JP2968064B2 - Manufacturing method of low alloy heat resistant steel with excellent high temperature low cycle fatigue characteristics and toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel material as a structural material for high-temperature equipment, and more particularly to a method for producing a steel material.
This is a method for producing steel materials for high-temperature equipment such as boilers, metal melting furnaces, heating furnaces, towers, and the like, which are operated in a medium to high temperature range of about 530 ° C.

[0002]

2. Description of the Related Art C-Mo steel (steel corresponding to ASTM standard A204B) is required for high temperature equipment from the viewpoint of high temperature strength, particularly from the viewpoint of tensile strength, proof stress and creep strength at high temperature.
And Mn-Mo steel (steel corresponding to ASTM standard A302B) are often used. However, there was a technical problem that weldability and toughness were not always sufficient.

That is, in terms of weldability, in addition to a high C content (in most cases, 0.22 to 0.25%), the alloy contains about 0.5% of Mo. Equivalent carbon equivalent Ceq (Ceq = C + Si / 24 + Mn
/ 6 + Cr / 5 + Mo / 4 + Ni / 40 + V / 14) is usually 0.45 or more, and a parameter Pcm (Pcm = C + Si / 30 + Mn / 2) indicating weld cracking susceptibility.
0 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 15 + 5B) is usually 0.30 or more, which is an extremely high value as a welded structure. Therefore, the welding crack sensitivity is extremely high, and the preheating temperature during welding work must be increased, which is not preferable from the viewpoint of energy saving and welding workability, and needs to be improved.

[0004] Regarding toughness, since high-temperature properties, particularly creep properties, are emphasized because the steel is a high-temperature steel material, coarse-grained steel is used, and the initial toughness, that is, the toughness at the start of operation is low. In addition, Mo added in a large amount precipitates and agglomerates as carbides due to the heat history during operation of the device, and further reduces toughness. Such low weldability and toughness lead to brittle fracture of the structure, which is not preferable from the viewpoint of safe operation of equipment, and needs to be improved.

[0005] From such a viewpoint, Japanese Patent Application No. Hei.
No. 55, (1) Ceq and P from the viewpoint of improving weldability
cm, and (2) improve the initial toughness by using Al, Nb, etc. from the viewpoint of toughness, and reduce the amount of Si in consideration of toughness reduction during use. (3) If necessary, V, N
(4) Invention of a method for producing a low-alloy heat-resistant steel that improves weldability and toughness by performing quenching and tempering immediately after high-temperature processing to compensate for a decrease in high-temperature strength, particularly to compensate for a decrease in high-temperature strength. did.

[0006] The steel produced by this production method has not only excellent high-temperature strength but also good weldability and toughness. However, depending on the manufacturing conditions, there is a problem that the characteristics with respect to fatigue damage due to temperature fluctuations at the start and stop of the operation, that is, the high-temperature low-cycle fatigue characteristics are reduced. When considering the damage in various high-temperature equipment operated in the medium-high temperature range, when cracks occur due to high-temperature low-cycle fatigue due to fluctuations in operating conditions, and when the toughness of steel exceeds the allowable upper limit of length, The crack propagates rapidly, forming a crack that penetrates the plate thickness, leading to a serious situation such as an explosion. Therefore, it is desirable to develop a manufacturing method capable of ensuring high temperature and low cycle fatigue strength and toughness in a high and stable manner.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a manufacturing method which has not only high-temperature strength but also good weldability and toughness, and which can secure stable high-temperature low-cycle fatigue characteristics even when manufacturing conditions fluctuate. And

[0008]

