JPH08232015A - Method for producing high Cr ferritic stainless steel - Google Patents

Abstract

PURPOSE: To produce a high Cr ferritic stainless steel having excellent toughness in the state of final products and also free from surface wrinkling in the state of hot extruded products. CONSTITUTION: At the final pass at the time of applying hot plastic working to a high Cr ferrtic stainless steel stock containing, by weight, 25.0-31.0% Cr, 0.5-5.0% Mo, <=4.0% Ni, <=0.010% C, and <=0.020% N or further containing <=1.0% Nb, the steel stock is heated and held at 800-900 deg.C for >=10min and successively forged or rolled at >=20% draft. Further, in succession to the thermomechanical treatment, solution heat treatment and/or extrusion is applied.

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 high Cr ferritic stainless steel material which has excellent toughness in the final product and has no surface wrinkles in the product after hot extrusion.

[0002]

2. Description of the Related Art As a highly corrosion-resistant ferritic stainless steel, a high-purity high-Cr ferritic stainless steel in which the contents of C and N have been reduced as much as possible has been developed and has become popular. For example, in addition to being used in petrochemical plants, heat exchangers of power plants, pollution prevention plants, and the like, in recent years, they have also attracted attention as building exterior wall materials and semiconductor manufacturing equipment materials.

Generally, ferritic stainless steel has 8
When heated and held in the temperature range of 00 to 900 ° C, σ phase and χ
Intermetallic compounds containing Fe, Cr, and Mo as main components such as phases are precipitated and the toughness is reduced. Further, when heated and held in the temperature range of 450 to 550 ° C., a brittleness phenomenon known as 475 ° C. brittleness is exhibited. Therefore, such a material is used after being subjected to solution treatment at a high temperature of 1000 ° C. or higher and then rapidly cooled.

However, since ferritic stainless steel grows grains faster than austenitic stainless steel,
The resulting material is, for example, ASTM GS No. It becomes coarse grain of <2 and becomes a cause of generation of surface wrinkles in the subsequent hot working or cold working. As a result, the number of man-hours required for surface wrinkles increases, and the yield decreases.

Various proposals have hitherto been made in order to eliminate the above-mentioned drawbacks and improve the toughness. For example, Japanese Patent Publication Sho 57-
No. 38655 discloses thermomechanical treatment conditions for preventing ridging of a high purity high Cr ferritic stainless steel sheet, and Japanese Patent Publication No. 58-19725 discloses improvement of toughness of a high purity high Cr ferritic stainless steel sheet. The thermo-mechanical treatment conditions for this are shown. All of these are aimed at obtaining a uniform completely recrystallized structure, and the following 2
The hot rolling conditions and the heat treatment temperature are limited so as to satisfy the conditions.

(1) The hot rolling temperature and its reduction rate for randomly forming a large number of latent recrystallization nucleation points in the processed material. (2) Intermediate annealing conditions for supplying thermal energy sufficient to drive most of the latent recrystallization nuclei at the same time, but to stop the growth of recrystallized grains when they contact each other.

On the other hand, N represented by Alloy 718
In the case of the i-based alloy, for example, “Proc.
ys 718,625 and Various De
rivatives (1991) P. As shown in 913 ", grain refinement by forging utilizing the in-phase pinning effect in the δ phase precipitation region has been reported. However, regarding ferritic stainless steel, there is concern about high temperature embrittlement and thermal stress cracking. , No such attempt was made.

[0008]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The inventions described in Japanese Patent No. 655 and Japanese Patent Publication No. 58-19725 are premised on the control of what can not be directly confirmed by an optical microscope or the like, which is a latent recrystallization nucleus, in the rolling of a plate material, and it is continuous like a steel pipe and a steel bar. It is difficult to apply to bulk-shaped products that cannot be processed and heat-treated.

In view of the above situation, the present invention can be easily applied to a bulk shape such as a steel pipe or a steel bar which cannot be continuously worked and heat treated, has excellent toughness in a final product, and has a hot workability. High C without surface wrinkles on the product after extrusion
The present invention provides a method for producing an r-ferritic stainless steel material.

