JPH11350031A - Production of high cr heat resistant steel excellent in low temperature toughness and creep strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for simultaneously satisfying creep characteristics and toughness at the time of producing a high Cr heat resistant steel by normalizing or tempering treatment. SOLUTION: A slab contg., by weight, 0.03 to 0.20%, C, 0.01 to 1.0% Si, 0.10 to 2.0% Mn, 0.002 to 0.1% Al, 0.005 to 0.1% N and 8 to 13% Cr, furthermore contg. one or two kinds of 0.5 to 2.0% Mo and 0.5 to 4.0% W, and the balance Fe with inevitable impurities is heated, hot rolling in which the cumulative draft is 30 to 90% is started in the temp. region of 800 to 1250 deg.C and is finished at >=700 deg.C, and it is cooled to <=300 deg.C, is reheated at 1150 to 1300 deg.C, is cooled to the temp. region of 1000 to 700 deg.C at the cooling rate of >=1 deg.C/min, is held in the temp. region for 10 to 120 min, is thereafter cooled to <=300 deg.C at the cooling rate of 0.1 to 50 deg.C/s and is moreover tempered at 600 deg.C to less than the Acl transformation point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a martensite to martensite to martensite + δ ferrite structure having a Cr content of 8 to 13%, which has both excellent creep strength and good low temperature toughness, as a structural material for high temperature equipment. The present invention relates to a method for producing a high Cr heat resistant steel.

[0002]

2. Description of the Related Art Generally, high Cr heat-resistant steel containing about 8 to 13% of Cr is produced by normalizing or quenching at a relatively high temperature after hot rolling. Because this heat-resistant steel has a high Cr content, it can be sufficiently quenched by normalizing by standing cooling, without using quenching by accelerated cooling or water cooling, and a martensite-based structure can be formed from the steel surface to the entire central part. Easy. For this reason, it is common to manufacture by normal normalization + tempering (because the metallurgical effect is almost the same for accelerated cooling, water cooling, and standing cooling, the reheating process for controlling the structure is accelerated thereafter. Cooling, water cooling (quenching), and cooling are sometimes referred to as normalizing).

[0003] Since high Cr steel as a structural material for high-temperature equipment is required to have improved high-temperature strength and creep strength, various technical developments have been made to improve such properties. As a technique for improving high-temperature strength and creep strength, a manufacturing method in which normalizing treatment is omitted and tempering is performed as hot rolling has recently been disclosed in, for example, JP-A-5-331.
No. 544, for example. This manufacturing method that omits the normalizing process is generally called a TMCP process, and is an excellent technique that can simultaneously improve creep characteristics and toughness by devising hot rolling and cooling conditions after rolling. However, in this TMCP process, since the effect of rolling is used, the load of rolling must be increased, and there is a limit in the production of thick materials. In addition, there is a problem that the effect of rolling is lost when hot working is performed, and a manufacturing method by normalizing is often required depending on the application.

With respect to the improvement in creep strength in the case of normalizing and tempering, if the normalizing temperature is increased, C
The precipitates such as r, Mo, W, Nb, V, etc., and the carbonitride constituent elements increase in the amount of solid solution in the normalizing stage and increase in the amount of precipitation strengthening in the tempering stage. Is effective for improving the high-temperature strength of steel. However, when exposed to a high temperature for a long time, such as the creep characteristics, the precipitates generated by tempering are unstable because they are fine, and coarsening of the precipitates occurs on the way, which is not necessarily effective in improving the creep strength. . This is a problem common to steels manufactured by the TMCP process. Furthermore, fine precipitates also have an adverse effect on low-temperature toughness. That is, it was very difficult to achieve both high-temperature strength and creep characteristics, or high-temperature strength, creep strength, and toughness by conventional manufacturing methods.

Furthermore, high C containing about 8 to 13% of Cr
r In heat-resistant steels, if the normalizing temperature is excessively increased, the δ ferrite phase appears in equilibrium, so the creep strength is not always improved, and the coarse δ ferrite phase significantly reduces toughness. Conventionally, the normalizing temperature has been limited to a narrow temperature range of about 1000 ° C. to 1100 ° C. For example, Japanese Patent Application Laid-Open No. 6-1
As disclosed in Japanese Patent No. 28640, there is disclosed a technique in which the normalizing temperature is limited to a low temperature of 850 ° C. to 980 ° C. for the production of a thin material. You are sacrificing some.

When the amount of Cr is increased for oxidation resistance and the like, δ ferrite is more likely to be formed. In this case, an expensive austenite stabilizing element such as Ni or Co is added. However, such measures may inevitably increase the manufacturing cost and deteriorate other characteristics.

As described above, according to the conventional technique, it was not possible to increase the normalizing temperature and simultaneously secure the creep strength and the toughness.

[0008]

As described above, in the prior art, it is difficult to achieve both high temperature strength, creep strength, and toughness in reheating quenching / tempering or normalizing / tempering. was there. INDUSTRIAL APPLICABILITY The present invention is to produce a high Cr heat resistant steel having a Cr content of 8 to 13% and having a martensite to martensite + δ ferrite structure, which can achieve both excellent high-temperature strength, creep strength and toughness by effectively utilizing additive elements. It provides a method.

[0009]

Means for Solving the Problems The present inventors mainly contain 8 to 13% of Cr from the viewpoint of oxidation resistance,
Furthermore, from the viewpoint of high-temperature strength and creep strength, M
A production method for simultaneously improving the high temperature strength, creep strength and toughness of a high Cr heat resistant steel having a martensite to martensite + δ ferrite structure containing one or both of o and W, and a stable precipitation even under creep conditions. The present invention was studied from the viewpoints of fine dispersion of the product and suppression of formation of δ ferrite, and the present invention was reached.

[0010] The requirement of the present invention is that when performing quenching or normalizing, first, high-temperature heating for solutionizing the element is performed, and then the material is further kept in an appropriate temperature range lower than the heating temperature. By cooling the temperature range at an appropriate rate, it is possible to suppress δ ferrite which adversely affects toughness and creep strength, and to achieve stable fine dispersion of precipitates even during creep effective for improving creep strength. . The summary is as follows.