Means for Solving the Problems The present inventors have repeated studies on the effects of manufacturing conditions on the high temperature and low cycle fatigue strength of a low alloy heat resistant steel used for direct quenching and tempering after high temperature processing such as rolling. As a result, it was found that if the rolling reduction in the finish rolling is high, the lower the finish rolling temperature, the more excellent the high temperature and low cycle fatigue strength can be obtained. The present invention has been made based on this finding. The main points are as follows: (1) C: 0.03 to 0.12% by weight, Si: 0.01 to 0.
15%, Mn: 0.2 to 1.6%, Mo: 0.15
0.45%, Al: 0.005 to 0.05%, N: 0.
001 to 0.01%, the balance being inevitable impurities and substantially Fe, steel is heated to a temperature range of 1000 to 1280 ° C., and the finish rolling finish temperature is 700 ° C. or more and less than 850 ° C .; Finish rolling is performed at a cumulative rolling reduction of 30 to 75% from a higher temperature of 30 ° C., and thereafter, from a temperature range of higher than (finish rolling end temperature−30 ° C.) 1 to 60
A method for producing a low-alloy heat-resistant steel having excellent high-temperature low-cycle fatigue characteristics and toughness, characterized by cooling to an average cooling rate of 300 ° C / sec to 300 ° C or lower and then tempering in a temperature range of 600 to 720 ° C. Yes, (2) V: 0.02% by weight
The present invention relates to a method for producing a low-alloy heat-resistant steel excellent in high-temperature low-cycle fatigue characteristics and toughness containing at least one kind of Nb: 0.005 to 0.04%.

[0009]

Hereinafter, the present invention will be described in more detail.
0.10% C-0.07% Si-1.02% Mn-0.
003% P-0.002% S-0.27% Mo-0.0
After heating 23% Al-0.0041% N steel to 1050 ° C, the rolling reduction is 8-15% or 35-40% at various temperatures.
Finish rolling, and finish rolling finish temperature -30 ° C to 3
Cool to 00 ° C at an average cooling rate of 2 ° C / sec.
Tempering was performed at 0 ° C. for 1 hour. The rolling reduction here refers to a cumulative rolling reduction from (finish rolling end temperature + 30 ° C.) (hereinafter simply rolling reduction).

FIG. 1 shows the results of a high-temperature low-cycle fatigue test at 500 ° C. and the Charpy fracture surface transition temperature. High temperature low cycle fatigue test conditions are: strain range: ± 1.5%, strain rate:
0.1% / sec. The fracture surface transition temperature in the Charpy test was determined from the temperature at which the ductile fracture ratio became 50%.

The aforementioned conventional steels such as C-Mo steel and M
In the n-Mo steel, the number of breaks in the low cycle fatigue test under the above conditions is at most about 900. Various devices have been designed and used based on this characteristic. In FIG. 1, the finish rolling temperature is approximately 900 ° C. and the number of fatigue ruptures is stable at about 900 cycles regardless of the rolling reduction in the finish rolling. When the rolling reduction temperature in the finish rolling is low, when the rolling end temperature decreases, the number of breaks gradually decreases from the reference value of 900 times. On the other hand, the rolling reduction in finish rolling is 3
When it is as high as 5 to 40%, the finish rolling temperature is higher than this reference value even at 900 ° C., and the lower the rolling finish temperature, the higher the number of breaks. Even when the rolling reduction in finish rolling is 8 to 15% or 35 to 40%, the lower the finish rolling end temperature, the more the fracture surface transition temperature in the Charpy test shifts to a lower temperature side, and the toughness is improved.

From FIG. 1, it can be seen that the amount of reduction in finish rolling is 35-4.
When it is as high as 0%, the lower the finish rolling end temperature is, the higher the high temperature and low cycle fatigue strength is.
If the temperature is lower than 0 ° C, the steel material is too hard and the rolling reaction force becomes too large. Therefore, it is necessary to perform rolling at a temperature of 700 ° C or higher. Further, when the finish rolling end temperature is higher than 850 ° C., the effect of improving the high temperature and low cycle fatigue characteristics by the finish rolling with a high rolling reduction is lost, so the upper limit of the finish rolling end temperature is set to less than 850 ° C.

FIG. 2 shows that 0.10% C-0.07% Si
-1.02% Mn-0.003% P-0.002% S-
0.27% Mo-0.023% Al-0.0041% N
4 shows the results of a high-temperature low-cycle fatigue test when steel is finish-rolled at 740 to 760 ° C. at various rolling reductions. Other conditions are the same as those in FIG. When the rolling reduction in the finish rolling is 30% or more, the number of breaks increases, and as the rolling reduction increases, the number of breaks increases. For this reason, the lower limit of the rolling reduction is set to 30% or more. On the other hand, when the rolling reduction is increased, the effect of improving the number of breaks tends to be saturated, and when the rolling is more than 75%, the rolling time is excessively long and the load on the rolling mill is increased. Is set to 75%.

