JPH0353045A - Low-alloy heat-resisting steel excellent in toughness at low temperature and strength at high temperature - Google Patents

Abstract

PURPOSE:To produce a low-alloy heat-resisting steel excellent in toughness at low temp. and strength at high temp. by incorporating specific percentages of C, Si, Mn, Cr, Mo, V, Al, and N to Fe and also specifying ferrite grain size. CONSTITUTION:A low-alloy heat-resisting steel which has a composition consisting of 0.04-0.18% C, <=0.6% Si, 0.1-1.0% Mn, 0.5-3.5% Cr, 0.1-0.8% Mo, 0.1-0.5% V, 0.004-0.050% Al, 0.0025-0.0150% N, and the balance Fe with inevitable impurities and in which ferrite grain size No. is regulated to >=8.0 is prepared. By this method, the heat resisting steel excellent in toughness at low temp. as well as in high strength characteristics and suitable for high temp. member, such as boiler tube used at high temp. and high pressure can be obtained.

Description

[Detailed Description of the Invention] Industrial Application Fields The present invention relates to (1) evaporation tubes and superheater tubes for high-temperature and high-pressure boilers. Reheater pipes, main steam pipes, superheater pipes for chemical industrial plants,
This article relates to a low-alloy heat-resistant steel that has both low-temperature toughness and high-temperature strength and is suitable for use in high-temperature parts such as heat exchanger tubes. <Conventional technology> Conventional boiler materials for thermal power generation include JTSMJ. Sugi's STB3
Carbon steel such as 5, 42, STBA12, 22. 24
A variety of heat-resistant, high-temperature steels are used, including low-alloy steels such as . In addition to the above, steels containing Cr, Mo+V have excellent high-temperature strength and are often used as boiler steel pipes. For example, British standard BS-HF660 steel and Soviet standard G
OSTI2X IMF, 15X IMF, and Chinese standard 12Cr IMoV are well known. On the other hand, boiler materials and heat exchanger tubes are subjected to pressure tests before being used for high-temperature heating, and low-temperature toughness is required in addition to high-temperature strength to ensure that bursts do not occur. However, in Cr-Mo-V steel, for example,
As can be seen from the examples of the low-alloy heat-resistant steel proposed in Publication No. 12017, there was a problem in that it had high strength but low metametal toughness. That is, in order to obtain high strength in CrNo-V steel, the normalizing temperature is carried out at a high temperature of 940 to 950°C or higher, and in this case, there is a problem that the higher the strength, the lower the toughness. Therefore, it was necessary to sacrifice some strength to ensure toughness. For this reason,
The development of Cr-Mo-V steel with excellent strength and toughness was expected. Problems to be Solved by the Invention> The object of the present invention is to provide a material whose basic component is Cr-Mo-V, which has a Charpy absorbed energy of 15 kg-m or more at 0°C and has excellent high-temperature strength. Our objective is to provide low-alloy heat-resistant steel. Means for Solving the Problems> The present invention provides C: 0.04 to 0.18%, Si: 0.
6% or less, Hn: 0.1-1.0%, Cr:
0.5 to 3.5%, Mo: 0.1 to 0.8%
, V: o. t~o. s%, At: 0
．． 004 to 0.050%, N: 0.0025
~0.0150%, the remainder consists of Fe and unavoidable impurities, and the grain size is 8.
It is a low-alloy heat-resistant steel with excellent low-temperature toughness and high-temperature strength, characterized by a toughness of 0 or more. Effects> In order to achieve the present invention, as a result of a detailed study of the effects of the addition of various elements on strength and toughness, it was found that the above objectives can be achieved without changing the basic components of CrMo-V. That is, by adding appropriate amounts of AI and N, Cr
It was found that the low ilJ property can be significantly improved while maintaining the high strength as -Mo-V. The reason for this is not clear, but since the addition of 8L and N makes the crystal grains finer even when heat-treated at high temperatures, grain refinement lowers the transition temperature and improves toughness. It is thought that the contribution is large. Below, I will explain the reasons for limiting the precepts of the present invention. C is an inexpensive element that contributes to improving strength (including high temperature strength and creep properties). If it exceeds 0.18%, hardenability and strength will increase significantly, but weldability and workability will deteriorate, so the upper limit was set at 0.18%.

Further, if it is less than 0.04%, it becomes difficult to ensure high temperature strength, so the lower limit is set to 0.04% or more.

Si is added as a deoxidizing agent, but if it is present in a large amount, the toughness of the steel will deteriorate, so the upper limit was set at 0.6%.

On the other hand, in order to improve high-temperature long-term strength and toughness, it is better to lower the Si content, so the lower limit is not particularly specified.

