JPS5943845A - Low-alloy heat-resistant steel with improved hot workability and strength - Google Patents

Abstract

PURPOSE:To obtain the titled heat resistant steel with improved hot workability and strength at high temp., by providing a composition contg. prescribed percentages of C, Si, Mn, P, S, Mo, Ni, V, Al and N and by specifying the relation between Al and N. CONSTITUTION:The titlted low-alloy heat-resistant steel consists of <=0.23% C, <=1.0% Si, 0.2-1.5% Mn, <=0.01% P, <=0.01% S, 0.3-1.2% Mo, 0.05-0.30% Ni, 0.005-0.10% V, 0.04-0.10% Al, <0.004% N and the balance Fe with impurities. The relation between Al and N in the steel is in the range bounded by points A, B, C, D and E in the figure. The resistance of the steel to oxidation and corrosion due to hydrogen can be improved by further adding 0.5-<1.8% Cr. The steel can be easily manufactured by conventional refining in a converter, vacuum degassing and charging or other treatment in vacuum or an Ar atmosphere.

Description

[Detailed description of the invention] The present invention provides M with improved high temperature strength and hot workability.

This relates to Cr-Mo series and Cr-Mo series low alloy steels.

IVIo-based and Cr-Mo-based low-alloy heat-resistant steels are often used as materials for low-temperature parts such as boiler units for thermal power generation and oil phase equipment. Most of the custom-made materials were inferior in tensile strength, yield strength, creep strength, and creep rupture strength at room temperature or both temperatures. It has been common knowledge that A1 to be fixed is not added or is reduced to a very small amount.

Incidentally, although strength is certainly an important element for structures, workability when manufacturing structures is also an important factor as part of manufacturing technology and from an economic standpoint. In particular, when extremely thick steel plates are used, such as in large cylinder temperature P6 pressure reaction vessels, hot working is often used. On the other hand, the cooling rate of extremely thick steel plates during heat treatment is slow, making it difficult to ensure high-temperature strength and sufficient toughness.
It has become increasingly common to add However, it is known that excessive inclusion of AI impairs hot workability.

The present inventors obtained A for lν1o steel and Cr-IVio steel.
After conducting a basic study on the effect of t, we discovered that Al itself has a new 11'1 property that is very effective in improving strength, and we have found that we can effectively utilize this knowledge without impairing thermal processability. I thought about it. To achieve this goal, we focused on N2 and developed LVLo series and Cr-Mo series low-alloy heat-resistant steels with improved high-temperature strength and hot workability by suppressing the N level to an extremely low level compared to normal N levels. It was successful in doing so.

Table 1 shows the range of non-inventive ingredients.

Next, the present invention will be explained in detail. First, the reasons for limiting each component as described above will be described.

Although C is necessary to maintain strength, the upper limit was set at 0.23% from the viewpoint of weldability.

Although Si is added as a deoxidizing agent, it is also an element that improves strength. However, in consideration of toughness, the upper limit was set at 1% in order to improve the strength without impairing it. In addition, from the balance of strength and toughness, 01 to 0.8
% is preferred.

Seed is a necessary component not only for deoxidizing but also for maintaining strength. The upper limit was set at 1.5% because exceeding this content would be unfavorable from the viewpoint of toughness and bath weldability, and the lower limit was set at 0.2% from the viewpoint of strength.

P is an element that segregates at grain boundaries in this type of low alloy steel and significantly promotes embrittlement due to tarry pull alpha-tempering. The above-mentioned embrittlement can cause cracks in the bath contact area during use at high temperatures, and decrease in toughness after long-term use, resulting in brittle fracture during pressure tests, so in order to ensure the safety of the container, , must be suppressed as much as possible.・0 For this purpose, it is effective to reduce P's past scenery to 0.01% or less, so the upper limit is set to 0.01%.

For S, a lower force is better for improving toughness, but considering the current steelmaking technology, the upper limit is set at 0.01 as a reasonable value.
%.