[0010]

In order to achieve the above object, the present inventor has conducted earnest research on conditions for precipitation of intermetallic compounds in high Cr ferritic stainless steel and conditions for thermomechanical treatment for grain refinement. . As a result, the inventors have found the range of components for precipitation, the heat treatment conditions before processing, the processing conditions for atomizing the precipitates, and the final heat treatment conditions for maintaining the fine grain structure.

Explaining in more detail, F called the χ phase
Cr and Mo are essential components for the purpose of precipitating an intermetallic compound containing e, Cr and Mo as main components. In addition, as a result of making and examining the TTT diagram of the component shown in FIG. 1, it was found that intermediate heating at 800 to 900 ° C. is required to precipitate this phase before the final pass of hot working. did. Furthermore, to obtain a product with sufficient toughness, χ
The phases were allowed to form a solid solution, but at the same time final heat treatment conditions were determined in order to prevent coarsening. The present invention has been completed as follows based on the above findings.

(1) Cr: 25.0-3 by weight%
A high Cr ferritic stainless steel material containing 1.0%, Mo: 0.5 to 5.0%, Ni: 4.0% or less, C: 0.010% or less, N: 0.020% or less. A method for producing a high Cr ferritic stainless steel material, which comprises heating and holding at 800 to 900 ° C. for 10 minutes or more and then forging or rolling at a working rate of 20% or more in a final pass during hot working.

(2) After being heated and held at 800 to 900 ° C. for 10 minutes or more, it is subsequently reheated to 800 to 950 ° C. and subjected to forging or rolling at a working rate of 20% or more (1). Item 5. A method for producing a high Cr ferritic stainless steel material according to item.

(3) After heating and holding at 800 to 900 ° C. for 10 minutes or more, and subsequently performing forging or rolling at a working rate of 20% or more, when the temperature of the steel material is lowered to 700 ° C., it is reheated to 800 to 950 ° C. The method for producing a high Cr ferritic stainless steel material according to the item (1), which is characterized by heating and continuing forging or rolling to finish.

(4) The high Cr ferritic stainless steel material contains Nb: 1.0% or less in addition to the chemical components described above, (1), (2) and (3). The method for producing a high Cr ferritic stainless steel material according to 1.

(5) In the final pass, the processing rate is 2
After subjecting to 0% or more forging or rolling, 900-105
The method for producing a high Cr ferritic stainless steel material according to the above items (1), (2), (3) and (4), characterized by heating and holding at 0 ° C. for 1 to 30 minutes and cooling. .

(6) In the final pass, the processing rate is 2
After subjecting to 0% or more forging or rolling, 900-105
Item (1), item (2), item (3) and item (4), characterized in that the material is heated and held at 0 ° C. for 1 to 30 minutes and subjected to hot extrusion at an extrusion ratio of 3 or more and cooled. The method for producing a high Cr ferritic stainless steel material according to 1.

[0018]

As shown in FIG. 2, the thermo-mechanical heat treatment pattern of the present invention is 3 corresponding to (A) corresponding to claim 1, (B) corresponding to claim 2, and (C) corresponding to claim 3. Divided into patterns. The heating temperature a at that time is 800 to 900 ° C.
The holding time is 10 minutes or more. This temperature setting is shown in Fig. 1.
As shown in the TTT diagram of the high Cr ferritic stainless steel shown in Fig. 3, it is decided to heat in the χ phase precipitation range and perform hot working in the χ phase precipitation state. To do that, T
From the TT diagram, it can be seen that it is necessary to heat and hold in the temperature range of 800 to 900 ° C. for 10 minutes or more. Then, even if the holding time is longer than 120 minutes, the precipitation effect is saturated and does not change. Therefore, it is desirable to keep 120 minutes as the upper limit and to keep the holding time within that range. The hot working in the χ phase precipitation state is to utilize the prevention of crystal grain coarsening due to the χ phase pinning effect.

The steel material brought into the χ phase precipitation state by the heating is subsequently subjected to hot working such as forging and rolling, and the working is finished at a minimum of 700 ° C. For large materials, heat to the heating temperature a, and
After soaking in the temperature range b of 50 ° C., hot working is performed. Then, in the case where the work heat treatment pattern of FIG. 2A cannot be finished, as shown in FIG.
When the temperature is lowered to near 0 ° C (indicated by symbol c), it is reheated in the middle of processing and soaked in the temperature range b'of 800 to 950 ° C to continue the processing and finish while the temperature is lowered to 700 ° C.