Mn is a useful element as a deoxidizing and desulfurizing agent, and for obtaining a suitable &U weave with improved strength and heat processability, but if it is less than 0.1%, it has no useful effect; If it exceeds 0.1% or more, but not more than 1.0%, the hardenability will increase and cause a decrease in workability and toughness.C' is 0.5 to ensure high temperature oxidation resistance and high melon strength. % or more, but adding too much will deteriorate weldability, so the upper limit was set at 3.5%.
．． It is necessary to add 1% or more, but adding a large amount increases cost and deteriorates weldability, so the upper limit is set at 0.8%.
It is necessary to do so. (2) requires addition of 0.1% or more to increase strength, but too much will impair toughness and weldability, so the upper limit should be 0.5%. At has the effect of improving toughness by refining austenite grains during heat treatment. In order to effectively obtain this effect, it is necessary to add 0.004% or more. However, o. Adding more than oso% will cause cracks to occur during solidification and hot rolling, so
．． It is necessary to keep it below 0.050%.

N has the effect of refining crystal grains during heat treatment, and for this purpose it is necessary to add 0.0025% or more.

However, addition of more than 0.0150% reduces hardenability and promotes strain aging embrittlement, so 0.0150%
It is necessary to do the following. The reason for limiting the grain size No. to 8.0 or more is as follows. The normalizing temperature for the CrMo-V steel of the present invention steel was set at 930 to 1020°C, and the tempering temperature was set at 7.
As a result of various studies on the relationship between grain size and toughness after heat treatment at 10 to 770"C, we obtained the results shown in Figure 1. That is, when the ferrite grain size NO is 8.0 or more, It was found that the Charpy absorbed energy at 0°C is sufficiently high due to the sufficiently excellent toughness, but when the grain size is smaller than that, the absorbed energy is small.Therefore, in order to obtain good toughness, the Charpy absorbed energy is small. Grain size N
o must be 8.0 or higher. Examples> Table 1 shows the chemical composition of the steel of the present invention, and Table 2 shows the chemical composition of the comparative steel.

The steel ingots of these chemical precepts are produced in a high frequency melting furnace, and the steel ingots are hot rolled into 201 ITI 1 thickness wJ temporary, 9
Normalize at 50-1000'C, 730-760'
Tempering was performed using C. Thereafter, a tensile test was conducted at room temperature and high temperature using a 6-inch diameter round bar, and a 2IIIV notch Charpy test was conducted at O'C using a JI No. 34 test piece.

The Kihatmon steel types include steels with varying C, Cr, Mo, V, and 81N. Among the comparative steels, A steel is ICr-0.3Mo-0.2V steel with an AI lower than the range of the present invention, and the mouth steel is the same. Cr-0.
3Mo-0.2V steel with N below the range of the present invention, and C steel with 21/4CrIMo-0.25V steel with At below the range of the present invention.

? The test results of the invention are shown in Table 3. It has excellent room-temperature strength and high-temperature strength, and has sufficiently high Charpy absorbed energy at 0°C. On the other hand, as shown in Table 4, although a certain degree of altitude is achieved with the comparative steel, the Charpy absorbed energy at 0°C is significantly lower. This is the ferrite grain size No.
This is thought to be due to the fact that it is less than 8.0. Table 5 shows the symbols A, B, and EF, which are wood invention steel precepts.
An example will be shown in which, although a certain degree of strength can be obtained when the ferrite grain size No. is less than 8.0, the Charpy absorbed energy at 0°C is significantly reduced. Thus, it is shown that excellent properties can be obtained by setting the ferrite grain size No. to 8.0 or more for the steel according to the present invention. <Effects of the Invention> As explained above, the present invention
Compared to o-V steel, it has high strength characteristics and low toughness, and is a heat-resistant steel suitable for high-temperature parts such as boiler tubes and heat exchanger tubes that are used at high temperatures and high pressures.

[Brief explanation of drawings]

Figure 1 shows the ferrite viscosity grain size NO and Chano-Levy absorbed energy at 0°C for various Cr-Mo-V steels of the composition system of the present invention. Applicant J Saki Steel Co., Ltd.

Claims (1)

[Claims] C: 0.04-0.18%, Si: 0.6% or less, Mn
:0.1~1.0%, Cr:0.5~3.5%, Mo:
0.1-0.8%, V: 0.1-0.5%, Al: 0.
004-0.050%, N: 0.0025-0.015
A low-alloy heat-resistant steel having excellent low-temperature toughness and high-temperature strength, characterized in that the remainder is Fe and unavoidable impurities, and has a ferrite grain size No. of 8.0 or more.