1VIo is an element that significantly improves high-temperature strength through solid solution hardening and carbide precipitation, so it is an indispensable element for heat-resistant steel, but 7Na less than 03% has little effect (1.
If it exceeds 2%, weldability and ISO 1 will be impaired, so the upper limit should be set at 1.
2%, the limit was set at 0.3%.

Ni is effective in improving hardenability and toughness, but if it is less than 0.005%, it will be fruitless;
If the ratio exceeds the ratio, the effect becomes disadvantageous in terms of the LILA* efficiency, so the upper limit was set at 0.3% and the lower limit was set at 0.05%.

(2) is an element that significantly affects high-temperature strength, but if it is less than 0005, it has almost no effect, and if it exceeds 010, it will adversely affect weldability such as reheat cracking, so the upper limit should be set to 0.1.
0%, and the lower limit was 0.005%.

AI, together with N, is the main focus of the present invention,
It has been found that if the content exceeds 0.04%, the amount of transformation products generated during cooling from the austenitizing temperature increases, resulting in a significant increase in strength. In addition, if it exceeds 0.10%, the toughness will be adversely affected, and the Ni
lj', even if the N amount is suppressed to less than 0.004%, hot workability will be significantly inhibited, so the upper primary is set to 0.
．． Section 10, the lower limit was set at 0.04%.

Regarding N, in normal steel manufacturing methods, it is 0.005 to 0.
．． It is normal for the steel to contain 006 or more, but in the steel of the present invention, the above-mentioned A1 of A j? Based on the knowledge that in order to compensate for the decrease in the hot working raft due to 1, it is essential to suppress it to less than 0,004%, the upper limit was set at less than 0,004%. However, not only does the relationship between AI and N7 need to be within the above range, but from the viewpoint of workability, if A1 exceeds 0.06%, it is necessary to reduce the amount of N accordingly. It is necessary to keep the polygon ABCDE within Φ[3111 (1-1 ABC does not include the part on the line).

That is, Figure 1 shows C021%, Si0.25%, flatness 0.82
%, WCo 0.54%, Ni 0.1%, Vo, 00
This is a high-temperature tensile compensation when the N and A1 values of the steel of Section 7 are changed repeatedly), toughness, and heat workability from the 1ced point, and the continuous use in this net range has the above properties. I am satisfied with all of them.

Takete A B Cl) The points of E are ('AI item,
N amount) (0,04,0,004,), (006,0,
004), (010,0,002), (0,1,01O
), (0,04,O), and line segment AB is the upper limit of the amount of N,
Pure CIJ.

AE is the upper and lower limits of the Al content, and the spinning fraction BC is the two limits for maintaining good hot workability in the relationship between AI and N bales.

The reasons for limiting the fractional components of the steel of the present invention have been described above, but
In the present invention, in addition to this, Cr can be contained in order to improve oxidation resistance and hydrogen attack resistance. In other words, if Cr is less than 0.5%, its effect will not be sufficiently exhibited;
A modulus of 8 or higher impairs toughness and weldability, so the upper limit is 18%.
The lower limit was set to 0.5.

The steel of the present invention can be obtained by conventional converter melting, vacuum degassing treatment, injection in a vacuum or under an argon atmosphere, and can be easily produced using recent steelmaking technology.

Next, the effects of the steel of the present invention will be described with reference to examples.

Table 2 shows the chemical composition of the test steel, the tensile properties of a 100 mm thick steel plate at room temperature and sieve temperature (450°C), the impact absorption energy at 0°C, and the creep rupture test at 550°C and 20 kg/-. The rupture time and the reduction of area in a hot impact test at 850°C, which are representative of hot workability, are shown.

The steel according to item 1 of the present invention has a standard heat treatment after bath welding (625℃ x 4
time), the steel in Section 2 is tempered at 630°C x 3
0 minutes) and post-weld heat treatment (675°C x 16 hours).

-2; Of the steels shown in Table 2, D, E, F, I, K, and L steels belong to the first term, and Q, S, U, and V steels belong to the second term, For A, B, C, G, H, and JXM steels, see Section 1.
N, OlP, and RXT steels are comparative steels for the steel in item 2.