(1) In weight%, C: 0.03 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.10 to 2.0%, Al: 0.002 to 0% 0.1%, N: 0.005 to 0.1%, Cr: 8 to 13%, Mo: 0.5 to 2.0%, W: 0.5 to 4.0% A steel slab containing one or two types, the balance being Fe and unavoidable impurities, is heated to 1000 ° C. to 1300 ° C., and hot-rolled at a cumulative reduction ratio of 30% or more and 90% or less to 800%.
Starting in a temperature range of not less than 1250 ° C. and not more than 1250 ° C., ending the hot rolling at a temperature of not less than 700 ° C., and after cooling to not more than 300 ° C., reheating to a temperature of not less than 1150 ° C. and not more than 1300 ° C., 1000 ° C at a cooling rate of 1 ° C / min or more
Cool to a temperature range of 700 ° C., and in this temperature range for 10 minutes to 120
After holding for 1 minute, 0.1 ° C / s to 50 ° C up to 300 ° C or less
/ S at a cooling rate of 600 ° C. or more and Ac
A method for producing a high Cr heat resistant steel having excellent low temperature toughness and creep strength, characterized by tempering at a temperature lower than one transformation point.

(2) In weight%, C: 0.03 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.10 to 2.0%, Al: 0.002 to 0 0.1%, N: 0.005 to 0.1%, Cr: 8 to 13%, Mo: 0.5 to 2.0%, W: 0.5 to 4.0% A steel slab containing one or two types, the balance being Fe and unavoidable impurities, is heated to 1000 ° C. to 1300 ° C., and hot-rolled at a cumulative reduction ratio of 30% or more and 90% or less to 800%.
Starting in a temperature range of not less than 1250 ° C. and not more than 1250 ° C., ending the hot rolling at a temperature of not less than 700 ° C., and after cooling to not more than 300 ° C., reheating to a temperature of not less than 1150 ° C. and not more than 1300 ° C., 900 ° C to 8 at a cooling rate of 1 ° C / min or more
Cool to a temperature range of 50 ° C.
After cooling at a cooling rate of 0.1 ° C./min to 2 ° C./min to −700 ° C., 0.1 ° C./s to 50 ° C./s until 300 ° C. or less
A method for producing a high-Cr heat-resistant steel having excellent low-temperature toughness and creep strength, characterized by cooling at a cooling rate of 600 ° C. and tempering at a temperature of 600 ° C. or higher and lower than the Ac1 transformation point.

(3) The steel slab is further expressed by weight%: V: 0.05 to 0.50%, Nb: 0.01 to 0.20%, Ta: 0.02 to 0.40%, Ti: The low-temperature toughness and creep strength according to (1) or (2), wherein one or two or more of 0.005 to 0.10% and Zr: 0.005 to 0.10% are contained. Method for producing high Cr heat resistant steel with excellent heat resistance.

(4) The steel slab further contains Ni: 0.05-3.0%, Cu: 0.05-1.5%, Co: 0.05-5.0%, B: The high Cr heat resistant steel according to (1), (2) or (3), which is excellent in low-temperature toughness and creep strength, which contains 0.0005 to 0.01% of one or two kinds. Production method.

(5) The steel slab is further expressed by weight%: Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, REM: 0.005 to 0.10% The method for producing a high-Cr heat-resistant steel excellent in low-temperature toughness and creep strength according to the above (1), (2), (3) or (4), further comprising one or more kinds.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. An object of the present invention is to produce fine precipitates mainly composed of carbonitride, which contribute to improvement in high-temperature strength and creep strength, at a high temperature for a long time when manufactured by normalizing and tempering treatment (including quenching and tempering). In order to obtain a stable material that does not coarsen even during creeping of time, and to make the most of the precipitates effectively, even under high-temperature reheating conditions where sufficient reheating solution is obtained, high-temperature strength and creep An object of the present invention is to provide a production method capable of suppressing the formation of δ ferrite, which has a significant adverse effect on strength, particularly on toughness. That is,
It is an object of the present invention to provide manufacturing conditions that provide the best balance between creep strength and toughness without changing the chemical composition or without unnecessarily increasing the amount of expensive alloying elements.

One of the most effective metallurgical means for increasing high-temperature strength, particularly creep strength, is precipitation strengthening and dispersion strengthening by precipitates. In order to exhibit the effect effectively, it is necessary to finely disperse the precipitate as high as possible on the premise that the precipitate is stable even during creep.
Usually, precipitation in the ferrite temperature region of the cooling or tempering step of normalization can disperse fine precipitates more densely than precipitation in austenite, so precipitates in austenite, that is, in the heating step of normalization, Is thought to be preferable for strengthening by precipitates. However, the present inventors have found that strengthening by such means is not always preferable in the following two points.

That is, as a first point, 500 ° C. to Ac
If it is held for a long time in the temperature range of about 1 transformation point or less, fine precipitates precipitated during the cooling or tempering step of normalizing are not always stable, and coarsen during holding, reducing the effectiveness for creep characteristics. Second, in a high Cr steel having a relatively high content of ferrite stabilizing elements such as Cr, Mo, W, etc., the normalizing temperature is simply increased for solution of precipitates. In some cases, δ ferrite is generated, and δ ferrite deteriorates both strength, creep characteristics, and toughness.Therefore, raising the normalizing temperature with steel having such a composition cannot be adopted as a means of improving characteristics. found.

The inventors of the present invention make the most of the use of the strengthening by the precipitates to save the amount of expensive alloying elements, minimize the restrictions on components, and stably secure the creep characteristics. Based on the belief that
High-Cr heat-resistant steel that solves the above-mentioned problems in high-temperature heating and achieves excellent creep characteristics and toughness at the same time on the premise that the normalizing temperature for solution of the precipitate-forming element is increased. As a result of exploring the manufacturing technology of, δ which is harmful to the high density and fine dispersion of stable precipitates, creep characteristics and toughness
The present inventors have found a completely new metallurgical means capable of simultaneously suppressing the formation of ferrite, and have reached the present invention. Hereinafter, the basic requirements of the metallurgical means will be described first.