The steel materials obtained by the three patterns of thermomechanical treatment are semi-finished products, as shown in FIG.
As shown in (B) and (C), each processing heat treatment is followed by 1 to 30 in a temperature range e of 950 to 1050 ° C.
The solution heat treatment is carried out by heating for minutes, and the parts such as valves and nozzles are machined and finished. In the above, the case where the processing end temperature c is reheated to the solution treatment temperature range e is shown. However, depending on the object, after the processing heat treatment is finished, it is once cooled to room temperature, and the solution treatment is performed alone at a later date. Sometimes I do it. In addition, as shown in FIGS. 4A, 4B, and 4C, after each processing heat treatment is finished, it is reheated and heated and held in a temperature range e ′ of 950 to 1050 ° C. for 1 to 30 minutes. Extruded at an extrusion ratio of 3 or more to finish into steel pipes and steel bars. In the above description, after the processing heat treatment is finished, it is once cooled to room temperature and reheated to the extrusion processing temperature range e ′. However, the processing end temperature c is continuously reheated to the extrusion processing temperature range e ′. It can also be extruded. Further, it is desirable that the cooling f after the solution treatment and the cooling f ′ after the extrusion processing are rapidly cooled at a cooling rate of 200 ° C./min or more by water quenching, spray cooling or the like.

By subjecting the high-purity, high-Cr ferritic stainless steel to the heat treatment as described above, a steel material having excellent toughness can be obtained, and the hot-extruded product is of high quality with no surface wrinkles. To be

Next, the reason for limiting the chemical composition of the steel of the present invention will be explained. Cr forms a stable passive film, is extremely effective in improving corrosion resistance, and is an essential element for χ phase precipitation. To fully impart these characteristics, the content of 25% or more is required. However, if it exceeds 31%, embrittlement phenomena peculiar to high Cr steel such as 475 ° C. embrittlement and σ phase embrittlement easily occur.
It was limited to the range of 31%.

Mo has a remarkable effect in improving corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance, and is an essential element for χ phase precipitation. In order to obtain these effects sufficiently,
It is necessary to contain at least 5%. However, if it exceeds 5.0%, the workability and mechanical properties are impaired, so the content is limited to the range of 0.5 to 5.0%.

Ni is effective in improving toughness and corrosion resistance against strong acids such as H ₂ SO ₄ and HCl, and has a maximum value of 4.0.
%, A sufficient effect can be obtained, and when it exceeds 4.0%, stress corrosion cracking susceptibility is enhanced and precipitation of brittle phases such as σ phase is promoted. Therefore, the content is limited to 4.0% or less. did.

C and N have a very small solid solution amount in high Cr ferritic stainless steel, and carbides such as Cr and Mo, nitrides or carbonitrides are easily formed on the grain boundaries. These grain boundary precipitates not only deteriorate the mechanical properties of the steel, especially the low temperature toughness, but also reduce the concentrations of Cr, Mo, etc. around these precipitates and also deteriorate the corrosion resistance. In consideration of this point, since C is more harmful than N, C is limited to 0.010% or less and N is limited to 0.020% or less.

The inclusion of Nb fixes C and N, which are harmful to corrosion resistance and toughness, and as a result, the characteristics are improved. However, excessive addition causes a deterioration in toughness due to solid solution of Nb in steel, so the content thereof is limited to 1.0% or less.
Note that Ti, Ta, and A, which have the same actions and effects as Nb,
It is also possible to add 1, V and the like.

The main component other than the above-mentioned components is Fe, which also contains unavoidable impurities. However, the present invention is intended for fine graining by hot working in the χ phase precipitation region, and other component elements can be added and contained as long as this objective is not impaired.

Further, the reason why the processing heat treatment conditions are limited will be described. In order to deposit a precipitate mainly containing the χ phase before the final pass in hot working, that is, forging or rolling, it is desirable to keep it at 800 to 900 ° C for 10 minutes or more, preferably 12
Heat in the range of 0 minutes or less. As can be seen from the TTT diagram of the high Cr ferritic stainless steel shown in FIG. 1, if the heating temperature is lower than 800 ° C. or higher than 900 ° C., the precipitation time becomes long and it is not practical. Also, the heating time is 10
If it is less than minutes, the amount of precipitation of the χ phase is insufficient and the effect of grain refining is small, while if it exceeds 120 minutes, the effect is saturated and it is uneconomical. Under this heating condition, although carbides and the like are simultaneously precipitated,
The precipitation-solid-solution behavior is almost equal to that of the χ phase and contributes to grain refinement. In order to fully utilize the pinning effect of these precipitation phases, it is necessary that the precipitation phase accounts for 20% or more of the area ratio before the final processing.