A steel is 1/2 Mo steel which is a normal boiler steel material,
l amount is kept low. In addition, the N content is higher than that of the present invention, which is a medium content that can be obtained by ordinary converter melting, and because the plate thickness is thick, the N content is higher than that of the present invention.
42.8 KV/mA, which is four times the allowable stress at 450°C of c vllll DiV 1, cannot be satisfied.

Steel C is a steel with a medium N content and a high Al content of 0043, but the effect of Al is not recognized in terms of strength, and the hot impact test, which is an index of hot workability, is The aperture value decreases to 50% or less. On the other hand, BXD-F,
H to M steels have N content reduced to 0.004% or less, but if the Al content is as low as 0.35q6 like B steel, this hardly leads to an increase in strength.

As can be seen from the comparison with Steel B, DXE, F, and Im have an At content within the range of the present invention, and remarkable increases in room temperature strength, madder temperature strength, and creep strength are observed. Furthermore, the aperture value in the hot impact tensile test is 50% or more, ensuring sufficient heat resistance of 1°.

On the other hand, G and H1J steels have deteriorated hot workability because the A1 and N amounts are outside the polygonal range of FIG. 1.

Especially when the Al content exceeds 0.10% like J steel, the N content becomes 0.
．． Even if it is as low as 0.025% for stacking, there is a problem in hot workability.

The K, L, and ■ steels have hl-it, N content, and other components all within the range of the present invention, and are excellent in strength, toughness, and hot workability. The M steel has a Mo content exceeding the upper limit of the present invention, indicating that the deterioration of toughness is significant.

Next, O steel is the normal 11/4 Cr -1/21VIo
4i1ft, the Al i content is low and the N content is rather high. In this way, current steel has low strength when the plate thickness reaches 100 mm, and ASTM 387-11-
450 which is sufficient to meet the allowable stress of ME5ection ■
℃ tensile strength of 44.6 Ky/- cannot be satisfied.

R steel has increased AI to 0.045%, but because the N content is as high as 0.0065%, the effect of AI on strength is almost negligible.Furthermore, since Al and N are simultaneously high, it cannot be heated easily. The reduction of area in the impact tensile test is low and the hot workability is poor.

Although the P steel has a N content lowered within the range of the present invention, the A1 content is not sufficiently high at 0.035%, so there is almost no increase in strength. Q &+ili, the amount of Al is 0.04
5%, the strength increases rapidly compared to P steel and satisfies the above-mentioned 4.4.6 K9/aj. For R steel, the relationship between AI ``4 (and N11) is outside the range of the polygon in Figure 1, so even if N fl (is low, it will have a negative effect on hot workability. 1.25
%, it can be seen that the toughness is significantly impaired. UX
V* is a steel whose Al content, N content, and other components are all within the scope of the present invention, and which has excellent strength, toughness, and hot workability.

In summary, the steel of the present invention is a steel that has excellent room temperature and high temperature strength, toughness, and thermal workability as a low-alloy steel extra-thick steel plate for which demand has increased in recent years.

[Brief explanation of the drawing]

FIG. 1 is a chart showing appropriate ranges of A1 and N amounts in the present invention.

Claims (1)

[Claims] 1.C0.23% or less, Si1.0% or less, seed 0.2~
1.5%, Po, 01% or less, 5oot% or less, Mo
0.3-1.2%, NiO, 05-0.30%, v
o, o O5~0. i o%, Al O, O11~0
．． 1.0%, N O, OO less than 4%, and the relationship between A1 and N is within the range of polygon ABCDE in Figure 1 (however, ABC does not include the line), and the remainder is F'e and heat consisting of impurities. Low-alloy heat-resistant steel with improved machinability and strength. 2C0, 23% or less, Si 1.0% or less, Mn 0.2
~1.5% or less, Po, 01% or less, 80.01% or less, l'vio 0.3-1.2%, Cr0.5-1.8
less than %, 1'Ji 0.05-0.30%, VO,0
05~0.10chi, At 0.04~0.10%, NO
,004%, and the relationship between Al and N is within the range of polygon ABCDE in Fig. 1 (however, ABC does not include the line), and the balance is Fe and impurities. Alloy heat resistant steel.