Usually, in order to precipitate precipitates in austenite with steel of one composition, the heating temperature for normalizing is lowered. In this case, however, since the temperature is lower than the solution heat-up temperature, the normalizing heating temperature is lowered. Since the precipitates and other inclusions, grain boundaries, etc. that have already been deposited are gradually deposited as precipitation sites during heating and holding, the number density of ferrite in the cooling and tempering stages of normalization is high. It is much smaller than in the case of precipitation in the temperature range, and since most of the precipitates are precipitated at this stage, the amount of fine precipitation in subsequent tempering is also small, so the precipitation strengthening amount and dispersion strengthening amount are also small. However, it is difficult to say that the precipitate is effectively used.

On the other hand, in the case where the normalizing temperature is equal to or higher than the complete solution solution temperature, the solution is left to be cooled or water-cooled and then tempered. In some cases, some may precipitate during cooling, but most will precipitate during the tempering stage. In this case, since the precipitate is dissolved in a supersaturated state in a ferrite temperature region at a relatively low temperature, the dispersion of the precipitate becomes very fine and high density. However, the precipitates in this case are very fine and because of some of their morphology and composition,
If it is kept for a long time in the temperature range of the c1 transformation point, coarsening starts to occur, and the composition, the structure, and the consistency with the matrix also change. Therefore, the effectiveness as an obstacle against dislocation movement is increased. Reduce. Therefore, although it is effective for improving short-time strength as in a normal tensile test, the creep strength cannot be improved as much as the increase in short-time strength, and the effect is limited.

In order to effectively utilize the precipitates to improve the creep properties as much as possible, it is necessary to raise the normalizing heating temperature to completely precipitate the elements forming the precipitates and then to devise the subsequent process. Therefore, it is necessary to precipitate fine and high density in austenite. As a method for this purpose, the present inventors have found a method in which after heating, during cooling, a temperature lower than the heating temperature is maintained at a temperature at which a precipitate-forming element can be deposited, or the temperature range is cooled at an appropriate rate. . The composition and morphology of the precipitates finely dispersed in austenite at a high density by the method of the present invention are not uniform, but the most typical ones precipitate in austenite, and thus have a spherical or horny carbon. It is a nitride. Further, in the tempering step, the same precipitate or another kind of carbonitride is precipitated in a complex form by the element which was not yet dissolved in the normalizing step, using the stable precipitate formed in the austenite as a nucleus. Is strengthened.

Further, as a result of investigating and examining the structural change of the process in detail, it was found that the process not only controls the dispersion of precipitates but also makes it possible to form δ ferrite by simple high-temperature normalization. It has been found that this is an extremely excellent means that can suppress or reduce the formation thereof, is effective in improving the creep strength, and can simultaneously improve the creep strength and toughness.

The above is the principle requirement of the present invention, and the present inventors have invented the following two specific means based on the principle. That is, the first means is that the steel slab having a chemical composition range satisfying the present invention is 1000 ° C.
~ 1300 ° C and the cumulative draft is 30% or more and 9
Hot rolling of 0% or less is started in a temperature range of 800 ° C. or more and 1250 ° C. or less, finished at a temperature of 700 ° C. or more, and then cooled to 300 ° C. or less.
Reheat to a temperature of 00 ° C. or less, cool to a temperature range of 1000 ° C. to 700 ° C. at a cooling rate of 1 ° C./min or more, hold at this temperature range for 10 minutes to 120 minutes, and then reduce the temperature to 300 ° C. or less. 1
At a cooling rate of 50 ° C./s to 50 ° C./s.
Tempering at a temperature of 0 ° C. or higher and lower than the Ac1 transformation point ”. Further, the second means is a method for producing a steel slab having a chemical composition range satisfying the present invention in a range of 1000 ° C. to 1300 ° C.
The hot rolling with a cumulative draft of 30% or more and 90% or less is started in a temperature range of 800 ° C or more and 1250 ° C or less, finished at a temperature of 700 ° C or more, and then cooled to 300 ° C or less. Then, reheat to a temperature of 1150 ° C or more and 1300 ° C or less, and at a cooling rate of 1 ° C / min or more, 900 ° C or more.
It is cooled to a temperature range of 850 ° C., and 800 ° C. to 7
After cooling at a cooling rate of 0.1 ° C / min to 2 ° C / min to 00 ° C, cooling at a cooling rate of 0.1 ° C / s to 50 ° C / s to 300 ° C or less, Tempering at a temperature lower than the Ac1 transformation point ".

The present invention is based on normalizing (quenching) and tempering treatments as a means of exhibiting its characteristics. Before the treatment, however, the steel slab is subjected to hot rolling in order to form the shape of the steel material. . The conditions are the same for both the first means and the second means. The slab is heated to 1000 ° C. to 1300 ° C., and hot rolling with a cumulative draft of 30% to 90% is performed.
Starting at a temperature range of not less than 1250 ° C and not more than 1250 ° C,
The process is completed at the above temperature, and then cooled to 300 ° C. or less.

The heating temperature of the slab is 1000 ° C. to 1300 ° C.
The reason for limiting the heating temperature to less than 1000 ° C. is that the coarse solidification structure is insufficiently eliminated and the toughness of the final steel material is deteriorated, so that the present invention is limited to the range. In steels containing a large amount of precipitate forming elements such as Cr, Mo, W, etc., the solution becomes insufficient, and the high-density and fine dispersion of precipitates effective for strengthening in subsequent normalizing and tempering treatments are shown. 1300 ° C
In the case of super, the structure after rolling is coarsened, which also leads to deterioration of toughness, and the amount of δ ferrite generated at the time of heating increases and becomes coarse. This is because it is difficult to eliminate ferrite, and there is also a problem in securing toughness in that respect.