Next, the reasons for limiting the components will be described. C requires at least 0.03% or more to secure strength,
Taking into account the weldability and toughness, the lower C is more advantageous. The upper limit is desirably 0.10%, but the upper limit is set to 0.12%, which does not significantly affect practically.

Si is added as a deoxidizing agent to improve toughness. However, if deoxidation is sufficiently performed by Al or the like, it adversely affects toughness, and in particular, prolongs heating such as heat history during use. Since the embrittlement has a bad effect, the lower limit that can be reduced economically is set to 0.01%. The upper limit is 0.15 as a range that does not adversely affect toughness.
%.

Mn is an element that increases the strength and toughness, and at the same time, an element that deteriorates the weldability when its amount is increased. The preferred range from the balance of strength, toughness and weldability is 0.6 to 1.35%, but the lower limit is set to 0.2% at which the effect of improving strength is somewhat remarkable, and the upper limit is set to a lower value at which the effect of improving toughness is reduced. In addition, it is set to 1.6% at which the weldability is significantly reduced.

Mo is an important component in the present invention,
It is an essential element as an element for increasing strength, particularly high-temperature strength. The role of Mo in the present invention is to stably maintain the structure obtained by direct quenching after high-temperature processing, in stress relief annealing, and in the heat history during use, and to assure high strength. is there. The effect is more remarkable as the Mo content is larger. However, an increase in the Mo content adversely affects weldability and toughness, particularly embrittlement during use, and the preferred range is 0.20 to 0.35%. The lower limit is set to 0.15% at which the strength improvement effect becomes slightly remarkable, and the upper limit is set to 0.45 of the upper limit at which the weldability and embrittlement during use do not matter.
%.

Al is an element having a strong deoxidizing effect,
As in the present invention, it is an essential element in steel in which Si is kept as low as possible, and the toughness is improved by grain refinement. However, when the amount of addition is large, inclusions are generated and the toughness is reduced. The preferred range is 0.015 to 0.035%, but the lower limit is 0.1%, where deoxidation is sufficiently performed and the toughness effect appears.
005%, and the upper limit value is not a problem of toughness.
%. As the content of N increases, the strength increases and the toughness decreases. Further, an appropriate amount of N forms Al and AlN in steel, and is effective in improving toughness through grain refinement. The preferred range of addition is 0.003 to 0.006%, but the lower limit is 0.001%, which is the lower limit of the required amount in which the toughness effect by grain refining can be expected, and the upper limit is the point of toughness decrease with increasing strength. To 0.01%.

V is one of the elements to be added as required. By directly quenching and tempering after high-temperature processing, the effect of remarkably improving the high-temperature strength can be obtained by adding a very small amount. A tendency to decrease the toughness is recognized, and a preferable range when added is 0.03 to 0.08%.
The lower limit is set to 0.02% or more at which the effect of improving strength is remarkable, and the upper limit is set to 0.12 at which toughness does not matter.
%.

Nb is also one of the elements to be added if necessary.
As in the case of V, direct quenching and tempering after high-temperature processing have the effect of significantly improving high-temperature strength with the addition of a very small amount. Further, the toughness is also improved by the effect of grain refinement. However, if the amount increases too much, the strength and toughness are rather reduced. The preferable range of addition is 0.015 to 0.030%, but the lower limit is set to 0.005% at which the strength improving effect is exhibited, and the upper limit is set to 0.04% at which the strength and toughness start to decrease.

Next, the manufacturing conditions in the present invention will be described. The heating temperature is 1000 ° C. at which each alloy element is sufficiently dissolved in austenite and good workability is obtained.
Was set to the lower limit temperature. The upper limit temperature is preferably higher from the viewpoint of solid solution, but from the viewpoint of energy saving and scale formation, a temperature at which these problems do not become noticeable is 128.
The upper limit was 0 ° C. Cooling after the finish rolling is preferably performed early after the rolling so that the effect of the rolling remains as much as possible, and the cooling is started at a temperature equal to or higher than the finishing rolling temperature -30 ° C.
Further, when the cooling was stopped at a temperature higher than 300 ° C., the transformation was sometimes not completed, and the lower limit temperature of the cooling was set to 300 ° C.