In the final pass in the hot working, the pinning effect of the precipitation phase is utilized, so that the working is performed mainly in the precipitation temperature range of the χ phase. If the processing temperature is lower than 700 ° C., the deformation efficiency is poor and there is a risk of cracking.
C is the lower limit of processing temperature. Further, when the temperature exceeds 950 ° C, the solid solution of the χ-phase precipitate starts and a sufficient pinning effect cannot be obtained, so 950 ° C was set as the upper limit of the processing temperature. In addition,
The cross-section reduction rate in the final pass is preferably 20% or more in order to sufficiently reduce the grain thickness of the entire workpiece.

If the thermo-mechanical treatment shown in FIGS. 2A to 2C is left as it is, undissolved precipitates remain and sufficient toughness cannot be obtained. The solution treatment shown is applied. The conditions for this solution treatment were a heating temperature of 950 to 1050 ° C. and a holding time of 1 to 30 minutes from the viewpoint of complete solid solution and prevention of coarsening. The particle size obtained under these conditions is A
STM GS, No. The target was ≧ 2. The cooling rate f after the solution treatment is 475 ° C. to prevent reprecipitation of χ phase and the like.
From the viewpoint of preventing embrittlement, 200 ° C / min or more is desirable. This cooling rate is realized by, for example, water quenching or spray cooling.

By subjecting a steel material obtained by forging or rolling according to the present invention to hot extrusion, a high-quality product without surface wrinkles can be finished. For that purpose, the processing conditions were a heating temperature of 950 to 1050 ° C. and a holding time of 1 to 30 minutes from the viewpoint of complete solid solution of precipitates such as χ phase and prevention of coarsening. The particle size obtained under these conditions is AS
TM GS, No. The target was ≧ 5. The cooling rate f'after the solution heat treatment was 475 ° C to prevent reprecipitation of χ phase and the like.
From the viewpoint of preventing embrittlement, 200 ° C / min or more is desirable.

[0032]

EXAMPLE As an example of the present invention, each 1 ton of the high Cr ferritic stainless steel test material 1 (Nb-free steel) and test material 2 (Nb-containing steel) shown in Table 1 was subjected to vacuum induction melting. It was melted in a furnace, and cast into 400 ^■ steel ingot. 1 of the steel ingot
Heated to 050 ℃ and forged (3 passes) under normal conditions 21
0 was ^■. It was then subjected to final forging 175 ^■ and (working ratio 31%) under the conditions shown in Table 2.

[0033]

[Table 1]

[0034] from the 175 ^■ forgings 174 ^■ × 68t
The billet of No. 1 was cut, and sample No. 9 except 9 and 16
Solution treatment was performed under the conditions of 80 ° C. × 5 minutes. Also, 1
Was heated at 000 ° C. × 5 minutes heating conditions, and immediately spray cooling after performing extrusion with 50 ^■ × 10t (extrusion ratio 18). In addition, the sample No. For comparison, sample No. 9 was processed at a solution treatment temperature e and an extrusion heating temperature e ′ of 1100 ° C., respectively. No. 16 was carried out at a solution treatment temperature e900 ° C. and was not extruded.

[0035]

[Table 2]

A test piece was cut out from each of the above samples, and a particle size determination, a Charpy impact test and a surface wrinkle observation determination were performed. The Charpy impact test was carried out at 0 ° C. with a JIS No. 1/2 size test piece. The surface wrinkles were evaluated by shaving the outer surface of the extruded steel pipe by 0.1 mm and checking for wrinkle residue at that time. The results of these tests are shown in Table 2 above.