After heating the steel slab at 1000 ° C. to 1300 ° C., the shape of the steel material is adjusted by hot rolling. In the rolling, hot rolling with a cumulative rolling reduction of 30% or more and 90% or less is performed at 800 ° C. or more and 1250 ° C. It is necessary to start at a temperature range of not higher than 700C and end at a temperature of not lower than 700C. If the cumulative draft is less than 30%, the rolling effect is not sufficient, and the solidification structure is not sufficiently eliminated or the pressure bonding of defects such as porosity is insufficient. Therefore, the cumulative draft must be 30% or more. The larger the cumulative rolling reduction, the better the material. However, if the cumulative rolling reduction exceeds 90%, the effect is saturated, the load on the rolling mill increases, and the thickness of the steel material after rolling is limited. The cumulative rolling reduction is limited to 90% or less.

However, in order to sufficiently exhibit the effect of rolling, not only the cumulative rolling reduction should be limited to this range, but also the rolling temperature range should be optimized. In the present invention, based on the experimental results, hot rolling with a cumulative draft of 30% or more and 90% or less is started in a temperature range of 800 ° C or more and 1250 ° C or less, and is ended at a temperature of 700 ° C or more. .

If the rolling start temperature is lower than 800 ° C., there is a concern that the deformation resistance of the steel material becomes excessive and an excessive load is applied to the rolling mill. The possibility of producing a coarse tissue that is unfavorable in properties is increased. On the other hand, when the temperature exceeds 1250 ° C., the possibility that grain refinement by recrystallization of austenite by rolling does not work sufficiently increases. For the above reasons, in the present invention,
The rolling start temperature is limited to the range of 800C to 1250C.

It is necessary to limit the rolling start temperature as described above and further limit the ending temperature. That is, it is necessary to prevent the rolling end temperature from being lower than 700 ° C. However, this is similar to the case where the starting temperature is limited to 800 ° C or higher. The reason is that there is a concern that the rolling mill becomes excessively large and an excessive load is applied to the rolling mill, and that there is a high possibility that a transformation occurs during rolling depending on the chemical composition to produce a coarse structure which is not preferable in mechanical properties.

After heating the slab at 1000 ° C. to 1300 ° C., hot rolling with a cumulative draft of 30% or more and 90% or less is started in a temperature range of 800 ° C. or more and 1250 ° C. or less,
By ending at a temperature of 700 ° C. or more, it is possible to eliminate undesirable defects and solidification structure in the material, and to apply an excessive load to the rolling mill or impair the rolling efficiency,
Sufficient solution formation is achieved so that the effect of precipitates in the subsequent heat treatment can be used most efficiently, and the austenite grain size is fine as a martensite-based structure, which is preferable for improving the quality of the material after normalization and tempering. It is possible to achieve the structure of the rolled material.

Cooling after rolling may be either cooling or accelerated cooling as long as the steel has a chemical composition within the range of the present invention.
Taking into account the adverse effect on precipitation strengthening during normalization and tempering, slow cooling in which precipitates become extremely coarse during cooling after rolling should be avoided. Specifically, the average cooling rate from the end of rolling to 400 ° C is 5 ° C /
There is no problem if it is more than minutes.

Incidentally, the steel slab in the present invention means a steel material having a relatively large thickness before being subjected to the above-mentioned hot rolling, and includes an ingot, a continuous cast slab, a billet, and a solidified molten steel as it is. Bloom and the like, and those obtained by further heat treating the ingot, slab, and the like for segregation diffusion and structure control, and those prepared by hot rolling beforehand are also included.

The above is the reason for the limitation on the hot rolling conditions before the heat treatment. Next, normalization or quenching, which is the most important requirement for the production method of the present invention, will be described. As for normalizing or quenching, two means are presented, but they will be individually described.

First, in the first means, the rolled material produced according to the rolling method of the present invention is heated to 1150 ° C.
Reheat to a temperature of 300 ° C. or less, cool at a cooling rate of 1 ° C./min or more to a temperature range of 1000 ° C. to 700 ° C., hold at that temperature range for 10 minutes to 120 minutes, and then reduce the temperature to 300 ° C. or less.
It is characterized by cooling at a cooling rate of 1 ° C / s to 50 ° C / s.

When the reheating temperature is 1150 ° C. or more and 1300
The reason why the temperature is limited to not more than 1 ° C. is that the first purpose is to form a solution of the precipitate forming element, and if the reheating temperature is less than 1150 ° C., Mo, W, Nb, Ta, Ti,
Since the solution formation of precipitate forming elements such as Zr becomes insufficient,
Not preferred. A higher reheating temperature is naturally preferable for solution, but if the reheating temperature is too high, the austenite grain size becomes extremely coarse, or the oxidation of the surface becomes remarkable, leading to deterioration of the surface properties of the steel material. However, even when other conditions are within the range of the present invention, problems such as the inability to sufficiently suppress coarse δ ferrite become apparent. In the present invention, the reheating temperature is in the range of 1150 ° C. to 1300 ° C. because these problems do not occur and the solution can be solution-treated. In the case of processing in a normal normalizing or quenching step which is not in accordance with the present invention, in a high Cr steel of about 8 to 13%, it is necessary to suppress a coarse δ ferrite which is not particularly preferable in toughness. Generally, the reheating temperature is at most 1100 ° C. or lower, and reheating at such a high temperature is possible only under the conditions specific to the present invention after reheating described later.

Under conditions in which the reheating temperature for normalizing or quenching is 1150 ° C. or more and 1300 ° C. or less, a precipitate stable under creep conditions, which is an object of the present invention, is deposited at an extremely high density. In addition, in order to minimize δ ferrite generated by high-temperature reheating, after reheating, at a cooling rate of 1 ° C./min or more, 1000
C. to a temperature range of 700 ° C. to 10 minutes to 1 minute in the temperature range.
Hold for 20 minutes.

By maintaining the temperature in a temperature range of 1000 ° C. to 700 ° C. for 10 minutes to 120 minutes after reheating, stable precipitates contributing to an improvement in creep strength are dispersed at a high density.
And since the temperature range becomes an austenite stable range, δ ferrite generated by high-temperature reheating is transformed again to austenite, and the ratio of δ ferrite in the final structure is reduced to a level that does not lead to deterioration of mechanical properties. But,
If the cooling from the reheating temperature to the temperature range is excessively slow, it is not effective for creep strength during cooling, and coarse precipitates that adversely affect toughness may be precipitated.
The cooling rate until reaching the temperature range of 1000 ° C. to 700 ° C. needs to be 1 ° C./min or more.