(Finishing finish temperature -30 ° C.) to 300 ° C.
The average cooling rate of is that the decrease in tensile strength is not remarkable 1
C / sec was the lower limit. The upper limit was set to 60 ° C./sec corresponding to water cooling of 10 mm thickness. The lower limit temperature of the tempering temperature was set at 600 ° C., which is the lower limit for improving the toughness, and the upper limit was set at 720 ° C. at which lowering of the tensile strength was not a problem.

When the steel obtained by the method of the present invention is processed into a structure, cold working, warm working, and welding are performed. Masashi may be implemented.

[0024]

EXAMPLES (Example 1) Rolling and heat treatment were performed under the conditions shown in Table 2 using steels having the chemical components shown in Table 1.

[0025]

[Table 1]

[0026]

[Table 2]

Table 2 shows the properties of the steel sheet.

The steel sheets 1, 3, 5, 7, 9, 11, and 13 are the steels of the present invention and have good tensile strength and toughness at room temperature and fracture in a high-temperature low-cycle test at 500 ° C. High repetition rate and excellent characteristics. On the other hand, in the steel plates 2 and 14, the rolling reduction is low, and the number of repeated fractures in the low cycle fatigue test at a high temperature is small. Since the heating temperature of the steel sheet 4 is lower than 1000 ° C., and the cooling start temperature of the steel sheet 6 is not higher than the finish rolling end temperature by 30 ° C. or more, the tensile strength (normal temperature and high temperature) and the high temperature low cycle fatigue strength are not compatible. In the steel plates 8 and 12, the finish rolling end temperature is high, and both toughness and high-temperature low-cycle fatigue strength are not compatible. In the steel sheet 10, since the cooling end temperature is higher than 300 ° C., the tensile strength and the toughness are low, and the high-temperature low-cycle fatigue strength is not good. In the steel sheet 15, the C content is low and the tensile strength cannot be obtained. Steels 16 and 17 have poor high-temperature low-cycle fatigue strength because the C content is too high.

[0029]

The steel sheet manufactured by the manufacturing method of the present invention has excellent toughness which has a significant effect on high-temperature low-cycle fatigue properties and crack propagation, which are important properties of steel materials used for the production of equipment that frequently starts and stops. ing. For this reason, in a boiler, a metal melting furnace, a heating furnace, towers, and the like made of the steel of the present invention, the high-temperature and low-cycle fatigue characteristics are good, and the toughness is good. Therefore, safe operation of these devices can be guaranteed, and the industrial value is great.

[Brief description of the drawings]

FIG. 1 is a chart showing the influence of the rolling end temperature on the number of repetitions of fracture and the transition temperature of Charpy fracture surface in a high temperature low cycle fatigue test.

FIG. 2 is a chart showing the effect of the cumulative rolling reduction from the finish rolling finish temperature + 30 ° C. on the number of repetitions of fracture in a high-temperature low-cycle fatigue test.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-52-71326 (JP, A) JP-A-4-272125 (JP, A) JP-A-4-165044 (JP, A) (58) Survey Field (Int.Cl. ⁶ , DB name) C21D 8/00-8/02

Claims (2)

(57) [Claims] C: 0.03 to 0.12% Si: 0.01 to 0.15% Mn: 0.2 to 1.6% Mo: 0.15 to 0.45% Al: 0.005 to 0.05% N: 0.001 to 0.01% The balance is unavoidable impurities and substantially 100% steel.
Heating to a temperature range of 0 to 1280 ° C., finish rolling at a finish rolling temperature of 700 ° C. or more and less than 850 ° C., and a cumulative rolling reduction from 30 ° C. higher than the finish rolling finishing temperature of 30 to 75%, After cooling from a temperature range higher than (finish rolling end temperature -30 ° C) to 300 ° C or lower at an average cooling rate of 1 to 60 ° C / sec, tempering is performed in a temperature range of 600 to 720 ° C. A method for producing a low-alloy heat-resistant steel having excellent high-temperature low-cycle fatigue characteristics and toughness. 2. The high-temperature low-cycle fatigue property and toughness according to claim 1, which contain at least one of V: 0.02 to 0.12% and Nb: 0.005 to 0.04% by weight%. Manufacturing method of excellent low alloy heat resistant steel.