JPH027369B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for manufacturing an austenitic stainless steel pipe material having a fine crystal structure, excellent corrosion resistance at high temperatures, and high high temperature creep strength. [Prior art] So-called stabilized austenitic stainless steels such as SUS321H and SUS347H generally have excellent high-temperature properties and are therefore often used as high-temperature strength members used for long periods in corrosive environments such as boiler tubes for thermal power generation. . The required performances of these steels include not only workability and weldability, but also high-temperature creep strength and high-temperature corrosion resistance. However, in general, means for improving both of these characteristics are often contradictory. For example, steam oxidation resistance improves as the crystal grain size decreases, but creep strength decreases as the crystal grain size decreases. In a boiler tube, if the internal surface has insufficient steam oxidation resistance and the internal scale easily peels off, the tube will become blocked and the area will become high temperature.
In addition to a substantial decrease in strength, the thinning of the outer surface due to high-temperature corrosion is promoted, making it more likely that problems such as pipe blowouts will occur. Steam oxidation resistance is
There is no problem if it is a fine grain with an ASTM grain size number of 7 or higher, but if the grain size is around this level, the high temperature strength may not reach the design standard. Furthermore, although the addition of Cr is effective in improving corrosion resistance at high temperatures, it deteriorates the structural stability and promotes the formation of phases such as σ phase that are harmful to creep strength. Furthermore, adjusting alloying elements or adding special ingredients not only increases costs, but it is also necessary to consider the effects on other properties such as workability and weldability. This is hardly an advantageous solution. Therefore, it is necessary to try to solve this problem within the conventional range of ingredients, and one method is to cold-work the inner surface of the boiler tube by shot peening, etc., to make only the surface layer fine. This is proposed in Japanese Patent Application Laid-Open No. 58-39733. However, this method may also lose its effectiveness due to grain growth caused by annealing performed after welding during boiler assembly. As described above, it is technically quite difficult to obtain an austenitic stainless steel that simultaneously satisfies high-temperature strength and high-temperature corrosion resistance. However, in the future, the operating conditions for heat engines such as boilers will tend to be higher in temperature and pressure in order to achieve higher efficiency, and the environment in which materials are used is expected to become even more severe. As a method for manufacturing stainless steel boiler tubes having a fine grain structure and excellent high-temperature strength, for example, a method described in JP-A-58-87224 has been proposed. This method is C: 0.06-0.09%, Si: 0.30
~0.90%, Mn: 0.5~2.0%, Ni: 9.00~13.00%,
Cr: 17.00~20.00%, Nb: 8 x C% + 0.03%~
Contains 1.0%, N: 0.040-0.080% as necessary
Austenitic stainless steel billet containing
After hot pressing at 1,100 to 1,300°C, cold working is performed by 10% or more, followed by heating and rapid cooling at 1,120 to 1,250°C to produce boiler tubes. However, since the cooling rate is not specified in this method, the precipitates may become coarse in some cases, and the effect of suppressing crystal grain growth may be insufficient. Furthermore, if the final solution temperature becomes higher than the temperature in the previous step, solid solution of the precipitate occurs again, and crystal grains become significantly more likely to grow. Furthermore, a method described in Japanese Patent Application Laid-open No. 167726/1983 has also been proposed. This method uses Ti: 0.15~0.5wt
%, Nb: In the cold working process of austenitic stainless steel containing one or two types of 0.3 to 1.5 wt%, after heating and cooling with the final softening temperature set at 1100 to 1350 °C, 20% or more Adding cold processing of
This is followed by a final heat treatment in which the tube is heated to 1070 to 1300°C, at least 30°C lower than the final softening temperature, and cooled at a cooling rate higher than that of air cooling, thereby producing boiler tubes. Since this method requires cold working at least three times, the process becomes complicated and the manufacturing cost becomes very high. [Problem to be solved by the invention] The steam oxidation resistance of austenitic stainless steel improves as the grain size becomes smaller. Therefore, in order to obtain fine-grained steel, the final solution temperature must be higher than the recrystallization temperature. The lower the better. On the other hand, in order to improve high-temperature creep strength, it is better to incorporate as many MC-type carbide-forming elements such as Nb and Ti as possible into the base material, so the higher the final solution temperature, the better. in this way,
The means for satisfying steam oxidation resistance and the means for satisfying high temperature creep strength are contradictory. The present invention aims to provide a method for manufacturing a high-temperature austenitic stainless steel pipe material that can sufficiently secure high-temperature creep strength through high-temperature solution treatment, and also has steam oxidation resistance due to fine grain steel. It is. [Means for solving the problem] The present invention contains 0.04 to 0.10% C by weight, and one or both of Nb and Ti (Nb+Ti) is 1.0% or more twice the C content. The following austenitic stainless steel slabs are heated to form a solid solution of carbides, and then the average cooling rate to 500℃ is 0.3
For high-temperature austenitic stainless steel pipes, which are cooled to less than ℃/sec., hot extruded at 1230℃ or higher, and cooled under conditions where carbides do not precipitate or become fine carbides even if they precipitate. The gist is the manufacturing method of the material. As mentioned above, the austenitic stainless steel referred to in the present invention refers to the so-called stabilized austenitic stainless steel, including SUS321H,
Any material that complies with the component range specified by JIS such as SUS347H can be covered by the present invention. After the target austenitic stainless steel slab is subjected to the solution heat treatment, it is heated and hot extruded without being subjected to processing such as blooming, so comparisons made by continuous casting etc. This is a slab with a small cross section. Cooling after hot extrusion is preferably carried out at an average cooling rate of 0.2°C/sec. or more up to 500°C. The material according to the present invention, which has been hot extruded and cooled, is descaled by known means, cold worked, and solution heat treated to produce a product. Cold working can be carried out to the final product size without intermediate heat treatment or with intermediate heat treatment. The final solution heat treatment after cold working is