The reason why the holding condition is limited to 1000 to 700 ° C. for 10 to 120 minutes is that if the holding temperature is higher than 1000 ° C., the amount of precipitates is small, and the amount of δ ferrite remaining in equilibrium is also small. When the temperature is lower than 700 ° C., on the other hand, if the temperature is lower than 700 ° C., transformation may occur depending on the holding time, which may cause a decrease in strength. , The carbides mainly of M23C6 type and the Laves phase precipitate coarsely and the toughness is impaired. Therefore, in the present invention, the holding temperature in the holding is 1000 ° C to 700 ° C.
Limited to. Also, the holding time at the holding temperature should be limited, and if it is less than 10 minutes, a sufficient amount of precipitate cannot be deposited,
On the other hand, if it is more than 120 minutes, the growth of precipitates occurs and the number density is reduced, and holding for an unnecessarily long time may hinder productivity. Limited to ~ 120 minutes.

For the above reasons, in order to optimize the precipitation in austenite, the temperature is set to 1150 ° C. or more and 1300 ° C.
After reheating at a temperature of 1 ° C or less, 1000
C. to a temperature range of 700 ° C. to 10 minutes to 1 minute in the temperature range.
Hold for 20 minutes, but 30 minutes to make the transformed structure a martensite-based structure, which is preferable for securing creep strength and toughness.
It is necessary to cool to 0 ° C. or lower at a cooling rate of 0.1 ° C./s to 50 ° C./s. That is, if the steel having a chemical composition within the range of the present invention is sufficiently cooled to a temperature of 300 ° C. or less at which the martensitic transformation is almost completed, the steel can have a sufficient martensite-based structure. There is
If the rate is less than 0.1 ° C./s, coarse bainite which adversely affects toughness and strength may be generated in some cases. Therefore, in order to surely suppress the bainite transformation and cause martensitic transformation, 0% is required. A cooling rate of at least 1 ° C / s should be ensured.

However, rapid cooling at a cooling rate exceeding 50 ° C./s has little effect on the structure control, and rather should be avoided because there is a concern that the residual stress increases or the steel sheet shape deteriorates. . Accordingly, the thickness of the steel material is not extremely large, and the cooling rate in the cooling is 0.1 ° C./s.
In the case described above, it is sufficient to allow cooling (normalization). If the lower limit cooling rate cannot be satisfied by cooling the thick material, quenching treatment by accelerated cooling such as water cooling is performed. It is necessary to achieve the cooling rate specified in the above.

The second means relating to normalization or quenching is to reheat a rolled material produced according to the rolling method of the present invention to a temperature of 1150 ° C. or more and 1300 ° C. or less, and a cooling rate of 1 ° C./min or more. After cooling to a temperature range of 900 ° C. to 850 ° C. from the temperature range to 800 ° C. to 700 ° C. at a cooling rate of 0.1 ° C./min to 2 ° C./min.
The gist of the invention is that cooling is performed at a cooling rate of 0.1 ° C / s to 50 ° C / s to 0 ° C or less.

As means for achieving the optimum precipitate size, morphology and distribution for creep strength and for suppressing δ ferrite, the first means is more reliable, but this method is a kind of two-step method. Since the heat treatment is performed, there is a possibility that various restrictions may be caused by the capacity of the furnace when industrially employed. Therefore, the second means is invented as a method for controlling precipitates and suppressing δ ferrite, which is an object of the present invention, by only devising a cooling pattern without using a two-step heat treatment. .

The reheating temperature is 1150 ° C. or more and 1300 ° C.
The reason why the temperature is limited to below ℃ is exactly the same as the first means. After reheating, it is cooled at a rate of 1 ° C./min or more to a temperature range of 900 ° C. to 850 ° C. This is because the cooling before holding for controlling precipitates and δ ferrite is performed in the first means. If the cooling from the reheating temperature to the temperature range is less than 1 ° C./min, as in the case of limiting to 1 ° C./min or more, coarse precipitates are not effective for creep strength during cooling and adversely affect toughness. Is likely to precipitate.

After cooling at a rate of 1 ° C./min or more to a temperature range of 900 ° C. to 850 ° C.,
It is necessary to cool to 00 ° C at a cooling rate of 0.1 ° C / min to 2 ° C / min. This is a condition corresponding to the control of the precipitates in the first means and the holding for suppressing the δ ferrite, and the controlled cooling at a cooling rate of 0.1 ° C./min. To 2 ° C./min. 800 ° C. starting from 800 ° C. to 850 ° C.
C. to 700.degree. C. 0.1 ° C /
Control cooling at a cooling rate of 900 to 850 min / min.
By starting from 800 ° C. and ending at 800 ° C. to 700 ° C., the optimum precipitate size, morphology and distribution for creep strength and suppression of δ ferrite are achieved, but the cooling rate in this temperature range is 0 °. If the temperature is less than 1 ° C./min, the precipitates are coarsened and the dispersion density is reduced, so that the effect of improving the creep strength does not occur. This is not preferable because the ratio of stable precipitates decreases during creep and the suppression of δ ferrite becomes insufficient.

The temperature range for cooling at a cooling rate of 0.1 ° C./min to 2 ° C./min is 900 ° C. to 850 ° C. as the starting temperature.
° C and the end temperature must be 800 ° C to 700 ° C. If the starting temperature is higher than 900 ° C., the precipitates become coarse.
Since the temperature range for cooling at a cooling rate of 2 ° C./min to 2 ° C./min becomes narrow, precipitation may not be sufficiently generated. Therefore,
The starting temperature range is limited to 900C to 850C. On the other hand, when the end temperature is higher than 800 ° C., the temperature is also 0.1 ° C./min.
Precipitation may not be sufficient because the temperature range for cooling at a cooling rate of ° C./min is too small. If the temperature is lower than 700 ° C., the cooling rate is slow and transformation occurs at a high temperature, resulting in a decrease in strength. In addition, although the amount of precipitates contributing to creep strength increases, carbides mainly composed of M23C6 type and Laves phase precipitate coarsely and the toughness is impaired. Therefore, in the present invention, 0.1 ° C./min.
The ending temperature of cooling at a cooling rate of 2 ° C./min is 800 ° C. to 7
Limit to 00 ° C.

