019] Furthermore, limiting the cooling rate during cooling after hot working, normalizing or quenching is a major component of the present invention, and AlN is The cooling rate was set at 5° C./min or higher to prevent coarsening and adverse effects on toughness and creep strength. The above are the reasons for limiting the basic components and cooling rate of the steel of the present invention.

[0020] Since the present invention provides a heat-resistant steel with excellent toughness and high creep rupture strength, the steel of the present invention can be subjected to various manufacturing methods and heat treatments depending on the purpose of use. This does not impede the effects of the present invention in any way. The steel of the invention can be provided not only in the form of steel tubes, but also in the form of thick plates and thin plates, and can be used in the form of various heat-resistant materials, either as hot-rolled or by using plates that have been subjected to the necessary heat treatment. It is possible to do so without affecting the effects of the present invention.

[0021] The above steel pipes, plates, and heat-resistant members of various shapes can be subjected to various heat treatments depending on their purposes and uses, and this is important in fully demonstrating the effects of the present invention. Normally, products are often produced through a normalizing and tempering process, but in addition to this, it is possible and useful to perform quenching, tempering, and normalizing processes singly or in combination. It is also possible to repeat each of the above steps multiple times within the range necessary to fully express the material properties, and this does not affect the effects of the present invention in any way.

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

[0023]

[Example] 50 kg of steel having the composition shown in Tables 1 and 2
Melted in a vacuum furnace, hot rolled to a thickness of 15 mm, and heated at 5°C.
A plate was manufactured by cooling at a rate of at least 100 ml/min. Each test material was normalized by holding at 1050°C for 1 hour and cooling at 5°C/min or more, and then tempered at 780°C for 1 hour.

[0024] Creep properties were determined by cutting out a creep test piece of 6 mmφ x 30 mm GL from the center of the plate thickness parallel to the rolling direction of a rolled steel plate, and conducting a creep rupture test at 600°C for up to 20,000 hours. The fracture stress was linearly extrapolated and evaluated. Further, the Charpy impact properties were evaluated by aging a rolled steel plate at 600°C for 3000 hours, cutting out a JIS No. 4 2mm V-notch Charpy impact test piece from the center of the thickness parallel to the rolling direction, and conducting the test at 0°C.

For comparison, as shown in Tables 3 and 4, steels having components that do not correspond to the steel of the present invention were melted and produced in the same manner and evaluated. Figure 1 shows the 600℃×30 with Al addition.
The influence on 0°C toughness after 00 hours aging is shown. The toughness test results in the figure are the average values of three Charpy test points at 0°C. When Al is 0.005% or more, toughness is significantly improved, and when Al exceeds 0.10%, toughness is decreased. Also, N>14/27×Al+0.0
25 relationship is not satisfied, and the cooling rate is 5°C/min.
A decrease in toughness is observed in steels with less than

FIG. 2 shows the influence of Al addition on creep rupture strength at 600° C. for 100,000 hours. Even with the addition of Al, the creep rupture strength did not decrease and exceeded the target value of 160 MPa. Also, N>14/27
Steels that do not satisfy the relationship xAl+0.025 and whose cooling rate is less than 5° C./min have a decrease in creep rupture strength.

Steels A, B and C of the comparative examples shown in Tables 3 and 4 have no addition of Al, so their creep strength is 160.
An example where the MPa is secured, but the toughness after aging is low; an example where D steel, E steel, and F steel have coarsening of AlN due to the addition of too much Al, and the toughness after aging deteriorates;
G steel, H steel and I steel have N>14/27×Al+0.
A because it does not satisfy the relationship 025 and the cooling rate is slow.
This is an example of coarsening of lN and deterioration of toughness after aging.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

[0032]

[Effects of the Invention] As described above, the steel of the present invention is a steel that achieves increased high-temperature strength that can cope with higher temperatures and pressures in equipment than conventional ferritic heat-resistant steels, and also has excellent practical properties such as toughness. The contribution to industry is extremely large.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of Al addition on toughness after aging at 600°C for 3000 hours.

FIG. 2 is a diagram showing the influence of Al addition on creep rupture strength at 600° C. for 100,000 hours.

Claims (2)

[Claims] Claim 1: C: 0.01 to 0.30% in mass %,
Si: 0.01-0.80%, Mn: 0.10-1.5
0%, Cr: 8.00-13.00%, W: 0.20-
3.00%, Mo: 0.005-1.00%, V: 0.
05-0.50%, Nb: 0.02-0.15%, B:
0.0003-0.008%, N: 0.03-0.11
%, Al: 0.001 to 0.10%, P: 0.
030% or less, S: 0.010% or less, O: 0.015
% or less, and N (mass%)>14/27×
A ferritic heat-resistant steel having excellent toughness and creep strength, which satisfies the relationship of Al (mass%) + 0.025, with the remainder consisting of Fe and unavoidable impurities. 2. Cooling after hot working or cooling during normalizing or quenching of the steel according to claim 1 is carried out at 500° C. from the austenitic region to the martensitic region.
A method for producing a heat-resistant ferritic steel having excellent toughness and creep strength, characterized in that the method is carried out at a cooling rate of ℃/min or more.