It is desirable to conduct the process at 1200°C or higher and to perform rapid cooling to prevent carbide precipitation. [Function] First, in the present invention, the range of components for C, Nb, and Ti is limited so that at least one of NbC and TiC can be precipitated during the manufacturing process and in the final product. In the case of composite addition, the atomic ratio of Nb/Ti is preferably 1. In this case, C is an indispensable element to ensure high-temperature strength and requires at least 0.04%, but on the other hand, if too much is added, Cr is consumed by forming Cr carbides, which reduces corrosion resistance. Therefore, the upper limit was set at 0.10%. Nb and Ti are high-temperature strengthening elements, and in order to ensure high-temperature strength, it is necessary to add at least 2×C% or more, but adding large amounts may deteriorate weldability and workability, and increase costs. upper limit to
It was set at 1.0%. Heating the slab is to dissolve the network-like giant carbonitrides generated during casting into the base material, and this treatment increases the amounts of Nb, Ti, and C, which are involved in high-temperature strength, and improves the creep strength of the product. let In the present invention, cooling of the slab after heating is performed by
Because the average cooling rate to 500℃ is less than 0.3℃/sec., relatively large carbides precipitate. Therefore, by performing the subsequent hot extrusion process at 1230℃ or higher, the carbides are redissolved. Cooling after hot extrusion is carried out under conditions where carbide does not precipitate or even if it precipitates, it becomes fine carbide. The preferred conditions at this time are that the material has a smaller cross-section than the slab, so the average cooling rate up to 500℃ is
0.2℃/sec. or more. When the material thus obtained after hot extrusion was cold worked and then subjected to solution heat treatment, almost no carbides of Nb and Ti were precipitated during cooling after hot extrusion. In this case, Nb,
Since fine carbides of Ti precipitate uniformly, recrystallization is delayed, and fine recrystallized grains can be obtained even when high-temperature solution heat treatment is performed. In addition, if fine carbides of Nb and Ti precipitate during cooling after hot extrusion processing, similarly fine recrystallized grains will be formed by the action of these fine carbides during solution heat treatment after cold working. is obtained. In solution heat treatment after cold working, whether cold working is performed in one step without intermediate heat treatment or in multiple steps with intermediate heat treatment in between, final solution heat treatment is required. The higher the temperature, the more the amount of solid solution of Nb, Ti, and C increases, and by subsequently performing rapid cooling to prevent carbide precipitation, a product with high high-temperature creep strength can be obtained. In the case of the material obtained by the present invention, as mentioned above, precipitation may occur during the temperature increase during solution heat treatment after cold working, or uniformly dispersed fine particles that exist before the heat treatment may precipitate. Since recrystallization is delayed by the action of carbides, even if the final solution heat treatment is performed at a high temperature that increases the amount of solid solution of Nb, Ti, and C, coarsening of crystal grains will not occur as in conventional methods. This does not occur, and fine recrystallized grains are obtained. Therefore, with the material obtained by the present invention, an austenitic stainless steel pipe with high high temperature creep strength, fine crystal grains, and excellent steam oxidation resistance can be obtained. [Example] The test materials were three steel types, S, T, and U, with chemical compositions shown in Table 1, all of which are target steels of the present invention. S, T
are SUS347H with components within JIS standards, respectively.
SUS321H and U are Nb and Ti composite added steels. For these three steel types, steel pipes with an outer diameter of 50 mmφ and a wall thickness of 8 mm were manufactured by the manufacturing process shown in FIG. In FIG. 1, a shows a conventional example, and b and c show an example of the present invention. Continuously cast slabs are heated to 1,300℃ to dissolve the network-like giant carbonitrides, and then furnace-cooled (500℃).
The average cooling rate was 0.08°C/sec.). Then,
Heating, hot extrusion processing at each temperature shown, air cooling (average cooling rate 2°C/sec. up to 500°C), descaling, 30% cold drawing, solution heat treatment at 1200°C, and water cooling. (Average cooling rate 100℃ to 500℃/
sec.) did. However, in case c, the cold drawing process was performed twice, including an intermediate heat treatment. Each sample material S11 to S15 after final solution heat treatment,
Creep rupture tests were conducted at 650℃ and 750℃ using specimens cut from T11 to T13 and U12 to U13, and the 10 ⁵ hr creep rupture strength was determined by extrapolating from the average value of the results. It is shown in Table 2 along with the particle size. SO and TO in Table 2 are ASME
Tp347H and
This is the standard value for Tp321H steel. All steel pipes manufactured from the material obtained according to the present invention have a fine grain structure with a grain size number of 7 or more and have good steam oxidation resistance. The creep strength of S11 and T11 manufactured by the conventional method is
Although it satisfies ASME standard values, the crystal grains become coarse during the solution heat treatment after cold drawing, and the steam oxidation resistance is poor. Although the steel pipes manufactured from the material obtained by the present invention all have fine grains with a grain size of No. 7 or more, in the case of S steel (SUS347H), the grain size is equal to or higher than the grain size of No. 4.7 made by the conventional method.
Steel (SUS321H) exhibits creep rupture strength equivalent to or higher than grain size No. 3.5 obtained by conventional method.
It far exceeds the 10 ⁵ hr breaking strength calculated from ASME's allowable tensile stress value. Furthermore, steel pipes manufactured from U steel to which the present invention is applied also have a grain size of No. 7.2 to 7.3.
Despite having a fine grain structure, it has a creep rupture strength equal to or higher than that of SUS347H with a grain size of No. 4.7 produced by the conventional method, and exceeds the ASME allowable tensile stress conversion value. Regarding FIGS. 1b and 1c of the method of the present invention, when water cooling was performed after hot extrusion, almost the same results as in the case of air cooling were obtained.