The rolled material manufactured according to the rolling method of the present invention was used at a temperature of 1150 ° C. and 13 ° C. for the above-mentioned reason.
Reheat to a temperature of 00 ° C. or less, cool to a temperature range of 900 ° C. to 850 ° C. at a cooling rate of 1 ° C./min or more, and 0.1 ° C./min to 2 ° C. from the temperature range to 800 ° C. to 700 ° C. After cooling at a cooling rate of / min, cooling is performed at a cooling rate of 0.1 ° C / s to 50 ° C / s to 300 ° C or less for the same reason as the first means.

In the present invention, a heat treatment is performed between the hot rolling step and the normalizing or quenching treatment for the purpose of preventing hydrogen cracking and the like as long as the effects of the present invention are not impaired. It doesn't matter. In that case, in order not to impair the effects of the present invention, it is necessary to set the heating temperature of the intermediate heat treatment so as not to exceed the reheating temperature of the subsequent normalizing or quenching. If you observe the conditions of
Other conditions are not limited.

By performing the normalizing or quenching treatment by the first means or the second means as described above, the present invention has both excellent creep strength and good low-temperature toughness. It is possible to achieve the basic organizational requirements for:
%, In the case of a high Cr heat resistant steel having a martensite to martensite + δ ferrite structure, the internal stress due to marlensite transformation, solid solution C, excess Although the strength is very high due to a high dislocation density and the like, the toughness is remarkably inferior to that extent. Therefore, in order to adjust the strength and secure the toughness, both the first means and the second means have a temperature of 600 ° C. As described above, tempering at a temperature lower than the Ac1 transformation point is essential. This is because if the tempering temperature is lower than 600 ° C., the effect of the tempering is not sufficient, so that the recovery of toughness is not sufficient. Is transformed again into high C martensite, which is very poor in toughness, so that both toughness and creep strength are deteriorated.

It is not necessary to limit the cooling and holding conditions of the tempering. However, in order to sufficiently exert the effect, the tempering holding time at a temperature of 600 ° C. or more and less than the Ac1 transformation point is 30 minutes or more. Is preferred. The tempering treatment does not need to be performed once. For example, repeating a heat treatment for the purpose of preventing hydrogen cracking after quenching, for example, requires a heating temperature of 600 ° C. or more and less than the Ac1 transformation point. It does not matter if it is included. That is, as long as the heating temperature is within the range of the present invention, the heat treatment corresponding to the tempering with different heating temperatures may be repeated any number of times.

The above are the reasons for limiting the requirements relating to the production method of the present invention. The creep strength and the excellent creep strength of a high Cr heat resistant steel having a Cr content of 8 to 13% and having a martensite to martensite + δ ferrite structure are described. In order to achieve good low-temperature toughness at the same time, not only the manufacturing method,
The chemical composition also needs to be limited for the following reasons.

C forms carbides as a solid solution strengthening element and improves the high temperature creep strength. In addition, the formation of δ ferrite is suppressed and the toughness is improved. According to the method of the present invention, the formation of δ ferrite can be suppressed despite high-temperature heat baking, but C is required to be 0.03% or more to suppress δ ferrite according to the present invention. On the other hand, 0.
If it exceeds 20%, while the effect of suppressing δ ferrite is saturated, the toughness deteriorates due to the adverse effect of C itself, and the weldability also deteriorates. Therefore, the content is limited to 0.03% to 0.20%.
In particular, when paying attention to ensuring weldability and toughness, the upper limit of C is set to 0.
It is preferably 15%.

Si is required as a deoxidizing element, and is required to be 0.01% or more in order to secure the soundness of steel.
On the other hand, if it exceeds 1.0%, the toughness is reduced.
% To 1.0%.

Mn must be added as a deoxidizing agent in an amount of 0.10% or more. On the other hand, if it exceeds 2.0%, Mn segregation becomes remarkable and the toughness is reduced.
Limited to.

Al is an effective element as a deoxidizing element, and is also effective from the viewpoints of tissue stability and suppression of void swelling. If less than 0.002%, the effect is not clear,
If it exceeds 0.1%, coarse oxides are formed and the toughness is deteriorated. Therefore, the content is limited to the range of 0.002 to 0.1%.

N suppresses the formation of δ ferrite and increases toughness, and forms fine precipitates such as TaN and VN to increase the high temperature creep strength. According to the production method of the present invention, since a large amount of stable precipitates can be dispersed during creep,
Although the effect is exhibited even when the content is lower than that of the normal normalizing treatment, it is still required to be 0.005% or more. On the other hand, if the addition exceeds 0.1%, castability and toughness are reduced, so the content is limited to 0.005 to 0.1%.

Cr is one of the properties that the heat-resistant steel should have.
It is an essential element for improving corrosion resistance and oxidation resistance at high temperature, which is the most important, as well as high temperature strength characteristics. The Cr content is preferably as large as possible in order to improve the corrosion resistance and oxidation resistance at high temperatures, but in order to secure the strength and toughness by forming a martensite-based structure, the Cr content must be 8 to 13%. is there. If the Cr content is less than 8%, the oxidation resistance is not sufficient, the structure is unstable, and there is a possibility that remarkable deterioration in toughness may occur due to heat treatment. On the other hand, excessive addition generates δ ferrite and lowers toughness.
Limited to 13%.

Mo and W are the most effective elements for improving the high-temperature strength and the creep strength, and are elements having almost the same effects. The Mo content is preferably 0.5% to 2.0%, and the W content is preferably 0.5% to 4.0%. If Mo is added in an amount of less than 0.5%, the effect of improving high-temperature strength and creep strength is not exhibited, and if it is more than 2.0%, coarse carbides and intermetallic compounds are formed and the toughness is remarkably deteriorated. Absent. As for W, the high-temperature creep strength is remarkably improved as in the case of Mo.
If the amount is less than 4.0%, the effect is not clear. Conversely, if the added amount exceeds 4.0% and becomes excessive, coarse carbides and intermetallic compounds are generated, and the toughness is remarkably reduced, so that the content is limited to 0.5 to 4.0%. I do. Since Mo and W have almost the same qualitative effect and are additive, it is possible to exert an effect even if either one of Mo and W is added. .