Of the above samples, No. 1, 4, 7, 10, 1
Nos. 2 and 14 are examples of the present invention, and the other samples are temperatures which are limited in the present invention by any one of the heating temperature during thermomechanical processing, the heating temperature during soaking, and the finishing temperature during hot working. It is out of range. From the test result, the heating temperature was out of the temperature range limited by the present invention, and No. 2, 5, 6, 8, 11, 13, 15 are
In both cases, the structure is coarse and the toughness is insufficient.
In addition, the sample No. 1 having a low machining finishing temperature c. For No. 3, cracking occurred during forging, and the subsequent tests were stopped. On the other hand, it can be seen that the above-mentioned examples of the present invention are all finely divided and have excellent toughness.

Further, as a result of the surface wrinkle observation test, in each of the examples of the present invention, a high quality steel pipe having no surface wrinkles was obtained, whereas in other samples, surface wrinkles were generated. Sample No. In No. 9, the solution treatment temperature and the extrusion processing temperature were both too high. As a result, the toughness was low and the surface wrinkles were generated. In addition, the sample No. No. 16 was the case where the solution treatment temperature was low, but the toughness was remarkably inferior to the other ones.

[0039]

According to the present invention, high C after solution treatment
The r-ferritic stainless steel material has excellent toughness, and the extruded steel pipe and bar steel can be manufactured with high quality without surface wrinkles.

[Brief description of drawings]

FIG. 1 is a TTT diagram of a sample material 1 of an example.

FIG. 2 is a diagram showing a heat pattern of the work heat treatment of the present invention, wherein (A) is the case of claim 1 and (B) is the case of claim 2;
In the case of, (C) is the case of claim 3.

FIG. 3 is a diagram showing a heat pattern in the case where the solution heat treatment is carried out subsequent to the thermomechanical treatment in the case where the solution heat treatment is applied to the case of claim 5, and (A) is the case where the solution heat treatment is added to the pattern of claim 1. , (B) is the case where the solution treatment is applied to the pattern of claim 2, and (C) is the case where the solution treatment is applied to the pattern of claim 3.

FIG. 4 is a diagram showing a heat pattern in the case where extrusion processing is performed subsequent to the heat treatment in the case of claim 6, (A) shows a case where extrusion processing is added to the pattern of claim 1, B) is the case where extrusion processing is added to the pattern of claim 2, and (C) is the case where extrusion processing is added to the pattern of claim 3.

[Explanation of symbols]

 a Heating temperature range of working heat treatment b, b'Temperature range of soaking heat treatment c Lower limit temperature of hot working d Cooling e Heating temperature range during solution treatment e'Heating temperature range during extrusion f, f'Cooling

Claims (6)

[Claims] 1. Cr: 25.0 to 31.0 in% by weight.
%, Mo: 0.5 to 5.0%, Ni: 4.0% or less,
In the final pass when hot working a high-Cr ferritic stainless steel material containing C: 0.010% or less and N: 0.020% or less, after heating and holding at 800 to 900 ° C. for 10 minutes or more, it is continued. A method for producing a high Cr ferritic stainless steel material, characterized by performing forging or rolling at a working rate of 20% or more. 2. The method according to claim 1, wherein after heating and holding at 800 to 900 ° C. for 10 minutes or more, it is subsequently reheated to 800 to 950 ° C. and forged or rolled at a working rate of 20% or more. A method for producing a high Cr ferritic stainless steel material. 3. When the steel material temperature is lowered to 700 ° C. during the process of forging or rolling at a working rate of 20% or more after heating and holding at 800 to 900 ° C. for 10 minutes or more, 800 to 950 ° C. The method for producing a high-Cr ferritic stainless steel material according to claim 1, wherein the hot-working is continued and the hot working is finished. 4. The high Cr ferritic stainless steel material according to claim 1, wherein the high Cr ferritic stainless steel material contains Nb: 1.0% or less in addition to the chemical components. Production method. 5. In the final pass, after forging or rolling with a working rate of 20% or more, heating and holding at 900 to 1050 ° C. for 1 to 30 minutes and cooling are performed. 3. The method for producing a high Cr ferritic stainless steel material described in 3 or 4. 6. In the final pass, after forging or rolling with a working rate of 20% or more, heating and holding at 900 to 1050 ° C. for 1 to 30 minutes and hot extrusion at an extrusion ratio of 3 or more and cooling. Claims 1 and 2, characterized in that
3. The method for producing a high Cr ferritic stainless steel material described in 3 or 4.