【table】

【table】

〔Effect of the invention〕

According to the material obtained by the present invention, after cold drawing, the final solution heat treatment, which is the same as the conventional method,
Since it is possible to fully dissolve MC carbide in the matrix and obtain a fine grain structure, the creep rupture strength is equal to or higher than that of the conventional method, and the austenitic type has good steam oxidation resistance. Since it can be used to manufacture stainless steel pipes, it has great industrial benefits.

[Brief explanation of drawings]

FIG. 1 shows an example, in which a is a conventional example and b and c are examples of the present invention.

Claims (1)

[Claims] 1. Contains 0.04 to 0.10% C by weight, and contains Nb and
An austenitic stainless steel slab containing one or two types of Ti (Nb+Ti) with a content of at least twice the C content but not more than 1.0% is heated to solidify the carbides.
It is characterized by cooling at an average cooling rate of less than 0.3°C/sec., hot extrusion processing at 1230°C or higher, and cooling under conditions where carbide does not precipitate, or even if it precipitates, it becomes fine carbide. A method for manufacturing high-temperature austenitic stainless steel pipe material. 2. A method for producing a high-temperature austenitic stainless steel pipe material according to claim 1, characterized in that cooling after hot extrusion is performed at an average cooling rate of 0.2°C/sec. or more up to 500°C. .