The above is the reason for limiting the basic components of the present invention. For the purpose of improving creep strength and toughness, one or more of V, Nb, Ta, Ti, and Zr may be contained as necessary. Can be done.

V enhances high temperature creep strength by solid solution strengthening and precipitation strengthening. The effect is remarkable at 0.05% or more. However, the addition exceeding 0.50% causes a decrease in toughness due to the formation of δ ferrite and lowers the weldability. Therefore, the addition is limited to 0.05 to 0.50%. .

Nb improves high-temperature creep strength mainly by precipitation strengthening. In addition, it effectively works to reduce the heated γ particle size and improves the base material toughness. For these, 0.
01% or more is required. On the other hand, if it exceeds 0.20%, the high-temperature creep strength is conversely reduced and the weldability is reduced, so the content is limited to 0.01 to 0.20%.

[0062] Ta also improves the high-temperature creep strength by precipitation strengthening, effectively works to reduce the heated γ grain size, and improves the base metal toughness. For these, 0.02% or more is required. On the other hand, if it exceeds 0.40%, the high-temperature creep strength is reduced, and the weldability is reduced.
Limited to 0.40%.

Ti also improves the high-temperature creep strength by precipitation strengthening, but is particularly effective in improving the base metal toughness because the heated γ particle size can be significantly reduced. For these, 0.005% or more is required. On the other hand, if it exceeds 0.10%, coarse oxides or carbonitrides are formed and the toughness is deteriorated, so the content is limited to 0.005 to 0.10%.

Zr has almost the same effect as Ti, but in order to exhibit its effect, it needs to be 0.005% or more. If it exceeds 0.10%, coarse oxides and precipitates are formed. Therefore, the content is limited to 0.005 to 0.10%.

Further, if necessary, Ni, Cu, Co,
One or more of B can be added. Ni improves toughness by solid solution toughening, and also improves strength and toughness due to the stable formation of a martensite structure and the effect of suppressing the formation of δ ferrite. 0.05% or more is required to achieve the effect,
If the content exceeds 3.0%, the creep strength tends to decrease, so the content is limited to the range of 0.05 to 3.0%.

Cu also has a qualitatively similar effect to Ni qualitatively, and for that purpose 0.05% or more must be added. On the other hand, if it exceeds 1.5%, a problem such as hot cracking of the steel slab occurs, so in the present invention, the upper limit is made 1.5%.

Co also has an effect similar to Ni, and contributes to improvement in toughness and creep strength through suppression of δ ferrite. For that purpose, it is necessary to contain 0.05% or more.
On the other hand, if it exceeds 5%, the effect is saturated, the hardenability decreases, the martensitic phase becomes unstable, and the strength and toughness may be deteriorated. C
When o is added, the content is limited to the range of 0.05 to 5.0%.

B is an element capable of increasing the hardenability of steel by segregating at the grain boundaries even if it is contained in a very small amount. If necessary, it is necessary to improve the strength and toughness by controlling the transformation structure. Can be added. However, if the content is less than 0.0005%, a sufficient amount of solid solution cannot be secured, and the effect of improving hardenability is not clear. Conversely, if the content exceeds 0.01%, a coarse compound is formed and the structure control effect is lost. At the same time, the compound itself becomes a starting point of fracture and significantly impairs toughness.
Limit to 1% range.

Further, one or more of Mg, Ca and REM can be added as necessary to improve the toughness, particularly the toughness of the welded joint.

Mg and Ca have almost the same action,
By forming fine oxides and sulfides to make the austenitic grain size of the heat-affected zone fine and fixing oxygen and sulfur, the weldability and the toughness of the welded joint can be improved. In order to exhibit the effect, 0.0005% or more is required in all cases. If it exceeds 0.01%, the oxides and sulfides become coarse, and conversely, the toughness is deteriorated. 0
Limited to 1%.

Although the qualitative effect of REM is almost the same as that of Mg and Ca, the effect is weaker than that of Mg and Ca.
It is necessary to contain 0.005% or more. On the other hand, the upper limit for preventing formation of coarse inclusions that adversely affect toughness is 0.10%.

[0072]

EXAMPLES The effects of the present invention were confirmed using steels having the chemical compositions shown in Table 1. In Table 1, steel numbers 1 to 13 have the chemical composition of the present invention, and steel numbers 14 to 18 are out of the chemical composition range of the present invention as comparative examples.

[0073]

[Table 1]

Tables 2, 3, and 4 show the production conditions and the mechanical properties of the produced steel sheet. Mechanical properties include strength (0.2% at room temperature and 600 ° C, proof stress and tensile strength (TS)), creep properties (650 ° C, 100MP)
a), and toughness (50% of Charpy impact test, fracture surface transition temperature (vTrs)).
All the test pieces were sampled from the center of the thickness of the steel sheet in the direction (C direction) perpendicular to the rolling direction.

[0075]

[Table 2]

[0076]

[Table 3]

[0077]

[Table 4]

Test No. A1 to A15 are steel sheets manufactured according to the present invention, and it is clear that the strength, creep characteristics, and toughness are far superior to the comparative steel. In particular, when a steel having a chemical composition satisfying the present invention is compared by a manufacturing method, the steel sheet according to the method of the present invention has the same chemical composition as the steel sheet manufactured according to the comparative example, but has the same strength and creep characteristics. , Toughness is significantly improved in any or all of them, and the superiority of the present invention is apparent.

Test No. of Comparative Example Since B1 to B11 do not satisfy the requirements of the present invention, their properties are inferior to those of the steel of the present invention. The reason for this will be described below. Test No. B1
Has a chemical composition that satisfies the present invention, but is manufactured by a normal normalizing process, and thus is compared with a steel plate manufactured by the method of the present invention using the same steel piece (test No. A4). Inferior characteristics. In particular, the creep characteristics are remarkably inferior.

Test No. B2 also satisfies the present invention in terms of chemical composition, but is manufactured by normal pattern normalization processing in which only the heating temperature is increased and appropriate holding at the cooling stage and control cooling processing is not performed. Compared with the steel sheet manufactured by the method of the present invention using a billet (test No. A5), the formation of δ ferrite became remarkable, and the proper control of the precipitates was not performed, so that the strength, creep strength and toughness were obtained. Are all inferior.

Test No. Test No. B3 was also tested. Like B2, the chemical composition satisfies the present invention, but since only the heating temperature is increased, and the production is performed by normal pattern normalization processing that does not perform appropriate holding or controlled cooling processing in the cooling stage, Compared with the steel plate manufactured by the method of the present invention using the same steel slab (Test No. A6), the formation of δ ferrite was remarkable, and the proper control of the precipitate was not performed. Poor toughness properties.

Test No. B4 is the test No. Like B1, the chemical composition satisfies the present invention, but since it is manufactured by a normal normalizing process, the steel plate (test No. A7) manufactured by the method of the present invention using the same steel slab is used. In comparison, the strength and creep strength are inferior.

Test No. B5 employs the same manufacturing method as in claim 1 in which holding is performed during cooling during normalization. However, since the holding temperature is too low, a coarse structure or a coarse precipitate is generated during holding. Therefore, the characteristics are significantly deteriorated.

Test No. B6 is subjected to the same slow cooling treatment as in claim 2 for proper dispersion of precipitates during cooling during normalization. However, since the cooling rate in the control cooling stage is excessive, the control The expected effect of cooling is not exhibited, and the process is simply the same as normalization by high-temperature heating, so that the characteristics cannot be improved.

Test No. Since the steel compositions of B7 to B11 have a chemical composition outside the scope of the present invention, good properties are not obtained despite the manufacturing method according to the present invention.

That is, the test No. Since B7 contains excessive C, the deterioration of toughness is particularly remarkable.

Test No. B8 has an excessive amount of Si,
Although the normalization is maintained during cooling, the suppression of δ ferrite is not sufficient, and the creep characteristics and toughness are also insufficient.

Test No. Since B9 does not contain both Mo and W, the heat resistance of the matrix is not sufficient, and the creep strength cannot be improved regardless of the production method.

Test No. B10 and B11 are examples in which good characteristics cannot be obtained because Mo or W is excessively contained.

From the above examples, the steel sheets produced by the method of the present invention all have superior high-temperature strength, creep characteristics and toughness as compared with the comparative examples, and the effects of the present invention are clear.

[0091]

As described above, the present invention provides a high Cr heat resistant steel having excellent toughness as well as strength and creep characteristics by maximizing the effect on alloy elements and precipitate materials. It enables production, is very useful as a structural material for high-temperature equipment, and has extremely high industrial value.

Claims (5)

[Claims] C .: 0.03 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.10 to 2.0%, Al: 0.002 to 0. 1%, N: 0.005 to 0.1%, Cr: 8 to 13%, Mo: 0.5 to 2.0%, W: 0.5 to 4.0% A steel slab containing one or two types, the balance being Fe and unavoidable impurities, is heated to 1000 ° C. to 1300 ° C., and hot-rolled at a cumulative reduction ratio of 30% or more and 90% or less to 800%.
Starting in a temperature range of not less than 1250 ° C. and not more than 1250 ° C., ending the hot rolling at a temperature of not less than 700 ° C., and after cooling to not more than 300 ° C., reheating to a temperature of not less than 1150 ° C. and not more than 1300 ° C., 1000 ° C at a cooling rate of 1 ° C / min or more
Cool to a temperature range of 700 ° C., and in this temperature range for 10 minutes to 120
After holding for 1 minute, 0.1 ° C / s to 50 ° C up to 300 ° C or less
/ S at a cooling rate of 600 ° C. or more and Ac
A method for producing a high Cr heat resistant steel having excellent low temperature toughness and creep strength, characterized by tempering at a temperature lower than one transformation point. 2. C: 0.03 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.10 to 2.0%, Al: 0.002 to 0. 1%, N: 0.005 to 0.1%, Cr: 8 to 13%, Mo: 0.5 to 2.0%, W: 0.5 to 4.0% A steel slab containing one or two kinds, the balance being Fe and unavoidable impurities, is heated to 1000 ° C. to 1300 ° C., and hot-rolled with a cumulative draft of 30% or more and 90% or less is performed.
Starting at a temperature range of not less than 1250 ° C. and not more than 1250 ° C., ending the hot rolling at a temperature of not less than 700 ° C., cooling to not more than 300 ° C., and reheating to a temperature of not less than 1150 ° C. and not more than 1300 ° C. 900 ° C to 850 ° C at a cooling rate of at least ° C / min
To a temperature range of 800 ° C. to 70 ° C.
After cooling to 0 ° C. at a cooling rate of 0.1 ° C./min to 2 ° C./min, cooling to 300 ° C. or less at a cooling rate of 0.1 ° C./s to 50 ° C./s. A method for producing a high Cr heat resistant steel having excellent low temperature toughness and creep strength, characterized by tempering at a temperature lower than the transformation point. 3. The steel slab further comprises, by weight%, V: 0.05 to 0.50%, Nb: 0.01 to 0.20%, Ta: 0.02 to 0.40%, Ti: 0. 3. Low temperature toughness and excellent creep strength according to claim 1 or 2, characterized by containing one or more of 0.005 to 0.10% and Zr: 0.005 to 0.10%. Manufacturing method of high Cr heat resistant steel. 4. The steel slab further comprises, by weight: Ni: 0.05 to 3.0%, Cu: 0.05 to 1.5%, Co: 0.05 to 5.0%, B: 0. 0.0005% to 0.01%.
4. The method for producing a high-Cr heat-resistant steel having excellent low-temperature toughness and creep strength according to any one of Items 3 to 3. 5. The steel slab further comprises, by weight%, Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM: 0.005 to 0.10%. The method for producing a high Cr heat-resistant steel excellent in low-temperature toughness and creep strength according to any one of claims 1 to 4, further comprising at least one kind or two or more kinds.